KEYWORDS: Polarization, Cameras, Time correlated single photon counting, Imaging systems, Reflection, Underwater imaging, Time of flight imaging, Polarimetry, Photon polarization, Optical systems
Underwater imaging is crucial for many applications, from marine biology research to industrial inspections, oil and gas exploration, search-and-rescue operations, and defense and security. Yet, underwater imaging poses numerous challenges, including backscatter from suspended particles, light absorption, and distortion caused by the medium's varying optical properties, in which traditional imaging methods often fall short. To address these challenges, we exploit the polarization properties of light by integrating a unique polarization-demultiplexing metasurface with an imager. Both direct imaging using a conventional CCD camera and a time-of-flight single-photon counting camera are used. By correlating the polarization states of emitted and reflected light, our approach enables us to develop means to enhance image contrast and achieve a more accurate estimation of the true target depth.
Polarimetry is pivotal in analyzing light's polarization properties, with vast potential in areas like remote sensing, material analysis, biological imaging, medical diagnostics, and defense. Metasurfaces, finely engineered to control light's phase, amplitude, and polarization, present an innovative platform to augment polarized light examination. This study delves into the design, fabrication, and practical demonstration of these metasurfaces, showcasing their use in polarization demultiplexing and subsequent imaging reconstruction. Experimentally, we used metasurfaces to observe an underwater scene under varied polarized light sources. This allows the selective capture of distinct polarizations simultaneously, elevating the accuracy of polarimetric measurements. A more in-depth discussion on image reconstruction via Stokes parameters is provided.
Active illumination with underwater laser imaging has unique advantages for the identification of underwater objects, especially in shallow waters, complex marine environments and inaccessible locations. However, backscattered light from the water particulates can blur the resulting laser images. To improve the quality of underwater laser images, we have examined a wide range of image enhancement (IE) and restoration (IR) techniques. In our recent prior work, we have experimentally evaluated the efficacy of over 20 IE/IR methods specifically for the underwater object recognition, examining the impact of artifacts introduced by IE/IR on the deep neural network (DNN) architecture required for optimal classification accuracy. This paper builds on this work by considering the effect of polarization on underwater image restoration and object recognition. Using a one-of-a-kind multi-polarization underwater laser image dataset, this paper examines the image of polarization on the efficacy of IE/IR algorithms and proposes a deep neural network (DNN) for fusing and jointly exploiting the multi-polarization data for improved underwater object recognition.
Polarization conversion devices which can manipulate the polarization status of electromagnetic waves are essential to various areas of photonic applications such as communication, imaging, and remote sensing. Due to its wide bandwidth and high penetration through dielectric materials, THz wave, range from 0.1 to 10 THz is increasingly popular for noninvasive screening, high-resolution imaging, and more precise data collection. Metamaterials (MM) are artificial materials fabricated from repeated arrays of subwavelength-sized meta-atoms, and MM-based THz polarization conversion devices can be thin, extremely compact, easy to integrate, and even flexible, unlike conventional polarization devices. In light of that information, we utilized a stereo-metamaterial (SMM) structure for new-generation THz polarizers converting the polarization from linearly to circularly and elliptically polarized wave at the THz frequency range in reflection mode. In this work, we present the processes and results of the inverse design of SMM using the artificial neural network (ANN), trained by various parameters, including polarization status and ellipticity angle, to achieve highly efficient device performance. Training and testing our ANN with the created datasets by simulation for the inverse design of the device, design parameters were obtained by giving an artificial EM response or ellipticity angle spectrum or vice versa more efficiently and rapidly. With the device fabricated based on the ANN-powered design, we demonstrated effective sensing of different polarization statuses using THz polarimetry spectroscopy.
Polarimetry imaging technology has progressed rapidly in recent years. It promises advances in various fields of application, including remote sensing, medical imaging, molecular sensing, and many areas of defense and homeland security application. Conventional polarimetry is not flexible and has remained difficult to implement due to the complexity of optics and moving parts, and generally, it is bulky and costly. Recent advances in the design, micro/nanofabrication, and testing of metasurfaces have opened tremendous opportunities by simplifying the optics pathway. These sub-wavelength and flat structures can be engineered to transform the propagation, phase, and polarization of light. It is now conceivable to replace the carefully aligned optical components with a single well-designed metasurface. In this work, we present the design, fabrication, and integration of a multiplexed dielectric metasurface operating at 532 nm, which is of great interest for underwater imaging. The metasurface developed in this work spatially diffracts polarizations, resulting in demultiplexing the polarization, and the intensity of each polarization was recorded to determine the Stokes parameters. We will discuss the optimization process of designing the dielectric metasurface to recover the Stokes parameters for imaging and the degree of polarization. With the FDTD simulation, we explored the metasurface design parameter space to achieve better transmission and phase control. The incorporation of Pancharatnam–Berry phase and cross-talk among the orthogonal components of linearly and circularly polarized light were evaluated. The designed metasurface was fabricated using electron beam lithography and ICP-RIE etching. Finally, the fabricated metasurface was integrated with a time-of-flight multi-pixel imager.
