Preparation and properties of Ga1-xAlxAs-GaAs heterostructure lasers grown by metalorganic chemical vapor deposition

Recently Ga1-xAlxAs-GaAs double-heterostructure lasers having low threshold current densities have been grown by metalorganic chemical vapor deposition. In addition to these conventional double-heterostructure lasers, unique laser structures have been grown, e.g., channel-guide, distributed-Bragg-confinement, and single- and multiple-quantum-well lasers. We describe here the preparation and performance of these devices.     

Band offset effect on transport in AlxGa1-xAs/GaAs heterojunction bipolar transistors grown by metalorganic chemical vapor deposition

AlxGa1-xAs/GaAs heterojunction bipolar transistors were grown with x in the emitter from 0 to 0.57 and the band offset effect on electron transport has been studied using a new technique. The data, though preliminary, indicate that the transport is dominated by electrons near the conduction-band minima when x < 0.45, but not when x = 0.57.     

Spectral broadening of pulsating AlxGa1-xAs double heterostructure lasers

The longitudinal mode spectrum of stripe geometry double heterostructure (DH) lasers exhibits a spectral broadening of ∼ 0.8 Å when the laser emits pulsations. The broadening results from a chirp to longer wavelengths which occurs during the ∼ 0.4 ns long pulse. The chirp is due to an increase of the refractive index resulting from the reduction in the carrier concentration during the pulse. Both plasma and band to band interactions affect the index change with the latter mechanism giving the dominant contribution. A comparison of the wavelength shift obtained during the pulse with the current dependent shift of the longitudinal modes at currents well below threshold confirms that the change in carrier concentration is responsible for the chirp.     

AlInAs/GaInAs heterostructure bipolar transistors grown by metalorganic chemical vapour deposition

Al/sub 0.48/In/sub 0.52/As/Ga/sub 0.47/In/sub 0.53/As heterostructure bipolar transistors are demonstrated using metalorganic chemical vapour deposition. The transistors have a cutoff frequency of 80 GHz and a common-emitter breakdown voltage of 5.5 V.<>     

Low threshold current AlGaAs/GaAs rib-waveguide separate-confinement-heterostructure distributed-feedback lasers grown by metalorganic chemical vapor deposition

AlGaAs/GaAs rib-waveguide separate-confinement-heterostructure (SCH) distributed-feedback (DFB) lasers emitting at 880 nm were fabricated by a two-step atmospheric pressure metalorganic chemical vapor deposition (MOCVD) growth technique. A CW threshold current as low as 18 mA and an output power of more than 10 mW per facet at room temperature were obtained. Also, single longitudinal and fundamental transverse modes were maintained up to more than twice the threshold current level.     

Efficiency calculations for AlxGa1-xAs-GaAs heteroface solar cells

A computer-assisted analysis of the AlxGa1-xAs-GaAs heteroface solar cell is done to find the dependence of cell efficiency on substrate doping level. Assumptions for carrier lifetime needed for the evaluation of efficiency are based on measurements of experimertal AlxGa1-xAs-GaAs heteroface cells. The results show the doping range 10¹⁶to 10¹⁷cm^-3to be the best for heteroface solar cells, because experimental evidence suggests that the lifetimes required for high-efficiency cells are difficult to obtain at very low and very high doping levels. Calculations based on a T^3/2temperature dependence for lifetimes agree well with early experimental efficiency versus temperature measurements on GaAs cells, but do not explain the results for an AlxGa1-xAs-GaAs heteroface cell reported by Hovel (1975).     

Influence of V/III variation on AlxGa1-xAs quantum well lasers grown by metalorganic chemical vapor deposition

The role of the V-III ratio during growth on the optical and electrical properties of Al_xGa_1-x As laser material grown by metalorganic chemical vapor deposition has been investigated. Controlled studies involving more than twenty growth runs show that this parameter has a profound influence on both device performance and reliability.     

High-Quality GaN heteroepitaxial films grown by metalorganic chemical vapor deposition

In this paper, we describe the growth and characterization of high-quality GaN heteroepitaxial films grown on basal-plane sapphire substrates using metalorganic chemical vapor deposition. The quality of these films is analyzed by a variety of methods, including high-resolution x-ray diffraction, optical transmission spectroscopy, transmission electron microscopy (TEM), room temperature photoluminescence, and room-temperature Hall measurements. The x-ray diffraction full width at half maximum value of ΔΘ ～37 arc s is the narrowest reported to date for any III-V nitride film on any substrate. The x-ray rocking curves for ～0.48 μm thick GaN/Al₂O₃ heteroepitaxial layers exhibit Pendellösung fringes, indicating that even relatively thin films can be of high quality. High-resolution TEM lattice images further attest to the excellent structural quality, showing the films to be completely free of stacking faults. Furthermore, no evidence of columnar growth is observed.     

