AlGaN Intermediate Layers Research Articles

Structural characterization of ELO-GaN film on mask-stripe patterned GaAs (0 0 1) substrate grown by metalorganic vapor phase epitaxy

GaN films were epitaxial laterally overgrown on the [110]-stripe-patterned GaAs (001) substrate by MOVPE. AlGaN intermediate layers and low-temperature GaN buffer layer were inserted in order to prevent structural defects. The epitaxial lateral overgrowth of GaN results in a trapezoidal shape with flat facets. Structural phases and defects distribution were studied by TEM. Selective-area diffraction pattern indicates the mixed phase of GaN and a high density of stacking faults at nucleation. However, with the deposition of dielectric SiNx masks, a lateral overgrowth resulted in nearly free of defects, and the cubic phase transformed into hexagonal despite a cubic template. Moreover, dark-field TEM images were used to demonstrate the structural phase transformation.

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.

Enhanced crystalline quality of non-polar a-plane AlGaN epitaxial film grown with Al-composition-graded AlGaN intermediate layer

The non-polar a-AlGaN epitaxial film was successfully grown on the semi-polar r-sapphire substrate by metal-organic chemical vapor deposition technique. An Al-composition-graded AlxGa1−xN (x = 0.0 to 1.0) intermediate layer with varying film thickness from 260 to 695 nm was deposited between the high-temperature AlN layer and the non-polar a-AlGaN epitaxial film to enhance the morphological and crystalline quality. The non-polar a-AlGaN epitaxial films were investigated by using atomic force microscopy (AFM), high-resolution x-ray diffraction, photoluminescence (PL) spectroscopy and the Hall effect measurement techniques. The characterisation results indicate substantial improvements in surface morphology and crystalline quality for the non-polar a- AlGaN epitaxial film grown by adding an Al-composition-graded AlGaN intermediate layer. The surface roughness measured from AFM and the defect-related emission (yellow band) relative to the near-band-edge emission from PL spectra were decreased significantly by optimizing the layer thickness of the Al-composition-graded AlGaN layer. A relatively low background carrier concentration down to −4.4 × cm−3 was achieved from Hall effect measurement for the non-polar a-AlGaN epitaxial film.

Influence of AlN nucleation layer on vertical breakdown characteristics for GaN‐on‐Si

A study of metal‐organic chemical vapor deposition (MOCVD) grown AlN nucleation layer (NL) on breakdown characteristics for GaN‐on‐Si is presented. It is widely believed that AlN NL can act as an insulator because of its large band gap ∼6.2 eV. On contrary, this study of AlN NL/Si reveals conductive nature and shows high vertical leakage. The structural examinations along with electrical characterization show AlN NL/Si quality depends on the growth temperature. The surface morphology and presence of unintentional oxygen impurities govern the vertical leakage of AlN NL/Si. Interestingly, the AlN NL influences the growth of subsequent epitaxial layers as well as their vertical breakdown voltages (BVs). Further, it is found that AlGaN intermediate layer and multipairs of AlGaN/AlN strained layer superlattice (SLS) grown over AlN NL with better surface properties enhances the vertical BV. A high BV of 1.3 kV is achieved for SLS multipairs with a total thickness of 4.4 μm and the translated breakdown field strength is 2.8 MV cm−1 for MOCVD grown GaN‐on‐Si.

Strain control of GaN grown on 3C‐SiC/Si substrate using AlGaN buffer layer

AbstractGaN layers were grown on 3C‐SiC/Si substrates with AlGaN intermediate layers with various thickness and AlN mole fraction by low‐pressure metal‐organic vapor phase epitaxy (LP‐MOVPE). The effects of the interlayer on the crystal quality and strain‐status of GaN epilayer were investigated systematically by varing the thickness and the Al mole fraction of the AlGaN. Raman scattering and X‐ray diffraction results show that the AlGaN with thinner thickness and higher Al mole fractions can improve the crystalline quality of the GaN epilayer and the redue the strain at the same time. This phenomenon should be attributed to three dimensional (3D) growth at the initial stage of (the) GaN epitaxy. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN grown on Si(1 1 1) with step-graded AlGaN intermediate layers

The authors report the growth of crack-free GaN on Si(1 1 1) substrate with step-graded AlGaN intermediate layers all grown at 1120 °C. By preparing all these layers at high-temperature, we can simplify the growth proceduce and minimize the growth time. Using X-ray diffraction and transmission electron microscopy, it was found that the high-temperature step-graded AlGaN intermediate layers can effectively reduce the tensile stress on GaN epitaxial layers. Photoluminescence and Raman measurements also indicate that we can improve the crystal quality of GaN by inserting the step-graded AlGaN intermediate layers.

