Radiation effects in III-V compound semiconductor heterostructure devices

The radiation effects in III-V heterojunction devices are investigated in this thesis. Two types of heterojunction devices studied are InGaP/GaAs single heterojunction bipolar transistors (SHBTs) and GaN-based heterojunction light emitting diodes (LEDs). InGaP/GaAS HBTs are investigated for high energy (67 and 105 MeV) proton irradiation effects while GaN heterojunction LEDs are studied for neutron irradiation effects. A compact model and the parameter extraction procedures for HBTs are developed, and hence the I[subscript C]--V[subscript CE] characteristics of pre- and post-irradiation HBTs can be simulated by employing the developed model. HBTs are electrically characterized before and after proton irradiation. Overall, the studied HBT devices are quite robust against high energy proton irradiation. The most pronounced radiation effect shown in SHBTs is gain degradation. Displacement damage in the bulk of base-emitter space-charge region, leading to excess base current, is the responsible mechanism for the proton-induced gain degradation. The performance degradation depends on the operating current and is generally less at higher currents. Compared to the MBE grown devices, the MOVPE grown HBTs show superior characteristics both in initial performance and in proton irradiation hardness. The 67 MeV protons cause more damage than 105 MeV protons due to their higher value of NIEL (non-ionizing energy loss). The HBT I-V characteristics of pre- and post-irradiated samples can be simulated successfully by employing the developed model. GaN heterojunction LEDs are electrically and optically characterized before and after neutron irradiation. Neutron irradiation causes changes in both the I-V characteristic and the light output. Atomic displacement is responsible for both electrical and optical degradation. Both electrical and optical properties degrade steadily with neutron fluence producing severe degradation after the highest fluence neutron irradiation. The light output degrades by more than 99% after 1.6x10¹⁵ n/cm² neutron irradiation, and the radiation damage depends on the operating current and is generally less at higher currents

