氮化铝单晶薄膜的ECR PEMOCVD低温生长研究

采用电子回旋共振等离子体增强金属有机物化学气相沉积（ECR－PEMOCVD）技术，在c轴取向的蓝宝石即α Al2O3（0001）衬底上，以氮化镓（GaN）缓冲层和外延层作为初始层，分别以高纯氮气（N2）和三甲基铝(TMAl)为氮源和铝源低温生长氮化铝（AlN）薄膜.并利用反射高能电子衍射（RHEED）、原子力显微镜（AFM）和x射线衍射（XRD）等测量结果，研究了氢等离子体清洗、氮化和GaN初始层对六方AlN外延层质量的影响，从而获得解理性与α Al2O3衬底一致的六方相AlN单晶薄膜，其XRD半高宽为1 关键词： AlN 氢等离子体清洗 氮化 GaN     

GaAs(001)衬底上分子束外延生长立方和六方GaN薄膜

采用分子束外延方法在GaAs(001)衬底上生长出了03微米厚的GaN薄膜,X射线双晶衍射和室温光荧光测试结果表明,采用GaAs氮化表面作为成核层可获得高纯度立方GaN薄膜而采用AlAs氮化表面作为成核层可获得高纯度六方GaN薄膜.这一研究结果表明在GaAs衬底上生长GaN薄膜的相结构可以通过选择不同的成核层来控制. 关键词：     

SiO2图形化蓝宝石衬底对GaN生长及LED发光性能的影响

为了提高氮化镓(GaN)基发光二极管(LEDs)的发光性能,采用等离子体增强化学气相沉积(PECVD)在蓝宝石衬底上沉积SiO_2薄膜,经过光刻和干法刻蚀技术制备了SiO_2图形化蓝宝石衬底(SiO_2 patterned sapphire substrate, SPSS),利用LED器件的外延生长和微纳加工技术获得了基于SPSS的GaN基LED器件。通过分析GaN外延层晶体质量、光提取效率和LED器件性能,重点研究了SPSS对GaN生长及LED发光性能的影响。实验及模拟仿真结果表明,与常规图形化蓝宝石衬底(Conventional patterned sapphire substrate, CPSS)相比,SPSS上生长的GaN外延层位错密度较低,晶体质量较高,SPSS-LED的光提取效率提高26%、光输出功率和亮度均提高约5%。     

AlN插入层对硅衬底GaN薄膜生长的影响

利用金属有机化合物气相外延沉积技术在2inch(5.08cm)Si(111)图形衬底上生长了GaN外延薄膜,在Al组分渐变AlGaN缓冲层与GaN成核层之间引入了AlN插入层,研究了AlN插入层对GaN薄膜生长的影响。结果表明,随着AlN插入层厚度的增加,GaN外延膜(002)面与(102)面X射线衍射摇摆曲线半峰全宽明显变小,晶体质量变好,同时外延膜在放置过程中所产生的裂纹密度逐渐减小直至不产生裂纹。原因在于AlN插入层的厚度对GaN成核层的生长模式有明显影响,较厚的AlN插入层使GaN成核层倾向于岛状生长,造成后续生长的nGaN外延膜具有更多的侧向外延成分,从而降低了GaN外延膜中的位错密度,减少了GaN外延膜中的残余张应力。同时还提出了一种利用荧光显微镜观察黄带发光形貌来表征GaN成核层形貌和生长模式的新方法。     

