Growth of InP/InGaAs multiple quantum well structures by chemical beam epitaxy

InP/InGaAs multiple quantum well structures with up to 200 periods have been grown by CBE. These structures exhibit exceptional lateral uniformity, measured as ±1 Å in period, ±13 ppm in lattice mismatch and ±0.5 nm in wavelength across a 2 inch wafer. Good surface morphology, sharp interfaces and excellent growth control have all been demonstrated.     

The growth of InGaAsP by CBE for SCH quantum well lasers operating at 1.55 and 1.4 μm

InGaAsP has been grown by CBE at compositions of 1.1, 1.2 and 1.4 μm for the development of MQW-SCH lasers. The observed incorporation coefficients for TMI and TEG show strong temperature sensitivity while the phosphorus and arsenic incorporation behavior is constant over the substrate temperature range explored, 530 to 580°C setpoint. For higher substrate temperatures the growth rate increases with the largest growth rates occurring for the 1.4 μm quaternary. Low temperature photoluminescence indicates the possibility of compositional grading or clustering for the 1.1 μm material and also for the 1.2 μm material grown at the lowest substrate temperature. The final laser structure was grown with the InP cladding regions grown at 580°C with the inner cladding and active regions grown at 555°C. Using this approach we have successfully grown MQW-SCH lasers with the composition of the active In_xGa_1−xAs ranging from x=0.33 to x=0.73. Threshold current densities as low as 689 A/cm² have been measured for an 800 μm×90 μm broad area device with x=0.68.     

Compositional control of CdxZn1−xSe grown by photoassisted organometallic vapor phase epitaxy

The photoassisted OMVPE growth technique is important for the fabrication of blue/green laser diodes based on Cd_xZn_1−xSe quantum wells. Low temperature growth with photoassistance is key to the fabrication of these devices, however, the compositional control of Cd_xZn_1−xSe becomes increasingly difficult as the growth temperature is reduced. We have studied the compositional control of Cd_xZn_1−xSe using the sources DMCd, DMZn, and DMSe, with irradiation from a Hg arc lamp. We studied the dependence of the composition on the growth temperature, irradiation intensity, and source mass flows. The composition x increases with increasing temperature and decreases with increasing irradiation intensity. The solid-phase composition is a non-linear function of the gas-phase composition X. The slope of this characteristic, dx/dX, should be minimized for good compositional control. At 475°C without photoassistance, dx/dX is 1.75 near a composition of 20%, as determined from the data of Parbrook et al. Decreasing the temperature increases dx/dX. At 370°C with 12 mW/cm², dx/dX ≈ 13 and at 350°C with 58 mW/cm² dx/dX ≈ 60. We have investigated this behavior at 370°C with 12 mW/cm² irradiation by studying both the composition and the growth rate as a function of the gas-phase composition. The growth rate is non-monotonic, and is minimum for a gas-phase composition of 0.20. The behavior is quite complex, and is not fully understood at the present time. Nonetheless, our results indicate that the Cd-bearing precursor is adsorbed much more strongly than the Zn-bearing precursor. In addition to this, the introduction of the DMCd strongly inhibits the growth of ZnSe. We have achieved sufficiently good compositional control at 370°C and 12 mW/cm² to grow ZnSe/Cd_xZn_1−xSe/ZnSe multiple quantum well structures. More work is necessary in order to clarify the roles of irradiation intensity and VI/II ratio so that good compositional control can be achieved at lower growth temperatures.     

Growth and MBMS studies of reaction mechanisms for InxGa1−xAs CBE

The reaction mechanism involved in the growth of In_xGa_1−xAs lattice matched to InP by chemical beam epitaxy (CBE) was investigated using growth and modulated beam mass spectrometry studies. Emphasis was placed on elucidating how variations in substrate temperature, indium composition and arsenic overpressure influence growth kinetics and how sensitive changes in experimental conditions bring about deviations in the ideal stoichiometry (In_0.53Ga_0.47As) required for lattice matching to InP. Our observations indicate that the compositional variations in the InGaAs stoichiometry at high temperatures (> 485°C) arise because of the changes in the DEG decomposition: desorption branching ratio which is controlled by a temperature- and arsenic pressure-dependent surface population of indium atoms. The low temperature behaviour is governed by the availability of metal surface sites for triethylgallium decomposition which is increased by the presence of surface indium atoms.     

