Quality Of GaN Epilayers Research Articles

Effect of AlN buffer thickness on GaN epilayer grown on Si(1 1 1)

We studied the influence of high temperature AlN buffer thickness on the property of GaN film on Si (1 1 1) substrate. Samples were grown by metal organic chemical vapor deposition. Optical microscopy, atomic force microscopy and X-ray diffraction were employed to characterize the samples. The results demonstrated that thickness of high temperature AlN buffer prominently influenced the morphology and the crystal quality of GaN epilayer. The optimized thickness of the AlN buffer is found to be about 150 nm. Under the optimized thickness, the largest crack-free range of GaN film is 10 mm×10 mm and the full width at half maximum of GaN (0 0 0 2) rocking curve peak is 621.7 arcsec. Using high temperature AlN/AlGaN multibuffer combined with AlN/GaN superlattices interlayer we have obtained 2 μm crack-free GaN epilayer on 2 in Si (1 1 1) substrates.

Influence of the misorientation of 6H–SiC substrate on the quality of GaN epilayer grown by MOVPE

The effect of misorientation of 6H–SiC substrate on the structural quality of GaN epilayer grown by organometallic vapor phase epitaxy (MOVPE) has been investigated. The results reveal that the structural quality of GaN epilayer is significantly influenced by the misorientation of 6H–SiC substrate. Larger misorientation angle of 6H–SiC substrate results in better structural quality of GaN epilayer, such as narrower atomic step widths of GaN surface, less yellow luminescence band/edge emission (Iy/Ib) ratio of PL, and lower dislocations density. It is also found that the Iy/Ib ratio of GaN epilayer is affected only by the misorientation angle of 6H–SiC substrate, but the screw dislocation density and edge dislocation density are influenced by both the misorientation angle and the misorientation direction of 6H–SiC substrate. GaN epilayer grown on 6H–SiC substrate with 2.3° misorientation angle towards [1 1 2¯ 0] direction has the best structural quality.

Effect of growth temperature of AlN interlayers on the properties of GaN epilayers grown on c‐plane sapphire by metal organic chemical vapor deposition

AbstractThe effect of growth temperature of AlN interlayers on the properties of GaN epilayers grown on c‐plane sapphire by metal organic chemical vapor deposition has been investigated by high resolution X‐ray diffraction (HRXRD) and Raman spectroscopy. It is concluded that the crystalline quality of GaN epilayers is improved significantly by using the high temperature AlN (HT‐AlN) interlayer in GaN buffers. The density of threading dislocation is reduced especially for edge type dislocations. Higher compressive stress exists in GaN epilayers with HT‐AlN interlayer than with low temperature AlN (LT‐AlN) interlayer, which is related to the reduction of strain relaxation caused by the formation of misfit dislocation. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of the growth temperature of AlN buffer on the quality and stress of GaN films grown on 6H–SiC substrate by MOVPE

The influence of AlN buffer growth temperature on the quality and stress of 4.5 μm GaN epilayer on 6H–SiC substrate by organometallic vapor phase epitaxy (MOVPE) has been investigated. The crystalline quality and the atomic surface morphology were improved, the density of the pits and the stress of the GaN epilayer were reduced by increasing the growth temperature of the AlN buffer in the range from 950 °C to 1100 °C. By employing the optimized 1100 °C growth temperature of AlN buffer, very high quality of GaN epilayer was achieved. The X-ray full width of half maximums (FWHMs) of (0 0 2) and (1 0 2) reflection rocking curves of the GaN epilayer have been improved to 159 arcsec and 194 arcsec, respectively, and the surface RMS to only 0.31 nm in the 5 μm × 5 μm atomic force microscopy (AFM) scan. The stress of GaN epilayer was investigated by X-ray diffraction and Raman scattering as well. The degree of the tensile stress in GaN epilayer could be suppressed by increasing the growth temperature of AlN buffer. Finally, a high quality of crack-free 4.5 μm thick GaN epilayer was obtained on 6H–SiC substrate using the optimized 1100 °C AlN growth temperature.

