SiGe Surface Research Articles

Enhanced Band-to-Band-Tunneling-Induced Hot-Electron Injection in p-Channel Flash by Band-gap Offset Modification

A novel p-channel flash device with a SiGe layer is proposed, which is based on the analysis made with the simulator MEDICI, to enhance the band-to-band-tunneling current and improve the programming speed. The programming biases of the p-channel flash device can be reduced with an equal programming speed. Simulation results show that more than one hundred times enhancement in the programming speed or 35% reduction of the drain voltage can be achieved in the proposed p-channel flash device with a 40% Ge content in the surface SiGe layer. In addition, a Si-cap layer is inserted between the SiGe and the tunneling oxide to obtain a high-quality interface and to optimize the cell structure

Chemical Sensing Properties of Electrolyte/SiGe/SiO2/Si Structure

To investigate the surface properties of silicon germanium (SiGe) as a chemically sensitive membrane, SiGe layers deposited at Si2H6 and GeH4 ambience by low-pressure chemical-vapor-deposition (LPCVD) at 475 °C were prepared. First, the effect of ac measuring frequency on capacitance–voltage (C–V) curves obtained from a SiGe electrolyte–insulator–semiconductor (EIS) structure was studied. The pH sensitivity of SiGe-EIS structure was 59.8 mV/pH, calculated from reference voltages in the pH range from 10 to 2. A voltage shift was observed in the same pH buffer solution for different measurement loops. To analyze the depth profile of a SiGe membrane, Auger electron spectroscopy (AES) was used. The formation of a native oxidation layer on the surface of SiGe was observed, and the layer had a thickness of 20 Å.

DFT study on dissociative adsorption of SiH 4 and GeH 4 on SiGe(1 0 0)-2 × 1 surface

SiH 4 and GeH 4 dissociative adsorptions on a buckled SiGe(1 0 0)-2 × 1 surface have been analyzed using density functional theory (DFT) at the B3LYP level. The Ge alloying in the Si(1 0 0)-2 × 1 surface affects the dimer buckling and its surface reactivity. Systematic Ge influences on the reaction energetics are found in SiH 4 and GeH 4 reactions with four dimers of Si ∗–Si, Ge ∗–Si, Ge ∗–Ge, and Si ∗–Ge (∗ denotes the protruded atom). On a half H-covered surface, the energy barriers for silane and germane adsorption are higher than those on the pristine surface. The energy barrier for silane adsorption is higher than the corresponding barrier for germane adsorption. Rate constants are also calculated using the transition-state theory. We conclude that the SiGe surface reactivity in adsorption reaction depends on the Ge presence in dimer form. If the surface Ge is present in form of Ge ∗–Ge, the surface reactivity decreases as the Ge ∗–Ge content increases. If the surface Ge prefers to be in form of Ge ∗–Si at low Ge contents, the surface reactivity increases first, then decreases at high Ge surface contents when Ge ∗–Ge prevails. The calculated rate constant ratio of GeH 4 adsorption on Si ∗–Si over Ge ∗–Ge at 650 °C is 2.1, which agrees with the experimental ratio of GeH 4 adsorption probability on Si(1 0 0) over Si(1 0 0) covered by one monolayer Ge. The experimental ratio is 1.7 measured through supersonic molecular beam techniques. This consistency between calculation and experimental results supports that one monolayer of Ge on Si(1 0 0) exists in form of Ge ∗–Ge dimer.

