Showing papers on "Chemical vapor deposition published in 2007"

METHOD OF FABRICATING ZnO FILM AND THIN FILM TRANSISTOR ADOPTING THE ZnO FILM

Abstract: Provided is a method of fabricating a low temperature ZnO polycrystalline film and a thin film transistor (TFT) adopting the low temperature ZnO polycrystalline film. The method includes growing ZnO on a substrate at a first temperature for a first time using Metal Organic Chemical Vapor Deposition (MOCVD) to form a ZnO buffer layer, and heating the substrate at a temperature lower than the first temperature to grow ZnO on the ZnO buffer layer for a second time longer than the first time so as to form a ZnO film.

Organic Glasses with Exceptional Thermodynamic and Kinetic Stability

Abstract: Vapor deposition has been used to create glassy materials with extraordinary thermodynamic and kinetic stability and high density. For glasses prepared from indomethacin or 1,3-bis-(1-naphthyl)-5-(2-naphthyl)benzene, stability is optimized when deposition occurs on substrates at a temperature of 50 K below the conventional glass transition temperature. We attribute the substantial improvement in thermodynamic and kinetic properties to enhanced mobility within a few nanometers of the glass surface during deposition. This technique provides an efficient means of producing glassy materials that are low on the energy landscape and could affect technologies such as amorphous pharmaceuticals.

Rational Synthesis of p-Type Zinc Oxide Nanowire Arrays Using Simple Chemical Vapor Deposition

Abstract: We report, for the first time, the synthesis of the high-quality p-type ZnO NWs using a simple chemical vapor deposition method, where phosphorus pentoxide has been used as the dopant source. Single-crystal phosphorus doped ZnO NWs have their growth axis along the 〈001〉 direction and form perfect vertical arrays on a-sapphire. P-type doping was confirmed by photoluminescence measurements at various temperatures and by studying the electrical transport in single NWs field-effect transistors. Comparisons of the low-temperature PL of unintentionally doped ZnO (n-type), as-grown phosphorus-doped ZnO, and annealed phosphorus-doped ZnO NWs show clear differences related to the presence of intragap donor and acceptor states. The electrical transport measurements of phosphorus-doped NW FETs indicate a transition from n-type to p-type conduction upon annealing at high temperature, in good agreement with the PL results. The synthesis of p-type ZnO NWs enables novel complementary ZnO NW devices and opens up enormous...

Porous Indium Oxide Nanotubes: Layer-by-Layer Assembly on Carbon-Nanotube Templates and Application for Room-Temperature NH3 Gas Sensors

