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Pair spectra in tetragonal zinc diphosphide (ZnP2) and cadmium diphosphide (CdP2) single crystals
Volume 29A, number 9
PAIR AND
PHYSICS LETTERS
SPECTRA CADMIUM
28 July 1969
IN T E T R A G O N A L
ZINC
DIPHOSPHIDE
( C d P 2)
DIPHOSPHIDE SINGLE
(ZnP2)
CRYSTALS
w. WARDZY~Sm Institute of Electron Technology, Polish Academy of Sciences, Warsaw, Poland A. WOJAKOWSKI Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Wroctaw, Pola~l and W. 7.DANOWIC Z Technical University, Wroc~aw , Poland Received 20 May 1969
The photoluminescence of single crystals of ZnP2 and CdP2 are measured at 4.2oK. The received spectra are interpreted in terms of pair spectra.
A n u m b e r of I I - VI and III-V compounds show at low t e m p e r a t u r e i n t e n s e b r o a d e m i s s i o n bands n e a r the a b s o r p t i o n edge s o - c a l l e d " e d g e " e m i s sion. This e m i s s i o n i s u s u a l l y e x p l a i n e d in t e r m s of r a d i a t i v e r e c o m b i n a t i o n of h o l e s and e l e c t r o n s t r a p p e d at i s o l a t e d d o n o r - a c c e p t o r p a i r s d i s pl a yi n g d i f f e r e n t d i s c r e t e s e p a r a t i o n s . T h i s m e c h a n i s m should l e a d to the a p p e a r a n c e of d i s crete lines for small donor-acceptor separation. A s r e s u l t of i n c r e a s i n g d o n o r - a c c e p t o r s e p a r a t i o n t h e s e l i n e s u n d er g o t r a n s f o r m a t i o n into b r o a d bands [1]. T h i s type of s p e c t r u m was found in G a P [2,3]. In o t h e r c r y s t a l s of I I - V I and HI-V compounds a b r o a d e m i s s i o n was only o b s e r v e d . T h i s r e p o r t p r e s e n t s the r e s u l t s of the l u m i n s c e n c e m e a s u r e m e n t s f o r the s i n g l e c r y s t a l s of I I - V compounds, e.g. Z n P 2 and CdP 2. The t e t r a g o n a l m o d i f i c a t i o n of ZnP2 and C d P 2 s i n g l e c r y s t a l s w e r e grown f r o m the v a p o u r p h a s e a c c o r d i n g to the m e t h o d p r e v i o u s l y d e s c r i b e d in [4]. The e x c i t a t i o n s o u r c e was a 200 W high p r e s s u r e m e r c u r y lamp. S p e c t r a w e r e obtained by m e a n s of H i l g e r D- 4 0 0 g r a t i n g m o n o c h r o m a t o r . A s a d e t e c t o r a p h o t o m u l t i p l i e r was used. T h e m e a s u r e m e n t s w e r e c a r r i e d out at 4.2°K. The l u m i n e s c e n c e of ZnP 2 and CdP 2 at 4.2°K c o n s i s t of two b r o a d bands at 1.92 and 2.06 eV f o r ZnP 2 and at 1.82 and 2.08 eV f o r C d P 2. The high e n e r g y bands f o r both compounds have a p r o n o u n c e d line s t r u c t u r e . T h i s s t r u c t u r e is shown in detail in fig. 1. B a s e d on the known a t o m i c p o s i t i o n s of ZnP 2
Y~O
2OBO
2:100
2~20
2~)
AE(W) 2eO
2~0
cd P,
Aj ZnP, .
