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INVESTIGATIONS OF THE NEGATIVE
DIFFERENTIAL CONDUCTIVITY AND CURRENT BISTABILITY IN DOUBLE BARRIER n+
GaAs/(AlGa)As/GaAs/(AlGa)As/n+ GaAs
RESONANT TUNNELLING DEVICES USING HIGH MAGNETIC FIELDS
C. Payling, E. Alves, L. Eaves, T. Foster, M. Henini, O. Hughes, P.
Simmonds, J. Portal, G. Hill, M. Pate
To cite this version:
C. Payling, E. Alves, L. Eaves, T. Foster, M. Henini, et al.. INVESTIGATIONS OF THE NEG- ATIVE DIFFERENTIAL CONDUCTIVITY AND CURRENT BISTABILITY IN DOUBLE BAR- RIER n+ GaAs/(AlGa)As/GaAs/(AlGa)As/n+ GaAs RESONANT TUNNELLING DEVICES US- ING HIGH MAGNETIC FIELDS. Journal de Physique Colloques, 1987, 48 (C5), pp.C5-289-C5-292.
�10.1051/jphyscol:1987563�. �jpa-00226767�
JOURNAL DE PHYSIQUE
Colloque C5, suppl6ment au n0ll, Tome 48, novembre 1987
INVESTIGATIONS OF THE NEGATIVE DIFFERENTIAL CONDUCTIVITY AND CURRENT BISTABILITY IN DOUBLE BARRIER n*GaAs/(AlGa)As/GaAs/(A1Ga)As/n+GaAs RESONANT TUNNELLING DEVICES USING HIGH MAGNETIC FIELDS
C.A. PAYLING, E. ALVES, L. EAVES, T.J. FOSTER, M. HENINI, O.H. HUGHES, P.E. SIMMONDS, J.C. PORTAL*, G. HILL**
and M.A. PATE"
Department of Physics, University o f Nottingham, GB-Nottingham, NG7 2RD, Great-Britain
*SNCI-CNRS, Grenoble, F-38042 Cedex, and LPS,
I . N . S . A . ,Toulouse, F-31077 Cedex, France
*
1Department of Electronic Engineering, University of Sheffield, GB-Sheffield, S1 3JD, Great-Britain
/
c o n d u c t i v i t g n g g a t i v e d i f f e r e n t i e l l e (CND) e t b i s t a b i l i t ; e n c o u r a n t d a n s une s t r u c t u r e t u n n e l r e s o n n a n t e o n t /et< /etudie$s p a r l ' e f f e t o s c i l l a t o i r e d a n s l e courant/ magne(totunnel a v e c
21 12.
Les mesures m o n t r e n t que l a b i s t a b i l i t g e s t a s s o c i e e a v e c l ' a c c u m u l a t i o n e t l l e / j e c t i o n de c h a r g e nggat;ve du p u i t s . CND a s s o c i e / a v e c l a d e w i g m e sous-bande du p u i t s e s t o b s e r v g e a p l u s g r a n d v o l t a g e (- 1 V).,
L e s m a g n g t o - o s c i l l a t i o n s i n d i q u e n t l a p r 6 s e n c e du t u n n e l s G q u e n t i e l2
c e t t e r e g i o n .
N e g a t i v e d i f f e r e n t i a l c o n d u c t i v i t y (NDC) and c u r r e n t b i s t a b i l i t y i n a r e s o n a n t t u n n e l l i n g d e v i c e a r e i n v e s t i g a t e d by examining t h e o s c i l l a t o r y s t r u c t u r e i n t h e m a g n e t o t u n n e l l i n g c u r r e n t f o r J J
1 ~ .
The d a t a s u p p o r t t h e c o n c l u s i o n t h a tb i s t a b i l i t y i s a s s o c i a t e d w i t h b u i l d - u p and e j e c t i o n o f n e g a t i v e s p a c e c h a r g e from t h e w e l l . The c h a r g e d e n s i t y o f 2D e l e c t r o n s i n t h e c o n t a c t a c c u m u l a t i o n l a y e r i s measured d i r e c t l y . NDC due t o t u n n e l l i n g i n t o t h e s e c o n d sub-band o f t h e w e l l i s o b s e r v e d a t h i g h b i a s (-1 V). The a s s o c i a t e d m a g n e t o - o s c i l l a t i o n s i n d i c a t e t h e p r e s e n c e o f s e q u e n t i a l t u n n e l l i n g i n t h i s r e g i o n o f t h e I ( V ) c u r v e s .