Abstract: Indium oxide (In2O3), as an n-type and wide-bandgap semiconductor, is of great interest for use in toxic-gas detectors, solar cells, and light-emitting diodes because of its high electrical conductivity and high transparency. In particular, In2O3 has been extensively applied in film-based chemical sensors for a long time. However, In2O3-film-based sensing devices possess several critical limitations such as a limited maximum sensitivity and high operation temperatures (200–600 °C). In2O3 nanostructured materials, possessing ultrahigh surface-to-volume ratios, are expected to be superior gas-sensor candidates that may overcome the fundamental limitations as mentioned above. Therefore, considerable efforts have been devoted to synthesizing In2O3 nanostructures such as nanoparticles, nanowires, nanotubes, and nanobelts. Among them, nanotubes are believed to be one of the most promising structures for chemical sensors because of their higher surface-to-volume ratios and, moreover, they do not aggregate as easily as nanoparticles. Up to now, template-assisted approaches have been widely used to synthesize metal oxide nanotubes. Metal oxide nanotubes prepared by template-assisted approaches possess higher surface-to-volume ratios than those prepared by template-free approaches because of their polycrystalline and porous structure, and, therefore, may display a more superior gas-sensor performance. As a result, owing to the simplicity in the synthesis of nanotubes and their availability, quite a few metal oxide nanotubes have been fabricated by nanoporous alumina template assisted approaches such as Ga2O3, In2O3, TiO2, and Fe2O3. [8] Nevertheless, there are some disadvantages in using nanoporous alumina as a template to synthesize metal oxide nanotubes. Firstly, mass production of metal oxide nanotubes by such an approach is impractical, which is one of the bottlenecks for their wide application. Secondly, it is very difficult to completely remove the nanoporous alumina template. Thirdly, the diameters of the prepared metal oxide nanotubes by such an approach are usually larger than 100 nm. Recently, carbon nanotubes (CNTs) have been considered to be an ideal template for the synthesis of metal oxide nanotubes, which can circumvent the disadvantages of nanoporous alumina as mentioned above. For example, Rao and co-workers first fabricated ZrO2, Al2O3, V2O5, SiO2, and MoO3 nanotubes by a metal-alkoxide-based sol–gel process using CNTs as templates in combination with subsequent calcination. However, the deliquescence, toxicity, and high cost of metal alkoxides, as well as the long reaction time, restrict the practical applications of this approach. Liu and co-workers reported the synthesis of Fe2O3/CNT core–shell nanostructures and polycrystalline Fe2O3 nanotubes by a supercritical-fluid-approach using CNTs as templates. Unfortunately, this approach needed to be carried out at high temperature and pressure. In addition, metal oxide/CNT core–shell nanostructures and metal oxide nanotubes have been obtained by CNT-template-assisted chemical vapor deposition (CVD), which was also carried out at high temperature and, moreover, only resulted in the deposition of oxides on the top surface of the CNTs. Metal oxide/CNT core–shell nanostructures were also fabricated by a chemical precipitation method. However, in this route, the formation of metal oxide nanoparticles in the solution or metal oxides with a very large grain size on the surface of the CNTs was inevitable, which made it difficult to form metal oxide nanotubes after oxidation of the CNTs. We report a novel and versatile approach to synthesize metal oxide nanotubes using layer-by-layer (LBL) assembly on the CNT templates in combination with subsequent calcination. LBL assembly is based on the electrostatic attraction between charged species and it has been widely used to synthesize polymeric multicomposites, inorganic and hybrid hollow spheres, polymer nanotubes, and core–shell nanostructures. We now present its use, for the first time, to synthesize metal oxide nanotubes including In2O3, NiO, SnO2, Fe2O3, and CuO. Of these, In2O3 nanotubes are used to illustrate the basic idea underlying the approach presented in this work. The as-synthesized In2O3 nanotubes were applied in an NH3 gas sensor operated at room temperature, which exhibits improved performance and thus promising applications. C O M M U N IC A IO N

Chemical vapor deposition of high quality flow-like silicon dioxide using a silicon containing precursor and atomic oxygen

Abstract: Methods of depositing a silicon oxide layer on a substrate are described. The methods may include the steps of providing a substrate to a deposition chamber, generating an atomic oxygen precursor outside the deposition chamber, and introducing the atomic oxygen precursor into the chamber. The methods may also include introducing a silicon precursor to the deposition chamber, where the silicon precursor and the atomic oxygen precursor are first mixed in the chamber. The silicon precursor and the atomic oxygen precursor react to form the silicon oxide layer on the substrate, and the deposited silicon oxide layer may be annealed. Systems to deposit a silicon oxide layer on a substrate are also described.

Strong broadband optical absorption in silicon nanowire films

Abstract: The broadband optical absorption properties of silicon nanowire (SiNW) films fabricated on glass substrates by wet etching and chemical vapor deposition (CVD) have been measured and found to be higher than solid thin films of equivalent thickness. The observed behavior is adequately explained by light scattering and light trapping though some of the observed absorption is due to a high density of surface states in the nanowires films, as evidenced by the partial reduction in high residual sub-bandgap absorption after hydrogen passivation. Finite difference time domain simulations show strong resonance within and between the nanowires in a vertically oriented array and describe the experimental absorption data well. These structures may be of interest in optical films and optoelectronic device applications.

Twin-free uniform epitaxial GaAs nanowires grown by a two-temperature process

Abstract: We demonstrate vertically aligned epitaxial GaAs nanowires of excellent crystallographic quality and optimal shape, grown by Au nanoparticle-catalyzed metalorganic chemical vapor deposition. This is achieved by a two-temperature growth procedure, consisting of a brief initial high-temperature growth step followed by prolonged growth at a lower temperature. The initial high-temperature step is essential for obtaining straight, vertically aligned epitaxial nanowires on the (111)B GaAs substrate. The lower temperature employed for subsequent growth imparts superior nanowire morphology and crystallographic quality by minimizing radial growth and eliminating twinning defects. Photoluminescence measurements confirm the excellent optical quality of these two-temperature grown nanowires. Two mechanisms are proposed to explain the success of this two-temperature growth process, one involving Au nanoparticle-GaAs interface conditions and the other involving melting-solidification temperature hysteresis of the Au-Ga nanoparticle alloy.

