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Room-temperature Er-related electroluminescence (EL) properties have been investigated in Er,O-codoped GaAs (GaAs:Er,O) light emitting diodes (LEDs) grown by organometallic vapor phase epitaxy (OMVPE). Under forward bias, characteristic emission due to a luminescence center consisting of Er coordinated by O and As was clearly observed at room temperature, while the Er-related EL was undetectable under reverse bias. At lower current densities, the EL intensity increased linearly with the current density. Subsequently, the intensity exhibited a tendency to saturate at higher current densities. By analyzing the behavior with a fitting according to rate equations, the excitation cross section of Er ions due to current injection was determined to be approximately 10-15 cm2, which is by five orders in magnitude larger than that for optical excitation in Er-doped fiber amplifiers (10-20∼10-21 cm2).

Transparent electrodes were formed on Eu-doped GaN-based red-light-emitting diode (GaN:Eu LED) substrates by the screen printing of indium tin oxide nanoparticle (ITO np) inks as a wet process. The ITO nps with a mean diameter of 25 nm were synthesized by the controlled thermolysis of a mixture of indium complexes and tin complexes. After the direct screen printing of ITO np inks on GaN:Eu LED substrates and sintering at 850 °C for 10 min under atmospheric conditions, the resistivity of the ITO film was 5.2 mΩ cm. The fabricated LED up to 3 mm square surface emitted red light when the on-voltage was exceeded.

An electrical resonance technique was developed to enhance the emission efficiency of a light-emitting diode (LED) with a low density of dopants as the luminescence centers. A rectangular pulse drive, tuned to the frequency corresponding to the electrical time constant of the LED active layer, intensified the emission of a GaN:Eu red LED. The injected charge carriers, which are transported back-and-forth in the active layer (“back-and-forth transport”), can effectively excite the Eu luminescence centers. A wide scan of the rectangular pulse frequencies revealed injected charge behavior in the active layer. At low frequencies, the injected charges penetrated through the active layer and were lost outside of it (“external loss”), whereas localized back-and-forth motion of the injected charges occurred at high frequencies without interaction with the Eu dopants in the active layer (“internal loss”). An intermediate frequency, at which the sum of the external and internal losses was minimized, yielded the optimal electrical resonance for intensification.

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

We have succeeded in growing Eu-doped GaN (GaN:Eu) layers with high crystalline quality by organometallic vapor phase epitaxy, and have demonstrated a low-voltage current-injected red emission from a light-emitting diode (LED) using the GaN:Eu layer with an applied voltage of as low as 3 V. By optimizing growth conditions and device structures, the light output power has increased steadily to up to sub-mW at an injected current of 20 mA. For further improvement of the output power, Eu luminescent centers have been spectroscopically investigated in GaN:Eu. A variety of Eu luminescent sites and a strongly site-dependent energy transfer process were clearly revealed. The energy-transfer efficiency was markedly enhanced by codoping of Mg with Eu. A proposed model for centers with high energy-transfer efficiencies is discussed in relation to the local structure.

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.

Precisely size-controlled and crystalline (La0.275Pr0.35Ca0.375)MnO3 (LPCMO) nanobox array structures were fabricated down to 30 nm in wall-width by the three-dimensional nanotemplate pulsed laser deposition technique. The hard X-ray photoemission spectroscopy in LPCMO nanobox array structures showed the existence of the satellite on the Mn 2p3/2 peak at 203 K with a higher intensity than that in the LPCMO film even at 153 K. This result indicated that the insulator-metal transition (IMT) in the nanobox array structures occurred at the higher IMT temperature.

The photoluminescence (PL) and electrical properties of Eu-doped ZnO (ZnO:Eu) grown by sputtering-assisted metalorganic chemical vapor deposition (SA-MOCVD) were investigated. Sharp PL lines due to intra-4f transition in Eu3+ ions were observed when ZnO:Eu was annealed at temperatures (T a) higher than 500 °C in O2 ambient. In Hall measurements, the as-grown ZnO:Eu showed a high electron density of 3×1019 cm-3, indicating that the doped Eu ions act as a donor in ZnO. The electron density decreased rapidly at T a≥500 °C and was 2×1017 cm-3 at T a = 600 °C. These results revealed that a complex consisting of a Eu3+ ion and an interstitial oxygen (Eu3+–Oi complex) is formed by high-temperature annealing and acts as a Eu3+ luminescent center in ZnO.

