以金属为缓冲层在Si(111)上分子束外延GaN及其表征

用电子束蒸发方法在Si(111)村底蒸发了Au/Cr和Au/Ti/AI/Ti两种金属缓冲层,然后在金属缓冲层上用气源分子束外延(GSMBE)生长GaN.两种缓冲层的表面部比较平整和均匀,都是具有Au(111)面掸优取向的立方相Au层.在Au/Cr/Si(111)上MBE生长的GaN,生长结束后出现剥离.在Au/Ti/Al/Ti/Si(111)上无AIN缓冲层直接生长GaN,得到的是多品GaN;先在800℃生长一层AIN缓冲层,然后在710℃生长GaN,得到的足沿GaN(0001)面择优取向的六方相GaN.将Au/Ti/Al/Ti/Si(111)在 800℃下退火20min,金属层收缩为网状结构,并且成为多晶,不再具有Au(111)方向择优取向.     

插入δAl/AlN缓冲层在Si(111)上生长GaN

采用MOCVD(metal organic chemical vapor deposition)生长方法,对比在AlN层上加入δAl/AlN缓冲层和不加入δAl/AlN缓冲层两种生长结构,在Si(111)衬底上生长GaN.实验结果表明,在加入δAl/AlN缓冲层后,GaN外延层的裂纹密度得到了有效的降低,晶体质量也得到了明显的提高.通过MOCVD生长方法,利用光学显微镜、XRD和Raman等分析测试手段,研究了δAl/AlN缓冲层对GaN外延层的影响,获得了裂纹密度小、晶体质量高的GaN材料.     

插入δAl/AlN缓冲层在Si(111)上生长GaN

采用MOCVD(metal organic chemical vapor deposition)生长方法,对比在AlN层上加入δAl/AlN缓冲层和不加入δAl/AlN缓冲层两种生长结构,在Si(111)衬底上生长GaN.实验结果表明,在加入δAl/AlN缓冲层后,GaN外延层的裂纹密度得到了有效的降低,晶体质量也得到了明显的提高.通过MOCVD生长方法,利用光学显微镜、XRD和Raman等分析测试手段,研究了δAl/AlN缓冲层对GaN外延层的影响,获得了裂纹密度小、晶体质量高的GaN材料.     

预辅Al及AlN缓冲层厚度对GaN/Si(111)材料特性的影响

主要研究了采用高温AlN缓冲层外延生长GaN/Si(111)材料的工艺技术。利用高分辨X射线双晶衍射(HRXRD)分析研究了GaN/Si(111)样品外延层的应变状态和晶体质量,通过原子力显微镜(AFM)分析研究了不同厚度的高温AlN缓冲层对GaN外延层的表面形貌的影响。实验结果表明,AlN缓冲层生长前预通三甲基铝(TMAl)的时间、AlN缓冲层的厚度对GaN外延层的应变状态、外延层的晶体质量以及表面形貌都有显著影响。得到最优的预辅Al时间为10s,AlN缓冲层的厚度为40nm。在此条件下外延生长的GaN样品(厚度约为1μm)表面形貌较好,X射线衍射(XRD)双晶摇摆曲线半峰全宽(FWHM)(0002)面和(10-12)面分别为452″和722″。     

在复合衬底γ-Al2O3/Si(001)上生长GaN

采用分子束外延(MBE)生长方法,使用γ-Al2O3材料作为新型过渡层,在Si(∞1)衬底上获得了没有裂纹的GaN外延层,实验结果表明使用γ-Al2O3过渡层有效地缓解了外延层中的应力.通过生长并测试分析几种不同结构的外延材料,研究了复合衬底γ-Al2O3/Si(001)生长GaN情况,得到了六方相GaN单晶材料,实现了GaN c面生长.预铺薄层Al及高温AlN层可以提高GaN晶体质量,低温AlN缓冲层可以改善GaN表面的粗糙度.为解决Si(001)衬底上GaN的生长问题提供了有益的探索.     