III-V족 화합물 반도체 터널 전계 효과 트랜지스터 개발

학위논문(박사) -- 서울대학교대학원 : 공과대학 전기·정보공학부, 2022. 8. 최우영.리소그래피 기술의 놀라운 발전은 10 nm 이하의 논리 트랜지스터를 상용화했다. 게이트 길이 스케일링은 모스펫 (MOSFET)의 전력 소비를 줄이기 위한 노력의 큰 부분을 차지한다. 그러나 이러한 접근 방식은 리소그래피의 물리적 한계와 누설 전류 제어와 같은 몇 가지 문제에 직면했다. 모스펫의 근본적인 문제는 현재 전송 메커니즘의 한계로 인해 60 mV/dec 미만의 임계값 기울기 (SS)에 도달할 수 없다는 것이다. Si 터널링 전계효과 트랜지스터 (TFET)의 여러 연구자들이 60 mV/dec 미만의 결과를 보고했지만, Si 동종 접합 터널링 전계효과 트랜지스터는 간접 대역 갭 물질의 터널링 확률이 낮아 전류상으로 불충분하다. P-I 접합부에서의 터널링 확률은 터널링 전계효과 트랜지스터의 동작전류에 영향을 미치기 때문에 작은 직접 밴드갭을 가지고 유효질량이 낮은 III-V 화합물 반도체는 임계값 기울기가 60 mV/dec 미만인 높은 터널링 전류를 달성할 수 있는 가장 유망한 재료이다. 또한 밴드 오프셋이 다른 재료를 선택함으로써, 스태거드 또는 브로큰 갭을 형성함으로써 터널링 전류를 현저하게 증가시킬 수 있다. P-I 접합부의 터널링이 터널 전계효과 트랜지스터 소자의 전류 공급원이기 때문에 많은 연구자들이 분자빔 에피택시 (MBE) 방식으로 성장한 p형 도핑 농도가 높은 III-V 웨이퍼로 제조된 터널 전계효과 트랜지스터의 성능을 보고해왔다. 그러나 높은 도핑 농도와 가파른 도펀트 프로파일을 갖는 p형 InGaAs를 성장하기가 까다롭기 때문에 금속-유기 화학 기상 증착 (MOCVD) 성장 에피택셜 층에서 제조된 InGaAs TFET 소자는 거의 보고되지 않았다. 이에 따라 본 연구는 TFET 소자 제작을 위한 고품질 에피택셜 층을 성장시키기 위한 MOCVD 성장 기술을 선보인다. 종래의 TFET 소자에 대해서는 동종 접합 p-i-n InGaAs 에피택셜층을 성장시키고, p++-Ge/i-InGaAs/n+-InAs 나노선을 성장시켜 TFET 소자 성능 향상 가능성을 확인하였다. MOCVD에 의해 성장한 에피택시 층에서 제조된 TFET 소자의 잠재성을 확인하기 위해 평판과 나노선 에피택셜 층에서 제작된 TFET 소자의 성능이 확인되었다. MOCVD 방법을 이용하여 고품질의 에피택셜 층이 성장되었다. MBE에 비해 가성비, 높은 처리량, 우수한 결정 품질이 MOCVD의 가장 큰 장점이다. 이에 여러 성장 조건을 변화시키면서 InP (001) 기판 위로 InGaAs 필름층의 성장이 연구되었다. 소스 유량, 온도 및 V/III 비율이 성장된 InGaAs 필름층의 품질에 끼치는 영향이 연구되었다. 또한 MOCVD InGaAs 성장 기술에서 n형 및 p형 도펀트의 농도를 높이는 것과 도펀트 프로파일을 가파르게 하는 것이 도전적이므로 탄소 및 텔루륨 도핑을 통해 가파른 도펀트 프로파일을 보이는 고농도의 p형 및 n형 InGaAs층을 성장하였다. 성장된 에피택셜 필름층은 TFET 소자를 제작하여 평가하였다. TFET 소자 제작 전에 우선 TFET 소자의 채널 길이가 전기적 시뮬레이션 결과에 의해 선택되었다. MOCVD를 이용하여 도핑 프로파일이 가파른 고품질의 수직 p-i-n 에피텍셜 구조가 한번에 성장되었다. 에피택셜 성장 후에 TFET 소자는 수직 방향의 습식 식각을 통해 제작되었다. 옴 (Ohmic) 공정과 에어브릿지 공정도 소자 제작을 위해 최적화되었다. P형 도핑 농도에 대한 영향과 MOCVD 성장 중에 생긴 전위에 대한 영향이 TFET 성능을 통하여 확인되었다. 제조된 TFET 소자는 60 mV/dec에 가까운 SS와 괜찮은 온/오프 전류 비율을 보여주었는데, 이는 최초로 보고되는 MBE에서 성장된 웨이퍼에서 만들어진 TFET 소자와 비교할 수 있는 소자이다. 이 결과는 고품질의 MOCVD로 성장한 III-V TFET 소자의 양산 가능성을 보여준다. 이 연구의 다음 부분은 나노선 TFET 제작이다. 전자소자 제작을 위한 III-V 나노선 성장에는 몇 가지 장점이 있다. 다양한 종류의 웨이퍼에 다양한 특성을 가지는 헤테로 구조를 형성할 수 있다는 것이 큰 장점이다. 충분히 작은 직경으로 성장된 나노선은 웨이퍼와 다른 격자 상수를 가지더라도 전위 없는 계면을 가진다. 다양한 유형의 밴드 정렬이 만들어질 수 있으며, 이는 TFET의 터널링 정류를 증가시키는 데에 있어 중요한 요소이다. 또한 직경이 작은 나노선은 칩으로 제작되었을 때 더 나은 소자 밀도, 향상된 게이트 제어성, 성장 시간 단축을 통한 처리량 향상이 가능하다. InGaAs 나노선은 선택적 영역 성장법 (SAG) 성장되었다. 하드마스크 층으로서 InP (111)B 및 Ge (111) 웨이퍼에 SiO2 층이 증착 되었다. 성장 모드가 다르기 때문에 InGaAs 평판 필름층 성장과는 크게 다른 성장 조건을 테스트하였다. 나노선의 선택적 성장은 온도, V/III 비율 및 소스 유량을 최적화하여 확인하였다. 그 결과 InP (111)B와 Ge (111) 웨이퍼에서 InAs와 InGaAs 나노선을 성공적으로 성장시켰다. P형 물질로는 p++도핑된 Ge (111) 웨이퍼를 사용하였다. 인트린식 InGaAs와 InAs 나노선이 그 위에 선택적으로 성장되었다. 마지막으로 실리콘 도펀트를 가진 n형 InAs 나노선이 후속적으로 성장되었다. 성장된 나노선은 수직 나노선 TFET을 제작하여 평가되었다. 높은 단계 커버리지와 양호한 인터페이스 상태 밀도를 위하여 ALD HfO2 및 ALD TiN 공정과정이 최적화되었다. 개발된 ALD 공정을 적용함으로써 수직형 나노선 Ge/InGaAs 헤테로 접합 TFET의 동작이 성공적으로 확인되었다.The remarkable development of lithography technology commercialized the sub-10 nm logic transistors. Gate length scaling is a large portion of the effort to reduce the power consumption of metal-oxide-semiconductor field-effect transistors (MOSFETs). However, this approach faces several problems, such as the physical limitation of lithography and leakage current control. The fundamental problem of MOSFETs is that they cannot reach subthreshold-slope (SS) below 60 mV/dec due to their current transport mechanism. Several researchers of Si tunneling