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Very high UV-visible selectivity in polycrystalline CVD diamond films
Diamond and Related Materials 13 (2004) 881–885
Very high UV-visible selectivity in polycrystalline CVD diamond films J. Alvareza,*, A. Godarda, J.P. Kleidera, P. Bergonzob, D. Tromsonb, E. Sniderob, C. Merb, E. Rzepkac, H. Cheverryc a
´ ´ ´ Universites ´ Paris VI et Paris XI, Laboratoire de Genie Electrique de Paris (UMR 8507 CNRS), Ecole Superieure d’Electricite, Plateau de Moulon, F-91192 Gif-sur-Yvette Cedex, France b LIST (CEA-Recherche Technologique)yDIMRIySIAR, CEAySaclay F-91191 Gif sur Yvette, France c ´ ` (UMR 8635 CNRS), Universite´ de Versailles Saint-Quentin-en-Yvelines, Laboratoire de Physique des Solides et de la Cristallogenese 1 place Aristide Briand, F-92195 Meudon Cedex, France
Abstract Polycrystalline CVD diamond is a very attractive material for the fabrication of UV detectors. However, good quality material is required to obtain a high UV photoresponse together with a low visible sensitivity. In this work, we present and correlate the results of Raman spectroscopy, spectral photoresponse and thermally stimulated current (TSC) achieved on polycrystalline CVD diamond films. All the layers show the typical Raman diamond line with very small FWHM values, a broad TSC peak in the 480–530 K range, and have a high UV-visible selectivity (defined here as the ratio of the photosensitivity at 200 and 657 nm). This selectivity is found to increase when the FWHM of the Raman line decreases. In the same time, the mobility-lifetime product increases, and we observe both a shift in the TSC peak from 530 to 480 K and a decrease in the integrated trapped charge. For the best sample, the FWHM of the Raman line is 1.88 cmy1, and the values of the mobility-lifetime product and of the UVvisible selectivity are as high as 1.5=104 cm2 Vy1 , and 2=107 , respectively, while the trap density estimated from the TSC experiment is in the range of 1017 cmy3. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Polycrystalline CVD diamond; UV detector; High selectivity
1. Introduction Spectroscopic studies in the UV region as those realized in the astrophysics fields w1,2x require powerful detectors, which can operate in hostile environments (high temperature and radiation hardness). The extreme properties of diamond and the recent improvements in the chemical vapour deposition (CVD) make polycrystalline CVD diamond a promising material for the fabrication of UV photodetectors w3–8x. In particular, its wide band gap (5.5 eV) ensures good transparency and weak photosensitivity at wavelengths in the visible range (solar blindness) and a high sensitivity in the UV range (l-225 nm). However, the CVD diamond performance is limited by the polycrystalline structure (particularly the grain boundaries) and impurities that induce defect states in the band gap, thus affecting the photoelectric properties and altering the detection char*Corresponding author. Tel.: q33-1-69851642; fax: q33-169418318. E-mail address: [email protected] (J. Alvarez).
acteristics w9–13x. Of particular importance are the defects related to deep levels which after exposure to UVyX-ray photons or ionising particles enhance the conductivity and the sub-gap photoconductivity w14– 18x. These effects can also lead to lower UV-visible selectivity values. In the present work, we examine CVD diamond layers in terms of spectral photoresponse measurements that are correlated to Raman and thermally stimulated current (TSC) spectroscopies w19x. 2. Experimental details Diamond films were synthesized using microwave plasma enhanced chemical vapour deposition (MWPECVD) on 2-inch silicon (100) wafers using a hydrogen–methane gas mixture. A post-deposition chemical treatment was used to reduce the surface related leakage current w20x. Three polycrystalline CVD diamond films denoted CVD1, CVD2 and CVD3 with thicknesses of 130 mm, 260 mm and 50 mm, respective-
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.12.007
J. Alvarez et al. / Diamond and Related Materials 13 (2004) 881–885
882 Table 1 Diamond deposition conditions
Substrate Microwave Pressure CH4 H2 flow temperature (8C) Power (kW) (Torr) (%) rate (sccm) CVD1 850 CVD2 800 CVD3 850
3 5–5.5 1.5
95 1 90–105 2.5 69 0.5
400 1000 200
