- Email: [email protected]
Spatially resolved imaging of charge collection efficiency in polycrystalline CVD diamond by the use of ion beam induced current
Beam Interactions with Materials &Atoms Nuclear Instruments
ELSEVIER
and Methods in Physics Research B 130 (1997) 518-523
Spatially resolved imaging of charge collection efficiency in polycrystalline CVD diamond by the use of ion beam induced current D.R. Beckman a,*, A. Saint a, P. Gonon a, D.N. Jamieson ‘, S. Prawer a, R. Kalish b a Schoni of Physics, ~i~r~a~ulyti~ul Reseursh Centre. The ~ffiz,er.~~~of ~elb~)ur~e, P~~k~il~e~Vie 3052, ~u.~tralia b Physics Department rind Sotid State i~.~titute,Technion, Huffs 3?000. Israel
Abstract Diamond based detectors have potential applications in high energy physics experiments. These detectors can be fabricated from synthetic Chemical Vapour Deposited (CVD) polycrystalline diamond films. Previously it has been shown by the Turin group and their coworkers in Zagreb that it is possible to investigate the electrical characteristics of high quality polycrystalline CVD diamond films by Ion Beam Induced Current (IBIC). The present work describes IBIC images obtained using 2 MeV He+ irradiation of 2.50 km thick polycrystalline diamond films through a thin gold surface contact layer biased positively relative to the grounded rear surface of the film. In contrast to previous expe~ments the present spectra of collected charge display a clearly defined peak from the induced charge. Images obtained by separating these spectra into different regions of interest allow the identi~cation of regions in the sample of different charge collection efficiency. In particular the presence of some grains in which no charge collection appears possible and the reduction in charge collection efficiency at the grain boundaries is evident. 0 1997 Elsevier Science B.V.
1. Introduction Diamond is a promising material for the realisation of radiation hard detectors for nuclear radiation [l]. Such detectors require large areas and fairly thick samples. Diamond slabs grown by Chemical Vapour Deposition allow the practical and cost-effective realisation of such detectors. These slabs, however, are comprised of many diamond crystallites of different orientations and separated by grain boundaries. These may affect the electrical properties of the material by acting as traps and/or recombina-
* Corresponding author. [email protected].
Fax:
+61-3-934’7-4783; email:
tion centres, thus degrading the charge collection efficiency [2]. Most techniques for monitoring charge collection cannot provide spatial and depth resolved images and thus cannot provide info~ation which will identify those regions and features in the poly~rystalline CVD diamond which are responsible for charge recombination and trapping. Pulsed UV photoexcitation has been used so far mainly to probe the time dependence (or lifetime) of the carriers in a shallow region near the surface (since UV light is heavily absorbed by the diamond) [3]. Electron Beam Induced Current (EBIC) [4] does offer spatial resolution; however, the probing depth is only of the order of a few microns, and it is not generally possible to
0168-583X/97/$17.~ 0 1997 Ekevier Science B.V. Ail rights reserved. PII SOl68-583X(97)~0243-7
D.R. Beckman ef al. /Nucl.
Instr. and Meth. in Phys. Res. 3 I30 ~19971518-523
probe beneath electrical contacts. In contrast to these, ion beam induced current (IBIC), generated by a spatially scanned focused energetic ion beam, can offer both high spatial and depth resolution [5]. By adjusting the ion energy and species it is possible to select the depth from which charge is collected. In addition IBIC offers the possibility of monitoring detector performance under realistic irradiation conditions similar to those found during detector operation, and finally IBIC also offers the possibility of the in-situ assessment of the durability of the detector under ion irradiation, information which is crucial for the determination of the lifetime of diamond nuclear detectors. Much pioneering work in this regard has been performed by Manfredotti and his group using MeV proton beams and the reader is referred to their many excellent publications for the background on IBIC as applied to diamond [6-IO]. In the present work, we report on the results of our initial IBIC investigation of high quality CVD diamond irradiated with 2 MeV He+. The advantage of such a beam over protons is that the He+ ions’ stopping power is several times greater at the surface than that of a proton beam and so generate more charge carriers near the sample surface where we find they can be efficiently collected, producing a superior signal-to-noise ratio. We show that the charge collection is spatially very inhomogeneous on the micron scale, and suggest that this may be a consequence of the effect of diamond facets on the electrical properties of the metal-diamond electrical contact.
