Chemically deposited se thin films and their use as a planar source of selenium for the formation of metal selenide layers

Article abstract of DOI:10.1149/1.2201471

Selenium thin films of thickness ～300 nm were deposited from a solution of sodium selenosulfate of pH 4.5. These films are amorphous, but they are crystalline and photoconductive through annealing for 15 min at 150- 200 C. In this paper we present the properties of these films and their use as a planar source of selenium vapor (1.7× 10-6 mol cm2 of the film) to react with metal films to form metal selenide layers. For this, metal films of Ag, Sn, In, Cu, Sb, etc., deposited by thermal evaporation, are kept in contact with the Se-thin film and are heated at temperatures typically < 350 C in nitrogen. This process leads to the formation of thin films of Ag2 Se, SnSe2, In2 Se3, CuSe Cu2-x Se, Sb2 Se3, etc. Se-metal reaction also takes place under microwave heating. In the case of an evaporated Ag film on a chemically deposited thin film of Sb2 S3, AgSbSe2 is produced through heating at 200- 300 C. Photovoltaic structures SnO2: F-CdS- Sb2 S3 - AgSbSe2 fabricated this way show open-circuit voltage >500 mV and short-circuit current density of 2-5 mA cm2.

Full text of DOI:10.1149/1.2201471

C526
Journal of The Electrochemical Society, 153 ͑7͒ C526-C534 ͑2006͒
0013-4651/2006/153͑7͒/C526/9/$20.00 © The Electrochemical Society
Chemically Deposited Se Thin Films and Their Use as a Planar
Source of Selenium for the Formation of Metal Selenide
Layers
a
,z
*
K. Bindu, M. T. S. Nair, and P. K. Nair
Department of Solar Energy Materials, Centro de Investigación en Energia, Universidad Nacional
Autónoma de México, Temixco, Morelos 62580, Mexico
Selenium thin films of thickness ϳ300 nm were deposited from a solution of sodium selenosulfate of pH 4.5. These films are
amorphous, but they are crystalline and photoconductive through annealing for 15 min at 150–200ؠC. In this paper we present the
properties of these films and their use as a planar source of selenium vapor ͑1.7 ϫ 10−6 mol/cm2 of the film͒ to react with metal
films to form metal selenide layers. For this, metal films of Ag, Sn, In, Cu, Sb, etc., deposited by thermal evaporation, are kept in
contact with the Se-thin film and are heated at temperatures typically Ͻ350ؠC in nitrogen. This process leads to the formation of
thin films of Ag2Se, SnSe2, In2Se3, CuSe/Cu2−xSe, Sb2Se3, etc. Se–metal reaction also takes place under microwave heating. In the
case of an evaporated Ag film on a chemically deposited thin film of Sb2S3, AgSbSe2 is produced through heating at 200–300ؠC.
Photovoltaic structures SnO2:F–CdS–Sb2S3–AgSbSe2 fabricated this way show open-circuit voltage Ͼ500 mV and short-circuit
current density of 2–5 mA/cm2.
© 2006 The Electrochemical Society. ͓DOI: 10.1149/1.2201471͔ All rights reserved.
Manuscript submitted December 20, 2005; revised manuscript received February 20, 2006. Available electronically May 25, 2006.
Different groups have reported on the chemical deposition of thin
layers,15 etc., open up new possibilities in producing ternary com-
pounds of metal selenides or solid solutions. The reaction between
the metal film and Se film can be initiated in a microwave oven, in
the same manner as reported in the production of silver selenide16 or
metal nitrides.17 The reactions that produce CuInSe2, CuGaSe2, etc.,
from elemental components are exothermic and may be selfpropa-
gating once they are initiated.18 Chemically deposited semiconduc-
tor thin films of more than 60 binary compounds are well investi-
gated so far,19 and many metal films can be integrated to these by
solution deposition.20 All these suggest that the use of Se thin films
as a planar source of Se in the preparation of novel semiconductor
compounds or semiconductor heterojunctions for large area applica-
tions is worthy of attention.
The results in this paper are organized as follows: ͑i͒ structural,
optical, and electrical properties of Se thin films; ͑ii͒ reaction of Se
thin film with metal films to form metal selenides; ͑iii͒ formation of
metal selenides by microwave heating; and ͑iv͒ a photovoltaic struc-
ture, SnO2:F–CdS–Sb2S3–AgSbSe2, formed when a CdS–Sb2S3–Ag
layer is heated in contact with a Se thin film.
films of selenium.1-3 Such films have found application in the prepa-
ration of semiconductor thin films of metal selenides either by heat-
ing stacked layers of selenium and evaporated metal films to form
4
5
In2Se3 and CuInSe2 or by heating separate substrates coated with
selenium and metal films held in contact facing each other to pro-
duce SnSe2.6 A Se thin film of thickness 350 nm may be deposited
at 10ؠC in 15 h from a dilute solution of sodium selenosulfate at pH
4.5 with a thin-film deposition yield of nearly 90%.6 This thin film
can produce sufficient Se vapor required for a reaction in Sb2S3–Ag
layers to form photoconductive AgSbSe2, which in a photovoltaic
structure ͑SnO2:F–CdS–Sb2S3–AgSbSe2͒ provided open-circuit
voltage ͑Voc͒ of 530 mV and short-circuit current density͑Jsc͒ of
1.7 mA/cm2.7 In this paper, we report the properties of chemically
deposited amorphous Se thin films made crystalline through anneal-
ing Se–ZnS or Se–Al films at 150–200ؠC for 15–30 min. It is known
that in the hexagonal or trigonal form, selenium films are excellent
photoconductors with spectral sensitivity in the entire visible range.8
The hexagonal Se phase is thermodynamically most stable, and in
this work it is formed through annealing chemically deposited Se
thin films.
