We present models for the optical functions of 11 metals used as mirrors and contacts in optoelectronic and optical devices: noble metals (Ag, Au, Cu), aluminum, beryllium, and transition metals (Cr, Ni, Pd, Pt, Ti, W). We used two simple phenomenological models, the Lorentz-Drude (LD) and the Brendel-Bormann (BB), to interpret both the free-electron and the interband parts of the dielectric response of metals in a wide spectral range from 0.1 to 6 eV. Our results show that the BB model was needed to describe appropriately the interband absorption in noble metals, while for Al, Be, and the transition metals both models exhibit good agreement with the experimental data. A comparison with measurements on surface normal structures confirmed that the reflectance and the phase change on reflection from semiconductor-metal interfaces (including the case of metallic multilayers) can be accurately described by use of the proposed models for the optical functions of metallic films and the matrix method for multilayer calculations.

Optical properties of metallic films for vertical-cavity optoelectronic devices.

Status and future outlook of III-V compound semiconductor visible-spectrum light-emitting diodes (LEDs) are presented. Light extraction techniques are reviewed and extraction efficiencies are quantified in the 60%+ (AlGaInP) and ~80% (InGaN) regimes for state-of-the-art devices. The phosphor-based white LED concept is reviewed and recent performance discussed, showing that high-power white LEDs now approach the 100-lm/W regime. Devices employing multiple phosphors for "warm" white color temperatures (~3000-4000 K) and high color rendering (CRI>80), which provide properties critical for many illumination applications, are discussed. Recent developments in chip design, packaging, and high current performance lead to very high luminance devices (~50 Mcd/m2 white at 1 A forward current in 1times1 mm2 chip) that are suitable for application to automotive forward lighting. A prognosis for future LED performance levels is considered given further improvements in internal quantum efficiency, which to date lag achievements in light extraction efficiency for InGaN LEDs

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Status and Future of High-Power Light-Emitting Diodes for Solid-State Lighting

We review the family of optoelectronic devices whose performance is enhanced by placing the active device structure inside a Fabry‐Perot resonantmicrocavity. Such resonantcavity enhanced (RCE) devices benefit from the wavelength selectivity and the large increase of the resonant optical field introduced by the cavity. The increased optical field allows RCE photodetector structures to be thinner and therefore faster, while simultaneously increasing the quantum efficiency at the resonant wavelengths. Off‐resonance wavelengths are rejected by the cavity making RCE photodetectors promising for low crosstalk wavelength division multiplexing(WDM) applications. RCE optical modulators require fewer quantum wells so are capable of reduced voltage operation. The spontaneous emission spectrum of RCE light emitting diodes(LED) is drastically altered, improving the spectral purity and directivity. RCE devices are also highly suitable for integrated detectors and emitters with applications as in optical logic and in communication networks. This review attempts an encyclopedic overview of RCE photonicdevices and systems. Considerable attention is devoted to the theoretical formulation and calculation of important RCE device parameters. Materials criteria are outlined and the suitability of common heteroepitaxial systems for RCE devices is examined. Arguments for the improved bandwidth in RCE detectors are presented intuitively, and results from advanced numerical simulations confirming the simple model are provided. An overview of experimental results on discrete RCE photodiodes, phototransistors, modulators, and LEDs is given. Work aimed at integrated RCE devices,optical logic and WDM systems is also covered. We conclude by speculating what remains to be accomplished to implement a practical RCE WDM system.

Resonant cavity enhanced photonic devices

Data are presented characterizing a new process for fabrication of vertical‐cavity surface‐emitting lasers based on the selective conversion of high Al composition epitaxial AlGaAs to a stable native oxide using ‘‘wet oxidation.’’ The native oxide is used to form a ring contact to the laser active region. The resulting laser active regions have dimensions of 8, 4, and 2 μm. The lowest threshold laser is achieved with the 8‐μm active region, with a minimum threshold current of 225‐μA continuous wave at room temperature.

