Si heterojunction prepared by pulsed Nd:YAG-laser deposition technique

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Materials Science in Semiconductor Processing 10 (2007) 19–23

Optoelectronic properties of CdTe/Si heterojunction prepared by pulsed Nd:YAG-laser deposition technique Raid A. Ismaila,, Khaki I. Hassana, Omar A. Abdulrazaqb,1, Wesam H. Abodec a

School of Applied Sciences, University of Technology, Baghdad, Iraq NASSR State Company, Ministry of Industry and Minerals, Baghdad, Iraq c Computer Engineering Department, University of Technology, Baghdad, Iraq b

Available online 29 January 2007

Abstract The present study is on the optoelectronic properties of isotype CdTe/c-Si heterojunction photodetector made by deposition of CdTe by pulsed laser deposition (PLD) technique on clean monocrystalline Si. Optical, electrical and structural properties of grown CdTe film were investigated. The optical data show that the optical band gap of CdTe was around 1.45 eV at 300 K. The CdTe/Si junction exhibits fair diode rectification and the soft breakdown occurred at VB49 V. Dark and illuminated I–V characteristics of the CdTe/Si photodetector are examined at room temperature. The photodetector showed good photosensitivity in the visible and near-infrared regions with a value as high as 0.5A/W at 950 nm. r 2007 Elsevier Ltd. All rights reserved. Keywords: CdTe; Thin film; PLD; Heterojunction; Photodetector

1. Introduction CdTe tandem structures have the potential of being an economically viable source of electrical energy. Its near-ideal bandgap (direct transition) has the optimum value for solar energy conversion because it has a gap of 1.44 eV at 300 K means an optimum value for solar energy conversion. Furthermore, CdTe is crystallographically stable [1], therefore, its tandem structure offers the possibility of forming efficient and stable photovoltaic devices [2]. Corresponding author.

E-mail address: [email protected] (R.A. Ismail). Present address: Faculty of Education, Hadhramout University, Seiyun, Yemen. 1

1369-8001/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2006.12.001

CdTe/(II–VI compound semiconductors) heterojunction can be sited in literature [3–7] and it acts as a narrow-bandgap absorber while CdTe/Si heterojunction is seldom found in literature [8–10]. The CdTe layer in this combination acts as a window material for visible and near-infrared regions (EgCdTe4EgSi). Thin films of high quality CdTe have been successfully grown by a variety of techniques [11–13]. One of these is the well-known pulsed laser deposition (PLD) technique. The main advantage of this process is the stoichiometry of the CdTe film is completely preserved [14]. On the other hand, this material does not create serious problems with particulate generation in the PLD plum [15]. This paper describes the feasibility of preparing a high responsivity CdTe/Si heterojunction photodetector device by PLD technique.

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2. Experiment Cadmium telluride/silicon photodetector was fabricated by depositing of undoped CdTe precursor thin film of 150 nm thick by PLD technique on p-type CZ monocrystalline mirror-like silicon wafer of 1–3 O cm resistivity and orientation of (1 1 1) with area of 1 mm2. Before deposition, the CdTe target was irradiated with 10 successive laser shots to remove the oxide and contamination. During irradiation process the target was rotated at 120 rpm. The pulsed laser system used in this study was Nd:Yag laser operating at 1064 nm wavelength and pulse duration of 0.4 ms. The deposition of CdTe film is carried out with 30 laser shots each of 20 J/cm2 laser flunce (was found to be optimum conditions) [16] under vacuum with pressure of 105 mbar at 150 1C substrate temperature. Fig. 1 presents the schematic diagram of Nd:Yag PLD system. Prior to the deposition of the CdTe film, the silicon substrates were washed with boiled acetone for 2 min, immersed in in CP-4 etchant for 2 min and in 12% HF acid for 2 min and finally rinsed in acetone and methanol solution. Ohmic contacts of these devices were made by evaporating a thin 300 nm gold strips on the CdTe film and a thick Al on back surface of Si. The Ohmic contact area on CdTe film was 0.2 mm2. Fig. 2 demonstrates a cross-sectional view of Au–CdTe/Si–Al structure. Seebeck measurement of CdTe film deposited on glass substrate was investigated to find out the conductivity type of grown CdTe film. K-type thermocouple was used to

monitor the substrate temperature. Optical transmission of CdTe films deposited on glass substrates was investigated by means of UV/VIS Shimadzu spectrophotometer (400–900 nm). X-ray diffractometer using (CuKa) source was used to study the structural properties of grown CdTe film. Photosensitivity measurement was achieved in the spectral range of 500–1000 nm with the aid of monochromator after making power calibration using accurate silicon power meter.

