Photoconductors based on ZnxCd1−xS thin films

Thin Solid Films, 207 (1992) 231-235

Photoconductors

231

based on Zn,Cdl_,S

thin films

J. Torres and G. Gordillo Departamento

de Fisica, Universidad National,

Bogotli (Colombia)

(Received February 27, 1991; revised May 21, 1991; accepted July 2, 1991)

Abstract Photoconductors based on evaporated Zn,Cd, _$S thin films with a spectral responsivity at room temperature in the range between 0.25 and 1.2 pm were fabricated. A special method for successivedeposition of several (Zn,Cd)S layers with different zinc concentrations without opening the evaporation chamber was developed. In this way it was possible to obtain photoresistors with different structures and thus with different spectral responsivities. The results indicated that the photosensitivity of the (Zn,Cd)S is basically associated with band-to-band photoexcitation and the presence of sensitizing centres caused by cadmium and zinc vacancies.

1. Introduction

enrichment uncontrollable

Suitably prepared cadmium sulphide (CdS) is one of the most sensitive photoconductors known today, especially for the detection of visible and near-IR radiation at room temperature. The high photosensitivity of these crystals is associated with the presence of a particular type of sensitizing centre caused by cadmium vacancies [l[; these centres are acceptor impurities of a type whose energy levels are located about 1.1 eV above the valence band [2, 31. The mixed crystals of (Zn,Cd)S, besides their high photosensitivity, have the advantage that one can obtain a wide range of photodetection either by using different activators or simply by using a different percentage ratio of zinc and cadmium ions. By a proper choice of composition one can shift the detection band from UV to near IR. Photoconductors made of II-VI single-crystal compounds and compressed and sinterized CdS have been reported [4]. In this paper we describe the fabrication of photoresistors made of polycrystalline (Zn,Cd)S thin films with special structures, which permit photodetection in the spectral region between 0.25 and 1.2 pm.

in the evaporation and thus leads to an gradient of composition with depth [5].

Two separate evaporation sources of CdS and ZnS always lead to a lateral gradient in the layer. Therefore we used an evaporation source specially designed for the deposition of ternary compounds [6]. The source includes two coaxial chambers, one for ZnS and one for CdS. The flux of ZnS and CdS from the single chamber is determined by nozzles as shown in the schematic drawing of Fig. 1.

- awit nozzle mlxtng chamber

-nozzle

for ZnS

.nozzle

for CdS

2. Zn,Cd,,S thin film deposition and photoconductor fabrication technology

An appropriate deposition of the Zn,Cd, _ .$ thin films is necessary in order to obtain good performance of the photoconductors. Evaporation of a homogenized mixture of ZnS and CdS or Zn,Cd, _ XSpowder results in zinc

0040-6090/92/%5.00

Fig. I. Coaxial

source for deposition

0

of ternary

compounds.

1992 ~ Elsevier Sequoia.

AI1 rights reserved

J. Torres, G. Gordillo I Photoconductors based on ZnxCd t_ ~S

232

Any desired composition o f the vapour can be achieved by varying the diameter of the openings of the chambers and choosing an appropriate evaporation temperature. Figure 2 shows the variation in optical gap and thus zinc concentration of the (Zn,Cd)S layers as a function of the evaporation temperature, the diameter of the openings of the chambers and the exit nozzle diameter. Using a high evaporation temperature, a large diameter o f the CdS nozzle and a small diameter of the exit nozzle, we can obtain (Zn,Cd)S layers with zinc concentrations lower than 5%, because under these conditions the pressure within the mixing chamber caused by the CdS vapour is greater than the pressure inside the ZnS chamber, thus limiting the exit of the ZnS vapour. In contrast, a combination o f low evaporation temperature, small diameter of the CdS nozzle and large diameter of the exit nozzle leads to (Zn,Cd)S layers with high zinc concentration. In principle it is possible to obtain (Zn,Cd)S layers with arbitrary zinc content simply by choosing an appropriate set of the above deposition parameters.

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~

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~

II OCd$=O.?mm, I~lex.--Bmm

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1¢

~2,6'

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.102

2b,

101

q

X"~

~ 100 0 0'1 012 0.3 014 015 Fig. 3. Variation in optical gap and resistivity ofZn~Cdl _~Sthin films as 0"

a function of zinc concentration, with Evaporation temperature as parameter• energy-dispersive X-ray spectroscopy and photoluminescence (allowing a correlation between the zinc content and optical gap of the (Zn,Cd)S films). For thermodynamic reasons the vapour becomes metal rich at elevated pressure and temperature. The resistivity of the film can also be adjusted by varying the nozzle diameters and the source temperature [6]. The resistivity of the film is also largely influenced by the zinc concentration. An example of the resistivity variation as a consequence of varying the zinc concentration and evaporation is shown in Fig. 3. 2.1. Structure of the photoconductors

2.4

t ,'V.

