Indium | EPFL Graph Search

L'indium est l'élément chimique de numéro atomique 49, de symbole In. Généralités sur l'élément indium et son corps simple Cet élément monoisotopique stable appartient au bloc p, au groupe 13 et à la période 5 et au groupe IIIA dit du bore du tableau périodique ainsi qu'à la famille des métaux pauvres. Le rayon atomique de l'indium avoisine 150 pm (). Le corps simple indium est un métal gris brillant, à bas point de fusion à peine supérieur à , résistant à la corrosion atmosphérique. Ce métal malléable présente une chimie assez analogue à l'aluminium et au gallium, mais aussi au cadmium et au thallium. L'électronégativité selon Pauling est de l'ordre de 1,7(̠8), alors que celles du Ga et Al avoisinent respectivement 1,6 et 1,5. Il est rare. L'indium est resté un « métal ou un élément de laboratoire » jusque dans le début de l'entre-deux-guerres. En 1924, on découvre que l'indium peut stabiliser les métaux non ferreux. En 1940, l'indium comme revêtement protecteur et a

A multi-technique approach to study the microstructural properties of tin-based transparent conductive oxides

Federica Landucci

Transparent conductive oxides (TCOs) are semiconductor-like materials that exhibit high electrical conductivity and high optical transparency combined. They are adopted in various applications ranging from gas sensors, to electrochromic windows, to photovoltaic cells. Indium-based TCOs represent the industry standard. Nevertheless, indium is among the less abundant elements in the earth crust and forecasts based on its current consumption envisage an urgent need to replace it. Tin-based TCOs are a promising alternative, since their opto- electronic characteristics mimic the ones of indium-based materials. This thesis aims to investigate the link between optoelectronic and microstructural properties of tin dioxide and zinc tin oxide (ZTO) with a composition Zn0.05Sn0.30O0.65 and their stability when submitted to thermal treatments. Indeed, lots of practi- cal applications require the TCO to operate in high temperature conditions. To conduct this study, a combination of analytical techniques, such as transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X- ray diffraction (XRD), electron paramagnetic resonance (EPR) and differential scanning calorimetry (DSC) was employed. Amorphous SnO2 and ZTO were deposited by RF sputtering and annealed up to 1050°C in different atmospheres. The influence of annealing temperature and atmosphere were decoupled and led us to an in-depth comprehension of the mechanisms governing the optoelectronic properties of both materials. When annealed in air, between room temperature and 300°C, ZTO exhibits increased mobility and carrier concentration with respect to the as-deposited state. This increase, investigated with DSC, was ascribed to a structural relaxation that allows point defects to release electrons in conduction band. Between 300°C and 500°C atmospheric oxygen passivates oxygen vacancies, drastically decreasing the carrier concentration and therefore causing a large drop of the conductivity. EPR experiments allowed to ascribe the drop in conductivity to the decrease of carrier concentration, which occurs slightly before the phase change. At 570°C (and 930°C for the case of vacuum annealing) the phase change occurs and the ZTO crystallizes in the rutile form of SnO2. The material becomes completely insulating. When the temperature is increased to 1050°C, evaporation of zinc is observed. In order to improve the electrical conductivity of ZTO at high temperature, a doping strategy was implemented starting from DFT calculations conducted by a partner group, who screened among the entire periodic table, which elements are the best candidates to act as n-dopants for ZTO. Bromine and iodine were retained, since they were found to be the most energetically favorable to become substitutional defects for a tin site. An exploratory doping route is therefore presented and the treated samples analyzed with TEM, EDX and UV-VIS-IR spectroscopy. Finally, the structural properties of an indium-based TCO (zirconium-doped indium oxide) were investigated and used as a benchmark to propose a crystallization model for the tin-based, as well as the indium-based materials. The influence of pa- rameters such as the material thickness, annealing atmosphere and temperature and deposition pressure are discussed for both materials.

EPFL2019

Low Temperature Hermetic Wafer Bonding and Microfabrication of Wall-Coated Alkali Vapor Cells for Chip-Scale Atomic Clocks

