Basal Plane Sapphire Research Articles

Study of GaN and InGaN films grown by metalorganic chemical vapour deposition

In this paper GaN and InGaN films are examined, which are grown on basal plane (0001) sapphire substrates. Growth is performed in a novel type of vertical rotating disk reactor. The effects of several growth parameters on the film quality are discussed. The GaN and InGaN layers were evaluated by surface morphology studies, DC X-ray diffraction, electrical and optical characterisation.

Pyroelectricity in gallium nitride thin films

We report on the measurements of the pyroeffect in wurtzite n-type GaN films deposited over basal plane sapphire substrates. We measured the voltage drop between the contacts while the sample was subjected to the uniform heating or cooling. The pyroelectric voltage coefficient extracted from our data is comparable to that of the pyroelectric ceramics (∼104 V/m K). Our results show that the pyroelectric effect in GaN is a combination of a fast response to an initial heat flow and a slower response related to a change in the sample temperature.

Cathodoluminescence of AlN–GaN short period superlattices

Cathodoluminescence of AlN–GaN short period superlattice films was measured at 6 K, 77 K, and room temperature. The superlattice films were deposited using a switched atomic layer metalorganic chemical vapor deposition process onto a buffer layer of either AlN or GaN, which was deposited on basal plane sapphire substrates. The individual AlN and GaN layers of the superlattice films ranged in thickness from 2.6 to 20.8 Å. The cathodoluminescence of these samples was measured at several electron acceleration voltages to allow depth profiling of the samples. This allows the region of the sample (superlattice film, buffer layer, and substrate) from which the spectral features originate to be determined. A spectral peak in the ultraviolet region above the 3.5 eV band gap of GaN has been observed in all the superlattice samples studied to date. Our results indicate that the location of this peak is determined by quantum confinement in the GaN layers.

Infrared reflectance of AlN–GaN short period superlattice films

The room-temperature infrared reflectance of AlN–GaN short period superlattice films has been measured. These superlattice films were deposited by switched atomic layer metalorganic chemical vapor deposition onto GaN or AlN buffer layers deposited on basal plane sapphire substrates. The measured reflectance spectra are compared to calculated spectra using an effective medium theory to model the dielectric function of the superlattice. The optical properties of the individual materials making up the samples are modeled with Lorentz oscillators using only bulk input parameters. The effects of film and substrate anisotropy and off-normal incidence are included in the calculation. Using this modeling technique, it is possible to obtain thickness estimates for the superlattice film and the buffer layer. The complicated structures seen in the reststrahl region reflectance of these films are also analyzed by comparison to the calculated spectra.

Photoluminescence study of high quality InGaN–GaN single heterojunctions

In this letter we report the results of room-temperature continuous wave and pulsed photoluminescence measurements on InGaN–GaN single heterojunctions. These InGaN–GaN heterojunctions were deposited over basal plane sapphire substrates using low pressure metalorganic chemical vapor deposition. We suggest the use of vertical cavity stimulated emission instead of spontaneous emission peak position as a good measure of the InGaN band edge.

Low temperature inorganic chemical vapor deposition of heteroepitaxial GaN

We have developed a highly efficient method of growing thin oriented films of GaN on basal plane sapphire and (100) Si substrates using an exclusively inorganic single-source precursor free of carbon and hydrogen. Cross sectional transmission electron microscopy of the highly conformal films revealed columnar material growth on Si and heteroepitaxial columnar growth of crystalline GaN on sapphire. Rutherford backscattering spectroscopy (RBS) of layers grown at 700 °C confirmed stoichiometric GaN. Auger and RBS oxygen and carbon resonance profiles indicated that the films were pure and highly homogeneous. With respect to current chemical vapor deposition processes for GaN growth, our approach offers a number of potentially important improvements. These include high growth rates of 5–350 nm/min, low deposition temperature of 650–700 °C, nearly ideal Ga–N stoichiometry, elimination of the highly inefficient use of toxic ammonia, and a carbon–hydrogen free growth environment that could prove to be beneficial to p-doping processes.

