Bulk AlN Research Articles

BAlGaN alloys nearly lattice-matched to AlN for efficient UV LEDs

The lattice mismatch between AlGaN and AlN substrates limits the design and efficiency of UV light-emitting diodes, but it can be mitigated by the coincorporation of boron. We employ hybrid density functional theory to investigate the thermodynamic, structural, and electronic properties of BAlGaN alloys. We show that BAlGaN can lattice match AlN with bandgaps that match AlGaN of the same gallium content. We predict that BAlGaN emits transverse-electric polarization for a gallium content of ∼45% or more. Our results indicate that BAlGaN alloys are promising materials for higher efficiency UV optoelectronic devices on bulk AlN substrates.

Carbon pair defects in aluminum nitride

AlN bulk single crystals grown by the physical vapor transport method may be beneficially applied as substrates for deep ultraviolet light emitting devices or as a basic material for piezoelectric resonators operating at high temperatures. Identification of point defects which deteriorate the optical, electrical, and electromechanical properties of AlN crystals for such applications is the subject of the present work. Using Raman spectroscopy, two local vibrational modes (LVMs) were discovered at wave numbers of 1189 cm−1 and 1148 cm−1. By analyzing an AlN crystal intentionally enriched with the carbon isotope 13C, it is unambiguously shown that the two LVMs originate from two different, but in each case carbon-related defects. Furthermore, it is evidenced that the defect underlying the LVM at 1189 cm−1 contains exactly two carbon atoms. The tricarbon defect-related LVM reported earlier in an infrared absorption study is found to be Raman active at 1772 cm−1. The Raman scattering intensity of all three LVMs strongly depends on the photon energy of the exciting light what is interpreted as a resonance Raman effect. This allows linking the identified defects with their contribution to the strong, carbon-related ultraviolet absorption around 4.7 eV and proves that these defects introduce optically and electrically active deep levels in the bandgap of AlN.

A New Method of Growing AlN, GaN, and AlGaN Bulk Crystals Using Hybrid SiC/Si Substrates

The main principles of a new method of growing bulk single-crystal AlN, AlGaN, and GaN films with thickness from 100 μm and more on silicon substrates with a buffer silicon carbide layer with its subsequent detachment from Si substrates are presented. The main substance of this method is a combination of the method of chloride-hydride epitaxy that determines high growth rates of III nitride layers and the use a Si substrate with a buffer layer of nanoscale SiC film grown by the atomic substitution method as the growth substrate. The Si substrate with a SiC layer grown by the atomic substitution method has a number of structural, physical, and chemical features as compared to SiC layers grown on Si by the standard methods. It is shown that it is precisely this feature that enables the growth on their surfaces of thick crack-free AlN, AlGaN, and GaN layers with subsequent and quite simple their detachment from the substrate. The single-crystal crack-free AlN layers with thickness to 300 μm, AlGaN layers with thickness to 400 μm, GaN layers with thickness to 200 μm, and GaN films of the semipolar ($$11\bar {2}4$$) orientations with thickness to 30 μm have been grown.

Growing Bulk Aluminum Nitride and Gallium Nitride Crystals by the Sublimation Sandwich Method

The results of the growth of bulk crystals of aluminum and gallium nitrides on foreign seeds by the sublimation sandwich method are reviewed. The kinetics and mechanism of sublimation and condensation are analyzed depending on the growth conditions, vapor phase composition, crystal orientation, and the distance between the source and the seed. It is experimentally found that during joint annealing of aluminum nitride and silicon carbide, the sublimation rate of aluminum nitride substantially increases due to the formation of a liquid phase on the crystal surface. The inhomogeneous distribution of the liquid phase, localized mainly near structural and morphological defects, leads to the selective nature of surface etching and causes a deterioration in the quality of the growing crystal. A process of growth of bulk AlN crystals with simultaneous seed evaporation was implemented, which yields crystals without cracks and with improved parameters. Bulk crystals of aluminum nitride and gallium nitride up to 2 inches in diameter were grown on SiC seeds.

