OPTOELECTRONIC APPARATUS AND FABRICATION METHOD OF THE SAME

Abstract

An optoelectronic apparatus, such as a photodetector apparatus comprising a substrate (1), a dielectric layer (2), a transport layer, and a photosensitizing layer (5). The transport layer comprises at least a 2-dimensional semiconductor 5 layer (3), and the photosensitizing layer (5) comprises colloidal quantum dots. Enhanced responsivity and extended spectral coverage are achieved with the disclosed structures.

Claims

1. An optoelectronic apparatus comprising: a substrate, a dielectric layer, a transport layer, and a photosensitizing layer, wherein: the transport layer comprises at least one 2-dimensional semiconductor layer, wherein the material of the 2-dimensional semiconductor layer is MoS.sub.2; the photosensitizing layer comprises colloidal quantum dots for absorbing light that, in response to incident light, generates pairs of electric carriers, traps a single type of electric carriers of said pairs therein, and transfers a distinct single type of electric carriers of said pairs to the transport layer, to be transported thereby, wherein the material of said colloidal quantum dots is selected from the group consisting of Ge, HgTe, and AgBiSe.sub.2; and wherein the optoelectronic apparatus further comprises: a first electrode and a second electrode connected to the transport layer, the transport layer being adapted to generate, and make flow through a transport channel, an electric current between the first electrode and the second electrode upon incidence of incoming light in the photosensitizing layer; and further wherein a type-II heterojunction is formed between the photosensitizing layer and the transport layer to thereby provide a photoconductive gain.

2. The optoelectronic apparatus according to claim 1, further comprising a third electrode connected to the substrate; and a voltage source connected to the third electrode and providing a bias voltage thereto to tune a conductivity of the transport layer by applying the bias voltage to the third electrode, to the point that the transport channel is depleted of free carriers in order to minimize its conductivity in dark.

3. The optoelectronic apparatus according to claim 2, wherein said bias voltage provided by the voltage source ranges from 0.1 V to 10 V or from −0.1 to −10V.

4. The optoelectronic apparatus according to claim 1, further comprising a top electrode on top of the photosensitizing layer or on top of a dielectric layer arranged on top of the photosensitizing layer.

5. The optoelectronic apparatus according to claim 1, wherein the substrate layer comprises a doped semiconductor selected from the group consisting of Si, ITO, aluminum doped zinc oxide (AZO), and graphene.

6. The optoelectronic apparatus according to claim 1, wherein the material of the dielectric layer is selected from the group consisting of SiO.sub.2, HfO.sub.2, Al.sub.2O.sub.3, parylene, and boron nitride.

7. The optoelectronic apparatus according to claim 1, wherein the transport layer consists of a number of 2-dimensional semiconductor layers ranging from one to one hundred.

8. The optoelectronic apparatus according to claim 1, further comprising an interlayer barrier between the transport layer and the photosensitizing layer.

9. The optoelectronic apparatus according to claim 8, wherein the interlayer barrier is selected from the group consisting of ZnO, TiO.sub.2, Alumina, Hafnia, and boron nitride.

10. The optoelectronic apparatus according to claim 8, wherein the interlayer barrier comprises a self-assembled monolayer of organic molecules selected from the group consisting of ethanedithiol, propanedithiol, butanedithiol, octanedithiol, and dodecanedithiol.

11. The optoelectronic apparatus according to claim 8, wherein the interlayer barrier has a thickness between 0.1 and 10 nm.

12. The optoelectronic apparatus according to claim 8, wherein the interlayer barrier forms a type-II heterojunction with the photosensitizing layer, and a type-II or type-I heterojunction with the transport layer.

13. An optoelectronic apparatus comprising: a substrate, a dielectric layer, a transport layer, and a photosensitizing layer, wherein: the transport layer comprises at least one 2-dimensional semiconductor layer, wherein the material of the 2-dimensional semiconductor layer is selected from the group consisting of MoSe.sub.2, WS.sub.2, WSe.sub.2, and SnS.sub.2; the photosensitizing layer comprises colloidal quantum dots for absorbing light that, in response to incident light, generates pairs of electric carriers, traps a single type of electric carriers of said pairs therein, and transfers a distinct single type of electric carriers of said pairs to the transport layer, to be transported thereby, wherein the material of said colloidal quantum dots is selected from the group consisting of Ge, HgTe, and AgBiSe.sub.2; and wherein the optoelectronic apparatus further comprises: a first electrode and a second electrode connected to the transport layer, the transport layer being adapted to generate, and make flow through a transport channel, an electric current between the first electrode and the second electrode upon incidence of incoming light in the photosensitizing layer; and further wherein a type-II heterojunction is formed between the photosensitizing layer and the transport layer to thereby provide a photoconductive gain.

