| 1. |
- Caleman, Carl, et al.
(författare)
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Radiation damage in biological material : electronic properties and electron impact ionization in urea
- 2009
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Ingår i: Europhysics letters. - : IOP. - 0295-5075 .- 1286-4854. ; 85:1, s. 18005-
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Tidskriftsartikel (refereegranskat)abstract
- Radiation damage is an unavoidable process when performing structural investigations of biological macromolecules with X-rays. In crystallography this process can be limited through damage distribution in a crystal, while for single molecular imaging it can be outrun by employing short intense pulses. Secondary electron generation is crucial during damage formation and we present a study of urea, as model for biomaterial. From first principles we calculate the band structure and energy loss function, and subsequently the inelastic electron cross-section in urea. Using Molecular Dynamics simulations, we quantify the damage and study the magnitude and spatial extent of the electron cloud coming from an incident electron, as well as the dependence with initial energy.
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| 2. |
- Egg, David, et al.
(författare)
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Therapeutic options for CTLA-4 insufficiency
- 2022
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Ingår i: Journal of Allergy and Clinical Immunology. - : MOSBY-ELSEVIER. - 0091-6749 .- 1097-6825. ; 149:2, s. 736-746
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Tidskriftsartikel (refereegranskat)abstract
- Background: Heterozygous germline mutations in cytotoxic T lymphocyte-associated antigen-4 (CTLA4) impair the immunomodulatory function of regulatory T cells. Affected individuals are prone to life-threatening autoimmune and lymphoproliferative complications. A number of therapeutic options are currently being used with variable effectiveness. Objective: Our aim was to characterize the responsiveness of patients with CTLA-4 insufficiency to specific therapies and provide recommendations for the diagnostic workup and therapy at an organ-specific level. Methods: Clinical features, laboratory findings, and response to treatment were reviewed retrospectively in an international cohort of 173 carriers of CTLA4 mutation. Patients were followed between 2014 and 2020 for a total of 2624 months from diagnosis. Clinical manifestations were grouped on the basis of organ-specific involvement. Medication use and response were recorded and evaluated. Results: Among the 173 CTLA4 mutation carriers, 123 (71%) had been treated for immune complications. Abatacept, rituximab, sirolimus, and corticosteroids ameliorated disease severity, especially in cases of cytopenias and lymphocytic organ infiltration of the gut, lungs, and central nervous system. Immunoglobulin replacement was effective in prevention of infection. Only 4 of 16 patients (25%) with cytopenia who underwent splenectomy had a sustained clinical response. Cure was achieved with stem cell transplantation in 13 of 18 patients (72%). As a result of the aforementioned methods, organ-specific treatment pathways were developed. Conclusion: Systemic immunosuppressants and abatacept may provide partial control but require ongoing administration. Allogeneic hematopoietic stem cell transplantation offers a possible cure for patients with CTLA-4 insufficiency.
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| 3. |
- Gabrysch, Markus, 1978-
(författare)
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Charge Transport in Single-crystalline CVD Diamond
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Diamond is a semiconductor with many superior material properties such as high breakdown field, high saturation velocity, high carrier mobilities and the highest thermal conductivity of all materials. These extreme properties, as compared to other (wide bandgap) semiconductors, make it desirable to develop single-crystalline epitaxial diamond films for electronic device and detector applications. Future diamond devices, such as power diodes, photoconductive switches and high-frequency field effect transistors, could in principle deliver outstanding performance due to diamond's excellent intrinsic properties. However, such electronic applications put severe demands on the crystalline quality of the material. Many fundamental electronic properties of diamond are still poorly understood, which severely holds back diamond-based electronic device and detector development. This problem is largely due to incomplete knowledge of the defects in the material and due to a lack of understanding of how these defects influence transport properties. Since diamond lacks a shallow dopant that is fully thermally activated at room temperature, the conventional silicon semiconductor technology cannot be transferred to diamond devices; instead, new concepts have to be developed. Some of the more promising device concepts contain thin delta-doped layers with a very high dopant concentration, which are fully activated in conjunction with undoped (intrinsic) layers where charges are transported. Thus, it is crucial to better understand transport in high-quality undoped layers with high carrier mobilities. The focus of this doctoral thesis is therefore the study of charge transport and related electronic properties of single-crystalline plasma-deposited (SC-CVD) diamond samples, in order to improve knowledge on charge creation and transport mechanisms. Fundamental characteristics such as drift mobilities, compensation ratios and average pair-creation energy were measured. Comparing them with theoretical predictions from simulations allows for verification of these models and improvement of the diamond deposition process.
