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Determination of nitrogen in semiconductor materials using the 14N(p, α)11C and 14N(d, n)15O nuclear reactions
19
Nuclear Instruments and Methods in Physics Research B50 (1990) 19-22 North-Holland
DETERMINATION OF NITROGEN IN SEMICONDUCTOR AND 14N(d, n)“O NUCLEAR REACTIONS F. KiiHL ‘), J. KRAUSKOPF and K. BETHGE 2,
‘), P. MISAELIDES
MATERIALS
3), R. MICHELMANN
USING THE 14N(p, a)“C
‘), G. WOLF 2,
‘) Wacker-Chemitronic, 8263 Burghausen, FRG ‘) Institute for Nuclear Physics, University of Frankfurt, 6000 Fronkfuri am Main 90, FRG ” Department of Chemistry, Aristotelian University, 54006 Thessaloniki, Greece
The excitation functions of the 14N(p, cr)“C and i4N(d, n)i50 nuclear reactions have been determined in the projectile energy region between 2 and 7 MeV in order to obtain accurate cross-section data for the direct quantitative determination of traces of nitrogen in different semiconductor matrices (Si, GaAs, Gap). The advantages and limitations of the use of the above nuclear reactions for nitrogen determination and their possible interferences due to the matrix activation are also discussed. The data obtained led to the determination of a factor F, = [N]/a = (1.77 k0.27) X 10” cmm2 correlating the infrared absorption coefficient o for the line at 963 cm-’ with the absolute concentration of nitrogen in silicon, [N], determined by charged-particle activation analysis.
1. Introduction Nitrogen is a common impurity in semiconductor materials introduced during the growth and processing procedures. Although nitrogen is an element of group V and can substitute other elements of the same group in semiconductor materials, its role needs further investigations. Most information found in the literature concerns the chemical bonding and behaviour of nitrogen in silicon. For a silicon matrix the solubility [1,2], the diffusivity [3], the equilibrium coefficient [2] and the predominant bonding [4,5] have already been studied as an attempt to understand the complex phenomena connected with the intentional and nonintentional presence of this element [6]. The phenomena connected with the presence of nitrogen in gallium arsenide and phosphide are rather poorly known. Only limited information is available about the formation of gallium nitride in nitrogen-implanted gallium arsenide [7] and the substitutional role of N, as well as the N-N pair formation in gallium phosphide [8,9]. Infrared spectroscopy is the dominant conventional method used for the determination of nitrogen in semiconductor materials, although in some cases the presence of infrared-insensitive forms of nitrogen in the samples causes an underestimation of the amount of this element present [lo]. For the measurement of nitrogen in silicon [5,10] vibrational oscillations at 767 and 963 cm-’ can be used. In gallium arsenide [7] and phosphide [8] matrices the active vibrational bands appear at 480 and 496 cm-‘, respectively. However, the use of infrared spectroscopy for the determination of
0168-583X/90/$03.50 (North-Holland)
