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We have investigated the formation and decay of hydrogen-related donors (HDs) and irradiation-induced intrinsic defects. N-type m:Cz and FZ silicon wafers, which were irradiated with 2 MeV protons and subsequently annealed at 100–600 °C, were analyzed using spreading resistance profiling and photoluminescence (PL). HDs formed at 260 °C and then disappeared in two stages at 400–440 and 500–540 °C. This decay behavior indicates the existence of two types of HDs with different thermal stabilities. PL measurements showed interstitial silicon clusters ( W and X center), a carbon–oxygen complex ( C center), and exciton lines bound to unknown shallow centers. The origin of the HDs was investigated based on the correlation of the formation and decay temperatures between HDs and irradiation-induced defects. The predominant defects at the early stage of annealing, such as the C and W centers, are ruled out as candidates for the core defects of HDs because annealing above 260 °C is indispensable for the HD formation. In contrast, the X center was found to be thermally generated above 200 °C and disappeared at 580 °C. The similarity of the formation and decay temperatures between the X and HD centers suggests that HDs are associated with the formation of the interstitial silicon-related defects attached to hydrogen. Our results suggest that controlling the formation of interstitial silicon-related defects is important for realizing desirable doping profiles with high accuracy and reproducibility for power devices. Annealing above 400 °C exclusively provides thermally more stable HDs, leading to the realization of more rugged power devices.

We investigated the effect of the concentration of carbon, oxygen, and irradiation-induced intrinsic defects on hydrogen-related donor (HD) concentration. Several n-type silicon wafers having different carbon and oxygen concentrations were irradiated with 2 MeV protons, subsequently annealed at 300–400 °C, and analyzed by spreading resistance profiling. The HD concentration had no correlation with carbon and oxygen concentration. Additionally, the HD concentration showed a strong increasing linear dependence with proton-irradiation dose at 350 and 400 °C and a square root dependence at 300 °C. In the decay process of HD concentration at 400 °C, fast- and slow-decay components were observed regardless of wafer type. Our results show that the HD formation is based on the interactive process of irradiation-induced intrinsic defects and hydrogen, rather than hydrogen-catalyzed thermal double donor formation. Magnetic-field-applied Czochralski (m:Cz) wafers with 300 mm diameter, which are critical for the production scaling of power devices, have a relatively higher oxygen concentration than conventional floating-zone wafers. Our results further suggest that controlling the intrinsic defect formation, rather than oxygen impurity concentration, is more important in realizing designed doping profiles with high accuracy and reproducibility for next-generation power devices using large-diameter m:Cz wafers as a standard starting material.

The growing demand for power devices has led to the use of magnetic field-applied Czochralski (m:Cz) wafers owing to the limited production capacity and available diameters of the traditionally used floating zone (FZ) wafers. Consequently, the influence of oxygen impurities in the wafers on the electrical properties of devices, regardless of the growth method, needs to be investigated to achieve a stable fabrication process for power devices. Using the proton irradiation doping process and spreading resistance profiling technique, we evaluated the effective diffusion coefficient (Deff) related to trap-limited diffusion of hydrogen and the effects of impurities on diffusivity. We irradiated n-type silicon wafers, which have different carbon, oxygen, and phosphorus concentrations, with 2 MeV protons and annealed them at 300–400 °C. By analyzing the width of the n-type region, where hydrogen-related shallow donors (HDs) are induced, we estimated Deff to be five to six orders of magnitude lower than the intrinsic diffusion coefficient, indicating that hydrogen motion is highly trap-limited. Deff was significantly dependent on the oxygen concentration, and the activation energy of hydrogen diffusion varied from 0.57 ± 0.15 eV (pure epitaxial wafer) to 2.19 ± 0.15 eV (m:Cz wafer). This trend suggests that oxygen-related defects preferentially trap the mobile hydrogen released from thermally dissociated HDs. This study also reveals that the diffusion coefficients of different materials when annealed at 400 °C are comparable. This information is essential to realize the cost-effective production of power devices because we can treat m:Cz and FZ wafers equivalently during the doping process.

