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We studied the structural and electronic properties of Zn1−xMnxO ( 0 x 0.5 ) layers grown by peroxide molecular beam epitaxy. Hall effect measurements showed that the layers were highly resistive, pointing to strong electrical compensation of the Zn1−xMnxO films by Mn incorporation. The lattice parameter c was found to increase linearly with increasing x for low Mn concentrations and tended to saturate for heavily doped films ( x ≥ 0.2 ) , suggesting a possible second-phase precipitation. Optical transmission measurements revealed an increase in the band gap of Zn1−xMnxO and an enhancement of the broad below-band-gap absorption associated with Mn ions with increasing Mn content. However, no blueshift in the near-band-edge emission was detected in the photoluminescence spectra.

The electrical properties of high-resistivity zinc oxide (ZnO) bulk and epi-samples are strongly influenced by the sample ambient. Bulk samples that are highly resistive in ambient air can be reversibly transformed into a high conducting state under vacuum. As an explanation we suggest a conducting electron channel at the ZnO surface. Under vacuum this channel appears upon annealing. Exposure to ambient air destroys the channel. The channel is evident only for samples showing a high bulk resistivity, and it seems to be the “natural” state of the ZnO surface. We have investigated a variety of surface passivation layers and coatings in order to preserve or avoid the surface conducting channel under either environment. Appropriate coatings that preserve the surface conducting channel have been used for fabrication of MOS structures. We investigated the nature of the conducting channel by modulating the free carrier concentration at the surface.

The growth of ZnO epitaxial layers by using molecular beam epitaxy (MBE) is presented in this paper. We employed a modified Varian Gen II MBE system using H2O2 as an oxidant. ZnO layers with thickness from 100 to 600 nm were grown on (0 0 0 1)-sapphire using an MgO buffer. The surface morphology of the samples was studied by atomic force microscopy (AFM), optical microscopy with Nomarski contrast and scanning electron microscopy (SEM). The crystalline quality of the layers was investigated using X-ray diffractometry (XRD) and transmission electron microscopy (TEM). Optical properties of the ZnO layers were studied by photoluminescence (PL). The surface roughness (rms) measured by AFM for the best layers is about 0.2 nm. The formation of hexagonal ZnO pyramids was also observed in some cases. XRD measurements of the obtained ZnO layers show the excellent quality of the single crystalline ZnO heteroepitaxially grown on sapphire. The FWHM of the XRD (0 0 0 2) rocking curve measured for our best layers is as low as 27 arcsec.The influence of growth parameters on structural properties as well as on surface morphology of the zinc oxide layers on sapphire is investigated and discussed. High-quality ZnO-on-sapphire epiwafers with a very good lateral uniformity revealed both by XRD and PL measurements could provide an alternative to bulk ZnO wafers.

There have been controversial reports about the type of magnetic coupling and hence ferromagnetism in ZnMnO. We have investigated the magnetic properties of ZnxMn1−xO layers grown by Molecular Beam Epitaxy (MBE) on sapphire substrates. At first sight, the investigated ferromagnetism in our ZnMnO layers deposited on Al2O3 substrates seemed trivial, but the results of extensive analyses of the layers and also data from the Al2O3 substrates used strongly indicate that the influence of the substrate on magnetic property measurements needs to be considered when confirming ferromagnetism in Mn-doped ZnO layers. Our ZnMnO layers showed a paramagnetic behaviour.

The development of Molecular Beam Epitaxy (MBE) of ZnO epilayers employing hydrogen peroxide (H2O2) as an oxidant is presented. ZnO layers were grown on (0001)Al2O3 in a modified Varian Gen II MBE system, with H2O2 as an oxygen precursor. Layers with thicknesses from 100 nm to 500 nm were obtained. The influence of growth parameters on structural properties as well as on surface morphology of the zinc oxide layers on sapphire is investigated and discussed. The quality of the layers was improved by employing a thin MgO buffer and a subsequently grown low-temperature ZnO buffer layer followed by the main ZnO layer at higher temperature. The surface roughness of the best ZnO layers is about 0.2 nm for a 2×2 μm2 AFM scan area. X-Ray Diffractometry (XRD) measurements of the ZnO layers obtained show excellent quality of the single-crystalline ZnO heteroepitaxially grown on sapphire. The FWHM of the XRD (0002) rocking curve is as low as 30 arcsec. The excellent quality of the ZnO epiwafers provides the possibility of eventually using them as an alternative to bulk ZnO wafers.