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.

The effects of Mg and Si codoping on Eu luminescence properties have been investigated in Eu-doped GaN (GaN:Eu). The Mg codoping into GaN:Eu produced novel luminescence centers consisting of Eu and Mg, and increased photoluminescence (PL) intensity in Eu,Mg-codoped GaN (GaN:Eu,Mg). However, this increased PL intensity was quenched by thermal annealing in N2 ambient, which is due to activation of Mg acceptors. In GaN:Eu,Mg codoped additionally with Si (GaN:Eu,Mg,Si), on the other hand, the Eu–Mg centers disappeared, while an additional luminescence center appeared. Furthermore, the additional luminescence center showed no quenching under N2 annealing because Si donors compensated for the Mg acceptors in GaN. Thermal quenching of the luminescence center was also approximately half of that in GaN:Eu. These results indicate that the codoping with additional impurities in GaN:Eu is a powerful technique to control Eu luminescence centers for realization of improved device performance in red light-emitting diodes using GaN:Eu.

We report a study on Eu the luminescence properties of Eu-doped GaN (GaN:Eu) grown on a GaN substrate by organometallic vapor phase epitaxy (OMVPE). Thermal quenching of the Eu luminescence was suppressed using the GaN substrate, which is due to the preferential formation of a Eu luminescent site with a local structure with high symmetry. The preferential formation of this luminescent site was supported by the observation of strong near-band-edge emission. The strong near band-edge emission occurred as a result of suppressed formation of the dominant Eu luminescent site in GaN:Eu on a sapphire substrate, which is known to be coupled with a defect and to have a large capture cross section of photogenerated carriers.

The effects of thermal annealing on Eu,Mg-codoped GaN (GaN:Eu,Mg) grown by organometallic vapor phase epitaxy were investigated. After annealing in nitrogen ambient, Eu-Mg related photoluminescence emission was quenched to 13% without a change in the spectral shape. The quenched emission recovered to 65% of the original intensity after a subsequent annealing in ammonia ambient. Combined excitation emission spectroscopy and time-resolved photoluminescence results revealed that the quenching behavior is attributed to a nonradiative process induced by unpassivated Mg acceptors in the relaxation of excited 4f electrons of Eu ions.

The International Conference on Thin Films is the most established conference for all researchers and persons interested in thin films and coatings. It is one of the tri-annual conference series endorsed and co-organized by the Thin Film Division of the International Union for Vacuum Science, Technique and Applications (IUVSTA), a union of national member societies whose role is to stimulate international collaboration in the fields of vacuum science, techniques and applications and related multi-disciplinary topics including solid-vacuum and other interfaces. The 15th International Conference on Thin Films (ICTF-15) is organized by The Vacuum Society of Japan (VSJ) and held at Kyoto TERRSA in Kyoto, Japan on 8–11 November 2011, following the 14th International Conference on Thin Films (ICTF-14), which was held in Ghent, Belgium in 2008. Thin films and coatings are daily becoming increasingly important in the fields of various industries. This International Conference provides a multi-disciplinary forum for recent advances in basic research, development and applications of thin films and coatings. This conference will present a unique opportunity for researchers, engineers and managers to acquire new knowledge of thin films and coatings. We hope that our understanding on thin films and coatings will be deepened through this conference. The conference site, 'Kyoto TERRSA' is located in the historical heart of the old capital Kyoto. Kyoto is an ancient city with a 1200-year history. It was established as Japan's capital under the name 'Heian-kyo' in the year 794. Although many transformations have taken place over the years, Kyoto has always embraced the most advanced standards of the times. It has greatly contributed to the nation's industrial, economic and cultural development. The dauntless spirit of leadership of Kyoto's past as a capital city is still felt here today. Kyoto also preserves the beloved examples of its culture as testimonials of time. This is shown in the ancient temples and shrines, as well as private houses, which are built in styles unique to Kyoto. Furthermore, many festivals, ceremonies and traditional activities reveal the will of this city to convey and develop its 1200-year-old culture. Participants of the conference will also be able to see many world heritage sites in the city. Moreover, November is the best time of year to visit Kyoto. We hope you will enjoy Kyoto very much. We would like to offer our thanks for all the contributions from the members of the International Advisory Committee and Organizing Committee, Symposium Chairs, the Secretary General, the Thin Film Division of IUVSTA (chair: Professor Dr Alberto Tagliaferro), IUVSTA, VSJ and other cooperating societies, and to all of the supporting organizations and enterprises. We would also like to express our thanks to all of the participants, secretariat members and members of the supporting staff. I am very pleased to welcome you to ICTF-15 and Kyoto! Director Professor Dr Osamu Takai Chairperson of ICTF-15 EcoTopia Science Institute, and Department of Materials, Physics and Energy Engineering Graduate School of Engineering Nagoya University Japan

