Oscillation of the mirror and fractional RHEED reflections during homoepitaxy on the (2×4)-reconstructed GaAs(001) surface

Oscillations of the intensity of mirror and fractional RHEED reflections during homoepitaxy on the GaAs(001)-(2×4) reconstructed surface were studied. A considerable difference was observed in the patterns of intensity variation for the mirror and the fractional (0 1/4) and (0 3/4) reflections corresponding to the α and β phases on the reconstructed surface. A kinetic scheme of elementary processes occurring on the Ga(001) surface upon the homoepitaxial growth initiation is proposed. The activation energy for the nucleation process was experimentally determined (5-eV). It is shown that the temperature dependence of the probability of critical nucleus formation is determined by the desorption of As₂ dimers.     

As4 incorporation kinetics in GaAs (001) molecular-beam epitaxy

A kinetic model of epitaxial growth on a Ga-stabilized GaAs (001) surface from As₄ and Ga beams is proposed. Elementary surface processes are studied: adsorption-desorption of As₄, bimolecular reaction of As₄*, and incorporation of As ₂ ^chem in lattice sites. The model correctly describes the experimental results for the growth rate at low and high As₄ pressures. The role of As₄ desorption from the surface in the epitaxial growth of GaAs crystals is analyzed. Pis’ma Zh. Tekh. Fiz. 24, 31–38 (April 12, 1998)     

Growth mode transitions induced by hydrogen-assisted MBE on vicinal GaAs(110)

The homoepitaxial growth of GaAs by hydrogen-assisted molecular beam epitaxy (H-MBE) on (110) substrates vicinal to (111)A has been studied by reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM) for different kinetic regimes. When the GaAs growth rate is limited by the kinetics of adatom incorporation to steps, the presence of chemisorbed H on the surface after oxide removal promotes the incorporation of adatoms to steps from the lower terraces, leading to the formation of multiatomic step arrays or ridge patterns by a combination of step propagation and two-dimensional layer-by-layer growth. Supply of atomic H during epitaxy favours three-dimensional growth, leading to Ga-induced surface roughening or mound formation. At high temperatures, the Ga–As interactions at step edges are faster and stable growth of GaAs occurs by step propagation, leading to a faceted surface when H is used both during oxide removal and/or MBE growth.     

不同应力下的InxGa1-xAs薄膜表面形貌

利用分子束外延技术,通过反射式高能电子衍射仪实时监控InGaAs薄膜生长状况,在InAs(001)基片上生长In0.86Ga0.14As,在GaAs(001)基片上生长In0.14Ga0.86As(厚度均为20原子层)单晶薄膜。采用扫描隧道显微镜对原位退火后的InGaAs样品进行扫描,发现不同组分的InGaAs呈现不同的表面形貌。虽然生长的初始表面都是原子级平坦,但是由于晶格常数差异触发不同类型的表面应力,促使In0.14Ga0.86As/GaAs薄膜中台阶边缘平滑扭曲,而在In0.86Ga0.14As/InAs薄膜表面台阶却呈锯齿状;同时,由于不同类型表面应力的作用,低In组分薄膜形成更多的二维(2D)岛。     

Observation of Ga droplet formation on (311)A and (511)A GaAs surfaces

Using (100) GaAs substrates as a reference, we present a study of the formation of Ga droplets on (311)A and (511)A GaAs substrates in which the effect of both the substrate temperature and the amount of Ga supplied on the droplet density and height for the three different surfaces have been investigated. Droplets on (100) substrates show a round shape; however, they appear as elongated balls with tails along the [Formula: see text] direction of the (311)A substrate and the [Formula: see text] direction of the (511)A substrate. It has been found that the Ga droplets on (511)A surfaces have lower densities and higher heights than those on (100) substrates. In contrast, Ga droplets on (311)A surfaces have lower heights and much higher densities compared to those for both (100) and (511)A. We observed that the decrease in the droplet density with increasing growth temperature for both (311)A and (511)A is more than twice that for the (100)GaAs surface due to the larger drop in the nucleation rate. Based on these observations, we offer a physical explanation based on the thermodynamics and the anisotropy of the high-index surfaces.     

