Optimization of AlGaN-based deep ultraviolet light emitting diodes with superlattice step doped electron blocking layers.

The superlattice electron blocking layer (EBL) has been proposed to reduce the electron leakage of the deep ultraviolet light emitting diodes (DUV-LEDs). However, the hole transport is hindered by the barriers of EBL and the improvement of hole injection efficiency still suffers enormous challenges. The superlattice step doped (SLSD) EBL is proposed to improve the hole injection efficiency while enhancing the electron confinement capability. The SLSD EBL enhances the electron confinement capability by multi-reflection effects on the electron wave function. And a built-in electric field towards the active region is generated by superlattice step doping, which facilitates the transport of holes into the multiple quantum wells. The Advaced Physical Model of Semiconductor Devices (APSYS) software is used to simulate the DUV-LEDs with conventional EBL, superlattice EBL, superlattice doped EBL, and SLSD EBL. The results indicate that the SLSD EBL contributes to the increased electron concentration in the multiple quantum wells, the reduced electron leakage in the p-type region, the increased hole injection current, and the increased radiative recombination rate. When the current is 60 mA, the external quantum efficiency of DUV-LED with SLSD EBL is increased to 5.27% and the output power is increased to 13.81 mW. The SLSD EBL provides a valuable reference for solving the problems of serious electron leakage and insufficient hole injection of the DUV-LEDs.

Non-heavy doped pnp-AlGaN tunnel junction for an efficient deep-ultraviolet light emitting diode with low conduction voltage.

While traditional tunnel junction (TJ) light-emitting diodes (LEDs) can enhance current diffusion and increase hole injection efficiency, their reliance on highly doped AlGaN layers to improve hole tunneling efficiency results in a higher conduction voltage, adversely impacting LED device performance. This paper proposes a non-heavy doped pnp-AlGaN TJ deep ultraviolet (DUV) LED with a low conduction voltage. By inserting the TJ near the active region, between the electron blocking layer and the hole supply layer, the need for heavily doped AlGaN is circumvented. Furthermore, the LED leverages the polarization charge in the pnp-AlGaN TJ layer to decrease the electric field strength, enhancing hole tunneling effects and reducing conduction voltage. The non-heavy doped pnp-AlGaN TJ LED effectively enhances carrier concentration in the quantum well, achieving a more uniform distribution of electrons and holes, thus improving radiative recombination efficiency. Consequently, at an injection current of 120 A/cm2, compared to the traditional structure LED (without TJ), the proposed LED exhibits a 190.7% increase in optical power, a 142.8% increase in maximum internal quantum efficiency (IQE) to 0.85, and a modest efficiency droop of only 5.8%, with a conduction voltage of just 4.1V. These findings offer valuable insights to address the challenges of high heavy doped TJ and elevated conduction voltage in high-performance TJ DUV LEDs.

Study of ultraviolet light emitting diodes with InGaN quantum dots and lattice matched superlattice electron blocking layers.

Ultraviolet light emitting diodes (UV-LEDs) face the challenges including insufficient hole injection and severe electron leakage. Quantum dots (QDs) have been proven to provide three-dimensionally localized states for carriers, thereby enhancing carrier confinement. Therefore, UV-LEDs employing InGaN QDs are designed and studied in this paper. The APSYs software is used to simulate UV-LEDs. Simulation results indicate that the QDs effectively improve the electron and hole concentration in the active region. However, UV-LEDs with QDs experience efficiency droop due to serious electron leakage. What's more, the lattice mismatch between last quantum barrier (LQB) and electron blocking layer (EBL) leads to the polarization field, which induces the downward band bending at the LQB/EBL interface and reduces effective barrier height of EBL for electrons. The AlInGaN/AlInGaN lattice matched superlattice (LMSL) EBL is designed to suppress electron leakage while mitigating lattice mismatch between LQB and EBL. The results indicate that the utilization of QDs and LMSL EBL contributes to increasing the electron and hole concentration in the active region, reducing electron leakage, enhancing radiative recombination rate, and reducing turn-on voltage. The efficiency droop caused by electron leakage is mitigated. When the injection current is 120 mA, the external quantum efficiency is increased to 9.3% and the output power is increased to 38.3 mW. This paper provides a valuable reference for addressing the challenges of insufficient hole injection and severe electron leakage.

Optimization of AlGaN-based deep UV LED performance by p-AlInGaN/AlInGaN graded superlattice electron blocking layer.

In this paper, we significantly improved the internal quantum efficiency and output power of AlGaN-based deep UV (DUV) LEDs by replacing the conventional p-AlGaN electron blocking layer (EBL) with the p-AlInGaN/AlInGaN graded superlattice (SL) EBL. Simulation results show that the introduction of the p-AlInGaN graded SL EBL improved the carrier distribution while having the lower electric field, thus increasing the radiative recombination rate in multiple quantum wells (MQWs). The highest IQE obtained by p-AlInGaN/AlInGaN graded SL EBL is 96.6%, which is 44.9% higher than the conventional p-AlGaN EBL with no efficiency droop. At the same time, the output power is 4.6 times that of the conventional p-AlGaN EBL. It is believed that the proposed p-AlInGaN graded SL EBL will be helpful in the development of high-performance DUV LEDs.

Performance improvement of nitride semiconductor-based deep-ultraviolet laser diodes with superlattice cladding layers.

A deep-ultraviolet (DUV) laser diode (LD) consisting of specifically designed cladding layers involving superlattice nitride alloy has been proposed. Simulation studies of different cladding layers were carried out using Crosslight software. It was found that the proposed structure effectively suppresses the leakage of the optical field from the active region and the optical confinement coefficient is 1.45 times higher than that of the conventional structure. The proposed structure has a significant increase in laser power with a low threshold current. Moreover, the introduction of novel cladding layer suppresses the electron and hole leakage from the multiple quantum well (MQW) region, which provides an attractive solution for increasing the stimulated recombination rate in the MQW region leading to the improvement in the performance of the DUV LD.

