AlGaN Epilayers Research Articles

Charge carrier transport properties of Mg-doped Al0.6Ga0.4N grown by molecular beam epitaxy

We have studied the charge carrier transport properties of Mg-doped AlGaN epilayers with Al composition ∼60% grown by plasma-assisted molecular beam epitaxy. At room temperature, free hole concentration up to 8.7 × 1017 cm−3 was measured, and hole mobility values in the range of 10–17 cm2 V−1 s−1 can be reliably achieved. Significantly, a minimum resistivity of 0.7 Ω cm was measured, and its dependence on the Mg dopant incorporation was identified. Detailed temperature-dependent charge carrier transport studies further revealed that the hole concentration exhibited a negligible dependence on temperature near room temperature, but increases drastically for temperatures above 300 °C, suggesting the importance of impurity band conduction at room temperature.

Cathodoluminescence study on local high-energy emissions at dark spots in AlGaN/AlGaN multiple quantum wells

We investigated the spatial distribution of luminescence near threading dislocations in AlGaN/AlGaN multiple quantum wells (MQWs) by cathodoluminescence mapping. Emission at the higher-energy side of the AlGaN MQW peak was locally observed near the threading dislocations, which were not accompanied by any surface V-pits. Such higher-energy emission was not observed in the AlGaN epilayers. The energy difference between the AlGaN MQW peak and the higher-energy emission peak increased with increasing barrier-layer Al composition. These results suggest that the origin of the higher-energy emission is likely local thickness fluctuation around dislocations in very thin AlGaN MQWs.

Study of dual nitridation processes in growth of non-polar a-plane AlGaN epi-layers

The effect of dual nitridation processes for both r-plane sapphire and low temperature-grown AlN (LT-AlN) nucleation layer on non-polar a-plane AlGaN epi-layer was studied intensively. A root-mean-square value as small as 1.54 nm for a-plane Al0.53Ga0.47N epi-layer was achieved. It was revealed that the generation of AlN grains as well as the coalescence and recrystallization of LT-AlN islands were the key factors for growing a-plane AlGaN epi-layers with smooth surface morphology. Meanwhile, the evolution of surface morphology with varied nitridation processes and the mechanisms for improving surface morphology of a-plane AlGaN epi-layers were also investigated.

Growth of High‐Quality AlN and AlGaN Films on Sputtered AlN/Sapphire Templates via High‐Temperature Annealing

We investigated the fabrication and characterization of AlN and AlGaN epilayers grown on sputtered and annealed AlN/sapphire templates by metal organic vapor phase epitaxy. The surfaces of the AlN homoepilayers exhibited clear step‐terrace structures with minimal root mean square roughness values. X‐ray rocking curve (XRC) measurements showed that the AlN homoepilayers had smaller (102)‐plane full width at half maximum (FWHM) values (192 arcsec) than the AlN/sapphire templates did (214 arcsec), and that they had larger (0002)‐plane FWHM values (97 arcsec) than the AlN/sapphire templates (22 arcsec). The latter results may be due to the wafer curvature increasing as the AlN layer thickness increases. We also present results for the growth of AlGaN heteroepilayers with different AlN compositions on AlN/sapphire templates that either did or did not contain AlN regrowth layers. High‐AlN mole fraction AlGaN heteroepilayers had larger (0002)‐plane FWHM values and smaller (102)‐plane FWHM values than low‐AlN mole fraction AlGaN. Furthermore, the presence of an AlN regrowth layer tended to have a negative effect on the high‐AlN mole fraction AlGaN layers. The above results are discussed using lattice mismatch, wafer curvature, surface condition, and growth temperature parameters.

