NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Optical Properties of Zn(O,S) 

Thin Films Deposited by RF 

Sputtering, Atomic Layer 

Deposition, and Chemical Bath 

Deposition

Preprint

Jian Li

National Renewable Energy Laboratory and Colorado 

School of Mines

Stephen Glynn, Steven Christensen, 

Jonathan Mann, Bobby To, Kannan 

Ramanathan, Rommel Noufi, and Dean Levi

National Renewable Energy Laboratory

Thomas E. Furtak

Colorado School of Mines

Presented at the 2012 IEEE Photovoltaic Specialists Conference

Austin, Texas

June 3–8, 2012

Conference Paper

NREL/CP-5200-54128

June 2012
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1

Optical Properties of Zn(O,S) Thin Films Deposited by RF Sputtering, 

Atomic Layer Deposition, and Chemical Bath Deposition

Jian Li1,2, Stephen Glynn1, Steven Christensen1, Jonathan Mann1, Bobby To1, Kannan Ramanathan1, 

Rommel Noufi1

, Thomas E. Furtak2, and Dean Levi1

1

National Renewable Energy Laboratory, Golden, CO 80401, USA

2

Department of Physics, Colorado School of Mines, Golden, CO 80401, USA

Abstract — Zn(O,S) thin films 27 – 100 nm thick were 

deposited on glass or Cu(InxGa1-x)Se2/Molybdenum/glass with RF 

sputtering, atomic layer deposition, and chemical bath 

deposition. The complex dielectric functions ε of these films were 

extracted by spectroscopic ellipsometry and transmission 

analyses. It is found that ε varies on a large scale, indicative of 

significant variations in the films’ chemical and physical 

properties, with the growth methods and deposition parameters. 

By fitting the ε spectra based on the parabolic band 

approximation, the E0 critical point energies and broadening 

parameters were quantified to provide insights on the band gap, 

defect density, and phase segregation.

Index Terms — dielectric constant, ellipsometry, nanocrystals,

photovoltaic cells, thin films, wide band gap semiconductors,

Zinc compounds.

I. INTRODUCTION

ZnS is an attractive material for photovoltaic (PV) 

applications. Potentially, it can replace the widely used n-type 

junction partner CdS with the advantages of wider band gaps 

and low toxicity. As-grown ZnS thin films could contain a 

significant amount of oxygen [1,2], which was found, i.e., in 

this study, to be beneficial to cell performance in some cases. 

Therefore, Zn(O,S) thin films with or without intentional 

addition of oxygen are of broad interest to PV technologies.

However, properties of near-intrinsic ZnS films vary with 

growth conditions, and the introduction of oxygen makes such 

variations substantially more complicated. In the Zn(O,S)

thickness regime that is suitable for fabricating solar cells, 

usually ≤ 50 nm, many characterization techniques, such as 

x-ray diffraction and band gap calculation from transmission 

and reflectance, cannot provide reliable information due to the 

small thickness, whereas ellipsometry is a powerful tool 

owning to its high sensitivity to the film thickness and optical 

properties [3], both of which are important for understanding 

the materials properties and optimizing the cell performance.

II. EXPERIMENTAL DETAILS

Zn(O,S) thin films were deposited on glass substrate by RF 

sputtering and atomic layer deposition (ALD), and on 

Cu(InxGa1-x)Se2/Mo/glass by chemical bath deposition (CBD). 

For RF sputtering, the key process variables include: 1. 

deposition temperature T in the range of 20 – 200°C; 2. Ar 

pressure pAr in the range of 4 – 16 mTorr; 3. O2/[O2+Ar] flow 

ratio in the range of 0 – 10%; 4. target power P in the range of 

100 – 200 watts. The baseline sputtering parameters are: 

T=20°C, pAr=16 mTorr, P=200 watts, and O2/[O2+Ar]=0. For 

ALD performed with diethylzinc and [H2S+H2O], the key 

process variables include: 1. T; 2. the H2O/H2S pulse ratio. 

For CBD using an aqueous solution containing zinc sulfate 

(16.3 mM), thiourea (0.6 M), and ammonia (1.8 M), the key 

process variable was the bath temperature in the range of 60-

85°C.

Spectroscopic ellipsometry (SE) and normal incidence 

transmission measurements were performed using a rotating 

compensator ellipsometer. For sputtered and ALD films, an

SE methodology was adopted from that established in our 

previous works [4]. Both the film side and through-glass SE 

were performed at multiple angles of incidence (AOI), using a 

pair of focusing optics. For CBD films, only the film side 

measurements were performed at multiple AOIs, without the 

focusing optics.

