The quest for higher modulation speed and lower energy consumption has inevitably promoted the rapid development of semiconductor-based solid lighting devices in recent years. GaN-based light-emitting diodes (LEDs) have emerged as promising candidates for achieving high efficiency and high intensity, and have received increasing attention among many researchers in this field. In this paper, we use a self-assembled array-patterned mask to fabricate InGaN/GaN multi-quantum well (MQW) LEDs with the intention of enhancing the light-emitting efficiency. By utilizing inductively coupled plasma etching with a self-assembled Ni cluster as the mask, nanopillar arrays are formed on the surface of the InGaN/GaN MQWs. We then observe the structure of the nanopillars and find that the V-defects on the surface of the conventional structure and the negative effects of threading dislocation are effectively reduced. Simultaneously, we make a comparison of the photoluminescence (PL) spectrum between the conventional structure and the nanopillar arrays, achieved under an experimental set-up with an excitation wavelength of 325 mm. The analysis demonstrates that MQW-LEDs with nanopillar arrays achieve a PL intensity 2.7 times that of conventional LEDs. In response to the PL spectrum, some reasons are proposed for the enhancement in the light-emitting efficiency as follows: 1) the improvement in crystal quality, namely the reduction in V-defects; 2) the roughened surface effect on the expansion of the critical angle and the attenuated total reflection; and 3) the enhancement of the light-extraction efficiency due to forward scattering by surface plasmon polariton modes in Ni particles deposited above the p-type GaN layer at the top of the nanopillars.
Effect of the V/III ratio during buffer layer growth on the yellow and blue luminescence in undoped GaN epilayer has been studied by means of photoluminescence spectroscopy and high resolution X-ray diffraction.It is found that the densities of screw and edge threading dislocations increase with the V/III ratio of the buffer layer,and the intensities of the yellow luminescence(YL) and blue luminescence(BL) emissions also increase dramatically.However,the density ratio of the edge threading dislocation to the screw threading dislocation remains invariant,as well as the intensity ratio of YL emission to BL emission.It can be concluded from these phenomena that the edge threading dislocation and screw threading dislocation can enhance the YL and BL emissions,respectively.
After immersion in hydrofluoric acid,the sheet resistance of a 220-nm-thick silicon nanomembrane,measured in dry air by van der Pauw method,drops around two orders of magnitude initially,then increases and reaches the level of a sample with a native oxide surface in about one month.The surface component and oxidation rate are also characterized by x-ray photo electronic spectroscopy measurement.Fluorine is found to play a significant role in improving conductivity and has no apparent influence on the oxidation rate after hydrofluoric acid treatment.
In this paper,we simulate a new style vertical HVPE reactor by using computational fluid dynamics program FLUENT.In order to find the best parameter on the growth rate of Gallium nitride(GaN),we change the distance between the inlet and the substrate,GaCl and NH3 inlets,and also we add substrate rotation separately.With the increase of the distance between the substrate and the gas inlet,GaN deposition rate decreases and the uniformity becomes better.The results show that the optimal distance in this new-style vertical hydride vapour phase epitaxy(HVPE) system is 4 cm.Besides,as the distance between the GaCl inlet and the NH3 inlet changes,the uniformity of GaN deposition varies.Our findings indicate that the optimal distance is 3 cm.Furthermore,it is found that substrate rotation also affects the growth rate of GaN.
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.
High-resolution X-ray diffraction has been used to analyze GaN(0001) epitaxial layers on sapphire substrates. Several structural properties of GaN, including the lattice constants, strains, and dislocation densities are revealed by the technique of X-ray dffraction (XRD). Lattice constants calculated from the omega/2theta scan are c=0.5185 nm and a=0.3157 nm. Also, the in-plane strain is -1.003%, while out of the plane, the epitaxial film is almost relaxed. Several methods are used to deduce the mosaicity and dislocation density of GaN, showing that the edge type dislocations are the overwhelming majority.
XIE ZiLi 1,2,3, ZHOU YuanJun 1 , SONG LiHong 1,2 , LIU Bin 1 , HUA XueMei 1,2 , Xiu XiangQian 1 , ZHANG Rong 1,2,3 & ZHENG YouDou 1,2,3 1 JiangSu Provincial Key Laboratory of Advanced Photonic and Electronic Materials and Department of Physics, Nanjing University, Nanjing 210093, China
The paper reports the setting up of a model of fluid dynamic for GaN HVPE system and the simulation. It is found that when the direction of gravity is opposite to the direction of GaCl flow inlet,there exits a distance at which the uniformity of the deposition is optimal. Here the good uniformity of the deposition is obtained when the distance between the substrate and GaCl inlet is 5 cm. The parameters of gas flow used in growing GaN are also optimized. In addition, the influence of gravity and buoyancy on the deposition of GaN is discussed, too. It is found that the angle between the direction of gravity and the direction of GaCl flow inlet has a major effect on the deposition rate and the uniformity of the growth. Compared with the situation when the direction of gravity is the same with the direction of GaCl flow inlet, although the deposition rate of GaN has decreased obviously, the uniformity of the deposition has improved largely when the direction of gravity is opposite to the direction of GaCl flow inlet.
ZHAO ChuanZhen, XIU XiangQian , ZHANG Rong , XIE ZiLi, LIU Bin, LIU ZhanHui, YAN HuaiYue & ZHENG YouDou Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, Department of Physics, Nanjing University, Nanjing 210093, China
We develop a model for the effect of thermal annealing on forming In-N clusters in GaInNP according to thermodynamics.The average energy variation for forming an In-N bond in the model is estimated according to the theoretical calculation.Using the model,the added number of In-N bonds per mol of InGaNP,the added number of nearest-neighbor In atoms per N atom and the average number of nearest-neighbor In atoms per N atom after annealing are calculated.The different function of In-N clusters in InGaNP and InGaN is also discussed,which is due to the different environments around the In-N clusters.
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 non-polar a-plane GaN is grown on an r-plane sapphire substrate directly without a buffer layer by metalorganic chemical vapour deposition and the effects of V/III ratio growth conditions are investigated. Atomic force microscopy results show that triangular pits are formed at a relatively high V/III ratio, while a relatively low V/III ratio can enhance the lateral growth rate along the c-axis direction. The higher V/III ratio leads to a high density of pits in comparison with the lower V/III ratio. The surface morphology is improved greatly by using a low V/III ratio of 500 and the roughness mean square of the surface is only 3.9 nm. The high resolution X-ray diffraction characterized crystal structural results show that the rocking curve full width at half maximum along the m axis decreases from 0.757 to 0.720 , while along the c axis increases from 0.220 to 0.251 with the V/III increasing from 500 μmol/min to 2000 μmol/min, which indicates that a relatively low V/III ratio is conducible to the c-axis growth of a-plane GaN.
