Epitaxial Aluminum Nitride Research Articles

Solidly mounted BAW resonators with layer-transferred AlN using sacrificial Si surfaces

We present a new method to manufacture solidly mounted bulk acoustic wave resonators. This new process introduces the use of wafer bonding techniques and sacrificial surface removal to manufacture solidly mounted resonators having special properties. With the proposed process, Aluminum Nitride (AlN) thin films are obtained having exceptional c-axis crystal orientation with XRD rocking curve FWHM of 1.36° and material electromechanical coupling constant of 6.8% exceeding that of the epitaxial AlN electromechanical coupling constant. Fully functional single-mask resonators were successfully fabricated with this process working around 2.35 GHz and enjoying Q-values as high as 1300.

Structural Properties of AlN Grown on Sapphire at Plasma Self-Heating Conditions Using Reactive Magnetron Sputter Deposition

Aluminum nitride (AlN) films were grown on sapphire by reactive magnetron sputter deposition in N2 discharges at plasma self-heating conditions. The growth temperature was as low as 94°C. The structural properties resulting from different substrate biases and growth pressures were investigated by atomic force microscopy, x-ray diffraction (XRD) measurements, and transmission electron microscopy (TEM). At 20 mTorr of N2 with most sputtered species thermalized, films exhibited both AlN (0002) and \( (10\overline{1} 1) \) XRD peaks, with the AlN (0002) intensity initially increasing with ion energy above 15 eV, showing enhanced film quality with an optimum of 25 eV. At a lower growth pressure of 5 mTorr with energetic sputtered species, the AlN \( (10\overline{1} 1) \) peak disappeared and the crystallinity of AlN improved, exhibiting relaxed epitaxial AlN. The measured lattice parameter was 0.4975 nm, which was 0.10% smaller than that of bulk. The epitaxial relationship of a single-crystal AlN film was confirmed by pole figure and cross-sectional TEM. These results demonstrate that control of ion energy and energy of the sputter-deposited species is critical for film deposition at low temperature.

Vapor phase epitaxy of aluminum nitride from trimethylaluminum and molecular nitrogen

We have studied the pyrolysis of trimethylaluminum (TMA) in a nitrogen-containing atmosphere of a vapor phase epitaxy reactor. It is established that, in the presence of gallium nitride coatings in the reactor, the main product of TMA pyrolysis in a nitrogen-hydrogen atmosphere is aluminum nitride. Using this process (without introducing ammonia), we obtained perfect epitaxial aluminum nitride layers.

Epitaxial aluminum nitride tunnel barriers grown by nitridation with a plasma source

High critical current-density (10to420kA∕cm2) superconductor-insulator-superconductor tunnel junctions with aluminum nitride barriers have been realized using a remote nitrogen plasma from an inductively coupled plasma source operated in a pressure range of 10−3–10−1mbar. We find a much better reproducibility and control compared to previous work. From the current-voltage characteristics and cross-sectional transmission electron microscopy images it is inferred that, compared to the commonly used AlOx barriers, the polycrystalline AlN barriers are much more uniform in transmissivity, leading to a better quality at high critical current densities.

6H-SiC 위에 형성한 에피택시 AIN 박막 구조에 대한 전기적 특성의 평가온도 의존성

Epitaxial aluminum nitride films on 6H-SiC (0001) were fabricated using reactive RF magnetron sputtering and post-deposition rapid thermal annealing. The electrical properties of AIN films depending on film thickness and measurement temperature have been observed. Full width at half maximum of AIN (0002) was <TEX>$0.1204^{\circ}$</TEX> (about 430 arcsec) X-ray rocking curve results. The equivalent oxide thickness (EOT) of AIN film was estimated as about 10 nm and the leakage current density was within the order of <TEX>$10^{-8} 4/cm^2$</TEX>. The dielectric constant of AIN film estimated from the accumulation region of C-V curve measured at <TEX>$300^{\circ}C$</TEX> was 8.3. The dynamic dielectric constant was obtained as 5.1 from J vs. 1/T plots at the temperature ranging from R.T. to <TEX>$300^{\circ}C$</TEX> From above, estimation temperature dependance of the electrical properties of Al/AIN/SiC MIS devices was affirmed and useful data compilation for the reliabilities of SiC MIS is expected.

