Shallow levels in the band gap of CdTe films deposited on metallic substrates

Solar Energy Materials & Solar Cells 70 (2001) 379–393

Shallow levels in the band gap of CdTe films deposited on metallic substrates Xavier Mathewa,*, J.R. Arizmendia, J. Camposa, P.J. Sebastiana, N.R. Mathewsa, Cristino R. Jim!enezb, Miguel G. Jim!enezb, R. Silva-Gonz!alezb, M.E. Hern!andez-Torresb, Ramesh Dherec a

Solar Materials Department, Solar-Hydrogen-Fuel Cell Group, Centro de Investigacio!n en Energ!ıa-UNAM, 62580 Temixco, Morelos, Mexico b Instituto de F!ısica, UAP, J-48, Col. San Manuel, Puebla 72570, Mexico c National Center for Photovoltaics, NREL, 1617 Cole Boulevard, Golden, CO 80401, USA

Abstract CdTe thin films were developed on flexible metallic substrates using close spaced sublimation and electrodeposition techniques. The films were nearly stiochiometric, highly uniform and exhibit good crystallinity. The films were characterized using XRD, SEM and AUGER. The shallow levels in the band gap of CdTe were determined using photoluminescence and photoinduced current transient spectroscopy. The photoluminescence studies revealed a defect dominated CdTe surface. The two lines detected at 1.587 and 1.589 eV at 15 K are assigned to the donor levels associated with Cl at the Te sites. The additional features observed in the photoluminescence spectra of the CdCl2 treated films revealed that the CdCl2 treatment improves the quality of the films and the close space sublimated films are better than the electrodeposited films. The photoinduced current transient spectroscopic technique was effectively used to identify the electron and hole traps. Two shallow levels with activation energy 0.056 and 0.13 eV were detected and assigned to electron and hole traps, respectively. r 2001 Elsevier Science B.V. All rights reserved. Keywords: CdTe; Electrodeposition; Close space sublimation; PICTS; Photoluminescence; Schottky devices

1. Introduction Electrodeposition (ED) and close space sublimation (CSS) are two successful and well-known techniques for the development of CdTe thin films. A good number of *Corresponding author. E-mail address: [email protected] (X. Mathew). 0927-0248/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 1 ) 0 0 0 7 9 - 4

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articles have been published recently about the deposition and characterization of CdTe films on metallic substrates [1–4]. Photoinduced current transient spectroscopy (PICTS), photoluminescence (PL), thermally stimulated current spectroscopy (TSCS) and deep level transient spectroscopy (DLTS) are the well-known techniques for the characterization of deep and shallow levels in the band gap of CdTe [5–9]. Understanding of the trapping centers in CdTe is of interest since they control the optical and electrical characteristics of the films. It is known that the high resistivity of the as deposited n-CdTe film is due to the compensation effect between a donor and an acceptor type deep levels [10,11]. It was believed that a singly ionized acceptor level located at 0.14 eV above the valence band acts as a hole trap and limit the performance of CdTe radiation detectors [12]. There are a good number of articles about the deep and shallow levels and numerous trapping centers with activation energies in the range of 0.009–0.9 eV in CdTe have been reported [5–13]. The data reported in the literature was obtained from either hetero-junction configurations or CdTe films deposited on conducting glass. In order to avoid the complications due to the hetero-junctions and the inter-diffusion between layers we have deposited the films on metallic substrates. In this article, we report the growth of CdTe films on flexible stainless steel (SS) and molybdenum (Mo) substrates using two techniques, ED and CSS.

2. Experimental 2.1. Development of CdTe films The SS substrate (thicknessF0.05 mm) was obtained from Good fellow (AISI 302). The Mo substrate was 99.95% pure foil with 0.1 mm thickness and was supplied by Johnson Mathey. In all the cases, the substrates were cleaned by a 4-step process, starting with detergent, acetone, hot dilute acid and finally ultrasonic cleaning for 15 min. After each step the substrates were washed with doubly distilled water. The substrates, immediately after the ultrasonic cleaning, were washed with de-ionized water, dried in nitrogen environment and introduced in the electrodeposition cell/CSS reactor. 2.1.1. Electrodeposition The electrolyte was a 1 M CdSO4 solution containing 100–200 ppm of TeO2. The temperature of the bath was maintained at 801C and was continuously agitated with a magnetic stirrer at 50 rpm. Prior to the addition of TeO2 the electrolyte was purified at 801C at a potential more negative than the deposition potential of CdTe. Later the pH of the solution was adjusted to 2 and the required amount of TeO2 was added in the powder form. The details of the electrodeposition and the supply of TeO2 during deposition can be found in the literature [14]. Uniform films of about 1 mm thickness and area of 20 cm2 were deposited in 4 h.

