Relative importance of the Si–Si bond and Si–H bond for the stability of amorphous silicon thin film transistors

Abstract

We investigate the mechanism for Si dangling bond defect creation in amorphous silicon thin film transistors as a result of bias stress. We show that the rate of defect creation does not depend on the total hydrogen content or the type of hydrogen bonding in the amorphous silicon. However, the rate of defect creation does show a clear correlation with the Urbach energy and the intrinsic stress in the film. These important results support a localized model for defect creation, i.e., where a Si–Si bond breaks and a nearby H atom switches to stabilize the broken bond, as opposed to models involving the long-range diffusion of hydrogen. Our experimental results demonstrate the importance of optimizing the intrinsic stress in the films to obtain maximum stability and mobility. An important implication is that a deposition process where intrinsic stress can be independently controlled, such as an ion-energy controlled deposition should be beneficial, particularly for deposition temperatures below 300 °C.

Structural origins of intrinsic stress in amorphous silicon thin films

“…These stresses are small compared to total film stresses of between -1230 and +393 MPa. This result is expected due to the similar coefficients of thermal expansion 13 of silicon and amorphous silicon. Thus, we conclude that the measured stress in the films is governed by intrinsic stress, calculated through the subtraction of calculated thermal stress from the experimentally measured total stress.…”

“…The origin of stress in our films is likely due to an ion bombardment effect, 13,20,21 which has been previously reported in films deposited by both ion sputtering 20,22 and PECVD, 13,18 relating internal stresses of films to the momentum of the depositing ions. Based on models put forth by Windischmann 22 and Smets, 23 we are able to describe the observed stress behavior purely through ion interactions with the depositing film and correlate these descriptive models to the observed experimental data.…”

“…The Poisson's ratio has been shown by Wehrspohn et al 13 to remain virtually invariant across films of varying hydrogen content, and, through our examination, across all possible values (0 -~0.3) imparts little change on the values obtained by Eq. 9.…”

“…14 Such changes in atomic arrangement can be tailored by specific growth conditions. The PECVD input variables observed to exert the greatest influence on thin-film stress are deposition temperature, 13 hydrogen dilution, 15 plasma frequency, 16 discharge power, [17][18][19] and ambient gas pressure. 13 The influence of these deposition parameters on film structure consequently alters the mechanical and optoelectronic properties of the cells.…”

Damage at hydrogenated amorphous/crystalline silicon interfaces by indium tin oxide overlayer sputtering

“…The energy required to remove a hydrogen atom from an isolated Si-H bond in an a-Si:H network is 3.55 eV (Ref. 26) and the energy required to break weak SiSi bonds is lower (2.5 eV for a strong Si-Si bond 27 ). 28 Thus, as many species are present during sputtering with higher energies, both bonds are susceptible to be affected.…”