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Non-destructive characterization of ion-implanted diamond
Vacuum 55 (1999) 207}217
Non-destructive characterization of ion-implanted diamond Z.Q. Ma!,",*, B.X. Liu", H. Naramoto#, Y. Aoki#, S. Yamamoto#, H. Takeshita#, P.C. Goppelt-Langer$ !Functional Materials Group, Department of Physics, Faculty of Science and Engineering, Xinjiang University, Urumqi City, XJ 830046, People's Republic of China "Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People's Republic of China #Functional Materials Lab.2, Department of Materials Development, Takasaki Radiation Chemistry Research Establishment, JAERI, 1233 Watanuki, Takasaki, Gunma 370-12, Japan $Hahn-Meitner Institute, D-1000 Berlin, Germany
Abstract A position-sensitive micro-spectrophotometer with slightly focused light in transmission mode has been employed to characterize the optical inhomogeneity of the original synthetic diamond crystal (1 0 0) surface and a modi"ed carbon layer and its defects induced by low-energy H` implantation at 100 K and room temperature. The distribution of strong absorption relative to di!erent positions, 2 which starts at around 470 nm down to 280 nm in the VIS-UV regions, re#ects a distinctive di!erence of nitrogen concentration and intrinsic defects in di!erent growth sectors. The typical morphology of the intrinsic or ion-induced defects is showed in a twodimensional topography adjusted at the near absorption edge (j"430 nm). The relative optical density (OD) and band gap (E ) 3,015 are deduced via the use of the normalized transmittance and are used to interpret the energetic ion-induced defect and its evolution depending on the implanted dose and annealing temperature. It is found that the gradual change from pale to deep reddish brown color in both the re#ection and refraction orientations is associated with the dosage levels injected into top submicrometer layer, but is independent of the implantation temperature. The critical dose for the conversion of the diamond structure into a disordered network and the migrating temperature for nitrogen atoms in H` ion radiation-damaged diamond are found to be more than 1.3]1017 H/cm2 2 and 12003C, respectively. ( 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction The unique properties of diamond crystals and diamond-like carbon (DLC) "lms such as extreme hardness, optical transparency, high electrical resistivity and chemical inertness [1}4] have made them critical materials in various advanced technologies. Besides the development of diamond-like carbon "lms with various chemical and/or physical methods [5,6], the ion implantation technique has been extensively utilized [7}10] for (1) incorporation of dopants into lattice sites of tetrahedral carbon coordination as excitable carrier of a semiconductor with a wide gap (5.4 eV) for electronic application, (2) producing an unusual electronic bonding of the amorphous solid-state structure involved in several allot-
* Corresponding author. Functional Materials Group, Department of Physics, Faculty of Science and Engineering, Xinjiang University, Urumqi City, XJ 830046, People's Republic of China. Tel: #86-09912863365; fax: #86-0991-2862006. E-mail address: [email protected] (Z.Q. Ma)
ropes of carbon, and (3) achieving a `tunablea optical constant in a wide range of values. However, accompanying the implantation of useful dopants into diamonds, many defects like interstitials, vacancies and impurity complexes are induced by energetic ion through energy deposition and, the diamond crystal is seriously damaged along the projectile tracks. For most applications, this kind of damage is an undesirable e!ect. It is therefore of considerable interest to have a clear understanding of the ion-induced defect in diamond and to develop a highly sensitive, contactless and nondestructive measurement technique to control and monitor the processes of diamond treatment with ion implantation and subsequent annealing. As well known in the past [2], naturally occurring crystalline allotropes of carbon are either the tetrahedral diamond or the layered graphitic structure. Tetrahedral and layered structures represent the extremes of nearly isotropic solids on the one hand and solids with nearly two-dimensional properties on the other. Amorphous states of carbon are still other allotropes of carbon like
0042-207X/99/$ - see front matter ( 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 9 9 ) 0 0 1 5 3 - 0
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the well-known silicon amorphous-semiconductors which are by de"nition isotropic. The desire to understand the extreme properties of amorphous carbon has stimulated a number of structural investigations and attempts at modeling the microstructure of the amorphous carbon matrix [11}13]. These studies uncovered a metastable phase of carbon consisting of a mixture of tetrahedral (diamond-like) and trigonal (graphitic) bonding of the matrix. In the study of radiation e!ects on diamond crystals, it is important to not only use samples with low impurity content but also necessary to pay attention to the inhomogeneous distribution of the impurities, which re#ects the growth conditions. In order to meet such a requirement, in this report, a photon absorption spectroscopy which includes a position-sensitive scanning and a two-dimensional topographical imaging capability is demonstrated for defect characterization of virgin and modi"ed diamonds, before and after ion implantation, respectively. The commercially produced synthetic diamond crystals (type Ib) by Japan-Sumitomo Elec.Co., under the conventional high-pressure, high-temperature techniques, are used to investigate the dependence of diamond optical properties on the ion #uences and implantation temperature and subsequent thermal annealing. Molecular hydrogen ions with relatively low energy have been used in the past to investigate the surface submicrometer scale hybridization of diamond, because it was considered that hydrogen played a crucial role in stabilizing the tetrahedral bonding [5,14}16]. In particular, the following hypothesis was considered for the purpose of understanding the variation of diamond optical absorption when exposed to ultraviolet light in the UVVIS range for di!erent locations of an original diamond crystal, as-implanted and thermal annealed diamond, and it was suggested that a change in the microstructure of the amorphous carbon matrix from a fourfold-tothreefold transition exists. This e!ect was discussed in terms of the in#uence of temperature on annealing of defect states in ion-implanted diamond. The role of nitrogen (as the main impurity in type-Ib diamond) in the optical absorption properties of diamond is demonstrated via a wavelength spectrum in either non-irradiated or irradiated diamond. A description of the measuring principle of optical transmission or absorption is given in Section 2.1, while detailed experimental procedures are discussed in Sections 2.2 and 2.3. The position dependence of relative transmission intensity on photon energy, optical density, relative optical band gap and thermal annealing e!ect of virgin and implanted diamonds are listed in Sections 3.1}3.4. The change of microstructure associated with the variation of optical absorption from the electronic transitions in the ionhybridized diamonds are tentatively attributed to the displacement damage caused by collision cascades and the introduction of the disorder-network compositions in
quasi-tetrahedral structure of carbon bonding, in the last section.
