GaN在Si(001)上的ECR等离子体增强MOCVD直接生长研究

研究了用电子回旋共振(ECR)等离子体增强金属有机物化学气相沉积(PEMOCVD)技术在Si(001)衬底上,低温(620～720℃)下GaN薄膜的直接外延生长及晶相结构.高分辨透射电镜(HRTEM)和X射线衍射(XRD)结果表明:在Si(001)衬底上外延出了高度c轴取向纤锌矿结构的GaN膜,但在GaN/Si(001)界面处自然形成了一层非晶层,其两个表面平坦而陡峭,厚度均匀(≈2nm).分析认为,在初始成核阶段N与Si之间反应所产生的这层SixNy非晶层使GaN的β相没有形成.XRD和原子力显微镜(AFM)结果表明,衬底表面的原位氢等离子体清洗,GaN初始成核及后续生长条件对GaN膜的晶体质量非常重要.     

GaN在Si(001)上的ECR等离子体增强MOCVD直接生长研究

研究了用电子回旋共振(ECR)等离子体增强金属有机物化学气相沉积(PEMOCVD)技术在Si(001)衬底上,低温(620～720℃)下GaN薄膜的直接外延生长及晶相结构.高分辨透射电镜(HRTEM)和X射线衍射(XRD)结果表明:在Si(001)衬底上外延出了高度c轴取向纤锌矿结构的GaN膜,但在GaN/Si(001)界面处自然形成了一层非晶层,其两个表面平坦而陡峭,厚度均匀(≈2nm).分析认为,在初始成核阶段N与Si之间反应所产生的这层SixNy非晶层使GaN的β相没有形成.XRD和原子力显微镜(AFM)结果表明,衬底表面的原位氢等离子体清洗,GaN初始成核及后续生长条件对GaN膜的晶体质量非常重要.     

硅衬底GaN基LED外延生长的研究

采用在AlN缓冲层后原位沉积SiN掩膜层,然后横向外延生长GaN薄膜.通过该法在硅衬底上获得了1.7 μm无裂纹的GaN薄膜,并在此基础上外延生长出了GaN基发光二极管(LED)外延片,其外延片的总厚度约为1.9 μm.采用高分辨率双晶X-射线衍射(DCXRD)、原子力显微镜(AFM)测试分析.结果表明,GaN薄膜(0002)面的半峰全宽(FWHM)降低到403 arcsec,其表面平整度得到了很大的改善;InGaN/GaN多量子阱的界面较平整,结晶质量良好.光致发光谱表明,GaN基LED峰值波长为469.2 nm.     

Structure of GaN films grown by molecular beam epitaxy on (0001) sapphire

GaN films grown by electron-cyclotron resonance plasma-assisted molecular beam epitaxy were studied by transmission electron microscopy and x-ray diffraction (XRD). Two sets of films were compared that were grown under identical conditions except for the ratio of the Ga to N flux. Films with a 30% higher Ga to N ratio (A films) were found to contain inversion domains (IDs). No IDs were found in films grown with a lower Ga to N ratio (B films), but instead the zinc-blende GaN was found near the film substrate interface. A narrower XRD rocking curve width along the (0002) direction and a broader rocking curve width along the asymmetric (1102) axis were found for A films compared to B films.     

Effect of In Situ Thermal Cycle Annealing on GaN Film Properties Grown on (001) and (111) GaAs, and Sapphire Substrates

The effect of in-situ thermal cycle annealing (TCA) has been investigated for GaN growth on GaAs(lOO), GaAs(111) and sapphire substrates. X-ray diffractometry (XRD) and surface morphology studies were performed for this purpose. Enhanced cubic phase characteristics were observed by employing annealingfor GaN layers grown on (001) GaAs. The thickness of the layer subject to annealing is critical in determining the phase of the subsequently grown layer. Thin initial layers appear to permit maintenance of the cubic phase characteristics shown by the substrate, while hexagonal phase characteristics are manifested for thick initial layers. Higher temperature of annealing of thick pre-annealed layers results in changes from mixed cubic/hexagonal phase to pure hexagonal phase. Growth on GaAs(111) substrates showed single cubic phase characteristics and similar enhancement of crystal quality by using TCA as for layers on GaAs(OOl). Micro-cracks were found to be present after TCA on GaAs(lll) substrates. Thermal cycling also appears to be beneficial for layers grown on sapphire substrates.     

