High-quality AlN Research Articles

Influences of Powder Source Porosity on Mass Transport during AlN Crystal Growth Using Physical Vapor Transport Method

We developed a two-dimensional (2D) transport model to investigate mass transport during bulk AlN crystal growth via the physical vapor transport (PVT) process using the finite element method (FEM), taking the powder source porosity, buoyancy, and vapor diffusion into account. The porosity effects of the powder source on mass transport under various growth conditions were investigated in detail. The simulation results show that the porosity of the powder source significantly affects the mass transport process during AlN sublimation growth. When the porosity of the powder source decreases, the growth rate becomes more uniform along the seed deposition surface, although the sublimation rate and crystal growth rate decrease, which can be attributed to the reduced specific surface area of the powder source and the reduced flow rate of Al vapor in the powder source. A flat growth interface can be achieved at a porosity of 0.2 under our specific growth conditions, which in turn facilitate the growth of high-quality AlN crystals and better yield. The decomposition of the powder source and the transport of Al vapor in the growth chamber can be suppressed by increasing the pressure. In addition, the AlN growth rate variation along the deposition surface can be attributed to the Al vapor pressure gradient caused by the temperature difference in the growth chamber.

Extremely high internal quantum efficiency of AlGaN-based quantum wells on face-to-face annealed sputter-deposited AlN templates

This study investigated the influence of high-quality AlN templates on the internal quantum efficiency (IQE) of AlGaN-based multiple quantum wells (MQWs) using photoluminescence spectroscopy. An extremely high IQE of 90% at room temperature was obtained from MQWs on face-to-face annealed sputter-deposited AlN/sapphire templates. The dependence of efficiency curves on temperature indicated that nonradiative recombination centers were almost fully saturated under higher excitation power densities even at 400 K. Moreover, analysis of the efficiency curves using a rate equation model based on exciton recombination processes suggested that radiative recombination was the dominant recombination process even at 400 K.

Transfer-free graphene-guided high-quality epitaxy of AlN film for deep ultraviolet light-emitting diodes

We report on quasi-van der Waals epitaxy of high-quality AlN film guided by transfer-free graphene (Gr) with low wrinkle density (LWD-Gr) on sapphire. The surface wrinkle density of LWD-Gr is greatly reduced by 94% compared to conventional Gr with high wrinkle density (HWD-Gr). Also noteworthy is that AlN nucleation islands grown on LWD-Gr exhibit better crystalline orientation consistency and more feasibility to coalesce with each other, thus forming AlN film with lower stress and dislocation density. Finally, 285 nm deep ultraviolet (DUV) light-emitting diodes are fabricated on the high-quality AlN template with LWD-Gr, which shows stronger electroluminescence intensity than its counterpart without and with HWD-Gr. The insights granted by this research pave a new pathway for improving the performance of nitride-based DUV optoelectronic devices through Gr engineering.

(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.

(Invited) Complex Relative Permittivity of UV-C Transparent AlN

The optical absorption coefficients of high-quality 2-inch AlN substrates grown by physical vapor transport (PVT) were accurately calculated from transmittance and absolute reflectance measurements in the ultraviolet-visible (UV-Vis) range. Spatially uniform UV absorption coefficients of 13-23 cm-1 at 4.68 eV (265 nm) were demonstrated. This approach also provided accurate values for the reflection coefficients, allowing a determination of the complex refractive index and relative permittivity, which showed good agreement with previous reports. Values for the high-frequency and static permittivities obtained in this study were 4.22 and 7.84, respectively. The data specific to PVT AlN substrates obtained using this method are useful in the design of electronic and optoelectronic devices grown on these substrates.

Polarity tuning of crystalline AlN films utilizing trace oxygen involved sputtering and post-high-temperature annealing

The polarity tuning of AlN films via introducing a period of trace oxygen supply in the initial sputtering process was demonstrated. High-temperature annealing was further implemented to improve the crystal quality of AlN. High-quality pure N-polar AlN film could be achieved by sputtering with pure N2 gas. When trace oxygen was intentionally inputted in the sputtering chamber and its supply time surpassed 150 s, the AlN surface polarity transmitted to pure Al-polar. After optimization, the full widths at half of maximum of X-ray rocking curves for (0002)/(10-12) reflection were improved to 41.3/132.5 arcsec for N-polarity and 38.2/158.7 arcsec for Al-polarity.

