J.M. Bonar

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.

Boron diffusion across silicon–silicon germanium boundaries

Most boron diffusion studies in Si–Ge have been made in regions of uniform germanium content. In this paper diffusion is observed from a boron-doped epitaxial silicon layer across surrounding Si–Ge layers. Pileup of boron in the Si–Ge layers shows that the activity coefficient for boron in Si–Ge is lower than that for pure silicon. A simple pairing model for Si–B interaction fitted the pileup quite well, with the same equilibrium constant applying to both Si0.9Ge0.1 and Si0.97Ge0.03 layers. The effect of this was simply to immobilize a significant fraction of the boron while retaining its acceptor qualities, the ratio of immobile boron to normal substitutional boron being proportional to the germanium content. Quasielectric field effects at the Si–SiGe interface have a strong effect on the results obtained.

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.

Dislocation glide in {110} planes in semiconductors with diamond or zinc‐blende structure

The activation of the secondary a/2〈110〉{110} glide systems as observed by transmission electron microscopy in epitaxial Ge(Si) and InGaAs layers grown on comparatively highly misfitting substrates, is rationalized in terms of a mechanical equilibrium analysis that includes a frictional force on the gliding dislocations. The conditions for occurrence of further secondary glide planes, such as {113} and {100}, are outlined.

Effect of fluorine implantation dose on boron thermal diffusion in silicon

This paper investigates how the thermal diffusion of boron in silicon is influenced by a high energy fluorine implant with a dose in the range 5?1014 to 2.3?1015cm-2. SIMS profiles of boron marker layers are presented for different fluorine doses and compared with fluorine profiles to establish the conditions under which thermal boron diffusion is suppressed. The SIMS profiles show significantly reduced boron thermal diffusion above a critical F+ dose of 0.9-1.4?1015cm-2. Fitting of the measured boron profiles gives suppressions of the boron thermal diffusion coefficient by factors of 1.9 and 3.7 for F+ implantation doses of 1.4?1015 and 2.3?1015cm-2 respectively. The suppression of boron thermal diffusion above the critical fluorine dose correlates with the appearance of a shallow fluorine peak on the SIMS profile in the vicinity of the boron marker layer. This shallow fluorine peak is present in samples with and without boron marker layers, and hence it is not due to a chemical interaction between the boron and the fluorine. Analysis of the SIMS profiles and cross-section TEM images suggests that it is due to the trapping of fluorine at vacancy-fluorine clusters, and that the suppression of the boron thermal diffusion is due to the effect of the clusters in suppressing the interstitial concentration in the vicinity of the boron profile.

SiGe HBTs on bonded SOI incorporating buried silicide layers

A technology is described for fabricating SiGe heterojunction bipolar transistors (HBTs) on wafer-bonded silicon-on-insulator (SOI) substrates that incorporate buried tungsten silicide layers for collector resistance reduction or buried groundplanes for crosstalk suppression. The physical structure of the devices is characterized using cross section transmission electron microscopy, and the electrical properties of the buried tungsten silicide layer are characterized using sheet resistance measurements as a function of bond temperature. Possible contamination issues associated with the buried tungsten silicide layer are investigated by measuring the collector/base reverse diode tics. A resistivity of 50 /spl mu//spl Omega/cm is obtained for the buried silicide layer for a bond anneal of 120 min at 1000/spl deg/C. Collector/base reverse diode tics show a voltage dependence of approximately V/sup 1/2/, indicating that the leakage current is due to Shockley-Read-Hall generation in the depletion region. Fitting of the current-voltage tics gives a generation lifetime of 90 ns, which is as expected for the collector doping of 7 /spl times/ 10/sup 17/ cm/sup -3/. These results indicate that the buried tungsten silicide layer does not have a serious impact on junction leakage.

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.

Hole transport through single and double SiGe quantum dots

We report on measurements of hole transport through a double quantum dot structure formed by trench isolation from a SiGe:Si heterostructure. A period change in the Coulomb oscillations is observed upon changing a gate bias, which is attributed to the lowering of one of the tunnel barriers, effectively changing the device to a single quantum dot. Accompanying the period change is a significant change in the level of noise associated with the oscillations. This is explained by a carrier energy filtering effect in the double dot compared with the single dot, caused by the slight difference in energy level spacing in each quantum dot.

Antimony and boron diffusion in SiGe and Si under the influence of injected point defects

The diffusion of Sb and B in both Si and SiGe is studied in this work. Injection of both interstitial- and vacancy-type point defects using rapid thermal annealing (RTA) in both NH3 and O2 atmospheres is calibrated for Sb diffusion in Si, before examining Sb diffusion in SiGe and B diffusion in Si and SiGe. Measurement of the diffusion retardation or enhancement under defect injection conditions will elucidate the diffusion mechanism and allow determination of the diffusivity, necessary for modeling of device fabrication processes. These experiments confirm the predominant mechanism for diffusion of Sb in Si and SiGe to be vacancy-mediated, while the predominant mechanism for B appears to be interstitial-mediated in Si and SiGe. The diffusivity values measured for B in Si and SiGe are reported.

Diffusion Mechanisms in Sige Alloys

Interest in diffusion processes in SiGe alloys arises from their potential in HBTs, HFETs, and optoelectronics devices, where migration over distances as small as a few nanometres can be significant. Successful modelling of these processes requires a much improved understanding of the mechanisms of self- and dopant diffusion in the alloy, although recent progress has been made. It is the purpose of this review to set this in the context of diffusion processes in elemental silicon and germanium, and to identify how this can help to elucidate behaviour in the alloy. Firstly, self diffusion processes are reviewed, from general agreement that self-diffusion in germanium is dominated by neutral and acceptor vacancies, to the position in silicon which is still uncertain. Germanium diffusion in silicon, however, appears to be via both vacancy and interstitial processes, and in the bulk alloy there is evidence for a change in dominant mechanism at around 35 percent germanium. Next, a review of dopant diffusion begins with Sb, which appears to diffuse in germanium by a mechanism similar to self-diffusion, and in silicon via monovacancies also, from marker layer evidence. In SiGe, the effects of composition and strain in epitaxial layers on Si substrates are also consistent with diffusion via vacancies, but questions still remain on the role of charged defects. The use of Sb to monitor vacancy effects such as grown-in defects by low temperature MBE, are discussed. Lastly, progress in assessing the role of vacancies and interstitials in the diffusion of boron is reviewed, which is dominated by interstitials in silicon-rich alloys, but appears to change to domination by vacancies at around 40 percent germanium, although studies in pure germanium are greatly needed.