Composition and strain determination of (Al,Ga)N alloy films
O. Caha1,2, P. Kostelník3, T. Novák3
1CETEC MU, Masaryk University, Kotlářská 2, 611 37 Brno
2Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno
3ON Semiconductor Czech republic, Rožnov pod Radhoštěm
caha@physics.muni.cz
Wide band gap semiconductors are widely used for blue LEDs in a modern light production. They are also of growing interest for high voltage power applications where high charge carrier mobility and electric field strength allows better high frequency performance of electronic devices than standard silicon technology. LED technology often use sapphire (0001) substrates. Our goal is to develop GaN growth technology using cheaper and easier available silicon (111) substrates.
As a part of those studies we have grown series of samples (Al,Ga)N/AlN ternary alloy layers on silicon (111) substrate within a full concentration range, i.e from pure AlN to GaN. Such series allowed a systematic study of the concentration dependence of the wurtzite lattice parameters, band gap, and phonon frequencies.
Figure 1. Representative reciprocal space map in the vicinity of 1124 reciprocal lattice point. The line represents position of an unstrained (Al,Ga)N alloy peaks with emphasized points corresponding pure AlN and GaN.
The measurement were performed in a couple of diffractions, typically 0004 and 1124 as a symmetric and asymmetric diffraction. A representative reciprocal space map in the vicinity of 1124 reciprocal lattice point is plotted in figure 1.
The concentration dependence of the lattice parameter was determined assuming linear Vegard's law in both a and c hexagonal lattice parameters [1]
|
. |
(1) |
|
. |
(2) |
We have tested the linear dependence of the (Al,Ga)N alloy using energy dispersive x- ray spectroscopy (EDXS) in scanning electron microscope with electron beam voltage of 4 keV, which does not penetrate below the (Al,Ga)N layer. The results are are shown in figure 2 and shows a good agreement within precision of 0.015 in chemical composition.
Figure 2. Chemical composition
of (Al,Ga)N alloy determined using EDS plotted with respect to the chemical
composition determined using Vagard's law from XRD data.
The x-ray diffraction allows us to decouple chemical composition with a reasonable precision of 0.01 and strain of all the layers in the structure. Therefore, x-ray diffraction was used as a primary reference method to determine (Al,Ga)N alloy composition. The layers were studied also using a couple of optical spectroscopic methods, especially VIS-UV reflectometry and Raman scattering. The advantage of the optical methods is speed and possibility of wafer mapping. On the other hand those methods lacked proper calibration and the concentration information is also affected by a strain. The calibration curves of optical band gap and Raman phonon frequencies are reported [2].