K. Lorenz

Metal-organic vapor phase epitaxy and properties of AlInN in the whole compositional range

The authors present a detailed study of Al1−xInxN layers covering the whole composition range of 0.09<x<1. All layers were grown on GaN on Si(111) templates using metal-organic vapor phase epitaxy. For 0.13<x<0.32 samples grow fully strained and without phase separation. At higher In concentrations, the crystalline quality starts to deteriorate and a transition to three-dimensional growth is observed. A comparison of their experimental data with theoretically predicted phase diagrams reveals that biaxial strain increases the stability of the alloy.

Selectively excited photoluminescence from Eu- implanted GaN

The intensity of Eu-related luminescence from ion-implanted GaN with a 10nm thick AlN cap, both grown epitaxially by metal organic chemical vapor deposition (MOCVD) is increased markedly by high-temperature annealing at 1300°C. Photoluminescence (PL) and PL excitation (PLE) studies reveal a variety of Eu centers with different excitation mechanisms. High-resolution PL spectra at low temperature clearly show that emission lines ascribed to D05-F27 (∼622nm), D05-F37 (∼664nm), and D05-F17 (∼602nm) transitions each consist of several peaks. PL excitation spectra of the spectrally resolved components of the D05-F27 multiplet contain contributions from above-bandedge absorption by the GaN host, a GaN exciton absorption at 356nm, and a broad subedge absorption band centred at ∼385nm. Marked differences in the shape of the D05-F27 PL multiplet are demonstrated by selective excitation via the continuum/exciton states and the below gap absorption band. The four strongest lines of the multiplet are shown to consist ...

Damage formation and annealing at low temperatures in ion implanted ZnO

N, Ar, and Er ions were implanted into ZnO at 15 K within a large fluence range. The Rutherford backscattering technique in the channeling mode was used to study in situ the damage built-up in the Zn sublattice at 15 K. Several stages in the damage formation were observed. From the linear increase of the damage for low implantation fluences, an upper limit of the Zn displacement energy of 65 eV could be estimated for [0001] oriented ZnO. Annealing measurements below room temperature show a significant recovery of the lattice starting at temperatures between 80 and 130 K for a sample implanted with low Er fluence. Samples with higher damage levels do not reveal any damage recovery up to room temperature, pointing to the formation of stable defect complexes.

Photoluminescence and lattice location of Eu and Pr implanted GaN samples

Rare earth (RE) ions implanted GaN films were studied by optical spectroscopy and RBS techniques. Sharp emission lines due to intra-4f n shell transitions can be observed even at room temperature for the Eu 3+ and Pr 3+ .Th e photoluminescence spectra recorded by the above band gap excitation reveal dominant transitions due to the 5 D0- 7 F1,2,3 lines at 6004, 6211 and 6632 ( A for the Eu 3+ and 3 P0,1- 3 F2,3 at 6450 and 6518 ( A, respectively, for the Pr 3+ . We report on the temperature dependence of the intra-ionic emissions as well as on the lattice site location of the RE detailed angular scans through the / 00 0 1S and / 10 % S axial directions; which indicates that for Pr, complete substitutionality on the Ga sites was achieved while for Eu a Ga displaced site was found. r 2001 Elsevier Science B.V. All rights reserved.

High-temperature annealing and optical activation of Eu-implanted GaN

Europium was implanted into GaN through a 10nm thick epitaxially grown AlN layer that protects the GaN surface during the implantation and also serves as a capping layer during the subsequent furnace annealing. Employing this AlN layer prevents the formation of an amorphous surface layer during the implantation. Furthermore, no dissociation of the crystal was observed by Rutherford backscattering and channeling measurements for annealing temperatures up to 1300°C. Remarkably, the intensity of the Eu related luminescence, as measured by cathodoluminescence at room temperature, increases by one order of magnitude within the studied annealing range between 1100 and 1300°C.

Optical energies of AlInN epilayers

Optical energy gaps are measured for high-quality Al1−xInxN-on-GaN epilayers with a range of compositions around the lattice match point using photoluminescence and photoluminescence excitation spectroscopy. These data are combined with structural data to determine the compositional dependence of emission and absorption energies. The trend indicates a very large bowing parameter of ≈6eV and differences with earlier reports are discussed. Very large Stokes’ shifts of 0.4–0.8eV are observed in the composition range 0.13<x<0.24, increasing approximately linearly with InN fraction despite the change of sign of the piezoelectric field.

A mechanism for damage formation in GaN during rare earth ion implantation at medium range energy and room temperature

A detailed investigation of the crystallographic damage has been carried out in GaN following 300 keV rare earth ion implantation at room temperature by varying the fluence from 7×1013 to 5×1016 at/cm2. It is shown that above a threshold fluence around 2×1015 at/cm2, nanocrystallization takes place from the surface, subsequent to the formation of a planar defects network consisting of basal and prismatic stacking faults. This network starts to form at the lowest analyzed fluence mostly around the mean projected range. When the fluence increases, it propagates toward the surface, reaching it just before the on-set of the nanocrystallization. A model based on the mechanical breakdown of the GaN wurtzite structure mediated by prismatic stacking faults is proposed.

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.

Transmission electron microscopy investigation of the structural damage formed in GaN by medium range energy rare earth ion implantation

The crystallographic nature of the damage created in GaN by 300keV rare earth ions has been investigated following implantation at room temperature by varying the fluence of Er, Eu, or Tm from 7×1013to2×1016at.∕cm2. There is a build up of point defects clusters, which increases in density and depth versus the ion fluence. When a threshold around 3×1015at.∕cm2 is reached, a nanocrystalline surface layer is observed. From the lowest fluence, we point out the formation of basal stacking faults, with a majority of I1. Their density also increases with the fluence, but it is seen to saturate at the onset of the observation of the surface nanocrystalline layer. Extrinsic E faults bounded by the c∕2 partials have also been identified; however, most of the E stacking faults transform to I1 which are noticed to fold easily from basal to prismatic planes by switching or not to the Drum atomic configuration.

Radiation damage formation and annealing in GaN and ZnO

The radiation damage formation upon low temperature ion implantation and neutron irradiation has been compared for GaN and ZnO. Both materials exhibit strong dynamic annealing effects during implantation, even at 15 K, leading to high amorphisation thresholds. The damage build-up with fluence was found to proceed in a similar way for GaN and ZnO, both showing two saturation regimes below the amorphisation level where, over wide fluence regions, the damage level increases only very slowly. For low fluences the damage accumulation rate is similar for both materials. For higher fluences, on the other hand, GaN shows considerably higher damage levels and finally collapses into an amorphous structure while ZnO remains single crystalline up to the highest fluence of 7×1016 Ar/cm2. Neutron irradiation produces similar defects as ion implantation but within the entire sample while the defect density is much lower. The main effect of irradiation on the structural properties of GaN is an expansion of the c-lattice parameter. Optical properties are significantly deteriorated after irradiation and only recover partially after annealing. ZnO does not suffer such a pronounced change of the lattice parameters but reveals a strong deterioration of the surface, possibly due to blistering and exfoliation. At the same time the optical properties are less affected than for GaN. The near band edge emission is partly quenched but recovers to a large extend after annealing while broad defect bands are observed below the bandgap for irradiated samples, before and after annealing.