AlGaN photonics: recent advances in materials and ultraviolet devices

AlGaN-based materials own direct transition energy bands and wide bandgap and thus can be used in high-efficiency ultraviolet (UV) emitters and detectors. Over the past two decades, AlGaN-based materials and devices experienced rapid development. Deep ultraviolet AlGaN-based light-emitting diodes (LEDs) with improved efficiency of 20.3% (at 275 nm) have been produced. An electron beam (EB)-pumped AlGaN-based UV light source at 238 nm, output power of 100 mW, and power conversion efficiency (PCE) of 40% has also been fabricated. UV stimulated emission from AlGaN multiple-quantum-wells laser diodes (LDs) using electrical pumping at room temperature has also been achieved at a wavelength of 336 nm. Compared with GaN-based blue and green LEDs and LDs, the efficiency of AlGaN-based UV LEDs and LDs is lower. Further optimization and improvements in both structure and fabrication are required to realize high-performance devices. In AlGaN-based UV photodetectors (PDs), gain as high as 104 orders of magnitude has been reported using the separated absorption and multiplication region avalanche photodiode structure but is still far from detecting the weak signal, and thus UV single-photon detectors with high detectivity is challenging. Recently, there has been extensive work in the nonlinear optical properties of AlGaN and AlGaN-based passive devices, such as waveguides and resonators. However, how to minimize the scattering and defect-related absorption needs to be further studied. In this review, first, approaches used to grow an AlGaN epilayer and p-type doping are introduced. Second, progress in AlGaN-based UV LEDs, EB-pumped light sources, LDs, PDs, passive devices, and the nonlinear optical properties are presented. Finally, an overview of potential future trends in AlGaN-based materials and UV devices is given.

Spatially-resolved optical and structural properties of semi-polar mathrm{(11}bar{2}mathrm{2)} AlxGa1\u2212xN with x up to 0.56

Pushing the emission wavelength of efficient ultraviolet (UV) emitters further into the deep-UV requires material with high crystal quality, while also reducing the detrimental effects of built-in electric fields. Crack-free semi-polar mathrm{(11}bar{2}mathrm{2)} AlxGa1−xN epilayers with AlN contents up to x = 0.56 and high crystal quality were achieved using an overgrowth method employing GaN microrods on m-sapphire. Two dominant emission peaks were identified using cathodoluminescence hyperspectral imaging. The longer wavelength peak originates near and around chevron-shaped features, whose density is greatly increased for higher contents. The emission from the majority of the surface is dominated by the shorter wavelength peak, influenced by the presence of basal-plane stacking faults (BSFs). Due to the overgrowth technique BSFs are bunched up in parallel stripes where the lower wavelength peak is broadened and hence appears slightly redshifted compared with the higher quality regions in-between. Additionally, the density of threading dislocations in these region is one order of magnitude lower compared with areas affected by BSFs as ascertained by electron channelling contrast imaging. Overall, the luminescence properties of semi-polar AlGaN epilayers are strongly influenced by the overgrowth method, which shows that reducing the density of extended defects improves the optical performance of high AlN content AlGaN structures.

Effect of Al composition and V/III ratio of AlGaN on GaN for distributed Bragg reflector

In this study, we have deposited the AlGaN thin films for distributdd Bragg reflector (DBR). We investigated effects of Al content (18.0% ~ 47.2%) and III/V ratio (1437 4792) on AlGaN thin film. We analyzed image of grown AlGaN epi-layer by FE-SEM. There are different growth behaviors depending on III/V ratio under the greatest Al contents. Therefore, we optimized the AlGaN epi-layer growth conditions that have the highest Al content and adjusted III/V ratio. Also, AlGaN thin films were analyzed. Finally, we fabricated DBR using optimized AlGaN epi-layer and characterized the optical properties and surface morphology.

Defect characterization of MOCVD grown AlN/AlGaN films on sapphire substrates by TEM and TKD

High resolution transmission electron microscopy (TEM) has been used to characterize defects structures in AlN/AlGaN epilayers grown by metal-organic chemical vapour deposition (MOCVD) on c-plane sapphire (Al2O3) substrates. The AlN buffer layer was shown to be epitaxially grown on the sapphire substrate with the two lattices rotated relatively through 30°. The AlN layer had a measured thickness of 20–30nm and was also shown to contain nano-sized voids. The misfit dislocations in the buffer layer have been shown to be pure edge with a spacing of 1.5nm. TEM characterization of the AlGaN epilayers was shown to contain a higher than expected threading dislocation density of the order 1010cm−2 as well as the existence of “nanopipes”. TEM analysis of the planar lamella for AlGaN has presented evidence for the possibility of columnar growth. The strain and misorientation mapping in the AlGaN epilayer by transmission Kikuchi diffraction (TKD) using the FIB lamella has also been demonstrated to be complimentary to data obtained by TEM imaging.

