| Abstract |
The aim of this thesis was to create new knowledge for material and device processing effects on the performance of novel III-Nitride Heterostructure Field Effect (HFET) transistors using either an AlN barrier or InN channel layers. The AlN/GaN heterojunction offers the highest polarization discontinuity for GaN two-dimensional electron gas (2DEG) channel transistors, and high electron mobility transistor (HEMT) devices can be realized with ultra-shallow channels and very high current density.
In this work, an extensive study of unpassivated HEMTs (with Lg~1 μm) based on thin double AlN/GaN/AlN heterostructures with 1 nm GaN cap, directly grown on sapphire subtrates by plasma-assisted molecular beam epitaxy (PAMBE) is reported. The analysis is based on dc, pulsed and breakdown measurements, which were carried out on the devices for an AlN top barrier thickness in the range of 2.2-4.5 nm. The 2DEG density (Ns) varied from 6.8 x 1012 to 2.1 x 1013 cm-2 as the AlN barrier thickness increased from 2.2 to 4.5 nm and the maximum dc drain-source current (Ids) was 1.1 A/mm for AlN barrier thickness of 3.0 and 3.7 nm. The 3.0 nm AlN barrier HEMT exhibited the best operation in terms of standard performance metrics such as transconductance and off-state breakdown voltage (Vbr). Moreover, the Vbr of the 3.0 nm AlN barrier HEMT was more than double (70 V) the value measured for a single AlN/GaN HEMT grown on a thick GaN buffer layer, due to improved electron confinement in the 2DEG channel. Pulsed measurements were performed with a 500 ns pulse-width and exhibited a current collapse varied between 6%–12% and 10%–15% under gate and drain lag conditions, respectively. Small positive shifts of threshold voltage (0.2-0.4 V) with negligible reduction of transconductance interpreted to suggest small electron trapping predominantly in the layers and interfaces underneath the Schottky gate contact. These results suggest that the double heterostructures may offer intrinsic advantages for the breakdown and current stability characteristics of high current HEMTs. AlN/GaN/AlN double heterostructures using a 5-nm-thick GaN quantum well were also tested for transistor normally-off operation. The fabricated devices exhibited very low maximum Ids currents, ranging between 0.16-0.60 mA/mm, due to very high on-resistances resulting from the absence of 2DEG across the entire source-drain region. These structures may offer promise well beyond the established power-related applications and could be useful for digital applications.
The potential of using in situ SiNx deposition by PAMBE on an AlN/GaN/AlN HEMT structure (with 1 nm GaN cap and 3.5 nm AlN top barrier thickness) as a passivation layer and gate dielectric, was also investigated. The 5 nm in situ SiNx dielectric resulted in a large increase in Ns, exhibiting a value of 3.8 x 1013 cm-2, when compared to a similar structure without SiNx cap, in which Ns was 1.9 x 1013 cm-2, suggesting the presence of an additional positive charge at the SiNx/GaN cap interface. HEMT devices with ~1 μm gate length exhibited drain-source currents directly comparable to the Ns values, being 1.15 and 0.43 A/mm at Vgs = 0 V for SiNx and Schottky-gate
AlN/GaN/AlN HEMTs, respectively. However, the SiNx/AlN/GaN/AlN HEMTs exhibited increased gate leakage currents and severe current collapse due to the presence of high interface trap state densities.
To boost performance over GaN-based devices and pave the way for terahertz frequency electronics, InN as channel material represents the best candidate due to its unique transport properties. SiNx deposited in situ in the PAMBE system could effectively modulate the electron concentration and work as a gate dielectric for 2 nm ultrathin InN channel field effect transistors. Operation of InN-on-GaN field effect transistors was demonstrated for the first time exhibiting a maximum Ids of about 60 mA/mm and a pinch-off voltage of -9.5 V and -15 V for 5 and 10 nm thick SiNx, respectively.
An increase of InN layer thickness, in the 4-10 nm thickness range, could increase significantly Ids up to 1.2 A/mm, however, the channel could not fully pinch-off. This was attributed to the increased conductivity of the InN layers, caused by the high density of dislocations formed due to the large lattice mismatch between InN and GaN. The charge conduction mechanisms of Ni/SiNx/InN metal-insulator-semiconductor capacitors were investigated and I-V analysis suggested ohmic conduction by hopping at low electric fields, while field emission of electrons from trap centers in SiNx located 1.1-1.3 eV below the conduction band was prevailed at high electric fields. These results emphasize the use of ultrathin InN layers and the growth optimization of SiNx dielectric and SiNx/InN interface formation as a prerequisite for the development of InN channel transistors for ultra-high frequency applications.
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