1078 Diamond and Related Materials, 2 (1993) 1078-1082
Aerosol doping of flame-grown diamond films
Kathleen Doverspike and James E. Butler Naval Research Laboratory, Washington, DC 20375-5000 (USA)
Jaime A. Freitas, Jr. Sachs~Freeman Assoc. Inc., Landover, MD 20785-5396 (USA)
Abstract
Diamond films have been deposited using a pre-mixed oxy-acetylene flame onto cooled substrates. Doping of these flame-grown diamond films was achieved by introducing: aerosol droplets containing the dopant species into the gas flow. The aerosol was generated by a commercial atomizer and was entrained into the oxygen flow before the flame front leading to a uniform and reproducible flame. Micro-Raman measurements show a decrease in the background fluorescence of the boron-doped films as Compared with samples containing other dopants. In addition, low-temperature photoluminescence measurements of both the boron- and silicon-doped films show a significant decrease in the defect bands associated with nitrogen vacancy complexes that are often observed in flame-grown diamond films. The boron-doped diamond samples were also characterized by photoluminescence using the UV 351 nm laser line of an argon ion laser and show both green and/or blue emission.
1. Introduction
The combustion technique has been demonstrated at several laboratories to be viable for obtaining high- quality diamond films with a high growth rate (greater than 100 ~tm h -1) [1-10]. In this paper, we report the growth of diamond films that have been deposited using a pre-mixed oxy-acetylene flame onto cooled substrates. When the oxy-acetylene torch is operated in a fuel:rich mode, unburnt hydrocarbons, reactive intermediates, CO and H2 form a region (feather) which can cause diamond growth on appropriate substrates. This feather region is bounded by another flame front caused by oxygen diffusion from the surrounding atmosphere.
Doping of these flame-grown diamond films was achieved by introducing aerosol droplets containing the dopant species into the gas flow. The aerosol was generated by a commercial atomizer and was entrained into the oxygen gas flow before the flame front. The oxygen flow through the atomizer combined with the primary oxygen flow at the torch handle. The acetylene combined with both oxygen flows in the mixing chamber of the torch, which led to well-mixed gases exiting the torch at the nozzle. Thus by combining the atomizer and a commercial pre-mixed oxy-acetylene torch, we had a viable method of obtaining, not only a mixture of oxygen and acetylene exiting the torch, but also sub- micrometer droplets containing the dopant species. In addition to the combustion products and intermediates normally present in the feather region, we had a flux of the potential dopant species present in the immediate
environment of the growing diamond film. This new method of doping flame-grown diamond films is versatile in that both volatile and non-volatile dopants can be used, therefore allowing th e investigation of a wide range of dopant species that could not easily be explored with other methods. The dopant concentration in the dia- mond films can be varied by changing the concentration of the dopant solution or the aerosol percentage in the oxygen stream~ An advantage of this technique is that non-hazardous dopant species can be easily used instead of sources that are highly toxic, such as silane, phosphine and diborane.
In this study, methanol was used as the solvent for the dopants investigated (boron and silicon). Boron was chosen, not only because it is the only established dopant (p-type) producing semiconducting diamond [11-15], but also because of the color change (clear to blue) that occurs as a result of the shallow acceptor level of boron. Silicon doping demonstrates the versatility of this aerosol method, and in a controlled manner provides a range of samples for studying the 1.681 eV zero phonon line due to the silicon defect center [16]. Others have observed this silicon defect center in their diamond films by ion implantation of diamond by silicon, silicon incorpora- tion into the diamond film from the silicon substrate, or as a result of plasma etching of the silica walls of the reactor [17-19].
K. Doverspike et al. / Aerosol doping of flame-grown diamond 1079
2. Experimental details
All of the diamond films were synthesized using a pre- mixed oxy-acetylene welding torch with a nozzle diame- ter of 1.17 mm. The flow rates of the oxygen (99.975%) and acetylene (99.6%) were controlled by mass flow controllers with the total flow rate being held constant at approximately 9 standard liters per minute (SLM). The polycrystalline films were grown on molybdenum screws that were placed in a threaded hole in a water- cooled copper block. The temperature of the screw was controlled by the depth of penetration into the copper block and was monitored using a two-color pyrometer [7]. In order to enhance the nucleation, the surface of the molybdenum screw was polished with 600 mesh silicon carbide followed by 1 pm diamond paste, and then ultrasonically cleaned in acetone and methanol. Because of the large difference in the thermal expansion of diamond and molybdenum, the film delaminates as the substrate cools.
