Forward Bias Regime Research Articles

Planarization of p-GaN surfaces on MOCVD grown V-defect engineered GaN-based LEDs

The large polarization barriers between the quantum wells and quantum barriers in long-wavelength GaN-based light-emitting diodes (LEDs) inhibit their performance by requiring excess driving voltages to reach standard operating current densities. Lateral injection of carriers directly into quantum wells is required to circumvent this issue. V-defects are naturally occurring inverted hexagonal defects with semipolar 101¯1-plane sidewalls generated on surface depressions from threading dislocations. LEDs engineered to intentionally generate V-defects below the active region of the LED can achieve lateral carrier injection through the V-defect sidewalls and have already been able to demonstrate world record wall-plug efficiencies for LEDs in the green-red wavelengths. V-defects can be enlarged during kinetically limited growth where the growth rate of the c-plane GaN is faster than that of their sidewalls, leaving them unfilled. We report on the metal organic chemical vapor deposition growth conditions required to fill in V-defects with p-GaN during epitaxial growth of the LED post the active region. Circular transmission length measurements of Pd/Au contacts processed on p-GaN surfaces with various amounts of unfilled V-defects showed no significant difference in their sheet resistance and specific contact resistance. J–V measurements of LEDs grown with varying unfilled V-defect densities showed no significant difference in the forward bias regime. However, in the reverse bias regime, catastrophic breakdown occurred at markedly lower voltages for samples with larger unfilled V-defect densities. This suggests that unfilled V-defects may act as hotspots for device failure, and planarizing LED surfaces may help prevent early degradation of LED devices.

Current transport characterization and photovoltaic performance of Si nanopencil-based Schottky junction assisted with VOx as a hole-injection layer

Pencil-shaped silicon nanowires (SiNPs) were utilized in Schottky junction solar cells covered by sub-stoichiometric vanadium dioxide (VO2-x) to work as a hole injection layer. The asymmetry of nanopencils is responsible for their many useful properties, such as their ability to absorb and trap light over a wide spectrum. Dark current-voltage (I–V) curves for an Ag/VO2-x/SiNPs/Ti/Ag Schottky junction device were measured and analyzed across a temperature range of 298–358 K. The junction parameters were calculated in terms of thermionic emission theory at different temperatures from the (I–V) curves, including the ideality factor (n) and the barrier height (φb), and were found to be 1.73 and 0.78 eV, respectively, at room temperature. In the forward bias regime, we found that thermionic emissions dominate at low voltages (V ≤ 0.12 V), space-charge-limited current controlled by a single trap state dominates at middle voltages (0.12 < V < 0.3 V), and space-charge-limited current regulated by a distribution of trap levels dominates at high voltages (V ≥ 0.3 V). The (C–V) measurements were used to calculate the built-in potential, which was discovered to be 0.62 eV. Unless encapsulation is provided, PEDOT:PSS/SiNPs hybrid solar cells rapidly degrade under ambient conditions, whereas VO2-x/SiNPs solar cells are far more stable.

Full quantum simulation of Shockley–Read–Hall recombination in p-i-n and tunnel diodes

We have included Shockley–Read–Hall (SRH) generation/recombination in Non-equilibrium Green’s function (NEGF) calculations via a multiphonon relaxation model. The model has been used to study how the presence of defects affects the current–voltage characteristics of GaAs p-i-n diodes, and of an InGaAs tunnel diode. Regarding p-i-n diodes, we show that SRH generation/recombination is responsible for ideality factors approaching the theoretical value of two in the forward bias regime, while in reverse bias the recombination current density varies slowly with the applied voltage. In all the considered cases, the defects located in the center of the active region proved to be the most effective in allowing trap-assisted tunneling from the valence to the conduction band. Finally, the inclusion of SRH recombination in NEGF simulations of Esaki tunnel diodes permits to predict a realistic degradation of the peak-to-valley current ratio.

Inspection of the Defect State Using the Mobility Spectrum Analysis Method

Mobility spectrum analysis (MSA) is a method that enables the carrier density (and mobility) separation of the majority and minority carriers in multicarrier semiconductors, respectively. In this paper, we use the p-GaAs layer in order to demonstrate that the MSA can perform unique facilities for the defect analysis by using its resolvable features for the carriers. Using two proven methods, we reveal that the defect state can be anticipated at the characteristic temperature in which the ratio () that is associated with the density of the minority carrier , to the density of the majority carrier , exceeds 50%. (1) Using a p-GaAs Schottky diode in a reverse bias regime, the position of the deep level transient spectroscopy (DLTS) peak is shown directly as the defect signal. (2) Furthermore, by examining the current–voltage–temperature (I–V–T) characteristics in the forward bias regime, this peak position has been indirectly revealed as the generation–recombination center. The DLTS signals are dominant around the , according to the window rate, and it has been shown that the peak variation range is consistent with the temperature range of the temperature-dependent generation–recombination peak. The is also consistent with the temperature-dependent thermionic emission peak position. By having only through the MSA, it is possible to intuitively determine the existence and the peak position of the DLTS signal, and the majority carrier’s density enables a more accurate extraction of the deep trap density in the DLTS analysis.

