Morphology Of GaN Research Articles

GaN growth process using GaP(1 1 1)A and (1 1 1)B surfaces as an initial substrate

GaN growth on GaP(1 1 1)A and (1 1 1)B surfaces are discussed using P desorption energy from the GaP(1 1 1) surfaces calculated by ab initio pseudopotential method and surface morphology of GaN layers on the GaP(1 1 1) substrates obtained by experiments. It was found that the activation energy for P desorption from GaP(1 1 1)A was smaller than that from GaP(1 1 1)B and these values depend on the surface morphology of GaN on GaP(1 1 1) surfaces. These results explain a finding that the GaP(1 1 1)A surface can promise an initial substrate for freestanding GaN substrate like GaAs(1 1 1)A surface.

Influence of etching condition on surface morphology of AlN and GaN layers

The surface morphology of GaN and AlN thin films after etching was examined with a scanning electron microscope to seek the optimum condition for estimation of the density and distribution of threading dislocations (TDs). The correspondence of the pits and TDs was also confirmed by transmission electron microscopy. By etching GaN in HCl + N 2 , a pit is formed one by one on the end of each TD. The etching in a relatively high concentration of HCl for a short time is found to be preferable for specimens with a high density of TDs. The Burgers vector can be identified from the morphology of the pit. No pits are formed on the surface of AlN by etching in (HCl + N 2 ). By etching with molten NaOH, distinct pits are formed on whole TDs in GaN and on (a + c)-type TDs in AIN. The preferable conditions of etching are 400 °C or lower for 6-9 min for GaN, and 320 °C for 10 sec for AIN. By a treatment of reactive ion etching with Cl 2 , whiskers remain on AlN. The whiskers contain a TD at the center.

Drastic Improvements of GaN Film Quality by Applying Si Irradiation during Growth Interruption in rf‐MBE

Si irradiation during growth interruption is applied to GaN growth in radio-frequency molecular beam epitaxy. It is found that the surface morphology of GaN is greatly improved. Atomic force microscopy observation illustrates that a flat surface with clear monolayer steps can be achieved without using metalorganic chemical vapor deposition-grown GaN templates. Furthermore, high-resolution X-ray diffraction (XRD) measurements of the Si-irradiated GaN sample show that the FWHM value of the (002) diffraction peak is less than 100 arcsec and that of the (102) diffraction peak is as narrow as 550 arcsec. The XRD results imply that the dislocation density in the Si-irradiated sample is greatly reduced, which is confirmed by cross-sectional scanning tunneling electron microscopy observations.

Morphological study of GaN layers grown on porous silicon

In the present work, we investigate the effect of growth temperature on the crystallinity and morphology of GaN layers grown on porous silicon (PS) by metalorganic vapour phase epitaxy (MOVPE). For this purpose, we use X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM) techniques. XRD analysis show two emerging polycrystalline phases related to the wurtzite and zinc-blende structures. The crystallinity and the dominance of these two phases were found to depend on growth temperature, while one can observe an overall crystallinity improvement at high temperatures. AFM images show a pyramidal growth of the GaN layers. A significant increase of the grain size dimensions from 150 to 800 nm was observed as the temperature rises from 450 to 800 °C. Surface and cross sectional SEM observations of the GaN layers reveal a good surface coverage at 500 °C. A columnar like structure appears at 600 °C. Good agreement was observed between XRD, AFM and SEM results; the temperature dependence of the morphology and crystallinity of GaN grown on PS was clearly evidenced.

Phase Transitions on Gan Surfaces

ABSTRACTRecent experimental and theoretical studies highlighted the importance of the growing surface structure on the final morphology of GaN. Actually, optimum morphology is achieved by growth in presence of a Ga bilayer adsorbed on the GaN surface. The threshold fluxes limiting the region of the Ga bilayer adsorption have been measured as a function of the GaN substrate temperature, giving rise to a Ga adsorption phase diagram. The Ga flux limiting the regions in the adsorption phase diagram exhibit a linear behavior in an Arrhenius plot. However, both energy activation (about 5 eV) and prefactor (in the 1025 range) are surprinsingly high. These questions were adressed by studying the adsorption/desorption of Ga adatoms and small islands (consisting of 2 and 3 Ga adatoms) on the Ga bilayer surface employing first principle density functional theory calculations. We find a desorption barrier of 2.1 eV and a binding energy between two Ga atoms of approximately 0.3 eV. Using these numbers we derived a simple growth model (based on rate equations). An analysis of the experimental data with the model revealed the origin of the large difference in the activation energies and the unusually large prefactor. We find that the nucleation of the droplets cannot be described by a simple Arrhenius behavior (as commonly assumed to fit experimental data) but that the nucleation energy is temperature dependent.

