新穎矽基奈米鐘乳石結構於光熱電轉換效率提升之研究

Abstract

本研究中，驗證及解釋奈米矽在光熱效應上之現象以及在單一PN接面的光二極體上奈米矽之光熱電行為。我們利用不同製程的N型和P型半導體製作奈米鐘乳石結構，在不同摻雜濃度及快速熱退火之不同製程去探討矽的光熱效應而不是單純的光電效應。由於矽屬於非直接能矽，塊材矽在光照下只能由光子所激發的載子產生，即使加高壓此載子產生的光電流較微弱，此現象意味著吸收的光子能量在光熱效應上扮演著重大的角色。根據我們的實驗結果，重摻雜的P型奈米鐘乳石結構在強光下可使自身溫度提升至400度，同時光學上反射率可達到最低值3%。製程上，我們利用不同蝕刻時間以及矽基板來探討濕蝕刻機制，由於P型半導體比N型半導體產生較多的電洞濃度，有利於電化學濕蝕刻的進行，同時，由有效介質理論可知奈米鐘乳石結構能使入射光產生的折射率逐漸增加，在光學上可產生極低的反射率，此現象有利於光熱轉換，使奈米矽的升溫速率達到最高。 傳統的光熱電材料大多由鉍或碲等貴重金屬為主要材料，提升材料之熱電優值為改善光熱電效率之首要條件。本實驗中，我們設計具有奈米鐘乳石結構的單一PN接面之光二極體去提升光熱電轉換效率，為了避免熱傳導、熱對流以及熱輻射對試片的影響，光強度至少設定在0.34至0.44瓦之間。若光強度設定超過0.44瓦，因光熱電效應影響而產生熱載子輸出的電流，我們可藉由在連續照光下所測得的電流以及光反覆開關而沒有熱產生測得的電流之差值來驗證光熱電效應。根據實驗結果，具有奈米結構的光二極體所產生的熱電流為無結構元件的102倍之大，除此之外，在相同光強度下有奈米結構的光二極體所產生的總電流也比無結構光二極體的總電流還大。由於光有效被矽奈米鐘乳石結構吸收而導致聲子大量產生，強烈的光熱轉換可使聲子間強烈的擴散而使熱快速地傳導，因此，我們可利用P系列之半導體來達到良好的光熱以及光熱電特性，可將其結構視為一個完美的光及熱吸收器。

In this study, photothermal effect in nanostructured silicon and enhanced photothermoelectric phenomenon by silicon nanostalacties in simple p-n junction based photodiode are investigated and demonstrated. We prepared nanostalactites based on different process conditions of wafer type (n-type or p-type wafers), surface concentration change (external implantation), the rapid thermal annealing of 950 ℃/10 sec or not to verify the photothermal effect in Si wafer, except optical-to-electrical conversion. Because there is no optical detecting area in simple Si wafer and the indirect energy bandgap, the photo generation carriers can be excited as Si wafer under illumination. The effect photo current is still insignificant, even though device is under larger biasing voltage, implying the photothermal effect may play an important role for relaxation of absorbed light energy. Based on our experimental results, the heavily-doped p-type nanostalactites provides high temperature of ~ 400 ℃ and the lowest optical reflectance of 3 % simultaneously. The rapid etching mechanism through different etching time and silicon surface is also discussed. The better nanostalactites can be formed in p-type wafer instead of n-type wafer because of more hole concentration generated by the electro-less etching mechanism. Meanwhile, the nanostalactites provide ultra-low optical reflectance because of the existence of gradually refraction index in effective medium theory benefiting photothermal conversion for obtaining higher temperature except thermal conductivity reduction (the efficiency of light-to-heat in p-series nanostalactites ~110 °C/ W and temperature rise per second, up to 2 °C/s). In conventional photothermoelectric devices, the precious metals like Bismuth (Bi) and Tellurium (Te) are necessary for improving thermoelectric figure of merit (ZT). In this work, we integrate nanostalactites with a simple p-n junction-based photodiode to demonstrate the high efficiency of optical-to -thermal -to-electrical conversion capability. For overcoming the shortcomings of thermal convection, thermal conduction, and radiation issue, the light intensity is 0.34-0.44 Watt (Halogen Lamp) at least. As the illuminating power is larger than 0.44 Watt (Halogen Lamp), the photothermoelectric effect is significant to generate the thermal generation current, that can be extracted through the difference between device under illumination continuously and that under light-on/light-off without temperature change as possible as it can. According to the measured results, device with nanostalactites provides 102 times the thermal-to-electrical current of device without nanostructure. Additionally, the total current of device with nanostalactites is also higher than that of simple p-n junction-based photodiode under the same lighting intensity. Strong phonon-phonon scattering makes heat diffuse rapidly because light is perfectly absorbed and phonons generate inside silicon. Therefore, p-series nanostalactites (p-type and heavily-doped boron atoms presented P++(B) nanostalactites) are also regarded as perfect light and heat absorbers.