Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J18: Focus Session: Wide Band Gap Semiconductors I |
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Sponsoring Units: DMP Chair: Alan Doolittle, Georgia Tech Room: LACC 406A |
Tuesday, March 22, 2005 11:15AM - 11:51AM |
J18.00001: Ferromagnetism in Transition Metal Doped GaN and Related Materials Invited Speaker: There is high current interest in the development of dilute magnetic semiconductor (DMS) materials exhibiting ferromagnetic behavior for spin-based light-emitting diodes, sensors, and transistors. Such materials are formed through the introduction of transition metal (TM) ions, such as Mn and Cr, into semiconductor hosts. Unfortunately many DMS materials, such as GaMnAs, have a relatively low magnetic ordering temperature ( 170 K for GaMnAs), which severely limits their usefulness. In the past few years, several groups have reported achieving ferromagnetism at room temperature in wide bandgap materials, such as GaMnN. This property makes these materials attractive for use as ultra-low-power switching elements, where the bit state of the device is determined through control of electron spin. Furthermore, these materials may also allow for the integration of photonic (laser and light-emitting diodes), electronic (field-effect and bipolar transistors) and magnetic (information storage) devices on a single substrate, leading to a new class of electronic devices that offer multi-purpose functionality. However, to realize such devices, several challenges remain. One concern to date has been the relatively low thermal stability of the III-Mn-N compounds. Doping with Cr in place of Mn, however, appears to greatly enhance the ability of the material to retain its magnetic properties even after annealing at temperatures up to 700C, easing the road to practical device fabrication. In addition, the ability to achieve magnetic behavior in a semi-insulating barrier material such as AlCrN opens new device possibilities. The most evident application of ferromagnetic AlN is as a ferromagnetic tunnel barrier, similar to EuS, but unlike EuS should allow for operation at room temperature. Growth of tunnel devices using Al-TM-N as a barrier and Ga-TM-N as a spin injector will be discussed. This work is supported by the Army Research Office under ARO-DAAD19-01-0-0701 and NSF under ECS-0224203. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:03PM |
J18.00002: Demonstration of carrier mediated ferromagnetism in GaMnN by co-doping and heterostructures Meredith Reed, Erdem Arkun, Mason Reed, Acar Berkman, Oliver Luen, Salah Bedair, Nadia El-Masry, John Zavada We demonstrate carrier mediated ferromagnetism via GaMnN films co-doped with Si and Mg, heterostructures, and p-i-n junction devices that were grown by metal-organic chemical vapor deposition. The magnetic properties of GaMnN are affected by intentional introduction of donor or acceptor states into the film. Si or Mg co-doping of GaMnN films led to either ferromagnetic or paramagnetic behavior depending on the concentration. The magnetic properties of the GaMnN material system correlates with the Fermi level. Ferromagnetism was observed only when the Fermi level was near the Mn energy band resulting in a partially occupied Mn energy level; a prerequisite for conduction within this band. This allows carriers to be present in this band to mediate ferromagnetic behavior. In addition to co-doping, the dependence of ferromagnetic properties of GaMnN films on carrier transfer across heterojunctions was also studied. The magnetic properties of GaMnN, as a part of GaMnN/GaN:Mg heterojunctions depend on the thickness of both the GaMnN film and the adjacent GaN:Mg layer. These results are explained based on the occupancy of Mn energy band and how this occupancy is altered by carrier transfer at the GaMnN/GaN:Mg interface. Thus, the ferromagnetic properties result from a solid solution of Mn in GaN. [Preview Abstract] |
Tuesday, March 22, 2005 12:03PM - 12:15PM |
J18.00003: Electronic structure and ferromagnetism of Mn $\delta$-doped GaN Joongoo Kang, Kee Joo Chang Recently, Mn-doped GaN has attracted much attention because of the ferromagnetism observed in this material. However, experimental data so far are quite controversial, reporting the Curie temperatures ranging from 10 to 940 K. Very recent experiments showed that Mn $\delta$-doped GaN films have high hole carrier concentrations, which lead to the high Curie temperature and enhanced magnetization. In this work, we study the electronic and magnetic properties of Mn doped GaN and the origin of p-type conductance especially for Mn $\delta$-doped films through first-principles spin-density-functional calculations. The nature of magnetic interactions between two Mn ions is investigated by varying the Mn-Mn distance and their orientation. The ferromagnetic coupling has a short-range nature, effective for Mn-Mn distances up to about 7 $\rm \AA$. We also investigate the doping effect on ferromagnetism, and the energetics and ferromagnetism of Mn nanoclusters. Finally, we find that Ga vacancies near the Mn $\delta$-doped layer are more stable than in the bulk region of GaN due to the charge transfer from the Mn to Ga vacancy. We suggest that Ga vacancies near the Mn $\delta$-doped layer are likely to be the origin of p-type conductance. [Preview Abstract] |
