Patent Application
20220093823
The present disclosure relates to generally to a method and device for cascading broadband emission.
Molecular vibrational and rotation modes provide an optical absorption fingerprint in the infrared band that are of particular interest for optical sensing technologies. Chemical identification through absorption spectra via Fourier Transform Infrared (FTIR) spectrometry is a ubiquitous method, and imaging in the infrared has distinct advantages over the visible, most notably in dark or low-visibility situations. For generalized applications, broadband sources are important. Because of the utility of infrared light, infrared sources for illumination and spectroscopy need to be developed. These and other shortcomings are addressed by the approaches set forth herein.
It is to be understood that both the following general description and the following detailed description are exemplary and explanatory only and are not restrictive. Methods and a device for cascading broadband emission are described. An example device can comprise a substrate, a bottom contact layer above at least a portion of the substrate, and a plurality of emission regions above the bottom contact layer. The plurality of emission regions can be disposed one above another. Each of the plurality of emission regions can be configured with different respective bandgaps to emit radiation of different wavelengths. The device can comprise a plurality of tunnel junctions. Each of the tunnel junctions can be disposed between at least two corresponding emission regions of the plurality of emission regions. The device can comprise a top contact layer above the plurality of emission regions.
In another aspect, an example method of fabrication can comprise forming a first contact layer on a substrate and forming a plurality of emission regions. The plurality of emission regions can be disposed one above another. The plurality of emission regions can be separated by a plurality of corresponding tunnel junctions. Each of the plurality of emission regions can be configured with different respective bandgaps to emit radiation of different wavelengths. The method can comprise forming a second contact layer above the plurality of emission regions.
In another aspect, an example method of operation a device can comprise receiving a current at a first contact layer and providing the current to a plurality of emission regions disposed one above another and separated by a plurality of corresponding tunnel junctions. Each of the plurality of emission regions can be configured with different respective bandgaps. The method can comprise emitting electromagnetic radiation having a spectrum range from the plurality of emission regions. Each of the plurality of emission regions can emit a respective portion of the spectrum range based on the respective bandgap of the emission region.
In yet another aspect, a method and a device for cascading broadband emission is described. The device may comprise a substrate, a bottom contact layer above at least a portion of the substrate, and a plurality of emission regions above the bottom contact layer. The plurality of emission regions may be disposed one above another. Each of the plurality of emission regions may be configured with different respective bandgaps to emit radiation of different wavelengths. The device may comprise a plurality of tunnel junctions. Each of the tunnel junctions can be disposed between at least two corresponding emission regions of the plurality of emission regions. The device can comprise a top contact layer above the plurality of emission regions. The plurality of emission regions may comprise a W-superlattice comprising an electron-well layer, a hole-well layer and an electron confinement layer.
Semiconductor layers of the W-superlattice may include AlAsSb, InAs, InGaSb, and InAs. Exemplary embodiments of the W-superlattice may comprise a W-quantum well.
Additional advantages will be set forth in part in the description which follows or may be learned by practice. The advantages, will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate exemplary embodiments of the invention, and, together with the general description given above and the detailed description given below, serve to explain the features of the invention.
Where the definition of terms departs from the commonly used meaning of the term, applicant intends to utilize the definitions provided below, unless specifically indicated.
It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of any subject matter claimed. In this application, the use of the singular includes the plural unless specifically stated otherwise. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. In this application, the use of “or” means “and/or” unless stated otherwise. Furthermore, use of the term “including” as well as other forms, such as “include”, “includes,” and “included,” is not limiting.
For purposes of the present disclosure, the term “comprising”, the term “having”, the term “including,” and variations of these words are intended to be open-ended and mean that there may be additional elements other than the listed elements.
For purposes of the present disclosure, directional terms such as “top,” “bottom,” “upper,” “lower,” “above,” “below,” “left,” “right,” “horizontal,” “vertical,” “up,” “down,” etc., are used merely for convenience in describing the various embodiments of the present disclosure. The embodiments of the present disclosure may be oriented in various ways. For example, the diagrams, apparatuses, etc., shown in the drawing figures may be flipped over, rotated by 90° in any direction, reversed, etc.
For purposes of the present disclosure, a value or property is “based” on a particular value, property, the satisfaction of a condition, or other factor, if that value is derived by performing a mathematical calculation or logical decision using that value, property or other factor.
