Growth Of AlN Films Research Articles

Effect of flux rate on the growth of AlN films on sapphire by MNVPE

Effect of flux rate on the growth of AlN films on sapphire by MNVPE

Growth of SiC, AlN, and GaN Films on Silicon Parts of Arbitrary Geometry for Microelectromechanical Applications

Growth of SiC, AlN, and GaN Films on Silicon Parts of Arbitrary Geometry for Microelectromechanical Applications

Three-step growth of AlN films on sapphire substrates by metal nitride vapor phase epitaxy

Control of the growth process is important for growing high-quality AlN films. In this work, AlN films were grown on c-plane sapphire substrates by metal nitride vapor phase epitaxy (MNVPE). Interlayer was added between the buffer layer and the formal layer, and the effect of the Ⅴ/Ⅲ ratio of the interlayer on the crystal quality, surface morphology and stress evolution of the films were analyzed. An interlayer with proper Ⅴ/Ⅲ ratio can provide a stable transition from 3D growth mode to 2D growth mode, while high Ⅴ/Ⅲ ratio induce film separation. The optimum Ⅴ/Ⅲ ratio for the interlayer was found to be 11050, and the films obtained under this parameter had a FWHM of the (002) and (102) reflections were 641 arcsec and 840 arcsec, respectively. The maximum transmission is 88%. And the film separation phenomenon was found on the samples with high V/III ratio in the second step, which was thought to be the result of growing in 3D mode for a long time.

Effect of two-step cubic GaN buffer layer on the radio-frequency plasma-assisted molecular beam epitaxy growth of cubic AlN films grown on MgO (001) substrates

Cubic AlN (c-AlN) films were grown on MgO (001) substrates through the use of a two-step cubic GaN (c-GaN) buffer layer using radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE) technique. The two-step c-GaN buffer layer was found to be crucial for the epitaxial growth of c-AlN, as it showed a specific epitaxial relationship between c-AlN and c-GaN along the MgO [100] zone axis. On the other hand, the c-AlN films grown with the one-step and nonGaN buffer layers showed columnar grain structure. The optical study found that the c-AlN films grown with the two-step c-GaN buffer layer had an indirect bandgap energy of 4.99 ± 0.02 eV, which is consistent with previous experimental data and theoretical calculations. The c-AlN films grown with the one-step and nonGaN buffer layers had indirect bandgap energies of 5.00 ± 0.02 eV and 5.18 ± 0.02 eV, respectively. These results highlight the importance of the two-step c-GaN buffer layer in enhancing the MBE growth of c-AlN on MgO (001) substrate.

High-Quality AlN Grown by a Combination of Substrate Pretreatment and Periodic Growth Mode Control

The growth of high-quality AlN films has always been the focus of research. Combined with NH3 in situ pretreatment before growth, with optimized growth conditions, we demonstrated a periodic growth method, which alternates between three-dimensional (3D) growth mode and two-dimensional (2D) growth mode, to grow a thick crack-free AlN film with good crystal quality on the on-axis 4H-SiC substrate by metal–organic chemical vapor deposition (MOCVD). X-ray rocking curves (XRCs) indicated that full width at half-maximum (FWHM) of the (0002) and (10–12) reflections were 241.3″ and 474.4″, respectively. The film thickness was over 1.4 μm. Lattice constant and residual stress results showed residual compressive stress in the sample. The periodic two-step growth and substrate pretreatment both play an important role in controlling the residual stress of films to avoid cracking. This work can serve as an important reference for growing high-quality crack-free AlN on SiC.

