Physics, MOVPE growth, and investigation of m-plane GaN films and InGaN/GaN quantum wells on gamma-LiAlO 2 substrates (2011)

The growth of InGaN/GaN quantum well structures along a nonpolar orientation avoids the negative effects of the so-called "Quantum Confined Stark Effect" and is therefore considered as promising approach to improve wavelength stability and efficiency of future optoelectronic devices. This work describes physical principles and experimental results on metal-organic vapor phase epitaxy and characterization of GaN layers and InGaN/GaN quantum well structures, which grow along the nonpolar (1-100) m-plane on (100) lithium aluminum oxide (LiAlO2) substrates. The limited thermal and chemical stability of the LiAlO2 substrate can be improved by a nitridation step, which causes the formation of a thin (1-100) AlN layer on the surface of the LiAlO2. This enables the phase-pure deposition of high-quality and smooth (1-100) GaN layers. The low lattice mismatch of (1-100) GaN to (100) LiAlO2 allows for a coherent growth of thin films, which show strong in-plane compressive strain. Due to the absence of a suitable slip plane, this strain relaxes only partly for layer thicknesses up to 1.7 µm. Low densities of line and planar defects compared to other heteroepitaxially deposited nonpolar GaN layers were assessed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron channelling contrast imaging microscopy (ECCI). The surface of the GaN layers is dominated by macroscopic hillocks, which are elongated along the c-axis direction and result in an average root mean square (RMS) roughness of ~ 20 nm in a 50 • 50 µm2 scan area. Spiral growth around line defects is seen as most likely cause for this effect. In a microscopic scale, one can detect a stripe pattern, which is formed by 2 - 3 nm high steps aligned parallel to the c-axis. An anisotropic growth mode is assumed responsible for this appearance. Between these steps, much smoother areas with typical RMS roughness of 0.2 nm (for a 0.5 • 0.5 µm2 scan) is found, which is also an indication for high quality on this small scale. As a consequence of the anisotropic growth mechanism, the line widths of XRD omega-scans taken with the incident direction perpendicular to the c-axis are strongly broadened compared to the perpendicular direction. The larger extension of coherent crystal regions along the c-axis is also reflected in the electron mobility, which is on average by 13% larger for carriers moving in this direction and takes values of up to 130 cm2/Vs. (1-100) GaN layers on LiAlO2 are always n-type conductive with a background doping in the range of 1 • 1019 cm-3. The introduction of large amounts of magnesium allows for an overcompensation to achieve p-type conductivity. The reason for the strong background doping is the incorporation of oxygen, which may evaporate from the heated substrate and effectively re-incorporate on the growing film since the (1-100) GaN surface exhibits a strong affinity to oxygen at the relatively low growth temperatures. The typical physical oxygen concentration of 1 • 1019 cm-3 is in agreement with the measured electron density. Lithium can also escape from the substrate and act as a crystal impurity, but the measured concentrations range only in the order of 1 • 1016 cm-3. (1-100) InGaN/GaN multi-quantum well structures (MQW) with different indium contents of 5 - 30% were successfully deposited and characterized. A lower indium incorporation efficiency compared to equally prepared MQW with (0001) orientation is in accordance to literature. All MQW exhibit smooth surfaces and abrupt interfaces. A few triangular-shaped pits with typical diameter of 100 nm are found on the surface, which arise from defects in the underlying GaN. The MQW are also deposited on the tilted facets of these pits, which is accompanied by a local change in MQW thickness and indium content. Photoluminescence spectra of InGaN/GaN MQW with indium fractions below 16% show strong, blue emission with excellent wavelength stability at increased excitation levels. For higher indium contents, the peaks become broader and weaker and exhibit a slight wavelength shift at higher intensities. Indium accumulation near defects or surface pits is seen as most likely origin. Higher indium contents on a nm scale are also blamed for the lower degree of polarization of emission compared to literature reports on nonpolar MQW. Indium clusters change the spatial distribution of holes within the valence subbands and therefore affect the recombination properties. LED based on (1-100) InGaN/GaN MQW were successfully fabricated. Although the light output is still significantly lower compared to devices based on layers deposited along the (0001) orientation, the strong blue emission at a forward voltage of only 4.1 V appears already quite promising. Main challenges for further improvement are the optimization of the upper p-type contact layer and the MQW layer stack.