| 摘要: | 此篇論文主旨為成長並製作以有機金屬反應氣相沈積所成長之摻雜鎂之氮化鎵和氮化銦鎵/氮化鎵多重量子井發光二極體，並且加以分析。 首先，我們利用霍爾量測、光激發光譜儀以及二次離子質譜儀等量測儀器來分析電洞型氮化鎵之特性和鎂在氮化鎵之分布情形。雖然在成長時，我們加入鎂的量是固定的，但從量測數據中，卻發現鎂含量隨著成長時間增加而增加；即越接近氮化鎵的表面，鎂含量越高。甚至在電洞型氮化鎵之厚度到達2.5 μm時，增加之趨勢也沒有減緩。我們也研究了改變電洞型氮化鎵之成長條件（成長溫度、五族和三族的比率以及承載氣體）對鎂在氮化鎵之摻雜行為的影響。除了在單層電洞型氮化鎵材料上發現鎂有擴散現象外，在氮化銦鎵/氮化鎵多重量子井發光二極體也發現了相同現象。 為了改善氮化銦鎵/氮化鎵多重量子井發光二極體的特性，首要工作就是要必須了解其行為。我們試著去調變氮化銦鎵/氮化鎵多重量子井發光二極體的操作溫度及電流，來了解氮化銦鎵/氮化鎵多重量子井發光二極體發光機制，而從中發現導致元件發光波長藍位移(即波長變短)的主因為能帶填滿效應，而非游離載子電場遮蔽效應，並且從其發光波長之偏移狀況，去解析其發光訊號是從量子井能階或是侷限能階發出。 在分析氮化銦鎵/氮化鎵多重量子井發光二極體發光機制的過程中，我們發現了摻雜鎂之氮化鎵的發光訊號，因此我們設計了另一個實驗來研究鎂摻雜物對氮化銦鎵/氮化鎵多重量子井發光二極體發光機制的影響。利用發光二極體操作溫度的變化所量測到的電激發光譜，我們知道量子井和摻雜鎂之氮化鎵所產生的訊號，其波長分別是在3.1 eV和2.7 eV。藉由rate equation來分析其行為，並利用此兩個訊號之強度隨著溫度變化之消長情形，進而可以推算出在氮化鎵中的鎂摻雜物活化能為126 meV，而這個數字，跟其他學術單位利用不同的量測方式所得到之數字有一致性。 為了提升氮化銦鎵/氮化鎵多重量子井發光二極體的發光效率，我們在其製程中應用了選擇性活化技術。利用這種技術對p型打線墊下方以外的摻雜鎂之氮化鎵進行活化，可維持p型打線墊下方摻雜鎂的氮化鎵之半絕緣特性，並進一步在元件操作時形成一層半絕緣的電流阻擋層。其用意在避免電流流至打線墊下方，因打線墊下方所產生的光會被厚金屬擋住而無法透出。由發光強度-電流圖，我們可以得知，和傳統的氮化銦鎵/氮化鎵多重量子井發光二極體相比，利用選擇性活化技術的氮化銦鎵/氮化鎵多重量子井發光二極體，有明顯的提升其發光效率。 此外，由於p型氮化鎵的電特性不好，因此傳統氮化銦鎵/氮化鎵多重量子井發光二極體都多加了一層透明導電層在p型氮化鎵上來幫助電流擴散，雖然此層名字叫做透明導電層，但實際上並非是完全透明，還是吸收/反射了一部分的發光。為了避免這個損耗，我們改變了一般傳統氮化銦鎵/氮化鎵多重量子井發光二極體的結構（p型材料在上，n型材料在下），將p型氮化鎵和n型氮化鎵掉換位置，並加入n+/p+的穿隧二極體，如此一來，就不須這層所謂的透明導電層來幫忙做電流擴散，少了這一層的影響，果然元件之發光效率幾乎提升了兩倍。 This dissertation includes the growth, fabrication, and characterization of Mg-doped GaN and InGaN/GaN multiple-quantum-well (MQW) light-emitting diode (LED) by metalorganic chemical vapor deposition (MOCVD). First, we utilized Hall measurement, photoluminescence (PL), and second ion mass spectrum (SIMS) to characterize p-type GaN and Mg distribution in GaN films. Although the Mg flow is constant during growth, the measurement data shows that the Mg concentration increases with growth time since the Mg concentration is higher when closer to the surface. Even when the thickness of p-type GaN reaches 2.5 μm, the increasing hasn’t slow down yet. The effect of different growth condition, such as growth temperature, III/V ratio and carrier gas, upon Mg incorporation in GaN is studied also. We found Mg diffusion not only in bulk p-type GaN but also in InGaN/GaN MQW LEDs. To improve the characteristics of InGaN/GaN MQW LED, we must understand its behavior first. So we tried to identify its emission mechanism by changing the operation temperature and current, and found the major cause of blue shift, which means peak emission wavelength shifts to shorter wavelength with higher injection current, is band-filling effect but not free carrier screen effect. In addition, we can identify whether the emission comes from well states or localized states from the way it shifts. When analyzing the emission mechanism of InGaN/GaN MQW LED, we also found the emission generated by Mg-doped GaN. Therefore, we designed another experiment to study the effect of Mg doping upon InGaN/GaN MQW LED. By the electroluminescence (EL) spectra measured under different operation temperatures, the peak emission wavelength of quantum wells and Mg-doped GaN is 3.1 eV and 2.7 eV respectively. When analyzing this emission behavior with rate equation and the relative intensity variation of these two peaks under different temperature, we can calculate and obtain the activation energy of Mg doping in GaN being 126 meV, which agrees with the results from other academic organization utilizing different otherwise method. We utilized selective activation technique in InGaN/GaN MQW LED process to improve its external quantum efficiency. Using this technique to activate Mg-doped GaN except the area underneath P-type bonding pad made us able to maintain its semi-insulating property, and further, it became a semi-insulating current blocking layer during device operation. This layer is utilized to prevent the injection current flowing through the area below P-pad since P-pad blocks the emission light from below and thus reduces the emission efficiency. From the luminescence intensity versus injection current chart, we did succeed in our effort to improve the external quantum efficiency when comparing to normal InGaN/GaN MQW LED. Since p-type GaN shows poor conductivity, normal InGaN/GaN MQW LED process utilizes a transparent conductive layer for current spreading. However, despite its name, this layer isn’t 100 % transparent, and thus would absorb/reflect certain portion of emission light. To prevent this loss, we changed the normal InGaN/GaN MQW LED structure, which is p-type above and n-type under MQW, by switching the p-type and n-type layers’ positions and adding a n+/p+ tunnel junction. By this change, there’s no need of this transparent conductive layer for current spreading, and the device external quantum efficiency become almost twice as high as before. |
