M. Kotha, A. Kaloyeros, S. Gallis
SUNY Polytechnic Institute,
Keywords: 2D materials, gallium telluride, polarization, anisotropy, photodetectors, hydrogen silsesquioxane, Raman spectroscopy, photoluminescence, pseudo-one-dimensional materials
Summary:Gallium telluride (GaTe) is an emerging and promising 2D layered semiconductor that can serve as a vital building block towards the creation of devices within the fields of nanoelectronics, optoelectronics, and quantum photonics. An important obstacle towards practical implementation of quasi-2D gallium chalcogenides for electronic and photonic applications is their surface instability under ambient conditions. Ambient exposure of GaTe leads to significant conduction band restructuring, low photoluminescence (PL) emission yield caused by carrier dissociation via surface states, and an anisotropic to isotropic structural transition. For the as-exfoliated flakes, we observe a significant reduction of the GaTe-related PL (~100×) and Raman (~4×) peak intensities for the few-layered flakes over a time span of few days. We attribute this ambient degradation to the formation of elemental polycrystalline tellurium and gallium oxide through a combination of optical (Raman Spectroscopy) and elemental (Auger and X-ray Photoelectron Spectroscopy) characterization techniques. We have developed a novel chemical passivation technique that results in complete encapsulation of the as-exfoliated GaTe flakes in ultrathin hydrogen–silsesquioxane (HSQ) film. By leveraging our novel passivation method for environmental-stable GaTe flakes, our focus has been to study the anisotropy in the optical properties of GaTe nanomaterials. The anisotropy is caused by the 1D-like nature of the GaTe layer, as the layer comprises of Ga-Ga chains extending along the b-axis crystal direction. The identification of the b-axis in such anisotropic materials is imperative for the fabrication of polarization-dependent devices based on the generation and detection of polarized light, such as polarized photodetectors and light sources. Our encapsulation process using HSQ serves as a platform for studying the anisotropic properties of pristine GaTe flakes through polarization-dependent Raman and PL spectroscopy. Although, we observed Raman polarization dependence of different phonon modes from monoclinic GaTe, the selection of the appropriate phonon mode to identify the crystal b-axis is very crucial. This Raman polarization response for a particular phonon is wavelength and thickness dependent. Conversely, we observed similar dependences of the GaTe room-temperature PL peak emission (1.66 eV) anisotropy using various excitation wavelengths (633 nm, 532 nm, 476 nm and 458 nm) and thicknesses. We observed a PL polarization anisotropy contrast (ρ) of 0.6 calculated using equation (i) for a 40-nm thick GaTe flake using 532 nm excitation wavelength. ρ= (I(∥)-I(⊥))/(I(∥)+I(⊥)) (i) where I(∥) and I(⊥) are the integrated PL intensity along a direction parallel and perpendicular to the b-axis respectively. This value is comparable to other pseudo-1D materials like ZrS3. This identification of the crystal b-axis in GaTe through PL spectroscopy was later confirmed using Transmission Electron Microscopy (TEM). Our novel surface-passivation has a dual role, while effectively passivating the flakes, it offers the capability to simplify the integration process for fabricating GaTe-based nanodevices using HSQ, a commonly used resist for electron beam lithography. Our current study of optical and electrical anisotropy in encapsulated GaTe-based nanodevices (photodetectors) is an extension of the exploratory research to study the direction-dependent light-matter interaction is such asymmetric crystal structures.