Interfacial sharpness and intermixing in a Ge-SiGe multiple quantum well structure

Abstract

A Ge-SiGe multiple quantum well structure created by low energy plasma enhanced chemical vapour deposition, with nominal well thickness of 5.4 nm separated by 3.6 nm SiGe spacers, is analysed quantitatively using scanning transmission electron microscopy. Both high angle annular dark field imaging and electron energy loss spectroscopy show that the interfaces are not completely sharp, suggesting that there is some intermixing of Si and Ge at each interface. Two methods are compared for the quantification of the spectroscopy datasets: a self-consistent approach that calculates binary substitutional trends without requiring experimental or computational k-factors from elsewhere and a standards-based cross sectional calculation. Whilst the cross section approach is shown to be ultimately more reliable, the self-consistent approach provides surprisingly good results. It is found that the Ge quantum wells are actually about 95% Ge and that the spacers, whilst apparently peaking at about 35% Si, contain significant interdiffused Ge at each side. This result is shown to be not just an artefact of electron beam spreading in the sample, but mostly arising from a real chemical interdiffusion resulting from the growth. Similar results are found by use of X-ray diffraction from a similar area of the sample. Putting the results together suggests a real interdiffusion with a standard deviation of about 0.87 nm, or put another way—a true width defined from 10%–90% of the compositional gradient of about 2.9 nm. This suggests an intrinsic limit on how sharp such interfaces can be grown by this method and, whilst 95% Ge quantum wells (QWs) still behave well enough to have good properties, any attempt to grow thinner QWs would require modifications to the growth procedure to reduce this interdiffusion, in order to maintain a composition of ≥95% Ge.

Control of Electron-State Coupling in Asymmetric Ge/Si−Ge Quantum Wells

“…A quantitative analysis of the image clearly indicates the presence of Si-Ge intermixing at the interfaces, resulting in a broadening of the well-barrier profile over a length scale of approximately 0.8 nm. This effect is obviously more relevant in thin layers (thickness <2.5 nm), well matching the results of Bashir et al on nominally identical Ge/Si-Ge MQWs [29], and thus leading to a thicknessdependent lowering of the confining potential. Nonetheless, taking this intermixing effect into account, we obtain a reasonable agreement of the measured layer thickness with the nominal one [see inset of Fig.…”

Atomic-Scale Insights into Semiconductor Heterostructures: From Experimental Three-Dimensional Analysis of the Interface to a Generalized Theory of Interfacial Roughness Scattering

“…[18], using the same technique. The value of ℒ we report here is also more than a factor ~2.5× smaller compared to those measured in Ge/Si0.2Ge0.8 multi-quantum well samples grown by reduced pressure CVD and plasma enhanced CVD reactors [29,30]. This suggests that UHV-CVD might be better suited for realizing sharper interfaces in high-Ge content heterostructures with layer thicknesses of the order of few nanometers.…”

Electron Population Dynamics in Optically Pumped Asymmetric Coupled Ge/SiGe Quantum Wells: Experiment and Models

“…The basic requirements that a semiconductor heterostructure hosting QWs has to comply with in order to be suitable for QCL development are a high quality of the heterostructure; a band offset compatible with the target emitted photon energy; and a small effective mass of carriers (holes or electrons). The small effective mass is beneficial both to obtaining a high laser gain and to avoiding extremely thin tunnelling barriers and confinement wells, which are difficult to grow due to unavoidable intermixing effects in alloy materials [2,3].…”

“…These conditions, however, are detrimental for photon emission at the 3→2 transition, as they lead to (1) high electron temperatures under optical pumping; (2) high non-radiative scattering rates; and (3) scarce wavefunction overlap between levels 2 and 3, respectively. Therefore, lasing action under optical pumping in Ge/SiGe ACQWs has not been observed to date [22] despite absorption saturation being clearly observed.…”