Freestanding AlN Substrate Research Articles

Fabrication of a freestanding AlN substrate via HVPE homoepitaxy on a PVT-AlN substrate

Hydride vapor phase epitaxy (HVPE) is employed for the homoepitaxial development of AlN thick films on AlN substrates grown via physical vapor transport (PVT). A freestanding AlN substrate with a 200 μm thickness is then obtained by mechanically grinding away the PVT-AlN substrate. The as-grown HVPE AlN layer has a smooth surface with long parallel atomic steps. The freestanding HVPE-AlN substrate is crack-free and stress-free. In comparison to PVT-AlN substrate, HVPE-AlN substrate not only has better crystal quality but also substantially lower C, O, and Si impurity concentrations. The deep ultraviolet (DUV) transmittance of the 200 μm thick freestanding AlN substrate is as high as 66% at 265 nm. This performance aligns perfectly with the demands of AlGaN-based DUV optoelectronic devices.

24.4 W/mm X-Band GaN HEMTs on AlN Substrates With the LPCVD-Grown High-Breakdown-Field SiN x Layer

This paper reports on an AlGaN/GaN high-electron-mobility transistor (HEMT) on free-standing AlN substrates with a record-high output power density of 24.4 W/mm at the X-band. A high-drain current operation of 1.4 A/mm was realized by employing high-density 2-dimensional electron gas channel and regrown Ohmic contacts. Furthermore, a high-voltage operation of 110 V was achieved owing to the high-density and high-breakdown SiNx layer grown by low-pressure/high-temperature chemical vapor deposition. Three-stage AlGaN buffer layers improved the electron mobility of 2-dimensional electron gas and allowed us to grow a GaN channel with a low carbon concentration. Furthermore, a regrown n-GaN contact layer was employed to minimize source/drain contact resistances. A high drain current of 1470 mA/mm and high breakdown voltage of 258 V were achieved. Our results demonstrate the potential of GaN HEMTs on the AlN substrate as next-generation high-power radio frequency devices.

Influence of growth rate on homoepitaxial growth of AlN at 1450 °C by hydride vapor phase epitaxy

The influence of growth rate on the homoepitaxial growth of AlN at 1450 °C by hydride vapor phase epitaxy on bulk AlN(0001) substrates was studied. X-ray diffraction and Raman spectroscopy revealed that high structural quality comparable to that of the initial substrate can be achieved even when the growth rate is increased to over 150 μm h−1. Although the concentration of Si impurities increased with increasing growth rate, a freestanding AlN substrate prepared from a homoepitaxial layer grown at 155.6 μm h−1 showed a steep optical absorption edge at 207 nm and high optical transmittance at longer wavelengths.

First demonstration of X-band AlGaN/GaN high electron mobility transistors using free-standing AlN substrate over 15 W mm−1 output power density

In this letter, we successfully achieved high-power radio frequency (RF) operation of AlGaN/GaN high electron mobility transistors (HEMTs) fabricated on free-standing AlN substrate at X-band. The developed HEMT on AlN substrate comprised a 200 nm thick GaN channel and AlGaN buffer with an Al composition of 30%. Thanks to high breakdown voltage of the HEMT on AlN substrate, we successfully demonstrated 15.2 W mm−1 output power density at operating voltages of 70 V even without device technologies such as source-field plate and optimization of device dimension. Our results show that the potential of GaN HEMTs on AlN substrate as next-generation high-power RF devices.

Homoepitaxial growth of AlN on a 2-in.-diameter AlN single crystal substrate by hydride vapor phase epitaxy

Homoepitaxial growth of a thick AlN layer on a 2-in.-diameter single crystal AlN(0 0 0 1) substrate prepared by physical vapor transport (PVT) was investigated by hydride vapor phase epitaxy (HVPE). A water-clear crack-free homoepitaxial layer with a thickness of 500 μm or more could be grown on the entire surface of the PVT-AlN substrate. No degradation of crystallinity was observed during homoepitaxial growth. X-ray rocking curve measurements of the homoepitaxial layer showed small full widths at half maximum of 11–16 arcsec, the same as those for the PVT substrates. A freestanding AlN substrate prepared from the homoepitaxial layer showed high optical transmittance in the deep ultraviolet region owing to low impurity concentrations.

In-plane optical polarization and dynamic properties of the near-band-edge emission of an m-plane freestanding AlN substrate and a homoepitaxial film

