D. M. Hwang

Single quantum wire semiconductor lasers

Single quantum wire GaAs/AlGaAs injection lasers were fabricated using organometallic chemical vapor deposition on V‐grooved GaAs substrates. The quantum wire active region has a crescent‐shaped cross section ∼100 A thick and less than 1000 A wide. Amplified spontaneous emission and lasing spectra of the quantum wire lasers exhibit effects due to transitions between quasi‐one‐dimensional subbands separated by ∼10 meV. Single quantum wire laser structures with tight optical confinement exhibited threshold currents as low as 3.5 mA for uncoated devices at room temperature.

Patterned quantum well heterostructures grown by OMCVD on non-planar substrates: Applications to extremely narrow SQW lasers

Abstract We have studied the OMCVD growth of GaAs/AlGaAs quantum well heterostructures on non-planar substrates in the temperature range of 625 to 750°C. The lateral variation in layer thickness and other growth features were found to depend not only on the growth temperature but also on the aluminum content of the layer. An example of the application of this technique of producing lateral thickness variations in quantum well heterostructures to a quantum well semiconductor laser is given. A unique feature of this laser is the formation of a quantum-wire-like crescent shaped active region at the center of a two-dimensional optical waveguide.

Bonding by atomic rearrangement of InP/InGaAsP 1.5 μm wavelength lasers on GaAs substrates

A technique, namely bonding by atomic rearrangement has been invented to realize high quality heteroepitaxy for lasers and optoelectronics. High performance lasers of 1.5 μm wavelength have been fabricated on GaAs substrates using this method. The laser has the same threshold current and quantum efficiency as lasers on InP substrates. No performance degradation has been observed. The transmission electron microscopic results show that the heteroepitaxy is excellent, without a single threading dislocation or stacking fault.

Orientation dependence of S,Zn,Si,Te and Sn doping in OMCVD growth of InP and GaAs : application to DH lasers and lateral p-n junction arrays grown on non-planar substrates

Abstract The orientation dependence of doping in organometallic chemical vapor deposition (OMCVD) is shown to be far more complex than previously believed, with the variation of doping with increasing misorientation from (100) towards the (111)A/B being non-monotonic. However, the ratio of the n-doping on the B face to that on the corresponding A face is always greater than 1, irrespective of whether the dopant is a group IV or VI element. For p-doping with Zn, the reverse is true. The orientation dependence of doping has been used to create current blocking layers in InP/InGaAsP double heterostructure (DH) and multiple quantum well (MQW) lasers grown in a single step on a mesa or in a V-groove.

Carrier capture and quantum confinement in GaAs/AlGaAs quantum wire lasers grown on V-grooved substrates

Carrier capture mechanisms in semiconductor quantum wire (QWR) lasers grown by organometallic chemical vapor deposition on nonplanar substrates were investigated by cathodoluminescence and photoluminescence (PL) spectroscopy. Efficient carrier capture into the QWRs via adjacent quantum wells is manifested by a complete transfer of luminescence intensity from the well‐ to the wire‐spectral lines at temperatures above ∼100 K. In addition, higher QWR subbands separated by 19 meV are observed in the PL spectra, in agreement with the calculated subband spacing. The quantum well assisted carrier capture in these wires is important for the efficient room temperature operation of lasers and other optoelectronic devices based on QWRs.

Structural characterization of GaAs/ZnSe interfaces

We have studied, using reflection high energy electron diffraction (RHEED), the initial growth stages of ZnSe layers grown by molecular beam epitaxy (MBE) on GaAs exhibiting various surface terminations. The structural quality of the ZnSe layers was assessed by transmission electron microscopy (TEM). We have observed a preference for two‐dimensional nucleation on As‐terminated GaAs substrates, whether epitaxial or bulk. On Ga‐terminated surfaces a transitional region of three‐dimensional growth forms initially. The different behaviors may reflect the electronic imbalance present in the growing GaAs/ZnSe interface. While defect free, thin layers of ZnSe are best obtained on As‐terminated GaAs, the initial three‐dimensional growth region on the Ga‐rich surfaces appears to reduce the density of extended faults formed in thick ZnSe layers due to the lattice mismatch with GaAs.

Optical properties and band structure of short-period GaAs/AlAs superlattices

Abstract We have studied six GaAs/AlAs superlattices with periods ranging from 18 to 60 A and different average aluminum composition. Three of these samples are shown to be direct bandgap materials whose band structure differs strongly from that of the corresponding alloy, but is correctly described by an envelope function calculation. The three remaining samples are shown to be indirect both in real and reciprocal space. The lowest energy transitions are found to arise from an exciton involving a heavy hole state mostly confined in the GaAs layer and at the Brillouin zone center (Λ), and an electronic state of X character confined in the AlAs layers. Analysis of the time decay of the luminescence shows that this is a momentum-forbidden exciton made allowed by disorder scattering, which leads to a luminescence efficiency comparable to that of the direct bandgap samples. Partial lifting of the degeneracy of the three X orbitals by the superlattice potential is also observed. Finally, we take advantage of the strong dependence of these indirect transition energies on the band discontinuities to estimate the valence band offset to be about 550 meV in this system.

Vertically stacked multiple‐quantum‐wire semiconductor diode lasers

We report the structure and lasing characteristics of GaAs/AlGaAs vertically stacked multiple‐quantum‐wire (QWR) semiconductor lasers grown by organometallic chemical vapor deposition on V‐grooved substrates. The active region in these lasers consists of three crescent‐shaped wires, placed at the center of a single‐mode optical waveguide. The higher optical confinement factor, compared to single‐QWR structures, leads to reduced threshold currents, as low as 0.6 mA for high‐reflection coated devices at room temperature. The lower threshold carrier density results in oscillation at a lower QWR subband as compared to single‐QWR laser structures.

Quantum wire lasers by OMCVD growth on nonplanar substrates

Abstract In this paper, we describe the progress made in the fabrication of quantum wire lasers using growth on nonplanar substrates as a lateral patterning technique. GaAs/AlGaAs quantum wire injection lasers with up to three crescent-spaped quantum wire active regions have been fabricated. The lowest threshold current of 2.4 mA was obtained for lasers with 2 quantum wires. We also report on two techniques for the fabrication of quantum wire arrays in GaAs/AlGaAs. Finally, we present our results on an attempt to fabricate quantum wire lasers based on InP, and propose possible solutions to the problems encountered.

InGaAs/GaAs strained quantum wire lasers grown by organometallic chemical vapor deposition on nonplanar substrates

The structure and device characteristics of InGaAs/GaAs strained quantum wire (QWR) lasers grown by organometallic chemical vapor deposition on 0.25‐μm pitch periodic corrugations are reported. The crescent‐shaped InGaAs wires, 14–17 nm thick and 70–80 nm in full width, are formed in situ due to surface diffusion of In and Ga species to the bottom of the grating grooves. Room‐temperature pulsed operation has been achieved with threshold current densities of 1.9 kA/cm2 for 1.1 mm long lasers with uncoated facets. For laser stripes aligned perpendicular to the wires, the lasing wavelength remains almost constant at 920 nm for temperatures 80<T<150 K due to second‐order Bragg reflection from the QWR array. The possibility of achieving gain‐coupled distributed feedback using the periodic gain modulation in these devices is discussed.