Jörg Pezoldt

Group III nitride and SiC based MEMS and NEMS: materials properties, technology and applications

With the increasing requirements for microelectromechanical systems (MEMS) regarding stability, miniaturization and integration, novel materials such as wide band gap semiconductors are attracting more attention. Polycrystalline SiC has first been implemented into Si micromachining techniques, mainly as etch stop and protective layers. However, the outstanding properties of wide band gap semiconductors offer many more possibilities for the implementation of new functionalities. Now, a variety of technologies for SiC and group III nitrides exist to fabricate fully wide band gap semiconductor based MEMS. In this paper we first review the basic technology (deposition and etching) for group III nitrides and SiC with a special focus on the fabrication of three-dimensional microstructures relevant for MEMS. The basic operation principle for MEMS with wide band gap semiconductors is described. Finally, the first applications of SiC based MEMS are demonstrated, and innovative MEMS and NEMS devices are reviewed.

Growth of cubic InN on r-plane sapphire

InN has been grown directly on r-plane sapphire substrates by plasma-enhanced molecular-beam epitaxy. X-ray diffraction investigations have shown that the InN layers consist of a predominant zinc blende (cubic) structure along with a fraction of the wurtzite (hexagonal) phase which content increases with proceeding growth. The lattice constant for zinc blende InN was found to be a=4.986 A. For this unusual growth of a metastable cubic phase on a noncubic substrate an epitaxial relationship was proposed where the metastable zinc blende phase grows directly on the r-plane sapphire while the wurtzite phase arises as the special case of twinning in the cubic structure.

Phase selective growth and properties of rhombohedral and cubic indium oxide

Phase selective growth of rhombohedral and cubic indium oxide polytypes was studied. The selective growth of different polytypes was achieved by adjusting substrate temperature and trimethylindium flow rate during metal organic chemical vapor deposition on c-plane sapphire. The optical band gaps of the cubic and rhombohedral phases were determined to be ∼3.7 and ∼3.0eV, respectively. On the basis of the performed structural investigations, a phenomenological model of the nucleation and growth of highly textured cubic In2O3 on Al2O3 (0001) is proposed.

Nanomechanics of single crystalline tungsten nanowires

Single crystalline tungsten nanowires were prepared from directionally solidified NiAl-W alloys by a chemical release from the resulting binary phase material. Electron back scatter diffraction (EBSD) proves that they are single crystals having identical crystallographic orientation. Mechanical investigations such as bending tests, lateral force measurements, and mechanical resonance measurements were performed on 100-300 nm diameter wires. The wires could be either directly employed using micro tweezers, as a singly clamped nanowire or in a doubly clamped nanobridge. The mechanical tests exhibit a surprisingly high flexibility for such a brittle material resulting from the small dimensions. Force displacement measurements on singly clamped W nanowires by an AFM measurement allowed the determination of a Youngs modulus of 332 GPa very close to the bulk value of 355 GPa. Doubly clamped W nanowires were employed as resonant oscillating nanowires in a magnetomotively driven resonator running at 117 kHz. The Youngs modulus determined from this setup was found to be higher 450 GPa which is likely to be an artefact resulting from the shift of the resonance frequency by an additional mass loading.

Effect of dislocations on electrical and electron transport properties of InN thin films. I. Strain relief and formation of a dislocation network

The strain-relaxation phenomena and the formation of a dislocation network in 2H‐InN epilayers during molecular beam epitaxy are reported. Plastic and elastic strain relaxations were studied by reflection high-energy electron diffraction, transmission electron microscopy, and high resolution x-ray diffraction. Characterization of the surface properties has been performed using atomic force microscopy and photoelectron spectroscopy. In the framework of the growth model the following stages of the strain relief have been proposed: plastic relaxation of strain by the introduction of geometric misfit dislocations, elastic strain relief during island growth, formation of threading dislocations induced by the coalescence of the islands, and relaxation of elastic strain by the introduction of secondary misfit dislocations. The model emphasizes the determining role of the coalescence process in the formation of a dislocation network in heteroepitaxially grown 2H‐InN. Edge-type threading dislocations and dislocati...