field-effect transistors (TFETs) reported sub-60 mV/dec, but Si homo-junction TFETs show insufficient on-current due to the poor tunneling probability of indirect-band gap materials. As tunneling probability at the p-i junction influences the on-current of TFETs, III-V compound semiconductors, which have a direct small band gap and low effective masses, are the most promising materials to achieve high tunneling current with SS below 60 mV/dec. Also, the tunneling current can be remarkably increased by forming a staggered or broken gap by choosing materials with different band offsets. Since the tunneling at a p-i junction is the current source of TFET devices, many researchers have reported the performance of TFETs fabricated from III-V wafers with high p-type doping concentration grown by the molecular beam epitaxy (MBE) method. However, very few InGaAs TFET devices fabricated on MOCVD-grown epitaxial layers have been reported due to the challenging techniques for achieving p-type InGaAs with high doping concentration and steep dopant profile. Accordingly, this work demonstrates the metal-organic chemical vapor deposition (MOCVD) growth techniques to grow a high-quality epitaxial layer for TFET device fabrication. Homo-junction p-i-n InGaAs epitaxial layers were grown for conventional TFET devices, and hetero-junction p++-Ge/i-InGaAs/n+-InAs nanowires were grown to confirm the possibility of boosting the TFET device performance. The TFET device performance at both epitaxial layers was characterized to confirm the potential of TFET devices fabricated on the epitaxy layers grown by the MOCVD method. The high-quality epitaxial layers were grown using the MOCVD method. Compared to the MBE method, cost-effectiveness, high throughput, and excellent crystal quality are the significant advantages of the MOCVD method. The growth of InGaAs film layers on InP (001) substrate with several growth conditions was studied. The effects of source flow rate, temperature, and V/III ratio on the quality of grown InGaAs film layers were studied. As the high-concentration and steep dopant profile of n-type and p-type dopants are challenging in MOCVD InGaAs growth technique, carbon and tellurium doping techniques were introduced to achieve highly-doped p-type and n-type InGaAs layer with steep dopant profile. The grown epitaxial film layers were evaluated by fabricating the TFET device. Before the TFET device fabrication, the dimensions of the TFET device were selected by electrical simulation results of TFET devices with different structures. For TFET device fabrication, a high-quality vertical p-i-n epitaxial structure with a steep doping profile was successively formed by MOCVD. After epitaxial growth, the TFET devices were fabricated by the vertical top-down wet etching method. The ohmic process and air-bridge process were also optimized for device fabrication. The effect of p-type doping concentration and the dislocations formed during MOCVD growth was confirmed by TFET performance. The fabricated TFET devices showed SS of near-60 mV/dec and sound on/off current ratio, which was by far the first reported device comparable to TFET devices fabricated on the MBE-grown wafers. This result represents the possible mass-production of high-quality MOCVD-grown III-V TFET devices. The next part of this study is nanowire TFET fabrication. The growth of III-V nanowires for electronic