Impact of the Growth Polar Direction on the Optical Properties of Gan Films Grown by Metalorganic Vapor Phase Epitaxy
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IMPACT OF THE GROWTH POLAR DIRECTION ON THE OPTICAL PROPERTIES OF GaN FILMS GROWN BY METALORGANIC VAPOR PHASE EPITAXY A. Setoguchi*, a), K. Yoshimura**, M. Sumiya**, A. Uedono*, S. F. Chichibu*,b) *
Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan ** Department of Electrical and Electronic Engineering, Shizuoka University,3-5-1 Johoku, Hamamatsu, Shizuoka 432-8561, Japan a) On leave from Department of Electrical Engineering, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278-8510, Japan b) Contacting author, [email protected]
Abstract The growth polar direction during metalorganic chemical vapor phase epitaxy of wurtzite GaN films was shown to affect the optical properties in terms of impurity and vacancy-type defect incorporation during the growth. The GaN film grown towards the Gaface (0001) (+c polarity) exhibited clear excitonic features in its optical absorption and luminescence spectra up to room temperature. Conversely, the film with the N-face (000-1) (-c polarity) exhibited a broad emission band, which is located in the broadened absorption tail. The Stokes shift remained even at 300 K. The difference between the two was explained in terms of the presence of impurity-induced band tail states in –c GaN due to increased impurity density and enhanced incorporation of large volume vacancy-type defects, which were confirmed by secondary ion mass spectrometry [Sumiya et al., Appl. Phys. Lett. 76, 2098 (2000)] and monoenergetic slow positron annihilation technique. Introduction GaN and related alloys1) are attracting special attention since they serve as base materials for light emitting devices1) operating in UV1) to amber2) spectral region. Different from other III-V semiconductors like GaAs, GaN-based devices are fabricated on the c-plane sapphire substrates along the polar axis of the wurtzite structure. Therefore the importance of spontaneous3) and piezoelectric4) polarization on the optical4,5) and electrical6) properties in AlGaN / GaN / InGaN structures has been pointed out.3) The polarity of the films has been identified by several groups7) and reviewed by Hellman.8) Sumiya et al.9) have observed GaN film polarity by coaxial impact collision ion scattering spectroscopy (CAISISS) concluding that GaN polarity has been influenced by the topmost polarity of the buffer layer deposited at low temperature rather than the growth method. They10) have investigated the films by secondary ion mass spectrometry (SIMS) to find that the concentration of trace impurities such as C, O, and Al in the GaN films having the N (000-1) face (-c polarity) was higher than that in the one with the Ga (0001) face (+c polarity). However, correlation between the growth polar direction and emission mechanisms is still unclear at present. In this paper, impact of the growth polar direction on the optical properties of GaN films during metalorganic vapor phase epitaxy (MOVPE) on sapphire is discussed in connection with impurity and vacancy incorporation. The -c GaN
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