Step-flow Morphology Research Articles

Epitaxial Growth of Boron Carbide on 4H-SiC

In this work, the successful heteroepitaxial growth of boron carbide (BxC) on 4HSiC(0001) 4° off substrate using chemical vapor deposition (CVD) is reported. Towards this end, a two-step procedure was developed, involving the 4H-SiC substrate boridation under BCl3 precursor at 1200°C, followed by conventional CVD under BCl3 + C3H8 at 1600°C. Such a procedure allowed obtaining reproducibly monocrystalline (0001) oriented films of BxC with a step flow morphology at a growth rate of 1.9 μm/h. Without the boridation step, the layers are systematically polycrystalline. The study of the epitaxial growth mechanism shows that a monocrystalline BxC layer is formed after boridation but covered with a B-and Si-containing amorphous layer. Upon heating up to 1600°C, under pure H2 atmosphere, the amorphous layer was converted into epitaxial BxC and transient surface SiBx and Si crystallites. These crystallites disappear upon CVD growth.

Epitaxial growth of 2.5-μm quaternary AlInGaN for n-cladding layer in GaN-based green laser diodes

The ridge morphology, which is related to random atomic step meandering, appears in thick AlInGaN films grown by metal organic chemical vapor deposition on both GaN templates and free-standing GaN substrates; this can be primarily attributed to the in-plane compressive strain in the thick layer. Therefore, a 2.5-μm Al0.08In0.0123GaN film with a slightly tensive strain was grown, with a regular and smooth step-flow morphology; the root mean square deviation of the film (with a size of 5 μm × 5 μm) was 0.56 nm.

Effect of flux rate on AlN epilayers grown by hydride vapor phase epitaxy

The surface morphology and crystalline quality of AlN epilayers grown by high-temperature hydride vapor phase epitaxy (HVPE) with various growth rate were investigated by AFM, XRD and cross-sectional TEM analyses. The growth rate of AlN films were controlled by regulating the flux rates of HCl and NH3 while keeping V/III ratio constant. The surface morphology of as-grown AlN films revealed that the control of the appropriate growth rate is beneficial to obtain uniform step-flow morphology. And the crystalline quality of AlN layers has been improved at an appropriate growth rate, since the growth mode of AlN under such condition is beneficial to the bending of dislocations. The dislocation bending and reduction mechanism has also been demonstrated.

Reduction of surface roughness by modification of step-bunched aluminum nitride layers towards step-flow morphology

Conventional aluminum nitride (AlN) template fabrication techniques, like hydride vapour phase epitaxy or AlN growth on patterned sapphire substrates, usually lead to step-bunched template surfaces. The resulting macrosteps cause emission broadening or even multiple-peak characteristics of ultraviolet light-emitting diodes (UV LEDs) fabricated on such AlN templates. In order to reduce these macrosteps and to provide a smoother surface, even without the need of a thick AlN deposition, a two-layer growth procedure is reported here. A three-dimensional (3D) – twodimensional (2D) sequential growth initiates a significant modification of the previous AlN surface morphology and simultaneously limits the evolving tensile strain. A primarily step-bunched surface with a surface roughness root mean square of 1.8 nm is successfully reduced by the two-layer growth procedure down to 0.8 nm, without any film cracking. This distinct roughness reduction of more than 50% is achieved within an AlN thickness of only 1.3 µm. With a smoother surface, the electroluminescence characteristic of a UV LED structure is substantially improved. Instead of a double-peak emission, typical for LEDs grown on step-bunched templates, a single-peak emission and lower spectral width were achieved, indicating the high potential of the suggested two-layer technique for improving performance.

Significant increase of quantum efficiency of green InGaN quantum well by realizing step-flow growth

Two-dimensional (2D) island morphologies have been widely reported for green light-emitting InGaN quantum well (QW) layers, but the step-flow morphology has not been obtained for a green InGaN QW layer to date. In this Letter, we first investigate the cause of the 2D island morphology of green InGaN QWs via a comparison study with blue InGaN QWs. The short diffusion lengths of adatoms at low growth temperatures were found to be the cause of the 2D island morphology for the green InGaN QW. Step-flow growth of green InGaN QWs was obtained by increasing the miscut angle of the c-plane GaN substrates from 0.20° to 0.48°, which reduces the atomic terrace width. Green InGaN/GaN multiple quantum wells (MQWs) with step-flow morphologies were found to have sharper well/barrier interfaces than MQWs with 2D island morphologies. The internal quantum efficiency of the green InGaN/GaN MQWs with the step-flow morphology is double that of the corresponding MQWs with the 2D island morphology at an excitation power density of 6.4 kW/cm2. Additionally, the emission linewidth of the green InGaN/GaN MQWs with the step-flow morphology is greatly reduced. As a result, the threshold currents of green laser diodes with larger miscut angles are greatly reduced.

