Although known since decades, only recently III-sesquioxides have received attention as a new class of wide-band-gap semiconductors. In the past polycrystalline highly doped In₂O₃:Sn was actually used as high-conductivity material for transparent electrodes in “smart windows” [1], photovoltaics [2] and chemical sensors [3]. Nowadays, the research is focusing on single-crystalline oxide layers with low defect densities and semiconducting behavior. β-Ga₂O₃ is the most attractive representative of this class of materials due to the large band-gap (4.9 eV) and high break-down field (8 MV/cm) promising applications in transparent electronics [4], short wavelength photonics, optoelectronics, etc.

We recently started a research program aiming at depositing epitaxial thin films of β-Ga₂O₃ on sapphire (0001) and also on native Ga₂O₃ substrates. Low-pressure MOCVD, with trimethylgallium as a source of gallium and water vapors as a source of oxygen, was employed for this study. The thickness of the samples was varied in the 80-250 nm range. For optimal growth a temperature 800^oC was selected. Different structural, compositional and optical methods were employed to characterize the properties of the material grown. The MOCVD-grown epitaxial films are very pure and insulating owing to the large band gap of β-Ga₂O₃. To make them semiconducting, impurities such as Sn or Si have to be incorporated on Ga sites. We have chosen to study the behavior of Si since the Si atomic radius is smaller than that of Sn, which should favor its incorporation. In the literature however the Si-doping is discussed controversially [5,6], although there is some consensus that Si substitutes Ga in the crystal lattice. A parallel activity at IKZ on bulk β-Ga₂O₃ grown by the Czochralski method evidenced that single crystals were contaminated by Si from the growth environment or residual in the Ga₂O₃ source powder [7]. The crystals were n-type with an electron concentration in the range from 5x10¹⁶ cm^-3 to 5x10¹⁷ cm^-3.

The present study aims at checking the Si doping potential in the MOCVD process. As silicon source we have taken tetra-ethyl-ortho-silicate since it is well-developed for SiO₂ dielectric thin films growth. By adjustment of Ga to Si molar fraction ratios, a series of β-Ga₂O₃ epilayers with Si in the range from 10¹⁶ to 10¹⁹ cm^-3 (determined by SIMS) were prepared. Hall effect measurements performed at room temperature demonstrated that the resulting material was not conductive. SIMS shows that Si is physically incorporated in the films homogeneously but it is obviously not electrically active. To activate the Si species the epitaxial layers were additionally annealed at temperatures from 800 to 1000^oC with steps of 50^oC. The structural, optical and electrical properties for as-grown and annealed samples have been studied in detail by different characterization methods. The interplay between temperature regimes, structural, optical and electrical properties will be discussed.

References:

[2] J. Herrero, C. Guillén, Thin Solid Films  451–452 (2004) 630.

[3] P. Sowti Khiabani,  E. Marzbanrad, C. Zamani, R. Riahifar,  B. Raissi, Sensors and Actuators B: Chemical, 166–167  (2012) 128.

[4] J. Wager, Science 300 (5623) (2003) 1245.

[5] Encarnación G. Víllora, Kiyoshi Shimamura, Yukio Yoshikawa, Takekazu Ujiie, and Kazuo Aoki, Appl. Phys. Lett. 92, 202120 (2008).

[6] Kenichiro Takakura, Suguru Funasaki, Isao Tsunoda, Hidenori Ohyama, Daisuke Takeuchi, Toshiyuki Nakashima, Mutsuo Shibuya, Katsuya Murakami, Eddy Simoen, Cor Claeys, Physica B: Cond. Matter 407 (2012) 2900.

[7] K. Irmscher, Z. Galazka, M. Pietsch, R. Uecker, and R. Fornari, J. Appl. Phys. 110, 063720 (2011).

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