Diamond Devices Based on Valley Polarization

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

Diamond Devices Based on Valley Polarization

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

Optical detection of valley-polarized electron diffusion in diamond

Using the state of valley-polarization of electrons in solids is a promising new paradigm for information storage and processing. The central challenge in utilizing valley-polarization for this purpose is to develop methods for manipulating and reading out the final valley state. Here, we demonstrate optical detection of valley-polarized electrons in diamond. It is achieved by capturing images of electroluminescence from nitrogen-vacancy centers at the surface of a diamond sample that are excited by electrons drifting and diffusing through the sample. Monte Carlo simulations are performed to interpret the resulting experimental diffusion patterns. Our results give insight into the drift-diffusion of valley-polarized electrons in diamond and yield a way of analyzing the valley-polarization of ensembles of electrons

Determination of the acoustic phonon deformation potentials in diamond

The interaction between acoustic phonons and electrons in diamond has been investigated by comparing state-of-the-art time-of-flight drift velocity measurements with Monte Carlo simulations. We use a multivariable anisotropic description of acoustic deformation potential scattering. The phonon-electron interaction is the limiting factor for the carrier mobility in ultrapure single crystal diamond. Hence, having a correct description is necessary for both device simulations and for predicting the maximum device performance. The experiments were performed at low temperature and using ultrapure diamond to minimize the influence of other scattering sources. The electronic valley polarization in diamond at low temperatures enables determination of both uniaxial and dilatation deformation potentials in the same experiment. The uniaxial and dilatation deformation potentials are found to be 18.5±0.2 and −5.7±0.3 eV, respectively

Investigation of Photoexcitation Energy Impact on Electron Mobility in Single Crystalline CdTe

The exceptional electronic properties of cadmium telluride (CdTe) allow the material to be used in a wide range of high energy radiation detection applications. Understanding the mechanisms of local carrier scattering is of fundamental importance to understand the charge transport in the material. Here, we investigate the effect of photoexcitation on electron transport properties in chlorine doped single crystalline cadmium telluride (SC-CdTe:Cl). For this purpose time of flight measurements were performed on SC-CdTe:Cl in order to study the electron drift mobility in the low injection regime. Measurements were made at the temperature intervals of 80 to 300 K, for an applied electric field between 270 and 1600 V/cm and for wavelengths of 532, 355 and 213 nm. We have found that the electron drift mobility was affected by the excitation energy for temperatures below 200 K. In addition, the measurements revealed that it is possible to determine impurity and shallow trap concentration by this method. The method proves to be extremely sensitive in measuring very low impurity levels and in identifying dominant scattering mechanisms.

Rapid direct growth of graphene on single-crystalline diamond using nickel as catalyst

Although theoretical investigations indicate that the successful combination of graphene and diamond would give interesting properties, only a limited number of reports dealing with the subject have been published. Here, we present a rapid thermal process (RTP) which involves nickel (Ni) as metal catalyst for a direct growth of graphene on diamond at a temperature of 1073 K for 60 s. This process operates with a combination of a lower temperature and for a shorter duration than what has previously been reported. Thin Ni films of different thicknesses were deposited on top of (100) single-crystalline diamond. After RTP, the coverage of monolayer graphene was found to be around 20% shown by the intensity ratio between the 2D- and G-peak using Raman spectroscopy on 50 nm thick Ni films. In addition, x-ray photoelectron spectroscopy and atomic force microscopy analysis were conducted. For electrical characterization, Hall-effect measurements were performed at temperatures between 80 and 360 K

Carrier Scattering Mechanisms : Identification via the Scaling Properties of the Boltzmann Transport Equation

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

Observation of transferred-electron oscillations in diamond

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

A Valleytronic Diamond Transistor : Electrostatic Control of Valley Currents and Charge-State Manipulation of NV Centers

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