High Injection Conditions Research Articles

Hot-carrier solar cells

As the importance of renewable energy sources grows, the development of highly-efficient solar cells is increasingly gaining relevance. Today, the most efficient laboratory prototypes for solar cells are based on thin film multi-layered structures. These cells have 40% solar-to-electricity conversion efficiencies, which are well below the theoretical limit of 87% and leave considerable room for improvement. However, the design of multilayered solar cells can be complicated, and their workings might fail to tolerate changes to their operational conditions, such as the cell temperature or the power of the incident sunlight. Hotcarrier solar cells (HCSC), with their simplicity of design and ability to approach limiting conversion efficiencies, provide an attractive alternative to the multi-layer approach.1 Heat dissipation occurs when a material absorbs photons with energies larger than its bandgap. To circumvent this problem, the photo-generated charge carriers have to be collected through specially designed contacts that are energy-selective. In this way, heat production can be minimized: carriers with large kinetic energies—‘hot-carriers’—reach these contacts before losing most of their energy as heat. In principle, efficiencies as high as 86% could be achieved.1 However, since hot-carriers normally transfer their kinetic energy to the material in sub-picosecond times, the collection through contacts should be fast. This could be achieved at high-injection conditions, under which the interaction between the absorbent material and the hot-carriers becomes inefficient.2 As for any solar cell design, conversion efficiency is expected to grow with the concentration of incoming light, as this increases the output voltage of the solar cell by increasing the extracted work per absorbed photon. However, an optimal coupling between the incident radiation and the solar cells will lead to the high-injection regime, where drops in the cell’s output Figure 1. Energy band diagram of a hot-carrier solar cell with bandgap Eg and voltage qV. Electron-hole pairs are photo-generated in the absorber and kept hot at a temperature of TH (TH > TC , where TC is the ambient temperature). They are subsequently extracted using energyselective contacts with a transmission range iE and an extraction energy Eext. The Fermi levels in the electrodes are n and p , and the electron and hole chemical potentials in the absorber are e and h. The difference e h D H is known as the quasi-Fermi level splitting.

Influence of depletion region width on performance of solar cell under sunlight concentration

Abstract A new interpretation of high injection effect of minority carriers,np, which is happened in solar cell under sunlight concentration and imposed a limitation on its expected efficiency- has been introduced. This interpretation declare that: increasing of minority carrier, np, recombination current Irecom in deplation region when sunlight concentration C increase,s due to the depletion layer widen because the increasing of drift electric field at the junction boundary correspond to population inversion phenomena which happens at the interfaces between the base region and the depletion region. Experiments was performed with operating solar cells under high injection conditions (sunlight concentration simulator) to get I-V characteristics, then we characterize the relation between the depletion region width w and Irecom, by suggesting a new model which enable us to optimize a solar sell under sunlight concentration. In consequence we cocnlude that: Irecom is responsible of depleting a significant part of minority carrier, consequently, Irecom causes the solar cell efficiency degradation.

Recombination processes controlling the carrier lifetime in n−4H–SiC epilayers with low Z1/2 concentrations

The dominant recombination processes controlling the carrier lifetime in n-type 4H–SiC epitaxial layers grown with low concentrations of the Z1/2 defect (the dominant bulk lifetime killer), where Z1/2 no longer determines the lifetime, have been investigated by studying the variation in the carrier lifetime with temperature. The temperature dependent lifetimes were obtained primarily by low-injection photoluminescence decay for several low-Z1/2 epilayers over a wide temperature range. The results were fitted to simulations of the temperature dependent recombination rate, where bulk, surface and interface recombination was considered. No significant contribution from other bulk defects was observed, and upper limits to the bulk recombination rate were found to be consistent with the low Z1/2 concentrations measured in these materials. There was also no significant contribution from carrier capture at the epilayer/substrate interface, which is consistent with behavior expected at low injection for low-doped epilayers grown on n+ substrates. Corresponding high-injection measurements exhibited very different behavior, consistent with the surface/interface under flat-band conditions. Consequently, it is concluded that for low-Z1/2 materials, control of the carrier lifetime has not been transferred from Z1/2 to another bulk defect, but is instead dominated by surface and interface recombination. Simulations suggest that further enhancement of the total lifetime under the high injection conditions of a device structure would require very thick epilayers, effectively passivated surface and interface recombination and a further reduction in the remaining Z1/2 concentrations. The temperature dependence of the low-injection carrier lifetime was also found to provide a method to estimate the surface band bending and the surface defect density.