A contrivance of 277 nm DUV LD with B0.313Ga0.687N/B0.40Ga0.60N QWs and AlxGa1–xN heterojunction grown on AlN substrate

In this paper, an ultraviolet C-band laser diode lasing at 277 nm composed of B0.313Ga0.687N/B0.40Ga0.60N QW/QB heterostructure on Mg and Si-doped AlxGa1–xN layers was designed, as well as a lowest reported substitutional accepter and donor concentration up to NA = 5.0 × 1017 cm–3 and ND = 9.0 × 1016 cm–3 for deep ultraviolet lasing was achieved. The structure was assumed to be grown over bulk AlN substrate and operate under a continuous wave at room temperature. Although there is an emphasizing of the suitability for using boron nitride wide band gap in the deep ultraviolet region, there is still a shortage of investigation about the ternary BGaN in aluminum-rich AlGaN alloys. Based on the simulation, an average local gain in quantum wells of 1946 cm–1, the maximum emitted power of 2.4 W, the threshold current of 500 mA, a slope efficiency of 1.91 W/A as well as an average DC resistance for the V–I curve of (0.336 Ω) had been observed. Along with an investigation regarding different EBL, designs were included with tapered and inverse tapered structure. Therefore, it had been found a good agreement with the published results for tapered EBL design, with an overweighting for a proposed inverse tapered EBL design.

Phonons in short-period (GaN)m(AlN)n superlattices: ab initio calculations and group-theoretical analysis of modes and their genesis

The results of experimental and theoretical studies of phonon modes in short-period (GaN)m(AlN)n superlattices (SLs) grown by MOVPE and PA MBE on the (0001) Al2O3 substrate are reported. Using a comprehensive group-theoretical analysis, the genesis of the SL vibrational modes from the modes of bulk AlN and GaN crystals has been established, which is important for interpreting the SL Raman spectrum. In the framework of Density Functional Theory, the lattice dynamics and the structural properties of (GaN)m(AlN)n SLs (m+n⩽12) were studied. An analysis of the eigenvectors of the phonon modes made it possible to reveal their microscopic nature. We established that the E(TO) modes are localized in the layers constituting the SL. It is shown that the localized nature of this mode is kept even in the SLs with the thinnest layers (m+n=4). In turn, the A1(TO) mode demonstrates a delocalized nature and reflects the averaged characteristics of the SL as a whole. A combined analysis of the ab initio calculations and Raman data was performed. Thus, the above studies open new possibilities for analyzing the structural properties of GaN/AlN SLs by Raman and IR spectroscopy.

Raman Scattering in AlN Crystals Grown by Sublimation on SiC and AlN Seeds

The Raman-scattering technique is used to analyze the structural quality of bulk AlN crystals grown by sublimation on SiC and AlN seeds. Growth on SiC seeds is conducted with retention of the SiC seed during growth (type 1) and with total evaporation of the SiC seed (type 2). Growth on AlN seeds is conducted in tungsten containers with no graphite parts (type 3). According to the analysis of Raman spectra, the highest quality is inherent in type-3 crystals that exhibit minimal full widths at half maximum of Raman lines. The experimentally observed specific features are defined by differences in the mechanism of growth and by the content of dopant impurities in the crystals grown.

Preparation and characterization of AlN seeds for homogeneous growth

Large size AlN bulk crystal has been grown on SiC heterogeneous seed by physical vapor transport (PVT). The properties of AlN wafer were characterized by high resolution X-ray diffraction (HRXRD), Raman spectroscopy, etched method and atomic force microscope (AFM). Growth mechanism of AlN crystal grown on heterogeneous SiC seeds was proposed. Crystallization quality of AlN samples were improved with the growth process, which is associated with the growth mechanism. AlN single wafer has excellent crystallization quality, which is indicated by HRXRD showing the (0002), ( ) XRD FWHM of 76.3, 52.5 arcsec, respectively. The surface of the AlN wafer is measured by AFM with a roughness of 0.15 nm, which is a promising seed for AlN homogeneous growth.

Optimization of the CMP Process with Colloidal Silica Performance for Bulk AlN Single Crystal Substrate

Chemical mechanical polishing (CMP) of bulk AlN was performed with colloidal silica slurry at pH 9 for different times. The result shows that colloidal silica slurry at pH 9, which has the relatively high surface charge of -50.7 mV is most stable, and it was selected as chemically optimum condition in this study. The ultra-smooth surface was shown in CMP 90 min with the roughness average (Ra) value of 0.172 nm. It was demonstrated that the damaged layers including subsurface defects and micro scratches in the whole machining process were successfully removed and atomically flat surface can be shown. With increasing process time, the zeta potential and mean particle size of the colloidal silica decreased and increased by -35.07 mV and 143.4 nm, respectively. While the silica particles agglomerated and densely packed slurry particles were formed by mechanical shearing. These increased the Ra value above 0.5 nm of AlN substrate and generated additional surface damages. In terms of the surface chemistry, the carbon compounds and organic impurities adsorbed on the substrate during mechanical polishing (MP) can be removed and aluminum oxidehydroxide; AlOOH and Al(OH)3 were observed during the CMP. It was determined that the chemically polished AlN substrate was continuously hydrated with generating the AlOOH and Al(OH)3 on the surface. (Received June 4, 2019; Accepted July 15, 2019)

