High energy ion implantation into diamond and cubic boron nitride

Materials Science and Engineering, BI1 (1992) 179-190

179

High energy ion implantation into diamond and cubic boron nitride Alexander M. Zaitsev Byelorussian State University, Minsk 220080 (Byelorussia)

Abstract The influence of ion implantation on diamond and cubic boron nitride is discussed. It is shown that high energy ion implantation (0.5 MeV nucleon- ~and higher) is a promising ion beam method of modification causing new effects in the superstrong semiconductors: (1) formation of multilayer impurity-defect structures which allows one to make three-dimensional doped structures; (2) stimulation of impurity and defect diffusion along the tracks of the high energy ions (using this effect the doping of the superstrong semiconductors with low temperature (about 1000 °C) diffusion at depths of several microns is possible);(3) formation of high pressure (several gigapascals) local regions.

1. Introduction

At present one of the aims in the development of solid state electronics is the exploitation of the superhard materials (diamond and cubic boron nitride) as semiconductors. On the one hand the great strength of the interatomic forces of these substances results in a very high hardness and makes them a leading candidate for mechanical uses. On the other hand the great interatomic strength also appears to be a very significant property when the material is used as a semiconductor. The attraction of new semiconducting ~naterials for electronics is characterized by the transition to "stronger" crystals consisting of lighter atoms: germanium ~ silicon -* carbon which are the A4 semiconductors; GaAs, InP--'AIN, A I P - B N which are the A3B 5 semiconductors. Here the term strong crystal is meant in a wide sense, i.e. a crystal with a high resistivity to different external influences (mechanical, chemical, radiation, thermal etc.). Such a tendency is not incidental and reflects a certain regularity. As the atomic number decreases, the strength of the interatomic force grows and the interatomic distance decreases. In consequence we have increases in the mechanical hardness, the energy gap, the thermal conductivity, the interdefect interaction and-the Debye temperature. So all those parameters of the material which stipulate the characteristics of an electronic device made from it improve. 0921-5107/92/$5.00

Since diamond and cubic boron nitride are the last in the rows of A 4 and A3B 5 semiconductors, they are considered to be the materials with the help of which the upper limit of solid state semiconducting devices can be reached. Assimilation of wide gap superhard semiconductors into electronics reflects the increasing ability of technology to meet the required needs in the growth of these materials with adequate quality, in their mechanical and chemical treatment, in the fabrication of p- and n-type conductivity regions with given impurity-structural parameters, in preparing interfaces and ohmic contacts and so on. As the crystals become increasingly harder, ensuring the technological operations becomes more and more difficult, because those properties which provide the highest resistivity of the material are found to be the obstacles to technological treatments. As a result, many of the usual methods of growth, doping, chemical and mechanical treatment are not applicable to the new semiconducting materials. The situation is most evident in the case of the superhard semiconductors diamond and cubic boron nitride. In fact, diamond and cubic boron nitride can be grown as perfect crystals only by using complicated high pressure, high temperature equipment. It is extremely difficult in these substances to realize the necessary diffusion for impurity doping. The mechanical and chemical treatments of diamond and cubic boron © Elsevier Sequoia/Printed in The Netherlands

