Impurity Modes Research Articles

Optical studies on GaN-based spintronics materials

Structural properties of GaN layers doped with magnetic impurities (Mn, Cr) were investigatedby Raman scattering. In the case of Mn-doping, an impurity mode was observed at around585 cm−1 with concentrations of Mn up to 1–2%. This mode was assigned to a local vibrational modeof Mn substituting the Ga site, and interpreted as spectral evidence of a uniform solidsolution in the samples. For Cr-doping, good crystalline quality was also confirmed up to[Cr] = 3–5%. Cr is more miscible in GaN than Mn. Resonance enhancement of LO phonon signal wasobserved in GaCrN layers when excited by a deep UV laser at 266 nm (4.7 eV). Thisindicates generation of photo-carriers.

Raman scattering studies on ZnO doped with Ga and N (codoping), and magneticimpurities

ZnO layers doped simultaneously with Ga and N (codoping), and magneticelements (V, Co) were characterized by Raman scattering to studytheir structural stability. Five impurity modes were observed in range200–900 cm−1 in the doped samples, and showed characteristic variation with the doping level. It is shownthat these modes can be used as a good measure of lattice defects induced by doping. TheRaman spectra showed that the magnetic elements were incorporated up to 5 mol% withoutserious deterioration in crystallinity.

Raman studies on spintronics materials based on wide bandgap semiconductors

Structural properties of GaN and ZnO layers doped with magnetic impurities wereinvestigated by Raman scattering. Long-range lattice ordering and local atomicarrangement around magnetic impurities were analysed, and their solubility limit wasconsidered.For this study, GaN layers doped with Mn and Cr, and ZnO layers doped with Coand V were prepared by molecular beam epitaxy and pulsed laser deposition,respectively. ZnO layers codoped with Ga and N were also studied for the purposeof p-type activation. These samples were observed using a Raman microprobeusing visible and deep UV lasers for excitation. The main results are as follows:In Ga1−xMnxN layers, a uniform solid solution was formed for Mn concentration up tox = 1–2%. An impuritymode was observed at 585 cm−1 and assigned to a local vibrational mode of Mn substituting the Ga site. Athigher Mn concentrations, rapid deterioration in lattice ordering occurred.Ga1−xCrxN layers showed goodlattice ordering up to x = 3–5%. The samples showed resonance enhancement of LO-phonon signals when excited by aUV laser at 266 nm (4.7 eV). This indicates the photo-injection of free carriers to a dilutedmagnetic semiconductor.ZnO layers codoped with Ga and N showed many impurity modes due to host lattice defects. A strongsignal at 580 cm−1 showed a characteristic broadening at high concentrations of N and Ga.This suggested the formation of complex centres with N or related defects.Zn1−xCoxO and Zn1−xVxO layers formed uniform solid solutions up to , but precipitation of the secondary phase was observed at . These samples presented common defect modes as observed in codoped samples. Ourresult suggests that the impurity modes in ZnO-based materials can be used as asensitive probe of host lattice defects induced by the impurity incorporation process.

Investigation of optical and electrical properties of Mg-doped p-In xGa 1−xN, p-GaN and p-Al yGa 1−yN grown by MOCVD

Mg-doped p-In x Ga 1− x N ( 0 ⩽ x ⩽ 0.045 ) and p-Al y Ga 1− y N ( 0 ⩽ y ⩽ 0.095 ) layers were grown on sapphire substrates by metalorganic chemical vapor deposition (MOCVD). We investigated the activation energy of Mg dopant in Mg-doped p-InGaN, p-GaN and p-AlGaN, as confirmed by PL emission lines of the band to impurity transition of free electrons with neutral Mg acceptors. Under the same Mg doping concentration of 2×10 19 cm −3, the band to acceptor emission energy E(e −, A 0) of Mg-doped p-type In x Ga 1− x N decreased with increasing the In composition, whereas the E(e −, A 0) of Mg-doped p-type Al y Ga 1− y N increased with Al composition. It suggests that the activation energy of Mg dopant has proportional relationship with the band gap energy of III-nitrides. The Hall measurements showed that the hole concentration of Mg-doped p-type In 0.045Ga 0.955N was 1.3×10 18 cm −3 and that of Mg-doped p-type Al 0.075Ga 0.925N was 8.9×10 16 cm −3.