Experimental
Chemically deposited Se thin films were held in contact with
evaporated metal thin films and heated at temperatures of 80–350ؠC.
Using evaporated metal films of Cu, Sn, Ag, Sb, and In,
Cu2−xSe–CuSe mixed phase, SnSe2, SnSe–SnSe2, Ag2Se, Sb2Se3,
and In2Se3 are formed by heating at distinct temperatures, typically
Ͻ400ؠC. Structural, optical, and electrical properties of these films
are presented. Among the possible application of chemically depos-
ited Se thin film as a planar source of selenium to produce metal–
selenide layers is solar cell technology. By heating a Sb2S3–Ag layer
deposited on CdS-coated transparent conducting oxide ͑TCO͒ glass,
in contact with a Se thin film of different thicknesses, an absorber
layer of AgSbSe2 is formed and a photovoltaic structure results. This
option offers an alternative to the conventional process of heating
metal films or alloys such as Cu–In–Ga in H2Se9,10 or in Se vapor
evolved from an elemental Se source11-14 used in preparing
CuIn1−xGaxSe2 absorber layers for solar cells.
Deposition of selenium thin films.— Thin films of Se were de-
posited as reported in Ref. 6 on microscope glass slides ͑Corning,
75 ϫ 25 ϫ 1 mm thickness͒ coated with a chemically deposited
ZnS thin film and heated at 250ؠC for 15 min, in air to stabilize it to
the substrate.21 To deposit a Se film, an approximately 0.1 M solu-
tion of Na2SeSO3 was prepared by refluxing 0.79 g of selenium
powder ͑99.5% Aldrich͒ with 100 mL of 0.3 M solution of Na2SO3
͑Baker Analyzed Reagent, 98–99%͒ for 3 h at about 100ؠC. This
solution was diluted to 0.01 M, and 80 mL of it was taken in a
100 mL beaker to which 2 mL of 4.4 M acetic acid was added to
attain a pH of 4.5. This pH of the solution is an essential condition
to deposit elemental Se thin film.1,3 The solution quickly turns
orange-red. The films were deposited on the substrate mentioned
above at 10ؠC. Se film thickness is 300 nm in 10 h and 350 nm in
15 h. Thickness of the films was measured using an Alpha Step 100
͑Tencor Instruments, CA͒.
In order to study the evolution of structural, optical, and electri-
cal properties of selenium thin films by heating, a very thin layer of
ZnS ͑50 nm, chemical deposition͒ or aluminum ͑50 nm, vacuum
thermal evaporation͒ was deposited over them. These layers are es-
sential to prevent the degradation of chemically deposited amor-
phous ͑red͒ selenium to powdery crystalline gray Se in air or subli-
mation in nitrogen under reduced pressure. The Se–ZnS and Se–Al
layers ͑substrate film of ZnS is neglected to simplify, to the notation͒
were heated at 150–250ؠC in air for 15 min to 4 h in a furnace or at
The formation of AgSbSe2 thin films by heating a Sb2S3–Ag
layer in Se vapor7 and of crystalline and photoconductive solid so-
lutions of Sb2SxSe3−x by heating chemically deposited Se–Sb2S3
*
Electrochemical Society Active Member.
a Present address: Facultad de Ingeniería Mecánica
Autónoma de Nuevo León, Nuevo León, México
z E-mail: mtsn@cie.unam.mx
y Eléctrica, Universidad
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Journal of The Electrochemical Society, 153 ͑7͒ C526-C534 ͑2006͒
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Table I. Design of metal–selenium reactions leading to the formation of semiconducting metal–selenide thin films.
Atomic mass
͑g/mol͒;
Selenium requirement for stoichiometric
reaction with metal film of 100 nm in
thickness
mass density Semiconducting Formula mass ͑g/mol͒;
22
Metal
Cu
͑g/cm3͒
material
mass density ͑g/cm3͒
Crystalline structure ͑Eg͒
63.546; 8.92
Cu2−xSe
196.52; 6.90
͑2 − x͒Cu + Se → Cu2−xSe
͑100 nm͒Cu + ͑123 nm͒Se → ͑199 nm͒Cu2−xSe
͑100 nm͒Cu + ͑245 nm͒Se → ͑133 nm͒CuSe
Sn + 2Se → SnSe2
Cubic; 1.1–1 eV for x = 0–0.75
CuSe
SnSe2
142.51; 5.99
276.61; 5.95
Hexagonal
Hexagonal; 1.16 eV
Sn
118.7; 7.31
͑100 nm͒Sn + ͑214 nm͒Se → ͑278 nm͒SnSe2
Sn + Se → SnSe
SnSe
Ag2Se
Sb2Se3
In2Se3
197.65; 6.19
294.70; 8.24
480.38; 5.84
466.52; 5.92
Orthorhombic; 0.9 eV
Orthorhombic; 0.07 eV
Orthorhombic; 1.1 eV
Rhombohedral; 1.2–1.5 eV
͑100 nm͒Sn + ͑107͒Se → ͑97 nm͒SnSe
2Ag + Se → Ag2Se
Ag
Sb
In
107.87; 10.49
121.75; 6.68
114.82; 7.31
͑100 nm͒Ag + ͑78 nm͒Se → ͑160 nm͒Ag2Se
2Sb + 3Se → Sb2Se3
͑100 nm͒Sb + ͑183 nm͒Se → ͑287 nm͒Sb2Se3
2In + 3Se → In2Se3
͑100 nm͒In + ͑166 nm͒Se → ͑244 nm͒In2Se3
150ؠC in nitrogen atmosphere ͑300 mTorr͒ for 1 h in a vacuum
oven to study the crystalline properties of the films.
puter were used for electric measurements. J−V characteristics of
photovoltaic structures were recorded by applying silver print con-
tacts of approximately 1 mm2 area.
