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Abstract
Hot electron photodetectors based on a planar structure of metal-insulator /semiconductor-metal (MIM/MSM) have attracted much attention due to the easy and cheap fabrication process and the possibility of detecting light with energy lower than the semiconductor band gap. For this type of device, however, hot electron photocurrent is restricted by the trade-off between the light absorption and the internal quantum efficiency (IQE) since high absorption usually occurs within thick metals and the IQE in this case is usually low. The trade-off is circumvented in this paper by proposing a new type of hot electron photodetector based on planar MIM structure and coupled dual Tamm plasmons (TPs), which has a structure of one-dimensional photonic crystals (1DPCs)/Au/TiO2/Au/1DPCs. The coupled modes of the dual TPs at the two 1DPCs/Au interfaces can lead to a high absorption of 98% in a 5 nm-thick Au layer. As a result, the responsivity of the conventional device with two Schottky junctions in series configuration reaches a high value of 9.78 mA/W at the wavelength of 800 nm. To further improve the device performance, devices with four Schottky junctions in parallel configuration are proposed to circumvent the hot electrons loss at the interface of the Au layer and the first TiO2 layer of the 1DPCs. Correspondingly, the hot electrons photocurrent doubles and reaches a higher value of 21.87 mA/W. Moreover, the bandwidth of the responsivity is less than 0.4 nm, the narrowest one when compared with that for the hot electron photodetectors reported so far in the published papers.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Photodetectors have become one of the most widely used core components in optical communication and optical information processing. Conventional photodetectors based on semiconductor materials are usually limited by the material bandgaps and can only detect photons with energy above the forbidden energy band. This problem is solved by developing hot electron photodetectors of which the operating wavelength is not limited by the semiconductor band gap. This type of photodetectors can be used to detect visible light and near-infrared light with lower energy than the semiconductor band gap and therefore have become a new topic to be studied intensely [1–5]. Present research mainly focuses on improving the device performance by designing different metal nanostructures such as nanoantennas [6,7], nanowires [8], nanorods [9], nanocones [10], gratings [11–13], nanoparticles [14–16], disordered nanocomposites [17], metamaterials [18,19] and metasurfaces [20] to excite surface plasmons. The excitation of the surface plasmons generates a large number of hot electrons in the metal and these hot electrons can be emitted into a semiconductor or oxide insulator to form a photocurrent that can be used for light detection. In recent research, photodetectors that combine plasmon and free carrier absorption in doped silicon have shown outstanding performance in infrared and near-infrared photodetection [21,22]. However, the techniques for fabricating these devices are complicated and costly.
To circumvent this problem, planar metal-insulator/semiconductor-metal (MIM/MSM) structure with the advantages of simple structure, easy and cheap fabrication process have been proposed to fabricate hot electron devices [23]. However, it is still considerably difficult for these types of planar structure to achieve high optical absorption under direct incidence. To improve the absorption in these structures, Kretschmann plasmon coupling [24], nanocavity [25,26], nanoparticles [27,28] and Tamm Plasmons (TPs) [29–31] are then proposed. Optical TPs are surface waves occurring at the interface between the one-dimensional photonic crystals (1DPCs) and metals [32,33], which was firstly proposed by Li et.al. to construct the TP-based hot-electron photodetector (TP-HE PD) [29,30]. It is demonstrated that this type of single TP-HE PD has such advantages as good tolerance to the change of incident angle, tunability in a wide wavelength range and low-cost preparation [29]. In this type of devices, however, a relatively thick metal layer is required to achieve high absorption. This leads to a relatively low internal quantum efficiency (IQE) and in turn results in a relatively low responsivity under zero bias conditions. At the same time, the coupled-TPs hybrid system has been a research hotspot since its proposal [34,35], such as the realization of strong coupling between Tamm plasmons and cavity mode [36], guided-mode resonance [37] and topological photonic state [38].
