T. Chung

Observations of near-zero linewidth enhancement factor in a quantum-well coupled quantum-dot laser

Amplified spontaneous emission (ASE) measurements of a quantum-well coupled quantum-dot (QW–QD) laser are investigated in our experimental study. Fabry–Perot ASE spectrum taken below threshold of this device allows the extraction of gain, index of refraction change, and linewidth enhancement factor. Our experimental study includes continuous wave and pulsed measurements. The QW–QD laser consists of an auxiliary QW which assists in carrier collection while tunneling of carriers takes place from the well to the dot region. Our experimental analysis reveals a low linewidth enhancement factor of 0.15 over a flat spectrum for these GaAs–InGaAs–InAs QW–QD lasers.

Coupled strained-layer InGaAs quantum-well improvement of an InAs quantum dot AlGaAs–GaAs–InGaAs–InAs heterostructure laser

Data are presented showing that, besides the improvement in carrier collection, it is advantageous to locate strain-matching auxiliary InGaAs layers [quantum wells (QWs)] within tunneling distance of a single-quantum-dot (QD) layer of an AlGaAs–GaAs–InGaAs–InAs QD heterostructure laser to realize also smaller size QDs of greater density and uniformity. The QD density is changed from 2×1010/cm2 for a 50 A GaAs coupling barrier (QW to QD) to 3×1010/cm2 for a 5 A barrier. The improved QD density and uniformity, as well as improved carrier collection, make possible room-temperature continuous-wave (cw) QD+QW laser operation (a single InAs QD layer) at reasonable diode length (∼1 mm), current density 586 A/cm2, and wavelength 1057 nm. The cw 300 K coupled InAs QD and InGaAs QW AlGaAs–GaAs–InGaA–InAs heterostructure lasers are grown by metalorganic chemical vapor deposition.

High-gain coupled InGaAs quantum well InAs quantum dot AlGaAs–GaAs–InGaAs–InAs heterostructure diode laser operation

Data are presented showing that a single-layer InAs quantum dot (QD) laser in the AlGaAs–GaAs–InGaAs–InAs heterostructure system is improved in gain and continuous wave (cw) room temperature operation by coupling, via tunneling, auxiliary strained-layer InGaAs quantum wells (QWs) to the single InAs QD layer to assist carrier collection and thermalization. A QW-assisted single-layer InAs QD laser, a QD+QW laser, is demonstrated that operates cw (300 K), and at diode length 150 μm in pulsed operation exhibits gain as high as ∼100 cm−1.

Growth mechanism of InAs quantum dots on GaAs by metal-organic chemical-vapor deposition

The growth parameters affecting the deposition of InAs quantum dots (QDs) by metal-organic chemical-vapor deposition are reported. Experiments with arsine pause, gas switching, and hydrogen shroud flow show that a low V∕III ratio is the key to obtaining three-dimensional InAs island formation with high density and uniformity. Based on atomic force microscopy images of InAs QDs deposited under different growth conditions, a physical model for the epitaxial growth of three-dimensional islands is proposed. In this model, the InAs QD growth is governed by two types of arsenic sources at the growth surface: free arsenic atoms arriving at the boundary layer and dangling arsenic bonds available at the GaAs wafer surface. At high V∕III ratio, free arsenic atoms arriving at the boundary layer are the dominant hydride species and produce a low density of InAs islands with irregular shape and polycrystalline defects. At low V∕III ratio arsenic bonds on the GaAs surface are the main sites for indium atoms to attach t...

Coupled-stripe quantum-well-assisted AlGaAs–GaAs–InGaAs–InAs quantum-dot laser

Data are presented on the coupled-stripe laser operation (continuous wave, 300 K) of a single InAs quantum-dot (QD) layer coupled via a thin (5 A) GaAs barrier to an auxiliary strained InGaAs quantum well (QW) grown (confined) in an AlGaAs–GaAs heterostructure. Because of strain-induced (QW strain) improvement of the QD growth and QD alignment along diagonal (reflecting) ridges, the InGaAs-QW+InAs-QD crystals exhibit high gain along and across laser stripes, which is advantageous for coupled-stripe laser operation. A twin-stripe single-QD-layer QW+QD laser (4 μm stripes on 6 μm centers) of usual cleaved length, 50 mW.

Minority carrier lifetime degradation in carbon-doped base of InGaP/GaAs heterojunction bipolar transistors grown by low-pressure metalorganic chemical vapor deposition

The effect of intermediate temperature annealing on the carbon-doped base region of InGaP/GaAs heterojunction bipolar transistors (HBTs) was studied. This work shows that the minority carrier lifetime in the samples doped at 5.5×1019 cm−3 decreases upon annealing at only 600 °C. Magnetotransport measurements were performed to obtain the minority carrier mobility, with which the minority carrier lifetime was extracted. The decrease in the direct current (dc) current gain upon annealing is attributed to the increase in the base bulk recombination. The correlation between the dc current gain and the magnetotransport measurements indicates that the annealing increases the carbon-related defects in the GaAs base, decreases the minority carrier lifetime in the carbon-doped base, and degrades the dc current gain of the InGaP/GaAs HBTs. These results are very important to the growth and postgrowth processing of InGaP/GaAs HBTs.

1.55-μm asymmetric Fabry-Perot modulator (AFPM) for high-speed applications

We explored the possibility of using an aysmmetric Fabry–Perot modulator (AFPM) for the application of high-speed external optical modulation at 1.55 μm. Using ten pairs of AlGaInAs–AlInAs as the bottom distributed Bragg mirror (DBR) and semiconductor–air interface as the top mirror, we demonstrated an AFPM with a 3-dB frequency response around 20 GHz and a dc extinction ratio of 4 dB at a reverse bias voltage of 0–5 V (and sharp extinction ratio drop from 0 to 2 V) at operating wavelength 1.55 μm. With a multiple quantum-well intrinsic region of 900 nm, which consists of 50 pairs of InGaAs–InAlAs (8–10 nm), the modulator has the potential for high-frequency applications.

Current gain dependence on subcollector and etch-stop doping in InGaP/GaAs HBTs

The DC current gain dependence of InGaP/GaAs heterojunction bipolar transistors (HBTs) on subcollector and etch-stop doping is examined. Samples of InGaP/GaAs HBTs having various combinations of subcollector doping and etch-stop doping are grown, and large area 60 /spl mu/m/spl times/60 (/spl mu/) HBTs are then fabricated for DC characterization. It is found that the DC current gain has a strong dependence on the doping concentration in the subcollector and the subcollector etch-stop. Maximum gain is achieved when the subcollector is doped at 6/spl sim/7/spl times/10/sup 18/ cm/sup -3/ while the subcollector etch-stop is doped either above 6/spl times/10/sup 18/ cm/sup -3/ (current gain/sheet resistance ratio, /spl beta//R/sub b/=0.435 at I/sub c/=1 mA) or below 3.5/spl times/10/sup 17/ cm/sup -3/ (/spl beta//R/sub b/=0.426/spl sim/0.438 at I/sub c/=1 mA). The data show that it is not necessary to heavily dope the subcollector etch-stop to reduce the conduction barrier and to obtain high current gain. The high current gain obtained with the low InGaP etch-stop doping concentration is attributed to the reduction of the effective energy barrier thickness due to band bending at the heterojunction between the InGaP etch-stop and the GaAs subcollector. These results show that the /spl beta//R/sub b/ of InGaP/GaAs HBTs can improve as much as 69% with the optimized doping concentration in subcollector and subcollector etch-stop.