Liquid-phase epitaxy of AlGaAs heterostructures on profiled substrates

A method of fabrication of planar local structures using the selective epitaxial growth of GaAs and AIGaAs layers from liquid phase on profiled GaAs substrates was developed. The planar regrowth of the recesses formed in GaAs substrates by local etching was performed using the anisotropy of epitaxial growth rates and also by providing the uniformity of mass flow to the surface of local epilayer. The developed method of localized structures fabrication was used for improving the characteristics of discrete light emitting diodes — LED and for fabrication of DLE monolithic arrays.     

Equilibrium dopant incorporation in CVD silicon epitaxy

In-situ doping of epitaxial silicon layer with arsenic is controlled either by means of a mixed equilibrium – kinetical mechanism characterised by the influence of layer growth rate, or by means of a pure equilibrium-related mechanism showing no layer growth rate effect. Layer growth rate can be selected to mark the transition from one mode to the other by introducing a characteristic rate v. Equilibrium controls dopant incorporation when v_epi 〈 v 〉 0 is ture. That critical rate depends on temperatur as well as on input partial pressure of arsine and can be expressed by the three different empirical quantities A, B, and E. The former two of them are defined by conventional dopant incorporation theory. The additional quantity E is a function of arsine input pressure. That function not only defines an upper limit E= E_∞ but also a lower limit E= 0 at which v= 0 is valid.     

On the deposition mechanism of boron in silicon CVD epitaxy

The deposition mechanism of boron doping in CVD silicon epitaxy has been investigated by exposing silicon substrates to B₂H₆ H₂ doping gas mixtures at epitaxy temperatures and examining the effect by dopant profile measuring in an afterwards intrinsically in-situ deposited epitaxial silicon layer. It has been shown that boron is deposited increasing its concentration on the surface linearly with prolonged exposition time and desorbed by purging the surface in pure hydrogen. In the latter case its content decreases linearly proportional to the predeposited concentration. The desorbed boron builds up a secondary doping source which maintains a parasitic boron flow for reincorporation during following layer growth.     

Novel reactor for high volume low-cost silicon epitaxy

The chemical vapor deposition of epitaxial layers of silicon is a widely used process in the electronic industry. It is a batch process and the relatively small capacity (i.e., 20–30 wafers) of epitaxial reactors significantly contributes to the expense of the process. We thus embarked on a research project aimed at a significant expansion of the reactor capacity. The first step was to conduct a complete characterization of the presently used reactors by means of flow visualization, temperature measurements and mass spectrometric studies; results of these studies will be briefly presented and discussed. The main conclusion from these studies was that up-scaling of present reactors is not economical. We thus designed and constructed a novel epitaxial reactor, radically different from current types. In this reactor the susceptor structure consists of parallel graphite discs. Wafers are fastened to one or both sides of these discs. The nutrient gaseous mixture is injected into spaces between discs by a specially designed gas distributor, which delivers the same amount of the mixture to all interdisc spacings, thus insuring the wafer-to-wafer thickness uniformity. A combination of the rotation of the susceptor discs with the gas distributor motion insures the on-the-wafer thickness uniformity. The above described parallel packing allows much higher reactor capacities (e.g., 50–100 wafers). It also results in a more economical reactor in terms of consumption of energy and chemicals. We shall illustrate the application of the novel reactor (known as the “RCA Rotary Disc Reactor”) to epitaxial deposition of silicon from SiCl₂H₂.     

Doping process control in silicon epitaxy (I). System identification

The transition behaviour of the phosphorus incorporation in silicon epitaxial layers grown in a CVD reactor has been investigated, considering the reactor as a linear control system with u = lg p⁰_PH3 (t) as the input and y = lg N(t) as the output. The response of system to both upward and downward step inputs has been studied experimentally, using SiH₄ and PH₃ sources. The dopant system of a horizontal silicon epitaxial reactor has been identified and a mathematical model relating to the transient behaviour of the system has been found. The parameters of the model have been estimated from layer growth experiments. The step response functions found can be approximated by an exponential function relating to n time constants T, all equal to each other. It was found for the system investigated that the second order model is of sufficient accuracy for the optimal control calculations described in the next part of this series.     

Improvements in silicon doping of InP and GaInAs in metalorganic molecular beam epitaxy

We have investigated the Si doping of InP and GaInAs in metalorganic molecular beam epitaxy (MOMBE) by using a conventional Si effusion cell. In order to reduce the formation of SiC promoted by the background gases in MOMBE, we introduced a liquid nitrogen cooled baffle between the cell and the mechanical shutter. The results show that the passivating reaction can be substantially suppressed by a proper treatment of the source cell. The doping efficiency remains constant over a long period of operation corresponding to a large total layer thickness (>100 μm). The comparison of SIMS analysis with Hall data reveals an electrical activation of Si in InP up to 100% and about 65% for Si in GaInAs. These results and the investigations on doping profiles show that Si is a suitable donor in InP and GaInAs in the MOMBE process.     

