Investigation of Growth, Structural and Optical properties of different phases of Ga2O3

Abstract

Among the semiconducting sesquioxides, Ga2O3 has attracted considerable research attention in recent years due to its excellent properties, including direct ultra wide band gap, optical transparency, high excitonic binding energy. These properties makes it a potential candidate for deep UV optoelectronics and power electronics applications. The Ga2O3 exhibits polymorphism which includes at least α-, β-, γ- and ϵ-/κ- phases. Among these phases most of the research has been carried out on thermodynamically stable β-polyphase, whose highly asymmetric crystal structure imparts highly non-isotropic optical and electronic properties. Aside from the fact that β-(AlxGa1–x)2O3 alloy is limited to an Aluminium mole fraction of 71 % thereby impeding the bandgap tuning, its non-polar crystal symmetry pose some challenges or would add additional steps to the device development process. These factor make it imperative to investigate other meta-stable polymorphs. There is a critical need for cost-effective and high-throughput methods for the deposition of semiconducting thin films in a wide range of industrial applications. In this research work optical and structural properties of metastable phases of Ga2O3 have been investigated which were deposited using cost-effective, easy to use and high-throughput techniques. In particular, an approach involving microwave-irradiation was employed to deposit polycrystalline thin films at sub 200 oC temperatures, and mist-CVD method was developed to achieve epitaxial thin films of high crystallinity at atmospheric pressure. The work begins by understanding the structural properties and optical reponse of the cubic spinel γ-Ga2O3. The polycrystalline film was deposited on the sapphire substrate at various microwave powers, following which the sample deposited at 300 W microwave power was annealed and a comparative study vis-` a-vis the optical and structural properties was done on annealed and as-deposited sample. A planar geometry MSM photodetector was fabricated with decent response. Finally, the carrier transport mechanism was investigated by analyzing temperature dependent I-V curves with Thermionic emission models at low electric field and hopping conduction mechanism at high electric field regime. The outcomes of the investigation renders microwave as the method of choice for deposition of conformal, high quality polycrystalline optical films. These results persuaded us to deposit Ga2O3 poly-film on GaN/AlGaN-HEMT stack for the realization of dual-band/broadband photodetector. In the final section of this part, nanocrystalline (In0.26Ga0.74)2O3 film was realized, and its defect span was studied using Urbach’s rule. This film demonstrated the high responsivity of ∼ 17 WA . In general, microwave irradiation is suitable for the fabrication of highly conformal polycrystalline thin film; however, deposition of epitaxial thin films present a great challenge. Low-defect epitaxial films are imperative for manufacturing highly efficient photodetectors, sensors, transistors and diodes. In addition, they make it possible to observe specific optical excitations, such as free excitons. A polycrystalline film could assist in converting free excitons into trapped excitons, thus hindering our ability to observe the existence of free excitons in the film. Nevertheless, one could detect these free excitons in poly films at cryogenic temperatures. To address these issues, this stage of the research involves building a hot-walled mist-CVD reactor and deciphering its underlying growth mechanism. The highly epitaxial α-Ga2O3 film was stabilized at a relatively lower temperature of 350 oC. The crystallinity of the films were studied using series of rocking curve scans and pole figure measurements. Through the application of Elliott-Toyozawa theory, optical charcterization of the film with emphasis on excitonic properties was conducted. Eventually, an MSM photodetector was fabricated on the film deposited at 450 oC and its optical response was studied. This is the first time excitonic fingerprint has been observed in spectral responsivity measurements. While hot-walled mist-CVD reactors are quite capable of depositing α-Ga2O3 films, they suffer from specific growth-related issues. The film deposition rates are slow in conjunction with high thickness variability over the substrate. Furthermore, a large tube diameter promotes homogeneous nucleation, facilitating germination of high-density denuded regions. Considering these factors when depositing films conducive to high-quality devices is essential. As a method of alleviating the problems mentioned above, a fine channel mist-CVD (FCM) reactor was developed. This reactor was employed to deposit κ-Ga2O3 film with high crystallinity of ∼ 104 arcsec FWHM of on-axis Rocking Curve (RC); the dilemma about its crystal structure was resolved with the help of diffraction simulation coupled with a pole figure scan of the uncommon pole germane to orthorhombic symmetry. Ultimately, an MSM device was fabricated on the κ- phase and its spectral response was studied within the framework of the parabolic WKB model to extract depletion width, unintentional doping level, and built-in electric field. Later, the work evaluated spectra of various optical functions such as refractive index, dielectric function, and high-frequency dielectric constant. This study was concluded by an in-depth investigation of Urbach’s tail using Codi’s rule. Hitherto the ongoing research focussed on the deposition of high-quality pure phase κ-Ga2O3. These objectives were achieved at low precursor flow rates leading to slow deposition rates. Low film thickness obstructs fully utilizing the optical potential of the material. To deal with these hurdles, a thin buffer layer of (111) oriented cubic MgO was employed in addition to a high precursor flow rate for realizing a thicker film of κ-Ga2O3 on the sapphire substrate. The asdeposited film possesses a high absorbance in conjunction with high film thickness, owing to a significant deposition rate. The fabricated photodetector on this film demonstrates ultra-high responsivity of ∼ 920 WA with rapid transients. Finally, the thermal stability of the films was assessed using temperature-dependent XRD measurements and an RSM scan. The film was found to be thermally stable until at least 950 oC.