Transparent oxide semiconducting films have continued to receive considerable attention, from a fundamental and application-based point of view, primarily because of their useful fundamental properties. Of particular interest is zinc oxide (ZnO), an n-type semiconductor that exhibits excellent optical, electrical, catalytic and gas-sensing properties, and has many applications in various fields. In this work, thin film transistor (TFT) arrays based on ZnO have been prepared by reactive radio frequency (RF) magnetron sputtering. Prior to the TFT fabrication, ZnO layers were sputtered on to glass and silicon substrates, and the deposition parameters optimised for electrical resistivities suitable for TFT applications. The sputtering process was carried out at room temperature with no intentional heating. The aim of this work is to prepare ZnO thin films with stable semiconducting electrical properties to be used as the active channel in TFTs; and to understand the role of intrinsic point defects in device performance and stability. The effect of oxygen (O2) adsorption on TFT device characteristics is also investigated. The structural quality of the material (defect type and concentration), electrical and optical properties (transmission/absorption) of semiconductor materials are usually closely correlated. Using the Vienna ab-initio simulation package (VASP), it is predicted that O2 adsorption may influence film transport properties only within a few atomic layers beneath the adsorption site. These findings were exploited to deposit thin films that are relatively stable in atmospheric ambient with improved TFT applications. TFTs incorporating the optimised layer were fabricated and demonstrated very impressive performance metrics, with effective channel mobilities as high as 30 cm2/V-1s-1, on-off current ratios of 107 and sub-threshold slopes of 0.9 – 3.2 V/dec. These were found to be dependent on film thickness (~15 – 60 nm) and the underlying dielectric (silicon dioxide (SiO2), gadolinium oxide (Gd2O3), yttrium oxide (Y2O3) and hafnium oxide (HfO2)). In this work, prior to sputtering the ZnO layer (using a ZnO target of 99.999 % purity), the sputtering chamber was evacuated to a base pressure ~4 x 10-6 Torr. Oxygen (O2) and argon (Ar) gas (with O2/Ar ratio of varying proportions) were then pumped into the chamber and the deposition process optimised by varying the RF power between 25 and 500 W and the O2/Ar ratio between 0.010 to 0.375. A two-level factorial design technique was implemented to test specific parameter combinations (i.e. RF power and O2/Ar ratio) and then statistical analysis was utilised to map out the responses. The ZnO films were sputtered on glass and silicon substrates for transparency and resistivity measurements, and TFT fabrication respectively. For TFT device fabrication, ZnO films were deposited onto thermally-grown silicon dioxide (SiO2) or a high-k dielectric layer (HfO2, Gd2O3 and Y2O3) deposited by a metal-organic chemical deposition (MOCVD) process. Also, by using ab initio simulation as implemented in the “Vienna ab initio simulation package (VASP)”, the role of oxygen adsorption on the electrical stability of ZnO thin film is also investigated. The results indicate that O2 adsorption on ZnO layers could modify both the electronic density of states in the vicinity of the Fermi level and the band gap of the film. This study is complemented by studying the effects of low temperature annealing in air on the properties of ZnO films. It is speculated that O2 adsorption/desorption at low temperatures (150 – 350 0C) induces variations in the electrical resistance, band gap and Urbach energy of the film, consistent with the trends predicted from DFT results.

