Increasing Injection Current Research Articles

Optimization of the Al Composition of the p‐AlGaN Electron Blocking Layer in GaInN/GaN Multiquantum‐Shell Nanowire LEDs

The aim is to develop highly efficient GaInN/GaN nanowire (NW)‐based light‐emitting diodes (LEDs), which are composed of GaN NWs and multiquantum shell (MQS) active regions. These regions incorporate the polar c‐plane, nonpolar r‐plane, and semipolar m‐plane. A challenge with MQS‐LEDs is that the current path through the c‐plane MQS tends to dominate under low‐current injection conditions. Given that the MQS on the c‐plane is very defective, this injection current is mainly subjected to nonradiative recombination. Therefore, this study explores various optimizations of the p‐AlGaN electron blocking layers (EBLs) to minimize the current injection into the MQS in the c‐plane region. The samples are subsequently grown using a specific process. This involves n‐GaN NWs, GaInN/GaN‐based quantum shells, p‐AlGaN EBLs with different Al compositions, and p‐GaN shells. All these are developed by metal–organic vapor phase epitaxy on an n‐GaN template featuring a SiO2 hole pattern. NW LEDs are fabricated and subsequently their device characteristics are investigated. Under low‐current injection, the sample with a lower Al composition exhibits higher luminescence intensity. However, this trend reverses when the injection current increases. The findings suggest that AI composition and thickness in the p‐AlGaN EBL significantly affect the output power and the emission wavelength.

Effect of junction temperature on 1.3 µm InAs/GaAs quantum dot lasers directly grown on silicon

Laser junction temperature (Tj) is an essential parameter that directly affects the light power and lifetime of semiconductor lasers. Here, we investigate the effect of Tj on an InAs/GaAs quantum dot (QD) laser grown on a Si(001) substrate. Under 1% low pulsed current (1 µs pulse width and 100 µs period), the pure temperature-induced mode shift rate is 0.084 nm/°C. By increasing the duty cycle and measuring the corresponding mode wavelength shift, the laser’s Tj under the continuous-wave (Tj-CW) mode is predicted to be from 31.1 to 81.6 °C when the injection current increases from 100 to 550 mA. Next, the average thermal resistance is 36.2 °C/W. Moreover, the non-negligible increase in Tj-CW is analyzed to significantly reduce the mean-time-to-failure of Si-based QD laser, especially for cases under high CW injection currents. These results provide an accurate reference for the thermal analysis of silicon-based QD lasers and point the way to high performance on-chip light sources by improving the laser heat accumulation.

Hybrid multilevel-multiwavelength signaling scheme for optical data transmission

We suggest an idea of data encoding scheme based on the switching from the ground state (GS) to excited state (ES) lasing in quantum dot (QD) lasers with increase in injection current. The groups of two bits are assigned to lasing spectra that comprise either one or both GS and/or ES lasing lines depending on injection current. We expect that the proposed encoding scheme can potentially combine some advantages of multilevel signaling and wavelength division multiplexing.

Nonlinear dynamics in a circular-sided square microcavity laser

Self-chaotic dual-mode and tri-mode microcavity lasers have been recently proposed and demonstrated for high-speed random number generation. Here, we report the characteristics of irregular pulse packages and self-chaos operation for a dual-mode circular-sided square microcavity laser. In addition to the mode interaction between the fundamental and first-order transverse modes, we observed irregular pulse packages due to the mode beating of near-degenerate modes for the first time to our best knowledge. Moreover, a successive route from periodic-one and periodic-three states to chaos is first experimentally illustrated by increasing injection current. The chaotic state is observed over a current range of 10 mA, and the maximum chaos effective bandwidth of 22.4 GHz is realized with a flatness of ±4 dB. Chaotic characteristics are also investigated for different longitudinal modes, which indicates that the self-chaotic microlaser can provide robust parallel chaotic outputs for practical application.

Radiative recombination of a high internal-quantum-efficiency 268 nm ultraviolet C-band light emitting diode

After assigning a thickness d to the carrier recombination region of a light emitting diode (LED), we show that the ABC model involving Shockley–Read–Hall non-radiative, radiative, and Auger recombination coefficients, i.e., A, B, and C, respectively, can bring new insight into the radiative recombination process. In order to fit external quantum efficiency (EQE) data of ultraviolet C-band (UVC) as well as blue LEDs, the ABC model requires the product d·B to be invariant of the injection current. This can be understood that as the thickness of the recombination region increases the radiative recombination coefficient decreases due to reduced electron–hole wavefunction overlaps. For an LED with high internal quantum efficiency (IQE), its quality factor Q (Q=BAC) usually undergoes a noticeable drop as the injection current increases to pass the current of maximal EQE. This is due to an increase in the thickness of the recombination region and, hence, a reduction in the radiative recombination coefficient as the injected carriers start to drift or diffuse to involve more quantum wells for light emission. Applying this ABC model, we analyze a high-efficiency 268 nm UVC LED, which delivers ∼199 mW optical power under a direct current of 350 mA and obtains a maximal IQE of ∼86.4% and an effective light extraction efficiency of ∼15.3%.

