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Ion implantation in insulators
Nuclear Instruments and Methods 182/183 (1981) 727-732 © North-Holland Publishing Company
727
Section VL Ion-implanted insulators
ION IMPLANTATION IN INSULATORS P.D. TOWNSEND The School of Mathematical and Physical Sciences, The University of Sussex, Brighton, BN1 9QH, UK
The modification of insulating materials by ion implantation is developing in several areas. In many cases the property changes just parallel those observed in semiconductors, but in at least one new area, namely integrated optics, the use of the ion beams has distinct advantages for the control of the properties needed for waveguides and other optical devices. Ion beams also fill a second function as diagnostic tools in the production and analysis of defect structures. A selection of experiments using luminescence and optical absorption are mentioned.
1. Introduction
2. Integrated optics
The applications, results and advantages of ion implantation for semiconductor production have been well documented for many years. However, for wider energy gap materials the literature of ion beam effects is rather limited. There is no a priori reason why ion implantation in insulators should not be developed, and early experiments clearly indicate that many interesting ideas are likely to be successful. The present literature can broadly be separated into two groups, namely experiments which use the ion beams in a diagnostic role and those which are concerned with device manufacture. The experience gained in the semiconductor industry of pattern generation, lithography, ion sources and processing are readily carried over to implantation of insulators. We are therefore in the situation of having a viable technique in search of a problem, and fortunately at least one suitable problem has been identified. Earlier reviews have suggested that a major area of technology is developing [1] which, over the course of the next decade, will definitely need ion implantation as a production technique [ 2 - 4 ] . This field stems from the use of optical fibre communication systems and is concerned with the optical processing units [5-10]. By analogy with semiconductor devices it is often called integrated optics. In the present summary we shall reiterate why implantation seems appropriate for integrated optics. Some reference will also be made to the more varied diagnostic uses of ion beams in insulating materials. The list provided is not exhaustive and additional possibilities should be considered.
Since this is an area of physics which may be unfamiliar to those working with ion implantation, we shaU offer a brief outline of the main ideas, but for details of the subject we can recommend many excellent reviews and books (e.g. [5-10]). Integrated optics describes the packages of circuitry in which the signal pathways are waveguides for light of optical frequencies. The primary circuit element is the waveguide, but to make active circuits then there must also be switches, modulators, mixers, tuned filters, sources, detectors, logic elements, delay fines etc. By using the high frequency of an optical carrier we instantly gain a very large signal capacity even though the conducting pathway is only a few microns in cross section. Multiplexing of signals by pulse coding and/or by different carrier wavelengths offers further scope for the future expansion of optical waveguide systems. Whilst it is true that at this present stage of development of the glass fibre communication networks the optical processing units are relatively standard, it is evident that this is only an interim situation and truly sophisticated integrated optic circuits are going to be developed in the future. The optical paths are written as regions of high refractive index which confine the light beam and modulation, etc., are accomplished by external modification of the index. Changes can be caused by electric, magnetic or stress fields, but most devices designed so far use the electro-optic effects. The physical size of the guides (i.e. a few microns) makes them ideally suited for a planar technology, and they can be fabricated by either diffusion or ion implantaVI. ION-IMPLANTED INSULATORS
728
P.D. Townsend/Ion implantation in semiconductors
tion with the aid of mask patterns. The ion beams offer some advantages [3] in that the lateral spread of the patterns formed beneath the masks may be less than those generated by diffusion. We may also make implantations with various ions and energies to give a predetermined refractive index profile (e.g. to bury or taper the depth of the guide), further, modifications may be made at room temperature rather than by a high temperature diffusion process.
Surface Impl.ant (aL
index X) (Q)
_
LiNbO3 (b)--~(b)
Fr //
!b_k_./
Depth 2~ X
3. Implantation in LiNbOs and LiTaOs The materials of major interest are those which have large electro-optic coefficients [6,8] and thus offer the possibility of making efficient low power switches and modulators. Two prime examples are LiNbOs and LiTaOa. They are also chemically stable, robust and transparent over a wide range of wavelengths. For LiNbO3 the normal diffusion method of controlling the refractive index is to introduce titanium ions by diffusion [11,12] near 900°C. This increases the value of the extraordinary refractive index by some 0.04. Not all attempts at Ti diffusion are successful, since the formation of a surface barrier, perhaps a titanium oxide, inhibits the movement of the titanium. We may bypass this problem by a low energy Ti implant, and then diffuse from this sub-surface layer. A more direct approach is to use the implantation to change the refractive index. With LiNbO3 [13,14], as with silica, (e.g. [15-18]) the index change is a function of the energy deposited by nuclear collisions and is only partially sensitive to the choice of ion. For LiNbOa the ordinary index alters by as much as 10% (0.23), but it is a decrease [13,14,19]. Thus we need to reconsider how to use this change in practical devices. Two solutions are sketched in fig. 1. The simplest, fig. I(A), is to define the boundaries exterior to the waveguide by reducing the index. This leaves a central unirnplanted channel with the original high index and all the electro-optic properties of the original material. Such a high index guide mounted on a high index substrate is referred to as a strip waveguide. Therefore as shown in fig. 1(A) the simplest treatment is direct implantation alongside the guide region. If in addition we wish to isolate the guide from the substrate it might be convenient to use a combination of diffusion and implantation techniques [3,20]. Firstly we form a planar high index layer by
(B)
ImplTiant(c/+ __~_~ LiNbO + Ti (d_) _
_~
. . . .
