Contactless electroreflectance study of band bending for undoped, Si- and Mg-doped GaN layers and AlGaN/GaN transistor heterostructures

The surface band bending in undoped, Si-doped and Mg-doped GaN layers with Ga-face polarity as well as AlGaN/GaN heterostructures with Ga(Al)-face polarity has been investigated by room temperature contactless electroreflectance (CER) spectroscopy. The opposite phase of CER resonance (i.e., opposite band bendings) has been observed for n-type (undoped and Si-doped) and p-type (Mg-doped) GaN layers. It means that for thick GaN layers the surface band bending results not from crystal polarity but from the Fermi-level pinning at the surface and carrier type/concentration inside the layer. The crystal polarity can influence the surface band bending for thin (Al)GaN layers for which the screening phenomena can be neglected or are very weak. Such a situation is typical of AlGaN/GaN transistor structures where the thickness of AlGaN layer is below 40 nm. In this case, the strong internal electric field in AlGaN layer is manifested in CER spectra by a resonance with a long period Franz-Keldysh oscillation.     

Characterization of AlGaN/GaN structures on various substrates grown by radio frequency-plasma assisted molecular beam epitaxy

The structural properties and surface morphology of AlGaN/GaN structures grown on LiGaO₂ (LGO), sapphire, and hydride vapor phase epitaxy (HVPE)-grown GaN templates are compared. AlGaN grown on LGO substrates shows the narrowest x-ray full width at half maximum (FWHM) for both symmetric 〈00.4〉 and asymmetric 〈10.5〉 reflections. Atomic force microscopy (AFM) analysis on AlGaN surfaces on LGO substrates also show the smoothest morphology as determined by grain size and rms roughness. The small lattice mismatch of LGO to nitrides and easily achievable Ga-polarity of the grown films are the primary reasons for the smoother surface of AlGaN/GaN structure on this alternative substrate. Optimizations of growth conditions and substrate preparation results in step flow growth for an AlGaN/GaN structure with 300 Å thick Al_0.25Ga_0.75N on 2.4 μm thick GaN. A high III/V flux ratio during growth and recently improved polishing of LGO substrates aids in promoting two dimensional step flow growth. The GaN nucleation layer directly on the LGO substrate showed no evidence of mixed phase cubic and hexagonal structure that is typically observed in the nucleation buffer on sapphire substrates. Cross-sectional high-resolution transmission electron microscopy (HRTEM) was performed on an AlGaN/GaN heterostructure grown on LGO. The atomic arrangement at the AlGaN/GaN interface was sharp and regular, with locally observed monolayer and bilayer steps.     

Low-Frequency Noise Characteristics in Ion-Implanted GaN-Based HEMTs

Low-frequency noise characteristics in ion-implanted GaN/AlGaN/GaN and AlGaN/GaN HEMTs were investigated. The normalized spectral noise density was about 6 dB lower in GaN/AlGaN/GaN HEMTs than in AlGaN/GaN HEMTs. The normalized spectral noise density dependence on the gate length Lg indicates that the main origin of low-frequency noise is at the region under the gate in both devices. The Hooge parameters alphaH for both devices are on the order of 10^-1-10^{- 2}. The ion implantation process introduces a lot of defects in the source/drain regions, but the values of alphaH are comparable with those for conventional GaN-based HEMT devices. The values of alphaH are also lower in GaN/AlGaN/GaN HEMTs than in AlGaN/GaN HEMTs, which is due to the decrease of surface potential fluctuations in GaN/AlGaN/GaN HEMTs.     

AlGaN/GaN HEMT材料外延技术研究

采用低压MOCVD技术在蓝宝石和SiC衬底上生长了本征GaN和AlGaN/GaN HEMT结构材料。生长过程中采用了EpiTUNEⅡ在位监测技术,对材料生长工艺进行了模型分析以及优化控制。在获得具有良好表面形貌、晶体质量以及光荧光谱的GaN本征材料基础上,生长了AlGaN/GaN HEMT结构材料,获得了良好的2DEG性能。     

Growth of GaN on Buffer Layers with Different Polarities by Hydride Vapor-Phase Epitaxy

This paper reports the properties of GaN grown by the hydride vapor-phase epitaxy (HVPE) technique on buffer layers with different polarities. The N-, mixed-, and Ga-polarity buffer layers were grown by molecular-beam epitaxy (MBE) on sapphire (0001) substrates; then, thicker GaN epilayers were grown on these by HVPE. The surface morphology, structural, and optical properties of these HVPE-GaN epilayers were characterized by atomic force microscopy (AFM), x-ray diffraction (XRD), scanning electron microscopy, and photoluminescence (PL) spectroscopy. The results indicate that the crystallinity of these HVPE-GaN epilayers depends on the polarity of the buffer layer.     

