C. G. Tuppen

Equilibrium critical thickness for Si1−xGex strained layers on (100) Si

The critical thickness for Si1−xGex strained layers for the alloy range 0 ∼104 , cm−2) whereas in thinner strained epilayers, below the thermodynamic stability curve, no misfit dislocations were found. Nomarski micr...

The effects of misfit dislocation nucleation and propagation on Si/Si1-xGex critical thickness values

A detailed study of the nucleation and propagation of misfit dislocations has been made in the Si/Si1-xGex materials system using a standard defect revealing etch. The onset of relaxation occured at a point just above the equilibrium critical thickness limit (he) set by Matthews and Blakeslee. However, the majority of the material remained coherently strained for thickness values far in excess of he. Micrographs of partially relaxed layers are presented which show that the relaxation occured in patches, with the patches gradually enlarging until they covered the complete surface. The density of these patches is shown to be directly controlled by the density of defects in the homoepitaxial silicon underlying the Si1-xGex alloy layer, which, in turn, has been related to the efficacy of the oxide removal process prior to epitaxial growth. A close examination of the misfit dislocations in a partially relaxed Si/Si0.95Ge0.05 structure showed that each misfit line could comprise a number of individual half loops on the same slip plane. The misfit lines generally appeared as an orthogonal pair of equal length lines lying along 〈110〉 directions and intersecting at the centre points. The individual half loops nucleate at the centre of the 〈110〉 misfit lines and glide with equal speed in both directions away from the point of nucleation.

Dislocation nucleation and propagation in Si0.95Ge0.05 layers on silicon

Defect reveal etching has been used to study the onset of relaxation in strained Si0.95 Ge0.05 layers grown by molecular beam epitaxy on (001) silicon substrates. Etch features corresponding to nucleation centers and to interfacial and threading segments of mismatch dislocations have been observed at thicknesses well below the expected critical thickness. From these it is deduced that mismatch dislocations take the form of half‐loops on {111} planes which glide approximately symmetrically outwards from existing defects along the 〈110〉 directions parallel to the interface.

Misfit dislocation multiplication processes in Si1−xGex alloys for x<0.15

The density of misfit dislocation sources in strained Si1−xGex layers grown on Si substrates is rarely sufficient to explain the observed extent of relaxation when layer thicknesses are in excess of the metastable critical thickness. This letter describes a process whereby a small, but finite number of misfit dislocation nucleation sources can lead to extensive strain relaxation across a complete wafer. Two novel mechanisms for misfit dislocation multiplication are presented and shown to be compatible with microscopic observations of chemically etched layers.

Asymmetric strain distributions resulting from deliberately induced misfit dislocations

Misfit dislocations oriented in a specific 〈110〉 direction have been produced in strained Si1−xGex epitaxial layers deposited on Si(001), using sites of localized crystallographic damage as dislocation sources. During a high‐temperature anneal, misfit dislocation propagation from a series of parallel saw lines oriented along a particular 〈110〉 direction led to asymmetrically strained material demonstrating an orthorhombic symmetry. Processing conditions required to maximize [110]/[110] asymmetry in the strain distribution are discussed. The distance of a dislocation front emanating from the sites of crystallographic damage during a high‐temperature anneal has been used to measure the misfit dislocation glide velocity.

Optical detection of biatomic sheets of silicon in Si/Ge superlattices

A distinctive Raman spectrum associated with biatomic sheets of silicon in Si/Ge superlattices has been found in the energy range 370–410 cm−1. This double‐peaked structure was obtained over an order of magnitude of germanium layer thickness, but was not found in the alloy control layers or structures with thicker Si layers. It is proposed that the signal is due to modes that are normally forbidden in this scattering configuration. Strong direct optical transitions have been predicted for certain Si/Ge superlattices incorporating biatomic sheets of Si and this distinctive Raman signal could be used to characterize the biatomic sheets of silicon in these structures.

Selective polycrystalline and epitaxial growth by silicon molecular beam epitaxy

Abstract Silicon molecular beam epitaxy (MBE) on Si(001) substrates patterned with oxide has been studied. The transition between polycrystalline silicon deposition on the oxide and in situ etching of the oxide was compared, using a thermodynamic model. Islanding was found at the transition, which appears to reduce the maximum oxide etching rate. Patterned substrates prepared by the LOCOS (localized oxidation of silicon) process were etched in situ to give a level surface. Selective polycrystalline and epitaxial growth was then used to produce wafers suitable for CMOS processing. The structure of these layers is discussed in detail.

Relaxation of Si/Si1-xGex Strained Layer Structures

Silicon-germanium strained layer structures have recently aroused much interest. High efficiency injection in heterojunction bipolar transistors and high electron mobilities in modulation doped FETs were first demonstrated in III–V systems. Similar devices in the Si-Ge materials system will have the added advantages of silicon processing.