Date of Award

Summer 2023

Document Type

Open Access Dissertation


Chemistry and Biochemistry

First Advisor

Andrew B. Greytak


Ultrawide Bandgap (UWBG) and Extreme Bandgap (EBG) AlxGa1-xN (x > 0.40) alloys with EG > 4.2 eV are potential candidates for high temperature and high voltage applications, compared to Wide Bandgap (WBG) GaN-based lateral channel devices. Since critical electric field (Ec) scales as (Eg)2.5, theoretically AlGaN alloys can sustain large electric field before breaking down. However, achieving linear ohmic contacts to the 2-dimensional electron gas (2DEG) formed at the interface of AlyGa1-xN/ AlxGa1-xN (y – x ≥ 0.20) High Electron Mobility Transistors (HEMTs) is challenging. Direct contact through the AlGaN barrier is difficult due to their low electron affinity (χs) and high metal-semiconductor schottky barrier height for electron tunneling. This parasitic resistance in series with channel resistance of the 2DEG modulates the output drain current (ID,max) and hence device performance.

State-of-the-art UWBG and EBG AlGaN channel HEMTs have been reported by our group via (i) n-doping of the AlyGa1-yN barrier, and (ii) reverse graded n-doped AlGaN layer. To reduce the channel resistance in EBG AlGaN channel HEMT, a high NS is required, which can be achieved using an AlN barrier. However, it is extremely difficult to apply this strategy on AlN barrier due to challenges associated with DX transition of Si-doped AlN. Additionally, this strategy demands precise etching of the doped graded layer. Plus, etching induces surface leakage currents. To suppress the leakage current, high-k dielectrics have been used which in turn modulates 2DEG sheet carriers due to interfacial/bulk charges. Selective Area Growth (SAG), on the other hand, has been successfully reported for making ohmic contacts to GaN-channel HEMTs. However, the success of SAG strategy on AlxGa1-xN channel HEMTs largely depends on (i) doping and (ii) conduction band matching between the regrown material and the channel to facilitate sidewall conduction. These 2 key metrics control the contact linearity in EBG AlGaN channel HEMTs.

In this work, we explore a 2-fold strategy to obtain low resistance ohmic contacts. First, we explore controlled Metalorganic Chemical Vapor Deposition (MOCVD) growth and composition modulation of [AlN]m/[GaN]n short period superlattices (SPSL), where m and n are the number of monolayers of AlN barrier and GaN quantum well. The SPSL forms artificial alloys, also known as digital-AlGaN alloys and can be used as a channel material in HEMTs. Second, in these engineered AlGaN channels, we explore the SAG process for making ohmic contacts via regrowing highly doped n-GaN in trenches and flat growth (except SiO2 masked access region mesas). Potentially, this strategy has 2 key advantages: (i) Si-being a shallow donor in GaN can be doped heavily, and (ii) the conduction band offset can be eliminated by n-GaN-to-GaN quantum well side-wall conduction. In this respect, we explore a high temperature (~1050 °C) conventional and low temperature (~650 °C) atomic layer epitaxy (ALE) growth methods to achieve degenerate ( high~ 1019 to low 1020 cm-3) doping of GaN. MOCVD growth of doped GaN via SAG will be explored for the possibility of side-wall conduction to reduce the ohmic contact resistance, RC. Transmission Line Model (TLM) measurements of metallized contacts will be studied to verify the potential nature of this strategy.


© 2023, Mohammad Kamal Hussain

Available for download on Saturday, August 31, 2024

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