Recombination Current Research Articles

An explanation for invalidity of working currents' derating on improving light-emitting diode devices' reliability

Derating of the working current level does not work for improving GaN-based light-emitting diode (LED) devices' reliability. The present work demonstrates that it is not the levels but the specific components of the applied electrical currents weighing more on LEDs' degradation. Existing defects are sources for tunneling currents and Shockley-Read-Hall (SRH) non-radiative recombination current, and the component of tunneling currents and SRH non-radiative recombination current in the applied electrical current will in turn induce fast increase of defect density. The current component from electron tunneling to deep levels in the vicinity of mixed/screw dislocations will affect more on LEDs' degradation than other components, such as heavy-hole tunneling via intermediate state. In a whole, the overflow leakage current from the active region and Auger recombination currents in the applied electrical current will generate positive effects to alleviate LEDs' degradation.

Aluminum-induced iso-epitaxy of silicon for low-temperature fabrication of centimeter-large p+n junctions

Aluminum-induced crystallization of Si is achieved on crystalline Si substrates in a manner that produces near-ideal p+n diodes for centimeter large sizes. A layer-stack of physical-vapor-deposited materials, amorphous Si on aluminum, is inverted at an anneal temperature of 400°C to form a monocrystalline p-doped Si layer by solid-phase epitaxy (SPE). The stages of the crystallization process are been reviewed here and studied with respect to the filling of the large-area SPE Si layers. It is shown that a complete iso-epitaxy coverage of large areas is possible if the starting c-Si substrate is free of nucleation centers. This can be achieved by appropriate wet-etching of the oxide to the Si followed by diluted HF dip-etching and Marangoni drying before deposition of the Al mediator layer and α-Si layer. Near-ideal p+n diodes have been fabricated at 400°C with areas up to 1×1cm2, having ideality factors down to 1.02 and low leakage currents of a few nA/cm2. From temperature-dependent measurements it can be concluded that the dominant origin of the leakage current is from ideal diffusion over the depletion regions and not from defect-related generation–recombination currents. The full coverage by p+ SPE-Si is confirmed by material analysis.

Temperature Dependence of the Component Currents and Internal Quantum Efficiency in Blue Light-Emitting Diodes

We have decomposed the blue light-emitting diode (LED) current into the constituent components and studied the temperature dependence of each component and the internal quantum efficiency (IQE) quantitatively over the temperature range from 0°C to 80°C. The most temperature-sensitive current component is the nonradiative recombination current that increases with the temperature rise, and the component plays a dominant role in determining the temperature dependence of the IQE and luminescence output. Therefore, high-efficiency LEDs achieve high power and low temperature sensitivity simultaneously. On the other hand, the portion of the loss current that is responsible for the efficiency droop decreases with the temperature rise, resulting in lower droop at higher temperature. Some consequences and implications of the temperature dependence are also discussed.

Recombination and Excess Currents in 4H-SiC Structure with Low-Doped n-Region

Presented in this paper are the results of a study of forward and reverse current-voltage characteristics of the 4H-SiC pn structures produced by implantation of Al+ ions in low-doped ((5-7)∙1014cm-3) n-type conductivity epitaxial layer. A forward current was identified which is consistent with the model of recombination in the space charge region of a pn junction via a deep level: J=Joexp(qU/nkT), where Jo=Jo*exp[-Ea/(kT)] (or J= Jo*exp[(qU-2Ea)/nkT)]) with the ideality factor n=2. Parameters of this current are as follows: Jo(293 K) ~ 8∙10-25 A/cm2, Ea 1.73-1.75 eV (2Ea3.46-3.5 eV), Jo*~7∙105 A/cm2. A comparison of the experimental and model-based values of Ea, with allowance for all clearly defined temperature dependences, both strong and weak, of the model of recombination in the space charge region shows that the effective lifetime increases and effective trapping cross section σeff of the recombination level decreases with increasing temperature: σeff ~ T-2÷σeff ~ T-2.5 or σeff ~exp(ΔE /kT) where ΔE=80-100 meV. Reverse current practically in whole temperature range is excess current, only at high temperatures (~800 K) reverse current became close to generation current in space charge region. Special interest are the excess currents as a result of the application of a reverse or forward voltage at high temperature. The barrier type excess currents is characteristic of SiC.

