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Charge collection distance measurements in single and polycrystalline CVD diamond
Diamond and Related Materials 13 (2004) 872–875
Charge collection distance measurements in single and polycrystalline CVD diamond J. Isberga,*, J. Hammersberga, H. Bernhoffa, D.J. Twitchenb, A.J. Whiteheadb a
Division for Electricity and Lightning Research, Uppsala University, Box 534, S-751 21, Uppsala, Sweden b Element Six Limited, King’s Ride Park, Ascot, Berkshire SL5 8BP, UK
Abstract Diamond is one of the most promising materials for extreme performance electronic applications, such as high power, high temperature, and high frequency devices. Recent advances in the synthesis of device-quality, single crystal diamond by chemical vapour deposition (CVD) have opened up the possibility to make efficient diamond-based electronic devices. Two important material parameters, which should be maximized, that characterise the performance of diamond in applications of this type are the carrier mobility and the free carrier lifetime. We report an experimental study comparing photocurrent mobility=lifetime (mt) products in polycrystalline and single crystal CVD diamond. It is shown that the mt products of the single crystal samples are 2–3 orders of magnitude higher than those of the polycrystalline samples. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Charge collection distance; Mobility; Lifetime; Single crystal diamond
1. Introduction Semiconducting diamond is one of the most promising materials for many extreme performance electronic applications, such as high power, high temperature, and high frequency devices. Diamond has excellent mechanical, thermal, electrical and chemical properties. It is the hardest known material and it has the highest electrical breakdown field, the highest room temperature thermal conductivity and the widest electromagnetic radiation transparency range of any material, as well as a wide bandgap and high carrier mobilities. The transport properties of free carriers and their lifetimes are important intrinsic parameters of semiconductor materials, and these parameters are used in evaluating their potential for various electronic applications. Two important material parameters to monitor are the carrier mobilities (me,h) and the free carrier lifetimes (te,h), which both should reach as high values as possible in the ‘intrinsic’ synthesised material, to be able to fulfil the requirements demanded by high performance electronic devices. *Corresponding author. Tel.: q46-18-471-5821; fax: q46-18-4715810. E-mail address: [email protected] (J. Isberg).
In previous work, polycrystalline samples were studied using a mobility=lifetime (mt) product measurement technique w1x. This method is more commonly referred to as a ‘charge collection distance’ (CCD) measurement and is commonly utilised to characterise particle detector materials w2–4x. In the work reported in Ref. w1x, the average grain size in the studied samples ranged from 25 mm up to 110 mm. These samples showed a clear correlation between mobility=lifetime product and average grain size between samples of similar point defect concentration (nitrogen). Here we extend this study to include high purity single crystal CVD diamond samples w5x. 2. Sample preparation The samples were synthesised using a microwave plasma-assisted CVD technique. For the single crystal (SC) samples homoepitaxial CVD diamond layers were deposited on specially prepared 4=4=0.5 mm N100M oriented HPHT synthetic diamond substrates. Care was taken to ensure that the surface quality of the substrates was as high as possible and that the substrates were free of metallic inclusions that can act as sources of dislocations in the epitaxial overlayer. A pre-growth etch phase was followed by epitaxial overgrowth under con-
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.11.065
J. Isberg et al. / Diamond and Related Materials 13 (2004) 872–875