Active illumination with underwater laser imaging has unique advantages for the identification of underwater objects, especially in shallow waters, complex marine environments and inaccessible locations. Laser intensity images embody valuable information that can be utilized for object recognition; however, backscattered light from the water column and other particulates blur the resulting laser images, rendering the objects in the images unintelligible. Although over the years a variety of deblurring and other image restoration and enhancement algorithms have been proposed, these works primarily consider optical images of scenery, not monotone underwater images of objects, for which contours are more critical. This work proposes the utilization of edge metrics to evaluate the efficacy of image restoration and enhancement algorithms for underwater laser images. Our results provide insight into the best methods for improving underwater laser image quality for object recognition.
Quantum devices have the potential to revolutionize applications in computing, communications, and sensing; however, current state-of-art resources must operate at extremely low temperatures, making the routing of microwave control and readout signals challenging to scale. Interest in microwave photonic solutions to this problem has grown in recent years, in which control signals are delivered to the cold stage via optical fiber, where they are converted to electrical signals through photodetection. Overall link performance depends strongly on the characteristics of the photodiode, yet detailed measurements of many detector properties remain lacking at cold temperatures. In this work, we examine and compare the performance of a modified uni-traveling carrier photodiode (MUTC-PD) at both room (300 K) and liquid nitrogen (80 K) temperatures, focusing in particular on responsivity, bandwidth, and linearity. In line with previous work, we find a sharp reduction in responsivity at 1550 nm as temperature decreases, while RF bandwidth remains steady. Interestingly, our linearity tests reveal that the RF output saturates more quickly at 80 K, suggesting reduced linearity with lower temperature, the cause of which is still under investigation. Our results should help contribute to the understanding and future design of highly linear cryogenic quantum links.
Brianna Smiley, Amanda Marotto, Soner Balci, Seung Jo Park, M. Zeki Güngördü, Alex Maleski, A. Shahab Mollah, Elizabath Philip, Patrick Kung, Yonghyun Kim, Seongsin Kim
Sub-wavelength metamaterial structures are of great fundamental and practical interest because of their ability to manipulate the propagation of electromagnetic waves. Here we investigate the metamaterials composed of titanium and copper split-ring resonators for use in detection of living cells. Terahertz spectroscopy was utilized to detect a change in resonance frequencies of the bio-sensor in the presence of MDA-MB-231 breast cancer cells in culture in real time. The shift in frequency showed dependency upon cell density. We applied circuit model to interpret the resonance peak shift observed, and not only do we see shifts in resonance frequency but also in capacitance and resistance as time progresses.
We report Independent Component Analysis (ICA) technique applied to THz spectroscopy and imaging to achieve a blind source separation. A reference water vapor absorption spectrum was extracted via ICA, then ICA was utilized on a THz spectroscopic image in order to clean the absorption of water molecules from each pixel. For this purpose, silica gel was chosen as the material of interest for its strong water absorption. The resulting image clearly showed that ICA effectively removed the water content in the detected signal allowing us to image the silica gel beads distinctively even though it was totally embedded in water before ICA was applied.
Sub-wavelength metamaterial structures are of great fundamental and practical interest because of their ability to manipulate the propagation of electromagnetic waves. We review here our recent work on the design, simulation, implementation and equivalent circuit modeling of metamaterial devices operating at Terahertz frequencies. THz metamaterials exhibiting plasmon-induced transparency are realized through the hybridization of double split ring resonators on either silicon or flexible polymer substrates and exhibiting slow light properties. THz metamaterials perfect absorbers and stereometamaterials are realized with multifunctional specifications such as broadband absorbing, switching, and incident light polarization selectivity.
Devices operating at THz frequencies have been continuously expanded in many areas of application and major research
field, which requires materials with suitable electromagnetic responses at THz frequency ranges. Unlike most naturally
occurring materials, novel THz metamaterials have proven to be well suited for use in various devices due to narrow and
tunable operating ranges. In this work, we present the results of two THz metamaterial absorber structures aiming two
important device aspects; polarization sensitivity and broad band absorption. The absorbers were simulated by finite
element method and fabricated through the combination of standard lift-off photolithography and electron beam metal
deposition. The fabricated devices were characterized by reflection mode THz time domain spectroscopy. The narrow
band absorber structures exhibit up to 95% absorption with a bandwidth of 0.1 THz to 0.15 THz.
Terahertz based spectroscopy and imaging has become an active field of research in the past decade for a plethora of
applications including security screening, biomedical imaging, chemical analysis, and investigation of carrier dynamics.