Design of metal–selenium reactions.— The thickness of sele-
nium thin film ͑Se atomic mass, 78.97 g/mol; mass density,
4.5 g/cm3 for red Se͒ required for reacting with Cu, Sn, Ag, Sb, In,
and Se metal films of 100 nm thickness to form corresponding metal
selenide thin films may be estimated as given in Table I. For
stoichiometric reactions involving Se vapor, a higher thickness of Se
thin film would be required to compensate for losses. A Se film of
thickness 300 nm was used; it would supply 1.7 ϫ 10−6 mol of Se
per cm2 of film surface. The crystal structure and the bandgap ͑Eg͒
values22 of the semiconductor compounds are listed in the table. The
materials chosen here are relevant to thin-film solar cells.
Results and Discussion
Selenium thin films.— XRD analysis.— Elemental selenium ex-
ists in several allotropic forms: amorphous, monoclinic, trigonal,
and hexagonal.23 The as-prepared Se–ZnS films are amorphous in
accordance with previous reports.1-3,21 Figure 1 shows the XRD pat-
terns recorded at grazing incidence of ␪ = 1.5ؠ for Se–ZnS thin films
annealed under different conditions. Figure 1a shows the pattern for
hexagonal-phase Se ͑PDF-73–0465; a = 4.3662 Å, c = 4.9536 Å͒,
and Fig. 1b shows the pattern of the film annealed at 150ؠC in air for
15 min. All the peaks present in this pattern, indicated by the planes
͑100͒, ͑101͒, ͑012͒, and ͑003͒ as well as the others, match the stan-
dard pattern in relative intensity and position. This film appears deep
orange in daylight. The role of the overlying film of ZnS is to pre-
vent the degradation of Se thin film to powdery deposit or its rapid
sublimation. Figure 1c shows the pattern for the Se–ZnS film an-
nealed at 150ؠC for 4 h. All the peaks of the hexagonal Se are still
present, though the relative intensity of reflection from ͑012͒ planes
has enhanced. When the film is annealed in air at 200ؠC for 15 min
an XRD pattern Fig. 1d similar to Fig. 1b is obtained. However, in
the case of the films annealed at 200ؠC for 4 h only remnants of
͑101͒ and ͑102͒ diffraction peaks are present ͑Fig. 1e͒, which indi-
cates substantial loss of Se. Figure 1f is a pattern recorded for a
Se–ZnS film annealed in air at 250ؠC for 15 min; it shows that all
the XRD peaks of Se have disappeared, indicating total loss of Se
from the film under this condition. Peaks corresponding to crystal-
line phases of ZnS or of other components are absent in these pat-
terns. Hence, sublimation of selenium through the ZnS layer is pos-
sible in air at 200–250ؠC. Figure 1g is an XRD pattern for Se–ZnS
film recorded after its annealing at 150ؠC for 1 h in N2 ͑300 mTorr͒.
This pattern is similar to the patterns in Fig. 1b-d, showing that Se
film crystallizes in hexagonal form under this condition of annealing
as well. The material loss due to sublimation at 150ؠC under the
reduced pressure is not appreciable.
Evaporation of metal thin films and metal–selenium reaction.—
A small quantity of Cu, Sn, Ag, Sb, and In ͑99.999%, Aldrich-
Puratronic͒ were placed in an appropriate Mo/W boat and evapo-
rated onto a clean microscope glass substrate ͑Corning͒ kept about
15 cm above, after reaching a pressure of 10−6 to 10−5 Torr in a
vacuum deposition unit. The amount of the metal used and the ap-
proximate thin-film thickness of the metals deposited on the glass
substrates were: Cu-20 mg ͑100 nm͒; Sn-10 mg ͑50 nm͒, 20 mg
͑100 nm͒;6 Ag-20 mg ͑100 nm͒, 40 mg ͑200 nm͒;7 Sb-40 mg
͑200 nm͒; and In-10 mg ͑60 nm͒. For metal–selenium reaction, the
metal thin films were held in contact with the Se film, wrapped in
Teflon sealing tape. It was heated at temperatures of 80–400ؠC for
1 h in 300 mTorr of nitrogen in a vacuum oven.
Characterization.— X-ray diffraction ͑XRD͒ patterns of the
films were recorded using a Rigaku D-Max diffractometer in grazing
incidence at fixed angles ͑␪ = 1.5ؠ͒ or in the standard mode ͑␪
− 2␪͒ using Cu K␣ ͑wavelength, ␭ = 1.5406 Å͒ radiation. X-ray
fluorescence ͑XRF͒ measurement on Se was done in a Philips Mag-
iXPRO X-ray fluorescence spectrometer having a rhodium anode
operating at 50 kV and 40 mA. Se K␣ XRF radiation was analyzed
using LiF ͑200͒ planes at 2␪ = 31.8476ؠ. Optical transmittance ͑T͒
and specular reflectance ͑R͒ spectra of the films were recorded using
a Shimadzu UV 3100 PC spectrophotometer, with air and front alu-
minized mirror, respectively, as references. For electrical measure-
ments a pair of silver electrodes was painted at 5 mm separation and
5 mm length on the surface of the films. A tungsten halogen lamp
providing illumination intensity 1 kW/m2 at the plane of the sample
was utilized to measure photoconductivity. The duration of illumi-
nation was 10 s under a bias of 1–100 V, depending on the resis-
tance of the samples. A Keithley 230 programmable voltage source
and a Keithley 619 electrometer/multimeter interfaced with a com-
A study similar to that for Se–ZnS film was done for Se–Al thin
films annealed in air at different temperatures, as shown in Fig. 2.
