硅衬底氮化镓基LED薄膜转移至柔性黏结层基板后其应力及发光性能变化的研究

本文将硅(Si)衬底上外延生长的氮化镓(GaN)基发光二极管(LED)薄膜转移至含有柔性黏结层的基板上, 获得了不受衬底和支撑基板束缚的LED薄膜. 利用高分辨率X射线衍射仪(HRXRD)研究了薄膜转移前后的应力变化, 同时对其光致发光(PL)光谱的特性进行了研究. 结果表明: 硅衬底GaN基LED薄膜转移至柔性基板后, GaN受到的应力会由转移前巨大的张应力变为转移后微小的压应力, InGaN/GaN量子阱受到的压应力则增大; 尽管LED薄膜室温无损转移至柔性基板其InGaN阱层的In组分不会改变, 然而按照HRXRD倒易空间图谱通用计算方法会得出平均铟组发生了变化; GaN基LED薄膜从外延片转移至柔性基板时其PL谱会发生明显红移.

Changes of stress and luminescence properties in GaN-based LED films before and after transferring the films to a flexible layer on a submount from the silicon epitaxial substrate

Due to the lack of GaN substrates, hetero-epitaxial growth of GaN thin films is usually carried out on a foreign substrate. There are three kinds of substrate for GaN: sapphire, silicon carbide, and silicon; the sapphire substrate is the chief one, currently. Due to the availability of large scale and low cost of Si substrates, in recent years, extensive research has been devoted to the development of gallium nitride (GaN) optoelectronic devices on silicon substrates. Because of the large lattice mismatch and thermal-expansion cofficient difference between Si and GaN, it is difficult to grow thick enough crack-free GaN LED film on Si substrates. The two main kinds of methods for overcoming the crack problem are using the patterned Si substate and the thick AlGaN buffer layer. Although the two techniques could solve the problem of crack by cooling after growth, they will lead to an increase in tensile stress for GaN on Si. When making vertical-structured LED devices by transferring the GaN-based LED thin films from Si substrate to a new submount, this tensile stress will be partially released; but few researches have been made about the stress change before and after the transfer of the film, although the stress in GaN is an important factor that alters the energy band structure and may influence the vibrational properties. In this paper, we grow the crack-free GaN-based LED films on patterned Si(111), then light-emitting diode (LED) thin films are successfully transferred from the original Si (111) substrate to the submount with a flexible layer, and then the LED films without the influence of the submount and substrate are fabricated. In the following experiments, the strain-stress variation of the LED film is determined by using nondestructive high resolution X-ray diffraction (HRXRD) in detail, and the variation of photoluminescence (PL) properties of the film is studied too. Results obtained are as follows: 1) When the LED film is transferred to the flexible submount, the huge tensile stress will turn into compressive stress, and the latter in the InGaN layers will increase. 2) The In concentration in the (InGaN/GaN) MQW (multi-quantum well) systems can be evaluated with the help of reciprocal space maps (RSM) around the symmetric (0002) and asymmetric (1015) Bragg reflections. The In concentration in (InGaN/GaN) MQW will reduce when the GaN-based LED film is transferred to the flexible submount. 3) The PL spectra of the LED films will obviously appear red shift, after they are transferred to the flexible submount.