Parasitic Recombination Research Articles

Improved room-temperature luminescence of core-shell InGaAs/GaAs nanopillars via lattice-matched passivation

Optical properties of GaAs/InGaAs/GaAs nanopillars (NPs) grown on GaAs(111)B were investigated. Employment of a mask-etching technique allowed for an accurate control over the geometry of NP arrays in terms of both their diameter and separation. This work describes both the steady-state and time-resolved photoluminescence of these structures as a function of the ensemble geometry, composition of the insert, and various shell compounds. The effects of the NP geometry on a parasitic radiative recombination channel, originating from an overgrown lateral sidewall layer, are discussed. Optical characterization reveals a profound influence of the core-shell lattice mismatch on the carrier lifetime and emission quenching at room temperature. When the lattice-matching conditions are satisfied, an efficient emission from the NP arrays at room temperature and below the band-gap of silicon is observed, clearly highlighting their potential application as emitters in optical interconnects integrated with silicon platforms.

Investigation of Parasitic Edge Recombination in High-Lifetime Oxidized <i>n</i>-Si

An investigation of the parasitic surface recombination mechanisms in high-lifetime oxidized n-Si is presented. An approximate analytical expression describing recombination at the edge of square shaped specimens is derived. This shows that edge recombination can have a significant effect on the effective lifetime as measured using the transient photo-conductance technique and that for well passivated high quality material edge recombination can be the dominant mechanism in reducing the effective lifetime below the intrinsic level. For 3 x 3 cm2 pieces of silicon measured using a Sinton photo-conductance lifetime instrument, it is shown that recombination at the edge of the sample results in an additional component to the measured lifetime of around 16 ms at an injection level of 1015 cm-3. When this effect is taken into account measurements of 1 Ωcm FZ-Si show that a SRV as low as 1.5 cm/s is possible when the surface is passivated using a corona charge concentration of +2.2 x 1012 q/cm2 deposited on a 100 nm oxide layer.

Interview

Dr Fedor Zubov of St Petersburg Academic University in Russia talks about the work behind the paper ‘Suppression of sublinearity of light–current curve in 850 nm quantum well laser with asymmetric barrier layers’ page 1106. Dr Fedor Zubov My research relates to the development of modern injection lasers and my current interests include diode lasers of novel design – lasers with asymmetric barriers (LABs). LAB-concept was proposed in 2003 by a theorist, a member of our research team and co-author of our Letter - Levon Asryan, a professor at Virginia Tech. In such lasers, special, thin (several nanometers) asymmetric barrier layers (ABLs) on both sides of the active region block undesired transport of charge carriers, suppressing parasitic recombination of charge carriers in the waveguide. After almost 10 years our research team has realised this concept for the first time. It was a part of my Ph.D. research that I carried out in the Nanophotonics Lab under supervision of Prof. A.E. Zhukov. Recently, in 2013, the scientific community celebrated the 50th anniversary of semiconductor laser invention and now there is no sphere where they are not used. Among the most significant applications are data transmission, material processing and various medical applications. Today, diode lasers continue to be intensively investigated, new promising spectral ranges are being developed, and lasers of nondiode type are being displaced by them. In respect to application of our short-wave 0.85 µm LABs, the most interesting application seems to be high-power laser diodes used for optical pumping of nondiode lasers. Here, the problem of parasitic recombination is especially challenging. Our previous calculations have shown that the implementation of ABLs should improve device power characteristics owing to a decrease of current component related to parasitic recombination. Actually, within this work we are confirming this thesis experimentally. Our LABs, in comparison to a reference laser of traditional design, have demonstrated a significant improvement of the LCC linearity, so that the maximal output power was reached with 60% less pumping. The concept of asymmetric barrier lasers is universal, in the sense that it can be applied to diode lasers of different material systems and active region types. We have shown that suppression of parasitic recombination due to ABLs allows enhancement of main device characteristics: threshold current, efficiency, output optical power and temperature stability. This approach does not complicate the technology. In fact, temperature stability improvement would significantly simplify diode laser temperature-stabilisation systems. Such devices will provide a basis for new applications of injection lasers under elevated temperature conditions. A previous approach for suppression of parasitic recombination used a thinner waveguide in combination with a larger bandgap, resulting in the increase of vertical beam divergence and the decrease of COMD threshold. The attributes that should be improved to meet toughening application requirements are the lasing power, efficiency, reliability, brightness and cost. If we consider a 100 µm aperture single emitter for continuous wave room temperature (RT) operation, the present demand is 20 W. In the last 10 years the annual growth was 15% per year, and an achievement of this magnitude in the next 10 years seems to be realistic. Much more challenging are future diode laser pump systems for fusion energy installations. To achieve the required ∼100 GW pump power, the system should consist of ∼200 Mbars with 500W/bar at $0.01/W price. This could be achieved by using uncooled diodes, while maintaining the high efficiency and output power. I believe, in this regard, our lasers of ABL-design with improved temperature stability and power characteristics look very attractive. One part of our research is the development of whispering gallery mode microcavity quantum-dot lasers. These tiny (∼1-10 µm) semiconductor lasers are promising light sources for application in photonic ICs, biological sensing and imaging. Recently, we demonstrated RT lasing in the 1.3 µm wavelength range for microlasers as small as 1 µm, achieved under optical pumping. Our current research is the study of lasing under electrical pumping in such lasers. Another research area is the development of novel waveguide designs. We are targeting the realisation of symmetrical far-field radiation profiles with low divergence, which is necessary for effective coupling of laser emission to optical fibres. The current structure uses additional tunnel-coupled passive waveguide, suppressing high-order optical modes.

