氮化矽層抑制氮化鎵成長於(111)矽基板之回熔蝕刻現象研究

Abstract

異質磊晶氮化鎵(GaN)於矽(Si)晶片可應用於高功率半導體元件，其中GaN/ (111)Si為重要的結構，但直接成長GaN於Si基板上，高溫時Ga與Si會發生回熔蝕刻(meltback etching)反應，破壞基板平整度及磊晶品質，目前多使用氮化鋁(AlN)以改善此狀況。本論文探討GaN/Si結構(111)採用氮化矽(Si3N4)中間層抑制回熔蝕刻之效果，在許多半導體製程中Si3N4為良好擴散阻障層的材料，且β相氮化矽(β-Si3N4)與Si(111)基板晶格非常匹配。 本論文形成Si3N4層的製程，皆在有機金屬化學氣相沉積(MOCVD)系統中進行，分別使用下列三種Si3N4層的製程與結構: (1)高溫氮化Si基板，(2)高溫氮化並鍍上AlN層，(3)矽烷(silane，SiH4)與氨氣(HN3)反應沉積Si3N4。後續在不同溫度成長GaN，觀察不同製程之Si3N4抑制回熔蝕刻的效果。透過掃描式電子顯微鏡(SEM)觀察表面及橫截面；X光繞射(XRD)分析GaN結晶狀況；掃描穿透式電子顯微鏡(STEM)觀察高溫氮化的Si3N4界面；X光反射率(XRR)及X光光電子能譜儀(XPS)分析Si3N4膜厚和化學組成。 從實驗結果發現高溫氮化Si基板可形成2.5 ~ 3.5 nm的Si3N4層，且低溫成長GaN時，不會有回熔蝕刻反應，然而在高溫成長GaN階段，基板部分區域皆有回熔蝕刻的現象發生。在高溫氮化Si基板並搭配低溫AlN，Si3N4層厚度為1.7 ~ 2.5 nm，低溫AlN厚度為30 nm，兩者結合則能完全抑制回熔蝕刻。反應沉積得到厚度為13 nm的Si3N4，不論是否加入高溫AlN，皆無觀察到回熔蝕刻，表示13 nm厚的Si3N4足以完全抑制此現象發生。

GaN on Si(111) is an important structure in high-power devices for which heteroepitaxy of GaN on Si plays a critical role in the device fabrication. However, direct growth of GaN on Si may accompany with meltback etching reaction on Si which deteriorate the film qualities with rough surface morphologies. Conventional approach to prevent Si from meltback etching has used an AlN buffer layer on Si prior to GaN growth. In this thesis, we explored the effect of Si3N4 on meltback etching for GaN on Si as Si3N4 has been proved to be an excellent diffusion barrier in many semiconductor processes, and the lattice parameter of β phase Si3N4 can match well with Si(111). In this work, the formation of Si3N4 layer on 2-inch Si(111) was carried out in an MOCVD system, and three different processing conditions with structure were studied : 1. thermal nitridation of Si(111) with nitrogen-containing gas, 2. thermal nitridation followed by deposition of an AlN layer, and 3. deposition of Si3N4 by reaction of SiH4 and NH3. The subsequent growth of GaN were done at different temperature to study the effect of Si3N4 on inhibiting meltback etching. The morphology and crystallinity of GaN were characterized with scanning electron microscopy and X-ray diffraction. The GaN/Si3N4/Si interfacial structure in the nitrided samples were examined with scanning transmission electron microscopy. The film thickness and chemical bonding of deposited Si3N4 were characterized with X-ray reflectivity (XRR) and X-ray photoelectron spectroscopy. The experimental results show that Si3N4 formed by nitridation has a thickness of 2.5 - 3.5 nm, on which GaN growth at low temperature can be obtained with no evidence for meltback etching, while high-temperature growth of GaN results in meltback etching to occur in some portion of the surface area on the substrate. The thermal nitridation may not effectively inhibit meltback etching. For the bilayer structure consisting of the thermally nitrided Si3N4 layer (~ 1.7 - 2.8 nm thick) covered with a 30 nm thick AlN layer, no meltback etching has been observed after GaN growth at high temperature. For GaN growth on the 13 nm thick amorphous Si3N4 layer deposited with reaction of SiH4 with NH3 on Si, the evidence for meltback etching has not been found even after high-temperature GaN growth without AlN insertion.