K. Lyutovich

New virtual substrate concept for vertical MOS transistors

We propose a new concept of thin SiGe virtual substrates in which the interactions of point defects with dislocations play a key role. Being purposely introduced in the thin SiGe buffer layers during their metastable growth, point defects promote the relaxation of strain. Firstly, they cause dislocations to climb which helps to annihilate threading dislocation arms with opposite Burgers vectors. Secondly, condensation of point defects results in prismatic dislocation loops inside the layers which avoids nucleation from the surface sites. As a consequence, point defects reduce the density of existing threading dislocations and prevent the generation of new ones. This solution should allow the formation of virtual substrates with thin relaxed SiGe buffer layers and low threading dislocation density. In this paper, we explain how point defects can be injected using modified MBE process techniques. These techniques utilize either the injection of low energy Si + ions or supersaturation of point defects resulting from very low temperature growth.

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 (

Composition and strain in thin Si1−xGex virtual substrates measured by micro-Raman spectroscopy and x-ray diffraction

Micro-Raman spectroscopy was employed for the determination of the germanium content, x and strain, e, in ultrathin SiGe virtual substrates grown directly on Si by molecular beam epitaxy. The growth of highly relaxed SiGe layers was achieved by the introduction of point defects at a very low temperature during the initial stage of growth. SiGe virtual substrates with thicknesses in the range 40–200 nm with a high Ge content (up to 50%) and degree of relaxation, r, in the range 20%–100% were investigated using micro-Raman spectroscopy and x-ray diffraction (XRD) techniques. The Ge content, x, and strain, e, were estimated from equations describing Si–Si, Si–Ge, and Ge–Ge Raman vibrational modes, modified in this study for application to thin SiGe layers. The alteration of the experimentally derived equations from previous studies was performed using independent data for x and r obtained from XRD reciprocal space maps. A number of samples consisting of a strained-silicon (s-Si) layer deposited on a SiGe vir...

Interplay of dislocation network and island arrangement in SiGe films grown on Si(001)

Abstract A two-temperature process has been applied to grow 80-nm Si 0.7 Ge 0.3 films on Si(001) by molecular-beam epitaxy (MBE). The first 30 nm were deposited at a reduced temperature of only 150–200°C (low-temperature stage). The subsequent growth was performed at 550°C, the temperature range conventionally applied for SiGe MBE. Using atomic-force microscopy, we observed that the misfit dislocation network introduced during sample heating after the low-temperature (LT) stage guides the arrangement of {105}-faceted pyramid-like islands. In the case of a very narrow dislocation network — induced by ion-assisted growth during the LT stage — a checkerboard array of {105}-faceted pits and pyramids evolves with a ‘lattice constant’ of approximately 200 nm.

Ion assisted MBE growth of SiGe nanostructures

Abstract The bombardment of thin SiGe buffers with 1-keV Si + ions during molecular beam epitaxial growth is a possible way for the injection of point defects in order to promote the relaxation and to reduce the dislocation density. For this purpose, the e-beam evaporator was optimized by increasing the emission current and decreasing the energy of the impinging electrons to create a high density Si + ion flux in our MBE system. To the isolated substrate holder a potential up to several kilovolts can be applied to direct, focus and accelerate Si + ions. A high efficiency Ge effusion cell ensures stable and controllable Ge fluxes for growth rates up to 2.5 A/s. Under these conditions, several sets of thin SiGe layers (65–300 nm) containing from 23 to 100% of Ge were grown and investigated comparatively with reference samples deposited without ions at 650°C. By the ‘ion growth program’, after the deposition of Si buffers, SiGe layers were grown in three stages. The first part of the layer (e.g. 1/8 of the nominal thickness) and the last one (e.g. 5/8 of the thickness) were grown without ion bombardment. The second part (e.g. 2/8 of the thickness) was deposited under 1-keV accelerated Si + ion bombardment. Ge content was kept constant during all three stages. Sharp interfaces and uniform Ge profiles were shown by SIMS. Strain relaxation in the thicker layers is nearly 100% as proven by XRD. In thin pseudomorphic layers with low Ge content, a bombardment may result in nucleation of stacking faults shown by TEM. AFM and preferential chemical etching of relaxed ion bombarded layers have shown higher surface smoothness and a reduction of etch pit densities.