device fabrication has several advantages. The significant advantage is that hetero-structures with various characteristics can be formed on various wafers. The nanowires grown by a sufficiently small diameter show a dislocation-free interface even if nanowires have a different lattice constant compared to the wafer. Various types of band-alignment can be formed, and this is a crucial factor in boosting the tunneling current of TFETs. Also, nanowires with a small diameter show better device density in a chip, improved gate controllability, and enhanced throughput by reducing growth time. The InGaAs nanowires were grown by the selective area growth (SAG) method. As a hard-mask layer, a SiO2 layer was deposited on InP (111)B and Ge (111) wafers. Growth conditions far different from InGaAs film layer growth were tested due to the different growth modes. Selective growth of nanowires was identified by optimizing temperature, V/III ratio, and source flow rate. As a result, InAs and InGaAs nanowires were successfully grown on InP (111)B and Ge (111) wafers. For p-type material, the p++-doped Ge (111) wafer was used. The intrinsic InGaAs and InAs nanowires were selectively grown on the patterned substrate. Finally, n-type InAs nanowires with silicon dopant were grown subsequently. The grown nanowires were evaluated by fabricating the vertical nanowire TFETs. ALD HfO2 and ALD TiN processes were optimized for high step coverage and good interface state density. By applying the developed ALD processes, a successful demonstration of vertical nanowire Ge/InGaAs hetero-junction TFET was observed.Contents List of Tables List of Figures Chapter 1. Introduction 1 1.1. Backgrounds 1 1.2. III-V TFETs for Low Power Device 5 1.3. Epitaxy of III-V Materials 12 1.4. Research Aims 17 1.5. References 20 Chapter 2. Epitaxial Growth of InGaAs on InP (001) Substrate 24 2.1. Introduction 24 2.2. Temperature Dependent Properties of Intrinsic-InGaAs on InP (001) Substrate 33 2.3. In-situ Doping Properties of InGaAs on InP (001) Substrate 37 2.4. Conclusion 51 2.5. References 52 Chapter 3. Demonstration of TFET Device Fabricated on InGaAs-on-InP (001) Substrate 56 3.1. Introduction 56 3.2. Simulation of Basic Operations of TFET Device 60 3.3. Process Optimization of TFET Fabrication 68 3.4. Process Flow 74 3.5. Characterization of TFETs Fabricated on MBE-grown and MOCVD-grown Wafers 78 3.6. Conclusion 96 3.7. References 97 Chapter 4. Selective Area Growth of In(Ga)As Nanowires 101 4.1. Introduction 101 4.2. Process Flow of Nanowire Growth 108 4.3. Impact of Different Growth Variables on the Growth of InAs Nanowires 113 4.4. Impact of Different Growth Variables on the Growth of InGaAs Nanowires 127 4.5. Conclusion 144 4.6. References 146 Chapter 5. Demonstration of Vertical Nanowire TFET 149 5.1. Introduction 149 5.2. Optimization of ALD HfO2 High-k Stack 152 5.3. Optimization of ALD TiN Gate Metal 169 5.4. Detailed Demonstration of Vertical Nanowire TFET Fabrication Processes 182 5.5. Characterization of Fabricated Vertical Nanowire TFETs 196 5.6. Conclusion 203 5.7. References 205 Chapter 6. Conclusions and Outlook 209 6.1. Conclusions 209 6.2. Outlook 211 Appendix. 213 A. n+-InAs Nanowire Doping Concentration Evaluation by TLM Method 213 B. n+-InAs Nanowire Doping Concentration Evaluation by C-V Method 219 C. References 205 Abstract in Korean 226 Research Achievements 230박

