HfO2 Passivation Layer Research Articles

Surface passivation properties of atomic-layer deposited hafnium oxide on a (100) β-Ga2O3 MOSFET

We report the effect of HfO2 passivation on the electrical characteristics of (100) β-Ga2O3 metal-oxide-semiconductor field-effect transistors (MOSFETs). The atomic-layer-deposited HfO2 layer with negative defect charges enhances the transconductance and subthreshold slope. A significant positive threshold voltage (V TH) shift of ∼32 V is induced after the passivation. Moreover, significantly less V TH shift of ∼2 V is observed under negative bias stress (NBS) for 3600 s in comparison with an Al2O3 passivated and unpassivated device. Physics-based technology computer aided design (TCAD) simulation is performed to demonstrate the surface depletion effect and the dependency on the density of negative fixed charges in the HfO2 passivation layer. Finally, the HfO2 passivated device maintains superior electrical characteristics compared to the unpassivated device at a temperature of up to 500 K.

All‐In‐One Hardware Devices with Event‐Based Vision Sensor Arrays for Image Sensing, Computing, and Learning

AbstractMetal oxide semiconductors (MOSs) are considered as potential candidates for the low‐cost, large‐area fabrication of flexible optoelectronic devices. However, the current optoelectronic devices based on MOSs are limited to unidirectional photoresponse, which constrains the performance of MOSs‐based vision sensors for artificial vision systems. Herein, for the first time, a flexible artificial vision system integrated with optical perception, computation, and learning functionalities is demonstrated using SnO optoelectronic synaptic transistor‐based event‐driven vision sensors to enable dynamic image perception, noise reduction, detection, and recognition. Specifically, an ambipolar SnO transistor is demonstrated by introducing HfO2 passivation layer, which facilitates the movement of O atoms around Sn‐vacancy sites to the HfO2 layer to achieve the transformation from p‐type to ambipolar transport behaviors. More importantly, the HfO2‐passivated SnO transistors exhibit gate‐tunable bidirectional photoresponse behavior, which is essential to simulate the neurobiological functionalities of bipolar cells. This way, the multilayer neural network learning circuit built from SnO transistors achieves fast recognition at a 16% Gaussian noise level and high recognition accuracy up to 95.2% for pattern letters. Under the bending states, recognition accuracies are still retained at 91.2%. These properties are well retained even under the influence of 100% offset of the synaptic programming value.

Monolayer WS2 Nanosheets Passivated with HfO2 for Enhanced Photodetectors

Two-dimensional transition metal dichalcogenides (TMDCs) are widely used in electronic and optoelectronic devices. However, the conventional chemical vapor deposition (CVD) method is difficult to synthesize large-area monolayer WS2 nanosheets stably, which limits the application of WS2 in the field of photoelectric detection. In this work, we propose an innovative NaCl-assisted CVD method that allows freely adjustable substrate positions for synthesizing monolayer WS2 nanosheets. The obtained maximum grain size of the monolayer WS2 nanosheets is up to 30 μm. Subsequently, we investigated the effect of the HfO2 passivation layer on the performance of the metal–semiconductor–metal (MSM) WS2-based photodetectors. The HfO2 passivation layer brought an overall improvement to the performance of the fabricated photodetectors, exhibiting a high responsivity of 1093.1 AW–1, a high specific detectivity of 2.6 × 1012 Jones, and a high external quantum efficiency of 2.1 × 105%. Furthermore, the physical mechanism of the fabricated photodetectors has been discussed to explain how the HfO2 passivation layer takes effect in the improvement of the WS2-based photodetectors. This result can accelerate the development of optoelectronic devices based on TMDCs.

Novel Stacked Passivation Structure for AlGaN/GaN HEMTs on Silicon With High Johnson’s Figures of Merit

We present a high-performance AlGaN/GaN high electron mobility transistors (HEMTs) on silicon substrate with novel stacked passivation layer (HfO2/SiO2). The stacked passivation structure can effectively modulate the electric field and reduce the electric field peak on the gate side, thus improving the breakdown voltage of the device. The prepared device with a gate length of 450 nm has a unit current gain cutoff frequency (fT) of 31.5 GHz, a maximum oscillation frequency (fMAX) of 46.3 GHz, and a three-terminal OFF-state breakdown voltage (BVgd) of 140 V at the gate-drain distance of 2.3 μm. The estimated Johnsonfs figure of merit (J-FOM=BVgd×fT) is 4.4 THz∙V, which is three and five times higher than that of the device with single HfO2 passivation layer and single SiO2 passivation layer, respectively. Furthermore, a significant suppression of the current collapse ( 5.7%) is observed due to the electric field redistribution near the drain region. The results show that the AlGaN/GaN HEMTs with stacked passivation layer proved to be a promising candidate for high-performance radio frequency (RF) power device applications.

