GaMnN Films Research Articles

Role of vacancy-type defects in magnetism of GaMnN**Project supported by the National Natural Science Foundation of China (Grant Nos. 61204008, 11075176, and 11505211) and the National Key Basic Research Special Foundation of China (Grant No. 2013CB328705).

Role of vacancy-type (N vacancy (VN) and Ga vacancy (VGa)) defects in magnetism of GaMnN is investigated by first-principle calculation. Theoretical results show that both the VN and VGa influence the ferromagnetic state of a system. The VN can induce antiferromagnetic state and the VGa indirectly modify the stability of the ferromagnetic state by depopulating the Mn levels in GaMnN. The transfer of electrons between the vacancy defects and Mn ions results in converting Mn3+ (d4) into Mn2+ (d5). The introduced VN and the ferromagnetism become stronger and then gradually weaker with Mn concentration increasing, as well as the coexistence of Mn3+ (d4) and Mn2+ (d5) are found in GaMnN films grown by metal–organic chemical vapor deposition. The analysis suggests that a big proportion of Mn3+ changing into Mn2+ will reduce the exchange interaction and magnetic correlation of Mn atoms and lead to the reduction of ferromagnetism of material.

Mid-gap photoluminescence and magnetic properties of GaMnN films grown by metal—organic chemical vapor deposition

Metal—organic chemical vapor deposition (MOCVD) grown ferromagnetic GaMnN films are investigated by photoluminescence (PL) measurement with a mid-gap excitation wavelength of 405 nm. A sharp PL peak at 1.8 eV is found and the PL intensity successively decreases with the addition of Mn, in which the Mn concentration of sample A is below 1% ([Mn]A = 0.75%) but its PL intensity is stronger than other samples'. The 1.8-eV PL peak is attributed to the recombination of electrons in the t2 state of the neutral Mn3+ acceptor with holes in the valence band. With Mn concentration increasing, the intensity of the PL peak decreases and the magnetic increment reduces in our samples. The correlation between the PL peak intensity and ferromagnetism of the samples is discussed in combination with the experimental results.

Structure Dependence of Magnetic Properties for Annealed GaMnN Films Grown by MOCVD

GaMnN/GaN multilayers and conventional GaMnN single layers are grown by metal-organic chemical vapor deposition. Both kinds of samples show room-temperature ferromagnetism. After thermal annealing, the sample with GaMnN/GaN multilayer structure displays a larger coercivity and better thermal stability compared to the GaMnN single layer. The annealing effects on VGa related defects are observed from photoluminescence measurements. Moreover, a different magnetic behavior is also found in the annealed GaMnN films grown on different (n-type GaN and p-type GaN) templates. These kinds of structure-dependent magnetic behaviors indicate that defects or carriers transformation introduced during annealing may have important effects on the electronic structure of Mn ions and on the ferromagnetism. Our work may be helpful for further understanding the origin of ferromagnetism in GaN-based diluted magnetic semiconductors.

Effect of annealing time on the structural and ferromagnetic properties of the GaMnN thin films

GaMnN thin films were deposited on a sapphire (0001) substrate by using laser assisted molecular beam epitaxy. Subsequently, the samples were annealed in the ammonia ambience at 1000 °C for different time lengths. The crystalline quality was improved gradually, and the room temperature ferromagnetism of our samples becomes stronger with the increase of the annealing time within 25 min. The X-ray photoelectron spectra analysis confirmed that the Mn3+ concentration in the GaMnN films increased after annealing. The stronger ferromagnetism was observed in the sample with the higher Mn3+ concentration. However, too long annealing time, such as 35 min, will lead to the degradation of the crystalline quality and the decrease of Mn3+ concentration, which results in the weakened ferromagnetism. The optimal annealing time is 25 min at 1000 °C in our experiments. Finally, the origin of the room temperature ferromagnetism in our samples was discussed preliminarily.

The effects of oxygen co‐doping on structural, magnetic and optical properties of Mn‐doped GaN

ABSTRACTThe GaMnN and GaMnN: O films were obtained by annealing at different NH3 flow rates. When the NH3 flow rate was decreased to 20 sccm, the oxygen was co‐doped into GaMnN film and substituted the few host atoms, which were confirmed by the X‐ray Photoelectron Spectra. Moreover, the oxygen co‐doping in GaMnN film drastically enhanced the gain of the ferromagnetic (FM) state but oxygen as shallow donor in GaN offers little carriers. Hence, the enhanced FM state is not due to increase of carrier concentration. On optical properties, the oxygen co‐doping in GaMnN film will suppress the blue luminescence centered near 2.8 eV and make a blue shift of the photoluminescence spectra. Copyright © 2013 John Wiley & Sons, Ltd.

