We succeeded in growing Ge homoepitaxial films by metal–organic chemical vapor deposition (MOCVD) using tertiarybutylgermane (t-C4H9GeH3). We synthesized and investigated the characteristics of t-C4H9GeH3. The vapor pressure was sufficiently high in the CVD process. The precursor was sufficiently safe as it did not have a pyrophoric and explosive nature. The Ge homoepitaxial growth was achieved at 360 °C under reduced pressure on an appropriately cleaned Ge(001) substrate.
Ge homoepitaxial films are grown at low growth temperature of 320°C by metal-organic chemical vapor deposition (MOCVD) using tertiarybutylgermane (t-C4H9GeH3). We also performed ab initio calculations in order to reveal the chemical reaction for the epitaxial growth. As the result, it was revealed that the t-C4H9GeH3 was most likely decomposed into germane (GeH4) and isobutene [CH2=C(CH3)2] through the β-hydrogen elimination. We considered that this chemical nature allowed the growth temperature as low as that obtained by GeH4 precursor with sufficiently suppressed C impurity incorporation.
Recent work on the research of layered materials has revealed the material’s unique physical properties, its applications in various fields, and fabrication processes to produce high quality samples. We have been focusing on transition metal dichalcogenides (TMD) and its alloys. Fabrication of TMD alloys enables bandgap/band offset tuning which expands the application of TMD materials in various fields to a further extent. Although it has been predicted the fabrication of TMD alloys is difficult since the alloys are expected to be thermodynamically unstable [1,2], we have so far fabricated metal alloy, Mo 1- x W x S 2 [3], and chalcogen alloy, MoS 2(1- x ) Te 2 x [4]. In these studies, we have confirmed the control of composition by adjusting sputtering condition as well as bandgap and band offset shift according to the composition. However, the stability of the material, especially the change of physical properties over certain period of time, has not been discussed in depth despite the fact that such alloy material are thermodynamically unstable, as mentioned above. In addition, although there are numerous results on the fabrication differences, there are not many reports discussing on the stability of the fabricated films for TMDs [5]. In this study, we focused on the stability of the materials and optimization of fabrication condition in order to prevent degradation. MoS 2(1- x ) Te 2 x alloys are fabricated by co-sputtering MoS 2 and MoTe 2 followed by tellurization by inorganic Te precursor, (i-C 3 H 7 ) 2 Te, in H 2 or N 2 ambient. The samples were stored in a vacuum dessicator for a period of time (which we will refer as “storage time”) and then evaluated with X-ray photoelectron spectroscopy (XPS). It was revealed that depending on the carrier gas, or the ambient, during tellurization, the amount by which Mo is oxidized after the storage time is different despite the fact that the samples showed almost identical chemical state right after the tellurization. This may be attributed to the difference in grain size where H 2 ambient may have produced films with smaller grain size, i.e. longer or more grain boundaries, resulting in more area to be oxidized compared to N 2 ambient. This work was partly supported by JST CREST Number JPMJCR16F4, Japan. This work was also partly supported by JSPS KAKENHI Grant Number 18F22879 and 16J11377. Reference H. P. Komsa, et al. , J. Phys. Chem. Lett. 3 , 3652 (2012) J. Kang, et al. , J. Appl. Phys. 113 , 143703 (2013) Y. Hibino, et al. , Jpn. J. Appl. Phys. 57 , 06HB04 (2018) Y. Hibino, et al. , J. Mater. Res. 32 , 3021 (2017) R. Samnakay, et al. , Appl. Phys. Lett. 106 , 023115 (2015) Figure caption: The Mo 3d spectra of MoS 2(1- x ) Te 2 x samples fabricated with different gas ambient, obtained by XPS. The bottom spectra show there is no or only subtle oxidation, whereas after the storage time, samples undergo oxidation with sample fabricated in H 2 ambient further oxidized than that fabricated in N 2 ambient. Figure 1
New optical fiber sensors for detecting leakage of vapor phase alkanes and gasoline have been studied. When exposed to these vapors, certain rubber-type polymers such as polyisoprene and polyisobutylene cause swelling and their refractive indexes decease depending on the vapor pressure of these substances. Based on this effect, the fiber-type sensor heads were fabricated by coating the swelling polymer as a cladding layer on the fiber core with slightly lower refractive index than that of cladding. When was exposed to vapor phase substances, the sensor head changed its fiber structure from leaky to guided one, and then a large change in the transmitted light intensity was observed in a wide range of the vapor pressure. The response was also found to be reversible and reproducible.
In recent years, the fabrication of transition metal dichalcogenide (TMD) alloys is drawing attention due to their controllable bandgap. Fabrication of MoS 2(1− x ) Te 2 x is expected to be difficult due to its thermal instability although it shows wide tunable bandgap range. In this study, MoS 2(1− x ) Te 2 x fabrication is carried out by sputtering and post-deposition thermal treatment in chalcogen ambient. Films without phase separation were successfully fabricated. It was revealed that the band structure changes according to the chalcogen ratio. The valence band maximum shifted non-linearly showing bowing effect, while the conduction band minimum remained almost unchanged. It was considered that such bowing behavior of valence band minimum is attributed to the electronegativity difference between S and Te. The invariant nature of the conduction band was attributed to the fact that there is no such competition of electronegativity for the metal side whose electron orbitals mainly contribute to the conduction band formation. The maximum shift in the valence band maximum was as large as 0.5 eV. It was also revealed that suppressing the chalcogen deficiency may prevent phase separation. The wide tunability in the band structure and the possibility of realizing the uniform alloy promises the materials high applicability to different electronic devices.
