GaAs initial growth on InAs(001) vicinal surfaces observed by scanning tunneling microscopy

GaAs initial growth on InAs surfaces misoriented by 2° toward the [110] and [1 0] directions was investigated by scanning tunneling microscopy (STM). In the STM images of both InAs vicinal surfaces after GaAs deposition, white lines running in the [1 0] direction, corresponding to the grown GaAs surface, were observed. Almost all of the lines were attached only to steps running in the [110] direction (B-type steps) on both InAs surfaces; that is, the lines were seldom attached to steps running in the [1 0] direction (A-type steps). These results indicate that the B-type steps are more favorable for the sticking of deposited Ga atoms than the A-type steps during GaAs initial growth on InAs vicinal surfaces.     

Mass spectrometric studies of adsorption of silver on tungsten (110) surfaces

The adsorption of silver on (110)tungsten was studied by a mass spectrometric molecular beam technique. The temporal behaviour of the flux of adatoms leaving the substrate was measured at fixed temperatures between 860 K and 1080 K for constant impinging rates over the range 0.2 · 10¹³ to 9.0 · 10¹³ cm⁻² s⁻¹. For moderate supersaturation the growth mode changed from the layer-by-layer growth of three monolayers to the growth of three-dimensional nuclei (Stranski-Krastanov mechanism). At equal rates of impinging and desorption flux, the equilibrium adatom concentration was determined and a set of adsorption isotherms was thus derived. The shape of the experimental isotherms was found to be broadly similar to that of the theoretical Fowler-Guggenheim isotherm for a localized adsorption model. The binding energy of an Ag atom to the substrate, the lateral binding energy of a pair of atoms, the coordination number, and the vibration frequency of the adatoms for all three monolayers were obtained by suitable fitting procedures.     

Silicon compensation and scattering mechanisms in two-dimensional electron gases on (110)GaAs

A study of the MBE growth of (001) and (110) (Al,Ga)As is reported, and the efficiency of Si as an n-type dopant in (110)GaAs is accessed. A 40 nm spacer two-dimensional electron gas (2DEG) structure grown on (110)GaAs gives a mobility of 540,000 cm² V⁻¹ s⁻¹ at 4 K after illumination. The dominant scattering mechanisms in 2DEGs on (110) and (001)GaAs grown under the separate optimum growth conditions for the two orientations are compared.     

Multiatomic step formation on GaAs(001) vicinal surfaces during thermal treatment

We observed the surface morphology of vicinal GaAs(001) after thermal treatment in AsH₃/H₂ atmosphere by atomic force microscopy (AFM). Clear multiatomic steps were formed under the high temperature thermal treatment. Next, we investigated the mechanism of step bunching during thermal treatment by two experiments from the view point of Ga atom evaporation. One is the selective thermal treatment using a partially masked GaAs wafer, and the evaporation amount of Ga atoms was estimated by AFM. The other is the investigation of photoluminescence (PL) peak energy shifts for AlGaAs/GaAs single quantum wells with a thermal treatment process at the top of the GaAs quantum well layer, compared to those without thermal treatment. These results indicate that the evaporation hardly occurs during the thermal treatment process. Therefore, step bunching phenomena on GaAs(001) vicinal surfaces during thermal treatment are probably caused by migration of the atoms detached from upside steps and their re-incorporation to downside steps.     

On the growth of Au on clean and contaminated GaAs(001) surfaces

Auger analysis and transmission electron microscope (TEM) analysis have been used to determine the influence of contamination on the structural properties of Au films grown on different chemically treated (001)GaAs surfaces. The Au films were evaporated on to the (001)GaAs surfaces at 320°C in a vacuum of 10^-8 Torr after an initial bake-out of 500°C for 1 h. The results revealed that carbon contamination of the surfaces resulted in completely polycrystalline Au overgrowth while clean surfaces resulted in perfectly alligned epitaxial Au overgrowths. The epitaxial relationship is given by (100)_Au 6(100)_GaAs and <100 > _Au6 <100 > _GaAs which is equivalent to a misfit of 32.24%.     

