Structural Investigations of (GaIn)(NAs)/GaAs Multi-Quantum-Wells by Transmission Electron Microscopy
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ABSTRACT The structure of compressively strained (Galn)(NAs)/GaAs multi-quantum wells (MQWs) grown by MOVPE is investigated using TEM. The quaternary, metastable material exhibits a high structural perfection if a N concentration of 4% is not exceeded. Phase separation or clustering effects are not observed, and the In is dispersed homogeneously throughout the quantum wells. The interface roughness of the quantum wells to the GaAs barriers is in the order of several monolayers. Increasing the N content to above 4.5% results in a deterioration of the structure and of the homogeneity of the wells.
INTRODUCTION Mixed III(NAs)/GaAs materials have attracted increasingly more interest recently as they offer the possibility of realizing 1.3 and 1.55 lim wavelength emission devices [1,2] based on GaAs substrates. The long wavelength emission is due to the large band gap bowing of this material system. However, the quaternary material (Galn)(NAs) examined in the present study exhibits a large miscibility gap under thermodynamic equilibrium conditions. Therefore, the MQWs are grown under extreme non-equilibrium conditions at low temperatures by using metal organic vapour phase epitaxy (MOVPE) [2]. Possible phase separation effects due to the large miscibility gap are of great importance as they could lead to local strains and to a deterioration of the well structure and hence of the optical properties. On the other hand, little is known about the incorporation behavior of N and In, which is expected to have strong influence on the structure and properties of the MQWs [3,4]. In the present work, cross-sectional transmission electron microscopy (TEM) in combination with energy filtering (EF) techniques has been applied to study the structure of and element distribution throughout the MQWs in dependence on varying growth conditions.
EXPERIMENTAL The MQW structures examined in this study were grown in a horizontal reactor system by MOVPE using triethylgallium (TEGa), trimethylindium (TMIn), tertiarybutylarsine (TBAs) and 1,1-dimethylhydrazine (UDMHy) as precursor substances [2]. The temperature of the (001) GaAs substrates achieved by IR heating was as low as 525 'C in order to suppress phase separation during epitaxial growth. Samples with In concentration up to 35% and Nconcentration up to 4.5% have been grown by varying the partial pressure of TMIn and TBAs. The growth rate was varied in the range of 0.125 to 0.5 jim/h. 291 Mat. Res. Soc. Symp. Proc. Vol. 618 © 2000 Materials Research Society
[110] cross-sectional and [001] plan-view samples for TEM analysis were prepared by mechanical polishing and subsequent Ar ion sputtering at 8 keV under low angles until perforation. A final 400 eV Ar ion polishing step was applied for improving the quality of the sample surfaces. The TEM investigations were performed in 300 keV JEM 3010 microscopes, equipped with a 2k x 2k slow-scan CCD Camera and a Gatan Imaging Filter (GIF), respectively. The roughness of the QWs is always given in the [001] growth direction in units of monolayers
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