In the absence of suitable methods for integrating III-V materials into standard microelectronic fabrication processes, Si has been actively explored as an alternative light emitter for silicon photonics. Although several proposals on how to increase the internal quantum efficiency of interband above bandgap (λ ≤1μm) luminescence in this indirect bandgap material were successful and are becoming fruitful, the luminescence mode is not without pitfalls. These drawbacks are low emission power, temperature quenching, and the need for additional technological steps, like doping by emissive centers or fabrication of quantum-confined structures. Below, we describe an innovatively different approach for extracting light from Si at below-bandgap wavelengths (λ >>1μm) by making use of thermal emission from a bulk material. We also suggest several new optoelectronic devices operating in this unconventional mode.
Although performance of LEDs has been raised dramatically, wall-plug efficiency (WPE) of commercial high-power large-area (I ≥ 1 A) devices remains low given that the LEDs performance is close to that of the other optoelectronic devices. Instead of numerous studies aimed to increase optical efficiency that tends to saturate, we are concerned about electrical efficiency (epsilonel). In this paper, we consider inevitable electrical losses paid for carrying electrons into the active region before they recombine. More specifically, we are interested in the inherent limitations imposed on the WPE and epsilonel by the series resistance, current crowding effect, dimensions of chips, and ideality factor (β). The study was performed on commercial vertical InGaN-on-SiC multiple-quantum well LEDs with rated currents (Ir) of about 1A. All parameters are obtained exclusively from I-V characteristics. We show that a) epsilonel losses remarkably affect WPE even at I << Ir, b) the Ir values fall into high-current domain, c) 2D current distribution suffers of severe crowding, d) voltage drop on series resistance cannot be neglected, e) the dominant mechanism of carrier transport across the junction is carrier recombination inside the depletion region (β ≈ 2). We discuss advantages and disadvantages of industrial GaInN/SiC technology from the point of view of electrical efficiency and consider an alternative approach to make high-power LEDs more efficient.
It is believed that low power conversion efficiency in commercial MQW LEDs occurs as a result of efficiency droop, current-induced dynamic degradation of the internal quantum efficiency, injection efficiency, and extraction efficiency. Broadly speaking, all these “quenching” mechanisms could be referred to as the optical losses. The vast advances of high-power InGaN and AlGaInP MQW LEDs have been achieved by addressing these losses. In contrast to these studies, in this paper we consider an alternative approach to make high-power LEDs more efficient. We identify current-induced electrical efficiency degradation (EED) as a strong limiting factor of power conversion efficiency. We found that EED is caused by current crowding followed by an increase in current-induced series resistance of a device. By decreasing the current spreading length, EED also causes the optical efficiency to degrade and stands for an important aspect of LED performance. This paper gives scientists the opportunity to look for different attributes of EED.
Aimed to promote light conversion processes for radiative cooling of solids, we consider operation principles and fundamentals of two recently discovered approaches to make semiconductors cooled through photo excitation of free carriers. These are light up conversion that is due to above bandgap thermal-assisted luminescence converting heat into light and light down conversion occurring due to the enhancement of the thermal emission output when the overall energy of multiple below bandgap photons escaping a semiconductor exceeds the energy of a single pumped photon. These two processes come at the cost of the internal energy of an object by causing therefore it cooled. Figures of merit will be the microscopy of the processes, cooler band structure, entropy limitations, cooling power, cooling and power conversion efficiencies, and cooling temperature range.
Aimed to pursue the development of infrared scene projection technology beyond the current state-of-the-art, we
consider advantages of all-silicon bulk pixelless photonic projectors by light down conversion in comparison with
thermal emitter micromachining devices available in the market. There are several reasons for this. First, there are firm
evidences that the technology and performance of thermal emitters have already plateaued and future advances in the
field do not seem assured. Second, we show that photonic devices by light down conversion evolved from scientific
curiosity into technology poised to offer new capabilities to broadband projector applications. Finally, we demonstrate
that silicon becomes enabling material for emitting structures operating in the short, mid, and long wave IR spectral
bands.
