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Identification of vacancy-type defects in as-grown InP by positron annihilation rate distribution measurements
Solid State Communications, Vol. 99, No. 10, pp. 745-749, 1996 Copyright 6 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/96 $12.00 + .OO
0038-1098(96)00851-9
IDENTIFICATION OF VACANCY-TYPE DEFECTS IN AS-GROWN InP BY POSITRON ANNIHILATION RATE DISTRIBUTION MEASUREMENTS* Z.Q. Chen, X.W. Hu, S.J. Wang and S. Q. Li Department
of Physics, Wuhan University, Wuhan 430072, P.R. China (Received 12 December 1995 by 2. Gun)
Positron annihilation lifetime spectra have been measured on a series of InP crystals with different conduction types and carrier concentrations and analysed by numerical Laplace inversion technique. Three noticeable features were observed: (1) the average positron annihilation rate in n and SI-type InP is nearly the same but smaller than that in p-type InP; (2) the width of positron annihilation rate distribution (ARD) in nand SI-type is much broader than that in p-type; (3) the upper limit of ARD was shifted from 3.96 forp-type to about 3.77 n s-’ for n-type and SI-type InP. These results indicate that in n- and SI-type InP, both indium and phosphorus vacancies ( Vtn, Vr) can trap positrons, but in p-type InP, only In-vacancy is the trapping site. The charge state of Vr, and VP is neutral as determined by the temperature experiment. Copyright 0 1996 Published by Elsevier Science Ltd Keywords: A. semiconductors, scopies.
C. point defects, E. positron spectro-
1. INTRODUCTION Defects play a major role on the electronic properties of semiconductor material because they interact with free carriers (acting as scattering centers, traps and recombination centers), and their effect is nonnegligible even when their concentration is very small compared to the free carrier concentration. For this reason, the study of defects in semiconductors has remained an active research field, especially for III-V based semiconductors [l-3]. There are many experimental techniques to study defects in semiconductors. Among these, EPR (electron paramagnetic resonance) is the most powerful method which had provided a wealth of information on silicon. Another powerful method is DLTS (deep level transient spectroscopy) which allows the determination of the concentrations and energy levels of each individual defect. But these methods have their shortcomings, for example, EPR is difficult for com-
* Supported by the National Natural Science Foundation of China.
pound semiconductors because this technique requires weak hyperflne interaction with nuclear spins in order to produce sufficiently narrow resonance lines. A further limitation is the requirement for low carrier concentration and the defect must have a non-zero electron spin. For DLTS, it does not give information on the actual structure of the defect, and this method needs a junction to operate and therefore is not a bulk method. Positron, the antiparticle of electron, when ejected into materials, is strongly repelled by positive ion cores due to the Coulomb repulsion. Vacancy-type defects, where ions are missing or their density is locally reduced, act as attractive centers and positrons are trapped at these sites with the annihilation characteristics different from the bulk. Due to the reduced electron density in these defect sites, the trapped positron lifetime increases and the electron-positron momentum distribution narrows [4]. In semiconductors, only neutral or negative vacancies can trap positrons. The larger average lifetime or S-parameter than the bulk value is a strong evidence of the existence of vacancy-type defects. Consequently,
745
746
VACANCY-TYPE
DEFECTS IN AS-GROWN InP
