INFLUENCE OF GROWTH CONDITIONS AND OF ALLOY COMPOSITION ON ELECTRICAL AND OPTICAL PROPERTIES OF MBE AlxGa1-xAs (0.2 = x = 0.4)

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INFLUENCE OF GROWTH CONDITIONS AND OF ALLOY COMPOSITION ON ELECTRICAL AND OPTICAL PROPERTIES OF MBE AlxGa1-xAs (0.2 =

x = 0.4)

H. Künzel, H. Jung, E. Schubert, K. Ploog

To cite this version:

H. Künzel, H. Jung, E. Schubert, K. Ploog. INFLUENCE OF GROWTH CONDITIONS AND OF ALLOY COMPOSITION ON ELECTRICAL AND OPTICAL PROPERTIES OF MBE AlxGa1-xAs (0.2 = x = 0.4). Journal de Physique Colloques, 1982, 43 (C5), pp.C5-175-C5-182.

�10.1051/jphyscol:1982521�. �jpa-00222240�

JOURNAL DE PHYSIQUE

Co Z Zoque CS, suppl4ment au n '22, Tome 43, ddeembre 1982 page C5-175

INFLUENCE OF GROWTH CONDITIONS AND OF ALLOY COMPOSITION ON ELECTRICAL AND OPTICAL PROPERTIES OF MBE A1xGal,xAs

(0.2 = x = 0 . 4 )

H. Kiinzel, H.

Jung,

E. Schubert and K. Ploog

Max-PZanek-Institut fiir Festkijrperforschung, Stuttgart, F. R. G .

Resume. - Des mesures ti 2K de photoluminescence (PL), e f f e t Hall, admittance e t DLTS ont &t@ u t i l i s e e s pour etudier en d e t a i l l ' i n f l u e n c e des conditions de croissance e t de l a composition de l ' a l l i a g e sur l e s propriet6s electriques e t optiques de couches, nominalement non dopees e t dopees n au silicium, AlxGal-xAs (0,15

<

x

<

0,45), dont l a croissance a et@ real i s e e par epi t a x i e par j e t s mol6culaires (MBE). Sur t a u t l e domai ne de composition etudi6 de 1 'a1 1 iage, 1 ' i n t e n s i t e

PL

observee depend de facon c r i t i q u e de l a tempgrature de croissance. Les principales propri6tes des porteurs minoritaires ne peuvent Otre obtenues que pour des temp6ratures du s u b s t r a t au-delti de 640°C e t une reconstruction de surface lggerement s t a b i l i s e e arsenic (2x4). Des compositions de 1 'a l l i a g e proches de l a t r a n s i t i o n d i r e c t - i n d i r e c t conduisent ti un 61 argissement considerabl e des t r a n s i t i o n s PL. Simul tanement, la separation d'energie des t r a n s i t i o n s e n t r e accepteur associe e t exciton augmente

ii

cause d'un niveau donneur devenu plus profond. Les mesures d'admittance conduisent directement l ' e n e r g i e d ' i o n i s a t i o n du niveau donneur superficiel e t montrent l ' e x i s t e n c e domi- nante d'un pisge ti electrons dans n-AlxGal-xAs:Si, &labor6 par MBE. I1 a e t 6 trouve que l a concentration de ce piege c r o i t de facon consid6rable avec l a proportion d'Al.

Dans n-AlxGal-xAs:Si obtenu par MBE avec x

<

0,25, l a concentration du piege ti elec- trons e s t infgrieure au s e u i l de detection des mesures d'admittance. Des mesures DLTS complementaires montrent que l a concentration du piege 5 electrons pour des a l l i a g e s obtenus par MBE peut maintenant regulierement 6 t r e a j u s t e e au pour cent de l a concentration de dopage desiree.

Abstract. - 2K photoluminescence (PL), Hall e f f e c t , admittance and

DLTS

measure- ments were used t o study i n d e t a i l the influence of growth conditions and of a l l o y composition on the e l e c t r i c a l and optical properties of nominally undoped and of Si-doped n-AlxGal-xAs (0.15

<

x

^<

0.45) layers grown by molecular beam epitaxy (HBE).

