H. Hardtdegen

MOVPE growth of GaAs using a N2 carrier

We have studied LP-MOVPE (low pressure metalorganic vapor phase epitaxy) growth of GaAs in a N2 carrier. Growth temperature, gas velocity, reactor pressure and V/III ratio were varied successively in order to attain an optimum set of growth parameters. Samples with excellent morphology and optical properties comparable to H2 grown samples were obtained. Intentionally Si-doped material with an electron concentration of 5.3 × 1014 cm-3 showed Hall mobilities of 58,340 and 7530 cm2/V·s at 77 and 300 K, respectively. Growth investigations on SiO2 masked substrates were carried out to test the suitability of the N2 process for selective growth. Selective growth was possible in a large parameter range. Furthermore a set of parameters was developed with which only growth parallel to the (100) substrate surface is observed.

Effect of Si-doping on InAs nanowire transport and morphology

The effect of Si-doping on the morphology, structure, and transport properties of nanowires was investigated. The nanowires were deposited by selective-area metal organic vapor phase epitaxy in an N2 ambient. It is observed that doping systematically affects the nanowire morphology but not the structure of the nanowires. However, the transport properties of the wires are greatly affected. Room-temperature four-terminal measurements show that with an increasing dopant supply the conductivity monotonously increases. For the highest doping level the conductivity is higher by a factor of 25 compared to only intrinsically doped reference nanowires. By means of back-gate field-effect transistor measurements it was confirmed that the doping results in an increased carrier concentration. Temperature dependent resistance measurements reveal, for lower doping concentrations, a thermally activated semiconductor-type increase of the conductivity. In contrast, the nanowires with the highest doping concentration show a...

Spin-orbit coupling and phase-coherence in InAs nanowires

We investigated the magnetotransport of InAs nanowires grown by selective area metal-organic vapor phase epitaxy. In the temperature range between 0.5 and 30 K reproducible fluctuations in the conductance upon variation of the magnetic field or the back-gate voltage are observed, which are attributed to electron interference effects in small disordered conductors. From the correlation field of the magnetoconductance fluctuations the phase-coherence length lis determined. At the lowest temperatures lis found to be at least 300 nm, while for temperatures exceeding 2 K a monotonous decrease of lwith temperature is observed. A direct observation of the weak antilocalization effect indicating the presence of spin-orbit coupling is masked by the strong magnetoconductance fluctua- tions. However, by averaging the magnetoconductance over a range of gate voltages a clear peak in the magnetoconductance due to the weak antilocalization effect was resolved. By comparison of the experimental data to simulations based on a recursive two-dimensional Greens function approach a spin-orbit scattering length of approximately 70 nm was extracted, indicating the presence of strong spin-orbit coupling.

Characterization of Interface Structure in GalnAs/lnP Superlattices by means of X-Ray Diffraction

Ga-In-As-P/InP superlattice structures grown by low-pressure metalorganic vapour phase epitaxy (LP-MOVPE) were analysed with high-resolution X-ray diffraction (HR-XRD). The XRD pattern of a Ga0.46In0.54As/InP multiple quantum well sample shows up to two orders in magnitude more intensity than is expected according to a simulation assuming abrupt transitions. The strong intensity of the high order satellite peaks is explained with a simple model based on discontinuity in the lattice constant at the interfaces themselves. Remaining deviations lead to the assumption of compositional grading near the interfaces. A carry-over of As into InP as well as of P into GaInAs on a scale of some nm can clearly be shown, whereas no hints for comparable problems with the group III elements are found. The results show the great sensitivity of X-ray diffraction to compositional changes when strain occurs.

On the magnetic properties of Gd implanted GaN

The wurzite type gallium nitride doped by gadolinium, Ga1−xGdxN (x∼0.01–0.07), was prepared by Gd ion implantation of the parent GaN thin films deposited on sapphire substrates. The material obtained exhibits a weak ferromagnetism (FM) persisting up to 700K. At higher Gd concentrations, the minute FM component coexists with much more pronounced Curie-type paramagnetism. In a dilute limit (x⩽0.01), the latter part is substantially reduced and the saturated FM moment reaches the value M∼2μB∕Gd atom.