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nitride are also specific and rather complicated processes. The main question which arises then is to find a method of fabricating the required impuritydefect structures during device formation. Obviously, of all the modern methods, ion implantation is the most convenient and the most applicable to superhard semiconductors. This is indicated by the ability of ion implantation to overcome the strength of the superhard matrix. Further ion implantation is the most accurate, precise and pure method of impurity doping. Also ion implantation allows one to realize almost any geometry of the doped region, including the formation of submicron regions near the surface. Finally, ion implantation results in unique defect structures in superhard semiconductors which can serve as the basis for new practical devices. The investigation of ion-implanted diamond and cubic boron nitride is a necessary step in the determination of the electronics on superhard semiconductors. The goal of this is information on the content, properties and spatial distribution of impurity-defect structures made by the ion beam. 2. Interaction of accelerated ions with a solid matrix When fast ions enter a solid target, they begin to stop via inelastic collisions with electrons and elastic collisions with the nuclei of the target [1, 2]. In Fig. 1 the nuclear stopping power (dE/dx)n and the electronic stopping power (dE/ dX)e vs. the depth of ion penetration is shown. At very high energies when the ion velocity VI well exceeds the velocity Vi of the deep shell electrons of the target atoms (VI'> V~), the ion loses all its electrons and moves as an entirely stripped nucleus. The fast ion interacts with separate, almost fixed electrons and nuclei of the matrix and is stopped mainly because of collisions with electrons. The nuclear stopping is extremely low. As the ion slows down, the electron stopping power reaches a maximum. This is the region where VI = Vi. The ion nucleus begins to capture electrons. The stopping of the ion due to electron exchange mechanisms between the deep shells of the ion and the matrix atoms becomes important. This process goes on until VI equals the velocity V,, of the outer electrons of the target atoms (V~= Vo). When

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VI < V,,, the ion has all electrons on its shells and then an almost neutral atom moves in the matrix. In this region the (dE/dx)n value increases drastically and well exceeds the (dE/dx)~ value. The energy transfer from the ion to the matrix occurs via collisions between neutral atoms. The energy of the ion implantation at all stages of the stopping transforms into heat, radiation and impurity and radiation defect production. The radiation defect production uses only a small part of the ion energy and so on the whole the concentration of the defect production does not follow the density of the ion energy loss. 3. Why should high energy ion implantation be applied to superhard semiconductors? Among the advantages of the superhard semiconductors is their high thermal conductivity and

181

low diffusivity. This is why one can consider these substances as ideal semiconductors for superdense powerful microelectronic devices. The way to make such devices is by the formation of multilayer three-dimensional structures consisting of very tiny elements. When using ion implantation with conventional energies (from tens to hundreds of kiloelectronvolts) one can obtain only very thin semiconducting layers near the surface of the substrate. The depth of this layer usually does not exceed 1 pm, and its impurity defect structure is qualitatively identical throughout the depth [3]. By increasing the energy of the ion implantation one can easily obtain buried doped layers at different depths up to hundreds of micrometres in the semiconductor substrates and thus form multilayer structures. Unfortunately obtaining small deeply buried ion-doped regions is not possible since there are physical restrictions to the ion implantation technique. Owing to the transverse and longitudinal scattering of ions moving through the solid matrix it is impossible to make ion-doped regions with dimensions less than double the values of the mean square spread of ranges 2 A R p and 2 A Y [4]. The situation is enhanced with increasing ion energy since straggling increases too. However, entirely new possibilities arise if one tries to use the impurity defect structures on the basis of high energy ion tracks as microelectronic units. Since the diameter of the track in diamond and cubic boron nitride is very probably several nanometres, the density of such units can reach a value of up to 10 ~4 cm -2. Because of the extremely small dimensions of the elements the prevention of their rapid destruction is an important problem. It is clear that such a structure can be formed only in very hard stable matrices where atom diffusivity does not exceed about 10-22 cm 2 s-1 at working temperatures.

4. Impurities and defects

zero-phonon lines and phonon replicas (Fig. 2). The spectral parameters of these lines are sensitive to external forces (mechanical, electrical and thermal) and this is why one can apply different luminescent methods (e.g. piezospectroscopy and the Stark effect) to determine the electronic and spatial structure of the defects. One of the easier ways to obtain information about the vacancy or interstitial nature of the luminescent defects is by

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Defects in diamond and cubic boron nitride created during ion implantation have been systematically investigated mainly by the luminescence technique. Luminescence is an effective method of studying superhard materials. Since diamond and cubic boron nitride have high Debye temperatures (about 2000 K), the optically active defects in these crystals cause M/bssbauer-type centres with very distinctive

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determination of the value of their thermal softness [5, 6]:

