Thin GaN Buffer Layer Research Articles

A CMOS-Compatible Process for ≥3 kV GaN Power HEMTs on 6-inch Sapphire Using In Situ SiN as the Gate Dielectric.

The application of GaN HEMTs on silicon substrates in high-voltage environments is significantly limited due to their complex buffer layer structure and the difficulty in controlling wafer warpage. In this work, we successfully fabricated GaN power HEMTs on 6-inch sapphire substrates using a CMOS-compatible process. A 1.5 µm thin GaN buffer layer with excellent uniformity and a 20 nm in situ SiN gate dielectric ensured uniformly distributed VTH and RON across the entire 6-inch wafer. The fabricated devices with an LGD of 30 µm and WG of 36 mm exhibited an RON of 18.06 Ω·mm and an off-state breakdown voltage of over 3 kV. The electrical mapping visualizes the high uniformity of RON and VTH distributed across the whole 6-inch wafer, which is of great significance in promoting the applications of GaN power HEMTs for medium-voltage power electronics in the future.

A GaN-Based LED With Perpendicular Structure Fabricated on a ZnO Substrate by MOCVD

A perpendicular InGaN/GaN multiple-quantum- wells structure on ZnO substrate for blue light emitting diode (LED) was successfully fabricated by use of Metal-organic Chemical Vapor Deposition (MOCVD). During the growing process of GaN-based materials on ZnO substrates, the low-temperature-grown GaN buffer layer, inserted between ZnO substrate and undoped GaN layer, prevented the Zn and O from diffusing from ZnO substrate into the n-GaN layer. This thin GaN buffer layer, mainly as a insulating layer, was grown at relatively low temperature of 530 $^{\circ}{\hbox{C}}$ . By using our method, an integrated LED with ZnO substrate can be fabricated with a crack-free GaN film on (0001) ZnO substrate by MOCVD using this method. The epilayer crystalline structure has been measured by atomic force microscopy (AFM), and the optical properties of the LED were also characterized by photoluminescence and Current-Voltage characteristic curve.

Single crystalline InxGa1−xN layers on germanium by molecular beam epitaxy

InxGa1−xN (InGaN) alloys are predominantly grown by heteroepitaxy on foreign substrates. Most often Al2O3, SiC and Si are used as substrates, however this complicates vertical conduction from the InGaN surface to the substrate backside. Therefore we investigate the heteroepitaxial growth of InGaN layers on Ge substrates. Single crystalline InGaN was obtained and domain formation was suppressed by using a thin GaN buffer layer. The InGaN shows compressive strain, which follows from the lattice mismatch with the GaN buffer layer. The In distribution is uniform throughout the InGaN layer, with no significant In segregation within the layer. Only at the surface, in a very thin layer of 20 nm, strong In segregation is observed with about 50% In. InGaN/GaN/Ge diodes show vertical current conduction of 1 A cm−2 at −2 V. InGaN grown on Ge is therefore promising for device applications with preferred vertical conduction.

Anisotropic Dielectric Properties of GaN Epilayers on Sapphire

ABSTRACTPolarized infrared reflectance from 300 to 1200 cm-1 at different incidence angles was measured at room temperature for sapphire and GaN/sapphire samples. A film of GaN was grown approximately 1.5 μm thick on c-plane sapphire by MOCVD with a thin GaN buffer layer. Because of the hexagonal structure of sapphire and GaN, their uniaxial optical properties are anisotropic. In the p-polarized reflective spectrum, we observed the mixed effect between the distinct infrared-active modes with dipole-moment oscillation perpendicular (E-mode) and parallel (A-mode) to the c axis. The contribution of the A-mode increased with increasing incidence angle. Therefore, we were able to obtain simultaneously the infrared dielectric functions parallel and perpendicular to the c axis, by fitting simultaneously three polarized reflectance spectra at three different incident angles with a suitable model. In the procedure, we adopted a new fitting technique, i.e., fitting the first numerical derivative of the polarized reflectance spectra to improve the accuracy of the phonon parameters and to overcome the inconsistency between the model and measurement in the whole frequency range. Excellent agreement has been obtained between the measured and fitted first derivative reflectance spectra for both the sapphire and GaN/sapphire samples. The dielectric information thus obtained for sapphire and GaN is of greater accuracy than those reported previously.

Chemical beam epitaxy of GaN on (0001) sapphire substrate

Gallium nitride films were grown on (0001) sapphire substrates by chemical beam epitaxy (CBE) using triethylgallium (TEGa) and ammonia (NH 3) precursors. Prior to the GaN epilayer growth at 850°C, a thin GaN buffer layer was deposited at 560°C. Structural and optical properties of the epilayers were investigated as a function of the anneal treatment of the buffer layer. Annealing of the buffer in NH 3 up to 900°C increases the roughness of the surface, resulting in a epilayer with higher crystallinity. Heating the buffer to 900°C results in partial desorption of the film leaving small grains on an exposed substrate. While the epitaxy on this thin buffer is two-dimensional the resulting surface consists of a hexagonal tile-structure. The level of unintentional carbon doping is high in all films, although the growth conditions need further optimization. CBE may become a promising candidate for the growth of nitride films only if the carbon incorporation is not an inherent problem of the technique.