Terahertz emission from electrically pumped gallium doped silicon devices

Current pumped terahertz (THz) emitting devices have been fabricated from gallium doped silicon. The time resolved peak power was 12μW per facet at a peak pumping current of 400mA, and the emission was observed up to temperatures near 30K. The spectra occurred in two distinct series at 7.9–8.5THz, and at 13.2–13.8THz. The emission was attributed to the radiative transitions of holes from the split sublevels of the 1Γ8 excited state to the sublevels of the 1Γ8+ ground state and the 1Γ7+ ground state, yielding an energy separation of 22±0.07meV between the two ground states. These results indicated that emitters based on Ga impurity transitions open up a range of THz frequencies, and the properties of their spectra can improve the understanding of impurity level physics.

Resonant excitation of off-channel localized impurity modes by a photonic crystalwaveguide

A theoretical treatment is presented of the electromagnetic transmission properties of aphotonic crystal waveguide that interacts with off-channel dielectric impurities, impurityclusters, neighbouring waveguides, or neighbouring waveguide networks. The photoniccrystal studied is a two-dimensional square lattice array of parallel axis dielectric cylinders(formed of linear dielectric medium) in vacuum, and impurities and waveguides arecreated by cylinder replacement. The waveguide whose transmission is calculated isformed from linear dielectric media, but the off-channel features that it interactswith may be formed from either linear or Kerr nonlinear dielectric media. Theoff-channel features may also be composed of dissipative or amplifying media.Waveguide transmission resonances, associated with resonant scattering fromelectromagnetic modes on the off-channel features, are found. Modes present onoff-channel features formed from linear dielectric media include both propagating andlocalized modes. Modes present on off-channel features formed from Kerr nonlineardielectric media include simple localized and intrinsic localized (soliton-like) modes.

Effects of buffer layer and nitrogen purification on optical properties of MBE-grown GaN on sapphire(0001)

The optical properties and the surface morphology of molecular beam epitaxy grown GaN on sapphire(0001) have been studied. The samples were grown under various growth conditions, such as Ga-flux, different buffer layer and different degree of oxygen contamination from the nitrogen gas. High-resolution scanning electron microscopy and reflection high-energy electron diffraction revealed information about the surface morphology. Low-temperature photoluminescence was used for the characterisation of the step-by-step improved optical properties to reveal impurity and defect related transitions. The results clearly show that the linewidth of the GaN emission gradually improved by increasing the Ga-flux while keeping all other growth parameters unchanged. We also found that both the oxygen and carbon contaminations from the nitrogen source and the resulted morphology are directly related to the linewidth of the spectra.

Understanding spectroscopic phonon-assisted defect features in CVD grown 3C-SiC/Si(1 0 0) by modeling and simulation

New phonon-assisted defect features are observed using photoluminescence (PL) and Raman scattering spectroscopy on 3C-SiC/Si(1 0 0) films grown by chemical vapor deposition (CVD) technique. The ultraviolet excitation room-temperature (RT) PL-Raman spectra show a luminescence band near 2.3 eV due to RT recombination over the 3C-SiC indirect band gap. In addition to the strong Raman lines characteristic of Si substrate and 3C-SiC we also observed weaker impurity modes near 620, 743 and 833 cm −1. These frequencies are compared with the results of Green's function simulations of impurity modes with plausible defect structures to best support the observed Raman features as well as modes of some prototypical defect center.

Electroluminescence at 7 terahertz from phosphorus donors in silicon

Terahertz (THz) emissions corresponding to intracenter transitions of phosphorus impurities in silicon have been observed up to 30K. Electrical pulses (250ns) with a repetition rate of 413Hz were used for excitation, and the peak power was calculated to be ∼20μW∕facet for a 190×120μm2 device with a peak pumping current of 400mA at 12K. THz emission intensity increased linearly with pumping current and quenched when the sample temperature was above 30K. The current–voltage characteristics suggested a conduction and excitation mechanism by injection of electrons from a Schottky barrier followed by impact ionization of the neutral impurities.

