Trap-assisted Tunneling Mechanism Research Articles

Trap-assisted tunneling in type II Ag2O/β-Ga2O3 self-powered solar blind photodetector

This study investigates a high-performance Ag2O/β-Ga2O3 self-powered photodiode through experimental and modeling approaches. Initially, a p-type Ag2O film, with a bandgap close to 4 eV and a hole density of approximately 6.35×1018 cm−3, was fabricated using faced two-target sputtering. The conduction and valence band offsets between Ag2O and β-Ga2O3 were determined via X-ray photoelectron spectroscopy, confirming a type II heterojunction. The device had a low on-voltage of 1.50 V and a low on-resistance of 5.40 mΩ.cm² in the dark. Subsequent illumination at 254 nm resulted in a notably high photocurrent, responsivity, and detectivity. To confirm the role of trap-assisted tunneling in the type II Ag2O/β-Ga2O3 heterojunction, Silvaco simulations were employed to model both the dark current and photocurrent. These simulations confirmed the prevalence of the trap-assisted tunneling mechanism, particularly through energy levels situated above the equilibrium Fermi level at the Ag2O/β-Ga2O3 interface, as described by the Danielsson model. Understanding the transport mechanism is paramount for the development of high-performance photodetectors. By comprehending how charge carriers navigate through the device and the influence of traps on their behavior, researchers can optimize device design and fabrication processes to enhance performance.

Encapsulation of Er-doped GeO2 nanofilms by Al2O3 atomic layers: Surficial sensitivity to trimethylaluminum and improvement on electroluminescence

Based on previous results of the Er-doped GeO2 nanofilms fabricated by atomic layer deposition (ALD), the interfacial reactions of Al2O3 precursor within the GeO2/Al2O3:Er nanolaminates are investigated. The ALD pulse of Al(CH3)3 etches the as-deposited GeO2 nanofilm, while the oxidation of Al(CH3)3 by ozone realizes the deposition of atomic Al2O3 layer onto GeO2 surface. The Al2O3 atomic layers with ≥4 ALD cycles improve the thermal stability of the ALD GeO2 nanofilms, which restrain the formation of nanograins and also the reduction of film thickness during high-temperature annealing, giving rise to smooth GeO2/Al2O3:Er nanolaminates. For the 1530 nm electroluminescence (EL) from the silicon-based devices fabricated using the GeO2/Al2O3:Er nanolaminates annealed at 800 °C, the excitation efficiency is tripled to 22.1 %. The conduction of the energetic electrons resulting in the EL emissions still confirms to the trap-assisted tunneling mechanism within these amorphous GeO2/Al2O3:Er nanolaminates. The prototype devices based on the Al2O3-inserted nanolaminates exhibit comparatively increased operation time. These GeO2/Al2O3:Er nanolaminates with enhanced stability and improved optoelectronics performance pave the way for the practical utilization of ALD GeO2 nanofilms.

Carrier conduction mechanisms in MIS capacitors with ultra-thin Al2O3 at cryogenic temperatures

In this work, a study comprising the electrical characterization and analysis of the electrical response of metal–insulator–semiconductor Al/Al2O3/Si capacitors in a temperature range from ambient temperature down to 3.6 K is presented. An ultra-thin 6 nm Al2O3 film, deposited by atomic layer deposition, was used as an insulating layer. Current–voltage and electrical stress measurements were performed on the capacitors in the specified temperature range, and the experimental data obtained were analyzed using current transport equations to model the conduction mechanisms that allow charge transport through the Al2O3. Energetic parameters associated with trap levels within the Al2O3 bandgap corresponding to the Poole–Frenkel emission and trap-assisted tunneling mechanisms were obtained, and their temperature dependances were studied and associated with the presence of physical material defects. The analysis of the modeling results points to trap-assisted tunneling as the dominant mechanism at low temperatures for intermediate electric field values. Additional phenomena that limit charge transport were also observed, such as charge trapping in the bulk of the Al2O3 upon the application of electrical stress at ambient temperature and silicon freeze out at cryogenic temperatures. Our findings constitute an effort at understanding the physical phenomena that govern the electrical behavior of thin-film Al2O3-based capacitors, especially at cryogenic temperatures, given that these materials and devices are of considerable importance for applications in CMOS-based cryoelectronics and quantum technologies, among others.

