譚仕林, 尹順達, 歐陽鋼

尺寸效應對MoS2/WSe2范德華異質(zhì)結(jié)構層間與俄歇復合的界面調(diào)控

譚仕林, 尹順達, 歐陽鋼

(湖南師范大學 物理與電子科學學院, 低維量子結(jié)構與調(diào)控教育部重點實驗室, 長沙 410081)

為探索界面工程對二維材料范德華異質(zhì)結(jié)構中載流子復合率的影響, 本工作基于界面鍵弛豫理論和費米黃金定則, 建立了范德華異質(zhì)結(jié)俄歇和層間復合率與各結(jié)構組元尺寸之間的理論模型。結(jié)果表明, MoS2/WSe2異質(zhì)結(jié)的俄歇復合壽命隨著組元尺寸的增大而增加, 且異質(zhì)結(jié)的俄歇復合率遠小于相應的單組元體系。在MoS2/WSe2雙層異質(zhì)結(jié)中引入薄h-BN插層后, 體系的層間復合率和俄歇復合率隨h-BN厚度的增加而分別呈現(xiàn)減小和增大的趨勢; 在組元處于單層MoS2和WSe2情況下, 當界面插層h-BN厚度達到9.1 nm時, 俄歇復合率將趨于5.3 ns–1。該研究結(jié)果為二維過渡金屬硫族化合物基異質(zhì)結(jié)光電器件的優(yōu)化設計提供了一種理論依據(jù)。

MoS2; WSe2; 異質(zhì)結(jié); 插層絕緣體; 層間復合; 俄歇復合

單層MoS2和WSe2是典型的二維過渡金屬硫族化合物(Transition metal dichalcogenides, TMDs)材料, 由于具有優(yōu)異的光吸收和尺寸可調(diào)的帶隙等性質(zhì)[1-3], 可組裝成具有II型能級排列的MoS2/WSe2雙層范德華異質(zhì)結(jié)構。然而, 由于體系中存在較高的層間載流子復合[4-6], 導致光電轉(zhuǎn)換效率較低(0.1%~2%)[4-6], 不能達到在光電/光伏器件中的實際應用要求。目前研究發(fā)現(xiàn)[7-12], 在異質(zhì)結(jié)中引入界面插層, 可有效提高器件性能。如在雙層異質(zhì)結(jié)中引入半導體插層可提升載流子遷移率; 而引入薄絕緣體插入層, 則可以抑制層間復合并增強光電轉(zhuǎn)換能力等。

俄歇復合(Auger recombination, AR)是一種以聲子形式釋放多余能量且與載流子濃度、尺寸和形貌有關的非輻射復合過程, 此外雙激子AR是一個三體過程, 可由正俄歇復合和負俄歇復合來描述[13-15]。近年來, 俄歇復合的研究體系主要是單組元體系材料[16-17]和雙層異質(zhì)結(jié)構[18-22], 而雙層范德華異質(zhì)結(jié)的界面調(diào)控研究集中于光譜特性及光電轉(zhuǎn)換方面(包括層間復合)[8,10-12]。在納米體系中, 由于動量和能量守恒帶來的運動約束及電子與空穴之間庫侖相互作用的增強效應, 俄歇復合過程會隨尺寸的減小而增強, 進而影響納米器件的光電性能[15,23]。一方面, 二維體系的雙激子俄歇復合壽命與厚度或橫截面積有關, 而與體積基本無關[16]。另一方面, 由于尺度效應及能帶偏移導致光生載流子的有效分離, 具有II型能級排列的雙層范德華異質(zhì)結(jié)的雙激子俄歇復合壽命將隨厚度的增加而變大[18]。此外, 在雙層異質(zhì)結(jié)中引入界面合金層, 可延長雙激子壽命[19]。將高透光率的插層絕緣體h-BN引入MoS2/WSe2范德華異質(zhì)結(jié)中, 則可抑制載流子的分離和層間復合, 其抑制作用隨著h-BN厚度的增大而明顯增強[8,10]。另外, 單組元體系材料的載流子遷移率可在三層膜體系中得以保持[24]。這些結(jié)果表明異質(zhì)結(jié)的材料尺寸、能級排列類型和界面性質(zhì)會顯著影響體系的層間和俄歇復合。

此外, 載流子復合也是影響范德華異質(zhì)結(jié)器件光電性質(zhì)的重要因素之一。如前所述, 高載流子復合令MoS2/WSe2的光電性能無法明顯提升, 導致其不能實際應用于光伏/光電器件。因此, 有效地調(diào)制異質(zhì)結(jié)的層間和俄歇復合以提升光電性能是目前迫切需要解決的問題之一。雖然在實驗上雙層范德華異質(zhì)結(jié)體系的俄歇和層間復合已取得很大進展[11,12,18-22], 但仍缺乏基于TMDs的半導體–半導體雙層范德華異質(zhì)結(jié)的界面調(diào)控及其層間和俄歇復合率的相應理論研究。此外, 諸如范德華異質(zhì)結(jié)的非輻射復合物理機制, 組元尺寸與層間和俄歇復合之間的理論關系等一些基本問題還有待闡明和探索。

為此, 為了探索異質(zhì)結(jié)體系的復合機制, 本研究基于界面鍵馳豫理論[25-28]和費米黃金定則[29], 從原子層次研究了TMDs與插層絕緣體的尺寸對TMDs/TMDs異質(zhì)結(jié)層間和俄歇復合的影響, 得到了尺寸依賴的帶隙漂移、俄歇復合率(壽命)和層間復合率的關系, 為TMDs基范德華異質(zhì)結(jié)的優(yōu)化設計提供了一種理論依據(jù)。

