ARTICLE Received 6 Jun 2014 | Accepted 22 Sep 2014 | Published 7 Nov 2014 DOI: 10.1038/ncomms6346 n-type chalcogenides by ion implantation Mark A. Hughes1, Yanina Fedorenko1, Behrad Gholipour2, Jin Yao2, Tae-Hoon Lee3, Russell M. Gwilliam1, Kevin P. Homewood1, Steven Hinder4, Daniel W. Hewak2, Stephen R. Elliott3 & Richard J. Curry1 Carrier-type reversal to enable the formation of semiconductor p-n junctions is a prerequisite for many electronic applications. Chalcogenide glasses are p-type semiconductors and their applications have been limited by the extraordinary difficulty in obtaining n-type conductivity. The ability to form chalcogenide glass p-n junctions could improve the performance of phase-change memory and thermoelectric devices and allow the direct electronic control of nonlinear optical devices. Previously, carrier-type reversal has been restricted to the GeCh (Ch ¼ S, Se, Te) family of glasses, with very high Bi or Pb ‘doping’ concentrations (B5–11 at.%), incorporated during high-temperature glass melting. Here we report the first n-type doping of chalcogenide glasses by ion implantation of Bi into GeTe and GaLaSO amorphous films, demonstrating rectification and photocurrent in a Bi-implanted GaLaSO device. The electrical doping effect of Bi is observed at a 100 times lower concentration than for Bi melt-doped GeCh glasses. 1 Department of Electronic Engineering, Advanced Technology Institute, University of Surrey, Guildford GU2 7XH, UK. 2 Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, UK. 3 Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK. 4 The Surface Analysis Laboratory, Department of Mechanical Engineering Sciences, University of Surrey, Guildford GU2 7XH, UK. Correspondence and requests for materials should be addressed to M.A.H. (email: [email protected]) or D.W.H. (email: [email protected]) or to S.R.E. (email: [email protected]) or to R.J.C. (email: [email protected]). NATURE COMMUNICATIONS | 5:5346 | DOI: 10.1038/ncomms6346 | www.nature.com/naturecommunications 1 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6346 halcogenide glass alloys are of significant technological into peaks at 157.1 eV, which corresponds to neutral Bi, and importance for nonlinear optics1, infrared (IR) optics2 and 158.1 eV, which corresponds to a positively charged species with Cphase-change memory (PCM)3; however, electronic an oxidation state o3 þ , although Bi þ and Bi2 þ cannot be applications are limited as a consequence of their almost distinguished at this binding energy. Supplementary Figure 2a,b universally unmodifiable p-type conductivity4. The melt-doping indicates that X-ray exposure does not induce a valence state technique previously used to obtain carrier-type reversal (CTR) in change in Bi during measurement. Bi-implanted GeTe also has a chalcogenide glass alloys is incompatible with the tooling used in single peak 157.9 eV corresponding to Bi þ or Bi2 þ only. integrated circuit (IC) fabrication; ion implantation, however, is Figure 1b shows the XPS depth profile of a 1 Â 1016 ions cm À 2 integral to current IC fabrication. Therefore, CTR by ion Bi-implanted GaLaSO thin film; the Bi 4f7/2 peak from the implantation in chalcogenide glasses could open up a new implantation can be seen appearing below the surface of the film. branch of electronics based on these materials. Amorphous Se can Figure 1c shows the Bi concentration, calculated by fitting be doped n-type with alkaline elements, which has allowed the photoelectron spectra, as a function of etching time. The high-performance X-ray detectors to be fabricated5; however, concentration peaks at 0.6 at.% at an etch time of 7,000 s; the film amorphous Se is limited in its PCM, nonlinear and IR composition was Ga25La32S28O15 and was constant throughout applications. GaLaSO glass is a high-performance PCM the etch. Rutherford backscattering measurements indicate a material that, compared with standard GeSbTe PCM materials, similar film composition and resolved a single buried layer of Bi offers significantly higher thermal stability, and therefore in the film with a concentration of 0.32 at.% to a depth of 84 nm potentially improved endurance, along with more than order of (errors indicated in Fig. 1c), which is consistent with the XPS magnitude lower set and reset currents6,7. GeTe is an ingredient measurement. of standard GeSbTe PCM materials and is itself a useful PCM The depth profile of the 2 Â 1016 ions cm À 2 Bi-implanted material, having a higher crystallization temperature and GeTe film in Fig. 2a shows that the peak Bi concentration was 1.4 B therefore the ability to operate at higher temperatures. A at.