For purposes of the present disclosure, it should be noted that to provide a more concise description, some of the quantitative expressions given herein are not qualified with the term “about.” It is understood that whether the term “about” is used explicitly or not, every quantity given herein is meant to refer to the actual given value, and it is also meant to refer to the approximation to such given value that would reasonably be inferred based on the ordinary skill in the art, including approximations due to the experimental and/or measurement conditions for such given value.
While the invention is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and the scope of the invention.
As used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.
“Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.
Throughout the description and claims of this specification, the word “comprise” and variations of the word, such as “comprising” and “comprises,” means “including but not limited to,” and is not intended to exclude, for example, other components, integers or steps. “Exemplary” means “an example of” and is not intended to convey an indication of a preferred or ideal embodiment. “Such as” is not used in a restrictive sense, but for explanatory purposes.
Disclosed are components that can be used to perform the disclosed methods and systems. These and other components are disclosed herein, and it is understood that when combinations, subsets, interactions, groups, etc. of these components are disclosed that while specific reference of each various individual and collective combinations and permutation of these may not be explicitly disclosed, each is specifically contemplated and described herein, for all methods and systems. This applies to all aspects of this application including, but not limited to, steps in disclosed methods. Thus, if there are a variety of additional steps that can be performed it is understood that each of these additional steps can be performed with any specific embodiment or combination of embodiments of the disclosed methods.
The present methods and systems may be understood more readily by reference to the following detailed description of preferred embodiments and the examples included therein and to the Figures and their previous and following description.
As will be appreciated by one skilled in the art, the methods and systems may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the methods and systems may take the form of a computer program product on a computer-readable storage medium having computer-readable program instructions (e.g., computer software) embodied in the storage medium. More particularly, the present methods and systems may take the form of web-implemented computer software. Any suitable computer-readable storage medium may be utilized including hard disks, CD-ROMs, optical storage devices, or magnetic storage devices.
Embodiments of the methods and systems are described below with reference to block diagrams and flowchart illustrations of methods, systems, apparatuses and computer program products. It will be understood that each block of the block diagrams and flowchart illustrations, and combinations of blocks in the block diagrams and flowchart illustrations, respectively, can be implemented by computer program instructions. These computer program instructions may be loaded onto a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions which execute on the computer or other programmable data processing apparatus create a means for implementing the functions specified in the flowchart block or blocks.
These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including computer-readable instructions for implementing the function specified in the flowchart block or blocks. The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions that execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks.
Accordingly, blocks of the block diagrams and flowchart illustrations support combinations of means for performing the specified functions, combinations of steps for performing the specified functions and program instruction means for performing the specified functions. It will also be understood that each block of the block diagrams and flowchart illustrations, and combinations of blocks in the block diagrams and flowchart illustrations, can be implemented by special purpose hardware-based computer systems that perform the specified functions or steps, or combinations of special purpose hardware and computer instructions.
The present disclosure relates to methods for increasing the emission bandwidth of an LED multiple times. The present disclosure has applications in Infrared optical sensing and monitoring (e.g., gases, biomolecules, plastic, etc.), infrared spectroscopy, as we as other fields.
Molecular vibrational and rotation modes provide an optical absorption fingerprint in the infrared band that are of particular interest for optical sensing technologies. Chemical identification through absorption spectra via Fourier Transform Infrared (FTIR) spectrometry is a ubiquitous method, and imaging in the infrared has distinct advantages over the visible, most notably in dark or low-visibility situations. For generalized applications, broadband sources are important. Because of the utility of infrared light, infrared sources for illumination and spectroscopy need to be developed. The most common infrared sources are thermal, providing blackbody radiation with spectra in accordance with Planck's law, but the maximum radiances are constrained by maximum temperature materials can handle without thermal degradation, i.e., in sublimation and oxidation.
Light emitting diodes (LEDs) are semiconductor light sources that emit light in a narrow band of wavelengths which can be centered in the ultraviolet to the far infrared. Typically, the emission band is a few kBT wide, where kB is the Boltzman constant and T the temperature of the LED, and is centered at a photon energy approximately equal the semiconductor bandgap. For example, in the mid-infrared at an emission wavelength centered at 4 μm, this corresponds to an emission bandwidth at 77K of about 0.3 μm. That is, emission may extend from about 3.85 μm to about 4.15 μm.
This disclosed methods and apparatus can increase the emission bandwidth multiple times, potentially converting a narrow band LED into a broadband one. A broadband. LED can be invaluable in many applications. For example, in infrared absorption spectroscopy of one or several complex molecules, a broadband light source is typically needed over the infrared optical fingerprint region.
