Pn Homojunction Research Articles

Electrostatically Induced Black Phosphorus Infrared Photodiodes

AbstractHomojunctions are key elements in many mainstream electronic devices. However, conventional dopant‐based “pn” homojunctions are not easily achievable in new material families, such as the 2D materials. Several recent 2D material studies have shown that lateral pn homojunctions can instead be electrostatically induced using back gates localized to either the source or drain contacts. Here, a hBN‐encapsulated black phosphorus dual‐gate device containing a lateral pn homojunction, whose orientation can be switched via application of back gate voltages, is demonstrated. Importantly, this study extends the state‐of‐the‐art for this architecture by characterizing the photoresponse under infrared (λ = 2.2 µm) illumination. It is shown that when biased to form a homojunction, the device exhibits the photovoltaic effect, resulting in a specific detectivity of 8.5 × 108 cm Hz1/2 W−1 at 77 K under short‐circuit conditions, and an open circuit photovoltage up to 175 mV at 77 K. Further, it is shown that the device can be operated in photoconductive mode, allowing a high responsivity of 0.55 A W−1. This device is thus highly reconfigurable as it can be switched between photovoltaic and photoconductive modes of operation to prioritize low noise and fast response or high responsivity.

Advances in Two-Dimensional Materials for p-n Homojunctions

There has been a lot of interest in two-dimensional materials with rich properties. Due to their distinctive features, 2D materials-based p-n homojunctions have a significant deal of potential in the construction of new electrical and optoelectronic devices, which attracts people's interest. This paper gives a review of 2D pn homojunctions. including their characteristics, classification, construction strategies, application, and challenges faced. The review focuses on the construction strategy and practical applications. The construction strategy section introduces several commonly used methods, such as chemical doping, elemental doping, etc., and also provides some examples of 2D p-n homojunctions made using these methods. The application section focuses on introducing typical devices fabricated with p-n homojunctions, explaining why 2D p-n homojunctions were favored in electronic devices, and this section also gives examples to illustrate the advantages of such devices. Finally, the article also introduces some challenges faced by 2D homojunctions, such as environmental degradation and immature large-scale manufacturing. Despite these limitations, 2D homojunctions still exhibit great potential in various applications.

(Invited) Hybrid Nanodevices for Technology and Fun

Two-dimensional materials like graphene or transition metal dichalcogenides have remarkable properties, making them very attractive from a technological point of view and incredibly fun to work with. The intrinsic properties of 2D materials change drastically when going from the bulk into the true 2D limit, where they exhibit exotic quantum properties. While many groups have focused on studying and exploiting these materials, very little work has been devoted to engineering their intrinsic properties. The “all-surface” nature makes device fabrication complex, and so far, only the most conventional nanofabrication or tailoring routes have been explored. In this talk, I will present novel routes currently under exploration in our group to tailor and exploit the properties of TMDS and MOF devices based on mechanical deformation or physical nanopatterning.Also, the integration of 2D materials into superconducting circuits can lead to novel technological applications of interest for photon and radiation detectors or quantum computing.In this talk, I will present some results concerning the development of retroreflectors based on MoS2 oscillating membranes and how they can be turned into electroluminescent diodes. I will also present the tailoring of conductive channels on MoS2 FET devices. Preliminary results on lateral PN homo-junctions, memristive field-effect transistors and photonic crystal cavities will also be presented. I will present preliminary results on the integration of MOF into Hall bar structures for metrology applications. To finish, I will also present results on the integration of Graphene and MoS2 onto Superconducting nanowire single photon detectors. Figure 1

Improved performance in MoS2 homogeneous junction field effect transistors by optimizing electrodes contact

Junction field effect transistor (JFET) operates by controlling the thickness of depletion layer in channel with a reverse biased pn junction without dielectric issues, leading to high performance in nanoscale. Two-dimensional MoS2 has shown excellent electronic properties in atomic scale. MoS2 is an n-type semiconductor, therefore JFET based on MoS2 either adopts heterogeneous structure or doping of MoS2 needs to be done. Heterogeneous JFET suffers from lattice mismatching and other interface limitations. Homogeneous JFET with stable doping stands out. Herein, we propose a homogeneous JFET with stable doping by toroidal-magnetic-field controlled nitrogen plasma. The gate-channel pn homojunction in JFET exhibits excellent pn diode rectifying behavior with low reverse current. This JFET shows n-channel characteristics, and by improving contact between electrodes and MoS2, 102 improvement in mobility is demonstrated. The ON/OFF ratio reaches 104 in optimized JFETs.