Suppression of sublinearity of light–current curve in 850 nm quantum well laser with asymmetric barrier layers

An AlGaAs/GaAs quantum well (QW) laser is fabricated with GaInP and AlGaInAs asymmetric barrier layers (ABLs) and its light–current characteristic (LCC) is compared with that of a reference conventional QW laser without ABLs. It was found that the use of the ABLs suppresses the sublinearity of the LCC at high current densities. As a result, the maximum lasing power of 9.2 W, being limited by catastrophic optical mirror damage, is achieved at a considerably lower operating current in the laser with ABLs as compared to the reference laser (12.5 against 20.2 A). The ABL effect is associated with the suppression of the parasitic recombination in the optical confinement layer, as confirmed by a decrease of the intensity of the spontaneous emission from the layer.

Improved open-circuit voltage in Cu(In,Ga)Se2 solar cells with high work function transparent electrodes

Hydrogenated indium oxide (IOH) is implemented as transparent front contact in Cu(In,Ga)Se2 (CIGS) solar cells, leading to an open circuit voltage VOC enhanced by ∼20 mV as compared to reference devices with ZnO:Al (AZO) electrodes. This effect is reproducible in a wide range of contact sheet resistances corresponding to various IOH thicknesses. We present the detailed electrical characterization of glass/Mo/CIGS/CdS/intrinsic ZnO (i-ZnO)/transparent conductive oxide (TCO) with different IOH/AZO ratios in the front TCO contact in order to identify possible reasons for the enhanced VOC. Temperature and illumination intensity-dependent current-voltage measurements indicate that the dominant recombination path does not change when AZO is replaced by IOH, and it is mainly limited to recombination in the space charge region and at the junction interface of the solar cell. The main finding is that the introduction of even a 5 nm-thin IOH layer at the i-ZnO/TCO interface already results in a step-like increase in VOC. Two possible explanations are proposed and verified by one-dimensional simulations using the SCAPS software. First, a higher work function of IOH as compared to AZO is simulated to yield an VOC increase by 21 mV. Second, a lower defect density in the i-ZnO layer as a result of the reduced sputter damage during milder sputter-deposition of IOH can also add to a maximum enhanced VOC of 25 mV. Our results demonstrate that the proper choice of the front TCO contact can reduce the parasitic recombination and boost the efficiency of CIGS cells with improved corrosion stability.

Performance enhancement of quantum dot sensitized solar cells by adding electrolyte additives

The parasitic charge recombination processes between TiO2/QDs/electrolyte interfaces were effectively suppressed with the addition of polyethylene glycol (PEG) additive in polysulfide electrolyte, giving enhancement of PCE from 5.80% to 6.74% in the corresponding cell devices.

