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Showing papers in "Applied Physics Express in 2017"


Journal ArticleDOI
TL;DR: In this article, a transparent AlGaN:Mg contact layer, a Rh mirror electrode, an AlN template on a patterned sapphire substrate, and encapsulation resin were introduced to improve the light extraction.
Abstract: Enhancing the light-extraction efficiency is the key issue for realizing highly efficient AlGaN-based ultraviolet light-emitting diodes (UV-LEDs). We introduced several features to improve the light extraction: a transparent AlGaN:Mg contact layer, a Rh mirror electrode, an AlN template on a patterned sapphire substrate, and encapsulation resin. The combination of the AlGaN:Mg contact layer and the Rh mirror electrode significantly improved the output power and the external quantum efficiency (EQE) of UV-LEDs. By introducing the aforementioned features, a maximum EQE of >20% at an emission wavelength of 275 nm and a 20-mA direct current was achieved.

381 citations


Journal ArticleDOI
TL;DR: In this paper, the electron concentration and mobility were investigated using Van der Pauw Hall patterns, and the Hall data showed an improved electron mobility for the same electron concentration when Ge is used instead of Sn as the dopant.
Abstract: The Ge doping of β-Ga2O3(010) films was investigated using plasma-assisted molecular beam epitaxy as the growth method. The dependences of the amount of Ge incorporated on the substrate temperature, Ge-cell temperature, and growth regime were studied by secondary ion mass spectrometry. The electron concentration and mobility were investigated using Van der Pauw Hall patterns. Hall measurement confirmed that Ge acts as an n-dopant in β-Ga2O3(010) films. These results were compared with similar films doped by Sn. The Hall data showed an improved electron mobility for the same electron concentration when Ge is used instead of Sn as the dopant.

188 citations


Journal ArticleDOI
TL;DR: In this article, the β-(Al x Ga 1− x )2O3 barrier was partially doped by Ge to achieve a two-dimensional electron gas (2DEG) in Ga 2O3, which was confirmed by capacitance-voltage measurements.
Abstract: β-(Al x Ga1− x )2O3/β-Ga2O3 heterostructures were grown via plasma-assisted molecular beam epitaxy. The β-(Al x Ga1− x )2O3 barrier was partially doped by Ge to achieve a two-dimensional electron gas (2DEG) in Ga2O3. The formation of the 2DEG was confirmed by capacitance–voltage measurements. The impact of Ga-polishing on both the surface morphology and the reduction of the unintentionally incorporated Si at the growth interface was investigated using atomic force microscopy and secondary-ion mass spectrometry. Modulation doped field-effect transistors were fabricated. A maximum current density of 20 mA/mm with a pinch-off voltage of −6 V was achieved on a sample with a 2DEG sheet charge density of 1.2 × 1013 cm−2.

163 citations


Journal ArticleDOI
TL;DR: An effect of learning on associative memory operations is successfully confirmed for several 3 × 3-block patterns and a discussion on the present approach for realizing spintronics-based artificial intelligence is given.
Abstract: We demonstrate associative memory operations reminiscent of the brain using nonvolatile spintronics devices. Antiferromagnet–ferromagnet bilayer-based Hall devices, which show analogue-like spin–orbit torque switching under zero magnetic fields and behave as artificial synapses, are used. An artificial neural network is used to associate memorized patterns from their noisy versions. We develop a network consisting of a field-programmable gate array and 36 spin–orbit torque devices. An effect of learning on associative memory operations is successfully confirmed for several 3 × 3-block patterns. A discussion on the present approach for realizing spintronics-based artificial intelligence is given.

157 citations


Journal ArticleDOI
TL;DR: In this article, ultrasmall blue InGaN micro-light-emitting diodes (µLEDs) with areas from 10−4 to 0.01 mm2 were fabricated to study their optical and electrical properties.
Abstract: Ultrasmall blue InGaN micro-light-emitting diodes (µLEDs) with areas from 10−4 to 0.01 mm2 were fabricated to study their optical and electrical properties. The peak external quantum efficiencies (EQEs) of the smallest and largest µLEDs were 40.2 and 48.6%, respectively. The difference in EQE was from nonradiative recombination originating from etching damage. This decrease is less severe than that in red AlInGaP LEDs. The efficiency droop at 900 A/cm2 of the smallest µLED was 45.7%, compared with 56.0% for the largest, and was lower because of improved current spreading. These results show that ultrasmall µLEDs may be fabricated without a significant loss in optical or electrical performance.