Temperature and field dependencies of current leakage mechanisms in IrOx contacts on InAlN/GaN heterostructures

This Letter reports on the mechanisms of reverse leakage current transport in InAlN/GaN heterostructure Schottky diodes with intentionally oxidized iridium oxide (IrOx) contacts across a wide temperature range. Current–voltage characteristics were experimentally measured from 25 to 500 °C (≈ 300 to 773 K). Three distinct regions in the reverse bias regime of operation and their corresponding dominant current transport mechanisms are identified. A trap-assisted tunneling mechanism is observed at low reverse bias, and trap energy levels are between 1.12 and 1.99 eV. At medium reverse bias, Poole–Frenkel emission is decomposed into low-field, mid-field, and high-field regions and the related trap activation energies vary from 0.38 to 2.04 eV. At high reverse bias, the Fowler–Nordheim model is applied and the effective barrier height to tunneling is 0.78 eV. The model of the reverse leakage current constructed using the parameters associated with these transport mechanisms closely aligns with the experimental data and supports the advancement of high-temperature electronics based on IrOx -gated InAlN/GaN heterostructure technology.

Interfacial trap charge and self-heating effect based reliability analysis of a Dual-Drain Vertical Tunnel FET

This manuscript exclusively addresses the reliability concern of a double-drain vertical TFET (DD-VTFET) by analysing the influence of interface trap charges and variation in ambient temperature on various DC and analog/RF figure of merits. For detailed analysis, performance of the device is evaluated by considering different densities and polarities of trap charges existing at gate-oxide/channel interface. TCAD-based simulation results reveal that trap charges significantly alter the flat-band voltage, which further causes variation in the device performance. It is observed from the transfer characteristic curve that the leakage current is degraded from 10−18 to 10−13A/μm when a positive (donor) charge density of 1013 cm−2 is introduced at oxide/channel interface. However, cut-off frequency is found to be improved under the influence of donor traps, which is mainly attributed due to improvement in transconductance of the device. Moreover, simulation results indicate that DD-VTFET is immune to the negative (acceptor) traps in comparison with the positive traps. Furthermore, the analysis carried out for temperature affectability on the device performance suggests that, at higher ambient temperature, the off-state current degrades more in subthreshold region (at low gate bias) due to dominance of the Shockley-Read-Hall (SRH) and trap-assisted tunneling (TAT) mechanisms, which exponentially depend on the temperature. Therefore, the leakage current increases approximately by an order 7 when temperature is increased from 200 K to 400 K. Since band-to-band tunneling starts dominating over SRH and TAT in the super-threshold region (at high gate bias), the impact of temperature variation along with presence of the trap charges is found to be insignificant on the transfer characteristics.

Design and modeling of high-performance mid-wave infrared InAsSb-based nBn photodetector using barrier band engineering approaches

We report a new nBn photodetector (nBn-PD) design based on the InAlSb/AlSb/InAlSb/InAsSb material systems for mid-wavelength infrared (MWIR) applications. In this structure, delta-doped compositionally graded barrier (δ-DCGB) layers are suggested, the advantage of which is creation of a near zero valence band offset in nBn photodetectors. The design of the δ-DCGB nBn-PD device includes a 3 µm absorber layer (n-InAs0.81Sb0.19), a unipolar barrier layer (AlSb), and 0.2 μm contact layer (n-InAs0.81Sb0.19) as well as a 0.116 µm linear grading region (InAlSb) from the contact to the barrier layer and also from the barrier to the absorber layer. The analysis includes various dark current contributions, such as the Shockley–Read–Hall (SRH), trap-assisted tunneling (TAT), Auger, and Radiative recombination mechanisms, to acquire more precise results. Consequently, we show that the method used in the nBn device design leads to diffusion-limited dark current so that the dark current density is 2.596 × 10−8 A/cm2 at 150 K and a bias voltage of − 0.2 V. The proposed nBn detector exhibits a 50% cutoff wavelength of more than 5 µm, the peak current responsivity is 1.6 A/W at a wavelength of 4.5 µm and a − 0.2 V bias with 0.05 W/cm2 backside illumination without anti-reflective coating. The maximum quantum efficiency at 4.5 µm is about 48.6%, and peak specific detectivity (D*) is of 3.37 × 1010 cm⋅Hz1/2/W. Next, to solve the reflection concern in this nBn devices, we use a BaF2 anti-reflection coating layer due to its high transmittance in the MWIR window. It leads to an increase of almost 100% in the optical response metrics, such as the current responsivity, quantum efficiency, and detectivity, compared to the optical response without an anti-reflection coating layer.Graphical

Electroluminescence from Silicon-Based Light-Emitting Devices with Erbium-Doped ZnO Films: Strong Enhancement Effect of Titanium Codoping.

