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Timestamp: 2015-10-10 15:38:46
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Matched Legal Cases: ['Application No. 60', 'Application No. 60', 'Application No. 60', 'Application No. 61', 'Application No. 61', 'Application No. 61', 'Application No. 61']

Patent US20110303897 - Materials, systems and methods for optoelectronic devices - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsA photodetector is described along with corresponding materials, systems, and methods. The photodetector comprises an integrated circuit and at least two optically sensitive layers. A first optically sensitive layer is over at least a portion of the integrated circuit, and a second optically sensitive...http://www.google.com/patents/US20110303897?utm_source=gb-gplus-sharePatent US20110303897 - Materials, systems and methods for optoelectronic devicesAdvanced Patent SearchPublication numberUS20110303897 A1Publication typeApplicationApplication numberUS 13/214,898Publication dateDec 15, 2011Priority dateApr 18, 2007Also published asCN102017147A, CN102017147B, CN103839955A, EP2143141A2, EP2143141A4, EP2432015A1, EP2667412A1, US7923801, US8004057, US8013412, US8269260, US8269302, US8274126, US8441090, US8466533, US8476727, US8482093, US8513758, US8530940, US8530991, US8530992, US8530993, US8546853, US8558286, US8643064, US20090152664, US20100187404, US20100187408, US20110297815, US20110297915, US20110303898, US20110309238, US20120037789, US20120037887, US20120043455, US20120049045, US20120049311, US20120056074, US20120056075, US20120056076, US20120056160, US20120056289, US20140175591, WO2008131313A2, WO2008131313A3Publication number13214898, 214898, US 2011/0303897 A1, US 2011/303897 A1, US 20110303897 A1, US 20110303897A1, US 2011303897 A1, US 2011303897A1, US-A1-20110303897, US-A1-2011303897, US2011/0303897A1, US2011/303897A1, US20110303897 A1, US20110303897A1, US2011303897 A1, US2011303897A1InventorsHui Tian, Edward SargentOriginal AssigneeInvisage Technologies, Inc.Export CitationBiBTeX, EndNote, RefManPatent Citations (8), Referenced by (24), Classifications (19), Legal Events (6) External Links: USPTO, USPTO Assignment, EspacenetMaterials, systems and methods for optoelectronic devices
US 20110303897 A1Abstract
A photodetector is described along with corresponding materials, systems, and methods. The photodetector comprises an integrated circuit and at least two optically sensitive layers. A first optically sensitive layer is over at least a portion of the integrated circuit, and a second optically sensitive layer is over the first optically sensitive layer. Each optically sensitive layer is interposed between two electrodes. The two electrodes include a respective first electrode and a respective second electrode. The integrated circuit selectively applies a bias to the electrodes and reads signals from the optically sensitive layers. The signal is related to the number of photons received by the respective optically sensitive layer.
Images(100) Claims(81)
at least one optically sensitive layer; and a circuit comprising at least one node in electrical communication with the optically sensitive layer, wherein the circuit stores an electrical signal proportional to the intensity of light incident on the optically sensitive layer during an integration period, wherein a non-linear relationship exists between electrical characteristics of the optically sensitive layer and the intensity of light absorbed by the optically sensitive layer. 2. The sensor of claim 1, wherein at least one optically sensitive layer comprises closely-packed semiconductor nanoparticle cores.
3. The sensor of claim 2, wherein each core is partially covered with an incomplete shell, where the shell produces trap states having substantially a single time constant.
4. The sensor of claim 3, wherein the nanoparticle cores comprise PbS partially covered with a shell comprising PbSO3.
5. The sensor of claim 2, wherein the nanoparticle cores are passivated using ligands of at least two substantially different lengths.
6. The sensor of claim 2, wherein the nanoparticle cores are passivated using at least one ligand of at least one length.
7. The sensor of claim 2, wherein the nanoparticle cores are passivated and crosslinked using at least one crosslinking molecule of at least one length.
8. The sensor of claim 7, wherein the crosslinking molecule is a conductive crosslinker.
9. The sensor of claim 2, wherein each nanoparticle core is covered with a shell, where the shell comprises PbSO3.
10. The sensor of claim 2, wherein the nanoparticle cores comprise PbS that is partially oxidized and substantially lacking in PbSO4 (lead sulfate).
11. The sensor of claim 1, wherein at least one optically sensitive layer comprises a nanocrystalline solid, wherein at least a portion of a surface of the nanocrystalline solid is oxidized.
12. The sensor of claim 11, wherein a composition of the nanocrystalline solid excludes a first set of native oxides and includes a second set of native oxides.
13. The sensor of claim 12, wherein the first set of native oxides includes PbSO4 (lead sulfate) and the second set of native oxides includes PbSO3.
14. The sensor of claim 11, wherein trap states of the nanocrystalline solid provide persistence, wherein an energy to escape from a predominant trap state is less than or equal to approximately 0.1 eV.
15. The sensor of claim 14, comprising a non-predominant trap state, wherein an energy to escape from the non-predominant trap state is greater than or equal to approximately 0.2 eV.
16. The sensor of claim 1, comprising a continuous transparent layer, the continuous transparent layer comprising substantially transparent material, wherein the continuous transparent layer at least partially covers the optically sensitive layer.
17. The sensor of claim 1, comprising an adhesion layer anchoring constituents of the optically sensitive layer to circuitry of the integrated circuit.
18. The sensor of claim 1, wherein the second optically sensitive layer comprises a wavelength-selective light-absorbing material, wherein the first optically sensitive layer comprises a photoconductive material.
19. The sensor of claim 1, comprising an array of curved optical elements that determine a distribution of intensity across the optically sensitive layers.
20. The sensor of claim 1, wherein at least one optically sensitive layer comprises substantially fused nanocrystal cores having a dark current density less than approximately 0.1 nA/cm2.
21. The sensor of claim 1, wherein the circuit is an integrated circuit.
22. The sensor of claim 21, wherein a minimum feature spacing of the integrated circuit is in a range of approximately 100 nm to 200 um.
23. The sensor of claim 1, wherein the circuit is a complementary metal oxide semiconductor (CMOS) integrated circuit.
24. The sensor of claim 1, wherein a rate of the current flow through the optically sensitive layer has a non-linear relationship with the intensity of light absorbed by the optically sensitive layer.
25. The sensor of claim 1, wherein a gain of the optically sensitive layer has a non-linear relationship with the intensity of light absorbed by the optically sensitive layer.
26. The sensor of claim 1, wherein the optically sensitive layer has photoconductive gain when a voltage difference is applied across the optically sensitive layer and the optically sensitive layer is exposed to light.
27. The sensor of claim 1, wherein persistence of the optically sensitive layer is approximately in a range of 1 ms to 200 ms.
28. The sensor of claim 1, wherein the sensor is a non-rectifying device.
29. The sensor of claim 1, wherein the optically sensitive layer has a surface area determined by a width dimension and a length dimension.
30. The sensor of claim 29, wherein the width dimension is approximately 2 um.
31. The sensor of claim 29, wherein the length dimension is approximately 2 um.
32. The sensor of claim 29, wherein the width dimension is approximately 2 um and the length dimension is approximately 2 um.
33. The sensor of claim 29, wherein the width dimension is less than approximately 2 um.
34. The sensor of claim 29, wherein the length dimension is less than approximately 2 um.
35. The sensor of claim 29, wherein the width dimension is less than approximately 2 um and the length dimension is less than approximately 2 um.
36. The sensor of claim 1, wherein the optically sensitive layer comprises a continuous film of interconnected nanocrystal particles.
37. The sensor of claim 36, wherein the nanocrystal particles comprise a plurality of nanocrystal cores and a shell over the plurality of nanocrystal cores.
38. The sensor of claim 37, wherein the plurality of nanocrystal cores are fused.
39. The sensor of claim 37, wherein a physical proximity of the nanocrystal cores of adjacent nanocrystal particles provides electrical communication between the adjacent nanocrystal particles.
40. The sensor of claim 39, wherein the physical proximity includes a separation distance of less than approximately 0.5 nm.
41. The sensor of claim 39, wherein the electrical communication includes a hole mobility of at least approximately 1E-5 square centimeter per volt-second across the nanocrystal particles.
42. The sensor of claim 37, wherein the plurality of nanocrystal cores are electrically interconnected with linker molecules.
43. The sensor of claim 42, wherein the linker molecules include bidentate linker molecules.
44. The sensor of claim 43, wherein the linker molecules include ethanedithiol.
45. The sensor of claim 43, wherein the linker molecules include benzenedithiol.
46. The sensor of claim 1, wherein the optically sensitive layer comprises a unipolar photoconductive layer including a first carrier type and a second carrier type, wherein a first mobility of the first carrier type is higher than a second mobility of the second carrier type.
47. The sensor of claim 46, wherein the first carrier type is electrons and the second carrier type is holes.
48. The sensor of claim 46, wherein the first carrier type is holes and the second carrier type is electrons.
49. The sensor of claim 1, wherein the optically sensitive layer comprises a nanocrystal material having photoconductive gain and a responsivity of at least approximately 0.4 amps/volt (A/V).
50. The sensor of claim 49, wherein the responsivity is achieved under a bias approximately in a range of 0.5 volts to 5 volts.
51. The sensor of claim 1, wherein the optically sensitive layer comprises nanocrystals of a material having a bulk bandgap, and wherein the nanocrystals are quantum confined to have an effective bandgap more than twice the bulk bandgap.
52. The sensor of claim 1, wherein the optically sensitive layer includes nanocrystals comprising nanoparticles, wherein a nanoparticle diameter of the nanoparticles is less than a Bohr exciton radius of bound electron-hole pairs within the nanoparticle.
53. The sensor of claim 1, wherein the optically sensitive layer comprises monodisperse nanocrystals.
54. The sensor of claim 1, wherein the optically sensitive layer comprises nanocrystals.
55. The sensor of claim 54, wherein the nanocrystals are colloidal quantum dots.
56. The sensor of claim 55, wherein the quantum dots include a first carrier type and a second carrier type, wherein the first carrier type is a flowing carrier and the second carrier type is one of a substantially blocked carrier and a trapped carrier.
57. The sensor of claim 56, wherein the colloidal quantum dots include organic ligands, wherein a flow of at least one of the first carrier type and the second carrier type is related to the organic ligands.
58. The sensor of claim 1, wherein the optically sensitive layer can be biased as both a current sink and a current source.
59. The sensor of claim 1, comprising at least a first metal layer and a second metal layer, the optically sensitive layer in electrical communication with the second metal layer.
60. The sensor of claim 59, wherein the at least two metal layers include metal interconnect layers.
61. The sensor of claim 59, wherein the second metal layer forms contacts in electrical communication with the optically sensitive layer.
62. The sensor of claim 61, wherein the contacts comprise an aluminum body, a first coating and a second coating, the first coating comprising titanium nitride and positioned between the aluminum body and the optically sensitive layer, the second coating comprising titanium oxynitride and positioned between the first coating and the optically sensitive layer.