Several advantages exist for the use of THz techniques since investigation of a sample can be performed without contact
or ionization; however, fine detail is difficult to determine due to the diffraction limit of the radiation. The resolution
limit of THz imaging and sensing can be overcome by the incorporation of near-field optical techniques; which can
allow image resolution as fine as tens of nanometers at THz frequencies. With this expanded resolution capability, THz
imaging can decipher micro- and nano-structural information which, when coupled with the non-contact features of
these techniques, makes THz spectroscopy ideal for the analysis micro and nano-optical devices. In this study, we
demonstrate the development and performance of an aperture-less near-field system which has been integrated to
perform highly-spatially resolved Terahertz Time-Domain Spectroscopic (THz-TDS) imaging.
In recent years, many applications have been recognized for biomedical imaging techniques utilizing terahertz frequency
radiation. This is largely due to the capability of unique tissue identification resulting from the nature of the interaction
between THz radiation and the molecular structure of the cells. By THz identification methods, tissue changes in tooth
enamel, cartilage, and malignant cancer cells have already been demonstrated. Terahertz Time-Domain Spectroscopy
(THz-TDS) remains one of the most versatile methods for spectroscopic image acquisition for its ability to
simultaneously determine amplitude and phase over a broad spectral range.
In this study we investigate the use of THz imaging techniques to uniquely identify damage types in tissue samples for
both forensic and treatment applications. Using THz-TDS imaging in both transmission and reflection schemes, we
examine tissue samples which have been damaged using a variety of acids. Each method of damage causes structural
deterioration to the tissue by a different mechanism, thus leaving the remaining tissue uniquely changed based on the
damage type. We correlate the change in frequency spectra, phase shift for each damage type to the mechanisms and
severity of injury.
Among electromagnetic spectrum, terahertz region has been utilized less due to the lack of appropriate devices that works well in these frequencies But recently growing interest has been focused to design devices with functionality in terahertz region because of potential terahertz applications. We present a novel structure that broadens bandwidth of terahertz metamaterial absorber. Our structure takes a benefit of multiband absorber by making the bands close enough to each other but in a multilayer pattern. The absorber has composed of two concentric copper rings in two different layers followed by polyimide and a metal back layer. Simulation shows 100 GHz bandwidth which is double of that of a single layer single ring absorber.
Imaging with electromagnetic radiation in the THz frequency regime, between 0.2 THz and 10 THz, has made
considerable progress in recent years due to the unique properties of THz radiation, such as being non-ionizing and
transparent through many materials. This makes THz imaging and sensing promising for a plethora of applications;
most notably for contraband detection and biomedical diagnostics. Though many methods of generation and detection
terahertz radiation exist, in this study we utilize Terahertz Time Domain Spectroscopy (THz TDS) and THz digital
holography using a coherent, tunable CW THz source. These methods enable access to both the amplitude and phase
information of the traveling THz waves. As a result of the direct time-resolved detection method of the THz electric
field, unique spectroscopic information about the objects traversed can be extracted from the measurements in addition
to being able to yield intensity imaging contrast. Utilizing such capabilities for THz based imaging can be useful for
both screening and diagnostic applications. In this work, we present the principles and applications of several
reconstruction algorithms applied to THz imaging and sensing. We demonstrate its ability to achieve multi-dimensional
imaging contrast of both soft tissues and concealed objects.
In recent years a great amount of research has been focused on metamaterials, initially for fabrication of left-handed
materials for use in devices such as superlenses or electromagnetic cloaking. Such devices have been developed and
demonstrated in regimes from the radio frequency all the way to infrared and near optical frequencies. More recently, it
has been shown that, by careful adjustment of the effective permittivity and permeability, near perfect electromagnetic
absorbers can be realized. High absorption occurs when transmission and reflection are simultaneously minimized. With
some clever tuning of the electric and magnetic responses, the electric and magnetic energy can therefore both be
absorbed by the same metamaterial structure.
In this work we present the design, simulation and characterization of a novel thin, flexible, polarization insensitive
metamaterial absorber. Finite-element simulation results show that this device achieves almost perfect absorption at THz
frequencies. Each unit cell of the absorber is made up of two metallic structures separated by a dielectric filler material.
The electric response can be tuned by adjusting the geometry of the top metallic electric ring resonator structure. We
demonstrate that a rotation about the axis of THz wave propagation at normal incidence does not change the absorption
or the resonance frequency by a significant amount. A value of absorption of 99.6 % at a resonance frequency of 0.84
THz can be achieved. We also demonstrate the characteristics of this absorber structure under various THz wave
incidence angles, with respect to both the incident electric and magnetic fields.
