Photodetector and sensor device

Abstract

A photodetector (10) comprises an array comprising a plurality of microwells (110) formed in a first main surface (101) of a substrate (100). The photodetector further comprises a first contact layer (121) formed over sidewalls (103) of the plurality of microwells (110), a first charge carrier transport layer (122) over the first contact layer (121) and a first quantum dot layer (123) of a first conductivity type over the first charge carrier transport layer (122). Additionally, the photodetector (10) comprises a second quantum dot layer (124) of a second conductivity type over the first quantum dot layer (123), a second charge carrier transport layer (125) over the second quantum dot layer (124), and a second contact layer (126) in contact with the second charge carrier transport layer (125).

Description

[0001] PHOTODETECTOR AND SENSOR DEVICE

[0002] Infrared photodetectors working in a wavelength range of 750 nm or more , e . g . 750 nm to 12 pm, are applied in a variety o f applications for modern information technology . For example , IR photodetectors are used for remote sensing, military and general surveillance , astronomy, medical examination, environmen- tal / hazardous gas monitoring and others . Generally, ef forts are taken to improve the performance and cost ef ficiency of IR photodetectors .

[0003] It is an obj ect to provide an improved photodetector and an improved sensor device .

[0004] SUMMARY

[0005] According to embodiments , the above obj ect is achieved by the claimed matter according to the independent claims . Further developments are defined in the dependent claims .

[0006] According to embodiments , a photodetector comprises an array comprising a plurality of microwells formed in a first main surface of a substrate . The photodetector further comprises a first contact layer formed over sidewalls of the plurality of microwells , a first charge carrier transport layer over the first contact layer, and a first quantum dot layer of a first conductivity type over the first charge carrier transport layer . Additionally, the photodetector comprises a second quantum dot layer of a second conductivity type over the first quantum dot layer, a second charge carrier transport layer over the second quantum dot layer, and a second contact layer in contact with the second charge carrier transport layer . For example, the microwells may have a shape of a cone, respectively. According to embodiments, an opening angle of the cone may be larger than 15°, e.g. larger than 45°. Further, the opening angle may be less than 80°, e.g. less than 60°.

[0007] For example, the microwells may overlap in a region of the first main surface.

[0008] According to embodiments, a material of the substrate may be selected from borosilicate glass, aluminum borosilicate glass, lithium aluminosilicate, aluminosilicate, silicon, quartz, silicon oxide, and sapphire.

[0009] For example, the first contact layer may comprise a metal.

[0010] According to further embodiments, the first contact layer is transparent for electromagnetic radiation to be detected by the photodetector. Examples of a material of the first contact layer may be selected from indium tin oxide (ITO) , Al doped zinc oxide (AlxZn2-xO2) , Indium gallium zinc oxide (IGZO) , transition metal doped SnO2, BaSnO3, SrVO3, CaVO3, Sr1-xLaxGeO3(x=0 to 0.2) , Ta- doped Sni-xGexO2(x=0 to 1) .

[0011] For example, the first and second quantum dot layers may comprise PbS and InAs embedded in an organic material.

[0012] According to embodiments, the photodetector may be configured to detect radiation having a wavelength larger than 700 nm.

[0013] For example, the photodetector may be implemented as a front side illumination or as a back side illumination photodetector. According to embodiments , a sensor device comprises the photodetector as defined above and a semiconductor substrate comprising a readout circuit .

[0014] For example , the sensor device may further comprise a silicon oxide substrate over the semiconductor substrate . The microwells may be formed in the silicon oxide substrate .

[0015] According to embodiments , a method of manufacturing a photodetector comprises forming a plurality of microwells in a first main surface of a substrate . The method further comprises forming a first contact layer over sidewalls of the plurality of microwells , forming a first charge carrier transport layer over the first contact layer, and forming a first quantum dot layer of a first conductivity type over the first charge carrier transport layer . Additionally, the method comprises forming a second quantum dot layer of a second conductivity type over the first quantum dot layer, forming a second charge carrier transport layer over the second quantum dot layer, and forming a second contact layer in contact with the second charge carrier transport layer .

[0016] The accompanying drawings are included to provide a further understanding of embodiments of the invention and are incorporated in and constitute a part of this speci fication . The drawings illustrate the embodiments of the present invention and together with the description serve to explain the principles . Other embodiments of the invention and many of the intended advantages will be readily appreciated, as they become better understood by reference to the following detailed description . The elements of the drawings are not necessarily to scale relative to each other . Like reference numbers designate corresponding similar parts .

