simposium 3. growth, caracterization and applications of thin films

simposium 3. growth, caracterization and applications of thin films

42
SIMPOSIUM
3.
GROWTH,
CARACTERIZATION
APPLICATIONS OF THIN FILMS AND COATINGS
AND
ORAL
S3.O1. ZnTe FILMS GROWN BY ISOTHERMAL CLOSED
SUBLIMATION AND EPITAXY (ICSSE) ON GaAs SUBSTRATES
SPACE
O. de Melo ([email protected]), L. C. Hernández, L. Hernández, M. Sánchez, E. Sánchez, R.
Peña-Sierra*, G. Romero-Paredes* and F. Rábago- Bernal+
Physics Faculty, University of Havana, Colina Universitaria, 10400 La Habana, Cuba
*
Electrical Engineering Departament, CINVESTAV- IPN, Ave. IPN 2508 07300, Mexico D.F.
+
Physics Institute, UASLP, Mexico
A novel technique (previously used for the growth of CdSe films1) have been applied for
the epitaxial growth of ZnTe films on (100) GaAs substrates. A substrate of GaAs(100) is exposed
to Zn and Te elemental sources alternatively. The whole system is at the same temperature, for that
reason the only driving force for the film growth is the compositional difference between the
elemental source and the substrate or the previously grown film surface. This leads to a difference
of chemical potentials, in view of the vapor pressure differences between the pure element and the
element in the compound. At the initial stage of the exposure, the elemental source sees a
compound (GaAs or ZnTe) at the growing surface. We expect that once the growing surface is fully
covered with the atoms of this elemental source, the difference in vapor pressure becomes
negligible because both the source and the growing surface will be in equilibrium with the vapor of
the pure element. Next the substrate is shifted to the other source concluding a cycle when
equilibrium be reached at this second stage time. From this point of view, the growth is selfcontrolled and independent of the time of exposure. This procedure is reminiscent of Atomic Layer
Epitaxy (ALE) with the difference that this is a quasi-equilibrium technique while ALE is fully out
of equilibrium2. In order to have an estimation about the adequate growth conditions for ZnTe films
using the above procedure, we calculate the diffusion fluxes from the source to the growing surface.
These calculations are presented. As a consequence of these results the growth temperature and the
exposure time in each source were set at 380 oC and 3 s, respectively. ZnTe films obtained by this
procedure were epitaxial as reveled from x- Rays measurements and shown mirror like surfaces.
Estimation of the thickness using reflectance and ellipsometry measurements led to a growth rate of
around 2 ML per cycle. 10 K Photoluminescence measurements were done and near band gap
transitions were observed. We hope this technique has potential as a low cost alternative to grow
low temperature, thickness controlled thin II-VI semiconductor films.
1
O. de Melo, E. Sanchez, H. Rodriguez, S. De Roux, F. Rabago Bernal, J. Ruiz- Garcia, Mat.
Chem. Phys. 59, No. 2, (1999) pp. 120- 124
2
F. Hauzenberger, W. Faschinger, P. Juza, A. Pesek, K. Lischka, H. Sitter, Thin Solid Films 225,
265 (1993)
S3.O2. PHOTOLUMINESCENCE SPECTROSCOPY ON LPCVD SILICON
OXYNITRIDE THIN FILMS
A. Morales-Acevedo1([email protected]), J. Carrillo-López2, M. Aceves2,
G. Contreras3 and C. Mejía-García3
1
CINVESTAV-IPN, Dpto. Ingeniería Eléctrica, Avenida IPN No. 2508, México, D. F. 07360.
43
2
Instituto Nacional de Astrofísica y Optoelectrónica, Tonantzintla, Puebla.
Escuela Superior de Física y Matemáticas del IPN, Zacatenco, México, D. F. 07360.
*
Also at CIDS del Instituto de Ciencias de la Universidad Autónoma de Puebla.
3
Low pressure chemical vapor deposition (LPCVD) silicon oxynitride (SiON) films of
various compositions between SiO2 and Si3N4 were grown by changing the relative ratio (Ro) of
nitrous oxide (N2O) to silane (SiH4) pressures while keeping constant the ammonia (NH4) pressure.
The SiON films were deposited at 700 oC on p-type silicon substrates (with a carrier concentration
of 1015 cm-3), varying Ro from 0.32 to 1.38. We made photoluminescence spectroscopy
measurements at low temperature (9-10 ºK) and found luminescence in the range between 1.4 eV
and 2.4 eV with a peak around 2 eV. This is an interesting result since similar spectra has also been
observed in porous silicon and silicon oxide implanted with silicon suggesting a likely similar cause
for the photoluminescence in all these cases. We also discuss the effect that a thermal annealing at
700 ºC for 60 minutes in nitrogen has upon the photoluminescence.
S3.O3. OPTICAL AND ELECTRICAL PROPERTIES OF HYDROGENATED Gd
AND Dy FILMS
D. E. Azofeifa ([email protected]), N. Clark ([email protected])), J. Araya Pochet
([email protected])
CICIMA and Escuela de Física, Universidad de Costa Rica, San José, Costa Rica
Rare earths in general absorb hydrogen readily forming solid solutions and/or dyhydrides
and trihydrides. In this work the change in transmittance, electrical resistivity and Hall coefficient
of films exposed to an atmosphere of hydrogen is studied. The films thickness are between 230 and
270 nm, and they are prepared in a high vacuum chamber at 4x10-6 Torr base pressure. Since both
Gd and Dy are very reactive to nitrogen and oxygen they are coated with a thin Pd layer (20 nm)
through which hydrogen may pass. This overlayer enables us to perform ex-situ measurements. For
comparison some Gd films are made and hydrogenated in UHV without the Pd coating.
In order to make the optical and electrical measurements, films with different geometry’s
were evaporated simultaneously. Once the proper set ups for each measurement are made, hydrogen
is allowed in the chamber up to pressures of 6 Torr, simultaneously with the hydrogen concentration
the electrical and optical properties are monitored. Even with these small pressures both Gd and Dy
films absorb large amounts of hydrogen achieving concentrations of more than 2 hydrogen atoms
per metal atom. Concentrations are measured by means of a quartz crystal microbalance. Both
electrical and optical properties indicate that a transition to a semiconductor state, and possibly to an
insulator state, occurs when hydrogen concentration nears 2. In most films the electrical resistance
increases by a factor of order 5 and the optical transmittance (measured with a laser beam, λ=632.8
nm) increases by a factor of the order of 100. For Dy the Hall coefficient shows an evolution from
an electron dominated conduction, at zero and low concentrations, to a hole dominated one at high
hydrogen concentrations. In Gd the Hall coefficient shows a decrease as hydrogen is absorbed. In
the films created in the UHV chamber thermal induced desorption and thickness effects are
explored.
S3.O4. CHARACTERIZATION OF INHOMOGENEOUS THIN FILMS BY
ELLIPSOMETRY
R. Machorro ([email protected]), F. Villa1 ([email protected])
(samano@ccmc. unam.mx), and G. Soto ([email protected])
E.
Samano
44
CCMC-UNAM, Apdo. Postal 2732, Ensenada, B.C. Mexico.
(1)
CIO, ApdoPostal I-48 Leon Gto. Mexico
A film with a variable refractive index along its thickness usually occurs during the
deposition process. This fact has been used to fabricate rugate filters; i.e., an inhomogeneous film
made in purpose to perform in a predictable and desirable manner. The in-situ monitoring and
characterization of the film evolution is convenient and necessary to attain the requested optical
properties prior to deposition. A non intrusive and highly sensitive technique is essential to
determine the refractive index profile during synthesis, spectral ellipsometry arises as the right
choice. A trial and error method might be used to follow a given ellipsometric trajectory, but this is
a time consuming and risky procedure. To reduce the chances of guessing the solution, it is
recommended to record the film evolution by kinetic ellipsometry during fabrication.
In this work we present results on the characterization of thin films made by reactive laser
ablation using kinetic and spectral ellipsometry for sensing the film. A Si3N4 target is ablated with
an excimer laser (? =248nm), while O2 gas is introduced in the chamber. The gas pressure is
continuously varied to obtain the desired refractive index profile. The O2 gas pressure is changed
from 10-9 Torr, vacuum, to 10 mTorr. It is our interest to produce inhomogeneous films with a given
index profile, to be applied as rugate filters. This is why the refractive index should be precisely
related to the deposition parameters in order to satisfy our requirements.