%. It also shows that the composition was Ge33Te33O33 at the significant issue with PCM is the problem of leakage paths, surface, Ge15Te60O15 at the centre of the film and Ge15Te85 at the which means that significant current can pass through unselected interface with the substrate. The O profile shows that O does not memory elements8. A solution is to insert a rectifying p-n reach the film/substrate interface, which indicates that the film junction in series with each memory element; p-n junctions in has oxidized due to exposure to atmospheric O2. Rutherford chalcogenide glasses could also lead to the development of novel backscattering and particle-induced X-ray emission measure- photodetectors, light-emitting diodes, optical amplifiers and ments of this film taken 1 week after deposition indicated that the lasers. The injection of carriers into a p-n junction can majority of oxidation had already occurred before ion implanta- 9 modulate the nonlinear properties of the semiconductor used ; tion. The composition of the sputter target was Ge50Te50, therefore, chalcogenide p-n junctions could be used for direct indicating that Te is preferentially sputtered during film electronic control of optical ultrafast nonlinear devices such as deposition. Sputter markers indicated a B40-nm sputter depth demultiplexers, wavelength converters and optical Kerr shutters. after 2 Â 1016 ions cm À 2 Bi implantation. In contrast to GeTe Among the chalcogenide glasses, GaLaSO is particularly notable and many other chalcogenide films, we found that GaLaSO is with respect to optical nonlinear devices as it has the highest remarkably stable against atmospheric oxidation and also against nonlinear figure of merit (FOM) of any glass reported to date10, sputtering during implantation. As shown in the depth profile of 16 À 2 with FOM ¼ n2/2bl, where l is the wavelength, n2 is the real part 1 Â 10 ions cm Bi-implanted GaLaSO in Fig. 2b, the O of the nonlinear refractive index and b is the two-photon profile is flat, indicating that oxidation due to atmospheric O2 did absorption coefficient. Ultraviolet-written waveguide11 and not occur, since this would increase the O content towards the microsphere12 lasers have also been demonstrated in rare-earth- surface of the film. The flat composition profile through the Bi doped GaLaSO. Chalcogens are important components of implant indicates that the penetration depth of collision cascades thermoelectric materials, particularly when alloyed with Bi or created by the implantation was low enough not to affect the Pb; therefore, Bi-modified chalcogenide glasses could have composition. The flat profile also indicates that sputter profiling applications in novel thermoelectric devices. during the XPS measurement, where components are preferen- In this work, we present, to the best of our knowledge, the first tially sputtered or driven into the film, creating a false profile, is demonstration of CTR by ion implantation of a chalcogenide unlikely because, to get a flat profile when sputter profiling is glass, the first demonstration of CTR by Bi doping in a non-GeCh occurring, the actual profile would have to, by chance, be the glass and the lowest doping concentration (0.6 at.%) for CTR in inverse of the sputter-induced profile. The composition of the any chalcogenide glass alloy. The two alloys we used represent sputter target was Ga28La12S18O42, indicating that La is different classes of chalcogenide glasses: GaLaSO is highly preferentially sputtered and O and S are unfavourably sputtered resistive but has excellent optical properties, whereas GeTe is during film deposition. Sputter markers indicated no sputtering conductive but has poor optical properties. of the GaLaSO film after implantation, even with a dose of 2 Â 1016 ions cm À 2. Extended X-ray absorption fine structure spectroscopy of bulk Results GaLaS glasses with compositions spanning the glass-forming Material characterization and simulation. Figure 1a shows region indicated that GaS4 tetrahedra are the main glass former the X-ray photoelectron spectroscopy (XPS) spectrum of units for all compositions15. XPS measurements of Ga and O, Bi-implanted GaLaSO and GeTe, along with those for Bi metal within and below the Bi-implanted region of GaLaSO, can be seen and Bi melt-doped GeSe and silicate glass. The binding energy of in Fig. 3a,b, respectively. In the implanted region, there is a Bi is determined mainly by its oxidation state, and tends to decrease in the high binding energy region of the Ga peak. increase with increasing oxidation state. The broad 4f7/2 level of Analysis of this Ga peak in Supplementary Fig. 3 indicates the Bi melt-doped silicate at 160.0 eV contains components of Bi5 þ presence of homopolar Ga–Ga bonds, and that Ga is primarily 4 þ 13 at 160.6 eV and Bi at 159.8 eV ;Bi2O3 has a binding energy of coordinated with S. It also shows that implantation decreases the 3 þ 158.9 eV corresponding to Bi (ref. 14). The peak for Bi metal at presence of Ga species in an environment similar to b-Ga2O3 and 156.9 eV corresponds to neutral Bi.