Role of the substrate deoxidation process in the growth of strained InAs/InP heterostructures

We have investigated the role of the arsenic flux used during the substrate deoxidation process in the MBE (molecular beam epitaxy) growth of strained InAs/InP heterostructures. Two different experiments were performed: (i) thermal cleaning of the InP wafer under an As flux at different exposure times and (ii) the growth of very thin InAs layers (3-9 ML). The samples grown were characterized by Raman spectroscopy and selected area X-ray photoelectron spectroscopy. The results obtained demonstrated the formation of an InAs_xP_1−x sublayer at the interface of the InAs/InP system. The annealing of InP under an As flux promotes not only As → P substitution on the surface, but also the subsequent diffusion of As atoms into the deeper subsurface region of InP.     

Growth of InGaAs/GaAs heterostructures with abrupt interfaces on the monolayer scale

Segregation processes entail severe deviations from the nominal composition profiles of heterostructures grown by molecular beam epitaxy for most semiconductor systems. It is, however, possible to compensate exactly these effects, as shown here for InGaAs/GaAs. The deposition of a one-monolayer-thick indium-rich prelayer of InGaAs (or of a sub-monolayer amount of InAs) prior to growth of In_xGa_1−xAs allows forming a perfectly abrupt In_xGa_1−xAs-on-GaAs interface. Thermal annealing can furthermore be performed at the GaAs-on-InGaAs inter face, so as to desorb surface indium atoms and suppress In incorporation in the GaAs overlayer. This powerful approach has been validated from a detailed study of the surface composition at various stages of the growth of InGaAs/GaAs quantum wells, as well as from high-resolution transmission electron microscopy and photoluminescence investigations.     

Thermal radiation and low-temperature-vapour growth of HgI2 crystal in production furnace

Heat exchanges in a sealed ampoule in the LTVG (low temperature vapour growth) furnace have been modelled in order to compute temperature fields and control the growth of HgI₂ crystals from vapour phase at low temperatures. We use a coupled conductive-radiative model to determine the shapes of the source and the crystal at different equilibrium states (i.e. without growth rate). The model involves conductivity anisotropy in the crystal and radiative exchanges between grey and diffuse surfaces (source and crystal interfaces, Pyrex walls), which are considered as opaque. Internal buoyancy effect is not taken into account as the pressure inside the ampoule is very small. The source temperature is fixed. For different undercoolings, i.e. for different cold finger temperatures, the “equilibrium” isotherm between the source/gas and crystal/gas interface has been numerically obtained. This “equilibrium” isotherm, which is associated with the stop of the growing process, gives a crystal shape. This shape is compared with experimental results given by the ETH-Zürich group. The model would permit a better understanding and control of the future HgI₂ crystal growth experiment. The computations are performed using a finite element package (FIDAP).     

Dynamic stability analysis of the solidification of binary melts in the presence of a mushy region: changeover of instability

A dynamic linear instability analysis of steady-state binary melt solidification with a mushy region has been carried out. Such an instability differs from a conventional morphological one of a planar solid–liquid interface and is connected with the perturbations in the steady-state solidification velocity. Solidification with a narrow mushy region has been revealed to be absolutely unstable with a monotonous instability of a “hard” type. An increase of the mushy region width leads to an instability changeover from the “hard” type to the oscillatory “soft” one. Both the critical changeover width and the neutral stability curves have been determined as the functions of relevant physical and operating parameters. The steady-state solidification regime with a broad mushy region is absolutely stable. Thus, the mushy region width has been shown to represent a stabilizing factor.     

Low-temperature synthesis of starting materials for β-barium metaborate (β-BBO) crystal growth

Synthesis of starting materials for β-barium metaborate (β-BBO) crysal growth by a low-temperature “carbonate” method has been investigated. Materials systhesized below the σ-β phase transformation temperature of BBO have been subjected to thermogravimetry, X-ray difraction and chemical analysis. The results show that an almost carbonless stoichiometric β-phase can be obtained without the need for the widely employed high-temperature “carbonate” synthesis. Some advantages of using the β-BBO material synthesized by the low-temperature “carbonate” method are emphasized.     

