LPE growth and characterization of InP/InGaAsP ridge-waveguide lasers at 1.3 μm-wavelength

This paper presents the fabrication and the lasing characteristics of 1.3 μm-wavelength ridge-waveguide laser. The epitaxial material used in this study was grown applying the step-cooling technique of liquid phase epitaxy (LPE). The growth conditions for InGaAsP layers lattice-matched to the (001) InP-substrate are reported for lattice compositions corresponding to photoluminescence peak wavelengths of 1.07, 1.14, and 1.31 μm. We have used a conventional multiple-bin sliding boat to grow the LPE layers and a second apparatus for achieving batches of melts of uniform compositions. In the LPE apparatus the various batch melts (In–Sn In–Zn; In–Ga–As of different composition) were saturated with phosphorus using the seed dissolution technique. The epitaxial layers were grown by a single phase technique at a constant temperature. This LPE growth technique is useful for the fabrication of double-heterostructure wafers with an uniform alloy composition and a well-defined layer thickness. Using these epitaxial materials, metal-clad ridge-waveguide (MCRW) lasers have been prepared with stripe widths of 3.5 μm. CW threshold currents of 18 mA at room temperature are achieved for 200 μm long cavities. These lasers have T₀ values ranging from 50 to 70 K and well linear L-I-characteristics.     

Large area lateral overgrowth of mismatched InGaP on GaAs(111)B substrates

Application of InGaAs/InGaP double‐heterostructure (DH) lasers increases the band offset between the cladding layer and the active layer more than the use of conventional 1.3 µm InGaAsP/InP lasers. As a first step in realizing 1.3 µm InGaP/InGaAs/InGaP DH lasers, we proposed InGaP lattice‐mismatched epitaxial lateral overgrowth (ELO) technique and successfully carried out the InGaP growth on both GaAs (100), (111)B and InP (100) substrates by liquid phase epitaxy. In this work, we grew the InGaP crystal on GaAs (111)B substrate by adjusting Ga and P composition in In solution, to obtain In_0.79Ga_0.21P (λ = 820 nm) virtual substrate for 1.3 µm InGaAs/InGaP DH lasers. To grow the InGaP all over the lateral surface of the substrate, the growth time was extended to 6 hours. The amount of InGaP lateral growth up to 2 hours was gradually increased, but the lateral growth was saturated. The InGaP lateral width was about 250 µm at the growth time of 6 hours. We report the result that optical microscope observation, CL and X‐ray rocking curve measurements and reciprocal lattice space mapping were carried out to evaluate the crystal quality of the grown InGaP layers. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation and Characterization of MBE-grown Si/CaF2 Heterostructures

Growth and fabrication of silicon-on-insulator structures, based on heteroepitaxial growth of insulating films on Si, is an area of research that has rapidly developed in recent years. Thin CaF₂ films and Si/CaF₂ multilayers were prepared on {111}-oriented Si substrates by molecular beam epitaxy (MBE) and were investigated by RHEED and RBS. For epitaxial growth the Si substrates were cleaned using the ultraviolet/ozone surface cleaning method which is an effective tool to remove contaminants from the surface by low-temperature in-vacuo preheating. The growth of CaF₂ on such Si{111} substrates provides epitaxial layers with a high structural perfection. When this layer system, however, is used as a substrate for epitaxial Si growth the Si layers show always twin formation. Si layers without twins could be obtained only after deposition of thin amorphous Si buffer layers at room temperature, immediately followed by Si growth at 700 °C.     

Si–Ge–Sn alloys: From growth to applications

In this review article, we address key material parameters as well as the fabrication and application of crystalline GeSn binary and SiGeSn ternary alloys. Here, the transition from an indirect to a fundamental direct bandgap material will be discussed. The main emphasis, however, is put on the Si–Ge–Sn epitaxy. The low solid solubility of α-Sn in Ge and Si of below 1 at.% along with the large lattice mismatch between α-Sn (6.489 Å) and Ge (5.646 Å) or Si (5.431 Å) of about 15% and 20%, respectively, requires non-equilibrium growth processes. The most commonly used approaches, i.e. molecular beam epitaxy (MBE) and chemical vapor deposition (CVD), will be reviewed in terms of crucial process parameters, structural as well as optical quality and employed precursor combinations including Germanium hydrides, Silicon hydrides and a variety of Sn compounds like SnD₄, SnCl₄ or C₆H₅SnD₃. Special attention is devoted to the growth temperature window and growth rates being the most important growth parameters concerning the substitutional incorporation of Sn atoms into the Ge diamond lattice. Furthermore, the mainly CVD-driven epitaxy of high quality SiGeSn ternary alloys, allowing the decoupling of band engineering and lattice constant, is presented. Since achieving fundamental direct bandgap Sn-based materials strongly depends on the applied strain within the epilayers, ways to control and modify the strain are shown, especially the plastic strain relaxation of (Si)GeSn layers grown on Ge.Based on recently achieved improvements of the crystalline quality, novel low power and high mobility GeSn electronic and photonic devices have been developed and are reviewed in this paper. The use of GeSn as optically active gain or channel material with its lower and potentially direct bandgap compared to fundamentally indirect Ge (0.66 eV) and Si (1.12 eV) provides a viable solution to overcome the obstacles in both fields photonics and electronics. Moreover, the epitaxial growth of Sn-based semiconductors using CMOS compatible substrates on the road toward a monolithically integrated and efficient group IV light emitter is presented.     

