Stacking-faults-free zinc blende GaAs/AlGaAs axial heterostructure nanowires during vapor-liquid-solid growth

Pure zinc blende structure GaAs/AlGaAs axial heterostructure nanowires (NWs) are grown by metal organic chemical vapor deposition on GaAs(111) B substrates using Au-catalyzed vapor-liquid-solid mechanism.Al adatom enhances the influence of diameters on NWs growth rate.NWs are grown mainly through the contributions from the direct impingement of the precursors onto the alloy droplets and not so much from adatom diffusion.The results indicate that the droplet acts as a catalyst rather than an adatom collector.     

Morphological anisotropy during migration enhanced epitaxy of GaAs(0 0 1)

The GaAs(0 0 1) surface morphology and structure during growth by migration enhanced epitaxy (MEE) has been studied by reflection high energy electron diffraction and scanning tunneling microscopy. Changes induced by varying the incident As/Ga flux ratio, growth temperature and the total amount of material deposited in each cycle have been studied and the results compared with GaAs(0 0 1) growth by conventional molecular beam epitaxy (MBE). Comparison of the surface morphology at the end of the Ga and As cycles indicates no clear evidence for any enhancement in the Ga adatom diffusion length during the Ga cycle. However, the morphological anisotropy of the growth front does change significantly and it is proposed that this changing anisotropy during MEE enables Ga adatom diffusion along both azimuths. The surface anisotropy during MEE growth is found to increase with the Ga/As ratio. Although there is a clear correlation between composition and morphology, we have also found that highly ordered and flat surfaces are not necessarily an indication of stoichiometric material. We also attempt to clarify a recent controversy on the structure of the c(4 × 4) reconstruction by studying the surface structure at the end of the As cycle as a function of the As/Ga ratio.     

Glancing-angle ion enhanced surface diffusion on gaAs(001) during molecular beam epitaxy

We describe the effects of glancing incidence 3-4 keV Ar ion bombardment on homoepitaxial growth on vicinal GaAs(001). The average adatom lifetime on surface terraces, measured during growth using specular ion scattering, decreased monotonically with increasing ion current density. The results indicated that surface diffusivity was increased by the ions. The ion beam also suppressed growth oscillations and decreased the film surface roughness. This indicates a change from two-dimensional island nucleation to step-flow growth due to increased adatom surface diffusivity. A simple model, involving direct momentum transfer from ions to adatoms, is shown to be consistent with the measured enhanced diffusion.     

Reentrant mound formation in GaAs(001) homoepitaxy observed by ex situ atomic force microscopy

A study of the surface morphology of homoepitaxial GaAs(001) by means of ex situ atomic force microscopy in air reveals the reentrance of mounding behavior at low growth temperatures. A transition from statistical roughening to organized mound formation is observed as the growth temperature is reduced. We show by means of growth simulations that the observed morphology is compatible with anisotropic adatom diffusion in the presence of an Ehrlich-Schwoebel barrier. The mechanism leading to this kind of adatom kinetics at low temperatures is interpreted in terms of surfactant acting arsenic condensing on the surface.     

关于RHEED振荡技术优化GaAs(110)量子阱生长的研究

在GaAs(110)衬底上生长的半导体材料有诸多优良性能，使得在非极性GaAs(110)衬底上获得高质量各类异质结材料，成为近年来分子束外延生长关注的课题.考虑GaAs(110)表面是Ga和As共面，最佳生长温度窗口很小；反射式高能电子衍射的(1×1)再构图案对生长温度和V/Ⅲ束流比不敏感，难于通过观察再构图案的变化，准确地找到最佳生长条件.作者在制备GaAs(110)量子阱过程中，观察到反射式高能电子衍射强度振荡呈现出的单双周期变化.这意味着不同工艺条件下，在 GaAs(110)衬底上量子阱有单层和双层两种生长模式.透射电子显微镜和室温光致荧光光谱测量结果表明：在双层生长模式下量子阱样品光学性能较差，而在单层生长模式下量子阱光学性能较好，但是界面会变粗糙.利用这一特点，我们采用反射式高能电子衍射强度振荡技术，找到了一种在GaAs(110)衬底上生长高质量量子阱的可行方法. 关键词： 反射高能电子衍射 量子阱 分子束外延     

Growth of Pure Zinc Blende GaAs Nanowires: Effect of Size and Density of Au Nanoparticles