Over the e n t i r e ranga of a l l o y composition investigated, the observed

PL

i n t e n s i t y depends c r i t i c a l l y on t h e growth temperature. Superior minority c a r r i e r properties can be achieved only a t substrate temperatures above 640% and s l i g h t l y As-stabi- 1 ized (2x4) surface reconstruction. A1 loy compositions close t o t h e direct-in- d i r e c t cross-over lead t o a considerable broadening of the PL t r a n s i t i o n s . Simul- taneously t h e energy separation between the acceptor associated and bound exciton t r a n s i t i o n s increases due t o the deepening of t h e donor level. The admittance measurements d i r e c t l y yielded the bindina energy of t h e shallow donor level and showeq the existence of one dominant electron t r a p i n I4BE n-Al,Gal,,As:Si. The t r a p concentration was found t o increase considerably with enhanced A1 content. In

:!BE

n-Alxl;al-xAs:Si with

x <

0.25, t h e t r a p concentration i s below the detection 1 im i t of the admittance technique. Additional DLTS measurements demonstrate t h a t t h e t r a p concentration i n

:1BE

grown a l l o y material i s routinely on the order of

of the intentional doping concentration.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982521

JOURNAL DE PHYSIQUE

1. I n t r o d u c t i o n .

-

The method o f molecular beam e p i t a x y (!IBE) makes f e a s i b l e t h e design o f novel e l e c t r i c a l and o p t i c a l devices based on s u p e r l a t t i c e s t r u c t u r e s i n - c l u d i n g mu1 t i p l e quantum w e l l (YObI) l a s e r s /1/ and h i g h e l e c t r o n m o b i l i t y t r a n s i s t o r s (HEMT) /2/. T h i s a r i s e s from t h e h i g h degree o f dimensional, compositional, and doping c o n t r o l i n t h e d i r e c t i o n o f l a y e r growth t h a t has now been achieved by t h i s growth technique down t o submicron dimensions. I n these new devices t h e t e r n a r y a l l o y AlxGal-xAs i s w i d e l y used as a h i g h e r band gap m a t e r i a l w i t h 0 . 1 5 < x < 0 . 3 0 i n HEIlT s t r u c t u r e s and w i t h 0 . 3 < x < 0.4 i n standard double h e t e r o s t r u c t u r e (DH) as w e l l as MQI.1 l a s e r s t r u c t u r e s . For t h i s study we have performed d e t a i l e d low-tempe- r a t u r e photoluminescence (PL), Hal 1 e f f e c t , admittance and DLTS measurements t o evaluate t h e e l e c t r i c a l and o p t i c a l p r o p e r t i e s o f n o m i n a l l y undoped and o f S i - doped n-type AlxGal,xAs (0.15 < x c 0.45) l a y e r s grown by 1BE. The r e s u l t s have been used t o o p t i m i z e t h e growth c o n d i t i o n s f o r t h i s important t e r n a r y a l l o y m a t e r i a l . 2. Experimental.

-

The AlxGal-xAs f i l m s were grown i n a WBE system o f t h e horizon- t a l evaporation t y p e which was equipped w i t h a sample t r a n s f e r device, a r o t a t i n g s u b s t r a t e h o l d e r arid an extended LN2 cryopanel e n c i r c l i n g t h e whcle growth area.

The e p i t a x i a l a l l o y m a t e r i a l w i t h a thickness o f 2

-

³

urn

was deposited w i t h o u t any b u f f e r l a y e r simultaneously onto (100) o r i e n t e d Cr-doped S I and h e a v i l y S i doped nt-GaAs s u b s t r a t e wafers soldered s i d e by s i d e w i t h l i q u i d I n on t h e Mo mounting and t r a n s f e r p l a t e . The temperature o f t h e Ga e f f u s i o n c e l l was k e p t constant r e s u l t i n g i n a growth r a t e o f 1.0 um/hr f o r GaAs when deposited a t 580cC.

The temperatures o f t h e A1 and S i e f f u s i o n c e l l s were s y s t e m a t i c a l l y v a r i e d t o o b t a i n t h e d e s i r e d a l l o y composition and donor concentration. The a r s e n i c f l u x was adjusted t o y i e l d a s l i g h t l y A s - s t a b i l i z e d (2x4) surface r e c o n s t r u c t i o n . The sub- s t r a t e temperature Ts, which v a r i e d from 5520C t o 670% i n d i f f e r e n t growth runs, was c a l i b r a t e d by observino t h e oxygen d e s o r p t i o n from t h e chemically etched and UHV heated s u b s t r a t e s u r f a c e i n t h e RItEED p a t t e r n .

