Substitutional Ga Sites Research Articles

Lattice site location of optical centers in GaN:Eu light emitting diode material grown by organometallic vapor phase epitaxy

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 Å) 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.

Structural and paramagnetic properties of diluteGa1−xMnxN

Systematic investigations of the structural and magnetic properties of single crystal (Ga,Mn)N films grown by metal organic vapor phase epitaxy are presented. High resolution transmission electron microscopy, synchrotron x-ray diffraction, and extended x-ray absorption fine structure studies do not reveal any crystallographic phase separation and indicate that Mn occupies Ga-substitutional sites in the Mn concentration range up to 1%. The magnetic properties as a function of temperature, magnetic field and its orientation with respect to the c-axis of the wurtzite structure can be quantitatively described by the paramagnetic theory of an ensemble of non-interacting Mn$^{3+}$ ions in the relevant crystal field, a conclusion consistent with the x-ray absorption near edge structure analysis. A negligible contribution of Mn in the 2+ charge state points to a low concentration of residual donors in the studied films. Studies on modulation doped p-type (Ga,Mn)N/(Ga,Al)N:Mg heterostructures do not reproduce the high temperature robust ferromagnetism reported recently for this system.

Mössbauer study of 57Fe in GaAs and GaP following 57Mn+ implantation

Ion implantation provides a precise method of incorporating dopant atoms in semiconductors, provided lattice damage due to the implantation process can be annealed and the dopant atoms located on regular lattice sites. We have undertaken 57Fe emission Mossbauer spectroscopy measurements on GaAs and GaP single crystals following implantation of radioactive 57Mn + ions, to study the lattice sites of the implanted ions, the annealing of implantation induced damage and impurity–vacancy complexes formed. The Mossbauer spectra were analyzed with four spectral components: an asymmetric doublet (D1) attributed to Fe atoms in distorted environments due to implantation damage, two single lines, S1 assigned to Fe on substitutional Ga sites, and S2 to Fe on interstitial sites, and a low intensity symmetric doublet (D2) assigned to impurity–vacancy complexes. The variations in the extracted hyperfine parameters of D1 for both materials at high temperatures (T > 400 K) suggests changes in the immediate environment of the Fe impurity atoms and different bonding mechanism to the Mossbauer probe atom. The results show that the annealing of the radiation induced damage is more prominent in GaAs compared to GaP.

Vanadium adsorption and incorporation at the GaN(0001) surface: A first-principles study

We have carried out first-principles spin-polarized calculations in order to study the energetics and electronic structure of vanadium adsorption and incorporation on $\text{GaN}(0001)\text{\ensuremath{-}}2\ifmmode\times\else\texttimes\fi{}2$ surface using density-functional theory within a plane-wave ultrasoft pseudopotential scheme. It was found that V atoms preferentially adsorb at the T4 sites at low and high coverages (from 1/4 up to 1 monolayer). In addition, calculating the relative surface energy of several configurations and various V concentrations, we constructed a phase diagram showing the energetically most stable surfaces as a function of the Ga chemical potential. Based on these results, we found that incorporation of V adatoms in the Ga-substitutional site is energetically more favorable compared with the adsorption on the top layers. Our calculations show that the vanadium incorporation is most favorable under a nitrogen environment, in agreement with recent experimental results.

