Showing papers by "Oliver Ambacher published in 2004"

Vertical transport in group III‐nitride heterostructures and application in AlN/GaN resonant tunneling diodes

Abstract: We present a systematic study of vertical transport in AlGaN/GaN heterostructures. The influence of barrier thickness and Al-concentration on the current across AlGaN barriers is investigated experimentally and compared to model calculations. The effect of polarization fields on the electronic properties of single- and double barrier heterostructures is discussed and experimental results are reviewed. AlN/GaN double barrier RTD structures are fabricated under optimized growth conditions. Experimental analysis of their electronic properties reveals indications for resonant tunneling in the low forward bias regime. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Comment on "mie resonances, infrared emission, and the band gap of InN".

Abstract: A Comment on the Letter by T. V. Shubina et al., Phys. Rev. Lett. 92, 117407 (2004). The authors of the Letter offer a Reply.

Sputter depth profiling of InN layers

Abstract: Epitaxial InN layers have been investigated by means of sputter depth profiling with low energy argon ions in conjunction with Auger electron spectroscopy. We have proven that the incidence angle of the ion beam plays a crucial role in the sputter behaviour of InN and is very important for the proper sputter depth profiling procedure. The sputtering under normal incidence and at 60° with respect to the surface normal leads to a strong nitrogen depletion at the InN surface and the surface becomes rougher due to the build-up of indium droplets. Due to the increase of the roughness during sputtering, the sputter depth profiling measurements performed under such conditions provided broad interface regions. By an increase of the ion beam incidence angle up to 80°, metallization of the InN surface and creation of droplets were hindered. However, the surface appeared to be nitrogen depleted due to the sputtering under oblique incidence, too. Under this condition, smoothing of the InN surface was observed. This fact results in a sharp interface region between InN and AlN. We established the sputtering under grazing incidence angle as an appropriate tool for characterization of InN layers and their interfaces by means of sputter depth profiling.

Two-dimensional electron gas recombination in undoped AlGaN/GaN heterostructures

Abstract: The radiative recombination of a two-dimensional electron gas (2DEG) was investigated in Al0.30Ga0.70N/GaN single heterostructures (SHs) without intentionally doping the barrier material, i.e., where the 2DEG appears at the interface due only to polarization effects. In addition to the typical excitonic transitions and the LO-phonon replicas originating from the GaN flat-band region, the photoluminescence spectra displayed three well-defined transitions. Their small binding energies and the observed blue shift with the excitation density suggested the association of these new emissions to quasi-2D excitons. On the basis of the thermal and excitation power dependences, the transitions were assigned to interface excitonic lines. Applying a weak electric field parallel to the growth direction, which depletes the triangular well, corroborated the 2DEG nature.

Study of pinholes and nanotubes in AlInGaN films by cathodoluminescence and atomic force microscopy

Abstract: Cathodoluminescence (CL) in the scanning electron microscope and atomic force microscopy (AFM) have been used to study the formation of pinholes in tensile and compressively strained AlInGaN films grown on Al2O3 substrates by plasma-induced molecular beam epitaxy. Nanotubes, pits, and V-shaped pinholes are observed in a tensile strained sample. CL images show an enhanced emission around the pits and a lower intensity at the V-shaped pinholes. Rounded pinholes appear in compressively strained samples in island-like regions with higher In concentration. The grain structure near the pinholes is resolved by AFM.

Etching of SiC with Fluorine ECR Plasma

Abstract: Electron cyclotron resonance (ECR) dry etching of 3C-SiC with different fluorinated gases, namely, sulfurhexafluoride (SF6) and tetrafluoromethane (CF4), was carried out. The influence of the gas flow, the etch gases and the applied bias voltages on the etch rate was studied. The maximum etch rates in the case of SF6 achieved were 1570 Å/min and 260 Å/min for Si and 3CSiC, respectively. In the case of CF4 the 260 Å/min (Si) and 160 Å/min (3C-SiC) were obtained. Furthermore, we investigate the selectivity of this dry etching process of SiC against Si. The residue free surface conditions were determined with Auger electron spectroscopy.

