Correlation between Sheet Carrier Density-Mobility Product and Persistent Photoconductivity in ALGAN/GAN Modulation Dope
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be a strong function of only one parameter, the product of sheet carrier density (ns) and the two-dimensional electron gas mobility (µ). Since nsµ is the most important intrinsic material parameter for the HFET structural design, the magnitude of PPC in turn can be utilized as a sensitive probe for monitoring the electronic qualities of the AlGaN/GaN MOD-HFET structures. EXPERIMENTAL DETAILS The inset of Fig. 1 is a schematic diagram showing the generic structure of samples used in this work which consisted of a 1.3 µm highly insulating GaN epilayer followed by an Al0.25Ga0.75N spacer layer and finally a Si-doped Al0.25Ga0.75N layer. The structures were deposited over basal plane sapphire substrates using a variable pressure MOCVD system at a growth temperature of 1050 0C. A total of seven samples with varying growth or structural parameters were studied. For the PPC measurements, a 1.5 V bias was supplied to the sample, a Hg lamp was used as an excitation source, and the conductivity was monitored through current (Ippc) by using an electrometer. To ensure that each set of data obtained under different temperatures have the same initial conditions, the system was always heated up to 300 K, then cooled down in darkness to the desired measurement temperatures. The excitation intensity and buildup time span are fixed for different temperatures and samples. Fig. 1 illustrates the temperature variations of the sheet carrier density and mobility for one of our optimized structures. The values of the sheet carrier density and mobility products (nsµ) at different temperatures shown in Fig. 1 are among those highest values reported for the AlGaN/GaN HFET structures grown on sapphire substrates [12-14]. For all seven structures investigated here, the conductivity of the 2DEG channel is enhanced after exposure to light. Moreover, the light enhanced conductivity persists for a
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T (K) Fig. 1 2DEG sheet carrier density (ns) and mobility (µ) versus temperature, T, measured in a dark state for one of our optimized Al0.2Ga0.8N/GaN MOD-HFET structures. The inset shows the generic structures for the samples used in this study.
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t (s) Fig. 2 Buildup and decay kinetics of PPC associated with the 2DEG system in three Al0.25Ga0.75N/GaN MOD-HFET structures measured at two representative temperatures, (a) T = 300 K and (b) T = 20 K. Here Ippc denotes the persistent photocurrent measured at time t and Id the initial dark current levels. long period of time after the removal of excitation source, an effect which is referred to as PPC. In Fig. 2, we present the PPC results obtained for three representative samples measured at two different temperatures. We can see that the decay time constants of the
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