Nitrided Sapphire Substrates Research Articles

Effect of Nitridation on Indium-Composition of InGaN Films

The Present Study Aims to Understand the Relation between the Nitridation and Indium-Composition of Ingan Grown on Sapphire Substrate Using the Metalorganic Vapor Phase Epitaxy through X-Ray Diffraction Reciprocal Space Mapping Measurements. In-Composition of InGaN on Nitrided Sapphire Substrate Increased to 13% which Is Higher than the Sample without Nitridation with 7%. Also, Flat Surface Was Observed in the Nitrided Sample. Two Times Larger in-Plane Strain Was Induced at the Nitired Sample. Ingan Grown on Low-Temperature Gan Buffer, however, Did Not Show Clear Effect of Nitridation. The Two Investigated Samples Showed Similar Indium Composition, Surface Flatness, and in-Plane Strain with and without Nitridation. Differences of Indium Incorporation and Relaxation of in-Plane Strain Were Attributed to the Effect of AIN Formed by Nitridation Process.

直流反应磁控溅射在氮化的蓝宝石衬底上制备AlN薄膜

Aluminum nitride ( AlN ) films were prepared successfully on sapphire and nitrided sapphire substrates by reactive DC magnetron sputtering. The effect of nitridation of sapphire substrate on the growth of AlN films was studied. The films were characterized by X-ray diffraction ( XRD ) , atomic force microscopy ( AFM ) and optical ab- sorption spectrum. XRD patterns of AlN films exhibited a strong preferential c -axis orientation , and nitridation of sap- phire substrate could improve the crystal quality of AlN films and also decrease the residual stress of films. But AFM results revealed that the grain size distribution of films deposited on nitrided sapphire substrates was not more homoge- nous than that of films deposited on sapphire substrates , and optical absorption results also showed nitridation of sap- phire substrate nearly had no effect on the optical behavior of AlN films.

Gallium Nitride Film Growth Using a Plasma Based Migration Enhanced Afterglow Chemical Vapor Deposition System

Gallium nitride layers were grown by a new migration enhanced epitaxy technique called MEAglow. Initial experiments were performed to characterize the plasma source used and to examine the surfaces of thin samples grown by the technique. Atomic force microscopy (AFM) results show root mean square (RMS) surface roughness values of less than 1 nm for samples grown at 650 °C, this is commensurate with Ga-face material grown directly on nitrided sapphire substrates.

Elimination of Rotational Domain in AlN Layers Grown from Ga–Al Flux and Effects of Growth Temperature on the Layers

We have been investigating crystal growth of AlN on nitrided sapphire substrates, and have grown c-axis oriented AlN layers using liquid phase epitaxy (LPE) with Ga­Al fluxes. However, rotational domains having 1-deg difference around the c-axis exist in the AlN layers. One of the purposes of this study is to eliminate the rotational domains. To do so, an annealing process at elevated temperatures was attempted before the LPE process. Then, its effectiveness was discussed. The origin of the rotational domain was explained using distortion of O 2¹ ions arrangement in (0002) sapphire surface. The mechanism of the eliminating the domains was discussed using the lattice vibration of the sapphire at elevated temperatures. Secondly, effects of growth temperature on the AlN layers were investigated in terms of the growth rate, surface morphology and crystal quality. The growth rate of the LPE AlN layer increased concomitantly with increasing growth temperature at 1373­ 1673K. The growth rate attained at 1673K was 0.52µm·h ¹1 . Crystal quality is almost independent of the growth temperature in that temperature range. [doi:10.2320/matertrans.MBW201112]

Gallium Nitride Film Growth Using a Plasma Based Migration Enhanced Afterglow Chemical Vapor Deposition System

Gallium nitride layers were grown by a new migration enhanced epitaxy technique called MEAglow. Initial experiments were performed to characterize the plasma source used and to examine the surfaces of thin samples grown by the technique. Atomic force microscopy (AFM) results show root mean square (RMS) surface roughness values of less than 1 nm for samples grown at 650 °C, this is commensurate with Ga-face material grown directly on nitrided sapphire substrates.