We report on the visible and near-infrared electroluminescence (EL) from the light-emitting device (LED) based on the erbium (Er)-doped ZnO (ZnO:Er)/SiO2/n+-Si heterostructure, wherein an ∼10 nm thick SiO2 intermediate layer serves as the energy plateau for producing hot electrons, which come from n+-Si via the trap-assisted tunneling mechanism. These hot electrons excite the doped Er3+ ions by inelastic collision, enabling the Er-related EL from the aforementioned LED. More importantly, by means of codoping the appropriate content of titanium (Ti) into the ZnO:Er film, the aforementioned Er-related emissions can be significantly enhanced. The density functional theory calculations indicate that the Ti-codoping improves rather than degrades the symmetry of the crystal field around the optically active Er3+ ions, hence not increasing the intra-4f transition probabilities of Er3+ ions. However, it is found that Ti-codoping nearly eliminates the segregation of Er3+ ions near the ZnO/SiO2 interface. Moreover, Ti-codoping is derived to result in a number of Zn vacancies, which provide the sites for incorporating Er3+ ions in the ZnO matrix. For the above two reasons, the Ti-codoping promotes the incorporation of optically active Er3+ ions into the ZnO matrix, thus enhancing the EL from the aforementioned LED.

MBE-Grown Hybrid Axial Core–Shell n-i-p GaAsSb Heterojunction Ensemble Nanowire-Based Near-Infrared Photodetectors up to 1.5 μm

In this paper, high-performance self-assisted molecular beam epitaxy (MBE)-grown conventional core–shell (C–S) n-i-p GaAsSb nanowires (NWs) and a novel hybrid axial C–S n-i-p GaAsSb ensemble NW-based near-infrared photodetector (NIRPD) on nonpatterned Si substrate are demonstrated. The conventional room-temperature (RT) C–S n-i-p GaAsSb NW with a high responsivity of 190 A/W and a higher detectivity of 1.1 × 1014 Jones at −1 V bias and wavelength of 1.1 μm is reported by optimizing the intrinsic region thickness and appropriately compensating the intrinsic p-type behavior with n-dopant Te. Furthermore, hybrid axial C–S n-i-p GaAsSb has been band-gap-engineered for wavelength up to 1.5 μm, exhibiting responsivity of 18 A/W and detectivity of 1.1 × 1013 Jones operating at RT. In this hybrid design, we have combined both axial and radial intrinsic (i-) segments of different Sb% compositions to enhance the photoabsorption in the NIR region; hence, the photogenerated current and also the high-band-gap axial i-region help to suppress the trap-assisted tunneling mechanism, which is found to be advantageous over conventional C–S NW architectures. In addition, high rectification ratio from current–voltage (I–V) measurements, suppression of low-frequency noise, lack of 1/f noise, a low corner frequency of ∼2.5 Hz beyond which there is the presence of only frequency-independent white noise from low-frequency noise (LFN) measurements, and bias- and frequency-dependent capacitance–voltage (C–V) measurements suggest the formation of a high-quality C–S junction in the hybrid structure. Thus, our findings reveal that the hybrid axial C–S NW architecture provides the flexibility of three-dimensional (3D) design, which offers an unprecedented prospect for expanding IRPD and other next-generation optoelectronic device applications.

Reverse gate leakage mechanism of AlGaN/GaN HEMTs with Au-free gate

The reverse gate leakage mechanism of W-gate and TiN-gate AlGaN/GaN high-electron-mobility transistors (HEMTs) with N2 plasma surface treatment is investigated using current–voltage (I–V) and capacitance–voltage (C–V) characteristics and theoretical calculation analysis. It is found that the main reverse gate leakage mechanism of both devices is the trap-assisted tunneling (TAT) mechanism in the entire reverse bias region (–30 V to 0 V). It is also found that the reverse gate leakage current of the W-gate AlGaN/GaN HEMTs is smaller than that of the TiN gate at high reverse gate bias voltage. Moreover, the activation energies of the extracted W-gate and TiN-gate AlGaN/GaN HEMTs are 0.0551 eV–0.127 eV and 0.112 eV–0.201 eV, respectively.

