非故意掺杂GaN层厚度对蓝光LED波长均匀性的影响

通过调整非故意掺杂氮化镓层的厚度,分析氮化镓基LED外延生长过程中应力的演变行为,以控制外延片表面的翘曲程度,从而获得高均匀性与一致性的外延片。由于衬底与外延层之间的热膨胀系数差别较大,在生长温度不断变化的过程中,外延片的翘曲状态也随之改变。在n型氮化镓生长结束时,外延片处于凹面变形状态。在随后的过程中,外延薄膜"凹面"变形程度不断减小,甚至转变为"凸面"变形,所以n型氮化镓生长结束时外延片的变形程度会直接影响多量子阱沉积时外延片的翘曲状态。当非掺杂氮化镓沉积厚度为3.63μm时,外延片在n型氮化镓层生长结束时变形程度最大,在沉积多量子阱时表面最为平整,这与PLmapping测试所得波长分布以及标准差值最小相一致。通过合理控制非故意掺杂氮化镓层的厚度以调节外延层中的应力状态,可获得均匀性与一致性良好的LED外延片。

Effect of Undoped GaN Layer Thickness on The Wavelength Uniformity of GaN Based Blue LEDs

InGaN based blue light emitting diodes(LED) with different thickness of undoped GaN (u-GaN) layer were grown on c-plane pattern sapphire substrates by metal-organic chemical vapor deposition (MOCVD).The structure and photoelectric properties were characterized by in-situ wafer bowing measurements, high-resolution X-ray diffraction(HRXRD) and photoluminescence (PL).Due to the different thermal expansion coefficients between the sapphire and the epitaxial film, the curvature of wafer varies constantly in the progress of temperature changing.It is found that the wafer presents concave bowing at the end of n-GaN growth directly affects the wafer status of bowing during the MQWs growth.The wafer with 3.63 μm u-GaN layer is under the biggest bowing status indicating that the stress value of n-GaN is the maximum.In the subsequent lower temperature MQWs deposition stage, the concave deformation constantly decreases, and even transforms into the convex deformation.The in-situ wafer bowing measurements results are in accordance with the PL-mapping tests.The wafer with better uniformity and consistency can be obtained by adjusting the thickness of u-GaN layer which can change the status of stress in the epitaxial structures, and finally be beneficial to reduce the processing time and the cost of the LED chips.