R. C. Jayasinghe

Optical properties of nanostructured TiO 2 thin films and their application as antireflection coatings on infrared detectors

Oblique-angle deposited titanium dioxide (TiO(2)) nanorods have attracted much attention as good antireflection (AR) coating material due to their low n profile. Therefore, it is necessary to better understand the optical properties of these nanorods. TiO(2) nanorods grown on glass and Si substrates were characterized in the visible (0.4-0.8 μm) and infrared (2-12 μm) regions to extract their complex n profiles empirically. Application of these nanorods in multilayer AR coatings on infrared detectors is also discussed. Optimization of graded index profile of these AR coatings in the broad infrared region (2-12 μm) even at oblique angles of incidence is discussed. The effective coupling between the incoming light and multiple nanorod layers for reducing the reflection is obtained by optimizing the effect from Fabry-Perot oscillations. An optimized five-layer AR coating on GaN shows the reflectance less than 3.3% for normal incidence and 10.5% at 60° across the whole 2-8 μm spectral range.

Simultaneous detection of ultraviolet and infrared radiation in a single GaN/GaAlN heterojunction

Results are presented for a dual-band detector that simultaneously detects UV radiation in the 250-360 nm and IR radiation in the 5-14 microm regions with near zero spectral cross talk. In this detector having separate UV- and IR-active regions with three contacts (one common contact for both regions) allows the separation of the UV and IR generated photocurrent components, identifying the relative strength of each component. This will be an important development in UV-IR dual-band applications such as fire-flame detection, solar astronomy, and military sensing, eliminating the difficulties of employing several individual detectors with separate electronics-cooling mechanisms.

Plasma frequency and dielectric function dependence on doping and temperature for p-type indium phosphide epitaxial films

The optical properties of p-type InP epitaxial films with different doping concentrations are investigated by infrared absorption measurements accompanied by reflection and transmission spectra taken from 25 to 300 K. A complete dielectric function (DF) model, including intervalence band (IVB) transitions, free-carrier and lattice absorption, is used to determine the optical constants with improved accuracy in the spectral range from 2 to 35 μm. The IVB transitions by free holes among the split-off, light-hole, and heavy-hole bands are studied using the DF model under the parabolic-band approximation. A good understanding of IVB transitions and the absorption coefficient is useful for designing high operating temperature and high detectivity infrared detectors and other optoelectronic devices. In addition, refractive index values reported here are useful for optoelectronic device designing, such as implementing p-InP waveguides in semiconductor quantum cascade lasers. The temperature dependence of hole effective mass and plasma frequency is also reported.

Negative capacitance in GaN/AlGaN heterojunction dual-band detectors

A study of trap states in n+-GaN∕AlGaN heterostructures using electrical, thermal, and optical analyses is reported. Capacitance-voltage-frequency measurements showed negative capacitance and dispersion, indicating interface trap states. Infrared spectra identified three impurity related absorption centers attributed to shallow Si-donor (pinned to the AlGaN barrier), N-vacancy/C-donor, and deep Si-donor (pinned to the GaN emitter) impurities with corresponding activation energies of 30.8±0.2, 125±1, and 140±2meV, respectively. The shallow Si-donor impurity had a relaxation time of 155±9μs, while the C-donor/N-vacancy and deep Si-donor impurities appear to behave as a single trap state with a relaxation time of 1.77±0.05μs. Multiple analysis techniques allowed the determination of the activation energies of these impurity related centers and the study of the effects of trap states on the electrical behavior of the detector.

Capacitance hysteresis in GaN/AlGaN heterostructures

Capacitance characteristics with voltage and frequency of n+-GaN/AlxGa1−xN heterojunction ultraviolet (UV)-infrared (IR) photodetectors are reported. A distinct capacitance step and capacitance hysteresis have been attributed to trap energy states located just above the Fermi level at the GaN/AlGaN interface, most likely due to N-vacancy and/or C-donor impurities. The presence of the hysteresis is due to the accumulation of charge at the heterointerface, which is dependent on the location of the continuum of interface trap states relative to the Fermi level. The Al fraction in the barrier layer has been found to significantly change the positions of the interface trap states relative to the Fermi level.

Dopant Migration-Induced Interface Dipole Effect in n-Doped GaAs/AlGaAs Terahertz Detectors

A heterojunction interfacial workfunction internal photoemission (HEIWIP) terahertz detector with ~1times1018cm-3 n-type doped GaAs emitters in a multilayer GaAs/Al0.13Ga0.87 As heterostructure is presented. The detection mechanism is based on free carrier absorption with a broad response extending to ~ 5.26 THz (57 mum), corresponding to an effective workfunction of ~ 21.8 meV, which is much smaller than the offset expected for an Al fraction of x = 0.13 at a 1times1018 cm-3 doping. This is attributed to a reduction of the conduction band offset by interface dipole formation between the accumulated negative charges at the interface states and migrated positively charged donors in the barrier. The device has a peak responsivity of 0.32 A/W at ~ 26 mum at 5 K. It is demonstrated that the dopant migration-induced interface dipole effect can be used to extend the zero response threshold frequency (f 0) of n-type HEIWIP detectors.

