Multiple Quantum Barrier Research Articles

Improved hole injection for AlGaN-based DUV LEDs with graded-composition multiple quantum barrier insertion layers

Improved hole injection for AlGaN-based DUV LEDs with graded-composition multiple quantum barrier insertion layers

Quaternary AlInGaN/AlInGaN Polarization-Engineered Green Lights-Emitting Diodes by Using Inverted Trapezoidal Quantum Barriers

In this paper, a structure design using grading quaternary AlInGaN as inverted trapezoidal multiple quantum barriers is shown. This paper proposes an inverted trapezoidal structure in quantum barriers to decelerate the transmission of the electron which will enhance efficient carrier injection into the quantum wells and get a more uniform distribution of carriers. The polarization-matched AlInGaN/AlInGaN structures have less polarization and increase the overlap of electron and hole wave function, effectively improving the internal quantum efficiency and wall-plug efficiency, and making the In composition adjustable. The results show that the Inverted trapezoidal polarization-matched AlInGaN structure significantly improves the phenomenon of “green gap” and “green droop.” As a result, the new structure mitigates the droop ratio from 25.5% to 7%, and the LED power is increased from 59.95 mW to 109.68 mW at 200 mA. Finally, the device exhibits a uniform current distribution and greater radiative recombination while reducing resistance.

Spin transport properties of phosphorene multiple quantum barrier structure under strain and Rashba spin-orbit interaction

The effect of the strain and Rashba spin-orbit coupling (RSOC) on the spin-dependent transport properties, is investigated in monolayer phosphorene with multiple quantum barrier structure. It is found that the spin-dependent conductance, without and with the spin-flip, has an oscillatory behavior with the RSOC, and the oscillating feature of the conductance increases by increasing the number of the barriers. The spin-dependent conductance presents high sensitivity to strain. The results showed that the spin-dependent conductance in terms of the strain diminished with increasing the number of the barriers. It is demonstrated that the full spin-polarized current can be realized in monolayer phosphorene with multiple quantum barrier structure under strain. Specifically, the magnitude and sign of the spin polarization can be efficiently adjusted mechanically and by an external electric field. These transport properties indicate that the phosphorene multiple quantum barrier structure can be an ideal candidate for future spintronics and straintronic applications.

Optoelectronic properties of multiple quantum barriers nano-scale avalanche photo diodes

The important optoelectronic properties like spectral response, excess noise characteristics, time and frequency response of multiple quantum barrier (MQB) nano-scale avalanche photodiodes (APDs) based on Si~3C-SiC material system have been studied. A self-consistent simulation method based on quantum drift-diffusion model has been presented. Simulation results show that the Si~3C-SiC MQB nano-APDs are capable of detecting significantly longer wavelengths as compared to infrared flat Si APDs. The multiplication gain and excess noise factor (ENF) of the MQB APDs have been calculated by varying the number of quantum barriers (QBs). The numerically calculated ENF values of MQB APDs have been compared with the ENF of Si flat conventional APDs of similar dimensions and it is observed that the use of QBs leads to significant reduction in ENF of the APDs. Simulation results also show that MQB nano-APDs possess significantly faster time response and wider frequency response as compared to the Si counterparts.

Greatly enhanced performance of AlGaN-based deep ultraviolet light emitting diodes by introducing a polarization modulated electron blocking layer.

Carrier transport in AlGaN-based deep ultraviolet (DUV) light emitting diodes (LEDs) with the wavelength of 273 nm has been investigated by introducing polarization modulated electron blocking layer (EBL) that adopts an Al composition and thickness graded multiple quantum barriers (MQB) structure. The experimental result shows that the maximum light output power and external quantum efficiency for the proposed structure at the current of 250 mA are 9.6 mW and 1.03% respectively, severally increasing by 405% and 249% compared to traditional one, meanwhile, the efficiency droop at 250 mA is also dramatically reduced from 42.2% to 16.6%. Further simulation analysis indicates that this graded MQB-EBL enhances the potential barrier height for electrons and meanwhile reduces that for holes, hence effectively suppresses the electron leakage, and at the same time significantly improves the hole injection efficiency. As a result, the whole performance of the LED with the proposed MQB-EBL is dramatically improved.

