III-Nitride Semiconductor Photonic Nanocavities on Silicon

Optical nanocavities enhance light-matter interaction due to their high quality factors (Q) and small modal volumes (V). The control of light-matter interaction lies at the heart of potential applications for integrated optical circuits, including optical communication, quantum computation, biophotonics, and optical projection. Many of these applications could benefit from wide band gap III-nitride semiconductor material, which already forms the basis of industrial lighting technology based on blue light-emitting diodes. Cleanroom fabrication and optical spectroscopy comprise the laboratory work in this thesis. Air-suspended III-nitride nanocavities are fabricated using electron beam lithography, reactive ion etching, and vapor phase etching from thin (< 300 nm) III-nitride epilayers grown on silicon (111) substrates by metal-organic vapor phase epitaxy. An embedded single InGaN/GaN quantum well serves as an internal light source during micro-photoluminescence experiments. Strain and heating effects are analyzed by micro-Raman spectroscopy. A dedicated quantum optics laboratory is constructed in order to study nanocavities and quantum emitters in III-nitrides at short wavelengths (lambda < 500 nm). Photonic crystal nanocavities exhibit the highest figure-of-merit for light-matter interaction enhancement, Q/V. However, in contrast to silicon and gallium arsenide nanocavities working at telecommunication wavelengths, III-nitride based nanocavities underperform theoretical Q estimates by nearly three orders of magnitude at short wavelengths (lambda = 430 - 500 nm). This thesis accounts for this discrepancy by combined experimental and theoretical studies of nanobeam fabrication statistics. Surface state absorption is found to be the primary factor limiting Q at short wavelengths. Surface effects are studied in the microdisk resonator geometry. UV photoinduced desorption of oxygen gas from the III-nitride surface redshifts and broadens whispering gallery mode resonances over minute timescales. Oxygen desorption incurs additional optical absorption losses up to 100 cm^{-1}. The redshift and broadening is nearly reversible upon the introduction of oxygen into the measurement environment, implicating oxygen in passivation of the III-nitride surface. Finally, optimized surface passivation techniques using rapid thermal processing and conformal chemical vapor deposition more than double state-of-the-art Q of GaN-based microdisk resonators to beyond Q=10,000 in the blue spectral range. The results demonstrate the capabilities and limitations of III-nitride materials for optical nanocavities and photonic integrated circuits at short wavelengths.

GaN-based photonic crystal cavities on silicon for visible and near infrared applications

Noelia Vico Triviño

Photonic crystal (PhC) cavities combine ultra-high quality (Q) factors with small mode volumes, resulting in an enhancement of the light-matter interaction at the nanoscale, which, beyond fundamental studies is advantageous for countless applications in photonics. In addition, III-nitride (III-N) semiconductors offer unique optical properties including a direct wide bandgap and tunable emission spanning the ultraviolet to near infrared (NIR) spectral region. Despite these advantages the development of III-N based PhC cavities has been hindered owing to several processing issues inherent to such hard and chemically inert materials. This work aims at investigating GaN-based PhC cavities operating at both visible and NIR frequencies. In the first part of this work a fabrication process that fulfills the low-defect requirements of PhCs, while remaining compatible with silicon for future integration prospects, was developed. It is based on the growth of GaN layers on Si(111) substrates, which are then patterned by e-beam lithography and dry etching techniques, including the substrate undercut in order to create an airgap below the PhC area, which guarantees light confinement into the resulting planar waveguide. This approach allowed achieving airgaps > 3 µm, which are essential for structures operating in the NIR, and hardly achievable by sacrificial layer techniques. The ultimate assessment of the optical quality of such nanostructures is given by the Q-factor of the cavity. In this regard, spectroscopic characterization of the fabricated samples was carried out. At visible wavelengths experimental Q-factors up to 5200 and 14000 have been ascertained in 2D PhC cavities and 1D nanobeam cavities, respectively, which represent the current state-of-the-art for both geometries at the time this dissertation was written. At IR frequencies the maximum Q which was measured amounts to 22500. Furthermore, it is important to estimate the fluctuations on both the Q-factor and the resonant wavelength of nominally identical replicas of the same cavity to account for the fabrication process reliability. This has been addressed by taking advantage of PhC cavities optimized by an effective genetic algorithm for Q-factor optimization (166000 at 1.3 µm in this case) [M.Minkov and V. Savona, Sci. Rep. 4, 5124 (2014)]. Thus, 20 groups of cavities were fabricated, to allow for lithographic tuning, that differ by 2 nm in the hole diameter from one group to another. Q-factor measurements were then carried out in a total of 60 cavities, namely 6 replicas of 10 different groups of cavities, demonstrating a high reproducibility and an average Q-factor ~17000. The second part of this thesis investigates the lasing characteristics of nanobeam cavities emitting in the violet-blue spectral range featuring a single InGaN quantum well (QW). In the high-absorption QW region lasing has been observed under continuous wave (cw) optical pumping at room temperature, demonstrating low threshold power densities ~740 W/cm2. The laser behavior was also analyzed by means of laser rate equations. The low cw lasing threshold is well accounted for by a large spontaneous emission coupling factor (>0.8), inherent to the nanobeam geometry. These results highlight the high potential of III-nitrides for the realization of nanophotonic devices and set a new step forward for the integration of wide bandgap semiconductors with silicon opening future prospects in biosensing and optogenetics.