Strong Fermi-Level Pinning in GeS-Metal Nanocontacts.

Germanium sulfide (GeS) is a layered monochalcogenide semiconductor with a band gap of about 1.6 eV. To verify the suitability of GeS for field-effect-based device applications, a detailed understanding of the electronic transport mechanisms of GeS–metal junctions is required. In this work, we have used conductive atomic force microscopy (c-AFM) to study charge carrier injection in metal–GeS nanocontacts. Using contact current–voltage spectroscopy, we identified three dominant charge carrier injection mechanisms: thermionic emission, direct tunneling, and Fowler–Nordheim tunneling. In the forward-bias regime, thermionic emission is the dominating current injection mechanism, whereas in the reverse-bias regime, the current injection mechanism is quantum mechanical tunneling. Using tips of different materials (platinum, n-type-doped silicon, and highly doped p-type diamond), we found that the Schottky barrier is almost independent of the work function of the metallic tip, which is indicative of a strong Fermi-level pinning. This strong Fermi-level pinning is caused by charged defects and impurities.

Controllable spin diode based on a semiconductor quantum dot

We theoretically propose an all-electrically controlled spin-current diode consisting of a quantum dot sandwiched between one normal electrode and one ferromagnetic electrode. By applying a spin bias V S across one electrode, the spin current distribution shows a rectification effect; that is, in the forward spin bias regime, a spin current can tunnel through the junction, while in the reverse bias regime, the current is tiny. Such asymmetry in the spin-current profile suggests diode-like behaviour with respect to the spin bias. Moreover, the polarity direction of this spin-current diode can be manipulated and reversed by adjusting the gate voltage, which is much more feasible than the approach with traditional charge-current diodes. The present device can be realized by current technologies and has potential applications in spintronics or quantum information processing.

Multilayer emitter of molybdenum oxide/silver/molybdenum oxide thin films for silicon heterojunction solar cells: Device fabrication and electrical characterization

In a simplified heterojunction crystalline silicon (c-Si) solar cell, a molybdenum oxide/silver/molybdenum oxide (MoOx/Ag/MoOx) stack has been investigated as an alternative to the well-known p-type amorphous silicon hydrogenated (a-Si: H) layer or a single layer of MoOx. The optical characteristics of the MoOx/Ag/MoOx multilayer were studied and found to have acceptable features for embedding on the solar cell's front side. Using a straightforward fabrication technique, thermal evaporation, a device structure composed of Al/Ag/MoOx/Ag/MoOx/n-Si/MoOx/Mg/Al has been fabricated. Impedance spectroscopy measurements were performed and Nyquist plots revealed single semicircles in the frequency range (102 Hz - 106 Hz) and at different temperatures, indicating the formation of a junction between MoOx/Ag/MoOx and n-Si substrate and the establishment of good Ohmic contacts for the front/rear electrodes. Moreover, capacitance-voltage characteristics were used to determine the temperature dependency of the built-in potential, doping concentration, and depletion width. The temperature-dependent current-voltage plots for the cell suggested that the predominant carrier transport mechanisms in the forward bias regime are trap-assisted tunneling at V> 0.35 V and e-h pair recombination at 0.35–0.75 V. For the reverse bias regime, carriers transport is dominated by the current generation in the space-charge region. Additionally, the photovoltaic performance of the solar cell was measured under illumination.

Dependence of electrical parameters of co/gold-chloride/p-Si diode on frequency and illumination

