- Email: [email protected]
Blistering in diamond implanted with hydrogen ions
ARTICLE IN PRESS
Vacuum 78 (2005) 273–279 www.elsevier.com/locate/vacuum
Blistering in diamond implanted with hydrogen ions R.A. Khmelnitskiya,, E.V. Zavedeevb, A.V. Khomichc, A.V. Gooskova, A.A. Gippiusa b
a P.N. Lebedev Physical Institute, Leninsky prospect 53, 119991 Moscow, Russian Federation Natural Science Studies Center of A.M. Prokhorov General Physics Institute, Vavilova str. 38, 119991 Moscow, Russian Federation c Institute of Radio Engineering & Electronics, 1 Vvedenskogo sq., 141190 Fryazino, Moscow district, Russian Federation
Abstract Annealing of diamond implanted with hydrogen results in ‘‘island’’ graphitization and formation of bubbles (blistering). Bubbles are formed at the depth of the graphitized layer and filled with hydrogen. Based on elasticity theory, the pressure of gas and the amount of gaseous matter in the bubble as well as tensile stress in the upper part of the bubble are calculated. r 2005 Elsevier Ltd. All rights reserved. Keywords: Diamond; Ion implantation; Hydrogen; Graphitization; Blistering
1. Introduction Blistering, i.e. formation of gas-filled cavities (bubbles) in solid matrix is typical of high-dose implantation of light ions (H, D, He) into semiconductors, such as Si, Ge, GaAs [1] and other materials. Blistering was thoroughly studied in silicon and was even employed in the technology of silicon on insulator (SOI) structures [1,2]. Bubbles are formed (after annealing) at the depth
Corresponding author. Tel.: +7 95 132 69 53; fax: +7 95 135 79 50. E-mail address: [email protected] (R.A. Khmelnitskiy).
corresponding to the maximum of concentration of the implants. Blistering in diamond was mentioned in [1,3] and briefly described in our papers [4,5]. It was established that some features of blistering in diamond are quite different from those found in other materials. First, blistering in diamonds is observed only in hydrogen-implanted samples; no such effect was found in deuterium- or heliumimplanted samples. Second, due to metastability of diamond, it transforms into graphite under irradiation and heat treatment. For H+ implantation, the graphitized layer is not uniform but consists of oval ‘‘islands’’ (this effect is much less pronounced in D+ implanted samples). It means that in this case blistering occurs in the material, which is a
0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.01.038
ARTICLE IN PRESS 274
R.A. Khmelnitskiy et al. / Vacuum 78 (2005) 273–279
mixture of diamond and graphite phases. As a matter of fact, in H+ implanted diamond, both blistering and high-temperature graphitization are caused by disruption of relatively stable associations of hydrogen with radiation defects and releasing the lattice defects and free hydrogen atoms. The former stimulates the process of graphitization while the latter can diffuse and form in the disordered layer (and only in this layer) the hydrogen-filled bubbles of peculiar shape with lateral dimensions two orders of magnitude larger than the thickness [4,5]. In this paper, detailed data on the ‘‘island’’ graphitization and blistering in H+ implanted diamond are presented. The gas pressure in the bubbles and mechanical stress in their thin film upper part are estimated. The nature of hydrogencontaining defects is discussed.
2. Experimental Polished (1 0 0) samples of natural monocrystalline diamond were implanted at room temperature. In most cases, single energy (350 keV) implantation with doses in the range 2212 1016 cm2 was performed. Implanted areas of 0:75 1:75 mm2 in size were specified using masks. Samples for infrared spectroscopy were prepared using H+ implantation with a set of 18 energies in the range 652350 keV with total dose of 2:45 1017 cm2 : In this way, at the depth 0:321:8 mm; we obtained the layer uniformly doped with hydrogen at the concentration of 1:6 1021 cm3 (10%) which is slightly less than that at 1 at%. Annealing of the implanted samples was performed for 1 h at temperatures up to 1600 1C in a graphite furnace. After annealing, the samples were etched in hot solution K2Cr2O7+H2SO4 to remove very thin uniform graphite layer. Optical absorption in the range 2202870 nm was measured with the spectrophotometer ‘‘Specord M400’’ (Carl Zeiss, Jena). Measurements of infrared absorption in the range 2:5250 mm intended to monitor the formation of C–H bonds were made with the spectrophotometer ‘‘Specord M80’’.
Microphotography of the samples both in transmitted and reflected light was performed after several characteristic stages of annealing. Quantitative data on the topography of the samples surface were obtained using interferometric microscope ‘‘Zygo New View 5000’’. For detailed studies of the surface, atomic force microscopy was employed.