III. RESULTS AND DISCUSSIONS

A. SE and transmission analyses

The ellipsometric angles (ψ, ∆) are defined as: 

tan(ψ)exp(i∆)≡rp/rs, where rp and rs are the complex amplitude 

reflection ratio for the p- and s- polarized incidence light, 

respectively [3]. In the analyses of the SE and transmission 

data, each Zn(O,S) film was assumed to have a “bulk” layer 

and a surface over-layer. This was found to be necessary in 

order to obtain a reasonable fit to the SE data measured on 

both sides of the sample and the transmission simultaneously, 

especially in the spectral region near the band gap. Figure 1 

demonstrates the experimental data and the model fit for the 

sample sputtered with the baseline parameters. The analysis of 

Fig. 1 returns a bulk layer thickness of 43.2 ± 0.1 nm, a 

surface over-layer thickness of 6.7 ± 0.1 nm, and the optical 

properties of the (“bulk”) Zn(O,S) film to be discussed in the 

following subsections.
2

B. RF Sputtering

The extracted complex dielectric functions ε=ε1+iε2 of 

selected sputtered Zn(O,S) films are compared in Fig. 2. In the 

spectral range of 1.0 – 4.0 eV, the dominant critical point (CP) 

feature in ε is the direct band gap transition E0 [5]. To deduce 

the E0 CP parameters, the second derivative spectra d2

ε/dE2

were fit to an expression based on a parabolic band 

approximation and Lorentzian broadening:

0 (E ) A [exp(i )][E E i(Γ / 2)] 0 0 0 0 0

μ ε = φ − − (1)

where A0, E0, Γ0, μ0, and φ0 are the amplitude, transition 

energy (also the fundamental band gap Eg), broadening 

parameter, exponent, and phase, respectively, of the E0 CP 

[5]. As an example, the fit to the d2

ε/dE2 spectra of the P=200 

watts sample in Fig. 2(e) is shown in Fig. 2(f).

Theoretically, Γ0 is related to the mean free path λ of 

excited carriers through the equation: 

Γ0 = Γb0 + (hυg0/λ) (2)

where Γb0 is the broadening parameter of the single crystal 

(with a perfect lattice), h is the Planck constant, and υg0 is the 

group speed of excited carriers associated with the E0

transition [6]. Therefore, if defects scattering is the dominant 

mechanism that reduces λ, then Γ0 is indicative of the average 

defect density; if grain boundary is the dominant scattering 

source, then Γ0 is indicative of the average grain size [6].

Fig. 1. The experimental SE and transmission data, and the model 

fit for the Zn(O,S) sample sputtered with the baseline parameters.

1 2 3 4

5

6

7

8

ε2

 T=20O

C 

 Eg=3.32 eV Γ0=0.51 eV

 T=200OC

 Eg=3.61 eV Γ0=0.28 eV

photon energy (eV)

ε1, ε2

ε1

0.0

0.5

1.0

1.5

2.0

2.5

1 2 3 4

5

6

7

8

ε2

 pAr=16 mTorr 

 Eg=3.32 eV Γ0=0.51 eV

 pAr=4 mTorr 

 Eg=3.37 eV Γ0=0.45 eV

photon energy (eV)

ε1, ε2

ε1

0.0

0.5

1.0

1.5

2.0

2.5

1 2 3 4

-2

-1

0

1

2

3

4

5

6

7

8

ε2

 O2/[O2+Ar]=0 

 Eg=3.32 eV Γ0=0.51 eV

 O2/[O2+Ar]=2% 

 Eg=3.58 eV Γ0=0.90 eV

photon energy (eV)

ε1, ε2

ε1

0.0

0.5

1.0

1.5

2.0

2.5

1 2 3 4

1

2

3

4

5

6

 pAr=8 mTorr

O2/[O2+Ar]=10%

ε2

 24.5 nm bottom layer

 23.9 nm top layer 

photon energy (eV)

ε1, ε2

ε1

0

1

2

3

4

5

1 2 3 4

5

6

7

8

ε2

 P=100 watts 

 Eg=3.29 eV Γ0=0.46 eV

 P=200 watts 

 Eg=3.30 eV Γ0=0.46 eV

photon energy (eV)

ε1, ε2

ε1

pAr=8 mTorr

0.0

0.5

1.0

1.5

2.0

2.5

3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7

-15

-10

-5

0

5

10

15

 d2ε1/dE2

 fit

 d2ε2/dE2

 fit

d2

ε/dE2 (eV-2

)

photon energy (eV)

(b)

(c)

(d)

(e)

(f)

(a)

Fig. 2 (a) – (e): comparison of the ε=ε1+iε2 spectra of the Zn(O,S) thin films RF sputtered under the specified conditions and otherwise 

identical baseline parameters; (f): d2ε/dE2 spectra of the P=200 watts sample in Fig. 2(e) and the fit based on Eq. (1). The deduced E0 critical 

point energy Eg and the broadening parameter Γ0 are also displayed in the relevant panels.
3

Information from Fig. 2 includes the following.

