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Stability of polarized states for diamond valleytronics
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
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2014 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 104, no 23, p. 232105-Article in journal (Refereed) Published
Abstract [en]

The stability of valley polarized electron states is crucial for the development of valleytronics. A long relaxation time of the valley polarization is required to enable operations to be performed on the polarized states. Here, we investigate the stability of valley polarized states in diamond, expressed as relaxation time. We have found that the stability of the states can be extremely long when we consider the electron-phonon scattering processes allowed by symmetry considerations. We determine electron-phonon coupling constants by Time-of-Flight measurements and Monte Carlo simulations and use these data to map out the relaxation time temperature dependency. The relaxation time for diamond can be microseconds or longer below 100 K and 100 V/cm due to the strong covalent bond, which is highly encouraging for future use in valleytronic applications. (C) 2014 AIP Publishing LLC.

Place, publisher, year, edition, pages
2014. Vol. 104, no 23, p. 232105-
National Category
Engineering and Technology Physical Sciences
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
URN: urn:nbn:se:uu:diva-229299DOI: 10.1063/1.4882649ISI: 000337891200043OAI: oai:DiVA.org:uu-229299DiVA, id: diva2:736380
Available from: 2014-08-06 Created: 2014-08-05 Last updated: 2018-04-15Bibliographically approved
In thesis
1. Diamond Based Electronics and Valleytronics: An experimental study
Open this publication in new window or tab >>Diamond Based Electronics and Valleytronics: An experimental study
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Diamond is a promising semiconductor material for high power, high voltage, high temperature and high frequency applications due to its remarkable material properties: it has the highest thermal conductivity, it is the hardest material, chemically inert, radiation hard and has the widest transparency in the electromagnetic spectrum. It also exhibits excellent electrical properties like high breakdown field, high mobilities and a wide bandgap.  Hence, it may find applications in extreme conditions out of reach for conventional semiconductor materials, e.g. in high power density systems, high temperature conditions, automotive and aerospace industries, and space applications. 

 

With the recent progress in the growth of high purity single-crystalline CVD diamond, the realization of electronic devices is now possible. Natural and HPHT diamonds inevitably have too high a concentration of impurities and defects for electrical applications. To develop efficient electronic devices based on diamond, it is crucial to understand charge transport properties. Time-of-flight is one of the most powerful methods used to study charge transport properties like mobility, drift velocity and charge collection efficiency in highly resistive semiconductors, such as diamond. For commercial diamond devices to become a reality, it is necessary to have an effective surface passivation since the passivation determines the ability of a device to withstand high surface electric fields. Surface passivation studies on intrinsic SC-CVD diamond using materials like silicon oxide, silicon nitride and high-k materials have been conducted and observations reveal an increase in measured hole mobilities. Planar MOS capacitor structures form the basic building block of MOSFETs. Consequently, the understanding of MOS structures is crucial to make MOSFETs based on diamond. Planar MOS structures with aluminum oxide as gate dielectric were fabricated on boron doped diamond. The phenomenon of inversion was observed for the first time in diamond. In addition, low temperature hole transport in the range of 10-80 K has been investigated and the results are used to identify the type of scattering mechanisms affecting hole transport at these temperatures.

To utilize the potential of diamonds properties and with diamond being the hardest and most chemically inert material, new processing technologies are needed to produce devices for electrical, optical or mechanical applications. Etching of diamond is one of the important processing steps required to make devices. Achieving an isotropic etch with a high etch rate is a challenge. Semi-isotropic etch profiles with smooth surfaces were obtained by using anisotropic etching technique by placing diamond samples in a Faraday cage and etch rates of approximately 80 nm/min were achieved.

Valleytronics, which is a novel concept to encode information based on the valley quantum number of electrons has been investigated for the first time in diamond. Valley-polarized electrons with the longest relaxation time ever recorded in any material (300 ns) were observed. This is a first step towards demonstrating valleytronic devices.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. p. 61
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1163
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-229948 (URN)978-91-554-8999-1 (ISBN)
Public defence
2014-09-29, Polhemsalen, Ångström laboratory, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2014-09-05 Created: 2014-08-18 Last updated: 2014-12-02
2. Valley-Polarized Charge Transport in Diamond
Open this publication in new window or tab >>Valley-Polarized Charge Transport in Diamond
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Diamond is a wide bandgap semiconductor with extreme properties such as high thermal conductivity, high breakdown field and high carrier mobilities. These properties together with the possibility to synthesize high purity Single-Crystalline (SC) diamond by Chemical Vapor Deposition (CVD), makes it a really interesting material for electronic devices. The low impurity concentration achieved when fabricating diamonds by CVD allows for a detailed study of the intrinsic electronic properties of diamond, especially at low temperatures when the carrier scattering rate is low.

