J. Woitok

Characterization of the interface abruptness of In0.53Ga0.47As/InP multi quantum wells by Raman spectroscopy, X-ray diffractometry and photoluminescence

Abstract For lattice-matched InGaAs/InP multi quantum well structures, the interface abruptness was investigated by a combination of X-ray diffractometry, Raman spectroscopy and photoluminescence. The focus was on the effects of the gas switching parameters at the InGaAs-to-InP interface, especially the PH 3 and H 2 purging times. Ternary InAsP and quaternary InGaAsP interface layers due to carry-over and exchange effects were directly identified. Their thicknesses drastically depend on the PH 3 purging time. H 2 purging affects the interface quality to some degree, but it has only minor effects on the chemical composition at the interfaces.

Optical, structural and lattice dynamical properties of epitaxial Zn1 − yMgySxSe1 − x layers grown by metalorganic vapour phase epitaxy

The optical, structural and lattice dynamical properties of Zn 1-y Mg y S x Se 1-x epilayers grown by low pressure metalorganic vapour phase epitaxy were investigated. The composition of these layers, determined by electron probe microanalysis, was varied in the range of 0≤x, y ≤ 0.2. Photoluminescence measurements in the temperature range between 14 and 300 K were performed. The alloy broadening of the bandedge emission depending on the composition of the layer was calculated on the basis of a binomial statistical model. The comparison between the measured bandedge photoluminescence halfwidth and the calculated alloy broadening showed high composition homogeneity for samples with low sulphur and magnesium contents (x, y < 0.16) and poor homogeneity for samples with large x and y. Scanning transmission electron microscopy investigations showed a low density of structural defects in samples with narrow bandedge emission and low x and y, whereas a high defect density was detected in samples with large bandedge broadening. These results are correlated to the observation of symmetric and asymmetric profile shapes of the broadened X-ray rocking curves, respectively. Four-mode behaviour of the ZnMgSSe lattice vibrations was testified bv Raman measurements.

Low pressure metalorganic vapor phase epitaxy and characterization of (Al,Ga)Sb/GaSb heterostructures

Ternary AlxGa1 − xSb layers and AlSb/GaSb multi-quantum-well structures were grown by low pressure metalorganic vapor phase epitaxy (LP-MOVPE). First, a comparative investigation on the GaSb and AlSb growth kinetics with the standard methyl and ethyl group-III precursors (TMGa, TMAl, TEGa, TEAl) was performed to establish which starting materials should be preferentially utilized. Our studies revealed that only the combination of the ethyl precursors TEGa, TEAl and TESb exhibit a well matched reactivity for the LP-MOVPE deposition of (Al,Ga)Sb/GaSb heterostructures. Ternary AlxGa1 − xSb layers (0 < x < 1) grown on GaSb substrates showed low temperature photoluminescence responses for Al contents smaller than the direct-indirect crossover concentration. Furthermore, GaSb/AlSb multi-quantum-well stacks with good structural and optical quality were deposited which could be verified by X-ray diffraction, Raman and photoluminescence measurements.

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.

Growth and characterization of Sb-based heterostructures grown by LP-MOVPE

Abstract GaSb AlSb , InAs AlSb and InAs InP 0.69 Sb 0.31 multi-quantum well structures were grown successfully by metalorganic vapor phase epitaxy. The good structural and optical quality was characterized by a combination of Raman spectroscopy and X-ray diffractometry. For the GaSb AlSb structures an unintentional incorporation of some percent Al in the GaSb well and of Ga in AlSb barrier layers was identified. At the interfaces of the material systems InAs AlSb and InAs InP 0.69 Sb 0.31 our studies revealed the existence of InAsSb layers, which are explained in terms of the surfactant effect of Sb. The thickness and composition of these interlayers depend strongly on the InAs growth rate.

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

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.

Optical investigation of Cl-doped ZnSe and ZnSxSe1 − x layers grown by metalorganic vapour phase epitaxy

Abstract We applied optical methods for a contactless analysis of the incorporation of Cl atoms and the activation of free carriers in MOVPE grown epitaxial ZnSe and ZnSe1 − xSx layers on (100) oriented GaAs. The n-type doping level was in the range of 1016 to 5 × 1017cm−3. At low temperatures we performed Raman spectroscopy to investigate the phonon properties as well as electronic excitations of the Cl atoms. For higher temperatures we analyzed the temperature dependence of the coupled plasmon-LO-phonon (PLP) modes. In addition, we applied far infrared reflectance spectroscopy under oblique incidence with p-polarized light at T = 300 K. Our results at low temperature reveal the interaction of the electronic levels of the Cl atoms for high concentrations. Furthermore, the temperature dependent PLP modes show that the thermal activation of the carriers is accompanied by a strong increase of the damping. The optically derived carrier concentration and mobility at 300 K are in good agreement with Hall data.

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.

Structural characterization of MOVPE grown ZnMgSSe/ZnSe heterostructures by HRXRD

Abstract The correlation of the properties of ZnMgSSe/ZnSe heterostructures to the Mg and S content as well as to the perpendicular strain of the ZnMgSSe layer was studied by high-resolution X-ray diffractometry (HRXRD) and photoluminescence spectroscopy (PL). ZnMgSSe/ZnSe heterostructures were grown on GaAs by MOVPE. The nominal thickness of the quaternary layer was about 250 nm. The S and Mg content ranged from about 10 to 40%. An in-depth compositional gradient was observed in the quaternary layer. Moreover, a significant broadening effect both in the X-ray rocking curve and in the PL emission was observed with increasing Mg and S contents. The FWHM of PL peaks increases nearly with the perpendicular strain of the quaternary layer.