Effect of HfO2 Passivation Layer on Light Extraction Efficiency of AlInN Nanowire Ultraviolet Light-Emitting Diodes

Diverse applications of ultraviolet (UV) light in the field of purification, sterilization, detection, surface disinfection, sensing, and many others have been studied mostly on AlGaN light-emitting-diodes (LEDs). However, the external quantum efficiency (EQE) of AlGaN deep UV LEDs is limited due to strong induced polarization fields, high dislocation density, severe electron leakage, difficulty in p-type doping, low internal quantum efficiency (IQE), IQE droop, and low light extraction efficiency (LEE). Alternatively, AlInN semiconductor has been recently gained tremendous attention for high power UV LEDs [1-3]. Our group has successfully demonstrated the first AlInN core-shell nanowire UV LEDs with a relatively high IQE of ~52%[1]. However, the light extraction study of such nanowire LEDs is unexplored. In this context, we have studied the LEE of a single AlInN nanowire UV LED at 220 nm - 400 nm wavelength regime. Moreover, we report that the LEE of a single AlInN nanowire UV LED could be increased by ~60% using HfO2 passivation compared to conventional nanowire LED at 282 nm.The schematic structure of AlInN nanowire UV LED is shown in Figure 1(a). The nanowire is composed of 200 nm n-GaN, 100 nm n-AlxIn1-xN quantum barrier (QB)/40 nm i-AlyIn1-yN quantum well (QW)/100 nm p-AlxIn1-xN QB, and 20 nm p-GaN layer. We have estimated the LEE of nanowire LEDs using the finite difference time domain (FDTD) simulation method. Silicon (Si) is used as a substrate, and the n-GaN layer is tapered with a diameter from 70 nm (bottom) to 100 nm (top). The diameter is set to be 100 nm for the rest of the regions. We have placed a single TM-polarized dipole source at the QW center to calculate LEE for a nanowire LED in the simulations. The estimated LEE of 282 nm wavelength single nanowire AlInN UV LED with various HfO2 passivation layers thickness is provided in Figure 1(b). The LEE of such LED without any passivation is ~25.2%, whereas the maximum recorded LEE is ~40.6% with the utilization of 35nm thickness HfO2 passivation layer. This study provides a promising approach for enhancing the light extraction efficiency of nanowire UV LEDs. Reference R. T. Velpula, B. Jain, M. R. Philip, et al., Sci. Rep., 10(1) 2547-2556 (2020).R. T. Velpula, B. Jain, H.Q.T. Bui, et al., Opt. Mater. Express, 10(2), 472-483 (2020).B. Jain, R. T. Velpula, M. Tumuna, et al., Opt. Express, 28(15), 22908-22918 (2020). Figure 1

Effect of HfO2 Passivation Layer on Light Extraction Efficiency of AlInN Nanowire Ultraviolet Light-Emitting Diodes

One of the probable reasons behind the limitation of light extraction efficiency (LEE) in III-nitride nanowire (NW) deep ultraviolet (UV) light-emitting diodes (LEDs) is the presence of the high surface density states that significantly contribute to the non-radiative recombination near the surface. Herein, we investigate the LEE of a single AlInN NW UV LED in the entire UV wavelength regime using finite-difference time-domain simulations. It is found that these LEDs favor transverse-magnetic (TM) polarized LEE over transverse-electric (TE) polarized emission. Further, we examine the role of the HfO2 surface passivation layer in the improvement of LEE of AlInN NW LED at ~282 nm wavelength. Our results show that the TM-polarized LEE of such LED without any passivation is only ~25.2%, whereas the maximum recorded LEE is ~40.6% with the utilization of 40 nm HfO2 passivation layer. This study provides a promising approach for enhancing the LEE of NW UV LEDs.

Impact of HfO₂ as a Passivation Layer in the Solar Cell Efficiency Enhancement in Passivated Emitter Rear Cell Type.

