Polarity determination of a GaN thin film on sapphire (0001) with x-ray standing waves (original) (raw)
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Morphology, polarity, and lateral molecular beam epitaxy growth of GaN on sapphire
Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 1999
Gallium nitride was grown on sapphire ͑0001͒ substrates by radio frequency plasma assisted molecular beam epitaxy. The surface morphology was characterized during growth by reflection high energy electron diffraction, and ex situ by scanning electron microscopy ͑SEM͒, atomic force microscopy ͑AFM͒ and x-ray diffraction. It is found that surface morphological features are linked to domains of specific wurtzite crystal polarity, ͑0001͒Ga face or (0001 )N face, for Ga-rich growth. For growth on AlN buffer layers, we commonly observe films which consist of largely ͑0001͒Ga polarity material, as confirmed by selective etch tests, with a varying coverage of (0001 )N-face inversion domains threading along the growth direction. For growth near stoichiometric conditions, the growth rate of the N-face domains is slightly lower than that for the Ga-face matrix, which results in the formation of pits with inversion domains at their centers. For samples grown by first depositing GaN under N-rich conditions, followed by growth under Ga-rich conditions, a different morphology is obtained, exhibiting large hexagonal flat terraces observable by SEM and AFM. The apparent grain size of these films is increased substantially over films grown using a single step approach. The cross sectional SEM images of the two-step films show a network of voids and columns at the interface between the N-rich and the Ga-rich layers, above which micron-scale islands form and coalesce via lateral growth. Lateral growth may result in reduced defect density and improved crystal quality. The asymmetric x-ray peak (112 4) width is reduced to approximately 280 arcsec in the two-stage GaN films.
Optical properties of GaN with Ga and N polarity
MRS Proceedings, 2001
We compared photoluminescence (PL) and cross-sectional transmission electron microscopy (TEM) characteristics of GaN samples with Ga and N polarities grown by molecular beam epitaxy (MBE) on sapphire substrates. Ga-polar films grown at low temperature typically have very smooth surfaces, which are extremely difficult to etch with acids or bases. In contrast, the N-polar films have rougher surfaces and can be easily etched in hot H3PO4 or KOH. The quality of the X-ray diffraction spectra is also much better in case of Ga-polar films. Surprisingly, PL efficiency is always much higher in the N-polar GaN, yet the features and shape of the PL spectra are comparable for both polarities. We concluded that, despite the excellent quality of the surface, MBE-grown Ga-polar GaN layers contain higher concentration of nonradiative defects. From the analyses of cross-sectional TEM investigations, we have found that Ga-polar films have high density of threading dislocations (5x109 cm-2) and low de...
Polarity of GaN Grown on Sapphire by Molecular Beam Epitaxy with Different Buffer Layers
Physica Status Solidi (a), 2001
We report on polarity of GaN films grown on sapphire substrates by molecular beam epitaxy using different buffer layers and growth conditions. On high temperature AlN or GaN buffer layers, the GaN films typically show Ga or N-polarity, respectively. When low temperature (either AlN or GaN) buffer layers are employed, GaN films of both polarities can be grown, but these films have high density of inversion domains. Insertion of additional GaN/AlN quantum dot layers between the buffer layers and the GaN films provides strain relief and a significant improvement in the quality of the GaN epilayers.
Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 2002
We report the surface, structural, and optical properties of typical Ga-and N-polar GaN films grown on sapphire substrates by molecular beam epitaxy. The Ga-polar films were grown on AlN buffer while the N-polar films were grown on GaN buffer layers. Atomic force microscopy imaging shows that the as-grown and chemically etched Ga-polar films have a flat and pitted surface while the N-polar surface is rougher with isolated columns or islands. Transmission electron microscopy demonstrates a low density of inversion domains in the Ga-polar films, while a much higher density of inversion domains was observed in the N-polar films. X-ray diffraction curves show a narrower ͑002͒ peak for Ga-polar films than that for N-polar films. On the other hand, both Ga-and N-polar films show a similar width of ͑104͒ peak. Despite their rough surfaces, high density of inversion domains, and broader ͑002͒ x-ray diffraction peaks, N-polar films with low dislocation density were demonstrated. In addition, higher PL efficiency for the N-polar films than that for the Ga-polar films was observed.
