F. Y. Juang

Electron and hole impact ionization coefficients in GaAs‐AlxGa1−xAs superlattices

Electron and hole multiplication and impact ionization coefficients have been measured with pure carrier injection in p+‐n−‐n+ and p+‐n diodes grown by molecular beam epitaxy. Values of the electron and hole ionization coefficient ratio α/β=2–5 are measured for superlattices with well width Lz≥100 A and α/β>10 is measured in a graded band‐gap superlattice with a total well and barrier width LB+LZ=120 A. The ratio decreases and becomes less than unity for smaller well sizes. This is caused by an increase in β(E) while α(E) remains fairly constant. The results have been interpreted by considering varying hole confinement and scattering in the coupled quantum wells.

Determination of the microscopic quality of InGaAs‐InAlAs interfaces by photoluminescence—Role of interrupted molecular beam epitaxial growth

Photoluminescence (PL) studies have been carried out on 120 A InGaAs/InAlAs single quantum well structures grown by molecular beam epitaxy. Three types of samples were grown with the growth being interrupted before interface formation. The interruption times were 0, 2, and 3 min. The corresponding linewidth of the main excitonic transition associated with the quantum well was found to be 20, 16, and 10 meV, respectively, while the PL intensity changed by the ratio 1:0.4:0.1. We believe this behavior is due to a steady improvement in the interface quality due to interruption accompanied by impurity accumulation during the interruption. Analysis of the 10 meV linewidth, which is among the smallest ever reported, suggests that the InAlAs/InGaAs interface can be described by two‐dimensional InAlAs and InGaAs islands which have a height of two monolayers and a lateral extent of about 100 A.

Field‐dependent linewidths and photoluminescence energies in GaAs‐AlGaAs multiquantum well modulators

Photoluminescence linewidths and transition energies have been measured in GaAs‐AlGaAs multiple quantum wells with large (≥160 A) barrier widths as a function of applied transverse electric field. The experimental data agree well with values calculated by using a recently developed variational technique. It is apparent that heterointerface roughness is the dominant line broadening mechanism. The emission intensity decreases rapidly with field, principally due to carrier tunneling at high fields. At 80 kV/cm a shift of 20 meV in the emission energy is observed.

Molecular beam epitaxial growth and photoluminescence of near‐ideal GaAs‐AlxGa1−xAs single quantum wells

Single quantum wells with ∼120‐A GaAs wells and Al0.3Ga0.7As or GaAs‐Al0.3Ga0.7As superlattice barriers were grown by molecular beam epitaxy under conditions known to produce very high‐purity material. Low‐temperature photoluminescence measurements indicate that the dominant recombination transitions are associated with free and bound excitons involving both light and heavy holes. A forbidden transition, possibly E21h, is also observed. The transition associated with electron‐heavy‐hole free excitons is most intense and has a linewidth of 0.3 meV at 2 K. The linewidths observed for these samples, grown with As4 species at 630 °C, are the smallest for 120‐A single quantum wells and are close to theoretically calculated limits.

MATERIAL PROPERTIES AND OPTICAL GUIDING IN InGaAs-GaAs STRAINED LAYER SUPERLATTICES-A BRIEF REVIEW

Due to the absence of lattice-matching requirements, strained-layer superlattices offer a large tunability in bandgap and other material properties suitable for device applications. Encouraging progress has been made in the molecular-beam epitaxial and metalorganic-vapor-phase-epitaxial growth of strained-layer superlattices and in their characterization. These have been briefly reviewed here. Since a strained-layer superlattice allows the use of InzGa,_,vAs layers with x-values up to - 1.0, a large variation of the refractive index from that in GaAs occurs due to mismatch strain and alloying. This variation in refractive index has been calculated. The increase in refractive index can be used to form optical guides in the SLS and such guides with good vertical confinement is demonstrated. Preliminary measurements of the impact-ionization parameters and deep-level traps in these materials are also reported. a/@ values close to and slightly greater than unity are measured. A single electron trap with thermal activation energy equal to 0.16 eV is identified.

Growth and properties of In0.52Al0.48As/In0.53Ga0.47As, GaAS: In and InGaAs/GaAs multilayers