Specific features of kinetics of molecular beam epitaxy of compounds in the GaN-AlN system

The effect of growth conditions (V/III ratio, substrate temperature) on the properties of materials in AlN-GaN systems is discussed. A concept of the growth of the AlN/AlGaN/GaN multilayer heterostructure, which provides the improvement of crystal quality and surface morphology of the layers, is suggested and realized. The improvement of the properties of GaN in the AlN/AlGaN/GaN/AlGaN multilayer heterostructure is confirmed by a considerable increase in electron mobility in the two-dimensional electron gas formed at the upper heterointerface GaN/Al_0.3Ga_0.7N.     

Low damage, Cl/sub 2/-based gate recess etching for 0.3-/spl mu/m gate-length AlGaN/GaN HEMT fabrication

The traditional dry etching for GaN using the Ar/Cl/sub 2/ mixture gas in the reactive ion etching system has been developed. In order to reduce the surface damage, the additional CH/sub 4/ gas is introduced. However, this approach still has the problems of the residual surface damage and low etching selectivity between the AlGaN and GaN materials. Therefore, the following rapid thermal annealing (RTA) at 700/spl deg/C is necessary to recover the surface properties. In this study, we proposed the Ar/Cl/sub 2//CH/sub 4//O/sub 2/ for the GaN gate-recess etching in AlGaN/GaN HEMTs fabrication, which achieves a low surface damage and a high etching selectivity simultaneously. The 0.3 /spl mu/m gate-length AlGaN/GaN HEMTs present a transconductance of 230 mS/mm, an f/sub T/ of 48 GHz, and f/sub max/ of 60 GHz, respectively.     

AlN阻挡层对AlGaN/GaNHEMT器件的影响

利用低压MOCVD技术在蓝宝石衬底上生长了AlGaN/GaN异质结和AlGaN/AlN/GaN异质结二维电子气材料,采用相同器件工艺制造出了AlGaN/GaN HEMT器件和AlGaN/AlN/GaN HEMT器件.通过对两种不同器件的比较和讨论,研究了AlN阻挡层的增加对AlGaN/GaN HEMT器件性能的影响.     

SiN钝化前的NF3等离子体处理对AlGaN/GaN HEMT性能的影响

研究了SiN钝化前利用感应耦合等离子体（ICP）对AlGaN/GaN HEMT表面进行NF3等离子体处理对器件性能的影响. 结果表明,运用低能量的NF3等离子体处理钝化前的AlGaN/GaN HEMT表面能有效抑制器件电流崩塌,而器件直流及微波小信号特性则未受影响. 微波功率测试表明,经过6min NF3等离子体处理的AlGaN/GaN HEMT在2GHz, 30V工作电压下达到6.15W/mm的输出功率密度,而未经过处理的器件只达到1.82W/mm的输出功率密度.     

AlGaN/GaN HFET的2DEG和电流崩塌研究(Ⅰ)

从不同的视角回顾和研究了A1GaN/GaN HFET的二维电子气(2DEG)和电流崩塌问题.阐述了非掺杂的AIGaN/GaN异质结界面存在2DEG的原动力是极化效应,电子来源是AlGaN上的施主表面态.2DEG浓度与AlGaN/GaN界面导带不连续性、AlGaN层厚和Al组分有密切关系.揭示了AlGaN/GaN HFET的2DEG电荷涨落受控于表面、界面和缓冲层中的各种缺陷及外加应力,表面空穴陷阱形成的虚栅对输入信号有旁路和延迟作用,它们导致高频及微波状态下的电流崩塌.指出由于构成电流崩塌因素的复杂性,各种不同的抑制电流崩塌方法都存在不足,因此实现该器件大功率密度和高可靠性还有很长的路要走.     

高击穿电压AlGaN/GaN HEMT电力开关器件研究进展

作为第三代宽禁带半导体材料的典型代表,GaN材料在各个应用领域的研究工作都受到了高度的重视。概述了基于AlGaN/GaN HEMT结构的新型高压、高频、低损耗电力开关器件的最新研究进展。从器件的结构特征入手,详细介绍了改善器件击穿特性的途径、高频开关特性的研究情况、Si衬底上AlGaN/GaN HEMT结构材料的生长、增强型器件的制备技术和功率集成电路的研究等几个国际上的热点问题。最后,对该项研究面临的问题及未来的发展趋势做了展望。     

Schottky Heterojunction Facilitates Osteosarcoma Ferroptosis and Enhances Bone Formation in a Switchable Mode

