AlN插入层对AlGaN/GaN外延材料电学性能影响

采用MOCVD制备了带有MN插入层AlGaN/GaN异质结构外延材料,对外延材料分别进行了原子力显微镜AFM、双晶XRD以及变温HALL测试.测试结果表明:具有AlN插入层的外延材料表面非常平整,10 μm × 10 μm范围样品的表面均方根粗糙度RMS仅为0.302 nm,AlGaN势垒层衍射峰更尖锐,材料结构特性良好,大大提高了AlGaN/GaN异质结的2DEG面密度和迁移率,280 K和300 K时沟道电子迁移率分别为4736 cm2/V·s和1785 cm2/V·s,比无MN插入层的传统结构得到的结果分别提高了45.7;和23.4;.     

生长条件对AlGaN/GaN HEMT势垒层表面形貌及二维电子气迁移率的影响

使用金属有机化学气相沉积法生长了AlGN/GaN高电子迁移率晶体管.研究了生长压力和载气组分对AlGaN势垒层及AlGaN高电子迁移率晶体管二维电子气迁移率的影响.原子力显微镜(AFM)光谱和SEM-EDS用于表征外延层的质量.结果表明随着反应室压力从50torr提高到200torr,AlGaN势垒层的表面形貌先变好,随后开始变差.在反应室压力100torr情况下得到最好的表面形貌.同时发现在生长AlGaN势垒层时,同时适当的氢气会提高AlGaN层的质量.在反应室压力100torr、氢气组分59;时得到AlGaNHEMT的最大二维电子气迁移率为1545 cm2/V·s.     

低温AlN插入层的生长温度对AlGaN/GaN量子阱应力弛豫作用的影响

利用高分辨X射线衍射仪(HRXRD)及原子力显微镜(AFM)研究了低温AlN插入层的生长温度对AlGaN/GaN量子阱应力弛豫作用的影响.结果表明,低温AlN插入层不同的生长温度会导致AlGaN/GaN量子阱不同的表面粗糙度及穿透位错密度,并且当生长温度达到640℃时样品中表面粗糙度及穿透位错密度达到最低,同时具有最高的载流子迁移率及带边发光峰强度.在不同的生长温度,低温AlN表面具有不同的表面形貌.不同的表面形貌将直接影响界面处位错主滑移系的开动及位错阻挡机制.通过分析可以得知,低温AlN不同的表面形貌是由于Al原子在不同温度下的不同的迁移机制造成.     

过渡层结构和生长工艺条件对Si基GaN的影响

为了提高MOCVD外延硅基GaN材料的质量，在硅（111）衬底上以HT-AlN为缓冲层，在缓冲层上再生长变组份过渡层后外延生长GaN。过渡层为多层复合结构，分为高温变组分AlGaN、GaN、低温AlN、高温变组分AlGaN。在高温生长AlGaN和GaN层中插入一层低温生长AlN以缓解降温过程中应力对厚GaN层的影响，为了缓慢释放热应力、采用合适的慢降温工艺。当外延层的厚度小于1．7微米时GaN外延层无龟裂，而厚度不断增加时，GaN外延层产生龟裂。本文研究了AlN缓冲层生长温度、高温变组分AlGaN生长过程中生长时间的变化对所生长GaN材料的影响。采用三维视频显微镜、高分辨率双晶X射线衍射（DCXRD）、扫描电子显微镜（SEM）、原子力显微镜（AFM）和室温光致荧光光谱（RT-PL）对样品进行了测试分析。测试结果表明所研制的硅基GaN表面光亮、平整，过渡层的引入有利于降低外延层中应力，提高GaN结晶质量。     

