ATOMIC STRUCTURE OF INTERFACES IN GaAs/Ga1-xAlxAs SUPERLATTICES

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1 ATOMIC STRUCTURE OF INTERFACES IN GaAs/Ga1-xAlxAs SUPERLATTICES J. Laval, C. Delamarre, A. Dubon, G. Schiffmacher, G. Teste de Sagey, B. Guenais, A. Regreny To cite this version: J. Laval, C. Delamarre, A. Dubon, G. Schiffmacher, G. Teste de Sagey, et al.. ATOMIC STRUCTURE OF INTERFACES IN GaAs/Ga1-xAlxAs SUPERLATTICES. Journal de Physique Colloques, 1987, 48 (C5), pp.c5-97-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1987 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C5, suppl6ment au null, Tome 48, novembre 1987 ATOMIC STRUCTURE OF INTERFACES IN GaAs/Ga,-,Al,As SUPERLATTICES J.Y. LAVAL, C. DELAMARRE, A. DUBON, G. SCHIFFMACHER*, G. TESTE de SAGEY*, B. GUENAIS' * and A. REGRENY" Laboratoire de Microstructures, CNRS-ESPCI. 10, Rue Vauquelin, F Paris, France *ER 210, CNRS, 1, Place Aristide Briand, F Meudon, France "centre National dcetudes des T6l~communications, DBpartement MPA, Route de Tr&gastel, BP 40, F Lamion Cedex, France The free exciton photoluminescence linewidth depends on the distribution and size of the interfacial growth islands. Therefore one sees that the determination of the atomic structure of the interfaces is greatly needed. We have shown previously that it was possible to visualize atomic steps at the GaAs/AlAs interfaces of MBE grown superlattices along Cl101 orientation. This paper considers whether it is possible to complete this information by considering the contrast obtained with other orientations on the one hand and with the binary/ternary interfaces on the other hand. Atomic structure of GaAsIAlAs superlattices We have shown that high resolution electron microscopy at 100 kv with the [l101 orientation leads to a highly contrasted structure image of GaAs/AlAs heterostructures (1). However such an image gives only one projection of the atomic structure. In order to ccmplete this information one needs to ccmpare structure images on different planes. Such an analysis can be carried out only if the electror microscope possesses both a top resolution and a high angle tilt facility. The JEOL ARM 1000 at NCEM (LBL Berkeley) which has a point resolution of 0.17 nm and a +45" tilt angle enabled us to observe simultaneously (loo), (110) and (310) planes on (3.10) cross sections at 800 kv. The interpretation of such HREM images implies a systematic comparison with simulated images, calculated with the multislice method by varying voltage, thickness and defocus. By means of these image-simulation comparative techniques we succeeded to identify atcmic steps at the reverse interface of GaAs/AlAs superlattices along [l101 orientation in HREM at 100 kv (2). The most favourable conditicns are a 15 nm thickness and a -70 nm defocus Af. The different simulated images obtained at 800 kv are displayed figure 2. The most favourable conditions occur for t= 20 nm and Af= -60 nm. It is seen that the optimal thickness is noticeably increased. Article published online by EDP Sciences and available at

3 JOURNAL DE PHYSIQUE Atomic structure of G ~ A S ~ C ; ~ superlattices ~ - ~ A ~ ~ A ~ The binarylternary interfaces of GaAs/Gal-,A1 As superlattices have been simulated for different X values at 800 kv. In the case of a step at the interface, the structure images have been computed with multiple cells comprising 8x8 elementary cells for the Cl001 direction and 8x12 elementary cells for the C1101 direction. They hold about 500 atoms and it js necessary to use beams. The objective aperture gives a d cut off value equal to 7.6 nm-. The image is then reconstructed by means of 4461 beams. The evolution of the contrast with X is shown on fig. 3 and 4. It can be seen that the visibility limit is quite different according to the imaging direction. The C1001 direction appears to be more favourable than the C1101 one. For x=0.3 heterolayers cannot be visualised anymore in the [110] direction whereas they are still clearly visible for x=0.2 in the [l001 direction. Finally fig.5 shows the observed and simulated images obtained with the C3101 orientation which corresponds to spacing8 between diatomic columns (Ga/Al)As equal to 0.17 nm and 0.28 nm. Conclusion [l101 cross sections lead to the identification of atomic steps at the GaAs/AlAs interfaces of MBE grown superlattices. In order to complete this information on the atomic structure at the interface, we have carried out experiments at 800 kv on C3101 cross sections which enable to observe simultaneously C1001, C1101 and C3101 planes. We have simulated the corresponding structure images for different defocus and thickness. The most favourable conditions occur for t=20 nm and bf=-60 nm. We have then considered the visibility of the interface in the case of the binarylternary heterostructures. It appears to be much better for C1001 direction than [110] for low X values. Heterolayers can be seen in C1101 for X > 0.4 but they are still visible in C1001 down to x=0.2. Acknowledgements The authors wish to thank CNRS (PIRMAT) for an ATP contract (Interfaces dans les semiconducteurs) as well as the NCEM staff (LBL Berkeley) and especially C. Nelson for his most valuable assistance with the ARM 1000 high resolution microscope. References 1) Lava1 J.Y. and al. Inst. Phys. Conf. Ser. no (1985) 2) Oelamarre C. and al. J. Micr. Spect. Elec. 12 (1987)

4 C- - -_ kv Direction [l101 Binarylbinary with steps GaAs/AlAs HREM image simulation t=15 nm Af=-70 nm FIGURE 1 Structure image and simulation of an interface. 800 kv Direction [l 101 Binary/binary Defocus Thickness 15 nm 20 nm FIGURE 2 Simulations of an interface without step.

5 C T)URNAL DE PHYSIQUE X = l X =.5 X = -3 1 nla - FIGURE 3 Simulation of an interface with ste~. 800 kv Direction [l001 Binarylternary with steps X = l X =.5 X = -3 l nm- FIGURE 4 SimuIation of an interface with step kv Direction C3101 Bi narylternary HREM image Simulation t= 20 nm Af= -60 nm FIGURE 5 Structure image and simulation of the heterostructure.