ditions of high purity. For synthesis, a total gas flow of 700 sccm was used (consisting of hydrogen with approx. 5.5% methane and approx. 10% argon), at a substrate temperature of ;830 8C. To obtain free-standing plates, the epitaxial overlayer was separated from its substrate by a laser cutting technique and these plates were polished using standard lapidary techniques. The thickness, L, of the samples ranged between 95 and 690 mm (Table 1). Finally, the surfaces were oxygen-terminated by exposure to an oxygen plasma and semi-transparent aluminium contacts, 4 mm in diameter, were formed on the {100} surfaces using physical vapour deposition (PVD) combined with standard lithography and wet chemical etching (Fig. 1). Aluminium contacts on oxygen-terminated surfaces were used to ensure that the contacts had good blocking behaviour. This diminishes measurement errors due to photocurrent gain effects (discussed in Section 4). The contacts are placed on opposite sides of the sample, which gives an electric field in the N100M direction. When a semiconductor is exposed to above-bandgap light and simultaneously biased by applying a voltage difference to the contacts, the electron-hole pairs generated separate and the carriers drift toward opposite electrodes. Charge trapping and recombination may, however, prevent some carriers from being collected by the electrodes. The collection efficiency can be described by a quality factor, the so-called charge collection distance, which is defined as mE, where E is the applied field. Assuming exponential carrier loss in the drift current and steady state conditions, the collected charge
873
Table 1 Summary of the samples studied
噛1 噛2 噛3 噛4 噛5 噛6 噛7 噛8
Average grain size (mm)
Sample thickness (mm)
N-content (cmy3)
25 32 39 48 110 SC SC SC
95 227 202 225 98 690 496 550
-1015 -1015 1015 –1016 1015 –1016 -1015 -1015 -1015 -1015
at the electrodes can be described using the Hecht model w2 x : QsN0e
B mtV w D2 Ez F| x1yexpCy 2 D D y mtV G~
(1)
where msme or mh depending on whether carriers are generated in the vicinity of the cathode or anode, respectively. V is the bias voltage and D the sample thickness. A steady-state condition is obtained when the excitation pulse is considerably longer than other dynamical time constants, such as the transit time, te,hs D 2 y me,hV, and free carrier lifetimes. N0e is the total charge generated by the exciting pulse. The validity of Eq. (1) also requires a uniform field across the sample, EsVyD. For this assumption to hold true, it is necessary that N0e is substantially smaller than the charge on the
Fig. 1. Optical micrograph of a single-crystal CVD sample with an aluminium mesh contact. The illuminated zone is 3 mm in diameter. Inset shows an enlargement of the contact, which has a mesh spacing of 40 mm. The dark appearance of the sample, which is in fact transparent, is due to the lighting conditions in this micrograph.
874
J. Isberg et al. / Diamond and Related Materials 13 (2004) 872–875
electrodes of the sample, i.e. N0e
Fig. 3. Experimental results from the mt-product measurements on polycrystalline sample 噛5. The solid lines show the best fit using the Hecht-model, Eq. (1).
Figs. 2 and 3, which show typical photocurrent vs. bias curves for the two types of samples. With increasing bias voltage the photocurrent increases. This is related to the increase of the CCD with bias voltage. At some bias voltage the CCD becomes much larger than the sample thickness D and the photocurrent saturates. This behaviour is expected with blocking contacts that prevent charge replenishment effects at the electrodes and thus gain. Note that in Figs. 2 and 3 the current saturates at a much lower bias voltage in the SC sample (噛7) than in the polycrystalline sample (噛5), even though sample 噛5 is thinner than 噛7. This shows that the CCD is much greater in the SC sample than in the polycrystalline sample. A more careful analysis, least-squares fitting the Hecht-expression (Eq. (1)) to the photocurrent data, yields mt-products 2–3 orders of magnitude higher in all the measured SC-CVD samples than in the best polycrystalline CVD diamond samples. Table 2 shows the extracted mt-products for samples 1–8. Both the polycrystalline and SC samples show somewhat different photocurrent vs. bias voltage curves when the cathode and anode are illuminated, with systematiTable 2 Measured mt products
Fig. 2. Experimental results from the mt-product measurements on SC-CVD sample 噛7. The solid lines show the best fit using the Hechtmodel, Eq. (1).