14. The optoelectronic apparatus according to claim 13, further comprising a third electrode connected to the substrate; and a voltage source connected to the third electrode and providing a bias voltage thereto to tune a conductivity of the transport layer by applying the bias voltage to the third electrode, to the point that the transport channel is depleted of free carriers in order to minimize its conductivity in dark.

15. The optoelectronic apparatus according to claim 14, wherein said bias voltage provided by the voltage source ranges from 0.1 V to 10 V or from −0.1 to −10V.

16. An optoelectronic apparatus comprising: a substrate, a dielectric layer, a transport layer, and a photosensitizing layer, wherein: the transport layer comprises at least one 2-dimensional semiconductor layer, wherein the material of the 2-dimensional semiconductor layer is selected from the group consisting of MoSe.sub.2, WS.sub.2, WSe.sub.2, and SnS.sub.2; the photosensitizing layer comprises colloidal quantum dots for absorbing light that, in response to incident light, generates pairs of electric carriers, traps a single type of electric carriers of said pairs therein, and transfers a distinct single type of electric carriers of said pairs to the transport layer, to be transported thereby, wherein the material of said colloidal quantum dots is PbS; and wherein the optoelectronic apparatus further comprises: a first electrode and a second electrode connected to the transport layer, the transport layer being adapted to generate, and make flow through a transport channel, an electric current between the first electrode and the second electrode upon incidence of incoming light in the photosensitizing layer; and further wherein a type-II heterojunction is formed between the photosensitizing layer and the transport layer to thereby provide a photoconductive gain.

17. The optoelectronic apparatus according to claim 16, further comprising a third electrode connected to the substrate; and a voltage source connected to the third electrode and providing a bias voltage thereto to tune a conductivity of the transport layer by applying the bias voltage to the third electrode, to the point that the transport channel is depleted of free carriers in order to minimize its conductivity in dark.

18. The optoelectronic apparatus according to claim 17, wherein said bias voltage provided by the voltage source ranges from 0.1 V to 10 V or from −0.1 to −10V.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] For the purpose of aiding the understanding of the characteristics of the invention, according to a preferred practical embodiment thereof and in order to complement this description, the following figures are attached as an integral part thereof, having an illustrative and non-limiting character:

[0043] FIG. 1 shows a cross-sectional view of a preferred embodiment of the invention.

[0044] FIG. 2 is a scheme exemplifying the operation of said preferred embodiment as a photodetector.

[0045] FIG. 3 presents another embodiment of the invention, comprising an interlayer barrier between the transport layer and the quantum dot layer.

[0046] FIG. 4 compares the responsivity of an embodiment of the invention based on MoS.sub.2 and a graphene/QD photodetector known in the state of the art.

[0047] FIG. 5 compares the responsivity of a MoS.sub.2 photodetector with and without a quantum dots layer, according to a preferred embodiment of the invention.

[0048] FIG. 6 shows the spectral responsivity of a MoS.sub.2-only phototransistor and of the equivalent hybrid MoS.sub.2-Pbs detector.

[0049] FIG. 7 is a plot showing simulation results obtained with the optoelectronic apparatus of the present invention, for an embodiment for which the transport layer is made of MoS.sub.2 and the material of the colloidal quantum dots is HgTe. The solid lines correspond to the conduction (upper) and valence (lower) bands, and the dashed line to the fermi level.

[0050] FIG. 8 is a plot showing simulation results obtained with the optoelectronic apparatus of the present invention, for an embodiment for which the transport layer is made of WS.sub.2 and the material of the colloidal quantum dots is PbS. The solid lines correspond to the conduction (upper) and valence (lower) bands, and the dashed line to the fermi level.

[0051] FIG. 9 is a plot showing simulation results obtained with the optoelectronic apparatus of the present invention, for an embodiment for which the transport layer is made of MoS.sub.2 and the material of the colloidal quantum dots is AgBiSe.sub.2. The solid lines correspond to the conduction (upper) and valence (lower) bands, and the dashed line to the fermi level.