2. Samples and experimental details 2.1. Measuring principle Considering the subtraction of a background signal and parasitic light (#uorescence and re#ectance) from the standard and object spectra, the dark current and parasitic e!ect are measured during high-voltage adjustment. Taking into account the fact that the instrument response arises from spectral distribution of the illumination, transmittance of the micro-spectrophotometer and sensitivity of the detector, a quotient value between the object spectrum and the standard spectrum is used in measurements, as a relative transmission (%¹), which results in a corrected spectrum. The measured absorbance value is calculated from the %¹ of transmittance from the following equation: Meas.absorb.value"2!log(%¹),
(1)
where the %¹ is a normalized ratio between the transmitted intensities for a virgin (or standard) and another virgin point or irradiated (object) areas. The %¹ is given by equation %¹"[I (j)!I (j)]/[I (j)!I (j)]]100. (2) 0 1 4 1 In Eq. (2), the intensities de"ned as I (j)" standard (s), * object (o), and parasitic (p) spectra, are expressed in percentage. On the other hand, the measured relative optical density is de"ned as Log (I /I ) as follows: 10 4 0 *OD"(OD) !(OD) (3) 0 4 which is also equal to the measured absorbance value. 2.2. Diamond sampling and ion implantation Commercially produced synthetic diamonds were con"rmed by absorption spectra at around 470 nm to be type-Ib diamond crystals with volume of 3.5]3.5] 0.5 mm3, and the (1 0 0) facet exposed to coming projectiles. The transmission intensity imaging (topology) at 430 nm was used to survey the distribution of transmitted light intensity as well as the concentration of nitrogen. Due to the considerable variations of nitrogen impurity in the di!erent growth sectors at the submicrometer scale, there is an e!ect on the corrected quotient value in the very sensitive ultraviolet-visible region during optical transmission measurement. All of the diamond samples were irradiated with 40 KeV H` ions, at 100 K, except for two samples irra2 diated at room temperature. The base pressure before implantation was less than 1]10~4 Pa. Implanted doses
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ranged from 1]1015 to 2]1017 H`/cm2 and half-area of the surface of each sample was covered along a diagonal line during ion implantation in order to make a comparison between virgin and implanted areas. By obtaining a standard spectrum from a virgin area and an objective spectrum from the implanted area of diamond, a ratio was derived from the two spectra, considering the instrument response. The projected ranges and longitudinal straggling of the implanted H` and the other collision damage distribu2 tion were calculated using the TRIM program [17] with surface binding energy of 4.5 eV [18]. The maximum range of the possible damage caused by projectiles in diamond is estimated to be 140 nm, which means any change of atomic structure and electronic states distinguished it from the original diamond crystals. 2.3. Transmission and absorption spectroscopy The optical absorption measurements were performed at room temperature using a micro-spectrophotometer. First, the wavelength dependence of the transmittance in the UV-VIS region was recorded using a single-pass monochromator, "tted with a holographic grating and a photomultiplier tube. The light source was a xenon lamp with output in the 230 } 850 nm wavelength range, coupled with light shutters and motorized 8] "lter changer. The whole procedure, including the position scanning and data acquisition was controlled by a wellset software. Then, conversion of the transmission values to absorbance ones dependent on Eq. (1), was carried out taking into account an automatic correction of dark current, instrument response and parasitic light. The irradiated diamond samples were annealed for 2 h at temperatures in the 300}12003C range, using a furnace under #ushing argon gas.
3. Results and discussions
209
Figs. 1(a)}(c) show a transmission intensity distribution of two-dimensional micrographs for three synthetic diamond specimens. For the sake of clarity, they are labeled as specimens 1, 2 and 3, respectively. The small pictures inserted in Fig. 1 are direct displays of transmission intensity with di!erent colors and in a range of 0}150%, which are symmetric with respect to the center of a circle. Each of the areas measured by a directly incident focused light 20 lm diameter was imaged using this automatic surface scanning. Each area was carefully chosen to be about 1800]1800 lm2. In Fig. 1(a), the transmission intensity is nearly constant, while a dramatic variation is observed in Figs. 1(b) and (c) with changes of 30, 120 and 150%, respectively. The optical inhomogeneity of original diamond crystals are well revealed by the two-dimensional topographic images obtained in the wavelength of j"430 nm. Furthermore, this circle-round transmission intensity variation shows the nitrogen impurity distribution in arti"cial diamond, even though we have learned that the concentration of nitrogen atoms in the specimens was about 10}120 ppm by IR absorprion. 3.1.2. Ion implantation and thermal annealing The specimens labeled above were implanted with 40 KeV-molecular hydrogen ion (H`) at 100 K and 2 room temperature, and their optical transmission characteristics are shown in Figs. 2(a)}(d) for di!erent processes. Fig. 2(a) is a typical two-dimensional image of as-irradiated diamond (No.1) at 100 K, with an irradiation #uence of 6.3]1016 H/cm2. Fig. 2(b) is for specimen 2, also corresponding to the dose of 6.3]1016 H/cm2, but the sample temperature during implantation was naturally kept at room temperature and the "xed wavelength of adjusted light was at 380 nm. Fig. 2(c) is corresponding to the dose of 1.3]1017 H/cm2 for specimen 3, at 100 K. Fig. 2(d) is an image of specimen 3 which experiences a successive thermal annealing at 300 and 5003C for 2 h in each step after implantation. But there is no obvious change during the annealing processes.
3.1. Topographical image of synthetic diamond 3.2. Relative absorbance dependent on position-scanning 3.1.1. Virgin specimens Synthetic diamonds are notorious for their optical inhomogeneity, which is caused by a non-uniform distribution of point defects [19}22]. To get an outline of the inhomogeneity distribution and select a more transparent position to obtain the standard spectrum, we obtained an optical transmission topographical image using the enhanced near UV-VIS absorption edge by the single-substitutional nitrogen centers, which provides the distribution of nitrogen impurities in diamond. The topographical images were obtained by photographing the specimen plates via an optical transmission microscope "tted with a monochromatic UV-VIS (j"380 and 430 nm) light source.
