Saurabh Chopra

Fermi level depinning and contact resistivity reduction using a reduced titania interlayer in n-silicon metal-insulator-semiconductor ohmic contacts

Experimental evidence of reduction of ultrathin TiO2 by Ti is presented and its effect on Fermi level depinning and contact resistivity reduction to Si is experimentally studied. A low effective barrier height of 0.15 V was measured with a Ti/10 A TiO2−x/n-Si MIS device, indicating 55% reduction compared to a metal/n-Si control contact. Ultra-low contact resistivity of 9.1 × 10−9 Ω-cm2 was obtained using Ti/10 A TiO2−x/n+ Si, which is a dramatic 13X reduction from conventional unannealed contacts on heavily doped Si. Transport through the MIS device incorporating the effect of barrier height reduction and insulator conductivity as a function of insulator thickness is comprehensively analyzed and correlated with change in contact resistivity. Low effective barrier height, high substrate doping, and high conductivity interfacial layer are identified as key requirements to obtain low contact resistivity using MIS contacts.

Analysis of boron strain compensation in silicon-germanium alloys by Raman spectroscopy

The impact of heavy boron doping on the biaxial compressive strain in Si1−xGex layers grown on Si has been investigated using Raman spectroscopy and theoretical calculations. It is shown that one boron atom is sufficient to compensate the strain due to approximately 6.9 Ge atoms. This effect is appreciably large for boron concentrations as low as 1%, typical for applications, which employ heavily boron doped layers. Using strain compensation, the Ge content can be substantially increased without increasing the stored strain energy. This phenomenon can be useful in applications, which require low-resistivity p-type strained Si1−xGex layers with high Ge content.

Quantitative nanoscale local strain profiling in embedded SiGe metal-oxide-semiconductor structures

Mechanical strain by strain engineering has been widely used in Si metal-oxide-semiconductor field effect transistors. Experimental convergent beam electron diffraction (CBED) strain measurements and finite element calculations to quantitatively correlate the strain in the transmission electron microscope (TEM) sample with the actual device. It was found that the magnitude of the longitudinal strain, ex, along the channel direction, is about 20% higher in the TEM sample than in the real device. This combined approach can be used to explain data from other CBED studies of strained Si devices.

The effects of nickel germanosilicide contacts on the biaxial compressive stress in thin epitaxial silicon-germanium alloys on silicon

When a thin Si1−xGex epitaxial layer is grown on Si, it is under biaxial compression. In this letter, it is shown that a nickel germanosilicide (NiSi1−xGex) layer formed on Si1−xGex can significantly reduce the in-plane compressive strain in Si1−xGex. It is proposed that the observed reduction is due to the biaxial tensile stress applied by the NiSi1−xGex layer. Because the Si1−xGex bandgap is a strong function of the strain, this is expected to have a strong impact on the metal-semiconductor barrier height and the contact resistivity of the interface if the metal Fermi level is pinned near the Si1−xGex midgap.

Erbium Silicide Formation on Si1 − x C x Epitaxial Layers

Erbium silicide (ErSi 2-x ) formation was investigated on Si 1-x C x epitaxial layers grown on Si substrates. Substitutional carbon incorporation in the epitaxial layers was in the range of 0.6-1.6%. The silicide films were formed by rapid thermal annealing of sputter-deposited erbium layers in the temperature range of 350-700°C. The sheet resistance of the silicide films formed on Si 1-x C x epitaxial layers was found to be equal to or less than the sheet resistance of the films formed on Si epitaxial layers. At 600°C, an average resistivity of 114 ± 4 μΩ cm was obtained. The silicide grains were found to be epitaxially aligned to the substrate along the (100) orientation, regardless of the carbon concentration in the underlying epitaxial layer. Compositional analysis of the films indicated carbon accumulation at the ErSi 2-x /Si 1-x C x interface with no carbon incorporation in the silicide. The films formed on Si 1-x C x epitaxial layers exhibited a smooth interface/surface morphology free of pinholes, contrary to the silicides formed on Si. The root-mean-square surface roughness was found to be less than 1.5 nm, which was found to be the case with both substitutional and interstitial incorporation of carbon atoms in the epitaxial layer.

