1550 IEEE TRANSAcTlONS ON NUCLEAR SCIENCE, VOL. 42 NO. 6, DECEMBER 1995 DEGMDATION OF Si1 -xGe, EPITAXIAL HETEROJUNCTION BIPOLAR TRANSISTORS BY 1-MeV zyxwv FAST NEUTRONS H. Ohyama, J. VanheHemont*, Y. Takami" zyxwvu *, K. Hayama, H. Sunaga* * *, J. Poortmans" and M. Caymax" Kuaamoto National College of Technology, 2659-2 Suya Ebuchi-gun, Kumamoto 861-1 1, Japan *I=, Kapeldreef zyxwvuts 75, B-3001 Leuven, Belgium **Institute for Atomic Energy, R~kkyo University, 2-5-1 Nagasaka Yokosuka Kanagawa, 24.0-01 Japan ***Takasalu Radiation Chemistry Research Establishment,Japan Atomic Energy Research Institute, 1233 Watanuki Takasaki, Gunma, 370-12 Japan zyxwv Abstract Irradiation damage in nf-Silp+-Si l-xGexln-Si epitaxial heterojunction bipolar transistors (HBTs) by 1- MeV fast neutrons is studed as a function of fluence and germanium content for the first time. The degradation of the electrical performance of HBTs by irradiation increases with increasing fluence, while it decreases with increasing germanium content. The induced latme defects in the base and the collector regions are studied by DLTS methods. In the base regon, electron capture levels associated with interstitial boron are induced by irradiation, while two electron capture levels corresponding to the E centers and the divacancy are formed in the collector region. The device degradation of performance is then correlated with simulations of numbers of knock-on atom. In order to examine the recovery behavior, isochronal themal annealingis carried out for temperatures ranging from 75 to 300 OC Based on the recovery of electrical performance, it is pointed out that the electron capture levels induced in the base and collector regions are m d y responsible for the increase of base current and the decrease of collector current. many studies of the lattm defects induced by irradiation in Si and Ge and recently also in Sil-xGex crystals have been published [e. g. 4, 51, little is known on the nature of the lattice defects formed in strained Sil..xGex epitaxial layers. The radmtion source dependence of the degradation of device performance has not been dscussed so far, although some results obtained on diodes have been published elsewhere [6- Sf. In the present paper, the induced lattice defects and the degradation of the electrical perfomance of Sil_xGex epitaxial heterojunction bipolar transistors (HBTs), which are irradiated at room temperature with 1-MeV fast neutrons, are investigated as a function of fluence and germanium content for the first time. The radiation source dependence is d~scussed by comparing between 1-MeV fast neutrons zyxwv and 2-MeV electrons with respect to the damage coefficient and the number of knock-on atoms Based the observed recovery behavior resulting from isochronal thermal annealing, some possible explanations on the degradation and its dependence on germanium content are also presented. 11. EXPERIMENTAL I. LNTRODUCTION n+-Si/p+-Si l-xGex/n-Si epitaxial HBTs, whch were fabricated on strained Sil-xGex epitaxial layers grown on The utilization of space for broadcast and weather CZ silicon substrates phosphorus doped with about 1015 forecast using satellites is now an essential factor in our cm-3, were used in this study, ne dopants and their information intensive society. In space, matter is concentrations in this si substrate were as follows. ne continuously affected by Cosmic rays. Based on these active boron conceneation of the strained s i l - x ~ x requirements, extensive studies concerning the development epitaxial layers, were grown on silicon substrates of semiconductor devices which can operate normalIy in a using an ultra high vaCUUm chemical vapour deposition radiation environment have been undertaken everywhere [l- reasonable procedures for de hardening of MOs deposition pressure of 0.26 Pa, was about 2 x 1618 cm-3. metal-Oxide-Silicon) and bipolar Si devices, which can be The germanium content of the Sil-xGex epitaxial layer easily integrated,have been carried out. with a nominal thickness of 100 nm was x = 0.08, 0.12 There are however few reports available on the and 0.16. After the growth of epitaxial layers, mesas are damage created by radiationin Sil_xGex devices which have d&ned by wet etching in an m o 3 - W - H 2 0 solution. To an excellent performance with respect to high speed, large Passivate the mesa surface, a tetraethYlorthosilicate mas) current and direct transition, and its recovery behaviour due layer of about 100 nm thickness was deposited at 670 OC to thermal annealing. It is necessary to consider the by atmospheric CVD. The implantation of boron at an degradation mechanism of the Sil-xGex devices by energy of 55 keV was performed to make the base contact. irradiation for an improvement of the reliability of such Holes for the emitter region were defined after, which 200 electronic devices in radiation-rich environments. Although nm undoped polysilicon is grown by atmospheric CVD at a 31. Many studies on fie induced hge and system ' Dl at a growth temperature Of 630 OC and 0018-9499/95$04.00 zyxwvut 0 1995 IEEE