It is possible that broadband LEDs can displace incandescent (e.g., blackbody) infrared light sources that are currently used in optical gas sensors, which is a very high volume market, and infrared spectrometers. Incandescent infrared sources can have good wallplug (power) efficiency in the mid-infrared for specific temperatures (−1500K), and are cheap. At hotter or lower temperatures, less light is emitted in the mid-wave infrared, and they become less efficient in the mid-infrared. Moreover, the spectrum of incandescent light sources change over time, which spoils sensor calibration; the maximum radiance is limited by the melting point of metals used; the filaments burn out after a short lifetime; and the on/oft rate is low, less than 400 Hz.
Infrared LEDs have higher maximum radiance, long lifetime, stable spectrum, and high on/off rates (˜1 MHz). On the downside, the efficiencies are currently low—but improving with research—and spectra of infrared LEDs can be narrow. The narrow emission spectra of the current infrared LEDs makes the infrared LEDs less suitable for many infrared absorption spectroscopy applications. However, the present methods and apparatus can increase the spectra of infrared LEDs.
As a general overview, the device 100 can comprise a substrate and contact layer, emission regions and tunnel junctions alternatively layered N−1 times, a final emission region, top contact layer, and/or the like. The tunnel junctions can comprise a reverse biased pair of diode layers compared to the forward LED bias. For example, if the bottom contact of the LED is n-biased and the top contact is p-biased, then the tunnel junction would p then n.
The device 100 can comprise a substrate 102, for example, disposed under and in connection with a bottom contact layer 104. The substrate 102 may be configured for back emission (e.g., emission of radiation through the substrate 102). The substrate 102 may be configured to allow the emitted radiation (e.g., light) to travel through the substrate. For example, the substrate 102 can be transparent. The device 100 can comprise a bottom contact layer 104. The bottom contact layer 104 can comprise doped GaSb layers.
The device 100 can comprise a cascaded region 106. The cascaded region can comprise one or more tunnel junctions 108. The cascaded region 106 can comprise one or more emission regions 110. The cascaded region 106 can comprise a plurality of cascaded sections. One or more (or each) of the cascaded sections can comprise one of the tunnel junctions 108 and one of the emissions regions 110. The number of the cascaded sections can be based on design specifications. For example, the number of cascaded sections can be based on a predefined emission spectrum range. Emission wavelengths can be chosen to suit the application. If the user wants a continuous spectrum, the user should select the wavelength emission range, and then take sufficiently small tuning steps from one emission region to the next that a continuous spectrum results. In the examples her, 0.2 μm steps are shown (e.g., 3.3 μm, 3.5 μm, 3.7 μm, 3.9 μm, 4.1 μm, 4.4 μm, 4.7 μm, and 5.0 μm), which resulted in a continuous emission spectrum at high drive current. The detuning step size may be larger or smaller depending on wavelength range and drive current. In general, the emission width of a single emission region is relatively constant in energy (width ΔE− a few kBT, and independent of photon energy E), but varies with center wavelength (Δλ=(ΔE/hc)λ2). This may be known by someone skilled in the art. The emission spectrum need not be continuous, but could a discrete set of wavelength ranges in infrared wavelengths between about 2 and about 30 μm.
Emission regions can comprise InAs/GaSb superlattices matched to GaSb with variable thickness InSb interfaces. InAs layer thicknesses can vary from about 9.7 to about 6 monolayers. GaSb can have a thickness fixed at 16 monolayers.
The tunnel junctions can be configured (e.g. as a gate) to allow electrons (e.g., and holes) to pass through to an adjacent emission region. The tunnel junctions can be configured to allow electrons (e.g., and holes) to pass through to an adjacent emission region after (e.g., in response to) radiative: recombination (e.g., of a hole with an electron) in the previous emission region. As a further explanation, a standard single stage LED (e.g., an LED with a single emission layer) can be configured to inject an electron and hole into the LED from the cathode and anode, respectively. The electron and hole can meet in the emission region and recombine to produce a photon at the characteristic bandgap of semiconductor emission region. In the cascading configuration of the present device 100, multiple emission regions can be stacked one upon the other separated by tunnel junctions, as shown schematically in
The way a tunnel junction works as a gate is shown schematically in
In an aspect, the stacked emission regions can have different bandgaps. By making the bandgap slightly different each stage, the: photon emitted by each stage will have slightly different energy, and the total emission by the cascaded structure will be broadened up to N times for N stages. For a back-emitting geometry, such as in
In an aspect, the emission regions can be tunable in bandgap. The emission regions can be tunable without substantially changing the crystal lattice constant in order to prevent plastic relaxation of the crystal. In the infrared range, this is possible with InAs/GaSb superlattices over the emission range 3-30 μm. If GaInAsSb alloys are included, the tunable range can be increased down to about 2 μm.
