Gallium nitride (GaN) is an extremely promising wide band gap semiconductor material for optoelectronics and high temperature, high power electronics. Actually, GaN is probably the most important semiconductor since silicon. However, achievement of its full potential has still been limited by a dramatic lack of suitable GaN bulk single crystals. GaN has a high melting temperature and a very high decomposition pressure; therefore it cannot be grown using conventional methods used for GaAs or Si like Czochraslski or Bridgman growths.Since there is no GaN bulk single crystal commercially available, all technological development of GaN-based devices relies on heteroepitaxy. Most of the current device structures are grown on sapphire or 6H-SiC. However, since their lattice parameters and thermal expansion coefficients are not well-matched to GaN, the epitaxial growth generates huge densities of defects, with threading dislocations (TDs) being the most prevalent (109–1011 cm−2). As a comparison, homoepitaxially grown GaAs exhibits ~102–104 dislocation cm−2, and homoepitaxial Si almost 0. Actually this large density of TDs in GaN drastically limits the performance and operating lifetime of nitride-based devices. Therefore, there is currently a tremendous technological effort to reduce these defects.Metal organic vapour phase epitaxy (MOVPE) is currently the most widely used technology. Actually, all optoelectronic commercial device structures are fabricated using MOVPE. In MOVPE, the most appropriate precursor for nitrogen is ammonia (NH3), whereas either trimethyl or triethylgallium may be used as a gallium source. MOVPE of GaN requires a high partial pressure of NH3, high growth temperatures (~1000–1100°C) and a growth chamber specially designed to avoid premature reactions between the ammonia and gallium alkyls. Since sapphire (or 6H-SiC) and GaN are highly mismatched, direct growth of GaN is impossible. Therefore, the growth of GaN on any substrate first requires the deposition of a buffer layer, which, to some extent, accommodates the mismatch. Using appropriate nucleation layers allows a reduction of the dislocation density to the low 108 cm−2 range.Though laser diodes (LDs) were demonstrated in the late 1990s with such defect layers, the real breakthrough in laser technology was the dramatic improvement of the LD lifetime at the end of 1997, with the lifetime reaching 10 000 h. This was made possible by implementation of epitaxial lateral overgrowth (ELO) technology, which significantly reduces the dislocation density to below 107 cm−2.In ELO technology, parts of the highly dislocated starting GaN are masked with a dielectric mask, after which growth is restarted. At the beginning of the second growth step, deposition only occurs within the openings, with no deposition observed on the mask. This is referred to as selective area epitaxy (SAE). The TDs are prevented from propagating into the overlayer by the dielectric mask, whereas GaN grown above the opening (coherent growth) keeps the same TD density as the template, at least during the early stages of growth.Currently, two main ELO technologies exist: the simpler one involves a single growth step on striped openings. In this one-step-ELO (1S-ELO), growth in the opening remains in registry with the GaN template underneath (coherent part), whereas the GaN over the mask extends laterally (wings). This leads to two grades, namely highly dislocated GaN, above the openings, and low dislocation density GaN, above the masks. With this technique, devices have to be fabricated on the wings. Conversely, in the two-step-ELO (2S-ELO) process, the growth conditions of the first step are monitored to obtain triangular stripes. Inside these stripes, the TDs arising from the templates are bent by 90° when they encounter the inclined lateral facet. In the second step, the growth conditions are modified to achieve full coalescence. In this 2S-ELO technology, only the coalescence boundaries are defective. ELO technology produces high quality GaN, with TD densities in the mid 106 cm−2, line widths of the low temperature photoluminescence near band gap recombination peaks below 1 meV, and deep electron trap concentration below 1014 cm−3 (compared with mid 1015 cm−3 in standard GaN). Numerous modifications of the ELO process have been proposed either to avoid technological steps (maskless ELO) or to improve it (pendeoepitaxy, PE). To further reduce the TD density, multiple-step-ELO and pendeo have also been implemented.However, even ELO quality GaN is not good enough for the next generation of LDs. ELO samples do not yet offer a full surface suitable for laser technology. What is needed for LDs with at least 30 mW output power is high quality freestanding GaN with TDs close to or even below 106 cm−2. To reach this crystalline perfection, elaborate technologies are currently being implemented. They, at some stage, involve TD reduction mechanisms occurring in the ELO process.Self-supported GaN with at least ELO quality at an affordable cost is believed to be the next breakthrough in GaN technology.