In this paper, we propose a new type of hot electron photodetector based on the coupled dual TPs between Au and 1DPCs (dual TP-HE PD). The proposed device has a structure of top 1DPCs/Au/TiO2/Au/bottom 1DPCs/Glass, where Au/TiO2/Au acts as the MIM structure and the thickness of two TiO2 layers adjacent to Au layer in 1DPCs are used to adjust the electric field distributions to optimize the optical absorption. As a result of the dual TPs excitation within the device, the absorption in top and bottom Au layer can reach a high value of over 98% and 97% respectively with the full width at half maximum (FWHM) being less than 0.4 nm at the given wavelength. Correspondingly, the responsivity of the device with two Schottky junctions in series configuration reaches a value of 9.78 mA/W and that for the device with four Schottky junctions in parallel configuration reaches a higher value of 21.87 mA/W.
2. Results and discussion
Figure 1(a) shows the schematic diagram of proposed hot-electron photodetector device based on coupled TPs. In this structure, an Au/TiO2/Au MIM structure is sandwiched between two 1DPCs. For the MIM structure, the thickness of top Au, intermediate TiO2 and bottom Au layers is set as 5 nm, respectively. The top and bottom 1DPCs respectively consists of Nt and Nb pairs of alternating SiO2/TiO2 layers. The optical thickness of each layer in 1DPCs is determined by λt/4 or λb/4, where λt and λb are the central wavelength of the bandgap of top and bottom 1DPCs, respectively. As is shown in Fig. 1(b), both the top and bottom Au layers form two Schottky junctions with adjacent TiO2 layers and thus there are four Schottky junctions occurring within the device. Hot electrons reaching the Au/TiO2 interfaces with sufficient energy have the probability to overcome the Schottky barriers and can be emitted into TiO2 layers. The Schottky barrier φB of the Au/TiO2 interface is set as 1 eV [4].
Fig. 1. (a) Schematic of dual TP-HE PD. (b) Energy band diagram of the dual TP-HE PD. EF and Eph are the Fermi level and the energy of the absorbed photon respectively. φB is the Schottky barrier.
2.1 Optical calculation
The optical properties of the device are first investigated by employing transfer matrix method (TMM) to calculate optical reflectance, optical absorption, the amplitude reflection coefficients and electric field distribution. The optical absorption as a function of wavelength (λ) in the top and bottom Au layers is respectively defined as At (λ) and Ab (λ). The forward (reverse) net absorption is defined as Af = At – Ab (Ar = Ab – At) and the total absorption is defined as Atot = At + Ab. To optimize Af and Atot, a computational method based on particle swarm optimization [39,40] is performed to maximize At in the top Au layer by tailoring the structure parameters such as λt, λb, Nt, Nb and the thickness of top and bottom TiO2 layers. The top (or bottom) TiO2 layer is referred to as the TiO2 layer in the top (or bottom) 1DPCs, which is adjacent to the Au layer (see Fig. 1(a)). In the optimization process, the wavelength is set at 800 nm.
As is shown in Fig. 2(a), absorption At in the top Au layer reaches a maximum of 98% at the wavelength of 800 nm when the structure parameters of λt, λb, Nt and Nb are respectively set as 740 nm, 785 nm, 13, 20. Correspondingly, the forward net absorption Af and the total absorption Atot reaches high values of 96% and 100%. For comparison, At of single TP-HE PD is also optimized, which has a structure of 1DPCs/Au/TiO2/Au with thickness of top Au, intermediate TiO2 and bottom Au layer set as 25 nm, 5 nm, 100 nm, respectively. At for the single TP-HE PD reaches the maximum value of 90% and Af is about 78% at the wavelength of 800 nm. It indicates that the net absorption Af for the dual TP-HE PD is about 18% higher than that of the single TP-HE PD. It is also shown in Fig. 2(a) that FWHM is less than 0.4 nm for the At spectrum of dual TP-HE PD, which is about 1/16 of that (6.6 nm) of single TP-HE PD. The quality factor (Q = λ/FWHM) [41] and dephasing time (T = 2ℏ/FWHM) [42] at the resonant wavelength of 800 nm are 2000 and 2.26 ps, respectively. These findings demonstrate that the dual TP-HE PD have higher net absorption and narrower bandwidth than the single TP-HE PD.
