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We have used plasma molecular beam epitaxy on (0 0 0 1) and (0 0 0 [1bar]) ZnO substrates to induce epitaxial growth of GaN of a known polarity. The polarity of the ZnO substrates can be easily and unambiguously determined by measuring the sign of the piezoelectric coefficient. If we assume that N-face GaN grows on O face ZnO and that Ga-face GaN g...
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... little is known about the polarity of GaN epitaxial films. GaN has the wurtzite structure, and typically grows with its hexagonal basal plane parallel to the substrate. The orientation normal to the substrate is either 〈 0 0 0 1 〉 or 〈 0 0 0 〉 . The two orientations are not equivalent; the Ga-N bonds along the hexagonal axis are all oriented in one direction, resulting in faces that are either nitrogen terminated or gallium terminated. The two faces, along with the terminology conventions used in this article, are illustrated in Figure 1. The surface chemistry, epitaxial growth, doping, and defect structure on the two faces should be quite distinct. The GaN research community has by and large neglected the distinction between these two orientations, both because it is hard to measure, and because most growth technechniques should select one or the other orientation. One exception is the work of Sasaki and Matsuoka, [1] who studied the growth of GaN on the carbon and silicon terminated faces of SiC. Using x-ray photoemision spectroscopy to measure the oxidation of Ga atoms on the surface, they determined that Ga-face GaN was grown on C-face SiC, and that N-face GaN was grown on Si-face SiC. It is almost a surprise, given the differences between the two surfaces, that GaN of reasonable quality could even be grown with both polarities. Recently, the polarity identification problem has attracted the attention of electron microscopists [2] [3] [4], but there have been conflicting ...
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One of the epitaxial issues pertaining to the growth of AlGaN/GaN HEMTs on Si is the decrease of parasitic losses that can adversely impact the RF device performances. We characterized the microwave losses in coplanar waveguides (CPWs) on GaN-based high-electron-mobility-transistors (HEMTs) and their buffer layers on Silicon substrate, up to 40 GHz...
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... 22,23 Main challenges that hindered ZnO from emerging as the common substrate for the growth of nitrides were H 2 back etching of the substrate at high temperatures in MOCVD reactors 21 and the interfacial reaction between nitrides and ZnO. [24][25][26] More recently, high quality nitride films [27][28][29][30] have been achieved on ZnO using pulsed laser deposition (PLD) as this growth technique allows for growth temperatures as low as room temperature, and therefore, the interfacial reaction of nitrides with ZnO can be fully suppressed. On the contrary, the optimum growth temperature for InGaN via MBE is typically between 500 ○ C and 600 ○ Cho et al. observed the (In,Ga)N interfacial reaction with ZnO at growth temperatures higher than 450 ○ C by PAMBE. ...
Epitaxial growth of (In,Ga)N films on O-face ZnO substrates was studied via plasma-assisted molecular beam epitaxy. Atomically smooth GaN films, showing step edges, were grown at low temperatures to suppress the interfacial reaction between nitrides and the ZnO substrate at elevated temperatures using metal-enhanced epitaxy. High-quality growth of ∼300 nm-thick (In,Ga)N films with the In content varying from 11% to 23% was demonstrated using ∼2 monolayer-thick low temperature GaN as the buffer layer. A clear redshift in (In,Ga)N photoluminescence was observed by decreasing the substrate temperature. For the first time, we achieved an atomically smooth surface on 300 nm-thick GaN grown on ZnO, showing step edges. The surface morphology, however, eventually degraded after exposure to the ambient due to strain, which was perhaps facilitated by the formation of an oxide layer. These results are promising for optoelectronics and electronics applications since the eventual degradation of the surface morphology can be mitigated via strain engineering or surface passivation.
... Additionally, ZnO substrate may decrease the usage of expensive gallium for buffer layers. Therefore, ZnO has been proposed to be an ideal substrate [21] or buffer layers [22] to grow GaN. ...