Recombination channels in 2.4–3.2 µm GaInAsSb quantum-well lasers

Spontaneous emission has been used to evaluate the proportion of the different recombination channels in 2.4–3.2 µm type I- GaInAsSb quantum-well lasers operating at room temperature. The method consists in using the square root of the integrated spontaneous emission rate as a value proportional to the carrier density. A second-degree polynomial relationship can then be used to determine the proportion of the different recombination currents (monomolecular, radiative and Auger) in a laser diode operated below threshold. A notable increase of the Auger recombination has been observed with increasing wavelength. Auger recombination currents account for only 26% of the total current at threshold at 2.4 µm, while the proportion reaches 55% at 2.8 µm and 64% at 3.2 µm.

Transient phenomena in Cu(In,Ga)Se2 solar modules investigated by electroluminescence imaging

Industrially fabricated thin-film modules are investigated by electroluminescence (EL) photography. We observe metastable transients in a series of successive images recorded during application of forward bias immediately after the Cu(In,Ga)Se2 modules were kept in the dark for several hours. Our experiments are conducted in the dark either under constant current (monitoring the module voltage) or vice versa. For both situations, the EL intensities increase with time, whereas we observe a decrease of the overall module voltage (at fixed current) or an increase of the current (at fixed voltage). We ascribe our observations to the simultaneous decrease of the bulk series resistance Rs and the reduction of recombination currents (an increase in the junction resistance Rj) during the forward bias soaking. A quantitative analysis of our data shows that the reduction of Rs we observe is much stronger than the increase of Rj. We also show that the bulk series resistance can strongly adulterate the determination of the sheet resistance of the front ZnO from EL images.

Investigation of emitter size effect in InP/InGaAsSb/InGaAs double heterojunction bipolar transistors

The emitter size effect of a series InGaAsSb base series of InP/InGaAsSb/InGaAs heterojunction bipolar transistors (HBTs) with different emitter sizes is investigated. Compared to the InGaAs base HBTs, these devices exhibit much lower base surface recombination current. This is attributed to the surface Fermi level pinning near the valence band of the antimonide base. The effect of Sb composition and doping concentration of the base on the surface recombination current is well explained by the postulate.

Effects of rapid thermal process on the junction properties of aluminum rear emitter solar cells

N-type silicon with aluminum emitters for rear junctions was studied; aluminum back surface fields were replaced with n-type silicon wafers. Aluminum rear emitters for n-type silicon solar cells were studied with various rapid thermal processing conditions. With fast ramping-up and fast cooling, an aluminum rear junction was formed uniformly with low emitter recombination current. The effects of junction quality on solar cell efficiency were investigated.

Lock-In Thermography and Related Topics in Photovoltaic Research

In this contribution the basic principles of DLIT (dark lock-in thermography) are introduced and typical application examples are shown. These results are compared with that of other solar cell characterization techniques like electroluminescence (EL) and photoluminescence (PL) imaging, which are also very popular in Germany. It will be shown that these techniques are largely complementary to each other. Luminescence techniques are most sensitive for the detection of local recombination centers in the bulk and of series resistance problems of the cells, whereas DLIT is most effective for investigating all problems being connected with the dark current of the cells. A new DLIT technique is introduced which allows a separate imaging of the so-called diffusion current and the recombination current. These two contributions of the dark current are based on different physical mechanisms (recombination in the bulk and in the depletion region, respectively), and their spatial distributions differ significantly. Such investigations are impossible by applying luminescence-based imaging techniques.

Characterization of vertical Si nanowire p-n diodes fabricated by metal-assisted etching and AAO templates

Vertical Si nanowire p-n diodes were fabricated utilizing both anodic aluminum oxide (AAO) templates and metal-assisted etching. AAO templates with different diameters were fabricated on a p(2.5 μm)-n junction substrate using a two-step anodization and pore widening process. The average diameters of the AAO templates were 36.3, 57.4, and 78.1 nm. Vertical Si nanowire p-n diodes were then fabricated by metal-assisted etching, with average diameters of 37.4, 53.3, and 62.8 nm. The lengths of the vertical Si nanowires were controlled by varying the etching times. For the fully etched (3 μm) Si nanowire, the p-n diodes with smaller diameters yielded higher current densities than those with larger diameters, due to mobility enhancement. However, such dependency was not observed for the partially etched (500 nm) Si nanowire p-n diodes. It was concluded that the recombination current is too dominant in the depletion region to improve mobility in one-dimensional transport.