Quantum dot (QD) functionalized nanowire arrays are attractive structures for low cost high efficiency solar cells. QDs
have the potential for higher quantum efficiency, increased stability and lifetime compared to traditional dyes, as well as
the potential for multiple electron generation per photon. Nanowire array scaffolds constitute efficient, low resistance
electron transport pathways which minimize the hopping mechanism in the charge transport process of quantum dot
solar cells. However, the use of liquid electrolytes as a hole transport medium within such scaffold device structures
have led to significant degradation of the QDs. In this work, we first present the synthesis uniform single crystalline ZnO
nanowire arrays and their functionalization with InP/ZnS core-shell quantum dots. The structures are characterized using
electron microscopy, optical absorption, photoluminescence and Raman spectroscopy. Complementing
photoluminescence, transmission electron microanalysis is used to reveal the successful QD attachment process and the
atomistic interface between the ZnO and the QD. Energy dispersive spectroscopy reveals the co-localized presence of
indium, phosphorus, and sulphur, suggestive of the core-shell nature of the QDs. The functionalized nanowire arrays are
subsequently embedded in a poly-3(hexylthiophene) hole transport matrix with a high degree of polymer infiltration to
complete the device structure prior to measurement.
Green-emissive textured Zn2SiO4:Mn2+ phosphor film was fabricated by a thermal diffusion of ZnO:Mn on
quartz glass. The characterization has been performed in terms of Mn2+ ions concentration (Mn/Zn=1~9 mol %). As an
increase of Mn2+ ions concentration in the Zn2SiO4:Mn2+ phosphor film, the emission peak was red shifted from 519 nm
to 526 nm, and the decay time to 10% of the maximum intensity was shorter from 20 ms to 0.5 ms. All annealed
Zn2SiO4:Mn2+
phosphor films became textured along some hexagonal directions on the amorphous quartz glass. The
brightest Zn2SiO4:Mn2+
film at optimal Mn2+ concentration of 5 % showed the photoluminescence brightness of 65 %
and the shortened decay time of 4.4 ms in comparison with a commercially Zn2SiO4: Mn2+ powder phosphor screen. The
excellencies can be attributed to a unique textured structure.
Ce3+/Mn2+-codoped Ba1.20Ca0.8-2x-ySiO4:xCe3+, xLi+, yMn2+ phosphors show two emission bands peaking at around 600
nm (red) and 400 nm (deep-blue) from the forbidden 4T1-6A1 transition of Mn2+ ions and the allowed 4f-5d transition of
Ce3+ ions, respectively. Eu2+-doped Ba1.20Ca0.7SiO4:0.1Eu2+ phosphor shows a broad green emission band from 430 to
550 nm from the allowed f-d transition in Eu2+ ions. The mixtures of both phosphors, excited by the near ultraviolet of
365 nm, show the various qualities of white lights depending on the mixture ratio; the correlated color temperatures from
3500 to 7000 K, and the color-rendering indices up to 95 %. Furthermore, they show a high quenching temperature of
about 225 °C.
KEYWORDS: Light emitting diodes, Luminescence, Solid state lighting, Diodes, Ions, Energy transfer, Absorption, Near ultraviolet, Crystals, Solid state physics
A novel blue-emitting phosphor, Sr3Ga2O5Cl2:Eu2+, was synthesized by a two-step solid-state reaction. It has the
monoclinic structure with six different cation sites. It shows an efficient broad absorption band around the 400 nm of the
commercial near ultraviolet light-emitting diodes (LEDs), and an intense broad blue emission. Thus, it can be a
promising blue phosphor for white LED for solid state lighting.
Although ZnO has recently gained much interest as an alternative to the III-Nitride material system, the development of
ZnO based optoelectonic devices is still in its infancy. Significant material breakthroughs in p-type doping of ZnO thin
films and improvements in crystal growth techniques have recently been achieved, making the development of
optoelectonic devices possible. ZnO is known to be an efficient UV-emitting material (~380 nm) at room temperature,
optical UV lasing of ZnO has been achieved, and both homojunction and hybrid heterojunction LEDs have been
demonstrated.
In this paper, processing techniques are explored towards the achievement of a homo-junction ZnO LED. First, a
survey of current ZnO processing methods is presented, followed by the results of our processing research.
Specifically, we have examined etching through an n-ZnO layer to expose and make contact to a p-ZnO layer.
Wide bandgap III-Nitride semiconductors are a promising material system for the development of ultraviolet
avalanche photodiodes (APDs) that could be a viable alternative to photomultiplier tubes. In this paper, we report the
epitaxial growth and physical properties of device quality GaN layers on high quality AlN templates for the first backilluminated
GaN p-i-n APD structures on transparent sapphire substrates. The 25 μm x 25 μm device characteristics
were measured, and compared with the same devices grown on GaN templates, under low bias and linear mode
avalanche operation where they exhibited gains near 1500 after undergoing avalanche breakdown. The breakdown
electric field in GaN was determined to be 2.73 MV/cm. The hole impact ionization coefficients were shown to be
greater than those of electrons. These APDs were also successfully operated under Geiger mode.
GaN nanotubular material is fabricated with aluminum oxide membrane in MOCVD. SEM, XRD, TEM and PL are
employed to characterize the fabricated GaN nanotubular material. An aluminum oxide membrane with ordered nano
holes is used as template. Gallium nitride is deposited at the inner wall of the nano holes in aluminum oxide template,
and the nanotubular material with high aspect ratio is synthesized using the precursors of TMG and ammonia gas.