Demonstration of Nonpolar m-Plane InGaN/GaN Laser Diodes

Abstract: The first nonpolar m-plane (1-100) nitride laser diodes (LDs) have been realized on low extended defect bulk m-plane GaN substrates The LDs were grown by metal organic chemical vapor deposition (MOCVD) using conditions similar to that of c-plane device growth Broad area lasers were fabricated and tested under pulsed conditions Lasing was observed at duty cycles as high as 10% These laser diodes had threshold current densities (Jth) as low as 75 kA/cm2 Stimulated emission was observed at 4055 nm, with a spectral line-width of 1 nm

Multi-gas concentric injection showerhead

Abstract: A method and apparatus that may be utilized for chemical vapor deposition and/or hydride vapor phase epitaxial (HVPE) deposition are provided. In one embodiment, a metal organic chemical vapor deposition (MOCVD) process is used to deposit a Group III-nitride film on a plurality of substrates. A Group III precursor, such as trimethyl gallium, trimethyl aluminum or trimethyl indium and a nitrogen-containing precursor, such as ammonia, are separately delivered to a plurality of concentric gas injection ports. The precursor gases are injected into mixing zones where the gases are mixed before entering a processing volume containing the substrates.

Fabrication of Boron-Doped TiO2 Nanotube Array Electrode and Investigation of Its Photoelectrochemical Capability

Abstract: Boron-doped TiO2 nanotube arrays were produced by forming a nanotube-like TiO2 film in an anodization process on a Ti sheet, followed by chemical vapor deposition treatment using trimethyl borate as the boron source with N2 as the carrier gas, and were characterized by ESEM, XPS, XRD, and UV−vis methods. The highly ordered vertically oriented nanotube arrays were obtained, and the nanotubes were open at the top end with an average diameter of approximately 80 nm. Analysis by XPS indicated that the introduced boron was probably incorporated into TiO2 and that the chemical environmental surrounding boron might be Ti−B−O. The boron-doped sample with a mixture of anatase and rutile was identified by X-ray diffraction. A shift of the absorption edge to a lower energy in the spectrum of the UV−vis absorption was observed. Under both UV and 400−620 nm visible light irradiation, the B-doped TiO2 nanotube array electrode exhibited a higher photoconversion efficiency than the non-doped one, a notable photoconversio...

Plasmon-assisted local temperature control to pattern individual semiconductor nanowires and carbon nanotubes.

Abstract: We demonstrate a new versatile strategy to rapidly heat and cool subdiffraction-limited volumes of material with a focused light beam. The local temperature rise is obtained by exploiting the unique optical properties of metallic nanostructures that facilitate efficient light-to-heat conversion through the excitation of surface plasmons (collective electron oscillations). By locally heating nanoscale metallic catalysts, growth of semiconductor nanowires and carbon nanotubes can be initiated and controlled at arbitrarily prespecified locations and down to the single nanostructure level in a room-temperature chamber. This local heating strategy can be orders of magnitude (>105) more energy efficient than conventional chemical vapor deposition (CVD) tools in which an entire chamber/substrate is heated. For these reasons, it has great potential for use in process- and energy-efficient assembly of nanowires into complementary metal-oxide-semiconductor (CMOS) compatible device architectures. In general, the hig...

Synthesis and Characterization of Carbon-Doped TiO2 Nanostructures with Enhanced Visible Light Response

Abstract: Carbon-doped TiO2 micro-/nanospheres and nanotubes have been synthesized via a single source chemical vapor deposition in an inert atmosphere. Organic compound Ti(OC4H9)4 was used as the titanium, oxygen, and carbon source, while argon served as the carrier gas. The effect of the temperature, substrate, and the flow rate of the carrier gas is investigated. The diameter of the formed carbon-doped TiO2 spheres can be adjusted from 100 nm to several micrometers by varying the flow rate of the carrier gas. The as-prepared TiO2 nanotubes are highly ordered with a diameter of about 100 nm and a wall thickness of around 15 nm. The estimated optical band gap is 2.78 eV for the formed carbon-doped TiO2 microspheres and 2.72 eV for the synthesized carbon-doped TiO2 nanotubes, both of which are much smaller than that of bulk anatase TiO2 (3.20 eV). The photocurrent of the carbon-doped TiO2 spheres is much higher than that of commercial P-25, which is currently considered as one of the best TiO2 photocatalysts, espec...

