Electron beam induced asymmetric indium segregation in LP-MOCVD
InGaN QWs studied by quantitative electron microscopy
S. Kret1, P.Dłuzewski1, A. Szczepańska1, J. Dąbrowski1, R. Czernecki 2,
M.Zak1,3,, J-Y Laval3 , M. Krysko2, M.Leszczynski2,
1
Institute of Physics, PAS, Al. Lotników 32/46, 02-668 Warsaw, Poland
High Pressure Research Center, PAS, Sokolowska 29/37, Warsaw 01-142, Poland
3
Laboratoire de Physique du Solide UPR 5 CNRS ,ESPCI, 10 rue Vauquelin, 75231
Paris,Cedex 05 , France
2
The indium distribution in InGaN quantum wells (QW) strongly influences
luminescence properties. Quantitative electron microscopy is usually used for direct
determination of the indium distribution in such structures. However, InGaN is a sensitive
material for electron beam. In this work we show that significant changing in indium
distribution occurred after the first 2 minutes of the in-situ electron beam irradiation in
electron microscope. InGaN/GaN QW samples used in the presented study were grown by
low-pressure metal-organic chemical vapor deposition (LP-MOCVD) on high-pressure bulk
GaN substrates. Ten periods InGaN/GaN QW were grown at 780ºC by using trimethylindium (TMIn), triethyl-gallium (TEGa), and NH3. QWs were grown under a N2 ambient to
increase the In composition of the InGaN well layers.
The cross-sectional TEM samples were prepared by mechanical dimpling down to 30 m.
Electron transparency was obtained by ion milling with 4kV accelerated argon ions without
cooling. We used microscope JEOL 2000EX equipped with a LaB6 electron gun as well as
JEOL 2100F equipped with a field emission gun (FEG). The lattice fringes images were taken
in optimized two and three beam conditions for [11.0] zone axis. Such images with 00.2 and
00.1 fringes were used to measure the local tetragonal distortion and to calculate local indium
fraction x. Relatively low dose with the LaB6 microscope allows us to obtain unperturbed
indium distribution mapping in the sample with composition as high as x=0.2 as it is shown
on the figure (a). Obtained results clearly show homogenous distribution of indium without
clustering. The indium segregation under electron beam was observed for the next 20 minutes
and final state is shown on figure 1(b). Indium reach clusters are formed on the “bottom”
interface, which could come from the fact that indium concentration at the first monolayer of
the well was higher than for the next.
Figure 1. lattice fringes image of
MQW In0.18Ga0.82N .
(a )first shot after 30 s of irradiation
(b) after 20 minutes
In FEG microscope, where electron irradiation is much stronger, the similar segregation
occurred even for concentration x=0.1
This work is partially supported by the KBN grant No. 4 T07A 01026.