Primary challenges in Si science and technology are centered on active device engineering issues with the impact made
at their photonic characteristics and applications. Low cost and mature Si technology is driving force toward making
optoelectronic devices like LEDs, lasers, modulator, wavelength converter, to name a few. However, to fully exploit
tremendous potential of these Si-based devices, which are not as efficient as their III-V-based counterparts, external
cooling units are required. In this work, we present a first step towards experimental realizing radiative cooling in Si. The
fundamentals behind this approach are in the light down-conversion process through thermal emission that releases the
energy from the cooling element in the form of multiple intraband red-shifted photons when it is pumped with interband
incoherent light. We demonstrate that a large-area Si wafer kept in an evacuated chamber becomes net cooled by >3.0 K
starting from 470 K or demonstrates power conversion efficiency of >100% when pumped with 1.06-μm light. One of
other attractive attributes of the device is that it can be easy integrated with other silicon photonic components. We also
discuss the way toward further improvement of the performance of these devices.
To date, the reason for high ideality factor, β, in GaN-based LEDs grown on sapphire substrate is not fully understood
and explained. It is believed that β-factor exceeding 2.0 originates from the trap-assisted tunneling and charge carrier
leakage inside the active MQW LED region or is due to additional junctions available in the LED circuit. In this
research, we demonstrate that β values higher than those predicted by the classical theory may be related to the current
crowding effect that is difficult to avoid in LEDs grown on the insulating substrates. By analyzing theoretical model and
testing commercial lateral blue LEDs with two different p-electrode pattern, we show that β -factor could increase from
2.0 (current spreading geometry) up to 3.5 (current crowding geometry). This modification of β-factor occurs in the
intermediate range of current (10 μA - 10 mA, the space charge region dominates in LED performance) and therefore
could be erroneously treated as the change of carrier transport mechanism and charge carrier recombination nature. At
higher current (series resistance dominates) even insignificant increase of β-factor makes the current value of efficiency
rollover to decrease (from 35 mA to 15 mA) and the efficiency droop to increase by 10%.
Although thermal IR microemitters making use of Honeywell planar technology
remain the devices of choice for the last decade, a significant disadvantage of these
devices is their two-level structure, which results in low fill-factor and causes
mechanical and thermal stresses between the layers. In this paper, the technology for
single-level polycrystalline SiGe thermal microemitters, their design, and performance
characteristics are presented. The 128-element linear arrays with a fill-factor of 88 %
and a 2.5-μm-thick resonant cavity have been grown by low-pressure chemical vapor
deposition and fabricated using surface micromachining technology. The 200-nm-thick
60 × 60 μm2 emitting pixels enforced with a U-shape profile pattern demonstrate
time response of 2-7 ms and an apparent temperature of 700 K in the 3-5 and 8-12 μm
atmospheric transparency windows. The SiGe device application to the infrared
dynamic scene simulation and critical factors that aid their competitiveness over
conventional planar two-level design are discussed.
Infrared imaging in the 3-5 and 8-12 μm bands is demonstrated to be extremely fast and spatially resolved
characterization technology for testing light and heat in micron-size light emitting devices. It is shown
how this high-speed contactless technology coupled with the CCD micro vision can be used to monitor
both light and parasitic heat evolution in space (10-μm resolution step) and time (~10 μs temporal scale)
in white, near IR, mid-wave IR, and long-wave IR LEDs. The technology appears to be the best way to
find out if and where the excess heat emerges and how it affect on the light pattern and device
performance. We experimentally demonstrate the non-uniformity in light pattern and local heat traps with
giant temperature gradients (>103 K/cm), which affect LED parameters and cause these devices to
degrade.
In this report, we examine whether photonic IR emitters are able to compete
with advanced thermal microemitter technology in testing and stimulating IR sensors,
including forward-looking IR missile warning systems, IR search-and-track devices,
and missile seekers. We consider fundamentals, technology, and parameters of
photonic devices as well as their pros and cons in respect to thermal emitters. In
particular, we show that photonic devices can from platform for next generation of
multi-spectral and hyper-spectral dynamic scene simulation devices operating inside
MWIR and LWIR bands with high spectral output density and able to simulate
dynamically cold scenes (without cryogenic cooling) and low observable with very
high frame rate.
