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Structural relationship between epitaxially-grown Zn3P2 and InP substrates
Volume 3, number 1,2
MA~R~ALS
LETTERS
November 1984
STRUCTURAL RELATIONSHIP BETWEEN EPITAXIALLY-GROWN Zn3PZ AND InP SUBSTRATES S. NAKAHARA ATLeTBell Laboratories, Murray Hill, NJ 07974, USA Received 23 August 1984
Nakahara et al. have recently reported that annealing of zinc-metallized (001) InP leads to the formation of tetragonal Zn3P2 with an epitaxiai relationship: (001)zn3P2 ll(OO1)~npand [ 110 ] zn3P2 IlflOO]~np. This epitaxial relationship is further analyzed here to obtain a probable mechanism for this phase formation. A close examination of the structural relationship has revealed that the replacement of indium atoms with zinc atoms and the positioning of zinc atoms into their interstitial sites precisely construct the structure of Zn3P2 phase in InP lattice. In addition, the phase forms with only a small (2.28-2.30s) lattice misfit. The lattice locations of zinc atoms strongly indicate that there might be both vacancy and interstitial diffusion mechanisms operative for the phase formation.
1. ~t~uction Zinc has widely been used as a pdopant in the fabrication of InP devices. The lattice locations of zinc atoms in the InP host lattice have been a subject of interest in relation to its electrical activity. It is generally observed [ 1,2] that not all zinc atoms are likely to be electrically active in InP. In fact, Mahajan et al. [3] have reported the direct observation of zinc-related precipitates in highly-zinc-doped InP crystals, supporting the presence of electrically-inactive zinc atoms. A similar conclusion was also reached by Williams et al. [4]. Using tr~smission electron microscopy (TEM), Nakahara et al. [S] found that short-time (5 min at 420%) annealing of a zinc film grown on the {001) InP substrate produced tetragonal Zn,P, with an epitaxial relationship: (001)~~s~~ ll(OOl), and [llol Zn3P2 11I1001,.This result has demonstrated that zinc has a strong tendency to form its phosphide phase during interdiffusion processes. Furthermore, this fmding was consistent with previous TEM observations [3 ] of zinc-related precipitates. A question as to why and how this phase forms inside the InP lattice, however, has not been answered. Underst~ding of this phenomenon appears to provide information on the role of zinc as a p-dopant. In this Communication, we will give a detailed ac40
count of the structural relationship between InP and epit~iall~grown Zn3P2 phase. From the epitaxial relation~p, it can be deduced that the Zn3P2 phase is formed primarily via a vacancy diffusion mechanism through the indium sublattice and partly via an interstitial diffusion mechanism. This whole process thus involves the in-diffusion of zinc atoms from the metallization and the out-diffusion of indium atoms from the InP substrate.
2, Structural relationship between InP aud Zng Pz The structural relation~p observed [.5] between InP and Zn, P2 presents an interesting picture of the underlying interdiffusion phenomenon. Since Zn3P2 has a tetragonal symmetry, the (001)Zn3p2 plane having a cubic symmetry tends to match the (001) face of the cubic InP. To achieve a good lattice match with a small misfit on this plane, Zn3P2 rotates by 45’. This results in the observed epitaxial relationship: (OOl)z,,p, ll(OOl), and [l 10]zn3p2 II[lOO]w. From the epitaxial geometry and the lattice parameters [6] of these crystals shown in fig. 1 and table 1, we note the following relation~p: 2f/2,,b’,
h%cc’.
It can be easily shown from the lattice mismatch that 0 167-577x/84/$ 03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
November 1984
MATERIALS LETTERS
Volume 3, number 1,2
a Fig. 1. Epitaxial relationship between the unit cells of cubic InP {a, b and c) and tetragonal Zn3P2 (Q’,b’ and ~‘1.
