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Room-temperature defect-engineered spin functionalities in Ga(In)NAs alloys
Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
##### Abstract [en]

Semiconductor spintronics is one of the most interesting research fields that exploits both charge and spin properties for future photonics and electronic devices. Among many challenges of using spin in semiconductors, efficient generation of electron spin polarization at room temperature (RT) remains difficult. Recently, a new approach using defect-mediated spin filtering effect, employing $Ga_{i}^2^+$-interstitial defects in Ga(In)NAs alloys, has been shown to turn the material into an efficient spin-polarized source capable of generating >40% conduction electron spin polarization at RT without an application of external fields. In order to fully explore the defectengineered spin functionalities, a better understanding and control of the spin filtering effects is required. This thesis work thus aims to advance our understanding, in terms of both physical and material insights, of the recently discovered spin filtering defects in Ga(In)NAs alloys. We have focused on the important issues of optimization and applications of the spin filtering effects.

To improve spin filtering efficiency, important material and defect parameters must be addressed. Therefore, in Papers I–III formation of the $Ga_{i}^2^+$ defects in Ga(In)NAs alloys has been examined under different growth and post-growth treatment conditions, as well as in different structures. We found that the $Ga_{i}^2^+$ defects were the dominant and important nonradiative recombination centers in Ga(In)NAs epilayers and GaNAs/GaAs multiple quantum wells, independent of growth conditions and post-growth annealing. However, by varying growth and post-growth conditions, up to four configurations of the $Ga_{i}^2^+$ defects, exhibiting different hyperfine  interaction (HFI) strengths between defect electron and nuclear (e-n) spins, have been found. This difference was attributed to different interstitial sites and/or complexes of $Ga_{i}^2^+$ . Further studiesfocused on the effect of post-growth hydrogen (H) irradiation on the spin filtering effect. Beside the roles of H passivation of N resulting in bandgap reopening of the alloys, H treatment was shown to lead to complete quenching of the spin filtering effect, accompanied by strong suppression in the concentrations of the $Ga_{i}^2^+$ defects. We concluded that the observed effect was due to the passivation of the $Ga_{i}^2^+$ defects by H, most probably due to the formation of H-$Ga_{i}^2^+$ complexes.

Optimizing spin filtering efficiency also requires detailed knowledge of spin interactions at the defect centers. This issue was addressed in Papers IV and V. From both experimental and theoretical studies, we were able to conclude that the HFI between e-n spins at the $Ga_{i}^2^+$ defects led to e-n spin mixing, which degraded spin filtering efficiency at zero field.  Moreover, we have identified the microscopic origin of electron spin relaxation (T1) at the defect centers, that is, hyperfine-induced e-n spin cross-relaxation. Our finding thus provided a guideline to improve spin filtering efficiency by selectively incorporating the $Ga_{i}^2^+$ defects with weak HFI by optimizing growth and post-growth treatment conditions, or by searching for new spin filtering defect centers containing zero nuclear spin.

The implementation of the defect-engineered spin filtering effect has been addressed in Papers VI–VIII. First, we experimentally demonstrated for the first time at RT an efficient electron spin amplifier employing the $Ga_{i}^2^+$ defects in Ga(In)NAs alloys, capable of amplifying a weak spin signal up to 27 times with a high cut-off frequency of 1 GHz. We further showed that the defectmediated spin amplification effect could turn the GaNAs alloy into an efficient RT optical spin detector. This enabled us to reliably conduct in-depth spin injection studies across a semiconductor heterointerface at RT. We found a strong reduction of electron spin polarization after optical spin injection from a GaAs layer into an adjacent GaNAs layer. This observation was attributed to severe spin loss across the heterointerface due to structural inversion asymmetry and probably also interfacial point defects.

Finally, we went beyond the generation of strongly polarized electron spins. In Paper IX we focused on an interesting aspect of using strongly polarized electron spins to induce strong nuclear spin polarization at RT, relevant to solid-state quantum computation using a defect nuclear spin of long spin memory as a quantum bit (qubit). By combining the spin filtering effect and the HFI, we obtained a sizeable nuclear spin polarization of ~15% at RT that could be sensed by conduction electrons. This demonstrated the feasibility of controlling defect nuclear spins via conduction electrons even at RT, the first case ever being demonstrated in a semiconductor.

##### Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1607
Natural Sciences
##### Identifiers
ISBN: 978-91-7519-293-2 (print)OAI: oai:DiVA.org:liu-107621DiVA: diva2:725969
##### Supervisors
Available from: 2014-06-17 Created: 2014-06-17 Last updated: 2014-06-17Bibliographically approved
##### List of papers
1. Dominant recombination centers in Ga(In)NAs alloys: Ga interstitials
Open this publication in new window or tab >>Dominant recombination centers in Ga(In)NAs alloys: Ga interstitials
2009 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 95, 241904- p.Article in journal (Refereed) Published
##### Abstract [en]

Opticallydetected magnetic resonance measurements are carried out to study formationof Ga interstitial-related defects in Ga(In)NAs alloys. The defects, whichare among dominant nonradiative recombination centers that control carrier lifetimein Ga(In)NAs, are unambiguously proven to be common grown-in defectsin these alloys independent of the employed growth methods. Thedefects formation is suggested to become thermodynamically favorable because ofthe presence of nitrogen, possibly due to local strain compensation.

##### Place, publisher, year, edition, pages
American Institute of Physics, 2009
##### National Category
Condensed Matter Physics
##### Identifiers
urn:nbn:se:liu:diva-52858 (URN)10.1063/1.3275703 (DOI)
##### Note
Original Publication: Xingjun Wang, Yuttapoom Puttisong, C. W. Tu, Aaron J. Ptak, V. K. Kalevich, A. Yu. Egorov, L. Geelhaar, H. Riechert, Weimin Chen and Irina Buyanova, Dominant recombination centers in Ga(In)NAs alloys: Ga interstitials, 2009, Applied Physics Letters, (95), 241904. http://dx.doi.org/10.1063/1.3275703 Copyright: American Institute of Physics http://www.aip.org/Available from: 2010-01-12 Created: 2010-01-12 Last updated: 2014-06-17Bibliographically approved
2. Electron spin filtering by thin GaNAs/GaAs multiquantum wells
Open this publication in new window or tab >>Electron spin filtering by thin GaNAs/GaAs multiquantum wells
2010 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 96, no 5, 052104- p.Article in journal (Refereed) Published
##### Abstract [en]

Effectiveness of the recently discovered defect-engineered spin-filtering effect is closely examined in GaNAs/GaAs multiquantum wells (QWs) as a function of QW width. In spite of narrow well widths of 3-9 nm, rather efficient spin filtering is achieved at room temperature. It leads to electron spin polarization larger than 18% and an increase in photoluminescence intensity by 65% in the 9 nm wide QWs. A weaker spin filtering effect is observed in the narrower QWs, mainly due to a reduced sheet concentration of spin-filtering defects (e.g., Ga-i interstitial defects).

##### Keyword
electron spin polarisation, gallium arsenide, III-V semiconductors, nitrogen compounds, photoluminescence, semiconductor quantum wells
##### National Category
Engineering and Technology Condensed Matter Physics
##### Identifiers
urn:nbn:se:liu:diva-54084 (URN)10.1063/1.3299015 (DOI)000274319500045 ()
Available from: 2010-02-22 Created: 2010-02-22 Last updated: 2014-06-17Bibliographically approved
3. Room temperature spin filtering effect in GaNAs: Role of hydrogen
Open this publication in new window or tab >>Room temperature spin filtering effect in GaNAs: Role of hydrogen
2011 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 99, no 15, 152109- p.Article in journal (Refereed) Published
##### Abstract [en]

Effects of hydrogen on the recently discovered defect-engineered spin filtering in GaNAs are investigated by optical spin orientation and optically detected magnetic resonance. Post-growth hydrogen treatments are shown to lead to nearly complete quenching of the room-temperature spin-filtering effect in both GaNAs epilayers and GaNAs/GaAs multiple quantum wells, accompanied by a reduction in concentrations of Ga(i) interstitial defects. Our finding provides strong evidence for efficient hydrogen passivation of these spin-filtering defects, likely via formation of complexes between Gai defects and hydrogen, as being responsible for the Observed strong suppression of the spin-filtering effect after the hydrogen treatments.

##### Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2011
##### Keyword
gallium arsenide, gallium compounds, hydrogen, III-V semiconductors, interstitials, magnetic resonance, passivation, quenching (thermal), semiconductor epitaxial layers, semiconductor quantum wells, wide band gap semiconductors
##### National Category
Engineering and Technology Condensed Matter Physics
##### Identifiers
urn:nbn:se:liu:diva-72139 (URN)10.1063/1.3651761 (DOI)000295883800045 ()
Available from: 2011-11-18 Created: 2011-11-18 Last updated: 2015-05-28Bibliographically approved
4. Effect of hyperfine-induced spin mixing on the defect-enabled spin blockade and spin filtering in GaNAs
Open this publication in new window or tab >>Effect of hyperfine-induced spin mixing on the defect-enabled spin blockade and spin filtering in GaNAs
2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 12Article in journal (Refereed) Published
##### Abstract [en]

The effect of hyperfine interaction (HFI) on the recently discovered room-temperature defect-enabled spin-filtering effect in GaNAs alloys is investigated both experimentally and theoretically based on a spin Hamiltonian analysis. We provide direct experimental evidence that the HFI between the electron and nuclear spin of the central Ga atom of the spin-filtering defect, namely, the Ga-i interstitials, causes strong mixing of the electron spin states of the defect, thereby degrading the efficiency of the spin-filtering effect. We also show that the HFI-induced spin mixing can be suppressed by an application of a longitudinal magnetic field such that the electronic Zeeman interaction overcomes the HFI, leading to well-defined electron spin states beneficial to the spin-filtering effect. The results provide a guideline for further optimization of the defect-engineered spin-filtering effect. DOI: 10.1103/PhysRevB.87.125202

##### Place, publisher, year, edition, pages
American Physical Society, 2013
##### National Category
Engineering and Technology
##### Identifiers
urn:nbn:se:liu:diva-90752 (URN)10.1103/PhysRevB.87.125202 (DOI)000316103800004 ()
##### Note

Funding Agencies|Linkoping University through the professor contract, Swedish Research Council|621-2011-4254|Linkoping University through the professor contract Swedish Energy Agency||Knut and Alice Wallenberg Foundation||

Available from: 2013-04-08 Created: 2013-04-05 Last updated: 2014-06-17Bibliographically approved
5. Limiting factor of defect-engineered spin-filtering effect at room temperature
Open this publication in new window or tab >>Limiting factor of defect-engineered spin-filtering effect at room temperature
2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 19, 195412- p.Article in journal (Refereed) Published
##### Abstract [en]

We identify hyperfine-induced electron and nuclear spin cross-relaxation as the dominant physical mechanism for the longitudinal electron spin relaxation time (T-1) of the spin-filtering Ga-i(2+) defects in GaNAs alloys. This conclusion is based on our experimental findings that T-1 is insensitive to temperature over 4-300 K, and its exact value is directly correlated with the hyperfine coupling strength of the defects that varies between different configurations of the Ga-i(2+) defects present in the alloys. These results thus provide a guideline for further improvements of the spin-filtering efficiency by optimizing growth and processing conditions to preferably incorporate the Ga-i(2+) defects with a weak hyperfine interaction and by searching for new spin-filtering defects with zero nuclear spin.

##### Place, publisher, year, edition, pages
American Physical Society, 2014
##### National Category
Engineering and Technology
##### Identifiers
urn:nbn:se:liu:diva-107449 (URN)10.1103/PhysRevB.89.195412 (DOI)000335913900007 ()
Available from: 2014-06-12 Created: 2014-06-12 Last updated: 2014-08-05Bibliographically approved
6. Room-Temperature Electron Spin Amplifier Base on Ga(In)NAs Alloys
Open this publication in new window or tab >>Room-Temperature Electron Spin Amplifier Base on Ga(In)NAs Alloys
2013 (English)In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 25, no 5, 738-742 p.Article in journal (Refereed) Published
##### Abstract [en]

The first experimental demonstration of a spin amplifier at room temperature is presented. An efficient, defect-enabled spin amplifier based on a non-magnetic semiconductor, Ga(In)NAs, is proposed and demonstrated, with a large spin gain (up to 2700% at zero field) for conduction electrons and a high cut-off frequency up to 1 GHz.