Tuesday, March 22, 2005 12:15PM - 12:27PM |
J18.00004: Transition from Ferromagnetism to Antiferromagnetism in Ga$_{1-x}$Mn$_x$N Gustavo Dalpian, Su-Huai Wei Ga$_{1-x}$Mn$_x$N has attracted much attention recently because previous theoretical predictions suggested that it has a ferromagnetic (FM) ground state with T$_c$ above the room temperature. However, available experimental data often contradict each other. Some reports show that high T$_c$ FM phase is achievable in GaMnN; others show that the magnetic coupling of the Mn ions in GaMnN is actually antiferromagnetic (AFM). The exact nature of the magnetism observed in this system is also under debate. To help unravel the magnetic properties of this system, we developed a new band-coupling model to show that FM order in this system is facilitated by coupling between occupied majority d states inside the gap, whereas AFM ordering in this system is caused by coupling between majority spin d band and minority spin $d$ band. At low Mn concentration Ga$_{1-x}$Mn$_x$N is FM, whereas at high Mn concentration, under pressure, or if the hole at the Mn $d$ band is compensated, Ga$_{1-x}$Mn$_x$N will change to AFM ground state. Our {\it ab initio} total energy calculation based on spin density functional theory support this model. [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 12:39PM |
J18.00005: Antiferromagnetic coupling driven by bond length contraction near Ga$_{1-x}$Mn$_x$N film surface Wang Qian, Qiang Sun, Puru Jena, Yoshi Kawazoe Following the discovery of ferromagnetism in (Ga,Mn)As and the subsequent theoretical prediction that Mn doped GaN could be ferromagnetic at or above room temperature, numerous attempts have been made to synthesize this promising DMS material. However, the results have been rather confusing. Not only the reported Curie temperatures vary over a wide range (10K-945K), but also it is uncertain whether the ground state of (Ga,Mn)N is ferromagnetic (FM) or antiferromagnetic (AF). An understanding of the controversy between FM and AF is both important and challenging. Using first principles calculations based on gradient corrected density functional theory we show that Mn atoms, which couple ferromagnetically in bulk Ga$_{1-x}$Mn$_{x}$N, couple antiferromagnetically on its surface. This change in magnetic behavior is brought about a contraction of the Mn-Mn and Mn-N bond lengths which is significantly smaller on the surface than in the bulk. The present study provides new insight for explaining the numerous conflicting experimental observations in Mn doped GaN systems. [Preview Abstract] |
Tuesday, March 22, 2005 12:39PM - 12:51PM |
J18.00006: Magnetism and Energetics of Mn Doped ZnO (10 0) Thin Film Bijan K. Rao, Qian Wang, Puru Jena First principles calculations based on gradient corrected density functional theory are performed on Mn doped ZnO thin film. Magnetism and energetics are studied for two Mn concentrations and varying Mn configurations. It is found that in the dilute limit when Mn atoms are far apart, the ferro- and anti-ferromagnetic states are energetically nearly degenerate. The resulting fluctuation would, therefore, make the system paramagnetic as found in the experiment. But, as the concentration of Mn atoms increases, there is a tendency for Mn atoms to form nearest neighbors and cluster around oxygen. For such a configuration, the anti-ferromagnetic coupling between Mn atoms is energetically more favorable. The results are compared with a diverse range of experiments on Mn doped ZnO thin film. [Preview Abstract] |
Tuesday, March 22, 2005 12:51PM - 1:03PM |
J18.00007: Carrier-mediated Ferromagnetism in N Co-doped (Zn, Mn)O (10 0) Thin Film Sun Qiang, Qian Wang, Puru Jena, Yoshi Kawazoe Considerable experimental work is available on the (Zn, Mn)O system. However, the results have been rather controversial. While some groups have reported ferromagnetism in (Zn, Mn)O systems, others report observations of anti-ferromagnetic or spin-glass behavior. Using first principles calculations based on the density functional theory and generalized gradient approximation we show that the ground state of Mn doped ZnO (10$\overline 1 $0) thin film changes from antiferromagnetic to ferromagnetic when co-doped with N. The ferromagnetic coupling between Mn spins arises due to the overlap between N 2p and Mn 3d electrons in the spin up band, rendering the system half-metallic character. [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:15PM |