衬底温度对Al/Zn_3N_2薄膜制备的影响

运用等离子体发射光谱,分析衬底温度对氮化锌薄膜制备过程中各等离子体活性基团的影响,随着温度升高,N*2第一正系B3Πg→A3Σ+u,N*2第二正系C3Πu→B3Πg,N+*2第一负系B2Σ+u→X2Σ+g,Zn*以及Zn+*活性基团等离子体发射光谱特征谱线强度逐渐增强;由于衬底温度升高,腔室中各离子动能增加,使得碰撞电离加剧,导致N*2,N+*2,Zn*以及Zn+*活性基团的等离子体离子密度增加;等离子体发射光谱分析结果表明衬底温度在一定范围内升高有利于氮化锌薄膜生长。采用离子源辅助磁控溅射技术在Al薄膜上制备Zn3N2薄膜;X射线衍射图谱(XRD)分析结果表明:室温下,反应生长出单一择优取向面(321)氮化锌薄膜;随着温度的升高,在Al膜上反应生长的氮化锌薄膜择优取向面逐渐丰富,出现(222),(400),(600),(411),(332),(431)以及(622)择优取向面,体现出随着衬底温度的升高,薄膜的结晶度逐渐增加。XP-1台阶仪分析的结果表明,随着衬底温度的升高,氮化锌薄膜的沉积率逐渐增大。场效应扫描电子显微镜(SEM)图表明氮化锌薄膜晶粒尺寸随着衬底温度的升高逐渐变小,表面结构更加致密,晶粒排列更加有序;SEM断面扫描显示Al膜和氮化锌薄膜结合非常紧密。衬底温度影响薄膜性能实验分析结果与等离子体发射光谱分析的结果基本一致,体现出等离子体发射光谱了解等离子体内在特性的有效、快捷性。     

采用N2-RF等离子体氮化GaAs(001)

研究了在MBE系统中,GaAs(001)表面的氮化过程。GaAs(001)表面直接和间接地暴露在等离子体激发的N₂气流下。两种氮化过程显示了完全不同的表面氮化结果。在打开N₂发生器挡板的情况下,氮化导致GaAs(001)表面损伤,并且形成多晶结构。当增加N₂气压时,损伤变得更严重。但是,在关闭N₂发生器挡板的情况下,在500℃下,经过氮化将观察到(3×3)再构的RHEED花样,表面仍保持原子级的平整度。上述结果表明,不开N₂发生器挡板,低温(500℃下)氮化将在GaN外延生长之前形成平整的薄层c-GaN。     

金属有机化合物气相外延生长GaN薄膜的电子微结构研究

研究了金属有机化合物气相外延(MOVPE)方法在(0001)氧化铝基底上生长的GaN薄膜的微结构,目的在于解释GaN缓冲层在二步法生长过程中的作用及其对外延层晶体质量的影响.在缓冲层中观察到了高密度的结构缺陷,并发现了两种晶体结构(立方和六角)的GaN.进而对两种结构GaN的成因进行讨论,并对缓冲层和外延层中结构缺陷的关系进行了研究.     

在6H-SiC衬底上用低压MOVPE法生长AlN和GaN的过程中用(AlN/GaN)多层缓冲层来控制残余应力

人们已提出用BAlGaN四元系材料制备紫外光谱区的光发射器件.GaN和AlN二元系是这种四元材料在器件应用中的基础材料.6H-SiC衬底在氮化物生长中因其晶格失配小是一大优势,而且SiC衬底的热膨胀系数也和AlN的很接近.然而,对于AlN外延层来说,需要控制其中的残余应力,因为在SiC衬底上直接生长的AlN外延层中存在着因晶格失配所产生的压缩应力.另一方面,在SiC衬底上直接生长的GaN外延层中存在着拉伸应力.这种拉伸应力起源于GaN比衬底有着更大的热膨胀系数.本文讨论了在6H-SiC衬底上生长的氮化物外延层中残余应力的类型、数量及控制.为此目的,提出了在6H-SiC衬底上,无论是生长AlN,还是生长GaN,都可以采用(GaN/AlN)多层缓冲层的办法,作为控制残余应力的有效方法.我们还讨论了AlN和GaN外延层的结晶质量和残余应力间的关系.     