Pure zinc blende GaAs nanowires were grown by metal organic chemical vapor deposition on GaAs（111）B substrates via Au catalyzed vapor-liquid-solid mechanism. The diameter, size distribution, and density of Au particles can be changed by varying the Au film thickness. We find that the grown nanowires are of rod-like shapes and pure zinc blende structure; moreover, the growth rate depends on the density of Au particles and it is independent of its diameters. It can be concluded that the nanowire was grown with main contributions from the direct impingement of vapor species onto the Au-Ga droplets and contributions from adatom diffusion can be negligible. The results indicate that the droplet acts as a catalyst rather than an adatom collector.     

Controlling anisotropy of GaSb(As)/GaAs quantum dots by self-assembled molecular beam epitaxy

A systematic study was performed to control the geometrical anisotropy of GaSb(As)/GaAs quantum dot structures formed by the Stranski–Krastanov growth mode of molecular beam epitaxy. In particular, effects of both the Sb₄ beam flux and the As₄ background pressure on the geometrical anisotropy were clarified and elongated QDs with lateral aspect ratio greater than 3 were successfully formed. Under a low As₄ background pressure, As₄ is found to act as surfactant to influence the adatom diffusion and change the density of QDs. By contrast, under high As₄ background pressure, the intermixing of As and Sb takes place and reduces strains induced by the lattice mismatch.     

Growth and characterization of straight InAs/GaAs nanowire heterostructures on Si substrate

Vertical InAs/GaAs nanowire (NW) heterostructures with a straight InAs segment have been successfully fabricated on Si (111) substrate by using AlGaAs/GaAs buffer layers coupled with a composition grading InGaAs segment. Both the GaAs and InAs segments are not limited by the misfit strain induced critical diameter. The low growth rate of InAs NWs is attributed to the AlGaAs/GaAs buffer layers which dramatically decrease the adatom diffusion contribution to the InAs NW growth. The crystal structure of InAs NW can be tuned from zincblende to wurtzite by controlling its diameter as well as the length of GaAs NWs. This work helps to open up a road for the integration of high-quality III-V NW heterostructures with Si.     

Simulation of multilayer homoepitaxial growth on Cu （100） surface

The processes of multilayer thin Cu films grown on Cu (100) surfaces at elevated temperature (250--400\,K) are simulated by mean of kinetic Monte Carlo (KMC) method, where the realistic growth model and physical parameters are used. The effects of small island (dimer and trimer) diffusion, edge diffusion along the islands, exchange of the adatom with an atom in the existing island, as well as mass transport between interlayers are included in the simulation model. Emphasis is placed on revealing the influence of the Ehrlich--Schwoebel (ES) barrier on growth mode and morphology during multilayer thin film growth. We present numerical evidence that the ES barrier does exist for the Cu/Cu(100) system and an ES barrier $E_{\rm B} >0.125$\,eV is estimated from a comparison of the KMC simulation with the realistic experimental images. The transitions of growth modes with growth conditions and the influence of exchange barrier on growth mode are also investigated.     

Simulation of island aggregation influenced by substrate temperature, incidence kinetic energy and intensity in pulsed laser deposition

Using kinetic Monte Carlo method, we have simulated a pulsed energetic growth process in pulsed laser deposition. During the growth of film, substrate temperature mainly influences upon film morphology by directly enhancing the adatom mobility through the temperature-dependent thermal vibration. By contrast, the effect of incidence kinetic energy on film growth is complex resulting from the collisions between the incident particles and the adatoms. The results show that improving incident kinetic energy cannot significantly accelerate the migration rate of adatom but change surface microstructure and promote single adatom formation resulting in more island aggregation density. Moreover, since pulse-influx characterizes pulsed laser deposition, the intensity per pulse contributes to the evolvement of nucleation density and the results illustrate that a general scaling law different from ordinary power law still exists in energetic growth of pulsed laser deposition.     

AFM morphological characterization and Raman study of germanium grown on (111)GaAs

In this communication we report on the growth of Ge heterolayers on (100), (111)Ga and (111)As surfaces of GaAs substrates. Arsenic can play a role as both dopant and surfactant, changing the growth mechanism for the Ge/GaAs growth on (001) oriented substrates. The use of substrates oriented in different directions can change the growth mode and produce different results, since surface polarity can induce different growth modes: we have compared the results on (111) substrates with respect to the non-polar surface case.The growth behavior on (001), (111)Ga and (111)As substrates is discussed. The layer morphology was investigated by Atomic Force Microscopy and Raman spectroscopy has been carried out as a function of the sample thickness.     

