Preparation And Electrical Properties Of Boron-doped Diamond Films

As a superhard material, diamond films possess many unique properties. Dueto their high hardness, high wear resistance, high electron and hole mobility, wideband gap, high optical transmittance, and chemical inertness, they can operate athostile environments while other materials can't. Pure diamond film is anexcellent insulating material with resistivity of more than 1010 ?·cm. Borondoping, however, can turn it into semiconductor or conductor, or evensuperconductor under low temperature (about 10K), which extends itsapplications in electrical fields. In recent years, the discovery of superconductivityin heavily boron-doped diamond films, and the excellent electrochemistryproperties of boron-doped diamond films have received considerable attention ofmany researchers.Diamond films were prepared by EACVD technique. Ethanol is carried intothe chamber by hydrogen, and we studied the effect of the hydroxyl on the growthof diamond films. Using B(OCH3)3 solution as boron source, we preparedboron-doped diamond films of different growth conditions. The effects of boronconcentration, bias, and growth time were studied on the quality and electricalproperties of diamond films. Superconductivity at low temperature, and opticalproperties at different temperatures in heavily boron-doped diamond films weremeasured, respectively. These diamond films were adopted as electrochemistryelectrodes, and cyclic voltammetry was employed to measure the electrochemistryproperties of them.This paper includes four parts, and the details are given below.Firstly, hydroxyl may largely affect the growth properties of the diamondfilms. Ethanol was introduced into the chamber, and we studied the effects of theethanol flow rate on the growth properties of the diamond films when keepingother conditions constant. Ethanol shows strong etching effects in the growthprocess of the diamond films, and largely affects their growth rate, morphology,and mechanical properties. The proper addition of ethanol may increase thegrowth rate of the diamond films. When the growth rate reaches a certain level,the etching effects, however, will enhance, and restrain the growth of the diamondfilms with further increasing the ethanol flow rate. Raman spectra of the diamondfilms do not change obviously with the addition of ethanol, and in them onlydiamond characteristic peak shows up, while the characteristic peaks of graphiteand non-diamond phase don't. The addition of ethanol increases the concentrationof carbon source, it, however, introduces the hydroxyl which may effectively etchthe graphite and non-diamond phase. With the increase of the ethanolconcentration, the diffraction peak of (111) weakens, while that of (220)strengthens. The addition of ethanol leads the diamond films to a good 〈110〉orientation. Small addition of ethanol may increase the abrasiveness ratio of thefilms, excessive ethanol, however, will largely decrease their abrasiveness ratio.The addition of ethanol largely influences the surface morphology of thefilms. Without the addition of the ethanol, clear helical growth steps can be seenon the crystal faces of the films from SEM images;small addition of the ethanolmay shallow the steps, and decrease the grain boundaries;along with the increaseof the ethanol concentration, helical growth steps disappear, and the crystal shapetends to integrity;excessive addition of the ethanol may enhance the etchingeffects of hydroxyl, which then etches holes on the crystal faces, and destroys theintegrity of the crystal shape.Secondly, the growth technics of boron-doped diamond films werepresented in detail from different aspects such as the choice of boron source,growth parameters, and etc.. Raman spectra, and SEM were adopted to analyzethe grow characteristics of boron-doped diamond films. The results show that thegrow rate of the films decreases with the increase of the flow rate of B(OCH3)3.Proper boron doping may make the crystal shape integral, and make the structurecompact. In Raman spectra of the films with low boron doping level, sharpdiamond characteristic peak at 1332cm-1 can be seen, while characteristic peak ofnon-diamond phase at 1560cm-1 doesn't present, which indicates that low borondoping level is favorable to increase the quality of the diamond films. In Ramanspectra of the films with high doping level, the peak at 1332cm-1 shifts towardslower frequency, and becomes asymmetric. And three interesting broad bandsappears around 570, 900, and 1200cm?1 in the low frequency part, and theirintensity increases with the increase of the boron concentration. When the borondoping level is high, the presentation of the broad band around 1560cm-1 is owingto the appearance of non-diamond carbon phase. Boron concentration at the grainboundaries is obviously lower than that in the diamond grains. With otherconditions kept constant, higher bias is favorable to increase the quality of thediamond films.Thirdly, carriers concentration, resistivity, and optical reflectance spectraand conductivity spectra were determined to discuss the superconductivity ofboron-doped diamond films. Hall effect measurements were carried out at roomtemperature to study the electrical properties of boron-doped diamond films, andthe