Search

4H-SiC trenches with depths of approximately 25 μm were filled under a special condition called the quasi-selective epitaxial growth mode that showed a refill-rate of 2.1 μm/h within 12 h, using a hot-wall chemical vapor deposition (CVD) method with the assistance of HCl gas. To develop a high-rate growth mode for the 4H-SiC trench filling process, several CVD growth parameters, such as the flow rates of source gases, HCl and H2 carrier gases, their pressures, and the applied C/Si ratio, were regulated and optimized, to enhance the partial pressures of the source species in the gas phase of the CVD reactor. A refill-rate of 11.4 μm/h in 2 h was achieved, which is nearly five times faster than the initial growth.

Heavily Al-doped 4H–SiC thick epilayers (∼90 μm) were grown on 3-in n + 4H–SiC wafers by using the hot-wall CVD method. On the purpose of enhancing incorporated Al-dopant concentration, the growth condition dependence of the Al incorporation behavior in the heavy doping range near Al solubility limit in 4H–SiC was investigated by varying the growth parameters, i.e ., growth rate, pressure, temperature and Al-dopant source flow rate. A series of thick epilayers possessing Al-dopant concentration from 9.6×10 19 to 4.7×10 20 cm −3 were obtained. Among them, the epilayer with Al-dopant concentration of 3.5×10 20 cm −3 demonstrates a comparably low resistivity of 16.5 mΩ cm as that of commercial n + 4H–SiC wafer. The incorporated Al-dopant concentration dependences on surface morphology, crystalline quality and crystal structures of the heavily Al-doped thick epilayers on n + 4H–SiC substrates were characterized and discussed.

InN/In 0.83 Ga 0.17 N multiple quantum well (MQW) structures have been fabricated on 3C–SiC (0 0 1) substrates by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). The surface morphology, structural properties, and optical properties of the samples were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL), respectively. XRD satellite peaks due to the periodic structure were clearly observed in the MQW structures grown at temperatures lower than 530 °C. PL emissions from the MQW were observed around 0.85 eV.

a -Plane InN films were grown on r -plane sapphire substrates with GaN underlayers by RF-N 2 plasma molecular beam epitaxy (MBE). X-ray diffraction (XRD) measurements and reflection high-energy electron diffraction (RHEED) observation revealed that the InN films were grown with InN (1 1 2¯ 0)∥GaN (1 1 2¯ 0)∥sapphire (2 2¯ 0 4). We have carried out micro-Raman scattering measurements for a -plane InN film. Raman peaks were observed at 448, 490 and 598 cm −1 , which can be identified as the A 1 transversal optical (TO), E 2 (high) and E 1 longitudinal optical (LO) mode phonon, respectively. We also carried out photoluminescence (PL) measurements for a -plane InN film. Strong PL was observed between 0.62 and 0.65 eV, which is the lowest ever reported for InN. It is suggested from the excitation intensity and temperature dependence of the PL spectra that the red shift of the PL peak is due to the Franz–Keldysh effect.

We have used in situ micro-Raman spectroscopy to clarify the mechanism of the luminescence efficiency improvement of GaAsN alloys by laser irradiation. Raman peak intensity of GaAs-like longitudinal optical (LO) mode phonon was observed to increase with the laser irradiation time. With increasing laser power density, the Raman intensity of GaAs-like LO mode phonon was found to increase more rapidly and to be saturated in a shorter time. Changes in the Raman intensity can be expressed by the same equation used for the luminescence efficiency improvement. Time constants of changes in the Raman intensity were almost in agreement with those of the luminescence efficiency improvement. This indicates that the increase in Raman intensity of GaAs-like LO phonon is closely related to the luminescence efficiency improvement.

We have studied the epitaxial growth of AlN layers by plasma-assisted molecular beam epitaxy (MBE) on 6H–SiC substrate. Reflection high-energy electron diffraction (RHEED) was used to monitor the growth by the observation of the 2D–3D growth transition, respectively, in Al- and N-rich conditions. Special attention was given to the elimination of the Al droplets which often form in Al-rich conditions. Different growth procedures are proposed to avoid the appearance of these droplets while keeping a 2D growth. Each of the procedure gives AlN epilayers with identical crystalline quality and low surface roughness as measured, respectively, by X-ray diffraction and atomic force microscopy.

AlN layers were grown on 3.5° off 6H-SiC substrate by plasma-assisted molecular beam epitaxy (MBE). A study of the growth mechanism has been carried out by varying the growth time and the III/V ratio, at a constant growth temperature of 800°C. The layers were characterized by means of Reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM) and X-ray diffraction (XRD). It is shown that nitrogen-rich conditions always give rise to a three-dimensional growth with spotty RHEED patterns and rough surface. In Al-rich conditions, the growth is much more two dimensional like with streaky RHEED patterns. By AFM, step bunching growth is observed. The size of the steps depends on the thickness of the layer as well as on the III/V ratio. All the samples grown in Al-rich conditions showed better crystalline quality than those grown in N-rich conditions.

