鑲嵌奈米矽晶與奈米碳化矽晶之非晶富矽碳化矽PIN接面發光二極體特性研究

Characteristics of Silicon Nanocrystals and Silicon Carbide Nanocrystals Embedded Amorphous Silicon-rich Silicon Carbide Based PIN Junction Light Emitting Diode

戴宏宇(Hung-Yu Tai)

指導教授 ： 林恭如

國立臺灣大學/電機資訊學院/光電工程學研究所/碩士(2011年)

https://doi.org/10.6342/NTU.2011.01006

摘要

在本論文中，為了修正其間接能隙的發光特性與增強薄膜的結晶性，我們利用電漿輔助化學氣相沉積法，藉由調變氣體流量比例與升高基板溫度以及1100度高溫退火下，製備具有奈米矽晶和奈米碳化矽晶之非晶富矽碳化矽薄膜，從X射線光電子能譜分析發現當基板溫度從450度上升到650度時，矽原子的濃度從64.7%增加到71.6%，然而，碳和氧的濃度則分別是從27.7%減少到22.8%與7.6%減少到5.5%，氧矽比也從0.12降低到0.07，這是因為薄膜的結晶性變好使其變得更緻密以致於減少氧的侵入。而在拉曼頻譜上，有個很明顯的訊號在波數510 cm-1，因而可以發現高溫退火後奈米矽晶的存在，其他兩個很強的訊號峰值相對於3C塊材碳化矽訊號為796和972 cm-1有紅移的現象分別在波數744與933 cm-1處，而這些是由於奈米碳化矽晶尺寸縮小到奈米尺度所造成。從X射線繞射圖中，我們可以估計出奈米矽晶與奈米碳化矽晶的平均尺寸大小分佈分別約為4.2±0.5 nm與2.4±0.3 nm。從FTIR頻譜上，我們也發現在高溫退火後產生的去氫化現象，因為Si-H3的延展模態變成Si-H的延展模態，這是由於矽氫鍵被打斷而使得氫原子從膜中跑掉，因此透過Si-H3自由基的去氫化可使得矽原子之間更容易聚集形成奈米矽晶。此外，我們發現波數頻帶上範圍從792到806 cm-1的訊號是屬於Si-C的延展模態，而且在高溫退火後也會發現其模態有藍位移現象，波數從792位移到802 cm-1，因為退火後使膜中的矽與碳原子鍵結變強。在流量比例為60%退火的試片中，我們可發現很強的光激螢光訊號在波長為485 nm處，因為在奈米碳化矽晶與周圍的環境中交互作用下所形成表面的狀態會產生自我捕捉的激子，而產生很強烈的螢光，另外，在波長為580 nm處也同樣觀察到一個很強的螢光訊號是由奈米矽晶所帶來之量子侷限效應造成，同時也分析了流量比例40到70%樣本的組成比例可從0.74變化到0.62，確認能同時合成出兩種奈米結晶的膜之組成比例。
