Deep levels in the band gap of CdTe films electrodeposited from an acidic bath—PICTS analysis

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Solar Energy Materials & Solar Cells 81 (2004) 397–405

Deep levels in the band gap of CdTe films electrodeposited from an acidic bath—PICTS analysis Xavier Mathewa,*, N.R. Mathewsa, P.J. Sebastiana,b, C. Osvaldo Floresc a

Centro de Investigacion en Energia-UNAM, 62580, Temixco, Morelos, Mexico b Instituto Mexicano del Petroleo, Eje Central 152, DF 07730, Mexico c CCF-UNAM, Av. Universidad, Chamilpa, Cuernavaca, Morelos 62251, Mexico

Abstract The deep traps detected in a CdTe thin film electrodeposited from an acidic bath are discussed. CdTe thin films were developed on flexible metallic substrates by electro deposition. The films were nearly stiochiometric, highly uniform and exhibit good crystallinity. The films were characterized using XRD, SEM, AUGER and AFM. The crystallites exhibited a strong preference for the (1 1 1) plane. The grain size of the film was in the range 0.2–0.4 mm. The photoinduced current transient spectroscopic technique was effectively used to identify the electron and hole traps. Two hole traps and one electron trap was detected. The activation energies of those deep traps were 0.37 and 0.42 eV for the hole traps and 0.41 eV for the electron trap. r 2003 Elsevier B.V. All rights reserved. Keywords: CdTe; Electro deposition; PICTS; Deep levels; Electron and hole traps

1. Introduction The performance of a semiconductor device is greatly influenced by the presence of deep levels in the forbidden gap. Photo-induced current transient spectroscopy (PICTS) is an efficient technique for the detection and characterization of deep levels in the band gap of highly resistive semiconductors [1,2]. In PICTS, the device is *Corresponding author. Tel.: 777-325-0052; fax: 777-325-0018. E-mail address: [email protected] (X. Mathew). 0927-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2003.11.015

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illuminated with light pulses and from the current transient under dark the trap parameters are calculated. In literature there are a good number of reports of the PICTS investigations on GaAs [1–5], Cu2O [6,7] and CdTe [8,9–15] and a review on the PICTS investigation of the trap levels in CdTe [16]. CdTe is one of the leading candidates among the thin film solar cell materials with reported efficiency for small area devices exceeding 16% [17–19] and more than 10% efficiency for large area modules [20]. CdTe, in uncompensated form, is a low resistivity semiconductor and this low resistivity is due to the intrinsic defects and residual impurities. It is known that the defect states and optical parameters of the absorber layer are crucial factors in determining the device parameters. The deep levels control the quantum efficiency and charge transport properties of CdTe devices. The efficiency and open circuit voltage of CdTe-based solar cells are found to be decreasing as the density of deep levels is increased [21]. The post deposition treatments such as annealing in CdCl2 vapor influence the trap levels; it was observed that the CdCl2 treatment eliminates the majority carrier deep levels in CdTe [21]. In this article the deep levels detected in a CdTe thin film electrodeposited from an acidic bath is discussed. The nature of the traps was identified by changing the polarity of the applied field across an Au/CdTe device and using a highly absorptive radiation as the exciting source.

2. Experimental The CdTe films were electrodeposited from an acidic bath containing 1 M CdSO4 and 100–200 mM TeO2. The temperature of the solution was maintained at 80
C and stirred at a constant speed of 70 rpm. The thickness of the films was estimated from the total charge flowed during deposition as well as from the interference fringes in the reflectance spectra. The films were deposited at a potential of 580 mV with respect to a saturated Ag/AgCl reference electrode. The details of the potentiostatic deposition of CdTe are given elsewhere [22–25]. The as deposited films were n-type with resistivities in the range 104–106 O cm; the high resistivity is due to the compensation effect where the residual impurities and native defects control the electrical properties [1]. The XRD analysis of the films showed very strong preferential orientation of the crystallites along the (1 1 1) plane. The SEM analysis revealed a rough, but uniform surface. There are lots of inter-grain voids and the grain size is in the range 0.2– 0.4 mm (Fig. 1). The AFM topographical view is showed in Fig. 2, the rough nature of the surface is very clear and the film surface contains large clusters of grains and voids. We have investigated the composition of the film using AUGER depth profile analysis and found that the chemical composition is constant throughout the thickness of the film. The estimated composition was 50.4% Cd and 49.6% Te. The Schottky devices were prepared by sputtering a 15 nm thin layer of Au over the CdTe surface; these electrodes were transparent enough to allow partial illumination of the device and at the same time maintaining good electrical conductivity. The Au/CdTe Schottky diode was studied using the I2V character-

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Fig. 1. The SEM picture of a typical CdTe film electrodeposited from an acidic bath.

