Effect of Electron Injection on Minority Carrier Transport Properties in Unintentionally Doped GaN
Authors: Modak Sushrut , Chernyak Leonid , Xian Minghan , Ren Fan , Pearton Stephen , Khodorov Sergey , Lubomirsky Igor , Ruzin Arie , Dashevsky Zinovi 
Published: December 2020

GaN is a robust and stable wide bandgap material with a bandgap of 3.4eV. It is extensively studied for applications in light emitting diodes, high-power electronics and optoelectronics. Radiation hardness is another attractive attribute of GaN due to its atomic structure and wide bandgap with possible electronic applications in lower Earth satellite orbits. Like other wide bandgap materials, GaN also suffers from short diffusion length, which has been earlier reported to be ~100-300 nm. Minority carrier transport properties such as carrier lifetime (τ) and diffusion length (L) are of vital importance in bipolar and minority-carrier devices. Previously, it was shown that low energy electron beam irradiation (up to 30 keV) for extended duration (up to 3000 seconds) can be used to enhance L by a factor of 4 at room temperature in other wide bandgap materials like p-ZnO and β-Ga2O3
[1-3]
. It was suggested that the increase in L is associated with trapping of non-equilibrium electrons on meta-stable trap levels acting as recombination centers. Dependence of L on τ is given by the relation L=√(Dτ) , where D is the diffusion coefficient. Therefore, removal of recombination centers, due to electron trapping on them would, in turn, increase τ, and hence L. Another effect of low energy electron irradiation, apart from increasing τ, is manifested in decrease in the continuous CL intensity. In this work, unintentionally doped GaN was studied using Electron Beam-Induced Current (EBIC), Continuous (CL) and Time-Resolved Cathodoluminescence (TRCL) techniques to independently measure L and τ, and verify the validity of the proposed model[4]. Measurements were conducted on GaN Schottky rectifiers (~400 µm GaN layer grown by Halide Vapor Phase Epitaxy on sapphire) with electron concentration of ~1x1017 cm-3. L was found to increase from 306 nm to 347 nm for a maximum electron injected charge density of 117.5 nC/µm3 and saturating at 60 nC/µm3. For the same maximum and saturation injected charge densities, τ was found to increase from 77 ps to 101 ps. Fig. 1a shows a plot of TRCL streaks for bare region and the region with maximum injected charge density. Slower decay of streak signal indicates longer lifetime in the electron-injected region. Fig. 1b depicts the plot of L and √τ as a function of injected charge density at room temperature. A clear linear dependence (L=√(Dτ)) is seen till saturation charge density of ~60 nC/µm3. Furthermore, mobility values (from the diffusion coefficient) for various charge injection doses were obtained for the ultrafast regime of TRCL spectroscopy.

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Figure Caption:

Fig. 1 (a) Recorded streak signal for the bare region (τ=77 ps) and electron injected region with maximum dose (τ=101 ps). Lifetime is higher for the injected region with lower decay rate. Inset shows photon counts in a raw streak image. (b) Linear dependence between measured L (left) and √τ (right) as a function of injected charge density. Both parameters saturate at ~60 nC/µm3 and further charge deposition produces no effect.

Figure 1