MEASUREMENT OF ELECTRON AND HOLE MOBILITIES PERPENDICULAR TO THE INTERFACE OF GaAs/GaAlAs SUPERLATTICES

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MEASUREMENT OF ELECTRON AND HOLE MOBILITIES PERPENDICULAR TO THE

INTERFACE OF GaAs/GaAlAs SUPERLATTICES

H. Le Person, C. Minot, F. Alexandre, J. Palmier

To cite this version:

H. Le Person, C. Minot, F. Alexandre, J. Palmier. MEASUREMENT OF ELECTRON AND HOLE MOBILITIES PERPENDICULAR TO THE INTERFACE OF GaAs/GaAlAs SUPERLATTICES.

Journal de Physique Colloques, 1987, 48 (C5), pp.C5-467-C5-470. �10.1051/jphyscol:1987598�. �jpa-

00226802�

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JOURNAL DE PHYSIQUE

Colloque C5, suppl6ment au nO1l, Tome 48, novembre 1987

MEASUREMENT OF ELECTRON AND HOLE MOBILITIES PERPENDICULAR TO THE INTERFACE OF GaAs/GaAlAs SUPERLATTICES

H. LE PERSON, C. MINOT, F. ALEXANDRE and J.F. PALMIER Centre N a t i o n a l d l E t u d e s d e s M 1 e c o m m u n i c a t i o n s , 1 9 6 Avenue Henri R a v e r a , F-92220 B a g n e u x , F r a n c e

R6sum6

:

I1 e s t rapport6 une s 6 r i e de m s u r e s de l a m b i l i t 6 des Qlec- trons e t des trous, perpendiculaire aux plans des couches dans des super- rPseaux GaAs/GaAlAs ayant des epaisseurs de barriGres de 24 a e t

67

a.

L8interpr6tation des r 6 s u l t a t s expirimentaux, obtenus par une technique de temps de vol, s'appuie sur un modCle u t i l i s a n t l ' i q u a t i o n de Poisson couplge aux Pquations de dPrive-diffusion. Les valeurs sont en accord avec une conduction de type minibande dans l e cas des f a i b l e s Ppaisseurs de ?arriPres

;

pour des grandes barriGres, plusieurs me'canismes sont suggeres .

Abstract

:

W e report on measurements of the electron and hole mobilities perpendicular t o interfaces of GaAs/GaAlAs superlattices. The super- l a t t i c e s are i n i n t r i n s i c region of nin or pin s t r u c t u r e s and have two values of b a r r i e r thickness, 24

A

and

67 A.

The experimental r e s u l t s are associated to a numerical simulation using Poisson and drift-diffusion equations. The obtained values, for a small b a r r i e r , are in agreement with a conduction by miniband and, f o r a large b a r r i e r ( a t mid e l e c t r i c f i e 1 d) several e f f e c t s are involved.

I

-

INTRODUCTION :

I n i t i a t e d by Esaki and Tsu [ I ] , the studies of the super- l a t t i c e (SL) s t r u c t u r e s bring up a great i n t e r e s t by the fondamental concepts which they use, as well as the important applications which they suggest. In the case of perpendicular transport a t low f i e l d , two theoretical approachs have been developed.

A

f i r s t is a Bloch's theory 121 f o r type I regular SLs, the other, v a l i d when the s t a t e s are localized i s a hopping theory 131.

p - i - n """.,., A ~ ~ C D N ~

n' GavjAIo,As p' 3000 A I 10'crn-)l

Ga,,,AlonAs undoped 1000 A

4

{

^I ^! ^:^s*

barriers : 67A ubstrate GaAs n' I 10'cm" I

Fig. 1 : S c h e m a t i c view o f t h e n / S L / n a n d p/SL/n s t r u c t u r e s

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987598

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C5-468 JOURNAL DE PHYSIQUE

11

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EXPERIMENTS AND MODEL : The samples are grown by molecular beam e p i t a x y on ( 1 0 0 ) n : GaAs l a s e r q u a l i t y substrates. The GaAs/Ga -,AlxAs undoped SL(x = 0.27 o r 0.3) i s deposited a f t e r a nt GaAs b u f f e r layer.

he

growth i s terminated by a Gal-,AlXAs p f ( ~ e ) o r n + ( ~ i ) window l a y e r (WL) f o l l o w e d by a p+ o r n GaAs c o n t a c t l a y e r , r e s p e c t i v e l y . The d e t a i l e d s t r u c t u r e s

-

c h a r a c t e r i z e d by X rays, Auger and s p e c t r a l p h o t o c o n d u c t i v i t y measurements

-

o f t h e two s t u d i e d samples are i n d i

-

c a t e d i n Fig. 1. For the n + / ~ ~ / n + s t r u c t u r e s (nSLS) t h e SL i s embedded i n two l a y e r s , gradual i n A1 concentration, which a v o i d a d i s c o n t i n u i t y between the SL's f i r s t conduction miniband and the conduction band o f t h e c o n t a c t layers. The ohmic c o n t a c t s on e i t h e r s i d e o f t h e wafer are achieved by an AuGeNi deposit. A t e c h n o l o g i c a l process etches an o p t i c a l window i n t h e t o p GaAs l a y e r and separa- t e s each device by a mesa. The t y p i c a l device area i s 5.10-5 cm2 and the zero b i a s capacitances are 0.4 pF f o r t h e nSLS and 1.0 pF f o r t h e p + / ~ ~ / n + s t r u c t u r e