Evolution of AlGaN deep level defects as a function of alloying and compositional grading and resultant impact on electrical conductivity

AlGaN:Si epilayers with uniform Al compositions of 60%, 70%, 80%, and 90% were grown by metal-organic vapor phase epitaxy along with a compositionally graded, unintentionally doped (UID) AlGaN epilayer with the Al composition varying linearly between 80% and 100%. The resistivity of AlGaN:Si with a uniform composition increased significantly for the Al content of 80% and greater, whereas the graded UID-AlGaN film exhibited resistivity equivalent to 60% and 70% AlGaN:Si owing to polarization-induced doping. Deep level defect studies of both types of AlGaN epilayers were performed to determine why the electronic properties of uniform-composition AlGaN:Si degraded with increased Al content, while the electronic properties of graded UID-AlGaN did not. The deep level density of uniform-composition AlGaN:Si increased monotonically and significantly with the Al mole fraction. Conversely, graded-UID AlGaN had the lowest deep level density of all the epilayers despite containing the highest Al composition. These findings indicate that Si doping is an impetus for point defect incorporation in AlGaN that becomes stronger with the increasing Al content. However, the increase in deep level density with the Al content in uniform-composition AlGaN:Si was small compared to the increase in resistivity. This implies that the primary cause for increasing resistivity in AlGaN:Si with the increasing Al mole fraction is not compensation by deep levels but rather increasing activation energy for the Si dopant. The graded UID-AlGaN films maintained low resistivity because they do not rely on thermal ionization of Si dopants.

Indium-surfactant-assisted epitaxial growth of semi-polar $$\left(11\overline{2}2\right)$$ 11 2 ¯ 2 plane Al0.42Ga0.58N films

The semi-polar $$\left(11\overline{2}2\right)$$ plane Al0.42Ga0.58N films were successfully grown on $$\left(10\overline{1}0\right)$$ -oriented m-plane sapphire substrates for the first time with an indium (In)-surfactant-assisted metal–organic chemical vapor deposition (MOCVD) technology. The crystal orientation, surface morphology, and electrical properties of the grown semi-polar $$\left(11\overline{2}2\right)$$ plane AlGaN epi-layers were characterized with high-resolution X-ray diffraction (HR-XRD), atomic force microscopy (AFM), and Hall effect measurements, respectively. The XRD scanning results showed that crystalline quality for the semi-polar $$\left(11\overline{2}2\right)$$ plane AlGaN epi-layer was improved with In-surfactant. The AFM results demonstrated that the root mean square value of the semi-polar AlGaN epi-layer samples decreased with increasing the TMIn mole flow rate. Furthermore, the native electron concentration of the unintentionally doped semi-polar $$\left(11\overline{2}2\right)$$ plane AlGaN epi-layers was decreased from 2.66 × 1017 to 2.97 × 1016 cm−3 due to the significant decrease in nitrogen vacancies (V N) with indium-surfactant-assisted growth process.

Epitaxial growth and characterization of non-polar a-plane AlGaN films with MgN/AlGaN insertion layers

The MgN/AlGaN insertion layers were applied for the first time in the growth of non-polar a-plane AlGaN epi-layers by metal organic chemical vapor deposition technology. The full-width-at-half-maximum value of X-ray rocking curve for the a-plane AlGaN epi-layer was decreased by approximately 50.6% and the root-mean-square value of the surface was reduced by 74% by inserting the MgN/AlGaN insertion layers with optimized number of insertion pairs, which revealed that the compressive strain within the a-plane AlGaN epi-layers was effectively reduced, leading to significant improvements in crystalline quality and surface morphology, which is very helpful to fabricate high quality AlGaN-based ultraviolet light-emitting-diodes.