519
2. Experiment IBIC measurements were ~rfo~ed using the Micro Analytical Research Centre (MARC) nuclear microprobe at the University of Melbourne [ 1 I]. The sample used for this investigation was a free standing 250 pm thick detector-grade Norton CVD diamond film measuring about 6 mm in diameter. The collection distance in the film was of the order of 20-40 pm [ 121. MO ( I5 nm)/Au (60 nm) electrical contact pads, measuring about 0.5 X 0.5 mm2 , were evaporated onto the growth side of the CVD diamond in a grid-like arrangement. Gold wires were then wire bonded to a number of pads. The diamond was mounted (substrate side down) using Ag paint to a gold pad on a standard chip holder. The experimental setup is depicted in Fig. 1. Pad A was positively biased while both pad B and the back of the diamond (C) were grounded. A 2 MeV He+ beam, focussed to a spot of around 1 pm, was initially scanned over an area 3 X 3 mm2; this scan size was later reduced to 300 X 300 pm2 in order to focus in on the edge of a pad. Charge collected at pad A was converted into a voltage signal and recorded using a multichannel analyser. By scanning the beam in X, y, it was possible to produce spatially resolved images of the charge collection efficiency. The projected range for 2 MeV He+ ions in diamond is 3.4 f 0. I Frn [ 131. We found that it was necessary to “condition” the detector before IBIC signals could be obtained. This was accomplished by applying a bias of the
572 Spectroscopic
Amplifier
foci
Fig. I. Experimental setup for IBIC measurements performed on a 250 pm thick Norton CVD diamond film using a 2 MeV He+ beam. “A” and “B” are the MO (15 nm>/Au (60 nm) electrical contact pads which were evaporated onto the growth side of the diamond while “C” is the grounded back surface. IBIC signals were only obtained when a positive bias (with respect to B and C> was applied to pad A.
X. JZLECTRONICS
AND PHOTONICS
520
D.R. Becknzun et al. /N&tcl. Insrr. and M.&I. in P&s.
order of 800 V. Once IBIC signals were observed, the voltage could be reduced and IBIC images could still be obtained at biases as low as 100 V. However, prior to conditioning, no IBIC signal at all could be obtained for biases below 400 V. The L‘conditioning” bias of 800 V is close to that at which breakdown across the CVD diamond was observed to occur. It is important to note that IBIC signals were only obtained for positive bias fie pad A positive with respect to C). We did not observe any IBIC signals if the bias on A was negative with respect to C (see Fig. I).
3. Results Fig. 2 shows the typical output of the charge collected from a 300 X 300 pm2 scan of the Norton CVD film with an applied bias of 700 V (i.e. about 3 V/pm). The charge collected and converted into a voltage signal is proportional to the channel number. Hence it is possible, in principle, to convert channel number to charge collection efficiency [s], although this has not been done here. Fig. 2 is thus a spectrum of the relative charge collection efficiency of the detector integrated over the whole of the contact pad. The lowest few channels have been omitted from the spectrum since noise from the experiment is accumu-
E 4000 5 s 3000 2000 IOOO0
0
20
40
60
80
100
120
140
160
180
Fig. 2. 2 MeV He+ IBIC spectrum obtained from a Norton CVD film with an applied bias of + 700 V, integrated over the whole of the contact pad. The charge collected is proportional to channel number which is directly proportional to charge collection efficiency. The markers at the top indicate the regions of interest in collection efficiency which were used to extract the images shown in Fig. 4.
Res.
