• Energy Research

The need for non-contact techniques to improve the characterization of electronic defects in materials and components is critical for the assessment of new photovoltaic technologies. Advanced characterization tools are required to study such defects and quantify their density within the material. The Modulated Photoluminescence (MPL) technique that can operate in a wide frequency range [10Hz-200MHz] has proven to be a promising candidate, showing its utility in characterizing defects in semiconductor materials. A previous study revealed singularities in the phase dependence of MPL as a function of frequency, called V-shapes, where the phase is not monotonously varying with frequency, but exhibits a local extremum. On this basis, in our study, we carried out a theoretical analysis by solving the continuity equations, as described in a previous article. This suggests that the existence of a V-shape may be related to the presence of minority carrier traps within the material, but this relationship is not always clear and requires additional explanation. In studying the variation of the V-shape with temperature, we extended previous theoretical work by identifying rules for the appearance and disappearance of the V-shape in the MPL bode diagrams of probed materials in the presence of Shockley-Read-Hall (SRH) recombination centers. From modelling of such materials in low injection, by varying the energy level position of the defect and the capture cross-section of minority and majority carriers, we show that the V-shape curves appear when the defect is a minority carrier trap. In this case, we propose a method to extract the information of the defect properties. However, for other conditions like higher injection and intrinsic material,V-shapes can appear even if the defect is a majority carrier trap. These results will be illustrated using modelling and experimental works.

Studying defects in semiconductors is, in practice, a very important topic for opto-electronic applications. It involves advanced characterization tools able to quantify and qualify the defect densities present in the materials. In the present article we focus on the use of a contactless frequency domain technique: modulated photoluminescence (MPL), and show its potential to detect defects. MPL has been used for the measurement of differential lifetime for several decades in silicon wafers. By extending it to low lifetime/highly defective materials we discovered its potential to become a defect spectroscopy method, measuring time constants close to the ones governing impedance spectroscopy measurements. Proofs of concept and an analytical model for doped materials have been presented already. Here, we reformulate the analytical model more explicitly and check its applicability by extensive numerical simulations for the case of a low illumination for a thin layer with a single defect. We present a parametric numerical study simulating the response of a single Shockley-Read-Hall center, showing the appearance of so-called V-Shapes in the MPL phase patterns as predicted by the analytical model, and valid beyond small-signal approximation. We discuss the difference between these two approaches and extend the analytical model and numerical investigations to intrinsic materials.

During the last years, we developed both an efficient High-Frequency Modulated Photoluminescence (MPL) setup, covering a frequency range of 10Hz-200MHz [1] and a corresponding analytical theory [2] allowing to explain the appearance of V-shaped phase patterns in the MPL bode diagrams of probed semiconductors layers by the presence of Shockley Read Hall (SRH) recombination. An example of V-shape in phase behavior can be seen in the bode diagram on figure 1a. We precedingly investigated a fitting strategy based on curves obtained at several illumination levels. However, experiments at several temperatures are also a promising way to explore recombination paths. This year, we performed theoretical calculations and analytical simulations in order to show that the variations of the V-shape corner frequencies w1, w2, w3 with respect to the temperature allow for recovering some trap parameters such as the energy position in the bandgap (activation energy) and the minority carrier capture cross section (see Figures 1b and 1c). The validity domain of this theory will be discussed for low and high injection as well as doped and intrinsic materials. We also extended the previous theory [2] by finding supplementary rules for the appearance and disappearance of V-shapes. We proofed that the MPL amplitude curve also contains information about the injection level during the experiment. Finally, we are currently performing MPL versus temperature experiments in order to validate our theory. We will present all these results during the conference. [1]W. Zhao et al., « Coupled time resolved and high frequency modulated photoluminescence probing surface passivation of highly doped n-type InP samples », J. Appl. Phys., vol. 129, no 21, p. 215305, juin 2021, doi: 10.1063/5.0033122.[2]N. Moron, B. Bérenguier, J. Alvarez, et J.-P. Kleider, « Analytical model of the modulated photoluminescence in semiconductor materials », J. Phys. Appl. Phys., vol. 55, no 10, p. 105103, mars 2022, doi: 10.1088/1361-6463/ac39c4.