R. J. H. Morris

Diffusion of boron in germanium at 800–900°C

Diffusion of B in Ge is studied in the temperature range 800–900°C using implantation doping and B doped epitaxial Ge layers. Concentration profiles before and after furnace annealing were obtained using high resolution secondary ion mass spectroscopy (SIMS). Diffusion coefficients were calculated by fitting the annealed profiles using TSUPREM. We obtained diffusivity values which are at least two orders of magnitude lower than the lowest values previously reported in the literature. Using our values an activation energy of 4.65(±0.3)eV is calculated. Present experimental results suggest that interstitial mediated mechanism should be considered for B diffusion in Ge in accordance with recent theoretical calculations. Annealed SIMS profiles also suggest that B solid solubility in Ge is ∼2×1018cm−3 at 875°C which agrees with literature values.

Diffusion of ion-implanted boron in germanium

The diffusion of boron (B) in germanium (Ge) is studied. B was introduced in Ge wafers by ion implantation and concentration profiles after furnace annealing were obtained using secondary ion mass spectroscopy. The diffusion coefficient and solid solubility of B in Ge has been calculated to be 1.5(+/-0.3)x10-16 cm2/s and 5.5(+/-1.0)x1018/cm3, respectively at 850 degrees c by fitting experimentally obtained profiles. The value of diffusion coeffienc is at least two orders of magnitude lower than the minimum value reported in the literature for B diffusion in Ge. The results are significant as they question the general agreement about vacancy diffusion as the mechanism responsible for diffusion of B in Ge.

Ultra-high hole mobility exceeding one million in a strained germanium quantum well

In this paper, we report a Hall mobility of one million in a germanium two-dimensional hole gas. The extremely high hole mobility of 1.1 × 106 cm2 V−1 s−1 at a carrier sheet density of 3 × 1011 cm−2 was observed at 12 K. This mobility is nearly an order of magnitude higher than any previously reported. From the structural analysis of the material and mobility modeling based on the relaxation time approximation, we attribute this result to the combination of a high purity Ge channel and a very low background impurity level that is achieved from the reduced-pressure chemical vapor deposition growth method.

High conductance Ge p-channel heterostructures realized by hybrid epitaxial growth

Strained Ge p-channel heterostructures have been realized by hybrid-epitaxial growth. Strain-tuning Si0.4Ge0.6 virtual substrates were grown by ultra-high vacuum chemical vapour deposition and active layers were deposited by solid-source molecular beam epitaxy at low temperature. Following ex situ annealing, Hall effect measurements revealed a hole mobility of 1900 cm2 V−1 s−1 at 300 K (27 000 cm2 V−1 s−1 at 10 K), with a density of 1.8 × 1012 cm−2, giving a conductance in excess of current Ge heterostructures. Using a maximum-entropy mobility-spectrum analysis, 1.0 × 1012 cm−2 of these holes were found to have a mobility of 2700 cm2 V−1 s−1 at 300 K.

Growth and electrical characterisation of δ-doped boron layers on (111) diamond surfaces

A plasma enhanced chemical vapor deposition protocol for the growth of δ-doping of boron in diamond is presented, using the (111) diamond plane as a substrate for diamond growth. AC Hall effect measurements have been performed on oxygen terminated δ-layers and desirable sheet carrier densities (∼1013 cm−2) for field-effect transistor application are reported with mobilities in excess of what would expected for equivalent but thicker heavily boron-doped diamond films. Temperature-dependent impedance spectroscopy and secondary ion mass spectroscopy measurements show that the grown layers have metallic-like electrical properties with high cut-off frequencies and low thermal impedance activation energies with estimated boron concentrations of approximately 1020 cm−3.

Ion-implantation and diffusion behaviour of boron in germanium

Results are presented of implantation and diffusion study of boron (B) in germanium (Ge). B implantation was carried out in Ge with different energies and to different doses. High-resolution secondary ion mass spectroscopy was used to obtain concentration profiles after furnace annealing. The as-implanted profiles show a long tail possibly due to enhanced diffusion. A limited diffusion has been observed after furnace annealing. Using T-SUPREM, diffusivity value of 1.5(±0.3)×10?16 cm2/s at 850°C has been extracted. This value is two orders of magnitude lower than previously reported values. The results question the change in diffusion mechanism of B diffusion in Si–Ge alloys from low Ge levels to high Ge levels.

Technique for producing highly planar Si/SiO0.64Ge0.36/Si metal–oxide–semiconductor field effect transistor channels

Si/Si0.64Ge0.36/Si heterostructures have been grown at low temperature (450 °C) to avoid the strain-induced roughening observed for growth temperatures of 550 °C and above. The electrical properties of these structures are poor, and thought to be associated with grown-in point defects as indicated in positron annihilation spectroscopy. However, after an in situ annealing procedure (800 °C for 30 min) the electrical properties dramatically improve, giving an optimum 4 K mobility of 2500 cm2 V – 1 s – 1 for a sheet density of 6.2 × 1011 cm – 2. The low temperature growth yields highly planar interfaces, which are maintained after anneal as evidenced from transmission electron microscopy. This and secondary ion mass spectroscopy measurements demonstrate that the metastably strained alloy layer can endure the in situ anneal procedure necessary for enhanced electrical properties. Further studies have shown that the layers can also withstand a 120 min thermal oxidation at 800 °C, commensurate with metal–oxide–semiconductor device fabrication.

Charge compensation using optical conductivity enhancement and simple analytical protocols for SIMS of resistive Si1−xGex alloy layers

In this paper we demonstrate the use of red laser illumination to stimulate charge carriers in semiconductor layers with very low residual carrier densities, to eliminate surface potential changes during SIMS depth profiling. We show that very simple quantification protocols can be used for Ge assay, at least up to Ge fractions of 30%.

Strain dependence of electron-phonon energy loss rate in many-valley semiconductors

We demonstrate significant modification of the electron-phonon energy loss rate in a many-valley semiconductor system due to lattice mismatch induced strain. We show that the thermal conductance from the electron system to the phonon bath in strained n+Si, at phonon temperatures between 200 and 480 mK, is more than an order of magnitude lower than that for a similar unstrained sample.

Influence of regrowth conditions on the hole mobility in strained Ge heterostructures produced by hybrid epitaxy

Strained Ge p-channel heterostructures have been produced using a hybrid-epitaxy method, which allows the advantages offered by different growth techniques to be exploited. Chemical vapor deposition of thick strain-tuned virtual substrates has been combined with growth of the active layers by solid-source molecular beam epitaxy. This paper discusses optimization of the regrowth conditions, to achieve a high hole mobility, and correlates structural characterization with electrical measurements. Initial ex situ chemical cleaning of the virtual substrate was found to be essential for successful regrowth. Structural analysis, using cross-sectional transmission electron microscopy and atomic force microscopy, showed that the regrowth temperature significantly affects the growth mode of the active layers and that planar growth was only achieved below 400°C. Samples with Ge channels from 8to30nm thick were analyzed with plan view transmission electron microscopy to study the formation of misfit dislocations and ...