Heterostructures of wurtzite based devices have attracted great research interests since the tremendous success of GaN in light emitting diodes (LED) industry. Among the possible heterostructure material candidates, high quality GaN thin films on inexpensive and lattice matched ZnO substrate are both commercially and technologically desirable. However, the energy of ZnO polar surfaces is much lower than that of GaN polar surfaces. Therefore, the intrinsic wetting condition forbids such heterostructures. As a result, poor crystal quality and 3D growth mode were obtained. To dramatically change the growth mode of the heterostructure, we propose to use hydrogen as a surfactant, confirmed by our first principles calculations. Stable H involved surface configurations and interfaces are investigated, with the help of newly developed algorithms. By applying the experimental Gibbs free energy of H$_2$, we also predict the temperature and chemical potential of H, which is critical in experimental realizations of our strategy. This novel approach will for the first time make the growth of high quality GaN thin films on ZnO substrates possible. We believe that our new strategy may reduce the manufactory cost and improve the crystal quality and the efficiency of GaN based devices.
... Therefore, substrates such as ZnO and SiC are experimented which have the same crystal structure as GaN (wurtzite). 9,10 The Ga-rich face of GaN [0001], tends to grow on Zn and C on the ZnO and SiC substrates, respectively. 10 Whereas, the N-rich face [0001] grows on the O and Si faces. ...
In this paper, we present a systematic approach to the gallium nitride (GaN) chemical mechanical planarization (CMP) process through evaluating the effect of crystallographic orientation, slurry chemistry and process variables on the removal rate and surface quality responses. A new CMP process and a complementary tool set-up are introduced to enhance GaN material removal rates. The key process variables are studied to set them at an optimal level, while a new slurry feed methodology is introduced in addition to a new tool set up to enable high material removal rates and acceptable surface quality through close control of the process chemistry. It is shown that the optimized settings can significantly improve the material removal rates as compared to the literature findings while simultaneously enabling a more sustainable process and potential removal selectivity against silica.
... With water being the solvent for the diamond nano-particle seeding solutions, the main region of interest for diamond growth are the values in the pH range 6 to 7. In this range N-face GaN shows higher negative zeta potential when compared to Ga-face GaN. This can be attributed to two different causes, i) the presence of higher concentration of adsorbed oxygen on the N-face GaN 31,32 . and, ii) the Ga-face GaN surface is generally smoother than the N-face GaN surface 33 , as is evident from the AFM images in panels A and E in figure 2 and leading to a minor inaccuracy in the calculation of the total surface area in contact with the streaming electrolyte. ...
Measurement of zeta potential of Ga and N-face gallium nitride has been carried out as function of pH. Both the faces show negative zeta potential in the pH range 5.5-9. The Ga face has an isoelectric point at pH 5.5. The N-face shows higher negative zeta potential due to larger concentration of adsorbed oxygen. Zeta potential data clearly showed that H-terminated diamond seed solution at pH 8 will be optimal for the self assembly of a monolayer of diamond nanoparticles on the GaN surface. Subsequent growth of thin diamond films on GaN seeded with H-terminated diamond seeds produced fully coalesced films confirming a seeding density in excess of 10$^{12}$ cm$^{-2}$. This technique removes the requirement for a low thermal conduction seeding layer like silicon nitride on GaN.
... Therefore, substrates such as ZnO and SiC are experimented which have the same crystal structure as GaN (wurtzite). 9,10 The Ga-rich face of GaN [0001], tends to grow on Zn and C on the ZnO and SiC substrates, respectively. 10 Whereas, the N-rich face [0001] grows on the O and Si faces. ...
Gallium nitride is a hard and chemically inert material demoting high material removal rates in the chemical mechanical planarization (CMP) applications. This paper focuses on the optimization of the process conditions to enhance material removal rates while controlling surface defectivity for GaN CMP. Two different crystallographic orientations of the GaN are characterized and compared to a commercial 2” GaN wafer to optimize the CMP performance on the basis of the wafer crystallographic nature, surface charge and topography. Slurry pH, the type of polishing pad and applied conditioning were evaluated to increase material removal rates of GaN while minimizing defect formation and enhancing the selectivity against silica.
