Interface engineering of transition metal dichalcogenide/GaN heterostructures: Modified broadband for photoelectronic performance

The GaN-based heterostructures are widely used in optoelectronic devices, but the complex surface reconstructions and lattice mismatch greatly limit the applications. The stacking of two-dimensional transition metal dichalcogenide (TMD = MoS₂, MoSSe and MoSe₂) monolayers on reconstructed GaN surface not only effectively overcomes the larger mismatch, but also brings about novel electronic and optical properties. By adopting the reconstructed GaN (0001) surface with adatoms (N-ter GaN and Ga-ter GaN), the influences of complicated surface conditions on the electronic properties of heterostructures have been investigated. The passivated N-ter and Ga-ter GaN surfaces push the mid-gap states to the valence bands, giving rise to small bandgaps in heterostructures. The charge transfer between Ga-ter GaN surface and TMD monolayers occurs much easier than that across the TMD/N-ter GaN interfaces, which induces stronger interfacial interaction and larger valence band offset (VBO). The band alignment can be switched between type-I and type-II by assembling different TMD monolayers, that is, MoS₂/N-ter GaN and MoS₂/Ga-ter GaN are type-II, and the others are type-I. The absorption of visible light is enhanced in all considered TMD/reconstructed GaN heterostructures. Additionally, MoSe₂/Ga-ter GaN and MoSSe/N-ter GaN have larger conductor band offset (CBO) of 1.32 eV and 1.29 eV, respectively, extending the range from deep ultraviolet to infrared regime. Our results revel that the TMD/reconstructed GaN heterostructures may be used for high-performance broadband photoelectronic devices.     

Control of polarity of heteroepitaxial ZnO films by interface engineering

Control of polarity of heteroepitaxial ZnO films has been examined by interface engineering. ZnO films were grown by plasma-assisted molecular beam epitaxy on Ga-polar GaN template and c-plane sapphire substrates. Polarity of all the samples is determined by coaxial impact collision ion scattering spectroscopy. Zn- and O-polar ZnO films have successfully grown by Zn- and O-plasma pre-exposures on Ga-polar GaN templates prior to ZnO growth. High-resolution transmission electron microscopy revealed formation of a single-crystalline monoclinic Ga₂O₃ interface layer by O-plasma pre-exposure on Ga-polar GaN templates, while no interface layer was observed for Zn pre-exposed ZnO films. The polarity of ZnO films grown under oxygen ambient on c-plane sapphire with MgO buffer is revealed as O-polar. Fabrication of polarity inverted ZnO heterostructure has been studied: polarity of ZnO films on Ga-polar GaN templates was changed from Zn-polar to O-polar by inserting a MgO layer. High-resolution transmission electron microscopy revealed atomically flat interfaces at both lower and upper ZnO/MgO interfaces and no inversion domain boundaries were detected in the upper ZnO layer.     

Studies on the nucleation of MBE grown III-nitride nanowires on Si

GaN and AlN nanowires (NWs) have attracted great interests for the fabrication of novel nano-sized devices. In this paper, the nucleation processes of GaN and AlN NWs grown on Si substrates by molecular beam epitaxy (MBE) are investigated. It is found that GaN NWs nucleated on in-situ formed Si₃N₄ fully release the stress upon the interface between GaN NW and amorphous Si₃N₄ layer, while AlN NWs nucleated by aluminization process gradually release the stress during growth. Depending on the strain status as well as the migration ability of III group adatoms, the different growth kinetics of GaN and AlN NWs result in different NW morphologies, i.e., GaN NWs with uniform radii and AlN NWs with tapered bases.     

高阻GaN的MOCVD外延生长

利用金属有机化合物气相沉积(MOCVD)在蓝宝石衬底上生长了高阻GaN薄膜。对GaN成核层生长的反应室压力、生长时间和载气类型对GaN缓冲层电学特性的影响进行了分析。实验结果表明,延长GaN成核层的生长时间,降低成核层生长时的反应室压力,载气由H₂换为N₂都会得到高阻的GaN缓冲层。样品的方块电阻R_s最高为2.49×10¹¹ Ω/□。以高阻GaN样品为衬底制备了AlGaN/AlN/GaN结构HEMT器件,迁移率最高达1 230 cm²/(V·s)。     