The tunnel junctions for the materials can be chosen to be wider gap than the emission regions to provide blocking layers to prevent electron/hole leakage between the emission regions prior to radiative recombination. For InAs/GaSb or GaInAsSb, this can be achieved for example by using p-GaSb/n-Al0.20In0.80As0.73Sb0.23, which have 77K bandgaps of 0.8 eV/0.75 eV (cut-off wavelengths 1.55 μm/1.65 μm). In general, the tunnel junctions should have wider bandgaps than the emission layers on either side to function optimally. The tunnel junction can comprise a n+ doped layer and a p+ doped layer. The n+ doped layer can be configured to block hole leakage in the valence band of an adjacent emission region, and the p+ doped layer can be configured to block electron leakage in the conduction band of an adjacent emission region. The leakage barrier is best achieved with a wider bandgap semiconductor.
The device 100 can comprise a final emission region 112. The final emission region can have the same properties as the one or more emission regions. The device 100 can comprise a top contact layer 114. The top contact layer can be disposed above (e.g., directly on top of) the final emission region 112. The top contact layer 114 can comprise doped GaSb layers.
The disclosed device represents an improvement over other devices. The present device improves over devices that have only a small wavelength range and are unable to transmit infrared radiation. For example, some devices may be tunable only over a very small wavelength range when used in a cascaded device. Even though a wide tuning range is in principle possible with some materials, in practice (e.g., with a ternary alloy), the lattice constant of the crystal changes with the emission wavelength. Thus, if attempts are made to tune the wavelength much, the crystal will not be lattice matched to its (virtual or non-virtual) substrate, and it will relax with misfit dislocations. The present device does not have such drawbacks. Additionally, the present device does not rely on phosphor encapsulants. In contrast, the materials disclosed herein allows tuning of the emission wavelength over a wide range (2-30 μm without changing the superlattice (or quaternary alloy) lattice constant. Superlattices such as InAs/GaSb, InAs/InAsSb/GaSb, and InAs/GaInAsSb have tunable wavelength from 3-30 μm while maintaining the same 6.1 Å lattice constant. GaInAsSb allows additional coverage from 2-3 μm while maintaining the same 6.1 Å lattice constant. Due to the ability to vary the superlattice emission wavelength (e.g., bandgap=hc/wavelength) while keeping the lattice constant (e.g., averaged over a period) unchanged, broadband emission can be achieved in a cascaded structure from 2-30 μm with the desired spectral uniformity or multispectral characteristics simply by using enough tuned emission regions separated by tunnel junctions. Due to the ability to vary the superlattice emission wavelength (e.g., bandgap=hc/wavelength) while keeping the lattice constant (averaged over a period) unchanged, the present methods and devices allow for cascading many more detuned emission regions each with a thicker emission region. This results in a much larger range of broadband emission. The presently disclosed approach allows broadband emission in a single monolithic structure without the use of phosphors. Additionally, the presently disclosed approach improves over other devices by allowing broadband emission in the infrared spectrum.
At step 304, a plurality of emission regions can be formed. The plurality of emission regions can be disposed one above another. The plurality of emission regions can be separated by a plurality of corresponding tunnel junctions. One or more (or each) of the plurality of emission regions can be configured with different respective bandgaps to emit radiation of different wavelengths. Each of the plurality of emission regions can be configured to emit electromagnetic radiation within the infrared spectrum.
The plurality of emission regions can comprise superlattices of a first semiconductor and a second semiconductor. The first semiconductor can comprise InAs. The second semiconductor comprise GaSb. A thickness of the first semiconductor can vary among the plurality of emission regions. A thickness of the second semiconductor can be uniform among the plurality of emission regions.
A respective bandgap of each emission region of the plurality of emission regions can decrease the further the emission region is from the substrate. The respective bandgaps can be incrementally different in size from a top emission region to a bottom emission region of the plurality of emission regions.
The plurality of tunnel junctions can be configured to prevent electron leakage between the plurality emission regions prior to radiative recombination. One or more of the plurality of tunnel junctions can comprise a pair of diode layers that are biased opposite to a bias of the plurality of emission regions.