EPFL2015

Fabrication defects and grating couplers in III-nitride photonic crystal nanobeam lasers (Conference Presentation)

Raphaël Butté, Jean-François Carlin, Nicolas Grandjean, Ian Michael Rousseau

We report a numerical and experimental investigation of fabrication tolerances and outcoupling in optically pumped III-nitride nanolasers operating near lambda = 460 nm, in which feedback is provided by a one-dimensional photonic crystal nanobeam cavity and gain is supplied by a single InGaN/GaN quantum well. Using this platform, we [1] and others [2] previously demonstrated single-mu W lasing thresholds due to the high beta Q-product inherent to the nanobeam geometry (beta is spontaneous emission coupling fraction into desired mode). In this work, we improved the fraction of emission emitted into our microscope's light cone by combining a redesigned photonic crystal cavity (c.f. [3]) with a cross-grating coupler with period approximately twice the photonic crystal lattice constant [4]. The samples were fabricated in epitaxial III-nitride layers grown on (111) silicon substrates using metal organic vapor phase epitaxy. The photonic crystal and output couplers were patterned using a single electron beam lithography exposure and subsequently transferred to the underlying III-nitride layers using dry etching. The nanobeams were then suspended via vapor phase etching of silicon in XeF2 (Figure 1) [5]. Scanning electron microscopy cross-sections revealed high-aspect ratio (>5), sub-70 nanometer diameter holes with near-vertical sidewalls. Fabrication-induced geometry errors were characterized by processing scanning electron micrographs with custom critical dimension software (Figure 2). Using UV micro-photoluminescence spectroscopy at room temperature, we measured the nanobeams' emission intensity, far-field profile, and quality factor. By comparing more than ten nominally identical nanobeams for each geometry with finite-difference time-domain simulations taking into account the geometrical deviations measured during fabrication [8], we characterized the role of fabrication-induced imperfections. Finally, we explored the trade-off between the quality factor and collected signal via lithographic variations of the output coupler grating amplitude. Our results demonstrate the robustness of III-nitrides for short-wavelength photonic crystal applications, such as photonic integrated circuits, optoelectronics, and cavity quantum electrodynamics.

Spie-Int Soc Optical Engineering2016

Light Confinement and Nonlinear Light-Matter Interaction in Semiconductor Photonic Crystal Cavities

Mohamed Sabry Abdel-Aliem Mohamed

The ability to control and manipulate light is a fundamental aspect that is at the very core of the development of integrated photonic circuits. It is desirable to achieve such control down to a scale that is comparable to or smaller than the wavelength of light, to enable highly efficient interaction with matter, which can eventually go down to single photon levels. Photonic crystals are particularly appealing in that respect, by providing optical bandgaps and facilitating dispersion shaping, through the precise patterning of layers. This is to be accomplished ideally in compact designs that are well-suited for fabrication with current technology and by utilizing material that has the capacity for integration. By exploiting existing light-matter interaction mechanisms, the available toolset for component design can be expanded, allowing to build optical chips with a wide range of functionality. In the current work, a platform for integrated III-nitride photonic components on silicon is designed and implemented. A refined fabrication process for III-nitride micro- and nanostructures is developed, leading to the realization of suspended optical devices with both high performance and structural stability. Thorough optical and material characterization of the semiconductor layers is conducted to identify potential limitations, particularly with epitaxial GaN on Si technology, with respect to current crystal growth techniques. AlN layers based on a sputter-deposition process are proposed as an alternative for passive optical devices. Two-dimensional photonic crystals are designed in gallium nitride and aluminum nitride, targeting operational wavelengths in the near-infrared telecom range. Optimized photonic crystal cavities are considered for maximizing light confinement in the semiconductor layers. By utilizing resonance enhancement, second and third harmonic generation are demonstrated in gallium nitride photonic crystal cavities featuring record-high quality factor values. This is achieved through a far-field coupling approach under low-power continuous-wave operation. High conversion efficiencies are attained in gallium nitride, exceeding previous demonstrations in the material. Propagation of the harmonic signals in the semiconductor layer is analyzed using finite-element time-domain modeling for the feasibility of an extraction mechanism. Moreover, slow light coupled cavity waveguides are developed in a silicon platform, relying on optimized cavity designs that feature wideband operation in the near-infrared telecom range. A record-high value for group-index bandwidth product is achieved in the devices through improved design implementation. An end-fire based Fourier-space imaging technique is applied for the characterization of the optical structures, through which spatially-selective reconstruction of dispersion maps enabled accurate extraction of slow light properties. The influence of finite-size effects on slow light behavior is elucidated by implementing extended cavity chains. Furthermore, light transport regimes across the dispersion band are identified including the transition towards diffusive light transport and subsequent localization. The performance of coupled-cavity waveguides in the presence of disorder is quantified, revealing constraints on slow light transport, considering state-of-the-art fabrication.

EPFL2018