Resonant t u n n e l l i n g d e v i c e s (RTD) based on d o u b l e b a r r i e r s e m i c o n d u c t o r h e t e r o s t r u c t u r e s e x h i b i t a r a n g e o f i n t e r e s t i n g p h y s i c a l p r o p e r t i e s : n e g a t i v e d i f f e r e n t i a l c o n d u c t i v i t y (NDC); h i g h f r e q u e n c y o s c i l l a t i o n s [ I ] ; c u r r e n t b i s t a b i l i t y [ 2 , 3 ] ; band s t r u c t u r e e f f e c t s C41; magnetoquantum e f f e c t s C51. I n a d d i t i o n , t h e y have s t i m u l a t e d c o n s i d e r a b l e t h e o r e t i c a l d i s c u s s i o n [6-81. The band d i a g r a m i s shown s c h e m a t i c a l l y i n F i g u r e I . The s t r u c t u r e s were grown a t Nottingham by MBE u s i n g a V a r i a n GEN I1 and c o m p r i s e d l a y e r s w i t h t h e f o l l o w i n g t h i c k n e s s e s and nominal d o p i n g s : n + G a ~ s - - s u b s t r a t e , 2 x 1018 ~ m - ~ ; n + ~ a ~ s b u f f e r l a y e r , 2 x
lo17
cm-3,
I pm; (A1Ga)As b a r r i e r , [ A l l = 0 . 3 3 , w i d t h b = 8 . f nm;undoped GaAs w e l l , w = 5.6 nm; (A1Ga)As b a r r i e r ,
C ~ l l
= 0 . 3 3 , 8 . 8 nm; n GaAs t o pl a y e r , 2 x
,
I Pm.The I ( V ) c h a r a c t e r i s t i c s shown i n F i g u r e 2 e x h i b i t pronounced NDC w i t h p e a k - t o - v a l l e y r a t i o o f up t o 16:l a t 4 K ( 1 0 : l a t 77 K ) . T h e r e i s a l s o
pronounced c u r r e n t b i s t a b i l i t y o v e r c e r t a i n v o l t a g e r a n g e s . The NDC and a s ' s o c i a t e d b i s t a b i l i t y below -0.4 V a r e v e r y s i m i l a r t o t h o s e r e p o r t e d r e c e n t l y [ 3 1 . The r i s e i n c u r r e n t a s V i s i n c r e a s e d t o 159 mV i s a t t r i b u t e d t o t u n n e l l i n g o f b u l k e l e c t r o n s i n t o t h e l o w e s t quantum w e l l s t a t e . The s h a r p f a l l - o f f and b i s t a b i l i t y beyond t h e peak a r e a t t r i b u t e d t o e j e c t i o n o f e l e c t r o n s from t h e w e l l C31.
I n a d d i t i o n , however, we o b s e r v e a f u r t h e r r e g i o n o f NDC a t h i g h e r v o l t a g e (-0.9-1.2 V). When a m a g n e t i c f i e l d ,
E,
i s a p p l i e d p e r p e n d i c u l a r t o t h e b a r r i e r s(JI I?),
w e l l - r e s o l v e d Shubnikov-de H a a s - l i k e (SdH) o s c i l l a t i o n s , p e r i o d i c i n l/B, a r e o b s e r v e d under c o n s t a n t v o l t a g e c o n d i t i o n s o v e r a l m o s t a l l o f t h e v o l t a g e r a n g e . The o s c i l l a t i o n s c a n b e s e e n i n t h e d i r e c t I(B) c u r v e s b u t a r e c o n s i d e r a b l yArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987563
JOURNAL
DE
PHYSIQUEF i g u r e I Schematic band diagram o f d e v i c e under l a r g e f o r w a r d b i a s i n t h e a b s e n c e o f c h a r g e i n t h e w e l l . The b i a s c o r r e s p o n d s t o t h a t p a r t o f t h e I(V) c u r v e (-0.6 V ) n e a r t o t h e o n s e t o f r e s o n a n c e between t h e o c c u p i e d c o n d u c t i o n band l e v e l s o f t h e b u f f e r l a y e r and t h e second sub-band (E2) o f t h e quantum w e l l .