B I30 (1997) 818-523
Channel Fig. 3. 2 MeV He+ IBIC spectra obtained from a Norton CVD film with increasing applied bias voltage. The spectra have been horizontally translated relative to each other by multiples of 200 channels for clarity and have been normalised to the incident beam flux. These spectra were collected using different scan sizes (a) and (b) are from a 3 X 3 mm2 scan of the CVD diamond. (c). (d) and (e) are from a 600X 600 pm’ scan and (f) is from a 300X308 &m2 scan). The shape of the spectra can be seen to change with applied voltage, with a double peak structure nppcaring only for bias voitages over 600 V.
lated in them. Two peaks appear in the spectrum. Fig. 3 shows that the shape of the charge collection curve changes with applied bias, with the double peak structure appearing only for an applied bias in excess of 600 V. Previous measurements have shown counts versus collection efficiency to be a monotonically decreasing curve with no discernible peaks (similar to those shown here for biases up to 500 V>. By dividing the overall spectrum into different regions of interest, as indicated in Fig. 2, the images presented in Fig. 4 were obtained. These images show the charge collection efficiency for a 300 x 300 pm2 area of the diamond with a section of the gold electrical contact pad appearing in the lower half of each image. Fig. 4a shows that the highest charge collection efficiency originated inside select grains, with the maximum efficiency occurring in the centres of these grains. Fig. 4b shows that regions of moderate charge coilection efficiency are more uniformly distributed in the diamond, but nevertheless the grain structure is still clearly observable. Grain boundaries were primary sources of low charge collection efficiency, as shown in Fig. 4c. Very low charge collection efficiency, as presented in Fig. 4d, was recorded for charges generated outside the met-
allised region of the diamond from up to a distance of 45 pm from the edge of the contact pad. The difference in charge collection efficiency for carriers generated outside the metallised region of the diamond and those generated beneath the gold electrical contact pad can be explained in terms of the different physical processes occurring. In the first case carriers would need to diffuse across numerous grain boundaries in order to reach the electrical contact for detection - while crossing these grain boundaries, many carriers would be trapped, hence reducing the overall collection efficiency. Carriers created beneath the gold pad, on the other hand, would drift under the applied field predominantly along the grain in which they were created towards the electrical contact pad. thereby inducing charge in the contact. By carefully selecting the bright regions of each image in Fig. 4 and extracting the corresponding spectra, the individual contributions from each part of the diamond, to the overall spectrum shown in
Fig 4. Images correspondrng to the collection efficiency regions of interest indicated in Fig. 2. Scan size was 300X300 hm’. A section of the gold electrical contact pad appears in the lower hlllf of each image. (a) shows regions of the highest charge collection efficiency, (h) shows regions of moderate collection efficiency, (c> shows regions of low collection efficiency, corresponding primaiiy to grain boundaries while (d) shows regions of very low charge collection efficiency which can be seen to be primarily off the contact pad, corresponding to diffusion of carriers from the nonmetallised region of the diamond to the contact pad. Coiour versions of these images can be found at http://www.ph.unimelh.edu.au/ - drb/ibic.htmi.
25000 m s 200008 2 15000,” 2: 100005000 -
OO Fig. 5. Spectra extracted from the bright regions of each of the images appearing in Fig. 3 and normalised to area. Curve {rt) shows the signal due to grains which recorded the highest charge collection efficiency, (b) shows the moderate collection efficiency response, (c) shows the grain boundary efficiency and (d) shows the charge collection efficiency for carriers generated outside the metctllised region of the diamond. The overnil spectrum presented in Fig. 3 is the sup~~osition of these individual signals.
Fig. 2, were obtained. These were normatised with respect to area and are presented in Fig. 5. Curve (a) is the signal due to the grains which recorded the highest charge collection efficiency. Curve (b) is moderate efficiency. Curve (c) shows the efficiency of the grain boundaries while curve (d) shows the collection efficiency for charge carriers generated outside the metallised region of the diamond. The overall spectrum in Fig . 2 is the weighted superposition of these individual signals, with the first peak of the doubfe peak structure appearing to be due primarily to carrier diffusion from the non-metallised region of the diamond and the second being due to carrier drift from under the gold electrical contact pad. Thus, the shape of the overall charge collection spectrum could vary for different polycrystalline diamond samples depending on the relative quantities of high and low collection efficiency grains within the film. The IBIC maps collected were compared to optical and SEM images of the same gold pad on the diamond. A strong correlation was found between those areas of high charge collection efficiency and grains whose surfaces were aligned parallel with the substrate, although areas of high charge collection efficiency were found in other regions as well.
