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Epitaxial growth of solid solutions of ZnSiP2 in Si
Journal of Crystal Growth 15 (1972) 129—132 © North-Holland Publishing Co.
EPITAXIAL GROWTH OF SOLID SOLUTIONS OF ZnSIP2 IN Si V. P. POPOV* and B. R. PAMPLIN School of Physics, Bath University, Claverton Down, Bath, BA2 7A Y, England Received 29 December 1971; revised manuscript received 1 February 1972 A new method of epitaxial growth of some solid solutions between group IV elements and Il—tV—V2 diamondlike semiconductors is described. The method involves traversing an appropriate solvent across a slice of germanium or silicon under a temperature gradient in the presence of the vapour of a 11—I V—V2 compound in a sealed ampoule. The technique has been applied to the growth of solid solutions of Si in ZnSiP2 with a content of 5°~ ZnS1P2. The best solvents for the liquid zone have been found to be gold and silver.
8). The scheme of the perature gradient zone melting experiment is shown in fig. I. A sealed evacuated anipoule contains a silicon slice with a previously alloyed
1. Introduction The ternary chalcopyrite structure compounds [1— TV—V 2 aret) promising semiconductors for various apbut the problems of preparation of good plications single crystal material and of pn junctions is not yet solved59). One way of obtaining pn junctions is by 6’7) have been making heterojunctions. Several papers published on the heteroepitaxial growth of germanium and silicon on 1E—IV—V 2 substrates. In this paper we present a technique for producing heteroepitaxial layers of solid solutions of Si:ZnSiP2 on silicon by a travelling solvent method under vapour pressure of the volatile components of ZnSiP2. This pair of semiconductors was chosen because of the siiriilarity of the lattice parameters, (for Si, a = 5.431 A; for ZnSiP2, a = 5.400 A, c = 10.441 A). We have found appropriate solvents for the liquid zone and measured the kinetics of the travelling zone. We have also studied the resulting heteroepitaxial structures.
dot metalwill on cause it. This a temperature dientofwhich the placed drop toinmove across the grasurface of the silicon. One end of the ampoule contains ZnSiP source at mean a temperature is either higher2 or lowermaterial than the value at which the slice, and controlled at that value.
ZnStP2 _____
~
Fig. 1.
2. The method
/
/
Silicon
Distance The experimental scheme.
When heated the composition of the drop is defined
Substantial decomposition 3) of have ZnSiP2 at shownbegins that the 350 cCS). Valov and Ushakova non-volatile dissociation product of the ternary phosphides is silicon. The idea of our experiment was to supply the silicon to a molten zone from the silicon slice and the volatile Zn and P from the vapour phase. The molten zone is made to move over the silicon slice by the travelling solvent method also known as tem-
by the solubility silicon with in thethe drop andasbywell theasrate of reaction of theofvapours liquid by diffusion in the liquid. The temperature gradient causes silicon dissolution at the hot side and deposition of silicon at the cooler side, assuming a positive slope of the liquidus with temperature. If the zone velocity is not too high a constant composition of the liquid phase will result in a solid solution of Si:ZnSiP 2 being deposited at the cooler side of the zone.
—
*
Zone
Also: Novocherkasski Polytechnical Institute, U.S.S.R.
129
130
V. P. POPOV AND B. R. PAMPLIN
could be grown on germanium and any silicon containing Il—tV—V2 compound could be grown on silicon
\Zn
\P
)
T2>
I
I Solid solution
Zone
v
3. Experimental results
-
‘1
( Y
Si
_________
T:
using this technique.
T,
d
T2
/
/~ Fig. 2.
7 /Zn
The diagram of the process.
