近年来，AlGaN基紫外发光二极管 (UV-LEDs) 因其在杀菌消毒、水/空气净化、紫外固化等众多领域的应用前景而受到广泛关注。然而，传统的AlGaN基UV-LEDs一般由极性材料制备而成，在其生长方向存在强度高达MV/cm的自发极化和压电极化电场。这些极化电场会引起电子和空穴波函数在空间上的分离，使得电子和空穴的辐射复合几率减小，最终导致LEDs的内量子效率 (internal quantum efficiency, IQE) 急剧下降，此即所谓的量子限制斯塔克效应 (quantum confinement stark effect, QCSE)。而半极性AlGaN基材料，如 (11-22) 面的AlGaN材料，由于其生长方向和极化电场的方向存在一定夹角，因而能够有效地抑制QCSE，并最终提高AlGaN基UV-LEDs的IQE和发光效率。因此，对半极性 (11-22) 面AlGaN材料的生长及性质的研究就显得至关重要。本论文首先采用低压金属有机化合物化学气相沉积 (low-pressure metal organic chemical vapor deposition, LP-MOCVD) 系统在非极性 (10-10) m面蓝宝石衬底上外延生长了半极性 (11-22) 面AlGaN薄膜。特别值得强调的是，本论文通过系统地优化外延层的生长条件参数，显著地提高了半极性 (11-22) 面AlGaN外延层的晶体质量。其次，本论文分别详细地研究了Si掺杂和Mg掺杂对半极性 (11-22) 面AlGaN外延层的结构、光学以及电学性质的影响。此外，本论文制备了发光波长为279 nm的半极性 (11-22) 面AlGaN基多量子阱 (multiple quantum wells, MQWs) 结构，并深入地研究了其表面形貌、晶体质量及光学性能。最后，本论文采用APSYS软件从理论上研究了非极性 (112 ?0) 面AlInGaN 四元材料的运用对于提高非极性 (11-20) 面AlGaN基UV-LEDs光电性能的作用。本论文的主要研究内容和取得的具体研究成果如下：
1. 采用两步生长法和单步生长法分别在非极性 (10-10) m面蓝宝石衬底上生长了半极性 (11-22) 面AlGaN薄膜，并且重点研究了外延层结构、氮化时间以及生长压力对 (11-22) 面AlGaN 外延层的表面形貌、晶体质量、光学和电学性质的影响。研究结果表明，采用单步生长法更有利于获得纯 (11-22) 面晶向的AlGaN薄膜。同时，当氮化时间为5 min时，样品的晶体质量和表面形貌最佳。此外，研究还发现增加反应室的压力能有效地降低 (11-22) 面AlGaN外延层的背景载流子浓度。
2. 采用单步生长法在非极性 (10-10) m面蓝宝石衬底上生长了Al组分变化范围为0.08-0.66的半极性 (11-22) 面AlGaN薄膜，并系统地表征了其表面形貌、晶体质量及光学性质。研究结果表明，外延层的生长速率和表面粗糙度随着Al组分的增加而逐渐降低，而外延层的晶体质量则随着Al组分的增加略微有所下降。
3. 在非极性 (10-10) m面蓝宝石衬底上生长了Si掺杂的半极性 (11-22) 面n-Al0.45Ga0.55N 外延薄膜，并且系统地研究了SiH4的?摩尔流量对 (11-22) 面n-Al0.45Ga0.55N薄膜的表面形貌、晶体质量、光学和电学性质的影响。研究结果表明，适宜浓度的Si掺杂有利于Al0.45Ga0.55N外延层中压应力的释放，从而有效地促进了位错运动，使得位错连接直至合并湮灭，因此能够提高半极性n-Al0.45Ga0.55N薄膜的晶体质量；但过量的Si掺杂会形成新的位错中心，并抑制位错运动，最终导致 (11-22) 面n-Al0.45Ga0.55N晶体质量的劣化。研究还发现，当SiH4的摩尔流量为7.14 nmol/min时，外延层样品的表面粗糙度可低至1.47 nm，而样品的电子浓度和迁移率可高达1.8×1019 cm?3和36.65 cm2V?1s?1。
4. 在非极性 (10-10) m面蓝宝石衬底上成功制备了阱宽5 nm、垒厚11 nm、发光波长为279 nm的半极性 (11-22) 面Al0.39Ga0.61N/Al0.55Ga0.45N MQWs，并且重点研究了In表面活性剂对量子阱界面质量和光学性能的影响。SEM和HR-XRD的测试结果表明，当采用In作表面活性剂时，能够获得平滑、陡峭的MQWs界面。同时，In表面活性剂还能够有效改善MQWs的表面形貌和晶体质量。此外，通过对变温光致发光 (time-dependent photoluminescence, TD-PL) 测试结果进行分析得知，采用In表面活性剂生长的 (11-22) 面AlGaN基MQWs的激子局域能较未采用In表面活性剂生长的MQWs有了明显的提高，这有利于缩短MQWs中发生快辐射复合的载流子的寿命，提高MQWs的发光强度，从而最终提高MQWs的IQE。
5. 采用delta掺杂技术在非极性 (10-10) m 面蓝宝石衬底上生长了Mg掺杂的半极性 (11-22) 面p-Al0.15Ga0.85N外延薄膜，并且系统地研究了Cp2Mg的?摩尔流量对 (11-22) 面p-Al0.15Ga0.85N薄膜的表面形貌、晶体质量、光学和电学性质的影响。研究结果表明，适宜浓度的Mg-delta掺杂有利于阻断位错的传播，提高外延层的晶体质量。然而，对于Mg过掺的p-Al0.15Ga0.85N薄膜，由于Mg原子与Ga原子在晶格尺寸上的巨大差异，会导致p-Al0.15Ga0.85N的晶格发生扭折，最终引起晶体质量的退化。另一方面，p-Al0.15Ga0.85N外延层的表面粗糙度会随着Mg掺杂浓度的升高而急剧上升。同时发现，当Cp2Mg的摩尔流量为0.22 μmol/min时，在p-Al0.15Ga0.85N外延层中可获得高达5.4×1017 cm?3的空穴浓度。最后，通过优化退火工艺，采用不同气体氛围的三步退火法将样品的空穴浓度进一步提高至7.9×1017 cm-3。
6. 采用APSYS模拟软件分析计算了 (11-20) 面p-AlInGaN四元材料以及p-AlInGaN/AlGaN超晶格 (superlattice, SL) 结构作为非极性 (11-20) 面AlGaN基UV-LED电子阻挡层 (electron blocking layer, EBL) 对UV-LED光电性能的影响。研究发现，与Al0.595Ga0.405N EBL, Al0.595Ga0.405N/Al0.5Ga0.5N SL EBL或者Al0.634In0.03Ga0.336N EBL相比较，采用与非极性 (11-20) a面Al0.5Ga0.5N在沿 [10-10] m轴方向晶格匹配的a面Al0.634In0.03Ga0.336N/Al0.5Ga0.5N SL作为EBL，可以有效地提高EBL的有效势垒高度以及空穴从p-Al0.5Ga0.5N层到有源区的注入效率，并显著地减小漏电流。理论模拟结果还表明，采用 Al0.634In0.03Ga0.336N/Al0.5Ga0.5N SL作为EBL结构的非极性AlGaN基UV-LED有源区的电子和空穴浓度获得了显著的增加，能有效地提高电子和空穴的辐射复合几率，从而提高UV-LED的发光效率，并抑制UV-LED的IQE在大注入电流下的“效率下降”现象。