Space‐charge‐limited current transport in nitride HFETs

AbstractHigh‐voltage microwave AlGaN/GaN HFETs operating under high‐power conditions suffer from degraded RF performance and linearity due to a nonlinear resistance effect in the gate‐source region. During RF operation, the nonlinear resistance is due to the onset a of space‐charge‐limited current (SCLC) transport mechanism within the device channel. Under high current injection conditions, SCLC transport can set in and, consequently, the source resistance becomes a function of the injected charge causing a rapid increase and limiting device performance. The threshold for SCL current is dependent on the donor‐like states on the AlGaN surface which are responsible for supplying electrons to the channel. To alleviate the effect of nonlinear source resistance we show on an un‐gated HFET channel model that the critical current density of space‐charge effects and thus the onset of nonlinear source resistance can be shifted above the normal operating current density of the device by modification of the charge on the AlGaN surface. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature and Injection Level Dependencies of Carrier Lifetimes in p-Type and n-Type 4H-SiC Epilayers

Temperature and injection level dependencies of carrier lifetimes in p-type and n-type 4H-SiC epilayers have been investigated. The carrier lifetimes have been measured by differential microwave photoconductance decay measurements at various injection levels and temperatures. In both p-type and n-type epilayers, the carrier lifetimes gradually increased with increasing the injection level except for the very high injection condition. And the carrier lifetimes exhibited continuous increase with elevating the temperature for both epilayers. The carrier lifetime reached 3.3 µs in p-type and 4.2 µs in n-type epilayers at 250°C and an injection level of 1.8x1016 cm-3.

A Theoretical Analysis of Steady-State Charge Collection in Simple Diodes Under High-Injection Conditions

A previous rigorous mathematical analysis of drift-diffusion equations was used to investigate collected charge in a simple reverse-biased p-n junction diode exposed to an ionization source that liberates carriers (electron-hole pairs) in a quasi-neutral region within the diode. Each of two simple models was found to agree with the more rigorous analysis when carrier liberation is sufficiently intense. One is the sensitive volume (SV) model, and the other was called ¿ambipolar diffusion with a cutoff¿ (ADC). The earlier rigorous analysis was worked out in detail only for a localized source, i.e., a point source of carrier liberation, so it was able to validate the applicability of each simple model only for that case. The present paper treats an arbitrary spatial distribution of carrier generation and concludes that the ADC model remains valid for this more general case, but the SV model does not.

On an AlGaInP Light-Emitting Diode With a Modulation-Doped Multiquantum-Well (MD-MQW) Structure

An interesting AlGalnP multiquantum-well (MQW) light-emitting diode (LED) with an n-type modulation-doped (MD) structure, grown by low-pressure metal-organic vapor-phase epitaxy (LP-MOVPE), is fabricated and studied. This n-type MD-MQW LED exhibits lower dynamic resistance, higher luminescence, and higher luminous efficiency than those of a conventional undoped-MQW LED. Experimental results show a higher luminous efficiency of 16.05 lm/W and higher luminescence of 2.53 lm are obtained for the MD-MQW LED which are superior to the corresponding values of 14.49 lm/W and 2.04 lm for the undoped-MQW LED under dc operation. In addition, the n-type MD-MQW LED exhibits a higher quantum efficiency of 7.2% under dc operation as compared with the 6.7% of the undoped-MQW LED. The reduced junction temperature of 12degC at 200 mA is also acquired for the MD-MQW LED. Moreover, the brightness reliability of this new device is found to be comparable to the undoped-MQW LED. These positive results could be attributed to the presence of a higher electron concentration in the active region of the MD-MQW structure which causes the suppression of electron thermal velocity especially at a high level injection condition, and a reduced junction heating effect.