X. ELECTRONICS
AND PHOTONICS
4. Discussion From the data presented above we can conclude that the charge collection efficiency in polycrystalline CVD diamond is very spatially inhomogeneous and that the observed collection efficiency spectrum will depend on which sections of the sample are probed by the beam. The low charge collection efficiency as observed here in the grain boundaries is to be expected. However, the puzzle remains as to why some grains are of such superior charge collection efficiency compared to others. In particular why is there such a large contrast even between adjoining grains (see Fig. 4a). One possible explanation relates to the effect of crystal orientation on the nature of the diamond-metal interface. Cheng et al. [ 141 found that Pt electrical contacts made on diamond displaying predominantly (100) morphology were Schottky in character, whilst similar contacts made on (111) diamond showed ohmic behaviour. These differences persisted despite cam being taken to ensure a similar surface treatment for the (100) and (ill) surfaces. The phenomenon may be related to the difference in negative electron affinity of the (111) and (100) surfaces as reported by Geis [ 151. The Norton films used in the experiments showed a rough growth surface dominated by (100) and (1101 surfaces. If indeed, the metal-diamond contact resistance depends on the crystallite orientation. then the contacts to a given grain may be ohmic, whilst the contact to an adjoining grain may be Schottky. The importance of this is that voltage drop across the Schottky barrier may cause the electric ftdd in the diamond to be less than the applied field. Since the collection of charges depends on the field extant in the region of charge creation and drift, such a reduction in local field would be observed as a reduction in charge collection efficiency. This would explain the IBIC! images in which one grain is bright and an adjoining grain is dark. The effect of reduced local electric field was invoked by Manfreclotti et al. [S] to explain why large regions of CVD films away from the contacts display low charge collection efficiency.
5. Concfusions ( 1) A conditioning
bias must be applied across the
detector of a voltage similar to that at which breakdown across the CVD diamond occurs before IBIC signals can be obtained. (2) Charge collection in CVD diamond is spatially inhomogeneous on the l-10 p.m scale. (3) Grain boundaries act as traps and recombination centres resulting in poor charge collection from these regions. (4) Adjoining grains can display large differences in charge collection efficiency. We beheve that this is at least partially attributable to the dependence of the electrical properties of the metal-diamond contact as a function of crystallite orientation. (5) The results suggest that control of the surface morphology of the CVD films (by using CVD techniques to produce highly oriented films) may result in improved charge collection efficiency by reducing the number of Schottky barrier type metal-diamond interfaces over the surface of the contact pad.
Acknowledgement This work was supported by an Australian Research Council grant. DRB would like to acknowledge the support of an Australian Postgraduate Award. Many thanks to A Howard, Imperial College, and E. Vittone, University of Turin, for fruitful discussions. RK gratefully acknowledges the hospitality accorded him at the University of Melbourne during his recent stay there as a Meigunyah Fellow.
References 111C. White, Nucl. Instr. and Meth. A 351 (1994) 217. S. Han, R.S. Wagner,
Appl. Phys. Lett. 68 (1996) 3016.
t3; L.S. Pan, D.R. Kania, S. Han. J.W. Ager 111. M. Landstrass. O.L. Landen and P. Pianetta, Science 255 (1992) 830.
I41 H.J. Leamy, J. Appl. Phys. 53 (1982) R51. 151M.B.H. Breese. P.J.C. King, G.W. Grime, F. Watt, J. Appt. Phys. 72 (6) (1992) 2097. F. Fizzotti, E. Vittone. M. Boero, P. Poles&o, S. Galassini, M. JakSiC, S. Fazinic. I. Bogdanovic, Nucl. Instr. and Meth. B tO0 (1995) 133. (71 C. Manfred~tti, F. Fizzotti, P. Polesello, E. Vittone. M. JakSiC, I. Bogdanovic and S. Fazinic. Mat. Res. Sot. Symp. Proc. Vol. 416, Diamond for Electronic Applications. eds. L. Dreifus. A.T. Collins, T. Humphreys. K. Das and P. Pehrsson. Boston, MA, Nov. 27-Dec. I (1995) p. 193.
161C. Manfredotti,
D.R. Beckman et al. /Nucl.
