Christopher I. Thomas

Demonstration of a 4H SiC betavoltaic cell

A betavoltaic cell in 4H SiC is demonstrated. A p-n diode structure was used to collect the charge from a 1mCi Ni-63 source. An open circuit voltage of 0.72V and a short circuit current density of 16.8nA∕cm2 were measured in a single p-n junction. A 6% lower bound on the power conversion efficiency was obtained. A simple photovoltaic-type model was used to explain the results. Fill factor and backscattering effects were included in the efficiency calculation. The performance of the device was limited by edge recombination.

Reduced trapping effects and improved electrical performance in buried-gate 4H-SiC MESFETs

Surface effects on the current instability of 4H-SiC MESFETs were studied by comparing different surface structures. The current instability phenomenon was illustrated by bias sweeping methods and current recovery time measurements. A reduction in the current instability was observed for gate-recessed and buried-gate devices compared to the nonrecessed and channel-recessed devices. In addition, the buried-gate devices were found to have higher current density and breakdown voltage compared to the gate-recessed devices, resulting from their shorter effective gate length and lower electric field distribution under the gate, respectively. With high saturation current, high breakdown voltage, and much reduced surface effects, the buried-gate structure is a candidate for high-power SiC MESFETs.

Annealing of ion implantation damage in SiC using a graphite mask

For p-type ion implanted SiC, temperatures in excess of 1,600 C are required to activate the dopant atoms and to reduce the crystal damage inherent in the implantation process. At these high temperatures, however, macrosteps (periodic welts) develop on the SiC surface. In this work, the authors investigate the use of a graphite mask as an anneal cap to eliminate the formation of macrosteps. N-type 4H- and 6H-SiC epilayers, both ion implanted with low energy (keV) Boron (B) schedules at 600 C, and 6H-SiC substrates, ion implanted with Aluminum (Al), were annealed using a Graphite mask as a cap. The anneals were done at 1,660 C for 20 and 40 minutes. Atomic force microscopy (AFM), capacitance-voltage (C-V) and secondary ion mass spectrometry (SIMS) measurements were then taken to investigate the effects of the anneal on the surface morphology and the substitutional activation of the samples. It is shown that, by using the Graphite cap for the 1,660 C anneals, neither polytype developed macrosteps for any of the dopant elements or anneal times. The substitutional activation of Boron in 6H-SiC was about 15%.

Electronic properties of a 3C∕4H SiC polytype heterojunction formed on the Si face

The authors report on the electronic properties of a rectifying Si face 3C∕4H SiC heteropolytype junction on n+ 4H SiC. Capacitance-voltage profiling of the junction at temperatures from 4–300K showed high apparent carrier concentration. A semiclassical model was used to explain the behavior. The model predicted a spontaneous polarization-induced valence band quantum well in the 3C, indicating a polarization charge of 9.7×1012cm−2 for 4H SiC, in good agreement with theory. The formation of a two-dimensional hole gas was predicted. Using a Poisson-Schrodinger solver to analyze the measurements, it was found that large (∼3.5×1012cm−2) mobile hole charge was induced in the n-doped 3C SiC.

Observation of a two dimensional electron gas formed in a polarization doped C-face 3C∕4H SiC heteropolytype junction

A two dimensional electron gas (2DEG) was observed in a C-face 3C∕4H SiC heteropolytype junction. Sheet carrier concentrations of ∼3×1013cm−2 and Hall mobility of ∼314cm2∕Vs were measured at 77K. The temperature dependences of mobility and carrier concentration clearly demonstrate the presence of the 2DEG. Comparison with theory indicates that the carriers originate from both spontaneous polarization and unintentional degenerate nitrogen doping of 3C-SiC, suggesting a 77K 2DEG mobility ∼700cm2∕Vs in parallel with bulk hopping conduction. Mobility at high temperatures was phonon limited, indicating a Debye temperature of 1600K. Transmission line measurements yielded similar mobilities, with saturation currents of ∼3A∕mm, suggesting the utility of SiC heteropolytypes in microwave devices.

Gate field emission induced breakdown in power SiC MESFETs

The breakdown mechanism of SiC MESFETs has been analyzed by careful investigation of gate leakage current characteristics. It is proposed that gate current-induced avalanche breakdown, rather than drain avalanche breakdown, is the dominant failure mechanism for SiC MESFETs: thermionic-field emission and field emission are dominant for the ON state (above pinch-off voltage) and the OFF state (below pinch-off voltage), respectively. The effect of Si/sub 3/N/sub 4/ passivation on breakdown voltage has been also investigated. Si/sub 3/N/sub 4/ passivation decreases the breakdown voltage due to higher electric field at the gate edge compared to edge fields before passivation. A reduction in surface trapping effects after passivation results in the higher electric field because the depletion region formed by trapped electrons is reduced significantly.