Calculation of output characteristics of semiconductor quantum-well lasers with account for both electrons and holes

Using an extended theoretical model, which includes the rate equations for both electrons and holes, we have studied the output characteristics of semiconductor quantum-well lasers. We have found non-trivial dependences of electron and hole concentrations in the waveguide region of the laser on the capture velocities of both types of carriers from the waveguide region into the quantum well. We have obtained the dependences of the internal differential quantum efficiency and optical output power of the laser on the capture velocities of electrons and holes. An increase in the capture velocities has been shown to result in suppression of parasitic recombination in the waveguide region and therefore in a substantial increase in the quantum efficiency and output power.

A Markov chain approach to simulate Atomic Layer Deposition chemistry and transport inside nanostructured substrates

In this work, we present a new theoretical framework to model the transport and surface chemistry under molecular (Knudsen) flow. Our approach is based on casting the transport inside nanostructures as a single-particle discrete Markov chain process. One of the advantages of this approach is that it allows us to decouple the complexity of the surface chemistry from the transport model, thus allowing its application under general surface chemistry conditions, including atomic layer deposition (ALD) and chemical vapor deposition (CVD). Our model also allows us to determine statistical information of the trajectory of individual molecules, such as the average interaction time or the number of wall collisions for molecules entering the nanostructures as well as to track the relative contributions to thin-film growth of different independent reaction pathways at each point of the feature. This offers a straightforward way of incorporating into ALD simulations non-ideal surface processes, such as parasitic CVD or surface recombination. By studying the asymptotic behavior of the Markov chain process, we were also able to establish a direct link between ballistic models, kinetic Monte Carlo simulations, and continuous models based on the use of the diffusion equation under Knudsen conditions. Finally, we show that, under certain approximations, the coverage profile inside a nanostructure under ALD conditions is controlled by the total exposure, and not by the details of the surface flux dependence with time during the exposure, as long as the reaction probabilities are pressure independent.

Light-current characteristic of a quantum well laser with asymmetric barrier layers

Light-current characteristic (LCC) of a novel type of quantum well (QW) lasers—QW lasers with asymmetric barrier layers (ABLs)—is studied. The ABLs (one on each side of the QW) prevent electrons from entering the hole-injecting side of the structure and holes from entering the electron-injecting side. The use of ABLs thus suppresses the parasitic electron-hole recombination outside the QW and eliminates the mechanism of sublinearity of the LCC in conventional lasers associated with this recombination and with the carrier capture delay into the QW. As a result, no matter how slow is the carrier capture into the QW, the LCC of an ABL QW laser is virtually linear. In an ABL laser containing indent layers between the QW and each of the ABLs (parasitic recombination still occurs in these thin layers), even in the case of slow capture of carriers into the QW, the LCC is also considerably more linear than in a reference conventional QW laser.

Capture of charge carriers and output power of a quantum well laser

The effect of noninstantaneous carrier capture by a nanoscale active region on the power characteristics of a semiconductor laser is studied. A laser structure based on a single quantum well is considered. It is shown that delayed carrier capture by the quantum well results in a decrease in the internal differential quantum efficiency and sublinearity of the light-current characteristic of the laser. The main parameter of the developed theoretical model is the velocity of carrier capture from the bulk (waveguide) region to the two-dimensional region (quantum well). The effect of the capture velocity on the dependence of the following laser characteristics on the pump current density is studied: the output optical power, internal quantum efficiency of stimulated emission, current of stimulated recombination in the quantum well, current of spontaneous recombination in the optical confinement layer, and carrier concentration in the optical confinement layer. A decrease in the carrier capture velocity results in a larger sublinearity of the light-current characteristic, which results from an increase in the injection current fraction expended to parasitic spontaneous recombination in the optical confinement layer and, hence, a decrease in the injection current fraction expended to stimulated recombination in the quantum well. A comparison of calculated and experimental light-current characteristics for a structure considered as an example shows that good agreement between them (up to a very high injection current density of 45 kA/cm2) is attained at a capture velocity of 2 × 106 cm/s. The results of this study can be used to optimize quantum well lasers for generating high optical powers.