3. Results and discussion 3.1. Hydrogen-related defects and effects Hydrogen, as chemically active, passivating impurity tends to form some associations with other impurities and/or defects in diamond. This tendency must be particularly strong in ionimplanted layers with high concentration of radiation defects. We undertook some attempts to find direct evidence of formation of hydrogenrelated defects. First of all it was natural to suggest that due to a large amount of broken C–C bonds in ion-implanted layers, C–H bonds can be formed similar to Si–H bonds observed in hydrogendoped silicon. C–H bonds can be monitored quantitatively by infrared absorption: various modes and types of bonding produce characteristic absorption bands in the spectral range 280023100 cm1 : These bands are observed in CVD polycrystalline diamonds [6]. Our IR measurements performed on diamond samples with 1:8 mm layers uniformly doped with 1:6 1021 cm3 hydrogen have shown that within the limits of sensitivity of our technique (which is 10% of implanted hydrogen dose), no C–H bonds are present in our samples at any annealing conditions. Similar negative results were obtained using luminescence and spin resonance techniques. So we can conclude that there is no direct evidence of the presence of optically active hydrogenrelated defects in our hydrogen-implanted samples. The apparent absence of hydrogen-related defects underlines the importance of hydrogenrelated effects, that is, blistering and high-temperature graphitization (above 1400 1C), both caused by the release of hydrogen atoms and
ARTICLE IN PRESS R.A. Khmelnitskiy et al. / Vacuum 78 (2005) 273–279
defects as a result of the breakup of relative stable associations of hydrogen atoms and defects. These associations (evidently, optically inactive) are formed either immediately after implantation or at the early stages of annealing. Blistering, i.e. formation of hydrogen-filled bubbles, implies diffusion of hydrogen atoms in diamond. It should be noted that so far no diffusion of hydrogen atoms (including those ion-implanted) was found in diamond [7], except the migration of a charged hydrogen-related defect in semiconducting p-type diamond [8]. Our data do not contradict these observations. The specific shape of the bubbles with lateral dimensions two orders of magnitude larger than the thickness means that no diffusion of hydrogen atoms takes place outside the implanted layer. The migration of implanted hydrogen atoms occurs only within this layer in the presence of high concentration of radiation defects (either initial or released by high-temperature annealing). The suggestion that bubbles formed in hydrogen-implanted diamond in the process of blistering are filled with hydrogen seems quite natural [1]. However it cannot be excluded that the bubbles formed in the carbon matrix also contain some hydrocarbons. To check the latter suggestion, infrared absorption measurements were performed in the spectral region of the known absorption lines of gaseous hydrocarbons. No traces of hydrocarbon-related absorption were found. This means that the bubbles are indeed filled with hydrogen.
275
mined (for 350 keV H+ ions this depth is 1:86 0:02 mm). Both the depth and the thickness of the islands are practically independent of the implantation dose. The increase in dose produces an increase in the amount of islands and their size. This is quite different from the case of uniform graphitization where the thickness of the graphitized layer increases with the increase of dose. Island graphitization is observed also for deuterium implantation, i.e. it is characteristic of hydrogen isotopes. However, contrary to H+ implantation, in diamond samples implanted with D+, island graphitization is observed only within a very narrow interval of doses: at higher doses, a uniform graphitized layer is formed, at lower doses no graphitization is observed, and the annealing restores the undamaged diamond lattice. Blistering is characterized by some thresholds, referring both to implantation dose and annealing temperature. For example in 350 keV H+-implanted samples, no blistering is observed at the dose 4 1016 cm2 ; while at 6 1016 cm2 the effect is quite pronounced, with considerable amount of big bubbles. On the other hand, annealing of H+-implanted samples at temperatures in the interval 700–1350 1C produces island graphitization, but no blistering. An increase of annealing temperature up to 1400 1C produces formation of bubbles easily identified by interference patterns (Fig. 1). One of the bubbles in Fig. 1 is broken (two cracks along crystallographic axes
3.2. ‘‘Island’’ graphitization and blistering ‘‘Island’’ graphitization is observed in hydrogenimplanted diamond in the annealing temperature range 700–1350 1C within broad interval of implantation doses. The graphitized layer consists of oval dark graphitized spots with lateral dimensions 10–50 mm separated by light ungraphitized areas. Quantitative absorption measurements are not possible at such conditions. However, in transmission spectra, strong well-resolved interference is seen, and this means that the upper boundaries of various graphitized islands are all at the same depth which can be accurately deter-
Fig. 1. Microphoto of a diamond sample implanted with H+ (350 keV, 12 1016 cm2 ) and annealed at 1450 1C.
