See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/236609528 PLD deposition of tungsten carbide contact for diamond photodiodes. Influence of process conditions on electronic and chemical aspects ARTICLE in APPLIED SURFACE SCIENCE · AUGUST 2013 Impact Factor: 2.71 · DOI: 10.1016/j.apsusc.2013.02.016 READS 59 6 AUTHORS, INCLUDING: Emilia. Cappelli Italian National Research Council 68 PUBLICATIONS 573 CITATIONS SEE PROFILE Daniele Maria Trucchi Italian National Research Council 57 PUBLICATIONS 255 CITATIONS SEE PROFILE Alessio Mezzi Italian National Research Council 104 PUBLICATIONS 643 CITATIONS SEE PROFILE Veronica Valentini Italian National Research Council 28 PUBLICATIONS 147 CITATIONS SEE PROFILE Available from: Daniele Maria Trucchi Retrieved on: 04 February 2016
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LD deposition of tungsten carbide contact for diamond photodiodes.nfluence of process conditions on electronic and chemical aspects
. Cappelli a,∗, A. Belluccia, S. Orlandob, D.M. Trucchia, A. Mezzi c, V. Valentinia
CNR-IMIP, Montelibretti, via Salaria Km 29.3, P.O.B. 10, 00016 Rome, ItalyCNR-IMIP sez. Potenza, 85050 Tito Scalo, Potenza, ItalyCNR-ISMN, Montelibretti, via Salaria Km 29.3, P.O.B. 10, 00016 Rome, Italy
r t i c l e i n f o
rticle history:vailable online 10 February 2013
eywords:C films
ulsed laser depositionPS and Raman characterization
–V dosimetric responsechottky contacts on diamond
a b s t r a c t
Tungsten carbide, WC, contacts behave as very reliable Schottky contacts for opto-electronic diamonddevices. Diamond is characterized by superior properties in high-power, high frequency and high-temperature applications, provided that thermally stable electrode contacts will be realized. Ohmiccontacts can be easily achieved by using carbide-forming metals, while is difficult to get stable Schottkycontacts at elevated temperatures, due to the interface reaction and/or inter-diffusion between metalsand diamond. Novel type of contacts, made of tungsten carbide, WC, seem to be the best solution, for theirexcellent thermal stability, high melting point, oxidation and radiation resistance and good electrical con-ductivity. Our research was aimed at using pulsed laser deposition for WC thin film deposition, optimizingexperimental parameters, to obtain a final device characterized by excellent electronic properties, as adetector for radiation in deep UV or as X-ray dosimeter. We deposited our films by laser ablation from atarget of pure WC, using different reaction conditions (i.e., substrate heating, vacuum or reactive atmo-
sphere (CH4/Ar), RF plasma activated), to optimize both the stoichiometry of the film and its structure.Trying to obtain a material with the best electronic response, we used also two sources of laser radiationfor target ablation, i.e., nano-second pulsed excimer laser ArF, and ultra-short fs Ti:Sapphire laser. Thestructure and chemical aspects have been evaluated by Raman and X-ray photoelectron spectroscopy(XPS), while the dosimeter photodiode response has been tested by the I–V measurements, under softX-ray irradiation.
. Introduction
Tungsten carbide (WC) has many potential applications bothechanical and electronic, due to its unique physical and chemical
roperties (like high hardness, wear resistance, high temperaturetability, good resistance to oxidation and radiation), making it use-ul for hard coating, cutting, erosion-resistant applications, but alsos thermally stable blocking contact (Schottky contact) or diffusionarrier layers in integrated circuits [1–3].