Benchmarking of Novel Contact Architectures on Silicon and Germanium

Novel contact architectures to n-Silicon (n-Si) and to n-Germanium (n-Ge) were benchmarked for the first time against the state-of-the-art contact architecture to n-Si. It was found that although the recently reported contact architectures to n-Ge exhibit markedly improved performance, more work must be done to match state-of the-art NiSi/n-Si contact architecture in terms of current-carrying capability.

Integrating Selective Epitaxy in Advanced Logic & Memory Devices

INTRODUCTION Selective epitaxy has gained increasing momentum in advanced high-performance logic as well as volatile and nonvolatile memory device fabrication. The advantages of this technique range from the well documented application of strained SiGe epitaxial films used to increase hole mobility and performance in pFET devices to intrinsic Si epitaxial layers used to prevent short channel effects in memory devices (DRAM). Other applications call upon the time tested strengths of epitaxy in ensuring abrupt, activated doped layers or junctions without the defectivity associated with implanted profiles.

Critical thickness of heavily boron-doped silicon-germanium alloys

In this work, the effect of boron concentration on the critical thickness of heavily boron doped Si1−xGex alloys (Si1−x−yGexBy) has been studied using Raman spectroscopy. The experimental results indicate that while boron decreases the stored strain energy, it can substantially increase the critical thickness for a given Ge concentration. The Si1−x−yGexBy critical thickness was calculated using two different models based on energy balance and kinetic considerations. The results show that the kinetic model provides a good estimate for the Si1−x−yGexBy critical thickness.

Epitaxial Growth of Si/Si1-xGex Films on Corrugated Substrates for Improved pMOSFET Performance

The quasi-planar segmented-channel MOSFET (SegFET) design provides an evolutionary pathway for continued CMOS technology scaling [1], and can be fabricated using a conventional process flow starting with a corrugated substrate [2]. Fig. 1 shows a schematic plan view and cross-sectional views of the SegFET structure, whose channel region consists of parallel stripes of equal width (Wstripe) isolated by very shallow trench isolation (VSTI) dielectric material which extends below the source/ drain extensions but which can be much shallower than the STI dielectric material used to isolate transistors. The fringing electric fields through the VSTI regions provide for enhanced gate control of the channel potential, so that the SegFET exhibits better short channel behavior compared to the conventional MOSFET [2]. To achieve improved on-state performance, mobility enhancement techniques can be employed; for example, silicon-germanium (Si1-xGex) can be used as the channel material to enhance p-channel MOSFET performance [3-4]. This work investigates the selective epitaxial growth of Si and Si1-xGex layers to form corrugated-Si/Si1-xGex substrates for enhanced p-channel SegFET performance.

Impact of Heavy Boron Doping and Nickel Germanosilicide Contacts on Biaxial Compressive Strain in Pseudomorphic Silicon-Germanium Alloys on Silicon

In recent years, the semiconductor industry has increasingly relied on strain as a performance enhancer for both n and p-MOSFETs. For p-MOSFETs, selectively grown SiGe alloys in recessed source/ drain regions are used to induce uniaxial compressive strain in the channel. In order to induce compressive strain effectively using this technology, a number of parameters including recess depth, SiGe thickness (junction thickness), sidewall thickness, dopant density, dislocation density, and contact materials have to be optimized. In this work, we have studied the effects of heavy boron doping and self-aligned germanosilicide formation on local strain. Raman spectroscopy has been used to study the impact of heavy boron doping on compressive stress in SiGe films. Strain energy calculations have been performed based on Vegards law for ternary alloys and the effect of boron on strain in SiGeB alloys modeled quantitatively. It will be shown that, owing to the smaller size of a boron atom, one substitutional boron atom compensates the strain due to 6.9 germanium atoms in the SiGeB film grown pseudomorphically on silicon. The critical thickness of SiGeB has been calculated for the first time based on kinetically limited critical thickness calculations for metastable SiGe films. It will be shown that the critical thickness of the alloy increases as the boron content in the alloy is increased, making boron concentration an additional parameter for optimizing strain in the MOSFET. Based on these conclusions, boron concentration can be used to preserve the strain for thicker SiGeB films (compared to SiGe films) while keeping the dislocation density low. Furthermore, we show that NiSiGe contacts can have a profound impact on the SiGe strain. Our results indicate that NiSiGe introduces additional stress in the underlying SiGe, which further affects the strain induced in the channel