ARTICLE IN PRESS 276
R.A. Khmelnitskiy et al. / Vacuum 78 (2005) 273–279
on its upper part are seen). Sometimes the upper part of a bubble is removed forming a pit with boundaries also oriented along crystallographic axes. With the increase of annealing temperature, the amount of broken bubbles increases due to the increase of internal gas pressure. The lateral size of bubbles is generally larger than that of graphitized islands, and so, as it is seen in Fig. 1, there are, usually, several islands ‘‘laterally inside’’ the bubble. To make sure that they are also ‘‘vertically inside’’ the bubble requires some additional arguments. Let us note, first of all, that there are no graphite islands at the bottom of the broken bubbles in the samples etched in hot solution K2Cr2O7+H2SO4, which dissolves graphite. On the other hand, the islands which are seen (in the unetched samples) at the bottom of the bubbles with the removed upper part are less dense than those outside the bubbles. It looks like the islands, which are thus ‘‘vertically inside’’ the bubbles, are torn into two parts in the process of growth of the bubble. The upper part of the island, bound to the lower surface of the upper part of the bubble is removed when the bubble is broken, while the lower part of the island remains fixed to the bottom of the bubble. This notion is confirmed by atomic force microscopy study of the bottom of the bubbles (see Fig. 2). It was shown that the surface of the bottom is quite smooth (just like the outer surface of the graphitized islands, which is evidence from the interference measurements) everywhere except the areas of graphitized islands. Here, on the contrary, the surface is very rough which can be expected of the boundary of rupture of a material, which is not supposed to be monocrystalline. 3.3. Characterization of bubbles In this section, we present the estimates of mechanical tension in the upper boundary of the bubbles, internal pressure of gas and the amount of gaseous material inside the bubbles. The bubbles are formed in the thin surface layer of the implanted samples. Clearly, their bottom is plane, so the internal pressure is retarded only by the tension in the thin diamond film which forms the upper part of the bubbles. If this tension is
Fig. 2. Diamond sample implanted with H+ (350 keV, 12 1016 cm2 ) and annealed at 1450 1C. (a) Microphoto of cracked bubble in transmitted light, (b) atomic force microscopy image of the bottom of the bubble in the marked area.
elastic, it is possible, based on elasticity theory, to calculate the pressure of gas in the bubble using the data of the bending of the film. Plastic deformation of the film (possible in diamond at high temperature) will be touched upon later. The shape of the bubbles was determined from measurements of the surface topography using optical interferometry. In Fig. 3, the topography of a sample with several bubbles of various sizes is shown. In both Figs. 1 and 3, it is seen that most bubbles are laterally circular. Profiles along the lines numbered 1–3 in Fig. 3 were determined from
ARTICLE IN PRESS R.A. Khmelnitskiy et al. / Vacuum 78 (2005) 273–279
277
central bending b (in our case R is 100 mm; b0:223 mm). The solution has an axial symmetry in isotropic material. According to this solution, the deflection w at a distance r from the center of the membrane is w ¼ bð1 r2 =R2 Þ2 .
Fig. 3. Topography (represented by various degrees of gray) of a sample (H+-implanted with the dose 6 1016 cm2 ; the energy 350 keV, annealed at 1400 1C) with several bubbles of various sizes. Image dimensions are 710 532 mm2 :
2000
Surface profile, nm
1450 °C 1400 °C
1500
Cracked bubble without gas 1000
500
0 50
100
In Fig. 4, it is seen that the profile of the bubble given by this formula is in good agreement with the experiment. It means that elastic approximation works fairly well. In this approximation, the gas pressure inside the bubble can be calculated using the following expression: " # t 4 46 18m b3 16 b p¼E þ , 2 R 21ð1 mÞ t 3ð1 m Þ t (2)
Theoretic model
0
(1)
150
200
250
300
350
Distance, µm Fig. 4. Profiles of the bubbles along the line 1 (see Fig. 3) corresponding to annealing temperatures 1400 and 1450 1C.
this data. Two profiles along the line 1 corresponding to various annealing temperatures are shown in Fig. 4. One of the two bubbles (the smaller one) cracked at higher (1450 1C) annealing temperature, and so the gas left the bubble (while the etching removed graphite islands within this bubble). However, the bending of the surface in the area of this (empty) bubble still exists which means that it is due to plastic deformation which took place at high annealing temperature. To calculate the profile of a bubble, we used the solution given in [9] for an elastically deformed circular membrane with radius R much larger than
where E and m are Young modulus and Poisson ratio, respectively, and t is the thickness of the upper part of the bubble (1.86 mm for our samples). Tensile stress at any point of the upper surface of a bubble can also be calculated. The maximal stress s developed in the center of a bubble is given by the formula: 2 5 3m b E s¼ . (3) 6ð1 mÞ R Anisotropy of diamond, which was not taken into account, is not likely to contribute any considerable errors because of weak anisotropy of the Young modulus and small value of the Poisson ratio. This is confirmed by axial symmetry of the shape of the bubbles. The anisotropy shows up only when bubbles crack along cleavage planes. Now, using the equation of state for ideal gas, we can calculate for a given bubble the amount of gaseous matter based on the volume of the bubble and gas pressure. In Table 1, we present for several bubbles the data on their diameter, central bending, gas pressure at two temperatures, tensile stress in the center and the amount of gaseous matter. It is seen that the gas pressure inside the bubbles at room temperature is several MPa, which means that the approximation of ideal gas is valid.
