Surface Morphology Of GaN Research Articles

III-nitride dry etching: Comparison of inductively coupled plasma chemistries

A systematic study of the etch characteristics of GaN, AlN, and InN has been performed with boron halide- (BI3 and BBr3) and interhalogen- (ICl and IBr) based inductively coupled plasmas. Maximum etch selectivities of ∼100:1 were achieved for InN over both GaN and AlN in the BI3 mixtures due to the relatively high volatility of the InIx etch products and the lower bond strength of InN. Maximum selectivities of ∼14 for InN over GaN and <25 for InN over AlN were obtained with ICl and IBr chemistries. The etched surface morphologies of GaN in these four mixtures are similar or better that those of the control sample.

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)].

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.

Relation between GaAs surface morphology and incorporation of hexagonal GaN into cubic GaN

Cubic GaN (c-GaN) layers were grown on GaAs (0 0 1) substrates by metalorganic vapor-phase epitaxy, using dimethylhydrazine as a nitrogen source. For the investigated growth temperature region of 600–820°C, only h-GaN whose c-axis is oriented in the [1 1 1] A or [ 1 ̄ 1 ̄ 1] A direction was incorporated into c-GaN. This orientation anisotropy of h-GaN caused the anisotropy of the width of c-GaN (0 0 2) X-ray diffraction (XRD) peaks. By flattening the GaAs surface to the atomic level, the hexagonal composition was effectively reduced and the anisotropy in the width of XRD was eliminated. The observed result was interpreted in terms of molecular steps appearing on the GaAs surface.

Photo-Assisted RIE of GaN in BCl3/Cl2/N2

ABSTRACTGallium nitride (GaN) has been under intense investigation due to its unique qualities (wide band gap, chemical and temperature stability) for optoelectronic and high temperature/high power applications. To this end, reactive ion etching (RIE) experiments were performed on GaN thin films using BCl3/Cl2/Ar. These resulted in etch rates of 1400 Å/min at −400 V dc bias. However, rough etched surfaces, nitrogen surface depletion and high chlorine content were observed. In order to remedy these shortcomings, a photo-assisted RIE process using a filtered Xe lamp beam was developed, resulting in higher etch rates but again in nitrogen depleted surfaces. Preliminary results on using nitrogen instead of argon in the process chemistry show a big improvement in photo-asssisted etch rates (50%) and Ga/N ratio (0.78 versus 1.25). In this paper, the effects of epilayer doping, dc bias, nitrogen flow rate and photo-irradiation flux on GaN etch rates, surface morphology and composition are presented. Finally, preliminary results on the use of a KrF excimer laser beam in the GaN photo-assisted RIE process are presented.

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 the growth rate and the barrier doping on the morphology and the properties of InGaN/GaN quantum wells

InGaN/GaN single and multi quantum wells have been grown by metal-organic chemical vapor deposition, varying the growth rate of well and barrier layers as well as the Si-doping of the GaN barriers. Separately, the effect of these growth parameters on the surface morphology of thin GaN and InGaN layers grown under the same conditions had been studied. The surface morphology of the layers strongly influenced the structural properties of the multi quantum wells. The optical properties seemed to be less affected by the observed layer thickness fluctuations in the 200–500 nm range rather than by variations in the indium composition on a shorter length scale.

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.

Chemical beam epitaxy of GaN using triethylgallium and ammonia

GaN films were successfully grown on Al 2O 3(0 0 0 1) and 6H-SiC(0 0 0 1) substrates by chemical beam epitaxy (CBE) using triethylgallium (TEG) and ammonia (NH 3) as group III and group V sources. Reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM) and X-ray diffraction (XRD) were used to characterize the growth processes, surface morphologies and the film qualities of GaN. It was found that the GaN growth mode was two-dimensional and that the GaN quality was improved by increasing the NH 3 supply. XRD measurements show that the quality of GaN films grown on 6H-SiC(0 0 0 1) substrates is better than that grown on Al 2O 3(0 0 0 1) substrates. AFM characterizations illustrate that the surface morphologies of GaN are greatly influenced by the NH 3 flow rates.

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.

ECR-MBE growth of GaN on LiGaO2

GaN thin films were grown at relatively low temperatures on multi-domain LiGaO2 substrates by electron-cyclotron-resonance plasma-excited molecular beam epitaxy (ECR-MBE). LiGaO2 is one of the most promising substrates for GaN epitaxial growth because of its small lattice mismatch of around 0.9%. The multi-domain LiGaO2 substrate has two different polarity domains in the (001) surface. Two different GaN surface morphologies depending on the surface polarities were observed by atomic force microscopy (AFM). The difference was also observed in the shift of photoluminescence (PL) spectra. Each peak corresponding to two inversion domains was shifted in parallel around 2nm. One possible explanation for the cause of the peak wavelength shift is due to the difference of the strains in GaN thin films.

Effects of arsenic in gas-source molecular beam epitaxy

The role of arsenic in gas-source molecular beam epitaxy of GaN(As) was investigated. We found that arsenic mainly acts as a surfactant which dramatically improves the surface morphology of GaN when the growth temperature is over 700 °C, while it is incorporated into GaN as high as 1% at lower growth temperature. The optical absorption of the sample GaN0.99As0.01 shows a direct-band gap optical absorption edge, but smaller band gap compared to that of GaN.