Doping process control in silicon epitaxy (II). Calculation of optimum control

A procedure is developed to obtain desired dopant profile in epitaxial layer growth. Based an the results of system identification, linear-quadratic optimal control theory is used to determine the optimal input PH₃ concentration as a function of time. In the performance index an auxiliary weighting coefficient must be incorporated. It is discussed how to select this weighting coefficient.     

Epitaxial growth of magnetic semiconductor EuO on silicon by molecular beam epitaxy

Functional oxides demonstrate a wide range of magnetic, optical and transport properties. Their integration with silicon promises significant advances in electronics. An important key in enabling brand‐new oxide technologies is the utilization of silicon/oxide epitaxy, thus making quality of the interface a critical issue. The progress depends on our ability to avoid formation of impurity phases at the interface and to tackle structural mismatch of the oxide and Si. We design a novel chemical protection of Si (001) surface on the submonolayer scale based on the surface metal silicide with the (1×5) reconstruction. This new technique is applied to the long‐standing problem of integration of a ferromagnetic semiconductor with Si. Direct epitaxial growth of EuO on Si without any buffer layer, so far inaccessible, is achieved by molecular beam epitaxy. The nucleation step, comprising first 10 monolayers of EuO, is followed by a distillation‐controlled growth. An alternative to standard capping procedures for EuO, based on controlled formation of an amorphous Eu₂O₃ layer, is devised. Crystal perfection of the films is established ex situ by x‐ray diffraction and Rutherford backscattering. Magnetic properties of the EuO films match those of the bulk.     

Optimisation of low-temperature silicon epitaxy on seeded glass substrates by ion-assisted deposition

Using single crystalline Si wafer substrates, ion-assisted deposition (IAD) has recently been shown [J. Crystal Growth 268 (2004) 41] to be capable of high-quality high-rate epitaxial Si growth in a non-ultra-high vacuum (non-UHV) environment at low temperatures of about 600 °C. In the present work the non-UHV IAD method is applied to planar borosilicate glass substrates featuring a polycrystalline silicon seed layer and carefully optimised. Using thin-film solar cells as test vehicle, the best trade-off between various contamination-related processes (seed layer surface as well as bulk contamination) is determined. In the optimised IAD process, the temperature of the glass substrate remains below 600 °C. The as-grown Si material is found to respond well to post-growth treatments (rapid thermal annealing, hydrogenation), enabling respectable open-circuit voltages of up to 420 mV under 1-Sun illumination. This proves that the non-UHV IAD method is capable of achieving device-grade polycrystalline silicon material on seeded borosilicate glass substrates.     

Study of ZnSe and CdSe thin film epitaxy on germanium and silicon substrates

The parameters of vaporization, mass-transfer, condensation, and epitaxial growth by hot wall technique (HWT) of ZnSe and CdSe thin films on monocrystalline Ge and Si substrates are studied (Bubnov et al.). It is shown, that the layers structure is improved as the mass transfer mechanism approaches to gasodinamical vapor flow. The influence of condensation temperature of the layers on their crystallographic structure is shown. The increase of the temperature gradient from the source towards the substrate as well as the substrate temperature conditions for growing layers of hexagonal modification. The decrease of the temperature gradient leads to cubic modification. The electron diffraction study revealed the stepwise character of the zinc selenide and cadmium selenide film growth. The knowledge of the parameters of ZnSe and CdSe thin films on monocrystalline Ge and Si by hot wall technique at relatively low substrate temperatures allows to obtain layers, suitable for formation of solid state devices for registration and reflection of optical information.     

Heterogeneous kinetics and mass transport in chemical vapour deposition processes: Part II application to silicon epitaxy

The results obtained for the epitaxial deposition of Si from SiC1₄ and H₂ in a rotating disc reactor are described by a single equation in terms of kinetics and transport. The kinetic parameters required to fit the experimental data suggest that the rate determining step for the CVD process is the homogeneous conversion of SiC1₄ to SiHCl₃. The kinetic analysis also shows that the commonly reported activation energy for the overall growth is too low as a result of measurements being made under conditions of combined transport and kinetics. Results of other workers obtained with reactors of very different geometries, when normalised, are in excellent quantitative agreement with both the experimental and theoretical curves found in this work.     

High-quality silicon/insulator heteroepitaxial structures formed by molecular beam epitaxy using Al2O3 and Si

Heteroepitaxial growth of γ-Al₂O₃ films on a Si substrate and the growth of Si films on the γ-Al₂O₃/Si structures by molecular beam epitaxy have been investigated. It has been found from AFM and RHEED observations that, γ-Al₂O₃ films with an atomically smooth surface with an RMS values of ∼3 Å and high crystalline quality can be grown on Si (1 1 1) substrates at substrate temperatures of 650–750°C. Al₂O₃ films grown at higher temperatures above 800°C, did not show good surface morphology due to etching of a Si surface by N₂O gas in the initial growth stage. It has also been found that it is possible to grow high-quality Si layers by the predeposition of Al layer followed by thermal treatment prior to the Si molecular beam epitaxy. Cross-sectional TEM observations have shown that the epitaxial Si had significantly improved crystalline quality and surface morphology when the Al predeposition layer thickness was 10 Å and the thermal treatment temperature was 900°C. The resulting improved crystalline quality of Si films grown on Al₂O₃ is believed to be due to the Al₂O₃ surface modification.     

