T.E. Whall

Temperature dependence of transport properties of high mobility holes in Ge quantum wells

The transport properties of a two-dimensional hole gas (2DHG), with very high room-temperature drift mobilities, formed in 7.5- (2540cm2V−1s−1) and 20‐nm (2940cm2V−1s−1) strained Ge quantum wells of multilayered Si∕Si0.33Ge0.67∕Ge∕Si0.33Ge0.67∕Si0.73Ge0.27∕Si(001) p-type modulation-doped heterostructures were investigated experimentally in the temperature range of 10–295K. For both samples the drift mobility of the 2DHG behaves in the same way as the Hall mobility, increases as the temperature decreases. The sheet carrier density of the 2DHG increases with decreasing temperature, which is opposite to the behavior of Hall-effect sheet carrier density. We found that the 2DHG formed in the thicker Ge quantum well has a higher drift mobility at 295K but a lower one at low temperatures. For both samples the Hall factor increases with decreasing temperature, reaching unity at 10K, when carriers in parallel conducting layers completely freeze-out. The interpretation of these results is carried out and the scatte...

The determination of valence band discontinuities in Si/Si1−xGex/Si heterojunctions by capacitance‐voltage techniques

Capacitance‐voltage (C‐V) profiling has been used to measure the apparent carrier concentration profiles in Si/Si1−x Ge x /Si structures for a range of Ge percentages. Using Kroemers analysis, good agreement has been found between theoreticalvalence band offsets and those determined from the experimental data. The validity of Kroemers analysis has been assessed in the presence of traps using a C‐V simulation program. Under certain circumstances, large errors occur in the extracted valence band offset due to distortion of the apparent carrier concentration profile by traps. It is proposed that the experimental data presented here falls into a regime where minimal distortion is to be expected and is reflected by the accuracy of the extracted valence band offsets.

Low frequency noise measurements of p-channel Si/sub 1-x/Ge/sub x/ MOSFET's

In this brief, we report an investigation of the low frequency noise in p-channel SiGe MOSFETs. At low gate bias the noise spectrum consists of several trap related generation-recombination (g-r) noise components. At higher gate bias, the noise spectrum is dominated by 1/f noise. The 1/f noise is attributed to a fluctuation in the number of free carriers and the effective slow state trap density at the Fermi energy calculated.

A comparison of the behaviour of Si0.5Ge0.5 alloy during dry and wet oxidation

Abstract We have studied the oxidation behaviour of 350 nm thick films of Si 0.5 Ge 0.5 alloy grown on Si(100) substrates by molecular beam epitaxy. The oxidation was performed at 1000 °C in both dry and wet oxygen environments. As a reference, bulk silicon oxidation was also studied. Oxidation rates and atomic redistribution were measured using Rutherford backscattering. The formation of SiO 2 bonding was indicated by IR transmission spectroscopy, and X-ray photoelectron spectroscopy was used to determine the silicon and germanium electronic states in the oxide layer. Two stages of oxide growth can be identified in our experiment. During the initial stage the dry oxidation rates for alloy and bulk silicon are the same whilst the wet oxidation rate for the alloy is about three times faster than that for the bulk. Germanium trapped in the near-surface region and accumult wet and dry oxidation was observed at this stage. Longer oxidation times are characterized by similar growth rates for both alloy and bulk silicon during wet oxidation, but during dry oxidation a significantly lower rate for the alloy compared with bulk silicon. The accumulated germanium diffused away from the interface of the oxide layer in the case of dry oxidation and the alloy layer transformed to a germanium-rich layer during wet oxidation. The above results demonstrate that the presence of germanium increases the rate during wet oxidation, but decreases the rate during dry oxidation. We explain these phenomena in terms of the mass transport, of either silicon or oxygen atoms, to the oxide front.

Si/SiGe/Si pMOS Performance - alloy scattering and other considerations

Charge-carrier scattering in pseudomorphic p-channel Si/SiGe/Si heterostructures is reviewed. It is argued that current room-temperature field-effect device performance is limited by materials quality, particularly interface roughness and compositional inhomogeneity, rather than random alloy scattering. The way ahead is discussed.

On the low-temperature mobility of holes in gated oxide Si/SiGe heterostructures

A detailed comparison is made between theory and experiment for the low-temperature mobility of holes in gated oxide, coherently strained Si/SiGe heterostructures. We conclude that the mobility is mainly limited by interface impurities, conventional surface roughness and strain fluctuations; by contrast, we argue that alloy scattering is comparatively weak. Comments regarding possible mobility degradation due to oxide formation are also made.

Temperature dependence of incorporation processes during heavy boron doping in silicon molecular beam epitaxy

Boron doped layers were grown by silicon molecular beam epitaxy to establish incorporation processes at temperatures between 900 and 450 °C. For temperatures exceeding 650 °C a surface accumulated phase of boron was formed when doping levels exceeded solid solubility limits. The properties of this surface phase were used to determine solubility limits for boron in silicon. Above 750 °C, the measured equilibrium solubility limit was in the 1019‐cm−3 range in good agreement with previously published annealing data and showing a gradual decrease with decreasing temperature. Below 650 °C, the processes leading to the formation of the surface phase were kinetically limited, manifested by a sharp increase in boron solubility limit, with completely activated levels above 1 × 1020 cm−3 realized. At intermediate growth temperatures the degree of dopant activation was found to be dependent on growth rate. The stability of fully activated highly‐doped boron layers, grown at low temperatures, to ex situ annealing is ...

Photoluminescence and photoreflectance study of Si/Si 0.91 Ge 0.09 and Si 9 /Ge 6 quantum dots

Nanometer-scale quantum dots based on a series of Si/Si0.91Ge0.09 strained layer superlattices and a Si9/Ge6 strain-symmetrized superlattice were fabricated using electron beam lithography and reactive ion etching. They were investigated by photoluminescence and photoreflectance. It was found for the first time that the quantum efficiency of optical emission from the quantum well layers increased by over two orders of magnitude when the quantum dot sizes were reduced to ≤100 nm.

SiGe – heterostructures for CMOS technology

A review is given of the 300 K electron and hole mobilities in Si/SiGe heterostructures in the light of potential applications in CMOS technology. Particular emphasis is placed on p-channel structures where the gains are likely to be highest. Prospects for further enhancements in hole mobility and the growth procedures and layer configurations needed to achieve this are discussed. Recent work on heterointerface quality, limited area growth of strain-tuning virtual substrates and carrier mobility is also reported.

Hole Mobility in Germanium as a Function of Substrate and Channel Orientation, Strain, Doping, and Temperature

We present a comprehensive study of hole transport in germanium layers on “virtual” substrates using a full band Monte Carlo simulation approach, considering alternate “virtual” substrate and channel orientations and including the impact of the corresponding biaxial strain, doping, and lattice temperature. The superior mobility in strained germanium channels with orientation on a (110) “virtual” substrate is confirmed, and the factors leading to this enhancement are evaluated. The significant decrease in strain-and-orientation-induced mobility enhancement due to impurity scattering in doped material and at increasing lattice temperature is also demonstrated. Both factors determine how efficiently the mobility enhancement translates into transistor performance enhancement. Additionally, we shine light on the question of which factor has stronger impact in mediating the increase in mobility due to strain-the breaking of degeneracy for the heavy- and light-hole bands at the point or the reduction in the density of states.