Nuclear Instruments and Methods in Physics Research B 253 (2006)

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1 Nuclear Instruments and Methods in Physics Research B 53 () NIM B Beam Interactions with Materials & Atoms Thermal donor formation in silicon enhanced by high-energy helium irradiation P. Hazdra *, V. Komarnitskyy Department of Microelectronics, Faculty of Electrical Engineering, Czech Technical University in Prague, Technická, Prague, CZ-17, Czech Republic Available online 7 November Abstract The enhanced thermal donor (TD) formation was investigated in the float-zone n-type silicon irradiated with 7 MeV helium ions at fluences from 5 9 to 1 1 cm and subsequently annealed up to 5 C. Results show that radiation damage produced by helium ions remarkably enhances TD formation when annealing temperature exceeds 375 C, i.e. when the majority of vacancy-related recombination centers anneals out. At low fluences (below 1 11 cm ), the profile of radiation enhanced TD follows well the distribution of primary damage vacancies. The excess concentration of TD is proportional to helium fluence and peaks at 1 1 cm 3 if annealing temperature reaches 75 C. At higher fluences of helium, annealing temperature must be increased to stimulate formation of excess TDs. In this case, the concentration profile of TDs is more complex. Its growth starts from the irradiated surface and, with increasing temperature, it gradually extends up to the end-of-range of helium ions. Again, the excess TDs reach its maximum concentration of about 1. 1 cm 3 at 75 C. Ó Elsevier B.V. All rights reserved. PACS: 1.7.Cc; 1..Jh; 7..Jv Keywords: Alphas; Silicon; Irradiation; Thermal donors; Lifetime control 1. Introduction Nowadays, irradiation of silicon power devices, e.g. free-wheeling diodes, gate-turn-off thyristors, or insulated gate bipolar transistors, with high-energy hydrogen or helium ions became a popular tool for optimization of their turn-off properties. Radiation damage produced in this way is spatially localized and can be placed only in these parts of devices where it is necessary. This leads to local reduction of carrier lifetime which improves device turnoff behavior while still maintaining low on-state losses [1]. However, this favorable effect of ion irradiation is accompanied by negative side effects: increased leakage [] and * Corresponding author. Tel.: ; fax: address: hazdra@fel.cvut.cz (P. Hazdra). parasitic doping [3]. Power devices are usually produced on high resistive silicon to provide high blocking voltages. Therefore, the latter effect has to be carefully considered, since device blocking capability can be lost when the radiation damage is placed into the highly resistive region of the device. This is the case of proton irradiation followed by low-temperature (< C) annealing (necessary for defect stabilization), when formation of shallow donor levels related to hydrogen-defect complexes is enhanced []. In contrast with protons, helium irradiation was assumed to be free of donor doping side-effects. However, increasing demands on lifetime reduction (application of high fluences 1 cm ) and radiation defect stability (higher annealing temperatures) made conditions which allowed observing the enhancement of donor doping also after helium irradiation. The effect is was reported for the first time in oxygen enriched float-zone (FZ) silicon irradiated with He + to the fluence of 7 cm and subsequently 1-53X/$ - see front matter Ó Elsevier B.V. All rights reserved. doi:.1/j.nimb...3

