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

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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.
Page 1 and 2: Diamond-based UV and soft X-ray pho
Page 4 and 5: PROPERTIES OF DIAMONDDiamond crysta
Page 7 and 8: the current saturates. The electric
Page 10 and 11: the diamond Bragg peaks (111), (220
Page 14 and 15: 11. C. Wild, P. Koidl, W. Muller-Se
Page 16 and 17: Figure captionsFigure 1 Absorption
Page 18 and 19: Mean free path (cm)1010.10.011E-31E
Page 20 and 21: Si (400)A(111)(400)(220)(311)Intens
Page 22 and 23: Photocurrent (nA)10 E = 2.5x10 4 V/
Page 24 and 25: 1.035Normalized Intensity (u .a .)0
Page 26 and 27: Planar electrodesDiamondSiliconBack
Page 28: UV photocurrent (nA)6040200-20-40(1
Page 31 and 32: Figure 10 Quantum efficiency for a
Page 33: Figure 3 Micro-Raman spectrum of a

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