A.J. Neves

Use of ZnO thin films as sacrificial templates for metal organic vapor phase epitaxy and chemical lift-off of GaN

Continued development of GaN-based light emitting diodes is being hampered by constraints imposed by current non-native substrates. ZnO is a promising alternative substrate but it decomposes under the conditions used in conventional GaN metal organic vapor phase epitaxy (MOVPE). In this work, GaN was grown on ZnO/c-Al2O3 using low temperature/pressure MOVPE with N2 as a carrier and dimethylhydrazine as a N source. Characterization confirmed the epitaxial growth of GaN. The GaN was lifted-off the c-Al2O3 by chemically etching away the ZnO underlayer. This approach opens up the way for bonding of the GaN onto a support of choice.

Synthesis, surface modification and optical properties of Tb3+-doped ZnO nanocrystals

We report the synthesis and characterization of colloidal nanocrystals consisting of ZnO doped with Tb3+ cations. Introducing the Tb3+ cations in ZnO nanosized hosts led to photoluminescent systems whose wavelength emission depends on the excitation line. Chemical surface modification of the doped ZnO nanocrystals associated with photoluminescence studies allows us to conclude that Tb3+ cations are located in the ZnO core.

Near-band-edge slow luminescence in nominally undoped bulk ZnO

We report the observation of slow emission bands overlapped with the near-band-edge steady-state luminescence of nominally undoped ZnO crystals. At low temperatures the time-resolved spectra are dominated by the emission of several high-energy bound exciton lines and the two-electron satellite spectral region. Furthermore, two donor-acceptor pair transitions at 3.22 and 3.238eV are clearly identified in temperature-dependent time-resolved spectroscopy. These donor-acceptor pairs involve a common shallow donor at 67meV and deep acceptor levels at 250 and 232meV.

Influence of nucleation density on film quality, growth rate and morphology of thick CVD diamond films

Abstract The optimum growth parameters of our 5 kW microwave plasma CVD reactor were obtained using CH4/H2/O2 plasma and high quality transparent films can be produced reproducibly. Among the films prepared in this system, the film of best quality has very smooth crystalline facets free of second nucleation and the full width at half maximum (FWHM) of the diamond Raman peak is 2.2 cm−1, as narrow as that of IIa natural diamond. For this study, diamond films were grown on silicon substrates with low (104–105 cm−2) and high nucleation densities (>1010 cm−2), respectively. From the same growth run, a highly 〈110〉 textured 300 μm thick white diamond film with a growth rate of 2.4 μm/h was obtained from high nucleation densities (>1010 cm−2), and a white diamond film of 370 μm in thickness with a higher growth rate of 3 μm/h was obtained from low nucleation densities (5×104–105 cm−2) too. The effect of nucleation density on film quality, growth rate, texture and morphology was studied and the mechanism was discussed. Our results suggest that under suitable growth conditions, nucleation density has little effect on film quality and low nucleation density results in higher growth rate than high nucleation density due to less intense grain growth competition.

Optical doping and damage formation in AlN by Eu implantation

AlN films grown on sapphire were implanted with 300 keV Eu ions to fluences from 3x1014 to 1.4x1017 atomscm2 in two different geometries: “channeled” along the c-axis and “random” with a 10° angle between the ion beam and the surface normal. A detailed study of implantation damage accumulation is presented. Strong ion channeling effects are observed leading to significantly decreased damage levels for the channeled implantation within the entire fluence range. For random implantation, a buried amorphous layer is formed at the highest fluences. Red Eu-related photoluminescence at room temperature is observed in all samples with highest intensities for low damage samples (low fluence and channeled implantation) after annealing. Implantation damage, once formed, is shown to be stable up to very high temperatures.

A new elegant technique for polishing CVD diamond films

Abstract It is well known that the columnar growth nature of CVD diamond results in a very rough growth surface and the surface roughness steeply increases with film thickness, especially for thick CVD diamond films. In this paper, we report the successful implementation of a new elegant technique for polishing thick polycrystalline CVD diamond films at high polishing rate of up to 10 μm/h. This technique involves polishing the as-grown polycrystalline diamond films with another thick as-grown polycrystalline diamond film, which acts as a polishing abrasive. Two types of diamond films were prepared using microwave plasma CVD and then polished for 2 h using the new polishing technique. A stylus profilometer, scanning electron microscopy and Fourier transform infrared spectroscopy were used to measure the surface roughness, characterize morphology and optical transmission of the samples before and after polishing, respectively. By polishing, thickness of 20–30 μm was removed from the top surface, and the mean surface roughness Ra of the films reduced significantly, e.g. for one film Ra reduced initially from 5.2 to 1.35 μm and the other from 3.2 to 0.55 μm. The principal advantages of this new polishing technique are simplicity, flexibility and time saving. This simple method can serve as ‘rough chipping’ to quickly remove the rough top surface and then combine with conventional polishing methods for precision machining to further reduce the surface roughness to a specific desired degree.

Up conversion from visible to ultraviolet in bulk ZnO implanted with Tm ions

We report on the up-converted ultraviolet near-band edge emission of bulk ZnO generated by visible and ultraviolet photons with energies below the band gap. This up-converted photoluminescence was observed in samples intentionally doped with Tm ions, suggesting that the energy levels introduced by the rare earth ion in the ZnO band gap are responsible for this process.

A green-emitting CdSe/Poly(butyl acrylate) nanocomposite

CdSe/poly(butyl acrylate) nanocomposites were synthesized by in situ miniemulsion polymerization. The hybrid nanomaterial is very stable and presents a bright green photoluminescence at 2.29 eV under ultraviolet excitation. With the excitation conditions used the intensity of the emission band keeps nearly constant from 7 K to room temperature. The morphological, structural and room temperature electrical properties of the CdSe/poly(butyl acrylate) nanocomposite have been investigated.

Infrared absorption study of hydrogen incorporation in thick nanocrystalline diamond films

We present an infrared (IR) optical absorbance study of hydrogen incorporation in nanocrystalline diamond films. The thick nanocrystalline diamond films were synthesized by microwave plasma-assisted chemical vapor deposition and a high growth rate about 3.0μm∕h was achieved. The morphology, phase quality, and hydrogen incorporation were assessed by means of scanning electron microscopy, Raman spectroscopy, and Fourier-transform infrared spectroscopy (FTIR). Large amount of hydrogen bonded to nanocrystalline diamond is clearly evidenced by the huge CH stretching band in the FTIR spectrum. The mechanism of hydrogen incorporation is discussed in light of the growth mechanism of nanocrystalline diamond. This suggests the potential of nanocrystalline diamond for IR electro-optical device applications.

Influence of nucleation on hydrogen incorporation in CVD diamond films

Abstract In this work, the effect of nucleation on hydrogen incorporation in Microwave Plasma CVD (MPCVD) deposited diamond films is studied for the first time. Free-standing diamond films with thickness of approximately 200 μm were grown on silicon wafers with different nucleation densities under the same MPCVD deposition. The bonded hydrogen content and incorporation forms were measured by Fourier Transformer Infrared Spectroscopy (FTIR). Our results show that the total hydrogen content and the intensity of the peak at 2828 cm−1 increases rapidly with decreasing nucleation density while the film quality decreases. The origin of the two peaks at 2818 and 2828 cm−1 is discussed.