Demonstration of lateral epitaxial growth of AlN on Si (1 1 1) at low temperatures by pulsed reactive sputter epitaxy

We present a pulsed reactive magnetron sputter process for high quality AlN on Si (111) beneficially avoiding any high-temperature growth. Initially, metallic aluminium with a nominal thickness of about one monolayer is deposited at a substrate temperature around 850 °C in an Ar plasma followed by sputtering in an Ar/N plasma. For 250 nm thick AlN layers a surface roughness below 0.2 nm rms is obtained as determined by atomic force microscopy (AFM). Using an Al nucleation step prior to AlN growth substantially improves the crystalline properties of AlN. The FWHM values of the AlN (0 0 0 2) and the AlN (1 0 1 0) diffraction peaks with 0.45° and 0.86°, respectively, are comparable to state-of-the-art AlN on Si layers grown by metalorganic vapour phase epitaxy (MOVPE). Two different N-precursor gases, namely N2 and NH3, lead to distinct layer qualities as revealed by atomic force microscopy and transmission electron microscopy. Only with NH3 substantial lateral growth can be achieved at T = 850 °C which is mandatory to obtain smooth surface morphologies. In MOVPE such lateral AlN growth is typically only achieved at high growth temperatures (T > 1000 °C).

Strain management and AlN crystal quality improvement with an alternating V/III ratio AlN superlattice

We report the metal-organic chemical vapor deposition growth of high-quality AlN on sapphire enabled by an alternating V/III ratio AlN superlattice. We demonstrated that the insertion of an alternating V/III ratio AlN superlattice facilitates the relaxation of tensile stress during growth and assists the annihilation of threading dislocations. Dislocation inclination was proposed to play a major role in the stress relaxation and dislocation reduction, which not only provides an effective misfit-dislocation component but also increases the dislocation reaction probability. By this method, crack-free 3.25 μm-thick AlN films were grown on flat sapphire substrate (FSS) and nano-patterned sapphire substrate (NPSS) at 1180 °C, and the full width at half maximum of (002)/(102) rocking curves was 204/408 and 152/323 arcsec for such AlN/FSS and AlN/NPSS templates.

2DEGs formed in AlN/GaN HEMT structures with AlN grown at low temperature

Integration of nitrides with other material systems has recently become of interest due to the high performance of GaN-based high-electron mobility transistors. However, the elevated growth temperatures often used to grow high quality AlN pose challenges toward metalorganic chemical vapor deposition (MOCVD) on temperature sensitive substrates such as processed wafers. In this work, the growth of AlN was conducted at temperatures below 550 °C via MOCVD using a flow-modulated epitaxy scheme, and their morphological, compositional, and electronic properties of these films were investigated. Sheet charges up to 2.1 × 1013 cm−2 and mobilities on the order of 400 cm2/V s were measured for two dimensional electron gases, which formed at the interface between the low temperature grown AlN layers and the semi-insulating GaN base layers deposited at high temperatures. Despite their low growth temperatures, nominally pure AlN barrier layers exhibited measurable unintentional gallium incorporation adjacent to the GaN interface. The result sets the stage for the integration of nitride-based electronics via epitaxy-based schemes on temperature sensitive substrates.

Morphology and crystalline property of an AlN single crystal grown on AlN seed

AlN single crystal grown by physical vapor transport (PVT) using homogeneous seed is considered as the most promising approach to obtain high-quality AlN boule. In this work, the morphology of AlN single crystals grown under different modes (3D islands and single spiral center) were investigated. It is proved that, within an optimized thermal distribution chamber system, the surface temperature of AlN seed plays an important role in crystal growth, revealing a direct relationship between growth mode and growth condition. Notably, a high-quality AlN crystal, with (002) and (102) reflection peaks of 65 and 36 arcsec at full width at half maximum (FWHM), was obtained grown under a single spiral center mode. And on which, a high-quality Al x Ga1– x N epitaxial layer with high Al content (x = 0.54) was also obtained. The FWHMs of (002) and (102) reflection of Al x Ga1– x N were 202 and 496 arcsec, respectively, which shows superiority over their counterpart grown on SiC or a sapphire substrate.

Annihilation and Regeneration of Defects in (112̅2) Semipolar AlN via High-Temperature Annealing and MOVPE Regrowth

Semipolar III-nitrides have attracted great attention due to their weak polarization field for optoelectronic devices. High-quality AlN is a perfect template in the epitaxial growth of AlGaN-based ...

