Gallium nanoparticles for improved III-V solar cells

Speaker: Dr. Sergio Catalán.
Organization: Instituto de Sistemas Optoelectrónicos y Microtecnología (ISOM), de la E.T.S.I. Telecomunicación, Universidad Politécnica de Madrid.

Data:  January 21st, 2022.
Hours: 10.00 hours.
Place: Room B-222 of the ETSI of Telecommunications of the UPM [Cómo llegar]

ABSTRACT

In the last decade, third generation solar cells have progressed very strongly in efficiency. The latter is a key metric in the development of photovoltaic systems for reducing the cost of electricity per kilowatt-hour. The short-circuit current (JSC) and the open-circuit voltage (VOC) of the solar cell are the two main factors that should be taken into account to have an optimized efficiency. Some materials show moderate JSC values that indicate that better light management must be applied to improve the solar cell. This includes better coupling and trapping of light above the cell and reduction of light absorption in inactive regions of the cell. In this work, we report on the improved performance of GaAsSb/GaAsN superlattices and reference bulk GaAs solar cells via light scattering from deposited gallium nanoparticles atop the devices. After minimal nanoparticle size optimization, JSC derived from external quantum efficiency measurements is increased around 20% for each case that reveal a higher light collection. Theoretical calculations and detailed optical measurements support the comprehension of the study. This simple method has potential to enable thinner photovoltaic layers in solar cells.

A novel design of a racetrack memory based on functional segments

Speaker: Dr. Javier Rial
Organization: Instituto de Sistemas Optoelectrónicos y Microtecnología (ISOM), de la E.T.S.I. Telecomunicación, Universidad Politécnica de Madrid.

Data:  December 17th, 2021.
Hours: 10.00 hours.
Place: Room B-222 of the ETSI of Telecommunications of the UPM [Cómo llegar]

ABSTRACT

A racetrack memory is a device where the information is stored as magnetic domains (bits) along a nanowire (track). To read and record the information, the bits are moved along the track by current pulses until they reach the reading/writing heads. In particular, 3D racetrack memory devices use arrays of vertically aligned wires (tracks), thus enhancing storage density. In this work, we propose a novel 3D racetrack memory configuration based on functional segments inside cylindrical nanowire arrays. The use of selective magnetic segments inside one nanowire allows the creation of writing and storage sections inside the sametrack, separated by chemical constraints identical to those separating the bits.

The epitaxial growth of InAs/GaAs quantum dots for optoelectronic devices

Speaker: D. Lazar Stanojevic
Organization: Instituto de Sistemas Optoelectrónicos y Microtecnología (ISOM), de la E.T.S.I. Telecomunicación, Universidad Politécnica de Madrid.

Data:  December 3rd, 2021.
Hours: 10.00 hours.
Place: Room B-222 of the ETSI of Telecommunications of the UPM [Cómo llegar]

ABSTRACT

Quantum dots (QDs), which are usually seen as artificial atoms (nanocrystals, zero-dimensional nanostructures) in general have attracted a lot of attention of the scientific community due to, on one hand, a wide range of possibilities to tune their optical and other properties, and on the other hand, their incredibly diverse potential for applications that includes biology and medicine, so as optoelectronics, or more specifically, LEDs, laser diodes, single electron transistors (SETs), quantum computing, displays, photodetectors, photovoltaics etc.. Apart from colloidal QDs, which are more commonly studied, the epitaxially grown semiconductors InAs/GaAs QDs have been specially investigated for optoelectronic applications. Here I present the key tunability features and the most recent breakthroughs in InAs QD tunability, so as their potential applications in the field of photovoltaics.

How tunnel junctions changed our perception of III-N optoelectronic devices. III-N micro-LEDs

Speakers: M.Sc. Julia Sławińska and Prof. Czesław Skierbiszewski.
Organism:  Institute of High Pressure Physics, Polish Academy of Sciences, Warsaw. Poland.
Date: 16th November, 2021
Time: 12.00h.
Place: Salon de Grados, Building A.

ABSTRACT

Poor conductivity of the ptype region and difficulties with processing of low resistance ohmic ptype contacts are the most challenging issues to address in nitride based devices. Recently, there has been increasing attention given to the interband tunnel junctions (TJs) for efficient carrier conversion between ntype and ptype material region in nitride devices. Application of TJs eliminates the need for ptype contact deposition and create more freedom in device design. It was clearly demonstrated that TJs resistance for wide bandgap semiconductors can be effectively reduced by making use of the piezoelectric fields in the region of the junction. However, for metalorganic vapour phase epitaxy (MOVPE) it is difficult to activate the ptype conductivity in the (In)GaN:Mg layers which are buried below ntype layers due to the fact that diffusion of hydrogen is completely blocked through ntype region. On the other hand, for plasma assisted molecular beam epitaxy (PAMBE) ptype doping is achieved without post growth activation process. Therefore, PAMBE seems to be better suited than MOVPE for practical realization of the vertical devices with buried ptype regions especially for devices containing interband TJs. In this work we will show current status of IIIN TJs grown by PAMBE. Application of TJs enabled us to demonstrate novel types of IIIN devices, like: (1) vertically integrated light emitting diodes (LEDs) or laser diodes (LDs), (2) distributed feedback LDs. We will discuss in detail the inverted LED structures, operating at cryogenic temperatures. These LEDs contain the TJs below active region, which allow to reverse electric polarization of the diode. The application of TJs opens a possibility for novel architecture of microLEDs. We show an alternative method of nitride microLEDs fabrication, where emission surface was defined by size of the TJ embedded inside diode. MicroLEDs and arrays of microLEDs will be presented.

