Aluminum Nitride Surface Research Articles

Surface modification of piezoelectric aluminum nitride with functionalizable organosilane adlayers

The world of biosensors is expanding at a rapid pace with an ever-increasing demand for more sensitive miniaturized devices. Acoustic wave biosensors are not spared from this trend. In this domain, the search for enhanced sensitivity is increasingly oriented toward the rational design of new piezoelectric materials with superior properties to substitute for prevalent quartz. With respect to surface chemistry, construction of the biorecognition element, more often than not, requires the use of bifunctional molecules that can spontaneously assemble on the substrate and form organic surfaces readily biofunctionalizable in a subsequent, ideally single step. In this context, we present herein the surface modification of aluminum nitride (AlN) with alkyltrichlorosilane cross-linking molecules bearing a functionalizable benzenethiosulfonate moiety. This latter feature is next demonstrated through the straightforward, preactivation-free immobilization of thiolated biotin probes. To date, AlN has only received little attention in the field of piezoelectric biosensors despite its many attractive properties and the perspective to operate devices at ultra-high frequencies (GHz) with unprecedented sensitivity. To our knowledge, this work describes one of the first examples of direct surface derivatization of AlN with bifunctional trichlorosilane molecules. It also constitutes a first step toward the development of electrodeless GHz piezoelectric biosensing platforms based on AlN and trichlorosilane surface chemistry.

Surface plasma pretreatment for enhanced diamond nucleation on AlN

The surface of polycrystalline aluminum nitride (AlN) thin films is exposed to different gas discharge plasmas (N2, O2, and CF4) followed by a water-based colloidal seeding of ultra-dispersed nanodiamond particles. Fluorination of the AlN surface enhances the seeding density, whereas the oxidized surface does not yield any nucleation sites. In the former case, the seeding density improves by almost three orders of magnitude as compared to the untreated and N2 pretreated samples, and allows to grow pinhole-free nanocrystalline diamond film on AlN. Finally, we demonstrate a route towards selective diamond growth on AlN thin films by employing CF4 plasma pretreatment together with photolithography.

NO2 detection by nanosized AlN sheet in the presence of NH3: DFT studies

Adsorption of NH3 and NO2 molecules on aluminum nitride (AlN) nanosheets was investigated by using density functional calculations. Equilibrium geometries, stabilities, and electronic properties of NH3 and NO2 adsorptions on the surface of an AlN surface were identified. The adsorption energies were calculated to be about −91.84 and −95.02kJ/mol for NH3 and NO2 corresponding to the most stable configurations, respectively. It was revealed that the electrical conductivity of the sheet may be increased upon the NO2 adsorption, being insensitive toward NH3 adsorption. Thus, the AlN sheet may selectively detect NO2 molecules in the presence of NH3 molecules.

Ultra-fast photo-patterning of hydroxamic acid layers adsorbed on TiAlN: The challenge of modeling thermally induced desorption

Long-chain n-alkyl terminated hydroxamic acids (HA) are used for the modification of titanium aluminum nitride (TiAlN) surfaces. HA coatings improve the hydrophobicity of this wear resistant and industrially relevant ceramic. Therefore, HAs with different structural properties are evaluated with respect to their wear resistance and their thermal desorption properties. In order to find new coatings for rewritable offset printing plates, the changes in the surface polarity, composition, and morphology are analyzed by contact angle measurements, X-ray photoemission spectroscopy (XPS), and scanning force microscopy (SFM), respectively. The results are referenced to the strongly bonding molecule n-dodecyl phosphonate (PO11M), which has been used for surface hydrophobization before but proved difficult to remove due to the high laser outputs required for thermal desorption. It is found that for certain HAs, an equally good hydrophobization compared to PO11M can be achieved. Contact angles obtained for different hydroxamic acid coatings can be correlated to their modes of adsorption. Only for selected HA species, resistance to mechanical wear is sufficient for further investigations. Photo-patterning of these hydroxamic acid layers is achieved using a high energy IR laser beam at different energy inputs. Fitting of the obtained data and further evaluation using finite element analysis (FEM) calculations reveal significantly reduced energy consumption of about 20% for the removal of a specific hydroxamic acid coating from the ceramic surface compared to PO11M.

