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     

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     

Numerical simulation and comparison of Si BJTs andSi1-xGex HBTs

Using the Monte Carlo method for the solution of the Boltzmann transport equation, the authors analyze the low-field carrier mobilities of strained layer and bulk Si and Si_1-xGe_x alloys. Strained alloy layers exhibit higher low-field mobility compared with bulk Si at doping levels >10¹⁸ cm^-3 and for a Ge mole fraction x⩽0.2, while the unstrained alloy bulk low-field mobility is always lower than that of Si for any doping level or mole fraction. These mobilities are then used in a two-dimensional drift-diffusion equation solver to simulate the performance of Si BJTs (bipolar junction transistors) and Si_1-xGe_x HBTs (heterojunction bipolar transistors). The substitution of a Si_0.8 Ge_0.2 layer for the base region leads to a significant improvement in current gain, turn-on voltage, and high-frequency performance. Maximum unity current gain frequency f_T increases two times over that of an Si BJT if the bulk alloy mobility is used for the alloy base layer; it increases three times if strained-layer mobility is used. Maximum frequency of oscillation also improves, but not as dramatically as f_T     

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     

Influence of high channel doping on the inversion layer electron mobility in strained silicon n-MOSFETs

In this letter, we investigate the dependence of electron inversion layer mobility on high-channel doping required for sub-50-nm MOSFETs in strained silicon (Si), and we compare it to co-processed unstrained Si. For high vertical effective electric field E/sub eff/, the electron mobility in strained Si displays universal behavior and shows enhancement of 1.5-1.7/spl times/ compared to unstrained Si. For low E/sub eff/, the mobility for strained Si devices decreases toward the unstrained Si data due to Coulomb scattering by channel dopants.     

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.     

High Mobility Strained Ge pMOSFETs With High-κ/Metal Gate

Compressively strained Ge long channel ring-type pMOSFETs with high-kappa Si/SiO₂/HfO₂/TiN gate stacks are fabricated on Si_0.2Ge_0.8 virtual substrates. Effective oxide thickness is approximately 1.4 nm with low gate leakage current. A peak hole mobility of 640 cm²/ Vldrs and up to a four times enhancement over the Si/SiO₂ universal curve are observed. Parasitic conduction within the Si-cap layers degrades the mobility at large vertical fields, although up to a 2.5 times enhancement over universal remains at a field of 0.9 MV/cm.     

Valence band structure of GexSi1-x for holetransport calculation

We have calculated the hole densities of states and the velocities as functions of energy in strained and relaxed p-type Ge_xSi _1-x layers grown on 〈001〉 Si substrates. It is shown that the nonparabolic and nonspherical effects are very large in the energy range of (0, 0.2 eV) measured from the heavy hole band edge. Deeper into the valence band, the bands gradually become parabolic and spherical. For most applications, the impurity doping concentration is below 10²⁰ cm^-3. For 10²⁰ cm^-3 p-type doped Si, the Fermi level is 77.3 meV at 77 K. It is therefore concluded that the nonparabolic and nonspherical effects must be taken into proper consideration when investigating the transport properties of p-type Ge_xSi_1-x samples. The calculated data of both relaxed and strained Ge_xSi_1-x valence band structures are curve fitted and a data library is built up for further study of the hole transport properties. The mobility and the diffusion coefficient are largely affected when the doping concentration is increased. It is found that at high doping concentration the contributions from the light hole and spin split-off bands become very important, they can become even larger than the contribution from the heavy hole band, even if their densities of states are smaller than that of the heavy hole band     

Improved Ge Surface Passivation With Ultrathin SiOX Enabling High-Mobility Surface Channel pMOSFETs Featuring a HfSiO/WN Gate Stack

To realize high-mobility surface channel pMOSFETs on Ge, a 1.6-nm-thick SiO_X passivation layer between the bulk Ge substrate and HfSiO gate dielectric was introduced. This approach provides a simple alternative to epitaxial Si deposition followed by selective oxidation and leads to one of the highest peak hole mobilities reported for unstrained surface channel pMOSFETs on Ge: 332 cm² middotV^-1middots^-1 at 0.05 MV/cm-a 2times enhancement over the universal Si/SiO₂ mobility. The devices show well-behaved output and transfer characteristics, an equivalent oxide thickness of 1.85 nm and an I_ON/I_OFF ratio of 3times10³ without detectable fast transient charging. The high hole mobility of these devices is attributed to adequate passivation of the Ge surface     