ly, were analysed. Details on the deposition conditions are given in Table 1. The grains are randomly oriented and the grain size is estimated by a Veeco threedimensional optical profiler to approximately 10–50 mm, 20–80 mm and 5–10 mm, respectively. The samples were fitted with top evaporated coplanar gold electrodes (f100-mm thick) on the unpolished polycrystalline side. The electrodes geometry consists of interdigitated fingers with 200 mm spacing, and a total electrode length of 2.1 cm. The Raman parameters of the samples were evaluated with a Jobin-Yvon (Horiba group) confocal microRaman HR800, using the red line at 632.8 nm of an He–Ne laser with a power of 10 mW for the spot light on the crystal surface. The spatial resolution (xy resolution) was of the order of 1 mm with the =100 objective. By using the grating with 1800 stymm and a confocal hole of 50 mm, the wave number resolution was 0.52 cmy1. The signalynoise ratio was very satisfactory with a counting time of 0.5 s during the mapping of the grains. At that time, the set-up was installed in a non air-conditioned room and a drift of the order of 1 cmy1 could occur along the day from each day morning calibration. Raman parameters (peak position and shape, FWHM) was fitted by using LabSpec4, a commercial software delivered with the machine. With this Raman spectrometer, the average value for the peak position of a great number of synthetic HPHT diamond substrates delivered by Sumitomo䉸 or de Beers䉸 and also from a lot of good quality natural ‘pure’ diamond was 1331.3"0.4 cmy1. The FWHM were currently ranging from 1.7 cmy1 for ‘high’ grade sample to 2.3 cmy1 for ‘commercial’ grade concerning the check diamonds we have in our possession in other respects. The measurements of spectral photosensitivity have been performed at room temperature in air in the 200– 657-nm wavelength range. For this purpose, we used a 1 kW Xenon lamp and a set of interference filters with a FWHM at approximately 10 nm. The size of the beam covered the whole area between the sample electrodes. The photon flux impinging the sample was analysed using a Hamamatsu silicon photodiode with an active area much smaller than the area between electrodes in our diamond samples. We found that the photon flux impinging the sample was quite homogeneous with values ranging from 1013 to 1015 cmy2 sy1, depending on the wavelength. The photocurrent was measured at
the electric fields between 500 and 1000 Vycm, where a linear behaviour between current and voltage was observed. TSC measurements were performed in a vacuum chamber (10y6 mbar) on a cooling–heating stage allowing to vary the temperature from 290 to 800 K. For all TSC experiments, the samples were illuminated in vacuum (through a Silica window) for the duration of 10 min at room temperature with a deuterium lamp (30 W) and then heated with a constant rate. The electrical measurements were carried out using a HP 4140B as both a picoammeter and a dc voltage source. 3. Results The Raman spectra are reported in Fig. 1. The three polycrystalline CVD diamond films present the line related to the diamond and no other component due for instance to graphitic phases was observed. The FWHM values are between 1.88 and 2.57 cmy1, as reported in Table 2. Let us underline that these values are among the narrowest found in diamond films deposited onto non-diamond substrates and they are comparable to that of high purity natural diamond w21,22x. They evidence that there are no crystallographic defects with a size of the order of the laser wavelength, or larger, in the volume light up by the laser spot. However, the grain sizes were large as regards to the spot size. This result is also in agreement with the fact that there were no graphitic phases in the grain boundaries, which usually enlarged the FWHM of the diamond line. It is now very well established that the highest crystallographic quality of diamond is narrowest in the Raman scattering line. The spectral photoresponse of these diamond films is shown in Fig. 2. In order to make the results independent of the electric field and sample geometry, we present the values of RsDIy(qFjl), where DI is the photocur-
Fig. 1. Raman spectra of the three polycrystalline CVD diamond films.
J. Alvarez et al. / Diamond and Related Materials 13 (2004) 881–885
883
Table 2 Structural and electronic properties
CVD1 CVD2 CVD3
FWHM (cmy1)
mt (cm2 Vy1)
UV-visible selectivity
Peak Tmax (K)
Ns (cmy2)
Nt (cmy3)
1.88 2.10 2.57
1.5=10y4 2=10y6 8=10y7
2=107 6=104 6=103
490 480 520
5=1014 1=1017 4=1017
1=1017 4=1019 1=1020
density. We define the UV-visible selectivity s by the ratios of R at 200 and 657 nm. We observe that the values of s are equal to 2=107, 6=104 and 6=103, for samples CVD1, CVD2, and CVD3, respectively (see Table 2). The TSC experiments reveal one high temperature (480–530 K) broad peak. As expected, the peak shifts to higher temperature with increasing heating rate, as can be seen for example in Fig. 3 for the CVD3 sample. For comparable heating rates, the TSC peak was found to depend on the sample, as illustrated in Fig. 4. 4. Discussion Fig. 2. Spectral photoresponses of the three samples.
