T. Hackbarth

Strain relaxation mechanism for hydrogen-implanted Si1−xGex/Si(100) heterostructures

A mechanism of strain relief of H+ ion implanted and annealed pseudomorphic Si1−xGex/Si(100) heterostructures grown by molecular beam epitaxy is proposed and analyzed. Complete strain relaxation was obtained at temperatures as low as 800 °C and the samples appeared free of threading dislocations within the SiGe layer to the limit of transmission electron microscopy analysis. In our model, H filled nanocracks are assumed to generate dislocation loops, which glide to the interface where they form strain relieving misfit segments. On the basis of this assumption, the conditions for efficient strain relaxation are discussed.

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.

Carrier mobilities in modulation doped Si1-xGex heterostructures with respect to FET applications

Abstract Room temperature carrier mobilities in both p- and n-type modulation doped SiGe heterostructures were investigated by the magnetic field dependent Hall (B-Hall) technique. B-Hall allows a selective determination of mobility and sheet carrier density in the channel and in parasitic parallel conducting layers. The heterostructures grown by MBE on Si (100) substrates consisted of a strained Si 1− x Ge x channel on a Si 1− y Ge y strain relieved buffer (SRB) with x − y =0.3. Structural assessment was done by high resolution X-ray diffraction (HR-XRD) and cross-sectional TEM (XTEM). The hole mobility in p-type heterostructures depends clearly on the Ge content x in the channel and increases from 635 cm 2 /Vs for a SiGe alloy layer with x =0.67 up to 1665 cm 2 /Vs for a pure Ge channel, which is close to the value of undoped bulk Ge. For the corresponding n-type structure with a Si channel on a Si 0.7 Ge 0.3 SRB, a room temperature electron mobility of 2700 cm 2 /Vs was measured.

Strain relaxation of epitaxial SiGe layers on Si(1 0 0) improved by hydrogen implantation

Abstract We propose a new method to fabricate strain relaxed high quality Si1−xGex layers on Si by hydrogen implantation and thermal annealing. Hydrogen implantation is used to form a narrow defect band slightly below the SiGe/Si interface. During subsequent annealing hydrogen platelets and cavities form, giving rise to strongly enhanced strain relaxation in the SiGe epilayer. As compared to thermally induced strain relaxed Si–Ge epilayers, the hydrogen implanted and annealed samples show a greatly reduced threading dislocation density and a much higher degree of strain relaxation (90%). We assume that the hydrogen induced defect band promotes strain relaxation via preferred nucleation of dislocation loops in the defect band which extend to the interface to form misfit segments. The samples have been investigated by X-ray diffraction, Rutherford backscattering spectrometry and transmission electron microscopy.

Alternatives to thick MBE-grown relaxed SiGe buffers

Abstract We have investigated several growth concepts for strain relieved SiGe buffers as basis for high frequency transistors. Modulation doped quantum wells (MODQWs) were realized by molecular beam epitaxy (MBE) on top of thick graded buffers prepared by MBE, ultra-high vacuum chemical vapor deposition (UHVCVD) and low-energy plasma-enhanced CVD (LEPECVD). Additionally, thin buffers including a specific layer grown at low temperature (LT) were realized entirely by MBE. The overgrown thick CVD samples show comparable transport properties and thermal stabilities to those on thick graded MBE buffers. Mobilities of up to 90 000 cm 2 /V s have been measured at 30 K. Thin LT-MBE structures show slightly worse properties but are superior to conventional constant composition buffers.

Effect of helium ion implantation and annealing on the relaxation behavior of pseudomorphic Si1−xGex buffer layers on Si (100) substrates

The influence of He implantation and annealing on the relaxation of Si0.7Ge0.3 layers on Si (100) substrates is investigated. Proper choice of the implantation energy results in a narrow defect band ≈100 nm underneath the substrate/epilayer interface. During annealing at 700–1000 °C, He-filled bubbles are created, which act as sources for misfit dislocations. Efficient annihilation of the threading dislocations is theoretically predicted, if a certain He bubble density with respect to the buffer layer thickness is maintained. The variation of the implantation dose and the annealing conditions changes density and size of spherical He bubbles, resulting in characteristic differences of the dislocation structure. Si1−xGex layers with Ge fractions up to 30 at. % relax the initial strain by 70% at an implantation dose of 2×1016 cm−2 and an annealing temperature as low as 850 °C. Simultaneously, a low threading dislocation density of 107 cm−2 is achieved. The strain relaxation mechanism in the presence of He fi...

SiGe-based FETs: buffer issues and device results

Abstract SiGe quantum well structures gain increasing interest in the Si technology. The preparation of a Si channel or a Ge-rich or even a pure Ge channel with a respective two-dimensional carrier gas opens the attractive possibility to fabricate high performance n - or p -type field effect transistors. For both device types, a virtual substrate surface is required which is created by a strain relieved buffer layer grown on a Si standard wafer. The paper reviews various approaches of SiGe buffers including special attempts to reduce the thickness and to improve the quality. N - and p -type modulation-doped field-effect transistors are presented which show comparably good device characteristics and cut-off frequencies in the range of 100–120 GHz.

Reduced self-heating in Si'SiGe field-effect transistors on thin virtual substrates prepared by low-energy plasma-enhanced chemical vapor deposition

This letter reports on the electrical performance of strained Si-based n-type heterostructure field-effect transistors prepared on 500 nm Si0.56Ge0.44 virtual substrates. The method of low-energy plasma-enhanced chemical vapor deposition at low temperature was used for the growth of the relaxed SiGe buffer. The active layers have been deposited by molecular-beam epitaxy. The thin buffer improves the thermal conductivity by a factor of 3 and shows a much lower surface roughness compared to control structures on conventional virtual substrate with a 5-μm-thick graded buffer. Cutoff frequencies of fT=55 GHz and fmax(U)=138 GHz have been achieved which are very close to the results of the control sample.

Low temperature analysis of 0.25 /spl mu/m T-gate strained Si/Si/sub 0.55/Ge/sub 0.45/ n-MODFET's

A low temperature dc and HF investigation of 0.25 /spl mu/m T-gate Si/Si/sub 0.55/Ge/sub 0.45/ n-MODFETs is presented. Outstanding maximum oscillation frequencies f/sub max/ range from 100-120 GHz at 300 K up to 195 GHz at 50 K. These high-frequency characteristics are the first reported at low temperature on Si/SiGe n-MODFETs and are also the highest room temperature data reported so far; physical modeling is used to explain the main trends observed when cooling down the n-MODFET. Many experimental data are presented. The dependence on temperature and biases of the important small-signal equivalent circuit parameters is investigated to analyze the device high-frequency performances and the minimum noise figure of the intrinsic device is determined.

Results of a Precrash Application Based on Laser Scanner and Short-Range Radars

In this paper, we present a vehicle safety application based on data gathered by a laser scanner and two short-range radars that recognize unavoidable collisions with stationary objects before they take place to trigger restraint systems. Two different software modules that perform the processing of raw data and deliver a description of the vehicles environment are compared. A comprehensive experimental evaluation based on relevant crash and noncrash scenarios is presented.