Study of High-k Dielectrics and their Interfaces on Semiconductors for Device Applications

This thesis has focused on two emerging applications of high-k dielectrics in Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) and in Metal-InsulatorSemiconductor High Electron Mobility Transistors (MIS-HEMTs). The key aim has been to propose the best routes for passivation of semiconductor/high-k oxide interfaces by investigating the band alignments and interface properties of several oxides, such as Tm2O3, Ta2O5, ZrO2, Al2O3 and MgO, deposited on different semiconductors: Si, Ge, GaN, InGaAs and InGaSb. The electrical characterisation of fabricated MIS capacitor and (MIS)-HEMT devices have also been performed. Thulium silicate (TmSiO) has been identified as a promising candidate for integration as interfacial layer (IL) in HfO2/TiN MOSFETs. The physical properties of Tm2O3/IL/Si interface have been elucidated, where IL (TmSiO) has been formed using different post-deposition annealing (PDA) temperatures, from 550 to 750 °C. It has been found that the best-scaled stack (sub-nm IL) is formed at 550 °C PDA with a graded interface layer and a strong SiOx (Si 3+) component. A large valence band offset (VBO) of 2.8 eV and a large conduction band offset (CBO) of 1.9 eV have been derived for Tm2O3/Si by X-ray photoelectron spectroscopy (XPS) and variable angle spectroscopic ellipsometry. Further increase of device performance can be achieved by replacing Si with GaN for high frequency, high power and high-temperature operation. In this thesis, several GaN cleaning procedures have been considered: 30% NH4OH, 20% (NH4)2S, and 37% HCl. It has been found that the HCl treatment shows the lowest oxygen contamination and Garich surface, and hence has been used prior sputtering of Ta2O5, Al2O3, ZrO2 and MgO on GaN. The large VBOs of 1.1 eV and 1.2 eV have been derived for Al2O3 and MgO on GaN respectively, using XPS and Kraut’s method; the corresponding CBOs are 2.0 eV and 2.8 eV respectively, taking into account the band gaps of Al2O3 (6.5 eV) and MgO (7.4 eV) determined from XPS O 1s electron energy spectra. The lowest leakage currents were obtained for devices with Al2O3 and MgO, i.e. 5.3 ×10-6 A/cm2 and 3.2 ×10-6 A/cm2 at 1 V, respectively in agreement with high band offsets (> 1 eV). Furthermore, the effect of different surface treatments (HCl, O2 plasma and 1-Octadecanethiol (ODT)) prior to atomic layer deposition of Al2O3 on the GaN/AlGaN/GaN heterostructure has been investigated. The MIS-HEMTs fabricated using the low-cost ODT GaN surface treatment have been found to exhibit superior performance for power switching applications such as a low threshold voltage, VT of -12.3 V, hysteresis of 0.12 V, a small subthreshold voltage slope (SS) of 73 mV/dec, and a low density of interface states, Dit of 3.0 x10^12 cm-2eV-1. A comprehensive novel study of HfO2/InGaAs and Al2O3/InGaSb interfaces have also been conducted for use in III-V based MOSFETs. The addition of the plasma H2/TMA/H2 pre-cleaning has been found to be very effective in recovering etch damage on InGaAs, especially for (110) orientation, and led to the improvement of electrical characteristics. Furthermore, the combination of H2 plasma exposure and forming gas anneal yielded significantly improved metrics for Al2O3/InGaSb over the control HCltreated sample, with the 150 W plasma treatment giving both the highest capacitance and the lowest stretch out