https://iopscience.iop.org/article/10.1088/0034-4885/67/5/R02/pdf

Metal organic vapour phase epitaxy of GaN and lateral overgrowth

Statement of problem After the development of a variety of ceramic restorative systems over the past 20 years, the fabrication of fixed dental prostheses has undergone considerable change. Esthetics and resistance to fracture are two of the main determinants of the success of a restoration; the third is marginal adaptation. Therefore, a systematic review of the literature dedicated to the marginal accuracy of ceramic systems is indicated. Purpose This study reviewed the current scientific literature that pertains to the marginal fit of ceramic crowns fabricated with different systems and investigated the factors that influence marginal adaptation. Material and methods An electronic search was completed by using the PubMed and Scopus databases with the following combination of key words: (discrepancy or fit or gaps or adaptation) and (disilicate or ceramic) and (copings or crowns). The search was limited to English-language peer-reviewed articles published before April 15, 2012. Titles and abstracts were read to identify articles that fulfilled the inclusion criteria designed for this review. Results Of 469 studies identified, 54 satisfied the selection criteria and were included in this review. All were published between 1994 and 2012. A total of 17 ceramic systems were tested in 48 in vitro and 6 in vivo studies. Of all the marginal gaps measured, 94.9% were in the range of clinical acceptability. Study heterogeneity made it impossible to conduct a proper meta-analysis of research findings and to compare and rank the various systems in terms of marginal fit. Four parameters were found to influence marginal adaptation: finish line configuration, value of the predefined cementing space, veneering process, and cementation. Conclusions The systems evaluated in the selected articles generally provide a clinically acceptable marginal fit. The current state of research does not allow for a proper comparison of the various systems in terms of marginal fit. The use of computer x-ray microtomography is recommended for further research on marginal adaptation.

Marginal adaptation of ceramic crowns: A systematic review

Objectives
Digital impression techniques are advertised as an alternative to conventional impressioning. The purpose of this in vitro study was to compare the accuracy of full ceramic crowns obtained from intraoral scans with Lava C.O.S. (3M ESPE), CEREC (Sirona), and iTero (Straumann) with conventional impression techniques.

Accuracy of digital and conventional impression techniques and workflow

After biomaterials are implanted into the human body, there are inevitable interactions between the biological environment and implant surfaces. Therefore, the surface of biomaterials has become one of the hottest research topics. Nanotechnology is a powerful tool in modern materials science and able to incorporate biomimicry on the nanoscale into materials engineering. Therefore, research on nanotechnology/nanostructured biomaterials has attracted much attention. A nano-functionalized surface has promising biological properties and clinical applications of biomaterials can be improved by producing a nanostructured surface. Many surface modification techniques have been adopted to produce nano-functionalized biomaterials surface, and in this paper, the fabrication, characterization, and properties of biomaterials such as ceramics, metals, and polymers with nanostructured surfaces are reviewed.

Surface nano-functionalization of biomaterials

Statement of problem Little evidence is available with regard to the marginal fit of crowns fabricated with digital impressions and computer-aided design/computer-aided manufacturing technology in comparison with crowns fabricated from conventional techniques. Purpose The purpose of this study was to determine and compare the marginal fit of crowns fabricated with digital and conventional methods. Material and methods The maxillary right second premolar was prepared for a ceramic crown in a typodont. The typodont was then digitized with a laboratory scanner, and the digital file was used to mill a replica of the maxillary arch from a monolithic block of yttria-stabilized zirconia to serve as the master model. Digital impressions of the prepared maxillary right second premolar were recorded with a scanning unit. Scan files were exported as .STL files and sent by e-mail to a dental laboratory. The files were input into a digital design workflow for digital articulation, digital waxing, and design of the definitive crown. Fifteen crowns were produced by milling computer-aided designed lithium disilicate glass ceramic blocks with a 5-axis milling. Fifteen lithium disilicate glass ceramic crowns were produced with a conventional impression and a laboratory fabrication method. The original zirconia die was removed from the zirconia master model to evaluate the crown margins. Circumferential marginal gap measurements were made at 8 measurement locations: mescal, distal, buccal, palatal and associated line angles (mesiobuccal, mesiolingual, distobuccal, and distolingual). Measurements were made to determine the vertical component of the marginal gap according to the definition of marginal fit. Results A total of 240 images (2 groups, 15 crowns per group, 8 sites per crown) were recorded and measured. The overall mean ±SD vertical gap measurement for the digitally made crowns was 48 ±25 μm, which was significantly smaller than that for the conventionally made crowns (74 ±47 μm). Conclusion The fully digital fabrication method provided better margin fit than the conventional method.

A comparison of the marginal fit of crowns fabricated with digital and conventional methods.

This study evaluated the accuracy of marginal and internal fit between the all-ceramic crowns manufactured by a conventional double-layer computer-aided design/computer-aided manufacturing (CAD/CAM) system and a single-layer system. Ten standardized crowns were fabricated from each of these two systems: conventional double-layer CAD/CAM system (Procera) and a single-layer system (Cerec 3D). The copings and completed crowns were seated on the abutments by a special device that facilitated uniform loading, and the marginal discrepancies were measured. Internal gaps were also measured using a low-viscosity silicone material. Marginal discrepancies of Procera copings were significantly smaller than those of Procera crowns and Cerec 3D crowns (p 0.05). On internal gaps, Cerec 3D crowns showed significantly larger internal gaps than Procera copings and crowns (p < 0.05). Within the limitations of this study, the single-layer system demonstrated acceptable marginal and internal fit.

