High-temperature AlN Buffer Layer Research Articles

Enhancing two-dimensional growth of high-temperature AlN buffer to improve the quality of GaN on Si grown by ex situ two-step method

GaN epitaxial films were grown on Si substrates by ex situ two-step method combining the technologies of pulsed laser deposition (PLD) and metal organic chemical vapor deposition (MOCVD). The N/Al ratio of high-temperature AlN (HT-AlN) buffer layer was optimized, and its influence on the HT-AlN growth mode as well as the quality of GaN epitaxial films was investigated. When the N/Al ratio was 500, the two-dimensional growth of HT-AlN was greatly enhanced, and it obtained a coalesced and smooth surface with the minimum RMS surface roughness as 1.63 nm. The as-grown GaN epitaxial film had the best crystalline quality with the minimum full-width at half maximums of GaN(0002) and GaN(10 1¯ 2) as 0.14° and 0.22°, respectively. Owing to the high energy of PLD, the ex situ low-temperature AlN template had an abrupt Si/AlN interface and flat surface, which was of significance to improve the quality of HT-AlN buffer and GaN film.

Relation of V/III ratio of AlN interlayer with the polarity of nitride

Abstract N-polar GaN film was obtained by using a high-temperature AlN buffer layer. It was found that the polarity could be inverted by a thin low-temperature AlN interlayer with the same V/III ratio as that of the high-temperature AlN layer. Continuing to increase the V/III ratio of the low-temperature AlN interlayer, the Ga-polarity of GaN film was inverted to N-polarity again but the crystal quality and surface roughness of GaN film greatly deteriorated. Finally, we analyzed the chemical environment of the AlN layer by x-ray photoelectron spectroscopy (XPS), which provides a new direction for the control of GaN polarity.

Wafer-Scale Transferrable GaN Enabled by Hexagonal Boron Nitride for Flexible Light-Emitting Diode.

Epitaxy growth and mechanical transfer of high-quality III-nitrides using 2D materials, weakly bonded by van der Waals force, becomes an important technology for semiconductor industry. In this work, wafer-scale transferrable GaN epilayer with low dislocation density is successfully achieved through AlN/h-BN composite buffer layer and its application in flexible InGaN-based light-emitting diodes (LEDs) is demonstrated. Guided by first-principles calculations, the nucleation and bonding mechanism of GaN and AlN on h-BN is presented, and it is confirmed that the adsorption energy of Al atoms on O2 -plasma-treated h-BN is over 1eV larger than that of Ga atoms. It is found that the introduced high-temperature AlN buffer layer induces sufficient tensile strain during rapid coalescence to compensate the compressive strain generated by the heteromismatch, and a strain-relaxation model for III-nitrides on h-BN is proposed. Eventually, the mechanical exfoliation of single-crystalline GaN film and LED through weak interaction between multilayer h-BN is realized. The flexible free-standing thin-film LED exhibits ≈66% luminescence enhancement with good reliability compared to that before transfer. This work proposes a new approach for the development of flexible semiconductor devices.

Role of low temperature Al(Ga)N interlayers on the polarity and quality control of GaN epitaxy

Polarity control is essential for lateral polarity heterostructures. N-polar GaN films were directly grown on high temperature AlN buffer layer. A thin low temperature AlN interlayer could invert the polarity of subsequent GaN layer to Ga-polarity but a low temperature GaN interlayer with the same condition could not do it rather deteriorate the quality of GaN film. In addition, a relationship between the yellow luminescence band and the surface roughness was discovered. The blue luminescence band was found only on the inverted Ga-polar GaN film and was connected to screw dislocation density.

N-polar GaN Film Epitaxy on Sapphire Substrate without Intentional Nitridation.

High-temperature nitridation is commonly thought of as a necessary process to obtain N-polar GaN films on a sapphire substrate. In this work, high-quality N-polar GaN films were grown on a vicinal sapphire substrate with a 100 nm high-temperature (HT) AlN buffer layer (high V/III ratio) and without an intentional nitriding process. The smallest X-ray full width at half maximum (FWHM) values of the (002)/(102) plane were 237/337 arcsec. On the contrary, N-polar GaN film with an intentional nitriding process had a lower crystal quality. In addition, we investigated the effect of different substrate treatments 1 min before the high-temperature AlN layer’s growth on the quality of the N-polar GaN films grown on different vicinal sapphire substrates.

