Crystal quality degradation in MoTe2 monolayers by a thermal annealing and its suppression by hexagonal boron nitride encapsulation
Authors: Hayashida Shunya , Saito Risa , Watanabe Kenji , Taniguchi Takashi , Sawano Kentarou , Hoshi Yusuke 
Published: December 2020

Two dimensional transition metal dichalcogenides (TMDs) have attracted great research interests owing to a crossover to a direct gap upon reduction of the layer number down to a monolayer (1L) and exciton formation at room temperature based on extraordinarily large binding energy. In particular, a semiconducting molybdenum ditelluride (MoTe2) emits photons in the near-infrared region around 1.1 eV. Furthermore, it has a strong spin-orbit coupling to result in the longer spin and valley relaxation, providing a path to practical Si-based optoelectronic device applications. However, the 1L-MoTe2 crystal has some drawbacks such as surface oxidation within several days owing to its poor structural stability and crystal defect formation by a thermal annealing at a low temperature of 200 °C. It is reported that encapsulation by hexagonal boron nitride (hBN) prevents the oxidation of 1L-MoTe2 in air. There are few reports on effects of a thermal annealing on structural stability in hBN-encapsulated 1L-MoTe2. In this study, we investigate the effects of a thermal annealing on the crystal quality of hBN-encapsulated 1L-MoTe2 by a photoluminescence (PL) measurement at low temperatures. We demonstrate that hBN encapsulation is effective for suppression of crystal defect formation in 1L-MoTe2, improving thermal stability.

Bulk hBN crystals were grown using a temperature-gradient method at a high pressure (4.0-5.5GPa) and high temperature (1500-1750 °C). Monolayer MoTe2 and multilayered hBN flakes were exfoliated from bulk crystals and were prepared on a polydimethylsiloxane (PDMS) sheet. We fabricated two types of sample structures on a 100-nm-thick SiO2/Si (100) substrate by a standard dry transfer technique using these flakes. The first sample comprised 1L-MoTe2 directly deposited on the SiO2/Si substrate. The second sample comprised 1L-MoTe2 encapsulated by hBN. The fabricated sample structures were thermally annealed at temperatures ranging from 100 to 200 °C for 15min in an atmosphere.

Figures 1(a) and 1(b) show the PL spectra at 25K of the SiO2-supported and hBN-encapslated 1L-MoTe2, respectively. These PL spectra are reproduced using multi-Voigt functions to evaluate PL peak intensities in each spectrum. PL emissions attributed to neutral exciton (X0), charged exciton (T), and defect-bound excitons (XB1, XB2) are observed in both samples of SiO2-supported and hBN-encapsulated 1L-MoTe2. We identify these MoTe2 crystals as monolayers from the X0 PL peak position of 1.19 eV. Figure 1(c) shows the trion spectral weight (I
T
/I
tot) plotted as a function of the annealing temperature for the SiO2-supported and hBN-encapsulated 1L-MoTe2. For both sample structures, the I
T
/I
tot values decreases by the thermal annealing at 200℃, implying the decrease in excess carriers in 1L-MoTe2. The excess carriers should originate from impurities and residues adsorbed on the 1L-MoTe2. Thus, the thermal annealing makes it possible to reduce the impurities and residues, realizing high-quality sample structures. Figure 1(d) shows the PL intensity ratio (I
XB1
+
I
XB2)/I
X0 plotted as a function of the annealing temperature. Here, I
XB1, I
XB2, and I
X0 are PL intensities of XB1, XB2, and X0 peaks, respectively. For the SiO2-supported 1L-MoTe2, the (I
XB1
+
I
XB2)/I
X0 value increases by increasing the annealing temperature while it is kept to be almost constant for the hBN-encapsulated 1L-MoTe2. It is reported that the XB1 and XB2 peaks are attributed to the formation of Te vacancies by a thermal annealing and adsorption of O2 and H2O molecules at the crystal defect sites, respectively. Therefore, an increase in the (I
XB1
+
I
XB2)/I
X0 value of the SiO2-supported samples should result from the crystal defect formation in 1L-MoTe2, indicating a large density of non-radiative recombination centers at 200 °C. In contrast, for the hBN-encapsulated samples, the Te vacancy formation and adsorption of O2 and H2O molecules at the crystal defect sites should be avoided owing to the top hBN acting as the capping layer to suppress Te atom desorption.

In summary, we investigated the effect of a thermal annealing on a PL spectrum for SiO2-supported and hBN-encapsulated samples. It was found that the thermal annealing at 200 °C were able to remove residues and impurities on the 1L-MoTe2. For the SiO2-supported samples, an increase in the (I
XB1
+I
XB2
)/I
X0 value was seen by the thermal annealing, indicating crystal defect formation due to desorption of Te atoms. In contrast, for the hBN-encapsulated samples, it was kept to be almost constant even after the thermal annealing at 200 °C. It was therefore concluded that the hBN encapsulation should improve the thermal stability in 1L-MoTe2.

This work was partly supported by JSPS KAKENHI Grant Numbers JP25107003, JP25107004, JP15K21722, JP26248061, JP25107002, JP16H00982, and JP20H00354 and the CREST (JPMJCR15F3), JST.

Figure 1