An Investigation of Long and Short Time-Constant Persistent Photoconductivity in Undoped GaN Grown By RF-Plasma Assisted
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Figure 1. Reflectance spectrum of sample 9824 with a free exciton peak.
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Downloaded from https://www.cambridge.org/core. University College London (UCL), on 26 May 2020 at 00:12:11, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/S1092578300005007
Table 1. Various properties of the ten samples included in this investigation. See text for details of each measurement. Sample τ50% a-H Exciton n Yellow Mobility τshort (Thickness) during in RT (1016 τ10% 2 PL (cm /Vs) (ms) growth cm-3) Refl. (µm) (s) 9824 4.0 yes 0.7 weak 202 1.7 (9.2) 246 strong 9821 4.8 0.5 weak 15 9.2 no (2.0) 622 9826 5.1 1.2 weak 9.0 1.8 yes (7.5) 4,044 9825 7.0 1.4 not det. 12 1.7 yes (2.6) 7,171 9910 13 1.2 weak 330 0.10 yes (1.72) 29,521 9671 64 2.3 yes mod. 120 77 no (1.0) 6,704 9730 214 2.3 yes not det. 4 7.2 no (0.98) 21,061 9710 293 1.8 yes weak 50 85 no (0.85) 32,564 9823 308 1.6 yes weak 232 140 yes (2.1) 8,703 9822 357 1.0 mod. 13 0.67 (?) (2.1) 52,922 electrical properties, as indicated in Table 1. Several of the samples were of high electrical and structural quality, as indicated by their high mobility and low carrier concentration. Samples 9824 and 9826 exhibited free excitonic photoluminescence (PL) at low temperature. Room temperature reflectance measurements, such as that shown in Figure 1, also indicated the presence of free excitons in about half the samples. Fitting this peak with a Lorentzian function appropriate to excitonic transitions indicates a transition energy of 3.412 eV, which is comparable to the value of 3.411 eV reported recently by Viswanath et al. [10] for the free exciton (A) transition at room temperature. This indicates a bandgap of 3.438-3.400 eV at room temperature based on reported values of the exciton binding energy.[10] PHOTOCONDUCTANCE DECAY MEASUREMENTS Persistent photoconductivity was measured by monitoring sample resistance with a Keithley 175A multimeter after exposure to light. Samples were stored in the dark for approximately three to four days prior to light exposure in order to determine a baseline dark resistance, followed by exposure to either a bright white light source with the infrared removed via filtering, or to a weak light source consisting of filtered light corresponding to the yellow PL peak position. Either source of light led to a PPC effect, similar to that shown in Figure 2. The intensity varied, but the temporal decay was similar. Many authors [1,4,6,11] have described PPC in GaN using a so-called “stretched exponential”
F99W11.45
Downloaded from https://www.cambridge.org/core. University College London (UCL), on 26 May 2020 at 00:12:11, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/S1092578300005007
C(t) = C(0)exp[(t/τppc)β] (1) The two fits shown in Figure 2 represent this function. By better fitting the shorter time response, we obtained a value of β of about 0.2, comparable to that reported in the studies by Li et al. [
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