本論文研究一個新的結構與新的發光機制：載子擴散注入多重量子井發光二極體，利用一個完全不同載子傳輸機制相對於傳統電流注入方式，將主動區MQW(Multi-Quantum Well，MQW)置於pn-junction之外，形成n-p-MQW或p-n-MQW結構，主要發光機制是藉由載子空間濃度差，讓載子擴散注入至MQW複合發光，此結構理論上由於MQW並非置於pn-junction中間，可能會降低由於傳統p-MQW-n結構所造成Efficiency Droop因素，如極化電荷、載子溢流…等；另一部分為n-p-MQW或p-n-MQW結構在大電流注入下，pn-junction載子濃度上升，造成更大空間濃度差，載子擴散注入至MQW電流也跟著上升，而MQW中也會形成載子濃度差，讓載子從上層MQW(靠近pn-junction側)擴散至更下層MQW(遠離pn-junction側)，基於以上理論機制，或許載子擴散注入多重量子井發光二極體能有效降低效率衰減(Efficiency Droop)。
實驗上先探討p-n-MQW結構，製程方式上分為標準p-n-MQW結構、選擇性再成p-GaN之p-n-MQW結構，標準p-n-MQW結構利用感應式耦合電漿(ICP)蝕刻出p-GaN高台，選擇性再成長p-GaN之p-n-MQW結構利用二氧化矽遮罩做選擇性再成長形成p-GaN高台；之後研究n-p-MQW結構與傳統p-MQW-n結構比較探討，n-p-MQW結構相同使用二氧化矽遮罩做選擇性再成長形成n-GaN高台，傳統p-MQW-n結構利用感應式耦合電漿(ICP)蝕刻出p-GaN高台；元件光罩設計上也分為兩種，第一種為在相同元件面積大小下，具有不同指叉數之高台(Mesa)，也就是具不同p、n間距載子擴散注入多重量子井發光二極體，探討各尺寸間其光電特性之比較，第二種為在不同元件面積大小下，具有不同載子擴散距離寬，元件形狀分為矩形與圓形，探討哪個尺寸下最適合載子擴散，具有最佳之光電特性。
將實驗結果搭配能帶模擬分析，得到n-p-MQW結構光電特性遠遠優於p-n-MQW結構，而n-p-MQW結構與傳統p-MQW-n結構比較所得重要結論如下圖，n-p-MQW結構隨著元件尺寸變化Efficiency Droop趨勢相較於傳統p-MQW-n結構大(紅線部分)，元件尺寸越小Efficiency Droop越小於傳統p-MQW-n結構，這也相對表示載子擴散注入多重量子井結構不利於橫向上電流散佈(因電子電洞擴散差異與元件單位μm變化)，但載子垂直擴散至MQW佳(因MQW單位為Å(Angstrom))，而小尺寸元件就是消除橫向上擴散不易；其他實驗結果於本論文四、五章分析解釋。

Selective-area Regrowth Technique Applied to GaN-based Light Emitting Diodes with Carrier Diffusion Injected into Multi-quantum Well Structure

Shih-Sian Wang
Prof. Jinn-Kong Sheu
Department of Photonics,National Cheng Kung University

SUMMARY
In this thesis, we demonstrate a new structure and a new light-emitting mechanisms: Carrier Diffusion Injected into Multi-quantum Well Light Emitting Diodes, which is based on completely different current transport mechanism compared to conventional way of current injection. The light-emitting active region is located outside the pn-junction formed p-n-MQW or n-p-MQW structure. The charge carriers are diffusion injected into the active region by the concentration difference of space. It is expected to reduce some efficiency droop factors, such as carrier overflow, polarization charge…etc, which related to current injection with conventional structures.
In this structure design, pn-junction carrier concentration increases when high current injection, resulting in greater spatial concentration difference between pn-junction and MQW. Therefor the greater diffusion curret injects into MQW, allowing carriers from the upper MQW near th pn-junction side spread to more lower MQW away from the pn-junction side. Based on the above theory mechanism, perhaps carrier diffusion injected multi-quantum well light emitting diodes could reduce the efficiency droop.