Abtstract Various types of Mn in epitaxially grown (Ga,Mn)As have been investigated. Ionized interstitial manganese donors Mn I 2 + have been found to occur in (Ga,Mn)As at dopant concentrations as low as 0.5%. A comparison with spectra from interstitial Mn I 2 + inside bulk-doped GaAs:Mn yields a slight decrease of 1.5% in the hyperfine splitting with increasing dopant concentration. This is attributed to an increase in the lattice constant of (Ga,Mn)As with increasing manganese concentration. Contrary to Mn interstitials, Mn dopants on Ga lattice sites acts as acceptors. It is shown that Mn on Ga lattice sites Mn Ga 2 + is non-uniformly distributed. A low percentage of isolated Mn acceptors can be distinguished from the exchange broadened Mn Ga 2 + signal. Thus, electron paramagnetic resonance is a promising tool for the investigation of (Ga,Mn)As and the classification of various types of Mn dopants responsible for the magnetic properties of (Ga,Mn)As.

In the last two decades, diluted magnetic semiconductors have attracted great attention as promising materials for spintronics applications. [K. Sato, H. Katyama-Yoshida, Jpn. J. Phys., Part 2 39 (2000) L555] theoretically predicted that ZnO doped with V, Cr, Fe, Co, and Ni can be ferromagnetic. This has been recently confirmed experimentally for vanadium doped ZnO films which were grown on sapphire substrates, using laser deposition technique [H. Saeki, H.N. Tabata, T. Kawai, Solid State Commun. 120 (2001) 439]. In the present work, high-dose vanadium implantation was used to produce Zn 1− x V x O ( x ∼ 0.10) thin film structures (250 nm thick) that had been epitaxially grown on sapphire substrates. Implantation with the dose 2 × 10 16 cm −2 was performed to reach a maximum vanadium concentration of 10 at%. To avoid ZnO film amorphization due to radiation damage accumulation [S.O. Kucheyev, J.S. Williams, C. Jagadish, J. Zou, C. Evans, A.J. Nelson, A.V. Hamza, Phys. Rev. B 67 (2003) 094115], all implants were done at elevated temperatures 300 and 400 °C and ion current density 10 μA/cm 2 . X-ray diffraction, SIMS and photoluminescence techniques were exploited to study the implanted samples. No luminescence was observed in the implanted samples after implantation procedures. However, annealing at 800 °C for 30 min gave rise to ZnO crystal structure improvement. This implies that healing of implantation induced defects is possible even after heavy-ion bombardment. As a result, the photoluminescence peak at 3.359 eV related to the donorbound exiton was detected.

We report on an experimental study of the temporal photoluminescence dynamics of high-quality ZnO nanopillars from 10 K to room temperature. We find that defect states play an important role in the time evolution of the photoluminescence signal. At low excitation intensities capture into defects dominates the time dependence of the PL, at higher intensities they are saturated and the intrinsic excitation decay is observed. We separate the intrinsic exciton decay from the fast nonlinear M-band with the method of decay associated spectra and obtain the temperature dependence of the intrinsic exciton decay. High excitation measurements show a reduced exciton–exciton scattering in these thin nanorods.

We report on the studies of linearly polarized photoluminescence (PL) in a (0001) oriented ZnO epitaxial film, grown by metal organic chemical vapor deposition on a GaN template. The emission of mixed longitudinal-transverse exciton polariton modes was observed up to 130 K that evidences polaritonic nature of the excitonic spectrum up to this elevated temperature.

The rapidly expanding research in Spintronics, the electronics utilizing the electron spin instead of its charge, is driven by the very interesting potential applications. The actual task is to develop principles for the spin manipulations in spintronic devices. In this Report we suggest and verify experimentally a concept of heteromagnetic semiconductor structures. It is based on spin diffusion between layers of the nanostructure with different magnetic properties and allows controlling the spin-switching rate for magnetic ions. A ten times increase of spin-lattice relaxation rate of magnetic Mn-ions is achieved in (Zn,Mn)Se/(Be,Mn)Te heteromagnetic structures with an inhomogeneous distribution of Mn-ions. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

ZnO layers were grown on (0 0 0 1) GaN/sapphire substrates by molecular beam epitaxy (MBE) with the use of hydrogen peroxide as an oxidant. Single-crystal ZnO films were found to grow at substrate temperatures from 400°C to 650°C, whereas textured ZnO layers were prepared at the temperatures below 400°C. Structural and optical properties as well as surface morphology of the single-crystal ZnO films were investigated as a function of growth parameters. The single-crystal material showed strong bound-exciton (BE) emission with a full-width at half-maximum down to 4.7 meV. The increase in both substrate temperature and Zn flux was found to result in higher crystal perfection and stronger BE emission. The data obtained prove the feasibility of the peroxide-assisted MBE for the growth of high-quality ZnO layers.