Influence of sputter power and N2 gas flow ratio on crystalline quality of AlN layers deposited at 823 K by RF reactive sputtering

AbstractAluminum nitride (AlN) is increasingly demanded for use as a substrate for ultraviolet light emitting diodes (UV‐LEDs). A previous study investigated the influence of sputter conditions on the crystalline quality of AlN layers sputtered on nitrided sapphire substrate at 573 and 773 K using radio‐frequency (RF) reactive sputtering. AlN layers were deposited at a higher substrate temperature of 823 K to investigate the influence of sputter power and N2 gas flow ratio on their crystalline quality in this study. The AlN layers were deposited in various conditions (sputter power, 200–900 W; N2 gas flow ratio, 30–70 vol%N2). The growth rate increased concomitantly with increasing sputter power and decreasing N2 gas flow ratio. The crystalline orientation improved with increasing sputter power, except for 900 W at 50 vol%N2. High‐quality c‐axis oriented AlN layers were sputtered with 700 W at 50 vol%N2, and with 800 W at 40 and 50 vol%N2. Results showed that edge dislocation was dominant in AlN layers sputtered with 800 W at 50 vol%N2 from cross‐sectional images observed using transmission electron microscopy (TEM). (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Homoepitaxial growth of AlN on nitrided sapphire by LPE method using Ga–Al binary solution

AbstractA novel liquid phase epitaxy method was proposed for growing an AlN layer using Ga–Al binary flux under normal pressure. In this method, nitrogen gas was injected into the flux. Then a nitrided sapphire substrate was used as a template to achieve homoepitaxial growth. Advantages of using a nitrided sapphire substrate were demonstrated; the optimum flux composition was investigated. We grew 1‐µm‐thick c‐axis oriented AlN layer for 5 h at 1573 K. The full width at half maximum values of X‐ray rocking curves for AlN (0002) and (10−12) were, respectively, 50 and 590 arcsec. Moreover, the surface morphology and interfacial structure were observed using a scanning electron microscope, laser microscope, and high‐resolution transmission electron microscope.

Influence of sputtering conditions on crystalline quality of AlN layers deposited by RF reactive sputtering

AbstractAluminum nitride (AlN) is increasingly demanded for use in semiconductor‐related industries. This study was undertaken for fabrication of high‐quality AlN thick layers using radio‐frequency (RF) reactive sputtering with a pure Al target. The AlN layers were sputtered homoepitaxially on high‐quality thin AlN layers on sapphire substrates for 1.5 h with 300 to 650 W at 573 and 773 K. The AlN thin layers were grown using sapphire nitridation before sputtering. A N2‐Ar gas mixture was used as the sputtering atmosphere and N2 gas flow ratio was changed from 20 to 60 vol%N2. In all sputtering conditions c‐axis oriented AlN layers were fabricated. AlN layer sputtered with 400 W under 40 to 60 vol%N2 at 773 K was grown by about 1 μm thickness. The AlN layer sputtered with 400 W under 40 and 50 vol%N2 at 573 and 773 K showed superior quality, with almost identical crystal structural quality to that of each nitrided sapphire substrate. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

MOVPE growth and characterization of polar, semipolar and nonpolar InN on sapphire substrate

AbstractWe have investigated the heteroepitaxy of indium nitride (InN) directly grown on nitridated sapphire substrates having different orientations. Growths were performed on C, A, M and R‐plane oriented sapphire in order to analyse the substrate orientation effect on the structural, optical and electronic properties of as‐grown InN. Atomic force microscopy imaging on the InN epilayers revealed different surface morphologies with crystallites well organized along a crystalline orientation of the layer. We have studied the structural anisotropy observed in these layers analysing high‐resolution X‐ray diffraction rocking curve experiments as a function of the in‐plane beam orientation. A‐plane oriented InN grown on R‐plane sapphire substrate shows a polarized photoluminescence anisotropy, with an anisotropy percentage of about 33%.

Effect of GaN interlayer on polarity control of epitaxial ZnO thin films grown by molecular beam epitaxy

Epitaxial ZnO thin films were grown on nitrided (0001) sapphire substrates with an intervening GaN layer by rf-plasma-assisted molecular beam epitaxy. It was found that polarity of the ZnO epilayer could be controlled by modifying the GaN interlayer. ZnO grown on a distorted 3-nm-thick GaN interlayer has Zn-polarity while ZnO on a 20-nm-thick GaN interlayer with a high structural quality has O-polarity. High resolution transmission electron microscopy analysis indicates that the polarity of ZnO epilayer is controlled by the atomic structure of the interface between the ZnO buffer layer and the intervening GaN layer.