(dl (d) ~<
Depth
x
Fig. I(A). The effect of ion implantation on the refractive index of LiNbO3 . Region (a) is original material, whereas for the implanted region (b) the index is reduced. The region bounded by the implant acts as an optical waveguide. In fig. I(B) the surface refractive index is increased by titanium diffusion before implantation.
diffusion and then isolate this into a localised three dimensional guide by boundary implants. In the example of fig. I(B) the increase is made by Ti diffusion and the implants cause the decrease. The depth profiles of the two methods are rather different. This is not important as the waveguide modes are confined if either the layer thickness and/or the index difference between the guide and surrounds are small. This has the advantage that the projected range of the implant can be much less than the diffusion depth of the Ti. A quite different approach has been used for undoped LiNbO3 to take advantage of both the change in the ordinary index and the larger ra 3 electro-optic coefficient [14]. Since the damage and index change are generated by the (dE/dx)nuclear energy loss of the beam, light ions at high velocity (e.g. 2 MeV He*) traverse the first few microns of the LiNbOs without causing any significant damage to the lattice. Damage only occurs at the end of the track and produces a low-index wall which separates the surface layer from the substrate. The index proFile follows the damage generation curve, and as
P.D. Townsend/Ion implantation in semiconductors 2.18
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both 77 and 300 K using helium ions [24,25]. We have concentrated on implantation in the LiNbO3 and LiTaO3 but there are many other materials which are relevant for integrated optic devices. Materials which also show semiconductor or LED properties are particularly interesting (e.g. ZnTe or GaAs). They offer the possibility of combining the electronic and optical circuitry [1] onto a single monolithic substrate, and with this aim in mind many waveguide components have been formed by ion beam treatment of GaAs, CdTe, ZnTe etc. (In ref. [4] many more examples are discussed than is possible in this brief article). In summary, we see that ion beam technology is very well suited to the specific needs of optical waveguide circuitry, and we can confidently expect that this area of research and appfications will expand over the next decade.
DEPTH, MICRONS Fig. 2. Fast ion implantation of LiNbO3 produces a low refractive index barrier between the surface waveguide and the substrate. For the example shown, 2 MeV He+ ions confine 7 optical modes in the surface as computed theoretically.
shown in fig. 2, the calculated and observed refractive index pattern can support many waveguide modes. For the data shown only a single ion energy was used, but wider barrier layers have been formed by multiple energy implants [13,14]. In such guides the electro-optic coefficients are still very favourable, although reduced from the values in perfect LiNbOa [14,21]. We may emphasise a further advantage of ion implantation for integrated optics by reference to waveguide formation in LiTaOa. As with lithium niobate, the metal ions have metastable sites within the unit cell of the lattice, but by application of an electric field at high temperature preferred sites are taken and the polarised material then shows electrooptic properties. For LiTaO~ the index is less than for LiNbO3 (e.g. no is 2.177 compared with 2.286), hence a high index layer can be formed by doping with niobium [22,23]. This is in-diffused at 900°C. Unfortunately in IATaOa this is above the Curie temperature of 600°C, so the crystal must be repoled after the doping. Such a treatment is inconvenient for a complex device. Ion beam processing can avoid this problem by making the low index barrier layers, and successful waveguides in LiTaOa have been formed at
4. Diagnostic uses of ion beams With insulators, as with semiconductors, a major consequence of ion implantation is to introduce new energy levels into the valence-to-conduction band gap. Such states change the electrical and optical properties of the material, and to a large extent we neither expect nor observe features which are not already amply discussed for semiconductors. For commercial reasons wider band gap semiconductors such as GaAs, CdTe or ZnTe have received most attention, and ion beams have been less popular with the more insulating systems such as MgO, A12Oa or LiF. To demonstrate that semiconducting properties can be introduced in even the most obvious of insulators, we can cite the results of ion implantation into diamond [26]. In the pure form the band gap is 5.4 eV. Implants with A1÷ or B+ ions turn this into a p-type semiconductor, whereas Li÷, C÷ or P* generate n-type. A form of impure natural diamond, type IIb, also exists as p-type semiconductor. The damage associated with room temperature implants causes an amorphisation of the carbon which is difficult to remove by heating; however, implants at high temperature can avoid this problem. Since the localised energy levels in the wide band gap insulators often define electronic transitions of high energy the resulting luminescence or optical absorption may be in the visible part of the spectrum, and for this reason the optical properties may be of more applied significance than the electrical VI. ION-IMPLANTED INSULATORS
730
P.D. Townsend lion implantation in semiconductors
changes [27]. As a research tool the impurities and disorder in the lattice can be used as a probe, and by isotopic control of the implant it is possible to maxiraise the information obtainable from luminescence, absorption, electron spin resonance (ESR) and electron nuclear double resonance (ENDOR).