In order to analyze effect of the dicing process on the GaN epitaxial layer, the GaN-wafer is cut in sizes of the 0.7 mm square and the 1.7 mm square. The crystal characteristics of the GaN-chips have been measured using X-ray measurements and Raman spectra measurements. The full-width half maximum (FWHM) values of the X-ray rocking curves of (0002), (10–13) and (10–12) of the 0.7 mm square GaN-chip become wider than that of before the dicing process. The E2 (high) peak of Raman spectra at the edge in the 0.7mm square GaN-chip is shifted to lower wave number. In consideration of crystallography, we infer from these results that both the crystal strains and the screw dislocations have been generated during the dicing process.

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.

We investigated the photoluminescence properties of Eu,Mg-codoped GaN grown by organometallic vapor phase epitaxy. Some emission due to intra-4f shell transition of 5D0-7F2 in Eu3+ ions in a center with Eu and Mg was observed together with typical Eu emission. The peak intensity of the Eu-Mg emission was about five times higher than that of the typical Eu emission. The Eu-Mg emission exhibited a maximum at around 180 K, while the typical Eu emission intensity decreased monotonically with increasing temperature. It was found that only one type of Eu-Mg center contributed to the enhanced intensity up to 180 K.

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 ...

We report studies of the excitation mechanism of Eu ions in situ doped during organometallic vapor-phase epitaxy (OMVPE) of GaN. We find that the bright red emission under above-band gap excitation originates primarily from an incorporation site that exhibits high excitation efficiency but occurs in low relative abundance ( 97%).

Eu-doped GaN was grown by organometallic vapor phase epitaxy at temperatures from 900 to 1100 °C. Eu incorporation is influenced by temperature with the highest concentration found for growth at 1000 °C. In all samples, Eu is incorporated entirely on substitutional Ga sites with a slight displacement which is highest (∼0.2 A) in the sample grown at 900 °C and mainly directed along the c-axis. The major optical Eu3+ centers are identical for in situ doped and ion-implanted samples after high temperature and pressure annealing. The dominant Eu3+ luminescence lines are attributed to isolated, substitutional Eu.

We investigated the luminescence properties of Eu-doped GaN (GaN:Eu) grown by atmospheric-pressure organometallic vapor phase epitaxy. The GaN:Eu exhibited radiant red emission due to the intra-4f shell transition of Eu3+ ions at room temperature. The intensity of the dominant peak was about 4 times higher than that in the sample grown at 10 kPa, even though the Eu concentration was only half that of the 10 kPa sample. This was mainly caused by the enhancement of the energy transfer from the GaN host to Eu ions. The enhanced energy transfer resulted in improved luminescence properties of a GaN:Eu light-emitting diode.

We have grown Eu-doped GaN on a freestanding GaN substrate by organometallic vapor phase epitaxy and investigated its red luminescence that is due to intra-4f shell transitions of Eu3+ ions. The optimum growth temperature for Eu luminescence was 50 °C higher than that of Eu-doped GaN on a sapphire substrate. The highest emission intensity was more intense than that on the sapphire substrate, while the Eu luminescence lifetime was identical for both substrates. These results indicate that energy transfer from the GaN host to the Eu3+ ions occurs more efficiently in Eu-doped GaN on the GaN substrate because of a low dislocation density.

