Development of Micron Sized Photonic Devices Based on Deep GaN Etching

In order to design and development efficient III-nitride based optoelectronic devices, technological processes require a major effort. We propose here a detailed review focussing on the etching procedure as a key step for enabling high date rate performances. In our reported research activity, dry etching of an InGaN/GaN heterogeneous structure was investigated by using an inductively coupled plasma reactive ion etching (ICP-RIE). We considered different combinations of etch mask (Ni, SiO2, resist), focussing on the optimization of the deep etching process. A GaN mesa process with an etching depth up to 6 µm was performed in Cl2/Ar-based plasmas using ICP reactors for LEDs dimen sions ranging from 5 to 150 µm². Our strategy was directed toward the mesa formation for vertical-type diode applications, where etch depths are relatively large. Etch characteristics were studied as a function of ICP parameters (RF power, chamber pressure, fixed total flow rate). Surface morphology, etch rates and sidewall profiles observed into InGaN/GaN structures were compared under different types of etching masks. For deep etching up to few microns into the GaN template, we state that a Ni or SiO2 mask is more suitable to obtain a good selectivity and vertical etch profiles. The optimized etch rate was about 200nm/min under moderate ICP conditions. We applied these conditions for the fabrication of micro/nano LEDs dedicated to LiFi applications.

Surface Nanostructuring during Selective Area Epitaxy of Heterostructures with InGaAs QWs in the Ultra-Wide Windows

Selective area epitaxy (SAE) is widely used in photonic integrated circuits, but there is little information on the use of this technique for the growth of heterostructures in ultra-wide windows. Samples of heterostructures with InGaAs quantum wells (QWs) on GaAs (100) substrates with a pattern of alternating stripes (100-μm-wide SiO2 mask/100-μm-wide window) were grown using metalorganic chemical vapour deposition (MOCVD). It was found that due to a local change in the growth rate of InGaAs QW in the window, the photoluminescence (PL) spectra measured from the edge to the center of the window exhibited maximum blueshifts of 14 and 19 meV at temperatures of 80 K and 300 K, respectively. Using atomic force microscopy, we have demonstrated that the surface morphologies of structures grown using standard epitaxy or SAE under identical MOCVD growth conditions correspond to a step flow growth with a step height of ~1.5 ML or a step bunching growth mode, respectively. In the structures grown with the use of SAE, a strong variation in the surface morphology in an ultra-wide window from its center to the edge was revealed, which is explained by a change in the local misorientation of the layer due to a local change in the growth rate over the width of the window.

Selective Area Epitaxy of GaAs/Ge/Si Nanomembranes: A Morphological Study

We demonstrate the feasibility of growing GaAs nanomembranes on a plastically-relaxed Ge layer deposited on Si (111) by exploiting selective area epitaxy in MBE. Our results are compared to the case of the GaAs homoepitaxy to highlight the criticalities arising by switching to heteroepitaxy. We found that the nanomembranes evolution strongly depends on the chosen growth parameters as well as mask pattern. The selectivity of III-V material with respect to the SiO2 mask can be obtained when the lifetime of Ga adatoms on SiO2 is reduced, so that the diffusion length of adsorbed Ga is high enough to drive the Ga adatoms towards the etched slits. The best condition for a heteroepitaxial selective area epitaxy is obtained using a growth rate equal to 0.3 ML/s of GaAs, with a As BEP pressure of about 2.5 × 10−6 torr and a temperature of 600 °C.

Non-edge-triggered inversion from Ga polarity to N polarity of c-GaN domains on an SiO2 mask during epitaxial lateral overgrowth

It was previously reported that N-polar c-GaN domains nucleated in window openings on c-plane sapphire were inverted to Ga-polar domains at the edge of an SiO2 mask during epitaxial lateral overgrowth, but it was asserted that polarity inversion of N-polar GaN domains could not occur beyond the edge of the SiO2 mask. However, that assertion was demonstrated only in the case of a-facet-exposed GaN. It is reported here that polarity inversion from Ga polarity to N polarity of m-facet-exposed c-GaN domains occurred during epitaxial lateral overgrowth on the flat region beyond the edge of a circular-patterned SiO2 mask. An increased flow rate of NH3 during the epitaxial lateral overgrowth is thought to induce this type of non-edge-triggered polarity inversion. Further investigation reveals that non-edge-triggered polarity inversion is also possible when the a facet is exposed at the lateral growth front of Ga-polar GaN domains.

Correction: Surface-enhanced Raman scattering on a silver film-modified Au nanoparticle-decorated SiO2 mask array

Correction for ‘Surface-enhanced Raman scattering on a silver film-modified Au nanoparticle-decorated SiO2 mask array’ by Chi-Ching Chang et al., RSC Adv., 2015, 5, 66096–66103.