the Zn3 P, cell is a little smaller than the expanded InP cell by 2.28-2.30%. The formation of the epitaxial Zn,P, within the InP lattice is then expected to result in a slight contraction in the InP matrix. In addition to the dimensional matching of the two unit cells, the atomic positions within the cells were also found to closely correlate to each other. In fig. 2, we illustrate the stacking sequences of the indium/ phosphorus (In/P) and z~c/pho~horus (Znlp) planes along the [OOl] directions for InP and Zn3P2 crystals. Fig. 2a shows the alternating In/P planes along the [OOI] in two unit cells of the InP lattice. Similarly, in fig. 2b, the Zn/P alternating planes are shown. We have intentionally drawn these planes so that all the phosphorus planes match between the two unit cells. It is immediately obvious that the positions and configuration of the phosphorus atoms in InP are essentially coincident to those of ZnjPZ. except a small atomic shift in the phosphorus sites of ZngP2. Comp~~g of the indium planes in InP with the zinc planes in ZngPzf we found an interesting structural relationship. Let us consider the second irtdium and zinc planes from the bottom. In the indium plane, we see the substitutional sites of indium atoms and several unoccupied interstitial sites. If all the indium atoms Table I Lattice parameters (in A) of InP and Zn3P2 crystals InP
Zn3P2
a=b=c=5.869
a’ = b’ = 8.113
c* = 11.47
lP
a In
(al InP
(b) ZnsPa
Fig. 2. A schematic illustration showing how atom positions are related between the structures of (a) InP and (b) Zn3Pz.
are replaced and two interstitial sites are occupied by zinc atoms, the exactly same zinc plane shown in fig. 2b can be reproduced. A similar const~ction scheme can be offered for all the rem~g planes. It can easily be shown that the whole Zn,P, unit cell is constructed by the exchange (with zinc atoms) and removal of indium atoms plus the positioning of zinc atoms at the interstitial sites. The precise locations of zinc atoms are slightly different from those obtained by our simple operation of an atom exchange but is shifted according to their atomic force balance. This positional relationship appears to provide useful information on the formation mechanism of this particular phase. A probable mech~i~ for the Zn3Pz phase formation can be deduced from the structural diagram shown in fig. 2. It is clear from the exact matching of the phosphorus planes that the phosphorus atoms are in effect immobile and thus do not contribute to the dynamic part of the phase formation. The indium atoms, on the other hand, are mobile and can diffuse out toward the zinc metallization via a vacancy mechanism. At the same time the zinc atoms move into the vacant indium substitutional sites. Additional zinc atoms will migrate into the InP lattice via an interstitial diffusion mechanism and eventually become inter41
Volume 3, number I,2 Table 2 Crystal structure of Zn&
MATERIALS LETTERS
(P4s/nmc)
[7] al
4Xc in
0, 0,s; 0, 0,2; $, $, : + 2; ;, :, ; -z with z = 0.25 xt 0.01
4Xd in
0, :, 2; $, 0, I; 0, $, : +z; ;, 0, $ - z withz = 0.239 f 0.01
SXf in
x,x,O;x,jZ,O;~+x,:+x,f;~-x,:-x,~; x,jt,o;x,x,o;:+x,~-x,~;;-.,:+x,: withx = 0.261 r 0.01
3 times 8Mg in 0, x, z;x, 0,2; fr, : +x, $ - z; i +x, $, i + z; o,ji,z;x,o,z”;f,$ -x,;-z;;-x,;,;+z with XI = 0.217 * 0.008, q = 0.103 f 0.012; xJJ = 0.283 f 0.008, zII = 0.386 r 0.012; XIII = 0.250 i 0.008, zIrI = 0.647 f 0.012 a) X = phosphorus atom and M = zinc atom. stitial atoms in the zinc planes as discussed above. In the vacancy diffusion process, zinc atoms are actually diffusing through the indium sublattice. The fact that the indium atoms move much faster than the phosphorus atoms is consistent with diffusion data obtained by Goldstein [8] using a radio-trace technique. We found from his data that diffusivity (x 1O-24 cm2 /s) of indium at 42O’C is several orders of magnitude larger than that (=10-31 cm2/s) of phosphorus in InP. It is thus reasonable to assume that the phosphorus atoms were indeed immobile in the phase formation process. From the above argument, it is possible to speculate that migration of zinc atoms in InP proceeds via both vacancy and interstitial diffusion mechanics. From Iig. 2b, the ratio of the number of substitutional zinc atoms to that of the interstitial ones is estimated to be I, which implies the equally s~i~c~t contributions of both the vacancy and interstitial mechanisms in the phase formation [3,9]. Zinc-related precipitates observed in InP can be correlated to the structure of the Zn3P2 phase. The fact [3] that no extra spots have been seen previously from such precipitates in the electron diffraction pattern suggests that these precipitates might have indeed contained zinc atoms but not all the sites are filled completely to produce additional spots characteristic of Zn3P2. It can be argued that those interstitial zinc atoms are mobile and may even be unoccupied in many cases. The structure of Zn, P2 in the absence of the interstitial zinc atoms is close to that of the InP lattice except some small relaxation of the
42
November 1984
atomic position. Such a relaxation has indeed been observed [3,9] only as a strain contrast around the precipitates but no extra spots were seen. A strong tendency for Zn3P2 formation inside InP can be understood from the magnitude of the heat of formation. The heat of formation for Zn3P, is -4.90 eV/mofecule, whereas that for InP is -0.92 eV/molecule [lo]. Zinc-related precipitation within InP is considered to be connected to this difference in the heat of formation, Finally, it should be mentioned that Zn, As2, Cd3P2, and Cd, As2 belong to the same crystal structure family (P42/nmc) as Zn3P2 (6,7]. It can be envisaged that zinc in GaAs, and cadmium in InP and GaAs may undergo a similar phase formation scheme discussed here. A recent transmissionelectron-microscope study by Dutt and Brasen [ 1l] indeed reported the presence of Cd3P2 in ~d~urn-~ff~ed InP. Although the epitaxial relationship was not discussed, it is expected to be similar to that of the ZngPz/InP case.
