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Submitted on 1 Jan 1981
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PHONON SPECTROSCOPY OF
CHROMIUM-DOPED GALLIUM ARSENIDE USING
SUPERCONDUCTING TUNNEL JUNCTIONS
M. Hamdache, P. King, D. Murphy, V. Rampton
To cite this version:
JOURNAL DE PHYSIQUE
CoZZoque C6, supple'ment au n o 12, Tome 42, de'cembre 2981
page
c6-664PHONON SPECTROSCOPY OF CHROMIUM-DOPED GALLIUM ARSENIDE USING
SUPERCONDUCTING TUNNEL JUNCTIONS
M.
Hamdache, P . J . King, D.T. Murphy and V.W. RamptonDepartment of Physics, University o f N o t t i n g h , University
Park,
Nottingham,
NG7 2RD, England
A b s t r a c t : - Phonon spectroscopy measurements have been made on a v a r i e t y of chromium-doped gallium a r s e n i d e samples over t h e range 3 cm-I (90 GHz) t o 24 cm-1 (720
GHz)
a t 1 K. I n s e m i - i n s u l a t i n g samples and i n weakly p-type samples a c l u s t e r of t h r e e p a r t i a l l y r e s o l v e d r e s o n a n t a b s o r p t i o n s a t 4.2 cm-t 4.9 cm-l and 5.6 cm-l a r e found, t o g e t h e r with a weaker but broader absorp- t i o n a t 1 0 cm-l. These a b s o r p t i o n s a r e n o t a f f e c t e d by sub-band gap i l l u m i n a t i o n . I n n-type samples t h e s e f e a t u r e s a r e n o t found under dark c o n d i t i o n s , but may b e induced by i l l u m i n a t i o n .Chromium h a s o f t e n been i n t r o d u c e d i n t o gallium a r s e n i d e t o compensate shallow donors i n t h e p r o d u c t i o n of s e m i - i n s u l a t i n g (SI) d e v i c e s u b s t r a t e s . The numerous charge s t a t e s of chromium have been s t u d i e d by a wide range of techniques which have included e l e c t r o n s p i n resonance ( e p r ) , o p t i c a l a b s o r p t i o n , photo- luminescence and f a r i n f r a - r e d spectroscopy. Techniques which i n v o l v e t h e coupling of chromium
ions t o 'phononsl haye included a c o u s t i c r e l a x a t i o n , a c o u s t i c para-
magnetic resonance ( a p r ) , thermal conduction and h e a t p u l s e t e c h n i q u e s .Many experimental r e s u l t s have been explained on a model where chromium s u b s t i t u t e s ( s ) f o r gallium i n t h e GaAs l a t t i c e . Cr;+ h a s been used t o e x p l a i n e p r , f a r i n f r a - r e d s p e c t r o s c o p y , t h e 0.82 eV o p t i c a l a b s o r p t i o n l i n e , and an a c o u s t i c r e l a x a t i o n peak. 3+
C r s and C r ? have been r e p o r t e d t o g i v e d i s t i n c t i v e
e p r l i n e s while ~r:+ h a s been observed o n l y under l a r g e h y d r o s t a t i c p r e s s u r e . The dominant s t a t e i s found t o depend on t h e n a t u r e of t h e m a t e r i a l , 2+
4+ Crs
dominating i n n-type m a t e r i a l , C r s dominating i n s t r o n g l y p-type m a t e r i a l , while
2+ 3+
SI m a t e r i a l c o n t a i n s a mixture of
C r and C r.
Valence changes can be induced by s u i t a b l e i l l u m i n a t i o n ( s e e 1, 2 , 3 , 5 and 6 and r e f e r e n c e s t h e r e i n ) .Dynamic Jahn-Teller models, which have r e p l a c e d t h e o r i g i n a l s t a t i c Jahn-
3+ 2
T e l l e r models of
cr2+
and C r,
a r e now w e l l developed.
Some phonon r e s u l t s have, however, n o t r e c e i v e d a s a t i s f a c t o r y e x p l a n a t i o n , t h e s e i n c l u d i n g a p r l and f o r example t h e frequency c r o s s i n g r e s u l t s of C h a l l i s
3 and Ramdane
.