Monolithic Multicolor Emissions of InGaN-Based Hybrid Light-Emitting Diodes Using CsPbBr3 Green Quantum Dots.

To address the increasing demand for multicolor light-emitting diodes (LEDs), a monolithic multicolor LED with a simple process and high reliability is desirable. In this study, organic-inorganic hybrid LEDs with violet and green wavelengths were fabricated by depositing CsPbBr3 perovskite green quantum dots (QDs) as the light-converting material on InGaN-based violet LEDs. As the injection current was increased, the total electroluminescence (EL) intensities of the hybrid LEDs increased, whereas the light-converted green emission efficiency of the CsPbBr3 QDs decreased. The maximum green-to-violet EL spectral intensity ratio of the hybrid LEDs with CsPbBr3 QDs was achieved with the injection current of <10 mA. Moreover, the EL spectral ratio of the green-to-violet emission decreased at an injection current of 100 mA. The light-conversion intensity of the CsPbBr3 QDs decreased linearly as the junction temperature of the hybrid LEDs was increased with increasing injection current, similar to the temperature-dependent photoluminescence degradation of CsPbBr3 QDs. In addition, the junction temperature of the hybrid LED was minimized by pulse injection to suppress the thermal degradation of QDs and increase the light conversion efficiency to green emission. Therefore, the overall emission spectrum color coordinates of the hybrid LEDs exhibited a red shift from violet to blue in the low-current region and a blue shift toward violet as the green emission of the QDs was decreased above 10 mA.

Quantitative nanoscale temperature mapping across the multi-quantum well of a light-emitting diode in operation using vacuum null-point scanning thermal microscopy to evaluate local energy conversion efficiency.

Electrical energy that is not converted into light in light emitting diodes (LEDs) is locally dissipated as heat in the active layers. Therefore, by measuring the temperature distribution with nanoscale resolution across the multi-quantum well (MQW) of an LED in operation, the effect of nanostructures inside the LED on the local energy conversion efficiency can be observed. In this study, we first demonstrated that vacuum null-point scanning thermal microscopy (VNP SThM) could be used to quantitatively map the two-dimensional temperature distribution across the MQW of an LED in operation with a sufficient signal-to-noise ratio. Subsequently, by increasing the injection current in four steps, we quantitatively mapped the temperature distribution across the MQW at each step and observed the shift in the temperature peak across the active layers due to the increase in injection current. The measurements of the temperature distribution around the MQW indicate that as the injection current increased, the overall temperature around the MQW increased significantly, and the temperature peak position shifted. These results show that the main cause of the dissipation of electrical energy into thermal energy inside an LED changes as the injection current increases, and the nanostructures inside an LED affect the dissipation of electrical energy into thermal energy. The high thermal sensitivity, nanoscale resolution, and convenience of VNP SThM may enable the direct observation of the effect of the nanostructures inside various types of nanophotonic devices on local energy conversion even under intense localized radiation.

Simple Approach to Mitigate the Emission Wavelength Instability of III-Nitride μLED Arrays.

III-nitride semiconductors and their heterojunctions exhibit intrinsic polarization due to the asymmetry of their wurtzite structure, which determines all the fundamental properties of III-nitride optoelectronics. The intrinsic polarization-induced quantum-confined Stark effect leads to an emission wavelength shift with increasing injection current for III-nitride visible LEDs, forming an insurmountable barrier for the fabrication of a full color display. For instance, a yellow LED designed to produce yellow light emits green or blue light at an elevated current, while a green (blue) LED gives off blue (violet) light with increasing current. This color instability becomes a serious issue for a microdisplay such as the displays for augmented reality (AR)/virtual reality (VR) typically utilized at proximity to the eye, where human eyes are sensitive to a tiny change in light color. It is well-known that an optical mode wavelength for a microcavity is insensitive to injection current. In this work, we have demonstrated an approach to epitaxially integrating microLEDs (green microLEDs as an example, one of the key components for a full color microdisplay) and a microcavity. This allows the emission from the microLEDs to be coupled with the microcavity, leading to a negligible emission wavelength shift with increasing injection current. In contrast, identical microLEDs but without a microcavity show a large emission wavelength shift from 560 nm down to 510 nm, measured under identical conditions. This approach provides a simple solution to resolving the 30-year issue in the field of III-nitride optoelectronics.