Acknowledgement The author would like to thank Dr. L.C. Feldman for his encouragement and constructive comments on this manuscript.
References 111V.V. Galavanov, S.G. Metrevel, N.V. Siukaev and S.P. Staroseltseva, Soviet Phys. Semicond. 3 (1969) 94.
[21 A. Hooper and B. Tuck, Solid State Electron. 19 (1976) 513. f31 S. Mahajan, W.A. Banner, AK. Chin and D.C. Miller, Appl. Phys. Letters 35 (1979) 165. [41 R.S. Williams, P.A. Barnes and L.C. Feldman, Appl. Phys. Letters 36 (1980) 760. 151 S. Nakahara, P.K. Gallagher, E.C. Felder and R.B. Lawry, Solid State Electron. 27 (1984) 557. 161 Powder Diffraction File, JCPDS, International Center for Diffraction Data, Swarthmore, PA (1981). 171 M.V. Stackelberg and R. Paulus, Z. Physik. Chem. 28 (1935) 427. PI B. Goldstein, Phys. Rev. 121 (1961) 1305. Pl S. Nakahara, P.R. Skeath, T. Boone, J.M. Gibson and R. McConville, unpublished work (1984). [lot N.C. Wyeth and A. Catalano, 3. Appl Phys. 51 (1980) 2286. [Ill B.V. Dutt and D. Brasen, J. Electrochem. Sot. 130 (1983) 207.
MA~R~ALS
LETTERS
November 1984
STRUCTURAL RELATIONSHIP BETWEEN EPITAXIALLY-GROWN Zn3PZ AND InP SUBSTRATES S. NAKAHARA ATLeTBell Laboratories, Murray Hill, NJ 07974, USA Received 23 August 1984
Nakahara et al. have recently reported that annealing of zinc-metallized (001) InP leads to the formation of tetragonal Zn3P2 with an epitaxiai relationship: (001)zn3P2 ll(OO1)~npand [ 110 ] zn3P2 IlflOO]~np. This epitaxial relationship is further analyzed here to obtain a probable mechanism for this phase formation. A close examination of the structural relationship has revealed that the replacement of indium atoms with zinc atoms and the positioning of zinc atoms into their interstitial sites precisely construct the structure of Zn3P2 phase in InP lattice. In addition, the phase forms with only a small (2.28-2.30s) lattice misfit. The lattice locations of zinc atoms strongly indicate that there might be both vacancy and interstitial diffusion mechanisms operative for the phase formation.
1. ~t~uction Zinc has widely been used as a pdopant in the fabrication of InP devices. The lattice locations of zinc atoms in the InP host lattice have been a subject of interest in relation to its electrical activity. It is generally observed [ 1,2] that not all zinc atoms are likely to be electrically active in InP. In fact, Mahajan et al. [3] have reported the direct observation of zinc-related precipitates in highly-zinc-doped InP crystals, supporting the presence of electrically-inactive zinc atoms. A similar conclusion was also reached by Williams et al. [4]. Using tr~smission electron microscopy (TEM), Nakahara et al. [S] found that short-time (5 min at 420%) annealing of a zinc film grown on the {001) InP substrate produced tetragonal Zn,P, with an epitaxial relationship: (001)~~s~~ ll(OOl), and [llol Zn3P2 11I1001,.This result has demonstrated that zinc has a strong tendency to form its phosphide phase during interdiffusion processes. Furthermore, this fmding was consistent with previous TEM observations [3 ] of zinc-related precipitates. A question as to why and how this phase forms inside the InP lattice, however, has not been answered. Underst~ding of this phenomenon appears to provide information on the role of zinc as a p-dopant. In this Communication, we will give a detailed ac40
count of the structural relationship between InP and epit~iall~grown Zn3P2 phase. From the epitaxial relation~p, it can be deduced that the Zn3P2 phase is formed primarily via a vacancy diffusion mechanism through the indium sublattice and partly via an interstitial diffusion mechanism. This whole process thus involves the in-diffusion of zinc atoms from the metallization and the out-diffusion of indium atoms from the InP substrate.