63. The sensor of claim 61, wherein the contacts comprise an aluminum body, a first coating and a second coating, the first coating comprising titanium nitride and positioned between the aluminum body and the optically sensitive layer, the second coating located between the first coating and the optically sensitive layer and comprising a metal selected from the group consisting of gold, platinum, palladium, nickel and tungsten.
64. The sensor of claim 61, wherein the contacts are formed from a plurality of metal sub-layers, each metal sub-layer comprising a constituent selected from the group consisting of titanium nitride, titanium oxy nitride, gold, platinum, palladium, nickel and tungsten.
65. The sensor of claim 59, wherein the second metal layer consists of metal other than aluminum, the metal including at least one layer selected from the group consisting of titanium nitride, titanium oxynitride, gold, platinum, palladium, nickel and tungsten.
66. The sensor of claim 59, wherein the second metal layer consists of metal other than copper, the metal including at least one layer selected from the group consisting of titanium nitride, titanium oxynitride, gold, platinum, palladium, nickel and tungsten.
67. The sensor of claim 59, wherein the second metal layer comprises a constituent selected from the group consisting of titanium nitride, titanium oxynitride, gold, platinum, palladium, nickel and tungsten.
68. The sensor of claim 59, wherein the optically sensitive layer makes direct contact with the second metal layer.
69. The sensor of claim 59, wherein the optically sensitive layer comprises a coating on the second metal layer.
70. The sensor of claim 59, wherein the metal layers comprise at least one additional metal layer between the first metal layer and the second metal layer.
71. The sensor of claim 70, wherein each of the first metal layer and the at least one additional metal layer comprises aluminum, wherein the at least one additional metal layer excludes aluminum.
72. The sensor of claim 70, wherein each of the first metal layer and the at least one additional metal layer comprises aluminum and titanium nitride, wherein the at least one additional metal layer excludes aluminum.
73. The sensor of claim 70, wherein each of the first metal layer and the at least one additional metal layer excludes aluminum.
74. The sensor of claim 70, wherein each of the first metal layer and the at least one additional metal layer excludes copper.
75. The sensor of claim 59, wherein the first metal layer has a first thickness dimension and the second metal layer has a second thickness dimension.
76. The sensor of claim 59, wherein the first metal layer has a first aspect ratio and the second metal layer has a second aspect ratio.
77. The sensor of claim 1, wherein a continuous function represents the non-linear relationship.
78. A sensor comprising:
a first region of an optically sensitive material; a second region of the optically sensitive material, the second region covering at least a portion of the first region; at least two electrodes adjacent the optically sensitive material; wherein the optically sensitive material with the electrodes is non-rectifying, wherein a non-linear relationship exists between electrical characteristics of the optically sensitive material and the intensity of light absorbed by the optically sensitive material; wherein the first region and the second region possess substantially different spectral onset of absorption. 79. A sensor comprising:
at least one optically sensitive layer; wherein a first region of the optically sensitive layer provides electronic signals corresponding to an intensity of light incident on the first region and lying within a first spectral region; wherein a second region of the optically sensitive layer provides electronic signals corresponding to the intensity of light incident on the second region and lying within a second spectral region. 80. The sensor of claim 79, wherein the first spectral region includes at least one of a visible spectral region, an X-ray spectral region, an ultraviolet (UV) spectral region, a near infrared (IR) (NIR) spectral region, a short-wavelength IR (SWIR) spectral region, and a mid-wavelength IR (MWIR) spectral region.
81. The sensor of claim 80, wherein the second spectral region includes at least one of a visible spectral region, an X-ray spectral region, an ultraviolet (UV) spectral region, a near infrared (IR) (NIR) spectral region, a short-wavelength IR (SWIR) spectral region, and a mid-wavelength IR (MWIR) spectral region
This application is a Continuation of U.S. application Ser. No. 12/728,181, filed Mar. 19, 2010, which is a Continuation of U.S. patent application Ser. No. 12/106,256, filed Apr. 18, 2008, all of which are incorporated herein by reference in their entireties.
This application claims the benefit of U.S. Patent Application No. 60/912,581, filed Apr. 18, 2007.
This application claims the benefit of U.S. Patent Application No. 60/958,846, filed Jul. 9, 2007.
This application claims the benefit of U.S. Patent Application No. 60/970,211, filed Sep. 5, 2007.
This application claims the benefit of U.S. Patent Application No. 61/026,440, filed Feb. 5, 2008.
This application claims the benefit of U.S. Patent Application No. 61/026,650, filed Feb. 6, 2008.
This application claims the benefit of U.S. Patent Application No. 61/028,481, filed Feb. 13, 2008.
This application claims the benefit of U.S. Patent Application No. 61/046,379, filed Apr. 18, 2008.
Optoelectronic device, such as image sensors and photovoltaic devices, may include optically sensitive material. Example image sensors include devices that use silicon both for the sensing function and for the read-out electronics and multiplexing functions. In some image sensors, optically sensitive silicon photodiodes and electronics may be formed on a single silicon wafer. Other example image sensors may employ a distinct material, such as InGaAs (for short-wave IR sensing), or amorphous selenium (for x-ray sensing), for the sensing (photon to electron conversion) function. Example photovoltaic devices include solar cells that use crystalline silicon wafers for photon to electron conversion. Other example photovoltaic devices may use a separate layer of material such as amorphous silicon or polycrystalline silicon or a distinct material for photon to electron conversion. However, these image sensors and photovoltaic devices have been known to have a number of limitations.
FIG. 2 a shows an example of a quantum dot 1200.
FIG. 2 b shows an array of quantum dots 1200.
FIG. 2 c shows colloidal quantum dots in an illustration taken from the Wikipedia website. Ultraviolet light is incident on various sizes of Cadmium selenide (CdSe) quantum dots 1200, resulting in fluoresence.
FIG. 2 d shows various quantum dots 1200 arrayed in a quantum dot material 200.
FIG. 2 e further illustrates the absorbance of colloidal quantum dot material 200.
FIG. 3 is a side view of a portion of an embodiment of imaging configured in a lateral planar structure;
FIG. 3 a shows radiation entering pixel layouts.
FIG. 3 b shows an aspect of a closed simple geometrical arrangement of pixels;
FIG. 3 c shows an aspect of a closed with interdigitation geometrical arrangement of pixels;
FIG. 3 d shows an aspect of a open simple geometrical arrangement of pixels;
FIG. 3 e shows an aspect of an open with interdigitation geometrical arrangement of pixels;
FIG. 3 f shows a two-row by three-column sub-region within a generally larger array of top-surface electrodes;
FIG. 3 g shows a quantum dot structure stack involving two separate layers of quantum dot materials and a layer of dialetric material between the two separate layers of quantum dot materials;
FIG. 3 h shows an example a multi-layered quantum dot structure with electrical interconnections and pixel circuitry.
FIG. 3 i shows absorption and TEM graphs of small PbS nanocrystals. (a) Absorption of solution of as-synthesized oleic acid capped nanocrystals and of solution of the nanocrystals after butylamine ligand exchange, the extended absorption into the infrared is due to loss of high confinement followed by the formation of nanorods. (b) TEM of oleic acid capped nanocrystals shows the synthesis of small nanocrystals with diameter ˜3 nm. (c) After ligand exchange the nanocrystals conglomerate to form nanorods;
FIG. 3 j shows performance of the PbS small nanocrystal photodetector. (a) Spectral responsivity and normalized detectivity D* taken at 15 Hz of modulation frequency. The inset shows the noise equivalent power of the 0.0015 mm2 detector in the visible range for modulation frequency of 15 frames per second. The NEP of a typical silicon photodiode of similar surface area is plotted for comparison. (b) Responsivity and noise current density versus modulation frequency. (c) The detector exhibits D*˜1013 Jones for modulation frequency up to 5 Hz whereas it has D* greater than 1012 Jones up to 50 Hz at wavelength of 400 nm. The inset shows the resultant NEP of the detector at 400 nm with active area 0.0015 mm2 as a function of modulation frequency;
FIG. 3 k shows effects of the trap states on the photodetector performance. (a) Responsivity vs modulation frequency at various illumination levels measured at optical wavelength of 632 nm. The long lived trap states dominate at low optical powers to provide with high photoconductive gain. As higher power impinges on the detector the effective traps are filled leading to decrease of the photoconductive gain and increase of the 3-dB bandwidth. The 3-dB bandwidth increases from 8 Hz at low power to over 300 Hz at high optical power. This phenomenon could provide the detector with a self-limiting gain mechanism to avoid electronic saturation at high power levels. (b) The photocurrent versus optical illumination reveals a dynamic range of 75-dB. The inset shows the responsivity as a function of optical intensity. The responsivity drops beyond 10−5 W cm−2 due to the filling of the high gain trap states to enable the self-limiting mechanism of gain. This measurement was taken with light at 830 nm.
FIG. 3 l shows spectral responsivity of the stacked device shown in the inset for dual-spectral detection. The small-quantum dot (QD) layer detects effectively the short wavelength light, whereas the longer wavelengths are detected from the large-quantum dot (QD) layer. The responsivity of the large-quantum dot device before the stacking is also shown for comparison. The responsivities of the large-quantum dot device in both cases have been normalized to their exciton peak value at 1230 nm. The structure shown in the inset consists of gold electrodes with length 3 mm, width 20 μm and height 100 nm. The spacing between the electrodes is 20 μm and the applied bias was 100V.
FIG. 3 m shows the device structure. PbS quantum dots were spincoated from solution on a pre-patterned interdigitated electrode. The electrodes were fabricated using standard photolithography on a gold-coated F45 glass substrate −0.5 mm in thickness—from Schott. 10 nm of chromium were evaporated prior to 100 nm gold sputtering to achieve adhesion of gold to the glass substrate. The width of the gold stripes is 20 μm, the length is 3 mm and the spacing is 5 μm.
FIG. 3 n shows the dark current density as a function of the applied field of the reported device. The applied field is considered as the applied voltage over the device width of 5 μm.
FIG. 3 o (a-f) shows scanning electron microscope (SEM) images of the surface of a textured indium tin oxide (ITO) substrate before nanocrystal deposition and the surface of a ITO substrate after nanocrystal deposition with and without the use of cross-linking molecules. a-b, Bare textured ITO substrate. c-d, textured ITO after nanocrystal deposition without cross-linker showing the exposed ITO which leads to short-circuited devices. Individual nanocrystals can be seen clustered in the crevices between sintered ITO particles. e-f, Surface of functional devices after nanocrystal deposition utilizing cross-linkers to obtain a continuous nanocrystal over-coating.
