Mn-doped GaN Films Research Articles

The important role of Ga vacancies in the ferromagnetic GaN thin films

Room-temperature ferromagnetism was observed in the unintentionally doped GaN films, which were fabricated using laser molecular beam epitaxy followed by annealing process at different temperatures from 800 to 1000 °C for 25 min in flowing nitrogen gas. The annealing temperature has a prominent effect on the crystalline quality, the surface morphology, the concentration of Ga vacancies and the magnetic properties of the GaN films. It is found that high temperature annealing process is an effective means of introducing Ga vacancies, which play an important role in the origin of room-temperature ferromagnetism in the GaN samples. The Ga vacancy-related ferromagnetism may be an assistant mechanism in the Mn-doped GaN films, but it is dominant in the unintentionally doped GaN films, which deserves further study to develop novel diluted magnetic semiconductors without dopants.

Effects of growth temperature on the valence, optical and magnetic properties of Mn-doped GaN films grown by metal organic chemical vapor deposition

Mn-doped GaN epitaxial films were grown at 900°C, 930°C, and 980°C on sapphire substrates (0001) by metal organic chemical vapor deposition. The valence of Mn ions in the GaN:Mn films was investigated by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. It is found that Mn ions exist at Ga substituting sites as Mn3+ in films grown at a relatively high temperature of 980°C and as Mn2+ in films grown at a relatively low temperature of 900°C. The Mn ions in the films grown at 930°C show mixed valence. Photoluminescence measurements were performed. Fine structures in ultraviolet band are observed for each film. However, broad aquamarine luminescence band around 484nm is pronounced only in spectra of the film grown at 900°C, which is attributed to excess Ga in GaN. In addition, Mn-related zero phonon line (ZPL) at 1.414eV and its phonon replicas were detected in the films grown at 930°C and 980°C. The ZPL and its replicas are assigned to the internal 5T2→5E transition of the neutral Mn3+ state. Magnetometry reveals that all Mn-doped GaN films show paramagnetic-like behaviors, not changed with growth temperature. These data indicate that temperature significantly influences the Mn valence and optical properties of Mn-doped GaN films.

Intrinsic Ferromagnetism of the GaMnN Thin Films

The Mn-doped GaN (GaMnN) films on c-plane sapphire substrates were prepared by using Laser Molecular Beam Epitaxy (LMBE) at different base nitrogen pressure, followed by annealing in the ammonia atmosphere at 950 °C for 30 min, to study the original reason of the room-temperature ferromagnetism of GaMnN films. We found the crystalline quality was sensitive to the base nitrogen pressure during growth. X-ray photoelectron spectra (XPS) analysis confirmed that the Mn3+ and Mn2+ coexist in our samples. The room-temperature ferromagnetic behavior can be explained by double exchange. The ferromagnetism would be weakened by the nitrogen vacancies, which plays a role of donor.

Application of a Continued-Fraction-Based Theory to Line-Profile in Mn-Doped GaN Film

Starting with the Kubo formalism and using the projection operator technique (POT) introduced by Kawabata, the optical quantum transition line-profiles (QTLPs) formula for a Mn-doped wurtzite GaN film was derived as a function of temperature at a frequency of 9.49 GHz (X-band), on the basis of continued fraction representation (CFR) which is a counterpart of the conventional series expansion (CSE). Utilizing this formula we obtained the fine-structure parameter a - F = 9.4 ×10-4 cm-1 and fitting parameter ζ= 4.1. The optical quantum transition half-widths (QTHWs) obtained with the use of these parameters agrees quite well with the existing experimental result in the temperature region T > 20 K. The QTHWs increase with increasing temperature due to the interaction of electrons with optical phonons. Thus, the present technique is considered to be more convenient to explain the resonant system as in the case of other optical transition problems.

