We investigate α-(AlxGa1-x)2O3 layers deposited by PLD for 0≤x≤1 on a- and m-plane sapphire. RSM measurements reveal a fundamental difference for these planes. Pseudomorphic α-(AlxGa1-x)2O3 on m-plane sapphire shows a shear strain e'5 along the c-axis vanishing on a-plane sapphire. Similarly, only relaxed m-plane α-(AlxGa1-x)2O3 exhibits a global lattice tilt in c-axis direction. Modeling of lattice constants and e'5 as function of x prove the shear strain to be due to the non-vanishing C14 component of the stress-strain tensor for α-(AlxGa1-x)2O3 contributing only for the m-plane. We further explain the occurrence of the lattice tilt and identify possible relaxation mechanisms.
We present κ-Ga2O3 layers grown by tin-assisted PLD on highly conductive Al-doped ZnO back contact layers. κ-Ga2O3 deposited on c-sapphire typically exhibits no lateral current flow. Significant currents can only be detected when a vertical current flow through the κ-Ga2O3 layer is enabled by the back contact confirming a strong conductivity anisotropy possibly due to suppressed transport across rotational domain boundaries. Pt/PtOx or Pd/PdOx Schottky contacts and NiO or ZnCo2O4 p-type contacts exhibit rectification ratios up to seven orders of magnitude. Further, we obtain a mean barrier height of ~2.1 eV and ideality factors as low as ~1.3 for Pt/PtOx/κ-Ga2O3 Schottky barrier diodes.
We have fabricated a novel type of ultracompact spectrometer without dispersive element(s) suitable for the use in smartphones. It comprises a waveguide with a linear photodetector array on top. The photosensitive layer features a steep chemical concentration gradient, fabricated from (MgxZn1-x)O, allowing for spectral detection in the UV. The dimensions of the device make it ultracompact with a volume of less than 1 cubic millimeter.
We investigate the specifics of trigonal materials and their heterostructures, in particular alpha-phase (corundum phase) alumina and gallia and their alloy alpha-(Al,Ga)2O3. Compared to more common hexagonal semiconductor materials (like group-III nitrides), the symmetry is reduced. Regarding the elastic properties, this leads to the occurrence of shear strains in heteroepitaxy and a difference between a- and m-planes. Particular care must be taken when evaluating Raman scattering for the determination of the Raman tensor; for thin films, also the thickness must be taken into account. Also results on electrical properties will be given.
Recently, immense interest in the semiconductor Ga₂O₃ arose due to its large bandgap and high predicted electrical breakdown field. By alloying Ga₂O₃ with In₂O₃ or Al₂O₃, its bandgap can be tuned over a large range, allowing possible applications in heterostructure devices or devices with an adjusted absorption energy. For this purpose, property screening over a large composition range is crucial.
In this contribution, we present electrical, optical and structural properties of (Ga,In)₂O₃ and (Ga,Al)₂O₃ thin films grown by continuous composition spread pulsed laser deposition. The influence of growth parameters on phase boundaries were investigated and unipolar devices were fabricated.
When investigating Schottky contacts on heteroepitaxial β-Ga2O3 thin films, several non-idealities are observed in the current voltage characteristics, which cannot be accounted for with the standard diode current models. In this article, we therefore employed a model for the rigorous calculation of the diode currents in order to understand the origin of this non-idealities. Using the model and a few parameters determined from the measurements, we were able to simulate the characteristics with good agreement to the measured data for temperatures between 30 °C and 150 °C. Fitting of the simulated curves to the measured curves allows a deeper insight into the microscopic origins of said non-idealities.
We demonstrate that exceptional points exist in fully transparent, optically ”effectively” biaxial, anisotropic micro-cavities, fabricated using an uniaxial cavity material with its axis inclined to the Bragg mirror growth direction. This is similar to the existence of singular (optic) axes in absorbing biaxial crystals, but the lack of time reversal symmetry is mediated by the mode broadening, i.e. the photon escape from the – in principle – open cavity system. As a consequence the eigenmodes are generally elliptically polarized, and completely circularly polarized eigenmodes are expected in certain directions. Via geometric and chemical composition design degrees of freedom, the spectral and angular position of these chiral modes can be rationally designed. Possible applications arise from the use of such directions for circularly polarized emission without the use of spin injection or internal or external magnetic fields. Also the coupling of such modes to excitons, adding oscillator strength to the system, seems a promising avenue of research.
