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Intermediate band (IB) materials have attracted considerable research interest since they can dramatically enhance the near infrared light absorption and lead to applications in the fields of so-called intermediate band solar cells or infrared photodetectors. Hyperdoping Si with deep level impurities is one of the most effective approaches to form an IB inside Si. In this thesis, titanium (Ti) or chalcogen doped Si with concentrations far exceeding the Mott transition limits (~ 5×10^19 cm-3 for Ti) are fabricated by ion implantation followed by pulsed laser annealing (PLA) or flash lamp annealing (FLA). The structural and electrical properties of the implanted layer are investigated by channeling Rutherford backscattering spectrometry (cRBS) and Hall measurements. For Si supersaturated with Ti, it is shown that Ti-implanted Si after liquid phase epitaxy shows cellular breakdown at high doping concentrations during the rapid solidification, preventing Ti incorporation into Si matrix. However, the out-diffusion and the cellular breakdown can be effectively suppressed by solid phase epitaxy during FLA, leading to a much higher Ti incorporation. In addition, the formed microstructure of cellular breakdown also complicates the interpretation of the electrical properties. After FLA, the samples remain insulating even with the highest Ti implantation fluence, whereas the sheet resistance decreases with increasing Ti concentration after PLA. According to the results from conductive atomic force microscopy (C-AFM), the decrease of the sheet resistance after PLA is attributed to the percolation of Ti-rich cellular walls, but not to the insulator-to-metal transition due to Ti-doping. Se-hyperdoped Si samples with different Se concentrations are fabricated by ion implantation followed by FLA. The study of the structural properties of the implanted layer reveals that most Se atoms are located at substitutional lattice sites. Temperature-dependent sheet resistance shows that the insulator-to-metal transition occurs at a Se peak concentration of around 6.3 × 10^20 cm-3, proving the formation of an IB in host semiconductors. The correlation between the structural and electrical properties under different annealing processes is also investigated. The results indicate that the degrees of crystalline lattice recovery of the implanted layers and the Se substitutional fraction depend on pulse duration and energy density of the flash. The sample annealed at short pulse durations (1.3 ms) shows better conductivity than long pulse durations (20 ms). The electrical properties of the hyperdoped layers can be well-correlated to the structural properties resulting from different annealing processes.:Chapter 1 Introduction 1 1.1 Shallow and Deep level impurities in semiconductors 1 1.2 Challenges for hyperdoping semiconductors with deep level Impurities 2 1.3 Solid vs. liquid phase epitaxy 5 1.4 Previous work 7 1.4.1 Transition metal in Si 7 1.4.2 Chalcogens in Si 10 1.5 The organization of this thesis 15 Chapter 2 Experimental methods 18 2.1 Ion implantation 18 2.1.1 Basic principle of ion implantation 18 2.1.2 Ion implantation equipment 19 2.1.3 Energy loss 20 2.2 Pulsed laser annealing (PLA) 23 2.3 Flash lamp annealing (FLA) 24 2.4 Rutherford backscattering and channeling spectrometry (RBS/C) 27 2.4.1 Basic principles 27 2.4.2 Analysis of the elements in the target 28 2.4.3 Channeling and RBS/C 29 2.4.4 Analysis of the impurity lattice location 31 2.5 Hall measurements 31 2.5.1 Sample preparation 32 2.5.2 Resistivity 32 2.5.3 Hall measurements 33 Chapter 3 Suppressing the cellular breakdown in silicon supersaturated with titanium 34 3.1 Introduction 34 3.2 Experimental 35 3.3 Results 36 3.4 Conclusions 42 Chapter 4 Titanium-implanted silicon: does the insulator-to-metal transition really happen? 44 4.1 Introduction 44 4.2 Experimental section 45 4.3 Results 47 4.3.1 Recrystallization of Ti-implanted Si 47 4.3.2 Lattice location of Ti impurities 48 4.3.3 Electrical conduction 50 4.3.4 Surface morphology 52 4.3.5 Spatially resolved conduction 53 4.4 Discussion 55 4.5 Conclusion 56 Chapter 5 Realizing the insulator-to-metal transition in Se hyperdoped Si via non-equilibrium material processing 57 5.1 Introduction 57 5.2 Experimental 59 5.3 Results 60 5.4 Conclusions 65 Chapter 6 Structural and electrical properties of Se-hyperdoped Si via ion implantation and flash lamp annealing 67 6.1 Introduction 67 6.2 Experimental 68 6.3 Results 69 6.4 Conclusions 76 Chapter 7 Summary and outlook 78 7.1 Summary 78 7.2 Outlook 81 References 83 Publications 89

