Growth Of High-quality AlN Films Research Articles

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.

Development of a flux-film-coated sputtering (FFC-sputtering) method for fabricating c-axis oriented AlN film

In this study, we developed a novel growth method named “Flux-Film-Coated (FFC) sputtering.” In this method, nitrogen radicals were supplied to the Al–Sn flux at around 600 °C followed by the deposition of an Al–Sn metal film on a sapphire substrate as flux, which resulted in the growth of high-quality AlN films. The nitrogen radical, which is the source of nitrogen for the growth of AlN, was generated by the radio frequency plasma in a nitrogen atmosphere. Using a zirconia target in the plasma generation process, nitrogen radicals were easily generated because the zirconia was not etched in a pure nitrogen atmosphere, which enabled us to fabricate the AlN film using the flux method in the sputtering chamber in a single step. The crystallinity of the synthesized AlN determined using the FFC-sputtering method was remarkably improved when compared with that of the AlN film deposited using the reactive sputtering method.

Growth of high-quality AlN films on sapphire substrate by introducing voids through growth-mode modification

We demonstrate the achieving of high-quality AlN films on flat sapphire substrate (FSS) by introducing voids during growth. Voids are embedded into AlN epilayers through a growth-mode transition from island growth to step flow growth. Such voids significantly facilitated the underlying dislocations annihilation as demonstrated by the transmission electron microscopy (TEM) image. For the 3 μm-thick AlN film grown on FSS, the full width at half maximum of the X-ray rocking curve was 57/260 arcsec for (0 0 2)/(1 0 2) reflection and a threading dislocation density of 1.7 × 108 cm−2 was determined from plain-view TEM image. Moreover, the voids provided an additional stress relief channel in the AlN film grown on FSS, resulting in a tensile stress comparable to that of grown on nano-patterned sapphire substrate (NPSS). The measured lattice constants and Raman shift of AlN-E2 (high) peak verified the 3 μm-thick AlN film grown on FSS is nearly stress free at room temperature. Taking advantages of the deliberately embedded voids, a crack-free and atomically flat AlN film was grown on FSS. The strategy put forward in this work to obtaining high-quality AlN films on FSS is much cost-efficient, which is believed to hold great promise for commercialization in AlN-based devices.

Improved Epitaxy of AlN Film for Deep-Ultraviolet Light-Emitting Diodes Enabled by Graphene.

The growth of single-crystal III-nitride films with a low stress and dislocation density is crucial for the semiconductor industry. In particular, AlN-derived deep-ultraviolet light-emitting diodes (DUV-LEDs) have important applications in microelectronic technologies and environmental sciences but are still limited by large lattice and thermal mismatches between the epilayer and substrate. Here, the quasi-van der Waals epitaxial (QvdWE) growth of high-quality AlN films on graphene/sapphire substrates is reported and their application in high-performance DUV-LEDs is demonstrated. Guided by density functional theory calculations, it is found that pyrrolic nitrogen in graphene introduced by a plasma treatment greatly facilitates the AlN nucleation and enables fast growth of a mirror-smooth single-crystal film in a very short time of ≈0.5 h (≈50% decrease compared with the conventional process), thus leading to a largely reduced cost. Additionally, graphene effectively releases the biaxial stress (0.11 GPa) and reduces the dislocation density in the epilayer. The as-fabricated DUV-LED shows a low turn-on voltage, good reliability, and high output power. This study may provide a revolutionary technology for the epitaxial growth of AlN films and provide opportunities for scalable applications of graphene films.

Graphene-assisted quasi-van der Waals epitaxy of AlN film for ultraviolet light emitting diodes on nano-patterned sapphire substrate

We report the growth of high-quality AlN films on nano-patterned sapphire substrates (NPSSs) by graphene-assisted quasi-van der Waals epitaxy, which enables rapid coalescence to shorten the growth time. Due to the presence of graphene (Gr), AlN tends to be two-dimensional laterally expanded on the NPSS, leading to the reduction of dislocation density and strain release in the AlN epitaxial layer. Using first-principles calculations, we confirm that Gr can reduce the surface migration barrier and promote the lateral migration of metal Al atoms. Furthermore, the electroluminescence results of deep ultraviolet light emitting diodes (DUV-LEDs) have exhibited greatly enhanced emission located at 280 nm by inserting the Gr interlayer. The present work may provide the potential to solve the bottleneck of high efficiency DUV-LED.

