GaN薄膜表面形貌与AlN成核层生长参数的关系

采用金属有机物化学气相淀积技术生长了以AlN为成核层的GaN薄膜,研究了成核层生长时三甲基铝(TMAl)流量对最终GaN薄膜表面形貌的影响.研究结果表明,高质量GaN薄膜只能在高TMAl流量下获得,采用充足的TMAl源才能形成满足需要的AlN成核点,这是生长高质量GaN薄膜的一个先决条件.     

AlGaN缓冲层结构对Si基GaN材料性能的影响

Si衬底与GaN之间较大的晶格失配和热失配引起的张应力使GaN外延层极易产生裂纹,如何补偿GaN所受到的张应力是进行Si基GaN外延生长面临的首要问题.采用金属有机化合物化学气相沉积(MOCVD)技术在4英寸(1英寸=2.54 cm)Si (111)衬底上制备了GaN外延材料并研究了不同AlGaN缓冲层结构对Si基GaN外延材料性能的影响,并采用高分辨X射线衍射仪(HRXRD)、原子力显微镜(AFM)、喇曼光谱以及光学显微镜对制备的GaN材料的性能进行了表征.采用3层A1GaN缓冲层结构制备了表面光亮、无裂纹的GaN外延材料,其(002)晶面半高宽为428 arcsec,表面粗糙度为0.194 nm.结果表明,采用3层A1GaN缓冲层结构可以有效地降低GaN材料的张应力和位错密度,进而遏制表面裂纹的出现,提高晶体质量.     

GaN、AlN的形变势和应变层GaN/AlN异质结带阶的计算

本文采用混合基矢从头赝势能带计算方法研究了（００１）界面应变对ＧａＮ、ＡｌＮ应变层的能带、平均键能Ｅｍ和带阶参数Ｅｍｖ的影响．借助于带阶参数形变势的计算,预言了不同生长厚度情况下ＧａＮ／ＡｌＮ应变层异质结价带带阶和导带带阶．     

含有低温AlN插入层的厚膜GaN的氢化物气相外延生长

研究了采用低温氮化铝(LT-AlN)插入层的厚膜GaN的氢化物气相外延生长(HVPE),并比较了7nm厚的LT-AlN插入层在经过不同退火时间后对GaN膜生长的影响.结果表明,退火后的LT-AlN插入层表面形貌发生很大变化.在一定的退火条件下,AlN插入层能有效地改善HVPE生长的GaN外延层的结晶质量.     

含有低温AlN插入层的厚膜GaN的氢化物气相外延生长

研究了采用低温氮化铝(LT-AlN)插入层的厚膜GaN的氰化物气相外延生长(HVPE),并比较了7nm厚的 LT-AIN插入层在经过不同退火时间后对GaN膜生长的影Ⅱm纳果表明,退火后的LT.舢N插入层表面形貌发生 很大变化.在一定的退火条件下,AlN插入层能有效地改善HVPE生长的GaN外延层的结晶质量.     

SiC衬底AlN缓冲层的应变对GaN外延层质量的影响

在3英寸(1英寸=2.54 cm)SiC衬底上采用金属有机物化学气相沉积(MOCVD)法生长GaN外延材料。研究了AlN缓冲层的应变状态对GaN外延层应变状态和质量的影响。使用原子力显微镜和高分辨率X射线双晶衍射仪观察样品表面形貌,表征外延材料质量的变化,使用高分辨喇曼光谱仪观察外延材料应力的变化,提出了基于外延生长的应变变化模型。实验表明,GaN外延层的张应变随着AlN缓冲层应变状态的由压变张逐渐减小,随着GaN张应力的逐渐减小,GaN位错密度也大大减少,表面形貌也逐渐变好。     

