High Forward Bias Research Articles

Germanium–Tin $\hbox{n}^{+}\hbox{/p}$ Junction Formed Using Phosphorus Ion Implant and 400 $^{\circ} \hbox{C}$ Rapid Thermal Anneal

AGe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.976</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.024</sub> n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> /p diode was formed using phosphorus ion (P <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ) implant and rapid thermal annealing at 400°C. Activation of P in Ge typically requires high temperatures (e.g., 700°C), and it was found that this is not needed in the presence of a small amount of Sn. A high forward bias current of 320 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at -1 V is achieved for the Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.976</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.024</sub> n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> /p diode. This is four times higher than that of the Ge n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> /p control diode, which received the same P <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> implant but activated at 700°C. The n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -GeSn region has a high active dopant concentration of 2.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> × cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , much higher than that in the Ge control. The increased active dopant concentration in GeSn reduces the width of the tunneling barrier between the Al contact and the n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -GeSn and increases the forward bias diode current. Enhancement of P activation in Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.976</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.024</sub> could possibly be as a result of passivation of vacancies in the Ge lattice due to Sn atoms.

Influence of injected charge carriers on photocurrents in polymer solar cells

We determine and analyze the photocurrent ${J}_{\mathrm{ph}}$ in polymer solar cells under conditions where, no, one, or two different charge carriers can be injected by choosing appropriate electrodes and compare the experimental results to simulations based on a drift-diffusion device model that accounts for photogeneration and Langevin recombination of electrons and holes. We demonstrate that accounting for the series resistance of the device is essential to determine ${J}_{\mathrm{ph}}$. Without such correction, the results are, even qualitatively, incorrect. We show that in solar cells with forward bias applied ${J}_{\mathrm{ph}}$ is reduced by recombination of photogenerated charge carriers with injected charge carriers. Self-selective contacts or band bending are not necessary to explain the effects. Without injecting contacts ${J}_{\mathrm{ph}}$ is symmetric around the compensation voltage. A simple analytical model shows that under high forward bias ${J}_{\mathrm{ph}}$ scales inversely with 1 $+$ $\ensuremath{\xi}{\ensuremath{\gamma}}_{\mathrm{pre}}$, in which ${\ensuremath{\gamma}}_{\mathrm{pre}}$ represents the extent of Langevin recombination and \ensuremath{\xi} is a positive constant. Under conditions where Langevin recombination is very low or when electron and hole mobility are a very different, photogenerated charge carriers can affect the space-charge field and modify the injection of charge carriers. We show by simulations and experimentally that under such conditions the photocurrent can exceed that charge generation such that, effectively, photocurrent multiplication occurs.

Conduction of Topologically Protected Charged Ferroelectric Domain Walls

We report on the observation of nanoscale conduction at ferroelectric domain walls in hexagonal HoMnO(3) protected by the topology of multiferroic vortices using in situ conductive atomic force microscopy, piezoresponse force microscopy, and Kelvin-probe force microscopy at low temperatures. In addition to previously observed Schottky-like rectification at low bias [Phys. Rev. Lett. 104, 217601 (2010)], conductance spectra reveal that negatively charged tail-to-tail walls exhibit enhanced conduction at high forward bias, while positively charged head-to-head walls exhibit suppressed conduction at high reverse bias. Our results pave the way for understanding the semiconducting properties of the domains and domain walls in small-gap ferroelectrics.

Forward and reverse bias dependence of radiative recombination efficiency in green (In,Ga)N quantum well diodes

The relative internal quantum efficiency in green (In,Ga)N quantum well diodes is investigated by ac photoluminescence (PL) experiments using a lock-in technique under direct photoexcitation conditions where photoexcited carriers are generated only in the wells. The radiative recombination efficiency is found to be significantly varied as a function of both forward and reverse bias voltages. When the reverse bias is applied, the enhanced junction field causes a reduction of the PL intensity due to tunneling escape of carriers, although a blue-shift of PL emission energy is observed owing to the compensation of internal polarization field in the wells. A reduction of the PL intensity is also observed by increasing the forward bias, which we attribute to the reduced radiative recombination rate due to the decreased electron–hole wave function overlap in the wells. This finding suggests that the electroluminescence droop problem may also be caused as a result of reduced radiative efficiency under the high forward bias.

A Very Robust AlGaN/GaN HEMT Technology to High Forward Gate Bias and Current

Reports to date of GaN HEMTs subjected to forward gate bias stress include varied extents of degradation. We report an extremely robust GaN HEMT technology that survived—contrary to conventional wisdom—high forward gate bias (+6 V) and current (&gt;1.8 A/mm) for &gt;17.5 hours exhibiting only a slight change in gate diode characteristic, little decrease in maximum drain current, with only a 0.1 V positive threshold voltage shift, and, remarkably, a persisting breakdown voltage exceeding 200 V.

