Ga(In)As 기반 박막과 InAs 나노세선의 THz 응용 연구 = THz Applications of Ga(In)As-based Epitaxial Layers and InAs Nanowires

다국어 초록 (Multilingual Abstract)

In this thesis, the terahertz (THz) characteristics of Ga(In)As, InGaAs with periodically-positioned inserting InAlAs layer (InGaAs PPIL), and InAs nanowires (NWs) are discussed. Before describing THz properties of the epilayers and NWs, their structural, electrical, and optical properties are first demonstrated. And then, the THz generation and detection are mentioned by using transmitters and receivers with Ga(In)As, InGaAs PPIL, and InAs NWs.
The peak-to-peak current signal (PPCS) from a THz transmitter with the InGaAs PPIL showed three times higher than that of a simple InGaAs epilayer without the InAlAs insertion layer. Moreover, the detection properties of a THz receiver with the low-temperature grown (LT) InGaAs PPIL showed more than twenty-five times higher than that of the LT-InGaAs epilayer. In addition, the PPCS of the LT-InGaAs PPIL was significantly improved with increasing the periods of the InAlAs insertion layer.
The THz characteristics of InAs NWs formed on Si(111) were investigated by changing the structural dimensions and by simply dipping into normal tap water (DTW). The PPCS of InAs NWs obtained from THz-time domain spectroscopy was increased from 1.54 to 4.40 nA with increasing the height from 2.1 to 10 μm. For the DTW process, the InAs NWs were dipped into normal tap water for 1 minute and were dried at room temperature. For the 2.1 μm-long InAs NWs, there was no merging effect between adjacent NWs after the DTW process. However, top region of several InAs NWs with the heights of 4.6 and 5.8 μm was merged into conical bundle structures, respectively, via van der Waals force. For the InAs NWs with the height of 10 μm, the conical bundle structures again merged into bigger bundle features, where the length contributing to the bundle region was 4 μm. After the DTW process, the PPCS for the InAs NWs with the height of 10 μm was increased to 5.04 nA, which was 1.15 times higher than that of the as-grown InAs NWs (4.40 nA). The significant improvement in THz characteristics of InAs NWs by the DTW process can be explained by the reduction in carrier trapping at the surface states of the NWs due to the formation of bundle features.

국문 초록 (Abstract)

본 논문은 분자선증착기 (Molecular beam epitaxy)를 이용하여 Ga(In)As 박막과 InAlAs을 주기적으로 삽입한 InGaAs (InGaAs PPIL) 구조 및 InAs 나노세선 (Nanowires, NWs)을 성장하고 테라헤르츠 (Terahertz, THz) 응용한 연구결과를 보고한다.
InGaAs 박막과 비교하여 InGaAs PPIL 구조에서 THz 발생 및 검출 특성이 크게 향상되었다. InGaAs PPIL 구조를 갖는THz 발생소자의 전류신호 첨두값 (peak-to-peak current signal, PPCS)은 InGaAs 박막 구조만 갖는 경우와 비교하여 3배 이상 증가 하였다. 또한 저온에서 성장한 (low-temperature, LT) InGaAs PPIL 구조를 갖는 THz 검출소자는 단일 LT-InGaAs 박막과 비교하여 25배 이상 큰 전류신호를 얻을 수 있었다. 그리고 LT-InGaAs PPIL 구조를 갖는 THz 검출소자에서 InAlAs 삽입층 주기가 증가할수록 전류신호도 증가하였다.
THz 소자를 생물공학 (biological technology) 분야에 응용하기 위해서는 필요한 주파수 확보 및 신뢰할 수 있는 동작 특성이 요구된다. 본 논문에서는 Si(111) 기판에 성장한 InAs NW를 이용하여 0.5 ~ 2 THz 범위에서 THz 특성을 성공적으로 확보하였다. InAs NW의 길이가 증가할수록 THz PPCS크기가 증가하였다. 구체적으로 2.1 μm 길이를 갖는 InAs NW의 PPCS은 1.54 nA였으며 10 μm로 증가할 때 4.40 nA를 보였다. 또한 InAs NWs를 간단하게 수돗물에 담그는 과정 (Dipping into tap water, DTW) 을 통해 발생특성을 현저히 개선하였다. DTW과정 후, 2.1 μm 길이를 갖는 InAs NW는 구조적으로 특별한 변화가 없었다. 하지만 4.6 μm와 5.8 μm의 길이를 갖는 InAs NW는 van der Waals결합을 통해 NW 윗부분이 붙어 원뿔 모양의 다발 (bundle)을 형성하였다. 10 μm 길이를 갖는 InAs NW의 경우 원뿔 모양의 구조체들끼리 다시 뭉쳐 상대적으로 큰 다발 모양을 형성하였다. 특히, 10 μm 길이를 갖는 InAs NW는 bundle 형성 후, THz 발생특성에서PPCS가 5.04 nA로 측정되었으며 이는 DTW과정 전과 비교할 때 1.15배 증가한 수치이다. DTW 후 InAs NW의 THz 특성이 개선된 이유는 InAs NW가 다발구조를 형성하며 표면 준위에 의한 운반자 Trapping이 감소했기 때문이다.

목차 (Table of Contents)

• Contents i,
• Abstract (Korean) iii,
• Abstract (English) vi,
• Chapter 1. Introduction 1,
• References 6,
• Chapter 2. Theoretical backgrounds 10,
• 2.1. Importance of THz wave 13,
• 2.2. Methods of THz generation and detection 15,
• 2.3. Photoconductive antenna for THz applications 18,
• 2.4. Properties of low-temperature grown Ga(In)As 20,
• 2.4. Properties of catalyst-free InAs nanowire 25,
• References 28,
• Chapter 3. Experimental details 33,
• 3.1. Growth reactor; Molecular beam epitaxy 33,
• 3.2. Material characterization 35,
• 3.3. Device fabrication 38,
• 3.4. THz characterization 40,
• Chapter 4. Results and discussion 42,
• 4.1. THz characteristics of Ga(In)As-based epilayers 42,
• 4.1.1. Structural characteristics of InGaAs/InAlAs epilayers 47,
• 4.1.2. Electrical characteristics of InGaAs/InAlAs epilayers 50,
• 4.1.3. Optical characteristics of InGaAs/InAlAs epilayers 52,
• 4.1.4. THz characteristics of InGaAs epilayers 55,
• 4.1.5. THz generation of InGaAs/InAlAs epilayers 63,
• 4.1.6. THz detection of InGaAs/InAlAs epilayers 65,
• 4.1.7. THz imaging by using LT-GaAs 69,
• References 71,
• 4.2. THz characteristics of InAs nanowires 74,
• 4.2.1. Formation of InAs NWs under various growth conditions 75,
• 4.2.2. Structural characteristics of InAs NWs 83,
• 4.2.3. Electrical characteristics of InAs NWs 89,
• 4.2.4. THz generation of InAs NWs 93,
• 4.2.5. Improvement in THz characteristics of InAs NWs by
• simply dipping NWs into normal tap water 99,
• References 109,
• Chapter 5. Conclusions 112,