S3.O5. MORPHOLOGY AND OPTICAL STUDIES OF HIGHLY Te-DOPED
GaInAsSb EPITAXIAL LAYERS
J. L. Herrera-Perez1,5, M. C. Rivera-Morales1, O. Zarate-Corona1, J. Mendoza-Alvarez2,5, L.
Gómez-Herrera2,3, M. A. Vidal4, H. Navarro-Contreras4, and M. Rojas-López4
1
Instituto de Física, Univ.Aut.de Puebla. Apdo. Postal J-48. Puebla, Pue. 72570. México
Depto. de Física. Cinvestav del IPN. Apdo. Postal 14-740. México DF 07000. México
3
Laboratorio de Optoelectrónica. Universidad del Quindío. Armenia, Q. Colombia
4
Instituto de Comunicaciones Opticas-UASLP. San Luis Potosí, SLP. México
5
CICATA-IPN. Legaria 694. Col. Irrigación. México, DF 11500
2
Quaternary InxGa1-xAsySb1-y semiconductor alloys are being studied because of their
application in medium infrared light emitting and detecting optoelectronic devices. In particular we
are in the process of fabricating IR photodetectors in the spectral region between 2 and 2.5 microns
to be used for quantitative detection of atmospheric pollutants.
Using the liquid phase epitaxy growth technique we have grown InGaAsSb epitaxial layers
on GaSb substrates, adjusting the stoichiometry of the quaternary alloy in such a way as to have an
energy bandgap in the active region of the photodetector of around 2.3 microns; also, we have used
Sb3Te2 pellets as the source of doping in order to have n-type material. In this way we obtained ntype InGaAsSb layers highly doped (~1020cm-3) with tellurium.
The composition of these InxGa1-xAsySb 1-y epitaxial layers was determined by an electron
dispersive spectroscopy (EDS) analysis from a JEOL scanning electron microscope. We show that
the inclusion of the highly Te doping changes the stoichiometry layer, as evidenced from the
photoreflectance spectra measured in these samples. We also present results on the analysis of high
resolution X-ray diffractograms measured in a double-crystal X-ray diffractometer in order to study
the possible presence of Te agglomerates due to the high Te doping. These samples have been also
characterized by Raman spectroscopy looking for the presence of the metallic Te signal, and also to
study the evolution of the Raman spectra as a function of the increasing Te doping. The results of
these morphological and optical characterizations are discussed and compared to those results
measured in samples non-intentionally doped (p-type) and lightly Te-doped (~1017cm-3).
45
POSTERS
S3.P1. ESTRUCTURAL AND MORPHOLOGICAL CHARACTERIZATION OF
CdSXTe1-X THIN FILMS DEPOSITED BY EVAPORATION
A. Dussán([email protected]),G. Cediel ([email protected]),
L.M.Caicedo([email protected]),G.Gordillo ([email protected])
Departamento de Física, Universidad Nacional de Colombia, Santafé de Bogotá.
CdSXTe1-X thin films with chemical composition varying between X=0 and X=1, deposited
by simultaneous evaporation of the CdS and CdTe precursors, from a evaporation source
constituted by two coaxial chambers, were characterized by x-ray diffraction and atomic force
microscopy (AFM).
The crystallographic phases present in each of the samples were identified with a high
degree of accuracy the crystallographic phases present in each of the samples, with the help of a
program developed specially for this purpose. Additionally the effect of the chemical composition
and deposition parameters on the lattice parameters and on the crystallographic phases, was
determined.
It was determined, through an AFM study, the effect of the chemical composition on the
morphology and grain size of the samples. The chemical composition was determined by EDS
analysis carried out with the microprobe of a SEM microscope.
S3.P2. INFLUENCE OF THIOUREA CONCENTRATION, DEPOSITION
TEMPERATURE AND MAGNETIC FIELD IN THE PROPERTIES OF CDS THIN
FILMS CHEMICALLY DEPOSITED
Y. Rodríguez-Lazcano(1,*) ([email protected]), O. Vigil(2,*) ([email protected]), F.
Cruz(2), and A. Arias(3)
1
Centro de Investigación en Energía, UNAM, Temixco, Morelos, México
Escuela Superior de Física y Matemáticas del Instituto Plitécnico Nacional, México D.F., México
3
Facultad de Química, Universidad de La Habana , Cuba
*Permanent address: Facultad de Física-IMRE, Universidad de La Habana, Cuba
2
The influence of a constant D.C. magnetic field, deposition temperature and thiourea
concentration in the properties of CdS films grown by chemical bath deposition (CBD) are
presented. The aim of this work is to find the optimal values of the above parameters in order to
obtain the highest value of the band gap in CdS-CBD. For solar cells applications using CdS as
window material with a band gap of 2.43 eV; photons below 510 nm are absorbed in the CdS films
and do not contribute to the photocurrent. In our case we report 2.6 eV for the CdS gap. From
optical and structural characterization this behavior is discussed.
S3.P3. PHOTOLUMINESCENCE AND RAMAN SCATTERING OF MG-DOPED
GAN FILMS GROWN BY MOCVD
G. Martínez-Criado ([email protected]), A. Cros ([email protected]), A. Cantarero
([email protected]), R. Dimitrov1 ([email protected]), M. Stutzmann1
([email protected])
46
Department of Applied Physics, Materials Science Institute, University of Valencia, Dr. Moliner 50,
E-46100 Burjasot (Valencia), Spain
1
Walter Schottky Institute, Technical University Munich, Am Coulombwall, E-85748 Garching,
Germany
Scanning electron microscopy, micro-Raman and photoluminescence measurements are
reported for Mg-doped GaN films grown on (0001) sapphire substrates by low-temperature metalorganic chemical vapor phase deposition. The surface morphology, structure and optical properties
of GaN samples with Mg concentration ranging from 1019 to 1021 cm-3 have been studied. In the
scanning micrographs large triangular pyramids are shown due to surface nucleation and threedimensional growth. The density and size of these structures increase with the amount of
magnesium incorporated in the sample. By micro-Raman measurements the presence of cubic
inclusions in the triangular structures was detected. In the photoluminescence spectra from the
triangular hillocks, intense lines were found at 3.36 and 3.31 eV These transitions are significantly
below the hexagonal GaN band-gap (3.50 eV) and slightly above the cubic GaN one (3.30 eV). The
excitation and temperature behavior enable us to identify its excitonic nature. We propose that these
emissions are the result of carrier recombination in the confined cubic-phase inclusions inside the
hexagonal matrix.
S3.P4.
PREPARATION
AND
STUDY
OF
OPTOELECTRIC
AND
CRISTALOGRAPHIC PROPERTIES OF ZnO THIN FILMS DEPOSITED BY
REACTIVE EVAPORATION
C. Calderón, G. Gordillo ([email protected])
Departamento de Física, Universidad Nacional de Colombia, Santafé de Bogotá, Colombia.
A novel procedure is described to deposit in a reproducible way, using the reactive
evaporation method, ZnO thin films with properties of electrical resistivity and transmittance
adequate for use as transparent electrical contact in CuInSe2 based solar cells. Were investigated
the influence on the electrical resistivity and the transmittance of the ZnO thin films of the principal
parameters of deposition, such as the evaporation temperature of zinc, glow discharge current,
oxygen content in the deposition chamber, separation between the electrodes and diameter orifice
of Zn exit. From this study, the conditions to prepare undoped ZnO thin films whose resistivities
are in the order of 10-4 Ωcm and transmittances to values greater than 85% were found.
From Hall voltage and electrical conductivity measurements, it was found that the
conductivity is mainly dominated by the resulting carrier concentration which is controlled by
oxygen vacancies, and in less degree by the carrier mobility. X-rays diffraction measurements
showed that the carrier mobility is affected by the grain size of the samples.
S3.P5. DETERMINATION OF NONRADIATIVE RECOMBINATION LIFETIME IN THE
InGaAsSb LAYERS
I. Riech1, P. Díaz1, J. Mendoza-Alvarez2 and E. Marín1
1
I.M.R.E.-Facultad de Física, Universidad de La Habana, San Lázaro y L, Plaza, Ciudad de La
Habana 10400, Cuba.