At step 306, a second contact layer can be formed above the plurality of emission regions. A second electrode can be electrically coupled to the second contact layer.
At step 406, electromagnetic radiation having a spectrum range can be emitted from the plurality of emission regions. Each of the plurality of emission regions can emit a respective portion of the spectrum range based on the respective bandgap of the emission region.
The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the compounds, compositions, articles, devices and/or methods claimed herein are made and evaluated, and are intended to be purely exemplary and are not intended to limit the scope of the methods and systems. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by-weight, temperature is in ° C. or is at ambient temperature, and pressure is at or near atmospheric.
The present methods and devices, are illustrated by example using InAs/GaSb cascaded superlattice light-emitting diodes (SLEDs). The example SLEDs are configured as flexible broadband infrared sources that have competitive radiance and higher modulation bandwidth than current thermal blackbody sources. These SLEDs can further be improved in radiance through employment of approaches such as light extraction and heat management strategies. The example SLEDs are not fundamentally limited by thermal degradation for achieving high radiances.
The present methods and devices improve over current infrared sources. Infrared sources are dominated by thermal emitters: most notably SiC rods (Globars) for FTIR analysis, although other sources include Nemst glowers, NiChrome resistors, and blackbody cavities. Globars are currently limited to 1650° C., with devices typically operated near 1200° C. for spectroscopy applications. LumaSense, Inc. offers a blackbody cavity that is capable of reaching 3000° C. While capable of providing adequate illumination, thermal sources are limited by Planck's law of blackbody radiation and material properties. Emissivity, phase transition, and chemical reactions limit thermal sources in atmosphere to below 2000° C. Blackbody cavities can achieve emissivities of nearly unity, and can be sheathed in noble gases, such as argon, to prevent degradation of the sources. However, even the most refractory materials are limited in peak temperature before undergoing a phase transition. Furthermore, thermal sources may take several minutes to reach a stable operating temperature, limiting spectroscopic techniques.
Solid state materials offer an alternative technology capable of overcoming the radiance and temporal limitations of thermal sources. In general, solid state sources are monochromatic, such as lasers, or narrow spectrum, such as light-emitting diodes (LEDs). Bright, broadband LED sources are useful for spectroscopy, medical, and projection applications. To create a broadband source, several LEDs or lasers can be operated in tandem. InAs/GaSb type-II SLEDs provide a promising source of bright, fast infrared sources that can cascade several emission colors into a single device, creating a monolithic broadband emitter. The InAs/GaSb system can emit from 3 μm to 30 μm by varying superlattice layer thicknesses, and can suppress non-radiative Auger recombination by increasing the gap between light- and heavy-hole valence bands. Auger recombination is the dominant recombination event at high currents. Reducing Auger processes is important to creating efficient sources. Cascading emission regions further increases the efficiency of the devices by recycling the carriers in each emission region. Cascading works by coupling superlattice emission regions by tunnel junctions, in which electrons can emit a photon in each emission region. By varying emission region composition, cascaded devices are capable of emitting several different colors in a monolithic structure.
Several multicolor devices have been demonstrated, particularly in the visible spectrum. However, due to the “green-yellow” gap in efficiency, typical broadband visible diodes are comprised of a blue LED exciting red and green phosphors. Multi-wavelength devices in the infrared have been fabricated, but are primarily two-color devices. Devices based on InGaSb/AlGaAsSb and InGaAsSb/AlInGaAsSb multiple quantum wells operated at 77K were able to achieve 3.6 mW/cm2-sr and 0.18 mW/cm2-sr in the 1.8-2.0 μm and 2.6-3.0 μm regions, respectively. Devices based on InAs/GaInSb/InAs W-quantum wells operated at 196K demonstrated 14 mW/cm2-sr in the 3-4 μm and 0.19 mW/cm2-sr in the 6-9 μm spectral regions. Type-II InAs/GaSb SLEDs operated at 77K demonstrated 5.5 mW/cm2-sr and 2.7 mW/cm2-sr in the 3.2-4.2 μm and 4.2-5.2 μm ranges. However, all these devices were designed to exhibit independent dual-color operation. Outside of blackbody sources, broadband infrared sources are limited.