results show that the resistivity decreases with the increase of carriersconcentration, while it almost keeps constant with carriers concentration morethan 6×1020cm-3. Hall mobility of the films decreases with the increase of boronconcentration, and it is inversely proportional to the thickness of the samples.Four probe method was employed to study the electrical properties ofheavily boron-doped diamond films at low temperature, and without externalmagnetic field, heavily boron-doped diamond films show superconductivity atlow temperature of about 10K;while under the external magnetic field, thesuperconductive onset temperature, and zero resistance temperature decrease andthe superconductivity finally disappears with the increase the intensity of themagnetic field. The superconductive transition temperature width broadens withthe increase of the magnetic field, and with the magnetic field reaching 0.5T, thetransition width broadens rapidly. Resistivity and diamagnetic measurementsshow that boron-doped diamond film is a typical type-Ⅱ superconductor. Themechanism of the presence of the superconductivity in boron-doped diamondfilms is similar to that in MgB2 system, i.e. the coupling of a few holes doped atthe top of the bonding σ valence band to optical bond stretching phonon modes(e-p coupling).The optical reflectance spectra and conductivity spectra of the pure andboron-doped diamond films measured in a broad frequency range at 300K and10K show that the spectral weight shifts from interband transition zone to lowfrequency zone, which provides a direct experiment proof to the fact that theelectronic states of the diamond change with the increase of the boronconcentration. The optical reflectance spectra show that the pure diamond film is astandard semiconductor with the valence band completely filled and conductionband completely empty, and the Fermi level locates close to the top of the valenceband, and boron-doping creates the acceptor impurity states (near the Fermi level)with a binding energy of 0.38 eV. When the doping levels are low, those energylevels are isolated, and the impurity states are completely localized. As the boronconcentration increases, the impurity energy levels broaden and form a band. Athigh enough doping level, the impurity band may overlap with the top states ofvalence band because of their very close energies. Metallic conduction could beformed as long as the impurity band crosses the Fermi level. We consider thatperhaps the superconductivity, also, stems from the impurity band.Fourthly, owing to their good conductivity, boron-doped diamond filmswere adopted as electrochemistry electrodes, and cyclic voltammetry wasemployed to measure the volt-ampere characteristics of the diamond electrodes inKCl solution, K4Fe(CN)6 solution, and CySH solution. The results show that thesediamond electrodes possess a wide electrochemistry window of about 3.7V and alow background current close to zero in water solution. When the resistivity ofdiamond films is similar to that of metal, the effects of boron-dopingconcentration isn't evident on the width of the electrochemistry window, thequality of the films, however, largely affect the electrochemistry stability of theelectrodes. When the characteristic peak of non-diamond carbon phase presents inthe Raman spectra of the diamond films, the redox peak of graphite phase willappear in the potential window of diamond electrodes.Standard measurements of diamond film electrodes were carried out inK4Fe(CN)6 solution. The results show that the oxidative peak, and reductive peakare symmetrical, and they are very close with the distance of 72mV between them.The current increases with the increase of the scan speed, and the redox potentialsalmost don't shift, and the intensity of the redox currents are linearly dependent ofthe root of the scan speed., which indicates that the redox reaction on the surfaceof the diamond film electrode has good reversibility, and its process is very rapid,and reaction kinetics of the electrode are mainly controlled by diffusing process.Comparing with glassy carbon electrode, diamond film electrode s havehigh sensitivity and good stability in testing biopreparates. The volt-amperecharacteristics measurements of the diamond film electrode were carried out forthree times, and they coincide perfectly, which shows that the surface of theelectrode almost doesn't change in the reaction process, and the surface activitywill not decrease with the time passed by.As has been mentioned above, on the basis of optimizing the preparationtechnics, using B(OCH3)3 solution as boron source, we prepared boron-dopeddiamond films by EACVD method. The boron concentration in the films werecontrolled between 1017～1021cm-3, and the highest boron-doping concentration is1.60×1021cm-3. The lowest resistivity of the boron doped diamond films is10-3?·cm, and heavily boron-doped diamond films exhibit superconductivity atlow temperature. Boron-doped diamond film electrodes possess a wideelectrochemistry window and a low background current close to zero, and theyhave high sensitivity and good stability in testing biopreparates. Our works lay thefoundation for the application and further research of the boron-doped diamondfilms in both the electrical and electrochemistry fields...