Laser-induced surface potential transients observed in III-nitride heterostructures

We report on very long surface potential transients induced by ultraviolet laser illumination, which have been observed in nitride heterostructures. These surface potential transients correlate with current transients when measured simultaneously. Under illumination, electron–hole pairs are generated, which screen the electric field in the AlGaN barrier layer causing the surface potential to decrease. The holes move toward the surface assisted by the electric field in the AlGaN while the electrons increase the sheet charge concentration at the AlGaN/GaN interface, decreasing the net charge dipole across the barrier layer. This reduction can cause the surface potential to vary as much as 1 V between the unilluminated and illuminated states. The long transient response observed after the laser is turned off is explained by the slow recombination of the holes with the electrons thermionically emitted from the potential well at the interface. The thermionic emission is modeled by a continuously varying barrie...

Measurement of the mean electron-hole pair ionization energy in 4H SiC

A measurement of the mean e-h pair creation energy ⟨Ee-h⟩ in SiC using a scanning electron microscope is presented. Uncertainties stemming from backscattering from high Z metal contacts, as well as from the semiconductor surface, are removed by explicit measurement through direct electron bombardment of the bare semiconductor surface. A reduced value of ⟨Ee-h⟩=5.05eV for 4H SiC is reported, which is significantly lower than previously reported values. Good correspondence with Monte Carlo simulations of impact ionization in 4H SiC was obtained.

Demonstration of a 4H SiC Betavoltaic Cell

A betavoltaic cell in 4H SiC is demonstrated. An abrupt p-n diode structure was used to collect the charge from a 1mCi Ni-63 source. An open circuit voltage of 0.95V and a short circuit current density of 8.8 nA/cm2 were measured in a single p-n junction. An efficiency of 3.7% was obtained. A simple photovoltaic type model was used to explain the results. Good correspondence with the model was obtained. Fill factor and backscattering effects were included. Efficiency was limited by edge recombination and poor fill factor.

Passivation Effect on Channel Recessed 4H-SiC MESFETs

Channel recessed 4H-SiC MESFETs were fabricated and have demonstrated excellent small signal characteristics, such as, Ft of 14.5 GHz and Fmax of 40 GHz (for MAG = 1). The effect of Si3N4 passivation on these devices has been studied in this work. Current instability and RF power performance were improved after passivation. From our measurements, we found out that the passivation of SiC MESFETs reduces the surface effects and improves the RF power performance by suppressing the instability in DC characteristics. Introduction SiC MESFETs have been developed for high power applications due to unique material properties, such as, high saturated electron velocity, high breakdown field, and high thermal conductivity. In this work, channel recessed 4H-SiC MESFETs were fabricated and have demonstrated excellent small signal characteristics, such as, Ft of 14.5 GHz and Fmax of 40 GHz. SiC MESFETs have been reported to have current instability and strong dispersion [1]. We present that Si3N4 passivation reduces surface trapping effects and improves the power performance. From our measurements, however, we infer that both the surface traps and deep traps in the substrate can be responsible for the instability in SiC MESFETs. Experiments and Results Device Fabrication. The material structure consisted of a V-doped semi-insulating 4H-SiC substrate, a 0.25 μm p-type buffer layer doped < 5 x 10 15 cm -3 , and a 0.26 μm n-type channel layer doped Nd = 2 x 10 17 cm -3 . The cross-sectional device designs are shown in Figure 1. Source and drain regions were implanted with phosphorous. Ni deposition followed by annealing at 980 o C resulted in a specific contact resistance of 1.5 x 10 -6 Ω-cm 2 . ECR etching with a Cl2/CH4/Ar gas mixture was used to define the mesa and etch the channel region. The channel region was etched 0.06 μm for the channel recessed device. T-shaped gates with a gate length of 0.45 μm were fabricated by using an e-beam lithography process. PECVD Si3N4 passivation was done before the air-bridge step. The source-to-gate spacing was fixed at 0.5 μm and the gate-todrain spacings varied from 1.0 μm to 1.5 μm. DC and Small Signal Characteristics. Figure 2 shows typical I-V characteristics for the channel recessed device. A saturated current of 250 270 mA/mm was measured at Vgs = 0 V and Vds = 20 V, and a maxium transconductance of 40 45 mS/mm was measured at Vds = 20 V. Negligible gate leakage (~nA/mm) was observed and a pinch-off voltage, Vgs ~ -8 V was measured at Vds = 40 V. The off-state 3-terminal breakdown voltage, Vds, ranges from 120 V to more than 150 V at Vgs = -14 V, depending on gate-drain spacing. The small signal measurement was done at Vds = 20V and Vgs = -2V. 2 x 200 μm devices demonstrated excellent Ft and Fmax of 14.5 GHz and 40 GHz, respectively. The excellent small Materials Science Forum Online: 2003-09-15 ISSN: 1662-9752, Vols. 433-436, pp 749-752 doi:10.4028/www.scientific.net/MSF.433-436.749