GaAs superlattices: structural and electronic properties

......... C N Y I E T A L ONOWTH ELSEVIER

Journal of Crystal Growth 164 (1996) 271-275

Strain balanced GaP/GaAs/InP/GaAs superlattices: structural and electronic properties A.H. Bensaoula b,., A. Freundlich a a

Space Vacuum Epitaxy Center, Universi~ of Houston, Houston, Texas 77204-5507, USA b Department ofChemisto', UniL,ersi~"of Houston, Houston, Texas 77204-5641, USA

Abstract We have previously reported the growth of highly mismatched InP/GaAs and GaP/GaAs heterostructures. In this work, we address the structural and electronic properties of chemical beam epitaxy (CBE) grown GaP/GaAs/InP/GaAs short period supeflattices. We present a thorough crystallographic and optical study, using high resolution X-ray diffraction (HRXRD), transmission electron microscopy (TEM), photoluminescence spectroscopy (PL) and excitation photoluminescence (PLE). The realization of high quality pseudomorphic structures presenting up to 100 periods is shown. The global strain in these superlattices ranges from - 1.5 to + 5.0 kbar and translates into a noticeable modification of their electronic properties.

1. Introduction During the past few years, strained layers superlattices have attracted an increasing interest. So far, most of the results obtained are related to materials presenting a mismatch of the order of 1%. Under such conditions, the critical thickness "thickness leading to the creation of the first dislocation fault in the deposited layer" was evaluated at few thousand [1]. However, the strain introduced by this mismatch is too low to yield an important modification of the band gap [2]. For a noticeable band gap change to be obtained, one has to consider materials presenting a mismatch level of the order of 4% which in turn leads to a critical thickness of only few monolayers [3,4]. We have shown in previously published works [3-5], that it is in fact possible to avoid the premature creation of dislocations and control the * Corresponding author. Fax: + 1 713 747 7724.

strain level by alternating layers respectively under an extensive and a compressive stress. For this purpose we have used GaP and InP layers "inserted" in a GaAs matrix. In the following work, we present a more comprehensive study of the structural and optical properties of these structures. 2. Experimental procedure Several samples presenting different layer thicknesses were grown in a RIBER-CBE 32 system. Trimethylindium (TMIn), triethylgallium (TEGa) and precracked arsine (AsH 3) and phosphine (PH 3) were used as growth precursors. The growth sequence was controlled by Quantum Controls QCRHEED ® automation software. Further details on the growth conditions are given elsewhere [4,5]. The structural analysis of the grown samples and the evaluation of the residual strain were made possible through high resolution X-ray diffraction (HRXRD) studies using

0022-0248/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S 0 0 2 2 - 0 2 4 8 ( 9 6 ) 0 0 0 3 1-0

272

A.H. Bensaoula, A. Freundlich / Journal of Co,stal Growth 164 (1996) 271-275

a BEDE model 200 (double crystal + channel cut collimator). Low temperature photoluminescence (PL) experiments were performed using a liquid helium closed loop circulation cryostat. The 514.5 nm argon laser line was used as the excitation source. The PL signal was analyzed by a 1 m focal length double monochromator and detected using a room temperature GaAs photomultiplier followed by a 10ck-in amplifier.

104

~

103

< ~

102

101

1-010000

-5000

0

5 0

10000

Bragg Angle (arcsec) 3. Results and discussion

3.1. Structural analysis

Fig. 1. (400) HRXRD pattern obtained for the perfectly strain balanced superlattice GAP(5 ML)/GaAs(27 ML)/InP(5 ML)/GaAs(32 ML).

GaP(k ML)/GaAs(I ML)/InP(m ML)/GaAs(n ML) superlattices were grown at 520°C at a growth rate of 1 m m / h as determined by HRXRD and reflection high energy electron diffraction (RHEED). The growth scheme yielding sharp interfaces with minimum interdiffusion and mixing, is described elsewhere [4,5]. The details pertaining to the choice of this growth sequence will not be discussed in this paper. We show in Fig. 1 the HRXRD obtained for a perfectly strained balanced structure. The fwhm of the fundamental and higher order satellite peaks proves the level of reproducibility of the interfaces.

Furthermore the TEM results (Fig. 2) obtained for the same sample show a highly organized structure with sharp interfaces. Several structures presenting different layer thicknesses were then grown and analyzed by HRXRD to evaluate the net stress applied to the GaAs matrix. The thicknesses of the different layers are controlled in real time by the QCRHEED oscillation software and ex situ dynamical theory based simulation of the (400) HRXRD patterns. The details of each sample are presented on Table 1. Both symmetric (400) and

GaAs (27ML)

GaAs (32ML)

I 0 nm Fig. 2. Dark field g = 002 cross section TEM image obtained for the perfectly strain balanced superlattice GAP(5 ML)/GaAs(27 ML)/InP(5 ML)/GaAs(32 ML).

