Si Cap Layer Research Articles

Variation of Contact Resistivity with Ge in TiW/P+ SiGe Contacts

The dependence of contact resistivity on the Ge content in Si1–xGex is examinedfor TiW/p+ Si1–xGex interfaces. Measurements are made on contacts with epitaxial Si1–xGex layers either at the surface or buried under a Si cap layer of various thicknesses. The contact resistivity is found to decrease by an order of magnitude with increasing Ge content from 0 to 30 at. %, which is attributed to an increase in the valence band energy of p+ Si1–xGex. The measured contact resistivity is compared with a theoretical model, and the experimental results agree well with the modelled ones.

Recessed Oxynitride Dots on Self-Assembled Ge Quantum Dots Grown by LPD

Recessed oxynitride dots deposited on self-assembled Ge dots are demonstrated using liquid-phase deposition (LPD). By adding ammonia into the solution, the nitrogen atoms can be incorporated into the deposited film. The tensile strain of the Si cap layer directly deposited on Ge dots can enhance the oxynitride nucleation and deposition on Si surface. The tensile strain may also increase the etching rate of the Si cap layer and the recessed dots are formed directly above the Ge dots. The LPD-SiON dots have a higher dot step height as compared to LPD-SiO 2 dots.

Increase in luminescence efficiency of GaPN layers by thermal annealing

The effect of rapid thermal annealing (RTA) on the luminescence properties of GaPN layers was investigated. GaPN layers were grown on GaP and Si substrates. The luminescence properties were evaluated by cathodeluminescence and low-temperature photoluminescence. It was found that the luminescence intensity was increased and the peak photon energy was shifted to the high-energy side in all samples after RTA. The luminescence intensity of the GaPN layers was increased with increasing the annealing temperature. It was also found that high-temperature RTA can be applied to the GaPN layer grown on the Si substrate with a thin Si cap layer rather than the GaPN layer on the GaP substrate. It was estimated that high-temperature RTA reduces the spatial fluctuation of nitrogen compositions and the density of defects attributed to nitrogen atoms. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Mobility and performance enhancement in compressively strained SiGe channel PMOSFETs

We report that drive current enhancement and higher mobilities than the universal mobility in compressively strained Si 1− x Ge x on Si surface-channel p-type metal-oxide–semiconductor field-effect-transistors (PMOSFETs) with HfO 2 gate dielectric, for gate lengths ( L G) down to 180 nm. 36% drive current enhancement was achieved for Si 0.8Ge 0.2 channel PMOSFETs compared to Si with HfO 2 gate dielectric. We demonstrate that using SiGe in the channel may be one way to recover the mobility degradation due to the use of HfO 2. Buried-channel PMOSFETs with a Si cap layer and SiO 2 gate dielectrics were also studied. 41% peak mobility enhancement in Si 1− x Ge x channel PMOSFETs was observed compared to Si channel PMOSFETs. 17% drive current enhancement was achieved for 70 nm channel length ( L G) Si 0.9Ge 0.1 PMOSFETs with SiO 2 gate dielectric. This shows the impact of increased hole mobility even for ultra-small geometry of MOSFETs and modest Ge mole fractions. Comparable short channel effects (SCE) were achieved for the buried-channel Si 1− x Ge x devices with L G=70 nm, by controlling Si cap thickness, compared to the Si channel devices. Drive current enhancement without significant SCE and leakage current degradation was observed in this work.

Damage-free extreme ultraviolet mask with TaBN absorber

This article presents the results of evaluation of process-induced damage and improved reflectivity of an extreme ultraviolet (EUV) mask fabricated using a blank consisting of a multilayer, a Si capping layer, a CrN buffer layer, and a TaBN absorber. Long-term storage causes a centroid wavelength shift and stress change in the multilayer. The multilayer blank annealed at 90 °C was quite stable in centroid wavelength and film stress against resist baking at 135 °C and air storage. After the CrN buffer layer was etched with a mixture of Cl2 and O2 gases, the mask featured reflectivity loss of 1.5% due to the additional oxide layer generated on the Si capping layer. The reflectivity loss was able to be completely restored to its original value by treatment with a diluted HF solution. An EUV mask with a high reflectivity of 65% and excellent reflectivity uniformity of 0.7% 3σ was demonstrated using a blank consisting of a 40-period multilayer and a Si capping layer through a newly developed damage-free process that includes HF treatment.