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En raison de l'attrait croissant pour les applications haute tension, haute tempé-rature et haute fréquence, le carbure de silicium (SiC) continue d'attirer l'attention du monde entier comme l'un des candidats les plus compétitifs pour remplacer le sili-cium dans le champ électrique de puissance. Entre-temps, il est important de carac-tériser les défauts des semi-conducteurs et d'évaluer leur influence sur les dispositifs de puissance puisqu'ils sont directement liés à la durée de vie du véhicule porteur. De plus, la fiabilité, qui est également affectée par les défauts, devient une question incontournable dans le domaine de l'électricité de puissance.Les défauts, y compris les défauts ponctuels et les défauts prolongés, peuvent introduire des niveaux d'énergie supplémentaires dans la bande passante du SiC en raison de divers métaux comme le Ti, le Fe ou le réseau imparfait lui-même. En tant que méthode de caractérisation des défauts largement utilisée, la spectroscopie à transitoires en profondeur (DLTS) est supérieure pour déterminer l'énergie d'activa-tion Ea , la section efficace de capture Sigma et la concentration des défauts Nt ainsi que le profil des défauts dans la région d'épuisement grâce à ses divers modes de test et son analyse numérique avancée. La détermination de la hauteur de la barrière Schottky (HBS) prête à confusion depuis longtemps. Outre les mesures expérimentales selon les caractéristiques I-V ou C-V, différents modèles ont été proposés, de la distribution gaussienne du HBS au modèle de fluctuation potentielle. Il s'est avéré que ces modèles sont reliés à l'aide d'une hauteur de barrière à bande plate Phi_BF . Le tracé de Richardson basé sur Phi_BF ainsi que le modèle de fluctuation potentielle deviennent un outil puissant pour la caractérisation HBS. Les HBSs avec différents contacts métalliques ont été caractéri-sés, et les diodes à barrières multiples sont vérifiées par différents modèles. Les défauts des électrons dans le SiC ont été étudiés avec des diodes Schottky et PiN, tandis que les défauts des trous ont été étudiés dans des conditions d'injec-tion forte sur des diodes PiN. 9 niveaux d'électrons et 4 niveaux de trous sont com-munément trouvés dans SiC-4H. Une relation linéaire entre le Ea extrait et le log(sigma) indique l'existence de la température intrinsèque de chaque défaut. Cependant, au-cune différence évidente n'a été constatée en ce qui concerne l'inhomogénéité de la barrière à l'oxyde d'éther ou le métal de contact. De plus, les pièges à électrons près de la surface et les charges positives fixes dans la couche d'oxyde ont été étudiés sur des MOSFET de puissance SiC par polarisation de porte à haute température (HTGB) et dose ionisante totale (TID) provoquées par irradiation. Un modèle HTGB-assist-TID a été établi afin d'ex-plain l'effet de synergie. / Due to the increasing appeal to the high voltage, high temperature and high fre-quency applications, Silicon Carbide (SiC) is continuing attracting world’s attention as one of the most competitive candidate for replacing silicon in power electric field. Meanwhile, it is important to characterize the defects in semiconductors and to in-vestigate their influences on power devices since they are directly linked to the car-rier lifetime. Moreover, reliability that is also affected by defects becomes an una-voidable issue now in power electrics. Defects, including point defects and extended defects, can introduce additional energy levels in the bandgap of SiC due to various metallic impurities such as Ti, Fe or intrinsic defects (vacancies, interstitial…) of the cristalline lattice itself. As one of the widely used defect characterization method, Deep Level Transient Spectroscopy (DLTS) is superior in determining the activation energy Ea , capture cross section sigma and defect concentration Nt as well as the defect profile in the depletion region thanks to its diverse testing modes and advanced numerical analysis. Determination of Schottky Barrier Height (SBH) has been confusing for long time. Apart from experimental measurement according to I-V or C-V characteristics, various models from Gaussian distribution of SBH to potential fluctuation model have been put forward. Now it was found that these models are connected with the help of flat-band barrier height Phi_BF . The Richardson plot based on Phi_BF along with the potential fluctuation model becomes a powerful tool for SBH characterization. SBHs with different metal contacts were characterized, and the diodes with multi-barrier are verified by different models. Electron traps in SiC were studied in Schottky and PiN diodes, while hole traps were investigated under strong injection conditions in PiN diodes. 9 electron traps and 4 hole traps have been found in our samples of 4H-SiC. A linear relationship between the extracted Ea and log(sigma) indicates the existence of the intrinsic temper-ature of each defects. However, no obvious difference has been found related to ei-ther barrier inhomogeneity or contact metal. Furthermore, the electron traps near in-terface and fixed positive charges in the oxide layer were investigated on SiC power MOSFETs by High Temperature Gate Bias (HTGB) and Total Ionizing Dose (TID) caused by irradiation. An HTGB-assist-TID model was established in order to ex-plain the synergetic effect.

