A controlled field-effect passivation by a well-defined density of fixed charges is crucial for modern solar cell surface passivation schemes Al2O3 nanolayers grown by atomic layer deposition contain negative fixed charges Electrical measurements on slant-etched layers reveal that these charges are located within a 1 nm distance to the interface with the Si substrate When inserting additional interface layers, the fixed charge density can be continuously adjusted from 35 × 1012 cm–2 (negative polarity) to 00 and up to 40 × 1012 cm–2 (positive polarity) A HfO2 interface layer of one or more monolayers reduces the negative fixed charges in Al2O3 to zero The role of HfO2 is described as an inert spacer controlling the distance between Al2O3 and the Si substrate It is suggested that this spacer alters the nonstoichiometric initial Al2O3 growth regime, which is responsible for the charge formation On the basis of this charge-free HfO2/Al2O3 stack, negative or positive fixed charges can be formed by i

On the Control of the Fixed Charge Densities in Al2O3-Based Silicon Surface Passivation Schemes

Al–O monolayers embedded in ultrathin SiO2 were shown previously to contain Al-induced acceptor states, which capture electrons from adjacent silicon wafers and generate a negative fixed charge that enables efficient Si-surface passivation. Here, we show that this surface passivation is just in part attributed to field-effect passivation, since the electrically active interface trap density Dit itself at the Si/SiO2 interface is reduced by the presence of the acceptor states. For sufficiently thin tunnel-SiO2 films between the Si-surface and the Al–O monolayers, Dit is reduced by more than one order of magnitude. This is attributed to an interface defect deactivation mechanism that involves the discharge of the singly-occupied dangling bonds (Pb0 defects) into the acceptor states, so that Shockley-Read-Hall-recombination is drastically reduced. We demonstrate that the combined electronic and field-effect passivation allows for minority carrier lifetimes in excess of 1 ms on n-type Si and that additional H2-passivation is not able to improve that lifetime significantly.

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Deactivation of silicon surface states by Al-induced acceptor states from Al–O monolayers in SiO2

In Al2O3-based passivation layers, the formation of fixed charges and trap sites can be strongly influenced by small modifications in the stack layout. Fixed and trapped charge densities are characterized with capacitance voltage profiling and trap spectroscopy by charge injection and sensing, respectively. Al2O3 layers are grown by atomic layer deposition with very thin (∼1 nm) SiO2 or HfO2 interlayers or interface layers. In SiO2/Al2O3 and HfO2/Al2O3 stacks, both fixed charges and trap sites are reduced by at least a factor of 5 compared with the value measured in pure Al2O3. In Al2O3/SiO2/Al2O3 or Al2O3/HfO2/Al2O3 stacks, very high total charge densities of up to 9 × 1012 cm−2 are achieved. These charge densities are described as functions of electrical stress voltage, time, and the Al2O3 layer thickness between silicon and the HfO2 or the SiO2 interlayer. Despite the strong variation of trap sites, all stacks reach very good effective carrier lifetimes of up to 8 and 20 ms on p- and n-type silicon substrates, respectively. Controlling the trap sites in Al2O3 layers opens the possibility to engineer the field-effect passivation in the solar cells.

Trapped charge densities in Al2O3-based silicon surface passivation layers

Titanium oxide (TiO2) films and TiO2/SiNx stacks have potential in surface passivation, anti-reflection coatings and carrier-selective contact layers for crystalline Si solar cells. A Si wafer, deposited with 8-nm-thick TiO2 film by atomic layer deposition, has a surface recombination velocity as low as 14.93 cm/s at the injection level of 1.0 × 1015 cm−3. However, the performance of silicon surface passivation of the deposited TiO2 film declines as its thickness increases, probably because of the stress effects, phase transformation, atomic hydrogen and thermal stability of amorphous TiO2 films. For the characterization of 66-nm-thick TiO2 film, the results of transmission electron microscopy show that the anatase TiO2 crystallinity forms close to the surface of the Si. Secondary ion mass spectrometry shows the atomic hydrogen at the interface of TiO2 and Si which serves for chemical passivation. The crystal size of anatase TiO2 and the homogeneity of TiO2 film can be deduced by the measurements of Raman spectroscopy and spectroscopic ellipsometry, respectively. For the passivating contacts of solar cells, in addition, a stack composed of 8-nm-thick TiO2 film and a plasma-enhanced chemical-vapor-deposited 72-nm-thick SiNx layer has been investigated. From the results of the measurement of the reflectivity and effective carrier lifetime, TiO2/SiNx stacks on Si wafers perform with low reflectivity and some degree of surface passivation for the Si wafer.

https://ir.nctu.edu.tw:443/bitstream/11536/132693/1/552a81d4687de490779fb4d654496af1.pdf

Atomic layer deposition TiO2 films and TiO2/SiNx stacks applied for silicon solar cells

Al2O3-TiO2 nanolaminates are very attractive candidates for future conductive passivation layers because they are purely based on dielectric materials, which allow a simple integration in the state-of-the-art manufacturing process. In this study, Al2O3-TiO2 double and multilayers are grown by atomic layer deposition and systematically investigated. The nanolaminates feature good silicon surface passivation and moderate electrical conductivity. The best performance is found for a double-layer stack consisting of a 5 nm Al2O3 interface layer and a 15 nm TiO2 capping layer after postdeposition annealing in N2 or forming gas. With this stack, a surface recombination velocity of 15 cm/s and a contact resistance of 20 Ω·cm2 are achieved. In Al2O3 -TiO2 nanolaminates, the electrical transport is strongly influenced by the interaction of TiO2 and Al2O3 during layer growth. Raman measurements reveal that high conductivity correlates with a phase transition of TiO2 from amorphous to anatase.

