HT-AlN Buffer Research Articles

Effect of nitrogen gas flow and growth temperature on extension of GaN layer on Si* *Project supported by the National Key Research and Development Program of China (Grant No. 2017YFB0402900) and the National Natural Sciences Foundation of China (Grant No. 62074144).

The effect of nitrogen flow and growth temperature on extension of GaN on Si substrate has been studied. By increasing the nitrogen flow whose outlet is located in the center of the MOCVD (metal–organic chemical vapor deposition) gas/particle screening flange and by increasing the growth temperature of HT-AlN and AlGaN buffer layers near the primary flat of the wafer, the GaN layer has extended more adequately on Si substrate. In the meantime, the surface morphology has been greatly improved. Both the AlN and GaN crystal quality uniformity has been improved. X-ray diffraction results showed that the GaN (0002) XRD FWHMs (full width at half maximum) decreased from 579 arcsec∼ 1655 arcsec to around 420 arcsec.

MOCVD生长Si衬底上HT-AlN缓冲层低生长压力对GaN薄膜的影响

MOCVD生长Si衬底上HT-AlN缓冲层低生长压力对GaN薄膜的影响

High-performance GaN-based light emitting diodes grown on 8-inch Si substrate by using a combined low-temperature and high-temperature-grown AlN buffer layer

A combined buffer layer growth process was developed to grow crack-free GaN layers on 8-inch Si(111) wafers and so light-emitting diodes (LEDs). The combined buffer layer consisted of 2 nm-thick low-temperature (LT, 850 °C)-AlN, 8 nm-thick graded-temperature AlN, and 200 nm-thick high-temperature (HT, 1100 °C)-AlN layers. The X-ray diffraction (XRD) results showed that the LT-HT-AlN buffer layer exhibited better crystal quality than the HT-AlN buffer layer. The atomic force microscopy (AFM) images revealed that compared to the LT-HT-AlN buffer layer, the HT-AlN buffer layer had a rough surface with numerous bright spots, which correspond to N-polar AlN hillocks. Scanning electron microscopy (SEM) results showed many pits in the HT-AlN buffer layer. Transmission electron microscopy (TEM) results showed that the HT-AlN buffer layer contained about 1.3 nm-thick amorphous SixNy layer at the interface, while the LT-HT-AlN buffer layer showed a relatively smooth interface. It was further shown that using the LT-HT-AlN buffer layer, high-quality crack-free n-GaN layers (2.5 μm-thick) were grown on the 8-inch Si(111) substrate, which was confirmed by the XRD and cathodoluminescence results. Subsequently, packaged vertical LEDs (chip size: 1400 × 1400 μm2) grown on the LT-HT-AlN buffer layers showed higher light output power and chip yield than LEDs with the HT-AlN buffer layer.

Proton Irradiation Effects on AlGaN/AlN/GaN Heterojunctions

AlGaN/AlN/GaN heterojunctions were irradiated by 3 MeV protons with different fluences. Hall and C-V measurements showed that the density and mobility of 2DEG for heterojunctions decreased after proton irradiation. The crystal quality and optical properties of AlGaN/GaN heterojunctions were characterized by the variations of the micro-Raman scattering spectra, XRD and photo luminescence (PL) spectra with proton fluence. It has been obtained that the strain and dislocation of GaN and AlGaN were not affected by the proton injected. The proton irradiation caused the increase of structural defects in HT-AlN buffer layer, which lead to the red shift of ${{\rm A}_1}$ (LO) mode. The higher tails of XRD rocking curve indicated the introduction of point defects by irradiation. The full-width at half-maximum (FWHM) of ${\rm E}_2^{\rm high}$ phonon mode broadened, which was consistent with change of FWHM of PL near-band-edge emission (BE). The spectra of yellow band normalized to the intensity of BE demonstrated a great increase of Ga vacancies related defects, which may be the main reason for the degradation of optical properties.