6H-SiC衬底上采用不同厚度AlN缓冲层对GaN外延层的影响

采用不同厚度AlN作为缓冲层在6H-SiC衬底上生长了GaN外延层,并利用X射线衍射,拉曼散射和透射电子显微镜等对GaN性质进行了研究。AlN缓冲层的应变状态对GaN的晶体质量和表面形貌有很大影响。较厚的AlN缓冲层会导致GaN表面出现裂纹,而太薄的AlN缓冲层会导致GaN层较高的位错密度,从而恶化器件性能。分析了GaN产生裂纹和高位错密度的机制,并采用较优厚度（100nm）的AlN缓冲层生长出高质量的GaN外延层。     

蓝宝石图形衬底上利用AlN缓冲层生长高质量GaN

在图形化衬底上以AlN作为缓冲层生长高质量的GaN薄膜,国内相关的报道较少。通过引入两步缓冲层生长方法,在蓝宝石图形衬底上生长基于AlN缓冲层的高质量GaN薄膜,利用低温AlN层生长时内部的缺陷,选择性进行腐蚀,形成衬底与外延层界面间的侧向倒斜角,提高光萃取效率;同时在其上继续生长高温AlN,为后续GaN薄膜提供高质量模板。从外延角度出发,以表面形貌及其上生长的GaN薄膜的晶体质量为衡量依据,优化了低温AlN缓冲层以及高温AlN缓冲层的生长参数,优化后LED样品在20 mA测试电流下的光输出功率较参考样品提升了4%。     

蓝宝石上高电子迁移率AlGaN/GaN材料的研究

通过优化蓝宝石衬底上GaN材料缓冲层和Mg掺杂生长工艺,获得了高质量的半绝缘GaN缓冲层,同时通过优化AlGaN/GaN异质结材料中AlN插入层的厚度,获得了性能优良的AlGaN/GaN HEMT材料.室温下2DEG载流子面密度为1.21×10013/cm2,迁移率为1970 cm2/V·s,77 K下2DEG迁移率达到13000 cm2/V·s.     

Enhancement of the Schottky Barrier Height using a Nitrogen-Rich Tungsten Nitride Thin Film for the Schottky Contacts on AlGaN/GaN Heterostructures

Tungsten, stoichiometric W₂N, and nitrogen-rich W₂N films were used as Schottky contacts on AlGaN/GaN heterostructures. The nitrogen content in the film was controlled by varying the nitrogen-to-argon gas flow ratio during the reactive sputter deposition. The diode with the nitrogen-rich film exhibited a higher Schottky barrier height and the leakage current was comparable to that of the Ni/Au Schottky contact. Analysis suggested that this was due to the increase of the tungsten nitride work function as the result of higher nitrogen incorporation. Furthermore, after 600°C thermal annealing, the diode was stable and showed no change in the leakage current.     

The influence of composition and unintentional doping on the two-dimensional electron gas density in AlGaN/GaN heterostructures

We have studied the influence of Al content, AlGaN layer thickness, and unintentional background doping by oxygen on the two-dimensional electron gas (2DEG) density in AlGaN/GaN heterostructures. Hall measurements were made on samples grown with molecular beam epitaxy. The 2DEG densities in the range 2–3×10¹³ cm^?2 were measured. A one-dimensional Schrödinger-Poisson model was used to describe the heterostructure. The calculations gave two-dimensional electron densities in accordance with measured values. The electron density is very sensitive to the Al concentration in the AlGaN layer, whereas the sensitivity to layer thickness is small. Our simulations also showed that the two-dimensional concentration increased 50% when the free-carrier concentration changed from 10¹⁵ cm^?3 to 10¹⁸ cm^?3. The relation between donor concentration and free-carrier concentration was found to agree when using oxygen ionization energy as a parameter.     