Surface photovoltage investigation of recombination at the a-Si/c-Si heterojunction

We investigate the use of time-resolved surface photovoltage (SPV) transients as a means to determine band bending and recombination properties at amorphous/crystalline silicon ( a-Si:H/ c-Si) heterojunctions. Experimentally, it is shown that for a-Si:H film thicknesses above ~ 6 nm, SPV transients do not depend on the film thickness anymore. On this basis, a simple numerical model is proposed that consists of a single rechargeable gap state on the c-Si wafer surface, into which the properties of the a-Si:H/ c-Si interface and the adjacent a-Si:H are lumped. It is shown that this model can reproduce all principal features of high excitation SPV transients, i.e. an initial fast decay shown to be due to Auger recombination, a plateau region for high injection conditions and a fast decay when the sample returns into low injection and the defect states are recharged. Under sufficiently high excitation, the SPV saturates at a value that is determined by the a-Si:H/ c-Si interface band bending in the dark. From the slope of the transient decay, defect parameters (density, energetic position) can be extracted.

Electronic and optical properties of InGaN quantum dot based light emitters for solid state lighting

In this paper, we have made a systematic study of the electronic and optical properties of InGaN based quantum dot light emitters. The valence force field model and 6×6k⋅p method have been applied to study the band structures in InGaN or InN quantum dot devices. Piezoelectric and spontaneous polarization effects are included. A comparison with InGaN quantum wells shows that InGaN quantum dots can provide better electron-hole overlap and reduce radiative lifetime. We also find that variation in dot sizes can lead to emission spectrum that can cover the whole visible light range. For high carrier density injection conditions, a self-consistent method for solving quantum dot devices is applied for better estimation of device performance. Consequences of variations in dot sizes, shapes, and composition have been studied in this paper. The results suggest that InGaN quantum dots would have superior performance in white light emitters.

Reliability of InP-based HBTs at High Current Density

AbstractInP-based HBTs for ultrahigh speed optical communications systems operation at over 40 GHz require a long-term stability under high current injection conditions, such as current densities of 2 or 5 mA/μm2. We achieved high reliability by suppressing surface recombination and emitter-metal-related crystalline degradation.Changes in the electric properties of devices due to temperature and bias stress were evaluated. The reduction in DC current gain due to surface recombination had the activation energy of 1.7 eV without current density dependence, and the lifetime of HBTs for this degradation mode is predicted to be over 1×108 hours at 125°C. The emitter metal diffusion and disruption of uniformity of the atomic composition were observed by transmission electron microscopy and energy dispersive X-ray spectroscopy in HBTs with the conventional Ti/Pt/Au emitter, whereas suppression of those degradations was observed in HBTs with refractory metal of Mo and W. The emitter resistance was estimated to evaluate the contact layer degradation. The critical time was one order larger for HBTs with refractory metal than for HBTs with conventional metal. The activation energies for resistance increases were 2.0 and 1.65 eV for the current density of 2 and 5 mA/μm2, respectively, for all types of emitter electrodes.The effectiveness of the refractory metal electrode for improving device reliability was confirmed, especially in high-current-density operation, which is essential for applying InP HBTs in high-speed ICs.

Investigation of the Nonthermal Mechanism of Efficiency Rolloff in InGaN Light-Emitting Diodes

We present a comparative study on the optical characteristics of InGaN-based multiple quantum well light-emitting diodes (LEDs) with peak emission ranging from green to ultraviolet (UV) over a wide injection range. It is found that by pulsing the LEDs with a duty cycle that is below 1%, thermally induced peak red shift and efficiency reduction are largely eliminated. The current dependence of both the quantum efficiency and peak shift appears to be a strong function of the indium content in the active region. The quantum efficiencies of the blue and green LEDs peak at very low currents and dramatically decrease at high currents, whereas the UV LED has a nearly constant quantum efficiency under high injection conditions. In contrast to the minimal current- induced energy shift in the UV LED, a monotonic blue shift of the peak energy, which has a total amount of ~110 meV-1 kA/cm2, is seen for the green LED. These results offer a strong support for the argument that the current overflow from localized states is the major nonthermal mechanism underlying the efficiency rolloff in InGaN-based visible LEDs.