rent, q the electron charge, F the photon flux, j the electric field and l the length of the electrodes. If the impinging light is fully absorbed (at)1, a being the absorption coefficient and t the sample thickness), this ratio actually corresponds to the mobility-lifetime product mt. This is likely to be the case at the shortest wavelength, owing to the expected values of a in diamond, of the order of 102 –103 cmy1 for ls200 nm w16x. The mobility-lifetime products are reported in Table 2. For at<1, which occurs for the long wavelengths, R corresponds to the product of m, a wavelength dependent lifetime, a and t. It thus contains information on the subgap absorption, which depends on the defect
Fig. 3. TSC curves of the CVD3 sample for different heating rates: 7, 15, 45 Kymin.
There is a clear correlation between the structural and electronic properties of the samples, as can be seen in Table 2. Indeed, the increase from 6=103 to 2=107 in the UV-visible selectivity is correlated to a decrease of the FWHM of the characteristic diamond Raman line from 2.57 to 1.88. Let us insist here that both the very high UV-visible selectivity and the very narrow Raman peak observed in sample CVD1 are among the best ever reported for a polycrystalline CVD sample grown onto a non-diamond substrate. Also, the value of the mobilitylifetime product of 1.5=10y4 cm2 Vy1 found for CVD1 is very high for this kind of sample. We note that high values of mobility-lifetime products and UV-visible selectivities above 106 were previously obtained by other authors following post-annealing treatments under specific gas conditions w3x. Comparable or even higher values are obtained here without any post-annealing
Fig. 4. TSC curves of the three samples. The insert shows the experimental deconvolution of the TSC peak of sample CVD2.
J. Alvarez et al. / Diamond and Related Materials 13 (2004) 881–885
884 Table 3 Energy depth of the TSC peaks
CVD1 CVD2 CVD3
Peak Tm (K)
E (eV) Initial rise
Chen (first order)
Variation heating rate
490 480 510 520
0.94 0.97 1.04 0.86
1.1 1.12 0.95 1.23
0.98 – – 1.51
treatment, and they can be related to a very high crystallographic quality of the sample, as suggested by the Raman data. The high temperature TSC peak is also correlated to the previous parameters. Indeed, we observe that it significantly shifts to higher temperatures for CVD3, as can be seen in Fig. 4. Such a shift with the quality of the material was already reported in Ref. w11x. The analysis of this peak in terms of trap energy depth is non-trivial. We analysed the curves using the ‘initial rise method’ w23x, Chen’s method w24x and the various heating rate w25,26x and of course the thermal cleanings were used to isolate the peaks before applying the previous methods. The first method assumes that at low temperatures, in a temperature range near to the initial temperature of trapped charge carriers, the density of trapped carriers remains practically constant and the TSC current varies as an exponential law, ITSC(T)Aexp(yEykT), where the energy depth, E, can be evaluated from the slope in the Arrhenius plot. From the Chen’s method, the energy is deduced from the position and the asymmetry of the peak. Finally, for a ‘first-order kinetics’ approximation (retrapping carriers being negligible) we can calculate the energy level E and the attempt-to-escape frequency n from different values of the heating rate b and peak temperature Tm according to: w bE E z snexp y x |, kT2m kTm ~ y
(1)
where k is the Boltzmann constant. The results of the three methods are summarised in Table 3. For all samples, n was found of the order of 108 sy1, and the energy depth was close to 1 eV. There is, however, a spreading in the values obtained from the different methods. This is probably due to the fact that the TSC peak is a superposition of at least two peaks. This is evident for sample CVD2, as seen in the insert of Fig. 4, and it was analysed by several authors, w9,27,28x but this investigation is beyond the scope of this work. It is more interesting here to estimate the total emitted charge from the integral of the TSC peak. We checked that the intensity of the TSC was already saturated for 10 min illumination time. Since we are in