Peculiarities of liquid phase epitaxy in GaInPAs/InP lattice-matched heterostructures

The temperature phase stability of Ga_xIn_1−xP_yAs_1−y solid solution has been analyzed. A simple solution theory with the temperature-independent interaction parameters in solid and liquid phases has been used. The absence of miscibility gaps for all the compositions of the solid solution, lattice-matched to InP at a growth temperature of 640°C, has been demonstrated both theoretically and experimentally. The influence of the elastic deformations on the Ga_xIn_1−xP_yAs_1−y (λ_g = 1.4 μm) solid solution parameters has been observed assuming the model of the layer coherent conjugation in heterostructures.     

Chemical beam epitaxy of AlGaAs and AlInAs using trimethylamine alane precursor

Al_xGa_1−xAs and Al_xIn_1−xAs alloys were grown on GaAs and InP, respectively, by chemical beam epitaxy, using trimethylamine alane (TMAA) as the source of aluminium. TMAA could be used properly only after some problems had been solved. Low carbon and oxygen concentrations were obtained in both alloys, leading to residual hole concentrations of 2 × 10¹⁶ cm^-3 in Al_0.3Ga_0.7As. The abruptness of the AlGaAs/GaAs interface proved the absence of TMAA memory effect. The control of Al_xIn_1−xAs solid composition was more difficult than for Ga_xIn_1−xAs, but was less sensitive to growth temperature. Photoluminescence intensities of Al_0.3Ga_0.7As and Al_0.48In_0.52As grown at 510°C were similar to those of MBE grown materials.     

Electronic traps in mixed Si1−xGexO2 films

Thermally stimulated luminescence (TSL) and infrared (IR) spectroscopy were measured in plasma grown Si_1−xGe_xO₂ (x=0, 0.08, 0.15, 0.25, 0.5) with different thicknesses (12–40 nm). A comparison with the TSL properties of thermally grown SiO₂ and GeO₂ was also performed. A main IR absorption structure was detected, due to the superposition of the peaks related to the asymmetric O stretching modes of (i) Si–O–Si (at ≈1060 cm⁻¹) and (ii) Si–O–Ge (at 1001 cm⁻¹). Another peak at ≈860 cm⁻¹ was observed only for Ge concentrations, x>0.15, corresponding to the asymmetric O stretching mode in Ge–O–Ge bonds. A TSL peak was observed at 70°C, and a smaller structure at around 200°C. The 70°C peak was more intense in all Ge rich layers than in plasma grown SiO₂. Based on the thickness dependence of the signal intensity we propose that at Ge concentrations 0.25x0.5 TSL active defects are localised at interfacial regions (oxide/semiconductor, Ge poor/Ge rich internal interface, oxide external surface/atmosphere). Based on similarities between TSL glow curves in plasma grown Si_1−xGe_xO₂, thermally grown GeO₂ and SiO₂ we propose that oxygen vacancy related defects are trapping states in Si_1−xGe_xO₂ and GeO₂.     

CBE growth of GaAs/GaAs, GaAs/Si and AlGaAs/GaAs using TEG, AsH3 and amine-alane precursors

The growth of high quality AlGaAs by CBE has been limited by the high levels of carbon and oxygen contamination. The use of alane based precursors offers a significant reduction in such contamination. We report for the first time the CBE growth of Al_xGa_1−xAs from triethylgallium, dimethylethylamine-alane and arsine, and compare with. growth from triethylgallium, trimethylamine-alane and arsine. Some preliminary results of work on the CBE growth of GaAs on silicon will also be reported.     

The polycarbosilane-to-SixC1−x conversion studied by inelastic neutron scattering and infrared absorption

Polycarbosilane (PC) is a precursor which is converted to Si_xC_1−x fibers by pyrolysis in an inert gas atmosphere. The changes in the atomic vibrational spectrum during the conversion process from PC to Si_xC_1−x have been examined by means of inelastic neutron scattering (INS) and infrared absorption (IR). The existence of transverse-optical(TO)- and longitudinal-optical(LO)-like phonon modes due to amorphous SiC clusters was established in the original and pyrolyzed PC by INS measurements. After pyrolyzing at 700–800°C, these modes appear distinctly around 730 and 930 cm⁻¹, respectively, in the INS spectra. Pyrolysis at 1000°C makes these TO- and LO-like phonon modes sharper.     

