D. C. Houghton

Strain relaxation kinetics in Si1−xGex/Si heterostructures

A semiempirical kinetic model is presented which maps out the thermal budget for processing of strained layer devices through epitaxial growth and postgrowth anneals. Misfit strain relaxation in Si1−xGex/Si heterostructures by the injection and propagation of a/2 〈110〉 60°‐type misfit dislocations has been studied for a range of geometries and dimensions. Strained layer superlattices, Si1−xGex alloy layers, 0<x<0.3, and alloy layers with unstrained Si capping layers of thickness 0 to 400 nm were grown by molecular‐beam epitaxy on (100) Si substrates and subjected to post‐growth thermal cycles. Velocity and nucleation rate data from Nomarski interference microscopy of defect‐etched surfaces were correlated with electron beam induced current microscopy transmission electron microscopy and x‐ray diffraction results to define Arrhenius relationships for misfit dislocation injection rates and propagation velocities. A unified kinetic model for misfit strain relaxation that incorporates both nucleation and prop...

Intense photoluminescence between 1.3 and 1.8 μm from strained Si1−xGex alloys

Intense photoluminescence (PL) from strained, epitaxial Si1−xGex alloys grown by molecular beam epitaxy is reported with measured internal quantum efficiencies up to 31% from random alloy layers, single buried strained layers, and multiple quantum wells. Samples deposited at ∼400u2009°C exhibited low PL intensity, whereas annealing at ∼600u2009°C enhanced the intensity by as much as two orders of magnitude. This anneal treatment was found to be optimal for removal of grown‐in defect complexes without creating a significant density of misfit dislocations. PL peak energies at 4.2 K varied from 620 to 990 meV for Ge fractions from 0.53 to 0.06, respectively. Efficient PL was due to exciton accumulation in the strained Si1−xGex layers of single and multiple quantum wells, where the band gap was locally reduced. Optical transitions associated with the PL occurred without phonon assistance.

Misfit strain relaxation in GexSi1−x/Si heterostructures: The structural stability of buried strained layers and strained‐layer superlattices

The critical thickness‐strain relationships for buried strained layers and strained‐layer superlattices (SLSs) are derived using an energy balance model. Relaxation of the entire heterostructure and individual strained layers by both 60° type a/2〈011〉 and pure edge dislocations is considered. GexSi1−x/Si heterostructures designed to investigate the stability regimes predicted by the model were grown by molecular‐beam epitaxy. The extent of relaxation and the detailed dislocation structure were assessed in annealed structures by x‐ray rocking curve analysis, transmission electron microscopy, and Nomarski microscopy of etched samples. Comparison of metastable as‐grown and post‐growth annealed microstructures revealed the evolution of misfit dislocation structure as equilibrium was approached on annealing in the temperature range 600–900u2009°C. The predominant relaxation mechanism for most GexSi1−x/Si heterostructures was by 60° a/2 〈011〉 misfit dislocations at the first strained‐layer/substrate interface. Howe...

Growth and characterization of Si1−xGex and Ge epilayers on (100) Si

Two approaches to the growth of high‐quality epitaxial Ge epilayers on (100) Si have been investigated. The first consisted of compositional‐grading Si1−xGex layers and the use of strained‐layer superlattices as dislocation filters. In general, this method produced unsatisfactory results, due to the difficulty in achieving good epitaxial growth in the Ge concentration interval 30%−70%. The second approach consisted of simply depositing pure Ge directly on (100) Si. Excellent epitaxial films with dislocation densities of less than 107 cm−2 and smooth morphology were obtained after optimization of the growth parameters. The initial growth temperature and post‐growth annealing were found to be critical in obtaining good epitaxial material.