Assuming rapid reaction of the vapours with the hot zone so that equilibrium is quickly established between vapour and liquid, we can estimate the upper limit of the zone velocity, V, for uniform deposition. Consider the in fig. 2, where I is to thegolength of the zone and model d the zone depth, assumed right through the silicon slice. Taking into account diffusive transport of atoms of the vapour species in the melt and neglecting convection in the melt it is easy to show that a constant liquid composition is established if V < 2D1’d~ (I) /
if or,the zone penetrates right through the slice (as shown) V
(2)
<
I
if the zone moves along the surface of the slice. D is the diffusion constant for vapour atoms in the molten zone. D does not vary by more than an order of magnitude from element element a typical is 2 sec forto the melt.andTaking 1 =value 1 mm, l0 ~ cm ci = 0.4 mm, we obtain limiting velocities of 14 and 7 x l0 ~cm/sec for the two cases above. The zone composition depends on the average zone temperature and the temperature gradient (G) and it depends on the vapour pressure of the volatile cornponents, hence the source temperature. The geometry of the epitaxial layer is determined by the initial zone volume, by the temperature conditions and by the time taken. The choice of these parameters thus permits some control of the thickness, width and length of the epitaxmal growth. Simultaneous use of several zones would allow the growth of complicated heterostructures. Any germanium containing Il—tV--V 2 compound .
Unfortunately the phase diagrams of the ternary compounds with silicon and gerrnaniuni are scarcely known, so a suitable solvent must be found by trial and error. The solvent must dissolve silicon and ZnSiP2. For this reason we have chosen and tested the following metals Sn, In, Al, Zn, Ag and Au as solvents for the system Si—ZnSiP2. All metals used were 99.999°~ pure and pieces 2—20mg in weight were alloyed onto the silicon in vacuum (Au and Ag) or hydrogen (Sn, In, Al, Zn) after etching in HCI and washing in distilled water. Silicon slices of (Ill) orientation, both n and p type, 3 were used after meof dimensions 0.4 x 9 x 20 mm chanical and chemical treatment in CP4. It proved difficult to fuse Zn, Au and Ag into silicon. These metals were fused by very short heating at 1200 °C Excessive spreading of the metal across the silicon slice would be prevented by careful choice of the temperature of fusion and the time for which it was applied. The ZnSiP 2 source material was previously 2) using prepared 99.999°, by thestarting tin solution growth pure materials. The process slice and ZnSiP 2 powder were placed in a 10 mm diameter silica ampoule and sealed under vacuum. The ampoule was placed in a temperature gradient furnace at a chosen temperature in the range 1000—1200 °Cfor 10—80 hr. Heating and cooling rates of 20 °C/minwere used. T ABLE I
Table of results
.
—
-
Sample Solvent No.
T~ (C)
T, ( C)
G t (C cm ‘) (hr)
V>~10° (cm sec
—
-
4 5 6
Sn In Au
1150 1150 1160
1120 1120 1130
25 25 25
67 21 23
1.1 0 10.0
7
Au
1120
1090
25
45
5.7
8 II 13
Au Al
Zn
1100 1120
070 1090
25 25
25
64 23 24
3.0 0 0
18
Au
1180
1150
25
23
15.0
20 21* 23
Au
1050
1030
8
45
0
Au Ag
1120 1150
1120
25 35
65 63
4.2 6.7
1130
—-
1100
—
-—
----
-
--
T~and T. are the temperatures of zone and source (temperature of the cold end of the ampoule). ~ Without source of vapour.
EPITAXTAL GROWTH
OF SOLID SOLUTIONS
/ /~
>
1120
1160
T ~c
1200
Fig. 3. Graph of zone velocity against Si—P—Zn; (~)Au—Si; Ag—Si—P—Zn.
temperature.