Impact of Si doping on dislocation behavior in MOVPE-grown AlN on high-temperature annealed AlN buffer layers

In this work, we compare the defect structure in unintentionally doped and Si-doped AlN layers grown by metalorganic vapor phase epitaxy (MOVPE) on high-temperature annealed (HTA) sputtered AlN templates on sapphire substrates. Since the HTA process leads to a reduction of the in-plane lattice constant of the AlN layers, further homoepitaxial overgrowth results in compressively strained AlN layers. With increasing MOVPE-AlN layer thickness, strain relaxation takes place mostly by formation of dislocation half-loops of an irregular shape, which accumulate at the homoepitaxial MOVPE-AlN/HTA-AlN interface. We suggest that these dislocations nucleate at the layer surface and move down to the homoepitaxial interface at high temperatures. The formation of these irregular and hardly controllable defects can be avoided by introduction of Si-doping into the MOVPE-AlN layers. Si-doping enlarges the inclination of threading dislocation lines stemming from the HTA-AlN template, producing an alternative mechanism for strain relaxation.

The influence of temperature of nitridation and AlN buffer layer on N-polar GaN

The influence of temperature of nitridation and AlN buffer layer growth on N-polar gallium nitride (GaN) grown on 2-in. vicinal sapphire substrate by metal organic chemical vapour deposition (MOCVD) is investigated in this study. A medium temperature (1000 °C) leads to a relatively rougher surface but improved crystalline quality, enhanced optical performance, and higher electron mobility. The relatively stronger yellow luminescence and weaker near-band-edge (NBE) emission of the N-polar GaN undergoing a high temperature (1110 °C) nitridation and AlN buffer layer growth may be attributed to the increase of gallium vacancies, which is further confirmed by the relatively lower electron concentration. The sublinear excitation power dependence of the NBE emission may refer to the trion emission formed by the defect-bounded excitons.

Effect of high‐temperature AlN buffer on anisotropy of semi‐polar (11‐22) GaN with two pressure growth stages

The low-temperature GaN (LT-GaN) and high-temperature AlN (HT-AlN) buffer layers were grown on m -plane sapphire by metal–organic chemical vapour deposition. The following semi-polar (11-22) GaN thin films were deposited under high- and low-pressure growth stages. Anisotropy of (11-22) GaN grown on HT-AlN buffer along two in-planar directions ([11-2-3] and [-1100]) were clearly suppressed, the maximum variation of the X-ray rocking curve full width at half maximum of (11-22) GaN on LT-GaN buffer was 0.2498°, that of (11-22) GaN on HT-AlN buffer was 0.0488°. The X-ray diffraction results of on-axis and off-axis both indicated that the crystal quality of the epitaxial GaN layer with HT-AlN buffer was obviously improved, and surface morphology was much smoother.

Stimulated emission of AlGaN layers grown on sapphire substrates using ammonia molecular beam epitaxy

By optimising the growth temperature of the AlGaN layers and using high-temperature AlN buffer layers, high-quality AlxGa1 – xN layers (x = 0.15, 0.21, 0.26, and 0.3) were obtained, in which stimulated emission in the UV spectral range 330 – 297 nm was implemented with the threshold intensity of excitation Ith ≈ 0.7 – 1.4 MW cm−2, respectively. It is found that the threshold value of the stimulated emission of AlGaN layers grown by molecular beam epitaxy is largely determined by the intensity of the process of thermal decomposition of GaN, which affects the surface morphology and, consequently, the amount of optical scattering loss. It is shown that no pronounced localisation of nonequilibrium charge carriers occurs in the AlGaN layers, which is manifested in the absence of a large Stokes shift and in the realisation of optical amplification at transitions in an electron-hole plasma, and also indicates a relatively homogeneous material composition.

Growth of III-N/graphene heterostructures in single vapor phase epitaxial process

We investigated the growth of III-N/graphene heterostructures as a single process in a MOVPE reactor. Raman spectra revealed that graphene can be successfully deposited on sapphire substrate by propane pyrolysis if temperature exceeds 1060 °C and hydrogen is used as a carrier gas. GaN epitaxial layers and heterostructures on such graphene using both high-temperature AlN buffer layer and low-temperature GaN nucleation layer was demonstrated. Analysis of surface morphology and X-Ray diffraction curves indicate that GaN quality depends on graphene thickness. Use of copper electroplated Ni-based contact layer combined with thermal shock allows exfoliation of large-area III-N LED structures from sapphire.

