저차원 구조 실리콘의 열전 특성 연구 = The study of thermoelectric properties in low-dimensional silicon

다국어 초록 (Multilingual Abstract)

This study confirms a lot of potential and vision that the silicon based on the low-dimensional structure can be used for thermoelectric material. Therefore, there are two topics in this study; the study of electric and thermoelectric properties for silicon nanowires (SiNWs) and silicon/silicide hetero-junction thin film.
In the first study, The SiNWs were manufactured using top-down process and the width of SiNWs is set to 50 nm, which is smaller than the mean free path of the phonon (∼ 100 nm) in silicon and still larger than that of electron (∼ 5 nm) in order to control the heat transport by phonon. In other to consider scaling and temperature condition for thermoelectric properties, Seebeck coefficient (α) and power factors () of n- and p-type SiNWs were evaluated by varying the nanowire length (10 ㎛, 40 ㎛) and temperature (310 K- 450 K). Seebeck coeffieicent increased with the temperature and showed the maximum value in the special temperature range because of phonon drag effect. Phonon drag effect cause the increase of thermoelectric power because carriers gain the appropriate momentum and energy by scattering with phonon. Therefore, the results show the peaked Seebeck coefficients and power factors are 146.37 μV/K, , 113.83 μV/K, and -113.25 μV/K, for 10 μm, 40 μm long p-type SiNWs and 40 μm n-type SiNWs, respectively in specific temperature. Seebeck coefficient of the 40 μm SiNWs is lower than that of 10 μm SiNWs with the same temperature difference because the internal resistance of 40 μm SiNWs is about 40 ㏀, which is about 4-fold increase of the resistance in 10 μm SiNWs. In other to reach maximum thermoelectric properties of SiNWs, the internal resistances can be optimized by scaling consideration in SiNWs. Based on the results of this study, the results show silicon can be a valuable thermoelectric material in the low dimensional structures, such as nanowires in 350 K- 500 K range.
Recently, the specially designed silicon thin films (SiTFs) including a repeating hole patterns exhibited lower thermal conductivity and higher ZT value because the phonon scattering increased at interfaces. In the second study, the silicon/silicide hetero-junction thin film which was formed by silicide nanoscale pattern was manufactured. The structure have the obvious advantage of being able to reduce the thermal conductivity by scattering the phonon and the high energy carriers which penetrate the energy barrier between silicon and silicide can enhance Seebeck coefficient. Therefore, the figure of merit ZT value can be increased by this structure. In this study, the thermoelectric properties of silicon thin film with thin film size and doping concentrations were first evaluated for the optimized thermoelectric and electric properties of silicon thin film. As a results, when the doped silicon thin film has 20 ㎛ × 40 ㎛ size, power factor increased with temperature and the maximum value was at 450 K. Based on these results of the study, the size width and length of silicon/silicide thin film was set to 20 ㎛ × 40 ㎛, thickness was 70 nm and the implantation dose was . And then, the 70 nm diameter cobalt-silicide patterns having interval 140 nm were formed in silicon thin film. As a results, the maximum seebeck coefficient was 232.03 ㎶/K and power factor was which are higher than that of 70 nm silicon thin film. This is the first study to show that the silicon/silicide thin film can be used for thermoelectric material. Furthermore, the optimization of the silicon/silicide thin film for the maximum power factor and the analysis of thermal conductivity are necessary in other that the silicon/silicide thin film use for the alternative thermoelectric material instead of Bi2Te3.

국문 초록 (Abstract)

