Heterogeneous, group iv-vi / ii-vi, photoconductive detectors

Abstract

An apparatus comprises: a photoconductive detector comprising: a substrate (260); a detector layer (220) disposed on the substrate (260), wherein the detector layer comprises a 3D interconnected conductive network comprising: a plurality of interconnected group IV-VI semiconductors (230) forming a group IV-VI semiconductor core; and a matrix comprising group ll-VI semiconductors (240) which at least partially surround the group IV-VI semiconductor core, wherein the group IV-VI semiconductor core and the matrix together form a heterogeneous group IV-VI / II-VI photoconductive structure; and contacts (210) coupled to the detector layer. A method comprises: obtaining a substrate (260); depositing a group IV-VI semiconductor (230) on the substrate; depositing a group ll-VI semiconductor (240) around the group IV-VI semiconductor to obtain a pre-annealed structure; performing annealing to obtain a post-annealed structure; and attaching contacts (210) to the post-annealed structure to obtain a photoconductive detector.

Description

Heterogeneous, Group IV-VI / H-VI, Photoconductive DetectorsCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This claims priority to U.S. Prov. Patent App. No. 63 / 589,855 filed on October 12, 2023, which is incorporated by reference.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with government support under Contract Number W911NF1810418 awarded by the Army Research Office (ARO), Department of Defense. The government has certain rights in the invention.BACKGROUND

[0003] The global commercial market for IR imaging equipment is estimated to be well over $2 billion per year, while the global military market for IR imaging equipment is estimated to be about $6 billion per year. Group IV- VI semiconductor detectors are currently used for high- temperature operation of IR imaging equipment. However, further improvement of device homogeneity, device performance, and yield of IR imaging equipment is desired.BRIEF DESCRIPTION OF THE DRAWINGS

[0004] For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts.

[0005] FIG. 1 is a schematic diagram of imaging equipment.

[0006] FIG. 2 is a 2D schematic diagram of a photoconductive detector according to a first embodiment.

[0007] FIG. 3 is a 2D schematic diagram of a photoconductive detector according to a second embodiment.

[0008] FIG. 4 is a schematic diagram of a substrate.

[0009] FIG. 5 is a 3D schematic diagram of a photoconductive detector prior to annealing.

[0010] FIG. 6 is a schematic diagram of one of the core-shell QDs in FIG. 5.

[0011] FIG. 7A is a 3D picture of a portion of a photoconductive detector prior to annealing.

[0012] FIG. 7B is a cross-section of the portion of the photoconductive detector in FIG. 7A.

[0013] FIG. 8 is a 3D schematic diagram of a photoconductive detector after annealing.

[0014] FIG. 9 is an energy band diagram of the detector layer in FIG. 8.

[0015] FIG. 10A is a 3D picture of a portion of a photoconductive detector after annealing.

[0016] FIG. 10B is a cross-section of the portion of the photoconductive detector in FIG. 10A.

[0017] FIG. 11 is a schematic diagram of the computing device in FIG 1.

[0018] FIG. 12 is a flowchart of a method of fabricating the photoconductive detector of FIG.2 or the photoconductive detector of FIG. 3.

[0019] FIG. 13 is a graph of absorbance and photoluminescence versus wavelength as observed in the experimental detector after annealing.

[0020] FIG. 14 is a graph of current versus bias as observed in the experimental detector.

[0021] FIG. 15 is a graph of signal and noise versus bias in the experimental detector.

[0022] FIG. 16 is a graph of SNR versus bias in the experimental detector.

[0023] FIG. 17 is a graph of responsivity versus bias in the experimental detector.

[0024] FIG. 18 is a graph of specific detectivity versus bias in the experimental detector.

[0025] FIG. 19 is a graph of peak detectivity versus bias in the experimental detector.

[0026] FIG. 20 is a table comparing performances of RT-operating MWIR QD PDs.DETAILED DESCRIPTION

[0027] It should be understood at the outset that, although an illustrative implementation of one or more embodiments are provided below, the disclosed systems and / or methods may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents.

[0028] Before describing various embodiments in more detail by way of exemplary description, examples, and results, it is to be understood that the present disclosure is not limited in application to the details of methods and compositions as set forth in the following description. The disclosure is capable of other embodiments or of being practiced or carried out in variousways. As such, the language used herein is intended to be given the broadest possible scope and meaning; and the embodiments are meant to be exemplary, not exhaustive. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting unless otherwise indicated as so. Moreover, in the following detailed description, numerous specific details are set forth in order to provide a more thorough understanding of the disclosure. However, it will be apparent to a person having ordinary skill in the art that the presently disclosed concepts may be practiced without these specific details. In other instances, features that are well known to persons of ordinary skill in the art have not been described in detail to avoid unnecessary complication of the description.

[0029] Unless otherwise defined herein, scientific and technical terms used herein shall have the meanings that are commonly understood by those having ordinary skill in the art. Further, unless otherwise required by context, singular terms shall include pluralities and plural terms shall include the singular.

[0030] All patents, published patent applications, and non-patent publications mentioned in the specification are indicative of the level of skill of those skilled in the art to which the present disclosure pertains. All patents, published patent applications, and non-patent publications referenced in any portion of this application are herein expressly incorporated by reference in their entirety to the same extent as if each individual patent or publication was specifically and individually indicated to be incorporated by reference.

[0031] All of the systems and methods of production and application thereof disclosed herein can be made and executed without undue experimentation in light of the present disclosure. While the systems and methods of the present disclosure have been described in terms of particular embodiments, it will be apparent to those of skill in the art that variations may be applied to the systems and / or methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit and scope of the inventive concepts. All such similar substitutes and modifications apparent to those of skilled in the art are deemed to be within the spirit and scope of the inventive concepts disclosed herein.