2, Structural relationship between InP aud Zng Pz The structural relation~p observed [.5] between InP and Zn, P2 presents an interesting picture of the underlying interdiffusion phenomenon. Since Zn3P2 has a tetragonal symmetry, the (001)Zn3p2 plane having a cubic symmetry tends to match the (001) face of the cubic InP. To achieve a good lattice match with a small misfit on this plane, Zn3P2 rotates by 45’. This results in the observed epitaxial relationship: (OOl)z,,p, ll(OOl), and [l 10]zn3p2 II[lOO]w. From the epitaxial geometry and the lattice parameters [6] of these crystals shown in fig. 1 and table 1, we note the following relation~p: 2f/2,,b’,
h%cc’.
It can be easily shown from the lattice mismatch that 0 167-577x/84/$ 03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
November 1984
MATERIALS LETTERS
Volume 3, number 1,2
a Fig. 1. Epitaxial relationship between the unit cells of cubic InP {a, b and c) and tetragonal Zn3P2 (Q’,b’ and ~‘1.
the Zn3 P, cell is a little smaller than the expanded InP cell by 2.28-2.30%. The formation of the epitaxial Zn,P, within the InP lattice is then expected to result in a slight contraction in the InP matrix. In addition to the dimensional matching of the two unit cells, the atomic positions within the cells were also found to closely correlate to each other. In fig. 2, we illustrate the stacking sequences of the indium/ phosphorus (In/P) and z~c/pho~horus (Znlp) planes along the [OOl] directions for InP and Zn3P2 crystals. Fig. 2a shows the alternating In/P planes along the [OOI] in two unit cells of the InP lattice. Similarly, in fig. 2b, the Zn/P alternating planes are shown. We have intentionally drawn these planes so that all the phosphorus planes match between the two unit cells. It is immediately obvious that the positions and configuration of the phosphorus atoms in InP are essentially coincident to those of ZnjPZ. except a small atomic shift in the phosphorus sites of ZngP2. Comp~~g of the indium planes in InP with the zinc planes in ZngPzf we found an interesting structural relationship. Let us consider the second irtdium and zinc planes from the bottom. In the indium plane, we see the substitutional sites of indium atoms and several unoccupied interstitial sites. If all the indium atoms Table I Lattice parameters (in A) of InP and Zn3P2 crystals InP
Zn3P2
a=b=c=5.869
a’ = b’ = 8.113
c* = 11.47
lP
a In
(al InP
(b) ZnsPa
Fig. 2. A schematic illustration showing how atom positions are related between the structures of (a) InP and (b) Zn3Pz.
are replaced and two interstitial sites are occupied by zinc atoms, the exactly same zinc plane shown in fig. 2b can be reproduced. A similar const~ction scheme can be offered for all the rem~g planes. It can easily be shown that the whole Zn,P, unit cell is constructed by the exchange (with zinc atoms) and removal of indium atoms plus the positioning of zinc atoms at the interstitial sites. The precise locations of zinc atoms are slightly different from those obtained by our simple operation of an atom exchange but is shifted according to their atomic force balance. This positional relationship appears to provide useful information on the formation mechanism of this particular phase. A probable mech~i~ for the Zn3Pz phase formation can be deduced from the structural diagram shown in fig. 2. It is clear from the exact matching of the phosphorus planes that the phosphorus atoms are in effect immobile and thus do not contribute to the dynamic part of the phase formation. The indium atoms, on the other hand, are mobile and can diffuse out toward the zinc metallization via a vacancy mechanism. At the same time the zinc atoms move into the vacant indium substitutional sites. Additional zinc atoms will migrate into the InP lattice via an interstitial diffusion mechanism and eventually become inter41
Volume 3, number I,2 Table 2 Crystal structure of Zn&
MATERIALS LETTERS
(P4s/nmc)
[7] al
4Xc in
0, 0,s; 0, 0,2; $, $, : + 2; ;, :, ; -z with z = 0.25 xt 0.01
4Xd in