AlGaN/GaN 전력소자의 특성 향상을 위한 식각과 절연막에 관한 연구

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 전기·정보공학부, 2020. 8. 서광석.최근 에너지 위기와 환경규제 강화, 친환경 녹색성장 등의 이슈가 대두되어 에너지 절감과 환경 보호 분야에 IT 기술을 접목, 활용하는 그린 IT 패러다임이 부각되고 있다. 현재 고유가 환경규제 강화에 대응하기 위해 하이브리드 자동차, 전기자동차 등 친환경 미래형 자동차 개발이 요구되고 있으며, 자동차에서 전장부품이 차지하는 원가비중은 약 40%까지 달할 것으로 전망되고 이 중 반도체가 차지하는 비용은 약 30% 정도로 추정된다. 이러한 자동차 전장부품에서 전력소자가 핵심부품으로 자리 잡을 전망이다. 지금까지는 실리콘 기반의 전력소자 기술이 전력반도체 시장의 대부분을 주도하고 있지만 전력기기 로드맵에 의하면 전력밀도가 해를 거듭하면서 지속적으로 증가하기 때문에 내열, 내압, 전력손실, 전력밀도 등에서 나타나는 많은 한계를 가지고 있는 현재의 실리콘 기반 전력시스템은 효율이 눈에 띄게 감소할 것이 자명하므로 전력시스템의 전력전송효율과 신뢰성의 중요성이 크게 대두되고 있다. 이 같은 사회적 요구로 볼 때 현재의 실리콘 전력소자의 기술적 한계를 뛰어넘는 고효율의 차세대 전력반도체 소자의 개발이 시급히 요구되며 SiC와 GaN와 같은 광대역 반도체가 차세대 전력반도체 소재로 유력해지고 있다. 또한 전력시스템에서는 시스템의 안전성과 회로의 간략화를 위하여 normally-off (증강형) 전력소자가 요구되기 때문에 normally-off (증강형) GaN 전력소자에 대한 개발이 필수적이다. 본 그룹에서는 gate-recess 공정을 이용하여 normally-off 동작을 실현하는 연구를 진행하였고, gate-recess 시 발생하는 식각 데미지를 줄이고 우수한 성능의 게이트 절연막을 개발하여 GaN 전력 반도체 소자의 전기적 특성 및 신뢰성을 개선하는 연구를 진행하였다. 식각 연구에서는 최종적으로 셀프 DC 바이어스가 낮은 O2, BCl3 플라즈마를 이용한 atomic layer etching을 개발하였고, 이를 통해 거칠기가 작고 표면 N vacancy가 적은 고품질의 (Al)GaN 표면을 얻을 수 있었다. 박막 연구에서는 Oxide 박막 증착 시, (Al)GaN 표면에 생성되어 계면 특성을 악화시키는 Ga2O3 생성을 막기위해 ALD AlN layer를 개발 및 적용하여 박막/(Al)GaN 계면 특성을 향상시켰다. 이로 인해 소자의 동작전류 증가 및 Dit 감소 결과를 얻을 수 있었고 스트레스에 따른 문턱전압 이동 특성의 감소로 소자의 신뢰성 또한 개선시킬 수 있었다. 이는 타 기관의 결과와 비교해도 뒤떨어지지 않는 우수한 특성을 보여주었다. 결론적으로 본 연구의 작은 플라즈마 데미지를 갖는 식각공정과 고품질 절연막 개발을 통해 우수한 특성의 GaN 전력소자를 구현할 수 있었고 향후 차세대 전력소자에 적용을 위한 가능성을 확보하였다.The Si technology for power devices have already approached its theoretical limitations due to its physical and material properties, despite the considerable efforts such as super junction MOSFET, trench gate, and insulated gate bipolar transistors. To overcome these limitations, many kinds of compound materials such as GaN, GaAs, SiC, Diamond and InP which have larger breakdown voltage and high electron velocity than Si also have been studied as future power devices. GaN has been considered as a breakthrough in power applications due to its high critical electric field, high saturation velocity and high electron mobility compared to Si, GaAs, and SiC. Especially, AlGaN/GaN heterostructure field-effect transistors (HFETs) have been considered as promising candidates for high power and high voltage applications. However, these AlGaN/GaN heterostructure field-effect transistors with the 2DEG are naturally normally-on, which makes the devices difficult to deplete the channel at zero gate bias. Among the various methods for normally-off operation of GaN devices, gate-recess method is a promising method because it can be easier to implement than other approaches and ensure normally-off operation. However, charge trapping at the interface between gate dielectric and (Al)GaN and in the gate dielectric is a big issue for recessed gate MIS-HEMTs. This problem leads to degradation of channel mobility, on-resistance and on-current of the devices. Especially, Vth hysteresis after a positive gate voltage sweep and Vth shift under a gate bias stress are important reliability challenges in gate recessed MIS-HEMTs. The scope of this work is mainly oriented to achieve high quality interface at dielectric/(Al)GaN MIS by studying low damage etching methods and the ALD process of various dielectric layers. In the etching study, various etching methods for normally-off operation have been studied. Also, etching damage was evaluated by various methods such as atomic force microscopy (AFM), photoluminescence (PL) measurements, X-ray photoelectron spectroscopy (XPS) measurements and electrical properties