Betavoltaic Battery Conversion Efficiency Improvement Based on Interlayer Structures

Significant differences among the doping densities of PN junctions in semiconductors cause lattice mismatch and lattice defects that increase the recombination current of betavoltaic batteries. This extensively decreases the open circuit voltage and the short current, which results in low conversion efficiency. This study proposes P+PINN+-structure based betavoltaic batteries by adding an interlayer to typical PIN structures to improve conversion efficiency. Numerical simulations are conducted for the energy deposition of beta particles along the thickness direction in semiconductors. Based on this, 63Ni-radiation GaAs batteries with PIN and P+PINN+ structures are designed and fabricated to experimentally verify the proposed design. It turns out that the conversion efficiency of the betavoltaic battery with the proposed P+PINN+ structure is about 1.45 times higher than that with the traditional PIN structure.

Charge pumping and DCIV currents in SOI FinFETs

Charge pumping (ACIV) and direct current–voltage (DCIV) measurements were performed on standard floating-body n-channel FinFETs. The resulting recombination currents are monitored directly at the drain and source contacts, and are found to depend strongly on frequency, gate voltage, and FinFET geometry. The DCIV current reaches a maximum when the body potential transitions from accumulation to inversion in response to the applied gate bias and the body-to-source and body-to-drain diodes become forward biased. The ACIV current is dominated by contributions from interface traps when the surface potential is swept from accumulation to inversion during the gate pulse. It is sensitive to recombination currents at the body-to-source and body-to-drain diodes at intermediate voltage levels. At the highest applied gate voltages, band-to-band tunneling is also observed. We demonstrate that the change in ACIV current with frequency before and after electrical stress allows charge-pumping estimates of stress-induced interface trap densities in standard SOI FinFETs without body contacts.

Analysis of the electrical characteristics of GaInP/GaAs HBTs including the recombination effect

An analytical model is used to predict the effects of surface recombination current on the gain and transit time of GaInP/GaAs heterojunction bipolar transistors (HBTs). The present analysis shows that consideration of the recombination current gives current gain values that are comparable to those of the experimental results. The dependence of current gain on temperature, base doping and emitter area are also analyzed, and the variation in collector current with emitter-base voltage, temperature and doping is considered.

Current limitation in A 3 B 5 nitride light-emitting diodes under forward bias

The results of investigation into forward-bias current-voltage characteristics of InGaN/GaN light-emitting diodes (LEDs) in the static and pulsed modes for current densities up to 1000 A/cm2are reported. It is shown that starting from 5–6 V, the voltage-current characteristics are described by a linear dependence at room temperature. The series resistance is in no excess of 1Ω for the area of diodes S ≈ 1 mm2 and is determined by the specific contact resistance. It is also found that in the voltage–current characteristic under forward bias, there is a temperature-dependent segment between those of recombination and ohmic currents.

Current Transport Characteristics of Quasi-AlxGa1-xN/SiC Heterojunction Bipolar Transistors with Various Band Discontinuities

The current transport characteristics of quasi-AlxGa1-xN/SiC heterojunction bipolar transistors (HBTs) with various band discontinuities were investigated in a low-current range using a Gummel plot. In the low-current range, the base currents of the HBTs were dominated by recombination currents. The collector current characteristics of the HBTs in the low-current range were almost the same in spite of the various band discontinuities, and the ideality factor n was 1.0. The band discontinuities at the heterojunction had no effect on electron injection in the low-current range. This is because the collector currents were dominated by diffusion process in the base region rather than by injection process at the AlGaN/SiC interface.

Performance modeling of MWIR InAs/GaSb/B–Al0.2Ga0.8Sb type-II superlattice nBn detector

This paper reports on the unipolar medium wavelength infrared (MWIR) InAs/GaSb/B–Al0.2Ga0.8Sb type-II superlattice (T2SL) nBn detector's photoelectrical performance. In our model, the heterojunction barrier-active region (absorber) was assumed to be decisive as the contributing dark current mechanism limiting nBn's detector performance. The voltage drop analysis on the nBn structure was introduced to estimate the bias drop on the heterojunction barrier-active region. It was assumed that the contact n+-barrier heterojunction's layer has an insignificant influence on the electrical properties of the detector. In addition, a bulk-based model with an effective band gap of T2SL material has been assumed in the device modeling. Both current–voltage (I–V) and differential resistance–area product RA(V,T), characteristics of nBn's detector were found to be dominated by diffusion and generation–recombination currents in the zero-bias and the low-bias regions. At medium values of reverse voltages, the dark current was mostly affected by trap-assisted tunneling, whereas the band-to-band tunneling revealed its contribution at high values of reverse bias (V > 0.7 V). The RA(V,T) characteristics' fitting procedure allowed estimation of both diffusion and generation–recombination lifetimes as well as the trap energy level temperature dependence within T2SL energy gap. It was predicted that at T = 77 K, the RA product and detectivity reached values of 1000 Ωcm2 and 4 × 1011 cm Hz1/2 W-1, respectively. The corresponding values at room temperature were 0.01 Ωcm2 and D* = 5 × 108 cmHz1/2 W−1, respectively. Finally, InAs/GaSb/B–Al0.2Ga0.8Sb T2SLs nBn's state of the art was compared to the performance of InAs/GaSb T2SLs PIN photodiodes and the HgCdTe bulk photodiodes operated at near-room temperature. It was shown that the RA product of the MWIR T2SLs nBn detector has reached a comparable level with the state of the art of the HgCdTe bulk photodiodes.

