Diamond-based UV and soft X-ray photodetectors E. Pace Dip. di ...

Diamond-based UV and soft X-ray photodetectorsE. PaceDip. di Astronomia e Scienza dello Spazio, Università di Firenze,Largo E. Fermi 5, 50125 Firenze, ItalyandINFM, Sezione di Firenze, Largo E. Fermi 2, 50125 Firenze, Italy

ABSTRACTOwing to its physical properties, diamond is avery promising candidate for UV and soft X-ray photon detection. Polycrystalline diamondthick films can be grown by Chemical VaporDeposition (CVD) technique and metaldiamondstructures can be produced bydepositing electric contacts on the top and backsurface. There is a considerable interest ininvestigating diamond properties in the 30-1100 nm wavelength range in order to attainhigh performance detectors having highquantum efficiency and rejection ratio forvisible photons, besides radiation hardness andchemical inertness. Raman spectroscopy and X-ray diffraction are used to test the overallcrystal quality and orientation. The amount oforiented grains, i.e. the degree of texture, andtheir morphological quality can be thusestimated and such parameters are related to thephotoconductive performances. Photodetectorshaving ohmic and rectifying junctions and bothplanar and sandwich geometry are produced.The spectral region around the energy gap mustbe carefully investigated in order to evaluatethe selectivity to UV radiation and thecontribution of the impurities to conductivity.Furthermore, the quantum efficiency ismeasured in the 30-1100 nm wavelength range.This review paper describes the status of the artof diamond-based photon detectors, thetechniques employed to investigate themorphological properties and the electroopticalperformances of photoconductorsproduced in Italy and measured under UVillumination, in vacuum.INTRODUCTIONIn the last years, a considerable effort hasbeen devoted to the study and application ofnew materials for UV and X-ray photondetection. The main purpose is to solve sometechnological problems due to applications inhostile environments, or the complexity ofsensor cooling systems or the efficiency of theheat transport from the electronic devices. Sucharguments are very important for spacetechnology, but also in the scientific, industrial,telecommunication and military fields.The physical properties of diamond make ita very promising material for photon or particledetection, especially when applications in harshenvironments are required. In particular, itswide band-gap (5.5 eV) results in a very lowleakage current and in a selective absorption oflight with wavelengths below 225 nm. Inaddition, electronic properties and radiation

PROPERTIES OF DIAMONDDiamond crystal has a cubic structure,formed by one carbon atom surrounded by fourtetrahedrally (sp 3 σ-type bonds) placed nearestneighbors. This crystal structure is found alsoin silicon and germanium. The bond distance indiamond between two nearest neighbors is0.154 nm, compared with 0.234 nm for siliconand 0.245 nm for germanium. As a result, theatomic number density in diamond is 1.77 x10 23 cm -3 , the highest of any material at normalpressure, and its mass density is 3.52 g cm -3 .The cohesive energy of diamond, the energyrequired to disassemble a solid into itsconstituent parts, is almost twice as large asthat of silicon and germanium. This densestructure and strong bonding give diamondextreme hardness and wear resistance.Thermal conductivity of diamond isextremely high: at room temperature, it is fivetimes that of copper. It is able to dissipate itsown heat or the heat generated elsewhere andcan tolerate high temperature. It is one of therare materials that naturally conduct heat yetelectrically insulates. Electronically, diamond isan insulator, but can be a semiconductor bydoping it. Like silicon, diamond has an indirectband gap. The maximum of the valence bandlies at the center of the Brillouin zone in thereciprocal space; the lowest point of theconduction band is near the zone boundary inthe K = (111) direction, where the band gap, E g ,is defined. For diamond E g = 5.470 ± 0.005 eVat 295 K.Diamond has a large breakdown electricfield (about 10 7 Vcm -1 ) and its saturationvelocity is approximately 10 7 cm s -1 . Theelectron and hole mobility is 1800 cm 2 V -1 s -1and 1200 cm 2 V -1 s -1 respectively. Table 1 listssome of the important material parameters ofdiamond and compares them with the siliconand germanium. The typical operatingparameters for devices based on diamond,silicon, and germanium are also included inTable 1. An interesting property of the devicesconstructed in diamond is that they attain thesaturation properties of the material at a highelectric field.Pure diamond exhibits no photonabsorption or luminescence in the visiblespectral region, at λ > 225 nm. This property isvery important for UV visible-blindphotodetectors. On the other hand, photonswith λ < 225 nm are absorbed when theyimpinge on diamond. This process gives rise tothe electron excitation from the valence band tothe conduction band and diamond exhibitsphoto-induced electrical conductivity. It will bediscussed below in a following paragraph. Theabsorption spectra and the measured reflectance[3,15,16] are reported in Figs.1,2.Pure diamond can absorb optical radiationalso by the vibration of the atoms. Atoms in acrystal vibrate at characteristic frequencies ν v(phonons). If monochromatic light (usuallyfrom a laser) with frequency ν l interact withphonons in the crystal, Raman scattering canoccur. In this process photons with frequency(ν l -ν v ) (Stokes lines) and frequency (ν l +ν v )(anti-Stokes lines) are produced. The anti-Stokes line is much weaker than the Stokesline, and then the latter is normally observed.Raman spectroscopy is widely used toinvestigate the quality of CVD diamond. There,a sharp line at 1332 cm -1 reveals the presence ofdiamond phase. Graphite gives rise to a broaderpeak at 1580 cm -1 . The ratio of the intensities ofthese two peaks indicates how much of eachphase is present, but is also dependent on thewavelength of excitation (v. Fig.3). The widthof the diamond line reveals how much randomstress is present, and any directional stresspresent may give rise to a shift or splitting ofthe line. A very stringent test for high-qualitydiamond is to measure the Raman spectra withinfrared excitation (1.06 µm) rather than usingthe more conventional argon ion laser at 488nm [17].