A Q-DLTS investigation of aluminum nitride surface termination

Abstract

An Aluminum Nitride-based chemical sensor using Q-DLTS

The wide band-gap semiconductor material, single crystal Aluminum Nitride (AlN), was used in the development of a chemical sensor based on the Quantum Fingerprint™ method. The carefully prepared AlN surface was interrogated following exposure to test vapors of various species using a Charge-based Deep Level Transient Spectroscopy (Q-DLTS) instrument to analyze the presence of surface charge states induced in the band-gap by adsorbed vapor molecules. The AlN was metallized with a titanium adhesion layer topped with a gold contact layer. This produced a contact with Schottky-like properties. The AlN surface was terminated with hydrogen and was tested for its responses to water vapor, several alcohols, ethyl acetate, toluene, and chloroform in a small enclosure. The effect on the Q-DTLS spectrum was compared in each case to the baseline spectrum obtained prior to the testing exposure. All Q-DLTS spectra were observed to return to the preceding spectrum background upon removal of the test vapor. Spectral differences were observed that allowed discrimination to be made between the species tested. The number of peaks varied between one and three and their locations spanned the entire available spectral range. Combinations of the peak locations, amplitude, and amplitude ratios may be used to discriminate between different species.

Thermal and insulating properties of epoxy/aluminum nitride composites used for thermal interface material

AbstractWe synthesized an epoxy matrix composite adhesive containing aluminum nitride (AlN) powder, which was used for thermal interface materials (TIM) in high power devices. The experimental results revealed that adding AlN fillers into epoxy resin was an effective way to boost thermal conductivity and maintain electrical insulation. We also discovered a proper coupling agent that reduced the viscosity of the epoxy‐AlN composite by AlN surface treatment and increased the solid loading to 60 vol %. For the TIM sample made with the composite adhesive, we obtained a thermal conductivity of 2.70 W/(m K), which was approximately 13 times larger than that of pure epoxy. The dielectric strength of the TIM was 10 to 11 kV/mm, which was large enough for applications in high power devices. Additionally, the thermal and insulating properties of the TIM did not degrade after thermal shock testing, indicating its reliability for use in power devices. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Y2O3함량과 소결조건에 따른 상압소결 AlN 세라믹스의 열전도도 고찰

Aluminum nitride (AlN) has excellent thermal conductivity, whereas it has some disadvantage such as low sinterability. In this study, the effects of sintering additive content and sintering condition on thermal conductivity of pressureless sintered AlN ceramics were examined on the variables of 1~3 wt% sintering additive (Y 2 O 3 ) content at 1900℃ in N 2 atmosphere with holding time of 2~10 h. All AlN specimens showed higher thermal conductivity as the Y 2 O 3 content and holding time increase. The formation of secondary phases (yttrium aluminates) by reaction of Y 2 O 3 and Al 2 O 3 from AlN surface promoted the thermal conductivity of AlN specimens, because the secondary phases could reduce the oxygen contents in AlN lattice. Also, thermal conductivity was increased by long sintering time because of the uniform distribution and the elimination of the secondary phases at the grain boundary by the evaporation effect during long holding time. A carbothermal reduction reaction was also affected on the thermal conductivity. The thermal conductivity of AlN specimens sintered at 1900℃ for 10 h showed 130~200W/mK according to the content of sintering additive.

Deposition of ultrathin AlN films for high frequency electroacoustic devices

The authors investigate the microstructure, crystal orientation, and residual stress of reactively sputtered aluminum nitride (AlN) films having thicknesses as low as 200 down to 25 nm. A two-step deposition process by the dual cathode ac (40 kHz) powered S-gun magnetron enabling better conditions for AlN nucleation on the surface of the molybdenum (Mo) bottom electrode was developed to enhance crystallinity of ultrathin AlN films. Using the two-step process, the residual in-plane stress as well as the stress gradient through the film thickness can be effectively controlled. X-ray rocking curve measurements have shown that ultrathin films grown on Mo using this technology are highly c-axis oriented with full widths at half maximum of 1.8° and 3.1° for 200- and 25-nm-thick films, respectively, which are equal to or even better than the results previously reported for relatively thick AlN films. High-resolution transmission electron microscopy and fast Fourier transform analyses have confirmed strong grain orientation in 25–100-nm-thick films. A fine columnar texture and a continuous lattice microstructure within a single grain from the interface with the Mo substrate through to the AlN surface have been elicited even in the 25-nm-thick film.