The tellurium ad- and de-sorption on the Ge(0 0 1) surface have been studied using high-resolution low energy electron diffraction and reflection high-energy electron diffraction. Upon adsorption of one monolayer of Te at 280 °C a streaky (2 × 1) reconstruction occurs, which can be explained by a missing-row model. Upon annealing the surface transforms to a c(2 × 2)-(2 × 1) mixed reconstruction. A splitting of the superstructure spots was found which indicates an ordered arrangement of strip-like domains on the surface. We address the question of both the atomic and the domain structure of the Te-adlayer prior to desorption and present a model for the entire desorption process. Furthermore, a (1 1 3)-faceting of the (0 0 1) surface which occurs upon Te-adsorption at a higher substrate temperature (380 °C) was investigated.

In this contribution we analyse the structural and optical properties of ZnO as well as ZnMgO nanopillars grown catalyst-free by metalorganic vapour-phase epitaxy. The nanostructures were grown directly onto different substrate materials with various orientations. The nanopillars deposited on a-plane sapphire show the best vertical c-axis alignment and have a typical diameter of about 50 nm and a height of several micrometres, depending on growth time. We achieved well ordered, almost completely c-axis oriented pillars, as confirmed by scanning electron microscopy and high resolution x-ray diffraction. Photoluminescence measurements revealed very narrow donor-bound exciton emission lines with half widths as small as 0.5 meV. In order to investigate the possibility of a combination of band gap engineering and nanopillar growth, ZnMgO nanopillars were also grown. The Mg incorporation was confirmed by photoluminescence measurements and a blue shift of the band gap of up to 170 meV could be achieved for the nanopillars with the highest Mg concentration.

Ga1−xMn x As layers with Mn fractions 0 ≤ x ≤ 2.8 % grown on GaAs(001) substrates by low-temperature molecular beam epitaxy were investigated using micro-Raman spectroscopy and far-infrared (FIR) reflectance spectroscopy. The Raman and FIR spectra are strongly affected by the formation of coupled modes of longitudinal optical phonons and hole plasmons. A full line-shape analysis of the spectra was performed within a Lindhard–Mermin model for the dielectric function, including intraband and interband hole transitions. Annealing at temperatures between 250 and 500°C results in a decrease of the hole density with increasing annealing temperature and total annealing time. Simultaneously, a reduction of the number of Mn atoms on Ga lattice sites is deduced from high-resolution X-ray diffraction. After annealing at 450°C the Raman lines of elemental As are observed, which are due to the precipitation of As on the sample surface.

Due to its band gap of 3.3 eV and exciton binding energy of 60 meV, ZnO is a very interesting candidate for UV optoelectronics. Beyond that, the incorporation of magnetic impurities allows fabrication of semimagnetic semiconductors, which are potentially ferromagnetic at room temperature. Here, we report on our results of the MOVPE growth of ZnO thins films grown on various substrates using i-PrOH and t-BuOH as oxygen precursors. Despite the fact that the lattice constant of ZnO is larger than that of GaN, ZnO films are often tensile strained. One reason for that is the difference in thermal expansion of ZnO relative to the substrate material. The low-temperature photoluminescence of ZnO layers is dominated by donor-bound excitons, with a half width below 1 meV at low temperatures for optimized growth parameters. By a thorough analysis of the strain state by reciprocal space mapping the Poisson number of ZnO is derived. The shift of the excitonic features (A- and B-excitons) as function of strain has been measured.

Gallium and nitrogen ions have been implanted into ZnO crystals and metal organic vapor phase epitaxy grown ZnO layers. Postimplantation annealing behavior in the temperature range between 200 and 900 °C has been studied by means of Raman scattering and low-temperature photoluminescence. The temperature for healing of the implantation-induced defects was found to be 800 °C. Implanted gallium acts as donor with a donor binding energy ED of 53 meV, thus allowing the control of n-type doping in ZnO. From photoluminescence measurements of the donor-acceptor pair transition of a series of nitrogen-implanted ZnO samples we estimate the binding energy EA of the nitrogen acceptor between 163 and 196 meV. Electrical characterization of nitrogen-implanted samples shows a behavior ranging from low n-type to highly compensated. But no unambiguous and reproducible type conversion could be achieved.