A commercial atomizer (TSI Instruments) was used to produce the aerosol containing the dopant species. Oxygen was forced through a small (0.343 mm) hole to form a high velocity jet, while the liquid was fed into the atomizer by pneumatic suction. The liquid was atomized by the high velocity oxygen jet stream. The larger droplets hit the wall of the atomizer and return to the bottle containing the dopant solution, while only the smallest droplets remain suspended in the gas stream. The oxygen gas flow through the atomizer was 1.30-1.40 SLM which was then combined with the pri- mary oxygen flow entering the torch (3.30-3.50 SLM) at the torch handle. The acetylene (4.55 SLM) was mixed with the oxygen in the mixing chamber.
The excitation wavelength used in the micro-Raman spectrometer was 514.5 nm and the spatial resolution was less than 1 ~tm [20]. The photoluminescence experi- ments were carried out at 6 K in a liquid helium continuous flow cryostat using both 488 nm [21] and 351 nm [22] laser excitation with a spot size of 100 I,tm. Unless otherwise noted, the photoluminescence spectra were taken at the central region of the samples. Scanning electron microscopy (SEM) was performed on these samples using a Cambridge $200 Instrument.
3. Results
A polycrystalline reference sample was grown at 920 + 20~'C in which the atomizer was used, but the methanol solution did not contain any dopant species. The optimum flow rates were 4.55 SLM for the acetylene flow and 3.33 SLM for the primary oxygen flow with 1.30 SLM as the oxygen flow through the atomizer. The photoluminescence spectrum of the reference sample,
excited with the 488 nm laser line (488 nm line), is shown in Fig. 1.
The second set of samples discussed involves boron doping. Boric acid dissolved in methanol was used as the dopant solution in the atomizer. Several boron films were made using solutions ranging from 8.4 x 10 4M to 1.4 x 10 2 M. The flow rates for the polycrystalline films were similar to those used for the reference sample and the substrate temperature was 920___ 20°C. The central portion of the boron-doped samples, approxi- mately 3 4 mm in diameter, was blue in color and a white ring was observed around the outside edge of the blue central region. Nearly all the samples grown in the open atmosphere (both undoped and doped) display a dark ring on the outer edge. Figure 2 shows a micro-Raman spectrum of a boron-doped sample
4 I I I I I
(D
C
o 0
E~ 2
(,9 c "
¢ -
- - 1 __1
a_
b
b* c d
2.4 2.2 2.0 1.8 ] .6 1.4 Energy (eV)
Fig. 1. Photoluminescence (PL) spectrum (488 nm line) of a polycrys- talline reference sample prepared using only methanol in the atomizer.
8000 1332.5
6000 -
Z 4ooo i
2000
_ . L
o I
7
1600 1400 1200 1000 800 600 iVAVENUMBER SHIFT
Fig. 2. Micro-Raman spectrum of a boron-doped sample (1.4 x l0 2 M solution).
(1.4 x 10-2 M solution). A photoluminescence spectrum, excited with the 488 nm laser line, of this boron-doped sample is shown in Fig. 3. Photoluminescence spectra, taken using the 351 nm laser line (351 nm line), of a lightly doped sample (8.4 x 10 4M solution) and a heavier doped sample (1.4 x 10 -2 M solution) are shown in Figs. 4 and 5 respectively.
The third set of samples was grown in flames doped with silicon species at temperatures in the range (900-1000) + 20°C. Tetramethylsilane was dissolved in methanol (7.0 x 10 -4 M) and used as the source solution. Polycrystalline samples were grown using similar flow rates as the reference sample and the boron-doped samples. A photoluminescence spectrum, excited with the 488 nm laser line, for a silicon-doped sample grown at 965 + 20 °C is shown in Fig. 6.
5
16
83 v
t - "
E
d I
I I I I I
J I
2.4
d
2.2 2.0 1.8 1.6 1.4 Energy (eV)
Fig. 3. Photoluminescence (PL) spectrum (488 nm line) of a boron- doped sample (l.4 x 10 -z M solution).
