Changhe Huang

Optical and electrical properties of Al1−xInxN films grown by plasma source molecular-beam epitaxy

Epitaxial Al1−xInxN thin films with 0⩽x⩽1 have been grown by plasma source molecular beam epitaxy on sapphire (0001) substrates at a low temperature of 375 °C. Both reflection high-energy electron diffraction and x-ray diffraction measurements confirm the c-plane growth with the following epitaxial relations: nitride [0001] ∥ sapphire [0001] and nitride 〈0110〉 ∥ sapphire 〈2110〉. However, the degree of crystalline mosaicity and the compositional fluctuation increase with increasing x. The observed direct energy band gap, determined using optical transmission and reflection measurements show relatively less bowing compared to some earlier studies. Electrical resistivity and Hall effect measurements show n-type electrical conductivity in these alloys with carrier concentrations n⩾1019 cm−3 for In-rich alloys and n⩽1010 cm−3 for Al-rich alloys.

Pd/AlN/SiC thin-film devices for selective hydrogen sensing

A hydrogen sensing device has been fabricated using a 1500- A-thick AlN layer interposed between a SiC substrate and a Pd gate. This device behaves as a rectifying diode. Hydrogen in the gas flow causes a shift in the forward conduction and the sensitivity of the device is found to increase with the total flow rate. At a device temperature of 300 °C and a flow rate of 1000 sccm, the typical forward current of ∼1 mA increases by 10% for 10 ppm hydrogen, independent of the presence of oxygen or CO in the gas flow. The shift is also independent of the presence of propane. However, for very large concentrations of propane a slight response can be discerned, due to the presence of some hydrogen resulting from the incipient decomposition of the hydrocarbon at elevated temperature.

Pd/AlN/Si or SiC Structure for Hydrogen Sensing Device

An AlN (insulator) MIS Hydrogen Sensor was created using plasma source molecular beam epitaxy (PSMBE) deposition on Si (111) and 6H-SiC. A Pd layer was deposited on top of the AlN film via magnetron sputtering technique utilizing a hard mask. Pd was chosen since H 2 readily diffuses within its bulk, thus Pd acts not only as a metal electrode of the MIS structure, but also as a catalyst for hydrogen dissociation. To optimize the design structure several sensors with different AlN and Pd thickness have been developed. RHEED and XRD measurements show that AlN film is epitaxial on both Si (111) and 6H-SiC substrates. The sensors were characterized using capacitance versus voltage C(V) and I(V) measurements, at different frequencies ranging from 1kHz to 1 MHz. Shifts in the C-V and I-V curves occurred with the introduction of hydrogen in the chamber. The temperature, hydrogen partial pressure, effects of oxygen and hydrocarbon gases, insulator and metal thicknesses on sensor response were analyzed.

Three-dimensional ultrasound imaging

In medical and other important applications, it is desirable to achieve transmissive ultrasonic 3D imaging in “real time.” Until now, transmissive ultrasonic images have been possible, but either in real time 2D or relatively slow 3D. By using an ultrasonic stereo camera and a real time viewing system, live and interactive 3D transmissive ultrasonic images have been produced. Fast transmissive ultrasound tomography has been achieved by using a single ultrasonic camera. Some advantages and applications are discussed.

Direct ultrasonic holography: Feasibility demo

Holography is potentially the most powerful method for high spatial resolution real time 3D ultrasound imaging and diagnostics. Although versions of ultrasound holography have existed since the 1970s, they use laser reconstruction, thus losing 3-D resolution. By using transmissive ultrasound with a scanning needle hydrophone, full 3D holographic imaging is proved to be possible, if both amplitude and time delay phase information can be obtained.

A Study of Pt/AlN/6H-SiC MIS Structures for Device Applications

AlN films with thicknesses from 53 to 79 nm were deposited on 6H-SiC substrates via Plasma Source Molecular Beam Epitaxy (PSMBE). The influence of deposition temperature on the growth mode and film roughness was assessed. The optical constants of the films in the range 0.73-8.75 eV were determined using spectroscopic ellipsometry. Pt/AlN/6H-SiC MIS structures were created and current-voltage (I-V) and capacitance-voltage (C-V) measurements were performed at room temperature and at 250°C. Most of the MIS structures showed rectifying I-V characteristics regardless of growth temperature. A 120-nm-thick AlN film was deposited at 500°C. MIS structures created on this film showed a very low leakage current densities of 6×10 −8 A/cm 2 . The dielectric constant of the film was estimated at approximately 9. The relation between film structure and electrical properties of the films is discussed.

Temperature Effect on the Quality of Ain Thin Films

AIN thin films were deposited at various substrate temperatures via Plasma Source Molecular Beam Epitaxy. The films were grown on 6H-SiC (0001) substrates. Reflection High Energy Electron Diffraction and Atomic Force Microscopy showed a dramatic change in the surface morphology of the film grown at 640°C. This is attributed to a change in the growth mechanism from pseudomorphic at lower temperatures to three-dimensional at higher than 640°C temperatures. Photoreflectance measurements showed an absorption shift toward 200 nm as the deposition temperature increases which is attributed to the change in the growth mechanism at higher temperatures. X-Ray Diffraction was unable to conclusively determine the AIN (0002) peak due to a significant diffuse intensity from the SiC (0002) peak. A MIS structure was created by depositing Pt contacts on the film grown at 500°C. I-V measurements showed that the Pt/AIN contact is Schottky.