DETAILED DESCRIPTION OF THE INVENTION

[0052] The matters defined in this detailed description are provided to assist in a comprehensive understanding of the invention. Accordingly, those of ordinary skill in the art will recognize that variation changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In particular, the preferred embodiments of the invention are described for an optoelectronic apparatus based on a MoS.sub.2 transport layer sensitized with PbS quantum dots.

[0053] Nevertheless, the description of the photonic structures and of their underlying mechanism can be generally applied to other materials. Specifically, as indicated in a previous section, for each of MoS.sub.2, MoSe.sub.2, WS.sub.2, WSe.sub.2, and SnS.sub.2, for the transport layer, combined with each of Ge, HgTe, AgBiSe2, and PbS, for the photosensitizing layer, in any possible combination, as indeed for all those combinations a type-II heterojunction is formed that enables trapping of a single type of electrical carrier (electrons or holes) in the photosensitizing layer and transferring a distinct type of electrical carrier (holes or electrons) to the 2D semiconductor transport layer, and thus photoconductive gain is provided.

[0054] Note that in this text, the term “comprises” and its derivations (such as “comprising”, etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc.

[0055] FIG. 1 shows a cross-sectional view of a preferred embodiment of the optoelectronic apparatus of the invention, fabricated according to a preferred embodiment of the method of the invention. The apparatus comprises a substrate 1 fabricated of a heavily doped semiconductor such as Silicon, on top of which is deposited a dielectric layer 2 of silicon oxide. The transport layer of the apparatus is implemented by two 2-dimensional semiconductor (2DS) layers 3 of any of the above mentioned materials (MoS.sub.2, MoSe.sub.2, WS.sub.2, WSe.sub.2, and SnS.sub.2). The 2DS layers 3 are sensitized by a quantum dot (QD) sensitizing layer 5 made of any of the above mentioned materials (Ge, HgTe, AgBiSe2, and PbS). Alternatively, the transport layer can be implemented by less or more than two 2-dimensional semiconductor (2DS) layers 3.

[0056] For an embodiment, each 2DS layer 3 is a monolayer of MoS2 defined by three atomic layers (SMo-S), as opposed to single-atomic layer graphene. Moreover, MoS.sub.2 possess a bandgap and therefore allows the operation of the device in the off state of the transport layer, determined by the application of a back gate voltage. This operation regime is not possible with graphene, due to the lack of the bandgap.

[0057] The 2DS layers 3 are sensitized by a PbS quantum dot (QD) sensitizing layer 5. Thus, the optical absorption of the apparatus and therefore its spectral sensitivity is determined by that of the QDs. The apparatus can hence detect photons that have lower energy than the bandgap of the transport layer, extending the spectral range for photodetection.

[0058] A conductor layer 4 partially covers the top 2DS layer 3, providing contact points for electrodes. The conductor layer 4 can be implemented, for example, with Ti, Au, or any other conductor with similar functionalities. The conduction layer 4 can be fabricated, for example, by selective deposition or by a complete deposition followed by a selective etching. Quantum dots are deposited in a two-step process involving treatment with 1,2-ethanedithiol (EDT) followed by PbS QD deposition. Initially the MoS2 layer becomes more n-type doped due to surface doping from EDT. The subsequent deposition of p-type PbS QDs turns the MoS.sub.2 film again less n-type doped due to the formation of the heterojunction between the n-type MoS.sub.2 transport layer and the p-type PbS QD sensitizing layer 5. The MoS.sub.2 layer in its final configuration is still more n-type doped than the initial stand-alone flake, an effect that reduces the on/off-ratio in the experimental range of V.sub.G.

[0059] Thicknesses of the layers of the apparatus preferably are selected from the following ranges: [0060] Substrate layer 1: 0.1 nm-10 mm [0061] Dielectric layer 2: 5 nm-400 nm [0062] Transport layer: between 1 and 100 MoS.sub.2 monolayers [0063] QD layer 5: 2 nm-2,000 nm [0064] Conductor layer 4: 0.1 nm-100,000 nm

[0065] Additional substrate layers 1 can be included to provide support to the whole apparatus, such as silicon substrates, glass substrates or flexible plastic substrates like polyethylene terephtalate (PET).

