Scanning near-field PL spectroscopy was applied to study spatial variations of the emission spectra of AlGaN epilayers with AlN molar fractions between 0.3 and 0.7. Experiments were performed at 300 K with 100 nm spatial resolution. In general, photoluminescence spectra were found to be highly uniform with the peak energy deviation of 2 to 6 meV for different alloy compositions. In the 30% and 42% Al layers, a slightly lower Al content and a higher point defect concentration at the boundaries of growth domains were detected. These features were attributed to the higher mobility of Ga adatoms during growth. The inhomogeneous broadening beyond the random alloy distribution was found negligible for the 30% and 42% Al samples, and about 40–50 meV for the layers with a larger Al content.
We discuss factors affecting the external quantum efficiency, droop and reliability of AlGaN deep ultraviolet (DUV) light emitting diodes (LED) grown on sapphire substrates. Improvement of LED performance is achieved by suppression of the nonradiative recombination in epitaxial structures with dislocation density reduced to below 5x10<sup>8</sup> cm<sup>-2</sup>, transparent LED structure design and optimized UV encapsulation for enhanced light extraction. Relatively low light extraction efficiency remains to be a key factor limiting LED output power and quantum efficiency.
We present the results of design, fabrication, and characterization of the room-temperature, low electron heat capacity
hot-electron THz microbolometers based on two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures. The
2DEG sensor is integrated with a broadband THz antenna and a coplanar waveguide. Devices with various patterning of
2DEG have been fabricated and tested. Optimizing the material properties, geometrical parameters of the 2DEG, and
antenna design, we match the impedances of the sensor and antenna to reach strong coupling of THz radiation to 2DEG
via the Drude absorption. Testing the detectors, we found that the THz-induced photocurrent, ΔI, is proportional to the
bias current, I, and the temperature derivative of the resistance and inversely proportional to the area of 2DEG sensor, S.
The analysis allowed us to identify the mechanism of the 2DEG response to THz radiation as electron heating. The
responsivity of our sensors, normalized to the bias current and to unit area of 2DEG, R*= ΔI•S/ (I∙P), is ~ 10<sup>3</sup> W<sup>-1</sup> μm<sup>2</sup>.
So, for our typical sensor with an area of 1000 μm<sup>2</sup> and bias currents of ~ 10 mA, the responsivity is ~ 0.01 A/W. The
measurements of mixing at sub-terahertz frequencies showed that the mixing bandwidth is above 2 GHz, which
corresponds to a characteristic electron relaxation time to be shorter than 0.7 ps. Further decrease of the size of 2DEG
sensors will increase the responsivity as well as allows for decreasing the local oscillator power in heterodyne
We present results on design, fabrication, and characterization of hot-electron bolometers based on low-mobility
two-dimensional electron gas (2DEG) in AlInN/GaN and AlGaN/GaN heterostructures. Electrical and optical
characterization of our Hot Electron Bolometers (HEBs) show that these sensors combine (i) high coupling to incident
THz radiation due to Drude absorption, (ii) significant electron heating by the THz radiation due to small value of the
electron heat capacity, (iii) substantial sensitivity of the device resistance to the heating effect. A low contact resistance
(below 0.5 Ω·mm) achieved in our devices ensures that the THz voltage primarily drops across the active region. Due to
a small electron momentum relaxation time, the inductive part of the impedance in our devices is large, so these sensors
can be combined with standard antennas or waveguides. In the capacity of the THz local oscillator (LO) for heterodyne
THz sensing, we fabricated AlGaAs/GaAs quantum cascade lasers (QCLs) with a stable continuous-wave single-mode
operation in the range of 2.5-3 THz. Spectral properties of the QCLs have been studied by means of Fourier transform
spectroscopy. It has been demonstrated that the spectral purity of the QCL emission line doesn't exceed the spectrometer
resolution limit at the level of 0.1 cm<sup>-1</sup> (3 GHz). Discrete spectral tuning can be achieved using selective devices; fine
tuning can be done by thermally changing the refractive index of the material and by applied voltage. Compatibility of
the low-mobility 2DEG microbolometers with QCLs in terms of LO power requirements, spectral coverage, and cooling
requirements makes this technology especially attractive for THz heterodyne sensing.