Comparative study of different properties of GaN films grown on (0001) sapphire using high and low temperature AlN interlayers

Comparative study of high and low temperature AlN interlayers and their roles in the properties of GaN epilayers prepared by means of metal organic chemical vapour deposition on (0001) plane sapphire substrates is carried out by high resolution x-ray diffraction, photoluminescence and Raman spectroscopy. It is found that the crystalline quality of GaN epilayers is improved significantly by using the high temperature AlN interlayers, which prevent the threading dislocations from extending, especially for the edge type dislocation. The analysis results based on photoluminescence and Raman measurements demonstrate that there exists more compressive stress in GaN epilayers with high temperature AlN interlayers. The band edge emission energy increases from 3.423 eV to 3.438 eV and the frequency of the Raman shift of E2(TO) moves from 571.3 cm−1 to 572.9 cm−1 when the temperature of AlN interlayers increases from 700 °C to 1050 °C. It is believed that the temperature of AlN interlayers effectively determines the size, the density and the coalescence rate of the islands, and the high temperature AlN interlayers provide large size and low density islands for GaN epilayer growth and the threading dislocations are bent and interactive easily. Due to the threading dislocation reduction in GaN epilayers with high temperature AlN interlayers, the approaches of strain relaxation reduce drastically, and thus the compressive stress in GaN epilayers with high temperature AlN interlayers is high compared with that in GaN epilayers with low temperature AlN interlayers.

Epitaxial growth of improved GaN epilayer on sapphire substrate with platinum nanocluster

We report the epitaxial growth of a GaN epilayer on a sapphire substrate coated with platinum nanocluster (SSPN) by metalorganic chemical vapor deposition. To form the platinum nanocluster on sapphire substrate, a 5-nm-thick platinum layer was deposited by the e-beam evaporation, and then a thermal annealing process was carried out. Optical and structural properties of the GaN epilayer grown on SSPN were estimated by photoluminescence (PL), atomic force microscopy (AFM), high-resolution X-ray diffraction (HR-XRD) and transmission electron microscopy (TEM). The XRD full-width at half-maximum (FWHM) value of the (102) asymmetry plane for the GaN layer with SSPN was decreased from 390 to 320arcsec compared to that of the GaN without SSPN, and the reduced defect-related pits on the GaN surface were observed from the AFM images. In PL, intense band-edge luminescence with narrower FWHM (38meV) was obtained. We believe that improved quality of GaN epilayer grown on SSPN is attributed to lateral growth mode induced by thickness fluctuation of buffer zone, which is confirmed by TEM observation.

Effect of double AlN buffer layer on the qualities of GaN films grown by radio-frequency molecular beam epitaxy

This paper reports that the GaN thin films with Ga-polarity and high quality were grown by radio-frequency molecular beam epitaxy on sapphire (0001) substrate with a double AlN buffer layer. The buffer layer consists of a high-temperature (HT) AlN layer and a low-temperature (LT) AlN layer grown at 800°C and 600°C, respectively. It is demonstrated that the HT-AlN layer can result in the growth of GaN epilayer in Ga-polarity and the LT-AlN layer is helpful for the improvement of the epilayer quality. It is observed that the carrier mobility of the GaN epilayer increases from 458 to 858cm2/V·s at room temperature when the thickness of LT-AlN layer varies from 0 to 20 nm. The full width at half maximum of x-ray rocking curves also demonstrates a substantial improvement in the quality of GaN epilayers by the utilization of LT-AlN layer.

Carrier recombination under one-photon and two-photon excitation in GaN epilayers

Luminescence properties of 100-μm thick GaN epilayers grown by hydride vapor phase epitaxy (HVPE) over three different substrates: high-pressure grown n-type bulk GaN (HP-n-GaN), high-pressure bulk GaN doped with magnesium (HP-GaN:Mg), and free-standing HVPE lifted-off from sapphire (FS-HVPE-GaN), were compared by means of one-photon and two-photon excitations. The contribution of carrier capture to nonradiative traps was estimated by the analysis of luminescence transients with carrier diffusion taken into account. The estimated values of carrier lifetime of about 3 ns and diffusion coefficient of 1 cm 2/s indicate the highest quality of GaN epilayers on FS-HVPE-GaN substrates.

Reduction of Dislocations in GaN Epilayer Grown on Si (111)Substrates using a GaN Intermedial Layer

GaN intermedial layers grown under different pressures are insertedbetween GaN epilayers and AlN/Si(111) substrates. In situ opticalreflectivity measurements show that a transition from thethree-dimensional (3D) mode to the 2D one occurs during the GaNepilayer growth when a higher growth pressure is used during thepreceding GaN intermedial layer growth, and an improvement of thecrystalline quality of GaN epilayer will be made. Combining the insitu reflectivity and transmission electron microscopy (TEM)measurements, it is suggested that the lateral growth at the transitionof growth mode is favourable for bending of dislocation lines, thusreducing the density of threading dislocations in the epilayer.

