GaN and related III-nitrides have attracted considerable attention as promising materials for application in optoelectronic devices, in particular, light-emitting diodes (LEDs). At present, sapphire is still the most popular commercial substrate for epitaxial growth of GaN-based LEDs. However, due to its relatively large lattice mismatch with GaN and low thermal conductivity, sapphire is not the most ideal substrate for GaN-based LEDs. Therefore, in order to obtain high-performance and high-power LEDs with relatively low cost, unconventional substrates, which are of low lattice mismatch with GaN, high thermal conductivity and low cost, have been tried as substitutes for sapphire. As a matter of fact, it is not easy to obtain high-quality III-nitride films on those substrates for various reasons. However, by developing a variety of techniques, distincts progress has been made during the past decade, with high-performance LEDs being successfully achieved on these unconventional substrates. This review focuses on state-of-the-art high-performance GaN-based LED materials and devices on unconventional substrates. The issues involved in the growth of GaN-based LED structures on each type of unconventional substrate are outlined, and the fundamental physics behind these issues is detailed. The corresponding solutions for III-nitride growth, defect control, and chip processing for each type of unconventional substrate are discussed in depth, together with a brief introduction to some newly developed techniques in order to realize LED structures on unconventional substrates. This is very useful for understanding the progress in this field of physics. In this review, we also speculate on the prospects for LEDs on unconventional substrates.

https://iopscience.iop.org/article/10.1088/0034-4885/79/5/056501/pdf

GaN-based light-emitting diodes on various substrates: a critical review.

Fabrication of high-temperature superconducting thin films (Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O, Tl-Ba-Ca-Cu-O systems) by chemical vapour deposition (CVD) is reviewed. The key to the use of the CVD method is to find volatile compounds for the metallic constituents of superconductors, beta -diketonate complexes such as acetylacetonates and dipivaloylmethanates have been successfully used as source materials. The quality of the films grown is affected by the deposition temperature, substrate material and post-deposition annealing procedure. Considerable progress has recently been made and the critical properties of CVD-grown films are now equal to those of physical vapour-deposited films.

https://iopscience.iop.org/article/10.1088/0953-2048/6/9/001/pdf

Chemical vapour deposition of high-Tc superconducting thin films

Spiral crystal growth

High critical current density is essential for most large-scale applications of high-temperature superconductors (HTSs). In addition to the weak link problem, weak flux pinning is a major cause of the rapid decline of with magnetic field at high temperatures. Through intensive research in the past 10 years the weak pinning problem has been partially overcome and has reached a level approaching the requirement for some commercial applications. In this article, a number of techniques by which effective pinning centres can be introduced to improve flux pinning in Bi - (Pb) - Sr - Ca - Cu - O HTSs are reviewed. These include surface pinning through a spiral growth, fine precipitates produced through phase transformation or phase formation - decomposition, inclusions through chemical doping or addition, cascades and columnar defects created by fast-neutron and heavy ion beam irradiation and various defects induced by mechanical deformation. In particular, the results of Bi-2223 formation - decomposition and cold deformation versus hot deformation have a significant implication for HTS wire fabrication. Comparative studies with comprehensive electrical, magnetic and microstructural characterizations have been carried out to assess the effectiveness of these techniques.

https://iopscience.iop.org/article/10.1088/0953-2048/10/7A/006/pdf

Introduction of pinning centres into Bi - (Pb) - Sr - Ca - Cu - O superconductors

High-temperature superconductors (HTS) and scanning probe microscopes have been developed at a rapid rate over the last decade and a half. The cross-linking between these two exciting fields has produced many beautiful results and new possibilities that are still being explored. The highlights of these results are reviewed here, with emphasis on scanning microscopes based on tunnelling (STM), atomic force (AFM), magnetic force (MFM), the Hall effect (SHPM), a SQUID sensor, microwaves, near-field optics, or magneto-optic techniques. These tools are used in the characterization of HTS from the routine evaluation of surface topography to the sophisticated elucidation of the symmetry of the order parameter. Some more recent techniques and possible new directions are also discussed.

https://iopscience.iop.org/article/10.1088/0953-2048/12/4/001/pdf

Scanning probe microscopy of high-temperature superconductors

A technologically useful chemical vapor deposition process with high growth rate (≳4 μm/h) was developed for the epitaxial growth of YBa2Cu3O7−x (YBCO) thin films. Even at the high growth rate used in this process, a spiral growth mechanism was observed and the films grown had electrical (Tc=92 K, Jc of 2×106A/cm2 at 77 K) and structural properties equal to films produced by physical vapor deposition.