According to the Eqs. (1)}(3) presented in Section 2, the transmission intensity for the standard spectrum (I ) was 4 obtained at the most transparent point as shown in Fig. 1(b) (position A). However, the transmission intensities for the object spectra (I ) were obtained in other points 0 di!erent from position A (in Fig. 1b) under an X}> coordination system (as shown in the inset diagram of Fig. 3a). Typical results for specimen 2 are shown in Figs. 3(a) and (b) in a scanning map of 16 points. It is obvious that the absorbance di!erence at any two positions on the whole sample seems to be near zero in the visible region over 500 nm but a dramatic variation of the values is present in the UV-near visible range. This means that
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Fig. 1. Near-visible (j"430 nm) transmission topographs of the synthetic diamond plates, (a) specimen 1 with a scanning area of 1800]1800 lm2, (b) specimen 2 with a scanning area of 1700]1800 lm2, (c) specimen 3 with a scanning area of 1800]1800 lm2.
nitrogen is the dominant impurity in diamonds and the samples used here are complete crystals. It is apparent that the inhomogeneity of the optical property of synthetic diamond has been veri"ed by the position-sensitive optical density measurement; moreover, the data presented indicate that the nitrogen atoms are in substitutional positions. Fig. 4 shows the wavelength dependence of absorbance via scanning "eld length in micrometer along the X-axis. The wavelengths were chosen around absorption edges from 430 to 500 nm, and the spectra were obtained after the specimen 2 was exposed to molecular hydrogen ion implantation at RT, with 40 keV and 6.3]1016 H/cm2. Typical results in Fig. 4 include three points: (i) a clear increase of optical density in ion-processed area was observed; (ii) within the absorption edge of type-Ib diamond, an enhanced e!ect on absorbance was observed for a decreasing of incident wavelength, especially for
as-implanted diamond; and (iii) the more sensitive wavelength for the characterization of optical inhomogeneity of synthetic diamond is 430 nm. 3.3. Estimation of damaged layer of diamond As well known, the ion-induced radiation damage along the ion projectile is produced during ion implantation at room or low temperature, by ion collision cascades with target atom and recoils. Defects such as interstitials, vacancies, and vacancy}impurity complexes are present in as-implanted diamonds [7,8,23], which are accompanied by the change of atomic con"guration and/or electronic states of carbon bondings. Naturally, the optical properties such as re#ection, refraction index and absorption coe$cient linked with electronic transitions are sensitively in#uenced by the atomic microstructure in diamond. Usually, the radiation
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Fig. 2. Transmission topographs of synthetic diamond implanted with low energy (40 keV) H` ion for specimens 1, 2 and 3 (denoted as Nos. 1,2,3), 2 respectively. (a) implantation on No. 1 with a dose of 6. 3]1016 H/cm2, at 100 K, j"430 nm, (b) implantation on No.2 with a dose of 6.3]1016 H/cm2, at R.T, j"380 nm, (c) implantation on No.3 with a dose of 1.3]1017 H/cm2, at 100 K, j"430 nm, (d) after thermal annealing at 300 and 5003C for 2 h each step, j"430 nm.
damage pro"le induced by energetic ion in solids and, specially in diamond, was determined by some experiments [8,24}26] as well as by predicting calculations. Therefore, in order to learn the depth range of modi"ed layer of diamond, and to correlate it with the e!ects that the impinging ions may induce on the layer, computer simulations of the collision cascades for the ion species
were performed, using the computer simulation code (TRIM-91) with a surface binding energy of 4.5 eV, mass density of 3.52 g/cm3 and atomic displacement energy of 45 eV [27], as shown in Fig. 5. In this "gure, the TRIM results are shown for the "nal ion and vacancy distribution for H` irradiation. Al2 though the projected range (R ) and the straggling (*R ) 1 1
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Fig. 3. The measuring position dependence of absorbance in specimen 2 in the range of ultraviolet to visible regions, (a) X}> coordinating distribution for eight points, (b) X}> coordinating distribution for other eight points.
of the ion are approximately 1200 and 150 As , respectively, the "gure makes it clear that the damage is not con"ned to a layer which is buried beneath the surface, but in fact extends from the surface to a depth of about 1400 As (+R #*R ), following a mechanism similar to 1 1 that discussed in Ref. [28]. On the other hand, because diamond possesses a very low di!usivity at normal conditions, the extension of damage pro"le into the deep should be neglected during optical measurement. Thus, consideration of an approximately 1400 As damaged layer from the diamond surface to the end of the range was reasonable for the optical band gap estimations.
3.4. Annealing ewect of defects 3.4.1. Relative optical gap of the modixed layer In order to investigate the microstructure of a modi"ed layer of diamond and its evolution trend with annealing temperature, a relative optical gap which re#ects the variation of energy band structure, has been introduced by plotting (aE)1@2, as a function of photon energy [29] (aE)1@2"i(E!E ), (4) 3,015 where E is the photon energy, i is a constant, and E is 3,015 the relative optical gap obtained by extrapolation of the
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213
Fig. 4. The variation of absorbance of specimen 2 depends on the wavelength of incident light, for implanted sample by H` at room temperature. 2
linear part of the curves to a"0. In Eq. (4), a is a relative optical absorption coe$cient de"ned as (1/t)ln(10)*OD, and t is the thickness of that modi"ed layer overlying diamond. From the plotting based on Eq. (4), the approximate values of relative optical gap of 1.88, 1.96 and 2.05 eV, corresponding to as-implanted, thermal annealing at 300 and 5003C, were obtained at room temperature, respectively. In this way, the relative optical gap for virgin diamond is about 2.4 eV. Fig. 6 is a plot of the relative optical gap vs. annealing temperature for the damaged layer. Based on the measured values with mean error (1%, an approximate correlation between the relative optical gap (E ) and 3,015 annealing temperature (¹ ) was derived as follows: ! E "m¹ #f, (5) 3,015 ! where m and f are the "tted parameters. Replacing the E with a value of 2.4 eV as the relative optical gap, 3,015 measured on a virgin area, a derived temperature of about 15503C is obtained. It is just the temperature value around which the isolate-substitutional nitrogen atom begins to move in electron-irradiated type Ib diamond [30]. The temperature dependence of the relative optical gap in Fig. 6, therefore, shows a relationship of some optical absorption components and atomic structure development in subsequently thermal annealing. A possible explanation for this microscopic thermodynamical process is that the clustered size of sp3 or sp2 con"gurations of nitrogen}carbon or nitrogen}vacancy complexes have been changed upon thermal driving. The fact that the relative optical gap increases with annealing temperature is contrary to the annealing result
of a-C : H "lm by Smith [31]. This means that the amorphous layer in diamond produced by energetic H` ion doping is obviously di!erent from plasma deposited a-C : H "lms. Probably, the modi"ed layer of diamond by H` implantation at low temperature possesses 2 both properties of i-C (ion-beam deposition carbon) [32,33] and a-C : H bondings [34}37] if hydrogen is present, based on the evolution trend of the optical band gap. The thermal excitation of point defects such as interstitial, vacancy and vacancy}impurity complex is certainly induced during thermal annealing at a stirring temperature. Usually, the critical temperature for the migration of many kinds of defect compositions in diamond, after undergoing an irradiation, is reduced, depending on the type of diamond and the irradiated conditions [22,38]. We could not make an unambiguous conclusion on what kind of defect was annealed in this stage, but the increase of the relative optical gap with temperature can support this view point. Moreover, the structural defects of carbon atom coordination is the most important color center in the amorphous diamond layer after interaction with energetic ions. The fraction of atomic coordination with fourfold (sp3) or threefold (sp2) electronic states is a predominant factor for the optical absorption property, which is consistent with the TBMD (tight-binding molecular dynamics) theory for the amorphous carbon network system [13,39}41] However, the presence of sp3 to sp2 ratio in the damaged layer of diamond depends strongly on the dose levels and processing temperature during ion implantation and subsequent annealing.