(o)
Au—
(i)
The zone velocity was calculated from the zone trace distance and the time at temperature. When the ZnSiP2 was placed at the hot end of the furnace the silicon slice was covered with small po1yhedral crystals of ZnSiP2 and needles of ZnSiP2 grew at the cold end of the tube by a VLS mechanism. Further study of this V.L.S. growth is in progress and will be reported separately. Table I shows the results obtained for different sot~
~
IN
Si
131
were obtained. Au was chosen for a detailed further investigation. The zone velocity increased sharply with temperature (fig. 3). Below 1050 °Cthe Au zone did not move at all. The volume of liquid phase was about
A
1080
ZnSiP2
vents in the zone. For In, Al and Zn the zone did not move so these metals are of no use in the technique. The Sn zone moved but the zone size quickly decreased and the composition of the growing layer was not established. With Au and Ag zones good movement and traces
/ / 7 /
14
OF
,-
three or four times larger than the initial volume of the Au drop in the temperature range used. The surface of the grown layer was usually smooth. After polishing and etching the interface between the initial silicon and the recrystallized layer was sharp and distinct. A typical photomicrograph of a layer is shown in fig. 4. The grown layer was silvery in colour contrasting with the darker silicon, and free from cracks. Experiments were performed using a gold zone without any ZnSiP2 vapour for comparison. We observed unstable movement of the drop, which spread out over the silicon surface and broke up into smaller droplets 100—200 .tm in diameter. The epitaxial nature of the regrown layer was estab-
———,‘—~
~L
F’.
Fig. 4.
Microphotograph of silicon sample with an epitaxial layer. Magn.
6.5.
132
V. P. POPOV AND B. R. PAMPLIN
lished by Laue back-reflection photographs. The grown layer had a cubic lattice and the same orientation as the substrate. X-ray powder photographs of ground up material from the epitaxial layer of sample showed a lattice parameter of 5.418 A. The composition of the layer was further investigated by electron probe microanalysis using a 2 mm electron beam with an accuracy 0.5 ~/. The layers were constant in composition for a given sample and consisted of 5°~ ZnSiP2 in Si. The composition was constant across and slightly increased in the ZnSiP2 content along the layer.
to reaction of P and Zn with Si. This process has been investigated in detail by Buehler and Wernick5). The lattice constant of the solid solution (5.718 A) corresponds to 6°~ of the ZnSiP 2 in Si assuming Vegard’s law applies. This is in a good agreement with the result (5°~ ZnSiP2) obtained by electron probe microanalysis. It is likely that an increased concentration of ZnSiP2 can be achieved by successive running of several zones through the silicon or by reversal and passing back of the first zone. Acknowledgements
4. Discussion The gold zone velocity was higher in the absence of ZnSiP2 vapour indicating probably that the increased concentration of solute atoms increases the liquidus temperature of the zon& 0) The temperature dependence of zone speed was exponential. The curve of fig. 3 can be fitted by a formula V = V0 exp (—H/RI), where H is 200000 J/mol and V0 is a constant depending on the zone composition and geometry. The higher stability of the liquid zone in the Au—-Si—P—Zn system compared to the Au—Si system can be explained by the larger surface tension in the first case. The lack of movement of In, Al and Zn zones is explained by the freezing of the zone by the formation of compounds such as AlP. The uniform composition of the layers is evidence for the rapid establishnient of equilibrium between the drop and the vapour. The formation of ZnSiP2 crystals on the silicon bar was due
The authors wish to thank Dr. V. Scott for the microanalysis and Mr. B. Chapman for the X-ray work. Also, thanks are due to R. C. Draper for his help with the apparatus of the investigation. References I) L. 1. Berger and V. D. Prochukhan, Ternary Diamond-like Semiconductors (Consultants Bureau, New York-London, 1969). 2) A. J. Spring-Thorpe and B. R. Pamplin, J. Crystal Growth 3,4 (1968) 313. 3) Ya. A. Valov and T. N. Ushakova, lzv. Akad. Nauk SSSR 4 (1968) 1054. 4) S. A. Mughal, A. J. Payne and B. Ray, J. Mater. Sci. 4 (1969) 895. 5) E. Buehler and J. H. Wernick, J. Crystal Growth 8 (1971) 324. 6) A. J. SpringThorpe, R. J. Harvey and B. R. Pamplin, J. Crystal Growth 6 (1969) 104. 7) J. Beroti, J. Mater. Sci. 5 (1970) 1073.
8) W. G. Pfann, Zone Melting (Wiley, New York, 1968). 9) V. N. Lozovskii, V. P. Popov, V. A. tvkov and G. S. Konstantinova, in: Crystal Growth (Akad. Nauk U.S.S.R., 1971). 10) W. A. Tiller, J. Appi. Phys. 36 (1965) 261.
