Hydrogenated amorphous silicon (a-Si:H) thin films deposited by HWCVD have been investigated by the surface/interface-sensitive nonlinear technique of optical second harmonic generation (SHG). In situ spectral SHG scans have revealed two resonance peaks at similar to 1.2 eV and similar to 1.45 eV corresponding with electronic states (e.g., Si dangling bonds, Si-Si strained bonds) at the surface/interface of the a-Si:H with an isotropic distribution. The data are compared to SHG results obtained ex situ for a-Si:H films deposited by an rf plasma. The first real-time SHG experiments during HWCVD a-Si:H film growth are presented. (c) 2005 Elsevier B.V. All rights reserved.