Fabrication and characterization of Quantum Materials: Graphene heterostructures and Topological Insulators

[ES]La tesis empieza con una descripción de la Sala blanca de Salamanca y de su equipamiento, instalado durante los primeros años de mi doctorado. Sigue una detallada explicación de los procesos de fabricación de dispositivos en grafeno y otros materiales bidimensionales. En particular el sistema de trasferencia y la realización de contactos de borde tienen un rol fundamental en la realización de dispositivos de alta calidad. En nuestros dispositivos de grafeno encapsulado en nitruro de boro hexagonal hemos observado efecto Hall cuántico (QHE) a temperatura ambiente bajo la aplicación de altos campos magnéticos. El QHE en nuestros dispositivos de alta movilidad tiene características diferentes del QHE en dispositivos de grafeno de baja movilidad. Hemos también estudiado el transporte balístico y casi balístico en constricciones de grafeno con media y alta movilidad. En particular en las constricciones de mayor movilidad hemos introducido un método de definición de la constricción a bajas temperaturas, por la primera vez aplicado a dispositivos de grafeno y que nos han permitido obtener bordes con muy baja rugosidad. Esto ha permitido obtener un comportamiento balístico cerca del ideal y la observación de cuantización de la conductancia. En la última parte de la tesis reportamos medidas de transporte en pozos cuánticos de InAs/GaSb con diferente configuración de bandas (aislante, invertida y crítica). En la muestra crítica hemos encontrado una resistencia longitudinal anormal que hemos justificado con la posible formación de un excitón en bajas temperaturas.[EN]Starting from a detailed description of the Clean Room facilities, installed during this thesis work, we report the fabrication processes based on graphene and other 2D materials in detail. In hBN-encapsulated graphene the Quantum Hall Effect (QHE) at room temperature and high magnetic field was observed. We found different features in the QHE respect a previous work on lower mobility graphene on silicon oxide (Novoselov et al. Science 315 1379 2007). A detalied study of transport properties in graphene nanoconstrictionsis also reported. In particular in encapsulated graphene we introduced a new cryo-etching method to obtain low roughness edges nanocostrictions, in which quantized conductance was observed. In the last part of the thesis we report transport measurements on InAs/GaSb double quantum wells with different bandgap configurations (inverted, normal or critical)

Room-Temperature Terahertz Detection and Imaging by Using Strained-Silicon MODFETs

This chapter reports on an experimental and theoretical study of Schottky-gated strained-Si modulation-doped field-effect transistors (MODFETs) with different sub-micron gate lengths (100, 250, and 500 nm). Room-temperature detection of terahertz (THz) radiation by the strained-Si MODFETs was performed at two frequencies (0.15 and 0.3 THz). A technology computer-aided design (TCAD) analysis based on a two-dimensional hydrodynamic model (HDM) was used to investigate the transistor response to THz radiation excitation. TCAD simulation was validated through comparison with DC and low-frequency AC measurements. It was found that the photoresponse of the transistors can be improved by applying a constant drain-to-source bias. This enhancement was observed both theoretically and experimentally. The HDM model satisfactorily describes the experimental dependence of the photoresponse on the excitation frequency, the gate bias, and the drain-to-source current bias. The coupling of the incoming THz radiation to the MODFETs was studied at 0.15 and 0.3 THz. Finally, to demonstrate the suitability of strained-Si MODFET for terahertz applications, an image sensor within a pixel-by-pixel terahertz imaging system for the inspection of hidden objects was used