##### Keyword
spin amplifiers; spintronics; room temperature; defects; semiconductors
##### National Category
Condensed Matter Physics
##### Identifiers
Available from: 2012-11-20 Created: 2012-11-20 Last updated: 2014-06-17
7. Efficient room-temperature spin detector based on GaNAs
Open this publication in new window or tab >>Efficient room-temperature spin detector based on GaNAs
2012 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 111, no 7, 07C303- p.Article in journal (Refereed) Published
##### Abstract [en]

Efficient and highly spin-dependent recombination processes are shown to not only turn GaNAs into an efficient spin filter but also to make it an excellent spin detector functional at room temperature (RT). By taking advantage of the defect-engineered spin-filtering effect, the spin detection efficiency is no longer limited by the fast spin relaxation of conduction electrons. This leads to a significant enhancement in the optical polarization of the spin detector, making it possible to reliably detect even very weak electron spin polarization at RT, as demonstrated by a study of spin loss during optical spin injection across a GaAs/GaNAs interface.

##### Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2012
##### National Category
Engineering and Technology
##### Identifiers
urn:nbn:se:liu:diva-78283 (URN)10.1063/1.3671781 (DOI)000303282401019 ()
Available from: 2012-06-08 Created: 2012-06-08 Last updated: 2014-06-17
8. Room-temperature spin injection and spin loss across a GaNAs/GaAs interface
Open this publication in new window or tab >>Room-temperature spin injection and spin loss across a GaNAs/GaAs interface
2011 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 98, no 1, 012112- p.Article in journal (Refereed) Published
##### Abstract [en]

Recently discovered effect of spin-filtering and spin amplification in GaNAs enables us to reliably obtain detailed information on the degree of spin loss during optical spin injection across a semiconductor heterointerface at room temperature. Spin polarization of electrons injected from GaAs into GaNAs is found to be less than half of what is generated in GaNAs by optical orientation. We show that the observed reduced spin injection efficiency is not only due to spin relaxation in GaAs, but more importantly due to spin loss across the interface due to structural inversion asymmetry and probably also interfacial point defects.

##### Place, publisher, year, edition, pages
American Institute of Physics, 2011
##### National Category
Engineering and Technology Condensed Matter Physics
##### Identifiers
urn:nbn:se:liu:diva-65721 (URN)10.1063/1.3535615 (DOI)000286009800041 ()
##### Note

Original Publication: Yuttapoom Puttisong, Xiangjun Wang, Irina Buyanova, C W Tu, L Geelhaar, H Riechert and Weimin Chen, Room-temperature spin injection and spin loss across a GaNAs/GaAs interface, 2011, APPLIED PHYSICS LETTERS, (98), 1, 012112. http://dx.doi.org/10.1063/1.3535615 Copyright: American Institute of Physics http://www.aip.org/

Available from: 2011-02-18 Created: 2011-02-18 Last updated: 2015-05-28
9. Efficient room-temperature nuclear spin hyperpolarization of a defect atom in a semiconductor
Open this publication in new window or tab >>Efficient room-temperature nuclear spin hyperpolarization of a defect atom in a semiconductor
2013 (English)In: Nature Communications, ISSN 2041-1723, Vol. 4, no 1751Article in journal (Refereed) Published
##### Abstract [en]

Nuclear spin hyperpolarization is essential to future solid-state quantum computation using nuclear spin qubits and in highly sensitive magnetic resonance imaging. Though efficient dynamic nuclear polarization in semiconductors has been demonstrated at low temperatures for decades, its realization at room temperature is largely lacking. Here we demonstrate that a combined effect of efficient spin-dependent recombination and hyperfine coupling can facilitate strong dynamic nuclear polarization of a defect atom in a semiconductor at room temperature. We provide direct evidence that a sizeable nuclear field (~150 Gauss) and nuclear spin polarization (~15%) sensed by conduction electrons in GaNAs originates from dynamic nuclear polarization of a Ga interstitial defect. We further show that the dynamic nuclear polarization process is remarkably fast and is completed in <5 μs at room temperature. The proposed new concept could pave a way to overcome a major obstacle in achieving strong dynamic nuclear polarization at room temperature, desirable for practical device applications.

##### Place, publisher, year, edition, pages
Nature Publishing Group, 2013
##### National Category
Condensed Matter Physics
##### Identifiers
urn:nbn:se:liu:diva-93850 (URN)10.1038/ncomms2776 (DOI)000318872100108 ()
Available from: 2013-06-11 Created: 2013-06-11 Last updated: 2014-06-17Bibliographically approved

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