In this study, metal-semiconductor contact with an organic interlayer was fabricated. Ideality factor (n), barrier height ( φ b ), and series resistance (R s ) values for Co/Gold-chloride/p-Si structure were calculated for dark and 100–400 mW/cm 2 light illumination intensities. The current-voltage (I–V) measurements were used to extract the electrical parameters of Co/Gold-chloride/p-Si device such as ideality factor, barrier height and series resistance using different methods like thermionic emission theory and Norde method. The n values were found in the range of 2.00–2.52 and the φ b were calculated in the range of 0.50–0.53 eV and under light power intensities. Additionally, frequency-dependent capacitance-voltage (C–V) measurements were done at room temperature and frequency range from 100 kHz to 1 MHz. The results show that at sufficiently high frequencies, the interface cannot flow A.C signals. Furthermore, barrier height was also calculated from the C −2 -V plot for given the frequency range for Co/Gold-chloride/p-Si device. With this study, it has been shown that the rectifier contact of the organic-inorganic structure formed with suitable organic and inorganic semiconductor can be formed and this structure can be used in optoelectronic applications. The electrical properties of the diodes were measured with different AC signal frequencies between −2 V and +2 V in dark. During the examination of the electrical properties, it was understood that the AC signal frequency caused significant changes in the capacitance-voltage characteristic of the diode. In the capacitance-voltage graph, it was observed that a high capacitance value was obtained at low AC frequency. The increase in the frequency value caused a decrease in the capacitance value. Increasing capacitance indicates that charges are being accumulated in the measured area. Capacity increase at low frequencies indicates that charges cannot move at low frequencies, but the high frequency can move charges. Various factors play a role in the formation of these mechanisms. These can be factors such as interface states, series resistances, charges leap, etc [ 52 ]. In addition, it seems that there is no change in capacitance values at negative voltages, that is, the voltage does not affect the capacitance values in this region. A.G.Imer et al. [ 53 ] examined the frequency dependency of C–V behavior for Au/Sunset Yellow/n-Si Schottky architecture and they found that the capacitance value increases with the frequency in the forward bias regime for the fabricated photodiode. A. Kocyigit et al. [ 54 ] carried out C–V characterizations of Al/AB/p-Si devices and they revealed that the electrical properties of those devices are a powerful function of voltage and frequency. • Gold-chloride thin films were deposited on the p-Si substrate by sol-gel spin coating method. • Co/Gold-chloride/p-Si structure was fabricated. • Photoconductivity properties of the photodiode were investigated by current-voltage measurements. • Electrical properties were analyzed by frequency-dependent capacitance-voltage measurements.

Electron transport characteristics of FeGa, Ni/n-Si junctions by impedance spectroscopy

In this work, we have investigated electron transport across Galfenol (Fe x Ga 1-x ) and Nickel-based Schottky contacts deposited onto n-Si substrates using conventional DC current-voltage (I–V) measurements and have uniquely studied the effects of improper back contact causing additional bias dependent Resistance-capacitance (R–C) components using AC Impedance spectroscopy by choosing proper equivalent circuit models. Junction barrier height and ideality factor were obtained from DC current-voltage characteristics, and bias dependent impedance measurements have been carried out. From Impedance spectroscopy analysis the magnitudes of depletion layer capacitance and barrier height have been calculated and have been compared for both the junctions, and the effect of additional components has been distinguished. Negative capacitance behaviour at lower frequencies and higher forward bias regime has been observed, which might be attributed to interface states in presence of injected charge carriers and by choosing an equivalent circuit having inductor-resistor (R-L) type relaxation, better fit of experimental Nyquist plots of FeGa Schottky junction have been achieved. • FeGa and Nickel based Schottky Junctions have been deposited onto n-Si substrates using RF magnetron Sputtering. • Junction parameters have been calculated using conventional DC and, AC Impedance measurements. • Using equivalent circuit; bias dependency of both Schottky and Ohmic interfaces for has been studied.

(Invited) Theoretical Frequency Limit of Organic Rectifier Diodes

In this communication, a new theoretical model for the ac transit frequency of organic rectifier diodes is proposed. The model is built upon a full description of the drift-diffusion current in the rectifier diode for the entire forward bias regime. Such a method precisely correlates the transit frequency to a number of materials, geometrical, and operational parameters in particular the charge-Injection barriers. By systematic variation of parameters, it is found that a rectifier operating beyond 1 GHz is observable only with a highly injecting anode. Other parameters to reach that regime of operation are discussed.This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2018R1D1AB07048877), and by Materials, Components & Equipment Research Program funded by the Gyeonggi Province.

Band-to-band tunneling mechanism observed at room temperature in lateral non-degenerately doped nanoscale p-n and p-i-n silicon devices

Non-degenerately doped lateral nanoscale p-n and p-i-n silicon-on-insulator devices have been fabricated and characterized at room temperature (297 K). In both types of devices, p-type Si substrate is used as a backgate to modify the potential in the top Si layer in both forward- and reverse-bias regimes. In the forward-bias regime, both types of devices exhibit negative differential transconductance (NDT), with the current peak position and level controlled by the backgate and anode voltage. In the reverse-bias regime, the devices exhibit a sharp current increase as a function of the backgate voltage, which is a signature of the band-to-band tunneling (BTBT) mechanism. These findings suggest that NDT and the sharp increase of current, induced by the contribution of the BTBT mechanism, can be achieved even in non-degenerately doped backgated diodes, which opens new possibilities for BTBT-based functionalities, benefiting from a simple design and CMOS compatibility.

