奈米音波合成與二維奈米影像建構

Abstract

由於光載子激發與壓電效應引發的大振福同調聲學聲子震盪，壓電半導體組成的量子井結構可視為一個光壓電換能器，配合超短脈衝的雷射光激發來產生與偵測聲波。本篇論文中我們利用具有強大應力壓電場的氮化銦鎵/氮化鎵單層量子井做為光壓電換能器，此單層量子井可視為聲波合成器來產生奈米波長的聲波，同時配合光學同調控制的方法，可以在一個固定的單層量子井結構上產生次兆赫波段可調頻的奈米聲波。透過飛秒雷射之穿透式激發探測法，我們實驗上量測到0.1兆赫至0.88兆赫的奈米聲波產生，並且藉由控制激發聲波的光強度大小，亦可以輕易控制所產生的聲波波形，這樣聲波在頻率與波形上的可調性對於各種聲波應用有相當大的幫助，而奈米等級波長的聲波產生更提供了奈米超音波研究的可能性。

在更進一步的研究中，我們發現利用光學同調控制從光壓電換能器中產生奈米聲波時，在短時間內的多重聲波脈衝產生會有飽和的現象。由於光壓電換能器產生聲波的主要機制為量子井內建壓電場的屏蔽效應，因此當量子井內帶電載子濃度夠高時，內建壓電場將會被完全屏蔽而無法再進一步產生聲波，我們利用一個帶電平行板模型來解釋此現象，並且透過這樣簡化後的模型，亦可以很方便地利用帶電載子濃度估計出量子井內的內建壓電場大小。另一方面，我們也利用暫態光反射量測研究了所產生奈米聲波在氮化鎵中傳播時的衰減生命期，實驗結果顯示0.5兆赫的聲波在氮化鎵中的等效生命期可達420皮秒以上，其所對應到的穿透深度約為3.4微米。

在研究過奈米聲波的產生與偵測之後，本篇論文的後半我們利用奈米聲波展示了二維奈米影像的建構，藉由奈米等級波長以及超音波技術非破壞性探測的優點，目標建構出一套奈米等級的影像系統。我們初步採用三個週期的氮化銦鎵/氮化鎵多重量子井來做為光壓電換能器，所產生的奈米聲波波長由量子井週期決定為14奈米，相當於約0.5兆赫的聲波。我們在待測物表面蝕刻出條紋狀之週期性奈米結構，藉由奈米聲波的產生與迴音的偵測，可以得到一維的高解析度深度資訊，再結合精密的橫向移動平台達到二奈米結構的探測。在我們系統中聲波半徑約為190奈米，而橫向的影像解析度即受限於產生聲波的範圍大小，實驗結果顯示我們可以清楚地解析出待測物的表面奈米結構，比照表面探測原子力顯微鏡的掃描結果，兩者量測結果相當吻合。

相較於一般高解析度結構量測技術，此項奈米超音波技術具有非破壞性以及材料內部奈米結構量測的優勢，結合可調頻及可控制波形的聲波波源，可以搭配現有超音波技術來提升訊雜比，提升系統準確度，未來在橫向解析度提升與三維奈米影像建構相當值得發展與進一步研究。

We have successfully developed a system to generate arbitrary-waveform nano-acoustic-waves (NAWs) with a piezoelectric InGaN/GaN single quantum well (SQW). Based on an optical coherent control technique, the piezoelectric SQW is regarded as an acoustic waveform synthesizer and acoustic frequency tunability in the sub-terahertz range is realized within only one fixed sample. By utilizing the transient transmission measurement, the detection of generated acoustic frequency varying from 0.1 THz to 0.88 THz has been demonstrated. It is also possible to manipulate the generated acoustic waveform by engineering the optical excitation intensities. This flexible system enabled further studies in nano-ultrasonics.

With NAWs composed of acoustic pulses generated from the piezoelectric SQW, a saturation phenomenon of acoustic generation was observed. The acoustic generation mechanisms, especially Coulomb screening of the piezoelectric field in SQW, were discussed with optical excitation power dependency. At high photo-excited carrier concentrations, the acoustic generation induced by screening of built-in piezoelectric field tends to be saturated. To analyze this phenomenon, a simplified charged-parallel-plate model was introduced and a 1.1 MV/cm built-in piezoelectric field can also be roughly estimated.

By means of transient reflectivity measurement, lifetime of acoustic waves in piezoelectric semiconductors can be estimated. With the generated NAWs propagating along the c-axis of a GaN thin film, the lifetime of the 500 GHz coherent longitudinal-acoustic phonons in GaN was measured to be >420 ps, corresponding to a penetration depth more than 3.4 micron.

In this thesis, we also performed 2-D nano-ultrasonic measurement with NAWs. We adopted a 3-period InGaN/GaN multiple-quantum-well (MQW) as our NAW source for a preliminary ultrasonic study with fixed single acoustic frequency of ~500 GHz. Based on 1-D nano-scan performed by acoustic generation and echo detection, we built a 2D image system for nano-ultrasonics. We took a GaN thin-layer etched with a striped pattern on the top surface for nano-ultrasonic demonstration. A 2D surface profile was clearly measured and an etching depth of ~35 nm can be resolved, which was in great agreement with the measurement of a commercial AFM. To make sure of the lateral resolution restricted by the spot size of NAWs, a resolving distance between two acoustic spots was measured to be ~190 nm in our system.