Anisotropic structural and optical properties of semi-polar (11-22) GaN grown on m-plane sapphire using double AlN buffer layers.

We report the anisotropic structural and optical properties of semi-polar (11–22) GaN grown on m-plane sapphire using a three-step growth method which consisted of a low temperature AlN buffer layer, followed by a high temperature AlN buffer layer and GaN growth. By introducing double AlN buffer layers, we substantially improve the crystal and optical qualities of semi-polar (11–22) GaN, and significantly reduce the density of stacking faults and dislocations. The high resolution x-ray diffraction measurement revealed that the in-plane anisotropic structural characteristics of GaN layer are azimuthal dependent. Transmission electron microscopy analysis showed that the majority of dislocations in the GaN epitaxial layer grown on m-sapphire are the mixed-type and the orientation of GaN layer was rotated 58.4° against the substrate. The room temperature photoluminescence (PL) spectra showed the PL intensity and wavelength have polarization dependence along parallel and perpendicular to the [1–100] axis (polarization degrees ~ 0.63). The realization of a high polarization semi-polar GaN would be useful to achieve III-nitride based lighting emission device for displays and backlighting.

Influence of the TMAl source flow rate of the high temperature AlN buffer on the properties of GaN grown on Si(111) substrate

GaN epilayers have been grown on Si(111) substrates with various high temperature AlN buffer layers by metal-organic chemical vapor deposition (MOCVD). It is found that, the surface morphology and structural and optical properties of the GaN epilayer strongly depends on the TMAl source flow rate. The properties of GaN films were characterized by X-ray diffraction, atomic force microscopy, Raman, photoluminescence and cathodoluminescence measurements. With the optimized TMAl source flow rate, we were able to obtain a 1-μm-thick crack-free GaN layer. The (0002) and (101¯2) XRD FWHM of the GaN film are 547 and 563 arcsec respectively, the tensile stress calculated from the Raman spectra is 0.4 GPa, and RMS roughness of AFM 5 μm × 5 μm scan is 0.539 nm.

Influence of AlN buffer layer thickness on material and electrical properties of InAlN/GaN high-electron-mobility transistors

The influence of the thickness of a high-temperature AlN (HT-AlN) buffer layer on the properties of an InAlN/GaN high-electron-mobility transistor (HEMT) grown on a sapphire substrate was investigated. As revealed by atomic force microscope analysis, a rougher surface and larger grain size were observed with a thicker buffer layer. The larger grains promoted the two-dimensional (2D) growth mode of the GaN layer at the initial growth stage. This suppressed oxygen incorporation at the GaN/HT-AlN interface and thus improved the resistivity of the GaN layer. Moreover, the lower grain density also resulted in enhanced GaN crystal quality of the GaN layer. As a consequence, the electrical properties of the InAlN/GaN HEMT device, such as output current, transconductance and off-state breakdown voltage, were improved by increasing the HT-AlN buffer layer thickness.

Specific features of the hydride vapor-phase epitaxy of nitride materials on a silicon substrate

Specific features of the growth of the GaN/AlN/Si heterocomposite in which layers of Group-III element nitrides are grown on a silicon substrate by hydride vapor-phase epitaxy are studied. The effect of the temperature at which the AlN buffer layer is grown on diffusion processes at the heterointerfaces and on the quality of the epitaxial layers being grown is considered. It is shown that, with the epitaxial technique used, the buffer layer should be grown at high temperatures (1080°C) because the thickness of the component-mixing region is minimized in this case and abrupt interfaces are formed in the GaN/AlN/Si heterocomposite. The double-stage growth of gallium nitride on the high-temperature AlN buffer layer with a thickness of 300–400 nm makes it possible to obtain GaN layers with thicknesses of up to 0.3 μm without crack formation.

Properties of N-Polar GaN Films Grown by MOCVD on C-Face 6H-SiC Substrate

High quality N-polar GaN films were grown by MOCVD on C-face 6H-SiC substrate etched by KOH eutectics at 300°C for 5min. A 50 nm high-temperature AlN buffer layer was used to release the lattice mismatch stress between GaN and SiC substrate. The N-polar GaN films grow on the C-face 6H-SiC substrate etched by KOH eutectics have a high crystalline quality and smooth surface. The full-width-at-half-maximum (FWHM) of (002) and (102) XRD spectra were 347.4 and 501.5 arcsec., respectively. The root-mean-square (RMS) of 3μm×3μm is 0.666 nm. The luminescence property of the N-polar GaN films was studied. The band-edge emission of photoluminescence spectra was at 364.6 nm with weak yellow luminescence near 560 nm. The stress in the films was also calculated from Raman spectra. The results indicated a tensile stress in the N-polar GaN films, which is mainly caused by mismatch of thermal expansion coefficient between GaN and SiC substrate.