0 Elsevier Science PublishersB.V.
nitrogen requires reliable calibration, that can be provided by absolute techniques, like the nuclear techniques and especially the charged-particle activation analysis. The determination of nitrogen by charged-particle activation analysis is possible using a number of nuclear reactions. Some suitable reactions are given in table 1. They have in common that the reaction products of these reactions are positron emitters and the measurement of the intensity of the annihilation radiation as a function of the time provides a means of quantitative determination of nitrogen. The selection of the most suitable reaction depends on the semiconductor matrix, its dopings and, of course, on the amount of nitrogen expected. In some cases the charged particle activation can be combined with chemical processing of the irradiated sample or hot extraction of the activities produced in order to increase the sensitivity of the determination. The nuclear reaction 14N(p, a)“C has already been used for the determination of nitrogen in silicon [2,10]. Cross sections for this reaction can be found in the Table 1 Selection of possible reactions for the determination of nitrogen by charged-particle activation analysis Reaction
Half-life of reaction product
Q-value
14N(p, a)“C 14N(d, n)“O 14N(3He, a)13N 14N(3He, d)“O 14N(a, n)“F
20.3 124 9.96 124 66
- 2.92 5.07 10.03 1.81 - 4.70
min s min s s
I. NRA. CPAA
20
F. K6hI et af. / Nitrogen in semiconductormaterials
literature determined in the projectile energy region up to 24 MeV [l&25]. The agreement among the published experimental cross sections is not always especially good. Considering the disadvantages of use of gas 123,251 and adenine (CsHsN,) targets [24] for the measurement of the cross sections of the reactions on nitrogen and aiming at the determination of a calibration factor correlating the infrared absorption with the absolute nitrogen concentration in the sample obtained by charged-particle activation analysis, it was necessary to measure the excitation function of the 14N(p, cu)“C in the energy region between 5.7 and 7 MeV using lowtemperature-CVD silicon nitride targets. Different sets of cross section data for the nuclear reaction 14N(d,n)150 covering th e energy region up to 14.4 MeV can be found in the literature [ll-14). The agreement among the published data is not satisfactory, indicating the need of a redetermination of the excitation function of the above reaction in order to obtain rehable cross-section data for the charged-particle activation analysis. During this work, the excitation reaction has been function of the i4N(d, n)“O determined in the projectile energy region up to 6.9 MeV.
kept short (2 min) in order to reduce the formation of i3N (T,,, = 9.96 mm) and “F (T,,, = 1.08 min) which are produced in (d, n) reactions on carbon and oxygen traces present. The contribution of the “0 activity produced by the initial nitrogen content of the tantalum foils to the total activity was determined by the irradiation of the nonimplanted side of the individual samples under the same conditions,
3. Results and discussion Fig. 1 gives the excitation function of the 14N(p, cy)“C reaction. The experimental data obtained during this work are connected with an overall uncertainty of f 12%. The experimental cross sections of fig. 1 are comphmented by literature data [23] for the region below 5.2 MeV multiplied by an active volume correction factor of 1.1 [15]. The curve passing through the experimental values of fig. 1 was obtained by Fourier analysis of the experimental data and can be calculated using the analytical expression a(E)
=A(O) f
;
A(k)
cos[(E--3.78)nk/1.8]
k=l
2. Experimental Since the products of both nuclear reactions studied during this work are positron emitters, the measurement of the intensity of the annihiIation radiation as a function of time yields the required cross sections. The ration radiation was measured using two 4 X 4 in.* NaI(Tl) detectors switched in coincidence in order to reduce the background counts. The excitation function of the 14N(p, cw)“C reaction was determined using 0.5 pm thick low-temp~atur~ CVD silicon nitride targets deposited on a high-purity silicon substrate [15]. The stoichiometry of the targets was checked by hot extraction of the nitrogen at the disintegration temperature of 1720 “C. The measured weight fraction for nitrogen amounted to (39.9 f 0.4)% compared to the theoretical value of 39.938%. The energy of the 7 MV Van de Graaff accelerator of the Institute for Nuclear Physics of the University of Frankfurt, used for the measurements, was varied between 5.2 and 7.0 MeV in steps of 100 keV. The beam current was measured by a current integrator, using the whole target arrangement, including the isolated irradiation chamber, as Faraday cup. The cross sections of the r4N(d, n)*‘O reaction were also measured in the energy region between 2 and 7 MeV in steps of 0.5 MeV at the same accelerator. The targets were prepared by nitrogen ~pl~tation in 0.25 mm thick tantalum foils (fluence: 5 X 10’6-10’7 atoms/cm’, energy: 30 keV). The irradiation time was
+
sin[(E-
5 B(k) k-l
3.78)&/1.8],
where A(k) and B(k) are the Fourier coefficients and E the energy in MeV. The individual Fourier coefficient values are given in ref. [15]. Following the anaIyticaI procedure of 45 min irradiation using 1 p A of beam current, removaI of the surface contamination of the samples by etching with a mixture of 1 part of 40% HF and 5 parts of 60% HNO, (etching rate: 10 + 0.5 pm in 45 s), incineration-hot extraction of the produced activities, counting of the “CO, collected in Askarit, detection limits of 2 X 1014 nitrogen
4
4.5
5
5.5
6
6.5
7
E [MeVI
Fig. 1. Excitation function of the 14N(p, a)“C nuclear reaction. The curve passing through the cross-section values is obtained by a Fourier analysis of the data.
