C-face SiC Substrates Research Articles

Effect of substrate misorientation angle on the structural properties of N-polar GaN grown by hot-wall MOCVD on 4H-SiC(000[formula omitted])

The effects of substrate misorientation angle direction and degree on the structural properties of N-polar GaN grown by a novel multi-step temperature epitaxial approach using hot-wall metal–organic chemical vapor deposition (MOCVD) on 4H-SiC (0001̄) substrates is investigated. The surface morphology and X-ray diffraction (XRD) rocking curves (RCs) for both symmetric and asymmetric Bragg peaks of the multi-step temperature N-polar GaN are compared to a material obtained in a two-step temperature process. In the latter the temperature in the second step was varied so that it corresponds to the growth temperatures in each of the steps of the multi-step process. Different step-flow patterns are obtained on the substrates with a misorientation angle of 4° depending on whether its direction is towards the a-plane or the m-plane. In contrast, for a misorientation angle of 1° towards the m-plane, the surface morphology of N-polar GaN is dominated by hexagonal hillocks when using the 2-step temperature process and a step meandering growth mode is observed when employing the multi-step temperature process. These results are discussed and explained in terms of kinetic and thermodynamic considerations. As the growth temperature of the GaN layer in the 2-step temperature process increases from 950 °C to 1100 °C, the surface roughness and RCs widths decrease for the three types of substrates indicating improved crystal quality at higher temperature. The multi-step epitaxial approach is shown to be beneficial for achieving smooth surface morphology and low defect density of N-polar GaN layers grown on C-face SiC substrates with a misorientation angle of 4° and an RMS value of 1.5 nm over an area of 20 μm × 20 μm is attained when the substrate mis-cut is towards the m-plane.

Porous AlN films grown on C-face SiC by hydride vapor phase epitaxy

We report the growth of porous AlN films on C-face SiC substrates by hydride vapor phase epitaxy (HVPE). The influences of growth condition on surface morphology, residual strain and crystalline quality of AlN films have been investigated. With the increase of the V/III ratio, the growth mode of AlN grown on C-face 6H-SiC substrates changes from step-flow to pit-hole morphology. Atomic force microscopy (AFM), scanning electron microscopy (SEM) and Raman analysis show that cracks appear due to tensile stress in the films with the lowest V/III ratio and the highest V/III ratio with a thickness of about 3 μm. In contrast, under the medium V/III ratio growth condition, the porous film can be obtained. Even when the thickness of the porous AlN film is further increased to 8 μm, the film remains porous and crack-free, and the crystal quality is improved.

Growth of high-quality 4H-SiC epitaxial layers on 4° off-axis C-face 4H-SiC substrates

• 4H-SiC epilayers are prepared on C-face substrates. • Surface morphologies show a strong dependence on the growth conditions. • A specular surface morphology was obtained. 4H-Silicon carbide (SiC) epitaxial layers on the C-face SiC substrates are potentially useful for fabricating high-performance power SiC MOSFET device applications. In this research, 4H-SiC epilayers are prepared on 4° off-angle C-face 4H-SiC substrates through a low pressure chemical vapor deposition (CVD). Surface morphologies of the epilayers show a strong dependence on C/Si ratio, growth temperature and etching time. A specular surface morphology was obtained at the temperature of about 1550 °C, with the etching time of 15 min and C/Si ratio of 1.2.

N-polar AlN nucleation layers grown by hot-wall MOCVD on SiC: Effects of substrate orientation on the polarity, surface morphology and crystal quality

Hot-wall metalorganic vapor phase epitaxy enables a superior quality of group-III nitride epitaxial layers and high electron mobility transistor structures, but has not yet been explored for N-polar growth. In this work, we aim at achieving N-polar AlN nucleation layers (NLs) with optimized properties for subsequent growth of GaN device heterostructures. The effects of substrate orientation on the polarity, surface morphology and crystalline quality of AlN NLs on on-axis C-face SiC (0001̄), C-face SiC (0001̄) off-cut towards the [112̄0] by 4°, and Si-face SiC (0001) are investigated. The results are discussed in view of growth mode evolution with growth temperature and substrate orientation. It is demonstrated that N-polar AlN NLs with step-flow growth mode and 0002 rocking curve widths below 20 arcsec can be achieved on off-axis C-face SiC substrates.

