Еvaluation of Thermophysical Properties of Semiconductors by Photoacoustic Phase Neural Network

The idea of this paper is to develop a method for determination thermal diffusivity, linear expansion coefficient and thickness of a semiconductor sample from photoacoustic phase measurement by using neural network. The neural network has been trained on photoacoustic phases obtained from a theoretical model of measured signal for Si n-type in the range of 20Hz to 20kHz. For the analysis of parameter determination from phases, we trained phase neural networks on a large database obtained from numerical experiments in the expected range of parameter changes. An analysis of a theoretical photoacoustic model with a phase neural network is demonstrated. The advantages of using a phase neural network with high accuracy and precision in prediction depending on the number of epochs are presented, as well as analyzes of the application of random Gaussian noise to the network in order to better predict the experimental photoacoustic signal.BPU11 : 11th International Conference of the Balkan Physical Union : Proceedings book; Aug 11 - Sep 1, 2022S07-OP Optics and Photonic

Influence of thin oxide layer to photoacoustic signal of nano-mechanical structures

Nano mechanical structures (NEMS) are being developed for application in pressure and temperature sensors. They consist of a very thin oxide layer on a silicon wafer. This layer is placed to improve their sensitivity to pressure changes. In this paper, it was investigated how much this layer affects the physical properties of such structures. In the research, the photoacoustic gas microphone experimental measurement method was applied and neural networks were developed for solving the inverse photoacoustic problem. The obtained results show that controlling the thickness of the oxide layer can be done by engineering the thermal elastic and optical properties of the NEMS.IX International School and Conference on Photonics : PHOTONICA2023 : book of abstracts; August 28 - September 1, 2023; Belgrad

THE SURFACE RECOMBINATION VELOCITY AND BULK LIFETIME INFLUENCES ON PHOTOGENERATED EXCESS CARRIER DENSITY AND TEMPERATURE DISTRIBUTIONS IN N-TYPE SILICON

The temperature distributions in the n-type silicon circular plate, excited by a frequency-modulated light source from one side, are investigated theoretically in the frequency domain. The influence of the photogenerated excess carrier density on the temperature distributions is considered with respect to the sample thickness, surface quality and carrier lifetime. The presence of the thermalization and non-radiative recombination processes are taken into account. The existence of the fast and slow heat sources in the sample is recognized. It is shown that the temperature distribution on sample surfaces is a sensitive function of an excess carrier density under a bulk and surface recombination. The most favorable values of surface velocities ratio and bulk lifetime are established, assigned for a simpler and more effective analysis of the carrier influence in semiconductors. The photothermal and photoacoustic transmission detection configuration is proposed as a most suitable experimental scheme for the investigation of the excess carrier influence on the silicon surface temperatures

Photoacoustic Characterization of TiO2 Thin-Films Deposited on Silicon Substrate Using Neural Networks

In this paper, the possibility of determining the thermal, elastic and geometric characteristics of a thin TiO2 film deposited on a silicon substrate, with a thickness of 30 μm, in the frequency range of 20 to 20 kHz with neural networks were analysed. For this purpose, the geometric (thickness), thermal (thermal diffusivity, coefficient of linear expansion) and electronic parameters of substrates were known and constant in the two-layer model, while the following nano-layer thin-film parameters were changed: thickness, expansion and thermal diffusivity. Predictions of these three parameters of the thin-film were analysed separately with three neural networks. All of them together were joined by a fourth neural network. It was shown that the neural network, which analysed all three parameters at the same time, achieved the highest accuracy, so the use of networks that provide predictions for only one parameter is less reliable. The obtained results showed that the application of neural networks in determining the thermoelastic properties of a thin film on a supporting substrate enables the estimation of its characteristics with great accuracy

Analysis of plasma-elastic component of time-domain photoacoustic response

The plasma-elastic component of the photoacoustic response in the time-domain of thin semiconductor samples excited by long electromagnetic radiation pulses is analyzed in detail. The plasma-elastic component model assumes that ambipolar diffusion can be approximated by the minority carrier diffusion. The results obtained show that the plasma-elastic component in thin semiconductor samples affects photoacoustic measurements in the time domain, which is important for the photoacoustic determination of semiconductor electronic properties. © 2023 Author(s)