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2.1.2. Close spaced sublimation The CdTe source was a thick coating of 5 N pure CdTe on borosilicate glass. The separation between the source and the substrate was about 2 mm. The typical experimental parameters were as follows: source temperatureF6601C, substrate temperatureF6001C, oxygen pressureF1 Torr, helium pressureF15 Torr, and deposition timeF4 min. Under these conditions uniform films of about 8 mm thickness were obtained.

2.1.3. PICTS set-up The PICTS experimental system consists of a liquid nitrogen cryostat, Keithley 428 current amplifier, Tektronix 430 digital storage oscilloscope, 100W tungsten halogen lamp, Oriel 57373 band pass filter and SR540 optical chopper. The PICTS measurements were done only for the ED films. Since the ED films were n-type, it forms a Schottky barrier with gold electrodes and the Au/CdTe devices were prepared by sputtering approximately 15 nm thick semitransparent gold films on the CdTe surface. The electrical connections were taken from the gold electrode and the substrate. In thermal equilibrium the band bending was upwards and hence the direction of the built-in electric field is from CdTe to the gold electrode. The device was illuminated repetitively through the semitransparent Au electrode at a frequency of 60 Hz and the signal from the device was amplified and converted into proportionate voltage using the Keithley 428 and fed to the oscilloscope. The oscilloscope was triggered using the trigger out put signal from the optical chopper. The transient signal in the dark interval was captured from the oscilloscope using a data acquisition program and the data was processed on-line using the double gate PICTS technique. The software for controlling the cryostat, data acquisition and processing was a simple program developed in our laboratory.

3. Results and discussion 3.1. Structure and morphology The films prepared by both ED and CSS techniques show very good crystallinity. Fig. 1 is the XRD profiles of the films prepared by ED (S73) and CSS (CSS-M1). It is clear from the figure that the films have strong preference for the (1 1 1) plane. A comparison of the intensities of (2 2 0), (3 1 1) and (4 0 0) peaks show that the electrodeposited film is more oriented along the (1 1 1) plane than the film prepared by CSS. Figs. 2a and b are the SEM pictures of the films S73 and CSS-M1, respectively. As expected, the grain size of the CSS film is much larger than the ED film. The surface of the CSS film is more closely packed compared to the ED film. The average grain size of the films estimated from SEM analysis is 0.2 mm (S73) and 3 mm (CSS-M1).

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Fig. 1. The XRD profiles of the ED (S73) and CSS (CSS-M1) prepared films.

3.2. PICTS results PICTS is an efficient technique to determine the shallow and deep levels in the band gap of semiconductors. In PICTS the traps are optically filled using a light pulse and the trap parameters can be obtained from the current transient in the dark. The transient signal in the dark can be written as [15,16]: IðtÞ ¼ Kei eei t ;

ð1Þ

where ei is the emission rate (s1) from the trap i with activation energy Ei : The constant K contains information about the device parameters and applied bias. The emission rate ei can be expressed as [15,16]: ei ðs1 Þ ¼ AT 2 si ðcm2 ÞeEi =kT ;

ð2Þ

where si is the capture cross-section, T is the temperature and the constant A contains the density of states and trap degeneracy factor. The emission rate ei at each temperature can be determined from the transient using Eq. (1) and the trap parameters can be calculated from Eq. (2). But this process is time consuming since each transient has to be captured individually in order to plot in a semi logarithmic scale. But using double gate technique the data can be analyzed on-line and the whole information can be obtained in a single temperature scan. In double gate PICTS two time gates tm and tn are selected ðtn > tm Þ on the transient and the PICTS signal is given by S ¼ Iðtm Þ  Iðtn Þ:

ð3Þ

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Fig. 2. (a) The SEM picture of the virgin ED (S73) film, (b) The SEM picture of the virgin CSS (CSS-M1) film.