Instr. and Meth. in Phyx
[S] C. Manfredotti. F. Fizzotti, E. Vittone, P. Polesello. M. JakXZ, S. Fazinic and I. Bogdanovic, Proc. of EIJRODIAMOND ‘96, Torino, Italy, 17-20 Jan. (19961, Nuovo Cim. D, to be published. [9] M. JakSie. I. Bogdanovic. M. Bogovac, S. Fazinic, S. Galassini, K. Kovacevic, C. Manfredotti, E. Vittone, Nucl. Instr. and Meth. B I I3 (1996) 378. [IO] C. Manfredotti, F. Fizzotti, E. Vittone, M. Boero. P. Polesetto, S. Galassini, M. JakSiC, S. Fazinicand 1. Bogdanovic, Nucl. Instr. and Meth. B 109/110 (1996) 555.
Res. B 130 Cl9971 518-523
523
[Ill
D.N. Jamieson, M.B.H. Breese. A. Saint, Nucl. Instr. and Meth. B 85 (1994) 676. [ 121K.J. Gray, private communication. 1131 J.F. Ziegler, J.P. Biersack and I-J. Littmark, The Stopping and Range of Ions in Solids (Pergamon, New York, 1985). [I41 Y.T. Cheng, S.J. Lin, J. Hwang, Appl. Phys. Len. 63 (1993) 3344. [I51 M.W. Geis, D.D. Rathman, D.J. Ehrlich, R.A. Murphy, W.T. Lindley, IEEE Electr. Dev. Lett. 8 (1987) 341.
X. ELECTRONICS
AND PHOTONICS
ELSEVIER
and Methods in Physics Research B 130 (1997) 518-523
Spatially resolved imaging of charge collection efficiency in polycrystalline CVD diamond by the use of ion beam induced current D.R. Beckman a,*, A. Saint a, P. Gonon a, D.N. Jamieson ‘, S. Prawer a, R. Kalish b a Schoni of Physics, ~i~r~a~ulyti~ul Reseursh Centre. The ~ffiz,er.~~~of ~elb~)ur~e, P~~k~il~e~Vie 3052, ~u.~tralia b Physics Department rind Sotid State i~.~titute,Technion, Huffs 3?000. Israel
Abstract Diamond based detectors have potential applications in high energy physics experiments. These detectors can be fabricated from synthetic Chemical Vapour Deposited (CVD) polycrystalline diamond films. Previously it has been shown by the Turin group and their coworkers in Zagreb that it is possible to investigate the electrical characteristics of high quality polycrystalline CVD diamond films by Ion Beam Induced Current (IBIC). The present work describes IBIC images obtained using 2 MeV He+ irradiation of 2.50 km thick polycrystalline diamond films through a thin gold surface contact layer biased positively relative to the grounded rear surface of the film. In contrast to previous expe~ments the present spectra of collected charge display a clearly defined peak from the induced charge. Images obtained by separating these spectra into different regions of interest allow the identi~cation of regions in the sample of different charge collection efficiency. In particular the presence of some grains in which no charge collection appears possible and the reduction in charge collection efficiency at the grain boundaries is evident. 0 1997 Elsevier Science B.V.
1. Introduction Diamond is a promising material for the realisation of radiation hard detectors for nuclear radiation [l]. Such detectors require large areas and fairly thick samples. Diamond slabs grown by Chemical Vapour Deposition allow the practical and cost-effective realisation of such detectors. These slabs, however, are comprised of many diamond crystallites of different orientations and separated by grain boundaries. These may affect the electrical properties of the material by acting as traps and/or recombina-
* Corresponding author. [email protected].
Fax:
+61-3-934’7-4783; email:
tion centres, thus degrading the charge collection efficiency [2]. Most techniques for monitoring charge collection cannot provide spatial and depth resolved images and thus cannot provide info~ation which will identify those regions and features in the poly~rystalline CVD diamond which are responsible for charge recombination and trapping. Pulsed UV photoexcitation has been used so far mainly to probe the time dependence (or lifetime) of the carriers in a shallow region near the surface (since UV light is heavily absorbed by the diamond) [3]. Electron Beam Induced Current (EBIC) [4] does offer spatial resolution; however, the probing depth is only of the order of a few microns, and it is not generally possible to
0168-583X/97/$17.~ 0 1997 Ekevier Science B.V. Ail rights reserved. PII SOl68-583X(97)~0243-7
D.R. Beckman ef al. /Nucl.