PHOTO-INDUCED CHARGE GENERATIONIt is well known that an electromagneticwave incident on the surface of a material canbe partially reflected and partially transmitted.A simple relation can describe the intensity ofthe transmitted part, I tI t = I o (1 - R)where R is the reflectance of the material and I ois the normal incident intensity.The transmitted wave, propagating througha dielectric medium, causes transitions betweenatomic states, if its energy is as high as atomiclevel separations. As a result of photonabsorption, the intensity of the wave attenuatesas it propagates. The energy absorption leads toa decrease of the light intensity:I(x) = I t exp[-α(ν) x]α(ν) = K k(ν) / n 2where 1/α is referred to the absorption length; xis the depth from the medium surface; ν is thelight frequency; K = ν [(µε) / c] 1/2 ; µ and ε arethe permeability and permittivity of themedium; n and k(ν) are the optical index andthe extinction coefficient, respectively. Theabsorption length is a function of photonfrequency; for example, it is about 1-2 µm (seeFig.1) in diamond for photon energy of 6 eV.When a photon excitation occurs, excesselectron-hole pairs are produced. Theprobability for excitation of a carrier by anincident photon is usually called quantumefficiency, η. Taking into account the averageenergy to form a free charge, γ, the quantumefficiency at a given photon frequency can beexpressed in terms of the previous formulas asη = hν/γ (1 - R) [1 - exp(-α(ν) x)]Two basic processes cause the electron-holerecombination [18]. The intrinsicrecombination process occurs when electrons inthe conduction band make direct transitions tothe vacant states in the valence band. In theother process, electrons and holes recombinethrough intermediate states in the band gap.Those states, called recombination centers,exist because of structural defects or chemicalimpurities. This process is called the extrinsicrecombination process. Because theserecombination processes are independent, theeffective lifetime τ is given by1/τ = 1/τ ex + 1/τ inThe extrinsic lifetime τ ex in diamond isobserved to be of the order of 10 -8 -10 -10 s fromtransient photo-induced conductivitymeasurements. Therefore, the band-to-bandintrinsic lifetime, τ in , which ranges from 10 -6 -1s does not play an important role in therecombination process of excess carriers whenthe material contains a considerable amount ofdefects and impurities.After illuminating diamond with pulsed UVlight (λ ≤ 225 nm), the generated excesselectrons and holes move under the appliedelectric field E and induce a current pulse. Thecharge collected on electric contacts can beobtained integrating the current pulse, while thephotocurrent decay provides the carrier lifetime[19].CHEMICAL VAPOR DEPOSITION OFDIAMOND FILMSAs denoted in the introduction, CVDtechnology is used to grow diamond films. Thisapproach is based on creating a hot plasmaregion (>1500°C) near a substrate (typically at700-800°C). There, in the vapor phase, thethermal decomposition of carbon containinggases such as methane occurs.

