S. Ruffenach

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 ...

Indium nitride quantum dots grown by metalorganic vapor phase epitaxy

With respect to growing indium nitride quantum dots with very low surface densities for quantum cryptography applications, we have studied the metalorganic vapor phase epitaxy of InN onto GaN buffer layers. From lattice mismatch results the formation of self-assembled dots. The effects of the growth temperature, V/III molar ratio, and deposition time are studied, and we demonstrate that quantum-sized dots of InN can be grown with a material crystalline quality similar to the quality of the GaN buffer layer, in densities of 107 to 108 cm−2. Such low densities of dots allow for the realization of experiments or devices in which a single dot is isolated, and may be used in the near future to produce single-photon sources.With respect to growing indium nitride quantum dots with very low surface densities for quantum cryptography applications, we have studied the metalorganic vapor phase epitaxy of InN onto GaN buffer layers. From lattice mismatch results the formation of self-assembled dots. The effects of the growth temperature, V/III molar ratio, and deposition time are studied, and we demonstrate that quantum-sized dots of InN can be grown with a material crystalline quality similar to the quality of the GaN buffer layer, in densities of 107 to 108 cm−2. Such low densities of dots allow for the realization of experiments or devices in which a single dot is isolated, and may be used in the near future to produce single-photon sources.

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.

Radiative and nonradiative recombination processes in InN films grown by metal organic chemical vapor deposition

The 800meV photoluminescence band in indium nitride is excited under pulsed excitation conditions and is investigated as a function of temperature and time. Our results are consistent with a composite photoluminescence feature composed of two overlapping bands separated by an ∼10meV splitting, with populations described by a thermal equilibrium model. Efficient nonradiative recombination channels rule both the temperature dependence of the time-integrated photoluminescence spectra and the recombination dynamics. At 10K, the radiative recombination time is of the order of 300ns, while the nonradiative recombination time, which is ruled by activation energy of 8meV, is about 100ps.

Misfit relaxation of InN quantum dots : Effect of the GaN capping layer

The strain state on InN quantum dots (QDs) over GaN/sapphire substrates was analyzed by transmission electron microscopy. Changes in the in-plane lattice parameter of uncapped and capped InN QD heterostructures have been measured using moire fringe analysis. The uncapped QDs are almost completely relaxed, due to a misfit dislocation network present at the InN∕GaN interface without generating any threading dislocations inside the QDs. In addition, a low-temperature-GaN capping process on InN QDs heterostructures was evaluated. Although this deposition avoids the InN decomposition, it modifies the QDs’ morphology, decreases both the aspect ratio and, consequently, the plastic relaxation of the heterostructure.

Control of InN quantum dot density using rare gases in metal organic vapor phase epitaxy

Indium nitride (InN) quantum dots have been grown on gallium nitride (GaN) templates with heights of 10 and 20nm. The authors demonstrate that the surface densities of the dots are strongly affected by the nature of the carrier gas used during the growth, which can be used to modulate the surface density. The authors show here that replacing nitrogen by helium leads to a decrease of the dot surface density, while argon induces a strong increase of the density. Although validated for the InN∕GaN system, this approach has a more general scope and can be extended to other material systems.

Failure mechanism of AlN nanocaps used to protect rare earth-implanted GaN during high temperature annealing

The structural properties of nanometric AlN caps, grown on GaN to prevent dissociation during high temperature annealing after Eu implantation, have been characterized by scanning electron microscopy and electron probe microanalysis. The caps provide good protection up to annealing temperatures of at least 1300°C, but show localized failure in the form of irregularly shaped holes with a lateral size of 1–2μm which extend through the cap into the GaN layer beneath. Compositional micrographs, obtained using wavelength dispersive x-ray analysis, suggest that these holes form when GaN dissociates and ejects through cracks already present in the as-grown AlN caps due to the large lattice mismatch between the two materials. Implantation damage enhances the formation of the holes during annealing. Simultaneous room temperature cathodoluminescence mapping showed that the Eu luminescence is reduced in N-poor regions. Hence, exposed GaN dissociates first by outdiffusion of nitrogen through AlN cracks, thereby opening a hole in the cap through which Ga subsequently evaporates.The structural properties of nanometric AlN caps, grown on GaN to prevent dissociation during high temperature annealing after Eu implantation, have been characterized by scanning electron microscopy and electron probe microanalysis. The caps provide good protection up to annealing temperatures of at least 1300°C, but show localized failure in the form of irregularly shaped holes with a lateral size of 1–2μm which extend through the cap into the GaN layer beneath. Compositional micrographs, obtained using wavelength dispersive x-ray analysis, suggest that these holes form when GaN dissociates and ejects through cracks already present in the as-grown AlN caps due to the large lattice mismatch between the two materials. Implantation damage enhances the formation of the holes during annealing. Simultaneous room temperature cathodoluminescence mapping showed that the Eu luminescence is reduced in N-poor regions. Hence, exposed GaN dissociates first by outdiffusion of nitrogen through AlN cracks, thereby openi...

Influence of V/III molar ratio on the formation of In vacancies in InN grown by metal-organic vapor-phase epitaxy

We have applied a slow positron beam to study InN samples grown by metal-organic vapor-phase epitaxy with different V/III molar ratios (3300–24 000) and at different growth temperatures (550–625°C). Indium vacancies were identified in samples grown at V/III ratios below 4000. Their concentration is in the 1017cm−3 range. No strong dependence of vacancy concentration on the molar ratio was observed. At low V/III ratios, however, In droplets and vacancy clusters are formed near the substrate interface. The elevated growth temperature enhances the In vacancy formation, possibly due to limited sticking of In on the growth surface close to the decomposition temperature.

Nucleation of InN quantum dots on GaN by metalorganic vapor phase epitaxy

InN quantum dots (QDs) on GaN (0001) grown by metalorganic vapor phase epitaxy onto a sapphire substrate were studied by transmission electron microscopy (TEM). We found that the nucleation of InN QDs on GaN is directly related to the presence of threading dislocations (TDs) in the center of the QDs. The TEM analysis revealed that the TDs finish at the InN∕GaN interface and they are pure edge dislocations. Therefore, spiral growth models cannot explain nucleation of these QDs. Although controlling edge TDs constitute a possible approach to determine the QD density, a better approach may be an increase in the material growth rate in order to enter the diffusion-limited growth mode, where growth is not sensitive to surface heterogeneities.

The determination of the bulk residual doping in indium nitride films using photoluminescence

We extend to any temperature, the sophisticated calculation of the evolution of the 2 K photoluminescence energy of InN proposed by Arnaudov et al. [Phys. Rev. B 69, 115216 (2004)], in view of determining the residual doping of thin films. From the detailed line shape modeling, we extract the full width at half maximum of the photoluminescence line which, in the first order, varies like n0.51 at low temperature. This allows us to propose a handy tool for rapid residual doping evaluation. Last, temperature and inhomogeneous broadening effects are analyzed. Ignoring the latter is shown to lead to an overestimation of the residual doping.