153 citations


Journal ArticleDOI
TL;DR: In this paper, Si-ion implantation doping of the source/drain contacts and access regions was used to achieve enhancement-mode β-Ga2O3 transistors with low series resistance.
Abstract: Enhancement-mode β-Ga2O3 metal–oxide–semiconductor field-effect transistors with low series resistance were achieved by Si-ion implantation doping of the source/drain contacts and access regions. An unintentionally doped Ga2O3 channel with low background carrier concentration that was fully depleted at a gate bias of 0 V gave rise to a positive threshold voltage without additional constraints on the channel dimensions or device architecture. Transistors with a channel length of 4 µm delivered a maximum drain current density (I DS) of 1.4 mA/mm and an I DS on/off ratio near 106. Nonidealities associated with the Al2O3 gate dielectric as well as their impact on enhancement-mode device performance are discussed.

136 citations


Journal ArticleDOI
TL;DR: In this paper, the authors proposed and fabricated a current-driven phase-change optical gate switch using a Ge2Sb2Te5 (GST225) thin film, an indium-tinoxide (ITO) heater, and a Si waveguide.
Abstract: We proposed and fabricated a current-driven phase-change optical gate switch using a Ge2Sb2Te5 (GST225) thin film, an indium–tin-oxide (ITO) heater, and a Si waveguide. Microfabrication technology compatible with CMOS fabrication was used for the fabrication of the Si waveguide. The repetitive phase changing of GST225 was obtained by injecting a current pulse into the ITO heater beneath the GST225 thin film. The switching operation was observed by injecting a 100-ns current pulse of 20 mA into the ITO heater. The average extinction ratio over the wavelength range of 1,525 to 1,625 nm was 1.2 dB.

129 citations


Journal ArticleDOI
TL;DR: In this article, the authors reported silicon delta doping in gallium oxide (β-Ga2O3) grown by plasma-assisted molecular beam epitaxy using a shutter pulsing technique.
Abstract: We report silicon delta doping in gallium oxide (β-Ga2O3) grown by plasma-assisted molecular beam epitaxy using a shutter pulsing technique. We describe the growth procedures that can be used to realize high Si incorporation in an oxidizing oxygen plasma environment. Delta doping was adopted to realize thin (12 nm) low-resistance layers with a sheet resistance of 320 Ω/square (mobility of 83 cm2 V−1 s−1, integrated sheet charge of 2.4 × 1014 cm−2). A single delta-doped sheet of carriers was employed as a channel to realize a field-effect transistor with current I D,max = 236 mA/mm and transconductance g m = 26 mS/mm.

120 citations


Journal ArticleDOI
TL;DR: In this paper, a β-(Al x Ga 1− x )2O3:Si/Ga 2O3 modulation-doped structure was fabricated by direct β-Ga 1−x 2O 3 epitaxial growth on a (010)-β-Ga2O 3 substrate, which exhibited a confined sheet carrier density of 3 × 1012 cm−2.
Abstract: A β-(Al x Ga1− x )2O3:Si/Ga2O3 modulation-doped structure was fabricated by direct β-(Al x Ga1− x )2O3 epitaxial growth on a (010) β-Ga2O3 substrate. Si on the order of 1018 cm−3 from adsorbed contaminants on the substrate surface was doped into the β-(Al x Ga1− x )2O3 layer. The heterojunction interface exhibited a confined sheet carrier density of ~3 × 1012 cm−2, which is on the same order as that of AlGaAs/GaAs. The successful modulation doping for the β-(Al x Ga1− x )2O3/Ga2O3 heterostructure encourages the development of β-Ga2O3-based heterojunction field-effect transistors.

107 citations


Journal ArticleDOI
TL;DR: In this paper, the currentvoltage characteristics of p-n diodes fabricated on a GaN substrate showed distinct rectification at a turn-on voltage of about 3 V.
Abstract: Magnesium ion implantation has been performed on a GaN substrate, whose surface has a high thermal stability, thus allowing postimplantation annealing without the use of a protective layer. The current–voltage characteristics of p–n diodes fabricated on GaN showed distinct rectification at a turn-on voltage of about 3 V, although the leakage current varied widely among the diodes. Coimplantation with magnesium and hydrogen ions effectively suppressed the leakage currents and device-to-device variations. In addition, an electroluminescence band was observed at wavelengths shorter than 450 nm for these diodes. These results provide strong evidence that implanted magnesium ions create acceptors in GaN.