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| 4. |
- Gabrysch, Markus, et al.
(författare)
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Compensation in boron-doped CVD diamond
- 2008
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Ingår i: Physica status solidi. A, Applications and Materials Science. - : Wiley. - 1862-6300. ; 205:9, s. 2190-2194
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Tidskriftsartikel (refereegranskat)abstract
- Hall-effect measurements on single crystal boron-doped CVD diamond in the temperature interval 80-450 K are presented together with SIMS measurements of the dopant concentration. Capacitance-voltage measurements on rectifying Schottky junctions manufactured on the boron-doped structures are also presented in this context. Evaluation of the compensating donor (N-D) and acceptor concentrations (N-A) show that in certain samples very low compensation ratios (N-D/N-A below 10(-4)) have been achieved. The influence of compensating donors on majority carrier transport and the significance for diamond device performance are briefly discussed.
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| 5. |
- Gabrysch, Markus, 1978-, et al.
(författare)
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Electron and hole drift velocity in chemical vapor deposition diamond
- 2011
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 109:6, s. 063719-
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Tidskriftsartikel (refereegranskat)abstract
- The time-of-flight technique has been used to measure the drift velocities for electrons and holes in high-purity single-crystalline CVD diamond. Measurements were made in the temperature interval 83 ≤ T ≤ 460 K and for electric fields between 90 and 4 × 103 V/cm, applied in the <100> crystallographic direction. The study includes low-field drift mobilities and is performed in the low-injection regime to perturb the applied electric field only minimally.
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| 6. |
- Gabrysch, Markus, 1978-
(författare)
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Electronic Properties of Diamond
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Diamond is a semiconductor with many superior material properties such as high breakdown field, high saturation velocity, high carrier mobilities and the highest thermal conductivity of all materials. These extreme properties compared to other (wide bandgap) semiconductors make it desirable to develop single-crystalline epitaxial diamond films for many electronic device and detector applications. Future devices, such as power diodes, photoconductive switches and high-frequency field effect transistors, could in principle deliver outstanding performance due to diamond’s excellent intrinsic properties. However, such electronic applications put severe demands on the crystalline quality of the material. Unfortunately, many fundamental electronic properties of diamond are still poorly understood, which severely holds back diamond-based electronic device and detector development. Such uncertainties are largely due to an incomplete knowledge of the types and concentrations of defects present in the material and also due to a lack of understanding of the influence that these defects have on transport properties. The focus of this licentiate thesis is therefore the study of certain electronic properties of single-crystalline plasma-deposited (SC-CVD) diamond samples in order to gain more information about the charge creation and transport mechanisms. By measuring characteristics such as drift mobilities, saturation velocities, compensation ratios or average pair-creation energy and comparing them with theoretical predictions from simulations allows for verification of these models and improvement of the diamond growth process.
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| 7. |
- Gabrysch, Markus, et al.
(författare)
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Formation of secondary electron cascades in single-crystalline plasma-deposited diamond upon exposure to femtosecond x-ray pulses
- 2008
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 103:6
-
Tidskriftsartikel (refereegranskat)abstract
- Secondary electron cascades were measured in high purity single-crystalline chemical vapor deposition (CVD) diamond, following exposure to ultrashort hard x-ray pulses (140 fs full width at half maximum, 8.9 keV energy) from the Sub-Picosecond Pulse Source at the Stanford Linear Accelerator Center. We report measurements of the pair creation energy and of drift mobility of carriers in two CVD diamond crystals. This was done for the first time using femtosecond x-ray excitation. Values for the average pair creation energy were found to be 12.17 +/- 0.57 and 11.81 +/- 0.59 eV for the two crystals, respectively. These values are in good agreement with recent theoretical predictions. The average drift mobility of carriers, obtained by the best fit to device simulations, was mu(h)= 2750 cm(2)/V s for holes and was mu(e)= 2760 cm(2) / V s for electrons. These mobility values represent lower bounds for charge mobilities due to possible polarization of the samples. The results demonstrate outstanding electric properties and the enormous potential of diamond in ultrafast x-ray detectors.
