Polarization-optimized heterostructures with quaternary AlInGaN layers for novel group III nitride devices

摘要

Owing to their unique material properties, group III nitrides are attractive for the application in devices, e.g. heterostructure field effect transistors (HFETs) and light-emitting diodes (LEDs). Group III nitrides exhibit an inherent macroscopic total polarization, which is the sum of the spontaneous polarization and the piezoelectric polarization. In a metal-polar heterostructure, in which a material with wider bandgap and larger absolute total polarization is deposited on GaN, the abrupt change of the polarization leads to fixed positive interface charges. As a consequence, a two-dimensional electron gas (2DEG) is formed in the GaN layer to compensate these charges. Such a 2DEG is widely exploited in HFETs, which are seen as possible candidates to replace conventional Si-based electronic devices in the power electronic sector. Sheet electron densities up to 3∙10^13 cm^-2 have been demonstrated. These large electron densities, however, result typically in transistors of depletion mode (d-mode) type. To allow for GaN-based HFETs which can be easily integrated in current power-switching circuit architectures, the realization of enhancement mode (e-mode) devices is of major interest.In the lighting sector, LEDs have reached a high level of maturity, which is manifested by large luminous efficacies and high device lifetimes. However, the inherent polarization in the nitrides results in an electric field in the InGaN/GaN multiple quantum well (MQW), which in turn leads to a spatial separation of electrons and holes. As a result, the internal quantum efficiency is reduced and the emission wavelength is drifting with LED current. Further, the polarization is considered to be one of the causes for the typical efficiency droop at higher currents. There is consequently significant scientific and technological interest to explore the possibility and the limits of engineering the polarization effects in electronic and optoelectronic devices. In this thesis, the method of polarization engineering with quaternary AlInGaN layers is studied. AlInGaN offers the highest possible flexibility in adjusting material properties independently, e.g. bandgap and lattice constant or strain state and polarization. While tensile strained AlInGaN on GaN exhibits a large total polarization, compressively strained layers feature a strongly reduced polarization. By changing the composition and, hence, the strain state, the polarization difference can be controlled.In the first part, a comprehensive study on the growth process by metal-organic vapor phase epitaxy (MOVPE) and on structural properties of AlInGaN is presented. Optimum reactor conditions for the growth of AlInGaN are identified. A wide composition range of AlInGaN layers grown on GaN is achieved, which results in a large range of strain conditions from high tensile strain, to the almost unstrained case for nearly lattice-matched compositions and finally compressive strain. While in the first and second case, a high crystal quality is achieved, in the latter case, an inferior quality is observed due to large In contents. To describe relaxation effects in AlInGaN layers with significant In content, a multi-layer model is developed.In the second part, polarization engineering is utilized to obtain both d- and e-mode HFETs. While d-mode behavior is achieved with tensile strained AlInGaN layers with a large total polarization, e-mode behavior is realized with compressively strained AlInGaN layers with low polarization. However, for compressively strained AlInGaN with larger In contents and, hence, degraded crystal quality, inferior electrical properties are observed. Improved e-mode HFET performance is obtained with almost lattice-matched AlInGaN barriers with low In and Al contents. High-polarization and low-polarization layers are combined to provide positive threshold voltage and simultaneously low parasitic sheet resistance, enabling higher drain current and large transconductance. In analogy with the n-channel HFETs, the method of polarization engineering is applied to p-channel HFETs with 2D hole gases (2DHGs). Despite the success of n-channel HFETs with 2DEGs, there are only sparse results on p-channel HFETs due to challenges on both the technology and material side. First, MOVPE nitrides feature a high n-type background doping. Second, the typical acceptor in GaN, Mg, exhibits a large ionization energy of several hundred meV. Third, the effective hole mass is one order of magnitude larger than the electron mass. To overcome these challenges, an AlInGaN backbarrier with a high total polarization is used for the generation of a polarization-induced 2DHG. Both d- and e-mode characteristics are shown and record performance figures for p-channel devices are reported. In the third part, improved LED structures with AlInGaN layers with variable polarization are investigated. First, conventional GaN barriers in a MQW are replaced by AlInGaN barriers. It is confirmed that by changing the polarization in the AlInGaN barrier, it is possible to influence the internal electric fields. A smaller polarization results in a decreased emission wavelength and in improved optical oscillator strength. The wavelength shift due to the quantum-confined Stark effect (QCSE) is found to depend on the AlInGaN composition. Finally, a novel concept for LEDs is introduced which utilizes the superior electrical properties of a 2DHG. An inversed p-n junction in a metal-polar layer stack is applied, which results in a reversed polarization. This is realized by using a 2DHG as a hole reservoir and as a current spreading layer at the bottom p-GaN side. With AlInGaN backbarriers featuring a high polarization, a large sheet hole density in the 2DHG is achieved. A proof of concept is provided by the demonstration of prototype devices with stable emission wavelength.