M. Kummer

A plasma process for ultrafast deposition of SiGe graded buffer layers

Low energy plasma enhanced chemical vapor deposition (LEPECVD) has been applied to the synthesis of Si-modulation doped field effect transistor structures, comprising a SiGe relaxed buffer layer and a modulation doped strained Si channel. A growth rate of at least 5 nm/s for the relaxed SiGe buffer layer is well above that obtainable by any other technique. Due to the low ion energies involved in LEPECVD, ion damage is absent, despite a huge plasma density. The structural quality of the LEPECVD grown SiGe buffer layers is comparable to that of state-of-the-art material. The electronic properties of the material were evaluated by growing modulation doped Si quantum wells on the buffer layers. We obtain a low temperature (2 K) Hall mobility of μH=2.5×104 cm2/Vs for the electrons in the Si channel at an electron sheet density of ns=8.6×1011 cm−2.

Very high hole mobilities in modulation-doped Ge quantum wells grown by low-energy plasma enhanced chemical vapor deposition

We report on the fabrication of modulation-doped compressively strained Ge quantum wells by low-energy plasma enhanced chemical vapor deposition. A virtual substrate consisting of a thick linearly graded SiGe buffer layer and a cap layer of constant composition is first grown at a high rate (>5 nm/s). The active layer stack, grown at a reduced rate, contains strain compensating cladding layers with modulation doping above the channel. Mobilities of up to 3000 cm2/V s and 87 000 cm2/V s have been achieved at room temperature and liquid He temperature, respectively.

Heterojunction photodiodes fabricated from Ge/Si (1 0 0) layers grown by low-energy plasma-enhanced CVD

We have fabricated a series of p-i-n Ge/Si heterojunction photodetectors with different thicknesses of the nominally intrinsic Ge layer. Epitaxial Ge was deposited on Si(1 0 0) using low-energy plasma-enhanced CVD (LEPECVD) followed by cyclic annealing. The residual tensile strain

Virtual substrates for the n- and p-type Si-MODFET grown at very high rates

Low-energy plasma-enhanced chemical vapour deposition (LEPECVD) has been applied to the synthesis of SiGe relaxed buffer layers with Ge end concentrations between 35% and pure Ge. A growth rate of several nanometres per second for relaxed buffer layers is well above that obtainable by any other growth technique. The structural quality of SiGe buffers graded to pure Ge is compared with that of a Ge buffer of constant composition. The structural quality of the pure Ge buffer is remarkably good compared with the much more complicated graded buffer. Complete n-type Si-modulation doped field effect transistor structures have been synthesized by LEPECVD, and the electric properties have been characterized by magneto transport measurements.

Crystalline silicon thin films with porous Si backside reflector

Thin film crystalline Si solar cells on cheap Si-based substrates have a large potential in PV technology. Optical light confinement is a very crucial point of such thin film structures. Porous Si (PS) as a perfect light diffuser could be used as a backside reflector if its multi-layer structure would be preserved during the deposition of a thin Si film. That is why low-energy plasma enhanced chemical vapor deposition (LEPECVD) was chosen to deposit a thin Si film on a PS multilayer structure at low temperature and high deposition rate. This technique allows one to deposit a Si film with epitaxial quality on the top of PS without destroying its multi-layer structure as revealed by high-resolution X-ray diffraction and cross-sectional transmission electron microscopy (TEM). The epi-layers of 10 μm are grown at very high deposition rates (approx. 3 nm/s) at 590°C. TEM-analysis reveals that during the deposition a high density of defects forms at the PS/epi-Si interface and spreads through the whole epi-layer. The defect density is decreased when the deposition temperature is increased to 645°C. LEPECVD appears to be an appropriate deposition technique to grow thin Si films on cheap Si based substrates with PS reflector.

Shape evolution of Ge domes on Si (0 0 1) during Si capping

Abstract Three-dimensional, coherently strained Ge/Si (0 0 1) islands were overgrown with thin Si layers and their shape evolution was studied by scanning tunneling microscopy. The Si cap, necessary for exploiting the clusters as self-assembled quantum dots, intermixes with the Ge layer leading the dome-shaped islands to transform first into {1 0 5} faceted pyramids and finally into stepped mounds with steps parallel to the 〈1 1 0〉 directions. The observed morphological transitions can be understood in a general picture in which the shape of an island mainly depends on its volume and composition.

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.

Acoustoelectric effects in very high-mobility p-SiGe/Ge/SiGe heterostructure

Measurement results of the acoustoelectric effects [surface acoustic waves (SAW) attenuation and velocity] in a high-mobility p-SiGe/Ge/SiGe structure are presented. The structure was low-energy plasma-enhanced chemical vapor deposition grown with a two-dimensional (2D) channel buried in the strained Ge layer. The measurements were performed as a function of temperature (1.5-4.2 K) and magnetic field (up to 8.4 T) at different SAW intensities at frequencies 28 and 87 MHz. Shubnikov-de Haas-like oscillations of both SAW attenuation and the velocity change have been observed. Hole density and mobility, effective mass, quantum and transport relaxation times, as well as the Dingle temperature were measured with a method free of electric contacts. The effect of heating of the 2D hole gas by the electric field of the SAW was investigated. Energy relaxation time tau(epsilon) and the deformation potential constant determined

Compressively strained Ge channels on relaxed SiGe buffer layers

Abstract Strain-induced roughening and dislocation formation has been studied by high-resolution transmission electron microscopy (HRTEM) in compressively strained Ge quantum wells on linearly graded SiGe buffer layers grown by low-energy plasma-enhanced chemical vapour deposition (LEPECVD). We show that for appropriately chosen plasma densities and substrate temperatures, abrupt interfaces can be achieved on both sides of the Ge channels, when additional hydrogen is supplied to the reactive gases, even for channel widths above the critical thickness for dislocation formation. Optimized modulation doped Ge quantum wells (MODQWs) exhibit the highest hole mobilities observed to date, approaching values of ∼90000 cm 2 V −1 s −1 for a sheet density of ∼6×10 11 cm −2 at liquid He temperatures.

Si/SiGe FETs grown by MBE on a LEPECVD grown virtual substrate

Abstract This paper reports on the preparation and assessment of Si/SiGe based n-type field-effect transistors (n-FET). The layer growth was carried out in a two step epitaxy procedure. First, a strain-relieved SiGe layer with a final Ge fraction of 40% was deposited on a Si(100) wafer by means of low energy plasma enhanced chemical vapor deposition (LEPECVD). On this virtual substrate the active layer stack was grown by molecular beam epitaxy (MBE) consisting of a 9 nm thick strained Si channel sandwiched between Sb modulation doped Si 0.6 Ge 0.4 cladding layers. The samples were structurally analyzed by atomic force microscopy (AFM), X-ray diffraction (XRD), and cross-section transmission electron microscopy (XTEM). FET structures were prepared and electrically characterized by conductivity and Hall measurements and by recording DC characteristics. Electron Hall mobilities as high as 760 cm 2 V −1 s −1 at a carrier density of 7.6×10 12 cm −2 has been obtained at room temperature (RT). A maximum transconductance of 230 mS mm −1 and a drain saturation current of 230 mA mm −1 have been achieved.