O. A. Mironov

RBS ANALYSIS OF MBE GROWN SiGe/(001)Si HETEROSTRUCTURES WITH THIN HIGH Ge CONTENT SiGe CHANNELS FOR HMOS TRANSISTORS

The growth of Si1-xGex quantum wells with high Ge composition (x>0.5) on Si(001) substrates by MBE is of great interest both for HMOS device applications and fundamental research. One of the possibilities to obtain a high Ge content Si1-xGex channel for mobile carriers, while retaining its strain, is to grow a relaxed Si1-yGey buffer layer on an underlying Si substrate. Such a buffer is termed a virtual substrate (VS), which is a constituent of SiGe metamorphic heterostructures. The effect of annealing on the structure of the Si(001)/VS/Si0.7Ge0.3/Si0.2Ge0.8/Si0.7Ge0.3 heterostructures was studied by grazing angle of incidence RBS. The thickness of the Si0.2Ge0.8 channel is inhomogeneous, which makes the analysis of the data by traditional means very hard. We have developed a model whereby the influence of the thickness inhomogeneity of each layer in the apparent energy resolution as a function of depth can be calculated. Automatic fits to the data were performed, and the roughness parameters, that is, the standard deviation of the thickness inhomogeneity of the relevant layers, were obtained, together with the thickness and stoichiometry of each layer. The results are compared with TEM and high resolution XRD experiments.

High conductance Ge p-channel heterostructures realized by hybrid epitaxial growth

Strained Ge p-channel heterostructures have been realized by hybrid-epitaxial growth. Strain-tuning Si0.4Ge0.6 virtual substrates were grown by ultra-high vacuum chemical vapour deposition and active layers were deposited by solid-source molecular beam epitaxy at low temperature. Following ex situ annealing, Hall effect measurements revealed a hole mobility of 1900 cm2 V−1 s−1 at 300 K (27 000 cm2 V−1 s−1 at 10 K), with a density of 1.8 × 1012 cm−2, giving a conductance in excess of current Ge heterostructures. Using a maximum-entropy mobility-spectrum analysis, 1.0 × 1012 cm−2 of these holes were found to have a mobility of 2700 cm2 V−1 s−1 at 300 K.

Observation of microwave-induced resistance oscillations in a high-mobility two-dimensional hole gas in a strained Ge/SiGe quantum well

Microwave-induced resistance oscillations (MIRO) have been extensively studied for more than a decade but, until now, have remained unique to GaAs/AlGaAs-based 2D electron systems. Here, we report on the first observation of MIRO in a 2D hole gas hosted in Ge/SiGe quantum well. Our findings confirm that MIRO is a universal phenomenon and demonstrate that microwave photoresistance can be utilized to probe the energy spectrum and the correlation effects of 2D holes in Ge/SiGe quantum wells.

Ultrahigh room-temperature hole mobility in a SiGe quantum well

We report an ultrahigh room-temperature hole drift mobility obtained in a Si0.2Ge0.8 quantum well with a parabolic-like Ge profile, which has at its core a p-type modulation-doped (MOD) Si∕Si0.2Ge0.8∕Si0.65Ge0.35∕Si(001) heterostructure. High-conductivity holes at 293 K with a drift mobility of 3600cm2V−1s−1 at a sheet carrier density of 4.94×1012cm−2 were obtained in the Si0.2Ge0.8 quantum well after optimum annealing at 750 °C for 30 min. Hall mobility and sheet carrier density of this heterostructure are 1776cm2V−1s−1 and 2.37×1013cm−2, respectively. Structural characterization of the as-grown and the annealed samples revealed that the annealing had caused Si0.2Ge0.8 channel broadening, smearing of interfaces, and formation of a parabolic-like Ge profile that significantly improved room-temperature hole transport properties. The reported values of hole mobility are much higher than in the bulk Ge.

The Peculiarities of Magnetic Flux Dynamics at Magnetothermal Instability in Textured Bi2Sr2CaCu2O8+δ

The magnetic flux dynamics was investigated in a textured Bi2Sr2CaCu2O8+δ sample using a miniature Hall probe placed on the surface of the superconductor. The process of magnetic flux penetration into the superconductor was found to have a complex structure and consist of two processes with different time scales. The first one has a length of tens of microseconds and is considered as locally adiabatic. The second one has a length comparable with the time of thermal diffusion. The importance of the above two processes in the development of magnetic instability is analyzed.

