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Chemical vapor deposition of hexagonal boron nitride and its use in electronic devices

Dielectrics are insulating materials used in many different electronic devices and play an important role in all of them. Current advanced electronic devices use dielectric materials with a high dielectric constant and avoid high leakage currents. However, these materials show several intrinsic problems, and also a bad interaction with adjacent materials. Therefore, the race for finding a suitable dielectric material for current and future electronic devices is still open. In this context two dimensional [2D] materials have become a serious option, not only thanks to their advanced properties, but also to the development of scalable synthesis methods. Graphene has been the most explored 2D material for electronic devices. However, graphene has no band gap, and therefore it cannot be used as dielectric. MoS2 and other 2D transition metal dichalcogenides (TMDs) are semiconducting 2D materials that can provide more versatility in electronic devices. In this PhD thesis I have investigated the use of monolayer and multilayer hexagonal boron nitride (h-BN) as dielectric for electronic devices, as it is a 2D material with a band gap of ~5.9 eV. My work has mainly focused on the synthesis of the h-BN using chemical vapor deposition, the study of its intrinsic morphological and electrical properties at the nanoscale, and its performance as dielectric in different electronic devices, such as capacitors and memristors. Overall, our experiments indicate that h-BN is a very reliable dielectric material, and that it can be successfully used in capacitors and memristors. Moreover, h-BN shows additional performances never observed in traditional dielectrics, such as volatile resistive switching, which may also open the door for new applications.


Abstract
Dielectrics are insulating materials used in many different electronic devices (e.g. capacitors, transistors, varistors), and play an important role in all of them. In fact, the dielectric is probably the most critical element in most devices, as it is exposed to electrical fields that can degrade its performance. Silicon dioxide (SiO2) has been traditionally the most widely used dielectric material in the industry; however, the scaling down of the devices required a reduction of the SiO2 thickness, which provoked a dramatic increase of the leakage current. This not only results in an increase of the power consumption, but also on the failure of the devices and circuits. Current advanced electronic devices use dielectric materials with a high dielectric constant (e.g. HfO2, Al2O3 and TiO2) so that their thickness doesn't need to be reduced so much, and high leakage currents are avoided. However, these materials show several intrinsic problems (e.g. large density of native defects, crystallization at high temperatures), and also a bad interaction with adjacent materials (e.g. rough interface with silicon, high diffusivity to polysilicon gate, difficulty to be deposited on two dimensional [2D] materials). Therefore, the race for finding a suitable dielectric material for current and future electronic devices is still open.
In this context 2D materials have become a serious option, not only thanks to their advanced properties, but also to the development of scalable synthesis methods.
Graphene has been the most explored 2D material for electronic devices, and it has been used as channel in transistors, and as electrode in capacitors and memristors (among others). However, graphene has no band gap, and therefore it cannot be used as dielectric. MoS2 and other 2D transition metal dichalcogenides (TMDs) are semiconducting 2D materials that can provide more versatility in electronic devices (i.e. they can increase the current ON/OFF ratio in transistors because the density of carriers can be tuned via electrical field), but their small band gap difficult their use dielectric.
In this PhD thesis I have investigated the use of monolayer and multilayer hexagonal boron nitride (h-BN) as dielectric for electronic devices, as it is a 2D material with a band gap of ~5.9 eV. My work has mainly focused on the synthesis of the h-BN using chemical vapor deposition, the study of its intrinsic morphological and electrical properties at the nanoscale, and its performance as dielectric in different electronic devices, such as capacitors and memristors. Overall, our experiments indicate that h-BN is a very reliable dielectric material, and that it can be successfully used in capacitors and memristors. Its performance depends on several parameters, like the substrate on which it is grown, the growth temperature, the growth time, the vacuum pressure, and even the adjacent electrodes deposited. Moreover, h-BN shows additional performances never observed in traditional dielectrics, such as volatile resistive switching, which may also open the door for new applications.