基于GaN同质衬底的高迁移率AlGaN/GaNHEMT材料

研究了表面预处理对GaN同质外延的影响,获得了高电子迁移率AlGaN/GaN异质结材料.通过NH_3/H_2混合气体与H_2交替通入反应室的方法对GaN模板和GaN半绝缘衬底进行高温预处理.研究结果表明,NH_3/H_2能够抑制GaN的分解,避免粗糙表面,但不利于去除表面的杂质,黄光带峰相对强度较高;H_2促进GaN分解,随时间延长GaN分解加剧,导致模板表面粗糙不平,AlGaN/GaN HEMT材料二维电子气迁移率降低.采用NH_3/H_2混合气体与H_2交替气氛模式处理模板或衬底表面,能够清洁表面,去除表面杂质,获得平滑的生长表面和外延材料表面,有利于提高AlGaN/GaN HEMT材料电学性能.在GaN衬底上外延AlGaN/GaN HEMT材料,2DEG迁移率达到2113 cm~2/V·s,电学性能良好.     

Half-metallicity and efficient spin injection in AlN/GaN:Cr (0001) heterostructure

First-principles investigations of the structural, electronic, and magnetic properties of Cr-doped AlN/GaN (0001) heterostructures reveal the possibility of efficient spin injection from a ferromagnetic GaN:Cr electrode through an AlN tunnel barrier. We demonstrate that Cr atoms segregate into the GaN region and that these interfaces retain their half-metallic behavior leading to a complete, i.e., 100%, spin polarization of the conduction electrons. This property makes the wide band-gap nitrides doped with Cr to be excellent candidates for high-efficiency magnetoelectronic devices.     

Initial interface study of Au deposition on GaN(0 0 0 1)

Synchrotron radiation photoelectron spectroscopy (SRPES) has been used to study the electronic structure of the Au/GaN(0 0 0 1) system at the initial growth stage. The peak fitting of Au4f_7/2 core-level and the energy shift of valence band indicate that Au–Ga alloy were formed in the interface reaction. According to the Ga3d signal intensity attenuation vs. the gold film thickness, the early growth mode is considered to be 3D mode above the reaction layer. By using the Linear Augmented Plane Wave method the density of states (DOS) for GaN and Au bulk are calculated within the framework of local functional theory. The theoretical results agree with the valence band structure quite well. The mechanism of interface reaction is discussed based on the experimental and theoretical results.     

表面预处理对石墨烯上范德瓦耳斯外延生长GaN材料的影响

基于范德瓦耳斯外延生长的氮化镓/石墨烯材料异质生长界面仅靠较弱的范德瓦耳斯力束缚,具有低位错、易剥离等优势,近年来引起了人们的广泛关注.采用NH_3/H_2混合气体对石墨烯表面进行预处理,研究了不同NH_3/H_2比对石墨烯表面形貌、拉曼散射的影响,探讨了石墨烯在NH_3和H_2混合气氛下的表面预处理机制,最后在石墨烯上外延生长了1.6μm厚的GaN薄膜材料.实验结果表明:石墨烯中褶皱处的C原子更容易与气体发生刻蚀反应,刻蚀方向沿着褶皱进行;适当NH_3/H_2比的混合气体对石墨烯进行表面预处理可有效改善石墨烯上GaN材料的晶体质量.本研究提供了一种可有效提高GaN晶体质量的石墨烯表面预处理方法,可为进一步研究二维材料上高质量的GaN外延生长提供参考.     

掺Fe高阻GaN缓冲层特性及其对AlGaN/GaN高电子迁移率晶体管器件的影响研究

利用金属有机物化学气相沉积技术在蓝宝石衬底上制备了掺Fe高阻Ga N以及Al Ga N/Ga N高电子迁移率晶体管(HEMT)结构.对Cp_2Fe流量不同的高阻Ga N特性进行了研究.研究结果表明,Fe杂质在Ga N材料中引入的Fe~(3+/2+)深受主能级能够补偿背景载流子浓度从而实现高阻,Fe杂质在Ga N材料中引入更多起受主作用的刃位错,也在一定程度上补偿了背景载流子浓度.在一定范围内,Ga N材料方块电阻随Cp_2Fe流量增加而增加,Cp_2Fe流量为100 sccm(1 sccm 1mL min)时,方块电阻增加不再明显;另外增加Cp_2Fe流量也会导致材料质量下降,表面更加粗糙.因此,优选Cp_2Fe流量为75 sccm,相应方块电阻高达×10?/,外延了不同掺Fe层厚度的Al Ga N/Ga N HEMT结构,并制备成器件.HEMT器件均具有良好的夹断以及栅控特性,并且增加掺Fe层厚度使得HEMT器件的击穿电压提高了39.3%,同时对器件的转移特性影响较小.     