[0066] FIG. 2 presents an optoelectronic apparatus with the aforementioned structure and materials operating as a transistor. A first electrode 6 (drain electrode) and a second electrode 7 (source electrode) are connected to the top 2DS layer 3 through the conductor layer 4. A third electrode 8 (back-gate electrode) is connected to the substrate layer 1. Incident light 9 is absorbed by the QD layer 5, resulting in the separation of photoexcited electron 11—hole 10 pairs at the p-n-interface between MoS.sub.2 and PbS, or between any of MoS.sub.2, MoSe.sub.2, WS.sub.2, WSe.sub.2, and SnS.sub.2 and any of Ge, HgTe, AgBiSe2, and PbS. While holes 10 remain within the QD layer 5, electrons 11 circulate through the MoS.sub.2 channel (or MoSe.sub.2, WS.sub.2, WSe.sub.2 channel) driven by an electric field VDS applied between the drain electrode 6 and the source electrode 7. Alternatively, the opposite can also happen, i.e. electrons 11 remain within the QD layer 5 and holes 10 circulate through the transport channel. The current flow can be controlled electrically by applying an appropriate back-gate voltage (V.sub.G) at the back-gate electrode 8. At strongly negative values of V.sub.G, or at values of V.sub.G ranging from 0.1 V to 10 V or from −0.1 to −10V (where the dielectric layer is thin and thus a large voltage is not necessary), the gating depletes the n-type MoS.sub.2 sheet (or MoSe.sub.2, WS.sub.2, WSe.sub.2 sheet), increasing the resistance of the device (operation in OFF mode). By increasing V.sub.G, the MoS.sub.2 channel (or MoSe.sub.2, WS.sub.2, WSe.sub.2 channel) falls in the accumulation region and the transistor is in the ON state.

[0067] FIG. 3 shows a variation of the optoelectronic apparatus and method in which a thin interlayer barrier 12 is deposited between the top 2DS layer 3 and the QD layer 5. The interlayer barrier comprises ZnO, TiO.sub.2, Alumina, Hafnia, boron nitride or a self-assembled monolayer of organic molecules including Ethane-, propane-, butane-, octane-or dodecane-dithiol molecules. The thickness of the interlayer barrier may vary from 0.1 nm up to 10 nm. The effect of the interlayer barrier is to tailor the electronic interface between the QD and 2DS layer to improve the performance of the device in achieving more efficient charge transfer, tailoring the temporal response and improve the stability of the device.

[0068] In all the optoelectronic apparatus, materials of the QD layer 5 and the transport layer are selected in order to ensure a high carrier mobility in the transport layer and hence a carrier transit time (t.sub.transit) that is orders of magnitude shorter than the trapping lifetime (t.sub.lifetime) in the quantum dots. Since the gain of the device is given by the ratio t.sub.lifetime/t.sub.transit, this selection of materials provides a highly responsive device. The temporal response of the hybrid photodetector is determined by t.sub.lifetime, showing a time constant of ˜0.3 s for the particular case of a MoS.sub.2/PbS device.

[0069] The existence of a bandgap in the channel of the transistor, which allows the facile tuning of the dark conductivity, is a powerful tool to increase the sensitivity of a detector implemented in the proposed optoelectronic platform, as the noise current in the shot noise limit scales as i.sub.n=(2qI.sub.dB).sup.1/2, where q is the electron charge, I.sub.d the dark current flowing in the device and B is the electrical bandwidth. The resultant sensitivity of the detector in the shot-noise limit is then expressed by the normalized detectivity as D*=R(AB).sup.1/2/i.sub.n where R is the responsivity, A the area of the device and B is the electrical bandwidth. At high negative back-gate bias, or at values of V.sub.G ranging from 0.1 V to 10 V or from −0.1 to −10V, the channel is depleted from free carriers in the dark state and therefore the detector exhibits high sensitivity with D* reaching up to 7×10.sup.14 Jones at V.sub.G of −100 V in the shot-noise limit. MoS.sub.2/PbS photodetectors show significant performance even at very low applied electric field of 3.3 mV/pm with corresponding responsivity of 10 A/W. The presence of the bandgap in the MoS2 channel and thus the offered opportunity to tune the dark current via the back gate allows the achievement of similar responsivity values achieved via previously reported structures relying on graphene, albeit with lower dark current values. This reduction in the dark current is apparent in FIG. 4, which presents experimental results of the responsivity vs dark current for a MoS.sub.2/PbS 13 and a graphene/QD 14 photodetectors. The MoS.sub.2/PbS 13 photodetector can achieve the same responsivity with more than an order of magnitude reduction in the dark current. Similar results are obtained for photodetectors with a transport layer made of MoS.sub.2, and a photosensitizing layer comprising colloidal quantum dots made of any of Ge, HgTe, and AgBiSe.sub.2, and for photodetectors with a transport layer made of any of MoSe.sub.2, WS.sub.2, WSe.sub.2, and a photosensitizing layer comprising colloidal quantum dots made of any of Ge, HgTe, AgBiSe.sub.2, and PbS.

