HigHighly strained InGaAs/GaAs quantum wells emitting beyond 1.2 µm

Highly strained In_xGa_1–xAs quantum wells (QWs) with GaAs barriers emitting around 1.2 µm are grown on GaAs substrates by metal organic vapour phase epitaxy (MOVPE) at low growth temperatures using conventional precursors. The effects of growth temperature, V/III ratio and growth rate on QW composition and luminescence properties are studied. The variation of indium incorporation with V/III ratio at a growth temperature of 510 °C is found to be opposite to the results reported for 700 °C. By an appropriate choice of the growth parameters, we could extend the room temperature photoluminescence (PL) wavelength of InGaAs/GaAs QWs up to about 1.24 µm which corresponds to an average indium content of 41% in the QW. The results of the growth study were applied to broad area laser diodes emitting at 1193 nm with low threshold current densities. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fabrication and characterization of GaAs quantum well buried in AlGaAs/GaAs heterostructure nanowires

We developed a growth method for forming a GaAs quantum well contained in an AlGaAs/GaAs heterostructure nanowire using selective-area metal organic vapor phase epitaxy. To find the optimum growth condition of AlGaAs nanowires, we changed the growth temperature between 800 and 850 °C and found that best uniformity of the shape and the size was obtained near 800 °C but lateral growth of AlGaAs became larger, which resulted in a wide GaAs quantum well grown on the top (1 1 1)B facet of the AlGaAs nanowire. To form the GaAs quantum well with a reduced lateral size atop the AlGaAs nanowire, a GaAs core nanowire about 100 nm in diameter was grown before the AlGaAs growth, which reduced the lateral size of AlGaAs to roughly half compared with that without the GaAs core. Photoluminescence measurement at 4.2 K indicated spectral peaks of the GaAs quantum wells about 60 meV higher than the acceptor-related recombination emission peak of GaAs near 1.5 eV. The photoluminescence peak energy showed a blue shift of about 15 meV, from 1.546 to 1.560 eV, as the growth time of the GaAs quantum well was decreased from 8 to 3 s. Transmission electron microscopy and energy dispersive X-ray analysis of an AlGaAs/GaAs heterostructure nanowire indicated a GaAs quantum well with a thickness of 5−20 nm buried along the 〈1 1 1〉 direction between the AlGaAs shells, showing a successful fabrication of the GaAs quantum well.     

Metalorganic vapour phase epitaxy growth of InGaAs/GaAs strained layer quantum wells beyond the pseudomorphic limit

Low pressure metalorganic vapour phase epitaxy growth and characterization of strained InGaAs/GaAs quantum well structures beyond the pseudomorphic limit are reported. Photoluminescence and X-ray diffraction measurements of these structures show considerable gallium/indium interdiffusion at the interfaces and partial strain relaxation in the layers.     

Growth mechanisms in excimer laser photolytic deposition of gallium nitride at 500°C

The mechanisms of controlling laser-induced chemical vapour deposition of GaN at substrate temperatures between 350 and 650°C have been investigated. Ultraviolet (193 nm) photolytic decomposition of trimethylgallium (TMGa) and ammonia (NH₃) precursors was examined in this range. Laser-induced fluorescence studies support the view that the dissociated intermediate fragments GaCH₃ and NH are the reacting species in GaN film formation, irrespective of substrate temperature. It was found that two crystal phases coexist in films grown at substrate temperatures below 500°C, wurtzite crystal structure with (0002) orientation forms at substrate temperatures above 500°C. The growth rate increases with both NH₃/TMGa ratio, and TMGa flow rate, while the temperature dependence shows a thermal activation energy of 0.2 eV which is smaller by a factor of five than that of films prepared by conventional thermal CVD. The large NH₃/TMGa ratios needed to achieve stoichiometry are interpreted in terms of the two-photon dissociation cross section of NH₃.     

CBE growth of low threshold 1.5 μm InGaAs/InGaAsP MQW lasers

This paper reports on low-threshold InGaAs/InGaAsP multiple quantum well (MQW) lasers emitting at a wavelength of 1.52 μm. Separate confinement heterostructure (SCH) lasers were grown using chemical beam epitaxy (CBE) with source material pressure-control systems. A continuous wave threshold current of 12 mA and internal quantum efficiency of 73% (both facets) are observed in uncoated double-channel planar buried heterostructure (DCPBH) lasers. The internal loss is 15 cm^-1. More than 90% of 50 laser chips have a threshold current of 15±3 mA.     

Study of calcium phosphate precipitation at 37 °C

The nature of the calcium phosphate formed when equal volumes of 20 mM CaCl₂ and 15 mM or 12 mM K₂HPO₄ with pH 7.4 are rapidly mixed at 37 °C has been investigated by following the pH of the suspension as a function of time, by calorimetric and light scattering methods, chemical analysis and TEM. It is found that in the early stages of this process, there appears a spherular amorphous form of calcium phosphate, called ACP1, which transforms into a floccular amorphous form, called ACP2. We suggest that this transformation is solution-mediated. ACP2 has not previously been clearly described as a separate phase and has not before been shown to be amorphous.     