F i g u r e 2 ( a ) F o u r - t e r m i n a l I(V) c h a r a c t e r - i s t i c s o f d e v i c e a t 4 K w i t h n e g a t i v e s u b s t r a t e b i a s . The low v o l t a g e p a r t i s m a g n i f i e d x48. The c o n d u c t a n c e n e a r 0.6 V i s a l s o shown. (b) Fundamental f i e l d s , Bf, v e r s u s b i a s f o r t h e magneto- o s c i l l a t i o n s i n d i f f e r e n t r e g i o n s o f t h e I(V) c u r v e . The I(V) c u r v e s a r e f o r a mesa w i t h 200 pm d i a m e t e r .
F i g u r e 3 Second d e r i v a t i v e o f I(B) showing t h e c h a n g i n g n a t u r e o f t h e m a g n e t o - o s c i l l a t i o n s a t v a r i o u s p a r t s o f t h e I(V) c u r v e .
enhanced i n t h e d e r i v a t i v e p l o t s , d 2 1 / d ~ 2 . S i m i l a r o s c i l l a t i o n s h a v e been o b s e r v e d and i n t e r p r e t e d i n m a g n e t o t u n n e l l i n g measurements on s i n g l e b a r r i e r s t r u c t u r e s [ 9 1 . T y p i c a l c u r v e s a r e shown i n F i g u r e 3 f o r v a r i o u s a p p l i e d v o l t a g e s a t i n v e r s e p e r i o d B f 4 K . Each s e r i e s o f o s c i l a t i o n s c a n b e d e f i n e d by a fundamental f i e l d o r = [ A ( l / ~ ) l
-1 .
These a r e p l o t t e d i n F i g u r e 2b.We employ t h i s o s c i l l a t o r y s t r u c t u r e t o i n v e s t i g a t e t h e r e s o n a n t t u n n e l l i n g and t h e l o c a t i o n and m a g n i t u d e o f s p a c e c h a r g e i n t h e v a r i o u s p a r t s o f t h e d e v i c e u n d e r o p e r a t i n g c o n d i t i o n s . A t low b i a s , ( ( 6 0 mV, s u b s t r a t e n e g a t i v e ) , r o t a t i o n o f t h e m a g n e t i c f i e l d r e l a t i v e t o t h e p l a n e o f t h e b a r r i e r i n d i c a t e s t h a t t h e o s c i l l a t o r y s t r u c t u r e h a s more o f a t h r e e - d i m e n s i o n a l c h a r a c t e r t h a n two- d i m e n s i o n a l . The fundamental f i e l d Bf = 9 T g i v e s a b u l k e l e c t r o n d e n s i t y o f
1.6 x 1017 ~ m - ~ , c l o s e t o t h e nominal c o n t a c t d o p i n g v a l u e . As V i s i n c r e a s e d t o w a r d s t h e f i r s t peak a t 159 mV, B i n c r e a s e s s l i g h t l y t o around 10 T. Around t h e r e s o n a n c e , t h e o s c i l l a t i o n s a p p e a r t o d e v e l o p a 2D c h a r a c t e r : by t i l t i n g f
B,
t h e o s c i l l a t i o n s show a c h a r a c t e r i s t i c f r e q u e n c y s h i f t , g i v e n a p p r o x i m a t e l y by B ( 0 ) =
B~ICOSO,
where cosO =J.B/JB.
