Intracenter Transitions Research Articles

Emission intensity of the λ = 1.54 μm line in ZnO films grown by magnetron sputtering, diffusion doped with Ce, Yb, Er

The effect of the Er3+-ion excitation type on the photoluminescence spectra of crystalline ZnO(ZnO〈Ce, Yb, Er〉) films is determined in the cases of resonant (λ = 532 nm, Er3+-ion transition from 4S3/2, 2H11/2 levels to 4I15/2) and non-resonant (λ = 325 nm, in the region near the ZnO band-edge emission) excitation. It is shown that resonant excitation gives rise to lines with various emission intensities, characteristic of the Er3+-ion intracenter 4f transition with λ = 1535 nm when doping crystalline ZnO films with three rare-earth ions (REIs, Ce, Yb, Er) or with two impurities (Ce, Er) or (Er, Yb), independently of the measurement temperature (T = 83 and 300 K). The doping of crystalline ZnO films with rare-earth impurities (Ce, Yb, Er) leads to the efficient transfer of energy to REIs, a consequence of which is the intense emission of an Er3+ ion in the IR spectral region at λmax = 1535 nm. The kick-out diffusion mechanism is used upon the sequential introduction of impurities into semiconductor matrices and during the postgrowth annealing of the ZnO films under study. The crystalline ZnO films doped with Ce, Yb, Er also exhibit intense emission in the visible spectral region at room temperature, which makes them promising materials for optoelectronics.

Behavior of the Fe impurity in Hg3In2Te6 crystals

Optical and photoelectric measurements reveal that doping with iron leads to the formation of a deep level at Ec–0.69 eV in Hg3In2Te6 crystals. When light is absorbed by Fe2+ impurity centers, both electronic transitions of the impurity-level–conduction-band type and optical transitions between ground and excited states of the aforementioned centers (intracenter transitions) are observed. Investigations of transport phenomena point to the acceptor properties of Fe2+ centers.

Spectral dependences of extrinsic optical absorption in sillenite crystals

The influence of laser irradiation at wavelengths of 532 and and annealing in air at temperatures from 200 to on optical absorption spectra of undoped bismuth silicon oxide and bismuth germanium oxide and aluminium-doped bismuth titanium oxide crystals has been studied experimentally. The experimental data have been interpreted in terms of a model for extrinsic absorption that takes into account not only the contribution of the photoexcitation of electrons from deep donor centres with a normal distribution of their concentration with respect to ionisation energy but also that of intracentre transitions.

Energy transfer from Tb3+ to Eu2+ in Ga2S3:(Eu2+, Tb3+) crystals

The photoluminescence of Ga2S3 crystals activated with Eu2+ and Tb3+ ions separately and with ions of both types is studied in the temperature range 77–300 K. It is established that, in the range 77–300 K, the observed broadband photoluminescence in (Ga2S3)0.95:(Eu2O3)0.05 crystals with a peak at 545 nm is defined by 4f 65d-4f 7(8 S 7/2) intracenter transitions in Eu2+ ions and the photoluminescence with peaks at 492, 544, 584, 625, and 680 nm in (Ga2S3)0.99(Tb2O3)0.01 crystals is due to the 5d → 2 F j (j = 6−2) intracenter transitions in Tb3+ ions. It is shown that the photoluminescence bands of Tb3+ ions in the (Ga2S3)0.94(Eu2O3)0.05(Tb2O3)0.01 crystals disappears because of excitation energy transfer from Tb3+ ions to Eu2+ ions; i.e., the Tb3+ ion is a sensitizer of the photoluminescence of the Eu2+ ion.

Transition Metal Ions in Semiconductors: LDA, LDA+U, and Experiment

and particularly drastic in transition metal (TM) oxides. A considerable improvement is obtained by adding the +U correction for particular atomic orbitals. While the impact of +U terms was extensively discussed for ideal crystals, its impact on the electronic structure of defects is less understood. We analysed the impact of the +U term for Cr, Mn, Fe, and Co ions in GaN and AlN. The +U term was treated as a free parameter, and it was applied to p(N) and d(TM) orbitals. The results of LDA+U calculations were compared to available experimental data. The band gap of GaN is correct with U(N)=4 eV. The +U terms strongly aect the electronic structure of TM impurities. Surprisingly, for U(TM)=0, the energies of the gap levels induced by these centers, and of the intra-center optical transitions, agree well with experiment. In contrast, for U(N)=U(TM)=4 eV, these energies are in substantial disagreement with experimental values by about 1-2 eV.

