Ion implantation of optical devices

MON 13

Nuclear Instruments and Methods in Physics Research B68 (1992) 355-360 North-Holland

Beam Intem1tions with Materials 6 Atoms

Ion implantation of optical devices Ch . Buchal Institut für Schicht- und lonentechnik, Forschungszentrum Jülich (KFA), D-5170 Jülich, Germany The use of accelerators for research and technology in the field of optical materials is outlined . We stress the application of ion beams of 1 to 10 MeV energy for optical waveguide formation and for waveguide doping . This includes ion beam mixing and doping of semiconductor heterostructures, doping of glass guides with laser active ions, guide formation in ceramic single crystals by MeV He implantation and the associated index modification by electronic and nuclear energy transfer . Recent results of RBS/channeling/PIXE studies of dopant lattice locations are also discussed. 1. Introduction In this review it will be outlined, in which areas accelerators are useful for research and development of modern optical devices. Different aspects of this field have been summarized before [1-41. There are a number of very different interesting approaches, which deserve attention . Let us first consider the "classical" optical communication setup, as it is in widespread use for long distance links, even crossing the oceans . Semiconductor laser diodes emit light, modulated by the applied electrical signals . The light is guided in glass

fibers and after typically 50 to 100 km a photodiode detects the light and converts it back to an electrical signal. The powerful laser diodes consist of complex heterostructures of III-V semiconductors [5-7). In the diode fabrication process accelerators are used for doping with electrically active ions. This "standard ion implantation" is well understood. In semiconductors, the change of the electrical carrier concentration also influences the optical index. The index may be lowered by increasing the carrier concentration, and this is achieved by proton implantation [81. A third application is the ballistic ion beam mixing of

MeV 0+ Ions . Photoresist - Au Mask AuZn Contact ~aynyywwWwwW~~ p - GaAs -p - Alo 5Gan.5As, p - AIGaAs GRIN - GaAs OW n - AIGaAs GRIN n - Alo sGa o sAs n - GaAs

Laser Active Region Fig. 1 . (Taken from ref. [11].) Cross-sectional views of (a) a masked and as-implanted single quantum well laser device chip and (b) an oxygen ion implantation fabricated laser device . The authors used 2X10 15 O+/cm 2 at 1 .8 MeV to form the implanted insulating regions, achieving lateral confinement for the laser light and also for the electrical current. The numerous epitaxial layers are needed to form a vertical p-n junction (diode) and to give vertical confinement for the laser light. Since the ion beam energy is very high, a thick lithographical mask was needed- 4 ltm of photoresist and 0.35 wm of Au . 0168-583X/92/$05.00 © 1992 - Elsevier Science Publishers B .V. All rights reserved

Vlll . ION BEAM MODIFICATION

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Ch. Buchal / Ion implantation of optical devices

the heterostructures . A high dose MeV oxygen implantation of typically 10 17 ions/em2 into a multiple InGaAs or ÀIGaAs heterostructure generates compositional disorder and semi-insulating areas. This changes the optical index in such a manner that an optical waveguide is formed . Prototype index-guided lasers have been fabricated by MeV ion implantation [9-11] . One example from ref. [111 is demonstrated in fig. 1. The 1.8 MeV oxygen beam has two useful effects: Firstly the oxygen doping forms deep traps for electrons and the semiconducting material becomes insulating. Secondly the implantation-induced lattice disordering provides an additional change in the optical index, resulting in good low loss lateral optical confinement . The three different classes of implantation experiments mentioned above have been performed into III-V semiconductors . Only the III-V or I1-VI semiconductor systems permit the fabrication of light emitting diodes or laser diodes, since efficient light generation needs a direct electronic band gap. The multiplecomponent III-V heterostructures have the potential to integrate all electronic and optical functions on one chip : lasers, photodetectors, optical waveguides, electro-optical modulators and all electronic components. Unfortunately the difficult and expensive technology has up to now hindered widespread applications. Definitely silicon technology dominates in the semiconductor industries, but silicon happens to have an indirect band gap and all the highly developed silicon technology is of little help for active, light emitting optocommunication components, although passive detectors can be fabricated in excellent quality with Si diodes . It is also fairly easy to fabricate a passive lightguide on silicon by oxidation, since Si02 (quartz glass) and transparent silicon nitrides are easily formed . 2. Implantation of laser active ions into glasses An interesting approach is the use of MeV ion implantation of rare earth (RE) ions into these oxide layers. The motivation stems from the laser effect of RE ions in dielectric glasses or crystals, such as Nd : YAG. The lasers are optically pumped from an external light source and provide intense coherent light at useful wavelenghts. In the modern glass fiber technology RE doped fibers are very ittportant, since they permit the construction of fiber amplifiers [12] . These optical amplifiers need a pump light source, as a laser does, but they have no cavity minors. An arriving optical signal, i.e . a bunch of photons, stimulates the emission of photons from the RE ions, thus amplifying and refreshing the incoming signal. For Er doped optical fiber amplifiers a gain of 39 dB has been reported [131. In this case each arriving photon induces an