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Fig. 5. The simulation of defect range in diamond with TRIM- 91. A scale of &140 nm from the surface to the end of damage pro"le was estimated.
Fig. 6. Relative optical band gap (E ) of modi"ed surface layer in diamond by H` (40 keV, 6.3]1016 H/cm2) vs annealing temperature ¹ (3C). 3,015 2 !
3.4.2. Thermal annealing Post-thermal annealing treatment was performed on the two synthetic diamond crystals (type-Ib) which were implanted by 40 KeV H` with dose ranges of 5]1015 2 and 1]1016 H/cm2, at 100 K and RT. A light brownish color appeared on the irradiated area for the two processed samples, and optical absorption measurements showed that uniform enhancement of absorption in the UV}visible wavelength region was induced, especially, more clearly in a two-dimensional topographical image.
Subsequently, the thermal annealing treatments of samples under argon-atmosphere furnace, as shown in Figs. 7(a) and (b), were performed at 300, 400, 500, 600, 800 and 12003C for 2 h at each temperature, respectively. In the near ultraviolet and 470}800 nm wavelength regions, the normalized transmission spectra show an obvious thermal annealing e!ect. From 300 to 5003C the transmission intensity gradually increased by 5%, while when the annealing temperature was increased to 600 and 8003C, the transmission value remained nearly at
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215
Fig. 7. Thermal annealing e!ect of H` ion processed diamond (type-Ib), and the relative transmission intensity ranged from 5 to 100% in 380}800 nm 2 intervals. (a) sample with 5]1015 H/cm2 at 100 K, (b) sample with 1.0]1016 H/cm2 at RT.
98% of the incident light intensity. At the same time, the distinguished colored areas were annealed at 8003C. However, when the annealing temperature was increased up to 12003C, the transmission intensity decreased by 10% compared to that annealed at 8003C. In particular, it is easy to understand that the di!erence in Figs. 7(a) and (b) for their tendency and the scale of normalized spectra is the same measured for standard and objective
spectra at reversal points, i.e. between the more transparent and the more absorbing areas for the two specimens, respectively. According to the measuring principle, the transmission intensity is a ratio of the object spectrum (irradiated area) to the standard spectrum (virgin area) intensity, and the two spectra for each time were measured at the same positions with an X}> coordination system. In this way,
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the information obtained by annealing treatment is from one source and the standard spectrum from virgin point was maintained identical at the present temperature. Thus, the dependence of transmission intensity on the annealing temperature indicates the development of damaged structure in the surface layer of diamond by H` irradiation. 2 From the results of thermal treatments on molecular hydrogen ion implantation of synthetic diamond crystals mentioned above, the migration of interstitial defect below 5003C could be expected during 2 h annealing for each temperature, which is based on the evidence that a reduced peak intensity around 400 nm in the sample implanted with 1]1016 H/cm2 at RT has been found in Fig. 7(b). But for this featured peak, a larger increase of transmission intensity was observed when annealing temperature was increased above 6003C. This considerable change can be explained by the migration of vacancies. With further increase of annealing temperature up to 8003C, the transmission intensity gradually saturated and the boundary between the virgin and the implanted area became unclear, which implies the recovery of the damaged area with an increase in the fraction of diamondlike structure (sp3). On the other hand, in Fig. 7, a reversed change of transmission value at 12003C indicates the appearance of other compositions in diamond, such as single-substitutional impurity nitrogen atom which may move to form complexes with vacancies or combine with neighbors to form nitrogen aggregates so as to produce a series of new color centers. Referring to the previous investigation by others [30], the temperature for the excitation of nitrogen moving in type-Ia,b diamonds is around 18003C and reduced to 15003C for electron-irradiated diamond, but here we got a smaller value for molecular hydrogen ion implantation either at low temperature (100 K) or RT.
4. Conclusions A submicron diamond-like amorphous carbon (DLC) layer was produced in synthetic diamond by 40 KeV H` ion implantation, at liquid nitrogen and room tem2 peratures. The diamond microstructure induced by ion implantation re#ects band-gap-like values depending on the ion #uence and the temperatures for post-irradiation thermal annealing. The irradiation e!ects were characterized through a derived micro-spectrophotometer, by which the located position-scanning measurement became possible. The gradual increase of the absorption coe$cient is in good correlation with the loss of the original transparency of the irradiated region which depends strongly on the variation of the implanted dosage but weakly on the substrate temperature, implying that the predominant displacement damage is the main process for the partial amorphization of the tetrahedral
coordination of the carbon lattice. The annealing temperature dependence of the relative optical gap E in the 3,015 absorption edge, indicates that the atomic structure in the amorphized layer can be attributed to the mixing site network of fourfold (sp3) and threefold (sp2) states. The thermal evolution of E was much di!erent from that 3,015 of a-C : H (DLC) "lms made by plasma deposition, indicating a transformation mechanism from the diamondlike structure (sp3) to graphite-like structure (sp2) of carbon bonding. However, the presence of C}H bonds and chemical e!ect during subsequent thermal annealing might have possibly resulted from the increase of E 3,015 if the hydrogen composition still remained in that buried layer till to 8003C or more. Furthermore, with this method, the two-dimensional topography of optical transmission intensity has been successfully applied to a survey of the optical inhomogeneity of defects in synthetic diamonds, and it is also possible for a limiting measurement of damage induced by energetic ion beam.
Acknowledgements We gratefully acknowledge the award under the STA Scientist Exchange Program of Japan and Natural Science Foundation of Xinjiang University. The authors would like to thank the sta! of the Takasaki-TIARA center for their invaluable assistance with ion-beam implantations.