the current saturates. The electric contact ondiamond surface must be ohmic. It is difficultto fabricate a good ohmic contact on diamonddue to its inert and wide band gap nature. It isnecessary to somehow modify the nature of aperfect diamond surface so that the voltagedrop across the contact is negligibly small ascompared to that across the active portion ofthe device. Choosing or intentionally creatingdamaged surfaces, good contacts can beaccomplished. This causes a high electric fieldand high recombination velocities between themetal and the diamond. This type of contacts,however, tend to be mechanically fragile andelectrically very noisy. Alternative ways toproceed are ion implantation of boron in thediamond region under the metal contact or theuse of carbide forming metals such as Ti, Mo,and Ta. In the first case, although the potentialbarrier height does not change, the barrierthickness is reduced and charge-carriers on thediamond side can easily communicate with thecontact by tunneling through the barrier. Theohmic behaviour of carbide forming metalcontacts after annealing has been attributed tothe formation of carbide at the interface. Theyare evaporated onto diamond and subsequentlyannealed at elevated temperatures (typically ~400°C) to enhance the carbide formation at themetal/diamond interface. The metals are oftencapped with a thin Au layer to avoid theiroxidation.The extremely high resistivity of intrinsicdiamond eliminates the need for reverse-biasedjunctions and the associated material doping tosuppress thermally generated currents. Inaddition, the large band gap produces anextremely reduced dark current and then verylow-level photo-generated currents can bedetected.The basic physics of charge generation andcollection in photoconductor can be illustratedby assuming a uniform device with mobilecharge carriers, electron and hole, generated byincident radiation [21,22]. Consider a flux F o (t)of photon at a given wavelength that is incidenton a photoconductive detector. Considering thequantum efficiency, η, as defined above, thecarrier generation rate is G(t) = η F o (t). Thenumber of charges, N, is related to thegeneration rate by the following:dN /dt = G(t) – N /τFor an impulse excitation and a carrierdensity N o at t = 0,N(t) = N o exp(– t /τ)while in the steady state, when the generationrate equals the recombination rate, isN = G τ = η F o τIf an external electric field E is applied theexcited carriers move. The two types of carrierscover an average drift distance, denoted ascollection distance, L, that is determined bytheir drift velocity v d and lifetime τ:L = v d τ = µ E τSince electrons and holes have differenteffective masses (m * e, m * h) and exist indifferent energy bands, the values of electronmobility (µ e ) and hole mobility (µ h ) may bedifferent. In addition, the electron lifetime (τ e )and hole lifetime (τ h ) may not be equal.Therefore, the collection distance L is equal towhereL = (µ e τ e + µ h τ h ) E = µ E τµ = (µ e τ e + µ h τ h ) / τand τ is the mobility weighed lifetime.Each one of these carriers drift under theelectric field influence at the mean velocity v d =µ(E)E giving rise to a current in the external

circuit of i = qv d / d, where d is the length of thematerial between the electrodes (Ramo-Shockley theorem [23]). The total current isthus the product of i and the number of carrierspresenti t = iN = qF o η v d τ /d = qF o η (τ /τ d ) = qF o η(L /d)where τ d = d /v d is the drift time for a carrieracross the length d, while L = vτ is the meandistance a charge moves in the detector. Thefactors (τ/τ d ) or (L /d) thus represent thefraction of the crystal length drifted by theaverage excited carrier before recombining.If the current is integrated over time, therelation between the photo-generated chargeQ ph and the collected charge Q c isQ c = Q ph (L /d) = Q ph µτE /dIf L /d >1, then all the charge Q ph will becollected and the factor (µτE /d) can beconsidered as a gain factor. If L /d 400°C) [24,25]. The diodesmanufactured on p-type CVD diamond sufferedfrom relatively large leakage currents. One wayto reduce this effect would be to place a thin,defect-free, insulating layer between the metalcontact and the diamond (MIS diode structure).In its intrinsic state, diamond is itself aninsulator, and can be easily inserted by CVDgrowth on top of the p-type-doped substrate. Athin layer of SiO 2 (about 2 nm) has alsosuccessfully been used to reduce leakage(metal-oxide-semiconductor structure, MOS),but the I(V) characteristics of this structuredegrades as the temperature increases [26,27].The MOS structure has an advantage over theSchottky and MIS structure, since it is possibleto drive the charge carriers to the accumulationmode. Thus it is expected that the MOSstructure will allow a higher degree of freedomin terms of operation of the Schottky contactbaseddiamond devices. On the other hand, theMIS structures have an advantage in terms ofdensity of interface states and then they allow alower leakage current.In general, the electrical properties ofSchottky contacts on diamond such as barrierheights have little dependence on the materialproperties of the metals such as work functionsor electronegativity. It has been observed thatthe preparation of diamond surface and thechoice of metal, its deposition parameters andsubsequent treatments, like annealing, stronglydetermine the I(V) characteristics of thecontacts. Usually Au and Al metal contacts ondoped or undoped diamond are used, but also