Improving the Quality of GaN on Si(111) Substrate with a Medium-Temperature/High-Temperature Bilayer AlN Buffer

A medium-temperature/high-temperature (MT/HT) bilayer AlN buffer was introduced for GaN grown on Si(111) by metal organic chemical vapor deposition. The properties of the GaN films with a MT/HT bilayer AlN buffer and those with a single-layer HT-AlN buffer were compared and the influence of the growth temperature of the MT-AlN layer was investigated. With a MT-AlN layer grown in the temperature range from 800 to 1000 °C, the crystalline qualities of the subsequent HT-AlN layer and the GaN film were improved. According to the X-ray diffraction results and the transmission electron microscopy images, the dislocation density in GaN film was reduced with a MT/HT bilayer AlN buffer as compared to those with a single-layer HT-AlN buffer. Moreover, photoluminescence and Raman spectra exhibit enhanced optical properties and less tensile stresses of the GaN film. Better surface morphology of GaN was also obtained with a MT/HT bilayer AlN buffer.

Effect of high-temperature buffer thickness on quality of AlN epilayer grown on sapphire substrate by metalorganic chemical vapor deposition

The effect of an initially grown high-temperature AlN buffer (HT-AlN) layer's thickness on the quality of an AlN epilayer grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD) in a two-step growth process is investigated. The characteristics of AlN epilayers are analyzed by using triple-axis crystal X-ray diffraction (XRD) and atomic force microscopy (AFM). It is shown that the crystal quality of the AlN epilayer is closely related to its correlation length. The correlation length is determined by the thickness of the initially grown HT-AlN buffer layer. We find that the optimal HT-AlN buffer thickness for obtaining a high-quality AlN epilayer grown on sapphire substrate is about 20 nm.

Modulation of anisotropic crystalline in a-plane GaN on HT-AlN buffer layer

(1 1 2¯ 0) a-plane gallium nitride has been investigated by Raman spectroscopy and X-ray diffraction. Strains of a-plane GaN grown on LT-GaN nucleation layer were both compressive along [1 1¯ 0 0] and [0002] orientations. However, strains of GaN epitaxied on HT-AlN buffer were tensile along [0002] and compressive along [1 1¯ 0 0] GaN orientation. The crystalline anisotropy and quality were also improved by the HT-AlN layer. Atomic force microscopy was utilized for analyzing the HT-AlN buffer layer. We ascribed this divergence of strain and improvement of crystalline to the thermal expansion and lattice mismatch effects of HT-AlN buffer during the metalorganic vapor phase epitaxy.

The structural and optical properties of selectively grown a-plane GaN with LT-GaN and HT-AlN buffer layers

This study examined the effects of HT-AlN and LT-GaN buffer layers on the optical properties, crystallinity and morphology of 3D a-plane GaN structures with InGaN/GaN MQWs. The 3D a-plane GaN on the HT-AlN buffer layer showed a rough surface and various facets, whereas the 3D a-plane GaN on the LT-GaN buffer layer contained mainly {1 0 1¯ 1} facets that were tilted 25° to the c-axis giving a pyramidal shape. Narrower full-width at half maximum and higher intensity of the cathodoluminescence spectra of 3D a-plane GaN with InGaN/GaN MQWs were observed with the LT-GaN buffer layer. The monochromatic cathodoluminescence images at a specific wavelength showed that the LT-GaN buffer layer can effectively improve the region of 3D a-plane GaN with InGaN/GaN MQWs.

Crack formation in GaN on Si(111) substrates grown by MOCVD using HT Al‐preseeding and HT AlN buffer layers

AbstractWe have investigated the crack formation in GaN layers grown on Si(111) substrates by MOCVD using an AlN buffer layers and Al‐preseeding process. With an appropriate AlN buffer thickness using optimal TMAl flow rate, we were able to counteract the crack formation usually observed in GaN layers deposited on Si(111) substrate. With a 120 nm thick AlN buffer layer, TMAl source flow rate 18 μmol/min and Al‐preseeding time 5 s, we were able to obtain nearly crack free 2.5 μm GaN surface and a (002) plane HR‐XRD rocking curve FWHM of385 arcsec is achieved. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Synthesis and characterization of heteroepitaxial GaN films on Si(111)