Effects of high-temperature AIN buffer on the microstructure of AlGaN/GaN HEMTs

Effects on AlGaN/GaN high-electron-mobility transistor structure of a high-temperature AlN buffer on sapphire substrate have been studied by high-resolution x-ray diffraction and atomic force microscopy techniques. The buffer improves the microstructural quality of GaN epilayer and reduces approximately one order of magnitude the edge-type threading dislocation density. As expected, the buffer also leads an atomically flat surface with a low root-mean-square of 0.25 nm and a step termination density in the range of 108 cm−2. Due to the high-temperature buffer layer, no change on the strain character of the GaN and AlGaN epitaxial layers has been observed. Both epilayers exhibit compressive strain in parallel to the growth direction and tensile strain in perpendicular to the growth direction. However, an high-temperature AlN buffer layer on sapphire substrate in the HEMT structure reduces the tensile stress in the AlGaN layer.

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.

Ultra-Low Dark Current AlGaN-Based Solar-Blind Metal–Semiconductor–Metal Photodetectors for High-Temperature Applications

Solar-blind metal-semiconductor-metal (MSM) photodetectors (PDs) with Ni/Au semi-transparent interdigitated contact electrodes are fabricated on Al0.4Ga0.6N epi-layer grown by metal-organic chemical vapor deposition on sapphire substrate. The PD exhibits ultra-low dark current in fA range at room temperature (RT) under 20-V bias and a corresponding breakdown voltage higher than 300 V. The PD also shows a maximum RT quantum efficiency of 64% at ~ 275 nm under 10-V bias with a solar-blind/ultraviolet (UV) rejection ratio up to four orders of magnitude. Even at a high temperature of 150°C, the dark current of the PD is still in fA range with a reasonable rejection ratio of more than 8000, suggesting its potential applications for high-temperature deep-UV detection. The ultra-low dark current achieved is believed to be related to the high-temperature AlN buffer layer used in the epi-structure as well as the coplanar configuration of the MSM PD itself.

TEM study of defects in Al xGa 1− xN layers with different polarity

Transmission electron microscopy (TEM) studies of defects in Al x Ga 1− x N layers with various Al mole fractions ( x=0.2, 0.4) and polarities were carried out. The samples were grown by ammonia molecular beam epitaxy on sapphire substrates and consisted of low-temperature AlN (LT-AlN) and high-temperature AlN (HT-AlN) buffer layers, a complex AlN/AlGaN superlattice (SL) and an Al x Ga 1− x N layer ( x=0.2, 0.4). It was observed that at the first growth stages a very high density of dislocations is introduced in both Al-polar and N-polar structures. Then, at the interface of the LT-AlN and HT-AlN layers half-loops are formed and the dislocation density considerably decreases in Al-polar structures, whereas in the N-polar structures such a behavior was not observed. The AlN/AlGaN superlattice efficiently promotes the bend and annihilation of threading dislocations and respectively the decrease of the dislocation density in the upper Al x Ga 1− x N layer with both polarities. The lattice relaxation of metal-polar Al 0.2Ga 0.8N was observed, while N-polar Al 0.2Ga 0.8N did not relax. The dislocation densities in the N-polar Al 0.2Ga 0.8N and Al 0.4Ga 0.6N layers were 5.5×10 9 cm −2 and 9×10 9 cm −2, respectively, and in metal-polar Al 0.2Ga 0.8N and Al 0.4Ga 0.6N layers these were 1×10 10 cm −2 and 6×10 9 cm −2, respectively. Moreover, from TEM images the presence of inversion domains (IDs) in N-polar structures has been observed. The widths of IDs varied from 10 to 30 nm. Some of the IDs widen during the growth of the AlN buffer layers. The IDs formed hills on the surface of the N-polar structures.

Correlations of a High-temperature AlN Buffer Layer and Al-Preseeding with the Structural and the Optical Properties of GaN on a Si(111) Substrate

Correlations of a High-temperature AlN Buffer Layer and Al-Preseeding with the Structural and the Optical Properties of GaN on a Si(111) Substrate