Luminescence origins in molecular beam epitaxial Si1−xGex

Interstitial‐type features smaller than ∼1.5 nm and in areal densities up to 7×108 cm−2 have been identified as the origin of a broad photoluminescence (PL) band from thick, fully strained layers of Si1−xGex alloys grown by molecular beam epitaxy. The strong PL band was predominant when the alloy layer thickness was greater than 4–10 nm, depending on x and the growth temperature. Thinner alloy layers exhibited phonon‐resolved transitions originating from shallow dopant bound excitons, similar to bulk material but shifted in energy due to strain and hole quantum confinement.

Heterogeneous nucleation sources in molecular beam epitaxy-grown GexSi1−x/Si strained layer superlattices

Abstract In the past few years, a number of studies have indicated the importance of heterogeneous sources in understanding defect nucleation mechanisms. Nonetheless, the majority of critical thickness research is still based on homogeneous misfit dislocation nucleation theory to predict the onset of plastic relaxation at strained layer interfaces. It will be shown that heterogeneous sources are plentiful even in molecular beam epitaxy (MBE)-grown material wherein the generation of defects depends on the oxygen/carbon contamination levels. Several metastable Ge x Si 1− x /Si strained layer superlattices have been grown under slightly different conditions in order to vary the concentration of oxygen/carbon at the substrate-buffer interface. Subsequent annealing treatments were carried out in order to accentuate any characteristic differences in the plastic relaxation behaviour of these metastable structures. Chemical defect etching, plan-view and cross-sectional transmission electron microscopy have been used extensively to characterize misfit dislocation nucleation behaviour at the strained layer interfaces based on differences in the number and type of heterogeneous sources.

High quantum efficiency photoluminescence from localized excitons in Si1−xGex

We report a new photoluminescence process in epitaxial Si1−xGex layers grown on Si by rapid thermal chemical vapor deposition which we attribute to the recombination of excitons localized at random alloy fluctuations. This luminescence is characterized by saturation at very low excitation densities (≂100 μW cm−2), very long decay times (≳1 ms), and high quantum efficiency at low excitation. We have directly measured an external photoluminescence quantum efficiency of 11.5±2%.

Electroluminescence and photoluminescence from Si1-xGex alloys

Electroluminescence has been observed from Si1−xGex/Siu2009p‐n heterostructures grown by molecular beam epitaxy and fabricated into mesa diodes. The luminescence from each sample was observed at temperatures up to 80 K with diodes forward biased at current densities up to 50 A/cm2. For x=0.18 and x=0.25, broad (∼80 meV) electroluminescence peaks were observed at 890 and 860 meV, respectively. These energies as well as the peak shapes and quantum efficiencies (∼1%) were the same as those from corresponding photoluminescence spectra.

On the electron diffraction contrast of coherently strained semiconductor layers

Abstract The electron diffraction contrast of coherently strained semiconductor layers viewed in cross-section has been investigated. A number of contrast effects have been observed which are associated with either structure factor and/or strain variations between heterostructure layers. Firstly, the relaxation of built-in elastic strains near thin foil free surfaces has been treated using two elasticity solutions which, from two-beam dynamical image simulations, are shown to accurately predict the observed diffraction contrast features in both single and multilayer structures. Secondly, it has been shown that the contrast of strained epitaxial layers is primarily due to structure factor differences which are manifest by: (i) extinction distance shifts and (ii) δ-fringe patterns when the layers are inclined relative to the electron beam. More generally, it has been pointed out that the visibility of any compositional modulation for a given reflection depends on the orientation of the interfaces with respe...

Design criteria for structurally stable, highly strained multiple quantum well devices

Strain compensation allows the synthesis of infinitely thick heterostructures with many highly strained quantum wells. Design criteria are given for optimized strain and thickness parameters in several device geometries. Strain compensation, using alternating layers of opposite strain, is quantitatively treated using an energy balance analysis. The upper bound to stability for strained multiple quantum wells with and without strain compensation is defined for geometries typically used in optoelectronic devices. Highly metastable structures (composed of many layers of high strain and/or thickness) require low epitaxy temperatures to avoid strain relaxation during growth of individual strained layers, prior to their stabilization in a strain compensated structure.