Terahertz intracenter photoluminescence of silicon with lithium at interband excitation

Terahertz radiation has been revealed at interband photoexcitation of lithium-doped silicon crystals at liquid helium temperatures. It has been shown that the lines caused by optical transitions of electrons from the 2P excited states of lithium centers to the 1S(A1) state of the impurity prevail in the radiation spectrum. The strong suppression of terahertz radiation associated with transitions to the lowest state of the donor 1S(E + T2) as compared to radiation associated with transitions to the 1S(A1) state is explained by the reabsorption of radiation. The radiation spectrum also includes weaker terahertz lines, which can be attributed to the intracenter transitions in donors that are caused by Li-O complexes. The radiation spectrum also exhibits lines at ∼12.7 and ∼15.3 meV, which are possibly due to intraexcitonic radiative transitions and transitions from continuum to the ground state of excitons.

Terahertz lasers based on intracentre transitions of group V donors in uniaxially deformed silicon

This paper presents a brief overview of available experimental data on the characteristics of stimulated terahertz emission from optically excited neutral group V donors (phosphorus, antimony, arsenic and bismuth) in crystalline silicon subjected to uniaxial compressive strain along the [100] axis. Strain is shown to have a significant effect on the characteristics in question. Optimal strain depends on the dopant and may reduce the threshold pump intensity and improve lasing efficiency. We discuss possible mechanisms behind this effect and estimate the limiting output emission parameters.

Far- and near-infrared photoluminescence from n-GaAs/AlGaAs multiple quantum wells

The results of experimental investigations of impurity-assisted photoluminescence at low temperatures from n-GaAs/AlGaAs multiple quantum wells both in near- and far-infrared (terahertz) spectral ranges are presented. The optical electron transitions from impurity ground state to heavy hole subband in near-infrared spectral range are revealed. The depopulation of the donor ground state due to these transitions allowed us to observe photoluminescence in terahertz spectral range related to electron transitions from the first electron subband to donor ground state as well as to intracenter optical transitions. Experimental results in far- and near-infrared spectral ranges are in good agreement with the results on THz photoconductivity and energy spectrum calculation.

THz photoluminescence from n-GaN layers at CW interband excitation

We report on experimental observation and studies of terahertz emission from n-doped GaN films under continuous wave interband photoexcitation at low temperatures. Properties of the terahertz emission testify that the emission is caused by intracenter optical transitions in shallow donors of GaN. The THz intracenter optical transitions are initiated by the interband photoexcitation.

Current injection induced terahertz emission from 4H-SiC p-n junctions

We report on current injection induced terahertz electroluminescence from 4H-SiC p-n junctions with operating temperature up to 270 K. The emission is assigned to intracenter optical transitions in donor centers, initiated by the injection of non-equilibrium carriers into the n-doped region of a SiC p-n junction. At a pumping current of 300 mA at 100 K, the integrated output power was 58 μW from the device surface with an area of 3 mm2. These results suggest that THz emitting devices can be fabricated with simple structures of SiC p-n junctions, with relatively high operating temperatures and reasonable output powers.

Vacancy-Related Defects in Ge Doped with Tin

It has been found that isolated V20and V20localized near tin atoms are formed in Ge doped with tin. Simultaneously with V20annealing, the appearance of absorption spectra consisting of sharp lines was observed. The defect to which the spectra found corresponds has hydrogen-like properties. The distances between the lines in spectrum are in good agreement with those predicted by effective-mass theory. The formation of the defect found does not depend on oxygen concentration. An appearance of Fano resonance in the region of continuum was detected in addition to intracenter transitions of the defect. The defect found was identified as SnV20Ga.

Absorption and photoionization of the donor level in CdF2 semiconductor crystals

A model of strong vibronic interaction is proposed to interpret the specific features of infrared absorption and photoionization in CdF2 semiconductor crystals. The model takes into account the polaronic nature of the conductivity in these crystals and the profound configuration shift of the free and bound polaron states. It is shown that the intense infrared absorption band in the crystals is not due to the transitions of charge carriers from hydrogen-like donor levels to the conduction band, but is caused by the phonon replicas of intracenter transitions. The low-temperature photoconductivity (in the temperature range 0–70 K) is a result of tunneling transitions between the phonon states of bound and free polarons, since these states are separated by rather high potential barriers. Overcoming the barriers in both directions is responsible for equilibration in the polaron subsystem upon the photoexcitation of charge carriers. The tunneling character of this process is responsible for the slight variation in the equilibration time in the above-indicated temperature range.