[0017] BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is a perspective view of a substrate in which a photodetector may be formed.

[0018] Fig. IB shows a further example of a perspective view of a substrate in which a photodetector may be formed.

[0019] Fig. 1C shows a cross-sectional view of a substrate.

[0020] Fig. ID shows a cross-sectional view of a microwell that may be formed in a substrate.

[0021] Fig. 2A is a vertical cross-sectional view of a photodetector according to embodiments.

[0022] Figs. 2B to 2E are cross-sectional views of a portion of a photodetector according to embodiments.

[0023] Fig. 3 shows a vertical cross-sectional view of a sensor device according to embodiments.

[0024] Fig. 4 summarizes a method according to embodiments.

[0025] DETAILED DESCRIPTION

[0026] In the following detailed description reference is made to the accompanying drawings, which form a part hereof and in which are illustrated by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology such as "top", "bottom", "front", "back", "over", "on", "above", "leading", "trailing" etc. is used with reference to the orientation of the Figures being described. Since components of embodiments of the invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope defined by the claims.

[0027] The description of the embodiments is not limiting. In particular, elements of the embodiments described hereinafter may be combined with elements of different embodiments.

[0028] The terms "wafer" or "semiconductor substrate" used in the following description may include any semiconductor-based structure that has a semiconductor surface. Wafer and structure are to be understood to include silicon, silicon-on-insulator (SOI) , silicon-on sapphire (SOS) , doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. The semiconductor need not be silicon-based. The semiconductor could as well be silicon-germanium, germanium, or gallium arsenide. According to other embodiments, silicon carbide (SiC) , gallium nitride (GaN) or gallium oxide (Ga2Os) may form the semiconductor substrate material .

[0029] Generally, the term "substrate" may refer to an insulating substrate, e.g. a glass substrate or a silicon oxide substrate, a semiconductor substrate or a conductive substrate.

[0030] The term "vertical" as used in this specification intends to describe an orientation which is arranged perpendicular to the first surface of a substrate or semiconductor body.

[0031] The terms "lateral" and "horizontal" as used in this specification intends to describe an orientation parallel to a first surface of a substrate or semiconductor body. This can be for instance the surface of a wafer or a die. Fig. 1A shows a perspective view of a portion of a substrate 100 in which a photodetector according to embodiments may be formed. A plurality of microwells 110 are formed in a first main surface 101 of the substrate 100. The substrate 100 may e.g. be made of a glass such as a borosilicate glass, aluminum borosilicate glass, lithium aluminumsilicate, aluminosilicate, quartz, SiCy, sapphire and others. According to further implementations, the substrate may also be a silicon or other semiconductor substrate. Depending on whether the photodetector to be formed is a backside illuminated (BSI) or a front-side illuminated (FSI) photodetector, the substrate 100 may be transparent to IR radiation or not.

[0032] The microwells 110 may e.g. have a shape of an inverted cone. For example, a circumference 102 of the microwell 110 may be circular. As is clearly to be understood, any other suitable shape of a circumference, e.g. polygonal, may be selected. Generally, the term "microwell" is intended to refer to a cavity formed in the first main surface 101 of the substrate 100. For example, the microwell 110 may have sidewalls 103 that extend in an oblique direction, e.g. a direction that is different from a vertical direction. For example, the microwell 110? may have a shape so that sidewalls 103 of the microwell intersect. For example, a bottom portion of the microwell may be absent. In this manner, incident radiation may be prevented from being reflected back, e.g. by the bottom side. Rather, due to reflections by the sidewalls 103, incident radiation may be "caught" inside the microwell 110. The microwells 110 may have an arbitrary symmetry .