On the reversion behaviour of an Al-15 at.% Zn alloy between 155°C and 175°C

The structural changes going on in an Al-15 at.% Zn alloy postaged in the vicinity of the kink of the C-curve (161°C) in the T.T.T.-diagram after pre-ageing at R. T. or 90°C (till start radii r_S between 1.0 and 4.0 nm were obtained) were followed by means of SAXS-investigations. The main results are the following:

• 1 No indication could be found that the reversion behaviour is remarkably changed by crossing the temperature of the kink of the C-curve in the T.T.T.-diagram, i.e. the reversion as well as the growth mechanism of the zones does not differ considerably below and above 161°C.
• 2 The decrease of the integral intensity Q₀ during the reversion process is mainly due to both the loss of zinc content of the zones and its rise in the matrix and not essentially by the diminution of the volume fraction of the precipitates.
• 3 Zones of a start radius of about 4 nm are much more stable against the decrease of the zine content during the reversion treatment that those of 2 nm.

     

Structural Changes during Post-Ageing of an Al-Zn (15 at.%) Alloy between 125 °C and 215 °C

The structural changes going on in an Al Zn (15 at.%) alloy postaged between 125 °C and 215 °C after pre-ageing between 20 °C and 90 °C (till start r_s = (20…︁ 55) Å are obtained) were followed by means of XSAS-investigations. The main results are the following:

• 1 An increase of r_s does not favour the growth of the precipitates during the reversion treatment.
• 2 At later reversion times the happening is obviously independent of the pre-history of the samples.
• 3 The integral intensity is decreasing with increase of T_rev, but the mean distance (d)-size (r_G) relation between the precipitates is not changed remarkably up to T_rev ≈ 200 °C in the early reversion stage.

Conclusions from this behaviour are drawn.     

Isothermal Ageing in the T Range from 0°C to 180°C of a Technical Al-2.0 at.% Zn-1.3 at.% Mg Alloy after Direct Quench

The variations of the structure and the mechanical properties had been investigated by means of TEM investigations and determination of the microhardness number in the T_a range between 0 °C and 180 °C. The main results are the following: – At the starting period of the isothermal ageing an incubation time, t_inc, occurs. t_inc plotted versus T_a yields two intersecting C curves at about 100 °C. – At T_a = 70 °C the only decomposition product are G.P. zones. – At 70 °C <T_a <100 °C the G.P. zones formed in the first period of decomposition can be transformed into η-phase or spherical hexagonal (S.H.) zones. – Above about 110 °C the precipitates present in the matrix are quasi-homogeneously formed at nucleation sites delivered by clusters of impurity atoms.     

Kinetics of crystallization of potassium chloride from aqueous solutions at 30 °C

The aim of this work is to use a diffusion layer model for the determination of individual rate constants of reaction and diffusion step of KCl at 30 °C. Crystal growth rate was measured by travelling microscope technique. From the measured values of the mean linear growth rate the resulting individual rate constants and the effectiveness factors (GARSIDE) are evaluated as a function of the driving force and of the flow velocity of solution. The significance of diffusional and surface reaction resistances against mass transfer is discussed in relation to the experimental conditions used.     

Structural and electrical properties of InAlAs/InGaAs/InAlAs HEMT heterostructures on InP substrates with InAs inserts in quantum well

A complex study of the influence of nanoscale InAs inserts with thicknesses from 1.7 to 3.0 nm introduced into In_0.53Ga_0.47As quantum wells (QWs) on the structural and electrical properties of In_0.52Al_0.48As/In^0.53Ga_0.47As/In_0.52Al_0.48As heterostructures with one-sided δ-Si-doping has been performed. The structural quality of a combined QW was investigated by transmission electron microscopy. A correlation between the electron mobility in QW with the thickness of InAs insert and the technology of its fabrication is established. Specific features of the InP(substrate)/InAlAs(buffer) interface are investigated by transmission electron microscopy and photoluminescence spectroscopy. A relationship between the energy positions of the peak in the photoluminescence spectra in the range of photon energies 1.24 eV < ?ω < 1.38 eV, which is due to the electronic transitions at the InP/InAlAs interface, and the structural features revealed in the interface region is established. It is found that an additional QW is unintentionally formed at the InP/InAlAs interface; the parameters of this QW depend on the heterostructure growth technology.     

