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Defects and Impurities in CdTe: An ab Initio Study
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. (Materials Theory)
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis defects and impurities in CdTe have been studied with ab initio methods. CdTe is a II-VI semiconductor with many important applications such as γ- and X-ray detectors, solar cells and medical imaging. Even though CdTe has been studied for more than 70 years, some of its properties connected with defects and impurities, are still shrouded in mystery. Todays experimental techniques are highly developed and can provide rather detailed data, but require elaborate theoretical analysis. Here ab initio modelling comes into play and in particular density functional theory (DFT). When reviewing different theoretical studies of defects and impurities in CdTe, one finds a vast number of discrepancies between experiment and theory. Mismatches appear even between different theoretical studies. Although many problems, such as, for example, the semiconductor band gap underestimation or the spurious interaction between charged defects, are avoided by employing corrections or implementing new functionals, some of them still remain. Employing the hybrid functional HSE06, the following topics were studied in this thesis:

- Te antisites: Experimental data predict the defect state to appear in the middle of the band gap, thus "pinning" the Fermi level. In contrast, our calculations show that Te antisite alone cannot be the reason for the Fermi level pinning, since it does not form a defect level in the middle of the band gap. Instead we propose that charge compensation between Te antisites in a (+2) state and Cd vacancies in a (-2) state explains the Fermi level pinning.

- Cd vacancy: Electron paramagnetic resonance experiments clearly show the existence of a hole polaron for the (-1) charged vacancy. But DFT studies report a completely delocalised hole. In our studies, for the first time, this state was found in its proper geometrical configuration with a hole localisation stabilised by a Jahn-Teller distortion, thereby removing the discrepancy between experiment and theory.

- Cd chalcogenides: Additionally, with particular focus on the hole localisation problem, the series of isovalent compounds (CdTe, CdSe and CdS) was studied to understand the mechanism of hole polaron formation. We explain the trend of the hole localisation in terms of Coulomb interaction, explicitly showing that the effect of electron correlation is negligible.

- Cl-doped CdTe: The formation of a Cl - Cd vacancy complex explains the selfcompensation and selfpurification mechanism. We find Cl to annihilate the hole polaron.

- Te antisite under deformation: In an attempt to tailor the energy position of the Te antisite defect level in the CdTe band gap, we studied CdTe under different deformations. It is shown that by a carefully chosen deformation the defect levels can be pushed closer to the valence and/or conduction band and hence the CdTe detector performance may be improved.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. , 62 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1229
Keyword [en]
Native defects, Compensation mechanisms, Semiconductor doping
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-244964ISBN: 978-91-554-9171-0 (print)OAI: oai:DiVA.org:uu-244964DiVA: diva2:790268
Public defence
2015-04-10, Häggsalen, Ångströmlaboratoriet, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2015-03-19 Created: 2015-02-23 Last updated: 2015-04-17
List of papers
1. Tailoring of defect levels by deformations: Te-antisite in CdTe
Open this publication in new window or tab >>Tailoring of defect levels by deformations: Te-antisite in CdTe
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2013 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, no 41, 415801- p.Article in journal (Refereed) Published
Abstract [en]

The properties of the Te-antisite defect in the neutral state in CdTe were examined using ab initio calculations. The influence of three types of deformations (1D, 2D and 3D) on the defect energy levels and formation energies was investigated. It was found that the 2D deformation is the most effective for pushing the defect levels towards the band edges and opening up the bandgap of the semiconductor, and hence may improve the performance of CdTe as a detector material. We studied the defect levels and their occupancies including Jahn-Teller distortions. The Jahn-Teller distorted configuration places the 2A(1)(a) defect level closer to the valence band and this defect level position coincides with the 'unknown deep donor' measured in some experiments. Partial densities of states and band structures have been analysed to understand the arrangement of the defect bonds.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-209843 (URN)10.1088/0953-8984/25/41/415801 (DOI)000324920400017 ()
Available from: 2013-10-28 Created: 2013-10-28 Last updated: 2017-12-06Bibliographically approved
2. Small hole polaron in CdTe: Cd-vacancy revisited
Open this publication in new window or tab >>Small hole polaron in CdTe: Cd-vacancy revisited
2015 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, 14509Article in journal (Other academic) Published
Abstract [en]

The characteristics of electronic states of Cd-vacancies in CdTe, an important semiconductor for various technological applications, are under debate both from theoretical and experimental points of view. Experimentally, the Cd-vacancy in its negative charge state is found to have C3v symmetry and a (−1/−2) transition level at 0.4 eV. Our first principles density functional calculations with hybrid functionals confirm for the first time these experimental findings. Additionally, we find that the C3v symmetry and the position of the (−1/−2) transition level are caused by the formation of a hole polaron localised at an anionic site around the vacancy.

