Reactive Pulsed Laser Deposition of Microcrystalline Ge-based Thin Films
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A6.11.1
Reactive Pulsed Laser Deposition of Microcrystalline Ge-based Thin Films Matthew R. Wills1, Ruth Shinar2, and Alan P. Constant1 1 Department of Materials Science and Engineering 2 Microelectronics Research Center Iowa State University, Ames Iowa 50011
Pulsed laser deposition (PLD) was used to grow microcrystalline thin films of germanium (Ge) and Ge-carbon (Ge,C) alloys on fused quartz and silicon substrates at substrate temperatures 25 oC ≤ Ts ≤ 325 oC. The films were analyzed structurally with x-ray diffraction (XRD), optically, electrically with four-point probe measurements, and chemically with x-ray photoelectron spectroscopy (XPS). XRD results displayed a varying degree of crystallinity, with the most crystalline films obtained at Ts > 150 oC. The resistivity of the Ge films decreased with increasing temperature, displaying a significant decrease for the films deposited at Ts ≥ 230 oC. The growth conditions for Ge films served as a starting point for low-temperature deposition of Ge,C alloys with up to 5% C. The effects of Ts and carbon concentration on film properties are discussed.
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A6.11.2
INTRODUCTION Germanium (Ge) is an attractive material for electronic and optoelectronic applications due to its high electron and hole mobilities, 3900 and 1900 cm2/Vs, respectively, as well as its higher absorption coefficient in comparison to silicon (Si). Low temperature growth of amorphous to epitaxial Ge-based thin films has received great attention for photovoltaic/optoelectronic applications such as band gap engineering and development of high-speed devices [1-3]. However, the widespread application of Ge in microelectronics is limited due to the lattice mismatch of Ge (5.6575 Ǻ) and Si (5.4307Ǻ), and the small bandgap of Ge (0.67 eV), making it more susceptible to thermal noise and less suitable for photovoltaic applications. Fabrication of high quality Ge-based films, such as Si-Ge with carbon, may offer solutions to these issues. By varying the concentration of substitutional carbon the Si-Ge-C bandgap can be extended by 21-26 meV per at. %C [4]. Growth methods of Ge and Ge-based films include chemical vapor deposition (CVD)-based techniques, molecular beam epitaxy (MBE), and pulsed laser ablation [5-7]. Pulsed laser deposition (PLD) has emerged as in important tool for deposition of high quality films, varying from amorphous to crystalline. Advantages of PLD include the ability to transfer a complex stoichiometry from a target to a growing film, as demonstrated for superconducting oxides [8]. Growth occurs under extreme nonequilibrium conditions with minimal surface reorganization of the atoms impinging on the substrate. Though low surface mobility can adversely affect the quality of the growing film, it may allow fabrication of crystalline films of thermodynamicall
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