三五族半導體微米捲管及其光電與熱電效應研究

Abstract

本實驗樣品以分子束磊晶技術成長，能精確控制樣品結構的品質、厚度和材料，我們幾近完美地控制三五族半導體微米捲管的製作，其形成始於將二種不同晶格常數的材料薄膜接續成長於同一基板上，若將雙層薄膜與基板分離，那麼應變鬆弛而形成3D半導體微米捲管。 先討論應變薄膜厚度對微米捲管曲率的影響，並由拉曼光譜的聲子位移量與量子井的發光特性，計算出捲管內的殘留應變量，與理論值模擬捲管內殘留應變的分佈甚吻合，我們更發現捲管能增強數倍的量子井發光強度，且能避免樣品氧化，也能當作一種簡易製作TEM樣品的方法。 我們成功製作出有效的傳導性微米捲管元件，於雙層應變薄膜參雜高濃度的雜質時，提升了至少五倍以上的導電率。使用聚焦的雷射光，於微米捲管元件一端局部照光，發現了類似太陽能電池的光伏特效應，將光能轉換成電能，最高的光電轉換效率為1.1E-4 %，仍具有很大的改善空間。 此外，微米捲管具有極差的導熱性，將是一種極具潛力的熱電材料，我們利用加熱探針對元件作局部加熱，使捲管兩端有溫度差而產生熱電效應，大略保守估計懸空捲管的熱電效率，最好的ZT值為0.41，實際上應該遠超過這個值。我們相信不久的將來一定能有效提升微米捲管的熱電轉換效率，以實際應用半導體3D微米捲管微區域的致冷器。

The samples in our research are grown by Molecular Beam Epitaxy (MBE), which can accurately control the property, thickness, and composition of the samples. The group Ⅲ-Ⅴsemiconductor rolled-up micro-tubes are formed by epitaxial growth of the two material layers with different lattice constants on a GaAs substrate. When the strained planar bilayers are released from the substrate by selective etching, they will roll up into a 3D micro-tube. We can almost perfectly fabricate the rolled-up micro-tubes. The diameter of rolled-up micro-tubes depends on the thickness of the strained bilayers and on the built-in strain. We can detect the residual strain of the micro-tubes through the peak-shift of LO phonon frequencies in Micro-Raman spectra as well as the peak-shift of quantum well (QW) in PL spectra in comparison with the tubes and the unreleased areas. The experimental results coincide with a simple elastic model. Moreover, the rolled-up micro-tubes can enhance the luminous intensity of QW and prevent the samples from oxidizing. We had successfully fabricated the devices of conducting InGaAs/GaAs rolled-up micro-tubes. The electrical conductivity of the devices can be increased by at least five times when the strain bilayers are doped with high impurity concentrations. The micro-tubes devices are locally illuminated by a focused laser beam, and can convert the energy of light directly into electricity by the photovoltaic effect just like solar cells. The best energy conversion efficiency is 1.1E-4%, and the devices still have some rooms for improvement. Last but not least, the rolled-up micro-tubes would be a great potential for thermoelectric materials because of their poor thermal conductivity. The micro-tubes devices are locally heated by a hot probe station, and can convert the temperature difference to the electric voltage by the thermoelectric effect. The best thermoelectric figure of merit (ZT) is 0.41 under a conservative estimation, and actually it is much higher than this value. We believe the microtubes devices will show higher thermoelectric conversion efficiency in the near future.