Optimization of Synthesis Method of Monodisperse Silver Chalcogenide in the SWIR Region

정윤혜 경상국립대학교 대학원 2024 국내석사

Short-wave infrared (SWIR) radiation is less affected by external conditions, nondestructive, and invisible, making it valuable for applications in medical, space, military, security, environmental monitoring, agriculture, and industrial inspection fields. Among the materials functioning in this region, chalcogenides, compounds formed by combining chalcogen anions with cationic elements, demonstrate unique properties under electrical and energy stimuli. Silver chalcogenides, such as Ag₂X (X = S, Se, Te), are versatile semiconductor materials with tunable band gaps. Notably, due to their narrow band gaps, they exhibit luminescence in the infrared (IR) range and are less toxic to humans because they are free of heavy metals, making them suitable for bioimaging and related applications. This study aimed to synthesize monodisperse silver chalcogenide nanocrystals that exhibit luminescence in the SWIR range near 1500 nm. Various synthesis methods were explored to determine the most suitable approach. The results demonstrated the successful synthesis of monodisperse silver chalcogenide nanocrystals and controlled particle size, opening new possibilities for their utilization in the SWIR region. The tunable band gap and SWIR luminescence of heavy metal-free silver chalcogenide nanocrystals offer significant advantages for infrared sensing applications, such as LIDAR sensors. Additionally, the variation in absorption properties with particle size provides valuable insights into the quantum confinement effect of silver chalcogenides.

Atomic layer deposited ge-based amorphous chalcogenide alloys for ovonic threshold switching selector in 3D cross-point memory scaling

김명섭 Graduate School, Yonsei University 2022 국내박사

비정질 칼코겐나이드 기반 OTS (Ovonic Threshold Switching) 선택기를 포함한 3 차원 (3D) 교차점 (X-point) 기술은 고성능 컴퓨팅 시스템의 메모리 계층 구조에 새로운 변화를 가져오고 있습니다. 미래의 3D 교차점 메모리 스케일링을 준비하기 위해 우리는 Ge1-xSx 비정질 칼코겐나이드 합금 박막의 플라즈마 강화 원자층 증착 (PE-ALD)을 연구했습니다. PE-ALD Ge1-xSx 박막은 GeCl4 전구체와 H2S 플라즈마 반응물을 사용하여 합성되었으며 ALD 단계의 노출 시간, 온도 및 플라즈마 전력의 함수로 자체 제한 성장 특성을 자세히 연구했습니다. PE-ALD GeS2 박막은 0.29 nm 의 RMS 거칠기와 수직 3D 구조에서 양호한 등각을 보였다. 또한, 50nm 하부전극접촉 (BEC)를 갖는 GeS2 및 Ge2S3 소자의 OTS 거동 및 문턱 전압 (1.9 V ~ 6.2 V)과 정규화된 오프 전류 (20 nA ~ 250 nA) 사이의 트레이드오프 관계를 조사했습니다. 특히, 수정된 Poole-Frenkel (PF)에 따르면 GeS2 장치는 더 높은 트랩 밀도 (2.1 × 1021 cm-3)로 인해 Ge2S3 장치보다 더 높은 임계값 필드 (~3.1 MV/cm)와 더 낮은 정규화 오프 전류 특성을 보였습니다. OTS 애플리케이션을 위한 새로운 바이너리 GeS2 비정질 칼코겐나이드에 대한 이 PE-ALD 연구에서 달성된 결과는 미래의 3D 교차점 메모리 스케일링 개발에 기여할 것입니다. 이 논문에서 우리는 이전 ALD 연구에 발표된 이원 Ge-Se 및 Ge-S보다 우수한 OTS 특성에 대한 기대에 의해 동기가 부여된 삼원 Ge-Se-S 비정질 칼코겐나이드 합금의 ALD 연구를 제시합니다. 여기에서 우리는 HGeCl3 및 GeCl4 전구체를 Se(SiMe3)2 전구체와 ALD 반응을 비교하기 위해 밀도 기능 이론 (DFT) 계산 및 ALD 실험을 수행합니다. 우리는 자기 제한적 거동을 갖는 ALD GeSe2 박막에 후황화 공정을 통하여 새로운 Ge-Se-S 합금을 생성한다는 것을 보여줍니다. 저온 플라즈마 후황화 공정의 온도와 시간을 변경하여 GeSe2-Ge2S 유사 바이너리 라인을 따라 10nm 두께의 Ge-Se-S 박막의 조성 변화를 제어합니다. Ge5Se3S2 합금은 ALD GeSe2와 유사하게 비정질상과 우수한 스텝 커버리지를 유지함을 확인하였다. 마지막으로, 50 nm 하부 전극이 있는 장치에서 10nm 두께의 ALD GeSe2와 Ge5Se3S2 비정질 칼코겐화물 박막의 OTS 전기적 특성을 비교합니다. 새로운 Ge5Se3S2는 GeSe2 보다 약간 더 큰 임계 전압 (Vth) 드리프트를 갖지만 최대 106 사이클까지 더 높은 임계 필드, 더 낮은 오프 전류 및 더 작은 Vth 변동의 이점을 나타내었다. Three-dimensional (3D) cross-point (X-point) technology, including amorphous chalcogenide-based ovonic threshold switching (OTS) selectors, is bringing new changes to the memory hierarchy for high-performance computing systems. To prepare for future 3D X-point memory scaling, we studied the atomic layer deposition (ALD) of Ge based amorphous chalcogenide alloy thin films. The selection of Ge-S, Ge-Se and Ge-S-Se alloys was motivated by their high optical bandgap and wide amorphous formation area. First, in the chapter the PE-ALD Ge1-xSx thin films were synthesized using a GeCl4 precursor and H2S plasma reactant, and its self-limited growth characteristics were studied in detail as a function of the exposure time of the ALD steps, temperature, and plasma power. The PE-ALD GeS2 thin film showed RMS roughness of 0.29 nm and good conformality in the vertical 3D structure. Moreover, the OTS behavior of GeS2 and Ge2S3 mushroom-type devices with a 50-nm bottom electrode contact (BEC) were investigated as well as the and trade-off relationship between the threshold voltage (1.9 V ~ 6.2 V) and normalized off current (20 nA ~ 250 nA) based on scaling the film thickness down from 30 nm to 5 nm. In particular, GeS2 device showed a higher threshold field (~3.1 MV/cm) and lower normalized off current characteristics than Ge2S3 device due to the higher trap density (2.1 × 1021 cm-3), according to the modified Poole-Frenkel (PF) model. The results achieved in this PE-ALD research on novel binary GeS2 amorphous chalcogenide for OTS applications will contribute to the development of future 3D cross-point memory scaling. Second, in the chapter, we present an ALD study of binary Ge-Se and ternary Ge-Se-S amorphous chalcogenide alloys motivated by the expectation of superior OTS properties over Ge-S in chapter 1. Here, we perform density-functional theory (DFT) calculations and ALD experiments to compare HGeCl3 and GeCl4 precursors with Se(SiMe3)2 precursors. We show that ALD of GeSe2 thin films with self-limited behavior followed by post-sulfurization produces novel Ge-Se-S alloys. By changing the temperature and time of the low-temperature plasma sulfurization process, the compositional change of 10-nm-thick Ge-Se-S thin films is controlled along the GeSe2-Ge2S pseudo binary line. It is confirmed that the Ge5Se3S2 alloys maintained an amorphous phase and excellent step coverage, similar to ALD GeSe2. Finally, we compare the OTS electrical characteristics of 10-nm-thick ALD GeSe2 with Ge5Se3S2 amorphous chalcogenide thin films in a mushroom-type device with a 50-nm bottom electrode. The novel Ge5Se3S2 has a slightly larger threshold voltage (Vth) drift than GeSe2 but exhibits the advantages of a higher threshold field, lower off-current, and smaller Vth fluctuation up to 10E6 cycles.