GaAs:Mn nanowires grown by molecular beam epitaxy of (Ga,Mn)as at MnAs segregation conditions

GaAs:Mn nanowires were obtained on GaAs(001) and GaAs(111)B substrates by molecular beam epitaxial growth of (Ga,Mn)As at conditions leading to MnAs phase separation. Their density is proportional to the density of catalyzing MnAs nanoislands, which can be controlled by the Mn flux and/or the substrate temperature. After deposition corresponding to a 200 nm thick (Ga,Mn)As layer the nanowires are around 700 nm long. Their shapes are tapered, with typical diameters around 30 nm at the base and 7 nm at the tip. The wires grow along the 111 direction, i.e., along the surface normal on GaAs(111)B and inclined on GaAs(001). In the latter case they tend to form branches. Being rooted in the ferromagnetic semiconductor (Ga,Mn)As, the nanowires combine one-dimensional properties with the magnetic properties of (Ga,Mn)As and provide natural, self-assembled structures for nanospintronics.     

MBE生长GaAs薄膜表面形貌的RHEED图样研究

本文通过在GaAs(100)单晶衬底上MBE生长GaAs过程中形成的RHEED衍射图样,对GaAs薄膜的表面形貌进行研究。分析GaAs表面粗糙和生长时不发生RHEED强度振荡的原因。讨论在生长GaAs时出现In(Ga)As/GaAs(100)体系的RHEED衍射图样这种异常现象的原因。     

GaAs(001)薄膜的表面形貌相变和表面重构

从原子级平坦的GaAs(001)-β2(2×4)重构表面出发,结合Reflection High Energy Electron Diffraction(RHEED)衍射图像演变和不同尺度的Scanning tunneling microscope(STM)实空间扫描图像,获取GaAs(001)薄膜表面形貌相变和表面重构的重要信息,深入地研究GaAs(001)表面形貌相变和表面重构的相互促进关系。研究发现表面重构的变化是促使表面形貌发生相变的主要动力,单一表面重构组成的GaAs(001)表面形貌更容易处于有序平坦相,GaAs(001)表面预粗糙相则是由两种同类型或者重构原胞差异很小的表面重构交织混合形成,当表面由两种完全不同类型的表面重构交错混合形成时GaAs(001)表面形貌将进入粗糙状态。研究结果表明GaAs(001)表面重构是表面形貌发生相变过程的微观内在原因,而GaAs(001)表面形貌相变是表面重构发生变化的宏观外在体现。     

STM studies of island formation and surface ordering of Si on GaAs (001), (2×4) and c(4×4): Implications for δ-doping

Scanning tunnelling microscopy (STM) and reflection high energy electron diffraction (RHEED) have been used to study the deposition of Si below 400C onto GaAs (001) surfaces grown in situ by molecular beam epitaxy (MBE). The emphasis is on the island formation and growth, as well as surface ordering, for submonolayer quantities of Si (up to 0.2 ML) deposited on two different As-rich reconstructions of GaAs (0 0 1) (2 × 4) and c(4 × 4). For deposition on the c(4 × 4) surface, an asymmetric (3 × 1) RHEED pattern is formed, a consequence of anisotropic needle-like islands, which grow adjacent to each other along the [1 1 0] direction and produce a three-fold periodic superstructure. Individual islands grow by a site exchange process in which the additional As layer of the c(4 × 4) structure acts as a surfactant and enables the Si atoms to occupy Ga sites in the GaAs lattice. In contrast, Si deposition on the (2 × 4) surface does not lead to any new surface periodicities as monitored by RHEED. The Si atoms form poorly ordered clusters distributed randomly across the surface. The site exchange process cannot occur in this case as the (2 × 4) surface is terminated with only one layer of arsenic. Instead, the Si atoms occupy sites on top of the outer arsenic layer.     

Methylthiolate adsorbed on as-rich GaAs (001) surface

The adsorption of the methylthiolate (MT) on the as-rich GaAs (001) surface has been studied by using density functional theory (DFT) calculations with a three-dimensional periodic boundary condition. A complete characterization of structures and binding energies of the system consisting of MT and As-rich GaAs (001) surface is obtained. It is found that the most reactive binding site is related to empty Ga dangling bonds located at the threefold-coordinated second-layer Ga atom. Moreover, electronic properties of these structures are also calculated to study the bonding characteristics of S–Ga and S–As bonding, which show that the covalent bonding of the former is stronger than that of the latter. The analysis for this shortest chain binding is helpful to realize the electrical passivation and chemical protection of GaAs surfaces.     