Growth of InGaN multiple quantum wells and GaN eplilayer on GaN substrate

We investigated that the surface morphology of GaN epilayers was significantly affected by the surface tilt orientation of GaN substrate. Surface morphologies of GaN epilayers on GaN substrates show three types: mirror, wavy, and hillock. These surface morphologies are dependent on the surface orientation of GaN substrates. It is found that the hillock morphology of GaN epilayer was formed on the GaN substrate with surface tilt orientation less than 0.1°. As the surface tilt angle increased to 0.35°, the surface morphology varied from hillock to wavy morphology. Above a surface tilt angle of 0.4°, surface morphology changed to the mirror-like type morphology. Additionally, these three types of GaN surface morphology also affected the optical quality of GaN epilayers as well as InGaN multiple quantum wells on GaN substrates by non-uniform In incorporation on the different surface morphologies of GaN epilayers.

Impact of H2-Preannealing of the Sapphire Substrate on the Crystallinity of Low-Temperature-Deposited AlN Buffer Layer

Effect of H2-preannealing of sapphire substrate on low-temperature-deposited (LT) AlN buffer layer deposited by metalorganic vapor epitaxy has been investigated. Crystallinity of LT-AlN buffer layer drastically changes with preannealing temperature variation. It is found that H2-preannealing of sapphire substrate is a requisite to get a better quality LT-AlN and as a consequence it leads to growth of better quality GaN epi-layer on it.

Design of the low-temperature AlN interlayer for GaN grown on Si (1 1 1) substrate

We have investigated the effect of the thickness and layer number of the low-temperature AlN interlayer (LT-AlN IL) on the stress relaxation and the crystal quality of GaN epilayers grown on Si (1 1 1) substrate by metalorganic chemical vapor deposition. It is found that the stress decreases with the increase of the LT-AlN IL thickness, but the crystal quality of the GaN epilayer goes worse quickly when the LT-AlN IL thickness is larger than 16 nm. This is because the increase of the LT-AlN IL thickness will increase the coalescence thickness of its upper GaN layer, which sensitively affects the crystal quality of the epilayer. Using multiple LT-AlN ILs is an effective method not only to reduce the stress, but also to improve the crystal quality of the GaN epilayer. With the increase of the interlayer number, the probability that dislocations are blocked increases and the probability that dislocations are produced at interfaces decreases. Thus, dislocations in the most upper part of GaN are reduced, resulting in the improvement of the crystal quality. Finally, it is suggested that when the total thickness of the epilayer is fixed, both the thickness and the number of the LT-AlN IL should be carefully designed to reduce the stress and improve the crystal quality of the epilayer simultaneously.

Properties of various ion-implanted sapphire substrates for GaN epilayers

The structural, electrical, and optical properties of GaN epilayers grown on various ion-implanted sapphire (0001) substrates by metal-organic chemical vapor deposition (MOCVD) were investigated. Sapphire is a typical substrate for GaN epilayers. However, there are many problems such as lattice mismatch and thermal coefficient difference between sapphire substrate and GaN. The ion-implanted substrate's surface had decreased internal free energies during the growth of the ion-implanted sapphire (0001) substrates. The crystal and optical properties of GaN epilayers grown in ion-implanted sapphire (0001) substrate were improved. Also, surfaces excessively roughened and modified by ions degraded the GaN epilayers. Not only the ionic radius but also the chemical species of implanted sapphire (0001) substrates could improve the properties of GaN epilayers grown by MOCVD. This result implies that higher quality of GaN epilayers can be obtained using ion-implanted sapphire (0001) substrate with various ions.

Growth of high quality GaN epilayer on AlInN/GaN/AlInN/GaN multilayer buffer and its device characteristics

The role of AlInN 1st /GaN/AlInN 2nd /GaN multi-layer buffer (MLB) on the growth of the high quality GaN epilayers was demonstrated by atomic force microscope (AFM), X-ray diffraction (XRD), photoluminescence, and Hall measurement. The surface morphology and crystalline quality of GaN epilayers were considerably dependent on AlInN layers thicknesses rather than those of GaN inter layers. With optimal thickness of 2 nd AlInN layer, the pit density of GaN epilayers was substantially reduced. Also, the RMS roughness of the well ordered terraces generated on the GaN surface was 1.8 A at 5 x 5 μm 2 . The omega-rocking width of GaN (0002) Bragg peak and Hall mobility of GaN epilayers grown on AlInN 1st /GaN/AlInN 2nd /GaN MLB were 190 arcsec and 500 cm 2 /Vs, while those values of GaN epilayers on single GaN buffer layer were 250 arcsec and 250 cm 2 /Vs, respectively. Especially, the light output power and operating voltage of the fabricated light emitting diodes with this new buffer layer was about 5 mW and 3.1 V (dominant luminous wavelength -460 nm) at 20 mA, respectively.