(Invited) Advances in Ion Implantation of GaN and AlN

Ion implantation is one of the basic tools for semiconductor device fabrication and as such any maturing semiconductor system should be capable of being processed in this form. Currently, III-nitrides dot not possess a robust ion implantation toolbox that allows for reliable implantation control and activation, thus precluding the possibility of pursuing advanced device architectures comparable to those in SiC. This is obvious when trying to advance the capabilities of the III-nitrides for power applications, where the need for proper field managing schemes require spatially distributed doping distributions that can be achieved by ion implantation. Recent advances in ion implantation for the realization n-type AlN and p-type GaN will be discussed. AlN is considered for deep-UV optoelectronics and next generation high power devices due to its ultra wide bandgap. So far, n-type AlN has been achieved by doping with silicon during epitaxial growth. However, achieving high conductivity in n-type AlN is still a major challenge due to self-compensation via DX and vacancy complex formation at equilibrium. This has made it necessary to consider other non-equilibrium based doping methods, such as ion implantation. In this work, doping AlN by Si implantation is considered. Si implantation was realized into AlN homoepitaxial films grown on bulk AlN substrates via metal organic chemical vapor deposition (MOCVD). Samples were implanted with doses ranging between 5x1013and 1x1015cm-2at 100 keV at RT. Although high-temperature annealing is necessary for damage removal, it enables compensating defect formation by vacancy-silicon complex formation favorable at equilibrium as observed in MOCVD grown AlN. Hence point defect control is necessary during annealing. We further demonstrate defect quasi Fermi level (dQFL) control based compensating point defect reduction in Si implanted AlN during damage recovery. Thus, the highest reported n-type conductivity in AlN was achieved. These results show a possible pathway for the realization of n-type AlN for future optoelectronic and power electronic devices. GaN-based high power switches are a promising avenue towards realizing advances in power management. Controlling the selective area doping of both n- and p-type regions is necessary to realize these device structures. Although high n-type carrier concentrations and conductivities in GaN have been reproducibly demonstrated, achieving high p-type conductivity after ion implantation remains a challenge. To prevent the surface decomposition during annealing at these elevated temperatures required for post-implantation anneals, previous efforts have focused on the use of capping layers (e.g. AlN) and/or complicated annealing procedures. In this work we demonstrate the ability to successfully achieve p-type conductivity in GaN films via room temperature Mg implantation and a post-implantation annealing procedure at high pressure (1 GPa). The highest recorded p-type conductivity (~0.1 Ω-1·cm-1) was measured on the Mg implanted GaN film grown on a sapphire substrate and annealed at 1300 °C and 1 GPa. The implantation and annealing conditions that resulted in the highest p-type conductivity were selected to fabricate p-i-njunction diodes by Mg implantation in n-type GaN grown homoepitaxially on Ammono wafers. All these results support the possibility of realizing successful ion implantation in III-nitrides adding to the growing toolbox of capabilities for this technology.

EthylenediamineAl(III)chloride: Potential precursor for preparation of AlN materials/thin films

Tris(ethylenediamine) Al(III) Cl3 has been used as single-source precursor for the preparation of bulk AlN and thin films. The precursor is stable for long periods of time and cleanly generated aluminum nitride upon pyrolysis. Pyrolysis of precursor was optimized by varying the parameters such as temperature, pressure and flow of gases (N2 and Ar2). Thin films of AlN were prepared by spin coating of precursor on Si(100) and quartz substrates by pyrolysing under optimized CVD conditions obtained for bulk pyrolysis. Pyrolysed powder and films were characterized comprehensively by a variety of techniques including X-ray diffraction (XRD), Infrared spectroscopy, UV–Vis spectroscopy, Thermogravimetry analysis X-ray photoelectron spectroscopy (XPS). Our results showed that the AlN films showed dominant 220 orientation and the band gap is ~5.7 eV which is close to bulk AlN (6.1 eV).

Characterization of 60 mm AlN Single Crystal Wafers Grown by the Physical Vapor Transport Method

Physical Vapor Transport Crack-free bulk AlN single crystals up to 60 mm in diameter are grown for the first time by the physical vapor transport method. The demonstrated entire wafer exhibits excellent ultraviolet transparency with absorption coefficients of 14–21 cm−1 in the range 260–280 nm, and the Raman spectra show an E2(high) FWHM of 2.86 cm−1. More details can be found in article number 1900118 by Qikun Wang, Jason Wu, and co-workers.