Gate Tunable Lateral 2D Pn Junctions: An Analytical Study of Its Electrostatics

The electrostatics of two-dimensional (2D) lateral pn homojunctions considering the impact of electrostatic doping by means of two split bottom-gates are studied here. Analytical expressions are obtained from the solution of the 2D Poisson equation considering a depletion approximation. Straightforward analytical models for the electrostatic potential and the depletion width within both the dielectric and the 2D semiconductor are obtained for both the symmetrical and asymmetrical cases. In contrast to the case of devices with chemical doping, the obtained depletion width model of devices with electrostatic doping do not depend on the dielectric constant but only on the electrostatic potential and oxide thickness. The models describe the electrostatics of gate-tunable 2D pn junctions at arbitrary bias. A benchmark against numerical device simulations of MoS2-based pn junctions validate the analytical models.

Analysis of Silicon PN Homojunctions Using Octave

Abstract The assembly formed by a pn semiconductor homojunction together with the metal contacts necessary to connect the junction to the external circuit is a diode. The article aims to analysed and determine the internal parameters of a pn homojunction using the Octave software.

(Invited, Digital Presentation) Photovoltaic Applications Using Energy Transfer Characteristics from Quantum Dots

Silicon-related materials are most commonly used in solar cells and become now an invaluable material. However, a reported maximum energy conversion efficiency of Si solar cell is close to reaching its theoretical limits. To further improve the cell performance and create new functions, it will become increasingly important to functionalize Si materials using nanostructures. One- and zero-dimensional Si nanostructures called Si nanowires (SiNWs) and Si quantum dots (Si QDs) are increasingly being used as new solar cell materials. Our group has shown that the conversion efficiency of solar cells can be increased by the non-radiation energy transfer (NRET) from Si QDs [1]. Recently, research has been developed on compound semiconductor QDs and perovskite QDs, and the conversion efficiency has been successfully increased [3-7].n-type SiNW arrays were fabricated by electroless etching and Bosch & nanoimprint lithography followed by CVD process. The CVD process was performed to form p-type Si layer for pn homojunction. Hybrid heterojunction cells of PEDOT:PSS and n-SiNWs were also fabricated. Passivation by ozone and hydrogen were applied to improve the solar cell properties [2]. After the cell fabrications, the solar cells were coated with Si QDs, CdZnS/ZnS QDs, CdZnSe/ZnS QDs and perovskite QDs. The fabrication methods and conditions of QDs have been reported elsewhere [1-7]Energy transfer effects such as NRET are new ways of increasing solar cell efficiency. To effectively use the NRET effect, the surface of Si QDs should be fully passivated by ligand molecules. The NRET process is a highly distance-dependent phenomenon, and its dependence on the length of the passivation ligands clearly showed this. NRET efficiency was increased by shortening the ligand length from 1-octadecene to 1-octene, resulting in higher JSC, ultimately providing higher energy conversion efficiency. The efficiency was increased about 1-2 % by adding SiQDs. We also observed the same effect for CdZnS/ZnS QDs, CdZnSe/ZnS QDs and perovskite CsPbCl3 QDs. In these cases, in addition to the NRET effect, the radiative energy transfer (RET) effect also contributes to the increase in conversion efficiency.

Selective doping of a single ambipolar organic semiconductor to obtain P- and N-type semiconductors