Estimation of internal quantum efficiency in InGaN‐based light emitting diodes using electroluminescence decay times

AbstractWe investigate the internal quantum efficiency of InGaN‐based Light Emitting Diodes (LEDs) from the semiconductor rate equation of pulse current injection. A method is presented for estimating the internal quantum efficiency based on data of electroluminescence decay times measured as a function of current in the pulse injection. For the screening of built‐in electric field, the pulse current was injected with bias voltage. For blue LED, the internal quantum efficiency was measured to be about 70% in high injection condition. It is found that the internal quantum efficiency increase with injection current, whereas the external quantum efficiency decreases at high injection. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

1153 傾斜機能型電波吸収体の電磁場-熱連成効果と熱応力解析(G03-5 材料力学(5)力学挙動1,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Functionally graded (FG) type electromagnetic (EM) wave absorbers have been proposed for the improvement in absorption performance and material strength. In this study, coupling effects on the performance of the FG type EM wave absorbers and thermal stresses in the absorbers are investigated. The coupling effects of electromagnetic field and temperature field related with the heat generation due to high power injection and temperature-dependency of electromagnetic properties are analyzed by weak-coupling method based on the iterative use of the analytical solutions for each field. The thermal stress field is also analyzed for a thin plate with arbitrary nonhomogeneous properties and temperature-dependent thermal and electromagnetic properties. Numerical calculations are carried out for the FG type EM wave absorbers composed of epoxy resin and conductive titanium oxide. The magnitude of electric field, temperature field and thermal stress field is evaluated. The effects of material composition distribution, high power injection and heat condition on the absorption performance and thermal stress are then discussed. The significance of the coupling effects and the thermal stress analysis is revealed.

Analytical description of the injection ratio of self-biased bipolar transistors under the very high injection conditions of ESD events

This paper proposes a 1D-analytical description of the injection ratio of a self-biased bipolar transistor under very high current injection conditions. Starting from an expression of the current gain based on the stored charge into the emitter and base regions, we derive a new analytical expression of the current injection ratio. This analytical description demonstrates the presence of an asymptotic limit for the injection ratio at very high current densities, as the ratio of electron/hole mobilities in the case of an NPN transistor and to the ratio of hole/electron saturation velocities for a PNP. Moreover, for the first time, a base narrowing effect is demonstrated and explained in the case of a self-biased PNP, in contrast with the base widening effect (Kirk effect [Kirk CT, A theory of transistor cutoff frequency (fT) falloff at high current densities, IRE Trans Electr Dev 1961: p. 164–73]) reported for lower current density. These results are validated by numerical simulation and show a good agreement with experimental characterizations of transistors especially designed to operate under extreme condition such as electrostatic discharge (ESD) events.

Influence of Temperature on Shockley Stacking Fault Expansion and Contraction in SiC PiN Diodes

While Shockley stacking fault (SSF) creation and expansion within 4H-SiC bipolar devices is well known, only recently was it observed that this expansion and the associated increase in the forward voltage drop (V f) could be completely reversed via low-temperature annealing. Here we report the temperature dependence of the recovery rate of the V f drift via annealing, reporting an activation energy of 1.3 ± 0.3 eV. The V f drift was observed to saturate following extended electrical stressing, and it was observed that the value of V f at this saturation was inversely proportional to the stressing temperature. We also observed that SSF and V f drift recovery could occur in highly stressed diodes at elevated temperatures even under high current injection conditions (14 A/cm2).