Characterization of amorphous-Si/1ML-Ge/Si(001) Interface Structure by X-ray Standing Waves

The interface structure of an amorphous-Si/1ML-Ge/Si(001) crystal was investigated by the X-ray standing wave (XSW) method. The sample was prepared by the molocular beam epitaxy (MBE) deposition of 1ML of Ge on a clean Si(001) surface followed by the deposition of an amorphous Si (a-Si) capping layer of 50 Å thickness. The intensity of the reflected X-rays and the yield of the fluorescent X-rays of GeKα were measured around the 111 Bragg point. The fluorescence yield curve, which indicates the interaction between the XSW field and Ge atoms, showed that most of the Ge atoms stay at a position very close to the ideal Si site. This result provides effective feedback for fabricating SixGe1-x/Si devices.

Evolution of shape, height, and in-plane lattice constant of Ge-rich islands during capping with Si

The surface morphology of Ge-rich islands on Si (001) substrates capped with 0 to 10 monolayers (MLs) of Si at 550 °C was investigated by atomic force microscopy. An evolution of the island shape from domes to pyramids was observed, which coincides with a dramatic decrease of the island height during overgrowth. The average lateral lattice constant 〈a∥〉 of the Ge-rich islands for a series of samples was obtained from grazing incidence x-ray diffraction. 〈a∥〉 decreases appreciably with deposition of the Si cap layer, even for a cap thickness as low as 1.3 MLs. At the beginning of overgrowth, Si incorporation promotes the shape evolution and the size variation of the islands.

Mechanisms of diffusion-enhanced thermal stability of Si/Si 1− xGe x/Si heterostructures grown by chemical vapor deposition

The thermal stability of doped Si/Si 0.8Ge 0.2/Si (n–p–n or p–n–p) structures grown by reduced pressure chemical vapor deposition has been studied in correlation with the dopant in- and out-diffusion, using high-resolution X-ray reciprocal lattice mapping and secondary ion mass spectrometry as the main characterization tools. Initially, by doping the strained Si 0.8Ge 0.2 layer with reasonable amounts of boron, phosphorus, or arsenic, the thermal stability of the structures is shown to be dramatically increased compared to intrinsic layers. Secondly, the results show that when the dopants are present only in the Si buffer and cap layers, intrinsic Si spacer layers are required to obtain a significant enhancement in the thermal stability. These spacers reduce the interfacial dopant concentration and act as barriers for direct injection of precipitates into the SiGe layers. Finally, p–n–p and n–p–n structures were studied, showing a very good thermal stability, due to enhanced out-diffusion of dopants from the SiGe layer upon in-diffusion from the adjacent layers. By employing i-Si spacers, the boron out-diffusion in a n–p–n structure was reduced, giving rise to a degradation of the thermal stability of this structure.

Effects of high-dose BF2+ implantation on the formation of Ti-germanosilicide on polycrystalline Si/Si0.87Ge0.13/Si layers