The Influence of Buffer Layer Growth Parameters on the Microstructure and Surface Morphology of GaN on Sapphire Substrates Correlated with in-situ Reflectivity

GaN low temperature nucleation and high temperature buffer layers have been grown by metalorganic vapour phase epitaxy. The effect of nucleation layer annealing temperature and buffer layer growth temperature on their microstructure and morphology has been examined by atomic force microscopy and X-ray diffraction and correlated to the in-situ reflectivity obtained from the growing layers. For the nucleation layer, the anneal was observed to lead to the formation of GaN nucleites on the substrate surface, the properties of which changed markedly with temperature. Surface pits resulting from dislocations with a screw component have been observed and the variation of the density with growth temperature ascertained.

The Influence of Buffer Layer Growth Parameters on the Microstructure and Surface Morphology of GaN on Sapphire Substrates Correlated with in-situ Reflectivity

GaN low temperature nucleation and high temperature buffer layers have been grown by metalorganic vapour phase epitaxy. The effect of nucleation layer annealing temperature and buffer layer growth temperature on their microstructure and morphology has been examined by atomic force microscopy and X-ray diffraction and correlated to the in-situ reflectivity obtained from the growing layers. For the nucleation layer, the anneal was observed to lead to the formation of GaN nucleites on the substrate surface, the properties of which changed markedly with temperature. Surface pits resulting from dislocations with a screw component have been observed and the variation of the density with growth temperature ascertained.

In as a surfactant for the growth of GaN (0001) by plasma-assisted molecular-beam epitaxy

The influence of indium on the surface morphology of GaN (0001) grown by plasma-assisted molecular-beam epitaxy (MBE) has been investigated. The rough and grain-like surface under nitrogen-rich growth conditions becomes smoother and similar to surfaces grown under gallium-rich conditions when a sufficiently high indium flux is used. However, the use of indium instead of gallium-rich conditions prevents the formation of gallium droplets on the surface which are associated with voids at their edges. Since indium is not incorporated into GaN for growth temperatures above 700 °C, it can be used as a surfactant in MBE growth of GaN.

Improvement of electrical property and surface morphology of GaN grown by RF-plasma assisted molecular beam epitaxy by introduction of multiple AlN intermediate layer

The crystal properties of Ga-polarity GaN layers grown by molecular beam epitaxy using RF-plasma nitrogen on (0001) Al 2O 3 substrates were remarkably improved by introduction of high-temperature grown AlN multiple intermediate layers (HT–AlN–MILs). The effects of HT–AlN–MIL on the improvement of crystal quality were found to be different for its thickness. The 8 nm-thick HT–AlN–MIL improved the electrical property and the 2 nm-thick HT–AlN–MIL improved the surface morphology. The combination of 8 nm- and 2 nm-thick HT–AlN–MILs brought about improvement of both electrical property and surface morphology, concurrently. The HT–AlN–MIL was used for RF-MBE re-growth on MOCVD–GaN template. Relatively fine step flow structure without spiral hillocks was obtained.

Surface morphology of GaN grown by molecular beam epitaxy

The surface morphology of GaN(0001) grown on Si(111) by molecular beam epitaxy using ammonia has been studied by near-field microscopy techniques. Two distinct morphologies are observed, depending on the growth kinetics. When using Ga-rich growth conditions, the roughness is independent of the epitaxial layer thickness, while it increases with growth time under N-rich growth conditions. Also, the morphological pattern is different for the two regimes of growth and is not related to the crystallographic subgrains delimited by edge dislocations. However, under Ga-rich growth conditions, screw-type dislocations give rise to a spiral growth mode that imposes both the morphological pattern and the surface roughness.