The signal S has a maximum when [16] dS dei ¼ 0:  dei dT

ð4Þ

Using the condition in Eq. (4), we can obtain a relation between tm ; tn and the emission rate ei for the maxima of the PICTS spectra:     1 1 ðtn tm Þei tn  : ð5Þ ¼ tm  e ei ei

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In our double gate analysis the gate positions were selected such that tn ¼ 2tm : Knowing the values of tm and tn ; Eq. (5) can be solved to get ei for each set of double gates. The PICTS signals for each set of double gates can be plotted against temperature and the temperature corresponding to the peak maximum can be determined. According to Eq. (2) a plot of Lnðei =T 2 Þ against 1/T will be a straight line with the slope and intercept giving the activation energy and capture crosssection, respectively. 3.2.1. Identification of hole and electron traps Using a highly absorptive radiation in PICTS measurements, the nature of the traps can be determined. Since the CdTe has a high absorption coefficient in the region above the band gap energy, most of the incident radiation below 500 nm will be absorbed in the top layer of the material. In our measurements, light from the halogen lamp was filtered using a 300–500 nm band pass filter. Thus, it was ensured that the incident radiation on the Au/CdTe device created electron–hole pairs only in the top layer of the CdTe. Depending on the polarity of the applied bias either electrons or holes will be injected into the bulk of the material and get trapped. The transient in the dark interval contains information about the traps and can be calculated as explained earlier. In our experimental system, since the illumination was through the semitransparent gold electrode the electron–hole pairs were created underneath the gold electrode. A positive bias applied to the gold electrode injects only the holes in to the bulk and hence the transient under dark is due to the hole current, similarly a negative bias applied to the gold electrode produces transient due to the detrapped electrons. The magnitude of the applied bias was 0.8 V in both cases and at this bias the space-charge-limited-current conduction was the dominant transport mechanism. Fig. 3 shows three typical PICTS signals recorded in the 100–275 K region, curve (a) was obtained with white light as the source of excitation whereas (b) and (c) were obtained using radiation in the 300–500 nm range. Curve (b) is the signal due to the detrapped holes and (c) is due to the detrapped electrons. From this figure it is clear that the signals due to the electron and hole traps are well separated along the temperature scale and demonstrates the usefulness of this technique in identifying the nature of the traps. Moreover if the electron and hole traps are active in the same temperature region, the white light signal will not be able to resolve it and instead show only a broad band as is evident from the figure. Fig. 4 is the double gate PICTS signals obtained by applying +0.8 V bias to the gold electrode. These curves correspond to the gate values tm ¼ 1; 2; 3; 4; 5; 6; and 7 ms. The temperature corresponding to the peak maximum was determined in each case and a graph of Lnðei =T 2 Þ against 1=T was plotted (Fig. 5). The activation energy and capture cross-section of this hole trap was estimated as 0.13 eV and 6  1020 cm2, respectively. Chlorine in CdTe usually acts as a donor substituting a Te atom. It has been reported that Cl acts as a singly ionizable acceptor by forming a complex Vcd ClTe [12]. Even though our solution is free from impurities, traces of Cl can come in to the solution from the Ag/AgCl reference electrode. The calculated

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Fig. 3. The PICTS signal obtained from a Au/CdTe Schottky device. (a) white light illumination and with +0.8 V bias, (b) with +0.8 V bias and under 300–500 nm broad band irradiation and (c) illumination with 300–500 nm range radiation and with 0.8 V bias.

Fig. 4. PICTS signal corresponding to the hole traps. The tm values of these curves are 1–7 ms, respectively, from top curve to the bottom curve. The applied bias was +0.8 V and the excitation was with 300–500 nm-range radiation.

value of 0.13 eV above the valance band edge is very close to the previously reported value of 0.135 eV [12]. Similarly the signal due to the electron traps was obtained by applying –0.8 V bias to the gold electrode (Fig. 6). The signals were not as good as that obtained with positive bias, but we were able to obtain curves with gate values

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Fig. 5. The Arrhenius plot of Lnðei =T 2 Þ against 1000/T corresponding to Fig. 4. The hole trap parameters are Ev þ 0:13 eV and s ¼ 6  1020 cm2.