Crystal field and Zeeman parameters of substitutional Yb 3+ ion in GaN

Cathodoluminescence (CL) and photoluminescence (PL) spectra of GaN grown by metalorganic chemical vapor deposition on (0 0 0 1) sapphire substrate and doped by implantation with ytterbium ions (Yb 3+) were critically examined. We report on the crystal field (CF) energy levels calculation of Yb 3+ ions in GaN host using CF theory. The CL and PL spectra are interpreted in terms of lattice-sum and crystal-field splitting model assuming that implanted Yb 3+ ions replace substitutionally Ga cation in a single site (Yb Ga) and that Yb Ga are associated with nitrogen vacancy in a complex defect center (V N–Yb). We demonstrate that assuming the existence of these two centers, good agreement between measured and calculated energy levels of Yb 3+ ion transition lines can be obtained. Careful analysis of the available experimental results revealed that transition lines assigned to V N–Yb defect center overlap with the emission lines originating from the intrinsic Fe 3+ ion in the GaN host. Implications of this evidence on the developed Yb 3+ ion energy level diagram and modeling of the Yb-doped GaN spectra are critically reviewed. Furthermore, on the basis of the derived wave functions, the Zeeman g || and g ⊥ parameters for Yb 3+ ion in a Ga substitutional sites are calculated. This center is characterized by g-values of | g ||| = 1.308 and | g ⊥| = 3.135.

Atomic structure of the (3 × 2) Si–GaAs (0 0 1) reconstructed surface: A clue to δ doping mechanism derived from in situ grazing incidence X-ray diffraction data

In view of understanding silicon incorporation in the δ doping process of GaAs (0 0 1), Si atoms have been deposited, under UHV, on a α(2 × 4) arsenic terminated substrate. In the low coverage regime, a transition to a less As rich (3 × 2) reconstructed Si–GaAs (0 0 1) surface was observed whose atomic structure has been investigated by grazing incidence X-ray diffraction performed in situ. Silicon is found to occupy not only a Ga substitutional site, precursor of a donor dopant but also to form nuclei for neutral clusters, on a template made by the (3 × 2) GaAs (0 0 1) reconstructed surface observed by Martrou et al. [Phys. Rev. B 72 (2005) 241307®]. The maximum surface concentration of donor-like silicon is estimated at 1.04 × 10 14 cm −2 (1/6th monolayer).

High pressure luminescence studies of europium doped GaN

We reported on the high pressure luminescence spectra of polycrystalline Eu-doped GaN material synthesized in the reaction between alloys of gallium, bismuth and europium in ammonia atmosphere. The integrated luminescence intensity of the dominant Eu 3+ ion transition ( 5D 0→ 7F 2) at 622 nm increased approximately one order of magnitude whereas its spectral position and line width did not change significantly between ambient and 6.8 GPa pressure, respectively. Moreover, material was characterized with photo- and cathodo-luminescence, and photoluminescence excitation spectra at different temperatures. It was found that the Eu 3+ ions occupying substitutional Ga site created different centers which could be effectively excited with above band gap excitation and from excitons resonantly photoexcited at the I 2 bound exciton energy. Furthermore, the less efficient Eu 3+ ions excitation path existed through intrinsic impurities and defects generating shallow energy levels in the forbidden gap. It was proposed that reduction of the thermal quenching and consequent enhancement of Eu 3+ ion emission intensity resulted from stronger localization of bound exciton on RESI trap induced by applied pressure.

Structural and optical characterization of Eu-implanted GaN

GaN was implanted with 300 keV Eu ions over a wide fluence range from 1 × 1013 to 1 × 1016 Eu cm−2 at room temperature (RT) or 500 °C. Detailed structural and optical characterizations of the samples were performed using Rutherford backscattering spectrometry and channelling, transmission and scanning electron microscopy, wavelength dispersive x-ray emission and RT cathodoluminescence (CL) spectroscopy. RT implantation results in a sigmoidal-shaped damage build-up curve with four regimes that were correlated with the formation of specific kinds of defects. After annealing at 1000 °C only samples implanted to fluences below 0.8 × 1015 Eu cm−2 showed near complete recovery of the crystal. Implantation at elevated temperature significantly decreases the implantation damage and increases the fraction of Eu incorporated on substitutional Ga-sites. The improved structural properties of samples implanted at elevated temperature are reflected in a higher intensity of Eu-related red light emission after annealing at 1000 °C. The RT CL intensity is correlated with the number of Eu ions on substitutional Ga-sites after annealing. Furthermore, a detailed study of optical activation shows that the optimum annealing temperature depends on the implantation fluence due to the sensitive balance of defects removed and created during high temperature annealing.

