電壓調控同調聲學聲子研究

近幾年來，在氮化鎵材料內所產生的同調聲學聲子已經被廣泛的討論。利用氮化銦鎵/氮化鎵為結構的多重量子井所具有的壓電電場，我們可以將此量子井視為一同調聲子之光壓轉換器，結合飛秒雷射的超快技術，在材料中產生奈米超音波。當雷射光在材料內激發載子，由於內部壓電電場的原因，會使得載子的產生在空間中被分開來形成和內部壓電電場方向相反的電場。進而對量子井內產生屏蔽效應。這種效應在壓電材料中就會對晶格產生應力。當材料如多重量子井做週期排列時，應力會形成週期性振盪的音波。其振盪的週期則決定於多重量子井內的成長週期。藉此，由光轉換所產生的音波頻率可以達到兆赫等級，也就是波長為奈米尺寸的音波。這樣的奈米音波在超音波的技術理論下，提供了奈米尺度的影像解析和研究發展。同樣的，當音波通過光壓轉換器，也就是多重量子井時，音波的應力會影響內部的壓電電場。這種電場的改變會造成材料對光吸收的特性變化。因此我們可以利用由音波調變光吸收的方式來偵測由量子井所產生的高頻音波。假設所使用的偵側光波長，接近材料本身的吸收能帶邊緣時，音波的對偵測光的調變影響就會相當劇烈。另一方面，偵測音波的頻率由多重量子井的週期結構決定，因此音波的產生源亦被視為用以觀察高頻音波的光電轉換器。在過去的研究可以發現，同調聲子由光所偵測的穿透調變大小決定於內部的壓電電場大小和載子準費米能階之位置。假設我們可以運用外加電壓的方式改變量子井內部電場的大小和能階位置時，就可以在不改變光的波長能量和實驗架構下，操控音波的偵測行為。在本篇論文中，我們首度的運用電的調控方式，改變了音波在壓電轉換器內對光的偵測行為。實驗結果發現，在運用氮化鎵為結構之發光二極體內的十層多重量子井做為奈米音波的光壓產生器時，我們可以產生和偵測到頻率約在0.54兆赫的高頻音波。音波在通過壓電材料時會改變內部的能帶進而調變電子和電洞的載子分布。透過調變載子的分布情形，對偵測光產生調變。因此，當用電壓改變電子電洞的準費米能階時，也就是載子分布的位置時，由音波通過量子井內所引起的穿透光的調變大小會有明顯的改變。這個研究結果成功的建立了氮化鎵多重量子井的壓電材料在電的調控下所改變的音波偵測行為，希望未來運用外加電壓的方式能夠在奈米超音波的影像技術上，提供更高的解析度、雜訊比，和進一步的應用。

關鍵字

同調聲子；飛秒雷射；壓電材料；發光二極體；奈米超音波

並列摘要

Coherent acoustic phonons (CAPs) in GaN-based materials have been widely discussed in recent years. Due to the large strain-induced piezoelectric fields in InGaN/GaN multiple-quantum-wells (MQWs), we can generate CAPs in MQWs using femtosecond laser techniques. The photo-induced carriers generated by femtosecond pulse excitation spatially separate and screen the piezoelectric field in the well region. This screening effect induces the lattice vibration and produces the longitudinal CAPs. The oscillation frequency in the terahertz (THz) region is decided by the periods of MQWs. When the acoustic waves propagate through piezoelectric materials, the strain affects the piezoelectric fields and thus modulates the optical absorption. The longitudinal CAPs induce a large absorption variation while the optical probe was near the energy gap of the materials. Therefore, the MQWs can be treated as an optical piezoelectric transducer (OPT) to detect the THz CAPs. For the optically detected oscillations of CAPs in MQWs, the sensitivity for CAP detection strongly depended on the piezoelectric field and the quasi-Fermi levels. As a result, if we change the piezoelectric field by external voltages, the detected oscillations of CAPs in MQWs can be altered by using electrical control while optical excitation condition remains the same. In this thesis, the author investigates the electrical control of the optical sensitivity for CAP detection by applying reverse voltages across InGaN/GaN MQWs light emitting diodes (LEDs). The author took 10-period MQWs in GaN-based p-i-n structure as the CAP source and detector simultaneously. The frequency of the CAPs generated in the MQWs was 0.54THz with well/barrier width 3/12nm. Through the piezoelectric effect, the CAPs modified the energy of subbands and thus modulated the transmission changes. By electrically controlling the quasi-Fermi levels, we can observe a large decrease of the strain-induced transmission modulation in the MQWs. As applying reverse bias voltages on the LEDs, the sensitivity for CAP detection drastically decreased and the CAP detection was possibly turned off.

並列關鍵字

Coherent Acoustic Phonons ； Femtosecond Laser ； Piezoelectric Materials ； Light Emitting Diodes ； Nano-Ultrasonics

參考文獻

[1.1] C. Kittel, Introduction to Solid State Physics. 8th edition. New York: Wiley. (2005).

[1.2] Neil W. Ashcroft, and N. David Mermin, Solid State Physics. Harcourt. (1976).

[1.3] J. Zucker and E.M. Conwell, “The recombination of hot carriers in germanium-experimental”, J. Phys. Chem. Solids 22, 141 (1961).

[1.4] J. Shah and R. C. C. Leitz, “Radiative Recombination from Photoexcited Hot Carriers in GaAs”, Phys. Rev. Lett. 22,1304 (1969).

[1.5] C. H. Yang, Jean M. Carlson-Swindle S. A. Lyon, and J. M. Worlock, “Hot-Electron Relaxation in GaAs Quantum Wells”, Phys. Rev. Lett. 55, 2359 (1985).

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電壓調控同調聲學聲子研究

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