第一性原理研究二维GaN的电子结构和光学性能

二维GaN是一种性能优良的半导体光电材料,用途广泛.因此,基于密度泛函理论采用广义梯度近似方法系统研究单层、双层和三层二维GaN的电子结构和光学性质,并与三维GaN体材料进行比较.结果表明:随着维数的降低,二维GaN的能带变宽,各能级的能量值起伏变大;不同于三维GaN,二维GaN量子尺寸效应明显,N的2s态和2p态相互作用增强,出现能带重叠,呈现较好的导电特性;分析费米能级发现导带底附近存在明显表面态,这是因为Ga的4s电子态的贡献;随着二维GaN层数的增加,对紫外光的反射特性越来越好,在特定的能量范围内,二维GaN的能量损失为零.由此可知,研究二维GaN的电子结构和光学性质有助于二维GaN在纳米光电器件中的应用.     

低温AlN插入层降低硅基GaN膜微裂

为了降低MOCVD外延硅基GaN膜层中的应力、减少硅基厚GaN层的微裂;在高温GaN层中插入低温AlN.低温AlN插入层可平衡HT-GaN生长和降温过程引起的张应力,降低厚膜外延层的微裂,已研制出厚度超过1.8微米无微裂GaN外延层.本文重点研究了低温AlN生长温度对HT-GaN材料的影响,给出了较佳的LT-AlN生长温度.采用扫描电子显微镜(SEM),原子力显微镜(AFM)和高分辨率双晶X射线衍射(DCXRD),对样品进行了测试分析.试验和测试结果表明低温AlN的生长温度至关重要,生长温度过低影响GaN晶体质量,甚至不能形成晶体;生长温度过高同样会影响GaN结晶质量,同时降低插入层的应力平衡作用;实验结果表明最佳的LT-AlN插入层的生长温度为680℃左右.     

苏州纳米所在增强型GaN HEMT器件研究方面取得进展

<正>由于GaN具有大禁带宽度、高电子迁移率、高击穿场强等优点,GaN HEMT成为新一代功率器件研究的热点。由于极化作用,AlGaN/GaN异质结界面会形成高浓度的二维电子气,浓度可达到1013/cm~2量级,因此一般的GaN HEMT都是耗尽型器件。如何实现增强型GaN HEMT一直是该领域研究的难点,目前几种主要用来制备增强型器件的方案包括:p型栅、凹槽栅、F处理和Cascode结构。其中,p型栅的方案已经被很     

高Al组份AlGaN/GaN异质外延材料生长及肖特基势垒二极管制备

采用MOCVD制备了带有AlN插入层的高Al组份AlGaN/GaN异质结构外延材料,在此外延材料的基础上利用磁控溅射Ti/Al/Ti/Au欧姆接触电极,利用EB蒸镀Ni/Au肖特基接触电极制备了AlGaN/GaN SBD,对外延材料和器件的性能进行了相关测试,测试结果表明:器件开启电压约为1.1V,-10 V时反向漏电流小于0.5 μA,反向击穿电压68.3 V,器件具有非常明显的整流特性,同时有AlN插入层的器件的正向、反向特性均优于不带AlN的器件,AlN插入层可以有效地提高器件的性能.     

缓冲层厚度对MOCVD法生长GaN外延薄膜性能的影响

本文研究了低温GaN(LT-GaN)缓冲层表面形貌,其随厚度的变化规律及对随后生长GaN外延膜各项性能的影响.用场发射扫描电镜(SEM)和原子力显微镜(AFM)研究LT-GaN缓冲层表面形貌,发现随着厚度的增加,其表面由疏松、粗糙变得致密、平整,六角GaN小晶粒的数量减少,且取向较为一致.用X光双晶衍射(XRD)、AFM和Hall测量研究1μm厚本征GaN外延薄膜的结晶质量、表面粗糙度、背底载流子浓度和迁移率等性能,发现随着LT-GaN缓冲层厚度的增加:XRD的半高宽FWHMs增大,表面粗糙度先减小后又略有增大,背底载流子浓度则随之减少,而迁移率的变化则不明显.通过分析进一步确认LT-GaN缓冲层的最优生长时间.     