positron annihilation technique can be used to obtain direct information, such as the nature, size, concentration, charge state and impurity surroundings of the vacancy-type defects on an atomic scale. InP, which is one of the most important III-V compound semiconductors, has been used in a number of potentially important optoelectronic devices with significant interest being shown particularly in its use in high-efficiency solar cells for space applications. Considerable attention has been paid in the study of defects in InP material especially in its asgrown state [5, 61. Up to now there has been little work on positron annihilation in InP semiconductors. Delubek et al. [7, 81 have found a single lifetime in various InP crystals (n-type doped with S, SI-type doped with Fe, p-type doped with Zn) varying from 242 to 252 ps except in a highly Zn-doped sample (4 x 10” cmp3) a second lifetime with a value of 320 ps was found which they attributed to Zn-divacancy complexes. Later, in 1993, Bretagnon [9] resolved two lifetimes (~1 = 19Ops, r2 = 27 ps) in a series of InP samples (undoped or doped with S, Fe or Zn), and found the lifetime was independent of the nature and the concentration of the dopants. Combined with the temperature experiment the positron trap is suggested to be a neutrally charged indium vacancy in all the samples. The identification of the vacancies in InP is still under discussion especially in experiment. The common result of these studies is that the lifetime is the same in all the samples with different conduction types. In this paper we first adopted a new analytical method (CONTIN program) to get some information about the distribution of the positron annihilation rate, and then studied systematically the native vacancy-type defects in several InP samples. A clear difference was observed between n-type, SI-type and p-type samples. 2. EXPERIMENTAL A series of InP crystals were chosen as samples with different conduction types (n-type undoped or doped with S, SI-type doped with Fe and p-type doped with Zn) and different dopant concentrations. Positron lifetime was measured using a fast-fast lifetime spectrometer with a time resolution of 240 ps in full width at half maximum (FWHM). Two identical pieces of each sample (10 x 10 x 0.5 mm3) were sandwiched with a 25,uCi 22Na prepared by evaporating 22NaCl solution onto a 5pm aluminium foil. The source correction was about 15% of 0.165 ns and 0.3% of 1 ns. Each spectrum contains 2 x lo6 total counts for PATFIT analysis and lo7 counts for CONTIN analysis.
Vol. 99, No. 10
3. RESULTS AND DISCUSSION 3.1. Extracting of positron-annihilation-rate distribution (ARD) In conventional data analysis, the positron lifetime spectrum was expressed as a sum of several negative exponentials convoluted with instrument resolution function R(t) plus background B as Y(t) = R(t)(Nt)
fJCliX+PA’i + B).
(1)
i=l
The analysis was performed by POSITRONFIT and RESOLUTION program in PATFIT package 1101. Recently we have employed a Laplace inversion technique to analyze positron lifetime spectrum to continuous lifetime distribution by using the CONTIN program [l 1, 121 when the lifetime spectrum is replaced by an integral Y(t) = R(t)(Nt)
r(Xa(X)e-“dX 5
+ B).
(2)
The solution X&(X) at every X value could be obtained by this program. This method avoids direct determination of the instrument resolution function by measuring a reference spectrum with a well-known single lifetime at the same experimental condition. In addition, it is model-independent, we do not need to know how many lifetime components exist prior to analysis. For CONTIN analysis, we measured the lifetime spectrum with a total count of 107. We used here the defect free single crystal silicon as a reference sample. Its bulk lifetime is known to be 220 ps. The positron annihilation rate distribution (ARD) is presented by its value Aa at all grid points X. Here the total grid point Ng is 60 points. As the defect lifetime is very close to the bulk lifetime in semiconductors, only one peak was obtained for each sample. The average lifetime (X-l) and the standard deviation of the distribution could be calculated through the values of the moments for the peak described as a2 = Mi/M_i
- (&/iK,)2,
(x-i) = M-z/M-r,
(3) (4)
where Mi is the ith moment of the solution given by
J
Las
Mi =
X’+‘a(X)dX. mm
x
The full width at the half maximum FWHM = 0 x [2sqrt(2 In 2)] could also be obtained. Four samples were measured. The results were presented in Table 1. The extracted solution of Aa( i.e., the annihilation