J18.00008: The role of Mn doping in ZnO based DMS studied by x-ray absorption and emission spectroscopy Jinghua Guo, Amita Gupta, Parmanand Sharma, K.V. Rao, J.M.O. Guillen, Rajeev Ahuja Independent control of spin and charge of doped carriers has attracted much interest in diluted magnetic semiconductors (DMSs) because the combination of the two degrees of freedom is expected to open up new functionalities in optoelectronic and magnetoelectric devices. Recently dilute Mn doped ZnO was shown to have such a property when processed at temperatures below 500$^{\circ}$C. In general, electronic structure ultimately determines the properties of matter, and therefore a detail description of the electronic structure of DMS will lead to a better understanding its magnetic properties. We studied the electronic structure of Mn-doped ZnO using X-ray absorption (XAS) and emission spectroscopy (XES). Upon Mn doping, the top of O $2p$ valence band extends into bandgap and a distinct absorption feature appears at the bottom of the conduction band, which suggests the strong hybridization of Mn $3d$ and O $2p$. The evidence of the ligand-hole states of Zn \textit{3d -- O 2p} is shown. Furthermore, Mn $2p$-absorption and $L-$emission spectra indicate Mn$^{2+}$ replacing Zn site in tetragonal symmetry. [Preview Abstract] |
Tuesday, March 22, 2005 1:15PM - 1:27PM |
J18.00009: Investigation of Magnetic Doping of High-Density GaN Nanorods J.E. Van Nostrand, J. Boeckl, J.D. Albrecht, R. Cortez We investigate GaN nanorods as host structures for transition metal doping and alloying. The epitaxial growth of bulk-like films of GaMnN, which has been predicted to exhibit ferromagnetism at or above 300K, has been the focus of many recent investigations. Epitaxial growth of GaMnN involves lowering the substrate temperature to allow the mobile Mn to incorporate while sacrificing the underlying GaN crystal quality. This delicate balance is difficult to achieve, and with increasing Mn flux the GaMnN often contains magnetic precipitates or serious structural inhomogeneities. We investigate the layers of vertical GaN nanorods having a width of 60$\pm$5 nm which were grown by MBE on Al$_2$O$_3$(0001) and Si(111) substrates. These rods appear to be fully-relaxed, low- defect structures which have PL with a narrow (2.05 {\AA} FWHM) peak centered at 3572.6 {\AA}. We report on nanorods with continuous Mn doping as well as Mn doped GaMnN/InGaN quantum wells incorporated into the nanorods. [Preview Abstract] |
Tuesday, March 22, 2005 1:27PM - 1:39PM |
J18.00010: Dielectric functions of as-grown and annealed Ga$_{1-x}$Mn$_{x}$As thin films Zachary Weber, Frank Peiris, X. Liu, J. K. Furdyna We have investigated the dielectric functions of a series of as-grown as well as annealed Ga$_{1-x}$Mn$_{x}$As thin films using spectroscopic ellipsometry. After determining the alloy compositions using x-ray diffraction experiments, a rotating analyzer spectroscopic ellipsometer was used to measure the complex reflection ratio for each of the films in the energy range between 0.9-6.5~eV. By modelling the ellipsometric data, the dielectric functions for each of the Ga$_{1-x}$Mn$_{x}$As samples were determined. All of the dielectric functions displayed the critical point structures related to the higher order electronic transitions, and showed differences between the as-grown and the annealed sample spectra. [Preview Abstract] |
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J18.00011: High Temperature Ferromagnetic and UV-Optic Properties of Co-Doped ZnO Nanoclusters Prepared under Different O2 Atmospheres Jiji Antony, Sweta Pendyala, Xiang-Bai Chen , Leah Bergman, David E. McCready , Mark Englehard , Amit Sharma , Daniel Meyer, You Qiang Co-doped ZnO nanocluster films are prepared at room temperature under different oxygen concentrations by, our novel nanocluster system, based on a technique that is a combination of high pressure sputtering and aggregation. Magnetic properties of the cluster films are measured by SQUID magnetometer. We measured hysteresis loops of these samples at various temperatures and with the increase of temperature the coercivity, remanence and saturation magnetization decreased. The UV-PL intensity of the samples prepared in high O$_{2}$ atmosphere is stronger, with low FWHM compared to the samples prepared in low O$_{2}$ atmosphere. The field cooling (FC) and zero-field cooling (ZFC) data are taken and analyzed. XRD pattern of these samples are quite similar to the bulk ZnO where as XPS data showed the presence of Co in the samples. [Preview Abstract] |
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