Wafer Scale Evaluation and Device Fabrication of MoS₂ Film Grown by MOCVD

The fabrication of wafer-scale FETs using CVD-grown MoS₂ has been the subject of intensive research for future integration. Although powder-source CVD, initially developed, has been extended to enable wafer-scale growth, its non-uniform powder source feeding prevents uniform growth on 300-mm wafers. Gas-source CVD, particularly MOCVD, offers a stable and switchable source supply, thereby yielding enhanced uniformity. MOCVD is anticipated to serve as an industrially compatible growth method. However, previous investigations of MoS₂ FET have suggested unsatisfying quality. Despite the importance of improving MOCVD film quality, the origin of degradation remains unrevealed. The objectives of this study are to evaluate the intrinsic quality of film MOCVD and to identify the factors influencing mobility degradation.<br/>From the <i>I</i>_d-<i>V</i>_g curve, showing <i>I</i>_on of ~10^-6 A and <i>I</i>_on/<i>I</i>_off ratio of ~10⁷. To extract <i>μ</i> without the influence of contact resistance, Y-function method was employed and yielded an estimated value of ~40 cm²/Vs. While this value is comparable to that reported by the other reports, it falls short of powder-source CVD standards. <i>I</i>_d-<i>V</i>_g characteristics at <i>T</i> = 20 - 300 K show that <i>I</i>_on decreased with decreasing <i>T</i>, and random teregraph noise (RTN) was observed within the temperature range of 20 - 100 K. <i>μ</i> was extracted by Y-function method at each temperature and it dropped with decreasing <i>T</i>, indicating the existence of thermally-activated trap sites in the band gap of MoS₂. In contrast to this result,<i> μ</i> of powder-source CVD increases with lowering <i>T</i> due to the suppression of phonon scattering. For mechanically exfoliated single grain MoS₂, the temperature dependence of <i>μ</i> has been reported to change from the thermally-activated behavior to the phonon limited behavior by healing sulfur vacancies. The critical mobility for this transition seems to locate around 50 cm²/Vs at 300 K. For further investigation of the thermally-activated behavior, linear polarized second harmonic generation (SHG) mapping will be effective. In the MoS₂ channel region, distinct black dots could be identified in the SHG mapping. However, no scratches and residues corresponding to black dots are discernible from the AFM image. There are two interpretations about SHG polarization dependencies of black dots. (i) Low quality grains and/or grain boundaries may exhibit black dots. (ii) Grain rotation may result in black dots. Although it is crucial to separate these two scenarios by polarization-dependent SHG measurements, this study suggests that SHG mapping is a convincing tool for detecting the origin of degradation.