Internal strains and crystal structure of the layers in AlGaN/GaN heterostructures grown on a sapphire substrate

In this paper, we investigate the structural properties of AlGaN/GaN heterostructures grown by metal organic chemical vapor deposition on sapphire substrates with different thicknesses using high-resolution x-ray diffraction and Raman scattering methods. We discuss the microscopic nature of spatial-inhomogeneous deformations and dislocation density in the structures. Microdeformations within mosaic blocks and the sizes of regions of coherent diffraction are determined. We reveal a gradient depth distribution of deformations in the mosaic structure of nitride layers, as well as at the interface regions of the sapphire substrate on the microscale level using confocal micro-Raman spectroscopy. We determine that an increase in substrate thickness leads to a reduction in dislocation density in the layers and an increase in the elastic deformations. The features of the block structure of nitrides layers are shown to have a significant influence on their elastic properties.

Modeling and experimental verification of deposition behavior during AlGaAs growth: a comparison for the carrier gases N2 and H2

Abstract A modeling and experimental study is carried out to understand why low-pressure metalorganic vapor-phase epitaxy (LP-MOVPE) of AlGaAs in nitrogen atmosphere differs from that in hydrogen in a horizontal tube type of reactor. To this end flow, heat transfer as well as the key chemical species’ mass transport are considered. The increased uniformity in N 2 atmosphere is related to the higher molecular weight and, therefore to the higher gas density of the carrier resulting in a flow structure that is more favorable for improved growth rate uniformity of AlGaAs on the substrate. Due to the so called “cold finger” [L. Stock, W. Richter, J. Crystal Growth 77 (1986) 144; D.F. Fotiadis, M. Boekholt, K.F. Jensen, W. Richter, J. Crystal Growth 100 (1990) 577.] effect as well as the enhanced inertia of the carrier gas and lower diffusion coefficients of the growth rate limiting chemical species in N 2 , lower total flow rates are found to be optimal for material quality and layer thickness uniformity when using N 2 as carrier gas. The dependence of growth rate uniformity on the carrier gas and total flow rate can only be understood by the detailed numerical modeling of three-dimensional flow, heat and species’ mass transfer with resulting layer deposition on the susceptor. The results of experiments are in good agreement with the modeling computations.

Electrical behaviour of the InPInGaAs based MSM-2DEG diode

Abstract The MSM-2DEG device consists of an MSM diode above a two-dimensional electron gas. It uses the same layer structure and processing procedures as needed for HEMT fabrication, offering new possibilities for the realization of monolithically integrated circuits. The electrical DC and RF properties of an InP InGaAs based MSM-2DEG device are investigated theoretically and experimentally. The effect of the 2DEG on the electrical behaviour of the diode is calculated analytically and numerically. Devices fabricated in InP InGaAs material exhibit high breakdown voltages up to 50 V for an electrode spacing of 2 μm. Capacitance-voltage measurements show an extremely high C max C min ratio up to 121, measured at 10 MHz and up to 7.8 at 5 GHz, demonstrating the potential of the diode as a varactor.

Nanoimprint and selective-area MOVPE for growth of GaAs/InAs core/shell nanowires

We report on the technology and growth optimization of GaAs/InAs core/shell nanowires. The GaAs nanowire cores were grown selectively by metal organic vapor phase epitaxy (SA-MOVPE) on SiO(2) masked GaAs (111)B templates. These were structured by a complete thermal nanoimprint lithography process, which is presented in detail. The influence of the subsequent InAs shell growth temperature on the shell morphology and crystal structure was investigated by scanning and transmission electron microscopy in order to obtain the desired homogeneous and uniform InAs overgrowth. At the optimal growth temperature, the InAs shell adopted the morphology and crystal structure of the underlying GaAs core and was perfectly uniform.

Weak antilocalization in a polarization-doped AlxGa1−xN∕GaN heterostructure with single subband occupation

Spin-orbit scattering in a polarization-doped Al0.30Ga0.70N∕GaN two-dimensional electron gas with one occupied subband is studied at low temperatures. At low magnetic fields weak antilocalization is observed, which proves that spin-orbit scattering occurs in the two-dimensional electron gas. From measurements at various temperatures the elastic scattering time τtr, the dephasing time τϕ, and the spin-orbit scattering time τso are extracted. Measurements in tilted magnetic fields were performed, in order to separate spin and orbital effects.