F. Kiihl et al. / Nitrogen in semiconductor
atoms/cm3 or about 4 ppba could be obtained for the determination of nitrogen in silicon. The etching rate has been determined experimentally covering a part of the sample surface by an acid-resistant tape and measuring the height of the step formed after the treatment. Applying the above-mentioned analytical procedure and using the data obtained during this work, the nitrogen concentration of 60 silicon samples has been determined. These samples were obtained from the end parts of 30 cylindrical samples of 10 mm length previously studied by IR spectroscopical techniques using an FT-IR spectrometer (Bio-Rad Lab. Digilab FTS-15/80) at room temperature [15]. These nitrogen determinations led to the calculation of a factor F, = [N]/a
= (1.77 f 0.26) X 10”
cm-‘,
correlating the infrared absorption coefficient a for the line at 963 cm-’ and the absolute concentration of nitrogen in silicon [N]. This value is in good agreement with the value of (1.83 f 0.24) X 1017 cm-’ given in ref.
WI.
Boron doping in a silicon matrix could cause a serious interference by the “B@, n)“C reaction which produces the same product nucleus as the (p, a)-reaction on 14N. Serious interference is also expected from the silicon matrix producing, via the 3oSi(p, n)30P and 29Si(p, Y)~‘P reactions, the 8+-emitting nucleus 3oP min). The threshold energy of the (T/2 = 2.54 “Si(p, n)“P reaction producing the main part of the 3oP activity is 5.174 f 0.030 MeV. No interference is expected in the case of determination of nitrogen in gallium arsenide matrices by the 14N@, a)“C reaction unless Te and Cr dopings are present. These two elements are producing, via the 12*Te(p, n)“*I and 52Cr(p, n)52mMn reactions, “‘1 and 52mMn with half-lives of 25 and 21 min, respectively, values very close to the 20.3 min half-life of “C. Both reactions have cross sections larger than the 14N(p, a) “C reaction [26-281. The excitation function of the 14N(d, n)“O reaction is given in fig. 2 together with some of the previously published cross-section data [11,13,14]. There is some disagreement between the data of this work and of ref. [ll]. There is also disagreement with the data of ref. [12], not appearing in fig. 2. The cross-section data of ref. [13] in the overlapping energy region up to 3.2 MeV seem to be systematically higher than the data of the present study, whereas the agreement with the data of ref. [14] is very good in the projectile energy region below 6.0 MeV. The 14N(d, n)150 reaction does not seem, however, very suitable for the determination of nitrogen in silicon matrices, because of the 3oP produced by the 29Si(d n)sc’P reaction. No such interference is expected in the’case of gallium arsenide and phosphide matrices, unless silicon contamination is present due to the growth
materials
oThls work AT Retz-Schmldt,J.L.WelI p K.Wohlleben,E.Schuster 0 H.Vero
I
0
I
I1
I
2.0
4.0
I
Ruiz.A.P.
I
I
6.0
8.0
I
I
10.0
Iref.111 (ref.131
Wolf (ref. 14 I
1 1
I,
,I
12.0
14.0
1
E [MeVl
Fig. 2. Excitation function of the 14N(d, n)150 nuclear reaction compared with the data of refs. [11,13,14].
of the material in a quartz crucible. Keeping the irradiation times short, no interference is expected from the Te and Cr dopings eventually present. The nuclides 1231 (Tl,2 = 13.2 h), “‘1 (Tl,2 = 60.4 d), 1261(Tl,2 = 13 d) and 51Mn (Tl,2 = 46.2 min) formed by the (d, n) reactions on the tellurium isotopes with mass numbers A = 122, 123, 125 and 50Cr, used as dopings, cannot be produced in considerable amounts by irradiation of 2 or 3 min duration. In the case of gallium arsenide no interference due to the matrix activation could be observed using projectiles of energies below 2.5 MeV. The variation of the activity of the annihilation radiation of a gallium arsenide sample irradiated for 2 min by 2.5 MeV deuterons is given in fig. 3. The three components observed with half-lives of 9.96, 2.03 and 1.08 min belong to the decay of 13N, 150 and “F produced by the (d, n) reactions on carbon, nitrogen and oxygen.
