Perturbation of charges in AlGaN/GaN heterostructures studied by nanoscale capacitance-voltage technique
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Perturbation of charges in AlGaN/GaN heterostructures studied by nanoscale capacitancevoltage technique G. Koley and L. Lakshmanan Department of Electrical and Computer Engineering University of South Carolina Columbia, SC 29208 ABSTRACT Perturbation of charges at the surface and interface of AlGaN/GaN heterostructures has been studied by quantitative nanoscale capacitance-voltage (C-V) measurements. The nanoscale C-V curves were found to have different slopes in the forward and reverse directions. These measurements indicate a change in confinement of the two-dimensional electron gas (2DEG) at the AlGaN/GaN interface depending on the direction of the dc voltage sweep during C-V measurements, which can be explained by surface state charging and discharging during the bias sweep. Under UV illumination, the density of the 2DEG increased significantly as inferred from the increase in threshold voltage of the nanoscale C-V scans, and no change in 2DEG confinement, depending on the direction of the bias sweep, was observed.
INTRODUCTION AlGaN/GaN heterostructures have been extensively studied in the last decade due to their applications in high power microwave devices.1, 2 It has been observed that the surface states of the AlGaN/GaN heterostructures affect the performance of the microwave devices to a significant extent as they strongly influence the 2DEG density present at the AlGaN/GaN interface (which is formed solely due to the piezoelectric properties of III-Nitrides, without any doping). Characterization of the effect of surface states is therefore important to optimize the performance of the AlGaN/GaN heterostructure based microwave devices. Scanning capacitance microscopy is commonly used as a characterization tool for the measurement of carrier concentration, with nanoscale resolution, in semiconductor samples.3-5 The advantage of this technique is that it is non-destructive, apart from being nanoscale, so that the characteristics of the surface under study does not get modified. Usually, in these measurements, the capacitance gradient signal dC/dz is measured rather than the capacitance itself. However, the capacitance signal itself, if measured, is much easier to interpret. There have been attempts in recent years to measure the tip-sample capacitance directly, using a lock-in amplifier and phase shifting circuitry,6 and a high precision capacitance bridge.7 In this work, we have demonstrated that a simple technique that is similar to the former, can be used to perform quantitative nanoscale C-V measurements on AlGaN/GaN based heterostructures. We have found that this simple technique is sensitive enough to yield nanoscale C-V curves for AlGaN/GaN heterostructure samples, which can yield important information regarding the surface and interface charged density variations.
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EXPERIMENTAL DETAILS The C-V measurements were performed using a schematic set up as shown in Fig. 1. A sinusoidal ac signal was applied to the probe tip in addition to a dc bias, and the current flowing be
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