Vol. 99, No. 10
VACANCY-TYPE
Positron
DEFECTS IN AS-GROWN InP
747
decompose the spectrum. But the average lifetime is a good parameter for monitoring changes in lifetime spectra [ 131and has a high statistical accuracy. Therefore, we calculated the average lifetime rav as
ARD in InP
7(IlJ= 711, + 7212,
4.00
3.50
4.50
5.00
A (ns-‘) Fig. 1. The positron-annihilation-rate distributions (ARD) obtained bv CONTIN urogram for InP &ystais with different conduction iypey rate distributions (ARDs) of three samples with different conduction type are shown in Fig. 1. As shown in Fig. 1 and Table 1: (1) The annihilatation rate distribution is nearly the same for n-type and SI-type samples, unaffected by the nature of the dopant or the carrier concentration. The average lifetime (X-l) has nearly the same value, which was about 237ps, and the widths of the distribution FWHM are all close to 0.3050ns-‘. (2) For p-type InP, the rate distribution showed a large difference. First, the peak position shifted to a larger value, the average lifetime dropped to 233~s. Second, its distribution is much narrower with FWHM only 0.2338ns-‘, (3) All the three distributions have nearly the same upper limit of 4.77 ns-’ . In order to study the effect of carrier concentration on the lifetime, we also measured a series of samples with different dopant concentrations and were analyzed by PATFIT program. Two components could be resolved. As the defect lifetime is very close to the bulk lifetime in semiconductors, it is often difficult to
(6)
which is insensitive to the uncertainties in the decomposition procedure and in this work we present only the result of the average lifetime. The result is shown in Table 2. The average lifetime given by equation (6) was clearly larger than the bulk lifetime (about 229~s) which provided evidence of existence of vacancy-type defects. For n-type and SI-type InP samples, the average lifetime is about 236ps, and this value is independent of the carrier concentration or the nature of the dopant. In n-type semiconductors, the Fermi level is located in the upper half of the band gap and moves upward with increasing dopant concentration. It is pinned in the middle of the gap in S&type materials. Our result indicated that the trapping sites in these two samples are the same and are independent of the fermi level position in the upper half of the gap. But in the case of p-type InP, the situation is different. The average lifetime r,, = 232.7~s also is smaller than the value for n and S&type samples, and this value is still unaffected by the carrier concentration. The result of average lifetime obtained by the PATFIT program was the same as obtained by CONTIN. 3.2. Identification of phosphorus vacancies In InP crystals, various intrinsic defects can be produced during crystal growth. These are In and P vacancies (VI,.,, VP), interstitials (Ini, Pi), and antisites (PI,, Inp). Positrons are sensitive to vacancy type defects. Therefore, positrons in InP may annihilate in these states: (1) The delocalized free positron state, i.e., the Bloch state; (2) The trap state of P vacancy; (3) The trap state of In vacancy. The single ARD peaks with different distributions shown in Fig. 1 represent these positron states. In P-type InP, the ARD had a region from 3.96ns-’ to 4.77ns-’ with FWHM
Table 1. CONTZN result of the lifetime spectra, including average annihilation rates (X), average lifetime rav, region of ARD, distribution width FWHM
Dopant
Type
Carrier cont. (cmp3) or resistivity (a-cm)
(A> (ns-‘)
%J (ns)
Region of ARD (ns-‘)
FWHM (ns-‘)
S Fe Fe Zn
n
1.4 x 10’9 2x 10’ 7x lo6 3.9 x 1oJ8
4.2126 4.2363 4.2365 4.2963
0.2378 0.2365 0.2365 0.2329
3.77-4.77 3.77-4.77 3.77-4.77 3.96-4.77
0.3019 0.3090 0.3065
SI SI P
0.2338
VACANCY-TYPE
748
Vol. 99, No. 10
DEFECTS IN AS-GROWN InP
Table 2. Characteristics of various InP samples and results of lifetime (by PATFITprogram)
Dopant
Type
1 2 3 4 5 6 7
None S S S S S S
n n n n n n n
8 9
Zn Zn
P P
10 11
Fe Fe
SI SI
Sample No.