~
r -~..2"K N3 N~ N,
............................... 6000
5gO0
~-~ ~(~3
Fig. 1. The line structure of 2.06 eV and 2.08 eV bands of ZnP 2 and CdP 2. [5,6] the suitable computation was p e r f o r m e d to obtain the n u m b e r of e q u i v a l e n t d o n o r - a c c e p t o r p a i r s l o c a t e d at Zn, P(1) and P(2) s i t e s a s a function of p a i r s s e p a r a t i o n . L i n e s denoted in fig. 1 a s N 1 to N15 could be i n t e r p r e t e d in t e r m s of r a d i a t i v e r e c o m b i n a t i o n of h o l e s and e l e c t r o n s t r a p p e d by d o n o r s and a c c e p t o r s l o c a t e d at Zn and P(2) s i t e s . A 0 , A 1 , A 2 and A 3 f o r m an a n o t h e r s e t of lines. The e n e r g y s e p a r a t i o n b et w een t h e s e l i n e s is equal. L i n e s A 1, A 2 and A3 a r e phonon r e p l i c a of line A 0 with phonon e n e r g y of 0.023 eV. The line of A o is p r o b a b l y c o n n e c t e d with a c a m 547
Volume 29A, number 9
PHYSICS LETTERS
p l e x f o r m e d by an ex c it o n bound to a r a t h e r deep center. The e n e r g y of l i n e s in eV a r e a s f o l l o w s :
N1 2.1829
Nll
2.1050
N2
2.1769
N12
2.0898
N3
2.1716
N13
2.0851
N4
2.1360
N14
2.0816
N 5 2.1347
N15
2.0666
N6
2.1321
A0
2.1446
N7
2.1285
A1
2.1216
N8
1.1158
A2
2.0986
N9
2.1136
A3
2.0759
N10
2.1097
28 July 1969
with l i n e s s t r u c t u r e have been found so f a r f o r phosphides only. A u t h o r s a r e g r a t e f u l to R. Kowalczyk f o r c o m putation.
References 1. E.E. Williams, J. Phys. Chem. Solids 12 (1960) 265. 2. J . J . Hopfleld, D.G. Thomas and M. Gershermon, Phys. Rev. Letters 1O (1963) 162. 3. D.G. Thomas, M. C-ershenzon and F. A. Trumbore, Phys. Rev. 140 (1964) A269. 4. W. Zdanowicz and A. Wojakowski, Phys. SLat. Sol. 10 (1965) K93. 5. I.J. Hegyi, E.E. Loebner, E.W. Poor Jr. and J. G. White, J. Phys. Chem. Solids 24 (1963) 333. 6. J.G. White, Acta Cryst. 18 (1965) 217.
It is i n t e r e s t i n g to note that the p a i r s p e c t r a * * * * *
CRYSTALLIZATION A
WITH COMPUTER
A
LENNARD-JONES EXPERIMENT*
POTENTIAL:
F. W. D E W E T T E , R. E. A L L E N and D. S. H U G H E S Department of Physics, University of Texas, Austin, Texas 78712, USA
and A. R A H M A N Argonne National Laboratory, Argonne, Illinois 60439, USA Received 10 June 1969
Crystallization of a two-dimensional system of particles interacting through a Lennard-Jones potential has been obtained in a molecular dynamics computer experiment,
We have obtained c r y s t a l l i z a t i o n of a f r e e , t w o - d i m e n s i o n a l s y s t e m of 400 p a r t i c l e s i n t e r a c t i n g t h r o u g h a L e n n a r d - J o n e s (LJ) p o t e n ti a l in a m o l e c u l a r d y n a m i c s c o m p u t e r e x p e r i m e n t . The c r y s t a l l i z a t i o n r e s u l t e d e n t i r e l y f r o m the i n t e r a c tion of the p a r t i c l e s ; i . e . , t h e r e w e r e n e i t h e r walls enclosing the system nor periodic boundary conditions. * Work supported by the U.S. Air Force Office of Scientific Research under Grant No. AF-AFOSR 125767 and performed in part under the auspices of the U. S. Atomic Energy Commission.
