Adrian R. Powell

New approach to the growth of low dislocation relaxed SiGe material

In this growth process a new strain relief mechanism operates, whereby the SiGe epitaxial layer relaxes without the generation of threading dislocations within the SiGe layer. This is achieved by depositing SiGe on an ultrathin silicon on insulator (SOI) substrate with a superficial silicon thickness less than the SiGe layer thickness. Initially, the thin Si layer is put under tension due to an equalization of the strain between the Si and SiGe layers. Thereafter, the strain created in the thin Si layer relaxes by plastic deformation. Since the dislocations are formed and glide in the thin Si layer, no threading dislocation is ever introduced in to the upper SiGe material, which appeared dislocation free to the limit of the cross sectional transmission electron microscopy analysis. We thus have a method for producing very low dislocation, relaxes SiGe films with the additional benefit of an SOI substrate.

Ge segregation in SiGe/Si heterostructures and its dependence on deposition technique and growth atmosphere

Ge segregation at SiGe/Si heterointerfaces has been studied for films deposited by atmospheric pressure chemical vapor deposition (APCVD), ultrahigh vacuum CVD (UHV/CVD) and molecular beam epitaxy (MBE). Profiles were taken by secondary‐ion‐mass‐spectroscopy (SIMS) of samples grown with these techniques at the same growth temperatures and Ge concentrations. The MBE grown profiles are dominated by segregation of Ge into the Si top layer in the temperature range from 450 to 800 °C. SiGe/Si interfaces deposited by UHV/CVD at elevated temperatures are smeared, but at 515 °C and below the interfaces are abrupt within the resolution of the SIMS. Heterostructures grown by APCVD show abrupt interfaces and no indication of Ge segregation in the investigated temperature range from 600 to 800 °C. Surface passivation by hydrogen appears to be responsible for the suppression of the Ge segregation in CVD processes.

RELAXATION OF SIGE THIN FILMS GROWN ON SI/SIO2 SUBSTRATES

The relaxation in Si/SiGe bilayers grown on top of SIMOX wafers has been studied. By judiciously choosing the thickness ratios of the Si and SiGe, it is possible to relax the bilayer through the glide of dislocations exclusively in the Si layer, leaving the top SiGe layer relaxed and (mostly) dislocation free. This approach is completely different from previously proposed ways of reducing the number of threading dislocations in SiGe films because at no stage during the relaxation process are new threads introduced in the top SiGe layer. It is shown that the Si/SiGe bilayer behaves as a free‐floating foil constrained to remain flat by the substrate, even at temperatures as low as 700 °C. The relaxation is shown to proceed until the strain left in the Si layer is too low for dislocations to glide. When the temperature is raised to 1050 °C, interdiffusion between the two layers forces the dislocation network to move into the SiGe through glide. The original network of 60° dislocations can then react to form ...

Formation of β‐SiC nanocrystals by the relaxation of Si1−yCy random alloy layers

In this work we consider the relaxation behavior of Si1−yCy random alloys grown epitaxially on Si, with 0.005≳y≳0.05. The Si1−yCy layers are under tensile strain as grown and at annealing temperatures below 900 °C the relaxation of strain is achieved by dislocation formation, in a fashion similar to SiGe relaxation. However, at temperatures in excess of 900 °C the C, which at lower temperatures remained in substitutional sites, precipitates out of the lattice, this removes all of the tensile strain from the layer. The nature of this precipitation is to form single crystal, nanoparticles of β‐SiC with the same lattice orientation as the Si lattice in which they are created. These nanoparticles are of uniform diameter (3±1 nm for y=0.005 Si1−yCy material) and randomly dispersed throughout the original Si1−yCy region. This ability to produce nanocrystals of wide band‐gap material within the Si matrix should enable the exploration of mesoscopic phenomena. The nanoparticles once formed also block the movement ...

SiCGe Ternarv Allovs - Extending Si-based Heterostructures

We have synthesized Si1-y Cyand Si1-x-yCyGexalloys using Molecular Beam Epitaxy. When combined with the Si-Ge system. the new ternary system offers greater versatility and freedom in strain and bandgap engineering. Unlike the Si-Ge system the Si-C system has a high misfit (52%) and low solubility (≪ 10-6), with a propensity to compound formation, therefore. the structures are kinetically stabilized by low temperature growth. In this paper. we first describe bandgap engineering applied to this system. We then consider the growth methodology and critical thickness. Strain compensation and strain engineering using the ternary system is then described. Finally we show that thermal degradation of these films does not occur till ≫ 800°C first by interdiffusion and subsequently at higher temperatures by silicon carbide precipitation.

Atomic layer doping for Si

Abstract We report on initial results of the potential of self-limiting surface reactions for the doping of Si layers using molecular beam epitaxy (MBE) and atmospheric pressure chemical vapor deposition (APCVD). In MBE experiments using Sb as an n-type dopant a self-limiting process is obtained at a coverage of half a monolayer. No evidence of a self-limiting process has yet been found for p-type doping using B2H6 above 400 °C. In the case of MBE growth at temperatures below 400 °C the B is only partly activated (10%–20%). In APCVD grown samples B surface coverage leads to significant growth inhibition of the subsequent deposition of Si from SiCl2H2. Finally, preliminary results of atomic layer doping using AsH3 in APCVD indicate a self-limitation of chemisorption of AsH3 at about 0.1 monolayer at a temperature of 600 °C; however, subsequent growth of Si leads to a smearing out of the As due to segregation and to the residence time of As in the system.

Growth and strain symmetrization of Si/Ge/C/Sn quaternary alloys by molecular beam epitaxy

We have synthesized, via molecular beam epitaxy alloys of SixSnyC1−x−y with symmetric strain. In this work we report the growth of systems with varying compositions/band gaps including the first silicon‐based quaternary (Si/Ge/Sn/C) system, which offers an additional degree of freedom for strain and band gap engineering in Si‐based alloys. We report the growth of Si.955Sn.03C.015 alloys up to 4500 A in thickness and quaternaries of composition in the neighborhood of Si.835Ge.125Sn.03C.01. Infrared absorption spectroscopy and photoluminescence data have provided evidence of the potential for significant band gap modification in these alloys.

Silicon-Germanium-Carbon Alloys Extending Si Based Heterostructure Engineering.

SiGe epitaxial growth on Si has been of interest for a number of years. In this work we consider the addition of 1 to 6% C into both Si and SiGe epitaxial material. We have used a solid, and a gas source, (acetylene), within a solid source SiGe molecular beam epitaxy system to produce a C flux. We have produced high-crystalline-quality Si1-y Cy and Si1-x -y Gex Cy material using both approaches. In addition we demonstrate strain-symmetrical short-period superlattice structures grown on (100) Si with high Ge compositions ranging from 20% up to 100% Ge, at 100% Ge the Ge/Si1-y Cy superlattice has an interface mismatch of 7%.

Thermally Induced Precipitation of Silicon Carbide in a Semiconductor Matrix-Application to Nanoparticle Fabrication.

In this work the Si1-y Cy random alloy is used as a starting point for the creation of nano particles of β-SiC with the same lattice orientation as the Si lattice in which they are grown. These nano particles are between 3 and 8 nm in diameter and are randomly dispersed throughout the Si1-y Cy region, where 0.005<y<0.05. This ability to produce quantum antidots of wide bandgap material within the Si matrix should enable the exploration of mesoscopic phenomena.