I I I I I I
o 14
tO E 12 0
~ 8
c 6 C
4 . . J
13_ 2
1080 K. Doverspike et al. / Aerosol doping of flame-grown diamond
I I I I I I
2.4 2.2 2.0 1.8 1.6 1.4 Energy (eV)
Fig. 4. Photoluminescence (PL) spectrum (351 nm line) of a lightly doped boron sample showing green emission.
7 i I I I I
~ 6
84
~ 3
d
o_ 1
0 i i i i 3.4 3.2 3.0 2.8 2.6 2.4
Energy (eV)
Fig. 5. Second-order photoluminescence (PL) spectrum (351 nm line) of a heavily doped boron sample showing blue emission.
5 j I I I I
C
o ° 3 b *
E
n
0 i I I I I 2.4 2.2 2.0 1.8 1.6 1.4
Energy (eV)
Fig. 6. Photoluminescence (PL) spectrum (488 nm line) of a silicon- doped sample.
4. Discussion
Before attempting to dope the diamond films by this aerosol technique, the effect on the diamond quality of adding pure methanol to the oxy-acetylene flame was studied. This was carried out in order to determine the growth conditions that are favorable for diamond growth when methanol is added to the flame. We were able to obtain high-quality diamond, as indicated by the color (white) and the linewidth of the Raman peak. Figure 1 shows the photoluminescence spectrum (488 nm line) of the methanol reference sample and can be used to monitor the quality of the film by examining the diamond first-order phonon line shape (first-order Raman scattered line) and the zero phonon line and phonon replicas of the defect bands [23]. The line
K. Doverspike et al. / Aerosol doping (~['flame-grown diamond I081
labeled "b" is the diamond first-order phonon line, and "b*" is indicative of amorphous carbon present in the sample. The line labeled "a" is a nitrogen complex band (N-N N). The line "c" at 2.16 eV is believed to be due to a double vacancy nitrogen (N V-V) complex, and the line "e" at 1.95 eV is associated with a vacancy-nitro- gen (N-V) pair. The broad band labeled "d" is due to the uncorrected spectrometer response. The nitrogen defect bands present in this reference sample are charac- teristic of films grown in the open atmosphere. The nitrogen incorporation into the films is believed to be from the diffusion of room air into the fuel-rich feather where diamond growth occurs. We have previously demonstrated that the nitrogen content in the films can be reduced by operating the torch in an enclosed cham- ber with a controlled atmosphere (argon oxygen) [24].
Boron doping resulted in crystallites that showed very small, well-faceted faces by SEM (typically, less than 5 gin). We have observed that the grain size of polycrys- talline films is often dependent on conditions such as the O= to CzH 2 ratio and the temperature, but in general the boron-doped samples show a smaller grain size than the undoped films. The dopant species containing boron are carried into the flame with the oxygen, exit through the torch, and account for the central blue color of the polycrystalline film. The white ring outside the inner blue region is probably the result of oxygen diffusion from the environment, diluting the boron species present, or possibly reacting with the boron to form stable BxOy compounds which would not be incor- porated into the diamond film. The dark ring around the outside portion of the film has previously been shown to contain lower quality diamond and also an increased amount of nitrogen [20]. This is probably the result of room air entrainment into the flame combined with sharp temperature changes at the edge of the screw. Figure 2 shows a micro-Raman spectrum of the heavier boron-doped polycrystalline film which reveals a low background fluorescence and no features associated with graphite or amorphous carbon. Figure 3 shows the photoluminescence spectrum (488 nm line) of this boron- doped sample. The first-order diamond phonon line "b" is very narrow and intense which is indicative of high- quality diamond. There is also evidence of only a very small amount of amorphous carbon "b*"present in the film. The presence of this amorphous carbon peak in the photoluminescence spectra may be related to the fact that we are probing about 100 lain in contrast with the micro-Raman spectrum where we are probing indivi- dual grains. This may lead to a larger contribution of intergranular regions which may contain amorphous carbon. The spectrum shows no evidence of the nitrogen defect bands (N-V or N V-V) that are normally seen in the undoped reference films. Photoluminescence spectra were also taken towards the outer edge of the
blue central region. The first-order diamond phonon line is still very narrow and intense, but the band due to the N-V-V complex is evident but very weak. There- fore a significant decrease in the nitrogen vacancy com- plexes is observed in the boron-doped region of the films. When a photoluminescence spectrum is taken in the white region outside the central blue area, the diamond phonon line is significantly smaller than the defect bands which now dominate the spectrum. Both the N-V and N-V V defect bands are very intense which is consistent with a larger amount of nitrogen in the outer regions of the film because of room air entrainment into the flame [25].