time ImInI
Fig. 3. The decay of the annihilation radiation activity of a Te-doped gallium arsenide sample irradiated by 2.5 MeV deuterons. The three components of the decay curve with half-lives of 9.96, 2.03 and 1.08 min belong to the decay of “N, 150 and i’F produced by the (d, n) reactions on “C, 14N and I60 traces present. I. NRA. CPAA
22
F. k”ahl et at. / Nitrogen in semirondrrctor materials
Concerning the limits of detection, concentrations of nitrogen of the order of 1 ppma in gallium arsenide could be determined under realistic conditions using the I4 N(d, n)l’ 0 reaction. A chemieaf or thermal processing of the irradiated samples could further improve the detection limits of the method. However, the short half-life of the reaction product is a serious disadvantage not &owing the remova of the contamination of &besurface layer of the sample by time-consuming etching procedures or the chemical processing of the irradiated samples. On the other hand, the deuteron irradiation offers the advantage of the simultaneous determination of traces of carbon, oxygen and nitragen without causing extreme radiation damage to the sample.
The reactions i4N(p, (Y)“C and 14N(dY n)% seem to be most suitable for the determination of traces of nitrogen in semiconductor materials (Si, GaAs, Gap). Good knowledge of the matrix material and its dopings are necessary in order to achieve a reliable determination. Chemical or thermal processing of the sample could lead to an increased sensitivity in most of the cases. However, the surface contamination of the sample must always be taken into account. Therefore absolute control over the procedures of removal of the surface layer after the irradiation of samples and good knowledge of the etching rates are requirements of a successful determination. The cooperation of the acceterator staff of the fnstitute for Nuclear Physics of the University of Frankfurt is acknowledged. This work was financially supported by the Stiftung Volkswagenwerk, the Federaf Ministry of Research and Technology and the Oreek General Secretariat of Research and Technology, as well as by the NATO Research Grant 0007/86.
References [I] W. Kaiser and CD. 7?mrmond, J. Appl. Phys. 30 (1959) 427.
[Z] Y. Yatsurugi, N. Akiyama, Y. Endo and T. Nozaki, J. Electrochem. Sot. 120 (1973) 975. [3) P.V. Pavlov, E.L. Zorin, D.L. Tetelbaum and A.F. Kbokhlov, Phys. Status Solidi A35 (1976) If. {4] H.J. Stein, AppJ. Phys. Lett. 43 [1983) 296. [5] H.J. Stein, J. Electrochem. Sot. 132 (1985) 668. [6] H.J. Stein, Proc. Mater. Res. Sot. vol. 59, eds. J.C. Mikkelsen, Jr. et at. (Mater. Res. Sot., Pittsburgh, PA, 1986) p_ 523. [7] A.H. Kachare, W.G. Spitzer, A. Kahan, F.K. Euler and T.A. Whatley, J. Appl. Phys. 44 (1973) 4393. [8] G. Ferenczi, T. Pavelka and M. Somagyi, Physica B116 (1983) 436. 191 F. Thompson and R.C. Newman, J. Phys. C4 (1971) 3249. [IO] Y. itoh, T. Nozaki, T. Masui and T. Abe, Appl. Phys. Lett. 47 (1985) 488. [II] T. Retz-Schmidt and J.L. Weil, Phys. Rev. 119 (1960) 1079. [12] H.W. Newson, Phys. Rev. 51 (1937) 620. [13] K. Wohllaben and E. Schuster, Radiachim. Acta 12 (1969) 75. (14] H. Vera Ruiz and A.P. Wolf, Radiocbim. Acta 24 (1977) 65. [15] F. Kishl, Doctoral Thesis, Univ. of Frankfurt (1988) to be published. [Id] J.P. Blaser, P. Marmier and M. Sempert, Helv. Pbys. Acta 25 (1952) 442. [I?] XL. Symonds, 3. Warren and J.D. Joung, Proc. Phys. Sot. (London) A70 (1957) 824. [IS] P.A. Benioff, Univ. of California Radiation Lab., Report UCRL-8780 (1959) (unpublished) and Phys. Rev. 119 (1960) 316. fl9] A.B. Clegg, K.J. Foley, G,L. Salmon and R.E. Segel, Pmt. Phys. Sot. (London) 78 (1961) 681. [20] L. Valentin, G. Albony, J.P. Cohen and M. Grusakow, J. Phys. (Parisf 25 (1964) 704. [Zl] A.M. MacLeod and J.M. Reid, Proc. Phys. Sot. (London) 87 (1966) 437. [22] M. Epherro and C. Seide, Phys. Rev. C3 (1971) 2147. 