High-resolution diffuse x-ray scattering from threading dislocations in heteroepitaxial layers

Abstract: Diffuse x-ray scattering from epitaxial layers with screw dislocations perpendicular to the surface is calculated assuming correlated dislocation positions. The resulting intensity distribution was compared with a reciprocal-space map measured in a symmetric diffraction from a hexagonal GaN(0001) layer, and a good correspondence was achieved. From the fit, both the dislocation density and their correlation length were determined.

SIMS investigation of the influence of Ge pre-deposition on the interface quality between SiC and Si

Abstract: SiC/Si(111) heterostructures formed by using an alternative method for stress relaxation were investigated by SIMS and AES. The applied stress reduction method is based on a theoretical approach, which predicts an improvement of the SiC layer quality if Group IV elements are incorporated into the interface between SiC and Si. Germanium was chosen to test this approach. The incorporation of Ge into the heterointerface was carried out by depositing different amounts of Ge prior to the SiC growth process and varying the Ge deposition temperature. SIMS investigations revealed that Ge remains near the SiC/Si interface independently of the pre-deposited amount of Ge. In the case of two monolayers (ML) Ge coverage (with respect to the silicon surface) a surface segregation was observed. This indicates a limited transport of Ge to the SiC surface through the growing SiC layer due to grain boundary diffusion. At Ge coverages between 0.5 ML and 2 ML a sharper interface between the SiC and Si was observed. In this case the tail of the carbon distribution remains within the region occupied by the Ge distribution. The incorporation of Ge at the interface suppresses the out-diffusion of Si from the substrate to the surface of the growing SiC layer and, therefore, impedes the formation of voids at the SiC/Si interface.

The role of Ge predeposition temperature in the MBE epitaxy of SiC on Ssilicon

Abstract: An alternative method for stress relaxation in the SiC/Si heteroepitaxial system based on the incorporation of a group-IV element (germanium) into the interface between SiC and Si is presented. We have investigated the effect of the temperature during the predeposition of 1 ML of Ge on a silicon (111) surface, prior to the MBE carbonization. The resulting structures were investigated by transmission electron microscopy (TEM), x-ray diffraction (XRD), secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectroscopy (FTIR). The crystalline quality of the 3C-SiC layer was improved by the Ge predeposition, as shown by XRD measurements and by the smaller damping constants of the TO and LO phonons in the FTIR analysis. As the temperature of Ge predeposition increases, the SiC layer exhibits lower residual strain and larger grain size. Furthermore, the incorporation of Ge at the interface suppresses the outdiffusion of Si from the substrate to the surface of the growing SiC layer and, therefore, impedes the formation of voids at the SiC/Si interface. TEM and SIMS results revealed a strong segregation of Ge at the interface leading to an increased stress relaxation, in agreement with theoretical predictions. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Linear alignment of SiC dots on silicon substrates

Abstract: A linear alignment of self-assembled, cubic SiC dots grown by molecular beam epitaxy on Si substrates is demonstrated. The formation of well-ordered biatomic steps on (111) Si was used to control the nucleation sites. The resulting terraces promote an alignment along their step edges. SiC on Si represents self-organization in a system with chemical interactions. The resulting instability of the Si surface during the nucleation requires a precise control of the process conditions. By atomic force microscopy we demonstrate the achieved linear chains of SiC dots.

Gap state absorption in AlGaN photoconductors and solar‐blind photodetectors

Abstract: AlGaN photoconducting layers and solar blind UV-detectors with high responsivities in a narrow range of photon energies were grown by molecular beam epitaxy on c-plane sapphire substrates. The applicability of photothermal deflection spectroscopy and the spectral photocurrent measurements for absorption studies in III-nitride thin films is investigated. Process dependent variations in direct band-gap energies, Urbach's tail widths, bulk and surface defect densities in the films are given as a measure of composition and structural quality of epitaxial layers. The difference of more than one order of magnitude in the photocurrent above the band gap has been measured in back- and top-illuminated UV-photodetector heterostructure demonstrating a functionality of the integrated filter (cut-off energy ∼ 4.85 eV). The UV/visible contrast of more than four orders of magnitude has been observed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Lateral alignment of SiC dots on Si