III-Nitride based deep ultraviolet (DUV) light emitting diodes (LEDs) rapidly penetrate into sensing market owing to
several advantages over traditional UV sources (i.e. mercury, xenon and deuterium lamps). Small size, a wide choice of
peak emission wavelengths, lower power consumption and reduced cost offer flexibility to system integrators. Short
emission wavelength offer advantages for gas detection and optical sensing systems based on UV induced fluorescence.
Large modulation bandwidth for these devices makes them attractive for frequency-domain spectroscopy. We will
review present status of DUV LED technology and discuss recent advances in short wavelength emitters and high power
Gate-voltage tunable plasmon resonances in the two dimensional electron gas of high electron mobility transistors
(HEMT) fabricated from the InGaAs/InP and AlGaN/GaN materials systems are reported. Gates were in the form of a
grating to couple normally incident THz radiation into 2D plasmons. Narrow-band resonant absorption of THz radiation
was observed in transmission for both systems in the frequency range 10 - 100 cm<sup>-1</sup>. The fundamental and harmonic
resonances shift toward lower frequencies with negative gate bias. Calculated spectra based on the theory developed for
MOSFETs by Schaich, Zheng, and McDonald (1990) agree well with the GaN results, but significant differences for the
InGaAs/InP device suggest that modification of the theory may be required for HEMTs in some circumstances.
Pronounced resonant absorption and frequency dispersion associated with an excitation of collective 2D plasmons have
been observed in terahertz (0.5-4THz) transmission spectra of grating-gate 2D electron gas AlGaN/GaN HEMT (high
electron mobility transistor) structures at cryogenic temperatures. The resonance frequencies correspond to plasmons
with wavevectors equal to the reciprocal-lattice vectors of the metal grating, which serves both as a gate electrode for the
HEMT and a coupler between plasmons and incident terahertz radiation. The resonances are tunable by changing the
applied gate voltage, which controls 2D electron gas concentration in the channel. The effect can be used for resonant
detection of terahertz radiation and for "on-chip" terahertz spectroscopy.
We review the physics of deep UV LEDs with emphasis on the features that differ from those for visible LEDs. We
discuss UV designs, novel growth process of light generating structures (MEMOCVD<sup>TM</sup>) that allows for reducing the
growth temperature and improving materials quality, and "phonon engineering" approach that takes advantage of high
polar optical energy in AlN/GaN/InN materials for confining electrons in the light emitting quantum wells. We then
review the characteristics of DUV LEDs grown on sapphire substrates with peak emission wavelength from 250 to 340
nm that demonstrate the lowest optical noise among all other UV light sources and, therefore, are well suited for the
detection of hazardous biological agents using fluorescence techniques. Finally, we describe high power multi-chip,
multi-wavelength deep UV light sources and review emerging applications of deep UV LED technology.
We demonstrate a compact system incorporating a 32-element linear array of ultraviolet (UV) light-emitting
diodes (LEDs) to the in-flight fluorescence detection of aerosolized particles. Custom electronics manage a standalone
system and enable real-time processing of spectral data, which is used to cue a miniaturized aerodynamic deflector for
physical particle separation. This front-end system improves the prospects for many second-stage analysis methods by
reducing the background particle burden and providing a suspicious-particle enriched sample. The performance of UV
LED arrays as an excitation source is established by the ability to detect emission from NADH and tryptophan in aerosol
samples. On-the-fly fluorescence collection, operation of a real-time spectral algorithm, and aerosol concentration is
demonstrated by separating particles that exhibit a specific spectral feature from a background of otherwise fluorescing
A set of UV light-emitting diodes (LEDs) with the peak wavelengths ranging from 255 nm to 375 nm was applied for
the investigation of spectral and decay-time fluorescence signatures in dry <i>B. globigii </i>spores and common airborne
interferants (albuminous, epithelium, and cellulosous materials as well as aromatic hydrocarbons). The fluorescence
decay signature was represented by a phase shift of the sinusoidal fluorescence waveform in respect of excitation
provided by high-frequency modulated LEDs. The obtained data matrix was used for the optimization a bioparticle
fluorescence sensor with a minimized number of excitation sources and detection channels and maximized
discrimination ability of bioparticles against common interferants. Based on the optimization, a new concept for a UV
LED based "detect-to-warn" bioparticle fluorescence sensor is proposed. The sensor contains a single deep-UV LED
emitting at 280 nm that is harmonically modulated at a high frequency (of about 70 MHz) and a dual-channel
fluorescence detector with the spectral windows peaked at 320 nm and 450 nm. The output parameters of the sensor are
the ratio of the fluorescence intensity in the two windows and the phase shift of the fluorescence waveform in the
320-nm detection channel in respect of the excitation one. Such a sensing scheme has a smaller number of optical
components and a potentially higher discrimination ability of bioparticles against common interferants in comparison
with the conventional approach based on just fluorescence intensity measurement under dual-wavelength excitation
(280 nm and 340 nm).
