The ${E}_{0}$, ${E}_{0}+{\ensuremath{\Delta}}_{0}$, ${E}_{1}$, ${E}_{1}+{\ensuremath{\Delta}}_{1}$, ${E}_{0}^{\ensuremath{'}}$, and ${E}_{2}$ optical transitions have been measured in ${\mathrm{Ge}}_{1\ensuremath{-}y}{\mathrm{Sn}}_{y}$ alloys $(yl0.2)$ using spectroscopic ellipsometry and photoreflectance. The results indicate a strong nonlinearity (bowing) in the compositional dependence of these quantities. Such behavior is not predicted by electronic structure calculations within the virtual crystal approximation. The bowing parameters for ${\mathrm{Ge}}_{1\ensuremath{-}y}{\mathrm{Sn}}_{y}$ alloys show an intriguing correlation with the corresponding bowing parameters in the ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x}$ system, suggesting a scaling behavior for the electronic properties that is the analog of the scaling behavior found earlier for the vibrational properties. A direct consequence of this scaling behavior is a significant reduction (relative to prior theoretical estimates within the virtual crystal approximation) of the concentration ${y}_{c}$ for a crossover from an indirect- to a direct-gap system.

Optical critical points of thin-film Ge 1-y Sn y alloys: A comparative Ge 1-y Sn y /Ge 1-x Si x study

▪ Abstract New classes of Sn-containing group IV semiconductors are described. Novel CVD routes lead to growth of a broad range of Ge1−ySny alloys and compounds directly on Si substrates. The direct bandgap (E0) and optical transitions E0+Δ0, E1, E1+Δ1, E0′, and E2 of Ge1−ySny exhibit strong nonlinearities in the compositional dependence, and their bowing parameters correlate with those in Ge1 −xSix, suggesting a scaling behavior for the electronic properties. The Ge1−ySny films can be used as “virtual substrates” for the subsequent growth of Ge1−x−ySixSny ternaries. These are created for the first time and exhibit unprecedented thermal stability, superior crystallinity and unique optical and strain properties such as adjustable bandgaps, and controllable strain states (compressive, relaxed, and tensile). The synthesis of Ge1−x−ySixSny makes it possible to decouple strain and bandgap and adds new levels of flexibility to the design of group IV devices. The Ge-Si-Sn system also represents a new class of “d...

TIN-BASED GROUP IV SEMICONDUCTORS: New Platforms for Opto- and Microelectronics on Silicon

Using a 820 nm-thick high-quality Ge0.97Sn0.03 alloy film grown on Si(001) by molecular beam epitaxy, GeSn p-i-n photodectectors have been fabricated. The detectors have relatively high responsivities, such as 0.52 A/W, 0.23 A/W, and 0.12 A/W at 1310 nm, 1540 nm, and 1640 nm, respectively, under a 1 V reverse bias. With a broad detection spectrum (800-1800 nm) covering the whole telecommunication windows and compatibility with conventional complementary metal-oxide-semiconductors (CMOS) technology, the GeSn devices are attractive for applications in both optical communications and optical interconnects.

GeSn p-i-n photodetector for all telecommunication bands detection

Alloys of silicon (Si), germanium (Ge) and tin (Sn) are continuously attracting research attention as possible direct band gap semiconductors with prospective applications in optoelectronics. The direct gap property may be brought about by the alloy composition alone or combined with the influence of strain, when an alloy layer is grown on a virtual substrate of different compositions. In search for direct gap materials, the electronic structure of relaxed or strained Ge1−xSnx and Si1−xSnx alloys, and of strained Ge grown on relaxed Ge1−x−ySixSny, was calculated by the self-consistent pseudo-potential plane wave method, within the mixed-atom supercell model of alloys, which was found to offer a much better accuracy than the virtual crystal approximation. Expressions are given for the direct and indirect band gaps in relaxed Ge1−xSnx, strained Ge grown on relaxed SixGe1−x−ySny and strained Ge1−xSnx grown on a relaxed Ge1−ySny substrate, and these constitute the criteria for achieving a (finite) direct band gap semiconductor. Roughly speaking, good-size (up to ~0.5 eV) direct gap materials are achievable by subjecting Ge or Ge1−xSnx alloy layers to an intermediately large tensile strain, but not excessive because this would result in a small or zero direct gap (detailed criteria are given in the text). Unstrained Ge1−xSnx bulk becomes a direct gap material for Sn content of >17%, but offers only smaller values of the direct gap, typically ≤0.2 eV. On the other hand, relaxed SnxSi1−x alloys do not show a finite direct band gap.

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Band structure calculations of Si Ge Sn alloys: achieving direct band gap materials

We have calculated the electronic properties of graphene and SiC, GeC and SnC monolayers in a two-dimensional graphene-like honeycomb structure under various strained conditions using first principles calculations based on density functional theory and the quasiparticle GW approximation. Our results show that the indirect–direct band gap transition of group-IV carbides can be tuned by strain, which indicates a possible new route for tailoring the electronic properties of ultrathin nanofilms through strain engineering.

Tuning the indirect–direct band gap transition of SiC, GeC and SnC monolayer in a graphene-like honeycomb structure by strain engineering: a quasiparticle GW study

We describe a class of Si-based semiconductors in the Ge1−xSnx system. Deuterium-stabilized Sn hydrides provide a low-temperature route to a broad range of highly metastable compositions and structures. Perfectly epitaxial diamond-cubic Ge1−xSnx alloys are grown directly on Si(100) and exhibit high thermal stability, superior crystallinity, and crystallographic and optical properties, such as adjustable band gaps and lattice constants. These properties are completely characterized by Rutherford backscattering, low-energy secondary ion mass spectrometry, high-resolution transmission electron microscopy, x-ray diffraction (rocking curves), as well as infrared and Raman spectroscopies and spectroscopic ellipsometry. Ab initio density functional theory simulations are also used to elucidate the structural and spectroscopic behavior.

M. Bauer

Papers

Ge–Sn semiconductors for band-gap and lattice engineering