the diamond Bragg peaks (111), (220), (311)and (400) has been investigated, and thepercentage p hkl of the grains oriented along the(hkl) direction has been reported in Tab. Itogether with the CH 4 content in the gasmixture. The patterns show an increase and achange of the in-plane texture from a toa .Raman spectra from our films typicallyreveal an intense peak at 1332 cm -1 having aline-width of about 3-3.5 cm -1 and with noother significant features in the range 1100-1650 cm -1 around the diamond peak.Fig.6 shows a sketch of a typical devicestructure. Photoconductive detector prototypeswere built by thermally evaporating planarinterdigitated Ag, Au or Ti-Au contacts(thickness 0.15 micron) on a 1.3 x 2 mm 2 areaof the diamond-film growth surface andprocessing them by a standard lift-offphotolithographic technique. The spacing andthe width of these electric contacts are alsoreported in Table 1. A thermal annealingtreatment at 400°C in air has been also used toobtain ohmic electric contacts after havingdeposited metals as Ag or Ti-Au. In Fig.7 aSEM image of the interdigitated Ag contacts ona textured film is shown. The electrical padshave been bonded by ultrasonically soldering20 µm Pt wires. A back contact, having an areaof about 5 mm 2 , is also deposited on the siliconside. A planar and sandwich arrangement onthe same film allows the comparison betweentheir electrical performances and UV response.EXPERIMENTAL METHODSDifferent experimental setups are normallyemployed to measure the spectral response ofthe diamond-based photodetectors in the visiblepart of the wavelength range and in the UVrange. Two main reasons lead to these differentsetups. Firstly, optical tests in the deep UVrange (at wavelengths below 180-200 nm)require evacuated systems, vacuum-gradematerials and sources, whilst at longerwavelengths, measurements can be performedin air. In addition, diamond is quite transparentat wavelengths above 225 nm and then aspecial instrumentation, involving heterodynedetection techniques, is required to observevery low level signals, mainly due to impuritiesand defects in the crystals.The schematic of the apparatus used fordetector characterization in the deep UV rangeis shown in Fig.8. There, a deuterium lamp (forλ > 120 nm) or a gas discharge source (for λ

superimposed to a continuous spectrumcovering the range from the infrared to the UV(about 200 nm). A chopper placed between thesource and the entrance slit of themonochromator produce an alternatingmonochromatic signal at the exit slit. A lock-inamplifier can detect the alternating currentinduced by the chopped illumination, while anyD.C. signal, such as the dark current, isrejected. This experimental method allows avery high sensitivity, such that signals over 7-8decades can be recorded [4,6,32,33].ELECTRO-OPTICAL PERFORMANCESIn order to define the performances ofphotodetectors, it is very important todetermine their quantum efficiency, as well asthe dark current. The quantum efficiency isdirectly related to the sensitivity of the detector,while the dark current is related to its thermalnoise. Therefore, these quantities also give anestimation of the signal-to-noise ratio of thedevice under test.The dark current of our detectors ismeasured at room temperature in air and invacuum. It is very important to study the darkcurrent at room temperature, since one of themain aims of our research is to produce highlysensitive UV detectors, having the dark currentat room temperature as lowest as possible.Moreover, the dark current behaviour, whilevarying the bias voltage, gives information onthe nature of the metal-diamond junction. Thedata, plotted in Figs.9a-d, show that the darkcurrent is very low and that the Ag-annealedelectric contacts are ohmic, even if the appliedelectric field is high. On the other hand, nonannealedAu and Ti-Au contacts on diamondare rectifying, even at low electric field for Aucontacts. The I(V) curves of devices tested inair show non-linear and higher values than thesame devices measured in an evacuatedenvironment: this is probably due to the airconductivity. Also water vapor can affect themeasurement in air, since some layersdeposited between the planar electric contactcan be responsible for a higher dark current ifcompared to the sandwich geometry.The quantum efficiency gives informationabout device spectral responsivity, determinesthe gain of the photoconductors or theavalanche regime of the photodiodes, allowsthe evaluation of visible/UV response ratio andthen the possibility to understand if the deviceis really visible blind. In addition, photoinducedcharge yield in the visible and infraredrange is used to identify intra-gap energy levelsdue to impurity species inside diamond films ordefects [18].The spectral response can be measuredputting the sample under test at the exit slit of amonochromator and interchanging it with acalibrated photodiode. In this way, afterrecording the photocurrent generated by theilluminated diamond-based device, it ispossible to monitor the photon flux emergingfrom the exit slit, by measuring the currentproduced by the calibrated photodiode whenexposed to the same illumination.Therefore, the absolute quantum efficiencycan be estimated by the followingηrelationship between the two measuredphotocurrents:η= η ph I d /I pwhere ηp is the quantum efficiency of thecalibrated photodiode at a given wavelength, I pand I d are the photocurrents measured by thephotodiode and the diamond detector,respectively. Figs.10a,b report the absolutequantum efficiency measured at wavelengthsabove 180 nm for a hot filament CVD-grownsample [32] and the quantum efficiencymeasured for a natural single-crystal diamond[3].To put in evidence the different sensitivitiesof diamond samples which have been grown

