三五族半導體發光元件結構優化之研究

Abstract

本論文旨在探討三五族光電半導體元件包含紫外線發光二極體(ultraviolet light-emitting diode, LED)、紅光共振腔發光二極體(resonant-cavity LED)、850-nm面射型雷射(vertical-cavity surface-emitting laser)以及1.3-um側射型雷射(edge-emitting laser)之操作性能改良相關研究。對於半導體發光元件而言要使其具有高輸出功率以及高穩定性的輸出性能，元件磊晶晶體品質(epitaxial crystal quality)以及元件磊晶結構設計上實屬於相當重要一環；其中要使發光元件能穩定地操作在高溫高輸入電流的情況下，磊晶結構的設計更扮演極重要的角色。本論文即以有機金屬液相沉積法(metalorganic chemical vapor deposition, MOCVD)成長以上四種元件，配合理論模擬方式進行結構設計上的改良來使電子能有效地被侷限於活性層量子井 (active region quantum well)裡，達到降低電子溢流(electron leakage)提升元件輸出性能的目的。 在紫外線發光二極體研究上，我們以四元氮化鋁鎵銦(AlGaInN)為活性層材料並製做出最大輸出功率達4 mW，外部量子效率(external quantum efficiency)為1.2%，波長為370 nm的發光二極體。而為使此一元件具有較高之輸出功率以及穩定的高溫高輸入電流特性，我們以理論模擬方式進行元件結構的改良設計，探討氮化鋁鎵銦量子井個數以及氮化鋁鎵電子阻礙層對元件輸出特性的影響。模擬結果顯示當氮化鋁鎵銦量子井個數為五至七個和氮化鋁鎵電子阻礙層中鋁組成為19%時，可以達到降低電子溢流作用並提供較佳之輸出特性。 在紅光共振腔發光二極體研究上，由於元件活性層量子井材料磷化鋁鎵銦(AlGaInP)的導電帶能帶間隙值只有約300 meV，當電子注入量子井中很容易產生電子溢流現象進而降低元件於高溫操作下的輸出性能。藉由將傳統共振腔發光二極體共振腔長為一個波長增加為三個波長，我們發現在溫度範圍25–95ºC變化下，傳統結構在注入電流固定為20 mA的輸出功率變化為-2.1 dB，而共振腔長為三個波長的共振腔發光二極體卻只有-0.6 dB，且其遠場圖(far field pattern)隨注入電流變化相當穩定，適合做為塑膠光纖傳輸的光主動元件。理論模擬結果也證明，增加共振腔長至三個波長亦即增加量子井個數至三倍，確實有助於降低電子溢流達到侷限電子、穩定元件高溫、高電流特性的目的。 在850-nm面射型雷射研究上，首先針對具有不同壓縮應力之量子井結構進行理論分析，發現以具有較高壓縮應力值的砷化鋁鎵銦(AlGaInAs)做為量子井比起砷化銦鎵(InGaAs)可以具有較高之材料增益(material gain) ，較低透明載子密度(transparency carrier concentration)與透明自發輻射電流密度(transparency radiative current density)。接著我們選擇以Al0.08Ga0.77In0.15As做為量子井實際成長並做成元件，我們得到在室溫操作下的850-nm面射型雷射，其臨界電流為1.47 mA，電光轉換效率(slope efficiency)為0.37 mW/mA，且隨操作溫度升高至95 ºC，臨界電流值增加至2.17 mA，電光轉換效率降為0.25 mW/mA。爲進一步改善雷射輸出性能，我們首次於活性層量子井中加入一電子阻礙層，發現於室溫操做的850-nm面射型雷射臨界電流降低至1.33 mA，電光轉換效率也達到0.53 mW/mA，且當溫度升高至95 ºC時臨界電流值只增加0.27 mA，電光轉換效率也仍有0.4 mW/mA的表現。同時我們也以數值模擬方式證明加入電子阻礙層確實可以有效阻擋電子溢流進而提升雷射輸出性能。 在1.3-um InGaAsN/GaAsN側射型雷射研究上，由於實際成長氮砷化銦鎵磊晶結構時，於砷化鎵位障層(barrier)裡加入氮可以平衡InGaAsN量子井裡的高壓縮應力(compressive strain)並有助於避免量子井中氮原子的流失(out-diffusion)，然而卻會因導電帶能帶間隙值降低而產生電子溢流現象。因此首先針對氮砷化銦鎵(In0.4Ga0.6As0.986N0.014)單量子井旁的氮砷化鎵位障層進行理論模擬分析，探討氮砷化鎵位障層中氮的成分對元件輸出性能的影響，我們提出以GaAs0.995N0.005做為位障層，不僅可以符合實際磊晶考量，更可防止電子溢流現象發生。經由實際磊晶成長及製程大小為4×1000 um2的雷射元件後，在室溫操作下此一元件的臨界電流值為84 mA，電光轉換效率為9%；當操作溫度增加至105 ºC，臨界電流值增加至188 mA，特徵溫度(characteristic temperature, T0)為118 K。為改善元件高溫高注入電流特性，我們更嘗試於活性層量子井中加入磷砷化鎵(GaAs0.9P0.1)電子阻礙層，發現雖然於室溫下臨界電流值為99 mA，但當操作溫度為105 ºC時臨界電流值降低至172 mA，特徵溫度(characteristic temperature, T0)為155 K。模擬結果亦證實加入磷砷化鎵做為電子阻礙層確實可以防止電子溢流，但卻也有電洞分布不均勻現象(hole inhomogeneity)產生而導致在低溫及低電流操作下元件之臨界電流值增加。而為防止電子溢流現象發生，我們更提出在磷砷化鎵電子阻礙層中增加磷成分至15–20%來得到較高特徵溫度值之雷射元件。 總而言之，在本論文中我們嘗試在半導體發光元件，包含紫外線發光二極體、紅光共振腔發光二極體、850-nm面射型雷射以及1.3-um側射型雷射的磊晶結構上，設計最佳化結構來防止電子溢流現象發生進而提升元件的操作性能。祈望這些觀念與經驗未來能對元件磊晶者在結構設計上有所裨益。

In this dissertation, the improvement in operation performance of III-V optoelectronic semiconductor light emitting devices, which include ultraviolet (UV) light-emitting diode (LED), 660-nm red resonant-cavity LED, 850-nm vertical-cavity surface-emitting laser (VCSEL), and 1.3-um edge-emitting laser (EEL), were studied. The key technologies for semiconductor light emitting devices to possess better output performance and high operation stabilities are the epitaxial crystal quality and the design of epitaxial structure. Noteworthily, the structure design is more important if we were to have a stable output performance in high temperature and high injection current operation. By using the