Carrier Collector Research Articles

Fabrication and Characterization Study of Porous Silicon for NIR-VIS Photodetector Applications

In this paper, photoelectrochemical etching of the n-type silicon (n-Si) wafers is used to prepare porous silicon (PSi) with current density of 10 mA.cm[Formula: see text] for 10 min. Moreover, the structural and morphological properties of the n-PSi were analyzed by using XRD and AFM, FTIR and PL. XRD patterns of PSi show that the films are single-crystalline, with cubic structure. The major peak characteristics are allocated to plane (004). The atomic force microscope image and the distribution chart of the grains of the n-PSi displayed that the grain sizes were −8.98. FTIR is a powerful and easy-to-use technique to obtain the surface chemical state of PSi. The convenience results from the transparency of silicon for IR light and the high surface area. The basic features begin from the knowledge of the bondings to hydrogen, [Si–H], and to oxygen [Si–O]. The model calculations sometimes provide useful information in the assignment. By examining FTIR, the active bonds showed the formation of PSi. PL spectra were measured in the range of (600–900) nm, the emission peak for the fixed excitation wavelength at 500 nm, and spectral 1.78 eV and 1.24 eV. PSi is recognized as an attractive building block for photonic devices because of its novel properties including high ratio of surface to volume and high light absorption. We first report NIR and VIS (PDs) fabricated by PSi as a carrier collector and a photoexcitation layer.

Performance promotion of aluminum oxide capping layer through interface engineering for tunnel oxide passivating contacts

High-efficiency silicon solar cells depend on passivating contact structure to decrease recombination losses at the crystalline silicon (c-Si) surface and the interface of metal/Si contact. Tunnel oxide passivating contact (TOPCon) technology is one of these structures, in which heavily doped poly-Si is placed over a SiO x layer grown directly on the c-Si wafer. The passivation performance can be further improved by depositing a thin aluminum oxide (AlO x ) layer on one side of the symmetric cell structure, which is attributed to the reduced interfacial defects, facilitated formation of carrier collector layer, and excellent field-effect passivation. Atomic layer deposition (ALD) may be utilized to produce high-quality and pinhole-free AlO x capping layers due to its controllability, homogeneity, and conformality. However, insufficient substrate surface activity is a challenge faced in AlO x deposition because it can impair Al(CH 3 ) 3 nucleation, resulting in an island-like growth over the first few ALD cycles, ultimately deteriorating the passivation effect of AlO x . In this study, we optimized the deposition process of ALD AlO x through a series of single-factor experiments. Subsequently, alternative substrate treatment methods were tested to compare the performance. In particular, we proposed a two-step interface engineering process (chemical mechanical polishing + RCA SC-2 treatment) to enhance the comprehensive performance of the ALD AlO x layer, achieving a high iV OC of 726 mV and low J 0 of 5.4 fA/cm 2 , compared with a reference poly-Si/c-Si passivating contact ( iV OC = 703 mV, J 0 = 19.4 fA/cm 2 ). The result was interpreted in terms of gradually improving substrate surface quality and wettability, together with the gradually decreasing thickness of the interfacial oxide layer. The circumvent of the island growth mode improved the growth quality of the subsequent film, effectively compressed the ALD cycle needed for stable growth, and enhanced the passivation property. Altogether, the present data may guide further efforts to obtain high-efficiency silicon-based solar cells by arising the interface engineering. • The optimum ALD AlO x process parameters are determined to optimize the AlO x coating process for industrial production. • A novel two-step interface engineering process (CMP + RCA SC-2 treatment) is proposed. • The overall passivation quality of the AlO x capped symmetric structure can be improved and the ρ c can be reduced simultaneously.

Carrier Diffusion in GaN : A Cathodoluminescence Study. II. Ambipolar versus Exciton Diffusion

We determine the diffusion length of excess carriers in $\mathrm{Ga}\mathrm{N}$ by spatially resolved cathodoluminescence spectroscopy utilizing a single quantum well as a carrier collector or carrier sink. Monochromatic intensity profiles across the quantum well are recorded for temperatures between 10 and 300 K. A classical diffusion model accounts for the profiles acquired between 120 and 300 K, while for temperatures lower than 120 K, a quantum capture process has to be taken into account in addition. Combining the diffusion length extracted from these profiles and the effective carrier lifetime measured by time-resolved photoluminescence experiments, we deduce the carrier diffusivity as a function of temperature. The experimental values are found to be close to theoretical ones for the ambipolar diffusivity of free carriers limited only by intrinsic phonon scattering. This agreement is shown to be fortuitous. The high diffusivity at low temperatures instead originates from an increasing participation of excitons in the diffusion process.