Al0.25Ga0.75N / GaN Schottky Barrier Photodetectors with an Al0.3Ga0.7N Intermediate Layer

AlGaN/GaN Schottky barrier photodetectors (PDs) with and without a low temperature (LT) AlGaN intermediate layer (IML) were both fabricated and characterized. It was found that we can reduce dark leakage current by around 4 orders of magnitude and enhance UV-to-visible rejection ratio by around 2 orders of magnitude by using an LT AlGaN IML. With −5 V applied bias, we can achieve lower noise equivalent power and higher normalized detectivity of the PD, which were and , respectively, with an LT AlGaN IML.

Effect of AlGaN intermediate layer on residual stress control and surface morphology of GaN grown on 6H-SiC substrate by metal organic vapour phase epitaxy

GaN layers with AlGaN intermediate layers of different Al mole fraction steps were grown on 6H-SiC by metal organic vapour phase epitaxy system. The residual stress and surface morphology of these samples were compared with GaN/AlN/SiC structure in detail. High resolution X-ray diffraction indicates that the c-axis constant increases with the increasing number of AlGaN steps, while low-temperature photoluminescence measurement shows a blue-shift of the GaN peak. These results should be attributed to the decreased residual stress in GaN. Furthermore, surface morphology of samples with AlGaN intermediate layers is improved according to the results of atomic force microscope.

Nitride-Based MSM Photodetectors with a HEMT Structure and a Low-Temperature AlGaN Intermediate Layer

UV metal–semiconductor–metal (MSM) photodetectors (PDs) based on the high-electron-mobility transistor (HEMT) structure with the low-temperature (LT) AlGaN intermediate layer atop were fabricated. A much lower dark current subsequently obtained was in fact benefited by the insertion of the LT AlGaN intermediate layer. In addition, the foregoing structure also rendered PDs with better gate controllability. For the MSM PD structure grown on the LT AlGaN intermediate layer, the resultant responsivity at varied from when the device was biased from while the UV/visible rejection was estimated to be around . With a applied bias, the corresponding noise equivalent power and normalized detectivity determined were and , respectively.

Buffer optimization for crack-free GaN epitaxial layers grown on Si(1 1 1) substrate by MOCVD

We report the growth of GaN films on the Si(1 1 1) substrate by metalorganic chemical vapour phase deposition (MOCVD). Different buffer layers were used to investigate their effects on the structural and optical properties of GaN layers. A series of GaN layers were grown on Si(1 1 1) with different buffer layers and buffer thicknesses and were characterized by Nomarski microscopy, atomic force microscopy, high-resolution x-ray diffraction (XRD) and photoluminescence (PL) measurements. We first discuss the optimization of the LT-AlN/HT-AlN/Si(1 1 1) templates and then the optimization of the graded AlGaN intermediate layers. In order to prevent stress relaxation, step-graded AlGaN layers were introduced along with a crack-free GaN layer of thickness exceeding 2.6 µm. The XRD and PL measurements results confirmed that a wurtzite GaN was successfully grown. The resulting GaN film surfaces were flat, mirror-like and crack-free. The mosaic structure in the GaN layers was investigated. With a combination of Williamson–Hall measurements and the fitting of twist angles, it was found that the buffer thickness determines the lateral coherence length, vertical coherence length, as well as the tilt and twist of the mosaic blocks in GaN films. The PL spectra at 8 K show that a strong band edge photoluminescence of GaN on Si (1 1 1) emits light at an energy of 3.449 eV with a full width at half maximum (FWHM) of approximately 16 meV. At room temperature, the peak position and FWHM of this emission become 3.390 eV and 58 meV, respectively. The origin of this peak was attributed to the neutral donor bound exciton. It was found that the optimized total thickness of the AlN and graded AlGaN layers played a very important role in the improvement of quality and in turn reduced the cracks during the growth of GaN/Si(1 1 1) epitaxial layers.

High quality GaN grown on silicon(111) using a SixNy interlayer by metal-organic vapor phase epitaxy

High quality GaN layers with dislocation density of (3.0±0.5)×108∕cm2 have been grown on silicon(111) substrates using a combination of AlGaN intermediate layers and a SixNy interlayer. A smooth and fully coalesced layer was obtained by virtue of a high temperature growth process which accelerates coalescence and improves at the same time the crystalline quality. This was confirmed by high resolution x-ray diffraction showing a full width of half maximum of 415arcsec for the asymmetric (−2201) rocking curve.

Epitaxial lateral overgrowth of GaN on 4 inch Si(111) by MOVPE

AbstractEpitaxial Lateral Overgrowth (ELOG) of GaN on 4 inch silicon(111) substrates by MOVPE was investigated in this study. ELOG was performed on a GaN template with a couple of AlGaN intermediate layers (IL) on an AlN nucleation layer. The AlGaN ILs supply compressive stress to the top GaN template and thereafter to the ELOG layer. Consequently, layer cracking is minimized. Two masks were used in this work: a 2 inch wagon wheel mask and a 4 inch mask with parallel stripes of various filling factors and periods. The filling factor is varied from 0.33 to 0.7. The periodic spacing is in the range of 6 μm to 10 μm. Temperature, V/III ratio, pressure and stripe orientation were optimized to achieve fastest lateral growth rate. The highest lateral to vertical ratio can be more than 4. A fully coalesced layer within the critical thickness for a crack‐free layer was achieved on 4 inch silicon substrates. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of SixNy interlayer on the quality of GaN epitaxial layers grown on Si(111) substrates by MOCVD