T h i s b e h a v i o u r presumably r e f l e c t s t h e n a t u r e o f t h e q u a n t i s e d bound s t a t e o f t h e w e l l f [ 5 ] . The s h a r p f a l l - o f f i n c u r r e n t and t h e narrow r e g i o n o f b i s t a b i l i t y beyond t h i s peak h a v e been a t t r i b u t e d t o e j e c t i o n o f e l e c t r o n s ( n e g a t i v e s p a c e c h a r g e ) from t h e w e l l C31. I n t h e s e c o n d r e g i o n o f b i s t a b i l i t y ( a b c d ) , i t i s g e n e r a l l y p o s s i b l e t o o b t a i n c l e a r SdHo s c i l l a t i o n s i n b o t h t h e h i g h and low c u r r e n t modes. On t h e c u r v e (a-+b) ( h i g h c u r r e n t mode o f b i s t a b i l i t y ) , o s c i l l a t i o n s c o r r e s p o n d i n g t o Bf = 10 T a r e s t i l l c l e a r l y o b s e r v e d ( i n d i c a t e d by x). Superimposed on t h e s e i s a s e c o n d s e r i e s w i t h B = 16 T ( i n d i c a t e d by +) i n a n a r r o w r e g i o n from 280 t o 320 mV. I n t h e low c u r r e n t mode (c'd) f a s i n g l e s e r i e s o f p e a k s i s o b s e r v e d w i t h a Bf ( i n d i c a t e d by o ) which i n c r e a s e s g r a d u a l l y w i t h b i a s . T h i s s e r i e s i s c l e a r l y o b s e r v e d f o r v o l t a g e s ( < 7 4 0 mV) w e l l i n t o t h e m o n o s t a b l e r e g i o n . By t i l t i n g
B,
t h e s e o s c i l l a t i o n s show a v e r y c l e a r 2D c h a r a c t e r . We a t t r i b u t e t h i s s e r i e s t o t h e Landau l e v e l s t r u c t u r e o f t h e d e g e n e r a t e two-dimensional e l e c t r o n g a s ( ~ D E G ) i n t h e a c c u m u l a t i o n s e c t i o n o f t h e b u f f e r l a y e r ( F i g u r e I ) . F o r t h e a c c u m u l a t i o n l a y e r , B i s r e l a t e d t o t h e 2D c a r r i e r d e n s i t y by n2D = 2eBf/h. T h i s t h e r e f o r e a l l o w s u s t o c a l c u l a t e t h e f a c c u m u l a t e d c h a r g e and t h e a s s o c i a t e d e l e c t r i c f i e l d s and p o t e n t i a l d i f f e r e n c e s t h r o u g h o u t t h e d e v i c e . S i m i l a r o s c i l l a t i o n s , a l s o o f 2D c h a r a c t e r and a t t r i b u t a b l e t o t h e 2DEG i n t h e a c c u m u l a t i o n l a y e r o f t h e b u f f e r c o n t a c t , a r e o b s e r v e d a t h i g h e r b i a s ( > 1.20 V). Assuming t h a t t h e a p p l i e d v o l t a g e V i s a s s o c i a t e d o n l y w i t h t h i s n e g a t i v e s p a c e c h a r g e i n t h e a c c u m u l a t i o n l a y e r ( g i v e n by B ) and w i t h t h e p o s i t i v e s p a c e c h a r g e due t o i o n i s e d d o n o r s (N t ) i n t h e d e p l e t i o n l a y e r o f f t h e t o p c o n t a c t , i t i s e a s y t o show t h a t f o r a s u f ? l c i e n t l y accumulated l a y e rHere R i s t h e s t a n d - o f f d i s t a n c e from t h e b a r r i e r o f t h e 2DEG i n t h e a c c u m u l a t i o n l a y e r . A t e r m f o r t h e c h a r g e i n t h e w e l l can a l s o b e i n c l u d e d . When comparing t h e Bf i n F i g u r e 2b w i t h t h e I ( V ) i n F i g u r e 2 a , t h e r e g i o n o f b i s t a b i l i t y abcd i s p a r t i c u l a r l y n o t e w o r t h y . The B v a l u e s ( 0 ) i n t h e low c u r r e n t r e g i o n cd a r e s i g n i f i c a n t l y l a r g e r t h a n t h o s e ( 1 0 T , marked f x) o f t h e dominant o s c i l l a t o r y s e r i e s i n t h e h i g h c u r r e n t mode. T h i s i n d i c a t e s t h a t i n t h e h i g h c u r r e n t ( a b ) mode t h e b u f f e r r e g i o n a d j a c e n t t o t h e b a r r i e r i s l e