Application of a Continued-Fraction-Based Theory to Line-Profile in Mn-Doped GaN Film

Starting with the Kubo formalism and using the projection operator technique (POT) introduced by Kawabata, the optical quantum transition line-profiles (QTLPs) formula for a Mn-doped wurtzite GaN film was derived as a function of temperature at a frequency of 9.49 GHz (X-band), on the basis of continued fraction representation (CFR) which is a counterpart of the conventional series expansion (CSE). Utilizing this formula we obtained the fine-structure parameter a - F = 9.4 ×10-4 cm-1 and fitting parameter ζ= 4.1. The optical quantum transition half-widths (QTHWs) obtained with the use of these parameters agrees quite well with the existing experimental result in the temperature region T > 20 K. The QTHWs increase with increasing temperature due to the interaction of electrons with optical phonons. Thus, the present technique is considered to be more convenient to explain the resonant system as in the case of other optical transition problems.

Nitrogen vacancy effects on the ferromagnetism of Mn doped GaN films

The amorphous gallium nitride thin films doped with Mn were deposited by Laser assisted Molecular Beam Epitaxy (LMBE). After annealing at different NH3 flow rates, the high-quality GaMnN crystalline films with different concentration of nitrogen vacancies (VN) were obtained, which were confirmed by the X-ray diffraction spectroscopy and Raman measurements. The magnetic behaviors of these films were also obtained to investigate the effects of nitrogen vacancies. It indicates that VN play a significant role in the origin of ferromagnetism. The stronger room-temperature ferromagnetism is given with the higher VN concentration when it is not beyond a critical concentration. Moreover, from our M(T) curves and Raman analysis, the films present high resistivity. The magnetism of films with high resistivity varies with concentration of nitrogen vacancies, which can be explained by the bound magnetic polarons (BMP) theory.

Magnetic behavior of Mn-doped GaN (11¯00) film from first-principles calculations

Using first principles calculations based on spin-polarized density functional theory, the magnetic behavior of Mn-doped GaN (11¯00) film is studied. The doping Mn atoms have an attractive pair interaction. Our results give the ground state with antiferromagnetic coupling for Mn-doped GaN (11¯00) film when the nearest neighbor Ga atoms on the surface layer are replaced by Mn atoms, which is contrary to the ferromagnetic coupling when Mn is doped in the bulk GaN. However, in-plane tension and hole doping can switch the magnetic ordering from antiferromagnetism to ferromagnetism, which is important for application in semiconductor spintronics.

The important role of Mn 3+ in the room-temperature ferromagnetism of Mn-doped GaN films

Mn-doped GaN films (Ga 1− x Mn x N) were grown on sapphire (0 0 0 1) using Laser assisted Molecular Beam Epitaxy (LMBE). High-quality nanocrystalline Ga 1− x Mn x N films with different Mn concentration were then obtained by thermal annealing treatment for 30 min in the ammonia atmosphere. Mn ions were incorporated into the wurtzite structure of the host lattice by substituting the Ga sites with Mn 3+ due to the thermal treatment. Mn 3+, which is confirmed by XPS analysis, is believed to be the decisive factor in the origin of room-temperature ferromagnetism. The better room-temperature ferromagnetism is given with the higher Mn 3+ concentration. The bound magnetic polarons (BMP) theory can be used to prove our room-temperature ferromagnetic properties. The film with the maximum concentration of Mn 3+ presents strongest ferromagnetic signal at annealing temperature 950 °C. Higher annealing temperature (such as 1150 °C) is not proper because of the second phase Mn x Ga y formation.

Experimental and theoretical investigations on ferromagnetic nature of Mn-doped dilute magnetic semiconductors

X-ray absorption fine structure (XAFS) and first-principles calculations are effectively combined to establish correlations of fabrications, atomic and electronic structures as well as ferromagnetism for the Mn-doped dilute magnetic semiconductors (DMS), and to shed light on the magnetism origin for a variety of ZnO-, GaN-, and Si-based DMSs. The results of Mn:ZnO and (Mn,N):ZnO thin films reveal that either the existence of Zn vacancy or N substitution of O sites can stabilize the ferromagnetic interactions between neighboring Mn-Mn pairs, and enhance the magnetic moment per Mn. In a 2.5at.% Mn-doped GaN film, a part of substantial Mn ions is found to locate at the interstitial sites near the substitutional Mn ions, forming Mn-Mn dimers that possess unique electronic and magnetic properties. Similar phenomena have also been found in the Mn-doped Si system in which the interstitial Mn atoms intend to assemble together via an intervening substitutional Mn ion. We have proposed a pathway to understand the microscopic origin of ferromagnetism in the DMS materials from the viewpoint of experimental determination and theoretical calculations.

