Values Of Surface Recombination Velocity Research Articles

Modeling Lattice Matched Dilute Nitride Triple and Quadruple Junction Solar Cells on Virtual SiGe Substrate

A lattice matched triple junction solar cell (TJSC) structure with a GaAs0.58 P0.42 top cell and bandgap tunable GaNxAs1-x-zPz middle and bottom cells on virtual SiGe substrate is proposed in this study. SiGe/Si substrate is preferred as it is a low-cost substrate and because it provides a lattice constant at which bandgap tunable dilute nitride materials that are appropriate for highly efficient multijunction solar cells can be obtained. By changing the nitrogen content in GaNxAs1-x-zPz, the bandgap of the middle and bottom subcells is adjusted to the optimum values. The bandgap of the top cell is constant at 1.95 eV. Three models with different values of surface recombination velocities and Shockley–Read–Hall recombination lifetimes are applied to the presented TJSC structure. Peak efficiencies of 48.9%, 40.6% and 33.7% are achieved at EG2 = 1.45 eV and EG3 = 1.04 eV for Model 1, EG2 = 1.45 eV and EG3 = 1.15 eV for Model 2, and EG2 = 1.5 eV and EG3 = 1.17 eV for Model 3, respectively. A fourth bandgap adjustable GaNxAs1-x-zPz junction is inserted into the system and a significant improvement is obtained under high sun concentration for Models 1 and 2. The presented original results are very promising because the variable bandgaps provide very efficient absorption of incoming spectrum.

Impact of different parameters on the performance of GaAs solar cell using PC1D simulation

In this work, GaAs solar cell has been analyzed by using PC1D simulation software. Optimization of thickness and doping concentration of emitter and absorber layer of solar cell is done. Effect of front and rear surface recombination velocity is analyzed in this work. When surface recombination velocity increases, the value of efficiency decreases due to increase in recombination process. The optimum value of surface recombination velocity are Sn = Sp = 102 cm/s. Front surface reflection is minimized by the use of TiO2 single layer ARC and double layer LiF/HfO2 ARC. Increase in efficiency around 2.71% is achieved as compared to cell without ARC after applying anti-reflection coating of optimum thickness. Simulation results shows GaAs solar cell with efficiency of 26.68%.

Effects of damages induced by indium-tin-oxide reactive plasma deposition on minority carrier lifetime in silicon crystal

A reactive-plasma deposition (RPD) process damage is evaluated for an indium-tin-oxide (ITO)/SiO2/Si structure in terms of the recombination properties at the interface. To calculate the surface recombination velocity (SRV), the defect density at the SiO2/Si interface (Dit), the effective total charge (Qtot) which is the sum of the charges in the SiO2 and the trapped charge at the SiO2/Si interface, and the workfunction of ITO at the ITO/SiO2 interface (ϕITO), are extracted by capacitance-voltage (C-V) analysis as a function of a postdeposition annealing (PDA) temperature. The Dit and Qtot significantly increase by the RPD process to 1.4×1012 cm-2eV-1 and 1.5×1012/q cm-2, respectively, and decrease by the PDA at 200 °C in N2 ambient to 1.4×1011 cm-2eV-1 and 1.1×1011/q cm-2, respectively. The ϕITO also varies from 4.80 eV to 4.35 eV by the PDA. The correlation is examined between the SRV values calculated based on the extended SRH model using the extracted Dit, Qtot, and ϕITO values [SRV(CV)], and the SRV ones obtained from the minority carrier lifetime measurements [SRV(LT)]. It is found that multiple sets of capture cross-sections were required for SRV(CV) values to coincide with SRV(LT) ones. The effect of variation in the Qtot and ϕITO on the band bending is almost canceled out in the present PDA condition. It is thus considered that the increase in Dit is the main cause to decrease the minority carrier lifetime.