2
Departamento de Física, CINVESTAV-IPN, D.F., México.
47
The photoacoustic (PA) technique, in the heat transmission configuration, has been used to
study the InGaAsSb/GaSb samples. We propose a theoretical model for the calculation of the PA
signals in two-layers semiconductor samples. The nonradiative recombination lifetime is
determined by fitting the theoretical curves to the experimental PA phase data. The obtained
nonradiative recombination lifetime agree reasonably well with the available reported values for
this material.
S3.P6. STRUCTURAL AND PHOTOELECTRICAL PROPERTIES OF PBS THIN
FILMS OBTAINED BY CHEMICAL BATH DEPOSITION (CBD) IN PRESENCE
OF MAGNETIC FIELD
E. M. Larramendi* ([email protected]), O. Calzadilla* ([email protected]), F. Rábago+
([email protected]), J. Ruiz + ([email protected])
*
+
Physics Faculty of Havana University, Cuba
Physics Institute, UASLP, Mexico
Thin PbS polycrystalline films were chemically deposited on glass from alkaline bath on
the influence of magnetic field. The samples were orientated parallel and perpendicular to the
magnetic field during the deposition of the films. The structural and photoelectrical properties of
these films were studied. It has been observed change in morphology of the films produced by
magnetic field, while the photoelectrical properties had little change.
S3.P7. IN SITU DIAGNOSTIC OF RF-SPUTTERING DEPOSITION OF CdTe
OXIDE FILMS
R. Castro-Rodríguez ([email protected]), A. Iribarren1 ([email protected]), and J.L.
Peña2 ([email protected])
Applied Physics Department, CINVESTAV-IPN Merida Unit, A.P. 73 Cordemex, Merida 97310,
Yucatan, Mexico.
1
DIEES, Instituto de Materiales y Reactivos, Universidad de La Habana, Zapata y G, Plaza de la
Revolución, La Habana 10400, Cuba.
2
Applied Physics Department, CINVESTAV-IPN Merida Unit, A.P. 73 Cordemex, Mérida 97310,
Yucatán, Mexico. CICATA-IPN, Legaria 694, Mexico 11500, D. F., Mexico
The transport of materials from target-to-substrate has been monitored in the rf-sputtering
deposition of CdTe oxide films (with controlled plasma of Ar-O-N) using in situ diagnostic namely
plasma ion mass spectroscopy (MS). The MS spectra shows that oxygen does not become
significative until 8.0 x 10-5 Torr of N2O, where the CdTe has almost reached the "saturation" at 60
at% of oxygen. The mechanism formation of CdTe oxides and the physical and chemistry reactions
plasma of CdTe-N2O are proposed.
S3.P8. NANOSTRUCTURES BASED ON Cd1-XMnXTe SEMICONDUCTOR
M. Becerril, R. Várgas, and O. Zelaya1
48
Departamento de Ingeniería Metalúrgica, Instituto Politécnico Nacional, Apartado Postal 75-874.
07300 México DF, México
1
Departamento de física, Centro de Investigación y de Estudios Avanzados del IPN, Apartado
Postal 14-740,07000 México DF, México.
CdMnTe nanoestructured thin films were prepared by radio frequency sputtering from a
target of CdTe and manganese compressed powders. The structural and electrical film properties
were studied as a function of the manganese atomic concentration in the films (x = 0.05, 0.1, and
15). X-ray diffraction patterns showed a cubic CdTe parent structure with a (111) preferential
orientation for x= 0.05 and amorphous for (x=0.1 and 0.15). The microcrystalline grain size of
around 50 nm for x = 0.05, decrease to an average of 30 nm for x = 0.15. From atomic force
microscopy micrographs, a fine granular morphology with a random distribution of grain sizes in
the films was observed. The electrical resistivity of the films were measured as a function of the
temperature in the 80-473 K range. The variations of the gap were measured as a functions of the
Mn concentrations as long as the mobility.
S3.P9. DIFFERENT TIO2 MESOPOROUS
PREPARATION OF PHOTOELECTRODES
POWDERS
USED
FOR
THE
J.A. Ayllon* ([email protected]), R. Espinosa†, I. Zumeta† H. Sadoun*, R. RodríguezClemente*, E. Vigil† ([email protected])
†
Instituto de Materiales y Reactivos, Univ. de La Habana, Cuba
*Instituto de Ciencia de los Materiales de Barcelona, CSIC, España
The importance of nanostructured TiO2 photoelectrodes is due to their perspective
applications in areas such as photocatalysis, sensitized solar cells and other uses. So far, their
physical behavior is not fully explained and this makes them also quite interesting. Different types
of mesoporous high surface titania powders have been sensitized using 1, 2 diolates of titanium as
suitable precursors. Nanostructured thin films on top of SnO2 conducting glass are obtained using
two commercially available TiO2 powders (Degussa P-25 and Hombikat UV-100), as well as, the
synthesized powders. The “dip-coating” method is employed for making them. These layer
structures are used as photoelectrodes in a two-contact photoelectrochemical cell. Their short-circuit
photocurrent, open circuit photovoltage and their time response are characterized. Their different
behaviors are explained on the basis of these powders properties (crystallites, aggregates and pore
dimensions; crystalline structure and surface area).
S3.P10. ELECTRICAL TRANSPORT PROPERTIES STUDY ON CdS THIN FILMS
THROUGH THERMOELECTRIC POWER MEASUREMENTS
C. E. Jácome1, J. M. Flórez†† ( [email protected] ), G. Gordillo
([email protected]), Y. G. Gurevich††† ([email protected])
†
Pontificia Universidad Javeriana
Universidad Distrital Francisco José de Caldas
†††
Centro de Investigación y Estudios Avanzados del IPN (CINVESTAV, México D.F.)
Departamento de Física, Universidad Nacional de Colombia
††
49
In this work it presents an electrical transport properties on CdS thin films prepared by CSS
(Close Space Sublimation Method) to be used in solar cell fabrication. The study was made through
thermoelectric power measurements as a function of the temperature and using a theoretical
calculation model of the thermopower (α) vs T. This model involves dimensional and
polycrystalline effects. The electrical transport in CdS semiconductor thin films is dominated by
electrons with mass of 0.22 m0 which was found from the comparison between theoretical and
experimental results. Besides, the comparison showed that the electrical transport in these films is
strongly influenced by sample’s dimensional effects, scattering processes with piezoacoustic
phonons (r= 1/2) and grain boundaries.
S3.P11. ETHANOL GAS DETECTOR USING ZnO-BASED-COATINGS
J. Morales, J.L. Solis ([email protected]) and W. Estrada ([email protected])
Facultad de Ciencias, Universidad Nacional de Ingeniería, Casilla 31-139, Lima, Peru
Undoped and doped ZnO-based-coatings were obtained by spray pyrolysis for gas sensor
applications. Both alumina and glass slides were used as substrates. Samples were doped with Al,
Cu, In and Sn.
X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive of xrays (EDS) were used for coatings characterization. The EDS measurements have shown that the
atomic percent ratios of the doping elements in the films are proportional to doping concentration in
the pyrolytic solution. XRD and SEM analysis show that grain size and preferential crystal growth
direction depend on the type and amount of dopants. Under similar preparation condition dopedcoatings with Sn and Al have bigger grain size than grain size of undoped samples, which in turn
are bigger than those doped with In and Cu. We have found as a general behavior that undoped
samples present better electric conduction than those doped one. This behavior is enhanced with
temperature.
Ethanol vapor detection (electrical response and adsorption/desorption process) depends on
the coating temperature during vapor-solid interaction. We have found that the optimized detection
conditions for ZnO-based coatings are around 400 ºC. All samples (doped and undoped) increase
their electrical conductance as vapor concentration increases in the air-vapor mixture, surrounding
the sample. Samples doped with Sn, AL and In showed more sensitivity for ethanol detection
respect to undoped and Cu-doped samples. ZnO:Sn samples with 15% (at.) tin-concentration
showed the best response. For instance, an ethanol vapor of 4 ppm in air a signal response ratio of
80:14 is found for Sn-doped 15 % at.: undoped sample.