Vertically standing Ge nanowires on GaAs(110) substrates

The growth of epitaxial Ge nanowires is investigated on (100), (111) B and (110) GaAs substrates in the growth temperature range from 300 to 380?°C. Unlike epitaxial Ge nanowires on Ge or Si substrates, Ge nanowires on GaAs substrates grow predominantly along the [Formula: see text] direction. Using this unique property, vertical [Formula: see text] Ge nanowires epitaxially grown on GaAs(110) surface are realized. In addition, these Ge nanowires exhibit minimal tapering and uniform diameters, regardless of growth temperatures, which is an advantageous property for device applications. Ge nanowires growing along the [Formula: see text] directions are particularly attractive candidates for forming nanobridge devices on conventional (100) surfaces.     

Reflection high energy electron diffraction and X-ray studies of AlN films grown on Si(111) and Si(001) by organometallic chemical vapour deposition

The crystallinity of AlN films on silicon substrates grown by organometallic chemical vapour deposition was investigated using X-ray diffraction and reflection high energy electron diffraction (RHEED). Single-crystal films of good quality with atomically smooth surfaces can be epitaxially grown on Si(111) substrates. Epitaxial films can also be grown on Si(001) substrates. These films have previously been reported to have a fibre structure. Different RHEED patterns were observed from the films on Si(111) and Si(001). It is established that the films grown on Si(001) consist of two types of crystallite with the following orientations: [11$\text{2}$0]_AlN//[110]_Si and [11$\text{2}$0]_AlN//[1$\text{1}$0]_Si The thickness dependence of the crystallinity was also investigated. The standard deviation σ of the X-ray rocking curve for the films grown on Si(111) is less than that for the films on Si(001) and is independent of the film thickness. The σ values for the films on Si(001) decrease markedly with increasing film thickness. On the basis of these observations, the growth mechanism of AlN epitaxial films on Si(111) and Si(001) is discussed.     

Chemistry and structure of GaAs surfaces cleaned by sulfur annealing

A new method for cleaning and passivating GaAs surfaces, sulfur annealing, is proposed. The GaAs surfaces are exposed to sulfur atoms generated by an Ag/AgI/Ag₂S/Pt electrochemical cell at elevated substrate temperatures without arsenic beam irradiation. The chemistry and structure of the GaAs surfaces cleaned by sulfur annealing are studied by synchrotron radiation photoemission spectroscopy and low energy electron diffraction (LEED). The S 2p and Ga 3d spectra indicate that Ga---S bonds are formed on the GaAs surfaces where no oxides remain. 4 × 1 and 2 × 1 LEED patterns were observed for the surfaces with an average sulfur layer thickness of 0.15 nm and 0.24 nm respectively. The surface band bending is found to be reduced by 0.2–0.3 eV for the sulfur-annealed surfaces, but slightly increased for the conventionally cleaned surfaces with arsenic beam irradiation. We conclude that the surface cleaning and the submonolayer sulfur passivation can be attained simultaneously by sulfur annealing without arsenic beam irradiation. The method is promising for pre-cleaning the GaAs surfaces before semiconductor crystal growth and thin film deposition.     

RHEED实时监控下MBE生长不同In组分的InGaAs薄膜

利用分子束外延技术,通过RHEED图像演变实时监控薄膜生长状况,采用RHEED强度振荡测量薄膜生长速率,固定Ga源温度、改变In源温度在GaAs（001）基片上外延生长了不同In组分（39%、29%、19%）的InGaAs薄膜。比较RHEED强度振荡以及RHEED衍射图像,发现随着In组分的增加In-GaAs的生长将很快进入三维粗糙表面生长模式,并指出In0.19Ga0.81As和In0.29Ga0.71As薄膜处于（2×3）表面重构相。In0.19Ga0.81As样品进行退火处理后完成STM扫描分析,证实样品为表面原子级平整的In-GaAs薄膜。     

Spatial Structure of a Single Mn Impurity State on GaAs (110) Surface

Using a low temperature scanning tunneling microscope (STM), we have manipulated individual Mn adatoms at the GaAs (110) surface to apparently bond with two surface As atoms. In this configuration the Mn atoms, which either are at an interstitial site or have substituted for a surface Ga atom, give rise to strong in-gap levels as probed by spatially resolved STM spectroscopy measurements. Mapping the Mn-induced in-gap bound state shows an unusual spatial structure, with highly anisotropic character. The bound state shares some characteristic features with subsurface Mn and Zn dopants.     