噛1 噛2 噛3 噛4 噛5 噛6 噛7 噛8
mete (=10y6 cm2 yV)
m ht h (=10y6 cm2 yV)
0.6 3.1 0.1 0.6 3.8 1700 3300 1700
0.6 1.8 0.8 1.7 3.7 650 1400 700
J. Isberg et al. / Diamond and Related Materials 13 (2004) 872–875
cally lower mt-products for the holes, except for sample 噛3. This reflects the difference in mobility of electrons and holes combined with effective lifetime variations as a consequence of competing trap mechanisms. 5. Discussion In a previous report w1x we pointed out the correlation between the grain size and the mt product in polycrystalline diamond samples. Similar correlations have also been noted previously w6–8x. In SC-CVD diamond, we find a dramatic increase in the measured mt products. In our experiments we use a contact geometry giving a field direction parallel with the growth axis. In the polycrystalline samples the grains have a columnar structure where most columns reach between the electrodes w1x and thus the grain boundaries are roughly parallel to the electric field. In this geometry, it is reasonable to assume that the measured drift current, in the field direction, mainly passes through the centre of the grains due to presumably lower carrier mobilities and higher density of defects at the grain boundaries. Any carrier motion transverse the field direction will on the other hand mainly be caused by diffusion. Due to the short diffusion length in the polycrystalline samples the obtained mt-products are necessarily limited by trapping and recombination at intra-grain point defects or dislocations, as noted in w1x. In the SC-CVD samples measured mt products are 2–3 orders of magnitude
875
higher, which may be partly attributed to the fact that these samples have a low dislocation density, typically -106 cmy2 w5x. However, it is clear that further work is needed to understand the details of the trapping mechanisms in these high quality samples. 6. Summary In this paper we have presented an experimental study of the mobility=lifetime products CVD grown single crystal diamond and compared the results with previously reported measurements on polycrystalline samples. We find dramatically increased mobility=lifetime products in the SC-CVD diamond, which we attribute mainly to the low point defect and dislocation densities in these samples. References w1x J. Hammersberg, J. Isberg, E. Johansson, T. Lundstrom, ¨ O. Hjortstam, H. Bernhoff, Diamond Relat. Mater. 10 (2001) 574. w2x K. Hecht, Z. Physik (Berlin) (1932) 235. w3x V.M. Gerrish, Semiconductors and Semimetals 43 (1995) 493. w4x S. Sciortino, Rivista del Nuovo Cimento 22 (1999) 1. w5x J. Isberg, J. Hammersberg, E. Johansson, T. Wikstrom, ¨ D.J. Twitchen, A.J. Whitehead, et al., Science 297 (5587) (2002) 1670. w6x M.A. Plano, M.I. Landstrass, L.S. Pan, S. Han, D.R. Kania, S. McWilliams, et al., Science 260 (1993) 1310. w7x S. Han, R.S. Wagner, Appl. Phys. Lett. 68 (1996) 3016. w8x H. Yoneda, K-I. Ueda, Y. Aikawa, K. Baba, N. Shohata, J. Appl. Phys. 83 (1998) 1730.
Charge collection distance measurements in single and polycrystalline CVD diamond J. Isberga,*, J. Hammersberga, H. Bernhoffa, D.J. Twitchenb, A.J. Whiteheadb a
Division for Electricity and Lightning Research, Uppsala University, Box 534, S-751 21, Uppsala, Sweden b Element Six Limited, King’s Ride Park, Ascot, Berkshire SL5 8BP, UK
Abstract Diamond is one of the most promising materials for extreme performance electronic applications, such as high power, high temperature, and high frequency devices. Recent advances in the synthesis of device-quality, single crystal diamond by chemical vapour deposition (CVD) have opened up the possibility to make efficient diamond-based electronic devices. Two important material parameters, which should be maximized, that characterise the performance of diamond in applications of this type are the carrier mobility and the free carrier lifetime. We report an experimental study comparing photocurrent mobility=lifetime (mt) products in polycrystalline and single crystal CVD diamond. It is shown that the mt products of the single crystal samples are 2–3 orders of magnitude higher than those of the polycrystalline samples. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Charge collection distance; Mobility; Lifetime; Single crystal diamond
1. Introduction Semiconducting diamond is one of the most promising materials for many extreme performance electronic applications, such as high power, high temperature, and high frequency devices. Diamond has excellent mechanical, thermal, electrical and chemical properties. It is the hardest known material and it has the highest electrical breakdown field, the highest room temperature thermal conductivity and the widest electromagnetic radiation transparency range of any material, as well as a wide bandgap and high carrier mobilities. The transport properties of free carriers and their lifetimes are important intrinsic parameters of semiconductor materials, and these parameters are used in evaluating their potential for various electronic applications. Two important material parameters to monitor are the carrier mobilities (me,h) and the free carrier lifetimes (te,h), which both should reach as high values as possible in the ‘intrinsic’ synthesised material, to be able to fulfil the requirements demanded by high performance electronic devices. *Corresponding author. Tel.: q46-18-471-5821; fax: q46-18-4715810. E-mail address: [email protected] (J. Isberg).
