Journal of Non-Crystalline Solids 114 (1989) 301-303 301 North-Holland ELECTROSTRICTION, ELECTROABSORPTION AND ELECTROREFLECrANCE IN a-Si:H Lazhar HADJERIS, Gilles DE ROSNY, and Bernard EQUER Laboratoire de Physique des Interfaces et des Couches Minces, (UPR A 0258 du CNRS), Ecole Polytechnique, F91128 PALAISEAU CEDEX, FRANCE The change in the amplitude of the light reflected by a "sandwich" structure (semi-transparent Cr / a-Si:H / Cr) as a consequence of electric field modulation in the film is studied by means of Michelson interferometry as a function of wavelength, ac and de voltages, and film thickness. Electrostriction and electroabsorption are evidenced. The a-Si:H electrostriction constant is measured and found to be of the order of -16 10 -21 m2/V 2. The field dependence of the a-Si:H electroabsorption coefficient is quantitatively studied. The origins of these phenomena are discussed, and the expected level of electrorefiectance is computed. 1. INTRODUCTION Electromodulation of the a-Si:H optical properties have been studied by a few groups either by transmissionI or by reflection 2, 3, 4. The observations may all be accounted for assuming that the absorption coefficient varies as the square of the electric field. It is also concluded that the modulation of the reflection coefficient is a consequence of the electroabsorption of the back surface reflected beam2, 3. It may be established, following computations proposed by Aspnes5, that there should also exist a contribution to the reflection coefficient coming from the internal electric field spatial dependence. In the limit of thick samples, only this last term contributes to the reflection modulation. Moreover, it may be shown6 that the measurement of the complex electroreflection coefficient wavelength dependence allows the determination of the internal electric field profile. An experiment has been designed to perform such measurements. The present contribution concerns new results on a-Si:H electromodulation and a conclusion concerning the possibility of measuring electric field profiles. 2. DESCRIPTION OF THE EXPERIMENT The complex modulated reflection amplitude is measured using a Michelson interferometer ¢. The mirror of one arm is the sample under study, submitted to a modulating electrical potential of the form : V(t) = Vdc + Vac Cos(tot) The optical length of the other arm may be varied continuously, using a piezoelectric translator. The light 0022-3093/89/$03.50 @ Elsevier Science Publishers B.V. (North-Holland) comes from either a Xe-arc lamp through a monochromator in the 350-850 nm range or a 632 nm He-Ne LASER source; it is finally sent on a photomultiplier whose response is analysed by a lock-in amplifier. The unmodulated R, fundamental ARo and first harmonic AR2t 0 reflected light intensities are measured. The complex reflection amplitudes are deduced from the study of the dependence of these intensities with the optical length variation.The sample consists in an a-Si:H film sandwiched between two Cr layers, the front one being possibly semi-transparent. 3. RESULTS The experiment allowed the observation of two distinct phenomena : electroabsorption as expected, and electrostriction, i.e. an electromodulation of the film thickness. Let us first present the electrostriction results. 3.1. Electrostriction Michelson interferometry is indeed a convenient way to measure minute thickness variations 7. In the present case, the wavelength dependence of the light modulation on samples having an opaque front electrode is to be attributed to electrostriction 4. This effect is quite general, the induced electrostrictive strain being proportional to the square of the electric field strength 8 E : AL 2 -L--= "t E (1) ~/being the electrostrictive constant of the material, and AL is the induced change in the thickness L, in the direction of the applied field.