We present measurements of a single-particle transport spectrum of a vertical GaAs/AlGaAs resonant tunneling device at high magnetic fields. We investigated the spin splitting of several quantum dot levels. A detailed analysis of the spin-resolved local density of states fluctuations revealed strong correlations in their pattern underlining the origin of the different energy levels belonging to the same spectrometer probing the local disorder of the emitter.

In this paper we give a brief overview of our studies on dynamical processes in diluted-magnetic-semiconductor heterostructures based on (Zn,Mn)Se and (Cd,Mn)Te. Presence of free carriers is an important factor which determines the energy- and spin transfer in a coupled systems of magnetic ions, lattice (the phonon system) and carriers. We report also new data on dynamical response of magnetic ions interacting with photogenerated electron-hole plasma. (Zn,Mn)Se/(Zn,Be)Se structures with relatively high Mn content of 11% provide spin-lattice relaxation time of about 20 ns, which is considerably shorter then the characterictic times of nonequilibrium phonons ranging to 1 μs. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigate ZnO epitaxial layers grown by MBE (Molecular Beam Epitaxy) and MOVPE (Metal Organic Vapor Phase Epitaxy) techniques. The samples show similar optical behavior in temperature dependent photoluminescence measurements, reflection and photoluminescence excitation spectroscopy in the low density regime. High excitation measurements show different behavior. While the MBE sample leads to stimulated emission from the exciton-exciton-scattering, an electron hole plasma is formed in the MOVPE sample which leads to stimulated emission at higher excitation intensities. The gain value measured by the variable stripe length method is much higher for the MBE grown sample.

The effect of short-time and long-time annealing at 250C on the conductivity, hole density, and Curie temperature of GaMnAs single layers and GaMnAs/InGaMnAs heterostructures is studied by in-situ conductivity measurements as well as Raman and SQUID measurements before and after annealing. Whereas the conductivity monotonously increases with increasing annealing time, the hole density and the Curie temperature show a saturation after annealing for 30 minutes. The incorporation of thin InGaMnAs layers drastically enhances the Curie temperature of the GaMnAs layers., 4 pages, 6 figures, submitted to Physica E

We report on the investigations of in-plane optical anisotropy in non-common-atom heterostructures: ZnSe/BeTe perfect quantum wells (QWs) and CdSe/BeTe rough QWs and quantum dots. A noticeable linear polarization of photoluminescence (PL) with respect to the in-plane [1–10] and [110] crystal axes was observed in the ZnSe/BeTe QWs with equivalent ZnTe-type interfaces due to the reduction of QW symmetry, induced by unintentional formation of BeSe chemical bonds at a “BeTe-ZnSe” interface. The BeSe bond concentration and, hence, the polarization degree depend on the Te/Be flux ratio during molecular beam epitaxy growth of the samples. Strongly linearly polarized (up to 80%) PL was detected in the CdSe/BeTe structures, evidencing QW-like flat symmetry of the emitting sites of carrier localization.

The II–VI compound semiconductor ZnO is already widely used for various applications such as UV resistive coatings, gas sensors and surface acoustic wave devices. Recently, the optoelectronic properties of ZnO are attracting increasing interest, which has been initiated by first reports on p-type doping. ZnO appears especially favorable for such applications because of its large exciton binding energy of about 60 meV, its low-power thresholds for optical pumping at room temperature and the availability of bulk ZnO substrates. Here, we report on the metal-organic vapor phase epitaxy growth of ZnO layers using diethylzinc as the zinc source and iso-propanol or tertiary-butanol as the oxygen precursors. The choice of an appropriate oxygen precursor is of great importance to minimize parasitic prereactions in the gas phase, that have a negative influence on layer quality. Epitaxial growth is conducted in a water-cooled, RF-heated horizontal quartz glass reactor. Growth rates up to 2.6 μm have been accomplished. The influence of the growth parameters as substrate temperature, reactor pressure, gas flow rates, and especially the impact of the II/VI-ratio on growth rates and quality of the epitaxial material will be discussed.

We report on ferromagnetic resonance measurements of Ga1−x Mn x As thin films with Mn contents 0.022 ≤ x ≤ 0.051. For x ≥ 0.036 and the external magnetic field normal to the thin film, we observe several resonances, which we identify as spin wave resonances. The non-quadratic mode spacing can be consistently explained by a linear gradient in the magnetic properties of the films. From the measurements, the exchange constant A can be deduced for different Mn contents x.

