Drift hole mobility in strained and unstrained doped Si1-xGex alloys

Results of the drift hole mobility in strained and unstrained SiGe alloys are reported for Ge fractions varying from 0 to 30% and doping levels of 10¹⁵-10¹⁹ cm^-3. The mobilities are calculated taking into account acoustic, optical, alloy, and ionized-impurity scattering. The mobilities are then compared with experimental results for a boron doping concentration of 2×10¹⁹ cm^-3. Good agreement between experimental and theoretical values is obtained. The results show an increase in the mobility relative to that of silicon     

Lattice mobility of holes in strained and unstrained Si1-xGex alloys

Results of the lattice drift mobility in strained and unstrained SiGe alloys are reported for Ge fractions, 0.0⩽x⩽1.0. The mobilities are calculated using acoustic, optical, and alloy scattering mechanisms. Due to the strain-induced symmetry reduction in the band structure of Si_1-xGe_x, the mobility is found to be a tensor with two distinct components parallel and perpendicular to the growth plane. Assuming that the scattering mechanisms are independent of the strain, the strained mobility increases exponentially with increasing Ge content, for x=0.3     

Effective mass and mobility of holes in strained Si1-xGex layers on (001) Si1-yGey substrate

The directional density-of-state effective masses of the valence bands of a strained Si_1-xGe_x layer for the (001) growth direction are calculated using k×p and strain Hamiltonians. The mobilities are then calculated as functions of temperature and doping concentration for various Ge contents using the relaxation time approximation and the known valence-band structure. The nonparabolicity and warped nature of the valence bands are included in the mobility calculation. Under the biaxial strain present in the film, all the directional effective masses except the longitudinal heavy hole mass at the Γ point are shown to be strongly affected by the strain. Comparatively, the strain effect becomes weak for large k values. The mobility of the strained layer becomes anisotropic under strain. Both the longitudinal and the transverse mobilities are higher than that of the relaxed alloy with the same Ge content     

Silicide/strained Si1-xGex Schottky-barrierinfrared detectors

By employing a thin silicon sacrificial cap layer for silicide formation, the authors successfully demonstrated Pd₂Si/strained Si_1-xGe_x Schottky-barrier infrared detectors with extended cutoff wavelengths. The sacrificial silicon eliminates the segregation effects and Fermi level pinning which occur if the metal reacts directly with Si_1-x Ge_x alloy. The Schottky barrier height of the silicide/strained Si_1-xGe_x detector decreases with increasing Ge fraction, allowing for tuning of the detector's cutoff wavelength. The cutoff wavelength was extended beyond 8 μm in PtSi/Si _0.85Ge_0.15 detectors. It is shown that high quantum efficiency and near-ideal dark current can be obtained from these detectors     

Low- and high-field electron-transport parameters for unstrainedand strained Si1-xGex

Ohmic minority and majority drift mobilities as well as saturation velocities are reported for unstrained and strained Si_1-xGe _x alloys up to z=0.31. The electron-transport model is verified by measurements of the in-plane majority drift mobility in strained Si_1-xGe_x samples for various dopant and Ge concentrations. Saturation velocities are determined by full-band Monte Carlo simulations. There is no substantial decrease in the mobility perpendicular to the Si/SiGe interface for doping concentrations above 10¹⁹ cm^-3 and growing x. In contrast, the saturation-drift velocity is strongly reduced with x     

Photoresponse model for Si1-xGex/Siheterojunction internal photoemission infrared detector

A photoresponse model has been developed for the Si_1-xGe_x/Si heterojunction internal photoemission (HIP) infrared detector at wavelengths corresponding to photon energies less than the Fermi energy. A Si_0.7Ge_0.3/Si HIP detector with a cutoff wavelength of 23 μm and an emission coefficient of 0.4 eV^-1 has been demonstrated. The model agrees with the measured detector response at λ>8 μm. The potential barrier determined by the model is in close agreement (difference ~4 meV) with the potential barrier determined by the Richardson plot, compared to the discrepancies of 20-50 meV usually observed for PtSi Schottky detectors     

Low-resistance bandgap-engineeredW/Si1-xGex/Si contacts

Bandgap-engineered W/Si_1-xGe_x/Si junctions (p⁺ and n⁺) with ultra-low contact resistivity and low leakage have been fabricated and characterized. The junctions are formed via outdiffusion from a selectively deposited Si_0.7Ge _0.3 layer which is implanted and annealed using RTA. The Si _1-xGe_x layer can then be selectively thinned using NH₄OH/H₂O₂/H₂O at 75°C with little change in characteristics or left as-deposited. Leakage currents were better than 1.6×10^-9 A/cm² (areal), 7.45×10^-12 A/cm (peripheral) for p⁺/n and 3.5×10^-10 A/cm² (peripheral) for n⁺/p. W contacts were formed using selective LPCVD on Si_1-xGe_x. A specific contact resistivity of better than 3.2×10^-8 Ω cm² for p ⁺/n and 2.2×10^-8 Ω cm² for n⁺/p is demonstrated-an order of magnitude n⁺ better than current TiSi₂ technology. W/Si_1-xGe _x/Si junctions show great potential for ULSI applications     