[0032] As utilized in accordance with the methods and compositions of the present disclosure, the following terms, unless otherwise indicated, shall be understood to have the following meanings:

[0033] The use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims and / or the specification may mean “one”, but it is also consistent with the meaning of “one or more”, “at least one”, and “one or more than one”. The use of the term “or” in the claims is used to mean “and / or” unless explicitly indicated to refer to alternatives only or when the alternatives are mutually exclusive, although the disclosure supports a definition that refers to only alternatives and “and / or”. The use of the term “at least one” will be understood to include one as well as any quantity more than one, including but not limited to, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, or more, or any integer inclusive therein. The term “at least one” may extend up to 1000 or more, depending on the term to which it is attached; in addition, the quantities of 100 / 1000 are not to be considered limiting, as higher limits may also produce satisfactory results. In addition, the use of the term “at least one of X, Y, and Z” will be understood to include X alone, Y alone, and Z alone, as well as any combination of X, Y, and Z.

[0034] As used in this specification and claims, the words “comprising” (and any form of comprising, such as “comprise” and “comprises”), “having” (and any form of having, such as “have” and “has”), “including” (and any form of including, such as “includes” and “include”) or “containing” (and any form of containing, such as “contains” and “contain”) are inclusive or open- ended and do not exclude additional, unrecited elements or method steps. For example, unless otherwise noted, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements, but may also include elements not expressly listed or inherent to such process, method, article or apparatus.

[0035] The term “or combinations thereof’ as used herein refers to all permutations and combinations of the listed items preceding the term. For example, “A, B, C, or combinations thereof’ is intended to include at least one of: A, B, C, AB, AC, BC, or ABC, and if order is important in a particular context, also BA, CA, CB, CBA, BCA, ACB, BAC, or CAB. Continuing with this example, expressly included are combinations that contain repeats of one or more item or term, such as BB, AAA, AAB, BBC, AAABCCCC, CBBAAA, CABABB, and so forth. The skilled artisan will understand that typically there is no limit on the number of items or terms in any combination, unless otherwise apparent from the context.

[0036] Throughout this application, the terms “about” or “approximately” are used to indicate that a value includes the inherent variation of error for the device, system, components, or method,or the variation that exists among the study subjects. Further, in this detailed description and the appended claims, each numerical value (e.g., error, time, distance, angle) should be read once as modified by the term "about" or “approximately” (unless already expressly so modified), and then read again as not so modified unless otherwise indicated in context. For example but not by way of limitation, when the term “about” or “approximately” is utilized, the designated value may vary by plus or minus 20 percent, plus or minus 15 percent, plus or minus 12 percent, or plus or minus 11 percent, or plus or minus ten percent, or plus or minus nine percent, or plus or minus eight percent, or plus or minus seven percent, or plus or minus six percent, or plus or minus five percent, or plus or minus four percent, or plus or minus three percent, or plus or minus two percent, or plus or minus one percent, or plus or minus one-half percent.

[0037] Also, any range listed or described herein is intended to include, implicitly or explicitly, any number within the range, particularly all integers, including the end points, and is to be considered as having been so stated. For example, "a range from 1 to 10" is to be read as indicating each possible number, particularly integers, along the continuum between about 1 and about 10, including for example 2, 3, 4, 5, 6, 7, 8, and 9. Similarly, fractional amounts between any two consecutive integers are intended to be included herein, such as, but not limited to, .05, .1, .15, .2, .25, .3, .35, .4, .45, .5, .55, .6, .65, .7, .75, .8, .85, .9, and .95. For example, the range 3 to 4 includes, but is not limited to, 3.05, 3.1, 3.15, 3.2, 3.25, 3.3, 3.35, 3.4, 3.45, 3.5, 3.55, 3.6, 3.65, 3.7, 3.75, 3.8, 3.85, 3.9, and 3.95. Thus, even if specific data points within the range, or even no data points within the range, are explicitly identified or specifically referred to, it is to be understood that any data points within the range are to be considered to have been specified, and that the inventors possessed knowledge of the entire range and the points within the range.

[0038] As used herein, the term “substantially” means that the subsequently described event or circumstance completely occurs or that the subsequently described event or circumstance occurs to a great extent or degree. For example, the term “substantially” means that the subsequently described event or circumstance occurs at least 90% of the time, or at least 95% of the time, or at least 98% of the time, or comprises at least 90%, 95%, or 98% of the reference quantity.

[0039] As used herein any reference to “one embodiment” or “an embodiment” means that a particular element, feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. The appearances of the phrase "in oneembodiment" in various places in the specification are not necessarily all referring to the same embodiment.

[0040] As used herein any reference to “we” as a pronoun herein refers generally to laboratory personnel or other contributors who assisted in the laboratory procedures and data collection and is not intended to represent an inventorship role by said laboratory personnel or other contributors in any subject matter disclosed herein.

[0041] The following abbreviations apply:A: ampere(s)Ag: silverAg2Se: silver selenideASIC: application-specific integrated circuitAu: goldAU: absorbance unit(s)CBD: chemical bath depositionCd: cadmiumCdSe: cadmium selenideCdSei-xSx: cadmium selenium sulfide cm: centimeter(s) cm2: square centimeter(s) cm'1: inverse centimeter(s)CPU: central processing unitCQD: colloidal quantum dotCU: copperDC: direct currentDSP: digital signal processorEO: electrical -to-opticalEu: europium eV: electronvolt(s)FE-SEM: field-emission scanning electron microscope FPA: focal plane arrayFPGA: field-programmable gate arrayFTIR: Fourier transform infraredGe: germaniumHg: mercuryHgSe: mercury selenideHgTe: mercury tellurideHz: hertzICDD: International Centre for Diffraction DataIn: indiumIR: infraredK: kelvin kV: kilovolt(s) kQ: kiloohm(s) mA: milliampere(s)MCT : mercury cadmium tellurideMg: magnesiumMWIR: mid-wave infrared mm: millimeter(s) mV: millivolt(s) nm: nanometer(s)NWIR: near- wave infrared0: oxygenO2: dioxygenOE: optical -to-electricalPa: pascal(s)Pb: leadPbxCdi.xSe: lead cadmium selenidePbS: lead sulfidePbSe: lead selenidePbTe: lead telluridePC: photoconductorPD: photodetectorPL: photoluminescenceQD: quantum dotQDIP: quantum dot infrared detectorRAM: random-access memoryRF : radio frequencyROIC: readout integrated circuitROM: read-only memoryRT . room temperatureRX: receiver unitS: sulfurSAQD: self-assembled quantum dotSCCM: standard cubic centimeters per minuteSe: seleniumSeO2: selenium dioxideSi: siliconSiO2: silicon dioxideSn: tinSNR: signal-to-noise ratioSr . strontiumSRAM: static RAMSWaP+C: size, weight, power, and costSWIR: short-wave infraredTCAM: ternary content-addressable memoryTe: telluriumTX: transmitter unitV: volt(s)VPD: vapor phase depositionW: watt(s)XRD: x-ray diffractionZn. zinc2D: two-dimensional3D: three-dimensional .m: micrometer(s)°C: degree(s) CelsiusA: angstrom(s).