FIG. 3 p shows a table of the effects sintering treatment has on the performance of photovoltaic devices. All measurements performed under 12 mW/cm2 975 nm illumination
FIG. 3 q shows current-voltage curves of sintered device made with nanocrystals having a first excitonic transition at 1340 nm. The AM 1.5 illumination intensity was 100 mW/cm2. Monochromatic illumination intensities of 12 and 70 mW/cm2 were with 975 nm light.
FIG. 3 r shows external quantum efficiency of sintered devices. Inset shows schematic of band positions of the materials used in devices. PbS conduction and valence bands are drawn for 1 eV first excitonic transition nanocrystals by employing the effective mass approximation to modify bulk energy levels.
FIG. 3 s is a stacked multilayer pixel having an electrode configuration wherein each respective first electrode (C1) is positioned laterally to at least a portion of the respective second electrode (c2), under an embodiment.
FIG. 3 t is a stacked multilayer pixel having an electrode configuration wherein one common electrode (CC) extends in the vertical direction over the height of more than one of the photosensitive layers (QF1+QF2); and wherein separate, electrically independent, electrodes (C1 AND C2) are used to bias and collect current substantially independently from the photosensitive layers (QF1 AND QF2), under an embodiment.
FIG. 3 u and FIG. 3 v is a side and top view, respectively, of a stacked multilayer pixel having an electrode configuration wherein a common electrode (CC) is disposed around an electrode (C1) in electrical contact with a first photosensitive layer (QF1); and the common electrode (CC) is disposed around an electrode (C2) in contact with a second photosensitive layer (QF2), under an embodiment.
FIG. 3 w depicts an image sensor, in cross-section, showing by illustration how two layers of optically sensitive material, stacked atop one another, can be independently read out electrically;
FIG. 4 is a diagram illustrating a transient response to modulated illumination at a 5V bias.
FIG. 6 a illustrates the 3T transistor configuration for interfacing with the quantum dot material;
FIG. 6 b; illustrates the 4T transistor configuration for interfacing with the quantum dot material;
FIG. 6 c shows the noise equivalent exposure of a QD layer device as compared with conventional Si CCD and CMOS sensor devices;
FIG. 7 a shows a quantum dot pixel 1800 structure according to an aspect of the present embodiment;
FIG. 7 b shows an example of an alternative (non-quantum dot) arrangement;
FIG. 7 c shows a focal plane array with a side view of an imaging system which includes an integrated circuit with an array of electrodes located on the top surface thereof;
FIG. 7 d shows a side view of a portion of an optical device configured in a vertical sandwich structure;
FIG. 7 e shows a side view of a portion of an optical device configured in a lateral planar structure;
FIG. 7 f shows a plan view of a portion of an optical device configured in a lateral interdigitated structure;
FIG. 8 shows an arrangement of quantum dot chip interconnections 2010. FIG. 20 shows an optical micrograph of a light sensitive layer formed on an electronic read-out chip;
FIG. 9 is an diagrammatic illustration of a scanning electron micrograph of an integrated circuit in which an optically sensitive film;
FIG. 10 illustrates 4 process flows (a, b, c, and d) that may be employed to produce integrated circuits similar to those described in FIG. 21;
FIG. 11 is a diagram of an alternative embodiment a scanning electron micrograph of an integrated circuit in which an optically sensitive film;
FIGS. 12-19 illustrate a “standard” single pixel shutter arrangement according to an embodiment;
FIG. 20 a is a schematic diagram for a 16-to-2 shared pixel circuit;
FIG. 20 b is a layout for the 16-to-2 shared pixel circuit of FIG. 31 a; FIG. 20 c shows a layout for pixel circuits for two pixel regions where each pixel circuit extends below both pixel regions;
FIGS. 21-36 illustrate a “global” pixel shutter arrangement;
FIG. 37 shows the vertical profile of an embodiment where metal interconnect layers of an integrated circuit shield the pixel circuitry on the semiconductor substrate from incident light;
FIG. 38 shows a layout (top view) of an embodiment where metal interconnect layers of an integrated circuit shield the pixel circuitry on the semiconductor substrate from incident light;
FIG. 39 is a block diagram of functional blocks of a lateral pixel according to an embodiment;
FIG. 40 is a block diagram of functional blocks of a vertical pixel according to an embodiment;
FIGS. 41-51 illustrate embodiments that include controlling temporal response of photoconductive photodetectors via selective introduction of surface trap states;
FIGS. 52-66 illustrate an embodiment including fast, spectrally-tunable solution-processed photodetectors;
FIGS. 67-69 illustrate an embodiment including smooth-morphology ultrasensetive solution-processed photoconductors; and
FIG. 70 is a block diagram of an example system configuration that may be used in combination with embodiments described herein.
Image sensors incorporate arrays of photodetectors. These photodetectors sense light, converting it from an optical to an electronic signal. It is often desired that such photodetectors possess the following features, individually or in combination(s):
be readily integrable with other circuitry related to the image sensing function, such as circuits which store charge, circuits which relay signal levels to the periphery of the array, circuits which manipulate these signal levels in the analog domain, circuits which convert analog into digital signals, and circuits which process image-related data in the digital domain.
provide a maximum of sensitivity to light within the wavelength band, or bands, of interest. Sensitivity is often quantified using the measure signal-to-noise (SNR) at a given level of illumination. Signal is maximized when the responsivity, quantum efficiency, or gain of the device is maximized. Noise is minimized when random fluctuations in electronic signals are minimized, subject to the limits prescribed by natural fluctuations in electrical currents and voltages at a given temperature. Relatedly, noise and other uncontrolled or difficult-to-predict variations in background signal are generally minimized when the magnitude of dark current is minimized. For this and other reasons, photodetectors having high sensitivity and low dark current are desired.
provide a response in time that is suitably fast. Applications such as video imaging and shutterless still-image acquisition typically require photodetectors whose signal levels change substantially completely in response to a transient within fewer than 100 milliseconds (10 frames per second), or fewer than 33 miliseconds (30 frames per second), or even 1 millisecond ( 1/1000 second exposure of a still image).
provide for the detection of a wide range of light intensities in a manner that can conveniently be processed by available electronic circuitry. This feature is known as providing high dynamic range. One means of providing high dynamic range is to compress the measured electronic response as a function of the incident optical stimulus. Such compression may be referred to as a sublinear, i.e. a nonlinear with decreasing slope, dependence of electrical signal on incident intensity. High dynamic range may also be facilitated by employing a photodetector whose gain may be controlled, such as through the selection of a voltage bias known to produce a specific gain.
provide for the convenient discrimination among different spectral bands of electromagnetic radiation. Of particular interest are the x-ray, ultraviolet, visible (including blue, green, and red), near-infrared, and short-wavelength infrared bands.
Herein are discussed means of creating, integrating with circuits, and exploiting in a variety of applications photodetectors, and arrays of photodetectors, that possess many of these properties in combination. Specifically:
the photodetectors described herein can readily be integrated with other portions of the image sensor circuit and system by straightforward, low-cost methods such as spin-coating, spray-coating, drop-coating, and self-assembly. Embodiments include exchanging ligands passivating nanoparticle surfaces for shorter ligands that will provide for appropriate charge carrier mobilities once films are formed. Embodiments include solution-phase exchanges which enable the realization of smooth-morphology films necessary to the realization of image sensors having acceptable consistent dark currents and photoresponses across an array.
The photodetectors described herein provide a maximum of sensitivity through a combination of means. They maximize signal by providing photoconductive gain. Typical values for photoconductive gain range from 1-10, resulting in responsivities in, for example, the visible wavelengths ranging from 0.4 A/W to 4 A/W. In embodiments, the photodetectors described herein minimize noise by fusing nanocrystal cores such as to ensure substantially non-noise-degrading electrical communication among the particles making up the optically sensitive layer through which current flows. In embodiments, the photodetectors described herein minimize dark current by minimizing the net doping of the active layer, thus ensuring that the dark carrier density, and thus the dark conductance, of these optically sensitive materials is minimized. In embodiments, the photodetectors described herein minimize dark current by providing an electrode-to-nanocrystalline-layer electrical connection that blocks typically one type of carrier, including potentially the majority carrier at equilibrium. In embodiments, cross-linking molecules are employed that utilize chemical functionalities that remove oxides, sulfates, and/or hydroxides responsible for p-type doping. Thus, in embodiments, a more intrinsic or even n-type optically sensitive layer may be provided, leading to lowered dark currents. In embodiments, many steps in quantum dot synthesis and/or processing and/or device packaging may be performed in a controlled environment such as a Schlenk line or Glove Box; and optically sensitive layers may be encapsulated using substantially impermeable layers such as oxides, oxynitrides, or polymers such as parylene, or epoxies, in order to prevent reactive gases such as oxygen or water from significantly permeating the optically sensitive layer. In this manner, desired combinations of properties such as gain, dark current, and lag may be preserved over the desired useful lifetime of an image sensor.
The photodetectors described herein provide a time-domain response that can be as rapid as sub-100-miliseconds, sub-30-milliseconds, and sub-1-millisecond. In embodiments, this is achieved by providing gain-providing (and persistence-providing) trap states associated with the optically sensitive layer that trap at least one type of carrier for only a limited time period such as 100 milliseconds, 30 milliseconds, or 1 millisecond. In embodiments, PbS nanoparticles are decorated with PbSO3, an oxide of PbS, which is shown to have a trap state lifetime in the vicinity of 20-30 milliseconds, providing for a transient response suited to many video imaging applications. In embodiments, photodiodes are instead provided based on colloidal quantum dot layers, wherein two electrical contacts having appreciably different work functions are employed to contact the active layer. In embodiments, dark currents may be minimized through operation of such devices without the application of an appreciable external voltage bias. In embodiments, cross-linking moieties such as benzenedithiol, a bidentate linker, may be employed to remove and/or passivate certain trap states that may be present, or may develop, in such materials.
The photodetectors described herein provide a means of enhancing dynamic range by producing a sublinear dependence of electrical signal (such as photocurrent). Over a region of low to middling intensities, trap states are available to become filled, and escape occurs following some moderate persistence, or trap state, lifetime, such as 30 milliseconds for example. At higher intensities, these trap states become substantially filled, such that charge carriers experience shorter lifetimes, or persistence times, corresponding to lower differential gains. As a result these devices exhibit substantially constant gains over a range of low to middling intensities, followed by a gentle roll-off in gain at higher intensities. Put another way, at low to middling intensities, photocurrent depends approximately linearly on intensity, but at higher intensities, photocurrent exhibits sublinear dependence on intensity. In embodiments, photodetectors are provided wherein photoconductive gain depends on the bias applied to a device. This arises because gain is proportional to carrier lifetime divided by carrier transit time, and transit time varies in inverse proportionality with applied field. In embodiments, circuits are developed that exploit this dependence of gain on bias to increase dyanamic range.