Multiple Quantum Barrier Nano-avalanche Photodiodes - Part I: Spectral Response

Background: The spectral response of Multiple Quantum Barrier (MQB) nano-scale avalanche photodiodes (APDs) based on Si~3C-SiC material system shows considerable responsivity of the device within a very wide wavelength range which includes some portion of Ultra- Violet (UV) spectrum (200- 90 nm), visible spectrum (390-770 nm), near-infrared (700-1400 nm), short-wavelength infrared (1400-3000 nm) and mid-infrared (3000-4000 nm) wavelengths. It has already been concluded from preceding studies that Si~3C-SiC MQB APDs shows better spectral response and excess noise characteristics as compared to equivalent conventional APDs based on Si. Moreover, the superiority of the illumination through p+-side (ITPS) structure has been observed among two probable optical illumination configurations such as illumination through n+- side (ITNS) and illumination through p+-side (ITPS) structures. Methods: In this paper, the time and frequency responses of Si~3C-SiC MQB APDs have been investigated. A very narrow rectangular pulse of pulse-width of 0.4 ps has been used as the input optical pulse having 850 nm wavelength incident on the p+-side of the MQB APD structures (i.e. ITPS is considered here) and corresponding current responses have been calculated by using a rigorous simulation method developed by the authors; finally the frequency responses of the devices are obtained via the Fourier transform of the corresponding pulse current responses in time domain. Results: The width of the current responses are limited to 4.7 and 3.1 ps in Si nano-APD and Si~3C-SiC MQB (consisting of five quantum barriers) nano-APD respectively for the input optical pulse of width 0.4 ps of 850 nm wavelength. On the other hand, the 3 dB upper cut-off frequencies of the above-mentioned diodes are obtained to be 68.63 and 82.64 GHz respectively. Conclusion: Simulation results show that MQB nano-APDs possess significantly faster time response and wider frequency response as compared to the flat Si nano-APDs under similar operating conditions.

Multiple Quantum Barrier Nano-avalanche Photodiodes - Part II: Excess Noise Characteristics

Background: Excess noise characteristics of Multiple Quantum Barrier (MQB) nanoscale avalanche photodiodes (APDs) based on Si~3C-SiC heterostructures have been studied in this part of the paper. The multiplication gain and Excess Noise Factor (ENF) of the MQB APDs have been calculated by varying the number of Quantum Barriers (QBs). Methods: The numerically calculated ENF values of MQB APDs have been compared with the ENF of Si flat conventional APDs of similar dimensions and it is observed that the use of QBs leads to significant reduction in ENF of the APDs under similar biasing and illumination conditions. Results: The enhanced ratio of hole to electron ionization rates in MQB structures as compared to the bulk Si APD structure has been found to be the primary cause of improvement in the noise performance of the MQB nano-APDs. Conclusion: Finally, the numerically calculated ENF of Si flat APD has been compared with the experimentally measured ENF of a commercially available Si APD and those are found to be in good agreement; this comparison validates the simulation methodology adopted by the authors in this paper.

Multiple Quantum Barrier Nano-avalanche Photodiodes - Part III: Time and Frequency Responses

Background: The time and frequency responses of Multiple Quantum Barrier (MQB) nano-scale Avalanche Photodiodes (APDs) based on Si~3C-SiC material system have been investigated in this final part. Methods: A very narrow rectangular pulse of pulse-width of 0.4 ps has been used as the input optical pulse having 850 nm wavelength incidents on the p+-side of the MQB APD structures and corresponding current responses have been calculated by using a simulation method developed by the authors. Results: Finally the frequency responses of the devices are obtained via the Fourier transform of the corresponding pulse current responses in time domain. Conclusion: Simulation results show that MQB nano-APDs possess significantly faster time response and wider frequency response as compared to the flat Si nano-APDs under similar operating conditions.

Improvement of Output Power of AlGaN-Based Ultraviolet Light Emitting Diodes with Sawtooth Barriers

A special structure with Al-composition graded barriers in the active region was designed to provide a sawtooth layer for the multiple quantum barriers, which were investigated numerically. The simulation demonstrates that the output power of optimized structure has reached to 50 mW and the efficiency droop also has significantly improved. By detailedly analyzing the results, the advantages of the ultraviolet light emitting diodes with sawtooth barriers are attributed to the design, which could enhance the ability of electrons reservoir, modulate carrier distribution and suppress electron spill out from the active region. As a result, it can enhance the rate of carrier radiation recombination and ameliorate internal quantum efficiency.

