D. M. Todorović

Photoacoustic frequency transmission technique: Electronic deformation mechanism in semiconductors

A quantitative analysis of the electronic deformation (electronic strain) contribution to the photoacoustic signal in semiconductor samples was done. The photoacoustic (PA) effect was investigated as a function of the modulation frequency in a transmission detection configuration. The theoretical model for this configuration was given for the thermal and elastic processes besides the carrier transport characteristic and the relation for electronic elastic deformation was derived by the theory of the elastic thin plate vibrations. Analysis showed that the electronic deformation mechanisms can have an important infleucne on the PA signal in the frequency range typical for gas-microphone detection. The experimental photoacoustic data for Si samples were measured and they exhibited a satisfactory agreement with the theoretical model.

Plasma, thermal, and elastic waves in semiconductors

The system of partially coupled plasma, thermal, and elastic wave equations and conditions for neglecting the coupling between them is analyzed. The treatment considers a semiconductor elastic medium with isotropic and homogeneous thermal and elastic properties. The solution of the coupled system of plasma, thermal and elastic equations is a very complex problem. In most practical cases partially coupled treatment is sufficient. Conditions where it is possible to neglect the coupling between the plasma, thermal, and elastic waves are considered in this work. Special cases for coupled thermal and elastic waves (the thermoelastic problem) and coupled plasma and elastic waves (the electronic deformation) are taken into consideration. An approximate quantitative analysis of the coupling effects is given.

Investigation of carrier transport processes in semiconductors by the photoacoustic frequency transmission method

The development of various methods for the investigation of transport phenomena in solids has attracted much attention. The pho- toacoustic method is a valuable tool in the study of various processes in semiconductors and electronic devices. Especially for the characteriza- tion of thermal and carrier transport processes in semiconductors, the transmission detection configuration is available for many applications. That the photoacoustic frequency transmission technique as a valuable tool in the study of carrier transport processes of the semiconductor is shown. The photoacoustic spectra of wide bandgap semiconductors (Si- wafers, Ge and GaAs), narrow bandgap semiconductors (InSb, PtSb2) and thin amorphous semiconductor (GeSe) films are measured as a function of modulation frequency.

Photothermal and electronic elastic effects in microelectromechanical structures

The dynamic thermoelastic and electronic deformation bending in various micromechanical systems were investigated. The elastic bending effect was investigated as a function of modulation frequency including the generation and propagation processes of the plasma, thermal, and elastic waves. The theoretical model for two microstructures (circular plate and cantilever beam) was given and the relations for dynamic elastic bending were derived. The elastic bending was calculated and analyzed for both type of microplates. This investigation is important for the analysis of the possibility to use these semiconducting optically driven microstructures as sensors and actuators.

Study of photothermal vibrations of semiconductor cantilevers near the resonant frequency

The elastic vibrations of semiconductor cantilevers, which were excited with a frequency-modulated laser, were studied theoretically and experimentally in this paper. The carrier density, temperature and elastic displacement distributions were obtained by using the theoretical model including plasma and thermoelastic effects. The results showed that the plasma wave had a significant influence on the vibrations. Using an interferometric setup the photothermal signals were investigated near the resonant frequency. The results showed that the experimental results were in good agreement with the theoretical ones.

Photoacoustic investigation of thermal and transport properties of amorphous GeSe thin films

In this work the possibility of simultaneous determination of thermal and transport properties of thin films using a photoacoustic method was investigated. The properties of amorphous GeSe thin films, which were evaporated on quartz substrata, were examined by measuring their amplitude and phase photoacoustic spectra as a function of the modulated optical laser beam frequency. The measurements were performed in a specially constructed photoacoustic cell which enabled excitation of the sample on one side and detection of the acoustic response on the other. Thermal diffusivity and transport properties (diffusivity coefficient, recombination time, and the surface recombination velocity) of the GeSe thin films were determined by comparing of the experimental results and the calculated theoretical photoacoustic spectra.

Electronic and thermal generation of vibrations of optically excited cantilevers

The results of the theoretical analysis of the dynamic effects in the optically excited cantilevers were given. Theoretical model for dynamic elastic bending for two-layer cantilevers was derived including electronic and thermal elastic deformation effects which have the main influence on the dynamics of the cantilevers. The influence of the carrier transport characteristics (the carrier diffusion coefficient, the lifetime of photogenerated carriers, and the carrier recombination velocities) to the elastic vibrations of cantilevers was analyzed. Theoretical model was verified by comparing with the experimental results. The results of these investigations are important for sensors, actuators, and resonators based on the cantilevers.

Effect of the absorption coefficient of aluminium plates on their thermoelastic bending in photoacoustic experiments

The open-cell photoacoustic signal measured in the transmission configuration for aluminum thin plates with thicknesses of 280 μm, 197 μm, and 112 μm is experimentally and theoretically analyzed, in the 20 Hz–7 kHz modulation frequency range. It is shown that the observed differences between the predictions of the standard thermoelastic model and the experiment data of both the amplitude and phase of the photoacoustic signal can be overcome by considering the aluminum samples coated with a thin layer of black paint as volume-absorber materials. This new approach provides a quite good agreement with the obtained experimental data, in the whole frequency range, and yields an effective absorption coefficient of (16 ± 2) mm−1, for a 280 μm-thick sample. The introduction of the finite absorption coefficient led to the correct ratio between the thermal diffusion and thermoelastic components of the photoacoustic signal. Furthermore, it is found that the “volume-absorber” approach accurately describes the behavior of the amplitude, but not that of the phase recorded for a 112 μm-thick sample, due to its relatively strong thermoelastic bending, which is not considered by this theory. Within the approximation of the small bending, the proposed “volume-absorber” model provides a reliable description of the photoacoustic signal for Al samples thicker than 112 μm, and extends the applicability of the classical “opaque” approach.

Energy spectra and dynamics of surface states on silicon investigated by the photoacoustic method

The spectral and dynamic characteristics of surface energy states (SES) on a Si wafer are investigated by photoacoustic (PA) spectroscopy. The PA spectra were measured as a function of the wavelength and modulation frequency of the excitation optical beam by the PA spectrometer. The PA amplitude spectra of Si samples of various surface quality were studied at room temperature in the spectral range of the excitation optical energy beam from 0.7 to 1.5 eV and for various modulation frequencies. The difference in PA amplitude spectra of an as-received Si wafer and a Si wafer with a mechanically roughened surface for various modulation frequencies indicates the energy distribution and the dynamics of the SES. A change in the PA amplitude signal for various modulation frequencies indicates the temporal distribution of the SES for a fixed energy of excitation.

Study of laser excited vibration of silicon cantilever

In this paper an interferometric setup called “thermoelastic microscopy” has been used to measure the vibration response of the semiconductor cantilevers under modulated laser thermal source. The small vibration amplitude has been detected well below 10 pm magnitude in accurate adjustment conditions. The results showed that experimental responses have a good agreement with the theoretical ones. Also from the analysis we could conclude that the signal amplitude has a power functional dependence on the modulation frequency and that the phase angle linearly depended on the square root of modulation frequency.