H a l l e f f e c t measurements a t 300 K and 77 K were made on r e c t a n g u l a r shaped samples u s i n g t h e van d e r Pauw technique. Ohmic contacts were formed by c a r e f u l l y a l l o y i n g small I n b a l l s i n t o t h e sample. Nominally undoped AlxGal-xAs l a y e r ~ ~ f x - - ~ , h i b i t p-type c o n d u c t i v i t y and have 300 K n e t c a r r i e r concentrations below 10 cm over t h e e n t i r e composition range studied. I n Si-doped n-type AlxGa -,As l a y e r s w i t h 0.15 < x < 0.25, t h e 300 K f r e e e l e c t r o n H a l l m o b i l i t y i s reducea compared t o n-GaAs and i s o n l y s l i g h t l y dependent on C a r r i e r concentration. The 300 K m o b i l i - t i e s thus decrease from 1600 cmz/~.sec t o o n l y 1215 cm2/~.sec, when t h e c a r r i e r c o n c e n t r a t i o n i s increased from 6x1016 cm-3 t o 1 . 2 ~ 1 0 1 8 cm-3. F o r t h i s a l l o y com- p o s i t i o n range no marked c a r r i e r f r e e z e o u t was observed down t o 77 K. F o r n-A1xGal-xAs:Si l a y e r s w i t h h i g h e r A1 c o n t e n t (x > 0.3), however, a f u r t h e r reduc- t i o n o f t h e 300 K H a l l m o b i l i t y and a s u b s t a n t i a l c a r r i e r f r e e z e o u t was detected.

I n t h i s composition range the s u b s t r a t e temperature Ts has o n l y a minor i n f l u e n c e on t h e H a l l m o b i l i t y f o r T,> 6000C, whereas t h e e l e c t r i c a l p r o p e r t i e s d r a s t i c a l l y d e t e r i o r a t e f o r Ts < 600°C. The behavior can be analyzed by studying t h e 1 umi- nescence p r o p e r t i e s and t h e n a t u r e o f t h e deep e l e c t r o n traps.

The luminescence experiments were performed on t h e (100) s u r f a c e w i t h t h e samples immersed i n l i q u i d He pumped t o T< 2 K. As e x c i t a t i o n source t h e l a s e r l i n e from a Kr+ l a s e r w i t h photon energy P1~=2.603 eV was used. The e x c i t a t i o n i n t e n s i t y could be v a r i e d by employing n e u t r a l d e n s i t y f i l t e r s . The luminescence l i g h t was analyzed w i t h a Spex

lm

g r a t i n g monochromator, and the d e t e c t i o n i n t h e range

n w

> 1.45 eV was made w i t h a cooled GaAs photocathode mu1 t i p l i e r .

Capacitance measurements performed on Schottky diodes were used t o evaluate t h e dependence o f deep e l e c t r o n t r a p s on t h e A1 c o n t e n t x o f t h e a l l o y and on growth temperature TS. I n a d d i t i o n we i n v e s t i g a t e d t h e deepening o f t h e shallow donor l e v e l . The Schottky diodes were f a b r i c a t e d by evaporating Au onto t h e as- grown s u r f a c e o f t h e n-A14Gal-xAs:Si l a y e r s i n a separate vacuum system. The ob- served diode c h a r a c t e r i s t i c s were as f o l l o w s : diode n-value n=1.1+0.5, s a t u r a t i o n c u r r e n t I s = 1.5x?0-8~/cm2, and b a r r i e r h e i g h t V 0.95 eV. V d e t e s i n e d by capaci- tance-vol tage measurements was found t o be sligR;ly h i g h e r Vvb=l.l eV). The tempe- r a t u r e and frequency dependence of t h e capacitance and conductance o f t h e Schottky diodes were measured f o r temperat r e s ranging from 10 K t o 300 K and frequencies ranging from 10 sec-1 t o 107 Sei-Y. The evaluated t r a p depth i s thus l i m i t e d t o

Et=0.5 eV. The technique of admittance spectroscopy

/3/

i s well suited t o study deep majority c a r r i e r traps i f t h e i r concentration i s on the order of o r even higher than the shallow level concentration.

3. Results and Discussion. -

3.1 Luminescence properties of A1,Gal-xAs

I n big.

