J. Soares

Green, red and infrared Er-related emission in implanted GaN:Er and GaN:Er,O samples

Er-related luminescence near 1.54 μm (∼805 meV) is observed under below band gap excitation at 4.2 K in GaN:Er and GaN:Er,O implanted samples. The spectrum of the recovered damage samples is a multiline structure. So far, these lines are the sharpest ones reported for GaN. Well-resolved green and red luminescences are observed in implanted samples. The dependence of luminescence on the excitation energy as well as the influence of different nominal fluence and annealing conditions is discussed. Combining the results obtained from photoluminescence and Rutherford backscattering spectrometry, different lattice sites for the optical active Er-related centers are identified.

High temperature annealing of Er implanted GaN

Abstract Defect recovery, optical activation and diffusion of Er implanted GaN epilayers grown on sapphire were studied after annealing at 1000°C with proximity cap and 1200°C under nitrogen atmosphere at high pressure (1GPa). The erbium ions with 160 keV were implanted at room temperature to nominal fluences of 5×10 14 cm −2 and 5×10 15 cm −2 . Some samples were co-implanted with oxygen ions to study its influence on the Er behaviour. During implantation a large fraction of Er is incorporated in Ga sites of the GaN lattice for the samples implanted with lower dose. The implantation damage recovers almost completely after rapid thermal annealing (120 s) at 1000°C with proximity cap. The annealing has no influence on the Er profile. The increase of the annealing time leads to the degradation of the surface due to nitrogen loss. The samples implanted with higher fluence and exposed to the same annealing procedure display distinct behaviour depending on the presence of oxygen. In samples without oxygen, the recovery is faster and accompanied by the segregation of Er towards the surface. For samples containing oxygen the damage recovery proceeds slowly and the Er profile remains stable. Annealing at 1200°C in nitrogen atmosphere at a pressure of 1GPa promotes the complete recovery of the damage in the sample without oxygen. During this annealing, a fraction of Er diffuses into the bulk. After annealing the optical spectra reveal the presence of several sharp lines the intensity of which increases significantly with the annihilation of the implantation damage.

Effect of crystal orientation on defect production and optical activation of Er-implanted sapphire

Abstract Defect production and optical activity of Er-implanted α-Al 2 O 3 were studied as a function of implanted dose and crystal orientation. Erbium fluences in the range of 8×10 13 –4×10 15 Er + /cm 2 were implanted into single crystalline samples with either the m -axis or c -axis normal to the surface. The energy of the Er ions was 200 keV. We found that the number of displaced O atoms was about two times higher on samples with the c -axis normal to the surface compared to the m -samples. The annealing stages of the damage follow a similar behaviour for the m -axis and c -axis samples and the complete recovery occurs for temperatures above 1200°C. The reconstruction of the lattice is accompanied by a reduction in the intensity of the photoluminescence signal above 825°C. Since the amount of Er and its lattice site is maintained, it is concluded that the change on its surroundings was responsible for the decrease of optical activity.

Structural and optical studies of Co and Ti implanted sapphire

Abstract Single crystals of α-Al 2 O 3 with different orientations were implanted with several fluences of Ti and Co ions. For low fluences both Ti and Co ions are fully incorporated in Al lattice sites and remain stable up to annealing temperatures of 1000 °C. For fluences of 5×10 16 cm −2 the implanted region becomes completely disordered (amorphous) for samples implanted with Ti while for Co the same condition is achieved only for higher fluences (2×10 17 Co + /cm 2 ). The recovery of the implantation damage is almost complete after annealing at 1000 °C in either oxidizing or reducing atmospheres for fluences below 5×10 16 cm −2 . For higher fluences annealing in a reducing ambient promotes the precipitation of crystalline metallic Co and Ti particles, as revealed by TEM and RBS. These precipitates retard the damage recovery. The presence of oxygen during annealing leads to the formation of mixed Co and Al oxides through the entire implanted region. Annealing at 1000 °C promotes the formation of a spinel phase (Al 2 CoO 4 ) and the blue or green coloration of sapphire, depending on the Co fluence. Moreover, narrow red emission lines were observed. For Ti the oxides concentrate at the surface and optical absorption measurements show the presence of a broad absorption band centered at 325 nm. This band is absent in the samples annealed in a reducing atmosphere. Photoluminescence measurements reveal the presence of an emission band centered near 840 nm.

Influence of annealing atmosphere on the behavior of titanium implanted sapphire

Abstract Sapphire single crystals with (0xa00xa00xa01) and (0xa02xa0 2 xa01) orientations were implanted at room temperature with several fluences of Ti at 150 keV. For low fluences (up to 1×1015 Ti+/cm2) the Ti ions are fully incorporated in Al sites of the lattice and remain stable up to temperatures of the order of 1000 °C as revealed by detailed channeling angular scans. The amorphous state is reached after implantation of 5×1016 Ti+/cm2. Annealing in oxidizing atmosphere leads to complete recovery of the amorphous region and the segregation of Ti to the surface. The RBS and TEM pictures show that Ti oxides concentrate at the surface after annealing at 1000 °C. The same annealing in a reducing atmosphere produces a buried layer of Ti precipitates centered at the range of the implanted ions. The damage recovery is hindered by the presence of this layer as shown by the 〈0 0 0 1〉 aligned RBS spectrum. The surface layer becomes polycrystalline as the TEM micrographs reveal. Optical absorption measurements show the presence of a broad band centered at 350 nm for the samples annealed in air. This band is absent in the samples annealed in a reducing atmosphere but the background absorption is much higher compared with the samples annealed on air. Photoluminescence (PL) measurements reveal the presence of an emission band centered around 800 nm.

Erbium implantation in strontium titanate

Abstract We report on the lattice location of Er in SrTiO3 single crystals using the emission channeling technique. The angular distribution of conversion electrons emitted from 167 m Er (T1/2=2.27 s) was monitored with a position-sensitive detector following the room-temperature 60 keV implantation of the precursor isotope 167 Tm (T1/2=9.25 d) to a dose of 2×1012 at./cm2. The results for the sample annealed in vacuum at 610 °C for 15 min provide direct evidence that Er occupies both Sr and Ti substitutional lattice sites. In addition, thermal recovery of lattice damage was also studied with Rutherford backscattering/channeling for SrTiO3 implanted to doses of 5×1014 and 5×1015 Er/cm2. We further comment on preliminary photoluminescence results from these samples.