Surface NH3 anneal on strained Si0.5Ge0.5 for metal-oxide-semiconductor applications with HfO2 as gate dielectric

In this Letter, metal-oxide-semiconductor capacitors were fabricated and characterized on compressively strained Si50Ge50 on Si0.8Ge0.2 virtual substrates by using metal-organic chemical vapor deposition HfO2 as gate dielectric and TaN as metal gate electrode. It is demonstrated that surface nitridation treatment using NH3 annealing prior to HfO2 deposition effectively prevents the growth of GeOx at the interfacial layer which exists on SiGe surface with direct deposition of HfO2. X-ray photoelectron spectroscopic analysis suggests that the nitridation process tends to occur to Si atoms rather than to Ge atoms and results in the formation of an interfacial layer of SiNxOy on the SiGe surface. Capacitors with the surface nitridation treatment show good capacitance-voltage characteristics with negligible hysteresis, much smaller interface trap charge density, and significantly improved gate leakage current in comparison with those of capacitors made without nitridation treatment. The surface nitridation seems to be a promising method for the formation of high quality high-κ gate stack on strained SiGe substrate.

Fabrication and mobility characteristics of SiGe surface channel pMOSFETs with a HfO/sub 2//TiN gate stack

This paper describes an extensive experimental study of TiN/HfO/sub 2//SiGe and TiN/HfO/sub 2//Si cap/SiGe gate stacked-transistors. Through a careful analysis of the interface quality (interface states and roughness), we demonstrate that an ultrathin silicon cap is mandatory to obtain high hole mobility enhancement. Based on quantum mechanical simulations and capacitance-voltage characterization, we show that this silicon cap is not contributing any silicon parasitic channel conduction and degrades by only 1 /spl Aring/ the electrical oxide thickness in inversion. Due to this interface optimization, Si/sub 0.72/Ge/sub 0.28/ pMOSFETs exhibit a 58% higher mobility at high effective field (1 MV/cm) than the universal SiO/sub 2//Si reference and a 90% higher mobility than the HfO/sub 2//Si reference. This represents one of the best hole mobility results at 1 MV/cm ever reported with a high-/spl kappa//metal gate stack. We thus validate a possible solution to drastically improve the hole mobility in Si MOSFETs with high-/spl kappa/ gate dielectrics.

Suppressed Boron Diffusion in Bulk Silicon below Strained (100) SiGe Surfaces during Nitrogen Annealing

not Available.

Polycrystalline Si1-xGex thin film deposition by rapid thermal chemical vapor deposition

Polycrystalline silicon germanium (poly-Si1−xGe x ) thin films on a-Si film have been deposited by rapid thermal chemical vapor deposition (RTCVD) with SiH4–GeH4–H2. Effect of GeH4/SiH4 and deposition temperature on stoichiometry (x), Si-Ge binding character, composition, hydrogen configuration, crystallinity, preferred orientation, grain size, and surface roughness of poly-Si1−xGe x films has been investigated. Poly-Si1−xGe x deposited on the substrate with amorphous silicon buffer layer on oxide shows better crystallinity and contains the less amount of oxygen than the one deposited directly on the oxide surface. At low temperature region, the Ge–H bond with the small amount of Si–H2 bond is dominant but all hydrogen bonds are desorbed at high temperature. All films have polycrystalline phase and the grain size and (111) orientation increased with increasing deposition temperature in which Ge content also increases at the fixed gas flow rate of GeH4 to total source gas. Poly-Si1−xGe x /Si thin film transistors (TFT) are fabricated and hydrogen during post-hydrogenation process preferentially is attached to Ge dangling bond and the TFT characteristics could be improved.

Comprehensive study on low-frequency noise and mobility in Si and SiGe pMOSFETs with high-/spl kappa/ gate dielectrics and TiN gate

Low-frequency noise and hole mobility are studied in Si and SiGe surface channel pMOSFETs with various types of high-/spl kappa/ dielectric stacks (Al/sub 2/O/sub 3/, Al/sub 2/O/sub 3//HfAlO/sub x//Al/sub 2/O/sub 3/ and Al/sub 2/O/sub 3//HfO/sub 2//Al/sub 2/O/sub 3/) and TiN as gate electrode material. Comparisons are made with poly-SiGe-gated pMOSFETs as well as poly-Si/SiO/sub 2//Si references. The choice of channel material (strained SiGe or Si), gate material (TiN or poly-SiGe), and high-/spl kappa/ material (Al/sub 2/O/sub 3/, HfO/sub 2/, HfAlO/sub x/) is discussed in terms of mobility and low-frequency noise. A TiN gate in combination with a surface SiGe channel is advantageous both for enhanced mobility and low 1/f noise. The dominant sources of carrier scattering are identified by analyzing the mobility measured at elevated temperatures. The 1/f noise is studied from subthreshold to strong inversion conditions and at different substrate biases. The mobility fluctuation noise model and the number fluctuation noise model are both used to investigate the 1/f-noise origin.