Hot plasma chemical vapor deposition of GaN on GaAs(100) substrate

GaN films have been deposited on GaAs(lOO) substrates by a novel growth technique, hot plasma chemical vapor deposition. A radio frequency N plasma source with high power, up to 5 kW, provides an abundance of nitrogen atoms during growth. In addition, strong ultraviolet emissions from the hot plasma irradiate onto the substrate and promote the dissociation of triethylgallium, this results in growth of GaN at very low temperature (even at room temperature). In this paper, we describe the characteristics of hot nitrogen plasma and present the results of the low temperature growth of GaN. In addition, we have investigated the effects of the nitridation of GaAs substrates. Reflection high energy electron diffraction indicates the formation of a surface cubic nitrided layer on the pretreated GaAs. The GaN films grown on fully nitrided GaAs(l00) substrates are of dominantly cubic structures.     

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.     

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.     

磁控溅射和氨化法生长GaN纳米棒及其特性

使用一种新奇的稀土元素铽(Tb)作催化剂,通过氨化磁控溅射在Si(111)衬底上的Ga2O3/Tb薄膜,合成了大量的GaN纳米棒,氨化温度为950℃,氨化时间为15min。该方法可以进行持续合成且制备的GaN纳米棒纯度较高、成本低廉。实验后分别用扫描电子显微镜(SEM)、X射线衍射(XRD)、透射电子显微镜(TEM)、高分辨透射电子显微镜(HRTEM)和X射线光电子能谱(XPS)对样品进行了结构、表面形态和成分测试。通过XRD和XPS测试分析,合成的纳米棒具有六方纤锌矿GaN结构;通过SEM、TEM和HRTEM观察分析得出合成的纳米棒为单晶GaN纳米棒。简单讨论了GaN纳米棒的生长机制。     

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)衬底上蒸发了Au/Cr和Au/Ti/Al/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. 两种缓冲层的表面都比较平整和均匀,都是具有Au(111)面择优取向的立方相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AlN缓冲层直接生长GaN,得到的是多晶GaN;先在800℃生长一层AlN缓冲层,然后在710℃生长GaN,得到的是沿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.     

HVPE法制备GaN过程中GaAs衬底的氮化

在自制的立式氢化物气相外延(HVPE)系统炉中,一定温度下通入一定流量的NH3使GaAs(111)衬底氮化一层GaN薄膜,以防止高温外延生长GaN时GaAs分解,进而提高了之后GaN外延生长的晶体质量。实验主要通过XRD检测氮化层的质量,研究了氮化温度和时间对氮化层的影响。实验发现,氮化温度过高会使GaAs表面分解,氮化层为多晶。氮化时间过短,氮化层致密性低,不能起到保护衬底的作用;时间过长则氮化层质量降低,GaN(002)半高宽(FWHM)较大。分析结果表明,在500℃氮化2min的工艺条件下,获得的氮化层质量相比其他条件较好,致密性高。     

立方相GaN的高温MOCVD生长

利用MOCVD技术在提高生长温度(900℃)下生长出了高质量的立方相GaN,生长速度提高到1.6μm/h.高温生长的GaN样品近带边峰室温光荧光半高宽为48meV,小于在830℃下生长的GaN样品.在ω扫描模式下,X射线衍射表明高温生长的GaN具有较小的(002)峰半高宽21′.可以看出,尽管立方GaN是亚稳态,高生长温度仍然有利于其晶体质量的提高.本文对GaN生长中生长温度和生长速度之间的关系作了讨论.     

立方相GaN的高温MOCVD生长

利用MOCVD技术在提高生长温度(900℃)下生长出了高质量的立方相GaN,生长速度提高到1.6μm/h.高温生长的GaN样品近带边峰室温光荧光半高宽为48meV,小于在830℃下生长的GaN样品.在ω扫描模式下,X射线衍射表明高温生长的GaN具有较小的(002)峰半高宽21′.可以看出,尽管立方GaN是亚稳态,高生长温度仍然有利于其晶体质量的提高.本文对GaN生长中生长温度和生长速度之间的关系作了讨论.     

High quality AlGaN grown on a high temperature A1N template by MOCVD

A high temperature A1N template was grown on sapphire substrate by metalorganic chemical vapor deposition. AFM results showed that the root mean square of the surface roughness was just 0.11 nm. Optical transmission spectrum and high resolution X-ray diffraction (XRD) characterization both proved the high quality of the A1N template. The XRD (002) rocking curve full width at half maximum (FWHM) was about 53.7 arcsec and (102) FWHM was about 625 arcsec. The densities of screw threading dislocations (TDs) and edge TDs were estimated to be～6×10~6 cm~(-2) and ～4.7×10~9 cm~(-2). A1GaN of A1 composition 80.2% was further grown on the A1N template. The RMS of the surface roughness was about 0.51 nm. XRD reciprocal space mapping was carried out to accurately determine the Al composition and relaxation status in the A1GaN epilayer. The XRD (002) rocking curve FWHM of the A1GaN epilayer was about 140 arcsec and (102) FWHM was about 537 arcsec. The density of screw TDs was estimated to be ～4×10~7 cm~(-2) and that of edge TDs was～3.3×10~9 cm~(-2). These values all prove the high quality of the A1N template and AlGaN epilayer.     

