Differences between Polar-Face and Non-Polar Face 4H-SiC /SiO 2 Interfaces Revealed by Magnetic Resonance Spectroscopy

. We performed electron-spin-resonance (ESR) and electrically-detected-magnetic-resonance (EDMR) spectroscopy on 4H-SiC


Introduction
4H-SiC metal-oxide-semiconductor field-effect-transistors (MOSFETs) are typically fabricated using a standard polar face, i.e., 4H-SiC(0001) Si-face.On the other hand, non-polar faces such as (112 � 0) a-face and (11 � 00) m-face may be also promising for MOSFETs, because of their high fieldeffect mobility (µFE) [1,2].However, a simple dry oxidation on these non-polar faces suffers from a high density of interface states (Dit), resulting in much worse MOS characteristics as compared to the Si-face MOS interface with the same oxidation.There are no microscopic data on such high-density Dit of the non-polar faces.Therefore, we tried to observe electron-spin-resonance (ESR) signals of such interface defects using two techniques.One is ESR measurements on free-standing epitaxial 4H-SiC(112 � 0) substrates with dry oxidation.Another is electrically-detected-magnetic-resonance (EDMR) measurements on 4H-SiC(112 � 0) MOSFETs under MOS gate biases.Both the former and latter experiments successfully revealed MOS interface defects on Si-face MOS substrates [3,5] or MOSFETs [4,5], respectively.In this paper, we compare ESR and EDMR results between the polar and non-polar faces.

(1) ESR measurements
For ESR measurements, we fabricated free-standing substrates of a high-purity and high-quality undoped epitaxially-grown 4H-SiC(112 � 0), as illustrated in Fig. 1(a).Previously, such a special substrate enabled us to observe ESR signals of the PbC center (interface carbon dangling-bond center) in Si-face MOS interfaces, by taking advantage of minimizing bulk ESR signals [3].We prepared such substrates for Si-face ("Si sub") and a-face ("a sub").In addition, "Si dry" and "a dry" substrates were prepared by the standard dry oxidation.The oxide thicknesses are 50 nm for "Si dry" and approximately 60 nm for "a dry".Table I summarizes the four ESR samples.
ESR measurements were carried out by Bruker E500 X-band spectrometer with/without UV light illumination (3.40±0.15eV, ~1 mW) in a wide temperature range (4 to 295 K).The UV illumination may help to excite interface defects to their different charge states (either paramagnetic or spin-less states).Figures 1(b) and (c) show typical ESR spectra of the "Si dry" and "a dry" samples, in addition to their reference spectra of the "Si sub" and "a sub" samples, respectively.If interface signals appeared, we could find any differences between the two spectra.In fact, for Si-face, we found the PbC signal with 3.8×10 12 cm -2 , overlapping over a sample-rod signal (the E' signal in SiO2) [6], at room temperature.In contrast, for a-face, we could not find any ESR signals at room temperature, suggesting that high-density interface states on a-face never retain singly-occupied states.To change electronic occupation (i.e., the charge state) of the interface states, we conducted low-temperature ESR experiments with a strong photo excitation.Nevertheless, we could not any differential ESR signals, despite a strong photo excitation generated photo-excited signals (broad signal and E'+H centers) in a quartz sample rod.This fact indicates that on a-face, all interface defects are stabilized into spin-less states.We suggest that non-polar faces such as a-face promote charge transfer between Si and C atoms at the interfaces, resulting in pairs of doubly-occupied and empty states (both are spinless) and eliminating electron spins.This is strikingly contrast with the case of polar face such as Siface which allow singly-occupied interface states (ESR-active states).