Hydrogen desorption from SiH 4 adsorbed SiGe(001) surfaces

Desorption rates of surface hydrogen from hydrogenated SiGe (001) surfaces with various Ge concentrations have been investigated with thermal desorption spectroscopy. The activation energy of the hydrogen desorption slightly decreases with an addition of Ge in the SiGe film where the frequency factor of the desorption increases. This implies that the desorption center is the Si site. Fourier transform infrared spectroscopy clearly indicates that the hydrogen desorbs not from Ge but mainly from Si at the substrate temperatures in excess of 430 °C, while the surface hydrogen desorbs both from Si and Ge sites below 430 °C.

Lattice relaxation and dislocation generation/annihilation in SiGe-on-insulator layers during Ge condensation process

Dislocation behaviors in SiGe-on-insulator (SGOI) layers during the Ge condensation process are systematically studied by employing transmission electron microscopy, X-ray diffraction, Raman spectroscopy and secondary ion mass spectroscopy, in order to obtain insights into sufficient relaxation of SGOI layers with good crystal quality. It is found that misfit dislocations (MDs) are generated in the ramping up process in Ge condensation, whereas they disappear and threading dislocations (TDs) are generated after the oxidation process. This TD generation is attributable to the fact that the MDs were fragmented into dislocation half-loops and then rose to the SiGe surface. Thus, suppression of the rising motion is a key to fabricating high-quality SGOI substrates by the Ge-condensation technique.

Study of Pathway of Hydrogen Migration and Desorption on SiGe(100) Surface Using Ab Initio Calculations

Ab initio calculations have been carried out to investigate the pathways of H-atom migration and H2 desorption on a mixed SiGe(100)-2×1 surface using the cluster model. The H2 recombinative desorption is the rate-determining step in hydrogen migration and desorption on SiGe(100) surfaces, since the energy barrier to H-atom migration is generally lower than that of H2 desorption. The energy barriers for H2 desorption from the interdimer, the Si–Ge pair, (52.8 kcal/mol), and the Ge–Ge pair, (45.1 kcal/mol), are lower than that for the Si–Si pair by 7.5 and 15.2 kcal/mol, respectively. Thus, the SiGe(100)-2×1 surface in chemical vapor deposition provides more dangling bonds than the Si(100)-2×1 surface because of Ge inclusion. In contrast, the chemisorbed H tends to stay on the Si-site, since the barrier for H-migration from the Ge-site to the Si-site is lower that in the opposite direction by 5.6 kcal/mol. Hence, a considerably higher percentage of Ge sites are dangling bonds, compared with Si sites on the SiGe(100) surface. Related transition state structures in the migration and desorption steps are also discussed.

Quantitative coverage and stability of hydrogen-passivation layers on HF-etched Si(1−x)Gex surfaces

This study investigates the stability of surface hydride layers passivating silicon-germanium alloys against contamination in ambient conditions after treatment in dilute hydrofluoric acid (HF), which is of central importance to the fabrication of SiGe-based semiconductor devices. We report quantitative hydrogen coverages on HF-etched Si(1−x)Gex surfaces (x=0,0.30,0.60,0.82), determined after air-to-vacuum transfer by H-specific H1(N15,αγ)C12 nuclear reaction analysis. Combination of this coverage information with analysis of the zero-point vibrational properties and N15-ion-induced desorption kinetics of the surface H atoms enables the distinction of well-passivated SiGe surfaces terminated exclusively by hydrides of Si and Ge from those partially covered by contaminating adsorbates. It is found that the resistance of HF-etched Si(1−x)Gex alloys against recontamination is drastically reduced at increasing Ge contents. Pure Si(100)–H is stably passivated at least up to 1week in air by a layer of 1.3±0.1 monolayer total H coverage. Si0.70Ge0.30 initially resembles H-passivated Si(100) but shows indications of moderate contamination after 1week in air. The HF treatment does not produce stable passivation layers on Ge-rich alloys (x⩾0.60), which suffer heavy recontamination within minutes after removal from the HF solution.