We report on the fundamentals and technology of Si-based linear all-optical light down-conversion process. The approach is in the possibility to enhance the thermal emission power of semiconductors in the spectral range of intraband electron transitions (mid- and long-wave infrared, free carrier absorption band) by the shorter wavelength optical pump (interband transitions, visible to near-infrared, fundamental absorption band). We experimentally realize conditions (the 1.15-μm-pump wavelength and 2 to 16-μm-signal wavelengths, T ≈ 500 K) when Si-based device demonstrated 220 % external power efficiency. As a matter of fact, we come up with new concept for high-temperature incoherent light amplifier (optical transistor) made of indirect bandgap semiconductors.
In a radical departure from conventional thermal emitter-based dynamic IR scene simulation devices, we have tested InAsSbP/InAs LEDs grown by liquid phase epitaxy and tuned at several peak-emitting wavelengths inside the mid-IR band. Light uniformity, radiation apparent temperature (Ta), thermal resistance, and self heating details were characterized at T=300°K in the microscale by calibrated infrared cameras in the 3-5 μm (light pattern) and 8-12 μm (heat pattern) bands. We show that LEDs are capable of simulating very hot (Ta ≥740°K) targets as well as cold objects and low observable with respect to a particular background. We resume that cost effective LEDs enable a platform for photonic scene projection devices able to compete with thermal microemitter MEMS technology in testing and stimulating very high-speed infrared sensors used for military and commercial applications. Proposals on how to further increase LEDs performance are given.
In this report, fundamentals, design, fabrication technology, and parameters are presented for contactless Si photonic emitter operated in the 3-5 um atmosphere transparency window at well above room temperature. To bypass the material band structure limitation, we utilized the above-bandgap light-induced free carrier thermal emission as a way to monitor the below-bandgap radiation that falls into 3 to 5 μm band (light down conversion). Two-facet external power conversion efficiency up to 5% is observed at T~500 K with further improvement to be expected. The device application to the IR dynamic scene simulation as well as it pros and cons in respect to thermal emitters and IR LEDs are also considered.
This paper presents new physical concept and Hardware-in-the-Loop Facility for simulating cold background and/or
target in 3-5 μm and 8-12 μm atmospheric transparency windows. The goal for this work is the demonstration of scene
projector capable of static and dynamic (>20 kHz frame rate) simulating both point and extended targets across low
apparent temperature background (≥210 K in 8-12 μm band) even if a device itself is kept at T≥300 K. We show that
these record parameters as well as a possibility to dynamically erase a target (low observable simulation) are easy to
achieve by manipulating the below-bandgap local emissivity of initially transparent scene made of Ge or Si and exposed
in front of cold screen.
The design, fabrication technology and parameters are presented for monolithic linear 16-element IR emitter bar and the 8 x 8 stack of bars. Both types of devices are based on the Si p+in+-diodes with the 0.86 x 0.86 mm2 emitting surface integrated into a single chip and operated at well above room temperatures by the contact double injection of free charge carrier. To bypass Si electronic band structure limitation, we utilized free carrier absorption as a way to monitor material below-bandgap IR thermal emission. At a device temperature T=453 K, nearly 1.0 mW output power and 420 K apparent temperature of IR (3 to 12 μkm spectral band) radiation could be achieved with ~0.8% external power efficiency and 0.1 ms rise-fall time. This represents the longer wavelengths, higher operating temperatures and output power from Si spontaneous emitters ever reported.
We report on basic principle and technology of Si high-temperature (T>300K) IR emitter based on all optical down conversion concept. The approach is based on the possibility to modulate semiconductor thermal emission power in the spectral range of intra-band electron transitions through shorter wavelength (inter-band transitions) optical pumping (light down conversion process). Device emission bands are matched to transparency windows in atmosphere (3-5 μm and 8-12 μm) by adjusting thin film coat parameters. The carrier lifetime is responsible for the device time response whereas its maximum power emitted (mW-range) activates with temperature increase. One of the major advantages of devices employing optical "read in" technology is that they are free of contacts and junctions, thus making them ideal for operation at high temperatures.