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where ST° is the reduced thermal softness, To is the Debye temperature of the crystal, T is the temperature during measurement and F is the spectral iinewidth. The results of the investigation of many luminescent centres in diamond, cubic boron nitride, beryllium oxide and silicon allow us to conclude that the vacancy-type defects have ST°>0.02 and interstitial-type defects have S~.° < 0.01. The determination of the interstitial or vacancy nature of the radiation defects is very important for understanding the nature of the ion-implanted layers generated. For example essentially different spatial distributions of the vacancies and the interstitials explain the formation of tracks during high energy ion implantation. In diamond the well-known GR1 centre is a vacancy centre. The centres TR12 and 3H also seem to be of vacancy type. The intrinsic interstitial-type radiation defects in diamond result in the luminescent centres with the zero-phonon lines 484.5, 510.8,412, 511.5,590, 509, 563,602 and 533.5 nm [7]. The regions of thermal stability of some centres are shown in Fig. 3. The most prominent impurity centres in diamond are the nitrogen-containing centres. Some of them at 503 nm (H3), 496 nm (H4), 415 nm (N3), 638 nm and 575 nm are of vacancy type [8]. The centres with zero-phonon lines at 389 and 441.5 nm are nitrogen centres of interstitial type [8, 9]. In ionimplanted diamond, besides nitrogen many other impurity atoms form optically active centres. A list of the elements with the wavelengths of the zero-phonon lines of the corresponding centres is as follows: helium (522.5, 536.5 and 560.5 nm), neon (716 and 719.5 nm; the NG centre and 518 nm), silicon (736 nm), nickel (882.7, 884.4 and 484 nm), silver (398.5 nm), xenon (816 nm), thallium (614 nm), chromium (741 nm) and zinc (518 nm) (Fig. 4). In cubic boron nitride, only the intrinsic radiation centres RC1 (546 nm), RC2 (577 nm), RC3 (623 nm), RC4 (668 nm) and C (591 nm) appear in the luminescence [5]. These defects appear to be of vacancy type, having just nitrogen vacancies. No impurity defects in cubic boron nitride have been detected for certain up to now.

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5. The ion-implanted layer An important purpose of ion implantation is impurity doping of the target. The spatial parameters of the ion-doped (implanted) layer (i.e. the depth and width) are determined by the mean values of the projective ranges Rp of ions and their longitudinal and transversal stragglings, ARp and A Y respectively. Figures 5 and 6 show examples of ionimplanted layers in diamond irradiated with high energy nitrogen and xenon ions. The ion-implanted layer after high energy ion implantation is buried and quite localized. It is an important advantage of the method. The conservation of the impurity-free layer near the irradiated surface is an obligatory condition when making multilayer three-dimensional electronic structures. A good theory for the description of conventional energy ion implantation has been developed. This theory predicts Rp and ARp values rather exactly. In this case the distribution of the implanted atoms through the depth depends mainly on the elastic collisions of the

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ions with the atoms of the target [10]. The theory for the passage of high energy ions has not been worked out as satisfactorily and overall gives only a qualitative picture of the ion-implanted layer [11]. The least satisfactory results are for the prediction of the ARp value and the shape of the implanted concentration distribution. The calculated ARp value is lower than the experimentally measured value and the asymmetry of the impurity profile is inverse to that predicted for asimplanted layers. These discrepancies can be seen in Figs. 5 and 6. The experimental impurity profiles have asymmetric tails in depth. The unsatisfactory theoretical description of high energy ion implantation results from inadequate accounting of the electronic stopping mechanisms on scattering of the ions when their velocity reaches Vi values. In particular sufficient correction to the theory can result from obligatory accounting for the statistical character of deep electron-ion interactions and the effects of channelling.

