B. Jobst

E0 band‐gap energy and lattice constant of ternary Zn1−xMgxSe as functions of composition

The E0 band gap energies and the lattice constants of zinc‐blende Zn1−xMgxSe alloys grown by molecular beam epitaxy in the composition range of 0≤x≤0.95 are determined. A nonlinear dependence on the composition is observed for both the band‐gap energies and the lattice con‐ stants of the ternary alloys. To our knowledge this is an initial report of a bowing in the lattice constant of a ternary II–VI alloy. Considering the bowings, the band‐gap energy and the lattice constant of zinc‐blende MgSe are extrapolated to be about 4.0 eV and 5.91 A, respectively.

FIRST ORDER DISTRIBUTED FEEDBACK OPERATION IN ZNSE BASED LASER STRUCTURES

First order distributed feedback laser with periods down to 94 nm based on ZnSe have been realized by electron beam lithography and wet chemical etching. Distributed feedback operation was demonstrated by optical excitation using a pulsed N2 laser. A threshold density of 80 kW/cm2 was found at room temperature for a resonator length of 225 μm. From the stop band width, a coupling coefficient of 120 cm−1 can be estimated. By varying the grating period the emission wavelength can be tuned over a wide spectral range of more than 130 meV. Measurements in the range between 20 and 300 K show that the shift of the emission wavelength with temperature is reduced by more than a factor of four as compared to the temperature shift of the spontaneous emission.

First order gain and index coupled distributed feedback lasers in ZnSe‐based structures with finely tunable emission wavelengths

First order gain and index coupled distributed feedback (DFB) gratings were realized in ZnSe‐based laser structures using direct implantation with a focused ion beam for gain‐modulated structures and conventional electron beam lithography for index modulation. With both technologies, gratings with periods below 90 nm could be achieved, permitting DFB emission in the blue spectral range. Fine tuning of the emission wavelength in steps of 0.14 nm is demonstrated by sampled DFB gratings based on a periodic modulation of the resonator period.

Radiative recombination centers induced by stacking-fault pairs in ZnSe/ZnMgSSe quantum-well structures

Stacking-fault pairs in ZnSe/ZnMgSSe quantum-well structures are found to induce enhanced radiative recombination visible as pairs of bright spots in microphotoluminescence intensity maps. Structural investigation by atomic-force microscopy and transmission electron microscopy (plan view as well as cross section) reveal that a widening and bending of quantum wells occurs when they are intersected by Frank-type stacking faults. The enlargement of the well width by up to 12 bilayers evokes an efficient localization of excitons. The localizing potential related to Shockley-type stacking-fault pairs is found to be much shallower.

Room temperature emission in narrow (14 nm) Cd0.35Zn0.65SeZnSe quantum wires with strong lateral confinement effects

We have fabricated deep etched Cd 0.35 Zn 0.65 Se/ZnSe quantum wires with lateral extensions down to 14 nm. The wires were defined by electron beam lithography and a low damage wet chemical etch process. Time-integrated and time-resolved photoluminescence spectroscopy was used to study excitonic properties in ultranarrow wide bandgap quantum wires. Lateral quantization causes a blue shift of the photoluminescence emission of up to 17 meV with respect to the 2D reference. A low temperature (T= 2 K) lifetime of about 110 ps was found, almost independent of the wire width. At room temperature, non-radiative sidewall recombination reduces the exciton lifetime from 330 ps for the 2D reference to about 21 ps in the case of a 28 nm wide wire.

Fabrication of dry etched CdZnSe/ZnSe quantum wires by thermally assisted electron cyclotron resonance etching

A low damage dry etch technology suitable for in situ processing was developed for the fabrication of ZnSe-based nanostructures. Thermally assisted electron cyclotron resonance etching combines plasma etching at low ion energies with process temperatures between 80°C and 210°C. Due to a variation of the process parameters, i.e., plasma power and sample temperature, a transition from partially physical to prevailing chemical etch properties is obtained. Therefore an accurate control of etch profile, surface morphology, and etch rate is possible. Optically active CdZnSe/ZnSe quantum wires with lateral sizes down to 20 nm were realized, indicating a significantly reduced influence of optically inactive layers compared to conventionally dry etched nanostructures. In narrow wires, a systematic blue shift of the photoluminescence signal with decreasing wire width clearly demonstrates lateral carrier confinement effects.

Gain-coupled distributed feedback lasers in the blue-green spectral range by focused ion beam implantation

We have observed gain-coupled distributed feedback (DFB) laser emission from CdZnSe/(Zn,Mg)(S,Se) structures at room temperature. First order DFB gratings with periods in the 90 nm range were defined by maskless Ga + -implantation using a focused ion beam technique. For optical pumping a lasing threshold of about 100 kW/cm 2 was found. The DFB emission is characterized by single mode emission and a temperature dependence of ΔE/ΔT = -0.11 meV/K, reduced by a factor of four compared to the spontaneous emission. From the Bragg condition a wavelength dispersion of the effective refractive index of Δn eff /Δλ = -0.23 x 10 -2 nm -1 and a temperature variation of the refractive index of Δn eff /ΔT = 1.7 x 10 -4 K -1 can be deduced.

A comparison of Mg and Ca for quaternary cladding layers in ZnSe based laser diodes

Abstract Calcium as a substitute for magnesium in Zn 1 − x Mg x S y Se 1 − x cladding layers for ZnSe-based laser diodes has been investigated. Ternary and quaternary Zn 1 − x Ca x S y Se 1 − y and Zn 1 − x Mg x S y Se 1 − y alloys were grown by molecular beam epitaxy on (100) GaAs substrates and characterized by various experimental methods. Epitaxial growth of Zn 1 − x Ca x S y Se 1 − y in zincblende structure was possible for Ca-concentrations x ≤ 0.1 only. The incorporation of sulphur is strongly enhanced by the presence of even small amounts of calcium. No similar behaviour is observed for Zn 1 − x Mg x S y Se 1 − y alloys, but the composition is very sensitive to the substrate temperature. The observed differences between the Ca- and the Mg-compounds will be discussed, considering the properties of the sulphides and selendies of calcium, magnesium and zinc.