A Comparison of N-Polar () GaN Surface Preparations for the Atomic Layer Deposition of Al₂O₃

Abstract

Nitrogen-polar gallium nitride offers several advantages over Ga-polar GaN for high frequency-high power electronic devices, but its processing has not been fully developed. Here we report on a systematic study of the effect of surface pretreatments on N-polar GaN for metal oxide semiconductor capacitors (MOSCAPs). Bulk n-type GaN (0001) substrates were prepared with a variety of treatments including: HF; HCl; base-piranha; H 2 plasma; and no pretreatment for comparison. Then 14nm thick Al 2 O 3 layers were deposited by atomic layer deposition (ALD). Both the original surfaces of GaN and ALD films were characterized by atomic force microscopy (AFM). MOSCAPs were fabricated and characterized by capacitance-voltage (C-V) and current voltage (I-V) measurements. The surface morphology and electrical performance was greatly affected by the pretreatments due to the reactive nature of N-polar GaN. The MOSCAP fabricated on GaN as-received with no additional preparation had the best performance including the smallest hysteresis (0.03V), lowest leakage current density (2.09 × 10 −8 A/cm 2 at +4V) and total trap density (2.47 × 10 10 cm −2 eV −1 ). This was correlated to the smoothest surface morphology (0.23 nm). The wide bandgap semiconductor gallium nitride (GaN) is of great interest not only for optoelectronic devices such as blue LEDs, 1 laser diodes, 2 and UV detectors, but also for highly efficient electronic devices operating at high frequency, temperature and power. After years of development, Ga-polar GaN based high electron mobility transistors (HEMTs) have been demonstrated with excellent performance and high RF output power.3,4 Increasing the maximum frequency by scaling down the GaN HEMT dimensions to reduce electron transit time, establishing control of the contact resistance and capacitive elements are critically important to prevent device delay times and to achieve the best possible device performance.5 Nitrogen-polar GaN has potential advantages over Ga-polar GaN for high-frequency applications due to its low contact resistance and a natural back barrier to improve electron confinement.6-8 For example, an extremely low contact resistance of 23 -μm was reported by Mishra et al. 9 To date, most of the N-polar GaN HEMT devices employ structures with a Schottky barrier as the gate contact. 6,10,11 However, a metaloxide-semiconductor structure is preferred, as it provides a higher input impedance, larger gate voltage swings, and lower gate leakage currents. 12,13 Aluminum oxide (Al 2 O 3 ) is an excellent gate dielectric for IIInitride based devices due to its large bandgap (7∼9 eV), relatively high dielectric constant (∼9) and high thermal stability (up to 1000• C). 14-20 Precise control of this dielectric's thickness is necessary to maintain the aspect ratio between the gate length and the oxide thickness for good high frequency performance. Atomic layer deposition (ALD) offers excellent thickness control for the deposition of such dielectrics, 21 but the initial condition of the substrate surface is crucia...

Band alignments of sputtered dielectrics on GaN

“…The band offset values (VBO = 1 eV and CBO = 2.2 eV) obtained by Jia et al [17] are comparable with this work (VBO = 1.1 eV and CBO = 2.2 eV) but the differing test conditions make it difficult to compare leakage currents directly. Wei et al [63], Irokawa et al [64] and Kim et al [65] report leakage currents similar to those of this work, but did not investigate conduction mechanisms or extract band offset(s). Finally it is important to note that further analysis of our results (not shown) using typical J-V characteristics plotted on log-log scales shows evidence of the space-chargelimited (SCL) conduction mechanism in the low electric field region (0-1.5 V) which could indicate that the current flow is inhomogeneous and bulk rather than electrode limited.…”

“…Similarly, the 20 nm Al 2 O 3 -based MIScapacitor shows a lower leakage current density than that of ZrO 2 at 5.3 × 10 −6 A cm −2 at 1 V gate bias. A value of 1 × 10 −4 A cm −2 at -10 V was reported for 25 nm Al 2 O 3 on HF treated GaN deposited by ALD [17] and 5.33 × 10 −2 A cm −2 at 4 V for the ALD-deposited 14 nm Al 2 O 3 on a HCl treated GaN substrate [63]. The leakage current density for the 20 nm MgO MIS-capacitor is 3.2 × 10 −6 A cm −2 at 1 V. A value of 5 × 10 −3 A cm −2 for 80 nm MgO in GaN MIS-FET (field effect transistor) device at 2.2 V has been reported [64].…”

Trap state characterization of Al2O3/AlInGaN/GaN metal-insulator-semiconductor heterostructures

“…The deterioration of surface quality occurred because of In segregation in the AlIn-GaN layers with higher In% [29]. Such segregation of In atoms on the quaternary surface and deterioration of surface quality may generate trap states in the Al 2 O 3 /AlInGaN/GaN MIS structures in presence of the higher composition of In [16,23,30]. However, such trap states were suppressed in wafer A because of lower In content during epitaxial growth of the quaternary nitride.…”

“…To fabricate the MIS structure, a 20-nm Al 2 O 3 layer was deposited as a gate dielectric by an atomic layer deposition (ALD) system at 300 °C using alternating precursor pulses of TMA, H 2 O vapor and O 3 . ALD has an excellent control on the gate dielectric deposition, but the initial surface condition of the substrate plays a crucial role on the nucleation of the dielectric layer and quality of the deposited film [16]. The deposited oxide layers were then subjected to RTA at 500 °C in an N 2 atmosphere for 60 s to improve the oxide/semiconductor interface.…”