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The possibility of using a minimal-volume photoacoustic cell to perform spectroscopy of samples is discussed. It is shown that this alternative signal-to-noise-enhanced photoacoustic configuration allows one to obtain both absorption and transmission spectra with minimal experimental arrangement and cell machining requirements. The theoretical model is presented, the use of which is exemplified by a complete optical and thermal characterization of leaves.

Using two different photoacoustic techniques for a two-layer system of variable thickness, we show that the thermal diffusivity and the thermal conductivity are completely determined, based upon the efFective-sample model widely used in heat-transfer problems. A procedure to establish a standard photothermal technique for measuring both the thermal diffusivity and the thermal conductivity is also discussed.

The band-gap energies of the CdS semiconductor are obtained by a photoacoustic spectroscopy (PAS) technique over a range of temperature of thermal annealing (TTA), in which the evolution of the sample structure is characterized by x-ray diffraction patterns. The PAS experiment gives a set of data for the band-gap shift in the region of the fundamental absorption edge. With increasing TTA the band-gap shift increases up to a critical TTA when its slope decreases in a roughly symmetrical way. It is suggested that at this temperature a cubic to hexagonal-lattice transition occurs.The use of photoacoustic spectroscopy (PAS) has become well established in the past few years mainly due to its importance as a guide in the study of optical properties of semiconductors.'-" PAS can lead, for instance, to the value of the band-gap energy, which is an important parameter in electronic and optoelectronic design.'-" It is worth saying that less attention has been paid to applications of this technique to investigate the band-gap shift (BGS) of intrinsic and extrinsic semiconductors. In particular, the CdS semiconductor, which presents a highly stable hexagonal structure (y_CdS,= can also be obtained in the metastable cubic phase ,8-CdS.'2-'5 Cardona et al. by means of reflectivity measurements at room temperature found the optical band gap of the two phases of CdS thin films, and they could not infer any other conclusions except that the energy difference between the cubic and hexagonal CdS energy gap differs less than 0

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A model for the effective thermal conductivity of metal-nonmetal particulate composites J. Appl. Phys. 111, 044319 (2012) Thermoelectric efficiency of topological insulators in a magnetic field J. Appl. Phys. 111, 07E319 (2012) Effective medium formulation for phonon transport analysis of nanograined polycrystals J. Appl. Phys. 111, 014307 (2012) On the accuracy of classical and long wavelength approximations for phonon transport in graphene J. Appl. Phys. 110, 113510 (2011) A study of the impact of dislocations on the thermoelectric properties of quantum wells in the Si/SiGe materials system J. Appl. Phys. 110, 114508 (2011) Additional information on J. Appl. Phys. In this paper the use of the so-called open photoacoustic cell for thermal characterization of two-layer systems of variable thickness is described. It is shown that the thermal diffusivity as well as the thermal conductivity are completely determined, based upon the effective sample model widely used in heat-transfer problems.

The room temperature thermal diffusivity evolution of electrochemically formed porous silicon as a function of the etching time is investigated. The measurements were carried out using the open-cell photoacoustic technique. The experimental data were analyzed using a composite two-layer model. The results obtained strongly support the existing studies, indicating the presence of a high percentage of SiO 2 in the composition of porous silicon material. [S0031-9007(97) PACS numbers: 44.30. + v, 61.43.Gt, 81.05.Cy, 81.05.Rm Since the discovery of its room-temperature visible luminescence [1][2][3][4][5], porous silicon (PS) has become a subject of considerable interest, especially for its promising use as an optoelectronic device [6,7]. There are several methods [8][9][10][11] for fabricating PS from crystalline silicon wafers. The electrochemical etching [1,8] is, however, the most extensively used so far. The morphology of the resulting porous layer is strongly dependent upon the fabrication controlling parameters such as electrolyte composition, current density, etching time, etc., as well as on the type of substrate used.In general, an electrochemically formed n-type PS layer consists essentially of a double-layer system on top of the silicon substrate [1,12]. The outermost thin layer, known as the microporous layer, is typically 10-15 mm thick and is responsible for the observed photoluminescence. Except for very small etching times, the inner layer adjacent to the crystalline substrate, designated as the macroporous layer, consists of a parallel array of airembedded free-standing n-PS columns.Despite the large body of literature that already exists on PS [13,14], so far there has been no reported detailed investigation of the thermophysical properties of this important system. In this Letter we apply the modern photothermal techniques to the evaluation of the thermal properties of electrochemically formed n-PS.The samples used in our experiments were prepared by electrochemical etching on (100) oriented, nondegenerated, n-type ͑2.1 3 10 18 cm 23 ͒ crystalline silicon. The samples had a thickness of roughly 300 mm and an electrical resistivity of 1-5 V cm. The electrochemical etching was carried out following the procedure outlined in Ref. [12]. The crystalline samples, with an appropriate Pt network electrode attached to them, were immersed in a 150 ml Becker filled with HF. A current density of 40 mA͞cm 2 was then applied to the samples using a HP-model 6206B dc power supply operating between 5-10 V. During the etching period the samples were always kept under the irradiation of a 250 W infrared lamp positioned roughly 20 cm away from the etching bath. By controlling the etching time, ranging from 10 to 83 min, we could fabricate samples with different macroporous thicknesses.In Fig. 1 we show the side view optical micrograph of a typical n-PS sample, produced with 60 min etching time. The three distinct regions mentioned above, namely, the microporous and macroporous layers on top of the crystalline sub...

A simple method is demonstrated for obtaining the thermal diffusivity of solids, by measuring t~e phase lag between a front and rear illumination, at a single chopping frequency. The method IS tested using some semiconductor and glass samples.