Pulsed-laser-deposited epitaxial aluminum nitride films on (111) Si for surface acoustic-wave applications

Epitaxial (001) aluminum nitride (AlN) thin films on (111) Si substrates are prepared using pulsed-laser deposition. The epitaxial structure of the as-prepared thin films is characterized by checking the X-ray-diffraction theta-2theta scan and pole-figure, using scanning electron microscopy, infrared radiation (IR) spectroscopy and Raman spectroscopy. The surface acoustic-wave resonance at 345 MHz for a 1.5 mum thick AlN film on a (111) Si substrate is observed using an interdigital electrode.

Pulsed low-energy ion-assisted growth of epitaxial aluminum nitride layer on 6H-silicon carbide by reactive magnetron sputtering

Epitaxial aluminum nitride thin films have been grown on silicon carbide (6H-SiC) substrates by pulsed low-energy ion-assisted reactive magnetron sputter deposition (+5/−20 V of bias pulses), with ion-assisted energy (Ei)≅22 eV, under ultrahigh-vacuum conditions. Surface ion interactions during the negative bias pulse gave rise to enhanced surface mobility of adatoms with beneficial effects, which extended over the limit of ion repelling in the positive pulse as the film thickness increased. High-resolution electron microscopy shows that a large (&amp;gt;90 nm) AlN domain width can form on the substrate. Domain-boundary annihilation and domain suppression during film growth have been observed. The growth rate also increased by a factor of ∼4 compared to growth conditions with no ion assistance (Ei=2 eV) and by a factor of 2 from dc ion-assisted growth. This indicates that the supply of nitrogen is a limiting factor for AlN formation and that the reactivity of nitrogen is increased on the growing AlN film surface for pulse ion-assisted deposition. High-resolution x-ray diffraction shows a reduction in the full width at half maximum of the rocking curve from 1490 to 1180 arcsec when pulsed ions are used. The cathodoluminescence shows high intensity of near-band edge emissions at wavelengths of 206 (6.02 eV) and 212 nm (5.84 eV) at a measured temperature of 5 K, with relatively low defect and oxygen and carbon impurity related emission, which is indicative of a high quality electronic material.

Ion beam assisted pulsed laser deposition of epitaxial aluminum nitride thin films on sapphire substrates

Epitaxial aluminum nitride films were grown on c- and r-plane oriented sapphire substrates by pulsed laser deposition technique. Additionally the influence of nitrogen/argon ion bombardment during film growth was studied. By X-ray diffraction measurements only the hexagonal wurzite structure of AlN was found. The epitaxial relationship between film and substrate was investigated by X-ray diffraction texture measurements. On c-plane oriented sapphire the wurzite c-axis is perpendicular to the substrate surface. On r-plane oriented sapphire not only an a-axis orientation but also a tilt of the hexagonal c-axis by 30° off the surface normal has been observed. Ion irradiation can improve epitaxy on c-plane oriented sapphire at a certain energy, depending on the angle of incidence. From the dechanneling dependence in Rutherford backscattering spectroscopy channeling spectra defect clusters, like stacking faults, are considered as dominating defects.

Near band-edge transitions in AlN thin films grown on different substrates

The optical properties of epitaxial aluminum nitride (AlN) films grown on sapphire, SiC, and Si substrates have been investigated in the (2–6.3) eV photon energy range using cathodoluminescence (CL) at liquid nitrogen temperature. Besides a broad luminescence band at 3.2 eV the CL spectra shows multiple, relatively narrow excitonic features in the near band-edge region at approximately 6.1 eV. The nature of the excitonic transitions is discussed and an estimate of the band-gap energy is given.

Epitaxial aluminum nitride films on sapphire formed by pulsed laser deposition

Highly oriented aluminum nitride thin films were grown on sapphire (0001)-substrate by pulsed laser deposition technique. Characterization was done by X-ray-diffraction, elastic recoil detection analysis and Rutherford backscattering/channeling measurements. The epitaxial properties were studied as function of the substrate temperature and the deposition rate. An epitaxial relation to the sapphire substrate is found to be AlN [0001] || Al 2O 3 [0001] and AlN [112̄0] || Al 2O 3 [101̄0]. XRD-texture-analysis on films deposited at 850°C shows a full width half maximum Δω of 0.13° (rocking curve) and Δϕ of 1.1° (in-plane).

Epitaxial aluminum nitride thin films grown by pulsed laser deposition in various nitrogen ambients

Abstract The effect of nitrogen ambient pressure on growth of AlN films has been examined. High-quality epitaxial AlN films were grown on (0001) sapphire substrates using pulsed laser deposition from a sintered AlN target in low nitrogen ambient of 9.0×10 −5 Torr. The orientation of AlN films can be controlled by nitrogen pressure. AlN films are c -axis oriented when grown in a nitrogen pressure of 9.0×10 −5 to 4.0×10 −2 Torr. Film orientation converted to a -axis as nitrogen pressure increased to 4.0×10 −1 Torr. The X-ray rocking curves of the AlN (0002) peak became narrower with decreasing ambient pressure and yielded a full width at half-maximum of 0.078°. The N/Al composition ratio increases with nitrogen pressure.