Al 2 O 3 -TiO 2 Nanolaminates for Conductive Silicon Surface Passivation

This article reports an improvement in the performance of the hafnium oxide-based (HfO2) ferroelectric field-effect transistors (FeFET) achieved by a synergistic approach of interfacial layer (IL) engineering and READ-voltage optimization. FeFET devices with silicon dioxide (SiO2) and silicon oxynitride (SiON) as IL were fabricated and characterized. Although the FeFETs with SiO2 interfaces demonstrated better low-frequency characteristics compared to the FeFETs with SiON interfaces, the latter demonstrated better WRITE endurance and retention. Finally, the neuromorphic simulation was conducted to evaluate the performance of FeFETs with SiO2 and SiON IL as synaptic devices. We observed that the WRITE endurance in both types of FeFETs was insufficient to carry out online neural network training. Therefore, we consider an inference-only operation with offline neural network training. The system-level simulation reveals that the impact of systematic degradation via retention degradation is much more significant for inference-only operation than low-frequency noise. The neural network with FeFETs based on SiON IL in the synaptic core shows 96% accuracy for the inference operation on the handwritten digit from the Modified National Institute of Standards and Technology (MNIST) data set in the presence of flicker noise and retention degradation, which is only a 2.5% deviation from the software baseline.

https://pubs.acs.org/doi/pdf/10.1021/acsaelm.2c00771

Synergistic Approach of Interfacial Layer Engineering and READ-Voltage Optimization in HfO2-Based FeFETs for In-Memory-Computing Applications

The degradation predicted by classical DC reliability methods, such as bias temperature instability (BTI) and hot carrier injection (HCI), might not translate sufficiently to the AC conditions, which are relevant on the circuit level. The direct analysis of circuit level reliability is therefore an essential task for hardware qualification in the near future. Ring oscillators (RO) offer a good model system, where both BTI and HCI contribute to the degradation. In this work, it is qualitatively shown that the additional off-state stress plays a crucial role at very high stress voltages, beyond upper usage boundaries. To yield an accurate RO lifetime prediction a frequency measurement setup with high resolution is used, which can resolve small changes in frequency during stress near operation conditions. An ACDC conversion model is developed predicting the resulting frequency change based on DC input data. From the extrapolation to 10 years of circuit lifetime the model predicts a very low frequency degradation below 0.2% under nominal operation conditions, where the off-state has a minor influence.

Off-state Impact on FDSOI Ring Oscillator Degradation under High Voltage Stress

Al2O3 based nanolaminates provide new functionalities for silicon surface passivation layers in future high-efficient cell concepts. This work presents two different applications: (I) Symmetrical passivation layers for application on p- and n-type Si are realized with thin HfO2 and Al-doped SiO2 interface layers between Al2O3 and Si. These stacks have excellent chemical passivation but zero fixed charges. Consequently, no surface inversion layer is formed in n-type Si and low surface recombination velocities below 2 cm/s are achieved in low injection. (II) Conductive passivation layers are realized with Al2O3-TiO2 double and multilayers. The optimum material combination and post deposition treatment results in a current density of 0.5 mA/cm2 at 10 mV with a surface recombination velocity of about 15 cm/s. This conductivity is more than eight orders of magnitude higher than for pure Al2O3.

ALD Al2O3 based nanolaminates for solar cell applications

The continuous evolution and physical scaling of RF CMOS devices empowers the high performance millimeter wave applications of modern technologies. The reliable operation of these devices is also becoming more and more important with the rapid scaling to achieve higher performance. To obtain in depth understanding of reliability of FD-SOI RF devices, the worst case hot carrier stress was induced and the corresponding impact on small signal parameters is analyzed using S-parameter and DC measurements. The physical origin of the variation and the link with DC parameter degradation is investigated.

Talha Chohan

Papers

Al 2 O 3 -TiO 2 Nanolaminates for Conductive Silicon Surface Passivation

Synergistic Approach of Interfacial Layer Engineering and READ-Voltage Optimization in HfO2-Based FeFETs for In-Memory-Computing Applications

Off-state Impact on FDSOI Ring Oscillator Degradation under High Voltage Stress

ALD Al2O3 based nanolaminates for solar cell applications

Impact of hot carrier stress on small-signal parameters of FD-SOI NMOSFETs