For accelerating the development of deep-ultraviolet light-emitting diodes based on high AlN mole fraction (x) AlxGa1-xN for sterilization, disinfection, and skin therapy applications, in-plane optical polarization and dynamic properties of the near-band edge (NBE) cathodoluminescence (CL) peak of a low threading dislocation density (<103 cm−2) m-plane freestanding AlN substrate and a homoepitaxial film are assessed. Consistent with the polarization selection rules, the electric field (E) component of the NBE emission was essentially polarized parallel to the c-axis (E∥c). Low-temperature CL spectra of the homoepitaxial film exhibited exciton fine structures: CL peaks at 6.0410 and 6.0279 eV, which were polarized E∥c and E perpendicular to the c-axis (E⊥c), respectively, are assigned as being due to the recombination of free A-excitons of irreducible representations Γ1 and Γ5. The hydrogenic binding energy of the Γ1 A-exciton being 51 meV is verified. Detectable CL peaks under E∥c polarization at 6.0315 and 6.0212 eV are tentatively assigned as Γ1-mixed Γ5-exciton-polaritons. The concentration of multiple vacancies consisting of an Al-vacancy (VAl) and N-vacancies (VNs), namely, VAlVN2−3, in the substrate was estimated by the positron annihilation measurement to be 2–3 × 1016 cm−3, while that in the epilayer was lower than the detection limit (<1016 cm−3). The NBE CL lifetime of 28 ps of the epilayer subsurface at 300 K is likely limited by the recombination at carbon deep-acceptors on nitrogen sites (3 × 1017 cm−3) and/or VAlVN2−3 Shockley-Read-Hall nonradiative recombination centers (∼1 × 1016 cm−3) with hole capture coefficients of approximately 1×10−7 and 3×10−6 cm3 s−1, respectively.

HVPE homoepitaxy on freestanding AlN substrate with trench pattern

AbstractConditions on chemical surface treatment and thermal treatment of sublimation‐freestanding AlN substrates were investigated to remove damage layers on surfaces for high‐quality and crack‐free AlN films grown by hydride vapour phase epitaxy (HVPE). By wet etching, the residue on the surface was removed and the polishing scratches were reduced. Atomic steps were formed on the surface by the subsequent thermal treatment at 1450 °C for 10 min, and surface layer of 220 nm in thickness was removed, Homoepitaxial growth on freestanding AlN substrate with trench pattern was also performed. A crack‐free AlN film with atomic steps was obtained on the trench‐patterned bulk AlN substrate, and the emission at the band edge near 206.9 nm was dominant. The wavelength of luminescence from cross‐sectional and surface of the HVPE‐AlN layer was maintained. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

AlN/AlGaN HEMTs on AlN substrate for stable high‐temperature operation

An AlN/AlGaN high-electron-mobility transistor (HEMT) fabricated on a free-standing AlN substrate is demonstrated. A metal stack, composed of Zr/Al/Mo/Au, was found to show low contact resistivity for source and drain ohmic contacts. The fabricated AlN/AlGaN HEMT exhibited a maximum drain current of 38 mA/mm with a threshold voltage of -3.4 V. Negligible drain current degradation was observed at temperatures from 300 to 573 K, demonstrating that an AlN/AlGaN approach on an AlN substrate is promising for stable high-temperature operation.

Development and Progress in Bulk c-Plane AlN Single-Crystalline Template Growth for Large-Area Native Seeds

28-mm diameter free-standing AlN substrates were obtained from single crystalline templates grown hetero-epitaxially on (0001) SiC substrates by the sublimation method. The grown template crystals have fairly high structural quality with X-ray rocking curve FWHM values of 120 and 200 arcsec for symmetric and asymmetric reflections, respectively and an average etch pit density of about 5×105 cm-2. In Raman spectroscopy, the E2(high) phonon mode peak FWHM is 18 cm-1 and its position shift shows a very low tensile strain of ∼1.5×10-4 in the crystals. The presence of Si and C impurity-related local vibrational modes is observed. These impurities might be responsible for lowering the optical absorption band edge to 4.3 eV. Homo-epitaxial growth of 5-mm-thick bulk crystals, using 10 mm diameter seeds prepared from these templates demonstrates their suitability as native seeds for further growth.

Deep-ultraviolet lasing at 243 nm from photo-pumped AlGaN/AlN heterostructure on AlN substrate

Deep-ultraviolet lasing was achieved at 243.5 nm from an AlxGa1−xN-based multi-quantum-well structure using a pulsed excimer laser for optical pumping. The threshold pump power density at room-temperature was 427 kW/cm2 with transverse electric (TE)-polarization-dominant emission. The structure was epitaxially grown by metalorganic chemical vapor deposition on an Al-polar free-standing AlN (0001) substrate. Stimulated emission is achieved by design of the active region, optimizing the growth, and the reduction in defect density afforded by homoepitaxial growth of AlN buffer layers on AlN substrates, demonstrating the feasibility of deep-ultraviolet diode lasers on free-standing AlN.

Shottky Barrier Diodes on AlN Free-Standing Substrates

Lateral Schottky rectifiers were fabricated on bulk single-crystal free-standing AlN substrates. The unintentionally doped substrates display n-type conductivity. The diode shows a low reverse leakage current of ∼0.1 nA at -40 V at room temperature. The ideality factor for forward characteristics is 11.7 at room temperature and shows temperature dependence, suggesting the lateral nonuniformities at the metal/semiconductor interface. The fabricated devices are stably operated even at 573 K, owing to the wide band gap (6.2 eV) of AlN. The reverse leakage current of the device is explained by either a trap-assisted tunneling process or one-dimensional variable-range-hopping conduction along the dislocations.

