A. Sereika

Visible–blind photoresponse of GaN-based surface acoustic wave oscillator

We describe the photoresponse of GaN-based surface acoustic wave (SAW) delay-line oscillator operating in the 200 MHz range. The decrease in oscillator frequency under ultraviolet illumination of GaN transducer is caused by the SAW velocity decrease due to the acoustoelectronic interaction with photoconductivity electrons. The oscillator frequency shift reaches its maximum value at 365 nm and drops to zero above 400 nm with visible/ultraviolet rejection ratio more than 100. The optical quenching of the photoconductivity in GaN was observed. These results demonstrate the potential of the GaN-based SAW oscillators for applications as visible–blind remote UV sensors.

AlGaN single-quantum-well light-emitting diodes with emission at 285 nm

We report on AlGaN single-quantum-well light-emitting diodes (LEDs) on sapphire with peak emission at 285 nm. A study is presented to identify the key material parameters controlling the device quantum efficiency. At room temperature, for a 200 μm×200 μm square geometry mesa type device, we obtain a power as high as 0.25 mW for 650 mA pulsed pumping. The LEDs show significantly higher output powers at temperatures below 100 K.

Acoustoelectric effects in reflection of leaky acoustic waves from LiTaO3 crystal surface coated with metal film

The strong variations in amplitude and phase of leaky acoustic wave excited on one surface of YX lithium tantalate crystal plate and reflected from the opposite parallel surface have been observed during thermal evaporation of copper film onto the reflection surface. These variations are caused by the acoustoelectric interaction in the reflection region, and they are described in terms of surface acoustic wave velocity and attenuation dependence on the film sheet conductivity. The electromechanical coupling constant value of 5% has been extracted from amplitude measurements, and that of 4% from the phase measurements.

AlGaN based highly sensitive radio-frequency UV sensor

The response of the AlGaN based radio-frequency (rf) sensor to deep ultraviolet (UV) illumination was investigated. Illumination by UV light emitting diodes with wavelengths from 280 to 375 nm significantly decreased rf oscillator frequency due to change in the impedance of AlGaN-based metal-semiconductor-metal structure. The UV-induced frequency shift attains 400 kHz from the oscillator dark frequency of 144.5 MHz with the highest sensitivity of 40 kHz/(μW/cm2) at 280 nm wavelength and UV power density less than 7.4 μW/cm2. The AlGaN-based rf oscillator allows for a wireless visible-blind and solar-blind UV sensing.

Ultraviolet-sensitive AlGaN-based surface acoustic wave devices

Two types of novel ultraviolet-sensitive III-nitride based surface acoustic wave devices are presented. We demonstrate the Al/sub x/Ga/sub l-x/N-based surface acoustic wave delay-line oscillator, which is applied as an ultraviolet sensor capable of solar-blind operation with remote wireless pickup of the output signal. Also, we report on the SAW-induced high-frequency resistance modulation effect, which is dramatically increased under ultraviolet illumination.

Impact of humidity on surface acoustic wave propagation in vanadium pentoxide xerogel-lithium niobate structure

The impact of ambient humidity on surface acoustic wave (SAW) propagation in the structure consisting of vanadium pentoxide xerogel (V2O5nH2O) layer deposited on a piezoelectric lithium niobate (LiNbO3) substrate has been studied. Thin V2O5nH2O layers were synthesized using sol–gel method and employed as humidity sensing materials. Freshly prepared layer drastically reduced the transmitted SAW signal, which later increased with time reaching saturation after more than 120 h at the level well below the free-surface value. The experimental observations of SAW behaviour were discussed and found related to the variations in resistance and dielectric permittivity of the V2O5nH2O.

Effect of substrate piezoelectricity on surface acoustic wave propagation in humidity-sensitive structures with porphyrin layers

The impact of air humidity on surface acoustic wave (SAW) propagation in different piezoelectric substrates (lithium niobate, lithium tantalate, and gallium nitride on sapphire) with nanostructured meso-tetra(4 sulfonatophenyl) porphyrin (TPPS4) overlays has been investigated. The humidity-induced increase in SAW attenuation is proportional to the electromechanical coupling constant of the substrate and is attributed to the viscoelastic loss due to interaction of water adsorbing TPPS4 material with electric field of the SAW. The humidity-induced decrease in SAW velocity does not depend on the substrate piezoelectric properties and is explained by the changes in TPPS4 layer mechanical properties due to adsorbed moisture.

Phase noises of GaN-based surface acoustic wave oscillator

Gallium nitride (GaN) is of interest with respect to electronics as it features both semiconducting and piezoelectric properties. We explore the possibilities for employing a GaN-based surface acoustic wave oscillator as a sensor of ultraviolet radiation, consider its thermal properties and phase noise.

III-Nitride Based Ultraviolet Surface Acoustic Wave Sensors

Large piezoelectric constants of AIN, GaN, and their alloys make these materials attractive for applications involving surface acoustic waves (SAW). The electrical conductivity and, thus, SAW velocity in these materials is affected by UV radiation. This allowed us to develop a SAW-based UV GaN sensor by placing a SAW element into an oscillator feedback loop. The output of such a sensor is a radio signal with UV radiation-dependent frequency, which makes this sensor especially attractive for remote sensing applications. In addition, the spectrum line width depends on the noise of the UV source and is especially narrow for solar radiation allowing for a solar-blind detection. We present the basic principles of the III-nitride based SAW UV sensors, summarize the results of theoretical simulations and experimental investigations of their properties, and discuss possible applications.

Highly sensitive radio-frequency UV sensor based on photocapacitive effect in GaN

We demonstrate a novel highly sensitive UV sensor based on RF LC oscillator with an optical frequency modulation due to the photocapacitive effect in an interdigitated aluminum-on-GaN/sapphire structure. In the range from 280 nm to 325 nm, the sensor exhibits extremely high sensitivity of 60 KHz/¿W/cm2 at low UV powers (¿30 ¿W). The maximum oscillator frequency shift of 4 MHz was observed at 375 nm corresponding to UV power density of 70 ¿W/cm2. This is at least two orders of magnitude higher than the values previously reported in literature for GaN-based SAW delay-line oscillators. The spectral response data from LC oscillator have been corroborated with direct measurements of photocapacitance and photoconductance values extracted from S-parameter measurements in both frequency and time domains. This sensor offers advantages of remote wireless signal transmission and high precision in frequency response measurements.