[0033] For example, the microwells 110 may be arranged in a hexagonal pattern. As is further shown in Fig. 1A, the microwells 110 may be arranged so that they do not overlap. According to further examples, as is e.g. shown in Fig. IB, the microwells 110 may overlap. In this way, planar surface 101 is reduced between the adjacent microwells resulting in higher absorption (lower reflection) of incoming radiation / photons . For example, a distance between adjacent microwells 110 may be less than 50 pm. A diameter of the circumference 102 at the first main surface 101 of the substrate may be larger than 2 pm and less than 100 pm. A height h of the microwells 110 measured in the vertical direction, e.g. the z-direction, may be larger than 2 pm. The height of the microwells 110 may be less than 100 pm. A ratio of diameter d to height h may be 1:0.1 to 1: 6. For example, the ratio of diameter to height may be in a range of approximately 1:1. Increasing the aspect ratio will generally improve the net absorption of IR light by the pixel, and therefore the quantum efficiency.

[0034] Fig. 1C shows a vertical cross-sectional view of the substrate 100 comprising a plurality of microwells 110. An opening angle a of the cones may be larger than 15° . The angle a may be less than 80°. For example, the angle a may be larger than 45° and less than 60°. For example, the microwells 110 may be formed on wafer level for mass production using an etching process, e.g. laser-induced deep etching (LIDE) . For example, a sidewall 103 of the microwells 110 may be oblique with respect to a vertical direction. As a consequence, the absorption of impinging photons may be increased.

[0035] Fig. 2A shows a vertical cross-sectional view of a photodetector 10 according to embodiments. As is illustrated, the photodetector 10 comprises a first contact layer 121 which is formed in the plurality of microwells. For example, the first contact layer 121 may comprise a metal or plurality of metals. According to further embodiments, the first contact layer comprises a transparent conductive material. Examples of a metal of the first contact layer 121 may comprise Ag, Au, TiN, Al, Cr, Ni, Ti, Sn, Cu and others. Examples of a transparent conductive material for the first contact layer 121 comprise indium tin oxide (ITO) , zinc oxide which is doped with Al, indium gallium zinc oxide (IGZO) , BaSnOa, SrVO3, CaVO3, SrGeO3, and Ta-doped Sn!-xGexO2. The photodetector further comprises a first majority charge carrier transport layer 122 which is formed over the first contact layer 121. The first majority charge carriers may for example be holes. Accordingly, the first charge carrier transport layer 122 may be implemented as a hole transport layer. Examples of the hole transport layer comprise NiOx, Poly (N, N ' -bis-4-butylphenyl-N, N ' - bisphenyl ) benzidine (poly-TPD) , Cu-doped ZnO and others.

[0036] The photodetector 10 further comprises a first quantum dot layer 123 of a first conductivity type, e.g. p-type which is formed over the first charge carrier transport layer 122. Moreover, the photodetector comprises a second quantum dot layer 124 of a second conductivity type, e.g. n-type, over the first quantum dot layer 123. For example, the first quantum dot layer 123 may comprise PbS or InAs quantum dots. For example, further ligands such as oleate, octylamine, dodecylamine, octadecylamine, 2- mercaptopropionic acid (MPA) , 1 , 2-ethanedithiol (EDT) , 1,4-ben- zenedithiol (BDT) , or halides, e.g. Br-, C1-, and I-based ligands may be attached to the PbS or InAs quantum dots. For example, the quantum dot layer may be exposed to oxygen to achieve p-type conductivity. Moreover, according to embodiments the second quantum dot layer 124 comprises PbS and optionally any of the above-mentioned ligands. For example, the second quantum dot layer 124 is not exposed to oxygen to achieve n-type conductivity. The size of the quantum dots and the process parameters, e.g. temperature, may be adjusted to set an absorption wavelength. For example, the quantum dots of layers 123, 124 may be embedded in an organic material. The band gap may be adjusted by changing or the size of the quantum dot. Within the present disclosure, the term "quantum dots" ("QDs" also referred to as semiconductor nanocrystals) refers to small crystals made of II-VI-, III-V-, IV-V-materials , which typically have a diameter of 1 nm to 20 nm, corresponding to the range of the de-Broglie wavelength of the charge carriers. The energy difference of the charge carrier states of a quantum dot is a function of the composition as well as of the physical size of the quantum dots. In other words, with a given material, by varying the size, the absorption spectrum of the quantum dots may be varied. As a consequence, by using quantum dots, a large range of wavelengths may be absorbed or detected.