In embodiments, photodetectors described herein may readily be altered, or ‘tuned’, to provide sensitivity to different spectral bands. One means of tuning is provided through the quantum size effect, whereby nanoparticle diameter is decreased, in cases through synthetic control, to increase the effective bandgap fo the resulting quantum dots. Another means of tuning is provided through the choice of materials composition, wherein the use of a material having a larger bulk bandgap generally facilitates the realization of a photodetector with responsivity onset at a higher photon energy. In embodiments, photodetectors having different absorption onsets may be superimposed to form vertical pixels, wherein pixel(s) closer to the source of optical signal absorb and sense higher-energy bands of electromagnetic radiation, whereas pixel(s) further frojm the source of optical signal absorb and sense lower-energy bands.
FIG. 1 shows structure of and areas relating to quantum dot pixel chip structures (QDPCs) 100, according to example embodiments. As illustrated in FIG. 1, the QDPC 100 may be adapted as a radiation 1000 receiver where quantum dot structures 1100 are presented to receive the radiation 1000, such as light. The QDPC 100 includes, as will be described in more detail herein, quantum dot pixels 1800 and a chip 2000 where the chip is adapted to process electrical signals received from the quantum dot pixel 1800. The quantum dot pixel 1800 includes the quantum dot structures 1100 include several components and sub components such as quantum dots 1200, quantum dot materials 200 and particular configurations or quantum dot layouts 300 related to the dots 1200 and materials 200. The quantum dot structures 1100 may be used to create photodetector structures 1400 where the quantum dot structures are associated with electrical interconnections 1404. The electrical connections 1404 are provided to receive electric signals from the quantum dot structures and communicate the electric signals on to pixel circuitry 1700 associated with pixel structures 1500. Just as the quantum dot structures 1100 may be laid out in various patterns, both planar and vertical, the photodetector structures 1400 may have particular photodetector geometric layouts 1402. The photodetector structures 1400 may be associated with pixel structures 1500 where the electrical interconnections 1404 of the photodetector structures are electrically associated with pixel circuitry 1700. The pixel structures 1500 may also be laid out in pixel layouts 1600 including vertical and planar layouts on a chip 2000 and the pixel circuitry 1700 may be associated with other components 1900, including memory for example. The pixel circuitry 1700 may include passive and active components for processing of signals at the pixel 1800 level. The pixel 1800 is associated both mechanically and electrically with the chip 2000. From an electrical viewpoint, the pixel circuitry 1700 may be in communication with other electronics (e.g. chip processor 2008). The other electronics may be adapted to process digital signals, analog signals, mixed signals and the like and it may be adapted to process and manipulate the signals received from the pixel circuitry 1700. In other embodiments, a chip processor 2008 or other electronics may be included on the same semiconductor substrate as the QDPCs and may be structured using a system-on-chip architecture. The chip 2000 also includes physical structures 2002 and other functional components 2004, which will also be described in more detail below.
The QDPC 100 may be manufactured using a number of manufacturing processes 300 such as quantum dot solution processing 400, quantum dot materials processing 500, photodetector processing 600, pixel processing 700, pixel/chip processing 800, overall system integrations 900, and other such processes. The QDPC 100 may also be incorporated into integrated systems 2200 where the integrated systems 2200 may be structured 2202 and have various features 2204. The structures 2202 and features 2204 may be adapted to serve particular applications and/or markets 2100. An integrated product may include conventional pixel structures (e.g. CCD or CMOS) along with quantum dot pixel structures 1800 in certain embodiments.
The photoconductive layer of the QDPC may have a resistance that changes in response to changes in radiation 1000. In embodiments, the QDPC is an image detector that monolithically sits on top of a semiconductor. The photoconductive coating (e.g. quantum dot material 200) may be transferred (e.g. spin coated) on top of the integrated circuit. A photon generates an electron-hole pair. The hole can participate in conduction. An electron is trapped for a trap time ranging from nanoseconds to microseconds to milliseconds depending on the nature of the traps. The hole is transported during a period on the order of microseconds. The increase in conductivity thus persists over many multiples of the transit time for the hole across the devices. As a result, multiple charges are, over the measurement interval, measured in an external circuit for a given absorbed photon. In an alternative embodiment, the electron is transported, with the increase in conductivity persisting over many multiples of the transit time for the electron across the device. In some embodiments, the QDPC is a generally linear device, which can benefit from a change in resistance for an extended period, generally ranging, on an order of magnitude, from one microsecond to one second with desirable periods in the range of approximately 100 microseconds to approximately one millisecond. In other embodiments, the current across the quantum dot material 200 has a non-linear relationship to the intensity of light absorbed by the optically sensitive layer of quantum dot material 200. In some embodiments, analog or digital circuitry on the chip 2000 or on another chip associated with the QDPC may be used to linearize the signals read from the pixel regions in order to produce digital pixel data that can be used by a digial camera or other system in which the QDPC is used. Embodiments described herein relate to measuring the persistent resistance change associated with the photoconductive layer. Certain of these embodiments involve low cost, largest dynamic range, most efficient, least noisy, etc. methods by which to measure this resistance. In some embodiments, the QDPC 100 consists of a photoconductive material contacted by at least two generally different contacts. Either of these contacts may be ohmic, or may form a Schottky contact. Upon absorption of a photon, an electron-hole pair is created. This may contribute to the photoconductive gain described above. In addition, both the electron and the hole may be electrically transported to some degree, either under an applied or no applied external bias. The electron and the hole may be transported, with generally different efficiencies, to their respective contacts, depending on the biasing of the device. The device may exhibit a generally nonlinear relationship between current and voltage in some embodiments. In certain biasing regimes it may produce photoconductive gain, and in other regimes the same device may not. In certain biasing regimes the device may produce much lower dark currents than in others. In some embodiments, there are regimes of significant photoconductive gain (e.g. 1, 10, or 100) but with low dark current (e.g. 1 nA/cm2, or 1 uA/cm2).
Radiation 1000 The QDPC 100 detects electromagnetic radiation 1000, which in embodiments may be any frequency of radiation from the electromagnetic spectrum. Although the electromagnetic spectrum is continuous, it is common to refer to ranges of frequencies as bands within the entire electromagnetic spectrum, such as the radio band, microwave band, infrared band (IR), visible band (VIS), ultraviolet band (UV), X-rays, gamma rays, and the like. The QDPC 100 may be capable of sensing any frequency within the entire electromagnetic spectrum; however, embodiments herein may reference certain bands or combinations of bands within the electromagnetic spectrum. It should be understood that the use of these bands in discussion is not meant to limit the range of frequencies that the QDPC 100 may sense, and are only used as examples. Additionally, some bands have common usage sub-bands, such as near infrared (NIR) and far infrared (FIR), and the use of the broader band term, such as IR, is not meant to limit the QDPCs 100 sensitivity to any band or sub-band. Additionally, in the following description, terms such as “electromagnetic radiation”, “radiation”, “electromagnetic spectrum”, “spectrum”, “radiation spectrum”, and the like are used interchangeably, and the term color is used to depict a select band of radiation 1000 that could be within any portion of the radiation 1000 spectrum, and is not meant to be limited to any specific range of radiation 1000 such as in visible ‘color’.
Quantum Dot 1200 A quantum dot 1200 may be a nanostructure, typically a semiconductor nanostructure, that confines a conduction band electrons, valence band holes, or excitons (bound pairs of conduction band electrons and valence band holes) in all three spatial directions. The confinement can be due to electrostatic potentials (e.g., generated by external electrodes, doping, strain, impurities), the presence of an interface between different semiconductor materials (e.g., in core-shell nanocrystal systems) or a semiconductor and another material (e.g., a semiconductor decorated by organic ligands; or by a dielectric such as an oxide such as PbO, a sulfite such as PbSO3, a sulfate such as PbSO4, or SiO2), the presence of a semiconductor surface, or a combination of one or more of these. A quantum dot exhibits in its absorption spectrum the effects of the discrete quantized energy spectrum of an idealized zero-dimensional system. The wave functions that correspond to this discrete energy spectrum are typically substantially spatially localized within the quantum dot, but extend over many periods of the crystal lattice of the material.
FIG. 2 a shows an example of a quantum dot 1200. In one example embodiment, the QD 1200 has a core 1220 of a semiconductor or compound semiconductor material, such as PbS. Ligands 1225 may be attached to some or all of the outer surface or may be removed in some embodiments as described further below. In some embodiments, the cores 1220 of adjacent QDs may be fused together to form a continuous film of nanocrystal material with nanoscale features. In other embodiments, cores may be connected to one another by linker molecules. In some embodiments, trap states may be formed on the outer surface of the nanocrystal material. In some example embodiments, the core may be PbS and trap states may be formed by an oxide such as PbSO3 formed on the outer surface of core 1220.
FIG. 2 b shows a two-dimensional representation of a portion of a QD layer. The layer includes a continuous network of fused QD cores 1220, having outer surfaces 1221 that are of a different composition than that in the core, e.g., oxidized core material such as PbS03, or a different kind of semiconductor. The individual QD cores in the film are in intimate contact, but continue to exhibit many of the properties of individual quantum dots. For example, a lone (unfused) quantum dot has a well-characterized excitonic absorbance wavelength peak that arises from quantum effects related to its size, e.g., 1-10 nm. The excitonic absorbance wavelength peak of the fused QDs in the film is not significantly shifted from the central absorbance wavelength the was present prior to fusing. For example, the central absorbance wavelength may change by about 10% or less when fused. Thus, the QDs in the film retain their quantum effects, despite the fact that they may be an integral part of a macroscopic structure.
Current is not generally thought of as “flowing” through a lone (unfused and unlinked) QD; instead, electrons simply occupy well-known quantum energy states in the quantum dot core. If two lone (unfused) QDs are brought near each other, current can flow between them by electron or hole hopping between the QDs. Current more readily flows between fused QD cores, even though the cores themselves generally retain their quantum energy states. Because the cores are in contact, electrons can more easily move between them. It may also be said that the wavefunctions of the quantum-confined electron or hole states in adjacent dots see increased overlap as the quantum dots are brought close together to form a fused quantum dot-based solid, while at the same time not fusing the QDs to such an extent that they lose their “identity,” namely their individual characteristics that provide quantum confinement, in turn resulting in their excitonic feature manifest in their absorption spectrum. In embodiments, mobilities may range from 10−7 cm2/Vs to 102 cm2/Vs. It is also possible to “overfuse” QDs, in which case they no longer exhibit many of the normal properties of individual quantum dots. In the overfused case, the cores of the QDs do not generally have their own quantum energy levels, but the energy levels are instead distributed over multiple QD cores. This results in a film with a very low electrical resistance, but which in many ways is effectively a bulk semiconductor material. “Overfused” QDs can also be recognized experimentally by their relatively large shift (e.g., greater than about 10%) to the red (longer wavelengths) in their absorption and/or emission spectra. Complete fusing, and complete loss of quantum confinement properties, may be recognized when the solid approaches taking on the absorption spectral characteristics of a bulk matrial, e.g. absorption begins near the bulk materials' bandgap.