In an aspect, the following examples illustrate InAs/GaSb type-II superlattice light-emitting diodes that were fabricated to form a device that provides emission over the entire 3-5 μm mid-infrared transmission window. In a type II superlattice, the conduction and valence subbands can be staggered in both real and reciprocal space, so that electrons and holes are confined in different layers. Variable bandgap emission regions were coupled together using tunnel junctions to emit at peak wavelengths of 3.3 μm, 3.5 μm, 3.7 μm, 3.9 μm, 4.1 μm, 4.4 μm, 4.7 μm, and 5.0 μm. Cascading the emission regions can recycle the electrons in each emission region to emit several wavelengths simultaneously. At high current densities, the light-emitting diode spectra broadened into a continuous, broadband spectrum that covered the entire mid-infrared band. When cooled to 77K, radiances of over 1 W/cm2-sr were achieved, demonstrating apparent temperatures above 1000K over the 3-5 μm band. InAs/GaSb type-II superlattices were configured to emit from about 3 μm to about 30 μm. The device design can be expanded to include longer emission wavelengths.
Broadband type-II InAs/GaSb superlattices designed to emit across the 3-5 μm atmospheric transmission window are demonstrated. By cascading emission regions of varying composition, broadband mid-wave emitter devices have been fabricated in a monolithic device. At low injection currents, individual peaks are discernible in the spectrum, but broaden to a continuous spectrum peaking at 3.68 μm with a full width at half-maximum of 1.4 μm, and peak radiances on 100 μm×100 μm devices of 1.04 W/cm2-sr.
An example fabrication of the device is described as follows. The structures were designed to emit across the 3-5 μm atmospheric transmission window. To accomplish design specification, eight emission region stages were grown to emit at different wavelengths. The emission regions were coupled with tunnel junctions to reuse electrons in each emission region. As previously explained, a band diagram for a two-stage device is given in
The example device was grown in a split panel Veeco Gen20 molecular beam epitaxy reactor. The: molecular beam epitaxy reactor can be equipped with valved cracker cells for the V-sources and dual filament SUMO cells for the HI-sources. A 1 μm-thick buffer of Te-doped (1e18 cm3) n-GaSb which also served as a bottom contact layer was grown on the (001) face of a GaSb wafer. The buffer was followed by eight superlattice emission regions coupled by tunnel junctions and a doped cap layer for a top contact. The superlattice emission regions, listed in Table 1, comprise: twenty periods each, separated by tunnel junctions of n-AlInAsS b/p-GaSb.
The n-AlInAsSb was grown digitally to prevent phase separation. The superlattice and tunnel junction are typically grown by molecular beam epitaxy between 420 and 450 C with true V/III ratios around 1.3-2. Lattice matching of the InAs/GaSb superlattices to GaSb substrates is achieved by the insertion of InSb interfaces. The AlInAsSb is a digitally grown alloy, and lattice matched to the (e.g., virtual or non-virtual) substrate. To achieve the target alloy, the As and Sb fluxes are set to achieve the desired concentration, while the In and Al shutters can be alternated in thin layers to achieve the correct concentration (e.g., but not so thick as to cause relaxation). At this time, these materials are typically grown by MBE, though the materials could be grown by MOCVD. MOCVD would have very different optimal growth conditions.
And following the eighth superlattice, a 140 nm p-GaSb layer doped (Be) to 5e18 cm−3 was grown. To pattern the devices, standard photolithography and wet etching was used to form square mesas with variable sizes. A mixture of citric acid, phosphoric acid, and hydrogen peroxide provided an anisotropic 45° etch in the structure, forming angled sidewalls on the diode mesas. Contacts of Ti/Pt/Au were electron-beam evaporated for both anode and cathode, on top of which a layer of indium was deposited for bonding. An 8-stage single-color device was also grown for comparison. This was identical in growth to the BILED structure, except every stage was fabricated to emit light around 4.1 μm.
Table I illustrates details of each emission region in the example BILED device. The layers are evenly spaced in energy to span the mid-wave infrared 3-5 μm transmission window.
The fabricated LEDs were flip-chipped to a silicon fanout header, which was wirebonded to a leadless chip carrier. This was mounted in a dewar against an aluminum mount in contact with an open flow of liquid nitrogen, with the temperature monitored by a silicon diode. Measurements were taken with a cooled mercury-cadmium-telluride detector with a cutoff at 10 μm. No efforts were made to collect light, thin the substrate, or apply an antireflective layer.
Using double-modulated FTIR, the output from the devices were spectrally resolved as a function of current density.
The electrical response of the diodes is given in
The results above are discussed in further detail as follows. While not the brightest mid-wave infrared devices demonstrated (highest reported radiance), the example device demonstrates broadband output (1.04 W/cm2) while producing a peak spectral radiance similar to comparable single color eight stage devices. Comprised of only eight stages, this device could be cascaded to include more emission regions, increasing radiance.