2 1 P. Hazdra, V. Komarnitskyy / Nucl. Instr. and Meth. in Phys. Res. B 53 () annealed at 3 C [3]. Irradiation induced extra donor doping was explained by formation of thermal double donors (TDDs). Thermally activated shallow donor doping is connected with the intestitial oxygen (O i ) which is the major form of oxygen atoms in oxygen rich silicon. During thermal treatments at temperatures higher than 35 C, O i centers are able to condense resulting in a variety of aggregates that depend on several conditions, e.g. annealing temperature, doping conditions, thermal history of the material, and impurity content (hydrogen, carbon, nitrogen, etc.). For annealing at temperatures lower than 5 C, small oxygen defects are formed, including dimers, trimers, as well as larger complexes such as thermal double donors (TDDs) and shallow thermal donors (STD) [5]. Although atomic structure of thermal donors is still in dispute, it is generally accepted that TDDs contain an electrically active core (a complex of silicon interstitial with two interstitial oxygen atoms) and additional oxygen atoms in chain along a h1i crystal direction []. At temperatures above C, oxygen aggregates in SiO precipitates and thermally activated donor doping disappears. The influence irradiation with different energetic particles on TD formation was already reported for electrons [,7], neutrons [], protons [,9], and ions [,], however, results were sometimes controversial concerning the stimulating effect of radiation. Up to now, little attention was paid to the effect of high-energy helium irradiation. The aim of this paper is to show a systematic analysis of the influence of helium irradiation on TD formation which is studied as a function of irradiation fluence and annealing temperature. Both the shallow TD and the deep acceptor states related to radiation defects are characterized by electrical methods: C V profiling and capacitance deep level transient spectroscopy (C-DLTS). In this way, the influence of radiation defects on enhanced TD formation is enlightened.. Experimental Radiation enhanced production of thermal donors was studied on the low-doped (phosphorous concentration below 1 cm 3 ) h1 i-oriented FZ n-type silicon substrate forming the n-base of the planar p + nn + diodes. Diodes had deep p + and n + emitters produced by long thermal diffusion which resulted in relatively high concentration of oxygen in samples. The diodes were irradiated from the anode side with 7 MeV He ions in the range of fluences from 5 9 to 1 1 cm. After irradiation, the diodes were subjected to isochronal 3 min furnace annealing in the temperature range up to 5 C. Deep and shallow levels produced by irradiation and subsequent annealing were studied by the C-DLTS using the DLS- E spectrometer and C V profiling. The C V measurement was performed by HP 1 MHz capacitance meter at elevated temperature (5 C) to eliminate effect of deep acceptors on measured profile of free carriers. 3. Results and discussion 3.1. Recombination centers At lower fluences, helium irradiation introduces a series of defects acting as deep acceptor levels in silicon bandgap. Their evolution during post-irradiation annealing is shown in Fig. 1 where C-DLTS spectra of majority carrier (electron) traps measured on irradiated diodes are presented. The figure contains spectra recorded after irradiation (n.a.) with a fluence of cm and subsequent annealing at, 35 and 3 C. The spectra reveal several peaks originating form different deep levels appearing in the silicon bandgap. Table 1 summarizes identification parameters of levels detected after irradiation E1 E3 and subsequent annealing A1 A together with their attribution to particular lattice defects. Only deep levels given by pure radiation defects, the divacancy (E, E3) and vacancy oxygen pair (E1), are clearly resolved in the spectrum taken on the diode irradiated with alphas prior to DLTS signal ΔC/C o (a.u.) A3 E1 A A1 A5 E He + 1x cm - X A Temperature (K) E3 3 o C 35 o C o C Fig. 1. Majority DLTS spectra of diodes irradiated with 7 MeV alphas to a fluence 1 cm. The spectra were recorded after irradiation (n.a.) and subsequent 3 min annealing at, 35 and 5 C, rate window s 1. Table 1 Survey of electron traps introduced in the FZ n-type silicon by helium irradiation (E1 E3) and subsequent annealing (A1 A) Level Bandgap position (ev) Capture cross-section (cm ) Identity E1 E C VO ( /) + C i -C ð=þ s E E C V ð = Þ E3 E C V ð =Þ A1 E C.3 15 V O ( / ) A E C V O ( /) A3 E C ? A E C ? A5 E C. 3 15? A E C. 15? n.a.