INTRODUCTION
In recent years, the blue light has been successfully developed, such as blue light diodes and laser diodes. They become necessary light for full-color displays and optical information storage systems at present. The blue light with short wavelength can improve the storage capacity of data. Materials currently used to produce blue light diodes are silicon carbide, zinc selenide and gallium nitride. Silicon carbide is an indirect bandgap, besides zinc selenide and nitriding gallium are a direct bandgap. The emitting efficiency is poor due to the indirect bandgap, which getting a high brightness light-emitting diodes are not easy. Zinc selenide is a direct band gap, but the light emitting devices using this material made have the question of not long liftetime.
Gallium nitride is the direct bandgap semiconductor material of III-V compounds. the energy gap (Eg) is 3.4eV, and it can form ternary or quaternary compounds whit AlN (Eg = 6.2eV) and In (Eg = 0.7 ~ 0.9eV). By adjusting the doping ratio between the compounds, the range of the gap can continuously change from 0.7eV to 6.2eV, as shown in fig.1. From visible light to ultraviolet light can be encompassed within the emission wavelength range. In addition, the GaN material also has high physical rigidity, high saturation drift velocity, high breakdown electric field, high thermal stability and resistance to radiation, etc. Gallium nitride is quite suitable for research as blu-ray diodes materials.
MATERIALS AND METHODS
First, we investigate the structure of p-n-MQWs in our experiments. Furthermore, our fabrication process also fall into two types of structure, standard p-n-MQWs and selective regrowth p-GaN of p-n-MQWs,respectively. The structure of standard p-n-MQWs is using inductively coupled plasma to define the mesa of p-GaN. The structure of selective regrowth p-GaN of p-n-MQWs is using patterned silicon dioxide mask and then we can obtain p-GaN mesa by selective regrowth.After we study optical characteristics between conventional p-MQWs-n structure and the selectively regrowth n-GaN of n-p-MQWs. Similarly the structure of selective regrowth n-GaN of n-p-MQWs is using patterned silicon dioxide mask and then we can obtain p-GaN mesa by selective regrowth, and the conventional p-MQW-n structure forms p-GaN mesa by inductively coupled plasma etching.
The mask designs are also divided into two types. The first one is the same chip size with different numbers of finger-shaped mesa, that is to say, with different spacings between p and n type GaN. Second, we design different chip sizes with circle-shaped and rectangle-shaped mesa, that is to say, with different carriers diffusion distances . We discusse which size best suited carrier diffusion, and has the best optical characteristics.

RESULTS AND DISCUSSION
For selective-area regrowth technique applied to GaN-based light emitting diodes with carrier diffusion injected into multi-quantum well structure, the optical characteristics of n-p-MQWs structure is far superior to p-n-MQWs structure. We can summarize in the following points:
1. n-p-MQWs structure completely eliminate the yellow band of pn-MQWs structure.
2. For p-n-MQWs structure, the loading effect would make the hole not spread evenly.
3. The EQE of n-p-MQWs structure is much higher than the EQE of p-n-MQWs structure. If the two structures have the same light extraction efficiency, the IQE of n-p-MQWs structure is much higher than the IQE of p-n-MQWs structure.
4. Selective regrowth technology does not make much impact on the doping concentration, which verified from transmission line mode, but selective regrowth technology would reduce the quality of the lattice, which verified from the reverse bias voltage-leakage current curve.

The comparison between selective regrowth n-GaN of n-p-MQWs structure and conventional p-MQWs-n structure summariz in the following points:
1. In the energy band simulation of n-p-MQWs structure, the Fermi level in the p-GaN is too far from valence band which cause the larger series resistance and 順向偏壓 than conventional p-MQWs-n structure Besides, the both structures are presented ohmic contact in TLM measurement.
2. As shown in Fig. 2, n-p-MQWs structure with component size changes has a larger efficiency droop trend compared to conventional p-MQW-n large structure. The n-p-MQWs structure has smaller efficiency droop than conventional p-MQWs-n structure when component size changing smaller. It also indicates the carriers diffusion injected MQWs structure is not conducive to lateral current spreading, which attributed to the different diffusion lengths of electron and hole and component size varied in micrometer. However the carrier perpendicularly diffuse into MQW efficiently, which attributed to the thickness of MQW are angstrom-unit. In the other words, the small size of elements eliminate the hardship of lateral diffusion.
3. As shown in Fig. 2, the smaller the size of the components, the lower the efficiency droop, but Max EQE value is also relatively low. If we design a large emitting area,and np-types are spaced in interdigital arrangement to shorten the carrier diffusion distance, in this way, we can not only significantly enhance the Max EQE value, but also effectively reduce efficiency droop.
CONCLUSION
Redesign finger-shaped mask: As shown in Fig. 3, this design allows n-type electrode adjacent to p-GaN being red line in Fig. 3, and let each size having same emitting area of 250000〖 μm〗^2, which havig the same Max EQE value in a small current, if the Light Extraction is approximately identical. According to the current results, we predict that as the number of the fork increase the efficiency droop would decrease.
Changing the p-type material: The first method is to use p-type MQW which means the barrier of GaN doped Mg, but this method will reduce the quality of the lattice. The second method is to enhance the concentration of p-type, such as p-type of AlGaN/ GaN Superlattice. The two methods are both used to improve the p-type energy level position, which caused the fermi level being closer to the valence band in the p-type of energy band. The purpose of above is to reduce series resistance and 順向偏壓. The primary purpose is to enhance the energy conversion efficiency and reduce the thermal generated by resistance, and the secondary purpose would try to increase Max EQE values.

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