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Author:Norrman, Vesa
Title:Calculation of positron annihilation characteristics in InGaN alloys
Positroniannihilaatio-ominaisuuksien laskeminen in InGaN seoksissa
Publication type:Master's thesis
Publication year:2013
Pages:52      Language:   eng
Department/School:Teknillisen fysiikan laitos
Main subject:Ydin- ja energiatekniikka   (Tfy-56)
Supervisor:Tuomisto, Filip
Instructor:Makkonen, Ilja
OEVS:
Electronic archive copy is available via Aalto Thesis Database.
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Location:P1 Ark Aalto  122   | Archive
Keywords:positron annihilation spectroscopy
InGaN
positroniannihilaatiospektroskopia
InGaN
Abstract (eng): Positron annihilation spectroscopy (PAS) is a way to get information about electronic and atomic structures of materials.
By measuring lifetimes and Doppler broadening spectra of annihilating positrons it is possible to detect vacancies and characterize their chemical environments.

By varying the concentration of In and Ga the band gap of InGaN alloys can be tuned to cover the whole visible spectrum of light.
Hence this materials family is interesting for optoelectronic devices, such as light emitting diodes (LEDs).
The quality of devices is affected by the existence of defects.

In this work the electronic and atomic structure of InGaN, vacancies in InGaN, and polar InN/GaN superlattices were calculated with the local density approximation (LDA) of the density functional theyry (DFT).
The wave function of the positron and PAS characteristics such as lifetime and Doppler broadening spectra were calculated for those systems.
I investigated how the indium mole fraction, chemical environment of vacancies, and thickness of the InN layer affect positron annihilation characteristics.
The InN/GaN superlattice was modeled by stacking 1 - 4 wurtzite unit cells of InN in the direction of the hexagonal c-axis.
Because of periodic boundary conditions the layer is surrounded by GaN on both sides.

There is no inversion symmetry in c-direction of InN and GaN which causes polarization in the superlattice.
In calculations the positron is attracted by the InN/GaN interface.
Electronic structures of superlattices were also studied with hybrid functionals which are computationally more demanding but give more accurate results.
Abstract (fin): Positroniannihilaatiospektroskopia (PAS) on tapa saada tietoa aineiden elektroni- ja atomirakenteista.
Mittaamalla annihiloituvien positronien elinaikoja ja Doppler-levenemis-spektrejä voidaan havaita vakansseja ja tunnistaa niiden kemiallisia ympäristöjä.

Muuttamalla In- tai Ga-konsentraatioita InGaN:in energia-aukko voidaan säätää näkyvän valon aallonpituusalueelle.
Tämän takia tämä materiaaliperhe on mielenkiintoinen optoelektronisten laitteiden kuten valoa lähettäviin diodien (LED) valmistukseen.
Hilavirheet vaikuttavat näiden laitteiden toimintaan.

Tässä työssä laskin elektroni- ja atomirakenteen InGaN:ssä ilman vakansseja, InGaN:n metallivakansseille ja polaarisissa InN/GaN superhiloissa käyttäen paikallisen tiheyden approksimaatiota (LDA).
Näille systeemeille laskettiin myös positronin aaltofunktiot ja PAS-ominaisuudet kuten elinaika ja Doppler-levenemisspektri.
Selvitin miten indiumatomien mooliosuus, vakanssien kemiallinen ympäristö ja InN-kerroksen paksuus vaikuttavat positroniannihilaatioominaisuuksiin.

InN/GaN superhilat mallinnettiin pinoamalla 1 - 4 wurtsiittiyksikkökoppia InN:ä heksagonaalisen c-akselin suuntaisesti.
Periodisten reunaehtojen takia kerroksen kummallakin puolella on GaN:ä.
InN:n ja GaN:n c-suunnalla ei ole inversiosymmetriaa, mikä aiheuttaa polarisaation superhilassa.
Laskuissa positroni hakeutuu lähelle InN/GaN rajapintaa.
Superhilojen elektronirakenteita tutkittiin myös hybridifunktionaaleilla jolloin tulokset ovat tarkempia mutta laskenta on raskaampaa.
ED:2013-03-25
INSSI record number: 46005
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