GaAsN Thin Film Growth by Chemical Beam Epitaxy with Source Gas Flow Rate Modulation
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0955-I15-51
GaAsN Thin Film Growth by Chemical Beam Epitaxy with Source Gas Flow Rate Modulation Yoshio Ohshita, Kenji Saito, Kenichi Nishimura, Toshihide Suzuki, and Masafumi Yamaguchi Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya, 468-8511, Japan
ABSTRACT The amount of residual carbon in the GaAsN film deposited by chemical beam epitaxy (CBE) is decreased by the flow rate modulation growth method (FM-CBE). The number of carbon atoms remained in the grown film increases as the growth temperature decreases. At low temperatures below 440ºC, the carbon atoms are mainly originated from the nitrogen source gas mono-methylhydrazine. However, increasing the substrate temperature during the growth causes the deterioration of film qualities and the high growth temperature is not the solution for reducing the impurities. On the other hand, by intermitting the supply of gallium source triethylgallium while the arsenic and nitrogen sources are continuously supplied, the carbon concentration drastically decreases as compared with that grown by the conventional CBE growth. The results of temperature programmed desorption and ab initio calculations suggest that the desorption of adsorbates that contain C atom, such as, NHCH3, is enhanced by the FM-CBE growth, resulting in the decrease of the residual C concentration. INTRODUCTION The band gaps of GaAsN and InGaAsN drastically decrease as increasing the nitrogen (N) content of the crystal because of the abnormal bowing effect, and around 1 eV band gap is realized [1,2]. Therefore, they have been considered to be promising candidates for materials of future multi-junction solar cells, such as, InGaP/GaAs/InGaAsN/Ge, with high conversion efficiency over 40% [3]. However, the minority carrier lifetime in the present crystal is too short to realize the tandem solar cells with high performance. GaAsN and its related materials have a large miscibility gap due to the different radii of N and other component atoms. Consequently, N concentration fluctuation in the grown film might occur, which induces the deterioration of electrical properties of the films. To suppress this problem, it is necessary to grow the crystal at low temperatures. To achieve this requirement, we have been developing the chemical beam epitaxy (CBE) method [4,5]. This growth method is carried out using organic gases as sources, and under a high vacuum (-10-2 Pa). Because of the ultra low pressure during the growth, the reactions between the source gas molecules in the gas phase are suppressed. Thus, the source gases that decompose at a low temperature can be used and the film growth at low temperature is realized. When monomethylhydrazine (H2N2H(CH3): MMHy), trisdimethylaminoarsenic (As(N(CH3)2)3: TDMAAs), and triethylgallium (Ga(C2H5)3: TEGa) were used as N, As and Ga source gases, high quality films with narrow X-ray diffraction peaks were grown at low growth temperatures in the 380 - 420ºC range. However, low-temperature growth increased carbon (C) incorporation in the grown films, which degra
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