Density functional theory study of adsorption and dissociation of HfCl4 and H2O on Ge∕Si(100)-(2×1): Initial stage of atomic layer deposition of HfO2 on SiGe surface

We have investigated adsorption and dissociation of water and HfCl4 on Ge∕Si(100)-(2×1) surface with density functional theory. The Si–Ge heterodimer and Ge–Ge homodimer are employed to represent the Si1−xGex surface. The activation energy for adsorption of water on Ge–Ge homodimer is much higher than that on Si–Ge heterodimer. No net activation barrier exists during the adsorption of HfCl4 on both SiGe surface dimers. The differences in the potential energy surface between reactions on Si–Ge and Ge–Ge dimers are due to different bond strengths. It should also be noticed that the activation energy for HfCl4 is quite flat, thus HfCl4 adsorbs and dissociates on Ge∕Si(100)-(2×1) easily.

Quantum chemical study of adsorption and dissociation of HfCl4 and H2O on Ge/Si(100) − (2×1): Initial stage of atomic layer deposition of HfO2 on SiGe surface

We investigated adsorption and dissociation of water and HfCl4 on a Ge/Si(100) −(2 × 1) surface with a density-functional theory. The Si–Ge and Ge–Ge homodimers are used to represent the Si1−xGex surface. (i) Water first adsorbs on the bare Ge/Si(100) − (2 × 1) surface and then dissociates into OH and H. The activation energy for adsorption of water on the Ge–Ge homodimer is much higher than that on the Si–Ge heterodimer. (ii) HfCl4 dissociates upon adsorption on the Ge/Si(100) − (2 × 1) surface into HfCl3 and Cl. No net activation barrier exists during the adsorption of HfCl4 on both SiGe surface dimers. The molecular adsorption state is found to be metastable according to the calculation, which implies that the reaction tends to move toward to the product rather than trapping in HfCl4 adsorbed state. The difference in the potential energy surface between reactions on Si–Ge and Ge–Ge dimers is due to different bond strengths.

Density functional theory study of initial stage of ZrO 2 atomic layer deposition on Ge/Si(100)-(2×1) surface

The reaction mechanisms of ZrCl 4 adsorption and dissociation on Ge/Si(100)-(2×1) surface as the initial stage of ZrO 2 atomic layer deposition process are investigated with density functional theory (DFT). The Si–Si, Si–Ge and Ge–Ge one-dimer cluster models are employed to represent Ge/Si(100)-(2×1) surface with different Ge composition. The reaction of ZrCl 4 with hydroxylated Ge/Si(100)-(2×1) surface forms a bridged ZrCl 2 site, while on H-passivated surface, ZrCl 3 site is formed. The reaction energy barrier of ZrCl 4 with H-passivated SiGe surface is much higher than ZrCl 4 with hydroxylated SiGe surface, which indicates that reaction proceeds more slowly on H-passivated surface than on OH-terminated surface. In addition, adsorption of ZrCl 4 is favorable on Ge surface atoms than on Si surface atoms.