Electrically pumped simultaneous ultraviolet and visible random laser actions from ZnO-CdO interdiffused film

We have demonstrated electrically pumped simultaneous ultraviolet (UV) and visible random laser (RL) actions from ZnO-CdO interdiffused film. The interdiffusion between ZnO and CdO films at 700 °C forms composition-graded CdxZn1−xO alloy within the ZnO-CdO interdiffused film, which is luminescent in both UV and visible regions. A device based on SiO2/ZnO-CdO/SiO2 double-barrier structure on silicon substrate, where SiO2 acts as the barrier, is constructed for electrical pumping of the ZnO-CdO interdiffused film. As the device is applied with sufficiently high forward bias with negative voltage connecting to the silicon substrate, the UV and visible RL actions simultaneously occur. The mechanism for such electrically pumped RL actions has been tentatively elucidated.

Electroluminescence from n–n isotype heterostructures of graded-band-gap ZnMgO : Al and ZnO films on platinized Si

We report on room temperature electroluminescence (EL) from n–n isotype heterostructures composed of Al-doped graded-band-gap Zn1−xMgxO (g-ZnMgO : Al) and ZnO films fabricated on Pt/Ti/SiO2/Si substrates. The isotype heterostructure device generated EL emission at operation voltages as low as 3–5 V, whose emission spectra covered visible and near infrared regions under the unipolar operation condition, with g-ZnMgO : Al as positive. The intensity of light emission increased nonlinearly and short-wavelength emissions in the visible region became appreciable at high injection currents. We also observed negative capacitance at high forward bias above the onset voltage, implying generation of holes by hot electrons. We discussed impact ionization as a possible origin of the EL from the heterostructure and the effect of quasi-electric field generated in the graded-band-gap layer in conjunction with the apparent upconversion EL by considering the electric field needed for impact ionization.

A Double-Heterojunction Bipolar Transistor Having a Degenerately Doped Emitter and Backward-Diode Base Contact

A double-heterojunction bipolar transistor having a degenerately doped emitter layer is investigated. The base-emitter Esaki diode introduces very low input impedance in the off- state but does not degrade the current gain at high forward bias. The heavily doped emitter layer makes it possible to scale the emitter-layer thickness considerably. The heavily doped emitter layer also allows contacting the base via the base-emitter backward diode. The high-frequency performance of the device is presented.

Small-signal response of nanocrystalline-titanium dioxide/poly(3-hexylthiophene) heterojunction solar cells

Admittance spectra of nanocrystalline titanium dioxide/poly(3-hexyl thiophene), heterojunction solar cells have been measured in air and under vacuum. In air, a 3-orders of magnitude increase in capacitance is observed when the measurement frequency is decreased from 1 MHz to 0.1 Hz. The frequency-dependence of the loss tangent with zero bias voltage indicates the presence of two processes, the weaker of which is attributed to adsorbed water molecules on the surface of the nanocrystalline titanium dioxide. The second process which becomes dominant in forward bias is believed to arise from the presence of an interfacial capacitance. In air, the capacitance is seen to depend strongly on applied forward bias consistent with the presence of an interfacial depletion region. However, a number of inconsistencies indicate that a mechanism related to carrier interaction with oxygen-related interface states may be the cause. At high forward bias and low frequencies, a negative incremental capacitance is observed similar to that seen in organic light emitting diodes following the onset of double-injection.

A Novel Fabrication of p–n Diode Based on ZnO Nanowire/p-NiO Heterojunction

We report the deposition of NiO onto ZnO nanowires prepared on ZnO:Ga/glass templates. With the sputtered NiO, the nanowires became mushroom-like. Experimental results point out that sputtered NiO also formed nanoshells surrounding ZnO nanowires. The p-NiO/n-ZnO heterostructure exhibits rectifying behavior with a sharp turn on at ∼1 V. Furthermore, the current vs voltage characteristic is dominated by space-charge-limited current (SCLC) at high (1.1 V) forward bias.

A Novel Fabrication of p–n Diode Based on ZnO Nanowire/p-NiO Heterojunction

We report the deposition of NiO onto ZnO nanowires prepared on ZnO:Ga/glass templates. With the sputtered NiO, the nanowires became mushroom-like. Experimental results point out that sputtered NiO also formed nanoshells surrounding ZnO nanowires. The p-NiO/n-ZnO heterostructure exhibits rectifying behavior with a sharp turn on at ∼1 V. Furthermore, the current vs voltage characteristic is dominated by space-charge-limited current (SCLC) at high (1.1 V) forward bias.

