M. Fehrer

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.

Plasma assisted molecular beam epitaxy growth of GaN

Abstract The growth of GaN on sapphire by plasma assisted molecular beam epitaxy (MBE) is investigated with the object of optimizing the material quality. The V/III flux ratio as well as the growth temperature are discussed in relation to their impact on electrical, optical and structural layer properties. Samples grown under nearly stoichiometric conditions exhibit both the highest mobilities and the highest photoluminescence efficiencies. Growth temperatures above 800°C were found to result in narrow reflections in X-ray diffraction. However, the chemical decomposition of GaN at temperatures above 850°C limits the suitable temperature range for the growth under high vacuum conditions.

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.

Buffer layers for the growth of GaN on sapphire by molecular beam epitaxy

Abstract The change of structural, optical and electrical properties of GaN grown by plasma assisted molecular beam epitaxy on sapphire due to the introduction of GaN and AlN buffer layers is investigated. Whereas the layer surface becomes smoother its structural features remain very small. Photoluminescence and resistivity measurements confirm an increased density of extended defects produced by the buffer layer. A predominant lateral growth mode as known from metalorganic vapour-phase epitaxy is not observed.

INTERNAL PHOTOLUMINESCENCE AND LIFETIME OF LIGHT-EMITTING DIODES ON CONDUCTIVE ZNSE SUBSTRATES

We report on the molecular beam epitaxial growth of green (508 nm) and blue (489 nm) light-emitting diodes (LEDs) on conductive ZnSe substrates. The resistivity of the (001)-oriented ZnSe wafers was drastically reduced by a zinc extraction treatment to typical values of 1×10−1 Ω cm. The intensity of an additional orange band around 600 nm observed in electroluminescence depends strongly on the wavelength of the multi-quantum-well emission. This can be explained by absorption and effective re-emission in the substrate material named internal photoluminescence and was confirmed by transmission and photoluminescence experiments with the bare substrates. The LEDs with lifetimes up to 100 hours proofed to be surprisingly stable compared to the structures on undoped ZnSe substrates grown before.

Quantum confined Stark effect of II-VI heterostructures suitable as modulators in the blue–green spectral region

We present a systematic and quantitative investigation of the quantum confined Stark effect in (Zn, Cd)Se/Zn(S,Se) quantum well structures at room temperature. For this purpose, differential transmission spectroscopy is performed on two samples with different thicknesses of the active layers (4×5 nm and 10 nm) and compared to model calculations. We observe a Stark shift of 18 meV in the heavy-hole exciton peak for an electric-field change from 82 to 175 kV/cm for the 4×5 nm sample. In contrast, the 10 nm sample shows a rather weak and more Franz–Keldysh-like signal. Furthermore, we have analyzed the spectral behavior of the linewidth enhancement factor αH. Values between 5.1 and 0.2 are found at the energy of absolute maximum of the absorption change for the 4×5 nm sample.

Correlations between Structural, Electrical and Optical Properties of GaN Layers Grown by Molecular Beam Epitaxy

Screw and edge dislocation densities were estimated from X-ray diffraction profiles of GaN layers grown by molecular beam epitaxy. The results were confirmed by transmission electron microscopy. The layers had varying thicknesses or were deposited on differently nitridated sapphire substrates. It was found that the low temperature Hall mobility correlates with the screw dislocation density whereas the full width at half maximum of the donor bound exciton emission line changes similarly as the edge dislocation density.

Homoepitaxial laser diodes grown on conducting and insulating ZnSe substrates

The different steps of ZnSe substrate preparation are described. In situ hydrogen plasma cleaning to get rid of contaminations like thermally stable oxides on ZnSe substrate surfaces before growth is the crucial point to fabricate reproducibly pseudomorphic (Mg,Zn) (S,Se) layers. It is found that the crystalline quality of the layers depends strongly on that of the ZnSe substrates itself as well as on the initial growth start conditions. Finally, ZnSe-based laser diodes were grown on insulating and different kinds of conducting ZnSe substrates. They operated at room temperature under pulsed conditions.

Influence of driving conditions on the stability of ZnSe-based cw-laser diodes

ZnSe-based laser diodes grown by molecular beam epitaxy are tested under various current driving conditions. We found that the driving conditions do not influence the degradation process itself. Lifetime measurements show no dependence on the pulse width of the applied current, whereas the duty cycle has a strong influence on the lifetime. This indicates that heat is one of the driving forces of the degradation process. It is found that stable high-power pulsed operation can be maintained over a long time when the delay between the current pulses is long enough. In pulsed mode light pulses with 128 mW and in continous wave (cw)-mode an output power of more than 37 mW have been obtained.

Optical Spectroscopy of Mg- and C-Related Donor and Acceptor Levels in GaN Grown by MBE

We present photoreflectance (PR) in combination with temperature and density dependent photoluminescence (PL) on undoped as well as on Mg- and C-doped GaN grown by molecular beam epitaxy (MBE). We determined the binding energy of the free A-exciton, the residual donor-bound exciton and the Mg-acceptor-bound exciton to (26 ± 1), (5 ± 1) and (18 ± 1) meV, respectively. In n-type GaN a residual DAP band appears independent of the particular type of dopant. Free-hole concentrations up to 1018 cm—3 in MBE grown p-GaN:Mg were achieved without generation of deep compensating donors. Moreover, we found evidence for a second DAP series involving a shallow compensating donor level built in along with Mg doping. The correlation of different DAP bands to the respective type of conductivity opens an alternative way to control the p-type doping in GaN:Mg without the need for electrical contacting.