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Non-destructive characterization of ion-implanted diamond Z.Q. Ma!,",*, B.X. Liu", H. Naramoto#, Y. Aoki#, S. Yamamoto#, H. Takeshita#, P.C. Goppelt-Langer$ !Functional Materials Group, Department of Physics, Faculty of Science and Engineering, Xinjiang University, Urumqi City, XJ 830046, People's Republic of China "Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People's Republic of China #Functional Materials Lab.2, Department of Materials Development, Takasaki Radiation Chemistry Research Establishment, JAERI, 1233 Watanuki, Takasaki, Gunma 370-12, Japan $Hahn-Meitner Institute, D-1000 Berlin, Germany
Abstract A position-sensitive micro-spectrophotometer with slightly focused light in transmission mode has been employed to characterize the optical inhomogeneity of the original synthetic diamond crystal (1 0 0) surface and a modi"ed carbon layer and its defects induced by low-energy H` implantation at 100 K and room temperature. The distribution of strong absorption relative to di!erent positions, 2 which starts at around 470 nm down to 280 nm in the VIS-UV regions, re#ects a distinctive di!erence of nitrogen concentration and intrinsic defects in di!erent growth sectors. The typical morphology of the intrinsic or ion-induced defects is showed in a twodimensional topography adjusted at the near absorption edge (j"430 nm). The relative optical density (OD) and band gap (E ) 3,015 are deduced via the use of the normalized transmittance and are used to interpret the energetic ion-induced defect and its evolution depending on the implanted dose and annealing temperature. It is found that the gradual change from pale to deep reddish brown color in both the re#ection and refraction orientations is associated with the dosage levels injected into top submicrometer layer, but is independent of the implantation temperature. The critical dose for the conversion of the diamond structure into a disordered network and the migrating temperature for nitrogen atoms in H` ion radiation-damaged diamond are found to be more than 1.3]1017 H/cm2 2 and 12003C, respectively. ( 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction The unique properties of diamond crystals and diamond-like carbon (DLC) "lms such as extreme hardness, optical transparency, high electrical resistivity and chemical inertness [1}4] have made them critical materials in various advanced technologies. Besides the development of diamond-like carbon "lms with various chemical and/or physical methods [5,6], the ion implantation technique has been extensively utilized [7}10] for (1) incorporation of dopants into lattice sites of tetrahedral carbon coordination as excitable carrier of a semiconductor with a wide gap (5.4 eV) for electronic application, (2) producing an unusual electronic bonding of the amorphous solid-state structure involved in several allot-
* Corresponding author. Functional Materials Group, Department of Physics, Faculty of Science and Engineering, Xinjiang University, Urumqi City, XJ 830046, People's Republic of China. Tel: #86-09912863365; fax: #86-0991-2862006. E-mail address: [email protected] (Z.Q. Ma)
ropes of carbon, and (3) achieving a `tunablea optical constant in a wide range of values. However, accompanying the implantation of useful dopants into diamonds, many defects like interstitials, vacancies and impurity complexes are induced by energetic ion through energy deposition and, the diamond crystal is seriously damaged along the projectile tracks. For most applications, this kind of damage is an undesirable e!ect. It is therefore of considerable interest to have a clear understanding of the ion-induced defect in diamond and to develop a highly sensitive, contactless and nondestructive measurement technique to control and monitor the processes of diamond treatment with ion implantation and subsequent annealing. As well known in the past [2], naturally occurring crystalline allotropes of carbon are either the tetrahedral diamond or the layered graphitic structure. Tetrahedral and layered structures represent the extremes of nearly isotropic solids on the one hand and solids with nearly two-dimensional properties on the other. Amorphous states of carbon are still other allotropes of carbon like
0042-207X/99/$ - see front matter ( 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 9 9 ) 0 0 1 5 3 - 0
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the well-known silicon amorphous-semiconductors which are by de"nition isotropic. The desire to understand the extreme properties of amorphous carbon has stimulated a number of structural investigations and attempts at modeling the microstructure of the amorphous carbon matrix [11}13]. These studies uncovered a metastable phase of carbon consisting of a mixture of tetrahedral (diamond-like) and trigonal (graphitic) bonding of the matrix. In the study of radiation e!ects on diamond crystals, it is important to not only use samples with low impurity content but also necessary to pay attention to the inhomogeneous distribution of the impurities, which re#ects the growth conditions. In order to meet such a requirement, in this report, a photon absorption spectroscopy which includes a position-sensitive scanning and a two-dimensional topographical imaging capability is demonstrated for defect characterization of virgin and modi"ed diamonds, before and after ion implantation, respectively. The commercially produced synthetic diamond crystals (type Ib) by Japan-Sumitomo Elec.Co., under the conventional high-pressure, high-temperature techniques, are used to investigate the dependence of diamond optical properties on the ion #uences and implantation temperature and subsequent thermal annealing. Molecular hydrogen ions with relatively low energy have been used in the past to investigate the surface submicrometer scale hybridization of diamond, because it was considered that hydrogen played a crucial role in stabilizing the tetrahedral bonding [5,14}16]. In particular, the following hypothesis was considered for the purpose of understanding the variation of diamond optical absorption when exposed to ultraviolet light in the UVVIS range for di!erent locations of an original diamond crystal, as-implanted and thermal annealed diamond, and it was suggested that a change in the microstructure of the amorphous carbon matrix from a fourfold-tothreefold transition exists. This e!ect was discussed in terms of the in#uence of temperature on annealing of defect states in ion-implanted diamond. The role of nitrogen (as the main impurity in type-Ib diamond) in the optical absorption properties of diamond is demonstrated via a wavelength spectrum in either non-irradiated or irradiated diamond. A description of the measuring principle of optical transmission or absorption is given in Section 2.1, while detailed experimental procedures are discussed in Sections 2.2 and 2.3. The position dependence of relative transmission intensity on photon energy, optical density, relative optical band gap and thermal annealing e!ect of virgin and implanted diamonds are listed in Sections 3.1}3.4. The change of microstructure associated with the variation of optical absorption from the electronic transitions in the ionhybridized diamonds are tentatively attributed to the displacement damage caused by collision cascades and the introduction of the disorder-network compositions in
quasi-tetrahedral structure of carbon bonding, in the last section.
