Molecular Dynamics Simulation of Germanium Nanoparticles
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Molecular Dynamics Simulation of Germanium Nanoparticles Sang H. Yang and Rajiv J. Berry Air Force Research Laboratory, Material and Manufacturing Directorate, WPAFB, OH 45433, USA ABSTRACT Nanoparticles are known to melt at temperatures well below the bulk melting point. This behavior is being exploited for the recrystallization of Germanium to form large-grain semiconductor thin films on flexible and low temperature substrates. The melting of Ge nanoparticles as a function of size was investigated using the ab-initio Harris functional method of density functional theory (DFT). The DFT code was initially evaluated for its ability to predict the bulk properties of crystalline Ge. A conjugate gradient method was employed for minimizing the multiphase atomic positional parameters of the diamond, BC8 and ST12 structures. The computed lattice constants, bulk moduli, and internal atomic positional parameters were found to agree well with other calculations and with reported experimental results. A constant temperature Nose-Hoover thermostat was added to the DFT code in order to compute thermal properties via molecular dynamics. The simulations were tested on a 13-atom Ge cluster, which was found to melt at 820 K. Further heating resulted in the cluster breaking up into two smaller clusters, which remained stable up to 1300K. INTRODUCTION Recent Air Force Research Laboratory advances in high-efficiency, multijunction space photovoltaic cells have been identified as enabling for upcoming Air Force GPS (Global Positioning System), SBIRS (Space-Based Infrared System), Advanced EHF (Extremely High Frequency of 30-300 GHz: 1cm-1mm) and WGS (World Geodetic System) spacecraft that require greater payload power and/or mass budgets. These solar cells are crystalline, rigid, evolutionary in design, and result in space solar array specific power of 50-100 W/kg. The development of a new class of space solar cell, namely the next generation flexible-thin-film-photovoltaics (FTFPV) has been proposed. It consists of thin layers of amorphous or polycrystalline semiconductor deposited on a flexible substrate, such as a thin layer of polymer or metal foil. Compared to conventional stateof-the art rigid solar arrays FTFPV technology holds the promise of producing improved space-radiation resistant, lighter, and cheaper arrays capable of generating power in the range 200-450 W/kg with efficiencies of 10-15%. The leading thin-film candidates under development are amorphous silicon (α-Si), polycrystalline copper indium gallium diselenide (CIGS) and its alloys. To date, crystalline thin film solar cells can be grown only on high temperature ceramic substrates incompatible with space applications, while only small grain polycrystalline or amorphous semiconductors can be grown on lightweight, flexible substrates. Grain
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