High Ge content photodetectors on thin SiGe buffers

Abstract PIN SiGe photodetectors (PD) are grown by MBE with Ge contents x =0, 0.10, 0.2, 0.27 and 0.5. For PDs with high Ge content, which are grown beyond the SiGe layer critical thickness, thin (sub 100 nm) strain relaxed, p + (B)-doped SiGe buffers are of special importance. The layer structure of the samples consists of a 5×10 18 cm −3 p + (B)-doped buffer layer as a bottom contact, followed by a 300 nm intrinsic active zone covered with a 3×10 20 cm −3 n ++ (Sb)-doped top contact layer. Extremely low temperatures (LT) during the first growth stage of the SiGe buffers are implemented. Process windows for high strain relaxation on different substrates are determined. The role of growth conditions in crystal structure formation is in situ monitored by time resolved reflectivity (TRR) measurements. For comparison, pseudomorphic PDs and that on conventional graded buffers were also realised. Secondary ion mass spectrometry (SIMS), μ-Raman-spectroscopy and energy dispersive X-ray (EDX) analysis are performed to measure Ge content, composition profiles and degree of relaxation. The microstructure is characterised by cross-section transmission electron microscopy (XTEM). By optical microscopy with Nomarski differential interference contrast (NIC) combined with defect etching technique, typical defects in the layers were studied. Electrical measurements are performed to determine the different current components. The DC current–voltage characteristics show a distinct diode ‘s behaviour of the detectors, which are optically characterised in terms of reverse current for different incident wavelengths.

Relaxed SiGe buffer layer growth with point defect injection

Abstract For virtual substrate preparation, we used thin relaxed SiGe buffer layer growth on Si (001) substrates by molecular beam epitaxy (MBE). Aiming to stimulate the strain relaxation and to procure the conditions for crystal structure improvement, we deliberately introduced point defects in situ, during layer growth. To generate point defects during MBE, we pursued two separate techniques, both applied during the stage of metastable pseudomorphic growth: (i) epitaxy at very low temperature and (ii) bombardment of the growth surface with Si+ ions. We studied the role of these measures and of other growth conditions in the strain relaxation processes and in the crystal structure of the SiGe buffer layers. This paper reports the successful relaxation of thin (

Control of Self-Heating in Thin Virtual Substrate Strained Si MOSFETs

This paper presents the first results and analysis of strained Si n-channel MOSFETs fabricated on thin SiGe virtual substrates. Significant improvements in electrical performance are demonstrated compared with Si control devices. The impact of SiGe device self-heating is compared for strained Si MOSFETs fabricated on thin and thick virtual substrates. This paper demonstrates that by using high-quality thin virtual substrates, the compromised performance enhancements commonly observed in short-gate-length MOSFETs and high-bias conditions due to self-heating in conventional thick virtual substrate devices are eradicated. The devices were fabricated with a 2.8-nm gate oxide and included NiSi to reduce the parasitic series resistance. The strained layers grown on the novel substrates comprising 20% Ge did not relax during fabrication. Good on-state performance, off-state performance, and cross-wafer uniformity are demonstrated. The results show that thin virtual substrates have the potential to circumvent the major issues associated with conventional virtual substrate technology. A promising solution for realizing high-performance strained Si devices suitable for a wide range of applications is thus presented

Strain relaxation of metastable SiGe/Si: Investigation with two complementary X-ray techniques

Metastable and strain relaxed SiGe layers with about 20% Ge content have been grown by molecular beam epitaxy on Si substrates at 550 °C. The thickness regime of metastability and the onset of strain relaxation were investigated on dust particle free surfaces obtained by careful chemical cleaning and epitaxy loading under clean room conditions. Compared to earlier results true metastable regime without misfit dislocations was obtained up to 140 nm thickness. The onset of strain relaxation started with heterogeneous nucleation sites of misfit dislocations. X-ray topography proved to be a unique monitoring tool to observe a low density of single dislocations. From these results we suggested to define a critical thickness band with lower bound tcl from dislocation nucleation to an upper bound tco (600 nm in our case) defined by the onset of considerable strain relaxation. The strain relief was measured by X-ray diffraction (reciprocal space mapping) and found to be very abrupt (76% strain relaxation at 800 n...

Micro-Raman investigations of the degree of relaxation in thin SiGe buffer layers with high Ge content

Virtual substrates with high Ge content x of 0.25<x<0.5 for metal oxide semiconductor field effect transistor (nMOSFET) structures are grown by molecular beam epitaxy (MBE). Thin strain-relaxed SiGe buffers are of special importance for this application, therefore, sub-100 nm layer growth procedures have been developed. Micro-Raman spectroscopy has been extensively employed for measurements of Ge content and the degree of relaxation (r) in our virtual substrates. Two growth stages – a very low temperature (VLT) stage and overgrowth at 550 °C (OT stage) were used to provide the high relaxation in thin SiGe layers. Implementation of this process under careful temperature control within 50 °C during the VLT growth stage allows us to regulate precisely the degree of relaxation. The role of low temperatures and of Sb surfactants during the VLT growth stage for process-window widening is studied. The Sb pre-buildup is found to increase the degree of relaxation in a higher temperature range during the VLT stage. Optimum processing conditions are determined and high uniformity of composition and residual-strain distribution on virtual substrate wafers are demonstrated.