Surface morphology of AlN buffer layer and its effect on GaN growthby metalorganic chemical vapor deposition

The in situ optical reflectivity measurements are employed to monitor the GaN epilayer growth process above low-temperature AlN buffer layer on c-plane sapphire substrate by metalorganic chemical vapor deposition. It is found that the lateral growth of GaN islands and their coalescence is promoted in the initial growth stage if the AlN buffer layer is treated with a long annealing time and has an optimal thickness: As confirmed by atomic force microscopy observations, the quality of GaN epilayers is closely dependent on the surface morphology of AlN buffer layer, especially the grain size and nuclei density after the annealing treatment.

The effects of strained sapphire (0001) substrate on the structural quality of GaN epilayer

AbstractWe investigated the structural properties of GaN epilayers grown on strained sapphire (0001) substrate by metal‐organic chemical vapor deposition (MOCVD). To minimize the mismatch in lattice constants and thermal expansion coefficients between the sapphire substrate and the GaN epilayer, we imposed the strain onto the sapphire (0001) substrate using high‐energy O+, Cl+, and As+ ion implantation. The strained sapphire surfaces result in the different behaviors of defect generation in the GaN epilayer. Not only the ionic species but also the thickness of the low‐temperature GaN buffer layer on sapphire affected the structural quality of the GaN epilayer. The present results show that O+ ion‐implanted substrate can provide an improved GaN epilayer using only a thin (∼5 nm) buffer layer rather than using a thicker one. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of pretreatment with ion implantation on substrate for GaN

The structural, electrical and optical properties of GaN epilayers grown on various ion-implanted sapphire(0001) substrates by MOCVD were investigated. Sapphire substrates have been widely adopted to grow high quality GaN epilayer despite the large differences of lattice constant and thermal expansion coefficient between them. So, GaN or AlN buffer layer and pre-treatment was indispensably introduced before the GaN epilayer growth. The ion-implanted substrate's surface had decreased internal free energies during the growth of the ions implanted sapphire(0001) substrates. The crystal and optical properties of GaN epilayers grown in ions implanted sapphire(0001) substrate were improved. Also, excessively roughened and modified surface by ions degraded the GaN epilyers. Not only the ionic radius but also the chemical species of implanted sapphire(0001) substrates could improve the properties of GaN epilayers grown by MOCVD. This result implies that higher quality of GaN epilayers was achieved by using ion-implanted sapphire(0001) substrate with various ions.

The influence of AlN buffer layer thickness on the properties of GaN epilayer

The influence of low-temperature AlN buffer layer thickness on GaN epilayer was investigated by triple-axis X-ray diffraction (XRD) and photoluminescence measurements. A method was proposed to measure the screw and edge dislocation densities by XRD. It was found that the buffer layer thickness was a key parameter to affect the quality of GaN epilayer and an appropriate thickness resulted in the best structural and optical properties except the lateral grain size. After the thickness exceeding the appropriate value, the compressive stress in the epilayer decreased as the thickness increased, which led to the redshift of the near-band edge luminescence. The experimental results showed the buffer layer thickness had more influence on edge dislocation than screw type and the former was perhaps the main source of the yellow band.

Effect of SiN treatment on GaN epilayer quality

High-temperature GaN films were grown at 1120 °C and 1080 °C by atmospheric pressure metalorganic vapor phase epitaxy on silicon nitride (SiN)-treated sapphire using a GaN buffer layer grown at 600 °C. The effect of a SiN interlayer has been investigated. We have varied the silane (SiH4) flow from 4 to 20 sccm and the duration of SiN treatment from 60 s to 180 s. These layers have been characterized in-situ by laser reflectometry and ex-situ by atomic force microscopy (AFM) and low-temperature photoluminescence. AFM revealed that GaN grown with optimized SiN treatment shows a terrace-like structure with a reduced density of dislocations of about 5 × 108 cm−2 compared to GaN without SiN treatment. The I2 line is very intense with a 4 meV full width at half maximum. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Impact of H2-preannealing of the Sapphire Substrate on the Crystallization of Low-Temperature-Deposited AlN Buffer Layer

ABSTRACTThe effect of H2-preannealing of sapphire substrate on low-temperature (LT- ) AlN buffer layer deposited by metalorganic vapor phase epitaxy is investigated. Crystallinity of LT-AlN drastically changes with preannealing temperature variation. It is found that H2-preannealing of sapphire substrate is a requisite to get a better quality LT-AlN and as a consequence it leads to growth of better quality GaN epi-layer on it.