1231 P.D. Ingalls, J.S. Schweizer and B.R. Anderson, Phys. Rev. Cl3 (1976) 524. f24] W.W. Jacobs, D. Bodansky, D. Chamberiin and D.L. Oberg, Phys. Rev. C9 (1974) 2134. [25] G.T. Bida. T.J. Ruth and A.P. Wolf, Radiochim. Acta 27 (1980) 181. [26] J.P* %ser, F. Boehm, P. Marmier and P. Seherrer, Hefv. Phys. Acta 24 (1951) 441. [27] H. Taketani and W. Parker Alford, Phys. Rev. 125 (1962) 291. [28] 3. Wing and J.R. Huizenga, Phys. Rev. 128 (1962) 280.
Nuclear Instruments and Methods in Physics Research B50 (1990) 19-22 North-Holland
DETERMINATION OF NITROGEN IN SEMICONDUCTOR AND 14N(d, n)“O NUCLEAR REACTIONS F. KiiHL ‘), J. KRAUSKOPF and K. BETHGE 2,
‘), P. MISAELIDES
MATERIALS
3), R. MICHELMANN
USING THE 14N(p, a)“C
‘), G. WOLF 2,
‘) Wacker-Chemitronic, 8263 Burghausen, FRG ‘) Institute for Nuclear Physics, University of Frankfurt, 6000 Fronkfuri am Main 90, FRG ” Department of Chemistry, Aristotelian University, 54006 Thessaloniki, Greece
The excitation functions of the 14N(p, cr)“C and i4N(d, n)i50 nuclear reactions have been determined in the projectile energy region between 2 and 7 MeV in order to obtain accurate cross-section data for the direct quantitative determination of traces of nitrogen in different semiconductor matrices (Si, GaAs, Gap). The advantages and limitations of the use of the above nuclear reactions for nitrogen determination and their possible interferences due to the matrix activation are also discussed. The data obtained led to the determination of a factor F, = [N]/a = (1.77 k0.27) X 10” cmm2 correlating the infrared absorption coefficient o for the line at 963 cm-’ with the absolute concentration of nitrogen in silicon, [N], determined by charged-particle activation analysis.
1. Introduction Nitrogen is a common impurity in semiconductor materials introduced during the growth and processing procedures. Although nitrogen is an element of group V and can substitute other elements of the same group in semiconductor materials, its role needs further investigations. Most information found in the literature concerns the chemical bonding and behaviour of nitrogen in silicon. For a silicon matrix the solubility [1,2], the diffusivity [3], the equilibrium coefficient [2] and the predominant bonding [4,5] have already been studied as an attempt to understand the complex phenomena connected with the intentional and nonintentional presence of this element [6]. The phenomena connected with the presence of nitrogen in gallium arsenide and phosphide are rather poorly known. Only limited information is available about the formation of gallium nitride in nitrogen-implanted gallium arsenide [7] and the substitutional role of N, as well as the N-N pair formation in gallium phosphide [8,9]. Infrared spectroscopy is the dominant conventional method used for the determination of nitrogen in semiconductor materials, although in some cases the presence of infrared-insensitive forms of nitrogen in the samples causes an underestimation of the amount of this element present [lo]. For the measurement of nitrogen in silicon [5,10] vibrational oscillations at 767 and 963 cm-’ can be used. In gallium arsenide [7] and phosphide [8] matrices the active vibrational bands appear at 480 and 496 cm-‘, respectively. However, the use of infrared spectroscopy for the determination of
0168-583X/90/$03.50 (North-Holland)