... 11,12) However, a low growth temperature between room temperature (RT) and 380°C is required because a high growth temperature forms a Ga 2 ZnO 4 interfacial layer, which degrades the GaN crystalline quality. 13) Unfortunately, such low-temperature growth is unacceptable in conventional growth techniques such as metal-organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy. ...
High-quality InGaN-based visible light-emitting diodes (LEDs) are demonstrated on ScAlMgO4 (SCAM) (0001) substrates. GaN grown on SCAM by metal-organic vapor phase epitaxy is nearly strain-free with an in-plane compressive strain of −1.26 × 10−3, which is much smaller than that in conventional GaN/sapphire owing to the smaller thermal expansion mismatch between GaN and SCAM. We fabricate InGaN/GaN quantum well LEDs on GaN/SCAM templates, and observe bright blue electroluminescence at ~470 nm wavelength. The device performances of LEDs on SCAM are comparable to those of LEDs on sapphire. Our achievements indicate that highly efficient InGaN-based light emitters are possible on SCAM substrates.
... 4,13) While these defects stimulate GaN nucleation, they also disrupt the structural ordering of the graphene surfaces or the interfacial reactions, and degrade the crystalline quality of the GaN films; this situation frequently occurs in GaN/ZnO. 5,[14][15][16] Therefore, to ensure the growth of high-quality GaN films, the interface between the group III nitrides and graphene should be clearly delineated. A sharp interface would improve the performance of GaN-based devices constructed on graphene. ...
GaN films were grown on a multilayer graphene (MLG)/amorphous SiO2 stack by pulsed sputtering deposition and their structural properties were investigated. The GaN films on MLG show high c-axis orientation. In addition, the GaN films exhibit coexisting zincblende and wurtzite phases, but the zincblende phase is suppressed by the insertion of AlN interlayers. The polarity control of the GaN films was demonstrated using AlN interlayers with and without surface oxidation. These results indicate that the proposed technique can yield high-quality Ga-polarity GaN films on MLG for potential use in large-area GaN-based optical and electronic devices.
... The most commonly used substrate, sapphire, has a very large misfit for GaN, -13.7%. Several alternatives to sapphire have been explored, including SiC [1], spinel [2], ZnO [3] [4], β-LiGaO 2 [5] [6], and γ -LiAlO 2 [7]. β-LiGaO 2 was an obviously promising possibility for GaN epitaxy, because its crystal structure is just a wurtzite superstructure and could be grown by the Czochralski technique. ...
The (1 0 0) face of γ-LiAlO 2 has attracted attention as a possible substrate for GaN epitaxial growth. This is partly because this face has an excellent lattice and structural match to (1 0 0) GaN. This orientation would have a misfit of only-1.4% along the c-direction and-0.1% along the b-direction of LiAlO 2. We find that in practice this orientation relationship does not occur; instead, (0 0 0 1) oriented GaN grows with a small tilt (0.6° towards the c-direction) between the film and substrate. Although the misfit along the substrate b direction is large (-6.3%) for this orientation, the tilt perfectly accommodates the-1.4% misfit in the c direction. We present characterization of these films by RHEED, X-ray diffraction, and TEM. We propose that the tilt is driven by a reduction of interface energy which occurs in polar, incoherent interfaces.
GaN-based light-emitting devices have the potential to realize all visible emissions with the same material system. These emitters are expected to be next-generation RGB displays and illumination tools. These emitting devices have been realized with highly efficient blue and green light-emitting diodes (LEDs) and laser diodes (LDs). Extending them to longer wavelength emissions remains challenging from an efficiency perspective. In the emerging research field of micro-LED displays, III-nitride red LEDs are in high demand to establish highly efficient devices like conventional blue and green systems. In this review, we describe fundamental issues in the development of red LEDs by III-nitrides. We also focus on the key role of growth techniques such as higher temperature growth, strain engineering, nanostructures, and Eu doping. The recent progress and prospect of developing III-nitride-based red light-emitting devices will be presented.
We present a novel approach for growth of surface-directed spinel ZnGa2O4 and β-Ga2O3 nanofins coated with non-polar GaN shell. Our results show that use of a binary compound such as...