1 理論方法

1.1 異質(zhì)結(jié)模型

由于MoS2、WSe2的強光吸收和h-BN的高透光率等性質(zhì), 以及h-BN不影響MoS2/h-BN/WSe2中TMDs的層內(nèi)激子結(jié)合能[9], 使得該異質(zhì)結(jié)構在光電器件中具有廣闊的應用前景。本工作首先采用Atomistix Toolkit (ATK)建立了垂直堆垛的MoS2/WSe2和MoS2/h-BN/WSe2范德華異質(zhì)結(jié)模型, 如圖1所示, 其中,1、2和3分別為MoS2、h-BN和WSe2的厚度。

1.2 理論與方法

1.2.1 鍵弛豫理論

圖1 (a) MoS2/WSe2和(b) MoS2/h-BN/WSe2兩種不同范德華異質(zhì)結(jié)構示意圖

1.2.2 費米黃金定則

2 結(jié)果與討論

2.1 帶隙漂移

各材料帶隙的尺寸效應如圖2所示, MoS2、WSe2和h-BN的帶隙寬度隨尺寸的增大而減小, 并迅速趨近于塊體值, 且其相應帶隙在小尺寸處變化明顯, 表明表面原子的影響隨厚度的減小而增大。根據(jù)鍵馳豫機理, 表/界面原子的配位缺陷隨體系尺寸的減小而增大, 導致原子鍵收縮, 體系趨近于能量最低的自平衡狀態(tài)[4], 影響能帶的漂移。近年來, 相關的實驗研究[2]和第一性原理計算[44]也表明, 帶隙隨尺寸的減小而增大。單層MoS2(WSe2)的帶隙為1.88 eV (1.68 eV), 而相應塊體則為1.29 eV (1.20 eV)[2,4], 我們的結(jié)果與實驗測量和第一性原理計算的結(jié)果一致。圖2中的插圖為三單層MoS2/h-BN/WSe2異質(zhì)結(jié)的能級排列圖。顯然, h-BN與MoS2之間有較大的電子勢壘, 可以有效地抑制層間復合, 從而提升光電轉(zhuǎn)換效率[11-12]。此外, 在保持TMDs厚度不變的情況下, h-BN/TMDs異質(zhì)界面處的導帶偏移與價帶偏移都會隨著h-BN厚度的增大而減小, 這是納米體系導帶底能量的減小、價帶頂能量的增大[39], 以及特殊的能級排列類型所導致。

圖2 厚度依賴的MoS2、WSe2和h-BN的帶隙

2.2 俄歇復合與層間復合

圖3 不同體系的厚度依賴的負俄歇復合壽命()和雙激子俄歇復合壽命()

圖4 厚度依賴的異質(zhì)結(jié)層間(R)和雙激子俄歇復合率()

因此, 在低維納米結(jié)構中, 增大光活性材料TMDs的尺寸和構成異質(zhì)結(jié)是抑制俄歇復合過程的有效方法。此外, 插層絕緣體有益于提升器件性能; 然而插層絕緣體削弱了光生載流子的分離, 并且增強了異質(zhì)結(jié)中的量子限域效應, 導致俄歇復合率增強。值得注意的是, 雖然在計算中假設低維納米體系具有一個理想的能帶結(jié)構, 并忽略了異質(zhì)結(jié)界面效應的影響, 但本工作的結(jié)果變化趨勢與實驗測量的變化趨勢非常吻合。

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Size Effect on the Interface Modulation of Interlayer and Auger Recombination Rates in MoS2/WSe2van der Waals Heterostructures

TAN Shilin, YIN Shunda, OUYANG Gang

(Key Laboratory of Low-Dimensional Quantum Structures and Quantum Control of Ministry of Education, Hunan Normal University, Changsha 410081, China)

To explore the interface engineering on the carrier recombination in two-dimensional (2D) van der Waals (vdW) heterostructures, we developed a theoretical model to address the size-dependent interlayer and Auger recombination rates in MoS2/WSe2in terms of interface bond relaxation method and Fermi's golden rule. It is found that the Auger recombination lifetime in MoS2/WSe2increases with increasing thickness due to the weakening of Coulomb interaction between holes and electrons, as well as the Auger recombination rate is much smaller than that of MoS2and WSe2units. However, when the thin h-BN layer is introduced into the MoS2/WSe2, the interlayer and Auger recombination rates show opposite trends as the h-BN thickness increases. When the thickness of h-BN reaches 9.1 nm under the condition of 1L MoS2/h-BN/1L WSe2, the Auger recombination rate approaches 5.3 ns–1. These results indicate that the relevant recombination processes can be tuned by interface and dimension. Therefore, our results provide a useful guidance for the optimal design of 2D transition metal dichalcogenides-based optoelectronic nanodevices.

MoS2; WSe2; heterostructure; intercalated insulator; interlayer recombination; Auger recombination

O484

A

1000-324X(2020)06-0682-07

10.15541/jim20190386

2019-07-22;

2019-08-19

國家自然科學基金(11574080, 91833302)National Natural Science Foundation of China (11574080, 91833302)

譚仕林(1994–), 男, 碩士研究生. E-mail: tanshilin_hnu@126.com

TAN Shilin (1994–), male, Master candidate. E-mail: tanshilin_hnu@126.com

歐陽鋼, 教授. E-mail: gangouy@hunnu.edu.cn

OUYANG Gang, professor. E-mail: gangouy@hunnu.edu.cn