the coplanar configuration with Ohmic contacts, one has to take account of the photoconductivity gain w29x, which can be estimated from the photoconductivity measurements at ls200 nm. We can thus obtain values of surface charge densities Ns, which are finally converted into a density of trapped charges Nt assuming as300 cmy1. Results are given in Table 2. They clearly show that the UV-visible selectivity and the mobilitylifetime products are correlated with the density of deep traps inferred from the TSC measurements. In particular, the value of Nt deduced in sample CVD1 is low compared to values reported in the literature w28x, which further confirms the quality of this sample. 5. Conclusions Polycrystalline CVD diamond samples have been analysed from Raman spectroscopy, spectral photoresponse and TSC experiments. Raman spectroscopy shows the typical Raman diamond line with narrow values of the FWHM and no signature of graphitic phase, indicating a high-structural quality. Spectral photoresponses indicate high mobility-lifetime products and high UV-visible selectivities. We find a close correlation between the FWHM and the electronic and photoresponse properties. The best sample with a FWHM of 1.88 cmy1 has outstanding values of the mobilitylifetime product (1.5=10y4 cm2 Vy1) and UV-visible selectivity (2=107). We also find that the high TSC peak is shifted to lower values for the best samples. From the emitted integrated charge, the density of deep traps was estimated to 1017 cmy3. Our results show that it is possible to grow large area polycrystalline CVD diamond onto non-diamond substrates with excellent structural and electronic properties. References w1x J.F. Hochedez, P. Bergonzo, M.C. Castex, P. Dhez, O. Hainaut, et al., Diamond Relat. Mater. 10 (2001) 673–680. w2x J.F. Hochedez, J. Alvarez, F.D. Fauret, P. Bergonzo, et al., Diamond Relat. Mater. 11 (2002) 427–432. w3x O. Gaudin, S. Watson, S.P. Lansley, H.J. Looi, M.D. Whitfield, R.B. Jackman, Diamond Relat. Mater. 8 (1998) 886–891. w4x R.D. McKeag, R.B. Jackman, Diamond Relat. Mater. 7 (1998) 513–518. w5x S.P. Lansley, O. Gaudin, M.D. Whitfield, R.D. McKeag, N. Rizvi, R.B. Jackman, Diamond Relat. Mater. 9 (2000) 195–200. w6x S. Salvatori, M. C. Rossi, F. Galluzzi, E. Pace, P. Ascarelli, M. Marinelli, Diamond Relat. Mater. 7 (1998) 811–816. w7x S. Salvatori, M.C. Rossi, F. Galluzzi, D. Riedel, M. C. Castex, Diamond Relat. Mater. 8 (1999) 871–876. w8x S. Salvatori, M. C. Rossi, F. Scotti, G. Conte, F. Galluzzi, V. Ralchenko, Diamond Relat. Mater. 9 (2000) 982–986. w9x D. Tromson, P. Bergonzo, A. Brambilla, C. Mer, F. Foulon, V.N. Amosov, Phys. Stat. Sol. A 174 (1999) 155. w10x D. Tromson, P. Bergonzo, A. Brambilla, C. Mer, F. Foulon, J. Appl. Phys. 90 (2001) 1608.
J. Alvarez et al. / Diamond and Related Materials 13 (2004) 881–885 w11x E. Gheeraert, A. Deneuville, P. Gonon, M. Benabdesselam, P. Iacconi, Phys. Stat. Sol. A 172 (1999) 183. w12x M. Benabdesselam, P. Iacconi, D. Briand, A. Berkane-Krachaı, ¨ J. Appl. Phys. 88 (2000) 4648. w13x E. Borchi, M. Bruzzi, L. Lombardi, D. Menichelli, S. Miglio, S. Pirollo, et al., Mat. Res. Soc. Symp. Proc. 588 (2000) 277. w14x R. Vaitkus, T. Inushima, S. Yamazaki, Appl. Phys. Lett. 62 (1993) 2384. w15x P. Gonon, S. Prawer, D. Jamieson, Appl. Phys. Lett. 68 (1995) 1238. w16x C.F.O. Graeff, E. Rohrer, C.E. Nebel, M. Stutzmann, H. Guttler, ¨ R. Zachai, Appl. Phys. Lett. 69 (1996) 3215. w17x T. Behnke, A. Oh, A. Wagner, W. Zeuner, A. Bluhm, C.P. Klages, et al., Diamond Relat. Mater. 7 (1998) 1553. w18x J. Alvarez, J.P. Kleider, P. Bergonzo, C. Mer, D. Tromson, A. Deneuville, et al., Diamond Relat. Mater. 11 (2002) 635–639.
