R-plane Sapphire Substrates Research Articles

Non-polar a-plane AlN epitaxial films on r-plane sapphire with greatly reduced defect densities obtained by high-temperature annealing

This study addresses the difficulty of obtaining high crystalline quality non-polar a-plane AlN films grown epitaxially on r-plane sapphire substrates by applying a high-temperature annealing process to films grown via standard-temperature metal organic chemical vapor deposition. Atomic force microscopy results demonstrate the occurrence of a significant recrystallization process in the AlN films during annealing. Raman spectroscopy measurements reveal that the AlN films are subject to a transition from tensile strain to compressive strain after the annealing process. Transmission electron microscopy analyses demonstrate that the non-polar a-plane AlN films provide an appropriately low dislocation density of 3.7 × 109 cm−2.

The In-Plane-Two-Folders Symmetric a-Plane AlN Epitaxy on r-Plane Sapphire Substrate

In the present work, a single-crystalline epitaxial nonpolar a-plane AlN film with in-plane two-folder symmetries was successfully achieved on an r-plane sapphire substrate, by combining physical vapor deposition and a high-temperature annealing technique. Moreover, by varying the AlN thickness, the evolution of crystalline quality and structure were systematically investigated using X-ray diffraction, Raman spectroscopy, and atomic force microscopy. The crystalline quality was much improved by the annealing treatment. Most importantly, when the thickness of AlN was increased up to 1000 nm, the AlN lattice was found to endure strong distortion along the out-of-plane direction, and the lattice showed an obvious expansion. The change of the surface morphology induced by high-temperature annealing was also tracked, and the morphology displayed structural anisotropy along the [11¯00] direction. Our results act as a crucial platform to better understand and employ the nonpolar AlN template; in particular, it is of importance for subsequent device fabrication.

Structural and optical properties of AlN sputtering deposited on sapphire substrates with various orientations

AlN thin films were deposited on c-, a- and r-plane sapphire substrates by the magnetron sputtering technique. The influence of high-temperature thermal annealing (HTTA) on the structural, optical properties as well as surface stoichiometry were comprehensively investigated. The significant narrowing of the (0002) diffraction peak to as low as 68 arcsec of AlN after HTTA implies a reduction of tilt component inside the AlN thin films, and consequently much-reduced dislocation densities. This is also supported by the appearance of E 2(high) Raman peak and better Al–N stoichiometry after HTTA. Furthermore, the increased absorption edge after HTTA suggests a reduction of point defects acting as the absorption centers. It is concluded that HTTA is a universal post-treatment technique in improving the crystalline quality of sputtered AlN regardless of sapphire orientation.

Low temperature deposition of Ga2O3 films on sapphire and epi-GaN substrates

Ga2O3 films were deposited on non-polar sapphire and epi-GaN substrates at low temperature by MOCVD. The ε-Ga2O3 films, which were preferentially grown along the (0001) crystal plane family, were obtained on a-, r-plane sapphire and GaN substrates, and contained a certain polycrystalline component. On the m-plane sapphire substrate, a polycrystalline film containing α/β-Ga2O3 was obtained. The grain size and surface fluctuations on m-plane sapphire substrate were large, while the film on r-plane sapphire and GaN substrates had more uniform surface and the film on GaN substrate had the flattest surface. Optical analysis indicated that the best quality Ga2O3 film on these sapphire substrates was deposited on r-plane substrate. XRD showed that the Ga2O3 film on GaN substrate had better crystalline quality than that on a-plane sapphire substrate.

Effect of NH3-flow modulation on the morphological properties of nonpolar a-plane AlGaN epilayers

In this article, the effect of ammonia (NH3) flow-modulation on the crystalline quality and morphology of the nonpolar a-plane AlGaN epilayer grown on the r-plane sapphire substrate was reported. The as-grown metal-organic chemical vapor deposition (MOCVD) epilayers have been diagnosed with high-resolution X-ray diffraction, atomic force microscopy, and scanning electron microscopy techniques. It was found that with the introduction of proper interruption of NH3-flow during the MOCVD growth process, enormous reduction in the threading dislocation density (TDD) and significant improvements in the crystalline quality and surface morphology could be realized. By optimizing the NH3-flow interruption time to 30 s during the growth of the nonpolar AlGaN intermediate layer, a small root-mean-square value of 1.8 nm for the surface roughness and a TDD of 2 × 1010 cm−2 were achieved. Moreover, the full width at half maximum values determined from the X-ray rocking curves were reduced from 2410 to 1512 arcsec and 2355 to 1645 arcsec along c- and m-plane directions, respectively.