Growth of Heterostructures Based on InAs–AlxGa1–xSb

It is reported on the liquid phase epitaxial (LPE) growth of heterostructures on the base of InAs–Al_xGa_1–xSb. The paper includes the investigation of epitaxial layers of Al_xGa_1–xSb alloys on InAs substrates and results of experiments for the determination of optimum growth regimes.     

Investigation of 3C-SiC growth on Si(111) by vapor–liquid–solid transport using a SiGe liquid phase

The crystal growth of 3C-SiC onto silicon substrate by Vapor–Liquid–Solid (VLS) transport, where a SiGe liquid phase is fed with propane, has been investigated. Three sample configurations were used. In a preliminary approach, the VLS growth of SiC was conducted directly onto Si substrate using a Ge film as liquid catalyst. It led to the growth of a thick continuous SiC polycrystalline layer which was floating over a SiGe alloy located between the silicon substrate and the topping SiC layer. In the second configuration, a thin seeding layer of 3C-SiC grown by chemical vapor deposition (CVD) was used and the VLS growth was localized using a SiO₂ mask. The liquid phase was a CVD deposited SiGe alloy. The growth of a few hundred nanometers thick 3C-SiC epitaxial layer was demonstrated but the process was apparently affected by the presence of the oxide which was dramatically etched at the end. In the last configuration, the silicon substrate was patterned down to 10 μm and a thin seeding layer of 3C-SiC was grown by CVD onto this patterned substrate. The liquid phase was again a CVD deposited SiGe alloy. In this last configuration, the presence of epitaxial SiC was evidenced but it grew as trapezoidal islands instead of an uniform layer.     

Hydrothermal Liquid Phase Epitaxy of Gallium Orthophosphate on Quartz Crystal Substrates

Gallium orthophosphate (GaPO₄) layers for surface acoustic wave (SAW) and sensor applications have been grown on quartz crystal substrates with sizes of about 30 x 30 mm² by hydrothermal liquid phase epitaxy (HLPE). The growth of epitaxial GaPO₄ layers is difficult because of a strong tendency for twinning. Besides, a retrograde solubility and an intense chemical aggressiveness of the solution has to be considered. Nevertheless, we found an effective crystal growth technique to deal with these problems using large and qualitatively good substrate crystals of quartz. The most important step of the epitaxy is the formation of an interlayer between the quartz substrate and the GaPO₄ deposit. Epitaxial layers with thickness up to 500 μm were obtained and characterised by means of X‐ray techniques.     

Liquid phase epitaxial growth of Ga1−xAlxAs on channeled substrates

The growth mechanism of liquid phase epitaxial layers of Ga_1?xAl_xAs on preferentially etched GaAs substrates has been investigated. It has been found that enhanced diffusion of As atoms due to a local concentration gradient, which is set up by non-uniform growth at channels, plays a critical role in determining the growth morphology. The relation between growth morphology and growth conditions is discussed by using a simple growth model.     

In-process monitoring and control of doping gas concentration during vapor phase silicon epitaxial growth by using a flame photometric detector

An in-process monitoring and control method of the doping gas concentration during epitaxial growth of Si was developed. A flame photometric detector (FPD) can be used as a monitor for the PH₃ and B₂H₆ dopant concentrations in the injected doping gases. A combination of this dopant monitor with an automatic control system of the silicon source (SiHCl₃) gas concentration using an infrared spectrophotometer as a monitor, makes possible an automatic in-process control of the concentrations of dopant and of silicon source gas supplied to the reactor. The present system provides an accurate and reproducible control of impurity concentrations in Si epitaxial layers. Good correlation between the monitored signal (or the doping gas concentration) and the impurity concentration incorporated into the growth layers was confirmed for PH₃ (n-type) and B₂H₆ (p-type) doping. For the B₂H₆ doping, a divergence from the linear relationship between the doping gas concentration and the impurity concentration in the layers was observed in the level of acceptor concentration below about 10¹⁵ atoms/cm³. The transient response of the present system was measured by growing epitaxial layers with increasing and decreasing step-changes in the dopant gas flow during continuous deposition of the layers. Some interesting, but complicated, transient responses of impurity concentration in the growth layer were observed. The responses are different between the PH₃ doping and the B₂H₆ doping, and also different between increasing and decreasing steps especially for the B₂H₆ doping.     

