P-TYPE ZNO:AS OBTAINED BY ION IMPLANTATION OF AS+ WITH POST-IMPLANTATION ANNEALING IN OXYGEN RADICALS A. N. GEORGOBIANI AND V. I. DEMIN P.N.Lebedev Physical Institute of RAS 53 Leninsky Prospekt, 119991 Moscow, Russia E-mail: firstname.lastname@example.org M. B. KOTLYAREVSKY, I. V. ROGOZIN AND A. V. MARAKHOVSKY Berdyansk State Pedagogical University 4 Shmidt Prospekt, 71100 Berdyansk, Ukraine E-mail: email@example.com Zinc oxide is the promising material for creation of the new generation of detectors for particle physics and radiation dosimetry. It has been shown that ion implantation of arsenic into zinc oxide film (arsenic is an acceptor impurity in ZnO) can result in formation of the p–type conductivity only in case of annealing in the flux of atomic oxygen. The ion implantation and the following annealing had influenced not only electrical properties of ZnO:As + layers, but also their photoluminescence spectra. The ultraviolet luminescence band with the maximum at 3.33. eV corresponding to the As O acceptor center had been clearly observed in the spectra of ZnO films implanted by As + ions. The optimal temperature range of annealing in the atomic oxygen flux, required for obtaining of p–type conductivity in ZnO films, had been determined. 1. Introduction Zinc oxide is the promising semiconductor for creation of the new generation of detectors for particle physics and radiation dosimetry. ZnO ,as well as diamond, differs from silicon because of the much higher bandgap energy. Consequently, the operating mode of the particle detectors based on them should be much different and much easier than that for the silicon detectors. Also radiation and chemical tolerance of ZnO is better than that of silicon. Creation of p-n junctions is one of the important problems in producing semiconductor detectors based on ZnO. The main difficulty in solving this problem is connected with the strong tendency of wide-gap II-VI compounds towards monopolar n–type conductivity. The introduction of acceptor impurities by means of traditional methods is accompanied by the process of self-compensation because of the oxygen vacancies generation, that act as donors in ZnO. The thermodynamic and kinetic analysis made by us [1-3] has shown that it is possible to suppress the self- 1 2 compensation in II-VI compounds only when the growth and doping temperature in the atmosphere of the saturated vapour of non-metal component of the compound don’t exceed the critical value. The expression determining the critical temperature (Tcr) has been obtained. The calculation shows that Tcr for ZnO is about 300 K. It is evident that at this temperature the processes of growth and doping are “frozen”. The small value of Tcr is due to the fact that dissociation energy of the oxygen molecule is rather high - about 5 eV, but only atomic oxygen is active in the mentioned processes. The method called Radical Beam Gettering Epitaxy (RBGE) has been elaborated on this basis [3-7]. The essence of this method is the artificial dissociation of molecules in the atmosphere of non-metal component, for example stimulated by radio frequency (RF) discharge. This results in the growth of Tcr . With the help of this method the p-type ZnO samples had been obtained [4, 8, 9] In this case the intrinsic defects zinc vacancies – served as acceptors. The goal of this work is to investigate the potentiality of the RBGE method in case of acceptor doping. Arsenic is one of the most promising acceptors for ZnO . We have used the low-temperature method – ion implantation – for its introduction. 2 Experimental methods The initial ZnO films with thickness 0.1 m doped by gallium were obtained by magnetron high frequency deposition on the amorphous SiO2 substrates at temperature equal to 620 K. Ga2O3 was used as a source of gallium impurity. According to the data obtained by Hall measurements, the layers possessed n- type of conductivity. At room temperature the carrier concentration and mobility were 2·1017 cm-3 and 120 cm2·V-1·s-1 respectively. The films were doped by As+ ion implantation with doses: 51014 ÷ 51015 cm-2. The energy of arsenic ions was 200 keV. Analysis of the impurity composition was performed by SIMS method. After implantation the films became amorphous, insulating, and non- luminescent. Post-implantation annealing of radiation defects was necessary to restore the crystal lattice and to control the composition of the point defects aimed at the stoichiometry shift to oxygen surplus. Such an annealing was realized with the help of RBGE method in the atmosphere of the atomic oxygen during 30 minutes at different temperatures. Elemental oxygen was generated by RF–discharge of 40 Wt power in oxygen atmosphere at pressure ~ 1 Pa. By passing molecular oxygen through the RF-discharge region we obtained a mixture of molecular and elemental oxygen and oxygen plasma. The charged particles were removed by the applied magnetic-field, so that only oxygen molecules and atoms reach the film surface. The concentration of atomic oxygen 3 was about 1017 cm-3. In case of thermal dissociation at 800 °C such a concentration of atomic oxygen corresponded to oxygen pressure about 105 MPa, a value which could not be attained because it is much greater than the saturated vapour pressure at this temperature. The photoluminescence (PL) spectra of the films under excitation by the pulse nitrogen laser at liquid helium temperature were measured. The spectra were analyzed by means of the double grading monochromator. The film thickness was measured by quartz thickness-meter during the deposition process. The type of conductivity was determined by the sign of the thermal EMF signal. 