ARTICLE IN PRESS R.A. Khmelnitskiy et al. / Vacuum 78 (2005) 273–279
278
Table 1 Characteristics of several bubbles for a sample implanted with 6 1016 cm2 of H+ and annealed at 1400 1C Label of the bubble
Diameter of the Maximum bubble, mm bending, mm
Gas pressure at room temperature, MPa
Gas pressure at Tensile stress in the Amount of H2 1400 1C, MPa center of the bubble molecules in the at room temperature, bubble, 1012 MPa
1 1 1 2 2 1 2 2 3 1 3 2
150 120 110 83 75 69
3.0 3.6 4.1 8.2 10 14
9.1 10 11 21 26 34
1.8 1.1 0.92 0.62 0.53 0.5
Summing the amount of gaseous matter inside the bubbles over a given area, we can calculate the amount of hydrogen for a unit of area to be compared with the hydrogen implantation dose. For the sample with the implantation dose 6 1016 cm2 ; the amount of hydrogen inside the bubbles is 2:3 1016 and 3:0 1016 cm2 for annealing temperatures 1400 and 1450 1C, respectively. It is seen (not quantitatively) that blistering increases with increase in temperature (for still higher temperatures most of bubbles are broken). Bearing in mind that no blistering was observed for hydrogen implantation dose 4 1016 cm2 (see Section 3.2), the latter looks like a limit of hydrogen solubility in the implanted layer. The increase of dose by 2 1016 cm2 (up to 6 1016 cm2 ) produces blistering with practically the same amount of (excess) hydrogen within bubbles. For implantation dose 10 1016 cm2 ; the amount of hydrogen inside the bubbles is 1:8 1016 and 2:2 1016 cm2 for annealing temperatures 1400 and 1450 1C, respectively, which is lower than for the dose 6 1016 cm2 : This is probably due to larger fraction of graphitized material (where hydrogen may be more soluble) for higher implantation dose. It is pertinent to discuss the role of plastic deformation in the process of formation of bubbles. It is manifested in Figs. 4 and 2. In the latter case, it is seen that most of the upper part of the broken bubble is gone, while some fragment still remains (which is evidenced by the interference pattern). The measured profile indicates
540 320 260 210 190 200
7.5 3.6 2.9 2.2 1.9 2.0
that this fragment is bent upward, most of its area is plain, and so the bending is concentrated at the very edge of the bubble. It is only in this region that plastic deformation occurs. Moreover, due to superlinear dependence of gas pressure in the bubble on its upper part bending (see (2)), the neglect of effects of plastic deformation in the gas pressure calculation does not contribute appreciable error, especially as during annealing (when plastic deformation occurs) the bubbles become larger.
4. Conclusions Optical absorption, interference and microscopy studies of diamond implanted with hydrogen ions revealed some important details of behavior of hydrogen in diamond. The absence of direct evidence of hydrogen-related defects is in contrast with very important role of hydrogen in the process of the diamond–graphite phase transition, accompanied by blistering. Some features of graphitization of diamond induced by hydrogen-ion implantation remain to be understood such as the absence of blistering in samples implanted with deuterium (chemically identical to hydrogen) which might be due to the different amounts of radiation damage produced by H+ and D+. Still less clear is the nature of ‘‘island’’ graphitization which is characteristic only of hydrogen isotopes.
ARTICLE IN PRESS R.A. Khmelnitskiy et al. / Vacuum 78 (2005) 273–279
Acknowledgments The authors gratefully acknowledge financial support by the Russian Foundation for Basic Research (Projects N 04-02-17060, 03-03-32396). References [1] Tong Q-Y, Gutjahr K, Hopfe S, Gosele U, Lee T-H. Appl Phys Lett 1997;70(11):1390. [2] Kozlovskiy VV, Kozlov VA, Lomasov VN. Fiz Teckh Poluprovodn 2000;34(2):129 [in Russian]. [3] Anderson GC, Prawer S, Manory R. Proceedings of applied diamond conference, 1990. p. 37–50.
279
[4] Gippius AA, Khmelnitskiy RA, Dravin VA, Khomich AV. Physica B 2001;308–310:573–6. [5] Gippius AA, Khmelnitskiy RA, Dravin VA, Khomich AV. Diamond Relat Mater 2003;12:538–41. [6] Khomich AV, Ralchenko VG, Vlasov AV, Khmelnitskiy RA, Vlasov II, Konnov VI. Diamond Relat Mater 2001;10:546. [7] Sideras-Hadat E, Conell SH, Sellschop JPF, Machi IZ, Rebuli D, Maclear RD, Doyle BP. Nucl Instrum Methods B 2001;181:419–25. [8] Saguy C, Cytermann C, Fizgeer B, Richter V, Avigal Y, Moriya N, Kalish R, Mathieu B, Deneuville A. Diamond Relat Mater 2003;12:623–31. [9] Volmir AC. Flexible membranes and casing. Mir 1956 [in Russian].