Photonic Readout of Superconducting Nanowire Single Photon Counting Detectors

Scalable, low power, high speed data transfer between cryogenic (0.1–4 K) and room temperature environments is essential for the realization of practical, large-scale systems based on superconducting technologies. A promising approach to overcome the limitations of conventional wire-based readout is the use of optical fiber communication. Optical fiber presents a 100–1,000x lower heat load than conventional electrical wiring, relaxing the requirements for thermal anchoring, and is also immune to electromagnetic interference, which allows routing of sensitive signals with improved robustness to noise and crosstalk. Most importantly, optical fibers allow for very high bandwidth densities (in the Tbps/mm2 range) by carrying multiple signals through the same physical fiber (Wavelength Division Multiplexing, WDM). Here, we demonstrate for the first time optical readout of a superconducting nanowire single-photon detector (SNSPD) directly coupled to a CMOS photonic modulator, without the need for an interfacing device. By operating the modulator in the forward bias regime at a temperature of 3.6 K, we achieve very high modulation efficiency (1,000–10,000 pm/V) and a low input impedance of 500 Ω with a low power dissipation of 40 μW. This allows us to obtain optical modulation with the low, millivolt-level signal generated by the SNSPD.

Two-dimensional electric-double-layer Esaki diode

Two-dimensional van der Waals materials offer unique advantages for the development of band-to-band tunneling devices given their lack of dangling bonds, atomically flat thickness and steep band edges. Here, we present the experimental demonstration of an electric double layer (EDL) Esaki junction in synthetic WSe2 thin films. A Si-compatible process is developed for the fabrication of nanoscale FETs utilizing molecular beam epitaxy of WSe2 performed directly on top of a high-κ dielectric at back-end-of-line-friendly temperatures (<550 °C). Degenerate and abrupt doping profiles are obtained by modulating the electron/cation and hole/anion EDLs formed at the interface between a tens-of-nanometer long WSe2 channel and a solid polymer electrolyte, polyethylene oxide:cesium perchlorate (PEO:CsClO4). Numerical simulations are used to determine the bias dependence of the equilibrium ion and carrier density profiles. The EDL-doped tunnel diode exhibits repeatable, gate-tunable band-to-band tunneling with negative differential resistance in the forward bias regime at temperatures up to 140 K, and strong conduction in reverse bias. A maximum peak-to-valley current ratio of 3.5 is measured at 110 K.

Usage of Low Cost Digital Camera for Silicon Solar Cell Electroluminescence Detecting

The goal of this article is focused on the developing of experimental measuring workspace used for diagnostic of solar cells and modules by electroluminescence detection. Electroluminescent imaging is increasingly used to detect defects in silicon solar cells. However, the cost of the conventional luminescence systems is a limiting factor for generalized use. A simple and reliable low-cost electroluminescence setup is presented. The developed system was tested on commercial silicon solar cells for the acquisition of electroluminescence images in the forward bias regimes.

Current linearity and operation stability in Al2O3-gate AlGaN/GaN MOS high electron mobility transistors

To investigate current linearity and operation stability of metal–oxide–semiconductor (MOS) AlGaN/GaN high electron mobility transistors (HEMTs), we have fabricated and characterized the Al2O3-gate MOS-HEMTs without and with a bias annealing in air at 300 °C. Compared with the as-fabricated (unannealed) MOS HEMTs, the bias-annealed devices showed improved linearity of ID–VG curves even in the forward bias regime, resulting in increased maximum drain current. Lower subthreshold slope was also observed after bias annealing. From the precise capacitance–voltage analysis on a MOS diode fabricated on the AlGaN/GaN heterostructure, it was found that the bias annealing effectively reduced the state density at the Al2O3/AlGaN interface. This led to efficient modulation of the AlGaN surface potential close to the conduction band edge, resulting in good gate control of two-dimensional electron gas density even at forward bias. In addition, the bias-annealed MOS HEMT showed small threshold voltage shift after applying forward bias stress and stable operation even at high temperatures.