As current density is raised, the peaks broaden and blueshift, as shown in
The spectral output exhibits several interesting experimental features. The first is the alternating efficiencies in the stages at low current densities. Additionally, one emission peak appears to be missing: only seven peaks appear in the spectrum, even though there are eight distinct emission regions. Fourteen-band k·p calculations, which factor in radiative and Auger recombination coefficients, indicate that the internal quantum efficiency for these structures is relatively constant compared to the fluctuations exhibited in the low current density electroluminescent data (e.g., lower spectra in
In conclusion, broadband type-II InAs/GaSb superlattice light-emitting diodes were demonstrated across the mid-wave infrared band. At half-max, the spectrum spans 3.24 μm-4.68 μm, achieving radiances of up to 1.04 W/cm2-sr and an apparent temperature of 1100K across the 3-5 μm band. At low current densities, periodic variations in the relative efficiency were revealed, suggesting a method to probe individual emission regions in cascaded devices. By cascading superlattices designed to emit anywhere from 3-30 μm, the mid-wave, long-wave, and very-long-wave infrared regions can be represented in a BILEDs device. By varying the number of single-color emission regions, the output spectrum has the ability to be tailored to over-represent longer wavelengths, providing a brighter mid-wave to long-wave source than is possible in thermal materials.
While the methods and systems have been described in connection with preferred embodiments and specific examples, it is not intended that the scope be limited to the particular embodiments set forth, as the embodiments herein are intended in all respects to be illustrative rather than restrictive.
Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is in no way intended that an order be inferred, in any respect. This holds for any possible non-express basis for interpretation, including: matters of logic with respect to arrangement of steps or operational flow; plain meaning derived from grammatical organization or punctuation; the number or type of embodiments described in the specification.
Thus, embodiments directed, for example, to
For purposes of the present disclosure, the term “superlattice” refers to a quasi-2D structure that may include a repetition of two or more semiconductor layers with an overall bandgap, bandstructure, and band offset determined by, but different from, the constituent layers. In a type II superlattice, band offsets of the constituent layers are such that the electron is more likely to be found in one layer, known as the electron well, and the hole is more likely to be found in a different layer, known as the hole-well. Some type II superlattices may be important to the mid- and long-wave infrared and may also be utilized by the disclosed invention to include InAs/GaSb, W-superlattice, InAs/InAsSb, AlGaInSb/InAs, as well as others. Note all these are type II. In some disclosed embodiments, the disclosed W-superlattices have shown the greatest promise for infrared LEDs due to their high quantum efficiency, and high gain, which is important for amplification of spontaneous emission and enhanced extraction of light.
In accordance with disclosed embodiments, superlattice semiconductor structures may be serially connected through tunnel junctions in an arrangement known as cascading. A current flowing through one superlattice structure must flow serially through each superlattice structure in the cascade to complete the circuit. A single electron-hole pair injected into the cascaded superlattice structure can produce a photon in each of the N cascaded superlattice structures. In other words, one injected electron-hole pair produces up to N photons in the structure due to cascading.
For purposes of the present disclosure, the term “W-superlattice” refers to repeats of four semiconductor layers, which may typically include AlAsSb, InAs, InGaSb, and InAs, respectively. The InAs layers may serve as or comprise an electron well, and the InGaSb layer may serve as or comprise a hole-well. In a select embodiment, confinement layers comprising, for example, wide-gap AlAsSb layers may be utilized to better confine the electrons and increase their penetration into the InGaSb layer for improved electron-hole overlap. Thus, embodiments of the disclosed W-superlattice may comprise electron-well layers (e.g., InAs layers); hole-well layer (e.g., InGaSb); and electron confinement layers (e.g., AlAsSb). The strained InGaSb produces a favorable bandstructure for suppression of Auger scattering. In accordance disclosed embodiments, the W-superlattice is favorable in an LED, because its added thickness (compared to a quantum well) reduces carrier density and Auger scattering. For example,
n=(I/A)(1/d)(τ)(1/q),
where A is the area of the device, d is the thickness of the emission layer, τ is the carrier lifetime, and q is the charge of an electron. Thus as the emission layer gets thicker it is evident that the carrier density goes down. As the carrier density drops for a given input current I, the Auger scattering rate drops.