Improvement of the detectivity in an Fe–Sn magnetic-field sensor with a large current injection

A ferromagnetic nanocrystalline Fe–Sn is an excellent platform for magnetic-field sensor based on anomalous Hall effect (AHE) owing to simple fabrication and superior thermal stability. For improvement of the magnetic-field sensitivity, doping impurity and increasing injection current are effective approaches. However, in the light of magnetic-field detectivity, the large current may increase the voltage noise. In this study, a maximum allowable current was improved by employing the overlayer electrode configuration on a Ta-doped Fe–Sn AHE sensor. In noise measurements, the 1/f noise becomes significant with increasing the current at low frequency, resulting in saturation of the detectivity to 240 nTHz−1/2 at 120 Hz. At high frequency, the detectivity reaches 48 nTHz−1/2 at 3.1 mA showing ten times improvement of the detectivity compared with the non-doped Fe–Sn AHE sensor. Material design and device structure optimization will accelerate further improvement of the sensing properties of the Fe–Sn-based AHE sensor.

Electroluminescence study of InGaN/GaN QW based p-i-n and inverted p-i-n junction based short-wavelength LED device using laser MBE technique

In the present work, the Laser Molecular Beam Epitaxy (Laser MBE) technique has been used for the fabrication of InGaN/GaN quantum well LEDs. A comparative study was performed for analysing the performance of LEDs with two different device structures i.e conventional (p-GaN/QW/i-GaN/n-GaN/Substrate) and inverted (n-GaN/QW/i-GaN/p-GaN/Substrate). The fabricated QW based Inverted p-i-n junction and conventional (p-i-n) LEDs exhibited non-linear rectifying current-voltage characteristics with a forward threshold voltage of about 0.7 V and 0.5 V, respectively. Room temperature electroluminescence was observed with increasing injection current in both conventional and inverted junction LED, where LEDs with 5 QW periods showed carrier localization effect and higher external quantum efficiency (EQE) at fixed injection current. The emission in fabricated QW LEDs was found to exhibit a spectral blue shift with increasing injection current. Convention LEDs (topmost layer: p-type GaN) exhibited higher luminous efficiency (i.e., 2.83 lm/W) than the Inverted LEDs (top most layer: n-type GaN) luminous efficiency (i.e., 1.56 lm/W). The efficiency droop observed is 14% in the conventional and 8% in the inverted LED device, respectively. This study focuses on the demonstration of room temperature electroluminescence in InGaN/GaN based LEDs grown using Laser MBE technique. The successful utilization of Laser MBE technique for growth of InGaN/GaN quantum wells can be a revolutionary step towards the establishment of a reliable, cost-effective, and ammonia-free process for fabrication of LED.

Explorations on Growth of Blue-Green-Yellow-Red InGaN Quantum Dots by Plasma-Assisted Molecular Beam Epitaxy.

Self-assembled growth of blue-green-yellow-red InGaN quantum dots (QDs) on GaN templates using plasma-assisted molecular beam epitaxy were investigated. We concluded that growth conditions, including small N2 flow and high growth temperature are beneficial to the formation of InGaN QDs and improve the crystal quality. The lower In/Ga flux ratio and lower growth temperature are favorable for the formation of QDs of long emission wavelength. Moreover, the nitrogen modulation epitaxy method can extend the wavelength of QDs from green to red. As a result, visible light emissions from 460 nm to 622 nm have been achieved. Furthermore, a 505 nm green light-emitting diode (LED) based on InGaN/GaN MQDs was prepared. The LED has a low external quantum efficiency of 0.14% and shows an efficiency droop with increasing injection current. However, electroluminescence spectra exhibited a strong wavelength stability, with a negligible shift of less than 1.0 nm as injection current density increased from 8 A/cm2 to 160 A/cm2, owing to the screening of polarization-related electric field in QDs.

Study on the effect of size on InGaN red micro-LEDs

In this research, five sizes (100 × 100, 75 × 75, 50 × 50, 25 × 25, 10 × 10 µm2) of InGaN red micro-light emitting diode (LED) dies are produced using laser-based direct writing and maskless technology. It is observed that with increasing injection current, the smaller the size of the micro-LED, the more obvious the blue shift of the emission wavelength. When the injection current is increased from 0.1 to 1 mA, the emission wavelength of the 10 × 10 μm2 micro-LED is shifted from 617.15 to 576.87 nm. The obvious blue shift is attributed to the stress release and high current density injection. Moreover, the output power density is very similar for smaller chip micro-LEDs at the same injection current density. This behavior is different from AlGaInP micro-LEDs. The sidewall defect is more easily repaired by passivation, which is similar to the behavior of blue micro-LEDs. The results indicate that the red InGaN epilayer structure provides an opportunity to realize the full color LEDs fabricated by GaN-based LEDs.