High-quality AlN films grown on chemical vapor-deposited graphene films

We report the growth of high-quality AlN films on graphene. The graphene films were synthesized by CVD and then transferred onto silicon substrates. Epitaxial aluminum nitride films were deposited by DC magnetron sputtering on both graphene as an intermediate layer and silicon as a substrate. The structural characteristics of the AlN films and graphene were investigated. Highly c-axis-oriented AlN crystal structures are investigated based on the XRDpatterns observations.

Effect of Al substrate nitridation on the properties of AlN films grown by pulsed laser deposition and its mechanism

AlN films have been grown on nitrided and non-nitrided Al substrates by pulsed laser deposition (PLD) technology at 450°С. The surface morphology, crystalline quality, and interfacial property of the as-grown AlN films on nitrided and non-nitrided Al substrates have been investigated systematically. It is found that the as-grown ∼300nm-thick AlN films on nitrided Al substrates show the very smooth surface with a root-mean-square (RMS) surface roughness of 1.5nm, high crystalline quality with full-width at half-maximum (FWHM) values for AlN(0002) and AlN(10−12) X-ray rocking curves (XRCs) of 0.6° and 0.9°, and sharp and abrupt interfaces. These results are in striking contrast to those of ∼300nm-thick AlN films grown on non-nitrided Al substrates. The as-grown ∼300nm-thick AlN films on non-nitrided Al substrates show relatively rough surface with a RMS surface roughness of 4.0nm, poor crystalline quality with FWHM values for AlN(0002) and AlN(10−12) XRCs of 1.1° and 1.5°, and a ∼1.5nm-thick interfacial layer. Based on the characterizations, the effect of Al substrates nitridation and the growth mechanism of AlN films grown on nitrided Al substrates by PLD are hence proposed. This work provides an effective approach for the growth of high-quality AlN films on Al substrates for the future application of AlN-based devices.

Effect of nitrogen pressure on the properties of AlN films grown on nitrided Al(1 1 1) substrates by pulsed laser deposition

AlN films have been epitaxially grown on nitrided Al(111) substrates with various nitrogen pressures by pulsed laser deposition with an in-plane alignment of AlN[11–20]//Al[1–10]. The effect of nitrogen pressure on the surface morphologies and structural properties of AlN films is studied in detail. It reveals that AlN films grown at appropriate pressure exhibit very flat and smooth surface with a surface root-mean-square roughness of 1.1nm, and abrupt interface; while AlN films grown at other pressures show poorer properties. This work presents an optimized growth conditions for the growth of high-quality AlN films for the application of film bulk acoustic wave resonators and surface acoustic wave devices that ask for the flat surface and abrupt interface.

Epitaxial growth of high-quality AlN films on metallic nickel substrates by pulsed laser deposition

Single-crystalline AlN films with smooth surface and abrupt interface have been grown on metallic nickel (Ni) substrates by pulsed laser deposition with an in-plane epitaxial relationship of AlN[112̄0]//Ni[11̄0].

Influence of the seed layer on structural and electro-acoustic properties of sputter-deposited AlN resonators

The growth of high-quality AlN films has been studied by reactive sputtering onto Mo electrodes with Ni, Ti, and TiW seed layers and subsequent integration into thin film bulk acoustic wave resonators. The crystalline structure and morphology of the Mo and c-axis oriented AlN films were found to vary strongly with seed layer material and thickness. The smoothest Mo electrodes were obtained on thin Ti films. Reactive sputtering of AlN on top of these optimized electrodes resulted in a dense columnar grain structure with a well-aligned (002) crystal orientation and good electro-acoustic properties, including an effective coupling coefficient of 6.89% and quality factor above 1000.