Gd-doped GaN studied with element specificity: Very small polarization of Ga, paramagnetism of Gd and the formation of magnetic clusters

Element specific X-ray linear dichroism (XLD), X-ray magnetic circular dichroism (XMCD) at the Ga K - and Ga and Gd L 3 -edges and magnetic resonance measurements have been carried out on Gd:GaN grown by molecular beam epitaxy with different Gd concentrations. XMCD studies at the Ga K -edge reveal only a very weak magnetic polarization, which is too small to account for the colossal moments reported before by SQUID. In the dilute limit we can show by XLD that the majority of the Gd dopant atoms go to substitutional Ga sites. XMCD studies demonstrate that the Gd sublattice behaves paramagnetic which is in contrast to integral SQUID measurements. The absence of ferromagnetic order at room temperature is corroborated by magnetic resonance studies. First signatures of phase separation are visible. At higher Gd concentrations phase separation is found by XLD. In such samples ferromagnetic-like order is consistently found by SQUID, XMCD and magnetic resonance which is characteristic for a blocked superparamagnetic ensemble. In turn, phase separated ferromagnetic Gd/GdN clusters cannot account for magnetic order observed at and above 300 K by SQUID.

Europium doping of zincblende GaN by ion implantation

Eu was implanted into high quality cubic (zincblende) GaN (ZB-GaN) layers grown by molecular beam epitaxy. Detailed structural characterization before and after implantation was performed by x-ray diffraction (XRD) and Rutherford backscattering/channeling spectrometry. A low concentration (<10%) of wurtzite phase inclusions was observed by XRD analysis in as-grown samples with their (0001) planes aligned with the {111} planes of the cubic lattice. Implantation of Eu causes an expansion of the lattice parameter in the implanted region similar to that observed for the c-lattice parameter of wurtzite GaN (W-GaN). For ZB-GaN:Eu, a large fraction of Eu ions is found on a high symmetry interstitial site aligned with the ⟨110⟩ direction, while a Ga substitutional site is observed for W-GaN:Eu. The implantation damage in ZB-GaN:Eu could partly be removed by thermal annealing, but an increase in the wurtzite phase fraction was observed at the same time. Cathodoluminescence, photoluminescence (PL), and PL excitation spectroscopy revealed several emission lines which can be attributed to distinct Eu-related optical centers in ZB-GaN and W-GaN inclusions.

Local structure of (Ga,Fe)N and (Ga,Fe)N:Si investigated by x-ray absorption fine structure spectroscopy

X-ray absorption fine-structure (XAFS) measurements supported by {\em ab initio} computations within the density functional theory (DFT) are employed to systematically characterize Fe-doped as well as Fe and Si-co-doped films grown by metalorganic vapour phase epitaxy. The analysis of extended-XAFS data shows that depending on the growth conditions, Fe atoms either occupy Ga substitutional sites in GaN or precipitate in the form of $\epsilon$-Fe$_3$N nanocrystals, which are ferromagnetic and metallic according to the DFT results. Precipitation can be hampered by reducing the Fe content, or by increasing the growth rate or by co-doping with Si. The near-edge region of the XAFS spectra provides information on the Fe charge state and shows its partial reduction from Fe$^{+3}$ to Fe$^{+2}$ upon Si co-doping, in agreement with the Fe electronic configurations expected within various implementations of DFT.

Optically active centers in Eu implanted, Eu in situ doped GaN, and Eu doped GaN quantum dots