3 P. Hazdra, V. Komarnitskyy / Nucl. Instr. and Meth. in Phys. Res. B 53 () annealing. For carrier recombination, the most significant deep level is the acceptor level of the vacancy oxygen pair VO / at E C.17 ev (E1) and the single-acceptor level of divacancy V = at E C.3 ev (E3) [1]. Annealing at C does not affect the signal of the level E1 (VO pair) while two divacancy-related peaks E and E3 shift to A1 and A. This shift is interpreted as annealing of V and formation of a new center with two charge states located close to V = and V =. The amplitude of the new center decreases after annealing at 35 C. This observation and identification parameters of A1 and A levels are in good agreement with literature [11], where the annealing of divacancies was studied in oxygenated FZ silicon irradiated with protons. The new center, which starts to form at C and anneals out at C, was tentatively identified and subsequently confirmed [1] as V O complex. This complex, which is formed by reaction of divacancies with interstitial oxygen according to the reaction V + O i! V O, is responsible for faster annealing of divacancies in oxygen rich silicon. Above 35 C, the annealing of VO and V O gives rise to multiple-vacancy V n and VO n complexes []. This is evidenced in DLTS spectra in Figs. 1 and by the transformation of the peaks corresponding to levels E1(VO / ), A1 and A (V O) to the new features labeled A3 A and one unidentified level X1. Increasing the fluence of helium ions (see Fig. ), i.e. excess generation of vacancies, makes the V O / level more pronounced and results in stronger signal of levels A A. Increasing strain in the damage layer also suppress the peak of the V O / level. This phenomenon was analogically observed for double-acceptor level E of divacancies [13]. The concentration profile of the level A (Fig. 3), which was measured in the sample irradiated to the fluence of 1 cm and annealed at 3 C, shows that residual defects (probably V n and VO n complexes) still remain localized in the region of primary damage maximum although a noticeable widening of the defect peak is registered. Deep levels ( 11 cm -3 ) 1 In summary, it can be concluded that helium irradiation followed by subsequent annealing in the 35 3 C range introduces various complexes containing both vacancies and oxygen. Concentration of these defects increases with helium fluence and, up to 3 C, their distribution follows in principle the distribution of primary damage. 3.. Shallow donors He + 7 MeV 1x cm - T an = 3 o C Fig. 3. Concentration profile of the level A measured in the sample irradiated with 7 MeV alphas to the fluence of 1 cm after 3 minutes annealing at 3 C. The simulated profile of primary defects (vacancies) is shown for reference. Free carrier profiles presented in Fig. show the evolution of the excess donor doping with annealing temperature for the sample irradiated to the fluence of 1 cm 7 MeV He +. Figure unambiguously confirms that radiation damage produced by helium ions remarkably enhances TD formation when annealing temperature exceeds 375 C, i.e. when the majority of vacancy-related recombination centers anneals out. The profile of radiation enhanced TDs follows well the distribution of primary 1 Primary vacancies ( 1 cm -3 ) DLTS signal ΔC/Co (a.u.) E1 A3 A A1 A5 He + x 11 cm A A Temperature (K) 3 o C 35 o C Fig.. Majority DLTS spectra of diodes irradiated with 7 MeV alphas to a fluence 11 cm. The spectra were recorded after 3 min annealing at 35 and 3 C, rate window s 1. Donor concentration ( 13 cm -3 ) 1 He + 7 MeV 1x cm - 5 o C o C 75 o C 5 o C 5 o C Fig.. Evolution of the excess donor doping with annealing temperature in FZ silicon irradiated with 7 MeV alphas to the fluence of 1 cm. The simulated profile of primary defects (vacancies) is shown for reference. Primary vacancies ( 1 cm -3 )

4 19 P. Hazdra, V. Komarnitskyy / Nucl. Instr. and Meth. in Phys. Res. B 53 () damage-vacancies. The full-width half maximum of the TDs peak is very close to that of the level A which is related to radiation damage (compare Figs. 3 and ). Increasing of annealing temperature stimulates the excess TD formation. However, while in the unirradiated area (beyond 5 lm), the TD formation increases and peaks at 75 C, at the damage maximum, the TD concentration saturates already at 5 C. Results of measurement on samples irradiated to higher fluences, i.e. 1 cm (see Fig. 5) show that annealing temperature must be increased to stimulate (or register) formation of excess TDs. Due to higher concentration of acceptor-like radiation centers, the damaged region is compensated up to C and excess donor doping is registered at T >5 C. The concentration profile of TDs is now more complex. Its growth starts from the irradiated surface and, with increasing temperature, gradually extends up to the end-of-range (R p ) of helium ions where it exhibits remarkable spread-out. Again, the excess TDs reach its maximum at 75 C when their concentration peaks at about 1. 