Kent M. Geib

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.

Epitaxial films of semiconducting FeSi2 on (001) silicon

Epitaxial thin films of the semiconducting transition metal silicide, beta‐FeSi2, were grown on (001) silicon wafers. The observed matching face relationship is FeSi2(100)/Si(001), with the azimuthal orientation being FeSi2[010]‖‖Si〈110〉. This heteroepitaxial relationship has a common unit mesh of 59 A2 area, with a mismatch of 2.1%. There is a strong tendency toward island formation within this heteroepitaxial system.

The C KLL first‐derivative x‐ray photoelectron spectroscopy spectra as a fingerprint of the carbon state and the characterization of diamondlike carbon films

The C KLL spectra from natural diamond, graphite, and single‐crystal β‐SiC have been investigated using x‐ray photoelectron spectroscopy and first‐derivative x‐ray excited Auger electron spectroscopy (XAES). It is shown that the XAES spectra is essentially the same as that obtained with conventional AES with the added benefits of no electron beam (e‐beam) damage, enhanced fine structure, and the simultaneous acquisition of the C 1s and valence‐band spectra. The C KLL XAES fine structure provides a fingerprint of the carbon bonding state and the C 1s and valence‐band spectra provide insight into the origin of this fine structure. Diamondlike carbon films fabricated by hydrogen gas reactive sputtering of graphite on Si were also investigated. The XAES C KLL spectra in conjunction with the valence‐band spectra verifies the tetrahedral C–C bonding in these films even though the AES spectra shows a graphitelike structure; indicative of e‐beam damage.

Fabrication and performance of selectively oxidized vertical-cavity lasers

We report the high yield fabrication and reproducible performance of selectively oxidized vertical-cavity surface emitting lasers. We show that linear oxidation rates of AlGaAs without an induction period allows reproducible fabrication of buried oxide current apertures within monolithic distributed Bragg reflectors. The oxide layers do not induce obvious crystalline defects, and continuous wave operation in excess of 650 h has been obtained. The high yield fabrication enables relatively high laser performance over a wide wavelength span. We observe submilliamp threshold currents over a wavelength range of up to 75 nm, and power conversion efficiencies at 1 mW output power of greater than 20% over a 50-nm wavelength range.<<ETX>>

Cavity characteristics of selectively oxidized vertical‐cavity lasers

We show that a buried oxide layer forming a current aperture in an all epitaxial vertical‐cavity surface emitting laser has a profound influence on the optical and electrical characteristics of the device. The lateral index variation formed around the oxide current aperture leads to a shift in the cavity resonance wavelength. The resonance wavelength under the oxide layer can thus be manipulated, independent of the as‐grown cavity resonance, by adjusting the oxide layer thickness and its placement relative to the active region. In addition, the electrical confinement afforded by the oxide layer enables record low threshold current densities and threshold voltages in these lasers.

Single-transverse-mode vertical-cavity lasers under continuous and pulsed operation

Using a hybrid ion implanted/selectively oxidized device structure, we report high-power single-mode operation of an 850-nm vertical-cavity laser. Under continuous-wave operation, >4 mW of single-mode power with 45 dB of side-mode suppression is achieved. The spectral behavior under pulsed modulation is determined to be influenced by thermal lensing. When biased to threshold, single-mode operation with >35-dB side-mode suppression is obtained for large signal modulation.

Scalability of small-aperture selectively oxidized vertical cavity lasers

We analyze the threshold properties of small area selectively oxidized vertical cavity lasers. Agreement for threshold gain versus laser size is found using the experimental intrinsic threshold voltage matched with a gain theory, as compared to a two-dimensional optical cavity simulation. Our analysis indicates the increasing threshold current density of small area lasers arises from both increasing threshold gain and the concomitant increasing leakage current. We further show that the optical loss can be reduced for lasers with areas as small as 0.25 μm2 while maintaining sufficient transverse optical confinement by displacing the apertures longitudinally away from the cavity and reducing the oxide thickness.

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.

A review of the geometrical fundamentals of reflection high‐energy electron diffraction with application to silicon surfaces

Reflection high‐energy electron diffraction (RHEED) is an experimentally simple technique, and yet a powerful one for examining the structure of a substrate surface and for monitoring the surface crystal structure and the crystallographic orientation of thin films during their growth. However, it can be difficult to learn to interpret the RHEED patterns of new materials, because a practical and adequately detailed introduction to the technique is not generally available. To address this need, we develop the geometrical principles of RHEED; using the kinematic approximation, we show how a particular point of the sample surface’s reciprocal net gives rise to a diffraction maximum at a particular location on the RHEED viewing screen. We explain the origins of ‘‘reciprocal lattice rods,’’ RHEED streaks, and Laue rings. We show how to calculate the streak spacing, and clarify the basic effect on the RHEED pattern of using a nonzero angle of incidence for the incident beam. Crystalline nets, reciprocal nets, an...

Two-element phased array of antiguided vertical-cavity lasers

We demonstrate antiguided coupling of two adjacent vertical-cavity surface-emitting lasers (VCSELs), obtaining a 1×2 phase-locked array at 869 nm. The lateral index modification required for antiguiding is achieved by a patterned 3 nm etch performed between two epitaxial growths. In contrast with prior coupled VCSELs, adjacent antiguided VCSELs can emit in phase and produce a single on-axis lobe in the far field. Greater than 2 mW of in-phase output power is demonstrated with two VCSELs separated by 8 μm. Moreover, phase locking of two VCSELs separated by 20 μm is observed, indicating the possibility of a promising class of optical circuits based upon VCSELs that interact horizontally and emit vertically.