Effects of pressure and temperature on epitaxial growth of InP on non-planar substrates using OMVPE

The effects of pressure and temperature on the epitaxial growth of InP on the mesastructured substrate have been investigated using OMVPE. At 550° C and 700 Torr, the reverse-mesa profile was observed. As the pressure decreases, the epitaxial growth profile changes from the reverse-mesa shape to the mesa shape. As the growth temperature increases, the growth profile follows the mesa-structured substrate even at high growth pressure. We propose that two major factors contributed to the formation of the reverse-mesa shape at low temperature and high pressure. We can explain the pressure effects on the epitaxial growth profile with these two factors at all growth temperatures ranging from 550 to 650° C. One factor is the growth rate enhancement on the (111)A facet compared to the growth rate on the (001) facet at low growth temperature, the other is the deficiency of constituent at the recess region of the epitaxial layer caused by reduced gas phase diffusivity at high pressure.     

Seeded growth of GaN at high N2 pressure on (0 0 0 1) polar surfaces of GaN single crystalline substrates

GaN single crystals obtained by the high nitrogen pressure solution method without an intentional seeding, show strong growth anisotropy which results in their platelet shape. The attempts to enhance the growth into (0 0 0 1) directions by the increase of the integral supercooling in the solution, often lead to the growth instabilities on both Ga-polar and N-polar (0 0 0 1) surfaces. This can be avoided only by the precise control of the growth conditions at the crystallization front on the particular surface.The results of the seeded growth on both Ga- and N-polar (0 0 0 1) surfaces in configuration enabling such a control is reported. It is shown that dominating mechanisms of the unstable growth such as the cellular growth or edge nucleation can be suppressed. Differences in nucleation and growth in dependence on surface polarity are discussed.     

CdTe and HgTe surface growth kinetics for molecular and metalorganic molecular beam epitaxy

The surface growth kinetics of CdTe and HgTe have been investigated during molecular and metalorganic molecular beam epitaxy. The surface growth kinetics was studied through in-situ measurements of the growth rate as a function of flux ratio and substrate temperature on the (001), (111)B, and (211)B CdTe surface orientations. For the (001) and (111)B CdTe growth kinetics, the existence of low binding energy surface precursor sites was proposed for both molecular and atomic growth species before lattice incorporation. Intensity oscillations were observed during HgTe growth on misoriented (111)B surfaces and during CdTe growth on the (211)B orientation. The (211)B surface reconstructions displayed both vicinal and singular surface characteristics, depending on the growth flux ratio.     

Atomic force microscopy studies of CdTe films grown by epitaxial lateral overgrowth

Epitaxial lateral overgrowth (ELO) of CdTe was carried out on GaAs using silicon nitride as the mask material. Windows were delineated on silicon, nitride mask deposited on GaAs substrates and CdTe was grown using metalorganic vapor phase epitaxy. The films were characterized by atomic force microscopy (AFM). It has been shown that highly selective growth of CdTe can be achieved at temperatures higher than 500 C and pressures lower than 25 torr using silicon nitride as the mask layer. Optimizing the growth conditions as well as the stripe directions on the substrates enables the growth of ELO-CdTe with a flattop surface and vertical sidewalls. AFM studies show that ELO-grown CdTe contains large grains with reduced defect densities, but there seems to be no difference on the films grown on the window region or on the masked region. The results suggest that the growth mechanism for CdTe growth on GaAs is different from that of ELO-grown GaN. A possible growth model for the patterned CdTe growth is also proposed.     

Low temperature growth of AIGaP and GaP on si substrates by atomic layer epitaxy

AIGaP and GaP films were deposited on the (100) Si substrates by atomic layer epitaxy (ALE) in the temperature range between 450 and 600°C. Under optimum growth conditions, the growth of GaP and AIGaP was observed to proceed in a two-dimensional (2-D) fashion in the initial growth stages. These ALE-grown films have better surface morphology when compared with the corresponding MOCVD-grown films. With an AIGaP buffer layer grown on Si, the subsequent growth of GaAs on the AlGaP-coated Si substrates tends to proceed as 2-D growth. This avoids island growth and the two-step growth process currently used.     

