N. Griffin

Electrical properties of two-dimensional electron gases grown on cleaned SiGe virtual substrates

The low temperature electrical properties of modulation-doped two-dimensional electron gases (2DEG) in the SiGe system were studied. The effect on the electrical properties of removing the substrate from the growth chamber after the growth of the virtual substrate, chemically cleaning the virtual substrate and then growing the 2DEG on a thin SiGe buffer were investigated. The results demonstrate that the carrier density and mobility decrease as the regrowth interface is moved closer to the 2DEG. Lower temperature desorption of the passivating chemical oxide improves the mobility and carrier density when a regrowth interface is close to the quantum well. The mismatch of Ge content in the virtual substrate and the heterolayers was also shown to produce a reduction in the carrier density and mobility.

Two-dimensional electron gas mobility as a function of virtual substrate quality in strained Si/SiGe heterojunctions

The electron mobilities of two-dimensional electron gases in tensile strained Si grown on relaxed cubic SiGe alloys on Si (001) substrates are reported. The effects of using high and low temperature growth for the relaxed buffer layers, in an ultrahigh vacuum compatible chemical vapor deposition system using SiH4 and GeH4 gases, were investigated. We have measured electron mobilities of up to 2.6×105 cm2 V−1 s−1 for 4.5×1011 cm−2 carrier densities at 1.5 K; there is a strong correlation between surface morphology and underlying misfit dislocation volume densities which is reflected in the electron mobility. The highest mobility was achieved with high growth temperatures and high growth rates for the relaxed layers, while lower temperatures and growth rates produced samples with lower mobilities. We present transmission electron microscopy images, together with optical micrographs of the sample surfaces to demonstrate that substrate growth technology plays an important part in device performance and manufa...

High-mobility two-dimensional electron gases in Si/SiGe heterostructures on relaxed SiGe layers grown at high temperature

Low-temperature mobilities for two-dimensional electron gases (2DEGs) formed in tensile-strained Si/SiGe heterostructures are reported, with values up to for a density of electrons. The strained layers were grown at in a ultra-high-vacuum chemical vapour deposition system using and operating at around 20 Pa. The surface morphology of the layers is also discussed and both the mobility and morphology are linked to the quality of the virtual substrates. The virtual substrate consists of strain-relaxed SiGe alloys grown on Si(001) substrates; we show that it is preferable to grow these substrates at higher temperatures and higher growth rates. For low growth rates and temperatures the 2DEG mobility as a function of sheet carrier density was found to be degraded.

Low temperature characterization of modulation doped SiGe grown on bonded silicon‐on‐insulator

Modulation doped pseudomorphic Si0.87Ge0.13 strained quantum wells were grown on bonded silicon‐on‐insulator (SOI) substrates. Comparison with similar structures grown on bulk Si(100) wafers shows that the SOI material has higher mobility at low temperatures with a maximum value of 16 810 cm 2/V s for 2.05×1011 cm−2 carries at 298 mK. Effective masses obtained from the temperature dependence of Shubnikov–de Haas oscillations have a value of (0.27±0.02) m0 compared to (0.23±0.02) m0 for quantum wells on Si(100) while the cyclotron resonance effective masses obtained at higher magnetic fields without consideration for nonparabolicity effects have values between 0.25 and 0.29 m0. Ratios of the transport and quantum lifetimes, τ/τq=2.13±0.10, were obtained for the SOI material that are, we believe, the highest reported for any pseudomorphic SiGe modulation doped structure and demonstrates that there is less interface roughness or charge scattering in the SOI material than in metal–oxide–semiconductor field ef...

Universality at a quantum Hall – Hall insulator transition in a Si/Si0.87Ge0.13 2D hole system

The temperature dependence of the magnetoresistivity of a Si/Si0.87Ge0.13 2D hole system with n=2.2×1011cm−2 and μ=16900cm2/Vs (at 50 mK) has been studied. An insulating phase is observed for ν<1, with ρxy remaining finite and close to h/e2, indicating a possible quantised Hall insulator state. The data on either side of the ν=1 to Hall insulator transition can be scaled to a single pair of curves with a scaling exponent of κ=0.68±0.05. Temperature-independent conductivities close to ν=0.69 and ν=1.5 are found to appear at values of σxx=0.5e2/h(σxy=0.5e2/h) and σxx=0.5e2/h(σxy=1.5e2/h), respectively, in agreement with theory.

Schottky gating high mobility Si/Si1−xGex 2D electron systems

Abstract A high mobility (3.6×105 cm2/V s) n-type Si/Si1−xGex heterostructure has been Schottky gated using Au metal to cover the whole of the device. Over the front-gate voltage (Vg) range for which there was negligible gate leakage (−0.1 V

Inter-edge-mode scattering in a high-mobility strained silicon two-dimensional electron system

The magnetoresistances of two-dimensional electron systems in strained silicon on a relaxed silicon-germanium buffer have been measured at low temperatures (50 mK). Samples, with Hall mobilities up to 3.61 × 105 cm2 V-1 s-1 , have shown a marked asymmetry between adjacent Shubnikov-de Haas peaks and a prominent overshoot on the low-field side of the odd-filling-factor quantum Hall plateaux. This effect persisted to unusually small magnetic fields. It is argued that both of these phenomena can be explained by a strong back-scattering of multiple edge modes which is suppressed at integer filling factors.

Gating high mobility silicon-germanium heterostructures

Abstract Possible methods for fabricating gates to modulate the carrier density of modulation-doped Si SiGe two-dimensional electron gas (2DEG) material have been investigated. Plasma anodised oxides with Al gates along with Pt Al Schottky barriers have demonstrated excellent modulation of the 2DEGs. For the plasma oxides, the subsequent low temperature mobility of the material depended on the thickness of oxide grown relative to the consumption of the capping layers of the Si SiGe material. For an oxide which just consumes the silicon cap, the gates do not adversely affect the as-grown mobility of the material. The application of a bias to the gates of the thicker oxide samples produced similar mobilites as the ungated wafer when the carrier densities were matched.

Cyclotron resonance measurements of Si/SiGe two-dimensional electron gases with differing strain

Far-infrared cyclotron resonance measurements have been used to investigate the effective mass in the strained silicon channels of modulation-doped, two-dimensional electron gases grown on relaxed Si1−xGex. By using a range of Ge fractions x, the effect of strain was investigated. Consistent results were obtained when the resonance positions were fitted to a model for zero-dimensional confinement, yielding m*≈0.196 me for most samples. The use of this formula was justified by invoking electron localization due to a disorder potential. The observed confinement effect was strongest in two samples where the Si channel was partially relaxed, suggesting this to be a possible mechanism. Qualitatively different results were obtained for a sample with a high background concentration of donor impurities, indicating that the type of disorder present can affect the nature of the resonances.

Investigation of the zero-field 2D “metallic” state with rS and kFl controlled independently

We compare the rise in resistivity with increasing temperature in the “metallic” regime for a range of electron densities and for several values of substrate bias. This allows the strength of Coulomb interactions (rS) and of disorder (1/kFl) to be controlled independently. For temperatures well below the Fermi temperature, our data obey a scaling law where kFl, and not rS, is the crucial parameter, suggesting that the “metallic” state is not a novel state caused by interactions.