Wed 22 Jan 16:15: Porous nitride semiconductors for novel light sources
Porous semiconducting nitrides are effectively a new class of semiconducting material, with properties distinct from the monolithic nitride layers from which devices from light emitting diodes (LEDs) to high electron mobility transistors are increasingly made. The introduction of porosity provides new opportunities to engineer a range of properties including refractive index, thermal and electrical conductivity, stiffness and piezoelectricity. Quantum structures may be created within porous architectures and novel composites may be created via the infiltration of other materials into porous nitride frameworks. A key example of the application of porous nitrides in photonics is the fabrication of high reflectivity distributed Bragg reflectors (DBRs) from alternating layers of porous and non-porous GaN. These reflectors are fabricated from epitaxial structures consisting of alternating doped and undoped layers, in which only the conductive, doped layers are electrochemically etched. Conventionally, trenches are formed using a dry-etching process, penetrating through the multilayer, and the electrochemical etch then proceeds laterally from the trench sidewalls. The need for these trenches then limits the device designs and manufacturing processes within which the resulting reflectors can be used. We have developed a novel alternative etching process, which removes the requirement for the dry-etched trenches, with etching proceeding vertically from the top surface through channels formed at naturally-occurring defects in the crystal structure of GaN (see Figure). This etch process leaves an undoped top surface layer almost unaltered and suitable for further epitaxy. This new defect-based etching process provides great flexibility for the creation of a variety of sub-surface porous architectures on top of which a range of devices may be grown. Whilst DBR structures enable improved light extraction from LEDs and the formation of resonant cavities for lasers and single photon sources, recent development also suggests that thick, sub-surface porous layers may enable strain relaxation to help improve the efficiency of red microLEDs for augmented reality displays. Meanwhile, the option of filling pores in nitride layers with other materials provides new opportunities for the integration of nitrides with emerging photonic materials, such as the hybrid-perovskite semiconductors, with perovskites encapsulated in porous nitride layers demonstrating greatly improved robustness against environmental degradation.
- Speaker: Rachel Oliver (DMSM)
- Wednesday 22 January 2025, 16:15-17:15
- Venue: Small Lecture Theatre, Cavendish Laboratory, J.J. Thomson Avenue.
- Series: Cavendish Quantum Colloquium; organiser: Lucas Sá.
Tue 18 Feb 11:00: Searching for Life in Stranger Seas Join Zoom Meeting https://us02web.zoom.us/j/89826306833?pwd=cnNHSG9OWHRjVngzMGVMc2F0NnA4dz09 Meeting ID: 898 2630 6833 Passcode: 662640
With a planet teaming with life all around us, it is tempting to think that any life on other worlds must be like Earth life. But is that true, and if it is not, what limits can we place on where we might look for life on other worlds? Starting from the still-controversial possibility of the presence of phosphine in the clouds of Venus, I will discuss what we know (not much) can model (some) and speculate about how the chemistry of life might work on other worlds under conditions very different from Earth. In particular, different atmospheres give different chemistries, possibly chemistries that use solvents other than water as their base. Alternative solvents open up the possibility of life on many bodies previously considered uninhabitable, such as the clouds of Venus, the surface of Mars, even the Moon. I will end with some thoughts on complex, even intelligent, life and where we might find it.
Join Zoom Meeting https://us02web.zoom.us/j/89826306833?pwd=cnNHSG9OWHRjVngzMGVMc2F0NnA4dz09 Meeting ID: 898 2630 6833 Passcode: 662640
- Speaker: William Bains
- Tuesday 18 February 2025, 11:00-12:00
- Venue: Chemistry Dept, Unilever Lecture Theatre and Zoom.
- Series: Centre for Atmospheric Science seminars, Chemistry Dept.; organiser: Dr Megan Brown.
Fri 21 Feb 14:00: A General Design-Based Framework and Estimator for Randomized Experiments
We describe a new design-based framework for drawing causal inference in randomized experiments. Causal effects in the framework are defined as linear functionals evaluated at potential outcome functions. Knowledge and assumptions about the potential outcome functions are encoded as function spaces. This makes the framework expressive, allowing experimenters to formulate and investigate a wide range of causal questions. We describe a class of estimators for estimands defined using the framework and investigate their properties. The construction of the estimators is based on the Riesz representation theorem. We provide necessary and sufficient conditions for unbiasedness and consistency. Finally, we provide conditions under which the estimators are asymptotically normal, and describe a conservative variance estimator to facilitate the construction of confidence intervals for the estimands.
- Speaker: Fredrik Sävje (Uppsala University)
- Friday 21 February 2025, 14:00-15:00
- Venue: Centre for Mathematical Sciences MR12, CMS.
- Series: Statistics; organiser: Qingyuan Zhao.
Fri 31 Jan 14:00: Metric and variable selection for functional nonparametric regression models
In this talk, we will consider nonparametric regression models with multiple functional covariates. The focus of the work is to identify relevant variables and useful metrics for the functional covariates, and to efficiently estimate the regression function. The proposed method is based on an extension of the Nadaraya-Watson estimator, where a kernel function is applied to a linear combination of distance measures, each computed on individual covariates, in combination with an adaptive thresholding step on the kernel weights. This data-driven least squares cross-validation method can asymptotically remove irrelevant noise variables and select relevant metrics for the functional covariates, as will be shown both by theory and numerical examples.
- Speaker: Torben Sell (University of Edinburgh)
- Friday 31 January 2025, 14:00-15:00
- Venue: Centre for Mathematical Sciences MR12, CMS.
- Series: Statistics; organiser: Qingyuan Zhao.
Fri 31 Jan 14:00: Metric and variable selection for functional nonparametric regression models
Abstract not available
- Speaker: Torben Sell (University of Edinburgh)
- Friday 31 January 2025, 14:00-15:00
- Venue: Centre for Mathematical Sciences MR12, CMS.
- Series: Statistics; organiser: Qingyuan Zhao.