P型非晶富矽碳化矽薄膜在流量比為50%與掺雜氣體乙硼烷莫爾分率比為2%時，電阻率降到最小為2.2×101 Ω-cm，對應到的雜質濃度與活化能分別為1.35×1016 cm-3和0.17 eV，由於有適當數量的硼原子佔到碳化矽材料中正確的位置，在經過熱處理的活化雜質後，使得有足夠的電洞可以被釋放。另一方面，因為各種氣體解離能的不同，利用加大三倍氣體流量下成長P型碳化矽材料，相當於稀釋薄膜之中的雜質濃度，使得電阻率可以從22再下降到0.72 Ω-cm對應到的雜質濃度為從1.35×1016增加到4.35×1017 cm-3，這是由於過掺雜現象被減少，此現象減少可以減小過量的雜質原子互相散射與電洞之間的碰撞之影響。當製程電漿功率為80 W時，N型碳化矽的電阻率很快地下降到11.3 Ω-cm對應到的磷化氫雜質濃度為1.46×1015 cm-3。
藉由合成具有鑲嵌奈米矽晶與奈米碳化矽晶的主動層之PIN薄膜發光二極體元件來加大輸出光功率、減小臨界電壓和增強電流注入效率。若有效地將P層碳化矽的載子濃度增加，可改善載子注入與電流傳輸特性。越厚的主動層會造成較大的串聯電阻，因此無論是臨界電壓或是注入電流在較厚的主動層中都會比薄的主動層大而且發光二極體的輸出光功率在流量比例為60%之主動層為50 nm元件中比在主動層厚度為25 nm時大了三倍。隨著主動層流量比例為50%的發光層厚度從25 nm增加到100 nm，將使得穿隧機率變小，因為電場變小而使載子注入變少，但在流量比為50%的主動層中擁有更富矽的環境，使其可產生很多以奈米矽晶主導的放光中心，輸出光功率在主動層流量比例為50%之主動層為50 nm元件中比在主動層厚度為25 nm時大了三點五倍，最佳化的功率轉換比率是在主動層為50 nm。當流量比為60%的主動層厚度從25 nm增加到50 nm時，外部量子效率也是大了兩倍以上，而功率-電流斜率大了四倍，所以要同時兼顧載子傳輸與穿隧進入主動層以及在主動層中產生很多的發光中心這兩件事是互相違背的，所以我們可以找出一個最佳化的主動層厚度約為50 nm。在流量比為50%的主動層厚度從25 nm增加到50 nm時，外部量子效率大了四倍，所以無論是在流量比例為60%與50%的主動層下，最佳化的厚度50 nm都適用。載子的電流傳輸機制被確認是由高電場引起的傳導帶到價帶之間的穿隧機制，而且輻射躍遷是透過載子穿隧進入主動層與其鄰近區域所產生。在主動層流量比例為60%的發光元件中，主要的電激螢光峰值在495 nm有個較窄的波包對應到藍白色的發光圖案為奈米碳化矽晶與周圍環境中產生之自我捕捉激子所貢獻，而在主動層流量比例為50%的發光元件中，主要的電激螢光峰值在570 nm有個較寬廣的波包對應到橘黃色的發光圖案被認為是由奈米矽晶造成之近似能帶間的輻射性躍遷。當電子濃度從1015 cm-3增加到1016 cm-3時，注入效率可以提升到原來的六倍，而當外部量子效率從7.84%增加到46%時，外部量子效率可提升到接近六倍。

