Properties of (Ga, Mn)N With and Without Detectable Second Phases

(Ga, Mn)N is a strong candidate for room-temperature dilute magnetic semiconductor (DMS) applications. Numerous groups have reported promising magnetic properties for GaN doped with Mn during growth or by implantation or diffusion.^{1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21} However, there are still strong variations in the reported magnetic behavior, with some films exhibiting only paramagnetism^{2 7 8 13} and even those with ferromagnetism showing a wide range of apparent Curie temperatures Particularly, the origin of this ferromagnetism is not clear. Current hypotheses include a mean-field model (based on the Ruderman-Kittel-Kasuya-Yoshida, RKKY interaction) in which the ferromagnetism results from carrier mediation by delocalized or weakly localized holes in p-type material^{22 23 24 25} or various types of small clusters of the Mn such as ^{26 27} Given that the acceptor level is deep eV),^{28 29 30} it is not expected that free-carrier mediated magnetism is significant but tightly bound carriers could play a role. The first class of approaches is based on mean-field theory which originates in the original model of Zener magnetism. The theories that fall into this general model implicitly assume that the dilute magnetic semiconductor is a more-or-less random alloy, e.g., (Ga, Mn)N, in which Mn substitutes for one of the lattice constituents. The second class of approaches suggests that the magnetic atoms form small (a few atoms) clusters that produce the observed ferromagnetism. A difficulty in experimentally verifying the mechanism responsible for the observed magnetic properties is that, depending on the growth conditions employed for growing the DMS material, it is likely that one could readily produce samples that span the entire spectrum of possibilities from single-phase random alloys to nanoclusters of the magnetic atoms to precipitates and second-phase formation. Therefore, it is necessary to decide on a case-by-case basis which mechanism is applicable. This can be achieved only by a careful correlation of the measured magnetic properties with materials analysis methods that are capable of detecting other phases or precipitates. If, for example, the magnetic behavior of the DMS is characteristic of that of a known ferromagnetic second phase (such as MnGa or in (Ga, Mn)N), then clearly the mean field models are not applicable.

There is clearly a need to examine the properties of (Ga, Mn)N with and without second phases, at least as detected by common analysis methods such as X-ray diffraction (XRD). In this article, we report on the magnetic properties of (Ga, Mn)N with a broad range of Mn concentrations (5 or 50 atom %) and which exhibit either one phase or multiple phases.

The layers were grown by radio-frequency (rf) plasma assisted molecular beam epitaxy (MBE) at 650-700 C on ∼3 μm thick GaN buffers grown by metalorganic chemical vapor deposition (MOCVD) on c plane sapphire substrates. The growth conditions have been described in detail previously.⁹ Three different samples, each ∼0.3 μm thick, were examined. The first had a Mn concentration of ∼5 atom % as determined by both Auger electron spectroscopy (AES) and Rutherford backscattering (RBS). The growth conditions (temperature, rate, and V/III ratio) were optimized to produce single-phase material. The second sample also had a Mn concentration of ∼5 atom %, but was grown under conditions where we observe second phases. The third sample had a Mn concentration of ∼50 atom % and was designed to contain large concentrations of second phases.

Figure 1 shows the resulting XRD powder scans from the three samples. Second phase peaks are observed for the unoptimized 5 atom % Mn sample and the 50 atom % sample. In the optimized 5 atom % Mn material, only peaks due to hexagonal c axis aligned GaN and GaMn were observed. In separate experiments we have observed a linear variation in (Ga, Mn)N lattice constant with Mn concentration provided the material does not develop secondary phases. The only second phases observed in the unoptimized or high Mn concentration samples are Within this family of compounds, most are reported to be ferromagnetic in bulk form, namely, K); K); K), and MnGa K). We have not observed or other phases under our growth conditions. In growth of (Ga, Mn)N by MBE using a single precursor of the dominant second phase was K).¹⁵

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All the samples exhibited hysteresis in 300 K magnetization vs. field loops (Fig. 2), with coercivities in the range of 100 G. A more instructive measurement is that of the temperature dependence of the field-cooled (FC) and zero field-cooled (ZFC) magnetization, performed in Quantum Design superconducting quantum interference device (SQUID) magnetometer. As shown in Fig. 3 (top), the single-phase (Ga,Mn)N is ferromagnetic to >300 K as evidenced by the separation in FC and ZFC curves. By sharp contrast, the sample shows behavior typical of a spin glass at <100 K (Fig. 3 center), while the multiphase material shows behavior consistent with the presence of at least two ferromagnetic phases (Fig. 3 bottom). Note that the 50% sample still exhibits a loop at 300 K, indicating a small difference in FC and ZFC magnetization at this temperature. All this behavior is consistent with the XRD data.

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In summary, (Ga, Mn)N with 5 atom % Mn, grown by MBE under optimum conditions, shows no detectable second phases in XRD spectra and exhibits ferromagnetism to >300 K. In material known to have second phases, the magnetization vs. temperature behavior shows either a spin-glass type transition or cusps corresponding to the presence of multiple phases.

Acknowledgments