Fig. 2. The AFM picture of an as deposited CdTe film.

istics in dark and under illumination. In our devices the current transport was dominated by the space charge limited conduction. The diode rectification ratio in dark, defined as the ratio of the forward to reverse current under 1 V bias was estimated in the range of 900. The PICTS set up consists of an open-cycle liquid nitrogen cryostat, a Keithley 428 current-amplifier/bias-source and a computer controlled digital oscilloscope. The cryostat was controlled using an Omega process controller and the temperature at each set point was maintained constant within an error limit of 71
C. At each set temperature the data corresponds to the transient signal was captured on-line from the oscilloscope and stored in the computer. The device was illuminated with pulses of a highly absorptive radiation obtained with a

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300–500 nm band pass filter. The PICTS theory and the technique applied in the case of CdTe can be found in the literature [16].

3. Results and discussion Assuming that the decaying current in the dark interval contains only one exponential with time constant tT ¼ ðen Þ1 ; the current transient can be written as [2,13,16]: IðtÞ ¼ Bmn tn en nTl een t ;

ð1Þ

where en is the electron emission rate, mn is the electron mobility, tn is the electron life time, nTd and nTl are, respectively, the steady-state densities of occupied traps in the dark and under illumination and B is a constant depending on the electric field and the device geometry. Eq. (1) is used in almost all the double gate PICTS analysis, where the PICTS signal is written as PðTÞ ¼ Iðtj Þ  Iðtf Þ;

ð2Þ

where Iðtj Þ and Iðtf Þ are the transient current at the time gates tj and tf : The gate positions were selected such that tf ¼ 2tj : Using Eq. (1): PðTÞ ¼ Kei ðeei tj  eei tf Þ;

ð3Þ

where K ¼ BmtnTl and ei is the emission rate from the trap i: The electron thermal emission rate can be written as [2,5,16]: 2 Ei =kT ; ei ¼ t1 i ¼ Asi T e

ð4Þ

where Ei and si are, respectively, the ionization energy and capture cross section of the trap i: Taking into consideration the effective mass of electrons and holes, the value of A in the case of CdTe is reported in the literature as 1.35  1020 s1 cm2 K2 for electron emission and 1.07  1021 s1 cm2 K2 for hole emission [16]. Hence, for an electron trap Eq. (4) can be written as ein ðs1 Þ ¼ 1:35  1020 T 2 s ðcm2 Þ eEin =kT :

ð5Þ

Similarly, for a hole trap, the emission rate can be written as eip ðs1 Þ ¼ 1:07  1021 T 2 s ðcm2 Þ eEip =kT :

ð6Þ

Eq. (6) can be written in a more useful form as follows: e  Eip ip : ð7Þ ln ¼ ln ð1:07  1021  sÞ  T2 kT The PICTS technique can be utilized to identify the electron and hole traps; this is achieved by exciting the device using a highly absorptive radiation. Since the CdTe has a high optical 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. Thus an excitation source emitting radiation below 500 nm can ensure that the incident radiation creates electron–hole pairs only in the top layer of the CdTe.

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Then depending on the polarity of the applied bias, either the holes or electrons will be injected into the bulk of the material and get trapped. A positive bias applied to the top Au electrode injects only the holes into the bulk and hence the transient under dark is due to the hole current, similarly a negative bias applied to the top electrode produces transient due to the de-trapped electrons. The PICTS signal PðTÞ was calculated from the current transients at different temperatures for various values of tj and tf and a graph was plotted for each pairs of tj and tf : A plot of the PðTÞ in the temperature range 250–400 K is shown in Figs. 3 and 4. The Fig. 3 corresponds to the PðTÞ obtained under +0.8 V bias and it corresponds to the hole traps and Fig. 4 corresponds to the PðTÞ under 0.8 V bias which implies that the signal is due to the de-trapped charge from the electron traps. Fig. 3 was obtained by taking tj ¼ 1; 2; 3; 4; 5; 6; 7 and 8 ms and the corresponding emission rates ðei Þ were calculated as 1445, 722.5, 481.6, 361.2, 289, 240.8, 206.4 and 180.6 s1, respectively. The tj values for the data in Fig. 4 was 0.5, 1, 1.5, 2 and 2.5 ms and the corresponding emission rates were 2890, 1445, 963.3, 722.5 and 578 s1, respectively. The spectra in Fig. 3 shows the presence of two hole traps and the PðTÞ peaks are labeled as A and B. From this graph the temperature corresponding to each peak for different pairs of tj and tf were determined and the trap parameters were calculated using Eq. (7). Figs. 5 and 6 are the best fit of Eq. (7) obtained for the peaks A and B, respectively, and the activation energy ðEip Þ and capture cross section ðsÞ were estimated as 0.37 eV and 1  1017 cm2 for peak A and 0.42 eV and 2  1017 cm2 for peak B. The trap parameters of the electron trap observed in Fig 4 were estimated by using Eq. (5) and the best fit is presented in Fig. 7. The activation energy and the capture cross-section were 0.41 eV and 1  1016 cm2, respectively.