During the last few years, our group has presented two new phenomena discovered in SC-CVD diamond at temperatures below 150 K. For the very first time, Negative Differential Mobility (NDM) and valley polarization have been observed in diamond. NDM occurs at a temperature range of 110 to 140 K and at an electric field range of 300 to 600 V/cm and has been explained by electron repopulation between different valleys. At temperatures below 100 K, stable valley polarization has been observed due to the low phonon scattering rate in diamond that enable electrons to reside in one valley.

This licentiate thesis will give a short review on electronic properties and charge transport in diamond. It will also present the two discovered phenomena and the methods used to observe them. There will be further discussions of how these discoveries can be used for making future devices, such as the Transferred-Electron Oscillator (TEO) and valleytronic devices.      

Place, publisher, year, edition, pages
Uppsala: Institutionen för teknikvetenskaper, 2015. p. 51
Series
UURIE / Uppsala University, Department of Engineering Sciences, ISSN 0349-8352
Keyword
CVD diamond, valleytronics, Negative Differential Mobility, NDM, electron polarization, Time-of-Flight, ToF, magnetotransport, carrier transport, drift velocity
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-261009 (URN)
Presentation
(English)
Supervisors
Available from: 2015-09-07 Created: 2015-08-28 Last updated: 2015-09-15Bibliographically approved
3. Diamond Devices Based on Valley Polarization
Open this publication in new window or tab >>Diamond Devices Based on Valley Polarization
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Diamond is a wide bandgap semiconductor with extreme properties such as high thermal conductivity, high breakdown field, high carrier mobilities and chemical inertness. These properties together with the possibility to synthesize high purity Single-Crystalline (SC) diamond by Chemical Vapor Deposition (CVD), make it a very interesting material and a candidate for use in power electronics and in hazardous environments. The low impurity concentration achieved when fabricating diamond by CVD allows for a detailed study of the intrinsic electronic properties.

Diamond has six equivalent conduction band valleys oriented along the {100} axes with a uniquely low scattering rate between them. At low temperatures, the intervalley phonon scattering rate in diamond becomes negligible, which leads to a stable valley polarization state. We have observed non-equilibrium valley populations (valley-polarized electron ensembles), which in turn have been found to result in a Negative Differential Mobility (NDM).

NDM is commonly only observed in direct bandgap materials such as GaAs, InP and CdTe but our group has also observed NDM in diamond at a temperature range of 100 to 150 K. The occurrence of this phenomenon can be explained by electron repopulation, which is the scattering of electrons between different valleys. If NDM is pronounced enough, electric current instabilities build up and give rise to oscillations. By exploiting this phenomenon, a Transferred-Electron Oscillator (TEO) can be constructed for microwave applications.

Further investigations into the valley-polarized electrons seen in diamond could bring it forward as an alternative material for use in electronic devices. This use, called valleytronics, is similar to spintronics but instead of using the electron spin, the polarization in the conduction band valleys is used to transfer information. Digital electronic circuits use the presence or absence of charge to encode information which relies on a rapid redistribution of mobile charge carriers. This requires energy which results in losses and thus sets a theoretical limit to the maximum switching frequency. This is one of the main issues of electronic devices and can be mitigated by using alternative technologies such as spintronics or valleytronics.

In order to get a better understanding of the electron valley repopulation effects, the focus of this doctoral thesis is the study of electron charge transport in SC-CVD diamond at low temperatures. The thesis also aims at using valley-polarized states as a foundation for the creation of electronic devices such as TEOs or valley-transistors, out of diamond.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 88
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1670
Keyword
CVD diamond, valleytronics, Negative Differential Mobility, NDM, electron polarization, Time-of-Flight, magnetotransport, carrier transport, drift velocity, valley-transistor, Transferred-Electron Oscillator, TEO, TED, Gunn diode
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:uu:diva-348551 (URN)978-91-513-0335-2 (ISBN)
Public defence
2018-06-08, Siegbahnsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2018-05-15 Created: 2018-04-15 Last updated: 2018-05-15Bibliographically approved

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Hammersberg, JohanMajdi, SamanKovi, Kiran KumarSuntornwipat, NattakarnGabrysch, MarkusIsberg, Jan

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