Photoelectron spectroscopy investigations of recombination contacts for tandem organic solar cells

Recombination contacts play an important role in highly efficient organic tandem solar cells. We present a photoelectron spectroscopy study on contact systems that have previously been shown to work efficiently as recombination contacts. Here, the conversion of an electron current into a hole current is realized either by insertion of gold clusters or by a highly doped pn-junction. From the measured energy level alignments, we show that the working principles of these two approaches are significantly different. For gold clusters, the recombination current is promoted by an accumulation of charge carriers, while for doped pn-junctions, it is achieved by tunneling through a depletion layer.

Approach for Al2O3 rear surface passivation of industrial p‐type Si PERC above 19%

ABSTRACTAtomic layer deposition (ALD) of thin Al2O3 (≤10 nm) films is used to improve the rear surface passivation of large‐area screen‐printed p‐type Si passivated emitter and rear cells (PERC). A blister‐free stack of Al2O3/SiOx/SiNx is developed, leading to an improved back reflection and a rear recombination current (J0,rear) of 92 ± 6 fA/cm2. The Al2O3/SiOx/SiNx stack is blister‐free if a 700°C anneal in N2 is performed after the Al2O3 deposition and prior to the SiOx/SiNx capping. A clear relationship between blistering density and lower open‐circuit voltage (VOC) due to increased rear contacting area is shown. In case of the blister‐free Al2O3/SiOx/SiNx rear surface passivation stack, an average cell efficiency of 19.0% is reached and independently confirmed by FhG‐ISE CalLab. Compared with SiOx/SiNx‐passivated PERC, there is an obvious gain in VOC and short‐circuit current (JSC) of 5 mV and 0.2 mA/cm2, respectively, thanks to improved rear surface passivation and rear internal reflection. Copyright © 2012 John Wiley & Sons, Ltd.

Can morphology tailoring improve the open circuit voltage of organic solar cells?

While the effect of interfacial morphology on the short circuit current (ISC) of organic photovoltaic devices (OPVs) is well known, its impact on open circuit voltage (VOC) and fill-factor (FF) are less clear. Since the output power of a solar cell Pout=ISCVOCFF, such understanding is critical for designing high-performance, morphology-engineered OPVs. In this letter, we provide an explicit analytical proof that any effort to radically improve VOC by tailoring bulk heterojunction morphology is futile, because any increase in ISC due to larger interface area is counterbalanced by corresponding increase in recombination current, so that the upper limit of VOCBHJ cannot exceed that of the corresponding planar heterojunction devices, i.e., VOCBHJ≤VOCPHJ. We discuss the implication of this VOC-constraint on the efficiency optimization of organic solar cells.

Integration of Al2O3 as Front and Rear Surface Passivation for Large-Area Screen-Printed P-Type Si PERC

Atomic layer deposition (ALD) of thin Al2O3 (≤ 10nm) films is used to improve both front and rear surface passivation of large-area screen-printed p-type CZ Si passivated emitter and rear cells (PERC). As emitter passivation, the SiNx anti reflection coating (ARC) is capped with Al2O3, giving improved hydrogenation during co-firing and a front recombination current (J0,front) of 128 ± 5 fA/cm2. As rear surface passivation, a blister-free stack of Al2O3/SiOx/SiNx is employed, leading to optimal back reflection and a rear recombination current (J0,rear) of 92 ± 6 fA/cm2. Internal quantum efficiency (IQE) measurements clearly confirm the improved passivation properties of both Al2O3-based stacks, even compared to passivation stacks based on thermally grown SiO2.