The in-plane optical anisotropic properties of the non-polar a-plane GaN films grown by metal organic chemical vapour deposition are investigated by using polarised photoluminescence(PL),optical transmission and Raman scattering measurements.Through polarised PL and transmission spectra,the in-plane optical anisotropic properties of a-plane GaN film are found,which are attributed to the topmost valance band(atΓpoint)split into three sub-bands under anisotropic strain.The PL spectra also exhibit that the light hole band moves up more rapidly than the spin-orbit crystal-field spilt-off hole band with the increasing in-plane anisotropic compressive strain.Raman scattering spectra under different configurations further indicate the in-plane anisotropy and the hexagonal crystalline structure of these a-plane GaN films.
The influence of dry etching damage on the internal quantum efficiency of InGaN/GaN nanorod multiple quantum wells (MQWs) is studied.The samples were etched by inductively coupled plasma (ICP) etching via a selfassembled nickel nanomask,and examined by room-temperature photoluminescence measurement.The key parameters in the etching process are rf power and ICP power.The internal quantum efficiency of nanorod MQWs shows a 5.6 times decrease substantially with the rf power increasing from 3W to 100W.However,it is slightly influenced by the ICP power,which shows 30% variation over a wide ICP power range between 30W and 600W.Under the optimized etching condition,the internal quantum efficiency of nanorod MQWs can be 40% that of the as-grown MQW sample,and the external quantum efficiency of nanorod MQWs can be about 4 times that of the as-grown one.
The m-plane InN (1100) epilayers have been grown on a LiAlO2 (100) substrate by a two-step growth method using a metal-organic chemical vapor deposition (MOCVD) system. The low temperature InN buffer layer (LT-InN) is introduced to overcome the drawbacks of thermal instability of LiAlO2 (LAO) and to relieve the strains due to a large thermal mismatch between LAO and InN. Then the high temperature m-plane InN (1100) epilayers (HT-InN) were grown. The results of X-ray diffraction (XRD) suggest that the m-plane InN (1100) epilayer is a single crystal. The X-ray rocking curves (scans) (XRC) and atomic force microscopy (AFM) indicate that the m-plane InN (1100) epilayer has anisotropic crystallographic properties. The PL studies of the materials reveal a remarkable energy band gap structure around 0.70 eV at 15 K.
A series of experiments were conducted to systematically study the effects of etching conditions on GaN by a convenient photo-assisted chemical(PAC) etching method. The solution concentration has an evident influence on the surface morphology of GaN and the optimal solution concentrations for GaN hexagonal pyramids have been identified. GaN with hexagonal pyramids have higher crystal quality and tensile strain relaxation compared with as-grown GaN. A detailed analysis about evolution of the size, density and optical property of GaN hexagonal pyramids is described as a function of light intensity. The intensity of photoluminescence spectra of GaN etched with hexagonal pyramids significantly increases compared to that of as-grown GaN due to multiple scattering events, high quality GaN with pyramids and the Bragg effect.
A significant enhancement in solar hydrogen generation efficiency has been achieved by inductive coupled etching(ICP) surface roughening treatment using nano-sized nickel mask. As much as 7 times improvement of photocurrent is demonstrated in comparison with a planar one fabricated from the same parent wafer. Under identical illumination conditions in HBr solution, the incident photon conversion efficiency(IPCE) shows an enhancement with a factor of 3, which even exceed 54% at 400 nm wavelength. We believe the enhancement is attributed to several facts including improvement in absorption, reacting area, carrier localization and carrier lifetime.
Roughened surfaces of light-emitting diodes(LEDs)provide substantial improvement in light extraction efficiency.By preparing the self-assemble nanoporous Ni template through rapid annealing of a thin Ni film,followed by a low damage dry etching process,a p-side-up LED with a roughened surface has been fabricated.Compared to a conventional LED with plane surface,the light output of LEDs with nanoporous p-GaN surface increases up to 71%and 36%at applied currents of 1 mA and 20 mA,respectively.Meanwhile,the electrical characteristics are not degraded obviously after surface roughening.
