Patent Application
20230231063
The present invention has its application within the optoelectronics sector and, especially, in the industrial area engaged in providing optoelectronic platforms with enhanced photoconductive gain.
Many optoelectronic applications, such as photodetectors and photovoltaic cells, rely on the generation of an electric current under incidence of incoming light upon the device. Light absorption at an active layer triggers the separation of electron-hole pairs, with free electrons circulating through a transport layer driven by an electric field applied by electrodes. In order to provide an efficient conversion under diverse conditions and wavelength regimes, many optoelectronic platforms have been proposed over the years.
For example, WO 2013017605 A1 discloses an optoelectronic platform in which the transport layer consists of a semi-metallic carbon based conductor, such as graphene. A quantum dot sensitizing layer is also comprised in order to induce a change in the conductivity of the transport layer. In view of the high mobility of graphene and long lifetime of carriers in the quantum dots, a large photoconductive gain was achieved. However, this apparatus requires a high dark current level in order to achieve the desired responsivity. This dark current level requirement further affects the sensitivity and the shot-noise limit of the apparatus.
On the other hand, 2-dimensional semiconductors have also been used to implement transport layers in photoresponsive optoelectronic devices. For example, WO 2013063399 A1 presents an optoelectronic platform incorporating MoS2 layers. However, this technology presents a limited spectral coverage, determined by the bandgap of the MoS2.
Therefore, there is still the need in the state of the art of an optoelectronic platform, capable of providing a high responsivity for low dark current levels for a broad spectral range of incoming light.
The current invention solves the aforementioned problems by disclosing an optoelectronic apparatus, and a method for its fabrication, which provide enhanced responsivity at low dark current levels and extended spectral coverage, due to the combination of a 2-dimensional semiconductor transport layer and a photosensitizing layer comprising colloidal quantum dots.
In a first aspect of the invention, an optoelectronic apparatus with enhanced responsivity and spectral coverage is disclosed. The optoelectronic apparatus comprises, from top to bottom, a photosensitizing layer, a transport layer, a dielectric layer and a substrate. Preferably, the optoelectronic apparatus is adapted to act as a photodetector by comprising a first electrode and a second electrode connected to the transport layer through two contacts of a conductor layer. Upon reception of incident light at the photosensitizing layer, an electric current between the first electrode and the second electrode is hence created through the transport layer. More preferably, the substrate is connected to a third electrode, therefore enabling to tune the conductivity of the transport by applying a bias voltage to said third electrode.
The photosensitizing layer comprises colloidal quantum dots for light absorption and transport layer conductivity modulation. Preferably, the photosensitizing layer comprise one or more of the following types of quantum dots: PbS, CIS (Copper indium disulfide), Ge, Si, HgTe, CIGS (Copper indium gallium selenide), CZTS (Copper zinc tin sulfide), AgBiS2, SnO2, ITO (indium tin oxide) and ZnO.
The transport layer comprises at least a 2-dimensional semiconductor layer, being the number of 2-dimensional semiconductor layers preferably comprised between two and ten. Preferably, the 2-dimensional semiconductor layer (or layers) comprises one or more of the following materials: MoS2, MoSe2, WS2, WSe2, black phosphorous and SnS2.
For an implementation of the first aspect of the invention, the optoelectronic apparatus comprises: a substrate, a dielectric layer, a transport layer, and a photosensitizing layer, wherein:
For another implementation of the first aspect of the invention, the optoelectronic apparatus comprises: a substrate, a dielectric layer, a transport layer, and a photosensitizing layer, wherein:
For still another implementation of the first aspect of the invention, the optoelectronic apparatus comprises: a substrate, a dielectric layer, a transport layer, and a photosensitizing layer, wherein:
At least all the materials listed for the 2-dimensional semiconductor layer for the implementations described above, i.e., MoS2, MoSe2, WS2, WSe2, and SnS2, have a band gap of >1 eV in their two-dimensional form, i.e., in the form they are included in the optoelectronic apparatus of the invention, while also, at least all the materials listed for the colloidal quantum dots of the photosensitizing layer for the implementations described above, i.e., Ge, HgTe, AgBiSe2, and PbS have a bandgap <1 eV.
Therefore, for at least those implementations, the above indicated condition 1 for forming a type-II heterojunction is met, as all the materials for the transport layer have a band gap >1 eV and all the materials for the photosensitizing layer have a band gap <1 eV.
The above indicated condition 2 is indeed also met by all the implementations described above, as indeed the bandgaps are aligned as illustrated above, so that only one type of photogenerated carriers (electrons or holes) is injected from the photosensitizing layer to the transport layer. This can be achieved by several known processes, such as by ligand engineering of the colloidal quantum dot materials in the photosensitizing layer as is done in the following reference: “Energy Level Modification in Lead Sulfide Quantum Dot Thin Films through Ligand Exchange”; Patrick R. Brown et al. American Chemical Society, 2014.
