THERMAL STABILITY OF ION-IMPLANTED HYDROGEN IN ZnO

K. Ip, M.E. Overberg,Y.W.Heo,D.P. Norton, and S.J. Pearton

Department of Materials Science and Engineering

University of Florida, Gainesville, FL 32611

 

S.O. Kucheyeva), C. Jagadish, and J.S. Williams

Department of Electronic Materials Engineering

            Research School of Physical Sciences and Engineering

            The Australian National University, Canberra, ACT 0200, Australia

 

            R.G. Wilson

            Consultant, Stevenson Ranch, CA 93181

 

            J.M. Zavada

            U.S. Army Research Office, Research Triangle Park, NC 27709

 

 

 

ABSTRACT

            The evolution of implanted 2H profiles in single-crystal ZnO was examined as a function of annealing temperature (500 – 700˚C) by Secondary Ion Mass Spectrometry(SIMS).  The as-implanted profiles show a peak concentration of ~2.7×1019 cm‑3 at a depth of ~0.96μm for a dose of 1015 cm-2.  Subsequent annealing causes outdiffusion of 2H from the ZnO, with the remaining hydrogen decorating the residual implant damage.  Only 0.2% of the original dose is retained after annealing at 600˚C.  Rutherford Backscattering/Channeling(RBS/C) of samples implanted with 1H at a dose of 1016cm-2 showed no change in backscattering yield near the ZnO surface but did result in an increase near the end-of-range from 6.5% of the random level before 1H implantation to ~7.8% after implantation.  Results of both cathodoluminescence and photoluminescence studies show that even for a 1H dose of 1015 cm-2, the intensity of the near gap emission from ZnO is reduced more than 2 orders of magnitude from the values in unimplanted samples.  This is due to the formation of effective non-radiative recombination centers associated with ion-beam-induced defects.

 

 

*Current address:  Lawrence Livermore National Laboratory, Livermore, CA.

 

            ZnO is attracting considerable attention for use in UV light-emitters, transparent high-power electronics, piezoelectric transducers, gas-sensing surface acoustic-wave devices and in conducting transparent windows for solar cells and displays.(1-27)  There is particular interest in the properties of hydrogen in ZnO, because of the predictions from density functional theory and total energy calculations that it should be a shallow donor.(24-27)  The generally observed n-type conductivity, therefore, may at least in fact be explained by the presence of residual hydrogen from the growth ambient, rather than to native defects such as Zn interstitials or O vacancies.  There is some experimental support for the predicted observations of its muonium counterpart (26, 29) and from electron paramagnetic resonance of single-crystal samples.(30)   There have been many other studies on the effects of hydrogen on the electrical and optical properties of ZnO, (32-39) but no detailed study on the thermal stability of implanted hydrogen.

            In this letter we report on an investigation into the retention of implanted hydrogen in single-crystal, bulk ZnO as a function of annealing temperature and the effects of the implantation on both the crystal quality and optical properties of the material.  The temperatures at which implanted hydrogen is evolved from the ZnO are considerably lower then for GaN which is a more developed materials system for visible and UV light-emitters.

            Bulk, wurtzite (0001)ZnO crystals from Eagle-Picher (grade I quality) were employed for all experiments.  The samples were nominally undoped (n~8x1016cm-3 ,electron mobility of 190 cm2/V.s at 300K) and were implanted into the Zn face with either 2H+ or 1H+ ions.  In the latter case, implantation was performed with a 1.7MV tandem accelerator (NEC, 5SDH-4) at 25˚C with 100keV H- ions using a beam flux of ~1.3×1013cm-2s-1 to a dose of either 1015 or 1016cm-2.  During implantation, samples were tilted by ~7˚ relative to the incident ion beam to minimize channeling.  After implantation, these samples were characterized by Rutherford Backscattering/Channeling (RBS/C) using a 1.7MV tandem accelerator (NEC, 5SDH) with 1.8MeV 4He+ ions incident along the [0001] direction and backscattered at ~168˚ relative to the incident beam direction.  The RBS/C spectra were accumulated for long enough that the random yield at the depth of the bulk defect peak corresponded to ~4000 counts. The 2H+ implantation was also performed of an energy of 100keV to a dose of 1015cm-2.  Annealing was performed for 5 mins at 500 – 700˚C under flowing N2 in a Heatpulse 610T rapid thermal annealing furnace with the samples in a face-to-face configuration.  These samples were examined by photoluminescence at 300K using a He-Cd laser and also by Secondary Ion Mass Spectrometry (SIMS).  The latter was performed in a Camera system using a Cs+ ion beam with 14.5keV energy and 24˚ incident angle.