Abstract: An alignment of self-assembled SiC dots grown by molecular beam epitaxy on Si substrates is demonstrated. Atomic force microscopy was applied showing the possibility to control the lateral ordering in linear chains and in dense dot arrays. The large lattice mismatch between Si and SiC of 20% stimulates a three-dimensional nucleation on the substrate. The formation of well ordered monoatomic, and biatomic steps as well as step bands on (100) and (111) Si was used, offering the advantage to not need additional processing steps to define the alignment. However, during the SiC nucleation on Si the substrate participates in the reaction to SiC by lateral Si diffusion resulting in an unstable surface during the growth. Thus, the reproducible control of the nucleation sites independent on the movement of the steps during the growth is a critical issue to achieve the lateral alignment. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Kinetic Monte Carlo simulation of SiC nucleation on Si(111)

Abstract: Solid source molecular beam epitaxy was applied to create silicon carbide nanoclusters on silicon. The island size distribution can be controlled by appropriate substrate temperature, carbon fluxes and process times. Nevertheless, up to now the earliest stages of the carbon interaction with silicon are not well understood. To have a deeper insight into the early nucleation stages and to investigate the growth laws in such systems the kinetic Monte Carlo computer simulation method was applied. The simulation enabled us to estimate the values of the growth parameters and to obtain the cluster size distribution function and nanocluster concentration in agreement with the experimental obtained data.

Stress Control in 3C-SiC Films Grown on Si(111)

Abstract: In this study we report on the effect of Ge coverage prior to carbonization, on the stress state in 3C-SiC thin films grown by solid source MBE on Si (111). Plan view μ-Raman technique was used to extract the residual stress in the 3C-SiC films. The obtained results showed that the stress depends strongly on the Ge amount and decreases linearly with increasing the Ge amount. The linear dependence of the residual stress allows to adjust the stress to a given value. For the obtained correlation between the predeposited amount of Ge and the residual stress a model will be proposed to explain the obtained results within the framework of the S-correlated theory of 3C-SiC on Si.

3C-SiC:Ge alloys grown on Si (111) substrates by SSMBE

Abstract: In this work for the first time a comprehensive research of (Si1-xC1-y)Gex+y thin films epitaxially grown on Si (111) substrates by solid source molecular beam epitaxy are presented. The layers were grown at substrate temperatures ranging from 900 °C to 1040 °C with a growth rate of 0.6 nm/min. They were analysed by atomic force microscopy, scanning electron microscopy, X-ray diffraction, Auger electron spectroscopy, secondary ion mass spectroscopy and transmission electron microscopy. The structural analysis revealed that the grown epitaxial layer consists of the cubic polytype. A maximum Ge incorporation of 0.16% was achieved in epitaxial layers grown at 900 °C. It was obtained that the Ge concentration decreases with increasing growth temperature. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of the Ge Coverage Prior to Carbonization on the Structure of SiC Grown on Si(111)

Abstract: A structural study of SiC layers grown by molecular beam epitaxy on carbonized Si(111) substrates with Ge modified interfaces is reported. Different quantities of Ge were predeposited prior to the SiC growth for comparison. X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS), Fourier-transform infrared spectroscopy (FTIR), transmission electron microscopy in both conventional (CTEM) and high-resolution modes (HREM) and selected-area electron diffraction (SAED) have been used to evaluate the structural quality of the SiC layers. SIMS measurements show that, after the epitaxial growth, the predeposited Ge remains at the interface and in the region of the silicon substrate closest to the interface. This observation was confirmed by HREM investigations. The 300 nm-thick SiC layers are subject to a residual in-plane tensile strain of 0.70.8 %. The defect structure indicates an enhanced lattice relaxation caused by Ge incorporation into the SiC/Si interface. The grain size tends to decrease as Ge coverage increases. Furthermore, the presence of Ge suppresses the formation of voids at the interface thanks to the formation of a Si/Ge/C alloy interlayer acting as a barrier for the outdiffusion of Si.

