InP interfaces grown by MOVPE

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Applied Surface Science 63 (1993) 197-201 North-Holland

applied surface science

Optical characterization of InP/InAIAs/InP interfaces grown by MOVPE T. B e n y a t t o u , M.A. Garcia, S. M o n 6 g e r , A. T a b a t a Laboratoire de Physique de la Mati~re (URA CNRS 358), INSA de Lyon, 20 avenue Albert Einstein, F-69621 Villeurbanne Cedex, France

M. Sacilotti, P. Abraham, Y. M o n t e i l Laboratoire de Physicochimie Min~rale (URA CNRS 116), Universitd Lyon I, 43 boulevard du 11 Novembre 1918, F-69622 Villeurbanne Cedex, France

and

R. L a n d e r s lnstituto de Fisica, Universidade de Campinas, Campinas, Brazil Received 2 June 1992; accepted for publicaton 31 July 1992

The InA1As alloy grown on InP has a great technological importance for applications to both high-speed devices and optoelectronic systems. In this paper, we present results of low-temperature optical studies carried out on I n P / l n A I A s / I n P heterostructures. Samples were grown by the atmospheric pressure metal organic vapor phase epitaxy (MOVPE) technique at 650°C, using TMAI, TMIn, PH 3 and A s H 3 sources. The interface formed by growing InAIAs on InP is called direct interface. The interface formed by growing InP on InAIAs is called the inverse interface. Photoluminescence (PL) experiments were carried out on these samples. The type II nature of the direct interface leads to a strong interface PL peak at 1.2 eV. This PL is very sensitive to the quality of the heterostructure and could be used as a probe for the M O V P E growth parameters. More surprising is the 1.3 eV PL peak which we ascribe to the inverse interface. From its dependence upon the excitation power (energy and full width at half maximum), it is clear that it originates from the interface recombination. We show that the two heterointerfaces I n P / I n A 1 A s and I n A l A s / I n P are not equivalent in the case of our M O V P E grown samples.

1. Introduction The InA1As/InP heterostructure is very promising for optoelectronic and high-speed devices applications. This system presents the advantage of a high electron mobility, high saturation velocity, high electron density and high breakdown voltage. Mobilities as high as 2300 cmZ/V • s at 300 K have already been observed in modulation-doped heterostructures [1,2]. Moreover, the type II nature of the interface gives rise

to a strong photoluminescence near 1.2 eV [3,4] that can be tuned in electroluminescent devices by varying the current density [3] or the InA1As alloy composition. For this work, we have used this interface emission as a probe and we show that the inverse interface I n P / A l I n A s exhibits different optical properties as compared to the direct one (AlInAs/InP) and we correlate these results to the difference of abruptness between these two types of interface.

0169-4332/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

198

)'~ Benyattou cl al. / Optical characterization ql I n l ' / lnA/As' /InP inteUhrc~' ,q,rown by MO [q¥'.

2. Materials and techniques The growth system is a h o m e m a d e atmospheric pressure M O V P E system, c q u i p p c d with a fast delivering T h o m a s Swan lincar manifold and a rectangular shape quartz reactor [5]. The carrier gas velocity over the substratc is 64 c m / s . This allows fast gas switching over the substratc. The l n P / A l l n A s / l n P layers were grown at 65()°(~ on lnP (Fe) substrates. The growth was started after a 5 min substrate annealing at 650°C under P H > (7are was taken to aw)id ternary alloy fluctuation frequently e n c o u n t e r e d for A P - M O V P E systems, due to M O relatively high vapor pressure sources [5-7]. A l l n A s thick layers are intrinsic n-type ( 3 × 10 > cm :) and InP is also intrinsic n-type (I × 1() 15 cm -~) doping. For thc present PL characterization experiment two types of samples were grown: (a) ,m lnP (0.25 # m ) buffer layer, covered by a 1.5 # m A l l n A s lop laycr. The interface formed by this sequence is called direct, (b) an haP (0.1 ,u.nl) buffcr layer followed by a 1.7 # m A l l n A s layer and c a p p e d by a 0. l /.tm lnP top laycr. In this sequcnce, a direct ( A l l n A s on lnP) and an inverse (lnP on A l l n A s ) interface arc fl)rmed. For both samples the A l l n A s lattice mismatch is 3 × 10 ; (indium rich composition). Both direct and inverse interfaces have a 4 s growth stop during which P H ; and AsH~ are switched on~off after 2 s of growth stop. Thc PL experiments were carried out at 4 K. The excitation was providcd by the 5145 ,~ linc of a C W argon ion laser focus on a 150 ,u,m diameter spot. The PL signal was detected with a 0.64 m H R S 2 J O B I N Y V O N m o n o c h r o m a t o r and a Ge p h o t o d e t e c t o r cooled down to 77 K.

T=4K

(a

v

>, E

[

..J I:1.

1.1

,.-?