본 연구는 실리콘을 기반으로 한 저차원 구조가 열전소재로써의 발전가능성을 확인하기 위하여 크게 두 가지 주제로 진행되었다. 따라서 실리콘 나노와이어와 실리콘/실리사이드 이종접합 박막을 열전소재에 적용하고 전기적 및 열전특성을 비교분석하였다.
첫 번째 연구는 포논의 평균자유행로보다 작은 50 nm 선폭의 실리콘 나노와이어를 top-down 방식으로 제작하고 포논의 열전달을 제어하여 열전특성을 향상시키고자 하였다. 따라서 나노와이어의 10 μm와 60 μm 길이를 가지는 실리콘 나노와이어를 도핑종류별로 제작하고 310 K- 450 K 온도에 따르는 전기적 특성 및 열전특성에 대하여 연구를 진행하였다. 실리콘 나노와이어의 Seebeck 계수는 phonon drag 현상에 기인하여 특정온도구간에서 온도에 따라 증가하면서 최대값을 보였으나 이후의 온도에서 점차 감소하였다. 전자가 고온부에서 저온부로 이동하는 동안 포논과의 상호작용에 의하여 추가적인 에너지를 얻게 되므로 실리콘 나노와이어에서 350 K 이상까지 온도에 따라 열전특성이 점차 증가하여 p-타입 10 μm 나노와이어는 360 K부근에서 146.37 μV/K Seebeck 계수의 최대값을 보이고 p-타입 40 μm 나노와이어는 395 K에서 113.83 μV/K을, n 타입 40 μm 나노와이어는 440 K에서–113.25 μV/K 값을 보였다. 10 μm, 40 μm 길이의 실리콘 나노와이어의 열전특성을 비교한 결과, 40 μm 나노와이어의 저항이 10 μm 나노와이어의 저항의 약 4배 이상으로 내부저항은 약 60 kΩ이다. 즉, 전압강하가 발생하여 Seebeck 전압의 손실로 인하여 40 μm 나노와이어의 Seebeck 계수가 10 μm 나노와이어보다 낮았다. 본 연구 결과를 바탕으로 열전소자의 출력전압에 영향을 미치지 않도록 내부저항이 고려된다면 실리콘 나노와이어가 중고온 (350 K- 500 K) 영역에서 열전소재로 활용될 가능성을 제시하였다.
최근에 들어서 2차원 실리콘 구조에 포논이 산란할 수 있도록 나노 홀 (hole) 패턴 등 특수한 구조를 반복적으로 형성한 박막 구조를 적용하여 열전도도를 감소시키고 우수한 ZT 값이 보고되고 있는데, 두 번째 연구에서는 top-down 공정을 통하여 실리사이드 나노패턴을 주기적으로 형성된 실리콘/실리사이드 이종접합 박막을 제작하였다. 이는 실리사이드를 형성하여 포논의 산란을 발생시키고 열전달을 최소화 할 수 있는 구조인 동시에, 실리콘과 실리사이드 사이에 생기는 에너지 장벽으로 장벽을 통과한 높은 에너지를 가지는 캐리어가 Seebeck계수를 높여 최종적으로 열전성능지수 ZT 값을 향상시킬 수 있는 구조이다. 따라서 본 연구에서는 먼저 실리콘 박막의 열전특성을 최적화하고자 박막의 크기 및 도핑농도에 다른 열전특성을 분석한 결과, 으로 도핑 된 실리콘 박막이 20 ㎛ × 40 ㎛ 크기를 가질 때, n과 p-타입의 열전특성이 가장 높았으며 파워펙터는 온도에 따라 증가하여 450 K에서 약 를 가졌다. 20 ㎛ × 40 ㎛ 크기를 가지고 70 nm 두께를 가지는 실리콘 박막에 지름 70 nm 코발트-실리사이드 패턴을 140 nm 간격으로 형성하고 전기적 특성 및 열전특성을 분석한 결과, 기존 70 nm 실리콘 박막의 Seebeck계수와 파워펙터보다 상승하여, Seebeck계수 232.03 ㎶/K과 Power factor , 최대값을 보였다. 이는 실리콘/실리사이드 이종접합 박막이 열전소재로서 적용될 수 있다는 가능성을 보여주는 최초 결과이다. 이 결과를 바탕으로 실리사이드의 구조를 최적화하여 파워펙터를 향상 시키고 포논의 열전달을 감소시키는 실리콘/실리사이드 이종접합 구조의 정확한 열전도도 분석을 통하여 Bi2Te3 열전소재를 대체할 수 있는 실리콘 박막의 가이드라인을 제시할 수 있다.

목차 (Table of Contents)

• 국문초록
• Abstract
• Ⅰ. 서 론··············································1
• 1. 연구의 목적··············································1
• 2. 열전소자 연구 동향·····································4
• Ⅱ. 이론적 배경···········································8
• 1. 열전변환 이론···········································8
• 1.1 Seebeck 효과········································10
• 1.2 Peltier 효과··········································11
• 1.3 Thomson 효과······································12
• 2. 열전성능지수의 정의····································14
• 2.1 열전발전효율과 성능지수 (ZT)···················14
• 2.2 열전냉동효율과 성능지수 (COP)·················16
• 3. 열전성능의 최적화······································19
• 3.1 Power factor ()의 향상························19
• 3.2 열전도도의 최소화··································21
• Ⅲ.본 론 Ⅰ··············································· 31
• 1.실리콘 나노와이어 열전소자 제작····················33
• 1.1 실리콘 나노와이어 열전소자 구조설계··········33
• 1.2 반도체 공정기반 소자 제작·······················34
• 2. 실리콘 나노와이어 열전소자 특성 평가·············38
• 2.1 실리콘 나노와이어 열전소자의 열전특성 평가 방법·······················································38
• 2.2 Seebeck 계수 측정 장비 및 평가 프로그램····40
• 3. 실리콘 나노와이어의 열전특성 결과 및 고찰·······44
• 3.1. 실리콘 나노와이어의 열전특성 평가 목적······44
• 3.2. 실리콘 나노와이어의 열전특성 결과 및 고찰··45
• 3.2.1. 온도에 따른 실리콘 나노와이어의 열전특성·45
• 3.2.2. 실리콘 나노와이어의 길이에 따른 열전특성·47
• 4. 결 론······················································56
• Ⅳ.본 론 Ⅱ···············································58
• 1. 실리콘/실리사이드 이종접합 박막 열전소자 제작··59
• 1.1. 실리콘/실리사이드 이종접합 박막 열전소자 구조설계··················································59
• 1.2. 반도체 공정기반 소자제작························59
• 2. 실리콘/실리사이드 이종접합 박막의 열전특성 평가··························································65
• 2.1. 실리콘/실리사이드 이종접합 박막의 열전특성 평가 방법·················································65
• 2.2. 실리콘/실리사이드 이종접합 박막의 열전특성 측정 시스템··············································66
• 3. 실리콘/실리사이드 이종접합 박막의 열전성능······67
• 3.1. 실리콘/실리사이드 이종접합 박막 열전특성 결과 및 고찰·············································67
• 3.1.1.
• 실리콘/실리사이드 이종접합 박막 열전특성 평가조건···········································67
• 3.1.2.
• 박막크기에 따른 실리콘 박막의 열전특성····68
• 3.1.3.
• 실리콘/실리사이드 이종접합 박막의 열전특성···················································69
• 4. 결 론······················································85
• Ⅴ.결 론···················································87
• 참고문헌···············································89
• 연구실적···············································95