I‐Hsing Tan

A self‐consistent solution of Schrödinger–Poisson equations using a nonuniform mesh

A self‐consistent, one‐dimensional solution of the Schrodinger and Poisson equations is obtained using the finite‐difference method with a nonuniform mesh size. The use of the proper matrix transformation allows preservation of the symmetry of the discretized Schrodinger equation, even with the use of a nonuniform mesh size, therefore reducing the computation time. This method is very efficient in finding eigenstates extending over relatively large spatial areas without loss of accuracy. For confirmation of the accuracy of this method, a comparison is made with the exactly calculated eigenstates of GaAs/AlGaAs rectangular wells. An example of the solution of the conduction band and the electron density distribution of a single‐heterostructure GaAs/AlGaAs is also presented.

Electron states in mesa‐etched one‐dimensional quantum well wires

Two‐dimensional, self‐consistent solutions of the Schrodinger and Poisson equations are used to find the electron states in GaAs/AlGaAs quantum well wires. Both deep and shallow mesa structures are simulated. Our results show that while these structures are capable of providing the single occupied subband and wide energy separations needed for a true quantum wire, the process tolerances allowed are very small, on the order of 200 A of width variation. Cutoff widths calculated are 1000 A for the shallow mesa and 2100 A for the deep mesa. The agreement with experimental results is good for the shallow mesa, but poor for the deep mesa. This suggests additional process‐induced sidewall depletion mechanisms contributing to the cutoff of the deep mesa structures.

High quantum efficiency and narrow absorption bandwidth of the wafer-fused resonant In/sub 0.53/Ga/sub 0.47/As photodetectors

We demonstrate greater than 90% quantum efficiency in an In/sub 0.53/Ga/sub 0.47/As photodetector with a thin (900 /spl Aring/) absorbing layer. This was achieved by inserting the In/sub 0.53/Ga/sub 0.47/As/InP epitaxial layer into a microcavity composed of a GaAs/AlAs quarter-wavelength stack (QWS) and a Si/SiO/sub 2/ dielectric mirror. The 900-/spl Aring/-thick In/sub 0.53/Ga/sub 0.47/As layer was wafer fused to a GaAs/AlAs mirror, having nearly 100% power reflectivity. A Si/SiO/sub 2/ dielectric mirror was subsequently deposited onto the wafer-fused photodiode to form an asymmetric Fabry-Perot cavity. The external quantum efficiency and absorption bandwidth for the wafer-fused RCE photodiodes were measured to be 94/spl plusmn/3% and 14 nm, respectively. To our knowledge, these wafer-fused RCE photodetectors have the highest external quantum efficiency and narrowest absorption bandwidth ever reported on the long-wavelength resonant-cavity-enhanced photodetectors.<<ETX>>

120-GHz long-wavelength low-capacitance photodetector with an air-bridged coplanar metal waveguide

We demonstrate a long-wavelength detector structure using an undercut mesa and an air-bridged coplanar metal waveguide to significantly reduce both the diode RC constant and the parasitic capacitance. Record electrical bandwidths of 120 GHz are demonstrated for long-wavelength photodetectors.

Photoluminescence study of strain‐induced quantum well dots by wet‐etching technique

Simple holographic lithography and wet etching have been used to fabricate strain‐induced quantum well dot structures. Lateral confinement was generated in a GaAs quantum well (QW) by etching a double‐exposed grating pattern into a pseudomorphic, strained layer of In0.3Ga0.7As which overlies the QW. By spacing three QWs of different widths at varying depth from the stressor, lateral strain confinement and vertical strain propagation are directly resolved. We have observed at 14 meV redshift in the photoluminescence spectra for the QW located 22 nm away from the stressors and have confirmed that the strain propagation depth along the material growth direction is comparable to the lateral dot dimension.

Reduced quantum efficiency of a near‐surface quantum well

The effect of the proximity of a bare barrier surface on the quantum efficiency of underlying GaAs/Al0.3Ga0.7As and In0.13Ga0.87As/GaAs quantum wells (QWs) is studied by low‐temperature photoluminescence. The quantum efficiency of the resonantly excited QWs diminishes with decreasing surface barrier thickness; the onset of the reduction in quantum efficiency of the InGaAs QW occurs for a barrier that is 50 A thicker than for the GaAs QW. A simple model of carrier tunneling to the surface is formulated to explain the dependence of the quantum efficiency on surface barrier thickness and well width and height. This model shows good agreement with both sets of experimental data.

Ultrafast transport dynamics of p-i-n photodetectors under high-power illumination

We studied ultrafast transport dynamics of highspeed p-i-n photodetectors under high illumination using an electrooptic sampling technique. Under high illumination, saturation nonlinearities were found to be dominated by a space-charge-screening effect. A transient forward external current was also observed, which was attributed to an underdamped plasma oscillation. The external bias required to compensate these nonlinear effects increased with increased illumination.

Luminescence efficiency of near‐surface quantum wells before and after ion‐gun hydrogenation

We have studied the effects of the proximity of a bare Al0.3Ga0.7As surface on the luminescence of an underlying GaAs quantum well (QW) before and after hydrogenation. The mechanism which is affected by H is tunneling to surface states through the surface barrier. Its thickness was varied by wet etching from 60 to 350 A. Our experiments reveal that the degradation of luminescence efficiency from the QW is dependent on the surface barrier thickness and the excitation energy used in the photoluminescence measurements. A complete recovery or even further enhancement of luminescence efficiency was observed in the near‐surface QW after low‐energy ion‐beam hydrogenation, even at room temperature.

Systematic observation of strain‐induced lateral quantum confinement in GaAs quantum well wires prepared by chemical dry etching

HCl radical beam etching has been used to produce strain‐induced lateral confinement in a GaAs quantum well. This confinement was generated in the GaAs quantum well by radical beam etching a grating pattern into a pseudomorphic, strained layer of In0.35Ga0.65As which overlies the GaAs quantum well. The photoluminescence spectrum showed two peaks, corresponding to the GaAs quantum well both beneath the strained layer and in regions where the strained overlayer had been etched away. The peak due to strain‐induced confinement displayed a redshift that increases with etch time; the maximum shift observed was 20 meV. The after‐etch photoluminescence intensity and the systematic peak shift with etch time are indicators of the degree of control and low‐damage nature of the etch process used.

InGaAs quantum well wires grown on patterned GaAs substrates

Strained InGaAs/GaAs quantum well wires have been grown on 2300 A period gratings etched into GaAs substrates. Both conventional molecular beam epitaxy and migration‐enhanced epitaxy have been used as growth techniques. The low‐temperature photoluminescence from the quantum well wires was compared to the photoluminescence from a planar quantum well sample grown simultaneously. The planar samples showed a sharp single peak, whereas the patterned samples displayed two peaks. The samples grown by conventional molecular‐beam epitaxy had two fairly broad, overlapping peaks of comparable intensity. The samples grown by migration‐enhanced epitaxy had two distinct peaks, with one peak an order of magnitude larger than the other. Polarization‐dependent photoluminescence was performed on the migration‐enhanced epitaxy grown samples. The larger of the two peaks on the patterned sample showed a 21% linear polarization dependence, while the other peak showed unpolarized emission. In addition, the quantum well grown on...