Surface Morphology of MBE-grown GaN on GaAs(001) as Function of the N/Ga-ratio

Molecular beam epitaxy growth utilising an RF-plasma nitrogen source was used to study surface reconstruction and surface morphology of GaN on GaAs (001) at 580 °C. While both the nitrogen flow and plasma excitation power were constant, the grown layers were characterised as a function of Ga-flux. In the initial growth stage a (3×3) surface reconstruction was observed. This surface periodicity only lasted up to a maximum thickness of 2.5 ML, followed by a transition to the unreconstructed surface. Samples grown under N-rich, Ga-rich and stoichiometric conditions were characterised by high-resolution scanning electron microscopy and atomic force microscopy. 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 defect formation at the GaN/GaAs interface also depended on the N/Ga-flux ratio.

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

AbstractThe 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 l nm RMS.

New plasma chemistries for etching GaN and InN: BI3 and BBr3

Smooth, anisotropic etching of InN and GaN is obtained in BI3- or BBr3-based Inductively Coupled Plasmas. Etch selectivities of 100:1 were achieved for InN over both GaN and AlN in the BI3 mixtures, while for BBr3 discharges values of 100:1 for InN over AlN and 25:1 for InN over GaN were measured. The etched surface morphologies of InN and GaN with both mixtures are similar or better than those of control samples.

Surface morphology and optical characterization of GaN grown on α-Al2O3 (0001) by radio-frequency-assisted molecular beam epitaxy

Wurtzitic GaN epilayers have been grown on sapphire (0001) substrates by radio-frequency atomic nitrogen plasma-assisted molecular beam epitaxy. Atomic force microscopy, reflectivity, and photoluminescence measurements have been performed in order to investigate the influence of in situ and ex situ annealing on the surface morphology and optical properties. We demonstrate a significant improvement in both the structural and optical quality of our GaN epilayers using in situ annealed low-temperature-deposited GaN buffer layers.

The Effect Of The Nucleation Layer On The Low Temperature Growth Of Gan Using A Remote Plasma Enhanced – Ultrahigh Vacuum Chemical Vapor Deposition (RPE-UHVCVD)

AbstractThis study investigated the low temperature growth of GaN on a nucleation layer in a remote plasma enhanced-ultrahigh vacuum chemical vapor deposition (RPE-UHVCVD) system which is equipped with an rf plasma cell for a nitrogen source. It was found that the growth temperature and the film thickness of the nucleation layer and the nitrogen flow rate for GaN growth play important roles in the improvement of crystallinity of the GaN layer. The nitridation of sapphire was also found to enhance the formation of facet shaped nuclei on the nucleation layer. As the temperature of the nucleation layer increased, islands with hexagonal and other facet shapes were formed on the grown GaN surface. This facet formation was related with the surface morphology and crystallinity of GaN. The best crystallinity was measured in a GaN layer with hexagonal facets on the surface and such GaN layers could be grown on a nucleation layer grown at 375 °C. Nitridation of sapphire and the growth temperature of the nucleation layer were also found to change the island shapes which enhances the formation of columnar structures in the GaN layer, resulting in the growth of a high crystalline GaN layer at low temperature.

High temperature surface degradation of III–V nitrides

The surface stoichiometry, surface morphology, and electrical conductivity of AlN, GaN, InN, InGaN, and InAlN were examined at rapid thermal annealing temperatures up to 1150 °C. The sheet resistance of the AlN dropped steadily with annealing, but the surface showed signs of roughening only above 1000 °C. Auger electron spectroscopy (AES) analysis showed little change in the surface stoichiometry even at 1150 °C. GaN root mean square (rms) surface roughness showed an overall improvement with annealing, but the surface became pitted at 1000 °C, at which point the sheet resistance also dropped by several orders of magnitude, and AES confirmed a loss of N from the surface. The InN surface had roughened considerably even at 650 °C, and scanning electron microscopy showed significant degradation. In contrast to the binary nitrides, the sheet resistance of InAlN was found to increase by ∼102 from the as grown value (3.2×10−3 Ω cm) after annealing at 800 °C and then remain constant up to 1000 °C, while that of InGaN increased by two orders of magnitude between 700 and 900 °C. The rms roughness increased above 800 and 700 °C, respectively, for InAlN and InGaN samples. In droplets began to form on the surface at 900 °C for InAlN and at 800 °C for InGaN, and then evaporate at 1000 °C, leaving pits. AES analysis showed a decrease in the N concentration in the top 500 Å of the sample for annealing ≥800 °C in both materials.

Inductively coupled plasma etching of GaN

Inductively coupled plasma (ICP) etch rates for GaN are reported as a function of plasma pressure, plasma chemistry, rf power, and ICP power. Using a Cl2/H2/Ar plasma chemistry, GaN etch rates as high as 6875 Å/min are reported. The GaN surface morphology remains smooth over a wide range of plasma conditions as quantified using atomic force microscopy. Several etch conditions yield highly anisotropic profiles with smooth sidewalls. These results have direct application to the fabrication of group-III nitride etched laser facets.

Growth of GaN by gas-source molecular beam epitaxy by ammonia and by plasma generated nitrogen radicals

We will present a comparison between GaN grown on c- and r-plane sapphire by gas-source molecular beam epitaxy (GSMBE) using solid source elemental Ga with uncracked ammonia (NH 3). Improved GaN film quality was found for samples grown at substrate temperatures above 700°C with optimized temperatures at 780°C. GaN deposited on a low-temperature (∼ 350°C) GaN buffer layer grown using an rf-plasma radical beam source exhibited poor photoluminescence (PL) intensity and carrier mobility while SEM analysis showed smooth GaN surface morphologies. GaN deposited on an ammonia-grown low-temperature (∼ 550°C) GaN or AlN buffer were found to have improved PL intensity and line widths, GaN surface morphology and carrier mobility. X-ray rocking curve (XRC) data showed that the GaN crystal quality improved as a function of increasing substrate growth temperature independent of buffer layer implementation.