•A well-balanced ambipolar organic semiconductor is synthesized as host semiconductor •Proper doping of the single host semiconductor yields both p- and n-type semiconductors •Organic PN homojunctions and corresponding photodiodes are constructed and characterized The idea of acquiring p- and n-type organic semiconductors (OSCs) through controllable doping of a single OSC is highly attractive. However, realization of this idea remains challenging due to the difficulties in achieving effective p- and n-doping on a single OSC simultaneously. Herein, by synthesizing a solution-processable conjugated polymer with well-balanced ambipolar transport as the host semiconductor and using appropriate dopants, we demonstrate the feasibility of obtaining p- and n-type OSCs from a single OSC. The successful doping control of the polarity and electrical property of the OSC is confirmed by electrical characterizations on doped films and devices. Then, solution-processed organic PN homojunctions based on the p- and n-doped OSC films are constructed, which provide good platforms for studying organic semiconductor physics and for fabricating functional devices like photodiodes. The results provide a paradigm for utilizing doping technique in organic semiconductor devices similar to that in silicon-based devices. The idea of acquiring p- and n-type organic semiconductors (OSCs) through controllable doping of a single OSC is highly attractive. However, realization of this idea remains challenging due to the difficulties in achieving effective p- and n-doping on a single OSC simultaneously. Herein, by synthesizing a solution-processable conjugated polymer with well-balanced ambipolar transport as the host semiconductor and using appropriate dopants, we demonstrate the feasibility of obtaining p- and n-type OSCs from a single OSC. The successful doping control of the polarity and electrical property of the OSC is confirmed by electrical characterizations on doped films and devices. Then, solution-processed organic PN homojunctions based on the p- and n-doped OSC films are constructed, which provide good platforms for studying organic semiconductor physics and for fabricating functional devices like photodiodes. The results provide a paradigm for utilizing doping technique in organic semiconductor devices similar to that in silicon-based devices.

Numerical modelling: Design and investigation of uniformly and non-uniformly doped absorber layer based PN homojunction perovskite solar cell variants

Perovskite Solar Cell (PSC) is the fastly emerging solar technology. The unexpected optoelectronic properties of perovskite material and high performance of these cells makes them an exceptional choice for solar photovoltaics. This work aims to propose and investigate four design variants of regular homojunction lead-based PSC namely uniform absorber PSC, uniform absorber optimum PSC, graded absorber PSC and graded absorber optimum PSC with structure as TCO/WS2/MAPbI3(n)/MAPbI3(p)/MoO3/Metal. The homojunction structure explored in this work identify absorber thickness, doping density, defect density, temperature, diffusion length, and lifetime of charge carriers as strong contestants in PSC design. In this paper, all the key parameters of uniform absorber PSC are evaluated, analyzed, discussed, and further fine-tuned to obtain optimum results with Power Conversion Efficiency (PCE) of 26.81%. Further, the impact of graded absorber layer on performance of PSC is investigated. Results show that grading in absorber layer will improve the performance parameters of PSC. Graded absorber optimum PSC shows efficiency of 28.26% which is higher than the uniform absorber PSC by 7.48%.

Analysis of piezoelectric PN homojunction and heterojunction considering flexoelectric effect and strain gradient

In this paper, a one-dimensional model of piezoelectric PN junction under static extension has been established. The combined influences of flexoelectricity and strain gradient are taken into account simultaneously. Closed-form expressions for the distributions of carrier concentrations and electromechanical fields of piezoelectric PN homojunction and heterojunction are given. It is observed that flexoelectricity and strain gradient have a significant impact on the distributions of electromechanical fields along the junction. The distributions of electromechanical fields for homojunction are origin-symmetric approximately, while the corresponding results for heterojunction show evident asymmetric behaviors. The electromechanical coupling behaviors of PN junction are enhanced under the action of external forces. Compared with homojunction, the distributions of electromechanical fields for heterojunction are more sensitive to the action of external force. This research offers a new idea to tune carrier transport and electromechanical characteristics of PN junction.

Low temperature annealing of nanocrystalline Si paste for pn junction formation

Low temperature annealing of p- and n-type Si paste films consisting of nanocrystal Si particles coated on Al substrates was investigated for application to low cost large-area electronic devices such as solar cells. This process based on aluminum-induced-crystallization enabled one to enhance recrystallization of the Si paste films with reduction of residual tensile stress while suppressing oxidation greatly even at low temperatures (400–550 °C), as confirmed by Raman spectroscopy, X-ray diffraction, and Fourier-transform infrared spectroscopy. pn homo-junction diodes were fabricated successfully using a p/n-type double layer Si paste film and even an n-type single layer Si paste film on Al substrates by annealing at temperatures less than 550 °C. The current-voltage characteristic of the diode using a p/n-type double layer Si paste film showed rectification with the on/off current ratio of about 3200 and the reverse current density of the order of 10−9 A/cm2 at room temperature.