We report crack-free and single-crystalline wurtzite GaN heteroepitaxy layers have been grown on Si (111) substrate by metal-organic chemical vapor deposition(MOCVD). Synthesized GaN epilayer was characterized by X-ray diffraction(XRD), atomic force microscope (AFM) and Raman spectrum. The test results show that the GaN crystal reveals a wurtzite structure with the <0001> crystal orientation and XRD ω-scans showed a full width at half maximum (FWHM) of around 583 arcsec for GaN grown on Si substrate with an HT-AlN buffer layer. In addition, the Raman peaks of E 2 high and A 1(LO) phonon mode in GaN films have an obvious redshit comparing to bulk GaN eigen-frequency, which most likely due to tensile strain in GaN layers. But the AO phonon mode of Si has a blueshit which shows that the Si substrate suffered a compressive strain. And we report that the AlN buffer layer plays a role for releasing the residual stress in GaN films.

Surface Morphology of Al0.3Ga0.7N/Al2O3-High Electron Mobility Transistor Structure

We present surface properties of buffer films (AIN and GaN) and Al0.3Gao.zN/Al2O3-High Electron Mobility Transistor (HEMT) structures with/without AIN interlayer grown on High Temperature (HT)-AIN buffer/Al2O3 substrate and Al2O3 substrate. We have found that the GaN surface morphology is step-flow in character and the density of dislocations was about 10(8)-10(9) cm(-2). The AFM measurements also exhibited that the presence of atomic steps with large lateral step dimension and the surface of samples was smooth. The lateral step sizes are in the range of 100-250 nm. The typical rms values of HEMT structures were found as 0.27, 0.30, and 0.70 nm. HT-AIN buffer layer can have a significant impact on the surface morphology of Al0.3Ga0.7N/Al2O3-HEMT structures.

Effect of III/V Ratio of HT-AlN Buffer Layer onPolarity Selection and Electrical Quality of GaN Films Grown by RadioFrequency Molecular Beam Epitaxy

We investigate the effect of Al/N ratio of the high temperature (HT)AlN buffer layer on polarity selection and electrical quality of GaNfilms grown by radio frequency molecular beam epitaxy. The results showthat low Al/N ratio results in N-polarity GaN films and intermediateAl/N ratio leads to mixed-polarity GaN films with poor electricalquality. GaN films tend to grow with Ga polarity on Al-rich AlN bufferlayers. GaN films with different polarities are confirmed by in-situ reflection high-energy electron diffraction during the growthprocess. Wet chemical etching, together with atomic force microscopy,also proves the polarity assignments. The optimum value forroom-temperature Hall mobility of the Ga-polarity GaN film is703 cm2/V.s, which is superior to the N-polarity andmixed-polarity GaN films.

Low current operation of GaN-based blue-violet laser diodes fabricated on sapphire substrate using high-temperature-grown single-crystal AlN buffer layer

Low current laser operation at 405 nm has been demonstrated for the first time for the devices fabricated on sapphire substrates by metalorganic chemical vapor deposition (MOCVD) using a high-temperature-grown single-crystal AlN buffer. The thick optical guiding layers were adopted to improve optical confinement. The device structure was the 2-μm-wide ridge-stripe type without facet coating. The minimum threshold current and current density were 60 mA and 3.8 kA/cm 2 for cavity lengths of 500 mm and 1 mm, respectively. These data were comparable to those reported using the special dislocation reduction techniques. The threshold current density linearly decreases with decreasing inverse of cavity length. It was expected that the low threshold current density ranging from 1 to 2 kA/cm 2 could be realized by adapting high reflection coating for laser facets. This expected current density was comparable to values realized for devices grown on the thick freestanding GaN as substrates. These findings support the promising potential of the HT-AlN buffer technique for production of advanced short-wavelength light-emitting devices on sapphire substrates.

High quality MOCVD GaN film grown on sapphire substrates using HT-AlN buffer layer

In this work we report on the growth and characterization of high quality MOCVD GaN film grown on Al2O3 substrates by using a HT (>1150°C)-AlN buffer layer. We have investigated the most favorable growth conditions in terms of temperature, thickness and growth rate of AlN buffer layer in order to optimize the high temperature GaN layer. The improved morphological and structural properties of GaN layer were verified by AFM and XRD measurements. The optimized GaN layer presents a smooth surface with a rms value of 1.4Å. The full width at half maximum (FWHM) for 800nm thick GaN films is 144″. Furthermore PL measurements and C–V analysis confirm that in GaN layer grown on HT-AlN buffer layer defect density is drastically reduced.

