The electron-phonon interaction in monolayer graphene is investigated using density-functional perturbation theory. The results indicate that the electron-phonon interaction strength is of comparable magnitude for all four in-plane phonon branches and must be considered simultaneously. Moreover, the calculated scattering rates suggest an acoustic-phonon contribution that is much weaker than previously thought, revealing an important role of optical phonons even at low energies. Accordingly it is predicted, in good agreement with a recent measurement, that the intrinsic mobility of graphene may be more than an order of magnitude larger than the already high values reported in suspended samples. DOI: 10.1103/PhysRevB.81.121412 PACS number͑s͒: 72.10.Di, 71.15.Mb, 72.80.Vp Graphene, a two-dimensional ͑2D͒ sheet of carbon atoms in a honeycomb lattice, continues to attract much attention due to its unique physical properties. Aside from a substantial academic interest resulting from the relativisticlike behavior of charge carriers, this material is considered very promising in device applications as it has an extremely high intrinsic mobility, even at room temperature. Although in realistic conditions ͑i.e., placed on a substrate͒ the mobility tends to decrease significantly due to the presence of additional scattering mechanisms at the interfaces, 1-3 much effort is currently being devoted to eliminate, or at least minimize, these effects which are detrimental to graphene transport characteristics. Therefore, it is crucial to develop an accurate knowledge of the electron-phonon scattering as it determines the ultimate limit of any electronic device performance. The strength of electron-phonon coupling is typically estimated using the deformation potential approximation ͑DPA͒; it has been applied for graphene by a number of authors. [4][5][6] When the corresponding deformation potential constant was estimated from the transport measurements, however, the results revealed a discrepancy that is too large to be ignored. 1,2,7 Moreover, a very recent observation of mobilities in excess of 10 7 cm 2 / V s at T Շ 50 K in the decoupled graphene 8 drastically departs from the conventionally accepted values, raising serious questions about the current understanding of the intrinsic transport characteristics of graphene. A detailed theoretical analysis of electron-phonon interaction beyond the DPA is clearly called for. In this work, we apply a first-principles approach based on density-functional theory ͑DFT͒ to calculate the electronphonon coupling strength in graphene. The obtained electron-scattering rates associated with all phonon modes are analyzed and the intrinsic resistivity and mobility of monolayer graphene are estimated as functions of temperature. The results clearly elucidate the role of different branches ͑particularly, the significance of optical phonons and intervalley scattering via acoustic phonons͒ as well as limitations of DPA. The obtained effective deformation potential constants suggest the possibil...

A spin field effect transistor (FET) is proposed by utilizing a graphene layer as the channel. Similar to the conventional spin FETs, the device involves spin injection and spin detection by ferromagnetic source and drain. Due to the negligible spin-orbit coupling in the carbon based materials, spin manipulation in the channel is achieved via electrical control of the electron exchange interaction with a ferromagnetic gate dielectric. Numerical estimates indicate the feasibility of the concept if the bias can induce a change in the exchange interaction energy of the order of meV. When nanoribbons are used for a finite channel width, those with armchair-type edges can maintain the device stability against the thermal dispersion.

As discussed in the accompanying articles in this issue of MRS Bulletin, the optical properties of rare-earth (RE) elements have led to many important photonic applications, including solid-state lasers, components for telecommunications (optical-fiber amplifiers, fiber lasers), optical storage devices, and displays. In most of these applications, the host materials for the RE elements are various forms of oxide and nonoxide glasses. The emission can occur at visible or infrared (IR) wavelengths, depending on the electronic transitions of the selected RE element and the excitation mechanism. Until recently, the study of semiconductors doped with RE elements such as Pr and Er has concentrated primarily on the lowest excited state as an optically active transition. The presence of transitions at IR wavelengths (1.3 and 1.54 μm) that are coincident with minima in the optical dispersion and the loss of silica-based glass fibers utilized in telecommunications, combined with the prospect of integration with semiconductor device technology, has sparked considerable interest.The status and prospects of obtaining stimulated emission in Si:Er are reviewed by Gregorkiewicz and Langer in this issue and by Coffa et al. in a previous MRS Bulletin issue. While great progress is being made in enhancing the emission intensity of Er-doped Si, it still experiences significant loss in luminescence efficiency at room temperature, as compared with low temperatures. This thermal quenching was shown by Favennec et al. to de crease with the bandgap energy of the semiconductor. Hence wide-bandgap semiconductors (WBGSs) are attractive candidates for investigation as hosts for RE doping.

Hydrogen incorporation depths of >25 μm were obtained in bulk, single-crystal ZnO during exposure to H2 plasmas for 0.5 h at 300 °C, producing an estimated diffusivity of ∼8×10−10 cm2/V⋅s at this temperature. The activation energy for diffusion was 0.17±0.12 eV, indicating an interstitial mechanism. Subsequent annealing at 500–600 °C was sufficient to evolve all of the hydrogen out of the ZnO, at least to the sensitivity of secondary ion mass spectrometry (<5×1015 cm−3). The thermal stability of hydrogen retention is slightly greater when the hydrogen is incorporated by direct implantation relative to plasma exposure, due to trapping at residual damage in the former case.