[0042] Introduction

[0043] For several decades, MWIR PDs have been intensively explored, and their capabilities are well proven for applications in environmental gas sensing, security and defense, medical diagnostics, and space and astronomy. Epitaxial MCT and Type-II superlattice structures are the leading MWIR PDs in detectivity performance. However, due to the requirement of cryogenic cooling to maintain high performance, these PDs are facing significant challenges in reducing SWaP+C. Therefore, there is a growing demand for uncooled MWIR PDs that can deliver decent performance with reduced size, weight, power, and cost. To date, the best low-cost solution for uncooled MWIR PDs is polycrystalline PbSe PCs. FPAs with very promising performance have been demonstrated, but some challenges remain, including film inhomogeneity.

[0044] MWIR-ab sorbing QDs have recently triggered substantial interest as potential semiconducting materials for MWIR cameras, with promises of reducing SWaP+C while maintaining good range and resolution for FPA imagers. QD MWIR PDs offer several potential advantages over conventional PDs. Firstly, QDs possess unique electronic properties, such as quantum confinement and tunable bandgaps. This allows for precise engineering of the absorption wavelength, enabling tailored detection within the MWIR range. Secondly, QDs exhibit superior photo-response characteristics. Their quantum confinement facilitates efficient light absorption, enhancing the overall quantum efficiency of the PDs and at the same time lowering dark current. The other advantage of QDs is their high-temperature operation capability. In theory, the inherent quantum-mechanical properties of QDs suggest the potential for increased operating temperatures in MWIR PDs. This is attributed to the substantial reduction in thermal carrier generation, owing to the energy quantization occurring in all three dimensions.

[0045] CQDs have emerged as promising materials for MWIR PDs due to their unique properties. There are three main groups of MWIR CQDs, Pb chalcogenide, Hg chalcogenide, and Ag chalcogenide. These materials exhibit excellent MWIR absorption characteristics and tunable bandgaps, enabling tailored optical responses within the MWIR range. Lead-chalcogenide CQD PDs have very promising responsivities, but have been studied mostly in NWIR and SWIRapplications due to synthesis limitations of large CQDs, such as low air stability and difficulty in cation exchange for growing shell cadmium chalcogenides. The main absorbing mechanism in Hg-chalcogenide CQD PDs is interband transitions, which have demonstrated an RT cutoff wavelength of around 5 pm. Intra-band transitions are an alternative method for achieving MWIR sensitivity. Ag-chalcogenides CQDs utilize the intra-band transitions in the conduction band to absorb the MWIR light due to the Ag-chalcogenides CQDs’ ability to self-dope with excess Ag. Many Ag-based CQDs can cover the MWIR region because of this characteristic. Although solution-processed CQDs are robust and controllable, their need for ligand exchange can make the process labor intensive and expensive. The cost of specialized ligands and purification techniques can impact overall production expenses, which is not desirable for mass production. Furthermore, in the MWIR range, thick CQD film is desirable to achieve sufficient absorbance. However, this poses a challenge due to crack formation in the film and low carrier mobility.

[0046] In addition to CQD MWIR photodetectors, epitaxial SAQDs have also been investigated for their potential in this field. They are formed by the island (Stranski-Krastanow) growth of QDs on mismatched substrates. The epitaxial growth of these QDs enables their integration with Si ROIC capability. However, limited absorption efficiency due to the small fill factor of QDIPs, as well as the trade-off between absorption efficiency and dark current, limits their performance, especially at high operating temperatures. Furthermore, their reliance on epitaxial growth methods significantly increases their cost of production, posing a hindrance to their large-scale commercialization. Despite their superior performance compared to quantum well counterparts, expensive SAQDs exhibit comparable performance to inexpensive bolometer technology, which hinders their commercialization potential. It is therefore desirable to overcome the deficiencies of the techniques above.

[0047] Embodiments

[0048] Disclosed herein are embodiments for heterogeneous, group IV-VI / II-VI, photoconductive detectors. To obtain the detectors, group II- VI semiconductors are deposited on group IV- VI semiconductors via VPD, and the structure is then annealed in O2 to make pathways for photogenerated carriers and to passivate defects. The group IV-VI semiconductors are connected to each other and form a core, and the group II- VI semiconductors at least partially surround the group IV-VI semiconductors and form a shell. Together, the group IV-VI semiconductors and the group II- VI semiconductors form a heterogeneous, group IV-VI / II-VI,photoconductive structure and form a 3D, interconnected, conductive network. The group IV-VI semiconductors absorb photons, generate electrons and holes from the absorbed photons, and pass the holes to a negative contact to create a photocurrent. The absorbed photons may be in the mid- IR range. The group II- VI semiconductors separate the electrons and the holes and pass the electrons to a positive contact to create the photocurrent. The detectors may implement a gas sensor, a fire detector, a pixel for a pixel array, or another suitable device.

