Ion beam induced charge gate rupture of oxide on 6H–SiC

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Abstract

Investigation of oxide charge trapping and interface state generation in SiO2 grown on 6H–SiC was performed using ion beam induced charge (IBIC), electroluminescence (EL) and the high frequency capacitance–voltage (CV) method. A large flatband voltage shift in CV measurements indicated a high density of positive charges trapped near the SiC/SiO2 interface. These trapped charges were related to defects either existing in the oxide or generated during alpha particle irradiation. EL indicated trap levels at 1.36, 1.6, 2.3 and 2.9 eV. Levels at 1.36 and 2.3 eV are defects existing in the SiC substrate, while the other two remaining levels are due to defects in the oxide layer. These defects affected the radiation hardness of the SiC electronic devices. Oxide rupture caused by alpha particle irradiation of the metal-oxide-p-type SiC device is observed.

Introduction

One of the keys to fabrication of successful metal-oxide–semiconductor (MOS) devices is the oxide layer. SiC is of special interest since SiO2 can be grown on it in a similar way to Si although the presence of carbon can lead to defects and poor electrical properties at the SiC/SiO2 interface which cannot be improved by annealing in a hydrogen ambient [1]. The generation of the interface states therefore seems to be qualitatively different from Si/SiO2. Defects in the SiO2 layer may be charged or uncharged, diamagnetic or paramagnetic. The presence of charged defects can perturb the operation of MOS devices. So far the reason for the large defect density that appears near the SiC/SiO2 interface is not well understood.

Oxide reliability is commonly tested with either constant voltage or current stressing which stresses the entire oxide. Oxides which cannot withstand prolonged electrical stressing are thought to possess gross defects. This leads to the conclusion that impurities in the oxide layer produce the dielectric breakdown [2]. Heavy ions injected into Si capacitors are known to cause localised stressing of the oxide layer leading to destruction of the devices [2]. This phenomenon is known as single event gate rupture (SEGR). When a heavy ion strikes a positively biased capacitor, a high density of holes accumulates at the interface of the oxide and Si. The holes at the interface and their image charge in the gate electrode cause the electric field in the gate oxide to increase. The increase in the electric field is a temporary, transient field that adds to the pre-existing steady-state field in the gate oxide. If the electric field in the gate becomes sufficiently large, then oxide breakdown occurs. Between ∼100 and ∼1000 localised ions on each oxide region are required to cause breakdown in thin oxide layers [2].

Studies in gate rupture caused by heavy particles indicated that only ions with LET greater than 3MeVcm2mg−1 will lower the breakdown voltage and cause the oxide to breakdown. For 2 MeV alpha particles, the LET is not greater than 1.7MeVcm2mg−1 in SiC [6]. Recently 200 MeV and 44 MeV proton-induced dielectric breakdown was reported by Titus and Wheatley [7]. They suggested that the recoil product (Si, Al, Mg) from a nuclear reaction or the primary knock-on atom has sufficient LET to cause the gate rupture but this is a negligible possibility for 2 MeV alpha particles in the present experiments.

All the SEGR reported so far in the literature are for the Si/SiO2 system and, to our knowledge, alpha induced gate rupture has not been reported before. We report in this work the observation of alpha particle-induced rupturing in SiO2 grown on SiC. IBIC microanalysis was used to measure, with high spatial resolution, both pre-existing and irradiation-induced local variations in the bulk defect structure. The CV method was used to determine the charge trapped in the oxide and at the interface. The electroluminescence measurements provided information on electrically active defects in the oxide.

Section snippets

Device fabrication and experimental details

The devices studied in this work were designed and fabricated at the National Institute of Advance Industrial Science and Technology on 5μm p-type epilayer 6H–SiC substrates purchased from Cree Research. The substrates were cleaned as described in [4] followed by oxide growth by pyrogenic oxidation at 1100°C. The thickness of the SiO2 layer was in the range 10–30 nm. Ni contacts were deposited onto the back of the substrates for ohmic connection. A Schottky electrode was fabricated by Al

Results and discussion

The typical IBIC map of a diode is shown in Fig. 1(b). This map shows the charge collection from regions which we denote the “inversion layer”, “noise from the charge collection system” and “site of the gate rupture”. The signal from the inversion region is interesting as it shows charge collection despite the fact that there is no electrode over the oxide layer in this region. Yet an electric field must exist in this region. The reason for this can be understood from the large difference

Conclusions

The development of reliable oxides on SiC has become an important issue for the fabrication of metal-oxide–semiconductor and their applications. IBIC, EL and CV techniques have provided direct and indirect evidences for the existence of defects in the oxide layer grown on a 6H–SiC material. Positive charges were found trapped within the oxide as indicated by IBIC and CV measurements. IBIC also revealed the site of ion-induced gate rupture. The rupture was possibly caused by an inhomogeneous

Acknowledgements

K.K. Lee wishes to acknowledge the support of a University of Melbourne Overseas Postgraduate Research Scholarship, and a Melbourne Research Scholarship. We acknowledge the expert assistance of Roland Szymanski in the maintenance of the accelerator and Jeff McCallum for the use of the Ocean Optics spectrometer.

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