We have succeeded in growing cubic AlGaN epilayers in the whole Al compositional region for the first time, on 3C-SiC (0 0 1) substrates by radio frequency N 2 plasma molecular beam epitaxy. The use of AIN buffer layers was found effective for the growth on 3C-SiC substrates. The lattice constant of obtained cubic AlGaN epilayers was confirmed to obey the Vegard's law in the whole compositional region, and several Raman modes were observed. The highest emission energies in the cathodoluminescence spectra of the cubic AlGaN epilayers increased linearly as Al composition increased, and no significant decrease in the emission intensity was observed. These results suggest the direct transition band structure of the cubic AlGaN, contrary to the theoretical prediction.

The results of spectral as well as temperature dependent photoconductivity measurements performed on molecular beam epitaxy (MBE) grown gallium nitride (GaN) epilayers are reported in this work. It is observed that persistent photoconductivity (PPC) is seen in only those materials which contain the yellow luminescence (YL) band. It is further revealed that there exists a clear threshold for the observation of PPC in our GaN epilayers both at 77 and 297 K. The relevance of this threshold with the YL band in GaN is discussed within the scope of our experimental observations.

We have measured micro-photoluminescence (PL) spectra of nitrogen delta (δ)-doped GaAs with various concentrations. In nitrogen δ-doped GaAs with low concentrations, the micro-PL spectra measured at different positions were individually different. In contrast, almost the same spectra were obtained for nitrogen 5-doped GaAs with higher concentrations and uniformly nitrogen-doped GaAs. The micro-PL study shows that there are no more than a few isoelectronic traps within a diameter of ∼1 μm. This indicates a good prospect for applications of isoelectronic traps to single photon emitting devices.

Cubic GaN, AlGaN and AlN epilayers were grown on 3C-SiC(0 0 1) substrates by gas source molecular beam epitaxy using radio-frequency N 2 plasma containing atomic nitrogen species. Due to the enhancement of growth rate by this plasma source, cubic GaN epilayers with the thickness of several micrometers were obtained, and the quality of epilayers was so much improved that they showed an X-ray diffraction peak width as small as 9 min. Cubic Al x Ga 1− x N and cubic AlN epilayers were also grown, and the variations of X-ray diffraction peak position and emission energy were observed according to the Al content.

The growth mode of hexagonal GaN epilayers on sapphire(0 0 0 1) surfaces during N 2 plasma-assisted molecular beam epitaxy was investigated using in situ reflection high-energy electron diffraction technique. It was found that the Ga-rich condition results in a flat surface showing streak patterns rather than the N-rich condition, and the intensities of the diffraction spots show behaviors that are different for Ga-rich and N-rich conditions. For the specular spot, oscillation-like intensity variations were observed according to the supply of Ga flux, which depends on Ga flux intensity and substrate temperature. At the lower temperature region, the growth of cubic GaN was observed with its [1 1 1] crystal axis along the normal direction of the sapphire(0 0 0 1) surface.

We have studied several growth initiation processes for heteroepitaxial growth of hexagonal GaN on (0 0 0 1) sapphire substrates by molecular beam epitaxy using radio-frequency N2 plasma. Based on the results of reflection high-energy electron diffraction, high-resolution X-ray diffraction and high-resolution transmission electron microscopy analyses, it was found that the nitridation process results in better structural quality of epilayers compared with the low-temperature buffer layer processes. By using nitrogen flux modulation during the initial period of main growth of GaN, very flat and smooth surfaces could be obtained. The best ever value of X-ray diffraction full-width at the half-maximum value of 51 arcseconds for MBE grown hexagonal GaN epilayers on sapphire was achieved.

Raman scattering characterizations for two types of group III-nitride epitaxial layers are presented. First, GaN layers where cubic and hexagonal phases are mixed have been studied by Raman microprobe. It is shown that the dominant phase can be estimated by the observation of the transverse-optic (TO) phonon band spectra. Planar distributions of the phase-mixing ratio are examined by a two-dimensional analysis. Second, cubic Al x Ga 1− x N mixed crystals have been studied. With the increase of AlN molar fraction x , the TO phonon shows up-shift in the frequency. However, the shifts are much smaller than those expected from the linear interpolation, which may suggest a two-mode behavior for the TO mode of mixed crystals.

We have grown high-quality cubic GaN epilayers on GaAs and 3C-SiC substrates by molecular beam epitaxy technique using dimethylhydrazine or ammonia/nitrogen plasma as a nitrogen source. An X-ray diffraction peak width of 16 min and a low-temperature photoluminescence peak width of 19 meV were achieved. Various surface reconstruction transitions have been observed for cubic GaN(0 0 1) surfaces, recently. These results, along with previously published studies on cubic nitrides, are summarized, and the current status of the growth and characterization of cubic nitrides including AlN and InN is discussed.

