Ion beam modification of diamond

Diamond and Related Materials, 2 (1993) 621 633

621

Ion beam modification of diamond R. K a l i s h Physics Department and Solid State Institute, Technion--lsrael Institute of Technology, HaiJa 32000 (Israel)

Abstract Diamond can be modified by ion implantation either through the damage inflicted on the crystal during the slowing down of the ions in the material or by doping effects due to the presence of the implanted foreign atoms in the matrix. Both are of basic and technological importance. The response of diamond to implantation-induced damage is most interesting and instructive, since information on the behaviour of native defects and on the competition between sp2 and sp3 bonding in the material can be obtained. The conclusions drawn from studies on implantation-damaged and annealed diamond are therefore relevant to the basic understanding of defects in diamond and hence also to the understanding of diamond film growth. The present paper reviews the current status of ion beam modifications of diamond, summarizing both fundamental and applied aspects of the field. The potential uses of ion implantation into diamond for electronic, tribological and material synthesis applications are discussed.

1. Introduction Ion implantation is a method by which energetic atoms (ions) are forced into solid targets as a result of their high kinetic energy. It is commonly used as the method of choice to modify many of the near-surface properties of materials. The major applications of ion implantation are in the field of microelectronics, where ion implantation has in m a n y cases replaced diffusion as a means of introducing dopant atoms into semiconductors. Such doping applications also apply in principle to diamond. However, as will be shown below, the doping of diamond by ion implantation is much more complicated than the doping of other semiconductors owing to the tendency of diamond to graphitize. Ion implantation is also used in fields other than semiconductors, e.g. to improve the mechanical properties of materials or to synthesize new phases. These uses have also been applied to diamond, yielding interesting and technologically promising results. In addition, ion implantation has been used as a method of introducing controlled amounts of lattice damage into diamond, thus allowing the study of the transformation of metastable crystalline diamond to an amorphous structure and eventually to the most stable form of carbon, namely graphite. The understanding of the effects that ion bombardment has on diamond, whether desirable (i.e. material hardening) or undesirable (i.e. degradation of semiconductor device performances), is important. Furthermore, the novel techniques of diamond film deposition and ion beam deposition of a m o r p h o u s carbon all hinge on the possibility of building carbon materials with predomi-

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nantly sp 3 (diamond) bonding while preferentially removing undesirable sp 2 (graphite) bonds. The role that ion impact has on the formation and removal of these bonds under various external conditions (ion energy, ion species and target temperature) needs to be understood. Information about it can be obtained from experiments in which known amounts of damage are inflicted on diamond by ion implantation. In this paper a short review of the current status of the science and technology of ion implantation of diam o n d in the various fields to which it has been applied is given. More complete reviews can be found in the recent literature [1, 2]. Emphasis is given to the fundamental physical effects that ion implantation has on diamond, since these are the basis of utilizing this method to modify diamond in a controlled way. Of particular interest in this respect is the desire to achieve electrical activation of n- or p-type dopants implanted into diamond, thus realizing electronic devices in this unique wide band gap semiconductor.

2. The basics of ion implantation Ion implantation is a violent process in which energetic ions (the implants) are forced into the target material. During their slowing down in the solid, large amounts of damage are inflicted upon the target structure until the implants come to rest. It is important to bear in mind that, unless the implantation-related damage is annealed out (or extremely large implantation doses are

© 1993 -- Elsevier Sequoia. All rights reserved

622

R. Kalish / Ion beam mod(fication of diamond

employed), most measurable changes in the implanted layer are due to the damage and not to the direct presence of the implanted species. The volume density of the energy deposited in the stopping medium by the ion during its slowing down is the important parameter which determines the damage inflicted on the material by each implanted ion. The general way to treat the slowing down of an ion in matter is through the "stopping power" dE/dx, defined as the energy dE lost by an ion traversing a distance dx [3, 4]. The stopping power is usually divided into two major independent components: (1) collisions with electrons in the solid (considered free) slow down the moving ions (called "electronic stopping" and denoted (dE/dx)e); (2) the moving ions undergo elastic collisions with the atoms of the target material (called "nuclear stopping" a n d denoted (dE/dx),). Of interest to the present discussion is mainly the nuclear stopping, since it is the one which is responsible for most damage which accompanies ion implantation. From the total energy loss the ion range can be calculated according to

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where the integration limits are from the initial ion energyto zero. The final ion distribution is assumed, to first order, to be of gaussian shape with a standard deviation in the projected range Rp referred to as the ion range straggling ARp. It should be noted that the assumption of a gaussian profile ignores diffusion, which may take place during the implantation, as well as channelling [5], More realistic information about the implant distribution in the target and the related damage inflicted is obtained from computer simulations, of which TR~M[6] is the most commonly used one. The TRIM programme is a Monte Carlo programme which follows exactly the collisions that individual ions undergo while in motion in t h e target material. The input parameters to the programme are the required experimental conditions (ion type and energy, target material, composition and density) and the intrinsic parameter Ed, which is the energy required to displace a target atom far enough from its lattice site so that it will not fall back into the vacancy that it has left behind. This parameter is usually not well known. An uncertainty in E d is directly reflected in uncertainties in the calculated numbers of vacancies and intersfitials. Typical damage cascades (as calculated by TRIM) created by a single light (C) and a single heavy (Sb) ion implanted at 100 and 320 keV respectively into diamond are.shown in Figs~ l(a) and l(b). It is important to note the large difference in the average number of vacancies