Optimal synthesis condition in MOCVD is obtained successfully for the gallium nitride nanotubular material in this
research. The diameter of GaN nanotube fabricated is approximately 200 ~ 250 nm and the wall thickness is about 40 ~
50 nm. GaN nanotubular material consists of numerous fine GaN particulates with sizes ranging 15 to 30 nm. The
composition of gallium nitride is confirmed to be stoichiometrically 1:1 for Ga and N by EDS. XRD and TEM analyses
indicate that grains in GaN nanotubular material have nano-crystalline structure. No blue shift is found in the PL
spectrum on the GaN nanotubular material fabricated in aluminum oxide template.
There is a need for semiconductor based UV photodetectors to support avalanche gain in order to realize better performance and more effectively compete with existing photomultiplier tubes. However, there are numerous technical issues associated with the realization of high-quality solar-blind avalanche photodiodes (APDs). In this paper, APDs operating at 280 nm, within the solar-blind region of the ultraviolet spectrum, are investigated. The devices consist of an Al0.38Ga0.62N active region grown atop a high quality AlN template layer designed to allow back illumination of the devices through the sapphire substrate. These devices perform well in the unbiased mode of operation. Under the application of large reverse bias these devices show a soft breakdown starting at relatively low electric fields. The devices achieve a maximum optical gain of ~1000 at a reverse bias of ~90 Volts, which corresponds to an electric field strength of 2.5 MV/cm. The origins of this gain are discussed in detail and modeling of the devices is used to investigate the electric field build up in the multiplication region.
We report on the growth and fabrication of AlGaN-based deep ultraviolet light-emitting diodes (LEDs) with peak emission of below 255 nm. In order to achieve such short wavelength UV LEDs, the Al mole fractions in the device layers should be greater than ~60%. This introdues serious challenges on the growth and doping of AlxGa1-xN epilayers. However, with the aid of a high-quality AlN template layer and refinement of the growth conditions we have been able to demonstrate UV LEDs emitting below 255 nm. The ratio of the intensity of the electroluminescence primary peak to that of the secondary peak (related to Mg deep levels) is ~ 18:1 at moderate injection current levels. Milliwatt level output powers have been achieved for these deep UV LEDs.
Back-illuminated solar-blind ultraviolet p-i-n photodetectors and focal plane arrays are investigated. We initially study single-pixel devices and then discuss the hybridization to a read-out integrated circuit to form focal plane arrays for solar-blind UV imaging. The photodetectors consist of an AlGaN p-i-n active region grown atop a high quality AlN template layer with a ~1 μm thick Al0.5Ga0.5N:Si-In co-doped low-resistance UV-transparent lateral conduction layer. The material is processed into a 320 x 256 array of 25 μm x 25 μm pixels using standard lithographic techniques. Typical pixels demonstrate a peak responsivity of 93 mA/W at 278 nm; this corresponds to an external quantum efficiency of 42%. The uniformity of the array is discussed, and a selection of sample images from the solar-blind focal plane array is included. In addition, recent attempts to achieve shorter wavelength deep UV back-illuminated p-i-n photodetector and focal plane arrays are also discussed.
GaN-based optoelectronic devices are plagued by a tendency to non-radiative transitions linked to defects in the active layers. This problem has its origin in (1) intrinsic factors such as GaN's relatively low exciton binding energy (~24meV) and (2) extrinsic factors including the poor availability of native substrates good enough to significantly suppress the defect density. Indeed, the quality and availability of large-area bulk GaN substrates is currently considered a key problem for the continuing development of improved GaN-based devices. Since development of bulk GaN substrates of suitable quality has proven very difficult, a considerable amount of effort is also being directed towards the development of alternative substrates which offer advantages compared to those in widespread use (c-sapphire and 6H SiC). ZnO is promising as a substrate material for GaN because it has the same wurtzite structure and a relatively small lattice mismatch (~1.8%). In this paper, we discuss use of ZnO thin films as templates for GaN based LED.
We demonstrate high power AlGaN based ultraviolet light-emitting diodes (UV LEDs) with an emission wavelength of 280 nm using an asymmetric single quantum well active layer configuration on top of a high-quality AlGaN/AlN template layer grown by metalorganic chemical vapor deposition (MOCVD). An output power of 1.8 mW at a pulsed current of 400 mA was achieved for a single 300 μm × 300 μm diode. This device reached a high peak external quantum efficiency of 0.24% at 40 mA. An array of four diodes produced 6.5 mW at 880 mA of pulsed current. We also demonstrate high output power operation of AlGaN-based UV LEDs at a short wavelength of 265 nm. An output power of 2.4 mW at a pulsed current of 360 mA was achieved for a single diode. A packaged array of four diodes produced 5.3 mW at 700 mA of pulsed current. The DC output power is 170 μW at 250 mA.