Cathodoluminescence (CL) topographs were taken at room temperature at 364 nm on undoped GaN films grown by plasma-enhanced molecular beam epitaxy at various V/III ratios. The CL topographs were compared with scanning electron microscope (SEM) images. The CL topograph was found to be significantly influenced by the V/III ratio during growth. For GaN films grown at low V/III ratio, spatially nonuniform luminescence intensity was observed. This nonuniformity in luminescence may be due to the local variation of stoichiometry of GaN induced by Ga microsegregation. At nitrogen-rich condition, almost flat surfaces in SEM image were observed and the uniform intensity of luminescence was obtained by CL topograph. Thus, the nitrogen-rich condition is favourable to get structually and optically uniform films.

Control of the reversible transformation between the (2 × 2) surface reconstruction and the unreconstructed (1 × 1) surface of hexagonal-phase GaN during growth by molecular beam epitaxy is examined with respect to substrate temperature, gallium flux, nitrogen flow rate, nitrogen plasma cell power and system pressure. The transition is considered to occur at a particular surface stoichiometry which depends on the net arrival rate of reactant species at the growth surface. The (2 × 2) surface reconstruction as observed by RHEED is stable under a nitrogen environment, whereas the disappearance of the half-order reconstruction occurs during Ga-rich growth yielding a (1 × 1) RHEED pattern and eventual accumulation of metallic Ga on the growth surface. Based on these observations, a model describing the flux balance at the transition stoichiometry is developed. It is shown to be useful for estimating the concentration of active nitrogen species available, determining the interdependence of growth parameters, and optimizing growth conditions.

Interface structures of cubic-GaN on GaAs and 3C-SiC substrates grown by gas-source molecular beam epitaxy using NH 3 plasma were examined by high resolution transmission electron microscopy. There is no intermediated layer at the interface for both the structures. It was found that there are pyramid-like structures formed by (111) facets at cubic-GaN/GaAs interfaces and its interface is fairly rough. On the contrary, interfaces of cubic-GaN/3C-SiC are quite flat without specific structures, which results in good quality GaN epilayers.

Cubic and hexagonal GaN epilayers were successfully grown on GaAs substrates by gas source molecular beam epitaxy using a microwave plasma nitrogen source. These GaN epilayers showed quite strong cathodoluminescence emission. X-ray and electron diffraction analyses indicated the improved crystalline quality, compared with GaN epilayers grown using dimetylhydrazine. GaN epilayers were also grown, by the same method, on 3C-SiC substrates, which have a lattice constant much closer to GaN than GaAs.

GaAs and GaN epilayers were grown on GaAs substrates by gas source molecular beam epitaxy technique using triethylarsine (TEAs) and diethylarsine (DEAsH) as As sources, and dimethylhydrazine (DMHy) as an N source. It was found that GaAs grows layer by layer even when organic arsine molecular sources are used. Cubic GaN was found to grow epitaxially on sufficiently nitrided surfaces of GaAs (001) substrates, in contrast with the growth of hexagonal GaN on GaAs (111) surfaces. It was also found that nitridation of GaAs surfaces does not occur when DEAsH and DMHy beams are supplied onto the GaAs substrates, simultaneously. Thus, GaN/GaAs multilayers were obtained only by intermittent supply of a DEAsH beam.

Gen/Sim strained-layer superlattices (SLSs) grown on Si(001) substrate by molecular beam epitaxy using phase-locked epitaxy method were investigated by X-ray diffraction, transmission electron micrograph (TEM), and photoluminescence. In the TEM image, neither threading dislocations nor Ge clusters were observed over the whole superlattice area. In the substrate, however, misfit dislocations due to lattice mismatch between superlattice and substrate were observed. The photoluminescence spectra of Gen/Si3n (n = 4, 5, 6) SLSs exhibited an intense emission peak, while Gen/Si4n (n = 2, 4, 6) SLSs showed no intense emission. This result indicates that the optical properties of the Gen/Sim SLS strongly depend not only on the superlattice periodicity but also on the thickness ratio of Ge to Si.

We report the fabrication of Ge m /Si n strained-layer superlattices (SLSs) using the phase-locked epitaxy method. During the growth of Ge m /Si n SLSs, reflection high-energy electron diffraction intensity oscillation was observed clearly up to the total thickness of 240 monatomic-layers. The samples were characterized using X-ray diffraction and Raman scattering. For a Ge 4 /Si 16 SLS, diffraction peaks originated from the superlattice structure and a Raman peak assigned to the zone-folding mode of the acoustic phonon branch were observed.