Vacuum 78 (2005) 273–279 www.elsevier.com/locate/vacuum
Blistering in diamond implanted with hydrogen ions R.A. Khmelnitskiya,, E.V. Zavedeevb, A.V. Khomichc, A.V. Gooskova, A.A. Gippiusa b
a P.N. Lebedev Physical Institute, Leninsky prospect 53, 119991 Moscow, Russian Federation Natural Science Studies Center of A.M. Prokhorov General Physics Institute, Vavilova str. 38, 119991 Moscow, Russian Federation c Institute of Radio Engineering & Electronics, 1 Vvedenskogo sq., 141190 Fryazino, Moscow district, Russian Federation
Abstract Annealing of diamond implanted with hydrogen results in ‘‘island’’ graphitization and formation of bubbles (blistering). Bubbles are formed at the depth of the graphitized layer and filled with hydrogen. Based on elasticity theory, the pressure of gas and the amount of gaseous matter in the bubble as well as tensile stress in the upper part of the bubble are calculated. r 2005 Elsevier Ltd. All rights reserved. Keywords: Diamond; Ion implantation; Hydrogen; Graphitization; Blistering
1. Introduction Blistering, i.e. formation of gas-filled cavities (bubbles) in solid matrix is typical of high-dose implantation of light ions (H, D, He) into semiconductors, such as Si, Ge, GaAs [1] and other materials. Blistering was thoroughly studied in silicon and was even employed in the technology of silicon on insulator (SOI) structures [1,2]. Bubbles are formed (after annealing) at the depth
Corresponding author. Tel.: +7 95 132 69 53; fax: +7 95 135 79 50. E-mail address: [email protected] (R.A. Khmelnitskiy).
corresponding to the maximum of concentration of the implants. Blistering in diamond was mentioned in [1,3] and briefly described in our papers [4,5]. It was established that some features of blistering in diamond are quite different from those found in other materials. First, blistering in diamonds is observed only in hydrogen-implanted samples; no such effect was found in deuterium- or heliumimplanted samples. Second, due to metastability of diamond, it transforms into graphite under irradiation and heat treatment. For H+ implantation, the graphitized layer is not uniform but consists of oval ‘‘islands’’ (this effect is much less pronounced in D+ implanted samples). It means that in this case blistering occurs in the material, which is a
0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.01.038
ARTICLE IN PRESS 274
R.A. Khmelnitskiy et al. / Vacuum 78 (2005) 273–279
mixture of diamond and graphite phases. As a matter of fact, in H+ implanted diamond, both blistering and high-temperature graphitization are caused by disruption of relatively stable associations of hydrogen with radiation defects and releasing the lattice defects and free hydrogen atoms. The former stimulates the process of graphitization while the latter can diffuse and form in the disordered layer (and only in this layer) the hydrogen-filled bubbles of peculiar shape with lateral dimensions two orders of magnitude larger than the thickness [4,5]. In this paper, detailed data on the ‘‘island’’ graphitization and blistering in H+ implanted diamond are presented. The gas pressure in the bubbles and mechanical stress in their thin film upper part are estimated. The nature of hydrogencontaining defects is discussed.
2. Experimental Polished (1 0 0) samples of natural monocrystalline diamond were implanted at room temperature. In most cases, single energy (350 keV) implantation with doses in the range 2212 1016 cm2 was performed. Implanted areas of 0:75 1:75 mm2 in size were specified using masks. Samples for infrared spectroscopy were prepared using H+ implantation with a set of 18 energies in the range 652350 keV with total dose of 2:45 1017 cm2 : In this way, at the depth 0:321:8 mm; we obtained the layer uniformly doped with hydrogen at the concentration of 1:6 1021 cm3 (10%) which is slightly less than that at 1 at%. Annealing of the implanted samples was performed for 1 h at temperatures up to 1600 1C in a graphite furnace. After annealing, the samples were etched in hot solution K2Cr2O7+H2SO4 to remove very thin uniform graphite layer. Optical absorption in the range 2202870 nm was measured with the spectrophotometer ‘‘Specord M400’’ (Carl Zeiss, Jena). Measurements of infrared absorption in the range 2:5250 mm intended to monitor the formation of C–H bonds were made with the spectrophotometer ‘‘Specord M80’’.
Microphotography of the samples both in transmitted and reflected light was performed after several characteristic stages of annealing. Quantitative data on the topography of the samples surface were obtained using interferometric microscope ‘‘Zygo New View 5000’’. For detailed studies of the surface, atomic force microscopy was employed.