MOVPE Growth of InP-based III-V compounds doped with transition metals (Fe,Mn)

The project of thesis has concerned the growth through phase epitassia vapor with metal-organic precursors (MOVPE) of semiconductors compounds III-V "InP-based" (InGaAsp/ InP o GaAs)and their structural optimization, electric and optics to the goals of the study of the effects of the introduction of metals of transition (Fe, Mn) obtained through ionic implantation after the growth or through doping during the same one. The first aspect is concerning the possibility of obtaining areas to high electrical resistivity for the electric insulation in integrated devices while the second is turned to the possibility of exploiting the magnetic properties induced in the semiconductor (Diluted magnetic semiconductors) by the incorporation of 3d metals.Il progetto di tesi ha riguardato la crescita mediante epitassia da fase vapore con precursori metallo-organici (MOVPE) di semiconduttori composti III-V “InP-based” (InGaAsP/ InP o GaAs) e la loro ottimizzazione strutturale, elettrica e ottica ai fini dello studio degli effetti dell’introduzione di metalli di transizione (Fe, Mn) ottenuta mediante impiantazione ionica dopo la crescita o mediante drogaggio durante la stessa.Il primo aspetto è relativo alla possibilità di ottenere regioni ad alta resistività elettrica per l’isolamento elettrico in dispositivi integrati mentre il secondo è rivolto alla possibilità di sfruttare le proprietà magnetiche indotte nel semiconduttore (semiconduttori magnetici diluiti) dall’incorporazione di metalli 3d

Advances in Solid State Circuit Technologies

This book brings together contributions from experts in the fields to describe the current status of important topics in solid-state circuit technologies. It consists of 20 chapters which are grouped under the following categories: general information, circuits and devices, materials, and characterization techniques. These chapters have been written by renowned experts in the respective fields making this book valuable to the integrated circuits and materials science communities. It is intended for a diverse readership including electrical engineers and material scientists in the industry and academic institutions. Readers will be able to familiarize themselves with the latest technologies in the various fields

MOCVD Emitter Regrowth Technology for Scaling InGaAs/InP HBTs to Sub-100nm Emitter Width

By scaling semiconductor thicknesses, lithographic dimensions, and contactresistivities, the bandwidth of InGaAs/InP Hetero-junction Bipolar Transistors(HBTs) has reached 550/1100 GHz ft/fmax at 128 nm emitter width (wE). Primary challenges faced in scaling the emitter width are: developing high aspectratio emitter metal process for wE < 100nm, reducing base contact resistivityρb,c, and maintaining high DC current gain β.The existing W/TiW emitter process for RF HBTs cannot scale below 100nm. Process modules for scaling the emitter width to 60 nm are demonstrated.High aspect ratio trenches are etched into a sacrificial Si layer and then filledwith metal via Atomic Layer Deposition (ALD). Metals with high melting pointsare chosen to withstand high emitter current densities (JE) at elevated junctiontemperatures without suffering from electromigration or thermal decompositionand is thus manufacturable. ALD deposition of TiN, Pt, and Ru are explored.Novel base epi designs are proposed for reducing Auger recombination current(IB,Auger). A dual doping layer in the base is proposed with a higher doping in theupper 5 nm of the base for lower ρb,c and a lower doping in the remainder of thebase for reducing IB,Auger. Presence of a quasi-electric field (4EC) in the upperdoping grade accelerates electrons away from the region towards the collector,thus further reducing IB,Auger.Selective regrowth of the emitter semiconductor via Metal-Organic ChemicalVapour Deposition (MOCVD) is proposed for decoupling the extrinsic base regionunder the base metal from the intrinsic region under the emitter-base junction,for increasing β,ft, and improving ρb,c. Carbon p-dopants in the InGaAs base arepassivated by H+ during regrowth. Annealing to reactivate carbon induces surfacedamage and increases base sheet resistance (Rb,sh) and ρb,c. Process techniquesfor minimizing Rb,sh and ρb,c in an emitter regrowth process are demonstratedand compared. ρb,c of 5.5 Ω.µm2 on p-InGaAs is demonstrated on TransmissionLine Measurement (TLM) structures after regrowth and anneal, by protecting thesemiconductor surface with tungsten. This is comparable to 2.9 Ω.µm2 measuredon TLM structures that do not undergo regrowth and anneal.Feasibility of emitter regrowth is demonstrated on Large Area Devices (LADs)with SiO2 as regrowth mask, and W cap during anneal. Emitter-regrowth andnon-regrowth devices of identical dimensions and epi design are compared. Emitterregrown HBTs yield higher β of 28 as compared to 13 for non-regrowth devices.Benefits of emitter regrowth cannot be ascertained on LADs due to high seriesresistance and large gap spacings between base metal and emitter-base junction.A process flow is proposed for scaling regrown HBTs to 60 nm emitter widths.The process incorporates ALD emitter metal technology that is demonstrated inthe first half of the dissertation. New epi designs for regrown-emitter HBTs areoptimized for maximizing β, ft. Scaling regrown-emitter HBTs is essential forrealizing their benefit over non-regrowth HBTs