548
The p r o c e d u r e was the following: 1) The p a r t i c l e s w e r e p l a c e d at r a n d o m p o s i t i o n s at t i m e t = 0, as shown in fig. l ( a) , and then w e r e a l lowed to m o v e f r e e l y u n d e r the influence of t h e i r i n t e r a c t i o n s . Each p a r t i c l e i n t e r a c t e d with e v e r y o t h e r p a r t i c l e in the s y s t e m . 2) As the p a r t i c l e s m o v e d to p o s i t i o n s of l o w e r potential e n e r g y , the r e s u l t i n g i n c r e a s e in the k i n et i c e n e r g y was r e m o v e d by damping the motion of the p a r t i c l e s . In this way the kinetic energy per particle was kept far enough below the value at the meRing temperature for further order to develop. 3) W h e n the particles had the positions shown in
PAIR AND
PHYSICS LETTERS
SPECTRA CADMIUM
28 July 1969
IN T E T R A G O N A L
ZINC
DIPHOSPHIDE
( C d P 2)
DIPHOSPHIDE SINGLE
(ZnP2)
CRYSTALS
w. WARDZY~Sm Institute of Electron Technology, Polish Academy of Sciences, Warsaw, Poland A. WOJAKOWSKI Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Wroctaw, Pola~l and W. 7.DANOWIC Z Technical University, Wroc~aw , Poland Received 20 May 1969
The photoluminescence of single crystals of ZnP2 and CdP2 are measured at 4.2oK. The received spectra are interpreted in terms of pair spectra.
A n u m b e r of I I - VI and III-V compounds show at low t e m p e r a t u r e i n t e n s e b r o a d e m i s s i o n bands n e a r the a b s o r p t i o n edge s o - c a l l e d " e d g e " e m i s sion. This e m i s s i o n i s u s u a l l y e x p l a i n e d in t e r m s of r a d i a t i v e r e c o m b i n a t i o n of h o l e s and e l e c t r o n s t r a p p e d at i s o l a t e d d o n o r - a c c e p t o r p a i r s d i s pl a yi n g d i f f e r e n t d i s c r e t e s e p a r a t i o n s . T h i s m e c h a n i s m should l e a d to the a p p e a r a n c e of d i s crete lines for small donor-acceptor separation. A s r e s u l t of i n c r e a s i n g d o n o r - a c c e p t o r s e p a r a t i o n t h e s e l i n e s u n d er g o t r a n s f o r m a t i o n into b r o a d bands [1]. T h i s type of s p e c t r u m was found in G a P [2,3]. In o t h e r c r y s t a l s of I I - V I and HI-V compounds a b r o a d e m i s s i o n was only o b s e r v e d . T h i s r e p o r t p r e s e n t s the r e s u l t s of the l u m i n s c e n c e m e a s u r e m e n t s f o r the s i n g l e c r y s t a l s of I I - V compounds, e.g. Z n P 2 and CdP 2. The t e t r a g o n a l m o d i f i c a t i o n of ZnP2 and C d P 2 s i n g l e c r y s t a l s w e r e grown f r o m the v a p o u r p h a s e a c c o r d i n g to the m e t h o d p r e v i o u s l y d e s c r i b e d in [4]. The e x c i t a t i o n s o u r c e was a 200 W high p r e s s u r e m e r c u r y lamp. S p e c t r a w e r e obtained by m e a n s of H i l g e r D- 4 0 0 g r a t i n g m o n o c h r o m a t o r . A s a d e t e c t o r a p h o t o m u l t i p l i e r was used. T h e m e a s u r e m e n t s w e r e c a r r i e d out at 4.2°K. The l u m i n e s c e n c e of ZnP 2 and CdP 2 at 4.2°K c o n s i s t of two b r o a d bands at 1.92 and 2.06 eV f o r ZnP 2 and at 1.82 and 2.08 eV f o r C d P 2. The high e n e r g y bands f o r both compounds have a p r o n o u n c e d line s t r u c t u r e . T h i s s t r u c t u r e is shown in detail in fig. 1. B a s e d on the known a t o m i c p o s i t i o n s of ZnP 2
Y~O
2OBO
2:100
2~20
2~)
AE(W) 2eO
2~0
cd P,
Aj ZnP, .