Selective epitaxial growth of Si1−xGex by cold-wall ultrahigh vacuum chemical vapor deposition using disilane and germane

Selective epitaxial growth of Si_1−xGe_x alloys in a cold-wall-type ultrahigh vacuum chamber using pure disilane and pure germane was studied at a constant temperature 550°C. The germanium content in the grown layer was increased as the germane flow rate increased. In all cases, the germanium content in the solid is less than the germane content in the gaseous source. The growth rate was enhanced by germane addition. It is conceivable that the growth rate enhancement is due to increasing the germanium content in the solid. Selectivity was constant at high disilane flow rate and increased at low disilane flow rate by germane addition. Selective epitaxial growth of Si_1−xGe_x was successfully achieved with high growth rate.     

Low-temperature growth of epitaxial layers of ZnSe1−xTex solid solutions

The layers of ZnSe_1−xTe_x (0 < x < 1.0) solid solutions have been grown by liquid-phase epitaxy in a closed tube at 620–680 °C. Zinc chloride served as a solvent. ZnTe and ZnSe crystals were used as sources and substrates with orienting surfaces (110) and (111) for ZnSe and (110) for ZnTe. The composition of the grown layer was specified by the relative content of the ZnSe and the ZnTe in the solution and was controlled by X-ray analysis. The position of the exciton bands in the photoluminescence spectra of ZnSe_1−xTe_x over the interval 0.3 < x < 1.0 is in agreement with the free exciton energies calculated for these compositions. Relatively low-ohmic (of about 10² Ω cm) epitaxial layers of ZnSe_1−xTe_x solid solutions were grown.     

Impedance spectroscopy analysis of AlGaN/GaN HFET structures

For HFET application a series of samples with 30 nm Al_xGa_1−xN (x=0.02–0.4) layers deposited at 1040°C onto optimised 2 μm thick undoped GaN buffers were fabricated. The Al_xGa_1−xN/GaN heterostructures were grown on c-plane sapphire in an atmospheric pressure, single wafer, vertical flow MOVPE system. Electrical properties of the Al_xGa_1−xN/GaN heterostructures and thick undoped GaN layers were evaluated by impedance spectroscopy method performed in the range of 80 Hz–10 MHz with an HP 4192A impedance meter using a mercury probe. The carrier concentration distribution through the layer thickness and the sheet carrier concentration were evaluated. A non-destructive, characterisation technique for verification of device heterostucture quality from the measured C–V and G–V versus frequency characteristics of the heterostructure is proposed.     

The structure of liquid Sb---Se alloys

The structure of liquid Sb_1-xSe_x alloys with x = 0.5, 0.6 and 0.7 was investigated in the temperature range of 600–800°C. The neutron diffraction measurements were carried out by using the high intensity total (HIT) scattering spectrometer of the booster synchrotron utilization facility (BSF) at the National Laboratory for High Energy Physics. A prepeak is observed in the structure factor for these three alloys. It suggests that the medium-range order stillo remains melting for liquid Sb_1-xSe_x alloys, with x = 0.5, 0.6 and 0.7. The structure factor liquid Sb₂Se₃ alloys shows no appreciable temperature variation. For liquid Sb_0.5Se_0.5 alloy, the intensity of the prepeak exhibits an apparent decrease in the temperature range in which the non-metal-metal transition occurs. The non-metal-metal transition in liquid Sb---Se alloys is due to the change in the medium range structure.     

Monitoring real-time CBE growth of GaAs and AlGaAs using dynamic optical reflectivity

Dynamic optical reflectivity (DOR) uses the interference oscillations arising from the multiple reflections, of a normally incident CW laser beam, between the surface of a growing film and the film-substrate interface. The oscillations have a period determined by the refractive index of the film and the laser wavelength. DOR measurements have been made, in real time, during the CBE growth of Al_xGa_1−xAs layers on a GaAs(100) substrate. The results show that the growth rate and the aluminum composition x can be monitored.     

Influence of the growth temperature and substrate orientation on the layer properties of MOVPE-growth (Ga,In)(As,P)/GaAs

The luminescence properties of In_1−xGa_xAs_yP_1−y layers and heterostructures grown lattice matched to GaAs by metalorganic vapour-phase epitaxy (MOVPE) were studied and correlated to the crystalline properties. For laser structures emitting around 800 nm a red-shift of the emission from the active layer (y = 0.72) grown at 680°C together with an anomalous temperature behaviour and excitation dependence of the bandgap is observed. Although some degree of ordering is observed for thick layers of this composition, polarization dependent photoluminescence does not indicate ordering of the quantum well to be the main reason for this excitation dependence. Instead, interfacial In-rich layers are found to be responsible. The thickness of these interfacial layers strongly depends on substrate misorientation and growth conditions.     