Impact of Gamma-Irradiation on the Characteristics of III-N/GaN Based High Electron Mobility Transistors

In this study, the fundamental properties of AlGaN/GaN based High Electron Mobility Transistors (HEMTs) have been investigated in order to optimize their performance in radiation harsh environment. AlGaN/GaN HEMTs were irradiated with 60Co gamma-rays to doses up to 1000 Gy, and the effects of irradiation on the devices\u27 transport and optical properties were analyzed. Understanding the radiation affects in HEMTs devices, on carrier transport, recombination rates and traps creation play a significant role in development and design of radiation resistant semiconductor components for different applications. Electrical testing combined with temperature dependent Electron Beam Induced Current (EBIC) that we used in our investigations, provided critical information on defects induced in the material because of gamma-irradiation. It was shown that low dose (below ~250 Gy) and high doses (above ~250 Gy) of gamma-irradiation affects the AlGaN/GaN HEMTs due to different mechanisms. For low doses of gamma-irradiation, the improvement in minority carrier diffusion length is likely associated with the irradiation-induced growing lifetime of the non-equilibrium carriers. However, with the increased dose of irradiation (above ~ 250 Gy), the concentration of point defects, such as nitrogen vacancies, as well as the complexes involving native defects increases which results in the non-equilibrium carrier scattering. The impact of defect scattering is more pronounced at higher radiation, which leads to the degradation in the mobility and therefore the diffusion length. In addition for each device under investigation, the temperature dependent minority carrier diffusion length measurements were carried out. These measurements allowed the extraction of the activation energy for the temperature-induced enhancement of the minority carrier transport, which (activation energy) bears a signature of defect levels involved the carrier recombination process. Comparing the activation energy before and after gamma-irradiation identified the radiation-induced defect levels and their dependences. To complement EBIC measurements, spatially resolved Cathodoluminescence (CL) measurements were carried out at variable temperatures. Similar to the EBIC measurements, CL probing before and after the gamma-irradiation allowed the identification of possible defect levels generated as a result of gamma-bombardment. The observed decrease in the CL peak intensity after gamma-irradiation provides the direct evidence of the decrease in the number of recombination events. Based on the findings, the decay in the near-band-edge intensity after low-dose of gamma-irradiation (below ~250 Gy) was explained as a consequence of increased non-equilibrium carrier lifetime. For high doses (above ~250 Gy), decay in the CL intensity was observed to be related to the reduction in the mobility of charge carriers. The results of EBIC are correlated with the CL measurements in order to demonstrate that same underlying process is responsible for the changes induced by the gamma-irradiation. DC current-voltage measurements were also conducted on the transistors to assess the impact of gamma-irradiation on transfer, gate and drain characteristics. Exposure of AlGaN/GaN HEMTs to high dose of 60Co gamma-irradiation (above ~ 250 Gy) resulted in significant device degradation. Gamma-rays doses up to 1000 Gy are shown to result in positive shift in threshold voltage, a reduction in the drain current and transconductance due to increased trapping of carriers and dispersion of charge. In addition, a significant increase in the gate leakage current was observed in both forward and reverse directions after irradiation. Post-irradiation annealing at relatively low temperature was shown to restore the minority carrier transport as well as the electrical characteristics of the devices. The level of recovery of gamma-irradiated devices after annealing treatment depends on the dose of the irradiation. The devices that show most recovery for a particular annealing temperature are those exposed to the low doses of gamma-irradiation, while those exposed to the highest doses results in no recovery of performance. The latter fact indicates that a higher device annealing temperature is needed for larger doses of gamma-irradiation