5. Luminescence studies Many defects cause luminescence but the most precise analysis is possible if we have a series of discrete energy states, as seen by phonon assisted transitions or from inner shell transitions. Rare earth impurities are ideal as the precise energy states of the free atoms are perturbed by the coupling to the crystal field. Therefore measurements of the modified energy states, together with their state of polarisation and the effects of pressure can give detailed information on the defect symmetry and the presence of other impurities. Implanted ions do not occupy unique lattice sites, and indeed may exist in forms which are unobtainable by thermal mechanisms of doping. Annealing of the crystal can result in a whole range of impurity sites as the defects associate or are removed. Luminescence studies follow these changes in a quantitative fashion, even in cases where actual identification of defects is not possible. This type of measurement is clearly demonstrated in the work of Bryant and his co-workers [28-30] where, for example, they implanted Yb ions into CdTe. The sharp line cathodoluminescence spectra observed at 4 K show a wealth of detail for atoms implanted at low temperature and with pulse annealing the spectra change both in intensity and line position. After annealing the spectra change dramatically in the relative intensities of the lines. They interpret their results in terms of Yb ions moving into various types of substitutional lattice sites. Measurements of erbium ions implanted into ZnS are even more interesting as we can sense [29] the location of the Er in both the original interstitial site, as implanted, and also follow the progression to the substitutional lattice sites as the samples are annealed. In similar work [30] with ion damage in ZnTe they expect the formation of Te vacancies. However these do not yield a luminescence but are recognisable because they act as traps for oxygen impurities which then give a characteristic light emission. Luminescence measurements are intrinsically very sensitive, and in principle we might detect as few as
109 defect centres in a sample. Therefore to identify trace impurities by their luminescence requires either a very high degree of purification (and an absence of defects made in the sample preparation), or a technique in which we alter the impurity concentration in a well characterised sample. In this instance implantation is a favourable technique since the quantitative addition of known impurities can be made at low temperature where thermal defect generation and reactions with the surrounding atmosphere can be avoided. It also avoids problems of secondary dopants which enter the sample during diffusion doping. An example of the latter problem [31 ] was noted in the luminescence of CaO. After thermal doping with Bi the luminescence showed lines characteristic of Gd. To decide if the secondary impurities had originated in the source materials or the crucible etc., was not trivial, a simple solution was to implant with either Gd or Bi. This confirmed that the original CaO did not contain the Gd, but it had entered with the bismuth doping. Care was necessary, as it is possible to sense one impurity only after the addition of a second. This multiple impurity type of problem occurred in the thermoluminescence dosimeter material based on LiF. The intentional additives to the melt are Mg and Ti, and by suitable heat cycles the system is sensitized for dosimetry. ESR measurements [32] determine that the Ti+4 ions occupy Li+1 lattice sites. The necessary charge compensation comes either from three adjacent O= ions in F- sites, or from charged vacancies. Oxygen is a particularly awkward impurity to control in the ppm range since during melt or high temperature treatments it can come into equilibrium with residual gas around the crystal. Implantation gave a direct route for oxygen doping [33,34]. The results confirmed the need for oxygen at the luminescence site. Control experiments with chemically inert ions of Ne showed that the damage associated with the implant was not a major factor in the luminescence enhancement.
6. ESR and ENDOR
The powerful resonance techniques of ESR and ENDOR produce very definite defect identification with site symmetry as well as the interactions with neighbouring atoms. To sense this we use the hyperfine lines caused by the nuclei, isotopic control of the impurities is thus an extremely fruitful addition to
P.D. Townsend ~Ion implantation in semiconductors
the ESR measurements, as we can select the strength and number of the hyperfine structure lines. In conjunction with measurements of luminescence or absorption the ESR data can provide us with detailed defect models, even for defects with no unpaired spins. To cite an example where alternative methods exist we can note that implants of Ce ions into CaF2 show many defect sites in crystalline environments which change with annealing [35]. Potentially we can confirm the assignments of these defect models by making low temperature luminescence studies of the sharp line features of the rare earth ion in the different sites. A combination of optical and ESR measurements using the technique of optically detected magnetic resonance (ODMR) has been applied to ZnS [36]. The separate effects of impurity and radiation damage are resolved by compazison of inert gas and active ion implants. In the case of Ar and P implants the ODMR reveals that there is clustering into P4 groups. The combination of ESR and implantation methods seems to be in its infancy, and we suspect that the major difficulty is in interpreting the wealth of data which appears, rather than in detecting a signal. For radiation damage measurements the confusion of many types of defect is clearly evident from the ESR data.
7. Optical absorption Optical absorption bands in solids are invariably rather broad and their :use in defect identification becomes an elaborate detective story. In a few cases sharp line features exist and here we may make use of both isotopic effects and ion implantation. One familar example occurs in the stretching vibration of the OH and OD bonds. These infrared features are clearly separated and have been used to follow bonding changes of hydrogen isotopes implanted into silica glass [37]. Similar vibrational modes of CH and CD bonds in SiC implanted with hydrogen are used to locate the sites of the impurity atoms [38]. In damage studies we may make use of ion beams to help distinguish between different mechanisms of defect formation, to create very high optical densities in thin layers [39], or to introduce dopant ions [40]. Such studies are important both for defects formed by the ion implantation as well as by other processes. To demonstrate the approach taken for a "classical" colour centre problem we shall quote the case of the
731
alkali halides. The normal halogen interstitial is formed by a hole trapped on a crowdion interstitial. This is called the H centre. The optical absorption band labelled V4 is most probably the di-interstitial. To test this proposal we could generate a higher V4 to H ratio if we could use a very high defect creation rate. By merely increasing the damage flux the desired result is not possible since the total power level would cause additional effects. A comparison of damage production rates with ion and molecular beams avoids the power problem, but along the track of the components of the dissociating molecule we reach the necessary local high flux condition. In the case of KBr the V4 model was successfully tested by comparing defect production from beams of hydrogen atoms and molecules [41]. Both the electronic and nuclear collision processes can generate defects, and because the two sources of energy deposition overlap within the ion track, we needs to consider both enhanced defect formation and annealing. In insulators such processes can make a significant contribution to the final defect equilibrium. The flexibility available in the choice of the relative fractions of electronic and nuclear energy loss rates achieved by controlling the mass and energy of the ions has led to a better understanding of defect formation in alkali halides,MgO, A1203, and SiO2 as well as with narrower gap materials. To illustrate the possibilities, we note that in MgO [42] a comparison was made of the lattice expansion and the F-band optical absorption caused by ion bombardment. The damage produced by heavy ion irradiation became modified and the lattice expansion decreased during subsequent ionisation. The changes in optical absorption are consistent with a model that the nuclear collision damage generated complex defects, whereas ionisation energy destroys the complexes and simpler point defects remain. A somewhat similar picture emerges in A120 a studies [43] and also from more recent studies of silica glass [44,45]. The latter authors chose to assess the rate of damage production not from equilibrium measurements of optical absorption but rather from the dynamic measurements of luminescence stimulated by the ion bombardment. For silica this has been very helpful, and we can sense results ofionisation induced annealing by the variations of luminescence intensity as a function of the ion energy. This type of measurement for wide gap insulators, has many possibilities. The same group even attempted to make measurements of local average "temperature" VI. ION-[MPLANTEDINSULATORS
732
P.D. Townsend ~Ion implantation in semiconductors
in the bombarded region by noting the shifts in the wavelength of the ruby emission lines [46]. Their data suggests temperature gradients of some 25 K existed between the implanted layer and the ruby substrates.
