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Applied Surface Science 254 (2008) 2233–2237

Effect of ambient hydrogen sulfide on the physical properties

of vacuum evaporated thin films of zinc sulfide

Beer Pal Singh a,*, Virendra Singh b, R.C. Tyagi a, T.P. Sharma a

a Department of Physics, C.C.S. University, Meerut 250004, Indiab Forensic Science Laboratory, Malviya Nagar, New Delhi 110017, India

Received 8 May 2007; received in revised form 27 August 2007; accepted 27 August 2007

Available online 6 September 2007

Abstract

Evaporated thin films of zinc sulfide (ZnS) have been deposited in a low ambient atmosphere of hydrogen sulfide (H2S �10�4 Torr). The H2S

atmosphere was obtained by a controlled thermal decomposition of thiourea [CS(NH2)2] inside the vacuum chamber. It has been observed that at

elevated substrates temperature of about 200 8C helps eject any sulfur atoms deposited due to thermal decomposition of ZnS during evaporation.

The zinc ions promptly recombine with H2S to give better stoichiometry of the deposited films. Optical spectroscopy, X-ray diffraction patterns and

scanning electron micrographs depict the better crystallites and uniformity of films deposited by this technique. These deposited films were found

to be more adherent to the substrates and are pinhole free, which is a very vital factor in device fabrication.

# 2007 Elsevier B.V. All rights reserved.

PACS : 78.20.Ci; 78.50.Ge; 78.66.�w; 78.66.Hf

Keywords: Zinc sulfide; Hydrogen sulfide; Evaporation; Optical properties; Structural properties

1. Introduction

The wide band gap II-VI semiconductors are of current

interest for optoelectronic applications such as blue lasers, light-

emitting diodes, photonic crystals and optical devices based on

nonlinear properties [1,2]. This is because in these materials (e.g.,

ZnxCd1�xS, x! 0–1) the band gap can be tuned and the lattice

parameters can be varied easily by controlling stoichiometric

composition. ZnS is a II–VI group semiconductor material with a

large direct band gap and due to its high bulk refractive index (b-

ZnS, n589 = 2.36) and lack of absorption in the visible and near IR

region, is a convenient material for photonic applications such as

in blue light-emitting diodes [3] and other optoelectronics

devices such as electroluminescent display [4], cathodolumi-

nescent display and multilayer dielectric filters [5,6]. ZnS has

become a commercially viable material in nano-powder, when

prepared in the forms of nanostructures like: bi-crystals

nanoribbons [7], nanobelts [8] and electroluminescent thin films

* Corresponding author at: Department of Physics, C.C.S. University, Meerut

250004, India. Tel.: +91 121 2762021.

E-mail address: drbeerpal@gmail.com (B.P. Singh).

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.08.090

[9]. The preparation methods for ZnS films in the devices are

generally magnetron sputtering [10–13], molecular beam

epitaxy (MBE) [14], atomic layer epitaxy (ALE) [15], metal

organic chemical vapor deposition (MOCVD) [16], sol–gel [17]

and chemical bath deposition (CBD) [18] and vacuum

evaporation [19,20] technique.

The properties of ZnS films produced by sulfurizing the

sputtered ZnO films in H2S or sulfur-vapor ambient have been

reported recently but vacuum evaporation of ZnS film in H2S

ambient has not been unveiled. As a matter of fact, the structural,

electrical and optical properties of vacuum deposited thin films of

sulfide semiconductors are very sensitive to the deposition

conditions [19]. The vacuum evaporated thin films of ZnS are

usually polycrystalline and have excess of zinc owing to the

dissociation of ZnS during evaporation. The stoichiometry can be

restored by co-deposition of sulfur together with ZnS. However,

compensation of sulfur deficiency can also be accomplished by

exposing evaporated ZnS vapors to a low hydrogen sulfide during

growth itself. Thus it can be very important to understand the

properties and growth mechanism of the ZnS films prepared by

vacuum evaporation in sulfidation ambient and to explore a cost-

effective deposition technology of ZnS films. In this paper, we

report alternative preparation method of ZnS films, using vacuum

Fig. 1. ZnS film deposition rate with and without H2S ambient. The deposition

time was 30 min and the thiourea concentration was 20 mg for creating H2S

ambient.

B.P. Singh et al. / Applied Surface Science 254 (2008) 2233–22372234

evaporation deposition in H2S ambient and have studied the

effect of ambient hydrogen sulfide on the physical properties of

these ZnS films.

2. Experimental procedure

The method deals with the deposition of ZnS by vacuum

evaporation in a controlled H2S atmosphere. From the point of

view of vacuum evaporation, the thermal decomposition of

thiourea [CS(NH2)2] is a convenient source of H2S, which was

controlled by regulating the temperature of the electrically

heated Borosil1 test tube inside the evaporation chamber. The

higher reactivity of H2S has ensured a better conversion of the

dissociated zinc ions into ZnS and also has not produce any

excess of sulfur at the substrate. Furthermore, keeping the

substrate at an elevated temperature has ejected the dissociated

sulfur ions deposited during growth.