1

typical low-temperature

PL

spectra obtained from three representative

MBE

AlxGal-xAs layers of d i f f e r e n t a l l o y composition and grown a t d i f f e r e n t sub- s t r a t e temperatures a r e shown. The A10,1gGa0:81As specimen was grown a t the highest

s u b s t r a t e temperature used in t h e present study. When we comare t h i s PL s ~ e c t r u m

a l l o y composition, the binding energies of the exciton and of the two acceptors Si and

C

i n A1xGal-xAs d i f f e r only s l i g h t l y from thosein GaAs

/5/.

Variing the exci- t a t i o n i n t e n s ~ t y , y i e l d e d no s h i f t of the emission energy confirmin? our assign- ment. A t present, the source of t h i s Si contanination (Al-effusion c e l l o r sub- s t r a t e holder?) i s not y e t unambiguously identified. Further i t i s important t o note t h a t t h i s (e, SiO) recombination

1

ine i s only detectable in nominally undoped sanples grown a t high (>650°c) s u b s t r a t e temperatures due t o the strongly amphoteric behavior of Si i n

?!BE

GaAs and MBE A1xGal-xAs.

kIhen we compare the f i r s t PL spectrum ( a ) with spectrum (b) obtained from A10.24Gao 76As layer which was grown a t a lower temperature (630°C), we observe t h a t the l i n e due t o (e,Sio) recombination has t o t a l l y disappeared. In t h i s spectrum (b), the half-width of the (e,CO) recombination l i n e i s extremely narrow (13 meV) ar.d the BE peak i s well separated. Both features demonstrate the excellent minority- c a r r i e r properties of the

MBE

grown material.

In t h e PL spectrum ( c ) obtained from intentionally Si doped n-A10 42Ga0,58As layer with a high A1 content close t o t h e i n d i r e c t t r a n s i t i o n , we o b s e h e a l s o three d i s t i n c t peaks. The high energy peak a t 2.046 eV can again be a t t r i b u t e d t o

BE

tran- s i t i o n , and the two other l i n e s a r e apparently associated with

C

and Si acceptors on As s i t e s . Compared w i t h t h e r e s u l t s a t lower A1 content, however, t h e i r ener- g e t i c distances t o t h e

BE

peak a r e now considerably larger.

In Fig. 2 we show, in addition, t h a t a t high A1 content t h e low energy peak labeled B i s s h i f t i n g strongly t o higher energy with increasing excitation intensity.

We have used the l a r g e r separation of the PL peaks a s well a s the enhanced l i n e s h i f t observed on a l l o y specimens with higher A1 content a s two p a r t i c u l a r features which can be used t o i d e n t i f y the

PL

l i n e s labeled A and

B

a s donor-acceptor (DA) p a i r recombination /6/. The luminescence energy

hw i s then given by the equation

Rw =

Eg - (EA + EE) +;- e2

(1)

ENERGY teV 1

( a ) with a PL spectrum from

1.6 1.8 2.0 2.2

GaAs /4/, we can assign the

observed high energy peak forming a shoulder a t 1.74 eV t o recombination of a

I

, I

0

bound exci ton (BE). Then

T = 2 K

the two intense peaks a t

g

^{a ]} x = O L Z

1.721 eV can be a t t r i b u t e d

U) x = 0 1 9 x = 0 2 ~ SI - doped

t o band-acceptor (BA) tran-

z nom undoped nom undoped

s i t i o n s , (e,Si") and (e.CO),

P

^{sub= 6.ro0c} Tsub= 630nC TQ,~= 660°C

z respectively, due t o r e s i -

-

I

dual carbon and s i l i c o n

-I

acceptors. In t h i s range of

a

Fig. 1: Low-temperature photolumi nescence spectra

of nominally undoped and of

7000 6000

Si-doped MBE AlxGal-xAs

WAVELENGTH [ A 1

1 ayers.

.-. M B E

AI, Gal_,As P L= 1 0 ~w emQ ~ ~ ~

JOURNAL DE PHYSIQUE

F i g . 2 : E x c i t a t i o n i n t e n s i t y dependence

1.8 1.9 2.0 2.1 o f t h e PL spectrum i n MBE

I I I I A1 0. 42Ga0, 5 8 A ~ : S i

.