epitaxial technology – metalorganic chemical vapor deposition (MOCVD) to grow the structures and advanced simulation programs to give theoretical analysis, the operation performances of semiconductor light emitting devices are investigated and improved. Mainly, we focus on confining the electrons effectively in the quantum well (QW) active region to reduce the electronic leakage current so as to improve the output performances. In the research of UV LED, to emit an emission wavelength of 370 nm, quaternary AlGaInN is utilized as QW material during the epitaxy of UV LED. The device after standard process as 300×300 um2 size chip can provide a maximal output power of 4 mW and an external quantum efficiency of 1.2%. With an aim to enhance the output power of UV LED, we then theoretically investigate the effect of the number of QWs and the aluminum content in AlGaN electron-blocking layer on the UV LED output performance. After fitting in with the experimentally demonstrated output performance of UV LED, we find that the UV LED can provide a better output performance when the aluminum content in AlGaN electron-blocking layer is in a range of 19–21% and the AlGaInN QW number is in a range of 5–7. In the research of 650-nm RCLED, it is known that the conduction band offset value in AlGaInP material QW active region is approximately 300 meV. When the device is under high temperature operation, the electron leakage problem may become more serious and consequently leading to the degradation of output performance. By means of widening the resonant cavity to a thickness of three wavelength (3 lambda), the degree of power variation between 25 and 95 ºC for the device biased at 20 mA is apparently reduced from -2.1 dB for the standard structure design (1-lambda□cavity) to -0.6 dB. The current dependent far field patterns also show that the emission always takes place perfectly in the normal direction, which is suitable for plastic fiber data transmission. To optimize the RCLED structure, we continue numerically studying the structure dependent output performance by using an advanced simulation program. After fitting in with the experimentally demonstrated output performances of RCLEDs, we analyze the percentage of electron leakage current of the two structures, and we find that the stable temperature dependent output performance of 3-lambda-cavity RCLED is attributed to the reduction of electron leakage current. In the research of 850-nm VCSEL, we first theoretically investigate the gain-carrier characteristics of In0.02Ga0.98As and InAlGaAs QWs of variant In and Al compositions. More compressive strain, caused by higher In and Al compositions in InAlGaAs QW, is found to provide higher material gain, lower transparency carrier concentration and transparency radiative current density over a temperature range of 25−95 ºC. Then we choose Al0.08Ga0.77In0.15As as QW material in the epitaxy of 850-nm VCSEL structure. After standard oxidation confinement process, this device can provide a threshold current of 1.47 mA with a slope efficiency of 0.37 mW/mA at 25 ºC, and the threshold current increases to 2.17 mA with a slope efficiency reduction of 32% when the device temperature is raised to 95 °C. To improve the