Photoacoustic characterization of carbon nanotube array thermal interfaces

Abstract: This work describes an experimental study of thermal conductance across multiwalled carbon nanotube (CNT) array interfaces, one sided (Si-CNT-Ag) and two sided (Si-CNT-CNT-Cu), using a photoacoustic technique (PA). Well-anchored, dense, and vertically oriented multiwalled CNT arrays have been directly synthesized on Si wafers and pure Cu sheets using plasma-enhanced chemical vapor deposition. With the PA technique, the small interface resistances of the highly conductive CNT interfaces can be measured with accuracy and precision. In addition, the PA technique can resolve the one-sided CNT interface component resistances (Si-CNT and CNT-Ag) and the two-sided CNT interface component resistances (Si-CNT, CNT-CNT, and CNT-Cu) and can estimate the thermal diffusivity of the CNT layers. The thermal contact resistances of the one- and two-sided CNT interfaces measured using the PA technique are 15.8±0.9 and 4.0±0.4mm2K∕W, respectively, at moderate pressure. These results compare favorably with those obtained using a steady state, one-dimensional reference bar method; however, the uncertainty range is much narrower. The one-sided CNT thermal interface resistance is dominated by the resistance between the free CNT array tips and their opposing substrate (CNT-Ag), which is measured to be 14.0±0.9mm2K∕W. The two-sided CNT thermal interface resistance is dominated by the resistance between the free tips of the mating CNT arrays (CNT-CNT), which is estimated to be 2.1±0.4mm2K∕W.

A Review of Carbon Nanotube Synthesis via Fluidized-Bed Chemical Vapor Deposition

Abstract: Carbon nanotubes (CNTs) are crystalline, tubular, carbon structures with extraordinary mechanical, chemical, optical, and electrical properties. These unique properties make CNTs potentially valuable in a wide range of end-use applications. Currently, research into nanotubes and their applications is hampered by the lack of a suitable technique for manufacturing them in large quantities, which we define here as 10 000 tons per plant per year. Consequently, research into large-scale manufacturing techniques is ongoing. There are three established methods of CNT synthesis: (i) arc discharge, (ii) laser ablation, and (iii) chemical vapor deposition (CVD). Of these, CVD techniques show the greatest promise for economically viable, large-scale synthesis, based upon yields reported in the literature and the inherent scalability of similar technologies, e.g., fluidized catalytic cracking. In particular, the fluidized-bed CVD (FBCVD) technique (where the CVD reaction occurs within a fluidized bed of catalyst par...

Carbon Nanotube Growth from Semiconductor Nanoparticles

Abstract: Nanoscale metal catalysts have been indispensable for carbon nanotube (CNT) synthesis by chemical vapor deposition (CVD). We show that even semiconductor nanoparicles of SiC, Ge, and Si produce single-walled and double-walled CNTs in CVD with ethanol. This implies that nanosize structures might act as a template for the formation of CNT caps composed of five- and six-membered rings. Providing a template for cap formation is the essential role of the catalysts.

Transition between grain boundary and intragrain scattering transport mechanisms in boron-doped zinc oxide thin films

Abstract: A comprehensive model for the electronic transport in polycrystalline ZnO:B thin films grown by low pressure chemical vapor deposition is presented. The optical mobilities and carrier concentration calculated from reflectance spectra using the Drude model were compared with the data obtained by Hall measurements. By analyzing the results for samples with large variation of grain size and doping level, the respective influences on the transport of potential barriers at grain boundaries and intragrain scattering could be separated unambiguously. A continuous transition from grain boundary scattering to intragrain scattering is observed for doping level increasing from 3×1019to2×1020cm−3.