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Fig. I. Collision cascade following the penetration of (a) a single 100 keV 12C+ ion and (b) a single 300 keV 12Sb+ ion in diamond (from TRIM simulations). The darker track (starting at the origin) denotes the track of the primary ion while the lighter tracks represent the trajectories of carbon recoils (primary as well as higher generations).

created by each Carbon or antimony ion, i.e. 94 and 310 vacancies per ion respectively (for 100 keV implantation energies). Hence the total damage and the density of the damage cascades of heavy ion implants are much higher than for light ion implants. Of particular relevance to ion implantation into diamond is the fact that the target material consists of carbon atoms, which are among the lightest atoms in nature. Most ions used as implants into diamond are therefore of masses equal to or heavier than the target atoms. This has important consequences on the collision kinematics, and through this on the spatial distributions of the vacancies, the carbon recoils (usually interstitials) and the implants.

3. Diamond

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The phase diagram of carbon is given in Fig. 2 [7]. It shows that under ambient conditions the graphite phase is the stable phase of carbon. Under the application of high pressure and high temperature, transformation to the diamond structure takes place. Once the pressure is released, diamond remains .essentially stable under ambient conditions, although in principle it will transform very slowly to the 4hermodynamically stable form of

R. Kalish ,; Ion beam modification o/diamond 1600

TABLE I. Properties of graphite and diamond

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Hexagonal 2.462, 6.708 1.14 × 102"~ 2.26 0.04 1582

Cubic 3.567 1.77 × I0 e3 3.515 5.47 1332

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solid carbon, which is graphite. However, when exposed to various perturbations, diamond will transform back to the equilibrium graphite phase. In the graphite structure, strong in-plane bonds exist between a carbon atom and its three nearest neighbours, f o r m i n g s p 2 bonds. The remaining electron provides only weak interplanar bonding but is responsible for the semimetallic electronic behaviour of graphite. In contrast, the carbon atoms in the diamond structure are tetrahedrally bonded to their four nearest neighbours in an sp 3 configuration. The difference in the structural arrangement of these allotropic forms gives rise to the wide differences in their physical properties. Those most relevant to the present discussion are listed in Table 1. As can be seen from the table, the physical (and chemical) properties of diamond and graphite are about as different as can be. Hence transformation of diamond to graphite, which occurs naturally but can be much enhanced when a sufficient number of sp 3 bonds are broken, is accompanied by dramatic changes to the material. These include large changes in density, electrical conductivity, optical transparency, hardness and chemical stability. Since the passage of energetic ions through matter is accompanied by massive bond breakage, it is obvious that there should be a threshold

damage level in implanted diamond beyond which the diamond structure can no longer recover but will instead "collapse" to the graphite phase. This transformation of the implantation-affected volume should be easily noticeable by the large increase in density, the tremendous increase in electrical conductivity, the mechanical weakening of the material, the appearance of new optical absorption lines and the change in chemical properties (graphite being etchable, while diamond is chemically extremely inert). These changes, all of which are enhanced upon heating, have indeed been observed in implanted diamond. The implantation-induced damage in diamond is influenced by the different diffusivities of the two major damage products: vacancies and interstitials. Carbon interstitials become mobile in diamond at about 300 K, while vacancies start to diffuse only at about 700 K. This causes an imbalance between interstitials and vacancies in the vicinity of the implant end-of-range, which depends on the implantation temperature. This imbalance has important implications regarding the possibility of annealing implanted diamond, as will be discussed below.

4. Structural modification of implanted diamond As already pointed out above, the passage of energetic ions through the diamond crystal breaks bonds and dislodges atoms. This process, which increases with increasing implantation dose, is more severe for heavy as compared with light implants. It can be observed experimentally by several different techniques. Rutherford-backscattering channelling (RBS-ch) experiments [8] are sensitive to the degree of blockage of the channels in the crystal structure by dislodged atoms, hence to the nature and amount of damage in the implantation-affected volume. Figure 3 displays RBS-ch spectra [9] which show how the damage due to the implantation of 350 keV Sb ions into (111 )-oriented diamond increases with ion dose (open circles in the figure). Two points should be noted: (1) the damage is manifested as a peak in the RBS spectrum, indicating

R. Kalish / Ion beam modification of diamond

624

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Energy (keY) Fig. 3. Random and aligned energy spectra of backscattered protons from Sb-implanted (350 keV) diamonds showing the effect of the damage and of its subsequent annealing (1150 ~C for 1 h) for different implantation doses: a, 0.9 x 10 ]4 cm-2; b, 1.8 × 10 ]4 cm-2; c, 2.3 x 1014 cm -2 [9].