MOCVD growth of AlGaAs/GaAs structures on nonplanar {111} substrates: Evidence for lateral gas phase diffusion

We investigate the MOCVD growth characteristics of AlGaAs on nonplanar {111}A and {111}B substrates. Growth over features etched into the {111} substrates is found to be highly anisotropic and asymmetric. The ratio of growth rates on adjacent facets is strongly dependent on the depth of the etched feature during growth, and is strikingly different between AlGaAs and GaAs layers. These observations suggest a large difference in the surface chemistry of Al and Ga species under these growth conditions and indicate that the column III element determines the relative growth rates of different facets during nonplanar growth. The results also provide strong evidence that lateral gas phase diffusion of reactants can be perhaps more significant than surface migration as a mechanism determining the incorporation sites of column III elements. Growth characteristics on nonplanar {111} substrates are markedly different than those observed for nonplanar growth on {100} substrates, creating a new set of design tools for the single step growth of guided wave devices such as lasers, modulators and waveguides.     

Selective area growth of GaN using gas source molecular beam epitaxy

Ammonia cracking efficiencies on various surfaces were examined. The following is an ordering of surfaces according to their ammonia cracking efficiencies: GaN (highest), Si₃N₄, SiO₂ (lowest). Selective area growth of GaN was performed over SiO₂ masks deposited on GaN previously grown on sapphire substrates using ammonia-based molecular beam epitaxy. GaN growth on patterned SiO₂/GaN is very selective at a growth temperature of 800°C. Good quality growth occurs in the window region with no deposits on the mask surface when growth is performed at 800°C, whereas some deposits on the SiO₂ masks accumulate when growth is performed at 700°C. The ratio of lateral growth rate to vertical growth rate is ≤1.     

Reliability growth models for hardware and software systems based on nonhomogeneous Poisson processes: A survey

We summarize the reliability growth models for hardware and software systems described by a stochastic process, where the underlying stochastic process is assumed to be a nonhomogeneous Poisson process (NHPP). The background of reliability growth modelling based on an NHPP is surveyed. The Duane model, which was first postulated as a reliability growth model and is commonly used, is first explained. Secondly, the Weibull growth and modified Weibull growth models for hardware systems and the exponential type growth and gamma type growth models for error detection for software systems are discussed. The parameter estimates can be obtained by maximum likelihood estimation. Finally, the goodness-of-fit tests based on chi-square, Cramér-von Mises and Kolmogorov-Smirnov statistics are presented for the reliability growth models based on an NHPP.     

Mechanisms of homo- and heteroepitaxial growth of SiC on α-SiC(0001) by solid-source molecular beam epitaxy

Epitaxial growth of SiC on hexagonal (or α)-SiC(0001) has been performed by means of solid-source molecular beam epitaxy (MBE). The solid-source MBE growth conditions have been analyzed concerning the supersaturation and the excess phase formation of silicon and carbon. In general, our results demonstrate that control of the Si/C ratio and the supersaturation (S) is essential for the growth mode and the kind of polytype grown. Low temperature (T<1450K) deposition on on-axis SiC substrates always results in the growth of 3C-SiC, which is significantly improved by an alternating supply of Si and C. On vicinal substrates, a step flow growth mode has been realized at T down to 1300K. In experiments performed at T>1450K under near surface equilibrium conditions, different growth modes, and conditions stabilizing the growth of certain polytypes have been found. With a step decrease of S, a step-flow growth mode of both 4H-and 6H-SiC was obtained and, for the first time in case of epitaxial SiC growth, a homogeneous nucleation of α-SiC at more C-rich conditions has been realized. Conditions stabilizing the growth of certain polytypes have been estimated by thermodynamic calculations considering the influence of polytype structure on the supersaturation and the surface energy. Based on these results, we have demonstrated the growth of a double-heterostructure by firstly growing a 3C-SiC layer on 4H-SiC(0001) at low temperature and a subsequent growth of 4H-SiC under near surface equilibrium conditions on a C-stabilized surface on top of this layer.     

Embedded epitaxial growth of 4H-SiC on trenched substrates and pn junction characteristics

Embedded epitaxial growth has been carried out on 4H-SiC substrates with very narrow and deep trenches. The growth behavior near trenches is investigated under various growth conditions. Epitaxial growth on the sidewalls and at the bottom of trenches was enhanced under a low C/Si ratio which may bring a larger surface diffusion length of reactant species. Pn diodes were fabricated by embedded epitaxial growth on trenched substrates. The crystallographic orientation of the trenches has been found to be important for device fabrication.     