Origin of efficiency droop in GaN-based light-emitting diodes

The efficiency droop in GaInN∕GaN multiple-quantum well (MQW) light-emitting diodes is investigated. Measurements show that the efficiency droop, occurring under high injection conditions, is unrelated to junction temperature. Furthermore, the photoluminescence output as a function of excitation power shows no droop, indicating that the droop is not related to MQW efficiency but rather to the recombination of carriers outside the MQW region. Simulations show that polarization fields in the MQW and electron blocking layer enable the escape of electrons from the MQW region and thus are the physical origin of the droop. It is shown that through the use of proper quaternary AlGaInN compositions, polarization effects are reduced, thereby minimizing droop and improving efficiency.

Optical characterization of a strained silicon quantum well on SiGe on insulator

An 8nm thick strained silicon layer embedded in relaxed Si0.8Ge0.2 has been grown on SiGe on insulator substrate in order to reduce the optical response of dislocations present in the SiGe virtual substrate. Photoreflectance measurement shows bandgap shrinkage at Γ point of 0.19eV which corresponds to a 0.94% strain value close to the one measured in Raman spectroscopy. The luminescence arising only from the strained Si quantum well in high injection conditions reveals clearly two optical transitions observed at 0.959 and 1.016eV.

A novel feedback method for optimizing the focus of an ultrafast photon source using the transient photocurrent of a high-speed device

A novel feedback method for focusing an ultrafast photon source is illustrated for the first time. The method relies on the pulse shape analysis (PSA) of the high-speed transient photocurrent (TP) measured as a function of relative distance between the objective lens and a photosensitive semiconductor device. Space-charge effects due to high-injection conditions result in a screened electron–hole pair plasma which exhibits an erosion time constant which depends on the density and ambipolar diffusivity of the plasma. Focusing the beam increases the average injection level thereby decreasing the ambipolar diffusivity and extending the transient photocurrent. A simple pulse shape analysis of the transient inside a feedback loop is used for optimizing the spot size. In theory, the method can be applied to any technique involving a photon source which delivers an ultrafast pulse which generates high-injection carrier levels when focused. Here, the method is outlined and demonstrated on a Si photodetector optimized for λ = 800 nm.

Heavy ion and pulsed laser SET measurements in ultrahigh speed MSM GaAs photodetectors

Single Event Transient (SET) measurements using both focused MeV heavy ions and pulsed picosecond lasers are employed to examine the injection and spatial dependence of SETs in high speed GaAs MSM photodetectors. Ion track structure and its influence on the average electron-hole pair density result in space-charge effects which determine the SET duration and possible BER contamination over more than one decision cycle of a GHz modulation based communication system. Laser probing is used to confirm the existence of space-charge effects and scanned SET measurements using an MeV ion microbeam are used to examine the spatial dependence of SET formation. Finally, ISE-TCAD simulations are performed to examine signal generation under high-injection conditions where space-charge effects prevail.

Polarization field effects on the recombination dynamics in low-In-content InGaN multi-quantum wells

The interplay of polarization fields and free carrier screening in In x Ga 1− x N/GaN ( 0.03 < x < 0.07 ) multiple quantum wells is studied by combining photoluminescence (time-integrated and time-resolved) and cathodoluminescence studies, in an excitation density range from 10 8 to 10 12 cm −2 of generated e–h pairs. For such low In content, the quantum-confined Stark effect is verified to rule the recombination dynamics, while effects of carrier localization in potential fluctuations have a minor role. Efficient field screening is demonstrated in CL steady-state high-injection conditions and in PL time-resolved experiments at the maximum excitation density. Under recovered nearly flat band conditions, quantum confinement effects are revealed and a high and possibly composition-dependent bowing parameter is extrapolated. Information on radiative and non-radiative rates for carrier recombination in the wells is obtained, both from steady-state and from time-resolved experiments, modelling the carrier dynamics in the framework of a theoretical rate equation model, which calculates electronic states and recombination rates in the nanostructure by coupling complete self-consistent solutions of Schrödinger and Poisson equations.