Fig. 2. (Taken from ref. [15].) Er part of RBS spectra of a Si02 film after implantation of 5 x 10 15 Er/cm2 at 3.5 MeV. Spectra for as-implanted (o) and annealed sample at 1200°C for 1 h (solid line) . There is no diffusion and the dopant profile is well preserved after annealing. average of nearly 10000 coherent emissions . As pump light source most favorably semiconductor laser diodes are employed. It is in the implementation of this optical laser and amplifier technology into the oxide waveguides on silicon wafers, where MeV RE doping becomes important . The Si02 glass does not permit diffusion doping. The optical waveguides have thicknesses of the order of a few micrometers into which the RE ions have to be introduced . MeV implantation is a good solution, since Er ions, implanted at 3.5 MeV, display a dopant profile of 1.3 Wm depth and 0.5 pm FWHM in Si02, see fig . 2. This dopant profile is well matched to the light intensity distribution within the waveguide, Since Er diffusion is not noticeable, even subsequent high temperature processing steps are possible . Optical fluorescence measurements show a sufficiently long fluorescence lifetime and a favorable fluorescence yield [14,15] . The integration of an ion implanted optical amplifier onto a silicon chip should be feasible . The need for an external "optical power supply", i.e . a pump light source, presents no problem, but presently a very serious missing link is the inability to electrically influence the Er amplifier gain. This optical amplifier will ride on a Si chip, but the chip cannot talk to its optical component ("silicon breadboard technology"). Up to now no reasonable and realistic idea hasemerged for solving this problem . In only a few test setups Si wafers were used as substrates for optical waveguides . For the commercial production of beam splitters, branches and some sensors, optical glasses are used as substrates . The waveguides are formed on them by ion exchange . This is a diffusion replacement of K' or Na' ions of the glass

Ch. Buchal / Ion implantation of optical devices

by Ag + from a melt bath [16]. We are presently investigating the additional implantation doping of laser active RE ions into these optical glasses. 3. Implantation into single crystalline materials The optical properties of a dielectric solid are determined by the polarizability of the valence electrons . This is reflected in the well-known relation n(.) = e(w) . In single crystalline materials the bonds are aligned along certain crystallographic directions and if individual bonds show specific polarizabilities, the index n becomes anisotropic . External electric fields may be used to shift the ions slightly within the unit cell, thereby deforming the bonds and changing the index. This electro-optic effect enables the construction of powerful high speed waveguide switches and modulators . If the bonds are "sensitive" and the light intensity is sufficiently high, the electronic response becomes nonlinear, giving rise to such effects as frequency doubling, frequency mixing, etc. [17,18]. Ion implantation may be employed to modify the delicate optical response of single crystals by three different means: 1) introduction of new constituents ("doping"), 2) generation of lattice disorder ("nuclear recoil damage"), 3) local energy deposition ("electronic stopping") . 3.1 . Laser active ions Laser active ions may also be introduced into oxide single crystals, either by diffusion or by ion implantation. We have studied diffusion and implantation doping of LiNbO3 with Nd and Er [19]. Local doping is especially useful, if an active laser/ amplifier region shall be incorporated into a complex electro-optic device . RE doped LiNb03 waveguide lasers/amplifiers have been reviewed very recently by Sohler [20] . There are several examples of operative RE : LiNb03 waveguide lasers published [21-25]. Presently there seems to be fairly little need to implant RE ions into LiNb03. The most important dopant, Er, diffuses sufficiently well in this crystal [19,24,25] . Nevertheless, future sophisticated dopant depth profiles may ask for implantation . In addition, transition metal dopants for the fabrication of tunable lasers in a host of LiNb03 have not yet been studied at all . Several attempts to implant Ti into A1 203 for the fabrication of a Ti :sapphire waveguide laser have failed, since a sufficient epitaxial regrowth of the sapphire could not be established [26]. On the other hand it is possible to implant carbon at 6 MeV into A1 203 , forming a stable optical guide [27] . This gives hope to establish a Ti : A] 203 waveguide laser by starting from