Although further studies are needed, these preliminary results are in agreement with the assumption
that ethanol is behaving as gas reductor increasing the electrical conductance of the samples (n-type
semiconductor).
S3.P12. INFRARED SPECTROSCOPY AND OPTICAL PROPERTIES OF LPCVD
SILICON OXYNITRIDE THIN FILMS
J. Carrillo-López1([email protected]), A. Morales-Acevedo2([email protected]),
W. Calleja1, and E. Díaz-Valdes3
1
Instituto Nacional de Astrofísica y Optoelectrónica, Tonantzintla, Puebla. (Also at CIDS del
Instituto de Ciencias de la Universidad Autónoma de Puebla).
50
2
Centro de Investigación y de Estudios Avanzados del IPN, Departamento de Ingeniería
Eléctrica, Avenida IPN No. 2508, México, D. F. 07360.
3
Escuela Superior de Física y Matemáticas del IPN, Zacatenco, México, D. F. 07360.
Low pressure chemical vapor deposition (LPCVD) silicon oxynitride (SiON) films of
various compositions between SiO2 and Si3N4 were grown by changing the relative ratio (Ro) of
nitrous oxide (N2O) to silane (SiH4) pressures while keeping constant the ammonia (NH4) pressure.
The SiON films were deposited at 700 oC on p-type silicon substrates (with a carrier concentration
of 1015 cm-3), varying Ro from 0.32 to 1.38. Some samples were subsequently annealed at 700 oC
for times up to 150 min, whereas other samples were annealed at 1000 oC for times up to 60 min in
nitrogen. In this work, we present results of the effect of Ro upon the refractive index, the infrared
spectrum, and the growth rate of the LPCVD SiON layers. In particular, we show that as Ro
increases, the refractive index decreases from values very close to those of SiN to values very close
to those of. SiO2. In addition, the infrared spectrum shows that the peak associated to the Si-O
stretching vibration mode decreases as compared to the Si-N stretching local mode when Ro
increases. The annealing processes seem not to modify the films as the thickness and refractive
index remain constant as a function of the annealing time.
S3.P13. THERMOELECTRIC CHARACTERIZATION OF POLYCRYSTALLINE
CdS THIN FILMS
J. M. Flórez a, B. A. Paez b ([email protected]), C. E. Jácome b, and G. Gordillo
a
Universidad Distrital Fco. De Caldas
Departamento de Física, Pontificia Universidad Javeriana
Departamentode Física Universidad Nacional de Colombia
(Santafé de Bogotá, Colombia)
b
Polycrystalline semiconducting thin films of CdS deposited by closed space sublimation
method (CSS) were characterized through temperature dependent thermoelectric power (α) and
resistivity (ρ) measurements. It was done to study the dominant scattering mechanisms in the
electric transport process. Additionally, by combining experimental measurements of α against
temperature with theoretical calculations it was found that the electric transport is swayed by
scattering in the grain and furthermore by surface and dimension effects.
S3.P14. STUDY OF ELECTRICAL
POLYCRYSTALLINE CdTe THIN FILMS
TRANSPORT
PROPERTIES
IN
B.A. Paez ([email protected]), C.E. Jácome, J. M. Flórez(a), and G. Gordillo(b)
Departamento de Física, Pontificia Universidad Javeriana
a
Universidad Distrital Fco. De Caldas
b
Departamentode Física Universidad Nacional de Colombia (Santafé de Bogotá, Colombia)
Polycrystalline CdTe thin films deposited by Close Space Sublimation method (CSS) were
characterized through temperature dependent thermoelectric power (α) and resistivity (ρ)
measurements, in order to determine the scattering mechanisms which mainly affect their electrical
transport properties. The results were interpreted with the help of theoretical calculations of α vs T,
carried out using a theoretical model developed specially for this purpose. This model includes
scattering processes within the grains and at the grain boundaries. Some of the parameters used in
51
the calculation of α were determined experimentally and the main approximations were justified
from experimental measurements.
The reliability of the model was tested by comparing the values of α calculated in
dependence of the temperature, with curves of α vs T obtained experimentally in the range between
90 and 330 K.
S3.P15. INFRARED RESPONSE OF SEMICONDUCTOR SUPERLATTICES: 45°
REFLECTOMETRY
A. Silva-Castillo ([email protected]), F. Pérez-Rodríguez (fperez@acuario.
ifuap.buap.mx)
Instituto de Física, Universidad Autónoma de Puebla,, Apdo. Post. J-48,Puebla, Pue. 72570,
México.
We study theoretically the spectra of infrared 45° reflectometry of semiconductor
superlattices, whose total thickness is much smaller than the wavelength of the incident light. This
optical spectroscopy is based upon the measurement of the difference Rp –Rs2 between the
reflectivity Rp for p-polarized light and the squared reflectivity Rs2 for s-polarized light with an
angle of incidence of 45°. The frequencies of both transverse and longitudinal optical phonons in
thin heterostructures on substrate can be directly measured by means of the 45° reflectometry.
Using the transfer-matrix method, we have calculated spectra of 45° reflectometry for
semiconductor superlattices composed of optically isotropic layers. These spectra display prominent
resonances at frequencies of transverse and longitudinal phonons in individual layers of
superlattice. These results agree with those obtained by using the long-wavelength approach, i.e. by
modeling the superlattice as an uniaxial crystal. Also, we have analyzed the effect of free-carrier
concentration of the substrate on the infrared spectra at 45° angle of incidence.
This work was partially supported by CONACYT (Grant 26184-E)
S3.P16. DAMAGE OF MBE GROWN CaF2 THIN FILMS AFTER ELECTRON BEAM
IRRADIATION, STUDIED BY RHEED AND RBS
I.Becerra1 ([email protected]), A. Ramírez1, A. Zehe2 ([email protected])
1
Instituto de Ciencias, Departamento de Microelectrónica, Universidad Autónoma de Puebla,
Apdo. Post. J-32, San Manuel, 72570 Puebla, Pue., México
2
Facultad de Cs. Físico-Matemáticas, Universidad Autónoma de Puebla, Apdo. Post. # 1505, 72570
Puebla, Pue., México
The effect of ion- and electron beam irradiation damage to pseudomorphic CaF2/Si(111)
structures was investigated. Based on known evolutionary approaches for feature sizes and
consequently a microelectronics to nanoelectronics transition, with the prospect of a threedimensional integration in thoroughly epitaxially grown heterosystems, CaF2 has bean singled out
as a crystalline model dielectric with electrical properties similar to SiO2, but ease to be grown
epitaxially on silicon. Even in the concept of the classical SiO2-based material system, the direct
electron beam irradiation for fabricating mesoscopic or conventional short gate-length transistors
was reported recently [1]. In situ electron-or ion beam lithography, using CaF2 layers grown on
silicon wafers by MBE implies the opportunity for a closed-loop fabrication of a complete device
structure. The application of CaF2 as on inorganic, even epitaxial resist, is based on the
transformation of CaF2 into CaO, and the utilization of their different solubilities or vapor pressures.
It has been found that the electron beam bombardment of CaF2 layers in oxygen ambient makes a
52
controlled generation of CaO possible. The kinetics of this reaction was studied by AES. The
application of ion instead of electron beam irradiation allows to reduce the exposure dose by four
orders of magnitude, leading to the same lithography effect. Characteristic solubility curves for ion
– or electron beam exposed CaF2 are given. RBS was applied in order to evaluate the chemical
composition prior and after irradiation of the films, with a clear indication of a fluorine substitution
by oxygen.
[1] D.J. Paul et al., J. Vac. Sci. Technol. B 14(6), 3834 (1996)
S3.P17. NITROGEN INCORPORATION TO CDTE THIN FILMS GROWN BY RF
SPUTTERING IN AN AR-NH3 PLASMA
F. Caballero-Briones1 ([email protected]) R. Castro-Rodríguez2 ([email protected]),
P. Bartolo-Pérez2 ([email protected]), W. Cauich2 ([email protected]), J. L.
Peña1,2 ([email protected])
1
1
CICATA-IPN, Legaria 694, Mexico 11500, D. F., Mexico
Applied Physics Department, CINVESTAV-IPN Merida Unit, A.P. 73 Cordemex,
We have grown CdTe films on glass substrates, by rf reactive sputtering in an Ar-NH3
controlled plasma. Semi- quantitative secondary-ion mass spectrometry analysis show evidence of
nitrogen and hydrogen presence in the films and that their content on the films seems to decrease
with the NH3 partial pressure. Auger electron spectroscopy shows that oxygen content increases
from 17 at% to 65 at% and Cd content decreases from 42 at% to 6 at% when NH3 partial pressure is
increased from 1x10-5 to 2x10-2 Torr. No nitrogen was detected with AES, but several deformation
of the Cd, Te and O peaks is registered. X-ray diffraction shows that films tend to be amorphous at
increased ammonia partial pressures.