We report on the fabrication and optical characteristics of microdisk (MD) resonators with Er,O-codoped GaAs (GaAs:Er,O) as an active component. MD resonators based on GaAs:Er,O with different sidewall angles are fabricated by electron beam lithography followed by two different processes: dry and wet etching. Numerical simulations using a finite-difference time-domain method demonstrate an increase in the cavity quality (Q) factor of the MDs when sharpening the taper angle of the MDs. Micro-photoluminescence characterization of the MD resonators with excitation by a He–Ne laser reveals an 11.4-fold enhancement of Er-related luminescence and a cavity Q factor of 4.2 × 103. For the MD resonators with tapered sidewalls, further enhancement of Er luminescence and increased Q factor (>9.0 × 103) are observed.

Near-infrared (NIR) light with a wavelength of 650–950 nm is used for various biomedical applications. Although NIR emitters are typically based on GaAs-related materials, they contain toxic elements, and the emission wavelength can easily shift during the device operation due to temperature changes and current injection levels. On the other hand, Tm 3 +, which is one of the rare-earth ions, can generate ultra-stable NIR luminescence with a wavelength of ∼ 800 nm, based on 3 H 4 – 3 H 6 transitions in a 4f shell, and we have recently focused on Tm-doped GaN (GaN:Tm) based light-emitting diodes (LEDs) as novel NIR emitters. In this paper, we present a demonstration of a NIR-LED based on GaN:Tm grown by the organometallic vapor phase epitaxy method with optimized growth conditions and structures, where the parasitic reaction is well suppressed. NIR luminescence from the GaN:Tm-based LED is derived from 3 H 4 – 3 H 6 transitions of Tm 3 + ions and consists of three dominant peaks at 795, 806, and 814 nm. The...

The relationship between the valence states of Eu ions in Eu-doped GaN and their magnetic properties is investigated by experiments and simulations. X-ray measurements have verified that the valence states of Eu ions can be controlled through growth temperature and codopants and that the highest concentration of divalent Eu ions is found in samples grown at 700 ° C by codoping with Si and O. According to our phenomenological analysis, magnetoresistance measurement implies the presence of Zener’s p–f exchange interaction. However, the magnetization dependence on an external magnetic field shows non-hysteretic sigmoidal curves in all the samples. Our simulations suggest that this is due to the formation of nanostructures of the magnetic impurities in the samples by spinodal decomposition.The relationship between the valence states of Eu ions in Eu-doped GaN and their magnetic properties is investigated by experiments and simulations. X-ray measurements have verified that the valence states of Eu ions can be controlled through growth temperature and codopants and that the highest concentration of divalent Eu ions is found in samples grown at 700 ° C by codoping with Si and O. According to our phenomenological analysis, magnetoresistance measurement implies the presence of Zener’s p–f exchange interaction. However, the magnetization dependence on an external magnetic field shows non-hysteretic sigmoidal curves in all the samples. Our simulations suggest that this is due to the formation of nanostructures of the magnetic impurities in the samples by spinodal decomposition.

The dopant distribution and surface and structural properties of Er- and Eu-doped GaN samples were investigated using atom probe tomography (APT) and atomic force microscopy (AFM). Erbium accumulation within host GaN threading dislocations was directly detected by APT allowing for the dislocations to be imaged in three dimensions. In addition, photoluminescence spectroscopy with high lateral resolution, by means of scanning near-field optical microscopy, was performed on Eu-doped GaN samples. By combining these results with AFM mappings of the same area, it was concluded that Eu3+ ions also accumulate at threading dislocations. Moreover, high-resolution surface profiles of both samples show that even dilute doping (

While InGaN/GaN blue and green light-emitting diodes (LEDs) are commercially available, the search for an efficient red LED based on GaN is ongoing. The realization of this LED is crucial for the monolithic integration of the three primary colors and the development of nitride-based full-color high-resolution displays. In this perspective, we will address the challenges of attaining red luminescence from GaN under current injection and the methods that have been developed to circumvent them. While several approaches will be mentioned, a large emphasis will be placed on the recent developments of doping GaN with Eu3+ to achieve an efficient red GaN-based LED. Finally, we will provide an outlook to the future of this material as a candidate for small scale displays such as mobile device screens or micro-LED displays.

In order to investigate the excitation processes in Eu,O-codoped GaN (GaN:Eu,O), the time-resolved photoluminescence signal including the rising part is analyzed. A rate equation is developed based upon a model for the excitation processes in GaN:Eu to fit the experimental data. The non-radiative recombination rate of the trap state in the GaN host, the energy transfer rate between the Eu3+ ions and the GaN host, the radiative transition probability of Eu3+ ion, as well as the ratio of the number of luminescent sites (OMVPE 4α and OMVPE 4β), are simultaneously determined. It is revealed and quantified that radiative transition probability of the Eu ion is the bottleneck for the enhancement of light output from GaN:Eu. We also evaluate the effect of the growth conditions on the luminescent efficiency of GaN:Eu quantitatively, and find the correlation between emission intensity of GaN:Eu and the fitting parameters introduced in our model.