Electrical characterization and morphological properties of AlN films prepared by dc reactive magnetron sputtering

Aluminum nitride (AlN) films were deposited by dc reactive magnetron sputtering on p-Si-(1 0 0) substrate in Ar–N 2 gas mixtures. The effects of nitrogen concentration and sputtering power on AlN films deposition rate, crystallographic orientation, refractive index, and surface morphology are investigated by means of several characterization techniques. The results show that AlN films reasonably textured in (0 0 2) orientation with low surface roughness can be obtained with the deposition rate as high as 70 nm/min by the control of either target power or N 2 concentration in the gas mixture. Increasing the dc discharge power, Al atoms are not completely nitridized and the Al phases appear, as well as the AlN phases. MIS (Metal–Insulator–Semiconductor) structures were fabricated and electrically evaluated by I– V (current–voltage) and C– V (capacitance–voltage) measurements at high frequency (1 MHz). The results obtained from C– V curves indicate that charges at the dielectric/semiconductor interface occur, and the dielectric constant values (extracted under strong accumulation region) are compatible with those found in literature.

Site-Selective Biofunctionalization of Aluminum Nitride Surfaces Using Patterned Organosilane Self-Assembled Monolayers

Surface biochemical functionalization of group-III nitride semiconductors has recently attracted much interest because of their biocompatibility, nontoxicity, and long-term chemical stability under demanding physiochemical conditions for chemical and biological sensing. Among III-nitrides, aluminum nitride (AlN) and aluminum gallium nitride (AlGaN) are particularly important because they are often used as the sensing surfaces for sensors based on field-effect transistor or surface acoustic wave (SAW) sensor structures. To demonstrate the possibility of site-selective biofunctionalization on AlN surfaces, we have fabricated two-dimensional antibody micropatterns on AlN surfaces by using patterned self-assembled monolayer (SAM) templates. Patterned SAM templates are composed of two types of organosilane molecules terminated with different functional groups (amino and methyl), which were fabricated on AlN/sapphire substrates by combining photolithography, lift-off process, and self-assembly technique. Because the patterned SAM templates have different surface properties on the same surface, clear imaging contrast of SAM micropatterns can be observed by field-emission scanning electron microscopy (FE-SEM) operating at a low accelerating voltage in the range of 0.5-1.5 kV. Furthermore, the contrast in surface potential of the binary SAM microstructures was confirmed by selective adsorption of negatively charged colloidal gold nanoparticles (AuNPs). The immobilization of AuNPs was limited on the positively charged amino-terminated regions, while they were scarcely found on the surface regions terminated by methyl groups. In this work, selective immobilization of green fluorescent protein (GFP) antibodies was demonstrated by the specific protein binding of enhanced GFP (EGFP) labeling. The observed strong fluorescent signal from antibody functionalized regions on the SAM-patterned AlN surface indicates the retained biological activity of specific molecular recognition resulting from the antibody-EGFP interaction. The results reported here show that micropatterning of organosilane SAMs by the combination of photolithographic process and lift-off technique is a practical approach for the fabrication of reaction regions on AlN-based bioanalytical microdevices.

Carbon attachment on the aluminum nitride gate dielectric in the pentacene-based organic thin-film transistors

This study presents carbon attachment on an aluminum nitride (AlN) gate dielectric to improve the device performance of pentacene-based organic thin-film transistors (OTFTs). This approach produces high OTFT performance on an aged AlN surface. A high mobility of 0.67 cm2/V s was achieved on an AlN surface aged for 14 days, compared to a mobility of 0.05 cm2/V s on an as-deposited AlN surface. This improvement in device performance is correlated with carbon attachment on the AlN surface, which lowers surface energy. The lowered surface energy made the surface less polar, as measured by a contact angle instrument. The chemical composition of the aged AlN surface was analyzed using x-ray photoelectron spectroscopy before pentacene deposition. Enhanced C=C bonding at 284.5 eV was observed on the aged AlN surface. These enhanced C=C bonds favored the growth of large pentacene islands in the initial growth stage, which may improve OTFT device performance.

