Stimulated emission from Ga1-xAlxAs diodes at 77°K

Ga1-xAlxAs was found to be a suitable material for semiconductor lasers. Stimulated emission from Fabry-Perot type of diodes has been observed at 77°K and 273°K. The highest peak energy of the laser line at 77°K so far is 1.65 eV (l = 7500Å). The Ga1-xAlxAs material was obtained by a liquid phase epitaxial method, described in a previous paper.     

A computer analysis of Ga1-xAlxAs-GaAs solar cells with multilayered window structure

Numerical calculations have been made of the spectral response and conversion efficiency of Ga1-xAlxAs-GaAs solar cells with multilayered window structures. Maximum conversion efficiencies of about 20 percent are predicted, equaling those achieved with graded band-gap solar cells, for the case of three-layered window structures.     

A Ga1-xAlxAs monolithic opto-isolator

The performance of a Ga1-xAlxAs monolithic opto-isolator incorporating identical emitter and detector structures is described. The device is fabricated from graded bandgap Ga1-xAlxAs grown by liquid phase epitaxy on a semi-insulating GaAs substrate. Three identical rectangular diode elements are fashioned on the chip; the center element acts as an edge-emitting LED and the outer two bars serve as edge-receiving photodiodes. Air-isolated prototype structures possess optical coupling nearly comparable to typical hybrid devices, and this coupling will be improved by dielectric isolation. Isolation voltages between the LED and the detector exceed 1400 volts. The devices achieve excellent performance in both digital and analog signalling tests.     

Substrate radiation losses in GaAs heterostructure lasers

Double-heterostructure (DH) diode lasers with a thin Ga1-xAlxAs layer between the active GaAs region and the GaAs substrate or superstrate are analyzed. In these devices power flows through the thin Ga1-xAlxAs layer and is radiated into the substrate or superstrate. Three methods for computing the laser thresholds are developed and compared. The first is an analytic perturbation technique, which yields accurate results in many cases of practical interest. The second and third are rapidly convergent numerical iteration techniques. The former utilizes overlap integrals to compute absorption losses and thresholds; the latter includes all losses and gains directly in the formulation. We show that conventional DH diode lasers can be designed with thick active GaAs layers and still achieve lowest order TE-mode operation. These devices will produce better collimated, higher power output beams than do similar devices with thinner active regions. Transverse-mode control is achieved because all higher order modes have increased penetration through the thin Ga1-xAlxAs , and therefore exhibit inereased radiation losses into the substrate or superstrate. A design example is included in which it is shown that with proper choice of the Ga1-xAlxAs-layer thickness the TE0-mode threshold increases by 5 percent compared with a 110-percent increase in the TE1threshold. These results are virtually independent of the substrate power-absorption coefficient. Threshold current densities are computed for a set of diodes studied experimentally by Casey and Panish and the results are shown to be in excellent agreement.     

Near-infrared high-power LED's with Ga1-xAlxAs epitaxially grown junctions

This paper reports on a study of several techniques of fabricating an efficient Ga1-xAlxAs IRED with high power. Smooth epitaxial layers are grown on a substrate containing a thick Ga1-xAlxAs epitaxial layer which is used to form a hemispherical emitting surface. This is achieved by using a melting-back technique without up-heating. An effect of the AlAs molefraction ratio between n- and p-type regions was clearly found by measurement of a large number of diode chips, and the best performances were obtained on the junctions with the ratioX_{n}/X_{p} simeq 1.5. Ohmic contacts to the p and n regions were applied on the same surface of the wafer using only one metallic source for the vacuum deposition. A cerium-oxide film serves as an antireflection coating when deposited on the hemispherical surface and increases the optical output power by more than 30 percent. External quantum efficiencies of 28 percent (current density: 400 A/ cm²) and optical output power of 96 mW (drive current: 300 mA, current density: 1500 A/cm²) have been observed from diodes emitting at 8300 Å under dc operation at room temperature.     

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.     

GaAs MESFET's with Ga1-xAlxAs buffer layers

MESFET's were fabricated with GaAs active layers and Ga1-xAlxAs buffer layers grown by metallorganic vapor-phase epitaxy on semi-insulating substrate Ga1-xAlxAS buffer layers with x = 0.2, 0.4, and 0.6 were used to determine the effect of buffer-layer composition on device performance. While very high transconductance values (175 mS/mm) were obtained with buffer layers with x = 0.2, the transconductance decreased with increase of aluminum composition in the buffer layer. The decrease in transconductance may be due to an observed reduction of electron mobility in the active layer with increasing aluminum concentration in the buffer layer.     