69 citations


Journal ArticleDOI
TL;DR: In this paper, a high activation ratio of Mg ion implantation by conventional rapid thermal annealing (RTA) was demonstrated and the dependence of hole concentration on Mg dose was investigated.
Abstract: A high activation ratio of Mg ion implantation by conventional rapid thermal annealing (RTA) was demonstrated. To obtain the high activation ratio of Mg ion implantation, the dependence of hole concentration on Mg dose was investigated. A maximum hole concentration and a high activation ratio of 2.3% were obtained at a Mg dose of 2.3 × 1014 cm−2 between 9.2 × 1013 and 2.3 × 1015 cm−2. The ratio is, to the best of our knowledge, the highest ever obtained by conventional RTA.

Journal ArticleDOI
TL;DR: In this article, the effects of nitridation on the density of traps at SiO2/SiC interfaces near the conduction band edge were qualitatively examined using a simple, newly developed characterization method that utilizes Hall effect measurements and split capacitance-voltage measurements.
Abstract: The effects of nitridation on the density of traps at SiO2/SiC interfaces near the conduction band edge were qualitatively examined using a simple, newly developed characterization method that utilizes Hall effect measurements and split capacitance–voltage measurements. The results showed a significant reduction in the density of interface traps near the conduction band edge as a result of nitridation, but the interface traps were not completely eliminated by nitridation.

Journal ArticleDOI
TL;DR: The photoluminescences of ion-implanted (I/I) and epitaxial Mg-doped GaN (GaN:Mg) are compared in this paper.
Abstract: The photoluminescences of ion-implanted (I/I) and epitaxial Mg-doped GaN (GaN:Mg) are compared. The intensities and lifetimes of the near-band-edge and ultraviolet luminescences associated with a MgGa acceptor of I/I GaN:Mg were significantly lower and shorter than those of the epilayers, respectively. Simultaneously, the green luminescence (GL) became dominant. These emissions were quenched far below room temperature. The results indicate the generation of point defects common to GL and nonradiative recombination centers (NRCs) by I/I. Taking the results of positron annihilation measurement into account, N vacancies are the prime candidate to emit GL and create NRCs with Ga vacancies, (VGa) m (VN) n , as well as to inhibit p-type conductivity.

Journal ArticleDOI
Na Kou1, Shixing Yu1, Long Li1
TL;DR: In this article, a high-order Bessel vortex beam carrying orbital angular momentum (OAM) is generated by using multilayer amplitude-phase-modulated surfaces (APMSs) at 10 GHz.
Abstract: A high-order Bessel vortex beam carrying orbital angular momentum (OAM) is generated by using multilayer amplitude-phase-modulated surfaces (APMSs) at 10 GHz. The APMS transmitarray is composed of four-layer conformal square-loop (FCSL) surfaces with both amplitude and phase modulation. The APMS can transform a quasi-spherical wave emitted from the feeding source into a pseudo non-diffractive high-order Bessel vortex beam with OAM. The APMS for a second-order Bessel beam carrying OAM in the n = 2 mode is designed, fabricated, and measured. Full-wave simulation and measurement results confirm that Bessel vortex beams with OAM can be effectively generated using the proposed APMS transmitarray.

Journal ArticleDOI
TL;DR: In this article, depletion mode vertical Ga2O3 trench metal-oxide-semiconductor field effect transistors were developed by using n+ contact and n− drift layers, which showed good DC characteristics with a specific on-resistance of 3.7 mΩ cm2 and clear current modulation.
Abstract: We developed depletion-mode vertical Ga2O3 trench metal–oxide–semiconductor field-effect transistors by using n+ contact and n− drift layers. These epilayers were grown on an n+ (001) Ga2O3 single-crystal substrate by halide vapor phase epitaxy. Cu and HfO2 were used for the gate metal and dielectric film, respectively. The mesa width and gate length were approximately 2 and 1 µm, respectively. The devices showed good DC characteristics, with a specific on-resistance of 3.7 mΩ cm2 and clear current modulation. An on–off ratio of approximately 103 was obtained.

