P. L. Gourley

Controversy of critical layer thickness for InGaAs/GaAs strained‐layer epitaxy

The critical layer thickness for InxGa1−xAs layers in InxGa1−xAs/GaAs single strained quantum wells (SSQW’s) and strained‐layer superlattices (SLS’s) are investigated. Photoluminescence microscopy (PLM) images and x‐ray rocking curves for two series of SSQW and SLS structures corresponding to many different layer thicknesses were obtained. We find that the PLM technique, which directly images dislocations and is sensitive to low dislocation densities, is much more suitable for determining the onset of dislocation creation. The x‐ray technique can detect lattice relaxation by dislocations but only at relatively high densities of dislocations. Using the former technique, we determine critical thicknesses of 190 A for SSQW’s and 250 A for SLS’s with x≊0.2. These results are near the theoretical predictions of J. W. Matthews, S. Mader, and T. B. Light [J. Appl. Phys. 41, 3800 (1970)] (150 and 300 A, respectively) and are much lower than results obtained by x‐ray or other techniques which sense lattice relaxation.

Critical layer thickness in In0.2Ga0.8As/GaAs single strained quantum well structures

We report accurate determination of the critical layer thickness (CLT) for single strained‐layer epitaxy in the InGaAs/GaAs system. Our samples were molecular beam epitaxially grown, selectively doped, single quantum well structures comprising a strained In0.2Ga0.8As layer imbedded in GaAs. We determined the CLT by two sensitive techniques: Hall‐effect measurements at 77 K and photoluminescence microscopy. Both techniques indicate a CLT of about 20 nm. This value is close to that determined previously (∼15 nm) for comparable strained‐layer superlattices, but considerably less than the value of ∼45 nm suggested by recent x‐ray rocking‐curve measurements. We show by a simple calculation that photoluminescence microscopy is more than two orders of magnitude more sensitive to dislocations than x‐ray diffraction. Our results re‐emphasize the necessity of using high‐sensitivity techniques for accurate determination of critical layer thicknesses.

A GaAsxP1−x/GaP strained‐layer superlattice

Strained−layer superlattices form a broad new class of semiconductor materials with tailorable electronic properties. We have succeeded in growing a GaAsxP1−x/GaP(100) strained−layer superlattice (SLS). The structure was grown by alternate metalorganic chemical vapor deposition of thin (60 A)layers (20 each) of GaAs0.4P0.6 and GaP. These layers were grown onto a GaAsxP1−x layer which was graded in composition from x = 0 (composition of underlying GaP substrate)to x = 0 (average composition of the SLS). Photoluminescense studies of the SLS were carried out to determine the optical band gap. At T = 78 K, the spectrum shows a dominant band−edge peak at 2.03 eV as well as weaker peaks at higher energies. Tight binding and effective mass calculations, also carried out, predict a direct band gap (due to zone folding) of 2.02 eV and higher lying transition energies which are in good agreement with these data.

Optical properties of two‐dimensional photonic lattices fabricated as honeycomb nanostructures in compound semiconductors

We have experimentally studied two‐dimensional photonic lattices, honeycomb nanostructures, fabricated by electron beam lithography with (Al,Ga)As materials. Surface normal optical properties were investigated by measuring reflectance to determine the effective index of refraction and lattice stability against degradation. Also, continuous wave and time‐resolved luminescence spectroscopy was used to assess electron‐hole recombination. Finally, light scattering was employed to study photon coupling and propagation through the lattice. These measurements show that the structures are stable, that nonradiative surface recombination is present, and that resonant coupling of light into/out of the lattice occurs at selected wavelengths satisfying a Bragg condition.

Gain characteristics of strained quantum well lasers

InGaAs/AlGaAs laser diode arrays fabricated with differing amounts of In in the quantum well active layer are characterized by threshold currents of 115 A/cm2, transparency currents of 50 A/cm2, optical losses of 3 cm−1, and wavelengths to 960 nm for In compositions of 20%. Gain coefficient measurements indicate an increase from 0.0535 to 0.0691 cm μm/A for quantum well lasers with 0% InAs and 10–20% InAs, respectively. The maximum output power achieved for a device with a 100 μm aperture is 3 W cw.