High performance Si/Si1-xGex resonanttunneling diodes

Resonant tunneling diodes (RTDs) with strained i-Si_0.4Ge_0.6 potential barriers and a strained i-Si quantum well, all on a relaxed Si_0.8Ge_0.2 virtual substrate were successfully grown by ultra high vacuum compatible chemical vapor deposition and fabricated using standard Si processing methods. A large peak to valley current ratio of 2.9 and a peak current density of 4.3 kA/cm² at room temperature were recorded from pulsed and continuous dc current-voltage measurements, the highest reported values to date for Si/Si_1-xGe_x RTDs. These dc figures of merit and material system render such structures suitable and highly compatible with present high speed and low power Si/Si_1-xGe_x heterojunction field effect transistor based integrated circuits     

Bidirectional bistability in n-p-nSi/Si1-xGex/Si structures

Several structures of n-Si/p-Si_1-xGe_x/n-Si double heterojunction bipolar transistors (DHBTs) with strained thin base, fabricated by molecular beam epitaxy (MBE), are described. Negative differential resistance (NDR) phenomena-a strong and symmetric bidirectional bistability modulated by base bias, together with a multistep characteristic in collector current versus emitter-collector bias voltage in the devices with very thin base-were observed at room temperature. The physical origins are analyzed. The results are compared with the characteristics of n-Ga_1-xA1_xAs/p-GaAs/n-GaAs single HBTs (SHBTs)     

Si/Si1-xGex heterojunction bipolartransistors with high breakdown voltage

Heterojunction bipolar transistors are desirable for microwave applications because a low base resistance can be achieved yielding high maximum frequency of oscillation. Here we report Si/Si_1-xGe _x heterojunction bipolar transistors with high breakdown voltages and excellent small-signal microwave characteristics. The transistors structures were grown by molecular beam epitaxy and fabricated by a double-mesa process. Measured f_T and f_max were 10 and 22 GHz, respectively, for transistors with BV_CBO of 40 V     

Nickel silicidation techniques for strained Si1?xGex, Si1?x?yGexCy, and Si1?yCy alloys material-device applications

A nickel silicide process for Si_1-xGe_x, Si_1-x-yGe_xC_y, and Si_1-yC_y alloy materials compatible with Si technology has been developed. Low-resistivity-phase (12–20 μΘ cm) nickel silicides have been obtained for these alloys with different low sheet-resistance temperature windows. The study shows that thin (15–18 nm) silicide layers with high crystalline quality, smooth silicide surface, and smooth interface between silicide and the underlying material are achievable. The technique could be used to combine the benefits of Ni silicide and Si_1-xGe_x, Si_1-x-yGe_xC_y, and Si_1-yC_y alloys. The technique is promising for Si or Si_1-xGe_x, Si_1-x-yGe_xC_y, and Si_1-yC_y alloy-based metal-oxide semiconductor, field-effect transistors (MOSFETs) or other device applications.     

Electron Mobility of Doped Strained Si1-x Gex Alloys Grown on Si(100) Substrate^①②

The electron mobility for strained Si1-xGex alloy layer grown on Si(100) substrate has been calculated for doping rage of 10^17 cm^-3 to 10^20 cm^-3with Ge contents of 0.0≤x≤1.0.The results show a decrease in the mobility with increasing of Ge content and doping concentration.The electron mobility in-plane is foud to be smaller than the perpendicular component.     

A drift-diffusion/Monte Carlo simulation methodology for Si1-xGex HBT design

An accurate and efficient simulation methodology for Si_1-x Ge_x HBTs is presented. A two-dimensional (2-D) drift-diffusion solver is employed for dc and ac characteristics, and one-dimensional (1-D) full-band Monte Carlo for transport in the base-collector high-electric-field region. Extrinsic parasitics are introduced as lumped circuit elements whose values are obtained from measurements and layout considerations. This approach not only reduces the computational cost of the simulation, but it also helps to differentiate the relevance of the intrinsic and extrinsic device parameters. We discuss the calibration of the simulation on a 0.25 μm process and use a 1-D regional analysis in the quasi-static approximation to identify the major source of delay. Results of the delay analysis were used to improve device performance for the 0.16 μm technology node     

The impact of germanium in strained Si/relaxed Si1-xGex on carrier performance in non-degenerate and degenerate regimes

The impact of the fraction of germanium on the carrier performance of two-dimensional strained silicon, which embraces both the non-degenerate and degenerate regimes,is developed.In this model,the Fermi integral of order zero is employed.The impact of the fraction of germanium on the relaxed Si_1-xGe_x substrate（x）,carrier concentration and temperature is reported.It is revealed that the effect of x on the hole concentration is dominant for a normalized Fermi energy of more than three,or in other words the non-degenerate regime.On the contrary, the x gradient has less influence in the degenerate regime.Furthermore,by increasing x there is an increase in the intrinsic velocity,particularly with high carrier concentration and temperature.     

