Journal of Nondestructive Evaluation, VoL 13, No. 2, 1994 Photoacoustic Investigation of Multilayer Semiconductor Materials Yao-chun Shen 1 and Shu-yi Zhang ~ Received March 23, 1993; revised March 1, 1994 A one-dimensional model to investigate the photoacoustic (PA) generation and depth profiling in multilayer semiconductor materials has been developed by considering the contributions of thermal and plasma waves. The theoretical results show that the contribution of the plasma wave induced by photo-generated carriers (PGC) to the photoacoustic signal through the electro-elastic effect is predominant at high modulation frequencies and the contribution of the thermal wave, which is caused by the recombination of PGC, to PA signal through the thermo-elastic effect is more important at low modulation frequencies. We also investigated the depth profiling ability of PA detection by changing either the modulation frequency or the phase shift of the reference signal. The theoretical analyses are consistent with the experimental results. KEY WORDS: Photoacoustic; thermal wave; plasma wave; acoustic wave; depth characterization. 1. INTRODUCTION In the past decade, the photoacoustic (PA) tech- nique has been successfully applied to study semicon- ductor materials and devices31~ One of the most promising applications of the technique is the laminated characterization of layered materials. (2) Opsal and Ron- sencwaig (3~ developed a one-dimensional (l-D) model (O-R theory) of thermal wave depth profiling in multilayer materials, in which they considered a semi- infinite layered sample with discontinuous thermal im- pedance between two neighboring layers and obtained expressions for the temperature at the surface and the elastic response beneath the surface of the sample. On the basis of O-R theory, we have investigated the mech- anism of depth profiling in piezoelectric PA detection for a multilayered sample, under thermally thick but acoustically thin conditions, in theory and experiment. (4~ In the theories above, the authors considered that the PA generation is predominantly due to the thermoelastic ef- Institute of Acoustics and Lab of Modern Acoustics, Nanjing Uni- versity, Nanjing 210008, China. fect. In fact, due to the diffusion and recombination of photo-generated carriers (PGC) in semiconductors, there are significant changes in the distribution of thermal and plasma wave sources, and moreover, the electro-elastic strain contribution to the PA generation cannot be ne- glected. (5) In this paper, we present experimental and the- oretical investigations of the PA generation and depth profiling in multilayer semi-conductor materials. In the experiment, the PA signal of a Si wafer containing Sb- doped areas is measured as a function of modulation frequency. The experimental results show that some dis- crepancy exists in comparing with previous theories, considering only the thermoelastic effect. We present here a 1-D multilayer theory in which both thermoelastic and electro-elastic effects have been taken into account. The PA signals caused by the elastic sources (thermal and plasma waves) at arbitrary depth have been calcu- lated, using a Green's function method. In addition, the influences of thermal and electronic properties of the subsurface layer on the surface temperature and PA sig- nal have also been calculated. Theoretical results are consistent with those of experiment. 55 0195-9298/94/0600-0055507.00/0 1994 PlenumPublishing Coq~oration