We report the simulation of high-efficiency c-silicon Passivated Emitter Rear Contact (PERC) type solar cell structure with rear side passivated with HfO₂ as a passivating material. Variation in the half length of pyramid has been carried out to investigate its effect on the solar cell electrical characteristics such as fill factor (FF), open circuit voltage (Voc) and efficiency. Aluminum back Surface Field (Al-BSF) and PERC type solar cell with Al₂O₃ passivation layer structures were also modeled for comparison. Effect of variation in passivation layer (HfO₂) thickness (10 and 15 nm) and permittivity (k = 14 and 25) on the solar cell electrical characteristics has been investigated. Result shows the efficiency improvement in the PERC solar cell with HfO₂ passivation layer by 0.5941% and 0.983% as compared to the Al-BSF and PERC with Al₂O₃ passivation layer at 8 μm pyramid half length. Increased series resistance and reduced FF has been observed with the incorporation of passivation layer at the solar cell structure. Negligible effect of passivation layer thickness has been observed on the solar cell electrical parameters whereas the permittivity value does have significant effect.

Enhanced energy storage capability of P(VDF-HFP) nanodielectrics by HfO2 passivation layer: Preparation, performance and simulation

Flexible dielectric nanocomposites have shown great potential for electrostatic capacitors due to their excellent energy storage properties. In an effort to eliminate the disadvantages induced by the interfacial polarization of two different components in the nanocomposite, in this work, HfO2@BaTiO3 nanoparticles (HfO2@BT NPs) with core-shell structure were synthesized and used to fabricate poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP))/HfO2@BT nanocomposite films via solution casting method. The passivation HfO2 layer with high resistivity and moderate dielectric permittivity could act as a buffering barrier and alleviate the dielectric mismatch between the BT nanoparticles (BT NPs) and P(VDF-HFP) matrix. The breakdown strength (Eb) and discharged energy density (Ue) of P(VDF-HFP)/HfO2@BT nanocomposites have been substantially enhanced. The nanocomposite filled with 1 wt% HfO2@BT presents a maximum Ue of 10.7 J/cm3 at 450 MV/m, which is 36% and 59% higher than that of the pristine P(VDF-HFP) and nanocomposite filled with the same BT NPs, respectively. Meanwhile, P(VDF-HFP)/HfO2@BT nanocomposites show a charge-discharge efficiency of 72%, which is superior to most of the reported values. The possible mechanism of HfO2 layer on above performance enhancements is proposed based on both experimental analysis and finite element simulation. This work demonstrates that utilizing dielectric fillers with passivation layer might be an effective way to improve the energy storage performance of the flexible dielectric nanodielectrics.

Coulomb scattering mechanism transition in 2D layered MoTe2: effect of high-κ passivation and Schottky barrier height

Clean interface and low contact resistance are crucial requirements in two-dimensional (2D) materials to preserve their intrinsic carrier mobility. However, atomically thin 2D materials are sensitive to undesired Coulomb scatterers such as surface/interface adsorbates, metal-to-semiconductor Schottky barrier (SB), and ionic charges in the gate oxides, which often limits the understanding of the charge scattering mechanism in 2D electronic systems. Here, we present the effects of hafnium dioxide (HfO2) high-κ passivation and SB height on the low-frequency (LF) noise characteristics of multilayer molybdenum ditelluride (MoTe2) transistors. The passivated HfO2 passivation layer significantly suppresses the surface reaction and enhances dielectric screening effect, resulting in an excess electron n-doping, zero hysteresis, and substantial improvement in carrier mobility. After the high-κ HfO2 passivation, the obtained LF noise data appropriately demonstrates the transition of the Coulomb scattering mechanism from the SB contact to the channel, revealing the significant SB noise contribution to the 1/f noise. The substantial excess LF noise in the subthreshold regime is mainly attributed to the excess metal-to-MoTe2 SB noise and is fully eliminated at the high drain bias regime. This study provides a clear insight into the origin of electronic signal perturbation in 2D electronic systems.

Simulation and Fabrication of HfO2 Thin Films Passivating Si from a Numerical Computer and Remote Plasma ALD

Recombination of charge carriers at silicon surfaces is one of the biggest loss mechanisms in crystalline silicon (c-Si) solar cells. Hafnium oxide (HfO2) has attracted much attention as a passivation layer for n-type c-Si because of its positive fixed charges and thermal stability. In this study, HfO2 films are deposited on n-type c-Si using remote plasma atomic layer deposition (RP-ALD). Post-annealing is performed using a rapid thermal processing system at different temperatures in nitrogen ambient for 10 min. The effects of post-annealing temperature on the passivation properties of the HfO2 films on c-Si are investigated. Personal computer one dimension numerical simulation for the passivated emitter and rear contact (PERC) solar cells with the HfO2 passivation layer is also presented. By means of modeling and numerical computer simulation, the influence of different front surface recombination velocity (SRV) and rear SRV on n-type silicon solar cell performance was investigated. Simulation results show that the n-type PERC solar cell with HfO2 single layer can have a conversion efficiency of 22.1%. The PERC using silicon nitride/HfO2 stacked passivation layer can further increase efficiency to 23.02% with an open-circuit voltage of 689 mV.

