Rajitha N. P. Vemuri

Investigation of defect generation and annihilation in IGZO TFTs during practical stress conditions: illumination and electrical bias

This investigation elucidates the influence of standalone-bias stress and standalone-illumination stress, and their combinatory effect on indium-gallium-zinc oxide (IGZO) thin-film transistors (TFTs). Each phenomenon associated with illumination (wavelengths of 410, 467, 532 and 632 nm) and bias (Vg = ±20 V; Vd = 0, 20 V) stresses is categorized individually. Wavelengths below 532 nm are responsible for ionized vacancy creation, carrier generation and interface state creation while only gate stresses of +20 V create excessive charge trap states. Failure conditions are identified as gate stresses of −20 V with 410 nm illumination. An improvement in the Ion/Ioff ratio from 106 to 107 is due to increased charge contribution for on currents, and trapping of holes at the intermetallic dielectric near the source–channel junction in the off region. Upon stress removal TFTs exhibit incomplete recovery due to slow trapping of excited carriers from dielectric layers and inability created by the passivation layer to absorb oxygen for vacancy regeneration. The low-temperature fabrication and optimized post-fabrication anneal have created reduced defect and vacancy densities that make the IGZO TFTs more stable than the previous generation TFTs.

Susceptor assisted microwave annealing for recrystallization and dopant activation of arsenic-implanted silicon

The increasing need for quicker and more efficient processing techniques motivates the study of the use of a single frequency applicator microwave cavity, along with an alumina-coated SiC susceptor, as an alternative to current post-implantation processing. The extent of Si recrystallization and repair of the damage caused by arsenic implantation into Si is determined by cross-section transmission electron microscopy and Raman spectroscopy. Dopant activation is evaluated by sheet resistance measurements. Secondary ion mass spectroscopy is used to compare the extent of diffusion that results from such microwave annealing with that experienced when using conventional rapid thermal annealing (RTA). The results show that, compared to susceptor-assisted microwave annealing, RTA caused undesired dopant diffusion. The SiC-alumina susceptor plays a significant role in supplying heat to the Si substrate and also acts as an assistor that helps a high-Z dopant, like arsenic, to absorb the microwave energy, using a m...

Effective dopant activation via low temperature microwave annealing of ion implanted silicon

Susceptor-assisted microwave annealing enables effective dopant activation, at low temperatures, in ion-implanted Si. Given similar thermal budgets for microwave annealing and rapid thermal annealing (RTA), sheet resistances of microwave annealed Si, with either B+ or P+ implants, are lower than the values obtained using RTA. The fraction of dopants activated is as high as 18% for B+ implants and 64% for P+ implants. Dopant diffusion is imperceptible after microwave annealing, but significant after RTA, for P+ implanted Si samples with the same dopant activation. Microwave annealing achieves such properties using shorter anneal times and lower peak temperatures compared to RTA.

Dopant Activation in Arsenic-Implanted Si by Susceptor-Assisted Low-Temperature Microwave Anneal

It is important for nanoscale transistors to have abrupt junctions, which are difficult to achieve via high-temperature anneals of implanted semiconductor layers due to undesired dopant diffusion. The use of a single-frequency microwave cavity applicator, along with a SiC-Alumina susceptor/assistor, is suggested as an alternative postimplantation process. Secondary ion mass spectroscopy analysis of microwave-annealed As-implanted Si samples show minimal diffusion, compared to rapid thermal annealed samples. Cross-sectional transmission electron microscopy and Raman spectroscopy confirm damage repair and Si recrystallization upon low-temperature microwave annealing (up to 650°C). Ion channeling and sheet resistance measurements validate dopant relocation and activation. The susceptor is used to provide surface heating to the high-atomic-number Z implanted sample to enable it to absorb microwaves and thereby recrystallize through volumetric heating.

Effective dopant activation by susceptor-assisted microwave annealing of low energy boron implanted and phosphorus implanted silicon

Rapid processing and reduced end-of-range diffusion result from susceptor-assisted microwave (MW) annealing, making this technique an efficient processing alternative for electrically activating dopants within ion-implanted semiconductors. Sheet resistance and Hall measurements provide evidence of electrical activation. Susceptor-assisted MW annealing, of ion-implanted Si, enables more effective dopant activation and at lower temperatures than required for rapid thermal annealing (RTA). Raman spectroscopy and ion channeling analyses are used to monitor the extent of ion implantation damage and recrystallization. The presence and behavior of extended defects are monitored by cross-section transmission electron microscopy. Phosphorus implanted Si samples experience effective electrical activation upon MW annealing. On the other hand, when boron implanted Si is MW annealed, the growth of extended defects results in reduced crystalline quality that hinders the electrical activation process. Further comparison of dopant diffusion resulting from MW annealing and rapid thermal annealing is performed using secondary ion mass spectroscopy. MW annealed ion implanted samples show less end-of-range diffusion when compared to RTA samples. In particular, MW annealed P+ implanted samples achieve no visible diffusion and equivalent electrical activation at a lower temperature and with a shorter time-duration of annealing compared to RTA. In this study, the peak temperature attained during annealing does not depend on the dopant species or dose, for susceptor-assisted MW annealing of ion-implanted Si.

Enhanced electrical performance of Ag–Cu thin films after low temperature microwave processing

In the present study, thin films of binary Ag–Cu alloys with different Cu content were prepared by a cosputtering technique and then annealed by microwave heating. The metallographic and electrical properties of the thin films were observed experimentally. It was found that the electrical performance of Ag–Cu thin films enhanced after microwave processing when compared to vacuum annealing. Based on the fact that Cu has a low solubility in Ag, it was chosen as the alloying element. The low solubility favored segregation of Cu at the surface and grain boundaries. This prevented Ag grain boundary diffusion and agglomeration. The as-deposited thin films were more resistive when compared to the microwave processed films. Comparison of the two postdeposition annealing techniques, microwave and vacuum, showed that microwave annealing is a clean, faster, and efficient process that can improve the electrical performance of Ag–Cu thin films. Results from this investigation demonstrated that microwave annealing is a...

Kinetic Stress Testing and the Influence of Long-Time Anneals on the Behavior of IZO Thin Film Transistors

Stability testing of indium-zinc oxide under extreme conditions is necessary before the metal oxide can be deemed a reliable alternative to a-Si as channel layer in thin film transistors (TFTs). In this paper, we apply thermal stress under positive and negative gate bias stress creating practical application conditions. This stress scenario gives greater insight into thermally activated defects in the presence of an electrical field. Operational temperatures of 20 °C, 50 °C, and 80 °C are used, and a more rapid degradation of the devices is seen with increased temperatures. Kink and dip are observed due to the donor-like trapped charges increasing the subthreshold swing and acceptor-like trapped charges reducing the ON current, respectively. Post fabrication, long-time anneals for 12-, 24-, 36-, 48-, and 60-h at low temperatures (150 °C) are performed to alleviate defects. The 48-h annealed TFTs show remarkable stability under extreme temperatures and electric fields maintaining a high ON-OFF ratio (~108) after 104 s. This is attributed to the reduced density of charges in acceptor-like trap states, modeled to reduce by 58% from a 12-h anneal and to insignificant extent over a 48-h anneal.