The XRD patterns recorded for the thin films annealed at 150–250ؠC
for 15 min are given in Fig. 2. At 150ؠC, selenium thin film crystal-
lizes in the hexagonal form as seen in Fig. 2b. The pattern matches
that of the standard, repeated in Fig. 2a. In the case of the films
annealed at 200ؠC, the peak intensity ͑Fig. 2c͒ is reduced, and for
the film annealed at 250ؠC for 15 min, the pattern is devoid of any
peaks. Thus, under this condition of heating Se sublimes through the
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C528
Journal of The Electrochemical Society, 153 ͑7͒ C526-C534 ͑2006͒
Figure 2. XRD patterns of Se–Al thin films subjected to heating: ͑a͒ stan-
dard pattern of hexagonal selenium; ͑b͒ 150ؠC, 15 min; ͑c͒ 200ؠC, 15 min;
and ͑d͒ 250ؠC, 15 min; ͑inset͒ Williamson–Hall plot for the film 200ؠC, 1 h.
Figure 1. XRD patterns of Se–ZnS thin films subjected to heating: ͑a͒ stan-
dard pattern of hexagonal selenium; ͑b͒ 150ؠC, 15 min; ͑c͒ 150ؠC, 4 h; ͑d͒
200ؠC, 15 min; ͑e͒ 200ؠC, 1 h; ͑f͒ 200ؠC, 4 h in air; and ͑g͒ 150ؠC in nitro-
gen ͑300 mTorr͒, 1 h.
due to annealing the films at 150 or 200ؠC for 15 min. This result is
in accordance with comparable peak intensities observed in the
XRD patterns of the films, given in Fig. 1 and 2. At these tempera-
tures the presence of ZnS or Al film over the Se film would greatly
inhibit the evolution of Se vapor for reaction with metal films. Pro-
longed reaction time of ϳ4 h at 200ؠC is required to release enough
Se, ϳ40–75% of the 1.7 ϫ 10−6 mol/cm2 of the film. At 250ؠC,
almost all the Se is lost from the films in 15 min. This conclusion is
in accordance with the absence of Se XRD peaks in Fig. 1f and 2d.
Hence, a planar source of Se–ZnS or Se–Al would be desirable for
reactions designed for this temperature to facilitate efficient use of
the Se vapor toward the intended reaction.
Al film, as was the case with Se–ZnS film. Peaks of aluminum or
aluminum selenide are not observed in these patterns.
Average crystallite grain size of selenium in the film was esti-
mated using the diffraction line broadening analysis, in the case of
Se–Al thin films annealed at 150ؠC for 15 min, of Fig. 2b. The line
broadening can occur due to small grain size as well as due to
microstrains associated with the thin films. The size and strain effect
can be separated by the Williamson–Hall plot,24 in which
͑␤ cos ␪͒/␭͒ vs ͑sin ␪/␭ are plotted, ␤ being the full width at half
maximum ͑fwhm͒ of the diffraction peak located at 2␪. This plot can
be fitted to a straight line. The inverse of the intercept of the line on
͑␤ cos ␪͒/␭ axis gives the crystalline grain diameter, and the slope
indicates the microstrain. Assuming Lorentzian shape for the diffrac-
tion peaks, the Williamson–Hall plots were constructed, taking into
account the prominent peaks due to ͑100͒, ͑101͒, and ͑012͒ reflec-
tions. The result is presented in the inset of Fig. 2. Grain size and
microstrain evaluated for the Se film from the plot are 19 nm and
3 ϫ 10−3, respectively. The grain diameter for hexagonal Se in
Se–ZnS film annealed under the same condition ͑Fig. 1b͒ is about
15 nm.
Optical properties
.— Optical transmittance ͑T͒ and reflectance ͑R͒
spectra of the ZnS thin films and those of Se–ZnS films are given in
Fig. 3a. Thin films of ZnS, as-prepared or annealed at 200ؠC, have
negligible optical absorption for ␭ Ͼ 400 nm due to their wide
bandgap, 3.7 eV.22 Optical absorption is significant at ␭ Ͼ 400 nm
Table II. XRF results of Se–ZnS and Se–Al thin films heated at
different temperatures.
Amount of Se ͑cps͒
Temperature of
heating ͑°C͒ in air
Time
Se–Al
Se–ZnS
XRF analysis.— Table II illustrates the loss of selenium in the films
in terms of the intensity of Se K␣ radiation in Se–ZnS and Se–Al
thin films annealed at different temperatures in air. Due to a slight
variation in the thickness of Se films, only the general trend in the
variation in the Se K␣ peak intensity ͑as prepared, 900 10% cps͒
of the annealed films is relevant. There is no significant loss of Se
As-prepared
150
200
0
904
888
815
540
41
-
15 min
15 min
4 h
958
876
232
104
250
15 min
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Journal of The Electrochemical Society, 153 ͑7͒ C526-C534 ͑2006͒
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Figure 5. ͑a͒ XRD patterns, ͑b͒transmittance ͑T͒, and reflectance ͑R͒ spectra
of Cu thin film heated in contact with a Se thin film at 200 and 300ؠC in
nitrogen ͑300 mTorr͒, for 1 h.