[0037] Usually, the quantum dots may comprise a core material, that is surrounded by a coating material. The band gap of the semiconductor core material may be smaller than the band gap of the semiconductor coating material. For example, the core may be made up of CdSe and the coating may comprise CdS as well as optionally further layers. According to further embodiments, the core may be composed of InP and the coating includes ZnS and, optionally, further layers. According to further embodiments, the coating may include an oxide. Powders made from these quantum dots nanoparticles are commercially available. Basically, quantum dots may be chalcogenide or perovskite type structures comprise one or more of the following materials: CdS, CdSe, CdTe, CdPo, ZnS, ZnSe, ZnTe, ZnPo, HgS, HgSe, HgTe, InTe, MgS, MgSe, MgTe, PbSe, PbS, PbTe, GaN, GaP, GaAs, InP, InAs, CuInS2, CuInSe2, CdSi-xSe, ABX3(A = Cs, formamidinium CH(NH2)2or methylammonium CH3NH3; B= Pb,Sn; X=C1, I, Br) , BaTiO3, PbZrO3, PbZrxTi2-xO3 , BaxSr2-x, CaTiOs, SrTiOs, LaMnOs, CaMnOs and Lai-xCaxMnO3.

[0038] For example, the quantum dots can be cured into silicone or another suitable matrix material and may appear as a yellow powder . The photodetector 10 further comprises a second charge carrier transport layer 125, e.g. an electron transport layer which is arranged over the second quantum dot layer 124. The second charge carrier transport layer 125 may be implemented as an electron transport layer. Examples of materials comprise Nb-doped TiCy, MO03and others. The photodetector 10 further comprises a second contact layer 126 over the second charge carrier transport layer 125. For example, the second contact layer 126 may comprise a transparent material or a metal or plurality of metals, e.g. Ag, Au, TiN, Al, Cr, Ni and others.

[0039] For example, the quantum dot layers 123, 124 may be formed by spin coating over the patterned substrate 100. According to further embodiments, different manufacturing methods such as physical vapor deposition e.g. e-beam or thermal evaporation may be employed .

[0040] According to further embodiments, a light-absorbing layer 130, e.g. a polymer film may be arranged over the layer stack, e.g. over the second contact layer 126. Further, random or engineered surface structures may be arranged over the substrate or over the layer stack for forming the photodetector. For example, nano- or micro-photonic structures may be formed over the layer stack of the photodetector, e.g. using inkjet printing, embossing, nano-imprinting, photolithography, micro-stereolithography, laser etching / machining and others. The photodetector 10 may further comprise a bandpass filter 131 which comprises a multilayer dielectric layer stack that may implement a distributed Bragg reflector. For example, the mirror may comprise an alternating sequence of layers having comparably high and low refractive indices. For example, the layers may be dielectric layers. The alternating sequence of layers may form a DBR ("distributed Bragg reflector") mirror. Accordingly, the rear mirror stack may be insulating and may have a high reflectivity.

[0041] The distributed Bragg reflector may be configured to selectively absorb or reflect specific wavelengths. In this manner, absorption in the desired wavelength range, e.g. infrared radiation, may be improved.

[0042] The photodetector may further comprise a reflector 128 neighboring the first contact layer 121. For example, the reflector 128 may comprise a metallic reflector layer, e.g. an Au coating which is arranged over the sidewalls 103 of the microwell 110. Due to the large reflectance of Au for IR radiation, of e.g. 97 to 98 %, multiple reflections within the microwells 110 may be achieved. In this way, radiation losses of the first contact element 120 may be reduced. According to further examples, the reflector 128 may comprise an Al layer having a thickness larger than 50 nm, e.g. instead of the Au coating. For example, the thickness of the Al layer may be less than 500 nm. Generally, a metallic reflector may be fabricated in an easier manner on the sidewalls 103 of the microwell 110 than a distributed Bragg reflector .

[0043] Moreover, the photodetector 10 may comprise coating 129 which may further reduce losses at the first contact layer 121. For example, the coating 129 may comprise a very thin metal layer, e.g. Al at a thickness of less than 5 nm, which is partially transparent to IR radiation so as to minimize optical losses to the stack. According to further examples, a thin dielectric layer, e.g. an SiCy layer having a thickness of 10 nm to 1000 nm may be used as the coating 129.

[0044] According to further implementations, the first contact layer 121 may comprise a material which is transparent for IR radiation and which is matched with the first charge carrier transport layer 122 to provide an ef ficient ohmic contact . For example , ITO or another transparent conductive metal oxide may be used as a material for the first contact layer 121 . In thi s case , the reflector 128 may comprise a multilayer Bragg reflector over the sidewalls 103 of the microwells 110 . The multilayer Bargg reflector may be arranged between the sidewall 103 and the first contact layer 121 .