Low-temperature fabrication of an HfO2 passivation layer for amorphous indium\u2013gallium\u2013zinc oxide thin film transistors using a solution process

We report low-temperature solution processing of hafnium oxide (HfO2) passivation layers for amorphous indium–gallium–zinc oxide (a-IGZO) thin-film transistors (TFTs). At 150 °C, the hafnium chloride (HfCl4) precursor readily hydrolyzed in deionized (DI) water and transformed into an HfO2 film. The fabricated HfO2 passivation layer prevented any interaction between the back surface of an a-IGZO TFT and ambient gas. Moreover, diffused Hf4+ in the back-channel layer of the a-IGZO TFT reduced the oxygen vacancy, which is the origin of the electrical instability in a-IGZO TFTs. Consequently, the a-IGZO TFT with the HfO2 passivation layer exhibited improved stability, showing a decrease in the threshold voltage shift from 4.83 to 1.68 V under a positive bias stress test conducted over 10,000 s.

Strong perpendicular magnetic anisotropy energy density at Fe alloy/HfO2 interfaces

We report on the perpendicular magnetic anisotropy (PMA) behavior of heavy metal (HM)/Fe alloy/MgO thin film heterostructures when an ultrathin HfO2 passivation layer is inserted between the Fe alloy and MgO. This is accomplished by depositing one to two atomic layers of Hf onto the Fe alloy before the subsequent rf sputter deposition of the MgO layer. This Hf layer is fully oxidized during the subsequent deposition of the MgO layer, as confirmed by X-ray photoelectron spectroscopy measurements. The HfO2 insertion generates a strong interfacial perpendicular anisotropy energy density without any post-fabrication annealing treatment, for example, 1.7 erg/cm2 for the Ta/Fe60Co20B20/HfO2/MgO heterostructure. We also demonstrate PMA even in Ni80Fe20/HfO2/MgO structures for low-damping, low-magnetostriction Ni80Fe20 thin films. Depending on the choice of the HM, further enhancements of the PMA can be realized by thermal annealing to at least 400 oC. We show that ultra-thin HfO2 layers offer a range of options for enhancing the PMA properties of magnetic heterostructures for spintronics applications.

Improved Reliability of InGaN-Based Light-Emitting Diodes by HfO2 Passivation Layer.

We utilized a passivation layer to improve the leakage current and reliability characteristics of GaN-based light-emitting diodes. The electrical and optical characteristics of the fabricated LEDs were characterized by current-voltage and optical power measurements. The HfO2 passivation layer showed no optical power degradation and suppressed leakage current. The low deposition temper- ature of sputtered HfO2 is responsible for the improved reliability of the LEDs because it suppresses the diffusion of hydrogen plasma into GaN to form harmful Mg-H complexes.

Effect of Surface Passivation on Photoelectrochemical Water Splitting Performance of WO3 Vertical Plate-Like Films

WO3 vertical plate-like arrays provide a direct pathway for charge transport, and thus hold great potential as working electrodes for photoelectrochemical (PEC) water splitting. However, surface recombination due to surface defects hinders the performance improvement. In this work, WO3 vertical plate-like arrays films with HfO2 passivation layer were fabricated via a simple dip-coating method. In the images of transmission electron microscope, a fluffy layer and some small sphere particles existed on the surface of WO3 plate. X-ray photoelectron spectroscopy (XPS) showed a higher concentration of Hf element than the result of energy-dispersive X-ray spectroscopy (EDX), which means that HfO2 is rich on the surface of WO3 plates. A higher photocurrent under visible light irradiation was gained with surface passivation. Meanwhile, the results of intensity modulated photocurrent spectrum (IMPS) and incident photon to current conversion efficiency (IPCE) indicate that HfO2 passivation layer, acting as a barrier for the interfacial recombination, is responsible for the improved photoelectrochemical performance of WO3 vertical plate-like arrays film.