3. Results and discussion 3.1. Hydrogen-related defects and effects Hydrogen, as chemically active, passivating impurity tends to form some associations with other impurities and/or defects in diamond. This tendency must be particularly strong in ionimplanted layers with high concentration of radiation defects. We undertook some attempts to find direct evidence of formation of hydrogenrelated defects. First of all it was natural to suggest that due to a large amount of broken C–C bonds in ion-implanted layers, C–H bonds can be formed similar to Si–H bonds observed in hydrogendoped silicon. C–H bonds can be monitored quantitatively by infrared absorption: various modes and types of bonding produce characteristic absorption bands in the spectral range 280023100 cm1 : These bands are observed in CVD polycrystalline diamonds [6]. Our IR measurements performed on diamond samples with 1:8 mm layers uniformly doped with 1:6 1021 cm3 hydrogen have shown that within the limits of sensitivity of our technique (which is 10% of implanted hydrogen dose), no C–H bonds are present in our samples at any annealing conditions. Similar negative results were obtained using luminescence and spin resonance techniques. So we can conclude that there is no direct evidence of the presence of optically active hydrogenrelated defects in our hydrogen-implanted samples. The apparent absence of hydrogen-related defects underlines the importance of hydrogenrelated effects, that is, blistering and high-temperature graphitization (above 1400 1C), both caused by the release of hydrogen atoms and
ARTICLE IN PRESS R.A. Khmelnitskiy et al. / Vacuum 78 (2005) 273–279
defects as a result of the breakup of relative stable associations of hydrogen atoms and defects. These associations (evidently, optically inactive) are formed either immediately after implantation or at the early stages of annealing. Blistering, i.e. formation of hydrogen-filled bubbles, implies diffusion of hydrogen atoms in diamond. It should be noted that so far no diffusion of hydrogen atoms (including those ion-implanted) was found in diamond [7], except the migration of a charged hydrogen-related defect in semiconducting p-type diamond [8]. Our data do not contradict these observations. The specific shape of the bubbles with lateral dimensions two orders of magnitude larger than the thickness means that no diffusion of hydrogen atoms takes place outside the implanted layer. The migration of implanted hydrogen atoms occurs only within this layer in the presence of high concentration of radiation defects (either initial or released by high-temperature annealing). The suggestion that bubbles formed in hydrogen-implanted diamond in the process of blistering are filled with hydrogen seems quite natural [1]. However it cannot be excluded that the bubbles formed in the carbon matrix also contain some hydrocarbons. To check the latter suggestion, infrared absorption measurements were performed in the spectral region of the known absorption lines of gaseous hydrocarbons. No traces of hydrocarbon-related absorption were found. This means that the bubbles are indeed filled with hydrogen.
275
mined (for 350 keV H+ ions this depth is 1:86 0:02 mm). Both the depth and the thickness of the islands are practically independent of the implantation dose. The increase in dose produces an increase in the amount of islands and their size. This is quite different from the case of uniform graphitization where the thickness of the graphitized layer increases with the increase of dose. Island graphitization is observed also for deuterium implantation, i.e. it is characteristic of hydrogen isotopes. However, contrary to H+ implantation, in diamond samples implanted with D+, island graphitization is observed only within a very narrow interval of doses: at higher doses, a uniform graphitized layer is formed, at lower doses no graphitization is observed, and the annealing restores the undamaged diamond lattice. Blistering is characterized by some thresholds, referring both to implantation dose and annealing temperature. For example in 350 keV H+-implanted samples, no blistering is observed at the dose 4 1016 cm2 ; while at 6 1016 cm2 the effect is quite pronounced, with considerable amount of big bubbles. On the other hand, annealing of H+-implanted samples at temperatures in the interval 700–1350 1C produces island graphitization, but no blistering. An increase of annealing temperature up to 1400 1C produces formation of bubbles easily identified by interference patterns (Fig. 1). One of the bubbles in Fig. 1 is broken (two cracks along crystallographic axes
3.2. ‘‘Island’’ graphitization and blistering ‘‘Island’’ graphitization is observed in hydrogenimplanted diamond in the annealing temperature range 700–1350 1C within broad interval of implantation doses. The graphitized layer consists of oval dark graphitized spots with lateral dimensions 10–50 mm separated by light ungraphitized areas. Quantitative absorption measurements are not possible at such conditions. However, in transmission spectra, strong well-resolved interference is seen, and this means that the upper boundaries of various graphitized islands are all at the same depth which can be accurately deter-
Fig. 1. Microphoto of a diamond sample implanted with H+ (350 keV, 12 1016 cm2 ) and annealed at 1450 1C.