MOVPE growth of single-crystal hexagonal AlN on cubic diamond

We have obtained single-crystal aluminum nitride (AlN) layers on diamond (1 1 1) substrates by metalorganic vapor-phase epitaxy (MOVPE). When the thermal cleaning temperature of the substrate and growth temperature of the AlN layer were below 1100 °C, the AlN layer had multi-domain structures mainly consisting of rotated domains. An interface layer, consisting of amorphous carbon and poly-crystal AlN, was formed between the AlN layer and the diamond substrate. On the other hand, when the thermal cleaning temperature and growth temperature were above 1200 °C, a single-crystal AlN layer was grown and no interface layer was formed. Therefore, we attribute the multi-domain structures to the interface layer. Even at the growth temperature of 1100 °C, by performing the thermal cleaning at 1200 °C, the single-crystal AlN layer was obtained, indicating that the thermal cleaning temperature of the substrate is a critical factor for the formation of the interface layer. The epitaxial relationship between the single-crystal AlN layer and the diamond (1 1 1) substrate was determined to be [0 0 0 1] AlN∥[1 1 1] diamond and [1 0 1¯ 0] AlN∥[1 1¯ 0] diamond. The AlN surface had Al polarity and no inversion domains were observed in the AlN layer.

The effect of AlN buffer growth parameters on the defect structure of GaN grown on sapphire by plasma-assisted molecular beam epitaxy

The defect structure of GaN film grown on sapphire by plasma-assisted molecular beam epitaxy (PAMBE) depends on the growth temperature and thickness of the aluminum nitride (AlN) buffer layer. High-resolution X-ray diffraction was used to measure symmetric (0 0 0 2) and asymmetric (1 0 1¯ 2) rocking curve (ω-scans) broadening, which allowed the estimation of screw threading dislocation (TD) and edge TD densities, respectively. For GaN grown on lower-temperature buffer, the density of screw TD was increased while the density of edge TD was decreased. Further examinations revealed that the edge TD was closely related to stress in GaN film and the screw TD was controlled by AlN surface roughness. Since the GaN defect was dominated by edge TD, the total TD was also effectively suppressed with the use of lower-temperature buffer with appropriate thickness.

Formation of Antibody Microarrays on Aluminum Nitride Surface Using Patterned Organosilane Self-Assembled Monolayers

AbstractSurface biofunctionalization of group-III nitride semiconductors has recently attracted much interest due to their biocompatibility, nontoxicity, and long-term chemical stability under demanding physiochemical conditions for chemical and biological sensing. Among III-nitrides, aluminum nitride (AlN) and aluminum gallium nitride (AlGaN) are particularly important because they are often used as the sensing surfaces for sensors based on field-effect transistor or surface acoustic wave sensor structures. Patterned self-assembled monolayer (SAM) templates are composed of two types of organosilane molecules terminated with different functional groups (amino and methyl), which were fabricated on AlN/sapphire substrates by combining photolithography, lift-off process, and self-assembly technique. Clear imaging contrast of SAM micropatterns can be observed by field emission scanning electron microscopy (FE-SEM) operating at a low accelerating voltage in the range of 0.5–1.5 kV. In this work, the formation of green fluorescent protein (GFP) antibody microarrays was demonstrated by the specific protein binding of enhanced GFP (EGFP) labeling. The observed strong fluorescent signal from antibody functionalized regions on the SAM-patterned AlN surface indicates the retained biological activity of specific molecular recognition resulting from the antibody–EGFP interaction. The results reported here show that micropatterning of organosilane SAMs by the combination of photolithographic process and lift-off technique is a practical approach for the fabrication of reaction regions on AlN-based bioanalytical microdevices.

Immobilization of DNA-Au nanoparticles on aminosilane-functionalized aluminum nitride epitaxial films for surface acoustic wave sensing

A generic method for immobilization of gold nanoparticle bioconjugates onto aluminum nitride (AlN) surfaces using aminosilane molecules as cross-linkers is demonstrated for surface acoustic wave (SAW) sensor applications. Electrostatic interaction between positively charged surface amine groups and negatively charged DNA-Au nanoparticle conjugates allows the self-assembly of a probe nanoparticle monolayer onto functionalized AlN surfaces under physiological conditions. Both 10 and 20 nm Au nanoparticles bound with thiolated oligonucleotides were employed in the detection scheme. We show that Au nanoparticles can play multiple roles in SAW sensing for probe immobilization, signal amplification, and labeling.