~
r -~..2"K N3 N~ N,
............................... 6000
5gO0
~-~ ~(~3
Fig. 1. The line structure of 2.06 eV and 2.08 eV bands of ZnP 2 and CdP 2. [5,6] the suitable computation was p e r f o r m e d to obtain the n u m b e r of e q u i v a l e n t d o n o r - a c c e p t o r p a i r s l o c a t e d at Zn, P(1) and P(2) s i t e s a s a function of p a i r s s e p a r a t i o n . L i n e s denoted in fig. 1 a s N 1 to N15 could be i n t e r p r e t e d in t e r m s of r a d i a t i v e r e c o m b i n a t i o n of h o l e s and e l e c t r o n s t r a p p e d by d o n o r s and a c c e p t o r s l o c a t e d at Zn and P(2) s i t e s . A 0 , A 1 , A 2 and A 3 f o r m an a n o t h e r s e t of lines. The e n e r g y s e p a r a t i o n b et w een t h e s e l i n e s is equal. L i n e s A 1, A 2 and A3 a r e phonon r e p l i c a of line A 0 with phonon e n e r g y of 0.023 eV. The line of A o is p r o b a b l y c o n n e c t e d with a c a m 547
Volume 29A, number 9
PHYSICS LETTERS
p l e x f o r m e d by an ex c it o n bound to a r a t h e r deep center. The e n e r g y of l i n e s in eV a r e a s f o l l o w s :
N1 2.1829
Nll
2.1050
N2
2.1769
N12
2.0898
N3
2.1716
N13
2.0851
N4
2.1360
N14
2.0816
N 5 2.1347
N15
2.0666
N6
2.1321
A0
2.1446
N7
2.1285
A1
2.1216
N8
1.1158
A2
2.0986
N9
2.1136
A3
2.0759
N10
2.1097
28 July 1969
with l i n e s s t r u c t u r e have been found so f a r f o r phosphides only. A u t h o r s a r e g r a t e f u l to R. Kowalczyk f o r c o m putation.
References 1. E.E. Williams, J. Phys. Chem. Solids 12 (1960) 265. 2. J . J . Hopfleld, D.G. Thomas and M. Gershermon, Phys. Rev. Letters 1O (1963) 162. 3. D.G. Thomas, M. C-ershenzon and F. A. Trumbore, Phys. Rev. 140 (1964) A269. 4. W. Zdanowicz and A. Wojakowski, Phys. SLat. Sol. 10 (1965) K93. 5. I.J. Hegyi, E.E. Loebner, E.W. Poor Jr. and J. G. White, J. Phys. Chem. Solids 24 (1963) 333. 6. J.G. White, Acta Cryst. 18 (1965) 217.
It is i n t e r e s t i n g to note that the p a i r s p e c t r a * * * * *
CRYSTALLIZATION A
WITH COMPUTER
A
LENNARD-JONES EXPERIMENT*
POTENTIAL:
F. W. D E W E T T E , R. E. A L L E N and D. S. H U G H E S Department of Physics, University of Texas, Austin, Texas 78712, USA
and A. R A H M A N Argonne National Laboratory, Argonne, Illinois 60439, USA Received 10 June 1969
Crystallization of a two-dimensional system of particles interacting through a Lennard-Jones potential has been obtained in a molecular dynamics computer experiment,
We have obtained c r y s t a l l i z a t i o n of a f r e e , t w o - d i m e n s i o n a l s y s t e m of 400 p a r t i c l e s i n t e r a c t i n g t h r o u g h a L e n n a r d - J o n e s (LJ) p o t e n ti a l in a m o l e c u l a r d y n a m i c s c o m p u t e r e x p e r i m e n t . The c r y s t a l l i z a t i o n r e s u l t e d e n t i r e l y f r o m the i n t e r a c tion of the p a r t i c l e s ; i . e . , t h e r e w e r e n e i t h e r walls enclosing the system nor periodic boundary conditions. * Work supported by the U.S. Air Force Office of Scientific Research under Grant No. AF-AFOSR 125767 and performed in part under the auspices of the U. S. Atomic Energy Commission.
548
The p r o c e d u r e was the following: 1) The p a r t i c l e s w e r e p l a c e d at r a n d o m p o s i t i o n s at t i m e t = 0, as shown in fig. l ( a) , and then w e r e a l lowed to m o v e f r e e l y u n d e r the influence of t h e i r i n t e r a c t i o n s . Each p a r t i c l e i n t e r a c t e d with e v e r y o t h e r p a r t i c l e in the s y s t e m . 2) As the p a r t i c l e s m o v e d to p o s i t i o n s of l o w e r potential e n e r g y , the r e s u l t i n g i n c r e a s e in the k i n et i c e n e r g y was r e m o v e d by damping the motion of the p a r t i c l e s . In this way the kinetic energy per particle was kept far enough below the value at the meRing temperature for further order to develop. 3) W h e n the particles had the positions shown in