R.L. Naone

Silicon hydride composition of plasma-deposited hydrogenated amorphous and nanocrystalline silicon films and surfaces

In situ attenuated total reflection Fourier transform infrared spectroscopy was used to study the H bonding on the surfaces of a-Si:H and nc-Si:H during plasma enhanced chemical vapor deposition from SiH4/H2/Ar containing discharges. Well-resolved SiHx (1⩽x⩽3) absorption lines that correspond to the vibrational frequencies commonly associated with surface silicon hydrides were detected. During deposition of a-Si:H films using SiH4 without H2 dilution, the surface coverage was primarily di- and trihydrides, and there are very few dangling bonds on the surface. In contrast, during deposition of nc-Si:H using SiH4 diluted with H2, the amount of di- and trihydrides on the surface is drastically reduced and monohydrides dominate the surface. Furthermore, the vibrational frequencies of the monohydrides on nc-Si:H film surfaces match well with the resonant frequencies of monohydrides on H terminated Si (111) and Si (100) surfaces. The decrease of higher hydrides on the surface upon H2 dilution is attributed to i...

High-speed characteristics of low-optical loss oxide-apertured vertical-cavity lasers

We characterize the high-speed modulation properties of thin-oxide-apertured vertical-cavity lasers. The modulation response scales with device diameter due to the negligible optical scattering loss present in these devices. A small diameter laser of 3.1 /spl mu/m has a maximum 3-dB bandwidth of 15.2 GHz at a bias of only 2.1 mA. Modeling indicates a no-parasitic bandwidth of 18.2 GHz at this current level, with an intrinsic 3-dB bandwidth limit of 45 GHz due to gain compression. The present devices are limited by parasitic capacitance across the thin oxide layer.

Reduced optical scattering loss in vertical-cavity lasers using a thin (300 /spl Aring/) oxide aperture

Calculations show that significant optical scattering loss persists as standard quarter-wave (800 /spl Aring/) thick, dielectrically apertured vertical cavity laser diameters are reduced below 4 /spl mu/m and that thinner apertures can reduce the scattering loss, By using a thin (300 /spl Aring/) AlAs-oxide defined aperture, optical scattering loss has been dramatically reduced over the quarter-wave AlAs-oxide defined vertical-cavity laser. The optical loss reduction results in 2.3 /spl mu/m diameter lasers with differential efficiencies of 0.43 (80% of the value of broad-area lasers) and continuous-wave single-mode powers of 1.2 mW.

LUMINESCENCE FROM PLASMA DEPOSITED SILICON FILMS

We report the observation of room-temperature and low-temperature visible photoluminescence from nanocrystalline silicon (nc-Si) thin films produced by plasma-enhanced chemical vapor deposition (PECVD) through a gas discharge containing SiH4 diluted in Ar and H2. The nanocrystalline silicon films were characterized using transmission electron microscopy, spectroscopic ellipsometry, infrared and Raman spectroscopy, and were examined for photoluminescence. Luminescent films consisted of dense silicon nanocrystals that grew in a columnar structure with approximately 20%–30% void space dispersed inside the film. Aside from having small crystalline silicon regions, the structure of the nc-Si films is different than that of porous Si, another luminescent Si material generally produced by electrochemical anodization. Yet, the photoluminescence spectra of the thin nc-Si films were found to be similar to those observed from porous silicon. This similarity suggests that the same mechanism responsible for light emis...

Visible luminescence from nanocrystalline silicon films produced by plasma enhanced chemical vapor deposition

Thin nanocrystalline silicon (nc‐Si) films deposited by plasma enhanced chemical vapor deposition (PECVD) exhibited room‐temperature photoluminescence in the visible range of the electromagnetic spectrum. High resolution transmission electron microscopy revealed that the films are made of Si crystals with dimensions 2–15 nm. The photoluminescence spectra of the nc‐Si films were similar to the spectra observed from porous silicon produced by anodization and electrochemical dissolution of crystalline Si. This similarity suggests that the luminescence mechanism of nc‐Si films is similar to the mechanism of light emission from porous silicon. The ability to manufacture luminescent Si films by methods which are compatible with the current Si based technology, such as PECVD, can provide new possibilities in the realization of optoelectronic devices.

