J. Söllner

Influence of compensation on the luminescence of nitrogen-doped ZnSe epilayers grown by MOVPE

Abstract The luminescence of ZnSe grown by metalorganic vapour phase epitaxy and doped by a DC nitrogen plasma is investigated. With increasing N 2 flux the donor-acceptor pair (DAP) band continuously develops into a structureless band peaking at 2.62 eV for highest doping levels. This broad band evolves back into a structured DAP band peaking at 2.698 eV with increasing excitation density. At high N concentrations and at large degree of compensation potential fluctuations become important for the spatially indirect DAP recombination. These fluctuations can easily be screened by optically excited carriers making the experimental conditions decisive for luminescence spectra of strongly doped ZnSe: N samples.

Saturation of optical gain in ZnSe heterostructures

Abstract The optical gain in ZnSe heterostructures is investigated by the variable stripe length method (VSLM). Saturation of the gain occurs due to the interaction of the carrier and photon densities which leads to an inhomogeneous electron—hole pair distribution along the excited stripe. This inhomogeneity is tested by spatially resolved pump—probe spectroscopy and compared to calculations based on a rate-equation model.

Thermalization of free excitons in ZnSe quantum wells

The thermalization dynamics of hot free excitons in ZnSe quantum wells is studied by photoluminescence excitation (PLE), steady-state (cw-PL) and time-resolved photoluminescence (TRPL). Formation of excitons by emission of LO phonons is very efficient and leads to the generation of narrow hot-exciton distributions. The LO phonon-assisted recombination of the excitons is a direct monitor of the hot-exciton dynamics. A thermalized exciton distribution is reached only after some 100 ps due to the slow exciton relaxation assisted by acoustic phonons. The hot-exciton features in PLE, cw-PL and TRPL are consistently reproduced by explicitly calculating the relevant scattering processes.

Nitrogen doping of ZnSe with trimethylsilylazide, triallylamine or bisditrimethylsilylamidozinc during metalorganic vapour phase epitaxy

Abstract Several nitrogen precursors without nitrogen hydrogen bonds were tested to dope ZnSe in a MOVPE process. Bisditrimethylsilylamidozinc (ZnBTM), trimethylsilylazide (TMSiN) and triallylamine (TAN) were used to grow ZnSe:N with ditertiarybutylselenide (DTBSe), diisopropylselenide (DIPSe) and dimethylzinc-triethylamine (DMZnue5f8TEN) at growth temperatures between 340 and 420°C. The samples were analysed by photoluminescence (PL), current-voltage ( I ue5f8 V ), capacitance-voltage ( C ue5f8 V ) and Hall measurements. The dependence of nitrogen incorporation and electrical activation on growth temperature, VI/II ratio and dopant flow were investigated. In the PL spectra of the layers grown with TMSiN an intense bound-exciton emission, correlated to Zn vacancies is observed, besides the I N 1 emission line for high dopant flows. The samples grown with ZnBTM and TAN as precursors exhibit a broadening of the excitonic emissions and a shift towards the energetic position of the I N 1 emission line for increasing nitrogen incorporation in the layers. The intensity of the excitonic emissions in comparison to the donor-acceptor pair emissions remains dominating. Although Hall measurements of several samples doped with ZnBTM and TAN indicate p-type conductivity, the analysis of the C ue5f8 V and I ue5f8 V measurements shows the n-type or semi-insulating character of the samples.

Thermal stability of (Zn,Cd)(Se,S) heterostructures grown on GaAs

We have analyzed the thermal stability of ZnSe‐based single quantum well structures grown on a GaAs substrate by applying a rapid thermal annealing process. The photoluminescence intensity of the quantum well was used as a monitor for the thermal changes induced by the annealing process. X‐ray diffractometry yields information about the crystal quality and the strain condition before and after the thermal treatment. As a main result, we found that the thermal stability of the quantum well photoluminescence signal critically depends on the thickness of the II–VI buffer layer, i.e., the distance between the active layer and the GaAs‐II–VI heterointerface. For a buffer layer thickness of about 38 nm, the quantum well signal is totally quenched after a 1 min annealing step at 500u2009°C, while clear luminescence signals can be observed in samples with a 1 μm buffer even for a 750u2009°C process. Additionally, by comparing CdZnSe/ZnSe and ZnSe/ZnSSe quantum wells, we found that the Cd–Zn interdiffusion seems to be mor...