Characterization of thin Al2O3/SiO2 dielectric stack for CMOS transistors

This paper systematically investigates the electrical properties of thin Al2O3/SiO2 (with a target equivalent oxide thickness of 4.9 nm) as gate dielectric stack in the metal–oxide–semiconductor (MOS) capacitor. Different deposition techniques, i.e. thermal oxidation, thermal atomic layer deposition, and plasma-enhanced atomic layer deposition, are employed in the fabrication of MOS capacitors. The second derivative, Terman and conductance methods are used to extract the fixed oxide charges and interface trap densities in the MOS capacitors. Our results demonstrate that the Al2O3/SiO2 stack presents better performance in terms of negative fixed oxide charges and low interface trap density compared with the single-layer SiO2 with the similar equivalent oxide thickness about 3.9 nm. Furthermore, to evaluate the reliability of the Al2O3/SiO2 gate dielectric stack, the leakage current is analyzed. Contributions from Pool-Frenkel emission, trap-assisted tunneling, and Fowler-Nordheim mechanisms to the leakage current are detailed. Our results indicate that Al2O3/SiO2 gate dielectric stacks fabricated by plasma-enhanced atomic layer deposition Al2O3 and thermal SiO2 feature the lowest leakage current.

A Study on Dominant Mechanism and Analytical Model of Low-Frequency Noise in FD-SOI pTFET

The origin of low-frequency noise (LFN) properties and the accuracy of LFN model were demonstrated in a fully depleted silicon-on-insulator p-type tunneling field-effect transistor (pTFET). We demonstrated that the origin of LFN properties in pTFET can be deduced and analyzed via the current fluctuation induced by tunneling in the pTFET operation region. Since the trap sites near the tunneling junction contributed to LFN, pTFET must consider the tunneling junction characteristics and channel transportation influences. The tunneling path was not formed at low |VGS| and the carrier transport in pTFET crossed the energy band by Shockley-Read-Hall (SRH) generation-recombination or the trap-assisted tunneling (TAT) mechanism and not through band-to-band tunneling (BTBT). However, with the increase in |VGS|, the tunneling path was formed and the carriers could flow through the energy band by BTBT. The correlation between the simulated model data and experimental data was examined, confirming that the proposed small-signal model accurately represents noise in pTFETs.

Crystallization and electroluminescence performance of the Er-doped polycrystalline Gd3Ga5O12 nanofilms fabricated by atomic layer deposition on silicon

Er-doped Gd2O3/Ga2O3 nanolaminates are fabricated by atomic layer deposition on silicon, which transform to polycrystalline Gd3Ga5O12 garnet (GGG) nanofilms after annealing above 900 °C. The Gd/Ga ratio approaching stoichiometry and Ga2O3 interlayers thinner than 1.5 nm are essential for the gallium garnet crystallization after annealing, while resulting in better film morphology in optoelectronic utilization, otherwise the Ga2O3 interlayers tend to segregate into micrometer grains on the surface. The impact-excitation of the doped Er3+ ions by hot electrons within the nanolaminate GGG:Er devices results in the characteristic Er3+ electroluminescence (EL) centered at 1530 nm, exhibiting the external quantum efficiency of 1.9% and decay lifetime of 1.27–2.47 ms. Stronger EL emissions and longer lifetime are obtained from the devices with the stoichiometry Gd/Ga ratio and better morphology. The electron conduction within the GGG matrix is ascribed to the trap-assisted tunneling mechanism above threshold electric field. This work further explores the deposition of nanolaminates with designed component and crystallinity, and the possibility of fabricating optoelectronic devices from Si-based garnet nanofilms.