0.2338 ns-‘. This peak was relatively narrower. In ntype and X-type samples, the ARD had the same upper limit at 4.77ns-’ but its lower limit shifted to 3.77 ns-’ with a broader distribution. The FWHM of ARD increased to about 0.3050ns-’ by a factor of about 32%. Bretagnon’s results [9] observed the Indium vacancies that trapped positrons in p-type Inp. In n- and SI-type samples, the shift of the ARD lower limit implies directly an increase in the fraction of positron annihilation in the trapping state with a smaller annihilation rate as the upper limit remained unchanged. This state was the trapping in P vacancy. Therefore both In and P vacancies trapped positrons in n-type and SI-type InP crystals. However, the position lifetime result gives no information on the chemical surroundings around the vacancy and therefore cannot determine whether the vacancy is isolated or complexed with impurity atoms. The theoretical results have also provided the possibility of positron trapping by P vacancy in nand SI-InP. There have been several theoretical calculations on the unrelaxed ideal defect energy level of InP sample [14-161. According to their result, the In vacancy in InP can have many different charge states and their energy levels are all located in the lower half of the band gap. The P vacancy, on the other hand, is always positively charged irrespective of the position of the Fermi level position. However, in semiconductors defects often undergo strong lattice relaxation when they change their charge state. The relaxation causes the change of the defect energy level, which is called negative-U behavior [17]. Recently, Alatalo et al. [18] reported the discovery of the negative-U effect in the compound semiconductor InP. They calculated the ionization levels of the relaxed indium and phosphorus vacancy and found
Carrier cont. (cme3) or resistivity (a-cm)
Average lifetime rm (ps)
; 3.2 (3-4) 3.89 1.4 (4-5)
;$ 10” 1018 lo’* lOI lOI9
235.5 235.6 235.0 235.1 236.2 235.8 235.9
2.68 x 1018 3.9 x lOus
232.8 232.6
; x x x x x
236.0 236.6
2x 10’ 7 x lo6
that the inclusion of the relaxation does not alter the picture for the In vacancy, but the ionization level of P vacancy is largely changed, and it has an ionization level from V; to I$ in the lower half of the band. This tells us that in n-type and SI-type InP both In and P vacancies may trap positrons, but in p-type InP, the P vacancy may be positively charged and cannot bind positrons. One of the samples (undoped n-type InP) was studied as a function of temperature between 95 and 350K. The spectra were analyzed by the PATFIT program. It is found that the average lifetime was constant over the temperature range, as shown in Fig. 2. Bretagnon et al. [9] has also found that the positron lifetime for all types of InP remained
n-type
210’
. 80
InP(undoped)
’ 170
Temperature
I 260
350
(K)
Fig. 2. The temperature dependence of the average lifetime TV,,in undoped n-type InP.
VACANCY-TYPE
Vol. 99, No. 10
DEFECTS IN AS-GROWN InP
unchanged over the measuring temperature range from 30 to 300K. The calculation by Puska showed that the trapping coefficient for a neutral vacancy is practically independent of temperature [ 191. These two temperature experiments strongly indicated that the vacancies in all the samples are neutrally charged. 4. CONCLUSION In summary, we have systematically studied the vacancy-type defects in InP semiconductors. Both indium and phosphorus vacancies attracted positrons in n-type and S&type InP as the Fermi level is located in the upper half of the gap. In p-type InP, however, only the indium vacancy trapped positrons. The temperature experiment showed that these vacancies that trapped positrons were neutrally charged. Our result from positron annihilation was in coincidence with the theoretical result on the defect energy level in InP material. Acknowledgements-This
work was supported by the National Natural Science Foundation of China. REFERENCES 1. 2. 3.
J.C. Bourgoin, H.J. von Bardeleben & D. Stievenard, J. Appl. Phys. 64, R65 (1988). M.O. Manasreh, D.W. Ficher & W.C. Mitchel, Phys. Status Solidi (b) 154, 11 (1989). P.M. Mooney, J. Appl. Phys. 67, RI (1990).
4. 5. 6. 7. 8.
9.
10. 11. 12. 13.
W. Brandt
8~ A. Dupasquier
16. 17. 18. 19.