Color‐Tunable 3D InGaN/GaN Multi‐Quantum‐Well Light‐Emitting‐Diode Based on Microfacet Emission and Programmable Driving Power Supply

AbstractColor‐tunable InGaN/GaN multi‐quantum‐well (MQW) light‐emitting diodes (LEDs) are reported based on GaN microfacet structure directly grown on c‐plane patterned sapphire substrate by metal organic vapor phase epitaxy (MOVPE) through promoting 3D growth. By adjusting GaN growth temperature and pattern arrangement, a GaN microfacet with almost pure {101} semipolar facets is obtained. The multifacetted InGaN/GaN MQW LED chip evolves three distinct emission peaks around 630, 530, and 450 nm in electroluminescence (EL) as injection current increases from 1 to 100 mA. The EL behavior originates from locally different facets of the complex 3D structure: MQWs grown on c‐planes and semipolar facets, respectively, which is confirmed by cathodoluminescence characterization in a scanning transmission electron microscope (STEM‐CL). Considering the dependence of emission wavelength and intensity on injection currents, a programmable power supply is designed to drive the LED. The specific color of the LED is tuned by time‐shared driving of the currents based on three channels with controllable magnitudes and duty cycle from the power supply, covering red, yellow, green, cyan, blue, and purple. Furthermore, white LEDs with high color rendering index (CRI) up to 96.1 and correlated color temperature (CCT) between 4000 and 10 000 K are achieved.

Ultraviolet Electrostatic Field Effect Light-Emitting Diode

The potential of ultraviolet (UV) AlGaN quantum well (QW) light-emitting diodes (LEDs) is stunted due to the lack of efficient p-type doping for this ultra wide bandgap material. In this work, a novel device - UV electrostatic field effect LED (EFELED) is demonstrated to address this issue by significantly improving hole injection into the QW active region which leads to a relative increase of external quantum efficiency (EQE) up to 42%. The UV EFELED is realized by integrating a capacitor layer on top of a conventional 267 nm AlGaN QW LED, which introduces band bending to assist hole injection under a forward bias. A 10x increase in injection current is achieved through the application of a positive bias to the integrated capacitor layer. With 20 V applied to the capacitor, a 42% increase of EQE is achieved. Photon intensity modulation is demonstrated with the application of varying capacitor biases, which presents a novel brightness control at UV wavelength. Thus, the proposed UV EFELED provides significantly improved hole injection through a capacitor integration, which leads to high-efficiency UV LEDs for important applications such as sterilization and purification.

Performance improvement of GaN-based blue and ultraviolet double quantum well laser diodes by using stepped-doped lower waveguide

We have proposed a new stepped-doped lower waveguide (LWG) and systematically investigated its effect on GaN-based blue and ultraviolet laser diodes (LDs) through the simulation software LASTIP. It is found that increasing doping concentration of the LWG is beneficial to decrease the threshold current and raise the output power. Furthermore, when a suitably designed stepped-doped LWG layer replaces the homogeneously heavily doped LWG layer, the performance of the blue and ultraviolet LDs is further improved. It is mainly attributed to the decrease of electron leakage current and the increase of hole injection current. In addition, the stepped-doped LWG improves ultraviolet LDs characteristics more significantly than the blue LDs, which is due to the reduction of the total optical loss.

Indium Localization‐Induced Red, Green, and Blue Emissions of Semipolar (11‐22) GaN‐Based Light‐Emitting Diodes

Red, green, and blue electroluminescence (EL) in semipolar (11‐22) GaN‐based light‐emitting diodes (LEDs) is achieved using SiO2 hexagonal patterns epitaxial lateral overgrowth (HP‐ELO). The size, density, and area of arrowhead‐like surface structure of the semipolar (11‐22) HP‐ELO GaN are significantly affected by increasing the SiO2 hexagonal patterns from 6.0 to 15.0 μm. The full width at half maximum and the blueshift wavelength of EL spectra increases with increasing from 6.0 to 9.0 μm, and then decreases with increasing the hexagonal pattern width to 15.0 μm. These results show a similar tendency to the surface area of arrowhead‐like structure. In addition, micro‐EL images show that indium localization‐induced low energy emission initiates at the front end of arrowhead‐like pattern, and then blueshifted emissions propagate to the side edges as the injection current increases. Therefore, it suggests that HP‐ELO can control the arrowhead‐like surface structure composed of different indium compositions, which can achieve red, green, and blue emissions of a semipolar (11‐22) HP‐ELO GaN‐based LED.