A comparison is presented between Eu implanted and Eu in situ doped GaN thin films showing that two predominant Eu sites are optically active around 620 nm in both types of samples with below and above bandgap excitation. One of these sites, identified as a Ga substitutional site, is common to both types of Eu doped GaN samples despite the difference in the GaN film growth method and in the doping technique. High-resolution photoluminescence (PL) spectra under resonant excitation reveal that in all samples these two host-sensitized sites are in small amount compared to the majority of Eu ions which occupy isolated Ga substitutional sites and thus cannot be excited through the GaN host. The relative concentrations of the two predominant host-sensitized Eu sites are strongly affected by the annealing temperature for Eu implanted samples and by the group III element time opening in the molecular beam epitaxy growth. Red luminescence decay characteristics for the two Eu sites reveal different excitation paths. PL dynamics under above bandgap excitation indicate that Eu ions occupying a Ga substitutional site are either excited directly into the D50 level or into higher excited levels such as D51, while Eu ions sitting in the other site are only directly excited into the D50 level. These differences are discussed in terms of the spectral overlap between the emission band of a nearby bound exciton and the absorption bands of Eu ions. The study of Eu doped GaN quantum dots reveals the existence of only one type of Eu site under above bandgap excitation, with Eu PL dynamics features similar to Eu ions in Ga substitutional sites.

Effect of fluence on the lattice site of implanted Er and implantation induced strain in GaN

A GaN thin film was implanted with 5×1014cm−2 of 60keV stable 166Er, followed by the implantation of 2×1013cm−2 radioactive 167Tm (t1/2=9.3d) and an annealing sequence up to 900°C. The emission channeling (EC) technique was applied to assess the lattice location of Er following the Tm decay from the conversion electrons emitted by 167mEr, which showed that more than 50% of 167mEr occupies substitutional Ga sites. The results are briefly compared to a 167mEr lattice location experiment in a GaN sample not pre-implanted with 166Er. In addition, high-resolution X-ray diffraction (HRXRD) was used to characterize the perpendicular strain in the high-fluence implanted film. The HRXRD experiments showed that the Er implantation resulted in an increase of the c-axis lattice constant of the GaN film around 0.5–0.7%. The presence of significant disorder within the implanted region was corroborated by the fact that the EC patterns for off-normal directions exhibit a pronounced angular broadening of the order of magnitude of 0.5°.

Element specific investigations of the structural and magnetic properties of Gd:GaN

The authors present element specific measurements of the x-ray linear dichroism and the x-ray magnetic circular dichroism (XMCD) on Gd:GaN samples. They can show that the majority of the Gd dopant atoms goes to substitutional Ga sites and that a small XMCD is detectable for Gd. There are significant deviations of the magnetic hysteresis recorded for Gd compared to superconducting quantum interference device measurements. Our measurements show that the magnetic signal from the Gd dopant atom itself is rather small highlighting the role of magnetic contributions of the GaN host crystal.

Intra-atomic photoluminescence at 1.41 eV of substitutional Mn in GaMnN of high optical quality

We report on a characteristic photoluminescence feature of the substitutional Mn in high quality GaMnN layers. The lattice site was identified using atom localization by channeling enhanced microanalysis with a transmission electron microscope. It shows that 96.5%±5.0% of the Mn atoms are incorporated on the substitutional Ga site. In photoluminescence a feature appears at 1.41 eV with a phonon sideband related to the GaN matrix. The temperature evolution is characteristic of an intra-atomic transition and it is assigned to the internal transition E5→T52 of the Mn3+ ion. The assignment is supported by absorption experiments. The persistence of the clear PL signal up to about 1% Mn concentration is proposed to be a fingerprint of high quality diluted GaMnN.

Enhanced magnetization by modulated Mn delta doping in GaAs

The magnetic property of GaMnAs can be enhanced by increasing the Mn concentration as well as hole concentration. The interstitial Mn is inevitably formed when Mn concentration is very high even if the growth condition is carefully controlled. They act as a double donor and compensates for Mn acceptors in GaAs. The interstitial Mn atoms also tend to be bound by those occupying the substitutional Ga site. The combination of interstitial and substitial Mn atom produces antiferromagnetic pairs. In order to reduce the interstitial Mn and increase hole concentration, we introduce Mn delta-doped layers separated by undoped GaAs layers. The magnetic properties are investigated as a function the spacing between Mn delta-doped layers. It is found that magnetization characteristics such as the magnetic susceptibility, coercive force and Curie temperature are enhanced when the spacing between Mn delta-doped layers is optimized.

