S. Blumstengel

Visible band-gap ZnCdO heterostructures grown by molecular beam epitaxy

Single-phase ZnCdO alloys with a band gap extending from the violet to yellow spectral range are fabricated by molecular beam epitaxy using extremely low growth temperatures in conjunction with O-rich growth conditions. The Cd concentration can be systematically adjusted via the Cd∕Zn beam pressure ratio. Despite growth temperatures as low as 150°C, layer-by-layer growth is accomplished allowing for the preparation of ZnCdO∕ZnO quantum well structures. Both epilayers and quantum wells exhibit strong band-gap-related emission at room temperature in the whole composition range.

Growth of high-quality ZnMgO epilayers and ZnO∕ZnMgO quantum well structures by radical-source molecular-beam epitaxy on sapphire

We report on a specific growth procedure combining low-temperature growth of ZnMgO and postgrowth annealing at intermediate temperatures. Despite the large lattice misfit induced by the sapphire substrate, layer-by-layer growth is accomplished up to the phase-separation limit found at a c-lattice constant of 0.5136nm and Mg mole fraction of 0.40. The procedure allows us to grow quantum wells with atomically smooth interfaces in a wide range of structural designs exhibiting prominent emission features up to room temperature.

Radical-source molecular beam epitaxy of ZnMgO and ZnCdO alloys on ZnO substrates

We report on a dramatic improvement of the crystalline quality of ZnMgO and ZnCdO epilayers using Bridgman-grown ZnO substrates. ZnMgO alloys grow pseudomorphically over several 100nm and the (0002) ω-rocking curve width is as low as 19arcsec. Strain inhomogeneities in low-temperature grown ZnCdO are significantly reduced and the rocking width is lowered down to 45arcsec. Despite the high crystalline perfection, the optical properties of the films are mainly determined by their ternary character.

Visible-wavelength laser action of ZnCdO∕(Zn,Mg)O multiple quantum well structures

We report on laser action of ZnCdO∕ZnO quantum well structures up to room temperature under optical pumping. Prerequisite is a novel annealing step increasing the radiative efficiency of the low-temperature grown structures by more than one order of magnitude. The carrier states involved are localized making the lasing properties temperature robust. The longest wavelength reached so far is 490nm.

Optical gain and lasing of ZnO∕ZnMgO multiple quantum wells: From low to room temperature

Optical gain and lasing properties of ZnO∕ZnMgO multiple quantum wells with and without separate optical confinement are investigated in the temperature range from 5to290K. The data signify that localized states are crucially involved in the laser action up to room temperature. The lasing threshold increases by about one order of magnitude and reaches 140kW∕cm2 at 290K. The room temperature material gain is in the 103cm−1 range.

Interface formation and electronic structure of α-sexithiophene on ZnO

Interface formation between the organic semiconductor α-sexithiophene (6T) and polar as well as nonpolar ZnO surfaces is investigated. The growth mode of the organic layer is strongly influenced by the orientation of the ZnO surface. No indication for chemisorption of 6T on ZnO is found by photoelectron spectroscopy. The energy level alignment at the 6T/ZnO interface is of type-II facilitating electron transfer from the organic to the inorganic part and hole transfer in the other direction, rendering this heterostructure interesting for photovoltaic applications.

Extreme low-temperature molecular beam epitaxy of ZnO-based quantum structures

We report on extreme low-temperature growth of ZnO by plasma-assisted molecular beam epitaxy. Epilayers and quantum well (QW) structures with very good structural and optical properties are prepared at substrate temperatures as low as 50 °C. The growth proceeds in a single crystalline layer-by-layer mode. ZnO QWs prepared on a-plane sapphire show bright excitonic luminescence with a very narrow linewidth of only 6 meV at 5 K. High-resolution transmission electron micrographs confirm that low-temperature single crystalline growth is not restricted to a particular surface termination of ZnO but works also for crystal growth along a nonpolar direction.

Surface excitons on a ZnO (000-1) thin film

Elementary excitations at the polar (000-1) surface of a 20 nm pseudomorphically grown ZnO thin film are examined by steady state and time-resolved photoluminescence spectroscopy at low temperature. We control the density of emission centers through the deposition of prototypical organic molecules with a carboxylic acid anchor group by the Langmuir-Blodgett technique. Knowledge of the precise film thickness, defect concentrations and number density of deposited molecules leads us to associate the surface exciton emission to defect-related localization centers that are generated through a photochemical process.

Formation of CdSe quantum dots on homoepitaxial ZnSe

CdSe quantum dots were prepared by molecular beam epitaxy on homoepitaxially grown ZnSe (001) via thermally activated reorganization of an initially two-dimensional film. In spite of the difficulties related to the ZnSe substrate treatment prior to the epitaxial growth, atomically smooth sample surfaces could be achieved with increasing growth time and layer-by-layer growth was realized. The photoluminescence quantum yield of the quantum dots grown on homoepitaxial ZnSe is only slightly reduced in comparison to the standard epitaxy using GaAs as substrate. Distinct single-dot features were identified using a micro-optical setup.

ZnCdO/ZnO hetero- and quantum well structures for light-emitting applications

Molecular-beam epitaxial growth far from thermal equilibrium allows us to overcome the standard solubility limit and to alloy ZnO with CdO in strict wurtzite phase up to mole fractions of several 10%. In this way, a band-gap range extending from 3.3 eV down to 2.3 eV can be covered. Strong improvement of the crystalline quality indicated by a rocking curve width of only 45 arc sec is achieved when growing the ternary on ZnO substrates. Despite very low growth temperatures (~150 °C), layer-by-layer growth indicated and controlled by RHEED oscillations is accomplished. This enables us the fabrication of atomically smooth heterointerfaces and well-defined quantum well structures exhibiting prominent band-gap related light emission in the whole composition range. Post-growth annealing increases the radiative efficiency up to two orders of magnitude and demonstrates thermal stability of the structures with respect to phase separation even up to temperatures of about 500°C. Low-energy shifts of the photoluminescence features reaching the order of 1 eV as well as a dramatic increase of the lifetime from the sub-ns to the 100-μs time-scale uncover the presence of huge polarization-induced electric fields of some 108 V/m in ZnCdO/ZnO single quantum well structures. Carrier injection by moderate optical excitation in the 10 kW/cm2 screens these fields and recovers practically the bare quantum-confined energy transitions. On appropriately designed structures, laser action from the UV down to the green wavelength range is observed under optical pumping. The threshold at low temperature is only 60 kW/cm2 and increases only moderately up to room temperatures. All these findings make ZnO-based heterostructures a promising alternative to group-III-nitrides for opto-electronic applications in the short-wavelength range.