High purity ZnS (Aldrich, 99.99%) powder was grounded.

This powder was evaporated using a molybdenum boat filament

in a high vacuum chamber (pressure �5 � 10�5 Torr). During

the deposition of ZnS films, the filament and substrate were

kept about 10 cm apart. This long distance results in the

formation of quite uniform films. The films were deposited on

the glass substrates held at 200 8C in a vacuum (�5 �10�4 Torr). Prior to deposition, the glass substrates were

cleaned in aqua-regia and washed in distilled water and

isopropyl alcohol (IPA). A Borosil1 test tube was used for

thermal decomposition of thiourea at 150 8C. For low ambient

atmosphere of H2S, this was separated from the ZnS

molybdenum boat by a steel heat shield. Two lots of samples

were deposited which include ZnS film deposited without the

H2S ambience and ZnS films deposited in H2S ambience.

Several samples of ZnS film were deposited for different H2S

concentration. In all samples, the deposition time of 20 min was

the maximum time duration for obtaining a film thickness in

400–500 nm in normal and H2S ambient.

The crystallinity and phase of the films on glass substrates

were characterized by X-ray diffraction (XRD) measurement

using an X-ray diffractometer with Cu Ka radiation. Standard

u/2u scans were performed with 2u ranging from 158 to 408. The

surface morphology and composition of the films on glass

substrates were investigated by scanning electron microscopy

(SEM). The optical transmission and absorption spectra of the

ZnS films on glass substrates were recorded at room

temperature with the help of ‘‘Hitachi spectrophotometer

model U-3400’’. In this spectrometer all the lenses were

replaces with the mirrors to eliminate any image deviation due

to chromatic aberration in the wavelength range 187–2600 nm.

The PbS detector was used for the detection of infra-red rays.

The visible wavelength light source was a long life WL lamp.

3. Results and discussion

3.1. Optimization of deposition time

The deposition time and total pressure were kept constant at

30 min and 5 � 10�4 Torr, respectively. The base pressure in

the vacuum chamber was pumped down to 5 � 10�5 Torr.

Then, the ZnS films together with 5 g thiourea of 99.5% purity

were heated. For the studies of the H2S ambience effects, the

same ZnS films were prepared without H2S ambience at 200 8Csubstrate temperature. Deposition time was optimized by

taking a substrate out of chamber at regular interval of 30 min.

Fig. 1 shows the variation of film thickness with deposition

time. Film thickness increases up to 25 min deposition time

and then remains nearly constant. The average rate of

deposition in H2S ambient was 17–19 nm/min depending

upon the H2S ambience volume. The resultant average

thickness obtained in H2S ambience was about 495 nm and

about 350 nm for deposition made without H2S ambience for

total 20 min duration. The maximum rate of deposition of

22 nm/min was witnessed in 10–15 min interval for deposition

in H2S ambience.

3.2. Structural characterization

The crystallinity and phase of the films on glass substrates

were characterized by X-ray diffraction (XRD) and scanning

electron microscope (SEM). It is well known that pure ZnS

exist in two crystalline phases, i.e., a cubic form with

sphalerite structure and a hexagonal form with wurtzite

structure. As to ZnS, the cubic form (c-ZnS) is the low

temperature phase and the hexagonal form (h-ZnS) is the

polymorph stable at high temperatures. Because there are

usually just a few peaks in the diffractograms, and many

d-values between the two phases are overlapping or only have

very small difference, it is difficult to distinguish these two

phases from the thin film samples [19].

Fig. 2 displays the superimposed u/2u spectrum obtained for

ZnS films deposited onto glass substrates with and without H2S

ambient. The XRD (Fig. 2) revealed that all ZnS/glass

deposited with and without H2S ambient had only cubic

crystal structure with preferred orientation in [1 1 1] direction

Fig. 2. XRD pattern of vacuum evaporated ZnS thin films in H2S ambient and

without H2S.

Fig. 3. SEM micrograph for ZnS (a) film deposited in low H2S ambient and (b)

for ZnS film without H2S ambient. ZnS film grown in H2S ambient shows better

morphology and crystalline nature.

B.P. Singh et al. / Applied Surface Science 254 (2008) 2233–2237 2235

as depicted by single diffraction peak located at 2u = 28.88.For the as-grown ZnS film, the lack of diffraction from the

hexagonal planes indicates that as-grown ZnS films are

polycrystalline having zinc sulfide cubic structure with

preferred orientation along the [1 1 1] direction. XRD

indicates that the ZnS films grown in H2S ambient are also

having Wurtzite phase polycrystalline ZnS. The peaks from

the hexagonal (a) and cubic (b) phases of ZnS in H2S

ambient overlap considerably. In order to differentiate

between them the presence of the [1 0 0] peak at 2u = 278was taken as evidence for the existence of the hexagonal

structure [21].