3. Results and discussion Fig. 3 shows the XRD spectrum of CdTe film grown on silicon substrate. It shows clearly that the film has cubic polycrystalline structure with (1 1 1), (3 1 1), and (5 1 1) orientations corresponding to Au-electrode

CdTe film

p-Si

Al-electrode Fig. 2. Side view of Au–CdTe/Si–Al heterostructure.

Substrate (Si or glass)

Ablated particles

Heater

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Pump Fig. 1. Schematic diagram of Nd:yag PLD system.

Pulsed Nd:YAG laser

ARTICLE IN PRESS R.A. Ismail et al. / Materials Science in Semiconductor Processing 10 (2007) 19–23

2y ¼ 23.81, 2y ¼ 46.41 and to 2y ¼ 76.21 and the lattice constant was 0.641 nm for (1 1 1) plane. Fig. 4 depicts a plot for the variation of a2 (absorption coefficient) with photon energy of poly-

crystalline CdTe deposited on glass substrate. The CdTe film acts as a window layer for near-infrared region. From Fig. 4, the band gap was calculated and found to be 1.45 eV which is less than the bulk crystal and this can be ascribed to internal layer strains [17]. Thermoelectric measurements revealed that the polycrystalline CdTe layer is p-type, which is in conformity with published results [18] and hence depositing of CdTe onto p-type silicon forms an isotype heterojunction. The DC electrical conductivity of CdTe film was measured and found to be 104 O1 cm1. I–V characteristics at the dark of (p–p) CdTe–Si heterojunction is presented in Fig. 5 where a good rectification characteristic was noticed. The forward current (in semi-log scale) follows a relation of I ¼ C exp(AV) where C and A are constants. The ideality factor b was calculated and found to be larger than 2.5 which confirmed the domination of recombination current. This may be due to large lattice mismatch (19%) between CdTe and Si materials and also due to high electrical resistivity of CdTe layer. Soft breakdown was noticed at reverse bias voltage greater than 9 V. Reverse I–V curve under white light illumination with different levels is shown in Fig. 6. It is evident from this plot that the generated photocurrent is high and increases significantly with light intensity which reflects a fair linear characteristics (see Fig. 7). At low power light (0.8 mW) the white light photocurrent density was around 3 mA/cm2 at bias voltage 4.5 V and 2.5 mA/cm2 at self bias. This explains that the external bias voltage improved the

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2θ Fig. 3. XRD spectrum of CdTe grown on Si.

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photocurrent. Spectral responsivity of heterojunction at 10 mV reverse bias in the spectral range of 500–1100 nm is illustrated in Fig. 8. The response exhibits a combined effect of photogenerated carriers in both the CdTe and Si sides of the heterojunction. Over this span of wavelengths there are two distinct regions, one represents a photoresponse corresponding to the short wavelengths that is generated in the diffusion length region of CdTe layer. Nearly flat response is observed between 600 and 800 nm wavelengths. The high photosensitivity of photodetector is appeared at the