Significantly improved surface morphology of N-polar GaN film grown on SiC substrate by the optimization of V/III ratio

In this paper, N-polar GaN films with different V/III ratios were grown on vicinal C-face SiC substrates by metalorganic chemical vapor deposition. During the growth of N-polar GaN film, the V/III ratio was controlled by adjusting the molar flow rate of ammonia while keeping the trimethylgallium flow rate unchanged. The influence of the V/III ratio on the surface morphology of N-polar GaN film has been studied. We find that the surface root mean square roughness of N-polar GaN film over an area of 20 × 20 μm2 can be reduced from 8.13 to 2.78 nm by optimization of the V/III ratio. Then, using the same growth conditions, N-polar InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes (LEDs) were grown on the rough and the smooth N-polar GaN templates, respectively. Compared with the LED grown on the rough N-polar GaN template, dramatically improved interface sharpness and luminescence uniformity of the InGaN/GaN MQWs are achieved for the LED grown on the smooth N-polar GaN template.

Simulation and fabrication of N-polar GaN-based blue-green light-emitting diodes with p-type AlGaN electron blocking layer

N-polar GaN-based blue-green light-emitting diodes (LEDs) with p-AlGaN electron blocking layer (EBL) were numerically investigated by simulation and experimentally grown on vicinal C-face SiC substrates by metal–organic chemical vapor deposition. By numerical simulation, we can find that p-AlGaN EBL in N-polar LEDs is able to play a more important role in blocking electron overflow than that in Ga-polar LEDs due to the reversed polarization, which leads to a high output power and internal quantum efficiency. Besides, the holes injection efficiency is enhanced in N-polar LED, resulting in a lower turn-on voltage. In experimental studies, N-polar LEDs based on different numbers of quantum wells were grown on vicinal C-face n-SiC substrates. When a forward bias is applied to the epitaxial N-polar LED with two quantum wells, a strong blue-green emission located at 480 nm can be observed. This work indicates that N-polar group-III nitrides have great potential in the application of optoelectronic devices.

Extremely uniform epitaxial growth of graphene from sputtered SiC films on SiC substrates

ABSTRACTA novel method to fabricate uniform epitaxial graphene on C-face SiC substrates was investigated. Graphene was grown on the C-face 6H-SiC substrates with a sputtered SiC film by annealing temperatures ranging from 1400 to 1900 °C under an Ar ambient. The fractional area of the graphene having the layer number of two was about 95% in a 75×75 μm square by a Raman mapping and a low energy electron microscopy. Graphene on the C-face SiC fabricated by this method is quite uniform compared to that made by a conventional method without the sputtered SiC films and is thus suitable for high frequency analog devices.

Effects of aluminum on epitaxial graphene grown on C-face SiC

The effects of Al layers deposited on graphene grown on C-face SiC substrates are investigated before and after subsequent annealing using low energy electron diffraction (LEED), photoelectron spectroscopy, and angle resolved photoemission. As-deposited layers appear inert. Annealing at a temperature of about 400 °C initiates migration of Al through the graphene into the graphene/SiC interface. Further annealing at temperatures from 500 °C to 700 °C induces formation of an ordered compound, producing a two domain √7× √7R19° LEED pattern and significant changes in the core level spectra that suggest formation of an Al-Si-C compound. Decomposition of this compound starts after annealing at 800 °C, and at 1000 °C, Al is no longer possible to detect at the surface. On Si-face graphene, deposited Al layers did not form such an Al-Si-C compound, and Al was still detectable after annealing above 1000 °C.

Influence of growth stress on the surface morphology of N-polar GaN films grown on vicinal C-face SiC substrates

In-situ stress measurements were used to monitor the growth of N-polar GaN films on vicinal C-face SiC substrates using a two-step temperature process. A reduction in compressive stress in the N-polar GaN and a corresponding decrease in surface roughness were observed as the initial growth temperature was reduced from 1000 °C to 900 °C. The results suggest that compressive stress in N-polar GaN promotes step bunching and macroscale roughness in films grown on vicinal substrates. The reduction in compressive stress is proposed to originate from tensile thermal stress induced by the temperature change in the two-step process.