The ensuring of reliability of piezoelectric thin films is crucial for a successful piezoelectric micro-electromechanical system (piezoMEMS) application. One of the most important limiting factors for reliability is resistance degradation, where the leakage current increases over time under electrical load. The understanding of resistance degradation in piezoelectric thin films requires knowledge about point defects inside the materials. In this dissertation, the resistance degradation mechanism in sputtered lead zirconate titanate (PZT) and lead-free alternative sodium potassium niobate (KNN) thin films is studied in both voltage polarities, and its relation to point defects is established. The conduction mechanism of both PZT and KNN thin films is found to be Schottky-limited. Furthermore, the resistance degradation is due to the reduction in Schottky barrier height, which results from the interfacial accumulation of additional charged defects. In order to study those defects, we use thermally stimulated depolarization current (TSDC) measurements and charge-based deep level transient spectroscopy (Q-DLTS) to characterize the defects in both PZT and KNN thin films. In PZT thin films, the resistance degradation take place in different waves of increasing leakage current. Both oxygen vacancies and lead vacancies contribute to the different waves of resistance degradation in both voltage polarities. A physical degradation model was developed based on hopping migration of oxygen vacancies at constant speed and exponent accumulation of lead vacancy trapping, where the natural logarithm of leakage current is proportional to the accumulated defect concentration to the power of 0.25. By using the oxygen vacancy concentration measured by TSDC and lead vacancy concentrations measured by Q-DLTS, the model successfully explained the resistance degradation behaviors of PZT films varying due to deposition non-uniformity and due to different process parameters. The accumulation of oxygen vacancies at cathode is supported by X-ray photoelectron spectroscopy (XPS), and the resistance degradation can be restored by proper heat and electrical treatment as predicted by the defect characterization results. In KNN thin films, oxygen vacancies contribute to the resistance degradation when a negative voltage is applied at the top electrode, whereas sodium and potassium vacancies contribute to the resistance degradation when a positive voltage is applied at the top electrode. The model developed for PZT can be applied also to KNN, where the model successfully explained the resistance degradation behaviors of KNN films varying due to the deposition non-uniformity by using the defect concentration measured by TSDC. The accumulation of oxygen vacancies at cathode and sodium plus potassium vacancies at anode are supported by transmission electron microscopy energy dispersive X-ray spectroscopy (TEM-EDX), and the resistance degradation can be restored also by proper heat and electrical treatment as predicted by the defect characterization results. This dissertation revealed the similarity of the resistance degradation between sputtered PZT and KNN thin films. The degradation is controlled by the crystallography point defects created during deposition process inside the material, indicating the significance of process control on material reliability. This dissertation also demonstrates the applicability of TSDC and Q-DLTS as alternative methods to assess the quality of the piezoelectric thin films. Both measurement techniques provide additional information regarding specific defects when comparing with conventional highly accelerated lifetime test (HALT) or other relevant tests. / Die Sicherstellung der Zuverlässigkeit piezoelektrischer Dünnschichten ist entscheidend für eine erfolgreiche Anwendung in piezoelektrischen mikro-elektromechanischen Systemen (piezoMEMS). Einer der wichtigsten limitierenden Faktoren für die Zuverlässigkeit ist die Widerstandsdegradation, bei der der Leckstrom mit der Zeit unter elektrischer Last zunimmt. Das Verständnis der Widerstandsdegradation in