For spin injection, detection and manipulation, diluted magnetic semiconductors are interesting materials. GaMnAs is one of these candidates, though the ferromagnetism occurs only at temperatures below 110 K. In order to possibly increase the Curie temperature, nitride based III–V semiconductors have been proposed. Here, we report results on the structural, magnetic and electronic properties of Ga1−xMnxNyAs1−y. In GaMnNAs, the valence band maximum is supposed to increase in energy with N concentration, and since Mn is a relative deep acceptor with an activation energy of 110 meV, this would lead to a reduction of the activation energy and hence to a higher doping level. GaMnNAs thin films have been grown by low-temperature molecular-beam epitaxy. We present experimental data on the transport properties of this compound, indicating that it is ferromagnetic at low nitrogen concentrations. Samples with higher N concentration show a decrease of the Curie temperature or even no ferromagnetism at all and a high ohmic resistance compared to GaMnAs reference layers. This is most probably due to a compensation of the holes, which are needed for the ferromagnetic coupling.

In this paper we report on the metal-organic chemical vapour deposition of ZnO layers. We focus on heteroepitaxial growth on c-plane Al 2 O 3 and the influence of the VI/II ratio during growth on the properties of the layers. The layer quality has been investigated by HRXRD, PL and reflectivity measurements. Under optimized growth conditions the photoluminescence is dominated by strong near band edge emission lines with half widths below 4 meV and the excitonic signals are clearly visible in reflectivity measurements. Hall effect measurements indicate an n-type background doping in the 10 17 cm -3 range, and mobilities of more than 100 cm 2 /Vs can be reached. Moreover, a post-growth annealing step is found to improve the quality of the layers grown under suboptimal conditions. The problem of strain will be addressed and a tensile strain can be confirmed in the heteroepitaxial ZnO layers.

Ga 1− x Mn x As layers with Mn concentrations 0%⩽ x ⩽2.7% grown on GaAs (0 0 1) substrates by low temperature molecular beam epitaxy were studied using micro-Raman spectroscopy. The layers were identified as p-type and an estimate of the hole densities was made by a full line shape analysis of the coupled plasmon-LO-phonon modes. The line width of the TO-phonon mode as a function of the Mn concentration was determined by spatially resolved measurements on cleaved (1 1 0) side faces.

Properties of interfaces with no-common atom in type-II ZnSe/BeTe heterostructures are studied with polarization spectroscopy. Structures with four possible configurations of normal and inverted interfaces have been investigated. Radiative recombination and its polarization anisotropy have been found to depend crucially on the interface configuration and excitation power. The comparison of interfaces formed by the growth on anion (Se, Te) and cation (Zn, Be) terminated layers has shown that the latter demonstrate a significantly higher nonradiative recombination rate. In agreement with the quantum well (QW) symmetry, the photoluminescence (PL) of the structures with nonequivalent normal and inverted interfaces is highly linearly polarized both at low and high excitation densities. Unexpectedly, a similarly strong PL polarization has been found for structures with equivalent interfaces up to carrier densities of 1012 cm−2 per QW. The polarization is explained by a built-in electric field, it decreases with...

We studied the diffusion of carriers in dilute magnetic semiconductors for conditions when an additional force caused by a spatially inhomogeneous exchange potential is active. The spatially inhomogeneous exchange potential arises from a creation of photocarriers in the Zn 0.988 Mn 0.012 Se/ Zn 0.94 Be 0.06 Se quantum well. The photocarriers enhance the spin temperature of the Mn ions by a spin-flip scattering with the magnetic ions, which results in a reduction of the giant Zeeman splitting. The gaussian like intensity profile of the exciting laser beam causes a locally varying exchange potential for the carriers. A strong influence of the gradient in the exchange potential on the diffusion of the carriers was found. The diffusion of the carriers was examined by a spatially resolved photoluminescence technique.

Magnetic properties of two new classes of highly p-type doped II-VI diluted magnetic semiconductors (DMS): thin layers of Be 1-x Mn x Te:N and bulk Zn 1-x Mn x Te:P are presented. Despite the fact that the samples are insulating, we observe clear indications of the low temperature ferromagnetic correlation, namely: (i) positive Curie-Weiss temperature (T CW 2 K), and (ii) open hysteresis loops at temperatures below T CW . A qualitative analysis of the results favours those models of the carrier-mediated ferromagnetism in DMS which allow for the coupling of only a part of the available spins.