Enhancing redshift phenomenon in time-resolved photoluminescence spectra of AlGaN epilayer**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0400101) and Beijing Science and Technology Project, China (Grant No. Z151100003315024).

AlGaN epitaxial layer has been studied by means of temperature-dependent time-integrated photoluminescence (PL) and time-resolved photoluminescence (TRPL). An enhancing redshift phenomenon in TRPL spectra with increasing temperature was observed, and the localized excitons behaved like quasi two-dimensional excitons between 6 K and 90 K. We demonstrated that these behaviors are caused by a change in the carrier dynamics with increasing temperature due to the competition of carriers’ localization and delocalization in the AlGaN alloy.

Gain mechanism and carrier transport in high responsivity AlGaN-based solar blind metal semiconductor metal photodetectors

We report on the highest responsivity for III-nitride Metal Semiconductor Metal solar-blind photodetectors on sapphire. Devices on unintentionally doped AlGaN epilayers grown by Metal Organic Chemical Vapor Deposition exhibited sharp absorption cut-off in the range of 245–290 nm. Very high responsivity >5 A/W at 10 V bias was achieved with visible rejection exceeding three orders of magnitude for front illumination. Compared to the responsivity values reported in the literature for state-of-the-art solar-blind photodetectors, this work presents the highest values of responsivity at a given bias and up to sub-250 nm detection threshold. The high responsivity is attributed to an internal gain mechanism operating on these devices. The reverse-bias leakage current across these samples was found to be dominated by thermionic field emission at low biases and Poole-Frenkel emission from a deep trap level (0.7 eV from the conduction band-edge for Al0.50Ga0.50 N) at high biases.

Influence of nitridation process on characteristics of N-polar AlGaN films grown by MOCVD

The N-polar AlGaN epi-layers with an Al composition of ~10% were grown on (0001)-oriented c-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD) technology. Special attention was paid on the nitridation process in the epitaxial growth of N-polar AlGaN films. The optical microscope (OM) measurement results demonstrated that the size of hexagonal defects on the surface of N-polar AlGaN epi-layer decreased dramatically with our improved nitridation process. Furthermore, the structural, electrical, and optical properties of N-polar AlGaN epi-layers were characterized extensively by means of X-ray rocking curves (XRCs), Hall effect, and photoluminescence (PL) spectroscopy. It was found that the defects-related blue-band (BB) emission was greatly suppressed and the threading dislocation (TD) density was significantly reduced by the optimized nitridation process. These characterization results reveal that the reformed nitridation process plays a vital role in the improvement in the crystalline quality as well as the electrical and optical properties of the N-polar AlGaN epi-layers.

Epitaxial growth and characterization of nonpolar a-plane AlGaN films with MgN/AlGaN insertion layers

MgN/AlGaN insertion layers were applied for the first time to the growth of nonpolar a-plane AlGaN epilayers by metal organic chemical vapor deposition technology. The full width at half maximum of the X-ray rocking curve for the a-plane AlGaN epilayers was reduced by approximately 50.6% and the root-mean-square roughness value of the surface was reduced by 74% by applying the MgN/AlGaN insertion layers with an optimized number of insertion pairs. These results reveal that the compressive strain within the a-plane AlGaN epilayers was effectively reduced, leading to significant improvements in crystalline quality and surface morphology. These improvements are very helpful in fabricating high-quality AlGaN-based ultraviolet light-emitting diodes.

Structural characterization of Al0.55Ga0.45N epitaxial layer determined by high resolution x-ray diffraction and transmission electron microscopy**Project supported by the National Key Research and Development Project of China (Grant No. 2016YFB0400100), the Hi-tech Research Project of China (Grant Nos. 2014AA032605 and 2015AA033305), the National Natural Science Foundation of China (Grant Nos. 61274003, 61422401, 51461135002, and 61334009), the Natural Science Foundation of

Structural characteristics of AlN epilayer were investigated by high resolution x-ray diffraction (HRXRD) and transmission electron microscopy (TEM); the epilayer was grown on GaN/sapphire substrates using a high-temperature AlN interlayer by metal organic chemical vapor deposition technique. The mosaic characteristics including tilt, twist, heterogeneous strain, and correlation lengths were extracted by symmetric and asymmetric XRD rocking curves as well as reciprocal space map (RSM). According to Williamson–Hall plots, the vertical coherence length of AlGaN epilayer was calculated, which is consistent with the thickness of AlGaN layer measured by cross section TEM. Besides, the lateral coherence length was determined from RSM as well. Deducing from the tilt and twist results, the screw-type and edge-type dislocation densities are 1.0 and 1.8, which agree with the results observed from TEM.