S3.P18. AES STUDY OF CdTe-Zn SYSTEM GROWN BY CSVT-FE TECHNIQUE
W. Cauich1 ([email protected]), P. Bartolo-Pérez1 ([email protected]),
R. Castro-Rodríguez1 ([email protected]), M. Zapata-Torres2 ([email protected]), F.
Caballero-Briones2 ([email protected]), J.L. Peña1,2 ([email protected])
1
Applied Physics Department, CINVESTAV-IPN Merida Unit, A.P. 73 Cordemex, Merida 97310,
Yucatan, Mexico
2
CICATA-IPN, Legaria 694, Mexico 11500, D. F., Mexico
In order to obtain CdTe-Zn system films grown on glass substrates by close spaced vapor
transport combined with free evaporation technique (CSVT-FE) we using coevaporation of CdTe
and ZnTe. Zn concentration was controlled by temperature of ZnTe source, and the composition of
the films was investigated by Auger Electron Spectroscopy (AES). The CdTe temperature was fixed
at 575 oC, and the temperature of ZnTe varies from 550 oC to 775 oC in order to obtain different
concentration of Zn in the films. We found, that the concentration of Te stay on ~ 45 at. % and Cd
decrease from 42 at% to 3 at% and the Zn increase from 13 at% to 52 at%
This work was partial support by CONACYT.
S3.P19. TIN OXIDE THIN FILMS DEPOSITED BY REACTIVE SPUTTERING
L.C. Jimenez ([email protected]), H. Méndez ([email protected]) and
B.A. Paez
53
Departamento de Física, Pontificia Universidad Javeriana (Santafé de Bogotá, Colombia)
High transmittance and conducting tin oxide (SnO2) thin films were prepared by reactive
sputtering, and studies of deposition parameters such as pressure, plasma current, and voltage were
carried out. Additionally, measurements of thermoelectric power (α), electrical resistivity, and
transmittance coefficient were done in order to find the parameters which best characterize the thin
films for applications as optical and electrical contact windows.
S3.P20. CdS FILMS GROWTH ON DIFFERENT ROUGHNESS ITO SUBSTRATES
A.I. Oliva ([email protected]), R. Castro-Rodríguez ([email protected]),
O. Ceh ([email protected]), F. Caballero-Briones1([email protected])
Applied Physics Department, CINVESTAV-IPN Merida Unit, A.P. 73 Cordemex,
Merida 97310, Yucatan, Mexico
1
CICATA-IPN, Legaria 694, Mexico 11500, D.F., Mexico
Cadmium sulfide, CdS, is an excellent window layer for solar applications when CdTe solar
cells are used. In this work we study CdS films deposited on indium tin oxide (ITO) substrate with
different roughness by chemical bath deposition (CBD). CdS films were studied by atomic force
microscopy (AFM), x-ray diffraction, Auger Spectroscopy and optical techniques. ITO substrates
with different surface roughness were prepared by chemical attack with hydrochloric acid at
different times. By this way, we obtained samples with rms-roughness values between 10 and 30
nm as characterized by AFM.
After that, CdS films were deposited during 40 minutes, such that similar film thickness
should be obtained. However, rms-roughness on CdS films, demonstrated that initial substrate
roughness plays an important role on the properties obtained on films.
A constant CdS final rms-roughness is rapidly reached on substrates with minor initial
roughness, as compared with substrates with higher initial roughness. However, structural and
optical properties change depending on the initial substrate conditions.
For higher CdS deposition times, a constant value of 15 nm on the roughness was reached
in all cases.
S3.P21. DETERMINATION OF THE TRAP DENSITY IN POLYCRYSTALLINE
CdTe FILMS FROM OPTICAL ABSORPTION PROFILES.
A. Iribarren ([email protected])
D.I.E.E.S., Instituto de Materiales y Reactivos, Universidad de La Habana
Zapata y G, Vedado, Plaza, Ciudad de La Habana 10400, Cuba
We report the linear behavior of the band-tail parameter as a function of the reciprocal of
the grain size in polycrystalline CdTe films. The theoretical analysis based on a novel tail parameter
model justifies that the behavior is ruled by the contribution of the grain boundary traps. From
absorption measurements at 77 and 300 K we calculated the grain-surface trap concentration to be
Qt=1.15x1011 cm-2 and the bulk defect density to be Ndef=2.4x1013 cm-3 . Both results allowed to
determine the strain parameter in the films Y=0.97.
S3.P22. COMPOSITIONAL AND STRUCTURAL CHARACTERISTICS OF CdTeO
FILMS GROWN BY RADIO FREQUENCY SPUTTERING
54
A. Iribarren1 ([email protected]), E. Menéndez-Proupin1, F. Caballero-Briones2, R. CastroRodríguez3and J. l. Peña2,3
1
I.M.R.E.-Facultad de Física, Universidad de La Habana, San Lázaro y L, Plaza, Ciudad de La Habana
10400, Cuba.
2
CICATA-IPN, Legaria 694, Col. Irrigación 11500, México D.F.
3
Departamento de Física Aplicada, CINVESTAV-IPN, Unidad Mérida, A.P. 73-Cordemex 97310, Mérida,
Yucatán, México.
An analysis of the absorption spectra of CdTeO films with different oxygen content grown
by radio frequency sputtering was carried out. The behavior and shape of the spectra, which exhibit
long tails extended for more than 500 meV, suggest the presence of CdxTeyOz variable-composition
compounds ranging between CdTe and CdTeO with 67 at.% oxygen, where x, y and z are ruled by
the valence balance 2x+4y-2z=0, taking into account that oxidation number of Te in CdTeO is +4.
We present a model based in bond statistics, which describes the absorption profile behavior of this
and other materials with similar optical characteristics and advances the real composition of such
films.
S3.P23. POSITRON ANNIHILATION AND
ANALYSIS OF AMORPHOUS CARBON FILMS
RAMAN
SPECTROSCOPY
E. Halac1 ([email protected]), A. Somoza2 ([email protected]), H. Huck1
([email protected]), R. S. Brusa3and M. Reinoso1.
1
Departamento de Física. Comisión Nacional de Energía Atómica, Avda Gral. Paz 1499, (1650)
San Martin, Buenos Aires, Argentina.
2
IFIMAT, Universidad Nacional del centro de la Pcia. de Buenos Aires, Pinto 399, 7000 Tandil.
Pcia. de Buenos Aires, Argentina.
3
Instituto Nazionale per la Fisica della Materia, Dipartamento di Fisica, Universitá di Trento,
38050 Povo, Italia.
Hard amorphous carbon films produced using high- energy ( 30keV) ion beam deposition of
CH3+ and CH4+ on silicon wafers, have been investigated by Positron Annihilation Spectroscopy,
Raman Spectroscopy and Electrical Resistivity measurements. The microstructural modifications of
the films as a function of the annealing temperature in the 300-600 o C range have been studied.
Changes in the open volume defects distribution are described and related to the graphitization
process.
S3.P23. RAMAN SPECTROSCOPY STUDY OF CdTe OXIDE THIN FILMS
GROWN BY RF SPUTTERING
F. Caballero-Briones,1,2 F. Rábago,3 A. Iribarren,4 R. Castro Rodríguez1, A. Zapata-Navarro,2
J. L. Peña.1,2 and S. Jiménez5.
1
Applied Physics Dept. CINVESTAV-IPN Mérida. A.P. 73, 97310, Mérida, Yucatán, México.
CICATA-IPN, Legaria 694, Col Irrigación, 11500 México D.F.
3
Instituto de Física, UASLP, Av. Alvaro Obregón 64, 78000, SLP. México
4
DIEES-IMRE, Universidad de La Habana, San Lázaro y L, Vedado, La Habana Cuba.