Laser pyrolisis for energy storage

Speaker: D. Andrés Velasco.
Organization: Instituto de Sistemas Optoelectrónicos y Microtecnología (ISOM), de la E.T.S.I. Telecomunicación, Universidad Politécnica de Madrid.

Data:  November 12th, 2021.
Hours: 10.00 hours.
Place: Room B-222 of the ETSI of Telecommunications of the UPM [Cómo llegar]

ABSTRACT

Energy storage is called to be one of the enablers of the full connectivity of the modern world. We all want all-day (or all-week!) battery life for our smartphones and gadgets, but there’s much more to it. Supercapacitors, in contrast to batteries, can be used to power low energy devices with a virtually infinite lifetime, and with new fabrication techniques such as laser pyrolysis, graphene electrodes can be made in a low cost and quick way. Interconnected sensor networks, wearable electronics and other IoT applications, including health, will benefit soon from the application of these graphene supercapacitors, but still some challenges must be overcome.

Cool ferromagnetic nanostrips

Speaker: D. Rodrigo Guedas. 
Organization: Instituto de Sistemas Optoelectrónicos y Microtecnología (ISOM), de la E.T.S.I. Telecomunicación, Universidad Politécnica de Madrid.

Data:  October 22th, 2021.
Hours: 10.00 hours.
Place: Room B-222 of the ETSI of Telecommunications of the UPM [Cómo llegar]

ABSTRACT

This work explores the problem of Joule heating on metallic micro or nanostrips, still one of the most popular geometries in modern spintronics. Many of the effects that result from the interaction of a spin polarized current and the local magnetization, require of a sizeable current density. This implies, quite often, an unneglectable Joule heating. Despite the few articles devoted to some aspects of Joule heating, there is still disinformation and many misconceptions in this topic, which is key for the correct interpretation of the scientific results. We highlight the material parameters that are important to keep the temperature of the strip under control and those that give only a marginal advantage. In the vast majority of papers, at least one of these parameters is missing. We also focus on some misconceptions, such as the belief that performing the measurement on a cryostat, rules Joule heating out. In fact, for a fixed current density, measuring in a cryostat decreases the temperature but not enough to justify the use of such a costly measuring set-up. At the practical level, we put forward a 1D model to calculate, in few seconds, if Joule heating is present and if it should be taken into account when interpreting the results. Finally, and importantly, we describe a simple fabrication route to enhance the dissipation of heat in the strip considerably. This fabrication strategy is more effective at keeping the temperature under control than performing the experiment at cryogenic temperatures

Metal-oxides based plasmonics for the mid-IR

Speaker: D. Eduardo Martínez.
Organization: Instituto de Sistemas Optoelectrónicos y Microtecnología (ISOM), de la E.T.S.I. Telecomunicación, Universidad Politécnica de Madrid.

Data:  October 8th, 2021.
Hours: 9.30 hours.
Place: Room B-222 of the ETSI of Telecommunications of the UPM [Cómo llegar]

ABSTRACT

IR photonics is currently a well stablished field with a wide range of commercial and experimental applications including medicine, biosensing, solar cells, astronomy and surveillance. Despite the huge advances in IR detection, commercial devices must struggle with unavoidable issues emerging from the low-energy range of the spectrum they target. IR photodetectors use materials and systems with low energy transitions for an effective optical absorption. When operating at room temperature, thermal energy of the carriers (kT) become comparable to the energy of the optical transition, resulting in an increased thermal noise.

In the last few decades, the field of plasmonics has provided an innovative approach to this problem, based on the field-enhancement properties of metallic nanostructures. Plasma oscillations in a metal-photodetector interface allow sub-wavelength light confinement that drives to improved absorption efficiency. However, due to their high plasma frequency noble metals are better suited for UV or visible-range photonics.

In this work we introduce CdZnO as a perfect substitute of metals for the mid-IR. Different plasmonic coatings and systems are presented for enhanced mid-IR detection, including gratings and nanoparticle systems. Here, we computationally study the potential gains in power absorption when different photodetectors are coupled with these coatings. In addition to that, experimental gains as high as 2.5x are reported in experimental plasmon-intersubband coupled systems.

Defense of the Doctoral Thesis of Amalia Luisa Fernando Saavedra (VIDEOCONFERENCE). 22-02-2021, 15.00 hours.

Títle: “Growth and characterization of GaN/AlN non-polar pseudo-substrates”. 

Thesis Director: Dr. Enrique Calleja Pardo

Place: SALA VIRTUAL. Date: 22 de Febrero a las 15:00 horas.

Added public link to assist defense:

https://us02web.zoom.us/j/87414464490?pwd=MUFmZ1BHQ0dxSHR0TDNwRmVFRFFIZz09

Meeting ID: 874 1446 4490

Access code: 119631