Electronic components, based on current semiconductor technologies and operating in radiation rich environments, suffer degradation of their performance as a result of radiation exposure. Silicon carbide (SiC) provides an alternate solution as a radiation hard material, because of its wide bandgap and higher atomic displacement energies, for devices intended for radiation environment applications. However, the radiation tolerance and reliability of SiC-based devices needs to be understood by testing devices  under controlled radiation environments. These kinds of studies have been previously performed on diodes and MESFETs, but multilayer devices such as bipolar junction transistors (BJT) have not yet been studied. In this thesis, SiC material, BJTs fabricated from SiC, and various dielectrics for SiC passivation are studied by exposure to high energy ion beams with selected energies and fluences. The studies reveal that the implantation induced crystal damage in SiC material can be partly recovered at relatively low temperatures, for damag elevels much lower than needed for amorphization. The implantation experiments performed on BJTs in the bulk of devices show that the degradation in deviceperformance produced by low dose ion implantations can be recovered at 420 oC, however, higher doses produce more resistant damage. Ion induced damage at the interface of passivation layer and SiC in BJT has also been examined in this thesis. It is found that damaging of the interface by ionizing radiation reduces the current gain as well. However, for this type of damage, annealing at low temperatures further reduces the gain. Silicon dioxide (SiO2) is today the dielectric material most often used for gate dielectric or passivation layers, also for SiC. However, in this thesis several alternate passivation materials are investigated, such as, AlN, Al2O3 and Ta2O5. These materials are deposited by atomic layer deposition (ALD) both as single layers and in stacks, combining several different layers. Al2O3 is further investigated with respect to thermalstability and radiation hardness. It is observed that high temperature treatment of Al2O3 can substantially improve the performance of the dielectric film. A radiation hardness study furthermore reveals that Al2O3 is more resistant to ionizing radiation than currently used SiO2 and it is a suitable candidate for devices in radiation rich applications. / QC 20120117

An experimental and theoretical approach is taken to determine the effect of a heterojunction on the effective work function in a metal/high-? gate stack, the characteristics of aqueous hydrochloric acid cleaned (aq-HCl) GaN surface and the interface between GaN and Al2O3, HfO2 and GaON. The investigation of the effect of a heterojunction on the effective work function in a metal/high-? gate stack found that when a Ge/Si heterostructure on silicon is lightly doped and sufficiently thin, the work function can be extracted in a manner similar to that for a simple silicon substrate. Modifications to the terraced oxide structure are proposed to remove oxidation effects of the alternate channel materials. The extracted work function of TiN with various thicknesses on HfSiO is found to be in agreement with that of TiN on a silicon substrate. X-ray and ultraviolet photoelectron spectroscopy are used to observe the interface electronic states at the GaN (0001) and Al2O3, HfO2 and GaON dielectric interfaces. The GaN is cleaned using aqueous HCl prior to thermal oxidation to form GaON and atomic layer deposition of Al2O3 and HfO2. This was followed by a post deposition anneal. The GaN/HfO2 and GaN/Al2O3 interfaces exhibited dipoles of 1.6 eV and 0.4 eV +/- 0.2 eV, respectively. It is determined that the formation of an interfacial layer at the GaN/HfO2 interface is the primary cause of the larger dipole. Due to the knowledge of the formation of an interfacial GaOx or GaON layer during atomic layer deposition of HfO2, a better understanding of the GaN/GaON interface is needed. To accomplish this task, the interface electronic states at the GaN(0001) and GaON interface are observed using X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). XPS and UPS analysis of the GaN/GaON interface resulted in the calculation of a -2.7 eV +/- 0.2 eV dipole assuming that the core level shifts are only representative of the GaN band bending at the interface. If it is assumed that the core level shifts are only due to the oxidation of GaN, then the exhibited dipole at the GaN/GaON interface is -1.8 eV +/- 0.2 eV. Results indicate that the observed dipole is primarily caused by the polarization of the GaN. A theoretical approach is taken to provide a more complete understanding of the underlying formation mechanisms of a GaON interfacial layer during atomic layer deposition of HfO2. First, density functional theory is used to calculate the interactions of oxygen and water with the Ga-face of GaN clusters. The GaN clusters could be used as testbeds for the actual Ga-face on GaN crystals of importance in electronics. The results reveal that the local spin plays an important role in these interactions. It is found that the most stable interactions of O2 and the GaN clusters results in the complete dissociation of the O2 molecule to form two Ga-O-Ga bonds, while the most stable interactions between a H2O molecule and the GaN clusters are the complete dissociation of one of the O-H bonds to form a Ga-O-H bond and a Ga-H bond. Second, density functional theory is used to calculate the interaction of the reactants used to deposit HfO2 and Al2O3 during atomic layer deposition with hydrolyzed Ga-face GaN clusters. The results suggest that while further research is needed in this area to grasp a better understanding of the interactions of Trimethylaluminum (TMA) or Tertrakis(EthylMethylAmino)Hafnium (TEMAH) with hydrolyzed GaN clusters, it is found that a Ga-N(CH3)(CH2CH3) bond can form during the deposition of HfO2 using ALD and TEMAH as the reactant without breaking the Hf-N bond. The formation of a Ga-N(CH3)(CH2CH3) bond is significant because with the introduction of water into the system, the methyl and ethylmethyl groups may react to form a Ga-N-O bond which is believed to be the interfacial oxide found during deposition of HfO2 using ALD on GaN. No Ga-C bond structure formed in any fully optimized stable structure when analyzing the interaction of TMA with hydrolyzed GaN.

