J.M. Fernández

New insights on SiGe growth instabilities

In this work we investigate the influence of the Si substrate orientation on the growth instability of strained Si1−xGex heterostructures. The work mainly consists in atomic force microscopy and grazing incidence x-ray diffraction analyses of the Si1−xGex layers deposited by gas source molecular beam epitaxy on vicinal Si substrates tilted from (001) to (111) surfaces. The major result is that the two- to three-dimensional growth transition is dramatically affected by the orientation of the substrate but also by the equilibrium shape of silicon. For instance, we evidence the layer by layer growth of Si1−xGex on Si (111) in contrast to the nucleation of three-dimensional islands on 2° off Si (111) in the same experimental conditions. We systematically verify that the homoepitaxial growth of unstressed Si on vicinal Si (111) consists in a regular array of single steps. Therefore, we propose that the stress induced by the heteroepitaxial growth destabilizes the regular step train by reducing the repulsive el...

Arsenic surface segregation and incorporation in Si and Si1-xGex during gas source molecular beam epitaxy

Abstract The surface segregation of As in Si and Si1 − xGex during gas source molecular beam epitaxy (GSMBE) has been investigated. It is shown that the segregation process is suppressed in the alloy compared with pure Si. The segregation energy is shown to be dependent on growth temperature and has been attributed to a change of surface hydrogen coverage. Surface hydrogen blocks surface sites, thus acting as a surfactant to suppress As segregation. Arsenic incorporation from AsH3 involves dissociative chemisorption via empty surface sites. The balance between the rate of dissociation, surface segregation and desorption results in a temperature and Ge concentration dependence of the net effective flux of As and consequently its concentration in the film. It has an upper limit of less than 1018 cm−3 in Si, but much higher concentrations can be achieved in SiGe alloys.

Arsenic doping kinetics in silicon during gas source molecular beam epitaxy

Abstract Arsenic doping from arsine during Si gas source molecular beam epitaxy (GSMBE) has been investigated. The As concentration in the epitaxial film has been measured by secondary ion mass spectroscopy (SIMS) and electrochemical capacitance-voltage (eCV) analysis. It has been found to relate to the surface coverage through a segregation process, while the surface coverage itself is determined by surface adsorption/desorption kinetics, whose dependence on the AsH 3 flux indicates a non-integral-order desorption process of As from Si(100) surfaces. The surface coverage of arsenic is found to decrease the growth rate of Si from Si 2 H 6 , which can be used as a measure of As surface concentration. This enables segregation parameters such as the segregation ratio and the Gibbs energy for segregation to be extracted and they are shown to be in good agreement with previously reported results.

Self-limiting segregation and incorporation during boron doping of Si and SiGe

We have studied the experimental conditions to obtain highly boron-doped thin layers (-doping) in Si and SiGe during gas source molecular beam epitaxy (GSMBE). Pre-deposition and the co-deposition of boron have been compared by secondary ion mass spectroscopy (SIMS) and electrochemical capacitance-voltage profiling eC(V). We have shown that provided a pre-deposition step is used before co-deposition, higher doping levels are obtained without degrading the abruptness of the interfaces. This is explained by a coverage limit of boron during the pre-deposition step, above which islanding occurs and degrades the crystalline quality of the film. In addition, a reflection high-energy electron diffraction (RHEED) oscillation study shows that there exists a virtually constant boron coverage of the surface during the Si and SiGe overgrowth, which produces a decrease of the growth rate.

Negative magnetoresistance and electron-electron interaction in Si:SiGe quantum wells

Abstract We have measured the magnetoresistance of n-channel Si:SiGe quantum wells in the temperature range 0.4–6.6 K. For magnetic fields less than 1 T and before the onset of Shubnikov-de Haas oscillations, there is a broad negative magnetoresistance. The change in resistance follows a B2 dependence and can be explained in terms of the 2D electron-electron interaction.

SiGe nMOSFETs with gate oxide grown by low temperature plasma anodisation

Abstract The characterisation of strained surface channel SiGe nMOSFETs with gate oxide grown by low temperature plasma anodisation is described. We report excellent off-state leakage currents and zero field mobilities of 640 and 420cm 2 /Vs for Si and SiGe devices respectively. Reduction in SiGe mobility is primarily attributed to increased interface state scattering in the absence of any increased surface roughness as observed in high resolution microscopy. The oxide composition is mixed and conservation of crystallinity of SiGe during oxidation is observed via RBS analysis.

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.

Investigations of electron-beam and optical induced damage in high mobility SiGe heterostructures

Abstract Modulation-doped two dimensional electron gases (2DEGs) grown on Si 0.7 Ge 0.3 virtual substrates were investigated. Low temperature measurements were used to characterise the uniformity of the wafers and annealing studies demonstrated that high annealing temperatures (above 600°C) destroyed the electrical properties. Studies of excimer irradiation of the 2DEG material demonstrated that only surface damage was induced, but the subsequent annealing of this damage reduced the carrier density in the material, suggesting strain relaxation of the strained Si cap. Electron beam irradiation experiments at 40 keV and PMMA doses showed charging effects at room temperature but little damage. Finally a number of narrow channel devices were fabricated using 300 keV electrons and characterised at low temperature to estimate the range of the electron-beam induced damage.

Magnetotransport of two-dimensional electron gas in SiSiGe modulation doped structures grown by gas source molecular beam epitaxy

Abstract We have characterized two-dimensional electron gases (2DEGs) in Si SiGe modulation doped structures grown by gas source molecular beam epitaxy. Hall bar structures were fabricated to characterize the structures and magnetotransport measurements were carried out at temperatures down to 0.4 K. Pronounced Shubnikov-de Haas oscillations were observed in the longitudinal magnetoresistance, indicative of high quality 2DEG in the channel. Electron mobilities up to 87,000 cm2 V−1 s−1 with sheet densities about 7 × 1011 cm−2 were obtained at low temperature. Dingle plots of the magnetoresistance vs reciprocal magnetic field were utilized to determine the single-particle relaxation times in order to investigate scattering mechanisms in these structure. The ratio of the transport scattering time derived from electron mobility to the single-particle relaxation time is of the order of 10, indicating that remote impurity scattering is a dominant factor limiting the mobility of our structures. The behaviour of the magnetotransport data is discussed using a model for parallel conduction.