Study on Fast Response of Semiconductor Laser Driven by Impulse Current

A simulation model based on single-mode rate equations of semiconductor laser and its typical parameters shows that, the photon stimulated emission rate at the early turn-on time of the semiconductor laser is increased with the increase of the injection current, and the corresponding rise time is reduced with the increase of injection current. However, for an output power is fixed semiconductor laser, increasing the injection current to reduce the rise time of the transmitted pulse leads to increase in temperature of semiconductor laser. We show that adding an impulse current in the front of the normal luminous pulse current can reduce the rise time of the transmitted pulse and ensure the stable output of the semiconductor laser. This method is verified by simulation and an experiment using PLTB450B semiconductor laser. The simulation and test results show that for an output power is fixed semiconductor laser, the rise time of the transmitted pulse can be reduced significantly by adding impulse current.

GaN phosphors converted white light‐emitting diodes for high luminous efficacy and improved thermal stability

This study analysed GaN nanophosphors based white light-emitting diodes (WLEDs) with ultraviolet (UV) excitation. Graphene quantum dots (GQDs) used as a charge transfer medium to enhance the performance in terms of luminous efficacy and colour quality. The improvement in colour rendering and colour temperature has been observed with the increase in injection current. The luminous efficacy of radiation also gets improved with injection current and maximises up to 255 lm/W at 260 mA along with 90 colour rendering index and 7100 K correlated colour temperature. Mapping of higher surface temperature for GQD-based devices shows a better thermal stability, indicating the good heat dissipation capability of GQDs because of excellent thermal conductivity. Therefore, proposed WLEDs were found more competent with improved thermal quenching of phosphors for rare-earth-free solid-state lighting as compared to the blue chip-excited yellow phosphors.

Room temperature electroluminescence from Laser MBE grown Gallium nitride LEDs

Gallium Nitride (GaN) based inverted p-i-n junction and InGaN/GaN quantum well LEDs have been demonstrated using Laser Molecular Beam Epitaxy technique. The fabricated p-i-n junction and QW LEDs exhibited rectifying I-V characteristics with a forward threshold voltage of about 3.4 V and 4.5 V, respectively. The electrically injected LEDs showed room temperature electroluminescence, wherein inverted p-i-n junction LED exhibited single peaked UV emission around 365 nm wavelength and the InGaN/GaN QW LED demonstrated both UV (368 nm) and visible (430 nm) spectral components. The emission in QW LED was found to exhibit a spectral blue shift with increasing injection current.

Photo- and electro-luminescence studies of semipolar (112¯2) InxAl1−xN

InxAl1−xN (0.15 &amp;lt; x &amp;lt; 0.4) single layers (thickness = 300 nm) and light emitting diodes (LEDs) with InxAl1−xN (x = 0.24 or 0.28) quantum wells (QWs) were grown on (112¯2) free-standing GaN substrates by metal organic chemical vapor deposition. The emission characteristics of InxAl1−xN films and QWs were then investigated by photoluminescence (PL), cathodoluminescence (CL), and electroluminescence (EL). PL emission down at 1.87 eV (∼660 nm) was observed from In0.37Al0.63N at room temperature. However, its emission wavelength and spectral width showed almost no temperature-dependence, strongly suggesting the radiative recombination process in In0.37Al0.63N is heavily affected by the potential fluctuations associated with the defect levels such as deep donor–acceptor pairs (DAPs). For In0.23Al0.77N, the emission peaks at 3.24 and 2.4 eV were observed at 300 K but CL showed no evidence of compositional fluctuations. Given the wide spectral width and weak In clustering, PL at 3.24 eV is likely from a shallow DAP transition involving an impurity-related shallow donor and valence band. PL emission at 2.4 eV, on the other hand, behaved more like In0.37Al0.63N, suggesting significant deep DAP transitions. Both LEDs with In0.24Al0.76N and In0.28Al0.72N QWs showed a discrete blueshift of the emission peak with increasing injection current, which is very different from the gradual shift observed in InGaN LEDs. The high-energy peak was much narrower than the low-energy peak, suggesting it could be the near-band edge emission. Si doping of In0.24Al0.76N QWs lowered the injection current level at which the high-energy peak appears.