Effect of Buffer Thickness on Degradation of AlGaN/ GaN HEMTs on Si

In this work, we have evaluated the electrical reliability for AlGaN/ GaN HEMTs on Si with varying buffer thickness to identify the critical buffer thickness. We have performed step-stress measurement under OFF-state condition. Raman techniques were studied to investigate the strain relaxation process on 1μm thick GaN grown on different buffer thickness (TBuf) on Si (111). We observed the device degradation was minimized with the negligible threshold shift (Vth) before and after step-stress test for TBuf = 2.5 μm. Also the calculated residual stress of GaN on Si was found very close to the unstrained GaN for TBuf = 2.5 μm. Introduction AlGaN/ GaN HEMTs are currently considered to be promising candidates in application for power device application because of the high breakdown voltage and low on-resistance [1]. Though different substrates are available for the GaN epi-growth, Si substrate is the most suitable one because of its low cost and large scale availability. However, reliability becomes a bigger concern because of its large lattice mismatch and thermal coefficient between Si and GaN [2]. Recently, fewer studies were reported on the electrical reliability for AlGaN/GaN HEMTs on Si substrate but still it needs better understanding of the device failure mechanism. The electric field drives the device degradation and it is commonly attributed to inverse piezoelectric field [3]. Also hot carrier-induced defect creation at the AlGaN barrier and GaN buffer have been considered as reasons for the increased gate leakage current or carrier trapping [4]. It is believed that GaN epitaxial layers grown uniformly on Si substrate suffer from distributed thermal cracks hence the growth of GaN on superlattice buffer and on prepatterned Si substrate have been demonstrated as efficient ways to control the geometrical distribution of the thermal crack [5]. Besides the strong voltage dependence of the degradation, a buffer thickness dependent has also been found. Experimental AlGaN/GaN HEMTs structure with different buffer thickness were grown on a 4” p-Si (111) substrate by Taiyo Nippon Sanso (SR4000) horizontal metal-organic chemical vapor deposition (MOCVD) system. The growth started with the nucleation layer of 100 nm AlN, followed by the 40 nm of AlGaN. In order to understand the contribution of the buffer thickness towards reliability of the device degradation, wafers with different buffer thickness (TBuf = 1.25, 2.5, 4.0 and 5.0μm) were grown. In order to study the step-stress reliability the un-intentionally doped GaN of thickness 1.0 μm and final 25 nm Al0.26Ga0. 74N top layers were fixed. All these layers were grown at a high temperature of 1130 o C and these samples are free from cracks. The schematic diagram of the MOCVD grown HEMT structure used in the current study is shown in Fig. 1. The AlGaN/ GaN HEMTs on Si substrate with device dimension of gate width (Wg) = 200 μm, the gate-drain distance (Lgd) = 4.0μm, the source-drain distance (Lsd) = 9. 5μm and the gate length (Lg) = 1. 5μm were studied. We have performed the step-stress test under OFF-state and VDS = 0 state condition for many devices. In a typical experiment, at OFF-state, VDS is stepped from 5V to 40V with 5V step keeping VGS = -10V, well below threshold voltage. The device is stressed for 600sec in each step voltage. Results and Discussion Fig. 2 compares the results of an OFF-state step-stress test for different buffer thickness for AlGaN/ GaN HEMTs on Si. This experiments stops at VDS = 40V. Although the device undergo various pattern of degradation from the beginning the TBuf = 2.5 μm shows a less degradation when compared to the other buffer thickness. Fig. 3. shows a change in the Vth shift before and after stress. The transfer characteristics measured before and after stress shows a negligible difference in the threshold voltage for TBuf = 2.5 μm. The positive Vth shift for TBuf = 1.25 μm observed due to the high dislocations which act as a trapping centers for electrons in the two-dimensional electron gas (2DEG) channel [6]. TBuf = 4.0 and 5.0 μm resulted in negative Vth shift, this is because at high electric field, electrons can be injected from the AlGaN layer to the buffer. For sufficiently high voltage levels, electrons may achieve enough energy to ionize the deep acceptor in the buffer or to promote the transfer of an electron from the valence band to the deep acceptor, thus leading to the generation of a free hole. Generated holes may accumulate at the AlGaN/GaN interface which in turn resulted in a negative Vth shift [7]. Using visible Raman technique E2 phonon frequency (Strain) and phonon stress relationship was calculated to identify the nature of stress (tensile/compressive) as shown in Fig. 4. It is believed that, the E2 high mode originated from the GaN buffer. As the E2 high mode frequency for unstrained GaN at 300 k is 567.5 cm -1 , the E2 high mode for grown GaN on TBuf = 2.5 μm was 567.22 cm -1 which almost very close to the relaxed GaN. As the E2 high mode is related to the biaxial stress, the residual stress based on the peak shift can be calculated from the Eq.1. Δω = 4.3 σxx cm -1 Gpa -1 (1) The residual stress of the GaN buffer calculated shows from tensile to compressive stress The calculated stress in the GaN buffer shows very minimum of +0.05 GPa for TBuf = 2.5 μm which believe to be the critical thickness for the i-GaN = 1.0 μm. Conclusion: We have evaluated the reliability of AlGaN/ GaN HEMTs on Si with various buffer thicknesses. We find that TBuf = 2.5 μm shows a better reliability performance. The residual stress calculated using Raman spectroscopy showed a very close value to the typical unstrained GaN -919Extended Abstracts of the 2012 International Conference on Solid State Devices and Materials, Kyoto, 2012, pp919-920 F-9-2