MOVPE growth of AlN and AlGaN films on N-polar annealed and sputtered AlN templates

We performed metalorganic vapor phase epitaxy (MOVPE) growth of AlN (MOVPE-AlN) and AlGaN films on N-polar face-to-face annealed and sputtered AlN templates (N-polar FFA Sp-AlN) on sapphire substrates. The off-cut angle of the sapphire substrates was varied from 0.2° to 6.0°. The effects of the off-cut angle on the surface morphology and crystallinity of the N-polar MOVPE-AlN and AlGaN were elucidated. The results showed that the crystallinities of N-polar FFA Sp-AlN and MOVPE-AlN were independent of the off-cut angle. The threading dislocation density (TDD) was estimated to be approximately 5.0 × 108cm−2 by using values of the X-ray rocking curve full-width at half maximum. In comparison, the density of the hillocks on the MOVPE-AlN surfaces was suppressed, and the surface flatness was improved by increasing the off-cut angle of the sapphire substrate. The crystallinity of the AlGaN was improved by increasing the off-cut angle. Our results prove that N-polar AlN and AlGaN films with smooth surfaces and low TDDs could be achieved by MOVPE on N-polar FFA Sp-AlN with a large substrate off-cut angle of 6.0°.

Heteroepitaxy Growth and Characterization of High-Quality AlN Films for Far-Ultraviolet Photodetection.

The ultra-wide bandgap (~6.2 eV), thermal stability and radiation tolerance of AlN make it an ideal choice for preparation of high-performance far-ultraviolet photodetectors (FUV PDs). However, the challenge of epitaxial crack-free AlN single-crystalline films (SCFs) on GaN templates with low defect density has limited its practical applications in vertical devices. Here, a novel preparation strategy of high-quality AlN films was proposed via the metal organic chemical vapor deposition (MOCVD) technique. Cross-sectional transmission electron microscopy (TEM) studies clearly indicate that sharp, crack-free AlN films in single-crystal configurations were achieved. We also constructed a p-graphene/i-AlN/n-GaN photovoltaic FUV PD with excellent spectral selectivity for the FUV/UV-C rejection ratio of >103, a sharp cutoff edge at 206 nm and a high responsivity of 25 mA/W. This work provides an important reference for device design of AlN materials for high-performance FUV PDs.

Thermodynamic Analysis of Group-III-Nitride Alloying with Yttrium by Hybrid Chemical Vapor Deposition.

Group-IIIb-transition-metal-alloyed wurtzite Group-IIIa-nitride (IIIb-IIIa-N) thin films have higher piezoelectric characteristics than binary IIIa-N for a broad range of applications in photonic, electronic, sensing, and energy harvesting systems. We perform theoretical thermodynamic analysis for the deposition and epitaxial growth of Y-alloyed GaN and AlN films by a newly introduced growth technique of hybrid chemical vapor deposition (HybCVD), which can overcome the limitations of the conventional techniques. We investigate the equilibrium vapor pressures in the source zones to determine the dominant precursors of cations for the input of the mixing zone. Then, we study the driving force for the vapor-solid phase reactions of cation precursors in the growth zone to calculate the relationship between the solid composition of YxGa1-xN and YxAl1-xN and the relative amount of input precursors (Y vs. GaCl and AlCl3) in different deposition conditions, such as temperature, V/III precursor input ratio, and H2/inert-gas mixture ratio in the carrier gas. The xY composition in YAlN changes nearly linearly with the input ratio of cation precursors regardless of the growth conditions. However, YGaN composition changes non-linearly and is also substantially affected by the conditions. The thermodynamic analysis provides insight into the chemistry involved in the epitaxial growth of IIIa-IIIb-N by the HybCVD, as well as the information for suitable growth conditions, which will guide the way for ongoing experimental efforts on the improvement of piezoelectricity of the lead-free piezoelectric materials.

The ultra-thin AlN epitaxy on monolayer WS2 by helicon sputtering at 400 °C

This work demonstrates the epitaxial growth of ultra-thin AlN films grown on monolayer WS2/Sapphire by helicon sputtering. In our work, WS2 acts as a template to facilitate the initial two-dimensional (2D) AlN growth via van der Waals epitaxy (VDWE), and this growth method provides a promising semiconductor manufacturing process with low thermal budget (<450 °C) than the required for the backend-of-the-line (BEOL). The ultrathin AlN epitaxy on WS2 is characterized by synchrotron-based grazing-incidence wide-angle x-ray scattering (GIWAXS). There are two stages in AlN epitaxy on the WS2; the first is the layered AlN growth aligned with the underlying WS2 lattice. The subsequent development of AlN transforms into 3D growth, which is similar to Stranski–Krastanov (SK) mode. The ultrathin AlN films achieve atomic level smoothness and significant piezoelectric properties as indicated by the technique of atomic- and piezo force microscopy (AFM/PFM). This research broadens the application of 2D materials in assisting AlN epitaxy and has a potential avenue for hetero-epitaxial integration in developing future electronic devices.