Fig. 6. PICTS signal corresponding to the electron traps. The tm values of these curves are 1, 1.5, 2 and 2.5 ms, respectively, from top curve to the bottom curve. The applied bias was 0.8 V and the excitation was with 300–500 nm range radiation.

tm ¼ 1; 1:5; 2 and 2.5 ms. It is known that for electron transport, even though the ionized impurity scattering dominates, there is a small contribution from the shallow levels located at 0.03 eV below the conduction band [12]. Theoretical calculations suggest that two substitutional chlorine atoms form a complex defect with the cadmium vacancy (Vcd 2ClTe ) giving a 0.05 eV level below the conduction band [12]. Fig. 7 is the Arrhenius plot of Lnðei =T 2 Þ against 1=T giving the activation energy and the capture cross-section of the electron trap as 0.056 eV and 1  1020 cm2,

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Fig. 7. The Arrhenius plot of Lnðei =T 2 Þ against 1000/T corresponding to Fig. 6. The electron trap parameters are Eb  0:05 eV and s ¼ 1  1020 cm2.

respectively. The assignments of these traps along with various shallow levels reported [5–9,12,13,17–24] for the CdTe are presented in Table 1. 3.3. Photoluminescence results The photoluminescence measurement was done with an argon laser using the 514 nm excitation line. The sample was placed in a helium gas cryostat and the measurement temperature was varied from 15 K to higher temperatures. A SPEX 1000 M monochromator with a resolution of 72 meV was used for the spectral analysis. A GaAs photo multiplier was used as the detector and the signal was analyzed using a computer controlled DATA-SCAN system. Both as-deposited as well as heat treated (in air and CdCl2) CdTe samples were analyzed to study their photoluminescence properties. Figs. 8 and 9 show the photoluminescence spectra of the films prepared by CSS and ED, respectively. Both the figures present the spectra of the as-deposited and heat treated samples. These figures indicate that the samples present photoluminescence emission irrespective of the way in which they were deposited. In both ED and CSS cases the photoluminescence spectra shows an emission line corresponding to the well known defect band [25] of CdTe in the region of 1.4–1.5 eV. This behavior is indicative of a defect or impurity dominated CdTe surface, which normally gives rise to transitions PDA, BI, IB and their respective phonon transitions [7,26]. The ionization energies of the various emission lines presented in Figs. 8 and 9 were calculated by assuming the band gap as 1.6 eV at 15 K and these results along with the assignments are presented in Table 1. As a consequence of the heat treatment of the samples, the emission intensity increased and new shoulder peaks appeared along with the principal line. This behavior was observed in both ED and CSS samples.

388

Table 1 Various shallow levels reported for CdTe along with the assignment and measurement techniques Activation energy (eV)

20

1  10 6  1020

Assignment

Measurement technique

Reference

Electron trap VCd 2ClTe Hole trap VCd ClTe Donor Clþ Te Donor Clþ Te Acceptor V2 Cd Defect Defect Defect Defect Defect Defect Defect

PICTS PICTS PL PL PL PL PL PL PL PL PL PL

This work

PL PL TSCS TSCS TSCS

[12]

0.03 0.135 0.084 0.19 0.24

8.9  1025 8.2  1024 1.5  1024

Electron trap VCd 2ClTe Hole trap VCd ClTe Hole trap Vcd complex Hole trap

0.086 0.17

8.9  1025 8.2  1024

Hole trap Vcd complex

0.11 0.17 0.24

8.9  1025 8.2  1024 1.5  1024

Hole trap Vcd complex Hole trap

[9,17]

[9,18]

0.04 0.06–0.08 0.095–0.115

Donor,Te vacancy Donor, substitutional Cd Acceptors, Cd vacancy, anti-site CdTe defect

PL PL PL

0.14

Acceptors, Cd vacancy, anti-site CdTe defect

PL

[8]

X. Mathew et al. / Solar Energy Materials & Solar Cells 70 (2001) 379–393

0.05 0.13 0.011 0.013 0.05 0.127 0.146 0.154 0.162 0.161 0.167 0.175

Capture cross-section (cm2)