We present a review of our work on the development and applications of AlInGaN-based deep ultraviolet light emitting diodes (LEDs) with peak emission in the spectral range from 247 nm to 365 nm. The devices demonstrate wall-plug efficiency in excess of 2%, modulation frequency in excess of 200 MHz and very low noise performance. Single wavelength device as well as multi-wavelength high power ultraviolet lamps have been developed for applications in sterilization industry, UV-curing, optical sensors, medical, biomedical and spectroscopic instrumentation.
Recently developed deep-UV light-emitting diodes (LEDs) are already used in prototype fluorescence sensors for detection of hazardous biological agents. However, increasing of the sensor ability of discrimination against common interferents requires further development of measurement technique. In particular, LED-based fluorescence lifetime measurements are to be considered as a technique supplementary to fluorescence spectral and excitation measurements. Here we report on application of UVTOP<sup>®</sup> series deep-UV LEDs developed by Sensor Electronic Technology, Inc. for real-time measurements of fluorescence lifetime in the frequency domain. LEDs with the wavelengths of 280 nm (targeted to protein excitation) and 340 nm (for excitation of coenzymes NADH and flavins) were used. The output of the LEDs was harmonically modulated at frequencies up to 100 MHz and fluorescence lifetime on the nanosecond and subnanosecond scale was estimated by measuring the phase angle of the fluorescence signal in respect of the LED output. Dual-wavelength LED-based phase-resolved measurement technique was tested for discrimination of <i>B. globigii</i> against a variety of interferents such as diesel fuel, paper, cotton, dust, etc. We conclude that fluorescence phase measurements have potential to improve the discrimination ability of the "detect-to-warn" optical bioparticle sensors.
Light emitting diodes (LEDs) are excellent candidates for the applications requiring low noise light sources with wavelengths ranging from 200 nm to 900 nm. These applications include the detection of fluorescence from protein molecules excited with the ultraviolet (UV) light (200-300nm) for identifying miniscule amounts of hazardous biological pathogens. The detection system including the light source must exhibit low noise and high stability over tens of minutes. In comparison with xenon, tungsten halogen lamps, lasers, and other conventional UV sources, UV LEDs are more stable, have lower noise, are smaller, cheaper, and easier to use. We report on the low frequency fluctuations of the current and light intensity of LEDs (fabricated by SET, Inc.) with wavelengths ranging from 265nm to 340nm. The results are compared with the noise properties of the halogen lamps and other commercially available LEDs with the wavelengths of 375nm, 505nm and 740nm. We show that the LEDs fabricated by Sensor Electronic technology, Inc. are suitable for studying steady state and time-varying UV fluorescence of biological materials. The correlation coefficient between the current and light intensity fluctuations varies with the LED current and load resistance. This dependence is explained in terms of the contributions to the 1/f noise from the active region and from the LED series resistance. The noise level could be reduced by operating the LEDs at a certain optimum current level and by using a large external series resistance (in the current source driving mode).
Recent progress in wide-bandgap semiconductor optoelectronics resulted in an appearance of deep-UV light-emitting diodes (LEDs), which can be used for fluorescence excitation in a variety of chemical and biological compounds. We used two generations of AlGaN-based UVTOP series deep ultraviolet LEDs developed by Sensor Electronic Technology, Inc. The peak wavelength of these fully packaged devices is 340 nm and 280 nm, line width at half maximum approximately 10 nm, wall-plug efficiency up to 0.9% and output power in the milliwatt range. The second-generation emitters are shown to have an extremely low level of unwanted long-wavelength emission what is important for fluorescence measurements. The UV LEDs were tested for fluorescence excitation in standard fluorophores (organic dyes), autofluorescent biological compounds (riboflavin, NADH, tryptophan, and tyrosine) and medical specimens (fluid secreted by prostate gland). Fluorescence lifetime measurements in the frequency domain were demonstrated using UVTOP-340 and -280 devices. The output of the LEDs was modulated at frequencies up to 200 MHz by high-frequency current drivers and the phase angle of the fluorescence signal was resolved using a radio-frequency lock-in amplifier. Nanosecond-scaled measurements of fluorescence lifetimes, which are the “fingerprints” of chemical and biological compounds, were demonstrated.