Surface energy model for the thickness dependence of the lateral oxidation of AlAs

The lateral oxidation rate of AlAs layers decreases dramatically for layers thinner than about 500 A, because the activation energies for the rate constant of the reaction at the oxidation front increases by an amount inversely proportional to the layer thickness. We derive a model for the thickness dependence of the lateral oxidation rate of AlAs based on the surface energy of the curvature observed at the oxide tip. From the model, we show that the linear oxidation rate has an exp(−θ0/θ) dependence on the AlAs layer thickness θ, and we can predict the slowing of oxidation when the AlAs layer is cladded with AlGaAs barriers. Also, we estimate the surface energy of the AlAs/oxide interface to be 50 eV/nm2.

Design parameters for lateral carrier confinement in quantum-dot lasers

Quantum-dot (QD) lasers have fallen short of their promise of ultralow threshold and high characteristic temperature. Here, we report that QDs show great promise for controlling lateral carrier leakage. While oxide apertures continue to enable improved performance in vertical cavity surface emitting lasers (VCSELs) by reducing optical losses and current spreading, lateral carrier losses remain uncontrolled. We investigate QD active material in which lateral diffusion is intentionally reduced. Cathodoluminescence results demonstrate reduced lateral diffusion in the material with which we expect >50% reduction in the threshold current for 1-μm-wide edge emitters or 5-μm-diam VCSELs. However, initial edge-emitter results demonstrated 10% reduction due to unintended current spreading and lasing from higher states.

Interdiffused quantum wells for lateral carrier confinement in VCSELs

We show that it is necessary to eliminate carrier diffusion in the active region for viable vertical-cavity lasers (VCLs) with small dimensions. However, methods that work well in reducing lateral carrier leakage in narrow ridge-waveguide lasers such as silicon induced disordering may be problematic in VCL structures. Encouraging results from novel methods for impurity free intermixing for VCL applications are presented.

Lateral carrier confinement in miniature lasers using quantum dots

Although quantum-dot (QD) lasers are yet to reach their promise of ultralow threshold and high characteristic temperature because of QD size nonuniformity, we have found that they can be used to effectively limit the lateral diffusion of carriers in the active region, enabling the scaling of lasers to small lateral dimensions. Although oxide apertures continue to enable improved performance in vertical-cavity surface-emitting lasers (VCSELs) by reducing optical losses and current spreading, lateral carrier losses remain uncontrolled. We investigate QD active material in which lateral diffusion is intentionally reduced. Cathodoluminescence (CL) results demonstrate reduced lateral diffusion in the material with which we expect 50% reduction in the threshold current for 1-/spl mu/m-wide edge-emitters or 5-/spl mu/m-diameter VCSELs. We have made QD stripe lasers with submicrometer widths that lase from the ground state and have quantified the lateral carrier reduction in the QD laser active region. We show empirically that the degree of lateral carrier confinement is dependent on the quantum state from which lasing occurs and demonstrate 63% reduction in lateral carrier leakage for the ground-state lasers. Finally, the scaling of threshold current in QD VCSELs is compared with that of quantum-well (QW) VCSELs by numerical modeling for future design considerations.

Tapered-apertures for high-efficiency miniature VCSELs

Improved performance of smaller vertical cavity lasers is important for applications which require arrays of vertical cavity lasers operating at low power such as free-space optical interconnections between computer boards. If one obtains ideal scaling in which threshold current density and slope efficiency remain constant as device size shrinks, overall threshold currents should scale inversely with the device area. By engineering the shape of intra-cavity dielectric apertures it is possible to better confine the current and provide low-loss guiding of the optical mode, thereby approaching the desired scalability. More importantly, we show that the power conversion efficiency is higher at the lower output powers because the voltage can be lower despite increased resistance due to current transport through the aperture.