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| 8. |
- Gabrysch, Markus, et al.
(författare)
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XUV-induced transient phase gratings for probing ultra-fast carrier generation and recombination processes in wide-bandgap semiconductors
- 2013
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Ingår i: Annalen der Physik. - : Wiley. - 0003-3804 .- 1521-3889. ; 525:1-2, s. 59-65
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Tidskriftsartikel (refereegranskat)abstract
- A method for probing the temporal evolution of ultra-fast carrier generation and recombination processes in wide-bandgap semiconductors, e.g. diamond, is described. Two extreme ultraviolet (pump) pulses produced by high-order harmonic generation in Argon gas (with a photon energy of 32 eV) are superimposed on a sample with a small angle between them, inducing periodic changes in the refractive index of the material causing it to act as a transient diffraction grating. A delayed synchronized infrared (probe) pulse gets diffracted on the induced phase grating and is detected in the first diffraction order. By varying the time-delay between pump and probe, the full temporal evolution of the free carrier generation and recombination processes can be resolved. Feasibility calculations and the first steps towards experimental implementation are presented.
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| 9. |
- Hammersberg, Johan, et al.
(författare)
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Stability of polarized states for diamond valleytronics
- 2014
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 104:23, s. 232105-
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Isberg, Jan, et al.
(författare)
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A lateral time-of-flight system for charge transport studies
- 2009
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Ingår i: Diamond and related materials. - Langford Lane, Kidlington, Oxford, OX5 1GB, United Kingdom : Elsevier Ltd. - 0925-9635 .- 1879-0062. ; 18:9, s. 1163-1166
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Tidskriftsartikel (refereegranskat)abstract
- A measurement system for lateral ToF charge carrier transport studies in intrinsic diamond is described. In the lateral ToF geometry, carriers travel close to the sample surface and the system is therefore particularly suited for studies of thin layers as well as the influence of different surface conditions on transport dynamics. A 213nm pulsed UV laser is used to create electron-hole pairs along a line focus between two parallel metal electrodes on one surface. The use of reflective UV-optics with short focal length allows for a narrow focal line and also for imaging the sample in UV or visible light without any dispersion. A clear hole transit was observed in one homoepitaxial single crystalline diamond film for which the substrate was treated by a Ar/Cl plasma etch prior to deposition. The hole transit signal was sufficiently clear to measure the near-surface hole drift mobility of about 860cm2/Vs across a contact spacing of 0.3mm.
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| 11. |
- Isberg, Jan, et al.
(författare)
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Generation, transport and detection of valley-polarized electrons in diamond
- 2013
-
Ingår i: Nature Materials. - 1476-1122 .- 1476-4660. ; 12:8, s. 760-764
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Tidskriftsartikel (refereegranskat)abstract
- Standard electronic devices encode bits of information by controlling the amount of electric charge in the circuits. Alternatively, it is possible to make devices that rely on other properties of electrons than their charge. For example, spintronic devices make use of the electron spin angular momentum as a carrier of information. A new concept is valleytronics in which information is encoded by the valley quantum number of the electron. The analogy between the valley and spin degrees of freedom also implies the possibility of valley-based quantum computing. In this Article, we demonstrate for the first time generation, transport ( across macroscopic distances) and detection of valley-polarized electrons in bulk diamond with a relaxation time of 300 ns at 77 K. We anticipate that these results will form the basis for the development of integrated valleytronic devices.
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| 12. |
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| 13. |
- Isberg, Jan, 1964-, et al.
(författare)
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Negative differential electron mobility and single valley transport in diamond
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Electron transport in isolated conduction band valleys across macroscopic distances has been observed in single-crystalline CVD diamond at 70 K by use of the time-of-flight technique. This is possible due to the very low scattering cross section for intervalley scattering in single-crystalline CVD diamond. This effect enables a precise determination of the ratio between longitudinal and transverse conduction band effective masses in diamond. We find ml/mt = 5.2. At temperatures in the interval 110-140 K, a negative differential mobility (NDM) has been observed for electrons with the electric field parallel to the crystallographic <100> direction. The NDM can be explained in terms of valley repopulation effects between the equivalent energy conduction band minima.
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| 14. |
- Isberg, Jan, et al.
