In situ observation of secondary phase formation in Fe implanted GaN annealed in low pressure $N_2$ atmosphere

Talut, G.; Shalimov, A.; Reuther, H.; Grenzer, J.; Novikov, D.; Walz, B.; Baehtz, C.

doi:10.1063/1.3271828

Journal Article

In situ observation of secondary phase formation in Fe implanted GaN annealed in low pressure $N_2$ atmosphere

Talut, G. (Corresponding author)Extern* ; Grenzer, J. ; Reuther, H. ; Shalimov, A. ; Baehtz, C. ; Novikov, D. ; Walz, B. ; DESY

2009
American Institute of Physics Melville, NY

Applied physics letters 95, 232506 (2009) [10.1063/1.3271828]

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Please use a persistent id in citations: doi:10.1063/1.3271828 doi:10.3204/PHPPUBDB-12678

Abstract: The formation of secondary phases in Fe implanted GaN upon annealing in low pressure N$_2$-atmosphere was detected by means of in situ x-ray diffraction and confirmed by magnetization measurements. A repeatable phase change from Fe$_3$N at room temperature and Fe$_3$−$_x$N at 1023 K was observed in situ. The phase transformation is explained by the change of lattice site and concentration of nitrogen within nitrides. The diffusion of Fe toward sample surface and oxidation with increasing annealing cycles limits the availability of secondary phase and hence the repeatability. At high temperature GaN dissolves and Ga as well as Fe oxidize due to presence of residual oxygen in the process gas. The ferromagnetism in the samples is related to nanometer sized interacting Fe$_3$−$_x$Ncrystallites.GaN is a wide band gap semiconductor that has been studied intensively in the last years because of its potential field of applications like in optoelectronics, plasmonics, as well as for high power electronics. By doping with transitional metals like Fe it might also be a diluted magnetic semiconductor (DMS) with a Curie temperature above room temperature (RT) and could then be used for spintronics.1 There are many experimental studies reporting ferromagnetism at RT in Fe doped GaN. In a real DMS, with magnetic atoms randomly substituting cation sites, ferromagnetic coupling is supposed to be due to the indirect exchange coupling between magnetic impurities mediated by holes.1–4 Experimental observation of strong-coupling effects in a DMS Ga$_1$−$_x$Fe$_x$Nwas reported by Pacuski et al.5 Robust ferromagnetism in the region of insulator-to-metal transition was predicted for high hole densities. However, there are also other possible sources of ferromagnetism like spinodal decomposition of Fe or ferromagnetic secondary phases. The detection of those is rather difficult. Bonanni et al.6 prepared GaN:Fe layers by metalorganic chemical vapor deposition (MOCVD) and observed ferromagnetism that was partially accounted to the spinodal decomposition and non-uniform distribution of Fe-rich magnetic nanocrystals. Kuwabara et al.7 reported the formation of nanoclusters and superparamagnetic behavior in GaN:Fe epilayers prepared by rf-plasma-assisted molecular beam epitaxy. In case of ion implantation the reports from different groups are quite controversial. Theodoropoulou et al.8 and Shon et al.9,10 did not relate ferromagnetic response to secondary phases after transition ion implantation into semiconductors. In our experiments, however, the formation of α-Fe nanoclusters, that were responsible for ferromagnetic response, was observed.11 Though the appearance of such precipitates is not desired in a DMS they might be useful for certain applications.12 Li et al.13 detected co-occurrence of α-Fe and ε-Fe$_3$Nin MOCVD prepared GaN:Fe films and pointed out the role of nitrogen pressure and structural disorder in the formation of Fe-rich phases. Bonanni et al.14 have shown that the controlled aggregation of magnetic ions in a semiconductor can be affected by the growth rate and doping with shallow impurities.Recently we reported predominant formation of epitaxially oriented α-Fenanoclusters if Fe-doped samples were annealed in a N$_2$ flow at 1.1 bar pressure.11 In this paper we report the formation of ε-Fe$_3$−$_x$N with x<1 that builds up during annealing at 1073 K in 0.5 bar N2 and the reversible transformation to ε-Fe$_3$Nduring cooling down to RT.P-type (Mg) doped (∼2×10$^{17}$cm$^{−3}$)single crystalline wurtzite GaN(001) films of about 3μm thickness epitaxially grown by metal organic vapor phase epitaxy on sapphire (001) were used. Samples, 7° tilted relative to the ion beam to avoid channeling, were implanted with 195 keV $^{57}$Fe ions with fluence Φ=4×10$^{16}$cm$^{−2}$ (peak Fe concentration of 4 at. % at the projected range R$_p$=85nm according to TRIM$^{15}$), keeping the samples at RT. In order to reduce the implantation damage and to investigate the formation of secondary phases the implanted samples were annealed at 1073 K in a low pressure N$_2$-atmosphere(0.5 bar) within several minutes. The annealing experiments along with the in situ x-ray diffraction characterization were performed at the Rossendorf beamline at the ESRF in Grenoble with a x-ray wavelength of λ=0.124nm. The annealing chamber was equipped with a boron nitride heater, controlled by a Eurotherm controller, gas inlet and a half sphere beryllium dome. The temperature was measured by a PtRh/Pt thermocouple placed on top of the heater. The gas pressure in the chamber was limited to 0.5 bar with a flow of about 40l⋅min$^{−1}$. The purity of the N$_2$gas (99.9999%) was limited by the setup with an oxygen contamination in the ppm range.A Pilatus 100 K two-dimensional (2D) pixel detector was used to record 2D diffraction pattern. Additionally, a scintillation counter was used for 2θ-ω-scans. Generally, for clusters in the range of some nm the signal to background ratio is very low. In order to increase the signal to background ratio to an acceptable level the acquisition time of the 2D detector was set to 10 s and those of the scintillation counter to 5 s per point. The investigations of the magnetic properties were performed with a Quantum Design MPMS superconducting quantum interference device magnetometer. For the evaluation of the 2D detector exposures rectangular areas with dimensions 2θ from 32.3° to 36.5° and χ=±0.2° (angle ⊥ to the scattering plane) were integrated over χ and assumed as line scans. Those line scans are represented over annealing/cooling time with the color/grayscale coded intensity in Fig. 1(a). Single examples of line scans are given in Figs. 1(b) and 1(c) for RT and 1073 K, respectively. Prior to the annealing procedure (time t=0 at RT) no reflexes from secondary phases were detected [see Fig. 1(a)]. After about 200 s annealing, at 1073 K, a broad reflex starts to evolve in the region between 32.5° and 35° with a local maximum at about 33.3°. The peak shift due to the lattice expansion from RT to 1073 K is in the order of 0.1° and can be neglected. The position of the maximum fits well to the pattern of FeN(200) (33.46° at RT). However, because of the broadness of the reflex, other nitrides like disordered ε-Fe$_3$−$_x$N(002) or ζ-Fe$_2$N(102) can also be taken into account. Disordered in this sense means the redistribution of mainly N atoms within the structure and can be described by the transfer of N from Fig. 2(b) and 2(c) Wyckoff site position in the ε-Fe$_3$−$_x$N-phase.16 From symmetry reasons the formation of ε-Fe$_3$−$_x$N is more probable since it features the same type of structure (wurtzite) as GaN. The strain caused by the lattice mismatch is supposed to relax by generating misfit dislocations, as was shown in Ref. 13.

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