Characterization Of G-R Noise In GaN Films Grown By RF-MBE On Intermediate-Temperature Buffer Layers
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Characterization Of G-R Noise In GaN Films Grown By RF-MBE On Intermediate-Temperature Buffer Layers W.K. Fong, B.H. Leung, C.F. Zhu, and Charles Surya The Hong Kong Polytechnic University Department Of Electronic And Information Engineering Hong Kong ABSTRACT We report detailed investigations of generation-recombination (G-R) noise in GaN films grown by rf-plasma assisted molecular beam epitaxy on intermediate-temperature buffer layer (ITBL) in addition to conventional low-temperature buffer layer. To characterize the film quality affected by the use of ITBL, low-frequency noise measurements were performed. The voltage noise power spectra show a strong dependence on the thickness of the ITBL. A model has been presented to explain the observed G-R noise, which stipulates that the phenomenon arises from the thermally activated trapping and detrapping of carriers by traps. The process leads to the correlated fluctuations in both the carrier number and the coulombic scattering rate. Detailed numerical evaluation shows that number fluctuation dominates in our samples. The calculated trap densities show that the use of ITBL can effectively reduce defect density by over an order of magnitude. INTRODUCTION Gallium nitride and related alloys have been under intense research because of its promising applications in optoelectronic devices such as blue light emitting diodes, blue laser diodes, UV photodetectors and microwave power amplifiers [1-3]. However, it has been well documented that the lack of native substrate remains to be the main bottleneck for the development of III-nitride based technology. (0001) oriented sapphire wafers are commonly used as a substrate for the growth of GaN. Due to the large mismatch in the lattice constants between GaN and sapphire, GaN epilayers typically exhibit high defect concentration and poor crystallinity [4]. The growth of high quality GaN films requires the deposition of a thin buffer layer between the GaN epilayer and the substrate at a relatively low temperature, providing a high density of nucleation centers. The improvement of film quality by the deposition of a thin GaN or AlN low-temperature buffer layer (LTBL) is well documented for metal organic chemical vapor deposition of GaN [5-6]. The implementation of LTBL in the molecular beam epitaxy (MBE) growth process proved beneficial to certain extent. However, the mechanisms by which the buffer layer relieves stress, and by which the stress relaxation affects defect formation are not well understood. There are ongoing efforts among different groups in the development of novel buffer layer systems suitable for MBE process. Recent works by the authors showed the incorporation of an intermediate-temperature buffer layer (ITBL) results in substantial improvements in both the optical and electronic properties [7-10]. A high room-temperature electron mobility of 460 cm2V−1s−1 has been reached [8]. It is important to characterize how the ITBL affects the defect concentration in GaN films because crystalline defects are known to have subst
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