In order to investigate the magnetic properties of GaAs:Er,O, we performed X-band ESR measurement on three different Er-concentration samples in the temperature region from T = 4.6 K to 18 K. Three types of Er-related ESR signals were observed below 18 K. The temperature dependence of the signal intensities and the result of the line shape analyses suggest that the luminescent related Er centers have a singlet-like ground state: that is, the Er-related centers are not homogenously distributed in GaAs:Er,O, and Er3+ ions can be antiferromagnetically coupled via the super exchange interaction. Moreover, we have shown that the Er-concentration dependence of intensities of ESR absorption has a strong correlation with the Er-concentration dependence of the PL intensities of GaAs:Er,O

We investigated the optical properties of Eu-implanted GaN and related-alloy semiconductors by photoluminescence (PL) and time resolved PL measurements at room temperature (RT). After thermal annealing, the red emission was observed in each sample due to the intra-4f transitions in Eu3+ ions. The PL intensity increases with Al incorporation into GaN while it decreases with In incorporation into GaN, indicating that the emission efficiency depends on the host materials while the emission wavelength is independent of the host materials. Since the decay times of Eu emission of the samples are found to be similar by time resolved PL measurements, the origin of the Eu emission of each sample is supposed to be same. Therefore, the difference in the PL intensity is attributed to the difference in the energy transfer efficiency because of the ion implantation damage and/or bandgap energy of the host materials.

Modifications of direct transition energies by crystal lattice deformations were confirmed in β-FeSi2 epitaxial films on Si(111) substrates. With an increasing of annealing temperature (Ta), lattice constants of a-axis expanded, and those of b- and c-axis shrank, resulting in the volume reduction in −0.2%. In photoreflectance measurements, the direct transition energy at Y point in the Brillouin zone of β-FeSi2 (∼0.92 eV) was shifted to lower photon energy with the increase in Ta. These results revealed that the band-gap energy was modulated systematically by the lattice deformation, which suggests a possibility of band-gap engineering by the lattice deformation in β-FeSi2 epitaxial films on Si(111) substrates.

Growth of GaAs by MOVPE (Metal-Organic Vapor-Phase Epitaxy) at a much lower temperature than conventional conditions of 650°C required to produce highly resistive buffer layers and molecular-layer-level abrupt heterojunctions and doped-layer interfaces. It has been widely recognized that growth at 550~600°C causes poor morphological surfaces on the grown layers. This paper describes a fundamental improvement of the low-temperature growth of GaAs, resulting in smooth surfaces with very low impurity concentrations. Excellent GaAs layers can be grown at 500°C by increasing partial pressure of AsH3 during growth. The method has been applied to high quality epitaxial layers for electronic device production.

We have fabricated two types of 1.5 μm light-emitting devices based on Er,O-codoped GaAs (GaAs:Er,O) using organometallic vapor phase epitaxy (OMVPE). In a device exhibiting a room-temperature lasing at the GaAs band-edge, the threshold current density increased with Er concentration. The Er intensity rose up steeply and then decreased gradually in the spontaneous emission region. In the stimulated emission region, it remained almost constant, reflecting ultrafast energy transfer to Er ions. In a device exhibiting a room-temperature lasing from strained GaInAs quantum wells (QWs) embedded in GaAs:Er,O, on the other hands, the lasing wavelength was designed to tune to the energy separation between the second excited states 4I11/2 and the ground state 4I15/2 of Er3+ ions. The Er intensity revealed initially steep increase with injected current density in the region for spontaneous emission from the GaInAs QWs. In the stimulated QW emission region, however, the intensity continued to increase with the current density.

Band structure of β-FeSi2 epitaxial and polycrystalline films was investigated by photoreflectance (PR) measurements. In the PR spectra, two critical points were obtained at Eg = 0.918 eV and Ei = 0.895 eV in the films. The Eg consistent with the direct transition energy at Y point in the Brillouin zone of β-FeSi2. The Ei was assigned as a transition-energy between conduction band and the acceptor-level originating from Si vacancies in β-FeSi2. The energy from top of valence band to the acceptor-level was determined to be 23 meV.