Marginal and internal fit of all-ceramic crowns fabricated with two different CAD/CAM systems.

Titanium is widely used as an implant material for artificial teeth. Furthermore, various studies have examined surface treatment with respect to the formation of a fine passive film on the surface of commercial titanium and its alloys and to improve the bioactivity with bone. However, there is insufficient data about the biocompatibility of implant materials in the body. The purpose of this study was to examine whether surface modification affects the precipitation of apatite on titanium metal. Specimens were chemically washed for 2 min in a 1∶1∶1.5 (vol.%) mixture of 48 %HF, 60%HNO3 and distilled water. The specimens were then chemically treated with a solution containing 97%H2SO4 and 30%H2O2 at the ratio of 1∶1 (vol.%) at 40°C for 1h, and subsequently heat-treated at 400°C for 1h. All the specimens were immersed in HBSS with pH 7.4 at 36.5°C for 15d, and the surface was examined with TF-XRD, SEM, EDX and XPS. In addition, specimens of commercial pure Ti, with and without surface treatment, were implanted in the abdominal connective tissue of mice for 28 d. Conventional aluminum and stainless steel 316L were also implanted for comparison. An amorphous titania gel layer was formed on the titanium surface after the titanium specimen was treated with a solution of H2SO4 and H2O2. The average roughness was 2.175 μm after chemical surface treatment. The amorphous titania was subsequently transformed into anatase by heat treatment at 400°C for 1h. The average thickness of the fibrous capsule surrounding the specimens implanted in the connective tissue was 47.1μm in the chemically treated Ti, and 52.2, 168.7 and 101.9μm, respectively, in the untreated commercial pure Ti, aluminum and stainless steel 316L.

Evaluation of in vitro and in vivo tests for surface-modified titanium by H2SO4 and H2O2 treatment

Structural and optical properties of high quality GaN films over the entire surfaces of sapphire substrates employing SiO2 mask-removed double Pendeo-epitaxy technique by metalorganic chemical vapor deposition (MOCVD) were investigated by scanning electron microscopy (SEM), tranmission electron microscopy (TEM), high-resolution X-ray diffraction (HRXRD) and cathodoluminescence (CL). By optimizing the ratio of the lateral to vertical growth rate for the lateral overgrowth of the first and the second window regions of double Pendeo-epitaxial GaN layers, the crystallographic tilt in wing regions was reduced and the propagation of threading dislocations up to the surface of the GaN layers was supressed in their entire region. The luminescence properties of the GaN lyers observed by CL measurement were also improved in their entire region compared to conventional Pendeo-epitaxial growth.

Structural and Optical Properties of Lateral Overgrown GaN Grown by Double Pendeo-Epitaxy Technique

The influence of antimony (Sb) as a surfactant on the structure and optical properties of InGaAsN/GaAs multi-quantum wells (MQWs) grown by metalorganic chemical vapor deposition (MOCVD) is studied. It was found that the photoluminescence (PL) peak intensity of as-grown MQWs increases strongly with introduction time of excess Sb flux during the growth interruption process. It seems to be due to the improvement of structure properties at the interface by a surfactant suppressing the surface diffusion phenomenon. After the annealing process, the PL intensity shows a strong dependence on Sb introduction time and annealing temperature. The PL peak wavelength is blue-shifted with increasing annealing temperature for all samples. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of Sb surfactant on the structural and optical properties of InGaAsN/GaAs multi‐quantum wells grown by metalorganic chemical vapor deposition

Hillocks formed on pendeo-epitaxial GaN (PE-GaN) films grown by metalorganic chemical vapor deposition were observed by scanning electron microscopy, atomic force microscopy and high-resolution transmission electron microscopy. In order to suppress hillock formation on PE-GaN films, a two-step growth technique was employed. In the first-step growth, whose growth temperature and V/III ratio were relatively low, it was necessary to suppress hillock formation on the surface, and reduce the crystallographic tilt in the wing regions of PE-GaN films. In the second-step growth, whose growth temperature and V/III ratio were relatively high, complete coalescence without tilt in the wing regions was achieved by a high lateral growth rate.

Charn-Woon Park

Papers

Marginal and internal fit of all-ceramic crowns fabricated with two different CAD/CAM systems.

Evaluation of in vitro and in vivo tests for surface-modified titanium by H2SO4 and H2O2 treatment

Structural and Optical Properties of Lateral Overgrown GaN Grown by Double Pendeo-Epitaxy Technique

Influence of Sb surfactant on the structural and optical properties of InGaAsN/GaAs multi‐quantum wells grown by metalorganic chemical vapor deposition

Direct observation of hillocks on pendeo‐epitaxial GaN films and stabilization of GaN seed layers for hillock‐free surface