One technique which h a s n o t been widely a p p l i e d t o t h i s system i s phonon spectroscopy u s i n g t u n n e l j u n c t i o n s , although Narayanamurti has d e s c r i b e d a
4
measurement made on s e m i - i n s u l a t i n g ( S I ) m a t e r i a l
.
We have made measurements on n-type, SI and lightly p-type material using
aluminium detectors and lead bismuth alloy generators both in the dark and in sub-
band gap illumination. The results reported here were obtained using low
frequency sine wave modulation of the generator junction, and detection of that
frequency using phase sensitive detection techniques, although pulse measurements
5
have also been made Csee for example Eisenmenger
).
The samples used originated either from Czechoslovakia or from the Plessey
Company in ~ n ~ l a n d l ' ~ ' ~ .
The results here were made on CS-5, an SI Czechoslovakian
sample, GA781,
a
lightly p-type sample containing zinc which is semi-insulating
at helium temperatures, GA735-i, an SI sample and GA735-1, an n-type sample which
remains n-type down to 1
K.
Figure 1:
Detected signal as a
function of generator bias.
The sample was cut for
<loo> propagation.
The results on the p-type and semi-insulating samples were essentially
similar and a spectrum obtained on CS-B is shown in Figure 1. The onset of
detector sensitivity at
%3 cm-' is masked by a group of three strong and
partially resolved resonant absorptions, indicated by arrows. A higher energy
-1
.
absorption in the region of 10 cm
is also indicated. In SI samples these
lines are little affected by sub-band gap illumination provided by an optic
light guide from a quartz iodine lamp through a silicon filter, this arrangement
providing a band pass of 0.8 eV to 1.1 eV.
JOURNAL
DE
PHYSIQIIEI
Sample GA735-1 CW F i g u r e 2: S u b t r a c t e d s p e c t r a a sa
f u n c t i o n of g e n e r a t o r b i a s . The sample was c u t f o r<loo>
propagation.of l i g h t
-
14
hours d a r k and I1 i s t h e r e s u l t of l i g h t-
3 hours dark. The t h r e e f e a t u r e s a t 4.2, 4.9 and 5.6 cm-' a r e c l e a r l y v i s i b l e and t h e broad band a t 10 cm-I i s reproduced. A lower shoulder on t h e t h r e e l i n e band i s a l s o i n evidence. A l l l i n e s appear t o be c r e a t e d and decay i n p r o p o r t i o n . The l i g h t dependence o f . t h e s e r e s u l t s resembles t h a t of t h e a p r on t h e same samples i n t h a t i n S I m a t e r i a l resonances a r e seen which can be l i g h t induced i n n-type samples. C h a l l i s and Ramdane have r e p o r t e d a phonon a b s o r p t i o n a t 5 cm-' a l t h o u g h they6
were unable t o r e s o l v e any s t r u c t u r e
.
Our lower s h o u l d e r may be t h e upper d n g of t h e i r l i n e a t 2.7 cm-l. They a t t r i b u t e t h e s e two l i n e s and one a t 13.3 cm-I t o~rz,
although i t seems improbable t h a t t h i s Kramers i o n i s a l s o r e s p o n s i b l e f o r t h e a p r .References
1. Bury, P., C h a l l i s , L. J . , King, P. J . , Honk, D. J . , Ramdane, A. Rampton, V.
W.
and Wiscombe, P., "Semi-Insulating 1 1 1 - V M a t e r i a l s " , Shiva P r e s s , 1980, p. 213. 2. Abhvani, A. S., Austen, S. P., B a t e s , C. A . , P a r k e r , L. W. and P o o l e r , D. R . ,"The Dynamic Jahn-Teller Theory f o r S u b s t i t u t i o n a l
cr2+
i n GaAs", submitted f o r p u b l i c a t i o n t o J. Phys. C: S o l i d S t a t e Phys., 1981.3. C h a l l i s , L. J. and Ramdane A . , "Phonon S c a t t e r i n g i n Condensed Matter", ed. H. J . Maris, Plenum P r e s s , 1980, p. 121.
4. Narayanamurti,V., "Phonon S c a t t e r i n g i n Condensed Matter", ed. H. J. Maris, Plenum P r e s s , 1980, p. 385.
5. Eisenmenger, W . , " P h y s i c a l Acoustics",