(författare)
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Negative electron mobility in diamond
- 2012
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 100:17, s. 172103-
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Tidskriftsartikel (refereegranskat)abstract
- By measuring the drift velocity of electrons in diamond as a function of applied electric field, wedemonstrate that ultra-pure diamond exhibits negative differential electron mobility in the [100] directionbelow 140 K. Negative electron mobility is normally associated with III–V or II–VI semiconductors withan energy difference between different conduction band valleys. The observation of negative mobility indiamond, an elemental group IV semiconductor, is explained in terms of repopulation effects betweendifferent equivalent conduction band valleys using a model based on the Boltzmann equation.
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| 15. |
- Isberg, Jan, et al.
(författare)
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On the transition between space-charge-free and space-charge-limited conduction in diamond
- 2011
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Ingår i: Solid State Sciences. - : Elsevier BV. - 1293-2558 .- 1873-3085. ; 13:5, s. 1065-1067
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Tidskriftsartikel (refereegranskat)abstract
- Carrier transport in a high-purity single-crystalline CVD diamond sample was studied using the Time-of-Flight technique with optical UV excitation. By varying the intensity of the optical excitation over four orders of magnitude, the transition between space-charge-free and space-charge-limited hole conduction in diamond is directly observed. Experimentally, we find that even a relatively small injected charge appreciably affects the drift velocity measurements. To achieve a relative error in drift velocity less than 1%, the injected charge has to be less than 0.01 CU, where C is the sample capacitance and U the applied bias.
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| 16. |
- Isberg, Jan, et al.
(författare)
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Transient current electric field profiling of single crystal CVD diamond
- 2006
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Ingår i: Semiconductor Science and Technology. - : IOP Publishing. - 0268-1242 .- 1361-6641. ; 21:8, s. 1193-1195
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Tidskriftsartikel (refereegranskat)abstract
- The transient current technique ( TCT) has been adapted for profiling of the electric field distribution in intrinsic single crystal CVD diamond. It was found that successive hole transits do not appreciably affect the electric field distribution within the sample. Transits of holes can therefore be used to probe the electric field distribution and also the distribution of trapped charge. Electron transits, on the other hand, cause an accumulation of negative charge in the sample. Illumination with blue or green light was shown to lead to accumulation of positive charge. Low concentrations of trapped charge can be detected in diamond using TCT, corresponding to an ionized impurity concentration below N = 10(10) cm(-3).
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| 17. |
- Kovi, Kiran Kumar, 1980-, et al.
(författare)
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A charge transport study in diamond, surface passivated by high-k dielectric oxides
- 2014
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 105:20, s. 202102-
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Tidskriftsartikel (refereegranskat)abstract
- The recent progress in the growth of high-quality single-crystalline diamond films has sparked interest in the realization of efficient diamond power electronic devices. However, finding a suitable passivation is essential to improve the reliability and electrical performance of devices. In the current work, high-k dielectric materials such as aluminum oxide and hafnium oxide were deposited by atomic layer deposition on intrinsic diamond as a surface passivation layer. The hole transport properties in the diamond films were evaluated and compared to unpassivated films using the lateral time-of-flight technique. An enhancement of the near surface hole mobility in diamond films of up to 27% is observed when using aluminum oxide passivation.
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| 18. |
- Kovi, Kiran Kumar, 1980-, et al.
(författare)
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Charge Transport Phenomena Unique to Diamond
- 2014
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Ingår i: MRS Online Proceedings Library. - : Cambridge Journals Online. ; 1591, s. null-null
-
Tidskriftsartikel (refereegranskat)abstract
- ABSTRACT Diamond is a unique material in many respects. One of the most well-known extreme properties of diamond is its ultrahardness. This property of diamond actually turns out to have interesting consequences for charge transport, in particular at low temperatures. In fact, the strong covalent bonds that give rise to the ultrahardness results in a lack of short wavelength lattice vibrations which has a strong impact on both electron and hole scattering. In some sense diamond behaves more like a vacuum than other semiconductor materials. In this paper we describe some interesting charge transport properties of diamond and discuss possible novel electronic applications.
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| 19. |
- Kovi, Kiran Kumar, et al.