(pSLS),

t h i s l a t t e r decreases t o 0.43 pF a t 20V reverse p o l a r i s a t i o n . This l a s t p o i n t i n d i c a t e s t h a t the e l e c t r i c f i e l d i s n o t homogeneous i n the SL a t zero- bias. A f t e r cleavage o f the processed sample each device i s stuck on an alumina m i c r o s t r i p l i n e o f 12 GHz frequency bandwith (FBI. The whole i s mounted i n a super h i g h frequency (SHF) case and biased by a 18 GHz network Fig. 2. The photo- c u r r e n t i s iwasured by a 14 GHz sampling o s c i l l o s c o p e and averaged by a m u l t i

-

channel analyser. The s i g n a l time constant (RC=40 ps) i s determined by the FB measurement o f each e l e c t r o n i c component. A p y r i d i n e dye l a s e r synchronously pumped by a frequency doubled CW N ~ ~ + : Y A G l a s e r photoexcites the samples w i t h 1- 3 ps l i g h t pulses o f 100 MHz r e p e t i t i o n r a t e . The photon energy (1.75 eV) i s a l i t t l e below t h e WL and SL b a r r i e r bandgap (1.8 eV). The focused l a s e r beam spot has a diameter o f 14 on t h e device WL. The temporal photoresponses versus the a p p l i e d voltage f o r t h e nSLS and pSLS are compared t o those c a l c u l a t e d by the simulation. The t h e o r e t i c a l responses are convolued by t h e RC Fig. 3 and 4.

- -

b

t

-

w ⁵⁰ n

6

^"7

-

_a

-

> *

t 25 m Z

W m

D l-

o ^ZV

a

3

U

-

7

-25

Time (seconds)

a b

Fig. 3.a) Experimental photocurrent o f t h e n/SL/n versus time a t d i f f e r e n t ap- p l i e d voltages. P o s i t i v e b i a s corresponds t o + on the upper contact.

b ) C u r r e n t d e n s i t y (A/cmz) vs. t i m e o b t a i n e d by simulation. No i s the maximal photogenerated c a r r i e r density, i n the SL, a t the WL-SL i n t e r f a c e , a i s t h e SL absorption c o e f f i c i e n t .

The model t r e a t s the SL l i k e an e f f e c t i v e medium having macroscopic parameters 141 which are : conduction and valence band p o s i t i o n s , an e f f e c t i v e mobi- l i t y f o r t h e e l e c t r o n s and l i k e w i s e f o r t h e holes. It assumes the existence of quasi instantaneously thermalised populations and solves n u m e r i c a l l y

-

by f i n i t e d i f f e r e n c e algorithms

-

Poisson's and c l a s s i c a l d r i f t - d i f f u s i o n equations i n one dimension. The recombination r a t e i s described by the sum o f Shockley-Read and r a d i a t i v e term. The l i g h t generation term has a 3 ps FWHM l o r e n t z i a n shape. We i n t r o d u c e a r e s i d u a l p- doping Na (1014

-

1015 cm-3), i n t h e

st,

confirmed by

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I

I : :

TIME I sec 1

a b

F i g . 4 : Photocurrent density o f the p/SL/n versus time a t d i f f e r e n t a p p l i e d voltages, reverse bias. a) Experiment

-

b ) Simulation.

a i s the SL absorption c o e f f i c i e n t : 0.78 104 cm-1.

s t a t i c c u r r e n t - v o l t a g e and DLTS measurements 151. The h e t e r o j u n c t i o n s are n o t taken i n t o account.

The nSLS photocurrent ( p o s i t i v e on the upper c o n t a c t ) i s i n t e r p r e t e d as the sum o f d i f f e r e n t c o n t r i b u t i o n s 6, 71 :

i

-

an e l e c t r o n c u r r e n t resu t i n g from the time o f f l i g h t o f t h e primary charges created by the l i g h t . I t s d u r a t i o n i s t e = ( l / a ) ( l / , ~ e E ) = 4 0 p s , where E i s t h e e f f e c t i v e e l e c t r i c f i e l d , ~ e the e l e c t r o n n o b i l i t y , a t h e absorption c o e f f i - c i e n t (a = 1.6 104 cm-1).

-

t h e holes, which have a weaker m o b i l i t y , stay longer i n t h e SL r e g i o n and modify i t s n e u t r a l i t y . They give r i s e t o an e l e c t r o n i n j e c t i o n c u r r e n t which increases d u r i n g the time necessary f o r the holes t o go t o the SL-GaAs i n t e r f a c e , where they are locked by the valence band d i s c o n t i n u i t y ( i f the a p p l i e d e l e c t r i c f i e l d i s n o t t o o strong, V a p p l i e d <2.5 t o 3. V). This e l e c t r o n c u r r e n t decays w i t h a time constant t h a t i s the h o l e recombination time.

