高温预生长对图形化蓝宝石衬底GaN薄膜质量的提高

Improvement of GaN Thin-film Quality Grown on Patterned Sapphire Substrate by High-temperature Pre-growth Treatment

在图形化蓝宝石衬底生长低温缓冲层之前,通入少量三甲基镓(TMGa)和大量氨气进行短时间的高温预生长,通过改变TMGa流量制备了4个蓝光LED样品。MOCVD外延生长时使用激光干涉仪实时监测薄膜反射率,外延片使用高分辨率X射线衍射(002)面和(102)面摇摆曲线估算位错密度,并使用光致发光谱表征发光性能,制备成芯片后测试了正向电压和输出光功率。结果表明,高温预生长可促进薄膜的横向外延,使得三维岛状GaN晶粒在较小的薄膜厚度内实现岛间合并,有利于降低位错密度,提高外延薄膜质量,LED芯片的输出光功率的增强幅度达29.1%,而电学性能无恶化迹象;但高温预生长工艺中TMGa的流量应适当控制,过量的TMGa导致GaN晶粒过大,将延长岛间合并时间,降低晶体质量。

Before the epitaxy of low-temperature buffer layer,the patterned sapphire substrate was prepared by a pre-growth treatment at a high temperature under a small amount of trimethyl gallium(TMGa) and a great quantity of ammonia(NH3).Four types of blue light-emitting diode(LED)wafers were fabricated using different flow-rate of TMGa.The laser interferometer in MOCVD was used to monitor the thin-film reflectance during the epitaxial growth process.The high-resolution Xray diffraction rocking curves of(002) and(102) crystal faces were utilized to estimate the threading dislocation density.The photoluminescence spectra were measured to feature the light-output performance.Moreover,LED chips were fabricated and tested.It is shown that the pre-growth treatment benefits the lateral growth of three-dimensional GaN islands in epitaxy process and leads to fast coalescence within a small thickness.Thus,the threading dislocation can be suppressed and the quality of epitaxial film would be improved.The enhancement of light-output power of LED chips can be up to 29.1% without degradation of electrical performance.However,the flow rate of TMGa should be tuned carefully,because excess quantity of TMGa would cause a large size of GaN grain crystal,which induces delayed coalescence time and low-quality of epitaxial film.