Enhanced photoelectrochemical performance of WO3 film with HfO2 passivation layer

In this article, Hafnium oxide (HfO2) overlayer was reported to be loaded on the surface of WO3 nanoparticles by a simple solvothermal method for the first time. HfO2 powders were dissolved in the concentrated sulfuric acid as raw material. The physicochemical properties of WO3 nanoparticles with and without HfO2 passivation layer were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). For the film electrode prepared with doctor-blade method, scanning electron microscopy (SEM) and UV–vis absorbance spectroscopy (UV–vis) were used to determine the morphological and optical properties. Meantime, the photoelectrochemical properties of two samples were evaluated by means of liner sweep voltammogram, electrochemical impedance spectroscopy (EIS), intensity modulated photocurrent spectrum (IMPS) and incident photon to current conversion efficiency (IPCE). The WO3 film with HfO2 passivation layer showed better photoelectrochemical performance which could be ascribed to the inhibition of the recombination of electron-holes.

Electrical performance of multilayer MoS2 transistors on high-κ Al2O3 coated Si substrates

The electrical performance of MoS2 can be engineered by introducing high-κ dielectrics, while the interactions between high-κ dielectrics and MoS2 need to be studied. In this study, multilayer MoS2 field-effect transistors (FETs) with a back-gated configuration were fabricated on high-κ Al2O3 coated Si substrates. Compared with MoS2 FETs on SiO2, the field-effect mobility (μFE) and subthreshold swing (SS) were remarkably improved in MoS2/Al2O3/Si. The improved μFE was thought to result from the dielectric screening effect from high-κ Al2O3. When a HfO2 passivation layer was introduced on the top of MoS2/Al2O3/Si, the field-effect mobility was further enhanced, which was thought to be concerned with the decreased contact resistance between the metal and MoS2. Meanwhile, the interface trap density increased from 2.4×1012 eV−1cm−2 to 6.3×1012 eV−1cm−2. The increase of the off-state current and the negative shift of the threshold voltage may be related to the increase of interface traps.

Metal-oxide-semiconductor field-effect-transistors on indium phosphide using HfO2 and silicon passivation layer with equivalent oxide thickness of 18 Å

In this letter, we demonstrate the electrical properties of metal-oxide-semiconductor capacitors and metal-oxide-semiconductor field-effect transistors (MOSFETs) on InP using atomic layer deposited HfO2 gate dielectric and a thin silicon interface passivation layer (Si IPL). Compared with single HfO2, the use of Si IPL results in better interface quality with InP substrate, as illustrated by smaller frequency dispersion and reduced hysteresis. MOSFETs with Si IPL show much higher drive current and transconductance, improved subthreshold swing, interface-trap density and gate leakage current with equivalent oxide thickness scaling down to 18 Å.

Influence of the substrate orientation on the electrical and material properties of GaAs metal-oxide-semiconductor capacitors and self-aligned transistors using HfO2 and silicon interface passivation layer

In this work, we studied the effects of postdeposition anneal (PDA) time and Si interface passivation layer on the material and electrical characteristics of the metal-oxide-semiconductor (MOS) capacitor with high-k (HfO2) material on different orientation substrates with (100), (110), and (311). The interfacial change of HfO2∕Si∕GaAs gate stacks after PDA has been characterized using x-ray photoelectron spectroscopy (XPS) and Dit measurement using conductance method and frequency dispersion. XPS measurement shows the formation of gallium and arsenic oxides with increasing annealing temperature. Unoxidized Si and gallium and arsenic oxides formation in the interface might act as traps. Self-aligned MOS field effect transistors using PDA at 600°C and post-metal-annealing at 800°C have also been fabricated and characterized. The (100) substrate has lower density of interface traps and higher mobility due to reduced Ga2O3 formation.

Self-aligned n-channel metal-oxide-semiconductor field effect transistor on high-indium-content In0.53Ga0.47As and InP using physical vapor deposition HfO2 and silicon interface passivation layer

In this work, we present the electrical and material characteristics of TaN∕HfO2∕In0.53Ga0.47As and InP substrate metal-oxide-semiconductor capacitors and self-aligned n-channel metal-oxide-semiconductor field effect transistor (n-MOSFET) with physical vapor deposition Si interface passivation layer. Excellent electrical characteristics, thin equivalent oxide thickness (∼1.7nm), and small frequency dispersion (<2%) were obtained. n-channel high-k InGaAs- and InP-MOSFETs with good transistor behavior and good split capacitance-voltage (C-V) characteristics on In0.53Ga0.47As and InP substrates have also been demonstrated.