8. C o n d u f i o n In this review we have sketched a few of the experiments where optical property changes have been made by ion implantation. The use of implanted impurities is obviously helpful in making detailed studies of particular defects and in some examples the ability to, change from normal isotopic abundances has been exploited. Alternatively the changes in electronic or nuclear damage rates and high flux conditions from molecular beams can set up defect production conditions which ere n o t possible by other techniques. Such ideas are equally appropriate for defect studies in semiconductors, b u t we have attempted to choose examples from wider gap materials. Electronic energy deposition creates d-erects in many insulators, and to a lesser extent changes damage rates in semiconductors, so we must be aware of this possibility. For the majority of our examples the ion beams have been used as a probe to study special situations in a crystal. Device manufacture by ion implantation into insulators is not yet a significant process for the materials we have considered but potentially there is a major industrial application in the area of integrated optics. Thus, over the coming years we can look confidently to an expansion of studies of ion implantation in insulators.
References [1] V. Evtuhov and A. Yafiv, IEEE Trans. Microwave theory Tech. 23 (1975) 44. [2] P.D. Townsend, J.C. Kelly and N.E.W. Hartley, Ion Implantation, Sputtering and their Applications (Academic, New York, 1976). [3] P.D. Townsend, J. Phys. E 10 (1977) 197. [4] P.D. Townsend and S. Valette, Optical effects of ion implantations Treatise on materials science and technology, Vol. 19, ed., J.K. Hirvonen (Academic, New York, 1980) ch. 11. [5] N.S. Kapany and J.J. Burke, Quantum Electronics (Optical Waveguides) (Academic, New York, 1972). [6] T. Tamir, eds., .Integrated Optics (Springer-Verlag, Berlin, 1975). [7] P.K. Tien, Rev. Mod. Phys. 49 (1977) 361. [8] H. Kogelnik, Fiber and Int. Opt. 1 (1978) 227.
[9] K.D. Mayne and J.P. Tomlinson, eds., Integrated Optics Bibliography (IEE, New York, 1978). [10] D.B. Ostrowsky, Optical Fibre Communications, eds., M.J. Howes and D.V. Morgan (Wiley, New York, 1980) ch. 4. [11] R.V. Schmidt and I.P. Kaminow, Appl. Phys. Lett. 25 (1974) 458. [12] H. Naitoh, M. Nunoshita and T. Nakayama, Appl. Opt. 16 (t977) 2546. [13] G.L. Destefanis, P.D. Townsend and J.P. Gailliard, Appl. Phys. Lett. 32 (1978) 293. [14] G.L. Destefanis, J.P. Gailliard, E.L. Ligeon, S. Valette, B.W. Earmery, P.D. Townsend and A. Perez, J. Appl. Phys. 50 (1979) 7898. [15] R.L. Hines and R. Arndt, Phys. Rev. 119 (1960) 623. [16] A.R. Bayly, Rad. Eft. 18 (1973) l l l . [17] A.P. Webb and P.D. Townsend, J. Phys. D 9 (1976) 1343. [18] J. Heibei and E. Voges, Phys. Star. Sol. 57a (1980) 609. [19] D.T.Y. Wei, W.W. Lee and L.R. Bloom, Appl. Phys. Lett. 25 (1974) 329. [20] J. Heibei and E. Voges, submitted to IEEE, Quan. Electron. [21] A. Ermolieff and S. Valette, to be published. [22] R.D. Standley and V. Ramaswany, Appl. Phys. Left. 25 (1974) 711. [23] J. Noda, T. Saku and N. Uchida, Appl. Phys. Lett. 25 (1974) 308. [24] J. O'Connor, A. Faik and P.D. Townsend, unpublished. [25] K. Wenzlik, J. Heibei and E. Voges, submitted to Phys. Star. Sol. (a). [26] V.S. Vavilov, Rad. Eft. 37 (1978) 229. [27] P.D. Townsend, Rad. Phys. Chem. 37 (1978) 205. [28] F.J. Bryant and 1. Nahum, Rad. Eft. 31 (1977) 106. [29] C.C. Yu and F.J. Bryant, J. Lumin. 18/19 (1979) 841. [30] D. Verity and E.J. Bryant, in press. [31] A.E. Hughes and G.P. Pells, J. Phys. C 7 (1974) 3997. [32] J.J. Davies, J. Phys. C 7 (1974) 599. [33] M.C. Wintersgill, P.D. Townsend and F. Cusso-Perez, J. de Phys, C 7 (1977) 123. [34] M.C. Wintersgill and P.D. Townsend, Phys. Stat. Sol. 47a (1978) K67. [35] U. Bangert, to be published. [36] D. Verity, J.J. Davies, J.E. Nicholls and F.J. Bryant, submitted to J. Appl. Phys. [37] P.L. Mattern, C.J. Thomas and W. Bauer, J. Vac. Sci. Technol. 13 (1976) 430. [38] L. Patrick and W.J. Choyke, Phys. Rev. B 8 (1973) 1660. [39] D. Pooley; Brit. J. Appl. Phys. 17 (1966) 855. [40] A.E. Hughes and D. Pooley, J. Phys. C 4 (1971) 1963. [41] M. Saidoh and P.D. Townsend, J. Phys. C 10 (1977) 1541. [42] G.B. Krefft, J. Vac. Sci. Technot. 14 (1977) 533. [43] G.W. Arnold, G.B. Krefft and C.B. Norris, Apph Phys. Lett. 25 (1974) 540. [44] P.J. Chandler, F. Jaque and P.D. Townsend, Rad. Eft. 42 (1979) 45. [45] F. Jaque and P.D. Townsend, these proceedings, p. 781. [46] P.J. Chandler and P.D. Townsend, Rad. Eft. Lett. 43 (1979) 61.
