ASSESSMENT OF A MULTIPLE QUANTUM WELL AND A SUPERLATTICE STRUCTURE BY SPECTROSCOPIC ELLIPSOMETRY, ELECTROREFLECTANCE AND PHOTOREFLECTANCE MODULATION SPECTROSCOPY

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ASSESSMENT OF A MULTIPLE QUANTUM WELL AND A SUPERLATTICE STRUCTURE BY

SPECTROSCOPIC ELLIPSOMETRY, ELECTROREFLECTANCE AND

PHOTOREFLECTANCE MODULATION SPECTROSCOPY

M. Erman, C. Alibert, J. Cavaillès, P. Frijlink, C. Bouche

To cite this version:

M. Erman, C. Alibert, J. Cavaillès, P. Frijlink, C. Bouche. ASSESSMENT OF A MULTIPLE QUAN- TUM WELL AND A SUPERLATTICE STRUCTURE BY SPECTROSCOPIC ELLIPSOMETRY, ELECTROREFLECTANCE AND PHOTOREFLECTANCE MODULATION SPECTROSCOPY.

Journal de Physique Colloques, 1987, 48 (C5), pp.C5-139-C5-142. �10.1051/jphyscol:1987526�. �jpa-

00226730�

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JOURNAL DE PHYSIQUE

Colloque C5, supplement au n°ll, Tome 48, novembre 1987 C5-139

ASSESSMENT OF A MULTIPLE QUANTUM WELL AND A SUPERLATTICE STRUCTURE BY SPECTROSCOPIC ELLIPSOMETRY, ELECTROREFLECTANCE AND PHOTOREFLECTANCE MODULATION SPECTROSCOPY

M. ERMAN, C. ALIBERT*, J . A . CAVAILLES, P . FRIJLINK a n d C. BOUCHE*

Laboratoires d'Electronique et de Physique Appliquée

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, 3, Avenue Descartes, F-94451 Limeil-Brévannes Cedex, France

* Université des Sciences et Techniques du Languedoc, Equipe Micro-Opto-Electronique, Place Eugène Bataillon,

F-34060 Montpellier Cedex, France

Résimé U t i l i s a n t V ellipsométrie spectroscopique, V é l e c t r o - réflexion et la photoréflexion, nous avons analysé une structure multicouche GaAlAs/GaAs. L'échantillon présente un gradient d'épaisseur parallèlement au diamètre de la plaquette. Grâce à cette non-uniformité, nous avons pu observer l ' é v o l u t i o n continue entre une structure type puits quantiques multiples et type super-réseau.

Abstract A GaAlAs/GaAs m u l t i p l e quantum w e l l s t r u c t u r e e x h i b i t i n g a thickness g r a d i e n t over the wafer surface has been analyzed using

spectroscopic e l l i p s o m e t r y , e l e c t r o r e f l e c t a n c e and p h o t o r e f l e c t a n c e . Due t o the sample non u n i f o r m i t y , we have been able t o observe the continuous t r a n s i t i o n from m u l t i p l e quantum well regime t o s u p e r l a t t i c e regime.

Introduction A GaAlAs/GaAs m u l t i p l e quantum well (MQW) s t r u c t u r e grown by organometallic chemical vapor phase e p i t a x y has been analyzed using

spectroscopic e l l i p s o m e t r y (SE), e l e c t r o r e f l e c t a n c e (ER) and p h o t o r e f l e c t a n c e (PR). The sample, c o n s i s t i n g o f 25 p e r i o d s o f GaAlAs and GaAs l a y e r s (Figure 1 ) , has been grown on h a l f o f a two inches n+ doped GaAs s u b s t r a t e . The growth c o n d i t i o n s have been s e t so t h a t the sample e x h i b i t s a thickness g r a d i e n t along the diameter o f the wafer, w h i l e the GaAlAs/GaAs i n t e r f a c e s are s t i l l sharp w i t h i n one monolayer. The purpose o f our study i s t o use t h i s thickness non u n i f o r m i t y t o observe the e v o l u t i o n o f the spectra vs quantum well and GaAlAs b a r r i e r t h i c k n e s s e s . As the b a r r i e r thickness decreases t h e sample should a l l o w f o r the observation o f the c o u p l i n g between quantum w e l l s . Ellipsowetry The sample has been f i r s t analyzed using high l a t e r a l l y resolved e l l i p s o m e t r y . This technique determines the r a t i o between the

r e f l e c t i o n c o e f f i c i e n t o f l i g h t p o l a r i z e d p a r a l l e l ( r p ) and perpendicular ( rs) to the plane o f i n c i d e n c e . The a c t u a l l y measured parameters are tant'VjJJand cos (/ft) wich are r e l a t e d t o p by :