In the past decades, there has been an outburst of the research on the AlGaN-based ultraviolet light emitting diodes (UV-LEDs) that have great potential in many application areas, such as sterilization, air and water purification, and resin/ink hardening. However, the conventional polar AlGaN-based UV-LEDs fabricated along  c-direction exhibit strong spontaneous and piezoelectric polarization fields with the order of magnitude up to several MV/cm. These polarization ?elds will result in a serious reduction in the overlapping of the electron and hole wave functions and thus a serious droop in the internal quantum efficiency (IQE) due to the quantum confinement Stark effect (QCSE). In contrast, the polarization effects can be significantly suppressed by employing the AlGaN-based UV-LEDs with semi-polar planes, such as (11-22) plane. As a result, superior performance is expected for the semi-polar (11-22) plane AlGaN-based UV-LEDs to their counterparts grown along the traditional polar c-direction. Therefore, it is vital to conduct systematic researches on the semi-polar (11-22) plane AlGaN materials. In this dissertation, the epitaxial growth of semi-polar (11-22) plane AlGaN materials on the non-polar (10-10) m-plane sapphire substrates in low-pressure metalorganic chemical vapor deposition (LP-MOCVD) system were investigated extensively. Firstly, the growth parameters for these semi-polar (11-22) plane AlGaN epi-layers were systematically optimized to improve the crystalline quality, surface morphology and to suppress the background electron concentration for the semi-polar (11-22) plane AlGaN epi-layers. Meanwhile, the effects of Si-doping and Mg-doping on the structural, electrical, and optical properties for the semi-polar (11-22) plane AlGaN films were studied in detail, respectively. Moreover, semi-polar (11-22) plane AlGaN-based multiple quantum wells (MQWs) with an emission wavelength of 279 nm were deposited, and the effects of Indium (In) surfactant on the surface morphology, crystalline quality and optical properties for the semi-polar (11-22) plane AlGaN-based MQWs were studied intensively. In addition, the influence of non-polar (11-20) a-plane AlInGaN materials on the optical and electrical properties of the a-plane AlGaN-based UV-LEDs was investigated in detail by the APSYS simulation software. The contents and major results achieved in this research were listed as follows:
The semi-polar (11-22) plane AlGaN films were grown on the non-polar (101 ?0) m-plane sapphire substrates with MOCVD by using traditional two-step and single-step growth methods, respectively. And the infulences of the growth conditions, such as layer structure, nitridation time, and reactor pressure, on the surface morphology, crystalline quality, optical, and electrical properties for the semi-polar (11-22) plane AlGaN epi-layers were studied systematically. It was found that the single-step growth method was more effective to obtain the AlGaN films with pure (11-22) crystal orientation than the traditional two-step growth method. Meanwhile, the semi-polar (11-22) plane AlGaN epi-layers with optimized surface morphology and crystalline quality were achieved when the nitridation time was 5 min. Furthermore, it was found that the background electron concentration for the semi-polar AlGaN films was decreased monotonously as the reactor pressure was increased.