Fig. 2. (a): Absorption At in the top Au layer for single and dual TP-HE PD; (b), (c) and (d): normalized electric field |E|2 for the single TP-HE PD, dual TP-HE PD and the MIM structure within the dual TP-HE PD. Z axis denotes the distance from the interface of air and top 1DPCs.
To understand this phenomenon, we calculate the normalized optical electric field |E|2 within the device and the calculated results are shown in Figs. 2(b), 2(c) and 2(d). For the single TP-HE PD, Fig. 2(b) shows that |E|2 reaches a high value of over 35 and the maximum occurs inside the TiO2 layer of 1DPCs close to the 1DPCs/Au interface. Unlike the single TP-HE PD, it is found in the dual TP-HE PD that there are two strong electric field maxima with high electric field |E|2 of over 500 and 400 for the dual TP-HE PD and the two maxima respectively occur inside the top and bottom TiO2 layers. This indicates that dual optical TPs are excited, which are localized on the two interfaces of the MIM-structure/top-1DPCs and MIM-structure/bottom-1DPCs. More specifically, the top branch of the TPs is stronger than the bottom one, since the electric maximum within the top 1DPCs is observed to be larger than that within the bottom one, as is suggested by Fig. 2(c). Correspondingly, |E|2 in the top Au layer is larger than that in the bottom Au layer since the latter is just positioned in the zero region of the electric field (see Fig. 2(d)). As a result, the net absorption between the top Au layer and bottom Au layer can be maximized.
Further analytical investigation is performed to verify the excitation of dual optical TPs by introducing four virtual interfaces between the two Au layers and two 1DPCs, as is shown in Fig. 3(a). The amplitude reflection coefficient of two propagating waves at virtual interfaces 1 and 4 are called as rt and rb, respectively. The t and r are the amplitude transmission and reflection coefficient of MIM. The condition of forming coupled TPs modes [34,43,44] is given by Eq. (1):
Fig. 3. (a) Schematic diagram used to analyse the coupling condition |F|. Four white lines are virtual interfaces. The amplitude reflection coefficient at virtual interfaces 1 and 4 are marked as rt and rb. t and r are the amplitude transmission and reflection coefficient of the MIM. (b) Coupling condition function |F| (Blue line) and the total absorption Atot spectrum (red line) as function of wavelength.
Equation (2) describes the excitation condition of the coupled TPs modes and the coupling condition is described by constructing a new function |F|:
|F| is calculated with r, rt, rb and t obtained from TMM method and the calculated results are shown in Fig. 3(b). It is found that |F| is nearly equal to 0 at the wavelength of 800 nm for the dual TP-HE PD. In other words, the excitation condition of the coupled TPs modes is satisfied. This indicates that the coupled dual TPs modes are excited and it is the excitation of the coupled TPs that leads to the nearly perfect absorption (100%) of Atot and the ultranarrow bandwidth for the dual TP-HE PD.
The judgment is further verified by exploring the effects of the thickness of top Au, intermediate TiO2 and bottom Au layers on the reflection spectra and coupling condition function |F|. Figure 4(a) shows that almost zero reflection is achieved with thickness of top Au, intermediate TiO2 and bottom Au layers set as 5 nm, i.e., most of the incident light is absorbed by the MIM structure. The reflection dramatically approaches to 100% as thickness of the top Au, TiO2 and bottom Au layers is varied. It happens because the coupled dual TPs is not excited, since the coupling condition |F| deviates from zero with thickness of these layers varying, as is shown in Fig. 4(b). In other words, the excitation of the coupled dual TPs plays the key role in maximizing the light absorption in MIM structure.
Fig. 4. (a) Reflection spectra and (b) coupling condition |F| as function of thickness of top Au, intermediate TiO2 and bottom Au layers.