6. Disordered regions and point defects High energy ions form defect structures in semiconductors qualitatively different from those which are characteristic of layers implanted with ions of conventional energy. The physical bases of this difference are, firstly, a much stronger electronic stopping of the high energy ions and, secondly, a change in the dominating defect production mechanisms. The value of the electronic stopping power for ions of mass 100 a.m.u, in a superhard semiconductor lattice reaches several kiloelectronvolts per fingstr6m [12], which is 103 times greater than that for nuclear stopping. New ionization mechanisms relevant to defect production which are absent for the stopping of ions with conventional energies become very important for high energy ion implantation. The experimental investigation of semiconductors irradiated with high energy ions shows that strong disordering of the lattice takes place only at the end of the ion paths [13]. The layer near the

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irradiated surface remains almost undamaged [14]. The distribution profile of the disordering in ion-implanted layers calculated with the Monte Carlo method taking into account only nuclear ion stopping and experimentally obtained profiles, coincides quite well [15]. The same result can be observed also in diamond implanted with cobalt ions of approximate energy 1 MeV a.m.u. -1 [16]. It is seen that the concentration change of the paramagnetic centres of amorphization through the depth follows the theoretical curve of the nuclear stopping power (Fig. 7). The distribution of point defects through the depth of the layer irradiated with high energy ions is complicated when compared with that of amorphous centres. Different types of point defect are introduced by different mechanisms and their distribution profiles can be described by (dE/dx)~ or (dE/dx). dependences or by some complex combination thereof. Figure 7 shows also the dis-

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tribution of the point paramagnetic O1 centres [17] through the depth of diamond irradiated with cobalt ions. The distribution is well described by (dE/dX)e. In cubic boron nitride irradiated with the high energy boron ions the RC centre distribution is explained by the dominant action of nuclear defect production mechanisms (Fig. 8). The distribution profiles of the GR1 centre and 3H centre intensities in diamond implanted with nitrogen ions are only quantitatively like the (dE/ dx)e dependence (Fig. 5). The distribution of the luminescence radiation centre N3 at 575 nm in nitrogen-containing Ia diamonds irradiated with copper ions of approximate energy 1 MeV a.m.u. -~ (Fig. 9) cannot be described either by (dE/dx)e or by (dE/dX)n curves, not even qualitatively. In the same samples the depth distribution of the centres at 638 nm and the B band qualitatively follow the curves of the electronic and nuclear stopping powers respectively. Some of the radiation defects for high energy ion implantation penetrate much deeper than the Rp value. For example one can see the very extended tail (up to 25 /~m) distribution of the

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575 nm centre and the H3 centre in copperimplanted diamonds (Fig. 9). One may note that Rp for copper ions of energy 58 MeV is only about 8/~m. This effect may occur for several reasons, the most probable of which are the channelling of a portion of the ions and the development of some microcracks behind the ion-implanted layer. If the ions have a high enough energy, the secondary irradiation with light particles due to nuclear reactions between the ion nuclei and the matrix nuclei can also stimulate the appearance of radiation damage at depths exceeding the penetration depth of the primary ion beam. 7. Track formation The most significant feature of defect structures of solids irradiated in high energy ion implantation is the appearance of tracks--thin

long highly damaged regions formed along the ion paths [18]. The model of ion track formation is as yet far from completion. Nevertheless a list of possible mechanisms initiating the tracks through the passing of energetic ions contains the following. (i) A high density of ionization leads to a significant reduction in the threshold energy E d for removing atoms out of the lattice sites. The smaller value of E d increases the probability of the displacement of those matrix atoms which have obtained small amounts of energy from the moving ion. (ii) The "Coulomb explosion" mechanism is very effective and takes place when the velocity of ions becomes equal to the velocity of K shell electrons of matrix atoms (i.e. carbon, boron or nitrogen atoms). At such ion velocities the ionization of K shells of lattice atoms occurs followed by multiple ionization of these atoms owing to Auger recombination [19]. The existence of multiply charged ions in the lattice results in repulsion. This effect strongly increases in the matrix containing the activated donors. (iii) The thermal mechanism is also considered as a track production mechanism. If the energy loss of the ion and consequent heating of the lattice around the ion path is high enough to

