Transport properties of III-V nitrides epilayers and aluminum gallium nitride/gallium nitride heterostructures

Persistent photoconductivity (PPC) effect has been observed and studied in p-type GAN epilayers, quaternary Ga1-xInxN yAs1-y alloys, and AlGaN/GaN heterostructures. Conductivity and Hall effect measurements have been employed to study the electrical transport properties of these GAN based material systems. PPC buildup and decay transients and the dependence of the decay time constant on the PPC buildup time have been systematically measured and formulated in the context of lattice relaxed deep centers. PPC results from MBE grown p-type epilayers have been compared with those grown by metal-organic chemical vapor deposition (MOCVD).;For quaternary GaInNAs alloy materials grown by MOCVD, PPC effect has been used to study the impurity properties and to exam the materials quality. Fundamental parameters, which characterize the deep centers in GaInNAs, have been determined and indicated that the lattice relaxed deep centers are responsible for the PPC and strongly influence the transport properties of this quaternary system.;PPC has also been used to study the transport properties in two-dimensional electron gas (2DEG) systems in the AlGaN/GaN heterostructures. Its unique features have been utilized to probe the interface quality and the transport property. For the non-optimized heterostructures, strong PPC effect has been observed. A transition of electrons from the first to the second subband in the 2DEG channel have been observed by monitoring the 2DEG electron mobility as a function of electron concentration through the use of PPC. The results suggested that the magnitude of the PPC and hence the photoinduced instability associated with these heterostructures are strong functions of n s x mu, the product of electron sheet density, ns, and the channel mobility, mu.;PPC effect on the performance of an AlGaN/GaN heterostructure ultraviolet detector has also been investigated. The photocurrent (PC) transient characteristics of the UV detector depend on the device operating history and are directly related to the effect of PPC caused by deep level centers. Such a PPC related effect could have a significant influence on the performance of III-nitride based UV detectors.;Finally, switching transient characteristics of AlGaN/GaN heterojunction field-effect transistors (HFETs) have been studied. Switching time constants have been systematically measured for varying gate-source and drain-source biases up to 40 GHz. The switching time constants, thus the cut-off frequency, depend strongly on the gate-source and drain-source biases with different switching responses in "switching-on" and "switching-off" processes. This was explained in terms of the variation of channel mobility and saturation velocity under different bias conditions, which are determined by material properties and structures. Implications of the switching transient characteristics on the performance of AlGaN/GaN HFETs were also discussed.