Chemical ordering in AlGaN layers grown by MOCVD
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Chemical ordering in AlGaN layers grown by MOCVD P. Ruterana1, F. Omnes* ESCTM-CRISMAT, Institut des Sciences de la Matière et du Rayonnement, 6 Bd Maréchal Juin, 14050 Caen, France * Centre de Recherche sur l' Hétéroépitaxie et ses Applications (CRHEA - CNRS), rue Bernard Gregory Sophia-Antipolis, 06560Valbonne, France
Abstract One of the applications of wurtzite gallium based nitride compounds will be to provide optoelectronic devices from 6.2eV (AlN) to 1.89eV (InN). This will depend on the possibility to grow wurtzite AlGaN and InGaN ternary alloys. As expected, the most difficult region is InGaN due to the large misfit between GaN and InN (= 10%), in this case, ordering, phase separation and growth instabilities have been reported. In the case of AlN and GaN, the misfit is smaller ( ≈ 2.5%) and one would expect more stable growth. However, it was in this system that ordering along the c axis between AlN and GaN was reported for the first time. In this work, we have found that the growth of AlGaN may be more complicated. Not only the wurtzite lattice can be decreased to simple hexagonal by AlN/GaN ordering along the c axis, but the growth can lead to other types of stackings. Even in the low Al composition range, 10 – 15%, we have found that three processes can operate : 1. Ordering into AlN/GaN as two simple hexagonal sublattices. 2. The 3 :1 ordering which has been recently reported to occur in InGaN. 3. A new type of ordering where diffraction experiments (XR and electron diffraction) detect superlattice reflection with a period close to 3 nm. The most adequate model which was found to take this into account shows that, in these growth conditions, the system has preferred to form one AlN cell in between 5 GaN cells, leading to a 5 :1 ordering.
1. Introduction Wurtzite GaN based semiconductors are under worlwide investigation due to their large direct band gap which allows the possibility to make optoelectronic devices from red (InN: 1.89 eV) to the deep ultraviolet range (AlN: 6.2 eV). Starting from GaN, one has to alloy with In for narrowing the gap, or with Al for broadening it. As expected, the most difficult part is InGaN due to the large mismatch value close to 10% between GaN and InN. In this case, ordering, phase separation and growth instabilities have been reported [1-4]. For AlN and GaN, the misfit is smaller (≈ 2.5%), but it was in this system that ordering along the c axis was reported for the first time by Korakakis et al.[5]. Another HREM report has shown that composition fluctuations along the c axis could take place with a periodicity of approximately 1nm, however no crystallographic analysis was provided [6]. A recent study has shown that 3 :1 ordering could take place inside InGaN layers grown by metalorganic chemical vapor deposition (MOCVD) [4]. The InxGa1-xN samples were in the composition range of 0 < x < 0.5. This ordering corresponds to In0.25Ga0.75N and was observed in samples with x = 0.49 and 0.21, implying that the ordered areas were small domains within the InxGa1-xN random 1
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