External Reverse Bias Research Articles

Free carrier induced dark pulse generation in microresonators.

We theoretically demonstrate a novel, to the best of our knowledge, mechanism for dark pulse excitation in normal dispersion microresonators exploiting free carrier dispersion and free carrier absorption effects due to multi-photon absorption. Dark pulses can be generated in the three- and four-photon absorption regimes in the presence or absence of external reverse bias to control the lifetime of free carriers, respectively. Direct generation of dark pulses is proven to be feasible in both regimes with a frequency fixed laser. The dynamics of their temporal and spectral evolution have also been investigated. Our findings establish a reliable path for dark pulse and Kerr microcomb generation in related platforms with simplified controlling and tuning techniques.

Stepwise tuning of the doping and thickness of a-Si:H(p) emitter layer to improve the performance of c-Si(n)/a-Si:H(p) heterojunction solar cells

Crystalline silicon (c-Si)/ hydrogenated amorphous silicon (a-Si:H) heterojunction (Ag/Al/c-Si(n)/a-Si:H(i)/a-Si:H(p)/ITO/Ag) solar cells were fabricated by RF-PECVD technique. The efficiency (η), short circuit current density (Jsc) and fill factor (FF) of the heterojunction (HJ) solar cells were improved from 10 to 17%, 24 to 32 mA/cm2 and 0.60 to 0.74 respectively through systematic tuning of processing steps; the thickness and doping of a-Si:H(p) layer respectively. High open circuit voltage (Voc) of 705 mV could be achieved by improving the interfaces between the a-Si:H layers and c-Si by inserting an ultrathin intrinsic a-Si:H film using H2 plasma treatment on c-Si(n) wafer and a-Si:H(i) layer respectively. Simultaneously, other solar parameters like Jsc and FF were also improved step by step by carefully adjusting the gas flow rate and deposition time of doped a-Si:H(p) layer. By increasing the doping of a-Si:H(p) layer, electric field at the interface could be enhanced, which led to the immediate separation of electron–hole pair and thus improving the solar cell parameters. This is also confirmed by external quantum efficiency (EQE) spectra measured with and without external reverse bias.

Photodoping of graphene/silicon van der Waals heterostructure observed by terahertz emission spectroscopy

Photodoping as a nonvolatile and reversible method can be used to modify the carrier distribution at the heterojunction interface. Herein, we explore the 2D/3D van der Waals (vdW) graphene/silicon (G/Si) heterostructure in real time by terahertz (THz) emission spectroscopy. Photoinduced doping is introduced by a continuous wave laser, which leads to a screening effect to the built-in electric field at the interface. The resulting decrease in transient photocurrent reduces the THz emission amplitude from the G/Si heterostructure. The photoinduced doping effect suggests a 40% THz intrinsic modulation depth at external reverse bias. This work provides an effective way to actively control the THz emission process from the G/Si interface and paves the way for analyzing the interfacial process under photoinduced doping in vdW heterostructures.

Multifunctional Self-Combustion Additives Strategy to Fabricate Highly Responsive Hybrid Perovskite Photodetectors.

To resolve the inherent trade-off issue between responsivity and detectivity in FA0.9Cs0.1PbI3 perovskite photodetectors, this paper proposes a novel strategy using multifunctional self-combustion additives (urea and ammonium nitrate). During the early stages of crystallization, urea allows for the formation of a strong Lewis complex-derived low-dimensional intermediate phase; this suppresses the formation of perovskite nuclei, while ammonium ions assist the preferred grain growth along the [110] direction. During the high-temperature annealing steps, a self-combusting exothermic reaction occurs between urea as a fuel and NH4NO3 as an oxidizer, through which a locally supplied heat facilitates the removal of residual urea and byproducts. These multifunctional roles of self-combustible additives facilitate the production of high-quality, enlarged grain-structured perovskite films with improved optoelectronic properties, as confirmed by various analyses, including impedance spectroscopy and intensity-modulated photocurrent spectroscopy. The resulting FA0.9Cs0.1PbI3-based photodiode-type photodetectors exhibit outstanding performance, such as a high responsivity (0.762 A W-1) and specific detectivity (over 5.08 × 1013 Jones) at a very low external reverse bias (-0.5 V). Our findings clearly suggest that the multifunctional self-combustion additives strategy could help realize the full potential of FA1-xCsxPbI3 as a photodiode-type photodetector.