727
Section VL Ion-implanted insulators
ION IMPLANTATION IN INSULATORS P.D. TOWNSEND The School of Mathematical and Physical Sciences, The University of Sussex, Brighton, BN1 9QH, UK
The modification of insulating materials by ion implantation is developing in several areas. In many cases the property changes just parallel those observed in semiconductors, but in at least one new area, namely integrated optics, the use of the ion beams has distinct advantages for the control of the properties needed for waveguides and other optical devices. Ion beams also fill a second function as diagnostic tools in the production and analysis of defect structures. A selection of experiments using luminescence and optical absorption are mentioned.
1. Introduction
2. Integrated optics
The applications, results and advantages of ion implantation for semiconductor production have been well documented for many years. However, for wider energy gap materials the literature of ion beam effects is rather limited. There is no a priori reason why ion implantation in insulators should not be developed, and early experiments clearly indicate that many interesting ideas are likely to be successful. The present literature can broadly be separated into two groups, namely experiments which use the ion beams in a diagnostic role and those which are concerned with device manufacture. The experience gained in the semiconductor industry of pattern generation, lithography, ion sources and processing are readily carried over to implantation of insulators. We are therefore in the situation of having a viable technique in search of a problem, and fortunately at least one suitable problem has been identified. Earlier reviews have suggested that a major area of technology is developing [1] which, over the course of the next decade, will definitely need ion implantation as a production technique [ 2 - 4 ] . This field stems from the use of optical fibre communication systems and is concerned with the optical processing units [5-10]. By analogy with semiconductor devices it is often called integrated optics. In the present summary we shall reiterate why implantation seems appropriate for integrated optics. Some reference will also be made to the more varied diagnostic uses of ion beams in insulating materials. The list provided is not exhaustive and additional possibilities should be considered.
Since this is an area of physics which may be unfamiliar to those working with ion implantation, we shaU offer a brief outline of the main ideas, but for details of the subject we can recommend many excellent reviews and books (e.g. [5-10]). Integrated optics describes the packages of circuitry in which the signal pathways are waveguides for light of optical frequencies. The primary circuit element is the waveguide, but to make active circuits then there must also be switches, modulators, mixers, tuned filters, sources, detectors, logic elements, delay fines etc. By using the high frequency of an optical carrier we instantly gain a very large signal capacity even though the conducting pathway is only a few microns in cross section. Multiplexing of signals by pulse coding and/or by different carrier wavelengths offers further scope for the future expansion of optical waveguide systems. Whilst it is true that at this present stage of development of the glass fibre communication networks the optical processing units are relatively standard, it is evident that this is only an interim situation and truly sophisticated integrated optic circuits are going to be developed in the future. The optical paths are written as regions of high refractive index which confine the light beam and modulation, etc., are accomplished by external modification of the index. Changes can be caused by electric, magnetic or stress fields, but most devices designed so far use the electro-optic effects. The physical size of the guides (i.e. a few microns) makes them ideally suited for a planar technology, and they can be fabricated by either diffusion or ion implantaVI. ION-IMPLANTED INSULATORS
728
P.D. Townsend/Ion implantation in semiconductors
tion with the aid of mask patterns. The ion beams offer some advantages [3] in that the lateral spread of the patterns formed beneath the masks may be less than those generated by diffusion. We may also make implantations with various ions and energies to give a predetermined refractive index profile (e.g. to bury or taper the depth of the guide), further, modifications may be made at room temperature rather than by a high temperature diffusion process.
Surface Impl.ant (aL
index X) (Q)
_
LiNbO3 (b)--~(b)
Fr //
!b_k_./
Depth 2~ X
3. Implantation in LiNbOs and LiTaOs The materials of major interest are those which have large electro-optic coefficients [6,8] and thus offer the possibility of making efficient low power switches and modulators. Two prime examples are LiNbOs and LiTaOa. They are also chemically stable, robust and transparent over a wide range of wavelengths. For LiNbO3 the normal diffusion method of controlling the refractive index is to introduce titanium ions by diffusion [11,12] near 900°C. This increases the value of the extraordinary refractive index by some 0.04. Not all attempts at Ti diffusion are successful, since the formation of a surface barrier, perhaps a titanium oxide, inhibits the movement of the titanium. We may bypass this problem by a low energy Ti implant, and then diffuse from this sub-surface layer. A more direct approach is to use the implantation to change the refractive index. With LiNbO3 [13,14], as with silica, (e.g. [15-18]) the index change is a function of the energy deposited by nuclear collisions and is only partially sensitive to the choice of ion. For LiNbOa the ordinary index alters by as much as 10% (0.23), but it is a decrease [13,14,19]. Thus we need to reconsider how to use this change in practical devices. Two solutions are sketched in fig. 1. The simplest, fig. I(A), is to define the boundaries exterior to the waveguide by reducing the index. This leaves a central unirnplanted channel with the original high index and all the electro-optic properties of the original material. Such a high index guide mounted on a high index substrate is referred to as a strip waveguide. Therefore as shown in fig. 1(A) the simplest treatment is direct implantation alongside the guide region. If in addition we wish to isolate the guide from the substrate it might be convenient to use a combination of diffusion and implantation techniques [3,20]. Firstly we form a planar high index layer by
(B)
ImplTiant(c/+ __~_~ LiNbO + Ti (d_) _
_~
. . . .