0, :, 2; $, 0, I; 0, $, : +z; ;, 0, $ - z withz = 0.239 f 0.01
SXf in
x,x,O;x,jZ,O;~+x,:+x,f;~-x,:-x,~; x,jt,o;x,x,o;:+x,~-x,~;;-.,:+x,: withx = 0.261 r 0.01
3 times 8Mg in 0, x, z;x, 0,2; fr, : +x, $ - z; i +x, $, i + z; o,ji,z;x,o,z”;f,$ -x,;-z;;-x,;,;+z with XI = 0.217 * 0.008, q = 0.103 f 0.012; xJJ = 0.283 f 0.008, zII = 0.386 r 0.012; XIII = 0.250 i 0.008, zIrI = 0.647 f 0.012 a) X = phosphorus atom and M = zinc atom. stitial atoms in the zinc planes as discussed above. In the vacancy diffusion process, zinc atoms are actually diffusing through the indium sublattice. The fact that the indium atoms move much faster than the phosphorus atoms is consistent with diffusion data obtained by Goldstein [8] using a radio-trace technique. We found from his data that diffusivity (x 1O-24 cm2 /s) of indium at 42O’C is several orders of magnitude larger than that (=10-31 cm2/s) of phosphorus in InP. It is thus reasonable to assume that the phosphorus atoms were indeed immobile in the phase formation process. From the above argument, it is possible to speculate that migration of zinc atoms in InP proceeds via both vacancy and interstitial diffusion mechanics. From Iig. 2b, the ratio of the number of substitutional zinc atoms to that of the interstitial ones is estimated to be I, which implies the equally s~i~c~t contributions of both the vacancy and interstitial mechanisms in the phase formation [3,9]. Zinc-related precipitates observed in InP can be correlated to the structure of the Zn3P2 phase. The fact [3] that no extra spots have been seen previously from such precipitates in the electron diffraction pattern suggests that these precipitates might have indeed contained zinc atoms but not all the sites are filled completely to produce additional spots characteristic of Zn3P2. It can be argued that those interstitial zinc atoms are mobile and may even be unoccupied in many cases. The structure of Zn, P2 in the absence of the interstitial zinc atoms is close to that of the InP lattice except some small relaxation of the
42
November 1984
atomic position. Such a relaxation has indeed been observed [3,9] only as a strain contrast around the precipitates but no extra spots were seen. A strong tendency for Zn3P2 formation inside InP can be understood from the magnitude of the heat of formation. The heat of formation for Zn3P, is -4.90 eV/mofecule, whereas that for InP is -0.92 eV/molecule [lo]. Zinc-related precipitation within InP is considered to be connected to this difference in the heat of formation, Finally, it should be mentioned that Zn, As2, Cd3P2, and Cd, As2 belong to the same crystal structure family (P42/nmc) as Zn3P2 (6,7]. It can be envisaged that zinc in GaAs, and cadmium in InP and GaAs may undergo a similar phase formation scheme discussed here. A recent transmissionelectron-microscope study by Dutt and Brasen [ 1l] indeed reported the presence of Cd3P2 in ~d~urn-~ff~ed InP. Although the epitaxial relationship was not discussed, it is expected to be similar to that of the ZngPz/InP case.
Acknowledgement The author would like to thank Dr. L.C. Feldman for his encouragement and constructive comments on this manuscript.
References 111V.V. Galavanov, S.G. Metrevel, N.V. Siukaev and S.P. Staroseltseva, Soviet Phys. Semicond. 3 (1969) 94.
[21 A. Hooper and B. Tuck, Solid State Electron. 19 (1976) 513. f31 S. Mahajan, W.A. Banner, AK. Chin and D.C. Miller, Appl. Phys. Letters 35 (1979) 165. [41 R.S. Williams, P.A. Barnes and L.C. Feldman, Appl. Phys. Letters 36 (1980) 760. 151 S. Nakahara, P.K. Gallagher, E.C. Felder and R.B. Lawry, Solid State Electron. 27 (1984) 557. 161 Powder Diffraction File, JCPDS, International Center for Diffraction Data, Swarthmore, PA (1981). 171 M.V. Stackelberg and R. Paulus, Z. Physik. Chem. 28 (1935) 427. PI B. Goldstein, Phys. Rev. 121 (1961) 1305. Pl S. Nakahara, P.R. Skeath, T. Boone, J.M. Gibson and R. McConville, unpublished work (1984). [lot N.C. Wyeth and A. Catalano, 3. Appl Phys. 51 (1980) 2286. [Ill B.V. Dutt and D. Brasen, J. Electrochem. Sot. 130 (1983) 207.