在复合衬底γ-Al2O3/Si(001)上生长GaN

采用分子束外延(MBE)生长方法,使用γ-Al2O3材料作为新型过渡层,在Si(∞1)衬底上获得了没有裂纹的GaN外延层,实验结果表明使用γ-Al2O3过渡层有效地缓解了外延层中的应力.通过生长并测试分析几种不同结构的外延材料,研究了复合衬底γ-Al2O3/Si(001)生长GaN情况,得到了六方相GaN单晶材料,实现了GaN c面生长.预铺薄层Al及高温AlN层可以提高GaN晶体质量,低温AlN缓冲层可以改善GaN表面的粗糙度.为解决Si(001)衬底上GaN的生长问题提供了有益的探索.     

复合氮化铝压电薄膜研制及其应用

采用中频(MF,40 kHz)双S枪磁控反应溅射制备出了氮化铝(AlN)压电薄膜;采用直流磁控溅射法制作了Mo电极薄膜;采用脉冲DC磁控溅射Au、Cr、Al靶分别制作Au/Cr底电极薄膜及Al/Cr顶电极薄膜。通过对AlN压电薄膜、Mo及Au电极薄膜进行了X线衍射(XRD)分析,结果表明,复合AlN压电薄膜(002)面、Mo薄膜(110)面及Au薄膜(111)面择优取向优良,说明选用Al/Cr/AlN/Au/Cr/YAG复合结构压电薄膜能研制出Ku波段及K波段声体波微波延迟线(BAWDL), 其Ku及K波段BAWDL器件插入损耗分别低至43.7 dB、54.6 dB。     

100mm直径硅衬底上MOCVD外延生长无裂纹GaN

为实现在100 mm直径硅衬底上金属有机物化学气相外延(MOCVD)生长无裂纹GaN,系统研究了预铺铝时间参数对AlN成核质量的影响、不同厚度条件下AlN层对GaN生长的影响以及Al组分渐变的AlGaN缓冲层对GaN裂纹抑制的效果。实验结果表明,适当的预铺铝时间可以显著提高AlN成核层晶体质量,合理的AlN层厚度有助于上层GaN的生长,AlN与GaN之间AlGaN缓冲层的引入可以有效抑制GaN裂纹的产生。最后,采用高分辨率X射线衍射(HRXRD)测试了MOCVD外延生长的无裂纹GaN材料,得到AlN(002)面、GaN(002)面和GaN(102)面的衍射峰半峰宽(FWHM)分别为1 382,550和746 arcsec。     

Si衬底GaN外延材料六角形缺陷分析

通过改变AlN形核层的生长温度分别在Si(111)衬底上生长了两个GaN样品,并对GaN外延材料表面的六角形缺陷进行了分析研究。通过显微镜和扫描电镜(SEM)观测发现,AlN形核层在高温下生长时,GaN材料表面会产生大量六角形缺陷。通过电子能谱(EDS)分析得出GaN六角形缺陷中含有大量的Si元素以及少量的Ga和Al元素,其中Si元素从Si衬底中高温扩散而来。在降低AlN形核层的生长温度后,GaN材料表面的六角形缺陷随之消失。表明AlN形核层在较低的温度下生长时可以有效地抑制Si衬底表面Si原子的扩散,减少外延层中由于衬底Si反扩散引起的缺陷。     

MOCVD生长AlGaN/GaN/Si(111)异质结材料

利用MOCVD在Si(111)衬底上生长了无裂纹的GaN外延薄膜和AlGaN/GaN异质结构。通过优化Si衬底的浸润处理时间、AlN层厚度等参数获得了无裂纹的GaN外延薄膜,研究了SiN缓冲层和插入层厚度对AlGaN/GaN异质结电学性质的影响,2DEG的迁移率和面密度分别达到1410cm2/V.s和1.16×1013cm-2。     

Spontaneous growth of GaN nanowire nuclei on N- and Al-polar AlN: A piezoresponse force microscopy study of crystallographic polarity