The high-dose BF2+ implantation, which is employed for forming resistors and lowering the extrinsic base resistance in the SiGe heterojunction bipolar transistor (HBT) integrated circuit (IC) fabrication process, is shown to have strong effects on the Ti-silicidation behavior of the polycrystalline Si/Si0.87Ge0.13/Si stack layer. As the dose of BF2+ increases from 4×1014 to 4.4×1015 cm−2, the growth and C49-to-C54 transformation of the silicide/germanosilicide layer and protrusions are highly enhanced, which is consistent with previous observations on the defect-assisted nucleation of silicide grains. However, the final sheet resistance increases with increasing dose of BF2+. Such an inconsistency between the amount of C54 phase and the sheet resistance is probably due to the formation of numerous surface voids on the silicide layer from the fluorine-containing gaseous complexes. As the number of germanosilicide protrusions increases with increasing dose of BF2+, the contact resistance also increases because the protrusions are readily etched away during contact opening and leave behind cavities within the Si/Si0.87Ge0.13/Si layer. To achieve lower values of extrinsic base and contact resistances in SiGe HBT ICs, it is desirable to replace the BF2+ with B+ as well as thicken the Si cap layer on the contact region using the selective growth process of Si.

Theory and simulation of dopant implantation and diffusion in SiGe

AbstractStrained Si‐based technology has imposed a new challenge for understanding dopant implantation and diffusion in SiGe that is often used as the buffer layer for a strained Si cap layer. In this work, we describe our latest modeling effort investigating the difference in dopant implantation and diffusion between Si and SiGe. A lattice expansion theory was developed to account for the volume change due to Ge in Si and its effect on defect formation enthalpy. The theory predicts that As diffusion in SiGe is enhanced by a factor of ∼10, P diffusion by a factor of ∼2, and B diffusion is retarded by a factor of ∼6, when compared to bulk Si. These predictions are consistent with experiment. Dopant profiles for As, P, and B were simulatedusing process simulators FLOOPS and DIOS. The simulated profiles are in good agreement with experiment. Dopant implantation was simulated using REED‐MD. The results showed a noticeable difference in peak and tail positions SiGe compared to Si. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrical properties of W/Si interfaces with embedded Ge/Si islands

In this work, we investigated electrical and morphological properties of W/p-type Si Schottky diodes with intentional inhomogeneities introduced by macroscopic Ge-islands embedded beneath the interface. The Si-cap layer thickness (or the island-distance to the interface) was progressively reduced by successive chemical etching cycles. Electrical characterizations were achieved through reverse current–voltage ( I– V) at room temperature and forward I– V measurements as a function of the temperature. In parallel, Rutherford backscattering spectroscopy analyses were performed to follow the Si-cap/Ge islands chemical thinning down with increasing the number of etching cycles. In addition, the comparison of topographical and electrical properties of the etched silicon-cap layer was carried out by conductive atomic force microscopy analyses with a nanometer-scale resolution. Our results indicate that the areas on the top of islands exhibit lower resistance than those which covered the wetting layer. This lateral variation of resistance at the surface of the semiconductor may correspond to Schottky barrier height inhomogeneities observed on broad area I– V characteristics of Schottky contacts.

Influence of boron-doped Si cap layer on the photoluminescence of β-FeSi2 particles embedded in Si matrix

The influence of a boron-doped silicon cap layer on the photoluminescence (PL) of β-FeSi2 particles embedded in a silicon p–n junction is investigated. PL is found to improve significantly by optimizing silicon growth temperature and boron concentration. Surface morphology is also analyzed by atomic force microscopy. Dislocations and point defects are found to be generated by oxygen incorporated into the heavily boron-doped silicon layer during the 14 h of thermal annealing at 900 °C, and are suggested to be responsible for the quenching of the 1.53 μm PL.

Formation and Properties of Epitaxial CoSi2 Layers on p-Si0.83Ge0.17/p-Si(001) using a Si Capping Layer by Metal-Organic Chemical Vapor Deposition

Epitaxial CoSi2 layers were in-situ grown using a sacrificial Si capping layer on p-Si0.83Ge0.17/p-Si(001) by metal organic vapor deposition (MOCVD) at 600°C using cyclopentadienyl cobalt, Co(η5-C5H5)(CO)2. The grown epitaxial CoSi2 layers remain thermally stable at the annealing temperature, TA≤800°C. The CoSi2/p-Si0.83Ge0.17/p-Si(001) samples annealed at TA≤850°C showed the sheet resistance values as low as \\cong5.6 Ω/□. At TA=900°C, the observed increase in the sheet resistance value to \\cong14.7 Ω/□ was attributed to the formation of CoSi phase with higher resistivity resulting from the direct interaction of Co with the SiGe alloy layer. The effective Schottky barrier heights (φBp), measured for the first time, ranged from 0.103 to 0.194 eV at the reverse bias of -5–0 V.

