B.E. Hammons

Advances in selective wet oxidation of AlGaAs alloys

We review the chemistry, microstructure, and processing of buried oxides converted from AlGaAs layers using wet oxidation. Hydrogen is shown to have a central role in the oxidation reaction as the oxidizing agent and to reduce the intermediate predict As/sub 2/O/sub 3/ to As. The stable oxide is amorphous (Al/sub x/Ga/sub 1-x/)/sub 2/O/sub 3/ which has no defects along the oxide/semiconductor interfaces but can exhibit strain at the oxide terminus due to volume shrinkage. The influence of gas flow, gas composition, temperature, Al-content, and layer thickness on the oxidation rate are characterized to establish a reproducible process. Linear oxidation rates with Arrhenius activation energies which strongly depend upon AlAs mole fraction are found. The latter produces strong oxidation selectivity between AlGaAs layers with slightly differing Al-content. Oxidation selectivity to thickness is also shown for layer thickness <60 nm. Differences between the properties of buried oxides converted from AlGaAs and AlAs layers and the impact on selectively oxidized vertical cavity laser lifetime are reported.

Selective oxidation of buried AlGaAs versus AlAs layers

We report significant differences between the properties of buried oxides converted from AlGaAs and AlAs layers using selective wet oxidation. Layers of AlxGa1−xAs with x≥0.96 exhibit crystallographic dependent oxidation rates, while for layers with x≤0.92 the oxidation rate is isotropic. Mesas containing partially oxidized layers of AlAs are unstable to rapid thermal cycling and exhibit excessive strain at the oxide terminus, while mesas containing partially oxidized layers of AlGaAs are robust and lack evidence of strain. Finally, the oxidation of AlGaAs layers, rather than AlAs, is found to provide robust oxide apertures for reliable vertical‐cavity surface emitting lasers.

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.

Reflection mass spectrometry of As incorporation during GaAs molecular beam epitaxy

An apertured and cryoshrouded mass spectrometer, which measures line‐of‐sight molecular fluxes from the surface, has been incorporated into a GaAs molecular beam epitaxy system. The spectrometer is simple to implement, yet is a powerful real‐time growth diagnostic. We have used the spectrometer to measure transient and steady‐state As incorporation from As4 during bilayer‐by‐bilayer growth of GaAs. We find, interestingly, that (1) the incorporation coefficient does not oscillate significantly; (2) transient incorporation coefficients depend on surface reconstruction, and may be higher than 0.5 at high Ga fluxes; and (3) in the absence of a Ga flux, excess Ga on the surface need not imply an incorporation coefficient of 0.5.

Surface‐emitting, multiple quantum well GaAs/AlGaAs laser with wavelength‐resonant periodic gain medium

A novel surface‐emitting semiconductor laser with a vertical resonator, extremely short gain length, and enhanced gain at a specific design wavelength has been demonstrated. The gain medium consists of a series of GaAs quantum wells separated by AlGaAs spacers whose thicknesses are chosen to be one‐half the wavelength of a particular transition in the quantum wells. This structure forces the antinodes of the standing‐wave optical field to coincide with the gain elements, enhancing the gain and frequency selectivity in the vertical direction and substantially reducing amplified spontaneous emission. We have achieved optically pumped lasing with a threshold of 6 MW/cm2 at room temperature in a molecular beam epitaxially grown structure of thickness 4.3 μm, of which only 320 nm provided gain.

Lasing threshold in quantum well surface‐emitting lasers: Many‐body effects and temperature dependence

The geometry of quantum well surface‐emitting lasers has several important consequences. The ultrashort (∼1 μm) vertical cavity defines longitudinal modes with energy separation greater than the bandwidth of spectral gain. The optical confinement of these modes can approach unity. To achieve lasing, high carrier densities (∼1012 cm−2) in the quantum well are required. The confined carriers interact through enhanced many‐body exchange which influences both the lasing wavelength and threshold characteristics. Indeed, the exchange interaction can facilitate the lasing process. We theoretically and experimentally study the role of the short cavity and exchange interaction on the cw lasing threshold as a function of temperature. In constrast to edge emitters, the lasing threshold in these surface emitters exhibits a well‐defined minimum at a particular temperature. The temperature of the minimum can be designed by merely changing the active layer thickness.

Integrated injection-locked high-power cw diode laser arrays

We report the first integrated injection‐locked high‐power continuous‐wave diode laser array with an on‐chip independently controlled master laser. This device emits a near‐diffraction‐limited (0.5° full width at half maximum) single‐lobed far‐field emission beam at single‐facet powers up to 125 mW. Also, by current tuning the emission wavelength of the master laser, we observe steering of the single‐lobed emission over an angular range of 0.50° at a rate of −1.2×10−2 deg/mA. Our work demonstrates the feasibility of incorporating active optical injection and control in the structure of high‐power diode laser devices.

On-line determination of alloy composition during ternary III/V molecular beam epitaxy

We describe a simple, new method for deducing surface alloy composition during ternary III/V molecular beam epitaxy. The method is based on on‐line reflection mass spectrometry of the group V flux ‘‘reflected’’ from the surface during momentary terminations of individual group III fluxes.

Waveguide-to-fiber coupling using a second-order grating and an anamorphic binary optic

Historically, obtaining efficient coupling from single-mode waveguides in high performance GaAs modulator devices to single-mode fiber has been difficult. The reasons are; (1) the large modal mismatch between the elliptical waveguide output and the gaussian profile of the optical fiber; and (2) the large NA difference (0.9 for the waveguide in one direction) and 0.16 for fiber. Despite this difficulty, there exists a need for packaging devices with multiple fiber outputs, that have been gang-aligned, efficiently coupled, and hermetically sealed. (The latter item will be very important in automotive or aerospace applications.) Instead of trying to have fiber penetrate the package wall, the SNL approach to efficient coupling and hermeticity has been to allow light to penetrate the package wall. This has been accomplished by sending out the light normal to the waveguides and collecting it with a binary optic that focuses it on to a fiber outside the package. The optical design of this system requires that the beam be nearly collimated as it leaves the surface of the device. To accomplish this, a second-order grating was etched into a 200 /spl mu/m long section of an adiabatically expanded single-mode waveguide.<<ETX>>

Implanted-planar-buried-heterostructure, graded-index, separate-confinement-heterostructure laser in GaAs/AlGaAs

For the implanted planar buried-heterostructure, graded-index, separate-confinement-heterostructure (IPBH-GRINSCH) laser in (AlGa)As/GaAs, ion implantation is used to form p-n-p-n current blocking layers and to create a buried-heterostructure waveguide. This results in significantly reduced fabrication complexity of high-quality, index-guided laser diodes compared to regrowth techniques, and in contrast to diffusion-induced disordering, allows for the creation of self-aligned, buried, blocking junctions. Kink-free, CW operation of single-stripe IPBH-GRINSCH lasers along with single-lobed near-field and far-field optical emission patterns, consistent with index-guided operation, is demonstrated.<<ETX>>