A.R. Powell

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.

X-ray diffraction and reflectivity characterization of SiGe superlattice structures

The authors demonstrate the effectiveness of both X-ray diffraction and X-ray reflectivity in the structural characterization of semiconductor structures. By combining information from both techniques the abruptness of the interfaces for Si1-xGex structures, with x=0.1-0.57, may be determined. For superlattice structures with x<0.3 both types of interface were found to have a root mean square (RMS) roughness of 0.5+or-0.3 nm. For a Si/Si0.45Ge0.55 superlattice structure the interfaces are found to have differing roughnesses. For the SiGe-on-Si interface the RMS roughness is found to be 0.5+or-0.2 nm; however, the Si-on-SiGe interface has a larger value of roughness, 1.0+or-0.3 nm. This roughness at the Si-on-SiGe interface is found to be dependent on the Ge content of the layer and it is shown by transmission electron microscopy analysis to be long ranged (about 70 nm) and wavy at the interface.

Structural and electrical properties of B delta layers in Si

X-ray diffraction has been used to deduce the width and strain fields of an elemental boron delta layer in (100) Si grown by MBE. It is found to be <1 nm thick and tetragonally distorted with a lattice contraction of 0.031 nm in the (100) direction. Hall measurements have been used to obtain the hole concentration in the layer and it is found that it is fully activated with a sheet carrier density of 3.5*1014 cm-2, one of the highest values reported to date. Cross-sectional TEM analysis confirms that it is a near-ideal delta layer, with no precipitation evident.

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.

Residual Strain and Defect Analysis in as Grown and Annealed SiGe Layers

In this paper we address the problem of producing SiGe buffer layers of acceptable quality for the growth of symmetrically strained SiGe structures. Initially we consider SiGe layers grown to well beyond the metastable critical thickness and examine the degree of residual strain both as - grown and post anneal. The defect levels in metastable SiGe layers following high temperature anneal were also studied. A buffer layer was grown consisting of stacked metastable SiGe layers each of which is annealed in situ prior to the growth of the next layer and terminating with a 0.45 SiGe alloy. This produces nearly fully relaxed 1.15pim thick structures with threading dislocation densities of 4 × 10 6 cm −2 . Limited area growth on Si suggests that elastically relaxed material free of both threading and misfit dislocations can be produced.

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.

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.

X-ray diffraction characterization of Sb delta doping in Si

This communication demonstrates the use of high precision X-ray diffraction together with dynamical X-ray simulation theory in the non-destructive characterization of an Sb delta doped layer in (100) Si. The width of the delta layer is determined to be <2 nm. Information is also obtained on the strain fields induced by the Sb delta layer, indicating a total lattice expansion in the (100) direction of 0.046 nm.

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.

Behavior of high dose O+-implanted Si/Ge/Si structures

The synthesis of a buried oxide layer in multilayer Si/Ge/Si structures by the implantation of high doses of 200 keV O+ ions is studied by Rutherford backscattering analysis. The presence of Ge is found to have a minimal effect upon the mass transport of excess oxygen and interstitial silicon. Infrared transmission spectroscopy and x‐ray photoelectron spectroscopy confirm that the oxygen atoms bond preferentially to silicon forming silicon dioxide and SiOx, where x<2, with no evidence for Ge—O bonding.