2. Samples and experimental details 2.1. Measuring principle Considering the subtraction of a background signal and parasitic light (#uorescence and re#ectance) from the standard and object spectra, the dark current and parasitic e!ect are measured during high-voltage adjustment. Taking into account the fact that the instrument response arises from spectral distribution of the illumination, transmittance of the micro-spectrophotometer and sensitivity of the detector, a quotient value between the object spectrum and the standard spectrum is used in measurements, as a relative transmission (%¹), which results in a corrected spectrum. The measured absorbance value is calculated from the %¹ of transmittance from the following equation: Meas.absorb.value"2!log(%¹),
(1)
where the %¹ is a normalized ratio between the transmitted intensities for a virgin (or standard) and another virgin point or irradiated (object) areas. The %¹ is given by equation %¹"[I (j)!I (j)]/[I (j)!I (j)]]100. (2) 0 1 4 1 In Eq. (2), the intensities de"ned as I (j)" standard (s), * object (o), and parasitic (p) spectra, are expressed in percentage. On the other hand, the measured relative optical density is de"ned as Log (I /I ) as follows: 10 4 0 *OD"(OD) !(OD) (3) 0 4 which is also equal to the measured absorbance value. 2.2. Diamond sampling and ion implantation Commercially produced synthetic diamonds were con"rmed by absorption spectra at around 470 nm to be type-Ib diamond crystals with volume of 3.5]3.5] 0.5 mm3, and the (1 0 0) facet exposed to coming projectiles. The transmission intensity imaging (topology) at 430 nm was used to survey the distribution of transmitted light intensity as well as the concentration of nitrogen. Due to the considerable variations of nitrogen impurity in the di!erent growth sectors at the submicrometer scale, there is an e!ect on the corrected quotient value in the very sensitive ultraviolet-visible region during optical transmission measurement. All of the diamond samples were irradiated with 40 KeV H` ions, at 100 K, except for two samples irra2 diated at room temperature. The base pressure before implantation was less than 1]10~4 Pa. Implanted doses
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ranged from 1]1015 to 2]1017 H`/cm2 and half-area of the surface of each sample was covered along a diagonal line during ion implantation in order to make a comparison between virgin and implanted areas. By obtaining a standard spectrum from a virgin area and an objective spectrum from the implanted area of diamond, a ratio was derived from the two spectra, considering the instrument response. The projected ranges and longitudinal straggling of the implanted H` and the other collision damage distribu2 tion were calculated using the TRIM program [17] with surface binding energy of 4.5 eV [18]. The maximum range of the possible damage caused by projectiles in diamond is estimated to be 140 nm, which means any change of atomic structure and electronic states distinguished it from the original diamond crystals. 2.3. Transmission and absorption spectroscopy The optical absorption measurements were performed at room temperature using a micro-spectrophotometer. First, the wavelength dependence of the transmittance in the UV-VIS region was recorded using a single-pass monochromator, "tted with a holographic grating and a photomultiplier tube. The light source was a xenon lamp with output in the 230 } 850 nm wavelength range, coupled with light shutters and motorized 8] "lter changer. The whole procedure, including the position scanning and data acquisition was controlled by a wellset software. Then, conversion of the transmission values to absorbance ones dependent on Eq. (1), was carried out taking into account an automatic correction of dark current, instrument response and parasitic light. The irradiated diamond samples were annealed for 2 h at temperatures in the 300}12003C range, using a furnace under #ushing argon gas.
3. Results and discussions
209
Figs. 1(a)}(c) show a transmission intensity distribution of two-dimensional micrographs for three synthetic diamond specimens. For the sake of clarity, they are labeled as specimens 1, 2 and 3, respectively. The small pictures inserted in Fig. 1 are direct displays of transmission intensity with di!erent colors and in a range of 0}150%, which are symmetric with respect to the center of a circle. Each of the areas measured by a directly incident focused light 20 lm diameter was imaged using this automatic surface scanning. Each area was carefully chosen to be about 1800]1800 lm2. In Fig. 1(a), the transmission intensity is nearly constant, while a dramatic variation is observed in Figs. 1(b) and (c) with changes of 30, 120 and 150%, respectively. The optical inhomogeneity of original diamond crystals are well revealed by the two-dimensional topographic images obtained in the wavelength of j"430 nm. Furthermore, this circle-round transmission intensity variation shows the nitrogen impurity distribution in arti"cial diamond, even though we have learned that the concentration of nitrogen atoms in the specimens was about 10}120 ppm by IR absorprion. 3.1.2. Ion implantation and thermal annealing The specimens labeled above were implanted with 40 KeV-molecular hydrogen ion (H`) at 100 K and 2 room temperature, and their optical transmission characteristics are shown in Figs. 2(a)}(d) for di!erent processes. Fig. 2(a) is a typical two-dimensional image of as-irradiated diamond (No.1) at 100 K, with an irradiation #uence of 6.3]1016 H/cm2. Fig. 2(b) is for specimen 2, also corresponding to the dose of 6.3]1016 H/cm2, but the sample temperature during implantation was naturally kept at room temperature and the "xed wavelength of adjusted light was at 380 nm. Fig. 2(c) is corresponding to the dose of 1.3]1017 H/cm2 for specimen 3, at 100 K. Fig. 2(d) is an image of specimen 3 which experiences a successive thermal annealing at 300 and 5003C for 2 h in each step after implantation. But there is no obvious change during the annealing processes.
3.1. Topographical image of synthetic diamond 3.2. Relative absorbance dependent on position-scanning 3.1.1. Virgin specimens Synthetic diamonds are notorious for their optical inhomogeneity, which is caused by a non-uniform distribution of point defects [19}22]. To get an outline of the inhomogeneity distribution and select a more transparent position to obtain the standard spectrum, we obtained an optical transmission topographical image using the enhanced near UV-VIS absorption edge by the single-substitutional nitrogen centers, which provides the distribution of nitrogen impurities in diamond. The topographical images were obtained by photographing the specimen plates via an optical transmission microscope "tted with a monochromatic UV-VIS (j"380 and 430 nm) light source.