The sample has been scanned i n energy from 1.4 eV t o 4 eV and i n p o s i t i o n over a 46 m\ l i n e . Figure 2 shows one t y p i c a l experimental spectrum . We have used such spectra f o r both analyzing the m u l t i l a y e r s t r u c t u r e o f the sample ( t h i c k n e s s e s , Al composition and i n t e r f a c e q u a l i t y ) and measuring the energy p o s i t i o n o f v a r i o u s e x c i t o n i c features o f the MQW. As observed on Figure 2 , experimental spectra e x h i b i t two regions o f i n t e r e s t :

- a t low energies (E < 2.3 eV) i n t e r f e r e n c e s due t o the t o t a l thickness o f the MQW are observed.

- a t high energies (2.8 < E <.3 eV) the absorption o f both GaAs and GaAlAs i s h i g h and only a few l a y e r s c l o s e t o t h e surface c o n t r i b u t e t o

*LEP : A member of the Philips Research organization

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987526

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C5-140 JOURNAL D E PHYSIQUE

o p t i c a l spectra ( 1 ) . M u l t i l a y e r m o d e l l i n g o f t h i s p a r t o f t h e spectra a l l o w s for t h e determination o f the A1 concentration, t h e l o c a l i s a t i o n energy o f the E l t r a n s i t i o n and t h e thicknesses of the t o p l a y e r s (2). I n p a r t i c u l a r , we found t h e r a t i o d2/d3 t o be 1.025 and the A1 c o n c e n t r a t i o n 64 %. These two parameters do n o t v a r y over t h e wafer. The v a r i a t i o n o f t h e quantum w e l l t h i c k n e s s v s t h e p o s i t i o n X i s r e p o r t e d on F i g u r e 1.

The e x c i t o n i c t r a n s i t i o n s associated w i t h each sub-band are r e s p o n s i b l e f o r sharp s t r u c t u r e s which i n SE spectra appear as p e r t u r b a t i o n s o f t h e low energy i n t e r f e r e n c e p a t t e r n s . We have detennined the energy of these t r a n s i t i o n s c o n s i d e r i n g the f i r s t d e r i v a t i v e o f t h e e f f e c t i v e d i e l e c t r i c f u n c t i o n 1 3 ) .

F i g u r e 3 summarizes our r e s u l t s . The f i r s t t h r e e heavy h o l e - e l e c t r o n t r a n s i t i o n s ( l a b e l e d 1,2 and 3 ) a r e observe when t h e quantum

1

w 1 1 thickness i s l a r g e enough. For a b a r r i e r w i d t h o f 30

,

the e l e c t r o n and t h e h o l e wave f u n c t i o n s o v e r l a p and the l e v e l s 1 and 1' ( i .e. 1 ig h t

h o l e - e l e c t r o n t r a n s i t i o n ) s p l i t t o form a sub-band

.

The l o w e s t energy l e v e l s correspond t o a symnetric wave f u n c t i o n ( 1 and 1 ' ~ ) and t h e h i g h e s t energy l e v e l ( i . e . the upper edge o f the sub-band! t o t h e a n t i - s y m n e t r i c wave

f u n c t i o n ( l a and 1

'

,)

.

The t h e o r e t i c a l curves on f i g u r e 3 have been c a l c u l a t e d using t h e c l a s s i c a l Kronig-Penney model ( 3 ) . A good agreement between our experimental data and t h e model i s found f o r a 60 % conduction band offset.

P h o t o r e f l e c t a n c e and e l e c t r o r e f l e c t a n c e

ER and PR have been proven t o be v e r y u s e f u l f o r t h e study o f QW s t r u c t u r e s (2,4). I n PR t h e modulation i s provided by a chopped beam (HeNe 1mW l a s e r ) w h i l e i n ER t h e e l e c t r i c f i e l d i n t h e s t r u c t u r e i s modulated v i a a t r a n s p a r e n t electrode. One o f the advantages o f PR over ER i s t h a t i t i s c o n t a c t l e s s . On t h e o t h e r hand, i n PR, complicated modulation mechanisms are associated w i t h c a r r i e r - i n j e c t i o n and are s t i l l n o t c a n p l e t e l y understood.

F i g u r e 4 shows two t y p i c a l Pr! s!,

a

c t r a : tile t o p spectrum corresponds t o t h e MQW p a r t o f the sanple ( d 2 = 77 ) w h i l e t h e bottom one has been recorded on t h e s u p e r l a t t i c e r e ion. The arrows i n d i c a t e the t r a n s i t i o n s already observed and assigned i n

SE

?see Fig. 3 ) . The agreement between SE and

PR

i s v e r y good. However, more and b e t t e r r e s o l v e d s t r u c t u r e s can be seen on PR spectra. The s t r u c t u r e s a t 1.42 eV can be associated t o GaAs ( b o t h c l a d d i n g 1 ayer and s u b s t r a t e )

.