In some embodiments, QD cores are linked by linker molecules as described further below. This allows current to flow more readily than through unlinked, unfused QDs. However, the use of linker molecules to form a continuous film of QDs instead of fusing the cores may reduce the dark current for some photoconductor and image sensor embodiments.
In certain embodiments the QD layer is exceptionally radiation 1000 sensitive. This sensitivity is particularly useful for low-radiation 1000 imaging applications. At the same time, the gain of the device can be dynamically adjusted so that the QDPC saturates, that is, additional photons continue to provide additional useful information that can be discerned by the read-out electronic circuit. Tuning of gain can be conveniently achieved by changing the voltage bias, and thus the resultant electric field, across a given device, e.g., a pixel. As discussed in greater detail below, photoconductive photovoltaic gain, and correspondingly the responsivity in A/W, can be made to vary approximately linearly with bias and field. Thus, in a given device, a bias of about 1 V may result in a gain of about 10, while a bias of about 10 V may result in a gain of about 100.
Some embodiments of QD devices include a QD layer and a custom-designed or pre-fabricated electronic read-out integrated circuit. The QD layer is then formed directly onto the custom-designed or pre-fabricated electronic read-out integrated circuit. The QD layer may additionally be patterned so that it forms individual islands. In some embodiments, wherever the QD layer overlies the circuit, it continuously overlaps and contacts at least some of the features of the circuit. In some embodiments, if the QD layer overlies three-dimensional features of the circuit, the QD layer may conform to these features. In other words, there exists a substantially contiguous interface between the QD layer and the underlying electronic read-out integrated circuit. One or more electrodes in the circuit contact the QD layer and are capable of relaying information about the QD layer, e.g., an electronic signal related to the amount of radiation 1000 on the QD layer, to a readout circuit. The QD layer can be provided in a continuous manner to cover the entire underlying circuit, such as a readout circuit, or patterned. If the QD layer is provided in a continuous manner, the fill factor can approach about 100%, with patterning, the fill factor is reduced, but can still be much greater than a typical 35% for some example CMOS sensors that use silicon photodiodes.
In many embodiments, the QD optical devices are readily fabricated using techniques available in a facility normally used to make conventional CMOS devices. For example, a layer of QDs can be solution-coated onto a pre-fabricated electronic read-out circuit using, e.g., spin-coating, which is a standard CMOS process, and optionally further processed with other CMOS-compatible techniques to provide the final QD layer for use in the device. Details of QD deposition and further processing are provided below. Because the QD layer need not require exotic or difficult techniques to fabricate, but can instead be made using standard CMOS processes, the QD optical devices can be made in high volumes, and with no significant increase in capital cost (other than materials) over current CMOS process steps.
FIG. 2 e further illustrates the absorbance of some example colloidal quantum dot materials 200.
Example nanocrystal materials that may be used as optically sensitive layers of QDs in example embodiments are described further below as well as methods for making the same, including those with fused or linked cores and trap states. These nanocrystal materials can be used to provide QD layers with photoconductive in example embodiments. Example embodiments of the QDPC 100 shown in FIG. 1 may use any of these nanocrystal materials as an optically sensitive layer to form a pixel region of a photosensor array. These nanocrystal materials may also be used to form the optically sensitive layer in embodiments of the other example image sensors, pixel regions and photoconductors described in this specification. In particular embodiments, the optically sensitive layer of the pixel regions is formed from compound semiconductor nanocrystal cores, such as PbS or other materials described below, that are fused or linked and oxides of the core material, such as PbS03, are formed on the outer surface to form trap states to provide photoconductive gain. These are examples only and other QDs or optically sensitive layers may be used in the pixel regions in other embodiments. In some embodiments, other optically sensitive layers such as silicon photodiodes may be used in combination with or instead of QDs for pixel regions or for particular layers in pixel regions.
Quantum Dot Structures 1100 An aspect of the example embodiment in FIG. 1 relates to quantum dot structures 1100. Quantum dot structures 1100 generally include quantum dots 1200, quantum dot materials 200 and quantum dot layouts 1300.
Example embodiments provide quantum dot (QD) 1200 devices and methods of making nanoscale crystalline material devices. Many embodiments are optical devices that possess enhanced gain and sensitivity, and which can be used in optical, including radiation 1000 imaging applications, photovoltaic applications, among other applications. The term “quantum dot” or “QD” is used interchangeably herein with the term “nanocrystal,” and it should be understood that the present embodiments are not limited exclusively to standalone quantum dots but rather to any nanoscale crystalline material, including continuous films having nanoscale features, such as those formed by networks of fused or linked nanocrystal cores.
The term ‘nanocrystal film’ is used to describe a material that includes crystalline components that can typically range in size from 0.5 nm to 10 nm, and wherein the constituent nanocrystals' axes of symmetry (or of periodicity) in their repetition of atoms is, in generally, not well-ordered across the film, i.e. from nanocrystal to nanocrystal. The term ‘colloidal quantum dot film’ is used to describe nanocrystalline films wherein the constituent nanocrystals are approximately same-sized, such as a film in which the nanocrystals are on average 2 nm and have a standard deviation in diameter of less than 0.5 nm. There exists another category of quantum dots, known as epitaxial, or Stranski-Krastanow-grown quantum dots, that are not encompassed in the term ‘nanocrystal film or ‘colloidal quantum dot film’ as defined herein. Epitaxial quantum dot solids are generally lattice-matched, and thus the constituent crystals poss-sess approximately the same lattice orientation as one another.
In many embodiments, the optically sensitive QD layer includes a plurality of QDs that have been specially processed to give the layer an enhanced gain and sensitivity. Specifically, a plurality of QDs may be fabricated using the methods described below, and in some embodiments may include a core as well as an outer surface that includes a plurality of ligands. The ligands may be exchanged for shorter ligands, in certain cases volatile ligands, and then the ligand-exchanged QDs are solution-deposited onto a substrate to form a QD precursor layer in some embodiments. Other methods for forming and/or depositing nanocrystal materials may also be used as described below. The substrate itself may include one or more electrodes, or the electrodes may be deposited in a later step. Subsequently, the short ligands are removed from the QD precursor layer in some embodiments. This may bring the QDs in the QD precursor layer into very close proximity. In embodiments, at least some of the QDs may make contact with their neighbors. This contact between QDs may be referred to as “necking” Bringing the nanoparticles closer together may produce an increased ease of movement of electrons and/or holes between nanoparticles, thus improving the mobility for charge carries traversing the electrical path within the layer.
In some embodiments, necked QDs may be annealed, which may fuse the necked QDs together. In this instance, the QD precursor layer may be maintained in an inert atmosphere after ligand removal, so that the outer surfaces of the individual QDs do not oxidize until annealing is complete. While two given fused QDs in the annealed QD layer retain a large portion of their original shape, and thus remain individually recognizable, after annealing the QDs may no longer be physically distinct from each other. Instead, the cores of the QDs together may form a continuous electrical path, which may thus further improve the conductivity of the electrical path within the layer. Thus, if many adjacent QDs neck, or fuse together into an annealed QD layer, they may form an electrical pathway with a physical extent that is substantially greater than that of the individual QDs, and through which current will readily flow. In some embodiments, a QD film may have a macroscopic extent, though the QDs themselves are nanoscopic. In some embodiments, the finished QD layer may essentially be considered a continuous inorganic film having nanoscale features. The general shapes of the individual QDs may still be recognizable, but their cores form a continuous electrical pathway that is mechanically robust. For example, a micrograph of the finished QD layer would show the general shape and size of the individual QDs from which the layer is formed, as well as robust joints between many adjacent QDs. In some embodiments, the nanocrystal cores may be linked by other molecules rather than fused together.
In many embodiments, the fused QD layer is subsequently processed to modify its outer surfaces. For example, a material such as a semiconductor shell can be coated on the outer surfaces of the fused quantum dots. Or, for example, defect states can be formed on the exposed outer surfaces of the QDs, e.g., by oxidizing the fused QDs layer. These defect states effectively trap electrons excited by photons, so that they recombine with holes far less readily and thus greatly enhance the amount of current that a given photon generates in the finished QD layer. This greatly enhances the photoconductive gain of the device. The fused QD cores, and the juncture between them, will generally not have defect states, so current will flow readily between them, in certain embodiments. Photoconductive gain occurs because the persistence of the trap state is longer than the transit time of carriers across the layer of nanocrystal material.
Photodetector Structures 1400 An aspect of the example embodiment in FIG. 1 relates to photodetector structures 1400. Photodetector structures 1400 generally include electrical connections 1404 that are connected to the quantum dot structures 1100 and adapted to carry an electric signal or charge from the quantum dot structures 1100 to be communicated to associated pixel circuitry 1700. The electrical interconnections 1404 may be laid out in a photodetector geometric layout 1402 that corresponds with a quantum dot layout 1300 as described herein. For example, if the quantum dot layout is a closed interdigitated pattern 1438 then the layout of the electrical interconnections may be laid out in a similar pattern such that the electrical charge or signal from the nodes can be transferred from the dots to the pixel circuitry 1700. In other embodiments, the electrical interconnections may be laid out in a particular pattern and the quantum dot structures 1100 may be laid out in another pattern (e.g. dispersed in a continuous film) such that the electrical interconnections 1404 draw charge or signal from only a portion of the quantum dot structure 1100.
In embodiments the photodetector structure includes a semiconductor film contacted with some number of electrodes. While many of the embodiments disclosed herein describe the use of photodetectors structures with image detection systems, it should be understood that the photodetector may be a photodiode (e.g., photovoltaic with gain or no gain) or a photoconductor in some embodiments. The photoconductor may generally pass holes and stop or fail to pass electrons. Some of the energetic relationships include Fermi energy, work function, valence energy, and conduction band edge. Photodetector structures 1400 may contain material combinations such as including or containing two or more types of quantum dots. In some example embodiments, the photodetector structures may form a unipolar device in which the transport of one carrier type (electrons or holes) predominates over the transport of the other type (holes or electrons). For example, in PbS photoconductive colloidal quantum dot detectors described herein, holes may be the flowing carrier and may have a mobility of at least 10� greater than electrons. For example, the hole mobility may equal 1E-4 cm2/Vs while the electron mobility may be inferior to 1E-5 cm2/Vs.
The photodetector structure 1400 is a device configured so that it can be used to detect radiation 1000 in example embodiments. The detector may be ‘tuned’ to detect prescribed wavelengths of radiation 1000 through the types of quantum dot structures 1100 that are used in the photodetector structure 1400. The photodetector structure can be described as a quantum dot structure 1100 with an I/O for some input/output ability imposed to access the quantum dot structures' 1100 state. Once the state can be read, the state can be communicated to pixel circuitry 1700 through an electrical interconnection 1404, wherein the pixel circuitry may include electronics (e.g., passive and/or active) to read the state. In an embodiment, the photodetector structure 1400 may be a quantum dot structure 1100 (e.g., film) plus electrical contact pads so the pads can be associated with electronics to read the state of the associated quantum dot structure 1100. In an embodiment, the electrical interconnections 1404 may include a KDP electric field detector or other field detection system. Once an electrical connection is made with the quantum dot structure and made available, it may be read through contact (e.g. charge, voltage, current, power, resistance) or non-contact (e.g. magnetic field, electric field, capacitance, inductance) measurement methods.