Temperature dependence of crystal growth behavior of AlN on Ni–Al using electromagnetic levitation and computer vision technique

Bulk AlN single crystal has a great demand as a substrate material for AlGaN-based optical and electronic devices such as deep ultraviolet light-emitting diodes and high-power transistors. We have previously proposed a novel solution-growth method using Ni–Al solution with an in-situ observation system for solution growth of AlN crystal using electromagnetic levitation. In this paper, to investigate AlN formation behavior on 40 mol%Al–Ni droplets, two precisely synchronized high-speed cameras from the horizontal and vertical directions were installed. AlN formation behavior was evaluated quantitatively using computer vision image processing techniques. Based on the results, we demonstrated the growth of thick AlN film at 2030 K for 1 h. A 3.8-μm-thick c-axis oriented AlN film successfully formed on the droplet, and the film was also oriented in-plane. The + c-direction AlN film grew towards the droplet center from the surface by reacting dissolved nitrogen and Al atoms.

Growth of Thin AlN Films on Si Wafers by Reactive Magnetron Sputtering: Role of Processing Pressure, Magnetron Power and Nitrogen/Argon Gas Flow Ratio

AlN is a wide band gap semiconductor that is of growing industrial interest due to its piezoelectric properties, high breakdown voltage and thermal conductivity. Using magnetron sputtering to grow AlN thin films allows for high deposition rates and uniform coverage of large substrates. One can also produce films at low substrate temperatures, which is required for many production processes. However, current models are inadequate in predicting the resulting structure of a thin film when different sputter parameters are varied. In this work, the growth of wurtzite AlN thin films has been carried out on Si(111) substrates using reactive direct current magnetron sputtering. The influence of the processing pressure, magnetron power and N2/Ar ratio on the structure of the grown films has been analyzed by investigating crystallinity, residual film stress and surface morphology using X-ray diffraction, profilometry, atomic force microscopy and scanning electron microscopy. In every case, the films were found to exhibit c-axis orientation and tensile stress. It was found that high-quality AlN films can be achieved at an N2/Ar ratio of 50% and a low pressure of 0.2 Pa. High magnetron powers (900–1200 W) were necessary for achieving high deposition rates, but they led to larger film stress.

Adsorption properties of Al, Ga, and N related particles on GaN substrate surface by first principle calculations

GaN is promising for fabricating high-quality substrate for growth of AlN and AlGaN film. To study the microscopic adsorption and bonding mechanism of AlN and AlGaN film growth on GaN substrate, the first principle calculations are applied to study the adsorption Al atom, Ga atom, N atom, AlN molecules, and GaN molecules on GaN substrate. The adsorption energies, adsorption states, bonding properties, electronic local functions, and density of states are analyzed. The results reveal that N atom has the lowest adsorption energy and the strongest capacity among above five adsorption particles. And the top side of N atom on GaN substrate surface is the favorable adsorption site for all particles. Moreover, the adsorbed Al atom, Ga atom, N atom, AlN molecules, and GaN molecules all form chemical bonds with substrate atoms. The adsorbed Al atom, Ga atom, and N atom all form a covalent bond connection with the atoms in GaN substrate while the AlN molecules and GaN molecules are connected by an ionic bond with GaN substrate. Furthermore, the total density of state distribution and band gap of GaN substrate have change after adsorption of particles, which further verified that the chemical adsorption occurred after the adsorption of the above particles. The density of states of AlN molecule with Al atom closer to GaN substrate and GaN molecule Ga atom closer to GaN substrate on GaN surface match the most with GaN substrate,indicating that the lattice matches well when AlN molecule with Al atom closer to GaN substrate and GaN molecule Ga atom closer to GaN substrate adsorbed on GaN substrate.