The calculation above demonstrates that large absorption is achieved in top Au layer by optimizing the top branch of coupled dual TPs. Besides, large absorption Ab can also be achieved in bottom Au layer by optimizing the bottom TPs branch. For this purpose, structure parameters of λt, λb, Nt and Nb are optimized to be 789 nm, 769 nm, 11 and 20, respectively. With these parameters, Ab reaches the maximum of over 97% at a resonant wavelength of 800 nm and the FWHM is also less than 0.4 nm. This happens because the coupled dual TPs mode is excited, since the coupling condition function |F| nearly equals to zero, as is shown in Fig. 5(a). In addition, the bottom branch of the coupled mode is optimized to be stronger than the top one. It is supported by Fig. 5(b) where the electric field maximum in the bottom 1DPCs is larger than the top one. As a result, larger electric field occurs within the bottom Au layer (see Fig. 5(c)) and a large absorption can be obtained in bottom Au layer.
Fig. 5. (a) Absorption Ab in bottom Au layer and coupling condition |F| for dual TP-HE PD with the bottom TPs branch optimized; (b) and (c) the normalized electric field |E|2 along z axis of dual TP-HE PD and MIM structure.
Furthermore, we demonstrate that the peak wavelength of the absorption spectrum can be tuned to any randomly given wavelength by properly designing the structure parameters. As is shown in Fig. 6, absorption spectra of At in top Au layer or Ab in bottom Au layer are tailored to peak at wavelength of 700 nm, 800 nm, 900 nm, 1000 nm and 1100 nm when the structure parameters such as λt, λb, Nt and Nb are set at proper values. In all these wavelengths, peak values for all the At spectra reach over 98% and those for Ab spectra reach over 97%. The FWHM for both cases is less than 0.4 nm.
Fig. 6. At (a) and Ab (b) spectra peaking at different wavelengths by tailoring the structure parameters.
Finally, for devices designed at a resonant wavelength of 800 nm, we investigate the effect of thickness on total absorption. As shown in Fig. 7(a), as the thickness of the top Au layer varies from 5 nm to 10 nm with a step of 1 nm, the resonant wavelength is red shifted from 800 nm to 803.9 nm with the total absorption Atot is decreased from 100% to 92.5%. A similar phenomenon occurs with increasing TiO2 thickness shown in Fig. 7(b). The resonant wavelength is red shifted from 800 nm to 804.5 nm with the total absorption Atot is decreased from 100% to 93.6%.
Fig. 7. Total absorption Atot spectrum as function of the thickness of top Au layer (a) and intermediate TiO2 layer (b).
2.2 Electrical calculation
Hot electrons are generated in Au layer after the Au layer absorbs photons under light illumination. The generation rate for hot electrons G(z, v) can be calculated as [29]:
where v is the frequency of the photon, ɛi is the imaginary part of the dielectric constant of metal, EE(z,v) is the electric field at position z, and h is the Planck constant. Pr is resistive loss assumed to be a constant value of 25% [17]. After a photon with energy hv is absorbed in Au layer, a hot electron is excited from an initial energy Ei (EF-hv < Ei<EF) to a higher energy state EF + E0, where EF is Fermi energy, and E0 is the excess energy above the Fermi level (0<E0<hv) [45]. The energy distribution of the hot electrons is critical for the collection because only the hot electrons with sufficiently high energy and momentum are able to cross the Au/TiO2 Schottky barrier. The initial energy density distribution of the hot electrons can be determined by the function of electron distribution joint density of states (EDJDOS) [45,46]: where E is the energy of the excited electron, D(E) is the electron density of states as a function of the electron energy, hv is the energy of the incident photon, E-hv is the initial electron energy, and f(E) is Fermi distribution function.Figure 8(a) shows the initial energy distribution of hot electrons. The green area indicates the hot electrons with energy higher than the Schottky barrier. Figure 8(b) shows the spatial distribution of generation rate of hot electrons for the coupled dual TP-HE PD of which the absorption in the top Au layer is optimized. The interfaces of top-Au/top-TiO2, top-Au/ intermediate-TiO2, bottom-Au/intermediate-TiO2 and bottom-Au/bottom-TiO2 are denoted as interfaces 1, 2, 3 and 4, respectively. It is found that almost all the hot electrons are generated in the top Au layer and most of the hot electrons are distributed near interface 1. In the coupled dual TP-HE PD, four Schottky junctions occur at the four Au/TiO2 interfaces, which are denoted as Schottky junctions 1, 2, 3 and 4, respectively.