Self-Nucleated Thin Films of GaN and AlxGa1−xN for Optoelectronic Devices: Structure and Morphology

Abstract There is increasing interest in the use of Group IIIA nitrides (A1N, GaN, InN, and their alloys) for a variety of optoelectronic applications, including short wavelength sources and detectors. Progress, while spectacular these past few years, has been hampered by the lack of bulk, lattice-matched material to serve as the substrate for homoepitaxial growth. The most common substrate, basal plane sapphire, with a 16% lattice mismatch, requires a predeposited nucleation layer to achieve pseudo two-dimensional layer-by-layer growth suitable for device processing. The surface morphology and mosaic dispersion of both unnucleated and self-nucleated GaN thin films have been studied by a combination of real space images from atomic force and scanning tunneling microscopies and reciprocal space intensity data from X-ray scattering measurements. The unnucleated GaN films show a large-grained hexagonal relief, typical of three-dimensional island growth, while the self-nucleated films are shown to be dense mo...

High responsitivity intrinsic photoconductors based on AlxGa1−xN

This letter reports on the fabrication and characterization of visible-blind ultraviolet photoconductors using single-crystal AlxGa1−xN layers deposited on basal plane sapphire substrates. With aluminum mole fractions ranging from 5% to 61%, the long-wavelength cutoff can be varied from 350 to 240 nm. Photoresponsitivities as high as several hundred amperes per watt were measured with 10 μm interelectrode spacing.

Cleaved GaN facets by wafer fusion of GaN to InP

Basal plane sapphire is a common substrate for the heteroepitaxy of GaN. This presents a challenge for fabrication of cleaved-facet GaN lasers because the natural cleavage planes in (0001) α-Al2O3 are not perpendicular to the wafer surface. This letter describes a method for achieving perpendicular cleaved facets through wafer fusion that can potentially be used to fabricate GaN based in-plane lasers. We demonstrate successful fusion of GaN to InP without voids or oxide at the interface and fabricate optically flat cleaved GaN facets that are parallel to the crystallographic planes of the host InP. I–V measurements have been performed across the n-N fused interface. These results show that the fused junction exhibits a barrier of several electron volts for electrons passing from the InP to the GaN and ohmic conduction of electrons moving in the opposite direction.

Growth defects in GaN films on sapphire: The probable origin of threading dislocations

Single crystal GaN films with a wurtzite structure were grown on the basal plane of sapphire. A high density of threading dislocations parallel to the c-axis crossed the film from the interface to the film surface. They were found to have a predominantly edge character with a Burgers vector. In addition, dislocation hal-loops, elongated along the c-axis of GaN, were also found on the prism planes. These dislocations had a mostly screw character with a [0001] Burgers vector. Substrate surface steps with a height of were found to be accommodated by localized elastic bending of GaN (0001)GaN planes in the vicinity of the film/substrate interface. Observations show that the region of the film, with a thickness of ∼100 nm, adjacent to the interface is highly defective. This region is thought to correspond to the low-temperature GaN “buffer” layer which is initially grown on the sapphire substrate. Based on the experimental observations, a model for the formation of the majority threading dislocations in the film is proposed. The analysis of the results leads us to conclude that the film is under residual biaxial compression.

Deposition of high quality wurtzite GaN films over cubic (111) MgAl2O4 substrates using low pressure metalorganic chemical vapor deposition

We report the deposition of high quality single-crystal wurtzite GaN films on cubic (111) spinel (MgAl2O4) substrates. The room-temperature electron mobility and the optically pumped stimulated emission threshold for these films are nearly identical to those of films deposited on basal plane sapphire. Using cross sectional high resolution TEM we have determined the following orientation relationship between the film and the substrate: [0001]GaN//[111]MgAl2O4 and [112̄0]GaN//[110]MgAl2O4. This should provide a common cleavage plane for (111) spinel and the wurtzite GaN films over it.

