Growth of the III-VI Two Dimensional Chalcogenide Layered Semiconductors for Electronic Device Applications by Molecular Beam Epitaxy
Restricted (Penn State Only)
- Author:
- Liu, Derrick Shao
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 11, 2024
- Committee Members:
- John Mauro, Program Head/Chair
Suzanne Mohney, Major Field Member
Joan Redwing, Chair & Dissertation Advisor
Joshua Robinson, Major Field Member
Ying Liu, Outside Unit & Field Member - Keywords:
- Molecular Beam Epitaxy
thin film
2D materials
indium selenide
growth kinetics
chalcogens - Abstract:
- Post-transitional metal chalcogenides have gained increasing attention since graphene ignited the exploration of 2D materials. In particular, the indium selenide family shows promising potential in electronics, optoelectronics, sensors, and catalyst applications thanks to its intriguing properties. For example, γ-InSe has been demonstrated as a high mobility channel material and broadband photodetector, and In2Se3 has been predicted and realized as 2D ferroelectrics for nonvolatile memory applications. In response to the properties investigation, large-scale synthesis is required for indium selenide to transit into practical utilization in future technologies. Molecular beam epitaxy (MBE) is one of the favorable deposition techniques for achieving high-quality, large-scale indium selenide thin films. However, due to the coexisting nature and structural complexity of the indium selenide family, challenges such as stoichiometry control and structure identification remain unsolved. In addition, how the performance of MBE grown indium selenide thin film compared against exfoliated crystal or thin film synthesized by other approaches is left unanswered. This dissertation aims to provide a universal reference for γ-InSe and β-In2Se3 growth under intuitive metrics, which can benefit indium selenide synthesis in MBE and other deposition methods. It also focuses on examining the electronic properties of γ-InSe and β-In2Se3, specifically, the carrier mobility of γ-InSe and ferroelectricity of β-In2Se3. The Se absorption and desorption behavior that predominately dictates the stoichiometric ratio of indium selenide films is studied as well to provide an advanced understanding of the chalcogen species’ growth kinetics. Se’s absorption behavior is determined as the sticking coefficient of Se on Se, which was measured by a heated quartz crystal microbalance as a function of temperature. Te was also measured as the complementary study of chalcogen species. Both sticking coefficients are found to drop sharply within a narrow temperature range of 20 and 30 °C from above 0.8 down to about 0.2 at film surface temperatures around 35 and 115 °C for selenium and tellurium, respectively. While the sticking coefficient of tellurium reaches zero at temperatures above 150 °C, the sticking coefficient of selenium remains about 0.2 up to a film surface temperature of 60 °C, suggesting that selenium was supplied in different chemical forms. The desorption behavior of Se and Te was investigated by desorbing Se and Te thin film that was deposited on the quartz crystal surface through effusion cells in MBE. Tellurium is found to exhibit one desorption peak around 190 °C, indicating the loss of the entire film irrespective of film thickness. Selenium is revealed that the thermal desorption takes place via a two-stage process with a smaller portion of the material desorbing within a narrower temperature window of 5 °C at a peak temperature of 65 °C, while most selenium desorbs within a temperature range of 10 °C around 90 °C. This two-stage behavior indicates the presence of at least two chemically distinct selenium species or binding states. These results provide important insights into the kinetics of chalcogenide-based film growth and expose the need for a more detailed understanding of the chemical composition state of atomic and molecular beams supplied from thermal evaporation sources during growth. Following the growth kinetics study of Se, the wafer-scale combinatorial approach was employed to map out the growth window as functions of the Se/In ratio and growth temperature for γ-InSe on the Si(111) 7 × 7 substrate. Four distinct surface morphologies of γ-InSe are found, enabling a discussion of the growth mechanisms associated with each morphology. Cross-sectional atomic resolution scanning transmission electron microscopy (STEM) confirmed that the film was of high crystalline quality and had nearly single-phase γ-InSe. Furthermore, migration enhanced epitaxy growth approach was utilized to expand the growth window of γ-InSe. Electrical measurement and band alignment calculations by X-ray photoelectron spectroscopy pointed out that the p-type Si hindered the transport mechanism of γ-InSe as carriers migrated away from the interface. This part of the work elucidates the indium selenide phase map for thin film growth parameters, providing invaluable landmarks for the reproducible synthesis of high-quality γ-InSe layers, yet an alternative platform is needed to extract the carrier mobility of MBE grown γ-InSe. Growth window mapping was conducted on β-In2Se3 as well for Si(111) 1 × 1 and Al2O3(0001) substrates. A characterization filtering process of performing X-ray diffraction and Raman spectroscopy in sequence was demonstrated to identify the β-In2Se3 film. Growth of β-In2Se3 on Si(111) is feasible at growth temperature as low as 150 °C, while β-In2Se3 can be grown on Al2O3(0001) with a wide range of parameters. The effects of growth parameters on β-In2Se3 surface morphology were studied by atomic force microscopy. High growth temperature and large flux ratio lead to island and pinhole formation. STEM imaging identified the β-In2Se3 as the 3R polytype on both substrates. Moreover, the STEM intensity profile indicated that the β-In2Se3 film on Al2O3(0001) exhibited in-plane Se atoms displacement, which resulted in in-plane polarization. Second harmonic generation measurement data supports the claim of in-plane polarization. Potential ferroelectric behaviors were observed in the polarization-voltage and positive-up-negative-down (PUND) measurements when the polar axis was parallel to the Se displacement direction. These findings provide intuitive growth metrics for β-In2Se3 synthesis in MBE and explore the suitable candidates for 2D ferroelectrics nonvolatile memory.