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the doped Ti : Al 20 3 crystal and constructing a carbon implanted optical guide on it, similar to the He im planted guides described in the next section. 3.2. Waveguide formation by He implantation This, technology has ben pioneered by Townsend, and several review articles have been published [1-3]. A He beam is used for implantation . The He ions enter the crystal with an energy of 1 to 10 MeV. They are continuously stopped by collisions with electrons. This "electronic stopping" amounts to 10 to 100 eV/Â all along the He path. If the ions are finally slowed down to below 100 keV energy, near the end of their track, they interact sufficiently strongly with the target nuclei to transfer small, but noticeable amounts of momentum to individual atoms and generate lattice defects. This process is called "nuclear stopping" . Thus at ion energies below 30 keV, 1 to 3 eV/A are transferred into nuclear collisions and generate the end-ofrange (EOR) lattice damage . This layer of damaged crystal is typically at a depth of a few wm. In all crystalline materials the FOR damage leads to a lowering of the optical index, generating an index "wall" or "barrier", which permits the guiding of light in the volume between the surface and the barrier. The formation of the index barrier itself depends on the dose and the nature of the crystal [3]. A rigorous understanding of the physics involved has not yet emerged, but clearly the following effects need attention : 1) bond breaking and local depoling due to the deposited energy, 2) lattice disorder due to momentum transfer, 3) dilution of the crystalline matrix due to He implantation, 4) onset of annealing and diffusion. Irmseher et al . have used RBS/channeling investigations to study the FOR lattice damage after He implantation and to correlate it with the measured index profiles in KNb0 3 and LiNb03 [28] . In LiNb03, they observed a clear correlation between index reduction and dechanneling (disorder) from onset to saturation . In KNb0 3, however, the :~ntical index responds much more sensitively and a strong index reduction is achieved before any lattice damage becomes visible in the RBS/channeling spectra. As mentioned before, the main energy loss along the path of the He ion is due to "electronic stopping" . The birefringent crystals (very many optically useful crystals are birefringent) respond to the influence of electronic excitation during implantation in an amazingly similar way: the higher index (generally n~) is lowered and the lower index (n e) is increased, as if the crystal tries to reduce its anisotropy. As the indices are dependingon the local asymmetry, this behaviour hints to the formation of a more symmetric microscopic VIII . ION BEAM MODIFICATION

Ch. Buchal / !on implantation of optical devices

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Square of Mode Number (m,1)2 40 60 20

2

3 Depth (p .)

Fig. 3. (Taken from ref. [33] .) Relative change of the refractive index as a function of depth for a Nd :YAG crystal after implantation of 3x 10 6 He/cm2 at 2 MeV. From the surface to a depth of 4 wm the index is increased by 0.15% dueto the energy dumped into electronic excitations . At the end of the He range a narrow optical barrier of reduced index is formed due to the collision-induced lattice damage . A number of optical modes may be guided between the surface and the barrier. These modes propagate with different effective indices, as marked by the black dots (0). The straight line (n vs (m t 1)2) is characteristic for a "flat bottom" of the "index well" and very steep "walls of the container" . The lowest lying mode is guided at an effective index exceeding the bulk value marked 0. This mode does not show tunneling losses. Higher index modes have afinite tunneling probability through the barrier, resulting in an increasingly lossy propagation. configuration, e.g. a loss of the local polarization . Other explanations have been the radiation-induced diffusion of Li or a lattice distortion effect [29] . The ion induced increase of the lowest index (LiNbO3: ne, KNb0 3: n,) can be used to guide light without the need of the low index barrier in the depth [29-32]. In contrast to the barrier guides it is the virtue of this configuration that there are no tunneling losses through the barrier into the substrate . A beautiful example of a "well-and-barrier guide" is shown in fig. 3 (from ref. [33]). The authors [33] have implanted an YAG crystal at a dose of 3 x 10 16 He/CM2 at 2 MeV . A small index enhancement ("well") of 0.1% is seen in the electronic stopping regime (0 to 4 Rm depth). A steep and narrow "barrier" is formed by "nuclear stopping" at a depth of 4.3 1im. The. black dots denote the measured indices ("mode iudax ) as a function of mode number . The straight line d^monstvates an ideal rectangular "bottom-and-wall configuration". The lowest lying mode is