O;

1 00

ill

1 20

i

i

T(OC) i

1050

i

I

1070

Fig. 2. Variation in optical gap of Zn,Cd I _,S layers as a function of evaporation temperature,with diameters of openings of CdS and ZnS chambers and of exit nozzle as parameters.

Thin-film-based photoconductors with a sandwichtype structure as shown in Fig. 4 were fabricated. The three ZnxCdl xS layers with different zinc concentrations were deposited one on the other in continuous form without opening the deposition chamber. Devices with a zinc concentration profile as shown in Fig. 4 were

X(~m) Changes in zinc content o f the (Zn,Cd)S layers lead to changes in optical gap of the (Zn,Cd)S as shown in Fig. 3. In this way it is possible to shift the optical gap o f the ZnxCdl _xS layers from 2.4 eV (at x = 0 ) up to 3.6 eV (at x = 1). A typical variation in zinc concentration as a function of the diameter of the CdS nozzle (tPcas) with the evaporation temperature as parameter is shown in the inset of Fig. 3. The composition of the films has been determined by the following methods [6]: polarography (an electrochemical method which requires partial dissolution of the film),

B

lnS-(ZnCd)S-cdS__

ii!i-°~: - "

240

] .... Ag~ ~

Gloss

Z.4

Z.7

Eg(eV)--~,3.3 3.6

3.0

Fig. 4. Structure and zinc concentration profile of ZnxCd I _xS-based

photoconductors.

233

J. Torres, G. Gordillo / Photoconductors based on ZnxCd1_xS

obtained by choosing a suitable set of deposition parameters. At the beginning of the deposition the evaporation temperature was 1070 °C in order to obtain a layer with a very low zinc concentration corresponding approximately to a CdS layer). After 2 min the evaporation temperature was decreased to 970 °C in order to obtain an increment in the zinc content of the (Zn,Cd)S layer (see Fig. 2). The ZnS layer on the top of the device was deposited at the end of the process when the whole CdS source material became completely evaporated. The purpose of fabricating photoresistors with the structure shown in Fig. 4 is to obtain a photodetector with a wide range of spectral photosensitivity, because in this way the spectral photosensitivity of the device is the superposition of the photosensitivities of the CdS, (Zn,Cd)S and ZnS layers. In order to determine the contributions of the three layers to the photocurrent, it is necessary to deposit an electrical contact on the top layer and another one on the bottom. If the two electrical contacts are deposited on the top layer or on the bottom layer, only the top layer or the bottom layer would contribute to the photosensitivity of the device, because the resistivity of the top layer is about eight orders of magnitude larger (see Fig. 3). On the other hand, if the electrical transport is perpendicular to the substrate, the photoconductors present a response velocity greater than in the case of electrical conduction parallel to the substrate, because the mobility of carriers moving perpendicularly to the substrate is larger than that of carriers moving parallel to the substrate [7].

3. Measurements of the spectral photosensitivity

Measurements at room temperature of the spectral photosensitivity of (Zn,Cd)S-based photoresistors were carried out using an ELH lamp of 300 W power as light source and applying a voltage difference of 1-10 V. Typical results of the spectral photosensitivity of the ZnxCdl - xS-based photoresistors are shown in Fig. 5. The inset shows the device structure. The results indicate that the devices present a high photosensitivity in the spectral region corresponding to the energy gap of the ZnxCdl-xS layers (region I in Fig. 5). This photosensitivity is caused by excitation of electrons from the valence band to the conduction band. By changing the zinc content of the (Zn,Cd)S layers, the photosensitivity peaks caused by band-to-band photoexitation in region I can be shifted from 2.4 eV (at x = 0) to 3.6 eV (at x = 1). There is experimental evidence obtained from luminescence studies [8] and EPR and ODMR measurements [9] carried out on monocrystals of ZnS and CdS which indicates that defects generated by zinc and cadmium vacancies and complexes formed by metal

