D. Biswas

Conduction‐ and valence‐band offsets in GaAs/Ga0.51In0.49P single quantum wells grown by metalorganic chemical vapor deposition

We have independently estimated the conduction‐ and valence‐band offsets ΔEc and ΔEv in GaAs/Ga0.51In0.49P quantum wells by measuring the capacitance transient resulting from thermal emission of carriers from the respective wells. The heterostructure samples were grown by low‐pressure metalorganic chemical vapor deposition. The band offsets are extrapolated from the emission activation energies with appropriate corrections. The estimated values of ΔEc and ΔEv are 0.198 and 0.285 eV, respectively.

Traps in molecular-beam epitaxial In0.53(GaxAl1−x)0.47As/InP

Deep‐level transient spectroscopy measurements have been made on molecular‐beam epitaxial In0.53(GaxAl1−x)0.47As lattice matched to InP. Several electron and hole traps, with activation energies ranging from 0.14 to 0.79 eV, have been identified and characterized. In particular, systems of electron traps (0.30≤ΔET≤0.79 eV) and hole traps (0.14≤ΔET≤0.31 eV) with monotonically changing activation energies have been identified in these alloys. We believe these traps are dominant in this alloy system.

Tunability of InGaN/GaN quantum well light emitting diodes through current

In the recent years, InGaN/GaN quantum well (QW) light emitting diodes (LEDs) have gathered much importance through the introduction of white LEDs and dual wavelength LEDs. However, the continuous tunability of InGaN/GaN QW LEDs has not been well addressed or discussed. In this paper, we introduce the tunability of an InGaN/GaN QW LED having a well width of 4 nm and In mole fraction of 0.3. The results, obtained from self-consistent solutions of the Schrodinger and Poisson equations, show that the transition energy of the LED may be continuously tuned by the device current. A prominent nonlinearity of the transition energy with the device current is generated, which should be of interest to the research workers in the field of optoelectronics.

Low‐temperature (10 K) photoluminescence of Ga1−xInxPyAs1−y quantum wells grown by metalorganic chemical vapor deposition

Ga1−x Inx As1−y Py /InP (x=0.72, y=0.39) lattice‐matched quantum wells (QWs) are grown by low‐pressure metalorganic chemical vapor deposition on (100) and 3° misoriented substrates, using different variations of growth technique. Low‐temperature (10 K) photoluminescence is used to characterize the QWs. We find that substrates oriented closely to (100) (no intentional misorientation) produce QWs of consistently higher quality as judged by the width of the n=1 photoluminescence peak. The use of growth interruptions at the well interfaces severely degrades the QW quality. The narrowest peak observed is 5.8 meV wide from a 70‐A‐wide well.

Recombination velocity at molecular‐beam‐epitaxial GaAs regrown interfaces

We have estimated the recombination velocity and minority‐carrier diffusion length at and near molecular‐beam‐epitaxial GaAs regrowth interfaces. The diffusion length in the regrown layers is ∼1–3 μm and is lowered to 0.3 μm at the interface. The interface recombination velocity is ∼105 cm/s. These parameters are better for a sample which was ion milled and lamp annealed before regrowth, compared to a sample which was wet‐chemical etched and annealed in the growth chamber under arsenic flux before regrowth.

Low-power exciton-based heterojunction bipolar transistors for thresholding logic applications

The principles of an integrated optoelectronic controller-modulator device, based on excitonic transitions and the enhanced Stark effect in quantum wells, are outlined. The device consists of a controller and a modulator as components. The controller is a heterojunction phototransistor with multiquantum wells incorporated in the base-collector depletion region. The amplified output of the controller enables switching of the modulator for low optical power levels. Experimental results on GaAs-AlGaAs based devices, realized by one-step molecular beam epitaxy and selective etching, are presented. The bipolar devices have current gains of approximately 35-40. The integrating-thresholding properties of the device are demonstrated and switching characteristics for 10 mu W input to the controller are measured. Cascadability, optoelectronic amplification, and multistage operation are demonstrated in terms of a fan out of eight devices. >

Optical and structural properties of molecular-beam epitaxial GaAs on sapphire

We have evaluated the structural and optical properties of GaAs grown by molecular‐beam epitaxy on (0001) and (0112)‐oriented sapphire substrates for possible direct deposition on sapphire optical fibers and optoelectronic device applications. X‐ray diffraction studies and cross‐sectional transmission electron microscopy measurements show that the GaAs films are single‐crystal and have a (111) orientation for both substrate orientations. Both x‐ray and temperature‐dependent photoluminescence measurements indicate that the quality of the GaAs films is better for the (0112) sapphire orientation. The best results are produced when growth is initiated at a very slow rate (∼0.1 μm/h) and low temperature (∼400 °C) followed by growth parameters for normal homoepitaxial GaAs (1 μm/h, 560–600 °C). The incorporation of a graded band gap strained layer in InGaAs/GaAs superlattice near the sapphire–GaAs interface also improves the optical quality of the GaAs films. Low‐temperature photoluminescence spectra is domin...

Noise suppression characteristics of InP-based monolithically integrated guided wave balanced photodiodes

Measurements were made of the noise suppression and signal enhancement properties of dual In/sub 0.53/Ga/sub 0.47/As-InP metal-semiconductor-metal (MSM) photodiodes integrated with a leaky mode In/sub 0.52/Al/sub 0.48/As waveguide directional coupler, and an electro-optic phase shifter. The experiments were done with a 1.3- mu m laser as the local oscillator. A noise suppression of 11 dB and signal enhancement of 1.3 dB were recorded for the MSM photodiodes.<<ETX>>

The influence of strain on the small signal gain and lasing threshold of GaInAs/GaAs and GaAs/GaInAlAs strained-layer quantum well lasers

Results of calculations on how band-structure changes introduced by strain affect the gain spectra in multi-quantum-well (MQW) lasers are presented. Reduction of threshold injection as well as increased TE mode emission occurs for the compressive-strain case. In tensile strain the thresholds for TE and TM modes approach each other. Experimental studies on the compressive-strain laser structures show that the spontaneous and lasing spectra shift to longer wavelengths, the threshold current density is reduced, and a more rapid rise of gain with increased injection occurs. The output spectrum shifted from the 959-962 nm range for the GaInAs devices with 20% In to the 1033.5-1041.0-nm range for devices with 30% In. The threshold current density decreased with increasing strain, as expected from the theory. The modal gain of the devices and the peak modal gain were shown to rise very quickly with increased injection. This qualitatively agrees with the expected decrease in hole density of states brought about by strain, which could cause the hole states to be more rapidly filled and result in a more rapid rise in the material gain.<<ETX>>

Effect of coherent strain on hydrogenic acceptor levels in InyGa1−yAs/AlxGa1−xAs quantum well structures

The biaxial strain produced in lattice‐mismatched epitaxy can have a substantial effect on the valence band structure. Theoretical results are presented for a hydrogenic acceptor in a quantum well under tensile and compressive strain. The acceptor level energy is a strong function of strain and could be used as a signature for the effect of strain on the valence band structure. Experimental studies are carried out on compressively strained InyGa1−yAs/ AlxGa1−xAs quantum well structures and the acceptor level energy is determined by photoluminescence measurements. Good agreement is found with the experiments.