(författare)
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(Invited) Surface Passivation of High-k Dielectric Materials on Diamond Thin Films
- 2015
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Ingår i: ECS Transactions. - : The Electrochemical Society. - 1938-5862 .- 1938-6737. ; 69, s. 61-65
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Tidskriftsartikel (refereegranskat)abstract
- Single-crystalline CVD diamond films have excellent electrical and material properties with potential in high power, high voltage and high frequency applications that are out of reach for conventional semiconductor materials. For realization of efficient devices (e.g. MOSFET), finding a suitable dielectric is essential to improve the reliability and electrical performance of devices. In the current study, we present results from surface passivation studies by high-k dielectric materials such as aluminum oxide and hafnium oxide deposited by ALD on intrinsic and boron doped diamond substrates. The hole transport properties in the intrinsic diamond films were evaluated and compared to unpassivated films using the lateral time-of-flight technique. The MOS capacitor structure, which forms the basic building block of the MOSFET, is discussed.
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| 20. |
- Kovi, Kiran Kumar, et al.
(författare)
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Silicon Oxide Passivation of Single-Crystalline CVD Diamond Evaluated by the Time-of-Flight Technique
- 2014
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Ingår i: ECS SOLID STATE LETT. - : The Electrochemical Society. - 2162-8742 .- 2162-8750. ; 3:5, s. P65-P68
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Tidskriftsartikel (refereegranskat)abstract
- The excellent material properties of diamond make it highly desirable for many extreme electronic applications that are out of reach of conventional electronic materials. For commercial diamond devices to become a reality, it is necessary to have an effective surface passivation since the passivation determines the ability of the device to withstand high surface electric fields. In this paper we present data from lateral Time-of-Flight studies on SiO2-passivated intrinsic single-crystalline CVD diamond. The SiO2 films were deposited using three different techniques. The influence of the passivation on hole transport was studied, which resulted in the increase of hole mobilities. The results from the three different passivations are compared. (C) 2014 The Electrochemical Society. All rights reserved.
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| 21. |
- Kumar Kovi, Kiran, et al.
(författare)
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Time-of-Flight Characterization of Single-crystalline CVD Diamond with Different Surface Passivation Layers
- 2011
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Ingår i: MRS Proceedings. - : Cambridge University Press. - 1946-4274 .- 0272-9172. ; 1282:mrsf10-1282-a09-01
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Tidskriftsartikel (refereegranskat)abstract
- The electronic properties of diamond, e.g. a high band-gap and high carrier mobilities, together with material properties such as a very high thermal conductivity, chemical inertness and a high radiation resistance makes diamond a unique material for many extreme electronic applications out of reach for silicon devices. This includes, e.g. microwave power devices, power devices and high temperature electronics. It is important to have an effective passivation of the surface of such devices since the passivation determines the ability of the device to withstand high surface electric fields. In addition, the passivation is used to control the surface charge which can strongly influence the electric field in the bulk of the device. It is possible to measure sample parameters such as electron and hole drift mobilities, charge carrier lifetimes or saturation velocities using Time-of-flight (ToF) method. The ToF technique has also been adapted for probing the electric field distribution and the distribution of trapped charge. In this paper we present new data from lateral ToF studies of high-purity single crystalline diamond with different surface passivations. Silicon oxide and silicon nitride are used as passivation layers in the current study. The effect of the passivation on charge transport is studied, and the results of different passivation materials are compared experimentally.
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| 22. |
- Majdi, Saman, 1977-, et al.
(författare)
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Carrier Scattering Mechanisms : Identification via the Scaling Properties of the Boltzmann Transport Equation
- 2021
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Ingår i: Advanced Theory and Simulations. - : John Wiley & Sons. - 2513-0390. ; 4:1
-
Tidskriftsartikel (refereegranskat)abstract
- A method based on the scaling properties of the Boltzmann transport equation is proposed to identify the dominant scattering mechanisms that affect charge transport in a semiconductor. This method uses drift velocity data of mobile charges at different lattice temperatures and applied electric fields and takes into account the effect of carrier heating. By performing time‐of‐flight measurements on single‐crystalline diamond, hole and electron drift velocities are measured under low‐injection conditions within the temperature range 10–300 K. Evaluation of the data using the proposed method identifies acoustic phonon scattering as the dominant scattering mechanism across the measured temperature range. The exception is electrons at 100–200 K where conduction‐band valley repopulation has a prominent effect. At temperatures below ≈80 K, where valley polarization is observed for electrons, transport dominated by acoustic phonon scattering is observed in different valleys separately. The scaling model is additionally tested on data from highly resistive gallium arsenide samples to demonstrate the versatility of the method. In this case, impurity scattering can be ruled out as the dominant scattering mechanism in the samples for the temperature range 80–120 K.
