Electrons and phonons in amorphous semiconductors

Abstract

The coupling between lattice vibrations and electrons is one of the central concepts of condensed matter physics. The subject has been deeply studied for crystalline materials, but far less so for amorphous and glassy materials, which are among the most important for applications. In this paper, we explore the electron-lattice coupling using current tools of a first-principles computer simulation. We choose three materials to illustrate the phenomena: amorphous silicon (a-Si), amorphous selenium (a-Se) and amorphous gallium nitride (a-GaN). In each case, we show that there is a strong correlation between the localization of electron states and the magnitude of thermally induced fluctuations in energy eigenvalues obtained from the density-functional theory (i.e. Kohn-Sham eigenvalues). We provide a heuristic theory to explain these observations. The case of a-GaN, a topologically disordered partly ionic insulator, is distinctive compared to the covalent amorphous examples. Next, we explore the consequences of changing the charge state of a system as a proxy for tracking photoinduced structural changes in the materials. Where transport is concerned, we lend insight into the Meyer-Neldel compensation rule and discuss a thermally averaged Kubo-Greenwood formula as a means to estimate electrical conductivity and especially its temperature dependence. We close by showing how the optical gap of an amorphous semiconductor can be computationally engineered with the judicious use of Hellmann-Feynman forces (associated with a few defect states) using molecular dynamics simulations. These forces can be used to close or open an optical gap, and identify a structure with a prescribed gap. We use the approach with plane-wave density functional methods to identify a low-energy amorphous phase of silicon including several coordination defects, yet with a gap close to that of good quality a-Si models.

Accurate Control of VS2 Nanosheets for Coexisting High Photoluminescence and Photothermal Conversion Efficiency

“…[29] Av ast of investigations proves that electric and optical properties of amorphous materials are totally different from the crystalline phase,like the theory of localization electronic state in amorphous semiconductor energy bands. [30] Herein, the optical properties of 2D amorphous VS 2 and partial crystallized 2D VO 2 (D) nanosheets are exploited through UV/Vis-NIR spectra and photoluminescence spectra to verify the underlying distinction with crystal VS 2 .A se xpected, navy-blue 2D amorphous VS 2 and green 2D VO 2 (D) nanosheets manifest am arked reinforce in visible and NIR absorption (Figure 5a)a nd photoluminescence (Figure 5c) compared with the crystal VS 2 (Supporting Information, Figure S9). Apart from apparent blue-shifts of absorption edge due to reduced dimensions and size,abroad absorption enhancement and reflectance decreased below the direct band (500 nm to 1200 nm) suggests an in-gap density of states motivated by the coexisting of interstitial V 3+ and oxygen vacancies in 2D amorphous nanosheets.…”

Density functional theory model of amorphous zinc oxide (a-ZnO) and a-X0.375Z0.625O (X= Al, Ga and In)

“…We calculated the coordination number for Zn and O given by the aver- The electronic structure is analysed by calculating the electronic density of states (EDOS) and inverse participation ratio (IPR) of the individual states. The EDOS is shown in Fig.3 (black) and the green vertical lines represents the electronic state localization measured by IPR [15,16]. The value of IPR is 1 for highly localized state and 1/N for extended state, where N is the number of atoms.…”

Realistic inversion of diffraction data for an amorphous solid: The case of amorphous silicon

“…The significant number of defects states clutters the gap in FEAR, which is a prediction in this case, since the EDOS has not to our knowledge been measured for the sample we are studying. Electronic localization is studied using the inverse participation ratio (IPR) [39] which is shown in Fig.8. Banding among the states in the gap leads to an expected delocalization [38].…”