Piezoresistive effect in wurtzite n-type GaN

We report on the measurements of the piezoresistive effect in the n-type wurtzite GaN films. The 3–5 μm thick GaN layers were deposited slightly off axis over basal plane sapphire substrates. The static and dynamic gauge factor (GF) of these structures was measured at room temperature for both longitudinal and transverse configurations. The dynamic effect is related to a strong piezoeffect in GaN. The maximum dynamic GF observed was ∼130 (approximately four times larger than for SiC).

New Pathways to Heteroepitaxial GaN by Inorganic CVD Synthesis and Characterization of Related Ga-C-N Novel Systems

ABSTRACTWe describe the development of a new deposition method for thin oriented films of GaN on basal plane sapphire using an exclusively inorganic single-source precursor free of carbon and hydrogen, Cl2GaN3. The films have been characterized by Rutherford backscattering spectroscopy (RBS) and cross sectional transmission electron microscopy (TEM) for composition morphology and structure. RBS analysis confirmed stoichiometric GaN and TEM observations of the highly conformal films revealed heteroepitaxial columnar growth of crystalline wurrtzite material on sapphire. Auger and RBS oxygen and carbon resonance profiles indicated that the films were pure and highly homogeneous. We also report the reactions of Cl2GaN3 with organometallic nitriles to yield a crystalline, novel gallium carbon nitride of composition GaC3N3. Quantitative X-ray powder diffraction has been used to refine the cubic structure of this material which consists of Ga atoms octahedrally surrounded by on the average three C and three N atoms. The structurally analogous LiGaC4N4 phase has also been prepared and characterized.

Forces between Crystalline Alumina (Sapphire) Surfaces in Aqueous Sodium Dodecyl Sulfate Surfactant Solutions

Using the surface forces apparatus, we have studied the adsorption of negatively charged sodium dodecyl sulfate (SDS) surfactant onto positively charged surfaces of Al2O3 and the resulting interactions between these surfaces in aqueous solutions. The adsorbed layer thicknesses, adhesion forces, and long range colloidal interactions were measured at SDS concentrations from 0.01 to 5 mM (below the critical micelle concentration, cmc = 8 mM). Our results show that an SDS bilayer of thickness ∼3.2 nm forms at bulk concentrations above 1 mM (≥1/10(cmc)) and that beyond bilayer−bilayer contact the measured forces are well described by the classical Derjaguin−Landau−Verwey−Overbeek (DLVO) theory (with a constant surface potential at low surfactant coverage and constant surface charge density at high coverage). The lack of any hydrophobic monolayer formation well below the cmc was noticed in the force measurements, confirmed indirectly by wettability studies, and attributed to the low surface charge density on the sapphire basal plane (52.7 nm2 per charge). With one exception, the results are in good agreement with previous data on similar systems obtained using other techniques. The results clarify how electrostatic binding interactions determine the stepped adsorption of, and transitions between, hydrophobic and hydrophilic surfactant layers. The results also provide a basis for understanding why certain surfactants (under appropriate solution conditions) can be used as effective additives in the colloidal processing of ceramic materials.

Material and Device Characteristics of MBE-Grown GaN Using a New rf Plasma Source

ABSTRACTA new rf plasma nitrogen source has been characterized for growth of GaN on basal-plane sapphire by molecular beam epitaxy. For rf power of 500 W and N2 flow rate of 2 sccm, a maximum GaN growth rate of 0.80 μm/hr is obtained, implying a source efficiency greater than 5%. It is found that the GaN surface roughness is extremely sensitive to V:Il ratio near unity and independent of growth rate in the range 0.3-0.8 μm/hr. Roughness as small as 1.0 nm (rms) is measured by atomic-force microscopy. Microstructure of the high-growth-rate films is similar to other GaN films, as observed in cross-section transmission electron microscope images. The electroluminescence spectra from homojunction light-emitting diodes exhibit a band of nearultraviolet emissions corresponding to the energy separation of the intentional donor and acceptor levels on the two sides of the junction. The intensity of these emissions relative to the visible spectrum increases with drive current density, implying saturation of deep trap levels responsible for the visible light output.