operation performance of 850-nm VCSEL, a 10-nm-thick Al0.75Ga0.25As electron-blocking layer is employed in the QW active region for the first time, and the threshold current at 25 ºC is found reducing to 1.33 mA with an increment of slope efficiency to 0.53 mW/mA. When the device temperature raises to 95 °C, the threshold current increases by only 0.27 mA and the slope efficiency drops by only 24.5%. Numerical simulation is also done to analyze the effect of the electron-blocking layer on the output performance of 850-nm VCSEL, and the results show that the output performance is improved by the reduction of electron leakage current. In the research of 1.3-um InGaAsN/GaAsN EEL, there has been several works investigating using strain GaAsN as direct barrier. Using GaAsN in the epitaxial growth can balance the highly compressive strain in InGaAsN QW and reduce the phenomenon of nitrogen out-diffusion from the well. However, it is a small bandgap material system, which indicates that the electron leakage may become more serious if adding more nitrogen into GaAsN barrier. Therefore, in the first instance, the temperature effects on the optical gain properties of single In0.4Ga0.6As0.986N0.014 QW with GaAsN barrier of different nitrogen compositions are studied for optimization. Theoretically, we suggest using GaAs0.995N0.005 as direct barrier can be a better choice with the considerations of epitaxial growth and electron confinement. Then we choose In0.4Ga0.6As0.986N0.014/GaAs0.995N0.005 as active region in the epitaxial growth of 1.3-um EEL structure. After standard process as 4×100 um2 size chip, this device can provide a threshold current of 84 mA with a slope efficiency of 0.09 mW/mA. When the device temperature increases to 105 °C, the threshold current becomes 188 mA and a characteristic temperature value (T0) of 118 K is obtained. To improve the operation performance of 1.3-um In0.4Ga0.6As0.986N0.014/GaAs0.995N0.005 EEL, we also try to insert a GaAs0.9P0.1 layer as electron blocking layer in the epitaxial growth of 1.3-um EEL. The threshold current of the device at 25 °C becomes 99 mA and a slope efficiency of 0.11 mW/mA is obtained. The threshold current at 105 °C only increases to 172 mA with a T0 value of 155 K and the reduction of slope efficiency becomes less. Numerical simulation is done to analyze the effect of the electron blocking layer on the output performance of 1.3-um EEL. The results show that the electron leakage current is reduced with the use of a high-bandgap GaAs0.9P0.1 layer. Further theoretical simulation work of investigating the effect of increasing the phosphide composition in GaAsP electron-blocking layer on the T0 value is also done. And, we find that increasing the phosphide composition in GaAsP to 15–20% can provide a better T0 value. In a summary, the III-V optoelectronic semiconductor light emitting devices, which include 370-nm UV LED, 660-nm RCLED, 850-nm VCSEL, and 1.3-um EEL, are experimentally demonstrated and theoretically analyzed for a purpose of reducing the electron leakage current and thus improving the operation performance. We hope those all will turn into useful information in the design and epitaxy of optoelectronic semiconductor light emitting devices.