We report AlGaN-based back-illuminated solar-blind p-i-n photodetectors with a record peak responsivity of 150 mA/W at 280 nm, corresponding to a high external quantum efficiency of 68%, increasing to 74% under 5 volts reverse bias. Through optimization of the p-AlGaN layer, we were able to remove the out-of-band negative photoresponse originating from the Schottky-like p-type metal contact, and hence significantly improve the degree of solar-blindness. We attribute the high efficiency of these devices to the use of very-high quality AlN and Al0.87Ga0.13N/AlN superlattice material, a highly conductive Si-In co-doped Al0.5Ga0.5N layer, and the elimination of the negative photoresponse through improvement of the p-type AlGaN.
Thanks to advances in the quality of wide bandgap AlxGa1-xN semiconductors, these materials have emerged as the most promising approach for the realization of photon detectors operating in the near ultraviolet from 200 to 365 nm. This has in turn spurred the need for such devices in an increasing number of applications ranging from water purification to early missile threat warning systems. Nevertheless, the control of the material quality and doping, and the device technology remain tremendous challenges in the quest for the realization of high performance photodetectors. Design of the photodetector structure is one of the key issues in obtaining high performance devices; especially the thickness of the intrinsic region for p-i-n photodiodes is a crucial value and needs to be optimized. We compare the performance of the p-i-n photodiodes with different widths for the depletion region, which shows a trade-off between speed and responsivity of the devices. Furthermore, another challenge at present is the realization of low resistivity wide bandgap p-type AlxGa1-xN semiconductors. We present here recent advances and propose future research efforts in the enhancement of the AlxGa1-xN p-type conductivity through the use of polarization fields in AlxGa1-xN/GaN superlattice structures.
There has been a growing interest for the development of solar blind ultraviolet (UV) photodetectors for use in a variety of applications, including early missile threat warning, flame monitoring, UV radiation monitoring and chemical/biological reagent detection. The AlxGa1-xN material system has emerged as the most promising approach for such devices. However, the control of the material quality and the device technology are still rather immature. We report here the metalorganic chemical vapor deposition, the n-type and the p-type doping of high quality AlxGa1-xN thin films on sapphire substrates over a wide range of Al concentration. The quality of this AlxGa1-xN material was verified through the demonstration of high performance visible and solar blind ultraviolet p-i-n photodiodes with a cut-off wavelength continuously tunable from 227 to 365 nm, internal quantum efficiencies up to 86% when operated in photovoltaic mode, and a ultraviolet-to-visible rejection ratio as high as six orders of magnitude. Both front and back side illuminated p-i-n photodiodes were realized. Photodetector devices were also demonstrated on GaN material obtained using lateral epitaxial overgrowth. The technology for such AlxGa1-xN based devices was improved in an effort to enhance their performance, including the development of ohmic metal contacts to both n-type and p-type AlxGa1-xN films with an Al concentration up to 40%.
Lateral epitaxial overgrowth (LEO) has recently become the method of choice to reduce the density of dislocations in heteroepitaxial GaN thin films, and is thus expected to lead to enhanced performance devices. We present here the LEO growth and characterization of GaN films by low pressure metalorganic chemical vapor deposition. Various substrates were used, including basal plane sapphire and oriented Si substrates. The steps in the LEO growth technology will be briefly reviewed. The characterization results will be discussed in detail. The structural, electrical and optical properties of the films were assessed through scanning, atomic and transmission electron microscopy, x-ray diffraction, capacitance-voltage, deep level transient spectroscopy, photoluminescence, and scanning cathodoluminenscence measurements. Single-step and double- step LEO GaN was achieved on sapphire. Similarly high quality LEO grown GaN films were obtained on sapphire and silicon substrates. Clear and dramatic reduction in the density of defects are observed in LEO grown materials using the various characterization techniques mentioned previously.
There is currently a strong interest in developing solid- state, UV photodetectors for a variety of applications. Some of these are early missile threat warning, covet space to space communications, flame monitoring, UV radiation monitoring and chemical/biological reagent detection. The III-Nitride material system is an excellent candidate for such applications due to its wide, reagent detection. The III-Nitride material system is an excellent candidate for such applications due to its wide, direct bandgaps and robust material nature. However, despite many inherent material advantages, the III-Nitride material system typically suffers from a large number of extended defects which degrade material quality and device performance. One technique aimed at reducing defect densities in these materials is lateral epitaxial overgrowth (LEO). In this work, we present a preliminary comparison between AlGaN UV, solar-blind p-i-n photodiodes fabricated form LEO GaN and non-LEO GaN. Improvements in both responsivity and rejection ratio are observed, however, further device improvements are necessary. For these, we focus on the optimization of the p- i-n structure and a reduction in contact resistivity to p- GaN and p-AlGaN layers. By improving the structure of the device, GaN p-i-n photodiodes were fabricated and demonstrate 86 percent internal quantum efficiency at 362 nm and a peak to visible rejection ratio of 105. Contact treatments have reduced the contact resistivity to p-GaN and p-AlGaN by over one order of magnitude form our previous results.