second region (beyond 850 nm). This region of photoresponse is related to the wavelengths that transmit through CdTe and well absorbed in the depletion and minority carriers diffusion regions of the bulk silicon. This window effect exhibits a satisfactory agreement with the results of optical transmittance of CdTe. The above mentioned results were reconfirmed by repeating the experiment after two months of storing the samples without observing any noticeable degradation. It is important to remark here that these results were obtained without post-deposition annealing. Furthermore, the high value of responsivity as compared with other CdTe/Si heterojunctions prepared by other methods can be attributed to high quality of CdTe layer grown by PLD which played an important role in improving the interface characteristics [19]. The photosensitivity of CdTe/ Si photodetector was found to be higher than other common heterojunction photodetectors such as CdS/Si [20], CdO/Si [21], and ZnO/Si [22]. This can be attributed to the high absorption coefficient of CdTe film in the visible region and due to high crystalline quality of CdTe grown by PLD. In addition, the photosensitivity of CdTe/Si near 1 mm is so high compared to its value for p–n and pin silicon photodetectors. Based on these results it is clear that the CdTe/Si photodetector can be used efficiently to detect 1064 nm laser pulses. 4. Conclusions CdTe/c-Si heterojunction photodetectors have been fabricated successfully by PLD of high quality

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CdTe thin film on silicon surface. The characteristics of grown film have been investigated. XRD pattern revealed that the film is polycrystalline in nature and non-stoichiometric phases have not been noticed. The photosensitivity of CdTe/Si photodetector was high (0.52 A/W) at 950 nm, a result confirms that the CdTe film prepared by PLD technique is promising for crystalline silicon optoelectronic devices. The high sensitivity of detector at 1064 nm suggests that it is a promising candidate to be used for Nd:YAG laser pulses detection. No degradation in the photodetector main characteristics was observed after two months of storing in air. The effect of post-deposition annealing on the device performance is underway. References [1] Ikegami S. Sol Cells 1988;23:89. [2] Seto S, Yamada S, Suzuki K. J Cryst Growth 2000;214:5. [3] Feredikes C, Britt J, Ma Y, Kilian L. In: 23rd IEEE PVSC, 1993. p. 83. [4] Zweibel K. Prog Photovoltaics 1995;3(69):279. [5] Chu H, Rohatgi A. J Electrochem Soc 1995;132:254. [6] Shao M. Appl Phys Lett 1996;69:3045. [7] Romeo N, Bosio A, Tedeschi R, Romeo A, Canevari V. Sol Energy Mater Sol Cells 1999;58:209. [8] Kot MV, Panasjuk LM. Sov Phys Semicond 1967;1:155.

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[9] Hampshire MJ, Tomlinson RD. In: Proceedings of the international conference on the physics and chemistry of semiconductor, heterojunction, vol. 2. Budapset; 1971. p. 323. [10] Seto S, Yamada S, Miyakawa T, Suzuki K. J Cryst Growth 2002;237:1585. [11] Neretina S, Sochinskii N, Mascher P. J Electron Mater 2005;34:786. [12] Uda H, Taniguchi H, Yoshida M, Yamashita T. Jpn J Appl Phys 1978;17:585. [13] Birkmire RW. In: Proceedings of the 18th IEEE photovoltaic specialists conference, Las Vegas; 1985. p. 1413. [14] Kuzma M, Bester M, Pyziak L, Stefaniuk I, Virt I. Appl Surf Sci 2000;168:132. [15] Alvin D, Voyles P, Spry D, Feng Z. In: SPIE proceedings, vol. 2403, Laser-induced thin film processing. San Jose, USA; 1995. p. 232. [16] Tariq E. PhD thesis, University of Technology, Baghdad, Iraq; 2006. [17] Bilevych Y, Boka A, Darchuk L, Gumenjuk J, Sizov F, Boelling O, et al. Semi Phys, Quantum Electron Optoelectron 2004;7:129. [18] Valdua V. Thin film chalcogenide materials, In: Symposium B, EMRS, France, 2002. p. 26. [19] Sharma BL, Purohit RK. Semiconductor heterojunctions. New York: Pergamon Press; 1974. [20] Ismail RA, Al-Majeed A, Sultan OA. Qatar Univ Sci J 2005;25:31. [21] Ismail RA, Al-Ani SK, Gaffar M. In: World renewable energy congress, Scotland, UK; 22–27, May 2005. p. 513. [22] Song D, Zhao J, Wang A, Widenborg P, Chin W, Aberle AG. 17th European PV conference, Munich; 2000. p. 1.