Introduction
Thin dielectric films are key elements in a wide range of electronic devices, as they can generate (for example) the capacitance effects required to form the conductive channel in field effect transistors (FETs), or the resistive switching (RS) phenomenon required to induce two logic (resistive) states in non-volatile memories (NVMs) [1][2].
With the scaling down of electronic devices, the traditionally used SiO2 dielectrics became too thin to withstand the electrical fields applied (that problem appeared first in FETs in the early 2000's, around the 45 nm technological node) [3], which threatened the reliability of the devices due to prohibitive leakage currents and dielectric breakdown (BD). The solution adopted by the industry was to replace the ultra thin SiO2 films by thicker high-k dielectric stacks (like HfO2, Al2O3 and TiO2); the thicker nature of the high-k blocked the leakage current, and its higher dielectric constant produced a similar capacitance effect than the thinner SiO2, which is required to make the devices work. However, the introduction of high-k dielectrics in the semiconductors industry generated several new problems, such as high density of native defects, interaction with the polysilicon gate and Si substrate, severe inhomogeneities, polycrystallization at the temperatures required during the manufacturing process of the devices (>400 °C), and high scattering at the channel region [4][5]. Furthermore, high-k dielectrics interact very badly with other advanced and very promising materials for future electronic devices, such as two dimensional (2D) materials [6][7]. Therefore, research on alternative dielectrics for high performance electronic devices is necessary.

Objectives of this PhD thesis
The main goal of this PhD thesis is to provide useful knowledge that clarifies if h-BN can be reliably used as dielectric in electronic devices. This task can be divided in three objectives. The first one is to develop a scalable method to synthesize h-BN that leads to high quality and low amount of defects. The second one is to characterize the intrinsic properties of the materials synthesized, such as thickness, surface roughness, density of defects, Raman signature, and percentage of B and N atoms. And the third is to study its performance as dielectric by applying electrical stresses, both at the nanoscale and at the device level. Another indirect goal of this thesis is to structure the knowledge available until now about the use of h-BN as dielectric. This is important because the first studies in this field didn't distinguish between h-BN stacks synthesized using different methods, and also because we have detected some literature that (in our opinion) reported irrelevant, unsupported and/or wrong claims. For this reason, this PhD thesis includes, not only four research articles (in the format of letters and full papers), but also two extensive review articles.

Key findings
In the first part of this thesis I present a deep literature review about the synthesis, characterization methods and performance of h-BN as dielectric in electronic devices. Article 1 is a review paper in which I analyze more than 179 references. We make critical comments related to the different performances shown by h-BN grown by different methods. Scalability of the synthesis process and its suitability for industrial applications is one of the main criteria when classifying the knowledge available on the use of h-BN as dielectric.
In the second part I describe the synthesis of multilayer h-BN on different metallic substrates (Pt, Cu, and Fe) using chemical vapor deposition (CVD) approach, and in all cases I check its performance as dielectric in real devices. In Article 2 and This article analyzes more than 364 references, and contains 12 tables comparing the structure, size, current window, endurance, retention, operation voltages, speed, power consumption, transparency and flexibility of the 2D materials based memristors. I also discuss the status and challenges to solve in this direction.

Thesis Outline
This thesis is divided in five chapters: Chapter 1 presents the dissertation summary, which introduces the most relevant aspects of this thesis.  dielectric and the entire BD process will be also described.

Properties of 2D layered h-BN
Boron nitride exists in multiple crystalline forms that differ in the arrangement of the B and N atoms, such as cubic boron nitride (c-BN), wurtzite boron nitride (w-BN) and hexagonal boron nitride (h-BN) [36]. Among them, h-BN is a typical sp 2 -hybridized 2D insulator, which is analogous to graphene in terms of their hexagonal lattice structure (see Figure [9], which allowed their use as anti-oxidation coating [9]. However, recent reports demonstrated that local defects can have a dramatic negative effect on the performances of h-BN. For example, it has been proved that local defects (i.e. lattice distortions) in h-BN sheets can serve as focus for local oxidation due to the presence of dangling bonds, where oxygen can easily bond [40]. This promotes oxygen migration towards the underlying (protected) substrate, which degrades the material below (and also the h-BN). Therefore, lattice defects can remarkably shorten the lifetime of h-BN, and it is expected that they also contribute negatively to the performance of h-BN based electronic devices. In fact, the presence of defects in a dielectric is something in most of cases unwanted from a reliability point of view, independently that it is a h-BN stack or a SiO2/high-k film.