低温GaN形核层的形核速率对GaN外延薄膜晶体质量的影响

采用金属有机化学气相沉积方法系统研究了在蓝宝石衬底上低温GaN形核层的形核速率对GaN外延薄膜晶体质量的影响机理.利用高分辨X射线衍射仪、原子力显微镜、光致发光光谱和Hall测试仪表征材料的位错密度、表面形貌以及光、电学性能.研究结果表明随着形核速率的增加,GaN形核层更倾向于三维生长模式;当形核速率达到1.92(A)/s时退火后生成尺寸为100 nm宽、32 nm高的均匀形核岛,随后生长的未掺杂GaN外延薄膜层的螺型和刃型位错密度以及黄带峰强度达到最小值,并且其具有最高的载流子迁移率和最低的载流子浓度.     

Low pressure MOVPE of GaN and GaInN/GaN heterostructures

GaN single layers and GaInN/GaN heterostructures have been grown by low pressure metalorganic vapor phase epitaxy on sapphire substrates. We found best growth conditions and the highest growth rate for GaN to be at about 1000°C, whereas the growth rate decreased for both, higher and lower temperatures. In contrast, GaInN with a significantly high In content could only be grown at lower temperatures around 700°C. Besides growth temperature and reactor pressure, the composition of the carrier gas was found to play an important role: the In incorporation rate is about doubled when reducing the hydrogen/nitrogen ratio. GaInN/GaN quantum wells show even higher In contents compared to bulk layers.     

Overgrowth of GaN on GaN nanowires produced by mask-less etching

We report on the generation of GaN nanowires (NWs) using mask-less reactive ion etching (RIE). The NWs are believed to be the result of a high etching rate in regions where a high dislocation density is present in the GaN films grown on sapphire substrates. We have studied the effect of defect densities in the original GaN films and its relation to the generation of these NWs. We show that defect reduction in the overgrown GaN is related to the presence of a network of embedded voids generated between these nanowires during the regrowth on the etched nanowires. We show that further reduction in dislocation density can be achieved by repeating the process of nanowire generation and overgrowth. Also we report on the residual strain and curvature in GaN after the first and second embedded voids approach (EVA).     

Growth of freestanding GaN using pillar-epitaxial lateral overgrowth from GaN nanocolumns

Dislocation-free and strain-free GaN nanopillars, grown on Si by molecular beam epitaxy, were used as nanoseeds for a new form of epitaxial lateral overgrowth (ELO) by metalorganic vapour phase epitaxy (MOVPE) until full coalescence. Such overgrown GaN films are almost relaxed and were used as templates for producing thick GaN layers by halide vapour phase epitaxy (HVPE). The final GaN film is easily separated from the starting Si substrate. This is henceforth a new technology to produce freestanding GaN. The GaN crystal quality was assessed by transmission electron microscopy (TEM), photo- and cathodoluminescence (PL, CL). It was seen that the pillar-ELO is produced from a limited number of nanopillars. Some dislocations and basal stacking faults are formed during the coalescence. However, those that propagate parallel to the substrate do not replicate in the top layer and it is expected that the thickened material present a reduced defect density.     

Overgrowth of GaN on Be-doped coalesced GaN nanocolumn layer by rf-plasma-assisted molecular-beam epitaxy—Formation of high-quality GaN microcolumns

GaN crystals were overgrown on GaN nanocolumn platforms with a Be-doped coalesced layer by rf-plasma-assisted molecular-beam epitaxy (rf-MBE). The overgrown GaN included large micrometer-scale hexagonal columnar crystals. These microcrystals were named ‘microcolumns’ and showed high optical properties comparable to those of GaN bulk crystals grown by hydride vapor phase epitaxy (HVPE).     

