Semiconductor quantum dots (QDs) as a quantum emitter can generate single photons which can deliver quantum information. In particular, III-N semiconductor QDs have potential advantages of scalability and current-driven operation at room temperature because of their small size, p-type doping and large valence band offset energy. Despite the advantages, QDs still struggle in achieving unity extraction efficiency, which is a prerequisite for their applications. For example, Stranski-Krastanov growth mode QDs grown in planar structures have difficulties in extracting light due to total internal reflection. Even though epitaxially grown three dimensional pyramidal structures exhibit the increase in light extraction, it is still required to have higher efficiency to applications. In this thesis, two approaches based on finite-difference time-domain (FDTD) simulations are proposed as upper and lower side light extraction.
First, upward light extraction is enhanced by using an immersion lens. An immersion lens on top of site-controlled QDs, fabricated by two photon polymerization, extracts and focuses emitted light from QDs. FDTD simulation predicts 8 times increase a light intensity and 3 times increase in light extraction efficiency at 450 nm emission wavelength and numerical aperture of 0.5.
Second, downward light extraction is enhanced by coupling between the source and an optical fiber with a metal mirror. A metal layer along pyramidal QDs directs the emitted light downwards. A dielectric layer between the metal layer and pyramids is introduced to reduce metal absorption. The emitted light of 450 nm wavelength is then coupled with an 800 nm diameter tapered optical fiber; FDTD simulation predicts a single mode coupling efficiency of 48 %.
FDTD simulation predicts that extraction efficiency of QDs on pyramids in upper and lower direction is improved. Its realization for a site-controlled III-nitride pyramidal QDs is expected to have a potential application in quantum communication, quantum cryptography, .and quantum simulation.
질화물 반도체 양자점은 그 작은 크기와, 전류 및 상온 구동가능성 덕분에, 향후 범용적으로 양자 암호 혹은 양자 정보 통신 등에 사용할 수 있다. 단일광자광원으로서 양자점은 그 응용을 위하여 높은 광 추출효율을 요구한다. 본 논문에서는 위치 조절 가능한 양자점에서 위, 아래로 추출하는 2가지 방법을 통해 광추출 효율을 높이고자 한다. 위로 추출하는 방법은 굴절률이 낮은 고분자를 이광자 중합을 이용해 마이크로 렌즈를 양자점 위에 형성하여, 더 많은 빛을 추출하며 빛의 방향성을 조절하는 방법이다. 유한요소시간영역법을 이용하여 시뮬레이션한 결과 파장이 450 nm 이며, NA가 0.5일 때 8배의 추출 효율이 향상되었다. 아래로 추출하는 방법은 금속을 거울로 이용하여 광섬유와 커플링을 하는 방법이다. 시뮬레이션 결과로 800 nm 지름의 광섬유에 60% 이상의 빛이 기본모드로 전파되었다.