Analysis of passivation property using thin Al2O3 layer and simulation for realization of high-efficiency TOPCon cell

Conventional p–n junction solar cells exhibit good thermal stability but poor carrier selectivity. The main aim of this study was to achieve improved passivation along with carrier-selective contact for a solar cell having a tunnel oxide passivated contact (TOPCon) structure. To this end, we attempted to optimize the polysilicon (poly-Si) layer deposited on an ultrathin (~1.3 nm) SiO2 tunnel oxide layer to achieve high carrier lifetime and low recombination. We observed the passivation properties under varied thicknesses of the poly-Si layer and the phosphine (PH3) gas flow rate at three different annealing temperatures. Our experimental approach was able to achieve the best passivation properties by use of a moderately thin poly-Si layer. The poly-Si layer thickness was varied from 55 to 113 nm, and the PH3 flow rate was varied from 10 to 80 sccm at three different annealing temperatures in the range of 800–950 °C. The 113-nm-thick poly-Si layer was able to yield an implied open-circuit voltage (i-VOC) of 733 mV along with a very low recombination current density (J0) of 5.2 fA/cm2 at a PH3 flow rate of 40 sccm and post-deposition annealing temperature of 900 °C. A higher annealing temperature resulted in damage to the substrate, which in turn led to poor passivation; hence, optimization of this temperature was necessary. The passivation properties were further improved via the deposition of a thin (10-nm thick) Al2O3 layer on one side (i.e., front side) of the symmetric cell structure. This deposition caused a decrease in J0 to 2.5 fA/cm2 and an increase in i-VOC to 742 mV. These results are attributed to a decrease in the number of interfacial defects, promoted formation of a carrier collector layer, and good field-effect passivation. We performed a simulation study for the realization of a TOPCon solar cell and employed therein the experimentally determined parameters, which yielded highly promising cell results. Our proposed simple and cost-effective approach has high potential for future large-scale production of high-efficiency TOPCon solar cells.

High performance broadband bismuth telluride tetradymite topological insulator photodiode

A small bulk gap and the presence of Dirac electrons due to conductive surface states make tetradymite topological insulators promising candidates for optoelectronic devices. In this work, we demonstrate a highly responsive Bi2Te3–Si heterostructure photodiode. The thermally evaporated Bi2Te3 film, exhibiting a nanocrystalline nature, shows p-type doping behavior due to bismuth vacancies. As a result of the work function difference between Bi2Te3 and p-type Si, charge transfer occurs and a Schottky barrier is formed. Using the thermionic emission model, the barrier height (ΦB) is extracted to be ∼0.405 eV. For minimizing the effect of extrinsic defects, the photodiodes were capped with graphene or Si3N4. Since graphene acts as an efficient photoexcited carrier collector, the graphene capped device outperforms the Si3N4 capped device. The higher quality Bi2Te3 nanocrystalline film of the Si3N4 capped photodiode contributes to a one-order-of-magnitude improvement in responsivity at 1550 nm wavelength, as compared to the graphene capped photodiode. The Si3N4 capped photodiode shows photoresponse even at zero bias for 1550 nm wavelength. Built-in potential due to charge transfer at the interface of Bi2Te3 and Si capped with a graphene electrode exhibits the highest responsivity (8.9 A W−1). Broadband photodetection is observed in both types of photodiodes.

Sb2Te3/graphene heterostructure for broadband photodetector: A first-principles calculation at the level of Cooper’s exchange functionals

