M. Scholl

Electroabsorption and light modulation with ZnSe/ZnSSe multiquantum wells grown by metalorganic vapor phase epitaxy

ZnSe/ZnSSe multi‐quantum wells were grown on GaAs by metalorganic vapor phase epitaxy and investigated by photocurrent and photoluminescence measurements. Photocurrent spectra show clear excitonic peaks. The observed peaks were identified as light and heavy hole excitonic transitions by comparing the data to theoretical ones. Field dependent photocurrent measurements show features useable for modulator applications in the blue spectral range at energies above 2.7 eV at 300 K.

Optical study of interdiffusion in CdTe and ZnSe based quantum wells

Abstract The interdiffusion in single quantum well structures was studied for a variety of II–VI semiconductor materials based on CdTe and ZnSe. In particular we have investigated CdTe/CdMnTe, CdTe/CdMgTe, Hg x Cd 1- x Te/Hg y Cd 1- y Te and ZnSe/CdZnSe structures in which an intermixing of column II elements can be induced as well as ZnSe/ZnSSe allowing an interdiffusion within the column VI sublattice. The diffusion was induced by rapid thermal annealing (RTA) for 1 min at different temperatures. The resulting blue shift of the characteristic emission spectrum was analyzed using photoluminescence spectroscopy. We observed a significant difference of the diffusion behavior between both groups of materials. While in all three CdTe based material system an almost complete interdiffusion within the column II sublattice could be obtained at a high optical quality of the structures, both ZnSe based quantum wells show only remarkably small diffusion lengths. For all three CdTe based quantum wells we derived an activation energy of the interdiffusion process from a simple Fickian diffusion model applied to our measurements. We obtained a value of 2.8 eV for CdTe/CdMnTe and CdTe/CdMgTe and a value 2.1 eV for Hg x Cd 1- x Te/Hg y Cd 1- y Te.

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.

Optimization of strained short-period ZnSSe/ZnSe superlattices grown by metalorganic vapour phase epitaxy

Abstract Series of ZnSe/ZnS x Se 1- x strained-layer superlattices with x =0.1-0.14 grown by MOVPE using either diethylsulphide (DES) or hydrogen sulphide (H 2 S) as the sulphur source have been studied with X-ray double-crystal diffractometry and photoluminescence. The effects of period thickness, number of periods, growth interruption and stabilization of the ZnSSe-to-ZnSe interface as well as buffer layers on the structural and optical properties were determined. These data serve as basis for optimization of the growth parameters. High quality material was obtained as indicated by narrow peak widths and numerous satellite peaks in the X-ray diffraction profiles for optimized samples grown with DES (120 periods, without stabilization, without buffer). Highly efficient blue luminescence at 2.7 eV observed at room temperature confirms the excellent properties of the heterostructures. Because of the high resolution of the double-crystal X-ray diffraction we were enabled to detect very small variations of layer thickness and composition in the superlattice systems.

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.

Homogeneity of ZnSSe/ZnSe multiquantum wells grown by metalorganic vapour phase epitaxy

The ZnS x Se 1−x (0 ≤ x ≤ 1) layers and multiquantum well structures studied here were grown in an atmospheric or in a low-pressure metalorganic vapour phase epitaxy (MOVPE) reactor, respectively. For industrial device production, an excellent homogeneity across the whole wafer is required, so both reactors are equipped with rotating susceptors for two-inch wafers. X-ray diffraction measurements of optimized multiquantum well structures grown with diethylsulphur (DES) in the atmospheric-pressure reactor show very good structural properties and high reproducibility of the layer sequence indicated by sharp satellite peaks and «pendellosung» fringes. Also the near-band-edge photoluminescence (PL) spectrum hints on the excellent layer quality by sharp PL peaks (FWHM = 2,6 meV). The homogeneity of a multiquantum well structure across a two-inch wafer was determined from the local dependence of the PL blue-shift across the wafer. At the edge of the sample grown with H 2 S the blue-shift increases, which can be explained by an increased sulphur content or a lower thickness owing to a decreased growth rate. However, Raman measurements taken at samples grown with DES show that the sulphur content is very homogeneous laterally, whereas the variation is much larger in samples grown with H 2 S. Homogeneity tests for ZnSSe grown with DES under low pressure or atmospheric pressure show no significant difference. Nevertheless, with H 2 S as a sulphur source, the gas phase depletion was reduced at low pressure. With increased gas flow velocity, both the sulphur content and the growth rate homogeneity were improved

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.

Crystalline quality and interface sharpness of ZnSe/ZnSxSe1-x superlattices on GaAs: analysis by Raman spectroscopy and X-ray diffractometry

Abstract ZnSe/ZnS x Se 1- x strained layer superlattices were grown by atmospheric-pressure metalorganic vapour phase epitaxy (MOVPE) on GaAs substrates. As group VI precursors for ZnS x Se 1- x the combination of diethyl-selenium (DESe) with either diethyl-sulphur (DES) or with H 2 S was applied, while we used diethyl-zinc (DEZn) for group II. We investigated the influence of sulphur stabilization during growth interruptions. For the analysis of the structural superlattice properties a combination of Raman spectroscopy and X-ray diffractometry was employed. DES-grown structures with 120 periods, grown at 480°C, show a very good lateral homogeneity of the composition, a high crystal quality and very regular periodicity, which leads in the DCXD profiles to narrow satellite peaks and even pendellosung fringes from the total stack thickness, while in the Raman spectrum the regular modulated structure results in narrow folded acoustical phonon peaks. These results demonstrate the capability of MOVPE to grow high quality ZnSe/ZnS x Se 1- x superlattices.

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.

Optical and X-ray analysis of ZnSxSe1−x/ZnSe superlattices grown on GaAs by metalorganic vapour phase epitaxy

Abstract X-ray diffractometry, Raman backscattering and photoluminescence were applied to characterize ZnS x Se 1- x /ZnSe superlattices, grown on GaAs(100) by metalorganic vapour phase epitaxy (MOVPE). As sulphur precursor materials diethylsulphide (DES) and H 2 S were compared. Furthermore, we investigated the influence of different kind of buffer layers, the effect of increasing the number of periods and the consequences of stabilization during growth interruptions. Superlattices with a high crystal quality and very regular periodicity were obtained for 120-period structures, grown with DES as precursor, without intentional buffer layer. They show very narrow folded acoustical phonons in the Raman spectrum and their X-ray diffraction pattern contains not only sharp satellite peaks, whose fine structure reveals monolayer fluctuations, but also fringes which are due to interference from the entire stack.