Characterization of 60 mm AlN Single Crystal Wafers Grown by the Physical Vapor Transport Method

Crack‐free bulk AlN single crystals up to 60 mm in diameter are successfully grown for the first time using a series of proprietary techniques by the physical vapor transport method. The single crystals are sliced into on‐axis (±0.2°) wafers and then lapped/polished following common wafering standards. The obtained wafers are characterized by Raman spectroscopy and high‐resolution X‐ray diffraction (HRXRD). The Raman spectra show an E2(high) full width at half maximum (FWHM) of 2.85–2.87 cm−1. The symmetric and asymmetric HRXRD rocking curves show FWHMs of 172–288 and 103–242 arcsec, respectively. The optical transmission spectra reveal that the entire wafers exhibit excellent ultraviolet (UV) transparency with absorption coefficients of 14–21 cm−1 in the UV range 4.43–4.77 eV (260–280 nm). The average etch pit density (EPD) determined by preferential chemical etching is about 2.3 × 105 cm−2. The major impurities determined by evolved gas analysis and glow discharge mass spectrometry are carbon at 7.4 × 1018 cm−3 (45 ppmw), oxygen at 1.2 × 1019 cm−3 (100 ppmw), and silicon at 6.8 × 1017 cm−3 (9.7 ppmw). The usable area of the 60 mm wafers exceeds 98%.

AlGaN avalanche Schottky diodes with high Al-content

AlxGa1−xN-based avalanche photodiodes with a Schottky-contact grown on AlN bulk substrate with an Al-content of x = 0.68 have been examined with respect to their structural and electro-optical properties. The Schottky diodes suitable for the solar-blind ultraviolet spectral regime show avalanche gain for voltages in excess of 25 V reverse bias in linear gain mode under frontside illumination. For 60 V reverse bias, avalanche multiplication exceeding 104 was obtained. The devices were operated well below breakdown; the measured current for frontside illuminated conditions exceeds the dark-condition characteristics by more than two orders of magnitude at this voltage. The Schottky-barrier height and the ideality n were extracted to eV and at room temperature, respectively. Temperature dependent measurements indicate a temperature insensitive dark current mechanism with an enhanced multiplication process towards lower temperatures.

Impact of metalorganic vapor phase epitaxy growth conditions on compressive strain relaxation in polar III-nitride heterostructures

A novel approach to estimating the critical thicknesses (CTs) of compressively strained III-nitride layers is suggested, based on a quasi-thermodynamic growth model and accounted for the effect of material decomposition during dislocation half-loop formation on the CT value. The approach provides good quantitative agreement with available data on CTs of MOVPE-grown InGaN/GaN and AlGaN/AlN epilayers. The extremely large CTs observed for high Al-content AlGaN alloys grown on bulk AlN substrates may be attributed, in particular, to the dominant AlGaN decomposition mechanism, producing group-III metallic liquid and gaseous nitrogen. The suggested approach may also be helpful for analysis of threading dislocation inclination in compressively strained layers and applicable to studying point defect formation in semiconductors and its dependence on growth conditions.

Highly radiative nature of ultra-thin c-plane Al-rich AlGaN/AlN quantum wells for deep ultraviolet emitters

Temperature-dependent photoluminescence (PL) lifetimes were measured for a series of ultra-thin c-plane Al0.61Ga0.39N/AlN multiple quantum wells (QWs) on bulk AlN substrates with the well thickness varying from 0.6 to 2 nm. At temperatures below 75 K, estimates of the internal quantum efficiency indicate that the recombination is primarily radiative, with a lifetime of ∼160 ps for the 0.6 nm QWs, comparable to the low temperature PL lifetime observed in bulk AlGaN films of a similar Al content. This short lifetime is observed despite the presence of layer thickness fluctuations and the quantum-confined Stark effect associated with the large polarization field in the heterostructures, which tend to increase the radiative lifetime. This behavior is explained using many-body calculations of radiative recombination rates that extend beyond the conventional ABC rate equation model by accounting for both excitons and free carriers within a nonequilibrium Green's function formalism. The results indicate that the combination of the large wave function overlap integral (∼0.65) and exciton binding energy (1.82 times the 3D Rydberg) for the 0.6 nm QWs leads to an ∼20-fold increase in the radiative recombination rate relative to that obtained for the 2 nm QWs. This greater radiative recombination rate competes favorably with trapping at interface fluctuations and defect-induced nonradiative recombination that dominates recombination at higher temperatures.