Fig. 8. (a) Initial energy distribution of hot electrons with a wavelength of 800 nm and 1100 nm. (b) Spatial distribution of generation rate of hot electrons.
In a single Au layer (e.g., top Au layer), half of the hot electrons will travel towards interface 1 with an incidence angle θ after being excited and the other half will travel towards interface 2, as is shown in Fig. 9. Hot electrons may travel back and forth due to the reflection at interfaces 1 and 2 [47]. The hot electrons with energy exceeding φB have the probability to be emitted into top TiO2 layer or intermediate TiO2 layer every time when they reach interface 1 or 2 [47].
To calculate the total emission probability, the hot electrons in top Au layer which move toward interface 2 initially is first considered. It is also considered that the initial angular distribution R(θ) of hot electrons is anisotropic [48,49]. For the hot electrons with a distance z to interface 2, the excess energy attenuates when they transport and they have a transport probability $P_{\textrm{t}_{1}}^{\prime}$ reaching the interface 2. Only the hot electrons with a kinetic energy perpendicular to the interface higher than φB are likely to be emitted into TiO2 layers and the others will be reflected back [47]. At the same time, the emission of hot electrons is limited within an escape cone by an “acceptance angle θa” [48,49], which is due to momentum mismatch of hot electrons between the Au and TiO2 but the momentum parallel to the interface direction needs to be conserved. This also affects the transmission probability $T_1^{\prime}$ of hot electrons at the interface 2 [50]. The corresponding emission $P_{1}^{\prime} = {R} \cdot {P}_{{\textrm{t}_{1}}}^{\prime} \cdot \textrm{T}_{1}^{\prime}$ and reflection probability $1 - P_{1}^{\prime}$ at the interface 2 can be calculated according to the method presented in Ref. [7,47]. The reflected hot electrons will travel toward interface 1 and the corresponding emission and reflection probabilities at this interface are $P_{2} = (1 - P_{1}^{\prime}) \cdot P_{2}^{\prime}$ and $1 - P_{2}^{\prime}$, respectively. After n times reflection, the emission probabilities of the hot electrons at interface 1 and 2 before the excess energy is reduced to be below φB are calculated by the expression $P_{2n} = P_{2n}^{\prime} \cdot \mathop \prod \nolimits_{k = 1}^{2n - 1} (1 - P_{k}^{\prime})$ and $P_{2n + 1} = P_{2n + 1}^{\prime} \cdot \mathop \prod \nolimits_{k = 1}^{2n} (1 - P_{k}^{\prime})$, respectively. The excess energy of the hot electrons reaches the interface 1 or 2 after n times reflection is approximated as:
In this equation, lMFP is the mean free path of hot electrons in Au which is energy-dependent and ln is the distance travelled by the hot electrons to the interface 1 or 2 for the nth time. Therefore, the maximum number nmax of hot electrons reaching the interface depends on En, which requires En > φB [47]. The emission probability of hot electrons in top Au layer with initial moving direction toward interface 2 and finally traveling across Schottky barrier 2 is given as: In a similar manner, the emission probability P1,2 of hot electrons in top Au layer moving toward interface 1 initially and finally travelling across Schottky barrier 2 can also be calculated. Therefore, the total emission probability of hot electrons travelling across Schottky barrier 2 is given as:The internal quantum efficiency (IQE) of the hot electrons in top Au layer travelling across the Schottky barrier 2 is given by:
Figure 10 demonstrates the proportion of Nt,1, Nt,2, Nb,3, and Nb,4 to the total hot electrons emitted into the TiO2 layers. The wavelength of the incident light is set as 800 nm and the structural parameters are optimized to maximize At. It is found that more than half of the generated hot electrons are emitted into the top TiO2 layer (Nt,1) and nearly half of the generated hot electrons are emitted into the immediate TiO2 layer (Nt,2). The proportion of Nt,1 is larger than that of Nt,2 because most of the hot electrons are generated near interface 1 and they need to travel a longer distance to reach interface 2, which results in a larger energy loss for the hot electron due to electron-electron and electron-phonon scatterings [4]. Therefore, the proportion of hot electrons emitted into the top TiO2 layer is larger than that of the hot electrons emitted into the intermediate TiO2 layer.
