Modification of the bandgap of hexagonal BN and deep-ultraviolet luminescence dynamics of excitons in them

2022 Fiscal Year Final Research Report

• Project/Area Number: 20K20993
• Research Category: Grant-in-Aid for Challenging Research (Exploratory)
• Allocation Type: Multi-year Fund
• Review Section: Medium-sized Section 21:Electrical and electronic engineering and related fields
• Research Institution: Tohoku University
• Principal Investigator: CHICHIBU Shigefusa 東北大学, 多元物質科学研究所, 教授 (80266907)
• Co-Investigator(Kenkyū-buntansha): 原 和彦 静岡大学, 電子工学研究所, 教授 (80202266) ; 小島 一信 東北大学, 多元物質科学研究所, 准教授 (30534250) ; 嶋 紘平 東北大学, 多元物質科学研究所, 助教 (40805173)
• Project Period (FY): 2020-07-30 – 2023-03-31
• Keywords: 窒化ボロン / 間接遷移 / 直接遷移 / 深紫外線 / 時間分解分光

Outline of Final Research Achievements

Hexagonal boron nitride (hBN) crystallizes in layers of a two-dimensional honeycomb BN structure. Since hBN exhibits high-efficiency emissions at 5.2-5.9 eV in spite of the indirect bandgap, hBN has a potential for the use in deep-ultraviolet light emitters. Recently, the presence of a direct bandgap has been reported in an isolated monolayer hBN (mBN). However, little is known about the luminescent properties of mBN. Here, temporally and spatially resolved luminescence measurements were carried out to elucidate the emission dynamics of indirect excitons (iXs) in hBN and direct excitons (dXs) in mBN epilayers. The room-temperature emission lifetimes of iXs in hBN were about 55 ps, which implies excellent radiative performance of hBN. Cathodoluminescence (CL) spectra at 13 K of mBN exhibited a distinct dX emission peak at around 6.04 eV superimposed on a broad 5.5-eV-band that originated from hBN. In addition, a CL peak at 6.035 eV that originated from graphitic BN was identified.

Free Research Field

化合物半導体光物性・光デバイス

Academic Significance and Societal Importance of the Research Achievements

hBNが間接遷移型半導体でありながら光子エネルギー約5.8 eVの高効率深紫外線(DUV)発光を呈すること、hBNを単一層まで薄くしたmBNの禁制帯が直接遷移型となり6.04 eVの発光を呈する事に加え、グラファイト状積層構造のBernal BNも直接遷移型ではないかと思われる禁制帯を持ち6.035 eVの発光を呈する事を明らかにした事は学術的意義が大きいだけでなく、BNを用いたDUV発光素子の開発に繋がり殺菌や消毒を通じて安全な水・空気・サニタリー環境の提供を通じて社会貢献できる。また、本研究に用いた自作の陰極線蛍光計測系は禁制帯幅に制限されないためBN以外の2次元材料の評価に有用である。