(dl (d) ~<
Depth
x
Fig. I(A). The effect of ion implantation on the refractive index of LiNbO3 . Region (a) is original material, whereas for the implanted region (b) the index is reduced. The region bounded by the implant acts as an optical waveguide. In fig. I(B) the surface refractive index is increased by titanium diffusion before implantation.
diffusion and then isolate this into a localised three dimensional guide by boundary implants. In the example of fig. I(B) the increase is made by Ti diffusion and the implants cause the decrease. The depth profiles of the two methods are rather different. This is not important as the waveguide modes are confined if either the layer thickness and/or the index difference between the guide and surrounds are small. This has the advantage that the projected range of the implant can be much less than the diffusion depth of the Ti. A quite different approach has been used for undoped LiNbO3 to take advantage of both the change in the ordinary index and the larger ra 3 electro-optic coefficient [14]. Since the damage and index change are generated by the (dE/dx)nuclear energy loss of the beam, light ions at high velocity (e.g. 2 MeV He*) traverse the first few microns of the LiNbOs without causing any significant damage to the lattice. Damage only occurs at the end of the track and produces a low-index wall which separates the surface layer from the substrate. The index proFile follows the damage generation curve, and as
P.D. Townsend/Ion implantation in semiconductors 2.18
-
n=1 MODES 2,ZO
-
2,22
-
2.24
-
i-
\
2,26
2,28 -
--~
I 3.5
I 3.0
I
I
2.5
2.0
~
r
729
both 77 and 300 K using helium ions [24,25]. We have concentrated on implantation in the LiNbO3 and LiTaO3 but there are many other materials which are relevant for integrated optic devices. Materials which also show semiconductor or LED properties are particularly interesting (e.g. ZnTe or GaAs). They offer the possibility of combining the electronic and optical circuitry [1] onto a single monolithic substrate, and with this aim in mind many waveguide components have been formed by ion beam treatment of GaAs, CdTe, ZnTe etc. (In ref. [4] many more examples are discussed than is possible in this brief article). In summary, we see that ion beam technology is very well suited to the specific needs of optical waveguide circuitry, and we can confidently expect that this area of research and appfications will expand over the next decade.
DEPTH, MICRONS Fig. 2. Fast ion implantation of LiNbO3 produces a low refractive index barrier between the surface waveguide and the substrate. For the example shown, 2 MeV He+ ions confine 7 optical modes in the surface as computed theoretically.
shown in fig. 2, the calculated and observed refractive index pattern can support many waveguide modes. For the data shown only a single ion energy was used, but wider barrier layers have been formed by multiple energy implants [13,14]. In such guides the electro-optic coefficients are still very favourable, although reduced from the values in perfect LiNbOa [14,21]. We may emphasise a further advantage of ion implantation for integrated optics by reference to waveguide formation in LiTaOa. As with lithium niobate, the metal ions have metastable sites within the unit cell of the lattice, but by application of an electric field at high temperature preferred sites are taken and the polarised material then shows electrooptic properties. For LiTaO~ the index is less than for LiNbO3 (e.g. no is 2.177 compared with 2.286), hence a high index layer can be formed by doping with niobium [22,23]. This is in-diffused at 900°C. Unfortunately in IATaOa this is above the Curie temperature of 600°C, so the crystal must be repoled after the doping. Such a treatment is inconvenient for a complex device. Ion beam processing can avoid this problem by making the low index barrier layers, and successful waveguides in LiTaOa have been formed at
4. Diagnostic uses of ion beams With insulators, as with semiconductors, a major consequence of ion implantation is to introduce new energy levels into the valence-to-conduction band gap. Such states change the electrical and optical properties of the material, and to a large extent we neither expect nor observe features which are not already amply discussed for semiconductors. For commercial reasons wider band gap semiconductors such as GaAs, CdTe or ZnTe have received most attention, and ion beams have been less popular with the more insulating systems such as MgO, A12Oa or LiF. To demonstrate that semiconducting properties can be introduced in even the most obvious of insulators, we can cite the results of ion implantation into diamond [26]. In the pure form the band gap is 5.4 eV. Implants with A1÷ or B+ ions turn this into a p-type semiconductor, whereas Li÷, C÷ or P* generate n-type. A form of impure natural diamond, type IIb, also exists as p-type semiconductor. The damage associated with room temperature implants causes an amorphisation of the carbon which is difficult to remove by heating; however, implants at high temperature can avoid this problem. Since the localised energy levels in the wide band gap insulators often define electronic transitions of high energy the resulting luminescence or optical absorption may be in the visible part of the spectrum, and for this reason the optical properties may be of more applied significance than the electrical VI. ION-IMPLANTED INSULATORS
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changes [27]. As a research tool the impurities and disorder in the lattice can be used as a probe, and by isotopic control of the implant it is possible to maxiraise the information obtainable from luminescence, absorption, electron spin resonance (ESR) and electron nuclear double resonance (ENDOR).