Alloy phase separation in InAs/InAlAs/InP nanostructure superlattices studied by finite element calculation

The alloy phase separation effect in InAlAs spacer layer of InAs/InAlAs nanostructure superlattices was studied by two-dimensional finite element calculation. The calculation results showed that InAs islands with wide top would prefer to induce “V”-like In-rich InAlAs arms above InAs islands in InAlAs spacer layer, while InAs islands with narrow top would promote the formation of “I”-like In-rich InAlAs arms above InAs islands in InAlAs spacer layer which corresponded well with the experimental results reported in Ref. [9].     

Effect of steady ampoule rotation on radial dopant segregation in vertical Bridgman growth of GaSe

For vertical Bridgman growth of the nonlinear optical material GaSe in an ampoule sufficiently long that flow and dopant transport are not significantly influenced by the upper free surface, we show computationally that steady rotation about the ampoule axis strongly affects the flow and radial solid-phase dopant segregation. Radial segregation depends strongly on both growth rate U and rotation rate Ω over the ranges 0.25 μms⁻¹U3.0 μms⁻¹ and 0Ω270 rpm. For each growth rate considered, the overall radial segregation passes through two local maxima as Ω increases, before ultimately decreasing at large Ω. Rotation has only modest effects on interface deflection. Radial segregation computed using a model with isotropic conductivity (one-third the trace of the conductivity tensor) predicts much less radial segregation than the “correct” model using the anisotropic conductivity, with the segregation decreasing monotonically with Ω. Consideration of a model in which centrifugal acceleration is deliberately omitted shows that, as Ω increases, diminution and ultimately disappearance of the “secondary” vortex lying immediately above the interface is due to centrifugal buoyancy, while axial distension of the larger “primary” vortex above is due to Coriolis effects. These results, which are qualitatively different from those accounting for centrifugal buoyancy, suggest that several earlier computational and analytical predictions of rotating vertical Bridgman growth are either limited to rotation rates sufficiently low that centrifugal buoyancy is unimportant, or are artifacts associated with its neglect. The overall radial segregation depends approximately linearly on the product of and the growth rate U for the conditions considered, where is the segregation coefficient.     

Growth of gallium antimonide epitaxial layers on indium arsenide substrates

In the present work the possibility is demonstrated of growing gallium antimonide epitaxial layers on indium arsenide substrates using the liquid phase epitaxial (LPE) method. The influence is nivestigated of the growth conditions on the morphology of the surface and interface of the epitaxial structures InAs GaSb. The optimum technological regimes for growth of heterostructures in the system InAs GaSb are found.     

Crystal structure, carrier concentration and IR-sensitivity of PbTe thin films doped with Ga by two different methods

The comparison of the results of chemical composition, crystal structure, electronic properties and infrared photoconductivity investigations of PbTe/Si and PbTe/SiO₂/Si heterostructures doped with Ga atoms by two different techniques is presented in this work. One of these techniques is principally based on the vapour-phase doping procedure of PbTe/Si and PbTe/SiO₂/Si heterostructures, which were previously formed by the modified “hot wall” technique. The second method of PbTe(Ga)/Si and PbTe(Ga)/SiO₂/Si heterostructure preparation is based upon the fabrication of lead telluride films, which have been doped with Ga atoms in the layer condensation process directly. The lattice parameter and charge carrier density evolutions with the Ga impurity concentration show principally the different character of PbTe(Ga)/Si films prepared by these techniques. It has been proposed that complicated amphoteric (donor or acceptor) behaviour of Ga atoms may be explained by different mechanisms of substitution or implantation of impurity atoms in the crystal structure of lead telluride.     

Highly uniform AlGaAs/GaAs and InGaAs(P)/InP structures grown in a multiwafer vertical rotating susceptor MOVPE reactor

Highly uniform AlGaAs/GaAs and InGaAs(P)/InP epitaxial layers have been grown in a vertical rotating susceptor MOVPE reactor capable of accommodating three 2′ wafers. The unique water-cooled “showerhead”-type injection distributor which is located 1.5 cm above the substrates ensures a uniform reactant distribution, resulting in uniform growth over a wide range of growth conditions. Periodic multilayer and single layer structures have been used to investigate the thickness and compositional uniformities. The thickness variations over a radial distance of 48 mm for three wafers grown in the same run are within ± 2% for both AlGaAs and InGaAs layers, resulting in a standard deviation of only 0.9%. The gallium concentration of an InGaAs layer varies from 46.88% to 47.01% over the same radial distance with the standard deviation of 0.043%. Measurements of InGaAsP layers grown onto 2′ InP wafers with different alloy compositions show good compositional uniformity yielding standard deviations within 4.4 nm in PL wavelength and 135 ppm in lattice mismatch over a 46 mm radial distance.     