High Breakdown Voltage and Low-Current Dispersion in AlGaN/GaN HEMTs With High-Quality AlN Buffer Layer

We have successfully grown AlGaN/GaN high electron mobility transistor (HEMT) structure on the high-quality undoped thick AlN buffer layer with large band offset to replace the conventional high-resistivity GaN buffer layer. The AlGaN/GaN HEMT fabricated on this AlN buffer layer exhibits low OFF-state leakage current with high ${I}_{\mathrm{\scriptscriptstyle ON}}/{I}_{\mathrm{\scriptscriptstyle OFF}}$ of $\sim 10^{{6}}$ due to enhanced confinement of the electrons in the 2-D electron gas (2-DEG) channel. The undoped AlN buffer layer is responsible for suppressing the trapping effects to greatly reduce the current dispersion in pulsed ${I}_{\text {D}}$ – ${V}_{\text {D}}$ characteristics, which is hardly avoided in conventional deep acceptor-doped GaN buffer layer. The device also demonstrates high breakdown voltage of 2154 V with very high figure of merit (FOM) of ~1.8 GV $^{2-1}$ cm $^{-{2}}$ , one of the highest ever reported, suggesting that the AlGaN/GaN-based Hemts WITH AlN buffer layer are promising for high-performance RF and power applications.

Void containing AlN layer grown on AlN nanorods fabricated by polarity selective epitaxy and etching method

Creating voids between thin films is a very effective method to improve thin film crystal quality. However, for AlN material systems, the AlN layer growth, including voids, is challenging because of the very high Al atom sticking coefficient. In this study, we demonstrated an AlN template with many voids grown on AlN nanorods made by polarity selective epitaxy and etching methods. We introduced a low V/III ratio and NH3 pulsed growth method to demonstrate high-quality coalesced AlN templates grown on AlN nanorods in a metal organic chemical vapor deposition reactor. The crystal quality and residual strain of AlN were enhanced by the void formations. It is expected that this growth method can contribute to the demonstration of high-performance deep UV LEDs and transistors.

Tris(dimethylamido)aluminum(III) and N2H4: Ideal precursors for the low-temperature deposition of large grain, oriented c-axis AlN on Si via atomic layer annealing

The low-temperature (≤400 oC) deposition of polycrystalline AlN films on silicon is demonstrated by atomic layer annealing (ALA) using either trimethyl aluminum (TMA) and anhydrous hydrazine (N2H4) or tris(dimethylamido) aluminum (TDMAA) and anhydrous N2H4 with an argon plasma treatment utilizing a DC bias to tune the ion energy. Using TDMAA and N2H4, 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.

Growth mechanism on graphene-regulated high-quality epitaxy of flexible AlN film

We report a novel diffusion–adsorption regulation growth method in the epitaxy of AlN on graphene for the high-quality and transferable large-size AlN film.

High-Quality A1GaN Epitaxy on Lattice-Engineerable AlN Template for High-Power UVC Light-Emitting Diodes

AlGaN-based UVC light-emitting diodes (LED) were fabricated on high-quality AlN templates with an engineerable in-plane lattice constant. The controllability of the in-plane strain originated from the vacancy formation in Si-doped AlN (AlN:Si) and their interaction with edge dislocations. The strain state of the AlN:Si top interface could be well depicted by a dislocation-tilt model depending on the buffer strain state, threading dislocation density (TDD), and regrown AlN:Si thickness. The validity of the model was verified by cross-sectional TEM analysis. With a gradually widened lattice constant of regrown AlN:Si layer, strain-induced defects of subsequently grown n-AlGaN was suppressed. Therefore, growing a current spreading layer which possesses a moderate Al content ( 1.5 μm), and a low TDD ( 200 mW) with a low forward voltage (V f =5.7 volt) were demonstrated at I=1.35 A. The low forward voltage under high current injection density was attributed to the success in preparation of a low series resistance and high-quality n-AlGaN current spreading layer.

Efficiency improvement analysis of nano-patterned sapphire substrates and semi-transparent superlattice contact layer in UVC light-emitting diodes

UVC light-emitting diodes (LEDs, λ = 275 nm) with different types of contact layers and sapphire substrates were demonstrated on high-quality AlN templates. For LEDs on flat sapphire substrates (FSSs), replacing the absorbing p-GaN contact with p-AlGaN short-period superlattices (p-SPSLs) strongly enhanced the emission along the substrate normal. The integrated external quantum efficiency (EQE) increased from 2.4% to 3.9% under I = 350 mA. For LEDs with a p-SPSL contact, replacing the FSS with nano-patterned sapphire substrates slightly deteriorated the quality of epitaxy, but the overall EQE is still enhanced to 4.4% under I &amp;gt; 350 mA without lens encapsulation. According to the far-field intensity measurement, the light extraction is better improved along the high emission angle to the substrate normal. The interplay among substrates, dipole polarization, and EQE enhancement factors was further analyzed and discussed in the context.