Quantum phase transition of Ising-coupled Kondo impurities

We investigate a model of two Kondo impurities coupled via an Ising interaction. Exploiting the mapping to a generalized single-impurity Anderson model, we establish that the model has a singlet and a (pseudospin) doublet phase separated by a Kosterlitz-Thouless quantum phase transition. Based on a strong-coupling analysis and renormalization-group arguments, we show that at this transition the conductance $G$ through the system either displays a zero-bias anomaly, $G\ensuremath{\sim}{\ensuremath{\mid}V\ensuremath{\mid}}^{\ensuremath{-}2(\sqrt{2}\ensuremath{-}1)}$, or takes a universal value, $G=({e}^{2}∕\ensuremath{\pi}\ensuremath{\hbar}){\mathrm{cos}}^{2}(\ensuremath{\pi}∕2\sqrt{2})$, depending on the experimental setup. Close to the Toulouse point of the individual Kondo impurities, the strong-coupling analysis allows us to obtain the location of the phase boundary analytically. For general model parameters, we determine the phase diagram and investigate the thermodynamics using numerical renormalization-group calculations. In the singlet phase close to the quantum phase transition, the entropy is quenched in two steps: first the two Ising-coupled spins form a magnetic minidomain which is, in a second step, screened by a Kondoesque collective resonance in an effective solitonic Fermi sea. In addition, we present a flow-equation analysis which provides a different mapping of the two-impurity model to a generalized single-impurity Anderson model in terms of fully renormalized couplings, which is applicable for the whole range of model parameters.

Excitonic transitions in β-FeSi2 epitaxial films and single crystals

Photoreflectance spectra were obtained from an epitaxial film and a bulk single crystal of β-FeSi2 at low temperatures (T⩽180 K). A model based on the results of low-temperature absorption [M. Rebien et al., Appl. Phys. Lett. 74, 970 (1999)] was used to describe the main features of the spectra. In agreement with the absorption results, transitions corresponding to the ground state and first excited state of the free exciton were observed in both the epitaxial film and single crystal. However, additional subband gap features are revealed in the photoreflectance spectra of the thin film. It is suggested that these may be related to impurity transitions or an impurity transition plus a bound exciton resonance. From the analysis of the spectra taken on the thin film, over a temperature range of 12–180 K, we extract a free exciton binding energy of (0.009±0.002) eV and a direct energy gap at T=0 K of (0.934±0.002) eV.

Strongly Localized Singular Bloch Modes Created in Dual-Periodic Microstrip Lines

One-dimensional dual-periodic photonic crystals were fabricated using periodic microstrip lines. Singular Bloch modes with an extremely high Q factor of 170 in the crystals were successfully obtained in the bandgap region. The measured transmission and reflection spectra were in very good accordance with those calculated by the method of moments. The spatial distribution of the electrical current in the singular Bloch modes of the electromagnetic wave was simulated using the method of moments, and the localization of the singular Bloch modes was clearly evident. The spatial distribution pattern of the electrical current in the modes was not localized in the restricted area of the line. This was not similar to that in impurity modes for a simple periodic photonic crystal, that is, the pattern expanded over the entire line.

On the magnetic hyperfine fields at nd impurities in Gd and Tb hosts

In this work we study the magnetic hyperfine fields at nd ( n=3, 4, 5) transition impurities diluted in ferromagnetic Gd and Tb hosts. The nd impurities are described within the Wolff–Clogston picture and the next-neighbor perturbation, due to the translational invariance breaking introduced by the impurity is also included in this work. The calculated hyperfine fields are in good agreement with the observed experimental data.

Relaxation Modes of Classical Hopping Protons in Perovskite Structures I

The relaxations by proton hopping in a perovskite lattice are theoretically investigated: the 24 sites available for protons in the primitive unit cell give rise to the 24 relaxation branches composed of 1 diffusive ( ε =1) and 23 nondiffusive bands ( ε =2–24). It turns out that two lowest nondiffusive modes ε =2,3, being degenerate at q =0, can work for a radio-frequency dielectric function. On the other hand, the cell including a dopant results in impurity modes when proton has a deep potential energy as a trapping state. The impurity modes consist of five radio frequency modes, two- and three-degenerate modes. It is discussed how these relaxation modes and impurity modes get involved in proton diffusions or two-peaks mechanism of the dielectric loss, experimentally observed.

Electronic transition of vanadium impurities in different alkali hosts

Abstract Thin quench-condensed films of Na, K, Rb and Cs are covered with 1 100 of a monolayer of V. Then the impurities are covered with several atomic layers of the host. Measured by means of the anomalous Hall effect, V impurities show a magnetic moment of 6– 7 μ B in Na and 4 μ B in Cs. This contradicts the favored atomic model (predicting 3 5 μ B ). Furthermore, the sign of the anomalous Hall resistance changes from negative in Na to positive in Cs. This indicates a transition of the electronic structure of the impurity and the host environment.