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evaporate the matrix substance, the appearance of the track can take place. The value of the energy deposition E T which is enough for track formation depends only on the type of matrix and does not depend on the ion species. A tentative E T value for diamond is about 0.3 keV A- 1. The structure of the track is not firmly established. However, one can imagine the track as a multivacancy cord surrounded by an interstitialrich envelope so that the atom density in the track centre is significantly lower than that at the periphery (Fig. 1). 8. Structure of ion-irradiated layer The experimental data show that in high energy ion implantation the radiation defects are introduced into the lattice at certain stages of the stopping of ions. It results in a multilayer defect structure of the irradiated layer. The most spe-

Fig. 10. The scheme of the buried local high pressure region: 1, high energy ions; 2, irradiated but non-disordered irradiated area; 3, disordered and ion-doped region; 4, mask. The shape of the GRI zero-phonon cathodolumineseence line recorded in the cases I and II (carbon ions at 82 MeV and a dose of 1.2 x 1016 cm- 2; d < 0.5 mm).

cific layers in this structure are the following (see Zig. 1). (i) A slightly damaged layer occurs at the surface. The layer contains, in the main, homogeneously distributed point radiation defects. The layer is formed when V, ,> Vi. (ii) A layer of increased concentration of point defects which are distributed non-homogeneously is formed because VI = V~. The layer is subjected to significant mechanical strains. If the electronic stopping is high enough (i.e. (dE/dx)e > E T ), this layer contains the tracks. (iii) The disordered layer lies at the depth of the maximum nuclear stopping power. (iv) The ion-implanted layer is doped with the implanted atoms. The defect structure of this layer is the same as the common ion-implanted layer after conventional ion implantation. (v) The layer of the secondary defect production spreads much deeper than the ion-implanted layer. The layers mentioned do not become strictly localized and may strongly overlap each other. The real position of these layers in diamond and

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cubic boron nitride implanted with high energy ions can be seen in Figs. 5, 6, 8, 11, 12 and 14. 9. Effect of high pressure due to the tracks

When the local area of the sample is irradiated with ions of high enough energy to provide the condition Rp "> 2 ARp, one can obtain in a crystal a buried impurity-defect region surrounded by an almost undisordered crystalline cover. It is clear that such a region contains many intrinsic defects and the additional implanted impurity atoms are subjected to mechanical compression [20]. The value of the pressure in this region depends on the geometry, the concentration and type of the implanted atoms, and the condition of the covering crystalline envelope. The crystalline envelope restrains the internal pressure if the minimal thickness h of the envelope exceeds the diameter d of the local region under pressure by at least twice, i.e. h > 2d. A region of high pressure was experimentally obtained in diamond by the implantation of carbon ions of energy 82 MeV at a dose of 1.2 x 1016 cm -2. The diameter of the irradiated area on the sample surface was less than 0.5 mm. The irradiated sample was studied using luminescence and Raman spectroscopy. From the shift of the zero-phonon line GR1 and the Raman line at 1330 cm -1 the pressure value in the implanted region is evaluated as 25 GPa. The localization of the implanted area is the principal condition for the appearance of the high pressure region (Fig. 10). Figure 10 shows that the mechanical strain in the sample detected by the broadening and the shift of the GR1 zero-phonon line is observed only for the sample with the irradiated local area. On the contrary, when this line is observed for the sample irradiated through the whole surface, it is found that the line has its usual shape. High pressure regions arise also in the layer containing the tracks. Since the track has a structure with an atom density gradient from the centre to the periphery, the high pressure occurs and tries to return the interstitial atoms into the vacancy-rich nucleus (Fig. 1). The high pressure in the tracks was detected in cubic boron nitride, irradiated with neon, cobalt and xenon ions of approximate energy 1 MeV a.m.u.-~. The pressure value was estimated through the spectral shift of the narrow zero-phonon line of the radiation centres RC (Fig. 1 l(a)). In first estimation the

pressure value depends on the charge of the nuclei of the irradiating ions, i.e. on the value of the electronic stopping (dE/dX)e. Figure 11(b) shows that the high pressure regions settle near the irradiated surface where (dE/dx)e reaches its maximum. The layer of maximal disordering penetrates deeper (from 2 to 7 gm) and does not contain regions of high pressure. One can also see that the tracks appear at velocities exceeding V~ for boron and nitrogen atoms. The formation of stable local areas under high pressure is possible only in materials having a strong crystalline structure and hence a low diffusivity of impurities and defects. The best

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substances in this case would seem to be the superhard semiconductors diamond and cubic boron nitride.