Intra-center and recombination luminescence of bismuth defects in fused and unfused amorphous silica fabricated by SPCVD

Photoluminescence (PL) of bismuth doped silicon dioxide excited by UV excimer lasers (ArF — 193nm, KrF — 248nm) and a green light laser diode (532nm) is studied in a wide spectral band at temperatures ranging from 12 to 750K. Two types of samples are investigated: unfused, 100μm in thickness amorphous layer immediately deposited on the inner surface of silica substrate tube, and the same material after profusion resulted from tube collapsing to a rod by external heating. PL bands centered at 620–650nm, 820nm and 1400nm wavelengths are observed in both fused and unfused samples. Under excitation by the green laser diode decay time constants for 650nm (orange) and 1400nm (NIR) PL bands measured at room temperature amount 3μs and 600μs respectively. These rather long decay times point to partly forbidden intra-center electron transitions. PL intensities of the orange and NIR bands are not temperature dependent within 12–450K range. At higher temperatures the orange band manifests an intra-center thermal quenching, activation energy and frequency factor being 0.42±0.04eV and 5·109s−1 respectively, while intensity of the NIR band weakly depends on temperature up to 700K. Electron-hole recombination excitation mechanism is found to contribute to the orange, but not to the NIR PL band under UV laser pumping. The specific feature of orange PL excited by recombination is the magnitude of decay time constant being about milliseconds at temperature of 12K and decreasing with the temperature increase. Localized state ionization and two-photon absorption of intense UV light excite electron-hole pairs in silica host. Some bismuth defects serve as traps for the electrons, while holes transmute into self-trapped state thus generating self-trapped holes (STHs). Thermally activated escape of the STHs followed by their subsequent recombination with trapped electrons forms the mechanism to transfer the excitation to particular bismuth defects responsible for orange PL. At the same time no signature of the impact of such recombination process on the excitation of NIR PL is observed. This permits one to conclude that the nature of bismuth defects responsible for these two PL bands varies, and these two types of defects are available in both fused and infused silicon dioxides.

The physical principles of terahertz silicon lasers based on intracenter transitions

AbstractThe first silicon laser was reported in the year 2000. It is based on impurity transitions of the hydrogen‐like phosphorus donor in monocrystalline silicon. Several lasers based on other group‐V donors in silicon have been demonstrated since then. These lasers operate at low lattice temperatures under optical pumping by a midinfrared laser and emit light at discrete wavelengths in the range from 50 to 230 µm (between 1.2 and 6.9 THz). Dipole‐allowed optical transitions between particular excited states of group‐V substitutional donors are utilized for donor‐type terahertz (THz) silicon lasers. Population inversion is achieved due to specific electron–phonon interactions of the impurity atom. This results in long‐living and short‐living excited states of the donor centers. Another type of THz laser utilizes stimulated resonant Raman‐type scattering of photons by a Raman‐active intracenter electronic transition. By varying the pump‐laser frequency, the frequency of the Raman intracenter silicon laser can be continuously changed between at least 4.5 and 6.4 THz. The gain of the donor and Raman‐type THz silicon lasers is of the order of 0.5 to 10 cm−1, which is similar to the net gain realized in THz quantum cascade lasers and infrared Raman silicon lasers. In addition, fundamental aspects of the laser process provide new information about the peculiarities of electronic capture by shallow impurity centers in silicon, lifetimes of nonequilibrium carriers in excited impurity states, and electron–phonon interaction.

Luminescence of Li6Gd(BO3)3 crystals upon ultraviolet and inner-shell excitations

The paper presents the results of the study on luminescence and electronic excitations in Li6Gd(BO3)3 single crystals. The optical and luminescence spectroscopy with a sub-nanosecond time resolution upon selective photoexcitation in the energy range from 3.0 to 650eV was used to investigate in detail the luminescence of both the Gd3+ host ions and Ce3+ impurity ions as well as the processes of energy transfer between them. The intrinsic ultraviolet emission at 3.95–3.97eV due to Pj6→S7/28 transitions in the Gd3+ host ion and the fast luminescence at 2.8–3.0eV due to 5d→4f1 dipole-allowed transitions in the Ce3+ impurity ion were studied upon excitations through the intracenter, charge transfer, band-to-band, and inner-shell transitions. The specificity of the energy transfer upon excitation of photoluminescence in the energy range of absorption of the inner shells of different atoms of the crystal, as well as in the energy region of the giant resonance of 4d–4f transitions was revealed. On the basis of the experimental data, the bandgap Eg=9.3eV, the minimum energy of the 5d→4f1 and charge transfer transitions were determined. The expected positions of the ground states of the 4fn and 4fn−15d levels of trivalent lanthanide ions as well as the 4fn+1 and 4fn5d levels of divalent ions were calculated for all the lanthanide ions in Li6Gd(BO3)3 host crystal.