位错形态与GaN外延薄膜电阻率之间的关系

利用金属有机化学气相沉积（MOCVD）,通过改变生长过程中成核层退火阶段的反应室压力,在蓝宝石衬底上制得了不同阻值的GaN外延薄膜。利用原子力显微镜（AFM）、X射线衍射（XRD）和透射电子显微镜（TEM）对所生长的GaN薄膜的表面形貌、位错密度和位错形态进行了研究。结果表明,GaN的电阻率与位错形态之间存在密切联系,由此建立了模型来解释两者之间的关系。由于刃型位错附近存在负电荷,因此可为电子提供传导通道。在低阻GaN中,绝大多数位错发生弯曲和相互作用,在平行于基底方向上形成负电荷的导通通道,GaN薄膜的电导率较高。在高阻GaN中,位错生长方向垂直于基底,负电荷很难在平行于基片方向上传导,GaN薄膜的电导率很低,由此得到高阻GaN。     

等离子增强原子层沉积低温生长GaN薄膜

采用等离子增强原子层沉积技术在低温下于单晶硅衬底上成功生长了Ga N多晶薄膜,利用椭圆偏振仪、低角度掠入射X射线衍射仪、X射线光电子能谱仪对薄膜样品的生长速率、晶体结构及薄膜成分进行了表征和分析.结果表明,等离子增强原子层沉积技术生长Ga N的温度窗口为210—270?C,薄膜在较高生长温度下呈多晶态,在较低温度下呈非晶态;薄膜中N元素与大部分Ga元素结合成N—Ga键生成Ga N,有少量的Ga元素以Ga—O键存在,多晶Ga N薄膜含有少量非晶态Ga_2O_3.     

GaN外延片中载流子浓度的纵向分布

采用适合宽禁带半导体材料的电化学电容电压(ECV)分析仪,对掺硅GaN外延片用硫酸逐层进行了精密腐蚀后,在此基础上得到了在进口MOCVD设备上生产的GaN基外延片的载流子浓度纵向分布.探讨了该分布与晶体生长过程及晶体质量的关系,测量分析结果可为生长工艺参数的优化提供参考.还采用主扩散模型对测量结果进行高斯拟合,得出了高温时(1030℃)硅在GaN中的扩散系数,并由此估算了硅在GaN外延片中的扩散宽度.该结果可为GaN外延层结构设计提供参考.     

First-principles study on improvement of two-dimensional hole gas concentration and confinement in AlN/GaN superlattices

Using first-principles calculations based on density functional theory,we have systematically studied the influence of in-plane lattice constant and thickness of slabs on the concentration and distribution of two-dimensional hole gas(2 DHG)in AlN/GaN superlattices.We show that the increase of in-plane lattice constant would increase the concentration of 2 DHG at interfaces and decrease the valence band offset,which may lead to a leak of current.Increasing the thickness of AlN and/or decreasing the thickness of GaN would remarkably strengthen the internal field in GaN layer,resulting in better confinement of 2 DHG at AlN/GaN interfaces.Therefore,a moderate larger in-plane lattice constant and thicker AlN layer could improve the concentration and confinement of 2 DHG at AlN/GaN interfaces.Our study could serve as a guide to control the properties of 2 DHG at Ⅲ-nitride interfaces and help to optimize the performance of p-type nitride-based devices.     

Modeling of trap-assisted tunneling in AlGaN/GaN heterostructure field effect transistors with different Al mole fractions

Although significant progress has been achieved in GaN based high power/high frequency electronic devices, surface-related problems still need an immediate solution. In particular, leakage currents through Schottky contacts not only impede device reliability but also degrade power efficiency and noise performance in such devices. This article discusses the mechanism of leakage currents through GaN Schottky and AlGaN/GaN Schottky interfaces for both forward and reverse biases. A theoretical model for the calculation of currents based on trap-assisted tunneling is discussed. In the calculation the trap energy has been assumed as a fitting parameter which is about 0.48 eV for different Al mole fractions. The comparison of the results obtained with the existing experimental data in the literature shows a good agreement.     