The polarity of gallium nitride (GaN) nanowire nuclei grown on AlN layers was studied by piezoresponse force microscopy (PFM). N- or Al-polar AlN layers were grown by molecular beam epitaxy (MBE) on Si (111) substrates by use of Al- or N-rich growth conditions, respectively. Short and low density GaN nanowires were then grown on each AlN polarity type. PFM measurements verified the expected AlN layer polarity and further indicated that predominantly N-polar nanowires are produced for growth on both AlN polarity types. Cross-section scanning transmission electron microscopy (STEM) images further reveal that the nanowires on Al-polar AlN films are nucleated on regions in the AlN layer that contain inversion domains, which propagate into the GaN nanowire nuclei. PFM measurements were found to be a convenient technique for mapping the polarity of a statistically significant number of individual GaN nanowires.     

Formation of Ta/Ti/Al/Mo/Au ohmic contacts to an AlGaN/AlN/GaN heterostructure grown on a silicon substrate

The features of the formation of Ta/Ti/Al/Mo/Au ohmic contacts to a Al_0.26Ga_0.74N/AlN/GaN heterostructure grown on semi-insulating Si(111) substrates are studied. The dependences of the contact resistance on the Al (90, 120, 150, 180 nm) and Ti (15, 30 nm) layer thickness and optimal temperature-time annealing conditions are determined for each studied metallization scheme. It is shown that the minimum achievable contact resistance monotonically increases from 0.43 to 0.58 Ω mm as the Al layer thickness increases from 90 to 180 nm at unchanged Ta, Ti, Mo, Au layer thicknesses. A change in the Ti layer thickness from 15 to 30 nm has no significant effect on the minimum contact resistance. The least contact resistance of 0.4 Ω mm is achieved for Ta/Ti/Al/Mo/Au layers with thicknesses of 10/15/90/40/25 nm, respectively. The optimal annealing temperature for this metallization variant is 825°C at a process duration of 30 s. The grown ohmic contacts have smooth contact-area edges and flat morphology of their surface.     

Comparison of various buffer schemes to grow GaN on large-area Si(111) substrates using metal-organic chemical-vapor deposition

A comparison of gallium-nitride (GaN) films grown on large-area Si(111) using a single aluminum-nitride (AlN) buffer, an AlN/graded-Al_xGa_1−xN buffer, and the introduction of additional low-temperature (LT)-grown AlN interlayers is reported. A graded-AlGaN buffer followed by additional LT-AlN interlayers is shown to completely eliminate cracking in nitride films of thickness >2 μm and also reduce the threading-dislocation density significantly. A partial compensation of GaN-tensile strain by the compressive-lattice strain induced by the AlGaN and AlN layers is responsible for this effect. The surface roughness is increased by the introduction of the LT-AlN buffers.     

可用于Ⅲ族氮化物生长的50mm 3C-SiC/Si(111)衬底的制备

利用新研制出的垂直式低压CVD(LPCVD)SiC生长系统,获得了高质量的50mm 3C-SiC/Si(111)衬底材料.系统研究了3C-SiC的n型和p型原位掺杂技术,获得了生长速率和表面形貌对反应气体中SiH4流量和C/Si原子比率的依赖关系.利用Hall测试技术、非接触式方块电阻测试方法和SIMS,分别研究了3C-SiC的电学特性、均匀性和故意调制掺杂的N浓度纵向分布.利用MBE方法,在原生长的50mm 3C-SiC/Si(111)衬底上进行了GaN的外延生长,并研究了GaN材料的表面、结构和光学特性.结果表明3C-SiC是一种适合于高质量无裂纹GaN外延生长的衬底或缓冲材料.     

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.     