The polarization properties of indirect photoluminescence (PL) in ZnSe/BeTe quantum well structures with a type-II band alignment were studied experimentally and analyzed theoretically. The linear polarization degree appears with increasing density of photoexcitation. It is caused by an electric field along the structure growth axis induced by a spatial separation of photocarriers. The PL circular polarization degree was induced by magnetic fields applied along the structure growth axis. Its maximum value was limited by the in-plane optical anisotropy of the optical matrix elements at the type-II heterointerface. In-plane oriented magnetic fields induce a linear polarization degree that is determined by a non-zero in-plane component of the heavy-hole g-factor in the BeTe layers.

Resonant Raman scattering and photoluminescence were studied in semimagnetic CdSe/ZnSe structures with self-organized flat quantum discs of CdZnMnSe composition. Different kinds of magnetic excitations were observed in Raman scattering under sharp resonance with excitons localized in the discs. They include spin-flip transitions from donor-bound electrons in the discs and collective multiple spin-flip transitions in the system of Mn 2+ ions.

This paper reports on the observation of optical gain due to charged excitons (trions) in quantum wells. The specific optical coupling of the trion gives rise to stimulated emission on the low-energy wing of the trion photoluminescence line without degeneracy and inversion in the total particle numbers. Gain values as large as 10 4 cm -1 are found for excitation intensities of a few kW/cm. Higher injection levels lead to carrier heating which limits the available gain. A calculation of the absorption-gain crossover based on a kinetically determined equilibrium of excitons, trions and electrons with a common carrier gas temperature describes the experimental data well.

Properties of interfaces with no-common atom in type-II ZnSe/BeTe heterostructures are studied with polarization spectroscopy. Radiative recombination and its polarization anisotropy have been found to depend crucially on the interface configuration and excitation power. Effects of external and internal electric fields on the in-plane polarization are studied both numerically and experimentally at low and high levels of photoexcitation density.

We report on optical and structural studies of the interface symmetry in CdSe/BeTe multiple-layer structures containing self-assembled quantum dots. Temperature and decay behavior of the broad photoluminescence (PL) band is consistent with the type-II transitions involving deeply localized electron states. Large linear in-plane polarization of the PL (up to 80%) is observed, implying the C-2v (or lower) symmetry of the individual places of the electron localization.

Properties of a two-dimensional electron gas with a density of 5×10 11 cm −2 in a ZnSe/(Zn,Be,Mg)Se quantum well structure have been investigated by means of polarized magneto-luminescence in fields up to 47 T and by an optically detected resonance (ODR) technique. With increasing magnetic field, the optical spectra of the electronic system changes its appearance from Landau-level-like with a linear shift of lines, to excitonic-like with a diamagnetic shift. The transition occurs at filling factor 2. Pronounced oscillatory features at even filling factors are found for the luminescence intensity and polarization degree, the energy of optical transitions, and in ODR signal.

Three dimensional GaN structures with different crystal facets and doping types have been investigated employing the surface photo-voltage (SPV) method to monitor illumination-induced surface charge behavior using Kelvin probe force microscopy. Various photon energies near and below the GaN bandgap were used to modify the generation of electron–hole pairs and their motion under the influence of the electric field near the GaN surface. Fast and slow processes for Ga-polar c-planes on both Si-doped n-type as well as Mg-doped p-type GaN truncated pyramid micro-structures were found and their origin is discussed. The immediate positive (for n-type) and negative (for p-type) SPV response dominates at band-to-band and near-bandgap excitation, while only the slow process is present at sub-bandgap excitation. The SPV behavior for the semi-polar facets of the p-type GaN truncated pyramids has a similar characteristic to that on its c-plane, which indicates that it has a comparable band bending and no strong influence of the polarity-induced charges is detectable. The SPV behavior of the non-polar m-facets of the Si-doped n-type part of a transferred GaN column is similar to that of a clean c-plane GaN surface during illumination. However, the SPV is smaller in magnitude, which is attributed to intrinsic surface states of m-plane surfaces and their influence on the band bending. The SPV behavior of the non-polar m-facet of the slightly Mg-doped part of this GaN column is found to behave differently. Compared to c- and r-facets of p-type surfaces of GaN-light–emitting diode micro-structures, the m-plane is more chemically stable.

Lattice-polarity dependence of InN surface photovoltage has been identified by an anomalous surface potential behavior observed via photoassisted Kelvin probe force microscopy. Upon above bandgap light illumination in the ambient atmosphere, the surface photovoltage of the In-polar InN shows a pronounced decrease, while that of the N-polar one keeps almost constant. Those different behaviors between N-polar and In-polar surfaces are attributed to a polarity-related surface reactivity, which is found not to be influenced by Mg-doping. These findings provide a simple and non-destructive approach to determine the lattice polarity and allow us to suggest that the In-polar InN, especially that with buried p-type conduction, should be chosen for sensing application.

