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Register a new userUniaxially stressed germanium with fundamental direct band gap
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We demonstrate the crossover from indirect- to direct band gap in tensile-strained germanium by temperature-dependent photoluminescence. The samples are strained microbridges that enhance a biaxial strain of 0.16% up to 3.6% uniaxial tensile strain. Cooling the bridges to 20 K increases the uniaxial strain up to a maximum of 5.4%. Temperature-dependent photoluminescence reveals the crossover to a fundamental direct band gap to occur between 4.0% and 4.5%. Our data are in good agreement with new theoretical computations that predict a strong bowing of the band parameters with strain.
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Germanium is a strong candidate as a laser source for silicon photonics. It is widely accepted that the band structure of germanium can be altered by tensile strain so as to reduce the energy difference between its direct and indirect band gaps. However, the conventional deformation potential model most widely adopted to describe this transformation happens to have been investigated only up to 1 % uniaxially loaded strains. In this work, we use a micro-bridge geometry to uniaxially stress germanium along [100] up to $varepsilon_{100}$=3.3 % longitudinal strain and then perform electro-absorption spectroscopy. We accurately measure the energy gap between the conduction band at the $Gamma$ point and the light- and heavy-hole valence bands. While the experimental results agree with the conventional linear deformation potential theory up to 2 % strain, a significantly nonlinear behavior is observed at higher strains. We measure the deformation potential of germanium to be a = -9.1 $pm$ 0.3 eV and b = -2.32 $pm$ 0.06 eV and introduce a second order deformation potential. The experimental results are found to be well described by tight-binding simulations. These new high strain coefficients will be suitable for the design of future CMOS-compatible lasers and opto-electronic devices based on highly strained germanium.
The monolayer Gallium sulfide (GaS) was demonstrated as a promising two-dimensional semiconductor material with considerable band gaps. The present work investigates the band gap modulation of GaS monolayer under biaxial or uniaxial strain by using Density functional theory calculation. We found that GaS monolayer shows an indirect band gap that limits its optical applications. The results show that GaS monolayer has a sizable band gap. The uniaxial strain shifts band gap from indirect to direct in Gallium monochalcogenides (GaS). This behavior, allowing applications such as electroluminescent devices and laser. The detailed reasons for the band gap modulation are also discussed by analyzing the projected density of states (PDOS). It indicates that due to the role of p$_y$ orbital through uniaxial strain become more significant than others near the Fermi level. The indirect to direct band gap transition happen at $varepsilon$=-10y$%$. Moreover, by investigating the strain energy and transverse response of structures under uniaxial strain, we show that the GaS monolayer has the Poissons ratio of 0.23 and 0.24 in the zigzag (x) and armchair (y) directions, respectively. Thus, we conclude that the isotropic nature of mechanical properties under strain.
Spin orientation of photoexcited carriers and their energy relaxation is investigated in bulk Ge by studying spin-polarized recombination across the direct band gap. The control over parameters such as doping and lattice temperature is shown to yield high polarization degree, namely larger than 40%, as well as a fine-tuning of the angular momentum of the emitted light with a complete reversal between right- and left-handed circular polarization. By combining the measurement of the optical polarization state of band-edge luminescence and Monte Carlo simulations of carrier dynamics, we show that these very rich and complex phenomena are the result of the electron thermalization and cooling in the multi-valley conduction band of Ge. The circular polarization of the direct-gap radiative recombination is indeed affected by energy relaxation of hot electrons via the X valleys and the Coulomb interaction with extrinsic carriers. Finally, thermal activation of unpolarized L valley electrons accounts for the luminescence depolarization in the high temperature regime.
We report a strain-induced direct-to-indirect band gap transition in mechanically deformed WS2 monolayers (MLs). The necessary amount of strain is attained by proton irradiation of bulk WS2 and the ensuing formation of one-ML-thick, H2-filled domes. The electronic properties of the curved MLs are mapped by spatially- and time-resolved micro-photoluminescence revealing the mechanical stress conditions that trigger the variation of the band gap character. This general phenomenon, also observed in MoS2 and WSe2, further increases our understanding of the electronic structure of transition metal dichalcogenide MLs and holds a great relevance for their optoelectronic applications.
Here we report two-dimensional (2D) single-crystalline holey-graphyne (HGY) created an interfacial two-solvent system through a Castro-Stephens coupling reaction from 1,3,5-tribromo-2,4,6-triethynylbenzene. HGY is a new type of 2D carbon allotrope whose structure is comprised of a pattern of six-vertex and eight-vertex rings. The carbon-carbon 2D network of HGY is alternately linked between benzene rings and sp (carbon-carbon triple bond) bonding. The ratio of the sp over sp2 bonding is 50%. It is confirmed that HGY is stable by DFT calculation. The vibrational, optic, and electric properties of HGY are investigated theoretically and experimentally. It is a p-type semiconductor that embraces a natural direct band gap (~ 1.0 eV) with high hole mobility and electron mobility at room temperature. This report is expected to help develop a new types of carbon-based semiconductor devices with high mobility.
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