abstract: High-k dielectrics have been employed in the metal-oxide semiconductor field effect transistors (MOSFETs) since 45 nm technology node. In this MOSFET industry, Moore’s law projects the feature size of MOSFET scales half within every 18 months. Such scaling down theory has not only led to the physical limit of manufacturing but also raised the reliability issues in MOSFETs. After the incorporation of HfO2 based high-k dielectrics, the stacked oxides based gate insulator is facing rather challenging reliability issues due to the vulnerable HfO2 layer, ultra-thin interfacial SiO2 layer, and even messy interface between SiO2 and HfO2. Bias temperature instabilities (BTI), hot channel electrons injections (HCI), stress-induced leakage current (SILC), and time dependent dielectric breakdown (TDDB) are the four most prominent reliability challenges impacting the lifetime of the chips under use. In order to fully understand the origins that could potentially challenge the reliability of the MOSFETs the defects induced aging and breakdown of the high-k dielectrics have been profoundly investigated here. BTI aging has been investigated to be related to charging effects from the bulk oxide traps and generations of Si-H bonds related interface traps. CVS and RVS induced dielectric breakdown studies have been performed and investigated. The breakdown process is regarded to be related to oxygen vacancies generations triggered by hot hole injections from anode. Post breakdown conduction study in the RRAM devices have shown irreversible characteristics of the dielectrics, although the resistance could be switched into high resistance state. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2018

Foram estudados filmes de oxinitreto de silício obtidos por PECVD à 320°C, a partir da mistura gasosa de N2O+SiH4+He, com diferentes valores de pressão e potência de deposição com o objetivo de produzir boa qualidade de interface deste material com o Si e de obter uma baixa densidade de carga efetiva visando a aplicação desses filmes em dispositivos semicondutores MOS. Os resultados mostraram que com uma pressão de deposição de 0,160 mbar e potências menores que 125 W/cm2 é possível obter um valor de densidade de estados de interface (Dit) de 4x1010 eV-1.cm-2, campo elétrico de ruptura (Ebd) de 13 MV/cm, valores comparáveis ao SiO2 térmico e uma densidade de carga efetiva (Nss) de 4x1011 cm-2. Segundo resultados experimentais esse valor de Nss é o mínimo possível que se pode atingir com a limpeza química utilizada em nosso laboratório. Pode-se dizer que estes são resultados bastante interessantes considerando que se trata de um material obtido por PECVD à baixa temperatura, porém viável para aplicação em dispositivos MOS. Iniciando os estudos com dielétricos de maiores valores de constante dielétrica optamos por estudar filmes de TiOx (k=40-100), obtidos por sputtering reativo, a partir da mistura gasosa de Ar+O2 e utilizando alvo de Ti. Foram fabricados capacitores MOS com estes filmes e obteve-se valores de constante dielétrica que variaram de 40-160. Porém esses materiais ainda apresentavam valores apreciáveis de corrente de fuga que foram minimizadas em ordens de grandeza quando utilizados dielétricos de dupla camada com SiO2 ou SiOxNy (otimizado neste trabalho) na interface, além de se observar uma melhora significativa da qualidade de interface. Utilizando dupla camada dielétrica com filmes de SiOxNy e SiO2, ainda espessos (³ 1nm) para camada intermediária, obteve-se uma constante dielétrica efetiva em torno de 20. Vale ressaltar que os dois filmes SiOxNy e TiOx, conseqüentemente a dupla camada, foram fabricados a baixas temperaturas. / Silicon oxynitride films obtained by the PECVD technique from N2O+SiH4+He gaseous mixtures, at 320°C, with different deposition pressure and RF power were studied intending to improve the interface quality with Si, decreasing the effective charge density and the interface state density in order to utilize them in MOS semiconductor devices. The results showed that with a deposition pressure of 0.160 mbar and a RF power density lower than 125 W/cm2 it is possible to obtain interface state density (Dit) values of 4x1010 eV-1.cm-2, Electrical Breakdown (Ebd) of 13 MV/cm, comparable with the obtained for thermally grown SiO2 , and an effective charge density (Nss) of 4x1011 cm-2. According with experimental results this Nss value is the minimum attainable with our chemical cleaning process. In this way it can be said that these results are very promising, considering that these materials were obtained by PECVD at low temperatures, but still viable for MOS devices application. In order to initiate studies with high dielectrics constant material, TiOx films (k= 40-180), obtained by reactive sputtering through the Ar+O2 gaseous mixture utilizing a Ti target, were chosen. MOS capacitors with these films were fabricated and dielectric constant values varying from 40 to 160 were obtained. However, until now, these materials have presented appreciable leakage current values, which were, minimize by orders of magnitude with the addition of a thin SiO2 or SiOxNy (optimized in this work) layer at the interface were utilized. This thin layer also resulted in a significant improvement of the interface quality. Utilizing double dielectric layer with SiOxNy or SiO2, still thick (³ 1nm) as intermediate layer a dielectric constant value of 20 was obtained. Its important to mention that the SiOxNy and TiOx films, and consequently the double layer, were deposited at low temperatures.