Thermal characterisation of miniature hotplates used in gas sensing technology

The reliability of micro-electronic devices depends on the device operating temperature and therefore self-heating can have an adverse effect on the performance and reliability of these devices. Hence, thermal measurement is crucial including accurate maximum operating temperature measurements to ensure optimum reliability and good electrical performance. In the research presented in this thesis, the high temperature thermal characterisation of novel micro-electro-mechanical systems (MEMS) infra-red (IR) emitter chips for use in gas sensing technology for stable long-term operation were studied, using both IR and a novel thermo-incandescence microscopy. The IR emitters were fabricated using complementary-metal-oxide semiconductor (CMOS) based processing technology and consisted of a miniature micro-heater, fabricated using tungsten metallisation. There is a commercial drive to include MEMS micro-heaters in portable electronic applications including gas sensors and miniaturised IR spectrometers where low power consumption is required. IR thermal microscopy was used to thermally characterise these miniature MEMS micro-heaters to temperatures approaching 700 °C. The research work has also enabled further development of novel thermal measurement techniques, using carbon microparticle infra-red sensors (MPIRS) with the IR thermal microscopy. These microparticle sensors, for the first time, have been used to make more accurate high temperature (approaching 700 °C) spot measurements on the IR transparent semiconductor membrane of the micro-heater. To substantially extend the temperature measurement range of the IR thermal microscope, and to obtain the thermal profiles at elevated temperatures (> 700 °C), a novel thermal measurement approach has been developed by calibrating emitted incandescence radiation in the optical region as a function of temperature. The calibration was carried out using the known melting point (MP) of metal microparticles. The method has been utilised to obtain the high temperature thermo-optical characterisation of the MEMS micro-heaters to temperatures in excess of 1200 °C. The measured temperature results using thermo-incandescence microscopy were compared with calculated electrical temperature results. The results indicated the thermo-incandescence measurements are in reasonable agreement (± 3.5 %) with the electrical temperature approach. Thus, the measurement technique using optical incandescent radiation extends the range of conventional IR microscopy and shows a great potential for making very high temperature spot measurements on electronic devices. The high power (> 500mW) electrical characterisation of the MEMS micro-heaters were also analysed to assess the reliability. The electrical performance results on the MEMS micro-heaters indicated failures at temperatures greater than 1300 °C and Scanning Electron Microscope (SEM) was used to analyse the failure modes

MOVPE Growth and Study of III-V Multi-Junction Structures for Advanced Photovoltaic Applications