Characterizations of n-type ferromagnetic GaMnN thin film grown on GaN/Al 2O 3 (0 0 0 1) by metal-organic chemical vapor deposition

A high-quality ferromagnetic GaMnN (Mn=2.8 at%) film was deposited onto a GaN buffer/Al 2O 3(0 0 0 1) at 885 °C using the metal-organic chemical vapor deposition (MOCVD) process. The GaMnN film shows a highly c-axis-oriented hexagonal wurtzite structure, implying that Mn doping into GaN does not influence the crystallinity of the film. No Mn-related secondary phases were found in the GaMnN film by means of a high-flux X-ray diffraction analysis. The composition profiles of Ga, Mn, and N maintain nearly constant levels in depth profiles of the GaMnN film. The binding energy peak of the Mn 2p 3/2 orbital was observed at 642.3 eV corresponding to the Mn (III) oxidation state of MnN. The presence of metallic Mn clusters (binding energy: 640.9 eV) in the GaMnN film was excluded. A broad yellow emission around 2.2 eV as well as a relatively weak near-band-edge emission at 3.39 eV was observed in a Mn-doped GaN film, while the undoped GaN film only shows a near-band-edge emission at 3.37 eV. The Mn-doped GaN film showed n-type semiconducting characteristics; the electron carrier concentration was 1.2×10 21/cm 3 and the resistivity was 3.9×10 −3 Ω cm. Ferromagnetic hysteresis loops were observed at 300 K with a magnetic field parallel and perpendicular to the ab plane. The zero-field-cooled and field-cooled curves at temperatures ranging from 10 to 350 K strongly indicate that the GaMnN film is ferromagnetic at least up to 350 K. A coercive field of 250 Oe and effective magnetic moment of 0.0003 μ B/Mn were obtained. The n-type semiconducting behavior plays a role in inducing ferromagnetism in the GaMnN film, and the observed ferromagnetism is appropriately explained by a double exchange mechanism.

Electrical properties of pinholes in GaN:Mn epitaxial films characterized by conductive AFM

Mn doped GaN films have been studied by conductive Atomic Force Microscopy (AFM), Cathodoluminescence (CL) and Electron Beam Induced Current (EBIC). AFM measurements revealed the presence of pinholes with diameters between 130 and 380 nm. The distribution, density and size of the pinholes depend on the Mn doping concentration. AFM Leakage Current images (LC) show a defined contrast at the pinhole planes { 101 1 ̄ } in the sample with Mn concentration of 6.2×10 20 cm −3. For the sample with an Mn concentration of 1.1×10 20 cm −3, LC contrast appears around the pinholes, while no LC contrast was observed for sample with lower Mn concentration. CL measurements indicate that the samples exhibit strain related to Mn incorporation. In correlation with LC measurements, EBIC images show that pinholes are recombination sites. The combination of these techniques enabled us to analyze the Frenkel–Poole conduction in the samples and its relationship with the residual strain and the doping concentration in the films, which would exclude the mechanism of conduction through dislocations.

Role of Defect Sites and Ga Polarization in the Magnetism of Mn-Doped GaN

We report a study of the Mn local structure, magnetism, and Ga moments in molecular beam epitaxy grown Mn-doped GaN films. Using x-ray absorption spectroscopy and magnetic circular dichroism, we find two distinct Mn sites and a Ga moment antiparallel to Mn. First-principles calculations reproduce this phenomenology and indicate that Mn preferentially populates Ga sites neighboring N split interstitial defects. These results show that defects may strongly affect the Mn ordering and magnetism, and that the GaN valence band is polarized, providing a long-range ferromagnetic ordering mechanism for Ga1-xMnxN.

