W. L. Li

Enhanced Output Power of GaN-Based LEDs With Nano-Patterned Sapphire Substrates

GaN-based light-emitting diodes (LEDs) with an emitting wavelength of 450 nm were grown on nano-patterned sapphire substrates (NPSS) fabricated by nanosphere lithography. The crystalline quality of the epitaxial film could be improved by using the NPSS technique. The output power of LED grown on NPSS was 1.3 and 1.11 times higher than those of LEDs grown on conventional and patterned sapphire substrates at the injection current of 20 mA, respectively. The enhancement in output power could be contributed to the efficiently scattering by NPSS. But some voids formed at the GaN/NPSS interface cause a thermal dissipation problem of NPSS LED operated at high injection current.

ESD engineering of nitride-based LEDs

GaN-based light emitting diodes (LEDs) with p-cap layers grown at various temperatures were fabricated. It was found that the LED with 900/spl deg/C-grown p-cap layer could only endure negative 1100 V electrostatic discharge (ESD) pulses while the LED with 1040/spl deg/C-grown p-cap layer could endure ESD pulses as high as negative 3500 V. It was also found that the ESD performances of the LEDs with 900 and 1040/spl deg/C-grown p-cap layers were limited by the V-shape defects and the bonding pad design, respectively.

Nitride-based MQW LEDs with multiple GaN-SiN nucleation layers

Nitride-based light emitting diodes (LEDs) separately prepared with a conventional single low-temperature (LT) GaN nucleation layer and multiple GaN-SiN nucleation layers were both prepared. It was found that we could reduce defect density and thus improve crystal quality of the GaN-based LEDs by using multiple GaN-SiN nucleation layers. With a 20-V applied reverse bias, it was found that the reverse leakage currents measured from the LED with a single LT GaN nucleation layer and the one with 10-pair GaN-SiN nucleation layers were 1.5/spl times/10/sup -4/ and 2.5/spl times/10/sup -6/ A, respectively. It was also determined that we could use the multiple GaN-SiN nucleation layers to enhance the output intensity of near ultraviolet (UV) LEDs and to improve the reliability of nitride-based LEDs.

The influences of refractive index dispersion on the modal gain of a quantum-well laser

A new self-consistent method (SCM) for single-quantum-well (SQW) AlGaAs-GaAs diode lasers is introduced to study systematically the influences of refractive-index dispersion on TE modal gain. The refractive-index dispersion of QW layers is calculated by the density matrix method. It is affected by the effective propagation constant of guided mode. Likewise, the transverse guided mode of QW lasers, as obtained by the transfer matrix method, is also influenced by the refractive-index dispersion. SCM, using the density matrix and transfer matrix methods self-consistently, provides the TE modal gain spectra. SCMs calculated results are compared with those of Dumkes approximation and show a decrease in energy of modal gain peak and a decline of modal gain values at high emission energies. The differences between these two methods are seen to increase with an increase of well width and to be unrelated to barrier height. Although not treated formally in this paper, we suggest that SCM results show a significantly superior match to real phenomena.

Nitride-based LEDs with MQW active regions grown by different temperature profiles

Nitride-based light-emitting diodes (LEDs) with multiple quantum-well active regions were separately prepared by metal-organic vapor phase epitaxy in different temperature profiles. Compared with conventional samples, the reduced reverse leakage current and improved electrostatic discharge characteristics of the LEDs can both be achieved using temperature ramping and temperature cycling methods. However, using the temperature ramping may degrade the optical properties of devices due to desorption of In atoms and/or impurity incorporation. With an emission wavelength of 465 nm, the 20-mA output powers measured were 5.5, 6.0, and 7.9 mW for temperature ramping LED, conventional LED, and temperature cycling LED, respectively.

On-chip reference oscillators with process, supply voltage and temperature compensation

Here we present the design and implementation of a 130-MHz on-chip reference oscillator in a 0.18-µm 1-ploy 6-metal digital CMOS process. To compensate for the influences on the oscillation frequency by process, supply voltage and temperature (PVT) variations, the oscillator uses a bias adjustment technique without BJT devices, on-chip inductors or external components. Measurements of 8 samples in the 0 to 100°C temperature range indicate an average deviation of ±4.99% in the oscillation frequency. The process-induced frequency deviation is ±1.13% across chips at room temperature. The deviation of frequency with 10% supply voltage variation is within ±5.4%.

Measurement of AlInAsSb/GaInAsSb heterojunction band offset by photoluminescence spectroscopy

We have grown unstrained Al0.66In0.34As0.85Sb0.15/Ga0.64In0.36As0.84Sb0.16 multiple-quantum-well (MQW) structures on InP substrates by metalorganic vapor phase epitaxy. Low-temperature photoluminescence was performed for these MQW structures. By comparing the luminescence peak energies with the theoretical calculations, we estimated the conduction-band offset ratio to be 0.75±0.10 for the Al0.66In0.34As0.85Sb0.15/Ga0.64In0.36As0.84Sb0.16 heterostructure.

DESIGN OF AlGaInP VISIBLE LASERS WITH A LOW VERTICAL DIVERGENCE ANGLE

Abstract AlGaInP visible lasers with depressed index cladding layers are studied theoretically and experimentally. Using the above low-refractive-index layer, we can significantly improve the characteristics of the transverse beam divergence. According to our experimental results, the transverse beam divergence can be reduced from 41.4° to 26.2°. These values also agree well with our theoretical calculations. In addition, we have optimized this structure theoretically. With a proper choice of structure parameters, we can design an AlGaInP laser diode with a transverse divergence angle as narrow as 8° while maintaining a low threshold current density.

High-performance 670-nm AlGaInP/GaInP visible strained quantum well lasers

We have grown high-performance AlGaInP/GaInP visible (670 mn) strained quantum well lasers by low-pressure metalorganic chemical vapor deposition. With AlInP cladding layer, a high-power AlGaInP/GaInP visible laser diode is achieved. Its threshold current is about 30 mA. The output power of this laser diode can maintain, at least, at 32 mW under continuous-wave (CW) operation at room temperature. High slope efficiency (0.8 mW/mA) and differential quantum efficiency (0.87) can be achieved. To improve beam quality, AlGaInP/GaInP visible lasers with and without depressed index cladding layer are theoretically and experimentally studied. From experimental results, the transverse beam divergence can be reduced from 41.4/spl deg/ to 26.2/spl deg/ while maintaining a low threshold current (from 36 mA to 46 mA). By using the transfer matrix method, our theoretical calculations are in good agreement with the experimental results.

A current compensated reference oscillator

This paper reports a reliable current compensated reference oscillator without any BJT or external component. To maintain a stable oscillation frequency, the discharging current of the proposed architecture varies with process, supply voltage and temperature (PVT) variations. Fabricated in a 0.18-µm digital CMOS process, this oscillator consumes 0.5 mW from a 1.8..V supply. The post-simulation results show the worst case is 12%. From 500 times Monte Carlo simulation results, the frequency variations are all limited within ±3%. The measurement result of ten samples at room temperature with 10% supply voltage deviation shows variation within ±10%. The maximum variation is –10.19% with process and supply voltage variations from measurement results.