The optoelectronic apparatus described by the invention is indeed providing photoconductive gain for all its implementations and embodiments. The type II heterojunction enables trapping of a single type of electrical carrier in the photosensitizing layer (for a duration tlifetime) and transferring a distinct type of electrical carrier to the 2D semiconductor transport layer. When a bias is applied over the transport layer, the transit time ttransit of that current will provide a photoconductive gain given by the ratio tlifetime/ttransit, i.e., a photoconductive gain will be provided.
The substrate layer preferably comprises a doped semiconductor selected between Si, ITO, AZO (Aluminum doped zinc oxide) and graphene. The dielectric layer preferably comprises at least one of the following materials: SiO2, AlO2, HfO2, parylene and boron nitride.
For an embodiment, the optoelectronic apparatus further comprises a top electrode on top of the photosensitizing layer or on top of a dielectric layer arranged on top of the photosensitizing layer.
In some preferred embodiments, the optoelectronic apparatus further comprises an interlayer barrier located between the transport layer and the photosensitizing layer. Preferably, the interlayer barrier comprises either one of the following materials: ZnO, TiO2, SiO2, AlO2, HfO2 and boron nitride; or a self-assembled monolayer of organic molecules such as ethanedithiol, propanedithiol, butanedithiol, octanedithiol and dodecanedithiol. The thickness of the interlayer barrier is preferably comprised between 0.1 and 10 nm. Preferably, the interlayer barrier forms a type-II heterojunction with the photosensitizing layer, and a type-II or type-I heterojunction with the transport layer.
For an embodiment, the optoelectronic apparatus further comprises 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.
For a variant of the above mentioned embodiment, the bias voltage provided by the voltage source ranges from 0.1 V to 10 V or from −0.1 to −10V. This variant corresponds to a practical implementation of the apparatus, where the dielectric layer will be thin and thus a large voltage is not necessary. Moreover, in a CMOS implementation, the max voltage sustained by the CMOS will be 10V. Hence, for this variant, the bias voltage is limited as indicated above. Depending on the polarity of the two-dimensional semiconductor, the gate voltage should be either positive or negative to be able to deplete the transport channel of free carriers.
In a second aspect of the present invention, a fabrication method of a photosensitive optoelectronic apparatus is disclosed. The method comprises depositing a dielectric layer, a transport layer and a photosensitizing layer on a substrate. The transport layer is deposited by depositing one or more 2-dimensional semiconductor layers. The photosensitizing layer comprises colloidal quantum dots. Preferably, the 2-dimensional semiconductor layers are either grown by chemical vapor deposition or exfoliated from a bulk crystal.
Note that all the preferred options of the apparatus of the invention (such as materials, structures, thicknesses, electrodes, etc.), can also be implemented as preferred options of the method of the invention by appropriately adapting any deposition and/or etching step thereof.
The disclosed apparatus and method provide an optoelectronic platform, which combines an enhanced responsivity under low dark current and an extended spectral range for photodetection. These and other advantages of the invention will become apparent from the following description and accompanying drawings.
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:
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 MoS2 transport layer sensitized with PbS quantum dots.
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 MoS2, MoSe2, WS2, WSe2, and SnS2, 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.
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.
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, MoS2 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.
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.
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 MoS2 film again less n-type doped due to the formation of the heterojunction between the n-type MoS2 transport layer and the p-type PbS QD sensitizing layer 5. The MoS2 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 VG.
Thicknesses of the layers of the apparatus preferably are selected from the following ranges:
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).
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 (ttransit) that is orders of magnitude shorter than the trapping lifetime (tlifetime) in the quantum dots. Since the gain of the device is given by the ratio tlifetime/ttransit, this selection of materials provides a highly responsive device. The temporal response of the hybrid photodetector is determined by tlifetime, showing a time constant of ˜0.3 s for the particular case of a MoS2/PbS device.
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 in=(2qIdB)1/2, where q is the electron charge, Id 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)1/2/in 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 VG 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 7×1014 Jones at VG of −100 V in the shot-noise limit. MoS2/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
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 MoS2 devices, or traditional MoSe2, WS2, WSe2 devices.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 14177172.5 | Jul 2014 | EP | regional |
This is a continuation in part of U.S. patent application Ser. No. 14/800,320 filed Jul. 15, 2015, now U.S. Pat. No. 11,527,662.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14800320 | Jul 2015 | US |
| Child | 17986548 | US |
