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

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