Nature of Low-Frequency Excess Noise in n-Type Gallium Nitride
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NATURE OF LOW-FREQUENCY EXCESS NOISE IN n-TYPE GALLIUM NITRIDE C. F. Zhu, W. K. Fong, B. H. Leung, C. C. Cheng and C. Surya Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hong Kong ABSTRACT Low-frequency noise is investigated in n-type GaN film grown by rf-plasma assisted molecular beam epitaxy. The temperature dependence of the voltage noise power spectra, SV(f), was examined from 400K to 80K in the frequency range between 30Hz and 100KHz, which can be modeled as the superposition of 1/f (flicker) noise G-R noise. At f > 500 Hz the noise is dominated by G-R noise with activation energies of 360meV and 65meV from the conduct band. The results clearly demonstrate the trap origin for both the 1/f noise and G-R noise. At the low-frequency range the fluctuation was dominated by 1/f noise. To determine the origin of the noise we considered both the bulk mobility fluctuation and the trap fluctuation models. Our experimental results showed that rapid thermal annealing (RTA) at 800°C resulted in over one order of magnitude decrease in the Hooge parameter. Annealing at temperatures in excess of 1000°C resulted in significant increase in the noise. Photoluminescence and x-ray diffraction measurements also showed that the crystallinity of the films improved with RTA at 800°C with an accompanying reduction in deep levels. Annealing at 900°C and 1000°C resulted in an increase in the FWHM of the x-ray diffraction indicative of thermal decomposition of the materials. The results are in excellent agreement with the trend of Hooge parameters as a function of annealing temperature, strongly indicating trap origin of the observed 1/f noise.
INTRODUCTION Wide bandgap GaN and its related semiconductor compounds have found important applications in blue and green LEDs[1], lasers[2], UV detectors[3], as well as high temperature and high power microwave electronics[4-5]. Due to significant mismatches in the lattice constants and coefficients of thermal expansion between the GaN material and the sapphire substrate, GaN films typically exhibit large defect concentration such as point defects, microtwins and stacking faults[6-8]. Low-frequency noise is an important figure of merit for the devices[9-10]. It is therefore important to have a correct knowledge of the underlying physical processes for the noise. Previous studies of low-frequency noise in various semiconductors showed that there are two possible origins for flicker noise. The bulk mobility fluctuation model states that the normalized voltage noise power spectral density for flicker noise is given by the following universal equation[11]:
T6.23.1
SV α = , 2 V fN
(1)
where N is the total number of carriers in the device. It is assumed that the bulk mobility fluctuation arises from lattice scattering and the Hooge parameter, α, which varies as (µ/µl)2, where µ is the carrier mobility and µl is the mobility due to lattice scattering alone. On the other hand, the trap model stipulates that 1/f noise arises from the capture and emission o
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