According to the Eqs. (1)}(3) presented in Section 2, the transmission intensity for the standard spectrum (I ) was 4 obtained at the most transparent point as shown in Fig. 1(b) (position A). However, the transmission intensities for the object spectra (I ) were obtained in other points 0 di!erent from position A (in Fig. 1b) under an X}> coordination system (as shown in the inset diagram of Fig. 3a). Typical results for specimen 2 are shown in Figs. 3(a) and (b) in a scanning map of 16 points. It is obvious that the absorbance di!erence at any two positions on the whole sample seems to be near zero in the visible region over 500 nm but a dramatic variation of the values is present in the UV-near visible range. This means that
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Fig. 1. Near-visible (j"430 nm) transmission topographs of the synthetic diamond plates, (a) specimen 1 with a scanning area of 1800]1800 lm2, (b) specimen 2 with a scanning area of 1700]1800 lm2, (c) specimen 3 with a scanning area of 1800]1800 lm2.
nitrogen is the dominant impurity in diamonds and the samples used here are complete crystals. It is apparent that the inhomogeneity of the optical property of synthetic diamond has been veri"ed by the position-sensitive optical density measurement; moreover, the data presented indicate that the nitrogen atoms are in substitutional positions. Fig. 4 shows the wavelength dependence of absorbance via scanning "eld length in micrometer along the X-axis. The wavelengths were chosen around absorption edges from 430 to 500 nm, and the spectra were obtained after the specimen 2 was exposed to molecular hydrogen ion implantation at RT, with 40 keV and 6.3]1016 H/cm2. Typical results in Fig. 4 include three points: (i) a clear increase of optical density in ion-processed area was observed; (ii) within the absorption edge of type-Ib diamond, an enhanced e!ect on absorbance was observed for a decreasing of incident wavelength, especially for
as-implanted diamond; and (iii) the more sensitive wavelength for the characterization of optical inhomogeneity of synthetic diamond is 430 nm. 3.3. Estimation of damaged layer of diamond As well known, the ion-induced radiation damage along the ion projectile is produced during ion implantation at room or low temperature, by ion collision cascades with target atom and recoils. Defects such as interstitials, vacancies, and vacancy}impurity complexes are present in as-implanted diamonds [7,8,23], which are accompanied by the change of atomic con"guration and/or electronic states of carbon bondings. Naturally, the optical properties such as re#ection, refraction index and absorption coe$cient linked with electronic transitions are sensitively in#uenced by the atomic microstructure in diamond. Usually, the radiation
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211
Fig. 2. Transmission topographs of synthetic diamond implanted with low energy (40 keV) H` ion for specimens 1, 2 and 3 (denoted as Nos. 1,2,3), 2 respectively. (a) implantation on No. 1 with a dose of 6. 3]1016 H/cm2, at 100 K, j"430 nm, (b) implantation on No.2 with a dose of 6.3]1016 H/cm2, at R.T, j"380 nm, (c) implantation on No.3 with a dose of 1.3]1017 H/cm2, at 100 K, j"430 nm, (d) after thermal annealing at 300 and 5003C for 2 h each step, j"430 nm.
damage pro"le induced by energetic ion in solids and, specially in diamond, was determined by some experiments [8,24}26] as well as by predicting calculations. Therefore, in order to learn the depth range of modi"ed layer of diamond, and to correlate it with the e!ects that the impinging ions may induce on the layer, computer simulations of the collision cascades for the ion species
were performed, using the computer simulation code (TRIM-91) with a surface binding energy of 4.5 eV, mass density of 3.52 g/cm3 and atomic displacement energy of 45 eV [27], as shown in Fig. 5. In this "gure, the TRIM results are shown for the "nal ion and vacancy distribution for H` irradiation. Al2 though the projected range (R ) and the straggling (*R ) 1 1
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Fig. 3. The measuring position dependence of absorbance in specimen 2 in the range of ultraviolet to visible regions, (a) X}> coordinating distribution for eight points, (b) X}> coordinating distribution for other eight points.
of the ion are approximately 1200 and 150 As , respectively, the "gure makes it clear that the damage is not con"ned to a layer which is buried beneath the surface, but in fact extends from the surface to a depth of about 1400 As (+R #*R ), following a mechanism similar to 1 1 that discussed in Ref. [28]. On the other hand, because diamond possesses a very low di!usivity at normal conditions, the extension of damage pro"le into the deep should be neglected during optical measurement. Thus, consideration of an approximately 1400 As damaged layer from the diamond surface to the end of the range was reasonable for the optical band gap estimations.
3.4. Annealing ewect of defects 3.4.1. Relative optical gap of the modixed layer In order to investigate the microstructure of a modi"ed layer of diamond and its evolution trend with annealing temperature, a relative optical gap which re#ects the variation of energy band structure, has been introduced by plotting (aE)1@2, as a function of photon energy [29] (aE)1@2"i(E!E ), (4) 3,015 where E is the photon energy, i is a constant, and E is 3,015 the relative optical gap obtained by extrapolation of the
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213
Fig. 4. The variation of absorbance of specimen 2 depends on the wavelength of incident light, for implanted sample by H` at room temperature. 2
linear part of the curves to a"0. In Eq. (4), a is a relative optical absorption coe$cient de"ned as (1/t)ln(10)*OD, and t is the thickness of that modi"ed layer overlying diamond. From the plotting based on Eq. (4), the approximate values of relative optical gap of 1.88, 1.96 and 2.05 eV, corresponding to as-implanted, thermal annealing at 300 and 5003C, were obtained at room temperature, respectively. In this way, the relative optical gap for virgin diamond is about 2.4 eV. Fig. 6 is a plot of the relative optical gap vs. annealing temperature for the damaged layer. Based on the measured values with mean error (1%, an approximate correlation between the relative optical gap (E ) and 3,015 annealing temperature (¹ ) was derived as follows: ! E "m¹ #f, (5) 3,015 ! where m and f are the "tted parameters. Replacing the E with a value of 2.4 eV as the relative optical gap, 3,015 measured on a virgin area, a derived temperature of about 15503C is obtained. It is just the temperature value around which the isolate-substitutional nitrogen atom begins to move in electron-irradiated type Ib diamond [30]. The temperature dependence of the relative optical gap in Fig. 6, therefore, shows a relationship of some optical absorption components and atomic structure development in subsequently thermal annealing. A possible explanation for this microscopic thermodynamical process is that the clustered size of sp3 or sp2 con"gurations of nitrogen}carbon or nitrogen}vacancy complexes have been changed upon thermal driving. The fact that the relative optical gap increases with annealing temperature is contrary to the annealing result
of a-C : H "lm by Smith [31]. This means that the amorphous layer in diamond produced by energetic H` ion doping is obviously di!erent from plasma deposited a-C : H "lms. Probably, the modi"ed layer of diamond by H` implantation at low temperature possesses 2 both properties of i-C (ion-beam deposition carbon) [32,33] and a-C : H bondings [34}37] if hydrogen is present, based on the evolution trend of the optical band gap. The thermal excitation of point defects such as interstitial, vacancy and vacancy}impurity complex is certainly induced during thermal annealing at a stirring temperature. Usually, the critical temperature for the migration of many kinds of defect compositions in diamond, after undergoing an irradiation, is reduced, depending on the type of diamond and the irradiated conditions [22,38]. We could not make an unambiguous conclusion on what kind of defect was annealed in this stage, but the increase of the relative optical gap with temperature can support this view point. Moreover, the structural defects of carbon atom coordination is the most important color center in the amorphous diamond layer after interaction with energetic ions. The fraction of atomic coordination with fourfold (sp3) or threefold (sp2) electronic states is a predominant factor for the optical absorption property, which is consistent with the TBMD (tight-binding molecular dynamics) theory for the amorphous carbon network system [13,39}41] However, the presence of sp3 to sp2 ratio in the damaged layer of diamond depends strongly on the dose levels and processing temperature during ion implantation and subsequent annealing.
