Characterization of a-plane grown GaN on sapphire substrates by electron microscopy

Gallium nitride (GaN) is one of the most interesting materials for devices applications such as blue light emitting diodes, laser diodes and high power and high temperature electronic applications, because of its large band gap (3.39 eV). Several growth techniques including metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) and hybrid vapor phase epitaxy (HVPE) are employed to synthesize high quality GaN. Sapphire, that is available in several orientations, is the common substrate to grow GaN. The GaN that grown on c-(0001) sapphire, has a wurtzite structure and suffers of spontaneous polarization effects. These effects create internal electric fields that affect the operation and the efficiency of the devices. This work is part of a large effort to grow defects free GaN layers in non-polar geometries undertaken at LASPE-IQEP-EPFL. Among the non-polar geometries, the a-(1120) GaN geometry has been selected. The specimens were produced using the HVPE and Epitaxial lateral Overgrowth (ELO) techniques and were fully characterized mainly by electron microscopy observations. Despite the ELO improvements and smaller lattice mismatch in a-GaN (≈1.1%) than c-GaN (≈16%), a high density of dislocations and stacking faults are still observed. They are results of the lattice mismatch and the difference of the thermal expansion coefficients between the components. The reduction of defect density due to ELO has been found to be approximately two orders of magnitude for dislocations (1 × 1010 cm-2 to 3 × 108 cm-2) and for stacking faults (1 × 106 cm-1 to 4 × 104 cm-1). In the ELO window areas, the threading dislocations have screw and mixed characters with Burgers vector b→ = 1/3 [1120] and b→ = 1/3 [1123] respectively. In the overgrown ELO areas, the dislocations have different characters such as screw character with b→ = 1/3 [1120], edge character with b→ = 1/3 [2110] and partial dislocations. Different types of stacking faults have been observed in the a-GaN film: basal (BSF) I1 and I2, prismatic (PSF) (1120) and (1010). The I2 BSF is bordered by two Shockley partial dislocations of Burgers vector b→ = 1/3 < 1010 > and its energy γ has been calculated: 50 ergs/cm2. Prismatic stacking faults, R→ = 1/2 [1101] are located at the end of the I1- BSFs, where stair rod dislocations are formed at the intersection of the two faults. In order to know the influence of these defects on the optical properties, cathodoluminescence (CL) and microphotoluminescence (µ-PL) experiments were performed. These observations demonstrate that the optical properties are affected by the high density of stacking faults. The CL spectra show 4 different emission peaks where the dominant emission at 3.42 eV is attributed to the I1 BSFs. In (µ-PL spectra, 4 emission peaks are also observed, where the dominant one at 3.44 eV in the window area of the ELO mask is also attributed to the SFs and that at 3.49 eV in the overgrown mask area is the typical NBE transition. This shows that locally the GaN film is of good quality. A single GaN quantum well (SQW) intercalated by two Al1-xGaxN layer were grown on HVPE-ELO templates by MBE. The structural characterization of six specimens have shown a high pits density on the sample surface (1 × 1012 cm-2) associated with the threading dislocations of screw character ending at the bottom of the pits. Moreover, the pits created at the intersection, near the surface, of several dislocations groups have been observed. A high density of BSFs (1 × 105 to 1 × 106 cm-1) was found in the samples, which are formed at Al1-xGaxN/GaN interface and propagated towards the specimen surface. We also observed that these stacking faults affect the optical properties of the SQW-GaN. Our results have shown that the inhomogeneity of the optical properties of a-(1120) GaN layers is associated with specific structural defects, in particular BSFs. These results provide guidelines to design new growth procedures aimed at improving the overall quality of such GaN films.

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