[0049] The detectors reduce dark current and increase sensitivity by reducing the carrier concentration during annealing. The detectors provide monolithic integration on a silicon ROIC, which is a relatively inexpensive product and allows for large-scale production. The detectors may fully perform without the need for cooling, thus further reducing cost. Compared to other growth methods, VPD offers lower cost, more precise control over growth, good uniformity, strong reproducibility, and CMOS compatibility. The group IV-VI semiconductors may comprise PbSe, which is a semiconductor with a narrow bandgap of 0.27 e and has a very large bulk Bohr exciton radius of 46 nm, which strongly confines quantum states in nanocrystals and enables the tuning of the bandgap and absorption edge from NWIR to MWIR. Low Auger recombination and low defect density of PbSe QDs enable them to exhibit high IR responses even at ambient temperatures. The group II- VI semiconductors may comprise CdSe. Wide-bandgap CdSe is nearly lattice matched to PbSe. The PbSe core and CdSe shell structure helps passivate the surface of PbSe QDs and reduce Auger recombination. Compared to a CQD film, the disclosed all-inorganic approach eliminates the need for ligand exchange and has no difficulty in producing large-size QDs to cover the MWIR range.

[0050] FIG. 1 is a schematic diagram of imaging equipment 100. The imaging equipment 100 may be a gas sensor, a fire detector, a pixel in a pixel array, or another suitable device. The imaging equipment 100 comprises an input port 110, a photoconductive detector 120, a computing device 130, and a display 140.

[0051] The input port 110 may comprise glass for accepting light and comprise a cavity for focusing the light and passing the light. The light may be IR light in the wavelength range of 700 nm-1 mm or more specifically mid-IR light in the wavelength range of 3-8 pm. The photoconductive detector 120 and the computing device 130 are described further below. The display 140 may comprise a single external screen or eyepieces with individual screens.

[0052] In operation, the input port 1 10 receives light and passes the light to the photoconductive detector 120. The photoconductive detector 120 converts the light into a photocurrent and passes the photocurrent to the computing device 130. The computing device 130 processes the photocurrent to create an image and passes the image to the display 140. The display 140 displays the image.

[0053] FIG. 2 is a 2D schematic diagram of a photoconductive detector 200 according to a first embodiment. The photoconductive detector 200 may be a semiconductor photoconductive detector. The photoconductive detector 200 comprises contacts 210, a detector layer 220, and a substrate 260.

[0054] The contacts 210 are disposed on top of the detector layer 220 and draw photocurrent from the detector layer 220 in response to an external bias. The contacts 210 may also be referred to as electrical contacts or electrodes. The contacts 210 comprise Au, Cu, In, or other conductive materials such as conductive metal oxide. One of the contacts 210 is a positive contact, and the other contact 210 is a negative contact.

[0055] The detector layer 220 comprises a plurality of interconnected group IV- VI semiconductors 230, comprises a matrix of group II- VI semiconductors 240, and optionally comprises one or more dead QDs 250. Together, the group IV-VI semiconductors 230 and the group II- VI semiconductors 240 form the detector layer 220 as a heterogeneous, group IV-VI / II- VI, photoconductive structure.

[0056] The group IV-VI semiconductors 230 are shown in dark gray. The group IV-VI semiconductors 230 are discrete structures or semi-discrete structures and are substantially spherical or cubical. Semi-discrete means that less than 50% of the surface area of a group IV-VI semiconductor 230 contacts another object such as another group IV-VI semiconductor 230. The group IV-VI semiconductors 230 are connected to each other to allow carrier transport in a chainlike fashion. The notation “IV-VI” indicates the group IV-VI semiconductors 230 comprise at least one group IV element and at least one group VI element. The group IV-VI semiconductors 230 may comprise Pb chalcogenides or Pb salts. The Pb chalcogenides may be PbS, PbXS, PbSe, PbXSe, PbTe, PbXTe, PbXSexTei-x, PbXSexSi-x, PbXSexSxTei-x.y, or related alloys. X represents a chemical element with a composition able to form ternary compound semiconductors. For instance, X may be Cd, Eu, Ge, Sn, or Sr. x and y are compositional variables. The Pb chalcogenides may be in mono-crystalline form, polycrystalline form, or amorphous form. ThePb salts may be in bulk form, micro-crystalline form, or nanostructure form. While QDs are discussed, the nanostructures may instead be quantum wires or quantum wells.

[0057] The group II- VI semiconductors 240 are shown in light gray. The group II- VI semiconductors 240 are not discrete structures, but instead form a matrix which completely or partially surrounds the group IV- VI semiconductors 230 to form the detector layer 220 as a 3D, interconnected, conductive network. The notation “II- VI” indicates the group II-VI semiconductors 240 comprise at least one group II element such as Cd, Mg, or Zn, and at least one group VI element such as S, Se, or Te. The group II-VI semiconductors 240 may be ternary compounds such as CdSei-xSx, quaternary compounds, or other suitable compounds.

[0058] Alternatively, the group IV- VI semiconductors 230 and the group II-VI semiconductors 240 are combined in a group (II, IV)- VI homogeneous compound. The notation “(II, IV)- VI” indicates the compound comprises at least one group II element, at least one group IV element, and at least one group VI element. The compound may be PbxCdi-xSe. x is a compositional variable for Pb and may be graded to form a graded compound.

[0059] The dead QD 250 is a discrete structure and may be substantially spherical or cubical. The dead QD 250 occurs when the group IV-VI semiconductors 230 do not fully connect to each other, which prevents tunneling of holes between the group IV-VI semiconductors 230 due to the barrier of the group II-VI semiconductors 240. Thus, the dead QD 250 may absorb the light, but may not create the photocurrent, though a high external bias may overcome the barrier and allow for the creation of the photocurrent. While one dead QD 250 is shown, the detector layer 220 may comprise multiple dead QDs 250, which may increase over time.

[0060] The substrate 260 provides a base for the photoconductive detector 200 and allows for deposition of the detector layer 220 on top. The substrate 260 comprises any material suitable for such deposition. The substrate 260 may be part of an ROIC.

