S.M. Newstead

On the practical applications of MBE surface phase diagrams

Abstract This paper reports detailed surface phase diagrams for (100) GaAs and (100)InAs. In the case of GaAs, growth rates from 0.07 to 3.0 μm/h, As 4 : Ga flux ratios from 0.25:1 to 100:1 and growth temperatures from 300°C to 800°C were used, covering the whole range of growth conditions of practical use in MBE. Results on the nucleation of the (100) InAs/GaAs heterojunction are also presented. The correlation between material properties, surface reconstruction and growth conditions is discussed for both GaAs and homo- and heteroepitaxial InAs. The emphasis of the paper is on the practical application of the results as an aid to optimising (and reproducing) MBE growth conditions with reasonable efficiency.

A comparison of the behaviour of Si0.5Ge0.5 alloy during dry and wet oxidation

Abstract We have studied the oxidation behaviour of 350 nm thick films of Si 0.5 Ge 0.5 alloy grown on Si(100) substrates by molecular beam epitaxy. The oxidation was performed at 1000 °C in both dry and wet oxygen environments. As a reference, bulk silicon oxidation was also studied. Oxidation rates and atomic redistribution were measured using Rutherford backscattering. The formation of SiO 2 bonding was indicated by IR transmission spectroscopy, and X-ray photoelectron spectroscopy was used to determine the silicon and germanium electronic states in the oxide layer. Two stages of oxide growth can be identified in our experiment. During the initial stage the dry oxidation rates for alloy and bulk silicon are the same whilst the wet oxidation rate for the alloy is about three times faster than that for the bulk. Germanium trapped in the near-surface region and accumult wet and dry oxidation was observed at this stage. Longer oxidation times are characterized by similar growth rates for both alloy and bulk silicon during wet oxidation, but during dry oxidation a significantly lower rate for the alloy compared with bulk silicon. The accumulated germanium diffused away from the interface of the oxide layer in the case of dry oxidation and the alloy layer transformed to a germanium-rich layer during wet oxidation. The above results demonstrate that the presence of germanium increases the rate during wet oxidation, but decreases the rate during dry oxidation. We explain these phenomena in terms of the mass transport, of either silicon or oxygen atoms, to the oxide front.

Elemental boron doping behavior in silicon molecular beam epitaxy

Boron-doped Si epilayers were grown by molecular beam epitaxy (MBE) using an elemental boron source, at levels up to 2×1020 cm−3, to elucidate profile control and electrical activation over the growth temperature range 450–900 °C. Precipitation and surface segregation effects were observed at doping levels of 2×1020 cm−3 for growth temperatures above 600 °C. At growth temperatures below 600 °C, excellent profile control was achieved with complete electrical activation at concentrations of 2×1020 cm−3, corresponding to the optimal MBE growth conditions for a range of Si/SixGe1−x heterostructures.

Temperature dependence of incorporation processes during heavy boron doping in silicon molecular beam epitaxy

Boron doped layers were grown by silicon molecular beam epitaxy to establish incorporation processes at temperatures between 900 and 450 °C. For temperatures exceeding 650 °C a surface accumulated phase of boron was formed when doping levels exceeded solid solubility limits. The properties of this surface phase were used to determine solubility limits for boron in silicon. Above 750 °C, the measured equilibrium solubility limit was in the 1019‐cm−3 range in good agreement with previously published annealing data and showing a gradual decrease with decreasing temperature. Below 650 °C, the processes leading to the formation of the surface phase were kinetically limited, manifested by a sharp increase in boron solubility limit, with completely activated levels above 1 × 1020 cm−3 realized. At intermediate growth temperatures the degree of dopant activation was found to be dependent on growth rate. The stability of fully activated highly‐doped boron layers, grown at low temperatures, to ex situ annealing is ...

SIMS response functions for MBE grown delta layers in silicon

Abstract SIMS response functions and depth resolution parameters have been measured using O 2 + primary ions at normal incidence and 45°, for a range of silicon MBE grown epilayers containing ultrathin buried layers or “deltas” doped with B. Growth and decay slopes and differential shifts are shown to be species dependent. For B, the depth resolution is superior at normal incidence, and has a linear energy dependence for both normal incidence and 45°. The differential shift is dependent on the depth of a feature with respect to the final crater depth, and is due to erosion rate changes across the pre-equilibrium region, species dependent probe-induced mass transport, and the effect of the residual altered layer on crater depth measurement.