(eds) Positron Amsterdam
Solid-State Physics North-Holland,
(1983). S.W.S. Mckeever, R.J. Walters, S.R. Messenger & G.P. Summers, J. Apply. Phys. 69, 1435 (1991). J.A. Walk, W. Walukiewicz, M.L.W. Thewalt & E.E. Haller, Phys. Rev. Lett. 68,3619 (1992). G. Delubek and 0. Briimmer, Ann. Physik Leipzig 43, 178 (1986). G. Delubek, 0. Briimmer, F. Plazaola, P. Hautojarvi & K. Naukarinen, Appl. Phys. Lett. 46, 1136 (1985). T. Bretagnon, S. Dannefaer & D. Kerr, J. Appl. Phys. 73,4697 (1993). PATFIT package Riser National Laboratory, Roskilde, Denmark (1989). R.B. Gregory & Y. Zhu, Nucl. Instrum. Methods Phys. Rex A290, 172 (1990). Z. Tang & S. J. Wang, Nucl. Znstr. & Meth. A355, 548 (1995). P. Hautojarvi, Mater. Sci. Forum 175-178, 47 (1995).
14. 15.
749
Hongqi Xu, Phys. Rev. B42, 11295 (1990). M.J. Puska, J. Phys. Condens. Mutt. 1, 7347 (1989). R.J. Jansen, Phys. Rev. J341,7666 (1990). P.W. Anderson, Phys. Rev. Lett. 34,953 (1975). M. Alatalo, R.M. Nieminen, M.J. Puska, A.P. Seitsonen & R.Virkkanen, Phys. Rev. B47,638 1 (1993). M.J. Puska, C. Corbel & R.M. Nieminen, Phys. Rev. B41, 9980 (1990).
0038-1098(96)00851-9
IDENTIFICATION OF VACANCY-TYPE DEFECTS IN AS-GROWN InP BY POSITRON ANNIHILATION RATE DISTRIBUTION MEASUREMENTS* Z.Q. Chen, X.W. Hu, S.J. Wang and S. Q. Li Department
of Physics, Wuhan University, Wuhan 430072, P.R. China (Received 12 December 1995 by 2. Gun)
Positron annihilation lifetime spectra have been measured on a series of InP crystals with different conduction types and carrier concentrations and analysed by numerical Laplace inversion technique. Three noticeable features were observed: (1) the average positron annihilation rate in n and SI-type InP is nearly the same but smaller than that in p-type InP; (2) the width of positron annihilation rate distribution (ARD) in nand SI-type is much broader than that in p-type; (3) the upper limit of ARD was shifted from 3.96 forp-type to about 3.77 n s-’ for n-type and SI-type InP. These results indicate that in n- and SI-type InP, both indium and phosphorus vacancies ( Vtn, Vr) can trap positrons, but in p-type InP, only In-vacancy is the trapping site. The charge state of Vr, and VP is neutral as determined by the temperature experiment. Copyright 0 1996 Published by Elsevier Science Ltd Keywords: A. semiconductors, scopies.
C. point defects, E. positron spectro-
1. INTRODUCTION Defects play a major role on the electronic properties of semiconductor material because they interact with free carriers (acting as scattering centers, traps and recombination centers), and their effect is nonnegligible even when their concentration is very small compared to the free carrier concentration. For this reason, the study of defects in semiconductors has remained an active research field, especially for III-V based semiconductors [l-3]. There are many experimental techniques to study defects in semiconductors. Among these, EPR (electron paramagnetic resonance) is the most powerful method which had provided a wealth of information on silicon. Another powerful method is DLTS (deep level transient spectroscopy) which allows the determination of the concentrations and energy levels of each individual defect. But these methods have their shortcomings, for example, EPR is difficult for com-
* Supported by the National Natural Science Foundation of China.