Implantation of nanoporous GaN with Eu ions

Nanoporous GaN samples were implanted with 150keV Eu+ ions with a fluence of 5×1015cm−2. Channelling and X-ray diffraction (XRD) measurements indicate a high crystalline quality. After implantation the optical activity of the GaN is quenched by the implantation damage. The 〈0001〉 aligned spectrum reveals broad damage distribution over the entire porous layer responsible for an expansion along the c-axis with a parameter increase of 0.3% as indicated by the XRD. Above band gap excitation photoluminescence at low temperature indicates the presence of two regions with different optical properties after annealing. We observe a transparent zone with UV and yellow–green broad emissions and a pale brown region where the UV emission is absent. In both regions the intraionic 4f6Eu3+ emissions are present. XRD and channelling confirm the good crystalline quality of the samples after the annealing and the incorporation of Eu into near substitutional Ga sites.

Optical and magnetic properties of Eu-doped GaN

GaN films were doped with Eu to a concentration of ∼0.12at.% during growth at 800°C by molecular beam epitaxy, with the Eu cell temperature held constant at 470°C. All samples were postannealed at 675°C. The films exhibited strong photoluminescence (PL) in the red (622nm) whose absolute intensity was a function of the Ga flux during growth, which ranged from 3.0×10−7to5.4×10−7Torr. The maximum PL intensity was obtained at a Ga flux of 3.6×10−7Torr. The samples showed room temperature ferromagnetism with saturation magnetization of ∼0.1–0.45emu∕cm3, consistent with past reports where the Eu was found to be predominantly occupying substitutional Ga sites. There was an inverse correlation between the PL intensity and the saturation magnetization in the films. X-ray diffraction showed the presence of EuGa phases under all the growth conditions but these cannot account for the observed magnetic properties.

Lattice site location and annealing behavior of implanted Ca and Sr in GaN

We report on the lattice location of ion-implanted Ca and Sr in thin films of single-crystalline wurtzite GaN. Using the emission channeling technique the angular distributions of β− particles emitted by the radioactive isotopes 45Ca (t1/2=163.8 d) and 89Sr (t1/2=50.53 d) were monitored with a position-sensitive detector following 60 keV room-temperature implantation. Our experiments give direct evidence that ∼90% of Ca and >60% of Sr atoms were occupying substitutional Ga sites with root mean square displacements of the order of 0.15–0.30 Å, i.e., larger than the expected thermal vibration amplitude of 0.074 Å. Annealing the Ca implanted samples at 1100–1350 °C in high-pressure N2 atmosphere resulted in a better incorporation into the substitutional Ga site. The Sr implanted sample showed a small decrease in rms displacements for vacuum annealing up to 900 °C, while the substitutional fraction remained nearly constant. The annealing behavior of the rms displacements can explain why annealing temperatures above 1100 °C are needed to achieve electrical and optical activations, despite the fact that the majority of the acceptors are already located on Ga sites immediately after ion implantation.

Magnetic and Optical Properties of Eu-doped GaN

ABSTRACTGaN films were doped with Eu to a concentration of ∼0.12 at. % during growth at 800°C by molecular beam epitaxy, with the Eu cell temperature held constant at 470°C. All samples were post-annealed at 675°C. The films exhibited strong photoluminescence (PL) in the red (622 nm) whose absolute intensity was a function of the Ga flux during growth, which ranged from 3-5.4×10−7 Torr. The maximum PL intensity was obtained at a Ga flux of 3.6×10−7 Torr. The samples showed room temperature ferromagnetism with saturation magnetization of ∼0.1-0.45 emu/cm3, consistent with past reports where the Eu was found to be predominantly occupying substitutional Ga sites. There was an inverse correlation between the PL intensity and the saturation magnetization in the films. X-ray diffraction showed the presence of EuGa phases under all of our growth conditions but these cannot account for the observed magnetic properties.