Due to the attractive features of diamond material, relatedevices have attracted significant interest for high power and highrequency applications. The Schottky contact is an essential partf these devices, acting as a metal gate to control the current [4].ntil recently, a number of metals such as Al, Au, Cr, Ni, Pt, Zn and W
ave been used as Schottky contacts to oxidized p-type diamonds5,6]. However, the reliability of these contacts is rather question-ble at elevated temperatures, mainly due to interface reactions
and inter-diffusion between metals and diamond. In order to real-ize the materials for high-temperature applications, high-qualitySchottky contacts stable at high temperature, without deteriorat-ing the performance of the device, are required. Refractory metalsand their carbides, such as tungsten, and tungsten carbide, how-ever, have recently shown better characteristic for Schottky contactapplications mainly due to their excellent chemical stability andhigh electrical conductivity [1,7].
WC thin films are usually produced by reactive magnetron sput-tering, mainly for electronic applications [7,8,9]; however, thinfilms with various stoichiometric compositions of tungsten carbide,for different applications, have been also deposited by pulsed laserablation (PLD), performed by short pulses, since 1998 [10–12] and,more recently, also by ultra-short pulsed lasers [13].
In our work we evaluated the possibility of using the PLD methodfor electronic applications, in particular, to grow rectifying contacts,tungsten/carbon based, on substrates of free-standing polycrys-
talline diamond.
We proceeded exploring and comparing the respective capa-bilities of two laser sources, short (ns UV pulsed excimer ArF,� = 193 nm) and ultra-short (fs Ti:Sapphire (� = 785 nm) to grow
ood quality tungsten carbide thin films. We tried to optimize theelevant experimental parameters governing the growth process,pecifically, laser energy and fluence, substrate temperature, and,or plasma reactive PLD, the Ar/CH4 pressure, RF plasma configura-ion and energy.
The performance of deposited film, working as “Schottky con-act”, has been evaluated on a diamond photodiode.
. Experimental
.1. Pulsed laser deposition
Carbon-based thin films were prepared by pulsed laser depo-ition (PLD) technique. We used a nanosecond pulsed ArF
xcimer laser (Lambda Physik COMPex 102), 10 Hz and energy50 mJ/pulse, with a corresponding fluence of ∼2.7 J/cm2 on the
arget and a femtosecond Ti:Sapphire, � = 785 nm, and energy3.5 mJ/pulse (∼20.0 J/cm2). The laser beam was focused at an angle
able 2PS atomic percentage from peak deconvolution.
ote: The peak assignment was made according to: J.F. Moulder, W.F. Stickle, P.E. Sobopectroscopy, Physical Electr. Inc., Eden Prairie, MN, USA, 1995.
37.0/3.7 530 60– Tamb 7– Tamb 5
of 45◦ on a WC target (99.999% purity). During the deposition,the target was rotated to ensure uniform erosion over the targetsurface. In order to keep under control the stoichiometry of thedeposited film and to prevent the dissociation of tungsten carbide,in the reaction chamber has also been introduced a reactive atmo-sphere of pure methane or a methane/argon plasma, activated byan RF system (see Table 1). Alumina, technical grade, small plateswere used as substrates, during optimization of prep-procedures;they were ultrasonically cleaned in n-hexane and then mounted ona heated holder, at a distance of ∼5 cm. Free-standing single crystaldiamond substrate (Element Six Ltd., British Isles) has been usedfor the preparation of the photodiode to be tested. Before startingdeposition the chamber was evacuated to ∼5.0 × 10−7 mbar.
2.2. SEM and Raman characterization
Surface morphology of deposited films has been characterizedby a field emission scanning electron microscopy apparatus (FE-SEM, Zeiss Supra 40).
l, K.D. Bomben, in: J. Chastain, R.C. King (Eds.), Handbook of X-ray Photoelectron
E. Cappelli et al. / Applied Surface Science 278 (2013) 111–116 113
Fa
edlOed
2
esspsrr0aAtttTL[
2
(Xctctfi[
3
b
Fig. 2. XPS spectra of sample (WC0) deposited in vacuum by ns ArF laser ablation:(a) W 4f peak: lower spectrum (a) as received upper and (b) depth profile. Peak com-ponents: (1) W 4f7/2 – WC or W0; (1′) W 4f5/2; (2) W 4f7/2 – WO2; and (3) W 4f7/2 –
ig. 1. SEM micrographs of thin film, grown by fs laser (WCFs2), showing the irregularnd porous granular structure.
