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。     

The effect of AlN/AlGaN superlattices on crystal and optical properties of AlGaN epitaxial layers

We investigate the effect of AlN/AlGaN superlattices (SLs) on crystal and optical properties of AlGaN epitaxial layers. The result indicates that the crystal quality of AlGaN layers is consistent within a wide range of SLs thicknesses, while the optical properties are opposite. With SLs thickness decreasing from 20/44 to 17/36 and 15/29 nm, the full-width at half maximum of X-ray rocking curves for (0002)-and (1012)-plane of n-AlGaN layers grown on SLs are consistent of around 250 arcsec and 700 arcsec, respectively. Meanwhile, the center of the low optical transmittance band decreases from 326 to 279 nm and less than 266 nm as the SLs thickness decreases. 280 nm deep ultraviolet light-emitting diodes (DUV-LEDs) structures are further regrown on the n-AlGaN layers. The electroluminescent intensities of samples are 30% higher than that of the sample whose low optical transmittance band appears around 279 nm. Optical simulations reveal that the SLs acts as distributed Bragg reflectors, thus less photons of the corresponding wavelength escape from the sapphire backside.     

Flat GaN epitaxial layers grown on Si(111) by metalorganic vapor phase epitaxy using step-graded AlGaN intermediate layers

In this work, we report on the growth by metalorganic vapor phase epitaxy (MOVPE) of GaN layers on AlN/Si(111) templates with step-graded AlGaN intermediate layers. First, we will discuss the optimization of the AlN/Si(111) templates and then we will discuss the incorporation of step-graded AlGaN intermediate layers. It is found that the growth stress in GaN on high-temperature (HT) AlN/Si(111) templates is compressive, although, due to relaxation, the stress we have measured is much lower than the theoretical value. In order to prevent the stress relaxation, step-graded AlGaN layers are introduced and a crack-free GaN epitaxial layer of thickness >1 μm is demonstrated. Under optimized growth conditions, the total layer stack, exceeding 2 μm in total, is kept under compressive stress, and the radius of the convex wafer bowing is as large as 119 m. The crystalline quality of the GaN layers is examined by high-resolution x-ray diffraction (HR-XRD), and the full-width-at-half maximums (FWHMs) of the x-ray rocking curve (0002) ω-scan and (−1015) ω-scan are 790 arc sec and 730 arc sec, respectively. It is found by cross-sectional transmission electron microscopy (TEM) that the step-graded AlGaN layers terminate or bend the dislocations at the interfaces.     

Effect of AlN buffer thickness on GaN epilayer grown on Si(1 1 1)

We studied the influence of high temperature AlN buffer thickness on the property of GaN film on Si (1 1 1) substrate. Samples were grown by metal organic chemical vapor deposition. Optical microscopy, atomic force microscopy and X-ray diffraction were employed to characterize the samples. The results demonstrated that thickness of high temperature AlN buffer prominently influenced the morphology and the crystal quality of GaN epilayer. The optimized thickness of the AlN buffer is found to be about 150 nm. Under the optimized thickness, the largest crack-free range of GaN film is 10 mm×10 mm and the full width at half maximum of GaN (0 0 0 2) rocking curve peak is 621.7 arcsec. Using high temperature AlN/AlGaN multibuffer combined with AlN/GaN superlattices interlayer we have obtained 2 μm crack-free GaN epilayer on 2 in Si (1 1 1) substrates.     

GaN基多层结构椭偏测试模型优化和光谱研究

利用椭圆偏振测试仪,对n+AlGaN/n-AlGaN/超晶格(SL)/AlN/蓝宝石多层复杂结构的椭偏测试方法进行了研究,其中SL层为AlGaN/GaN多层量子阱结构,将其简化为单层处理。通过改变测试条件进行多次测量,分析了结构模型、算法模型及不同入射角度对测试结果的影响,得到了最优测试结构模型和算法模型:粗糙度层(有效介质近似EMA模型)/n+AlGaN(柯西Cauchy模型)/n-AlGaN(Cauchy)/SL(EMA)/AlN(Cauchy)/蓝宝石。选择55°为入射角,在300～800nm波长测试得到了各层膜厚和光学常数谱。结果表明,椭偏测试所得各层膜厚度与工艺给出数据相符,但SL层厚度相对而言有较大偏差,这与其结构复杂性有关,尚待进一步研究。各层折射率色散关系正常,n+AlGaN层折射率与n-AlGaN层折射率相比偏小,这与其较高浓度和较大损伤有关。     

Impact of low-temperature buffer layers on nitride-based optoelectronics

A historical overview and recent trends in the research and development of nitride-based light emitters are presented. The growth of GaN using a low-temperature-deposited buffer layer conductivity control, and the use of GaInN alloys, by which nonradiative recombination centers in nitrides are screened, have been employed to fabricate high-efficiency blue and green light-emitting diodes. Today, luminous flux efficiency of up to 50 lm/W at a specific wavelength is available. Electrical pumping has realized commercial laser diodes in the violet and blue regions. Several milestones in the realization of these achievements are reviewed. Future prospects of the nitride-based light emitters are also discussed.     