Impact of sandwiched strain periodic multilayer AlN/GaN on strain and crystalline quality of a-plane GaN

We demonstrated high-quality single crystalline a-plane undoped-gallium nitride grown on a nonpatterned r-plane sapphire substrate via metal–organic chemical vapor deposition. The effect of four different numbers of sandwiched strain-periodic AlN/GaN multilayers on the strain state, crystal quality, optical and electrical properties was investigated. Field emission scanning electron microscopy and atomic force microscopy showed that the surface morphology was improved upon insertion of 120 pairs of AlN/GaN thin layers with a root-mean-square roughness of 2.15 nm. On-axis X-ray ω-scan rocking curves showed enhanced crystalline quality: the full width at half maximum decreased from 1224 to 756 arcsec along the [0001] direction and from 2628 to 1360 arcsec along the [1–100] direction for a-GaN grown with 120 pairs of AlN/GaN compared to a-GaN without AlN/GaN pairs. Reciprocal space mapping showed that a-plane GaN with a high number of AlN/GaN pairs exhibits near-relaxation strain states. Room-temperature photoluminescence spectra showed that the sample with the highest number of AlN/GaN pairs exhibited the lowest-intensity yellow and blue luminescence bands, indicating a reduction in defects and dislocations. The a-plane InGaN/GaN LEDs with 120 pairs of SSPM-L AlN/GaN exhibited a significant increase (~ 250%) in light output power compared to that of LEDs without SSPM-L AlN/GaN pairs.

High quality non-polar a-plane AlN template grown on semi-polar r-plane sapphire substrate by three-step pulsed flow growth method

The structural and optical properties of the non-polar a-plane (112̅0) AlN template grown on semi-polar r-plane (11̅02) sapphire substrate with three-step pulsed flow growth method in a metalorganic chemical vapor deposition system were investigated extensively. It was found that the crystalline quality, optical property, and surface morphology of the a-plane AlN template grown by the three-step pulsed flow growth method featured with variable temperature were greatly improved as compared with the a-plane AlN template grown by conventional one-step pulsed flow growth method with a fixed temperature. In fact, besides the remarkably reduced full width at half maximum values of both the symmetrical X-ray rocking curve and the A1(TO) Raman scattering peak, strong relative light transmittance and steep absorption edge were observed in the ultraviolet-visible absorption spectrum for the a-plane AlN template grown by the three-step pulsed flow growth method. Furthermore, a superior surface morphology and a decreased surface deformation were achieved with a root mean square value as small as 3.4 nm in an atomic force microscope detection area of 5 × 5 µm2. These results reveal that the three-step pulsed flow growth method should be powerful to grow high crystalline quality non-polar a-plane AlN template which plays a crucial role in the epitaxial growth of the AlGaN-based III-nitrides.

Structure and luminescence of a-plane GaN on r-plane sapphire substrate modified by Si implantation* *Project supported by the Key-Area Research and Development Program of Guangdong Province, China (Grant Nos. 2019B010132001, 2020B010174003, and 2019B121204004), the Basic and Application Basic Research Foundation of Guangdong Province, China (Grant Nos. 2020A1515110891 and 2019A1515111053), and the Fund from the Ion Beam Center (IBC) at HZDR.

We show the structural and optical properties of non-polar a-plane GaN epitaxial films modified by Si ion implantation. Upon gradually raising Si fluences from 5 × 1013 cm−2 to 5 × 1015 cm−2, the n-type dopant concentration gradually increases from 4.6 × 1018 cm−2 to 4.5 × 1020 cm−2, while the generated vacancy density accordingly raises from 3.7 × 1013 cm−2 to 3.8 × 1015 cm−2. Moreover, despite that the implantation enhances structural disorder, the epitaxial structure of the implanted region is still well preserved which is confirmed by Rutherford backscattering channeling spectrometry measurements. The monotonical uniaxial lattice expansion along the a direction (out-of-plane direction) is observed as a function of fluences till 1 × 1015 cm−2, which ceases at the overdose of 5 × 1015 cm−2 due to the partial amorphization in the surface region. Upon raising irradiation dose, a yellow emission in the as-grown sample is gradually quenched, probably due to the irradiation-induced generation of non-radiative recombination centers.