2. The semi-polar (11-22) plane AlxGa1-xN films with the Al composition varied in the range of 0.08-0.66 were grown on the non-polar (10-10) m-plane sapphire substrates with MOCVD by using the single-step growth method. The structural and optical properties for the (11-22) plane AlxGa1-xN films were characterized. It was revealed that the growth rates and the surface roughness values for the as-deposited semi-polar (11-22) plane AlxGa1-xN films were decreased continuously as the Al composition was increased. In contrast, the crystalline quality for the semi-polar (112 ?2) plane AlxGa1-xN films was degraded with increasing the Al content.
3. The semi-polar (11-22) plane Al0.45Ga0.55N epi-layers with various Si-doping levels were successfully deposited on the non-polar (10-10) m-plane sapphire substrates with MOCVD technique. The characterization results showed that the compressive strain in the as-deposited Al0.45Ga0.55N epi-layers could be completely relaxed by Si-doping with appropriate SiH4 mole ?ow rate owing to the enhanced dislocation movements induced by Si dopants. This fact implies that Si-doping to the semi-polar (11-22) plane Al0.45Ga0.55N epi-layer is especially bene?cial to the interaction and annihilation of the dislocations, resulting in a signi?cant improvement in crystalline quality. Moreover, the minimum root mean square (RMS) value detected by AFM was as small as 1.47 nm, and the electron concentration and mobility determined by the room temperature (RT) Hall e?ect measurement were 1.8×1019 cm?3 and 36.65 cm2V?1s?1for the same sample, respectively.
4. High quality semi-polar (11-22) plane AlGaN-based multiple quantum wells (MQWs) with an emission wavelength of 279 nm were successfully deposited on the non-polar (10-10) m-plane sapphire substrates with MOCVD technology and the effects of indium (In) surfactant on the structural and optical properties of the AlGaN-based MQWs were investigated intensively. The characterization results revealed that the surface morphology as well as the crystalline quality for the semi-polar (11-22) plane AlGaN MQWs could be improved remarkably by adopting In as surfactant during the MOCVD growth process. Furthermore, the integrated MQWs-related excition emission peak intensity and the radiative recombination probabilities in MQWs could be increased as well with the help of In-surfactant, resulting in an enhanced IQE.
5. The Mg-delta-doped semi-polar (11-22) plane Al0.15Ga0.85N films were successfully deposited on non-polar (10-10) m-plane sapphire substrates with MOCVD technique and the effects of Cp2Mg mole flow rate on the characteristics of the p-AlGaN films were investigated intensively and systematically. The measurement results for the X-ray rocking curves indicated that the crystalline quality of the semi-polar (11-22) plane p-AlGaN films could be improved to a certain extent by optimizing the Mg-delta-doping process as a result of the suppression of the dislocation propagation along the growth direction. It was also found that the surface morphology for the semi-polar (11-22) plane p-AlGaN samples was monotonously degraded with increasing the Mg-doping level due to the serious lattice distortion and the three dimensional growth induced by Mg-doping. Furthermore, a hole concentration as high as 5.4?1017 cm-3 and a hole mobility of 2.5 cm2/V.s were achieved at room temperature by optimizing the Cp2Mg mole flow rate. In addition, the rapid thermal annealing (RTA) technique for the semi-polar (11-22) plane p-Al0.15Ga0.85N epi-layers was optimized. It was found that the hole concentration for the semi-polar (11-22) plane p-Al0.15Ga0.85N films could be further improved to 7.9×1017 cm-3 with multiple-step RTA technique in various gas ambient.
6. The optical and electrical properties of non-polar (11-20) a-plane AlGaN-based UV-LEDs with various kinds of electron blocking layers (EBLs) were analyzed numerically with the APSYS simulation software. The simulation results revealed that the (11-20) a-plane AlGaN-based UV-LED with the Al0.634In0.03Ga0.336N/Al0.5Ga0.5N superlattice (SL) EBL, which was lattice matched with the Al0.5Ga0.5N barrier layer along [10-10] axis, displays obviously superior characteristics in reducing electron leakage and enhancing hole injection efficiency as compared to the traditional non-polar (11-20) a-plane UV-LEDs with Al0.595Ga0.405N EBL, Al0.595Ga0.405N/Al0.5Ga0.5N SL EBL, and Al0.634In0.03Ga0.336N. Moreover, it was found that both the electron and hole concentrations in the active region could be significantly increased for the non-polar (11-20) a-plane AlGaN-based UV-LED with p-Al0.634In0.03Ga0.336N/Al0.5Ga0.5N SL EBL, which results in the enhancement of radiative recombination in the active region and the improvement in the efficiency droop at high injection current density.