Growth and characterization of GaN films on (0001) sapphire substrates by alternate supply of trimethylgallium and NH 3

GaN films were grown on (0001) sapphire substrates in a temperature range of 500∼950°C by exposing the substrates to trimethylgallium (TMG) and NH 3 one at a time. High quality GaN films were achieved at 800∼900°C with a thin GaN buffer layer predeposited at 500°C. The linewidth of the (0004) double-crystal rocking curve X-ray diffraction (DCXRD) of a 2.0 μm thick GaN film is about 250∼500 arcsec. Increment in growth temperature over a temperature range of 800∼900°C is helpful to suppress the intensity of yellow luminescence and to enhance the luminescence intensity of the near band-to-band emission. Typically, the room temperature photoluminescence intensity ratio of the near band edge emission to the yellow luminescence for an as-grown high quality GaN film is more than one order of magnitude. The best GaN films with superior optical behavior were achieved at ∼900°C with a V/III ratio of 5000. Photoluminescence measurements of these GaN films at 9 K show strong and sharp near band-to-band emission with almost no yellow luminescence.

The growth behavior of GaN film on Si(111) substrate by an ion-beam assisted evaporation process

Highly oriented GaN thin films were grown on Si(111) substrate using an ion beam assisted evaporation method. Nitrogen ions, with a kinetic energy of about 40 eV, was supplied by a Kaufman ion source; and Ga vapor was supplied by thermal evaporation. The surface morphology of the nucleation layer, and the crystalline properties of 200–300 nm thick GaN epi-layer were investigated by atomic force microscopy, transmission electron microscopy, and X-ray diffraction. Film grown under a Ga-rich flux condition produced film growth behavior of large islands of hexagonal configuration. Crystallinity on such film, however, was of poorer quality than other films with smaller islands, grown under high nitrogen ion flux conditions. The full width at half-maximum of (0002) diffraction peak was measured at 52 arcminutes for the GaN epilayer single-stepwise grown at 660°C. Ion-enhanced decomposition occurred, causing no film formation at substrate temperatures above 710°C. Additionally, the effect of predeposition of a buffer layer on GaN crystallinity was investigated for surface roughness. AFM measurement revealed that the GaN buffer layer grown on Si(111) showed smooth surface under the relatively N 2 +-sufficient condition. The introduction of thin GaN buffer layer, grown at 600°C under N 2 +-sufficient condition, worked on reducing the lattice-mismatch stress and in-plane misorientation of grains, and thus enhancing the crystallinity of the two-stepwise grown GaN epi-layer. Characteristic behavior of GaN epi-layers, single or two stepwise grown on Si(111), show a type of granular (columnar) epitaxy.

Effect Of Atomic-Hydrogen Treatment Of (001) Gaas Substrate At “High Temperatures” On Rf Plasma-Assisted Molecular Beam Epitaxy Of Cubic Gan

AbstractsGrowth of high quality c-GaN on atomic-H treated (001) GaAs was examined by rf plasma-assisted MBE. First the initial growth stages (atomic-H treatment of GaAs, nitridation/deposition of a thin GaN buffer layer, the post deposition annealing of the buffer layer, and epitaxy of c-GaN) were studied by RHEED and AFM observations. It was found that atomically flat GaAs surface with one monolayer-height steps and ragged step edges could be obtained by the atomic-H treatment at high temperatures. It was found that the atomic-H treated GaAs surface was preferable to grow high quality c-GaN; the FWHM of the X-ray rocking curve for the (002) c-GaN was as small as 70 – 90 arcsec.

Vapor Phase Epitaxy of GaN Using Gallium Tri-Chloride and Ammonia

AbstractGaN films with good crystalline quality are grown on sapphire by atmospheric pressure vapor phase epitaxy using gallium tri-chloride (GaCl3) and ammonia (NH3). Epitaxial growth is carried out over temperature and V/III-ratio ranges of 800–1000°C and 100–1000, respectively. Typical growth rate obtained is in the range of 5–20 μm/hr. The films grown below 925°C typically show three dimensional (island) growth, while above that temperature, continuous films are obtained. Films grown at 975°C with a V/III ratio > 300 exhibit a smooth surface. XRD analysis shows that the films are single crystal with hexagonal polytype. Strong band-edge photoluminescence is observed with a FWHM of 60 meV at room temperature and 25 meV at 77K. The results indicate that this simple growth technique is effective for growing high quality bulk GaN, which can be used as a substrate for subsequent epitaxy. In order to further improve the surface morphology, a preliminary experiment on GaN growth on a thin GaN buffer layer prepared by gas source MBE is also presented.

Reduction of p-ZnSe/p-GaAs non-ohmic barrier by inserting a GaN buffer layer

Nitrogen (N) radical irradiated GaAs was investigated by Auger analysis and reflection high-energy electron diffraction. N radical beam irradiation on GaAs causes an exchange of As atoms for N atoms, consequently forming a GaN layer onto the GaAs surface without a Ga source. No degradation of the ZnSe layers was observed when they were grown on GaAs substrates with GaN buffer layers. The current-voltage and capacitance-voltage characteristics of p-ZnSe/p-GaAs heterodiodes are examined. The necessary voltage for hole injection across the p-ZnSe/p-GaAs heterojunction is reduced to about half when a thin GaN buffer layer is inserted into the junction.