Comparison between blue lasers and light‐emitting diodes for future solid‐state lighting

Solid‐state lighting (SSL) is now the most efficient source of high color quality white light ever created. Nevertheless, the blue InGaN light‐emitting diodes (LEDs) that are the light engine of SSL still have significant performance limitations. Foremost among these is the decrease in efficiency at high input current densities widely known as “efficiency droop.” Efficiency droop limits input power densities, contrary to the desire to produce more photons per unit LED chip area and to make SSL more affordable. Pending a solution to efficiency droop, an alternative device could be a blue laser diode (LD). LDs, operated in stimulated emission, can have high efficiencies at much higher input power densities than LEDs can. In this article, LEDs and LDs for future SSL are explored by comparing: their current state‐of‐the‐art input‐power‐density‐dependent power‐conversion efficiencies; potential improvements both in their peak power‐conversion efficiencies and in the input power densities at which those efficiencies peak; and their economics for practical SSL.     

Ir-based diffusion barriers for Ohmic contacts to p-GaN

Ir-based electrical contacts to p-type GaN have been fabricated and characterized. Both GaN//Ni/Au/Ir/Au and GaN//Ni/Ir/Au contact structures were deposited, however, only the former produced Ohmic current-voltage characteristics. At an anneal temperature of 500 °C, the Ni/Au/Ir/Au contact had a specific contact resistance of ∼2 × 10⁻⁴ Ω cm², comparable or superior to conventional Ni/Au contacts that are less thermally stable. Anneal temperatures above 500 °C caused the Ir-based contact to fail. Auger electron spectroscopy was used to obtain depth profiles of both types of contacts at a variety of temperatures in order to provide insight into the mechanism of Ohmic formation as well as potential reasons for failure. A comparison to other metallization schemes on p-GaN is also given.     

GaN-based light-emitting diode with ZnO nanotexture layer prepared using hydrogen gas

ZnO films were deposited on indium tin oxide (ITO), which formed the transparent conductive layer (TCL) of a GaN-based light-emitting diode (LED), by ultrasonic spraying pyrolysis to increase the light output power. The ZnO nanotexture was formed by treating the as-deposited ZnO films with hydrogen. The root mean square (RMS) roughness increased from 4.47 to 7.89 nm before hydrogen treatment to 10.82-15.81 nm after hydrogen treatment for 20 min. Typical current-voltage (I-V) characteristics of the GaN-based LEDs with a ZnO nanotexture layer have a forward-bias voltage of 3.25 V at an injection current of 20 mA. The light output power of a GaN-based LED with a ZnO nanotexture layer improved to as much as about 27.5% at a forward current of 20 mA.     

Time-resolved spectroscopy of freestanding GaN layers grown by halide vapour phase epitaxy

Freestanding GaN layers of various thicknesses grown by HVPE have been studied by time-resolved spectroscopy combined with structural and electrical measurements. We have observed an increase of the PL lifetime with increasing layer thickness; however, a saturation of the recombination times has been detected for the GaN layers thicker than 400 μm. We explain the observed thickness-dependent behavior of the decay times by competition of two nonradiative mechanisms; namely, for layers with thickness less than 400 μm the main nonradiative channel is related to the structural defects, while in thicker layers the recombination decay time is limited by impurities and/or vacancies.     

Growth and characterization of pine-needle-shaped GaN nanorods by sputtering and ammoniating process

Pine-needle-shaped GaN nanorods have been successfully synthesized on Si(111) substrates by ammoniating Ga₂O₃/Nb films at 950 ^°C in a quartz tube. The products are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and field-emission transmission electron microscope (FETEM). The results show that the pine-needle-shaped nanorods have a pure hexagonal GaN wurtzite with a diameter ranging from 100 to 200 nm and a length up to several microns. The photoluminescence spectra (PL) measured at room temperature only exhibit a strong emission peak at 368 nm. Finally, the growth mechanism of GaN nanorods is also briefly explored.     

氧气氛中p-GaN/Ni/Au电极在相同温度不同合金时间下的欧姆接触形成机制和扩散行为

用卢瑟福背散射/沟道技术研究了p-GaN上的Ni/Au电极在氧气氛下相同合金温度(500℃)不同合金时间后的微结构演化,以揭示欧姆接触的形成机制.利用背散射随机谱和RUMP模拟程序研究了电极金属之间的互扩散,用沟道谱探测了电极金属中的氧分布.结合不同合金时间下比接触电阻ρ_c的变化,发现随着合金时间的延长比接触电阻持续降低,在合金时间60 s后降低的速度减慢, Au扩散到GaN的表面,在p-GaN上形成外延结构,O向电极内部扩散反应生成NiO对降低ρ_{关键词： GaN 卢瑟福背散射/沟道 欧姆接触}     

用逆压电极化模型对AlGaN/GaN 高电子迁移率晶体管电流崩塌现象的研究

通过自洽求解一维Poisson-Schrdinger方程,模拟了AlGaN/GaN高电子迁移率晶体管在工作时等效外电场对AlGaN/GaN异质结沟道处二维电子气(2DEG)浓度的影响.分析了逆压电极化效应的作用,从正-逆压电极化现象出发,提出了逆压电极化模型.计算结果显示：逆压电极化明显影响2DEG性质,当Al组分x=0.3,AlGaN层厚度为20 nm时,不考虑逆压电极化,2DEG浓度为1.53×10¹³cm^-2;当等效外电压分别为10和15V 关键词： AlGaN/GaN高电子迁移率晶体管 Poisson-Schrdinger方程 逆压电极化模型 电流崩塌     

Al，Mg掺杂GaN电子结构及光学性质的第一性原理研究

基于密度泛函理论,采用广义梯度近似方法,计算了Al,Mg掺杂的闪锌矿型GaN的电子结构和光学性质,分析了其电子态分布与结构的关系,给出了掺杂前后GaN体系的介电函数和复折射率函数.计算结果表明掺有Mg的GaN晶体空穴浓度增大,会明显提高材料的电导率,而Al掺杂GaN晶体的载流子浓度不变,只是光学带隙变宽;通过分析掺杂前后GaN晶体的介电函数和复折射率函数,解释了体系的发光机理,为GaN材料光电性能的进一步开发与应用提供了理论依据.通过比较可知,所得出的计算结果与现有文献符合得很好. 关键词： GaN晶体 电子结构 光学性质 掺杂     

p-GaN层厚度对GaN基p-i-n结构紫外探测器性能的影响

研究了p-GaN层厚度对GaN基pin结构紫外探测器性能的影响.模拟计算表明：较厚的p-GaN层会减小器件的量子效率,然而同时也会减小器件的暗电流,较薄的p-GaN层会增加器件的量子效率,但是同时也增加了器件的暗电流.进一步的分析表明,金属和p-GaN之间的结电场是出现这种现象的根本原因.在实际的器件设计中,应该根据实际需要选择p型层的厚度. 关键词： GaN 紫外探测器 量子效率 暗电流     

m面GaN平面内结构和光学各向异性研究

采用金属有机物化学气相淀积方法在铝酸锂LiAlO₂衬底上外延生长m面GaN薄膜.X射线衍射测量的结果表明所得薄膜具有较理想的m面晶体取向,并对其各向异性的应变进行了计算,摇摆曲线的测量发现样品存在明显的面内结构各向异性.采用偏振光致发光研究材料的面内光学各向异性,发现随着偏振角度的改变,发光峰的峰位和强度均有明显变化,并用对称性破缺导致价带子带劈裂的理论对结果进行了解释. 关键词： m面GaN 结构各向异性 偏振光致发光