Surface modifications of AlGaN/GaN sensors for water based nano- and picodroplets

Abstract: Modifications of AlGaN surfaces have been carried out in order to tune their wetting properties. A hydrophilic surface is achieved by wet and dry thermal oxidation, whereas the deposition of fluorocarbon layers leads to a hydrophobic behaviour. It was found, that both surfaces still change their properties in the first days/hours after the modification. For the fluorocarbon layers differences are observed in dependence of the deposition method, which are explained by different ratios of the various carbon-flourine bonds.

Pt/GaN based Schottky diodes for gas sensing applications

Abstract: The performance of Pt/GaN Schottky diodes with different thickness of the catalytic metal were investigated as hydrogen gas detectors. The area as well as the thickness of the Pt were varied between 250 /spl mu/m/sup 2/ and 1000 /spl mu/m/sup 2/, 8 and 40 nm, respectively. The sensitivity to hydrogen gas was investigated in dependence on the active area, the Pt thickness and the operating temperature for 1 vol% hydrogen in synthetic air. We observed a significant increase of the sensitivity and a decrease of the response and recovery times by increasing the temperature of operation to about 350/spl deg/C and by decreasing the Pt thickness down to 8 nm. Electron microscopy of the microstructure showed that the thinner platinum had a higher grain boundary density. The increase in sensitivity with decreasing Pt thickness points to the dissociation of molecular hydrogen on the surface, the diffusion of atomic hydrogen along the platinum grain boundaries, and the adsorption of hydrogen at the Pt/GaN interface as a possible mechanism of sensing hydrogen by Schottky diodes.

Surface modifications of AlGaN/GaN sensors for water based nano- and picodroplets [chemical and biosensors]

Abstract: Modifications of AlGaN surfaces have been carried out in order to tune their wetting properties. A hydrophilic surface is achieved by wet and dry thermal oxidation, whereas the deposition of fluorocarbon layers leads to a hydrophobic behaviour. It was found that both surfaces still change their properties in the first days/hours after the modification. For the fluorocarbon layers, differences are observed in dependence of the deposition method, which are explained by different ratios of the various carbon-fluorine bonds.

A novel class of sensors for system integrative concepts in biotechnological applications.

Abstract: Miniaturisation of analytical and technical devices for biotechnological, medical and chemical applications shows high potential for accelerating the discovery of new biological relevant substances and drugs. Especially nanotechnological approaches promise a new quality with respect to time, sensitivity, and specificity in the field of biology driven sensor technology. New sensor concepts on the nanoscale demand for new system integrative concepts including the manipulation of picoand nanofluid volumes on chemical or structural modified surfaces and defining suitable interfaces to the macroworld.

Auger electron spectroscopy of silicon carbide

Abstract: The paper is devoted to evaluating the concentration of silicon on the surface of SiC exposed to bombardment by Ar + ions. For quantitative interpretation of the measured Auger spectra, the method was used in which the Auger current is determined as the area below the Auger peak in the direct N(E) spectrum after subtracting the background of inelastically scattered Auger electrons of the given peak and of the so-called true secondary electrons. We have found that bombardment of SiC by Ar + ions with energy 1 keV at an angle of 68 ◦ with respect to the surface normal and with the sample rotating at 6 rpm leads to Si depletion of SiC in comparison with its stoichiometric composition. The retrieved concentration of Si was 35.4at/nm 3 in the case of using the Si LVV spectra, and 39.7at/nm 3 if Si KLL spectra were used. K e y w o r d s: AES, quantitative analysis, sputtering, SiC, AES depth profiling

SiC/Si and AlN/Si heterostructures for microelectromechanical RF sensors

Abstract: Micromechanical SiC and AlN resonators with resonant frequencies between 170 kHz and 1.8 MHz have been prepared on silicon substrates by heteroepitaxial deposition of thin layers (/spl sim/200 nm) and subsequent dry etching. The resonators were operated by magnetomotive actuation and detection using a thin (/spl sim/50 nm) metallisation on top of the resonant beams. The strong RF driving signal masked the much weaker response of the resonators at resonance. Different methods to overcome this problem are proposed, namely (i) differential measurements, (ii) separation of excitation and detection by using coupled resonators, or an additional (e.g., capacitive) read-out electrode, and (iii) piezoelectric actuation.