The study of the AlN/Si(111) epitaxial structures grown by PA MBE via coalescence overgrowth of AlN nanocolumns

The AlN/Si(111) epitaxial structures were synthesized by coalescence overgrowth of AlN nanocolumns using PA MBE technique. Such epitaxial structures can be used as a buffer layer for obtaining high quality AlN and GaN layers. Structural, electrical and chemical properties of these samples were studied. For the first time it was demonstrated that the etching of the obtained type of AlN/Si(111) structures in KOH can become a promising method for obtaining high quality free-standing AlN and GaN.

Sec-Eliminating the SARS-CoV-2 by AlGaN Based High Power Deep Ultraviolet Light Source.

The world‐wide spreading of coronavirus disease (COVID‐19) has greatly shaken human society, thus effective and fast‐speed methods of non‐daily‐life‐disturbance sterilization have become extremely significant. In this work, by fully benefitting from high‐quality AlN template (with threading dislocation density as low as ≈6×108 cm−2) as well as outstanding deep ultraviolet (UVC‐less than 280 nm) light‐emitting diodes (LEDs) structure design and epitaxy optimization, high power UVC LEDs and ultra‐high‐power sterilization irradiation source are achieved. Moreover, for the first time, a result in which a fast and complete elimination of SARS‐CoV‐2 (the virus causes COVID‐19) within only 1 s is achieved by the nearly whole industry‐chain‐covered product. These results advance the promising potential in UVC‐LED disinfection particularly in the shadow of COVID‐19.

Low-Temperature Deposition of Large-Grained Polycrystalline AlN Using Trisdimethylamido Aluminum (TDMAA) and Anhydrous Hydrazine

High quality, large-grained AlN films are of interest for use as either heat spreading layers or as a buffer/templating layer for further film growth. As the typical deposition temperature for high-quality crystalline AlN is usually in excess of 800 oC, there is a large amount of thermal strain in the films and it is advantageous to develop processes to lower the deposition temperature so that it is compatible with back end of line (BEOL) processing. In this work, the low-temperature (<400 oC) deposition of polycrystalline AlN films is demonstrated by atomic layer annealing (ALA) which is a variant of ALD that utilizes a third pulse of low-energy inert gas ions in addition to the usual metal and co-reactant pulses [1].Using trimethyl aluminum (TMA) or tris(dimethylamido) aluminum (TDMAA) with the highly reactive nitrogen-containing precursor hydrazine (N2H4), AlN can be deposited at ~200 oC [2]; however, these films are amorphous and would have low thermal conductivity due to a high degree of phonon scattering. Using TDMAA with N2H4 or NH3 at temperatures >350 oC, polycrystalline films can be deposited in a purely thermal process; however, the reported grain sizes are small (<5 nm) or there is a mixture of polycrystalline and amorphous phases [3-4]. ALA has been used to deposit crystalline films such as AlN [1,5] and GaN [6] on a near lattice-matched substrate (sapphire) at low temperatures, but a nitrogen-containing plasma was used.In the present study of AlN ALA on Si (111) (a non-lattice matched substrate), two metal precursors (TMA and TDMAA) were compared using ultra-high purity anhydrous N2H4 as a co-reactant and argon ions with tuned energy for the third pulse. It was found that deposition using TDMAA as the Al precursor resulted in high-quality AlN films with large grain size (>20 nm) and low C/O contamination (<1.5%) whereas films deposited using TMA had much higher carbon content (>5%), owing to its thermal instability at 400 oC. Transmission electron micrographs for a 40 nm film grown using TDMAA and N2H4 show vertical grain structure with grains spanning the entire thickness of the film. As heat spreading layers often need to be in excess of 1 micron thick in order to be relevant for use in high volume manufacturing, it is also demonstrated that these 40 nm films successfully transfer the large grain size of the ALA film to a thick sputtered AlN film (which can be deposited relatively quickly) by acting as a template layer.[1] H-Y. Shih et al, Scientific Reports 7:39717.[2] M. Mizuta et al, Japanese J. Appl. Phys. 25(12), L945-L948 (1986).[3] R. G. Gordon, U. Riaz, and D. M. Hoffman, J. Mater. Res., 7(7) (1992).[4] A. I. Abdulagatov et al Russian Microelec. 47(2), 118-130 (2018).[5] W-C. Kao et al, RSC Adv. 9, 12226-12231 (2019).[6] W-H. Lee et al ACS Sustainable Chem. Eng. 7,1, 487-495 (2019).This work was supported in part by the Semiconductor Research Corporation. Figure 1