Samiul Hasan

Date of Award

Fall 2023

Document Type

Open Access Dissertation

Department

Electrical Engineering

First Advisor

Iftikhar Ahmad

Abstract

III-Nitride-based compound semiconductors have unique properties such as high bandgap and high breakdown field, which make them attractive for a variety of applications, including high-power and high-frequency electronics and optoelectronics. The most common types of III-Nitride-based field effect transistors (FETs) are aluminum gallium nitride (AlGaN)/gallium nitride (GaN) based, which suffer from some inherent problems such as virtual gate effect, current collapse, gate leakage, etc. The solution to this problem can be the inclusion of a dielectric passivation layer under the gate. However, the addition of the dielectric layer impacts one of the most critical device-controlling parameters, “threshold voltage”, which suffers significantly due to the shift to a higher value. In this dissertation, we have developed a new approach to improve the threshold voltage of the metal oxide semiconductor heterostructure field effect transistor (MOSHFET) structure by in-situ deposition of oxide (e.g., gallium oxide (Ga2O3)) dielectric starting from the sapphire substrate in one step without breaking the vacuum inside the deposition chamber. The overall process shows ~75%-88% improvement of interfacial trap density between the oxide and the AlGaN barrier layer, which ultimately helped to reduce the threshold voltage shift. Integration of oxide precursors in the III-Nitride growth reactor is challenging as the precursors may react, leading to catastrophic damage to the metal-organic chemical vapor deposition (MOCVD) system. However, a systemic approach was used to avoid this problem by mainly using nitrogen as the carrier gas instead of using the most common, hydrogen gas. Switching the growth method to nitrogen carrier gas changes the growth dynamics; hence few of the epilayers’ processes require optimization. In this regard, first, a high-quality aluminum nitride (AlN) epitaxial growth process was developed in the nitrogen environment. The epilayer showed low X-ray diffraction (10¯12) rocking curve FWHM of 289 arcsecs with E2 (high) phonon peak linewidth of 3.4 cm-1 at around 659 cm-1 and low compressive stress of 0.59 GPa. To further understand the differences between the AlN layers developed using these two types of carrier gases, a point defect study was done of the same. The Al0.3Ga0.7N/GaN heterostructure field effect transistor (HFET) structures with a Ga2O3 passivation layer were developed in two ways, which include ex-situ and in-situ oxide deposition processes. X-ray diffraction (XRD) showed the crystalline (¯201) orientation peaks of -Ga2O3 in the device structure. The van der Pauw and Hall measurements yield the electron density of ~ 4 x1018 cm-3 and mobility of ~770 cm2V-1s-1 in the 2-dimensional electron gas (2DEG) channel at room temperature. Capacitance-voltage measurement for the on-state 2DEG density for the MOSHFET structure was found to be of the order of ~1.5 x1013 cm-2. While the leakage current for the ex-situ and in-situ remained similar, it was a 2-order improvement compared to the HFET structure. The interface charge density between the -Ga2O3 and Al0.3Ga0.7N barrier layer in the in-situ process was found to be ~75%-88% lower than in the ex-situ process. The annealing experiment showed that, compared to atomic layer deposited oxides, the MOCVD-grown dielectric oxide Ga2O3 is thermally stable, leading to its use for extreme environment applications. Overall, this dissertation work leads to the in-situ MOCVD oxide dielectric deposition, which tremendously reduces the density of interface states and improves the threshold voltage performance of the MOSHFET structures.

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© 2024, Samiul Hasan

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