Effect of V–III Ratio‐Based Growth Mode on the Surface Morphology, Strain Relaxation, and Dislocation Density of AlN Films Grown by Metal‐Organic Chemical Vapor Deposition

Herein, regulation of the growth mode through manipulating the V–III ratio during metal‐organic chemical vapor deposition (MOCVD) growth of AlN films on c‐plane sapphire substrates and optimization of the crystalline quality and surface morphology of AlN films are demonstrated. Based on detailed analyses of the evolution process of surface morphology, dislocation annihilation, and stress state of the AlN film during MOCVD growth, it is found that 2D rather than 3D growth mode introduces tensile stress as well as promotes edge‐type threading dislocation (ETD) propagation and crack generation, while 3D growth mode produces rough surface. These results would help to further understand the AlN growth mechanism and provide important guidance on the preparation of AlN‐related epitaxial structure.

Magnetron sputtering growth of AlN film for photocatalytic CO2 reduction

Magnetron sputtering growth of AlN film for photocatalytic CO2 reduction

Effect of reactive gas condition on nonpolar AlN film growth on MnS/Si (100) by reactive DC sputtering

The effects of reactive gas flow conditions on nonpolar AlN film growth on MnS/Si (100) substrates using reactive DC magnetron sputtering were investigated. During AlN deposition at a substrate temperature of 750 °C, the MnS surface can be unintentionally nitrided, resulting in a decrease in the crystallinity of the AlN. Low-temperature growth of the AlN layer at 300 °C prevents this nitridation and results in the crystallization of nonpolar AlN. A N2 flow equal to 30% of the Ar sputtering gas flow was found to improve the crystallinity of the nonpolar AlN and to reduce nitrogen defects, which play an important role in interfacial reactions. Nitrogen defects promote the formation of alloys such as AlMn and MnSi that degrade the interface and can significantly decompose the MnS. A higher proportion of N2 improves the nonpolar AlN crystallinity, reduces the concentration of defects and suppresses reactions at the AlN/MnS interface.

Porous AlN films grown on C-face SiC by hydride vapor phase epitaxy

We report the growth of porous AlN films on C-face SiC substrates by hydride vapor phase epitaxy (HVPE). The influences of growth condition on surface morphology, residual strain and crystalline quality of AlN films have been investigated. With the increase of the V/III ratio, the growth mode of AlN grown on C-face 6H-SiC substrates changes from step-flow to pit-hole morphology. Atomic force microscopy (AFM), scanning electron microscopy (SEM) and Raman analysis show that cracks appear due to tensile stress in the films with the lowest V/III ratio and the highest V/III ratio with a thickness of about 3 μm. In contrast, under the medium V/III ratio growth condition, the porous film can be obtained. Even when the thickness of the porous AlN film is further increased to 8 μm, the film remains porous and crack-free, and the crystal quality is improved.

Possible ferromagnetism or antiferromagnetism in defective AlN via strain engineering: Hybrid versus GGA calculations

Extensive calculations with hybrid and GGA functionals of the electronic and magnetic properties in cubic AlN for the charged Al and N vacancies are carried out to study the formation energies, the defect equilibrium the induced localized magnetism, the type and range of the magnetic interactions and the possible realization of the magnetic percolation via epitaxial strain engineering during the coherent growth of AlN films. Our calculations are based on the plane wave pseudo-potential spin polarized density functional theory. Our calculations with hybrid and GGA show that only Al vacancies can induce localized magnetic moments. Point defect equilibrium calculation shows that among all the charged Al vacancies, the charge-neutral is the stablest moment carrying state. Study of the collective magnetism between vacancy pairs using the Heisenberg model shows that these defects couple ferromagnetically for the first and third FCC distances and antiferromagnetically for the fourth distance. Thermodynamic considerations show that we cannot achieve magnetic percolation at equilibrium conditions since the minimal vacancy concentration needed for wall to wall percolation is many order of magnitude higher that the equilibrium calculated concentration. Nevertheless, our work shows that we can drastically enhance the vacancy concentration during the coherent growth of AlN films via a tensile epitaxial strain.