Synthesis of 2D layered h-BN
The first synthesis of ultra thin 2D h-BN stacks was achieved in 2005 via mechanical exfoliation of an h-BN crystal [41]. Taking a piece of scotch tape and repeatedly peeling single crystal or bulk h-BN materials, atomically thin h-BN nanosheets can be easily recognized on the tape using an optical microscope (see Figure   2.2a). This method is based on a mechanical stripping process, i.e. break the weak van der Waals forces between each two adjacent layers, and doesn't involve the introduction of any type of chemical, nor other alien species. Therefore, the obtained h-BN nanosheets retain their original (nearly perfect) crystal structure. However, the size of exfoliated h-BN flakes becomes smaller and smaller with the number of peelings, resulting in small lateral size of (in the best cases) few micrometers. Furthermore, exfoliating h-BN with a specific number of layers is not doable, and very severe thickness inhomogeneities are always present (see Figure 2.2b) [42], which is not acceptable for the industry. The need of (expensive) human labor is also an important drawback.
Liquid phase exfoliation (LPE) is a scalable synthesis process that consists on applying mechanical stresses to an h-BN crystal by sonication [42][43], with the assistance of organic reagents. After sonication, centrifugation to remove large size h- films containing a rough network of flakes with folds and random orientations [44].
Therefore, LPE h-BN may be suitable for some very specific applications (e.g. coatings), but not for others (e.g. ultra thin gate dielectric in FETs).
It's known that large-area h-BN sheets can be grown by physical vapor deposition (PVD) methods. Among them, magnetron sputtering has been successfully used to produce monolayer h-BN sheets on a 100 nm Ru/α-Al2O3 substrate [45]. The sheets produced by this method show decent layered structure with low density of defects, as confirmed by cross sectional transmission electron microscopy (TEM, see  When picking up a method to grow scalable h-BN with controllable thickness, CVD is the approach that has produced the best results until now [18]. cm, as shown in Ref. [47]. When using CVD, as well as when using PVD, the main problem is the high temperatures used during the growth (> 900 °C ), which make impossible its direct growth on patterned samples (i.e. the high temperatures would destroy any device patterned due to severe diffusion). For this reason, CVD always uses metallic foils as substrate. Ref. [48] reported the growth of h-BN on Fe-coated wafers by using a cold walled CVD system (which is 10 times more expensive than a normal one), but no discussion on metal de-wetting and diffusion into the wafer is available. Also, no devices have been reported using this method. In order to avoid this problem, the h-BN is grown by CVD on an independent substrate (so far it has been only synthesized on metals) and later it has been transferred to any arbitrary substrate using different methods (e.g. wet transfer [49], dry transfer [50], imprint techniques [51], electrostatic transfer [52], among others). This can be seen as an advantage but also as a problem, as the transfer process may produce cracks, wrinkles and contamination. Interestingly, when fabricating prototype devices aimed to just test the performance of h-BN as dielectric, the substrate used during the h-BN growth (metallic foil) can be also used as bottom electrode. This strategy does not require the use of a transfer process.

Use of 2D layered h-BN as dielectric
When a thin dielectric is placed between two electrodes under polarization the electrical field can generate defects in its microstructure, mainly imperfect bonding (i.e. broken bonds between the atoms that form the dielectric or atoms that penetrate from adjacent electrodes) [53]. If the stress is enough aggressive (high voltage or long time), the density of defects increases prohibitively until forming one/few effective percolation path across the dielectric, leading to the complete loss of the insulating properties, and the circulation of very high currents across it (namely BD) [54]. The currents increase the local temperature, which further increases the current in a self-accelerated manner.
Therefore, the BD process strongly depends on several properties of the dielectric material, such as: i) density of native bulk defects, ii) number of defects at the interfaces with the electrodes, iii) chemical stability, iv) mechanical stability, and v) thermal conductivity [55]. As an example, the BD process in high-k dielectrics show slightly differences compared to SiO2, as the density of defects in high-k materials is much higher, which accentuates charge trapping and de-trapping, producing the observation of random telegraph noise (RTN) signals [56].
2D layered h-BN is an insulating material with an energy band gap of ~5.9 eV [8] (measured in exfoliated samples) and a dielectric constant between 2 and 4 [57] (measured in CVD grown samples). Therefore, h-BN may be suitable for being used as dielectric in electronic devices. In fact, it is expected that h-BN shows an excellent performance as dielectric, because it holds many wanted properties that are relevant during the degradation and BD of a dielectric, such as chemical stability, high mechanical stability, and high thermal conductivity (see also section 2.1). In this context, the strong covalent bonds of h-BN may slow down the speed for defect generation, the isolation between planes may difficult the propagation of the defects during the electrical stress, and the high thermal conductivity may avoid the formation of hot spots (which also slows down the BD process). Furthermore, h-BN adheres to the adjacent electrodes by van der Waals forces, minimizing the formation of interface defects. In this regard, h-BN is very promising for the interaction with graphene and MoS2, two materials that form very bad interfaces with SiO2 and high-k dielectrics [6][7].
Initially, 2D layered h-BN was used as substrate to enhance the carrier mobility of graphene FETs [15]. Due to the atomically flat surface of 2D layered h-BN (which is free of dangling bonds), the mobility of the carriers in the graphene channel was improved one order of magnitude compared to FETs using traditional SiO2 as substrate.
It's also known that the use of h-BN in high-mobility graphene devices can enhance the energy gap in multi-terminal measurements of fractional quantum Hall effect [58][59]. and reported that the tunneling current across it is extremely homogeneous (see Figure   2.3a). Ref. [62] showed that increasing the thickness of h-BN in one layer reduces the current in a factor 50 (see Figure 2.3b), and Ref. [62] claimed that in monolayer, bilayer and trilayer h-BN the tunneling current flows by Direct tunneling at low fields and by Fowler Nordheim tunneling at high fields, and that in thicker h-BN stacks the current always flows by Fowler Nordheim tunneling [62]. Ref. [63] suggested that exfoliated stacks of h-BN reache the BD layer-by-layer, producing dramatic physical removal of material after the BD of each layer (see Figure 2.3c). In that article the authors observed the formation of a hole with increasing depth after sequences of current vs. voltage (I-V) curves.  insulating properties when exposed to an electrical stress in a device. In this case the reliability of the entire h-BN stack is defined by the weakest location, as the BD event is a stochastic process. In this chapter these two properties are analyzed for h-BN stacks grown on different metallic substrates.