that the implantation leads to the creation of point defects in the diamond; (2) the damage peak reaches the random level at a well-defined dose (1.8 x 1014 Sb cm -2 in the displayed case). Further implantation broadens only the damage region, which eventually extends all the way from the crystal surface to a depth of approximately Rp +ARp. Once the damage has reached the random level, recovery of the diamond structure by annealing is no longer possible; instead, the whole damaged region turns graphitic. Reflection high energy electron diffraction (RHEED) experiments on 40 keV Ar-implanted diamond [10] describe the evolution of the damage with increasing dose as follows. At small doses (less than 7 × 1013 cm -2) the RHEED pictures reveal the presence of small damage clusters in the form of disks 2-6 A thick, buried about 100 A below the diamond surface. At somewhat higher doses (7 x 1013-3 × 1014 cm -2) RHEED shows that the implantation-affected volume consists of small, weakly misoriented diamond blocks (about 1000 A in size) and twins. These break up upon further implantation. At a dose of 3.7 x 1014 c m - 2 the electron diffraction pattern practically disappears (at this dose an anomaly in electrical conductivity occurs--see below); it reappears at a somewhat higher dose in patterns which are characteristic for weakly oriented graphite crystallites with an average size L a ~100 ,~, Lc ,~30 A. Low energy (1 keV) Ar implantations followed by in situ electron spectroscopies [11] have provided the

opportunity to follow the transformation of diamond to graphite. The analysis of the carbon Auger electron line shape, measurements of the total secondary electron yield and the observation that an abrupt shift in Auger electron energy occurs at a particular ion dose (attributed to charging effects) all lead to the following picture about the transformation of diamond to graphite. Below a certain critical dose the diamond remains essentially single crystalline but with increasing levels of disorder. At a critical dose the damaged structure abruptly transforms to an amorphous phase (possibly a-sp 3) which still maintains many diamond-like features. Further irradiation results in the transformation of the bonding from sp 3 to sp 2, which eventually leads to the formation of graphitic crystallites. Electron spin resonance (ESR) measurements, being sensitive to the number and nature of dangling bonds, should reveal bond breakage in implanted diamond. Indeed, abrupt jumps in both ESR line intensity and linewidth were reported [12] for 300 keV Sb-implanted diamond to take place in the dose range from about 5 x 1013 to 1014 cm -2. This is the range where sharp changes in electrical and optical properties of implanted diamond have been observed (see below). Swelling of the diamond lattice has been observed by several researchers [13-15] as a clearly measurable step between implanted and non-implanted regions. Three factors may in principle contribute to increases in volume due to the implantation: (1) the addition of extra atoms

R. Kalish

625

Ion beam modification of diamond

forced into the lattice by the implantation: (2) the creation of vacancies and interstitials in the collision cascade; (3) the possible phase transformation from compact diamond to some less compact but more thermodynamically stable form of carbon (i.e. graphite) enhanced by sp 3 bond breakage by the traversing ion. Maby et al. [13] have implanted diamond with boron ions and have reported an irreversible volume expansion which sets in above a certain critical dose (q5>5 x 1015 cm-2). They attribute their observation to the creation of an amorphous region due to the ioninduced damage which has a density less than or equal to that of graphite, thereby accounting for the observed swelling. A different approach to the swelling of light-ionimplanted diamond has been given in a series of recent publications by the South African group [14]. The basic points in explaining the volume expansion measured for ion-implanted diamonds are, according to this work, the difference in the spatial distribution of the vacancies and interstitials created in the damage cascades and the difference in their mobilities. While substantial diffusion of self-interstitials already occurs at about 50 °C during implantation [16], the vacancies formed during the collision cascade are immobile until about 500°C. As a result, diamonds implanted at low temperatures (T<50°C) should show little swelling, while those implanted at elevated temperatures (50< T<500°C) should exhibit volume expansion due to the escape of interstitials from the damage region with the vacancies still locked in. Indeed, the Johannesburg group demonstrated that higher steps are formed in diamonds implanted with 170 keV 19F ions at elevated temperatures [14] than at low temperatures (77 K) and that 13C diffusion is correlated with this step height [17]. At high temperatures (about 400 °C) steps could no longer be measured between the implanted and unimplanted regions and substantial 13C in- and out-diffusion were observed. Prins et al. [14] also give a series of arguments to explain why graphitization is an unlikely process for explaining the swelling of implanted diamond. When the increased volume due to the addition of the implanted atoms to the matrix (assumed to be linear with dose) is subtracted from the swelling data, a saturation in the volume expansion is found as shown in Fig. 4. The swelling, being determined by the diffusivities of vacancies and interstitials, depends on the implantation temperature [15]. It does not exist for very low temperature implantation (77 K), when both interstitials and vacancies are immobile, it is strong at room temperature, when only the interstitials can diffuse, and it disappears again at higher temperatures at which both interstitials and vacancies are mobile (see Fig. 4). Implantations at elevated temperatures (Ti) and the resulting diffusion of the point defects may lead either

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Fig. 4. Dose dependenceof the rapid volumeexpansion observed at low ion doses after subtraction of the linear expansion which is prevalent at high ion doses [I4]. to instantaneous damage annealing or to enhanced graphitization. These have been used to electrically activate B implants in diamond or to "grow" diamond by very heavy dose C implantations into heated diamond. Electrical contacts to diamond have also been realized using heavy C implantations, but at lower target temperatures (see below). A statistical model, based on the assumption that the volume expansion at low ion doses should be a direct measure of the number of immobile vacancies remaining in the damaged layer after out-diffusion of the interstitials, allowed Prins et al. [14] to deduce the displacement energy Ed for a C atom in diamond. This parameter, which is an important input parameter in all simulation programmes (e.g. ~RIM), is found by the above analysis to be Eo ~ 55 eV in diamond. The physical picture of the swelling of the diamond [17], namely that following implantation at ambient temperature the implanted ion may reside in a vacancy-rich environment, has important implications for the possibility of annealing the crystal. In order to obtain perfect annealing, as is for example needed for electrical activation of implanted donor or acceptor species, it may be important to supply the vacancy-rich regions with extra carbon atoms to annihilate the vacancies. This point is discussed in more detail below. It is interesting to note that purely theoretical calculations on self-diffusion mechanisms in diamond [18] indicate that the self-diffusion in diamond is dominated by vacancies, predicted to be the most mobile species in diamond. This is apparently in contrast with the findings of the South African group [17] described above, which are based on studies of the swelling of diamond [14] and on tracer experiments on the diffusion of 13C [19], both of which can be understood if self-interstitials are the most mobile defects in diamond. The difference between these two orthogonal ideas may have to do with the fact that the theory [18] deals with an undam-

