The effects of vicinal sapphire substrates on the properties of AlGaN/GaN heterostructures

AlGaN/GaN heterostructures on vicinal sapphire substrates and just-oriented sapphire substrates (0001) are grown by the metalorganic chemical vapor deposition method. Samples are studied by high-resolution x-ray diffraction, atomic force microscopy, capacitance--voltage measurement and the Van der Pauw Hall-effect technique. The investigation reveals that better crystal quality and surface morphology of the sample are obtained on the vicinal substrate. Furthermore, the electrical properties are also improved when the sample is grown on the vicinal substrate. This is due to the fact that the use of vicinal substrate can promote the step-flow mode of crystal growth, so many macro-steps are formed during crystal growth, which causes a reduction of threading dislocations in the crystal and an improvement in the electrical properties of the AlGaN/GaN heterostructure.     

利用选择性外延法生长单芯片双波长白光InGaN/GaN多量子阱结构

为了制备单芯片无荧光粉白光InGaN/GaN多量子阱发光结构,利用选择性外延生长法在SiO2条纹掩膜板上生长出具有梯形形貌的GaN微面结构,并在该GaN微面结构上生长InGaN/GaN多量子阱结构,最终在单芯片上获得了双波长发光.结果表明:梯形GaN微面由(0001)和(11-22)面组成,两者的表面能和极性不同,并且在InGaN/GaN多量子阱生长过程中,In原子和Ga原子迁移速率不同,从而使得(0001)和(11-22)面上的多量子阱具有不同的发光波长;该性质可以使(11-22)面的微面量子阱发出蓝光(峰值波长为420nm),而(0001)面的量子阱发出黄光(峰值波长为525nm),最终形成双波长的复合白光外延结构.     

镓源对氮化镓纳米线形貌的影响

用催化剂辅助的化学气相沉积法合成了单晶氮化镓纳米线。通过调节镓原子的供应量从而影响过饱和度,研究不同过饱和度对氮化镓纳米线生长形貌的影响。用金作为催化剂辅助纳米线的生长,纳米线是按照气-液-固生长机制形成的。利用扫描电镜（SEM）、透射电镜（TEM）、X射线衍射（XRD）、能量色散X射线谱（EDS）和光致发光（PL）对产物的形貌、结构和发光性质进行了表征和分析。发现维持氨气流量不变,随着生长的进行,镓原子浓度降低,从而导致体系的过饱和度下降,由其驱动的纳米线的形貌发生变化,出现直径变细的现象。     

Effects of GaN cap layer thickness on an AlN/GaN heterostructure

In this study, we investigate the effects of GaN cap layer thickness on the two-dimensional electron gas (2DEG) electron density and 2DEG electron mobility of AlN/GaN heterostructures by using the temperature-dependent Hall measurement and theoretical fitting method. The results of our analysis clearly indicate that the GaN cap layer thickness of an AlN/GaN heterostructure has influences on the 2DEG electron density and the electron mobility. For the AlN/GaN heterostructures with a 3-nm AlN barrier layer, the optimized thickness of the GaN cap layer is around 4 nm and the strained a-axis lattice constant of the AlN barrier layer is less than that of GaN.     

Effects of V/Ⅲ ratio on a-plane GaN epilayers with an InGaN interlayer

The effects of V/Ⅲgrowth flux ratio on a-plane GaN films grown on r-plane sapphire substrates with an InGaN interlayer are investigated.The surface morphology,crystalline quality,strain states,and density of basal stacking faults were found to depend heavily upon the V/Ⅲratio.With decreasing V/Ⅲratio,the surface morphology and crystal quality first improved and then deteriorated,and the density of the basal-plane stacking faults also first decreased and then increased.The optimal V/Ⅲratio growth condition for the best surface morphology and crystalline quality and the smallest basalplane stacking fault density of a-GaN films are found.We also found that the formation of basal-plane stacking faults is an effective way to release strain.     

Selective Growth of GaN on Slope Cone-shaped Patterned Sapphire Substrate

Cone-shaped patterned sapphire substrate was prepared by inductively coupled plasma etching and GaN nucleation layer was grown on it by metal-organic chemical vapor deposition.A selective growth of GaN nucleation layer was found on the slope of the cone-shaped patterned sapphire substrat,and the distribution morphology of GaN had significantly changed after it was recrystallized.GaN selective growth and redistribution were analyzed by investigating the distribution of crystallographic planes on the cone surface and the atom array of specific planes at atom level.     

表面势垒对梯度掺杂GaN光电阴极电子逸出几率的影响

研究表面势垒对梯度掺杂GaN光电阴极电子逸出几率的影响.计算梯度掺杂透射式GaN光电阴极的电子能量分布及逸出几率,结果显示梯度掺杂与均匀掺杂相比,可以获得更大的电子逸出几率;I势垒对电子逸出几率的影响显著,而Ⅱ势垒影响较小.利用GaN光电阴极多信息量测试系统,测试两种GaN阴极样品的光电流.实验结果表明,梯度掺杂GaN样品比均匀掺杂电子逸出几率更大;单独进行Cs激活形成的I势垒对电子逸出几率有显著影响,而Cs/O共同激活形成的Ⅱ势垒对其影响较小.     

透射式变掺杂GaAs光电阴极研究

为了将变掺杂GaAs材料应用于微光像增强器,开展了透射式变掺杂GaAs光电阴极实验研究,制备了2种反转结构透射式变掺杂GaAs光电阴极。测试了玻璃粘接前后GaAs光电阴极载流子浓度变化,发现高温粘接后载流子浓度增加现象。通过测试高温激活的透射式变掺杂GaAs光电阴极发现,在450 nm～550 nm波段内,变掺杂GaAs光电阴极仍然具有较高的光谱响应。     

Optically pumped GaN/AlGaN quantum well intersubband terahertz laser

We propose an optically pumped nonpolar GaN/AlGaN quantum well (QW) active region design for terahertz (THz) lasing in the wavelength range of 30 μm～ 40 μm and operating at room temperature. The fast longitudinal optical (LO) phonon scattering in GaN/AlGaN QWs is used to depopulate the lower laser state, and more importantly, the large LO phonon energy is utilized to reduce the thermal population of the lasing states at high temperatures. The influences of temperature and pump intensity on gain and electron densities are investigated. Based on our simulations, we predict that with a sufficiently high pump intensity, a room temperature operated THz laser using a nonpolar GaN/AlGaN structure is realizable.     

Dual-blue light-emitting diode based on strain-compensated InGaN-AlGaN/GaN quantum wells

Strain-compensated InGaN quantum well (QW) active region employing tensile AlGaN barrier is analyzed. Its spectral stability and efficiency droop for dual-blue light-emitting diode (LED) are improved compared with those of the conventional InGaN/GaN QW dual-blue LED based on stacking structure of two In_0.18Ga_0.82N/GaN QWs and two In_0.12Ga_0.88N/GaN QWs on the same sapphire substrate. It is found that the optimal performance is achieved when the Al composition of strain-compensated AlGaN layer is 0.12 in blue QW and 0.21 in blue-violet QW. The improvement performance can be attributed to the strain-compensated InGaN-AlGaN/GaN QW that can provide a better carrier confinement and effectively reduce leakage current.