A.H. Bensaoula, A. Freundlich / Journal of Co,stal Growth 164 (1996) 271-275

273

3.2. Optical analysis

asymmetric (511) diffractions were used to allow for an independent determination of the in plane average lattice parameter. The in plane and perpendicular deformations were calculated in relation to the bulk GaAs lattice constant (matrix element). The results obtained are shown in Table 2. The difference is well within the detection limit of the experimental technique.

In general, the stress due to lattice mismatch can be decomposed into two terms; a uniaxial term and a hydrostatic one. Uniaxiai stress lowers the symmetry of the lattice and splits some degenerate states. In cubic semiconductors [2], such as GaAs, the 4-fold degeneracy of the valence band is lifted resulting in

Table 1 Summary of the samples considered in this study Sample #

CBE208 CBE210 CBE213 CBE214 CBE298 CBE303 CBE306

[400] HRXRD results used in the simulation

Layer thicknesses as determined by dynamical theory based simulation

(AO) (arcsec)

( 1 ) (A)

(a±)

k M L GaP

! M L GaAs

m M L InP

n M L GaAs

- 90 - 126 117 450 18 27 9

212 122 268 73 158 162 158

5.657 5.659 5.648 5.634 5.652 5.652 5.653

4 4 5 4 5 4 4

40 24 40 4 30 29 27

4 4 4 4 4 4 4

40 24 40 4 30 30 30

(A)

Table 2 Summary of the symmetric 400 and asymmetric 511 H R X R D results Sample #

[400] H R X R D results

[511] and [511] HRXRD results

A all/aGaAs

A a ±/aGaAs

A all/aGaA~

A a ±/aGaAs

- 1.000 -2.000 1.000 6.000 3.895 4.548 1.206

6.347 9.115 -7.776 -3.000 -9.810 - 1.634 -3.735

~11' GaAs CBE208 CBE210 CBE213 CBE214 CBE298 CBE303 CBE306

6.713 9.398 -8.727 -3.000 -1.343 -2.014 -6.713

X X x X X )< X

10 - 4

-7.460 - - 1.000 9.698 4.000 1.492 2.238 7.460

10 -4

10 -4 10 -3 10 -4 10 -4 10 -5

X x x x x X X

10 -4 10-3 10 - 4 10 -3 10 - 4 10 - 4 10 -5

X X X X X X X

10 - 3 10-3 10 -3 10 -3 10 -4 10 - 4 10 - 4

X X x X X x x

10 - 4 10 - 4 10 - 4 10 -3 10 -5 10 -4 10 - 5

Table 3 Summary of the results obtained from the structural analysis of the different samples Sample #

k (ML)

1 (ML)

m (ML)

n (ML)

~11' GaAs

X (kbar)

~V3/2 (meV)

~VI/2 (meV)

AEg (meV)

CBE208 CBE210 CBE213 CBE214 CBE298 CBE303 CBE306

4 4 5 4 5 4 4

40 24 40 4 30 29 27

4 4 4 4 4 4 4

40 24 40 4 30 30 30

- 7 . 4 6 0 ) < 10 - 4 - 1.000 X 10 -3 9.69 x 10 -4 4.000 x 10 -3 1.492 x 10 -4 2.238 x 1 0 - 4 7.460 x 10 -5

-0.9198 - 1.2330 1.1958 4.9321 0.1837 0.2749 0.0919

-3.955 - 5.302 5.142 21.2 0.791 1.182 0.395

-8.922 - 11.960 11.599 47.842 1.784 2.660 0.892

3.955 5.302 - 11.599 - 47.842 - 1.784 -2.660 -0.894

A.H. Bensaoula, A. Freundlich / Journal of Crystal Growth 164 (1996) 271-275

274

a J = 3 / 2 mj = 3 / 2 and J = 3 / 2 ml = 1 / 2 heavy hole and light hole bands. The hydrostatic contribution shifts the center of gravity of these energy bands. The resultant splitting of the valence band due to both contributions is given by aVe ,/2 = [ - 2 a ( S , , + 2S,2 ) -

b(S,i

S,2)] X,

-

1.52

1.50

~ ~ O

1.48

1.46

aV± 3/2 = [ - 2a( all + 2S,2 ) + b(S,l - S,2)] X,

(1) where a is the uniaxial deformation potential ( - 9.77 eV for GaAs), b is the hydrostatic deformation potential ( - 1 . 7 eV for GaAs), S~j are the elastic compliances (Sil and $12 respectively equal to 1.176 × 10 -6 and - 0 . 3 6 5 × 10 -6 bar -~ for GaAs) and X is the stress value given in kbar. Applying Eq. (1) for GaAs, we obtain (meV) = 9.7X,

8 V + 1/2

8V± ,/2 (meV) -- 4.3X.