Figure 3. ͑a͒ Transmittance ͑T͒ and reflectance ͑R͒ spectra of ZnS and
Se–ZnS as-prepared or heated at 200ؠC, 15 min; ͑b͒ plots for energy band-
gap, Eg, evaluation of amorphous Se ͑as-prepared Se–ZnS͒; ͑c͒ bandgap
evaluation for hexagonal Se ͑Se–ZnS, 200ؠC, 15 min͒.
observe photoconductivity.21 Photocurrent response is observable in
for the as-prepared and annealed Se–ZnS films. The optical absorp-
as-prepared Se–ZnS film ͑curve b͒. Dark conductivity ͑␴ ͒ of the
d
tion coefficients ͑␣͒ at different wavelengths ͑photon energies, h␯͒
film was evaluated from the dark current and is about
10−10 ͑⍀ cm͒−1. Photosensitivity, s = ͑␴ − ␴ ͒/␴d, where ␴ is
for such films were calculated using the equation,
␣
ph
d
ph
= ͑1/d͒ln͑100 − R͒/T, where, d is the thickness of the Se film,
the electrical conductivity under illumination, and has been com-
puted for the films. The as-prepared Se–ZnS film has s Ϸ 0.1. The
value of s increases to Ͼ10 after the Se–ZnS films are annealed at
150ؠC in air for 15 min or for 4 h ͑curves c and d͒. For films an-
nealed at 200ؠC for 15 min in air ͑curve e͒ or at 150ؠC in N2 for 1 h
͑curve f͒, s Ϸ 100.
1/2
300 nm. We observe in Fig. 3b and c that plots of ͑␣h␯͒ against
h␯ fit into straight lines. The intercepts of the straight lines on pho-
ton energy axes are at 1.75 and 1.5 eV, and hence, both optical gaps
are indirect. The optical gap values are marked by arrows in the
transmittance curves in Fig. 3a. These values are lower than the
optical gap reported for amorphous ͑2 eV͒3 and hexagonal ͑1.8 eV͒
forms of selenium.2
Metal–selenide thin films.— We consider chemical deposition19
as the simplest method to prepare many semiconductor metal–
selenide thin films, including the ones presented here. By using ei-
Electrical properties.— Figure 4 shows the photocurrent response
recorded in Se ͑300 nm͒–ZnS ͑50 nm͒ films annealed under differ-
ent conditions. In curve a it is seen that the underlying film of ZnS
of 150 nm thickness has negligible contribution to photocurrent;
such a film requires annealing in air at temperatures Ͼ300ؠC to
ther
sodium
selenosulphate,25
selenourea,26
or
N,N-dimethylselenourea27 as a source of selenide, several metal–
selenide thin films have been prepared, as reviewed in previous
reports.19,28,29 The advantage of the present approach is that metal
films may be deposited on a chemically deposited semiconductor
film and reacted with Se-vapor from a Se film to produce semicon-
ductor heterojunctions or ternary and quaternary semiconductor thin
films.7 The metal films can be patterned using appropriate masks to
produce patterned heterojunctions. Evaporation of metal films in a
vacuum chamber and then reacting in Se vapor ensures cleaner at-
mosphere in the evaporation chamber and could lead to better repro-
ducibility of film or device characteristics. Alternatively, metal films
such as Ag and Cu, which are important components of thin-film
photovoltaic materials and well-investigated for silver mirror
production30 and for Cu contacts for integrated circuits,31 can be
deposited by solution deposition.20 In this latter approach the depo-
sition process becomes a nonvacuum process, just as in chemical
deposition. The simplicity and cost consideration of the process
would make it attractive for large-area production of the films.
Copper selenide ͑Cu2ϪxSe and CuSe͒
.— Figure 5a shows the XRD
patterns of the thin films formed by heating copper thin films of
90 nm thickness held in contact with the Se film at 200ؠC ͑curve b͒
or 300ؠC ͑curve c͒ in nitrogen ͑300 mTorr͒. Copper–selenide film
thickness, as measured in Alpha Step, is close to 100 nm. The film
formed at both the temperatures consists of mixed phases of
Cu2−xSe ͑berzelianite; cubic with a = 5.739 Å; curve a͒ and CuSe
͑klockmannite; hexagonal with a = 3.98 Å, c = 17.254 Å͒. Relative
higher intensity of Cu2−xSe Peaks denoted by the planes ͑111͒,
͑200͒, 220͒, and ͑311͒ at 2␪ positions 26.94, 31.21, 44.84, and 53ؠ,
respectively, suggests that the main reaction product is Cu2−xSe. In
Figure 4. Photocurrent response of thin films of: ͑a͒ ZnS; ͑b͒ Se–ZnS; ͑c͒
Se–ZnS, 150ؠC, 15 min; ͑d͒ Se–ZnS 150ؠC, 4 h; ͑e͒ Se–ZnS 200ؠC, 15 min
in air; and ͑f͒ Se–ZnS 150ؠC in nitrogen ͑300 mTorr͒, for 1 h.