Unidimensional photocurrent model for induced-junction photodiodes

At present, numerical simulations are the common method to predict the response of quantum efficient detectors based on induced-junction diodes. As alternative, this work proposes an analytical model based on photocurrent analysis where surface and bulk losses are modelled considering design and operation photodiode parameters and the characteristics of the incident beam. The model shows how, at short wavelengths, the surface recombination velocity dominates the losses that are directly proportional, whereas the bulk doping and reverse bias voltage determine the losses at long wavelengths. The results obtained by the here proposed analytical model are discussed for different values of surface recombination velocity and compared with simulations reported by other authors. For wavelengths from 400 nm up to 700 nm, the losses calculated by the analytical model at room temperature are in the order of tens of ppm for values between 1000 cm/s and 10000 cm/s of surface recombination velocity. Some simulations agree with a maximum difference of 80 ppm up to 700 nm, where the abrupt rise of bulk losses starts for the analytical model but it is shifted around 800 nm for reported simulations.

Excitation-dependent carrier lifetime and diffusion length in bulk CdTe determined by time-resolved optical pump-probe techniques

We applied time-resolved pump-probe spectroscopy based on free carrier absorption and light diffraction on a transient grating for direct measurements of the carrier lifetime and diffusion coefficient D in high-resistivity single crystal CdTe (codoped with In and Er). The bulk carrier lifetime τ decreased from 670 ± 50 ns to 60 ± 10 ns with increase of excess carrier density N from 1016 to 5 × 1018 cm−3 due to the excitation-dependent radiative recombination rate. In this N range, the carrier diffusion length dropped from 14 μm to 6 μm due to lifetime decrease. Modeling of in-depth (axial) and in-plane (lateral) carrier diffusion provided the value of surface recombination velocity S = 6 × 105 cm/s for the untreated surface. At even higher excitations, in the 1019–3 × 1020 cm−3 density range, D increase from 5 to 20 cm2/s due to carrier degeneracy was observed.

Surface recombination velocity imaging of wet-cleaned silicon wafers using quantitative heterodyne lock-in carrierography

InGaAs-camera based heterodyne lock-in carrierography (HeLIC) is developed for surface recombination velocity (SRV) imaging characterization of bare (oxide-free) hydrogen passivated Si wafer surfaces. Samples prepared using four different hydrofluoric special-solution etching conditions were tested, and a quantitative assessment of their surface quality vs. queue-time after the hydrogen passivation process was made. The data acquisition time for an SRV image was about 3 min. A “round-trip” frequency-scan mode was introduced to minimize the effects of signal transients on data self-consistency. Simultaneous best fitting of HeLIC amplitude-frequency dependencies at various queue-times was used to guarantee the reliability of resolving surface and bulk carrier recombination/transport properties. The dynamic range of the measured SRV values was established from 0.1 to 100 m/s.

Synthesis and Characterization of Alkylamine-Functionalized Si(111) for Perovskite Adhesion With Minimal Interfacial Oxidation or Electronic Defects.

We investigated synthetic strategies for the functionalization of Si(111) surfaces with organic species containing amine moieties. We employed the functionalized surfaces to chemically "glue" perovskites to silicon with efficient electron transfer and minimal oxidation leading to deleterious recombination at the silicon substrate. A two-step halogenation-alkylation reaction produced a mixed allyl-methyl monolayer on Si(111). Subsequent reactions utilized multiple methods of brominating the allyl double bond including reaction with HBr in acetic acid, HBr in THF, and molecular bromine in dichloromethane. Reaction with ammonia in methanol effected conversion of the bromide to the amine. X-ray photoelectron spectroscopy (XPS) quantified chemical states and coverages, transient-microwave photoconductivity ascertained photogenerated carrier lifetimes, atomic force microscopy (AFM) quantified perovskite-silicon adhesion, and nonaqueous photoelectrochemistry explored solar-energy-conversion performance. The HBr bromination followed by the amination yielded a surface with ∼10% amine sites on the Si(111) with minimal oxide and surface recombination velocity values below 120 cm s-1, following extended exposures to air. Importantly, conversion of amine sites to ammonium and deposition of methylammonium lead halide via spin coating and annealing did not degrade carrier lifetimes. AFM experiments quantified adhesion between perovskite films and alkylammonium-functionalized or native-oxide silicon surfaces. Adhesion forces/interactions between the perovskite and the alkylammonium-functionalized films were comparable to the interaction between the perovskite and native-oxide silicon surface. Photoelectrochemistry of perovskite thin films on alkylammonium-functionalized n+-Si showed significantly higher Voc than n+-Si with a native oxide when in contact with a nonaqueous ferrocene+/0 redox couple. We discuss the present results in the context of utilizing molecular organic recognition to attach perovskites to silicon utilizing organic linkers so as to inexpensively modify silicon for future tandem-junction photovoltaics.