Liquid-phase epitaxy of ZnSe from Zn-Ga solution

Undoped or Ga doped ZnSe single crystal layers with thicknesses between 3 and 13 μm were grown on ZnS_xSe_1?x (0 ? x ? 1) single crystal substrates by liquid phase epitaxy in a sealed tube system from a Zn or Zn-Ga alloy solution. Smooth and uniform epitaxial layers with high crystal perfection were obtained on the (111)- and (100)-oriented substrates. All the epitaxial layers were n-type and highly conductive (0.1 < ? < 10 Ω cm). A remarkable enhancement of the blue part of the photoluminescence spectra at room temperature was observed for the epitaxial layers grown from the Zn-Ga alloy solution.     

Lateral confined epitaxy of GaN layers on Si substrates

We developed a novel, simple procedure for achieving lateral confined epitaxy (LCE). This procedure enables the growth of uncracked GaN layers on a Si substrate, using a single, continuous metalorganic chemical vapor deposition (MOCVD) run. The epitaxial growth of GaN is confined to mesas, defined by etching into the Si substrate prior to the growth. The LCE-GaN layers exhibit improved morphological and optical properties compared to the plain GaN-on-Si layers grown in the same MOCVD system. By performing a set of LCE growth runs on mesas of varying lateral dimensions, we specified the crack-free range of GaN on Si as 14.0±0.3 μm.     

Catalyzed-assisted growth of well-aligned silicon oxide nanowires

Bulk quantity and ultra-long silicon oxide nanowires on micrometer-sized solid tin balls have been synthesized by typical chemical vapor deposition via a vapor–liquid–solid process. Low melting point tin droplets can be used as an effective catalyst for the large-scale growth of highly aligned silicon oxide nanowires. Observations using scanning electron microscopy indicate that numerous nanowires simultaneously nucleate, grow at nearly the same rate, and simultaneously stop growing. The silicon oxide nanowires have a uniform diameter distribution about 60 nm and are well-aligned. A model for the growth of silicon oxide nanowires on the surface of the tin balls was proposed. The Sn balls on the substrate come from the thermal evaporating SnCl₂ powders, and one of the reactants, Si, on the surface of a Sn ball come from the silicon wafer. Silicon reacts with oxygen to form silicon oxide nanowires on the surface of a liquid Sn ball.     

Model for defect-free epitaxial lateral overgrowth of Si over SiO2 by liquid phase epitaxy

A growth model for defect-free epitaxial lateral overgrowth by liquid phase epitaxy is presented. Growth in the pure step flow mode and a preferential development of (111) planes in liquid phase epitaxy permit one to quantitatively predict the overgrowth. The shape and size of the Si lamellae which grow over SiO₂ depend on the crystallographic orientations of the substrate growth face and of the seeding windows. Overgrowth experiments with Si on oxidized, (111)- and (100)-oriented Si wafers serve to verify the model. Growth experiments from In and Bi solutions in temperature intervals between 950 and 800°C yield overgrowth widths up to 120 μm and aspect ratios of 40:1 on (111) oriented wafers.     

MOVPE of AlN-free hexagonal GaN/cubic SiC/Si heterostructures for vertical devices

The heterostructures of GaN/SiC/Si were prepared without using AlN or AlGaN buffer layers (AlN buffers) in the metalorganic vapor phase epitaxy of GaN on SiC. GaN (0 0 0 1) with specular surface was obtained. The AlN buffers are usually used in the conventional growth of GaN on SiC due to the poor nucleation of GaN on SiC. Instead, the nucleation of GaN was controlled by varying the partial pressure of H₂ in the carrier gas, the mixture of H₂ and N₂, during the low-temperature (600 °C) growth of GaN (LT-GaN). After the LT-GaN, the high-temperature (1000 °C) growth of GaN was performed using pure H₂ as the carrier gas. The epitaxial film of cubic SiC (1 1 1) on a Si (1 1 1) substrate was used as the SiC template. Increasing the partial pressure of H₂ in the carrier gas decreased the coverage of SiC surface by LT-GaN. It is suggested that the hydrogen atoms adsorbed on the surface of SiC is preventing the nucleation of GaN.     