Quantitative analysis of small amounts of cubic GaN phase in GaN films grown on sapphire

Thin GaN films, grown by metal organic chemical vapor deposition on the basal plane of sapphire substrates, were characterized by x-ray pole figures, high-resolution x-ray diffraction and transmission electron microscopy. This combination was found sensitive to small amounts (down to 0.1%) of cubic GaN phase in specimens subjected to surface nitridation treatment prior to epitaxial growth. The presence of the cubic phase and its orientation relations to the hexagonal GaN matrix was established by means of pole figures and selected area electron diffraction. The amount of cubic phase was determined by comparing the integrated x-ray diffraction intensities of the (311) cubic GaN and the (11.2) hexagonal GaN reflections. Optimum nitridation duration was found, which corresponds to almost complete suppression of the cubic phase formation.     

Effect of Growth Time on Properties of GaN Micro-grains Grown by Hot-wall CVD Method

GaN microcrystalline grains were grown by hot- wall chemical vapor deposition on Si (111) substrate. These grains with diameters of 2- 4 tim were detected by scanning electron microscopy. X-ray diffraction, Fourier transformation infrared transmission spectroscopy and photoluminescence were used to analyze the structure, composition and the optical properties of the samples. The results show that the microcrystalline grains are hexagonal wurtzite GaN, and the property of the grains was greatly affected by the growth time.     

Liquid-target reactive magnetron sputter epitaxy of High quality GaN(0001̄) nanorods on Si(111)

Direct current magnetron sputter epitaxy with a liquid Ga sputtering target has been used to grow single-crystal GaN(0001̄) nanorods directly on Si(111) substrates at different working pressures ranging from 5 to 20 mTorr of pure N₂. The as-grown GaN nanorods exhibit very good crystal quality from bottom to top without stacking faults, as determined by transmission electron microscopy. The crystal quality is found to increase with increasing working pressure. X-ray diffraction results show that the rods are highly (0001)-oriented. The nanorods exhibit an N-polarity, as determined by convergent beam electron diffraction methods. Sharp and well-resolved 4 K photoluminescence peaks at ~3.474 eV with a FWHM ranging from 1.7 meV to 35 meV are attributed to the intrinsic GaN band edge emission and corroborate the superior structural properties of the material. Texture measurements reveal that the rods have random in-plane orientation when grown on Si(111) with native oxide, while they have an in-plane epitaxial relationship of GaN[112̅0]//Si[11̄0] when grown on substrates without surface oxide.     

蓝宝石衬底上HVPE-GaN厚膜生长

采用氢化物气相外延(HVPE)方法,以蓝宝石作衬底,分别在MOCVD-GaN模板和蓝宝石衬底上直接外延生长GaN.模板上的GaN生长表面平整、光亮,但开裂严重.其(0002)的双晶衍射半高宽最低为141″;蓝宝石衬底上直接生长GaN外延层质量较差,其双晶衍射半高宽为1688″,但不发生开裂.HCl的载气流量对预反应有很大的影响.应力产生于外延层和衬底之间的界面处,界面孔洞的存在可以释放应力,减少开裂.光致发光(PL)谱中氧杂质引起强黄光发射.     

Si衬底GaN基多量子阱外延材料的TEM研究

本文采用高分辨透射电子显微技术对在Si衬底生长的GaN基多量子阱外延材料的位错特征、外延层与衬底的晶体取向关系及界面的结晶形态等微观结构进行了分析和研究.结果表明:Si衬底生长的GaN与衬底有一定的取向关系;材料在MQW附近的穿透位错密度达108 cm-2量级,且多数为刃型位错;样品A的多量子阱下方可见平行于界面方向的...     

蓝宝石衬底上HVPE-GaN厚膜生长

采用氢化物气相外延(HVPE)方法,以蓝宝石作衬底,分别在MOCVD-GaN模板和蓝宝石衬底上直接外延生长GaN.模板上的GaN生长表面平整、光亮,但开裂严重.其(0002)的双晶衍射半高宽最低为141″;蓝宝石衬底上直接生长GaN外延层质量较差,其双晶衍射半高宽为1688″,但不发生开裂.HCl的载气流量对预反应有很大的影响.应力产生于外延层和衬底之间的界面处,界面孔洞的存在可以释放应力,减少开裂.光致发光(PL)谱中氧杂质引起强黄光发射.