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w19x Thermally Stimulated Relaxation in Solids, in: P. Braunlich ¨ (Ed.), Topics in Applied Physics, 37, Springer-Verlag, Berlin, Heidelberg, New York, 1979. w20x C. Jany, F. Foulon, P. Bergonzo, R.D. Marshall, Diamond Relat. Mater. 7 (1988) 951. w21x A. Mainwood, Phys. Stat. Sol. A 25 (1999) 172. w22x C. Jany, A. Tardieu, A. Gicquel, P. Bergonzo, F. Foulon, Diamond Relat. Mater. 9 (2000) 1086. w23x G.F. Garlick, A.F. Gibson, Proc. Phys. Soc. 60 (1948) 574–590. w24x R. Chen, J. Appl. Phys. 49 (1969) 570–585. w25x W. Hoogenstraaten, Philips Res. Repts. 13 (1958) 515. w26x J.G. Simmons, G.W. Taylor, Phys. Rev. B 4 (1972) 1619. w27x M. Bruzzi, D. Menichelli, S. Pirollo, S. Sciortino, Diamond Relat. Mater. 9 (2000) 1081. w28x M. Bruzzi, D. Menichelli, S. Sciortino, L. Lombardi, J. Appl. Phys. 91 (2002) 5765. w29x C. Manfredotti, R. Murri, A. Quirini, L. Vasanelli, Phys. Stat. Sol. 38 (1976) 685.
Very high UV-visible selectivity in polycrystalline CVD diamond films J. Alvareza,*, A. Godarda, J.P. Kleidera, P. Bergonzob, D. Tromsonb, E. Sniderob, C. Merb, E. Rzepkac, H. Cheverryc a
´ ´ ´ Universites ´ Paris VI et Paris XI, Laboratoire de Genie Electrique de Paris (UMR 8507 CNRS), Ecole Superieure d’Electricite, Plateau de Moulon, F-91192 Gif-sur-Yvette Cedex, France b LIST (CEA-Recherche Technologique)yDIMRIySIAR, CEAySaclay F-91191 Gif sur Yvette, France c ´ ` (UMR 8635 CNRS), Universite´ de Versailles Saint-Quentin-en-Yvelines, Laboratoire de Physique des Solides et de la Cristallogenese 1 place Aristide Briand, F-92195 Meudon Cedex, France
Abstract Polycrystalline CVD diamond is a very attractive material for the fabrication of UV detectors. However, good quality material is required to obtain a high UV photoresponse together with a low visible sensitivity. In this work, we present and correlate the results of Raman spectroscopy, spectral photoresponse and thermally stimulated current (TSC) achieved on polycrystalline CVD diamond films. All the layers show the typical Raman diamond line with very small FWHM values, a broad TSC peak in the 480–530 K range, and have a high UV-visible selectivity (defined here as the ratio of the photosensitivity at 200 and 657 nm). This selectivity is found to increase when the FWHM of the Raman line decreases. In the same time, the mobility-lifetime product increases, and we observe both a shift in the TSC peak from 530 to 480 K and a decrease in the integrated trapped charge. For the best sample, the FWHM of the Raman line is 1.88 cmy1, and the values of the mobility-lifetime product and of the UVvisible selectivity are as high as 1.5=104 cm2 Vy1 , and 2=107 , respectively, while the trap density estimated from the TSC experiment is in the range of 1017 cmy3. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Polycrystalline CVD diamond; UV detector; High selectivity
1. Introduction Spectroscopic studies in the UV region as those realized in the astrophysics fields w1,2x require powerful detectors, which can operate in hostile environments (high temperature and radiation hardness). The extreme properties of diamond and the recent improvements in the chemical vapour deposition (CVD) make polycrystalline CVD diamond a promising material for the fabrication of UV photodetectors w3–8x. In particular, its wide band gap (5.5 eV) ensures good transparency and weak photosensitivity at wavelengths in the visible range (solar blindness) and a high sensitivity in the UV range (l-225 nm). However, the CVD diamond performance is limited by the polycrystalline structure (particularly the grain boundaries) and impurities that induce defect states in the band gap, thus affecting the photoelectric properties and altering the detection char*Corresponding author. Tel.: q33-1-69851642; fax: q33-169418318. E-mail address: [email protected] (J. Alvarez).