Growth of InP in chemical beam epitaxy with high purity tertiarybutylphosphine

InP layers were grown by chemical beam epitaxy (CBE) using high purity thermally precracked tertiarybutylphosphine (TBP) and trimethylindium (TMI) as the source of the group III element. For optimized substrate temperature and V/III ratio, InP films of good electrical and optical quality have been obtained; the n-type background carrier concentration is (1–2) × 10¹⁵ cm^-3, with a Hall mobility at 77 K being μ₇₇ = 45,000 cm² V^-1 s^-1. Given the low value of the V/III ratio, and according to mass spectrosc measurements, the phosphorus species giving rise to epitaxy is expected to be the dimer P₂. The TBP consumption in CBE is very low when compared to organometallic vapour phase epitaxy (OMVPE), typicaly below 0.25 g/μm of InP layer.     

A multi-wafer growth technique for GaAs vapor phase epitaxy using the AsCl3-Ga-N2 system with a horizontal reactor

A multi-wafer growth technique for vapor phase epitaxial GaAs has been developed using the open-tube AsCl₃-Ga-N₂ system with a conventional horizontal reactor. Use of this technique allows to process 4 wafers in a run with each wafer being 6 cm². The uniformities of growth rate, carrier concentration, and Hall mobility with wafer-to-wafer are typically ∽±3%, ∽±4%, and ∽±3%, respectively, and are shown to be sufficient for demanding microwave device applications.     

New frontiers of molecular beam epitaxy with in-situ processing

The in-situ process combines film growth and device fabrication steps which take place under an ultra-high vacuum (UHV) or a controlled ambient environment. Processing, materials, and devices produced using this technique with molecular beam epitaxy (MBE) are reviewed.     

Morphological and structural properties of InP/Gd2O3 nanowires grown by molecular beam epitaxy on silicon substrate

InP/Gd₂O₃ heterostructures have been prepared by molecular beam epitaxy of Gd₂O₃ on InP nanowires grown on silicon substrates by molecular beam epitaxy assisted with the vapor–liquid–solid method. Transmission electron microscopy showed Gd₂O₃ nanocrystals, having diameters between 3 and 7 nm, decorating the sidewalls of InP nanowires. No epitaxial relationship was observed between Gd₂O₃ nanocrystals and InP nanowires due an amorphous interfacial layer. Depending on the Gd₂O₃ growth temperature, two morphologies have been highlighted. For Gd₂O₃ grown at 30 °C, anisotropic heterostructures made of oxide nanocrystals covering just one side of the nanowires were observed, while at 250 °C Gd₂O₃/InP core/shell nanowires were identified.     

Control of ridge shape for the formation of nanometer-scale GaAs ridge quantum wires by molecular beam epitaxy

We have investigated the molecular beam epitaxial (MBE) growth mechanisms of nanometer scale GaAs ridge structures formed on patterned substrates and studied the way to control the widths of ridges and those of quantum wires grown on them. It is found that the width of the ridge structure decreases, as the growth temperature is reduced, reaching about 20 nm when grown below 580°C. The width of an AlAs ridge (10 nm at 570°C) is always found to be narrower than that of GaAs. A Monte Carlo simulation is performed to investigate the diffusion process of atoms in these ridge structures and indicates the important role of thermodynamical stability on the shape of a nanometer structure.     

Effective reduction of bowing in free-standing GaN by N-face regrowth with hydride vapor-phase epitaxy

Free-standing GaN films prepared with hydride vapor-phase epitaxy (HVPE) technique usually show bowing resulting from the high densities of defects near the N-polar face after separation from the original substrates. To solve the problem, a simple technique has been developed. A GaN layer was regrown on the N-polar face of the free-standing GaN by HVPE. High-resolution X-ray diffraction (HRXRD) measurements were performed to compare the bowings among GaN films before laser lift-off (LLO), after LLO, and after regrowth. The apparent reductions of XRD full-width at half-maximum (FWHM), along with the increase of XRD peak intensity, after regrowth clearly demonstrate the effectiveness of this method to eliminate bowings of the free-standing GaN films.     

Indium surface segregation in InGaAs-based structures prepared by molecular beam epitaxy and atomic layer molecular beam epitaxy

We present a comparative study on In surface segregation in InGaAs/GaAs structures prepared by molecular beam epitaxy (MBE) and atomic layer MBE (ALMBE) at different growth temperatures. The effect of segregation is evaluated by the energy position of exciton transitions in pseudomorphic 10 ML thick In_xGa_1−xAs/GaAs (0.15≤x≤0.30) and in 1 ML thick InAs/GaAs quantum wells. We show that: (i) In segregation decreases with the growth temperatures and is minimized at ALMBE and MBE growth temperatures lower than 260 and 340°C, respectively, and (ii) the segregation is more effective in ALMBE structures than in the MBE counterparts. The growth conditions that have been singled out allow the preparation of structures with high photoluminescence efficiencies even at the low growth temperatures required to minimize In segregation.