Surface diffusion length of Ga adatoms in molecular-beam epitaxy on GaAs(100)-(110) facet structures

By the molecular-beam epitaxial (MBE) growth of GaAs on [001]-mesa stripes patterned on GaAs(100) substrates, (110) facets were formed on the mesa edges defining (100)-(110) facet structures. The surface diffusion length of Ga adatoms along the [010] direction on the mesa stripes was obtained for a variety of growth conditions by in-situ scanning microprobe reflection high-energy electron diffraction (μ-RHEED). Using these values and the corresponding growth rate on the GaAs(110) facets, the diffusion length on the (110) plane was estimated. We found that the Ga diffusion length on the (110) plane is longer than that on the (100) and (111)B planes. The long diffusion length on the (110) plane is discussed in terms of the particular surface reconstruction on this plane.     

Structural analysis of AlGaAs quantum wires on vicinal (110)GaAs by transmission electron microscopy and energy dispersive X-ray spectroscopy

Giant step structures consisting of coherently aligned multi-atomic steps were naturally formed during the molecular beam epitaxy growth of Al_0.5Ga_0.5As/GaAs superlattices (SLs) on vicinal (110)GaAs surfaces misoriented 6° toward (111)A. The growth of AlAs/Al_xGa_1−xAs/AlAs quantum wells (QWs) on the giant step structures realized Al_x0Ga_1−x0As (x₀<x) quantum wires (QWRs). We studied the giant step structures and the QWRs by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). TEM observations revealed that the QWRs were formed at the step edges. The cross sections of the QWRs were as small as 10 nm×20 nm and the lateral distances between them were about 0.15 μm. We clarified the roles of the SLs to form the coherent giant step structures. From EDX analysis, it was estimated that the AlAs composition in the Al_0.5Ga_0.5As layers varied from 0.5 (terrace) to 0.41 (step edge). In the AlAs/Al_xGa_1−xAs/AlAs QWs, the AlAs compositional modulation and the confinement by the AlAs barriers led to the embedded Al_x0Ga_1−x0As regions. These results supported the existence of the Al_x0Ga_1−x0As QWRs on the giant step structures.     

Ga assisted oxide desorption on GaAs(0 0 1) studied by scanning tunnelling microscopy

Native oxide removal on GaAs wafers under conventional thermal desorption causes severe surface degradation. Recently a new method of Ga assisted oxide removal has reported improved initial surface conditions. A precise dosing of Ga is required to optimise the oxide removal, however the effects of alternate temperatures on the desorption process effects the reaction kinetics. By using selected bias imaging, scanning tunnelling microscopy (STM) can be used to probe the underlying bulk whilst the native oxide is still present. Hence the effects of oxide removal on the surface can be identified during the native oxide desorption. By comparing Ga assisted oxide removal on both vicinal and off cut samples, the Ga adatom kinetics are shown to underpin the oxide removal process and a sample temperature in excess of 500 °C is necessary to optimise the procedure.     

Oxygen depletion in electron beam bombarded glass surfaces studied by XPS

XPS has been utilized to study the compositional changes which occur in glass after electron beam irradiation. In the bombarded surfaces a reduction in Na and O signals is observed, while Ca signal shows negligible variations. Two O_1s signals, assigned to bridging and non-bridging oxygen were found. Irradiation causes a decrease of these two types of oxygen. This is at variance with Lineweaver's mechanism which accounts for the decrease of the non-bridging oxygens only. After irradiation a second component in the Si_2p line is found. It has been attributed to the formation of SiSi bonds induced by oxygen outgassing. The formation of these bonds leads to a more compact structure and this might explain the observed density change in irradiated glass.     

采用MOCVD方法在GaAs衬底上生长ZnO(002)和ZnO(100)薄膜

采用金属有机化学汽相沉积生长法（MOCVD）,在不同的衬底表面处理条件和生长温度下,在GaAs衬底上生长出了ZnO薄膜。随着化学腐蚀条件的不同,可生长出优先定位不同的ZnO（100）和ZnO（002）薄膜。该薄膜的晶体结构特性是由X光衍射谱仪（XRD）所获得的,而其光学特性是由光荧光谱仪（PL）来测的。与ZnO（002）相比,ZnO（100）薄膜具有更优越的晶体结构特性,并且在同样的生长温度下都具有相似的光学特性。对于腐蚀条件不同的GaAs衬底所进行的XPS分析结果表明,ZnO薄膜优先定位变化的主要原因在于腐蚀过程中形成的富As层。     