Fig. 10. Proportion of Nt,1, Nt,2, Nb,3, and Nb,4 to the total number of hot electrons emitted into the TiO2 layer.
The traditional way to collect hot electrons is connecting Schottky junctions 2 and 3 in series configuration [7,12,24], as is shown in Fig. 11(a). In this case, hot electrons generated in top Au layer are emitted into the intermediate TiO2 layer and can be collected by the bottom Au layer. A forward photocurrent can be formed from the bottom Au layer to top Au layer. On the other hand, hot electrons generated in the bottom Au layer experience a reverse moving process and form reverse photocurrent. The photocurrent of the device is calculated as the difference between the forward and reverse photocurrent:
where e is element charge. The calculated results are shown in Fig. 11(c). A peak responsivity of 9.78 mA/W with an ultranarrow-bandwidth of less than 0.4 nm is achieved at the resonant wavelength of 800 nm. At the wavelength of 1100 nm, the responsivity still remains at a relatively high value of 0.18 mA/W. In this traditional connection method, however, more than half of the generated hot electrons are not collected (Nt,1, and Nb,4).Fig. 11. Schematic for dual TP-HE PD with two Schottky junctions in series configuration (a) and four Schottky junctions in parallel configuration (b) Balls with red and blue edges respectively denote the hot electrons generated in the top and bottom Au layers. Responsivity of devices with two Schottky junctions in series configuration (c) and with four Schottky junctions in parallel configuration (d) when At in the top Au layer is optimized at different wavelengths by tailoring the structural parameters.
To circumvent this problem, dual TP-HE PD with four Schottky junctions (Schottky junctions 1, 2, 3 and 4) in parallel configuration is proposed, as is shown in Fig. 11(b). In this case, the hot electrons in top and bottom Au layers emitted into both adjacent TiO2 layers in contact are collected and the total photocurrent can be expressed as:
Figures 11(c) and 11(d) demonstrate the responsivity of the dual TP-HE PDs with absorption At in the top Au layer maximized (the absorption spectra are shown in Fig. 6(a)). It is found that the responsivity of the device with peak wavelength of 800 nm reaches a high value of 21.87 mA/W for the device with four Schottky junctions in parallel configuration, which is more than 2 times that (9.78 mA/W) of the device with two Schottky junctions in series configuration. Assume that shot noise is dominant in the contribution of dark current, the noise equivalent power (NEP) and the photodetectivity (D*) [51,52] at the wavelength of 800 nm are 0.33 pw Hz1/2 and 2.97×1012 Jones, respectively.
In addition, the FWHM of the responsivity spectra is less than 0.4 nm, which is the narrowest compared with the hot electron photodetectors reported so far in previously published papers [11,12,29–31]. By tailoring the structure parameters, the ultranarrow response can be tuned to 700 nm, 800 nm, 900 nm, 1000 nm and 1100 nm and a responsivity of 36.65 mA/W, 21.87 mA/W, 10.48 mA/W, 3.48 mA/W and 0.57 mA/W is achieved.