Surface reconstruction phase diagrams for InAs, AlSb, and GaSb

We present experimental flux-temperature phase diagrams for surface reconstruction transitions on the 6.1 Å compound semiconductors. The phase transitions occur within or near typical substrate temperature ranges for growth of these materials by molecular beam epitaxy and therefore provide a convenient temperature standard for optimizing growth conditions. Phase boundaries for InAs (0 0 1) [(2×4)→(4×2)], AlSb (0 0 1) [c(4×4)→(1×3)], and GaSb (0 0 1) [(2×5)→(1×3)] are presented as a function of substrate temperature and Group V-limited growth rate (proportional to flux), for both cracked and uncracked Group V species. We discuss differences between materials in the slopes and offsets of the phase boundaries for both types of Group V species.     

Low lying electronic tail spectrum of the attractive δ-potential random system by the path integral method and coherent state representation variational method

By using the path integral method to study the “partition function” and therefore the density of states of the attractive Frisch-Lloyd random system, it is found that the “partition function” is divergent. Therefore, we modify the Frisch-Lloyd model to a system with regular lattice sites on which the atoms are randomly distributed. Path integral and coherent state representation variational methods are applied to this random system; by using the effective Hamiltonian theory of a periodic potential with slowly varying envelope, the low lying tail spectrum which is consistent with the Lifshitz conjecture for a random system is obtained.     

Stress measurements in sol-gel films

Thin solid films of a wide variety of materials have received increased attention during the past decade. These films have been instrumental in the growth of numerous technologies. Until recently, “thin films” have primarily described layers of metallic or dielectric materials deposited onto substrates by evaporation, electron beam or ion beam techniques.

Advances in sol-gel technology have extended film research to include “glassy” materials of either crystalline, or amorphous nature. Sol-gel films can be tailor-made to accommodate a diverse range of applications due primarily to flexibility in chemical make up which determines the respective film's structure. One important characteristic of such films is their inherent residual stress. This inherent stress, and the stress the film introduces to the substrate as it is deposited, can result in a complex stress profile. While “thin” in the case of sol-gel films generally means <1 μm in thickness, large (10–100s of nm of retardance) inherent stress per unit thickness can severely limit a film's performance and subsequent application.

We describe our efforts to quantify the residual stress in silica-based sol-gel films as a function of several processing parameters.     

Glass composition for spinning fibre

The performance of polymer composites reinforced with neutral glass fibres was compared with that of “E” glass reinforced composites. The strength and fatigue characteristics of composites, and the polymerisation kinetics at the glass-polymer interface, were studied. The results show that “N” glass fibre composites possess superior wet strength and are suitable for general application.     

Microstructure and growth rate of chemical vapour deposited TiN films in a vertical cold wall reactor

TiN films were grown on SUS304 substrates heated by an induction furnace in a vertical cold wall reactor. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction were used to characterize the microstructures of films obtained at different deposition conditions (temperature, gas flow rate and gas composition). Film structures obtained in the present vertical reactor had the following features compared with those in the tubular reactor: (1) Abnormally grown “star-shaped” crystals were observed on the surfaces of films deposited in the following ranges of total gas flow rate (Q_T), temperature (T) and partial pressures (P): 9.0×10⁻⁶ ≤ Q_T ≤ 1.6×10⁻⁵ m³ s⁻¹, 1223 ≤ T ≤ 1273 K, 0.92 ≤ P_TiCl4 ≤ 6.18 kPa, P_H2 = P_N2. The matrix grains were responsible for (211) preferred orientation. (2) Surface morphologies did not vary so much with P_TiCl4. On the other hand, a drastic change was brought about by adding HCl to the source gas, i.e., plate-shaped crystals dominated and the large “star-shaped” crystals were no longer present. (3) The apparent activation energy for deposition reaction was 230 kJ/mol (1173 ≤ T ≤ 1273 K) and 76.5 kJ/mol (1273 ≤ T ≤ 1373 K) at P_TiCl4 = 2.43 kPa and P_H2 = P_N2 = 49.45 kPa.     