Comparison of Ga2O3 Films Grown on m- and r-plane Sapphire Substrates by MOCVD

Ga2O3 films were respectively deposited on m- and r-plane sapphire substrates by LP-MOCVD. The growth pressure greatly influenced the surface morphology and the grain shape, and the grain size obviously decreased with the increasing growth pressure. XRD results indicated that a higher growth pressure helped to suppress the polycrystalline orientation of β-Ga2O3 films grown on m-plane sapphire substrates, but was not conducive to the formation of α-Ga2O3. Ellipsometer measurement shows that the higher growth pressure will slow down the growth rate, and the deposition rate on the r-plane was significantly faster than the m-plane under the lower growth pressure. The bandgap obtained by fitting the optical absorption spectrum was also consistent with the previous reports.

Enhanced tunability and temperature-dependent dielectric characteristics at microwaves of K0.5Na0.5NbO3 thin films epitaxially grown on (100)MgO substrates

K0.5Na0.5NbO3 thin films were deposited by pulsed laser deposition on (100)MgO substrates for microwave device applications. A fine epitaxial growth of pure perovskite phase was evidenced by X-ray diffraction. Dielectric characterizations were performed from 1 to 40 GHz using coplanar microwave devices printed on the 500 nm-thick K0.5Na0.5NbO3 thin films. Dielectric permittivity εr = 355 and loss tangent tanδ = 0.35 at 10 GHz were retrieved without biasing. A comparison of the results with those retrieved from the resonant cavity method (to characterize as-deposited films) showed no deleterious influence neither from the device patterning nor the thin film-device interface. A frequency tunability up to 22% was measured under a moderate external DC bias electric field Ebias = 94 kV/cm. Temperature measurements from 20° to 240°C exhibited a permittivity increase up to εr = 975 coupled to a loss decrease tanδ = 0.25 at 10 GHz. According to such measurements, an orthorhombic-tetragonal phase transition was evidenced close to 220 °C with an increase of the frequency tunability up to 34%. Comparison of the properties of such films with those grown on R-plane sapphire substrates demonstrated the benefit brought by the epitaxial growth of K0.5Na0.5NbO3 films on (100) MgO.

Triethylamine sensing characteristics of nonpolar (11-20) and polar (0001) GaN thin films

Intrinsically n-doped a-plane and c-plane GaN thin films were successfully deposited on r-plane and c-plane sapphire substrates, respectively, by metalorganic vapor phase epitaxy (MOVPE). The c-plane GaN sensor shows a response value of 5.23–200 ppm triethylamine (TEA) at 480 °C, much higher than the a-plane one at the same condition. The TEA vapor is detectable down to its threshold value of 1 ppm for the c-plane sensor at 480 °C. Excellent selectivity was obtained for the c-plane sensor, by distinguishing TEA from interfering gases according to a combined signal of response, response time and recovery time. Meanwhile, we investigated pre-adsorption of O2 molecules on the two GaN surface facets using density functional theory (DFT) simulations. The much lower adsorption energy and much more accumulated electrons around the O2 molecules for the c-plane facet explains the above-mentioned better TEA sensing performance of the c-plane sensor as compared to its counterpart.

Letter to the Editor Regarding “Tactile Skill-Based Neurosurgical Simulators Are Effective and Inexpensive”

Nonpolar ZnO films were grown on r-plane sapphire substrates through the reaction between dimethylzinc (DMZn) and high-temperature H2O, the latter produced by the Pt-catalyzed reaction between H2 and O2. The resulting ZnO films were characterized using atomic force microscopy, X-ray diffraction, and photoluminescence (PL) measurements. They were found to have an anisotropic surface morphology with stripe arrays, and exhibited a diffraction peak associated with ZnO (11-20) index planes. The PL spectra indicated anisotropy in polarization between the directions parallel and perpendicular to the c-axis. The band-edge luminescence at 3.3 eV exhibited a maximum when the electric field vector E was perpendicular to the c-axis (parallel to the [1-100] direction) and another minimum when E was parallel to the c-axis. The angular dependence of the linear polarization of the band-edge luminescence was large for ZnO films grown at low temperatures. The large degree of polarization observed for low-temperature growth is thought to be due to the film geometry.

