Quantitative photopyroelectric out-of-phase spectroscopy of amorphous silicon thin films deposited on crystalline silicon CONSTANTINOS CHRISTOFIDES, ANDREAS MANDELIS, AND ALBERT ENGEL Phorouco~tsric utzcl Phororhertnal Sciences Lnborc~ronl, Depnrrtnerzt c$ Mechunicul Erlgirzc.erirlg, utzd Otztnrio Luser urzd Lightlvuve Reseurch Center, Utziversity of Toronto, Toronto, Or~t. . Cnrlcldcl M5S / A 4 AND MlTEL, Setnicotid~tcror Divisiotz, 18 Bortl. De I'Aeroporr, Brotnotzt, Que., Cutzadu JOE ILO Received August 14, 1990 A photopyroelectric spectrometer with real-time and(or) self-normalization capability was used for both conventional trans- mission and thermal-wave spectroscopic measurements of amorphous Si thin films, deposited on crystalline Si substrates. Optical-absorption-coefficient spectra were obtained from these measurements and the superior dynamic range of the out-of- phase (quadrature) photopyroelectric signal was established as the preferred measurement method, owing to its zero-background compensation capability. An extension of a photopyroelectric theoretical model was established and successfullly tested in the determination of the optical absorption coefficient and the thermal diffusivity of the sample under investigation. Instrumental sensitivity limits of pl = S X 10- ' were demonstrated. Un spectrometre photopyroelectrique avec capacitC de temps reel etlou d'auto-normalisation a CtC utilisC pour des mesures spectroscopiques, par transmission conventionnelle et par ondes thermiques, sur des couches m i n c e ~ ~ d e Si amorphe dCposCes sur des substrats de Si cristallin. Des spectres du coefficient d'absorption optique ont CtC obtenus a partir de ces mesures, et la partie dynamique supCrieure du signal photopyroClectrique dCphasC (en quadrature) a CtC Ctablie comme mCthode de mesure prCfCrCe, B cause de sa capacite de compensation du fond. Une extension d'un modele thCorique photopyroClectrique a CtC Ctablie et testCe avec succes dans la dttermination du coefficient d'absorption optique et de la diffusivitk thermique de 1'Cchan- tillon CtudiC. Des limites de sensibilite instrumentale de pt = S x I0 -' ont CtC atteintes. [Traduit par le rCdaction] Can. J. Phys 69. 117 (1991) 1. Introduction As is well known, during the last 20 years researchers and semiconductor industries have shown a great interest in the development of amorphous semiconducting materials. Many experimental techniques have already been used for the char- acterization of noncrystalline materials and several transport theories have been established (I). However, the interest in improving the experimental techniques, as well as the desire to obtain more information concerning the structural, electrical, and optical properties of amorphous materials has led to the development of new characterization techniques affording spe- cific advantages over conventional methods. Recently, frequency-domain photopyroelectric spectroscopy (PPES) has been used for the characterization of thin semi- conducting films. This technique is capable of out-of-phase thermal-wave lock-in detection with complete suppression of synchronous background signals owing to direct optical trans- mission to the pyroelectric sensor (2, 3). Mandelis et al. (4) first performed spectroscopic PPE measurements of an a-Si: H thin film on quartz. Christofides er al. (5, 6) have extended the use of PPES to spectroscopic measurements of thick crystalline Ge and Si wafer substrates. In this work we present the first application of PPES on very thin absorbing amorphous layers deposited on absorbing crys- talline Si substrates using the geometries designed for micro- electronic device fabrication. In Sect. 2 of this paper we present a PPE theoretical model pertinent to such geometries. In Sect. 3 the experimental set-up for PPES will be discussed briefly, since the spectrometer has already been presented elsewhere in detail (7). Experimental results and comparisons between PPE and conventional transmission spectra will be presented in Sect. 4. Section 5 includes our preliminary assessment of the dynamic range of PPES with respect to the product of pt ( p is the optical absorption coefficient, and t the thin-film thickness), in com- parison with that reported for photothermal beam deflection spectroscopy (PDS), a related zero-background technique. Finally, we present some$onclusions and future perspectives of the photopyroelectric tEhni+e. 2. Theoretical model Figures la and I b present a one-dimensional geometry of the photopyroelectric system. A crystal substrate of thickness L assumed entirely transparent, on which an amorphous layer of thickness t is deposited, is irradiated with monochromatic light of wavelength A, the intensity I of which is modulated at fre- quency f. As is shown in Fig. Ici, where the sample is in contact with the pyroelectric polyvinylidene fluoride (PVDF), the induced signal, S, is the sum of a therma'l component, ST, and an optical component, So. If the substrate is assumed trans- parent, the surface reflectance is included in the form I-R,(A), and the pyroelectric detector surface is blackened (i.e., photo- thermally saturated) then the normalized detected total PPE sig- nal S, in this case is given by (2, 4)