R. Kalish / Ion beam modification of diamond

626

aged diamond crystal whereas the experiment refers to the dynamics of diffusion under ion bombardment, which corresponds to a severely damaged crystal. Nevertheless, this apparent discrepancy is worth further consideration.

5. D a m a g e - r e l a t e d electrical conductivity

The structural changes that diamond undergoes as a result of the damage which accompanies ion implantation is most clearly reflected in changes in the electrical properties of the implanted layer. Regardless of implant mass or energy (if within the range of ion implantation energies), the conductivity a of diamond changes with increasing ion dose D in the peculiar way shown in Fig. 5. Three distinct regimes are noticeable in the log a vs. log D curve. At low doses a exhibits a gentle rise, followed by a drop with increasing dose. However, at a

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was done at normal incidence or at some other angle (___60° with respect to the normal) 1-20].

well-defined higher (critical) dose a sharp rise in conductivity sets in, eventually reaching saturation characterized by a very high conductivity. The different regimes in the a(D) curve have been correlated by Vavilov et al. [10] with the structural modifications of diamond as observed by RHEED. More recently, other explanations of some parts in the a vs. D curve have been offered. A statistical percolation approach [20] has shown that the sharp rise in conductivity can be explained by a graded percolation transition that the damaged diamond undergoes, gradually changing from highly insulating to more and more conductive regions with increasing overlap of damage cascades. The conduction in this transition region is described by variable-range hopping, characterized by a T- 1/4 dependence of log a on measurement temperature T [21]. The hopping of charge carriers is presumably between highly conductive small graphitic islands. The very high conductivity eventually reached in the saturation region is of metallic nature, since it does not exhibit any dependence on measurement temperature (see Fig. 6). The reasons for the non-monotonic behaviour of the conductivity in the low dose range is still not completely understood. Its origin seems to be the presence of isolated, very small clusters of sp2-bonded carbon. The justification for this interpretation comes from the similarity of the onset of conduction of C-implanted SiOz and C-implanted diamond [22]. Both a vs. D curves look rather similar, but the diamond curve is shifted to doses smaller by a factor of about 50 as compared with SiO2. This shift is due to the liberation of carbon recoils in the damage cascade in diamond which contribute to the conduction. Hence the comparison between Cimplanted diamond and SiO2 indicates that of the approximately 100 recoils generated by each implanted 100 keV C ion in diamond, about one-half annihilate, while the remaining recoils contribute to the electrical conductivity, just like primary carbon implants in SiO2 do. The results of electron spectroscopy studies on low energy Ar-damaged diamond confirm these findings [11]. The dependence of the a(T) curves on the implantation temperature Ti sheds more light on the transformation that damaged diamond undergoes with increasing damage density. The phenomena described above for room temperature implanted diamond are much enhanced and sharpened [23] when the implantations are performed even into moderately heated diamond (about 200°C) [24]. The percolation transition in the a(D) curve becomes much steeper, indicating that the transition from diamond to graphite is sharper at elevated temperatures. Furthermore, the saturation conductivity sets in at lower doses [25] as shown in Fig. 7. The observed changes in conductivity in hot-implanted diamond have been correlated with defect mobility argu-

R. Kalish / Ion beam modification of diamond

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diamond heat sinking may, in the light of this finding, cause a very large spread in the properties of ionimplanted diamond when measured on different samples implanted under the same nominal implantation conditions.

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Fig. 6. Temperature dependence of the resistivity for various a-C (amorphous carbon) layers: 1100 A a-C sputtered at 95 K and annealed at 300 K; a-C film evaporated at 300 K; implanted diamond with increasing implantation doses. The doses listed represent total dosage in C + cm-2 [21].

ments similar to those used to explain the swelling of diamond caused by ion implantation [17]. The fact that the dependence of the resistivity on Ti is steepest just around room temperature, with the measured resistivity dropping by over two orders of magnitude between 15 and 35 °C for identical implantations (2.5 x 1016Ar + cm -2) is unfortunate. This high sensitivity to implantation temperature near 300K makes the quantitative comparison between different experiments carried out at so-called "room temperature" extremely difficult. Slight variations in implantation power (i.e. beam current or energy) or differences in

6. Post-implantation annealing The common way to restore the structure of an implantation-damaged single-crystal layer is by postimplantation annealing. However, while most damaged crystals will eventually recover to their original structure upon heat treatment, diamond will not necessarily do so because of the instability of the diamond phase under ambient conditions (see Fig. 2). Post-implantation annealing of damaged diamond restores the crystallinity perfectly only if the damage density does not exceed a certain value. The data in Fig. 3 (full large circles) display the channelled spectra of 350 keV protons following annealing (l h at 1150°C in vacuum) of diamonds implanted at room temperature with 350 keV 121Sb ions to different doses. While complete annealing is obtained following implantation of 0.9 x 1014cm -2, the damage which results from an implantation to 1.80 x 1014cm -2 does not disappear but rather remains as a sharp peak at approximately