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Journal of The Electrochemical Society, 153 ͑7͒ C526-C534 ͑2006͒
Figure 6. XRD patterns: ͑a͒ standard pattern of Ag2Se ͑orthorhombic͒; ͑b͒
Ag2Se thin film formed by heating Ag thin film of thickness 50 nm; and ͑c͒
100 nm, in contact with Se thin film at 200ؠC in nitrogen ͑300 mTorr͒, for
1 h.
chemical deposition using sodium selenosulfate, the material depos-
ited is Cu2−xSe or Cu3Se2,32,33 and it is CuSe when
N,N-dimethylselenourea is used as the source of selenide ions.33
Electrical conductivity of the Cu2−xSe/CuSe film is 2–5
ϫ 103 ⍀−1 cm−1 ͑p-type by hot probe test͒. Such conductivity is
typical in chemically deposited films of these materials as well.33
The nature of the transmittance and reflectance spectra of these films
shown in Fig. 5b is in accordance with the behavior explained in
Ref. 33: the low optical transmittance, 10%, and the high reflec-
tance, Ͼ50%, in the near-infrared region arise from free carrier
processes in the degenerate semiconductor.
Figure 7. XRD patterns ͑a͒ standard orthorhombic Sb2Se3; Sb2Se3 thin films
formed by heating Sb thin film of thickness 200 nm in contact with selenium,
͑b͒ 350ؠC, and ͑c͒ 400ؠC, in nitrogen ͑300 mTorr͒, for 1 h.
reported in electrodeposited Ag2Se thin films from an aqueous so-
lution of KSCN, AgNO3, KNO3, and SeO2.34 The electrical conduc-
−1
tivity of the silver selenide reported here is 10−1
⍀
cm−1
.
Antimony selenide ͑Sb Se ͒
.— XRD patterns of films formed by
2
3
Tin selenide ͑SnSe , SnSe –SnSe͒
.— The use of chemically depos-
2
2
heating a thin film of Sb ͑200 nm͒ in contact with Se film at 350 and
400ؠC in nitrogen ͑300 mTorr͒ are given in Fig. 7b and c, respec-
tively. The peak positions in these patterns are in agreement with
that of orthorhombic Sb2Se3 ͑PDF-15-0861͒ given in Fig. 7a. Crys-
talline grain diameter evaluated utilizing ͑020͒ diffraction peak for
the film ͑b͒ formed at 350ؠC is 25 nm. From the overall reduction in
peak intensities, it is apparent that there is loss of material through
sublimation when the film is processed at 400ؠC.
Formation of Sb2Se3 films by this approach has not been previ-
ously reported. This meterial in bulk crystalline form has an optical
bandgap near 1 eV.22 Figure 8a-c shows the transmittance ͑T͒ and
reflectance ͑R͒ spectra, plot of optical obsorption coefficient ͑␣͒ vs
h␯, and the plot to evaluate Eg of Sb2Se3 thin films formed at 350ؠC,
respectively. In order to evaluate the absorption coefficient, experi-
mental data on optical transmittance was corrected by considering
reflection losses from the front surface ͑␣:Tcorr͒: ␣ = ͑1/d͒ln͑100
− R%͒/T% or by considering multiple reflections within the thin
film ͑␣:MR͒
ited Se thin films of 80–350 nm in thickness in the preparation of
SnSe and SnSe2 thin films either by heating a vacuum-deposited Sn
thin film in close contact with the Se thin film at 200ؠC in a nitrogen
atmosphere for 1 h or by heating a Se-Sn stack in air at 400ؠC for
about 5 min has been reported in Ref. 6. By controlling individual
film thicknesses and the conditions of heating, SnSe2 or composite
films of SnSe2 and SnSe are formed. Optical bandgap of 1–1.27 eV
and electrical conductivity of 0.01–0.2 ⍀−1 cm−1 has been observed
in these materials. In bulk form, SnSe2 is an n-type and SnSe a
p-type semiconductor.22
Silver selenide ͑Ag Se͒
.— A reaction between a thin film of Ag held
2
in contact with Se thin film to form Ag2Se occurs at 80ؠC in vacuum
͑100 mTorr͒.7 XRD patterns of the films shown in Fig. 6b and c are
for Ag2Se films formed from silver films of approximately 50 and
100 nm thickness. Ag2Se crystallizes in orthorhombic phase ͑a
= 4.333 Å, b = 7.062 Å, c = 7.764 Å, PDF 24-1041͒, which is the
stable phase at temperatures Ͻ135ؠC. The cubic phase ͑a
= 4.99 Å, PDF 27-0619͒ is stable at temperatures above 135ؠC.
There are differences between the relative intensities of the peaks in
the recorded patterns and that of Ag2Se ͑PDF 24-1041͒ given in Fig.
6a. However, the peak positions match. For both films the most
intense XRD peaks match 2␪ = 22 . 919ؠ for ͑002͒ reflection. The
intensity of this peak is only 6% in the standard pattern; the major
peaks due to diffraction planes ͑112͒ and ͑102͒ are completely sup-
pressed in the patterns obtained for these films. Oriented film growth
of this type along the ͑002͒ plane is common in thin films—it was
2
2
2
1/2
1
␣ = ln
d
1
͑100 − R%͒
͑2T%͒
͑100 − R%͒
2T%
+
+ R%2
ͩ
ͪ
ͫ
ͬ
ͭ
ͮ
100
The absorption spectra evaluated by both estimates are nearly the
same, as illustrated in Fig. 8b. The linear fit in the ͑␣h␯͒ vs h␯
2/3
plot in Fig. 8c suggests that the optical bandgap of this material is
1.03 eV ͑direct͒, involving forbidden transitions. This value closely
corresponds to the onset of strong optical absorption shown in Fig.