Journal of Crystal Growth, 2008
Thick GaN films, with (11 2 0) or (11 2 6) planes parallel to the r-plane of sapphire, were grown by molecular beam epitaxy using AlN or GaN buffer layers. Characterization by transmission electron microscopy revealed a high density of basal-plane stacking faults (BSFs) in the (11 2 0) non-polar GaN (a-GaN) films. {11 2 0} and {1 0 1 0} prismatic-plane and {11 0 2} pyramidal-plane stacking faults (SFs) were observed to terminate some BSFs. The SFs on {1 0 1 0} and {11 0 2} planes generally formed closed domains. For (11 2 6) semi-polar GaN (s-GaN) films, most threading dislocations were located at smallangle grain boundaries. Many BSFs were observed close to the AlN/GaN interface but, in comparison with a-GaN, the s-GaN films had much lower BSF density at the top surface. Inversion domain boundaries (IDBs) on {1 0 1 0} planes were observed to form closed domains. GaN/AlGaN multiple quantum wells (MQWs) grown on s-GaN or a-GaN followed the morphology of the GaN surface. Some IDBs in the s-GaN propagated through the GaN/AlGaN MQWs to the top surface.
Structure of GaN Films Grown by Molecular ,Beam Epitaxy on (0001) Sapphire
GaN films grown by electron-cyclotron resonance plasma-assisted molecular beam epitaxy were studied by transmission electron microscopy and x-ray diffraction (XRD). Two sets of films were compared that were grown under identical conditions except for the ratio of the Ga to N flux. Films with a 30% higher Ga to N ratio (A films) were found to contain inversion domains (IDs). No IDs were found in films grown with a lower Ga to N ratio (B films), but instead the zinc-blende GaN was found near the film substrate interface. A narrower XRD rocking curve width along the (0002) direction and a broader rocking curve width along the asymmetric (1 102) axis were found for A films compared to B films.
GaN quantum dot polarity determination by X-ray photoelectron diffraction
Applied Surface Science, 2016
Growth of GaN quantum dots (QDs) on polar and semipolar GaN substrates is a promising technology for efficient nitride-based light emitting diodes (LED) due to suppressed dislocation density in the active region of the devices. The QDs crystal orientation typically repeats the polarity of the substrate. In case of non-polar or semipolar substrates, the polarity of QDs is not obvious. In this article, the polarity of GaN QDs and of underlying layers was investigated nondestructively by X-ray photoelectron diffraction (XPD). Polar and semipolar GaN/Al 0.5 Ga 0.5 N heterostructures were grown on the sapphire substrates with (0001) and (1100) orientations by molecular beam epitaxy (MBE). Polar angle dependence of N 1s core-level photoelectron intensities were measured from GaN QDs and compared with the corresponding experimental curves from freestanding GaN crystals. It is confirmed experimentally, that the crystalline orientation of polar (0001) GaN QDs follows the orientation of the (0001) sapphire substrate. In case of semipolar GaN QDs grown on (1100) sapphire substrate, the (1122) polarity of QDs was determined.
X-ray absorption spectroscopy in the analysis of GaN thin films
Surface and Interface Analysis, 2003
Stoichiometric amorphous GaN thin films have been grown by an ion-assisted deposition method and examined by x-ray photoelectron spectroscopy and x-ray absorption near-edge spectroscopy (XANES). The crucial question is the nature of the local structure around the N and Ga in the x-ray amorphous films. The N K edge XANES has been used to determine coordination around the N centre and reveals substantial differences to crystalline GaN. Although the transitions observed mirror those of the crystalline material and are consistent with density of states calculations, the low-energy peak at ∼402 eV is dominant in all films less than ∼150 nm in thickness. This peak, initially attributed to an sp 2 environment, is associated with the presence of molecular nitrogen. For thicker films, a duplex-type structure is observed with a surface layer much closer to the structure of the crystalline material.
Journal of Applied Physics, 2003
Achieving nitride-based device structures unaffected by polarization-induced electric fields can be realized with nonpolar GaN, although polarity plays a key role in the growth. (112¯0) a-plane GaN films were grown on (11¯02) r-plane sapphire substrates and subsequently laterally overgrown using metalorganic chemical vapor deposition. Convergent beam electron diffraction analysis was used to determine the a-GaN polarity to explicitly define the film/substrate relationship, and subsequently to identify various growth features and surfaces observed throughout our studies of a-plane GaN. In particular, the effects of polarity on (1) lateral overgrowth from mask stripe openings aligned along [1¯100]GaN and (2) pit formation in heteroepitaxial films grown under nonoptimized conditions were investigated. The fundamental differences between the polar surfaces are clearly observed; analysis of the lateral epitaxial overgrowth stripes revealed that (0001) surfaces grew faster than (0001¯) surfaces by approximately an order of magnitude, and these stable, slow-growing (0001¯) surfaces are a likely cause of pitting in a-GaN films. The growth features under investigation were imaged using scanning and transmission electron microscopy.