628

R. Kalish / Ion beam modification of diamond

the implant range. This peak may be associated with a buried narrow graphitic layer unresolved by the RBS experiment. For even higher doses (2.3 × 1014 cm -2) no damage removal occurs whatsoever other than a slight epitaxial growth from the diamond-damage interface. The "annealed layer" for this and higher dose implantations exhibits many graphitic properties, including etchability by acids which are known to attack graphite and not diamond, a fact that has been used to remove thin diamond layers in a well-controlled way [26]. The annealing of diamond implanted under conditions which damage it below the critical level beyond which annealing is no longer possible (i.e. 1 × 1015 cm -2 for 90 keV C) has been studied by channelling experiments [23]. The results of these measurements indicate that complete damage removal can be achieved only when extremely high annealing temperatures (1450°C) are employed. In contrast to the results of ref. 23, which apply to the annealing of (111) diamond implanted at room temperature, Liu et al. [27] have studied the annealing behaviour of (100) diamond implanted at 77 K and subsequently annealed. In this work 200 keV carbon ions have been implanted into (100) natural diamonds to doses ranging from 1015 to 3 x 1015 cm -2. The implanted diamonds have been subjected either to rapid thermal annealing (1100 °C for 2 min) or to isochronal annealing for 1 h at temperatures ranging from 450 to 900°C. RBS-ch analysis has shown clearly that (a) prolonged annealing in a furnace yields superior results to those obtained following rapid annealing, (b) an anneal at 900°C for 1 h is sufficient to remove the damage following cold implantations and (c) there is a critical dose for amorphization of diamond which is around 2 × 10is cm -2 for the cold C implantations employed in ref. 27. Optical transmission measurements covering the wavelength range 200-800 nm have verified the annealing behaviour deduced from the RBS-ch experiments, but indicate that some damage, undetectable by channelling, remains even after annealing at 900 °C for 1 h. Very similar conclusions were drawn by Lee et al. [28] from their studies of hot implantations into diamond using ESR techniques. They found that the ESR signal attributed to amorphous carbon (which probably corresponds to what is identified in channelling experiments as point defects) is reduced as the implantation temperature is increased; however, implantation above 600 °C results in the formation of two new ESR-active centres attributable to distorted hexavacancy rings or to some other vacancy clusters. Such multivacancy clusters can account very well for the typical dechannelling behaviour observed in hot-implanted diamond. Two non-conventional annealing schemes should be mentioned here. The first relates to the possibility of annealing diamond by the "thermal spike" locally

imparted to the regions affected by ion impact. It has been shown [29] that light-ion-beam-induced annealing occurs in diamond when damaged below the same critical dose beyond which its structure cannot be restored even by thermal annealing. (111) diamonds damaged by implanting 300 keV Sb ions to a dose of 1014 cm -2 were subjected to high doses of 2.3 MeV He + or 320 keV H + beams impinging on the diamond under either channelling or random incidence conditions. Damage removal due to the passage of the light ions through the damaged layer has clearly been observed by these experiments. The annealing has been shown to be caused predominantly by nuclear and not electronic collisions. In this respect diamond does not differ from other covalent crystals for which ion-beam-induced annealing has been observed. The second point relates to the possibility of annealing (and eventually graphitizing and possibly liquifying) deeply buried damage layers in diamond by laser irradiation. Damaged layers buried beneath the surface of type IIa diamond slabs by 2.8 MeV C + ion implantation have been subjected to irradiation with 14 ns pulses from a focused Nd-glass laser [30]. At high powers ablation of the implanted surface was observed. However, the correct choice of laser power and wavelength results in annealing of the implanted layer without any disruption to the surface morphology. Annealing was confirmed by optical measurements and RBS-ch spectroscopy on the small (10 ~tm diameter) laser-irradiated spots. The results suggest that an undamaged diamond cap can be utilized to promote damage annealing of diamond by pulsed laser beams. The surface cap is believed to aid in the production of extremely high internal pressures in the damaged layer during the laser pulse, which prevents graphitization and promotes diamond regrowth. At somewhat higher laser power densities than required for annealing, graphitization of the buried damage layer followed by melting have been observed, in agreement with the phase diagram of diamond near the diamond-graphite-molten carbon triple point (Fig. 2).

7. Doping of diamond In order to be able to utilize the semiconducting properties of diamond, n- and p-type conductivities are required. Natural type IIb diamonds exhibit p-type conductivities with an activation energy of about 0.37 eV, attributed to B impurities [31]. Natural type Ib diamonds contain N, a potential donor in diamond; nevertheless, no n-type conductivity could be related to its presence, possibly because of the deep level (about 2 eV) it has within the wide gap of diamond [32]. Interestingly, radiation-damaged diamonds exhibit n-type conductivi-

