[Paper Review] Heteroepitaxial growth of high optical quality, wafer-scale van der Waals heterostrucutres

Katarzyna Ludwiczak, Aleksandra K. Dąbrowska+10 more|arXiv (Cornell University)|Sep 4, 2021

2D Materials and Applications67 references31 citations

TL;DR

This study demonstrates a hybrid MOVPE-MBE epitaxial growth method to produce wafer-scale, high-optical-quality MoSe2 monolayers directly on epitaxial hBN substrates on 2-inch sapphire wafers. The technique achieves narrow, well-resolved excitonic peaks in photoluminescence across the entire wafer, with only ±0.14 meV energy deviation, proving exceptional homogeneity and optical quality for large-scale van der Waals heterostructures.

ABSTRACT

Transition metal dichalcogenides (TMDs) are materials that can exhibit intriguing optical properties like a change of the bandgap from indirect to direct when being thinned down to a monolayer. Well-resolved narrow excitonic resonances can be observed for such monolayers, however only for materials of sufficient crystalline quality, so far mostly available in the form of micrometer-sized flakes. A further significant improvement of optical and electrical properties can be achieved by transferring the TMD on hexagonal boron nitride (hBN). To exploit the full potential of TMDs in future applications, epitaxial techniques have to be developed that not only allow to growlarge-scale, high-quality TMD monolayers, but allow to perform the growth directly on large-scale epitaxial hBN. In this work we address this problem and demonstrate that MoSe2 of high optical quality can be directly grown on epitaxial hBN on an entire two-inch wafer. We developed a combined growth theme for which hBN is first synthesized at high temperature by Metal Organic Vapor Phase Epitaxy (MOVPE) and as a second step MoSe2 is deposited on top by Molecular Beam Epitaxy (MBE) at much lower temperatures. We show that this structure exhibits excellent optical properties, manifested by narrow excitonic lines in the photoluminescence spectra. Moreover, the material is homogeneous on the area of the whole two-inch wafer, with only +/-0.14 meV deviation of excitonic energy. Our mixed growth technique may guide the way for future large-scale production of high quality TMD/hBN heterostructures.

Motivation & Objective

• To develop a scalable, epitaxial method for growing high-quality transition metal dichalcogenides (TMDs) on large-area substrates.
• To overcome the limitations of mechanical exfoliation and post-transfer processing by enabling direct, wafer-scale growth of TMDs on epitaxial hexagonal boron nitride (hBN).
• To achieve high optical quality in MoSe2 monolayers by combining MOVPE-grown hBN with MBE-deposited MoSe2.
• To demonstrate uniform, high-quality van der Waals heterostructures across an entire 2-inch wafer for scalable optoelectronic applications.

Proposed method

• First, high-quality hBN layers (1–13.6 nm thick) were grown on 2-inch sapphire wafers using Metalorganic Vapor Phase Epitaxy (MOVPE) with two growth modes: Continuous Flow Growth (CFG) and two-stage epitaxy.
• The two-stage epitaxy mode enabled thicker hBN layers (>6 nm) with excellent structural quality by combining a CFG buffer with pulsed ammonia and TEB flow switching.
• Subsequently, MoSe2 monolayers were grown on the MOVPE-hBN substrates using Molecular Beam Epitaxy (MBE) at lower temperatures, with optimized growth and annealing cycles.
• Three MBE growth variants were tested: long-duration growth (MoSe2A), faster growth with single annealing (MoSe2B), and multi-cycle growth with stepwise heating (MoSe2C).
• Optical and structural characterization was performed using low-temperature photoluminescence, Raman spectroscopy, XRD, FTIR reflectance, SEM, AFM, and transmission electron microscopy (TEM) with FIB cross-sectioning.
• Photoluminescence and Raman mapping were conducted at room temperature across the 2-inch wafer to assess spatial homogeneity and optical quality.

Experimental results

Research questions

• RQ1 Can high optical quality MoSe2 monolayers be directly grown on large-area epitaxial hBN substrates using a hybrid epitaxial approach?
• RQ2 What is the impact of hBN thickness and growth mode (CFG vs. two-stage) on the optical and structural quality of subsequently grown MoSe2?
• RQ3 To what extent can the optical properties of MoSe2/hBN heterostructures be homogenized across a 2-inch wafer using this method?
• RQ4 Can well-resolved, narrow excitonic lines be achieved in large-area heterostructures, comparable to those in mechanically exfoliated flakes?

Key findings

• The hybrid MOVPE-MBE approach successfully produced wafer-scale MoSe2/hBN heterostructures on 2-inch sapphire wafers with high optical quality.
• Low-temperature photoluminescence revealed well-resolved neutral A-exciton and trion peaks, indicating high crystalline quality and low defect density.
• The excitonic energy exhibited a standard deviation of only ±0.14 meV across the entire 2-inch wafer, confirming exceptional homogeneity.
• Photoluminescence mapping at room temperature showed a peak width of 67.9 ± 6.1 meV and an energy of 1569.1 ± 2.1 meV, with low standard deviation indicating uniform optical response.
• The X0A exciton peak width distribution had a standard deviation of 24.04 ± 0.84 meV, and the XC peak width had 21.5 ± 1.8 meV, further confirming consistent optical properties.
• TEM and AFM analysis confirmed high structural quality, with minimal surface defects and uniform layer thickness, even in thicker hBN samples.