(Invited) Thermal Stability of 1×1020 cm-3 Al+ Implanted 4H-SiC after Electrical Activation at Temperature ≥ 1850°C
Authors: Nipoti Roberta , Canino Mariaconcetta , Puzzanghera Maurizio , Sozzi Giovanna 
Published: February 2020

In 4H-SiC electronic devices, Al+ ion implantation is often used to obtain selected regions of desired p-type conductivity in n-type epitaxial layers. A mandatory high temperature annealing is necessary after the implantation process for the electrical activation of the implanted Al. The temperature of the former and the density of the latter vary in the range 1600-1950°C and 1017-1020 cm-3, respectively. The switching properties of the so obtained bipolar junctions depend on carrier life-time in the n-type 4H-SiC epitaxial layers after processing.

In the literature, it has been shown that carbon vacancy VC in n-type 4H-SiC epi-layers is a carrier life-time killer defect and that its concentration, after high temperature treatments in the range 1500-1950°C, saturates on values that are higher the higher are the applied temperature and cooling rate [1]. It has been also shown that, in the case the thermally treated 4H-SiC epi-layers were ion implanted, and whatever is the implanted ion species, VC annihilation took place from 1000°C till 1500°C, while at 1600°C the phenomena of VC annihilation and VC formation seem to compete [2], suggesting that up to 1500°C, a carbon interstitial IC’s injection from the implanted near-surface layer into the epi-layer can be sufficient to make dominant VCannihilation, but this is not the case at 1600°C.

Our previous studies on carrier life-time in Al+ implanted p+-n-n+
diodes, with 6×1019 cm-3 and 2×1020 cm-3 Al+ implanted emitters after 1600 °C/20 min without carbon-cap (C-cap) and 1950 °C/5 min with C-cap post implantation annealing, respectively, have shown longer carrier life-time for diodes annealed at the lower temperature, about 700 ns against 170 ns [3], as measured by open circuit voltage decay (OCVD). The diodes annealed at the higher temperature have shown, by DLTS spectroscopy, a VC concentration lower by a factor ~ 3 relative to that in as-grown epi-layers subjected to almost the same thermal annealing (1950 °C /3 min) [1]. If the ion dose is sufficiently high, it is reasonable that injection of IC’s from the implanted layer into the epi-layer bulk takes place even at temperatures as high as 1950 °C but thermal generation of VCstill remains the prevailing process.

Taking into account the above results, it sounds reasonable to deserve efforts to the investigation of an ion implantation technology for bipolar junction in 4H-SiC where the electrical activation of the implanted species is achieved by a high temperature treatment and the degradation of the carrier life-time in the device drift layer is rescued by a subsequent low temperature treatment. The first step towards such a technology is to verify that during the thermal treatment for carrier life-time rescue, none or negligible deactivation of the electrically activated dopant takes place. This is the purpose of this study.

4H-SiC materials, both high-purity-semi-insulating (HPSI) and low doped n-type, have been implanted with Al+ ions so to obtain a homogenous doped layer next of the wafer surface. Post implantation annealing has been performed at 1950°C for different times in the range 5-40 min. Some samples have been also thermally treated at 1500°C for 40 min or for 240 min. During every annealing, the sample were protected by a pyrolyzed resist film (C-cap), that was removed after the last thermal treatment. The implanted Al concentration was 1×1020 cm-3, that is lower than Al solubility in 4H-SiC at 1950°C, and higher than the expected, but never measured, Al solubility in 4H-SiC at 1500°C [4].

Van der Pauw devices were used for Hall effect and sheet resistance measurements in the temperature range 180-680 K. The temperature dependences of these data show that the 1500°C treatment has an effect ranging from “neutral” to “beneficial” on the electrical transport of the Al+implanted 4H-SiC layer submitted to a 1950°C annealing immediately after implantation.

This result opens the way towards the fabrication of bipolar diodes with p-type emitters of almost identical hole density but different thermal treatments after implantation, thus with drift layers that might have very different current transport properties.

[1] H. M. Ayedh, V. Bobal, R. Nipoti, A. Hallén, B. G. Svensson, J. Appl. Phys.
115, 012005 (2014)

[2] H. M. Ayedh, A. Hallén, B. G. Svensson,
J. Appl. Phys.
118, 175701 (2015).

[3] H. M. Ayedh, M. Puzzanghera, B. G. Svensson, R. Nipoti, Mater. Sci. Forum
897, 279-282 (2017)

[4] M. K. Linnarson, U. Zimmermann, J. Wong-Leung, A. Shoener, M.S. Janson, C. Jagadish, B. G. Svensson, Appl. Surf. Sci.
203-204, 427-432 (2003).