We report on the study of heat 2D-distribution in InGaN LEDs with the stress made on local device overheating and temperature gradients inside the structure. The MQW InGaN/GaN/sapphire blue LEDs are designed as bottom emitting devices where light escapes the structure through the transparent GaN current spreading layer and sapphire substrate, whereas the LED structure with high-reflectivity Ni/Ag p-contact is bonded to the thermally conductive Si submount by a flip-chip method. The measurements are performed with an IR microscope operating in a time-resolved mode (3-5 um spectral range, <20 μm spatial and 10 μs temporal resolution), while scanning a heat emission map through a transparent sapphire substrate. We show how current crowding (which is difficult to avoid) causes a local hot region near the n-contact pads and affects the performance of the device at a high injection level.
In this report, we show both theoretically and experimentally how the IR signature of a semiconductor scene (with band gap energy Eg) can be monitored through contactless emissivity control even if this scene thermometric temperature is kept constant. More specifically, we show how a scene emissivity in the spectral band beyond the fundamental absorption range (ω2 < Eg / h, 3 to 5 μm and 8 to 12 μm transparency windows) can be dynamically (frame frequency > 20 kHz) monitored by a shorter wavelength photo excitation of non-equilibrium charge carriers (ω1 > Eg/h, "visible range"). Experimental tests performed on Si and Ge scenes (300 < T < 600 K), demonstrate optically generated cold and hot images and, what is more important, negligible temperature contrast between an object and a background (Stealth effect in IR).
The technology for the small-size focal plane arrays and linear arrays of polycrystalline SiGe microbolometers is developed at IMEC and successfully transferred to its industrial partner XenICs. A NETD of about 100 mK is achievable at the readout level on 14×14 and 200×1 arrays with 50 - 60 μm pixel pitch at a time constant of 20 - 25 ms. The design of pixels provides very precise tuning of the infrared resonant cavity. The resistance and TCR nonuniformity with σ/μ better than 0.2% combined with about 1% noise nonuniformity and 100% pixel operability are demonstrated. The first lot of arrays has been characterized, the arrays have been assembled with hybrid readout chips, supplied with the dedicated evaluation board and a software, and the results of system testing are being reported. The possibility to use the SiGe arrays as infrared emitters has been investigated for the first time and the results are presented as well.
The concept for a new high spatial resolution, high-temperature, Dynamic Infrared Scene Projector (DISP) for generating high-speed (microsecond range) broadband (3-16 microns) IR scenery through visible pumping of DISP semiconductor scene (visible-to-infrared conversion) was developed, fabricated and tested. The principle of this new device operation and the results of our initial experimental study are reported for the first time. Key potential operating parameters of the new device prototype (based on a Germanium screen) are compared to that of modern conventional DISP engine (SBIR Emitter Array Projector).
We present further progress in high-resolution time-resolved thermal imaging of electronic and optoelectronic devices. We show that concurrent multi-spectral mapping of light, emissivity, and heat patterns a single device produces may hold the key of the device performance improvement by visualizing current carrier distribution and heat flows. To demonstrate advantage of this approach, thermal heaters, light emitting devices, and Peltier coolers are tested with emphasis laid on uniformity of carrier distribution and thermal control.
The concept for a new high spatial resolution, high-temperature, Dynamic Infrared Scene Projector (DISP) for generating high-speed (microsecond range) broadband (3-16 microns) IR scenery through visible pumping of DISP semiconductor scene (visible-to-infrared conversion) was developed, fabricated and tested. The principle of this new device operation and the results of our initial experimental study are reported for the first time. Key potential operating parameters of the new device prototype (based on a Germanium screen) are compared to that of modern conventional DISP engine (SBIR Emitter Array Projector).