Raman spectra were recorded in 1808 – backscattered geom-try (on samples oriented perpendicularly to the excitation beamirection (z) with a DILOR XY-triple spectrometer, equipped with a
iquid nitrogen cooled CCD-multichannel detector and an adaptedlympus microscope in a confocal mode. The deposited films werexcited with an Ar + laser (514.5 nm), focused to a spot of 2 mm iniameter.
.3. XPS characterization
XPS experiments were carried out by an ESCALAB Mk II (VG Sci-ntific Ltd., U.K.) spectrometer, equipped with non monochromatictandard Al K� excitation source and a 5-channeltron detectionystem. Photoelectron spectra were collected at 20 eV constantass energy of the analyser and a base pressure in the analy-is chamber of 10−8 Pa. For the depth profiling, an Ar+ beam,astered over an area of 3 mm × 3 mm, set to 2.0 keV, sample cur-ent of 3 × 10−3 mA/cm2, and an average sputtering rate of about.2 nm/min. XPS depth profiling was carried out at 90◦ take-offngle. The binding energy (BE) scale was calibrated by measuringu 4f spectrum from sputter-cleaned Au of 99.99% foil and set-
ing Au 4f7/2 peak at BE = 84.0 eV. The samples were mounted onhe holder by using an Au mask, which ensured the grounding ofhe samples. The accuracy of experimental BE scale was ±0.1 eV.he curve fitting is carried out with a mixture of Gaussian andorentzian functions. The peak assignment was made according to14].
.4. Electronic measurements
Tungsten carbide has been deposited as a blocking contactSchottky contact) on a free-standing diamond film, working as-ray dosimeter, the Ohm contact being a Ti/Ag bi-layer. Theurrent–voltage (I–V) characteristics were measured in a condi-ion of continuous and modulated X-ray irradiation. A completeharacterization was performed analyzing the dependence of pho-ocurrent from bias voltage variation, radiation dose-rate andrequency. More specific details on the preparation of device andts characterization are given in a paper submitted to IEEE Letters15].
. Results and discussion
As the obtainment of carbide films, specifically tungsten car-ide, via laser ablation is quite complex, since often the interaction
WO3. (b) C1s peak: lower spectrum (a) as received upper and (b) depth profile. Peakcomponents: (1) 285 eV C C bond; (2) 286.5 eV; C O (3) 288.7 eV C O or RCOOH;(4) 290.0 eV CO3– and (5) 283.0 eV C (carbide).
of laser radiation can easily give rise to a decomposition and dis-proportionation of carbide itself to metal, with carbon release, anda subsequent re-deposition in the form of amorphous material, weconsidered appropriate to study and optimize the ablation from theWC target on a substrate of convenience, cheap and easy to obtain,as a technical grade alumina. We then exported the process, at opti-mized conditions, to the substrate of interest, that, being a singlecrystal of diamond, very expensive and complex to handle, can-not be used for testing optimization. Alumina, however, is quiteporous and with uneven surface, for which some morphologicalaspects of deposited layers, in particular the roughness of the film,are strongly influenced by the substrate itself. Analysing by SEMthe films obtained, under vacuum, by direct sputtering from thepure WC target, both by ns ArF laser and fs Ti:Sapphire (samplesWC0, WCFs1, WCFs2) (see Table 1), the morphology is characterizedby a granular and porous nano-structured features, with macro-scopic dimensions of the particles, varying in the range 20–150 nm,(Fig. 1). This inhomogeneity and porosity of the film turns out tobe also cause of a quite high level of oxidation, detected by XPS(Fig. 2a,b, and Fig. 5a,b) (Table 2), despite the films are depositedalways in high vacuum.