Numerical study of enhanced performance in ZnO-based ultraviolet light-emitting diodes with step graded-composition MgZnO multiple quantum barriers

ZnO-based ultraviolet (UV) light-emitting diodes (LEDs) are attractive for potential applications. In the study, ZnO-based UV LEDs were numerically investigated to determine the effects of the different step graded Mg compositions in multiple quantum barriers (QBs) on their electrical and optical properties. With the increase of Mg composition for QBs from n-side to p-side, the maximum internal quantum efficiency of various LEDs increase from 32.6% to 91.9%. These improvements can be explained in terms of the modified energy band structures which improve hole injection from p-side to the active region and change the carrier-concentration distributions in multiple quantum wells, then lead to an improvement of radiative recombination rates. One can expect that by the elaborate design of device structure, the performance of ZnO-based LEDs can be further improved.

Dark current reduction in nano-avalanche photodiodes by incorporating multiple quantum barriers

ABSTRACTMultiple quantum barriers have been used to suppress the dark current of nanoscale avalanche photodiodes (APDs). The n+–π–p+-structured Si–3C-SiC heterojunction-based multiple quantum barrier (MQB) APDs are considered and a detailed model of dark current has been developed from the self-consistent solution of the coupled Schrödinger–Poisson equations. Four major types of electron–hole pair (EHP) generation mechanisms such as (1) thermal generation, (2) band-to-band tunnelling generation, (3) trap-assisted tunnelling generation and (4) avalanche generation are considered for calculating variation of the total dark current with reverse bias voltage. It is observed that the dark current can be suppressed significantly by increasing both the number and thickness of quantum barriers. However, the authors have also admitted that both the number and thickness of quantum barriers cannot be increased indefinitely, since it will cause deterioration in spectral response of the device in near-infrared range (λ < 1100 nm).

Improved carrier injection in GaN-based VCSEL via AlGaN/GaN multiple quantum barrier electron blocking layer.

In this report, the improved lasing performance of the III-nitride based vertical-cavity surface-emitting laser (VCSEL) has been demonstrated by replacing the bulk AlGaN electron blocking layer (EBL) in the conventional VCSEL structure with an AlGaN/GaN multiple quantum barrier (MQB) EBL. The output power can be enhanced up to three times from 0.3 mW to 0.9 mW. In addition, the threshold current density of the fabricated device with the MQB-EBL was reduced from 12 kA/cm2 (9.5 mA) to 10.6 kA/cm2 (8.5 mA) compared with the use of the bulk AlGaN EBL. Theoretical calculation results suggest that the improved carrier injection efficiency can be mainly attributed to the partial release of the strain and the effect of quantum interference by using the MQB structure, hence increasing the effective barrier height of the conduction band.

Low drive voltage optical phase modulator with novel InGaAlAs/InAlAs multiple-quantum-barrier based n-i-n heterostructure

An optical phase modulator based on novel n-i-n InGaAlAs/InAlAs multiple-quantum-barrier (MQBs) is proposed to reduce the waveguide loss and enhance the modulation efficiency. Efficient bias loading with low leakage current is realized by taking advantages of the large conduction band discontinuity in InGaAlAs/InAlAs heterostructure and high electron reflection by MQBs. Low optical transmission loss of 2.3 dB/cm and improved modulation performance with a voltage length product of 1.06 V⋅cm are demonstrated for TE mode at 1550 nm. The linear and quadratic electro-optic coefficients of the fabricated device are estimated to be 1.0 pm/V and 1.4 × 10⁻¹⁹ m²/V², respectively.

Enhancement of Radiative Recombination by Different Indium Composition of Multiple Quantum Barriers in GaN-Based Light-Emitting Diodes

In this study, the characteristics of the nitride-based blue light emitting diodes (LEDs) having different indium contents multiple quantum barriers were analyzed numerically. The carrier concentrations in the quantum wells (QWs), energy band diagrams, radiative recombinations were investigated. InxGa1-xN with different indium-composition multiple quantum barriers (QBs) instead of GaN QBs were designed to improve hole transport and radiative recombination. The simulation results indicate that LEDs have a better hole transport in the active regions and uniformity of hole concentration over the conventional LED with GaN QBs. Consequently, the luminescence power of proposed structure was enhanced about 60 times at 3.0 V.

