Central to the operation of organic electronic and optoelectronic devices is the transport of charge and energy in the organic semiconductor, and to understand the nature and dynamics of charge carriers is at the focus of intense research efforts. As a basic transport property of solids, the Seebeck coefficient S provides deep insight as it is given by the entropy transported by thermally excited charge carriers and involves in the simplest case only electronic contributions where the transported entropy is determined by details of the band structure and scattering events. We have succeeded for the first time to measure the temperature- and carrier-density-dependent thermopower in single crystals and thin films of two prototypical organic semiconductors by a controlled modulation of the chemical potential in a field-effect geometry. Surprisingly, we find the Seebeck coefficient to be well within the range of the electronic contribution in conventional inorganic semiconductors, highlighting the similarity of transport mechanisms in organic and inorganic semiconductors. Charge and entropy transport is best described as band-like transport of quasiparticles that are subjected to scattering, with exponentially distributed in-gap trap states, and without further contributions to S.

The temperature dependent magnetoresistance of high mobility p-SiGe/Ge quantum wells is studied with hole densities ranging from 1.7 to 5.9×1011cm−2. At magnetic fields below the onset of quantum oscillations that reflect the high mobility values (up to 75000cm2/Vs), we observe the clear signatures of electron–electron interaction. We compare our experiment with the theory of electron–electron interaction including the Zeeman band splitting. The observed magnetoresistance is well explained as a superposition of band structure induced positive magnetoresistance and the negative magntoresistance due to the electron–electron interaction effect.

We present a method to produce thin SiGe virtual substrates suitable for electronic applications. This method is based on the gas phase process of low-energy plasma-enhanced chemical vapor deposition. The strain-relaxed buffers are characterized by X-ray diffractometry, transmission electron microscopy and atomic force microscopy. We find threading dislocation densities lower than 3×108cm−2 and a surface rms roughness of 1.8nm, for a buffer thickness of 500nm. Room temperature electrical results are also presented, which are competitive with those obtained on SiGe buffers produced by other methods.

D. Chrastina, B. Rossner, G. Isella, H. von K¨ ¨anelINFM and L-NESS Dipartimento di Fisica, Politecnico di MilanoPolo Regionale di Como, Via Anzani 52, I-22100 Como, ItalyJ. P. HagueDepartment of Physics & Astronomy, University of Leicester,Leicester LE1 7RH, UKT. Hackbarth, H.-J. Herzog, K.-H. Hieber, U. Konig¨DaimlerChrysler AG, Research and Technology, D-89081 Ulm,Germany

We report on the low-temperature mobility in remotely doped p-type strained Ge layers on relaxed Si0.3Ge0.7 virtual substrates, grown by low-energy plasma-enhanced chemical vapor deposition. A maximum mobility of 120 000 cm2 V−1 s−1 has been reached at 2 K, at a carrier sheet density of 8.5×1011 cm−2. Analysis of the mobility and Dingle ratio τ/τq as a function of sheet density suggests that remote impurity scattering is the limiting factor at low sheet densities, but that interface impurities become more important as the sheet density increases.

We review the potential of low-energy plasma-enhanced chemical vapor deposition (LEPECVD) for the fabrication of strained Si and Ge heterostructures and devices. The technique is shown to be equally applicable to the formation of relaxed SiGe buffer layers, and to entire heterostructures including strained modulation doped channels. Pure Ge channels on Ge-rich linearly graded buffers are shown to exhibit low-temperature hole mobilities up to 120,000 cm2V−1s−1, limited by remote impurity and background impurity scattering rather than interface roughness scattering. Strained-Si modulation-doped field-effect transistors (n-MODFETs) with excellent frequency response have been fabricated by combining LEPECVD and MBE for buffer layer and active layer growth, respectively. Maximum oscillation frequencies of n-MODFETs above 140 GHz have been achieved for active layer stacks both on buffers linearly graded to a Ge fraction of 40% at a rate of 10% per micron, and on constant composition buffers which are 10 times thinner. The use of a thin buffer results in significantly less device self-heating.

We present results obtained on modulation-doped quantum wells with compressively strained Ge channels. The heterostructures have been grown by low-energy plasma-enhanced chemical vapor deposition (LEPECVD). Magnetotransport measurements were carried out both on van der Pauw squares and lithographically defined Hall bars. The 2 K hole mobility increases with sheet density, and exceeds 105 cm2V−1s−1 below 40 K for carrier densities above 8×1011 cm−2. It is dominated by remote impurity scattering, according to linear transport theory calculations. The same conclusion follows from the Dingle ratios of 5–10 derived from the low-field longitudinal magnetoresistance. Room-temperature channel mobilities were extracted from the magnetic field dependence of longitudinal and transverse magnetoresistance by means of mobility spectrum analysis. We obtained 2940 cm2V−1s−1 at a sheet hole density of 5.7×1011 cm−2.

Low energy plasma enhanced chemical vapour deposition (LEPECVD) is a relatively new growth method, which has been used to create high quality epitaxial silicon germanium material on conventional Si(001) wafers. This material is eminently suitable for electronic devices. The best performance for n-type and p-type conduction is seen in tensile-strained Si and compressively strained Ge quantum wells, respectively. Since such quantum wells cannot be grown directly on a silicon substrate, a virtual substrate (VS) is first grown. The reactive conditions within the plasma make it possible to grow the VS at rates of up to 10 nms(-1) independent of substrate temperature. The quantum wells were grown using a lower plasma intensity, at growth rates of approximately 0.3 nms(-1). The electrical properties of the material compare very well with molecular beam epitaxy (MBE) references, and hybrid material where the buffer is grown by LEPECVD and the electrically active layers are grown by MBE. In addition, the structural quality of the material is analysed by atomic force microscopy, transmission electron microscopy and defect etching. (C) 2003 Elsevier B.V. All rights reserved.

We report on the dependence of the effective masses on hole density in remotely doped strained Ge layers on relaxed Si0.3Ge0.7 buffers with sheet densities from 2.9×1011 cm−2 to 1.9×1012 cm−2. The masses have been determined using temperature dependent Shubnikov–de Haas oscillations. No noticeable dependence of the mass on the magnetic field has been found. The extrapolated Γ point effective mass has been found to be 0.080 times the free electron mass. From the measured data the variation of the mass with kinetic energy and the shape of the topmost heavy hole subband have been calculated. The results are in good agreement with theoretical predictions.