Optical and electrical characterization of GaN layers grown on silicon and sapphire substrates

Characterization of the structural, optical and electrical properties of GaN layers grown by two epitaxial techniques (ECR-MBE and MOCVD) using different substrates (vicinal Si111 and sapphire) has been performed. The quality of the samples grown by MOCVD seems to be influenced by the nitrogen source used for the growth. Unintentionally doped MBE samples with n-type concentrations around 10¹⁸ cm⁻³ and Hall mobility of 15 cm² (V s)⁻¹ were studied. GaN films doped with Mg and grown using AlN buffer layers have also been analyzed to study the influence of the thickness of the buffer layer on the optical properties of the GaN epilayer. In the samples with low Mg doping, a thin AlN buffer layer improved the optical quality of the film. In general, all the MBE samples doped with Mg were highly resistive, probably due to a low activation or high ionization energy of the Mg acceptors. Technological issues related to the formation of ohmic contacts on GaN layers are also presented.     

Pendeo-epitaxial growth of gallium nitride on silicon substrates

Pendeo-epitaxy (PE)¹ from raised, [0001] oriented GaN stripes covered with silicon nitride masks has been employed for the growth of coalesced films of GaN(0001) with markedly reduced densities of line and planar defects on Si(111)-based substrates. Each substrate contained previously deposited 3C-SiC(111) and AlN(0001) transition layers and a GaN seed layer from which the stripes were etched. The 3C-SiC transition layer eliminated chemical reactions between the Si and the NH₃ and the Ga metal from the decomposition of triethylgallium. The 3C-SiC and the GaN seed layers, each 0.5 μm thick, were also used to minimize the cracking and warping of the GaN/SiC/silicon assembly caused primarily by the stresses generated on cooling due to the mismatches in the coefficients of thermal expansion. Tilting in the coalesced GaN epilayers of 0.2° was confined to areas of lateral overgrowth over the masks; no tilting was observed in the material suspended above the trenches. The strong, low-temperature PL band-edge peak at 3.456 eV with a FWHM of 17 meV was comparable to that observed in PE GaN films grown on AlN/6H-SiC(0001) substrates.     

Growth optimization and doping with Si and Be of high quality GaN on Si(111) by molecular beam epitaxy

GaN layers have been grown by plasma-assisted molecular beam epitaxy on AlN-buffered Si(111) substrates. An initial Al coverage of the Si substrate of aproximately 3 nm lead to the best AlN layers in terms of x-ray diffraction data, with values of full-width at half-maximum down to 10 arcmin. A (2×2) surface reconstruction of the AlN layer can be observed when growing under stoichiometry conditions and for substrate temperatures up to 850°C. Atomic force microscopy reveals that an optimal roughness of 4.6 nm is obtained for AlN layers grown at 850°C. Optimization in the subsequent growth of the GaN determined that a reduced growth rate at the beginning of the growth favors the coalescence of the grains on the surface and improves the optical quality of the film. Following this procedure, an optimum x-ray full-width at half-maximum value of 8.5 arcmin for the GaN layer was obtained. Si-doped GaN layers were grown with doping concentrations up to 1.7×10¹⁹ cm⁻³ and mobilities approximately 100 cm²/V s. Secondary ion mass spectroscopy measurements of Be-doped GaN films indicate that Be is incorporated in the film covering more than two orders of magnitude by increasing the Be-cell temperature. Optical activation energy of Be acceptors between 90 and 100 meV was derived from photoluminescence experiments.     

磁控溅射生长Cu/Si薄膜

用溅射方法在Si(111)上生长Cu/Si,Ti/Si,Cu/Ti/Si薄膜。用XRD,红外吸收光谱,台阶仪对薄膜进行分析和测量。结果表明:在150℃溅射生长出的Cu/Ti/Si薄膜的缓冲层为硅化物TiSi2(311);Cu薄膜的主要成分是晶粒大小为17nm的Cu(111);Cu/Ti/Si(111)平均厚度为462nm,粗糙度为薄膜厚度的3%。在以TiSi2薄膜为缓冲层的Si(111)衬底上生长出的Cu薄膜抗氧化性较强、薄膜均匀性和致密性较好。