MoTe2 PN Homojunction Constructed on a Silicon Photonic Crystal Cavity for High-Performance Photodetector

We demonstrate a straightforward construction of MoTe2 PN homojunction on a silicon photonic crystal cavity, which promises the realizations of cavity-enhanced optoelectronic devices integrated on silicon photonic chips. The employed silicon photonic crystal cavity has an air-slot in the middle to split it into two parts, which directly function as two individual back-gate electrodes of the top-coated few-layer MoTe2. Beneficial from MoTe2’s ambipolar property, reconfigured (PN, NN, PP, NP) homojunctions are realized by controlling the gate voltages of the two separated silicon electrodes. For instance, on/off ratios exceeding 104 are obtained from the PN or NP homojunctions, and the ideality factors could approach 1.00. On the other hand, the silicon photonic crystal cavity could enhance light absorption in the coated MoTe2, promising high-performance photodetection. By coupling the resonant mode with the MoTe2 PN junction, a high photoresponsivity of 156 mA/W is obtained at the wavelength of 1353.7 nm, though it is the limit of MoTe2’s absorption bandedge. As attributes of the PN junction, the photodetector has a dark current as low as 0.14 nA and a response bandwidth exceeding 1.1 GHz. The proposed device geometry has advantages of employing silicon photonic crystal cavity as the back gates of MoTe2 PN junction directly and simultaneously to enhance light–MoTe2 interactions, which might open a new avenue to construct MoTe2-based laser diodes and electro-optic modulators on silicon photonic chips.

Solar cells based on n+-AZO/p-BaSi2 heterojunction: Advanced opto-electrical modelling and experimental demonstration

We performed advanced opto-electrical simulations on thin-film BaSi 2 solar cells. First, absorption spectra of BaSi 2 - pn homojunction solar cells on Si substrate were calculated based on flat and/or pyramidally-textured surfaces, wherein 20-nm-thick n + -BaSi 2 was the topmost electron transport layer. By changing the front surface structure from flat to texture, the reflectance decreased in the wavelength ( λ ) range 700–1200 nm and the photocurrent density ( J ph ) delivered by the photogenerated carriers in the 500-nm thick p -BaSi 2 layer increased by 1.2 mA/cm 2 . Simulations revealed that the key factor inhibiting light absorption in the p -BaSi 2 layer was parasitic absorption in the n + -BaSi 2 and in the c-Si substrate. To solve these optical issues, we propose a new device structure, Al-doped n + -ZnO (AZO, 50 nm)/ i -ZnO (20 nm)/ p -BaSi 2 (500 nm) heterojunction solar cell (HJSC). In this device structure, the parasitic absorption reduced drastically, and J ph reached 30.23 mA/cm 2 . Furthermore, by replacing the Si substrate with a glass substrate, the light trapping worked more effectively, and the absorber layer thickness required for J ph to saturate was reduced to 1 μm, yielding 32.06 mA/cm 2 . Based on these simulation results, we manufactured n + -AZO/ p -BaSi 2 HJSC. The internal quantum efficiency exceeded 30% at λ = 600 nm, meaning that we demonstrated the operation of n + -AZO/ p -BaSi 2 HJSC for the first time. We investigated origins of small efficiencies compared to those simulated, and found that the passivation of defects in the p -BaSi 2 layer and the reduction of carrier recombination at the i -ZnO/ p -BaSi 2 interface would significantly improve the solar cell performance. This paper presents n + -AZO/p-BaSi 2 heterojunction solar cells (HJSC) to solve parasitic absorption in the n + -BaSi 2 and in the c-Si substrate of a BaSi 2 -pn homojunction solar cell. By replacing the Si substrate with a glass substrate, the light trapping works more effectively, and the absorber layer thickness required for photocurrent density to saturate can be reduced to 1 μm, yielding 32.06 mA/cm 2 . We manufactured n + -AZO/p-BaSi 2 HJSC, and demonstrated its operation for the first time. • We performed opto-electrical simulation on BaSi 2 pn-junction solar cells with flat and/or pyramidally-textured structure. • Parasitic absorption in the topmost layer significantly decreases the photocurrent density. • A new device structure, Al-doped n-ZnO/p-BaSi 2 , is proposed and demonstrated. • The importance of the carrier accumulation reduction at the ZnO/BaSi 2 interface is discussed.