AlN homoepitaxial growth on sublimation-AlN substrate by low-pressure HVPE

Crack-free thick AlN layers with low impurity concentrations were grown on free-standing AlN substrates fabricated by a sublimation method. Cracks due to tensile stresses were generated in the overgrowth layer when using on-axis AlN (0001) substrates, as indicated by Raman scattering spectroscopy. In contrast, cracks were not generated when using 5° off-angle AlN (0001) substrates. High crystalline quality was indicated by X-ray rocking curve (XRC) analysis. The full width at half maximum (FWHM) values of the (0002) and (10–10) diffractions were 277 and 306arcsec, respectively. Secondary ion mass spectrometry (SIMS) measurements indicated that the Si and C impurity concentrations were reduced to half of those in the sublimation-grown AlN substrates.

Preparation of freestanding AlN substrates by hydride vapor phase epitaxy using hybrid seed substrates

A novel hybrid seed substrate for preparing a freestanding (0001) AlN substrate was proposed. The seed substrate consists of thin single-crystalline AlN layer and thick poly-crystalline AlN base, which is expected to remove the differences of thermal expansion coefficient and lattice constant between a subsequently grown thick AlN epitaxial layer and the seed substrate. A freestanding AlN substrate (diameter: 20mm; thickness: 180μm) was successfully prepared, although the freestanding AlN substrate included a number of inner cracks. The freestanding substrate had no strong specific absorption in the wavelength range 210–1000nm and showed an absorption number of 52cm−1 at 265nm.

Investigation of void formation beneath thin AlN layers by decomposition of sapphire substrates for self-separation of thick AlN layers grown by HVPE

Void formation at the interface between thick AlN layers and (0 0 0 1) sapphire substrates was investigated to form a predefined separation point of the thick AlN layers for the preparation of freestanding AlN substrates by hydride vapor phase epitaxy (HVPE). By heating 50–200 nm thick intermediate AlN layers above 1400 °C in a gas flow containing H 2 and NH 3, voids were formed beneath the AlN layers by the decomposition reaction of sapphire with hydrogen diffusing to the interface. The volume of the sapphire decomposed at the interface increased as the temperature and time of the heat treatment was increased and as the thickness of the AlN layer decreased. Thick AlN layers subsequently grown at 1450 °C after the formation of voids beneath the intermediate AlN layer with a thickness of 100 nm or above self-separated from the sapphire substrates during post-growth cooling with the aid of voids. The 79 μm thick freestanding AlN substrate obtained using a 200 nm thick intermediate AlN layer had a flat surface with no pits, high optical transparency at wavelengths above 208.1 nm, and a dislocation density of 1.5×10 8 cm −2.

Characterization of a freestanding AlN substrate prepared by hydride vapor phase epitaxy

AbstractA thick AlN layer was grown on a (111)Si starting substrate at 1230 ºC by hydride vapor phase epitaxy (HVPE). A 112‐μm‐thick freestanding AlN substrate was successfully prepared by removing the Si substrate using a chemical etchant. The AlN substrate was transparent with a smooth (0001) surface. Immersion of the AlN substrate in an aqueous KOH solution revealed that the AlN layer grown on the (111)Si substrate predominately has Al‐polarity. Owing to improvement of crystalline quality by thick layer growth, a near‐band‐edge (NBE) emission could be obtained at 5.92 eV in the room‐temperature photoluminescence (PL) spectrum taken at the top surface of the AlN substrate. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of high‐quality 1‐inch diameter AlN single crystal by sublimation method

Abstract1‐inch diameter AlN single crystals with the thickness of 0.025–2 mm were grown on SiC substrates by the sublimation method. Crystalline qualities were evaluated by X‐ray diffraction and EPD measurements. The FWHM of X‐ray rocking curve for (0002) reflection was 281 arcsec with 1.2‐mm thick crystal and dislocation density measured by EPD was 1.6×106/cm2 with 0.8‐mm thick crystal. These results were well consistent with the dependence of those properties on the thickness of the crystal, which was found in our previous work on 10‐mm diameter crystals. According to this dependence, growing the thick AlN crystal on SiC substrates by the sublimation method is expected to lead to high‐quality free‐standing 1‐inch AlN substrates. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Direct heteroepitaxial growth of thick AlN layers on sapphire substrates by hydride vapor phase epitaxy

Thick AlN layers were grown directly on sapphire substrates by hydride vapor phase epitaxy at growth rates of 40–60 μm/h. The resulting films were colorless, smooth and specular. Subsurface cracking, attributed to the plastic relief of tensile strain from island coalescence, was observed by Nomarski optical contrast microscopy and cross-sectional scanning electron microscopy. Typical full-widths at half-maximum of X-ray rocking curves for the on-axis (0 0 0 2) and off-axis (2 0 2¯ 1) reflections of the AlN films were 310–640′′ and 630–800′′, respectively. The threading dislocation density was 2×10 9 cm −2 as determined by plan-view transmission electron microcopy. Convergent beam electron diffraction was used to determine that the surfaces of our AlN films are Al-face. These results represent substantial progress towards rapid production of AlN templates and free-standing AlN substrates of high optical quality.