Rigid wing sailboats: A state of the art survey

Properties and perspectives of ultrawide bandgap Ga2O3 in optoelectronic applications

Autonomous sailboat design: A review from the performance perspective

Heterogeneous integration of β-(SnxGa1-x)2O3 (TGO) UV-C photodetectors on silicon substrates by molecular beam epitaxy is demonstrated. Multimodal electron microscopy and spectroscopy techniques reveal a direct correlation between structural, compositional and optical properties of the TGO and the functional properties of the photodetectors. Wavelength dispersive X-ray spectroscopy results accurately determine the Sn concentrations (x) in the region of 0.020, and room temperature cathodoluminescence (CL) hyperspectral imaging shows changes in CL emission intensity in the TGO compared with a Ga2O3 sample with no Sn. Alloying Ga2O3 with Sn is shown to quench the red emission and enhance the blue emission. The increase in blue emission corresponds to the rise in VGa-related deep acceptors responsible for the high gain observed in the TGO detectors. A Ga2O3 nucleation layer is shown to improve the TGO surface quality and give better device properties compared to TGO grown directly onto the Si substrate, including a higher specific detectivity on the order of 1012 Jones.

Correlation between deep-level defects and functional properties of β-(SnxGa1-x)2O3 on Si photodetectors

An alternative wing sail section shape was developed as a potential improvement to either a soft sail or the common NACA00XX section used on some small autonomous sailing craft. The prototype wing was developed as an evolution of a traditional square sail. Evaluation included wind tunnel and on-the-water tests followed by performance estimates from a Velocity Prediction Program. The test vessel was the 1.2 m long Version 2 MaxiMOOP. Advantages of the wing sail over the current voyaging soft sail design include: higher speeds in some conditions, increased durability, increased buoyancy in the case of capsizing, and easier implementation of control systems. Some potential theoretical advantage was found over the NACA0009.

An Alternative Wing Sail Concept for Small Autonomous Sailing Craft

β-Ga2O3 is an ultra-wide bandgap semiconductor and is thus expected to be optically transparent to light of sub-bandgap wavelengths well into the ultraviolet. Contrary to this expectation, it is found here that free electrons in n-doped β-Ga2O3 absorb light from the IR to the UV wavelength range via intra- and inter-conduction band optical transitions. Intra-conduction band absorption occurs via an indirect optical phonon mediated process with 1 / ω 3 dependence in the visible to near-IR wavelength range. This frequency dependence markedly differs from the 1 / ω 2 dependence predicted by the Drude model of free-carrier absorption. The inter-conduction band absorption between the lowest conduction band and a higher conduction band occurs via a direct optical process at λ ∼ 349 nm (3.55 eV). Steady state and ultrafast optical spectroscopy measurements unambiguously identify both these absorption processes and enable quantitative measurements of the inter-conduction band energy and the frequency dependence of absorption. Whereas the intra-conduction band absorption does not depend on light polarization, inter-conduction band absorption is found to be strongly polarization dependent. The experimental observations, in excellent agreement with recent theoretical predictions for β-Ga2O3, provide important limits of sub-bandgap transparency for optoelectronics in the deep-UV to visible wavelength range and are also of importance for high electric field transport effects in this emerging semiconductor.

Intra- and inter-conduction band optical absorption processes in β-Ga2O3

We are designing a cheap, mass producible 1 m semi-autonomous robotic sailboat that can navigate the oceans for months using only intermittent external supervision. The boat should efficiently collect environmental data such as salinity, turbidity, fluorescence, and animal sounds. The boat has a symmetric airfoil sail, a thin, bulbed keel, and most notably, an apparently new means of steering: a tail-vane rudder replacing the water rudder. The tail-vane rudder is to the main sail as an elevator is to the wing on an airplane, controlling both the angle-of-attack of the wing and the vehicle orientation. The angle-of-attack of the main wing is set by the tail-vane rudder, and the direction of the boat relative to the wind is set by the mast-rotation motor; the tail-vane rudder turns the boat. 2D and 3D dynamic simulations indicate that the tail-vane rudder design yields both (1) a stable wing angle-of-attack (like other wing-sail boats with hinged main sails); and also (2) boat directional stability relative to the wind (like boats with auto-steer wind-vanes). With fixed control-surface angles, the boat finds and maintains a stable heading, regardless of initial conditions. This directional stability allows the boat to operate intermittently with neither electrical power nor a complex wind-vane, thus reducing demands on batteries and solar cells. Tests show that in light winds the boat can sail stably within approximately $\pm 45^{\circ }$ of the wind direction. Because of the air tail-vane-, instead of water-, rudder, the boat requires new tacking techniques which we are currently developing. As predicted by 3D simulations, experiments show that in heavy winds the boat has an oscillatory instability. It then finds a stable backwards sailing mode. We have yet-untested ideas for correcting this high-wind instability.

Using a Controlled Sail and Tail to Steer an Autonomous Sailboat

The authors use optical supercontinuum spectroscopy to probe the time evolution of the absorption spectrum of gallium vacancy charge states in \ensuremath{\beta}- Ga${}_{2}$O${}_{3}$ at ultrafast time scales.

Ultrafast dynamics of gallium vacancy charge states in β − Ga 2 O 3

Point defects in crystalline materials often occur in multiple charge states. Although many experimental methods to study and explore point defects are available, techniques to explore the non-equilibrium dynamics of the charge states of these defects at ultrafast (sub-nanosecond) time scales have not been discussed before. We present results from ultrafast optical-pump supercontinuum-probe spectroscopy measurements on $\beta$-Ga$_2$O$_3$. The study of point defects in $\beta$-Ga$_2$O$_3$ is essential for its establishment as a material platform for high-power electronics and deep-UV optoelectronics. Use of a supercontinuum probe allows us to obtain the time-resolved absorption spectra of material defects under non-equilibrium conditions with picosecond time resolution. The probe absorption spectra shows defect absorption peaks at two energies, $\sim$2.2 eV and $\sim$1.63 eV, within the 1.3-2.5 eV probe energy bandwidth. The strength of the absorption associated with each peak is time-dependent and the spectral weight shifts from the lower energy peak to the higher energy peak with pump-probe delay. Further, maximum defect absorption is seen for probe polarized along the crystal c-axis. The time-dependent probe absorption spectra and the observed dynamics for all probe wavelengths at all pump-probe delays can be fit with a set of rate equations for a single multi-level defect. Based on first-principles calculations within hybrid density functional theory we attribute the observed absorption features to optical transitions from the valence band to different charge states of Gallium vacancies. Our results demonstrate that broadband ultrafast supercontinuum spectroscopy can be a useful tool to explore charge states of defects and defect dynamics in semiconductors.

Arjan Singh

Papers

Intra- and inter-conduction band optical absorption processes in β-Ga2O3

Using a Controlled Sail and Tail to Steer an Autonomous Sailboat

Ultrafast dynamics of gallium vacancy charge states in β − Ga 2 O 3

Ultrafast Dynamics of Gallium Vacancy Charge States in $\beta$-Ga$_2$O$_3$