High spatial resolution infrared (3-5 and 8-12 micrometers ) scanning microscopy study shows that light and heat signatures of conventional planar light emitting diodes for 3-5 micrometers spectral range. (InGaAs, InAs, InAsSb) are drastically affected by current crowding effect. As a result, Joule heating causes unavoidable heat traps in the vicinity of point contact (those are most pronounced in substrate down structures), whereas large uniform emitting areas are difficult to produce. Contrary to this, emitters based on magnetoconcentration effect (InSb) are free of current crowding and could be made of larger areas (of some mm2). For the diode stripe (width of 100 micrometers ) lasers (AlGaAs/GaAs, InGaAsP) we show that heat concentrates at lateral stripe sides that are difficult to penetrate. Some details of an infrared micromapping system characterized by 20 micrometers spatial resolution and 10 microsecond(s) time resolved interval are also given.
Negative luminescence (NL) operation at ∼4 µm for p-InAsSbP/n-InAs and 4÷5 µm for p-InAsSb/n-InAsSb(P) diodes with FWHM∼0.1⋅λmax is reported for a reverse bias operation mode. NL output at 180oC is as high as 3÷5 µW and negative apparent temperature contrast is as strong as ∆T= - (6÷10°C) which show advantages of InAs and InAsSb based NL devices for high temperature applications. The remarkable feature is the uniformity of spatial NL output distribution, which is a confirmation of the existence of a potential barrier in narrow band p-n junction at elevated temperatures.
M. Aidaraliev, Nonna Zotova, Sergey Karandashev, Boris Matveev, Maxim Remennyi, Nikolai Stus', Georgii Talalakin, Volodymyr Malyutenko, Oleg Malyutenko
Negative luminescence (NL) operation at approximately 4 micrometer is reported for p-InAsSbP/n-InAs reverse biased diodes with efficiency of about 60% (180 degrees Celsius). High NL conversion efficiency (25 mWcm-2A-1, 180 degrees Celsius) and remarkable value of negative apparent temperature ((Delta) T approximately equals -6 degrees Celsius) show advantages of p-InAsSbP/n-InAs NL devices for high temperature applications.
Determination of the interstitial oxygen concentration in silicon is one. of important elements of its characterization. In the room temperature interstitial oxygen has an absorption level for the —1 wavelength X=9.05 im Cv = 1106 cm ). This level is very well seen because defectless silicon has low absorption in this range. Thus, mainly used method for interstitial oxygen concentration determination is based on dependence between the absorption coefficient of the oxygen peak as measured and interstitial oxygen concentration N [1, 2, 3]. In measurements of N some difficulties associated with absorption on phonons appear. At low silicon resistivities additional difficulties connected with the absorption on free carriers appear [4, 5] and the measurements at very low temperatures are necessary [6, 7]. For the absorption measurements an external source of infrared radiation (IR) of high stability (e.g. CO2 - laser) and thick samples with very well polished both surfaces [3] are required, which is often impossible in the case of commercial wafers. In this work a new optical method enabling determination of the interstitial oxygen concentration N, as well as its spatial distribution in silicon wafers is described. This method is based on the thermal radiation (TR)[4] measurement of the investigated material in the range of the interstitial oxygen absorption [81. According to the Kirchhoff's law the TR emission depends on the absorption properties of the material which enables in certain conditions to estimate the oxygen concentration. For this method an external source of IR is not necessary, and measurements can be carried out on wafers with metallized or rough back surfaces. Creating conditions enabling the measurement of TR radiation of the wafer is realized by heating or cooling a screen placed behind the wafer. With this method one can measure temperature dependence of interstitial oxygen concentration. The measurements can be carried outwith the use of the spectrophotometer, a scanning IR microscope or a thermovision camera with suitable adjusted spectral range.
Fundamental limitation on interband luminescence's quantum efficiency value in narrow gap semiconductors prevent practical use of orthodox light emitting diodes for optical processing in IR (5-25 µm). Use of thermal emission resulting from intraband transitions in wide gap semiconductors seems to be alternative approach to decide the problem. Pros and cons of the proposal supported by theoretical calculations and experimental study are being discussed in details. Subject areas: Submillimeter and Infrared Devices and Technology
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