Quantum nanoconstrictions fabricated by cryo-etching in encapsulated graphene

More than a decade after the discovery of graphene, ballistic transport in nanostructures based on this intriguing material still represents a challenging field of research in two-dimensional electronics. The presence of rough edges in nanostructures based on this material prevents the appearance of truly ballistic electron transport as theo\-re\-tically predicted and, therefore, not well-developed plateaus of conductance have been revealed to date. In this work we report on a novel implementation of the cryo-etching method, which enabled us to fabricate graphene nanoconstrictions encapsulated between hexagonal boron nitride thin films with unprecedented control of the structure edges. High quality smooth nanometer-rough edges are characterized by atomic force microscopy and a clear correlation between low roughness and the existence of well-developed quantized conductance steps with the concomitant occurrence of ballistic transport is found at low temperature. In par\-ti\-cu\-lar, we come upon exact 2$e^{2}/h$ quantization steps of conductance at zero magnetic field due to size quantization, as it has been theoretically predicted for truly ballistic electron transport through graphene nanoconstrictions

Excitons, trions and Rydberg states in monolayer MoS2 revealed by low temperature photocurrent spectroscopy

We investigate excitonic transitions in a h-BN encapsulated monolayer $\textrm{MoS}_2$ phototransistor by photocurrent spectroscopy at cryogenic temperature (T = 5 K). The spectra presents excitonic peaks with linewidths as low as 8 meV, one order of magnitude lower than in earlier photocurrent spectroscopy measurements. We observe four spectral features corresponding to the ground states of neutral excitons ($\textrm{X}_{\textrm{1s}}^\textrm{A}$ and $\textrm{X}_{\textrm{1s}}^\textrm{B}$) and charged trions ($\textrm{T}^\textrm{A}$ and $\textrm{T}^\textrm{B}$) as well as up to eight additional spectral lines at energies above the $\textrm{X}_{\textrm{1s}}^\textrm{B}$ transition, which we attribute to the Rydberg series of excited states of $\textrm{X}^\textrm{A}$ and $\textrm{X}^\textrm{B}$. The relative intensities of the different spectral features can be tuned by the applied gate and drain-source voltages, with trions and Rydberg excited states becoming more prominent at large gate voltages. Using an effective-mass theory for excitons in two-dimensional transition-metal dichalcogenides we are able to accurately fit the measured spectral lines and unambiguously associate them with their corresponding Rydberg states. The fit also allows us to determine the quasiparticle bandgap and spin-orbit splitting of monolayer $\textrm{MoS}_2$, as well as the exciton binding energies of $\textrm{X}^\textrm{A}$ and $\textrm{X}^\textrm{B}$

Fast response photogating in monolayer MoS_2 phototransistors

Two-dimensional transition metal dichalcogenide (TMD) phototransistors have been the object of intensive research during the last years due to their potential for photodetection. Photoresponse in these devices is typically caused by a combination of two physical mechanisms: the photoconductive effect (PCE) and photogating effect (PGE). In earlier literature for monolayer (1L) MoS_2 phototransistors, PGE is generally attributed to charge trapping by polar molecules adsorbed to the semiconductor channel, giving rise to a very slow photoresponse. Thus, the photoresponse of 1L-MoS_2 phototransistors at high-frequency light modulation is assigned to PCE alone. Here we investigate the photoresponse of a fully h-BN encapsulated monolayer (1L) MoS_2 phototransistor. In contrast with previous understanding, we identify a rapidly-responding PGE mechanism that becomes the dominant contribution to photoresponse under high-frequency light modulation. Using a Hornbeck-Haynes model for the photocarrier dynamics, we fit the illumination power dependence of this PGE and estimate the energy level of the involved traps. The resulting energies are compatible with shallow traps in MoS2 caused by the presence of sulfur vacancies

Comprehensive characterization of Gunn oscillations in In0.53Ga0.47As planar diodes