8a. Photocurrent response of Sb2Se3 thin film produced at 350ؠC is
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Journal of The Electrochemical Society, 153 ͑7͒ C526-C534 ͑2006͒
C531
Figure 8. ͑a͒ Transmittance ͑T͒, reflectance ͑R͒, and corrected transmittance
͑Tcorr͒ spectra of Sb2Se3 thin film formed at 350ؠC; ͑b͒ optical absorption ͑␣͒
spectra calculated using data on Tcorr and ͑T and R͒ for the film; ͑c͒ plots for
bandgap ͑Eg͒ evaluation of the film.
Figure 10. ͑a͒ XRD patterns of In2Se3 thin film formed by heating indium
thin film ͑60 nm͒ in contact with Se thin film at 200 and 300ؠC and standard
pattern of rhombohedral In2Se3; ͑b͒ Transmittance ͑T͒, reflectance ͑R͒, and
2/3
corrected transmittance Tcorr spectra of In2Se3; ͑Inset͒ plot of ͑␣h␯͒ vs h␯.
given in Fig. 9. Current–Voltage ͑I–V͒ characteristics of the film in
the dark and under illumination, shown here, illustrate the ohmic
nature of silver contacts on it. Electrical conductivity of the film in
the dark is 10−4 ͑⍀ cm͒−1. Photosensitivity is ϳ4 under an illumi-
nation intensity of 1 kW/m2.
1.2–1.5 eV.22 In a previous paper, a bandgap of 1.8 eV was reported
for In2Se3 films formed by heating the Se–In layer.4
The photocurrent response of In2Se3 film formed at 300ؠC is
given in Fig. 11. Electrical conductivity of the film in the dark is
ϳ10−5 ͑⍀ cm͒−1; the photosensitivity is Ͼ10. The photocurrent
measurements made under different bias voltages show that silver
contacts to In2Se3 films are near-ohmic.
Indium selenide ͑In Se ͒
.— Figure 10a shows the XRD pattern ob-
3
2
tained for the films formed by heating at 200 and 300ؠC in nitrogen
for 1 h an indium thin film in contact with Se film. The peak posi-
tions correspond to that of rhombohedral In2Se3 ͑PDF 45-1041͒. The
optical transmittance and reflectance spectra of the film formed at
300ؠC are given in Fig. 10b. The optical absorption coefficient was
evaluated from the Tcorr data and plotted as shown in the inset for
the evaluation of optical bandgap. The intercept is at 1.6 eV, corre-
sponding to direct ͑forbidden͒ transitions. This value is indicated in
the Tcorr plot and is found to correspond to the onset of strong
optical absorption. The value reported for bulk In2Se3 is
Microwave processing of Se–Ag and Se ϩ Sb layers.— A
stacked layer of Se–Ag ͑Ag film thickness, 100 nm͒ deposited on
glass substrate was introduced into a commercial microwave oven
rated at 900 W. The oven was operated for 5 min without resorting
to any further refinement as a technique, such as those reported in
Ref. 17 and 18. For the formation of Sb2Se3, a selenium thin film
was held in contact with Sb ͑100 nm͒ and heated in the microwave
oven for 10 min. Figure 12a shows an XRD pattern of microwave-
processed Ag–Se thin film. The formation of orthorhombic Ag2Se
͑PDF 24-1041 given in the figure͒ and crystallization of Se into
Figure 9. Photocurrent response of Sb2Se3 thin film formed at 350ؠC re-
corded with different bias voltages; ͑Inset͒ I–V characteristics of the film in
the dark and under illumination.
Figure 11. Photocurrent response of In2Se3 thin film measured with different
bias voltages.
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C532
Journal of The Electrochemical Society, 153 ͑7͒ C526-C534 ͑2006͒
Figure 13. Schematic representation of ͑a͒ process of fabrication of a PV
structure using a Se thin film ͑deposited on a ZnS-coated glass substrate, and
shown facing down͒, which serves as a planar source of selenium when
heated in contact with silver-coated Sb2S3 thin film forming the AgSbSe2
absorber, and ͑b͒ the completed device structure.
in an evacuated oven with 300 mTorr of nitrogen, Ag2Se is formed
first, which reacts with Sb2S3 in the presence of excess Se to pro-
duce AgSbSe2. This layer was further heated at 300ؠC for 5 min in
air under atmospheric pressure. With silver contacts applied to
AgSbSe2, a photovoltaic structure results, shown in Fig. 13b. The
cell is illuminated from the SnO2:F side.
Optical transmittance spectra of the photovoltaic cell structures
formed using Se film of 60 and 350 nm in thickness are shown in
Fig. 14. The low transmittance in the near-infrared region in the
spectra is due to the degenerate semiconductor film, SnO2-F; the
optical interference causes the wavy curves. The abundance of Se
vapor for the cell processed with a Se film of 350 nm thickness
results in a higher optical absorption in this cell structure. This is
due to the formation of Sb2SxSe3−x ͑optical bandgap, 1 eV
upward͒15 underneath the AgSbSe2 film during heating under the Se
vapor. Figure 15a shows the dark current and photocurrent response
of Sb2S3 thin films deposited on glass slides subjected to heating at
200ؠC in nitrogen with a selenium thin film ͑60 nm͒ in contact and
Figure 12. ͑a͒ XRD patterns of Ag2Se and Sb2Se3 thin films formed by
microwave processing of Ag–Se stack layer and ͑b͒ Sb layer in contact with
Se thin film.