Growth h-BN on Pt, Cu and Fe substrates by CVD
The  After cleaning, the substrates are placed in the center of the tube furnace, which is connected to two gas lines: the first one (line 1) drives H2 inside the tube, and the second one (line 2) drives the precursor. For the growth of graphene the precursor is typically a gas (methane, Argon), but in the case of h-BN the precursor we used is liquid borazine. Other works used solid ammonia borane powder heated [67]. Therefore, in the precursor line (line 2) a gas was necessary to drive the borazine molecules (seeds) inside the tube. We also used H2 because it has provided good results in previous literature [68]. Both lines are controlled by a valve, and both of them are closed initially.
After introducing the substrates in the center of the tube furnace, the system was closed and a pump working at a certain pressure (P) was used to remove the air molecules inside the tube. Once the desired vacuum level is reached, a constant flow of H2 was immersed in the tube using line 1 (namely F1), and the temperature of the tube furnace was ramped up from room temperature (RT) to the annealing temperature (TA).
TA was maintained constant during the annealing time (tA). Despite the flow of H2 in the tube furnace during the annealing step may not be strictly necessary, it has been demonstrated that this helps to the formation of 2D materials [68]. After the annealing, a constant flow of H2 was introduced in the tube furnace using line 2 (F2), which carried borazine molecules into the tube for reaction with the metallic substrate. The time that the metallic substrate was exposed to the precursor is called growth time (tG), and during tG the temperature of the furnace was set at a suitable value for h-BN growth (TG), which depends on the metallic substrate used. After that, the furnace was cooled to RT. When using Pt substrate the cooling down time (tC) was quite fast, and when using Fe substrates this time needed to be enlarged.
The value of these parameters strongly affects the quality of the h-BN stack, and needed to be found experimentally. We are aware that world leading scientists in the field of CVD-grown 2D materials suffered strong difficulties to tune these parameters, especially when moving to a new lab. By empirical experience and collaborations with other groups we have corroborated that the use of identical parameters in two different labs may lead to different thicknesses and amount of defects. Most probably experimental factors like the brand of the equipment used and the purity and cleanness of the tube lines and metallic foils may play a role. Table 3.1 summarizes the values of the CVD parameters that we used to produce the best h-BN quality on Cu, Pt and Fe substrates. In order to find them, I repeated the process > 20 times for each material. Mainly two different chemical mechanisms producing the growth of layered h-BN at the surface of the metallic foils have been detected. The first one is surface-mediated growth mechanism, which happens in the metallic substrates with high solubility of boron but with/low nitrogen solubility under high temperature, such as Pt and Cu substrates [69]. The h-BN layers form on the surface of the catalytic substrates by decomposition of precursor and boron and nitrogen atoms deposited on the catalytic substrate. The second one is precipitation reaction, in which the metallic substrates have low solubility of boron and nitrogen; then both atoms will segregate from the substrate and form the h-BN layers during the cooling down process [69]. This mechanism is related to the use of Fe substrates. However, in some cases, mixed mechanisms with both surface-mediated and precipitation may contribute to the formation of thick h-BN stacks [70].
Generally speaking, the use of larger F2, tG and tC result in thicker h-BN stacks. F2 tunes the number of seeds on the metallic substrate, which will also control the domain size. Ref. [71] achieved centimeter scale single crystal graphene using a single seed deposited suing an external pipette but, to the best of our knowledge, this strategy has never been tried using h-BN. TA and TG are normally the same, and if TG is too low the borazine seeds cannot decompose properly to achieve the conformal growth of h-BN layers. Therefore, TG has a deep influence on the amount of defects within the h-BN stack. TG is selected mainly depending on the melting temperature of the substrate used, i.e. the substrate needs to be heated at a relative high temperature (lower that its melting point) in order to facilitate atomic rearrangements. It is also worth noting that under high temperature, the H2 gas could dissolve into the substrate, which facilitates the h-BN growth. The pressure used during the entire process was set at a constant value between 35 and 70 mtorr. This modest vacuum level is already enough to extract the air inside the tube and provide a reasonable clean atmosphere for the CVD process. Some groups used a CVD system working in ultra high vacuum (UHV) [72], but the complexity of the process increased dramatically, and the quality improvement is not remarkable. Independently of the material used, the growth of h-BN has been always carried out on metallic foils. Moreover, it has been reported that monolayer h-BN stacks can also be synthesized on wafers coated with 500 nm Fe metal films [73]. However, this has been only reported by one group, which didn't repeat the experiments.