Ge-Se-Bi Chalcogenide Glass의 결정화와 전기적 특성에 관한 연구

이명원 檀國大學校 大學院 1992 국내석사

Amorphous Semiconductor로서 Chalcogenide계의 Ge-Se-Bi 계 비정질화와 결정화 실험을 통하여 전기전도도를 평가코자 하였다. 시료의 조성 범위는 Ge_15-25 Se_65-85 Bi₂.5-15의 범위에서 5 N 의 Ge, Se, Bi metal 분말을 사용하였다. 시료는 석영관에 진공 장입후 용융시켜 비정질화하였다. 이때 열처리 조건은 1000℃에서 10시간 동안 가열하였으며 급냉 조건은 3834℃/sec로 처리 하였다. 비정질 sample의 결정화는 결정핵을 형성 시킨 후 온도 변화 및 시간의 변화를 주면서 결정을 성장 시켰으며, 이때 Bi₂Se₃와 GeSe₂결정상을 관찰할 수 있었다. 박막화는 위의 실험에 사용된 Bulk sample을 사용하여 박막을 제작 하였으며 유리화 영역은 Ge 15 at%, Se 70 at% 이상, Bi가 10 at% 이하일 때 비정질화가 용이하였다. Bulk의 경우 Ge를 20 at%로 고정시 Bi의 at% 함량이 증가함에 따라 전기전도도가 증가했으며 Bi가 7.5 at% 이상일때 급격한 전도도의 증가를 가져왔다. 박막의 경우엔 Bulk sample 보다 Bi의 함량이 증가시 더욱 큰 전기 전도도의 증가를 가져왔다. Ge₂0Se77.5Bi₂.5 조성의 결정화 경우 330℃에서 4 hr 유지시킨 경우의 전기 전도도가 가장 양호 하였다. The purpose of this study was to evaluate electrical conductivity of Ge-Se-Bi system Chalcogenide glass as a amorphous semiconductor by observing its dissolution and crystallization. In this experiment, Ge,Se,Bi metal powders in the range of Ge_12-25, Se_65-85, Bi₂.5-15 was used. The mixed metal powder were put into a vaccous quartz tube and then melted. The condition of heat treatment was to dispose it to 1000℃ heat for 10 hours and then rapidly quenched it at 3834℃/sec. The crystallization of the fused sample ripened as the change of temperature and time, after the crystal core was formed. At that time it was possible to observe the state that Bi₂Se₃and GeSe₂were crystallized. In the experiment of making thin film, thin film was produced by using the previously experimented bulk sample. And the amorphousness was well progressed when Ge was over 15 at%, Se was over 70 at%, and Bi was under 10 at%. As for bulk, when Ge was fixed to 20 at%, the conducting of electricity was increased as Bi gained at%. In the case of this film, the conductivity was much more increased than that of bulk sample as the increase of at% of Bi. In the experiment on Ge₂0, Se_77.5, and Bi₂.5, the crystalization was most vigorous when they were kept at 330℃ for 4 hours.