ELECTRON-BEAM-PUMPED LASING IN ZNSE EPITAXIAL LAYERS GROWN BY METAL-ORGANIC VAPOR-PHASE EPITAXY

Stimulated emission and laser action under N2 laser radiation and electron‐beam excitation at room temperature was studied in ZnSe epitaxial layers grown by metal‐organic vapor‐phase epitaxy. The laser threshold was about Ithr=1 MW/cm2, the pulse energy was Eem=0.3 μJ at the excitation intensity of Iexc=26 MW/cm2. The duration was tem=2 ns and the wavelength was λem=473.2–475.2 nm. Stimulated emission and laser action are due to the recombinations in a high‐density electron‐hole plasma at Iexc≳Ithr. The light amplification takes place in both the Fabry–Perot cavity and in the scheme of the zig‐zag ray propagation inside the crystal without feedback.

Stabilization and growth interruption effects at ZnSxSe1−x/ZnSe quantum‐well interfaces grown by metalorganic vapor‐phase epitaxy

ZnSxSe1−x/ZnSe quantum wells (QW) with and without growth interruption at the interface were grown using atmospheric pressure metalorganic vapor‐phase epitaxy. It has been shown that growth interruptions have a major influence on the optical properties of the QW. An interruption of growth in ternary ZnSSe layers causes a quasi‐QW photoluminescence (PL), explained by sulfur depletion of the layer. A stabilization is necessary to avoid a quasi‐QW. High‐quality ZnSSe/ZnSe QWs were achieved by suitable stabilization with H2S and DESe during the growth interruption at the interface, so a sulfur diffusion out of the barrier can be avoided. The QW thicknesses grown under these conditions were in agreement with the nominal well thicknesses, and for stabilized 1‐nm QWs we obtain a PL blueshift of 158 meV for x=0.68 and 60 meV for x=0.4 compared to the ZnSe band‐edge emission. Characteristic QW‐PL transitions at room temperature were observed for unstabilized QW grown with a long interruption time, tp=90 s. The the...

Photo- and cathodoluminescence of ZnSSe quantum well heterostructures grown by MOVPE

Abstract Lateral- and depth-resolved cathodoluminescence as well as photoluminescence of MOVPE-grown single- and multi quantum well ZnSSe-based heterostructures in the temperature range of 14–300 K were investigated. The intensity ratios between different emission peaks and their dependencies on the electron penetration depth together with the temperature dependence of photoluminescence spectra permit concluding that the transport of non-equilibrium carriers or excitons in the structure is hindered by potential fluctuations created by microvariations of the sulphur content. The deep centres are localized mostly in the near-substrate region which is indicated by an increase of the self-activated emission at 2.2 eV with increasing electron beam penetration depth. Further improvement of layer homogeneity as well as a sufficient distance between the quantum well and the substrate together with minimization of the interlayer strain is necessary to improve the radiative properties of ZnSe-based heterostructures.

Correlation of room temperature photoluminescence to structural properties of ZnSSe/ZnSe superlattices grown by metalorganic vapor phase epitaxy

ZnS0.1Se0.9/ZnSe strained layer superlattices with up to 120 periods were grown by metalorganic vapor phase epitaxy. We demonstrate the sensitivity of room temperature photoluminescence of these structures for the assessment of the dependence of structural properties on growth conditions. Low temperature photoluminescence (PL) and x‐ray diffraction data confirm the results of the room temperature PL measurements. In optimized samples showing highly efficient blue luminescence (2.71 eV) at 300 K heavy‐hole and light‐hole free exciton recombinations at 11 K and higher‐order satellite reflections in the x‐ray diffraction profiles were observed.

Hot Excitons in ZnSe Quantum Wells

Abstract The optical and acoustic phonon assisted relaxation of free hot excitons in ZnSe quantum wells is reflected in photoluminescence excitation spectra. Slow thermalization of excitons on a 100 ps time scale is directly observed in time-resolved studies of luminescence in the spectral region of the phonon sideband.