Enhanced performance in deep-ultraviolet laser diodes with an undoped BGaN electron blocking layer.

Aluminum-rich p-AlGaN electron blocking layers (EBLs) are typically used for preventing overflow of electrons from the active region in AlGaN-based deep ultraviolet (DUV) laser diode (LD). However, these cannot effectively prevent electron leakage and form barrier layers, which affects the hole injection efficiency. Herein, the traditional p-AlGaN EBL in LD is replaced with an undoped BGaN EBL. The undoped BGaN EBL LD increases the effective barrier height of the conduction band to prevent the leakage of electrons and decreases the energy loss caused by the polarization induced electric field, enhancing the hole injection. The slope efficiency of the undoped BGaN EBL LD is 289% higher than that of the highly doped AlGaN EBL LD, and its threshold current is 51% lower. Therefore, the findings of this study provide insights for solving the problems of electron leakage and insufficient hole injection in high-performance and undoped EBL DUV LDs.

Growth of Tellurium Nanobelts on h-BN for p-type Transistors with Ultrahigh Hole Mobility.

The lack of stable p-type van der Waals (vdW) semiconductors with high hole mobility severely impedes the step of low-dimensional materials entering the industrial circle. Although p-type black phosphorus (bP) and tellurium (Te) have shown promising hole mobilities, the instability under ambient conditions of bP and relatively low hole mobility of Te remain as daunting issues. Here we report the growth of high-quality Te nanobelts on atomically flat hexagonal boron nitride (h-BN) for high-performance p-type field-effect transistors (FETs). Importantly, the Te-based FET exhibits an ultrahigh hole mobility up to 1370 cm2 V-1 s-1 at room temperature, that may lay the foundation for the future high-performance p-type 2D FET and metal-oxide-semiconductor (p-MOS) inverter. The vdW h-BN dielectric substrate not only provides an ultra-flat surface without dangling bonds for growth of high-quality Te nanobelts, but also reduces the scattering centers at the interface between the channel material and the dielectric layer, thus resulting in the ultrahigh hole mobility .

Compositionally graded AlGaN hole source layer for deep-ultraviolet nanowire light-emitting diode without electron blocking layer.

The electron blocking layer (EBL) plays a vital role in blocking the electron overflow from an active region in the AlGaN-based deep-ultraviolet light-emitting diode (DUV-LED). Besides the blocking of electron overflow, EBL reduces hole injection toward the active region. In this work, we proposed a DUV nanowire (NW) LED structure without EBL by replacing it with a compositionally continuous graded hole source layer (HSL). Our proposed graded HSL without EBL provides a better electron blocking effect and enhanced hole injection efficiency. As a result, optical power is improved by 48% and series resistance is reduced by 50% with 4.8 V threshold voltage. Moreover, graded HSL without EBL offer reduced electric field within the active region, which leads to a significant increment in radiative recombination rate and enhancement of spontaneous emission by 34% at 254 nm wavelength, as a result, 52% maximum internal quantum efficiency with 24% efficiency drop is reported.

Improving ESD Protection Robustness Using SiGe Source/Drain Regions in Tunnel FET.

Currently, a tunnel field-effect transistor (TFET) is being considered as a suitable electrostatic discharge (ESD) protection device in advanced technology. In addition, silicon-germanium (SiGe) engineering is shown to improve the performance of TFET-based ESD protection devices. In this paper, a new TFET with SiGe source/drain (S/D) regions is proposed, and its ESD characteristics are evaluated using technology computer aided design (TCAD) simulations. Under a transmission line pulsing (TLP) stressing condition, the triggering voltage of the SiGe S/D TFET is reduced by 35% and the failure current is increased by 17% in comparison with the conventional Si S/D TFET. Physical insights relevant to the ESD enhancement of the SiGe S/D TFET are provided and discussed.

Application of multiscale Poincaré short-time computation versus multiscale entropy in analyzing fingertip photoplethysmogram amplitudes to differentiate diabetic from non-diabetic subjects.

BACKGROUND AND OBJECTIVES: Multiscale Poincaré (MSP) plots have recently been introduced to facilitate the visualization of time series of physiological signals. This study aimed at investigating the feasibility of MSP application in distinguishing subjects with and without diabetes. METHODS: Using photoplethysmogram (PPG) waveform amplitudes acquired from unilateral fingertip of non-diabetic (n = 34) and diabetic (n = 30) subjects, MSP indices (MSPI) of the two groups were compared using 1000, 500, 250, 100 data points. Data from Poincaré index (short-term variability/long-term variability [i.e. SD1/SD2] ratio, SSR) and multiscale entropy (MSE) were also obtained with the four corresponding data points for comparison. RESULTS: SSR and MSPI were both negatively related to glycated hemoglobin (HbA1c) and fasting blood sugar levels. Significant negative correlation was also noted between MSPI and pulse pressure. When only 500 and 250 data points were included, significant elevations in the non-diabetic group were only noted in MSPI (both p < 0.01). Furthermore, MSPI was significantly higher in non-diabetic than that in diabetic subjects on all scales (i.e., 1-10) but not using MSE when utilizing 1000 data points. CONCLUSIONS: The results demonstrated enhanced sensitivity of MSP in differentiating between non-diabetic and diabetic subjects compared to SSR and MSE, highlighting the feasibility of MSP application in biomedical data analysis to reduce computational time and enhance sensitivity.