Electroluminescence from the light-emitting devices with erbium-doped SrTiO3 films on oxidized silicon substrate

We report on the visible and near-infrared (NIR) electroluminescence (EL) from the light-emitting devices (LEDs) with erbium-doped SrTiO3 (SrTiO3:Er) films on oxidized silicon substrate, namely, with the structure of SrTiO3:Er/SiO2/Si in abbreviation. It is found that the 900 °C-annealed SrTiO3:Er films are most desirable for the SrTiO3:Er/SiO2/Si structured LEDs. In the EL-enabling voltage range, the electron transportation through the SiO2 layer of SrTiO3:Er/SiO2/Si structured LED is revealed to be compliant with the trap-assisted tunneling (TAT) mechanism. Based on the energy band diagram of the SrTiO3:Er/SiO2/Si structured LED under the EL-enabling forward bias where the negative voltage is connected with Si substrate, it is believed that a number of conduction electrons in Si can tunnel into the conduction band of SiO2 layer via the aforementioned TAT mechanism and then jump down to the conduction band of SrTiO3 host. Because the conduction band offset of SiO2 and SrTiO3 host is larger than 3 eV, the conduction electrons reaching the SrTiO3 host gain considerably high energies so that they become the so-called hot electrons. The Er3+ ions incorporated into the SrTiO3 host are impact-excited by the hot electrons, ultimately emanating the characteristic visible and NIR light.

Modeling dislocation-related reverse bias leakage in GaN p–n diodes

Finite element analysis software was used to model and visualize two p-n junction models: one with a single threading dislocation (TD) and a control without one. TDs are modeled as a Gaussian distribution of trap states with a full width at half maximum value of 5 nm localized around the line in a cylindrical coordination such that the linear trap state density was 1 trap c−1-translation; this model allows the cylindrical symmetry of the c-plane GaN crystal orientation to be used to avoid more computationally intensive 3D models. In this work, a vertical p–n diode with typical doping characteristics and an equivalent threading dislocation density of was modeled in reverse bias. Our simulations show that the dislocation-mediated leakage mechanism for reverse bias leakage in GaN p–n diodes is the generation of electron–hole pairs via a trap-assisted tunneling mechanism whereby electrons from the valence band use the intermediate trap state to traverse the band gap. This mechanism results in electron–hole pairs that are swept out of the junction by the reverse bias electric field. This behavior results in a measurable leakage current within the model with behavior consistent with experimental values.

The reverse bias current–voltage–temperature (I–V–T) characteristics of the (Au/Ti)/Al2O3/n-GaAs Schottky barrier diodes (SBDs) in temperature range of 80–380 K

We analyzed current conduction mechanisms (CCMs) of the (Au/Ti)/Al2O3/n-GaAs (MIS) type SBDs in the wide temperature range (80–380 K) by 30 K steps using reverse bias current-voltage (IR–VR) characteristics. In general, the values of barrier-height (BH) obtained from the forward bias current–voltage (IF–VF) and reverse bias current-voltage (IR–VR) characteristics increase with increasing temperature and this change is in-agreement with the reported negative-temperature coefficient bandgap of semiconductor (α = ΔEg/ΔT) or BH for ideal SDs. However, the change in BH at lower temperatures becomes more pronounced. The value of BH obtained from the IR–VR data is lower than that obtained from IF–VF especially under room temperatures. The necessary barrier height (Φt) for electron emission from the trap state was obtained from the ln (IR) − E0.5, (E = VR/d), plot as 1.25 eV to (at 80 K) 0.91 eV (at 380 K), respectively. This change in the Φt with temperature is in agreement with the reported α value of the Eg for GaAs. The reverse leakage current mechanism in this study was assessd for various temperatures using Schottky emission (SE), Poole–Frenkel emission (PFE), Trap-Assisted Tunneling (TAT), and ohmic conduction mechanisms. The evaluation of the IR–VR characteristics shows that the ohmic and TAT are the most dominant conduction mechanisms rather than others.

Experimental Verification of Current Conduction Mechanism for a Lithium Niobate Based Memristor

This work presents electrical characterization and analysis of the dominant charge transport mechanism suggesting inhomogeneous, filamentary conduction for a lithium niobate switching layer based memristor for use in neuromorphic computing. Memristor conductivity has been investigated both for the high and low resistance states. It is suggested that when the device is in a high resistance state, deep trap energy level within the switching layer initiate the device conduction process. The elastic trap assisted tunneling mechanism with a simple steady state approach agrees with the experimental measurements in the high resistance state. This work considers existence of inhomogeneously distributed positively charged oxygen ions/vacancies (within the oxygen deficient switching layer) as the deep trap energy level, required for electron tunneling from memristor electrode. Alternatively, ohmic conduction was found to be the main mechanism for the memristor on state conductivity at room temperature. Existence of intermediate resistive states in the memristor’s high resistive region was experimentally investigated and the elastic trap assisted tunneling mechanism for such phenomena was validated through simulation.