Low optical absorption and large energy band gap in the material utilized as an absorbing layer prevented a report of high-performance broadband photodetector. However, integrating antimony telluride (Sb2Te3) with graphene in a heterostructure appear to be the more promising approach to overcome the said issues. In this heterostructure, optical absorption takes place in Sb2Te3 while graphene acts as charge carrier collector owing to its high carrier mobility. Hence, detailed knowledge of the electronic and optical properties of Sb2Te3/graphene heterostructure, is essential to expose its hidden potentials. In this study, electronic and properties of Sb2Te3 surface states ad Sb2Te3/graphene heterostructure are presented using first-principles approach based on density functional theory (DFT) framework within the most recent developed Coope’s exchange (vdW-DFC09x). Our surface states band structure calculations show that Sb2Te3 has metallic behaviour as its valence and conduction bands are found to be overlapped when the film thickness is four quintuple layers (QLs). The energy analysis of Sb2Te3/graphene heterostructure reveals that the most stable configuration is the one in which the Te-1 atom of Sb2Te3 facing to graphene is above the hole center of graphene’s hexagonal lattice. More attractively, the system of Sb2Te3/graphene heterostructure shows that strong hybridization between Sb2Te3 and graphene at smaller interlayer distance resulted in an energy gap at the Dirac states. The Results of optical absorption show that Sb2Te3/graphene heterostructure has strong absorption in the near infrared to ultraviolet wavelenghts, therefore, it can be used in optoelectronics devices like optical communication, biomedical imaging, remote sensing, and gas sensing.

High performance, self-powered photodetectors based on a graphene/silicon Schottky junction diode

In this work, we design and demonstrate a graphene/silicon (Gr/Si) van der Walls (vdW) heterostructure for high-performance photodetectors, where graphene acts as an efficient carrier collector and Si as a photon absorption layer. The Gr/Si heterojunction exhibits superior Schottky diode characteristics with a barrier height of 0.76 eV and performs well as a self-powered detector responding to 532 nm at zero bias.

Enhancement of carrier-collection by electrospun Nb-doped TiO$$_2$$ nanofiber-network in photoelectrode of dye-sensitized solar cells

Conductive nanofiber(NF)-network in nano scale is expected to act as a collector of excited carriers in a medium with short diffusion length of carriers. For the effective usage of conductive NF-network in mesoporous media, we tried to actualize the composite structure of electrospun NF-network of Nb-doped TiO $$_2$$ (TNO) in a mesoporous TiO $$_2$$ layer keeping the network and its direct connection to conductive substrate. The effects of inserted NF-network with different mesh size were examined for the performance of dye-sensitized solar cells. By the insertion of NF-network, the power conversion efficiency $$\eta$$ was increased by 29.3% resulting from the increase of the short circuit current density $$J_\text{sc}$$ by 31.3%. From the time-resolved spectra of photovoltage and photocurrent, and electrochemical impedance spectra, it was found that photoelectrodes with NF-network had long lifetime of the electrons $$\tau _\text{e}$$ , small recombination rate of electrons $$k_\text{eff}$$ , and large steady state electron density $$n_\text{s}$$ . The enhancement of photocurrent and the external quantum efficiency also shows that TNO NF-network acts as a good carrier collector with the low leakage rate of photoexcited carriers.

Solution assembly MoS2nanopetals/GaAs n–n homotype heterojunction with ultrafast and low noise photoresponse using graphene as carrier collector

A MoS2nanopetal/GaAs homotype heterojunction with ultrafast and low noise photoresponse was solution assembled using graphene as the carrier collector.

Flexible High Performance Hybrid Azo/Ag-Nanowire/Azo Sandwich Structured Transparent Conductors for Flexible Cu(In,Ga)Se2 Solar Cell Applications