R. Kalish / Ion beam modification of diamond

ties which have been utilized to realize simple devices in type IIb diamonds (see below). The exact nature of this donor activity is as yet uncertain. It has been proposed by Prins [33] to be due to "vacloids"--aggregates of vacancies arranged in a semiordered "superlattice" in the diamond matrix, thus giving rise to an energy band approximately 4 eV below the conduction band. p-Type doping of diamond has been achieved either by introducing B during synthetic diamond growth (i.e. epitaxial growth of diamond films) or by ion implantation followed by proper annealing. In contrast, impurity n-type doping of diamond is still much an open problem and only a few recent reports on successful donor activity due to doping by Li or P exist in the open literature. Since the implantation process in diamond is accompanied by much damage, which may by itself give rise to electrical conductivity, appropriate annealing conditions must be found to remove the lattice damage and to drive the implants to the desired lattice sites. The procedure for obtaining conductivities due only to the dopants and not to lattice damage is thus a complex one and many misleading results have been published on this subject. Most convincing are results of experiments in which control experiments on identically treated diamond but subjected to non-active implants have been carried out and have not shown any electrical activity. Attempts to implant diamond and subsequently anneal out the implantation-induced damage have centred around a limited number of potential donor (Li, P, As and Sb) and acceptor (B) ions. Pioneering work has already been carried out by the Lebedev group in the 1970s [34, 35], but some results of this early work were later shown to be due to damage-related effects and not to chemical doping. More recent attempts to implant diamond, anneal the damage and activate the implants, while avoiding graphitization, concentrate on three different approaches: (1) hot implantations [36], (2) high dose implantations followed by annealing and graphite removal [23] and (3) C + co-implantations [37, 38]. The first approach relies on the instantaneous annealing which takes place during implantation into hot (T>1000'C) diamond. The Harwell group showed in 1977 that diamond growth can be achieved by high dose C implantation into hot diamond [39, 40], hence the extension of this method to dope diamond is obvious. Indeed, both the Harwell [41] and Technion groups [36, 42] have employed hot implantations to dope diamond p and n type by B and Li implantations. Of particular interest are the recent results of the Harwell group [41] on successful n-type doping of diamond by implantation of 30 keV Li ions at different doses into diamonds held at 800, 1000 and 1200"C. Interestingly, the best results as far as sheet resistivity are concerned were obtained

629

for the lowest implant temperature (800 'C). This has been attributed by the authors to in situ annealing at the higher temperatures of defects in the diamond which would otherwise contribute to conduction. The activation energies deduced in that work from log a vs. l I T curves depend on implant dose and exhibit metallic conductivity for the highest dose (2.5 x 1016cm 2) employed. Despite the fact that no direct proof for donor activities which can be attributed directly to the Li impurity is given in that work, p-n junctions in type IIb diamonds have been realized by this method. The second method for achieving implant activation in diamond relies on high dose implantations (exceeding the graphitization limit) followed by high temperature (1400 c'C) annealing and subsequent chemical removal of the graphite top layer [23]. This process has been shown to leave, after graphite removal, a surface layer of intact diamond which contains only the "tail" of the implant distribution, with the implants residing on lattice sites where they are expected to be electrically active [36] (B on substitutional and Li on interstitial sites). Indeed, by employing this method and by performing control experiments on identically treated C-implanted samples, Braunstein and Kalish [23] were able to show unambiguously that p-type activity due to B doping has been achieved. As for n-type doping, increased conductivities which cannot be related to damage were measured for Li-implanted samples [42]: however, just as in the Harwell work [41], no Hall effect measurements could be carried out, possibly owing to the low mobilities of the carriers. The third approach to anneal the implantation damage and to activate the implants electrically has been developed by Prins and coworkers [37, 38]. On the basis of extensive work by the South African group on the temperature dependence of the volume expansion and electrical conductivity of C-implanted diamond [ 16], the important role that the implantation temperature plays in the nature and spatial distribution of the defects in the implantation-affected layer has been clarified. Hence Prins and coworkers have argued that if dopant atoms such as B are co-implanted with C ions into the same volume at low temperatures and if the system is then annealed rapidly, there should be a chance for the implanted foreign atoms to compete with the selfinterstitials in the filling of vacancies. The result may thus be that an appreciable number of dopant atoms will reside on substitutional sites in an undisturbed diamond lattice, hence becoming electrically active. This idea was tried out by Prins [37], who co-implanted C and B ions into type IIa diamond held at liquid nitrogen temperature using different carbon-to-boron dose ratios so that the total dose was always 5 x 1016 cm 2. The diamond was then annealed rapidly to 500 ~C followed by further annealing at 1200 ~C for 2 h. The properties

R. Kalish / Ion beam modification of diamond

630

of diamonds treated in this way were characterized both optically and electrically. Most convincing are the results of the sheet resistivity vs. inverse temperature of the B-implanted layers for different B fractions in the B + C implantations (Fig. 8). While the specimen implanted with pure B showed a very low sheet resistance with a slow temperature dependence corresponding to an activation energy of 0.02 eV (similar to that measured by Braunstein and Kalish [23] for a heavily doped B layer), increases in resistivity and increased activation energies were observed with decreasing B fraction. Of particular interest is the slope of the 70% line which corresponds to an activation energy of 0.37 eV, known to be that for substitutional B in type lib diamonds. This was taken by Prins as proof that substitutional B acceptors are present in this particular sample [37]. Optical absorption measurements and identically (B + C)-implanted and annealed samples [38, 43] have shown lines which could be identified with substitutional B. More recent work by Prins [33] has shown that many of the complications involved in the dual-implantation procedure may be unnecessary. Pure B implantation into cold diamond followed by standard furnace annealing (1160°C for 30 min) has yielded results comparable with or even better than those reported for dual implantation. The dependence of the resistivity on 1/T for a B dose of 4.7 × 1015 cm -2 showed a very convincing activation energy of 0.38 + 0.02 eV. Data for slightly higher doses showed much lower activation energies, either

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because of overdoping or owing to the formation of damage-related graphitic islands. No control experiments or Hall measurement were reported in that work. While the search for n-type dopants in diamond is still on, theoretical work [44] predicts Na to be the donor of choice in diamond (being superior to Li and P). However, no experimental confirmation of this prediction has been published so far. It should be noted here that most recently P ions introduced into chemical vapour deposition (CVD) diamond films during growth seem to act as good donors. This is based on the p-n devices produced from polycrystalline diamond structures grown with B and P impurities [45].