8英寸Si衬底上高质量AlGaN/GaN HEMT结构的生长

在8英寸(1英寸=2.54 cm)的Si衬底上采用金属有机化学气相沉积(MOCVD)生长了高质量、无龟裂的GaN薄膜和AlGaN/GaN高电子迁移率晶体管(HEMT)结构.通过调节应力调控层的结构,厚度为5 μm的GaN膜层翘曲度低于50 μm.采用X射线衍射(XRD)对GaN薄膜的(002)和(102)衍射峰进行扫描,其半峰全宽(FWHM)分别为182和291 arcsec.透射电子显微镜(TEM)截面图显示GaN外延层的位错密度达到了3.5×107/cm2,证实了在大尺寸Si衬底上可以制作高质量的GaN薄膜.AlGaN/GaN HEMT结构的二维电子气浓度和载流子迁移率分别为9.29×1012/cm2和2 230 cm2/(V·s).基于这些半绝缘AlGaN/GaNHEMT结构所制作的功率电子器件的输出电流可达20 A,横向击穿电压可达1 200 V.     

Technology of the production of laser diodes based on GaAs/InGaAs/AlGaAs structures grown on a Ge/Si substrate

InGaAs/GaAs/AlGaAs laser diodes with quantum wells are grown by the metal-organic chemical vapor deposition (MOCVD) method on an exact Si (001) substrate with a Ge buffer layer. The diodes generate stimulated emission in the pulsed mode at room temperature in the spectral range from 1.09 to 1.11 μm.     

AlGaN/GaN/InGaN/GaN DH-HEMTs with an InGaN notch for enhanced carrier confinement

We report an AlGaN/GaN/InGaN/GaN double heterojunction high electron mobility transistors (DH-HEMTs) with high-mobility two-dimensional electron gas (2-DEG) and reduced buffer leakage. The device features a 3-nm thin In/sub x/Ga/sub 1-x/N(x=0.1) layer inserted into the conventional AlGaN/GaN HEMT structure. Assisted by the InGaN layers polarization field that is opposite to that in the AlGaN layer, an additional potential barrier is introduced between the 2-DEG channel and buffer, leading to enhanced carrier confinement and improved buffer isolation. For a sample grown on sapphire substrate with MOCVD-grown GaN buffer, a 2-DEG mobility of around 1300 cm/sup 2//V/spl middot/s and a sheet resistance of 420 /spl Omega//sq were obtained on this new DH-HEMT structure at room temperature. A peak transconductance of 230 mS/mm, a peak current gain cutoff frequency (f/sub T/) of 14.5 GHz, and a peak power gain cutoff frequency (f/sub max/) of 45.4 GHz were achieved on a 1/spl times/100 /spl mu/m device. The off-state source-drain leakage current is as low as /spl sim/5 /spl mu/ A/mm at V/sub DS/=10 V. For the devices on sapphire substrate, maximum power density of 3.4 W/mm and PAE of 41% were obtained at 2 GHz.     

6英寸硅基GaN HEMT外延材料的制备

采用金属有机物化学气相沉积(MOCVD)方法,在6英寸(1英寸=2.54 cm)Si(111)衬底上,使用多层不同Al摩尔组分的AlGaN插入层技术,成功生长出厚度为2.9 μm无裂纹(扣除边缘2mm)的GaN外延层,解决了大尺寸外延片的翘曲度问题,并在此基础上生长了全结构的高电子迁移率晶体管(HEMT)外延片.采用X射线双晶衍射对外延材料结构进行了表征.Hall测试结果表明,HEMT外延材料的迁移率为2 080 cm2/(V·s),方块电阻为279.8 Ω/□,电荷面密度为1.07×1013 cm-2.采用喇曼光谱仪对GaN的应力进行了表征,GaN的喇曼E2(h)峰位于567.02 cm-1,表面受到的张应力为0.170 6 GPa,由于GaN外延层受到的张应力很小,说明插入多层AlGaN后应力已经释放.汞探针C-V测试二维电子气浓度较Hall测试结果偏低,可能是在C-V测试时肖特基势垒接触会降低载流子浓度.     