10. Migration of impurities and defects through the tracks One can argue that the appearance of channels such as the tracks and the strong non-homogeneity of the atom density through the crystal after high energy ion implantation must result in acceleration of diffusion. This effect indeed exists and clearly becomes apparent in diamond. It is common knowledge that there is no impurity diffusion in diamond, at least until the temperature is above 2000 °C. So the appearance of any diffusion through long distances at lower temperatures can be explained only as a result of increasing the mobility of atoms by some unusual defect structure. Some experimental data on the redistribution of impurities and defects in diamond and cubic boron nitride irradiated with energetic ions are presented below. The most convincing manifestation of the socalled "track migration" is shown in Fig. 12(a) where one can see the influence of the ion tracks on the migration of helium through diamond. Two profiles of the distribution of helium implanted into diamond with energy of several hundred kiloelectronvolts and subsequently annealed at 1000 °C are compared. One of the samples before helium implantation was preliminarily irradiated with 1 MeV a.m.u.-~ xenon ions. The second was not subjected to any preliminary irradiation. In the first case, helium atoms migrate at a temperature of 1000 °C to a depth of 6/~m, i.e. up to the depth of the xenonimplanted layer. The observed effect cannot be explained by the known stimulation of diffusion by radiation defects; hence the penetration of helium atoms does not occur at a depth below the beginning of the xenon-implanted layer which is rich in defects. That is to say the migration took place only in the track-containing layer. The helium distribution profile has two maxima: one of these is near the surface and is characterized by the depth of the initial helium-implanted layer; the second at a depth of 4/~m reflects the distribution of those helium atoms which were captured into the tracks and then migrated through them. The "track migration" is also a very probable mechanism for explaining the redistribution of

xenon atoms implanted into diamond with a high energy at a high temperature of the sample (Fig. 12(b)). In this case the xenon profile reveals a long tail to the surface. If xenon implantation of the sample was carried out at room temperature, the subsequent heating does not redistribute xenon atoms out of the ion-implanted layer. One can suppose that some xenon atoms implanted into the heated lattice do not have enough time to get into stable positions and hence migrate through the tracks back to the surface. Figure 13 represents the redistribution of the GC2 centres in cubic boron nitride after high energy ion implantation with cobalt and xenon and subsequent annealing. The GC2 centres were distributed homogeneously through the depth before implantation. After implantation some centres are in the high pressure regions (see Fig.

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-

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!

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Fig. 12. The migration through the tracks in diamond irradiated with 124 MeV xenon ions, showing the depth distribution of helium-containing centres at 536.5 nm (o) and 560.5 nm (tz) and a xenon-containing centre at 816 nm (e) in the samples (a) irradiated at room temperature first with xenon ions and then with 300 keV helium ions and subsequently annealed at 1000 °C and (b) irradiated at 1350 °C with xenon ions and then at room temperature with 200 keV helium ions and subsequently annealed at 1000 °C: ii, the layer containing the tracks; - - -, calculated helium-ion-implanted layers.

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,

,

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o

o

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¢

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Fig. 13. The depth distribution of the GC2 centres in cubic boron nitride irradiated with (a) 67 MeV cobalt ions (6 X l014 cm-2), (b) 124 MeV xenon ions (1.2 x 1014 cm -2) and annealed at 1350 °C: - - - , initial level of the GC2 centre intensity.