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| 23. |
- Majdi, Saman, et al.
(författare)
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Characterization by Internal Photoemission Spectroscopy of Single-Crystal CVD Diamond Schottky Barrier Diodes
- 2010
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Ingår i: Journal of Electronic Materials. - : Springer Science and Business Media LLC. - 0361-5235 .- 1543-186X. ; 39:8, s. 1203-1208
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Tidskriftsartikel (refereegranskat)abstract
- Internal photoemission spectroscopy measurements have been performed to study the electrical characteristics of Schottky diodes on boron-doped single-crystalline chemical vapor deposited (SC-CVD) diamond. These measurements were compared with current-voltage (I-V) and current-temperature (I-T) measurements. Schottky contact barrier heights and ideality factors have been measured on Schottky contacts formed on four samples with Au, Ni, and Al contact metallizations. I-V and I-T measurements were performed in the temperature range from 300 K to 500 K. The internal photoemission method, which is less influenced by local variations in the Schottky barrier height than the other two methods, yielded the highest values of Schottky barrier heights to p-type material: I broken vertical bar(B) = 1.78 eV to 2.10 eV, depending on the choice of contact metal and sample boron concentration.
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| 24. |
- Majdi, Saman, 1977-, et al.
(författare)
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High-temperature deep-level transient spectroscopy system for defect studies in wide-bandgap semiconductors
- 2019
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Ingår i: Review of Scientific Instruments. - : AMER INST PHYSICS. - 0034-6748 .- 1089-7623. ; 90:6
-
Tidskriftsartikel (refereegranskat)abstract
- Full investigation of deep defect states and impurities in wide-bandgap materials by employing commercial transient capacitance spectroscopy is a challenge, demanding very high temperatures. Therefore, a high-temperature deep-level transient spectroscopy (HT-DLTS) system was developed for measurements up to 1100 K. The upper limit of the temperature range allows for the study of deep defects and trap centers in the bandgap, deeper than previously reported by DLTS characterization in any material. Performance of the system was tested by carrying out measurements on the well-known intrinsic defects in n-type 4H-SiC in the temperature range 300-950 K. Experimental observations performed on 4H-SiC Schottky diodes were in good agreement with the literature. However, the DLTS measurements were restricted by the operation and quality of the electrodes.
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| 25. |
- Majdi, Saman, et al.
(författare)
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Low temperature conduction-band transport in diamond
- 2016
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 109:16
-
Tidskriftsartikel (refereegranskat)abstract
- By performing Time-of-Flight measurements on high-purity single-crystalline chemical vapor deposited diamond, we are able to extract the electron drift velocity of valley-polarized electrons in the low-injection regime. The aim of this study is to improve the understanding of the mechanisms involved in the conduction-band transport of valley-polarized electrons. The measurements were carried out within the temperature range of 10-80 K, and the experimental results are systematically compared with Monte Carlo charge transport simulations. We observe a rapid enhancement of the electron mobility with decreasing temperature, which reveals that inelastic effects in electron-phonon scattering become important below similar to 40 K. In addition, we obtain the momentum relaxation rate for electrons with different valley polarizations.
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| 26. |
- Soman, Deepak Elamalayil, et al.
(författare)
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Analysis of Three-level Buck-Boost Converter Operation for Improved Renewable Energy Conversion and Smart Grid Integration
- 2014
-
Ingår i: 2014 IEEE International Energy Conference (ENERGYCON 2014). - : IEEE conference proceedings. - 9781479924493 ; , s. 76-81
-
Konferensbidrag (refereegranskat)abstract
- The increased smart grid integration of renewable energy sources demands high power handling and wide controllability for the enabling power conversion technologies. The conventional energy conversion techniques are inadequate to efficiently handle the highly varying nature of renewable energy sources like wave, solar, tidal and wind. The present work examines the advantages of using a three-level buck-boost DC-DC converter to aid three-level neutral-point-clamped inverter based grid integration. There are two main reasons for using this converter. It can provide the conventional buck-boost capability at higher power levels for absorbing and conditioning the renewable source output. Besides, it can be used as a voltage balancing device to satisfy the input requirement for the three-level neutral-point-clamped inverter. The work includes complete operating range analysis of the converter for the combined buck-boost action and voltage balancing effects to understand its suitability for various applications. The converter switching modes of operation are also presented in detail along with essential example waveforms. The final results show good controllability bandwidth for the converter which makes it an attractive solution for smart grid integration of renewable energy sources.
