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Self-compensation effect of the silicon impurity in heavily doped AlGaN layers
|Igor V. Osinnykh 1,2, Konstantin S. Zhuravlev 2, Dmitry Y. Protasov 2, Timur V. Malin 2, Valentin V. Ratnikov 3, V. Y. Davydov 3, Alexander N. Smirnov 3, Reginold N. Kyutt 3
1. Novosibirsk State University (NSU), Pirogov 2, Novosibirsk 630090, Russian Federation
Due to wide and direct band gap AlGaN alloys have a wide application in light emitting diodes, high electron mobility transistors, high-density optical storage devices, ultraviolet emitters, and photodetectors. Doping of AlGaN layers is needed to fabricate multilayer device structures. Silicon is usually used in molecular beam epitaxy (MBE) for n-type doping of AlGaN layers. However, influence of silicon on AlGaN properties is very complicated and complex. Impurity causes formation of defects in epitaxial layers. Aim of this work was to investigate effect of silicon on electron concentration in AlGaN layers.Sets of GaN and Al0.3Ga0.7N layers were grown by MBE using ammonia as source of active nitrogen and silane (SiH4) as source of Si on (0001)-oriented sapphire substrates. Thickness of the layers was about 1.1-1.3 mm. In order to characterize the structural properties of the AlGaN layers, atomic force microscopy (AFM) and X-ray diffraction (XRD) were used. Concentration of silicon atoms in layers was found from secondary ions mass spectrometry (SIMS) data. Electron concentration (n) was evaluated by Hall measurements and Raman spectroscopy. Optical properties were studied by photoluminescence (PL) techniques. All measurements were carried out at 300K. PL of GaN layers was excited by laser with photon energy of 3.81 eV, and for excitation of PL of Al0.3Ga0.7N layers two types of lasers with photon energy of 466 and 5.08 eV respectively.
In both materials, the concentration of silicon atoms in the layers increases linearly with the silane flux. The dependence of the electron concentration on the concentration of impurity atoms is different in the case of GaN and Al0.3Ga0.7N: if the layers of GaN observed a linear increase with increasing concentrations of silicon, in Al0.3Ga0.7N electron concentration saturates. The difference is explained by a high degree of self-compensation Al0.3Ga0.7N which is caused by either an increase in the number of defects, point or extended, or embedding the silicon atoms in place of nitrogen in the crystal lattice, which is why he is an acceptor. Gallium and aluminum vacancies form deep acceptor levels in AlGaN, the energy of formation of cation vacancies decreases with increasing Fermi energy. As Al0.3Ga0.7N having a wide band gap, it has more cation vacancies. Transitions with vacancy’s deep levels lead to appearance of wide yellow band in PL spectra. Experimental results show that in the case of GaN increase of doping leads to suppression of the defect band in the PL spectra induced cation vacancies, while the growth of silicon concentration in AlGaN its intensity increases. However, with increasing concentration of the donor concentration of cation vacancies in AlGaN is growing more slowly than in GaN, so the saturation of the electron concentration on the contrary should occur faster in GaN. Concentration of edge dislocations in AlGaN increase faster with increasing silicon content than in GaN, therefore we assume that rise of concentration of this type defects leads to decline of efficiency of dopind in Al0.3Ga0.7N. In addition, the visible surface of the AlGaN extended defects in heavily doped, they are replaced by defects of hexagonal shape, with the doping concentration of defects increases strongly. Presumably, these defects occur at the grain boundaries during the growth layers and may make compensation centers.
Presentation: Poster at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, General Session 10, by Igor V. Osinnykh
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17
Submitted: 2013-04-15 15:27 Revised: 2013-04-15 15:27