Si/Si1-xGex heterojunction bipolartransistors produced by limited reaction processing

Si/Si_1-xGe_x heterojunction transistors (HBTs) fabricated by a chemical vapor deposition (CVD) technique are reported. A rapid thermal CVD limited-reaction processing (LRP) technique was used for the in situ growth of all three device layers, including a 20-mm Si_1-xGe_x layer in the base. The highest current gains observed (β=400) were for a Si/Si_1-x Ge_x HBT with a base doping of 7×10¹⁸ cm^-3 near the junction and a shallow arsenic implant to form ohmic contacts and increase current gain. Ideal base currents were observed for over six decades of current and the collector current remained ideal for nearly nine current decades starting at 1 pA. The bandgap difference between a p-type Si layer doped to 5×10¹⁷ cm^-3 and the Si_1-xGe_x(x=0.31) base measured 0.27 eV. This value was deduced from the measurements of the temperature dependence of the base current and is in good agreement with published calculations for strained Si_1-xGe_x layers on Si     

A p-channel poly-Si/Si1-xGex/Si sandwichedconductivity modulated thin-film transistor

A p-channel poly-Si/Si_1-xGe_x/Si sandwiched conductivity modulated thin-film transistor (CMTFT) is proposed and demonstrated in this paper for the first time. This structure uses a poly-Si/Si_1-xGe_x/Si sandwiched structure as the active layer to avoid the poor interface between the gate oxide and the poly-Si_1-xGe_x material. Also an offset region placed between the channel and the drain is used to reduce the leakage current. Furthermore, the concept of conductivity modulation in the offset region is used to provide high on-state current. Results show that this structure provides high on-state current as well as low leakage current as compared to that of conventional offset drain TFTs. The on-state current of the structure is 1.3-3 orders of magnitude higher than that of a conventional offset drain TFT at a gate voltage of -24 V and drain voltage ranging from -15 to -5 V while maintaining comparable leakage current     

The effects of base dopant outdiffusion and undopedSi1-xGex junction spacer layers in Si/Si1-xGex/Si heterojunction bipolar transistors

The effects of base dopant outdiffusion and nominally undoped Si _1-xGe_x spacer layers at the junction interfaces of Si/Si_1-xGe_x/Si n-p-n heterojunction bipolar transistors (HBTs) have been studied. It has been found that small amounts of boron outdiffusion from heavily doped bases of nonabrupt interfaces cause parasitic barriers in the conduction band, which drastically reduce the collector current enhancement in the HBTs. Undoped interface spacers can remove the parasitic barriers, resulting in a strongly improved collector current enhancement     

Graded-base Si/Si1-xGex/Si heterojunctionbipolar transistors grown by rapid thermal chemical vapor depositionwith near-ideal electrical characteristics

Graded-base and uniform-base Si/Si_1-xGe_x/Si heterojunction bipolar transistors with near-ideal base and collector currents have been fabricated by rapid thermal chemical vapor deposition. The temperature dependences of the collector currents are shown to obey a simple analytical model of an effective Gummel number. The model can be applied to devices which have arbitrary base profiles. The base currents are independent of base composition, and current gains in excess of 11000 have been observed at 133 K     

Optimization of Si1-xGex/Si waveguidephotodetectors operating at λ=1.3 μm

This paper analyzes the influence of various design parameters in the external quantum efficiency (QE) of waveguide detectors based on Si/Si_1-xGe_x strained-layer superlattices (SLSs), for use in optical communications at λ=1.3 μm. The study presents an algorithm that automatically generates structurally stable SLSs. This generation is completed by intensive simulation of the generated SLSs to calculate the external QE. The simulation embraces optical waveguiding, absorption, quantum size effect, as well as thermodynamics of the strained layers. Two sets of data were created using two different models for the SiGe layer critical thickness, h_c(x). A conservative model for h_c, corresponding to the equilibrium regime, yielded discrete maximum values for QE (around 12%) that were mainly dependent on the alloy absorption. A second model for h_c, corresponding to the metastable regime, produced considerably higher QEs (around 60%), and shows the great importance of fiber-to-waveguide coupling efficiency. The importance of the passive-waveguide coupler geometry is investigated using the beam propagation method     

A deep submicron Si1-xGex/Si vertical PMOSFETfabricated by Ge ion implantation

We report a deep submicron vertical PMOS transistor using strained Si_1-xGe_x channel formed by Ge ion implantation and solid phase epitaxy. These vertical structure Si_1-xGe_x /Si transistors can be fabricated with channel lengths below 0.2 μm without using any sophisticated lithographic techniques and with a regular MOS process. The enhancement of hole mobility in a direction normal to the growth plane of strained Si_1-xGe_x over that of bulk Si has been experimentally demonstrated for the first time using this vertical MOSFET. The drain current of these vertical MOS devices has been found to be enhanced by as much as 100% over control Si devices. The presence of the built-in electric field due to a graded SiGe channel has also been found to be effective in further enhancement of the drive current in implanted-channel MOSFET's