A new chemical etching technique for formation of cavity facets of (GaAl)As lasers

A new chemical etching technique which offers excellent cavity facets of Ga1-xAlxAs lasers is reported. This technique is based on our finding that the crystallographic anisotropy in the conventional etching process of Ga1-xAlxAs multilayers depends strongly on the AlAs mode fractionxin every layer. A suitable combination of the mole fractions in the multilayer is therefore a key factor for obtaining practically vertical walls with sufficient smoothness and flatness as laser cavity facets. In fact, the reflectivity of the etched facet obtained is 28 percent, being compatible to that in the conventional cleaved facets. As a result, a CW operation with threshold current as low as 28 mA and external quantum efficiency as high as 24 percent per facet has been attained with high reproducibility.     

Ga1-xAlxSb avalanche photodiodes: Resonant impact ionization with very high ratio of ionization coefficients

The liquid-phase epitaxy and device fabrication of p-n and p-i-n Ga1-xAlxSb avalanche photodiodes is described. Breakdown voltages up to 95 V and dark currents of 10^-4A/cm²have been obtained. With p-i-n diodes we have measured the impact ionization coefficients α (electrons) and β (holes) with different composition and temperature. A resonant enhancement of the hole ionization coefficient is found forx = 0.065(300 K) where the ratiobeta/alphaexceeds values of 20. This effect is attributed to impact ionization initiated by holes from the split-off valence band: if the spin orbit splitting Δ is equal to the bandgap energy Eg, the threshold energy for hole initiated impact ionization reaches the smallest possible value (E_{i} = E_{g}) and the ionization process occurs with zero momentum. This leads to a strong increase of β atDelta/E_{g} = 1. The experimentally determined dependence of ionization coefficients on threshold energy is compared with theoretical expectations.     

An experimental study on improvement of performance for hemispherically shaped high-power IRED's with Ga1-xAlxAs grown junctions

This paper describes the structure and performance of a high-power infrared emitting diode (IRED) designed as a high speed optical beam source for optoelectronic applications. The heterostructured junction is formed on a thick Ga1-xAlxAs liquid phase epitaxy (LPE) grown layer which is used to shape hemispherical emitting surfaces. Dislocation density in recombination region was considerably decreased by the thick layer growth on a GaAs wafer used as a primary substrate. Under dc operations, external quantum efficiencies of around 45 percent at a current density of 0.6 kA/cm²and about 110 mW of optical output power at 200 mA (1 kA/cm²) have been obtained from the diodes with a 160-µm junction diameter. The tendency to reach power saturation with increased current has been decreased by means of reducing of thermal resistance of the mount, and the diodes with 240- µm junction diameter have shown about 180 mW at 600 mA dc and 1.4 W at a 4-A pulse (60 Hz, 50 µs). A large improvement in high frequency response has been obtained and the bandwidth at -3-dB intensity has reached above 120 MHz.     

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.     

Frequency response of Ga1-xAlxAs light-emitting diodes

Frequency responses of Ga1-xAlxAs light-emitting diodes with various Al composition are measured. When the emission peaks become shorter than 7000 Å, the cutoff frequencies decrease abruptly from 15 MHz to 5 MHz. A conclusion is that the frequency response is determined mainly by the lifetime of injected electrons which changes along with the Al composition because of the distribution of electrons between the Γ and X valleys.     

Temperature dependence of threshold current in (GaAl)as double-heterostructure lasers with emission wavelengths of 0.74-0.9µm

The threshold-current variation with temperature has been measured for Ga1-xAlxAs double-heterostructure (DH) lasers with AlAs mole fraction in the active layerxof 0.08 and 0.2, and with several heterojunction step heightsDeltax. The threshold-temperature coefficient Jth(350 K)/Jth(300 K), which generally increases with decreasingDeltax, is found to be larger forx = 0.2than that forx = 0.08at the same value ofDeltax, and also to be larger for the lasers with smaller effective electron diffusion length in thePcladding layer, in the case ofx = 0.2. These characteristics are well explained by a model of carrier leakage due to unconfined carriers in the active layer. It is confirmed by a good fit of the experimental results with the calculated values that the electron leakage in theGammaconduction band of thePcladding layer dominates forx leq 0.1, but the hole leakage in theNcladding layer increases withxand becomes comparable in magnitude with the electron leakage atx sim 0.2.     