In previous work, polycrystalline samples were studied using a mobility=lifetime (mt) product measurement technique w1x. This method is more commonly referred to as a ‘charge collection distance’ (CCD) measurement and is commonly utilised to characterise particle detector materials w2–4x. In the work reported in Ref. w1x, the average grain size in the studied samples ranged from 25 mm up to 110 mm. These samples showed a clear correlation between mobility=lifetime product and average grain size between samples of similar point defect concentration (nitrogen). Here we extend this study to include high purity single crystal CVD diamond samples w5x. 2. Sample preparation The samples were synthesised using a microwave plasma-assisted CVD technique. For the single crystal (SC) samples homoepitaxial CVD diamond layers were deposited on specially prepared 4=4=0.5 mm N100M oriented HPHT synthetic diamond substrates. Care was taken to ensure that the surface quality of the substrates was as high as possible and that the substrates were free of metallic inclusions that can act as sources of dislocations in the epitaxial overlayer. A pre-growth etch phase was followed by epitaxial overgrowth under con-
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.11.065
J. Isberg et al. / Diamond and Related Materials 13 (2004) 872–875
ditions of high purity. For synthesis, a total gas flow of 700 sccm was used (consisting of hydrogen with approx. 5.5% methane and approx. 10% argon), at a substrate temperature of ;830 8C. To obtain free-standing plates, the epitaxial overlayer was separated from its substrate by a laser cutting technique and these plates were polished using standard lapidary techniques. The thickness, L, of the samples ranged between 95 and 690 mm (Table 1). Finally, the surfaces were oxygen-terminated by exposure to an oxygen plasma and semi-transparent aluminium contacts, 4 mm in diameter, were formed on the {100} surfaces using physical vapour deposition (PVD) combined with standard lithography and wet chemical etching (Fig. 1). Aluminium contacts on oxygen-terminated surfaces were used to ensure that the contacts had good blocking behaviour. This diminishes measurement errors due to photocurrent gain effects (discussed in Section 4). The contacts are placed on opposite sides of the sample, which gives an electric field in the N100M direction. When a semiconductor is exposed to above-bandgap light and simultaneously biased by applying a voltage difference to the contacts, the electron-hole pairs generated separate and the carriers drift toward opposite electrodes. Charge trapping and recombination may, however, prevent some carriers from being collected by the electrodes. The collection efficiency can be described by a quality factor, the so-called charge collection distance, which is defined as mE, where E is the applied field. Assuming exponential carrier loss in the drift current and steady state conditions, the collected charge
873
Table 1 Summary of the samples studied
噛1 噛2 噛3 噛4 噛5 噛6 噛7 噛8
Average grain size (mm)
Sample thickness (mm)
N-content (cmy3)
25 32 39 48 110 SC SC SC
95 227 202 225 98 690 496 550
-1015 -1015 1015 –1016 1015 –1016 -1015 -1015 -1015 -1015
at the electrodes can be described using the Hecht model w2 x : QsN0e
B mtV w D2 Ez F| x1yexpCy 2 D D y mtV G~
(1)
where msme or mh depending on whether carriers are generated in the vicinity of the cathode or anode, respectively. V is the bias voltage and D the sample thickness. A steady-state condition is obtained when the excitation pulse is considerably longer than other dynamical time constants, such as the transit time, te,hs D 2 y me,hV, and free carrier lifetimes. N0e is the total charge generated by the exciting pulse. The validity of Eq. (1) also requires a uniform field across the sample, EsVyD. For this assumption to hold true, it is necessary that N0e is substantially smaller than the charge on the
Fig. 1. Optical micrograph of a single-crystal CVD sample with an aluminium mesh contact. The illuminated zone is 3 mm in diameter. Inset shows an enlargement of the contact, which has a mesh spacing of 40 mm. The dark appearance of the sample, which is in fact transparent, is due to the lighting conditions in this micrograph.
874
J. Isberg et al. / Diamond and Related Materials 13 (2004) 872–875
electrodes of the sample, i.e. N0e
Fig. 3. Experimental results from the mt-product measurements on polycrystalline sample 噛5. The solid lines show the best fit using the Hecht-model, Eq. (1).