Measurement of the room temperature forward bias current-voltage behavior of InGaN/AlGaN double heterostructure blue light-emitting diodes demonstrates a significant departure from the usual Is exp(qV/ nkT) behavior where n is the ideality factor which varies between 1 and 2. The observed current-voltage behavior at room temperature may be represented as I=2.7×10−11 exp(5.7V) which suggests a tunneling mechanism. Measurement of the electroluminescence for currents from 0.5 to 100 mA demonstrates that the emission peak shifts to higher energy while increasing in intensity. The shifting peak spectra is due to band filling, a process which results from the injection of holes via tunneling into an empty acceptor impurity band and vacant valence band tails. At currents near 100 mA, a non-shifting band-to-band emission approaches the intensity of the shifting peak spectra. The active layer of these diodes is codoped with both the donor Si and the acceptor Zn.

We report on spectral and time-resolved photoluminescence ͑PL͒ studies performed on Eu-doped GaN prepared by solid-source molecular-beam epitaxy. Using above-gap excitation, the integrated PL intensity of the main Eu 3ϩ line at 622.3 nm ( 5 D 0 → 7 F 2 transition͒ decreased by nearly 90% between 14 K and room temperature. Using below-gap excitation, the integrated intensity of this line decreased by only ϳ50% for the same temperature range. In addition, the Eu 3ϩ PL spectrum and decay dynamics changed significantly compared to above-gap excitation. These results suggest the existence of different Eu 3ϩ centers with distinct optical properties. Photoluminescence excitation measurements revealed resonant intra-4 f absorption lines of Eu 3ϩ ions, as well as a broad excitation band centered at ϳ400 nm. This broad excitation band overlaps higher lying intra-4 f Eu 3ϩ energy levels, providing an efficient pathway for carrier-mediated excitation of Eu 3ϩ ions in The visible and infrared light emissions from rare-earthdoped GaN ͑GaN:RE͒ are of significant current interest for applications in thin-film electroluminescence ͑EL͒ devices. [1][2][3][4] For achieving red light emission, the 5 D 0 → 7 F 2 intra-4 f transition of trivalent Eu 3ϩ ions seems most promising. Intense red photoluminescence ͑PL͒ around 622 nm from GaN:Eu ͑as-grown and ion-implanted͒ has been reported from several research groups. [1][2][3][4][5][6][7][8][9] In addition, several EL device structures based on GaN:Eu have been demonstrated. [1][2][3][4][5] The optimization of present EL devices, however, requires a more detailed understanding of the incorporation, excitation, and emission properties of Eu 3ϩ ions in the GaN host matrix.Several studies have recently appeared focusing on the preparation and optical properties of GaN:Eu. 4 -11 Based on the comparison to RE ions in other III-V semiconductors ͑e.g., InP:Yb, 12 GaAs:Er 13 ͒, the most probable lattice location for Eu 3ϩ ions in GaN are ͑substitutional͒ Ga sites, which have C 3V symmetry. However, significant differences in the Eu 3ϩ PL properties have been observed depending on the material preparation. Monteiro et al. 7 studied Euimplanted GaN and Eu in situ doped GaN grown by metalorganic chemical vapor deposition. They observed significant differences in the Eu 3ϩ PL properties, including the number of emission lines associated with the 5 D 0 → 7 F 2 transition. Based on optical spectroscopy and Rutherford backscattering studies, the authors concluded that the local symmetry of the Eu 3ϩ ions has to be lower than C 3V symmetry. 7 Bang et al. 9 studied Eu-doped GaN prepared by gas-source molecularbeam epitaxy ͑MBE͒ and concluded, based on extended x-ray absorption fine-structure data, that Eu 3ϩ occupies Ga sites with C 3V symmetry. It was also suggested that more than one local environment of Eu 3ϩ ions may exist in the investigated GaN samples.In this letter, we present PL results on GaN:Eu prepared by solid-source MBE, which provide spectroscopic evidence for the existence of different Eu...

We report the observation of the 1.54-μm luminescence of optically excited Er3+ in ion-implanted epitaxially grown GaN and AlN films using below band-gap excitation. The Er-implanted layers were co-implanted with oxygen. At room temperature, this luminescence for GaN grown on sapphire is nearly as intense as it is at 6 or 77 K and exhibits many resolved transitions between crystal-field levels of the 4I13/2 first excited multiplet and the 4I15/2 ground multiplet.

Effect of chiral property on hot phonon distribution and energy loss rate due to surface polar phonons in a bilayer graphene J. Appl. Phys. 113, 063705 (2013); 10.1063/1.4790309Phonon-limited electron mobility in graphene calculated using tight-binding Bloch waves J. Appl. Phys. 112, 053702 (2012); 10.1063/1.4747930 Two-dimensional electron gases: Theory of ultrafast dynamics of electron-phonon interactions in graphene, surfaces, and quantum wellsThe effects of surface polar phonons on the electronic transport properties of monolayer graphene are studied by using a Monte Carlo simulation. Specifically, the low-field electron mobility and saturation velocity are examined for different substrates ͑SiC, SiO 2 , and HfO 2 ͒ in comparison to the intrinsic case. While the results show that the low-field mobility can be substantially reduced by the introduction of surface polar phonon scattering, corresponding degradation of the saturation velocity is not observed for all three substrates at room temperature. It is also found that surface polar phonons can influence graphene's electrical resistivity even at low temperature, leading potentially to inaccurate estimation of the acoustic phonon deformation potential constant.