s s s t r o n g l y accumulated t h a n i n t h e low c u r r e n t mode ( c d ) . These o b s e r v a t i o n s o f f e r s u p p o r t t o t h e model f o r t h e b i s t a b i l i t y g i v e n i n [ 3 1 . A t t h e s h a r p t r a n s i t i o n b*, n e g a t i v e s p a c e c h a r g e i s r e l e a s e d from t h e w e l l , t h u s i n c r e a s i n g a c c u m u l a t i o n i n t h e b u f f e r l a y e r and g i v i n g r i s e t o t h e o b s e r v e d i n c r e a s e i n Bf i n t h e low c u r r e n t mode a t c . Note i n t h e low c u r r e n t mode c+d, t h e a c c u m u l a t i o n r e m a i n s a t t h i s h i g h e r l e v e l ,
i n d i c a t i n g a much lower l e v e l of e l e c t r o n s p a c e c h a r g e i n t h e w e l l . E q u a t i o n ( 1 ) g i v e s a r e a s o n a b l e f i t t o t h e o s c i l l a t i o n s a s s o c i a t e d w i t h t h e a c c u m u l a t e d 2DEG ( d e n o t e d by o i n F i g u r e 2b) f o r v o l t a g e above -400 mV. The f i t i s p a r t i c u l a r l y good f o r t h e o s c i l l a t i o n s above 1.20 V , which we a l s o a t t r i b u t e t o Landau l e v e l e f f e c t s i n t h e a c c u m u l a t i o n l a y e r . We s p e c u l a t e t h a t t h e s e c o ~ t d s e r i e s w i t h B = 16 T i n t h e h i g h c u r r e n t mode may b e r e l a t e d t o Landau l e v e l s o f t h e E l subband i n t h e w e l l . f
A t i n t e r m e d i a t e v o l t a g e s (660-860 m ~ ) , n e a r t h e p r i n c i p a l peak i n t h e c u r r e n t , a n a d d i t i o n a l s e r i e s o f o s c i l l a t i o n s i s o b s e r v e d w i t h v e r y h i g h Bf
=
70-80 T, a sC5-292
JOURNALDE
PHYSIQUEshown i n F i g u r e 2b ( i n d i c a t e d by 0 ) . These a l s o h a v e two-dimensional c h a r a c t e r . They c a n n o t b e e x p l a i n e d i n t e r m s o f Landau l e v e l s t r u c t u r e p a s s i n g t h r o u g h a n e q u i l i b r a t e d Fermi e n e r g y s i n c e t h e a s s o c i a t e d e l e c t r o n s h e e t d e n s i t y would g i v e r i s e t o e l e c t r i c f i e l d s much t o o l a r g e t o b e c o n s i s t e n t w i t h t h e a p p l i e d v o l t a g e . We a t t r i b u t e t h e peak i n t h e c u r r e n t a t 880 mV t o r e s o n a n t t u n n e l l i n g from t h e bound s t a t e o f t h e a c c u m u l a t i o n l a y e r E i n t o t h e second bound s t a t e o f t h e w e l l E ( s e e F i g u r e 3 ) . A t t h i s r e s o n a n c e (E2 = Eo) a n a p p r e c i a b l e d e n s i t y o f
2
e l e c t r o n s b u i l d s up i n t h e w e l l . T h i s i s e v i d e n t from t h e f l a t t e n i n g o f f of t h e B - v a l u e s ( 1 7 T ) o f t h e a c c u m u l a t i o n l a y e r s e r i e s ( 0 ) between 600 and 740 meV. We
f
t h e r e b y e s t i m a t e t h a t t h e e l e c t r o n d e n s i t y i n t h e w e l l b u i l d s up t o around I x 1 0 " cm-2 a t t h e 880 mV peak. The r e s o n a n t s t a t e E2 i s o v e r l a p p e d i n e n e r g y by t h e h i g h e r n Landau l e v e l s o f t h e lower w e l l s t a t e E l . I n t e r s u b b a n d s c a t t e r i n g p r o c e s s e s ( i m p u r i t i e s , e l e c t r o n - e l e c t r o n , phonons) c a n t r a n s f e r e l e c t r o n s o u t o f t h e E r e s o n a n c e i n t o t h e s e Landau s t a t e s . F o r e l a s t i c t u n n e l l i n g and ( q u a s i - )
2
e l a s t i c s c a t t e r i n g p r o c e s s e s , s c a t t e r i n g w i l l b e a maximum when
T h i s g i v e s r i s e t o o s c i l l a t o r y s t r u c t u r e (2D i n n a t u r e ) w i t h Bf 5 ( E ~ - El)m*/fie.