We investigated the relationship between optical properties and crystal quality of Eu-implanted GaN-based semiconductors. Emission peaks that are due to intra-4f shell transitions in Eu3+ ions were observed in Eu-implanted GaN, InGaN, and AlGaN. Emission wavelengths from the Eu ions were constant regardless of the matrix materials. The emission peak intensity in the Eu-implanted AlGaN was higher than that in the Eu-implanted InGaN or GaN because of less matrix damage by ion implantation.

Er-doped ZnO (ZnO:Er) thin films were grown by metalorganic chemical vapor deposition. Subsequent annealing improved the crystallinity of the ZnO:Er by suppressing nonradiative centers in the films. The annealed ZnO:Er showed clear 1.54 μm PL originating from the 4I13/2 → 4I15/2 transitions of Er3+ ions. This was in addition to luminescence from the ZnO host. Er-related PL intensity increased when prepared with annealing temperatures of 800–900 °C, while annealing at 1000 °C reduced the intensity. PL of 1.54 μm with high intensity was obtained in the ZnO:Er film that exhibited band-edge emission of free- or bound-excitons.

Optical properties of GaInP/GaAs:Er,O/GaInP double heterostructure (DH) laser diodes (LDs) grown by organometallic vapor phase epitaxy (OMVPE) were investigated. The Er-doped LDs showed laser emission of the GaAs band-edge. The threshold current density (Jth) was 3.6 kA/cm2 on the n-type GaAs substrates and 15.8 kA/cm2 on the p-type at 77 K. The Er-doped LDs on the n-type substrate showed a strong sulfur (S)-related emission in the near-infrared region. Conversely, the S-related emission was strongly suppressed in the LDs on the p-type substrates. This was achieved by preventing the incorporation of S atoms into the GaAs active layer. For the electroluminescence (EL) properties, 1.54 μm-emission from Er3+ ions was clearly observed in the Er-doped LDs on p-type substrates. The dependence of EL intensities on the current density showed that the suppression of an energy transfer from the Er3+ ion to the S-related emission was necessary for efficient Er3+ emission at the stimulated emission region of the GaAs band-edge.

Structural and luminescent properties of Er-doped ZnO (ZnO:Er) grown by metalorganic chemical vapor deposition were investigated. In comparison to the undoped ZnO, the c-axis crystallographic orientation was deteriorated and the photoluminescence (PL) intensity of the band-edge became lower in the ZnO:Er. The weak band-edge PL in the ZnO:Er was due to the nonradiative recombination centers induced by the Er-doping processes. The band-edge PL intensity of ZnO:Er was drastically increased by the temperature annealing at 800°C in O2 ambient. The annealed ZnO:Er showed clear 1.54μm PL originating from the I13∕24→I15∕24 transitions of the Er3+ ions. These PL properties showed that the local arrangement of the Er ions was changed by the thermal annealing, resulting in a transformation from Er-related nonradiative centers in the as-grown sample to luminescent centers for 1.54μm PL in the annealed one.

Carrier capturing in GaInP/Er,O-codoped GaAs (GaAs:Er,O)/GaInP laser diodes (LDs) was investigated by means of the threshold current density (Jth) of laser emission. The Er-doped LDs showed laser emission of the GaAs band edge at room temperature, and its Jth increases with the Er flow rate during the growth. In the dependence of Jth on reciprocal cavity length, the effective gain factor of the LDs decreased by the codoping of Er and O. These results reveal that ultrafast capturing of injected carriers at an Er-related trap level is accomplished even in the stimulated emission region of GaAs. The fast capturing process allowed us to observe both the spontaneous intra-4f-shell emission due to Er3+ ions at 1.54 μm and the stimulated emission at the GaAs band edge (∼840 nm) in the Er-doped LDs.

Ultrafast carrier dynamics and terahertz radiation from Er,O-codoped GaAs (GaAs:Er,O) have been investigated by pump and probe reflectance and time-domain terahertz spectroscopy. In pump and probe reflectance measurements, GaAs:Er,O showed faster relaxation time (0.37–0.56ps) of photoexcited carriers than undoped GaAs. In terahertz spectroscopy, the radiated terahertz amplitude decreased and the decay time of transient photocurrent became long with increasing Er concentration. The Er concentration dependence was understood by additional electron scattering due to the Er doping. The fast relaxation time and the terahertz radiation properties suggest new applications of GaAs:Er,O for the terahertz frequency region.

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.