Correlational of device performance and defects in AlGaN/GaN high-electron mobility transistors

Journal of Electronic Materials, May 2003 by Zhang, A P, Rowland, L B, Kaminsky, E B, Tilak, V, Et al

Device performance and defects in AlGaN/GaN high-electron mobility transistors (HEMTs) have been correlated. Surface depressions and threading dislocations, revealed by optical-defect mapping and atomic force microscopy (AFM), compromised the effectiveness of the SiN^sub x^ surface-passivation effect as evidenced by the gate-lag measurements. The residual carriers in the GaN-buffer layer observed from the capacitance-voltage depth profile have been attributed to the point defects and threading dislocations either acting as donors or causing local charge accumulations. Deep-level transient-spectroscopy measurements showed the existence of several traps corresponding to surface states and bulk-dislocation defects. The formation of electron-accumulation regions on the surface or (and) in the GaN-buffer layer was confirmed by current-voltage measurements. This second, virtual gate formed by electron accumulations can deplete the channel and cause a large-signal gain collapse leading to degraded output power. A good correlation was established between the device performance and defects in AlGaN/GaN HEMT structure.

Key words: AlGaN/GaN HEMTs, defects, surface passivation, AFM, DLTS, capacitance-voltage measurements, gate-lag measurements, current-voltage measurements, large signal measurements

INTRODUCTION

The AlGaN/GaN high-electron mobility transistors (HEMTs) have demonstrated respectable device performance and are highly promising for microwave power transmitter applications as required by next-generation phased-array radars and wireless base stations.1-3 Because of the unavailability of large-size, bulk-GaN substrates, semi-insulating 4H-SiC substrates have been commonly used. Although the device performance can benefit from the high thermal conductivity of SiC, the lattice mismatch (3.5%) and thermal-expansion difference (3.2%) of SiC relative to GaN have produced a high density of threading dislocations in epitaxial films, typically in the range 10^sup 8^-10^sup 10^ cm^sup -2^. These dislocations and other point defects present in AlGaN/GaN HEMTs degrade the device performance and raise the questions for device long-term reliability.

As in the early development stage of GaAs transistors, drain current collapse under large signal conditions are commonly observed in AlGaN/GaN HEMTs.^sup 4-10^ Current collapse occurs when a high drain-source voltage is applied to the device, resulting in the transfer of hot carriers from the 2-dimensional electron gas (2DEG) channel to either the AlGaN surface or the GaN-buffer layer that contains a high concentration of deep traps. The loss of channel carriers and the resulting large transverse-electric field lead to reduced drain current and increased knee voltage.5 Gate-lag measurements have shown that the traps on the surface between gate and drain are responsible for drain current collapse. Previous drain-lag measurements also suggested the existence of deep levels in the GaN-buffer layer when the 2DEG channel showed a poor carrier confinement.6

The accumulation of charges either on the AlGaN surface or in the GaN-buffer layer can form a second parasitic-distributed virtual gate that is loosely coupled to the gate metal by lateral injection and charge transport.9 To mitigate the surface traps, both SiN^sub x-^ and Sc^sub 2^O^sub 3^-surface passivations have been developed, and the surface states have been effectively alleviated.11-13

Threading dislocations (crew, edge, and mixed) propagate from the GaN/SiC interface into the epitaxial films and terminate on the surface. These dislocations can be directly observed using transmission electron microscopy.14 To determine the density of dislocations, photoelectrochemical hot-KOH or H^sub 3^PO^sub 4^ wet etch is commonly used, and counts are taken directly from atomic force microscopy (AFM) or scanning-electron microscopy images.15 Monte Carlo calculations have been performed to determine the charge accumulation on threading dislocations in GaN as a function of the dislocation density and background-dopant density.16 Among all the point defects (Ga vacancies, N vacancies, and antisite defects), N vacancies have been reported to be the major source of the background doping in GaN when the carrier concentration is 2 x 10(17) cm^sup -3^.18

In the absence of dislocations, the step-terrace structure should be uniform with a terrace width inversely proportional to the substrate miscut. However, the observed-terrace morphologies were highly nonuniform. Dislocation-mediated structures dominated the surfaces because of the high densities of dislocations that intersected the free surfaces. Small depressions were found at pure-edge dislocations, and large depressions were found at the crew or mixed dislocations.19

In this paper, we report the surface and defect characterizations of AlGaN/GaN wafers. A correlation of defects in the AlGaN/GaN HEMT structure and the device performance was evaluated. We found that the surface morphology induced by the dislocations directly affected the effectiveness of the SiN^sub x^ surface-passivation effect. The surface states and bulk defects present in AlGaN/GaN HEMTs structure strongly influenced the device power performance.