Design and fabrication of metal chalcogenide thin films for applications in optoelectronic devices including infrared detectors and windows

안학영 Graduate School, Korea University 2020 국내박사

The infrared (IR) field has been greatly expanded in military, industrial, aerospace and commercial applications and it has gained a great deal of interest. The active layer of infrared photodetector and infrared optics are key components of Infrared optical instruments. Recentrly, as the demand is increasing in the private market, research to lower the process cost is essential. In this thesis, several processes have been proposed to improve the properties of the metal chalcogenide thin film, which is the active layer of the detector, and the performance of the infrared detector. Firstly, synthesis within a short period of time through an environmentally-friendly and simple mechanochemical process resulted in the significant improvement in the properties of powders which was indicated by the mechanism analysis. Secondly, densification of the thin film was performed through compression. And then, the properties of the metal chalcogenide thin film were improved and the band gap was controlled by a simple method of one-step and one-chamber selenization process. Here, we studied the PbSe thin film which is most widely used as a mid-infrared absorber among metal calcogenides, and an environmentally friendly CGSe thin film that can replace it. In addition, for the fabrication of windows for infrared transmission, ZnS powder was synthesized through the mechanochemical process, and the effects of LiF and CaF2 additives in the sintering process were analyzed. Firstly, facile and low-cost method of forming a dense PbS thin films using a mechanochemically synthesized PbS powders and fabricating PbSe thin films by a selenization process is proposed. As IV–VI group semiconductors, lead chalcogenides such as PbS and PbSe are of great interest because of their applications in thermoelectric and optoelectronic devices. For their application for the infrared photodetectors, PbSe is more desirable light-absorbing materials than PbS since it has a direct bandgap of 0.28 eV at room temperature, which enables it to detect the mid-infrared radiation of 3 – 5 μm, where the high temperature inspection is possible. PbSe thin films can be prepared by a variety of methods, but most processes are expensive, complex and time consuming. Among many methods, the powder process for fabricating PbSe thin films is advantageous, due to its low-cost, simplicity and minimal usage of materials. However, thin films produced by the powder process often result in high properties, which has been an important barrier for application in a wider range of fields. In this thesis, we demonstrate the mechanochemical synthesis of PbS and PbSe particles and optimize synthetic conditions through mechanism analysis. We also discuss a simple compression of the PbS powder coated films and the transformation of a thin film of PbS to that of PbSe through selenization. The morphological, structural, optical and electrical properties of PbS, PbSe powders and thin films and optoelectronic properties of PbSe detector were investigated. The results of the photoresponse measurements of PbSe photodetector demonstrate that the proposed processes are promising candidates for improving detector performance. This approach is very meaningful in that it has proposed a new environmentally-friendly, simple and low-cost method of preparing an active layer suitable for optoelectronic applications, and it is expected to significantly expand the range of infrared applications in the future. Secondly, we propose Cu2GeSe3 (CGSe) materials and thin films, an environmentally-friendly infrared substitute for lead chalcogenide. Cu2GeS3 (CGS) nanoparticles were successfully synthesized by mechanochemical process, and CGSe absorber layers were fabricated by selenization of CGS thin films; the morphological, optical and electrical properties of the thin films were analyzed. Moreover, we present an alternative facile new concept of a one-step and one-chamber selenization system for obtaining high quality powder coated films, by introducing a new Se source by mixing Al2O3 with Se. In this chapter, we investigated the effect of controlling the mixing ratio of Se and Al2O3 as well as the thermal behavior and stability of Se in the Se-Al2O3 source. In particular, we investigated the band gap of CGSe thin films produced through selenization of Cu2GeS3 thin films and analyzed whether the properties of the thin films were improved. These approaches provide a low-cost and simple alternative for manufacturing CGSe infrared photodetectors. Finally, we report a low-cost environmentally-friendly synthesis method for ZnS powders and its sintering for its application as a window material. One of the important components of such IR detectors is the IR window, which requires high transparency in the infrared region. ZnS is a most desirable substance for such windows due to its high transparency and high thermal stability. Using only elemental precursors, Zn and S, submicrometer-sized ZnS powder with high phase purity was obtained. Spark plasma sintering was applied to compact the ZnS powder, and the resulting ceramic was analyzed by SEM, FTIR, Raman spectroscopy, and XRD. We also examined the effect of an additive during sintering and found that LiF functions well as a sintering aid for the ZnS compact.