Foram estudados filmes de oxinitreto de silício obtidos por PECVD à 320°C, a partir da mistura gasosa de N2O+SiH4+He, com diferentes valores de pressão e potência de deposição com o objetivo de produzir boa qualidade de interface deste material com o Si e de obter uma baixa densidade de carga efetiva visando a aplicação desses filmes em dispositivos semicondutores MOS. Os resultados mostraram que com uma pressão de deposição de 0,160 mbar e potências menores que 125 W/cm2 é possível obter um valor de densidade de estados de interface (Dit) de 4x1010 eV-1.cm-2, campo elétrico de ruptura (Ebd) de 13 MV/cm, valores comparáveis ao SiO2 térmico e uma densidade de carga efetiva (Nss) de 4x1011 cm-2. Segundo resultados experimentais esse valor de Nss é o mínimo possível que se pode atingir com a limpeza química utilizada em nosso laboratório. Pode-se dizer que estes são resultados bastante interessantes considerando que se trata de um material obtido por PECVD à baixa temperatura, porém viável para aplicação em dispositivos MOS. Iniciando os estudos com dielétricos de maiores valores de constante dielétrica optamos por estudar filmes de TiOx (k=40-100), obtidos por sputtering reativo, a partir da mistura gasosa de Ar+O2 e utilizando alvo de Ti. Foram fabricados capacitores MOS com estes filmes e obteve-se valores de constante dielétrica que variaram de 40-160. Porém esses materiais ainda apresentavam valores apreciáveis de corrente de fuga que foram minimizadas em ordens de grandeza quando utilizados dielétricos de dupla camada com SiO2 ou SiOxNy (otimizado neste trabalho) na interface, além de se observar uma melhora significativa da qualidade de interface. Utilizando dupla camada dielétrica com filmes de SiOxNy e SiO2, ainda espessos (³ 1nm) para camada intermediária, obteve-se uma constante dielétrica efetiva em torno de 20. Vale ressaltar que os dois filmes SiOxNy e TiOx, conseqüentemente a dupla camada, foram fabricados a baixas temperaturas. / Silicon oxynitride films obtained by the PECVD technique from N2O+SiH4+He gaseous mixtures, at 320°C, with different deposition pressure and RF power were studied intending to improve the interface quality with Si, decreasing the effective charge density and the interface state density in order to utilize them in MOS semiconductor devices. The results showed that with a deposition pressure of 0.160 mbar and a RF power density lower than 125 W/cm2 it is possible to obtain interface state density (Dit) values of 4x1010 eV-1.cm-2, Electrical Breakdown (Ebd) of 13 MV/cm, comparable with the obtained for thermally grown SiO2 , and an effective charge density (Nss) of 4x1011 cm-2. According with experimental results this Nss value is the minimum attainable with our chemical cleaning process. In this way it can be said that these results are very promising, considering that these materials were obtained by PECVD at low temperatures, but still viable for MOS devices application. In order to initiate studies with high dielectrics constant material, TiOx films (k= 40-180), obtained by reactive sputtering through the Ar+O2 gaseous mixture utilizing a Ti target, were chosen. MOS capacitors with these films were fabricated and dielectric constant values varying from 40 to 160 were obtained. However, until now, these materials have presented appreciable leakage current values, which were, minimize by orders of magnitude with the addition of a thin SiO2 or SiOxNy (optimized in this work) layer at the interface were utilized. This thin layer also resulted in a significant improvement of the interface quality. Utilizing double dielectric layer with SiOxNy or SiO2, still thick (³ 1nm) as intermediate layer a dielectric constant value of 20 was obtained. Its important to mention that the SiOxNy and TiOx films, and consequently the double layer, were deposited at low temperatures.