Effective antitumor agents with concurrent osteogenic properties are essential for comprehensive osteosarcoma (OS) treatment. However, the current clinical therapeutic strategies of OS fail to completely eradicate tumors while simultaneously encouraging bone formation. To address this issue, a switchable strategy for dynamic OS ablation and static bone regeneration is developed by integrating piezoelectric BaTiO₃ (BTO) with atomic-thin Ti₃C₂ (TC) through a Schottky heterojunction, resulting in the formation of TC@BTO. Under sequential ultrasound and near-infrared irradiation, the optimized carrier transport of TC@BTO, based on Schottky heterojunction, exhibits excellent characteristics of photothermal conversion and reactive oxygen species generation. This results in ferroptosis of tumor cells and eventual elimination of OS. Moreover, in the static state, the interfacial Schottky heterojunction facilitates the carriers’ directed transfer from the semiconductor to the metal. The Schottky heterojunction-enhanced static electrical stimulation enhances the osteogenic differentiation of bone marrow-derived mesenchymal stem cells and repair of bone defects. Furthermore, RNA-sequencing analysis reveals that static TC@BTO promotes bone regeneration by activating Wnt signaling pathway, and remarkably, pharmacological inhibition of Wnt signaling suppresses the TC@BTO-induced osteogenesis. Overall, this work broadens the biomedical potential of Schottky heterojunction-based therapies and provides a comprehensive strategy for overall OS ablation and bone regeneration.     

含有Al组分阶变AlGaN过渡层的Si基AlGaN/GaN HEMT

采用一个AlN缓冲层和两个Al组分阶变的AlGaN过渡层作为中间层,在76.2mm Si衬底上外延生长出1.7μm厚无裂纹AlGaN/GaN异质结材料,利用原子力显微镜、X射线衍射、Hall效应测量和CV测量等手段对材料的结构特性和电学性能进行了表征。材料表面平整光滑,晶体质量和电学性能良好,2DEG面密度为1.12×1013cm-2,迁移率为1 208cm2/(V.s)。由该材料研制的栅长为1μm的AlGaN/GaN HEMT器件,电流增益截止频率fT达到10.4GHz,这些结果表明组分阶变AlGaN过渡层技术可用于实现高性能Si基GaN HEMT。     

InGaN背势垒AlGaN/GaN HEMT中的电流崩塌研究

Current collapses were studied,which were observed in AlGaN/GaN high electron mobility transistors(HEMTs) with and without InGaN back barrier(BB) as a result of short-term bias stress.More serious drain current collapses were observed in InGaN BB AlGaN/GaN HEMTs compared with the traditional HEMTs.The results indicate that the defects and surface states induced by the InGaN BB layer may enhance the current collapse.The surface states may be the primary mechanism of the origination of current collapse in AlGaN/GaN HEMTs for short-term direct current stress.     

Modeling on oxide dependent 2DEG sheet charge density and threshold voltage in AlGaN/GaN MOSHEMT

We have developed a physics based analytical model for the calculation of threshold voltage, two dimensional electron gas (2DEG) density and surface potential for AlGaN/GaN metal oxide semiconductor high electron mobility transistors (MOSHEMT). The developed model includes important parameters like polarization charge density at oxide/AlGaN and AlGaN/GaN interfaces, interfacial defect oxide charges and donor charges at the surface of the AlGaN barrier. The effects of two different gate oxides (Al₂O₃ and HfO₂) are compared for the performance evaluation of the proposed MOSHEMT. The MOSHEMTs with Al₂O₃ dielectric have an advantage of significant increase in 2DEG up to 1.2×10¹³ cm^-2 with an increase in oxide thickness up to 10 nm as compared to HfO₂ dielectric MOSHEMT. The surface potential for HfO₂ based device decreases from 2 to -1.6 eV within 10 nm of oxide thickness whereas for the Al₂O₃ based device a sharp transition of surface potential occurs from 2.8 to -8.3 eV. The variation in oxide thickness and gate metal work function of the proposed MOSHEMT shifts the threshold voltage from negative to positive realizing the enhanced mode operation. Further to validate the model, the device is simulated in Silvaco Technology Computer Aided Design (TCAD) showing good agreement with the proposed model results. The accuracy of the developed calculations of the proposed model can be used to develop a complete physics based 2DEG sheet charge density and threshold voltage model for GaN MOSHEMT devices for performance analysis.     