GaN-Based heterojunction structures for ultraviolet/infrared dual-band detection

Ultraviolet and infrared (UV/IR) dual-band photodetectors based on GaN/AlGaN heterojunction structures are presented. Since the UV/IR dual-band detectors do not respond to solar radiation or another artificial visible lighting, these detectors are highly applicable for tracking and surveillance of targets. A dual-band detector which simultaneously detects UV in the 250 – 360 nm and IR in the 3–14 µm regions, showing a near zero spectral crosstalk between the two spectral bands, is discussed. A UV/IR detector design to further improve the detection capabilities is also reported.

GaAs and GaN based high operating temperature spin split-off band infrared detectors

Recently developed high operating temperature (up to 330 K) GaAs/AlGaAs detectors responding in the 3-5 μm wavelength range and based on split-off (SO) transitions followed by escape by scattering to the light/heavy hole(LH/HH) band or by direct quantum mixing of the states offer a viable alternative to present day detectors operating at cryogenic temperatures. This paper presents a theoretical model to predict the response of SO detectors. The model calculates the dark current and illuminated currents from the photoabsorption, carrier escape, and transport, explaining the experimental response. Using this model, different strategies to improve the performance of the GaAs based SO detectors are presented. A graded barrier improves the performance by reducing the space charge build up, and the double barrier resonant structure by enhanced escape of holes from the SO to the light/heavy hole bands by bringing the two bands into resonance. A detailed analysis of the effect of detector parameters on responsivity and D* is made. The change of material system to GaN/AlGaN should extend the response to longer wavelengths (THz) as its zinc blende and wurtzite crystal structures have SO transition energies of 20meV and 8meV respectively. Experimental measurement of SO absorption in GaN and potential THz detector designs are discussed.

Spin split-off band-based high operating temperature IR detectors in 3-5 µm and beyond

Overcoming the stringent cooling requirement for the operation of most of the infrared (IR) detectors is one of the major challenges towards capturing their full potential. Split-off (SO) transitions based detector exhibit encouraging results and gives hope to provide a novel alternative to the conventional IR detectors operating with cryogenic aid. Recently, a GaAs/AlGaAs SO detector operating up to 330 K in the 3-5 μm spectral region was developed. This paper presents various design modifications including graded barrier (in place of flat barrier), and double barrier resonant structure (in place of a single barrier) to improve the performance of these detectors. The graded barrier improves the detector performance by reducing the space charge buildup due to the trapping of charge carriers at the emitter-barrier interface; additionally, the model implementation on GaAs/AlGaAs based detectors also suggests that a barrier offset of 20 meV approximately doubles the responsivity. The implementation of a double barrier resonant structure increases the escape of holes from the SO to the light/heavy hole (LH/HH) bands by bringing the two bands into resonance and increases the response by a factor of ~ 85. The results from our ongoing efforts to extend the concept of SO mechanism based IR detection towards longer wavelength are also presented. This should be possible by exploiting SO absorption in alternative material systems such as phosphides and nitrides. The successful utilization of SO mechanism can result in the high operating temperature detectors operating in mid-IR and terahertz (THz) region.

Al fraction induced effects on the capacitance characteristics of n+-GaN/AlxGa1-xN IR detectors

Capacitance-voltage-frequency measurements on n+-GaN/AlxGa1-xN Heterojunction Interfacial Workfunction Internal Photoemission (HEIWIP) detectors were used to analyze the effects of Al fraction induced heterojunction barrier and its effect on the electrical characteristics at the heterointerface. The detectors IR threshold can be modified by changing the barrier Al concentration. A sample with an Al fraction of 0.1 shows a distinct capacitance step and capacitance hysteresis, which is attributed to N-vacancies and/or C-donor electron trap states located just above the Fermi level (200 meV) at the GaN/AlGaN interface, with activation energies of 149±1 and ~189 meV, respectively. A sample with an Al fraction of 0.026 showed negative capacitance and dispersion, indicating interface electron trap states located below the Fermi level (88 meV), most likely due to C-donor and/or N-vacancy with activation energies of 125±1 and 140±2 meV, respectively. Additional impurity related absorption centers were identified in both samples, however these shallow Si-donor sites (~30.9±0.2 meV) did not affect the capacitance as these states were located in the barrier layer and not in the vicinity of the Fermi level. The Al fraction in the barrier layer was found to significantly change the positions of the interface trap states relative to the Fermi level, resulting in the observed capacitance characteristics.