Structural, optical and electrical properties of GaN films grown by metalorganic chemical vapor deposition on sapphire.
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Structural, optical and electrical properties of GaN films grown by metalorganic chemical vapor deposition on sapphire. P. Visconti 1,2, M. A. Reshchikov, F. Yun, K. M. Jones and H. Morkoç Virginia Commonwealth University, Dept. of Electrical Engineering, Richmond, VA, 23284
A. Passaseo, E. Piscopiello, A. Pomarico and R. Cingolani 1
Istituto Nazionale di Fisica della Materia, Unita’ di Lecce and Dipartimento di Ingegneria dell’Innovazione, Universita’ di Lecce, 73100, Lecce, ITALY.
M. Lomascolo, M. Catalano 2
Istituto per lo Studio di Nuovi Materiali per l’Elettronica, CNR, Via Arnesano, 73100, Lecce, ITALY.
ABSTRACT Properties of GaN layers grown by metalorganic chemical vapor deposition (MOCVD) on cplane of sapphire have been investigated using atomic force microscopy (AFM), wet etching for defect investigation, transmission electron microscopy (TEM), high-resolution X-ray diffraction, Hall effect measurements and low-temperature photoluminescence (PL). Tapping-mode AFM images of the as-grown samples showed atomically smooth surfaces (rms roughness ≈ 0.2 nm) consisting of terraces separated by about 3Å bi-layer steps. Hot H3PO4 chemical etching was used to produce hexagonal-shaped etch pits at the surface defect sites as revealed by AFM imaging. The obtained etch pit densities (9x108 - 2 x109 cm-2) were in agreement with the dislocation density found by plan-view and cross-sectional TEM observations. The full-width at half-maximum (FWHM) of the X-ray diffraction rocking curve was about 4.8 and 3.9 arcmin for the symmetric (002) and asymmetric (104) directions, respectively. PL spectrum at 15 K demonstrated sharp peaks (FWHM ≈ 4 meV) in the excitonic region, which were attributed to free and bound excitons. The spectrum contained also weak PL bands with maxima at about 2.2, 2.9 and 3.27 eV, which have been attributed to three different acceptors. INTRODUCTION Nitride semiconductors and their heterostructures are very promising materials for optical emitters and detectors, and high power/temperature electronic devices [1,2]. They have been deposited by hydride vapor phase epitaxy (HVPE) [3], metalorganic chemical vapor deposition (MOCVD) [4], and by molecular beam epitaxy (MBE) [5]. The most commonly used substrates, such as sapphire and silicon carbide poorly match with GaN material resulting in a high density of threading dislocations. These defects are believed to affect both optical and electrical properties and therefore to be responsible, for instance, for the high threshold-current required to achieve lasing in diodes. For this reason, to get to a better understanding of the material quality, a thorough characterization should be made. We have carried out a comprehensive analysis of the structural, optical and electrical properties of GaN films grown by MOCVD on sapphire substrates. E3.8.1
EXPERIMENTAL DETAILS The epitaxial growth of the unintentionally doped GaN layers was performed in a horizontal low pressure (LP)-MOCVD system (AIXTRON 200 AIX RF), equipped with a rotating substrate holder, heated
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