A s i m p l e e s t i m a t e o f (E
-
E l ) from t h e p o t e n t i a l p r o f i l e i n t h e d e v i c e ( s e e F i g u r e 3 ) a t a n a p p l i e d v o l t a g g a r o u n d 0 . 8 V g i v e s a v a l u e o f Bf = 90 T , i n r e a s o n a b l e agreement w i t h t h a t o b s e r v e d . Note t h a t a s V i s i n c r e a s e d , Eo-
E l w i l l a l s o i n c r e a s e , b u t r a t h e r s l o w l y , a s i s o b s e r v e d , due t o t h e b u i l d - u p o f n e g a t i v e c h a r g e i n t h e w e l l . A p r o c e s s of t h e t y p e p r o p o s e d would m o d u l a t e t h e c u r r e n t a s a f u n c t i o n o f B s i n c e t r a n s f e r o f e l e c t r o n s from E2 t o E l m o d i f i e s t h e t u n n e l l i n g p r o b a b i l i t y and e l e c t r o n s p a c e c h a r g e d e n s i t y i n t h e w e l l . To c o n c l u d e t h i s s e c t i o n , i t i s w o r t h n o t i n g t h a t t h e dI/dV c u r v e shows a weak f e a t u r e around 0 . 6 V due p o s s i b l y t o t h e r e s o n a n c e between E2 and t h e c o n d u c t i o n band edge o f t h e b u f f e r c o n t a c t .Our o b s e r v a t i o n o f t h e h i g h Bf s e r i e s and i t s i n t e r p e t a t i o n imply t h a t t h e t u n n e l c u r r e n t n e a r t h e E - E2 r e s o n a n c e i n v o l v e s , a t l e a s t p a r t i a l l y , e l e c t r o n s which a r e u n d e r g o i n g i n t e g s u b b a n d s c a t t e r i n g i n t h e w e l l , i . e . t h e y a r e t u n n e l l i n g
s e q u e n t i a l l y [ 6 , 7 1 . The mechanism which we p r o p o s e i s q u i t e d i s t i n c t from t h e one d e s c r i b e d i n r e f e r e n c e [ 5 ] which r e l a t e s t o b u l k (3D) e l e c t r o n s t u n n e l l i n g i n t o E l i n t h e p r e s e n c e o f a q u a n t i s i n g
~1 1.J
and. f o r which Landau l e v e l number n andk
a r e c o n s e r v e d .F i n a l l y , we n o t e b r i e f l y t h a t f o r
g J ,
t h e NDC i s p a r t i a l l y s u p p r e s s e d a t h i g h m a g n e t i c f i e l d s (B>
5 T ) . I n a d d i t i o n , t h e r e s o n a n t s t r u c t u r e s h i f t s t o h i g h e r v o l t a g e . Both f e a t u r e s c a n be e x p l a i n e d by i n c l u d i n g t h e e f f e c t o f t h e m a g n e t i c v e c t o r p o t e n t i a l on t h e e l e c t r o n H a m i l t o n i a n .T h i s work i s s u p p o r t e d by SERC and CNRS. E. A l v e s i s s u p p o r t e d by CNPq, B r a z i l . We acknowledge u s e f u l d i s c u s s i o n s w i t h K . Chan, J . R . F l e t c h e r , F.W. S h e a r d , G . A . Toombs and K.W.H. S t e v e n s .
P.E. Simmonds: permanent a d d r e s s , U n i v e r s i t y o f Wollongong, N.S.W., A u s t r a l i a . ' References
[ I ] S o l l n e r T.C.L.G. e t a l ; Appl. Phys. L e t t .
5
(1984) 1319.[21 Shewchuk T . J . e t a l ; Appl. Phys. L e t t .
47
( 1 9 8 5 ) 986.E31 Goldman V . J . , T s u i D . C . and Cunningham J . E . ; Phys. Rev. L e t t .
58
(1987) 1256.C41 Mendez E.E. e t a l ; Phys. Rev. B
33
( 1 9 8 6 ) 7368.[ 5 ] Mendez E.E., E s a k i L. and Wang W . I . ; P h y s . Rev. B
33
(1986) 2893.[61 Weil T. and V i n t e r B.; Appl. Phys. L e t t . 50 ( 1 9 8 7 ) 1281.
[ 7 1 L u r y i S.; Appl. Phys. L e t t .
47
( 1 9 8 5 ) 4 9 0 7C81 R i c c o B . and Azbel M.Ya.; Phys. Rev. B
2
(1984) 1970.[ 9 ] Eaves L. e t a l ; P r o c . 1 8 t h I n t . Conf. on P h y s i c s o f S e m i c o n d u c t o r s , Stockholm (World S c i e n t i f i c ) (1986) 1615.