SnS Homojunction Solar Cell with n‐Type Single Crystal and p‐Type Thin Film

Herein, a pn homojunction SnS solar cell is fabricated for the first time by the deposition of p‐type SnS polycrystalline thin films on the recently reported large n‐type SnS single crystals. The p‐type thin films consist of columnar grains that grow along the <100> direction, which is the same orientation as the n‐type single crystal. In addition, the interface of the pn homojunctions is void‐free and compositionally sharp. The SnS homojunction solar cell achieves an open‐circuit voltage (VOC) of 360 mV, which is as large as the highest VOC of previously reported SnS‐based heterojunction solar cells. The built‐in potential of the homojunction cell is 0.92 eV, which is close to the bandgap energy of SnS (≈1.1 eV), and larger than reported for heterojunctions (≈0.7 eV). The resulting 1.4% conversion efficiency (η) of the homojunction solar cell is smaller than the record 4–5% in heterojunctions, mainly due to the low short‐circuit current density (JSC) of 7.5 mA cm−2. Once the device structure of the homojunction cell is optimized to efficiently collect the photogenerated carriers and achieve a comparable JSC as the conventional heterojunction cells (≈25 mA cm−2), high η exceeding 4–5% will be realized with improving the VOC.

Developing hematite homojunction with titanium and magnesium dopants for photocatalyzing water oxidation

Developing homojunction can promote preferable charge cascade and improve electron transfer. Doping different metals in hematite is able to create pn homojunction, which cannot only increase electrical conductivity but also accelerate oxidation evolution kinetics. In this study, titanium and magnesium are separately doped in the first and second layers of hematite to create pn homojunction and to improve the photocatalytic ability toward water oxidation. The ratio of titanium and magnesium dopants incorporated in hematite is optimized regarding to the morphology, optical property and electrochemical feature. The highest photocurrent density of 2.99 mA cm−2 at 1.60 VRHE is obtained for the hematite homojunction photoanode prepared using magnesium to titanium ratio of 30–70. This optimized hematite homojunction photoanode also shows the smallest charge-transfer resistance, the highest carrier density and the longest electron lifetime. Also, nearly no decay on the photocurrent density is also achieved for the optimized hematite homojunction photoanode after 3600 s continuous light illumination. It is verified that the ratio of two dopants in hematite plays important roles on the photocatalytic ability toward water oxidation, since the hematite homojunction photoanode with the magnesium to titanium ratio of 50% only presents one-half of the photocurrent density and one two-thousand of the carrier density of those for the optimized photoanode. It is expected to establish more efficient hematite photocatalysts via creating more beneficial homojunction with suitable dopants.

Heteroatom Doping Strategy for Establishing Hematite Homojunction as Efficient Photocatalyst for Accelerating Water Splitting.

Hematite (α-Fe2 O3 ) is one of the promising photocatalysts for water oxidation, owing to its stable, abundant and visible-light responsive features. Enhancing electrical conductivity and accelerating oxidation evolution kinetics are expected to improve photocatalytic ability of hematite toward water oxidation. In this work, strategies of doping heteroatoms and developing pn homojunction are adopted to enhance the photocatalytic ability of hematite electrodes. The Ti and Mg dopants are separately incorporated in two layers of hematite electrodes via two-step hydrothermal reaction and one-step annealing process. The effect of regrowth time for synthesizing Mg-doped hematite on the photoelectrochemical performance of Mg-doped and Ti-doped hematite (Mg-Fe2 O3 /Ti-Fe2 O3 ) electrode is studied. The size of rod-like structure and gaps in-between play important roles on the photocatalytic ability of Mg-Fe2 O3 /Ti-Fe2 O3 . The optimized Mg-Fe2 O3 /Ti-Fe2 O3 electrode is prepared by using merely 10 min for synthesizing the Mg-doped hematite top layer, which shows the highest photocurrent density of 2.83 mA/cm2 at 1.60 VRHE along with the highest carrier density of 5.89×1016 cm-3 and the smallest charge-transfer resistance. This largely improved photoelectrochemical performance is attributed to the more donor generation with heteroatom-doping and more efficient charge cascade with homojunction establishment. Other p-type metals are encouraged to dope in hematite as the second layer to couple with the n-type Ti-doped hematite for developing efficient pn homojunction and improve the photocatalytic ability of hematite in the near future.