pound semiconductors because this technique requires weak hyperflne interaction with nuclear spins in order to produce sufficiently narrow resonance lines. A further limitation is the requirement for low carrier concentration and the defect must have a non-zero electron spin. For DLTS, it does not give information on the actual structure of the defect, and this method needs a junction to operate and therefore is not a bulk method. Positron, the antiparticle of electron, when ejected into materials, is strongly repelled by positive ion cores due to the Coulomb repulsion. Vacancy-type defects, where ions are missing or their density is locally reduced, act as attractive centers and positrons are trapped at these sites with the annihilation characteristics different from the bulk. Due to the reduced electron density in these defect sites, the trapped positron lifetime increases and the electron-positron momentum distribution narrows [4]. In semiconductors, only neutral or negative vacancies can trap positrons. The larger average lifetime or S-parameter than the bulk value is a strong evidence of the existence of vacancy-type defects. Consequently,
745
746
VACANCY-TYPE
DEFECTS IN AS-GROWN InP
positron annihilation technique can be used to obtain direct information, such as the nature, size, concentration, charge state and impurity surroundings of the vacancy-type defects on an atomic scale. InP, which is one of the most important III-V compound semiconductors, has been used in a number of potentially important optoelectronic devices with significant interest being shown particularly in its use in high-efficiency solar cells for space applications. Considerable attention has been paid in the study of defects in InP material especially in its asgrown state [5, 61. Up to now there has been little work on positron annihilation in InP semiconductors. Delubek et al. [7, 81 have found a single lifetime in various InP crystals (n-type doped with S, SI-type doped with Fe, p-type doped with Zn) varying from 242 to 252 ps except in a highly Zn-doped sample (4 x 10” cmp3) a second lifetime with a value of 320 ps was found which they attributed to Zn-divacancy complexes. Later, in 1993, Bretagnon [9] resolved two lifetimes (~1 = 19Ops, r2 = 27 ps) in a series of InP samples (undoped or doped with S, Fe or Zn), and found the lifetime was independent of the nature and the concentration of the dopants. Combined with the temperature experiment the positron trap is suggested to be a neutrally charged indium vacancy in all the samples. The identification of the vacancies in InP is still under discussion especially in experiment. The common result of these studies is that the lifetime is the same in all the samples with different conduction types. In this paper we first adopted a new analytical method (CONTIN program) to get some information about the distribution of the positron annihilation rate, and then studied systematically the native vacancy-type defects in several InP samples. A clear difference was observed between n-type, SI-type and p-type samples. 2. EXPERIMENTAL A series of InP crystals were chosen as samples with different conduction types (n-type undoped or doped with S, SI-type doped with Fe and p-type doped with Zn) and different dopant concentrations. Positron lifetime was measured using a fast-fast lifetime spectrometer with a time resolution of 240 ps in full width at half maximum (FWHM). Two identical pieces of each sample (10 x 10 x 0.5 mm3) were sandwiched with a 25,uCi 22Na prepared by evaporating 22NaCl solution onto a 5pm aluminium foil. The source correction was about 15% of 0.165 ns and 0.3% of 1 ns. Each spectrum contains 2 x lo6 total counts for PATFIT analysis and lo7 counts for CONTIN analysis.
Vol. 99, No. 10
3. RESULTS AND DISCUSSION 3.1. Extracting of positron-annihilation-rate distribution (ARD) In conventional data analysis, the positron lifetime spectrum was expressed as a sum of several negative exponentials convoluted with instrument resolution function R(t) plus background B as Y(t) = R(t)(Nt)
fJCliX+PA’i + B).
(1)
i=l
The analysis was performed by POSITRONFIT and RESOLUTION program in PATFIT package 1101. Recently we have employed a Laplace inversion technique to analyze positron lifetime spectrum to continuous lifetime distribution by using the CONTIN program [l 1, 121 when the lifetime spectrum is replaced by an integral Y(t) = R(t)(Nt)
r(Xa(X)e-“dX 5
+ B).