<p>The development and implementation of a metal gate technology (alloy, compound, or silicide) into metal-oxide-semiconductor field effect transistors (MOSFETs) is necessary to extend the life of planar CMOS devices and enable further downscaling. This thesis examines possible metal gate materials for improving the performance of the gate stack and discusses process integration as well as improved electrical and physical measurement methodologies, tested on capacitor structures and transistors. </p><p>By using reactive PVD and gradually increasing the N₂/Ar flow ratio, it was found that the work function (on SiO₂) of the TiN_x and ZrN_x metal systems could be modulated ~0.7 eV from low near nMOS work functions to high pMOS work functions. After high-temperature anneals corresponding to junction activation, both metals systems reached mid-gap work function values. The mechanisms behind the work function changes are explained with XPS data and discussed in terms of metal gradients and Fermi level pinning due to extrinsic interface states.</p><p>A modified scheme for improved Fowler-Nordheim tunnelling is also shown, using degenerately doped silicon substrates. In that case, the work functions of ALD/PVD TaN were accurately determined on both SiO₂ and HfO₂ and benchmarked against IPE (Internal Photoemission) results. KFM (Kelvin Force Microscopy) was also used to physically measure the work functions of PVD TiN and Mo deposited on SiO₂; the results agreed well with <i>C-V</i> and <i>I-V</i> data.</p><p>Finally, an appealing combination of novel materials is demonstrated with ALD TiN/Al₂O₃/HfAlO_x/Al₂O₃/strained-SiGe surface channel pMOS devices. The drive current and transconductance were measured to be 30% higher than the Si reference, clearly demonstrating increased mobility and the absence of polydepletion. Finally, using similarly processed transistors with Al₂O₃ dielectric instead, low-temperature water vapour annealing was shown to improve the device characteristics by reducing the negative charge within the ALD Al₂O₃.</p>

The development and implementation of a metal gate technology (alloy, compound, or silicide) into metal-oxide-semiconductor field effect transistors (MOSFETs) is necessary to extend the life of planar CMOS devices and enable further downscaling. This thesis examines possible metal gate materials for improving the performance of the gate stack and discusses process integration as well as improved electrical and physical measurement methodologies, tested on capacitor structures and transistors. By using reactive PVD and gradually increasing the N2/Ar flow ratio, it was found that the work function (on SiO2) of the TiNx and ZrNx metal systems could be modulated ~0.7 eV from low near nMOS work functions to high pMOS work functions. After high-temperature anneals corresponding to junction activation, both metals systems reached mid-gap work function values. The mechanisms behind the work function changes are explained with XPS data and discussed in terms of metal gradients and Fermi level pinning due to extrinsic interface states. A modified scheme for improved Fowler-Nordheim tunnelling is also shown, using degenerately doped silicon substrates. In that case, the work functions of ALD/PVD TaN were accurately determined on both SiO2 and HfO2 and benchmarked against IPE (Internal Photoemission) results. KFM (Kelvin Force Microscopy) was also used to physically measure the work functions of PVD TiN and Mo deposited on SiO2; the results agreed well with C-V and I-V data. Finally, an appealing combination of novel materials is demonstrated with ALD TiN/Al2O3/HfAlOx/Al2O3/strained-SiGe surface channel pMOS devices. The drive current and transconductance were measured to be 30% higher than the Si reference, clearly demonstrating increased mobility and the absence of polydepletion. Finally, using similarly processed transistors with Al2O3 dielectric instead, low-temperature water vapour annealing was shown to improve the device characteristics by reducing the negative charge within the ALD Al2O3.