0 Elsevier Science PublishersB.V.
nitrogen requires reliable calibration, that can be provided by absolute techniques, like the nuclear techniques and especially the charged-particle activation analysis. The determination of nitrogen by charged-particle activation analysis is possible using a number of nuclear reactions. Some suitable reactions are given in table 1. They have in common that the reaction products of these reactions are positron emitters and the measurement of the intensity of the annihilation radiation as a function of the time provides a means of quantitative determination of nitrogen. The selection of the most suitable reaction depends on the semiconductor matrix, its dopings and, of course, on the amount of nitrogen expected. In some cases the charged particle activation can be combined with chemical processing of the irradiated sample or hot extraction of the activities produced in order to increase the sensitivity of the determination. The nuclear reaction 14N(p, a)“C has already been used for the determination of nitrogen in silicon [2,10]. Cross sections for this reaction can be found in the Table 1 Selection of possible reactions for the determination of nitrogen by charged-particle activation analysis Reaction
Half-life of reaction product
Q-value
14N(p, a)“C 14N(d, n)“O 14N(3He, a)13N 14N(3He, d)“O 14N(a, n)“F
20.3 124 9.96 124 66
- 2.92 5.07 10.03 1.81 - 4.70
min s min s s
I. NRA. CPAA
20
F. K6hI et af. / Nitrogen in semiconductormaterials
literature determined in the projectile energy region up to 24 MeV [l&25]. The agreement among the published experimental cross sections is not always especially good. Considering the disadvantages of use of gas 123,251 and adenine (CsHsN,) targets [24] for the measurement of the cross sections of the reactions on nitrogen and aiming at the determination of a calibration factor correlating the infrared absorption with the absolute nitrogen concentration in the sample obtained by charged-particle activation analysis, it was necessary to measure the excitation function of the 14N(p, cu)“C in the energy region between 5.7 and 7 MeV using lowtemperature-CVD silicon nitride targets. Different sets of cross section data for the nuclear reaction 14N(d,n)150 covering th e energy region up to 14.4 MeV can be found in the literature [ll-14). The agreement among the published data is not satisfactory, indicating the need of a redetermination of the excitation function of the above reaction in order to obtain rehable cross-section data for the charged-particle activation analysis. During this work, the excitation reaction has been function of the i4N(d, n)“O determined in the projectile energy region up to 6.9 MeV.