acteristics w9–13x. Of particular importance are the defects related to deep levels which after exposure to UVyX-ray photons or ionising particles enhance the conductivity and the sub-gap photoconductivity w14– 18x. These effects can also lead to lower UV-visible selectivity values. In the present work, we examine CVD diamond layers in terms of spectral photoresponse measurements that are correlated to Raman and thermally stimulated current (TSC) spectroscopies w19x. 2. Experimental details Diamond films were synthesized using microwave plasma enhanced chemical vapour deposition (MWPECVD) on 2-inch silicon (100) wafers using a hydrogen–methane gas mixture. A post-deposition chemical treatment was used to reduce the surface related leakage current w20x. Three polycrystalline CVD diamond films denoted CVD1, CVD2 and CVD3 with thicknesses of 130 mm, 260 mm and 50 mm, respective-
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.12.007
J. Alvarez et al. / Diamond and Related Materials 13 (2004) 881–885
882 Table 1 Diamond deposition conditions
Substrate Microwave Pressure CH4 H2 flow temperature (8C) Power (kW) (Torr) (%) rate (sccm) CVD1 850 CVD2 800 CVD3 850
3 5–5.5 1.5
95 1 90–105 2.5 69 0.5
400 1000 200
ly, were analysed. Details on the deposition conditions are given in Table 1. The grains are randomly oriented and the grain size is estimated by a Veeco threedimensional optical profiler to approximately 10–50 mm, 20–80 mm and 5–10 mm, respectively. The samples were fitted with top evaporated coplanar gold electrodes (f100-mm thick) on the unpolished polycrystalline side. The electrodes geometry consists of interdigitated fingers with 200 mm spacing, and a total electrode length of 2.1 cm. The Raman parameters of the samples were evaluated with a Jobin-Yvon (Horiba group) confocal microRaman HR800, using the red line at 632.8 nm of an He–Ne laser with a power of 10 mW for the spot light on the crystal surface. The spatial resolution (xy resolution) was of the order of 1 mm with the =100 objective. By using the grating with 1800 stymm and a confocal hole of 50 mm, the wave number resolution was 0.52 cmy1. The signalynoise ratio was very satisfactory with a counting time of 0.5 s during the mapping of the grains. At that time, the set-up was installed in a non air-conditioned room and a drift of the order of 1 cmy1 could occur along the day from each day morning calibration. Raman parameters (peak position and shape, FWHM) was fitted by using LabSpec4, a commercial software delivered with the machine. With this Raman spectrometer, the average value for the peak position of a great number of synthetic HPHT diamond substrates delivered by Sumitomo䉸 or de Beers䉸 and also from a lot of good quality natural ‘pure’ diamond was 1331.3"0.4 cmy1. The FWHM were currently ranging from 1.7 cmy1 for ‘high’ grade sample to 2.3 cmy1 for ‘commercial’ grade concerning the check diamonds we have in our possession in other respects. The measurements of spectral photosensitivity have been performed at room temperature in air in the 200– 657-nm wavelength range. For this purpose, we used a 1 kW Xenon lamp and a set of interference filters with a FWHM at approximately 10 nm. The size of the beam covered the whole area between the sample electrodes. The photon flux impinging the sample was analysed using a Hamamatsu silicon photodiode with an active area much smaller than the area between electrodes in our diamond samples. We found that the photon flux impinging the sample was quite homogeneous with values ranging from 1013 to 1015 cmy2 sy1, depending on the wavelength. The photocurrent was measured at
the electric fields between 500 and 1000 Vycm, where a linear behaviour between current and voltage was observed. TSC measurements were performed in a vacuum chamber (10y6 mbar) on a cooling–heating stage allowing to vary the temperature from 290 to 800 K. For all TSC experiments, the samples were illuminated in vacuum (through a Silica window) for the duration of 10 min at room temperature with a deuterium lamp (30 W) and then heated with a constant rate. The electrical measurements were carried out using a HP 4140B as both a picoammeter and a dc voltage source. 3. Results The Raman spectra are reported in Fig. 1. The three polycrystalline CVD diamond films present the line related to the diamond and no other component due for instance to graphitic phases was observed. The FWHM values are between 1.88 and 2.57 cmy1, as reported in Table 2. Let us underline that these values are among the narrowest found in diamond films deposited onto non-diamond substrates and they are comparable to that of high purity natural diamond w21,22x. They evidence that there are no crystallographic defects with a size of the order of the laser wavelength, or larger, in the volume light up by the laser spot. However, the grain sizes were large as regards to the spot size. This result is also in agreement with the fact that there were no graphitic phases in the grain boundaries, which usually enlarged the FWHM of the diamond line. It is now very well established that the highest crystallographic quality of diamond is narrowest in the Raman scattering line. The spectral photoresponse of these diamond films is shown in Fig. 2. In order to make the results independent of the electric field and sample geometry, we present the values of RsDIy(qFjl), where DI is the photocur-
Fig. 1. Raman spectra of the three polycrystalline CVD diamond films.