Transparent conducting oxides (TCOs) are an important class of materials used in a wide range of optoelectronic technologies, including solar cells, flat-panel displays and organic light emitting diodes, and presently there is a strong drive toward the development of solar cell technologies on flexible platforms that are robust, portable and flexible enough to conform to unusual shapes, particularly building structures. Tin-doped indium oxide (ITO) is currently the commonly chosen TCO for use in non-flexible applications; however, its brittleness limits its application on flexible substrates, whilst the scarcity of indium maintains an undesirably high cost for use in solar cell applications. As such, it is important that ITO alternatives are developed with suitably flexible mechanical and electronic properties, which can also be fabricated at reasonable costs. The chalcopyrite absorber, copper indium gallium selenide, Cu(In,Ga)Se2, has a very high absorption coefficient (~104 cm-1), requiring only thin-films (~1 μm) for efficient solar cell fabrication, making it thin enough for flexible applications. However the lack of a high quality flexible TCO has, to date, limited its use in such applications. In this work, we present a new approach for the fabrication of high performance transparent electrodes suitable for flexible CIGS solar cell applications, using a stacked Al-doped ZnO/Ag-nanowire/Al-doped ZnO (AAA) sandwich structure.1 In our AAA electrode the Al-doped ZnO (AZO) layers act as a broad area charge carrier collector and protective encapsulant for the Ag-nanowire network. Concurrently, the Ag-nanowires provide low-resistance, long-range pathways that aid the extraction of charge carriers, but more importantly, they aid in maintaining a low resistivity for our total electrode structure, by maintaining charge transport pathways across cracks that develop in the AZO during repeated strenuous bending. By optimising of our Ag-nanowire network density, AZO deposition and AAA stack structure we are able produce a transparent electrode with comparable resistivity and transparency performance to that of ITO. These combined benefits enable us to fabricate CIGS solar cells that are able to maintain 95% of their initial power conversion efficiency, following 1000 bending cycles. In comparison, devices fabricated using equivalent thickness AZO and ITO electrodes are only able to maintain 57 and 5%, respectively, due to crack formation and delamination of the films. This AAA sandwich structure electrode could therefore serve as a high-performance electrode for numerous flexible optoelectronic applications. 1. W.-C. Tsai, S. R. Thomas, C.-H. Hsu, Y.-C. Huang, J.-Y. Tseng, T.-T. Wu, C.-h. Chang, Z. M. Wang, J.-M. Shieh, C.-H. Shen and Y.-L. Chueh, Journal of Materials Chemistry A, 2016.

Bias-Free Operational UTC-PD above 110 GHz and Its Application to High Baud Rate Fixed-Fiber Communication and W-Band Photonic Wireless Communication

We present the design of a bias-free uni-travelling-carrier photodiode (UTC-PD) that is operational above 110 GHz, and its application to 100-GBd fixed-fiber communication and 109-GHz carrier-photonic wireless communication for advanced optical networks. The design and analysis for bias-free operation were explained in detail using the carrier concentration in the photo-absorption layer and carrier collector layer. In the experimental results, the developed UTC-PD with back-illuminated structure exhibited a wide 3-dB bandwidth of over 110 GHz at 0 V. For fixed-fiber communications, a high-baud-rate photoreceiver based on the UTC-PD was fabricated. Eye diagrams showed clear openings at up to 107 GBd and high photo-currents of 3–7 mA at 100 GHz signal light were observed at zero-bias. Assuming photonic wireless communication using photonic power supply through optical fiber, the developed UTC-PD contributed to reducing bias feeds to the photoreceiver module. Additionally, low power consumption was achieved when using the newly developed 110 GHz InP-PHEMT amplifier with the UTC-PD.

Near-ultraviolet-sensitive graphene/porous silicon photodetectors.

Porous silicon (PSi) is recognized as an attractive building block for photonic devices because of its novel properties including high ratio of surface to volume and high light absorption. We first report near-ultraviolet (UV)-sensitive graphene/PSi photodetectors (PDs) fabricated by utilizing graphene and PSi as a carrier collector and a photoexcitation layer, respectively. Thanks to high light absorption and enlarged energy-band gap of PSi, the responsivity (Ri) and quantum efficiency (QE) of the PDs are markedly enhanced in the near-UV range. The performances of PDs are systemically studied for various porosities of PSi, controlled by varying the electroless-deposition time (td) of Ag nanoparticles for the use of Si etching. Largest gain is obtained at td = 3 s, consistent with the maximal enhancement of Ri and QE in the near UV range, which originates from the well-defined interface at the graphene/PSi junction, as proved by atomic- and electrostatic-force microscopies. Optimized response speed is ∼10 times faster compared to graphene/single-crystalline Si PDs. These and other unique PD characteristics prove to be governed by typical Schottky diode-like transport of charge carriers at the graphene/PSi junctions, based on bias-dependent variations of the band profiles, resulting in novel dark- and photocurrent behaviors.

Remote collection and measurement of photogenerated carriers swept by surface acoustic waves in GaN

The interaction of surface acoustic waves and photogenerated carriers in GaN has been used for the fabrication of a remote ultraviolet detector where the carrier collector electrode is far away from the illuminated region. In this device, the recombination of the photogenerated carriers at the region where they are created is prevented by the potential fields associated with the acoustic wave, and the carriers are swept by the acoustic wave to the collector electrode. This effect is strongly dependent on the frequency and power of the acoustic waves and therefore of the input radio frequency signal. New optoelectronic devices based on the combination of the acoustic and electronic properties of the semiconductors can be envisaged.