The Emission of Terahertz Radiation from Doped Silicon Devices

ABSTRACTRecent interest in biological imaging, remote sensing, and biochemical spectroscopy has made nanoscale Terahertz (THz) emitters based on semiconductors become extremely attractive. THz lasers with good performance (emitting over 1 milliWatt CW at 10 K) have been fabricated from the group III-V compounds by molecular beam epitaxy, but such devices are complex with over 500 quantum wells and barriers and are incompatible with low cost Si processing. Similar devices from the SiGe system have had difficulties because the strain limits the total number of active layers, and have produced relatively poor performance (peak powers near 5 nW), even using strain symmetric active regions on virtual substrates of relaxed SiGe buffers. We report here on the characteristics of a new type of THz emitting device with higher powers (above 0.1 milliWatt) based on radiative impurity transitions in doped silicon, which can be simply fabricated without Ge alloying.The THz emitters were fabricated from both p-type and n-type doped silicon wafers with typical resistivities of 5 Ω-cm, using conventional photolithography and metal contact lift off. Samples were electrically pulsed and the electroluminescence was measured by Fourier Transform Infrared spectrometry. At 4.2 K, the emission spectra showed several peaks centered around 8.1 THz for the boron doped device, 7 to 13 THz for the gallium doped device, and 6.6 THz for the phosphorus doped device. The electroluminescence was attributed to radiative transitions from excited p-like to s-like hydrogenic dopant states, with emission energies in remarkable agreement with the known dopant absorption levels. Current-voltage measurement suggested an excitation mechanism based on impact ionization of neutral dopants by hot carriers. The net quantum efficiency (emitted photons per injected electron) was estimated to be up to 2 × 10-3. The temperature dependence will be reported with operation above 30 K.

Transition metal ion implantation into AlGaN

n- and p-type AlxGa1−xN (x=0.38 for n-type, x=0.13 for p-type) layers grown on Al2O3 substrates were ion implanted with the transition metals Mn, Cr, and Co at high concentrations (peak doping levels ∼3 at. %). After implantation and annealing at 1000 °C, only impurity transitions at ∼2.9 and 3.9 eV and no band-edge photoluminescence could be observed in all the samples. X-ray diffraction did not detect any peaks associated with second phase formation. Room-temperature hysteresis loops were obtained for Co-implanted n-type AlGaN, while there was no convincing evidence for ferromagnetism in the Mn- or Cr-implanted n-AlGaN. By sharp contrast, Mn implantation in p-AlGaN did produce ferromagnetic behavior and 300 K hysteresis. Both carrier type and crystalline quality can influence the resulting magnetic properties.

Properties of Co-, Cr-, or Mn-implanted AlN

AlN layers grown on Al2O3 substrates by metalorganic chemical vapor desposition were implanted with high doses (3×1016 cm−2, 250 keV) of Co+, Cr+, or Mn+. Band-edge photoluminescence intensity at ∼6 eV was significantly reduced by the implant process and was not restored by 950 °C annealing. A peak was observed at 5.89 eV in all the implanted samples. Impurity transitions at 3.0 and 4.3 eV were observed both in implanted and unimplanted AlN. X-ray diffraction showed good crystal quality for the 950 °C annealed implanted samples, with no ferromagnetic second phases detected. The Cr- and Co-implanted AlN showed hysteresis present at 300 K from magnetometry measurements, while the Mn-implanted samples showed clear loops up to ∼100 K. The coercive field was <250 Oe in all cases.

The fabrication of photonic band gap materials with a two-dimensional defect

Colloidal crystals with three-dimensional periodicities in the refractive index have a photonic band gap (PBG) in which electromagnetic waves are forbidden. We present a method to fabricate stacked colloidal crystals containing a two-dimensional defect as a middle layer by combining vertical deposition method with the Langmuir–Blodgett (LB) technique. The defect layer introduces an impurity mode within the optical stop band, which is observed as a defect peak (pass band) in the optical density spectrum. The result shows that the combination of vertical deposition with LB technique provides a way for introducing defect modes in PBG materials.

Impurity absorption spectroscopy in 28Si: the importance of inhomogeneous isotope broadening.

We report high-resolution infrared absorption spectra of the neutral donors phosphorus and lithium, and the neutral acceptor boron, in isotopically pure 28Si crystals. Surprisingly, many of the transitions are much sharper than previously reported in natural Si. In particular, the 2p(0) line of phosphorus in 28Si has a full width at half maximum of only 4.2 microeV, about 5 times less than the narrowest 2p(0) line previously reported for natural Si, making it the narrowest shallow impurity transition yet observed. The widely held assumptions that the impurity transitions previously reported in high quality samples of natural Si revealed the true, homogeneous linewidths, are thus shown to be incorrect. The sharper transitions in 28Si also reveal new substructures in the boron and lithium spectra.