Cascade Organic Solar Cells

We demonstrate planar organic solar cells consisting of a series of complementary donor materials with cascading exciton energies, incorporated in the following structure: glass/indium-tin-oxide/donor cascade/C60/bathocuproine/Al. Using a tetracene layer grown in a descending energy cascade on 5,6-diphenyl-tetracene and capped with 5,6,11,12-tetraphenyl-tetracene, where the accessibility of the π-system in each material is expected to influence the rate of parasitic carrier leakage and charge recombination at the donor/acceptor interface, we observe an increase in open circuit voltage (Voc) of approximately 40% (corresponding to a change of +200 mV) compared to that of a single tetracene donor. Little change is observed in other parameters such as fill factor and short circuit current density (FF = 0.50 ± 0.02 and Jsc = 2.55 ± 0.23 mA/cm2) compared to those of the control tetracene–C60 solar cells (FF = 0.54 ± 0.02 and Jsc = 2.86 ± 0.23 mA/cm2). We demonstrate that this cascade architecture is effective i...

Output power of a double tunneling-injection quantum dot laser

We develop a comprehensive theoretical model for a double tunneling-injection(DTI) quantum dot (QD) laser. Both electrons and holes are injected into QDsby tunneling from two separate quantum wells (QWs). Ideally, out-tunneling ofeach type of carriers from QDs into the opposite-to-injection-side QW should becompletely blocked; as a result, the parasitic electron–hole recombination outsideQDs will be suppressed and the light–current characteristic (LCC) of a laser willbe strictly linear. To scrutinize the potential of a DTI QD laser for high-poweroperation and the robustness of an actual device, our model includes out-tunnelingleakage of carriers from QDs. We complement our calculations by an analyticalmodel and derive closed-form expressions for the LCC and carrier populationacross the layered structure. We show that, even in the presence of out-tunnelingleakage, the flux of parasitic recombination outside QDs remains restricted withincreasing injection current. As a consequence, the LCC exhibits a remarkablefeature distinguishing the DTI QD laser from other types of injection lasers—itbecomes increasingly linear and the slope efficiency grows closer to unity at highinjection currents. The linearity is due to the fact that the current paths connectingthe opposite sides of the structure lie entirely within the QDs—in view of thethree-dimensional confinement in QDs, the out-tunneling fluxes of carriers from dots arelimited.

Characteristic temperature of a tunneling-injection quantum dot laser: Effect of out-tunneling from quantum dots

A laser structure is studied, which exploits tunneling-injection of electrons and holes into quantum dots (QDs) from two separate quantum wells (QWs). An extended theoretical model is developed allowing for out-tunneling leakage of carriers from QDs into the opposite-to-injection-side QWs (electrons into the p-side QW and holes into the n-side QW). Due to out-tunneling leakage, parasitic recombination of electron-hole pairs occurs outside QDs – in the QWs and optical confinement layer. The threshold current density j th and the characteristic temperature T 0 are shown to be mainly controlled by the recombination in the QWs. Even in the presence of out-tunneling from QDs and recombination outside QDs, a tunneling-injection laser shows potential for significant improvement of temperature stability of j th – the characteristic temperature T 0 remains very high (above 300 K at room temperature) and not significantly affected by the QD size fluctuations.

Tunneling-injection of electrons and holes into quantum dots: A tool for high-power lasing

We study the optical output power of a semiconductor laser, which exploits tunneling-injection of electrons and holes into quantum dots (QDs) from two separate quantum wells. Even if there is out-tunneling leakage of carriers from QDs, the intensity of parasitic recombination outside QDs remains restricted with increasing injection current. As a result, the light-current characteristic becomes increasingly linear, and the slope efficiency grows closer to unity at high injection currents—a fascinating feature favoring the use of tunneling-injection of both electrons and holes into QDs for high-power lasing.