用MOVPE和RF-MBE方法极性控制生长GaN

本文报道了用低压MOVPE和RF-分子束外延法在蓝宝石衬底上作极性控制的GaN生长.以“双Al单层”模型讨论了用MOVPE和MBE法在蓝宝石衬底上生长GaN的极性选择的机理,并对AlN在极性转换过程中的作用给出了适当的解释.通过极性控制的生长,使MBE法生长的GaN的表面形貌和电学特性都得到了改善;并对LP-MOVPE生长开发出了一种“三步生长法”,这样就可以用更多的外延方式在蓝宝石衬底上生长出高质量的GaN膜.     

金属有机物化学气相沉积同质外延GaN薄膜表面形貌的改善

为了获得高质量的GaN薄膜材料,研究了金属有机物气相沉积系统中GaN插入层对GaN衬底同质外延层表面宏观缺陷和晶体质量的影响.研究发现,插入层生长温度是影响GaN同质外延膜表面形貌和晶体质量的关键因素.由于生长模式与插入层生长温度相关,随着插入层生长温度的降低,外延膜生长模式由准台阶流模式转变为层状模式,GaN同质外延膜表面丘壑状宏观缺陷逐渐减少,但微观位错密度逐渐增大.通过对插入层温度和厚度的优化,进一步调控外延层的生长模式,最终有效降低了外延层表面的宏观缺陷,获得了表面原子级光滑平整、位错密度极低的GaN同质外延膜,其X射线衍射摇摆曲线(002),(102)晶面半峰宽分别为125arcsec和85arcsec,表面粗糙度均方根大小为0.23nm.     

Passivation of GaAs(001) surface by the growth of high quality c-GaN ultra-thin film using low power glow discharge nitrogen plasma source

The benefits of using a low power glow discharge nitrogen plasma source to create high quality GaN layers on GaAs(001) surface are first highlighted. This uncommon type of plasma source has the particularity of working at a low power (3–10 W) and a low pressure (10^? 1 Pa) which induce creation of a small quantity of active nitrogen species. We put in evidence that this distinctiveness allows the growth of a stoichiometric and As-free GaN ultra-thin film on a GaAs(001) substrate by the mean of the inter-diffusion of As and N atoms. XPS, EELS, AFM are used to monitor surface composition and structure changes and to estimate the GaN thickness. A near saturation of the nitride layer thickness versus plasma exposure time is found. Furthermore, the possibility to crystallize the amorphous GaN layer by an annealing at 620 °C in a cubic structure with a lattice parameter close to that of c-GaN is put in evidence by means of TEM and LEED measurements. These measurements also show the homogeneity of the GaN thickness. In addition, the passivating effect of the GaN ultra-thin film to protect the GaAs surface is proved with the monitoring by XPS of the surface oxidation during several days of air exposure.     

Surfactant properties of cesium in molecular beam epitaxy of GaAs(100)

The experimental and ab initio investigations of the effect of a decrease in the binding energy of surface arsenic atoms under the cesium adsorption on an As-stabilized GaAs(001)-(2 × 4) surface have been performed. The cesium-induced redistribution of the charge on the surface atoms reduces the electron density in the As-Ga bond of the upper layer of the GaAs(001) surface; thus, the As-Ga binding energy decreases and, as a result, the diffusion activation energy, as well as the arsenic atom desorption, decreases. An increase in the diffusion coefficient of surface atoms, along with the property of Cs to segregate on the surface of a growing semiconductor film, makes it possible to use cesium as a surfactant in the low-temperature growth of GaAs by molecular beam epitaxy.     

Low-energy electron energy-loss spectroscopy with Ge:GaAs (110) heterostructures

Electronic properties and chemical composition of Ge films grown by molecular beam epitaxy on GaAs (110) surfaces were studied in situ by electron energy-loss spectroscopy. The loss peaks involving core-level excitations proved that As atoms segregate at the surface of the growing film. The well known 20 eV loss peak of the clean GaAs (110) surface, being attributed to transitions from Ga(3d) to Frenkel-type excitons of dangling-bond surface states, was found to persist, slightly shifted to 19.7 eV, with the growing Ge (110) film. Since the Ga coverage amounts to below approximately 0.05 of a monolayer this transition seems to contain a strong intraatom contribution.     