Bright Visible Light Extraction from Amorphous Silicon Nitride Heterojunction Pin Diode

We report the transformation of an ordinary hydrogenated amorphous silicon nitride based heterojunction diode into a strong visible light emitting electroluminescent device. This forming process (FP) comprises Joule heating induced crystallization during the application of sufficiently high forward bias to the fresh diode. FP starts at an arbitrary point and continues until the accompanying visible light is uniformly emitted from the whole diode. This remarkable phenomenon is presented by real-time photography of the lateral propagation of the formed region. Finally, both the role of window electrode for a successful FP and the luminescence mechanism after FP are briefly discussed.

A new growth method of roughed p-GaN in GaN-based light emitting diodes

A new growth method of roughed p-GaN has been demonstrated in this paper. First, some crystal seeds of p-GaN are obtained by utilizing low-temperature growth. Then, a p-GaN high-temperature expitaxy layer is grown on it subsequently with a fast growth rate, which will enlarge the roughness degree. Compared with the luminous flux of the conventional light emitting diode with flat p-GaN, the luminous flux is improved by 45%. Meanwhile, it is found that the problems of large reverse current and high forward bias aroused by the low-temperature epitaxy are also solved.

Illumination effect on electrical characteristics of organic-based Schottky barrier diodes

The forward and reverse bias capacitance–voltage (C−V) and conductance–voltage (G/ω−V) characteristics of Au/polyvinyl alcohol (Co, Zn-doped)/n-Si Schottky barrier diodes have been investigated depending on illumination intensity at room temperature and 1 MHz. These experimental C−V and G/ω−V characteristics show fairly large illumination dispersion especially in the weak inversion and depletion regions and they increase with the increasing illumination intensity because of the illumination induced interface states and electron-hole pair. The C−V plots show that peaks are the results of the particular distribution density of the interface states (Nss), interfacial polymer layer, and series resistance (Rs) of device. The magnitude of the peaks increases with the increasing illumination intensity and their positions shift from the high forward bias voltage to low forward bias voltages. The C−2−V plots give a straight line in a wide bias voltage region for each illumination intensity. The variation in doping concentration (ND), depletion layer width (WD), and barrier height [ΦB(C−V)] were obtained from these C−2−V plots. In addition, voltage dependent density distribution profile of Nss was obtained from both low-high capacitance (CLF−CHF) and Hill–Coleman methods. It is observed that there is a good agreement between the results obtained by these methods. In addition, voltage dependent Rs profile was obtained from C−V and G/ω−V data by using Nicollian and Brews method.

Electrical and dielectric characteristics of Al/polyindole Schottky barrier diodes. II. Frequency dependence

AbstractThe dielectric properties and ac electrical conductivity of Al/polyindole (Al/PIN) Schottky barrier diodes (SBDs) were investigated by using admittance spectroscopy (capacitance–voltage [C‐V] and conductance–voltage [G/ω‐V]) method. These C‐V and G/ω‐V characterizations were performed in the frequency range of 1 kHz to 10 MHz by applying a small ac signal of 40 mV amplitude from the external pulse generator, whereas the dc bias voltage was swept from (−10 V) to (+10 V) at room temperature. The values of dielectric constant (ε′), dielectric loss (ε″), dielectric loss tangent (tanδ), real and imaginary part of electrical modulus (M′ and M″), ac electrical conductivity (σac), and series resistance (Rs) of the Al/PIN SBDs were found to be quite sensitive to frequency and applied bias voltage at relatively low frequencies. Although the values of the ε′, ε″, tanδ, and Rs of the device were observed to decrease with increasing frequencies, the electric modulus and σac increased with increasing frequency for the high forward bias voltages. These results revealed that the interfacial polarization can more easily occur at low frequencies and that the majority of interface states (Nss) between Al and PIN, consequently, contribute to deviation of dielectric properties of the Al/PIN SBDs. Furthermore, the voltage‐dependent profile of both Rs and Nss were obtained from the C‐V and G/ω‐V characteristics of the Al/PIN SBDs at room temperature. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