piezoelektrischen Dünnschichten erfordert laut Literatur Kenntnisse über Punkt-Defekte innerhalb der Materialien. In dieser Dissertation wird der Mechanismus der Widerstandsdegradation in gesputterten Blei-Zirkonat-Titanat (PZT) Dünnschichten und dessen bleifreier alternative Kalium-Natrium-Niobat (KNN) in beiden Spannungspolaritäten untersucht und deren Zusammenhang mit Punkt-Defekte hergestellt. Der Leitungsmechanismus von PZT- und KNN-Dünnschichten ist Schottky-begrenzt. Außerdem ist die Widerstandsdegradation auf die Reduzierung der Schottky-Barrierhöhe zurückzuführen, die von der Akkumulation zusätzlicher aufgeladener -Defekte an der Grenzfläche stammt. Um diese -Defekte zu untersuchen, verwenden wir thermisch stimulierte Depolarisationsstrommessungen (Thermally stimulated depolarization current, TSDC) und ladungsbasierte Deep-Level-Transientenspektroskopie (Charge-based deep level transient spectroscopy, Q-DLTS), um die Defekte sowohl in PZT- als auch in KNN-Dünnschichten zu charakterisieren.Die Wiederstandsdegradation in PZT-Dünnschichten findet in unterschiedlichen Wellen des erhöhenden Leckstroms statt. Sowohl Sauerstofffehlstellen als auch Bleifehlstellen tragen zu den unterschiedlichen Wellen der Widerstandsdegradation in beiden Spannungspolaritäten bei. Ein physikalisches Degradationsmodell wurde entwickelt, basierend auf der Hopping-Migration von Sauerstofffehlstellen bei konstanter Geschwindigkeit und exponentieller Akkumulation von Ladungseinfang durch Bleifehlstellen, wobei der natürliche Logarithmus des Leckstroms proportional zur akkumulierten Defektkonzentration hoch 0,25 ist. Durch die Verwendung der Sauerstofffehlstellen- und Bleifehlstellenkonzentrationen konnte das Modell das Widerstandsdegradationsverhalten von PZT-Dünnschichten erklären, das wegen der Ungleichmäßigkeit der Deposition und wegen der verschiedenen Prozessparameters variiert. Die Sauerstofffehlstellenkonzentration wird durch TSDC gemessen und die Bleifehlstellenkonzentrationen wird durch Q-DLTS gemessen. Die Akkumulation von Sauerstofffehlstellen an der Kathode wird durch die Röntgen-Photoelektronenspektroskopie (X-ray photoelectron spectroscopy, XPS) unterstützt und die Widerstandsdegradation kann durch eine ordnungsgemäße Wärme- und elektrische Behandlung wiederhergestellt werden, wie durch die Ergebnisse von Defektecharakterisierung vorhergesagt wurde. Bei KNN-Dünnschichten tragen Sauerstofffehlstellen zu der Widerstandsdegradation bei, wenn eine negative Spannung an der oberen Elektrode anliegt, und Natrium- und Kaliumfehlstellen tragen zu der Widerstandsdegradation bei, wenn eine positive Spannung an der oberen Elektrode anliegt. Das für PZT entwickelte Modell kann auch auf KNN angewendet werden. Das Modell erklärt erfolgreich das Widerstandsdegradationverhalten von KNN-Dünnschichten, das durch die Ungleichmäßigkeit der Deposition variiert werden, was mithilfe der mit TSDC gemessenen Defektkonzentrationen erklärt werden kann. Die Akkumulation von Sauerstofffehlstellen an Kathode und Natrium- und Kaliumfehlstellen an der Anode wird durch die transmissionselektronenmikroskopische energiedispersive Röntgenspektroskopie (transmission electron microscopy energy dispersive X-ray spectroscopy, TEM-EDX) unterstützt, und die Widerstandsdegradation kann auch durch eine ordnungsgemäße Wärme- und elektrische Behandlung wiederhergestellt werden, wie durch die Ergebnisse von Defektecharakterisierung vorhergesagt wurde. Diese Dissertation zeigt die Ähnlichkeit der Widerstandsdegradation zwischen gesputterten PZT- und KNN-Dünnschichten. Die Degradation wird durch die kristallographischen Punkt-Defekte gesteuert, die während des Abscheidungsprozesses im Material entstehen. Das weist auf die Bedeutung der Prozesskontrolle für die Zuverlässigkeit des Materials hin. Diese Dissertation zeigt auch die Anwendbarkeit von TSDC und Q-DLTS als alternative Methoden zur Beurteilung der Qualität der piezoelektrischen Dünnschichten. Beide Messtechniken liefern zusätzliche Informationen zu spezifischen Defekte im Vergleich zu traditionellen HALT-Prüfungen (highly accelerated lifetime test).

