Louis J. Guido

Stimulated emission and lasing in whispering-gallery modes of GaN microdisk cavities

We report optically pumped, pulsed lasing action in whispering-gallery modes of GaN microdisk cavities at room temperature. The microdisk structure was fabricated by reactive-ion etching a 2-μm-thick GaN epitaxial layer grown via metalorganic chemical vapor deposition. Below the lasing threshold, stimulated emission with superlinear pump-intensity dependence is observed. Spontaneous-to-stimulated emission transition occurs at a pump intensity that is 10× lower than that for a GaN sample without a cavity structure. Above the lasing threshold, the pump-intensity dependence is almost linear and gain pinning is observed. In addition, whispering-gallery modes are observed with the linewidth of individual peaks being as narrow as 0.1 nm.

Magnitude, origin, and evolution of piezoelectric optical nonlinearities in strained [111]B InGaAs/GaAs quantum wells

The magnitudes of excitonic nonlinearities were compared at 12 K in InGaAs/GaAs multiple quantum well structures with growth directions oriented along the [100] and [111] crystal axes by measuring both the steady‐state and time‐resolved differential transmission spectra. As expected, the spectra for the [100] sample are indicative of excitonic bleaching at all times and for all excitation levels, and a carrier recombination time of 0.8 ns and a nonlinear cross section (change in absorption coefficient per carrier pair) of ∼8×10−14 cm2 are extracted for the [100] sample. By comparison, for low excitation levels, the spectra for the [111] sample are consistent with a blueshift of the exciton, indicating a screening of the strain‐induced piezoelectric field. At higher excitation levels, the spectra are dominated by excitonic bleaching. Under identical 1 ps pulsed excitation conditions, the magnitudes of the changes in the absorption coefficient caused by screening in the [111] sample are comparable to those ...

Threshold lowering in GaN micropillar lasers by means of spatially selective optical pumping

A new optical pumping method is demonstrated for creating a population inversion in GaN micropillar cavities with circular symmetry. This approach employs an axicon lens to produce a ring-shaped optical beam and provides for independent control of ring thickness and diameter. By bringing the pump beam into coincidence with whispering gallery modes confined near the outer rim of the cavity, we achieve a significant reduction in the threshold pump intensity for room-temperature operation of GaN micropillar lasers.

Electronic properties of arsenic-doped gallium nitride

Arsenic-doped GaN films were grown via metalorganic chemical vapor deposition using trimethylgallium, ammonia, and arsine precursors. The arsenic concentration increases from 3×1016 to 5×1017 cm−3 in response to a change in arsine mole fraction from 3.3×102 to 3.2×104 ppm. The electron mobility increases with arsenic content reaching a maximum value of 374 cm2/V s at 300 K. In addition, the integrated photoluminescence intensity exhibits a 35-fold increase in magnitude at 300 K. To explain these findings, a simple physical model is proposed in which arsenic “impurities” occupy otherwise vacant sites on both the gallium and nitrogen sublattices.

Screening effects in (111)B AlGaAs-InGaAs single quantum well heterostructures

A reduction in luminescence decay time and a shift toward higher optical transition energy is observed in response to an increase in photogenerated carrier density for a p‐i‐n (111)B Al0.15Ga0.85As‐In0.055Ga0.945As strained‐layer single quantum well heterostructure. These effects, which are attributed to free‐carrier screening of the strain‐induced electric field, are expected to be useful for designing novel optoelectronic devices that exploit the unique electro‐optic properties of (111) strained quantum wells.

Criteria for formation of interface dislocations in a finite thickness epilayer deposited on a substrate

The critical epilayer thickness for the formation of misfit dislocations at the interface between an epilayer and a substrate with a finite thickness is derived in the present study. The analysis is based on the energy approach, in which the self-energy of dislocation, the interaction energy between the dislocation and free surfaces, and the lattice mismatch energy of substrate and epilayer are calculated. To satisfy the free surface condition, the methodology of superposition principle and Fourier transformation are used in analyzing the stress field due to the interface dislocation. The critical epilayer thickness is compared with those reported in the literature.

Optical properties of strained layer (111)B Al 0.15 Ga 0.85 As-In 0.04 Ga 0.96 As quantum well heterostructures

Data presented here demonstrate that strained-layer (111)B Al0.15Ga0.85As-In0.04Ga0.96As quantum wells exhibit unique optical properties when compared to otherwise identical (100) oriented strained layers. Photoluminescence measurements identify a strain-induced electric field of order 6.7 Vµm-1 within the (111)B quantum well that is not present for the (100) case. Photoluminescence excitation spectroscopy measurements show that the heavy-hole to light-hole energy band splitting is approximately 7 meV larger for the (111)B structure than for the (100) structure.

Evidence of two‐species nucleation of InAs islands on (100) and (111)B GaAs substrates

The nucleation of InAs islands on (100) and (111)B GaAs substrates during metalorganic chemical vapor deposition has been investigated via scanning electron microscopy. The measured values of island density are 4–5 orders of magnitude smaller than those predicted using conventional one‐species nucleation theory. In addition, the effect of an increase in arsine flow on island density is opposite in sense for the two substrate orientations. To explain these observations, we further develop a two‐species picture of nucleation—originally proposed by Stoyanov [Appl. Phys. A 50, 349 (1990)]—in which the InAs island density is an explicit function of both the indium flux and the arsenic partial pressure at the growth interface. This more realistic physical model is in good agreement with the complete set of experimental data.

Annealed indium oxide transparent ohmic contacts to GaAs

The current–voltage characteristics of annealed In2O3–GaAs devices have been investigated for the purpose of fabricating transparent ohmic contacts to GaAs. A detailed study of the reverse current for as‐deposited structures with degenerately doped n‐type In2O3 on lightly doped n‐type GaAs yields a 0.67‐V high Schottky barrier. After annealing, the barrier height is only slightly modified, but the reverse current increases dramatically. This reverse current follows a voltage dependence that is consistent with a Fowler–Nordheim tunneling mechanism, for which the electric field is enhanced around localized asperities. These data suggest that an optimized annealing schedule might be used to fabricate low‐resistance, transparent In2O3 ohmic contacts to GaAs‐based optoelectronic devices.

An array of dislocations in a strained epitaxial layer. I. Elastic energy

The total energy of an array of dislocations in a strained epitaxial layer is composed of the self energy of the dislocations, the strain energy which arises from the lattice mismatch between the layer and its substrate and the interaction energy between the dislocations and the mismatch strains. The sum of the self energy and the interaction energy represents the formation energy of the dislocations. In this study, the self energy is formulated using complex potentials. Two limiting conditions are used to check the solution. The first is that the self energy of the array reduces to that for an isolated single dislocation as the dislocation spacing in the array approaches infinity. Secondly, as the layer thickness approaches infinity, the self energy reduces to that for a dislocation wall. A negative formation energy promotes dislocation generation while a positive formation energy implies a suppression of dislocation generation. A critical thickness required for the generation of an isolated dislocation ...