Influence of high-temperature AlN intermediate layer on the optical properties of MOCVD grown AlGaN films

By combining spectroscopic ellipsometry (SE) and optical transmission (OT) characterization methods we have systematically investigated the influence of AlN intermediate layer and AlN transition layer on the optical properties of AlGaN epilayers grown on sapphire by metalorganic chemical vapor deposition (MOCVD) method. Most dielectric functions of III-nitrides obtained by different research groups show significant band-tail absorption—which is not anticipated for such a direct band gap material. The dielectric functions are studied for a series of AlGaN/AlN/Al2O3 structures, with a four-layer model taking into account both high temperature grown AlN layer and low temperature grown AlN layer. The results obtained by fitting the optical parameters to experimental data show that the band-tail absorption should originate from the transition layer. AlGaN film without high temperature AlN epilayer exhibited a redshift of band gap around 0.24 eV.

High quality thin AlN epilayers grown on Si(110) substrates by metal-organic chemical vapor deposition

Thin AlN epilayers are grown on Si(110) by metal-organic chemical vapor deposition, and are intentionally treated not only as conventional seed layers but also as base layers for high quality AlGaN epilayer growth. By controlling the growth conditions, a V-shape pit free AlN surface with atomic steps is achieved. Characterization results of high resolution X-ray diffraction show that the structural qualities of the thin AlN epilayers grown on Si(110) are excellent, and the epilayers are superior to those grown on Si(111) with the same thickness in the literature. In addition, the structural quality of the AlGaN epilayers grown on AlN/Si(110) is confirmed to strongly depend on the underlying AlN epilayer. Furthermore, it is important to control the AlN thickness to prevent the generation of large cracks during the growth process. Our results present potential applications of high quality III-nitrides grown on Si(110) for optical (UV region) and power device applications.

AlN and AlGaN layers grown on Si(111) substrate by mixed-source hydride vapor phase epitaxy method

High Al-composition AlGaN and AlN epilayers were grown directly on Si(111) substrate by a hydride vapor phase epitaxy (HVPE) method with a melted mixed source in a graphite boat set in a source zone with high temperatures of T = 700 and 800 °C, respectively. The presence of the Ga material in the mixed source of Ga and Al promoted the growth of AlN and AlGaN epilayers in the growth zone. When the temperature in the source zone was 800 °C, the crystalline quality of the AlN and AlGaN epilayers increased as the ratio of Ga to Al increased, and the optimum mix ratio of Ga to Al for the growth of AlN epilayers was approximately 0.35–0.42, obtained from a numerical fitting analysis of the X-ray diffraction (XRD) data for these epilayers. It appears that they can be grown directly by our melted-mixed-source HVPE method in a high-temperature source zone.

Defects reduction in a-plane AlGaN epi-layers grown on r-plane sapphire substrates by metal organic chemical vapor deposition

Nonpolar a-plane AlGaN epi-layers were grown on a semi-polar r-plane sapphire substrate with an innovative two-way pulsed-flows metal organic chemical vapor deposition growth technology. A root-mean-square value of 1.79 nm was achieved, and the relative light transmittance of the a-plane AlGaN epi-layer was enhanced by 36.9%. These results reveal that the innovative growth method is able to improve the surface morphology and reduce the defect density in nonpolar a-plane AlxGa1–xN epi-layers, particularly those with an Al composition greater than 0.5, which are key materials for the fabrication of nonpolar AlGaN-based high light emission efficiency deep-ultraviolet light-emitting diodes.