0.25

PICTS signal (a.u)

0.20 0.15 0.10 0.05 0.00

(A) 250

(B) 300

350

400

T (K) Fig. 3. The PICTS signal obtained due to the de trapped holes in the CdTe film. The applied bias was +0.8 V and the excitation was with radiation having wavelength in the range 300–500 nm. The tj values were 1, 2, 3, 4, 5, 6, 7 and 8 ms for the graphs starting from top to bottom. The two sets of peaks A and B correspond to two distinct trap levels. The peak B was resolved only for tj values 4, 5, 6, 7 and 8 ms.

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PICTS Signal (a.u)

0.015

0.010

0.005

0.000 250

300

350

400

T (K) Fig. 4. The PICTS signal corresponding to the electron trap. The applied bias was 0.8 V and the excitation was with radiation having wavelength in the range 300–500 nm. The tj values were 0.5, 1, 1.5, 2 and 2.5 ms for the graphs starting from top to bottom.

-4.0

-5.0

2

-1

-2

Ln(e/T ) (s K )

-4.5

-5.5

-6.0

3.0

3.2

3.4

3.6

3.8

4.0

-1

1000/T (K ) Fig. 5. The best fit of the Eq. (7) applied to peak A in Fig. 3. The activation energy and capture cross section were determined from the slope and intercept as 0.37 eV and 1  1017 cm2, respectively.

It is known that in majority of cadmium-based compounds; Cd vacancies are the predominant defects in the material. These vacancies are present both with singly and doubly charged states and produce defect states near the mid-gap [26]. In CdTe, the cadmium vacancy ðV2 Cd Þ sites behave as doubly ionized acceptor levels. The activation energies of these levels were reported in the range 0.05–0.76 eV [8,12–14,21,27–29]. In electrodeposited films, it was observed that the energy levels

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-5.5 -5.6

-5.8

-1

-2

Ln(e/T ) (s K )

-5.7

2

-5.9 -6.0 -6.1 -6.2 -6.3 3.20

3.22

3.24

3.26

3.28

3.30

3.32

3.34

-1

1000/T (K ) Fig. 6. The best fit of the Eq. (7) applied to peak B in Fig. 3. The activation energy and capture cross section of this hole trap were 0.42 eV and 2  1017 cm2, respectively. -3.6 -3.8

-4.2

2

-1

-2

Ln(e/T ) (s K )

-4.0

-4.4 -4.6 -4.8 -5.0 2.80

2.85

2.90

2.95

3.00

3.05

3.10

-1

1000/T (K ) Fig. 7. The fit of Eq. (5) to the PICTS peak shown in Fig. 4. The activation energy and capture cross section of this electron trap was 0.41 eV and 1  1016 cm2, respectively. 2þ associated with the native defects V2 and the grain boundary defects fall Cd and Cdi within two energy bands located symmetrically around the mid-band gap intrinsic level [16]. Copper is known to create complexes with native defects giving rise to a band of defect levels with energies in the range 0.11–0.23 eV. These levels cause selfcompensation of the material [14,26,30]. Copper is also known to create acceptor levels by substituting into Cd sites. The ionization energy of these levels range from 0.3 to 0.35 eV [14,26,30].

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Kim et al. [31] have used PICTS to investigate the effect of hydrogenation on the deep levels in p-CdTe epilayers. Before hydrogenation, five deep hole traps were detected with activation energies 0.22, 0.38, 0.45, 0.63 and 0.74 eV. The 0.22 eV level was considered to be due to the Au and Ag impurities. The 0.38 eV level was assigned to the Te vacancies and the complexes of the shallow impurities; the 0.45 and 0.63 eV levels were treated as the native defects or Te precipitates and the 0.74 eV trap was due to the Cd vacancies. After hydrogenation it was observed that the intensity of the PICTS peaks corresponding to the 0.63 and 0.74 eV levels have almost disappeared and the peaks corresponding to the 0.38 and 0.45 eV levels decreased in intensity. The decrease in intensity or the disappearance of the peaks after hydrogenation was due to the fact that those levels originate from complexes of vacancies and impurities [31].

4. Conclusions Good quality CdTe films were electrodeposited on metallic substrates and the structure and morphology were studied. The usefulness of the PICTS technique in distinguishing between electron and hole traps is demonstrated. Two hole traps and one electron trap was identified and the activation energy and capture cross-section were estimated. The activation energies and capture cross sections of the two hole traps were 0.37 and 0.42 eV and 1  1017 and 1  1017 cm2, respectively. The activation energy and capture cross section of the electron trap were 0.41 eV and 1  1016 cm2, respectively.

Acknowledgements This work was supported by CONACYT and PAPIIT-UNAM through the projects 38542-U, G38618-U and IN115102-3, respectively. The AFM and AUGER analysis were done at NREL, Colorado and we thankfully acknowledge the help of Dr. H.R. Moutinho and Dr. Craig Perkins.

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