關鍵字

奈米矽晶 ； 奈米碳化矽晶 ； 非晶富矽碳化矽 ； 電阻率 ； 雜質濃度 ； PIN接面薄膜發光二極體 ； 注入效率

並列摘要

In this thesis, the synthesis of a-SixC1-x films embedded with Si-ncs and SiC-ncs by fluence-ratio detuned PECVD at high-temperature growth is investigated to modify its luminescent property and to enrich the crystallinity after thermal annealing at 1100oC. With changing the deposition temperature from 450oC to 650oC, the Si concentration increases from 64.7% to 71.6%. However, the carbon and oxygen contents decrease from 27.7% to 22.8% and 7.6% to 5.5% and the O/Si ratio is reduced from 0.12 to 0.07 from the XPS analysis due to growth of better crystallinity to prevent the oxygen invasion in a-SixC1-x films. A significant signal at 510 cm-1 is shown to confirm the existence of Si-ncs after post-annealing. The other two intensive peaks at 744 and 933 cm-1 are red-shifted than bulk 3C-SiC Raman peaks at 796 (TO) and 972 cm-1 (LO) and ascribed to reduced nanograin size of SiC-ncs, respectively. From the results of XRD spectra, the average crystallite sizes of Si and 3C-SiC nanocrystals are around 4.2±0.5 nm and 2.4±0.3 nm, respectively. On the basis of FTIR analysis, the Si-H3 stretching mode is transformed into Si-H stretching mode after annealing since the hydrogen bond is broken up to diffuse out. Accordingly, Si-ncs can be easily aggregated by dehydrogenation in Si-H3 radical. A distinct band at 792-806 cm-1 is ascribed to Si-C stretching mode and the blue-shifted peak from 792 to 802 cm-1 is due to enhanced strength of bonds between Si and C atoms. The intense visible PL centered at 485 nm is found in annealed sample of g=60% and it attributed to the luminescence of SiC-ncs due to the self-trapped excitons at the surface states between SiC-ncs and surrounding. In addition, the PL peak at 580 nm is also observed from the contribution of Si-ncs in view of quantum confinement effect. Composition ratio x in SixC1-x is detuned from 0.74 to 0.62 with increasing fluence ratio from 40 to 70% by XPS spectra.
The resistivity of P type a-SixC1-x network at g of 50% reduces to 2.2×101 Ω-cm when B2H6 doping mole fraction increases to 2% since the appropriate amounts of boron atoms occupy the position in tetrahedral SiC network to release enough holes to form the electrically active dopant after thermal process of 650oC. The dopant density is also increased to 1.35×1016 cm-3 when doping mole fraction is at 2% corresponding to activation energy of 0.17 eV. On the other hand, the resistivity is reduced from 22 to 0.72 Ω-cm and dopant density is increased from 1.35×1016 to 4.35×1017 cm-3 due to the minimization of the overdoping phenomenon to reduce the influence on excess impurity atoms scattering and collision between released carriers when enlarged triply doping gaseous fluence and it is equivalent to gaseous dilution owing to various dissociation energy for different process gas. The resistivity of N-SiC is decreased abruptly to 11.3 Ω-cm when RF power changes to 80 W corresponding PH3 dopant density of 1.46×1015 cm-3.
The PIN thin film light emitting diode with intrinsic layer embedded with Si-ncs and SiC-ncs is fabricated to enlarge optical power, reduce turn on voltage and enhance carrier injection efficiency. Carrier injection and transport properties can be improved with the higher doping concentration P-SiC layer. The thicker intrinsic layer caused the larger series resistance thus whether turn on voltage or injection current is larger than the thinner I-layer and the optical power emitted from PIN TFLED with I layer thickness of 50 nm is triple than I layer thickness of 25 nm at g of 60%. With increasing thickness of intrinsic layer from 25 nm to 100 nm at g of 50% since the carrier tunneling probability is decreased with enlarging the intrinsic layer thickness since insufficient electric field across the I-SiC film is to reduce the carrier injection and the luminescent centers are more plentiful owing to the Si-rich SiC matrix at g of 50% with embedded more quantity of Si-ncs. The optical power from PIN LED with g of 50% at I layer thickness of 50 nm is three point five times than I layer at 25 nm. The PCR increasing trend is due to the production of more luminescent centers at thickness of 50 nm but subsequently decreased is owing to too thicker I-layer caused the carrier tunneling probability reduction. The EQE is larger twice than others with increasing intrinsic layer thickness from 25 to 50 nm at g of 60% and the P-I slope is four times than others since the trade off relation between carrier transport and tunneling into active layer and the more luminescent centers in active layer are observed thus the optimized thickness of intrinsic layer is 50 nm. The EQE is four times than others with increasing g of 50% intrinsic layer thickness from 25 to 50 nm and the P-I slope is six times than others since the appropriate intrinsic thickness at 50 nm is needed to enhance light emission and preserve the carrier injection ability. Carrier transport via band to band tunneling is confirmed owing to high electric field and then radiative transition is also occurred due to the carrier tunneling into intrinsic region and its neighborhood. The principal EL peak at 495 nm with narrower shape is assigned to self-trapped excitons at the surface states between SiC-ncs and surrounding at g of 60% corresponding to blue-white EL emission pattern. Moreover, the main EL wavelength centered at 570 nm with broader shape is attributed to nearly direct band to band transition by Si-ncs at g of 50% and consistent with orange-yellow EL emission pattern. The injection efficiency is six times than N-SiC with 1015 cm-3 when dopant density increased to 1016 cm-3. The EQE is enhanced nearly six times when injection efficiency is increased from 7.84% to 46 %.

並列關鍵字

Si nanocrystals ； SiC nanocrystals ； Si-rich SiC ； Resistivity ； Dopant density ； PIN junction thin film light emitting diode ； Injection efficiency

參考文獻

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