Preparation of single phase tungsten carbide by annealing of sputtered tungsten-carbon layers

Abstract Tungsten carbide layers were prepared by sputtering from a stoichiometric WC target and subsequent annealing. Carbide formation was found at temperatures above 800°C. Annealing in pure hydrogen ambient results in a carbon depletion in the layers and the formation of a dominant W 2 C phase. We demonstrate that propane added to the annealing ambient stimulates a transformation of the tungsten-carbon layers to a stoichiometric WC phase. The variation of the propane concentration allows a continuously alteration of the layer structure between single phase WC and a mixed layer with dominant W 2 C and the adjustment of different values of the electrical resistance and the optical constants.

Initial stages in the carbonization of (111)Si by solid-source molecular beam epitaxy

Silicon carbide can be reproducibly grown on (111)Si below 600 °C by carbonization using an elemental solid carbon source in molecular beam epitaxy. The initial stages were observed by in situ reflection high-energy electron diffraction. Prior to silicon carbide growth, the continuous carbon flux lead to a transition from the (7×7) reconstruction of clean (111)Si to a carbon-induced (∛×∛)R30° structure. Above 660 °C, the silicon carbide growth starts directly on the silicon surface via three-dimensional nucleation. Below 660 °C, first a thin silicon–carbon alloy was formed by diffusion of carbon into the surface near the region with a concentration exceeding the bulk solubility in silicon.

Growth of thin β-SiC layers by carbonization of Si surfaces by rapid thermal processing

The growth of thin SiC layers by carbonization via rapid thermal chemical vapour deposition at atmospheric pressure on Si(100) and Si(111) surfaces using propane (C 3 H 8 ) and hydrogen (H 2 ) at 11 min -1 as a carrier gas was investigated. The dependences of the growth kinetics, the crystal structure and the surface morphology of the SiC on C 3 H 8 concentration and ramp rate were determine by reflection high energy electron diffraction, scanning electron microscopy and ellipsometry. No Fundamental differences in growth kinetics were found between (100) or (111) substrates and n- or p-type Si. The propane concentration in the flowing gas shows the strongest influence on SiC thickness and morphology. The maximal layer thickness connected with a disturbed structure was maintained at 1330 o C and 0.025% C 3 H 8 . The best growth conditions regarding crystallinity were found at lower temperatures (1240 o C) and higher concentrations (above 0.1% C 3 H 8 ). At long growth cycles above 60 s and concentrations greater than 0.6% C 3 H 8 a graphite-like carbon phase on top of a single-crystal SiC layer occurred. Possible growth mechanisms were discussed. The observed ability of self-limited growth was used for large-area thin β-SiC film growth

Raman studies of Ge-promoted stress modulation in 3C–SiC grown on Si(111)

We present a study of the stress state in cubic silicon carbide (3C–SiC) thin films (120 and 300 nm) grown by solid-source molecular-beam epitaxy (SSMBE) on Si(111) substrates modified by the deposition of germanium prior to the carbonization of Si. μ-Raman measurements were used to determine the residual stress existing in the 3C–SiC layers. The stress is found to decrease linearly with increasing Ge quantity but with different strength depending on the 3C–SiC thickness deposited after the introduction of Ge. Based on secondary ions mass spectroscopy (SIMS) and transmission electron microscopy (TEM) analyses it is suggested that the Ge introduced prior to the carbonization step remains in the near-interface region and reduces the Si outdiffusion, which further reduces the stress state of the 3C–SiC layers.

Sputtering effects in hexagonal silicon carbide

Abstract Sputtering yields of α-SiC crystals in the energy range of argon ions from 1 to 2.5 keV as a function of substrate temperature were determined. For a normal incident beam the sputtering yield was in the range 0.45 – 0.71 for acceleration voltages of 1–2.6 keV. The temperature dependence of the sputtering yield was determined at 2 keV in the temperature range 20–1000 °C. A drop in sputtering yield was observed between 400 °C and 700 °C. Auger electron spectroscopy studies and computer simulation of Ar + sputtering as a function of the ion energy at room temperature showed changes in composition under argon sputtering, which were due to preferential sputtering of silicon. At room temperature sputtering led to the formation of a thin amorphized layer on the surface, observed by reflection high energy electron diffraction. At a substrate temperature of 200 °C a partial phase transition of the type GH → 3C was obtained, whereas at 400 °C a partial transition of the type 6H → 15R occurred. At higher substrate temperatures no changes in the polytype structure were observed. However, increasing temperatures led to a decrease and at higher temperatures to elimination of the amorphized fraction at the silicon carbide surface.