The energy question is one of the main problem of modern society and is particularly urgent because of the drawbacks of fossil fuel exploitation, which provide 80% of the total energy we currently consume. The only realistic and far-seeing solution to current energy crisis is represented by the employment of renewable energy sources, assisted by global energy conservation policies. Photovoltaic represents one of the most interesting renewable technologies, since it’s the only one that can convert solar energy directly into electricity, without the use of any moving parts. Solar radiation, moreover, is abundant, inexhaustible and diffuse all over the world. Since the 1950s, when the first silicon solar cell was produced, three generations of devices were conceived with the purpose of improving the production cost/conversion efficiency ($/W) ratio to become market-competitive. In particular, the third generation is based on innovative devices that mean to exceed the theoretical efficiency limit for single junction p-n solar cells, by reducing their main energy loss mechanisms. This PhD thesis deals with the study of two types of third generation structures based on multiple band gaps. The first structure is based on a InGaP p-i-n junction with an intrinsic region consisting of 30 periods of 8 nm thick GaAs quantum wells (QW) and 12 nm thick InGaP barriers, conceived to be part of a quantum well solar cell (QWSC). This heterostructure was grown by a low pressure MOVPE reactor, with the employment of liquid alternative metalorganic precursors for the group V elements, terbutylarsine (TBAs) and terbutylphosphine (TBP). In particular, it was investigated the light response of the structure by an accurate photoelectric spectroscopy (PES) study: both the photocurrent (PC) and photovoltage (PV) signals were detected by a standard lock-in technique, as a function of the wavelength, at different sample temperatures and for different frequencies of the exciting light, modulated by a chopper. A second type of photovoltaic structure was designed and realized, consisting in a relatively simple monolithic GaAs-based tandem structure grown on GaSb substrates. By taking advantage of the high temperature of the growth process (T=600-650° C), the deposition of a highly Zn doped GaAs layer enabled the Zn diffusion into the Te-doped (n-type) GaSb substrate, forming a buried GaSb p-n homojunction. By depositing additional GaAs layers with appropriate doping levels, a tunnel and a top junction were stacked to obtain the final tandem structure. The originality of the proposal is related both to the method employed to activate the Zn diffusion in GaSb, and also to the assessment of the GaAs-on-GaSb epitaxial growth. The possibility to realize a tandem cell by properly modulating the doping of the same compound (GaAs), thus making the fabrication process very simple, is the main advantage of this structure

Phosphide-based optical emitters for monolithic integration with GaAs MESFETs

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 1995.Includes bibliographical references (p. 137-144).by Joseph F. Ahadian.M.S

Design, Growth, and Characterization of III-Sb and III-N Materials for Photovoltaic Applications

abstract: Photovoltaic (PV) energy has shown tremendous improvements in the past few decades showing great promises for future sustainable energy sources. Among all PV energy sources, III-V-based solar cells have demonstrated the highest efficiencies. This dissertation investigates the two different III-V solar cells with low (III-antimonide) and high (III-nitride) bandgaps. III-antimonide semiconductors, particularly aluminum (indium) gallium antimonide alloys, with relatively low bandgaps, are promising candidates for the absorption of long wavelength photons and thermophotovoltaic applications. GaSb and its alloys can be grown metamorphically on non-native substrates such as GaAs allowing for the understanding of different multijunction solar cell designs. The work in this dissertation presents the molecular beam epitaxy growth, crystal quality, and device performance of AlxGa1−xSb solar cells grown on GaAs substrates. The motivation is on the optimization of the growth of AlxGa1−xSb on GaAs (001) substrates to decrease the threading dislocation density resulting from the significant lattice mismatch between GaSb and GaAs. GaSb, Al0.15Ga0.85Sb, and Al0.5Ga0.5Sb cells grown on GaAs substrates demonstrate open-circuit voltages of 0.16, 0.17, and 0.35 V, respectively. In addition, a detailed study is presented to demonstrate the temperature dependence of (Al)GaSb PV cells. III-nitride semiconductors are promising candidates for high-efficiency solar cells due to their inherent properties and pre-existing infrastructures that can be used as a leverage to improve future nitride-based solar cells. However, to unleash the full potential of III-nitride alloys for PV and PV-thermal (PVT) applications, significant progress in growth, design, and device fabrication are required. In this dissertation, first, the performance of ii InGaN solar cells designed for high temperature application (such as PVT) are presented showing robust cell performance up to 600 ⁰C with no significant degradation. In the final section, extremely low-resistance GaN-based tunnel junctions with different structures are demonstrated showing highly efficient tunneling characteristics with negative differential resistance (NDR). To improve the efficiency of optoelectronic devices such as UV emitters the first AlGaN tunnel diode with Zener characteristic is presented. Finally, enabled by GaN tunnel junction, the first tunnel contacted InGaN solar cell with a high VOC value of 2.22 V is demonstrated.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201