J. Alvarez et al. / Diamond and Related Materials 13 (2004) 881–885
883
Table 2 Structural and electronic properties
CVD1 CVD2 CVD3
FWHM (cmy1)
mt (cm2 Vy1)
UV-visible selectivity
Peak Tmax (K)
Ns (cmy2)
Nt (cmy3)
1.88 2.10 2.57
1.5=10y4 2=10y6 8=10y7
2=107 6=104 6=103
490 480 520
5=1014 1=1017 4=1017
1=1017 4=1019 1=1020
density. We define the UV-visible selectivity s by the ratios of R at 200 and 657 nm. We observe that the values of s are equal to 2=107, 6=104 and 6=103, for samples CVD1, CVD2, and CVD3, respectively (see Table 2). The TSC experiments reveal one high temperature (480–530 K) broad peak. As expected, the peak shifts to higher temperature with increasing heating rate, as can be seen for example in Fig. 3 for the CVD3 sample. For comparable heating rates, the TSC peak was found to depend on the sample, as illustrated in Fig. 4. 4. Discussion Fig. 2. Spectral photoresponses of the three samples.
rent, q the electron charge, F the photon flux, j the electric field and l the length of the electrodes. If the impinging light is fully absorbed (at)1, a being the absorption coefficient and t the sample thickness), this ratio actually corresponds to the mobility-lifetime product mt. This is likely to be the case at the shortest wavelength, owing to the expected values of a in diamond, of the order of 102 –103 cmy1 for ls200 nm w16x. The mobility-lifetime products are reported in Table 2. For at<1, which occurs for the long wavelengths, R corresponds to the product of m, a wavelength dependent lifetime, a and t. It thus contains information on the subgap absorption, which depends on the defect
Fig. 3. TSC curves of the CVD3 sample for different heating rates: 7, 15, 45 Kymin.
There is a clear correlation between the structural and electronic properties of the samples, as can be seen in Table 2. Indeed, the increase from 6=103 to 2=107 in the UV-visible selectivity is correlated to a decrease of the FWHM of the characteristic diamond Raman line from 2.57 to 1.88. Let us insist here that both the very high UV-visible selectivity and the very narrow Raman peak observed in sample CVD1 are among the best ever reported for a polycrystalline CVD sample grown onto a non-diamond substrate. Also, the value of the mobilitylifetime product of 1.5=10y4 cm2 Vy1 found for CVD1 is very high for this kind of sample. We note that high values of mobility-lifetime products and UV-visible selectivities above 106 were previously obtained by other authors following post-annealing treatments under specific gas conditions w3x. Comparable or even higher values are obtained here without any post-annealing
Fig. 4. TSC curves of the three samples. The insert shows the experimental deconvolution of the TSC peak of sample CVD2.