Study of GaN films grown by metalorganic chemical vapour deposition

In this paper GaN films are examined, which are grown on basal plane (0001) sapphire substrates. Growth is performed in a novel type of vertical rotating disk reactor. The effects of several growth parameters on the film quality are discussed. The results on n-type doping of GaN with SiH4 are presented. The GaN layers were evaluated by surface morphology studies, DC X-ray diffraction, electrical and optical characterisation.

Growth of Niobium Films at High Temperatures on Sapphire

AbstractThe growth and microstructural evolution of Nb thin films on the basal plane of α-Al2O3were studied at different growth temperatures. The influence of island orientation, density, and misfit strain energy on the growth behavior of Nb films on (0001)α-Al2O3at high temperatures has been investigated. The films were grown by MBE at 900°C and 1100°C. At these temperatures the Nb grows in the Volmer-Weber growth mode on the basal plane.In-situreflection high energy electron diffraction (RHEED), Auger electron spectroscopy (AES) and transmission electron microscopy (TEM) investigations revealed that in the initial growth stage, Nb nuclei with different epitaxial orientations were formed. This leads to different orientations of thicker Nb films at different growth temperatures. At a growth temperature of 900°C the Nb{111} planes are parallel to the sapphire basal plane whereas at 1100°C Nb grows with the {110) planes parallel to the basal plane of sapphire. The formation of two different epitaxial orientations of thick Nb films can only be explained by considering both the change in the total density of Nb islands with temperature and the influence of island size on their total energy.

GaN Film Growth by a Supersonic Arcjet Plasma

AbstractThis paper reports on preliminary studies of GaN growth obtained using a supersonic nitrogen arcjet plasma expanding into low pressure. A hydrogen-free and carbon-free growth environment was achieved by use of a Ga vapor source positioned downstream of the expanding plume. GaN growth rates of around 8 μm/hour were obtained in these preliminary studies. We believe these growth rates are the highest reported for a non-chemical decomposition process. The growth rates are attributed to the very high nitrogen atom fluxes estimated to be on the order of 1019 atoms/cm2/sec. The initial growth rates are believed to be well below those possible with a fully optimized system. The GaN films are characterized by Raman spectroscopy, visible-IR transmission, and x-ray diffraction. The deposited films appear to be single crystal and epitaxial on basal plane sapphire substrates.

Pyroelectric Effect in Wurtzite Gallium Nitride

AbstractWe report on the measurements of the pyroeffect in wurtzite n-type GaN films deposited over basal plane sapphire substrates. The voltage drop between the contacts was measured while the sample was subjected to uniform heating. Our results show that the pyroelectric effect in GaN can be partially attributed to the secondary pyroelectricity, caused by the development of strain in the material due to thermal expansion.

Effect of the Nitridation of the Sapphire (0001) Substrate on the GaN Growth

ABSTRACTThe analysis of the sapphire surface nitridation by in situ reflection high-energy electron diffraction evidences the formation of a relaxed AIN layer. Its role on the early stage of the GaN growth is investigated by transmission electron microscopy (TEM). GaN crystallites of high structural quality, with the c axis perpendicular to the sapphire basal plane, are observed when the starting surface is nitridated. On the other hand, the growth of GaN on a bare substrate involves the formation of larger islands with numerous defects. TEM study reveals that the c axis of these latter crystallites is systematically tilted by about 19° with respect to the sapphire basal plane. Actually, this orientation corresponds to a particular epitaxial relationship between GaN and sapphire (0001) substrates. Finally, the optical properties of GaN thin layers are shown to be strongly dependent on the nitridation state of the sapphire surface.