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h(nm)--~

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600

800

1000

1200

Fig. 5. Normalized curves of typical spectral photosensitivity of photoresistorsmadeofpolycrystalline(Zn,Cd)Sthinfilmswithdifferent zinc concentrations. vacancy~clonor nearest-neighbour pairs give rise to deep acceptor states located 1.1 eV above the valence band. On the other hand, photoluminescence studies on (Zn,Cd)S thin films [10] indicate the presence of deep acceptor states located 1.6 and 1.1 eV above the valence band, caused by zinc and cadmium vacancies, and of shallow donor states located 0.1 eV below the conduction band, caused by sulphur vacancies. The photosensitivity in the IR region (regions II and III of Fig. 5) shown by polycrystalline (Zn,Cd)S thin films indicates that the above defects could be responsible for the presence of sensitizing hole centres located 1.1 and 1.5 eV above the valence band of (Zn,Cd)S thin films. The CdS layer presents a low density of cadmium vacancies and thus its photosensitivity (in region III) is lower than that of (Zn,Cd)S-based photoresistors. An increment in the zinc concentration leads to an increment in the number of zinc vacancies [7, 11] and thus in the sensitivity in the IR region. At zinc concentrations greater than 40% new photosensitive centres are created about 1.5 eV above the valence band, causing the maximum of photosensitivity in region II of Fig. 5. Figure 6 shows the energy band diagram of the (Zn,Cd)S layers, indicating some possible transitions caused by photoexcitation. Transitions between valence band and shallow donor levels and between levels in the gap, followed by thermally activated transitions, could also be possible. Figure 7 shows the photosensitivity of a typical photoresistor with a structure Ag/CdS/Zno.3Cdo.7S/In. The indium contact on the top of the device was deposited

234

J. Torres, G. Gordillo / Photoconductors based on Zn~Cd l_~S

improved by the contribution of the two layers to the photocurrent. Figure 8 shows the photosensitivity of photoresistors made with a sandwich structure formed by three (Zn,Cd)S layers (see Inset). In this case the (Zn,Cd)S layer has a zinc concentration of 30%.

Ec

Et

--•ev

"--'~ev Ev

II

IIi

Fig. 6. Energy band diagram of polycrystalline(Zn,Cd)S thin films indicating transitions caused by photoexcitationwith white light.

6

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• " ......

~ 400

600

800

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400

I

l

600

I

I

800

l

l

1000

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1200

Fig. 8. Photosensitivityof typicalphotoresistorswith a structure formed by three(Zn,Cd)Slayers.The zincconcentrationof the (Zn,Cd)Slayeris 30%.

hr.

Q,

o

I

°.•

• "'.

•

i . XInrnl

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1000

1200

Fig. 7. Photosensitivityof typicalphotoresistorswitha structure formed by two (Zn,Cd)Slayers.The zincconcentrationof the (Zn,Cd)Slayer is 30%. in order to determine the electrical transport perpendicular to the substrate (see inset of Fig. 7). When the CdS layer is deposited on the top, the device shows a spectral photosensitivity similar to that of photoresistors made with only one CdS layer (see Fig. 5), because the CdS has a high absorption coefficient and it is necessary to have a thickness greater than 2 gm in order to obtain good results. When the (Zn,Cd)S layer is deposited on the top, the spectral photosensitivity of the device is wider in the visible region, because in this case both layers contribute to the photocurrent since the (Zn,Cd)S has a larger optical gap than the CdS layer. The shoulder presented at 700 nm in Fig. 7 could be explained by the existence of photoquenching [12, 13] in (Zn,Cd)S layers deposited on metallized glass substrates but not in systems with structures Ag/CdS/(Zn,Cd)S/In illuminated on the (Zn,Cd)S side, because in this case the density and capture cross-section of the states responsible for the photoquenching decrease, leading to an increase in photosensitivity around 700 nm. The IR response is also

The maximum at 340 nm is caused by photoexcitation of carriers in the ZnS layer and the wide peak around 500 nm is due to photoexcitation of carriers in the CdS and (Zn,Cd)S layers. A high response in the IR region is presented since in this case the three layers contribute to an enhancement of the photocurrent. Responsivity values of 0.21 AW -~ at 2=1.240 nm/Es(eV ) were obtained with the (Zn,Cd)S photoresistors.

4. Conclusions

Photoresistors based on polycrystalline ZnxCdt_x S thin films were fabricated by evaporation of CdS and ZnS powders from a coaxial evaporation source. Using sandwich structures formed by three ZnxCd 1 -xS layers with zinc concentrations between x = 0 and 1, we obtained a spectral photosensitivity in the near-Uv, visible and near-IR regions at room temperature which is wider than that obtained with monocrystalline (Zn,Cd)S-based photoresistors. The improvement in the results was obtained via a preparation method consisting of the deposition of several (Zn,Cd)S layers (up to three) with different zinc concentrations without breaking the evaporation process. The devices show a high spectral response in the near-

J. Torres, G. Gordillo / Photoconductors based on ZnxCd ~_xS

IR region as a result of the creation of photosensitive centres 1.1 eV above the valence band.

Acknowledgments

This work was supported by COLCIENCIAS, Fundaci6n para el Desarrollo de la Ciencia y la Tecnologia del Banco de la Repfiblica and the Third World Academy of Sciences (TWAS).

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235

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