TEM characterization of catalyst- and mask-free grown GaN nanorods

Catalyst- and mask-free grown GaN nanorods have been investigated using transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy filtered transmission electron microscopy (EFTEM). The nanorods were grown on nitridated r-plane sapphire substrates in a molecular beam epitaxy reactor. We investigated samples directly after the nitridation and after the overgrowth of the structure with GaN. High resolution transmission electron microscopy (HRTEM) and EFTEM revealed that AlN islands have formed due to nitridation. After overgrowth, the AlN islands could not be observed any more, neither by EFTEM nor by Z-contrast imaging. Instead, a smooth layer consisting of AlGaN was found. The investigation of the overgrown sample revealed that an a-plane GaN layer and GaN nanorods on top of the a-plane GaN have formed. The nanorods reduced from top of the a-plane GaN towards the a-plane GaN/sapphire interface suggesting that the nanorods originate at the AlN islands found after nitridation. However, this could not be shown unambiguously. The number of threading dislocations in the nanorods was very low. The analysis of the epitaxial relationship to the a-plane GaN showed that the nanorods grew along the [000-1] direction, and the [1-100] direction of the rods was parallel to the [0001] direction of the a-plane GaN.

Growth of ZnO crystal on sapphire and nitridated sapphire substrates at 1000 °C by halide vapor phase epitaxy

Using a halide vapor phase epitaxy (HVPE) technique in which the starting materials are ZnCl 2 generated by the reaction between high purity Zn metal (7 N grade) and Cl 2 gas, and H 2O, ZnO crystals have been grown at a high temperature of 1000 °C on sapphire substrates with and without surface nitridation treatment. It was found that the nitridation treatment resulted in a change of the (112̅0) sapphire surface to a (0001) AlN structure, leading to two possible sets of orientations for (0001) ZnO crystals. In addition, the nitridation treatment leads to a smaller average ZnO grain size and a higher density of nuclei.

Effect of Refined Nitridation of Sapphire Substrates in Hydride Vapor Phase Epitaxy: Definite Correlation of Structural Characteristics between a Low-Temperature-Grown Buffer Layer and a Subsequent High-Temperature-Grown Layer of GaN

Effect of Refined Nitridation of Sapphire Substrates in Hydride Vapor Phase Epitaxy: Definite Correlation of Structural Characteristics between a Low-Temperature-Grown Buffer Layer and a Subsequent High-Temperature-Grown Layer of GaN

Correlation Between Threading Dislocations and Nonradiative Recombination Centers in InN Observed by IR Cathodoluminescence

The correlation between threading dislocations (TDs) and nonradiative recombination centers in InN films was investigated by infrared cathodoluminescence (CL). Samples were grown on nitridated (0001) sapphire substrates with a low-temperature-grown InN buffer layer by radio frequency molecular-beam epitaxy (RF-MBE). Panchromatic CL images of the InN films showed a high density of dark spots in a range of 108 cm−2 to 109 cm−2. The sample with a higher density of TDs had a higher density of CL dark spots. A depth-dependent CL measurement confirmed that CL dark spots aligned almost vertically in the film like TDs. Reasonable correlation between TDs and the nonradiative regions was also observed by a cross-sectional CL image of the InN film regrown on a microfaceted InN template, in which the TD density was dramatically reduced in part. These results suggest that threading dislocations act as nonradiative recombination centers in InN.

Growth of In-Rich In x Al1−x N Films on (0001) Sapphire by RF-MBE and their Properties

In-rich In x Al1−x N films (0.47 ≤ x ≤ 1) were grown directly on nitridated (0001) sapphire substrates without employing a buffer layer by radiofrequency molecular-beam epitaxy. Both photoluminescence peak energy and optical absorption-edge energy of the In x Al1−x N films decreased monotonically with increasing In composition, which was consistent with the recently reported InN bandgap energies of ∼0.7 eV. The bowing parameter of this alloy was estimated to lie between 2.9 eV and 6.2 eV. Substrate nitridation with lower temperature and longer period conditions not only reduced misoriented In x Al1−x N phases remarkably but also produced narrower tilt distribution in the c-axis-oriented In x Al1−x N matrix.