[0070] FIG. 5 displays the field effect transistor (FET) characteristics of a bilayer MoS.sub.2 transistor 15 and its MoS.sub.2/PbS hybrid device fabricated on a Si/SiO.sub.2 substrate. All measurements were performed in two-probe configuration and carried out under ambient conditions. The source-drain current (I.sub.DS) modulation characteristic as a function of V.sub.G and under bias voltage V.sub.DS=50 mV is presented in linear scale. The bilayer MoS.sub.2 transistor 15 shows a field effect mobility of 10-20 cm.sup.2V.sup.−1 s.sup.−1 in the linear regime and on/off-ratios in the range of 10.sup.5-10.sup.6. A significant increase in the drain current of MoS.sub.2/PbS transistors is observed for the MoS.sub.2/PbS hybrid device, both for light 16 and dark 17 states. Similar results are obtained for photodetectors with a transport layer made of MoS.sub.2, and a photosensitizing layer comprising colloidal quantum dots made of any of Ge, HgTe, and AgBiSe.sub.2, and for photodetectors with a transport layer made of any of MoSe.sub.2, WS.sub.2, WSe.sub.2, and a photosensitizing layer comprising colloidal quantum dots made of any of Ge, HgTe, AgBiSe.sub.2, and PbS.

[0071] FIG. 6 shows the spectral responsivity of a MoS.sub.2-only 19 phototransistor that exhibits a responsivity up to 5 A/W, being its spectral sensitivity determined by the bandgap of a 2-layer flake of around 1.8 eV. The equivalent hybrid MoS.sub.2-Pbs 18 detector shows dramatically higher responsivity on the order of 10.sup.5-10.sup.6 A/W and its spectral sensitivity is now extended to near infrared, as dictated by the bandgap of the PbS QDs. While the MoS.sub.2 device absorbs only until a wavelength of ˜700 nm, the hybrid follows clearly the expected PbS absorption with a exciton peak at 980 nm, which can be tuned by controlling the quantum dot species and size. This allows the development of detectors that have sensitivity further into the short-wave infrared using larger PbS QDs and/or alternative QD species. Similar results and conclusions can be made for photodetectors with a transport layer made of MoS.sub.2, and a photosensitizing layer comprising colloidal quantum dots made of any of Ge, HgTe, and AgBiSe.sub.2, and for photodetectors with a transport layer made of any of MoSe.sub.2, WS.sub.2, WSe.sub.2, and a photosensitizing layer comprising colloidal quantum dots made of any of Ge, HgTe, AgBiSe.sub.2, and PbS.

[0072] Experimental results therefore prove an increased responsivity under similar dark currents than graphene-based photodetectors, as well as a more extended spectral range than traditional MoS.sub.2 devices, or traditional MoSe.sub.2, WS2, WSe.sub.2 devices.

[0073] FIGS. 7-9 show simulation results obtained with the optoelectronic apparatus of the present invention, for three combinations of the above mentioned materials, showing that each of them indeed form a type II heterojunction, particularly for MoS.sub.2/HgTe (FIG. 7), WS.sub.2/PbS (FIG. 8), and MoS.sub.2/AgBiSe.sub.2 (FIG. 9).

TABLE-US-00001 MoS.sub.2 HgTe Bandgap [eV] 1.8 0.3 Electron affinity 4.3 4.2

TABLE-US-00002 WS.sub.2 PbS Bandgap [eV] 2.3 0.7 Electron affinity 4.7 4.5

TABLE-US-00003 MoS.sub.2 AgBiSe.sub.2 Bandgap [eV] 1.8 0.85 Electron affinity 4.3 4

[0074] The simulations have been made with the apparatuses modelled with SOAPS, a 1-D simulator for thin film semiconductor devices: M. Burgelman, K. Decock, S. Khelifi and A. Abass, “Advanced electrical simulation of thin film solar cells”, Thin Solid Films, 535 (2013) 296-301.