Figs. 2 and 3, which show typical photocurrent vs. bias curves for the two types of samples. With increasing bias voltage the photocurrent increases. This is related to the increase of the CCD with bias voltage. At some bias voltage the CCD becomes much larger than the sample thickness D and the photocurrent saturates. This behaviour is expected with blocking contacts that prevent charge replenishment effects at the electrodes and thus gain. Note that in Figs. 2 and 3 the current saturates at a much lower bias voltage in the SC sample (噛7) than in the polycrystalline sample (噛5), even though sample 噛5 is thinner than 噛7. This shows that the CCD is much greater in the SC sample than in the polycrystalline sample. A more careful analysis, least-squares fitting the Hecht-expression (Eq. (1)) to the photocurrent data, yields mt-products 2–3 orders of magnitude higher in all the measured SC-CVD samples than in the best polycrystalline CVD diamond samples. Table 2 shows the extracted mt-products for samples 1–8. Both the polycrystalline and SC samples show somewhat different photocurrent vs. bias voltage curves when the cathode and anode are illuminated, with systematiTable 2 Measured mt products
Fig. 2. Experimental results from the mt-product measurements on SC-CVD sample 噛7. The solid lines show the best fit using the Hechtmodel, Eq. (1).
噛1 噛2 噛3 噛4 噛5 噛6 噛7 噛8
mete (=10y6 cm2 yV)
m ht h (=10y6 cm2 yV)
0.6 3.1 0.1 0.6 3.8 1700 3300 1700
0.6 1.8 0.8 1.7 3.7 650 1400 700
J. Isberg et al. / Diamond and Related Materials 13 (2004) 872–875
cally lower mt-products for the holes, except for sample 噛3. This reflects the difference in mobility of electrons and holes combined with effective lifetime variations as a consequence of competing trap mechanisms. 5. Discussion In a previous report w1x we pointed out the correlation between the grain size and the mt product in polycrystalline diamond samples. Similar correlations have also been noted previously w6–8x. In SC-CVD diamond, we find a dramatic increase in the measured mt products. In our experiments we use a contact geometry giving a field direction parallel with the growth axis. In the polycrystalline samples the grains have a columnar structure where most columns reach between the electrodes w1x and thus the grain boundaries are roughly parallel to the electric field. In this geometry, it is reasonable to assume that the measured drift current, in the field direction, mainly passes through the centre of the grains due to presumably lower carrier mobilities and higher density of defects at the grain boundaries. Any carrier motion transverse the field direction will on the other hand mainly be caused by diffusion. Due to the short diffusion length in the polycrystalline samples the obtained mt-products are necessarily limited by trapping and recombination at intra-grain point defects or dislocations, as noted in w1x. In the SC-CVD samples measured mt products are 2–3 orders of magnitude
875
higher, which may be partly attributed to the fact that these samples have a low dislocation density, typically -106 cmy2 w5x. However, it is clear that further work is needed to understand the details of the trapping mechanisms in these high quality samples. 6. Summary In this paper we have presented an experimental study of the mobility=lifetime products CVD grown single crystal diamond and compared the results with previously reported measurements on polycrystalline samples. We find dramatically increased mobility=lifetime products in the SC-CVD diamond, which we attribute mainly to the low point defect and dislocation densities in these samples. References w1x J. Hammersberg, J. Isberg, E. Johansson, T. Lundstrom, ¨ O. Hjortstam, H. Bernhoff, Diamond Relat. Mater. 10 (2001) 574. w2x K. Hecht, Z. Physik (Berlin) (1932) 235. w3x V.M. Gerrish, Semiconductors and Semimetals 43 (1995) 493. w4x S. Sciortino, Rivista del Nuovo Cimento 22 (1999) 1. w5x J. Isberg, J. Hammersberg, E. Johansson, T. Wikstrom, ¨ D.J. Twitchen, A.J. Whitehead, et al., Science 297 (5587) (2002) 1670. w6x M.A. Plano, M.I. Landstrass, L.S. Pan, S. Han, D.R. Kania, S. McWilliams, et al., Science 260 (1993) 1310. w7x S. Han, R.S. Wagner, Appl. Phys. Lett. 68 (1996) 3016. w8x H. Yoneda, K-I. Ueda, Y. Aikawa, K. Baba, N. Shohata, J. Appl. Phys. 83 (1998) 1730.