AlGaN/GaN横向肖特基势垒二极管的仿真与制作

基于GaN横向肖特基势垒二极管(SBD)的频率特性和应用的需要,设计了一种基于AlGaN/GaN异质结的横向SBD.利用Silvaco Atlas软件研究了 AlGaN势垒层的厚度和Al摩尔组分对异质结AlχGaN1-χ/GaN SBD电学性能的影响.仿真结果表明,SBD器件截止频率随Al摩尔组分的增加先增大再减小,当AlχGaN,1-χ层中Al摩尔组分为0.2～0.25,其厚度为20～30nm时,AlGaN/GaN SBD器件的频率特性最好.在仿真的基础上,设计制作出了肖特基接触直径为2 μm的非凹槽和凹槽型AlGaN/GaN横向空气桥SBD.通过直流I-V[测试和射频S参数测试,提取了两种SBD器件的理想因子、串联电阻、结电容、截止频率和品质因子等关键参数,该平面 SBD可应用于片上集成和混合集成的太赫兹电路的设计与制造.     

Effects of high-temperature AIN buffer on the microstructure of AlGaN/GaN HEMTs

Effects on AlGaN/GaN high-electron-mobility transistor structure of a high-temperature AlN buffer on sapphire substrate have been studied by high-resolution x-ray diffraction and atomic force microscopy techniques. The buffer improves the microstructural quality of GaN epilayer and reduces approximately one order of magnitude the edge-type threading dislocation density. As expected, the buffer also leads an atomically flat surface with a low root-mean-square of 0.25 nm and a step termination density in the range of 10⁸ cm^?2. Due to the high-temperature buffer layer, no change on the strain character of the GaN and AlGaN epitaxial layers has been observed. Both epilayers exhibit compressive strain in parallel to the growth direction and tensile strain in perpendicular to the growth direction. However, an high-temperature AlN buffer layer on sapphire substrate in the HEMT structure reduces the tensile stress in the AlGaN layer.     

Improvement of near-ultraviolet InGaN-GaN light-emitting diodes with an AlGaN electron-blocking layer grown at low temperature

The 400-nm near-ultraviolet InGaN-GaN multiple quantum well light-emitting diodes (LEDs) with Mg-doped AlGaN electron-blocking (EB) layers of various configurations and grown under various conditions, were grown on sapphire substrates by metal-organic vapor phase epitaxy system. LEDs with AlGaN EB layers grown at low temperature (LT) were found more effectively to prevent electron overflow than conventional LEDs with an AlGaN one grown at high temperature (HT). The electroluminescent intensity of LEDs with an LT-grown AlGaN layer was nearly three times greater than that of LEDs with an HT-grown AlGaN. Additionally, the LEDs with an LT-grown AlGaN layer in H/sub 2/ ambient were found to increase the leakage current by three orders of magnitude and reduce the efficiency of emission.     

Effect of Buffer Layer Structure on Drain Leakage Current and Current Collapse Phenomena in High-Voltage GaN-HEMTs

High-voltage (> 400 V) GaN high-electron mobility transistors were fabricated using two types of heterostructures with different buffer layer structures. The buffer layer structure affected the crystal defect density in grown AlGaN/GaN heterostructure. The static on-resistance under low applied voltage was independent of the buffer layer structure because it has no influence on the 2-D electron-gas density. On the other hand, the drain leakage current through the grown layers and the dynamic on-resistance increase caused by the current collapse phenomena depended on the buffer layer structure. The leakage current was reduced by the AlN/n-GaN/AlN layers because of the potential barrier at the AlN/n-GaN interface and no-depletion of the n-GaN layer. In addition, the experimental results showed that the dynamic on-resistance was increased with the edge dislocation density and was not influenced by the screw dislocation density. From these results, it can be expected that edge dislocation is related to the electron trapping center, which must be reduced to suppress the current collapse phenomena.     

A1.3 μm strained quantum well laser on a graded InGaAs buffer with a GaAs substrate

We grew a 1.3 μm strained-layer quantum well (SL-QW) laser with InGaP cladding layers on a lattice-relaxation buffer layer by metalorganic vapor phase epitaxy. For the lattice-relaxation buffer, we used a compositionally graded InGaAs/GaAs structure. The significantly reduced surface roughness of the InGaP cladding layers achieved by supplying a large amount of H₂Se enables CW operation of our 1.3 μm SL-QW laser. We achieved a low threshold current of less than 10 mA and a high characteristic temperature of 100K around room temperature.     