Mn doping and p-type conductivity in zinc-blende GaMnN layers grown by molecular beam epitaxy

Mn-doped cubic GaN films were grown by plasma-assisted molecular beam epitaxy on GaAs (001) substrates. There was a strong influence of the Ga:N ratio on the Mn incorporation and a dramatic increase in the Mn concentration was observed for the layers grown under N-rich conditions. Hall-effect measurements unambiguously showed that the GaMnN samples had strong p-type conductivity. For both N-rich and Ga-rich growth conditions a bulk hole density of about 1018cm−3 was achieved with a corresponding mobility of >300cm2V−1s−1. This compares very favorably with other values reported for p doping of cubic GaN doped with C, suggesting that Mn may be a very suitable dopant for GaN-based electronic structures. The apparent values of ionization energy for Mn were around 50 meV, which is much shallower than for C or Mg in cubic GaN. In addition to a small high temperature ferromagnetic signal, a ferromagnetic correlation was detected among the remaining Mn ions at low temperatures, which is assigned to the onset of hole-mediated ferromagnetism in cubic GaMnN.

Structural and compositional analysis of transition-metal-doped ZnO and GaN PLD thin films

We have studied the structural and magnetic properties of thin films of ZnO and GaN semiconductors doped with magnetic and non-magnetic transition-metals. The films were prepared on sapphire substrates by pulsed laser deposition from doped ceramic targets. Room temperature ferromagnetism was observed in ZnO (doped with Sc, Ti, V, Fe, Co or Ni) and in Mn-doped GaN films. In both cases, single crystal epitaxial growth was observed. The higher dopant:Zn ratio observed in the films is attributed to the preferential sputtering of Zn by energetic ions in the laser ablation plasma plume.

Local electronic structure around Mn and Ga in Mn-doped GaN films showing room temperature ferromagnetism

Local electronic structures around Ga and Mn in Mn-doped GaN film with T c of 940 K are investigated by K X-ray absorption near edge structure (XANES) analysis. It was found that the shape of the Ga XANES spectrum is remarkably similar to that of the un-doped GaN film indicating that the local electronic structure around Ga is not disturbed with Mn doping. As for the Mn XANES spectra, obvious pre-edge peaks were observed: the fine structures in the pre-edges correspond with calculated Mn 3d partial density of states which predict impurity band formation with the Fermi energy stays in the spin-up band. These findings imply that Mn 3d levels stay within the gap with the Fermi energy stays in the spin-up band.

Manganese distribution in ferromagnetic gallium manganese nitride epitaxial film grown by plasma enhanced molecular beam epitaxy

Systematic transmission electron microscopy (TEM) study was performed to determine the change in lattice parameter of epitaxial Mn-doped GaN films with low Mn contents (0.06–0.5 at.%) grown by plasma-enhanced molecular beam epitaxy (PEMBE) by which added Mn distribution can be investigated. Secondary ion mass spectroscopy (SIMS) reveals that the Mn profiles for the films are uniform throughout the entire thickness range of 0.7–1.0 μm with no appreciable segregation. The lattice parameter for the plasma-enhanced molecular beam epitaxy grown GaMnN is found to be a=0.31865 nm, larger than those for the metal organic chemical vapor deposition grown GaN used as a substrate and plasma-enhanced molecular beam epitaxy grown GaN on metal organic chemical vapor deposition GaN, reflecting the expansion of unit lattice due to Mn ion substitution for Ga ion in the wurtzite (Ga x Mn 1− x )N structure. Lattice parameter measurement is believed to give useful information on the crystalline quality of (Ga x Mn 1− x )N structure grown by plasma-enhanced molecular beam epitaxy.