1 cm 3. The effect of irradiation fluence on generation of excess TD for annealing temperature of 75 C (the maximal TD introduction) is summarized in Fig.. According to results presented above, it can be concluded that strong enhancement of thermal donor doping by preliminary irradiation with helium is caused by a transformation of oxygenand vacancy-related radiation defects to thermal donors. Since the TDs enhancement is limited by oxygen content in the target [], the profiles shows spread-out and saturation at the defect maximum if irradiating fluence increases (see Figs. and 7). For higher fluences of helium ions, TDs change their distribution since vacancy-related defects start to accumulate closer to the irradiation surface. Now, let us discuss the effect of helium-induced TDs on the breakdown voltage of the PiN diode, the basic building element of bipolar power devices. We showed, that TDs cause a non-negligible increase of donor concentration Donor concentration ( 13 cm -3 ) o C 5 o C R P 5 o C 75 o C He + 7 MeV 1x 1 cm Fig. 5. Evolution of the excess donor doping with annealing temperature in FZ silicon irradiated with 7 MeV alphas to the fluence of 1 1 cm. Donor concentration ( 13 cm -3 ) x and, according to Gauss law, this extra positive charge increases electrical field intensity of about DE = (q/e Si ) Æ N TD where q is electron charge, e Si silicon permittivity and N TD is the sheet concentration of TDs. When the local lifetime reduction is made in the optimum depth, i.e. at pn junction, the resulting increase of electric field intensity DE reduces the breakdown voltage V BR proportionally to the N-base width w B (DV BR DE Æ w B ). For the irradiation process presented (see Fig. 7), the maximal achievable N TD is about 11 cm. In this limit case, DE is about one tenth of the breakdown field (E crit 3 5 Vcm 1 ). Therefore, the helium irradiation at fluences of 1 cm followed by annealing above C can significantly decrease breakdown voltage V BR.. Conclusions He + 7 MeV Fluence (cm - ) 3x 11 1x 1 1x 11 5x 1x T a = 75 o C Fig.. Evolution of the excess donor doping in FZ silicon irradiated with 7 MeV alphas to different fluences ranging from 5 9 to 1 1 cm, annealing temperature 75 C. TD sheet concentration (cm - ) 11 Results unambiguously show that radiation damage produced by helium irradiation significantly stimulates R P He + 7 MeV annealed at 75 o C 11 1 Alphas fluence (cm - ) Fig. 7. Maximal achievable sheet concentration of excess donors versus fluence of alphas samples annealed at 75 C.

5 P. Hazdra, V. Komarnitskyy / Nucl. Instr. and Meth. in Phys. Res. B 53 () thermal donor formation during post-irradiation annealing at temperatures from 375 to 5 C. While at low fluences of helium ions (< 11 cm ), the concentration of TD is proportional to helium fluence and peaks at 1. 1 cm 3 for annealing temperature of 75 C, for higher fluences, annealing temperature must be increased to stimulate formation of excess TDs. Strong enhancement of TD doping by preliminary irradiation with helium is caused by a transformation of oxygen- and vacancy-related radiation defects to TD. The excess TD generation, which is both proportional to irradiation fluence and oxygen content in the target, can substantially reduce blocking capability of irradiated devices. Acknowledgements This work was supported by the Research Programme MSM 771 and the project of the Ministry of Education, Youth and Sports of the Czech Republic LC1. Authors also acknowledge ABB Switzerland Ltd., Semiconductors for diode preparation and FZ Rossendorf for sample irradiation. References [1] P. Hazdra, J. Vobecký, K. Brand, Nucl. Instr. and Meth. B 1 () 1. [] P. Hazdra, V. Komarnitskyy, Microelectron. J. 37 () 197. [3] R. Siemieniec, H.-J. Schulze, F.-J. Niedernostheide, W. Südkamp, J. Lutz, Microelectron. J. 37 (). [] Y. Ohmura, Y. Zohta, M. Kanazawa, Solid State Commun. 11 (197) 3. [5] V.J.B. Torres, J. Coutinho, R. Jones, M. Barroso, S. Öberg, P.R. Briddon, Physica B () 9. [] E.P. Neustroev, I.V. Antonova, V.P. Popov, D.V. Kilanov, A. Misiuk, Physica B 93 (). [7] I.V. Antonova, M.B. Gulyev, L.N. Safronov, S.A. Smagulova, Microelectron. Eng. (3) 35. [] Y.X. Li, H.Y. Guo, B.D. Liu, T.J. Liu, Q.Y. Hao, C.C. Liu, D.R. Yang, D.L. Que, J. Cryst. Growth 53 (3). [9] E. Ntsoenzok, P. Desgardin, J.F. Barbot, J. Vernois, D.B. Isabelle, Proc. RADECS 95 (1995) 5. [] E.P. Neustoev, I.V. Antonova, V.P. Popov, V.F. Stas, V.A. Skuratov, A.Yu. Didyk, Nucl. Instr. and Meth. B 171 () 3. [11] E.V. Monakhov, B.S. Avset, A. Hallén, B.G. Svensson, Phys. Rev. B 5 () 337. [1] G. Alfieri, E.V. Monakhov, B.S. Avset, B.G. Svensson, Phys. Rev. B (3) 33. [13] B.G. Svensson, B. Mohadjeri, A. Hallén, J.H. Svensson, J.W. Corbett, Phys. Rev. B 3 (1991) 9.