Magnetic characteristics of nanocrystalline GaMnN films deposited by reactive sputtering

The magnetic characteristics of Ga1−xMnxN nanocrystalline films (x = 0.08 and x = 0.18), grown by reactive sputtering onto amorphous silica substrates (a-SiO2), are shown. Further than the dominant paramagnetic-like behaviour, both field- and temperature-dependent magnetization curves presented some particular features indicating the presence of secondary magnetic phases. A simple and qualitative analysis based on the Brillouin function assisted the interpretation of these secondary magnetic contributions, which were tentatively attributed to antiferromagnetic and ferromagnetic phases.

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.

The properties of GaMnN films grown by metalorganic chemical vapour deposition using Raman spectroscopy

An investigation of room-temperature Raman scattering is carried out on ferromagnetic semiconductor GaMnN films grown by metalorganic chemical vapour deposition with different Mn content values. New bands around 300 and 669 cm−1, that are not observed in undoped GaN, are found. They are assigned to disorder-activated mode and local vibration mode (LVM), respectively. After annealing, the intensity ratio between the LVM and E2(high) mode, i.e., ILVM/IE2(high), increases. The LO phonon-plasmon coupled (LOPC) mode is found in GaMnN, and the frequency of the LOPC mode of GaMnN shifting toward higher side is observed with the increase in the Mn doping in GaN. The ferromagnetic character and the carrier density of our GaMnN sample are discussed.

Optical properties of GaMnN films grown by metal-organic chemical vapor deposition

Optical absorption spectra for a series of GaMnN samples with different Mn content grown by metal-organic chemical vapor deposition have been investigated. Compared with GaN, the absorption coefficient of Ga1-xMnxN increases in the UV region and an absorption peak can be observed near 144 eV. In addition, the absorption coefficient increases with the increase of the Mn content. The experimental results are in excellent agreement with theoretical data based on density functional theory. Associated with theoretical data, the absorption peak near 144 eV is assigned to 5T2→5E internal transition between the e state and the t2 state of Mn3+ ion.

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.

Manipulation of Room Temperature Ferromagnetic behavior of GaMnN Epilayers

AbstractWe report on metal organic chemical vapor deposition growth of GaMnN/p-GaN/n-GaN multilayer structures and manipulation of room temperature (RT) ferromagnetism (FM) in a GaMnN layer. The GaMnN layer was grown on top of a n-GaN substrate and found to be almost always paramagnetic. However, when grown on a p-type GaN layer, a strong saturation magnetization (Ms) was observed. Ms was almost doubled after annealing demonstrating that the FM observed in GaMnN film is carrier-mediated. To control the hole concentration of the p-GaN layer by depletion, GaMnN/p-GaN/n-GaN multilayer structures of different p-GaN thickness (Xp) were grown on sapphire substrates. We have demonstrated that the FM depends on the Xp and the applied bias to the GaN p-n junction. The FM of these multilayer is independent on the top GaMnN layer thickness (tGaMnN) for tGaMnN >200 nm and decreases for tGaMnN < 200 nm. Thus the room temperature FM of GaMnN i-p-n structure can also be controlled by changing Xp and tGaMnN in the GaMnN i-p-n structures.

Role of hydrogen in room temperature ferromagnetism of GaMnN films

X-ray photoemission of the valence band and Mn 3s of hydrogenated/dehydrogenated GaMnNfilms and as-grown ferromagnetic GaMnN films is investigated. The Fermi level is shiftedupward by the hydrogenation. Larger energy splitting in Mn 3s peaks after the hydrogenationindicates that Mn ions obtain additional electrons in 3d states. The ferromagnetism of thehydrogenated film disappears after dehydrogenation. As-grown ferromagnetic films contain1021 cm−3 hydrogen. We conclude that hydrogen is responsible for the ferromagnetism of GaMnNfilms.

Role of Mn and H in formation of cubic and hexagonal GaMnN

Electron microscopy studies are carried out to determine the structure of GaMnN films grown on 6H-SiC(0001) and MgO(111) substrates by plasma-assisted molecular beam epitaxy. For the Mn∕GaN multilayer growth, cubic zinc blende GaMnN is observed in the nominal Mn∕GaN multilayer region and surface microprotrusions, indicative of high Mn mobility. In contrast, for the H-assisted growth of Mn-doped GaN, single phase hexagonal wurtzite GaMnN is obtained, but with columnar structure, indicative of reduced Mn mobility during growth.

Analysis of diluted magnetic semiconductor GaMnN grown by electron cyclotron resonance-plasma enhanced metal organic chemical vapor deposition

Diluted magnetic semiconductor film GaMnN was grown on sapphire (α-Al2O3) substrate using biscyclopentyldienyl manganese (Cp2Mn), N2 and TEGa by electron cyclotron resonance-plasma enhanced metal organic chemical vapor deposition(ECR-PEMOCVD) . The crystal structure and surface topography of the GaMnN films were characterized by RHEED, XRD and AFM. GaMnN films exhibit good (0002) preferred orientation, showing the films are inclined to c-axis growth and retain good wurtzite structure. The surface topography of GaMnN film is composed of many submicron grains piled in the consistent orientation. The magnetism of films is characterized by SQUID. SQUID shows that the film is ferromagnetic, which comes probably only from the ternary phase GaMnN and the Curie temperature of GaMnN film is higher than 350 K. Moreover, higher Mn concentration can enhance the Curie temperature of the film.