Abstract In the present paper, the effects of nitridation on the quality of GaN epitaxial films grown on Si(1 1 1) substrates by metal–organic chemical vapor phase deposition (MOCVD) are discussed. A series of GaN layers were grown on Si(1 1 1) under various conditions and characterized by Nomarski microscopy (NM), atomic force microscopy (AFM), high resolution X-ray diffraction (HRXRD), and room temperature (RT) photoluminescence (PL) measurements. Firstly, we optimized LT-AlN/HT-AlN/Si(1 1 1) templates and graded AlGaN intermediate layers thicknesses. In order to prevent stress relaxation, step-graded AlGaN layers were introduced along with a crack-free GaN layer of thickness exceeding 2.2 μm. Secondly, the effect of in situ substrate nitridation and the insertion of an Si x N y intermediate layer on the GaN crystalline quality was investigated. Our measurements show that the nitridation position greatly influences the surface morphology and PL and XRD spectra of GaN grown atop the Si x N y layer. The X-ray diffraction and PL measurements results confirmed that the single-crystalline wurtzite GaN was successfully grown in samples A (without Si x N y layer) and B (with Si x N y layer on Si(1 1 1)). The resulting GaN film surfaces were flat, mirror-like, and crack-free. The full-width-at-half maximum (FWHM) of the X-ray rocking curve for (0 0 0 2) diffraction from the GaN epilayer of the sample B in ω -scan was 492 arcsec. The PL spectrum at room temperature showed that the GaN epilayer had a light emission at a wavelength of 365 nm with a FWHM of 6.6 nm (33.2 meV). In sample B, the insertion of a Si x N y intermediate layer significantly improved the optical and structural properties. In sample C (with Si x N y layer on Al 0.11 Ga 0.89 N interlayer). The in situ depositing of the, however, we did not obtain any improvements in the optical or structural properties.

Series resistance in a GaN/AlGaN/n‐Si structure grown by MOVPE

GaN film was grown on a (111)Si substrate using AlGaN intermediate layer by selective metalorganic vapor phase epitaxy. The series resistance of the GaN/AlGaN/Si diode was evaluated as a function of the thickness and the alloy composition of the AlGaN intermediate layer. The resistance was decreased by reducing the thickness of the intermediate layer but showed a minimum value. The minimum resistance was evaluated as a function of the alloy composition. Assuming an exponential increase of the minimum resistance as a function of the conduction band offset in the AlGaN/Si interface, the resistance for a GaN/Si hetero interface was estimated to be 0.0018 Ω cm2. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Flat GaN epitaxial layers grown on Si(111) by metalorganic vapor phase epitaxy using step-graded AlGaN intermediate layers

In this work, we report on the growth by metalorganic vapor phase epitaxy (MOVPE) of GaN layers on AlN/Si(111) templates with step-graded AlGaN intermediate layers. First, we will discuss the optimization of the AlN/Si(111) templates and then we will discuss the incorporation of step-graded AlGaN intermediate layers. It is found that the growth stress in GaN on high-temperature (HT) AlN/Si(111) templates is compressive, although, due to relaxation, the stress we have measured is much lower than the theoretical value. In order to prevent the stress relaxation, step-graded AlGaN layers are introduced and a crack-free GaN epitaxial layer of thickness >1 µm is demonstrated. Under optimized growth conditions, the total layer stack, exceeding 2 µm in total, is kept under compressive stress, and the radius of the convex wafer bowing is as large as 119 m. The crystalline quality of the GaN layers is examined by high-resolution x-ray diffraction (HR-XRD), and the full-width-at-half maximums (FWHMs) of the x-ray rocking curve (0002) ω-scan and (−1015) ω-scan are 790 arc sec and 730 arc sec, respectively. It is found by cross-sectional transmission electron microscopy (TEM) that the step-graded AlGaN layers terminate or bend the dislocations at the interfaces.

Photocurrent spectroscopy of a (0 0 0 1)GaN/AlGaN/(1 1 1)Si heterostructure

A crack-free GaN/AlGaN sample was grown on (1 1 1) Si by selective area metal-organic vapor-phase epitaxy method using an AlGaN intermediate layer. The electrical properties of the GaN/AlGaN/Si heterojunction diode were investigated with photocurrent spectroscopy at 77 K . It was found that the current–voltage characteristics depend on the thickness/composition of the intermediate layer. The photocurrent spectra indicated that the energy band of the n-type-doped Si substrate is lifted at the heterointerface, while it is flat in case of the sample grown on a p-type-doped Si. The depletion of the energy band in the n-Si at the heterointerface is attributed to the diffusion of Al during the growth.