1) and then move out of the layer with the track into the ion-implanted layer which is the most friable. The experimental data presented for diamond and cubic boron nitride strongly support the idea of the existence of "track migration" in solids irradiated with high energy ions. Certainly these facts obtained using only the luminescence technique are not sufficient to determine the general regularities of atom migration along the tracks. Nevertheless, at present, one can try to predict the direction and depth of the "track migration" taking into consideration the species of high energy ions and hence the size of the tracks, the size of the migrating atoms and their starting location.

11. Role of nuclear reactions Nuclear reactions between high energy ions and the matrix atoms may modify the defect structure appreciably. When using light high energy ions then the Coulomb interaction barriers E c of the colliding nuclei are rather low. For example Ec values in the centre-of-mass system are 18.9 MeV for two carbon nuclei and 22 MeV for nitrogen and carbon nuclei. This means that, if the ion energy is several times greater than Ec, the ions may interact in nuclear reactions for most of their paths. On the assumption that the cross-section of the nuclear reaction is about l b, the portion of the ions which have

interacted with the matrix atoms is about 10-4 to 10 -3. In the majority of cases the nuclear reactions result in the emission of neutrons, protons and Z particles. For example the output of such particles in 160--'2.8Si reactions takes place in 74% of events. So diamond or boron nitride irradiated with high energy boron, carbon or nitrogen ions at a dose of about 1015 cm -2 is subjected to secondary irradiation with light particles at a dose of about 1011-1012 cm -2. The depth of the secondary irradiation can significantly exceed the path value of the primary ions. For instance, when diamond is being irradiated with high energy carbon ions, then protons of energy up to 10 MeV appear. The penetration depth of such protons is about 400/~m. Figure 14 shows the depth distribution of some radiation defects in very pure nitrogen-free diamond irradiated with 82 MeV carbon ions. The distribution of the radiation centre with the zero-phonon line at 575 nm is observed to be up to a depth of 220 /~m, although the carbon ion path is only 72/~m.

12. Determination of high energy ion implantation for superhard semiconductors One can now see that high energy ion implantation of superhard semiconductors causes the generation of specific defect structures which occur when the ions have a high enough energy to experience strong inelastic electronic stopping and to excite some defect production mechanisms via the ionization of K shells of carbon, boron or nitrogen atoms. The probability of lattice ionization reaches a maximum when the ion velocity V~ equals the K shell electron velocity V~. From this relation one can set the limits of high energy ion implantation ( V I > V~) and the conventional ion implantation ( VI < Vi). Using the velocity parameter tables from ref. 21 the low energy limit E h of the high energy ion implantation into diamond and cubic boron nitride can be written as

E h --- 0.5 MeV a.m.u.- 1

13. Conclusion The data presented show that high energy ion implantation is a new way of investigating superhard materials physics. High energy ions result in impurity-defect structures which qualitatively differ from those appearing after conventional

1<71i

allows us to exploit in practice some advantages of the superhard semiconductors as materials taking advantage of the upper limits of solid state electronics. The efforts made in this field promise to be fruitful.

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References

I1I

t

I I I

'i.," / \\ oo

o

50

1o0

~50

200

2~0

deo~,, p m Fig. 14. The depth distribution of the luminescent radiation centres in type IIa diamond irradiated with 82 MeV carbon ions: m, the GR1 centre before annealing; o, centre at 511.5 nm, after annealing at 1000°C; e, centre at 575 nm, after annealing at 1000 °C.

ion implantation. The role of the radiation defects in transformation of the substance properties seems to be dominant. As generally acknowledged, the radiation defects formed in Ion implantation with conventional energies are regarded as harmful. Contrary to this the defect structures due to high energy ion implantation will doubtless appear to offer a constructive physical basis for the development of new methods in superhard semiconductor technology. Of particular interest are the tracks. Using these tracks, one can find new possibilities for controlled doping of very small areas. The high pressure localized regions due to these tracks can also offer a new technological technique (e.g. for providing synthesis of metastable substances). In this connection the ion doping itself seems to be only a small part of the potential future applications arising from irradiation with high energy ions. The high energy ion implantation method could prove to be an important technology which

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