The temperature dependence of diffusion length and lifetime or diffusivity of the free exciton is measured in a
commercial ZnO-substrate and in an epitaxial ZnO quantum well using nm-spatially and ps-time resolved
cathodoluminescence (CL) spectroscopy. The characteristic temperature dependence of the exciton mobility is a
fingerprint of the underlying excitonic scattering processes. Since excitons are neutral particles scattering at ionized
impurities should be not effective. With decreasing temperature diffusion lengths and lifetimes give rise to a monotonous
increase of the excitonic mobility. Two different methods for determining the excitonic transport parameters will be
presented. On the one hand we are able to perform completely pulsed excitation experiments and on the other hand a
combination of cw- and pulsed excitation in two independent measurements are needed.
Homoepitaxial ZnO thin films doped with phosphorus (0.01% to 1% P) and/or alloyed with magnesium (1% to 4% Mg) show pseudomorphic growth with compressive or tensile strain in dependence on the dopant concentration. The structural quality of the used O-face ZnO(001) substrates was inspected by the rocking curves of the symmetric (002) and the skew-symmetric (101) peaks. Preselection of the substrate batches by the supplier decreased the twist dislocation density and increased the structural homogeneity within the batches considerably. TEM cross sections show increasing density of c-plane defects with increasing phosphorus concentration in the films. ZnO(002) rocking curves of MgZnO:P films on ZnO were as narrow as 27 arcsec with a FWHM of the substrate peak of 23 arcsec. The in-plane lattice match was confirmed for all dopant concentrations by HR-XRD triple axis scans of the (002) and (101) peaks. The results show the balance between tensile strain induced by Mg and compressive strain by P in ZnO. Two-dimensional growth with terrace-like surface structure is most prominent for the Mg-alloyed films without P. High electron mobilities up to 190 cm2/Vs at 300K and up to 800 cm2/Vs at 70 K were found in the homoepitaxial MgZnO:P thin films.
An optically pumped ZnO distributed feedback laser operating at 383 nm has been designed, fabricated and
characterized. Single mode operation was observed for a wide temperature range between 10 and 270 K. In order to
avoid technologically difficult etching of ZnO, a 3rd order diffraction grating was dry-etched into an additional 120 nm-thick
Si3N4 layer deposited on the ZnO active region. The spectral linewidth of the laser emission was 0.4 nm, whereas
an optical pump threshold intensity of 0.12 MW/cm2 and a peak output power of 14 mW were seen. The temperature
tuning coefficient of the ZnO refractive index was determined from wavelength vs. temperature measurements; a value
of 9 × 10-5 K-1 was found, in good agreement with literature values.
We have investigated the morphology, crystalline quality, the transport and electronic properties of homoepitaxial ZnO
and ZnO:P thin films grown by pulsed-laser deposition. Atomic surface steps are visible for growth temperatures of
650°C and higher. The unit cell volume of undoped thin films is smaller than that of the hydrothermal substrates.
Phosphorous doping increases the unit cell volume such that a perfect lattice match is achieved for a nominal
phosphorous content of 0.01 wt.%. Undoped thin films have a net doping concentration below 1015 cm-3, whereas the
phosphorous doping increases the free electron concentration at room temperature to 1017 cm-3 and above. Temperature
dependent Hall effect measurements show that interstitial zinc with a thermal activation energy of 34 meV is a dominant
donor in homoepitaxial ZnO:P thin films. The Hall mobility of such samples is similar to ZnO single crystals grown by
seeded chemical vapor transport. Low temperature photoluminescence measurements reveal recombination of free
excitons and excitons bound to interstitial zinc and excitons bound to neutral and ionized aluminum donors. Defect
related deep luminescence is not observed for undoped homoepitaxial thin films. In contrast phosphorous doping
introduces two broad recombination bands centered at 2.9 eV and 1.9 eV.