Journal ArticleDOI
TL;DR: In this article, the impact of [Mg] on the MOS channel properties was investigated and it was demonstrated that the threshold voltage (V th) can be controlled by [mg along with the theoretical curve, and a maximum field effect mobility of 123 cm2 V−1 s−1 is achieved on [M g] = 6.5 × 1016 cm−3 layer with V th = 3.0 V.
Abstract: Lateral GaN MOSFETs on homoepitaxial p-GaN layers with different Mg doping concentrations ([Mg]) have been evaluated to investigate the impact of [Mg] on MOS channel properties. It is demonstrated that the threshold voltage (V th) can be controlled by [Mg] along with the theoretical curve. The field effect mobility also shows [Mg] dependence and a maximum field effect mobility of 123 cm2 V−1 s−1 is achieved on [Mg] = 6.5 × 1016 cm−3 layer with V th = 3.0 V. The obtained results indicate that GaN MOSFETs can be designed on the basis of the doping concentration of the p-GaN layer with promising characteristics for the realization of power MOSFETs.

Journal ArticleDOI
TL;DR: In this article, the authors successfully fabricated vertical GaN merged PiN Schottky (MPS) diodes and comparatively investigated the cyclic p-GaN width dependence of their electrical characteristics, including turn-on voltage and reverse leakage current.
Abstract: In this study, we successfully fabricated vertical GaN merged PiN Schottky (MPS) diodes and comparatively investigated the cyclic p-GaN width (W p) dependence of their electrical characteristics, including turn-on voltage and reverse leakage current. The MPS diodes with W p of more than 6 µm can turn on at around 3 V. Increasing W p can suppress the reverse leakage current. Moreover, the vertical GaN MPS diode with the breakdown voltage of 2 kV was realized for the first time.

Journal ArticleDOI
TL;DR: In this article, the effect of crystallized ZrO2 seed layer, amorphous Hf0.43Zr0.57O2 (HZO; HZO-seed), and amorphized Al2O3 seed layers on the ferroelectricity of HZ O films was investigated.
Abstract: The effect of crystallized ZrO2 (ZrO2-seed), amorphous Hf0.43Zr0.57O2 (HZO; HZO-seed), and amorphous Al2O3 (Al2O3-seed) seed layers on the ferroelectricity of HZO films was investigated. The remanent polarization () of a TiN-electroded capacitor with a ZrO2-seed layer was much larger than that of capacitors with a HZO-seed, Al2O3-seed, or no seed layer. Furthermore, the maximum 2P r was exhibited when the thickness of the ZrO2-seed layer was 2 nm. Large grain growth was observed, which satisfied the same lattice pattern between ZrO2 and HZO films, and indicates that the ZrO2 seed layer plays an important role in the nucleation of the HZO film.

Journal ArticleDOI
TL;DR: In this article, Nitrogen-doped diamonds were grown on (111) substrates by microwave plasma chemical vapor deposition to perform the selective alignment of high-density nitrogen-vacancy (NV) ensembles with a density greater than 1016 cm−3 and spin relaxation time of 2 µs.
Abstract: Selectively aligning a nitrogen-vacancy (NV) ensemble in diamond is an important technique for obtaining a high-sensitivity magnetic sensor. Nitrogen-doped diamonds were grown on (111) substrates by microwave plasma chemical vapor deposition to perform the selective alignment of high-density NV ensembles, yielding perfectly aligned NV ensembles along the [111] direction with a density greater than 1016 cm−3 and a spin relaxation time of 2 µs. Such alignment results in a high signal contrast with an optical magnetic resonance close to the typical value reported with an isolated NV center. These results indicate the possibility of achieving a high sensitivity through the selective alignment of NV ensembles.

Journal ArticleDOI
TL;DR: In this article, the authors demonstrate highly stable NIR organic light-emitting diodes (OLEDs) based on a system using excitonic energy transfer from thermally activated delay fluorescence molecules to NIR fluorophores, showing an electroluminescence peak at 780 nm and robust operational stability with 2% loss of the initial radiant flux after 1000 h under a constant current density of 10 mA/cm2.
Abstract: We demonstrate highly stable NIR organic light-emitting diodes (OLEDs) based on a system using excitonic energy transfer from thermally activated delay fluorescence molecules to NIR fluorophores. The NIR OLEDs showed an electroluminescence peak at 780 nm and robust operational stability with 2% loss of the initial radiant flux after 1000 h under a constant current density of 10 mA/cm2. The variation of hemoglobin oxygen saturation can be detected using the NIR OLEDs as a light source.

