Chemical characterization of extra layers at the interfaces in MOCVD InGaP/GaAs junctions by electron beam methods
© Frigeri et al; licensee Springer. 2011
Received: 10 September 2010
Accepted: 3 March 2011
Published: 3 March 2011
Electron beam methods, such as cathodoluminescence (CL) that is based on an electron-probe microanalyser, and (200) dark field and high angle annular dark field (HAADF) in a scanning transmission electron microscope, are used to study the deterioration of interfaces in InGaP/GaAs system with the GaAs QW on top of InGaP. A CL emission peak different from that of the QW was detected. By using HAADF, it is found that the GaAs QW does not exist any longer, being replaced by extra interlayer(s) that are different from GaAs and InGaP because of atomic rearrangements at the interface. The nature and composition of the interlayer(s) are determined by HAADF. Such changes of the nominal GaAs QW can account for the emission observed by CL.
Several devices, such as HBTs, HEMTs, solar cells and LEDs, are currently based on InGaP/GaAs heterojunction because of its superior properties with respect to AlGaAs [1–4]. The InGaP/GaAs system, especially if it is grown by metal organic vapour phase deposition (MOCVD), has, however, the drawback that the interfaces between InGaP and GaAs are deteriorated, as shown by photoluminescence, X-ray diffraction and transmission electron microscopy (TEM), because there is no common group V element across the interface . This mostly 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 [5–10]. The normal InGaP-on-GaAs interface is always good, but this is not sufficient to guarantee reliable device performance. The deterioration of the inverted GaAs-on-InGaP interface has been seen to occur in practically every MOCVD InGaP/GaAs heterostructure containing such an interface, to a more or less great extent depending on the growth conditions [5–10]. It could sometimes be avoided by the use of growth interruption between the layers , the growth on top of InGaP of a thin (1 nm) intentional interfacial layer of GaP [5, 7, 9] or GaAlAs , or the application of a preflow of trimethylgallium on the InGaP surface before switching on the AsH3 flow .
A recent contribution to this field was based on cathodoluminescence (CL) measurements [12, 13]. The difference between the two interfaces was confirmed by comparing two InGaP/GaAs systems containing a GaAs QW and either one of the two interfaces [12, 13]. One sample had the layer sequence GaAs substrate/GaAs buffer/AlGaAs/GaAs/InGaP with the normal interface. It showed the expected GaAs QW emission (1.56 eV at 77 K). The other sample had the sequence GaAs substrate/GaAs buffer/InGaP/GaAs/AlGaAs with the inverted GaAs-on-InGaP interface. This sample did not exhibit the expected QW emission. On the contrary, a CL peak was seen at 1.48 eV, which suggested that the GaAs QW was absent, having been replaced by a transition layer of InGaAsP with mixed composition [12, 13]. The aim of this study is to check by TEM whether the CL results can be related to structural modifications of the GaAs QW, such as the presence of an interlayer of the type described earlier. An additional objective is to determine the composition of any extra layer that could have been formed by using the innovative chemically sensitive high angle annular dark field (HAADF) method in a scanning TEM thanks to its square dependence on the atomic number.
The InGaP/GaAs structures were grown by MOCVD at 973 K using an Emcore GS3100 reactor, and they had the following layer sequence: (100) GaAs substrate/GaAs buffer (180 nm)/InGaP (130 nm)/GaAs QW (10 nm)/AlGaAs (370 nm) cap. The expected layer thickness is given in brackets. Both CL and TEM gave 160 nm for InGaP, 360 nm for AlGaAs and 10 ± 1 nm for QW. They were analysed by spectroscopic CL and TEM. CL was done at temperatures of 300 and 77 K in an electron-probe microanalyser Camebax supplied with the CL system. TEM observations were done in an FEG 2200FS JEOL instrument on <011> cross-sectional specimens prepared by the standard sandwich procedure and finally thinned with Ar ion bombardment. The (200) dark field (DF) mode and the HAADF method in association with the scanning operation of the TEM (STEM) were used for detection of interface modifications and composition.
Results and discussion
The alloy whose R matches the experimental ratio R exp is the one that a sublayer is made of.
Values of the experimental ratio R exp of the HAADF intensity I HAADF of sublayer #1 to those of the three alloys (GaAs substrate, In0.51Ga0.49P and Al0.26Ga0.74As) contained in the sample and used as standards.
Values of the experimental ratio R exp of the HAADF intensity I HAADF of sublayer #2 to those of the three alloys (GaAs substrate, In0.51Ga0.49P and Al0.26Ga0.74As) contained in the sample and used as standards
The deterioration of the structure of the GaAs QW in an InGaP/GaAs/AlGaAs heterostructure grown by MOCVD has been studied by CL and (S)TEM. The chemically sensitive (200) DF and HAADF methods of (S)TEM helped us to establish that the nominal GaAs QW has changed its structure, being replaced by two sublayers made of InGaAsP with different compositions. The sublayer closer to the inverted GaAs-on-InGaP interface is more In and P rich than the one on the side of the AlGaAs-on-GaAs interface. The composition of the extra layer of InGaAsP closer to the inverted GaAs-on-InGaP interface, as determined by STEM-HAADF, reasonably accounts for the anomalous emission measured by CL. The formation of the extra layers during growth was ascribed to the rearrangement of the atoms available at the inverted GaAs-on-InGaP interface caused by In segregation in the growth direction and P/As intermixing during the early stages of the GaAs QW growth.
high-angle annular dark field
metal organic vapour phase deposition
transmission electron microscopy.
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