The ReaxFF reactive force-field : development, applications and future directions

The reactive force-field (ReaxFF) interatomic potential is a powerful computational tool for exploring, developing and optimizing material properties. Methods based on the principles of quantum mechanics (QM), while offering valuable theoretical guidance at the electronic level, are often too computationally intense for simulations that consider the full dynamic evolution of a system. Alternatively, empirical interatomic potentials that are based on classical principles require significantly fewer computational resources, which enables simulations to better describe dynamic processes over longer timeframes and on larger scales. Such methods, however, typically require a predefined connectivity between atoms, precluding simulations that involve reactive events. The ReaxFF method was developed to help bridge this gap. Approaching the gap from the classical side, ReaxFF casts the empirical interatomic potential within a bond-order formalism, thus implicitly describing chemical bonding without expensive QM calculations. This article provides an overview of the development, application, and future directions of the ReaxFF method

The interface states in gate-all-around transistors (GAAFETs)

The atomic-level structural detail and the quantum effects are becoming crucial to device performance as the emerging advanced transistors, representatively GAAFETs, are scaling down towards sub-3nm nodes. However, a multiscale simulation framework based on atomistic models and ab initio quantum simulation is still absent. Here, we propose such a simulation framework by fulfilling three challenging tasks, i.e., building atomistic all-around interfaces between semiconductor and amorphous gate-oxide, conducting large-scale first-principles calculations on the interface models containing up to 2796 atoms, and finally bridging the state-of-the-art atomic level calculation to commercial TCAD. With this framework, two unnoticed origins of interface states are demonstrated, and their tunability by changing channel size, orientation and geometry is confirmed. The quantitative study of interface states and their effects on device performance explains why the nanosheet channel is preferred in industry. We believe such a bottom-up framework is necessary and promising for the accurate simulation of emerging advanced transistors

First-principles study of oxygen vacancy defects in orthorhombic Hf0.5_{0.5}Zr0.5_{0.5}O2_2/SiO2_2/Si gate stack

The gate defect of the ferroelectric HfO$_2$-based Si field-effect transistor (Si FeFET) plays a dominant role in its reliability issue. The first-principles calculations are an effective method for the atomic-scale understanding of gate defects. However, the first-principles study on the defects of FeFET gate stacks, i.e., metal/orthorhombic-Hf$_{0.5}$Zr$_{0.5}$O$_2$/SiO$_2$/Si structure, has not been reported so far. The key challenge is the construction of metal/orthorhombic-Hf$_{0.5}$Zr$_{0.5}$O$_2$/SiO$_2$/Si gate stack models. Here, we use the Hf$_{0.5}$Zr$_{0.5}$O$_2$(130) high-index crystal face as the orthorhombic ferroelectric layer and construct a robust atomic structure of the orthorhombic-Hf$_{0.5}$Zr$_{0.5}$O$_2$/SiO$_2$/Si gate stack without any gap states. Its high structural stability is ascribed to the insulated interface. The calculated band offsets show that this gate structure is of the type-I band alignment. Furthermore, the formation energies and charge transition levels (CTLs) of defects reveal that the oxygen vacancy defects are more favorable to form compared with other defects such as oxygen interstitial and Hf/Zr vacancy, and their CTLs are mainly localized near the Si conduction band minimum and valence band maximum, in agreement with the reported experimental results. The oxygen vacancy defects are responsible for charge trapping/de-trapping behavior in Si FeFET. This work provides an insight into gate defects and paves the way to carry out the first-principles study of ferroelectric HfO$_2$-based Si FeFET.Comment: 18 pages, 5 figure

Insulators for 2D nanoelectronics: the gap to bridge

Nanoelectronic devices based on 2D materials are far from delivering their full theoretical performance potential due to the lack of scalable insulators. Amorphous oxides that work well in silicon technology have ill-defined interfaces with 2D materials and numerous defects, while 2D hexagonal boron nitride does not meet required dielectric specifications. The list of suitable alternative insulators is currently very limited. Thus, a radically different mindset with respect to suitable insulators for 2D technologies may be required. We review possible solution scenarios like the creation of clean interfaces, production of native oxides from 2D semiconductors and more intensive studies on crystalline insulators