Journal ArticleDOI
TL;DR: In this article, the effects of long-term heat-light soaking on CIGS solar cells with KF postdeposition treatment (KF-PDT) have been investigated.
Abstract: The effects of long-term heat-light soaking on Cu(In,Ga)Se2 (CIGS) solar cells with KF postdeposition treatment (KF-PDT) have been investigated. CIGS solar cells with KF-PDT frequently deteriorate after storage in the dark because of decreasing hole concentrations in the CIGS layers. Although light soaking improves hole concentrations, the resulting increase in conversion efficiency is not significant. In contrast, we found that long-term heat-light soaking effectively increases conversion efficiency, since the saturation current density and ideality factor are clearly improved by passivating the recombination centers at CdS/CIGS interfaces.

Journal ArticleDOI
TL;DR: In this article, the spectral filtering (SF-STAMP) system consists of a 25-beam-generating diffractive optical element, a band-pass filter, and two Fourier transform lenses.
Abstract: We captured ultrafast two-dimensional (2D)-burst images of the crystalline-to-amorphous phase transition of Ge2Sb2Te5. These transitions were induced by a femtosecond laser pulse, and the images, with a sub-picosecond temporal resolution, were acquired on a single-shot basis through the change in local optical transmittance. We employed a 2D-burst imaging method of sequentially timed all-optical mapping photography utilizing spectral filtering (SF-STAMP). The SF-STAMP system consists of a 25-beam-generating diffractive optical element, a band-pass filter, and two Fourier transform lenses. We used a frequency-chirped broadband pulse and achieved 25-frame burst imaging with an interval of 133 fs in a single-shot time window of 3.2 ps.

Journal ArticleDOI
TL;DR: In this paper, high-temperature Hall effect measurements revealed p-type conductivity and a deep acceptor level in the material with an activation energy of 1 eV, in good agreement with density functional theory calculations based on hybrid functionals as presented by the Van de Walle group.
Abstract: GaN crystals were grown by hydride vapor phase epitaxy (HVPE) and doped with C. The seeds were high-structural-quality ammonothermally crystallized GaN. The grown crystals were highly resistive at 296 K and of high structural quality. High-temperature Hall effect measurements revealed p-type conductivity and a deep acceptor level in the material with an activation energy of 1 eV. This is in good agreement with density functional theory calculations based on hybrid functionals as presented by the Van de Walle group. They obtained an ionization energy of 0.9 eV when C was substituted for N in GaN and acted as a deep acceptor.

Journal ArticleDOI
TL;DR: In this paper, an electrically coupled spin-Hall oscillator (SHO) array was proposed for associative memory, in which multiple SHOs exhibit synchronization by interacting with each other through selffeedback spin torques.
Abstract: An oscillator array has been proposed for associative memory, in which the synchronization of multiple oscillators is utilized for pattern-matching operations. An input pattern is represented by a set of frequency shifts of the oscillators and the matching result is attributed to the degree of synchronization. Here, we propose an electrically coupled spin-Hall oscillator (SHO) array in which multiple SHOs exhibit synchronization by interacting with each other through self-feedback spin torques. We numerically demonstrate the pattern matching functionality of the proposed SHO array.

Journal ArticleDOI
TL;DR: In this paper, the authors analyzed the spatio-temporal/spectral coupling in compressors induced by wavefront errors of gratings, which could dramatically distort ultra-intense pulses.
Abstract: Recently, several petawatt (PW, 1015 W) lasers with pulse duration of ~20–30 fs have been introduced throughout the world, pushing the upper limit on laser peak power. However, besides well-known spatio-temporal coupling effects, such as residual spatial/angular chirps and pulsefront tilt/curvature, the spatio-temporal/spectral coupling in compressors induced by wavefront errors of gratings, which could dramatically distort ultra-intense pulses, has been neglected. In this work, for the first time we analyzed this phenomenon and the peak power/intensity degradation induced by it. Our results suggest that the actual performance of femtosecond PW lasers may be worse than previously estimated.