2
55
5
Laboratorio de Investigación en Materiales, CINVESTAV, Querétaro, Qro., Libramiento
Norponiente 2000, Querétaro, Qro., México.
We present the Raman spectra obtained for CdTe oxide thin films with different growing
conditions. We prepared by rf sputtering a set of samples of CdTe oxide thin films using a
controlled Ar-N2O plasma. This study was carried on when using different N2O pressures and
plasma powers. Our results show great dependence in the Raman spectra with the growing
conditions and great discrepancy with those reported in the literature for telluritic glasses. Our
results are correlated with previous proposed models for the growing and structure of these
materials.
S3.P24. HALL EFFECT IN QUASI-CRYSTALLINE AlCr THIN FILMS
A. Sáenz R.1 ([email protected]) and V. M. Castaño M.2 ([email protected])
1
2
Escuela de Física-CICIMA, Universidad de Costa Rica, San José, COSTA RICA
Instituto de Física, U.N.A.M., México D.F., MEXICO
Hall effect and resistivity measurements on AlCr multilayers prepared by successive
evaporation onto heated substrates and which show the icosahedral phase under TEM electron
diffraction are reported.
Both Hall effect and electrical resistivity as a function of temperature in the range 80300 K, show an interesting maximum at about 150 K.
Twin samples except for substrate temperature were prepared at temperatures close to
Room Temperature. They do not show icosahedra diffraction lines nor they show the
aforementioned maximum in neither resistivity nor Hall effect; they show the expected behavior
for metals or alloys.
Samples have three Al and two Cr layers of thickness as to yield the desired
concentrations.
S3.P25. CONTROL OF THE MICROSTRUCTURE OF HYDROTHERMALELECTROCHEMICAL BaTiO3 FILMS
V. Fuenzalida1 ([email protected]), T. Vargas (tvargas @cec.uchile.cl)1, A. Zárate1
([email protected]), C. Silva ([email protected])2, M. Baeza3 and P. Valverde1
1Universidad de Chile, Facultad de Ciencias Físicas y Matemáticas, Santiago, Chile
2Comisión Chilena de Energía Nuclear, Santiago, Chile
3Universidad de Santiago, Santiago, Chile
The hydrothermal-electrochemical method is a low temperature route for the preparation of
ceramic thin films. A transition metal substrate, or a metallized non-metal substrate, is immersed in
an alkaline solution containing alkaline earth ions in the temperature range 80-250 ºC. This leads to
a thin ceramic film on the substrate. The process is accelerated by the application of an electric
current [1].
The properties of hydrothermal- films BaTiO3 films should be influenced by its
morphological and crystallographic properties. In the case of formation under hydrothermalelectrochemical procedures it is possible to control the film growth rate through an adequate control
of deposition potential. This approach can be used as a method to modify the film characteristics.
56
The formation of BaTiO3 films on titanium wafers by a hydrothermal-electrochemical
procedure in Ba(OH)2 solutions, was studied. The experimental approach focused on the study the
influence of deposition potential and Ba(OH)2 concentration on the morphological and
crystallographic characteristics of the BaTiO3 films. Experiments were conducted at 80ºC with
Ba(OH)2 concentrations of 0.5, 0.2, 0.1, 0.05 M, and films were formed at 0, -0.3, -0.5, -0.8 V vs.
Ag/AgCl (sat., 80ºC). The deposition time was 30 min. in each case.
Results showed that the formation of continuous BaTiO3 films can be only accomplished in
the highly concentrated solutions, i.e. 0.5 M Ba(OH)2. Formation of this films was evidenced from
SEM, X-ray diffraction and Raman. At lower Ba(OH)2 concentrations there was no formation of a
continuous films, and BaTiO3 was mainly forming as isolated islands. The surface density number
of BaTiO3 islands decreases with Ba(OH)2 concentration, but it was not clearly influenced by the
variations of the potential deposition.
[1] S.E. Yoo, N. Ishizawa, M. Hayasi, and M. Yoshimura, Report of the Research Laboratory of
Engineering Materials, Tokyo Institute of Technology, 16, 39-53 (1991)
[2] Tomás Vargas, Hugo Díaz, Carmen I. Silva, and Víctor M. Fuenzalida, J. Am. Ceram. Soc. 80,
213-18 (1997)
S3.P26. GROWTH OF ZnS FILMS ON SINGLE CRYSTAL SUBSTRATES BY
CHEMICAL BATH DEPOSITION TECHNIQUE
J. Vidal, O. Vigil, N. López, O. de Melo and O. Zelaya-Angel(a)
Facultad de Física-IMRE, Universidad de la Habana, San Lázaro y L. Vedado, C. Habana 10400,
Cuba. (a) Dpto. de Física, CINVESTAV - IPN, Apdo. Postal 14-740, México 07000. D.F. México
Results in the growth and characterization of ZnS films on Si and GaAs substrates by
chemical bath deposition (CBD) technique are presented. X- ray diffraction (XRD) patterns, and
Atomic Force Microscopy (AFM) images show very different features when compared with films
deposited onto glass substrates. In particular X- ray peaks in as grown samples are observed in films
deposited on GaAs substrates. Results on structural, morphological, optical and electrical properties
are reported. The influence of magnetic field during growth is discussed.
S3.P27. PHOTOREFLECTANCE STUDY OF THE SURFACE QUALITY AND
THE EFFECTS OF ANODIZATION ON GaInAsSb
M.C. Rivera-Morales1([email protected]), J.L.Herrera-Pérez1, O. Zarate-Corona2,
M.I. Reyes-Valderrama1and J. G. Mendoza-Alvarez3,4
1
Instituto de Física, Universidad Autónoma de Puebla. Apdo. Postal J-48, Puebla 72570. México.
CIDS-ICUAP, BUAP. Apdo. Postal 1651, Puebla, Pue. 72000 México.
3
Departamento de Física. Cinvestav del IPN. Apdo. Postal 14-740. México D.F. 07000. México
4
CICATA-IPN.Legaria 694.Col. Irrigación. Mexico DF 11500. México
2
In this work we show that the photoreflectance (PR) spectroscopy can be used as a
sensitivity probe to study the surface quality and the effects of controlled erosion on the surface of
Ga0.18In0.82AsySb1-y epitaxial films. GaInAsSb layers were grown onto GaSb single crystal substrates
by the liquid phase epitaxy technique. The stoichiometry of the quaternary layer was latticematched to the substrate. The low temperature PR spectra for as-grown high crystalline quality
films show Franz-Keldish oscillations in the high energy side of the PR spectrum, characteristic of
the high electric field regime. When the layers are anodized using a mixture of tartaric and
fluorhydric acid our results show that the film may suffer damage at a small depth.
57
The PR spectra measured in the anodized samples show a lineshape characteristic of the
low electric field regime. We present results on the study of both types of behavior (high and low
electric fields) in the PR spectra as a function of temperature and discuss the mechanism of
anodization and the erosion processes for these GaInAsSb epitaxial layers.
S3.P28. FABRICATION AND CHARACTERIZATION OF SOLAR CELLS OF
ZnO/CdS/CdTe
M. Grizalez, L. C. Moreno*, and G Gordillo ([email protected])
Universidad Nacional de Colombia, Departamento de Física, Santafé de Bogotá.
*Universidad Nacional de Colombia, Departamento de Química, Santafé de Bogotá.
ZnO/CdS/CdTe/Cu solar cell structures were fabricated. The CdTe deposited by CSS
method acts as the active layer of the cell, while the bilayer ZnO/CdS acts as window layer. The
ZnO layer was deposited using the reactive evaporation method and the CdS was grown by CBD. In
this work we describe the relevant aspects of the solar cells fabrication. I-V and spectral response
measurements are discussed. These measurement allowed us to determine the influence of the
deposition parameters and the post deposition thermal annealing on the performance of the cells.
Atomic Force Microscopy was used to study the morphological characteristic of the surface.
This measurement shows the influence of the substrate on the CdS layer, and hence on the
efficiency of the cell.
S3.P29. OPTICAL AND MORPHOLOGICAL PROPERTIES OF Cu2S CRYSTALS
PREPARED BY THE SOL-GEL METHOD
H. Arizpe-Chavez1, R. Ramirez-Bon1,2, F.J. Espinoza-Beltran1,2, and O. Zelaya-Angel3
1
CIFUS, Universidad de Sonora, P.O.Box 5-88, 83190, Son., México.
