A. Dobbie

Reverse graded relaxed buffers for high Ge content SiGe virtual substrates

An innovative approach is proposed for epitaxial growth of high Ge content, relaxed Si1−xGex buffer layers on a Si(001) substrate. The advantages of the technique are demonstrated by growing such structures via chemical vapor deposition and their characterization. Relaxed Ge is first grown on the substrate followed by the reverse grading approach to reach a final buffer composition of 0.78. The optimized buffer structure is only 2.8μm thick and demonstrates a low surface threading dislocation density of 4×106cm−2, with a surface roughness of 2.6nm. The buffers demonstrate a relaxation of up to 107%.

Reverse graded SiGe/Ge/Si buffers for high-composition virtual substrates

The effect of compositional grading rate on reverse linear graded silicon germanium virtual substrates, grown by reduced pressure chemical vapor deposition, is investigated. For a Si(001)/Ge/RLG/Si0.22Ge0.78 buffer of 2.4 μm total thickness the threading dislocation density (TDD) within the top, fully relaxed, Si0.22Ge0.78 layer is 4×106 cm−2, with a surface roughness of 3 nm. For a thicker buffer, where the grading rate is reduced, a lower TDD of 3×106 cm−2 and a surface roughness of 2 nm can be achieved. The characteristics of reverse graded Si0.22Ge0.78 virtual substrates are shown to be comparable to, or exceed, conventional buffer techniques, leading to thinner high-quality high Ge composition SiGe virtual substrates.

Ultra-high hole mobility exceeding one million in a strained germanium quantum well

In this paper, we report a Hall mobility of one million in a germanium two-dimensional hole gas. The extremely high hole mobility of 1.1 × 106 cm2 V−1 s−1 at a carrier sheet density of 3 × 1011 cm−2 was observed at 12 K. This mobility is nearly an order of magnitude higher than any previously reported. From the structural analysis of the material and mobility modeling based on the relaxation time approximation, we attribute this result to the combination of a high purity Ge channel and a very low background impurity level that is achieved from the reduced-pressure chemical vapor deposition growth method.

High Quality Strained Ge Epilayers on a Si0.2Ge0.8/Ge/Si(100) Global Strain-Tuning Platform

High structural quality, compressively strained Ge surface epilayers have been grown on Si(100) substrates of up to 200 mm diameter. The epitaxial growth by industrially compatible reduced pressure chemical vapor deposition proceeded uninterruptedly via an intermediate relaxed Si0.2Ge0.8/Ge buffer. In-depth characterization of the epilayers revealed a relatively smooth surface and a low threading dislocation density. The Ge layers were demonstrated to remain fully strained for thicknesses of more than 100 nm. The high quality of the material and flexibility to choose the Ge layer thickness over a wide range make these heterostructures very attractive for fabricating various electronic and photonic devices

Low-Frequency Noise Characterization of Strained Germanium pMOSFETs

Low-frequency noise in strained Ge epitaxial layers, which are grown on a reverse-graded relaxed SiGe buffer layer, has been evaluated for different front-end processing conditions. It has been shown that the 1/f noise in strong inversion is governed by trapping in the gate oxide (number fluctuations) and not affected by the presence of compressive strain in the channel. However, some impact has been found from the type of halo implantation used, whereby the lowest noise spectral density and the highest hole mobility are obtained by replacing the standard As halo by P implantation. At the same time, omitting the junction anneal results in poor device characteristics, which can be understood by considering the presence of a high density of nonannealed implantation damage in the channel and the gate stack near the source and the drain.

Strain dependence of electron-phonon energy loss rate in many-valley semiconductors

We demonstrate significant modification of the electron-phonon energy loss rate in a many-valley semiconductor system due to lattice mismatch induced strain. We show that the thermal conductance from the electron system to the phonon bath in strained n+Si, at phonon temperatures between 200 and 480 mK, is more than an order of magnitude lower than that for a similar unstrained sample.

High quality relaxed germanium layers grown on (110) and (111) silicon substrates with reduced stacking fault formation

Epitaxial growth of Ge on Si has been investigated in order to produce high quality Ge layers on (110)- and (111)-orientated Si substrates, which are of considerable interest for their predicted superior electronic properties compared to (100) orientation. Using the low temperature/high temperature growth technique in reduced pressure chemical vapour deposition, high quality (111) Ge layers have been demonstrated almost entirely suppressing the formation of stacking faults (< 107 cm−2) with a very low rms roughness of less than 2 nm and a reduction in threading dislocation density (TDD) (∼ 3 × 108 cm−2). The leading factor in improving the buffer quality was use of a thin, partially relaxed Ge seed layer, where the residual compressive strain promotes an intermediate islanding step between the low temperature and high temperature growth phases. (110)-oriented layers were also examined and found to have similar low rms roughness (1.6 nm) and TDD below 108 cm−2, although use of a thin seed layer did not offer the same relative improvement seen for (111).

High-resolution x-ray diffraction investigation of relaxation and dislocations in SiGe layers grown on (001), (011), and (111) Si substrates

This work presents a detailed characterization, using high-resolution x-ray diffraction, of multilayered Si1-xGex heterostructures grown on (001), (011), and (111) Si substrates by reduced pressure chemical vapor deposition. Reciprocal space mapping has been used to determine both the strain and Ge concentration depth profiles within each layer of the heterostructures after initially determining the crystallographic tilt of all the layers. Both symmetric and asymmetric reciprocal space maps were measured on each sample, and the evaluation was performed simultaneously for the whole data set. The ratio of misfit to threading dislocation densities has been estimated for each individual layer based on an analysis of diffuse x-ray scattering from the defects.

Strain enhanced electron cooling in a degenerately doped semiconductor

Enhanced electron cooling is demonstrated in a strained-silicon/superconductor tunnel junction refrigerator of volume 40 μm3. The electron temperature is reduced from 300 mK to 174 mK, with the enhancement over an unstrained silicon control (300 mK–258 mK) being attributed to the smaller electron-phonon coupling in the strained case. Modeling and the resulting predictions of silicon-based cooler performance are presented. Further reductions in the minimum temperature are expected if the junction sub-gap leakage and tunnel resistance can be reduced. However, if only tunnel resistance is reduced, Joule heating is predicted to dominate.

High-quality Ge/Si0.4Ge0.6multiple quantum wells for photonic applications : growth by reduced pressure chemical vapour deposition and structural characteristics

Strain-symmetrized Ge/SiGe multiple quantum wells have been grown on a thin (2.1 µm) relaxed Si0.2Ge0.8/Ge/Si(1 0 0) virtual substrate (VS) by reduced pressure chemical vapour deposition. Such structures are of interest in optoelectronic applications for which the structural integrity of the quantum well layers, after processing, is critical. The layer composition, thickness and interface quality have been studied for wafers both as-grown and after annealing between 550 and 700 °C. Transmission electron microscopy indicated precise thickness control of ±0.1 nm and sharp abruptness between the Ge QWs and SiGe barrier layers. A smooth surface was observed, with an average rms roughness of 1.5 ± 0.1 nm determined by atomic force microscopy. High-resolution x-ray diffraction (HR-XRD) indicated that both the QWs and barriers were fully strained compared with the relaxed VS. The thermal stability of the epilayers was investigated both by ultra low energy secondary ion mass spectroscopy of post-growth annealed layers and by in situ annealing in a high temperature HR-XRD stage. No obvious interdiffusion and strain relaxation was observed provided the annealing temperature was below 600 °C, but significant atomic rearrangement was evident for greater thermal budgets, thereby setting an upper processing temperature for this type of structure.