Diamond for bio-sensor applications

Abstract: A summary of photo- and electrochemical surface modifications applied on single-crystalline chemical vapour deposition (CVD) diamond films is given. The covalently bonded formation of amine- and phenyl-linker molecule layers is characterized using x-ray photoelectron spectroscopy, atomic force microscopy (AFM), cyclic voltammetry and field-effect transistor characterization experiments. Amine- and phenyl-layers are very different with respect to formation, growth, thickness and molecule arrangement. We detect a single-molecular layer of amine-linker molecules on diamond with a density of about 1014?cm?2 (10% of carbon bonds). Amine molecules are bonded only on initially H-terminated surface areas to carbon. In the case of electrochemical deposition of phenyl-layers, multi-layer formation is detected due to three-dimensional (3D) growths. This gives rise to the formation of typically 25?? thick layers. The electrochemical grafting of boron-doped diamond works on H-terminated and oxidized surfaces.After reacting such films with hetero-bifunctional crosslinker molecules, thiol-modified ss-DNA markers are bonded to the organic system. Application of fluorescence and AFM on hybridized DNA films shows dense arrangements with densities of up to 1013?cm?2. The DNA is tilted by an angle of about 35? with respect to the diamond surface. Shortening the bonding time of thiol-modified ss-DNA to 10?min causes a decrease of DNA density to about 1012?cm?2. Application of AFM scratching experiments shows threshold removal forces of around 75?nN for DNA bonded on phenyl-linker molecules and of about 45?nN for DNA bonded to amine-linker molecules. DNA sensor applications using Fe(CN6)3?/4? mediator redox molecules, impedance spectroscopy and DNA-field effect transistor devices performances are introduced and discussed.

Multi-gas straight channel showerhead

Abstract: A method and apparatus that may be utilized for chemical vapor deposition and/or hydride vapor phase epitaxial (HVPE) deposition are provided. In one embodiment, a metal organic chemical vapor deposition (MOCVD) process is used to deposit a Group III-nitride film on a plurality of substrates. A Group III precursor, such as trimethyl gallium, trimethyl aluminum or trimethyl indium and a nitrogen-containing precursor, such as ammonia, are delivered to a plurality of straight channels which isolate the precursor gases. The precursor gases are injected into mixing channels where the gases are mixed before entering a processing volume containing the substrates. Heat exchanging channels are provided for temperature control of the mixing channels to prevent undesirable condensation and reaction of the precursors.

Multi-step dep-etch-dep high density plasma chemical vapor deposition processes for dielectric gapfills

Abstract: A method of forming a dielectric material in a substrate gap using a high-density plasma is described. The method may include depositing a first portion of the dielectric material into the gap with the high-density plasma. The deposition may form a protruding structure that at least partially blocks the deposition of the dielectric material into the gap. The first portion of dielectric material is exposed to an etchant that includes reactive species from a mixture that includes NH3 and NF3. The etchant forms a solid reaction product with the protruding structure, and the solid reaction product may be removed from the substrate. A final portion of the dielectric material may be deposited in the gap with the high-density plasma.

Systematic control of the electrical conductivity of poly(3,4-ethylenedioxythiophene) via oxidative chemical vapor deposition

Abstract: Systematic variation in the electrical conductivity of poly(3,4-ethylenedioxythiophene) (PEDOT) was achieved by oxidative chemical vapor deposition (oCVD). For oCVD, both the oxidant, Fe(III)Cl 3 , and 3,4-ethylenedioxythiophene (EDOT) monomer are introduced in the vapor phase. A heated crucible allows for sublimation of the oxidant directly into the reactor chamber operating at 150 mTorr. Spontaneous reaction of the oxidant with the monomer introduced though a feedback-controlled mass flow system results in the rapid (>200 nm thick film in 30 min) formation of π-conjugated PEDOT thin films directly onto a temperature-controlled substrate. As the substrate temperature is increased from 15 to 110 °C, increasing conjugation length, doping level, and electrical conductivity of the PEDOT chains are observed by UV-vis absorption spectroscopy (UV-vis), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. Concomitantly, the measured electrical conductivity of the PEDOT films increases systematically with an apparent activation energy of 28.2 ±1.1 kcal/mol.