Heavily carbon-doped p-type (In)GaAs grown by gas-source molecular beam epitaxy using diiodomethane

Heavily carbon-doped p-type In_xGa_1−xAs (0≤x<0.49) was successfully grown by gas-source molecular beam epitaxy using diiodomethane (CH₂I₂), triethylindium (TEIn), triethylgallium (TEGa) and AsH₃. Hole concentrations as high as 2.1×10²⁰ cm⁻³ were achieved in GaAs at an electrical activation efficiency of 100%. For In_xGa_1−xAs, both the hole and the atomic carbon concentrations gradually decreased as the InAs mole fraction, x, increased from 0.41 to 0.49. Hole concentrations of 5.1×10¹⁸ and 1.5×10¹⁹ cm⁻³ for x = 0.49 and x = 0.41, respectively, were obtained by a preliminary experiment. After post-growth annealing (500°C, 5 min under As₄ pressure), the hole concentration increased to 6.2×10¹⁸ cm⁻³ for x = 0.49, probably due to the activation of hydrogen-passivated carbon accepters.     

Electrical conductivities of mixed cation glasses

The electrical conductivities of (1−x) Li₂O · x BaO · 2 SiO₂, (1−x) Na₂O · x MgO ·2 SiO₂, (1−x) Na₂O · x CaO · SiO₂ and (1−x) Na₂O · x BaO · 2SiO₂ glasses were measured at temperature ranging from room temperature to 450°C. The transport numbers for Na⁺ ion in (1−x) Na₂O · x BaO · 2 SiO₂ glasses were measured. It was found that the alkali ion carried a significant part of the current in these glasses except one that had no alkali ions, and the conductivity decreased markedly as the alkali oxide was substituted by an alkaline earth oxide. The results of conductivity measurements combined with the data hitherto reported on mixed alkali glasses led to the proposal that the so-called mixed alkali effect could be explained on the basis of the independent path model in which it is assumed that cations can move only through vacant sites left by those of the same type.     

Solubility of YBa2Cu3O7−δ and Nd1+xBa2−xCu3O7±δ in the BaO/CuO flux

The knowledge of the phase relations and solubilities in the Y–Ba–Cu–O and Nd–Ba–Cu–O systems are of fundamental importance for crystal growth and liquid-phase epitaxy of YBa₂Cu₃O_7−δ (YBCO) and Nd_1+xBa_2−xCu₃O_7±δ (NdBCO). The determination of the solubility curve of YBCO and NdBCO in a BaO/CuO flux containing 31 mol% BaO was done by observation of the formation and dissolution of crystals on the surface of the high-temperature solution. The heat of the solution of YBCO at 1000°C was found to be 34.7 kcal/mol, and for NdBCO at 1060°C, it was found to be 28.1 kcal/mol. The determination of the solubility curves requires special care, and the problems of the time-dependent shift of the solution composition due to the corrosion of the crucible is discussed. The scatter of the solubility data published by different authors could be due to the use of solutions with different Ba : Cu ratios, different determination methods, i.e. different crystallization mechanisms, different crucibles and starting chemicals.     

Characteristic crystal growth from amorphous WO3 film by vacuum heating

Selective growth of WO₂, W and WO_3−x crystals from amorphous WO₃ film by vacuum heating at 400–900°C was clarified. The grown WO_3−x crystals were incommensurate structure based on crystallographic share structure. The growth process of WO₂ crystal in the amorphous film was directly observed at high temperature in the electron microscope. The growth front of the WO₂ crystal consumes WO₃ microcrystallites with various orientations. The growth speed of the WO₂ depended on WO₃ microcrystallites orientation. The origin of the wavy growth front of WO₂ was due to an orientation dependence of the WO₃ microcrystallites.     

Strain-compensated quantum cascade lasers operating at room temperature

Quantum cascade (QC) lasers based on strain-compensated In_xGa_(1−x)As/In_yAl_(1−y)As grown on InP substrate using molecular beam epitaxy is reported. The epitaxial quality is demonstrated by the abundant narrow satellite peaks of double-crystal X-ray diffraction and cross-section transmission electron microscopy of the QC laser wafer. Laser action in quasi-continuous wave operation is achieved at λ≈3.6–3.7μm at room temperature (34°C) for 20 μm×1.6 mm devices, with peak output powers of 10.6 mW and threshold current density of 2.7 kA/cm² at this temperature.