P‐83: Tuning the External Quantum Efficiency and Reverse Bias of PIN Photodiode

The boron implantation and intrinsic layer thickness effects on the external quantum efficiency of pin photo sensor are investigated. It is found that the light absorption by p+ layer and asymmetric carrier loss phenomenon will be serious in the high implantation voltage condition, which will lead to the decrease of external quantum efficiency (EQE) of photodiode. The dark current at the range of reverse bias can be reduced through controlling the hydrogen content, film stress and intrinsic layer thickness of pin photodiode. The EQE of photodiode under red light illumination (680 nm) can be enhanced from 35.8% to 45.2% by increasing the intrinsic layer thickness from 200 nm to 300 nm.

Realization of In:ZnO/PEDOT:PSS based multifunctional device for ultraviolet (UV) light detection and resistive switching memory applications

The present research reports on a hybrid multifunctional device for UV light detection and non-volatile resistive switching memory based on n-In:ZnO/poly 3,4-ethylene dioxythiophene:polystyrene sulfonate (PEDOT:PSS) junctions. Using a spray pyrolysis method, indium (1–5 at. %) doped ZnO thin films (IZO) were deposited on the pre-heated glass substrate. The structural analysis of IZO thin films shows that all the prepared samples exhibit a hexagonal wurtzite structure with preferential orientation along the (101) plane. The morphological analysis shows a uniform distribution of grains without any voids. The optical transmission spectra reveal that IZO thin films show higher transparency (>90%) in the visible region. With an optimum doping concentration of In (4 at. %), the deposited IZO thin films exhibit high carrier concentration and low electrical resistivity value of 4.58 × 1020 cm−3 and 4.01 × 10−2 Ω cm, respectively. The current–voltage (I–V), photoresponse, and resistive switching behavior of the fabricated n-IZO/PEDOT:PSS-based hybrid device was studied. Under an external reverse bias, the device exhibits a high photoresponsivity (R) value of 0.31 A/W and fast photoresponse switching speed with the measured rise and fall time of 0.08 and 1 s, respectively. It was proposed that the formation/rupture of both anionic and cationic conductive filaments plays a crucial role in the obtained resistive characteristics of the fabricated hybrid device.

Self-biasing of carrier depletion based silicon microring modulators.

We report on the self-biasing effect of carrier depletion based silicon microring modulators (MRM) by demonstrating that a silicon MRM can generate open eye diagrams for non-return-to-zero (NRZ) on-off keying (OOK) modulation without an external reverse bias supplied to it. Two modulator configurations are investigated namely single-ended drive in a ground-signal-ground and differential drive in a ground-signal-signal-ground pad configurations. The single-ended modulator is designed with an on photonic integrated circuit (PIC) 50 Ω termination. Open eye diagrams are obtained at 25 Gbit/s and 36 Gbit/s NRZ OOK modulations. We carry-out thorough experimental characterization of the self-biasing of single-ended MRM under various operating conditions of input optical power, carrier wavelength, ring quality factor and extinction ratio as well as modulation speeds, driving voltage swing and pattern length. We demonstrate that the self-biasing is robust and works well in almost all tested conditions. The differential drive MRM is designed with a high impedance without an on-PIC 50 Ω termination. Open eye diagrams are obtained at 30 Gbit/s and 60 Gbit/s NRZ OOK modulations for modulating voltage swing of ∼2.5 Vpp. As demonstrated, the self-biasing works well in both single-ended and differential drive configurations as well as for on-PIC 50 Ω terminated and non-terminated MRMs. The electrical passive parts are all co-designed and fabricated on the same silicon chip as the PIC. The reported self-biasing eliminates the need of having bipolar DC biases supplied to the anode and cathode of the differential drive modulator and allows for simpler driver / modulator interfaces without inductive bias tees.

Electroreflectance study on optical anisotropy in β-Ga2O3

Electroreflectance (ER) spectra of β-Ga2O3 were measured using Schottky barrier diodes. The ER spectra were well fitted to a third derivative line shape function by assuming three excitonic transitions. The transition energies exhibited moderate blueshifts with an external reverse bias by reflecting the contribution of transitions at M0 type critical points. Optical anisotropy was selectively observed in the polarized ER spectra. The fittings for the third derivative line shape function gave relatively large broadening parameters (0.11–0.89 eV) due mainly to the large exciton-longitudinal-optical-phonon interaction in β-Ga2O3. The ER spectra exhibited a third derivative line shape with a Franz-Keldysh oscillation superimposed on it. The results demonstrate that ER measurements are useful for investigating optical anisotropy in β-Ga2O3 and related alloys.