The formation of surface arsenic oxide crystals on GaAs

The growth of small crystals of dimensions up to approximately ten microns was observed on the surface of GaAs(100) wafers upon air exposure. The composition and the formation mechanism of these crystals were studied by secondary electron micros-copy, scanning Auger microscopy and X-ray photoelecton spectroscopy. It was found that the crystals consist mainly of As₄O₆. The growth of these surface oxide crystals is attributed to the surface migration and nucleation of the arsenic oxides.     

Growth of oriented Ag nanocrystals on air-oxidized Si surfaces: An in-situ reflection high energy electron diffraction study

Growth of Ag nanoislands on air-oxidized Si(001), (111) and (110) surfaces has been investigated by reflection high energy electron diffraction (RHEED), scanning tunneling microscopy (STM) and cross-sectional transmission electron microscopy. We have shown that the oriented nanocrystalline Ag, similar to the epitaxial growth of Ag on clean Si surfaces, can be grown on oxide-covered Si surfaces. A thin oxide layer (~ 2-3 nm thick) is formed on ultra-high vacuum (UHV)-cleaned Si surfaces via exposure of the clean reconstructed surface to air. Deposition of Ag was carried out under UHV at different substrate temperatures and monitored by RHEED. RHEED results reveal that Ag deposition at room temperature leads to the growth of randomly oriented Ag islands while, in spite of the presence of the oxide layer between Ag islands and Si, preferred orientations with an epitaxial relationship with the substrate evolve when Ag is deposited at higher substrate temperatures. STM images of the oxidized surfaces, prior to Ag deposition, apparently do not show any order. However, Fourier transforms of STM images show the presence of a short range order on the oxidized surface following the unit cells of the underlying reconstructed Si surface. It is intriguing that Ag nanoislands follow an epitaxial orientational relationship with the substrate in spite of the presence of a 2-3 nm thick oxide layer between Ag and Si. Apparently, the short range order existing on the oxide surface influences the orientation of the Ag nanoislands.     

自组织InAs/GaAs与InGaAs/GaAs量子点生长及退火情况的比较

本文采用MBE进行InAs/GaAs与InGaAs/GaAs量子点的生长,利用RHEED进行实时监测,并利用RHEED强度振荡测量生长速率。对生长的InAs/GaAs和InGaAs/GaAs两种量子点生长过程与退火情况进行对比,观察到当RHEED衍射图像由条纹状变为网格斑点时,InAs所需要的时间远小于InGaAs;高温退火下RHEED衍射图像恢复到条纹状所需要的时间InAs比InGaAs要长。     

Different shape of GaAs quantum structures under various growth conditions

We report the influences of growth parameters on the characteristics of GaAs quantum rings (QRs) and quantum dots (QDs) formed on AlGaAs/GaAs by the droplet epitaxy (DE) method. After forming Ga droplets on the AlGaAs/GaAs surface, varying amounts of arsenic (As) flux were introduced to fabricate the GaAs quantum structures. By decreasing the As flux from 8 × 10^− 5 to 3 × 10^− 5 Torr, the shape of the GaAs quantum structures was changed from QDs to elongated QRs. With further decreasing As flux, the shape of the elongated QRs became symmetric. The formation characteristics of the GaAs QRs from the QDs with the amount of As flux were discussed in terms of migration behaviors of the gallium (Ga) atoms on the GaAs(001)-c(4 × 4) surface. The effects of the amount of Ga supply and the growth temperature for the deposition of Ga droplets on the formation of the GaAs quantum structures were also considered.     

Morphology and optical properties of GaAs layers grown on (1 1 1)A GaAs substrates in N2 ambient by metalorganic chemical vapor deposition

Using tertiary-butyl arsine as a group V precursor, GaAs epitaxial layers have been grown on both (1 1 1)A- and (1 0 0)-oriented GaAs substrates in N₂ ambient using the atmospheric metalorganic chemical deposition technique. The surface morphology and optical properties of the GaAs homoepitaxial layers were studied in detail. It is found that both the surface morphologies and optical properties of GaAs layers on (1 1 1)A GaAs substrates depend much more strongly on the combination of growth temperature and V/III input molar ratio, compared to the epitaxial layers on (1 0 0) plane GaAs. The photoluminescence (PL) emission efficiency increases with increasing growth temperature in the region 550–650 °C for both (1 1 1)A and (1 0 0) GaAs epitaxial layers. At room temperature, the PL emission intensity of the (1 1 1)A GaAs epilayer grown under optimum growth conditions is 67 times as strong as that of the same run-grown (1 0 0) GaAs epilayer.