TCAD Studies on the Determination of Diffusion Length for the Planar-Collector EBIC Configuration With Any Size of the Schottky Contact

In this brief, the transient single-contact electron-beam-induced current (SC-EBIC) and the conventional steady-state EBIC modes of the planar-collector configuration that were studied using a Technology Computer Aided Design device simulator are presented. The feasibility of these EBIC data in the extraction of the diffusion length of the planar-collector configuration with any values of surface recombination velocities and any size of the Schottky contact is also presented. The effect of the size of the Schottky contact on steady state and transient EBIC signals as well as the extracted diffusion length and linearization coefficient is discussed in this brief. The EBIC information obtained from the SC-EBIC and the conventional EBIC is found to be able to evaluate the diffusion length accurately regardless of the size of the Schottky contact.

Deposition temperature dependence of material and Si surface passivation properties of O3-based atomic layer deposited Al2O3-based films and stacks

The material composition and the Si surface passivation of aluminum oxide (Al2O3) films prepared by atomic layer deposition using Al(CH3)3 and O3 as precursors were investigated for deposition temperatures (TDep) between 200 °C and 500 °C. The growth per cycle decreased with increasing deposition temperature due to a lower Al deposition rate. In contrast the material composition was hardly affected except for the hydrogen concentration, which decreased from [H] = 3 at. % at 200 °C to [H] < 0.5 at. % at 400 °C and 500 °C. The surface passivation performance was investigated after annealing at 300 °C–450 °C and also after firing steps in the typical temperature range of 800 °C–925 °C. A similar high level of the surface passivation performance, i.e., surface recombination velocity values <10 cm/s, was obtained after annealing and firing. Investigations of Al2O3/SiNx stacks complemented the work and revealed similar levels of surface passivation as single-layer Al2O3 films, both for the chemical and field-effect passivation. The fixed charge density in the Al2O3/SiNx stacks, reflecting the field-effect passivation, was reduced by one order of magnitude from 3·1012 cm−2 to 3·1011 cm−2 when TDep was increased from 300 °C to 500 °C. The level of the chemical passivation changed as well, but the total level of the surface passivation was hardly affected by the value of TDep. When firing films prepared at of low TDep, blistering of the films occurred and this strongly reduced the surface passivation. These results presented in this work demonstrate that a high level of surface passivation can be achieved for Al2O3-based films and stacks over a wide range of conditions when the combination of deposition temperature and annealing or firing temperature is carefully chosen.

Passivation of Zn3P2 substrates by aqueous chemical etching and air oxidation

Surface recombination velocities measured by time-resolved photoluminescence and compositions of Zn3P2 surfaces measured by x-ray photoelectron spectroscopy (XPS) have been correlated for a series of wet chemical etches of Zn3P2 substrates. Zn3P2 substrates that were etched with Br2 in methanol exhibited surface recombination velocity values of 2.8 × 104 cm s−1, whereas substrates that were further treated by aqueous HF–H2O2 exhibited surface recombination velocity values of 1.0 × 104 cm s−1. Zn3P2 substrates that were etched with Br2 in methanol and exposed to air for 1 week exhibited surface recombination velocity values of 1.8 × 103 cm s−1, as well as improved ideality in metal/insulator/semiconductor devices.

Ultrathin Oxide Passivation Layer by Rapid Thermal Oxidation for the Silicon Heterojunction Solar Cell Applications

It is difficult to deposit extremely thin a-Si:H layer in heterojunction with intrinsic thin layer (HIT) solar cell due to thermal damage and tough process control. This study aims to understand oxide passivation mechanism of silicon surface using rapid thermal oxidation (RTO) process by examining surface effective lifetime and surface recombination velocity. The presence of thin insulating a-Si:H layer is the key to get highVocby lowering the leakage current (I0) which improves the efficiency of HIT solar cell. The ultrathin thermal passivation silicon oxide (SiO2) layer was deposited by RTO system in the temperature range 500–950°C for 2 to 6 minutes. The thickness of the silicon oxide layer was affected by RTO annealing temperature and treatment time. The best value of surface recombination velocity was recorded for the sample treated at a temperature of 850°C for 6 minutes at O2flow rate of 3 Lpm. A surface recombination velocity below 25 cm/s was obtained for the silicon oxide layer of 4 nm thickness. This ultrathin SiO2layer was employed for the fabrication of HIT solar cell structure instead of a-Si:H, (i) layer and the passivation and tunneling effects of the silicon oxide layer were exploited. The photocurrent was decreased with the increase of illumination intensity and SiO2thickness.