Native oxide removal from SiGe using mixtures of HF and water delivered by aqueous, gas, and supercritical CO 2 processes

HF and water delivered using liquid, gas, and supercritical CO 2 phases were used to remove native oxides and suboxides from SiGe surfaces containing 15–30% Ge. X-ray photoelectron spectroscopy was used to monitor the Si 2p and Ge 2p 3/2 peaks as a function of the process type, HF concentration, and Ge fraction. A larger percentage of Si oxides were removed compared to Ge oxides by all three processes. The threshold HF concentration in aqueous-phase solutions for complete removal of Si and Ge oxides was approximately 28 millimole/l (mM) on Si 0.85Ge 0.15 and increased by approximately an order of magnitude on the surfaces with the higher Ge concentrations. The threshold concentration increased from 0.007 mM on Si 0.85Ge 0.15 to at least 0.14 mM on Si 0.70Ge 0.30 to obtain significant removal of both Si and Ge oxides using supercritical CO 2 to carry the HF and water etching solution. Although air exposure of the liquid water and supercritical CO 2 etched samples provided an opportunity for reoxidization, the trends in the metal and oxide XPS peak areas with HF concentration suggest that chemical treatment had the largest effect. Gas-phase HF/vapor processes removed all of the SiO 2, Si suboxides, and GeO 2 in less than 30 s offering the best performance of the processes studied without roughening the surface.

Observation of strain field fluctuation in SiGe-relaxed buffer layers and its influence on overgrown structures

We investigated in-plane strain field distribution in strained-Si/strain-relaxed SiGe buffer layers by spatially resolved micro-Raman spectroscopy. Crosshatch-like strain distribution was observed in both the strained-Si and SiGe layer, indicating that strain field coming from underlying misfit dislocations reaches the SiGe surface and causes the strain fluctuation in the strained-Si layer. The wavelength of the distribution was found to strongly depend on the SiGe thickness. The amount of the fluctuation corresponded to 13% of the strain that strained-Si layer inherently had. It was also found that the strain field modulated the local growth rate of overgrown layer and caused the enhancement of the surface roughness. The strain fluctuation, therefore, should be carefully taken into account for device applications.

Fabrication of high-quality strain-relaxed thin SiGe layers on ion-implanted Si substrates

We fabricated high-quality strain-relaxed thin SiGe layers by Ar ion implantation into Si substrates before epitaxial growth. The surface of 100-nm-thick Si0.8Ge0.2 layers, the relaxation ratio of which was more than 80%, was found to be very smooth, with a rms roughness of 0.34 nm. Cross-sectional transmission electron microscopy analysis confirmed that strain-relieving dislocations were effectively generated due to the ion-implantation-induced defects and confined in the vicinity of the heterointerface, resulting in a dislocation-free SiGe surface. Moreover, in-plane strain-field fluctuation was found to be largely reduced by this ion implantation method.

The effect of Sb surfactant assisted growth on SiGe surface morphology

A scanning tunneling microscopy (STM) investigation into the surface morphology of Si and Si 0.8Ge 0.2 films grown at 500 °C on Si (100) with and without 0.8 monolayers (ML) of an Sb surfactant is presented. The presence of Sb induces the formation of large three-dimensional islands in Si and Si 0.8Ge 0.2 leading to a rougher surface morphology than films grown without Sb. The roughness of the Sb/SiGe surface is greater than Sb/Si surface, which we propose is due to more favorable exchange energetics between Sb and Ge than between Sb and Si. Encapsulation of the SiGe surface by an Si top layer results in the burial of the SiGe islands leading to compositional abrupt but physically rough interfaces.

Impact of the Ge concentration on the Ge-ionisation probability and the matrix sputter yield for a SiGe matrix under oxygen irradiation

As SiGe becomes a building block of advanced devices, depth profiling of SiGe gains in importance, requiring an understanding of its basic behavior under oxygen bombardment. In this study, the sputter yield and ionization probability of Si and Ge sputtered from SiGe-substrates are studied as a function of the incidence angle. The altered layer formation and the element redistribution are studied with in situ sputter/RBS and the oxidation of the bombarded SiGe surface is analyzed with XPS. The results show that the altered layer formation in the SiGe system is much more complex than in the Si case and strongly dependent of the incident angle/primary energy. In addition to sputter related effects, thermodynamics and volatility of the Ge-oxide are important mechanisms.