kept short (2 min) in order to reduce the formation of i3N (T,,, = 9.96 mm) and “F (T,,, = 1.08 min) which are produced in (d, n) reactions on carbon and oxygen traces present. The contribution of the “0 activity produced by the initial nitrogen content of the tantalum foils to the total activity was determined by the irradiation of the nonimplanted side of the individual samples under the same conditions,
3. Results and discussion Fig. 1 gives the excitation function of the 14N(p, cy)“C reaction. The experimental data obtained during this work are connected with an overall uncertainty of f 12%. The experimental cross sections of fig. 1 are comphmented by literature data [23] for the region below 5.2 MeV multiplied by an active volume correction factor of 1.1 [15]. The curve passing through the experimental values of fig. 1 was obtained by Fourier analysis of the experimental data and can be calculated using the analytical expression a(E)
=A(O) f
;
A(k)
cos[(E--3.78)nk/1.8]
k=l
2. Experimental Since the products of both nuclear reactions studied during this work are positron emitters, the measurement of the intensity of the annihiIation radiation as a function of time yields the required cross sections. The ration radiation was measured using two 4 X 4 in.* NaI(Tl) detectors switched in coincidence in order to reduce the background counts. The excitation function of the 14N(p, cw)“C reaction was determined using 0.5 pm thick low-temp~atur~ CVD silicon nitride targets deposited on a high-purity silicon substrate [15]. The stoichiometry of the targets was checked by hot extraction of the nitrogen at the disintegration temperature of 1720 “C. The measured weight fraction for nitrogen amounted to (39.9 f 0.4)% compared to the theoretical value of 39.938%. The energy of the 7 MV Van de Graaff accelerator of the Institute for Nuclear Physics of the University of Frankfurt, used for the measurements, was varied between 5.2 and 7.0 MeV in steps of 100 keV. The beam current was measured by a current integrator, using the whole target arrangement, including the isolated irradiation chamber, as Faraday cup. The cross sections of the r4N(d, n)*‘O reaction were also measured in the energy region between 2 and 7 MeV in steps of 0.5 MeV at the same accelerator. The targets were prepared by nitrogen ~pl~tation in 0.25 mm thick tantalum foils (fluence: 5 X 10’6-10’7 atoms/cm’, energy: 30 keV). The irradiation time was
+
sin[(E-
5 B(k) k-l
3.78)&/1.8],
where A(k) and B(k) are the Fourier coefficients and E the energy in MeV. The individual Fourier coefficient values are given in ref. [15]. Following the anaIyticaI procedure of 45 min irradiation using 1 p A of beam current, removaI of the surface contamination of the samples by etching with a mixture of 1 part of 40% HF and 5 parts of 60% HNO, (etching rate: 10 + 0.5 pm in 45 s), incineration-hot extraction of the produced activities, counting of the “CO, collected in Askarit, detection limits of 2 X 1014 nitrogen
4
4.5
5
5.5
6
6.5
7
E [MeVI
Fig. 1. Excitation function of the 14N(p, a)“C nuclear reaction. The curve passing through the cross-section values is obtained by a Fourier analysis of the data.
F. Kiihl et al. / Nitrogen in semiconductor
atoms/cm3 or about 4 ppba could be obtained for the determination of nitrogen in silicon. The etching rate has been determined experimentally covering a part of the sample surface by an acid-resistant tape and measuring the height of the step formed after the treatment. Applying the above-mentioned analytical procedure and using the data obtained during this work, the nitrogen concentration of 60 silicon samples has been determined. These samples were obtained from the end parts of 30 cylindrical samples of 10 mm length previously studied by IR spectroscopical techniques using an FT-IR spectrometer (Bio-Rad Lab. Digilab FTS-15/80) at room temperature [15]. These nitrogen determinations led to the calculation of a factor F, = [N]/a
= (1.77 f 0.26) X 10”
cm-‘,
correlating the infrared absorption coefficient a for the line at 963 cm-’ and the absolute concentration of nitrogen in silicon [N]. This value is in good agreement with the value of (1.83 f 0.24) X 1017 cm-’ given in ref.
WI.
Boron doping in a silicon matrix could cause a serious interference by the “B@, n)“C reaction which produces the same product nucleus as the (p, a)-reaction on 14N. Serious interference is also expected from the silicon matrix producing, via the 3oSi(p, n)30P and 29Si(p, Y)~‘P reactions, the 8+-emitting nucleus 3oP min). The threshold energy of the (T/2 = 2.54 “Si(p, n)“P reaction producing the main part of the 3oP activity is 5.174 f 0.030 MeV. No interference is expected in the case of determination of nitrogen in gallium arsenide matrices by the 14N@, a)“C reaction unless Te and Cr dopings are present. These two elements are producing, via the 12*Te(p, n)“*I and 52Cr(p, n)52mMn reactions, “‘1 and 52mMn with half-lives of 25 and 21 min, respectively, values very close to the 20.3 min half-life of “C. Both reactions have cross sections larger than the 14N(p, a) “C reaction [26-281. The excitation function of the 14N(d, n)“O reaction is given in fig. 2 together with some of the previously published cross-section data [11,13,14]. There is some disagreement between the data of this work and of ref. [ll]. There is also disagreement with the data of ref. [12], not appearing in fig. 2. The cross-section data of ref. [13] in the overlapping energy region up to 3.2 MeV seem to be systematically higher than the data of the present study, whereas the agreement with the data of ref. [14] is very good in the projectile energy region below 6.0 MeV. The 14N(d, n)150 reaction does not seem, however, very suitable for the determination of nitrogen in silicon matrices, because of the 3oP produced by the 29Si(d n)sc’P reaction. No such interference is expected in the’case of gallium arsenide and phosphide matrices, unless silicon contamination is present due to the growth
materials
oThls work AT Retz-Schmldt,J.L.WelI p K.Wohlleben,E.Schuster 0 H.Vero
I
0
I
I1
I
2.0
4.0
I
Ruiz.A.P.