Anomalous paramagnetism in pressure quenched CdS

Very large “paramagnetic” on positive magnetization has been observed in “pressure quenched” samples of CdS. Pressure quenching is a formative process involving pressure release rates ≈10⁶ bar s⁻¹.

The pressure quenched samples were prepared by pressure quenching at room temperature from above 30 kbar, i.e. from above the insulation-to-metal like transition. magnetization as a function of magnetic field was measured at 293 and 77 K using a vibrating specimen magnetometer. A linear M versus H behavior is observed in fields above a few hundred gauss, with values of χ = (M/H) ≈ 10^-4 cgs units. In some specimens saturation occurs, while in others the magnetization passes through a maximum. The maximum value of the magnetic moment M observed is of the order of tens of gauss.     

LPE growth of GaAs by yo-yo solute feeding method

The effect of gravity on both dissolution and growth of GaAs in the Ga---As system has been investigated using a horizontal “sandwich” system consisting of a substrate-solution-substrate configuration. Remarkable differences were observed in growth and dissolution between upper and lower substrates. These phenomena were attributed to the solutal convection driven by a concentration gradient. Based on these facts, a layer with a thickness of about 80 μm was successively grown by the yo-yo solute feeding method with 8 yo-yo repetition times between 700 and 650°C.     

Highly uniform growth in a low-pressure MOVPE multiple wafer system

A novel horizontal metal organic vapor phase epitaxy (MOVPE) system, which is capable of handling six 3 inch wafers or eighteen 2 inch wafers mounted on a 10 inch diameter susceptor, has been developed for the growth of III–V compound semiconductors. The characteristic features in this system are “triple flow channel” gas injection and “face-down” wafer setting configuration. The inlet for the source gas flow is divided into three zones (upper, middle and lower flows for hydrides, organometals and hydrogen, respectively) to control the concentration boundary layer and the growth area. The wafers are placed inversely to prevent thermal convection and particles on the growing surface. The independent controlled three-part heating system is also adopted to achieve a uniform temperature distribution over an 8 inch growing surface. The thickness and the doping of GaAs, Al_0.3Ga_0.7As, In_0.48Ga_0.52P and In_0.2Ga_0.8As grown by this system are uniform within ± 2% over all 3 inch wafers.     

Structural and electrical properties of GaSb, AlGaSb and their heterostructures grown on GaAs by metalorganic chemical vapor deposition

A systematic study of structural and electrical properties of GaSb and AlGaSb grown on GaAs by metalorganic chemical vapor deposition is reported. In general, the results obtained from surface morphologies, X-ray linewidths and Hall properties are consistent with each other and indicate that the optimal growth conditions for GaSb are at 525°C around V/III = 1. A highest hole mobility of 652 cm²/V · s at RT (3208 cm²/V · s at 77 K) and a lowest concentration of 2.8 × 10¹⁶ cm⁻³ (1.2 × 10¹⁵ cm⁻³ at 77 K) were obtained for GaSb grown under this optimal condition. Compared to the GaSb growth, a smaller V/III ratio is needed for the AlGaSb growth to protect the surface morphology. When Al was incorporated into GaSb growth, mobility decreased and carrier concentration increased sharply. The AlGaSb grown at 600°C had a background concentration about one order of magnitude lower than the AlGaSb grown at 680°C. Room-temperature current-voltage characteristics of GaSb/Al_xGa_{1 − x}Sb/GaSb show a rectifying feature when Al composition x is higher than 0.3, suggesting a valence-band discontinuity at the AlGaSb/GaSb interface. A leakage current much higher than the value predicted by the thermionic emission theory is observed at 77 K, presumably due to a large number of dislocations generated by the huge lattice mismatch between GaSb and GaAs.     

In-situ and ex-situ characterization of GaAs/AlAs quantum well structures using spectroscopic ellipsometry

Spectroscopic ellipsometer is used to monitor the MBE growth of quantum well structures. Real time monitoring of the growth enabled the measurement of growth rate and correlation with RHEED oscillations. The growth of a single GaAs/AlAs quantum well is also monitored in real time using multiple wavelengths. Interface roughness of the interrupted “inverted” AlAs/GaAs interface was also monitored with SE. Under our growth conditions, we measure approximately a 2 ML interfacial region at the inverted interface. A correlation with photoluminescence is also discussed.