Effect of AlN buffer layers on the surface morphology and structural properties of N-polar GaN films grown on vicinal C-face SiC substrates

We investigated the effect of AlN buffer layers on hexagonal hillock formation and the crystallinity of N-polar GaN films grown by metalorganic chemical vapor deposition on vicinal C-face SiC substrates misoriented toward <112¯0> by 3.6° and <101¯0> by 4°. As the source input group V/III ratio increased from 650 to 16310 and growth temperature was raised from 1100°C to 1150°C during AlN buffer layer growth on the substrates with <112¯0> miscut direction, the threading dislocation (TD) density of the AlN buffer layers and subsequent N-polar GaN films decreased. TD density of AlN and GaN films grown on <101¯0> misoriented substrates was higher in comparison to that grown on <112¯0> miscut substrates. The hexagonal hillock density of the N-polar GaN films was reduced by increasing the AlN V/III ratio up to 16310 and decreasing the AlN thickness from 90 to 30nm, indicating that nucleation of inversion domain occurs during AlN buffer growth. Hexagonal hillocks were completely suppressed in N-polar GaN films grown on both substrates when the growth temperature of the 30nm thick AlN was increased from 1100°C to 1150°C at a V/III ratio of 16310.

Is the Registry Between Adjacent Graphene Layers Grown on C-Face SiC Different Compared to That on Si-Face SiC

Graphene grown on C-face SiC substrates using two procedures, high and low growth temperature and different ambients, was investigated using Low Energy Electron Microscopy (LEEM), X-ray Photo Electron Electron Microscopy (XPEEM), selected area Low Energy Electron Diffraction (μ-LEED) and selected area Photo Electron Spectroscopy (μ-PES). Both types of samples showed formation of μm-sized grains of graphene. The sharp (1 × 1) μ-LEED pattern and six Dirac cones observed in constant energy photoelectron angular distribution patterns from a grain showed that adjacent layers are not rotated relative to each other, but that adjacent grains in general have different azimuthal orientations. Diffraction spots from the SiC substrate appeared in μ-LEED patterns collected at higher energies, showing that the rotation angle between graphene and SiC varied. C 1s spectra collected did not show any hint of a carbon interface layer. A hydrogen treatment applied was found to have a detrimental effect on the graphene quality for both types of samples, since the graphene domain/grain size was drastically reduced. From hydrogen treated samples, μ-LEED showed at first a clear (1 × 1) pattern, but within minutes, a pattern containing strong superstructure spots, indicating the presence of twisted graphene layers. The LEED electron beam was found to induce local desorption of hydrogen. Heating a hydrogenated C-face graphene sample did not restore the quality of the original as-grown sample.

Buffer layer induced band gap and surface low energy optical phonon scattering in epitaxial graphene on SiC(0001)

The temperature dependent electrical properties of epitaxial graphene grown on Si-face and C-face SiC substrates were investigated by Hall measurements, respectively. Quasi-free-standing epitaxial graphene by H2 intercalation was adopted as the control. Due to a ∼200 meV band gap, the electrical conductivities of graphene on Si-face SiC showed a great increase at temperatures above 350 K compared to the other two. The opened band gap was found attributed to the existent buffer layer. The fitting results of Hall mobility indicates that the buffer layer also limits the carrier transportation of graphene grown on Si-face SiC, as it introduced low energy optical phonon scattering to its epilayer.

Fabrication of Ti ohmic contact to n-type 6H-SiC without high-temperature annealing

The effect of surface morphology of 6H-SiC substrate on the ohmic contact properties of Ti/6H-SiC structure is studied. The H-terminated surface on Si-face 6H-SiC is obtained by both dipping SiC into HF acid solution for 15 s and thermal heating SiC in hydrogen atmosphere at 1100 °C for 10 min, while the H-terminated surface on C-face 6H-SiC could be obtained only by the latter method. Ti is deposited on Si-face and C-face SiC substrates with H-terminated surfaces and ohmic contact is obtained without high-temperature annealing.