Enhancement of ovonic threshold switch performance by nitrogen doping on silicon telluride for selector applications

이수봉 Graduate School, Yonsei University 2025 국내박사

인공 지능 및 자율 주행 차량 등의 데이터 기반 산업의 급속한 성장은 빅데이터와 빠른 연산 속도에 대한 수요의 증가로 반도체 산업에 큰 영향을 미쳤다. 상변화 메모리와 저항 메모리로 대표되는 뉴메모리는 기존의 디램과 낸드 각각의 장점인 비휘발성과 빠른 동작속도 모두를 가지고 있어 폰 노이만 메모리 병목현상을 해소할 수 있을 뿐만 아니라, 차세대 컴퓨팅 아키텍처로의 확장성도 제시한다. 특히, 이러한 가변 저항 메모리들은 기존의 메모리와 달리 2단자로 구성되어 있어 집적도를 극대화하기 위한 3차원 크로스포인트 아키텍쳐에 대한 가능성을 열었다. 하지만, 이 어레이 구조는 선택된 셀과 선택되지 않은 셀들이 워드라인 또는 비트라인을 공유하기에 본질적으로 선택되지 않은 셀에도 누설 전류가 흐르는 문제점이 있다. 이를 해결하기위해 선택소자를 각 셀에 통합하여 누설 전류를 막을 수 있으며, 여러 선택소자 중 오보닉 문턱 스위치(OTS) 현상을 보이는 칼코겐화합물은 여러 장점으로 인해 가장 촉망받는 재료이다. 하지만, OTS 현상의 경우 아직 동작 원리가 명확히 규명되지 않았기 때문에 여러 물질에 대해 선택소자의 요구 성능을 모두 만족하는지 연구가 활발히 수행되고 있다. 칼코겐화합물 중 Te 기반의 비정질 화합물은 Se, S 기반 화합물 대비 저전력으로 구동되는 것으로 알려져 있다. 본 박사 학위논문에서는 기본 재료로써 SiTe 비정질 칼코겐화합물에 N 도핑을 통하여 선택소자의 전기적 성능 향상을 도모하였으며, 여러 물질 분석을 통해 OTS 현상에 대한 이해를 도모하였다. 최초로 발표된 이 N-SiTe는 N 도핑을 통해 18 nA의 낮은 누설 전류와 106 펄스 동작 내구성, 400도의 열적 안정성을 보인 한편, Te 기반의 소재로써 1 V 미만의 저전력 작동과 1.89 MA/cm2 이상의 높은 on 전류밀도를 유지할 수 있다. 또한, Raman, XPS 등의 분광 분석법과 트랩 상태, 밴드갭 에너지 관점에서의 분석, 제일원리 계산을 통한 분석으로 본 기본 재료인 SiTe과 N 도핑을 통한 성능 향상을 해석하고 나아가 OTS 현상에 대한 이해를 확장하였다. 3장에서는 SiTe 2원소 물질에 대해 조성에 따른 전기적 특성을 조사하였고 Si47Te53 조성에서, 여러 요구 조건을 동시에 만족시키는 최적의 선택소자를 제작하였다. 조성에 따라 Te 함량이 많아질수록 저전력 구동과 충분한 on 전류를 보인 반면, 누설 전류를 충분히 억제하지 못한다는 한계를 보였다. XPS 분석과 trap 상태 분석으로 Te에 의해 짧아진 트랩 간 거리와 높아진 trap 농도로 인해 임계전압 이하에서의 전도가 증가함으로 I-V 특성을 해석하였다. 4장에서는 보다 심층적인 재료 분석을 위해 온도에 따른 임계전압 이하의 I-V 특성을 추가적으로 측정하여 활성화에너지를 포함한 trap 상태 변수들을 분석하였다. SiTe 2원소 물질에 N를 도입할 시 누설 전류를 29 μA에서 1.3 μA 수준으로 효과적으로 낮출 수 있다는 것을 확인하였고, 온도에 따른 I-V 커브 분석을 통해 N 도핑 시엔 증가된 trap 간 거리 (1.98에서 2.69 nm), 감소된 trap 농도 (5.11에서 1.29 × 1020 traps/cm3), 증가된 활성화에너지 (0.15에서 0.33 eV)로 향상된 선택소자 성능을 해석하였다. 5장에서는 N 도핑에 의한 효과를 분광분석과 밴드갭 분석으로 더욱 다방면으로 조사하였으며, N-SiTe 선택소자의 공정 및 소자 구조의 최적화로 18 nA의 낮은 누설전류, 5 ns의 빠른 지연속도, <2.97 mV/dec의 빠른 스위칭, 2배가량 높아진 읽기 마진뿐만 아니라 향상된 전기적 내구성과 열적 안정성을 확보하면서도 1 V 미만의 저전력 구동 특성을 유지하였다. 이러한 효과들에 대한 해석을 위해 라만, XPS 분광법을 통한 재료 내 결합 상태 분석과 광학 밴드갭 분석을 수행하였고, trap 상태에 해당하는 peak의 감소와 증가된 밴드갭이 누설 전류 억제와 약간의 임계 전압 증가에 기여하는 것으로 규명하였다. 6장에서는 비정질 OTS 칼코겐화합물에 대해 제일원리계산을 수행하여 계의 바닥상태 에너지, 전하 밀도 및 전자의 국소화, 상태밀도함수를 분석하였다. 먼저 실험적으로 OTS 동작이 확인된 조성에서의 비정질 구조는 빠르게 용융과 냉각을 반복하는 분자동역학 계산으로 모사하였고, RDF 계산을 통해 장주기 규칙이 사라진 것으로 비정질 상태가 잘 모사되었는지 확인하였다. 범밀도함수 이론을 통한 전하 밀도 계산에서는 SiTe 물질에 N 도입 시 전자가 N 원자들 주위에 ρmax 값이 8배 강하게 밀집되는 것을 확인하였으며, 이는 N의 Si, Te 대비 강한 전기음성도 값에 의한 것으로 해석하였다. 따라서, 재료 내 강하게 밀집된 전자 상태로 인해 N 도핑 시 재료를 통한 전도가 억제된 것으로 선택소자의 성능 향상을 해석하였다. 마지막으로 OTS 현상에 대한 이해를 도모하기 위해 강한 전기장이 있는 조건과 전기장이 가해지지 않은 조건에서의 SiTe 비정질 상과 결정상 각각에서 상태밀도함수를 비교하였으며, 그 결과로 trap 상태들이 비정질 칼코겐 원소들의 결함들에 기인한 것임을 확인하였고, 전기장이 있을 때 비정질 SiTe의 밴드갭이 사라진 높은 상태밀도함수를 통해 OTS on 상태에서 금속과 같은 높은 전도성을 보이는 것으로 비정질 칼코겐화합물의 OTS 현상을 해석하였다. 결론적으로, 본 학위논문에서는 비정질 SiTe 물질에 N 도핑을 통해 OTS 선택소자의 성능을 향상시켰고, 이를 전기적, 분광학적, 제일원리 계산 분석들을 통해 통합적으로 해석하였다. 