Lighting based on sources of light composed of colored light-emitting diodes (LEDs) offers versatile control of color and a possibility of trade-off between efficiency and color rendering. However, psychophysical issues related to such polychromatic solid-state sources have to be addressed. In this work, studies of the perception of standard colors under illumination with a quadrichromatic red-amber-green-blue (RAGB) solid-state source were carried out. An RAGB lamp containing primary LEDs with the emission peaks at 638 nm, 594 nm, 523 nm, and 441 nm and optimized for the highest value of the general color rendering index (86 points) was investigated and compared to a tungsten lamp. 40 standard Munsell samples of value 6, chroma /6, and hue incremented by 2.5 were used in the investigation. Changes in the saturation and hue of the Munsell samples illuminated by the RAGB lamp versus tungsten lamp (both with the correlated temperature of 2600 K) were obtained by colorimetric calculation comparisons and by psychophysical experiments on subjective matching of the samples. Subjective differences in hue and subjective color discrimination differences under the tungsten and RAGB lamps were found in the wavelength range of 440-500 nm and 560-580 nm. We attribute these differences to non-optimal peak wavelengths of the primary LEDs and to the narrow-band components of the RAGB spectrum.
White light with high color rendering indices can be produced by additive color mixing of emissions from several light-emitting diodes (LEDs) having different primary colors. White Versatile Solid-State Lamps (VSSLs) with variable color temperature, constant-chromaticity dimming, and efficiency/color-rendering trade-off can be developed using pulse-width modulation (PWM) driving technique. However, such lamps exhibit chromaticity shifts caused by different temperature and aging coefficients of the optical output for primary LEDs of different colors. To overcome this drawback, we developed a polychromatic white solid-state lamp with an internal digital feedback. The lamp features a quadrichromatic (red-amber-green-blue) design based on commercially available high-power LEDs. The design is optimized to achieve high values of the general color rendering index (69 to 79 points) in the color-temperature range of 2856 to 6504 K. A computer-controlled driving circuit contains a pulse-width modulator and a photodiode-based meter. The software performs periodical measurement of the radiant flux from primary LEDs of each color and adjusts the widths of the driving pulses. These VSSLs with feedback found application in phototherapy of seasonal affective disorder (SAD).
We report on fabrication, characterization, and properties of nanocrystalline semiconductor films and thin-film devices chemically deposited on fibers, cloth, and large area flexible substrates at low temperatures (close to room temperature). We also describe the photovoltaic effect in CdS/CuS films deposited on viewfoils and trylene threads. CdS films deposited on viewfoils exhibit unique behavior under stress and UV radiation exposure with reproducible resistance changes of several orders of magnitude with bending up to 10 mm curvature. The measurements of the 1/<i>f</i> noise in these nanocrystalline structures indicate a high quality of nanocrystallites.
Generation-recombination (GR) noise in GaN and AlGaN thin films, GaN based Metal Semiconductor Field Effect Transistors (MESFETs), Heterostructure Field Effect Transistors (HFETs) and Schottky diode photodetectors was investigated. AlGaN thin films, AlGaN/GaN HFETs and Schottky barrier Al<sub>0.4</sub>Ga<sub>0.6</sub>N diodes exhibited GR noise with activation energies of 0.8 - 1 eV. AlGaN/GaN HFETs also presented GR noise with activation energies of 1 - 3 meV and 0.24 eV at cryogenic temperatures. No such noise was observed in thin doped GaN films and GaN MESFETs. GR noise with the largest reported activation energy of 1.6 eV was measured in AlGaN/InGaN/GaN Double Heterostructure Field Effect Transistors (DHFETs). We conclude that the local levels responsible for the observed noise in HFETs and DHFETs could be located in AlGaN barrier layers.