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Fig. 5. The simulation of defect range in diamond with TRIM- 91. A scale of &140 nm from the surface to the end of damage pro"le was estimated.
Fig. 6. Relative optical band gap (E ) of modi"ed surface layer in diamond by H` (40 keV, 6.3]1016 H/cm2) vs annealing temperature ¹ (3C). 3,015 2 !
3.4.2. Thermal annealing Post-thermal annealing treatment was performed on the two synthetic diamond crystals (type-Ib) which were implanted by 40 KeV H` with dose ranges of 5]1015 2 and 1]1016 H/cm2, at 100 K and RT. A light brownish color appeared on the irradiated area for the two processed samples, and optical absorption measurements showed that uniform enhancement of absorption in the UV}visible wavelength region was induced, especially, more clearly in a two-dimensional topographical image.
Subsequently, the thermal annealing treatments of samples under argon-atmosphere furnace, as shown in Figs. 7(a) and (b), were performed at 300, 400, 500, 600, 800 and 12003C for 2 h at each temperature, respectively. In the near ultraviolet and 470}800 nm wavelength regions, the normalized transmission spectra show an obvious thermal annealing e!ect. From 300 to 5003C the transmission intensity gradually increased by 5%, while when the annealing temperature was increased to 600 and 8003C, the transmission value remained nearly at
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215
Fig. 7. Thermal annealing e!ect of H` ion processed diamond (type-Ib), and the relative transmission intensity ranged from 5 to 100% in 380}800 nm 2 intervals. (a) sample with 5]1015 H/cm2 at 100 K, (b) sample with 1.0]1016 H/cm2 at RT.
98% of the incident light intensity. At the same time, the distinguished colored areas were annealed at 8003C. However, when the annealing temperature was increased up to 12003C, the transmission intensity decreased by 10% compared to that annealed at 8003C. In particular, it is easy to understand that the di!erence in Figs. 7(a) and (b) for their tendency and the scale of normalized spectra is the same measured for standard and objective
spectra at reversal points, i.e. between the more transparent and the more absorbing areas for the two specimens, respectively. According to the measuring principle, the transmission intensity is a ratio of the object spectrum (irradiated area) to the standard spectrum (virgin area) intensity, and the two spectra for each time were measured at the same positions with an X}> coordination system. In this way,
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the information obtained by annealing treatment is from one source and the standard spectrum from virgin point was maintained identical at the present temperature. Thus, the dependence of transmission intensity on the annealing temperature indicates the development of damaged structure in the surface layer of diamond by H` irradiation. 2 From the results of thermal treatments on molecular hydrogen ion implantation of synthetic diamond crystals mentioned above, the migration of interstitial defect below 5003C could be expected during 2 h annealing for each temperature, which is based on the evidence that a reduced peak intensity around 400 nm in the sample implanted with 1]1016 H/cm2 at RT has been found in Fig. 7(b). But for this featured peak, a larger increase of transmission intensity was observed when annealing temperature was increased above 6003C. This considerable change can be explained by the migration of vacancies. With further increase of annealing temperature up to 8003C, the transmission intensity gradually saturated and the boundary between the virgin and the implanted area became unclear, which implies the recovery of the damaged area with an increase in the fraction of diamondlike structure (sp3). On the other hand, in Fig. 7, a reversed change of transmission value at 12003C indicates the appearance of other compositions in diamond, such as single-substitutional impurity nitrogen atom which may move to form complexes with vacancies or combine with neighbors to form nitrogen aggregates so as to produce a series of new color centers. Referring to the previous investigation by others [30], the temperature for the excitation of nitrogen moving in type-Ia,b diamonds is around 18003C and reduced to 15003C for electron-irradiated diamond, but here we got a smaller value for molecular hydrogen ion implantation either at low temperature (100 K) or RT.
4. Conclusions A submicron diamond-like amorphous carbon (DLC) layer was produced in synthetic diamond by 40 KeV H` ion implantation, at liquid nitrogen and room tem2 peratures. The diamond microstructure induced by ion implantation re#ects band-gap-like values depending on the ion #uence and the temperatures for post-irradiation thermal annealing. The irradiation e!ects were characterized through a derived micro-spectrophotometer, by which the located position-scanning measurement became possible. The gradual increase of the absorption coe$cient is in good correlation with the loss of the original transparency of the irradiated region which depends strongly on the variation of the implanted dosage but weakly on the substrate temperature, implying that the predominant displacement damage is the main process for the partial amorphization of the tetrahedral
coordination of the carbon lattice. The annealing temperature dependence of the relative optical gap E in the 3,015 absorption edge, indicates that the atomic structure in the amorphized layer can be attributed to the mixing site network of fourfold (sp3) and threefold (sp2) states. The thermal evolution of E was much di!erent from that 3,015 of a-C : H (DLC) "lms made by plasma deposition, indicating a transformation mechanism from the diamondlike structure (sp3) to graphite-like structure (sp2) of carbon bonding. However, the presence of C}H bonds and chemical e!ect during subsequent thermal annealing might have possibly resulted from the increase of E 3,015 if the hydrogen composition still remained in that buried layer till to 8003C or more. Furthermore, with this method, the two-dimensional topography of optical transmission intensity has been successfully applied to a survey of the optical inhomogeneity of defects in synthetic diamonds, and it is also possible for a limiting measurement of damage induced by energetic ion beam.
Acknowledgements We gratefully acknowledge the award under the STA Scientist Exchange Program of Japan and Natural Science Foundation of Xinjiang University. The authors would like to thank the sta! of the Takasaki-TIARA center for their invaluable assistance with ion-beam implantations.
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