H.-J. Lugauer

CdSe fractional-monolayer active region of molecular beam epitaxy grown green ZnSe-based lasers

This letter reports on the self-organized growth of nanoscale dot-like CdSe-based islands during molecular beam epitaxy of CdSe/ZnSe nanostructures with a CdSe thickness between 0.75 and 3.0 monolayers. An increase in the nominal CdSe thickness results in a higher density of islands (up to 2×1010 cm−2) and is accompanied by dramatic enhancement of the photoluminescence efficiency. The density of large relaxed islands appears to saturate at a value of (3–4)×109 cm−2. Room temperature (Zn, Mg)(S, Se)-based optically pumped lasers with an extremely low threshold (less than 4 kW/cm2), as well as (Be, Mg, Zn)Se-based injection laser diodes using a single (2.5–2.8) monolayer thick CdSe active region, both demonstrating significantly enhanced degradation stability, have been fabricated and studied.

ZnSe-based blue-green lasers with a short-period superlattice waveguide

We report the successful application of alternatively strained short-period superlattices for the waveguide region of optically pumped and injection room-temperature ZnSe-based lasers operating within the 470–523 nm spectral range. The design of optically pumped ZnMgSSe/ZnSSe/ZnCdSe lasers provides extremely low threshold power densities due to the enhanced electronic and optical confinement. Room-temperature BeMgZnSe/ZnCdSe injection lasers with threshold current density of about 750 A/cm2 and characteristic temperature as high as 366 K are demonstrated. The peculiarities of carrier transport across the short-period superlattices are explained by a thermally activated mechanism.

Cross-sectional atomic force microscopy of ZnMgSSe- and BeMgZnSe-based laser diodes

Atomic force microscopy (AFM) of cleaved facets of ZnSe-based lasers with various active region designs is reported. Different AFM probe friction on the materials forming the laser structures are exploited for imaging their basic layers. Unlike ZnMgSSe-based lasers, the cleaved surface of cladding layers in BeMgZnSe-based structures is atomically flat, which is attributed to hardening of the II–VI materials by Be incorporation. Nanometer-high steps and undulations are observed at the laser heterointerfaces on cleaved facets. The shape and height of such topographic singularities located in the vicinity of a (Zn,Cd)Se quantum well active region depend on the strain distribution in the laser waveguide.

Strongly index-guided II-VI laser diodes

Ridge-waveguide laser diodes based on Be-chalcogenides have been fabricated by reactive ion etching and planarization with polyimide. Etching close to or even through the active layer is demonstrated to suppress the current spreading efficiently; resulting in a significant reduction of the threshold current as compared to gain-guided structures. This allows the fabrication of narrow, strongly index-guided II-VI laser diodes with a ratio between the vertical and the lateral far-field angle of, e.g., 1.2:1 for L/sub m/=1 /spl mu/m.

Blue-green ZnSe lasers with a new type of active region

We report the results of an experimental study of molecular-beam epitaxy of ZnSe-based laser heterostructures with a new structure of the active region, which contains a fractional-monolayer CdSe recombination region in an expanded ZnSe quantum well and a waveguide based on a variably-strained, short-period superlattice are reported. Growth of a fractional-monolayer CdSe region with a nominal thickness of 2–3 ML, i.e., less than the critical thickness, on a ZnSe surface (Δa/a∼7%) leads to the formation of self-organized, pseudomorphic, CdSe-enriched islands with lateral dimensions ∼10–30 nm and density ∼2×1010 cm−2, which serve as efficient centers of carrier localization, giving rise to effective spatial separation of defective regions and regions of radiative recombination and, as a result, a higher quantum efficiency. Laser structures for optical pumping in the (Zn, Mg) (S, Se) system with a record-low threshold power density (less than 4 kW/cm2 at 300 K) and continuous-wave laser diodes in the system (Be, Mg, Zn) Se with a 2.5 to 2.8-ML-thick, fractional-monolayer CdSe active region have been obtained. The laser structures and diodes have an improved degradation resistance.

ZnSe/BeTe type-II LEDs emitting between 640 and 515 nm

We report the growth and characterization of LEDs with wavelengths from 640 to 515 nm based on ZnSe/BeTe type-II transitions. The spatially indirect transition shows bright luminescence at room temperature probably due to a strong carrier confinement at the interface. In addition, the device lifetime reaches values above 1000 h. This relatively good device stability can be explained by a reduction of stacking faults at the growth start with BeTe, the reduction of strain in the active zone, the absence of nitrogen-doped ZnSe, and the fully pseudomorphic p-BeTe contact.

VUV Ellipsometry on Beryllium Chalcogenides

We have determined the dielectric function (DF) of beryllium chalcogenides between 2.5 and 25 eV using an ellipsometer operating with synchrotron radiation in the VUV range. The VUV ellipsometer is, in particular, useful for the characterization of wide band gap semiconductors since the spectral range of spectrometers equipped with conventional light sources is limited to below 6 eV. Beryllium chalcogenides are a relatively new class of material with still unknown optical and electronic properties. Here we determine the dielectric function of BeTe and BexZn1—xSe for the full composition range (0 ≤ x ≤ 1). We verify that they are wide band gap semiconductors with their direct band gaps at 4.2 eV (BeTe) and 5.55 eV (BeSe).

Wavelength-dependent optical degradation of green II–VI laser diodes

In this letter we report on optical degradation studies on BeMgZnSe separate confinement quantum well laser structures for the blue-green spectral region. The wavelength of the incident light has been tuned from 3.81 down to 2.10 eV, corresponding to an energy range from above the band gap of the cladding layers down to below the band gap of the quantum well. The dominant degradation mechanism is initiated when electron hole pairs are created in the quantum well. Absorption of light in deep defect bands, e.g., of the p-type cladding material is negligible in these structures. The strain state of the quantum well is one possible driving force for the degradation. In this context, point defect propagation as well as a structural phase transition of the ZnCdSe quantum well are discussed.

Optical and acoustical phonon properties of BeTe

Abstract We present an investigation of the vibrational properties, the elastic constant c 11 and the infrared dielectric constant e ∞ of BeTe. For this purpose we applied far-infrared reflectance and Raman spectroscopy to single BeTe layers and BeTe/ZnSe superlattices. The experimental data are used to access the bond ionicity in the framework of an empirical tight binding approach. A low bond polarity of 0.61 is determined for BeTe, which corresponds to a high lattice stability. Furthermore, the elastic constant c 11 is determined as to be 8.9 ± 0.9.10 10 N/m 2 from the frequency positions of folded acoustic phonon (FAP) modes in BeTe/ZnSe superlattices. The experimental results are correlated to data obtained by first-principles local density approximation (LDA) calculations.

Optical properties of laser diodes and heterostructures based on beryllium chalcogenides

Abstract Laser diodes and heterostructures based on beryllium chalcogenides have been characterized optically by means of photoluminescence and photoluminescence excitation spectroscopy in the temperature range from 1.6 to 300 K. The study was performed on real lasing structures. An efficient photoluminescence at room temperature was observed in heterostructures with (ZnCd)Se and (BeZnCd)Se quantum wells ( L z = 100 A). The decrease of the luminescence intensity with increasing temperature is similar for both quantum well materials. Analyzing the temperature-dependent photoluminescence excitation spectra of laser structures, we have determined the efficiency of carrier collection into the active layer. At temperatures above 150 K all carriers from the 2000 A thick (BeZn)Se waveguide are collected into the active layer.