EPITAXIAL GROWTH OF SOLID SOLUTIONS OF ZnSIP2 IN Si V. P. POPOV* and B. R. PAMPLIN School of Physics, Bath University, Claverton Down, Bath, BA2 7A Y, England Received 29 December 1971; revised manuscript received 1 February 1972 A new method of epitaxial growth of some solid solutions between group IV elements and Il—tV—V2 diamondlike semiconductors is described. The method involves traversing an appropriate solvent across a slice of germanium or silicon under a temperature gradient in the presence of the vapour of a 11—I V—V2 compound in a sealed ampoule. The technique has been applied to the growth of solid solutions of Si in ZnSiP2 with a content of 5°~ ZnS1P2. The best solvents for the liquid zone have been found to be gold and silver.
8). The scheme of the perature gradient zone melting experiment is shown in fig. I. A sealed evacuated anipoule contains a silicon slice with a previously alloyed
1. Introduction The ternary chalcopyrite structure compounds [1— TV—V 2 aret) promising semiconductors for various apbut the problems of preparation of good plications single crystal material and of pn junctions is not yet solved59). One way of obtaining pn junctions is by 6’7) have been making heterojunctions. Several papers published on the heteroepitaxial growth of germanium and silicon on 1E—IV—V 2 substrates. In this paper we present a technique for producing heteroepitaxial layers of solid solutions of Si:ZnSiP2 on silicon by a travelling solvent method under vapour pressure of the volatile components of ZnSiP2. This pair of semiconductors was chosen because of the siiriilarity of the lattice parameters, (for Si, a = 5.431 A; for ZnSiP2, a = 5.400 A, c = 10.441 A). We have found appropriate solvents for the liquid zone and measured the kinetics of the travelling zone. We have also studied the resulting heteroepitaxial structures.
dot metalwill on cause it. This a temperature dientofwhich the placed drop toinmove across the grasurface of the silicon. One end of the ampoule contains ZnSiP source at mean a temperature is either higher2 or lowermaterial than the value at which the slice, and controlled at that value.
ZnStP2 _____
~
Fig. 1.
2. The method
/
/
Silicon
Distance The experimental scheme.
When heated the composition of the drop is defined
Substantial decomposition 3) of have ZnSiP2 at shownbegins that the 350 cCS). Valov and Ushakova non-volatile dissociation product of the ternary phosphides is silicon. The idea of our experiment was to supply the silicon to a molten zone from the silicon slice and the volatile Zn and P from the vapour phase. The molten zone is made to move over the silicon slice by the travelling solvent method also known as tem-
by the solubility silicon with in thethe drop andasbywell theasrate of reaction of theofvapours liquid by diffusion in the liquid. The temperature gradient causes silicon dissolution at the hot side and deposition of silicon at the cooler side, assuming a positive slope of the liquidus with temperature. If the zone velocity is not too high a constant composition of the liquid phase will result in a solid solution of Si:ZnSiP 2 being deposited at the cooler side of the zone.
—
*
Zone
Also: Novocherkasski Polytechnical Institute, U.S.S.R.
129
130
V. P. POPOV AND B. R. PAMPLIN
could be grown on germanium and any silicon containing Il—tV—V2 compound could be grown on silicon
\Zn
\P
)
T2>
I
I Solid solution
Zone
v
3. Experimental results
-
‘1
( Y
Si
_________
T:
using this technique.
T,
d
T2
/
/~ Fig. 2.
7 /Zn
The diagram of the process.