5. Luminescence studies Many defects cause luminescence but the most precise analysis is possible if we have a series of discrete energy states, as seen by phonon assisted transitions or from inner shell transitions. Rare earth impurities are ideal as the precise energy states of the free atoms are perturbed by the coupling to the crystal field. Therefore measurements of the modified energy states, together with their state of polarisation and the effects of pressure can give detailed information on the defect symmetry and the presence of other impurities. Implanted ions do not occupy unique lattice sites, and indeed may exist in forms which are unobtainable by thermal mechanisms of doping. Annealing of the crystal can result in a whole range of impurity sites as the defects associate or are removed. Luminescence studies follow these changes in a quantitative fashion, even in cases where actual identification of defects is not possible. This type of measurement is clearly demonstrated in the work of Bryant and his co-workers [28-30] where, for example, they implanted Yb ions into CdTe. The sharp line cathodoluminescence spectra observed at 4 K show a wealth of detail for atoms implanted at low temperature and with pulse annealing the spectra change both in intensity and line position. After annealing the spectra change dramatically in the relative intensities of the lines. They interpret their results in terms of Yb ions moving into various types of substitutional lattice sites. Measurements of erbium ions implanted into ZnS are even more interesting as we can sense [29] the location of the Er in both the original interstitial site, as implanted, and also follow the progression to the substitutional lattice sites as the samples are annealed. In similar work [30] with ion damage in ZnTe they expect the formation of Te vacancies. However these do not yield a luminescence but are recognisable because they act as traps for oxygen impurities which then give a characteristic light emission. Luminescence measurements are intrinsically very sensitive, and in principle we might detect as few as
109 defect centres in a sample. Therefore to identify trace impurities by their luminescence requires either a very high degree of purification (and an absence of defects made in the sample preparation), or a technique in which we alter the impurity concentration in a well characterised sample. In this instance implantation is a favourable technique since the quantitative addition of known impurities can be made at low temperature where thermal defect generation and reactions with the surrounding atmosphere can be avoided. It also avoids problems of secondary dopants which enter the sample during diffusion doping. An example of the latter problem [31 ] was noted in the luminescence of CaO. After thermal doping with Bi the luminescence showed lines characteristic of Gd. To decide if the secondary impurities had originated in the source materials or the crucible etc., was not trivial, a simple solution was to implant with either Gd or Bi. This confirmed that the original CaO did not contain the Gd, but it had entered with the bismuth doping. Care was necessary, as it is possible to sense one impurity only after the addition of a second. This multiple impurity type of problem occurred in the thermoluminescence dosimeter material based on LiF. The intentional additives to the melt are Mg and Ti, and by suitable heat cycles the system is sensitized for dosimetry. ESR measurements [32] determine that the Ti+4 ions occupy Li+1 lattice sites. The necessary charge compensation comes either from three adjacent O= ions in F- sites, or from charged vacancies. Oxygen is a particularly awkward impurity to control in the ppm range since during melt or high temperature treatments it can come into equilibrium with residual gas around the crystal. Implantation gave a direct route for oxygen doping [33,34]. The results confirmed the need for oxygen at the luminescence site. Control experiments with chemically inert ions of Ne showed that the damage associated with the implant was not a major factor in the luminescence enhancement.
6. ESR and ENDOR
The powerful resonance techniques of ESR and ENDOR produce very definite defect identification with site symmetry as well as the interactions with neighbouring atoms. To sense this we use the hyperfine lines caused by the nuclei, isotopic control of the impurities is thus an extremely fruitful addition to
P.D. Townsend ~Ion implantation in semiconductors
the ESR measurements, as we can select the strength and number of the hyperfine structure lines. In conjunction with measurements of luminescence or absorption the ESR data can provide us with detailed defect models, even for defects with no unpaired spins. To cite an example where alternative methods exist we can note that implants of Ce ions into CaF2 show many defect sites in crystalline environments which change with annealing [35]. Potentially we can confirm the assignments of these defect models by making low temperature luminescence studies of the sharp line features of the rare earth ion in the different sites. A combination of optical and ESR measurements using the technique of optically detected magnetic resonance (ODMR) has been applied to ZnS [36]. The separate effects of impurity and radiation damage are resolved by compazison of inert gas and active ion implants. In the case of Ar and P implants the ODMR reveals that there is clustering into P4 groups. The combination of ESR and implantation methods seems to be in its infancy, and we suspect that the major difficulty is in interpreting the wealth of data which appears, rather than in detecting a signal. For radiation damage measurements the confusion of many types of defect is clearly evident from the ESR data.
7. Optical absorption Optical absorption bands in solids are invariably rather broad and their :use in defect identification becomes an elaborate detective story. In a few cases sharp line features exist and here we may make use of both isotopic effects and ion implantation. One familar example occurs in the stretching vibration of the OH and OD bonds. These infrared features are clearly separated and have been used to follow bonding changes of hydrogen isotopes implanted into silica glass [37]. Similar vibrational modes of CH and CD bonds in SiC implanted with hydrogen are used to locate the sites of the impurity atoms [38]. In damage studies we may make use of ion beams to help distinguish between different mechanisms of defect formation, to create very high optical densities in thin layers [39], or to introduce dopant ions [40]. Such studies are important both for defects formed by the ion implantation as well as by other processes. To demonstrate the approach taken for a "classical" colour centre problem we shall quote the case of the
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alkali halides. The normal halogen interstitial is formed by a hole trapped on a crowdion interstitial. This is called the H centre. The optical absorption band labelled V4 is most probably the di-interstitial. To test this proposal we could generate a higher V4 to H ratio if we could use a very high defect creation rate. By merely increasing the damage flux the desired result is not possible since the total power level would cause additional effects. A comparison of damage production rates with ion and molecular beams avoids the power problem, but along the track of the components of the dissociating molecule we reach the necessary local high flux condition. In the case of KBr the V4 model was successfully tested by comparing defect production from beams of hydrogen atoms and molecules [41]. Both the electronic and nuclear collision processes can generate defects, and because the two sources of energy deposition overlap within the ion track, we needs to consider both enhanced defect formation and annealing. In insulators such processes can make a significant contribution to the final defect equilibrium. The flexibility available in the choice of the relative fractions of electronic and nuclear energy loss rates achieved by controlling the mass and energy of the ions has led to a better understanding of defect formation in alkali halides,MgO, A1203, and SiO2 as well as with narrower gap materials. To illustrate the possibilities, we note that in MgO [42] a comparison was made of the lattice expansion and the F-band optical absorption caused by ion bombardment. The damage produced by heavy ion irradiation became modified and the lattice expansion decreased during subsequent ionisation. The changes in optical absorption are consistent with a model that the nuclear collision damage generated complex defects, whereas ionisation energy destroys the complexes and simpler point defects remain. A somewhat similar picture emerges in A120 a studies [43] and also from more recent studies of silica glass [44,45]. The latter authors chose to assess the rate of damage production not from equilibrium measurements of optical absorption but rather from the dynamic measurements of luminescence stimulated by the ion bombardment. For silica this has been very helpful, and we can sense results ofionisation induced annealing by the variations of luminescence intensity as a function of the ion energy. This type of measurement for wide gap insulators, has many possibilities. The same group even attempted to make measurements of local average "temperature" VI. ION-[MPLANTEDINSULATORS
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in the bombarded region by noting the shifts in the wavelength of the ruby emission lines [46]. Their data suggests temperature gradients of some 25 K existed between the implanted layer and the ruby substrates.