Digitally Alloyed Modulated Precursor Flow Epitaxial Growth of Ternary AlGaN with Binary AlN and GaN Sub-Layers and Observation of Compositional Inhomogeneity

We report the growth of ternary aluminum gallium nitride (AlGaN) layers on AlN/sapphire template/substrates by digitally alloyed modulated precursor flow epitaxial growth (DA-MPEG), which combined an MPEG AlN sub-layer with a conventional metalorganic chemical vapor deposition (MOCVD)-grown GaN sub-layer. The overall composition in DA-MPEG Al _x Ga_1−x N was controlled by adjustment of the growth time (i.e., the thickness) of the GaN sub-layer. As the GaN sub-layer growth time increased, the Al composition in AlGaN decreased to 50%, but the surface morphology of the AlGaN layer became rough, and a three-dimensional structure with islands appeared for the DA-MPEG AlGaN with relatively thick GaN sub-layers, possibly resulting from the Ga adatom surface migration behavior and/or the strain built up from lattice mismatch between AlN and GaN sub-layers with increasing GaN sub-layer growth time. Through strain analysis by high-resolution x-ray diffraction, reciprocal space mapping, and scanning transmission electron microscopy, it was found that there was compositional inhomogeneity in the DA-MPEG AlGaN with AlN and GaN binary sub-layers for the case of the layer with relatively thick GaN sub-layers.     

氮化对蓝宝石衬底上AlGaN/GaN异质结

研究了不同氮化条件对蓝宝石衬底上生长的AlGaN/GaN异质结材料特性的影响。研究表明,随着氮化过程NH₃流量的增大,GaN外延层的晶体质量得到了改善,GaN内应力释放,但是AlGaN/GaN异质结构中二维电子气的迁移率有所恶化。讨论了上述现象出现的原因。     

C波段0.75mm AlGaN/GaN功率器件

研制并测试了以蓝宝石作衬底的10×75μm×0.8μm AlGaN/GaN微波器件,采用等离子增强气相化学沉积的方法生长了250nm的Si3N4形成钝化层,直流特性从0.56A/mm上升到0.66A/mm,跨导从158mS/mm增为170mS/mm,截止频率由10.7GHz增大到13.7GHz,同时在4GHz下,Vds＝25V, Vgs＝-2.5V,输出功率由0.90W增至1.79W,输出功率密度达到2.4W/mm. 钝化有效地改善了器件的输出特性,减小和消除了表面寄生栅对器件的影响.     

Investigations of GaN growth on the sapphire substrate by MOCVD method with different AlN buffer deposition temperatures

The effects of different AlN buffer deposition temperatures on the GaN material properties grown on sapphire substrate was investigated. At relatively higher AlN buffer growth temperature, the surface morphology of subsequent grown GaN layer was decorated with island-like structure and revealed the mixed-polarity characteristics. In addition, the density of screw TD and leakage current in the GaN film was also increased. The occurrence of mixed-polarity GaN material result could be from unintentional nitridation of the sapphire substrate by ammonia (NH₃) precursor at the beginning of the AlN buffer layer growth. By using two-step temperature growth process for the buffer layer, the unintentional nitridation could be effectively suppressed. The GaN film grown on this buffer layer exhibited a smooth surface, single polarity, high crystalline quality and high resistivity. AlGaN/GaN high electron-mobility transistor (HEMT) devices were also successfully fabricated by using the two-step AlN buffer layer.     

Cover Picture: Biomineralized Polysaccharide Capsules for Encapsulation,Organization, and Delivery of Human Cell Types and Growth Factors (Adv. Funct. Mater. 6/2005)

The cover shows biomineralized polysaccharide capsules with specifiable make‐up, which can provide microenvironments for stabilization, growth, and differentiation of human cell types, as reported by Oreffo and co‐workers on p. 917. The capsules are amenable to complexation with a range of bioactive molecules and cells, offering tremendous potential as multifunctional scaffolds and delivery vehicles in tissue regeneration of hard and soft tissues. The construction of biomimetic microenvironments with specific chemical and physical cues for the organization and modulation of a variety of cell populations is of key importance in tissue engineering. We show that a range of human cell types, including promyoblasts, chondrocytes, adipocytes, adenovirally transduced osteoprogenitors, immunoselected mesenchymal stem cells, and the osteogenic factor, rhBMP‐2 (BMP: bone morphogenic protein), can be successfully encapsulated within mineralized polysaccharide capsules without loss of function in vivo. By controlling the extent of mineralization within the alginate/chitosan shell membrane, degradation of the shell wall and release of cells or rhBMP‐2 into the surrounding medium can be regulated. In addition, we describe for the first time the ability to generate bead‐in‐bead capsules consisting of spatially separated cell populations and temporally separated biomolecule release, entrapped within alginate/chitosan shells of variable thickness, mineralization, and stability. Such materials offer significant potential as multifunctional scaffolds and delivery vehicles in tissue regeneration of hard and soft tissues.