본 논문에서 발표하는 우수한 성능의 N-SiTe OTS 물질로 말미암아 차세대 반도체 소자 개발뿐만 아니라 반도체 재료에 대한 이해의 확장성에 본 학위논문이 기여하기를 희망한다. The rapid growth of data-based industries, such as the artificial intelligence (AI) and self-driving vehicle industries, has impacted semiconductor industries via the increased demands for large data capacities and high computing speeds. Emerging memories like phase change memory (PCM) and resistive random-access memory (ReRAM) can not only bridge conventional memories of Von Neumann architecture but also extend to next computing architectures for those increased demands. Notably, emerging memories with two-terminal structures can be integrated into a cross-point architecture to maximize scalability. However, a selector device must be incorporated into the memory cell, as the array exhibits an inherent leakage current through sneak paths. Among the existing selectors, Ovonic threshold switching (OTS) chalcogenides are promising candidates owing to various advantages. Therefore, various amorphous chalcogenides have been widely investigated as OTS selectors that can satisfy requirements of an ideal selector, as the mechanism is only partially known. Among the explored chalcogenides, Te-based candidates generally operate in low-voltage ranges, yielding to low-power devices compared with those of Se- and S-based chalcogenides. In this Ph.D. dissertation aimed at gaining deeper insights into the OTS phenomenon, strategic binary silicon telluride (SiTe) based amorphous OTS chalcogenides were investigated to determine the effects of systematically doping SiTe with nitrogen (N–SiTe) on enhancing electrical performance of the selector for the first time. N-SiTe exhibited several advantages, such as suppressing the leakage current of 18 nA and enhancing electrical and thermal stabilities (106 pulse endurance and 400 ℃ annealing, respectively) via its N content as well as ensuring low-power operation below 1 V of threshold voltage (Vth) and on current density above 1.89 MA/cm2 via its Te content. Furthermore, material analysis including spectroscopic, energy state, and computational analysis enhanced understandings of the OTS phenomenon and offered insights into exploring and developing material candidates for OTS selectors. Decreased trap states which act as electrons hopping center, increased activation/bandgap energies, and electron localization were observed by doping nitrogen (N) on SiTe that were revealed to the origin of enhanced selector performances. In Chapter 3, the effects of chemical composition on SiTe OTS selector devices were investigated to determine the optimal stoichiometry which was Si47Te53. The Te-rich compositions ensured low-power operation and sufficient driving current with compared to Si-rich compositions. However, they failed to sufficiently suppress the leakage current. This might be due to the abundant trap states in Te-rich compositions, as revealed by Te 3d spectra of X-ray photoelectron spectroscopy (XPS) and Poole-Frenkel trap state analysis. The results confirmed that the slightly Te-rich composition of SiTe selector devices suppressed the leakage current and ensured the high driving-current for memory cells in cross-point architectures. In Chapter 4, the subthreshold regions of selector devices were investigated to achieve a deeper material analysis of the amorphous chalcogenides using trap states analysis based on electrical behaviors at different temperatures. Here, the off current (Ioff) was effectively lowered from 29 μA to 1.3 μA, thereby improving selectivity from 34 to 770 by doping amorphous SiTe with N. Furthermore, the effect of N doping was analyzed by using direct current-voltage (DC I-V) curves at various temperatures to derive trap-state parameters. Employing the Poole-Frenkel