CHARGE BURSTS THROUGH DIELECTRIC LAYERS OF 4H-SIC/SIO 2 METAL OXIDE SEMICONDUCTOR CAPACITORS M.J. Marinella 1* , D.K. Schroder 1 , G.Y. Chung 2 , M.J. Loboda 2 , T. Isaacs-Smith 3 , and J.R. Williams 3 1 Department of Electrical Engineering and Center for Solid State Engineering Research, Arizona State University, Tempe, AZ 85287-5706 ( * email: m@asu.edu) 2 Dow Corning Compound Semiconductor Solutions, LLC, Midland, MI 48611 3 Physics Department, Auburn University, Auburn, AL 36849 ABSTRACT Small bursts of inversion layer charge in 4H-SiC metal oxide semiconductor capacitors leak through the oxide layer leading to discontinuities during capacitance-time measurements. These, in turn lead to difficulties in generation lifetime extraction. This behavior has been observed using non-equilibrium capacitance-time (C-t) and current-time (I-t) measurements, at room temperature and at 400 °C. INTRODUCTION Silicon carbide is a unique compound semiconductor because, in addition to being ideally suited for high temperatures, extreme environments, and high power devices, it has the same native oxide as silicon – silicon dioxide. Consequently, intense research has been pursued toward developing a commercially viable SiC MOSFET, as it would offer better performance than its silicon counterpart in many situations. Although problems still exist, it is reasonable to predict that a SiC MOSFET will become commercially available in the near future. For this reason, reliability of SiC MOSFETs is an important concern [1]. Many of SiC’s useful properties are due to its wide band gap. Paradoxically, the wide band gap also causes a major reliability concern for the dielectric, since it gives substantially smaller oxide- semiconductor energy barriers than those of the standard Si/SiO 2 system. Thus, dielectric breakdown is worse for SiC/SiO 2 , both by time-zero [2] and time-dependent dielectric breakdown (TDDB) [3] measurements. In addition, F-N tunneling current is higher than in silicon [4], and is worse at high temperatures [5]. Furthermore, it has been found that SiC/SiO 2 devices suffer from negative bias temperature instability (NBTI), which is also a major reliability issue for silicon devices [6]. Reliability troubles at elevated temperatures are especially concerning, as SiC is desirable for its high temperature properties. In the following work, we present evidence which suggests that sudden bursts of inversion charge flow through the dielectric layer of 4H-SiC/SiO 2 MOS capacitors (MOS-C) in deep-depletion, without causing destructive breakdown. Evidence consistent with this theory is presented in the time-dependent capacitance (C-t) and current (I-t) behavior, at room temperature and at 400 °C. Phenomena involving the nondestructive breakdown of silicon MOS capacitors have been studied for many years [7-9]. The transient current observed by Jackson et al. during TDDB measurements includes current spikes in a silicon MOS-C which are fundamentally similar to the behavior we are reporting [9]. However, to the best of the authors’ knowledge, this behavior has not been previously reported in either SiC/SiO 2 or Si/SiO 2 MOS capacitors during the transition from deep-depletion to inversion. EXPERIMENTAL MOS capacitors (MOS-C) were formed from n-type 4H-SiC samples, grown with Dow Corning chlorosilane/propane chemistry. The 20 μm epitaxial layers were nitrogen doped to an approximate concentration of 10 16 cm -3 . A 45 nm SiO 2 layer was grown thermally, and passivated with NO. Molybdenum gate contacts with gold overlayers (each ~150 nm) were sputtered in Ar and circular contacts of approximate diameters of 377 and 675 μm were patterned using photolithography. The devices were measured in an electrically isolated probe station with a maximum temperature of 400 °C. The fundamental purpose of our work is to characterize carrier generation lifetime (τ g ) of 4H-SiC epitaxial layers using the pulsed MOS-C technique, originally suggested by Zerbst [10], and detailed in [11]. The basic procedure starts with an MOS capacitor in accumulation (V G = 5 V); and the gate voltage is switched to an inversion bias level (V G = -10 V). The inversion layer does not form immediately, since minority carriers must be thermally generated. Thus, as the inversion layer forms, the space charge region (scr) width below the gate decreases and the capacitance increases. The speed and behavior of this recovery contain information about the generation lifetime in the scr of the device. The MOS-C recovery time depends on the generation rate, which is proportional to the intrinsic concentration, n i . At room temperature, n i in 4H-SiC is about 500 cm -3 and the generation rate is negligible. Thus we use the probe station’s maximum temperature of 400 °C, which brings n i to about 10 8 cm -3 , and reduces the pulsed MOS-C recovery time to several minutes. RESULTS AND DISCUSSION It was observed that some of our SiC MOS capacitors have a discontinuity in the C-t curve as indicated by points (a) and (b) in Fig. 1. The proposed explanation for this behavior is the following: during the recovery from deep depletion, inversion charge slowly builds up at the oxide-semiconductor interface. The electric field across the oxide continually increases, due to the increasing inversion and gate charge. Eventually, most of the gate voltage is dropped across the oxide and the oxide field becomes very high, as illustrated in the band diagram of Fig 2 (a). For example, the oxide electric field of the device pulsed to V G = -10 V is about 2 MV/cm in strong inversion. At points (a) and (b) in Fig. 1, a momentary charge flow occurs, which results in some of the inversion charge drifting through the dielectric. This is indicated by the sudden lowering of the capacitance, which is directly proportional to the charge in the inversion layer. Since the gate voltage is constant, this loss of inversion charge redistributes the potential within the MOS-C such that the highest electric field is now located in the substrate and not in the oxide, as illustrated by Fig. 2 (b). This is why the breakdown is not destructive: the instant the inversion charge begins flowing through the oxide, the electric field in the oxide lowers. Furthermore, since only limited amount of inversion charge exists, only a small amount of charge can move through the oxide until additional inversion charge is generated – a process which takes a relatively long time. Thus, the event has a built-in negative feedback which protects the device, in a similar manner to a circuit breaker. If the