Instr. and Meth. in Phys. Res. 3 I30 ~19971518-523
probe beneath electrical contacts. In contrast to these, ion beam induced current (IBIC), generated by a spatially scanned focused energetic ion beam, can offer both high spatial and depth resolution [5]. By adjusting the ion energy and species it is possible to select the depth from which charge is collected. In addition IBIC offers the possibility of monitoring detector performance under realistic irradiation conditions similar to those found during detector operation, and finally IBIC also offers the possibility of the in-situ assessment of the durability of the detector under ion irradiation, information which is crucial for the determination of the lifetime of diamond nuclear detectors. Much pioneering work in this regard has been performed by Manfredotti and his group using MeV proton beams and the reader is referred to their many excellent publications for the background on IBIC as applied to diamond [6-IO]. In the present work, we report on the results of our initial IBIC investigation of high quality CVD diamond irradiated with 2 MeV He+. The advantage of such a beam over protons is that the He+ ions’ stopping power is several times greater at the surface than that of a proton beam and so generate more charge carriers near the sample surface where we find they can be efficiently collected, producing a superior signal-to-noise ratio. We show that the charge collection is spatially very inhomogeneous on the micron scale, and suggest that this may be a consequence of the effect of diamond facets on the electrical properties of the metal-diamond electrical contact.
519
2. Experiment IBIC measurements were ~rfo~ed using the Micro Analytical Research Centre (MARC) nuclear microprobe at the University of Melbourne [ 1 I]. The sample used for this investigation was a free standing 250 pm thick detector-grade Norton CVD diamond film measuring about 6 mm in diameter. The collection distance in the film was of the order of 20-40 pm [ 121. MO ( I5 nm)/Au (60 nm) electrical contact pads, measuring about 0.5 X 0.5 mm2 , were evaporated onto the growth side of the CVD diamond in a grid-like arrangement. Gold wires were then wire bonded to a number of pads. The diamond was mounted (substrate side down) using Ag paint to a gold pad on a standard chip holder. The experimental setup is depicted in Fig. 1. Pad A was positively biased while both pad B and the back of the diamond (C) were grounded. A 2 MeV He+ beam, focussed to a spot of around 1 pm, was initially scanned over an area 3 X 3 mm2; this scan size was later reduced to 300 X 300 pm2 in order to focus in on the edge of a pad. Charge collected at pad A was converted into a voltage signal and recorded using a multichannel analyser. By scanning the beam in X, y, it was possible to produce spatially resolved images of the charge collection efficiency. The projected range for 2 MeV He+ ions in diamond is 3.4 f 0. I Frn [ 131. We found that it was necessary to “condition” the detector before IBIC signals could be obtained. This was accomplished by applying a bias of the
572 Spectroscopic
Amplifier
foci
Fig. I. Experimental setup for IBIC measurements performed on a 250 pm thick Norton CVD diamond film using a 2 MeV He+ beam. “A” and “B” are the MO (15 nm>/Au (60 nm) electrical contact pads which were evaporated onto the growth side of the diamond while “C” is the grounded back surface. IBIC signals were only obtained when a positive bias (with respect to B and C> was applied to pad A.
X. JZLECTRONICS
AND PHOTONICS
520
D.R. Becknzun et al. /N&tcl. Insrr. and M.&I. in P&s.
order of 800 V. Once IBIC signals were observed, the voltage could be reduced and IBIC images could still be obtained at biases as low as 100 V. However, prior to conditioning, no IBIC signal at all could be obtained for biases below 400 V. The L‘conditioning” bias of 800 V is close to that at which breakdown across the CVD diamond was observed to occur. It is important to note that IBIC signals were only obtained for positive bias fie pad A positive with respect to C). We did not observe any IBIC signals if the bias on A was negative with respect to C (see Fig. I).