Selective area molecular beam epitaxy of InAs on GaAs (110) masked substrates for direct fabrication of planar nanowire field-effect transistors

We have synthesized InAs nanowires (NWs) by selective area molecular beam epitaxy (SA-MBE) on GaAs masked substrates. In particular, we have obtained in-plane-oriented NWs on the (110) plane, and then directly applied the NWs to planar nanowire field-effect transistors (NWFETs) using conventional electron beam lithography without a NW dispersion process. We have measured output and transfer characteristics of the NWFETs at room temperature, and obtained a current swing but no turning off, and a field-effect mobility peak of 150 cm²/V-s. We have also observed almost no temperature influence on field-effect mobility between 2 K and 300 K, suggesting a high-dense surface accumulation layer even at low temperatures.     

基于液滴外延法的Al(In)纳米结构在GaAs(001)的形成机制

采用液滴外延法在GaAs(001)衬底上同时沉积In、Al液滴形成纳米结构,利用原子力显微镜(AFM)对实验样品进行形貌表征,并通过X射线光电子能谱(XPS)与扫描电子显微镜分析In、Al组分比样品表面元素分布。实验结果显示,混合沉积后的表面InAlAs纳米结构密度随着In组分的降低而降低,而单个纳米结构的尺寸变大。SEM与XPS测试结果证明表面的In并没有因为衬底温度过高而全部偏析。根据实验结果推测,In&Al液滴同时沉积到表面形成InAl混合液滴。当液滴完全晶化后纳米结构中心出现孔洞,而产生这一现象的主要原因是液滴向下刻蚀。     

Se-doped AlGaAs grown on GaAs(111)A by molecular beam epitaxy

The electrical properties of Se-doped Al_0.3Ga_0.7As layers grown by molecular beam epitaxy (MBE) on GaAs(111)A substrates have been investigated by Hall-effect and deep level transient spectroscopy (DLTS) measurements. In Se-doped GaAs layers, the carrier concentration depends on the misorientation angle of the substrates; it decreases drastically on the exact (111)A surface due to the re-evaporation of Se atoms. By contrast, in Se-doped AlGaAs layers, the decrease is not observed even on exact oriented (111)A. This is caused by the suppression of the re-evaporation of Se atoms, by Se---Al bonds formed during the Se-doped AlGaAs growth. An AlGaAs/GaAs high electron mobility transistor (HEMT) structure has been grown. The Hall mobility of the sample on a (111)A 5° off substrate is 5.9×10⁴ cm²/V·s at 77 K. This result shows that using Se as the n-type dopant is effective in fabricating devices on GaAs(111)A.     

Growth of GaAs/AlGaAs HBTs by MOMBE (CBE)

In this paper, we will discuss how the unique growth chemistry of MOMBE can be used to produce high speed GaAs/AlGaAs heterojunction bipolar transistors (HBTs). The ability to grow heavily doped, well-confined layers with carbon doping from trimethylgallium (TMG) is a significant advantage for this device. However, in addition to high p-type doping, high n-type doping is also required. While elemental Sn can be used to achieve doping levels up to 1.5×10¹⁹ cm^-3, severe segregation limits its use to surface contact layers. With tetraethyltin (TESn), however, segregation does not occur and Sn doping can be used throughout the device. Using these sources along with triethylgallium (TEG), trimethylamine alane (TMAA), and AsH₃, we have fabricated Npn devices with 2 μm×10 μm emitter stripes which show gains of ≥ 20 with either ƒ_t = 55 GHz and ƒ_max = 70 GHz or ƒ_t = 70 GHz and ƒ_max = 50 GHz, depending upon the structure. These are among the best RF values reported for carbon doped HBTs grown by any method, and are the first reported for an all-gas source MOMBE process. In addition, we have fabricated a 70 transistor decision circuit whose performance at 10 Gb/s equals or exceeds that of similar circuits made from other device technologies and growth methods. These are the first integrated circuits reported from MOMBE grown material.     