MOCVD-Grown AlGaN Buffer GaN HEMTs With V-Gates for Microwave Power Applications

We report the performance of AlGaN buffer GaN high-electron mobility transistors (HEMTs) grown by metal–organic chemical vapor deposition. GaN HEMTs on high-quality AlGaN buffer were grown on SiC substrates. The incorporation of an AlGaN buffer into the GaN HEMT significantly improves channel confinement and suppresses the short-channel effect. Advanced deep-recess V-gate structures were employed to optimize the device for better microwave power performance. With a 10-nm GaN channel layer sandwiched between the AlGaN barrier and buffer, excellent power performance was achieved. The output power density is 13.1 W/mm, and the associated power-added efficiency is 72% at 4-GHz frequency and 48-V drain bias. This power performance is comparable to the state-of-the-art GaN HEMTs grown on GaN buffers, indicating that the AlGaN buffer in our optimized device structure does not introduce any noticeable trapping.      

蓝宝石衬底上生长AlGaN/AlN结构的应变分析

采用低温和高温AlN复合缓冲层的方法在蓝宝石衬底上外延生长AlGaN/AlN结构,并进行了应变分析.通过X射线双晶衍射ω-2θ扫描曲线和拟合曲线分析发现,AlN是由两个不同弛豫度的应变缓冲层组成,弛豫度分别为96%和97.2%.AlN缓冲层在低温下不完全弛豫,导致高温下存在一个继续弛豫过程.X射线双晶衍射和透射光谱分析发现,AlGaN层Al组分由于未知的AlGaN弛豫度而无法确定.对AlGaN带隙公式和实验结果进行拟合,推算出弯曲系数为1.05 eV,这与已有文献相吻合.     

Analysis of the growth of GaN epitaxy on silicon

Due to the great potential of GaN based devices,the analysis of the growth of crack-free GaN with high quality has always been a research hotspot.In this paper,two methods for improving the property of the GaN epitaxial layer on Si (111) substrate are researched.Sample A,as a reference,only has an AlN buffer between the Si substrate and the epitaxy.In the following two samples,a GaN transition layer (sample B) and an AlGaN buffer (sample C) are grown on the AlN buffer separately.Both methods improve the quality of GaN.Meanwhile,using the second method,the residual tensile thermal stress decreases.To further study the impact of the two introduced layers,we investigate the stress condition of GaN epitaxial layer by Raman spectrum.According to the Raman spectrum,the calculated residual stress in the GaN epitaxial layer is approximately 0.72 GPa for sample B and 0.42 GPa for sample C.The photoluminescence property of GaN epitaxy is also investigated by room temperature PL spectrum.     

Scanning force microscopy studies of GaAs films grown on offcut Ge substrates

The surface morphology of GaAs films grown on offcut Ge substrates is studied using a scanning force microscope (SFM). We investigated the effects of the Ge buffer layer, growth temperature, film thickness, and prelayer on the GaAs surface morphology. The starting Ge substrates are offcut 6° toward the [110] direction to minimize single steps on the substrates before molecular beam epitaxial film growth. We find that comparing with GaAs samples grown without Ge buffer layers or with unannealed Ge buffer layers, samples with annealed Ge buffer layers are much smoother and contain no antiphase boundaries (APBs) on the surface. For thick (≥1 μm) GaAs films with an annealed Ge buffer layer, the surfaces display crosshatch lines and elongated mounds (along , which are associated with the substrate offcut direction. As the film thickness increases, the crosshatch lines become shorter, denser and rougher, and the mounds grow bigger (an indication of GaAs homoepitaxial growth). We conclude that annealed Ge buffer layers are crucial for growing high quality GaAs films with few APBs generated during the growth. In addition, under optimal conditions, different prelayers make little difference for thick GaAs films with annealed Ge buffer layers.     

Nitride Laser Diodes With Nonepitaxial Cladding Layers

Semiconductor laser diodes (LDs) typically use lattice-matched epitaxial layers as waveguide cladding materials. We describe a new LD architecture in which the upper cladding layer is replaced with an evaporated or sputtered nonepitaxial material. Designs and results are presented for 415-nm InGaN LDs that use indium tin oxide, silver, or a silver–palladium–copper alloy as the cladding material, chosen because the material exhibits requisite optical and electrical properties. The nonepitaxial cladding layer offers several advantages, such as eliminating the need to expose vulnerable InGaN active layers to the high temperatures required for growing conventional p-AlGaN cladding layers subsequent to the active layer growth.