            Figure 1 shows the SIMS profiles of implanted 2H as a function of subsequent annealing temperature.  Note that the effects of the annealing is an evolution of 2H out of the ZnO crystal, with the remaining deuterium atoms of each temperature decorating the residual implant damage. The peak in the as-implanted profile occurred at 0.96μm, in good agreement with the projected range from Transfer-of-Ion-in-Matter (TRIM) simulations.  The thermal stability of the implanted 2H is considerably lower in ZnO than in GaN(40), where temperatures of ~900˚C are needed to remove deuterium to below the detection limit (~3×1015 cm-3) of SIMS and this suggests that slow-diffusing H2 molecules or larger clusters do not form during the anneal.  Since we did not observe conventional out-diffusion profiles we were unable to estimate a diffusion coefficient for the 2H in ZnO.  Our results are consistent with an implant-damaged trap-controlled release of 2H from the ZnO lattice for temperatures >500˚C.

            Figure 2 shows the percentage of 2H remaining in the ZnO as a function of anneal temperature.  The 2H concentrations were obtained by integrating the area under the curves in the SIMS data.  It is evident that the thermal stability of implanted deuterium is not that high with 12% of the initial dose retained after 500˚C anneals and ~0.2% after 600˚C anneals.

            RBS/C showed that implantation of 1H, even to much higher doses (1016 cm-2), did not affect the backscattering yield near the ZnO surface, as shown in Figure 3.  However, there was a small (but detectable)increase in scattering peak deeper in the sample, in the region where the nuclear energy loss profile of 100keV H+ is a maximum. The RBS/C yield at this depth was ~6.5% of the random level before H+ implantation and ~7.8% after implantation to the dose of 1016cm-2.

            While the structural properties of the ZnO were minimally affected by the hydrogen (or deuterium) implantation, the optical properties were severely degraded.  Room temperature cathodoluminescence showed that even for a dose of 1015cm-2 1H+ or  2H+ ions, the intensity of near-gap emission has reduced by more than 2 orders of magnitude as compared to control values.  This is due to the formation of effective non-radiative recombination centers associated with ion-beam produced defects.  Similar results were obtained from PL measurements.   Figure 4 shows the 300K spectra from the 2H+ implanted samples annealed at different temperatures.  The band-edge luminescence is  still severely degraded even after 700˚C anneals where the 2H has been completely evolved from the crystal.  This indicates that point defect recombination centers are still controlling the optical quality under these conditions.  Kucheyev et al (42) have found that resistance of ZnO can be increased by about 7 orders of magnitude as a result of trap introduction by ion irradiation.

            In conclusion, the thermal stability of implanted hydrogen in ZnO has been examined.  Annealing at ≤700˚C completely evolves the hydrogen from the ZnO, but the optical properties are still significantly degraded under these conditions because of residual implant-induced defects.

 

ACKNOWLEDMENTS

            The work at UF is partially supported by NSF (DMR0101438) and ARO.

 

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FIGURE CAPTIONS

            Figure 1.  SIMS profiles of 2H implanted into ZnO (100 keV, 1015 cm-2) before and after annealing at different temperatures (5 min anneals).

            Figure 2.  Percentage of retained 2H implanted into ZnO (100 keV, 1015 cm-2) as a function of annealing temperature (5 min anneals).The inset shows the data on a log scale.

            Figure 3.  RBS spectra of bulk, single-crystal ZnO before and after 100 keV 1H+ implantation to a dose of 1016 cm-2.

            Figure 4.  PL spectra at 300K of ZnO implanted with 2H+ ions (100 keV, 1015 cm-2) as a function of post-implanted annealing temperature (5 min anneals).