Mater. Res. Soc. Symp. Proc. Vol. 1039 © 2008 Materials Research Society 1039-P10-03 Single Crystal CVD Diamond Growth for Detection Device Fabrication Nicolas Tranchant, Dominique Tromson, Philippe Bergonzo, and Milos Nesladek CEA\LIST Saclay, DRT\LIST\SSTM\LTD, Bat 451, Pce 74, Gif Sur Yvette, 91191, France, Metropolitan Abstract Single crystal (SC) CVD diamond is known to exhibit superior electronic properties compared to polycrystalline diamond for detection applications. In our study, samples were grown using the Chemical Vapor Deposition (CVD) technique in an ASTEX AX 5400 reactor, at various microwave powers and keeping all other parameters constant. The crystalline quality and purity of samples were investigated using Raman spectroscopy and cathodo-luminescence measurements. The diamonds layers were chemically cleaned and oxidized, prior to their fabrication as ionization chambers using Ni and Au contacts for rectifying properties. Device electronic and detection properties were then characterized: Leakage currents were probed from I(V) measurements and contact properties were tested with 60Co source at various dose rates. Time of Flight (TOF) and Charge Collection Efficiency (CCE) measurements were evaluated with 241Am alpha particles irradiation, which enabled the measurements of mobility, carrier diffusion length and lifetime as a function of growth parameters. These measurements demonstrate the importance of growth condition optimization on the detection quality of these samples. 1. Introduction Due to superior electronic properties of diamond such as high carrier mobilities (higher than 1000 cm2/Vs) which are required for detection applications in radiotherapy, single crystalline (SC) diamonds have been grown homo-epitaxially on Ib HPHT substrates. A microwave plasma enhanced chemical vapor deposition (MPCVD) technique was used to reduce defect incorporation into these samples during growth. In order to reduce the number of defects and to obtain a surface as smooth as possible, we have applied oxygen plasma pre-treatment to the HPHT substrates [1]. The aim of this study is to correlate the growth conditions, the crystalline quality of the samples and their charge transport properties. Two growth parameters have been investigated in particular, namely the methane concentration and the plasma density. To vary the plasma density two ways can be applied, either changing the pressure applied or modifying the microwave power applied. By convenience, we choose to change the applied microwave power. 2. Experimental details Samples were grown in an AX5400 ASTEX reactor, with a base pressure of 10-7mbar, on commercial Ib HPHT single crystal substrates (100) oriented surfaces. To study the influence of methane, we operated at a microwave power of 500 W and at a pressure of 240 mbar. The methane concentration has been varied from 1 to 10 % while all other parameters were kept constant. To study the influence of microwave power, we operated at a constant pressure of 240 mbar keeping the methane concentration constant at 2 %. Indeed, using this low methane admixture allowed to investigate a wider range of settings for the other set of parameters. The microwave power has been studied from 500 W upwards, which was the minimum for the plasma to reach the substrate holder in our reactor configuration. After each deposition, the substrate was oxidized in a hot mixture of H2S04 and KNO3 during 30 minutes to remove all surface impurities. After this chemical treatment, substrates were rinsed in an ultrasonic bath of deionized water. The CVD diamond plates were then removed from the HPHT substrates by laser cutting. After mechanical polishing of both sides of the sample, all samples exhibited a better quality on their growth side than on the side which was in contact with the HPHT substrate (Fig. 1). In all the following, we only focus on the growth side of the samples. The quality of the HPHT side can be improved by polishing further into the sample. Intensity (a.u) 100 Side Position FWHM Growth 1331.98 1.47 Substrate 1332.03 1.70 50 0 Substrate side Growth side -50 1260 1350 1440 -1 Wave number (cm ) Fig. 1: Raman peak characterizations of both sides of the SC-CVD sample with 632 nm excitation 3. Results and discussion The first investigated parameter was the growth rate. It is well established that at very low methane concentrations, one can grow a very low concentration of defects [2]. The main drawback of this method is the very low growth rate applied. On the