||In this dissertation, we investigates the electrical stress induced degradation in low-temperature polycrystalline-silicon thin film transistors (LTPS TFTs) applied for system-on-panel (SOP), including the electrical degradations of device for switch operation in active matrix flat-panel displays, driving circuit and nonvolatile memory. Finally, we also present the reliability of LTPS TFTs applied for flexible displays. |
In first part, electrical degradation of conventional and pattered metal-shielding LTPS TFTs under darkened and illuminated dynamic AC stresses are investigated. Experimental results reveal that competitive mechanisms are generated in conventional LTPS TFTs during illuminated stress, namely, carrier increase and electric field weakening. This phenomenon is verified by stressing the patterned source/drain open metal-shielding LTPS TFTs, which determines that the electric field weakening dominates; conversely, the carrier increase is dominated the electrical degradation in channel open metal-shielding device under illuminated stress. In addition, an improvement in anomalous on-current and subthreshold swing (S.S.) in n-channel LTPS TFTs after positive gate bias stress are studied. These improved electric properties are due to the hole trapping at SiO2 above the lightly doped drain regions, which causes a strong electric field at the gate corners. The effect of the hole trapping is to reduce the effective channel length and the S.S.. Besides, the stress-related electric field was also simulated by TCAD software to verify the mechanism above.
Secondly, a mechanism of anomalous capacitance in p-channel LTPS TFTs was investigated. In general, the effective capacitance of the LTPS TFTs was only dependent with the overlap area between gate and source/drain under the off-state. However, the experimental results reveal that the off-state capacitance was increased with decreasing measurement frequency and/or with increasing measurement temperature. Besides, by fitting the curve of drain current versus electric field under off-state region, it was verified that the TAGIDL is consisted of the Pool-Frenkel emission and Thermal-Field emission. In addition, the charge density calculated from the Cch-Vg measurement also the same dependence with electric field. This result demonstrates that the anomalous capacitance is mainly due to the trap-assisted-gate-induced-drain-leakage (TAGIDL). In order to suppress the anomalous capacitance, a band-to-band hot electron (BTBHE) stress was utilized to reduce the vertical electric field between the gate and the drain.
In third part, in order to realize the reliability in p-channel TFTs under illuminated environment operation, the degradation of negative bias temperature instability (NBTI) with illumination effect is investigated. The generations of interface state density (Nit) are identical under various illuminated intensity DC NBTI stresses. Nevertheless, the degradation of the grain boundary trap (Ntrap) under illumination was more significant than for the darkened environment, with degradation increasing as illumination intensity increases. This phenomenon is mainly caused by the extra number of holes generated during the illuminated NBTI stress. The increased Ntrap degradation leads to an increase in the darkened environment leakage current. This indicates that more traps are generated in the drain junction region that from carrier tunneling via the trap, resulting in leakage current. Conversely, an increase of Ntrap degradation results in a decrease in the photoleakage current. This indicates that the number of recombination centers increases in poly-Si bulk, affecting photosensitivity in LTPS TFTs. Besides, the transient effect assisted NBTI degradation in p-channel LTPS TFTs under dynamic stress is also presented, in which the degradation of the Ntrap becomes more significant as rise time decreases to 1 μs. Because the surface inversion layer cannot form during the short rise time, transient bulk voltage will cause excess holes to diffuse into the poly-Si bulk. Therefore, the significant Ntrap increase is assisted by this transient effect.
Fourthly, we study the electric properties of n- and p-channel LTPS TFTs under the mechanical tensile strain. The improved on-current for tensile strained n-channel TFTs is originated form an increase in energy difference between 2- and 4-fold valleys, reducing the inter-valley scattering and further improving the carrier mobility. On the contrary, the hole mobility decreases in p-channel, suggesting the split between the light hole and heavy hole energy bands and an increase in hole population on the heavy hole energy band of poly-Si when the uniaxial tensile strain is parallel to the channel direction. In addition, the Nit and Ntrap degradations induced by NBTI for tensile strained LTPS TFTs are more pronounced than in the unstrained. Extracted density-of-states (DOS) and conduction activation energy (EA) both show increases due to the strained Si-Si bonds, which implies that strained Si-Si bonds are able to react with dissociated H during the NBTI stress. Therefore, the NBTI degradation is more significant after tensile strain than in an unstrained condition.
Finally, the SONOS-TFT applied to nonvolatile memory is prepared and studied. In the gate disturb stress, a parasitic capacitance and resistance in off-state region are identified as electrons trapped in the gate-insulator (GI) near the defined gate region. Meanwhile, these trapping electrons induced depletions in source/drain also degraded the I-V characteristic when the gate bias is larger than the threshold voltage. However, these degradations slightly recover when the trapped electrons are removed after negative bias stress. The electric field in the undefined gate region is also verified by TCAD simulation software.