3. Results Fig. 2 shows the typical output of the charge collected from a 300 X 300 pm2 scan of the Norton CVD film with an applied bias of 700 V (i.e. about 3 V/pm). The charge collected and converted into a voltage signal is proportional to the channel number. Hence it is possible, in principle, to convert channel number to charge collection efficiency [s], although this has not been done here. Fig. 2 is thus a spectrum of the relative charge collection efficiency of the detector integrated over the whole of the contact pad. The lowest few channels have been omitted from the spectrum since noise from the experiment is accumu-
E 4000 5 s 3000 2000 IOOO0
0
20
40
60
80
100
120
140
160
180
Fig. 2. 2 MeV He+ IBIC spectrum obtained from a Norton CVD film with an applied bias of + 700 V, integrated over the whole of the contact pad. The charge collected is proportional to channel number which is directly proportional to charge collection efficiency. The markers at the top indicate the regions of interest in collection efficiency which were used to extract the images shown in Fig. 4.
Res.
B I30 (1997) 818-523
Channel Fig. 3. 2 MeV He+ IBIC spectra obtained from a Norton CVD film with increasing applied bias voltage. The spectra have been horizontally translated relative to each other by multiples of 200 channels for clarity and have been normalised to the incident beam flux. These spectra were collected using different scan sizes (a) and (b) are from a 3 X 3 mm2 scan of the CVD diamond. (c). (d) and (e) are from a 600X 600 pm’ scan and (f) is from a 300X308 &m2 scan). The shape of the spectra can be seen to change with applied voltage, with a double peak structure nppcaring only for bias voitages over 600 V.
lated in them. Two peaks appear in the spectrum. Fig. 3 shows that the shape of the charge collection curve changes with applied bias, with the double peak structure appearing only for an applied bias in excess of 600 V. Previous measurements have shown counts versus collection efficiency to be a monotonically decreasing curve with no discernible peaks (similar to those shown here for biases up to 500 V>. By dividing the overall spectrum into different regions of interest, as indicated in Fig. 2, the images presented in Fig. 4 were obtained. These images show the charge collection efficiency for a 300 x 300 pm2 area of the diamond with a section of the gold electrical contact pad appearing in the lower half of each image. Fig. 4a shows that the highest charge collection efficiency originated inside select grains, with the maximum efficiency occurring in the centres of these grains. Fig. 4b shows that regions of moderate charge coilection efficiency are more uniformly distributed in the diamond, but nevertheless the grain structure is still clearly observable. Grain boundaries were primary sources of low charge collection efficiency, as shown in Fig. 4c. Very low charge collection efficiency, as presented in Fig. 4d, was recorded for charges generated outside the met-
allised region of the diamond from up to a distance of 45 pm from the edge of the contact pad. The difference in charge collection efficiency for carriers generated outside the metallised region of the diamond and those generated beneath the gold electrical contact pad can be explained in terms of the different physical processes occurring. In the first case carriers would need to diffuse across numerous grain boundaries in order to reach the electrical contact for detection - while crossing these grain boundaries, many carriers would be trapped, hence reducing the overall collection efficiency. Carriers created beneath the gold pad, on the other hand, would drift under the applied field predominantly along the grain in which they were created towards the electrical contact pad. thereby inducing charge in the contact. By carefully selecting the bright regions of each image in Fig. 4 and extracting the corresponding spectra, the individual contributions from each part of the diamond, to the overall spectrum shown in
Fig 4. Images correspondrng to the collection efficiency regions of interest indicated in Fig. 2. Scan size was 300X300 hm’. A section of the gold electrical contact pad appears in the lower hlllf of each image. (a) shows regions of the highest charge collection efficiency, (h) shows regions of moderate collection efficiency, (c> shows regions of low collection efficiency, corresponding primaiiy to grain boundaries while (d) shows regions of very low charge collection efficiency which can be seen to be primarily off the contact pad, corresponding to diffusion of carriers from the nonmetallised region of the diamond to the contact pad. Coiour versions of these images can be found at http://www.ph.unimelh.edu.au/ - drb/ibic.htmi.