using different processes and different gasmixtures, Figs.11a-d show the spectrum of adeuterium lamp in the 120-300 nm wavelengthregion as detected by three diamondphotodetectors having planar electric contacts.The same spectrum, recorded by an UVsensitive phosphor coated photomultiplier,corrected for the phosphor and photomultipliersensitivities, and normalized to the 160 nm line,has been reported for comparison. The twomain peaks of the line spectrum and thecontinuum in the 170-220 nm region can beobserved in the spectra recorded by diamonddetectors. As the excitation wavelength fallsbeyond the diamond band-gap energy, thephotocurrent is strongly reduced, due to theexpected lack of sensitivity. The large linewidthsand the very poor resolution are due tothe wide entrance and exit slits of themonochromator.The relative quantum efficiency of diamonddetectors with respect to the photomultiplier isalso shown in Fig.12. The enhanced responseof the device having Ti-Au contacts and grownusing the CH 4 -H 2 gas mixture can be observed.Analyzing the absorption edge at 225 nmreported in Fig.13 for several samples, it ispossible to observe that it is not really sharp asreported in literature for specially treated CVDdiamond and for natural diamond [32]. This isdue to the great concentration of crystal defectand impurities and also to the presence ofnitrogen. The latter is particularly pronouncedin the sample (i), where the spectral response inthe region around 300 nm shows a small peakdue to a high concentration of nitrogen atoms inthe diamond crystals.The diamond film quality and itsmorphology can influence the photoresponse[34]. To demonstrate this correlation, the set ofdiamond samples, having the CH 4 contentvarying in the gas mixture from 47% to 53%,was used. The UV response of devicesproduced with these films was measured,without any spectral resolution.The UV photocurrent curves relative toboth planar and sandwich electrodes, measuredin air and in vacuum, are reported in Fig.14a-d.In all cases a correlation between thephotoresponse and the CH 4 -induced degree oftexture can be observed. A higher CH 4concentration leads to a higher crystalorientation, and the growth conditions leadingto the lowest preferential crystal orientationfavour the UV photoresponse. In order toremark such a correlation, the UV photocurrentversus the CH 4 concentration has been reportedin Fig.15. This plot shows the sharp decrease ofthe photoresponse when the CH 4 concentrationincreases. The correlation is observed in thecase of planar contacts as well as of sandwichcontacts, as expected. In addition, the dataplotted in Figs.14a-d show no evidence of acorrelation between the photocurrent itself andthe film thickness. As far as the dark current isconcerned, there is no evidence of a correlationbetween its values and the film texture.The main open problem for UV-sensitivediamond detectors is that under UVillumination the current increases several orderof magnitudes. A slow slope (see Fig.16)characterizes this increasing current and itsmagnitude depends on the applied electric field.After illumination a phenomenon of persistentphotoconductivity is observed [3,6,35]. Thispersistence consists of a decreasing currenthaving a much slower slope than the increasingpart. This effect is detrimental to UVphotodetector applications. A simpleexplanation of this phenomenon is that UVillumination creates electron-hole pairsinvolving also the intra-gap states in theexcitation and de-excitation processes. Afterillumination, electrons excited in theconduction band can be trapped by intermediatestates and holes are free-charge carriers in thevalence band. There are some evidences forfaster responses when Schottky junctions areemployed as photoconductors [36], or when