Influence of the growth temperature of the high-temperature AlN buffer on the properties of GaN grown on Si(1 1 1) substrate

We have studied the influence of the growth temperature of the high-temperature (HT) AlN buffer layer on the properties of the GaN epilayer which was grown on Si(111) substrate by metalorganic chemical vapor deposition (MOCVD). It was found that the crystal quality of the GaN epilayer strongly depends on the growth temperature of the HT-AlN buffer. The growth temperature of the AlN buffer to obtain high-quality GaN epilayers lies in a narrow window of several tens of degrees. When the temperature is lower than a certain temperature range, the appearance of AlN polycrystals results in the deterioration of the crystal quality of the AlN buffer layer, which is greatly disadvantageous to the coalescence of the GaN epilayer. Although the AlN buffer's crystal quality is improved as the growth temperature increases, the Si outdiffusion from the substrate is also enhanced when the temperature is higher than a certain temperature range, which will demolish the subsequent growth of the GaN epilayer. Therefore, there exists an optimum growth temperature range of the AlN buffer around 1080°C for the growth of high-quality GaN epilayers.

Room-temperature operation of GaN-based blue-violet laser diodes fabricated on sapphire substrates using high-temperature-grown single-crystal AlN buffer layers

The laser operation has been demonstrated for the first time for the test devices fabricated on GaN layers grown on sapphire substrates by metalorganic chemical vapor deposition using high-temperature-grown single-crystal AlN buffer layers (HT-AlN buffer layers). The device structure was the simple electrode-stripe type with a 1-mm-long and 10-μm-wide laser cavity. The wavelength was 413 nm. The threshold current and current density were 760 mA and 7.6 kA/cm2, respectively. The operation voltage at the threshold current was 8 V. These characteristics were comparable to one of the best values reported using conventional low-temperature grown buffer layers, considering the used simple device structure. This fact was thought to support the promising potential of the HT-AlN buffer to realize high performance practical devices on sapphire substrates. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of the Buffer Layers on Lattice Polarity of GaN Epilayers Grown on the N‐Face of Bulk GaN Single Crystals by Molecular‐Beam Epitaxy

Lattice polarity of GaN epilayers grown on N-faces of bulk GaN single crystals has been investigated. The determinations of lattice polarity have been carried out by observation of surface reconstruction and surface morphology change by alkaline etching. As a result, it is found that GaN epilayers grown on N-faces using In-doped GaN buffer layers have Ga-polarity; on the other hand, GaN epilayers grown on N-faces using high-temperature (HT) AlN buffer layers have N-polarity. These results suggest that In-doping has the role of reversing the polarity of the GaN epilayers from N- to Ga-polarity, whereas HT-AlN buffer layers itself do not act as the layer reversing the polarity from N- to Al-polarity.

Surface etching of 6H-SiC (0001) and surface morphology of the subsequently grown GaN via MOCVD

The correlation between surface morphological properties of the GaN epilayers and the surface conditions of 6H-SiC (0001) substrates etched in H2, C2H4/H2, and HCl/H2 was studied. Etching 6H-SiC in H2 produced a high quality surface with steps and terraces, while etching in HCl/H2 produced either a rough surface with many pits and hillocks or a smooth surface similar to that etched in H2, depending on the HCl concentration and temperature. The GaN epilayers were subsequently deposited on these etched substrates using either a low temperature GaN or a high temperature AlN buffer layer via MOCVD. The substrate surface defects increased the density and size of the “giant” pinholes (2–4 µm) on GaN epilayers grown on a LT-GaN buffer layer. Small pinholes (<100 nm) were frequently observed on the samples grown on a HT-AlN buffer layer, and their density decreased with the improved surface quality. The non-uniform GaN nucleation caused by substrate surface defects and the slow growth rate of \(\{ 1\bar 101\} \) planes of the islands were responsible for the formation of “giant” pinholes, while the small pinholes were believed to be caused by misfit dislocations.