High‐forward‐bias transport mechanism in a‐Si:H/c‐Si heterojunction solar cells

AbstractIn order to elucidate the transport mechanism in a‐Si:H/c‐Si heterojunction solar cells under high forward bias (U &gt; 0.5 V), we conducted temperature‐dependent measurements of current–voltage (I–V) curves in the dark and under illumination. ZnO:Al/(p)a‐Si:H/(n)c‐Si/(n+)a‐Si:H cells are compared with inversely doped structures and the impact of thin undoped a‐Si:H buffer layers on charge carrier transport is explored. The solar cell I–V curves are analyzed employing a generalized two‐diode model which allows fitting I–V data for a broad range of samples. The fitting results are complemented with numerical simulations using AFORS‐HET under consideration of microscopic a‐Si:H parameters as determined by constant‐final‐state‐yield photoelectron spectroscopy (CFSYS) to identify possible origins for a systematic increase of the high‐forward‐bias ideality factor along with the open‐circuit voltage (Voc). It is further shown that also for a‐Si:H/c‐Si heterojunctions, dark I–V curve fit parameters can unequivocally be linked to Voc under illumination, which may prove helpful for device assessment.

Physical device modeling of carbon nanotube/GaAs photovoltaic cells

Photovoltaic response from semiconducting single-walled carbon nanotubes/n-type GaAs heterojunctions has been investigated. We propose a detailed device model of the photovoltaic cell, in which electron transport can be described as follows. The thermionic emission and tunneling through the spike barrier caused by the conduction band discontinuity dominate electron transport under a low and high-forward bias, respectively. In contrast, the dominant transport mechanisms at low and high-reverse bias could be attributed to the direct and Fowler–Nordheim tunneling though a molecular junction, respectively. Within this framework, the current-voltage characteristics of the photovoltaic response can be quantitatively described.

H 2 sensing properties of diode-type gas sensors fabricated with Ti- and/or Nb-based materials

A thermally oxidized TiO 2 or Nb 2O 5 film equipped with a top Pd film electrode and a bottom Ti or Nb plate electrode (Pd/MO( n)/M, MO: oxide film, M: metal plate, n: annealing temperature (°C)) has been investigated as a diode-type H 2 sensor under air or N 2 atmosphere. Pd/TiO 2( n)/Ti sensors showed relatively poor H 2 sensing properties in air, in comparison with Pd/anodic-TiO 2( n)/Ti sensors constructed with an anodized TiO 2 film equipped with a top Pd film electrode and a bottom Ti plate electrode, which were reported in our previous studies. On the other hand, Pd/Nb 2O 5( n)/Nb sensors showed relatively larger H 2 response with fast response and recovery speeds than Pd/TiO 2( n)/Ti sensors in air under high forward bias conditions. A Pd/Nb 2O 5(450)/Ti sensor, which was fabricated by radio-frequency magnetron sputtering of Nb metal on a Ti substrate followed by thermal oxidation at 450 °C, showed the largest H 2 response and relatively fast response and recovery speeds in air, among the sensors tested. In addition, H 2 response of the Pd/Nb 2O 5(450)/Ti sensor in air was much lower than that in N 2, but the logarithm of H 2 response was almost proportional to the logarithm of H 2 concentration in a wide range of H 2 concentration (10–8000 ppm) in air, and the H 2 sensitivity in air was much higher than that in N 2.

Normal incidence detection of ultraviolet, visible, and mid-infrared radiation in a single GaAs/AlGaAs device

A GaAs/AlGaAs detector is demonstrated showing multiple detection capabilities. This detector exhibits a broad spectral response in the 200-870 nm (ultraviolet-visible) range for forward bias and in the 590-870 nm (visible) range for reverse bias. In the mid-IR region, two peaks at 5 and 8.9 microm can be observed for low and high forward bias voltages, respectively. In addition, the peak at 8.9 microm is sensitive to the polarization of the incoming radiation.

Temperature‐dependent droop of electroluminescence efficiency in blue (In,Ga)N quantum‐well diodes

AbstractTemperature‐dependent electroluminescence (EL) intensity of a blue (In,Ga)N single quantum‐well diode has been extensively investigated as a function of current between 2 nA and 10 mA at 20‐300 K. At 300 K the EL external quantum efficiency (ηex) shows a maximum at a current density of ∼1 A/cm2 and droops under higher injection. However, the current density for the ηex maximum significantly decreases with decreasing temperature, for example, to ∼5 × 10–4 A/cm2 at 20 K, with improved maximum EL efficiency due to reduced nonradiative recombination. This low‐temperature enhancement of the ηex droop at lower injection indicates that the droop phenomena is not involved with high‐density carrier effects like Auger nonradiative recombination, but rather linked with the carrier escape under high forward bias conditions due to the presence of internal piezo‐fields in (In,Ga)N quantum‐well heterostructures. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)