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     

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     

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     

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     

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     

Thermo-optical switching in Si/Si1-xGexdistributed Bragg reflectors

The authors report thermo-optical switching in a 92 layer Si/Si _0.7Ge_0.3 distributed Bragg reflector (DBR) grown by molecular beam epitaxy. Depending on the layer periodicity, this structure exhibited a positive or negative reflectivity switching λ=1.06 μm, with a switch-on time of less than 20 ns and reflectivity contrast ratios greater than 50%     

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     

High-speed performance of Si/Si1-xGexheterojunction bipolar transistors

A theoretical investigation of Si/Si_1-xGe_x heterojunction bipolar transistors (HBTs) undertaken in an attempt to determine their speed potential is discussed. The analysis is based on a compact transistor model, and devices with self-aligned geometry, including both extrinsic and intrinsic parameters, are considered. For an emitter area of 1×5 μm², an f_t of over 75 GHz and f_max of over 35 GHz were computed at a collector current density of 1×10 ⁵ A/cm² and V_CB of 5 V     

Characteristics of p- and n-channelpoly-Si/Si1-xGex/Si sandwiched conductivitymodulated thin-film transistors

Both p- and n-channel poly-Si/Si_1-xGe_x/Si sandwiched conductivity modulated thin-film transistors (CMTFTs) are demonstrated and experimentally characterized. The transistors use 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 a high on-state current. Results show that the transistors provide a high on-state current as well as a low leakage current compared to those of conventional offset drain TFTs. The p- and n-channel CMTFTs can be combined to form CMOS drivers, which are very suitable for use in low temperature large area electronic systems on glass applications     

The scaled performance of Si/Si1-xGexresonant tunneling diodes

Small area resonant tunneling diodes (RTDs) with strained Si_0.4Ge_0.6 potential barriers and a strained Si quantum well grown on a relaxed Si_0.8Ge_0.2 virtual substrate were fabricated and characterized. A room temperature peak current density (J_P) of 282 kA/cm² with a peak to valley current ratio (PVCR) of 2.43 were recorded for a 5×5 μm ² sample, the highest values reported to date for Si/Si_1-xGe_x RTDs. Scaling of the device size demonstrated a decrease in J_P proportional to an increase in the lateral area of the tunnel junctions, whereas the PVCR remained approximately constant. This observation suggests that the dc behavior of such Si/Si_1-xGe_x RTD design is presently limited by thermal effects     

Si/Si1-x-yGexCy/Si heterojunctionbipolar transistors

We report the first Si/Si_1-x-yGe_xC_y /Si n-p-n heterojunction bipolar transistors and the first electrical bandgap measurements of strained Si_1-x-yGe_x C_y on Si (100) substrates. The carbon compositions were measured by the shift between the Si_1-x-yGe_xC_y and Si_1-xGe_x X-ray diffraction peaks. The temperature dependence of the HBT collector current demonstrates that carbon causes a shift in bandgap of +26 meV/%C for germanium fractions of x=0.2 and x=0.25. These results show that carbon reduces the strain in Si_1-x Ge_x at a faster rate than it increases the bandgap (compared to reducing x in Si_1-xGe_x), so that a Si _1-x-yGe_xC_y film will have less strain than a Si_1-xGe_x film with the same bandgap     

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     

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     

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     

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     

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.     

选择性腐蚀Si1-xGex与Si制备绝缘体上超薄应变硅

基于应变硅以及绝缘体上超薄应变硅(SSOI)工艺,使用氢氟酸、硝酸和醋酸的混合溶液与质量分数为25%的四甲基氢氧化铵(TMAH)溶液选择性腐蚀Si1-xGex与Si以制备绝缘体上超薄应变硅。研究了质量分数为0.5%～5%的HF和Si1-xGex中Ge的含量对选择性腐蚀的腐蚀速度与选择比的影响,优化了选择性腐蚀工艺。采用氨水、过氧化氢和水的混合溶液处理选择性腐蚀后的Si1-xGex与Si表面,得到了高应变度、高晶体质量的超薄SSOI。采用原子力显微镜(AFM)测试腐蚀速度以及腐蚀后的表面粗糙度;使用喇曼光谱仪表征Si1-xGex以及应变硅的组分以及应变度;使用透射电子显微镜(TEM)对SSOI的晶体质量进行了表征。结果表明,超薄SSOI的表面粗糙度(RMS)为0.446 nm,顶层Si的应变度为0.91%,顶层应变硅层厚度为18 nm,且具有高的晶体质量。