Structure and luminescence of gadolinium-doped cubic boron nitride powder

The structural characteristics and chemical, morphological, and optical properties of cBN and cBN:Gd micropowders are studied by x-ray diffraction, energy-dispersive electron probe microanalysis (x-ray spectral microanalysis), and photoluminescence techniques. Cubic boron nitride (cBN) micropowders were synthesized at high pressures and temperatures from hexagonal boron nitride (hBN) micropowder and Li3N catalyst. cBN:Gd micropowders were synthesized from mixtures of hBN, Li3N, and GdF3 micropowders. A lattice parameter of a~3.615 A is calculated for both types of powder (cBN and cBN:Gd). The photoluminescence spectra of the cBN:Gd powder are found to contain emission lines attributable to intracenter optical transitions of Gd3+ ions.

Intensity of emission from intracenter 4f-transitions in a-Si:H, ZnO, and GaN films doped with rare-earth ions

It is shown that the intensity of emission from intracenter 4f-transitions in amorphous a-Si:H films and crystalline (GaN, ZnO) films doped with rare-earth ions is governed by the local environment of doping impurity ions. In the case of a-Si:H, a pseudo-octahedron with the C4V point group is present due to nanocrystallites, which provides a local environment for rare-earth ions. In the case of a hexagonal crystal lattice in crystalline GaN and ZnO films, the local symmetry of rare-earth ions introduced into the semiconductor matrix by diffusion, with a pseudo-octahedron with the C4V point group, is formed by stresses due to rare-earth ion-oxygen complexes with a radius exceeding that of host ions incorporated at crystal lattice sites. In contrast to GaN films, ZnO films exhibit, on being doped with Tm, Sm, and Yb, both high-intensity emission in the long-wavelength spectral region, characteristic of intracenter 4f transitions in rare-earth ions, and a substantial increase in intensity in the short-wavelength spectral region (λ = 368–370 nm). GaN films doped with rare-earth ions exhibit in this spectral range only an inhomogeneously broadened emission spectrum due to the presence of an emission band characteristic of donor-acceptor recombination.

Luminescent vacuum ultraviolet spectroscopy of Cr3+ ions in nanostructured aluminum oxide

Impurity Cr3+ centers in submicron and nanostructured Al2O3 crystals of different phase compositions at temperatures of 300 and 7.5K were studied by a luminescent vacuum ultraviolet (VUV) spectroscopy method. Photoluminescence (PL) spectra and the energies of 2E, 4T2, and 4T1 excited states of Cr3+ ion depend on the type of crystalline samples phase. The PL excitation spectrum of R-line in α-Al2O3 nanoscale crystals is formed by intracenter transitions (2.5–5.5eV region), by charge transfer band (6.9eV) and by effective formation of impurity-bound excitons (9.0eV region). Such impurity-bound excitons correspond to O2p→Al3s electron transition in surroundings of an impurity Cr3+ center. The efficiency of impurity-bound excitons formation decreases with the increase of the grain size above 100nm. The size dependence is noticeably shown in PL excitation spectra in VUV region. Excitons bound to impurity centers do not appear in nanostructured δ+θ-Al2O3 crystals. The effect of the electron excitation multiplication is observed distinctly in nanostrucured α-Al2O3 at an excitation energy above 19eV (more than 2Eg).

Photoluminescence spectra of intracenter 4f transitions of rare-earth metal dopants in crystalline ZnO films

The influence of annealing in the medium of an ionized nitrogen, additionally introduced impurities, and regimes of post-growth annealing on photoluminescence spectra of intracenter 4f transitions of rare-earth metal dopants (Ce, Eu, Sm, Er, Tm, Yb) in crystalline ZnO films has been investigated. The films have been prepared using molecular-beam epitaxial growth and magnetron sputtering. According to the X-ray diffraction analysis, the films have a single-crystal structure. It has been shown that the annealing in the medium of an ionized nitrogen, regardless of the method used for producing the ZnO films, leads to significant changes in the photoluminescence spectrum, i.e., to a decrease in the emission intensity and a shift in the position of the emission maximum toward the long-wavelength range of the spectrum. The nitrogen concentration has been determined by the nuclear reaction method. It has been revealed that the spectra contain intense emission lines due to the intracenter 4f transitions of rare-earth elements (Sm, Er, Tm, Yb) in the crystalline ZnO films prepared by magnetron sputtering, and the intensity of the emission lines increases upon introduction of codopants, namely, Ce and Er.

Optical transitions in MnGa2Se4

The dependence of the absorption coefficient on incident photon energy in a MnGa2Se4 single crystal has been investigated in the temperature range 110–295 K. Using group-theory analysis of the electron state symmetry and comparison of the symmetry of the energy spectrum of MnGa2Se4 and its isoelectronic analogs, a conclusion about the character of optical transitions has been drawn. It is shown that the features observed at 2.31 and 2.45 eV are related to the intracenter transitions 6A11 → 4T2(4G) and 6A12 → 4T2(4G). The 6A1 state is split by the crystal field.