Doping of vanadium to nanocrystalline diamond films by hot filament chemical vapor deposition
© Zhang et al.; licensee Springer. 2012
Received: 29 June 2012
Accepted: 22 July 2012
Published: 8 August 2012
Doping an impure element with a larger atomic volume into crystalline structure of buck crystals is normally blocked because the rigid crystalline structure could not tolerate a larger distortion. However, this difficulty may be weakened for nanocrystalline structures. Diamonds, as well as many semiconductors, have a difficulty in effective doping. Theoretical calculations carried out by DFT indicate that vanadium (V) is a dopant element for the n-type diamond semiconductor, and their several donor state levels are distributed between the conduction band and middle bandgap position in the V-doped band structure of diamond. Experimental investigation of doping vanadium into nanocrystalline diamond films (NDFs) was first attempted by hot filament chemical vapor deposition technique. Acetone/H2 gas mixtures and vanadium oxytripropoxide (VO(OCH2CH2CH3)3) solutions of acetone with V and C elemental ratios of 1:5,000, 1:2,000, and 1:1,000 were used as carbon and vanadium sources, respectively. The resistivity of the V-doped NDFs decreased two orders with the increasing V/C ratios.
KeywordsNanocrystalline diamond Vanadium dopant Donor state levels Structural distortion toleration
Doping an impure element with a larger atomic volume into crystalline structure of buck crystals is normally blocked because the rigid structure could not tolerate a larger distortion. However, this difficulty may be overcome to some extent for nanocrystalline materials due to its weakened structure with large a surface-to-volume ratio. As a typical example, carbon nanotubes have been doped with silicon, sulfur , etc. Diamond is a super-functional material with many promising properties, which has been utilized in many commercial applications such as electrochemical electrodes, heterojunction, photodiode, radiation detectors, and high-frequency SAW devices [2–5]. Diamond films have high electrical resistivity when undoped and could be effectively p-type doped by boron [6, 7]. However, the realization of n-type doping of diamond films, based on device application, has met a serious obstacle of tough impurity doping problem . The ideal n-type diamond films for electronic applications are hard to be acquired in experiments with the doping of Li, Na, N, P, S, As, Sb, etc [9–12]. Finding a well-established substitutional donor for n-type diamond films is a worldwide issue, owing to the extremely small lattice space between C-C atoms within the diamond structure. Previous studies on most impurity elements which have been reported are in the main group, and the subgroup element is rarely seen in papers because of its complicated atom structure with larger atomic sizes and its acquisition difficulty.
The research purpose of this paper is to perform first principle calculations to study the electronic properties of V-doped diamond, and attempt to dope vanadium into nanocrystalline diamond films (NDFs), and to test if the V element can be used as dopant species which could be carried through bubbling in acetone by hydrogen during chemical vapor deposition process.
Changes of the atomic structure of V-doped diamond
Type of diamond film
Bond length (Å)
Bond angle (degrees)
1.544, 1.544, and 1.544
109.5, 109.5, and 109.5
1.804, 1.804, and 1.804
110.2, 110.2, and 110.2
For doping experiments, vanadyl acetylacetonate (VO(C5H7O2)2) powder, which was dissolved in acetone, was used at first as the source of vanadium for dissolubility and less toxicity. However, we found that during the experiment, the color of the liquid mixture changed from light green to dark green, and we measured the weight of the residual vanadyl acetylacetonate only to find that it was almost the same as that of the powder prior to deposition. It clearly means that little powder was carried out into the chamber. Based on the above consideration, the vanadium source selected was vanadium oxytripropoxide (VO(OCH2CH2CH3)3), which was in a liquid state at room temperature and was diffluent in acetone. However, we also found that when the V/C ratio was 1:1,000, it had already been saturated, so this ratio was the maximum in the experiment. At this time, the color of the liquid mixture was clarified as red-brown, and it never changed throughout the process, which could confirm that V was successfully being introduced into the chamber in sufficient amounts.
Different dilutions were used to vary the V/C ratio in the reactor gas phase, which was 0 (sample 1, undoped), 1:5,000 (sample 2), 1:2,000 (sample 3), 1:1,000 (sample 4). All these samples were grown on 2 × 2-cm2 silicon substrates, and the depositional conditions were also the same.
The deposited NDF morphology had been characterized using field emission scanning electron microscopy (ULTRA 55, Carl Zeiss AG, Oberkochen, Germany). Laser Raman spectroscopy (325-nm excitation) was used to evaluate the average sizes of nanocrystals in NDFs. Resistivity measurements were carried out by an Agilent voltmeter (Agilent Technologies, Inc., Santa Clara, CA, USA). X-ray fluorescence measurements were to probe the V concentration in the sample films.
Results and discussion
Theoretical calculation reveals that V is a dopant element for the n-type diamond semiconductor, but strong structural distortion is a significant difficulty for doping V into the diamond lattice. Experiments have demonstrated a way to doping V into NDFs in HFCVD conditions. The results from experiments indicated that doping an impure element with a larger atomic volume into the nanocrystalline-structured materials may be a possible way to synthesize normally difficult-to-dope semiconductors and that nanocrystalline structures could tolerate impurity induced by larger distortions.
This work is supported by the National High-Tech R&D Program of China (863, no. 2011AA050504), Natural Science Foundation of Shanghai (no. 10ZR1416300), the Foundation for SMC Excellent Young Teacher, and the Analytical and Testing Center in Shanghai Jiao Tong University. The authors also thank the support of theoretical calculations from Prof. XS Chen and Prof. W Lu of National Laboratory for Infrared Physics, SITP, CAS.
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