[0061] In operation, the detector layer 220 receives and absorbs the light, or photons. Specifically, the group IV-VI semiconductors 230 absorb the light. While IR light in the wavelength range of 700 nm-1 mm and mid-IR light in the wavelength range of 3-8 pm may enter the detector layer 220, the wavelength range of absorbed light depends on materials in the detector layer 220. For instance, at room temperature the detector layer 220 may absorb light in the wavelength range of 1-5 pm for PbSe-based materials or 1-14 pm for other materials. The group IV-VI semiconductors 230 generate photon-induced electrons and holes from the absorbed light.Each photon creates one electron and one hole. The electrons and holes decrease a resistance of the detector layer 220. The group II- VI semiconductors 240 spatially separate the electrons and the holes in the depletion region, which reduces the probability of the electrons and the holes recombining, which in turn enhances the lifetimes of those electrons and holes and thus improves performance of the photoconductive detector 200. Under voltage from the contacts 210, the electrons and holes are extracted towards the contacts 210 to create a photocurrent. Specifically, as shown by arrows, the holes travel within the group IV- VI semiconductors 230 towards the left contact 210, which would therefore be a negative contact, and the electrons travel within the group II- VI semiconductors 240 towards the right contact 210, which would therefore be a positive contact. The contacts 210 draw the photocurrent.

[0062] FIG. 3 is a 2D schematic diagram of a photoconductive detector 300 according to a second embodiment. The photoconductive detector 300 is similar to the photoconductive detector 200. Specifically, the photoconductive detector 300 comprises contacts 310, a detector layer 320, and a substrate 360, which are similar to the contacts 210, the detector layer 220, and the substrate 260; and the detector layer 320 comprises group IV-VI semiconductors 330, comprises group II- VI semiconductors 340, and optionally comprises a dead QD 350, which are similar to the group IV-VI semiconductors 230, the group II- VI semiconductors 240, and the dead QD 250. However, unlike the photoconductive detector 200 in which the contacts 210 are disposed on top of the detector layer 220, in the photoconductive detector 300, the contacts 310 are disposed on the sides of the detector layer 320.

[0063] FIG. 4 is a schematic diagram of a substrate 400. The substrate 400 may implement the substrates 260, 360. The substrate 400 comprises an SiC>2 layer 410 disposed on an Si layer 420. Alternatively, the Si layer 420 coats the SiCh layer 410. Alternatively, the substrate 400 comprises quartz, silica, sapphire, fluoride, a group III-V semiconductor, a group II- VI semiconductor, or another suitable material.

[0064] FIG. 5 is a 3D schematic diagram of a photoconductive detector 500 prior to annealing. The photoconductive detector 500 comprises core-shell QDs 510 and a substrate 530. The coreshell QDs 510 are substantially hemispherical or cubical because they have not been annealed. Though dead QDs 520 are shown in a legend at the top of FIG. 5, the photoconductive detector 500 has not been annealed and therefore dead QDs 520 are irrelevant. The substrate 530 comprisesan SiC>2 layer 540 and an Si layer 550, which are respectively similar to the SiCh layer 410 and the Si layer 420.

[0065] FIG. 6 is a schematic diagram of one of the core-shell QDs 510 in FIG. 5. The coreshell QD 510 comprises a core 630 and a shell 640. The core 630 comprises a group IV- VI semiconductor similar to the group IV- VI semiconductors 230. The shell 640 comprises a group II- VI semiconductor similar to the group II- VI semiconductors 240. The core-shell QD 510 has a conduction band 610 and a valence band 620. FIG. 6 shows how electrons move from the core to the shell by the built-in potential and shows how the holes remain in the core.

[0066] FIG. 7A is a 3D picture of a portion 700 of a photoconductive detector prior to annealing. The portion 700 comprises a detector layer 710, an SiCh layer 720, and an Si layer 730, which are respectively similar to the detector layers 220 and 320, the SiCh layer 410, and the Si layer 420. FIG. 7 shows that the portion 700 is dense, has grain sizes of about 100-300 nm, and has a thickness of about 531.5 nm. The portion 700 has almost the same atomic ratio between Pb and Se.

[0067] FIG. 7B is a cross-section of the portion 700 of the photoconductive detector in FIG. 7A. FIG. 7B shows the detector layer 710, the SiCh layer 720, and the Si layer 730. FIG. 7B also shows a relatively smooth surface resulting from VPD. The smooth surface contrasts PbSe films grown with CBD, which often have rough surfaces. Rough surfaces cause non-uniformity, which in turn necessitates individual device calibration and correction.

[0068] FIG. 8 is a 3D schematic diagram of a photoconductive detector 800 after annealing. The photoconductive detector 800 comprises Au contacts 810, a detector layer 820, and a substrate 850, which are respectively similar to the contacts 210, the detector layer 220, and the substrate 260. The detector layer 820 comprises interconnected PbSe nanocrystals 830 and interconnected CdSe fillings 840, which are respectively similar to the group IV-VI semiconductors 230 and the group II-VI semiconductors 240. The interconnected PbSe nanocrystals 830 and the interconnected CdSe fillings 840 are distorted from being substantially spherical to being substantially ovoidal due to the annealing. The detector layer 820 may be referred to as a heteronanocrystal. The substrate 850 comprises an SiO2 layer 860 and an Si layer 870, which are respectively similar to the SiCh layer 410 and the Si layer 420.

[0069] FIG. 9 is an energy band diagram 900 of the detector layer 820 in FIG. 8. The energy band diagram 900 comprises photogenerated holes as empty circles and photogenerated electronsas solid circles. The energy band diagram 900 shows good band alignment of the detector layer 820. Specifically, the PbSe nanocrystals 830 and the CdSe fillings 840 form a type-II band alignment, which allows electrons to transport in the PbSe nanocrystals 830 and holes to transport in the CdSe fillings 840,

[0070] FIG. 10A is a 3D picture of a portion 1000 of a photoconductive detector after annealing. The portion comprises a detector layer 1010, an SiCh layer 1020, and an Si layer 1030, which are respectively similar to the detector layer 710, the SiCE layer 720, and the Si layer 730. Like FIG. 7B, FIG. 10 shows a relatively smooth surface resulting from VPD. However, unlike FIG. 7B, FIG. 10 shows that the portion 1000 has grain sizes over about 1 pm due to crystallization from annealing.