Bipolar pulsed reactive magnetron sputtering of epitaxial AlN- films on Si(111) utilizing a technology suitable for 8″ substrates

This paper discusses the development of a reactive sputtering process for the deposition of epitaxially grown AlN films on Si(111) using a technology suitable for 8″ substrates. X-ray diffraction (XRD) and rocking curves (RC) are used to determine the crystalline orientation. The influence of substrate temperature, target-substrate distance and target voltage on the full width at half maximum (FWHM) of the RC is investigated. The film deposition was performed on Si(111) wafers and on AlN-on-Si(111) templates prepared by metal organic chemical vapor deposition (MOCVD). Based on the sixfold symmetry identifiable in the pole figure of the AlN(302) plane reflections, oriented in-plane film growth on Si(111) was verified for the process. Best oriented films sputtered directly on Si(111) achieved for 500 nm AlN films featured a RC-FWHM of 0.93° of AlN(100) and 0.77° of AlN(002). Root mean square roughness (RMS) of the samples varied between 2.3 nm and 2.8 nm. The deposition rate was between 70 nm/min and 100 nm/min. Films deposited onto the MOCVD templates grew maintaining the crystalline quality of the MOCVD substrate, verified by the FWHM of the RC of in-plane and out-of-plane reflections. • Sputter epitaxy of AlN on Si(111) using a a technology suitable for 8″ substrates • Process at low temperature (850 °C) featuring high deposition rates (1.3 nm/s) • Investigating influences of temperature, target-substrate distance, target voltage • Improvement of crystalline orientation achieved • Indication of local optimum of crystalline quality regarding target voltage

(Invited) Deposition of Crystalline AlN on Si and SiC Using Atomic Layer Annealing and Pulsed ALD/CVD

The low-temperature (< 400 oC) deposition of polycrystalline AlN films on silicon and silicon carbide is demonstrated by atomic layer annealing (ALA) using tris(dimethylamido) aluminum (TDMAA) and anhydrous N2H4 with an argon plasma treatment utilizing a DC or RF bias to tune the ion energy. ALA is a variant of ALD in which after the N2H4 and TDMAA are dose, a pulse of low energy rare ions ions is employed to induce crystalinity. Using TDMAA and N2H4 + Ar+ or Kr+ high-quality AlN films are deposited with large grain size and low oxygen/carbon contamination which can be used as a templating layer for further high speed AlN film growth via sputtering. The deposition of high quality AlN films deposited by ALA are successfully used as templates for sputtered AlN resulting in a >2x improvement in average grain size when compared to an analogous amorphous template layer. TEM imaging shows a crystalline interface between the AlN and Si(111) which is ideal for heat transfer. For SiC, DC bias is not effective due to the large bandgap so ALA must be performed with RF bias to accelerate the ions. Alternative high temperature (580oC thermal ALD with a CVD component using N2H4 and TDMAA can be employed to grow extremely large grain on SiC which can act as a template or rapid growth of highly crystalline AlN by sputtering.

Effect of MOVPE growth conditions on AlN films on annealed sputtered AlN templates with nano-striped patterns

The metalorganic vapor phase epitaxial (MOVPE) growth of AlN films was performed on face-to-face annealed sputtered AlN templates (FFA Sp-AlN) having a nano-striped pattern with a period of 300 nm and depth of 120 nm. Then, the effect of MOVPE growth conditions on the crystallinity was elucidated. The faceted structure clearly changed with the growth temperature (Tg), following the same trend as the epitaxial lateral overgrowth of GaN film on micro-patterned GaN. High Tg levels enhanced the lateral growth, and thus with a high Tg of 1300 °C, the coalescence in the early growth stage resulted in AlN films with coalesced surfaces and a thickness of 1 µm. For the AlN film grown at a Tg of 1300 °C, the threading dislocation density (TDD) was estimated to be 6.0 × 108 cm−2. This value proved that coalesced AlN films on nano-patterned FFA Sp-AlN templates with low TDDs could be achieved with the appropriate MOVPE growth conditions. The compressive strain in the AlN film on nano-patterned FFA Sp-AlN was lower than that of the film grown on FFA Sp-AlN without a pattern.