Characterization of the samples and devices
The characterization of the samples has been mostly carried out at the laboratory of Prof. Mario Lanza at Soochow University, although few data have been also collected at the laboratory of Prof. Jing Kong at Massachusetts Institute of Technology. Furthermore, for the first time we analyze the BD spot using adhesion and deformation maps, which can give information about the sign of the traps inside the dielectric, as well as their mobility.
Finally, we also carry out the device level electrical characterization using probestation. To do so, we fabricated real devices by depositing top electrodes on the h-BN stacks (as grown on the metallic substrates) using a laser-patterned shadow mask and an electron beam evaporator. The use of a mask patterned by laser is essential to avoid large variability on the electrode size. Using this process, we fabricated Au/Ti/h-BN/Pt and Au/Ag/h-BN/Fe devices.

Results and discussion
By using the parameters described in Table 3  Chapter 4:

On the use of h-BN and other 2D materials in memristors
In Chapter 3 the fabrication of Au/Ag/h-BN/Fe memristors with very attractive performances has been described. The development of memristors using 2D materials is a strategy that has recently gained a lot of interest [76][77][78], as they may enhance some RS performances and provide flexibility and transparency capabilities. However, the integration of 2D materials in the structure of a memristor is not an easy task, and it brings associated several challenges [79]. Moreover, introducing a 2D materials in the structure of a memristor does not necessarily improves its performance, and we have detected some reports in which the real usefulness of the 2D material in the device is highly questionable. In this chapter we review the fabrication of memristors using 2D materials and summarize their most remarkable performances.

Fabrication of memristors using 2D materials
The integration of 2D materials in memristive devices is complex because their synthesis and deposition methods are remarkably different than those used in the microelectronics industry. As mentioned in Chapter 3, the scalable synthesis method that produces the best quality so far is CVD. However, the temperatures used during the growth are too high (>1000 ºC ). If graphene or h-BN would be grown on wafers patterned with devices, the high temperatures would unavoidably damage all the devices due to severe atomic migration. Therefore, a transfer process is necessary in order to integrate the 2D material on the patterned wafers.
Different processes have been developed to transfer 2D materials on arbitrary substrates, such as dry transfer [50], wet transfer assisted by different solid scaffolds (like poly(methyl methacrylate) [PMMA], perfluoropolymer-hyflon or rubber film) [49,80], roll-to-roll transfer [81], and face-to-face transfer [82], among others [83]. Among Another possibility to integrate the 2D materials in memristors is to use LPE 2D materials spin coated on the wafers, but that may bring associated other problems, such as incontrollable thickness fluctuations that may lead to large device-to-device variability, and even uncovered areas or pinholes that reduce the yield (e.g. many devices would be initially shorted).
Another recognized problem is that graphene forms a bad interface with TMO materials, which in many cases are required to build the memristor. For example, Ref.
[6] shows that, due to the absence of dangling bonds in graphene, TMOs cannot be directly deposited on it using standard methods, such as atomic layer deposition (ALD).
After that, several reports claimed that this problem does not takes place on the surface of MoS2 [85][86][87], and therefore it was believed that the mechanism for TMO growth by ALD on MoS2 was different than that on graphene. However, Ref. [7] proved all them wrong by using an in situ characterization. Using that setup it was observed that HfO2 aggregates at the local defects of the MoS2, leading to a multi-island pattern instead a conformal coating with uniform thickness (i.e. similar to what happens on graphene).
Ref. [7] demonstrated that the samples in Ref. [86] may have been exposed to contamination and for that reason the HfO2 film could have been grown due to the presence of dangling bonds in the moisture on the MoS2.