The spectra shown in Figs. 4 and 5 are photolumin- escence spectra of two of the boron-doped samples taken using the 351 nm laser line. Both show emission in the energy region 2.0 3.4 eV which is designated as band A emission in natural and synthetic diamond [26]. Figure 4 shows the photoluminescence spectrum of the lightly doped boron sample which appears very light blue in color to the naked eye. The spectrum shows a very intense green emission from about 2.0 eV 2.4 eV. There is also evidence of an intense red emission at 1.8 eV which has not yet been identified. Dean [27] has pro- posed that the mechanism of band A may be explained by the donor acceptor pair recombination process. It is speculated that close donor acceptor pairs give rise to higher energy blue emission compared with the lower energy green emission which may arise from more dilute donor-acceptor pairs. The green emission observed in Fig. 4 may be indicative of regions where the donor acceptor pairs exhibit large internal separations and a relatively long lifetime against radiative recombination. This would be consistent with our sample if both the donor (nitrogen) and acceptor (boron) are present in low concentrations. Figure 5 shows the photoluminescence spectrum, taken in the second order, of a heavier boron- doped sample that was dark blue in color and shows blue emission (2.4 3.4 eV), which is much weaker than the green emission in Fig. 4. This sample also shows some green and red emission which can be excluded using an appropriate glass filter. This higher energy blue emission is observed only in the heavier doped sample, and occurs in the same spectral range as the blue band A emission. This suggests that this blue emission may be due to recombination of donor acceptor pairs with smaller internal separation.
The photoluminescence spectrum of a silicon-doped film is shown in Fig. 6. It is taken from the center of the film and clearly shows the silicon related defect center "f" at 1.68eV and the phonon replicas on the low- energy side. This sample was grown at 965 + 20 'C and shows a broad weak feature associated with the double vacancy nitrogen complex band "c". A photolumin- escence spectrum of a film grown at 920 _+ 20 ~C shows
1082 K. Doverspike et al. / Aerosol doping of flame-grown diamond
a weaker silicon defect line and much stronger double vacancy-nitrogen and vacancy-nitrogen complex bands. We have observed that, in the silicon doping experiments, higher temperatures appear to favor a larger incorpora- tion of silicon (by monitoring the silicon defect line at 1.68 eV in the photoluminescence spectra), and a corre- sponding decrease in the incorporation of nitrogen- vacancy complexes. Photoluminescence spectra that were taken by progressively moving out from the center of these films show a similar trend in the nitrogen defect bands as was seen in the boron-doped samples. Moving towards the outer portion of the sample results in an increase in the nitrogen defect band intensities and a corresponding decrease in the silicon defect line intensities.
5. Conclusions
We have demonstrated that the aerosol method of doping flame-grown diamond films is a versatile and useful technique that has much potential. We have successfully incorporated boron and silicon into the diamond films and have used photoluminescence spectroscopy to characterize the nitrogen defect bands present in these films. There appears to be a significant decrease in the nitrogen defect bands when doping with boron, and also when doping with silicon at higher temperatures. We have previously demonstrated that the nitrogen content in the films can be reduced by operating the torch in an enclosed chamber with a controlled atmosphere (argon-oxygen) [24], and we plan to con- tinue these doping studies in the controlled atmosphere in the near future. We are also using this technique and currently investigating potential n-type dopants such as phosphorus, sodium and lithium.
Acknowledgements
This work was supported in part by the Office of Naval Research and the Defense Advanced Research Projects Agency. We would also like to acknowledge Dan Vestyck for the micro-Raman measurements, Bob Gorman for the machining, and Steve Binari and Kurt Gaskill.