In embodiments, the photodetector structure 1400 may include multiple layers of quantum dot material 200, spin-coated on top of one another. There may be insulating materials laid down between these layers in order to reduce crosstalk between the layers. In embodiments, this material may be a very thin dielectric material, with a thickness of 1000 angstroms for instance. The application of this dielectric may involve process steps to assure that the dielectric does not interfere with the electrical contacts intended to connect with the layers of quantum dot material 200, such as by masking off or removal from the electrical connections. For instance, removal may involve an etching process. In embodiment, the dielectric may be laid down vertically to avoid making contact with the top of the electrodes.
In an embodiment, post-processing may involve sensitization of a photodetector 1400 by a combination of illumination and temperature. The photodetector 1400 sensitivity may freeze in place as long as there is uniform illumination; all other parameters may reach an equilibrium value. For example, a photodetector 1400 may be sensitized by heating to a temperature for a period of time. A combination of time and temperature may affect the sensitivity. Power from the photodetector 1400 may be dissipated on a pixel-by-pixel basis. Power may be inversely proportional to the response. Dissipation may normalize the response. Post-processing may involve illumination with a flat field that results in a consistent photodetector response across an array. The self-flattening may involve uniform illumination of the whole sensor: if there were pixels not having the desired level of gain, the uniform illumination will serve to increase their gain. Large currents may go through “hot” pixels, which may diminish their response. If the sensor is exposed to a uniform optical field, it may anneal to a flat response.
The quantum dot structures 1100 may also have planar layouts in some embodiments. The planar layouts may be provided to increase the sensitivity of a QDPC to a certain wavelength or range of wavelengths for example. Patterns of quantum dot structures in the horizontal plane may be provided for a number of reasons including, for example, increasing the sensitivity to a wavelength, decreasing the sensitivity to a wavelength, and compensating for the absorption, refraction, reflection or other properties associated with the materials.
In embodiments, a charge region is produced in the quantum dot structure 1100 such that charges produced in the charge region as a result of incident radiation 1000 are isolated from those associated with adjacent charge regions, and the effect of illumination only over the region of interest can be measured. This method can be used to create a map of charge response areas that define the pixels or portions of the pixels. For example, within a layer of quantum dot structure 1100 it may be desired to define areas (e.g., millions of areas) that may be used as separate pixel portions. A matrix or set of positively biased electrodes and ground plane or negatively charged electrodes may be associated with the layer (e.g. through a set or matrix of electrical interconnects). When radiation 1000 to which the layer is responsive falls on the sub portion of the layer a charge in a corresponding area develops and is moved towards the positively biased electrode in the area. By monitoring the electrodes the area of activity can be identified, and the intensity can be measured and associated with a particular pixel or group of pixels. Several planar electrical interconnection patterns are illustrated in FIGS. 3 b-3 e including examples of such patterns. Each of the patterns presented in FIGS. 3 b-3 e illustrates a positively biased electrical interconnection 1452 and a ground or negatively biased interconnection 1450 as an example of how such an arrangement may operate, but other configurations and biases may be used in other embodiments.
FIG. 3 b represents closed—simple patterns 1430 (e.g., conceptual illustration) and 1432 (e.g., vias used to create photodetector structures). In the closed-simple illustrations 1430-1432 the positively biased electrical interconnect 1452 is provided in the center area of a grounded contained square electrical interconnect 1450. Square electrical interconnect 1450 may be grounded or may be at another reference potential to provide a bias across the optically sensitive material in the pixel region. For example, inteconnect 1452 may be biased with a positive voltage and interconnect may be biased with a negative voltage to provide a desired voltage drop across a nanocrytal material in the pixel region between the electrodes. In this configuration, when radiation 1000 to which the layer is responsive falls within the square area a charge is developed and the charge is attracted to and move towards the center positively biased electrical interconnect 1452. If these closed-simple patterns are replicated over an area of the layer, each closed simple pattern forms a portion or a whole pixel where they capture charge associated with incident radiation 1000 that falls on the internal square area. In example embodiments, the electrical interconnect 1450 may be part of a grid that forms a common electrode for an array of pixel regions. Each side of the interconnect 1450 may be shared with the adjacent pixel region to form part of the electrical interconnect around the adjacent pixel. In this embodiment, the voltage on this electrode may be the same for all of the pixel regions (or for sets of adjacent pixel regions) whereas the voltage on the interconnect 1452 varies over an integration period of time based on the light intensity absorbed by the optically sensitive material in the pixel region and can be read out to generate a pixel signal for each pixel region. In example embodiments, interconnect 1450 may form a boundary around the electrical interconnect 1452 for each pixel region. The common electrode may be formed on the same layer as interconnect 1452 and be positioned laterally around the interconnect 1450. In some embodiments, the grid may be formed above or below the layer of optically sensitive material in the pixel region, but the bias on the electrode may still provide a boundary condition around the pixel region to reduce cross over with adjacent pixel regions.
FIG. 3 c illustrates closed patterns of electrical interconnections with interdigitation 1434 (e.g., conceptual illustration) and 1438 (e.g., vias used to create photodetector structures). The interdigitation creates a pattern where the electrical interconnections are intertwined in some manner. Similar to the closed-simple patterns, these closed patterns may be used to capture charge within the confines of the outer grounded electrical interconnect 1450 and move the charges to the positively biased electrical interconnect 1452. As with FIG. 3 b, outer electrical interconnect 1450 may be grounded or be at some other reference potential and may form a common electrode around an array of pixels or set of adjacent pixels.
FIG. 3 d illustrates open simple patterns of electrical interconnects. The open simple patterns do not, generally, form a closed pattern. The open simple pattern does not enclose a charge that is produced as the result of incident radiation 1000 with the area between the positively biased electrical interconnect 1452 and the ground 1450; however, charge developed within the area between the two electrical interconnects will be attracted and move to the positively biased electrical interconnect 1452. An array including separated open simple structures may provide a charge isolation system that may be used to identify a position of incident radiation 1000 and therefore corresponding pixel assignment. As above, electrical interconnect 1450 may be grounded or be at some other reference potential. In some embodiments, electrical interconnect 1450 may be electrically connected with the corresponding electrode of other pixels (for example, through underlying layers of interconnect) so the voltage may be applied across the pixel array. In other embodiments, the interconnect 1450 may extend linearly across multiple pixel regions to form a common electrode across a row or column.
FIG. 3 e illustrates open interdigitated patterns where the patterns do not generally form a closed pattern and possess some form of interdigitation. The open interdigitated patterns may include a positive interconnection 1452 and ground (or other reference potential) interconnection 1450. This configuration may capture charge developed within the area between the two electrical interconnects and attract and move the charge to the positively-biased electrical interconnect 1452. An array including separated open interdigitated structures may provide a charge isolation system that may be used to identify a position of incident radiation 1000 and therefore corresponding pixel assignment. As above, the interconnect 1450 may form a common electrode with adjacent pixel regions in some embodiments.
FIG. 3 f shows a two-row by three-column sub-region within a generally larger array of top-surface electrodes. The array of electrical contacts provides electrical communication to an overlying layer of optically sensitive material. 1401 represents a common grid of electrodes used to provide one shared contact to the optically sensitive layer. 1402 represents the pixel-electrodes which provide the other contact for electrical communication with the optically sensitive layer. In embodiments, a voltage bias of −2 V may be applied to the common grid 1401, and a voltage of +2.5 V may be applied at the beginning of each integration period to each pixel electrode 1402. Whereas the common contact 1401 is at a single electrical potential across the array at a given time, the pixel electrodes 1402 may vary in time and space across the array. For example if a circuit is configured such that the bias at 1402 varies in relation to current flowing into or out of 1402, then different electrodes 1402 may be at different biases throughout the progress of the integration period. Region 1403 represents the non-contacting region that lies between 1401 and 1402 within the lateral plane. 1403 is generally an insulating material in order to minimize dark current flowing between 1401 and 1402. 1401 and 1402 may generally consist of different materials. Each may for example be chosen for example from the list: TiN; TiN/Al/TiN; Cu; TaN; Ni; Pt; and from the preceding list there may reside superimposed on one or both contacts a further layer or set of layers chosen from: Pt, alkanethiols, Pd, Ru, Au, ITO, or other conductive or partially conductive materials.
In some embodiments, the optically sensitive material in each pixel region is a nanocrystal material and has symmetric properties when the polarity of the bias is reversed. In some embodiments, the voltage on interconnect 1450 (which may be a common electrode for the pixel array) may be varied during the read out cycle rather than held at ground or a constant reference potential. For example, the voltage may be varied to reset or control the pixel circuit during transitions of the read out cycle as described further below.
The above photoconductor structures are examples only and other structures may be used on other embodiments.
Some of the properties and charateristics of example embodiments of photodetectors will now be described. These properties and characteristics may be tailored based on the nanocrystal materials that are used, the bias that is applied and the size and geometry of the pixel regions. Example nanocrystal materials and methods of making the same are described further below. Example embodiments may include any of these optically sensitive layers used in the photodetector structures described above or elsewhere in this specification and the resulting photodetector structures may be used in example embodiments with any of the pixel circuits and system circuitry described below in this specification. These combinations are examples only and the nanocrystal materials and photodetector structures may be used in other embodiments as well.
In some example embodiments, photodetectors of the type described above may have one or more of the following properties:
provide a maximum of sensitivity to light within the wavelength band, or bands, of interest. Sensitivity is often quantified using the measure signal-to-noise (SNR) at a given level of illumination. Signal is maximized when the responsivity, quantum efficiency, or gain of the device is maximized. Noise is minimized when random fluctuations in electronic signals are minimized, subject to the limits prescribed by natural fluctuations in electrical currents and voltages at a given temperature. Relatedly, noise and other uncontrolled or difficult-to-predict variations in background signal are generally minimized when the magnitude of dark current is minimized. For this and other reasons, photodetectors having high sensitivity and low dark current may be desired in many embodiments.
Example embodiments may provide photoconductors and arrays of photodetectors, that possess one or more of these properties in combination as follows:
the photodetectors in example embodiments can readily be integrated with other portions of the image sensor circuit and system by straightforward, low-cost methods such as spin-coating, spray-coating, drop-coating, and self-assembly. Embodiments include exchanging ligands passivating nanoparticle surfaces for shorter ligands that will provide for appropriate charge carrier mobilities once films are formed. Embodiments include solution-phase exchanges which enable the realization of smooth-morphology films necessary to the realization of image sensors having acceptable consistent dark currents and photoresponses across an array.