Study of the composition of hydrogenated silicon nitride SiN x:H for efficient surface and bulk passivation of silicon

This work is a contribution towards the understanding of the properties of hydrogenated silicon nitride (SiN x :H) that lead to efficient surface and bulk passivation of the silicon substrate. Considering the deposition system used (low-frequency plasma-enhanced chemical vapour deposition (PECVD)), we report very low values of surface recombination velocity S eff. As-deposited Si-rich SiN x :H leads to the best results (n-type Si: S eff=4 cm/s – p-type Si: S eff=14 cm/s). After annealing, the surface passivation quality is drastically deteriorated for Si-rich SiN x :H whereas it is lightly improved for low refractive index SiN x :H ( n∼2–2.1). The chemical analysis of the layers highlighted a high hydrogen concentration, regardless the SiN x :H stoichiometry. However, the involved H-bond types as well as the hydrogen desorption kinetics are strongly dependent on the SiN x :H composition. Furthermore, “N-rich” SiN x :H appears to be denser and thermally more stable than Si-rich SiN x :H. When subjected to a high-temperature treatment, such a layer is believed to induce the release of hydrogen in its atomic form, which consequently leads to an efficient passivation of surface and bulk defects of the Si substrate. The results are discussed and compared with the literature data reported for the different configurations of PECVD reactors.

Sensitivity analysis of laterally resolved free carrier absorption determination of electronic transport properties of silicon wafers

Simulations are performed to investigate the uniqueness of simultaneous determination of electronic transport properties (the carrier lifetime, the carrier diffusivity, and the front surface recombination velocity) of silicon wafers by laterally resolved modulated free carrier absorption (MFCA) and multiparameter fitting. The dependences of MFCA amplitude and phase on these transport properties at different pump-probe-beam separations and modulation frequencies are analyzed. The uncertainties of the fitted parameter values are analyzed by investigating the dependences of a mean square variance including both the amplitude error and phase error on corresponding electronic transport parameters. Simulation results show that the electronic transport parameters can be determined accurately through fitting experimental MFCA data carrying both frequency- and space-domain information of carrier diffusion to a rigorous MFCA model. Among the three transport parameters, the carrier diffusivity can be determined most precisely, with an uncertainty of less than ±5%, due to the highest sensitivity of the laterally resolved MFCA signal to the diffusivity. The highly accurate determination of the diffusivity further improves the precision of the carrier lifetime and the front surface recombination velocity values simultaneously determined via multiparameter fitting. Experiments were performed with a silicon wafer and the results were in good agreement with the theoretical simulations.

An analysis of the factors affecting the alpha parameter used for extracting surface recombination velocity in EBIC measurements

This paper gives an in-depth analysis of the factors affecting the alpha parameter which is used for extracting the surface recombination velocity in electron beam induced current (EBIC) line scan measurements. The analysis shows that the alpha versus normalized surface recombination velocity curve is a function of both the normalized beam depth as well as the normalized scanning range. Variations in the normalized beam depth affect the accuracy only when extracting high surface recombination velocity values. On the other hand, the variation in the normalized scanning range affects the accuracy in extracting the middle range values of the surface recombination velocity only slightly. Conditions for accurate extraction are given in this paper. The analysis was further verified with the use of computer simulation.

Phosphorus-diffused silicon solar cell emitters with plasma enhanced chemical vapor deposited silicon carbide

We propose the use of annealed phosphorus doped amorphous silicon carbide layers (a-SiC:H(n)) deposited by PECVD as emitters in solar cells and explore the effect of the annealing time on the emitter saturation current density (). We use the quasy-steady state photoconductance method to determine the dependence of effective lifetime on excess carrier density and from these measurements we obtain values in the range of 300 fA cm-2 for sheet resistances around 100 O?sq. Finally, we obtain effective surface recombination velocity values around 104 cm s-1 by fitting the measured values with PC1D simulated ones.