8. C o n d u f i o n In this review we have sketched a few of the experiments where optical property changes have been made by ion implantation. The use of implanted impurities is obviously helpful in making detailed studies of particular defects and in some examples the ability to, change from normal isotopic abundances has been exploited. Alternatively the changes in electronic or nuclear damage rates and high flux conditions from molecular beams can set up defect production conditions which ere n o t possible by other techniques. Such ideas are equally appropriate for defect studies in semiconductors, b u t we have attempted to choose examples from wider gap materials. Electronic energy deposition creates d-erects in many insulators, and to a lesser extent changes damage rates in semiconductors, so we must be aware of this possibility. For the majority of our examples the ion beams have been used as a probe to study special situations in a crystal. Device manufacture by ion implantation into insulators is not yet a significant process for the materials we have considered but potentially there is a major industrial application in the area of integrated optics. Thus, over the coming years we can look confidently to an expansion of studies of ion implantation in insulators.
References [1] V. Evtuhov and A. Yafiv, IEEE Trans. Microwave theory Tech. 23 (1975) 44. [2] P.D. Townsend, J.C. Kelly and N.E.W. Hartley, Ion Implantation, Sputtering and their Applications (Academic, New York, 1976). [3] P.D. Townsend, J. Phys. E 10 (1977) 197. [4] P.D. Townsend and S. Valette, Optical effects of ion implantations Treatise on materials science and technology, Vol. 19, ed., J.K. Hirvonen (Academic, New York, 1980) ch. 11. [5] N.S. Kapany and J.J. Burke, Quantum Electronics (Optical Waveguides) (Academic, New York, 1972). [6] T. Tamir, eds., .Integrated Optics (Springer-Verlag, Berlin, 1975). [7] P.K. Tien, Rev. Mod. Phys. 49 (1977) 361. [8] H. Kogelnik, Fiber and Int. Opt. 1 (1978) 227.
[9] K.D. Mayne and J.P. Tomlinson, eds., Integrated Optics Bibliography (IEE, New York, 1978). [10] D.B. Ostrowsky, Optical Fibre Communications, eds., M.J. Howes and D.V. Morgan (Wiley, New York, 1980) ch. 4. [11] R.V. Schmidt and I.P. Kaminow, Appl. Phys. Lett. 25 (1974) 458. [12] H. Naitoh, M. Nunoshita and T. Nakayama, Appl. Opt. 16 (t977) 2546. [13] G.L. Destefanis, P.D. Townsend and J.P. Gailliard, Appl. Phys. Lett. 32 (1978) 293. [14] G.L. Destefanis, J.P. Gailliard, E.L. Ligeon, S. Valette, B.W. Earmery, P.D. Townsend and A. Perez, J. Appl. Phys. 50 (1979) 7898. [15] R.L. Hines and R. Arndt, Phys. Rev. 119 (1960) 623. [16] A.R. Bayly, Rad. Eft. 18 (1973) l l l . [17] A.P. Webb and P.D. Townsend, J. Phys. D 9 (1976) 1343. [18] J. Heibei and E. Voges, Phys. Star. Sol. 57a (1980) 609. [19] D.T.Y. Wei, W.W. Lee and L.R. Bloom, Appl. Phys. Lett. 25 (1974) 329. [20] J. Heibei and E. Voges, submitted to IEEE, Quan. Electron. [21] A. Ermolieff and S. Valette, to be published. [22] R.D. Standley and V. Ramaswany, Appl. Phys. Left. 25 (1974) 711. [23] J. Noda, T. Saku and N. Uchida, Appl. Phys. Lett. 25 (1974) 308. [24] J. O'Connor, A. Faik and P.D. Townsend, unpublished. [25] K. Wenzlik, J. Heibei and E. Voges, submitted to Phys. Star. Sol. (a). [26] V.S. Vavilov, Rad. Eft. 37 (1978) 229. [27] P.D. Townsend, Rad. Phys. Chem. 37 (1978) 205. [28] F.J. Bryant and 1. Nahum, Rad. Eft. 31 (1977) 106. [29] C.C. Yu and F.J. Bryant, J. Lumin. 18/19 (1979) 841. [30] D. Verity and E.J. Bryant, in press. [31] A.E. Hughes and G.P. Pells, J. Phys. C 7 (1974) 3997. [32] J.J. Davies, J. Phys. C 7 (1974) 599. [33] M.C. Wintersgill, P.D. Townsend and F. Cusso-Perez, J. de Phys, C 7 (1977) 123. [34] M.C. Wintersgill and P.D. Townsend, Phys. Stat. Sol. 47a (1978) K67. [35] U. Bangert, to be published. [36] D. Verity, J.J. Davies, J.E. Nicholls and F.J. Bryant, submitted to J. Appl. Phys. [37] P.L. Mattern, C.J. Thomas and W. Bauer, J. Vac. Sci. Technol. 13 (1976) 430. [38] L. Patrick and W.J. Choyke, Phys. Rev. B 8 (1973) 1660. [39] D. Pooley; Brit. J. Appl. Phys. 17 (1966) 855. [40] A.E. Hughes and D. Pooley, J. Phys. C 4 (1971) 1963. [41] M. Saidoh and P.D. Townsend, J. Phys. C 10 (1977) 1541. [42] G.B. Krefft, J. Vac. Sci. Technot. 14 (1977) 533. [43] G.W. Arnold, G.B. Krefft and C.B. Norris, Apph Phys. Lett. 25 (1974) 540. [44] P.J. Chandler, F. Jaque and P.D. Townsend, Rad. Eft. 42 (1979) 45. [45] F. Jaque and P.D. Townsend, these proceedings, p. 781. [46] P.J. Chandler and P.D. Townsend, Rad. Eft. Lett. 43 (1979) 61.