3 Experimental results After annealing of the implanted films with the help of the RBGE method they became polycrystal, that allowed to conduct PL measurements. The investigation of PL is one of the main stages necessary for the optimization of the treatment regimes, of the conditions of introduction and activation of the acceptor impurities. Figure 1. PL spectra of the initial ZnO film (a) and of ZnO:As+ layers implanted with the doses 5·1014 cm-2 (b) and 5·1015 cm-2 (c) after the annealing in oxygen radicals at 670 K for 30 minutes. Figure 1 presents the influence of the arsenic ion implantation at PL spectra of the ZnO films measured after post-implantation annealing at 670 K. In the spectra of initial ZnO:Ga films the intense peak with the maximum at 3.36 eV, and the green band at 2.4 eV are observed (curve a). The peak at 3.36 nm is connected with the annihilation of excitons bound at neutral donors. The estimation of energy location of the donor (Ed) according to the formulae (D0,X) 4 = Eg – Eex – 0.15Ed , where Eg = 3.43 eV, Eex = 60 meV, gives the value of Ed ~ 65 meV. The peaks at 3.33 and 3.25 eV appear in the spectra of the samples implanted with the dose 5·1014 cm-2 (curve b). Increase of the dose up to 5·1015 cm-2 leads to the significant growth of green emission (curve c). Figure 2. PL spectra of ZnO:As+ layers implanted with the dose 1015 cm-2 after annealing for 30 minutes in oxygen radicals flux at different temperatures: 670 K (a), 920 K (b) and 1170 K (c). The dependence of PL spectra of the layers implanted with the dose 1015 cm-2 on annealing temperature varying in the range 670 – 1170 K is shown in Fig 2. In the range 670 – 970 K the annealing results in the growth of the intensity of the 3.33 eV peak (curves a,b). The increasing of the annealing temperature up to 1170 K leads to the diminishing of its intensity. The SIMS profiles of arsenic distribution in the implanted layers in the dependence on the annealing temperature are presented in Fig. 3. The rising of the temperature results in the widening the profile (curve b). At the highest used annealing temperature (1170 K) the considerable diffusion of the impurity into the substrate bulk and the exhaustion of the surface region is observed (curve c). We have checked the conductivity type of the treated ZnO samples. The p– type of conductivity was obtained in the samples annealed in oxygen radicals in the temperature range 870 –970 K. 5 Figure 3. SIMS profiles of As distribution in ZnO film in the dependence on the annealing temperature: 670K (a), 870 K (b) and 1170 K (c). 4 Discussion According to the experimental results presented above, it is possible to attribute the peak at 3.33 eV to the annihilation of the exciton bound at the acceptor center – arsenic in oxygen sublattice – AsO. The estimation of energy location of the acceptor according to the formulae (A0,X) = Eg – Eex – 0.08Ea, where Eex = 89 meV  gives the value of Ea ~ 140 meV. This result is in good concordance with the data of , in which the depth of the acceptor level connected with arsenic was estimated. The peak with the maximum at 3.25 eV could be connected with the annihilation of excitons at the defect complexes (VZn – VO)+. Such a supposition is done because the shifting of the stoichiometry of ZnO to the side of oxygen surplus results in the diminishing of the number of its vacancies. In this case, the electro-neutrality occurs due to re-charging of oxygen vacancies to the positive double-charged state, and consequently, to the formation of defect complexes of (VZn – VO)+ type . The peaks at 3.33 and 3.25 eV are present only in the arsenic implanted samples. The intensity of 3.33 eV peak reaches its maximum value at the annealing temperature 920 K (Fig. 2, curve b). The existence of exciton luminescence testifies to the restoration of the crystal structure. It is likely that at low annealing temperatures As ions are located not only in the oxygen sites. The increasing of the annealing temperature leads to the effective immersion of As 6 into the latter. The diminishing of the intensity of the mentioned peaks at the annealing temperature 1170 K is evidently due to the evaporation of As from the layers, that is proved by the data of SIMS analysis (Fig. 3). Acknowledgments This work was supported by RFBR – NSFCC grant No 02-02-39007, by the Ministry of Industry, Science and Technology of Russian Federation – Programs “Inequilibrium phenomena in semiconductor nanostructures”, “Elaboration of compact sources of coherent emission” and by Russian Academy of Sciences. – Programs “Physics of solid-state nanostructures”, “Semiconductor lasers”, “New materials and structures”. References 1. A.N. Georgobiani and M.B. Kotlyarevsky, Sov. J. Izv. Akad. Nauk SSSR, Neorg. Mater. 17, 1153 (1981). 2. A.N. Georgobiani, M.B. Kotlyarevsky and V.N. Mikhalenko, Proc. P.N. Lebedev Phys. Inst. 138, 79 (1983). 3. A.N. Georgobiani, J. of Luminescence. 48&49, 839 (1991). 4. T.V. Butkhuzi, A.V. Bureev, A.N. Georgobiani, et al, J. 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