1

^MBE Alo,42Gao.58 As:Si p~aser

where E i s t h e band gap energy, EA and 1.3 r n ~ c r n "

ED

a r e !he i o n i z a t i o n energies f o r t h e

acceptor and t h e donor, r e s p e c t i v e l y ,

L.. and t h e l a s t term describes t h e depen-

- 0.1

w

ern" dence o f t h e emission energy on t h e

o e x c i t a t i o n i n t e n s i t y . An increase o f

-

t h e e x c i t a t i o n i n t e n s i t y leads t o a

>-

5

-

1.3 W cm-2 s a t u r a t i o n o f more d i s t a n t donor-

V) acceptor p a i r recombination and thus

z

r e s u l t s i n a l o w e r i n g o f r i n Eq. (1).

e

As a consequence t h e luminescence l i n e

3

s h i f t s t o h i g h e r energy. I n s p e c t i o n o f

I

A F i g . 2 r e v e a l s t h a t a s h i f t o f about

a. 6 meV i s observed when t h e e x c i t a t i o n

i n t e n s i t y i s changed by one order o f magnitude. T h i s r e s u l t i s i n good T = 2 K agreement w i t h t h e data r e p o r t e d i n

Ref. 7.

7000 6500 6000 A t t h e lowest e x c i t a t i o n i n t e n s i t y o f WAVELENGTH [ A l 1.3 mll. cm-2, t h e separation between t h e

bound-exciton recombination BE and t h e S i acceptor r e l a t e d t r a n s i t i o n B i s found t o be 76 meV, whereas t h e i n t e n - s i t y o f l i n e A r e l a t e d t o t h e C acceptor i s a l r e a d y below t h e d e t e c t i o n 1 im i t . F u r t h e r r e d u c t i o n o f e x c i t a t i o n i n t e n - s i t y r e s u l t s i n a separation o f t h e BE and B f e a t u r e s up t o about 100 meV.

According t o Ref. 5 t h e S i acceptor b i n d i n g energy should be on t h e order o f 50 meV f o r n-A1xGal-xAs:Si w i t h x = 0.4. Thus, t a k i n g t h i s v a l u e i n t o account, a tremendous deepening o f t h e S i donor l e v e l can be deduced f o l l o w i n g t h e equation

AE =

~8~ + ~ 2 ' -

^EEx

w i t h AE being t h e e n e r g e t i c separation o . t h e BE and B t r a n s i t i o n and E2', Eil, EEx t h e S i donor, S i acceptor and e x c i t o n b i n d i n g energies, r e s p e c t i v e l y . As a r e s u l t we o b t a i n a v a l u e o f b e t - ween 40 and 50 meV f o r t h e S i donor l e v e l assuming an e x c i t o n b i n d i n g energy o f 6 meV. /8/.

Next we i n v e s t i g a t e d t h e i n f l u e n c e o f t h e growth temperature on t h e i n t e n s i t y of t h e emission peak a t t r i b u t e d t o t h e S i acceptor which dominates t h e 2 K PL spectra o f n-Aloe 35Ga0. &AS. I n s p e c t i o n of F i g . 3 shows t h a t t h e i n t e n s i t y o f

SUBSTRATE TEMPERATURE tK1

Fig. 3 : Growth temperature dependence of t h e PL spectrum i n WBE A1 0. 42Ga0, 5 8 A ~ : S i

.

t h i s peak increases d r a s t i c a l l y by about two orders of magnitude when t h e substrate temperature i s increased from Ts

=

550°C t o Ts

>

6500C. In AlOe35Ga -65As layers grown a t lower substrate temperatures p a r t of t h e o p t i c a l l y excl te! c a r r i e r s recom- bine nonradiatively over deep t r a p centers. Our detai1.ed admittance measurements have revealed t h a t t h e i r concentration increases with decreasing growth temperature.

The observed reduction of

PL

i n t e n s i t y a t lower growth temperatures i s d i r e c t l y correlated with t h e occurence of an additional broad l i n e in the PL spectrum a t aD energetic position about 40 meV below the Si acceptor related peak. The i n t e n s i t y of t h i s broad

PL

f e a t u r e increases monotonically when the growth temperature i s lowered from 610°C and i t dominates the whole spectrum a t Ts

=

550%. The physical- chemical nature of t h i s deep recombination center i s a t present unknown.

3.2 Behavior of deep electron traps and shallow donors i n AlxGal,xAs

An example of the temperature depen ence o capacitance an con uctance a t a fre- quenc.v of 10 s - l f o r a rebresentati:e HBE :l.Gal -.Ar:Si (n!7x10P6cm-3) laver of A1

...

..\.