Temperature-insensitive semiconductor quantum dot laser

Different approaches to the design of a genuinely temperature-insensitive quantum dot (QD) laser are proposed. Suppression of the parasitic recombination outside the QDs, which is the dominant source of the temperature dependence of the threshold current in the conventional design of a QD laser, is accomplished either by tunneling injection of carriers into the QDs or by band-gap engineering. Elimination of this recombination channel alone enhances the characteristic temperatures T 0 above 1000 K. Remaining sources of temperature dependence (recombination from higher QD levels, inhomogeneous line broadening, and violation of charge neutrality in QDs) are studied. Tunneling-injection structures are shown to offer an additional advantage of suppressed effects of inhomogeneous broadening and neutrality violation.

High Efficiency Bulk Crystalline Silicon Light Emitting Diodes

ABSTRACTA high power conversion efficiency above 1% from a bulk crystalline silicon (c-Si) light-emitting diode (LED) has been demonstrated at near room temperature. These devices are based on normally weak one- and two-phonon assisted sub-bandgap light emission processes. Their improved performance results from device designs that take advantage of enhanced light absorption by a light trapping scheme which was developed for high efficiency silicon solar cells, and from reducing scope for parasitic non-radiative recombination within the diode. Each feature individually is shown to improve efficiency by a factor of ten, accounting for an improvement by factor of one hundred compared to baseline devices.Also demonstrated is a greatly improved band-edge light emission and detection using bulk c-Si diodes. A bulk c-Si LED is combined with a similar diode used as a detector that collects the light emitted with a high quantum collection efficiency of 33%, to produce a silicon to silicon optically coupled system that demonstrates 0.18% coupling quantum efficiency. The crystalline silicon LED demonstrates similarly high performance at very low power levels, where it has even higher power efficiency than a high efficiency GaAlAs LED.

Efficient silicon light-emitting diodes.

Considerable effort is being expended on the development of efficient silicon light-emitting devices compatible with silicon-based integrated circuit technology. Although several approaches are being explored, all presently suffer from low emission efficiencies, with values in the 0.01-0.1% range regarded as high. Here we report a large increase in silicon light-emitting diode power conversion efficiency to values above 1% near room temperature-close to the values of representative direct bandgap emitters of a little more than a decade ago. Our devices are based on normally weak one- and two-phonon assisted sub-bandgap light-emission processes. Their design takes advantage of the reciprocity between light absorption and emission by maximizing absorption at relevant sub-bandgap wavelengths while reducing the scope for parasitic non-radiative recombination within the diode. Each feature individually is shown to improve the emission efficiency by a factor of ten, which accounts for the improvement by a factor of one hundred on the efficiency of baseline devices.

Erbium in Silicon: Possible Light Source for 1.5 μm and Challenge for Defect Physics

The trivalent erbium ion emits at 1.54 gm, independent of the host crystal and temperature. This fact makes Si : Er an interesting candidate for integrated optics in the optimum wavelength regime for fiber optic communication systems. Recent progress in improving the luminescence yield is reviewed and the limiting factors are discussed, namely: the low solubility of different Er centers, thermal quenching of the luminescence above 100 K, the mechanisms for energy transfer from the host crystal to the Er 4f shell and the process induced parasitic recombination channels.

Analysis of geometric charge-pumping components in a thin-film SOI device

A numerical analysis of the charge-pumping experiment in thin-film SOI pin-diodes is presented. The approach is based on the self-consistent two-dimensional numerical solution of the time-dependent semiconductor equations including the interface trap dynamic equations, which are solved for interface traps on the front and on the back interface of the SOI device. We can identify two distinct mechanisms of parasitic recombination on the back interface which show up for steep edges of the front gate pulse train. The influence of accumulation or inversion of the back interface on the appearance of these parasitic effects is analyzed numerically and comparisons of simulation results with measurement data are shown.

Temperature dependence of current gain in AlGaAs/GaAs heterojunction bipolar transistors

Details of the temperature dependence of current gain in AlGaAs/GaAs heterojunction bipolar transistors (HBT’s) are reported for the first time. Unlike homojunction transistors, the gain initially increases with decreasing temperature from 425 K followed by a decrease towards 77 K at a temperature which is dependent on AlAs mole fraction in the emitter. Variation of gain with temperature is much smaller than what is normally seen in homojunction transistors adding another advantage in favor of the HBT’s. Hole injection current into the emitter and parasitic recombination currents are shown to be the reasons for the observed temperature dependence of gain.