other hand, the enhancement of the growth rate should not be done at the detriment of the crystalline quality. At first, we probed the values of the growth rate according to the microwave power used, keeping all other parameters constant as well as the methane concentration and the substrate temperature. To keep temperature constant, we played with the substrate geometry. In figure 2a, it clearly appears that increasing the microwave power leads to a slow increase in growth rate. A plateau is observed when the microwave power used reaches 600 W. Thus, adjusting the microwave power is not a suitable way to increase the growth rate. On the other hand, increasing the methane concentration, in the range of 1 to 10%, leads to a linear increase of growth rate. In order to obtain an increase of the growth rate one can try to increase the methane concentration instead of changing the microwave power. 25 6 V = -4.4 + Cmethane x 2.5 Growth rate (µm/h) Growth rate (µm/h) 20 5 4 3 F ig. 2.a 2 2 r =0.98 15 10 Fig. 2.b 5 Experimental data 2 Linear fit (r = 0,9999) 0 500 550 600 650 700 0 750 2 M icrowave power (W ) 4 6 8 10 12 Methane concentration (%) Fig. 2 Evolution of the sample growth rate according to the microwave power injected (2a) or to the methane concentration into the gas phase (2b) 3 1331,4 1 Position FWHM 1331,0 0 560 580 600 620 640 660 680 700 720 740 760 Micro-wave power (W) -1 -1 1332,6 Peak position 1332,4 b) FWHM 1332,2 1,8 1332,0 1331,8 1331,6 1331,4 1,6 1331,2 1331,0 2 3 4 5 FWHM of Raman peak (cm ) 2 1331,6 Raman peak postion (cm ) a) 1331,8 1331,2 2,0 -1 1332,0 FWHM of Raman peak (cm ) -1 Position of the Raman peak (cm ) Figure 2b shows that operating at low methane concentrations (< 4 %) or at a methane concentration higher than 6 % leads to a drop of crystal quality. The same trend is obtained for a tuning of the microwave power. The optimum condition is between 600 and 700 W. For lower or higher power, the Raman peak position is shifted due to increased internal stress, which is incorporated during growth. These results are in good agreement with cathodo-luminescence measurements. In the case of optimized experimental growth conditions, no GR1 band (740.9 and 744.4 nm) due to graphite inclusion and no interstitial peaks (270.6; 285.3 and 301.6 nm) are observable. 6 7 8 9 Methane concentration (%) Fig. 3 Evolution of position and FWHM of the Raman peak according to the microwave power injected (3a) or to the methane concentration into the gas phase (3b) Then we investigated the charge transport properties of the synthesized CVD diamond single crystals. At first, we focused on Time of Flight (TOF) measurements using 241Am alpha particles irradiation (Fig 4). The TOF set-up has already been presented in refs. 3, 4. Here, an electric field is applied to the detector in order to collect created charge. Electron-hole pairs are generated in the vicinity of one of the electrodes. So, only one kind of carrier propagation can be monitored, defined by the applied bias potential. We operated at various bias potentials and signals have been recorded by a fast oscilloscope. Then we use equation 1 to fit the experimental data [4] using a least squared method. This fit allows determine the drift carrier saturation velocity (vs) and the mobility at zero field (µ 0) for both types of carriers [5]. µ( Er ) = 1+ Fig. 4 Principle of the TOF technique using 241Am irradiation. µ0 µ0 E vs Eq. 1 Variation of the mobility as a function of electric field. Figure 5a shows mobilities as detected at zero field as a function of microwave powers used during growth of samples. Figure 5b shows the same results for variation of the methane concentration. These results are compared with data obtained for the crystalline quality shown in figure 3. Optimized values are obtained again for samples grown by 4 to 6% of methane and by 550 to 700 W microwave power. Moreover, the same trend in mobility variation is observed for both electrons and holes when growth conditions are varied. 