High hole mobility in p-strained Si grown on relaxed SiC virtual substrate by low-pressure chemical vapor deposition

Abstract The strained Si layer has been grown on the relaxed 3C–SiC virtual substrate by a hot-wall low-pressure chemical vapor deposition (LPCVD) system. The relaxed SiC virtual substrate was grown on Si(1 1 1) substrate by LPCVD and the crystal quality of the layer is characterized by X-ray diffraction and Fourier transform infrared absorption spectroscopy. It is apparent to find three regions in the Auger electron spectroscopy depth profile graph of the sample, which refer to the cap Si layer, the relaxed 3C–SiC virtual substrate and the Si(1 1 1) substrate, respectively. The bonding states of the relaxed 3C–SiC virtual substrate in the sample are analyzed by X-ray photoelectron spectroscopy. The Raman spectra of the sample indicate the cap Si layer is strained. A high hole Hall mobility value of 889 cm 2 /V s (300 K) is obtained in the strained Si layer, which is due to the in-plane compressive biaxial strain in this layer.

Influence of doping on thermal stability of Si/Si 1− xGe x/Si heterostructures

The thermal stability of Si/Si 0.8Ge 0.2/Si npn and pnp structures grown by reduced pressure chemical vapor deposition has been studied. The influences of the diffusion of dopants in SiGe or adjacent layers on the thermal stability of the structure are also discussed. The thermal stability of the structures is shown to be dramatically increased compared to that of intrinsic layers when doping the strained Si 0.8Ge 0.2 layer with boron, phosphorus or arsenic. This performance is attributed to competition between dopant diffusion/precipitation phenomena and misfit dislocation development in the doped SiGe. Moreover, it is observed that doping only the Si buffer and cap layers in the Si/SiGe/Si structure also results in an enhancement in the thermal stability of the SiGe layer. Finally, when pnp or npn structures are grown, the structures exhibit significantly enhanced thermal stability. In addition, the boron out-diffusion from the SiGe layer is retarded in the sample where the Si buffer and cap layers were doped with phosphorus, presumably by in-diffusion of phosphorus into the SiGe layer. In this study, high-resolution X-ray reciprocal lattice mapping and secondary ion mass spectroscopy have been used as the main analysis tools.

Electrical characterization of germanium p-channel MOSFETs

In this letter, we report germanium (Ge) p-channel MOSFETs with a thin gate stack of Ge oxynitride and low-temperature oxide (LTO) on bulk Ge substrate without a silicon (Si) cap layer. The fabricated devices show 2 /spl times/ higher transconductance and /spl sim/ 40% hole mobility enhancement over the Si control with a thermal SiO/sub 2/ gate dielectric, as well as the excellent subthreshold characteristics. For the first time, we demonstrate Ge MOSFETs with less than 100-mV/dec subthreshold slope.

Composition of self assembled Ge hut clusters

Determining the exact composition of self-assembled nanostructures is crucial for any fundamental investigation as well as for any potential application. Here, we present data on the interdiffusion behavior and segregation during growth of single and multiple layers of self assembled Ge hut clusters. Our results suggest that the critical thickness is reduced in upper island layers due to the strain fields of buried hut clusters in the underlying layers. Applying selective wet-chemical etching, we provide direct experimental evidence that the base of the Ge clusters intermixes with Si if hut clusters are stacked vertically. The inhomogeneous strain from the buried islands can explain the enhanced intermixing. Photoluminescence spectra for Si capped single and stacked hut cluster island layers are presented and we find that intermixing with Si during overgrowth is significant even in single layers of hut cluster islands. By reducing the substrate temperature during growth of the Si cap layer, the photoluminescence peak energy of the hut clusters is redshifted from about 800 to less than 640 meV. It is noticeable that this recombination energy is smaller than the fundamental bandgap of bulk Ge (0.74 eV) and is smaller than any emission energy yet detected from a pseudomorphic SiGe heterostructure grown on Si(001).