High-power and high-speed characteristics of modified heterostructure IRED's

A modified heterostucture Ga1-xAlxAs near infrared emitting diode is described with single (SH) and double heterostructures (DH) adopted for carrier confinement. The active region is arranged in a layer for best crystal quality as shown by photoluminescence measurement. The maximum 3-dB bandwidth is 175 MHz and optical output power reaches 7 mW in a surface emitting diode. The experimental results are compared with optical characteristics for SH and DH IRED's.     

Radiative recombination via direct optical transitions in In1 −x GaxAs (0≤x≤0.16) solid solutions

Photoluminescence from In_1?x Ga_xAs (0≤x≤0.16) solid solution epilayers LPE-grown on (111)InAs substrates and electroluminescence from p-n junctions on their bases have been studied in the temperature range 77–450 K. Despite the negative lattice mismatch between epilayer and substrate, radiative recombination in epilayers occurs via direct optical transitions ensuring a high internal quantum efficiency of luminescence (6% at 295 K).     

Improved molecular beam epitaxial growth of AlxGa1 - xAs/GaAs high-radiance LED's for optical communications

We report some improvements made in molecular beam epitaxial growth of AlxGa1-xAs/GaAs high-radiance LED's for optical communications. These improvements have been achieved by growing these wafers in a system which includes an air-lock wafer-exchange chamber. Interfacial recombination velocity as low as6 times 10^{2}cm/s was obtained. The series resistance of the Burrus-type LED's was reduced by increasing the acceptor concentrations in the p-layers to a density as high as 10¹⁹cm^-3using Be as the dopant. CW output of 5.8 mW, or a radiance of 92 W/sr. cm², at a safe operating current of 150 mA has been obtained in these devices. The device performance is comparable to the best obtained in LPE-grown diodes of the same geometry.     

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).     

Luminescence and electrical properties of MgxZn1-xTe alloys

The luminescence and transport properties of high-quality undoped and phosphorus-doped Zn1-xMgxTe alloys (x ≤ 0.50) have been investigated. At 4.2 and 300 K, the photoluminescence of unintentionally doped crystals is dominated by near-band-edge recombination mechanisms. In phosphorus-doped samples, the luminescence spectra exhibit free-bound transitions involving shallow acceptor centers (phosphorus in tellurium sites). Whenxand/orTincreases, a broad luminescence band also appears at lower energy (1.7-1.9 eV) which decreases the near-band-edge luminescence efficiency. This low-energy band could be due to phosphorus atoms occupying other sites in the lattice (metal sites, for example) and then acting like deep recombination centers. Such a behavior could also explain the electrical properties of phosphorus-doped crystals. The introduction of phosphorus leads to an increase of the free hole concentration p as compared to undoped crystals but all phosphorus atoms do not behave as shallow acceptors; an increasing fraction of these atoms would act like donors in sites other than the tellurium sites as x increases. In undoped materials, p decreases drastically when we add more magnesium and the hole mobility remains approximately constant. We think that this effect is due to compensation by residual donor impurities. On these undoped samples, light-emitting diodes (LED's) have been successfully obtained for the first time. Their quantum efficiency is reasonable if we take into account the low carrier concentration of the material. The emission peak position is 5390 Å for Zn0.9Mg0.1Te instead of 5550 Å for undoped ZnTe.     

Numerical modeling of energy balance equations in quantum well AlxGa1-xAs/GaAs p-i-n photodiodes

The energy balance equations coupled with drift diffusion transport equations in heterojunction semiconductor devices are solved modeling hot electron effects in single quantum well p-i-n photodiodes. The transports across the heterojunction boundary and through quantum wells are modeled by thermionic emission theory. The simulation and experimental current-voltage characteristics of a single p-i-n GaAs/Al _xGa_1-xAs quantum well agree over a wide range of current and voltage, The GaAs/Al_xGa_1-xAs p-i-n structures with multi quantum wells are simulated and the dark current voltage characteristics, short circuit current, and open circuit voltage results are compared with the available experimental data, In agreement with the experimental data, simulated results show that by adding GaAs quantum wells to the conventional cell made of wider bandgap Al_x Ga_1-xAs, short circuit current is improved, but there is a loss of the voltage of the host cell, In the limit of radiative recombination, the maximum power point of an Al_0.35Ga_0.65As/GaAs p-i-n photodiode with 30-quantum-well periods is higher than the maximum power point of similar conventional bulk p-i-n cells made out of either host Al_0.35Ga_0.65As or bulk GaAs material