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