The photodetectors in example embodiments can provide a maximum of sensitivity through a combination of means. They maximize signal by providing photoconductive gain. Typical values for photoconductive gain range from 1-10, resulting in responsivities in, for example, the visible wavelengths ranging from 0.4 A/W to 4 A/W. In embodiments, the photodetectors described herein minimize noise by fusing nanocrystal cores such as to ensure substantially non-noise-degrading electrical communication among the particles making up the optically sensitive layer through which current flows. In embodiments, the photodetectors described herein minimize dark current by minimizing the net doping of the active layer, thus ensuring that the dark carrier density, and thus the dark conductance, of these optically sensitive materials is minimized. In embodiments, the photodetectors described herein minimize dark current by providing an electrode-to-nanocrystalline-layer electrical connection that blocks typically one type of carrier, including potentially the majority carrier at equilibrium. In embodiments, cross-linking molecules are employed that utilize chemical functionalities that remove oxides, sulfates, and/or hydroxides responsible for p-type doping. Thus, in embodiments, a more intrinsic or even n-type optically sensitive layer may be provided, leading to lowered dark currents. In embodiments, many steps in quantum dot synthesis and/or processing and/or device packaging may be performed in a controlled environment such as a Schlenk line or Glove Box; and optically sensitive layers may be encapsulated using substantially impermeable layers such as oxides, oxynitrides, or polymers such as parylene, or epoxies, in order to prevent reactive gases such as oxygen or water from significantly permeating the optically sensitive layer. In this manner, desired combinations of properties such as gain, dark current, and lag may be preserved over the desired useful lifetime of an image sensor.
The photodetectors in example embodiments provide a time-domain response that can be as rapid as sub-100-miliseconds, sub-30-milliseconds, and sub-1-millisecond. In embodiments, this is achieved by providing gain-providing (and persistence-providing) trap states associated with the optically sensitive layer that trap at least one type of carrier for only a limited time period such as 100 milliseconds, 30 milliseconds, or 1 millisecond. In some embodiments, PbS nanoparticles are decorated with PbSO3, an oxide of PbS, which is shown to have a trap state lifetime in the vicinity of 20-30 milliseconds, providing for a transient response suited to many video imaging applications. In some embodiments, photodiodes are instead provided based on colloidal quantum dot layers, wherein two electrical contacts having appreciably different work functions are employed to contact the active layer. In some embodiments, dark currents may be minimized through operation of such devices without the application of an appreciable external voltage bias. In some embodiments, cross-linking moieties such as benzenedithiol, a bidentate linker, may be employed to remove and/or passivate certain trap states that may be present, or may develop, in such materials.
The photodetectors in example embodiments provide a means of enhancing dynamic range by producing a sublinear dependence of electrical signal (such as photocurrent). Over a region of low to middling intensities, trap states are available to become filled, and escape occurs following some moderate persistence, or trap state, lifetime, such as 30 milliseconds for example. At higher intensities, these trap states become substantially filled, such that charge carriers experience shorter lifetimes, or persistence times, corresponding to lower differential gains. As a result these devices exhibit substantially constant gains over a range of low to middling intensities, followed by a gentle roll-off in gain at higher intensities. Put another way, at low to middling intensities, photocurrent depends approximately linearly on intensity, but at higher intensities, photocurrent exhibits sublinear dependence on intensity. In embodiments, photodetectors are provided wherein photoconductive gain depends on the bias applied to a device. This arises because gain is proportional to carrier lifetime divided by carrier transit time, and transit time varies in inverse proportionality with applied field. In embodiments, circuits are developed that exploit this dependence of gain on bias to increase dynamic range.
In example embodiments, photodetectors may readily be altered, or ‘tuned’, to provide sensitivity to different spectral bands. One means of tuning is provided through the quantum size effect, whereby nanoparticle diameter is decreased, in cases through synthetic control, to increase the effective bandgap of the resulting quantum dots. Another means of tuning is provided through the choice of materials composition, wherein the use of a material having a larger bulk bandgap generally facilitates the realization of a photodetector with responsivity onset at a higher photon energy. In embodiments, photodetectors having different absorption onsets may be superimposed to form vertical pixels, wherein pixel(s) closer to the source of optical signal absorb and sense higher-energy bands of electromagnetic radiation, whereas pixel(s) further frojm the source of optical signal absorb and sense lower-energy bands.
In a particular example embodiment of a photodetector 1400, a solution-processed route to ultrasensitive photodetectors 1400 (e.g., D* ˜1013 Jones) comprises synthesizing PbS colloidal quantum dots, exchanging their organic capping ligands in solution, forming thin film photodetectors by spin-coating, and implementing a developing step to modify conductivity and modify trap states, resulting in photoconductive gains such as 1.5, 2, 5, 10, 20, 50, or 100 (See for example: Konstantatos, G. et al. Ultrasensitive solution-cast quantum dot photodetectors. Nature 442, 180-183 (2006)). The photodetector 1400 may employ an infrared or a visible bandgap.
In another embodiment of a photodetector 1400, a thin-film photodetector 1400 may be coated onto a chip. The thin-film photodetector 1400 may be more sensitive than example silicon photodiodes and may provide built-in gain which may simplify read-out circuit design. The use of a bias-dependent and/or intensity-dependent gain may enable enhanced dynamic range. The photodetector 1400 may employ colloidal quantum dots 1200 which may offer an advantage over conventional bulk semiconductors, such as crystalline, amorphous, or organic, in that the quantum dots are quantum size-effect tunable. The thin-film photodetector 1400 may comprise a stacked multi-spectral pixel which may incorporate use of the quantum size-effect tenability of the quantum dots 1200. The thin-film photodetector 1400 may be formed from quantum dot materials 200 subject to a post-film-formation treatment wherein long, insulating ligands may be exchanged for short ones, which may lead to highly sensitive visible photodetectors 1400. Thin-film post-processing may enable the integration of top-surface thin-film photodetectors 1400 with crystalline silicon. The photodetector 1400, across the entire visible spectrum, may exhibit a normalized detectivity D* ˜1013 Jones. The photodetector 1400 may exhibit photoconductive gains greater than unity, and ranging from 1.5 to 100, facilitating high-fidelity electronic read-out. The photodetector 1400 may enable between 3 dB and 20 dB greater dynamic range than provided in a constant-gain photodetector. The photodetector 1400 exploits quantum size effect tunability to demonstrate a stacked multicolor pixel.
The thin-film photodetector 1400 of example embodiments may meet demanding performance requirements including but not limited to sensitivity, gain, tunability, and dynamic range to name a few. The performance characteristics of some example embodiments are described in turn below.
Regarding sensitivity, a top-surface visible-wavelength photodetector 1400 for visible imaging may ideally meet or exceed silicon's sensitivity. One measure of sensitivity may be the noise-equivalent power NEP, the minimum incident optical power that a device can distinguish from noise. However, since noise current in a photodetector may not scale linearly with area, a quantity known as the normalized detectivity D*, with units of cmHz1/2 W−1 (“Jones”), is defined. The figure of merit D* is equal to the square root of the optically active area of the detector divided by its noise equivalent power (NEP). D* enables comparison between photodetectors of different areas.
Regarding gain, example silicon photodiodes may provide up to one electron's worth of current per incident photon. Under low-light conditions, and especially in small pixels, this may necessitate the use of extremely low-noise electronic amplifying circuits. Ideally, a photodetector would provide built-in gain, allowing for multiple carriers to be collected in pixel circuitry per absorbed photon, thus lessening demands on read-out circuitry. For example, in photoconductive photodetectors 1400, the creation of an electron-hole pair upon absorption of a photon may result in an increase in conductivity that persists for the excited carrier lifetime.
As for tunability, quantum size-effect tuning may enable selecting the semiconductor bandgap according to the needs of the application. For visible imaging, the optimal-bandgap sensor material would absorb only visible light, obviating the need for an infrared cutoff filter and, by maximizing bandgap, minimizing recombination-generation noise. A tunable materials system also enables a stacked pixel architecture, wherein a large-bandgap photodetector senses, and also filters out, higher-energy light, passing only the longer-wavelength light to the independently-read pixel elements beneath. Such architectures offer improved low-light sensitivity compared to (lossy) absorbent color filter arrays.
For dynamic range, the ratio of full-well capacity of CMOS image sensors to the number of dark electrons limits dynamic range to about 3 orders of magnitude in intensity (conventionally expressed as 60 dB in imaging). A photodetector 1400 may provide at least this much dynamic range. A photodetector 1400 may, by virtue of its bias-tunable gain and/or its intensity-dependent gain, allow controlled signal compression for dynamic range enhancement.
Nanocrystal material exhibiting the above properties and methods of making the same are described in further detail below.
Quantum Dot Pixel 1800 In the example embodiment of FIG. 1, the nanocrystal materials and photodetector structures described above may be used to provide quantum dot pixels 1800 for a photosensor array, image sensor or other optoelectronic device. In example embodiments, the pixels 1800 include quantum dot structures 1100 capable of receiving radiation 1000, photodetectors structures adapted to receive energy from the quantum dot structures 1100 and pixel structures. The quantum dot pixels described herein can be used to provide the following in some embodiments: high fill factor, potential to bin, potential to stack, potential to go to small pixel sizes, high performance from larger pixel sizes, simplify color filter array, elimination of de-mosaicing, self-gain setting/automatic gain control, high dynamic range, global shutter capability, auto-exposure, local contrast, speed of readout, low noise readout at pixel level, ability to use larger process geometries (lower cost), ability to use generic fabrication processes, use digital fabrication processes to build analog circuits, adding other functions below the pixel such as memory, A to D, true correlated double sampling, binning, etc. Example embodiments may provide some or all of these features. However, some embodiments may not use these features.
FIG. 7 c shows a schematic cross-section of an example imaging system shown in FIG. 8. In an example embodiment, the QD Layer 1820 may include a continuous film of cross-linked nanocrystal cores, such as linked PbS nanocrystal cores, with trap states formed using oxidized core material, such as PbSO3. The QD Layer 1820 may provide photoconductive gain and have a non-linear relationship between the rate of current flow through QD Layer 1820 and the intensity of light absorbed by the QD Layer 1820.
In example embodiments, the imaging system of FIG. 7 c includes a read-out structure that includes a substrate 1812, an optically sensitive layer 1820, e.g., QD layer 1820 and a transparent electrode 1818. Substrate 1812 includes a read-out integrated circuit (ROIC) having a top surface with an array of pixel electrodes 1814 located in the top surface thereof with counter-electrodes 1818 located outside the array, i.e., transparent electrode 1818 overlaying QD layer 1820. Electrodes 1814 shown in FIG. 7 c correspond to square electrode pads 300 shown in FIG. 8. The array of electrodes 1814, which together form focal plane array 1810, provide voltages for biasing and current collection from a given pixel 1814 of the array, and convey signals from the array to an input/output electrode 1812 (connection not shown). Optically sensitive layer 1820, e.g., QD layer, is formed on the top surface of the integrated circuit. More specifically, QD layer 1820 overlays the array of pixel electrodes 1814 on the top surface of the integrated circuit. The optically sensitive layer 1820 defines an array of imaging pixels for collecting radiation 1000 incident thereon.
