M. Klude

560-nm-continuous wave laser emission from ZnSe-based laser diodes on GaAs

Room-temperature continuous wave (cw)-lasing emission at wavelengths around 560 nm was obtained from ZnSe-based laser diodes grown on GaAs substrates. The wavelengths of the devices are most suitable as lightsources for plastic optical fibers. To achieve this emission wavelength, CdZnSSe quantum wells with high Cd content were employed as active region. The growth of such quantum wells requires Se-rich growth conditions.

500–560 nm Laser Emission from Quaternary CdZnSSe Quantum Wells

ZnSe-based laser diodes with emission wavelength from 500 to 560 nm are studied. The long wavelength operation of these laser diodes requires careful optimization of the CdZnSSe quantum well material. It is shown that under stoichiometric growth conditions quantum wells with high optical and structural qualtity can be realized. Employed in laser structures room-temperature cw-operation around 560 nm is obtained. In comparison with laser diodes emitting at 505 nm it is found that the high Cd content of the quantum well does not degrade the operational characteristics of the devices. In pulsed mode more than 1100 mW output power at 560 nm is achieved.

Internal photoluminescence in ZnSe homoepitaxy and application in blue–green–orange mixed-color light-emitting diodes

We discuss the controllable color-range in ZnSe-based light-emitting diodes (LEDs) realized by ZnSe homoepitaxy and internal photoluminescence. ZnSe-based LED structures were grown by molecular-beam epitaxy (MBE) on mostly conductive ZnSe substrates, which exhibit under short wavelength light excitation at room temperature strong orange emission around 600 nm. This fact is exploited to fabricate integrated mixed-color LED chips, where light from the active layer sandwiched in a p}n-junction acts as internal excitation source. We named this e!ect recently ‘Internal Photoluminescencea (Wenisch et al., J. Appl. Phys. 82 (1997) 4690). It leads to electroluminescence spectra with two distinct emission peaks originated from the active layer and from the ZnSe substrate, respectively. In view of color impression, just by varying the Cd x Zn 1~x Se quantum-well composition and the radiant recombination rate in the substrate by it’s choice, as much as two thirds of the visible color space is covered. Under conditions, when only the substrate emission is present, Commission Internationale d’Eclairage (CIE) chromaticity coordinates for orange color LEDs of (0.54, 0.45, 0.01) for the red, green and blue color, respectively, were determined. 490-nm quantum-well-emitting LEDs were found to be best suited in reaching the technologically important balanced white emission (‘White Pointa) and a value of (0.31, 0.39, 0.30) for the color coordinates close to it was experimentally achieved. ( 2000 Elsevier Science B.V. All rights reserved.

Electro-Optical Characterization of CdSe Quantum Dot Laser Diodes

We report the electroluminescence characteristics at room-temperature of CdSe quantum dot laser diodes. In comparison with quantum well laser diodes emitting in the same wavelength region a different dynamical behaviour under pulsed operation is found. Under short pulsed (0.1 μs) current injection a blue shift of the emission and an intensity increase are found with increasing driving current. Electrically pumped lasing is realized for 50 ns pulse width.

Device Properties of Homo- and Heteroepitaxial ZnSe-Based Laser Diodes

The characteristics of ZnSe-based laser diodes grown on GaAs and ZnSe substrates are discussed. There is no significant difference observed in the dynamic behavior and in the operating voltages between the two cases. The degradation mechanism is similar with the developing of dark line defects and a 1/t-like decrease in light intensity at constant current for t →∞. The width of the dark line defects is in homoepitaxy almost constant in time, although their number is higher. This difference is also reflected in the lifetimes of our devices during lasing, which is in heteroepitaxy three minutes and about one second in homoepitaxy, for both in cw operation at room temperature.

High Reflectivity p‐Type Doped Distributed Bragg Reflectors Using ZnSe/MgS Superlattices

Undoped and p-type doped distributed Bragg reflectors (DBRs) have been grown by molecular beam epitaxy using ZnSe layers for the high refractive index material and ZnSe/MgS superlattices (SLs) for the low index material. The ZnSe/MgS SLs consist of 22.5 periods with a period length of 2.5 nm as confirmed by high-resolution X-ray diffraction measurements. A reflectivity of the p-type doped DBR higher than 99% at 522 nm has been achieved. The low temperature photoluminescence spectrum of the p-type doped DBR shows a strong donor-acceptor pair recombination peak at 2.686 eV.

Post-growth p-type doping enhancement for ZnSe-based lasers using a Li3N interlayer

A method to increase decisively the p-type doping level in ZnSe-based laser diodes is described. Upon Li3N indiffusion, the formation of a stable acceptor complex is observed. Free hole concentrations of 8×1018 cm−3 are obtained. This value is at least one order of magnitude larger than typical p-type doping levels achieved by molecular-beam epitaxy of ZnSe. In addition, no compensation effects occur, as usually observed for p-type doping using either Li or N. ZnSe-based lasers processed by applying this post-growth p-doping enhancement technology show significantly improved properties.

Operation and catastrophic optical degradation of II-VI laser diodes at output powers larger than 1 W

An output power of 1.55 W, the highest ever reported, at 20 °C has been achieved for a ZnCdSSe/ ZnSSe/ZnMgSSe separate-confinement heterostructure laser diode under short-pulsed operation. A differential quantum efficiency of 28% and a maximum conversion efficiency of 1.5% were obtained. The output power was limited by catastrophic optical damage (COD) of the facets, as proved by atomic force microscopy (AFM).

Optical Gain of CdSe Quantum Dot Stacks

Systematic gain measurements of CdSe quantum dot stacks were carried out using the variable-stripe-length method at various pump densities and temperatures. The fivefold quantum dot stacks show an effective optical gain at double threshold of the order of 125 cm -1 (150 cm -1 ) for the doped (undoped) sample. A maximum optical gain of 400 cm -1 was achieved at a pump density of 980 kW/cm -2 and a temperature of 10 K. High-resolution transmission electron microscope and micro-photoluminescence measurements were carried out. Temperature depending measurements yield a strong increase of the threshold densities for temperatures above 100 K.

A novel approach for improved green-emitting II-VI lasers

New concepts to improve the performance of green-emitting laser diodes, based on the ZnSe system, are presented. The benefits of implantation-induced disordering (IID) and a novel alleged contact structure are discussed. Using IID, index-guided lasers with low thresholds are fabricated. The introduction of Li/sub 3/N-containing contacts leads to an acceptor indiffusion resulting in an increased p-type doping level and thereby extremely reduced turn-on voltages, threshold current densities, increased wall-plug efficiencies, and extended continuous-wave lifetimes.