Comparative Study of AlGaN / GaN HEMTs with LPCVD-and PECVD-SiN x Passivation

Low-Pressure Chemical Vapor Deposition (LPCVD) technique is utilized for growth of SiNx for passivation of AlGaN/GaN HEMTs. Compared with conventional PECVD-SiNx passivation, effective current collapse suppression and remarkably improved RF power performance are achieved by LPCVD-SiNx passivation. TEM-EDS mapping analysis suggests that the improved passivation from LPCVD-SiNx is due to the reduced oxidation of the AlGaN barrier surface for HEMT devices in comparison to PECVD-SiNx passivation. The strategy of “passivation-prior-to-ohmic” with LPCVDSiNx has shown improved performance for AlGaN/GaN HEMTs. INTRODUCTION AlGaN/GaN high-electron-mobility transistors (HEMTs) have enjoyed great success as the key components in RF/microwave power amplifiers and high-voltage power switches, primarily owing to the polarization-induced high mobility and high density 2-D electron gases (2DEG) at AlGaN/GaN hetero-interface and high breakdown E-field of (Al)GaN [1]. However, the high-electrical-conductivity 2DEG channel is generally at a short distance (e.g., ~20 nm) from the polarized AlGaN barrier surface, and could be easily affected by the depletion effect of surface traps. The slow detrapping behavior of the deep states would result in severe current collapse in GaN-based power amplifiers and power switches [2, 3]. To suppress the current collapse, plasma-enhanced chemical-vapor-deposited SiNx layer is commonly adopted to passivate the surface states, except for the recently developed AlN passivation featuring polarization-chargecompensation concept [4]. There are several challenging issues for PECVD-SiNx passivation, including: 1) relatively low film quality due to low growth temperature (typically below 350 C); 2) inescapable plasma-induced damages to (Al)GaN surface; and 3) long-term reliability induced by moisture erosion due to its loose texture [5]. In these regards, low-pressure chemical vapor deposition becomes an attractive technique for the growth of high quality SiNx passivation layer for AlGaN/GaN HEMTs by virtue of its higher growth temperature (typically at above 600 C), nonplasma-activated sources, and moreover, compact film quality of LPCVD-SiNx with less N-H and N-O bonds [6, 7]. Because the higher quality LPCVD-SiNx film can now withstand the high thermal stress from the ohmic rapid thermal annealing step, this allows the device fabrication flow to take advantage of the more attractive “passivationprior-to-ohmic” strategy. In addition, since LPCVD is an already mature technology in CMOS fabs, this approach is very attractive for mass production of AlGaN/GaN HEMTs because it can take advantage of the existing capability in CMOS line to further reduce fabrication cost. Recently, LPCVD-SiNx films have been implemented as gate dielectrics, as well as, passivation layer in CMOScompatible process for GaN-based MIS-HEMTs on Silicon [8, 9]. Low leakage and low current collapse were realized simultaneously [10]. In this work, a standard CMOS LPCVD-SiNx layer is successfully demonstrated for the passivation of AlGaN/GaN HEMTs. In comparison to conventional PECVD-SiNx passivation, improved current collapse suppression and improved RF power performance has been achieved. The enhanced performance of LPCVDSiNx passivation has been demonstrated by this comparison study of the two AlGaN/passivation interfaces (LPCVD and PECVD). DEVICE STRUCTURE AND FABRICATION The AlGaN/GaN heterostructure used in this work consists of a 24-nm undoped Al0.2Ga0.8N barrier, a 1-nm AlN interface enhancement layer, and a composite Al0.02Ga0. 98N /GaN (0.8 μm/0.8 μm) buffer layer. A 120-nm LPCVD-SiNx passivation layer was deposited on the fresh epi layer at 780 °C and a low pressure of 300 mTorr after a standard RCA cleaning process. After dry etching openings of the LPCVD-SiNx layer, ohmic contacts were formed with a Ti/Al/Ni/Au metal stack annealed at 850 °C for 50 s in ambient N2. The contact resistance is measured to be 0.85 Fig. 1. X-ray diffraction pattern of (a) PECVD-SiNx and (b) LPCVD-SiNx. “d” is the interplanar spacing. 9 173 CS MANTECH Conference, May 18th 21st, 2015, Scottsdale, Arizona, USA Ω·mm using transfer length method. Then boron ion implantation was used for device isolation. T-shape Ni/Au gate electrode is fabricated after gate opening, followed by a post-gate annealing at 350 °C for 5 min. For the reference PECVD-SiNx-passivated AlGaN/GaN HEMTs, RCA wet cleaning was conducted on the fresh epi layer. However, the PECVD-SiNx passivation was performed after ohmic contact formation, since the 120-nm low-temperature-grown PECVD-SiNx (~300 C) will crack from rapid thermal annealing process for ohmic metal. Note that an extra wet cleaning of the sample in a diluted NH4OH solution and a 1min in-situ N2 plasma pretreatment were adopted before PECVD-SiNx deposition. The followed fabrication process is similar to that of the LPCVSD-SiNx-passivated AlGaN/GaN HEMTs. The gate length (LG), gate-to-drain distance (LGD), gate-to-source distance (LGS), and gate width (WG) of the fabricated HEMTs are 1, 2.75, 1.5 and 2×50 μm, respectively. The refractive index of the LPCVD-SiNx and PECVDSiNx passivation layer are measured to be 2.0 and 1.95, respectively. In addition, the interplanar spacings of the LPCVD-SiNx passivation layer are smaller than the counterparts of the PECVD-SiNx, as determined by X-ray diffraction technique equipped in TEM measurement system (Fig. 1). Both results suggest the standard-CMOS LPCVDSiNx features a more compact structure than conventional PECVD-SiNx passivation, which provides better protection of the passivation/(Al)GaN interface from being oxidized, by H2O or oxygen. RESULTS AND DISCUSSION Transfer characteristics of both LPCVD-SiNx and PECVD-SiNx passivated AlGaN/GaN HEMTs are compared and shown in Fig. 2(a). The LPCVD-SiNx passivated sample shows higher peak drain current density and higher peak extrinsic transconductance than the PECVD-SiNx passivated sample, although with similar threshold voltage (VTH ~ -2 V). Moreover, the output power at 3 dB compression point and associated PAE of the LPCVD-SiNx passivated samples, measured at 4 GHz with VDS = 30 V, are 6.8 W/mm and 63.2%, respectively, which are 1.8 W/mm and 9.6% higher than that of the PECVD-SiNx passivated ones (Fig. 2(b)). The effectiveness of LPCVD-SiNx passivation is also verified by pulse I-V characterization, as shown in Fig. 3. The pulse width and period were 1 μs and 1 ms, respectively. The collapse ratio was calculated using the expression of [1ID(VGS, VDS)/ID(0,0)], where ID(VGS,VDS) is measured after pulsed from the quiescent bias: (VGS,VDS). Compared with the PECVD-SiNx-passivated HEMTs, the collapse ratio of the LPCVD-SiNx-passivated ones is much smaller at a high 0 2 4 6 8 10 0.0 0.2 0.4 0.6 0.8 PECVD (a) V GS : -3 V ~ 3 V, 3 V step