Instability of vicinal crystal surfaces with transparent steps: Transient kinetics and non-local electromigration

The steps at the crystal surfaces could be partially transparent in the sense that the migrating adatoms can cross the steps without visiting a kink position. In the case of significant transparency the velocity of a given step in a given moment is affected by atomic processes which took place at rather distant terraces in rather earlier moments. The reason is that an adatom needs time to cross many terraces before attaching to a kink position. That is why the theory of crystallization with non-local kinetics should account for the time dependence of the adatom concentrations on the terraces. Such a transient growth model (going beyond the quasi-static approximation of the BCF theory of crystal growth) is developed here for the limit of slow kinetics at the steps and fast surface diffusion. This model predicts instability of the vicinal surface (in agreement with the earlier studies) but rather different expression for the wavelength of the most unstable mode, which depends only on the ratio between the step transparency and the step kinetic coefficients.     

Labyrinthine island growth during Pd/Ru(0001) heteroepitaxy

Using low energy electron microscopy we observe that Pd deposited on Ru only attaches to small sections of the atomic step edges surrounding Pd islands. This causes a novel epitaxial growth mode in which islands advance in a snakelike motion, giving rise to labyrinthine patterns. Based on density functional theory together with scanning tunneling microscopy and low energy electron microscopy we propose that this growth mode is caused by a surface alloy forming around growing islands. This alloy gradually reduces step attachment rates, resulting in an instability that favors adatom attachment at fast advancing step sections.     

Dynamic RHEED observations of the MBE growth of GaAs

Detailed measurements have been made of the specular beam intensity in RHEED patterns from static and growing GaAs surfaces. The basic parameters investigated were substrate temperature and electron beam azimuth. The results have provided further understanding of growth dynamics and surface disorder, respectively. There is a significant trend away from two-dimensional growth at the higher temperatures, which also correspond to more Ga-rich surface structures. Conversely, surface disorder is apparently greater during growth at the lower temperatures, where the structure is As-rich. The static As-stable 2×4 surface is, however, the most ordered and the most closely two-dimensional. It has also been shown that ordered, two-dimensional growth can be initiated from excess Ga adatom populations.     

A first-principles surface phase diagram study for Si-adsorption processes on GaAs(1 1 1)A surfaces

The adsorption processes of an Si atom on GaAs(1 1 1)A surfaces under growth conditions are investigated on the basis of first-principles surface phase diagrams, in which adsorption-desorption behavior is described by comparing the calculated adsorption energy obtained by total-energy electronic-structure calculations with vapor-phase chemical potential estimated by quantum statistical mechanics. The calculated surface phase diagram as functions of temperature and As₂ pressure demonstrates that both Ga and As atoms are adsorbed on the Ga-vacancy site of GaAs(1 1 1)A-(2×2) surface under low As-pressure conditions, resulting in the formation of (2×2) surface with an As adatom. The surface phase diagrams as functions of temperature and Si pressure also reveal that an Si atom can be adsorbed on the (2×2) surface with an As adatom for temperatures less than ∼1160 K and this Si atom can occupy one of As-lattice sites after the incorporation of another As atom, leading to p-type conductivity. In contrast, the (2×2) surface with an As trimer is found to be stabilized under high As-pressure conditions. The surface phase diagram for Si incorporation clarify that an Si atom can be adsorbed at one of Ga-lattice sites of the (2×2) surface with an As trimer for temperatures less than ∼870 K. These calculated results provide one of possible explanations for the formation of p-type and n-type GaAs on GaAs(1 1 1)A surfaces under low and high As-pressure conditions, respectively.     

A3B5 nanowhiskers: MBE growth and properties

The structural properties of GaAs nanowhiskers (NWs) grown by molecular beam epitaxy (MBE) are investigated. Under optimal growth conditions, the aspect ratio of MBE grown GaAs NWs is higher than 100. The maximum length of NWs is several times (up to 10) larger than the effective thickness of deposited GaAs. A kinetic model of the diffusion-induced NW rowth is used to predict the dependence of NW length on the technologically controlled MBE growth conditions. The obtained results demonstrate that the NW growth is controlled by the adatom diffusion towards their tip rather than by the conventional vapor-liquid-solid mechanism. The growth conditions influence on the NW morphology may be used for the controlled fabrication of NWs by MBE for different applications. Presented at the X-th Symposium on Suface Physics, Prague, Czech Republic, July 11–15, 2005.     