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At present, focus of the SiC crystal growth development is on improving the crystalline quality without polytype inclusions, micropipes and the occurrence of extended defects. The purity of the grown material, as well as intentional doping must be well controlled and the processes understood. High-quality substrates will significantly improve device performance and yield. One of the aims of the thesis is further understanding of polytype inclusion formation as well as impurity control in SiC bulk crystals grown using PVT method also termed seeded sublimation method. Carbonization of the source was identified as a major reason behind the polytype inclusion occurrence during the growth. The aim of this work was further understanding of sublimation growth process of 4H-SiC bulk crystals in vacuum, in absence of an inert gas. For comparison growth in argon atmosphere (at 5 mbar) was performed. The effect of the ambient on the impurity incorporation was studied for different growth temperatures. For better control of the process in vacuum, tantalum as a carbon getter was utilized. The focus of the SiC part of the thesis was put on further understanding of the PVT epitaxy with an emphasis on the high growth rate and purity of grown layers. High resistivity 4H-SiC samples grown by sublimation with high growth rate were studied. The measurements show resistivity values up to high 104 cm. By correlation between the growth conditions and SIMS results, a model was applied in which it is proposed that an isolated carbon vacancy donor-like level is a possible candidate responsible for compensation of the shallow acceptors in p-type 4H-SiC. A relation between cathodoluminescence (CL) and DLTS data is taken into account to support the model. To meet the requirements for high voltage blocking devices such as high voltage Schottky diodes and MOSFETs, 4H-SiC epitaxial layers have to exhibit low doping concentration in order to block reverse voltages up to few keV and at the same time have a low on-state resistance (Ron). High Ron leads to enhanced power consumption in the operation mode of the devices. In growth of thick layers for high voltage blocking devices, the conditions to achieve good on-state characteristics become more challenging due to the low doping and pronounced thicknesses needed, preferably in short growth periods. In case of high-speed epitaxy such as the sublimation, the need to apply higher growth temperature to yield the high growth rate, results in an increased concentration of background impurities in the layers as well as an influence on the intrinsic defects. On-state resistance Ron estimated from current density-voltage characteristics of Schottky diodes on thick sublimation layers exhibits variations from tens of mΩ.cm2 to tens of Ω.cm2 for different doping levels. In order to understand the occurrence of high on-state resistance, Schottky barrier heights were first estimated for both forward and reverse bias with the application of thermionic emission theory and were in agreement with literature reported values. Decrease in mobility with increasing temperature was observed and its dependencies of T–1.3 and T–2.0 for moderately doped and low doped samples, respectively, were estimated. From deep level measurements by Minority Carrier Transient Spectroscopy (MCTS), an influence of shallow boron related levels and D-center on the on-state resistance was observed, being more pronounced in low doped samples. Similar tendency was observed in depth profiling of Ron. This suggests a major role of boron in a compensation mechanism. In the second part of the thesis growth and characterization of GaN is presented. Excellent electron transport properties with high electron saturate drift velocity make GaN an excellent candidate for electronic devices. Especially, AlGaN/GaN based high electron mobility transistors (HEMT) have received an increased attention in last years due to their attractive properties. The presence of strong spontaneous and piezoelectric polarization due to the lattice mismatch between AlGaN and GaN is responsible for high free electrons concentrations present in the vicinity of the interface. Due to the spatial separation of electrons and ionized donors or surface states, 2DEG electron gas formed near the interface of the heterostructure exhibits high sheet carrier density and high mobility of electrons. Al0.23Ga0.77N/GaN based HEMT structures with an AlN exclusion layer on 100 mm semiinsulating 4H-SiC substrates have been grown by hot-wall MOCVD. The electrical properties of the two-dimensional electron gas (2DEG) such as electron mobility, sheet carrier density and sheet resistance were obtained from Hall measurements, capacitance-voltage and contact-less eddy-current techniques. The effect of different scattering mechanisms on the mobility have been taken into account and compared to the experimental data. Hall measurements were performed in the range of 80 to 600 K. Hall electron mobility is equal to 17140 cm2(Vs)-1 at 80 K, 2310 cm2(Vs)-1 at room temperature, and as high as 800 cm2(Vs)-1 at 450 K, while the sheet carrier density is 1.04x1013 cm-2 at room temperature and does not vary very much with temperature. Estimation of different electron scattering mechanisms reveals that at temperatures higher than room temperature, experimental mobility data is mainly limited by optical phonon scattering. At relevant high power device temperature (450 K) there is still an increase of mobility due to the AlN exclusion layer. We have studied the behaviour of Ga-face GaN epilayers after in-situ thermal treatment in different gas mixtures in a hot-wall MOCVD reactor. Influence of N2, N2+NH3 and N2+NH3+H2 ambient on the morphology was investigated in this work. The most stable thermal treatment conditions were obtained in the case of N2+NH3 gas ambients. We have also studied the effect of the increased molar ratio of hydrogen in order to establish proper etching conditions for hot-wall MOCVD growth.

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Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第17581号 / 工博第3740号 / 新制||工||1570(附属図書館) / 30347 / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 木本 恒暢, 教授 藤田 静雄, 准教授 浅野 卓 / 学位規則第4条第1項該当