-

a h o l e current, always very weak, o f a d u r a t i o n equal t o : te(ph/ire).

For negative p o l a r i s a t i o n , t h e phenomenon i s roughly the same b u t the primary e l e c t r o n c u r r e n t i s weaker because the charges are created near the WL-SL zone where the e l e c t r i c f i e l d i s weak. The holes are trapped a t t h e WL-SL i n t e r f a c e . The e l e c t r o n i n j e c t i o n c u r r e n t decreases w i t h an e f f e c t i v e time constant g r e a t e r than f o r a p o s i v i t i v e bias.

111

-

RESULTS : The f i t , between experimental r e s u l t s and t h e modeling, w i t h t h e t h r e e a d j u s t a b l e parameters ~ e , ph and Na, enables us t o deduce t h e pe, ph and Na values t o 110

+

20 cm2V-1s-1, 12.5 i 2 cm2V-1s-1 and 1.5 1015 cm-3, r e s p e c t i v e l y . The e f f e c t i v e l i f e t i m e , used i n t h e equations, i s r e a d i l y obtained from the measurements (2.2 10-9 s).

The p i n s t r u c t u r e s , w i t h a SL i n i n t r i n s i c region, have been s t u d i e d by Capasso and a l . ( 8 1 f o r the couple AlInAs/GaInAs and Larsson and a l . 191 f o r GaAs/GaAlAs.

The Na concentration (2.5 1015 cm-31, f o r the pSLS, i s measured by DLTS. The f i t gives : ~ e = 10 and ,Jh = 15 ( f o r a SL o f 53

a -

Wells and 67

a -

B a r r i e r s ) , w i t h an e l e c t r i c f i e l d ( E l o f 4 104 Vcm-1. A t forward-bias

(E

l e s s than 104 Vcm-1) t h e t o t a l time response leads t o a lower m o b i l i t y value about 4 cm2V-1s-1 f i g . 4.

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C5-470 JOURNAL DE PHYSIQUE

I V

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CONCLUSION : The agreement between experiment and s i m u l a t i o n i s good f o r b o t h s t u d i e d s t r u c t u r e s as l o n g as the e l e c t r i c f i e l d i s n o t too s t r o n g ((4. 10-4 Vcm-1). The h i g h m o b i l i t y values, f o r t h e SL having small b a r r i e r (24 A), g r e a t e r than those c a l c u l a t e d i n hopping conduction regime, are i n f a v o r t o miniband conduction and agree w i t h t h e previous r e s u l t s obtained i n t r a n s i s t o r s t r u c t u r e s 1101. The e f f e c t i v e h o l e m o b i l i t y i n t e g r a t e s the c o n t r i b u t i o n s o f the heavy and l i g h t holes which have very d i f f e r e n t m o b i l i t i e s i n Bloch's regime. I n t h e case o f t h i c k b a r r i e r s (67 A ) , t h e c a r r i e r s are more l o c a l i z e d and we expect a conduction o f phonon-assisted t u n n e l i n g type (hopping). The measured m o b i l i t y values f o r mid e l e c t r i c f i e l d s ( 1 t o 4 104 Vcm'l) i n d i c a t e t h a t o t h e r e f f e c t s t a k e place, such as h o t c a r r i e r e f f e c t s and thermoionic emission y i e l d i n g a conduction above t h e b a r r i e r s .

REFERENCES

1

-

L. Esaki and R. Tsu, IBM. Res. Dev. 14, 61-65 (1970)

2

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J.F. Palmier and A. Chomette, Journal de Physique 43, 381 (1982)

3

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D. Calecki, J.F. Palmier and A. Chomette, J. o f Physics C 17, 5017 (1984) 4

-

J.F. Palmier, Proceedings o f the Winter School Les Houches, FRANCE, 21

(1985) Springer-Verlag

5

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^A. S i b i l l e , J.F. Palmier and Le Person, Defects i n Semiconductors, M a t e r i a l s Science Forum 10-12, 193-198 (1986)

6

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^C. Minot, H. Le Person, F. Alexandre and J.F. Palmier, Physica 134 B, 514

(1985) ^,

7

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^3.F. ^Palmier, ^H. Le Person, C. Minot, A. S i b i l j e and F. Alexandre, Proc. 18 t h I n t . Conf. Phys. Semiconductors, Stockholm, 235 (1986)

8

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F. Capasso, K. Mohammed and A.Y. Cho, IEEE-QE, 22, 1853 (1986)

9

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A. Larsson, A. Yariv, R. T e l l , J. Maserjian and S.T. Eng. Appl

.

Phys. L e t t . 47 (81, 866-868 (1985)

10

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^J.F. ^Palmier, ^C. Minot, J.L. LiGvin, F. Alexandre, J.C. Harmand, J. Dangia, C. Dubon-Chevallier and D. Ankri, Appl. Phys. L e t t . 49 (191, 1260-1262

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