Spectroscopic Studies of III-V Semiconductor Materials for Improved Devices

Defects in semiconductor crystals and at their interfaces usually impair the properties and the performance of devices. These defects include, for example, vacancies (i.e., missing crystal atoms), interstitials (i.e., extra atoms between the host crystal sites), and impurities such as oxygen atoms. The defects can decrease (i) the rate of the radiative electron transition from the conduction band to the valence band, (ii) the amount of charge carriers, and (iii) the mobility of the electrons in the conduction band. It is a common situation that the presence of crystal defects can be readily concluded as a decrease in the luminescence intensity or in the current flow for example. However, the identification of the harmful defects is not straightforward at all because it is challenging to characterize local defects with atomic resolution and identification. Such atomic-scale knowledge is however essential to find methods for reducing the amount of defects in energy-efficient semiconductor devices. The defects formed in thin interface layers of semiconductors are particularly difficult to characterize due to their buried and amorphous structures. Characterization methods which are sensitive to defects often require well-defined samples with long range order. Photoelectron spectroscopy (PES) combined with photoluminescence (PL) or electrical measurements is a potential approach to elucidate the structure and defects of the interface. It is essential to combine the PES with complementary measurements of similar samples to relate the PES changes to changes in the interface defect density. Understanding of the nature of defects related to III-V materials is relevant to developing for example field-effect transistors which include a III-V channel, but research is still far from complete. In this thesis, PES measurements are utilized in studies of various III-V compound semiconductor materials. PES is combined with photoluminescence measurements to study the SiO2/GaAs, SiNx/GaAs and BaO/GaAs interfaces. Also the formation of novel materials InN and photoluminescent GaAs nanoparticles are studied. Finally, the formation of Ga interstitial defects in GaAsN is elucidated by combining calculational results with PES measurements.Kidevirheet puolijohdekiteissä ja rajapinnoissa yleensä heikentävät laitteiden ominaisuuksia. Kidevirhe voi olla esimerkiksi vakanssi (puuttuva atomi), välisija-atomi (ylimääräinen atomi hilapaikkojen välissä) ja epäpuhtausatomi. Kidevirheet voivat alentaa (i) säteilytehoa, (ii) varauksen kuljettajien määrää ja (iii) elektronien nopeutta johtovyöllä. On yleistä, että kidevirheiden olemassaolo voidaan päätellä esimerkiksi heikentyneestä luminesenssistä tai virrankulusta. Ei kuitenkaan ole yksinkertaista tunnistaa minkälaatuisista virheistä on kyse, sillä on haastavaa karakterisoida paikallisia virheitä atomitarkkuudella. Sellainen tieto on kuitenkin välttämätöntä, että voidaan löytää menetelmiä kidevirheiden vähentämiseksi. Kidevirheet, joita muodostuu ohuissa rajapintakerroksissa, ovat erityisen haastavia tunnistaa, sillä ne sijatsevat näytteen pinnan alla ja ovat amorfisia. Karakterisointimenetelmät, jotka ovat hyödyllisiä kidevirheiden tutkimisessa, usein vaativat hyvin järjestäytyneen näytteen. Fotoelektronispektroskopia yhdistettynä fotoluminesenssimittaukseen tai sähköisiin mittauksiin on potentiaalinen lähestymistapa rajapinnan kidevirheiden tunnistamiseen. On välttämätöntä yhdistää fotoelektronispektroskopiamittaus muihin mittausmenetelmiin, jotta muutokset spektrissä voidaan ymmärtää paremmin. Parempi ymmärrys III-V kiteisiin liittyvistä kidevirheistä on välttämätöntä III-V kanavan sisältävien transistorien kehitystyön kannalta ja paljon on vielä opittavaa. Tässä väitöskirjatutkimuksessa fotoelektronispektroskopiaa hyödynnettiin III-V yhdistepuolijohteiden tutkimisessa. Se yhdistettiin fotolumisenssimittauksiin, kun tutkittiin SiO2/GaAs, SiNx/GaAs ja BaO/GaAs rajapintoja. Myös InN:n ja valoa tuottavien GaAs nanopartikkelien valmistamista tutkittiin. Lopuksi esitellään laskennallisia ja kokeellisia tuloksia Ga välisija-atomien muodostumisesta GaAsN:ssä.Siirretty Doriast

Nucleation and crystallisation of hafnium compounds and thin films

Hafnia and hafnium silicate are leading high-κ materials to replace SiO2 in CMOS devices. In this thesis the results of a study of bulk powders and thin films of these materials are reported. Bulk powders were investigated to provide a greater understanding of the crystallisation process by which HfO2 and HfSiO4 are formed. Investigation using thermal analysis, xray diffraction and electron microscopy techniques revealed that starting materials, heating conditions and atmosphere significantly affected the crystallisation pathway. In particular three mechanisms for tetragonal hafnia (t-HfO2) stabilisation were identified: (1) oxygen vacancies; (2) the critical particle size effect; and (3) the surface energy effect. Electron energy-loss spectroscopy (EELS) was used to try to obtain a standard O K edge for t-HfO2 from the powders and to better understand experimental EELS spectra obtained from thin films. A standard t-HfO2 edge was not found and many of the spectra obtained did not match existing standard edge shapes. The local atomic environment has a large effect on the edge shape in these samples, leading to the conclusion that a ‘standard’ edge shape may be impossible to obtain. Combining the EELS spectra from bulk and thin film samples, with modelled data it was found that the atoms within ~6Å from the excited atom had the largest effect on the edge shape. Consequently EELS spectra taken at a distance from an interface greater than ~6Å will give a bulk-like signal. 20nm HfxSi1-xO2 thin films were also investigated using TEM having been subjected to different thermal anneals and deposition conditions. It was found that the electron beam caused significant growth of SiO2 layers due to oxygen diffusion, and crystallisation within the high-κ layer. Furthermore, the higher the SiO2 content in the sample the more crystallisation was inhibited, though segregation into HfO2 and SiO2 rich regions occurred in all samples.

Insulators for 2D nanoelectronics: the gap to bridge

Nanoelectronic devices based on 2D materials are far from delivering their full theoretical performance potential due to the lack of scalable insulators. Amorphous oxides that work well in silicon technology have ill-defined interfaces with 2D materials and numerous defects, while 2D hexagonal boron nitride does not meet required dielectric specifications. The list of suitable alternative insulators is currently very limited. Thus, a radically different mindset with respect to suitable insulators for 2D technologies may be required. We review possible solution scenarios like the creation of clean interfaces, production of native oxides from 2D semiconductors and more intensive studies on crystalline insulators