Electrical and material quality ofSi1-xGex/Si p-N heterojunctions produced bylimited reaction processing

Si_1-xGe_x/Si p-N heterojunctions prepared by a chemical vapor deposition technique, limited reaction processing (LRP) were characterized using DC electrical measurements, transmission electron microscopy (TEM), and X-ray topography. Heterojunctions with Si _1-xGe_x layer thickness ranging from 52 to 295 nm and a constant Ge fraction of 23% were fabricated to study the effect of increasing the number of misfit dislocations on the device characteristics. Devices with the thinnest layers (⩽120 nm) display forward characteristics with ideality factors of 1.01 and reverse leakage current densities of less than 4 nA/cm for a 5-V reverse bias. These thin-layer devices have dislocation spacings greater than 10 μm. Devices utilizing Si_1-xGe_x layers thicker than 200 nm have forward characteristics which clearly display the presence of recombination currents, and reverse leakage current densities greater than 290 nA/cm² at -5 V. The dislocation spacing in these devices is less than 1 μm. Ideal characteristics were found at room temperature in devices known to contain dislocations     

Ga1-xAlxSb avalanche photodiodes: Resonant impact ionization with very high ratio of ionization coefficients

The liquid-phase epitaxy and device fabrication of p-n and p-i-n Ga1-xAlxSb avalanche photodiodes is described. Breakdown voltages up to 95 V and dark currents of 10^-4A/cm²have been obtained. With p-i-n diodes we have measured the impact ionization coefficients α (electrons) and β (holes) with different composition and temperature. A resonant enhancement of the hole ionization coefficient is found forx = 0.065(300 K) where the ratiobeta/alphaexceeds values of 20. This effect is attributed to impact ionization initiated by holes from the split-off valence band: if the spin orbit splitting Δ is equal to the bandgap energy Eg, the threshold energy for hole initiated impact ionization reaches the smallest possible value (E_{i} = E_{g}) and the ionization process occurs with zero momentum. This leads to a strong increase of β atDelta/E_{g} = 1. The experimentally determined dependence of ionization coefficients on threshold energy is compared with theoretical expectations.     

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-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     

Investigation of poly-Si1-xGex for dual-gateCMOS technology

Poly-Si_1-xGe_x-gated MOS capacitors were fabricated with x varying from 0 to 0.5. NMOS and PMOS C-V characteristics were measured. Reduced poly-gate depletion effect (PDE) was observed in PMOS devices with increasing Ge mole fraction; while for NMOS, devices with a Ge content ~20% exhibit the least PDE. Higher active dopant concentration and reduced gate-depletion width for devices featuring less PDE were confirmed. Work function difference (Φ_MS) was found to decrease slightly in N⁺ films and significantly in P⁺ films as Ge content increases. The shift in Φ_MS for N⁺ poly-Si_1-xGe_x is negligible while it is -0.13 V for P⁺Si_0.8Ge_0.2 and -0.32 V for P⁺Si_0.5Ge_0.5. The reduction in energy bandgap (ΔE_g) was also determined to increase from 0 to 0.26 eV as Ge content increases from 0 to 50%. For deep submicron dual-gate CMOS application, the shift in Φ_MS should be minimized for low and symmetrical V_th as well as improved short-channel effect (SCE). A Ge content of ~20% therefore seems to offer the best tradeoff between SCE and PDE     

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     

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     

Low-temperature annealing of polycrystallineSi1-xGex after dopant implantation

Hall effect measurement was employed to study the isothermal annealing of boron or phosphorus implanted polycrystalline Si_1-x Ge_x thin films, with x varying from 0.3-0.55. X-ray diffraction and cross-sectional transmission electron microscopy were used to study the crystal structure, whereas X-ray photoelectron spectroscopy was used to determine the film composition and the chemical bonding states of the elements. In low-temperature (⩽600°C) annealing, the conductivity, the dopant activation, and the Hall effect mobility decreased during extended annealing. The effective activation of phosphorus was less than 20% and decreased with increasing Ge content. Boron activation could reach above 70%. It was also found that Si_1-xGe_x could be oxidized at 600°C in a conventional furnace even with N₂ protection, especially for phosphorus doped films with high Ge content. Consequently, a low-temperature SiO₂ capping layer is necessary during extended annealing     

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)     

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     

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     

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     

A variable-work-function polycrystalline-Si1-xGex gate material for submicrometer CMOS technologies

P⁺ poly-Si_1-xGe_x is a promising candidate for the gate material in submicrometer CMOS technologies due to its improved resistivity and its work function (which can be modified to achieve more-scalable NMOS and PMOS devices). The work function of P ⁺ poly-Si_1-xGe_x decreases with increasing Ge content, by more than 0.3 V from 0 to 60%. Because of its ease of formation and compatibility with VLSI fabrication techniques, assimilating poly-Si_1-xGe_x into an existing CMOS process should be relatively simple     

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     

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     

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     

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     

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