2(a). Higher T decreases the defect density, possibly by 

increasing the grain size. Higher T also increases Eg, possibly 

by reducing the near-gap defect states.

2(b). Lower pAr decreases the defect density, possibly by 

increasing the grain size. Although this effect is not as clear as 

higher T in Fig. 2(a), it is consistent with the atomic force 

microscopy (AFM) images shown in Fig. 3.

2(c). The introduction of 2% O2 flow significantly decreases

the index of refraction n and increases Eg of the Zn(O,S) film.

The lower n is consistent with adding O into ZnS, as n of ZnO 

is significantly lower than that of ZnS. However, a higher Eg

is opposite the expectation of adding O into ZnS. One 

possible reason for increased Eg is the quantum confinement 

effect for nano-sized or amorphous grains with very small λ, 

consistent with the observed very large Γ0 value.

2(d). If O2 flow was further increased to 10%, the film became 

absorbing and hence not suitable as the buffer layer. SE + 

transmission analysis indicates that the film can be 

approximately divided into two layers with similar 

thicknesses. Their ε have a main CP feature near 3.0 eV, close 

to the Eg of ZnO. Another CP feature near 1.0 eV, however, is 

not present in the ε of any other samples in this study. The 

spectral range of this CP feature resembles that of the 

interband transition in single crystal Zn [7], indicating the 

existence of metallic Zn throughout the film. It is also clear 

that the main CP features in the bottom layer are broader as 

well as blue shifted compared to the top layer. A possible 

reason is the smaller grains in the bottom layer that broaden 

the CP according to Eq. (2) and at the same time blue shift the 

CP energies due to the quantum confinement effect.

2(e). P does not have a clear effect on the film properties.

C. Atomic Layer Deposition

The ε spectra of selected ALD Zn(O,S) films are compared 

in Fig. 4. Information can be drawn as follows.

4(a,b). In the range of 120°C ≤ T ≤200°C, there is a weak 

increase of Eg with increasing T. The trend in Γ0, however, is 

not clear. There may exist an intermediate T around 150°C, at 

which the grain size is the smallest.

Fig. 3 Atomic force microscopy (AFM) images of the two 

Zn(O,S) films in Fig. 2(b). The left panel corresponds to the 16 

mTorr film and the right panel corresponds to the 4 mTorr film.

(a)

(e)

(f)

(b)

(c)

(d)

Fig. 4 (a) – (d): comparison of the dielectric functions ε of the Zn(O,S) thin films deposited by atomic layer deposition (ALD) under the 

shown conditions; (e) and (f): second derivative spectra for the films in (c) and (d).
4

4(c,d). In the range of H2O/H2S ≤ 50%, E g decreases 

significantly, from that of ZnS toward that of ZnO, with 

increasing H2O. At H2O/H2S=80%, there is a ZnS-ZnO phase 

segregation, indicated by the two, instead of one, CP features 

in ε, one corresponding to ZnO and the other to ZnS. This is 

even clearer in the d2

ε/dE2 spectra shown in Figs. 4(e) and 

4(f), where the H2O/H2S≤20% films have a dominant peak 

and the H2O/H2S=80% film has two isolated peaks. The 

H2O/H2S=50% film appears to be at the onset of the phase 

segregation.

D. Chemical Bath Deposition

In this study, CBD films make excellent solar cells, with 

performances significantly better than cells with sputtered or 

ALD Zn(O,S), approaching the performance of cells with 

CdS. The ε spectra of two CBD Zn(O,S) films are compared 

in Fig. 5, together with the T=120°C ALD film with no H2O. 

Some distinctive characteristics can be seen for the CBD 

films. 

1. High Eg. Although X-ray photoelectron spectroscopy 

indicates that there is more O than S in the CBD films, their 

band gaps are the highest among all the films in this study. 

2. Low index of refraction. N of the CBD films are ~ 20% less 

than that of the ALD film, even lower than pure ZnO, 

indicating porous structure. One immediate impact of low n 

on the cell fabrication is the necessary modification to the 

anti-reflection design. This topic will be discussed in detail in 

another IEEE-PVSC-38 paper.

3. Long absorption tail below the band gap. This indicates the 

existence of a significant amount of states within the gap.

ACKNOWLEDGEMENT

This work was supported by the U.S. Department of Energy 

under Contract No. DE-AC36-08-GO28308 with NREL and 

by the Colorado School of Mines under Subcontract No. 

UGA-0-41025-17.

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Fig. 5 Comparison of the ε spectra of the Zn(O,S) thin films 

deposited by chemical bath deposition (CBD) under the shown 

conditions and the ALD film with H2O/H2S=0 in Figs. 4(c) and 4(d).