Optical anisotropy of interface-bound exciton emission in a-plane ZnO/Zn1-xMgxO MQWs with Zn1-yMgyO buffer layer

The anisotropic exciton emission in a-plane nonpolar ZnO/ZnMgO MQWs offers opportunity for anisotropic photonic devices. By introducing Zn1-yMgyO buffer layer between r-plane sapphire substrate and MQWs, we investigated the structural and optical properties of ten-period a-plane ZnO/Zn1−xMgxO (ZnO/ZnMgO) multiple quantum wells (MQWs) with different widths of Zn1−xMgxO barrier layers grown by pulsed-laser deposition on Zn1-yMgyO buffered r-plane sapphire. Polarization-dependent photoluminescence spectra reveal that two additional linear polarizations of interface-bound exciton emission with E⊥c orienting at ~± 50° with respect to the regular a-plane MQWs are attributed to the generation of (0 0 0 2)-oriented nano-domains (NDs). As evidenced by XRD and TEM, we found four sets of (0 0 0 2)-oriented MQWs NDs are embedded in regular a-plane ZnO/ZnMgO MQWs with 90° in-plane rotation from one another which are inclined by 27.6° towards the [1 1 2¯ 0]-direction of the ZnO/ZnMgO MQWs. The existence of (0 0 0 2)-oriented NDs is further confirmed by the phase-contrast atomic force microscopy that modulates the optical anisotropy of interface-bound exciton emission in a-plane nonpolar ZnO/ZnMgO MQWs.

GaAs epitaxial growth on R-plane sapphire substrate

Achieving epitaxial growth of III-As on r-plane sapphire would potentially allow the integration of both laser and amplifier with corresponding RF electronics. Here we report on the growth of high-quality GaAs on an r-plane sapphire substrate by molecular beam epitaxy. The epitaxial relationship between GaAs and r-plane sapphire is observed and explained. GaAs on r-plane sapphire resulted in (111) orientation, similar to growth orientation observed on c-plane sapphire. However, in comparison with growth of GaAs growth on a c-plane sapphire substrate, a stronger interaction is observed between GaAs on r-plane sapphire. The effect of growth temperature is also investigated for GaAs growth on r-plane sapphire. It is found that GaAs island size, density, and orientation can be tuned by varying the growth temperature. Finally, a thin AlAs nucleation layer on r-plane sapphire has been introduced to study its effect on the growth of GaAs. The introduction of the AlAs nucleation layer is found to enhance the wetting of GaAs but at the expense of introducing twinning defects.

Epitaxial stabilization of rutile germanium oxide thin film by molecular beam epitaxy

Ultrawide bandgap (UWBG) semiconductors (Eg >3 eV) have tremendous potential for power-electronic applications. The current state-of-the-art UWBG materials such as β-Ga2O3, diamond, and AlN/AlGaN, however, show fundamental doping and thermal conductivity limitations that complicate technological adaption and motivate the search for alternative materials with superior properties. Rutile GeO2 (r-GeO2) has been theoretically established to have an ultrawide bandgap (4.64 eV), high electron mobility, high thermal conductivity (51 W m−1 K−1), and ambipolar dopability. While single-crystal r-GeO2 has been synthesized in bulk, the synthesis of r-GeO2 thin films has not been previously reported but is critical to enable microelectronics applications. Here, we report the growth of single-crystalline r-GeO2 thin films on commercially available R-plane sapphire substrates using molecular beam epitaxy. Due to a deeply metastable glass phase and high vapor pressure of GeO, the growth reaction involves the competition between absorption and desorption as well as rutile and amorphous formation. We control the competing reactions and stabilize the rutile-phase growth by utilizing (1) a buffer layer with reduced lattice misfit to reduce epitaxial strain and (2) the growth condition that allows the condensation of the preoxidized molecular precursor yet provides sufficient adatom mobility. The findings advance the synthesis of single-crystalline films of materials prone to glass formation and provide opportunities to realize promising ultra-wide-bandgap semiconductors.

Short-Period CdO/MgO Superlattices as Cubic CdMgO Quasi-Alloys

New perspective cubic quasi-alloy {CdO/MgO}m short-period superlattices (SLs) were grown on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy. The quality of crystals was charac...