8. Electronic device realization

The most attractive application of ion implantation into diamond is the realization of electronic devices with this unique semiconductor. These should, for special applications, be superior to any other solid-state-based devices, owing to the excellent thermal conductivity of diamond, its wide band gap and its resistance to radiation. As early as 1973 some rectifying effects were found by Glover [46] in Schottky devices based on natural and synthetic boron-containing diamonds. The extensive work carried out in the USSR in the 1970s on the topic of implanting diamond to obtain devices is reviewed in the papers of Vul [47] and Vavilov [48]. Both reviews show I - V curves for p-n junctions obtained in diamond by either B + P or B + Sb implantations. The best results were obtained following annealing at 1400°C. Some of the devices even showed a photoresponse to irradiation with UV light. However, no follow-up of these early results has appeared in the literature. Furthermore, in the light of extensive studies on the basic science relevant to the implantation and annealing of implanted diamond, reviewed in the previous sections, it is possible that these early researchers were misled by other implantation- and annealing-induced phenomena (e.g. graphitization, etc.). The n-type conductivity obtained in damaged diamond has been utilized by Prins to fabricate a bipolar transistor in type lib diamond in which the emitter and collector were made n type by C ion implantation [49]. This device has demonstrated typical diode and transistor characteristics, but with a current amplification factor of only about 0.11, possibly owing to its unfavourable geometry. Unfortunately, this unique device has never been reproduced or improved, either by Prins or by anyone else. A p-n junction based on real chemical doping by activating implants has been realized by the Harwell

R. Kalish ,/ Ion beam modiJication of diamond

group [-41]. Here again use was made of p-type type IIb diamonds which were subjected to high dose (1.4 x l016 cm -2) Li ion implantations (at 30 keV) performed into heated diamond (T,=800°C). No postimplantation annealing was applied, relying on instantaneous damage annealing and on the assumption that some of the implanted Li may penetrate the diamond to a depth which is greater than the damage profile (an idea which actually combines the two implantation-annealing modes employed by Braunstein and Kalish [23]). Nice rectifying features were obtained for this device as shown in Fig. 9. It is very encouraging to note that the 1---V characteristic remained good up to 350 °C, justifying the expectations of high temperature devices based on diamond. Even though outside the scope of this review, the success of realizing devices in thin film CVD-grown diamond doped n type (by P) and p type (by B) during growth should be mentioned [45]. The pioneering work of Geis [50] on realization of devices in diamond layers is also worthy of note.

9. Improving mechanical properties It is well known that the mechanical properties of steels and other superhard materials can be improved by ion implantation, the major implant species being nitrogen. It is also known that type I diamonds, which contain as much as 0.25% nitrogen, exhibit superior hardness. Therefore it is not surprising that improvement in the mechanical properties of diamond by ion (mostly nitrogen) implantation has been attempted. The goal of these studies has been to enhance the frictional and

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scratch resistance of diamond as well as to improve its wear behaviour and its machining performance as a cutting edge. Hartley [40] implanted (at room temperature) various diamond plates, tips and tools with N +, B + and C + ions at 100-300keV to doses of about (3--5) X 1015 cm 2 and exposed these specimens to a series of scratch and wear tests under a variety of load conditions. Interestingly, some marked improvements were found in the mechanical properties of N +implanted diamond as compared with non-implanted or identically C +-implanted diamond tools. Of particular interest in this respect is the finding that N+-implanted diamond-finished cutting tools showed substantial improvement in their lifetime in machining acrylic plastic. A few experiments where B was implanted seem to show the same trend, in contrast to carbon implantation which did not leave any marked effect on the mechanical properties in the implanted diamond layers. Since the wear of the diamond is believed to be due to loss of material caused by cleavage or peeling of platelets from the diamond surface, the effect caused by N (or B) implantation must suppress this cleavage through both structural and chemical modifications, thus hardening the diamond. It is interesting to note that even though the largest commercial market for diamond is in the cutting, machining and drilling tool industry, no extensive scientific work has been published either on systematic investigations of the effect of ion implantation on the improvement in the mechanical properties of diamond or on the basic understanding of these hardening effects on an atomistic scale. The theoretical prediction [51] about the expected preferential growth of the diamond structure when containing 1% 8% vacancies is worth experimental verification. Ion implantation is the obvious method of choice for the controlled introduction of defects in diamond to check this point.