References
1 Y. Hirose and N. Kondo, Program and Book of Abstracts, Japan Applied Physics 1988 Spring Meeting, March 29, 1988, p. 95.
2 L. M. Hanssen, W. A. Carrington, J. E. Butler and K. A. Snail, Mater. Lett., 7 (1988) 289.
3 Y. Hirose, Proc. 1st Int. Conf. on New Diamond Science and Technology, October 1988, Tokyo, Japan, 1988 p. 51.
4 W. A. Carrington, L. M+ Hanssen, K. A. Snail, D. B. Oakes and J. E. Butler, Metall. Trans., 20A (1989) 1282.
5 W. Yarbrough, M. A. Stewart and J. A. Cooper, Surf Coat. Technol., 39/40 (1989) 241.
6 P. Kosky and D. S. McAtee, Mater. Lett., 8 (1989) 369. 7 L. M. Hanssen, K. A. Snail, W. A. Carrington, J. E. Butler,
S+ Kellogg and D. B. Oakes, Thin Solid Films, 196 (1991) 271. 8 Y. Tzeng, C. Cutshaw, R. Phillips, T. Srivinyunon, A. lbrahimand
and B. H. Loo, Appl. Phys. Lett., 56 (2) (1990) 134. 9 M. A. Cappelli and P. H. Paul, J. Appl. Phys., 67 (5) (1990) 2596.
10 K. V. Ravi, C. A. Koch, H. S. Hu and A. Joshi, J. Mater. Res., 5 (11) (1990) 2356.
I 1 D. J. Poferi, N. C. Gardner and J. C. Angus, J. Appl. Phys., 44 (4) (1973) 1428.
12 A. T. Collins and E. C. Lightowler in J. E. Field (ed.), The Properties of Diamond, Academic Press, London, 1979, Chapter 3, pp. 79-105.
13 B. V. Spitsyn, L. L. Bovilov and B. V. Derjaguin, J. Cryst. Growth, 52 (1981) 219.
14 K. Okano, H. Naruki, Y. Akiba, T. Kurosu, M. Iida, Y. Hirose and T. Nakamura, Jpn. J. Appl. Phys., 28 (1989) 1066.
15 K. Nishimura, K. Das and J. T. Glass, J. Appl. Phys., 69 (5) (1991) 3142.
16 T. G. Bilodeau, J. A. Freitas, Jr., K. Doverspike, U. Strom and R. Ramesham, Diamond Relat. Mater., 2 (1993) 699.
17 V. S. Vavilov, A. A. Gippius, A. M. Zaitsev, B. V. Derjaguin, B. V. Spitsyn and A. E. Alesanko, Soy. Phys. Semicond., 14 (1980) 1078.
18 C. D. Clark and C. B. Dickerson, J. Phys. Condensed. Matter. 4 (1992) 869.
19 A. T. Collins, M. Kamo and Y. Sato, Mater. Res. Soc. Symp. Proc., 162 (1990) 225.
20 D. B. Oakes, J. E. Butler, K. A. Snail, W. A. Carrington and L. M. Hanssen, J. Appl. Phys., 69 (4) (1991) 2602.
21 J. A. Freitas, Jr., J. E. Butler and U. Strom, J. Mater. Res., 5 (11) (1990) 2502.
22 J. A. Freitas, U. Strom and A. T. Collins, Diamond Relat. Mater., 2 (1993) 87.
23 J. A. Freitas, Jr., U. Strom, K. Doverspike, C. M. Marks and K. A. Snail, Mater. Res. Soc. Syrup. Proc., 242 (1991) 139.
24 K. Doverspike, J. E. Butler and J. A. Freitas, Jr., Mater. Res. Soc. Symp. Proc., 242 (1991) 37.
25 J. A. Freitas, Jr., U. Strom, J. E. Butler and K. A. Snail, Proc. 2nd Int. Conf. on New Diamond Science and Technology, MRS Proceed- ings, September 1990, Washington, DC, p. 723.
26 A. T. Collins, Proc. 2nd Int. Symp. on Diamond Materials, Electro- chemical Society Proceedings, May 1991, Washington, DC, Electro- chemical Society, Pennington, N J, p. 408.
