Border Trap Density Research Articles

Deposition of Tantalum Oxide Films by UV Laser Reactive Ablation in O3 Ambient

Tantalum oxide films have been deposited on n-type silicon substrate by 355 nm laser ablation of Ta2O5 in the presence of O3. The dielectric and electrical properties of Ta2O5 films after annealing treatment have been studied. The results suggest that the film has dielectric constant of 28, and leakage current of 10-7 A/cm2 at an applied electric field of 500 kV/cm. The capacitance-voltage characteristics of the Al/Ta2O5/n+Si capacitors exhibit an interface trap density and border trap density of 3×1011 eV-1 cm-2 and 1.2×1011 cm-2, respectively. The effect of ambient gases during laser deposition on the border trap densities of annealed films are also discussed.

A method for separating the effects of interface from border and oxide trapped charge densities in mos transistors

This paper presents a procedure for a more accurate separation of interface trap effects in the presence of large border trap densities after irradiation of MOS devices. It is based on the standard subthreshold technique, but a special measurement procedure is applied which eliminates the drifts produced by border traps via the tunneling effect. The procedure is demonstrated on pMOS dosimetric transistors, and it is shown that it gives different and, we claim, better estimates of interface trap density than standard techniques.

Correlation between latent interface trap buildup and 1/f noise in metal–oxide–semiconductor transistors

A long-term delayed increase in the 1/f noise of p-channel metal–oxide–semiconductor (MOS) transistors is observed in devices that show significant latent interface-trap buildup after exposure to ionizing radiation. During positive-bias postirradiation anneal, the noise increases by more than an order of magnitude above the level observed after irradiation. The increase in noise precedes the latent buildup of interface traps by at least 4.5 days during room-temperature annealing, and by ∼1 h during 100 °C annealing. The time and temperature dependencies of the increases in noise and interface trap buildup are consistent with the thermally activated motion of protons into the near-interfacial region of the oxide, followed by increases in border trap and interface trap densities. These results suggest hydrogen-related species can significantly affect the 1/f noise of MOS devices.

Oxide, interface, and border traps in thermal, N2O, and N2O-nitrided oxides

We have combined thermally stimulated-current (TSC) and capacitance–voltage (C–V) measurements to estimate oxide, interface, and effective border trap densities in 6–23 nm thermal, N2O, and N2O-nitrided oxides exposed to ionizing radiation or high-field electron injection. Defect densities depend strongly on oxide processing, but radiation exposure and moderate high-field stress lead to similar trapped hole peak thermal energy distributions (between ∼1.7 and ∼2.0 eV) for all processes. This suggests that similar defects dominate the oxide charge trapping properties in these devices. Radiation-induced hole and interface trap generation efficiencies (0.1%–1%) in the best N2O and N2O-nitrided oxides are comparable to the best radiation hardened oxides in the literature. After ∼10 Mrad(SiO2) x-ray irradiation or ∼10 mC/cm2 constant current Fowler–Nordheim injection, effective border trap densities as high as ∼5×1011 cm−2 are inferred from C–V hysteresis. These measurements suggest irradiation and high-field stress cause similar border trap energy distributions. In each case, even higher densities of compensating trapped electrons in the oxides (up to 2×1012 cm−2) are inferred from combined TSC and C–V measurements. These trapped electrons prevent conventional C–V methods from providing accurate estimates of the total oxide trap charge density in many irradiation or high-field stress studies. Fewer compensating electrons per trapped hole (∼26%±5%) are found for irradiation of N2O and N2O-nitrided oxides than for thermal oxides (∼46%±7%). More compensating electrons are also found for high-field electron injection than radiation exposure, emphasizing the significance of border traps to metal-oxide-semiconductor long term reliability. The primary effect of nitrogen on charge trapping in these oxides appears to be improvement of the near interfacial oxide in which border traps are found.