Femtosecond nonlinear optical techniques have been employed in the study of carrier dynamics and transport in UV detector materials. Visible femtosecond pulses derived from the signal beam of a 250 kHz regenerative amplifier-pumped optical parametric amplifier were frequency doubled to obtain pulses tunable from 250 nm to 375 nm. Time-resolved reflectivity experiments indicate that the room-temperature carrier lifetime in GaN grown by double lateral epitaxial overgrowth is about 3 times longer than that of GaN grown on sapphire without benefit of this technique. The electron velocity-field characteristics and saturation velocity in GaN have been obtained form time-resolved studies of electroabsorption in a GaN p-i-n diode. The peak steady- state velocity of 1.9 X 107 cm/s in this device occurs at 225 kV/cm. Time-resolved transmission measurements have been used to monitor ultrafast carrier relaxation phenomena in a thin AlGaN layer with bandgap in the solar blind region of the spectrum. Excitation intensity and wavelength dependent studies of the photoinduced bleaching decays suggest that they are primarily governed by trapping in a high density of sub-bandgap defect levels.
We report the lateral epitaxial overgrowth (LEO) of GaN films on (00.1) Al2O3 and (111) Si substrates by metalorganic chemical vapor deposition. The LEO on Si substrates was possible after achieving quasi monocrystalline GaN template films on (111) Si substrates. X-ray diffraction, photoluminescence, scanning electron microscopy and atomic force microscopy were used to assess the quality of the LEO films. Lateral growth rates more than 5 times as high as vertical growth rates were achieved for both LEO growths of GaN on sapphire and silicon substrates.
We report the growth and characterization of Schottky based metal-semiconductor-metal ultraviolet photodetectors fabricated on lateral epitaxially overgrown GaN films. The lateral epitaxial overgrowth of GaN was carried out on basal plane sapphire substrates by low pressure metalorganic chemical vapor deposition and exhibited lateral growth rates more than 5 times as high as vertical growth rates. The spectral responsivity, the dependence on bias voltage, on incident optical power, and the time response of these photodetectors have been characterized. Two detector orientations were investigated: one with the interdigitated finger pattern parallel and the other perpendicular to the underlying SiOx mask stripes.
Ultraviolet photodetectors have many military and commercial applications. However, for many of these applications, the photodetectors must be solar blind. This means that the photodetectors must have a cutoff wavelength of less than about 270 nm. Semiconductor based devices would then need energy gaps of over 4.6 eV. In the AlxGa1-xN system, the aluminum mole fraction, x, required is over 40%. As the energy gap is increased, doping becomes much more difficult, especially p-type doping. This report is a study of the electrical properties of AlxGa1-xN to enable better control of the doping. Magnesium doped p-type AlxGa1- xN has been studied using high-temperature Hall effect measurements. The acceptor ionization energy has been found to increase substantially with the aluminum content. Short-period superlattices consisting of alternating layers of GaN:Mg and AlGaN:Mg were also grown by low-pressure organometallic vapor phase epitaxy. The electrical properties of these superlattices were measured as a function of temperature and compared to conventional AlGaN:Mg layers. It is shown that the optical absorption edge can be shifted to shorter wavelengths while lowering the acceptor ionization energy by using short- period superlattice structures instead of bulk-like AlGaN:Mg. Silicon doped n-type films have also been studied.
We report the fabrication and characterization of AlxGa1-xN p-i-n photodiodes (0.05 less than or equal to X less than or equal to 0.30) grown on sapphire by low-pressure metalorganic chemical vapor deposition. The devices present a visible-rejection of about four orders of magnitude with a cutoff wavelength that shifts from 350 nm to 291 nm. They also exhibit a constant responsivity for five decades (30 mW/m2 to 1 kW/m2) of optical power density. Using capacitance measurements, the values for the acceptor concentration in the p-AlxGa1-xN region and the unintentional donor concentration in the intrinsic region are found. Photocurrent decays are exponential for high load resistances, with a time constant that corresponds to the RC product of the system. For low load resistances the transient response becomes non- exponential, with a decay time longer than the RC constant.
UV photodetectors are critical components in many applications, including UV astronomy, flame sensors, early missile threat warning and space-to-space communications. Because of the presence of strong IR radiation in these situations, the photodetectors have to be solar blind, i.e. able to detect UV radiation while not being sensitive to IR. AlxGa1-xN is a promising material system for such devices. AlxGa1-xN materials are wide bandgap semiconductors, with a direct bandgap whose corresponding wavelength can be continuously tuned from 200 to 365 nm. AlxGa1-xN materials are thus insensitive to visible and IR radiation whose wavelengths are higher than 365 nm. We have already reported the fabrication and characterization of AlxGa1-xN- based photoconductors with a cut-off wavelength tunable from 200 to 365 nm by adjusting the ternary alloy composition. Here, we present the growth and characterization of GaN p-i- n photodiodes which exhibit a visible-to-UV rejection ratio of 6 orders or magnitude. The thin films were grown by low pressure metalorganic chemical vapor deposition. Square mesa structures were fabricated using dry etching, followed by contact metallization. The spectral response, rejection ratio and transient response of these photodiodes is reported.