_{MBE # 4130}

ll

-

&0,35&10,65A~:Si

....

content x=0.35 which was grown a t a medium s^ubsir^ate temperature i s depicted i n Fig

4. The

temperature dependence of t h e capacitance (dark points) shows two c h a r a c t e r i s t i c steps. A t low temperature, the measured capacitance drops nearly t o zero due t o the freeze out of the shallow donor level. Any remaining capacitance i s limited by t h e geometry and spacing of t h e Schottky contact with respect t o t h e ohmic return contact /3/. The observed increase of the capacitance a t about 150°C i s caused by the emission of electrons from the dominant deep level. I t can be used d i r e c t l y t o determine the concentration of shallow donors and deep levels.

The increase of the capacitance i s d i r e c t l y

deep level i s broad and

1 asymmetric because i t re- sul t s from two closely

, spaced levels. The depth from the conduction band edge can be determi ed bv simplv drawing

w

T$ vs T-1 on a semi-logarithmic p l o t , where Tp i s the con- ductance peak temperature and

w

t h e measurement f r e - auencv. The r e s u l t s f o r

-

^300-

Lh -

w

200-

E

t h e 4 m p l e investigated

..-.. a r e given in Fig.

5.

The

...

a) f

=I0

Hz

...

u = o v

_.e4--''

0 .

* .

goo.o 0 OO-O.a e . e.

1 3-

^{0. O . e.}

0.

O.

bC =175pF % oo Be OOOO..OOe..

- I

0

1

^{I I}

1 quency of 10 sec-1

0

100 200 300

a ) capacitance of

TEMPERATURE [K]

the sample in the

dark (01 and a f t e r connected

w i t h

a peak i n t h e temperature dependence

o f t h e

conductance when the emission r a t e i s in phase with the o s c i l l a t i n g measurement voltage a t a fixed frequency. !Je de- monstrate t h i s behavior i n Fig.

4.

The conductance neak a t t r i b u t e d t o the

. . ...

^{MBE # 4130}

...

...

8

_n

-

A~0.356a0,65A~:!3

Fig. 4: Temperature depen-

dence of admittance

of A u / A ~ 1. gGa0.65

As:Si Sc o ky

b a r r i e r diode a t a

measurement f r e -

JOURNAL DE PHYSIQUE

b=1.35 eV ( 0 ) b) conductance of

10 3 the sample in the

MBE # 4130 dark.

n -

AL0.35G~0.65 AS : Si

Fig. 5:Thermal emission r a t e versus inverse temperature f o r l e v e l s detected in

CV the upper half of

Y on =6.3 x m 2 the band gap in blBE

10

V) A1 0. 356a0. 65A~:

SI.

U

I- donor binding energy was

3 Et =0.38 eV thus found t o be 50 meV.

0,=1.6 x 1 0 - l ~ m 2 This deepening of the donor

level with

x

i s consistent with Hall-effect data a s well a s with the photo- 10 2 0 30 luminescence r e s u l t s . In

1000/T ^[K-l] p a r t i c u l a r i t elucidates

the change from a band- acceptor l i k e emission struc- ture ( e , s i o ) in the

PL

spectrd a t 0.2 <

x

⁴ 0.25 t o a donor-acceptor recombination (DA) a t 0 . 3 < x < 0 . 3 5 indicated by t h e increase of

the

separation between the luminescence l i n e

B

and the bound exciton l i n e

BE

with increasing

x

(see Fig. 1).

Nhen t h e c a r r i e r concentration in the sample i s increased, the low-temperature capacitance s t e p s h i f t s t o lower temperatures. This menas t h a t the donor binding energy decreases. Fig. 6 shows t h i s dependence of the c a r r i e r concentration on the donor binding energy. For a c a r r i e r concentration of n=2

x

1017cm-3 and x=0.35, a binding energy of 13 meV i s evaluated. This i s in good agreement with recent r e s u l t s obtained from Hall-effect measurements on MBE A10 Ga0.65As:Si / 9 / . In addition the l o g o ~ $ vs T-1 plot y i e l d s a value of 0.38 eV for'38e depth of the dominant electron trap. This value has t o be corrected because t h e b a r r i e r f o r the emission of c a r r i e r s from the t r a p i s lowered due t o t h e Pool

-

Frenkel e f f e c t

/ l o / .

Usually t h i s correc- tion i s on the order of 10% f o r t h i s trap depth ( E ) and f o r t h i s deep level t o shallow level r a t i o (Nt/n) /11/. Using t h i s proce6ure we obtain a corrected value of 0.42 eV f o r the t r a p depth which agrees well with recent DLTS data f o r the do- minant t r a p obtained from the same material /12/.