Transmission pulse photoacoustic set-up for characterization of solids

Laboratory-built measuring device, based on pulse photoacoustic (PA) effect, which allows the measurement of optical and acoustic signals in time domain, is presented. The experimental setup is described and the device specifications are given in detail. Optical and PA signals are measured in a time range from 10 μs to 10 s, for different samples and conditions. As an example, the experimental results for Si square membrane (Si chip with 5 x 5 mm frame and 3 × 3 mm square membrane, 32 μm thick, used in micromechanic devices) are presented. The square membrane Si chips were prepared by wet anisotropic bulk micromachining process. The applicability of the measured PA signal in the characterization of semiconductors and micromechanical structures is discussed.InfoTeh-Jahorina : 21st International Symposium INFOTEH-JAHORINA, March 16-18, 2022; Jahorin

Investigation of the influence of the applied dye layer on surface temperature variations of laser sintered polyamide

U ovom radu ispitivan je uticaj nanetog sloja boje na površinske temperaturske varijacije uzorka poliamida. Uzorak, dobijen laserskim sinterovanjem industrijskog praha PA2200, se obasjava mnodulisanim laserskim snopom, dok se naneti sloj boje ne pobuđuje direktno. Razmatra se dinamička komponenta temperaturske promene na površini uzorka u transmisionoj i refleksionoj konfiguraciji, kao ključna komponenta formiranja fotoaksutičkog odziva.In this paper, the influence of the applied dye layer on surface temperature variations of polyamide sampleis investigated. The sample, obtained by laser sintering of PA2200 industrial powder, is irradiated by a modulated laser beam, while the applied layer is not excited directly. Dynamic component of temperature change at sample surface is considered, in both transmission and reflexion configuration, as the key componetn in the formation of photoacoustic resopnse.InfoTeh-Jahorina : 21st International Symposium INFOTEH-JAHORINA, March 16-18, 2022; Jahorin

Thermal characterization of n-type silicon based on an electro-acoustic analogy

Based on the analogy between resistor-capacitor (RC) filters and the thermoelastic component of a photoacoustic signal, the complex thermoelastic response of an open photoacoustic cell is described as a simple linear time-invariant system of a low-pass RC filter. This description is done by finding a linear relation between the thermoelastic cut-off frequency and sample material thickness within the range of 10-1000 μm. Based on the theory of a composite piston, we run numerical simulations of the proposed method for n-type silicon, described as either a surface or volume absorber. Theoretical predictions are experimentally validated by using an open photoacoustic cell setup to record the signal of an 850-μm-thick n-type silicon wafer illuminated by a blue light source modulated with frequencies from 20 Hz to 20 kHz. The obtained experimental results confirm the linear dependence of the thermoelastic cut-off frequency on the sample thickness, and, therefore, they lay the foundation of a new method for the thermal characterization of materials. © 2023 Author(s).Peer-reviewed manuscript: [https://vinar.vin.bg.ac.rs/handle/123456789/11212

Enhancement of the thermoelastic component of the photoacoustic signal of silicon membranes coated with a thin TiO2 film

The reduction of the photogenerated charge carriers' influence in periodically illuminated thin silicon membranes is investigated by using the experimental setup of an open photoacoustic cell in the standard range of modulation frequencies from 20 Hz to 20 kHz. It is confirmed that the deposition of a 200 nm thin film of titanium dioxide on the 30- and 50 μm silicon membrane leads to a large increase of the thermoelastic component of the photoacoustic signal, which restores the flexibility lost to the membrane under the influence of photogenerated carriers. The effect of the thermoelastic component enhancement is analyzed by observing the displacement of the tested samples along the heat propagation axis, depending on the carrier density and temperature differences on the illuminated and unilluminated sides, for different membrane thicknesses and a constant film thickness. It is found that the effect of enhancement of several orders of magnitude is more visible in thinner membranes due to higher ratios between the film and membrane thicknesses

Time-domain minimum-volume cell photoacoustic of thin semiconductor layer. I. Theory

The model of a photoacoustic gas-microphone signal in the time domain recorded in a transmission configuration using a minimum volume cell is derived. This model takes into account the inertial thermal relaxations of both the sample and the gas filling the cell by means of the generalized hyperbolic theory of heat conduction. With the introduction of electro-thermal analogy for the thin sample and short microphone length cavity, characteristic quantities are defined, which can be used in solving the inverse problem in time-domain photoacoustic in both cases, when thermal relaxations are neglected as well as when they are considered. The derived model that includes thermal relaxation explains the experimentally observed occurrence of overshoots and undershoots as well as an oscillatory approach to the steady values of the recorded signal in high-resolution time-domain photoacoustic measurements of thin semiconductor membranes, which will be presented in detail in Paper II of the paper. © 2023 Author(s)