Lateral monolayer MoS2 homojunction devices prepared by nitrogen plasma doping

Monolayer MoS2 possesses good electron mobility, structural flexibility and a direct band gap, enabling it to be a promising candidate for flexible and wearable optoelectronic devices. In this article, the lateral monolayer MoS2 homojunctions were prepared by a nitrogen plasma selective doping technique. The monolayer MoS2 thin films were synthesized by chemical vapor deposition and characterized by photoluminescence, atom force microscope and Raman spectroscopy. The electronic and photoelectric properties of the lateral pn and npn homojunctions were discussed. The results showed that the rectifying ratio of the pn homojunction diode is ∼103. As a photodetector of pn homojunction, the optical responsivity is up to 48.5 A W−1, the external quantum efficiency is 11 301%, the detectivity is ∼109 Jones and the response time is 20 ms with the laser of 532 nm and the reverse bias voltage of 10 V. As a bipolar junction transistor of npn homojunction, the amplification coefficient reached ∼102. A controllable plasma doping technique, compatible with traditional CMOS process, is utilized to realize the monolayer MoS2 based pn and npn homojunctions, and it propels the potential applications of 2D materials in the electronic, optoelectronic devices and circuits.

Si-nanocrystals embedded in SiNWs to reduce thermalisation in solar cells

Despite their interesting efficiencies and stabilities, first and second generations solar cells suffer from thermalisation that reduces the photovoltaic performances under operating conditions. In third generation solar cells scenarios, the use of Nano-Crystals (NCs) with suitable dimensions can be a realistic alternative to overcome the thermalisation phenomena. Due to the quantum confinement, silicon NCs with sizes less than 5 nm could be an efficient tool for down conversion and/or carriers multiplication by converting high energy photons (UV) to visible ones via photoluminescence (PL).This paper gives interesting results by stacking silicon NCs in Silicon NanoWires (SiNWs) which are elaborated at the top of a silicon pn homo-junction. NCs are obtained by annealing an amorphous silicon layer deposited on the SiNWs layer. Room temperature photoluminescence measurement at an incident wavelength of 250 nm shows three PL peaks at 378 nm, 480 nm and 587 nm. The latter is attributed to silicon NCs. Furthermore, the photovoltaic efficiency is considerably enhanced by an amount of 36.3 % from 6.9 % to 9.4 % which is due to an important enhancement of 30.3 % in the absolute value of the short-circuit current density values in presence of Si-NCs. We discuss these results with regard to the physical characterizations performed on fabricated samples.

Rapid thermal annealing of Si paste film and pn-junction formation

A Si nanoparticle paste has been studied to form a Si film on a substrate. Rapid thermal annealing (RTA) was conducted in order to recrystallize the Si paste which were prepared by a planetary ball milling grinding n-doped or p-doped Si chips. It was possible to minimize the oxidation during the melting process of Si nanoparticles with this RTA even at 1200 °C in 1 s. Lowering of the melting temperature appears to be due to the size effect and release of surface energy from the Si nanoparticles. RTA was conducted in an infrared furnace with temperatures varying from 1150 to 1300 °C. Si pn homo-junction structure was also fabricated by coating p-type followed by n-type Si pastes on a carbon substrate. Typical rectifying characteristics and slight photo-induced current was observed.

Band structure and diode characteristics of transparent pn-homojunction using delafossite CuInO2

Technologically relevant highly rectifying all transparent delafossite pn homojunctions with CuInO2 as n-type layer and Ca doped CuInO2 as p-type layer are fabricated by oxygen plasma assisted thermal evaporation method. The best diode gives a forward to reverse current ratio ~619, which is about 62 times than that reported to date in a delafossite pn homojunction, with an ideality factor of 2.42 and a cut-in voltage of 0.87 V. Optical transmittance is 50%–70% in the visible region. Junction capacitance is in picoFarads and it is independent of frequency in the range 2 kHz to 2 MHz at a reverse voltage of 5 V. The band structure of the transparent delafossite diode is deduced for the first time by a combined analysis of the valence band spectra from x-ray photoelectron spectroscopy and the optical data.