Continuous-wave (CW) room temperature operation of InGaN/GaN multi-quantum well (MQW) lasers is reported. Far-field beam divergence as narrow as 13 degrees and 20 degrees for parallel and perpendicular directions to epilayer planes were measured, respectively. The MQW lasers showed strong beam polarization anisotropy as consistent with QW laser gain theory. Dependencies of threshold current on cavity-length and temperature are also consistent with conventional laser theory. No significant degradation in laser characteristics was observed during lifetime testing for over 140 hours of CW room temperature operation.
High-quality AlN, GaN, AlGaN have been grown on sapphire substrate by low-pressure metalorganic chemical vapor deposition (LP-MOCVD). The x-ray rocking curve of AlN and GaN were 100 arcsecs and 30 arcsecs respectively with Pendelloesung oscillations, which are the best reported to date. GaN with high crystallinity simultaneously exhibited high optical and electrical quality. Photoluminescence linewidth of GaN at 77K was as low as 17 meV, which is the best reported to date. Si-doped GaN had a mobility higher than 300 cm2/V.s. GaN has been also successfully grown on LiGaO2 substrate with LP-MOCVD for the first time. AlGaN for the entire composition range has been grown. These layers exhibited the lowest x-ray FWHM reported to date. The excellent optical quality of these layers have been characterized by room temperature UV transmission and photoluminescence. N-type doping of AlGaN with Si has ben achieved up to 60 percent Al with mobility as high as 78 cm2/V.s. AlxGa1-xN/AlyGa1-yN superlattice with atomically sharp interface have been demonstrated. Optically-pumped stimulated emission in GaN:Ge and GaN:Si has been observed with threshold optical power density as low as 0.4 MW/cm2. AlGaN photoconductors with cut-off wavelengths from 200 nm to 365 nm have been achieved for the first time. GaN p-n junction photovoltaic detector with very selective photoresponse have been demonstrated and theoretically modeled. Ti/AlN/Si metal-insulator- semiconductor capacitor with high capacitance-voltage performances at both low and high frequencies and low interface trap level density have been demonstrated for the first time in this material system.
AlxGa1-x ultraviolet photoconductors with cut- off wavelengths from 365 nm to 200 nm have been fabricated and characterized. Various characteristics of the devices, such as photoresponse, voltage-dependent responsivity, frequency-dependent responsivity and noise spectral density, were measured and cross-referenced with optical, electrical and structural characteristics of the material to provide information about the mechanisms taking place during detection. The maximum detectivity reached 5.5 X 108 cm X Hz1/2/W at a modulating frequency of 14 Hz. The effective majority carrier lifetime in AlxGa1-xN materials, derived from frequency-dependent photoconductivity measurements, has been estimated to be from 6 to 35 msec. The frequency-dependent noise-spectrum shows that it is dominated by Johnson-noise at high frequencies for low Al-composition samples.
Metalorganic chemical vapor deposition was used to deposit AlxGa1-xN active layers with varying aluminum compositions on basal plane sapphire substrate. AlxGa1-xN (x < 0.5) ultraviolet photodetectors have been fabricated and characterized with cut-off wavelengths as short as 260 nm. Carrier lifetimes on the order of 10 milliseconds were estimated from frequency dependent measurements of the responsivity.
In SPIE Proceeding 2397 we demonstrated that there is a large payoff still to be gained by further improvements in the performance of solar blind UV detectors for astronomical purposes. We suggested that a particularly promising future technology is one based on the ability of investigators to produce high-quality films made of wide bandgap III-IV semiconductors. Here we report on significant progress we have made over the past year to fabricate and test single-pixel devices. The next step will be to measure and improve detective efficiency, measure the solar blindness over a larger dynamic range, and begin developing multiple-pixel designs.
High quality AlN and GaN epilayers have been grown on basal plane sapphire by low pressure metalorganic chemical vapor deposition. The X-ray rocking curve linewidth of the AlN and GaN films were about 100 and 30 arcsecs respectively. Sharp absorption edges were determined at 6.1 and 3.4 eV respectively. Successful donor-bound excitonic luminescence emissions were detected for GaN films grown on sapphire and silicon. Two additional lines at 3.37 and 3.31 eV were observed on GaN on sapphire and assumed to be impurity-related. Doping of GaN layers was achieved with magnesium. Mg-related photoluminescence emissions were successfully detected on as-grown samples, without any post-growth treatment.
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