Native-Oxide Defined Ring Contact for Low Threshold Vertical-Cavity Lasers

The authors have designed, fabricated, and tested vertical-cavity surface-emitting lasers (VCSEL) with diameters ranging from 0.5 mu m to>50 mu m. Design issues, molecular beam epitaxial growth, fabrication, and lasing characteristics are discussed. The topics considered in fabrication of VCSELs are microlaser geometries; ion implementation and masks; ion beam etching packaging and arrays, and ultrasmall devices. >

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Vertical-cavity surface-emitting lasers: Design, growth, fabrication, characterization

Carrier transport can significantly affect the high-speed properties of quantum-well lasers. The authors have developed a model and derived analytical expressions for the modulation response, resonance frequency, damping rate, and K factor to include these effects. They show theoretically and experimentally that carrier transport can lead to significant low-frequency parasitic-like rolloff that reduces the modulation response by as much as a factor of six in quantum-well lasers. They also show that, in addition, it leads to a reduction in the effective differential gain and thus the resonance frequency, while the nonlinear gain compression factor remains largely unaffected by it. The authors present the temperature dependence data for the K factor as further evidence for the effects of carrier transport. >

High speed quantum-well lasers and carrier transport effects

A detailed analysis of a Fabry-Perot surface emitting laser (FP-SEL) which utilizes the recently proposed concept of periodic gain is presented. It is shown that by using the periodic gain concept, close to a factor-of-two reduction in threshold current should be possible; the ideal reduction of a factor of two is only limited by the internal loss of the cavity. Multiple quantum-well active regions are also considered and shown to provide greater than a factor-of-two improvement over bulk GaAs periodic and uniform gain configurations. The effects of index perturbations within the cavity created by interleaving active and passive segments are treated for different Al mole fractions within the passive segments. The effects are found to be small for x >

Design of Fabry-Perot surface-emitting lasers with a periodic gain structure

Two-dimensional physical models for single-mode index guided vertical cavity surface emitting lasers (VCSELs) are developed and compared with experimental measurements on state-of-the-art devices. Starting with the steady-state electron and photon rate equations, the model calculates the above threshold light-current (LI) characteristics. Included are temperature effects, spatial hole burning effects, carrier diffusion, surface recombination, and an estimation of optical losses. The model shows that the saturation of output power in the experimental devices is due to carrier leakage over the heterojunction and not simply the shifting of the gain peak relative to the cavity mode. Using the verified model new designs are analyzed, showing that output powers greater than 15 mW and power efficiencies above 20% should be achievable with existing processing technology. >

Modeling temperature effects and spatial hole burning to optimize vertical-cavity surface-emitting laser performance

Vertical-cavity surface-emitting lasers fabricated utilizing a self-aligned process to provide planarized contacts are discussed. A single 80-AA In/sub 0.2/Ga/sub 0.8/As strained quantum well was used in the active region. Emission was at 963 nm. Threshold currents under continuous-wave room temperature operation of 1.1 mA, at 4.0-V bias, were measured for numerous 12- mu m*12- mu m devices. Corresponding threshold current densities were 800 A/cm/sup 2/ (600 A/cm/sup 2/ for broad area devices). These are the lowest figures yet reported for this type of device. It was found that grading of the mirror had a marked effect on mirror resistance. >

Low threshold planarized vertical-cavity surface-emitting lasers

The authors give theoretical and experimental results for vertical-cavity surface-emitting lasers (VCSELs). The modeling is applied to the design of InGaAs VCSELs. A simple method to calculate the reflectivity of semiconductor stack mirrors with graded interfaces and compound metal/semiconductor stack mirrors is introduced. The theoretical predictions are compared to results from actual device measurements. A novel technique is introduced to determine material parameters: fabrication of in-plane lasers from VCSEL material. The procedure used to determine the optical mode in such an in-plane laser is described. Using the insight gained from modeling, the external efficiency was increased to >30% with a threshold current density of 1 kA/cm/sup 2/. >

Randall S. Geels

Papers

High speed quantum-well lasers and carrier transport effects

Design of Fabry-Perot surface-emitting lasers with a periodic gain structure

Modeling temperature effects and spatial hole burning to optimize vertical-cavity surface-emitting laser performance

Low threshold planarized vertical-cavity surface-emitting lasers

InGaAs vertical-cavity surface-emitting lasers