Density control of GaN quantum dots on AlN single crystal

Full control over the density and emission properties of GaN quantum dots (QDs) should be feasible, provided that the growth proceeds in the Stranski-Krastanov (SK) growth mode. In this work, we derive the phase diagram for GaN QD formation on AlN by NH3-molecular beam epitaxy and analyze the corresponding optical signature by micro-photoluminescence (μ-PL). Interestingly, the growth window for SK-GaN QDs is very narrow due to the relatively small lattice mismatch of the GaN/AlN system (2.5%), constituting a fundamental challenge for QD growth control. By relying on bulk AlN single crystal substrates, we demonstrate QD density control over three orders of magnitude, from 108 to 1011 cm−2 by changing the growth rate. In contrast, the QD density is pinned to 2 × 1010 cm−2 when growing on AlN/sapphire templates, which exhibit dislocation densities on the order of 1010 cm−2. Thanks to QD densities as low as 108 cm−2 on bulk AlN, we can probe the emission of spatially isolated single GaN QDs by μ-PL on unprocessed samples.

High quality 10.6 μm AlN grown on pyramidal patterned sapphire substrate by MOCVD

In this letter, we demonstrate a crack and strain free AlN epilayer with a thickness of 10.6 μm grown on a pyramidal patterned sapphire substrate by metalorganic chemical vapor deposition. The full width at half maximum of the X-ray rocking curve was 165/185 arcsec for (002)/(102) planes, respectively. The total threading dislocation density was less than 3 × 108 cm−2. The dislocation evolution and the coalescence process were probed by transmission electron microscopy and scanning electron microscopy. A dual coalescence of the AlN epilayer was observed, which can effectively relax strain during the heteroepitaxy process. Owing to the approximately entire strain relaxation demonstrated by reciprocal space mapping and Raman shift, the surface morphology was crack-free and atomically smooth with a root-mean-square roughness of 0.14 nm. Temperature dependent Raman spectra showed the Raman linewidth of 4.3 cm−1 at 300 K which was comparable to that of bulk AlN; it also demonstrated good crystalline quality of the AlN epilayer.

Structural, electronic and mechanical properties of single-walled AlN and GaN nanotubes via DFT/B3LYP

Density functional theory with B3LYP hybrid functional and all-electron basis set was applied to study the AlN (SWAlNNTs) and GaN (SWGaNNTs) single-walled nanotubes.The structural and electronic properties were analyzed in function of its diameter and chiralities. Additionally, the elastic and piezoelectric constants were calculated for armchair, zigzag and chiral nanotubes. The simulations showed that both, SWAlNNTs and SWGaNNTs, are easily formed from the graphene-like surface than from the respective bulk. As the diameter increases, the band gap energy also increases, but converges to the band gap energy of its precursor surface. The calculated elastic constants for bulk, graphene-like surface and nanotubes of AlN and GaN show that AlN, in all configurations, is more rigid than GaN. This effect can be related to the more pronounced ionic character of Al–N bond, which confers the stiffness of material. This stiffness affects the AlN nanotube formation, especially that with small diameter, that has the higher energy strain and formation energy for all chiralities. The AlN configurations have piezoelectric response ~ 25% greater than GaN. The AlN zigzag nanotube has the higher piezoelectric constant e11, i.e., 0.84 C/m2. Compared to AlN bulk, the e11 of nanotube is less than the e33 of its bulk, 1.44 C/m2, but is higher when compared with the others’ piezoelectric constants of bulk and surface. Therefore, although the nanotubes present the same stability in diameters above 20 A, AlN and GaN differ in their band gap energy, piezoelectric response and elastic constant, which will interfere directly with their application in electronic and piezoelectric devices, besides a possible functionalization, such as doping or molecule adsorption.

Выращивание объёмных кристаллов AlN и GaN сублимационным сандвич-методом

The review of the results of growth of bulk Al and Ga nitride crystals on foreign substrates by the sublimation sandwich method (SSM) is presented. The kinetics and the mechanism of sublimation and condensation nitrides depending on the growth conditions, structure of the vapor phase, crystal orientation and the distance between the source and the seed are analyzed. It is experimentally established that by joint annealing of AlN and SiC the rate of AlN sublimation significantly increases due to formation of a liquid phase on the crystal surface. Non-uniform distribution of the liquid phase localized mainly near structural and morphological defects results in the selective nature of the surface etching, and also is the reason of deterioration of the growing crystal quality. The process of bulk AlN crystals growth with simultaneous evaporation of the seed allowing to grow crystals without cracks and with the improved parameters is realized. On SiC seeds bulk AlN crystals from 0.5 to 2 inch in diameter are grown.