D.G. Yarkin

Charge carrier transport in thermally oxidized metal/PS/p-Si and metal/PS/n-Si structures

Metal/PS/c-Si structures with porous silicon (PS) layers of 55?75% porosity were fabricated on moderately doped p- and n-type c-Si substrates and were thermally oxidized at 400?960 ?C. We studied the effect of oxidation on the charge carrier transport in these structures. It was demonstrated that trap-filled space charge limited current (SCLC) is the dominant transport mechanism at large forward bias. The analysis of the current?voltage characteristics in the SCLC region allowed us to determine the oxidation dependence of the effective thickness deff of the trap-rich tissue isthmuses, in which space charge is mostly accumulated. The exponential deff-dependence of the conductance in the ohmic region observed at low bias is explained by carrier tunnelling through potential barriers formed by SiOX isthmuses between silicon crystallites. The results can be used for fabrication of electronic devices based on oxidized PS and similar materials.

On the origin of ferromagnetism in semiconducting TiO2−δ:Co oxide

In Co-doped TiO2−δ oxide films deposited on SrTiO3(100) substrates, a room-temperature ferromagnetism is found to occur only in a limited charge-carrier concentration interval from 2×1018− 5×1022 cm−3. This indirectly testifies that ferromagnetism in the aforementioned n-type semiconductor is associated with the exchange interaction of magnetic ions via conduction electrons rather than with the formation of Co clusters in the material. The magnetic moment per Co atom is 0.8μB in the TiO cubic phase and 0.5μB in the anatase tetragonal phase of TiO2.

Low noise photosensitive device structures based on porous silicon

Abstract Photosensitive metal/PS/c-Si structures with oxidized and non-oxidized porous silicon (PS) layers were fabricated. The structures were made from moderately doped p-type c-Si substrates with dielectric windows and “stop” rings prepared using standard c-Si photodiode fabrication technology. Using this technology allowed orders of magnitude decrease the reverse currents of metal/PS/c-Si device structures. The best oxidized and non-oxidized structures have a dark reverse current of 20 nA/cm 2 at 10 V bias and a noise current of 0.16×10 −13 A/Hz 1/2 . Two types of photosensitive structures were obtained––photodiode structures with PS as an antireflection coating/optical window and bipolar phototransistor-like structures. Photodiodes are characterized by a quantum efficiency of 75% while “phototransistors” have a gain up to 11.

Giant magnetic moments in ferromagnetic oxide semiconductors

Concentration dependence of the specific magnetic moment of Co-and Fe-doped titanium oxide semiconductors has been studied at room temperature. A sharp increase in the magnetic moment has been found at low concentrations of a magnetic impurity. A giant value of 22.9 μB per impurity atom has been detected in TiO2 with a Co concentration of 0.15 at %, which has never been observed in oxide systems. The giant magnetic moments observed at low impurity concentration are attributed to the polarization of a crystal lattice. Comparison with the literature data indicates that the concentration dependences of magnetization are different in different oxide matrices.

Ferromagnetism of Mn-Implanted Silicon: Magnetization and Magnetooptic Faraday Effect

Manganese-implanted silicon plates of both n and p types have been obtained by implanting 195-keV manganese ions with doses from 1 × 1015 to 2 × 1016 cm−2. According to magnetic measurements by a vibrating sample magnetometer and a SQUID magnetometer, all of the samples exhibit a ferromagnetic ordering at room temperature. The magnetooptic Faraday effect is manifested in the spectral region 1–6 μm in the temperature interval 80–305 K. The characteristic features of the field and temperature dependences of magnetization and the spectrum of the Faraday effect indicate a percolation type of magnetic ordering at low temperatures and a crucial role of the exchange between delocalized p-type carriers and Mn ions at temperatures above 100 K.

Impact of vacuum thermal treatments on the structure and magnetic properties of titanium oxide films doped with Co

The structure and magnetic properties of different titanium oxide films doped with magnetic impurities are investigated both in the as-deposited state and after thermal treatments in a vacuum. The samples were characterized by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), x-ray absorption spectroscopy (XAS) and vibrating sample magnetometry. In the as-deposited films, ferromagnetism was shown to be caused by the formation of magnetic clusters during the deposition. Heating of Ti1−xCoxO2−δ films in a vacuum up to high temperatures and the following quenching thereof result in partial dissolution of the clusters with the magnetic ions being incorporated into the matrix. The low-temperature vacuum annealing leads to an increase of weak ferromagnetism in the semiconductor solid solution.

Formation of porous silicon on a non-conductive substrate and its use as a sacrificial layer

Porous silicon (PS) is an excellent material to be used as a sacrificial layer (SL) for the fabrication of membranes, bridges, cantilevers and other complicated micro-sensor structures. Major advantages of this material are its smooth surface and large area of pores that provide a high rate of PS SL removal. In this work, electrochemical and chemical etching processes of porous silicon formation from polycrystalline silicon deposited on insulating substrates have been studied. It was shown that a sufficiently thick (about 2 µm), uniform PS with a smooth surface and high etching rate in buffered HF solution can be made by both electrochemical and chemical etching from low resistive p-type poly-Si only. Finally, the obtained PS layers have been used as sacrificial layers in a fabrication process of microbolometer array.

Boundary Conditions for the Formation of a Ferromagnetic Phase during the Deposition of Ti{sub 1-x}Co{sub x}O{sub 2-{delta}} Thin Films

The conditions and the mechanism of the formation of a ferromagnetic phase in a Ti1-xCoxO2-δ oxide semiconductor are studied. It is found that the ferromagnetism manifests itself at room temperature in the films of Co-doped TiO2-δ oxide deposited on SrTiO3 (100) substrates only within a limited range of charge carrier densities: 2 × 1018−3 × 1022 cm−3. The minimum concentration of charge carriers corresponding to the formation of the ferromagnetic phase increases with a decrease in the cobalt content in the material under study. The ferromagnetism in Ti1-xCoxO2-δ thin films can be attributed to Co-enriched clusters with above critical sizes.

Study of the porous polycrystalline silicon formation process

Electrochemical dissolution (in hydrofluoric acid) of p-type polycrystalline silicon layers deposited on crystalline silicon substrates has been studied. Some properties of the porous silicon (PS) layers obtained are described. The dissolution occurs via two processes that take place simultaneously, at close rates. The first one is the formation of macropores, which results in the development of a columnar structure; the second is the transformation of the columns into a mesoporous medium. The PS layers obtained have a smooth surface, a sufficiently high porosity and a high etching rate in aqueous KOH solutions, which is important for the use of PS as sacrificial layers. Valuable information on the process is obtained by in situ capacitance measurements during electrochemical dissolution.

Formation of Porous Silicon Layers on Insulating Substrate for Microbridge – Type Sensor Applications

Formation processes of porous silicon on insulating substrate were studied. It was demonstrated that both electrochemical and chemical formation methods allow to transform heavily doped p-type polycrystalline silicon into homogeneous porous silicon. Porous silicon was successfully used as sacrificial layer in the fabrication process of microbridge structures.