Assuming rapid reaction of the vapours with the hot zone so that equilibrium is quickly established between vapour and liquid, we can estimate the upper limit of the zone velocity, V, for uniform deposition. Consider the in fig. 2, where I is to thegolength of the zone and model d the zone depth, assumed right through the silicon slice. Taking into account diffusive transport of atoms of the vapour species in the melt and neglecting convection in the melt it is easy to show that a constant liquid composition is established if V < 2D1’d~ (I) /
if or,the zone penetrates right through the slice (as shown) V
(2)
<
I
if the zone moves along the surface of the slice. D is the diffusion constant for vapour atoms in the molten zone. D does not vary by more than an order of magnitude from element element a typical is 2 sec forto the melt.andTaking 1 =value 1 mm, l0 ~ cm ci = 0.4 mm, we obtain limiting velocities of 14 and 7 x l0 ~cm/sec for the two cases above. The zone composition depends on the average zone temperature and the temperature gradient (G) and it depends on the vapour pressure of the volatile cornponents, hence the source temperature. The geometry of the epitaxial layer is determined by the initial zone volume, by the temperature conditions and by the time taken. The choice of these parameters thus permits some control of the thickness, width and length of the epitaxmal growth. Simultaneous use of several zones would allow the growth of complicated heterostructures. Any germanium containing Il—tV--V 2 compound .
Unfortunately the phase diagrams of the ternary compounds with silicon and gerrnaniuni are scarcely known, so a suitable solvent must be found by trial and error. The solvent must dissolve silicon and ZnSiP2. For this reason we have chosen and tested the following metals Sn, In, Al, Zn, Ag and Au as solvents for the system Si—ZnSiP2. All metals used were 99.999°~ pure and pieces 2—20mg in weight were alloyed onto the silicon in vacuum (Au and Ag) or hydrogen (Sn, In, Al, Zn) after etching in HCI and washing in distilled water. Silicon slices of (Ill) orientation, both n and p type, 3 were used after meof dimensions 0.4 x 9 x 20 mm chanical and chemical treatment in CP4. It proved difficult to fuse Zn, Au and Ag into silicon. These metals were fused by very short heating at 1200 °C Excessive spreading of the metal across the silicon slice would be prevented by careful choice of the temperature of fusion and the time for which it was applied. The ZnSiP 2 source material was previously 2) using prepared 99.999°, by thestarting tin solution growth pure materials. The process slice and ZnSiP 2 powder were placed in a 10 mm diameter silica ampoule and sealed under vacuum. The ampoule was placed in a temperature gradient furnace at a chosen temperature in the range 1000—1200 °Cfor 10—80 hr. Heating and cooling rates of 20 °C/minwere used. T ABLE I
Table of results
.
—
-
Sample Solvent No.
T~ (C)
T, ( C)
G t (C cm ‘) (hr)
V>~10° (cm sec
—
-
4 5 6
Sn In Au
1150 1150 1160
1120 1120 1130
25 25 25
67 21 23
1.1 0 10.0
7
Au
1120
1090
25
45
5.7
8 II 13
Au Al
Zn
1100 1120
070 1090
25 25
25
64 23 24
3.0 0 0
18
Au
1180
1150
25
23
15.0
20 21* 23
Au
1050
1030
8
45
0
Au Ag
1120 1150
1120
25 35
65 63
4.2 6.7
1130
—-
1100
—
-—
----
-
--
T~and T. are the temperatures of zone and source (temperature of the cold end of the ampoule). ~ Without source of vapour.
EPITAXTAL GROWTH
OF SOLID SOLUTIONS
/ /~
>
1120
1160
T ~c
1200
Fig. 3. Graph of zone velocity against Si—P—Zn; (~)Au—Si; Ag—Si—P—Zn.
temperature.