Method for depositing thin films by mixed pulsed CVD and ALD

Abstract: Films are deposited on a substrate by a process in which atomic layer deposition (ALD) is used to deposit one layer of the film and pulsed chemical vapor deposition (CVD) is used to deposit another layer of the film. During the ALD part of the process, a layer is formed by flowing sequential and alternating pulses of mutually reactive reactants that deposit self-limitingly on a substrate. During the pulsed CVD part of the process, another layer is deposited by flowing two CVD reactants into a reaction chamber, with at least a first of the CVD reactants flowed into the reaction chamber in pulses, with those pulses overlapping at least partially with the flow of a second of the CVD reactants. The ALD and CVD parts of the process ca be used to deposit layers with different compositions, thereby forming, e.g., nanolaminate films. Preferably, high quality layers are formed by flowing the second CVD reactant into the reaction chamber for a longer total duration than the first CVD reactant. In some embodiments, the pulses of the third reactant at separated by a duration at least about 1.75 times the length of the pulse. Preferably, less than about 8 monolayers of material are deposited per pulse of the first CVD reactant.

Process for producing silicon oxide films from organoaminosilane precursors

Abstract: A silicon oxide layer is deposited on a substrate by chemical vapor deposition (CVD) by reacting an organoaminosilane precursor, selected from specified categories, with an oxidizing agent under conditions for the formation of a silicon oxide film. Diisopropylaminosilane is the preferred organoaminosilane precursor for the formation of the silicon oxide film.

Method of eliminating a lithography operation

Abstract: Methods of semiconductor device fabrication are disclosed. An exemplary method includes processes of depositing a first pattern on a semiconductor substrate, wherein the first pattern defines wide and narrow spaces; depositing spacer material over the first pattern on the substrate; etching the spacer material such that the spacer material is removed from horizontal surfaces of the substrate and the first pattern but remains adjacent to vertical surfaces of a wide space defined by the first pattern and remains within narrow a space defined by the first pattern; and removing the first pattern from the substrate. In one embodiment, the first pattern can comprise sacrificial material, which can include, for example, polysilicon material. The deposition can comprise physical vapor deposition, chemical vapor deposition, electrochemical deposition, molecular beam epitaxy, atomic layer deposition or other deposition techniques. According to another embodiment, features for lines and logic device components having a width greater than that of the lines are formed in the spacer material in the same mask layer.

Vapor deposition reactor

Abstract: A vapor deposition reactor has a configuration where a substrate or a vapor deposition reactor moves in a non-contact state with each other to allow the substrate to pass by the reactor and an injection unit and an exhaust unit are installed as a basic module of the reactor for receiving a precursor or a reactant and for receiving and pumping a purge gas, respectively. With the use of a small-size inlet for the reactor, homogeneous film properties are obtained, the deposition efficiency of precursors is improved, and an amount of time required for a purge/pumping process can be reduced. In addition, since the reactor itself is configured to reflect each step of ALD, it does not need a valve. Moreover, the reactor makes it easier for users to apply remote plasma, use super high frequencies including microwave, and UV irradiation.

Defect reduction of GaAs epitaxy on Si (001) using selective aspect ratio trapping

Abstract: Metal-organic chemical vapor deposition growth of GaAs on Si was studied using the selective aspect ratio trapping method. Vertical propagation of threading dislocations generated at the GaAs∕Si interface was suppressed within an initial thin GaAs layer inside SiO2 trenches with aspect ratio >1, leading to defect-free GaAs regions up to 300nm in width. Cross-sectional and plan-view transmission electron microscopies were used to characterize the defect reduction. Material quality was confirmed by room temperature photoluminescence measurements. This approach shows great promise for the fabrication of optoelectronic integrated circuits on Si substrates.

Influence of the substrate misorientation on the properties of N-polar GaN films grown by metal organic chemical vapor deposition

Abstract: Smooth, high quality N-polar GaN films were realized by metal organic chemical vapor deposition (MOCVD) through growth on misoriented (0001) sapphire substrates and the development of a high temperature nucleation process. Misorientation angles from 0.5° to 4° toward the a and the m plane of the sapphire substrate were investigated. Whereas GaN films grown on substrates with a misorientation angle of only 0.5° or 1° exhibited high densities of hexagonal surface features as commonly observed for N-polar GaN films grown by MOCVD, smooth GaN layers were obtained on sapphire substrates with misorientation angles of 2° or larger. In addition, the structural and optical properties of the GaN films significantly improved with increasing misorientation angle, as evaluated by high resolution x-ray diffraction, atomic force microscopy, transmission electron microscopy, and photoluminescence measurements. The properties of GaN layers grown on (0001) sapphire with a misorientation of 4° were comparable to Ga-polar GaN films grown in the same reactor.