(2)

The results obtained from Eq. (2) based on the HRXRD results are summarized in Table 3. The stress values directly calculated from the HRXRD span the - 1.2 to the + 5 kbar range. Photoluminescence (PL) analysis results obtained at 10 K for this series of samples are given in Fig. 3. We note a direct correlation between the position of the exciton transition peaks and the measured stress given in Table 3. These results are further compared with the theoretically expected values. The plot showing both the experimental and the theoretical

~

1,44

1.42 I1

I

-

I 0

I 1

I

t 2

I 3

I 4

J

I 5

Biaxial Stress (Kbar) Fig. 4. Experimental and theoretical plots of the exciton transition energies versus stress measured.

results is given in Fig. 4. The fine structure of the exciton lines was investigated through excitation photoluminescence (PLE). By an adequate choice of the excitation energy, we are able to pinpoint the different energy levels. We present in Fig. 5, the results obtained for sample CBE210. The PL spectra shown in the same figure, exhibit three distinct peaks. The 1.5145 eV line corresponds to the neutral donor bound exciton (D°X) from the unstrained GaAs layer. The free exciton recombination (F-X) is represented by the 1.5152 eV peak. The 1.5164 eV band, labeled (a), slightly blue shifted and wider than

I

G~$

buffer

I

F~X

C210 2K

D°XI Heterostructure

0.14

0.12

~

0.10

..~ 0.08 0.06

8 ~ o

~ --

0.04

0.02!

""

=

~ - ----

-

- -- CBE21 CBE201 - " CBE291 - CBE30~

. . . . . . . -

~

-

cB~a0i

-

0.~ i 1.35

I

I

I

I

1.40

1.45

1.50

1.55

i

CBI~H CBE2P I 1.60

Energy (eV) Fig. 3. 10 K PL spectra obtained for the different samples. Note the shift in the exciton peaks for different stress values.

1.48

1.50 1.52 P h o t o n E n e r g y (eV)

1.54

Fig. 5. PL spectrum obtained at 2 K for CBE210 (a) strained GaAs in the superlattice. PLE spectrum obtained at 2 K for CBE210.

A.H. Bensaoula, A. Freundlich / Journal of Crystal Growth 164 (1996) 271-275

275

layer thicknesses leading to a change in the supeflattice's band edges given by PL measurements.

~, 40

4. Conclusion

. l x l 0 -3

0

l x l 0 "3

2x10-3

3xl 0 -3

4x10"3

Deformation {delta(a)/a} Fig. 6. Applied biaxial stress versus resonance peaks energies, (line) theoretical expectation, (filled circles) experimental results.

the previous ones corresponds to the response of the strained GaAs in the superlattice. The PLE spectrum obtained for 1.5164 eV excitation energy, presented in Fig. 5, shows clearly two strong absorption lines necessarily associated with excitons of the heterostructure. The first, observed at 1.5211 eV (b), represents the free exciton's ground state. The second, observed at 1.5281 eV (c), is the corresponding excited state. Furthermore, the PLE spectrum shows that the heterostructure's excitonic emission is Stokes shifted by 4.7 meV from its absorption threshold. This proves a moderately strong localization of excitons. In fact, using Eq. (2) it can be noted that the positions of the (b) and (c) absorption lines correspond exactly to the expected energies of the GaAs ground states splitted by a 1.2 kbar compressive biaxial stress. A plot summarizing the results obtained for the different samples is presented in Fig. 6. We note at this point the perfect correspondence between the experimental data and those expected through the application of Eq. (2) (shown in continuous lines). As a consequence, we can conclude that the presence of GaP and InP layers does not yield strong confinement in these GaAs based heterostructures. Furthermore, the shift (Stokes shift) between the experimental and the theoretical data shown in Fig. 5, can be due to minor variations of the different

Preliminary results on the electronic properties of G a P / G a A s / I n P / G a A s structures have been given. The layers of GaAs included in the superlattice are shown to be subjected to stress levels corresponding to those evaluated by HRXRD. Finally, we have shown that the presence of the GaP and InP layers does not yield strong confinement in these GaAs based heterostructures. However, it is not excluded that the minibands are organizing the band structure of the superlattice at higher energies. Further experiments are being conducted to investigate this possibility. So far, the results obtained show the existence of wide absorption bands at higher energies that could be due to such a confinement within the GaAs layers.

Acknowledgements

We would like to thank Dr. A. Ponchet for the TEM measurements and Dr. G. Neu for the PLE analysis and the fruitful discussions.

References [1] J.W. Matthews, Epitaxial Growth, Part B (Academic Press, New York, 1975). [2] T.P. Pearsall, in: Strained-Layer Superlattices: Physics (Academic Press, New York, 1990). [3] A.H. Bensaoula, A. Freundlich, A. Bensaoula and V. Rossignol, J. Vac. Sci. Technol. B 11 (1992) 843. [4] A.H. Bensaoula, A. Freundlich, A. Bensaoula, V. Rossignol and A. Ponchet, J. Vac. Sci. Technol. B 12 (1994) 1110. [5] A. Freundlich, A.H. Bensaoula, A. Bensaoula and V. Rossignol, InP and Related Compounds Int. Conf. Proc., Paris (France), 1993, WB4, pp. 489-492.