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Efficient room-temperature nuclear spin hyperpolarization of a defect atom in a semiconductor
Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, China .
Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.ORCID iD: 0000-0001-7155-7103
Paul-Drude-Institut fur Festkörpelektronik, Berlin, Germany.
2013 (English)In: Nature Communications, ISSN 2041-1723, E-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. Vol. 4, no 1751
##### National Category
Condensed Matter Physics
##### Identifiers
ISI: 000318872100108OAI: oai:DiVA.org:liu-93850DiVA, id: diva2:627289
Available from: 2013-06-11 Created: 2013-06-11 Last updated: 2017-12-06Bibliographically approved
##### In thesis
1. Room-temperature defect-engineered spin functionalities in Ga(In)NAs alloys
Open this publication in new window or tab >>Room-temperature defect-engineered spin functionalities in Ga(In)NAs alloys
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
urn:nbn:se:liu:diva-107621 (URN)10.3384/diss.diva-107621 (DOI)978-91-7519-293-2 (ISBN)
##### Supervisors
Available from: 2014-06-17 Created: 2014-06-17 Last updated: 2017-03-27Bibliographically approved

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Cite
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