Effects of interface traps and border traps on MOS postirradiation annealing response

Threshold-voltage and charge-pumping measurements are combined to estimate densities of radiation induced bulk-oxide, interface, and border traps in transistors with soft 45-nm oxides. Immediately after irradiation, nearly all effects usually attributed to interface traps are actually due to border traps in these devices. During positive-bias anneal at 80/spl deg/C, the interface-trap density grows by more than a factor of 10, and the border-trap density changes by less than 30%. The increase in interface-trap density is matched by a decrease in bulk-oxide-trap charge. This raises the possibility that slowly transporting or trapped protons in the oxide may be responsible for this effect. An alternate explanation is offered by H-cracking models. Latent "interface-trap" growth in harder 27.7-nm oxides is associated with (true) interface traps, not border traps. Switched-bias annealing of the soft 45-nm oxides reveals fast and slow border traps with different annealing responses. Trivalent Si defects associated with O vacancies in SiO/sub 2/, the E/sub /spl gamma//' center and the O/sub 3-x/Si/sub x/Si/spl middot/ family, are excellent candidates for slow and fast border traps, respectively. For O/sub 3-x/Si/sub x/Si, x=0 is the E/sub s/' defect; x=3 is the D center; and x=1 or 2 have been proposed as candidates for the "P/sub b1/" defect on (100) Si. A hydrogen-related complex (e.g. OH/sup -/) may also be a border trap. The practical significance of these results is discussed for (1) bias-temperature instabilities in thin oxides, (2) effects of burn-in on MOS radiation response, and (3) enhanced bipolar gain degradation at low dose rates.

Microscopic nature of border traps in MOS oxides

We show that enhanced hole-, electron-, interface-, and border-trap generation in irradiated Si/SiO/sub 2//Si systems that have received a high-temperature anneal during device fabrication is related either directly, or indirectly, to the presence of anneal-created oxygen vacancies. The high-temperature anneal results are shown to be relevant to understanding defect creation in zone-melt-recrystallized silicon on insulator materials. We observe the electron paramagnetic resonance (EPR) of trap-assisted hole transfer between two different oxygen vacancy-type defects (E'/sub /spl delta///spl rarr/3 E'/sub /spl gamma// precursor) in hole injected thermal SiO/sub 2/ films. Upon annealing the hole injected Si/SiO/sub 2/ structures at room temperature, the E'/sub /spl delta// center transfers its hole to a previously neutral oxygen vacancy (O/sub 3//spl equiv/Si-Si/spl equiv/O/sub 3/) site forming an E'/sub /spl gamma// center. This process, also monitored electrically, shows a concomitant increase in the border-trap density that mimics the growth kinetics of the transfer-activated E'/sub /spl gamma// centers. This suggests that both effects are correlated and that some of the transfer-created E'/sub /spl gamma// centers are the entities responsible for the border traps in these devices. One implication of these results is that delayed defect growth processes can occur via slow trap-assisted hole motion in SiO/sub 2/. >

Defect-defect hole transfer and the identity of border traps in SiO2 films.

We have found evidence for hole-transfer events between oxygen-vacancy-related defects in thermal ${\mathrm{SiO}}_{2}$ films selectively injected with holes using electron paramagnetic resonance (EPR) and capacitance-voltage measurements. We find that hole injection into ${\mathrm{SiO}}_{2}$ films reveals two types of EPR centers: oxygen-vacancy-related ${\mathit{E}}_{\mathrm{\ensuremath{\delta}}}^{\mathcal{'}}$ and ${\mathit{E}}_{\ensuremath{\gamma}}^{\mathcal{'}}$ centers; both centers are positively charged. Upon annealing the Si/${\mathrm{SiO}}_{2}$ structures at room temperature, the ${\mathit{E}}_{\mathrm{\ensuremath{\delta}}}^{\mathcal{'}}$ defect transfers its hole to a neutral oxygen-vacancy (${\mathrm{O}}_{3}$\ensuremath{\equiv}Si---Si\ensuremath{\equiv}${\mathrm{O}}_{3}$) site forming the classic ${\mathit{E}}_{\ensuremath{\gamma}}^{\mathcal{'}}$ center. Electrically, we observe a concomitant growth in the border-trap density, suggesting that some of the transfer-created ${\mathit{E}}_{\ensuremath{\gamma}}^{\mathcal{'}}$ centers may be the microscopic entities responsible for the border traps. These results constitute spectroscopic evidence that hole transfer between defect sites can occur over extremely long time scales (hours) in ${\mathrm{SiO}}_{2}$ films.