Nucleation of islands in GaAs molecular beam epitaxy studied by in-situ scanning electron microscopy

We studied two-dimensional nucleation of islands in the molecular beam epitaxy (MBE) of GaAs by in-situ scanning electron microscopy and found that the islands do not appear until about 1/3 monolayer is deposited. In contrast to what is expected from the previous simple nucleation and growth model, we observed islands to form continuously.     

Extremely flat interfaces in GaAs/AlGaAs quantum wells with high Al content(0.7) grown on GaAs (411)A substrates by molecular beam epitaxy

Effectively atomically flat interfaces over a macroscopic area (200 μm diameter) have been achieved in GaAs/Al_0.7Ga_0.3As quantum wells (QWs) with well widths of 3.6-12 nm grown on (411)A GaAs substrates by molecular beam epitaxy (MBE) for the first time. A single and very narrow photoluminescence peak (FWHM, full width at half maximum, is 6.1 meV) was observed at 717.4 nm for the QW with a well width of 3.6 nm at 4.2 K. The linewidth is comparable to that of growth-interrupted QWs grown on (100)-oriented GaAs substrates by MBE. A 1.5 μm thick Al_0.7Ga_0.3As layer with good surface morphology also could be grown on (411)A GaAs substrates in the entire growth temperature region of 580-700°C, while rough surfaces were observed in Al_0.7Ga_0.3As layers simultaneously grown on (100) GaAs substrates at 640-700°C. These results indicate that the surface of GaAs and Al_0.7Ga_0.3As grown on the (411)A GaAs substrates are extremely flat and stable on the (411)A plane.     

Strain relaxation in InGaAs/GaAs quantum wells grown on GaAs (111)A substrates

We have grown In_0.2Ga_0.8As strained quantum wells (SQWs) on GaAs (111)A just and off-angled substrates by molecular beam epitaxy (MBE). The photoluminescence (PL) peak energy of SQWs grown on (111)A related substrates shows a large redshift as compared with the calculated values. The red-shift observed in SQWs grown on a (111)A 5° off toward [001] substrate can be explained by the presence of a built-in electric field E = 154 kV/cm due to piezoelectric effect. The larger red-shift observed in samples grown on the other substrates is partially due to strain relaxation. A strain relaxation mechanism that consists of coherently grown islands when InGaAs growth begins and the generation of misfit dislocations when these islands coalesce, gives a qualitative explanation of the observed results.     

Growth of ZnTe layers on (111) GaAs substrates by metalorganic vapor phase epitaxy

ZnTe layers were grown on (111) GaAs substrates by metalorganic vapor phase epitaxy using dimethylzinc and diethyltelluride as the source materials. X-ray diffraction analysis revealed that epitaxial ZnTe layers can be obtained on (111) GaAs substrates. X-ray rocking curves, Raman spectroscopy, and photoluminescence measurements showed that the crystal quality of ZnTe layers depends on the substrate temperature during the growth. A high-crystalline quality (111) ZnTe heteroepitaxial layer with strong near-band-edge emission at 550 nm was obtained at a substrate temperature of 440 °C.     

利用Al/AlAs中间层调控GaAs在Si(111)上外延生长的研究

为了实现Ⅲ-V器件在硅基平台上单片集成,近年来Ⅲ-V半导体在硅衬底上的异质外延得到了广泛研究。由于Ⅲ-V半导体与Si之间大的晶格失配以及晶格结构不同,在Si上生长的Ⅲ-V半导体中存在较多的失配位错及反相畴,对器件性能造成严重影响。而Si(111)表面的双原子台阶可以避免Ⅲ-V异质外延过程中形成反相畴。本文利用分子束外延技术通过Al/AlAs作为中间层首次在Si(111)衬底上外延生长了GaAs(111)薄膜。通过一系列对比实验验证了Al/AlAs中间层的插入对GaAs薄膜质量的调控作用,并在此基础上通过低温-高温两步法优化了GaAs的生长条件。结果表明Al/AlAs插层可以为GaAs外延生长提供模板,并在一定程度上释放GaAs与Si之间的失配应力,从而使GaAs薄膜的晶体质量得到提高。以上工作为Ⅲ-V半导体在硅上的生长提供了新思路。