Photoluminescence characterization of Mg implanted GaN

Single crystalline (0001) gallium nitride layers, capped with a thin epitaxial aluminum nitride layer, were implanted with magnesium and subsequently annealed in vacuum to 1150-1300 °C for 10-60 minutes. Photoluminescence (PL) measurements showed the typical donor acceptor pair (DAP) transition at 3.25 eV after annealing at high temperatures, which is related to optically active Mg acceptors in GaN. After annealing at 1300 °C a high degree of optical activation of the implanted Mg atoms was reached in the case of low implantation doses. Electrical measurements, performed after removing the AlN-cap and the deposition of Pd/Au contacts, showed no p-type behavior of the GaN samples due to the compensation of the Mg acceptors with native n-type defects.

Epitaxial growth of AlN and GaN on Si(1 1 1) by plasma-assisted molecular beam epitaxy

Epitaxial wurtzite aluminum nitride (AlN) and gallium nitride (GaN) films have been grown on Si(1 1 1) by plasma-assisted molecular beam epitaxy (PA-MBE). Two-dimensional growth (2DG) of single-crystalline AlN films is achieved on Si(1 1 1) near stoichiometric supply of aluminum and atomic nitrogen. Two distinct categories of AlN-surface reconstructions have been observed. GaN films exhibit 1×1 high-energy electron diffraction (RHEED) pattern if grown between 650°C and 770°C substrate temperature on AlN buffers. Stable Ga- and N-adlayer-induced surface reconstructions have been studied below 600°C after the growth. The X-ray diffraction (XRD) pattern show sharp and well separated (0 0 0 l) reflections of wurtzite GaN and AlN indicating complete texture with GaN[0 0 0 1]‖AlN[0 0 01]‖Si[1 1 1]. From the determined GaN lattice constant complete strain relaxation can be concluded which is further confirmed by the temperature-dependent photoluminescence (PL) investigations.

Growth of atomically smooth AlN films with a 5:4 coincidence interface on Si(111) by MBE

Epitaxial aluminum nitride AlN(0001) thin films have been grown by plasma-assisted molecular beam epitaxy (PA-MBE) on Si(111). The influence of the composition of the nitrogen plasma on the crystal quality, as judged by X-ray diffractometry (XRD) and atomic force microscopy (AFM), has been investigated. Under an Al/N vapor phase ratio close to unity atomically smooth AlN films have been grown at 850°C substrate temperature with maximum growth rates of 2.5 nm min −1. A √3×√3 and a more Al-rich 2×6-surface reconstruction have been observed. Transmission electron microscopy (TEM) investigations show that these films are homogeneous 2H–AlN single crystals. Their defect structure consists of threading dislocations mostly. The hetero-interface is abrupt and flat. Processed high-resolution (HR) TEM images demonstrate a 4× d Si( 1 10) to 5× d AlN( 2 110) coincidence between substrate and epilayer. The XRD FWHM of the (0002)-diffraction peak of 0.5 μm AlN is 0.06° in the ω/2 ϑ scan and 0.32° in the ω scan. Phonon modes of AlN have been detected by Raman and infra-red spectroscopy.

Photoluminescence characterization of Mg implanted GaN

AbstractSingle crystalline (0001) gallium nitride layers, capped with a thin epitaxial aluminum nitride layer, were implanted with magnesium and subsequently annealed in vacuum to 1150-1300 oC for 10-60 minutes. Photoluminescence (PL) measurements showed the typical donor acceptor pair (DAP) transition at 3.25 eV after annealing at high temperatures, which is related to optically active Mg acceptors in GaN. After annealing at 1300 °C a high degree of optical activation of the implanted Mg atoms was reached in the case of low implantation doses. Electrical measurements, performed after removing the AlN-cap and the deposition of Pd/Au contacts, showed no p-type behavior of the GaN samples due to the compensation of the Mg acceptors with native n-type defects.