Vertically aligned gallium nitride (GaN) nanowire (NW) arrays have attracted a lot of attention because of their potential for novel devices in the fields of optoelectronics and nanoelectronics. In this work, GaN NW arrays have been designed and fabricated by combining suitable nanomachining processes including dry and wet etching. After inductively coupled plasma dry reactive ion etching, the GaN NWs are subsequently treated in wet chemical etching using AZ400K developer (i.e., with an activation energy of 0.69 ± 0.02 eV and a Cr mask) to form hexagonal and smooth a-plane sidewalls. Etching experiments using potassium hydroxide (KOH) water solution reveal that the sidewall orientation preference depends on etchant concentration. A model concerning surface bonding configuration on crystallography facets has been proposed to understand the anisotropic wet etching mechanism. Finally, NW array-based vertical field-effect transistors with wrap-gated structure have been fabricated. A device composed of 99 NWs exhibits enhancement mode operation with a threshold voltage of 1.5 V, a superior electrostatic control, and a high current output of >10 mA, which prevail potential applications in next-generation power switches and high-temperature digital circuits.

We present a novel way for depositing material on the top surface of three-dimensional (3D) structures. This process supplies the possibility of applying a wide range of materials on the top of structures which are too high to be processed with conventional techniques. A planarization process or vertical sidewalls of the structures are not necessary.

The main challenge in fabrication of ZnO-based devices is the absence of reliable p-type material. This is mostly caused by insufficient crystalline quality of the material and not well-enough-developed native point defect control of ZnO. At present high-quality ZnO wafers are still expensive and ZnO heteroepitaxial layers on sapphire are the most reasonable alternative to homoepitaxial layers. But it is still necessary to improve the crystalline quality of the heteroepitaxial layers. One of the approaches to reduce defect density in heteroepitaxial layers is to introduce a strained-layer superlattice (SL) that could stop dislocation propagation from the substrate-layer interface. In the present paper we have employed fifteen periods of a highly strained SL structure. The structure was grown on a conventional double buffer layer comprising of high-temperature MgO/low-temperature ZnO on sapphire. The influence of the SLs on the properties of the heteroepitaxial ZnO layers is investigated. Electrical measurements of the structure with SL revealed very high values of the carrier mobility up to 210 cm 2 /Vs at room temperature. Structural characterization of the obtained samples showed that the dislocation density in the following ZnO layer was not reduced. The high mobility signal appears to come from the SL structure or the SL/ZnO interface.

Polarity dependence (N-polar (000-1) and Ga-polar (0001)) of surface photovoltage of epitaxially grown, vertically aligned GaN nanorods has been investigated by photo-assisted Kelvin probe force microscopy (KPFM). Commercial GaN substrates with known polarities are taken as reference samples. The polarity of GaN substrates can be well distinguished by the change in surface photovoltage upon UV illumination in air ambient. These different behaviors of Ga- and N-polar surfaces are attributed to the polarity-related surface-bound charges and photochemical reactivity. GaN nanorods were grown on patterned SiO2/sapphire templates by metal-organic vapor phase epitaxy (MOVPE). In order to analyze the bottom surface of the grown GaN nanorods, a technique known from high power electronics and joining techniques is applied to remove the substrate. The top and bottom surfaces of the GaN nanorods are identified to be N-polar and Ga-polar according to the KPFM results, respectively. Our experiments demonstrate that KPFM is a simple and suitable method capable to identify the polarity of GaN nanorods. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present a numerical optimization of nanorod geometries with respect to the optical properties of an electrically driven LED emitting in the green spectral range. It is shown that an overall Purcell enhancement as well as directional emission can be achieved at an emission wavelength of 550 nm with nanorods of 110 nm radius. Position-controlled growth on patterned substrates demonstrates that the required dimensions are accessible by varying growth parameters and growth time in a large volume MOVPE reactor. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Selective area growth of GaN nanorods by metalorganic vapor phase epitaxy is highly demanding for novel applications in nano-optoelectronic and nanophotonics. Recently, we report the successful selective area growth of GaN nanorods in a continuous-flow mode. In this work, as examples, we show the morphology dependence of GaN rods with µm or sub-µm in diameters on growth conditions. Firstly, we found that the nitridation time is critical for the growth, with an optimum from 90 to 180 seconds. This leads to more homogeneous N-polar GaN rods growth. A higher temperature during GaN rod growth tends to increase the aspect ratio of the GaN rods. This is due to the enhanced surface diffusion of growth species. The V/III ratio is also an important parameter for the GaN rod growth. Its increase causes reduction of the aspect ratio of GaN rods, which could be explained by the relatively lower growth rate on (000-1) N-polar top surface than it on {1-100} m-planes by supplying more NH3 (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Energy conversion technologies are aiming to extremely high power capacities per year. Nontoxicity and abundance of the materials are the key requirements to a sustainable photovoltaic technology. Oxides are among the key materials to reach these goals. We investigate the influence of thin buffer layers on the performance of an ZnO:Al/buffer/Cu2O solar cells. Introduction of a thin ZnO or Al2O3 buffer layer, grown by thermal ALD, between ZnO:Al and Cu2O resulted in 45% increase of the solar cell efficiency. VPE growth of Cu2O employing elemental copper and pure oxygen as precursor materials is presented. The growth is performed on MgO substrates with the (001) orientation. On- and off- oriented substrates have been employed and the growth results are compared. XRD investigations show the growth of the (110) oriented Cu2O for all temperatures, whereas at a high substrate temperature additional (001) Cu2O growth occurs. An increase of the oxygen partial pressure leads to a more pronounced 2D growth mode, whereby pores between the islands still remain. The implementation of off-axis substrates with 3.5° and 5° does not lead to an improvement of the layer quality. The (110) orientation remains predominant, the grain size decreases and the FWHM of the (220) peak increases. From the AFM images it is concluded, that the (110) surface grows with a tilt angle to the substrate surface.