Comparison between Silicon-Carbide and diamond for fast neutron detection at room temperature

Neutron radiation detector for nuclear reactor applications plays an important role in getting information about the actual neutron yield and reactor environment. Such detector must be able to operate at high temperature (up to 600° C) and high neutron flux levels. It is worth nothing that a detector for industrial environment applications must have fast and stable response over considerable long period of use as well as high energy resolution. Silicon Carbide is one of the most attractive materials for neutron detection. Thanks to its outstanding properties, such as high displacement threshold energy (20-35 eV), wide band gap energy (3.27 eV) and high thermal conductivity (4.9 W/cm·K), SiC can operate in harsh environment (high temperature, high pressure and high radiation level) without additional cooling system. Our previous analyses reveal that SiC detectors, under irradiation and at elevated temperature, respond to neutrons showing consistent counting rates as function of external reverse bias voltages and radiation intensity. The counting-rate of the thermal neutron-induced peak increases with the area of the detector, and appears to be linear with respect to the reactor power. Diamond is another semi-conductor considered as one of most promising materials for radiation detection. Diamond possesses several advantages in comparison to other semiconductors such as a wider band gap (5.5 eV), higher threshold displacement energy (40-50 eV) and thermal conductivity (22 W/cm·K), which leads to low leakage current values and make it more radiation resistant that its competitors. A comparison is proposed between these two semiconductors for the ability and efficiency to detect fast neutrons. For this purpose the deuterium-tritium neutron generator of Technical University of Dresden with 14 MeV neutron output of 1010 n·s-1 is used. In the present work, we interpret the first measurements and results with both 4H-SiC and chemical vapor deposition (CVD) diamond detectors irradiated with 14 MeV neutrons at room temperature.

Nuclear Radiation Detectors Based on 4H-SiC p<sup>+</sup>-n Junction

Silicon carbide (SiC) radiation detectors were realized by 10B implantation into the metal contact in order to avoid implantation-related defects within the sensitive area of the 4H-SiC pn junction. No post implantation annealing was performed. Such detectors respond to thermal neutrons showing consistent counting rates as function of external reverse bias voltages and radiation intensity.

Electro-optical effects in strain-compensated InGaAs/InAlAs coupled quantum wells with modified potential

We design and experimentally report strain-compensated InGaAs/InAlAs coupled quantum wells with modified potential. At a lower applied electric field (F=50 kV/cm), the calculated blueshift of the lowest excitonic peak is 40.6 meV. In the room-temperature photocurrent experiments, a maximum upward shift of the apparent peak position of more than 35 meV is observed with an external reverse bias of -4 V. Furthermore, a lower absorption loss (alpha=9.8 cm(-1)) and a large negative refractive index change (Deltan=-0.0095) are obtained at 1.55 mum. This indicates that the strain-compensated InGaAs/InAlAs coupled quantum wells with modified potential have a great potential for application to reflection type electro-optical switches.

A spectroscopic study of the piezoelectric effect in ZnSe/ZnCdSe single quantum wells grown on (2 1 1)B GaAs

The piezoelectric effect has been studied in ZnSe/ZnCdSe single quantum wells grown on (2 1 1)B GaAs substrates by molecular beam epitaxy. The effect of the internal electric field on the exciton spectra has been investigated using photocurrent and photoluminescence spectroscopy on Schottky structures where an external reverse bias can be applied. The strength of the piezoelectric field has been determined from the flat band condition in each quantum well.

Observation of anomalously large blue shift of the heavy-hole photocurrent peak and optical bistability in narrow asymmetric coupled quantum wells

Blue shift of the heavy-hole peak of the photocurrent spectra has been observed, for the first time, in narrow GaAs/Al0.4Ga0.6As asymmetric coupled quantum wells near the heavy-hole resonance. With an external reverse bias of only −2.35 V, a maximum upward shift of the apparent peak position of ∼6.1 meV has been measured at 78 K. Sharp change of the inhomogeneous linewidth of the heavy-hole peak has also been observed. cw optical bistability has been observed with the external feedback.

Low frequency characterization of a photo-diode self-biased by a high frequency pump†

Abstract In an ideal photo-diode, with external reverse bias, the low frequency impedance presented to the photo-current generator (representing amplitude-modulated incident light) is very high, approaching infinity for moderate reverse voltages ; however, this is not the case for a self-biased photo-diode. It is shown that the low frequency impedance of a photo-diode, self-biased by an r.f. pump, is of the order of the forward resistance of the diode and may become equal to the forward resistance in certain oases. The results are of interest for any photo-parametric circuit in which one is concerned with the low frequency signal impedance