guided at an higher index than the bulk value (line at 0). Therefore this mode cannot tunnel through the barrier. The higher modes will show tunneling losses through the narrow barrier, unless multiple energy implants are used to increase the barrier width. It is the beauty of He implantation, that it permits waveguide formation even in complicated doped crystals, for instance laser rods. The formation of optical waveguides in laser active media has been demonstrated [34], and a Nd : YAG crystal with a He implanted waveguide has demonstrated guided-wave laser action [35] . Very recently . ion implanted optical waveguides in single crystals or the strongly nonlinear KNb03 have been used to convert red laser light (A = 860 nm) to blue light at 430 nm [35] . 3.3. Waveguide formation by Ti implantation LiNb03 is a powerful electro-optic crystal and it is possible to raise its optical index by replacing some Nb by Ti . Physically one uses the increased bond polarizability of the Ti06 configuration and a useful material demands good crystalline order [17,18]. The standard approach hasbeen to diffuse Ti at 1000°C into LiNb03. For strip guides lithographically defined Ti metal strips are deposited onto and diffused into the LiNb03. This process leads to stable guides, used for many electrooptical devices [7,16] . Even higher concentrations and steeper Ti gradients can be achieved by Ti ion implantation . The typical dose of 2.5 x 10" Ti/cm= provides a huge concentration of Ti in the as-implanted sample, but amorphizes the LiNb03, even if the sample is heated to 620 K during implantation [36,37] . An additional annealing step at typically 1000°C in an oxygen atmosphere causes the underlying undamaged crystal to regrow towards the surface. The details of this process are complex, since they involve diffusion of implanted Ti, diffusion of oxygen from the annealing atmosphere and Li diffusion out of the bulk. Tb~: process has been identified as solid phase epitaxy (SPE). The SPE is performed at temperatures below the ferro-electric Curie temperature, and it is amazing that the Ti-doped crystal rcgrows into a single ferroelectric domain without additional poling. The maximum Ti concentration for good guides in LiNb03 is 18 mole%, independent of implantation target temperature (620 to 77 K) [36-39]. Even Ti-indiffused devices profit from additional Ti implantation . Recently a Ti-implanted Bragg reflector on a Ti diffused guide in LiNb03 has been constructed [40] . A Bragg reflector is a wavelength-selective mirror. This mirror is incorporated into a waveguide and it permits the design of a waveguide laser cavity without interruption of the guide, which would be necessary if an external mirror cavity had to be used .

Ch. Buchal / Ion implantation of optical devices

4 . Lattice location of dopants in crystalline materials As mentioned before, the electronic bonds are responsible for the optical properties of a dielectric. Therefore it is of great interest to study the lattice location of dopants in crystalline hosts. We have used RBS/channeling measurements to observe the lattice recovery during annealing of Ti-implanted LiNb0 3 and to establish the substitutionality of Ti [36] . The additional use of particle induced X-ray emission (PIXE) spectroscopy during channeling seemed to confirm the perfect substitutionality of Ti" for Nbs+ in LiNbO, [41] . This observation was in agreement with EXAFS studies [42], but it is presently seriously doubted by other authors. According to refs. [43,44], the Ti °+ is found on a Li + site, and refs . [45,46] have located a Hf °+ on a Li + site also. Only Tay + seems to unambiguously substitute for Nb" [45] . It seems that the entire field presently is open for surprises. A general remark seems appropriate : If foreign ions are introduced into an ionic single crystal (LiNb0 31 e .g.), charge neutrality has to be established on a reasonably short length scale . Therefore the introduction of a "wrong" valence atom has to alter the neighbour configuration. If a foreign ion is introduced into Li + Nb` + Os - , we shall understand the details of its incorporation only if we know how the charge balance is achieved . To give some examples of useful doping: Mgt+ ions are added to LiNb0 3 for reduced photorefractive susceptibility (lower optical damage), the addition of Fe Z+ /Fe3+ increases the photorefractive effect (useful for holography), Ti °+ increases the optical index (waveguides), Nd3+ , Pr 3+ and Er a + are used for laser applications, Eu 3 + and Hf °+ have been studied for general interest. Sofar we do not know how the crystal configures itself around these dopants, although the Ti0 t, octahedron is established (its charge balance could be provided possibly by an additional Li + ion or by "sharing" an OZ- vacancy" with another Ti 4+ further out) . Therefore careful detailed impurity configuration studies by EXAFS and other locally sensitive methods are urgently needed . 5 . Conclusions We have outlined various applications of accelerators for research and development of optical devices. In integrated optics there is being made use of very different materials, much more so than in electronics, where silicon satisfies the majority of the needs. The accelerator oriented physicist may enter the field with analytical experiments as RBS/channeling/PIXE studies for lattice perfection and impurity atom location, or doping and diffusion profile analysis, or he may

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work on the basic research needed for device fabrication: ion beam mixing of heterostructures, implantation doping of semiconductors, glasses or oxide single crystals and the very interesting waveguide formation by damage patterning (MeV He implantation). Due to the wavelength of light the optical guides have cross sections of Wm dimensions and these depths call for MeV energies, if direct doping is intended . We therefore predict an increasing demand for MeV beam time not only for analytical experiments, but also for ion beam patterning and ion implantation.

Acknowledgements It is a great pleasure to acknowledge the numerous fruitful co-operations and the countless stimulating discussions with colleagues at Oak Ridge, Huntsville, Zurich, Osnabriick, Paderborn and Augsburg. I am especially grateful for the creative exciting spirit at KFA Jülich.

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