In the imaging system of FIG. 7 c, QD layer 1820 is monolithically integrated directly onto the electronic read-out chip.
Referring now to FIG. 7 d, there is shown at 1822 a side view of a basic optical device structure, which in certain embodiments can be used as an individual pixel in the completed integrated array shown in FIGS. 7 d-f. Device 1822 includes substrate 1824, which may be glass or other compatible substrate; contact/electrode 1828; optically sensitive layer, e.g., QD layer 1832; and at least partially transparent contact 1830 that overlays the QD layer. As above, QD layer 1832 may be a continuous film of cross-linked nanocrystal cores with trap states formed using oxidized core material in example embodiments. Contacts 1828 and 1830 may include, e.g., aluminum, gold, platinum, silver, magnesium, copper, indium tin oxide (ITO), tin oxide, tungsten oxide, and combinations and layer structures thereof, and may include band-pass or band-block filters that selectively transmit or attenuate particular regions of the spectrum which are appropriate to the end use of the device. The device has an overall “vertical sandwich” architecture, where different components of the device generally overlay other components. In operation, the amount of current that flows and/or the voltage between contact 1830 and contact 1828 is related to a number of photons received by QD layer 1832. In operation, current generally flows in the vertical direction. The embodiment shown in FIG. 7 d may also include one or more additional optional layers for electron/hole injection and/or blocking. The layer(s) allows at least one carrier to be transported, or blocked, from an electrode into the QD layer. Examples of suitable layers include a QD layer including QDs of a different size and/or composition, semiconducting polymers, and semiconductors such as ITO and Si.
Referring now to FIG. 7 e, there is shown at 1822 a a side view of a basic device structure that has a different configuration than each pixel in the completed integrated array shown in FIGS. 7 d-7 f, but which could be used to form a similarly functioning optical device. As above, the QD layer 1832 a may be a continuous film of cross-linked nanocrystal cores with trap states formed using oxidized core material in example embodiments. The configuration in FIG. 7 e corresponds to a lateral planar structure in which the optically sensitive layer 1832 a is deposited across two spaced contacts/electrodes 1828 a and 1832. Contacts 1828 and 1832 are deposited on a substrate, e.g., glass substrate 1824 b. The integrated circuit, including contacts 1828 a, 1832, and substrate 1824 b may include any appropriate system with which the optically sensitive material is compatible (e.g., Si, glass, plastic, etc.). Contacts 1828 b and 1832 may include aluminum, gold, platinum, silver, magnesium, copper, tungsten, tantalum, tungsten nitride, tantalum nitride, indium tin oxide (ITO), tin oxide, tungsten oxide, or combinations or layer structures thereof. The device has an overall “lateral planar” architecture, where at least some of the components of the device are generally laterally displaced from other components, forming a planar electrode structure. In operation, the amount of current that flows and/or the voltage between contact 1828 a and contact 1832 is related to a number of photons received by the QD layer 1832 a. In operation, current generally flows in the lateral direction.
FIG. 7 f shows a plan view of another basic device structure 1822 b that includes interdigitated electrodes, and which also can be used to form an optical device. The materials may be selected from those provided above regarding FIGS. 7 d-7 e. As above, QD layer 1832 b may be a continuous film of cross-linked nanocrystal cores with trap states formed using oxidized core material in example embodiments.
Each basic device 1822, 1822 a, and 1822 b as shown in FIGS. 7 d-7 f, among other possible architectures, can be thought of as representing a single device or an element in a larger device, such as in a linear array or a two-dimensional array. The basic devices can be used in many kinds of devices, such as detection and signal processing, as discussed above, as well as in emission and photovoltaic devices. Not all embodiments need be optical devices. Many QD layers have optical characteristics that can be useful for optical devices such as one or more of: image sensors useful in one or more radiation 1000 bands of the electromagnetic spectrum; optical spectrometers including multispectral and hyperspectral; communications photodetecting optical receivers as well as free-space optical interconnection photoreceivers; and environmental sensors. Some QD layers also have electrical characteristics that may be useful for other kinds of devices, such as transistors used in signal processing, computing, power conversion, and communications.
In one embodiment, the underlying electrodes on the integrated circuit define imaging pixels in an imaging device. The QD layers formed on the electrodes supply optical-to-electrical conversion of incident radiation 1000.
Further layers may be included in the layers atop the structure, such as electrical layers for making electrical contact (e.g. an at least partially transparent contact such as indium tin oxide, tin oxide, tungsten oxide, aluminum, gold, platinum, silver, magnesium, copper, or combinations or layer structures thereof), antireflection coatings (e.g. a series of dielectric layers), or the formation of a microcavity (e.g. two mirrors, at least one formed using nonabsorbing dielectric layers.), encapsulation (E.G. a substantially transparent dielectric such as silicon nitride (Si3N4) or silicon dioxide, or e.g. an epoxy or other material to protect various materials from environmental oxygen or humidity), or optical filtering (e.g. to allow one radiation 1000 band to pass and a second radiation 1000 band not to)
The above quantum dot pixels are examples only and other embodiments may use other pixel structures.
Pixel Circuitry 1700 (Including Other Components 1900)
Pixel circuitry that may be used to read out signals from the photoconductive pixel regions will now be described. As described above, in embodiments, pixel structures 1500 within the QDPC 100 of FIG. 1 may have pixel layouts 1600, where pixel layouts 1600 may have a plurality of layout configurations such as vertical, planar, diagonal, or the like. Pixel structures 1500 may also have embedded pixel circuitry 1700. Pixel structures may also be associated with the electrical interconnections 1404 between the photodetector structures 1400 and pixel circuitry 1700.
Pixel circuitry 1700 within the quantum dot pixel 1800 may take a plurality of forms, ranging from no circuitry at all, just interconnecting electrodes, to circuitry that provides functions such as biasing, resetting, buffering, sampling, conversion, addressing, memory, and the like. In embodiments, electronics to condition or process the electrical signal may be located and configured in a plurality of ways. For instance, amplification of the signal may be performed at each pixel, group of pixels, at the end of each column or row, after the signal has been transferred off the array, just prior to when the signal is to be transferred off the chip 2000, or the like. In another instance, analog-to-digital conversion may be provided at each pixel, group of pixels, at the end of each column or row, within the chip's 2000 functional components 2004, after the signal has been transferred off the chip 2000, or the like. In addition, processing at any level may be performed in steps, where a portion of the processing is performed in one location and a second portion of the processing is performed in another location. An example may be the performing analog-to-digital conversion in two steps, say with an analog combining at the pixel 1800 and a higher-rate analog-to-digital conversion as a part of the chip's 2000 functional components 2004.
Pixel circuitry may be defined to include components beginning at the electrodes in contact with the quantum dot material 200 and ending when signals or information is transferred from the pixel to other processing facilities, such as the functional components 2004 of the underlying chip 200 or another quantum dot pixel 1800. Beginning at the electrodes on the quantum dot material 200, the signal is translated or read. In embodiments, the photoconductive photovoltaic quantum dot material 200 may act as a resistor that changes its resistance in response to radiation 1000. As such, the quantum dot pixel 1800 may require bias circuitry 1700 in order to translate the resistance to a readable signal. This signal in turn may then be amplified and selected for readout. One embodiment of a pixel circuit shown in FIG. 6 a uses a reset-bias transistor 1802, amplifier transistor 1804, and column address transistor 1808. This three-transistor circuit configuration may also be referred to as a 3T circuit. Here, the reset-bias transistor 1802 connects the bias voltage 1702 to the photoconductive photovoltaic quantum dot material 200 when reset 1704 is asserted, thus resetting the electrical state of the quantum dot material 200. After reset 1704, the quantum dot material 200 may be exposed to radiation 1000, resulting in a change in the electrical state of the quantum dot material 200, in this instance a change in voltage leading into the gate of the amplifier 1804. This voltage is then boosted by the amplifier transistor 1804 and presented to the address selection transistor 1808, which then appears at the column output of the address selection transistor 1808 when selected. In some embodiments, additional circuitry may be added to the pixel circuit to help subtract out dark signal contributions. In other embodiments, adjustments for dark signal can be made after the signal is read out of the pixel circuit.
In embodiments, the impedance of the quantum dot material 200 may be very high, such as in the one teraohm range, making integration times for a dark scene in the milliseconds range, while the integration times for a bright scene may be in the microseconds range. Additionally, the response curve may not be linear, resulting in an S-shaped response curve. This type of response curve may require modification of how the 3T circuit is used, relative to what is conventional for a photodiode configuration. For instance, a conventional 3T reset is digital, with the reset off during integration and the totally on during reset. The reset of the QDPC 100 3T circuit, is analog, varying in voltage, for example in some embodiments from 0V to 1.8V continuously, biasing to get a particular shape and optimization of response. Reset for the QDPC 100 circuit may also be turned on harder during reset, and weaker during integration, but may always be supplying a bias voltage, which may provide greater control of the circuit. Also, as mentioned previously, a photodiode may have a very large capacitance, say 50-100 femtofarads, with a gain of less than unity. The QDPCs 100 quantum dot material 200 has a very low capacitance, say less than one femtofarad, with a large range of gain, say between 0.1 and 10,000, depending on the bias.
Control timing of the conventional 3T circuit is characterized by fixed voltages and timing, such as the reset voltage turning on hard and then back down to zero at the end of a frame interval, and the timing between frames being fixed. In the QDPC 100 3T circuit, reset voltages may vary. Also, the timing is variable for the QDPC because the quantum dot material 200 response curve is not linear as it is for a photodiode, where the slope is directly dependent on the amount of radiation 1000 and saturation can be reached. In the quantum dot material 200 response, the relationship is exponential, where strong radiation 1000 may produce a sharp decay and a weak radiation 1000 may produce a very slow decay. Because of this response, there is not voltage level where saturation is reached, and thus there is no limit on the dynamic range. For instance, over a range of one nanowatt to one milliwatt of radiation 1000 intensity, the QDPC 100 may experience a 360 millivolt output range, which is easy to read.
Although additional circuitry may not be required for the low noise operation of the QDPC 100 in some embodiments, a fourth transistor circuit group may be added to increase sensitivity. FIG. 6 b shows a QDPC 100 4T circuit, with the fourth transistor 1708 configured as a sample and hold. This configuration may also be referred to as a global shutter, where the entire pixel 1800 array may be sampled at the same time, but not necessarily at a f