Correlation between photoluminescence and magnetic properties of GaMnN films

GaMnN films grown by metal-organic chemical vapor deposition were studied by photoluminescence (PL) spectroscopy and hysteresis measurements. Depending on the growth conditions of these GaMnN films, hysteresis measurements along the easy axis of magnetization show a transformation from magnetic to nonmagnetic behavior. The PL spectra of both magnetic and nonmagnetic GaMnN films exhibited GaN band edge and deep-level impurity transitions at 3.4 and 1.3eV, respectively. The PL emission intensity of the 1.3eV emission peak is stronger considerably for magnetic GaMnN films and is believed to be due to the Mn3+ intraband transition.

Growth and its Properties of GaMnN Film Based on ECR-PEMOCVD

The DMS GaMnN film with certain concentration of Mn and good crystal qualities has been successfully grown on the substrate of sapphire (α-Al2O3) by ECR-PEMOCVD. The graphs of RHEED presented a clear spot-like lattice and the surface was not very glossy, which showed that the GaMnN film was single crystalline and its growth model was three -dimensional island. XRD analysis showed that the film was hexagonal structure with c -Axis oriented and the crystallinity was very well. The AFM test result showed that the GaMnN films were composed of many submicron grains with the same orientation. SQUID(superconducting quantum interference device) measurement showed an apparent ferromagnetic hysteresis at room temperature, and the Curie temperature of the film was about 400k.

Fermi Level and Magnetic GaMnN Thin Films

Spin based electronics is an emerging field where the spin of electrons in addition to the charge of electrons is manipulated and different functionalities in devices are achieved. Developments of new materials, that are ferromagnetic at room temperature such as GaMnN, will allow us to create spin- polarized electrons which are prerequisite for progress in this field. Theory suggests that GaN when alloyed with transition metals such as Mn at low concentrations (5 % Mn) will be ferromagnetic. The growth and the origin of the magnetic property in this crystal are still under investigation. We demonstrate the growth and characterization of GaMnN films on (0001) sapphire substrates by MOCVD. We have found that the magnetization in these films correlate with the position of the Fermi level in the crystal. Moreover, changing the position of the Fermi level by co-doping changes the magnetization observed in these films . Our experiments reveal a possible mechanism of the observed magnetic property in this material system. We have also found that charge transfer across ferromagnetic and non magnetic interfaces resulted in changes in the magnetic properties

Magnetic and magnetotransport properties of (AlGaN∕GaN):Mg∕(GaMnN) heterostructures at room temperature

Dilute magnetic semiconductor films (GaMnN) are highly resistive, making transport measurements difficult to achieve. However, when GaMnN films are sandwiched between p-type doped (AlGaN∕GaN) strained-layer superlattices, holes from the superlattice interact with the Mn3+∕2+ ions and transport measurements were realized. The authors have found also that the ferromagnetic properties of GaMnN critically depend on the level of p-type doping in the superlattice. They report anomalous Hall effect measurements in this (AlGaN∕GaN):Mg∕(GaMnN) multilayered structure. The current results also demonstrate the role of carriers, especially holes, in mediating the ferromagnetic properties of GaMnN dilute magnetic semiconductor films.

Magnetic behavior of Mn 3GaN precipitates in ferromagnetic Ga 1− xMn xN layers

Ga 1− x Mn x N layers (where x ≈ 10.4–12.5%) containing Mn 3GaN precipitates were grown on (0 0 0 1) sapphire substrates using molecular beam epitaxy, and were shown to exhibit two ferromagnetic phases. These p-type GaMnN films were revealed to have a well defined hysteresis loop up to T ∼ 200 K, which originates from the appreciable number of Mn 3GaN precipitates imbedded in the GaN matrix. The precipitated GaMnN samples exhibited a ferromagnetism due to Mn 3Ga clusters mixed with Mn 3GaN precipitates in the high temperature region (above 200 K). The Mn 3GaN precipitates and Mn 3Ga clusters were identified by X-ray diffraction and transmission electron microscopy. Mn 3GaN precipitates in the samples with a high Mn concentration have a characteristic transition temperature at T ∼ 200 K, which was characterized from field-cooled and zero-field-cooled magnetization curves.

Study of structures and magnetic properties of single crystalline HVPE–GaMnN films

We have grown single crystalline GaMnN films by a horizontal hydride vapor-phase epitaxy (HVPE) system. The structures and crystal quality were confirmed by XRD and Raman scattering measurements. The structure analysis indicated that we obtained uniform GaMnN solid-solution films; the Mn atoms were substituted for Ga sites and did not form Mn precipitates or clusters. The superconductor quantum interference device (SQUID) results show that the GaMnN films without the phase separation exhibit paramagnetic properties.