We report on the epitaxial growth and the optical properties of Tm,Yb-codoped ZnO (ZnO:Tm,Yb) thin films by sputtering-assisted metalorganic chemical vapor deposition. The characteristic light emission due to the intra-4f shell transitions of Tm3+ and Yb3+ ions in the ZnO host is observed at 4 K by using photoluminescence (PL) characterization. The time-resolved PL measurements on the Tm3+ and Yb3+ emission reveals that the carrier decay time of each ion can vary by changing the concentration of both Tm3+ and Yb3+ ions, which depends on a mixing ratio of Tm2O3 and Yb2O3 in a sputtering target. The energy transfer from Tm3+ to Yb3+ ions occurs in the sample with the high mixing ratio, and the reversed energy transfer process occurs with the lower mixing ratio. These results indicate that the concentration, and thus the carrier dynamics of Tm3+ and Yb3+ ions in the ZnO:Tm,Yb host, can be controlled by altering the mixing ratio of the sputter target and the frequency power.

Electron spin resonance (ESR) measurements of GaAs:Er,O grown by organometallic vapor phase epitaxy have been performed at 9.49GHz from 4.5to13K. Several anisotropic ESR signals (A, B, C, and D) are observed, which is consistent with the previous reports. We reexamined the angular dependence and temperature dependence of the signals in detail. Our ESR results indicate that the direction of alignment of two oxygen atoms or two arsenic atoms for an Er-2O center deviates from the ⟨110⟩ direction in various ways, resulting in the various Er-2O centers. Our ESR results also indicate that the paramagnetic states of the Er centers in GaAs change to a singletlike nonmagnetic state below 8K. The ESR measurements under illumination have been performed to investigate the effect of photoexcitation of the host. This result indicates that the energy transfer efficiency from the host to the B center is especially large. We found that the luminescent Er-2O center B and another Er-2O center C considerably differ in the co...

A systematic experimental study was performed on the Fano line shape exhibited by multilayer structures consisting of an Al layer, a SiO2 spacer layer, and an Al2O3 waveguide layer. In the structures studied, a sharp Fano resonance appears on the background of broad asymmetric resonance attributed to the excitation of a surface plasmon polariton at the Al/SiO2 interface. It is shown that the background asymmetric surface plasmon resonance can be well fitted to a single Fano function, and the sharp Fano line shape can be well fitted to a double Fano function expressed as a product of two single Fano functions. The results of measurements performed by varying the spacer layer thickness indicate that the width (Q factor) of the sharp Fano resonance decreases (increases) monotonously as the thickness increases. The Q factor achieved in the present study is as high as ∼1500. A comparison with the results of electromagnetic calculations suggests that not only the spacer layer thickness but also the imaginary part of the dielectric constant of the waveguide layer plays an important role in the Fano line shape engineering.

New insulating Bi14(Sr0.33Ca0.67)5O26 layered single crystals have been grown by a flux method and their crystallographic properties have been characterized. The crystals exhibited a smooth cleaved surface perpendicular to [001] orientation. The x‐ray precession camera analysis indicated the crystals have rhombohedral symmetry, belonging to a Laue group of 3m. Furthermore, some kinds of rare‐earth elements have been successfully substituted for lattice sites of the crystal. The variation of the c‐axis length reflected ionic radii of the rare‐earth elements substituted. Radiant luminescence has been observed in visible and infrared regions in the Er‐doped crystals, which was assigned to electron transition between spin‐orbit levels of trivalent Er ions in the crystal.

We found an anomalous Eu-related luminescence peak in the temperature dependence of photoluminescence (PL) spectra in Eu-doped gallium nitride (GaN:Eu) grown by organometallic vapor phase epitaxy. Its PL intensity increased with increasing temperature and reached a maximum around 140 K, while the main peak intensities decreased monotonically in this region. Time-resolved PL measurements (TR-PL) revealed a delayed increase of PL intensity after the laser pulse. An analysis of the characteristics hereof indicated that the mechanism behind this afterglow is thermally activated and its transfer efficiency is dependent on the excitation intensity. We developed a model that explains these results, where excitation of the Eu ions associated to this peak takes place via two different carrier traps.