Carrier-density-wave transport property depth profilometry using spectroscopic photothermal radiometry of silicon wafers II: Experimental and computational aspects

The experimental verification of a previously presented theoretical model for the photothermal radiometric (PTR) signal from an Si wafer excited by a laser of arbitrary wavelength is presented. A multiparameter fitting algorithm is developed and is used to fit experimental frequency scans to the theoretical model. The recombination lifetime and surface recombination velocity values extracted from the fits are consistent for all of the experiments performed. The diffusion coefficients for the more strongly absorbed excitation wavelengths are greater than those measured when using deeper penetrating excitation wavelengths. This discrepancy is discussed in terms of the dependence of the PTR signal on injected carrier densities and the nonlinearity of the PTR signal with temperature. The sensitivity of the PTR signal to a localized defect is shown to increase with the proximity of the defect to the centroid of the injected carrier density. The method amounts to carrier-density-wave depth profilometry of the relevant electronic transport parameters.

All-optical multiwavelength technique for the simultaneous measurement of bulk recombination lifetimes and front/rear surface recombination velocity in single crystal silicon samples

In this article, a contactless, all-optical, nondestructive method for separating the minority carrier recombination lifetime and surface recombination velocities in silicon samples at a low injection level is presented. The technique can be described as a pump–probe method in which the excess carrier density is probed by analyzing the free carrier absorption transient following laser pulse excitation that have several wavelengths. An attractive characteristic of the proposed technique is its capability to measure different values of surface recombination velocity on front and back surfaces of the silicon sample. The theoretical foundation of the method is also analyzed. Moreover, numerical simulations which validate the proposed methodology and preliminary experimental results that prove the applicability of scheme are presented.

Determination of bulk and surface transport properties by photocurrent spectral measurements

Reliable minority carrier diffusion length and surface recombination velocity values have been obtained from stationary photocurrent measurements. A modified surface photovoltage method has been used to determine diffusion lengths longer than the wafer thickness in high-purity Si, whereas the spectral variation of the photocurrent has been employed to measure the surface recombination velocity. The novelty presented in this paper is that a Schottky diode has been employed in both the methods to collect generated charged carriers. Moreover the same Schottky diode has been employed in both the methods in order to avoid any a priori assumptions on the material transport parameters. This combined application of the two methods at the same device enables the determination of highly reliable results.

Investigation of the bulk and surface electronic properties of HgCdTe epitaxial layers using photoelectromagnetic, Hall, and photoconductivity measurements

In this work a method is presented that permits the evaluation of the bulk and surface electronic properties of p-type Hg0.77Cd0.23Te epilayers grown by metalorganic chemical-vapor-deposition and liquid-phase-epitaxy growth techniques. The method is based on fitting the generalized photoelectromagnetic expression to the experimental results obtained from photoelectromagnetic, Hall, and photoconductivity measurements. Values of electron mobility, electron diffusion length, bulk lifetime, surface recombination velocities at the front and at the back surfaces of the HgCdTe layer, and the absorption coefficient were derived as a function of temperature. It is found that the Shockley–Read–Hall recombination process is the dominant recombination mechanism both in the bulk and at the surface of the HgCdTe layers. The recombination centers are most likely related to metal vacancies. It is shown that a low value of surface recombination velocity is a fundamental property of the CdTe/HgCdTe interface. In particular, a surface recombination velocity of less than 5000 cm/s was measured at 77 K for HgCdTe with a CdTe cap, which is the lowest value reported for narrow-gap p-type HgCdTe.

Effect of surface recombination on the decay of excess minority carriers in semiconductors induced by finely focused picosecond pulsed laser beams

A study is carried out of the effect of surface recombination on the decay of the excess minority carrier population induced in a semiconductor sample by a very short pulse of a finely focused laser beam. An expression is obtained for this decay in a substantially simplified form, which can be readily computed numerically. Some sample calculations are presented, which show that, in agreement with previous experimental observations, this effect is important only for the initial part of the decay and only for large values of surface recombination velocity (>104 cm/sec) and absorption coefficient. If the initial fast portion of the decay is neglected, the minority carrier lifetime can be determined fairly accurately from the average slope of the remaining part of the decay, the resultant error being typically up to 15%.