Surface morphology of ZnS films deposited on glass

substrates with and without H2S atmosphere was examined by

scanning electron microscope (SEM). The scanning electron

micrographs of the films deposited with and without H2S

atmosphere are shown in Fig. 3. SEM analysis suggests that the

surface of ZnS films deposited without H2S atmosphere is

rough and grains are not distributed uniformly. On the other

hand the surface of ZnS films grown in H2S atmosphere

indicate that the film is rather smooth and consists of granular

particles, probably due to grain boundaries are possibly formed

because of the limited surface mobility at the deposited

substrate temperature.

Further for ZnS/glass films prepared from ZnS films in the

H2S ambient, appreciable phase transformation or changes in

the texture were not observed. The XRD patterns of ZnS films

deposited in H2S atmosphere (5 g thiourea concentration was

decomposed) did not show any appreciable changes in crystal

structure. However, the results suggested that ZnS films

formed H2S ambient has larger grains than those as-deposited

ZnS film (without H2S ambient), because the solid-phase re-

crystallization during the sulfidation process seems favorable

in H2S-vapor. This suggests a better surface morphology

achieved in the films grown in the H2S atmosphere. Thus the

surface morphology of ZnS thin films grown in low ambient

atmosphere of H2S has improved. The films deposited in the

H2S atmosphere were also found to be more adherent to the

substrate as observed by their resistance to erosion by

physical rubbing and also by their resistance to peeling by an

adhesive tape.

3.3. Optical characterization

Optical properties of ZnS films both with and without H2S

atmosphere were studied with the help of absorption and

transmission spectra. The ZnS film deposited in H2S ambient

atmosphere has sharp more absorption and the absorption edge

shifts towards the lower wavelength, this shifts towards the

lower wavelength indicates the increase of optical band gap.

The energy band gap of these films was calculated with the help

of absorption spectra and comes out as 3.70 and 3.66 eV with

and without H2S atmosphere, respectively [22]. The slightly

higher band-gap value of 3.70 eV was obtained for the ZnS

films produced in the H2S ambient. The slightly higher band-

gap energies of ZnS films prepared in H2S ambient compared to

that of as-grown ZnS films can be attributed due to the

formation of S rich composition in ZnS film.

The optical transmittance spectra of as-deposited ZnS films

and films deposited with H2S anbient atmosphere in the

wavelength range 300–850 nm are shown in Fig. 4. It can be

seen that ZnS films produced in H2S, with the good

transparency of about 80% in the visible region, have somewhat

steeper absorption edge than as-grown ZnS, implying the good

homogeneity in the distribution of the composition for the

former.

The optical constants (refractive index n and the extinction

coefficient k) of these films have been determined from

transmittance measurements by using Manifacier’s envelope

Fig. 4. Transmission spectra of vacuum evaporated ZnS thin films for both with

and without H2S atmosphere.

Fig. 5. Variation of refractive index (n) and extinction coefficient (k) with

wavelength (l) of vacuum evaporated ZnS thin films for both with and without

H2S atmosphere.

B.P. Singh et al. / Applied Surface Science 254 (2008) 2233–22372236

method [23]. The transmission spectra of ZnS films both with

and without H2S atmosphere in the spectral range 300–850 nm,

as shown in Fig. 4 were used to determine the refractive index

(n) and extinction coefficient (k).

The refractive index (n) and extinction coefficient (k) were

calculated using the formula [23].

n ¼ ½N þ ðN2 þ n20n2

1Þ1=2�

1=2

where n0 is the refractive index of air; n1 the refractive index of

substrate (glass) and the number N is given by

N ¼ n20 þ n2

1

2þ 2n0n1ðTmax � TminÞ=TmaxTmin

where Tmax is the upper extreme transmission point and Tmin is

the lower extreme transmission point for particular wavelength.

The extinction coefficient k is given by

k ¼��l

4pt

�lnð pÞ

where t is the thickness of the film, calculated by a computer-

ized program through reflection spectroscopic method with the

help of spectrophotometer and

p ¼ ðnþ n0Þðn1 þ nÞðn� n0Þðn1 � nÞ �

1� ðTmax=TminÞ1þ ðTmax=TminÞ

:

Fig. 5 shows the variation the optical constants (n and k)

versus l for vacuum evaporated zinc sulfide films both with

and without H2S atmosphere. As we move away from the

absorption edge towards the longer wavelength, transitions

involving donor level, acceptor level and traps and even

intraband transitions would become more pronounced. These

would results in more losses, which is reflected in the increased

k with the increasing wavelength in Fig. 5. The decreased

absorption will also reflected in lower value of n as is also

evident from the mathematical expressions for n and k as given

in this section.

4. Conclusion

It is found that ZnS films deposited in a low ambient

atmosphere of H2S are of dark color, more uniform, pin hole

free, have better adhesion to the substrates, have better

crystallinity and have better transparency compared to the films

deposited without H2S atmosphere and would be better suited

for device fabrication.

Acknowledgement

One of the authors Beer Pal Singh would like to thank to

Rakesh Kumar, Department of Physics, C.C.S. University,

Meerut, for encouraging and giving valuable suggestions

during this course of research work.

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