Indium incorporation dynamics in N-polar InAlN thin films grown by plasma-assisted molecular beam epitaxy on freestanding GaN substrates

N-polar InAlN thin films were grown by plasma-assisted molecular beam epitaxy on freestanding GaN substrates under N-rich conditions. Indium and aluminum fluxes were varied independently at substrate temperatures below and above the onset of thermal desorption of indium. At low temperatures, the InAlN composition and growth rate are determined by the group-III fluxes. With increasing substrate temperature, the surface morphology transitions from quasi-3D to a smooth, 2D morphology at temperatures significantly above the onset of indium loss. At higher temperatures, we observe increased indium evaporation with higher indium fluxes and a suppression of indium evaporation with increased aluminum flux. The final optimized InAlN thin film results in step-flow morphology with rms roughness of 0.19nm and high interfacial quality.

Step-flow growth mode instability of N-polar GaN under N-excess

GaN layers were grown on N-polar GaN substrates by plasma-assisted molecular beam epitaxy under different III/V ratios. Ga-rich conditions assure step-flow growth with atomically flat surface covered by doubly-bunched steps, as for Ga-polar GaN. Growth under N-excess however leads to an unstable step-flow morphology. Particularly, for substrates slightly miscut towards ⟨101¯0⟩, interlacing fingers are covered by atomic steps pinned on both sides by small hexagonal pits. In contrast, a three-dimensional island morphology is observed on the Ga-polar equivalent sample. We attribute this result to lower diffusion barriers on N-polar compared to Ga-polar GaN under N-rich conditions.

Optimization of AlAs/AlGaAs quantum well heterostructures on on-axis and misoriented GaAs (111)B

We report systematic growth optimization of high Al-content AlGaAs, AlAs, and associated modulation-doped quantum well (QW) heterostructures on on-axis and misoriented GaAs (111)B by molecular beam epitaxy. Growth temperatures TG > 690 °C and low As4 fluxes close to group III-rich growth significantly suppress twin defects in high-Al content AlGaAs on on-axis GaAs (111)B, as quantified by atomic force and transmission electron microscopy as well as x-ray diffraction. Mirror-smooth and defect-free AlAs with pronounced step-flow morphology was further achieved by growth on 2° misoriented GaAs (111)B toward [01¯1] and [21¯1¯] orientations. Successful fabrication of modulation-doped AlAs QW structures on these misoriented substrates yielded record electron mobilities (at 1.15 K) in excess of 13 000 cm2/Vs at sheet carrier densities of 5 × 1011 cm−2.

Impact of Small Miscuts of (0001) Sapphire on the Growth of AlxGa1-xN/AlN

AbstractIn this paper, using chemical etching, atomic force microscope (AFM) and High- resolution X-ray diffraction (HRXRD), we report a study of the effect of various small miscuts of (0001) sapphire substrate (<1°) and the way to further improve the material quality. A set of AlN epilayers and AlN/AlxGa1-xN Superlattices (SLs) were grown by Migration-enhanced Metalorganic Chemical Vapor Deposition (MEMOCVD) on vicinal (0001) sapphire substrates. The threading dislocation density was found to be very sensitive to the miscut angles. The etch pit density reduced to 7×106 cm-2 for normal-oriented (0°-off) from the starting value of 7×107 cm-2 for 0.5°-off. We found the surface morphologies can be easily controlled by the different substrate miscut angles. The 1-2 Monolayers (MLs) step flow morphology for normal- oriented substrate changed to step bunches of 10 MLs height for 0.5°-off substrate. Correspondingly, AFM Root Mean Square (RMS) increased from 1.52 to 9.15 Å with a 5um×5um scan. This finding may help enhance the quality of full structure UVLED material and eventually improve the lifetime of UVLEDs.