Impacts of Thermal Atomic Layer-Deposited AlN Passivation Layer on GaN-on-Si High Electron Mobility Transistors
© Zhao et al. 2016
Received: 8 October 2015
Accepted: 23 February 2016
Published: 10 March 2016
Thermal atomic layer deposition (ALD)-grown AlN passivation layer is applied on AlGaN/GaN-on-Si HEMT, and the impacts on drive current and leakage current are investigated. The thermal ALD-grown 30-nm amorphous AlN results in a suppressed off-state leakage; however, its drive current is unchanged. It was also observed by nano-beam diffraction method that thermal ALD-amorphous AlN layer barely enhanced the polarization. On the other hand, the plasma-enhanced chemical vapor deposition (PECVD)-deposited SiN layer enhanced the polarization and resulted in an improved drive current. The capacitance-voltage (C-V) measurement also indicates that thermal ALD passivation results in a better interface quality compared with the SiN passivation.
AlGaN/GaN high electron mobility transistor (HEMT) is promising for high frequency, high power density, and high temperature applications owing to its superior material properties such as wide bandgap (3.4 eV), high breakdown field (2 × 106 V/cm), high thermal stability, and its 2-D electron gas (2DEG) channel [1, 2]. In order to realize GaN HEMTs on the mainstream 8-in. wafer, GaN-on-Si HEMTs have been intensively investigated. In the typical structure of GaN-on-Si HEMTs, the crystalline defect density is high because of the lattice mismatch of two material systems. The electron trapped at the AlGaN surface can cause surface leakage current and drain current collapse effects [2–4]. Therefore, passivation techniques have been widely applied for filling these traps [5–8].
ALD-grown AlN is a wide band-gap material recently reported as a new choice to passivate AlGaN/GaN HEMTs for its good isolation ability and high quality interface with AlGaN/GaN [3, 9]. However, most of the studies of the AlN passivation layer were grown by plasma-enhanced ALD (PEALD). In this work, we develop the thermal ALD technique and use it as a passivation layer. The plasma-free process leads to a simpler growth process and prevent possibly plasma-induced damage. A relatively larger film thickness is also applied to keep the traps on the new surface away from the 2DEG channel. For comparison, plasma-enhanced chemical vapor deposition (PECVD)-deposited SiN passivation layer is also grown on AlGaN/GaN HEMTs. The output characteristics of devices using the two different passivation techniques are compared and discussed in this work.
AlGaN/GaN HEMTs were fabricated on samples on Si(111) substrate with GaN buffer layer. A 25-nm Al0.25Ga0.75N layer barrier and a 3-nm undoped GaN cap layer were used. First, mesas were defined by BCl3/Ar-based reactive ion etching (RIE). After that, Ti/Al/Ni/Au (20/120/60/50 nm) stack were deposited by electron beam evaporation (EBE) followed by a rapid thermal anneal at 840 °C for 30 s to form the ohmic contact. Then, Schottky metal gates of Ni/Au (20/200 nm) were also deposited by EBE. The gate length, gate-to-drain spacing, gate-to-source spacing of fabricated HEMTs are all 1.5 μm. The output and transfer characteristics of the basic HEMT without passivation were then measured.
Results and Discussion
Varieties of c axis of AlGaN/GaN layer after passivation
SiN passivated (%)
AlN passivated (%)
In Fig. 4, the drive current of AlN-passivated device is almost unchanged compared with the sample before AlN passivation. That means the increase of sheet concentration caused by the polarization-induced positive charge at the AlN/capGaN interface was not obvious . As indicated in Table 1, the strain caused by the AlN passivation layer is negligible. In the previous work of Huang et al., crystalline AlN layer was used . The band gap and charge concentration of amorphous AlN are both lower than the crystalline AlN [12, 13], so that there are lower polarization and fewer charges at the interface. However, the lower polarization also means higher barrier and lower leakage current at the AlN/cap-GaN interface. The leakage current of amorphous AlN layer is lower than crystalline AlN layer as well .
On the other hand, an enhancement of the drive current in the SiN-passivated devices is observed. Depending on the deposition conditions, the SiN deposition process can apply tensile or stress strain . As shown in Table 1, the NBD analyses results indicate that a tensile strain was applied on the AlGaN layer. Therefore, piezoelectric polarization is enhanced, and the carrier density of 2DEG is increased [9, 16].
Capacitance-voltage (C-V) test was applied on the two types of passivation layers to detect the difference in filling interface traps and the qualities of passivation layers to describe this difference . 30-nm thermal ALD-grown AlN and 30-nm PECVD-deposited SiN were tested.
The thermal ALD-grown amorphous AlN passivation layer owns a satisfactory interface state and film quality. This passivation technique performs better in sealing interface traps and reducing the off-state leakage than the PECVD-deposited SiN. Low polarization is observed for the thermal ALD-grown amorphous AlN passivation layer. Amorphous AlN is also promising as gate dielectric for AlGaN/GaN HEMT because of its predictable lower leakage than the crystalline one.
This work was supported in part by the National Nature Science Foundation of China (Grant 61322404), Shanghai 13DZ1108100, and in part by the National Science and Technology Major Project of China (Grant 2013ZX02308004).
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