Synthesis and Properties of GaxMn1-xN films

ABSTRATCT:Mn-doped GaN films were grown by hydride vapor phase epitaxy(HVPE). structural measurements show that Mn may substitute Ga atoms in the GaN lattice. Ferromagnetism is observed in these HVPE grown Mn-doped GaN films, which may come from the GaxMn1−xN phase in the films.

Spin resonance investigations ofMn2+in wurtzite GaN and AlN films

High quality Mn-doped GaN and AlN films grown by molecular beam epitaxy have been investigated with X-band electron spin resonance (ESR). The observed resonance patterns are well described by the spin Hamiltonian for isolated ${}^{55}{\mathrm{Mn}}^{2+}$ centers with electronic spin $S=5/2$ and nuclear spin $I=5/2.$ Isotropic g factors $g=2.000$ and hyperfine parameters $A=\ensuremath{-}69\mathrm{G}\ifmmode\times\else\texttimes\fi{}g{\ensuremath{\mu}}_{B}$ are observed both in GaN and AlN, while the fine-structure parameters vary from ${D}_{\mathrm{GaN}}=\ensuremath{-}218\mathrm{G}\ifmmode\times\else\texttimes\fi{}g{\ensuremath{\mu}}_{B}$ for strongly strained GaN films to ${D}_{\mathrm{GaN}}=\ensuremath{-}236\mathrm{G}\ifmmode\times\else\texttimes\fi{}g{\ensuremath{\mu}}_{B}$ for almost relaxed GaN films, and to ${D}_{\mathrm{AlN}}=\ensuremath{-}648\mathrm{G}\ifmmode\times\else\texttimes\fi{}g{\ensuremath{\mu}}_{B}$ for relaxed AlN films. At intermediate orientations of the crystalline c axis with respect to the magnetic field, intermixing occurs between the nuclear spin eigenstates due to off-diagonal elements in the spin Hamiltonian, which strongly enhances the transition probabilities of usually forbidden ESR transitions with $|\ensuremath{\Delta}{m}_{I}|>0.$ This is confirmed experimentally as well as by numerical simulations. It is concluded that ${\mathrm{Mn}}^{2+}$ impurities are present as isolated, paramagnetic centers at the investigated doping concentration of ${10}^{20}{\mathrm{cm}}^{\ensuremath{-}3},$ without any evidence for electrostatic or magnetic coupling to extended valence band states, which is a prerequisite of ferromagnetic exchange required for spintronic devices.

Magnetic properties of Mn-doped GaN, InGaN, and AlGaN

ABSTRACTWe report on the growth and magnetic properties of single crystal Mn-doped GaN, InGaN, and AlGaN films. The III-Nitride films were grown by MOCVD, while the Mn doping was performed by solid-state diffusion of a surface Mn layer deposited by pulsed laser ablation. Mn-doped InxGa1-xN films were grown with x < 0.15, where the easy axis of magnetization rotates from in-plane to out-of-plane by changing the InxGa1-xN thickness/strain-state of the film from compressively strained to relaxed. Mn-doped AlxGa1-xN films were grown with x < 0.40 showing ferromagnetic behavior above room temperature. SQUID measurements ruled out superparamagnetism within these films. By optimizing the growth and annealing conditions of Mn-doped III-Nitrides, we have achieved Curie temperatures in the range of 228 to 500K. These ferromagnetic Mn-doped III-Nitride films exhibit hysteresis with a coercivity of 100–500 Oe. TEM analysis showed no secondary phases within these films.

Magnetic and transport characteristics on high Curie temperature ferromagnet of Mn-doped GaN

Mn-doped GaN films on sapphire (0001) substrates were grown by molecular beam epitaxy system using ammonia as nitrogen source. The result of magnetization measurement gives Curie temperature as high as 940 K. The field and temperature dependencies of the magnetization show coexistence of ferromagnetic and paramagnetic phases. In addition, the temperature dependencies of electrical resistance and carrier concentration were measured to investigate the relation between the ferromagnetism and transport property. Below about 10 K, a similar anomalous increase of magnetization and resistance is observed.