[EN]In this work, In0.53Ga0.47As planar Gunn diodes specifically designed for providing oscillations at frequencies below 30 GHz have been fabricated and characterized. Different types of measurements were used to define a set of consistent methods for the characterization of the oscillations that can be extended to the sub-THz frequency range. First, negative differential resistance and a current drop are found in the I–V curve, indicating the potential presence of Gunn oscillations (GOs), which is then confirmed by means of a vector network analyzer, used to measure both the S11 parameter and the noise power density. The onset of unstable GOs at applied voltages where the negative differential resistance is hardly visible in the I–V curve is evidenced by the observation of a noise bump at very low frequency for the same applied voltage range. Subsequently, the formation of stable oscillations with an almost constant frequency of 8.8 GHz is observed for voltages beyond the current drop. These results have been corroborated by measurements performed with a spectrum analyzer, which are fully consistent with the findings achieved by the other techniques, all of them applicable to Gunn diodes oscillating at much higher frequencies, even above 300 GHz.Spanish MINECO through project TEC2017-83910-R and the Junta de Castilla y León and FEDER through projects SA022U16 and SA254P18

Phonon-mediated room-temperature quantum Hall transport in graphene

The quantum Hall (QH) effect in two-dimensional electron systems (2DESs) is conventionally observed at liquid-helium temperatures, where lattice vibrations are strongly suppressed and bulk carrier scattering is dominated by disorder. However, due to large Landau level (LL) separation (~2000 K at B = 30 T), graphene can support the QH effect up to room temperature (RT), concomitant with a non-negligible population of acoustic phonons with a wave-vector commensurate to the inverse electronic magnetic length. Here, we demonstrate that graphene encapsulated in hexagonal boron nitride (hBN) realizes a novel transport regime, where dissipation in the QH phase is governed predominantly by electron-phonon scattering. Investigating thermally-activated transport at filling factor 2 up to RT in an ensemble of back-gated devices, we show that the high B-field behaviour correlates with their zero B-field transport mobility. By this means, we extend the well-accepted notion of phonon-limited resistivity in ultra-clean graphene to a hitherto unexplored high-field realm.Comment: 17 pages, 4 figures. Supplementary information available at https://doi.org/10.1038/s41467-023-35986-

The Low-Temperature Photocurrent Spectrum of Monolayer MoSe2: Excitonic Features and Gate Voltage Dependence

Two-dimensional transition metal dichalcogenides (2D-TMDs) are among the most promising materials for exploring and exploiting exciton transitions. Excitons in 2D-TMDs present remarkably long lifetimes, even at room temperature. The spectral response of exciton transitions in 2D-TMDs has been thoroughly characterized over the past decade by means of photoluminescence spectroscopy, transmittance spectroscopy, and related techniques; however, the spectral dependence of their electronic response is still not fully characterized. In this work, we investigate the electronic response of exciton transitions in monolayer MoSe2 via low-temperature photocurrent spectroscopy. We identify the spectral features associated with the main exciton and trion transitions, with spectral bandwidths down to 15 meV. We also investigate the effect of the Fermi level on the position and intensity of excitonic spectral features, observing a very strong modulation of the photocurrent, which even undergoes a change in sign when the Fermi level crosses the charge neutrality point. Our results demonstrate the unexploited potential of low-temperature photocurrent spectroscopy for studying excitons in low-dimensional materials, and provide new insight into excitonic transitions in 1L-MoSe2

Ionic-Liquid Gating in Two-Dimensional TMDs: The Operation Principles and Spectroscopic Capabilities

Ionic-liquid gating (ILG) is able to enhance carrier densities well above the achievable values in traditional field-effect transistors (FETs), revealing it to be a promising technique for exploring the electronic phases of materials in extreme doping regimes. Due to their chemical stability, transition metal dichalcogenides (TMDs) are ideal candidates to produce ionic-liquid-gated FETs. Furthermore, as recently discovered, ILG can be used to obtain the band gap of two-dimensional semiconductors directly from the simple transfer characteristics. In this work, we present an overview of the operation principles of ionic liquid gating in TMD-based transistors, establishing the importance of the reference voltage to obtain hysteresis-free transfer characteristics, and hence, precisely determine the band gap. We produced ILG-based bilayer WSe2 FETs and demonstrated their ambipolar behavior. We estimated the band gap directly from the transfer characteristics, demonstrating the potential of ILG as a spectroscopy technique