Istituto dei materiali per l'elettronica ed il magnetismo     
Frigeri C., Shakhmin A. A., Vinokurov D. A., Zamoryanskaya M. V. CL and Dark Field TEM Analysis of Composition Change at Interfaces in InGaP/GaAs Junctions Grown by MOCVD. In: BIAMS 2010 - 10th International Workshop on Beam Injection Assessment of Microstructures in Semiconductor (Halle (D), July 04 - 08 2010). Abstract, p. 09. MPI, Halle (D), 2010.
Because of its superior properties with respect to AlGaAs, InGaP is a key material for several devices, like HBTs, HEMTs, solar cells and LEDs, which are currenly based on the InGaP/GaAs heterojunction. For MOVPE grown InGaP/GaAs it has been shown by PL, X-ray diffraction and TEM that it is difficult to grow abrupt interfaces between InGaP and GaAs because there is no common group V element across the interface. This especially affects the inverted GaAs-on-InGaP interface where an unwanted extra interlayer forms which recombines the minority carriers more efficiently than the GaAs quantum well. Here we report on a study of the composition change at interfaces in InGaP/GaAs heterojunctions grown on GaAs (100) substrate by low pressure MOCVD using "Emcore GS3100" installation by means of cathodoluminescence (CL) and dark field TEM. The growth temperature was 700C. After the GaAs buffer the samples contained an InGaP layer nominally followed by a 10 nm thick GaAs layer capped with AlGaAs at the top. They were analysed by spectroscopic CL at the temperature of 300 K and 77 K in an electron-probe microanalyser Camebax supplied with the CL system. Cross section TEM observations in the dark field mode were carried out in a 2200 FS JEOL machine. Cathodoluminescence studies at various electronic beam energies allow receiving spectra of structures from various depths. In the CL spectra of the structure at 77K there are bands corresponding to AlGaAs layer at 1.89 eV, and InGaP layer at 1.94 eV. For this structure the only wide band of luminescence at 1.49eV corresponding to bulk GaAs was observed. In this connection we assume that instead of the 10 nm GaAs QW layer the quaternary layer of the mixed composition In0-0.15GaAsP0.02 was formed. By (002) dark field TEM it was observed that the nominal 10 nm thick GaAs layer on top of InGaP had contrast darker than the GaAs buffer layer which indicates that it is not made of GaAs. Theoretical calculation of the (002) dark field contrast suggested that such layer can be InxGa1-xAs1-yPy for some composition ranges of x and y that agree with the composition estimated by CL. The asymmetry of the TEM intensity profile across this layer is evidence that its composition is not homogeneous, being closer to the expected GaAs one far from the interface with InGaP. Therefore, the existence of a transition interlayer at the inverted GaAs-on-InGaP interface is confirmed. The formation of InGaAsP requires that In segregation from InGaP into GaAs took place. It is envisaged that the first step for the formation of the quaternary layer might be the formation of GaAsP by P/As intermixing at the inverted interface when the incoming As atoms merge with the residual P ones. The tensile strain field created by GaAsP can then be the driving force for the incorporation of In because the large atoms (like In) easily segregate in regions of high tensile strain. As the thickness of the nominal 10 nm thick GaAs layer increases fewer and fewer residual P atoms are available thus making the formation of GaAsP, and accordingly the segregation of In and the formation of InGaAsP, more and more difficult and the growth of pure GaAs possible, as expected. Such mechanism would be confirmed by the asymmetry of the dark field TEM contrast mentioned above.
Subject TEM

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