Pistevirheiden tutkiminen nanokiteissä ja puolijohteissa positroniannihilaatiospektroskopialla

The optical properties of bulk silicon are notoriously poor due to its indirect band gap. Byreducing the crystal size to the nanometer scale, the band gap becomes direct. This leads to aconsiderable improvement in the optical properties of the crystal. Silicon nanocrystalsembedded in silica are studied with positron annihilation and photoluminescencespectroscopies. The results indicate that the interface between the nanocrystal and thesurrounding silica lattice acts as a strong trap for positrons. The lattice defects located at theinterface were noted to passivate after annealing performed in the hydrogen gas. In this work, strongly phosphorous doped silicon samples are studied. A lattice strain isinduced in highly phosphorous doped silicon due to the formation of defect structures.According to earlier reports the cause for the strain are Si3P4 clusters. Calculations based ondensity functional theory indicates that the formation enthalpy of these clusters is higher thanthe thermal energy available during the growth of the sample. Unlike Si3P4, the formation enthalpy of V-Pn complexes is negative, which would indicate that these structures arespontaneously formed in the growth process. The theoretical calculations are consistent withthe Doppler broadening results, which state that the dominating lattice defect is amonovacancy decorated with several impurity atoms. Thanks to its lattice constant, GaSb is a suitable substrate material for various group III-Vsemiconductors whose band gaps cover a wide energy range. Undoped GaSb is p-typeirrespective of its growth method. In this work, undoped and tellurium doped GaSb are studied.Doppler broadening and lifetime spectroscopy measurements show that the dominatingvacancy defect is gallium vacancy. However, the primary cause, for the undoped GaSbbeing p-type, is the ion-type defect in which a Sb sublattice site is occupied by Ga atom. Highly arsenic, phosphorous and antimony doped germanium samples are studied with thehelp of Doppler broadening and lifetime measurements. The results indicate, that thedominating defect type in all samples is a complex consisting of a monovacancy and threeimpurity atoms. In the case of arsenic and phosphorous doped germanium samples, thepositron trapping is in saturation or in a saturation-like state, whereas in antimony dopedgermanium sample only a fraction of the positrons are trapped to complexes prior toannihilation.Bulkkipiin optiset ominaisuudet ovat tunnetusti huonoja johtuen sen epäsuorasta energia-aukosta. Pienentämällä kiteen kokoa nanometriluokkaan, jolloin sen energia-aukosta tulee suora, nähdään huomattava parannus optisissa ominaisuuksissa. Piioksidiin upotettuja piinanokiteitä on tutkittu positroniannihilaatio- ja fotoluminesenssispektroskopioiden avulla. Tuloksista voidaan todeta, että rajapinta piinanokiteen ja ympäröivän piioksidihilan välissä toimii voimakkaana loukkuna positroneille. Rajapinnassa sijaitsevien hilavirheiden todettiin passivoituvan typpivety-kaasussa tehdyn toivutuksen myötä. Voimakkaasti fosforilla seostettuun piihilaan aiheutuu venymä, joka johtuu piihin muodostuvista hilavirherakenteista. Aikaisempien tutkimusten perusteella venymän syynä on pidetty Si3P4-ryppäitä. Tiheysfunktionaaliteoriaan pohjautuvat elektronitiheyslaskut kuitenkin osoittavat, että tällaisten ryppäiden muodostumisentalpia on suurempi kuin näytteen kasvatuksen aikana käytettävissä oleva terminen energia. Sen sijaan, V-Pn-tyyppisillä komplekseilla muodostumisentalpia on negatiivinen, joka viittaisi siihen, että tällaisia rakenteita syntyy spontaanisti kasvatuksen aikana. Teoreettiset laskut ovat sopusoinnussa Doppler-levenemätulosten kanssa joiden mukaan hallitsevin hilavirhe on monovakanssi, jonka ympärillä on useampi epäpuhtausatomi. GaSb on hilavakionsa ansiosta sopiva alustamateriaali useammalle ryhmän III-V puolijohteelle, joiden energia-aukot kattavat laajan energiavälin. Seostamaton GaSb on p-tyyppistä riippumatta sen kasvatustavasta. Tässä työssä on tutkittu seostamatonta ja telluurilla seostettua GaSb näytettä. Doppler-levenemä- ja elinaikamittausten perusteella hallitsevin vakanssityyppinen hilavirhe on galliumvakanssi, mutta pääasiallinen syy sille, että seostamaton GaSb on p-tyyppistä, on ioni-tyyppinen virhe, jossa Ga-atomi on Sb-atomin hilapaikalla. Voimakkaasti arseenilla, fosforilla ja antimonilla seostettuja germaniumnäytteitä on tutkittu Doppler-levenemä- ja elinaikamittausten avulla. Tuloksista nähdään, että hallitsevinvirhetyyppi kaikissa näytteissä on kompleksi, joka koostuu vakanssista ja kolmesta epäpuhtausatomista. Arseenilla ja fosforilla seostettujen germaniumnäytteiden tapauksessa positroniannihilaatio on saturaatiossa tai saturaation tyyppisessä tilassa, kun taas antimonilla seostetussa germaniumnäytteessä vain osa positroneista loukkuuntuu komplekseihin ennen annihilaatiota