of the recessed schottky devices. Among the etching methods, the ALE shows the smoothest etched surface, the highest PL intensity and N/(Al+Ga) ratio of the etched AlGaN surface and the lowest leakage current of the gate recessed schottky devices. It is suggested that the ALE is a promising etching technique for normally-off gate recessed AlGaN/GaN MIS-FETs. In the study of dielectrics, excellent electrical characteristics and small threshold volt¬age drift under positive gate bias stress are achieved by employing the SiON interfacial layer. However, considerable threshold voltage drift is observed under the higher positive gate bias stress even at the devices using the SiON interfacial layer. For further improvement of interface and reliability of devices, we develop and optimize an ALD AlN as an interfacial layer to avoid the formation of poor-quality oxide at the dielectric/(Al)GaN interface. We also develop an ALD AlHfON as a bulk layer, which have a high dielectric constant and low leakage current and high breakdown field characteristics. Devices with AlN/AlON/AlHfON layer show smaller I-V hysteresis of ~10 mV than that of devices with AlON/AlHfON layer. The extracted static Ron values of devices with AlN/AlON/AlHfON and AlON/AlHfON are 1.35 and 1.69 mΩ·cm2, respectively. Besides, the effective mobility, Dit and threshold voltage instability characteristics are all improved by employing the ALD AlN. In conclusion, for high performance and improvement of reliability of normally-off AlGaN/GaN MIS-FETs, this thesis presents an etching technique for low damage etching and high-quality gate dielectric layer and suggests that the ALE and ALD AlN/AlON/AlHfON gate dielectric are very promising for the future normally-off AlGaN/GaN MIS-FETsChapter 1. Introduction 1 1.1. Backgrounds 1 1.2. Normally-off Operation in AlGaN/GaN HFETs 3 1.3. Issues and Feasible Strategies in AlGaN/GaN MIS-HFETs 11 1.4. Research Aims 15 1.5. References 17 Chapter 2. Development and Evaluation of Low Damage Etching processes 22 2.1. Introduction 22 2.2. Various Evaluation Methods of Etching Damage 24 2.3. Low-Damage Dry Etching Methods 29 2.3.1. Inductively Coupled Plasma-Reactive Ion Etching Using BCl3/Cl2 Gas Mixture 29 2.3.2. Digital Etching Using Plasma Asher and HCl 34 2.3.3. Atomic Layer Etching Using Inductively Coupled Plasma–Reactive Ion Etching System (ICP-RIE) 50 2.4. Conclusion 75 2.5. References 76 Chapter 3. SiON/HfON Gate Dielectric Layer by ALD for AlGaN/GaN MIS-FETs 80 3.1. Introduction 80 3.2. ALD Processes for SiON and HfON 83 3.3. Electrical Characteristics of ALD SiON, HfON and SiON/HfON Dual Layer on n-GaN 87 3.4. Device Characteristics of Normally-off AlGaN/GaN MIS-FETs with SiON/HfON Dual Layer 95 3.5. Conclusion 113 3.6. References 114 Chapter 4. High Quality AlN/AlON/AlHfON Gate Dielectric Layer by ALD for AlGaN/GaN MIS-FETs 120 4.1. Introduction 120 4.2. Development of ALD AlN/AlON/AlHfON Gate Stack 122 4.2.1. Process Optimization for ALD AlN 122 4.2.2. ALD AlN as an Interfacial Layer 144 4.2.3. Thickness Optimization of AlN/AlON/ AlHfON Layer 149 4.2.4. ALD AlHfON Optimization 159 4.2.5. Material Characteristics of AlN/AlON/AlHfON Layer 167 4.3. Device Characteristics of Normally-off AlGaN/GaN MIS-FETs with AlN/AlON/AlHfON Layer 171 4.4. Conclusion 182 4.5. References 183 Chapter 5. Concluding Remarks 188 Appendix. 190 A. N2 Plasma Treatment Before Dielectric Deposition 190 B. Tri-gate Normally-on/off AlGaN/GaN MIS-FETs 200 C. AlGaN/GaN Diode with MIS-gated Hybrid Anode and Edge termination 214 Abstract in Korean 219 Research Achievements 221Docto