25000 m s 200008 2 15000,” 2: 100005000 -
OO Fig. 5. Spectra extracted from the bright regions of each of the images appearing in Fig. 3 and normalised to area. Curve {rt) shows the signal due to grains which recorded the highest charge collection efficiency, (b) shows the moderate collection efficiency response, (c) shows the grain boundary efficiency and (d) shows the charge collection efficiency for carriers generated outside the metctllised region of the diamond. The overnil spectrum presented in Fig. 3 is the sup~~osition of these individual signals.
Fig. 2, were obtained. These were normatised with respect to area and are presented in Fig. 5. Curve (a) is the signal due to the grains which recorded the highest charge collection efficiency. Curve (b) is moderate efficiency. Curve (c) shows the efficiency of the grain boundaries while curve (d) shows the collection efficiency for charge carriers generated outside the metallised region of the diamond. The overall spectrum in Fig . 2 is the weighted superposition of these individual signals, with the first peak of the doubfe peak structure appearing to be due primarily to carrier diffusion from the non-metallised region of the diamond and the second being due to carrier drift from under the gold electrical contact pad. Thus, the shape of the overall charge collection spectrum could vary for different polycrystalline diamond samples depending on the relative quantities of high and low collection efficiency grains within the film. The IBIC maps collected were compared to optical and SEM images of the same gold pad on the diamond. A strong correlation was found between those areas of high charge collection efficiency and grains whose surfaces were aligned parallel with the substrate, although areas of high charge collection efficiency were found in other regions as well.
X. ELECTRONICS
AND PHOTONICS
4. Discussion From the data presented above we can conclude that the charge collection efficiency in polycrystalline CVD diamond is very spatially inhomogeneous and that the observed collection efficiency spectrum will depend on which sections of the sample are probed by the beam. The low charge collection efficiency as observed here in the grain boundaries is to be expected. However, the puzzle remains as to why some grains are of such superior charge collection efficiency compared to others. In particular why is there such a large contrast even between adjoining grains (see Fig. 4a). One possible explanation relates to the effect of crystal orientation on the nature of the diamond-metal interface. Cheng et al. [ 141 found that Pt electrical contacts made on diamond displaying predominantly (100) morphology were Schottky in character, whilst similar contacts made on (111) diamond showed ohmic behaviour. These differences persisted despite cam being taken to ensure a similar surface treatment for the (100) and (ill) surfaces. The phenomenon may be related to the difference in negative electron affinity of the (111) and (100) surfaces as reported by Geis [ 151. The Norton films used in the experiments showed a rough growth surface dominated by (100) and (1101 surfaces. If indeed, the metal-diamond contact resistance depends on the crystallite orientation. then the contacts to a given grain may be ohmic, whilst the contact to an adjoining grain may be Schottky. The importance of this is that voltage drop across the Schottky barrier may cause the electric ftdd in the diamond to be less than the applied field. Since the collection of charges depends on the field extant in the region of charge creation and drift, such a reduction in local field would be observed as a reduction in charge collection efficiency. This would explain the IBIC! images in which one grain is bright and an adjoining grain is dark. The effect of reduced local electric field was invoked by Manfreclotti et al. [S] to explain why large regions of CVD films away from the contacts display low charge collection efficiency.
5. Concfusions ( 1) A conditioning
bias must be applied across the
detector of a voltage similar to that at which breakdown across the CVD diamond occurs before IBIC signals can be obtained. (2) Charge collection in CVD diamond is spatially inhomogeneous on the l-10 p.m scale. (3) Grain boundaries act as traps and recombination centres resulting in poor charge collection from these regions. (4) Adjoining grains can display large differences in charge collection efficiency. We beheve that this is at least partially attributable to the dependence of the electrical properties of the metal-diamond contact as a function of crystallite orientation. (5) The results suggest that control of the surface morphology of the CVD films (by using CVD techniques to produce highly oriented films) may result in improved charge collection efficiency by reducing the number of Schottky barrier type metal-diamond interfaces over the surface of the contact pad.
Acknowledgement This work was supported by an Australian Research Council grant. DRB would like to acknowledge the support of an Australian Postgraduate Award. Many thanks to A Howard, Imperial College, and E. Vittone, University of Turin, for fruitful discussions. RK gratefully acknowledges the hospitality accorded him at the University of Melbourne during his recent stay there as a Meigunyah Fellow.
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X. ELECTRONICS
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