Effect of carbon doping on GaN:Er

The effect of carbon doping on the photoluminescence (PL) and morphology of GaN:Er has been investigated. GaN:Er,C with [Er] ∼8.5 ×10 20 cm −3, was grown using elemental Ga, elemental Er and CBr 4 via gas-source molecular beam epitaxy (GSMBE). The optimum room temperature 1.54 μm PL intensity was obtained for a carbon concentration of ∼7.7 ×10 20 cm −3. Further increase in the carbon concentration led to a decrease in overall PL intensity, but slightly decreased the degree of thermal quenching between 20 and 300 K. Comparison of thermal quenching data of GaN:Er grown with triethylgallium (TEG) and that grown with elemental Ga suggests that oxygen is more effective than carbon at suppressing thermal quenching. Material grown without high impurity backgrounds shows 75% quenching while that grown with TEG shows no quenching. Carbon-doped material falls in between these two values depending upon the amount of carbon in the film. Annealing improved the 300 K PL intensity from carbon-doped material and decreased the intensity from low impurity material.

Influence of N/Ga-flux ratio on optical properties and surface morphology of GaN grown on sapphire(0001) by MBE

We studied the correlation between the low-temperature optical properties and the surface morphology of molecular beam epitaxy (MBE) grown GaN on sapphire(0001). The samples were grown under Ga-rich conditions, with the Ga-flux as a parameter, and with all other growth parameters constant. High-resolution scanning electron microscopy (SEM) provided a measure of the surface morphology, while the optical properties were characterised by low-temperature photoluminescence. These spectra were dominated by the exciton bound to neutral donor transition, at 3.472–3.474 eV, indicating fairly strain relaxed layers. This peak width is increased when the surface morphology improved. Our results also showed a clear correlation between the optical properties and the purity of the nitrogen source, as improved oxygen purification improved the photoluminescence linewidth from 41 to 20 meV.

Nanocrystals at MBE-grown GaN/GaAs(001) interfaces

Molecular beam epitaxy (MBE) growth utilising an RF-plasma nitrogen source was used to study surface reconstruction and the effects of nitridation damage on the surface and interface morphology of GaN on GaAs(001) at 580°C. Keeping both the N-flow and plasma excitation power constant, the grown layers were studied with the Ga-flux as a parameter. In the initial growth stage, a (3×3) surface reconstruction was observed. Samples grown under N-rich, Ga-rich and stoichiometric conditions were characterised by high-resolution scanning electron microscopy (SEM) and atomic force microscopy (AFM) to reveal the surface morphology. The nitridation damage was characterised by Auger-electron spectroscopy (AES), X-ray diffraction (XRD) spectroscopy and high-resolution SEM, which revealed separated GaN and GaAs phases at interfaces deeply in the substrate region, bordered by {101} and {111} facets. The defect formation at the GaN/GaAs interface depended on the N/Ga-flux ratio.

Morphology of GaN in ammonia

chinese acad sci, inst phys, beijing 100080, peoples r china. chinese acad sci, ctr condensed matter phys, beijing 100080, peoples r china. acad sinica, int ctr mat phys, shenyang 110015, peoples r china.;lan, yc (reprint author), chinese acad sci, inst phys, pob 603, beijing 100080, peoples r china

Growth and morphology of Er-doped GaN on sapphire and hydride vapor phase epitaxy substrates

We report the morphological and compositional characteristics and their effect on optical properties of Er-doped GaN grown by solid source molecular beam epitaxy on sapphire and hydride vapor phase epitaxy GaN substrates. The GaN was grown by molecular beam epitaxy on sapphire substrates using solid sources (for Ga, Al, and Er) and a plasma gas source for N2. The emission spectrum of the GaN:Er films consists of two unique narrow green lines at 537 and 558 nm along with typical Er3+ emission in the infrared at 1.5 μm. The narrow lines have been identified as Er3+ transitions from the H11/22 and S3/24 levels to the I15/24 ground state. The morphology of the GaN:Er films showed that the growth resulted in either a columnar or more compact structure with no effect on green light emission intensity.

