A. C. Churchill

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...

Raman studies of plasmon modes in a drifting two‐dimensional electron gas

We present the first Raman‐scattering studies of the behavior of the intrasubband plasmon mode of a two‐dimensional electron gas which is undergoing lateral drift in an applied electric field. The data clearly show the expected Doppler shifts of the modes traveling up‐ and downstream, together with the expected dependence on the wave vector, but at higher drift velocities, the expected linear shift is distorted because of electron heating effects.

Experimental study of the acoustoelectric effects in GaAs-AlGaAs heterostructures

We present the results of a detailed experimental study of the electric current and voltage induced in the 2DEG of a GaAs-AlGaAs heterostructure by a surface acoustic wave (SAW). New results are obtained for these acoustoelectric effects at zero and low (<0.1 T) magnetic fields. At zero magnetic field the acoustoelectric current (voltage) was found to show a non-monotonic temperature dependence with a maximum at 40-50 K. Measurements on high-mobility 2DEGs where the electron mean free path is comparable with the SAW wavelength reveal geometric resonances of the cyclotron orbit with the SAW wavelength. With increasing magnetic field the acoustoelectric effects increase significantly and display rich oscillatory structure. We compare our data for the high-magnetic-field regime with that published in the literature.

FABRICATION OF INDEPENDENT CONTACTS TO TWO CLOSELY SPACED TWO-DIMENSIONAL ELECTRON GASES USING MOLECULAR BEAM EPITAXY REGROWTH AND IN SITU FOCUSED ION BEAM LITHOGRAPHY

Lateral patterning of a buried GaAs epilayer can be achieved during molecular beam epitaxy growth by in situ implantation with a high energy focused ion beam. Applying this technique, 30 keV Ga ions have been used to form small, highly resistive, regions in the backgate layer of a double two‐dimensional electron gas (2DEG) structure. Independent contacts to the two 2DEGs were then achieved by selectively depleting out regions of the upper and lower 2DEGs with potentials on patterned front and backgates. In the resulting devices, the magnetoresistance of the two 2DEG layers was measured both together and separately. It was demonstrated that a bias of up to ±50 mV could be applied, across the 20 nm AlGaAs barrier separating the 2DEGs, with leakage currents of <0.01 nA at 4.2 K. Finally, resonant tunneling between the two 2DEGs was observed when the barrier thickness was reduced to 7 nm. These results have demonstrated the success of this novel in situ fabrication route.

Anisotropic magnetotransport in two-dimensional electron gases on (311)B GaAs substrates

Interest in electron transport on high-index GaAs surfaces is increasing, especially since the advent of patterned substrate regrowth, in which high index surfaces are revealed on (001) GaAs after etching. In this paper we observe anisotropic mobility in orthogonal directions in two-dimensional electron gases grown on (311)B GaAs substrates. The mobility in the (233) direction is found to be up to 50 in the (011) direction. The lower mobility is accompanied by a large anomalous negative magnetoresistance. These effects are studied as a function of temperature and carrier density. It is suggested that interface roughness scattering could be a cause for the large anisotropies in mobility and a simple calculation is performed to demonstrate this hypothesis.

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

The low temperature electrical properties of modulation-doped two dimensional electron gases (2DEGs) in the SiGe system were studied. The effects 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 modulation-doped structure 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. The uniformity of the regrown wafers was also investigated. A monotonic increase in carrier density and a decrease in mobility were observed towards the edge of the wafers. Appropriate mechanisms will be discussed.

Optimization of high mobility two‐dimensional hole gases

Modulation‐doped (Al,Ga)As two‐dimensional hole gas (2DHG) structures have been grown by molecular‐beam epitaxy (MBE) on the (311)A surface of GaAs using silicon as a p‐type dopant. Systematic variations of carrier density ps and mobility μ with undoped spacer thickness were observed at 1.7 K, with a peak mobility of 4.95×105 cm2 V−1 s−1 at a sheet carrier density of 2.2×1011 cm−2 occurring at a spacer thickness of 200 A. The mobility and carrier density of all samples were also measured over the temperature range 300 mK to 4.2 K. These results lead to the conclusion that for samples with spacer thicknesses greater than 400 A, acoustic phonon scattering limits the mobility at low temperatures, whereas for samples with spacer thicknesses less than this value, ionized impurity scattering was observed to be the dominant scattering mechanism. As observed with electron gases, an increase in mobility was achieved by using a thicker region of lightly doped (Al,Ga)As, giving a sample with a mobility of 7.2×105 cm...

Effect of ion energy on Sn donor activation and defect production in molecular beam epitaxy GaAs doped with Sn ions during growth

The production of electrically active defect centers in molecular beam epitaxy (MBE) GaAs irradiated with low energy (50–500 eV) Sn ions during growth has been investigated as a function of ion energy. GaAs was doped n type during growth with a specially designed Sn focused ion beam column mounted on a MBE growth chamber. The 77 and 300 K Hall mobility and carrier concentration of the GaAs depended strongly on the ion energy, thus providing a sensitive measure of the concentration of ion‐induced acceptorlike defect centers. The material was found to be nonconducting for ion energies greater than 200 eV, while a systematic decrease in the acceptor concentration, and consequent increase in the mobility, was observed as the ion energy was decreased below this value. A peak mobility of 90 000 cm2 V−1 s−1 at a carrier concentration of 1×1014 cm−3 was achieved (at 60 K) which is in excess of that obtained in other reports of ion‐doped GaAs. A similar dependence on ion energy was found in the 4.2 K photoluminesc...

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.

The growth of high mobility heterostructures on (311)B GaAs

Abstract We demonstrate that the (311)B surface of GaAs can be used for the fabrication of high mobility (μ ≅ 2.4 × 106 cm2 V−1 s−1) two-dimensional electron gases, in which the mobility is found to be anisotropic with μ[233] > μ[011]. This paper reviews the magneto-transport properties of the (311)B system and sheds light on the nature of the scattering mechanisms determining the electron mobility. These results are of particular relevance to the current discussion of the nature of the {311} surface. It is well known that a similar mobility anisotropy exists in hole gases grown on the (311)A surface, although attempts to interpret such results are complicated by the anisotropic and non-parabolic nature of the valence band structure. For electron gases grown on the (311)B surface we demonstrate experimentally (with ballistic focusing) that the Fermi surface is isotropic, leading to the conclusion that the most likely cause of the mobility anisotropy is anisotropic interface roughness scattering. This is also confirmed by measurements of mobility as a function of carrier density, which can be fitted by a simple interface roughness scattering theory. Further experiments have demonstrated that ballistic quantization can be observed in both [233] and [011] directions, despite the large differences in anisotropic mobility.