H.-J. Herzog

The n-channel SiGe/Si modulation-doped field-effect transistor

At the heterointerface of Si1-xGex/Si the existence of two-dimensional carrier gas has recently been demonstrated. The electrons are confined inside the large-gap material Si. We report the first fabrication of n-channel modulation-doped SiGe/Si hetero field-effect transistors by use of molecular-beam epitaxial growth. Though neither layer sequence nor parasitic resistances were optimized, these first transistors exhibit an extrinsic transconductance of 40 mS/mm for a gate length of 1.6 µm. This value is higher than that of conventional Si MESFETs of comparable carrier concentration. Technological processing steps and device evaluation are described.

High hole mobility in Si0.17Ge0.83 channel metal–oxide–semiconductor field-effect transistors grown by plasma-enhanced chemical vapor deposition

We report on effective hole mobility in SiGe-based metal–oxide–semiconductor (MOS) field-effect transistors grown by low-energy plasma-enhanced chemical vapor deposition. The heterostructure layer stack consists of a strained Si0.17Ge0.83 alloy channel on a thick compositionally-graded Si0.52Ge0.48 buffer. Structural assessment was done by high resolution x-ray diffraction. Maximum effective hole mobilities of 760 and 4400 cm2/Vs have been measured at 300 and 77 K, respectively. These values exceed the hole mobility in a conventional Si p-MOS device by a factor of 4 and reach the mobility data of conventional Si n-MOS transistors.

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.

Secondary implantation of Sb into Si molecular beam epitaxy layers

We report on the influence of low‐energy Si+ ions on the incorporation of Sb adatoms existing on growing (100) Si molecular beam epitaxy layers. At a growth temperature of 650 °C employed for these experiments an increase of incorporation of about three orders of magnitude compared to the spontaneous incorporation is obtained at ion flux densities of typically 1012 cm−2 s−1. Dopant activation coefficients of almost unity are established up to 1019 cm−3. The number of incorporated adatoms is found to increase proportionally with preadjusted adatom density as well as with Si+ ion dose. At an ion energy of 500 eV the constant of proportionality is estimated to be σI =(5±2)×10−16 cm2.

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.

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.

Relaxed SiGe buffers with thicknesses below 0.1 μm

Abstract Virtual substrates with relaxed SiGe buffers on Si substrates are needed for strain adjusted heterodevices with high Ge content. We have investigated the degree of relaxation in thin (

Properties of Si layers grown by molecular beam epitaxy at very low temperatures

(100) silicon molecular beam epitaxy films with etch pit densities below 103 cm−2 and χmin values of 3.3–3.9% were grown at very low temperatures (Ts =250–350 °C). Although dopant activation is significantly below unity at n=1018 Sb atoms/cm3 Hall mobilities of homogeneously Sb‐doped samples (Ts =250 °C, 300 °C) are found to match reasonably bulk values. δ doping at a monolayer Sb deposition shows a dopant activation of 0.45–0.81 with no detectable broadening at Ts =200 °C.