Spiral growth in epitaxial YBa2Cu3O7−x thin films produced by high deposition rate chemical vapor deposition

Y1Ba2Cu3O7−x superconducting thin films were grown in situ on polycrystalline MgO substrates by a chemical vapor deposition process that closely couples the sublimation temperatures of the elemental sources to that of the substrate during deposition. It was found that the best quality films were achieved with a controlled ramping of the substrate temperature from 850  °C, at the onset of deposition, down to 750 °C at the end of deposition. The films were analyzed by Rutherford backscattering (RBS), scanning electron microscopy (SEM), x‐ray diffractometry (XRD), and four‐point resistivity probe. The results of these studies showed that the films were highly c‐axis oriented, had near‐stoichiometric composition, and exhibited Tc,onset=90 K, Tco=85 K, and had a Jc=2×105 A/cm2 at 77 K in zero magnetic field. A model is proposed for the effect of such temperature control on the CVD growth mechanism of high‐quality YBCO films.

High‐quality Y1Ba2Cu3O7−x thin films on polycrystalline MgO by temperature‐controlled chemical vapor deposition

Results are reported from an investigation of the effects of selected processing parameters on the morphology and properties of YBa2Cu3O7−δ (YBCO) superconducting thin films grown directly on polycrystalline silver substrates by chemical‐vapor deposition (CVD). These results were achieved through a set of experimental studies which examined: (i) recrystallization mechanisms of polycrystalline silver and their effect on the deposition of YBCO thin films; and (ii) CVD processing conditions leading to the growth of high‐quality YBCO films. The samples were analyzed using dynamic impedance, four‐point resistivity probe, x‐ray diffraction, Rutherford backscattering, and scanning electron microscopy. These studies showed that substrate temperature played a critical role not only in the formation of YBCO films, but also in the recrystallization of silver substrates, which in turn greatly influenced film growth. The studies also led to the identification of a two‐stage processing scheme for the growth of YBCO films on silver. The first processing stage consisted of a substrate conditioning cycle which involved a 10 min ramping from room temperature to deposition temperature where the substrates were held for an additional 10 min in a flow of 70 sccm O2 at a reactor working pressure of 2 Torr. The second processing stage involved actual film deposition at 760–800 °C for 3–10 min (depending on desired film thickness) in a mixed flow of 70 sccm O2 and 210 sccm N2O at a reactor working pressure of 4 Torr. Samples thus produced were highly oriented along the c axis perpendicular to the substrate with a zero resistance transition temperature of 87 K and a critical current density of 2×104 A/cm2 (77 K, B=0). The films had a thickness of 200–700 nm depending on the length of the growth cycle, which corresponded to the growth rates in the range 65–130 nm/min. A growth mechanism for YBCO on polycrystalline silver, which emphasized the role of silver recrystallization, was consequently proposed and discussed.

On the effect of processing parameters in the chemical‐vapor deposition of YBa2Cu3O7−δ thin films on polycrystalline silver

Work by the present investigators has focused on the growth of HTS films on technologically useful substrates for energy‐related and cryoelectronic applications. In this paper, results from two such studies are presented: (1) Temperature‐controlled chemical vapor deposition (TC‐CVD) processing was used to grow YBa2Cu3O7−δ (YBCO) thin films directly on sapphire substrates. Films were grown by employing a three‐step substrate temperature ramping scheme: first, film was deposited at 730 °C for 5 minutes; then the substrates were heated up to 825 °C rapidly and held at that temperature for 5 minutes with no deposition; and finally the film was deposited for 10 more minutes using the substrate temperature ramping from 825 °C to 730 °C. The films were characterized utilizing dc transport four point measurements, and scanning electron microscopy (SEM). These measurements showed the film of highly c‐axis oriented with Tc around 90K. It is postulated the intermediate high deposition temperature helps to form a hig...

Vapor phase processing of HTS films for technological application

Temperature‐controlled chemical vapor deposition (TC‐CVD) processing was employed to grow highly c‐axis oriented YBa2Cu3O7−x (YBCO) superconducting thin films directly on silver substrates. Films were grown in situ utilizing either an isothermal or a three stage fabrication scheme. While comparable Jc values were measured for the two different films, the three‐stage TC‐CVD processed film was much denser. It is postulated that in the first step, a seed layer of YBCO is nucleated on the AG substrate. In the second step, the deposition is discontinued and the substrate temperature is quickly raised with a short hold to promote densification and oriented growth of the YBCO film. In the third step, the substrate temperature is lowered and the deposition continued with the additional film growth driven by the underlying oriented (c‐axis perpendicular) YBCO seed layer. The samples were analyzed using dynamic impedance (DI), four point resistivity probe, x‐ray diffraction (XRD) and scanning electron microscopy (S...

High Tc conductors by MOCVD on silver substrates

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