Bulk ammonothermal GaN

In this work, results of structural characterization of high-quality ammonothermal GaN are presented. Besides expected low dislocation density (being of the order of 10³ cm⁻²) the most interesting feature seems perfect flatness of the crystal lattice of studied crystals. Regardless the size of crystals, lattice curvature radius exceeds 100 m, whereas better crystals reveal radius of several hundred meters and the best above 1000 m. Excellent crystallinity manifests in very narrow X-ray diffraction peaks of full-width at half-maximum (FWHM) values about 16 arcsec.     

Defect reduction of laterally regrown GaN on GaN/patterned sapphire substrates

Structural properties of GaN epilayers on wet-etched protruding and recess-patterned sapphire substrates (PSSs) have been investigated in detail using high-resolution double-crystal X-ray diffraction (DCXRD) and etch-pit density methods. The DCXRD results reveal various dislocation configurations on both types of PSSs. The etch pits of GaN on the recess PSS exhibit a regular distribution, i.e. less etch pits or threading dislocation density (TDD) onto the recess area than those onto the sapphire mesas. On the contrary, an irregular distribution is observed for the etch pits of GaN on the protruding PSS. A higher crystal quality of the GaN epilayer grown onto the recess PSS can be achieved as compared with that onto the protruding PSS. These data reflect that the GaN epilayer on the recess PSS could be a better template for the second epitaxial lateral overgrowth (ELOG) of GaN. As a result, the GaN epilayers after the ELOG process display the TDDs of around ∼10⁶ cm⁻².     

Influence of Predeposition on the Properties of GaN

Growth experiments with GaN in the system Ga/HCl/NH₃/He or H₂ were carried out in a reactor with two deposition zones. The extent of the reaction in the first zone, called the predeposition zone, is controlled by the NH₃ flow rate. In the second zone GaN is deposited epitaxially onto sapphire substrates. The variation of the carrier concentration of these epilayers indicates the dominating effect of impurities especially oxygen in opposition to the widely accepted vacancy model. Due to the high incorporation ratio of donor impurities into the solid a predeposition reduces the impurity content in the vapour phase. A reduction of the free carrier concentration in the epitaxial layers could be achieved. Different behaviour in the two carrier gas systems could be established.     

氮化镓晶体的氨热法生长进展

GaN作为性能最为优异的第三代半导体材料,其高质量的衬底材料的研发是目前乃至近5年的研究热点,而最好的衬底材料即为GaN体单晶.在为数不多的GaN体单晶的几种生长方法中,氨热法被普遍认为是生长GaN体单晶的一种很有前途的方法.本文主要论述了在不同矿化剂生长条件下GaN晶体的氨热法生长进展,指出了存在的问题,并给出了一些解决办法.     

Vapor phase growth of GaN using GaN powder sources and thermogravimetric investigations of the evaporating behaviour of the source material

The growth of GaN from the vapor phase is a promising technique for producing both bulk GaN crystals and GaN layers. For establishing a growth method from the vapor phase the source material and reactor setup are of great importance. Highly pure and self synthesized GaN powder was chosen as source material. The evaporation behaviour of the GaN powder was studied by means of thermogravimetry (TG). A vertical growth reactor was set up according to the results of numerical simulations of the temperature distributions and flow patterns. Freely nucleated GaN platelets of some millimetres in length were grown. Furthermore, thin GaN layers were deposited directly on a sapphire substrate. This nucleation layer was successfully overgrown by low pressure solution growth. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

采用溶胶凝胶前驱物制备氮化镓纳米晶体

采用溶胶凝胶法制备的前驱物进一步在900℃,氨气气氛中氮化得到粒径相对均匀、平均粒径为11.2nm的六方相氮化镓纳米晶体.XRD, HRTEM, SAED, EDS, FTIR被用于表征产物的微结构及组成.室温光致发光光谱显示产物位于3.46eV的带边发光峰和从 2.6～3.2eV 的宽的发光带.产物可直接用于制备氮化镓量子点复合材料和制备高质量的一维氮化镓晶体.