post-fabricated gas treated devices are used in alow-gain mode [33].Long-time response effect is not presentwhen pulsed-light is used to excite electrons.Very fast responses have been obtained byseveral diamond photoconductors detectinglaser pulses at wavelengths of 255, 532 and1064 nm [37]. Detectors were also fabricatedexhibiting response times of less than 70 psFWHM at 125 nm [38].CONCLUSIONSDiamond-based photodetector propertieshave been reviewed in this article. The electroopticalproperties and the structures normallyused have been discussed. This type ofdetectors is a promising candidate for UVphoton detection at room temperature, offeringenhanced efficiency and a real solar blindness.Some problems remain open and work is inprogress to solve them. The main problem is togrow very pure diamond crystals and to controlthe vapor phase to avoid contamination of thegrowing film. Moreover grain boundaries andnitrogen inclusion cause detrimentalperformances, due to the high density of intragaptrapping states. Therefore technicalimprovements in the growing process or in theexperimental methods should provide enhancedUV response and solar blindness.Actually, a big effort is done to improve thegain and the time response of diamonddetectors. In particular, the time response is toolow to allow an employment of diamonddevices as UV photoconductor. Recently, someexperiments have obtained preliminary resultsshowing that faster devices can be produced,but they are still slow with respect to silicondevices.On the other hand, very good photondetectors for faster laser pulses can be producedusing diamond as sensitive material. It hasprovided interesting results also in the visibleregion, where only extrinsic conductivity isinvolved.Once these problems will find a solution, afurther development can be foreseen in thearrays of MOS diamond-based photodiodes.Actually, such structures have been producedfor particle detection and are employed in theexperiments at CERN. Diamond-based UVimager may have many applications, such asmedicine, astronomy from space or chemicalenvironments.REFERENCES1. L. S. Pan, D. R. Kania, P. Pianetta and O. L.Landen, Appl. Phys. Lett., 57 (1990) 623.2. D.R. Kania, M.I. Landstrass, M.A. Plano,L.S. Pan, S. Han, Diam. Rel. Mat., 2 (1993)1012.3. S.C. Binari, M. Marckywka, D.A.Koolbeck, H.B. Dietrich, D. Moses, Diam.Rel. Mat., 2 (1993) 1020.4. R.D. Mckeag, R.D. Marshall, B. Baral,S.S.M. Chan, R.B. Jackman, Diam. Rel.Mat., 6 (1997) 374.5. S. Salvatori, R. Vincenzoni, M. C. Rossi, F.Galluzzi, F. Pinzari, E. Cappelli, P.Ascarelli, Diam. Rel. Mat. 6 (1996) 775.6. S. Salvatori, E. Pace, M.C. Rossi, F.Galluzzi, Diam. Rel. Mat., 6 (1997) 361.7. E. Pace, F. Galluzzi, M.C. Rossi, S.Salvatori, M. Marinelli, P. Paroli, Nucl.Instr. Meth. A, 387 (1997) 255.8. F. Galluzzi, M. Marinelli, L. Masotti, E.Milani, E. Pace, M.C. Rossi, S. Salvatori,M. Santoro, S. Sciortino, G. Verona Rinati,Nucl. Instrum. Meth. A, 409, 1-3 (1998)423.9. L.S. Pan, D.R. Kania, S. Han, J.W. Ager,M.I. Landstrass, O.L. Landen, P. Pianetta,Science, 255 (1992) 830.10. L. Allers, A.T. Collins, J. Appl. Phys., 77(1995) 3879.