An investigation of Si0.5Ge0.5 alloy oxidation by high dose oxygen implantation

Abstract An attempt to implant a high dose (up to 1.8 × 1018 cm−2) O+ ions into a Si0.5Ge0.5 alloy grown by molecular beam epitaxy (MBE) was made in this work, and the oxidation of the alloy by the implantation before and after thermal treatment was studied using X-ray photoelectron spectroscopy (XPS). The changes of the composition distribution in the sample were observed from the XPS depth profiles. The chemical states of Si and Ge as well as the location of their oxides were obtained from the spectrum fitting. The results indicate that compared to the implantation made on single crystal Si or Ge, this alloy seems to have more in common with the bulk Si and the reason is attributed to the different reactivities between Si and Ge with oxygen and the different stabilities of their oxides. A possible way to improve the experiment to achieve the SIMOX (separation by implanted oxygen) structure in this material is also suggested.

Synthesis of oxides in Si0.5Ge0.5 alloy by high dose oxygen ion implantation

Abstract A Si0.5Ge0.5 alloy layer was implanted at a temperature of about 500 °C with doses of 0.6 × 1018, 1.2 × 1018 and 1.8 × 1018 O+ cm−2 at an energy of 200 keV. The alloy layer was prepared by molecular beam expitaxy (MBE), with an 800 nm thick film of Si0.5Ge0.5 alloy followed by a 75 nm thick top silicon layer on an n-type Si(100) ( ϱ = 5–20 Ω cm ) substrate. X-ray photoelectron spectroscopy measurements and Rutherford backscattering indicated that SiO and GeO compounds were formed during the oxygen bombardment. The sample implanted with a dose of 1.8 × 1018 O+ cm−2 was annealed at a temperature of 1000°C, which caused further growth of the SiO2 phase with a concomitant reduction in the oxides of germanium and rejection of germanium from the synthesis layer. The different oxidation rates for silicon and germanium can be attributed to thermodynamic effects.

Elemental boron and antimony doping of MBE Si and SiGe structures grown at temperatures below 600°C

Abstract This paper considers the low temperature doping of (100) Si and SiGe structures with elemental B and Sb sources particularly with regard to obtaining very narrow delta doping spikes. B is found to be an excellent dopant at SiGe growth temperatures incorporating in an active state at concentrations up to 10%. B delta layers of 1 nm or less have also been grown. Sb is also shown to be capable of providing delta doped layers less than 2 nm wide. The B deltalayers have been incorporated into modulation doped structures yielding an order of magnitude increase in mobility at 77 K.

The determination of strain in Si-Ge superlattices by electron diffraction in a scanning transmission electron microscope

Abstract The nanometre scale of the novel strained layer electronic devices now being grown requires characterisation techniques of a corresponding resolution. This work employs the subnanometre probe of a dedicated scanning transmission electron microscope to investigate individual layers in a cross-sectioned SiGe superlattice. Using recently developed instrumentation, microdiffraction patterns have been obtained at very high resolution and the strains in each layer quantified by analysing the position of the deficit higher order Laue zone lines in the zero order beam. The experimental patterns are fitted to computer simulations incorporating possible dynamical effects. The results from a 10 nm SiGe layer are shown to be in good agreement with bulk X-ray diffraction analysis, with an accuracy limited only by the fundamental constraints of diffraction from a laterally finite sample. Hence surface relaxation, a major complication with previous applications of electron microscopy to strain measurement, can be ignored for the specimen geometry that the small probe allows. It is anticipated that the technique can in future be applied to multilayer structures which are not amenable to bulk characterisation.

The study of Si0.5Ge0.5 alloy implanted by high dose oxygen

The effects of high dose O+ implantation into a Si0.5Ge0.5 alloy, studied by Rutherford backscattering, infrared spectrophoiometry and X-ray photoelectron spectroscopy, are reported for the first time. The alloy layers were prepared by molecular beam epitaxy (MBE) when a thick (900 nm) film of Si0.5Ge0.5 alloy followed by a 75 nm top Si layer was grown on a n-type (100) Si (ρ = 5–20 Ω cm) Substrate. This material was, subsequently, implanted with doses of 0.6 × 1018 O+ cm−2, 1.2 × 1018 O+ cm−2 and 1.8 × 1018 O+ cm−2 at an energy of 200 keV with a substrate temperature of about 500°C. Selected samples have been annealed in flowing nitrogen at temperatures of 800° C, 900° C or 1000° C for l h. The redistribution of the implanted oxygen and the composition of the resulting structure has been investigated before and after thermal treatment. The results show that Si-O and Ge-O compound formation depends strongly on the dose of oxygen and annealing temperature. Most of the implanted oxygen reacts with Si to form SiO2. The Ge-O bonding was only observed in the buried oxide layer where the concentration of oxygen atoms was saturated. In the high dose sample and at the highest annealing temperature the SiOx converted to stoichiometric SiO2 with this dielectric also containing some Ge and GeOx (x ≤ 2) trapped in the silicon dioxide layer. With increasing anneal temperature decomposition of Ge-O bonding was observed and, generally, Ge atoms were found to be rejected from the oxide and segregated in the alloy layers above and below the buried oxide. The different behaviours of the Si and Ge atoms can be described in terms of the thermodynamics of the systems.