Properties Of GaN Epitaxial Films Research Articles

Effect of pulsed ion-beam treatment on the electronic and optical properties of GaN epitaxial films on sapphire

GaN films epitaxially grown on sapphire substrates and stored for more than a year under normal conditions were treated using pulsed mixed carbon-hydrogen ion beam. The electronic structure transformations of GaN on sapphire samples were studied using X-ray photoelectron spectroscopy (XPS), optical absorption, photoluminescence and density functional theory (DFT)-based simulations after such treatment. Weak acidification in the form of Ga—N…O was revealed by XPS method for both as-grown and ion-irradiated GaN on sapphire samples. GaN decomposition and Ga-loss effects were not detected after pulsed ion-beam treatment (PIBT), either experimentally or theoretically. It was shown that PIBT promotes a narrowing of the bandgap, changes the vacancy concentration in the surface region, and provides hydrogenation of the surface with the formation of hydroxyl groups in nondefective areas of the surface and near surface layers. PIBT-induced densification of the layers is observed due to annealing of vacancies; no detectable changes in the surface area morphology were found. The weak shift of the valence band edge corresponds to the contribution from areas with more nitrogen defects. The presence of these states in the bandgap could be the possible source of an increase in the electro and photocatalytic activity of GaN epitaxial films.

Study on the effects of growth rate on GaN films properties grown by plasma-assisted molecular beam epitaxy

Abstract This work systematically investigates the effects of growth rate on GaN films properties grown on thick GaN-on-sapphire templates by plasma-assisted molecular beam epitaxy (PA-MBE) under Ga-rich conditions without formation of large metal droplets. The nitrogen-limited growth rate is varied by modulating the nitrogen flows while maintaining the power of radio frequency plasma source constant, and the thickness of epitaxial GaN films is determined by Capacitance-Voltage and verified by second ion mass spectroscopy simultaneously. It is found that GaN films grown at high growth rate exhibit an improved crystal quality measured by X-ray diffraction, and display a rough surface morphology along with higher hillocks and clearer step terrace resolved by atomic force microscopy. Photoluminescence spectroscopy characterization indicates the full width at half maximum of near band emission peak and intensity of yellow band emission are decreased for GaN epilayers grown at high growth rate, corresponding to a reduced density of defect states and a lowered concentration of residual carbon impurity, respectively. The presented results suggest that the growth rate strong influences the structural, morphological and optical properties of GaN epitaxial films, and high growth rate facilitates realizing a better GaN film quality, giving a conducive insight for PA-MBE-grown III-nitride device-quality materials for electronic and optoelectronic applications.

Effect of growth temperature on the properties of GaN epitaxial films grown on magnesium aluminate scandium oxide substrates by pulsed laser deposition

High-quality GaN epitaxial films have been grown on magnesium aluminate scandium oxide (ScMgAlO4) substrates at low temperature of 450°C with an in-plane epitaxial relationship of GaN[1-100]//ScMgAlO4[1-100]. It is found that the ~300nm-thick GaN epitaxial films grown at 450°C show the full-width at half-maximums (FWHMs) for GaN(0002) and GaN (10-12) of 0.18° and 0.40°, respectively, and very smooth surface with a root-mean-square (RMS) surface roughness of 1.5nm. There is no interfacial layer existing between GaN and ScMgAlO4. However, as the growth temperature increases, the crystalline quality, surface morphology, and interfacial properties of as-grown GaN epitaxial films are deteriorated gradually. These GaN epitaxial films are of great importance for the fabrication of highly-efficient GaN-based devices.