Defects of Solid Semiconductor Structures
In order to visualize the hidden interface defects, we examined UV excitation at low temperatures (4 to 20 K) for a-face.Because of a long spin relaxation time at low temperatures, we used a rapidpassage (RP) ESR detection which can focus on long-relaxation-time spins.The results are shown in Fig. 1(c).A strong UV illumination damaged a sample rod and a cryostat tube (SiO2), creating the E' and E'+H centers as well as a growing broad signal [6].In spite of UV irradiation, no interface signals were found.
(2) EDMR measurements EDMR measurements on a MOSFETF enable us to tune spin states of interface defects via a MOS gate bias (Vg).Therefore, we performed EDMR measurements on a-face MOSFETs.Unfortunately, the "a dry" condition (see Table I) cannot allow its MOSFET to activate the channel current.Alternatively, we prepared lightly-nitrided a-face MOSFETs ("a NO10") for EDMR studies.Table II shows specifications of EDMR samples.The "a NO10" sample can activate the channel current, but its maximum µFE is much lower than the "Si dry" sample, suggesting a much higher Dit.
Figure 2 compares room-temperature EDMR spectra of n-channel 4H-SiC MOSFETs of "Si-dry" and "a NO10".For the "Si-dry" MOSFET, we detected an EDMR signal of the PbC center under negative Vg (e.g., -10 V).For "a NO10", we found a weak isotropic EDMR signal under negative Vg.This means that the observed center generates a singly-occupied state in the valence-band side.Although "NO10" should have a much higher Dit as compared to "Si dry", its EDMR signal was 1/25 or less of the PbC signal in "Si dry".No other signals were observed.The weak signal of "a NO10" shows g = 2.0019, suggesting that it arises from a carbon-related interface defect.
We therefore speculate that, on non-polar a-faces 4H-SiC MOS interfaces, a high-density interface states may be mainly located in the conduction-band side.When a singly-occupied state is close to the conduction band, such a shallow level becomes invisible to ESR (likewise shallow donors and acceptors), due to the lifetime broadening effect at room temperature.Such shallow interface states may interact more effectively with carries, resulting in larger mobility degradation.Thus, we performed EDMR measurements at 20 K in order to visualize the interface states close to the conduction band.This temperature makes it possible to observe ESR signals of shallow donors in 4H-SiC [5].
Figure 3(a) shows low-temperature EDMR spectra of "a NO10" under positive gate biases, focusing on the conduction-band-side interface states.We found an abnormal large hysteresis in Id-Vg curve of "a NO10", as shown in Fig. 3(b).This hysteresis was only visible at low temperatures (< 100 K) and is most probably related to a high-density shallow interface states close to the conduction band.Its origin will be studied elsewhere.Although we varied Vg over a wide range in order to cover both sides of the abnormal hysteresis as well as tried additional UV illumination, we did not find any EDMR signals of the shallow states, even we were able to reduce the EDMR noise level sufficiently.
Table II.EDMR samples of 4H-SiC MOSFETs.NO post-oxidation anneal (POA) was carried out at 1250 °C.When NO POA time was elongated to 60 min., the "a NO60" MOSFET showed a maximum µFE of 65 cm 2 V -1 s -1 .

Label
Gate oxide thickness

Summary
We carried out ESR and EDMR spectroscopy on a-face MOS interfaces and compared their interface defects with those in the standard Si-face MOS interfaces.We could not detect ESR signals of very-high-density interfacial defects in the "a dry" sample (dry oxidized a-face), indicating that 102 Defects of Solid Semiconductor Structures those defects are stabilized into spin-less states (doubly-occupied or empty states).We suggest that this phenomenon may be characteristic of non-polar faces because such faces may easily allow the charge transfer between coexisting Si and C atoms at the interfaces.In order to visualize the spinless interface defects, we also performed EDMR measurements on lightly-nitrided a-face MOSFETs ("a NO10") with a help of MOS gate bias.However, we only detected a weak EDMR signal (carbonrelated defect) in the valence-band side.The ESR/EDMR detection of a-face and other non-polar faces MOS interface defects with very high densities still remains an open issue.

Fig. 1 .
Fig. 1.(a) Preparation of free-standing epitaxial 4H-SiC substrates for ESR studies.ESR spectra of dry-oxidized 4H-SiC/SiO2 interfaces on (b) Si-face ("Si dry") and (c) a-face ("a dry")."Si sub" and "a sub" were measured for the same substrate before oxidation.Microwave excitation was 0.2 mW for (b) and 2 mW for (c).100-kHz magnetic-field modulation was used for (b) and (c).

Fig. 3 .
Fig. 3. (a) EDMR spectra of "a NO10" at 20 K under positive Vg.(b) Drain-to-source currents (Id-s) versus Vg in EDMR measurements (plotted by "×" symbols).A solid line is an Id-s-Vg curve at 20 K, showing an abnormal large hysteresis, possibly due to a high-density shallow interface states close to the conduction band.EDMR measurements were carried out with a pre-stress of either Vg = 0 V or Vg = +20 V for surveying the left-or right-hand-sides of the hysteresis, respectively.