Structural Studies on Ni75(SnSi)25 and Ni75(SnBi)25 Alloys

Ternary alloys on the quasi-binary system Ni₃Sn(r) in equilibrium condition. It has been observed that these phases undergo transformations at high temperatures but upto 500 °C room temperature modifications are stable. The two phases do not have any appreciable solid solubility in either of them. The phase Ni₃Si(r) crystallizes in to Cu₃Au (Li₂) structure where as Ni₃Sn(r) is based on c.p.h. structure with a = 5.305 A°, c = 4.254 A°. No new ternary phase has been detected in Ni₃Sn Ni₃Si section. The investigated alloys of the Ni Sn Bi system contain 75 at.% Ni. All the ternary alloys show simultaneous occurrence of three phases, namely Ni₃Sn(r), NiBi and Ni(Sn) in equilibrium state. The phase NiBi has NiAs(B8) type of structure. Due to non-existence of isostructural phases in the two binary systems (Ni Sn and Ni Bi), single solid solution phases are not formed. Widely differing atomic sizes of nickel and bismuth atoms restrict the formation of solid solution of bismuth in nickel in contrast to Ni(Sn) where atomic size factor is favourable.     

Preparation of Cu2ZnSnS4 single crystals from Sn solutions

We investigated the phase diagrams of the Cu₂ZnSnS₄ (CZTS)–Sn pseudobinary system in order to obtain knowledge useful for the growth of high-quality CZTS single crystals using a solution-based method. For Sn solutions saturated with less than ～60 mol% CZTS, the solutes are separated into two phases (CZTS phase+SnS_x phase+liquid phase). On the other hand, for solutions with more than 60 mol% CZTS, the solutes are single phase (CZTS phase+liquid phase). The CZTS single crystals were obtained from a 70 mol% CZTS solution (liquid temperature 850 °C) at 900 °C. The powder X-ray diffraction (XRD) pattern of the CZTS single crystal shows preferred orientations of (112), (220) and (312) planes, confirming the Kesterite structure of CZTS. The Raman spectrum shows three peaks at 287, 338, 371 cm^?1, which corresponded to CZTS peaks. The composition of the CZTS single crystal along the growth direction is found to be slightly Cu-poor, Zn-rich and S-rich. Therefore, it is assumed that the Cu vacancy is the dominant p-type conduction mechanism.     

Analysis of epitaxial laterally overgrown silicon structures by high resolution x‐ray rocking curve imaging

Spatially resolved rocking curve imaging has been used to analyze laterally overgrown silicon layers grown by liquid phase epitaxy. We were able to study both the overgrown layer as well as the strain fluctuations of the Si substrate underneath by means of a tabletop x‐ray topographic setup. The strain‐field analysis reveals relative changes of the lattice parameter up to 3.5×10^‐6 in the silicon substrate underneath the overgrown layer in particular regions and a down bending of both wings of the epitaxial overgrown layers. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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 temperature dependence in molecular beam epitaxy of gallium arsenide

Epitaxial layers of GaAs were grown on GaAs(100) at substrate temperatures ranging from 400° to 600°C by molecular beam epitaxy. Surface structures of the substrate and the epitaxial layers were investigated by means of low-energy electron diffraction. Two new structures of c(4 × 4) and c(8 × 8) were observed from layers grown at the low temperature of 400°C. The electrical and optical properties of layers doped with Si were investigated by measurement of Hall effect and photoluminescence as a function of growth temperature. It is found that a semi-insulating layer is grown below a critical temperature, and the layer is useful as a buffer layer for GaAs FET's. Variation of carrier concentration was observed near the interface between layers grown at different temperatures under a constant Sn beam flux. The effect is attributed to defect-induced segregation of Sn.     

Silicon nanowire growth on poly‐silicon‐on‐quartz substrates formed by aluminum‐induced crystallization

The vertical growth of Si nanowires on non‐monocrystalline substrates is of significant interest for photovoltaics and other energy harvesting applications. In this paper, we present results on using poly‐Si layers formed by aluminum‐induced crystallization (AIC) on fused quartz wafers as an alternative substrate for the vapor‐liquid‐solid (VLS) growth of vertical Si nanowires. Oxidation of the Al surface to Al₂O₃ before the a‐Si deposition was shown to be a key requirement in the formation of the poly‐Si template since it promotes the crystallization of the a‐Si into Si(111) which is required for vertical silicon nanowire growth. The effect of Al deposition technique (DC sputtering versus thermal evaporation) on a‐Si crystallization and Si nanowire growth was investigated. The use of Al thermal evaporation yielded AIC poly‐Si layers with the highest fraction of 〈111〉 grains as measured by orientation imaging microscopy (OIM) which enabled the growth of vertical Si nanowires. Cross‐sectional transmission electron microscopy analysis confirmed that the 〈111〉 Si nanowires grew epitaxially off of {111}poly‐Si grains in the AIC layer. This study demonstrates the potential of using AIC poly‐Si as a template layer for the vertical growth of silicon nanowires on amorphous substrates.