W. D. Goodhue

Oscillations up to 420 GHz in GaAs/AlAs resonant tunneling diodes

We report room‐temperature oscillations up to frequencies of 420 GHz in a GaAs resonant tunneling diode containing two 1.1‐nm‐thick AlAs barriers. These results are consistent with a recently proposed equivalent circuit model for these diodes in which an inductance accounts for the temporal delay associated with the quasibound‐state lifetime. They are also in accordance with a generalized impedance model, described here, that includes the effect of the transit time delay across the depletion layer. Although the peak‐to‐valley ratio of the 420 GHz diode is only 1.5:1 at room temperature, we show that its speed is limited by the parasitic series resistance rather than by the low negative conductance. A threefold reduction in this resistance, along with a comparable increase in the peak‐to‐valley ratio, should allow oscillations up to about 1 THz.

GaN avalanche photodiodes grown by hydride vapor-phase epitaxy

Avalanche photodiodes have been demonstrated utilizing GaN grown by hydride vapor-phase epitaxy. Spatially uniform gain regions were achieved in devices fabricated on low-defect-density GaN layers that exhibit no microplasma behavior. A uniform multiplication gain up to 10 has been measured in the 320–360 nm wavelength range. The external quantum efficiency at unity gain is measured to be 35%. The electric field in the avalanche region has been determined from high-voltage C–V measurements to be ∼1.6 MV/cm at the onset of the multiplication gain. Electric fields as high as 4 MV/cm have been measured in these devices. Response times are found to be less than 5 μs, limited by the measurement system.

AlGaAs-InGaAs slab-coupled optical waveguide lasers

The slab-coupled optical waveguide laser (SCOWL) concept, recently proposed and demonstrated, is extended to the AlGaAs-InGaAs-GaAs material system. Both 980- and 915-nm SCOWL devices feature a nearly circular large-diameter single-spatial mode that can be butt coupled with high efficiency to a single-mode fiber. Single-ended continuous-wave output powers of greater than 1 W have been obtained at 980 nm.

Monolithic two‐dimensional surface‐emitting arrays of GaAs/AlGaAs diode lasers

Monolithic two‐dimensional arrays with light emission normal to the surface have been obtained by fabricating edge‐emitting quantum well GaAs/AlGaAs lasers with deflecting mirrors adjacent to both laser facets. The facets and mirrors were formed by ion beam assisted etching. Proton bombardment between adjoining lasers was used to prevent lasing in the transverse direction. At the highest pulsed current used in these experiments, 10.5 A, the power output of a 22‐element array was 1.6 W, which corresponds to a power density of 160 W cm−2. At this level, the power output was still linear with current.

Microwave and Millimeter-Wave Resonant Tunneling Diodes

Resonant tunneling through double-barrier heterostructures has attracted increasing interest recently, largely because of the fast charge transport [1] it provides. In addition, the negative differential resistance regions which exist in the current-voltage (I–V) curve (peak-to- valley ratios of 3.5:1 at room temperature [2–4] and nearly 10:1 at 77 K have been measured) suggest that high-speed devices based on the peculiarities of the I–V curve should be possible. For example, the negative differential resistance region is capable of providing the gain necessary for high-frequency oscillations [5]. In our laboratory we have been attempting to increase the frequency and power of these oscillators [6], and others have worked toward a better understanding of the equivalent circuit of the device [7] and the underlying processes responsible for the frequency response [8–10]. Three-terminal devices using resonant tunneling in various ways have also been proposed and fabricated [11–13]. In this paper we will describe our most recent results for oscillators as well as some new resonant-tunneling devices that have application in the millimeter and submillimeter-wave spectrum.

