Localized vibrational mode infrared absorption of BH pair in silicon

Solid State Communications,Vo1.55,No.6, pp.501-503, 1985. Printed in Great Britain.

LOCALIZED

VIBRATIONAL

MODE INFRARED ABSORPTION

0038-1098/85 $3.00 + .OO Pergamon Press Ltd.

OF BH PAIR IN SILICON

Du Yonq-Chanq, Zhanq Yu-Fenq and Qin Guo-Gang Department of Physics, Beijing University, Beijing, China and Wenq Shi-Fu Department of Chemistry, Beijing University, Beijing, China (Received 20 Jun.1985 by W.Y. Kuan) High density of BH pair was produced by interaction between atomic hydrogen and boron in B-diffused silicon. The absorption band of BH pair, at 1873cm-', was first observed usins FT-IR soectroscopy. After thermal treatment at 240°C for 30min. the BH pair-dissolved and the carrier concentration increased, the hole mobility decreased owing to the increase in ionized impurity scattering, and the 1873cm-1 absorption band disappeared. Recently Sah et a1.[1,21 observed "bulk compensating donor" in P-type silicon after avalanche hole injection or after irradiation by Kev electrons. They demonstrated the role of hydrogen in deactivating the boron acceptors by forming the BIH+ pair. Pankove et a1.[31 observed that the boron acceptors in P-type silicon.were neutralized by atomic hydrogen generated by hydrogen plasma. Localized vibration infrared absorption measurement is a powerful technique to study impurities and defects in semiconductors[l]. In this communication we report an observation of localized vibrational mode infrared absorption spectrum of BH pair. The boron-diffused silicon samples were exposed to hydrogen plasmas in order to produce high density of BH pairs. The infrared absorption spectrum was measured using a Fourier transform infrared spectrometer. The silicon samples used in this study are about 5Qacm. N-type floatins-zone silicon. The sample-dimensions are 25X15X0.5mma for IR measurement and 8X8XO.5mm' for Hall measurement. Diffusion parameters are 950°C, 2Omin. in N7 ambient for pre-diffusion and 1125'C, 30mln. in 02 ambient for princeple diffusion. the depth of P-type region after diffusion is about Wm. An undiffused silicon sample employed as a reference in infrared absorption measurement. Hydrogen plasmas were produced by a ZOMH, RF oscillator with an output power of 80W. Treatment temperature is 140°C and treatment time is 20h at 0.5 Torr. Infrared spectra were run at room temperature on a Nicolet 7199B FT-IR spectrometer. Al dots were evaporated at nearambient temp8ratu:e for Hall measurement. The undiffused sample treated by hydrogen plasma shows no distinct absorption bdnd, but in boron-diffused silicon neutralized by atomic hydrogen there is a strong absorption band at 1873cm-1 which is in the wavenumber range of .9+-Hs&etching vibrational bands[51, hOWever, the free:carrier absorption is not very strong. Figure 1 illustrates the IR 8 ectra in wavenumber ranB and 1800-2340cm-1 for a ges of 850-1300cm501

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Fig. 1. FT-IR spectra of boron-diffused silicon neutralized by atomic hydrogen (A) and after annealing (B) B-diffused silicon sample neutralized by atomic hydrogen and for the Sam8 sample an: nealed attemperature 240°C for 30min.. The band width of 1873cm-1 at half maximum is 35cm-l. After annealin at temperature 240°C for 30min. the 1873cm- 9 band disappeared and the free-carrier absorption increased considerably. These imply that the 1873cm-1 band is the localized vibrational band aris8d from BH pair. Figure 2 shows a log-log plot of absorbance vs wavelength obtained from fiq.1. The relationship between the free-carrier absorption coefficient d, and the wavelength &can be written by[61

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LOCALIZEDVIBRATIONAL MODE INFRARED ABSORPTION OF BH PAIR IN SILICON

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Fig. 3. Isochronal annealing curves (lOmin.1 of the carrier concentration per cma and the hole mobility in borondiffused silicon treated by hydrogen plasma

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the effective hole mobility decreased simultaneously. While the annealing temperature was higher than 270°C, the carrier concenWavelength (pm) tration and the effective hole mobilit changed to the constant values 1.9X10 1%cmm2 Fig. 2. Free-carrier absorption in Band 36cm2/vs, respectively. The dissolving diffused silicon neutralized by atomic temperature of BH pair is in agreement with hydrogen before and after annealing that of Sah et al.111. The reduction of the hole mobility is owing to that the average relaxation time determined by ionized impuwhere N is the carrier concentration, n is rity scattering centers is smaller than that the real refractive index, m* is the effecby neutral impurity scattering centers. tive mass of free carrier and ?; is the relaThe concentration of BH pair got from xation time. However, the&' law may or may fig.3 is 1.7X1016cm-2. According this and not be obeyed depending on what is the prethe absorbance the cross section per BH pair dominant scattering mechanism. In general, is estimated to be 6X10-'*cm2. Noting the the free-carrier absorption coefficient is conductivity 6 and the free-carrier absorpdescribed by[61 tion coefficient dare both proportional to the carrier concentration N and 6 is propor&=const&p (2) tional to the average relaxation time% too, but & is inversely proportional tot, so From fig.2 it can be deduced that the p are that the change factor of the free-carrier 1.9 and 2.3 correspending to before and afabsorption coefficient (30-45) is much larter annealing, respectively. After annealing ger than that of the conductivity @6) can at 24O'C for 30min. the absorbance at a certain wavenumber in the range of 1300-2340cm-1 be accounted for. In summary, high density of BH pairs increased by a factor as high as 30-45. have been got using atomic hydrogen to hyAfter hydrogen-plasma treatment the drogenize the boron in B-diffused layer of carrier concentration per cm', N,, and the silicon. We observe for the first time the effective hole mobility were measured using localized vibrational mode infrared absorpvan der Pauw-Hall technique as a function of tion band of BH pair, it is located at 1873 annealing temperature and shown in figure 3. cm-l. AS the BH pairs dissolved at thermal Before thermal annealing the hole concentratreatment temperature above 240°C, the cartion and the effective hole mobility are rier concentration increased and the hole 2.0X1015cm-2 and 57cm2/VS, respectively. Afmobility decreased and the 1873cm-1 band diter annealing at temperature about 180 C for sappeared simultaneously. the hole concentration increased and 1 Omin., 0.04

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References

1. C.T.Sah, J.Y.C.Sun and J.J.Tsou,Appl. Phys. Lett., 43, 204(1983) 2. C.T.Sah, J.Y.C.Sun, J.J.Tsou and S.C.S. Pan, Appl. Phys. Lett., 43, 962(1983) 3. J.I>Pankove, D.E.Carlson, J.E.Berkeyheiser and R.O.Wance, Phys. Rev. Lett., Sl, 2224(1983)

4. A.S.Baker and A.J.Sivers, Rev. Mod. Phys., 47, 9~~~1.2, (1975) 5. H.J.Stein, J. Electra. Mater., 3, 159 (1975) 6. C.R.Pidgeon, Handbook on Semiconductors, Series ed. by T.S.Moss, North-Holland, (1980), Vol 2, ~223