I
I
6.0
8.0
I
I
10.0
Iref.111 (ref.131
Wolf (ref. 14 I
1 1
I,
,I
12.0
14.0
1
E [MeVl
Fig. 2. Excitation function of the 14N(d, n)150 nuclear reaction compared with the data of refs. [11,13,14].
of the material in a quartz crucible. Keeping the irradiation times short, no interference is expected from the Te and Cr dopings eventually present. The nuclides 1231 (Tl,2 = 13.2 h), “‘1 (Tl,2 = 60.4 d), 1261(Tl,2 = 13 d) and 51Mn (Tl,2 = 46.2 min) formed by the (d, n) reactions on the tellurium isotopes with mass numbers A = 122, 123, 125 and 50Cr, used as dopings, cannot be produced in considerable amounts by irradiation of 2 or 3 min duration. In the case of gallium arsenide no interference due to the matrix activation could be observed using projectiles of energies below 2.5 MeV. The variation of the activity of the annihilation radiation of a gallium arsenide sample irradiated for 2 min by 2.5 MeV deuterons is given in fig. 3. The three components observed with half-lives of 9.96, 2.03 and 1.08 min belong to the decay of 13N, 150 and “F produced by the (d, n) reactions on carbon, nitrogen and oxygen.
time ImInI
Fig. 3. The decay of the annihilation radiation activity of a Te-doped gallium arsenide sample irradiated by 2.5 MeV deuterons. The three components of the decay curve with half-lives of 9.96, 2.03 and 1.08 min belong to the decay of “N, 150 and i’F produced by the (d, n) reactions on “C, 14N and I60 traces present. I. NRA. CPAA
22
F. k”ahl et at. / Nitrogen in semirondrrctor materials
Concerning the limits of detection, concentrations of nitrogen of the order of 1 ppma in gallium arsenide could be determined under realistic conditions using the I4 N(d, n)l’ 0 reaction. A chemieaf or thermal processing of the irradiated samples could further improve the detection limits of the method. However, the short half-life of the reaction product is a serious disadvantage not &owing the remova of the contamination of &besurface layer of the sample by time-consuming etching procedures or the chemical processing of the irradiated samples. On the other hand, the deuteron irradiation offers the advantage of the simultaneous determination of traces of carbon, oxygen and nitragen without causing extreme radiation damage to the sample.
The reactions i4N(p, (Y)“C and 14N(dY n)% seem to be most suitable for the determination of traces of nitrogen in semiconductor materials (Si, GaAs, Gap). Good knowledge of the matrix material and its dopings are necessary in order to achieve a reliable determination. Chemical or thermal processing of the sample could lead to an increased sensitivity in most of the cases. However, the surface contamination of the sample must always be taken into account. Therefore absolute control over the procedures of removal of the surface layer after the irradiation of samples and good knowledge of the etching rates are requirements of a successful determination. The cooperation of the acceterator staff of the fnstitute for Nuclear Physics of the University of Frankfurt is acknowledged. This work was financially supported by the Stiftung Volkswagenwerk, the Federaf Ministry of Research and Technology and the Oreek General Secretariat of Research and Technology, as well as by the NATO Research Grant 0007/86.
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