1.2

1.3

-f,- iK

1.4

1.5

1.6

(b)

d >, e'i e.-

....J r'~

,]

1.1

1.2

1.3 Energy

: i

1.4

1.5

1.6

(eV)

Fig. 1. I , o v , - l c m p e r a t u r ¢ PI, s r ) c c l r u m : (a) single h c t c r o s l r u c |tire ( s a m p l e a), (b) d o u b l e h c l c r o s l r u c h l r c ( s a m p l e b).

cxcitonic and the d o n o r - a c c c p t o r pairs rccomhination in lnP at 1.416 and 1.37(~ cV. We can also see the LO p h o n o n replica of the d o n o r - a c c c p t o r transition at 1.335 eV. We can note t h a l the

(1)

>.,

(2)

I

(3)

(4) (5)

J

o3 ¢-,

[

/

", /,,

r-

3. Experimental results

It] fig. la wc have reported the low-temperature PL spectrum of the sample a (single heterojunction). We can easily distinguish three groups of peaks. The 1.535 and 1.223 eV ones are related to lnAIAs and interface recombinations, respectively. A n d around 1.4 eV we havc the

.d n

1.25

1.30

Energy

1.35

1.40

(eV)

Fig, 2. PL spectrmn of the inverse mtcrlacc a! different excitation powers: (1) 2 ,uW, (2) 2 roW, (3) 20 mW, (4) 100

rnW. (5) 200 roW.

72 Benyattou et al. / Optical characterization of lnP / In,41As / lnP interfaces grown by MOVPE (a)

....... I ........ I ........ I ........ I ........ I ........ i ...... 1012

T = 4K

199

//4/+

A I I n A s ~

1o,o ,~

10 ~

( 1 0 -7

1 0 "5

10"3

(b)

10-1

Power (W) Fig. 3. Integrated PL intensity of the inverse interface recombination versus the excitation power.

intensity of the interface p e a k is of the same o r d e r o f m a g n i t u d e as the others. T h e P L s p e c t r u m o f the d o u b l e h e t e r o s t r u c ture ( s a m p l e b) is similar to the p r e v i o u s s p e c t r a except for an extra p e a k at 1.305 e V (fig. lb). This p e a k is very sensitive to the excitation power. A s o n e can see in fig. 2, with increasing the

1.38

~" '

&

' '"~'l

........

i

........

i

........

i

T:4K

1.36

........

~

........

r

1.3 1.28 ........ I ........ t ........ I ........ i 10-S

........

10-3

I

........

~

t

.

.i

10-1

Power (W) 4O T : 4 K

35

( b)

30 ~--,

25

20 ~:

is 10

~

5

+

Fig.

5.

Schema

of

the

direction

observed

PL

transitions:

(a)

direct

interface, (b) inverse interface;" where (1) and (3) represent AllnAs and InP recombination, respectively, and (2) and (4) the interface PL transition for direct and inverse interface, respectively.

......

1.32

10-7

growth

4

As 1

(a) / /

1.34

1.26

3InP~

+

excitation p o w e r it shifts t o w a r d s h i g h e r e n e r g y and b r o a d e n s , all the rest o f the s p e c t r u m rem a i n i n g u n c h a n g e d . In fig. 3 we can see that the i n t e g r a t e d P L intensity varies almost linearly with the excitation power. This indicates that the transition is not i m p u r i t y r e l a t e d a n d is likely an excitonic r e c o m b i n a t i o n . Fig. 4a shows the variation of the P L p e a k p o s i t i o n with the power. B e t w e e n 10 7 a n d 10 4 W we can see a small variation a r o u n d 2.5 m e V / d e c a d e , w h e r e a s above 10 -3 W the shift i n c r e a s e s strongly with a slope o f 35 m e V / d e c a d e . T h e full width at half m a x i m u m ( F W H M ) shows a similar b e h a v i o r (fig. 4b). Below 10 - 4 W its value is almost c o n s t a n t ( ~ 10 m e V ) a n d starts to i n c r e a s e above 10 3 W at a rate of 13 m e V / decade.

+

........ I ........ i ........ ~ ........ b ........ q . . . . .ill , ~. 10-7

lO-S

Power

10-3

10-1

(W)

Fig. 4. Influence of the excitation power on (a) the PL peak energy and (b) the FWHM of the inverse interface recombination.

4. Discussion T h e PL results o b t a i n e d on s a m p l e a are similar to t h o s e r e p o r t e d in the l i t e r a t u r e in single