Recombination dynamics in GaAs/AlxGa1- xAs quantum well structures

Time-resolved and excitation-dependent photoluminescence of GaAs/AlxGa1-xAs quantum well structures reveal that recombination takes place between free carriers, not excitons, at room temperature for carrier densities at and above the mid -10¹⁶cm^-3level. Other samples show trapping and release of carriers from traps, evidence of dynamic Shockley, Hall, and Read recombination for optically active traps. The traps can be saturated to a large extent. Further studies show that they are associated with interfaces between different materials and that they become active at a temperature around 150 K. Results from all samples indicate that the bimolecular radiative recombination coefficientBfor quantum wells is no larger than the value ofBfor bulk GaAs, and may in fact be smaller. This is one of the first studies of time-resolved luminescence of impurities in quantum well structures. Impurity decays at low temperatures are found to be quite slow. A spectral line which appeared to be longitudinal optical phonon-shifted emission is shown to be due to an acceptor impurity.     

Characteristics of GaN stripes grown by selective-area metalorganic chemical vapor deposition

We report on the selective-area metalorganic chemical vapor deposition of GaN stripes in the size range of 50 to 125 urn and the characterization of the morphology, topography, and optical properties of these stripes. GaN films (∼1–3 μm) grown on (0001) sapphire are used as the substrates. Excellent surface morphology is achieved under optimized growth conditions which include a higher V/III ratio than broad area growth. It is found that, under certain growth conditions, (0001) terraces of ~5 μm in width develop at the edges of all stripes, independent of stripe size and orientation. The selectively grown GaN yields stronger band-edge emission than the “substrate” GaN which indicates an improvement in optical quality. However, the donor-acceptor pair recombination (or conduction band to acceptor transition) and yellow emission are also enhanced in certain areas of the stripes. The spatial correlation of these emission bands is established by cathodoluminescence wavelength imaging, and the origin of these emissions is speculated.     

High efficiency InGaAs/GaAs single-quantum-well lasers usingsingle-step metalorganic chemical vapor deposition

Low-threshold-current single-quantum-well InGaAs/GaAs lasers are fabricated by metalorganic chemical vapor deposition on a nonplanar substrate. By taking advantage of the growth rate and doping differences on different crystal facets during the growth, a buried heterostructure laser with natural current blocking p-n-p-n junction is formed by a single growth step. Threshold currents as low as 1.0 mA under pulsed operation and 1.2 mA under continuous-wave operation are obtained for uncoated lasers at room-temperature. The lasers showed high external quantum efficiency (80%) and high T₀ (150 K). High reflection coated laser (95%/95%) have CW threshold current as low as 0.28 mA     

Observation of compensation in GaN films grown by metalorganic chemical vapor deposition

Hall-effect measurements were conducted on an unintentionally doped n-type GaN films grown on (0 0 0 1) sapphire substrates in a low-pressure metalorganic chemical vapor deposition (LP-MOCVD) system. Variable temperature Hall-effect measurements reveal a different dependence of the electron concentration and the Hall mobility on temperature for the two samples. In the sample with a background concentration of 10¹⁸ cm⁻³, the electron concentration shows a non-monotonous relationship with increasing temperature from 90 to 400 K. However, in the normal sample with a background concentration of <10¹⁷ cm⁻³, the dependence of the electron concentration on temperature is monotonous between 90 and 400 K. The different behavior was also found in the dependence of the Hall mobility on temperature for the sample. The experimental data were analyzed by considering two donor levels and one acceptor level model. The good agreement between measured data and calculation demonstrates that the abnormal dependence is indicative of a compensation effect in GaN film grown by LP-MOCVD.     

Electron microscope studies of InxGav1−xAs/GaAs/Si grown by metalorganic chemical vapor deposition

The microstructure of In_xGa_1−xAs/GaAs (5 nm/5 nm, x < 0 to 1.0), as grown by a metalorganic chemical vapor deposition two-step growth technique on Si(100) at 450‡C, and subsequently annealed at 750‡C, is investigated using plan-view and cross-sectional transmission electron microscopy. The variations in resultant island morphology and strain as a function of the In content were examined through the comparison of the misfit dislocation arrays and moirés observed. The results are discussed in relation to the ways in which the island relaxation process changes for high In content.     