Laboratorio de Investigación en Materiales. CINVESTAV-IPN, Unidad Querétaro
3
Departamento de Física, CINVESTAV-IPN, P.O.Box 14-740 México 07360 D.F.
2
Cu2S-embedded SiO2 glasses were obtained by the sol-gel process. The glasses synthetized
through hydrolysis of Si(OC2H5)4 and hydrated CuCl (30% wt Cu), after annealed were exposed to
a bath of thiourea (NH2)2CS to precipitate sulphuryzed Cu crystals. Raman spectroscopy on the
obtained films showed an intense peak around 460 nm, corresponding to Cu2S compound. Also,
optical absorption in the visible region, showed an increasing for this absorption at 500 nm.
Transmission spectra showed a maximum value of 20% for wavelengths in the order of 700 nm.
Atomic force microscopy (AFM) were made after quenching these films in a bath of HF. The AFM
images show morphological changes in the surface, which depended on the quenching time. From
these figures we can observe surface pores with a diameter of 0.1 mm, surrounding granular objets
which could be identified as Cu2S crystals, too large to show confinements effects. These films
showed no photoluminescence signals.
S3.P30. GETTERING EFFECT OF ALUMINIUM UPON ELECTRICAL AND
TRANSPORT PROPIERTIES OF THE n+-p-p+ SILICON SOLAR CELLS
G. Santana ([email protected]), A. Morales-Acevedo*([email protected]), A.
Martel ([email protected]) and L. Hernández ([email protected])
Facultad de Física Universidad de La Habana. 10400 La Habana Cuba
58
*
Centro de Investigación y de Estudios Avanzados del IPN. Departamento de Ingeniería Eléctrica
México, D. F. 07360. México
Aluminum is usually deposited and alloyed at the back of n+-p-p+ silicon solar cells for
making a good ohmic contact and establishing a back electric field which avoids carrier
recombination at the back surface. Typically, the aluminum thermal treatment is made at
temperatures around 600º C for short periods (10-30 minutes). However, recently it has been
suggested that the alloyed region could act as a metallic impurity “getterer”, but still little work has
been done on this phenomenon. To study the effect that different annealing treatments of the
aluminum have on the characteristics of solar cells we carried out dark and illuminated I-V
measurements at different temperatures and we studied the solar cell transport mechanisms.
We show that both, short-circuit current and open-circuit voltage both improve when
aluminum is annealed at temperatures of 800º C for 40 minutes, in comparison to the typical
aluminum thermal annealing (600º C for 10 minutes). This improvement is due to the reduction of
the recombination centers in the band gap and the increase of the diffusion length of the minority
carriers in the base region. Furthermore, from DLTS measurements we have determined that the
iron concentration in the base is reduced when the annealing is made at 800º C for 40 minutes. In
other words, there appears to be a real metallic “gettering” caused by the high temperature
treatments, since the improvement seems to be due to the reduction of the recombination centers in
the base when iron is extracted from it.
S3.P31. FRACTURE FOR TENSIL STRESS IN CdTe:O THIN FILMS
H. Arizpe-Chavez1, A. Picos-Vega1, O.Zelaya-Angel2, R. Ramirez-Bon1,3, and F.J. EspinozaBeltran1,3
1
CIFUS, Universidad de Sonora, P.O.Box 5-88, 83190 Hermosillo, Son., México.
Departamento de Fisica, CINVESTAV-IPN, P.O.Box 14-740, México 07360 D.F.
3
Laboratorio de Investigación en Materiales, CINVESTAV-IPN, Querétaro, México.
2
Polycrystalline oxygenated CdTe thin fims were grown by r.f. sputtering on 7059 Corning
glass substrates. Films looked shine and with good adhesion to the substrate. After a couple of days
layers become scratched in some regions of their surface. After a week this region turns with
scratchs like a mesh of micrometric dimensions. By employing an optical microscope these scratchs
were identified as fractures. AFM images show these fractures were originated by tensil stress on
the volume of the film, because the aspect of them are like hills with a lowland along the central
region. We are identified the origin of that tensil stress with the presence of oxygen into the bulk of
the films by means of Auger measurements. Oxygen at low concentrations (<2%) increases the
lattice parameter and, consequently, shifts the energy band gap to higher energy as observed by
optical absorption spectra. For concentrations above 2% the films become amorphous.
S3.P32. A THEORETICAL REACTION-DIFFUSION MODEL FOR EUTECTIC
METAL ALLOY/n-GaAs.
M.P. Hernández1, Inti Garcés1, Carlos F. Alonso2, P. Bartolo3 and J.L. Peña3
1
IMRE, University of Havana, 10400 Vedado, La Habana, Cuba.
Physics Faculty, University of Havana, 10400 Vedado, La Habana, Cuba.
3
Applied Physics Department, CINVESTAV-IPN Merida Unit, A.P. 73 Cordemex,
Merida 97310, Yucatan, Mexico.
2
Theoretical and experimental study of a competitive chemical reaction in Al-Ge/n-GaAs
interface is presented. The reaction of each alloy component is well explained by a simple
theoretical reaction-diffusion model. Concentration of each element is the solution of the
inhomogeneous diffusion equation. This equation is subject to the initial separation condition along
59
the x-axis; where a Heaviside step function represents that one element is initially uniform
distributed on the left side (x<0) and the other on the right side (x>0). The mass interchange at
boundary (x=0) is the fundamental condition which is expressed as a concentration gradient. Such
solutions were observed in the obtained concentration profiles from Auger Electron Spectroscopy of
Al-Ge eutectic alloys on n-GaAs. Al-Ge/n-GaAs junctions were thermally treated for one minute in
forming gas at 500 and 540 °C. The concentration obtained by quantitative analysis of the
Dispersive Energy Spectrum also confirms these results.
S3.P33. CHARACTERIZATION OF STAIRCASE AlGaAs STRUCTURES FOR
PHOTODETECTION APPLICATIONS
A. Esparza-García, M. Galván-Arellano, R. Peña-Sierra ([email protected]), A.
Escobosa and M. Sánchez1
Electrical Engineering Department, CINVESTAV- IPN, Ave. IPN 2508 07300, Mexico D.F.
1
Faculty of Physics. University of Havana, San Lázaro y L, Vedado, 10400, La Habana. Cuba
Optical and Electrical characterization of multilayer graded-gap structures were carried out.
The structures were grown on n-type (100) GaAs by MOCVD system.
The basic element of the structure is the pair AlxGa1-xAs/GaAs (0<x<0.3). Each element extends
about 200 nm in thickness with the graded -gap layer occupying more than 90% of this extension.
At the end of the last AlxGa1-xAs layer, an undoped GaAs layer was grown, prior to the growth of
the p+GaAs contact layer. The samples was processed in to mesas for characterization.
Current-voltage and Capacitance-Voltage measurements are presented for structures with
different number of elements. The spectral response of the short circuit photocurrent was also
analyzed. An unusual negative response around 900 nm was found. This is probably caused by the
existence of the conduction band spike in the basic element.
S3.P34. EVIDENCE OF MULTIPLE PHASES IN CdTe/CdS/ITO/GLASS DEVICES
A. Martel1,2([email protected]), F. Caballero-Briones2, R. Castro-Rodríguez2, W. Cauich2
and J. L.Peña.2,3
1
Facultad de Física, Universidad de la Habana. San Lázaro y L. Vedado, 10400. La Habana,
Cuba.
2
Applied Physics Dept. CINVESTAV-IPN, AP 73, Cordemex, 97310 Mérida, Yucatán, México.
3
CICATA-IPN, Legaria 694, Col. Irrigación, 11500 México D.F.