[0071] FIG. 10B is a cross-section of the portion 1000 of the photoconductive detector in FIG. 10 A. FIG. 10B shows the detector layer 1010, the SiCL layer 1020, and the Si layer 1030. Like FIG. 7B, FIG. 10B also shows a relatively smooth surface resulting from VPD.

[0072] FIG. 11 is a schematic diagram of the computing device 130 in FIG. 1. The computing device 130 comprises ingress ports 1110 and an RX 1120 to receive data; a processor 1130, or logic unit, baseband unit, or CPU, to process the data; a TX 1140 and egress ports 1150 to transmit the data; and a memory 1160 to store the data. The computing device 130 may also comprise OE components, EO components, or RF components coupled to the ingress ports 1110, the RX 1120, the TX 1140, and the egress ports 1150 to provide ingress or egress of optical signals, electrical signals, or RF signals.

[0073] The processor 1130 is any combination of hardware, middleware, firmware, or software. The processor 1130 comprises any combination of one or more CPU chips, cores, FPGAs, ASICs, or DSPs. The processor 1130 communicates with the ingress ports 1110, the RX 1120, the TX 1140, the egress ports 1150, and the memory 1160. The processor 1130 comprises a photoconductive detector component 11 0, which implements the disclosed embodiments. The inclusion of the photoconductive detector component 1170 therefore provides a substantial improvement to the functionality of the computing device 130 and effects a transformation of the computing device 130 to a different state. Alternatively, the memory 1160 stores the photoconductive detector component 1170 as instructions, and the processor 1130 executes those instructions.

[0074] The memory 1160 comprises any combination of disks, tape drives, or solid-state drives. The computing device 130 may use the memory 1160 as an overflow data storage device to store programs when the computing device 130 selects those programs for execution and to store instructions and data that the computing device 130 reads during execution of those programs. The memory 1160 may be volatile or non-volatile and may be any combination of ROM, RAM, TCAM, or SRAM.

[0075] A computer program product may comprise computer-executable instructions that are stored on a computer-readable medium and that, when executed by a processor, cause an apparatus to perform any of the embodiments. The non-transitory medium may be the memory 1160, the processor may be the processor 1130, and the apparatus may be the computing device 130.

[0076] FIG. 12 is a flowchart of a method 1200 of fabricating the photoconductive detector 200 of FIG. 2 or the photoconductive detector 300 of FIG. 3. At step 1210, a substrate is obtained. For instance, the substrate 260, 360, 400, 530, or 850 is obtained through fabrication.

[0077] At step 1220, a group IV-VI semiconductor is deposited on the substrate. For instance, the group IV-VI semiconductors 230 or 330 are deposited on the substrate via VPD or another suitable method to form a nano-structured or micro-structured core. The group IV-VI semiconductor is, for example, PbSe. At step 1230, a group II- VI semiconductor is deposited around the group IV-VI semiconductor to obtain a pre-annealed structure. For instance, the group II- VI semiconductors 240 or 340 are deposited on the group IV-VI semiconductor via VPD or another suitable method to form a shell at least partially around the core. The group IV-VI semiconductor is, for example, CdSe. Alternatively, the group II- VI semiconductor could be deposited first and the group IV-VI semiconductor could be deposited second. Alternatively, wet chemical methods could be used instead of standard deposition.

[0078] At step 1240, annealing is performed on the pre-annealed structure to obtain a postannealed structure. The annealing is performed in an atmosphere, for instance an O2 atmosphere in a temperature range of 200-500 °C, suitable for producing the desired core-shell structure. In one example, the annealing is performed at 430 °C, though with device modifications that temperature could be lowered to enhance compatibility with the silicon ROIC. The annealing breaks the CdSe shell and allows the CdSe to reshape and create a heterogenous material comprising both the PbSe and the CdSe. The heterogeneous material may be referred to as a heteronanocrystal, for instance, the heteronanocrystal shown in FIG. 8. During the annealing, O2molecules increase in the pre-annealed structure, while Se decreases in the pre-annealed structure likely in the form of gaseous SeCh or elemental Se. The annealing also reduces defects and introduces mechanisms to enhance sensitivity. Such mechanisms, including photoluminescence peaks, are described in Jijun Qiu, et al., “Study of sensitization process on mid-infrared uncooled PbSe photoconductive detectors leads to high detectivity,” Journal of Applied Physics 113, 103102, March 11, 2013, which is incorporated by reference. At step 1250, contacts are attached to the post-annealed structure to obtain a photoconductive detector. For instance, the contacts 210 or 310 are attached via thermal deposition.

[0079] Experiment

[0080] An experimental detector was grown with a custom-designed thermal evaporator. High purity (99.999%) source materials were used to grow a PbSe-CdSe core-shell structure on a commercial SiCh-Si substrate together with a sapphire substrate for transmission measurements. An automatic shutter control system was used to ensure precise multi-stacked growth. Sixty layers of alternating PbSe with a deposition rate of 13.5 A per second for 48 seconds and CdSe with a deposition rate of 0.37 A per second for 60 seconds were grown at a 350 °C substrate temperature with a 2 x 10’4Pa background vapor pressure, resulting in a total thickness of 850 nm. Then the samples were cut into 2 mm x 4 mm portions. Later the as-grown films were annealed at the temperature range of 425-450 °C with a flow rate of 40 SCCM of ultra-high-purity O gas. Then 200 nm gold contacts were deposited on the two sides of the photoconductor with a thermal evaporator, leaving a 2 mm x 2 mm active area for light absorption.

[0081] The morphologies of the annealed and as-grown films were characterized by a high- resolution FE-SEM. A diffractometer was used for a high-resolution XRD analysis. Data were collected using Cu K radiation at 40 kV and 44 mA in parallel beam mode, and the data were analyzed using MDI Jade 2010 software and the ICDD PDF4 + database. An FTIR spectrometer with a 16 cm’1resolution was used to measure PL emissions and transmissions in the MWIR range. A PL module was paired with an external lock-in amplifier to reduce the heat background. A 980 nm diode laser was used to pump the QD sample.