3400 2800 2600 2400 2200 electron mobility hole mobility 2000 500 550 600 650 700 -1 2 2800 2600 2 -1 Mobility (cm ,V ,s ) 3000 -1 -1 Mobility (cm ,V ,s ) 3000 Fig 5.a 3200 2400 Fig 5.b 2200 electron mobility hole mobility 2000 1800 1600 750 Micro-wave power (W) 800 0 2 4 6 8 10 12 14 16 18 Methane concentration (%) Fig. 5 Variation of mobility of electrons and holes as a function of microwave power (5a) and methane concentration (5b) The charge collection efficiency was also measured, using the 241Am irradiation under otherwise identical conditions as presented in figure 4. Here the focus was to characterize lateral homogeneities of detectors and to measure the total amount of collected charge. To calibrate, we have calculated the ratio of collected charge in diamond with respect to collected charge in a calibrated Silicon diode, taking into account the number of electron-hole pairs created in each material. Best results are achieved for growth parameters as discussed above for both electrons and holes. When the microwave power is increased, the hole mobility is higher than the electron mobility, whereas tuning the methane concentration had no influence on the mobility difference between both types of carriers, as shown in Figure 6. 105 100 95 100 a) CCE (%) CCE (%) 90 85 80 75 b) 90 80 70 Positive bias Negative bias Hole collection Electron collection 65 70 60 55 500 550 600 650 700 2 3 Micro-wave power (W) 4 5 6 7 Methane concentration (%) 8 9 Fig. 6 Variation of the CCE of both carriers as a function of microwave power (6a) and of methane concentration (6b) Using the Hecht equation (Eq. 2), the deep trapping lifetime can be calculated taking into account mobilities as calculated from Eq. 1 and a least squared method. This has been applied for electrons and holes as a function of microwave power applied during growth. The results are summarized in Table 1. It demonstrates that the longest deep-trapping lifetime is obtained at 620 W for both electrons and holes which is consistent with data discussed above. In this case, the deep-trapping lifetime of holes is longer than that of electrons meaning that a higher amount of electron traps is present in the detector. CCE = ⎡ ⎛ x ⎞ ⎛ L2 µ.τ .V ⎧⎪ 1 − exp ⎨ ⎢− ⎜1 − ⎟.⎜⎜ L2 ⎪ ⎣ ⎝ L ⎠ ⎝ µ.τ .V ⎩ ⎞⎤ ⎫⎪ x ⎟⎟⎥ ⎬ + ⎠⎦ ⎪⎭ L Eq. 2 : Hecht equation [6] τ h+ (ns) τ e- (ns) 700 W 9 6 620 W 22 15 550 W 6 5 500 W 5,5 5 Table 1: Variation of deep trapping lifetime of electrons and holes as a function of microwave power. 4. Conclusions In this study, we have demonstrated that improved growth rate can give rise to very good crystal quality of single-crystalline CVD diamonds, synthesized in our laboratory. To do so, we have characterized the influence of methane concentration and of microwave power used to grow diamond. The crystal quality has been investigated using confocal Raman measurements and cathodo-luminescence. The mobilities of electrons and holes have been determined by use of TOF. This study shows that the mobility of holes is higher than that of electrons when the microwave power is increased. On the other hand, the difference in mobilities of electrons and holes does not depend on methane concentrations. From charge collection experiments and TOF measurements, we deduce the deep-trapping lifetime for each type of carrier as a function of applied microwave power. From these investigations we find, that a higher density of electron traps is present in diamond films than for holes. As a summary of these investigations we conclude a window of optimized growth parameters which is for methane concentrations 4 to 6% and for microwave power 600 to 700 W. Acknowledgements The authors acknowledge the European Commission for funding part of this research from I3HP JRA11 NORHDIA (Project # RII3-CT-2004-506078) and the GeMAC laboratory (CNRS, Meudon, France) for their kindly collaboration in Raman and cathodoluminescence spectroscopy. References [1] Single crystal diamond growth and characterizations, N. Tranchant, D. Tromson, Z. Remes, L. Rocha, M. Nesladek, P. Bergonzo, Proceeding of the MRS Fall 2006 conference, Symposium J, Paper # 0956-J07-02. [2] Homoepitaxial diamond growth by high-power microwave plasma chemical vapor deposition, T. Teraji, T. Ito, Journal of Crystal Growth, Volume 271, Issues 3-4, November 2004, Pages 409419. [3] Time Of Flight study of high performances CVD diamond detector devices, N. Tranchant, M. Nesladek, D. Tromson, Z. Remes, A. Bogdan, P. Bergonzo, physica status solidi (a), Volume 204, Issue 9, September 2007, Pages 3023-3029. [4] Charge transport in high mobility single crystal diamond, M. Nesladek, A. Bogdan, N. Tranchant, K. Haenen, P. Bergonzo, Diamond and Related Materials, in press, 2007. [5] Charge transport properties of single crystal CVD-Diamond particle detectors, M. Pomorski, E. Berdermann, W. de Boer, A. Furgeri, C. Sander, J. Morse, Diamond and Related Materials, Volume 16(4-7), pages 1066-9, 2007. [6] Currents induced by electron motion, S. Ramo, Proceedings of the IRE 584, 1939.