Journal ArticleDOI
TL;DR: In this paper, an Er-doped fiber laser (EDFL) was demonstrated with a MoS2 saturable absorber (SA), achieving a 256 fs, 2 nJ soliton pulse at 1563.4 nm.
Abstract: We demonstrate an Er-doped fiber laser (EDFL) mode-locked by a MoS2 saturable absorber (SA), delivering a 256 fs, 2 nJ soliton pulse at 1563.4 nm. The nonlinear property of the SA prepared by magnetron sputtering deposition (MSD) is measured with a modulation depth (MD) of ~19.48% and a saturable intensity of 4.14 MW/cm2. To the best of our knowledge, the generated soliton pulse has the highest pulse energy of 2 nJ among the reported mode-locked EDFLs based on transition metal dichalcogenides (TMDs). Our results indicate that MSD-grown SAs could offer an exciting platform for high pulse energy and ultrashort pulse generation.

Journal ArticleDOI
TL;DR: In this paper, the authors proposed a metamaterial that behaves like a thermal blackbody at wavelengths from 8 to 16 µm, and more interestingly, is transparent at visible frequencies.
Abstract: We propose a metamaterial that behaves like a thermal blackbody at wavelengths from 8 to 16 µm, and more interestingly, is transparent at visible frequencies. Our design is simple and can be fabricated cost-effectively in large scale. More than 90% absorptivity in far infrared and more than 80% average transmittance at visible frequencies are achieved experimentally with the metamaterial fabricated on an 80-cm-diameter substrate. We believe that this optically transparent infrared absorber with nanoscale thickness is promising for applications such as visible-radiation-controlled infrared absorption or solar cell radiative cooling.

Journal ArticleDOI
TL;DR: In this article, the temperature dependence of barrier height in a Ni/n-GaN Schottky barrier diode fabricated on a GaN homoepitaxial layer was investigated by capacitance, current, and internal photo-emission measurements in the range of 223-573 K.
Abstract: The temperature dependence of barrier height in a Ni/n-GaN Schottky barrier diode fabricated on a GaN homoepitaxial layer was investigated by capacitance–voltage, current–voltage, and internal photoemission measurements in the range of 223–573 K. The barrier height obtained by these methods linearly decreased with increasing temperature. The temperature coefficient was −(1.7–2.3) × 10−4 eV/K, which is about half of the temperature coefficient of the band gap reported previously. This indicates that the decrease in the barrier height may mainly reflect the shrinkage of the band gap (lowering of the conduction band edge) in GaN with increasing temperature.

Journal ArticleDOI
TL;DR: In this article, the vertical Schottky barrier diodes (SBDs) were fabricated from the metalorganic chemical vapor deposition (MOCVD)-grown GaN epitaxial layer on free-standing GaN substrates.
Abstract: Vertical Schottky barrier diodes (SBDs) were fabricated from the metal–organic chemical vapor deposition (MOCVD)-grown GaN epitaxial layer on free-standing GaN substrates. It was found that the quality of GaN drift layers and SBD properties were strongly dependent on the growth rates. The step-flow surface morphology, near-unity ideality factor (n ~ 1.04), and high Schottky barrier height (~0.97 eV) were achieved at a relatively low growth rate of 2.61 µm/h. An extremely low turn-on voltage (0.73 V), together with a low on-resistance of 0.72 mΩcm2, was obtained.

Journal ArticleDOI
TL;DR: In this article, the anomalous Nernst coefficient (ANE) in highly ordered Fe4N films was examined in the measurement configuration with an in-plane temperature gradient (∇Tx ).
Abstract: The anomalous Nernst effect (ANE) in highly ordered Fe4N films was examined in the measurement configuration with an in-plane temperature gradient (∇Tx ). The anomalous Nernst coefficient (Q s) in the case of ∇Tx ∥ Fe4N[110] was clearly larger than that in the case of ∇Tx ∥ Fe4N[100]. Note here that no reports on the Q s in Fe4N films have been presented. This anisotropic Q s (Q ani) was also observed in a different configuration with an out-of-plane temperature gradient. Unlike Fe4N, pure Fe and A2-FeAl systems showed a negligible Q ani. This Q ani might be attributed to the specific hybridization effects between the 3d orbitals of Fe and the 2p orbitals of N.