Στην παρούσα Εργασία λεπτά υμένια (5 -25 nm) ZrO2 έχουν εναποτεθεί με τη μέθοδο ALD σε μη αδρανοποιημένο (100) Ge τύπου-p, με ειδική αντίσταση 0.2-0.5 Ω-cm. Η εναπόθεση του ZrO2 πραγματοποιήθηκε στους 2500C, με τη χρήση διαδοχικών παλμών H2O και Tetrakis (Dimethylamido) Zirconium που ήταν και οι πρόδρομες ενώσεις. Ο δομικός χαρακτηρισμός των υμενίων (στοιχειομετρία, σύνθεση και τραχύτητα της διεπιφάνειας, κρυσταλλογραφική φάση του διηλεκτρικού κλπ) πραγματοποιήθηκε μέσω των μεθόδων XPS και ΤΕΜ. Ο λεπτομερής ηλεκτρικός χαρακτηρισμός των υμενίων έγινε με παράμετρο τη θερμοκρασία σε δομές (πυκνωτές) MOS που έφεραν λευκόχρυσο, Pt, ως μέταλλο πύλης. Πραγματοποιήθηκαν μετρήσεις C-V, C-f με παράμετρο τη θερμοκρασία και για θερμοκρασίες από 300Κ έως 80 Κ. Σύμφωνα με τα αποτελέσματα που προέκυψαν από την παρούσα μελέτη τα υμένια με πάχος μικρότερο των 15 nm εμφανίζουν πολύ φτωχή ηλεκτρική συμπεριφορά, η οποία βελτιώνεται με την αύξηση του πάχους. Σχετικά παχιά (25 nm) υμένια ZrO2 εμφανίζουν πυκνότητα διεπιφανειακών παγίδων της τάξης των 1011 eV-1cm-2, όπως προκύπτουν από μετρήσεις που πραγματοποιήθηκαν στους 80K. Από μετρήσεις I-V με παράμετρο τη θερμοκρασία προκύπτουν οι μηχανισμοί αγωγιμότητας που διέπουν τις μελετούμενες δομές. Η επίδραση της ανόπτησης σε περιβάλλον Forming Gas μετά την εναπόθεση του μετάλλου μελετάται επίσης. Τέλος, μελετώνται οι ηλεκτρικές ιδιότητες δομών Pt/ZrO2 (25 nm)/p-Ge, σε υποστρώματα που περιέχουν ταυτόχρονα περιοχές που έχουν υποστεί ανόπτηση με Laser και περιοχές που δεν έχουν υποστεί ανόπτηση. Η ανόπτηση με Laser φαίνεται να υποβαθμίζει την ηλεκτρική συμπεριφορά της δομής. Ωστόσο, σύμφωνα με τα αποτελέσματα της παρούσας εργασίας, υπάρχουν ενδείξεις ότι οι δομές σε περιοχές που γειτνιάζουν με αυτές που έχουν υποστεί ανόπτηση με Laser εμφανίζουν βελτιωμένες ηλεκτρικές ιδιότητες ακόμη και σε σχέση με τα δείγματα αναφοράς που περιλαμβάνουν δομές που αναπτύχθηκαν σε μη ακτινοβολημένο υπόστρωμα. / In the present Thesis, thin (5 -25 nm) films of ZrO2 have been deposited by Atomic Layer Deposition (ALD) on non-passivated p-type (100) Germanium substrates with resistivity 0.2-0.5 Ω-cm. ZrO2 deposition has been performed at 2500C using a series of alternating pulses of H2O and Tetrakis(Dimethylamido) Zirconium, which were the deposition precursors. Structural characterization of the films in terms of stoichiometry, interface composition and roughness, crystallographic phase of the dielectric etc., has been performed using XPS and TEM analysis. Detailed electrical characterization [C-V, and C-f measurements] of the films as a function of temperature has been performed in MOS capacitors using Pt as gate metal. It has been observed that the electrical behaviour of the films is extremely poor in thickness range below 15 nm, while they show an improvement in higher thickness regime. Thick (25 nm) ZrO2 showed an interface trap density of the order of 1011 eV-1cm-2 extracted at 80K. The conductivity mechanisms of the structures are revealed by I-V measurement at various temperatures. Finally the effect of post-metallization annealing in Forming Gas ambient has been studied. In parallel the electrical properties of structures Pt/ZrO2 (25 nm)/p-Ge, on substrates containing simultaneously laser annealed and non-annealed areas has been studied. It has been obtained that laser annealing of the substrate deteriorates the electrical behaviour of the structure, while it seems that structures on the areas in proximity to the annealed ones revealed superior electrical properties as compared to the corresponding deposited on non-annealed (reference) samples.