(2)
The solution X&(X) at every X value could be obtained by this program. This method avoids direct determination of the instrument resolution function by measuring a reference spectrum with a well-known single lifetime at the same experimental condition. In addition, it is model-independent, we do not need to know how many lifetime components exist prior to analysis. For CONTIN analysis, we measured the lifetime spectrum with a total count of 107. We used here the defect free single crystal silicon as a reference sample. Its bulk lifetime is known to be 220 ps. The positron annihilation rate distribution (ARD) is presented by its value Aa at all grid points X. Here the total grid point Ng is 60 points. As the defect lifetime is very close to the bulk lifetime in semiconductors, only one peak was obtained for each sample. The average lifetime (X-l) and the standard deviation of the distribution could be calculated through the values of the moments for the peak described as a2 = Mi/M_i
- (&/iK,)2,
(x-i) = M-z/M-r,
(3) (4)
where Mi is the ith moment of the solution given by
J
Las
Mi =
X’+‘a(X)dX. mm
x
The full width at the half maximum FWHM = 0 x [2sqrt(2 In 2)] could also be obtained. Four samples were measured. The results were presented in Table 1. The extracted solution of Aa( i.e., the annihilation
Vol. 99, No. 10
VACANCY-TYPE
Positron
DEFECTS IN AS-GROWN InP
747
decompose the spectrum. But the average lifetime is a good parameter for monitoring changes in lifetime spectra [ 131and has a high statistical accuracy. Therefore, we calculated the average lifetime rav as
ARD in InP
7(IlJ= 711, + 7212,
4.00
3.50
4.50
5.00
A (ns-‘) Fig. 1. The positron-annihilation-rate distributions (ARD) obtained bv CONTIN urogram for InP &ystais with different conduction iypey rate distributions (ARDs) of three samples with different conduction type are shown in Fig. 1. As shown in Fig. 1 and Table 1: (1) The annihilatation rate distribution is nearly the same for n-type and SI-type samples, unaffected by the nature of the dopant or the carrier concentration. The average lifetime (X-l) has nearly the same value, which was about 237ps, and the widths of the distribution FWHM are all close to 0.3050ns-‘. (2) For p-type InP, the rate distribution showed a large difference. First, the peak position shifted to a larger value, the average lifetime dropped to 233~s. Second, its distribution is much narrower with FWHM only 0.2338ns-‘, (3) All the three distributions have nearly the same upper limit of 4.77 ns-’ . In order to study the effect of carrier concentration on the lifetime, we also measured a series of samples with different dopant concentrations and were analyzed by PATFIT program. Two components could be resolved. As the defect lifetime is very close to the bulk lifetime in semiconductors, it is often difficult to
(6)
which is insensitive to the uncertainties in the decomposition procedure and in this work we present only the result of the average lifetime. The result is shown in Table 2. The average lifetime given by equation (6) was clearly larger than the bulk lifetime (about 229~s) which provided evidence of existence of vacancy-type defects. For n-type and SI-type InP samples, the average lifetime is about 236ps, and this value is independent of the carrier concentration or the nature of the dopant. In n-type semiconductors, the Fermi level is located in the upper half of the band gap and moves upward with increasing dopant concentration. It is pinned in the middle of the gap in S&type materials. Our result indicated that the trapping sites in these two samples are the same and are independent of the fermi level position in the upper half of the gap. But in the case of p-type InP, the situation is different. The average lifetime r,, = 232.7~s also is smaller than the value for n and S&type samples, and this value is still unaffected by the carrier concentration. The result of average lifetime obtained by the PATFIT program was the same as obtained by CONTIN. 3.2. Identification of phosphorus vacancies In InP crystals, various intrinsic defects can be produced during crystal growth. These are In and P vacancies (VI,.,, VP), interstitials (Ini, Pi), and antisites (PI,, Inp). Positrons are sensitive to vacancy type defects. Therefore, positrons in InP may annihilate in these states: (1) The delocalized free positron state, i.e., the Bloch state; (2) The trap state of P vacancy; (3) The trap state of In vacancy. The single ARD peaks with different distributions shown in Fig. 1 represent these positron states. In P-type InP, the ARD had a region from 3.96ns-’ to 4.77ns-’ with FWHM
Table 1. CONTZN result of the lifetime spectra, including average annihilation rates (X), average lifetime rav, region of ARD, distribution width FWHM
Dopant
Type
Carrier cont. (cmp3) or resistivity (a-cm)
(A> (ns-‘)
%J (ns)
Region of ARD (ns-‘)
FWHM (ns-‘)
S Fe Fe Zn
n
1.4 x 10’9 2x 10’ 7x lo6 3.9 x 1oJ8
4.2126 4.2363 4.2365 4.2963
0.2378 0.2365 0.2365 0.2329
3.77-4.77 3.77-4.77 3.77-4.77 3.96-4.77
0.3019 0.3090 0.3065
SI SI P
0.2338
VACANCY-TYPE
748
Vol. 99, No. 10
DEFECTS IN AS-GROWN InP
Table 2. Characteristics of various InP samples and results of lifetime (by PATFITprogram)
Dopant
Type
1 2 3 4 5 6 7
None S S S S S S
n n n n n n n
8 9
Zn Zn
P P
10 11
Fe Fe
SI SI
Sample No.