Effect of Ge-dots buried below the interface on the transport properties of Schottky diodes

In this work, we investigated the Schottky barrier height at W/p-type Si contacts with the presence of Ge-dots located below the interface. The effect of the dots is controlled by reducing the thickness of the Si-cap layer by successive chemical oxidation/etching cycles. The surface morphology of the samples is investigated by using atomic force microscopy and the Ge-content is determined by Rutherford backscattering spectroscopy. Nonideal behaviors in current–voltage characteristics are observed and explained in terms of inhomogeneities due to the presence of the Ge-dots.

Effect of Si cap layer on parasitic channel operation in Si/SiGe metal–oxide–semiconductor structures

We investigate the effects of silicon cap layer thinning on channel carrier confinement in silicon/strained silicon-germanium (Si/SiGe) metal–oxide–semiconductor (MOS) structures. The silicon cap thickness is shown to have a critical influence on the induced parasitic channel in the silicon cap, which lowers the transconductance value of the buried SiGe channel in hole carrier channel p metal–oxide–semiconductor field-effect transistor devices. This can have serious consequences on future implementation of SiGe ultrashort channel devices where gate induced parasitic channel and short channel effects can be pronounced. An exact methodology is devised for consumption of silicon cap layer using a modified Radio Corporation of America surface standard clean self-terminating chemical oxide and rapid thermal oxide growth. This provides (i) precise control over the silicon cap and thermal oxide thickness and (ii) a limit on the amount of thermal budget induced strain relaxation in the buried SiGe layer. The threshold voltages for inversion of carriers at the SiO2/Si and the Si/SiGe interfaces are extracted as functions of Si cap thickness. A transition from dual to single channel operation in MOS devices at room temperature on thinning the silicon cap layer is observed with capacitance–voltage measurements. Cross-section transmission electron microscopy and secondary ion mass spectroscopy techniques are used to calibrate and support the developed methodology for Si cap layer etch control and the effect of Ge species at the oxide–heterostructure interface. There is a high fixed charge density, which indicates Ge pileup at the SiO2/SiGe interface. Our analysis suggests that about 1 nm of silicon cap retention is necessary to minimize the gate induced parasitic channel and to decouple the oxide interface trap influence on the channel carriers in the SiGe.

Molecular-beam epitaxy of Ge on GaAs(001) and Si capping

Epitaxial quality of Ge layers on GaAs(001), as well as the quality of the Si capping layers, were investigated in situ by reflection high-energy electron diffraction during growth and, subsequently, by scanning electron and scanning probe microscopies. Ge was grown on the (1×1)-GaAs(001) surface prepared by oxide desorption at 580 °C in an As-free ultrahigh vacuum; its morphology, varying from reasonably flat layers with only atomic scale roughness to relatively large three-dimensional asperities, was found to crucially depend on the GaAs surface quality and growth temperature. The data presented in this work also account for the apparent discrepancies between various groups regarding the Ge/GaAs reconstruction; our detailed analysis proves that, at least under the experimental conditions described herein, it is a mixture of (1×2) and (2×1), rather than a (2×2) or c(2×2). Smooth Si growth was mainly impeded by a large lattice mismatch with the underlying Ge, initially replicating the morphology of the Ge layer and eventually forming discrete three-dimensional islands and continuous undulations. The study shows that flat epitaxial Si capping of GaAs should be possible by employing graded silicon–germanium buffers.