Taking steps from results of our previous work, concerningnano-second laser treatment of ceramics, specifically, sintered SiCand WC/Co [16,17], where it has been observed a clear tendency todecomposition of carbides with depletion of carbon, we decided to
114 E. Cappelli et al. / Applied Surface Science 278 (2013) 111–116
Fig. 3. XPS spectra of sample (WC1) deposited in CH4 flux, by ns ArF laser ablation:(a) W 4f peak: lower spectrum (a) as received upper and (b) depth profile. Peak com-ponents: (1) W 4f7/2 – WC or W0; (1′) W 4f5/2; (2) W 4f7/2 – WO2; and (3) W 4f7/2
– WO3. (b) C1s peak: lower spectrum (a) as received upper (b) depth profile. Peakc(
iol
tmiaWtcctpcsiosn
erw
Fig. 4. Raman spectra of samples deposited on alumina substrate at experimentalcondition reported in Table 1. The spectra are showing the appearance of G peak(∼1580 cm−1) in sample WC1 and also of D peak (at ∼1350 cm−1) in sample WC2RF,with the introduction of a C source (CH4 flow) in the deposition chamber (a). Theconstant presence of D and G peaks, typical finger-prints of nano-structured amor-phous carbon, when using a CH4/Ar plasma atmosphere (samples WC2RF, WC3RF and
omponents: (1) 285 eV C C bond; (2) 286.5 eV C O 3) 288.7 eV C O or RCOOH;
4) 290.0 eV CO3 and (5) 283.0 eV C (carbide).
ntroduce a source of carbon, in the form of CH4, as pure flowing gasr in a mixture with Ar, activated with RF plasma, to invert or, ateast, to limit the reaction of disproportion/elimination of carbon.
This concept appeared to give good results, especially as regardshe surface morphology of the films, which are smoother. Under a
ore detailed analysis, using both Raman spectroscopy and XPS, its clear that the introduction of methane has enough positive effect,lbeit carbide formation is small, compared to ordinary ablation ofC in vacuum (see Fig. 2a,b versus Fig. 3a,b), while, on the con-
rary, the presence of the RF activated CH4/Ar plasma appears toause the formation, on the surface, of amorphous nano-structuredarbon. The Raman spectra, reported in Fig. 4a and b, indeed showhe appearance and gradual increase of the well-resolved D and Geaks, typical of the formation of nano-structures of amorphousarbon [18,19]. The atomic percentage of carbon (by XPS) for theseamples is very high, more than 85% (Table 2), the majority of whichs constituted by aliphatic bonds of C–C type, and to a lesser amountf oxidized carbon, thus confirming the Raman outputs that theurface of these samples consists almost exclusively of amorphousano-structured carbon.
Examining these initial and introductory results obtained, it was
vident that a systematic study should be carried on, both on theole of methane flux and of RF plasma activation. A reactive ablationith the fs laser should also be considered, since the process of
WC4RF) in the deposition chamber (b).
decomposition of ablated material is much less likely, given thehigh kinetic energy of the process.
The most promising results, for the deposition of carbide, in fact,have proved to be those obtained with the processes carried outwith the fs laser ablation (Table 2).
Samples grown using the ultra-short laser source gave the bestresults, as regards the formation of carbide, especially evident inthe XPS spectra, after a depth profile of the film, which shows aconsistent presence of W and C in the form of carbide (see Fig. 5aand b).
Using the experimental conditions as referred for sample WCFs2,a layer of WC-based material has been deposited, to work as arectifying contact, on a free-standing single crystal diamond film,electronic grade, while TiC/Ti/Ag, deposited by PLD, works as Ohmcontact on the other side.
It is crucial at this stage, to keep in mind that the presence of asubstrate, with completely flat surface and consisting of an infinite
source of pure carbon, strongly favours the reaction of formation oftungsten carbide, minimizing also the effects of any inhomogeneityat the interface.