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40. Courtesy from G. Messina, University ofReggio Calabria, Reggio Calabria (Italy).TABLE 1. Deposition parameters of the analyzed diamond films. p 100 , p 110 , p 111 and p 311 are thecalculated percentages of each XRD peak related to the integral of all the peaks except the siliconpeak, renormalized to a diamond powder spectrum (see ref. [39]).Sample Mixture Thickness(µm)p 111 p 110 p 311 p 100A 52.7 % CH 4 in CO 2 25 0.07 0.03 0.07 0.82B 52.0 % CH 4 in CO 2 30 0.05 0.20 0.09 0.66C 49.2 % CH 4 in CO 2 50 0.13 0.63 0.16 0.08D 48.5 % CH 4 in CO 2 33 0.22 0.59 0.13

Figure captionsFigure 1 Absorption spectrum of CVDdiamond in the deep UV up in energies to thesoft X-ray region (data from ref. [2]).Figure 2 Reflectance of a single crystal naturaldiamond (data from ref. [16]).Figure 3 Micro-Raman spectrum of a nontexturedCVD diamond film obtainedilluminating a single grain at wavelength 514.5nm. The inset shows the linewidth of thediamond line (ref.[40]).Figure 4 Photocurrent versus the electric fieldfor a diamond-based photoconductor. The samebehaviour has been reported for two differentwavengths. At high electric field thephotocurrent saturates, since photogeneratedcharge is fully collected.Figure 5 X-Ray Diffraction patterns for 4different CVD diamond films, described in theTable 1. Table 1 explains also the meaning ofthe parameter p.Figure 6 Skecth of a typical diamondphotoconductor.Figure 7 SEM image of interdigitated Agcontacts on a textured diamond film. Thedistance between two metal strips is 12.5 µm.Figure 8 Schematic of the experimentalapparatus used for measurements in the vacuumUV (λ < 200 nm).Figure 9 Dark currents measured for diamondfilms having Ag ohmic contact (a,b), Au (c)and Ti-Au (d) rectifying contacts. Dark currentsin (a) and (b) are reported for planar andsandwich devices and they can be compared tothe same measurements performed in air.Figure 10 Quantum efficiency for a singlecrystal natural diamond (a) and a CVD-grownpolycrystalline diamond film (b) (data fromrefs. [3,32])Figure 11 Spectrum of a D 2 lamp in the 120-300 nm wavelength region (b,c,d) as detectedby three diamond photodetectors having planarelectric contacts. The same spectrum (a),recorded by an UV sensitive phosphor coatedphotomultiplier, has been reported forcomparison.Figure 12 Relative quantum efficiency of threediamond detectors with respect to thephotomultiplier. Quantum efficiency curve fora sandwich diamond device has been reportedfor comparison.Figure 13 Normalized responsivity of somediamond-based detectors: a microwave plasmaCVD-grown sample (i), a hot filament CVDgrownfilm (ii) and another microwave plasmaCVD-grown sample but nitrogen doped (iii).The curve (iv) is relative to sample (ii) after along UV exposure (data from ref. [32]).Figure 14 Comparison among photocurrentsproduced by several microwave plasma CVDgrownfilms. The preferential orientation hasbeen indicated to show the correlation betweenthe photocurrent and the film texture both inplanar and sandwich configurations.Figure 15 Plot showing the correlationbetween the photocurrent and the preferentialorientation of crystals in the diamond films(parameter p).Figure 16 Temporal behaviour ofphotocurrent.

Figure 7 SEM image of interdigitated Ag contacts on a textured diamond film. Thedistance between two metal strips is 12.5 µm.

Mean free path (cm)1010.10.011E-31E-41E-51E-61E-7Wavelength (nm)1000 100 10 1 0.11 10 100 1000 10000Energy (eV)Figure 1 Absorption spectrum of CVD diamond in the deep UV up in energies tothe soft X-ray region (data from ref. [2]).

0.7Wavelength (nm)200 150 1000.6Reflectance0.50.40.30.26 8 10 12 14 16 18 20Energy (eV)Figure 2 Reflectance of a single crystal natural diamond (data from ref. [16]).

Si (400)A(111)(400)(220)(311)Intensity (arb. units)(111)(111)Si (400)Si (400)(220)(220)(311)(311)BC(400)(400)(111)(220)Si (400)(311)D(400)40 60 80 100 1202ΘFigure 5 X-Ray Diffraction patterns for 4 different CVD diamond films, describedin the Table 1. Table 1 explains also the meaning of the parameter p.

5040Ti-Au planarPhotocurrent (pA)3020100160 nm120 nm-2 0 2 4 6 8 10 12 14 16Electric field (V/cm)Figure 4 Photocurrent versus the electric field for a diamond-based photoconductor.The same behaviour has been reported for two different wavelengths. At highelectric field the photocurrent saturates, since photogenerated charge is fullycollected.