Er-concentration effect in GaAs;Er,O containing charge carriers (n-type, high resistance, p-type) has been studied by X-band Electron spin resonance (ESR) at low temperature (4.7 K

The energy transfer and back transfer processes of GaAs co-doped with Er and O (GaAs:Er,O) were experimentally distinguished by using a frequency response analysis of the AC photocurrent. The results were achieved by using the difference in the frequency dispersion between (1) the dispersion of the energy transfer, which is triggered by the trapping of free charges in the GaAs host and is represented with the Debye relaxation response and (2) the dispersion of the energy back transfer, which is induced by non-radiative transition of 4f bound electrons in the Er dopants and is described with a Lorentzian. The Debye relaxation response found in GaAs:Er,O provided a charge trapping time that was dependent on temperature, which was well correlated with the thermal quenching property of intense intra-4f-shell luminescence. The spectral shape of the Lorentzian dependence on the temperature was explained with the thermal excitation of Er 4f electrons and release of trapped charges in GaAs. The thermal excitation and release of charges consistently explained the characteristics of weak 4f luminescence in low- and high-temperature regions, respectively.

We report a study on the Eu luminescence properties of Eu-doped GaN grown on a GaN substrate by organometallic vapor phase epitaxy. The site-selective excitation of Eu ions revealed the concentration of each luminescent site using the luminescence properties under resonant excitation. The quantitative evaluation of the Eu luminescent sites showed that more than 80% of Eu ions are incorporated into a high-symmetry site. However, the photoluminescence spectrum under indirect excitation is markedly different from that under resonant excitation, which indicates that the luminescent site with high symmetry exhibits low-efficiency energy transfer from the GaN host to the luminescent site.

Unintentionally doped β-FeSi2 epitaxial films were grown on silicon-on-insulator substrates by molecular beam epitaxy using a high-purity (5N) Fe source to investigate the effect of residual impurities on the transport properties of β-FeSi2. From secondary ion mass spectroscopy analysis, impurities of As, Al, and Mn (∼1017 cm−3); P and B (∼1016 cm−3); and Cr and Pb (∼1015 cm−3) were detected in the epitaxial layer. In Hall measurements at room temperature, the films exhibited n-type conduction with a carrier density of 4–6 × 1016 cm−3 and a Hall mobility of 400–440 cm2/Vs. In the temperature (T) dependence of the transport properties, a transition from band conduction to hopping conduction was observed at approximately T = 230 K. At temperatures of 110–150 K, both negative and positive magnetoresistance (MR) were observed depending on the temperature and magnetic field. The MR exhibits mixed conduction of defect band conduction and band conduction in this temperature range.

Although the photoluminescent material GaAs:Er,O has attracted much attention, the mechanism underlying its luminescence is unclear. To elucidate this mechanism, we have performed X-band electron spin resonance (ESR) measurements for different Er concentration samples. Several anisotropic ESR signals (A, B, and C) were observed, which is consistent with the previous reports. When the Er concentration is increased, the angular dependence of the resonance field is not affected, but the linewidth increases. From the Er concentration dependence of linewidth and the lineshape analysis, the dominant interaction between spins is not the magnetic dipole interaction but the exchange interaction. From the temperature dependence of integrated intensity, the exchange interaction between the nearest Er ions is antiferromagnetic, and this system has a singletlike ground state. We conclude that the Er-2O center is not distributed homogeneously in GaAs. A possible model is that the luminescent Er-related center consists ...

Electron spin resonance (ESR) measurements of Zn-codoped GaAs:Er,O grown by organometallic vapor phase epitaxy have been performed at 9.49GHz. Several anisotropic ESR signals (named as A, B, and C) are observed. The angular and temperature dependences of the A, B, and C signals are quite consistent with those observed in GaAs:Er,O. We found that the intensity of the C center decreases together with the decrease of the photoluminescence (PL) intensity by changing the doping amount of Zn, while the change of the ESR intensity of the B center is relatively small. This indicates that the Zn codoping selectively disturbed the formation of C center. Furthermore, the PL intensity seems to be scaled to the intensity of the signal C. This finding indicates that the C center plays an important role in the host excited PL of GaAs:Er,O.