Dislocation mediated surface morphology of GaN

The surfaces of GaN films grown by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were studied using atomic force microscopy (AFM). Due to the high dislocation densities in the films (108 cm−2), the typical surface morphologies of layers grown by both techniques were dominated by three dislocation mediated surface structures—pinned steps, spiral hillocks, and surface depressions. The characteristics of these surface structures were found to depend on growth technique (MOCVD vs MBE) and the group-III to group-V ratio used in the growth of MBE GaN films. Pinned steps, created by the intersections of mixed character dislocations with the free surface, were found on all GaN films. The pinned steps were observed to be predominantly straight on the MOCVD GaN and curved into spiral hillock formations on the MBE GaN. Spiral growth hillocks form when pinned steps grow outward and around the dislocation under step-flow growth conditions. The tightness of the spiral hillocks on MBE GaN surfaces was found to increases with III/V ratio. Surface depressions, caused by the high strain-energy density near dislocations, were also observed on the surfaces of the GaN films. Two characteristic depression sizes were found on all MOCVD GaN films whereas depressions were observed only on MBE GaN films grown with low III/V ratios. These observations are explained using theories developed by Burton, Cabrera, and Frank [Philos. Trans. R. Soc. London, Ser. A 243, 299 (1951)] and Frank [Acta Crystallogr. 4, 497 (1951)].

Surface reconstruction and surface morphology of GaN grown by MBE on GaAs (001)

RF-plasma assisted molecular beam epitaxy growth was used to study surface reconstruction and surface morphology as a function of Ga-flux and the plasma excitation power for growth of GaN on GaAs (001) at 580°C. In the initial growth stage a (3 × 3) surface reconstruction was observed. This surface periodicity only lasted up to a maximum thickness of 2.5ML, which was followed by a new transition to the unreconstructed surface. We found that the smoothest surfaces were provided by the N/Ga-ratio giving the thickest layer at the (3 × 3) ⇒ (1 × 1) transition. The growth rate providing optimum growth conditions was shown to increase with the activation power of the RF-source, i.e. the amount of active nitrogen. Using this information, a series of samples were grown at maximum RF-source power in order to find the optimum N/Ga-flux ratio at highest possible growth rate. Characterisation by SEM indicted that the best surface morphology was obtained at a growth rate of approximately 0.3 µm/h.

Effect of Buffer Layer and III/V Ratio on the Surface Morphology of Gan Grown by MBE

The surface morphology of GaN is observed by atomic force microscopy for growth on GaN and AlN buffer layers and as a function of III/V flux ratio. Films are grown on sapphire substrates by molecular beam epitaxy using a radio frequency nitrogen plasma source. Growth using GaN buffer layers leads to N-polar films, with surfaces strongly dependent on the flux conditions used. Flat surfaces can be obtained by growing as Ga-rich as possible, although Ga droplets tend to form. Ga-polar films can be grown on AlN buffer layers, with the surface morphology determined by the conditions of buffer layer deposition as well as the III/V ratio for growth of the GaN layer. Near-stoichiometric buffer layer growth conditions appear to support the flattest surfaces in this case. Three defect types are typically observed in GaN films on AlN buffers, including large and small pits and “loop” defects. It is possible to produce surfaces free from large pit defects by growing thicker films under more Ga-rich conditions. In such cases the surface roughness can be reduced to less than 1 nm RMS.

Real-time observations of the GaN dot formation by controlling growth mode on the AlGaN surface in gas-source molecular beam epitaxy

In situ reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM) observations were performed to monitor and characterize the growth processes and surface morphology of GaN on AlxGa1−xN surface in gas-source molecular beam epitaxy. It was found that the growth mode can be changed by introducing Si before GaN growth, where the Si is believed to play an important role in the change of the AlxGa1−xN surface free energy. Without introducing Si, the GaN growth mode was two-dimensional and (1×3) reconstruction was observed. The growth mode of GaN was changed from two-dimensional to three-dimensional by introducing Si on the AlxGa1−xN surface. Nanoscale GaN dots were successfully formed on AlxGa1−xN/6H-SiC(0001) surfaces. Furthermore, the density of the GaN dots was found to be dependent on the amount of Si dose and the growth temperature.

Growth of single crystal GaN on a Si substrate using oxidized AlAs as an intermediate layer

We demonstrate that single-crystal hexagonal GaN can be grown by metal-organic chemical vapor deposition on an aluminum oxide compound (AlOx) layer utilized as an intermediate layer between GaN and a Si(111) substrate. The surface morphology of GaN grown on the AlOx/Si(111) substrate is found to be very sensitive to both the GaN buffer-layer thickness and the AlOx thickness. Contact mode atomic force microscopy (C-AFM) observation indicates that the AlOx surface is composed of domain-like features with varying surface heights. A possible clue for a mechanism by which single crystal GaN grows on AlOx is discussed by comparing the domain-like surface features observed by C-AFM.