Fabrication of laser-annealed poly-TFT by forming aSi1-xGex thermal barrier

Germanium is ion-implanted deeply into the bottom of a Si film before excimer laser annealing begins. During the solidification step, the implanted Ges form a high thermal resistive Si_1-xGe_x alloy, which reduces the thermal extraction rate of laser energy and the grain growth rate. Laterally larger but double-stacked grains were achieved with a higher Ge implant dose and a slower grain growth. The performance of fabricated poly-TFTs has been enhanced with a Ge 5×10¹⁵/cm² at 80 keV implant but deteriorated at a higher dose. We attribute this enhancement to a laterally enlarged grain and show that the performance of TFT is deteriorated more dominantly by other Ge-related factors than by surface roughening and Ge-induced defect creation     

Fabrication and analysis of deep submicron strained-Si n-MOSFET's

Deep submicron strained-Si n-MOSFETs were fabricated on strained Si/relaxed Si_0.8Ge_0.2 heterostructures. Epitaxial layer structures were designed to yield well-matched channel doping profiles after processing, allowing comparison of strained and unstrained Si surface channel devices. In spite of the high substrate doping and high vertical fields, the MOSFET mobility of the strained-Si devices is enhanced by 75% compared to that of the unstrained-Si control devices and the state-of-the-art universal MOSFET mobility. Although the strained and unstrained-Si MOSFETs exhibit very similar short-channel effects, the intrinsic transconductance of the strained Si devices is enhanced by roughly 60% for the entire channel length range investigated (1 to 0.1 μm) when self-heating is reduced by an ac measurement technique. Comparison of the measured transconductance to hydrodynamic device simulations indicates that in addition to the increased low-field mobility, improved high-field transport in strained Si is necessary to explain the observed performance improvement. Reduced carrier-phonon scattering for electrons with average energies less than a few hundred meV accounts for the enhanced high-field electron transport in strained Si. Since strained Si provides device performance enhancements through changes in material properties rather than changes in device geometry and doping, strained Si is a promising candidate for improving the performance of Si CMOS technology without compromising the control of short channel effects     

Electrical properties of heavily doped polycrystallinesilicon-germanium films

The electrical properties of polycrystalline silicon-germanium (poly-Si_1-xGe_x) films with germanium mole fractions up to 0.56 doped by high-dose ion implantation are presented. The resistivity of heavily doped p-type (P⁺) poly-Si_1-x Ge_x is much lower than that of comparably doped poly-Si, because higher levels of boron activation and higher hole mobilities are achieved in poly-Si_1-xGe_x. The resistivity of heavily doped n-type (N⁺) poly-S_1-xGe_x is similar to that of comparably doped poly-Si for x<0.45; however, it is considerably higher for larger Ge mole fractions due to significant reductions in phosphorus activation. Lower temperatures (~500°C), as well as lower implant doses, are sufficient to achieve low resistivities in boron-implanted poly-Si_1-xGe_x films, compared to poly-Si films. The work function of P⁺ poly-Si_1-xGe_x decreases significantly (by up to ~0.4 Volts), whereas the work function of N⁺ poly-Si_1-xGe_x decreases only slightly, as Ge content is increased. Estimates of the energy bandgap of poly-Si_1-xGe_x show a reduction (relative to the bandgap of poly-Si) similar to that observed for unstrained single-crystalline Si_1-xGe_x for a 26% Ge film, and a reduction closer to that observed for strained single-crystalline Si _1-xGe_x for a 56% Ge film. The electrical properties of poly-Si_1-xGe_x make it a potentially favorable alternative to poly-Si for P⁺ gate-material applications in metal-oxide-semiconductor technologies and also for p-channel thin-film transistor applications     

Photoresistance of Si/Ge/Si structures with germanium quantum dots

An exponential decrease in the resistance of a Si/Ge/Si structure containing germanium quantum dots with an increase in the band-to-band optical excitation intensity is observed at 4.2 K. Two different exponential regions in the dependence of structure resistance on the optical excitation intensity are observed in elastically strained structures, but only one such region is observed in unstrained structures. The experimental results obtained are explained within the model of the hopping conduction of nonequilibrium electrons, which are localized at and between quantum dots in the strained structures, but are localized only between quantum dots in the unstrained structures.     