Phosphorous-doped ZnO (ZnO:P) nanowires were prepared by a high-pressure pulsed laser deposition process. To
extend the size range of available wires, μm-thick ZnO:P microwires were grown additionally by a direct carbothermal
deposition process. Low-temperature cathodoluminescence of single ZnO:P nanowires grown by both processes exhibit
characteristic phosphorus acceptor-related peaks: neutral acceptor-bound exciton emission ((A0, X), 3.356 eV), free-electron
to neutral-acceptor emission ((e, A0), 3.314 eV), and donor-to-acceptor pair emission (DAP, ~3.24 and ~3.04
eV). This proves that stable phosphorus acceptor levels have been induced into the ZnO:P nano- and microwires. From
the quantitative evaluation of the spectroscopic features we deduct an acceptor binding energy of 122 meV. The ZnO:P
microwires were used as channels in bottom-gate field effect transistors (FET) built on Si substrates with SiO2 gate
oxide. The electrical FET-characteristics of several wires show reproducibly clear qualitative indication for p-type
conductivity for variation of gate voltage. This behavior is opposite to that of nominally undoped, n-type conducting
wires investigated for comparison. The p-type conductivity was found to be stable over more than six months.
Strong polarization coupling is expected by combining ferroelectric materials with switchable polarization and wurtzite
layers exhibiting a permanent spontaneous polarization. To demonstrate these charge coupling effects, current-voltage,
conductivity-frequency and capacitance-frequency (admittance) characteristics have been measured on epitaxial
heterostructures grown of ferroelectric BaTiO3 (001) films on conducting SrRuO3 layers on SrTiO3 (100) substrates with
oxide SrRuOx, metallic Pt and semiconducting ZnO top electrodes. The electrical measurements show clear indications
for polarization coupling of the ferroelectric perovskite BaTiO3 and the piezoelectric wurtzite ZnO thin films.
We present numerical solutions for low order hexagonal whispering gallery modes to simulate the resonant behaviour of single zinc oxide (ZnO) nanopillars. Experimental resonance spectra of such nanocavities, determined by polarization-resolved micro-photoluminescence spectroscopy, are well described by the results of our numerics. The spectral analysis yields the particular birefringence of every investigated nanopillar, consistent with current literature values for ZnO bulk material. Hence, the whispering gallery effect has been utilized to detect optical constants of individual nanostructures.
Lateral confinement for cylindrical micro-resonator light emitters improves the ratio of the number of the axial resonant modes to the number of the spontaneous emitting lateral modes. We have observed resonator-behaviour of cylindrical micro-structures, whose lateral surfaces were coated with coaxial MgO/ZrO2 and a-Si/SiOχ Braggre flectors. Glass rods with circularly shaped basal planes and ZnO wires with hexagonally shaped basal planes were used as cavity material. Bragg-reflectors were deposited using pulsed laser deposition and plasma enhanced chemical vapour deposition at the lateral surface of the ZnO wires and the glass rods. The optical properties of the Bragg-reflectors were investigated using a confocal micro-reflectometer, spatially resolved spectroscopic ellipsometry technique, and spatially resolved cathodoluminescence measurements.
Inter-sublevel transitions in InGaAs/AlGaAs quantum dots (QDs) in the mid-infrared (MIR) wavelength range are investigated by means of absorption and optically and electrically pumped emission spectroscopy. Charging dependent energy shifts of inter-sublevel transitions observed in calorimetric absorption spectra are attributed to few-particle effects in the QDs. MIR emission from near-infrared QD lasers is observed in the MIR lasing mode below threshold, which is confirmed by a theoretical modelling of such a bipolar lasing device. In contrast, spontaneous MIR emission is recorded for optically pumped Qds.
Theoretical study of threshold characteristics of a quantum dot (QD) laser in the presence of excited-state transitions is given. The effect of microscopic parameters (degeneracy factor and overlap integral for a transition) on the gain is discussed. An analytical equation for the gain spectrum is derived in an explicit form. Transformation of the gain spectrum with the injection current is analyzed. The threshold current density is calculated as a function of the total losses. The conditions for a smooth or step-like change in the lasing wavelength with the losses are formulated. Threshold characteristics of a laser based on self-assembled pyramidal InAs QDs in GaAs matrix are simulated. A small overlap integral for transitions in such QDs (and hence large spontaneous radiative lifetime) is shown to be a main possible reason for a low value of the maximum single-layer modal gain of the respective structure which is deficient to attain lasing at moderately short (several hundreds of micrometers) cavity lengths.
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