The o t h e r 1 i nes correspond t o exci t o n i c t r a n s i t i o n s and c l e a r l y s h i f t w i t h p o s i t i o n on the sanple ( i .e. w i t h t h e quantum w e l l and b a r r i e r t h i c k n e s s ) . The s t r o n g l i n e a t 1.96 eV i s the s c a t t e r e d HeNe l i g h t . On the bottom GaAs 1 ine, Franz-Keldysh o s c i l l a t i o n s (FKO) i n d i c a t e a s t r o n g b u i l t - i n e l e c t r i c f i e 1 d. This complicates the lineshape a n a l y s i s and makes the c l a s s i c a l " t h i r d d e r i v a t i v e " curve f i t t i n g inadequate ( 5 ) .

F i g u r e 5 shows ER spectra recorded a t t h e same p o s i t i o n s as previous PR spectra. The e l e c t r i c f i e l d was modulated by v a r y i n g t h e v o l t a g e between -25 and OV. A -25 V b i a s corresponds t o l o w e l e c t r i c f i e l d whereas a s t r o n g b u i l t - i n e l e c t r i c f i e 1 8 i s present a t zero b i a s (see above). The spectrum obtained f o r d2 = 77A ( F i g . 5a) compares w e l l w i t h the PR one and t h e SE data. However, ER spectra have been recorded up t o 3.2 eV a l l o w i n g f o r t h e observation o f Ga.36Al .6 AS Eo s t r u c t u r e as w e l l as t h e E l t r a n s i t i o n o f t h e

QY. The second spectrum 462 = 18.7A) e x h i b i t s sane d i f f e r e n c e s w i t h PR. The o r i g i n o f these discrepancies can o n l y be undestood by comparing spectra obtained a t . d i f f e r e n t p o s i t i o n s and by v a r y i n g t h e s t a t i c canponent of the e l e c t r i c f i e l d i n t h e s t r u c t u r e ( i . e . changing t h e o f f s e t i n t h e a p p l i e d voltage)

.

Corresponding r e s u l t s w i l l be pub1 i shed e l sewhere

.

References :

1/ M. Erman, J.B. Theeten, N. Vodjdani and Y. Demay, J. Vac. Sci. Technol. B, (1983) 328

2/ M. Erman, J.B. Theeten, P. F r i j l i n k , S. G a i l l a r d , Fan J i a Hia and C. A l i b e r t , J. Appl. Phys. 56, (1984) 3241

3 / M. Erman, C. A l i b e r t , J.B. Theeten, P. F r i j l i n k and B. Catte, t o be p u b l i s h e d i n J. Appl

.

^Phys.

4 1 0.5. Glembocki, B.V. Shanabrook, N. Bottka, W.T. Beard and J. Comas, SPIE Vol. 524, (1985) 86

5/ P. Parayanthal, H. Shen, F.H. Pollak, O.J. Glembocki, B.V. Shenabrook and W.T. Beard, Appl. Phys. L e t t . 48, (1986) 1261

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axlde

-

m I A s

-

G A S

-

GOALAS

-

GaAs

-

G a A l A s

-

G a A r

-

G a A r

F i g u r e l : Schematic r e p r e s e n t a t i o n o f the sample (a) and t h e quantum w e l l t h i c k n e s s c@ v s p o s i t i o n X along t h e s t r o n g e s t thickness g r a d i e n t l i n e , as deduced fran SE a n a l y s i s ( b ) .

Figure2: One example o f masured SE spectra. Dotted l i n e s represent a m u l t i l a y e r model f i t o f t h e i n t e r f e r e n c e p a t t e r n s a t low energy (1.2

-

^2.2eV). ^Weak s t r u c t u r e s , superimposed t o i n t e r f e r e n c e regime are due t o the o p t i c a l t r a n s i t i o n s i n t h e MQW. The El and E l + a l s t r u c t u r e s o f GaAs and Ga. 36Al.64As a r e observed a t h i g h e r energies.

Figure3: Energy o f the

o p t i c a l t r a n s i t i o n s as a f u n c t i o n o f t h e quantum well thickness d2 f o r 60% ( f u l l l i n e ) and 85% (dashed l i n e ) . conduction band o f f s e t . Dots i n d i c a t e experfmentall y measured s t r u c t u r e s .

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JOURNAL DE PHY SlQUE ENERGY (eV)

Figure 4 :

Photoreflectance sp c t r a record d a t positions where the well thickness i s

77E ( a ) and 191 ( b ) . Arrows