GaAs半导体表面的等离子氮钝化特性研究

采用射频(RF)等离子方法,对Ga As样品进行了150 W高功率等离子氮钝化及快速退火处理。经过该方法钝化后的样品,光致发光(PL)强度上升了91%。XPS分析得出,Ga As样品表面的氮化效果随着氮等离子体功率的增加而逐渐趋于明显。氮化后的样品表面未发现氧化物残余。样品在空气中加热放置30 d,PL强度下降不明显,说明表面钝化层具有良好的稳定性。     

Monte Carlo investigation of the influence of V/III flux ratio on GaAs/GaAs(001) submonolayer epitaxy

The influence of the V/III flux ratio on the submonolayer growth of GaAs on the GaAs (001) surface is simulated by the Monte Carlo method. Growth is carried out using the method of molecular beam epitaxy at different values of process parameters. The surface density of islands against the V/III flux ratio is calculated. The saturation value of the surface density at 580°C is found to be 2 × 10¹² cm^–2, which is in agreement with the experimental data. The V/III flux ratio influences the island density most strongly at a reduced temperature (550°C) and an elevated growth rate (a monolayer per second), when the arsenic desorption is weak. The fraction of arsenic atoms in the growing film is estimated under different process conditions. It has been shown that, as the surface coverage rises, the influence of the V/III flux ratio on the fraction of arsenic atoms becomes weaker.     

XPS study of the formation of ultrathin GaN film on GaAs(1 0 0)

The nitridation of GaAs(1 0 0) surfaces has been studied using XPS spectroscopy, one of the best surface sensitive techniques. A glow discharge cell was used to produce a continuous plasma with a majority of N atomic species. We used the Ga3d and As3d core levels to monitor the chemical state of the surface and the coverage of the species. A theoretical model based on stacked layers allows to determine the optimal temperature of nitridation. Moreover, this model permits the determination of the thickness of the GaN layer. Varying time of nitridation from 10 min to 1 h, it is possible to obtain GaN layers with a thickness between 0.5 nm and 3 nm.     

Improvement of Atomic-Layer-Deposited Al2O3/GaAs Interface Property by Sulfuration and NH3 Thermal Nitridation

Fermi level pinning at the interface between high-h gate dielectric and GaAs induced by unstable native oxides is a major obstacle for high performance GaAs-based metal-oxide-semiconductor （MOS） devices. We demonstrate the improved Al2O3/GaAs interracial characteristics by （NH4）2S immersion and NH3 thermal pretreatment prior to A1203 deposition. X-ray photoelectron spectroscopy （XPS） analysis confirms that sulfuration of GaAs surface by （NH4 ）2S solution can effectively reduce As-O bonds while Ga-O bonds and elemental As still exist at Al2O3 /GaAs interface. However, it is found that N incorporation during the further thermal nitridation on sulfurated GaAs can effectively suppress the native oxides and elemental As in the sequent deposition of Al2O3. Atomic force microscopy （AFM） shows that the further thermal nitridation on sulfurated GaAs surface can also improve the surface roughness.     

Surface chemistry and growth mechanisms studies of homo epitaxial (1 0 0) GaAs by laser molecular beam epitaxy

In this paper, GaAs thin film has been deposited on thermally desorbed (1 0 0) GaAs substrate using laser molecular beam epitaxy. Scanning electron microscopy, in situ reflection high energy electron diffraction and in situ X-ray photoelectron spectroscopy are applied for evaluation of the surface morphology and chemistry during growth process. The results show that a high density of pits is formed on the surface of GaAs substrate after thermal treatment and the epitaxial thin film heals itself by a step flow growth, resulting in a smoother surface morphology. Moreover, it is found that the incorporation of As species into GaAs epilayer is more efficient in laser molecular beam epitaxy than conventional molecular beam epitaxy. We suggest the growth process is impacted by surface chemistry and morphology of GaAs substrate after thermal treatment and the growth mechanisms are discussed in details.