equations to the DC I-V curves, the subthreshold current was controlled by electron hopping through the trap states, increased trap spacing (Δz), decreased trap density (NT,tot), and increased activation energy (EA) owing to N doping. Decreased trap states NT,tot: 1.286 and 5.112 × 1020 traps/cm3, Δz: 1.98 and 2.69 nm, and EA: 0.15 and 0.33 eV of SiTe and N-SiTe, respectively, effectively explained the suppressed Ioff and increased Vth of the N-SiTe selector device. In Chapter 5, the improved performances and stabilities of the N-SiTe OTS selectors were demonstrated. Additionally, the origin of the effect of N doping was investigated by Raman spectroscopy, XPS, and optical bandgap analyses. The selector with N-SiTe exhibited potentially faster response of 5 ns with lower off-current of 18 nA, faster switching transitions of < 2.97 mV/dec, and twice larger read margins as well as shorter off-transition times of 15 ns compared to SiTe while maintaining low power states of < 1 V. The effective trap concentration of N-SiTe was reduced to lower electron hopping through traps in subthreshold ranges, and the increased optical bandgap (Eopt) by N doping correlated well with suppressed off current and increased threshold voltage. Furthermore, N-SiTe selectors had enhanced thermal stability up to 400 ℃ for back-end-of-line compatibility and electrical stability up to 106 pulse endurance for long-term reliability by stabilized amorphous states by N doping. In Chapter 6, the origin of the amorphous OTS chalcogenide was elucidated via first-principles observation of its electronic structure, such as the system energy, charge density, and density of states (DoS). The amorphous phase mimicking the SiTe and N-SiTe chalcogenides with experimentally observed OTS phenomena was verified based on the long-range order with the radial distribution function (RDF). By calculating the charge densities on N-SiTe, the 8 times higher ρmax localized electronic states than SiTe observed in the Si-N bond, especially near N atoms in first-principles calculations. These localized electrons were attributed to high electronegativity of N, and considered the source of the N-doping enhanced performance of the OTS chalcogenides. By calculating the density of states of amorphous and crystalline SiTe structures showed that bandgap tails were originated from Te lone pairs and VAPs, while midgap trap states were originated from dangling bonds of Te atoms in the amorphous phase and local structures. Finally, electrons in trap states of the midgap and band tails were excited resulting in DoS increment and closed mobility gap under electric field only in amorphous chalcogenides, and this were originated from delocalization of lone pair electrons in Te 5p resulting in metallic conduction after the threshold switching (OTS ON state). In conclusion, enhanced OTS selector performances by N doping on amorphous SiTe chalcogenides were studied in this dissertation with systematic approaches covering various aspects of electrical characteristics, spectroscopic analysis, and first-principles computational analysis. It is anticipated that the novel N-SiTe enhanced OTS chalcogenides for selector devices would extend the material candidates for sustainable, environmentally friendly, and low-power devices in the era of big data and AI.

