Obtainment of the density of states in the band tails of hydrogenated amorphous silicon

Kopprio, Leonardo; Longeaud, Christophe; Schmidt, Javier Alejandro

doi:10.1063/1.4999626

Cited by 8 publications

(6 citation statements)

References 22 publications

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“…When MGT is combined with other photoconductivity-based techniques, it can also be used for the estimation of the DOS. 9,11 Ventosinos et al 8 suggested that MGT is equivalent to the Oscillating Photocarrier Grating technique (OPG), i.e., the MGT current could be obtained by using another experimental configuration that produces a slightly different illumination. OPG also uses an interference pattern superimposed on a uniform background illumination of much higher intensity, although its movement is periodic: it moves at constant velocity for half a period, whereas in the next semiperiod it moves at the same speed but in the opposite direction.…”

Section: Articlementioning

confidence: 99%

The chopped moving photocarrier grating technique

Kopprio

Ventosinos

Schmidt

2019

Review of Scientific Instruments

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The Moving photocarrier Grating Technique (MGT) allows the simultaneous determination of the photocarrier drift mobilities and the small-signal recombination lifetime of photoconductive semiconductors. The technique measures the direct current (DC) induced by a monochromatic illumination consisting of a moving interference pattern superimposed on a uniform background of much higher intensity. A drawback of the technique is the low level of the signal to be measured, which can be masked by the noise at low temperatures or low light intensities. In this work, we propose implementing an alternating current (AC) version of the MGT by chopping the weak beam in the standard configuration. We call this new technique the Chopped Moving photocarrier Grating (CMG). In CMG, the AC signal can be measured with a lock-in amplifier for electrical noise removal. In this way, the signal-to-noise ratio can be increased compared to the standard DC technique. Assuming a multiple-trapping model for charge transport, we find the theoretical expression for the current density induced by CMG at fundamental frequency. By using a numerical simulation with parameters typical for hydrogenated amorphous silicon, we verify the expected equivalence between both techniques for low enough chopping frequencies. Then, we test experimentally this equivalence for an undoped hydrogenated amorphous silicon sample. For low signal levels, we demonstrate the superior performance of CMG.

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Section: Articlementioning

confidence: 99%

The chopped moving photocarrier grating technique

Kopprio

Ventosinos

Schmidt

2019

Review of Scientific Instruments

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“…For that purpose, using typical material parameters for undoped a-Si:H (listed in the first two columns of table 1), the fundamental equations of section 2 and appendix A are solved without approximations. By solving the charge neutrality equation for a certain temperature under dark conditions we find the Fermi level, and with this value we obtain the thermal equilibrium free carrier concentrations, n e and p e [27]. For the same temperature and different generation rates, we calculate the steady-state free carrier concentrations n and p by solving simultaneously the continuity and charge neutrality equations.…”

Section: Basics Of the Methodsmentioning

confidence: 99%

Hydrogenated amorphous silicon characterization from steady state photoconductive measurements

Kopprio¹,

Longeaud²,

Schmidt³

2019

Semicond. Sci. Technol.

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We propose to use steady state measurements and a teaching-learning-based optimization (TLBO) algorithm to get the complete set of material transport parameters of disordered semiconductors, taking undoped hydrogenated amorphous silicon as an example. First, the steady-state conductivity under illumination and the ambipolar diffusion length (L amb) are measured for several temperatures and generation rates. The steady-state photocarrier grating (SSPG) technique is used for the evaluation of L amb. Then, the TLBO algorithm is used for the obtainment of the material parameters that best satisfy the charge neutrality and the continuity equations. The use of this algorithm allowed us to get an excellent estimation of the valence band tail slope, as compared to the one obtained from measurements of the absorption coefficient by Fourier transform photocurrent spectroscopy and transmittance/reflectance. The dangling bonds and the conduction band tail parameters were also found to be in very good agreement with those measured from high frequency modulated photocurrent experiments (MPC). Numerical simulations show that the capture coefficients of the band tails and defects states can also be estimated, although with less precision than the DOS parameters.

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“…Above room temperature, the overestimation does not reach 41%, but at the lowest temperature, it exceeds 200%. Some papers [6,16,29,30] make the approximation γ n + γ p ∼ 2 in order to determine the minority carrier diffusion length from the ambipolar diffusion length, L 2 p = L 2 amb / (γ n + γ p ) ≃ L 2 amb /2, which is an extra but generally a lower source of errors [11].…”

Section: Parametermentioning

confidence: 99%

“…This one was the procedure used for extracting the points presented in figure 14. A simpler and more popular version of this approach is obtained by approximating γ n + γ p by 2 in the previous formula to obtain L 2 p directly from L 2 D [6,11,16,29,30]. With this extra approximation, the L 2 p values are sometimes higher than those obtained from the multi-voltage approach.…”

Section: Estimation Of Lmentioning

confidence: 99%

Increasing the scope and precision of the steady-state photocarrier grating technique by measuring the photocurrents at several voltages

Kopprio¹,

Longeaud²

2021

Semicond. Sci. Technol.

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The steady-state photocarrier grating (SSPG) experiment is a popular technique for extracting the minority carrier diffusion length of photoconductive thin films in coplanar configuration. The diffusion length is basically obtained from the measurement of the steady-state photocurrent produced by a low applied voltage while the material is illuminated by two monochromatic laser beams of different intensities that interfere between the electrical contacts of the sample. Despite its simplicity and popularity, it is well known that the technique can overestimate the minority carrier diffusion length in some samples. In this paper, we show that the precision of the technique can be substantially increased by performing the same experiment at different voltages. Additionally, we show how to estimate fundamental material parameters from the experiment, such as the density of states at the majority carrier quasi-Fermi energy and the ratio between the recombination states’ capture coefficient and mobility of majority carriers. First, we show that the procedures found in the literature for correcting the overestimation produced by the standard technique do not work properly due to an oversimplification in the modeling. Then, we use a numerical simulation of an unintentionally-doped hydrogenated-amorphous-silicon-like material to evaluate the precision of the new formulas and procedures presented. We clarify the conditions under which the standard SSPG technique produces large overestimations. In these cases, we show that the precision of the new procedure can be more than ten times higher. Finally, we use the standard and the new method to characterize a hydrogenated amorphous (a-Si:H) and a hydrogenated polymorphous (pm-Si:H) silicon sample at different temperatures. We observe that the overestimations produced by the standard technique increase with the ratio between the majority and minority carrier diffusion lengths and the ratio between the recombination states’ capture coefficient and mobility of majority carriers.

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Obtainment of the density of states in the band tails of hydrogenated amorphous silicon

Cited by 8 publications

References 22 publications

The chopped moving photocarrier grating technique

The chopped moving photocarrier grating technique

Hydrogenated amorphous silicon characterization from steady state photoconductive measurements

Increasing the scope and precision of the steady-state photocarrier grating technique by measuring the photocurrents at several voltages

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