Feature Papers in Electronic Materials Section

This book entitled "Feature Papers in Electronic Materials Section" is a collection of selected papers recently published on the journal Materials, focusing on the latest advances in electronic materials and devices in different fields (e.g., power- and high-frequency electronics, optoelectronic devices, detectors, etc.). In the first part of the book, many articles are dedicated to wide band gap semiconductors (e.g., SiC, GaN, Ga2O3, diamond), focusing on the current relevant materials and devices technology issues. The second part of the book is a miscellaneous of other electronics materials for various applications, including two-dimensional materials for optoelectronic and high-frequency devices. Finally, some recent advances in materials and flexible sensors for bioelectronics and medical applications are presented at the end of the book

Leakage current in AlGaN Schottky diode in terms of the phonon-assisted tunneling model

The leakage current in the AlGaN Schottky diode under a reverse bias is simulated and compared within the frameworks of the thermionic emission–diffusion and phonon-assisted tunneling models. It is shown that the phonon-assisted tunneling model is suitable to describe the reverse-bias characteristic of the AlGaN Schottky contact and can also be applied to calculate the gate leakage current in the AlGaN/GaN high electron mobility transistor

Accurate band alignment of sputtered Sc₂O₃ on GaN for high electron mobility transistor applications

Abstract Sc2O3 is a promising gate dielectric for surface passivation in GaN-based devices. However, the interface quality and band alignment of sputtered Sc2O3 on GaN has not been fully explored. In this work, x-ray photoelectron spectroscopy (XPS) and variable angle spectroscopic ellipsometry were performed to extract the discontinuities in the valence and conduction bands of the Sc2O3/GaN system. Sc2O3 films were deposited on GaN using radio frequency sputtering. The valence band offset of Sc2O3/GaN was determined to be 0.76 ± 0.1 eV using Kraut’s method. The Sc2O3 band gap of 6.03 ± 0.25 eV was measured using O 1s energy loss spectroscopy. The electron affinity measurements of GaN and Sc2O3 using XPS secondary electron cut-off spectra provided an additional degree of accuracy to the derived band line-up for the Sc2O3/GaN interface. The band alignment results were compared with literature values of band offsets determined experimentally and theoretically for differently grown Sc2O3 films on GaN.</jats:p

Applications of Vapor Deposition in Microelectronics and Dye-Sensitized Solar Cells

Over the past decades, vapor deposition of thin films has gained wide interest in both industry and academia, and a variety of its applications have been demonstrated. As one of the most promising vapor deposition techniques, atomic layer deposition (ALD) and its applications in microelectronics and dye-sensitized solar cells are extensively investigated in this dissertation. ALD has many distinct features including low temperature processing, self-limiting growth, and precise control of film composition and thickness. Thus, ALD is considered to be suitable for conformal coating of 3D nanostructures, such as nanoporous structures, high aspect-ratio trench or hole structures, and so forth. Additionally, pulsed chemical vapor deposition (CVD) and its applications in microelectronics are explored in this dissertation. Ruthenium (Ru) is a promising electrode material for next generation microelectronic devices. The ALD and pulsed CVD processes discussed in Chapter 2 provide several approaches to produce smooth, conformal, pin-hole free Ru metal thin films. High-quality Ru films can be made under either oxidizing ambient or reducing ambient, which provides more flexibility for applications in microelectronics. Conductive ruthenium dioxide $(RuO_2)$ is also considered as a promising microelectrode material. Chapter 3 demonstrates a pulsed CVD process of depositing pure, smooth $(RuO_2)$ films with reasonably low resistivity. Chapter 3 also demonstrates that $(RuO_2)$ can be epitaxially grown on rutile $TiO_2(011)$ with a high-quality coherent heteroepitaxy structure. III-V MOSFET is now a very active area of growing interest to researchers and engineers in electronic industry and academia. Applications of ALD WN and high-k oxide materials for GaAs and GaN based devices are investigated in Chapters 4 and 5. Taking advantage of the conformal-coating feature of ALD, a stack of gate dielectric and metal gate can be coated uniformly around suspended nanowire structures, which is crucial for well-behaved gate-all-around MOSFETs. III-V MOSFETs also generally lack a suitable dielectric layer that has low interface trap density $(D_{it})$. Epitaxial ALD high-k dielectric lanthanum yttrium oxide, grown on GaAs(111)A, is found to have a fairly low $(D_{it})$, and therefore, the electrical properties are dramatically improved with its inclusion. This finding is very insightful for the applications of next generation III-V MOSFETs. In addition, a few ALD processes of candidate dielectric materials for GaN based devices are discussed. Dye-sensitized solar cells have great potential to compete with conventional p-n junction solar cells due to their relatively low cost. However, their efficiency is limited by the ease with which electrons collected by the nanoparticle framework can recombine with ions in solution. As discussed in Chapter 6, by depositing insulating and transparent $SiO_2$ selectively onto the open areas of nanoparticulate $TiO_2$ surface, while avoiding any deposition of $SiO_2$ over or under the organic dye molecules, the solar cell efficiency can be significantly improved

