The Investigation Of Non-polar A-plane GaN Epi-layers Directly Grown On Micro/nano Hole-array Patterned R-sapphire Substrates

In recent years,GaN-based materials are widely used in optoelectronics and power devices due to their excellent performance,such as wide direct bandgap,high breakdown field strength and high carrier mobility.At present,the c-plane GaN-based material grown along the [0001] direction is generally used commercially,but the further development of these devices is greatly hindered by the intrinsic spontaneous and extrinsic piezoelectric polarizations in c-plane polar GaN-based materials.In cplane GaN-based light-emitting diodes(LED),the quantum confined Stark effect(QCSE)due to the polarization field leads to separation of electron and hole wave functions,thereby reducing the oscillator strength for optical transitions.Especially in longer-wavelength devices such as green LED and laser diodes(LD),the luminous efficiency is lower than that of blue LED due to more significant QCSE.Another deterioration effect is observed in AlGaN/GaN high electron mobility transistors(HEMT)built in the [0001] growth direction.A two-dimensional electron gas(2DEG)induced by a polarization-generated charge in turn makes the enhancement mode(e-mode)devices difficult to achieve.In this case,a-plane GaN films have attracted considerable attention for the potential of free of polarization-induced electric fields.In this paper,we propose a method for high-quality non-polar a-plane GaN directly grown on micro/nano-sized patterned substrates.The main research contents are as follows:1.We fabricate micro-and nano-sized SiO2 hole array patterns on the r-plane sapphire substrate by UV lithography and laser interference exposure.In the laser interference exposure technology,the transfer of nanoporous pattern is mainly used.Due to the translucency of sapphire,it is necessary to evaporate a layer of metal chromium Cr to obtain a well-formed,uniform-sized array of holes.The choice of metal needs to meet the selection ratio in the etching process to ensure accurate transfer of the pattern.2.We investigate the growth and characteristics of a-plane GaN directly grown on SiO2 hole-array patterned r-sapphire substrates(HPSS)and flat sapphire substrate(FSS).The in-plane crystal quality anisotropy and stress state of each sample are characterized by high-resolution X-ray diffraction.It is found that the full-width at a half maximum(FWHM)of the rocking curve along the m-axis direction decreases significantly with the increase of the period.When the pattern is 3 ?m,the anisotropy behavior of crystal quality is basically eliminated,and the patterned substrate can effectively reduce the residual stress in a-plane GaN.The morphology of GaN islands is characterized by scanning electron microscopy(SEM).The Raman spectroscopy results show that the different sizes of pattern govern the lateral growth length of aplane GaN islands,thereby the larger pattern would increase the mosaic size along m direction and eliminate anisotropic behavior.In addition,the basal stacking fault(BSF)density of each sample is also examined by the low temperature photoluminescence(LT-PL)and the modified Williamson–Hall(W-H)analysis,and it is found that theBSF density in a-plane GaN grown on the micron-sized pattern substrate can be reduced by an order of magnitude.3.We explore the cause of the abnormal morphology of a-plane GaN islands on the patterned substrate.We separately study the evolution process of GaN islands,the influence of growth parameters and the change of morphology after corrosion.It is found that a-plane GaN islands only have a single polarity and there are two(000-1)N-polar planes along the c-axis.The N-polar plane may be caused by the presence of twins in the nucleation process.In addition,the difference between the epitaxial and secondary epitaxial a-plane GaN island morphology and crystal quality are also compared.