Status and best performances of 2D materials based memristors
The main application of memristors is as non-volatile memories (NVM) for information storage, and the companies in that field (e.g. Intel, Samsung, Micron) have been the main players boosting its research. According to the International Technology Roadmap of Semiconductor (ITRS) [54], the preformance requirements for any NVM technology are: small operating voltages (< 1 V), low power consumption (~ 10 pJ per transition), high operation speed or switching time (< 10 ns per transition), high endurance (more than 10 9 cycles), long data retention time (> 10 years), small MIM cell sizes (< 600 nm 2 ) and high ON/OFF current ratio (ION/IOFF > 10 6 ).
Different 2D materials can be used to carry out different functions in memristive devices. Generally, graphene has been used as top or bottom electrode to provide flexibility and transparency [88], and as interface layer between the metallic electrodes and the RS medium to avoid atomic diffusions, resulting in an effective decrease of the cycle-to-cycle variability [89]. Moreover, the high out-of-plane contact resistance of graphene also contributes to reduce the power consumption [90]. Other insulating 2D materials, like GO, h-BN and black phosphorous (BP), can serve as active RS medium to induce the RS either by migration of intrinsic species or by penetration of metallic ions from adjacent electrodes [1]. The best performances reported so far for 2D materials based memristors are summarized in Table 4.1. temperatures, as well as to observe the relationship between thermal conductivity and the degradation. However, the biggest improvement would be to grow h-BN using a single seed. This method has been used in the past to grow graphene, but it has never been applied to h-BN. In addition, not only h-BN but also other 2D insulating material (like graphene oxide, black phosphorus) should be explored and studied as dielectric.
The range of possibilities is very wide, and the experiments and findings that will come in the next years very exciting !  Los memristores con estructura Ag/h-BN/Fe muestran modulación de la resistividad de tipo volátil cuando la ruptura dieléctrica es generada aplicando tensión positiva en el electrodo de plata, y de tipo bipolar cuando la ruptura dieléctrica y la recuperación son generadas aplicando tensiones negativa y positiva (respectivamente) en el electrodo de plata. La razón es que en modo volátil el filamento está formado por iones de plata que penetran en la capa h-BN, los cuales tienen una alta difusividad y pueden retroceder a su estado de reposo cuando la tensión es desactivada. En el caso del modo bipolar, los átomos que forman el filamento son de hierro, que tienen una menor difusividad, y por lo tanto requieren la aplicación de una tensión extra para romper el filamento.
A parte de las habilidades técnicas adquiridas durante los experimentos, durante el desarrollo de esta tesis doctoral he hecho un esfuerzo muy importante en revisar la literatura relacionada y organizar la información. En mi caso, esta contribución es mayor que en otras tesis doctorales, ya que he escrito dos artículos de revisión, y en total he estudiado más de 543 artículos. En el primer artículo, publicado en la revista Microelectronics Engineering, he analizado el uso de h-BN como dieléctrico en dispositivos electrónicos (estado previo a esta tesis). Y en el segundo he analizado el uso de materiales bidimensionales para memristores. Este segundo artículo de revisión ha sido escrito en colaboración con el profesor Andrea Ferrari de la Universidad de Cambridge, y ha sido seleccionado como portada en la revista Advanced Electronic Materials. Esto me ha dado una visión muy amplia sobre el uso de materiales bidimensionales como dieléctrico, que es una habilidad que espero explotar en el futuro.