In this work we use grazing incidence x-ray diffraction (GIXRD) to identify the phases
present in CdS/ITO/glass, CdS/glass and the entire CdTe/CdS/ITO/glass opto-device at different
conditions, as well as in the as-grown CdTe/CdS/ITO/glass heterostructures prepared by CSS,
without lifting off the overlying CdTe. To achieve this study, we used a conventional diffractometer
with independent θ and 2θ motion. The results obtained by this technique are correlated for the
different CdS/ITO, CdS/glass treatments and the conditions of growth of the CdTe/CdS/ITO/glass
heterostructure. By this way we show evidence of multiple phases in the above structures. Our
results suggest that phases such as CdInS, CdSnO3, Sn2S3, SnO2 could be formed ever since the CdS
deposition. These species may diffuse towards the CdTe/CdS interface, and up to the CdTe surface
in strong dependence with the CdTe growing conditions. The morphology of the as-deposited CdTe
layers, studied by scanning electron microscopy is also dependent on the its growing parameters,
specially on the oxygen partial pressure. We also study the CdTe layer by scanning electron
microscopy (SEM) to see the influence in the morphology of the films caused by the different CdTe
growth parameters varied.
S3.P35. STUDY OF THE SURFACE OXIDE REMOVAL OF GaAs (100)
SUBSTRATES PRIOR TO THE MBE GROWTH
60
A. Guillén Cervantes, Z. Rivera-Alvarez, M. López and J. Huerta1
Departamento de Física, CINVESTAV-IPN, Apdo Postal 14-740, 07000 México D.F.
1
CICATA-IPN, Unidad Querétaro, José Siurob 10, Querétaro, Qro76040, México
In this work the surface oxide removal process of GaAs (100) substrates in the growth
chamber of a II-VI MBE system was studied by in-situ reflection high energy electron diffraction
(RHEED), diffused reflectivity (DR) and ex-situ by atomic force microscopy (AFM). Additionally,
the GaAs (100) surface oxides desorption was monitored by recording the growth chamber pressure
variations. The results showed important variations in the RHEED images during the deoxidation
process which are related to the DR behavior and pressure changes in the MBE chamber. We have
attributed these changes to the evaporation of oxide species from the samples. Based on these
results a model intended to explain the GaAs (100) desorption mechanism is proposed. By the other
hand RHEED patterns of desorbed GaAs (100) surfaces showed different reconstructions, we
discuss these reconstructions in terms of the incorporation of elements of the group VI on the
sample surface.
S3.P36. SINGLE CRYSTALLINE ALUMINIUM THIN FILM GROWTH ON Si(111)
BY MBE: A RHEED STUDY
A. Zehe1([email protected]), A. Ramírez2 ([email protected]) and A. Thomas3
1
Universidad Autónoma de Puebla, FCFM, Apdo. Post. 1505, 72000, Puebla, México.
Universidad Autónoma de Puebla. ICUAP, 72500, Puebla, México.
3
Disetronik GmbH, Sulzbach/Ts., Germany.
2
Aluminum thin films were prepared on Si(111) wafers under UHV conditions by a
KNUDSEN-type evaporation. On clean substrate surfaces with a (7x7) reconstruction, epitaxial
growth can be achieved obeying the epitaxial relationship of {111}Si || {001}Al; <110>Si ||
<110>Al; which was established from in situ RHEED pattern. A twelve-fold macroscopic
symmetry of the deposit is found arising from a combination of the 4-fold and the 3-fold symmetry
of the {001} deposit plane, and of the substrate surface, respectively.
S3.P37. STUDY AND CHARACTERIZATION OF ZnXCd1-XO THIN FILMS
OBTAINED BY SPRAY PYROLYSIS TECHNIQUE.
L. Vaillantb ([email protected]), G. Santanac,1, O. Vigila,1, F. Cruza, A. Morales-Acevedoc and
G. Contreras-Puentea
a
Escuela Superior de Física y Matemática del IPN, Edifício 9 U.P. ALM. Lindavista. México C.P.
07738, México D.F.
b
Facultad de Física-IMRE, Universidad de la Havana, 43100, La Havana, Cuba.
c
Seccóin de Electórnica del Estado Sólido (SEES), Departamento de Ingeniería Eléctrica,
CINVESTAV del IPN, Apt. Postal 14-740,. México D.F.
1
Permanent address: Facultad de Física-IMRE, Universidad de la Habana, 43100, La Habana,
Cuba.
ZnxCd1-xO mixed thin films oxides were deposited on glass substrates by spray pyrolysis.
The effect of growth parameters on the films property is presented. Sample structure was studied by
X-ray diffraction (XRD) having founds the presence of the cubic phase resembling the CdO XRD
pattern, for low Zn concentrations. For x=0.75 a low crystalline of the ZnO structure was observed.
Optical band-gap and electrical resistivity were also studied from optical transmittance and four
points measurements respectively. As expected, the band-gap and resistivity values change between
those of pure CdO and those of ZnO.
61
S3.P38. OPTICAL AND ELECTRONIC PROPERTIES OF ORDERED ALLOYS
OF ZINCBLENDE Zn0.5Cd0.5Se(100)*
J. C. Salcedo1 ([email protected]), A. Camacho2
([email protected]), I. Hernández-Calderón3 ([email protected]), J.A.
Rodriguez1, J. Giraldo1 (jgiraldo@)ciencias.ciencias.unal.edu.co), L. M. HernándezRamírez3,([email protected]) and E. López-Luna3 ([email protected])
1
Departamento de Física, Universidad Nacional de Colombia, Santafé de Bogotá, Colombia.
Departamento de Física, Universidad de los Andes, Santafé de Bogotá, Colombia.
3
Departamento de Física, CINVESTAV, Av. IPN 2508, 07300 México, DF., México.
2
The zincblende ternary compound Zn0.5Cd0.5Se(100) has been grown following different
approaches: 1) by the typical molecular beam epitaxy technique (MBE), with simultaneous
exposure of the substrate to Zn, Cd and Se with the appropriate fluxes. 2) By atomic layer epitaxy
(ALE), where a layer of each element is (in principle)individually grown. By ALE it is also possible
to modify the layer sequences, for example, Zn-Se-Cd-Se... or Zn-Se-Zn-Se-Cd-Se-Cd-Se...., etc.
Then we have several ways to obtain the ternary compound with the same nominal composition and
different degrees of crystalline order. We present the initial results of the theoretical and
experimental investigation of these different structures. We use a semi-empirical tight binding
formalism to calculate the electronic band structure of the bulk alloy and build up the layer
Hamiltonian
following
the
surface-Green-function-matching
(SGFM)
formalism.
Photoluminescence and photoreflectance spectroscopies are employed for the investigation of the
optical properties, and Auger spectroscopy and reflection high energy electron diffraction (RHEED)
are employed to obtain chemical and structural information.. We discuss the effects of surface
reconstruction on the composition of the ternary compound.
*Work partially supported by Conacyt-Mexico.
S3.P39. PHOTOLUMINESCENCE AND PHOTOREFLECTANCE STUDIES OF
MBE GROWN ZnSe/CdTe/GaAs(100) HETEROSTRUCTURES.*
C.
Vargas-Hernándeza,#
([email protected]),
I.
([email protected]), O. de Melo+ ([email protected]),
([email protected])
Hernández-Calderóna
and E. López-Luna
Departamento de Física, CINVESTAV-IPN, Apdo. Postal 14-740, 07000 México, D. F.
In this work we presented results obtained from the growth by molecular beam epitaxy
(MBE) and characterization by low temperature photoluminescence spectroscopy (PL),
photoreflectance spectroscopy(PR), Auger spectroscopy (AES) and reflection high energy electron
diffraction (RHEED) of ZnSe/CdTe/GaAs(100) heterostructures. Layer between ∼ 8 Å (3 sec
growth) and ∼ 100 Å CdTe were grown on the cleaned GaAs(100) substrate. On these CdTe/GaAs
buffers we grew a series of samples with ZnSe film thickness in the range from 12 Å to 1 µm. The
analysis of the Auger spectra showed important reactivity of the CdTe film with the impinging Zn
and Se atoms. Exposition of CdTe to a Se flux produces a drastic reduction of the Te Auger signal.
These results suggest that a graded interface forms during the initial stages of growth of ZnSe on
CdTe. PR and PL were employed for the determination of band gaps and the characterization of the
structural properties through the analysis of the excitonic peaks and defect-related bands. The strong
self-activated band observed in the PL measurements indicated that the film contains an important
number of structural defects.
*Work partially supported by CONACyT.
+Permanent address: Faculty of Physics, University of Havana, Cuba.
# Permanent address: Universidad Nacional de Colombia-Manizales.