中科院半导体照明研发中心知识库

深紫外LED应用广泛，但是价格高昂，这主要是由于目前主流深紫外外延片使用的生产装备多为小型MOCVD。但是大型MOCVD在蓝宝石衬底上外延生长AlN时，除了面临晶格失配和热失配的困难外，由于反应炉较大，炉内热场和流场难以控制，而且预反应强烈，因此目前还很难使用大型MOCVD生长出高质量的AlN外延层。本文为了寻找可以实现大产量工业MOCVD设备生产高性能深紫外LED的工艺，探索了不同缓冲层或缓冲层组合上外延生长高温AlN，不同厚度溅射成核层上生长高温AlN，以及在溅射成核层生长深紫外LED。取得的主要研究成果如下：

1.        使用大型工业MOCVD设备，在不同缓冲层及缓冲层组合上外延生长1200℃的高温AlN。通过比较模板表面粗糙度和样品的摇摆曲线半高宽，结果表明，如果生长在蓝宝石衬底上的缓冲层质量较好，然后再用大型工业MOCVD在缓冲层上生长高温AlN外延层，有可能生长出高质量的AlN材料。基本可以确定，缓冲层（成核层）对生长环境非常敏感，另外高温AlN的晶体质量对缓冲层（成核层）质量非常敏感，值得庆幸的是高温AlN的晶体质量对生长环境的要求较为宽松，这为通过提高缓冲层（成核层）质量实现大型工业MOCVD外延高质量的AlN材料提供了可行性保障。

2.        使用大型工业MOCVD，在原位成核层上生长一层1250℃的高温AlN外延层，螺位错密度和刃位错密度分别高达 2.54×10⁹ cm^-2 和 2.88× 10¹⁰ cm^-2，结果表明，采用两步法外延，难以使大型工业MOCVD设备生长出高质量的AlN外延层。当在25nm、50nm和100nm厚的溅射成核层上，使用大型工业MOCVD，生长一层900nm厚的1250℃的高温AlN外延层， AlN模板的螺位错密度分别为9.77 × 10⁶、 1.65 × 10⁷和 1.11 × 10⁸ cm^-2 ，刃位错密度分别为1.46 × 10¹⁰、8.91 × 10⁹和 6.70 × 10⁹ cm^-2。溅射成核层的厚度增加，螺位错随之增加，但是刃位错密度却是降低的，两种位错密度之和随着溅射成核层厚度增加而降低。本实验结果表明，相比原位形核层，溅射成核层可以有效地降低预反应等带来的危害，提供了高质量的成核层。采用溅射成核层可以实现使用大型工业MOCVD设备生长出质量相对较好的AlN外延层。

3.        使用大型工业MOCVD设备在50nm厚溅射成核层上生长深紫外LED外延片，成功地制备出可以商业化应用的282nm深紫外LED芯片。该深紫外LED芯片，在注入电流为20mA时，光输出功率达到1.65mW，对应的外量子效率为1.87%。在注入电流为60mA时，达到饱和光输出功率4.31mW。此外，还在50nm厚溅射成核层上，使用大型工业MOCVD制备出波长为274nm的UVC-LED。在注入电流为20mA时，单颗UVC-LED的光输出功率达到0.51mW，对应的外量子效率为0.56%。综上，本文所述方法，非常有潜力成为实现深紫外LED低成本大规模量产的重要方法。

Deep UV-LEDs are widely used, but their price is very high. This is mainly because the production equipment of commercial deep UV-LEDs is mainly low-yield MOCVD system. However, the high-yield industrial MOCVDs are difficult to grow high-quality AlN due to harmful pre-reaction and hardly controlling the thermal field and gas flow fields in huge reactors, especially during AlN growth. In order to find a process to realize producing commercial deep UV-LEDs by high-yield industrial MOCVD equipment, this thesis studies the high-temperature AlN grown on different buffers or combinations of buffers, high-temperature AlN grown on sputtering nucleation layers (SNL) of different thickness, and deep UV-LEDs grown on SNL. The achievements mainly include:

1.        Using high-yield industrial MOCVD, 1200℃ high-temperature AlN layers were grown on different buffers or combinations of buffers. By comparing the surface roughness of the templates and the rocking curve full width at half maximum (FWHM) of the templates, the results show that it is possible to grow high-quality AlN on good buffer layer, which is grown on sapphire substrate, by the high-yield industrial MOCVD. The growth of AlN buffer is very sensitive to the growth environment. In addition, the crystal-quality of high-temperature AlN is very sensitive to the quality of buffer layer (nucleation layer). However, the environment requirements for growing high-temperature AlN is relaxed. It provides a viability guarantee for improving the quality of the buffer layer (nucleation layer) to achieve growing high-quality AlN materials by high-yield industrial MOCVD.

2.   A 1250°C high-temperature AlN layer was grown on in-situ nucleation layer by the high-yield industrial MOCVD. The calculated screw and edge dislocation densities of the AlN template are 2.54×10⁹ cm^-2 and 2.88× 10¹⁰ cm^-2. The results show that it is difficult to grow high-quality AlN layer by high-yield industrial MOCVD under general two-step process. Growing 1250°C high-temperature AlN layers with a thickness of 900nm on 25nm, 50nm and 100nm thick sputtering nucleation layers, respectively, the calculated screw dislocation densities of these AlN templates are 9.77 × 10⁶, 1.65 × 10⁷ and 1.11 × 10⁸ cm^-2, respectively, and the calculated edge dislocation densities are 1.46 × 10¹⁰、8.91 × 10⁹ and 6.70 × 10⁹ cm^-2, respectively. With the increasing of the thickness of the sputtering nucleation layer, the screw dislocation increases, but the edge dislocation density is reduced, and the sum of the screw and edge dislocation densities is reduced as well. Compared with the in-situ nucleation layer, the results show that the sputtering nucleation layer can effectively reduce the negative influence of pre-reaction, and provide a high-quality nucleation layer. Utilizing a sputtering nucleation layer, relatively high-quality AlN material can be grown on sapphire through the high-yield industrial MOCVD. The use of can be achieved using large-scale industrial MOCVD equipment to grow a relatively good quality AlN epitaxial layer.

3.        A 282nm commercial deep UV-LED was realized on 50nm-thick sputtered AlN nucleation layer by a high-yield industrial MOCVD. The light-output power (LOP) of the deep UV-LED reaches 1.65mW at 20 mA with external quantum efficiency of 1.87%. In addition, the saturation LOP of the deep UV-LED is 4.31mW at an injection current of 60 mA. In addition, utilizing a 50nm-thick sputtered AlN nucleation layer to enhance the quality of the epitaxial layer, commercial UVC-LEDs at 274nm were successfully realized on sapphire substrate by the high-yield industrial MOCVD system. The light-output power (LOP) of an UVC-LED reaches 0.51mW at 20 mA with external quantum efficiency of 0.56%. Hence, our studies supply a possible process to grow commercial UVC-LEDs in high throughput industrial MOCVD, which can increase yield, all at lower cost.

Keywords: high-yield industrial metalorganic chemical vapor deposition (MOCVD), aluminum nitride (AlN), deep ultraviolet light emitting diode (UV-LED), nucleation layer