The epitaxial growth of IV-VI heterostructures and superlattices on (001) Si

We report for the first time the use of rare-earth chalcogenides as buffer layers on silicon substrates prior to the high vacuum epitaxy of IV–VI compounds for the growth of thin film heterostructures and superlattices. The results of X-ray diffraction characterisation showed that YbS grows at 900–950 °C on (001) Si as a high-quality single crystal layer with an X-ray rocking curve half width δ = 300–500 arcsec. Such buffer layers allow us to grow single-crystal IV–VI films and various types of heterostructures and superlattices with crystal quality better than that achievable in the buffer layer (with δ = 100–300 arcsec), over a large area with good uniformity.

Large magnetoresistance of a dilute p-Si/SiGe/Si quantum well in a parallel magnetic field

We report the results of an experimental study of the magnetoresistance rho(xx) in two samples of p-Si/SiGe/Si with low carrier concentrations p=8.2x10(10) cm(-2) and p=2x10(11) cm(-2). The research was performed in the temperature range of 0.3-2 K in the magnetic fields of up to 18 T, parallel to the two-dimensional (2D) channel plane at two orientations of the in-plane magnetic field B-parallel to against the current I: B-parallel to perpendicular to I and B-parallel to parallel to I. In the sample with the lowest density in the magnetic field range of 0-7.2 T, the temperature dependence of rho(xx) demonstrates the metallic characteristics (d rho(xx)/dT>0). However, at B-parallel to=7.2 T the derivative d rho(xx)/dT reverses the sign. Moreover, the resistance depends on the current orientation with respect to the in-plane magnetic field. At B-parallel to congruent to 13 T there is a transition from the dependence ln(Delta rho(xx)/rho(0))proportional to B-parallel to(2) to the dependence ln(Delta rho(xx)/rho(0))proportional to B-parallel to. The observed effects can be explained by the influence of the in-plane magnetic field on the orbital motion of the charge carriers in the quasi-2D system.

Structural and optical characterisation of undoped SiSi0.78Ge0.22Si(00l) superlattices grown by MBE

We report the results of structural characterisation of five-period SiSi1−xGexSi(001), (x = 0.22) st superlattices (SL) by SIMS with an ultra-low energy (500 eV) O+2 primary beam and by 1.0 MeV 4He+ RBS together with optical and Raman spectroscopies. The SLs were grown by solid source MBE in a VG Semicon V90S machine at five different substrate temperatures in the range 550 °C < Ts < 810 °C, which corresponds to the ‘equilibrium regime’ of Ge segregation near the SiSiGe interfac The results obtained give information on material and interface quality, layer thicknesses, and state of the strain in the heterostructure. A good agreement is shown between all characterisation methods used.

Acoustoelectric effects in very high-mobility p-SiGe/Ge/SiGe heterostructure at low temperatures in high magnetic fields

The contactless Surface Acoustic Wave (SAW) technique was implemented to probe the high-frequency (ac) conductivity in a high-mobility p-SiGe/Ge/SiGe structure in the integer quantum Hall (IQHE) regime. The structure was grown by low-energy plasma-enhanced chemical vapor deposition and comprised a two-dimensional channel formed in a compressively strained Ge layer. It was investigated at temperatures of 0.3–5.8 K and magnetic fields up to 18 T at various SAW intensities. In the IQHE regime, in minima of the conductivity oscillations with small filling factors, holes are localized. The ac conductivity is of the hopping nature and can be described within the “two-site” model. Furthermore, the dependence of the ac conductivity on the electric field of the SAW was determined. The manifestation of non-linear effects is interpreted in terms of nonlinear percolation-based conductivity.

Magnetotransport studies of SiGe-based p-type heterostructures: Problems with the determination of effective mass

The use of Shubnikov-de Haas oscillations for determining effective mass is illustrated by a study of the magnetotransport properties of the two-dimensional hole gas in Si1− x Ge x (x = 0.13, 0.36, 0.95, 0.98) quantum wells. For some samples the data cannot be fitted to standard theoretical curves in which the scattering of charge carriers is described by the conventional Dingle factor. The reasons for the discrepancies between the experiment the theory are: (i) the effect of spin splitting on the amplitude of the peak in the SdH oscillations; (ii) extra broadening of the Landau levels attributable to an inhomogeneous distribution of the carrier concentration; (iii) the coexistence of short and long-range scattering potentials; and, (iv) population of the second energy level in the quantum well. Ways of calculating the effective hole masses m* for all these cases are presented and values of m* are found for the heterostructures studied here.