We review two complementary approaches to the development of white light solid-state sources. The first approach, which involves polychromatic LED modules, is targeted at advanced optimization of spectral power distribution in order to establish an optimal trade-off between luminous efficacy and color rendering. We apply a stochastic method of optimization of a white-light source that relies on additive color mixing of the emissions from colored primary LEDs. We present the results on optimized spectra for all-semiconductor lamps composed of four primary LEDs with the line widths typical of present AlGaInP and AlInGaN technologies. We point out the problem of the lack of efficient yellow-green (570 nm) emitters required for polychromatic lamps with four and more primary LEDs. The second approach is based on the development of AlInGaN-based UV emitters that can be tailored to directly excite different phosphors without sensitizers. AlInGaN materials system demonstrated potential for making UV LEDs with a high power and short wavelengths required for such applications. This has been achieved by using Strain Energy Band Engineering (SEBE) and Pulsed Atomic Epitaxy (PALE) techniques. SEBE relies on quaternary AlGaInN compounds for controlling strain and band offset and for producing UV emitters with improved device performance. PALE allows us to incorporate the required significant amount of indium (few percent) in AlGaN, since it can be performed at lower growth temperatures required for In incorporation. Further improvements in materials quality of AlInGaN layers with a high molar fraction of Al will be achieved by using bulk AlN substrates.
Polychromatic solid-state lamps that produce white light by additive mixing of the emissions from primary colored light emitting diodes (LEDs) should have a higher luminous efficiency that those using phosphors. These lamps require emission spectra that feature an optimal trade-off between luminous efficacy and color rendering. We developed a mathematical technique that allows us to maximize the luminous efficacy and general color rendering index (CRI) for the white solid-state lamp composed of an arbitrary number of primary LEDs with given spectra. We use this method in order to compare the optimal efficacy and general CRI for 4 and 5 primary LEDs with that for 2 and 3 LEDs. For a particular color temperature, the required number of primary LEDs depends on the trade-off between efficacy and general CRI. The quadrichromatic lamp is shown to meet requirements for most practical applications. Quintichromatic lamps and lamps with a higher number of primary LEDs yield negligible benefit in improving color rendering. However, quintichromatic LED lamps are capable of producing quasi-continuous spectra that might meet special lighting needs.
Wide energy gap and strong piezoelectric effects in A1GaN-based materials are very attractive for the development of visible-ultraviolet spectral range optoelectronic devices, such as optical waveguides and light modulators. In this paper, we report on the experimental studies ofthe acousto-optical diffraction in GaN-based layered structures grown by low-pressure MOCVD over sapphire substrates. We present the extracted values of the acoustooptic figures of merit and effective photoelastic constants for red (633 nm) and blue (442 nm) wavelengths. Our results demonstrate the potential of GaN-based structures for the development ofblue-ultraviolet acousto-optical devices.
Strain Energy Band Engineering of Group III-N heterostructures should allow us to prevent defect formation at the heterointerfaces ad to reduce the built-in electric field in the quantum wells. The strain, caused by lattice mismatch, may be decreased by incorporation of In into AlGaN. To monitor structural perfection of the quaternary compound AlInGaN and to evaluate electronic potential profile, we employed optical methods: reflectivity, site- selectively excited photoluminescence, photoluminescence excitation and time-resolved luminescence. AlGaN with the molar fraction of Al of 9% and two samples with the lattice mismatch reduced by partial substitution of Al by 1% and 2% of In were investigated. In AlGaN, the luminescence excited resonantly with the exciton position is red shifted. The photoluminescence excitation spectra indicate that the mobility edge is above the optical band gap, and the localization vanishes. These results show that the incorporation of approximately equals 2% indium into AlGaN leads to the disappearance of the band tail states and smoothing of the potential profile.
Ensemble Monte Carlo simulations of hot nonequilibrium electron relaxation in rectangular GaAs quantum wires is carried out. The simulations demonstrate that the initial stage of hot photoexcited electron cooling dynamics is determined by cascade emission of optical phonons. The second relaxation stage is controlled by inelastic electron interaction with acoustic phonons and exhibits strong dependence on the cross-section of a quantum wire. If electron concentration exceeds 10<SUP>5</SUP> cm<SUP>-1</SUP> nonequilibrium (hot) phonon effects come into play and hot phonon thermalization time defines the characteristic electron gas cooling time. In contrast to bulk materials and quantum wells, hot phonon effects in quantum wires are strongly dependent on the initial broadening of energy distribution of photoexcited electrons.