Θέμα της παρούσας ερευνητικής εργασίας είναι η μελέτη διατάξεων MOS σε υπόστρωμα Ge τύπου –p. Ως διηλεκτρικό πύλης χρησιμοποιήθηκε Al2O3 και ως μέταλλο πύλης Pt. Τέτοιες διατάξεις οι οποίες αποτελούνται από υπόστρωμα Ge στο οποίο εναποτίθεται διηλεκτρικό υψηλής διηλεκτρικής σταθεράς (high-k dielectric) εμφανίζουν ιδιαίτερο ερευνητικό και τεχνολογικό ενδιαφέρον για τους παρακάτω κυρίως λόγους: (i) Το Ge εμφανίζει υψηλότερη ευκινησία φορέων έναντι αυτής του Si. Eπομένως η χρήση υποστρωμάτων Ge στις διατάξεις MOS θεωρείται πλεονεκτική έναντι της χρήσης υποστρωμάτων Si, τα οποία μέχρι σήμερα έχουν μονοπωλήσει τις τεχνολογικές εφαρμογές και κατ’ επέκταση την έρευνα γύρω από αυτές. (ii) Η χρήση υλικών υψηλής διηλεκτρικής σταθεράς, όπως το Al2O3, ως διηλεκτρικά πύλης φέρεται πλέον ως ιδιαίτερα ελπιδοφόρα για την μελλοντική κατασκευή λειτουργικών διατάξεων MOS. (iii) Πρόσφατες μελέτες έχουν αποδείξει ότι, κατά την ανάπτυξη Al2O3 στα υποστρώματα Ge, στη διεπιφάνεια Ge/Al2O3, δημιουργείται ένα λεπτό στρώμα οξειδίου του γερμανίου, το οποίο αποτελεί βασική προϋπόθεση για την κατασκευή λειτουργικών CMOS δομών. Η ανάπτυξη των υμενίων Al2O3 στα υποστρώματα Ge έγινε με την τεχνική Eναπόθεσης Ατομικού Στρώματος (Atomic Layer Deposition- ALD), η οποία είναι μια από τις πιο διαδεδομένες και πολλά υποσχόμενες τεχνικές στον τομέα της Μικροηλεκτρονικής. Βασικά πλεονεκτήματα της μεθόδου αυτής έναντι άλλων μεθόδων εναπόθεσης (CVD, MBE κ.λπ.), είναι η άριστη ποιότητα και ομοιογένεια των αναπτυσσόμενων υμενίων, καθώς και ο απόλυτος έλεγχος του πάχους τους. Στόχος της εργασίας αυτής, είναι η μελέτη των ηλεκτρικών ιδιοτήτων δομών p-Ge/Al2O3/Pt, καθώς και της διεπιφάνειας Ge/Al2O3. Παρασκευάστηκαν δομές με πάχος διηλεκτρικού (Al2O3) 5nm, 10nm,15nm και 25nm σε θερμοκρασία εναπόθεσης 300oC. Ο δομικός χαρακτηρισμός των δειγμάτων έγινε με φασματοσκοπία XPS (X-ray Photoelectron Spectroscopy), ενώ ο ηλεκτρικός τους χαρακτηρισμός έγινε με τη μέθοδο της Διηλεκτρικής Φασματοσκοπίας Ευρέως Φάσματος (Broadband Dielectric Spectroscopy-BDS) στην περιοχή συχνοτήτων από 100Ηz έως 1ΜΗz. Τα αποτελέσματα της μελέτης του δομικού χαρακτηρισμού έδειξαν, ότι αυξανομένου του πάχους του υμενίου Al2O3, το πάχος του αναπτυσσόμενου Οξειδίου του Γερμανίου (GeOx) αυξάνεται. Παράλληλα υπάρχει ένδειξη πιθανής αλλαγής της στοιχειομετρίας του GeOx. Ο ηλεκτρικός χαρακτηρισμός των δομών αυτών, πραγματοποιήθηκε με παράμετρο αφενός μεν το πάχος του Al2O3 σε θερμοκρασία περιβάλλοντος, αφετέρου δε με παράμετρο τη θερμοκρασία, στην περιοχή θερμοκρασιών από 78Κ - 200Κ. Ελήφθησαν οι χαρακτηριστικές C-V και C-f, από τις οποίες προκύπτουν τα συμπεράσματα που αφορούν στην ηλεκτρική συμπεριφορά των δομών αλλά και στην ποιότητα της διεπιφάνειας Ge / Al2O3. Σε όλες τις δομές, ανεξαρτήτως του πάχους του Al2O3, οι χαρακτηριστικές C-V παρουσιάζουν την τυπική συμπεριφορά της δομής MOS, με διάκριτες τις τρεις περιοχές συσσώρευσης, απογύμνωσης και αναστροφής φορέων. Οι χαρακτηριστικές C-V σε θερμοκρασία περιβάλλοντος παρουσίασαν, σε όλα τα πάχη, φαινόμενα γένεσης – επανασύνδεσης φορέων, τα οποία εμφανίζονται υπό τη μορφή “γονάτων” στην περιοχή απογύμνωσης/ασθενούς αναστροφής. Τα φαινόμενα αυτά συνδέονται άμεσα με το μικρό ενεργειακό χάσμα του Ge και το μεγάλο πλήθος ενδογενών φορέων αγωγιμότητας που χαρακτηρίζει το Ge στη θερμοκρασία περιβάλλοντος. Αυτά τα φαινόμενα δεν παρατηρούνται στις χαμηλές θερμοκρασίες. Επιπλέον, από τις μετρήσεις C-f και εφαρμόζοντας τη μεθόδου αγωγιμότητας (conductance method), η οποία εφαρμόστηκε σε όλες τις θερμοκρασίες, προέκυψαν οι τιμές της πυκνότητας των διεπιφανειακών καταστάσεων, Dit’s, των δομών αυτών. Από τα αποτελέσματα αυτά, προκύπτει το συμπέρασμα ότι οι τιμές των Dit’s στη θερμοκρασία περιβάλλοντος εμφανίζονται αυξημένες σε σχέση με τις αντίστοιχες στις χαμηλές θερμοκρασίες. Η υπερεκτίμηση αυτή, η οποία μπορεί να φτάσει και τις 2 τάξεις μεγέθους, οφείλεται στην εμφάνιση των “γονάτων” στη χαρακτηριστική C-V. Επομένως το πραγματικό πλήθος των διεπιφανειακών καταστάσεων υπολογίζεται από την ανάλυση των πειραματικών μετρήσεων στις χαμηλές θερμοκρασίες. Επιπλέον, προκύπτει το συμπέρασμα ότι, η πυκνότητα διεπιφανειακών καταστάσεων, Dit’s στις δομές με το μικρότερο πάχος είναι μικρότερη από την αντίστοιχη σε παχύτερες δομές. Η συμπεριφορά αυτή έρχεται σε αντίθεση με την αναμενόμενη και η ερμηνεία της, πιθανώς να