hexagonal form are evident here. The relative intensities of XRD
peaks of Ag2Se films produced by this method are distinct from
those produced from Ag films deposited on glass slides and heated
in Se vapor given in Fig. 6a. For example, reflections due to ͑102͒,
͑120͒, ͑112͒, and ͑121͒ planes which are absent in Fig. 6a are ob-
served in the microwave-processed film. Overall, the Ag2Se peaks in
this case are well-defined as compared with patterns reported for this
material resulting from microwave processing of a mixture of Ag
and Se powder, reported in Ref. 16. The crystalline grain size evalu-
ated from the ͑101͒ planes of Se is 18 nm and from the ͑013͒ planes
of Ag2Se is 19 nm. Figure 12b shows the XRD pattern of thin film
formed by microwave heating of Sb thin film in Se vapor. Polycrys-
talline Sb2Se3 film with orthorhombie structure is produced in this
process, similar to that obtained by normal heating in nitrogen for a
longer duration ͑Fig. 7͒.
without selenium. Dark conductivity ͑␴ ͒ and the photoconductivity
d
͑␴ ͒ of the films are enhanced by the presence of selenium in the
p
annealing atmosphere. Also, the transmittance ͑T͒ spectra of these
films ͑Fig. 15b͒ show that optical absorption is enhanced by the
presence of selenium during the heating.
Fast material processing is the basic advantage here. Refinement
of the technique through the use of controlled atmosphere, pressure,
and temperature may lead to the use of Se film in microwave pro-
cessing of metal selenides.
Chemically deposited Se film as a planar source for photovoltaic
cell processing.— Se thin films reported here may be produced in
batch on glass substrates coated with ZnS films and used as a planar
source for producing photovoltaic structures. In this section we
present results to underline the need to optimize the Se film thick-
ness and process conditions to achieve improved characteristics of
SnO2:F–CdS–Sb2S3–AgSbSe2 cells reported previously.7 Figure
13a and b shows the scheme of preparing the solar cell structure.
Thin CdS films of thickness 80–100 nm are deposited at 60ؠC
from a citratocadmium bath35 on sheet glass of 3 mm thickness with
SnO2:F thin film of sheet resistance 8 ⍀, available commercially as
TEC-8 provided by Pilkington ͑Toledo͒. Thin film of Sb2S3 of thick-
ness 450 nm was deposited at 10ؠC in three successive depositions,
as described in Ref. 36. A silver film of 100 nm was thermally
evaporated on this film and then placed facing the Se thin film of
either 60 nm ͑3 h deposition͒ or 350 nm ͑14 h deposition͒, with the
edges sealed with Teflon sealing tape. When heated at 200ؠC for 2 h
Figure 14. Transmittance ͑T͒ spectra of the PV device glass–
SnO2:F–CdS–Sb2S3–Sb2SxSe3−x–AgSbSe2–Ag.
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Journal of The Electrochemical Society, 153 ͑7͒ C526-C534 ͑2006͒
C533
thin film in nitrogen ͑or air͒ in a vacuum oven or by microwave
processing. Formation of photovoltaic structures by this approach
has been illustrated, demonstrating the role of optimizing the supply
of Se vapor for the cell formation to achieve improved characteris-
tics.
Some salient features of Se chemical deposition are worth high-
lighting here: Deposition of Se film is possible with laboratory re-
agents of typically 98% assay; the use of high-purity chemicals is
required only to deposit metal films. The chemical deposition of Se
thin films has a deposition yield of nearly 50%.6 The precipitate is
reusable in the preparation of sodium selenosulfate, the starting ma-
terial for Se film formation. The chemically deposited Se thin film
may be covered by a thin film of ZnS 50 nm in thickness for stor-
age. Heating of such a layer at 150ؠC would retain the Se in crys-
talline form, but heating at 200–250ؠC would release Se for reacting
with metal films either to form metal selenides or related semicon-
ductor films of ternary ͑AgSbSe2͒ composition. This approach es-
sentially eliminates the need for high-purity metal selenides as
evaporation or sputtering source, which would require an additional
source of elemental Se or H2Se in the vacuum chamber to adjust
stoichiometry.
Figure 15. ͑a͒ Photocurrent response and ͑b͒transmittance ͑T͒ of Sb2Se3 thin
film heated at 200ؠC in nitrogen for 1 h with and without the selenium
source.
Acknowledgments
The authors are grateful to DGAPA-UNAM and CONACYT-
Mexico for the financial support provided for this work. Maria Luisa
Ramón helped with recording the XRD patterns and Patricia Altuzar
C with the XRF measurements. José Campos and Oscar Gomez-
Daza provided technical assistance for the electrical characterization
and chemical deposition. We are grateful to Pilkington ͑Toledo͒ for
the donation of TEC-TCO glass for our work.
J–V characteristics recorded in the dark and under illumination
͑1 kW/m2͒ with tungsten halogen radiation for both cell structures
are shown in Fig. 16. The cell formed using the thicker Se film
shows a higher short-circuit current, Jsc = 5.3 mA/cm2. This in-
crease is directly associated with efficient optical absorption in
Sb2SxSe3−x compared with the Sb2S3-only layer, Eg: 1.88 eV,22 un-
der the AgSbSe2 film. Both cells have open-circuit voltages ͑Voc͒
Ͼ 500 mV. The optical absorption in both cells has to be enhanced
by increasing the film thicknesses so that values of Jsc could be
improved. The overall thickness of the absorber layer
͑Sb2S3–Sb2SxSe3−x–AgSbSe2͒ in the present cells is ϳ0.5 ␮m;
most well-researched thin-film solar cells have absorber layer thick-
ness Ͼ2 ␮m. These issues need to be addressed in future work.
Universidad Nacional Autonoma de Mexico assisted in meeting the pub-
lication costs of this article.
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