75-

- 5

E

U

0 25

w

1016 10'7 1018 ted dominan't electron trap. This i s fur-

No

-

^NA ~ c r n - ~ l t h e r supported by the capacitance increase t h a t corresponds t o the increase a t high temperatures in t h e dark. This finding strongly suggests t h a t the 0.42 eV electron t r a ? i s responsible f o r the p e r s i s t e n t MBE A1 0.35Ga0.65As : Si

TSub= 600eC

\ *

- I J

Fig. 6: Dependence of Si-donor binding energy on doping concentration in WBE A10.356a0.65As:Si deduced from admittance measurements.

In addition, in Fig. 4a, the tempera- t u r e dependence of t h e capacitance of the Schottky diode illuminated with l i g h t of energy below the bandgap (TI ~ 1 . 3 5 eY) f o r a short time a t low temperatures i s shown (open symbols). The observed p e r s i s t e n t increase a t low temperatures indicates the emission of f r e e e7ectrons from the detec-

p h o t o c o n d u c t i v i t y e f f e c t observed. Photo-Hall e f f e c t measurements r e c e n t l y performed on t h e same m a t e r i a l i n o r d e r t o measure t h e f r e e - e l e c t r o n c o n c e n t r a t i o n a r e i n ex- c e l 1 e n t agreement w i t h r e s p e c t t o donor b i n d i n g energy, s h a l l o ~ l e v e l concentration, and deep l e v e l concentration. I n a d d i t i o n , these H a l l e f f e c t measurements y i e l d e d an increase o f t h e e l e c t r o n m o b i l i t y compared t o t h e measurements performed i n t h e dark a t low temperatures. Consequently i o n i z e d i m p u r i t y s c a t t e r i n g i s reduced i n t h e i l l u m i n a t e d sample. The r e s u l t s s t r o n g l y i n d i c a t e t h a t t h e deep e l e c t r o n t r a p i n f a c t i s a double acceptor changing i t s charged s t a t e from -2 t o -1 r a t h e r than a deep donor changing t h e charged s t a t e from 0 t o +1 d u r i n g i l l u m i n a t i o n /13,14/.

Since t h i s c e n t e r i s thus s t i l l charged n e g a t i v e l y , i t a c t s as a Coulomb b a r r i e r t o t h e e l e c t r o n s i n t h e conduction band, g i v i n g r i s e t o t h e observed ~ e r s i s t e n t photo- c o n d u c t i v i t y . Any p e r s i s t e n t p h o t o c o n d u c t i v i t y which m i g h t be r e l a t e d t o t h e

n -

A1xGal-xAs/GaAs s u b s t r a t e i n t e r f a c e can be excluded from these experiments, s i n c e our, p r o f i l i n g measurements performed a t low temperatures i n t h e dark as w e l l as i n t h e i l l u m i n a t e d s t a t e c l e a r l y demonstrate t h e homogeneous d i s t r i b u t i o n o f ca- pacitance enhancement i n t h e d i r e c t i o n o f l a y e r growth.

A d d i t i o n a l systematic s t u d i e s revealed a s t r o n g dependence o f deep t r a p concen- t r a t i o n on t h e A1 content, doping concentration, and growth temperature. A v a r i a t i o n o f t h e n e t c a r r i e r c o n c e n t r a t i o n from 7 x 1016cm-3 t o 2 x 1017cm-3 leads t o an i n - crease o f t h e deep l e v e l c o n c e n t r a t i o n from 2.4 x 1 0 l ~ c m - ~ t o 5.8 x 1017cm-~ f o r an A1 content o f x = 0.35. T h i s d i r e c t c o r r e l a t i o n o f donor and deep l e v e l concentra- t i o n s t r o n g l y suggests t h a t t h e deep l e v e l i s d i r e c t l y c o r r e l a t e d w i t h t h e donor species as has been proposed p r e v i o u s l y f o r LPE AlxGal-xAs:Si /15/. I n agreement w i t h o u r PL data o f Sect. 3.1 t h e s u b s t r a t e temperature has a marked i n f l u e n c e on t h e 0.42 eV t r a p concentration. A decrease o f Nt/n from 3.4 t o 1.2 i s observed f o r x = 0.35 when t h e s u b s t r a t e temperature i s increased from 615°C t o 6400C. A reduc- t i o n o f the A1 content from x = 0.4 t o x = 0.33 r e s u l t s i n a decrease o f t h e deep t r a p t o fr e e l e c t r o n r a t i o Nt/n from 4.8 t o 1.4 a t a f r e e c a r r i e r c o n c e n t r a t i o n o f n = 7 x l0J6cm-3.