Organic doped/undoped interface based diode structure: Distinct mechanisms underlying forward and reverse bias

Abstract Organic semiconductor diodes fabricated using doped/undoped (high-low) homojunction has the potential of providing controlled and high quality current density-voltage (J-V) characteristics based on majority carrier transport. We study mechanisms of transport underlying such characteristics both in forward and reverse bias regimes of typical doped/undoped homojunction organic diodes fabricated using 4,4′,4″-tris( N -3-methylphenyl- N -phenyl-amino) triphenylamine ( m -MTDATA). We study the J-V characteristics over a wide temperature range (200–300 K), and by varying the intrinsic layer thickness between 10 and 100 nm. The forward bias current, before entering into the space charge limited regime, is exponential over several orders of magnitude with the slope being temperature independent for all intrinsic layer thickness down to 10 nm. The reverse bias characteristics, on the other hand, are highly sensitive to the thickness of the intrinsic layer. While the forward bias is controlled by tunneling at the homojunction interface, the reverse bias is controlled by the interface of cathode (aluminum in this case) and the intrinsic layer. We show that the reverse current is due to Fowler-Nordheim tunneling across a barrier height, which is temperature independent but is sensitive to the layer thickness of the intrinsic layer. The origin of the thickness dependence of barrier height (0.45–0.72 eV for 10–20 nm) is attributed to the change in background carrier concentration in the intrinsic layer due to diffusion of carriers from the highly doped side. The results clearly show that the diffusion length of the majority carriers is approximately 1 nm and is comparable to the nearest neighbor jump distance.

Understanding the degradation processes of GaN based LEDs submitted to extremely high current density

This work presents the first analysis of the degradation mechanisms induced by extremely high current densities on green InGaN-based LEDs. To this aim, bare-chip LEDs were submitted to current step-stress, until catastrophic failure. We identified a current density threshold, around 260A/cm2, beyond which the devices start degrading, showing an increase in leakage current both in reverse and forward bias regimes. Moreover, a current crowding effect was detected on the degraded samples by electroluminescence measurements. A lumped element model was developed to describe the effects of stress. Results suggest that localized power dissipation, due to high current density flow under the anode pad, induces the formation of shunt paths and, eventually, leads to the catastrophic failure of the LEDs.

Electronic Properties of Thermal Oxides on Ti and Their Influence on Impedance and Photoelectrochemical Behavior of TiO2 Nanotubes

Thermal oxidation of titanium was carried out at 350°C, 450°C, and 550°C for 2 h or 12 h. X-ray Diffraction and Raman Spectroscopy suggest that the thermal oxides are scarcely crystalline when the annealing temperature is low, while both anatase and rutile are present for high annealing temperature and time. Photoelectrochemical measurements allowed estimation of a bandgap decreasing from 3.35 eV to 3.15 eV with increasing annealing temperature. The impedance spectra confirmed the formation of n-type semiconductors, with an impedance strongly decreasing on going from a reverse bias toward a forward bias regime. TiO2 nanotubes grown by anodizing Ti in NH4F and water containing ethylene glycol solution were annealed at different temperature and for different times. X-ray Diffraction and Raman Spectroscopy proved that rutile is present at the barrier layer beneath the tubular array, while the tubes are mainly constituted by anatase. The incident photon to current conversion efficiency increased by increasing the annealing temperature and time. The impedance spectra recorded under reverse bias regime for thermal planar oxides and for annealed TiO2 NTs are almost coincident, confirming that the predominant contribution arises from the barrier layer beneath the tubular array. Only under forward regime do the tube walls contribute to the measured impedance.

Graphene-based side-polished optical fiber amplifier.

We demonstrate a novel design for optical fiber amplifiers, utilizing side-polished fibers with a single-layer graphene overlay as the active medium and carrier injection in the graphene layer to provide the required inversion. We study the effects of an electrically induced graphene p-i-n heterojunction in the forward bias regime on optical modes of side-polished fibers and show that gain values of 0.51, 1.81, and 1.79 dB/cm for wavelengths 1064, 1330, and 1550 nm can be obtained for single-mode side-polished fibers. Our results show that in multi-mode side-polished fibers, higher order modes experience higher values of gain, and gain can be increased by increasing polished depth. The proposed system is a tunable wideband optical amplifier that can operate for wavelengths larger than 1000 nm.

Electroluminescence of silicon solar cells using a consumer grade digital camera

Abstract Electroluminescent imaging is increasingly used to detect defects in silicon solar cells. However, the cost of the conventional luminescence systems is a limiting factor for generalized use. A simple and reliable low-cost electroluminescence setup is presented. The developed system was tested on commercial silicon solar cells for the acquisition of electroluminescence images in the forward and reverse bias regimes. In forward bias the temperature was varied whilst in reverse bias the applied voltage was varied. The results used in conjunction allowed for the detection of defective areas and identification of their type and cause. The simplicity and low-cost nature of the developed setup should enable a more widespread use of luminescence techniques for the characterization of crystalline silicon solar cells.