电子产品研制阶段可靠性增长试验研究

结合工程实际经验,深入讨论了可靠性增长过程及实现途径,在保持试验条件和改进过程不变的条件下,实施了对具体型号电子产品的可靠性增长试验,达到了预期的可靠性增长目标,并且利用可靠性增长试验的数学模型(AMSAA模型)来评估产品的可靠性增长,对开展可靠性增长与可靠性增长试验工作具有重要的实际意义.     

Filling of light emitting diode via-holes by MOCVD

The growth of epitaxial films on featured substrates has an important device application in junction-confinement, double hetero-structure light emitting diodes. These devices are presently grown by a liquid phase epitaxy process but growth by metalorganic chemical vapor deposition is desirable because of MOCVD's superior surface quality, uniformity, and throughput. This paper describes the effect of growth parameters on AlGaAs films deposited by atmospheric-pressure MOCVD into substrate holes typically made in the fabrication of junction-confinement LEDs. MOCVD growth replicates the substrate features; it does not give a planar surface over the holes. The behavior of epitaxy filling into holes is strongly dependent on growth temperature and total gas flow and largely independent of substrate misorientation and the thickness of the layer grown. Wet-etched holes formed (ll0)-oriented V-groove and dovetail-groove features on the hole circumference. Faceting of the MOCVD growth was seen on the wall with the (111)A feature while smooth growth was seen on the etched (111)B surface. Deceased.     

Effect of nucleation layer morphology on crystal quality, surface morphology and electrical properties of AlGaN/GaN heterostructures

Nucleation layer formation is a key factor for high quality gallium nitride (GaN) growth on a sapphire substrate. We found that the growth rate substantially affected the nucleation layer morphology, thereby having a great impact on the crystal quality, surface morphology and electrical properties of AIGaN/GaN heterostructures on sapphire substrates. A nucleation layer with a low growth rate of 2.5 nm/min is larger and has better coalescence than one grown at a high growth rate of 5 nm/min. AIGaN/GaN heterostructures on a nucleation layer with low growth rate have better crystal quality, surface morphology and electrical properties.     

The contraction of lattice constant and the reduction of growth rate in p-InGaAs grown by organometallic vapor phase epitaxy

We have investigated the effect of diethylzinc (DEZn) on the lattice constant and the growth rate of InGaAs. Introducing DEZn for p-type doping induces the contraction of lattice constant and the reduction of growth rate compared to undoped InGaAs. Depletion of indium is responsible for these effects. These effects are reduced at lower growth temperatures or at lower growth pressures. From the observed effects of the growth temperatures and the growth pressures on the contraction of the lattice constant, it is concluded that depletion of indium occurs in the gas phase.     

Atomic‐Scale Mechanism of Unidirectional Oxide Growth

A fundamental knowledge of the unidirectional growth mechanisms is required for precise control on size, shape, and thereby functionalities of nanostructures. The oxidation of many metals results in oxide nanowire growth with a bicrystal grain boundary along the axial direction. Using transmission electron microscopy that spatially and temporally resolves CuO nanowire growth during the oxidation of copper, herein, direct evidence of the correlation between unidirectional crystal growth and bicrystal grain boundary diffusion is provided. Based on atomic scale observations of the upward growth at the nanowire tip, oscillatory downward growth of atomic layers on the nanowire sidewall and the parabolic kinetics of lengthening, it is shown that bicrystal grain boundary diffusion is the mechanism by which Cu ions are delivered from the nanowire root to the tip. Together with density‐functional theory calculations, it is further shown that the asymmetry in the corner‐crossing barriers promotes the unidirectional oxide growth by hindering the transport of Cu ions from the nanowire tip to the sidewall facets. The broader applicability of these results in manipulating the growth of nanostructured oxides by controlling the bicrystal grain boundary structure that favors anisotropic diffusion for unidirectional, 1D crystal growth for nanowires or isotropic diffusion for 2D platelet growth is expected.     