Temperature dependence improvement of polycrystalline-silicon tunnel field-effect thin-film transistor

Trap-assisted tunneling (TAT) mechanism dominates the subthreshold leakage current and ambipolar transport behavior of tunnel field-effect transistor (TFET) with polycrystalline-silicon (poly-Si) channel. The unwanted ambipolar transport behavior of poly-Si TFET can be suppressed by the employment of gate-to-drain underlap structure to increase the tunneling distance near the drain side when the TFET is operated at the negative gate voltage. The gate-to-drain underlap length of TFET would also exhibit series resistance effect and strong temperature effect on the on-state current (ION). The strong temperature dependent series resistance effect and bandgap narrowing effect cause the poly-Si TFET to exhibit temperature instability of the on-state current. In order to improve the temperature instability of the on-state current, the ammonia plasma treatment has been performed to passivate the defects and reduce the trap state density of poly-Si film. The trap state density reduction of poly-Si can improve the field-effect mobility of transport carrier and reduce the series resistance effect, resulting in the reduction of the temperature dependence of ION. Consequently, the improved temperature dependent ION of poly-Si TFET can be obtained, and it is useful for the development of poly-Si thin-film transistor and its applications in the high resolution display industry and three-dimensional integrated-circuit.

Effect of Nitrogen Content in Tunneling Dielectric on Cell Properties of 3-D NAND Flash Cells

The effect of band engineering by controlling nitrogen content in the tunneling dielectric of the 3-D NAND flash cells is investigated by comparing various cell properties of the two devices. These measured devices have the same device dimensions and different nitrogen contents. The device with higher nitrogen content shows larger trap density profile and improved erase characteristics. After the same P/E cycling stress, the device with higher nitrogen content shows larger increase of trap density and more significant degradation in its erase characteristics. The electrons stored in the device with higher nitrogen content more easily escape through the trap-assisted tunneling mechanism due to higher trap density, resulting in worse retention characteristics at room temperature.

Barrier engineering for HgCdTe unipolar detectors on alternative substrates

Delta-doped layers together with compositionally grading have been utilized to get nBn configurations for the HgCdTe material system in all the short-wave (SWIR), medium-wave (MWIR) and long-wave (LWIR) infrared bands. Shockley Read Hall (SRH), trap-assisted tunneling (TAT), Auger and radiative recombination mechanisms have been included in the analyses and strong suppression of SRH and TAT currents have been demonstrated with the designed structures. This methodology is especially useful when the carrier lifetime is limited due to alternative substrate usage. No degradation in photo-response has been observed as adjusting the valence band offset is quite flexible with the delta-doped nano-layers and the valence band barrier can be completely removed. Calculations have been performed for 1–3 μs lifetime targeting the alternative substrate applications and up to 60 degrees of increase in the operation has been shown to be possible.

Analysis injection area-dark current characteristics for mid-wavelength HgCdTe photodiodes

In this paper, we study the relationship between dark current mechanism and the injection area of mid-wavelength infrared (MWIR) HgCdTe photovoltaic detectors. A simultaneous-mode nonlinear fitting program for n-on-p mid-wavelength HgCdTe infrared detectors is reported. It is found that the impact of diffusion mechanism gradually weakens and the effect of generation-recombination mechanism becomes more significant as the area of injection increasing under forward bias. The effect of trap-assisted tunneling mechanism gradually weakens as the area of injection increasing under middle reverse bias and band-to-band tunneling mechanism has less impact on dark current of MWIR HgCdTe photodiodes. And as the area of injection increasing, the effect of surface leak mechanism is gradually decrease. Finally, we find the reversed welding pressure and the arrangement of common electrode for MWIR HgCdTe Photodiodes also impacts diffusion mechanism and generation-recombination mechanism under forward bias.