For purposes of the present disclosure, the term “W-quantum well” refers to a single period of the disclosed W-superlattice. Disclosed embodiments of the W-superlattice comprising, for example, the W-quantum well may be regarded as a truer 2D structure, leading to an improved bandedge density of states and higher peak gain, which provide characteristics of particular importance in laser structures of the present disclosure. The aforementioned high peak gain may also be favorable for amplified spontaneous emission in LEDs.
Disclosed embodiments recognize that in an LED, there is no reason the bandgap of each cascaded superlattice has to be the same. With type II superlattices, the bandgaps can be independently changed without changing the overall superlattice lattice constant, so the superlattices can stay lattice matched to the substrate. The advantage of such an arrangement is that as current serially flows through each superlattice, light of different wavelength is emitted due to differing superlattice bandgaps, which can lead to ultra-broadband, or multi-spectral emission of the infrared LED. Disclosed embodiments provide this importance for many applications. Spectroscopic analysis of gases, surfaces, solids can be done with a single LED emitting at multiple wavelengths rather than multiple LEDs emitting at different single wavelengths.
A key advantage of the disclosed W-superlattice, for example, over InAs/GaSb is the improved electron-hole overlap, brought about from the presence of the wide-gap AlAsSb layers. Disclosed embodiments have shown electron-hole overlap evident in bandstructure calculations.1 This is also evident in the high radiative B coefficient; a comparison shows the W-superlattice coefficient is about 4-5 times higher than the InAs/GaSb.1 This leads to higher quantum efficiency in an LED and higher gain.1 If these two layers are taken away, the structure would revert to an InAs/GaSb superlattice (or InAs/InGaSb).
Accordingly, the disclosed W-superlattice will comprise elemental layers conducive to providing improved quantum efficiency from improved electron-hole overlap, and higher gain for amplified spontaneous emission. In some disclosed embodiments, other variants may be utilized as layers of the disclosed W-superlattice structure. For example, in one exemplary configuration, the AlAsSb layer could use instead a quinternary AlInGaAsSb layer.
Turning again to the W-superlattice of
In an exemplary embodiment, the disclosed W-superlattices may include a plurality of layers. For example, in one disclosed embodiment, the W-superlattice may include four layers AlSbAs/InAs/InGaSb/InAs. It is readily appreciated that the disclosed W-superlattice may utilize the core concept of cascading superlattices with varying bandgaps.
Thus, disclosed embodiments may include any superlattice consisting of thin layers of compound semiconductor III-V layers. Other examples may include W-superlattices (AlSb/InAs/InGaSb/InAs), a W-quantum well (i.e., a single period of the superlattice), InAs/InAsSb, and AlGaInSb/InAs, though the aforementioned embodiment(s) may not be limited to these.
In accordance with disclosed embodiments, bandgaps for type II superlattices are generally useful from about 3-30 microns, or 40 meV to 400 meV. Embodiments of the disclosed invention are most focused on about 3-12 microns. In one exemplary embodiment, sample layer thicknesses for wavelength=3.7 μm (77K)−4.2 μm (300K) emitting W-superlattice are provided in Table 2. In accordance with some disclosed embodiments, bandgap may vary with temperature. Thicknesses are expressed in Angstroms.
Having described the many embodiments of the present disclosure in detail, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. Furthermore, it should be appreciated that all examples in the present disclosure, while illustrating many embodiments of the invention, are provided as non-limiting examples and are, therefore, not to be taken as limiting the various aspects so illustrated.
The following references are referred to above and are incorporated herein by reference:
All documents, patents, journal articles and other materials cited in the present application are incorporated herein by reference.
While the present disclosure has been disclosed with references to certain embodiments, numerous modification, alterations, and changes to the described embodiments are possible without departing from the sphere and scope of the present disclosure, as defined in the appended claims. Accordingly, it is intended that the present disclosure not be limited to the described embodiments, but that it has the full scope defined by the language of the following claims, and equivalents thereof.
This application claims benefit of priority of U.S. patent application Ser. No. 16/489,013 entitled, “CASCADED BROADBAND EMISSION,” filed Aug. 27, 2019 of which the present application is a continuation-in-part application, which is a 371 of PCT/US18/20296, filed Feb. 28, 2018 which claims benefit of priority to U.S. Provisional Patent Application 62/464,766 filed Feb. 28, 2017, which is hereby incorporated by reference in its entirety.
This invention was made with the United States government support under FA865114M0727 awarded by the Department of the Air Force. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 62464766 | Feb 2017 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 16489013 | Aug 2019 | US |
| Child | 17539260 | US |