[0082] A current-voltage analysis was performed using a sourcemeter. The detectivity measurements were carried out at room temperature (298 K) with a lock-in amplifier and a collimated blackbody at 823 K. A source and probe station were used to bias the devices, a load resistor was matched to the detector resistance, and a high-pass filter was used to block the DCsignal. Responsivity (R) and specific detectivity (D*) were calculated using the following formulas:R =Vs / P. V / W) (1)'1inVi is a signal voltage from the lock-in amplifier, vnis a noise from the lock-in amplifier, Adet is a detector active area, A / is a noise measurement bandwidth and is 1 Hz, and Pm is an incident radiant power. Pin was calculated as follows: / > is a blackbody chopper modulation coefficient and is 0.45, a is the Stefan-Boltzmann constant, T is a blackbody temperature and is 300 K, To is the room temperature, t is a transmission coefficient and is 0.7 based on the refractive index of PbSe and CdSe, Adet is a detector active area and is 0.04 cm2, ABB is a blackbody aperture and is 0.2 cm2, and d is a detector-to-blackbody distance and is 40 cm.

[0083] Results

[0084] FIG. 13 is a graph 1300 of absorbance and photoluminescence versus wavelength as observed in the experimental detector after annealing. The wavelength is in units of pm, the absorbance is shown on a black line in units of AU, and the photoluminescence is shown on a red line in units of AU. The graph 1300 shows strong photoluminescence at 3.52 pm. The photoluminescence peak position was used to estimate the average size of QDs using the reported size-dependent photoluminescence of PbSe QDs. The core diameter of the QD is estimated to be 16.9 nm. When growing on dissimilar substrates, QDs tend to accumulate to grow large islands rather than a flat layer. The other reason is oriented attachment and Ostwald ripening during oxygen annealing as discussed before. The graph 1300 shows that the detector exhibited approximately an 88-times higher photoluminescence intensity after annealing, indicating that O played a substantial role in defect passivation.

[0085] FIG. 14 is a graph 1400 of current versus bias as observed in the experimental detector. The bias is in units of V, and the current is in units of mA. The graph 1400 shows symmetric and linear characteristics with a resistance of 850 k£l.

[0086] FIG. 15 is a graph 1500 of signal and noise versus bias in the experimental detector. The bias is in units of V, the signal is shown on a black line in units of mV, and the noise is shownon a red line in units of mV. The bias is the total bias on both a load resistor and the experimental device. The bias on the experimental device is half the total bias. The signal increases linearly from 0.163 mV to 0.908 mV without any indication of signal saturation as the bias ranges from 15 V to 75 V. The noise, on the other hand, does not increase as rapidly as the signal up to 60 V. However, beyond that point, a significant increase in the noise is observed at 60 V. The underlying reason for the increase in the noise with bias will be discussed in the following paragraphs.

[0087] FIG. 16 is a graph 1600 of SNR versus bias in the experimental detector. The bias is in units of V, and the SNR is dimensionless. The graph 1600 was obtained from the graph 1500. At 60 V, barrier shot noise has not yet attained dominance and the signal remains high, showcasing a promising peak SNR of 5,978.

[0088] FIG. 17 is a graph 1700 of responsivity versus bias in the experimental detector. The bias is in units of V, and the responsivity is in units of V / W. The responsivity spans from 438 V / W to 2,430 V / W.

[0089] FIG. 18 is a graph 1800 of specific detectivity versus bias in the experimental detector. The bias is in units of V, and the specific detectivity is in units of Jones. The specific detectivity reaches 2.5 x 109Jones. However, at 60 V, the dominance of barrier shot noise results in a decrease of specific detectivity.

[0090] FIG. 19 is a graph 1900 of peak detectivity versus bias in the experimental detector. The bias is in units of V, and the peak detectivity is in units of Jones. The peak detectivity reaches 7.2 x 109Jones, which approaches the maximum performance of CBD-grown photoconductors.

[0091] FIG. 20 is a table 2000 comparing performances of RT-operating MWIR QD PDs. The table 2000 compares QD MWIR absorption at room temperature. The disclosed PbSe / CdSe material exhibits the best performance in terms of specific detectivity, specifically, 2.5 x 109Jones as discussed above. Reducing both the number of barriers and the barrier height could lead to an increase in signal and a decrease in noise.

[0092] A first aspect relates to an apparatus comprising: a photoconductive detector comprising: a substrate; a detector layer disposed on the substrate, wherein the detector layer comprises a 3D interconnected conductive network comprising: a plurality of interconnected group IV-VI semiconductors forming a group IV-VI semiconductor core; and a matrix comprising group II- VI semiconductors which at least partially surround the group IV-VI semiconductor core,wherein the group IV-VI semiconductor core and the matrix together form a heterogeneous group IV-VI / II-VI photoconductive structure; and contacts coupled to the detector layer.

[0093] In a first implementation of the first aspect, the substrate comprises an Si layer.

[0094] In a second implementation or any preceding implementation of the first aspect, the substrate further comprises an SiCh layer disposed on the Si layer.

[0095] In a third implementation or any preceding implementation of the first aspect, the plurality of interconnected group IV-VI semiconductors comprises discrete structures or semidiscrete structures that are substantially spherical or substantially ovoidal.

[0096] In a fourth implementation or any preceding implementation of the first aspect, the plurality of interconnected group IV-VI semiconductors comprises lead chalcogenides or lead salts.

[0097] In a fifth implementation or any preceding implementation of the first aspect, the plurality of interconnected group IV-VI semiconductors comprises PbSe.

[0098] In a sixth implementation or any preceding implementation of the first aspect, the plurality of interconnected group IV-VI semiconductors comprises CdSe.

[0099] In a seventh implementation or any preceding implementation of the first aspect, the contacts are disposed on the detector layer.