Metalorganic chemical vapor deposition of N-polar GaN films on vicinal SiC substrates using indium surfactants

The effect of indium surfactants on the growth of N-polar GaN films on vicinal C-face SiC substrates by metalorganic chemical vapor deposition was investigated. Triangular hillocks formed on the surface of N-polar GaN without indium, resulting in a rough surface. When indium surfactants were introduced during GaN growth, the surface roughness was reduced from 18.1 to 3.5 nm over a 20 × 20 μm2 area. The photoluminescence characteristics of N-polar GaN film were also improved because of a reduction of carbon caused by the presence of indium, demonstrating that indium is a useful surfactant in the growth of N-polar GaN.

Electrical properties of N-polar AlGaN/GaN high electron mobility transistors grown on SiC by metalorganic chemical vapor deposition

N-polar high electron mobility transistors (HEMTs) were fabricated from GaN/AlGaN/GaN heterostructures grown on n-type vicinal C-face SiC substrates by metalorganic chemical vapor deposition. The heterostructures had a sheet charge density and mobility of 6.6×1012 cm−2 and 1370 cm2 V−1 s−1, respectively. HEMTs with a gate length of 0.7 μm had a peak transconductance of 135 mS/mm, a peak drain current of 0.65 A/mm, and a three-terminal breakdown voltage greater than 150 V. At a drain bias of 20 V, the current-gain and power-gain cutoff frequencies with the pad capacitances de-embedded were 17 and 33 GHz, respectively.

Growth and characterization of N-polar GaN films on SiC by metal organic chemical vapor deposition

Smooth, high-quality N-polar GaN films were grown on C-face SiC substrates by metal organic chemical vapor deposition (MOCVD). Growth on substrates misoriented at 4° toward the m-plane suppressed the formation of hexagonal hillocks commonly observed for the growth of N-polar GaN by MOCVD. Aside from the misorientation, the growth temperature of the initial GaN was observed to have a strong impact on the structural and morphological properties of films grown on vicinal substrates as characterized by x-ray diffraction, atomic force microscopy, photoluminescence, and transmission electron microscopy. This strong temperature dependence was discovered to be a consequence of the island growth mode of the initial GaN. A two-step process was developed, which resulted in GaN films that exhibited XRD rocking curves with a full width at half maximum of 203 arc sec for a (0002¯) scan and 497 arc sec for a (202¯1¯) scan.

N-polar GaN∕AlGaN∕GaN high electron mobility transistors

We describe the development of N-polar GaN-based high electron mobility transistors grown by N2 plasma-assisted molecular beam epitaxy on C-face SiC substrates. High mobility AlGaN∕GaN modulation-doped two-dimensional electron gas channels were grown, and transistors with excellent dc and small-signal performance were fabricated on these wafers. Large-signal dispersion was observed, and the trap states responsible for this were identified, and layer designs to remove the dispersive effects of these traps were demonstrated. Finally, an AlGaN-cap layer was used to reduce gate leakage in these devices, and a low-dispersion high breakdown voltage device was achieved. This detailed study of dispersion and leakage in N-polar GaN-based transistors establishes a technological base for further development of field effect devices based on N-polar III-nitrides.

The effect of substrate polarity on the growth of InN by RF-MBE

This paper describes studies of the polarity of InN layers grown by radio-frequency plasma-assisted molecular-beam epitaxy. The InN layers in this study were grown on a variety of substrates: (0 0 0 1) sapphire substrates; (0 0 0 1) Ga-face and (0 0 0 1 ̄ ) N-face free-standing GaN substrates, and (0 0 0 1) Si-face and (0 0 0 1 ̄ ) C-face 6H-SiC substrates. It was found that the InN layers grown on the sapphire substrates had N-polarity. Furthermore, the growth temperature for successful growth of InN was strongly dependent on the substrate polarity: InN could be grown at 550°C on N-face GaN and C-face SiC substrates, while InN growth was only realized at a decreased temperature of 450°C on Ga-face GaN and Si-face SiC substrates. The polarity of the InN layers grown on the SiC substrates was further examined by etching the InN surface using KOH solution. The InN layers grown on Si-face and C-face SiC substrates had In-polarity and N-polarity, respectively. As a result, InN with N-polarity can be grown at higher temperature than InN with In-polarity, this can be explained by the different bonding configuration of the surface N atoms with the underlying In atoms for the two growth processes with In-polarity and N-polarity.