Nucleation of AlN on the (7×7) Reconstructed Silicon (1 1 1) Surface

The (7×7) reconstructed (1 1 1) surface of silicon is found to be an excellent surface for the nucleation of epitaxial aluminum nitride, despite the +23.4% misfit in the AlN/Si system. AlN nucleated above the (7×7) to (1×1) transition temperature (830°C) is found to contain 30° misoriented grains, while films nucleated below the transition temperature are single orientation. Optimized aluminum nitride films grown on (7×7) silicon surfaces make excellent substrates for GaN heteroepitaxy.

Pulsed laser deposition of epitaxial AlN, GaN, and InN thin films on sapphire(0001)

Epitaxial aluminum nitride (AlN), gallium nitride (GaN), and indium nitride (InN) films were grown by pulsed laser deposition on sapphire(0001) substrates in high vacuum and in a nitrogen ambient of a few mTorr. The films were analyzed using X-ray diffraction, scanning electron microscopy, atomic force microscopy, and room temperature van der Pauw-Hall measurements. The AlN films were single crystalline with AlN(0001)‖sapphire(0001) and AlN[11̄00]‖sapphire[112̄0]. The GaN films were also single crystalline with the same orientation as the AlN films. The InN films were found to have a preferred crystalline orientation with InN(0001)‖sapphire(0001) and InN[11̄00]‖sapphire[101̄0]. The AlN films were insulating. The GaN film grown in high vacuum had an n-type carrier concentration of 1 × 10 18 cm −3 and a mobility of 300 cm 2/V · s. The GaN film grown in 10 mTorr of N 2 had a p-type carrier concentration of 7.5 × 10 18 cm −3 and a mobility of 150 cm 2/V · s. The InN films grown in high vacuum and 5 mTorr of N 2 were both p-type with carrier concentrations of 6.5 × 10 20 and 4.7 × 10 19 cm −3, and mobilities of 30 and 240 cm 2/V · s, respectively.

High quality epitaxial aluminum nitride layers on sapphire by pulsed laser deposition

We have grown high quality epitaxial AlN layers on sapphire substrates by pulsed laser ablation of a stoichiometric AlN target. The AlN films deposited at 800 °C and laser energy densities in the range of 2–3 J/cm2 were found to be epitaxial with the c axis normal to the Al2O3(0001) surface. The x-ray rocking curve of epitaxial AlN films yielded a full width at half maximum of 0.21°. The selected area electron diffraction patterns and high resolution transmission electron microscopy also revealed that the films were epitaxial with an orientational relationship of AlN[0001]∥Al2O3[0001] and in-plain alignment of AlN[1̄21̄0]∥Al2O3[01̄10] and AlN[101̄0]∥Al2O3[2̄110]. This is equivalent to 30° rotation in the basal plane of the AlN film with respect to the sapphire substrate. The absorption edge measured by ultraviolet-visible spectroscopy for the epitaxial AlN film was sharp and the band gap was found to be 6.1 eV. The electrical resistivity of the films was about 5–6×1013 ohm cm with a breakdown field of 5×106 V/cm. At higher laser energy densities ≥10 J/cm2 and lower temperatures ≤650 °C, the deposited films were nitrogen deficient and contained free metallic aluminum, both of which degrade the microstructural, electrical, and optical properties of the AlN films.

Epitaxial AlN Thin Films Grown on α-Al2O3 Substrates by ECR Dual Ion Beam Sputtering

Aluminum nitride (AlN) thin films have been epitaxially grown on (001), (110) and (11̄2) Al2O3 substrates by ECR dual ion beam sputtering. It was found that (001)AlN could be epitaxially grown on (110)Al2O3 as well as on (001)Al2O3. Not only (110)AlN but also (116)AlN was epitaxially grown on (11̄2)Al2O3. The lattice mismatch in the two types of epitaxial growth was investigated by considering the chemical bonding between the oxygen atoms of α-Al2O3 and the aluminum atoms of hexagonal AlN. The lattice mismatches of the (116)AlNł(11̄2)Al2O3 and (110)AlNł(11̄2)Al2O3 were +1.7% and -4.4%, respectively. The lattice mismatch suggests that epitaxial growth of (116)AlN as well as (110)AlN on (11̄2)AlO3 is feasible.

Measurement of intrinsic stresses during growth of aluminum nitride thin films by reactive sputter deposition

Real-time measurements of intrinsic stresses during growth of polycrystalline and epitaxial aluminum nitride (AlN) thin films on Si(111) are reported. Our room-temperature measurements on polycrystalline films corroborate previous post-growth measurements. Our high-temperature measurements provide evidence of large intrinsic stresses during epitaxial growth of AlN on Si(111) and insignificant stress relaxation during growth.