Bright‐field analysis of field‐emission cones using high‐resolution transmission electron microscopy and the effect of structural properties on current stability

High‐resolution transmission electron microscopy has been used to analyze 150 nm diameter by 150 nm high polycrystalline molybdenum field‐emission cones. The analysis shows that the cones comprise 5 to 10 nm thick grains with tips having gross radii of curvature of about 5 nm and protrusions having radii of curvature of about 1 nm. Such small protrusions may explain why analysis of experimental emission data indicates that the effective emission area of such tips is only 0.1 to 0.5 nm.2 Furthermore, the fact that the structure is composed of small grains indicates that there is a substantial number of molybdenum atoms at grain boundaries and that many configurations of grains and boundaries are possible with minimal free energy. A qualitative model is proposed which links the structural properties to current stabilization and hydrogen passivation effects.

High-power nearly diffraction-limited AlGaAs-InGaAs semiconductor slab-coupled optical waveguide laser

Beam-quality measurements on the output of a 915-nm AlGaAs-InGaAs-GaAs slab-coupled optical waveguide laser (SCOWL) are reported. This device had a nearly circular mode (3.8 /spl mu/m by 3.4 /spl mu/m 1/e/sup 2/ widths in the near-field) and was capable of a single-ended continuous-wave output power of greater than 1 W. Measurements of M/sup 2/ indicate that the SCOWL output beam is nearly diffraction-limited in both directions with M/sub x//sup 2/ /spl sim/ M/sub y//sup 2/ /spl sim/ 1.1 over the entire range of output powers measured.

Monolithic two‐dimensional surface‐emitting strained‐layer InGaAs/AlGaAs and AlInGaAs/AlGaAs diode laser arrays with over 50% differential quantum efficiencies

Monolithic two‐dimensional surface‐emitting arrays of strained‐layer InGaAs/AlGaAs and AlInGaAs/AlGaAs diode lasers have been fabricated and operated pulsed with low‐threshold current densities and differential quantum efficiencies greater than 50%. The InGaAs/AlGaAs arrays emit at 1.03 μm, while the AlInGaAs/AlGaAs arrays emit at 0.815 μm. Thus, it should be possible to fabricate monolithic arrays with comparable performance over a wide wavelength range. The individual lasers of the arrays are horizontal folded‐cavity devices with two 45° internal reflectors and two top‐surface facets. The design is simple to implement using optical pattern‐generator masks, optical projection printing, and chlorine ion‐beam‐assisted etching in key fabrication steps.

Monolithic two-dimensional GaAs/AlGaAs laser arrays fabricated by chlorine ion-beam-assisted micromachining

Chlorine ion-beam-assisted etching (IBAE) has been used to micromachine laser facets and deflecting mirrors for monolithic two-dimensional GaAs/AIGaAs laser arrays. Three laser cavity/deflector designs have been successfully implemented. The first utilizes a parabolic deflecting mirror to directly focus the laser radiation; the second consists of a folded cavity with a vertical facet, a top surface facet, and an internal 45° reflector; and the third has a folded cavity with an internal Al0.2Ga0.8As/Al0.8Ga0.2As dielectric mirror stack and a top surface facet formed in a single etch step with two internal 45° reflectors. The parabolic deflecting mirrors are currently modeled forf- 0.8 collection efficiency, making the first design attractive in incoherent arrays for high-power applications such as pumping Nd:YAG lasers. The other two structures are of interest for incoherent or coherent arrays used in high- and medium-power applications, since the top surface facets can easily be antireflection coated. The design with a dielectric mirror stack is particularly simple to fabricate.

Sidewall growth by atomic layer epitaxy

Atomic layer epitaxy (ALE) has successfully been used to grow epitaxial layers over chemically etched grooves and dry etched sidewalls formed on GaAs(100) substrates. GaAs/InGaAs multilayers were deposited on V‐shaped and inverted trapezoid‐shaped grooves that were 4–5 μm deep and 8–20 μm wide. Growth conforming to the original sidewall surfaces was accomplished both on the chemically etched and on ion beam assisted etched surfaces. These features show distinct improvement over similar attempts by metalorganic chemical vapor deposition or molecular beam epitaxy. The ability of ALE to proceed in a self‐limiting fashion on such structures is expected to lead to the realization of novel device concepts.