200

72 Benyattou et al. / Optical characterization of hzP / h~4lAs / lnP interfaces grown by MOVPI:"

heterojunction [3,4] and superlattices [8,9]. As can be seen in fig. 5, the 1.2 eV luminescence originates from the spatially indirect recombination of electron-hole pairs located at the interface. It is surprising to see that, although this recombination is indirect in the real space, the observed luminescence is intense. Maybe it is because non-radiative recombinations are less effective in the case of spatially separated elect r o n - h o l e pairs. The F W H M of this transition is around 27 meV, which is larger than the lnAIAs one (18 meV). This indicates that the interface roughness contributes predominantly to the broadening. This is because the electrons and the holes are on each side of the interface. The intensity and the F W H M of this PL could be used to probe the quality of the interface. Indeed, the PL peak at 1.2 eV is very weak or even not observed in a sample without InP buffer layer. For the double heterostructure (sample b) we have detected a new peak at 1.3 eV. The slope of the energy shift versus the excitation power is too high to attribute this transition to d o n o r - a c c e p tor pairs. Since this peak is very sensitive to the excitation power, we attribute it to a recombination located at the inverse interface ( I n P / AIInAs). Similar behavior has already been observed in G a I n A s / I n P heterostructures [10,11]. Our samples show a residual n-doping so it is very likely that there is a confined electron gas at the interfaces (direct and inverse) and, thus, the recombination occurs between an electron of the bidimensional gas and a hole. In the case of the direct interface the hole is located in the InA1As layer whereas for the inverse interface the situation is not clear. However, this could explain the PL dependence on the excitation power. At low intensity the photogenerated electron-hole pairs just modify the band bending at the interface leading to the observed value of 2.5 m e V / d e c a d e for the blueshift. Above 10 3 W the intensity is high enough for the band filling effect to occur. This explains the increase of the F W H M of the PL transition and the higher value of 35 m e V / d e c a d e for the blueshift. Such behavior is not observed for the direct interface. Only the studies on the electroluminescence of the direct interface reported by Caine et al. [3] show similar

results. Further studies are under investigation to clarify this point. Moreover, the PL peak being at a higher energy for the inverse interface, all these results lead us to think that these interfaces (direct and inverse) are not equivalent. Indeed, this has been confirmed by Auger measurements which show that the inverse interface is not abrupt. There is some As diffusion in the InP top layer leading to the formation of an InAsP transition layer. This could be due to the high As sticking coefficient compared to that of P. If we assume that the recombination in the inverse interface is similar to that of the direct one, we would expect that the transition takes place at a lower energy because of the smaller gap of the InAsP transition layer. We can therefore make the hypothesis that the photogenerated hole is located in the InP layer and recombines with an electron of the interface gas. In this case, the transition takes place at an energy slightly lower than the InP gap as we have observed (see fig. 5 for illustration). In our hypothesis, the F W H M of the PL transition for the inverse interface should be smaller than for the direct one, because neither the electron, nor the hole, experience the AllnAs alloy disorder (both wave function are mostly located in the InP top layer). It is the case with a PL width of 10 meV. The reason why the non-abrupt interface leads to this kind of recombination is still not clear. We could speculate on the possibility that holes are trapped at the interface in the lnP layer because of the layer degradation.

5. Conclusion

In this paper, we have presented experimental results from photoluminescence studies on lnA I A s / I n P single and double heterostructures lattice matched on InP. The direct interface InA 1 A s / I n P shows a strong luminescence at 1.2 eV which can be explained by the type II nature of the heterostructure. For the inverse interface a new peak at 1.3 eV is detected. From its dependence upon the excitation power we attribute it to an interface recombination and we make the hypothesis that it involves an electron from the

T. Benyattou et al. / Optical characterization of lnP / InAlAs / InP interfaces grown by MOVPE

interface gas and a photogenerated hole in the InP top layer. We explained the difference between these two interfaces by the formation during the growth of an InAsP transition layer at the inverse interface. These results show that these photoluminescences could be used to probe the interface.

References [1] L. Aina and M. Mattingly, J. Appl. Phys. 64 (1988) 525. [2] L. Aina, M. Mattingly and B. Potter, Appl. Phys. Lett. 57 (1990) 492. [3] E.J. Caine, S. Subbana, H. Kroemer, J.L. Merz and A.Y. Cho, Appl. Phys. Lett. 45 (1984) 1123.

201

[4] L. Aina, M. Maningly, A. Fathimulla, E.A. Martin, T. Loughran and L. Stecker, J. Cryst. Growth 93 (1988) 911. [5] M. Sacilotti, L. Horiuchi, J. Decobert, M. Brasil, L. Cardoso, P. Ossart "and J. Ganiere, J. Appl. Phys. 71 (1992) 179. [6] A. Katz, W. Hobson, S. Chu, B. Weir, S. Pearton and W. Savin, Semicond. Sci. Technol. 6 (1991) 1158. [7] R. Glew, P. Greene, G. Henshall, C. Lowney, J. Stagg, J. Whiteway, B. Ganett and A. Norman, J. Cryst. Growth 107 (1991) 784. [8] L. Aina, M. Mattingly and L. Stecker, Appl. Phys. Lett. 53 (1988) 1620. [9] E. Lugagne-Delpon, P. Voisin, J.P. Vieren, M. Voos, J.P. Andre and J.N. Patillon, Semicond. Sci. Technol. 7 (1992) 524. [10] D. Bimberg, R. Bauer, D. Oerte|, J. Mycielski, K.H. Goetz and M. Razeghi, Physica B 134 (1985) 399. [11] P.W. Yu, C.K. Peng and H. Morkoc, Appl. Phys. Lett. 54 (1989) 1546.