συνδέεται με τα αποτελέσματα της φασματοσκοπίας XPS σύμφωνα με τα οποία, αυξανομένου του πάχους του Al2O3, υπάρχουν ενδείξεις μεταβολής της στοιχειομετρίας του Οξειδίου του Γερμανίου. / The subject of the present research work is the study of MOS structures on p- type Ge substrates. Al2O3 was used as gate dielectric and Pt as metal gate. Such devices, which are comprised by Ge substrate on which a high -k dielectric material is deposited, are of high scientific and technological interest for the following reasons: (i) Ge shows higher carrier mobility compared to that of Si. Therefore, the use of Ge substrates in MOS devices is considered advantageous compared to Si substrates, which up to date have been used exclusively for technological applications. (ii) The use of high -k materials seems to be more promising for the construction of functional MOS structures. (iii) Recent results have shown that a thin layer of Ge oxide builds up at the Ge/Al2O3 interface during the deposition of Al2O3 on Ge. This is a basic requirement for the operation of CMOS devises. In the current work, Al2O3 films were deposited on Ge substrates by Atomic Layer Deposition (ALD). ALD is one of the most widespread and very promising techniques in microelectronics. Basic advantages of this method, compared to other deposition techniques (e.g. CVD, MBE and others), are the quality and homogeneity of the films as well as the absolute control of their thickness. The purpose of the present work is the study of the electrical properties of Ge/ Al2O3 /Pt structures as well as of the quality of the Ge/ Al2O3 interface. Structures with Al2O3 thickness of 5 nm, 10 nm, 15 nm and 25 nm were prepared, at deposition temperature of 3000C. The structural characterization of the samples was performed by means of X-Ray Photoelectron Spectroscopy –XPS whereas the electrical characterization was performed with the Broadband Dielectric Spectroscopy- BDS in the frequency range from 100 Hz to 1 MHz. The XPS results suggest that the thickness of the Ge oxide (GeOx), grown during deposition, increases by increasing the thickness of the deposited Al2O3 films. Furthermore, there is evidence of possible change in the stoichiometry of GeOx. The electrical behaviour of these structures was determined using as parameters either the thickness of Al2O3 at room temperature or the temperature at constant thickness. Measurements were performed in the range of 78 oC to 200 oC. C-V and C-f characteristics were constructed, from which conclusions are drawn regarding the electrical behaviour of the structures and the quality of the interface Ge/ Al2O3. The C-V characteristics of all samples, show the typical behaviour of a MOS structure with the three distinct regions of accumulation, depletion and inversion, regardless the thickness of Al2O3. The C-V characteristics at room temperature show generation-recombination phenomena, which are demonstrated through “humps” in depletion / weak inversion regime. These phenomena are intimately connected with the small energy gap and the large density of intrinsic conductivity carriers of Ge. These “humps” do not appear at low temperatures, below 170K, indicating that “generation-recombination” phenomena have been suppressed by reducing temperatures. Furthermore, from C-f measurements the values of the interfacial trap density (Dit) were determined through the conductance method, in all temperatures. The Dit values at room temperature seem to be overestimated compared to those at low temperatures due to the “generation- recombination” phenomena. Therefore, the actual density of interfacial traps is determined from the analysis at low temperatures. Furthermore, the Dit values of the Al2O3 -10nm structure are lower compared to those of the Al2O3 -25nm structure. This behavior, which is in contradiction to the expected one, is connected with the XPS results, according to which there is evidence of change in the stoichiometry of Ge oxide as the thickness of Al2O3 is increased.