Growth of highly resistive Ga‐polar GaN by LP‐MOVPE

Electrical characteristics of 1.3 µm thick GaN films were investigated with respect to the polar type. Unintentionally doped N-polar GaN grown on nitrided sapphire substrates had a hexagonal facetted morphology and consistently exhibited a high n-type conductivity with a sheet resistance of ∼10 Ω/sq, corresponding to a carrier concentration on the order of 1019 cm–3. In contrast, Ga-polar GaN films grown on low temperature AlN buffer layers showed a high sheet resistance of ∼109 Ω/sq. GaN films of mixed polarity exhibited sheet resistance in-between the two extreme values obtained for polarity pure films. The results of this work show that the existence of N-polar domains (even on the nano-scale) in an otherwise Ga-polar matrix significantly decreases the insulating properties of unintentionally doped GaN films. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Controlled growth of Zn-polar ZnO epitaxial film by nitridation of sapphire substrate

Surface nitridation is used to eliminate O-polar inversion domains and control the growth of single-domain Zn-polar ZnO film on sapphire (0001) substrate by rf-plasma-assisted molecular-beam epitaxy. It is found that the nitridation temperature is crucial for achieving quality AlN buffer layers and ZnO films with cation polarity, as demonstrated by ex situ transmission electron microscopy. Under optimal growth conditions, a 4×4 surface reconstruction was observed, which is confirmed to be a characteristic surface structure of the Zn-polar films, and can be used as a fingerprint to optimize the ZnO growth.

Growth of InN layers by MOVPE using different substrates

This study presents the MOVPE growth of InN films onto different substrate materials, including sapphire, nitrided or not, GaN and AlN buffer layers deposited onto sapphire, and Si(111). For InN growth onto nitrided sapphire, different growth parameters were investigated in order to determine the best growth conditions. We found that a low V/III molar ratio has to be used in order to increase the growth rate. A light nitridation treatment gives the best electrical properties: mirror like layers with a mobility of 800 cm 2/V s were obtained. At room temperature, reflectivity experiments show the existence of a transition at 1.2 eV, while photoluminescence appears around 0.8 eV. Using the same growth conditions onto GaN buffers (with thicknesses ranging from 15 to 1000 Å), we found that the best mobilities are obtained above a given buffer thickness. By comparing also with AlN buffer layers and silicon substrates, we found that our previous conclusion still holds; lightly nitrided sapphire substrate leads to the best electrical properties and morphology.

Interplay between GaN polarity and surface reactivity towards atomic hydrogen

GaN epitaxial layers have been grown by molecular beam epitaxy (MBE). Low (200 °C) and high (700 °C) temperature nitrided sapphire substrates, and GaN and AlN buffer layers have been used as the platforms for investigating polarity selection processes of GaN grown by RF-MBE, aimed at presenting a comprehensive understanding of the issues of GaN polarity and growth conditions. The results show that Ga-polar GaN results from the use of AlN buffer layers, and when the substrate/bulk interface has a Ga- or Al-rich interface. In contrast, a large density of N-polar domains is found when GaN epitaxial films are grown on GaN buffer layers on sapphire when nitrided at 200 °C. The impact of the polarity of GaN on the interaction of GaN surfaces with atomic hydrogen is also studied. A different reaction rate and reaction extent is found for N- and Ga-polar GaN with atomic hydrogen, with N-polar GaN exhibiting greater reactivity. It is also demonstrated that the reactivity of GaN with atomic hydrogen coupled with surface potential measurements exploiting Kelvin probe microscopy is an alternative approach to determining the polarity of GaN films, and identifying the presence of inversion domains. Finally, the impact of GaN polarity on electrical properties of AlGaN/GaN heterojunction structures is also shown.

Effect of nitridation on polarity, microstructure, and morphology of AlN films

The polarity of AlN epitaxial layers grown on (0001) sapphire, SiC, and nitrided sapphire substrates was examined by convergent beam electron diffraction, and the morphology and microstructure were characterized by atomic force microscopy and scattering contrast transmission electron microscopy (TEM). The AlN films grown on sapphire and SiC without a buffer layer or nitridation of the substrate were flat and had Al polarity. From TEM studies in both cross section and plan view, it was found that the threading dislocation (TD) density was ∼1×108 and 2×1010 cm−2 for screw-component and pure edge component dislocations, respectively. N-face AlN was realized by pregrowth nitridation of sapphire substrates, but these films contained a small volume fraction of Al-face inversion domains which were related to hexagonal pyramids defined by {11̄02} facets. The density of screw-component TDs was significantly reduced due to nitridation. Cross-sectional TEM showed that the film grown on nitrided sapphire was almost free of screw-component TDs and that the density of pure edge TDs was ∼4×109 cm−2.