0.23 0.14 0.16–0.17 0.1 0.17 0.18 0.16 0.19 0.28

PL PL PL PL PL PL PL PL PL 1.6  1012 1  1017 (1–3)  1016 1  1020 1.3  1016 6.3  1018

[6]

Defects (Tei, TeCd) Compensation defect ðClþ Te Þ þ V2 Cd ClTe complex Compensation defect ðClþ Te Þ Defects (Tei ; TeCd ) Defects, ðV2 Cd Þ Hole trap Hole trap Hole trap

PICTS PICTS PICTS PICTS PICTS PICTS TSC TSC TSC

0.18 0.25

Hole trap Hole trap

TSC TSC

[21]

0.2

Hole trap

PL

[22]

0.0495 0.0516 0.0558 0.0581 0.0688 0.1084 0.1472

Acceptor, Acceptor, Acceptor, Acceptor, Acceptor, Acceptor, Acceptor,

PL PL PL PL PL PL PL

[23]

DLTS

[24]

0.09

ClTe–CdV–ClTe ClTe–CdV N Li P Ag Cu

[5]

[20]

389

(continued)

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0.0018 0.0049 0.014 0.009 0.032 0.0017 0.0064 0.026 0.0062

390

Activation energy (eV)

Capture cross-section (cm2)

Assignment

Measurement technique

Reference

0.0604 0.0989 0.1731 0.2003 0.1957

2.6  1018 2.8  1016 5.5  1012 4.8  1013 4.3  1013

þ V2 Cd 2BrTe complex Compensating defects Compensating defects Compensating defects Compensating defects

PICTS PICTS PICTS PICTS PICTS

[13]

Neutral donor Impurity level Acceptor level Donor level Acceptor level

PL PL PL PL PL

0.009 0.032 0.0614 0.015 0.07–0.08

[7]

X. Mathew et al. / Solar Energy Materials & Solar Cells 70 (2001) 379–393

Table 1 (continued)

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391

Fig. 8. Photoluminescence spectra of the films prepared by close spaced sublimation. CSS-M1: virgin sample, CSS-M2 and CSS-SS2: annealed in CdCl2 vapor at 4001C for 5 min.

Fig. 9. Photoluminescence spectra of the films prepared by electrodeposition. S73 is the virgin sample, S68-1 is annealed in air at 3501C for 15 min, S68-2 is the one, first annealed in air at 3501C for 15 min and then annealed in CdCl2 vapor at 3501C for 5 min. The substrates are stainless steel.

Additional peaks were observed (Fig. 8) for samples prepared by CSS and subjected to post-deposition treatments (CSS-M2 and CSS-SS2) at 1.550, 1.587 and 1.589 eV, which are near to the forbidden band. The position of the two lines, 1.587 eV (CSSM2) and 1.589 eV (CSS-SS2) with respect to the band gap of CdTe at 15 K (1.6 eV) is 13 and 11 meV, respectively. These two lines are assigned to the donor levels

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associated with the Cl in the sites of Te. Considering the experimental resolution, this assignment is in good agreement with the previously reported value of 14 meV [27]. This identification is further supported by the fact that the post-deposition treatment was in CdCl2 environment. On the other hand the line observed at 1.550 eV (CSS-SS2) is in good agreement with the value reported for the transition from conduction band to an acceptor level created due to the ionized Cd vacancies [28]. In general, by observing the appearance of additional features in the CdCl2 treated films, we can say that the CdCl2 treatment improves the quality of the films and the CSS prepared films are better than the electrodeposited films.

4. Conclusion The PICTS technique was effectively used to distinguish between the electron and hole traps. Two shallow levels with activation energies 0.056 and 0.13 eV were detected and assigned to electron and hole traps, respectively. The two PL lines detected at 1.587 and 1.589 eV at 15 K are assigned to the donor levels associated with the Cl in Te sites. The emission line at 1.550 eV originates from an acceptor level having ionization energy of 0.05 eV. The additional features observed in the photoluminescence spectra of the CdCl2 treated films reveal that the CdCl2 treatments improves the quality of the films and the CSS prepared films are better than the electrodeposited films.

Acknowledgements Ing. Rene Guardian (CCF-UNAM) prepared the Schottky devices. We thank Perkins Craig NREL Colorado for the AUGER analysis. DGAPA-UNAM supported this work through the project IN114599.

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