E. Colas

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.

In situ determination of free‐carrier concentrations by reflectance difference spectroscopy

We determine types and concentrations of free carriers in GaAs layers under organometallic chemical vapor deposition growth conditions from the linear electro‐optic structure observed near 3 eV in reflectance‐difference spectroscopy. The sensitivity is about 1017 cm−3 at 400 °C and 1018 cm−3 at 600 °C, sufficient to measure common doping levels at or near growth temperatures. We observed the transition between n‐ and p‐type doping during atomic layer epitaxy of a carbon‐doped p‐type layer on an n‐type substrate at 470 °C.

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.

Luminescence characteristics of quantum wires grown by organometallic chemical vapor deposition on nonplanar substrates

Luminescence properties of GaAs/AlGaAs quantum wire (QWR) heterostructures grown by organometallic chemical vapor deposition on V‐grooved substrates are reported. A model of the crescent‐shaped wires yields parabolic QWR potential wells with subbands separated by 21.7, 3.9, and 16.7 meV for electrons, heavy holes, and light holes and effective width of 16 nm for the ground electron state. Spectrally and spatially resolved cathodoluminescence images reveal highly uniform emission from the QWR regions. Photoluminescence excitation spectra exhibit enhanced absorption at the QWR subbands, with subband separations in good agreement with the model.

Integrated external cavity GaAs/AlGaAs lasers using selective quantum well disordering

Integrated external cavity GaAs/AlGaAs single quantum well lasers were fabricated by selective quantum well disordering. Lasers with 2.07‐mm‐long passive sections and 0.48‐mm‐long active sections had threshold currents of 33 mA, compared to 9.8 mA for lasers without passive sections. Lasing data indicate a residual modal loss of 11 cm−1 in the passive sections, consistent with direct waveguide loss measurements. Control composite structures with a nondisordered quantum well in the passive sections showed significantly higher threshold currents and a large red shift of as much as 11.4 nm in the lasing wavelength compared to lasers without a passive cavity. This red shift is the main reason for the reduced resonant losses in integrated external cavity lasers with a nondisordered quantum well in the passive section.

Lateral and longitudinal patterning of semiconductor structures by crystal growth on nonplanar and dielectric-masked GaAs substrates : application to thickness-modulated waveguide structures

Abstract Two methods which achieve both lateral and longitudinal patterning of semiconductor properties by organometallic chemical vapor deposition (OMCVD) are presented. Both approaches utilize diffusion of reactant species from a nongrowth surface to an adjacent growth surface, presumably via the gas phase. In the first method, the nongrowth surface is the (111)B facet which develops during deposition on a GaAs substrate where mesas in the (011) orientation have been etched prior to growth. Low-loss single mode rib-type waveguides (0.6 dB/cm at 1.52 μm wavelength) were fabricated with this approach. In the second method, the nongrowth surface is a dielectric mask deposited on the GaAs surface prior to growth. Growth rate enhancements can be controlled, and can be as high as 280% with this second approach, which appears to be more practical than the first one. This new capability for OMCVD will offer a wide range of applications.

Generation of macroscopic steps on patterned (100) vicinal GaAs surfaces

We show that macroscopic, as opposed to microscopic, steps can be obtained on a semiconductor vicinal surface when a perturbation has been ‘‘printed’’ on it, prior to epitaxial growth. This generic crystal growth concept has been studied here with the GaAs/AlGaAs system using organometallic chemical vapor deposition. The details of step formation, stabilization, and subsequent propagation have been investigated with scanning electron microscopy. Regular, sawtooth‐like growth patterns have been obtained, with periodic growth rate differences at the step edges. This novel lateral patterning technique was employed to fabricate arrays of quantum wire‐like heterostructures.

In situ monitoring of crystal growth by reflectance difference spectroscopy

Reflectance difference spectroscopy (RDS) is a surface analysis technique that was invented in 1985 by Aspnes. Here, we give a summary of its application to crystal growth techniques which gave new and valuable real time information about the growth process. Also, this information was obtained in-situ, in molecular beam epitaxy (MBE) and organometallic chemical vapor deposition (OMCVD) crystal growing setups which are routinely used to produce high quality device structures. The application of RDS to OMCVD allowed us to develop a “textbook” like model of growth kinetics, which includes two independent microscopic mechanisms, i.e. adsorption (at -26 kcal/mol) of the reacting molecule (trimethylgallium (TMG) in the case of GaAs), followed by its decomposition (at 39 kcal/mol) on the growing GaAs surface. Our model includes an effect called steric hindrance, associated with the large size of the TMG molecule. This study represents the first direct quantitative evaluation of the catalytic effect of the GaAs surface for the decomposition of TMG. We discuss implications of the model both for growth in the ALE mode as well as for conventional OMCVD growth and comment on the relative importance of surface and gas phase reactions. The application of RDS to MBE revealed remarkable details about the complex intermediate steps that surfaces undergo during growth and enabled to extract directly surface dielectric functions. Finally, applications of the technique as well as results obtained in a number of laboratories where RDS is currently being developed are discussed.

In situ definition of semiconductor structures by selective area growth and etching

Selective area growth (etching) by low‐pressure organometallic chemical vapor deposition (LP‐OMCVD) is utilized to intentionally modulate the local growth (etch) rate by choosing the pattern of dielectric‐masked areas, thereby defining III‐V semiconductor structures in situ. This technique is applied to tune the emission wavelength of a GaAs/AlGaAs quantum well structure, and to obtain InP/InGaAs superlattice structures tapered in thickness with growth rate increases as high as 800%, suitable for integrated optics applications. In contrast, selective deposition by organometallic molecular beam epitaxy (OMMBE) does not produce growth rate enhancements, thereby preventing similar in situ definition schemes but allowing to integrate structures with optimized nominal thicknesses.

Growth of GaAs quantum wire arrays by organometallic chemical vapor deposition on submicron gratings

We report on the growth of dense lateral arrays of GaAs quantum wire structures, obtained by organometallic chemical vapor deposition (OMCVD) on GaAs substrates where a submicron grating has been lithographically defined and etched prior to deposition. The experiments were performed simultaneously on (100) oriented substrates, where the wires are virtually ‘‘isolated’’ from each other, and on substrates that are vicinal with respect to the (100) orientation, where the wires are ‘‘smoothly connected’’ by quantum wells. Transmission electron microscopy investigations allowed the study of the morphology of the resulting structures, which was related to the microscopic step nature of the starting surfaces and revealed important basic aspects of growth dynamics.