Finally, we investigate the effects of Au thickness on the performance of the dual TP-HE PDs device. For this purpose, the thickness of the top Au layer varies from 5 nm to 20 nm with a step of 5 nm. λt, λb, Nt, Nb and the thickness of top and bottom TiO2 layers are optimized to maximize At in the top Au layer. In the optimization process, the wavelength is also set at 800 nm. For the optimized device with two Schottky junctions in series configuration, the forward net absorption increases and approaches to 100% when thickness of the top Au layer is increased up to 20 nm, as is shown in Fig. 12(a). However, the responsivity is decreased from 9.78 mA/W to 0.74 mA/W since the IQE reduces when a thicker Au layer is used, as is shown in Fig. 12(b). As to the device with four Schottky junctions in parallel configuration, an increase in the thickness of Au layer also decreases the responsivity from 21.87 mA/W to 6.41 mA/W due to the reduced IQE. This happens because the lifetime of hot electrons in metals is only a few tens of femtoseconds [53–55]. Correspondingly, the mean free path of hot electrons is energy-dependent, which are usually only several of nanometers [55–57]. For a device with a thicker Au layer, less hot electrons can reach the Au/TiO2, which leading to a lower IQE and responsivity.
Fig. 12. (a) Forward net absorption Af and total absorption Atot of the optimized dual TP-HE PDs with different thickness of top Au layer. (b) Responsivity as function of the thickness of top Au layer for the TP-HE PDs with two Schottky junctions in series configuration and with four Schottky junctions in parallel configuration.
So far, we have proposed a new type of planar hot electron photodetector, which can achieve a high responsivity with ultranarrow bandwidth at a specific wavelength by designing appropriate structural parameters. In practical fabrication of the devices, the thickness of the films in the fabricated device may deviate from the values of the designed structural parameters due to the fabrication accuracy. The effects of the deviation can be evaluated by Fig. 7, where the absorption peaks are redshifted with increase in the thickness of the top Au layer or the intermediate TiO2 layer. Fortunately, the total absorption remains at a high value of above 90% as the absorption peak shifts. The inclusion of the fabrication accuracy in the optical simulation may restrict the deviation of the absorption peak in a controlled wavelength region. Another assumption in the simulation is that the interfaces in the device is flat. For the practical fabricated device, roughness occurs at the Au/TiO2 interfaces. The roughness may lead to higher injection efficiency compared with a flat surface since the momentum mismatch can be relaxed by interface roughness scattering [58].
3. Conclusions
In summary, a new type of hot electron photodetector based on coupled dual TPs is proposed by sandwiching the MIM structure of Au/TiO2/Au between two 1DPCs. By tailoring thickness of the top and bottom TiO2 layers adjacent to Au layer, central wavelength and pairs of top and bottom 1DPCs, the two TPs at the interfaces of the Au layer and two 1DPCs can be coupled, leading to large electric field on both sides of the MIM structure. As a result, a high absorption of over 98% in the top Au layer with FWHM less than 0.4 nm is achieved at a randomly given wavelength by optimizing the structural parameters of 1DPCs. It is also demonstrated that light absorption in the bottom Au layer can also reach a high value of over 97% when the bottom branch of the coupled TP mode is optimized. Finally, coupled dual TP-HE PDs with four Schottky junctions in parallel configuration are proposed to circumvent the hot electrons loss. It is revealed that the responsivity of this type of devices reaches a high value of 21.87 mA/W at the wavelength 800 nm, which is more than 2 times that of the conventional devices with two Schottky junctions in series configuration. Furthermore, the effect of the thickness of Au layer on responsivity is investigated, which indicates a thinner Au layer used in the coupled dual TP-HE PD can lead to a higher responsivity.
Funding
National Natural Science Foundation of China (11774099); Guangdong Natural Science Funds for Distinguished Young Scholar (2014A030306005); Characteristic Innovation Project of Ordinary University of Guangdong Province (2019KTSCX018); Natural Science Foundation of Guangdong Province (2016A030313398); Key-Area Research and Development Program of Guangdong Province (2019B020214005, 2019B020219002); Guangdong Provincial Rural strategic revitalization project in 2019 (YCN (2019) No. 73).
Disclosures
The authors declare no conflicts of interest.
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