Keyword
Cd vacancy, hole polaron, symmetry of defect, CdTe, ab initio study
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-244951 (URN)10.1038/srep14509 (DOI)000361803600001 ()26411338 (PubMedID)
Funder
Swedish Research Council
Available from: 2015-02-23 Created: 2015-02-23 Last updated: 2017-12-04Bibliographically approved
3. Native Defects in CdTe. Te Antisite and Cd Vacancy
Open this publication in new window or tab >>Native Defects in CdTe. Te Antisite and Cd Vacancy
(English)In: Journal of Physics: Condensed MatterArticle in journal (Other academic) Submitted
Abstract [en]

In this paper, we have studied Te antisites and Cd vacancies in CdTe by local density approximation (LDA) and hybrid functionals (HSE06) within density functional theory. We have shown that LDA, in spite of successful predictions of Te antisite geometries, fails in case of Cd vacancy. C3v and D2d symmetries of the VCd−1 and VCd0 defects were found only by HSE06, while LDA calculations resulted in Td symmetry for all of these defects. This mismatch consequently resulted in severe differences in the defects formation energies calculated by HSE06 and LDA. In contrast to LDA, our hybrid functional calculations agree well with experimental observations. For Te-rich conditions considering only native defects, our calculations predict CdTe to have a Fermi level of about 0.6 eV, because at that level the vacancy double acceptor and antisite double donor are compensated. We propose that the defect compensation may be the reason for high resistivities in undoped CdTe. 

Keyword
CdTe, Te antisite, Cd vacancy, Formation energies, HSE06
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-244955 (URN)
Available from: 2015-02-23 Created: 2015-02-23 Last updated: 2015-04-17
4. Cl-doping of CdTe. Selfcompensation and Selfpurification From First Principles.
Open this publication in new window or tab >>Cl-doping of CdTe. Selfcompensation and Selfpurification From First Principles.
(English)In: Physical Review BArticle in journal (Other academic) Submitted
Abstract [en]

The coexistence of Cl substitutional defects and Cd vacancies in CdTe was studied from first principles utilizing the HSE06 hybrid functional. We find the Cl and Cd vacancy to bind and form an acceptor complex explaining the well known effect of selfcompensation. Moreover, we find the acceptor complex to be stable almost within the entire range of possible Fermi energies. The defect level of the (-1) charged Cd vacancy disappears when forming a complex with Cl thus explaining the so called selfpurification.

Keyword
CdTe, Cl-doping, Cl-Cd vacancy complex, A-center
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-244957 (URN)
Available from: 2015-02-23 Created: 2015-02-23 Last updated: 2015-04-17
5. Hole Polaron Formation in CdTe, CdSe and CdS
Open this publication in new window or tab >>Hole Polaron Formation in CdTe, CdSe and CdS
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We use a hybrid (HSE06) density functional theory approach to perform a systematic study of Cd vacancies in a Cd chalcogenide (CdTe, CdSe and CdS) host. For all Cd chalcogenides we find the (-1) charged Cd vacancy to be stabilized by a Jahn-Teller effect and the formation of a localized hole polaron. The polaron becomes more stable going from the bottom of the periodic table to the top. We calculate the polaron formation to be accompanied by a metal-semiconductor transition which is not based on electron correlation but on the direct Coulomb interaction. We find the neutral Cd vacancy in the Cd chalcogenids to be stabilized without the involvement of hole polarons. 

Keyword
CdTe, CdSe, CdS, correlation, polaron, Cd vacancy, symmetry
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-244962 (URN)
Available from: 2015-02-24 Created: 2015-02-23 Last updated: 2015-04-17

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