Some ternary and binary chalcogenides : KYSe_(2), EuY_(2)Se_(4), SmSe_(2-x)

차송이 이화여자대학교 대학원 1997 국내석사

Deposition and characterization of threshold-switching Ge-Sb-Bi-Te and Si-As-Te thin films for the application of next-generation non-volatile memory

이종호 서울대학교 대학원 2012 국내박사

Threshold-switching of chalcogenide – abrupt decrease of resistance when the applied electric field exceeds a critical value – has attracted wide attention since its discovery and enabled unique application for non-volatile memory. For the threshold switching, group VI elements such as Se or Te primarily constitute the chalcogenide alloy, but Se or Te alone cannot be used for non-volatile memory because these elements easily crystallizes at room temperature. To enhance the stability of the amorphous phase, group IV and V elements are added to cross-link the atomic network within amorphous phase. In this thesis, group IV and V elements are varied for different application of chalcogenides; Ge, Sb, and Bi for phase change memory and Si and As for threshold-switch application. Bi is added to Ge2Sb2Te5 – the most intensely studied material for phase change memory – by cosputtering Bi2Te3 and Ge2Sb2Te5 compound targets to incorporate Bi atoms in Ge/Sb sites of Ge2Sb2Te5. This study revealed that incorporated Bi increased the crystallization speed of both amorphous and liquid phase. This was attributed to the decrease of the activation energy of crystallization and reduction of the thermal conductivity by doping Bi to Ge2Sb2Te5. It is suggested that Bi-doped Ge2Sb2Te5 shows a guideline for the development of future phase change memory; thermal engineering of phase change material by substitutional doping. Amorphous Si-As-Te thin films were sputter-deposited to make threshold switch for selector devices of crossbar array of resistive switching memory. Bi-directional flow of current is possible in threshold switch, which is required for selector device of bipolar resistive switching memory. Instead of Ge and Sb used for phase-changing chalcogenides, Si and As are used for threshold-switch application because bonding enthalpy of Si-Te and As-Te are larger than that of Ge-Te and Sb-Te (bonding enthalpy: Si-Te 38.5 kcal/mol, As-Te 32.7 kcal/mol, Ge-Te 35.5 kcal/mol, and Sb-Te 31.6 kcal/mol). Higher thermal stability of Si-As-Te enabled the construction of stable threshold switch; endurance test using 200 ns-long pulse showed little degradation of the device performance after 1,000 times of ON-OFF switching. Suppression of the leakage current by serial connection of TiO2 unipolar memory and Si-As-Te threshold-switching thin film was observed, which indicates that Si-As-Te threshold switch is a promising candidate for selector devices of crossbar array of resistive switching memory.

Metal chalcogenide nanostructures : materials synthesis and applications for energy/transistor devices

Zhang, Fangfang Sungkyunkwan University 2022 국내박사

Metal chalcogenides have attracted significant attention due to their excellent physical and chemical properties. Until now, numerous efforts have been devoted to the synthesis of metal chalcogenides and their potential applications in many research fields, such as energy storage and conversion devices, (opto)electronics, etc. However, the performances of metal chalcogenides-based devices such as electrochemical capacitance of supercapacitors (SCs), hydrogen evolution reaction (HER) activity of electrocatalysts, or mobility of thin-film transistors (TFTs) need further improvement. Metal sulfides such as CoS, MnS, NiS, etc, have been proven excellent candidates for energy storage and conversion devices because of their good conductivity and chemical stability. Compared with single metal sulfides, bi-metal sulfides-based electrodes show the enhanced electrochemical capacitance for SCs because of the synergetic effect of bi-metal. Taking the advantages of cobalt ion, manganese ion, and nanoparticles (NPs) morphology, I synthesized manganese cobalt sulfide NPs by a hydrothermal method and explored the electrochemical performance. Otherwise, to enhance the catalytic behavior of NiS, I used a cost-effective method to synthesize NiS nanorods (NRs)/graphene (Gr) heterostructure on a three-dimensional (3D) substrate that can offer more surface active sites and improve the structural stability. As a result, the synthesized NiS NRs/Gr presented superior catalytic activity and cycling stability in both acidic and alkaline media. Metal tellurides are considered promising candidates for energy storage and conversion systems because of their superior metallic conductivity. However, the construction of metal tellurides as a bifunctional material for SCs and HER electrocatalysts remains challenging. As reported recently, materials with hollow structures show more advantages for energy storage and conversion systems because the unique morphology can boost the carrier charge transfer. Hence, I attempted to synthesize the hollow nickel telluride (NiTe2) with a simple strategy and employed it as a bifunctional material for SCs and HER. Nowadays, developing high-performance TFTs based on metal tellurides is a hot research topic. However, the poor electric performance of metal tellurides-based TFTs limited their commercial application. Based on the recent reports, long nanowire shows great potential in optimizing the electric property, because of the large length-to-diameter aspect ratio, high transmittance, good conductivity and excellent mechanical compliancy. Hence, I used a facile method to fabricate high-quality and uniform tellurium (Te) and metal tellurides nanowires and further investigated their electric performance for TFTs.