Coherent beams from high efficiency two‐dimensional surface‐emitting semiconductor laser arrays

We have fabricated and operated large two‐dimensional (2D) arrays of phase‐locked surface‐emitting semiconductor lasers. The arrays were fabricated by reactive ion beam etching of epitaxial Fabry–Perot resonators comprising GaAs/AlGaAs quantum wells surrounded by AlAs‐AlGaAs quarter‐wave mirrors. Different arrays corresponding to different pixel size (2–5 μm) and spacing (1–2 μm) were produced to investigate evanescent coupling between pixels. The arrays were photopumped so that the array size could be conveniently varied from 1×1, 2×2,... up to 20×20. Except for the 1×1 which emits a circular pattern, all arrays exhibit a well‐defined four‐lobed far‐field pattern in agreement with our theoretical analysis of the optical modes which predicts domination by the 2D out‐of‐phase eigenmode. As a consequence this pattern can be understood with simple Fraunhofer diffraction theory. The angular spread of the lobes, determined by the periodicity of the array elements, is 10° for the array with element size/spacing...

On-axis far-field emission from two-dimensional phase-locked vertical cavity surface-emitting laser arrays with an integrated phase-corrector

We have fabricated large, two‐dimensional (2D) arrays of optically pumped, phase‐locked vertical cavity surface‐emitting lasers that emit more than 50% of their light in a central on‐axis lobe. The emission of the arrays was modified from the usual four‐lobed far‐field of 2D coupled arrays by incorporation of a binary phase‐shift mask on the surface of the array. The array consists of Fabry–Perot resonators comprising GaAs/AlGaAs quantum wells surrounded by AlAs/AlGaAs quarterwave mirrors with a multiple order AlGaAs phase‐delay layer on the top mirror stack. The phase‐shift layer was etched away on alternating elements of the array. The resulting on‐axis emission had an angular width of 2° for an array of approximately 100 elements.

Visible, room‐temperature, surface‐emitting laser using an epitaxial Fabry–Perot resonator with AlGaAs/AlAs quarter‐wave high reflectors and AlGaAs/GaAs multiple quantum wells

We report operation of a new surface‐emitting laser. This epitaxial laser is fabricated with molecular beam epitaxy by the growth of quarter‐wave high reflectors of AlAs/Al0.4Ga0.6As (710 A/630 A) which surround a 4.5‐μm‐thick multiple quantum well of GaAs/Al0.4Ga0.6As (100 A/200 A). We characterize the structure with cw spectroscopy (absorption, reflection, and luminescence) and investigate stimulated emission spectra under pulsed photopumping. When photopumped, the structure lases in its as‐grown condition without need of substrate removal, cleaving, or heatsinking. The lasing wavelength is as short as 7400 A and can be tuned to as long as 8400 A by positioning the pump spot to different regions across the wafer. The pulsed threshold irradiance has a very weak temperature dependence varying from 6×105 W/cm2 at 4.2 K to 1.6×106 W/cm2 at 295 K.

Biocavity laser for high-speed cell and tumour biology

Through recent interdisciplinary scientific research, modern medicine has significantly advanced the diagnosis and treatment of disease. However, little progress has been made in reducing the death rate due to cancer, which remains the leading cause of death in much of the world. Pathologists rely on microscopic examination of cell morphology using methods that originated over a hundred years ago. These staining methods are labour-intensive, time-consuming, and sometimes in error. New micro-analytical methods for high speed (real-time) automated screening of tissues and cells could advance pathology and minimize cancer deaths. By teaming experts in physical/chemical sciences and engineering with those in medicine, it may be possible to develop micro-analytical cell spectral/imaging techniques to rapidly distinguish normal and abnormal cells. In this paper, we review the physics and applications of the biocavity laser which may enable these advances in the near future.

High‐efficiency TEM00 continuous‐wave (Al,Ga)As epitaxial surface‐emitting lasers and effect of half‐wave periodic gain

We report room‐temperature, continuous‐wave (cw), photopumped operation of (Al,Ga)As surface‐emitting lasers grown by molecular beam epitaxy. These monolithic semiconductor lasers comprise two multilayer semiconductor mirrors surrounding a layered active region. In the active region, GaAs quantum wells are spaced with half‐wave periodicity to center on standing‐wave maxima of the cavity optical field. By comparing threshold data for different lasers grown with and without half‐wave periodicity, we observe the first experimental evidence for reduced cw lasing threshold (as low as 2×104 W/cm2 ) with periodic gain in an epitaxial surface‐emitting laser. Up to 50 mW with high efficiency (35% total, 80% differential) and narrow spectral linewidth (2 A) have been measured. A very high quality beam with low divergence (2.5°) and circular TEM00 profile has been observed. All of these observations represent significant advances for surface‐emitting laser technology.