Properties of Si-doped a-plane AlGaN layers

The non-polar a-plane (112¯0) n-type AlGaN epi-layers with high electron concentration were successfully grown on r-plane (11¯02) sapphire substrates by metal organic chemical vapor deposition. High-resolution X-ray diffraction, room temperature photoluminescence spectrum, and Hall effect measurement were used to examine the influence of Si-doping on the crystalline quality, and the optical and electrical characteristics of the non-polar a-plane n-AlGaN epi-layer, respectively. The characterization results indicate that the electron concentration (EC), the electron mobility, and the crystalline quality for the non-polar a-plane n-AlGaN epi-layers could be improved significantly by optimizing the epitaxial growth conditions, especially the mole flow rate of SiH4 in the Si-doping process. In fact, at a SiH4 mole flow rate of 10.2 nmol/min, an EC as high as 1.25 × 1018 cm−3 and an electron mobility up to 3.86 cm2/V were achieved with the non-polar n-AlGaN epi-layer sample with the Al content of 41%.

Enhanced electrocatalytic oxygen evolution activity in geometrically designed SrRuO3 thin films

For generation of sustainable, clean and highly efficient energy, the electrocatalytic oxygen evolution reaction represents an attractive platform, thus inviting immense research activities in recent years. However, designing the catalyst with enhanced electrocatalytic activity remains one of the major challenges. Here, we examined the oxygen evolution reaction activities of geometrically designed (with and without step-textured morphology) thin films of an electrocatalytically active correlated metallic SrRuO3 perovskite grown on c- and r-plane sapphire substrates. On c-plane sapphire, as compared to the uniform surface, the step-textured films endowed with active Ru-sites show remarkable decrease in the overpotential (~25 mV). Interestingly, the behavior is opposite for the r-plane case, highlighting the significance of the active sites, in addition with the polar surface termination of selective crystal facets. Density functional theory calculation confirms the favorable energy reaction pathway for the active site dependent enhancement in OER. Our strategy might pave the way towards designing the surfaces of various oxide thin films for high performance energy conversion based devices.

Effects of Ⅴ/Ⅲ ratio and Cp2Mg flow rate on characteristics of non-polar a-plane Mg-delta-doped p-AlGaN epi-layer

Abstract The non-polar a-plane Mg-delta-doped p-AlGaN epi-layers with excellent electrical conduction were achieved on r-plane sapphire substrates via metal organic chemical vapor deposition technology. The impacts of Ⅴ/Ⅲ ratio and Cp2Mg flow rate on the properties of the non-polar p-AlGaN epi-layers were investigated with scanning electron microscopy, high-resolution X-ray diffraction, Raman spectroscopy, and Hall effect measurement. It was discovered that the crystalline quality and the electrical conductivity were extremely relied upon the Ⅴ/Ⅲ ratio and Mg-doping level. Actually, a hole concentration of 3.7 × 1017 cm−3 and an electrical resistivity of 2.6 Ω ⋅ cm were obtained for non-polar p-Al0.12Ga0.88N epi-layer by carefully optimizing the Ⅴ/Ⅲ ratio and Cp2Mg flow rate during the epitaxial procedure. In addition, the results revealed that improvement on electrical conductivity was ascribed to the evident repression of self-compensation effect generated by decreasing in the densities of the nitrogen vacancy (VN) as well as the VN-related complexes and the Mg-related defects via the employment of the proper Ⅴ/Ⅲ ratio and the Mg-doping concentration.

Improvement in structural and electrical characteristics of nonpolar a-plane Si-doped n-AlGaN

Nonpolar Si-doped AlGaN with high electron concentration (EC) and superior surface morphology (SM) layers were successfully grown on r-plane sapphire substrates for the first time with metal organic chemical vapor deposition technology. X-ray diffraction, cross-sectional scanning electron microscope, ultraviolet–visible absorption spectroscopy, atomic force microscopy, and Hall measurements were used to examine the influence of indium surfactant and an extra AlGaN buffer on crystalline quality (CQ), SM, and electrical characteristics of the nonpolar Si-doped n-AlGaN, respectively. The characterization results revealed that the anisotropy in CQ of the nonpolar Si-doped n-AlGaN epi-layers was suppressed effectively, and the SM, as well as the electrical properties including EC, electron mobility (EM), and electron resistivity (ER), had been significantly improved by using indium surfactant and an extra AlGaN buffer layer. In fact, an EC as high as 4.8 × 1017 cm−3, an EM up to 3.42 cm2/V s, an ER down to 4.78 $$\Omega \bullet $$cm , and a root-mean-square value as small as 8.2 nm for surface roughness was achieved for the nonpolar Si-doped n-AlGaN with a relatively high Al composition of 50%.