We have investigated the properties of some In-containing materials and heterostructures grown by molecular beam epitaxy on GaAs and InP substrates. Photoluminescence spectra of InGaAs/InAlAs quantum wells have been related to growth kinetics and it is seen that growth interruption or the incorporation of a few periods of superlattices at the heterointerfaces smooths the growth front. Experiments have been performed to determine the properties of In0 52A1048Asgrown on InP. This lattice-matched alloy In0 52Al048As may be clustered under normal growth conditions at a substrate temperature 500°C. Addition of small amounts of In (0.2—1.2%)to GaAs reduces trap and defect densities in these materials, as seen from DLTS and low-temperature photoluminescence data. The improvement may be related to the higher surface migration rate of In compared to Ga and a subsequent reduction of point defects. Single-mode optical guides and direction couplers with losses as low as 1—3dB/cm have been fabricated with GaAs: In and In02Gao gAs/GaAs strained-layer superlattices. In5Ga1 _~As and In5Al1 ~As are important The different materials and heterostructures alloy semiconductors for electronic and opto-elec- were grown in a three-chamber RIBER 2300 epitronic heterostructure devices. Together with In taxy system. Extensive source and system baking Ga1 LAs—GaAsstrained-layer superlattice (SLS), were carried out [1] to grow high-purity materials. a large and important spectral energy range can be Undoped 120 A GaAs/A103Ga07As single quancovered. Molecular beam epitaxy (MBE) is a tum wells with 0.3 meV excitonic linewidths in the growth technique which is principally controlled 2 K photoluminescence spectra and lattice-matchby surface kinetics, and therefore, the electrical ed undoped In0 53Ga047As with fL3~K = 12,050 and optical properties of the grown films and cm2/V’ s and ~s77~ = 53,000 cm2/V. s assert the heterointerfaces can be non-ideal. The intent of purity of the growth ambient. In-doped GaAs and this paper is to highlight some electrical and opti- the InGaAs/GaAs SLS were grown on undoped cal properties in these alloys and heterostructures, or doped GaAs substrates at 560—580°C at a which can result from the growth kinetics. More growth rate of 1.2 ~u rn/h. InAlAs and InGaAs/in specificaly, results of photoluminescence measure- AlAs were usually grown in the temperature range ments on In 052Al048As/In 053Ga047As single 480—520°C on (001) Fe-doped semi-insulating or quantum wells lattice matched to InP substrates S-doped N + InP substrates at 0.5—1.3~tm/h. suggest clustered growth in the Al-containing alloy and interface roughness at the heterointerface under normal MBE growth conditions. Results from ~ Phototuminescence in In ~Ga0 7As/ high temperature Hall and carrier impact ioniza- In052Al0 ~As single quantum wells 0. .4 tion measurements are presented, which also indicate the presence of clustering in the InAlAs a!ioys. Finally, the growth of In~Ga1 - ~As (x ~ 0.01) Photoluminescence measurements were done at and In~Ga1 5As/GaAs SLS on GaAs substrates 10 K and higher temperatures and with variable and the properties of optical components made excitation intensity. The luminescence was excited with these materials are described,

High-speed multiquantum well avalanche photodiodes

Synthetically modulated structures offer the capability of tunability of bandgap and other material parameters. In particular, the GaAs/AlGaAs and Ino.53Ga0.47As/ Ino.52A10.48As systems offer these advantages in the infrared and optical communication wavelength ranges. We report here the properties of superlattices and the properties of avalanche and p-i-n photodiodes made with thes materials. The multiquantum well struc-tures (MQW) have LZ and LB varying from 25-500Å. Impact ionization phenomena in the different structures have also been determined from carrier multiplication and noise measurements and have been analyzed by Monte Carlo techniques. Techniques for obtaining enhanced optical absorption in very high-speed photodiodes are demonstrated.

Demonstration of dual gain mechanism in an InGaAs/InAlAs superlattice photodiode

A high‐gain photodiode in which the internal gain can result from either potential barrier lowering or mass filtering action, depending on device geometry and bias conditions, is proposed and demonstrated. The photodiode structure is similar to a modulated barrier diode and uses In0.53Ga0.47As and InGaAs/InAlAs superlattice absorption regions. The superlattice helps to reduce the dark current and aids in mass filtering. The devices reported here were made with multilayered InP‐based materials grown by molecular beam epitaxy and exhibit responsivity as high as 1000 A/W.

Electric field effects on intersubband transitions in quantum well structures

Abstract We report on a Raman scattering study of the electric-field dependence of c 0 → c 1 intersubband transitions of photoexcited electrons in a 264 A GaAs- Al 0.3 Ga 0.7 As quantum-well structure. The measured Stark shifts are in very good agreement with theoretical predictions. The intensity of the intersubband peak increases rapidly with applied field due to parity-mixing. In contrast to the enhanced broadening shown by excitation resonances, the width of c 0 → c 1 is nearly independent of the field. This feature is attributed to effects of structural disorder.

Growth and properties of quasiperiodic heterostructures

Abstract We have recently demonstrated the MBE growth of heterostructures in which layers of GaAs and AlAs were deposited in a Fibonacci sequence. This yields a quasiperiodic structure with the ratio of incommensurate periods equal to the golden mean, γ. We present an overview of the unique structural, electronic, and vibrational properties of this new class of materials emphasizing the role of the incommensurate structure normal to the layers. Inevitably, defects are introduced by growth fluctuations but do not appear to disrupt significantly the special characteristics which originate from the quasiperiodic ordering.