Acceptors in undoped GaSb; the role of vacancy defects

The conventional lifetime setup was used to study Czochralski grown unintentionally p- and n- type ([n] ≈ 6 × 1017cm−3) GaSb bulk samples. Several approaches were used to analyze the data. However, it was not possible to successfully analyze the obtained spectrums with the conventional trapping model. From the analyzed data it was derived that the reason for p-type behavior of GaSb was not VGa. Additionally, the role of gallium vacancy was studied and it's effect to lifetime values are shortly discussed.Peer reviewe

Defect studies in MBE grown GaSbBi layers

Gallium antimonide is an interesting material both from a material and a device point of view. Thedirect, narrow band gap and high electron mobility makes the compound semiconductor a suitablecandidate for high speed electronics and optoelectric devices. It can also be used as a substratematerial for other ternary or quaternary III–V compounds whose band gaps cover a wide spectralrange from 0.8 to 4.3 \u10021dm. [1]Incorporating Bi into GaSb has shown to have several advantages compared to, for example, GaNSb.Not only is the band gap reduced [2], but the width of the gap depends very weakly on temperature [3]and the electron mobility is higher than that of GaNSb [4]. The spin-orbit splitting is also larger thanthe actual band gap which could be used for suppressing Auger-recombinations [5].Using positron annihilation spectroscopy (PAS) in Doppler broadening mode, we have studiedsamples of GaSbBi epitaxial layers on GaSb substrates. The PAS technique is based on the interactionbetween positrons and electrons in solids and can be used for e.g. vacancy defect characterization inthin layers. The studied samples were MBE-grown and the main varied growth parameter wastemperature, which lead to different Bi concentrations. The Bi concentrations were 0 - 0.7 %, theepitaxial layer thickness was 200 nm. The substrate was Te-doped (n-type) GaSb.From the measured results, differences between the samples grown under different conditions can beclearly observed. A short diffusion length for the positrons is observed in all of the epitaxial layers,which indicates an increase in positron trapping defects in the layers, compared to the substrate.Furthermore, the Doppler broadening annihilation parameters in the epitaxial layers also seem todepend on the growth temperature and hence, also on the Bi concentration. In order to be able todistinguish the influence of the Bi concentration from the influence of vacancy defects on the Dopplerbroadening parameters, more accurate measurements need to be conducted. We hope to achieve abetter understanding of the positron trapping defect in the epitaxial layers by using coincidenceDoppler broadening

Annealing effect on donor-acceptor interface and its impact on the performance of organic photovoltaic devices based on PSiF-DBT copolymer and C60

In this work, poly[2,7-(9,9-bis(2-ethylhexyl)-dibenzosilole)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PSiF-DBT) was used as active layer in bilayer solar cell with C60 as electron acceptor. As cast devices already show reasonable power conversion efficiency (PCE) that increases to 4% upon annealing at 100 °C. Space charge limited measurements of the hole mobility (μ) in PSiF-DBT give μ ∼ 1.0 × 10−4 cm2/(V s) which does not depend on the temperature of the annealing treatment. Moreover, positron annihilation spectroscopy experiments revealed that PSiF-DBT films are well stacked even without the thermal treatment. The variations in the transport of holes upon annealing are then small. As a consequence, the PCE rise was mainly induced by the increase of the polymer surface roughness that leads to a more effective interface for exciton dissociation at the PSiF-DBT/fullerene heterojunction.Peer reviewe

On the Evolution of Strain and Electrical Properties in As-Grown and Annealed Si

Heavily P doped Si:P epitaxial layers have gained interest in recent times as a promising source-drain stressor material for n type FinFETs (Fin Field Effect Transistors). They are touted to provide excellent conductivity as well as tensile strain. Although the as-grown layers do provide tensile strain, their conductivity exhibits an unfavorable behavior. It reduces with increasing P concentration (P > 1E21 at/cm(3)), accompanied by a saturation in the active carrier concentration. Subjecting the layers to laser annealing increases the conductivity and activates a fraction of P atoms. However, there is also a concurrent reduction in tensile strain (Peer reviewe

On the manifestation of phosphorus-vacancy complexes in epitaxial Si:P films

In situ doped epitaxial Si:P films with P concentrations >1 × 1021 at./cm3 are suitable for source-drain stressors of n-FinFETs. These films combine the advantages of high conductivity derived from the high P doping with the creation of tensile strain in the Si channel. It has been suggested that the tensile strain developed in the Si:P films is due to the presence of local Si3P4 clusters, which however do not contribute to the electrical conductivity. During laser annealing, the Si3P4 clusters are expected to disperse resulting in an increased conductivity while the strain reduces slightly. However, the existence of Si3P4 is not proven. Based on first-principles simulations, we demonstrate that the formation of vacancy centered Si3P4 clusters, in the form of four P atoms bonded to a Si vacancy, is thermodynamically favorable at such high P concentrations. We suggest that during post epi-growth annealing, a fraction of the P atoms from these clusters are activated, while the remaining part goes into interstitial sites, thereby reducing strain. We corroborate our conjecture experimentally using positron annihilation spectroscopy, electron spin resonance, and Rutherford backscattering ion channeling studies.Peer reviewe