Optical characteristics of m-plane InGaN/GaN multiple quantum well grown on LiAlO2 (1 0 0) by MOVPE

We investigate the optical properties of m-plane InGaN/GaN multiple quantum well grown on LiAlO₂ substrate by metal organic vapor phase epitaxy. Polarization-dependent photoluminescence and polarization-dependent photoluminescence excitation measurements have been performed at low temperature to study the optical absorption and emission characteristics. The main emission band possesses large polarization anisotropy which may be attributed to the anisotropic biaxial strain. We found the optical emission is not influenced by the polarization-induced electric field from the excitation-dependent photoluminescence measurements. From our results, we attribute the low-temperature emission band around 3.2 eV to interband transition in the quantum well. Besides, the mechanism of the main emission band is associated with interband transition and subsequent carrier localization. The realization of good-quality non-polar GaN-based devices can then be expected in near future.     

Growth of CdS/ZnS strained layer superlattices on GaAs(0 0 1) by molecular-beam epitaxy with special reference to their structural properties and lattice dynamics

Strained layer CdS/ZnS superlattices have been grown on GaAs(0 0 1) by molecular-beam epitaxy using CdS and ZnS compound sources. The samples were investigated with special reference to their structural properties and lattice dynamics by means of X-ray diffraction and Raman spectroscopy. The results of four superlattices with different period length are discussed in detail. X-ray diffraction profiles show superlattice satellite peaks up to the fourth order indicating a high degree of periodicity. The lateral and in-depth homogeneity of the period length is also confirmed by Raman investigations. Folded longitudinal acoustic phonons in CdS/ZnS superlattices were observed for the first time. The experimental values agree very well with theoretical calculations based on the Rytov model and show the expected dependency on the superlattice period. The behaviour of the optical phonons is mainly determined by strain induced shifts caused by the high lattice mismatch (−7%) for this system. A good agreement between theoretical predictions and detected frequencies is obtained.     

The properties of hydrogen forms of zeolite T obtained by the treatment with hydrochloric acid at 22°C

Hydrogen forms of zeolite T obtained by the treatment of synthetic zeolite of erionite-offretite type with hydrochloric acid at 22°C were tested with the methods of chemical analysis, sorption and X-ray diffraction. The influence of the variations in the number of treatments with acid, the concentration of acid and the calcination at 500°C on the properties of the samples was observed. It was found that the modification runs in two steps. The existence of the threshold of stability of the zeolite T structure under the treatment with 0.1 N HCl was indicated at the degree of decationization of 45–48% and the degree of dealumination of 25–28%. It was also found that about 30–33% of alkali cations in zeolite T are at the more accessible positions.     

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.     

Phase Equilibrium in the System HgSeO4 – H2SeO4 – H2O at 100°C

The solubility of the system HgSeO₄ – H₂SeO₄ – H₂O was studied at 100 °C. The fields of crystallization of HgSeO₄ H₂O and HgSeO₄ were established. The compounds obtamed were identified by means of chemical, X-ray and crystal-optical analysis. The mechanism of the thermal dissociation of the compounds obtained was studied.     

The role of Sb in the molecular beam epitaxy growth of 1.30–1.55 μm wavelength GaInNAs/GaAs quantum well with high indium content

Sb-assisted GaInNAs/GaAs quantum wells (QWs) with high (42.5%) indium content were investigated systematically. Transmission electron microscopy, reflection high-energy electron diffraction and photoluminescence (PL) measurements reveal that Sb acts as a surfactant to suppress three-dimensional growth. The improvement in the 1.55 μm range is much more apparent than that in the 1.3 μm range, which can be attributed to the difference in N composition. The PL intensity and the full-width at half maximum of the 1.55 μm single-QW were comparable with that of the 1.3 μm QWs.     

Photoluminescence characterization of GaxIn1−xAs (0 ≤ x ≤ 0.32) strained quantum wells grown on InP by chemical beam epitaxy

We have investigated some characteristics of Ga_xIn_1−xAs/InP (x = 0, 0.2, 0.32) strained materials by growing multi-quantum wells with various well widths from 3 to 60 Å by chemical beam epitaxy. Growth conditions such as valve sequences and growth interruptions were optimised to have single-peaked photoluminescence spectra for all strained samples. Measured room temperature emission wavelengths were compared with theoretically estimated values. Photoluminescence linewidths were around 16 meV for most samples at 77 K, but broadened for well widths of less than 20 Å. Photoluminescence intensity also peaked at well width of 20 Å for all compositions. Performances of optical devices may be closely related to this “critical thickness” determined probably by the growth system limitation.     

The influence of sodium chloride on the driving force of the crystallization of potassium chloride from aqueous solutions at 25 °C

Admixtures at higher concentrations affect the growth rate in two kinds by changing the kinetic phenomena and also the driving force. Based on measurements of the osmotic coefficient as a function of the total ionic strength of the solution at 25 °C different kinds of driving force are calculated. Usefull for the discussion of the admixture problem is the mean molal free enthalpy of crystallization. NaCl increases the driving force of the crystallization of KCl. It shows that the reason for the decreasing effect of NaCl in growth rate is a kinetic one.