Diamond and its thin films have acquired an amazing charm and great potential for industrial applications due to their highly valuable characteristics, such as high carrier mobility, a large energy gap that makes them good insulators, and high thermal conductivity. Moreover, the doping of various elements such as boron, nitrogen, oxygen, and phosphorus in diamond further enhances its utility for electronics and electrochemical devices. Such interesting characteristics have significantly attracted the attention of researchers and opened new ways for handling conductivity issues caused by impurity-induced metallization in diamonds. Hence, the present study on the characterization of diamond thin films under various conditions/morphologies has been performed and divided into two parts: experimental work and computational calculations based on density functional theory (DFT) using the pseudo-potential method. This involved spin-polarized GGA-PW91 functionals on plane wave basis sets using VASP codes. Micro- and nanocrystalline diamond films have been deposited on silicon (100) wafers using the hot-filament chemical vapor deposition method with source gases (CH4 + H2) and by doping with different elements such as boron, oxygen, and nitrogen under different deposition conditions, including chamber pressure, doping concentration, and gas flow rate. These films have been characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and van der Pauw technique to measure and analyze structure, crystal quality, morphology, and electrical resistivity of the deposited diamond films. Results have shown that diamond growth rate decreases at low methane concentrations but increases (~3 orders) at high methane content in the deposition chamber. It also decreases on increasing chamber pressure, showing improvement in the quality of diamond crystals. The optimum CH4 content for well-faceted crystals is 3.0 ml/min with minimum resistivity of 1.79 × 107 Ω-cm. The effect of doping on diamond characteristics demonstrates that as concentration of O2 or B increases, high-quality large-sized {111} grains are deposited yielding low resistivity (~ 105 Ω-cm). However, such good-quality micro-diamond grains are noticed at low N2 concentrations. In addition, superposition of both nano- and micro-sized grains is observed at higher N2 content. Resistivity of such films decreased significantly to 107 Ω-cm due to increased sp2 bonds. Theoretical aspects of doped diamond films demonstrate that both O2 and B-atoms support the splitting of diamond energy band by forming impurity states near/at Fermi level and hence enhance conductivities of these thin films. This fact supports our experimental results of doped diamond films. Moreover, high doping concentration affects significantly on atomic charge making it positive (1.89 eV). On the other hand, dopants of transition metals (TMs) become spin polarized by magnetizing p electrons of C-atoms through p-d hybridization. Such hybridization is strong for Cu doping as compared to other TM dopants and involves FM coupling state rather than AFM. The appearance of specific O (2p) band at −18 eV in the energy band spectrum of O-doped diamond signifies the existence of donor levels between valence and conduction bands, which were very deep and lead to reduction of the energy gap (~0.865 eV). However, the incorporation of oxygen atoms into the diamond (100) surface was partially favorable.

Electrical Conductivity of CVD Diamond Thin Films

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发表日期：

2025年

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Mahtab Ullah,
R. A. Manzoor,
E. Ahmed

摘要

Diamond and its thin films have acquired an amazing charm and great potential for industrial applications due to their highly valuable characteristics, such as high carrier mobility, a large energy gap that makes them good insulators, and high thermal conductivity. Moreover, the doping of various elements such as boron, nitrogen, oxygen, and phosphorus in diamond further enhances its utility for electronics and electrochemical devices. Such interesting characteristics have significantly attracted the attention of researchers and opened new ways for handling conductivity issues caused by impurity-induced metallization in diamonds. Hence, the present study on the characterization of diamond thin films under various conditions/morphologies has been performed and divided into two parts: experimental work and computational calculations based on density functional theory (DFT) using the pseudo-potential method. This involved spin-polarized GGA-PW91 functionals on plane wave basis sets using VASP codes. Micro- and nanocrystalline diamond films have been deposited on silicon (100) wafers using the hot-filament chemical vapor deposition method with source gases (CH4 + H2) and by doping with different elements such as boron, oxygen, and nitrogen under different deposition conditions, including chamber pressure, doping concentration, and gas flow rate. These films have been characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and van der Pauw technique to measure and analyze structure, crystal quality, morphology, and electrical resistivity of the deposited diamond films. Results have shown that diamond growth rate decreases at low methane concentrations but increases (~3 orders) at high methane content in the deposition chamber. It also decreases on increasing chamber pressure, showing improvement in the quality of diamond crystals. The optimum CH4 content for well-faceted crystals is 3.0 ml/min with minimum resistivity of 1.79 × 107 Ω-cm. The effect of doping on diamond characteristics demonstrates that as concentration of O2 or B increases, high-quality large-sized {111} grains are deposited yielding low resistivity (~ 105 Ω-cm). However, such good-quality micro-diamond grains are noticed at low N2 concentrations. In addition, superposition of both nano- and micro-sized grains is observed at higher N2 content. Resistivity of such films decreased significantly to 107 Ω-cm due to increased sp2 bonds. Theoretical aspects of doped diamond films demonstrate that both O2 and B-atoms support the splitting of diamond energy band by forming impurity states near/at Fermi level and hence enhance conductivities of these thin films. This fact supports our experimental results of doped diamond films. Moreover, high doping concentration affects significantly on atomic charge making it positive (1.89 eV). On the other hand, dopants of transition metals (TMs) become spin polarized by magnetizing p electrons of C-atoms through p-d hybridization. Such hybridization is strong for Cu doping as compared to other TM dopants and involves FM coupling state rather than AFM. The appearance of specific O (2p) band at −18 eV in the energy band spectrum of O-doped diamond signifies the existence of donor levels between valence and conduction bands, which were very deep and lead to reduction of the energy gap (~0.865 eV). However, the incorporation of oxygen atoms into the diamond (100) surface was partially favorable.