Thermoelectric and electrical properties of Si-doped InSb thin films. University, Japan

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1 / The Electrochemical Society Thermoelectric and electrical properties of Si-doped InSb thin films H. Nagata a and S. Yamaguchi a,b a Department of Electrical, Electronic and Information Engineering, Kanagawa University, Japan b High-Tech research center, Kanagawa University, Japan We studied the thermoelectric and electrical properties of Si-doped InSb thin films grown by Metalorganic Chemical Vapor Deposition. Their thermoelectric properties were evaluated using power factor (P f =α 2 /ρ), which is an important criterion, and a value of 10-3 W/mK 2 is standard for practical use. Maximum value of power factor was W/mK 2 at mol/min for Si flow rate dependence. Power factor of wholly Si-doped InSb thin film was W/mK 2, and power factor of partially Si-doped InSb thin film was W/mK 2. Si-doping may strongly influence the interface between InSb and substrate, and defects in the InSb layer near the interface could be decreased by Si doping. Introduction InSb has high electron mobility (78000cm 2 /Vs at room temperature (RT)), narrow energy band gap (0.18eV at RT), and middle infrared spectral region (6.89µm at RT). While InSb has been mainly used for infrared detectors and hall sensors, we have demonstrated that InSb has much potential as thermoelectric devices [1,2]. Thermoelectric conversion can be evaluated using figure of merit (Z). Figure of merit (Z) can be estimated by Z=α 2 /ρκ µm *3/2 /κ L [1] where α is Seebeck coefficient, ρ is electrical resistivity, κ is thermal conductivity, µ is mobility, m * is effective mass. But, in general, thermoelectric properties of thin films can be evaluated using power factor (P f ), which is an important criterion, and value of 10-3 W/mK 2 is standard for practical use. Power factor (P f ) can be estimated by P f =α 2 /ρ. [2] In this study, we measured the thermoelectric properties of InSb thin films with Si doping, and evaluated their power factor. 7

2 ECS Transactions, 16 (24) 7-12 (2009) Experiment In this study, InSb thin films were deposited on sapphire (0001) substrate with InAsSb buffer layer by metalorganic chemical vapor deposition (MOCVD). Trimethylindium (TMIn), Trimetyhlantimony (TMSb) and Tertiarybutylarsine (TBAs) were used as sources, and H2 was used as a carrier gas. The InAsSb buffer layer served as wetting layer because the wetting of InSb to sapphire is very poor. InSb thin film with a thickness of 0.5µm was grown at 490 C, and InAsSb buffer layer with a thickness of several nm was grown at 400 C. InSb layer was doped with Si. First, we studied the thermoelectric properties of Si-doped InSb thin films regarding Si flow rate. The schematics of a grown sample were shown in Fig.1. (a) (b) Fig.1 InSb thin film with Si doping (1) Next, we studied in terms of Si-doped layer thickness dependence. The schematics of a grown sample was shown in Fig.2. (a) (b) (c) (d) Fig.2 InSb thin film with Si doping (2) Electrical resistivity was measured by van der Pauw method, and Seebeck coefficient was measured using direct current (DC) method in order to obtain power factor (Pf =α2/ρ) of samples. Typical values of thermoelectric and electrical properties of InSb thin films were measured in room temperature. 8

3 Discussion (1) Si flow rate dependence Figure 3 shows the electron mobility and carrier concentration at RT. Maximum value of electron mobility was 10600cm 2 /Vs at mol/min, and minimum value of carrier concentration was cm -3 at mol/min. Figure 4 shows the electrical resistivity at RT. Minimum value of electrical resistivity was Ωcm at mol/min. Figure 5 shows the Seebeck coefficient at RT. Minimum value of Seebeck coefficient was -205µV/K at mol/min. Figure 6 shows the power factor at RT. Maximum value of power factor was W/mK 2 at mol/min. Electron mobility (cm 2 /Vs) Carrier concentration (cm -3 ) Fig.3 Electron mobility and carrier concentration Electrical resistivity (Ωcm) Fig.4 Electron resistivity 9

4 Seebeck coefficient (μv/k) Fig.5 Seebeck coefficient Power factor (W/mK 2 ) Fig.6 Power factor Figure 7 shows the compressive stress and full width at half maximum (FWHM) of the rocking curve in the (111) plane of our InSb thin films. Minimum value of compressive stress was 0.19GPa at mol/min. With increasing Si flow rate, lattice constant decreases. This is because lattice constant of Si smaller than that of InSb. This suggests that as Si flow rate increases, Si atoms that replace In atoms increase. 10

5 compressing stress (GPa) FWHM (degree) Fig.7 compressing stress and FWHM of the rocking curve in the (111) plane 0.4 (2) Si-doped layer thickness dependence Table1 shows the typical thermoelectric and electrical properties of InSb thin films at RT for (a) undoped InSb thin film, (b) wholly Si-doped InSb thin film, (c) InSb thin film with Si-doped near the interface, and (d) InSb thin film with Si-doped near the surface of InSb. Power factor for case (b) was W/mK 2, and that for case (c) was W/mK 2. Si-doping may well influence the interface between InSb and substrate, and it follows that defects both in the InSb and near the interface were decreased by Si-doping. Table1 Typical thermoelectric and electrical properties of InSb thin films at room temperature Electron mobility (cm 2 /Vs) Carrier concentration (cm -3 ) Electrical resistivity 10-3 (Ωcm) Seebeck coefficient (µv/k) Power factor 10-4 (W/mK 2 ) (a) (b) (c) (d)

6 Conclusion We studied the thermoelectric and electrical properties of Si-doped InSb thin films grown by MOCVD. Si doping has possibilities that influence the interface between InSb and substrate, and defects in the InSb near the interface decreased by Si-doping. Acknowledgements This study was supported by the Takahashi Industrial and Economic Research Foundation and the High-Tech Research Center Project from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References [1] S. Yamaguchi, Y. Nagawa, N. Kaiwa, and A. Yamamoto, Appl. Phys. Lett., 86, (2005). [2] S. Yamaguchi, T. Matsumoto, J. Yamazaki, N. Kaiwa, and A. Yamamoto, Appl. Phys. Lett., 87, (2005). 12