(o)
Au—
(i)
The zone velocity was calculated from the zone trace distance and the time at temperature. When the ZnSiP2 was placed at the hot end of the furnace the silicon slice was covered with small po1yhedral crystals of ZnSiP2 and needles of ZnSiP2 grew at the cold end of the tube by a VLS mechanism. Further study of this V.L.S. growth is in progress and will be reported separately. Table I shows the results obtained for different sot~
~
IN
Si
131
were obtained. Au was chosen for a detailed further investigation. The zone velocity increased sharply with temperature (fig. 3). Below 1050 °Cthe Au zone did not move at all. The volume of liquid phase was about
A
1080
ZnSiP2
vents in the zone. For In, Al and Zn the zone did not move so these metals are of no use in the technique. The Sn zone moved but the zone size quickly decreased and the composition of the growing layer was not established. With Au and Ag zones good movement and traces
/ / 7 /
14
OF
,-
three or four times larger than the initial volume of the Au drop in the temperature range used. The surface of the grown layer was usually smooth. After polishing and etching the interface between the initial silicon and the recrystallized layer was sharp and distinct. A typical photomicrograph of a layer is shown in fig. 4. The grown layer was silvery in colour contrasting with the darker silicon, and free from cracks. Experiments were performed using a gold zone without any ZnSiP2 vapour for comparison. We observed unstable movement of the drop, which spread out over the silicon surface and broke up into smaller droplets 100—200 .tm in diameter. The epitaxial nature of the regrown layer was estab-
———,‘—~
~L
F’.
Fig. 4.
Microphotograph of silicon sample with an epitaxial layer. Magn.
6.5.
132
V. P. POPOV AND B. R. PAMPLIN
lished by Laue back-reflection photographs. The grown layer had a cubic lattice and the same orientation as the substrate. X-ray powder photographs of ground up material from the epitaxial layer of sample showed a lattice parameter of 5.418 A. The composition of the layer was further investigated by electron probe microanalysis using a 2 mm electron beam with an accuracy 0.5 ~/. The layers were constant in composition for a given sample and consisted of 5°~ ZnSiP2 in Si. The composition was constant across and slightly increased in the ZnSiP2 content along the layer.
to reaction of P and Zn with Si. This process has been investigated in detail by Buehler and Wernick5). The lattice constant of the solid solution (5.718 A) corresponds to 6°~ of the ZnSiP 2 in Si assuming Vegard’s law applies. This is in a good agreement with the result (5°~ ZnSiP2) obtained by electron probe microanalysis. It is likely that an increased concentration of ZnSiP2 can be achieved by successive running of several zones through the silicon or by reversal and passing back of the first zone. Acknowledgements
4. Discussion The gold zone velocity was higher in the absence of ZnSiP2 vapour indicating probably that the increased concentration of solute atoms increases the liquidus temperature of the zon& 0) The temperature dependence of zone speed was exponential. The curve of fig. 3 can be fitted by a formula V = V0 exp (—H/RI), where H is 200000 J/mol and V0 is a constant depending on the zone composition and geometry. The higher stability of the liquid zone in the Au—-Si—P—Zn system compared to the Au—Si system can be explained by the larger surface tension in the first case. The lack of movement of In, Al and Zn zones is explained by the freezing of the zone by the formation of compounds such as AlP. The uniform composition of the layers is evidence for the rapid establishnient of equilibrium between the drop and the vapour. The formation of ZnSiP2 crystals on the silicon bar was due
The authors wish to thank Dr. V. Scott for the microanalysis and Mr. B. Chapman for the X-ray work. Also, thanks are due to R. C. Draper for his help with the apparatus of the investigation. References I) L. 1. Berger and V. D. Prochukhan, Ternary Diamond-like Semiconductors (Consultants Bureau, New York-London, 1969). 2) A. J. Spring-Thorpe and B. R. Pamplin, J. Crystal Growth 3,4 (1968) 313. 3) Ya. A. Valov and T. N. Ushakova, lzv. Akad. Nauk SSSR 4 (1968) 1054. 4) S. A. Mughal, A. J. Payne and B. Ray, J. Mater. Sci. 4 (1969) 895. 5) E. Buehler and J. H. Wernick, J. Crystal Growth 8 (1971) 324. 6) A. J. SpringThorpe, R. J. Harvey and B. R. Pamplin, J. Crystal Growth 6 (1969) 104. 7) J. Beroti, J. Mater. Sci. 5 (1970) 1073.
8) W. G. Pfann, Zone Melting (Wiley, New York, 1968). 9) V. N. Lozovskii, V. P. Popov, V. A. tvkov and G. S. Konstantinova, in: Crystal Growth (Akad. Nauk U.S.S.R., 1971). 10) W. A. Tiller, J. Appi. Phys. 36 (1965) 261.