E. Cappelli et al. / Applied Surface S
Fig. 5. XPS spectra of sample (WCFs2) deposited in vacuum by fs Ti:Sapphire laserablation: (a) W 4f peak: lower spectrum (a) as received upper and (b) depth profile.Peak components: (1) W 4f7/2 – WC or W0; (1′) W 4f5/2; (2) W 4f7/2 – WO2; and (3)W 4f7/2 – WO3. (b) C1s peak: lower spectrum (a) as received upper and (b) depthprofile. Peak components: (1) 285 eV C C bond; (2) 286.5 eV C O; (3) 288.7 eV C Oor RCOOH; (4) 290.0 eV CO3 and (5) 283.0 eV C (carbide).
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
-80 -60 -40 -20 0 20 40 60 80
@8.6x10-1
Gy/h
@2.5x10-1
Gy/h
@8.02x10-2
Gy/h
@5.04x10-1 Gy/h
@1.63x10-1
Gy/h
I ph_dc (A
)
Bias Voltage (V)
Fig. 6. Log-linear plot for the reverse region of Iph dc(Vbias) characteristics under DCX-ray irradiation, at maximum and minimum DR (dose-rate) (shown in the inset).
cience 278 (2013) 111–116 115
The characterization of the detector was initially performedunder DC irradiation by varying dose-rate DR from 0.016 to0.860 Gy/h (minimum and maximum values of the Cu tube (X-raysource) operating stability), and bias voltage, Vbias, in the range±75 V. DC photocurrent-to-voltage characteristics under irradia-tion show (see Fig. 6) the typical behaviour of a Schottky photodiode[1,6,20], characterized by a 3 orders of magnitude rectification ratiofor |Vbias| < 2.5 V.
Positive Vbias identifies the reverse operative region, while neg-ative refers to the forward one. DC signal Iph dc(A) is defined as thedifference between the total current under irradiation and the darkcurrent.
The analysis of the dynamic response of the device has con-firmed a good realization process, since the response follows veryquickly the variations of the modulation signal, under photovoltaic(at 0 V) and low bias voltage operations (±75 V range). The devicepresents also interesting dosimetric performances, which havebeen deeply investigated and reported with detailed and specificinformation in a paper submitted to IEEE letters [15].
4. Conclusion
The fs laser ablation method appears to be promising for thedeposition of thin layers of tungsten carbide, even if the structureappears granular, especially if compared with material achiev-able by thermal CVD. The ultra-short laser source, showed bettercapacity than the short, especially for a better maintenance of thestoichiometry of the film. The presence of CH4 flux in the reactionchamber prevents partially the trend to decomposition of WC, butthis part needs insights and a more detailed study.
The introduction of a RF plasma of a gaseous mixture CH4/Ardid not give the desired and expected results, namely, promotethe chemical equilibrium towards WC deposition, but rather hascaused the formation of nano-structured carbon, giving rise to asort of composite material: amorphous nano-structured C at thesurface and tungsten inside, when using technical grade Alumina,as a pre-survey sacrificial substrate.
The films obtained with ultra-short laser source have provedto be of better characteristics and composition, although partiallyoxidized, since the phase change mechanisms, including high non-equilibrium thermal process, specifically melting and vaporization[21], does not allow the system sufficient time to decompose intocomponents.
The fs deposited WC contact on single crystal diamond showedgood and promising rectifying properties, exploitable both in appli-cations like high frequency photodiodes, owing to the optimumdynamic response to AC irradiation, but also for dosimeter appli-cation, encouraging further research in the field.
Acknowledgements
S. Orlando and E.Cappelli would like to thank very much Dr.A. Lettino, IMAA- CNR, Tito Scalo (PZ), for the study, by FE-SEMmicroscopy, of samples deposited by femtosecond laser. Manythanks also to G. Piciacchia for the considerable technical help inthe ArF excimer laser and Raman operation management.
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