High-Temperature Electroluminescence of InGaN/GaN Light-Emitting Devices with Multiple Quantum Barriers

We investigate the high-temperature characteristics of InGaN/GaN multiple quantum well light-emitting devices with and without multiple quantum barriers (MQBs) in depth. The electroluminescence measurements were carried out over a temperature range from 200 to 380 K and an injection current level from 1 to 100 mA. Enhanced carrier confinement and stronger carrier localization in the active layer are achieved for the sample with MQBs. Furthermore, it is found that the external quantum efficiency of the sample possessing MQBs is higher than that of the sample with GaN barriers. The MQB structure improves the high-temperature operation of light-emitting devices.

Hole transport improvement in InGaN/GaN light-emitting diodes by graded-composition multiple quantum barriers

Graded-composition multiple quantum barriers (GQB) were designed and incorporated in c-plane InGaN/GaN light-emitting diodes (LEDs) grown on c-plane sapphire substrate to improve hole transport and efficiency droop. The simulation of GQB LED design predicts enhancement of the hole transport in the active region at both low and high current densities. The fabricated LED with GQB structure exhibits lower series resistance and substantially reduced droop behavior of only 6% in comparison with 34% for conventional LED, supporting the improvement of hole transport in our design.

A coherent semiclassical transport model for pure-state quantum scattering

We present a time-dependent semiclassical transport model for coherent pure-state scattering with quantum barriers. The model is based on a complex-valued Liouville equation, with interface conditions at quantum barriers computed from the steady-state Schrodinger equation. By retaining the phase information at the barrier, this coherent model adequately describes quantum scattering and interference at quantum barriers, with a computational cost comparable to that of classical mechanics. We construct both Eulerian and Lagrangian numerical methods for this model, and validate it using several numerical examples, including multiple quantum barriers.

Anomalous excitation dependence of electroluminescence in InGaN∕GaN light-emitting diodes

We have systematically investigated the anomalous excitation dependence of the electroluminescence (EL) in InGaN∕GaN multiple-quantum-well light-emitting diodes over a temperature range from 300to20K. Initially, an increase in the emission intensity occurred upon decreasing the temperature, until a maximum was reached at the temperature Tm. A blueshift in the position of the EL peak was followed by a redshift that occurred at the crossover temperature Tc. Both of these characteristic temperatures correlate with the presence of statistic microbarriers arising from potential inhomogeneity. The higher the In content incorporated into the heterobarriers, named multiple quantum barriers, the lower the values of Tm and Tc obtained from the spectral observations; this phenomenon implies an augmentation in the microscopic nonradiative transport through the microbarriers. An increase in the injection current also led to decreases in both of these characteristic temperatures. In addition, a functional correlation exists between the values of Tc and Tm. All of these experimental results suggest that InGaN∕GaN microstructures can be viewed as disordered collections of InGaN nanocrystallites. Further experimental verification will be necessary if this phenomenological model is to be used to account for the properties of any other disorderlike heteromaterials.

Modeling multiple quantum barrier effects and reduced electron leakage in red-emitting laser diodes

Severe electron leakage impedes the full exploitation of AlGaInP laser diodes in the 630nm regime. Such thermally activated currents are attributed to inherently small conduction band offsets and intervalley transfer between the Γ and X conduction band minima. To negate the detrimental effect of these two intrinsic material issues a theoretical model is proposed. A multi-quantum-barrier (MQB) structure able to inhibit both Γ- and X-band transmissions is inserted in the p-doped region adjacent to the active region of the device, allowing a greater percentage of injected electrons to be reflected back within the active region. The design of the MQB follows a strict optimization procedure that takes into account fluctuations of superlattice layer width and composition. This model is used in conjunction with a dual conduction band drift-diffusion simulator to enable the design of the MQB at an operating voltage and hence account for nonlinear charge distribution across it. Initial results indicate strong agreement between experimentally determined effective enhancements and those predicted theoretically.

Characterization of Berthelot-type behaviors of InGaN/GaN semiconductor heterosystems

Berthelot-type optical and electrical properties of InGaN/GaN multiple quantum well p–i–n heterostructures composed of InGaN/GaN multiple quantum barriers with different InN molar fraction have been systematically investigated. Indium incorporation in the nanostructures augmented the increase in crystalline randomization. The so-called Berthelot behavior was observed, that is, the EL peak energy of the blue emission increased with temperature and then decreased monotonically. The electrical conductances of these samples were also contingent on the Berthelot temperatures. The thermal-related characterization for the spectral and transport measurement both corroborated that the Berthelot-type response was attributed to the microstructure disordering arising from the In nonstoichiometry.