An In0.15Ga0.85As/GaAs pseudomorphic single quantum well HEMT

This letter describes high electron mobility transistors (HEMT's) utilizing a conducting channel which is a single In0.15Ga0.85AS quantum well grown pseudomorphically on a GaAs substrate. A Hall mobility of 40 000 cm²/V.s has been observed at 77 K. Shubnikov-de Haas oscillations have been observed at 4.2 K which verify the existence of a two-dimensional electron gas at the In0.15Ga0.85As/GaAs interface. HEMT's fabricated with 2-µm gate lengths show an extrinsic transconductance of 90 and 140 mS/mm at 300 and 77 K, respectively-significantly larger than that previously reported for strained-layer superlattice InxGa1-xAs structures which are nonpseudomorphic to GaAs substrates. HEMT's with 1-µm gate lengths have been fabricated, which show an extrinsic transconductance of 175 mS/mm at 300 K which is higher than previously reported values for both strained and unstrained InxGa1-xAs FET's. The absence of AlxGa1-xAs in these structures has eliminated both the persistent photoconductivity effect and drain current collapse at 77 K.     

Bulk and surface 1/f noise

The 1/f noise of an n-type silicon MOSFET has been studied under conditions ranging from accumulation to depletion at 300 K. The experimental results are interpreted in terms of a bulk phenomenon and are characterized by Hooge's empirical 1/f noise parameter α with values between 10^-7 and 10^-5. The α value for surface conduction at strong accumulation can be at least one order of magnitude larger than the value for bulk conduction     

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     

Electrical characterization of heavily doped polycrystalline silicon for high-frequency bipolar transistor application

Electrical conduction data from heavily doped p-type polysilicon thin films at room temperature and above are presented. Specifically, the sheet resistance, in the range from 1 kΩ/□ to 100 Ω/□ for a doping level of 10¹⁹cm^-3to 10²⁰cm^-3, is characterized over temperatures from 300 to 450 K. It is shown that the polysilicon resistivity, larger than the corresponding crystalline value by a factor ∼ 10, is flat over the entire temperature range used for measurement. This large resistivity is correlated to the degree of dopant activation and the mobility in polysiUcon. The measured mobility varying from 8 to 20 cm²/V . s is shown to be smaller than the crystalline mobility at the same doping level by a factor 7 ∼ 3. These data are comprehensively discussed and quantified, based on a distributed resistivity model.     

Electron transport in bipolar transistors with biaxially strainedbase layers

In silicon npn bipolar junction transistors grown on (100) oriented substrate, at base doping levels in excess of 10²⁰ boron atoms/cm³, strain induced splitting of the normally sixfold degenerated conduction band minimum becomes important and needs to be considered in modeling of injection currents. The biaxial tensile strain, originating in the smaller covalent radius of boron compared to silicon, induces a lowering of two valleys with heavy effective mass in vertical direction whereas the remaining four valleys are raised in energy. Using a coupled set of equations for the electron gas systems in the twofold and fourfold degenerated valleys, emitter and collector current formulas are derived. In the relevant case of strong f-type intervalley scattering rates compared to Auger recombination rates (which holds at least up to about 10²¹ cm^-3) collector currents are described by (V_BC=0 V) j_C=-e(D_n4n_4,0+D_n2n_2,0 )/w(e^V(BE/V(th))-1) provided that the electron diffusion length is large compared to the base width w. D_n4 D _n2, and n_4,0, n_2,0 are diffusion constants and equilibrium minority carrier concentrations in the two electron gas systems, respectively. In Si/SiGe heterojunction bipolar transistors the conduction band situation in the base is similar to that in extremely heavily boron doped (homojunction) base layers as presence of Ge also causes the conduction band minimum to split (splitting is, however, of opposite sign). Thus, the transport model discussed here applies also to that kind of device     

Single and dual p-doped channel In0.52Al0.48As/InxGa1-xAs (x=0.53, 0.65) FET's andthe role of doping

The properties of lattice-matched (x=0.53) and strained ( x=0.65) In_0.52Al_0.48As/In_xGa _1-xAs p-doped channel FETs are reported. The role of doping density is studied with the help of two designs (dual-channel with low doping and single-channel with high doping). The strained dual-channel devices demonstrated an improvement of mobility from 108 cm²/V-s (53% In) to 265 cm²/V-s (65% In) at 300 K. The corresponding intrinsic transconductance enhancement is from 23 Ms/mm (53% In) to 46.5 mS/mm (65% In) using 1.0 μm-long gates. The cutoff frequency (f_t) also improves from 1.0 to 1.4 GHz. The impact of strain in the highly-doped single-channel device is small. The band structure under lattice-matched and strained conditions and the position of the Fermi level according to doping seem to be the main factors determining the reported features