Modelling of the reactive interface formation on Bi2Sr2CaCu2O8

We have applied the phenomenological model of diffused interfaces, developed originally to describe the Fe growth on Bi₂Sr₂CaCu₂O₈, to interpret quantitatively the core photoemission spectra of two reactive interfaces, namely Cr and Ge on Bi₂Sr₂CaCu₂O₈. At room temperature the interfaces with Fe and Cr have the same kinetics of growth suggesting similar adatom condensation, while the semiconductor-superconductor interface grows layer-by-layer according to the Franck-van der Merwe kinetics. Differences in the parameters of the model resulting from the simultaneous fit of the core-level decays are related to differences in the microscopic processes occurring during the growth.     

The fundamental surface science of wurtzite gallium nitride

A review is presented that covers the experimental and theoretical literature relating to the preparation, electronic structure and chemical and physical properties of the surfaces of the wurtzite form of GaN. The discussion includes the adsorption of various chemical elements and of inorganic, organometallic and organic species. The focus is on work that contributes to a microscopic, atomistic understanding of GaN surfaces and interfaces, and the review concludes with an assessment of possible future directions.     

蓝光激光器结构中InGaN/GaN多量子阱界面效应的精细光致发光光谱研究

金属有机化学气相沉积(MOCVD)方法制备InGaN/GaN多量子阱结构时,在GaN势垒层生长的N2载气中引入适量H2,能够有效改善阱/垒界面质量从而提升发光效率.本工作利用光致发光(PL)光谱技术,对蓝光激光器结构中的InGaN/GaN多量子阱的发光性能进行了精细的光谱学测量与表征,研究了通H2生长对量子阱界面的调控...     

氮化物抛物量子阱中类氢杂质态能量

采用变分方法研究氮化物抛物量子阱(GaN/Al_xGa_1-xN)材料中类氢杂质态的能级,给出基态能量、第一激发态能量、结合能和跃迁能量等物理量随抛物量子阱宽度变化的函数关系.研究结果表明,基态能量、第一激发态能量、基态结合能和1s→2p±跃迁能量随着阱宽L的增大而减小,最后接近于GaN中3D值.GaN/Al_0.3Ga_0.7N抛物量子阱对杂质态的束缚程度比GaAs/Al_0.3Ga_0.7As抛物量子阱强,因此,在GaN/Al_0.3-Ga_0.7N抛物量子阱中束缚于杂质中心处的电子比在GaAs/Al_0.3Ga_0.7As抛物量子阱中束缚于杂质中心处的电子稳定.     

Direct growth of graphene on gallium nitride using C2H2 as carbon source

Growing graphene on gallium nitride (GaN) at temperatures greater than 900°C is a challenge that must be overcome to obtain high quality of GaN epi-layers. We successfully met this challenge using C₂H₂ as the carbon source. We demonstrated that graphene can be grown both on copper and directly on GaN epi-layers. The Raman spectra indicated that the graphene films were about 4–5 layers thick. Meanwhile, the effects of the growth temperature on the growth of the graphene films were systematically studied, and 830°C was found to be the optimum growth temperature. We successfully grew high-quality graphene films directly on gallium nitride.     

XPS, electric and photoluminescence-based analysis of the GaAs (1 0 0) nitridation

In this paper, nitridation process of GaAs (1 0 0) substrates was studied in-situ using X-ray photoelectron spectroscopy (XPS) and ex-situ by means of electrical method I-V and photoluminescence surface state spectroscopy (PLS³) in order to determine chemical, electrical and electronic properties of the elaborated GaN/GaAs interfaces.The elaborated structures were characterised by I-V analysis. The saturation current I_S, the ideality factor n, the barrier height Φ_Bn and the serial resistance R_S are determined.The elaborated GaN/GaAs structures are also exhibited a high PL intensity at room temperature. From the computer-aided analysis of the power-dependent PL efficiency measurements (PLS³ technique), the value of the interface state density N_SS(E) close to the mid-gap was estimated to be in the range of 2-4 × 10¹¹ eV⁻¹ cm⁻², indicating a good electronic quality of the obtained interfaces.Correlation among chemical, electronic and electrical properties of the GaN/GaAs interface was discussed.