Photoluminescence studies on InGaAlP layers grown by low-pressure metalorganic chemical vapor deposition

The optical properties for In_0.5(Ga_1-x Al _x )_0.5P (0 <x < 0.4) layers, grown by low-pressure Metalorganic Chemical Vapor Deposition, have been studied with photolominescence (PL) measurement. The PL intensity decreases with the increase of the Al composition (0 <x < 0.4). This dependence could not be accounted for only by the electron overflow from theΓ band to the X band. And the PL intensity is directly proportional to the excitation power at low temperature, below 50 K. On the other hand, the PL intensity is proportional to the second power of the excitation power at a high temperature range (>200 K). These results indicate that non-radiative recombination centers bound to theΓ band in In_0.5(Ga_1-x Al _x )_0.5P play a very important role in the radiation mechanism. PL dependence also shows these non-radiative recombination centers are thought to have strong relation to the aluminum substitution for In_0.5(Ga_1-x Al _x )_0.5P.     

AlInGaAs/AlGaAs strained quantum-well ridge waveguide lasers grownby metalorganic chemical vapor deposition

AlInGaAs/AlGaAs strained quantum-well ridge waveguide diode lasers with an emission wavelength of 890 nm are presented. These devices exhibit both single spatial and longitudinal mode operation up to 30 mW of optical output power. A CW threshold current of 13 mA was obtained for a 5-μm-wide ridge waveguide having a cavity length of 500 μm. The differential quantum efficiency was 52%. The lateral and perpendicular far-field radiation patterns (FWHM) from the laser were 6° and 51°, respectively. Reliability testing on uncoated gain-guided lasers made from the same wafer showed no sudden death failures and degradation rates as low as 4.6%/kh     

Nonlinear absorption properties of AlGaAs/GaAs multiple quantumwells grown by metalorganic chemical vapor deposition

The nonlinear absorption properties of the excitonic resonances associated with multiple quantum wells (MQWs) in AlGaAs/GaAs grown by metalorganic chemical vapor deposition are reported. The dependence of the saturation properties on growth parameters, especially growth temperature, and the well width are described. The minimum measured saturation intensity for these materials is 250 W/cm², the lowest reported value to date. The low saturation intensities are the result of excellent minority carrier properties. A systematic study of minority carrier lifetimes in quantum wells are reported. Lifetimes range from 50-350 ns depending on growth temperature and well width     

Characteristics of high Al content AlxGa1−xN grown by metalorganic chemical vapor deposition

The epitaxial growth of Al_xGa_1−xN film with high Al content by metalorganic chemical vapor deposition (MOCVD) has been accomplished. The resulting Al content was determined to be 54% by high resolution X-ray diffraction (HRXRD) and Vegard's law. The full width at half maximum (FWHM) of the AlGaN (0002) HRXRD rocking curve was about 597 arcsec. Atomic force microscopy (AFM) image showed a relatively rough surface with grain-like islands, mainly coming from the low surface mobility of adsorbed Al-species. From transmittance measurement, the cut-off wavelength was around 280 nm and Fabry–Perot fringes were clearly visible in the transmission region. Cathodoluminescence (CL) measurement indicated that there existed a uniformity in the growth direction and a non-uniformity in the lateral direction.     

Self-focusing effects in pulsating AlxGa1-xAs double-heterostructure lasers

Using proton bombarded stripe geometry lasers which emit intense optical pulses, we have measured the width of the optical beam in the plane of the junction as a function of time during the pulse. The width of the beam is qualitatively proportional to the change in the carrier density. The width increases during the quiescent period between pulses where the carrier density increases by current injection and decreases during the emission of the pulse. For one laser studied in considerable detail, the full width at half intensity decreases from 9 μm at the start of the pulse to 7.2 μm at the end of the pulse. The reduction in the width results from the self-focusing of the beam. It is due to an increase in the refractive index and the decrease in the gain distributions near the center of the stripe. The reduction in the beamwidth concentrates the mode to a region of sufficiently higher average gain to compensate for the reduction in spatial gain distribution. The self-focusing acts to reduce the damping of the relaxation oscillations, and thus enhances the effect of other nonlinearities such as saturable absorption in causing pulsations. The thermal induced refractive index distribution across the stripe is shown to play a crucial role in the gain instability caused by self-focusing.     

Carbon and hydrogen incorporation in ZnTe layers grown by metalorganic chemical vapor deposition

The metalorganic chemical vapor deposition growth of ZnTe has been performed at atmospheric pressure under helium and hydrogen carrier gases. Epitaxial growth was achieved on GaAs (100) substrates with the combination of diethylzinc and diethyltellurium as precursors. We have studied the incorporation of carbon and hydrogen in as-grown layers of ZnTe by secondary ion mass spectroscopy analysis and out-diffusion experiments with different carrier gases and growth temperatures. The amount of carbon and hydrogen incorporated in the ZnTe layers greatly depends on the nature of the gas considered. Under helium atmosphere, the amount of carbon and hydrogen incorporated are greater than under H₂ with an origin from organometallic precursors.