An ab initio-based approach to adsorption-desorption behavior of Si adatoms on GaAs(1 1 1)A-(2 × 2) surfaces

The adsorption-desorption behavior of Si adatoms on GaAs(1 1 1)A-(2 × 2) surfaces is investigated using our ab initio-based approach, in which adsorption and desorption behavior of Si adatoms is described by comparing the calculated desorption energy obtained by total-energy electronic-structure calculations with the chemical potential estimated by quantum statistical mechanics. We find that the Si adsorption at the Ga-vacancy site on the (2 × 2) surfaces with As adatoms occurs less than 1140-1590 K while the adsorption without As adatom does less than 630-900 K. The change in adsorption temperature of Si adatoms by As adatoms is due to self-surfactant effects of As adatoms: the promotion of the Si adsorption triggered by As adatoms is found to be interpreted in terms of the band-energy stabilization. Furthermore, the stable temperature range for Si adsorbed surfaces with As adatoms agrees with the experimental results. The obtained results provide a firm theoretical framework to clarify n-type doping processes during GaAs epitaxial growth.     

表面形貌对GaAs生长速率测量的影响

本文利用Reflection High Energy Electron Diffraction (RHEED)强度振荡测量GaAs同质外延生长,发现其生长速率随生长厚度按一定指数函数关系衰减.这种衰减与GaAs表面形貌的变化密切相关,表面台阶数量的增加使层状生长模式由2D成核模式逐渐转变为台阶流模式.由于RHEED强度振荡所测的生长速率与表面的粗糙程度密切相关,表面情况改变对生长速率会有一定的影响,导致测量的生长速率逐渐的衰减.根据生长速率随生长厚度的增加而衰减的拟合曲线,可以获得一个准确的生长速率.     

On the 2D-3D transition in epitaxial thin film growth

The theory is based on the fact that the equilibrium concentration of single atoms adsorbed on the surface of a monolayer island placed on a foreign substrate such that the substrate-deposit interaction is weaker than the deposit-deposit interaction is higher than the equilibrium concentration of atoms adsorbed on the surface of the same island now placed on a substrate of the same material. This higher adatom concentration leads to 2D nucleation on top of the monolayer island. The difference of the above equilibrium adatom concentrations appears as a driving force for the process of transformation of the initial monolayer island into a 3D island by detachment of atoms from the first monolayer island edges and their subsequent attachment to the edges of the second layer nucleus. The kinetics of this process are studied in detail, the following two cases being considered. The first case consists of breaking up and agglomeration of an initially continuous film into 3D crystals upon heating. The second case consists of change of the growth mode from layer to island mode during the vapour deposition when the substrate temperature increases. Expressions for the critical temperatures for these two phenomena to occur are derived. It is shown that they depend strongly on the substrate orientation, the critical temperature being higher for the 〈111〉 orientation in comparison with the 〈110〉 orientation, if substrate and deposit with a fcc lattice are considered. The theoretical results are compared with experimental data for deposition of Au on Mo{110}, Cu on W{110} and W{100} and Fe on Cu{111}.     

金辅助催化方法制备GaAs和GaAs/InGaAs纳米线结构的形貌表征及生长机理研究

利用金（Au）辅助催化的方法,通过金属有机化学气相沉积技术制备了GaAs纳米线及GaAs/InGaAs纳米线异质结构.通过对扫描电子显微镜（SEM）测试结果分析,发现温度会改变纳米线的生长机理,进而影响形貌特征.在GaAs纳米线的基础上制备了高质量的纳米线轴、径向异质结构,并对生长机理进行分析.SEM测试显示,GaAs/InGaAs异质结构呈现明显的“柱状”形貌与衬底垂直,InGaAs与GaAs段之间的界面清晰可见.通过X射线能谱对异质结样品进行了线分析,结果表明在GaAs/InGaAs轴向纳米线异质结构样品中,未发现明显的径向生长.从生长机理出发分析了在GaAs/InGaAs径向纳米线结构制备过程中伴随有少许轴向生长的现象.