A comparative study on the properties of c-plane and a-plane GaN epitaxial films grown on sapphire substrates by pulsed laser deposition

∼500 nm-thick c-plane and a-plane GaN epitaxial films on c-plane and r-plane sapphire substrates by pulsed laser deposition have been grown and explored. The ∼500 nm-thick c-plane GaN epitaxial films grown on c-plane sapphire substrates show high crystalline quality with full-width at half-maximum (FWHM) for GaN(0002) and GaN(10–12) X-ray rocking curves (XRCs) of 0.20° and 0.37°, respectively, very smooth GaN surface with a root-mean-square (RMS) surface roughness of 1.4 nm, sharp and abrupt c-plane GaN/c-plane sapphire hetero-interfaces, a −0.116 GPa residual stress, and high optical property. These results are in striking contrast to those of ∼500 nm-thick a-plane GaN epitaxial films grown on r-plane sapphire substrates. The ∼500 nm-thick a-plane GaN epitaxial films grown on r-plane sapphire substrates show poor crystalline quality with FWHM for GaN(11–20) and GaN(10–11) XRC of 0.72° and 0.81°, respectively, rough GaN surface with an RMS surface roughness of 7.2 nm, indistinct a-plane GaN/r-plane sapphire hetero-interface with a ∼1.5 nm-thick interfacial layer, a −0.884 GPa residual stress, and poor optical property. This work brings further insight into fully understanding of the properties of GaN epitaxial films grown on c-plane and r-plane sapphire substrates for obtaining high-quality GaN epitaxial films in the application of GaN-based devices.

Structures and optical properties of -implanted GaN epi-layers

The implantation damage build-up and optical properties of GaN epitaxial films under ion implantation have been investigated by a combination of Rutherford backscattering in channeling geometry, Raman spectroscopy, UV–visible spectroscopy and transmission electron microscopy. GaN epitaxial films were implanted with 134 keV ions to doses ranging from 3.75 × 1016 to 1.75 × 1017 cm−2 at room temperature or the same dose of 1.5 × 1017 cm−2 at room temperature, 573 and 723 K. The dependence of lattice disorder induced by -implantation on the ion dose can be divided into a three-step damage process. A strong influence of the H concentration on the defect accumulation is discussed. The decrease in relative Ga disorder induced by -implantation is linear with increasing implantation temperature. The absorption coefficient of GaN epitaxial films increases with increasing ion dose, leading to the decrease in Raman scattering spectra of Ga–N vibration. With increasing implantation doses up to 5 × 1016 cm−2, nanoscale hydrogen bubbles are observed in the H deposition peak region. Interstitial-type dislocation loops are observed in the damaged layer located near the damage peak region, and the geometry of the dislocation loops produced by H implantation is analyzed. The surface layer is almost free of lattice disorder induced by -implantation.

Influence of the nucleation layer annealing atmosphere on the resistivity of GaN grown by metalorganic chemical vapor deposition

High-resistance (HR) GaN with sheet resistance of 1.0×108Ω/sq was grown on sapphire substrates using metal organic chemical vapor deposition. Sheet resistance of the GaN film increases 4 orders of magnitude by changing the nucleation layer (NL) annealing atmosphere from H2 to N2. It is observed that the morphology of the NLs strongly depends on the annealing atmosphere. The analysis results based on high-resolution X-ray diffraction (HR-XRD) and etch pit density (EPD) measurements demonstrate that the density of edge-type threading dislocations increases with the proportion of the N2 in the annealing atmosphere. Photoluminescence (PL) spectra is employed to analyze the optical properties of GaN films. The XRD and PL results indicate the primary compensating mechanism is due to acceptor levels introduced by the increase in edge-type threading dislocations density. It is concluded that the annealing atmosphere of the NL controls sizes and densities of the nucleation islands, which affect electrical properties of GaN epitaxial films through changing the ratio of edge to screw/mixed-type threading dislocations.