Ohmic contact using the Si nano-interlayer for undoped-AlGaN/GaN heterostructures

Excellent ohmic characteristics for undoped-AlGaN/GaN heterostructures have been achieved by using a Si nano-interlayer: a 1-nm Si layer has been evaporated followed by Ti/Al/Mo/Au evaporation. A contact transfer resistance of 0.18 Ω-mm and a specific contact resistivity of 1 × 10⁻⁶ Ω-cm² have been achieved along with excellent surface morphology at an optimized annealing temperature (800°C). Both ohmic contact characteristics and surface morphology are significantly better than those obtained without the Si nano-interlayer. Auger depth profiles and temperature-dependent current-voltage characteristics were investigated to understand ohmic formation. It is suggested that titanium silicide formation at the interface during rapid thermal annealing lowers the barrier height and enhances thermionic emission current.     

Local electronic properties of AlGaN/GaN heterostructures probed by scanning capacitance microscopy

Local electronic properties in Al_xGa_1−xN/GaN heterostructure field-effect transistor epitaxial layer structures are probed using scanning capacitance microscopy. Acquisition of scanning capacitance images over a wide range of bias voltages combined with theoretical analysis and numerical simulation allows the presence, detailed nature, and possible structural origins of nanometer- to micronscale inhomogeneities in electronic structure to be elucidated. Substantial lateral variations in local threshold voltages for transistor channel formation are observed, at length scales ranging from submicron to >2 μm, and found to arise primarily from local variations in Al_xGa_1−xN layer thickness. Features in electronic structure are also observed that are consistent with the existence of networks of negatively charged threading edge dislocations, as might be formed at island coalescence boundaries during epitaxial growth. The negative charge associated with these structures appears to lead to local depletion of carriers from the channel in the Al_xGa_1−xN/GaN transistor epitaxial layer structure.     

Role of barrier and buffer layer defect states in AlGaN/GaN HEMT structures

We have used low energy electron-excited nanoscale luminescence spectroscopy (LEEN) to detect the defects in each layer of AlGaN/GaN HEMT device structures to correlate their effect on two-dimensional electron gas (2DEG) confinement. Also, we have used Auger electron spectroscopy (AES) to detect the chemical composition as a function of lateral position on a growth wafer and to correlate chemical effects with electronic properties. We investigated several high-quality AlGaN/GaN heterostructures of varying electrical properties using incident electron beam energies of 0.5–15 keV to probe electronic state transitions within each of the heterostructure layers. The LEEN depth profiles reveal differences between sucessful and failed structures and highlight the importance of acceptor deep defect levels in the near 2DEG region. Variations in the GaN and AlGaN band edge emissions, as well as the yellow defect emission across an AlGaN/GaN heterostructure growth wafer have been observed. AES and LEEN spectroscopy of the growth wafer suggest that variation in the cation concentration may play a role in the mechanism responsible for the deep aceceptor level emission in the AlGaN barrier layer.     

基于国产外延材料的SiC基AlGaN/GaN HEMT器件研制

本文报道了基于国产外延材料的SiC基Al GaN/GaN高迁移率晶体管（HEMT）器件的研制,外延材料利用金属有机物化学气相淀积技术（MOCVD）生长,器件栅长0.8μm,输出电流密度达到0.94A/mm,在5.4GHz下,单指型管芯获得了3.1W/mm的连续波测试功率,与蓝宝石衬底的器件相比,由于自热效应的有效改善,输出功率能力大大提高.     

Surface morphology and electronic properties of dislocations in AlGaN/GaN heterostructures

Scanning force microscopy was used to examine the surfaces of AlGaN/GaN heterostructures grown by molecular beam epitaxy (MBE) on GaN templates prepared by hydride vapor phase epitaxy (HVPE). Away from dislocations, the MBE growth replicates the surface morphology of the HVPE film, with monolayer steps clarly visible in topographic images. However, the surface morphology near dislocations depends strongly on the MBE growth conditions. Under Ga rich growth the dislocations appear as hillocks, while under stoichiometric growth they appear as pits. A dependence on Al concentration is also observed. Surface contact potential variation near dislocations is consistent with excess negative charges surrounding by a depletion region, but this was observed only for the film grown under stoichiometric conditions.