ARTICLE IN PRESS 276
R.A. Khmelnitskiy et al. / Vacuum 78 (2005) 273–279
on its upper part are seen). Sometimes the upper part of a bubble is removed forming a pit with boundaries also oriented along crystallographic axes. With the increase of annealing temperature, the amount of broken bubbles increases due to the increase of internal gas pressure. The lateral size of bubbles is generally larger than that of graphitized islands, and so, as it is seen in Fig. 1, there are, usually, several islands ‘‘laterally inside’’ the bubble. To make sure that they are also ‘‘vertically inside’’ the bubble requires some additional arguments. Let us note, first of all, that there are no graphite islands at the bottom of the broken bubbles in the samples etched in hot solution K2Cr2O7+H2SO4, which dissolves graphite. On the other hand, the islands which are seen (in the unetched samples) at the bottom of the bubbles with the removed upper part are less dense than those outside the bubbles. It looks like the islands, which are thus ‘‘vertically inside’’ the bubbles, are torn into two parts in the process of growth of the bubble. The upper part of the island, bound to the lower surface of the upper part of the bubble is removed when the bubble is broken, while the lower part of the island remains fixed to the bottom of the bubble. This notion is confirmed by atomic force microscopy study of the bottom of the bubbles (see Fig. 2). It was shown that the surface of the bottom is quite smooth (just like the outer surface of the graphitized islands, which is evidence from the interference measurements) everywhere except the areas of graphitized islands. Here, on the contrary, the surface is very rough which can be expected of the boundary of rupture of a material, which is not supposed to be monocrystalline. 3.3. Characterization of bubbles In this section, we present the estimates of mechanical tension in the upper boundary of the bubbles, internal pressure of gas and the amount of gaseous material inside the bubbles. The bubbles are formed in the thin surface layer of the implanted samples. Clearly, their bottom is plane, so the internal pressure is retarded only by the tension in the thin diamond film which forms the upper part of the bubbles. If this tension is
Fig. 2. Diamond sample implanted with H+ (350 keV, 12 1016 cm2 ) and annealed at 1450 1C. (a) Microphoto of cracked bubble in transmitted light, (b) atomic force microscopy image of the bottom of the bubble in the marked area.
elastic, it is possible, based on elasticity theory, to calculate the pressure of gas in the bubble using the data of the bending of the film. Plastic deformation of the film (possible in diamond at high temperature) will be touched upon later. The shape of the bubbles was determined from measurements of the surface topography using optical interferometry. In Fig. 3, the topography of a sample with several bubbles of various sizes is shown. In both Figs. 1 and 3, it is seen that most bubbles are laterally circular. Profiles along the lines numbered 1–3 in Fig. 3 were determined from
ARTICLE IN PRESS R.A. Khmelnitskiy et al. / Vacuum 78 (2005) 273–279
277
central bending b (in our case R is 100 mm; b0:223 mm). The solution has an axial symmetry in isotropic material. According to this solution, the deflection w at a distance r from the center of the membrane is w ¼ bð1 r2 =R2 Þ2 .
Fig. 3. Topography (represented by various degrees of gray) of a sample (H+-implanted with the dose 6 1016 cm2 ; the energy 350 keV, annealed at 1400 1C) with several bubbles of various sizes. Image dimensions are 710 532 mm2 :
2000
Surface profile, nm
1450 °C 1400 °C
1500
Cracked bubble without gas 1000
500
0 50
100
In Fig. 4, it is seen that the profile of the bubble given by this formula is in good agreement with the experiment. It means that elastic approximation works fairly well. In this approximation, the gas pressure inside the bubble can be calculated using the following expression: " # t 4 46 18m b3 16 b p¼E þ , 2 R 21ð1 mÞ t 3ð1 m Þ t (2)
Theoretic model
0
(1)
150
200
250
300
350
Distance, µm Fig. 4. Profiles of the bubbles along the line 1 (see Fig. 3) corresponding to annealing temperatures 1400 and 1450 1C.
this data. Two profiles along the line 1 corresponding to various annealing temperatures are shown in Fig. 4. One of the two bubbles (the smaller one) cracked at higher (1450 1C) annealing temperature, and so the gas left the bubble (while the etching removed graphite islands within this bubble). However, the bending of the surface in the area of this (empty) bubble still exists which means that it is due to plastic deformation which took place at high annealing temperature. To calculate the profile of a bubble, we used the solution given in [9] for an elastically deformed circular membrane with radius R much larger than
where E and m are Young modulus and Poisson ratio, respectively, and t is the thickness of the upper part of the bubble (1.86 mm for our samples). Tensile stress at any point of the upper surface of a bubble can also be calculated. The maximal stress s developed in the center of a bubble is given by the formula: 2 5 3m b E s¼ . (3) 6ð1 mÞ R Anisotropy of diamond, which was not taken into account, is not likely to contribute any considerable errors because of weak anisotropy of the Young modulus and small value of the Poisson ratio. This is confirmed by axial symmetry of the shape of the bubbles. The anisotropy shows up only when bubbles crack along cleavage planes. Now, using the equation of state for ideal gas, we can calculate for a given bubble the amount of gaseous matter based on the volume of the bubble and gas pressure. In Table 1, we present for several bubbles the data on their diameter, central bending, gas pressure at two temperatures, tensile stress in the center and the amount of gaseous matter. It is seen that the gas pressure inside the bubbles at room temperature is several MPa, which means that the approximation of ideal gas is valid.
