Optical and electrical characterization of 4H-SiC detectors R. Schifano, A. Vinattieri INFM - Dipartimento di Fisica, Un

1. Optical and electrical characterization of 4H-SiC detectors R. Schifano, A. Vinattieri INFM - Dipartimento di Fisica, Universita ’di Firenze ( Italy) S. Miglio,M. Bruzzi, S. Lagomarsino, S. Sciortino INFM - Dipartimento di Energetica, Universita ’ di Firenze ( Italy) F. Nava Dipartimento di Fisica, Universita ’ di Modena( Italy) We present optical and electrical characterization of 4H-SiC schottky diodes prepared for ionising radiation detection. Capacity vs. voltage (C-V) measurements have been performed on a set of commercial samples of different nominal nitrogen doping (Neff from about 1013/cm3 to 5 1015/cm3) and epilayer thickness(from 10 mm to 40 mm).This technique has been used to determine the depletion voltage and the Neff actual values. These measurements are in fairly good agreement with a charge collection efficiency characterization, performed on the higher quality samples. Subsequently, the samples have been studied by means of Photo-Luminescence (PL) characterisation. We analysed in particular the near-band edge recombination and compared the free excitons (FE-I ) and nitrogen-bound excitons (BE-Q) signatures, measured in the range 10-100 K. The temperature of the samples has been determined from a best fit of the FE I76 peak. The ratio between the Q0 BE peak and the I76 FE peak intensity, measured at about 20 K, has been compared with the measured Neff values obtaining a good correlation. This method has also been used to study the dependence of the Neff on the sample thickness by a proper tuning of the laser wavelength. The results have been compared with the Neff profile obtained from the electrical characterization. Characterization Procedure The samples tested were grown onto 4H-SiC n+ type substrates by CREE Research and by the Institut fur Kristallzüchtung of Berlin (IKZ). They are characterized by different values of the layer thickness (10-40 mm) and of the nominal nitrogen doping (6x1013-1015 cm-3). CCE measurements: The sensor signal is generated by a collimated 0.1 mCi 90Sr source emitting minimum ionizing b particles. The characterisation box contains an Amptek A225 charge integrating preamplifier (Noise level ENC~ 280e+10e/pF ). The trigger signal is given by a miniature scintillator connected with a photomultiplier. C-V measurementshave been performed by means of a HP 4284A connected to a probe station in the range 0-600V, in the whole range of frequency of the test signal (100 kHz). PL measurementsSamples were excited by a frequency-doubled ps dye-laser (the average intensity at 300 nm is in the range 4-30 W/cm2, repetition rate 76 MHz). A standard time-integrated photon counting has been used to detect PL signal. Comparison between ccd and C-V measurements Landau distibution and pedestal in a CCE measurement Doping profile in a high quality sample • The first batch of high quality samples in which the Landau distribution is clearly separated from pedestal • The ccd value of sample A is (37.6 2)mm,corresponding to a mean value of the deconvoluted Landau of 2100 electrons • Low-doped samples (Sample A: Neff= 6 ·1013 cm-3 ; sample B: Neff = 9.4 ·1014 cm-3 ): we have determined the thickness of the epilayer by means of C-V measurements • Thickness (w ) values obtained with this procedure (sample A: w=34.1mmand sample B: w=21.6mm) have to be corrected for the diffusion length of electrons from the substrate • Taking into account these corrections we obtained: sample A, w 40mm; sample B, w  23mm • The charge collected vs. applied voltage measurements were reproducible and no priming effect has been observed Sample A High quality SiC: charge collected vs bias voltage The charge collection efficiency (CCE) measurements under minimum ionizing  particles, performed on our samples, are in good agreement with the C-V measurements, yielding a 100 % efficiency over the total thickness W. Best fit of the FE (I76) peak intensity • By a proper tuning of the photon energy of the incoming radiation we can scan different depths of the epilayer along the direction of growth, from near the surface to the deep inside of the substrate bulk. We have observed a steep rise of the bound exciton signal and the occurrence of a strong luminescence band as the penetration depth became higher than the epitaxial layer thickness. • The spectral signature corresponding to the FE I76.4 line has been fitted with an asymmetric curve which takes into account the density of states distribution, the thermal broadening and an inhomogeneous broadening effect. • The fitting procedure clearly showed that the temperature of the thermalized excitons was at least 10 K higher than that of the crystalline lattice(hot excitons). Substrate contribuition to the PL spectra CONCLUSIONS BE and FE peak intensity ratio (R) vs nitrogen effective doping We stated the potential of epitaxial 4H-SiC as a particle detector by measuring the MIP (Minimun Ionising Particle) signal on Schottky devices, clearly separated from the noise. We performed a thourough optical characterisation by means of exciton recombination photoluminescence. C-V, CCE and photoluminescence results are in very good agreement. We developed an optical setup which can be used to evaluate the quality of a 4H-SiC wafer with no need of electrical measurements. Further study on hot excitons will most likely give important information on the kinetics of the carriers induced by the ionising radiation, in SiC-based soft X and particle detectors. A good correlation has been found between the ratio R (Intensity of the nitrogen-bound exciton line Q0 , and the free exciton signature I76) vs. the effective doping Neff. Our data are consistent with the only work on the subject reported in literature: Ivanov et al. J. Appl. Phys. Lett. 80, 3504, 1996 (red markers in the figure). It is of note that we have scanned a range of lower effective doping, corresponding to higher quality epilayers specially prepared for soft X rays and particle detectors. In brief: we have calibrated our optical characterisation in order that we can check the effective doping of a SiC wafer with a very high spatial resolution; future developments should allow to check the epilayer thickness by means of optical measurements. It is possible to use this technique for a complete characterisation ot the SiC wafer before preparing the devices (cutting, cleaning, metallisation).