H. Wenisch

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.

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.

(Cd,Zn)Se multi-quantum-well LEDs: homoepitaxy on ZnSe substrates and heteroepitaxy on (In,Ga) AsGaAs buffer layers

Abstract The properties of light emitting diodes (LEDs) based on (Cd,Zn)Se multi-quantum-wells in ZnSe grown by molecular beam epitaxy are compared for different substrate materials. ZnSe single crystals of high structural quality grown by the iodine transport, by the sublimation method in sealed ampoules and from the melt were used as substrates for ZnSe homoepitaxy in this work. Based on high resolution X-ray diffraction (XRD), transmission electron microscopy (TEM), and electro-optical measurements, it is shown that they all are superior to similar structures grown on (001) GaAs substrates with a 5 μm thick In0.04Ga0.96 As buffer which is nearly lattice matched to ZnSe. Cross sectional TEM studies indicate the III–V II–VI interface to be the origin of dislocations observed in the LED structure, while at the homoepitaxial ZnSe ZnSe interface hardly any new dislocations are generated under optimized conditions. The full width at half maximum (FWHM) in XRD (004 reflection) of a 4 μm thick multi-quantum-well diode structure was 47 arc sec which is near to the values of the substrate material. For epilayers, values as low as 21 arc sec have been observed. In reciprocal space mapping no significant broadening due to mosaicity can be found. The LED structures were tested as edge emitters in a laser like configuration under pulsed and continuous current injection at room temperature. They emit blue-green light at 490 nm with a FWHM of only 56 meV.

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.

Investigation of the interfacial quality and the influence of different substrates in ZnSe homoepitaxy

ZnSe substrates grown by the seeded chemical vapour transport and recrystallization as well as by the Bridgman method are compared for their use in molecular beam epitaxy. Intrinsic and extrinsic defects were analysed by low-temperature photoluminescence and especially by electron-paramagnetic-resonance. The transition region between substrate and epitaxial layers is studied by cross-sectional transmission electron microscopy. The defect densities in the interface are found to be lower than for heteroepitaxial layers on GaAs substrates. The strained multi-quantum-well region of homoepitaxial light-emitting diodes is also investigated. They were tested at room temperature and emit sharply at 490 nm with a full-width at half-maximum of 51 meV.

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.

Interplay of the Trion Singlet and Triplet State Transitions in Magnetooptical and Time-Resolved Investigation of ZnSe/Zn(S,Se) Single Quantum Wells

Negatively charged trions are investigated for ZnSe single quantum wells embedded in ternary barriers. Polarization and excitation dependent photoluminescence measurements were performed in magnetic fields up to 11.8 T. From these measurements we are able to develop an energy-term scheme for the spin-singlet and the spin-triplet state of the trion.

Structural properties of homoepitaxial and heteroepitaxial ZnSe-based laser structures

High-resolution X-ray diffraction has been used for the structural characterization of hetero- and homoepitaxial ZnSe-based laser structures. A common problem of both kinds of conventional laser structures containing sulphur is the formation of composition gradients in the thick quaternary layers. In addition to depth gradients within the single cladding layers, a composition shift between the n-type and the p-type side of the laser structures has been observed. Clear differences between homoepitaxial and heteroepitaxial samples were found with respect to the strain relaxation.

Gain characteristics of ZnSe/(Zn,Mg)(S,Se)/(Zn,Mg)(S,Se) quantum well lasers

A systematic investigation of the gain dependence on temperature, well width (3, 5, 7 nm), and excitation intensity in MBE grown ZnSe/(Zn,Mg)(S,Se)/(Zn,Mg)(S,Se) quantum-well lasers has been performed. At low lattice temperature (100 K) we find a rather low threshold density of 30 kW/cm 2 . We observe an exponential increase of the threshold density with increasing temperature for the 5 and 7 nm samples and an even stronger increase for the 3 nm sample. The characteristic temperatures (T O ) are 105 and 118 K for the 5 and 7 nm samples, respectively. The low T O values are mainly attributed to thermionic emission of carriers out of the quantum well due to the small total band offset of 156 meV between the strained ZnSe layer and the (Zn,Mg)(S,Se) barriers. At high temperatures and carrier densities contributions from higher subbands can clearly be seen in the gain spectra. The experimental results are in good agreement with calculations in the framework of a microscopic theory which includes the detailed coupled band structure and many-particle effects.

Investigations of ZnSe based laser structures on ZnSe substrates by high resolution x-ray diffraction

The structural quality of homoepitaxially grown ZnSe based layers depends sensitively on the ZnSe substrate preparation prior to the growth. In this paper the consequences of an in situ hydrogen plasma cleaning of ZnSe substrates to the crystalline quality of II-VI based laser structures are discussed. The crystalline properties of the resulting laser structures are compared with those of homoepitaxially grown structures on non-hydrogen cleaned substrates as well as with the characteristics of heteroepitaxially grown lasers on GaAs. The samples were grown by molecular beam epitaxy and characterized by high-resolution x-ray diffraction. Remarkable differences in the strain state of the homoepitaxially grown samples are observed. While the structures grown on hydrogen cleaned ZnSe substrates are pseudomorphic, layers of comparable thickness and lattice mismatch on non-hydrogen cleaned substrates show clear strain relaxation. Triple axis (004) rocking curves reveal distinct differences between the three sample types investigated. According to these measurements, the in situ hydrogen plasma cleaning of ZnSe substrates results in a drastic improvement of crystalline perfection of the homoepitaxial laser structures. However, even the defect density of the optimized homoepitaxial samples exceeds that of heteroepitaxial reference lasers by two orders of magnitude as estimated from the diffuse scattered intensity.