Characterization of Charge Trapping Phenomena in GaN-based HEMTs

This dissertation reports on charge-trapping phenomena and related parasitic effects in AlGaN/GaN high electron mobility transistors. By means of static and pulsed I-V measurements and deep-level transient spectroscopy, the main charge-trapping mechanisms affecting the dynamic performance of GaN-based HEMTs devoted to microwave and power switching applications have been comprehensively characterized, identifying the nature and the localization of the deep-levels responsible for the electrically active trap-states. A high-voltage measurement system capable for double-pulsed ID-VD, ID-VG and drain-current transient spectroscopy has been successfully designed and implemented. A characterization methodology, including the analysis of static I-V measurements, pulsed I-V measurements, and deep-level transient spectroscopy, has been developed to investigate the impact of voltage, current, and temperature on the parasitic effects of charge-trapping (threshold voltage instabilities, dynamic on-resistance increase, and transconductance reduction), and on trapping/detrapping kinetics. Experimental results gathered on transistor structures are supported by complementary capacitance deep-level transient spectroscopy (C-DLTS) performed on 2-terminal diode (FATFET) structures. Two main case-studies have been investigated. Schottky-gated AlGaN/GaN HEMTs grown on silicon carbide substrate employing iron and/or carbon doped buffers devoted to microwave applications, and MIS-gated double-heterostructure AlGaN/GaN/AlGaN HEMTs grown on silicon substrate devoted to power switching applications. The devices under test have been exposed to the complete set of current-voltage regimes experienced during the real life operations, including off-state, semi-on-state, and on-state. The main novel results are reported in the following: • Identification of a charge-trapping mechanism promoted by hot-electrons. This mechanism is critical in semi-on-state, with the combination of relatively high electric-field and relatively high drain-source current. • Identification of a positive temperature-dependent charge-trapping mechanism localized in the buffer-layer, potentially promoted by the vertical drain to substrate potential. This mechanism is critical in high drain-voltage off-state bias in high temperature operations. • Identification of deep-levels and charge-trapping related to the presence of doping compensation agents (iron and carbon) within the GaN buffer layer. • Identification of charge-trapping mechanism ascribed to the SiNX and/or Al2O3 insulating layers of MIS-gated HEMTs. This mechanism is promoted in the on-state with positive gate-voltage and positive gate leakage current. • Identification of a potential charge-trapping mechanism ascribed to reverse gate leakage current in Schottky-gate HEMTs exposed to high-voltage off-state. • The characterization of surface-traps in ungated and unpassivated devices by means of drain-current transient spectroscopy reveals a non-exponential and weakly thermally-activated detrapping behaviour. • Preliminary synthesis of a degradation mechanism characterized by the generation of defect-states, the worsening of parasitic charge-trapping effects, and the degradation of rf performance of AlGaN/GaN HEMTs devoted to microwave operations. The evidence of this degradation mechanism is appreciable only by means of rf or pulsed I-V measurements: no apparent degradation is found by means of DC analysis