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| 27. |
- Suntornwipat, Nattakarn, et al.
(författare)
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A Valleytronic Diamond Transistor : Electrostatic Control of Valley Currents and Charge-State Manipulation of NV Centers
- 2021
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Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 21:1, s. 868-874
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Tidskriftsartikel (refereegranskat)abstract
- The valley degree of freedom in many-valley semiconductors provides a new paradigm for storing and processing information in valleytronic and quantum-computing applications. Achieving practical devices requires all-electric control of long-lived valley-polarized states, without the use of strong external magnetic fields. Because of the extreme strength of the carbon–carbon bond, diamond possesses exceptionally stable valley states that provide a useful platform for valleytronic devices. Using ultrapure single-crystalline diamond, we demonstrate electrostatic control of valley currents in a dual-gate field-effect transistor, where the electrons are generated with a short ultraviolet pulse. The charge current and the valley current measured at the receiving electrodes are controlled separately by varying the gate voltages. We propose a model to interpret experimental data, based on drift-diffusion equations coupled through rate terms, with the rates computed by microscopic Monte Carlo simulations. As an application, we demonstrate valley-current charge-state modulation of nitrogen-vacancy centers.
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| 28. |
- Suntornwipat, Nattakarn
(författare)
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Diamond Devices Based on Valley Polarization
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 29. |
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| 30. |
- Suntornwipat, Nattakarn, et al.
(författare)
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Investigation of transferred-electron oscillations in diamond
- 2016
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 108:21
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Tidskriftsartikel (refereegranskat)abstract
- The recent discovery of Negative Differential Mobility (NDM) in intrinsic single-crystalline diamond enables the development of devices for high frequency applications. The Transferred-Electron Oscillator (TEO) is one example of such devices that uses the benefit of NDM to generate continuous oscillations. This paper presents theoretical investigations of a diamond TEO in the temperature range of 110 to 140K where NDM has been observed. Our simulations map out the parameter space in which transferred-electron oscillations are expected to occur for a specific device geometry. The results are promising and indicate that it is possible to fabricate diamond based TEO devices.
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| 31. |
- Suntornwipat, Nattakarn, et al.
(författare)
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Magnetotransport study of valley-polarized electrons in synthetic diamond
- 2016
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 94:3
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate that the highly stable valley-polarized electron states in ultrapure single-crystalline diamond allow for investigation of charge transport, magnetoresistivity, and determination of the dominant scattering mechanism. The Hall effect gives rise to nonisotropic contributions in the mobility tensor that were measured at a temperature of 70 K in a time-of-flight setup with an added magnetic field. The observations of the magnetotransport of valley-polarized electrons in diamond are compared with both Monte Carlo simulations and an analytical model based on the Boltzmann transport equation. We establish that acoustic phonon scattering is the dominant electron scattering mechanism at 70 K for each of the valley polarizations in the investigated samples.
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| 32. |
- Suntornwipat, Nattakarn, et al.
(författare)
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Observation of transferred-electron oscillations in diamond
- 2019
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 115
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Tidskriftsartikel (refereegranskat)abstract
- The transferred-electron oscillator (TEO), or Gunn oscillator, is a device used in microwave applications, which utilizes the negative differential mobility (NDM) effect to generate continuous oscillations. Recently, NDM was observed in intrinsic single-crystalline chemical vapor deposition (SC-CVD) diamond. The occurrence was explained by the electron repopulation between its different conduction band valleys. This paper presents the results of constructing a diamond TEO based on the NDM effect. A series of experiments have been performed for varying voltages, temperatures, and resonator parameters on three SC-CVD diamond samples of different thicknesses. For the temperature range of 90–300 K, we observe transferred-electron oscillations in diamond.
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| 33. |
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