J. Alvarez et al. / Diamond and Related Materials 13 (2004) 881–885
884 Table 3 Energy depth of the TSC peaks
CVD1 CVD2 CVD3
Peak Tm (K)
E (eV) Initial rise
Chen (first order)
Variation heating rate
490 480 510 520
0.94 0.97 1.04 0.86
1.1 1.12 0.95 1.23
0.98 – – 1.51
treatment, and they can be related to a very high crystallographic quality of the sample, as suggested by the Raman data. The high temperature TSC peak is also correlated to the previous parameters. Indeed, we observe that it significantly shifts to higher temperatures for CVD3, as can be seen in Fig. 4. Such a shift with the quality of the material was already reported in Ref. w11x. The analysis of this peak in terms of trap energy depth is non-trivial. We analysed the curves using the ‘initial rise method’ w23x, Chen’s method w24x and the various heating rate w25,26x and of course the thermal cleanings were used to isolate the peaks before applying the previous methods. The first method assumes that at low temperatures, in a temperature range near to the initial temperature of trapped charge carriers, the density of trapped carriers remains practically constant and the TSC current varies as an exponential law, ITSC(T)Aexp(yEykT), where the energy depth, E, can be evaluated from the slope in the Arrhenius plot. From the Chen’s method, the energy is deduced from the position and the asymmetry of the peak. Finally, for a ‘first-order kinetics’ approximation (retrapping carriers being negligible) we can calculate the energy level E and the attempt-to-escape frequency n from different values of the heating rate b and peak temperature Tm according to: w bE E z snexp y x |, kT2m kTm ~ y
(1)
where k is the Boltzmann constant. The results of the three methods are summarised in Table 3. For all samples, n was found of the order of 108 sy1, and the energy depth was close to 1 eV. There is, however, a spreading in the values obtained from the different methods. This is probably due to the fact that the TSC peak is a superposition of at least two peaks. This is evident for sample CVD2, as seen in the insert of Fig. 4, and it was analysed by several authors, w9,27,28x but this investigation is beyond the scope of this work. It is more interesting here to estimate the total emitted charge from the integral of the TSC peak. We checked that the intensity of the TSC was already saturated for 10 min illumination time. Since we are in
the coplanar configuration with Ohmic contacts, one has to take account of the photoconductivity gain w29x, which can be estimated from the photoconductivity measurements at ls200 nm. We can thus obtain values of surface charge densities Ns, which are finally converted into a density of trapped charges Nt assuming as300 cmy1. Results are given in Table 2. They clearly show that the UV-visible selectivity and the mobilitylifetime products are correlated with the density of deep traps inferred from the TSC measurements. In particular, the value of Nt deduced in sample CVD1 is low compared to values reported in the literature w28x, which further confirms the quality of this sample. 5. Conclusions Polycrystalline CVD diamond samples have been analysed from Raman spectroscopy, spectral photoresponse and TSC experiments. Raman spectroscopy shows the typical Raman diamond line with narrow values of the FWHM and no signature of graphitic phase, indicating a high-structural quality. Spectral photoresponses indicate high mobility-lifetime products and high UV-visible selectivities. We find a close correlation between the FWHM and the electronic and photoresponse properties. The best sample with a FWHM of 1.88 cmy1 has outstanding values of the mobilitylifetime product (1.5=10y4 cm2 Vy1) and UV-visible selectivity (2=107). We also find that the high TSC peak is shifted to lower values for the best samples. From the emitted integrated charge, the density of deep traps was estimated to 1017 cmy3. Our results show that it is possible to grow large area polycrystalline CVD diamond onto non-diamond substrates with excellent structural and electronic properties. References w1x J.F. Hochedez, P. Bergonzo, M.C. Castex, P. Dhez, O. Hainaut, et al., Diamond Relat. Mater. 10 (2001) 673–680. w2x J.F. Hochedez, J. Alvarez, F.D. Fauret, P. Bergonzo, et al., Diamond Relat. Mater. 11 (2002) 427–432. w3x O. Gaudin, S. Watson, S.P. Lansley, H.J. Looi, M.D. Whitfield, R.B. Jackman, Diamond Relat. Mater. 8 (1998) 886–891. w4x R.D. McKeag, R.B. Jackman, Diamond Relat. Mater. 7 (1998) 513–518. w5x S.P. Lansley, O. Gaudin, M.D. Whitfield, R.D. McKeag, N. Rizvi, R.B. Jackman, Diamond Relat. Mater. 9 (2000) 195–200. w6x S. Salvatori, M. C. Rossi, F. Galluzzi, E. Pace, P. Ascarelli, M. Marinelli, Diamond Relat. Mater. 7 (1998) 811–816. w7x S. Salvatori, M.C. Rossi, F. Galluzzi, D. Riedel, M. C. Castex, Diamond Relat. Mater. 8 (1999) 871–876. w8x S. Salvatori, M. C. Rossi, F. Scotti, G. Conte, F. Galluzzi, V. Ralchenko, Diamond Relat. Mater. 9 (2000) 982–986. w9x D. Tromson, P. Bergonzo, A. Brambilla, C. Mer, F. Foulon, V.N. Amosov, Phys. Stat. Sol. A 174 (1999) 155. w10x D. Tromson, P. Bergonzo, A. Brambilla, C. Mer, F. Foulon, J. Appl. Phys. 90 (2001) 1608.
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