[0045] As is further illustrated in Fig . 2A, the photodetector 10 may further comprise a first contact element 120 in electrical contact with the first contact layer 121 , and a second contact element 127 in electrical contact with the second contact layer 126 . A bias voltage may be applied to the photodetector 10 via the first and second contact elements 120 , 127 . Further, an electrical signal corresponding to an intensity of electromagnetic radiation detected by the photodetector 10 may be sensed via the first and the second contact elements 120 and 127 , respectively . For example , according to embodiments all pixels of the photodetector may be biased using one first contact element 120 and one second contact element 127 .

[0046] As is illustrated in Fig . 2B, the first contact element 120 and the second contact element 127 may be implemented as metal electrodes , e . g . Au electrodes which may be deposited as contact probes by developing photoresist masks . For example , such a photodetector could be bonded to a silicon substrate including a readout circuit , e . g . a CMOS readout circuit when the layer stack is e . g . reversed for individual pixel control . For example , the photodetector 10 may implement a backside illumination device wherein electromagnetic radiation 15 incident from a side of the substrate 100 is detected . Fig. 2C shows a further example of a portion of a photodetector, in which the first contact element 120 is implemented as a metal electrode which may be e.g. deposited from the bottom using laser-induced deep etching (LIDE) by creating through via holes. The second contact element 127 may be arbitrarily implemented. This configuration of a photodetector could be bonded to a substrate, e.g. a silicon substrate including a readout circuit.

[0047] Fig. 2D shows a further example of a photodetector, in which an array of microwells 110 is connected and operated via common electrode contacts. For example, as is shown, there may be a first contact element 120, and a second contact element 127 for addressing an array of single pixels 11 or microwells.

[0048] Fig. 2E shows a further example of a portion of a photodetector. As is illustrated, an array of microwells 110 or pixels 11 is connected and operated via a common second contact element 127 which may be implemented as an electrode, e.g. a metal electrode contact. The photodetector 10 may further comprise a plurality of first contact elements 120. The plurality of first contact elements may be assigned to single pixels 11, respectively. Accordingly, by applying a voltage to corresponding ones of the first contact elements 120, single pixels 11 may be addressed. This configuration may also be bonded to a substrate including a readout circuit.

[0049] Fig. 3 shows a cross-sectional view of a portion of a sensor device 20 comprising the photodetector 10 which has been described above. The photodetector 10 may be implemented as a front side illumination sensor, wherein electromagnetic radiation 15 is incident from a side remote from the substrate 100. The sensor device 20 may further comprise a semiconductor substrate 108, e.g. a silicon substrate. A readout circuit 107 may be arranged in the semiconductor substrate 108. For example, the readout circuit 107 may be implemented as a CMOS readout circuit. The readout circuit 107 may e.g. comprise a floating diffusion 109 and other components used in CMOS readout circuitry. For example, the photodetector 10 may be formed in an SiO2 substrate 104. The SiO2 substrate 104 may be attached, e.g. bonded to the semiconductor substrate 108. As is illustrated in Fig. 3, the photodetector 10 may be implemented in a manner as e.g. illustrated in Fig. 2E. In this manner, a contact to the first contact element 120 may be easily accomplished.

[0050] As has been described above with reference to e.g. Fig. 2B, the photodetector may also be reversed and may be attached to the semiconductor substrate 108. A plurality of metal layers 105 may e.g. be arranged in the SiCy substrate 100. For example, the sensor device 20 may further comprise a section including circuitry 22. Moreover, the sensor device 20 may comprise a readout portion 23 comprising e.g. contact pads 111 that may be electrically connected to the readout circuit 107 arranged in the circuitry portion 22, e.g. via metal layers 105.

[0051] Fig. 4 summarizes a method according to embodiments. A method of manufacturing a photodetector comprises forming (S100) a plurality of microwells in a first main surface of a substrate, forming (S110) a first contact layer in the plurality of microwells, forming (S120) a first charge carrier transport layer over the first contact layer, and forming (S130) a first quantum dot layer of a first conductivity type over the first charge carrier transport layer. The method further comprises forming (S140) a second quantum dot layer of a second conductivity type over the first quantum dot layer, forming (S150) a second charge carrier transport layer over the second quantum dot layer, and forming (S160) a second contact layer in contact with the second charge carrier transport layer. While embodiments of the invention have been described above , it is obvious that further embodiments may be implemented . For example , further embodiments may comprise any subcombination of features recited in the claims or any subcombination of elements described in the examples given above . Accordingly, this spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein .

[0052] LIST OF REFERENCES photodetector pixel electromagnetic radiation sensor device circuitry readout portion substrate first main surface circumference of microwell sidewall of microwell silicon oxide layer metal layers readout circuit semiconductor substrate floating di f fusion microwell contact pad first contact element first contact layer first charge carrier transport layer first quantum dot layer second quantum dot layer second charge carrier transport layer second contact layer second contact element reflector coating light-absorbing layer bandpass filter

Claims

CLAIMS1. A photodetector (10) comprising: an array comprising a plurality of microwells (110) formed in a first main surface (101) of a substrate (100) ; a first contact layer (121) formed over sidewalls (103) of the plurality of microwells (110) ; a first charge carrier transport layer (122) over the first contact layer (121) ; a first quantum dot layer (123) of a first conductivity type over the first charge carrier transport layer (122) ; a second quantum dot layer (124) of a second conductivity type over the first quantum dot layer (123) ; a second charge carrier transport layer (125) over the second quantum dot layer (124) , and a second contact layer (126) in contact with the second charge carrier transport layer (125) .

2. The photodetector (10) according to claim 1, wherein the microwells (110) have a shape of a cone, respectively.

3. The photodetector (10) according to claim 2, wherein an opening angle of the cone is larger than 45° and less than60° .

4. The photodetector (10) according to any of the preceding claims, wherein the microwells (110) overlap in a region of the first main surface (101) .

5. The photodetector (10) according to any of the preceding claims, wherein a material of the substrate (100) is selected from borosilicate glass, aluminum borosilicate glass, lithium aluminumsilicate, aluminosilicate, silicon, quartz, silicon oxide, and sapphire.

6. The photodetector (10) according to any of the preceding claims, wherein the first contact layer (121) comprises a metal .

7. The photodetector (10) according to any of claims 1 to 5, wherein the first contact layer (121) is transparent for electromagnetic radiation (15) to be detected by the photodetector (10) .

8. The photodetector (10) according to claim 7, wherein a material of the first contact layer (121) is selected from indium tin oxide (ITO) , Al doped zinc oxide (AlxZn!-x02) , Indium gallium zinc oxide (IGZO) , transition metal doped SnO2, BaSnOa, SrVO3, CaVOa, Sr1-xLaxGeO3(x=0 to 0.2) , Ta-doped Sn!-xGexO2(x=0 to 1 ) .

9. The photodetector (10) according to any of the preceding claims, wherein the first and second quantum dot layers (123, 124) comprise PbS and InAs embedded in an organic material .

10. The photodetector (10) according to any of the preced- ing claims being configured to detect radiation (15) having a wavelength larger than 700 nm.

11. The photodetector (10) according to any of the preced- ing claims, being implemented as a front side illumination photodetector .

12. The photodetector (10) according to any of claims 1 to10, being implemented as a back side illumination photodetector .

13. A sensor device (20) comprising the photodetector (10) according to any of the preceding claims and a semiconductor substrate (108) comprising a readout circuit (107) .

14. The sensor device (20) according to claim 13, further comprising a silicon oxide substrate (104) over the semiconductor substrate, wherein the microwells (110) are formed in the silicon oxide substrate (104) .

15. A method of manufacturing a photodetector (10) , comprising : forming (S100) a plurality of microwells (110) in a first main surface (101) of a substrate (100) ; forming (S110) a first contact layer (121) over sidewalls (103) of the plurality of microwells (110) ; forming (S120) a first charge carrier transport layer (122) over the first contact layer (121) ; forming (S130) a first quantum dot layer (123) of a first conductivity type over the first charge carrier transport layer (122) ; forming (S140) a second quantum dot layer (124) of a second conductivity type over the first quantum dot layer (123) ; forming (S150) a second charge carrier transport layer (125) over the second quantum dot layer (124) , and forming (S160) a second contact layer (126) in contact with the second charge carrier transport layer (125) .

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