ARTICLE IN PRESS R.A. Khmelnitskiy et al. / Vacuum 78 (2005) 273–279
278
Table 1 Characteristics of several bubbles for a sample implanted with 6 1016 cm2 of H+ and annealed at 1400 1C Label of the bubble
Diameter of the Maximum bubble, mm bending, mm
Gas pressure at room temperature, MPa
Gas pressure at Tensile stress in the Amount of H2 1400 1C, MPa center of the bubble molecules in the at room temperature, bubble, 1012 MPa
1 1 1 2 2 1 2 2 3 1 3 2
150 120 110 83 75 69
3.0 3.6 4.1 8.2 10 14
9.1 10 11 21 26 34
1.8 1.1 0.92 0.62 0.53 0.5
Summing the amount of gaseous matter inside the bubbles over a given area, we can calculate the amount of hydrogen for a unit of area to be compared with the hydrogen implantation dose. For the sample with the implantation dose 6 1016 cm2 ; the amount of hydrogen inside the bubbles is 2:3 1016 and 3:0 1016 cm2 for annealing temperatures 1400 and 1450 1C, respectively. It is seen (not quantitatively) that blistering increases with increase in temperature (for still higher temperatures most of bubbles are broken). Bearing in mind that no blistering was observed for hydrogen implantation dose 4 1016 cm2 (see Section 3.2), the latter looks like a limit of hydrogen solubility in the implanted layer. The increase of dose by 2 1016 cm2 (up to 6 1016 cm2 ) produces blistering with practically the same amount of (excess) hydrogen within bubbles. For implantation dose 10 1016 cm2 ; the amount of hydrogen inside the bubbles is 1:8 1016 and 2:2 1016 cm2 for annealing temperatures 1400 and 1450 1C, respectively, which is lower than for the dose 6 1016 cm2 : This is probably due to larger fraction of graphitized material (where hydrogen may be more soluble) for higher implantation dose. It is pertinent to discuss the role of plastic deformation in the process of formation of bubbles. It is manifested in Figs. 4 and 2. In the latter case, it is seen that most of the upper part of the broken bubble is gone, while some fragment still remains (which is evidenced by the interference pattern). The measured profile indicates
540 320 260 210 190 200
7.5 3.6 2.9 2.2 1.9 2.0
that this fragment is bent upward, most of its area is plain, and so the bending is concentrated at the very edge of the bubble. It is only in this region that plastic deformation occurs. Moreover, due to superlinear dependence of gas pressure in the bubble on its upper part bending (see (2)), the neglect of effects of plastic deformation in the gas pressure calculation does not contribute appreciable error, especially as during annealing (when plastic deformation occurs) the bubbles become larger.
4. Conclusions Optical absorption, interference and microscopy studies of diamond implanted with hydrogen ions revealed some important details of behavior of hydrogen in diamond. The absence of direct evidence of hydrogen-related defects is in contrast with very important role of hydrogen in the process of the diamond–graphite phase transition, accompanied by blistering. Some features of graphitization of diamond induced by hydrogen-ion implantation remain to be understood such as the absence of blistering in samples implanted with deuterium (chemically identical to hydrogen) which might be due to the different amounts of radiation damage produced by H+ and D+. Still less clear is the nature of ‘‘island’’ graphitization which is characteristic only of hydrogen isotopes.
ARTICLE IN PRESS R.A. Khmelnitskiy et al. / Vacuum 78 (2005) 273–279
Acknowledgments The authors gratefully acknowledge financial support by the Russian Foundation for Basic Research (Projects N 04-02-17060, 03-03-32396). References [1] Tong Q-Y, Gutjahr K, Hopfe S, Gosele U, Lee T-H. Appl Phys Lett 1997;70(11):1390. [2] Kozlovskiy VV, Kozlov VA, Lomasov VN. Fiz Teckh Poluprovodn 2000;34(2):129 [in Russian]. [3] Anderson GC, Prawer S, Manory R. Proceedings of applied diamond conference, 1990. p. 37–50.
279
[4] Gippius AA, Khmelnitskiy RA, Dravin VA, Khomich AV. Physica B 2001;308–310:573–6. [5] Gippius AA, Khmelnitskiy RA, Dravin VA, Khomich AV. Diamond Relat Mater 2003;12:538–41. [6] Khomich AV, Ralchenko VG, Vlasov AV, Khmelnitskiy RA, Vlasov II, Konnov VI. Diamond Relat Mater 2001;10:546. [7] Sideras-Hadat E, Conell SH, Sellschop JPF, Machi IZ, Rebuli D, Maclear RD, Doyle BP. Nucl Instrum Methods B 2001;181:419–25. [8] Saguy C, Cytermann C, Fizgeer B, Richter V, Avigal Y, Moriya N, Kalish R, Mathieu B, Deneuville A. Diamond Relat Mater 2003;12:623–31. [9] Volmir AC. Flexible membranes and casing. Mir 1956 [in Russian].