Pentacene Patterning on Aluminum Nitride by Water Dipping

This study reports a pentacene patterning method that can be combined with conventional lithography to pattern pentacene film. The aluminum nitride (AlN) surface was patterned using a conventional photolithography process and then treated with oxygen plasma on uncovered AlN to modify surface polarity. Following pentacene deposition, the sample was dipped in water to remove pentacene from the plasma-treated area. The plasma-treated AlN surface was analyzed using X-ray photoelectron spectroscopy (XPS) before pentacene deposition. The polar surface energy changed from when the AlN surface was treated with plasma at for . The polar surface energy was attributed to the increase of Al–O bonds on the surface based on XPS measurements. The intrusion energy of water was enhanced from due to the polar surface energy induced by the plasma treatment. The enhancement of water intrusion energy and the polar surface energy explains the water-removable pentacene patterning mechanism.

Improved Method of CO2 Laser Cutting of Aluminum Nitride

The traditional “evaporation∕melt and blow” mechanism of CO2 laser cutting of aluminum nitride (AlN) chip carriers and heat sinks suffers from energy losses due to its high thermal conductivity, formation of dross, decomposition to aluminum, and uncontrolled thermal cracking. In order to overcome these limitations, a thermochemical method that uses a defocused laser beam to melt a thin layer of AlN surface in oxygen environment was utilized. Subsequent solidification of the melt layer generated shrinkage and thermal gradient stresses that, in turn, created a crack along the middle path of laser beam and caused material separation through unstable crack propagation. The benefits associated with thermal stress fracture method over the traditional method are improved cut quality, higher cutting speed, and lower energy losses.

Thermal stress fracture mode of CO2 laser cutting of aluminum nitride

A thermal stress fracture mode of material removal by laser cutting was conducted in 1-mm thick wafers of aluminum nitride (AlN) using a continuous wave CO2 laser with a defocused beam. In this mode, a thin layer (10–20 μm) of AlN surface was melted in an oxygen environment to form aluminum oxide. Solidification of the melt layer coupled with thermal expansion mismatch generated thermal stresses that in turn created a crack along the middle path of the laser beam, resulting in material separation. Thermochemical modeling of laser heating, oxide forming, and subsequent cooling of AlN was performed to validate the formation of cracks as well as material separation through unstable crack propagation. A comparison with the conventional “evaporation/melt and blow” laser cutting method showed that the thermal stress method offers significant benefits such as improved precision, better cut quality, higher cutting speed, and lower energy losses.

Improved dispersion of aluminum nitride particles in epoxy resin by adsorption of two-layer surfactants

The purpose of this research is to enhance the thermal conductivity of polymeric protective coatings on electronic components. Thermally conductive but electrically insulating solid powders of aluminum oxide or aluminum nitride (AlN) are commonly dispersed into liquid epoxy resin, and this dispersion is applied as a coating, before the complete polymerization that occurs afterwards. The viscosity cannot be high initially, or damage to the delicate component might result during application. In the present study, the dispersibility of aluminum oxide powder in epoxy monomer resin was found to be better than that of aluminum nitride powder, because of the weaker basicity of the oxide surface compared with the nitride. To improve the dispersibility of higher conductivity AlN in liquid epoxy monomer, the solid AlN surface was modified by pretreatment with silane coupling agents. Methylsilane gave lower viscosities than chloro- or methoxysilane, while pretreatments using organic acids increased the viscosity of the AlN dispersion. The viscosity changes and FTIR peak intensity trends suggested that the silane molecules could be adsorbed on AlN surfaces in the form of a monolayer during optimization experiments, and the best silane monolayer coverage on the AlN powder surfaces was achieved with 2 wt.% amounts in a carrier solvent during a 3 h pretreatment. In addition, a particular phosphate ester was a good second layer dispersant for the AlN-plus-epoxy system. When that additional dispersant was added onto the silane-treated filler surfaces, the degree of viscosity reduction was dependent on the types of silane coupling agent functional groups. The conclusion was that silane pretreatment followed by the second dispersant was better than either dispersant alone.