Photocurrent (nA)10 E = 2.5x10 4 V/cm1transverseco-planar0.6 0.7 0.8P(ikl)Figure 15 Plot showing the correlation between the photocurrent and thepreferential orientation of crystals in the diamond films (parameter p).

Concave gratingUV SourceVacuummonochromatorDiamondsampleFigure 8 Schematic of the experimental apparatus used for measurements in the vacuum UV(λ < 200 nm).

1.035Normalized Intensity (u .a .)0.80.60.40.20.0PM T +TPBgain 10 7Photocurrent (pA )302520151050T i-A u planarE = 3 x 10 4 V/cm-0 .2100 150 200 250 300W avelength (nm )-5100 150 200 250 300W avelength (n m )P hotocurrent (nA )3.02.52.01.51.00.5Ti-A u planarE = 5 x 10 3 V/cmPhotocurrent (pA )80604020Ag planarE =8.3x10 4 V/cm80 100 120 140 160 180 200 220 240 260W avelength (nm )0100 150 200 250 300W a v e le n g th (n m )Figure 11 Spectrum of a D 2 lamp in the 120-300 nm wavelength region (b,c,d) as detected by threediamond photodetectors having planar electric contacts. The same spectrum (a), recorded by an UVsensitive phosphor coated photomultiplier, has been reported for comparison.

Photocurrent (nA)6005004003002001000Ti-Au planar30 V160 nm0 20 40 60 80Time (min.)Figure 16 Temporal behaviour of photocurrent.

Planar electrodesDiamondSiliconBack -contactFigure 6 Sketch of a typical diamond photoconductor.

10000 E=5 x 10 3Relative efficiency (u.a.)1000100101Ti-Au planar (a)Ti-Au sandwich (a)Ti-Au planar (b)Ag planar100 150 200 250 300Wavelength (nm)Figure 12 Relative quantum efficiency of three diamond detectors withrespect to the photomultiplier. Quantum efficiency curve for a sandwichdiamond device has been reported for comparison.

UV photocurrent (nA)6040200-20-40(100)(110)(111)randomplanarairUV photocurrent (nA)4003002001000-100-200-300(111)(110)(111)randomSandwichAir-60-4 -2 0 2 4E (10 4 V/cm)-400-4 -3 -2 -1 0 1 2 3 4E (10 5 V/cm)Photocurrent (nA)806040200-20-40-60-80(111)(110)randomsandwichvacuum-100-10 -5 0 5 10E (10 4 V/cm)Photocurrent (nA)150100500-50-100(100)(110)randomplanarvacuum-150-10 -8 -6 -4 -2 0 2 4 6 8 10E (10 4 V/cm)Figure 14 Comparison among photocurrents produced by several microwave plasma CVDgrownfilms. The preferential orientation has been indicated to show the correlation betweenthe photocurrent and the film texture both in planar and sandwich configurations.

Current (pA)100500-50(111)(110)randomrandom in air(x 0.05)planar-100-10 -8 -6 -4 -2 0 2 4 6 8 10E (10 4 V/cm)C u rren t (p A )150100500-5 0-1 0 0-1 5 0(100)(110)ran d o mra n d o m in air(x 0 .4 )san d w ich-1 0 -5 0 5 1 0E (10 4 V/cm)200100500400Current (pA)0-1 0 0C u rren t (p A )300200100-2 0 00-3 0 0-4 0 -2 0 0 2 0V oltage (V )-100-6 0 -5 0 -4 0 -3 0 -2 0 -1 0 0 1 0 2 0 3 0V oltage (V )Figure 9 Dark currents measured for diamond films having Ag ohmic contact (a,b), Au (c) andTi-Au (d) rectifying contacts. Dark currents in (a) and (b) are reported for planar and sandwichdevices and they can be compared to the same measurements performed in air.

Figure 10 Quantum efficiency for a single crystal natural diamond (a) and a CVDgrownpolycrystalline diamond film (b) (data from refs. [3,32])

Figure 13 Normalized responsivity of some diamond-based detectors: a microwaveplasma CVD-grown sample (i), a hot filament CVD-grown film (ii) and anothermicrowave plasma CVD-grown sample but nitrogen doped (iii). The curve (iv) isrelative to sample (ii) after a long UV exposure (data from ref. [32]).

Figure 3 Micro-Raman spectrum of a non-textured CVD diamond film obtained illuminating asingle grain at wavelength 514.5 nm. The inset shows the linewidth of the diamond line (ref.[40]).