Applications of thermodynamical modeling in molecular beam epitaxy of CdxHg1-xTe

It is well known that the crystalline quality of Cd_xHg_1-xTe grown by molecular beam epitaxy is critically dependent on the substrate temperature. The optimal growth temperature has been identified immediately below the crossing of the Te-rich phase boundary, that is just below the temperature range where Te precipitation occurs in the layer. It is potentially very useful to be able to predict the optimal temperature and its variation with other growth parameters, but no general guidelines for this can be found in the literature. We have studied experimentally the variation of the optimal growth temperature with Hg flux, Cd mole fraction and growth rate. These results are compared with a thermodynamical model published previously by Gailliard. We find that the modeled position of the phase boundary coincides well with the observed variations in optimal growth temperature for growth on Te-terminated surfaces, within the uncertainties of available thermodynamical constants. We show that the optimal substrate temperature depends mainly on the Hg flux and Cd mole fraction, while the dependence on growth rate can be neglected in practical molecular beam epitaxy conditions. The experimental observation of optimal layer quality at the phase boundary could suggest the existence of an adsorbed layer of Te, acting as a reservoir for Te atoms and reducing the supersaturation of the growth reaction. Simultaneous growth on the (211)B and (100) orientations reveals a clear, although not very large, difference in optimal growth temperature and Cd incorporation, indicating a difference in growth kinetics. This can be accounted for in the thermodynamical model by condensation and evaporation coefficients.     

成核层形貌对AlGaN/GaN异质结晶体质量，表面形貌以及电学特性的影响

Nucleation layer formation is a key factor for high quality gallium nitride（GaN）growth on a sapphire substrate.We found that the growth rate substantially affected the nucleation layer morphology,thereby having a great impact on the crystal quality,surface morphology and electrical properties of AlGaN/GaN heterostructures on sapphire substrates.A nucleation layer with a low growth rate of 2.5 nm/min is larger and has better coalescence than one grown at a high growth rate of 5 nm/min.AlGaN/GaN heterostructures on a nucleation layer with low growth rate have better crystal quality,surface morphology and electrical properties.     

初始层生长条件和Mg活性剂对MOVPE方法生长GaN薄膜质量的影响

The initial growth conditions of a 100 nm thick GaN layer and Mg-surfactant on the quality of the GaN epilayer grown on a 6H-SiC substrate by metal-organic vapor phase epitaxy have been investigated in this research. Experimental results have shown that a high Ⅴ/Ⅲ ratio and the initially low growth rate of the GaN layer are favorable for two-dimension growth and surface morphology of GaN and the formation of a smoother growth surface. Mg-surfactant occurring during GaN growth can reduce the dislocations density of the GaN epilayer but increase the surface RMS, which are attributed to the change of growth mode.     

氯基条件下4H-SiC衬底的同质外延生长研究

利用课题组自主研发的热壁低压化学气相沉积(HWLPCVD)系统,在朝[11-20]方向偏转4°的(0001)Si面4H-SiC衬底上进行快速同质外延生长,研究了生长温度及氯硅比(Cl/Si比)对外延生长速率的影响机理.研究发现,外延生长速率随生长温度的提高呈线性增加,而Cl/Si比的改变对生长速率的影响不大.文章进一步探究了Cl/Si比对4H-SiC外延层表面缺陷的影响.较低的Cl/Si比(0.4～2)可以减少或消除三角缺陷,Cl/Si比较高(大于5)时,表面质量反而下降,因而,适当的Cl/Si比对于获得表面形貌良好的4H-SiC外延层至关重要.     

Self-ordered nanostructures grown by organometallic chemical vapor deposition on V-grooved substrates: experiments and Monte-Carlo simulations

The growth the GaAs/GaAlAs quantum wires by organometallic chemical vapor deposition on V-grooved substrates relies on the formation of a self-limiting AlGaAs surface profile and the thickness modulation in the form of a crescent of the GaAs layer. In order to gain understanding on the growth process at the atomic level, we developed a two-dimensional Monte-Carlo simulation based on the solid-on-solid model. In good agreement with experimental results, our kinetic model shows that the self-limited profile results from the competition between the growth rate anisotropy on the different facets of the groove and the surface diffusion of adatoms. The predictions of the growth modeling are experimentally employed to control at the nanometer scale the shape of quantum wires using successive changes in the growth conditions. This understanding of the growth mechanisms opens the way to an accurate control of the quantum confinement in quantum wires.