0.2338 ns-‘. This peak was relatively narrower. In ntype and X-type samples, the ARD had the same upper limit at 4.77ns-’ but its lower limit shifted to 3.77 ns-’ with a broader distribution. The FWHM of ARD increased to about 0.3050ns-’ by a factor of about 32%. Bretagnon’s results [9] observed the Indium vacancies that trapped positrons in p-type Inp. In n- and SI-type samples, the shift of the ARD lower limit implies directly an increase in the fraction of positron annihilation in the trapping state with a smaller annihilation rate as the upper limit remained unchanged. This state was the trapping in P vacancy. Therefore both In and P vacancies trapped positrons in n-type and SI-type InP crystals. However, the position lifetime result gives no information on the chemical surroundings around the vacancy and therefore cannot determine whether the vacancy is isolated or complexed with impurity atoms. The theoretical results have also provided the possibility of positron trapping by P vacancy in nand SI-InP. There have been several theoretical calculations on the unrelaxed ideal defect energy level of InP sample [14-161. According to their result, the In vacancy in InP can have many different charge states and their energy levels are all located in the lower half of the band gap. The P vacancy, on the other hand, is always positively charged irrespective of the position of the Fermi level position. However, in semiconductors defects often undergo strong lattice relaxation when they change their charge state. The relaxation causes the change of the defect energy level, which is called negative-U behavior [17]. Recently, Alatalo et al. [18] reported the discovery of the negative-U effect in the compound semiconductor InP. They calculated the ionization levels of the relaxed indium and phosphorus vacancy and found
Carrier cont. (cme3) or resistivity (a-cm)
Average lifetime rm (ps)
; 3.2 (3-4) 3.89 1.4 (4-5)
;$ 10” 1018 lo’* lOI lOI9
235.5 235.6 235.0 235.1 236.2 235.8 235.9
2.68 x 1018 3.9 x lOus
232.8 232.6
; x x x x x
236.0 236.6
2x 10’ 7 x lo6
that the inclusion of the relaxation does not alter the picture for the In vacancy, but the ionization level of P vacancy is largely changed, and it has an ionization level from V; to I$ in the lower half of the band. This tells us that in n-type and SI-type InP both In and P vacancies may trap positrons, but in p-type InP, the P vacancy may be positively charged and cannot bind positrons. One of the samples (undoped n-type InP) was studied as a function of temperature between 95 and 350K. The spectra were analyzed by the PATFIT program. It is found that the average lifetime was constant over the temperature range, as shown in Fig. 2. Bretagnon et al. [9] has also found that the positron lifetime for all types of InP remained
n-type
210’
. 80
InP(undoped)
’ 170
Temperature
I 260
350
(K)
Fig. 2. The temperature dependence of the average lifetime TV,,in undoped n-type InP.
VACANCY-TYPE
Vol. 99, No. 10
DEFECTS IN AS-GROWN InP
unchanged over the measuring temperature range from 30 to 300K. The calculation by Puska showed that the trapping coefficient for a neutral vacancy is practically independent of temperature [ 191. These two temperature experiments strongly indicated that the vacancies in all the samples are neutrally charged. 4. CONCLUSION In summary, we have systematically studied the vacancy-type defects in InP semiconductors. Both indium and phosphorus vacancies attracted positrons in n-type and S&type InP as the Fermi level is located in the upper half of the gap. In p-type InP, however, only the indium vacancy trapped positrons. The temperature experiment showed that these vacancies that trapped positrons were neutrally charged. Our result from positron annihilation was in coincidence with the theoretical result on the defect energy level in InP material. Acknowledgements-This
work was supported by the National Natural Science Foundation of China. REFERENCES 1. 2. 3.
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