3D single-crystalline, well-aligned GaN-InGaN rod arrays are fabricated by selective area growth (SAG) metal-organic vapor phase epitaxy (MOVPE) for visible-light water splitting. Epitaxial InGaN layer grows successfully on 3D GaN rods to minimize defects within the GaN-InGaN heterojunctions. The indium concentration (In ∼ 0.30 ± 0.04) is rather homogeneous in InGaN shells along the radial and longitudinal directions. The growing strategy allows us to tune the band gap of the InGaN layer in order to match the visible absorption with the solar spectrum as well as to align the semiconductor bands close to the water redox potentials to achieve high efficiency. The relation between structure, surface, and photoelectrochemical property of GaN-InGaN is explored by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES), current-voltage, and open circuit potential (OCP) measurements. The epitaxial GaN-InGaN interface, pseudomorphic InGaN thin films, homogeneous and suitable indium concentration and defined surface orientation are properties demanded for systematic study and efficient photoanodes based on III-nitride heterojunctions.

Organic-inorganic hybrid gas sensors can offer outstanding performance in terms of selectivity and sensitivity towards single gas species. The enormous variety of organic functionalities enables novel flexibility of active sensor surfaces compared to commonly used pure inorganic materials, but goes along with an increase of system complexity that usually hinders a predictable sensor design. In this work, an ultra-selective NO2 sensor is realized based on self-assembled monolayer (SAM)-modified semiconductor nanowires (NWs). The crucial chemical and electronic parameters for an effective interaction between the sensor and different gas species are identified using density functional theory simulations. The theoretical findings are consistent with the experimentally observed extraordinary selectivity and sensitivity of the amine-terminated SnO2 NW towards NO2. The energetic position of the SAM-gas frontier orbitals with respect to the NW Fermi level is the key to ensure or impede an efficient charge transfer between the NW and the gas. As this condition strongly depends on the gas species and the sensor system, these insights into the charge transfer mechanisms can have a substantial impact on the development of highly selective hybrid gas sensors.

In n-type GaN, an upward band bending of about 1 eV is caused by negative charge at the surface. UV light reduces the band bending by creating a surface photovoltage (SPV), which can be measured by a Kelvin probe. Previously, we reported a fast SPV signal of about 0.6 eV in undoped and moderately doped GaN. In this work, we have studied degenerate GaN co-doped with Zn and Si, with a Si concentration of about 1019 cm−3 and a Zn concentration of 6×1017 cm−3. At room temperature, a fast component of about 0.6 eV was observed. However, after preheating the sample at 600 K for one hour and subsequently cooling the sample to 300 K (all steps performed in vacuum), the fast component disappeared. Instead, a very slow (minutes) and logarithmic in time rise of the SPV was observed with UV illumination. The total change in SPV was about 0.4 eV. This slow SPV transient can be reversibly converted into the “normal” fast (subsecond) rise by letting air or dry oxygen in at room temperature. Possible explanations of the ...