A f u r t h e r r e d u c t i o n o f t h e A1 c o n t e n t t o below x = 0.25 suppresses t h e t o t a l deep l e v e l c o n c e n t r a t i o n below t h e d e t e c t i o n l i m i t o f t h e admittance technique. I n Fig. 7 we show t h e r e s u l t s o f a d d i t i o n a l DLTS measurements which revealed f i v e elec- t r o n traps. Though s t i l l detectable, t h e 0.4 eV t r a p i s now no l o n g e r t h e most i n - tense emission. Rather, t h e e l e c t r o n t r a p s a t 0.6 and 0.7 eV dominate t h e spectrum.

A t t h i s growth temperature o f 630% t h e o v e r a l l r a t i o Nt/n i s reduced t o 2 x A f u r t h e r increase o f TS t o >6700C makes i t f e a s i b l e t o decrease t h e t o t a l t r a p c o n c e n t r a t i o n now r o u t i n e l y t o below o f t h e i n t e n t i o n a l c a r r i e r concentration.

On t h e o t h e r hand, as shown i n t h e PL spectra, t h e undesired S i autocompensation r a t i o i s s t r o n g l y enhanced i n t h i s case.

MBE AI0,23Ga0.77A~: Si

I I I I I

8x 1014 ern-3

lo0 200 300

TEMPERATURE I K I

4. Conclusion.

-

Ye have combined 2 K Photoluminescence, H a l l effect, admittance, and DLTS measurements t o i n v e s t i g a t e t h e i n f l u e n c e o f growth c o n d i t i o n s and a l l o y com- p o s i t i o n on t h e e l e c t r i c a l and o p t i c a l p r o p e r t i e s o f nominally undoped and c f S i doped n-type A1xGal-xAs (0.2 < x < 0.4) l a y e r s grown by molecular beam e p i t a x y (MBE) a t s u b s t r a t e temperatures ranging from 570 t o 6700C. The PL measurements demonstrate t h a t h i g h growth temperatures y i e l d F i g . 7: DLTS spectrum o f e l e c t r o n

t r a p s i n FlBE A ~ Q 23 Ga0.77As:Si i n d i t a t i n g t r a p depth and concentra- t i o n .

C5-182 JOURNAL DE PHYSIQUE

an improved luminescence i n t e n s i t y b u t a l s o an enhanced autocompensation r a t i o o f t h e amphoteric S i dopant. A v a r i a t i o n o f t h e A1 c o n t e n t from x l 0 . 2 5 t o x > 0.30 leads t o a conversion o f t h e band acceptor l i k e emission s t r u c t u r e (e, SiO) t o a donor acceptor (DA) s t r u c t u r e i n t h e PL spectrum. T h i s i s caused by t h e i n c r e a s i n g donor b i n d i n g energy as i s confirmed b y o u r admittance measurements. I n a d d i t i o n , f o r x > 0.30, one e l e c t r o n t r a p i s found t o dominate. The c o n c e n t r a t i o n o f t h i s t r a p l e v e l i s on t h e order o f o r even higher than t h e doping c o n c e n t r a t i o n a t t h i s a l l o y composition. The t r a p c o n c e n t r a t i o n can be reduced s i g n i f i c a n t l y by i n c r e a s i n g t h e growth temperature and reducinq A1 content. A t A1 contents below x = 0.25, t h e t o t a l t r a p c o n c e n t r a t i o n can now r o u t i n e l y be reduced below 10-2 o f t h e i n t e n t i o n a l c a r r i e r concentration.

Acknowledgement.

-

T h i s work was sponsored by t h e Bundesministerium f u r Forschung und Technologic. The authors a r e indebted t o A. F i s c h e r and J. Knecht f o r e x p e r t h e l p i n sample p r e p a r a t i o n and t o K. bliinstel f o r he1 p w i t h t h e DLTS measurements.

The t e c h n i c a l assistance o f B. Kiibler f o r metal c o n t a c t f a b r i c a t i o n and o f

!.I.

Heinz d u r i n g 1 umi nescence measurements i s g r a t e f u l 1 y acknowledged.

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