[0100] In an eighth implementation or any preceding implementation of the first aspect, the detector layer comprises sides, and the contacts are disposed on the sides of the detector layer.

[0101] In a ninth implementation or any preceding implementation of the first aspect, the contacts comprise Au, Cu, or In.

[0102] In a tenth implementation or any preceding implementation of the first aspect, the apparatus further comprises: an input port coupled to the photoconductive detector; a computing device coupled to the photoconductive detector; and a display coupled to the computing device.

[0103] A second aspect relates to an apparatus comprising: a photoconductive detector comprising: contacts comprising: a negative contact; and a positive contact; and a detector layer comprising: first semiconductors configured to: absorb photons as absorbed photons; generate electrons and holes from the absorbed photons; and pass the holes to the negative contact to create a photocurrent; and second semiconductors configured to: separate the electrons and the holes; and pass the electrons to the positive contact to create the photocurrent.

[0104] In a first implementation of the second aspect, the first semiconductors are further configured to further absorb the photons in a wavelength range of 2-5 pm.

[0105] In a second implementation or any preceding implementation of the second aspect, the apparatus further comprises an input port configured to: receive light; and pass the light to the photoconductive detector, and the light comprises the photons.

[0106] In a third implementation or any preceding implementation of the second aspect, the apparatus further comprises a computing device configured to process the photocurrent to create an image.

[0107] In a fourth implementation or any preceding implementation of the second aspect, the apparatus further comprises a display configured to display the image.

[0108] In a fifth implementation or any preceding implementation of the second aspect, the apparatus is a gas sensor, a fire detector, or a pixel for a pixel array.

[0109] A third aspect relates to a method comprising: obtaining a substrate; depositing a group IV-VI semiconductor on the substrate; depositing a group II- VI semiconductor around the group IV-VI semiconductor to obtain a pre-annealed structure; performing annealing to obtain a postannealed structure; and attaching contacts to the post-annealed structure to obtain a photoconductive detector.

[0110] In a first implementation of the third aspect, the method further comprises further depositing the group IV-VI semiconductor or further depositing the group II- VI semiconductor via VPD.[0U1] In a second implementation or any preceding implementation of the third aspect, the method further comprises further performing the annealing in an oxygen atmosphere and in a temperature range of 200-500 °C.

[0112] While several embodiments have been provided, the disclosed systems and methods might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented. Likewise, where single components, apparatuses, or systems are described as performing functions, multiple such components, apparatuses, or systems may implement the functions.

[0113] In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, components, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled may be directly coupled or may be indirectly coupled or communicating through some interface, device, or intermediate component whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and may be made without departing from the spirit and scope disclosed herein.

Claims

CLAIMSWhat is claimed is:

1. An apparatus comprising: a photoconductive detector comprising: a substrate; a detector layer disposed on the substrate, wherein the detector layer comprises a three-dimensional (3D) interconnected conductive network comprising: a plurality of interconnected group IV- VI semiconductors forming a group IV- VI semiconductor core; and a matrix comprising group II- VI semiconductors which at least partially surround the group IV-VI semiconductor core, wherein the group IV-VI semiconductor core and the matrix together form a heterogeneous group IV-VI / II- VI photoconductive structure; and contacts coupled to the detector layer.

2. The apparatus of claim 1, wherein the substrate comprises a silicon (Si) layer.

3. The apparatus of claim 2, wherein the substrate further comprises a silicon dioxide (SiCh) layer disposed on the Si layer.

4. The apparatus of claim 1, wherein the plurality of interconnected group IV-VI semiconductors comprises discrete structures or semi-discrete structures that are substantially spherical or substantially ovoidal.

5. The apparatus of claim 1, wherein the plurality of interconnected group IV-VI semiconductors comprises lead chalcogenides or lead salts.

6. The apparatus of claim 5, wherein the plurality of interconnected group IV-VI semiconductors comprises lead selenide (PbSe).

7. The apparatus of claim 1 , wherein the plurality of interconnected group IV-VT semiconductors comprises cadmium selenide (CdSe).

8. The apparatus of claim 1, wherein the contacts are disposed on the detector layer.

9. The apparatus of claim 1, wherein the detector layer comprises sides, and wherein the contacts are disposed on the sides of the detector layer.

10. The apparatus of claim 1, wherein the contacts comprise gold (Au), copper (Cu), or indium (In).

11. The apparatus of claim 1, further comprising: an input port coupled to the photoconductive detector; a computing device coupled to the photoconductive detector; and a display coupled to the computing device.

12. An apparatus comprising: a photoconductive detector comprising: contacts comprising: a negative contact; and a positive contact; and a detector layer comprising: first semiconductors configured to: absorb photons as absorbed photons; generate electrons and holes from the absorbed photons; and pass the holes to the negative contact to create a photocurrent; and second semiconductors configured to: separate the electrons and the holes; and pass the electrons to the positive contact to create the photocurrent.

13. The apparatus of claim 12, wherein the first semiconductors are further configured to further absorb the photons in a wavelength range of 2-5 micrometers (pm).

14. The apparatus of claim 12, further comprising an input port configured to: receive light; and pass the light to the photoconductive detector, wherein the light comprises the photons.

15. The apparatus of claim 12, further comprising a computing device configured to process the photocurrent to create an image.

16. The apparatus of claim 15, further comprising a display configured to display the image.

17. The apparatus of claim 12, wherein the apparatus is a gas sensor, a fire detector, or a pixel for a pixel array.

18. A method comprising: obtaining a substrate; depositing a group IV- VI semiconductor on the substrate; depositing a group II- VI semiconductor around the group IV-VI semiconductor to obtain a pre-annealed structure; performing annealing to obtain a post-annealed structure; and attaching contacts to the post-annealed structure to obtain a photoconductive detector.

19. The method of claim 18, further comprising further depositing the group IV-VI semiconductor or further depositing the group II- VI semiconductor via vapor phase deposition (VPD).

20. The method of claim 18, further comprising further performing the annealing in an oxygen atmosphere and in a temperature range of 200-500 degrees Celsius (°C).

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