Rahel Strässle

The increasing demand for highly precise, yet compact and low power timing devices pushes the research in the field of chip scale atomic clocks (CSACs). At the heart of a CSAC, there is a vapor cell containing a few micrograms of alkali metal, such as rubidium or cesium. Hermetic sealing of this cell is essential not just to avoid the strong reaction of the alkali metal to oxygen and hydrogen, but also to maintain a stable operation atmosphere that ensures minimized clock shifts. Since alkali atoms loose their spin polarization when colliding with the cell walls, a buffer gas is commonly added in alkali cells to reduce the mean free path. An alternative approach to increase the polarization lifetime is to use antirelaxation wall-coatings, which have been studied extensively for larger glass-blown alkali vapor cells but not yet for microfabricated cells. This is mainly because anodic bonding, which is the standard process for sealing microfabricated alkali vapor cells, degrades the known antirelaxation coatings with its elevated temperatures and the electric field strength. In this work, the problem of implementing the wall-coatings in microfabricated cells is addressed. In a first step, a hermetic and low temperature thin-film indium-bonding technique was developed, which is compatible with the most common antirelaxation coatings and meets the requirements to fabricate alkali vapor cells. The technique is based on the thermocompression of indium and can be applied on wafer level with temperatures ranging from room temperature to temperatures below indium’s melting point at 156 ◦ C. It yields a leak rate below 1.5 × 10−13 mbar l/s and a tensile strength between 10 and 25 MPa, depending on the substrates and the bonding parameters. Wafer-level bonding at such low temperatures with the reported level of hermeticity is unique in the field of MEMS packaging. This novel bonding technique was then successfully applied to microfabricate alkali vapor cells at the lowest temperature alkali cells have been sealed before. The gas contamination in an evacuated cell was lower than 1 mbar and the suitability of indium-bonded cells for atomic clock applications was demonstrated with a buffer-gas filled cell by recording the double-resonance signal. The obtained clock signal linewidth was 2.02 kHz, only slightly higher than the theoretically predicted intrinsic linewidth for this cell design. After validation of the bonding technique for alkali vapor cells, a wall-coating for the cell walls was integrated in the microfabrication process. The bulk and surface properties of selected antirelaxation wall-coatings materials, such as octadecyltrichlorosilane (OTS), paraffin, an alkene mix, and Parylene, were investigated for compatibility with alkali cells and microfabrication. The gathered knowledge led to the fabrication of the first microfabricated rubidium vapor cell with an OTS coating that shows antirelaxation properties.

EPFL2013

Elucidating the role of the InGaN UL in the efficiency of InGaN based light-emitting diodes

Camille Haller

Blue III-nitride light-emitting diodes (LEDs) are widely used nowadays in solid-state lighting, as white light can be produced by combining yellow phosphors and blue LEDs. This technology has the advantage to have a higher luminous efficiency than incandescent light bulbs and fluorescent tubes. This high efficiency is attributed to InGaN quantum wells (QWs), which exhibit an internal quantum efficiency (IQE) higher than 90 % at 300 K. However, even if blue LEDs are industrially produced, the high IQE of InGaN QWs is still barely understood. In InGaN/GaN-based LEDs, an InGaN layer grown before the InGaN/GaN QW is often found. This InGaN layer named an underlayer (UL) is used in all commercial devices and is known to increase drastically the efficiency of LEDs. However, the physical mechanism behind the efficiency improvement is debated in the literature. In the first part of this work, a study of the different existing hypotheses proposed is conducted. The conclusion is that the role of the InGaN UL is to capture point defects before they reach the InGaN QWs. Indeed, points defects react with InGaN and are transformed into non-radiative centers when they are buried in the InGaN alloy. We call such point defects reacting with indium "surface defects" (SDs). To explore the SD capture by the InGaN UL, we studied different kinds of UL: InGaN UL, InAlN UL, and low-temperature GaN UL. We found that indium atoms play a key role in the capture of SDs. Indeed, the number of SDs buried in the UL depends on its thickness and indium composition. As a consequence, GaN layers are not able to bury SDs as efficiently. A model based on segregation of SDs was proposed based on our observations. As InGaN UL are present in all LEDs, we concluded that SDs are problematic for all LEDs independently of the type of reactor. The growth temperature of GaN was identified as the parameter which generates SDs. Indeed, for temperatures higher than 820 \degres C in MOVPE, SDs are created. Different impurities were searched by secondary ion mass spectrometry measurements, but no correlation was found between the sample efficiency and the impurity concentration. Our hypothesis is that the stability of the GaN surface is affected by the temperature and therefore leads to the formation of SDs, which could be intrinsic defects such as nitrogen vacancies. However, we do not have any experimental proof about the actual nature of SDs yet. In the last part of this work, near UV LEDs with InAlN UL were designed and characterized. Single QW (SQW) LEDs emitting around 405 nm with an InAlN/GaN superlattice UL exhibit an IQE around

\sim

70 %, which is higher than for a SQW LED without UL (

\sim

10 %). The

I

V

characteristic is similar for both samples thanks to a highly doped region placed at the interface between the InAlN/GaN superlattice UL and the GaN spacer.

EPFL2019

A multi-technique approach to study the microstructural properties of tin-based transparent conductive oxides

Federica Landucci

EPFL2019

Low Temperature Hermetic Wafer Bonding and Microfabrication of Wall-Coated Alkali Vapor Cells for Chip-Scale Atomic Clocks

Rahel Strässle

EPFL2013

Elucidating the role of the InGaN UL in the efficiency of InGaN based light-emitting diodes

Camille Haller

\sim

70 %, which is higher than for a SQW LED without UL (

\sim

10 %). The

I

V

characteristic is similar for both samples thanks to a highly doped region placed at the interface between the InAlN/GaN superlattice UL and the GaN spacer.

EPFL2019