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Very high dose implantation (above 1017 cm 2) can introduce large concentrations of foreign (implanted) atoms into the substrate centred around Rp so that new materials can sometimes be synthesized. This procedure has also been applied to diamond in an attempt to stimulate SiC or diamond growth by Si or C implantations respectively into diamond. Most efforts to obtain an SiC layer in diamond by Si implantation have been made by the Lebedev group [52], who have employed X-ray diffraction, IR absorption and ion-scattering techniques to evaluate their materials. In their work 30-40 keV 2sSi ions were implanted into diamond to very high doses (up to

632

R. Kalish / Ion beam mod(fication of diamond

4.4 × 1017 cm -2) and the composition and structure of the Si + C mixed layer so formed were studied following different implantation doses and annealing procedures. RHEED analysis [-53] has shown the existence of small, randomly oriented 30-50 A crystallites concentrated around the projected range of the Si implants, identified as having the 13-SIC phase. It is claimed that graphitization, which on the basis of today's knowledge should take place at the high dose implantation levels employed, has been prevented by the existence of the tetrahedral Si-C bonds. Nevertheless, the fact that the implanted layer could be attacked by acids, which should not be the case for both diamond and SiC, raises the question of whether the phases reported [-54] were indeed SiC embedded in diamond. A patent by Nelson claiming that diamond growth can be achieved by high dose carbon ion implantation into heated (400-1200 °C) diamond [55] has generated enthusiasm, and attempts to increase the size of natural diamonds by this method have followed. The diamond layer grown by this method was, however, found to be defective, containing large amounts of extended defects which manifested themselves through enhanced dechannelling of the RBS probe ions. These defects were attributed to the agglomeration of migrating point defects into a dislocation array during the hot implantation. Auger electron spectroscopy verified that the material grown by high dose C + implantation consisted predominantly of diamond. While the mechanical and chemical properties of the as-grown diamond are identical to those of single-crystal diamond, their optical properties indicate the presence of defects which give the implanted diamond a yellowish or light brownish appearance. Therefore this method of increasing the diamond size has up until now had no real value to the gemstone industry, in particular in the light of the competition from the recently developed CVD diamond growth techniques.

11. Conclusions

Ion beam modification of diamond, reviewed here, is of particular basic and technological importance. The two major topics related to ion implantation studies of diamond are (1) the damage inflicted on the crystal by the implants during their slowing down in the material and (2) the attempts to achieve chemical doping of diamond by the implantation of the appropriate impurity ions. As has been shown above, both these are complicated by the unique property of diamond which is its tendency to undergo a phase transition and convert into graphite. These topics have also attracted renewed theoretical attention. Recent publications based on ab initio calcula-

tions address both the structure and properties of defects in diamond [51] and the role of impurities in it in attempts to predict suitable n-type dopants in diamond [44]. The results of these theories have not yet been put to extensive experimental test, but publications on these will most certainly appear very shortly. The recent fields of CVD diamond film growth and ion beam deposition of diamond-like coatings are intimately related to the topic of the present review on ion beam modification of diamond. They too share the same problem of reducing sp 2 in favour of sp 3 bonding of the carbon, hence many of the conclusions deduced from ion implantation studies are of relevance to these novel techniques for carbon film growth. Furthermore, it has been shown recently [-24] that the response of polycrystalline CVD diamond films to ion damaging, as far as electrical conductivity is concerned, is remarkably similar to that of crystalline diamond discussed here. Hence many results obtained for diamond stones may be directly applicable also to CVD diamond films.

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39 N. E. W. Hartley and M. J. Poole, Mater. Sci., 8 (1973) 900. 40 N. E. W. Hartley, in S. T. Picraux and W. J. Choyke (eds.), Metastable Materials Formation by Ion Implantation, NorthHolland, Amsterdam, 1982, p. 295. 41 I. M. Buckley-Golder, R. Bullough, M. R. Hayns, J. R. Willis, R. C. Piller, N. G. Blamires, G. Gard and J. Stephen, Diamond Relat. Mate., 1 ( 1991 ) 43. 42 G. Braunstein, Ph.D. Thesis, Technion, 1982. 43 G. S. Sandhu, M. L. Swanson and W. K. Chu, MRS Symp. Proc., 128 (1989) 707. 44 S. A. Kajihara, A. Antonelli, J. Bernholc and R. Car, Phys. Rev. Lett., 66 (1991) 2010. 45 K. Okino, H. Kiyota, T. Iwasaki, T. Kurosu, M. lida and T. Nakamura, Appl. Phys. Lett., 58 (19911 840. 46 G. H. Glover, Solid State Electron., 16 {1973) 973. 47 B. M. Vul, Jpn. J. Appl. Phys., 43 (1974} 183. 48 V. S. Vavilov, Phys. Status Solidi A, 31 (1975) 11. 49 J. F. Prins, Appl. Phys. Lett., 41 (1982) 950. 50 M. W. Gels, MRS Syrup. Proc., 162 (1990) 15. 51 Y. Bar-Yam and T. D. Moustakas, Nature, 342 (1989) 786. 52 J. P. Akimchenko, K. V. Kisseleva, V. V. Krasnopevtsev, Y. V. Milyutin, A. G. Touryansky and V. S. Vavilov, Radiat. Eff~,ct~s,33 (1977) 75. 53 V. V. Krasnopevtsev, Y. V. Milyutin, V. S. Vavilov, A. E. Gorodetskii, A. N. Khodan and A. P. Zakharov, in F. Chernow, J, A. Borders and D. K. Brice (eds.), Proc. 5th Int. Conf on Ion Implantation in Semiconductors, Plenum, New York, 1977, p. 295. 54 R. S. Nelson, J. A. Hudson, D, J. Mazey and R. C. Piller, Proc. R. Soc. London A, 386 (1983) 211. 55 R. S. Nelson, Br. Patent 1,476313, 1977.