Influence of the nucleation layer morphology and epilayer structure on the resistivity of GaN films grown on c-plane sapphire by MOCVD

The influence of hydrogen and nitrogen carrier gases used during the preparation of the nucleation layer on the structural and electrical properties of GaN layers has been investigated. The GaN were grown on sapphire substrates using metal organic chemical vapor deposition. The nucleation layer morphology strongly depends on the carrier gas affecting the electrical properties of GaN epitaxial films through changes of the ratio of edge to mixed and screw-type threading dislocations. X-ray diffractometry, X-ray reflectometry, atomic force microscopy, and defect selective etching were employed to study the structural properties of both the nucleation layer and the GaN epilayers deposited on top of this. It is found that the density of edge-type dislocations determines the resistivity of GaN epilayers and that one key factor for varying the density of these dislocations is the morphology of the nucleation layer, i.e. the morphology of the GaN epilayer can be controlled by the type of carrier gas used in the preparation of the nucleation layer. The electrical resistivity of our GaN epilayers is typically about 0.5 Ω cm and more than 3×10 4 Ω cm with nucleation layers grown using hydrogen and nitrogen carrier gases, respectively.

Role of sapphire nitridation temperature on GaN growth by plasma assisted molecular beam epitaxy: Part I. Impact of the nitridation chemistry on material characteristics

The impact of the nitridation temperature on sapphire/GaN interface modifications and the structural, chemical, and optical properties of GaN epitaxial thin films with N plasma radicals is investigated. Based on ex situ spectroscopic ellipsometry and x-ray photoelectron spectroscopy analysis, it is found that the sapphire nitridation chemistry, specifically AlN versus oxynitride (NO) production, depends on the surface temperature. Nitridation at 200 °C produces a very thin AlN layer with 90% coverage, while high temperature nitridation leads to a 70% coverage of AlN layer containing NO. These initial stages of growth significantly impact the characteristics of the layers following the nitridation step, specifically the low temperature buffer, annealed buffer, and the GaN epitaxial layer. The annealed buffer on a 200 °C nitridation provides a homogeneous GaN thin layer covering most of the sapphire surface. This homogeneous GaN layer after annealing produces a superior template for subsequent growth, resulting in improved structural and optical properties of GaN epitaxial films. On the other hand, the annealed buffer grown on sapphire nitrided at temperatures lower or higher than 200 °C, has islands of GaN nuclei revealing the sapphire substrate, and ultimately, resulting in degraded GaN epitaxial film quality as demonstrated by photoluminescence and x-ray diffraction measurements. The results can be traced back to the chemistry of the nitridation process.

Substrate effects on the structure and optical properties of GaN epitaxial films

The phase composition and luminescent properties of GaN films grown by molecular beam epitaxy on (0001) sapphire and 6H-SiC substrates were studied. The films grown on SiC were found to consist only of the hexagonal phase and contain a lower concentration of impurities. Grains of cubic GaN, as well as donor and acceptor impurities, were found in the GaN film grown on sapphire. The formation of impurity centers is caused by the diffusion of oxygen and aluminum from the sapphire substrate during crystal growth.

Optical properties of GaN epitaxial films grown by low-pressure chemical vapor epitaxy using a new nitrogen source: Hydrazoic acid (HN3)

We report results of our growth and characterization of GaN films using low-pressure chemical vapor epitaxy with a new nitrogen source, hydrazoic acid (HN3). This growth technique allows for low-temperature deposition, low III/V ratios, and increased deposition rates (up to ∼2–3 μm/h). The deposited films show Ga:N atomic ratios of 1±0.25 based on our x-ray photoelectron spectroscopy analyses, and the He(II) UPS (ultraviolet photoelectron spectroscopy) spectra compare favorably with the semi-ab initio calculations for the GaN valence bands and with the reported UPS data for single crystal GaN films. X-ray and Raman spectra show deposited films crystallized in the expected wurtzite structure. We find these epitaxial films to be efficient light emitters in the blue or yellow region of the spectrum, depending upon growth conditions. Our photoluminescence time-decay kinetics confirm the excitonic nature of the blue emission. Lastly, far infrared time-domain spectroscopy shows the low carrier concentration of this material.