I-Ⅲ-Ⅳ2 박막 태양전지를 위한 Chalcogenide 나노물질의 제조 및 특성

손영석 계명대학교 대학원 2010 국내석사

화합물 태양전지는 차세대 태양전지 중 가장 높은 변환효율을 가지고 있다. 그리고 태양전지에서 흡수층으로서 역할을 하는 chalcogenide계는 Co-evaporation, sputtering 등의 건식법을 통해 기판위에 박막을 증착시켜왔다. 고가의 건식법을 재제하고 습식법을 통하여 CuInTe2 나노화합물을 제조 하고 제조된 나노화합물을 코팅방법을 이용하여 기판위에 박막을 형성하는 연구를 하였다. copper(II) chloride dihydrate(CuCl2․2H2O), indium(III) chloride tetrahydrate(InCl3․4H2O), tellurium(Te)을 출발물질로 하여 용액열 합성법으로 나노 크기의 CuInTe2 분말을 제조하였다. 반응 조건은 교반시 120℃, 반응 온도 260℃, 반응시간 24시간, 비율 Cu:In:Te=1:1:2의 최적 조건을 얻었다. 합성된 CuInTe2의 특성을 분석하기 위하여 XRD, SEM, UVDRS, EPMA를 측정하였다. 그리고 UVDRS 결과에서 가시광영역뿐만아니라 적외선 영역까지 광범위한 빛을 흡수하고 있을을 확인할 수 있었다. 또한 합성된 CuInTe2 나노분말을 기판위에 박막을 형성하기 위해 전착법과 paste coating법을 사용하였으며, paste coating법으로 박막을 형성하였을때 균일한 박막을 얻을 수 있었다. Compound solar cell is the highest convertion efficiency among next generation solar cells. Chalcogenide structure materials which is a part as a absorbance layer in solar cell have been coated by dry coating process such as a Co-evaporation and sputtering. In this study, CuInTe2 nanoparticles were experimented about preparing and coating on a substrate, except an expensive dry process, through a wet process. CuInTe2 nanoparticles have been prepared by a solvothemal method using copper(II) chloride dihydrate(CuCl2․2H2O), indium(III) chloride tetrahydrate(InCl3․4H2O), tellurium(Te) as a starting materials. In this experiment, we could find optimum condition of a reaction with stirring temperature 120℃, reaction temperature 260℃ and component ratio(Cu:In:Te=1:1:2). CuInTe2 nanoparticles were characterized using XRD, SEM, UVDRS and EPMA. In UVDRS results, CuInTe2 nanoparticles absorbed a wide region which is not only visible region but also infra-red. CuInTe2 nanopaticles formed thin layer on a substrate using electrochemical deposition method and paste coating method, and using paste coating method, CuInTe2 nanoparticles were formed uniform thin layer.

Growth, Characterization and Assembly of Lead Chalcogenide Nanosemiconductors for Ionizing Radiation Sensors

Davis, Brandon J ProQuest Dissertations & Theses University of Mich 2022 해외박사(DDOD)

The demand for precise and efficient nuclear radiation detection has increased dramatically, particularly in numerous scientific disciplines, as well as homeland security and medical imaging applications. Modern methods by which ionizing radiation was sensed has now heavily considered using nano-scale materials for the sake of effective counting. Nanocrystalline (NC), or quantum dot (QD), semiconductors themselves exhibit exploitable properties—such as tunable energy band gap and charge carrier multiplication (multi-exciton generation) which arise due to strong quantum confinement. With this, fabricating a quantum-dot-based semiconductor to operate as a high-performance detector, via a low-cost solution-based manufacturing method, can truly alter the capabilities of radiation detectors. Using this NC approach which primarily focuses on high atomic number and density materials, was investigated as a means to maximize charge creation while minimizing the uncertainty in that conversion, as the approach is based on favorable features of NC materials for their application to the detection of ionizing radiation. The intrinsically high charge mobility combined with high atomic number and density of the lead chalcogenides makes them attractive for sensing applications with highly penetrating quanta, such as x-rays and gamma-rays, as the lead chalcogenide materials possesses an extensive literature of synthetic routes with which one can explore the strong confinement regime in quantum dots. By varying the reaction conditions, NCs of various sizes and shapes were synthesized, and their physical and opto-electric properties were investigated. Drop-, float-, or dip-coating NC dispersions on various metal contacts resulted in close-packed NC assemblies of lead chalcogenides. However, in sensing architectures, the exploitation of various properties for each individual nanocrystallite (NC) is hampered by the need to transport the charge carriers throughout the active volume, a motion that can be retarded by energetic surface barriers typically in the form of insulating oxides. Various synthetic routes are investigated to fabricate lead chalcogenide QDs while the feasibility of utilizing NC materials as a basis for detecting ionizing radiation is also explored. QDs and their assembled structures were carefully investigated through characterization to determine their overall quality. Methods to improve NC interconnectivity were studied and mentioned. The prevention of surface oxidation through the fabrication of NCs was also explored, resulting in chemically and optically stable NCs for at least 1.6 years. Overall, this study focuses on using solution-based methods to fabricate nano-semiconductor nuclear radiation detectors. Various recipes will be presented, as well as the electrical results of developed NC assembly samples, with the focus on improving the charge carrier transport properties of these NC assemblies.