Multiple Quantum Well Region Research Articles

Selective area grown photonic integrated chips for completely suppressing the Stokes shift

In this work, we report on the selective area growth (SAG) of InGaN multiple quantum well (MQW) structures to completely suppress the phenomenon of the Stokes shift in monolithically integrated photonic chips. The original green MQW region is designed as the integrated photodetector (PD), while the SAG blue MQW region acts as the integrated light-emitting diode (LED). The detection spectra of the PD can completely cover the emission spectra of the LED, greatly improving the on-chip optical connection by the complete suppression of the Stokes shift. Thus, the bottleneck of on-chip optical connection based on spectra-tail overlap in integrated photonic chips has been broken. Under the same operating current, the photocurrent of the SAG integrated PD reaches 11.8 μA, while the conventional chip achieves only 0.6 μA. By SAG method, the photo-to-dark current ratio of integrated PD exhibits about two orders of magnitude increase under 0 V bias. Undoubtedly, the SAG technology provides a strategy to further improve the on-chip optical signal transmission efficiency of the MQW structure integrated photonic chips.

Temperature-Dependent Optical Behaviors and Demonstration of Carrier Localization in Polar and Semipolar AlGaN Multiple Quantum Wells

Semipolar AlGaN multiple quantum wells (MQWs) have unique advantages in deep ultraviolet light emitters due to the weak Quantum-Confined Stark Effect. However, their applications are hampered by the poor crystalline quality of semipolar AlGaN thin films. Different treatments were developed to improve the crystal quality of semipolar AlGaN, including a multistep in situ thermal annealing technique proposed by our group. In this work, temperature-dependent and time-resolved photoluminescence characterizations were performed to reveal the carrier localization in the MQW region. The degree of carrier localization in semipolar AlGaN MQWs grown on top of the in situ-annealed AlN is similar to that of conventional ex situ face-to-face annealing, both of which are significantly stronger than that of the c-plane counterpart. Moreover, MQWs on in situ-annealed AlN show drastically reduced dislocation densities, demonstrating its great potential for the future development of high-efficiency optoelectronic devices.

Multilevel Simultaneous Lighting-Imaging System.

In a III-nitride multiple quantum well (MQW) diode biased with a forward voltage, electrons recombine with holes inside the MQW region to emit light; meanwhile, the MQW diode utilizes the photoelectric effect to sense light when higher-energy photons hit the device to displace electrons in the diode. Both the injected electrons and the liberated electrons are gathered inside the diode, thereby giving rise to a simultaneous emission-detection phenomenon. The 4 × 4 MQW diodes could translate optical signals into electrical ones for image construction in the wavelength range from 320 to 440 nm. This technology will change the role of MQW diode-based displays since it can simultaneously transmit and receive optical signals, which is of crucial importance to the accelerating trend of multifunctional, intelligent displays using MQW diode technology.

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.Graphical abstract

Direct observation of charge accumulation in quantum well solar cells by cross-sectional Kelvin probe force microscopy

We report here the direct observation of charge accumulation in GaAs/AlGaAs multiple quantum well (MQW) solar cells by employing cross-sectional Kelvin probe force microscopy (KPFM). This sample is characterized by thin barrier layers that enable miniband formation. The contact potential difference, or potential between the tip and the semiconductor sample, was measured along the p–i–n junction. We observed, under illuminated conditions, a change in the potential gradient, or bending, at a position of the MQW layer, but not in the reference sample without quantum well. This clearly shows that charge is accumulated in the MQW region. We also found that electron accumulation in the MQW layer and the density measured on the surface is about 1 × 1011 cm−2. Our experimental results show that KPFM is a powerful way of understanding the device physics of nanostructure-based solar cells.

A secret luminescence killer in deepest QWs of InGaN/GaN multiple quantum well structures

This work suggests new alternative explanation why a single InGaN quantum well (QW) or the deepest QWs in the multiple quantum well (MQW) structures suffer with a high non-radiative recombination rate. According to SIMS results, positron annihilation spectroscopy and photoluminescence measurements we suggest that vacancy of Ga in complex with hydrogen atoms can play a dominant role in non-radiative Shockley-Read-Hall recombination of the deepest QWs in InGaN/GaN MQW structures. Vacancy of gallium originate dominantly in GaN buffer layers grown at higher temperatures in H2 atmosphere and are transported to the InGaN/GaN MQW region by diffusion, where they are very effectively trapped in InGaN layers and form complex defects with hydrogen atoms during epitaxy of InGaN layers. Trapping of gallium vacancies is another suggested mechanism explaining why the widely used In containing prelayers help to increase the luminescence efficiency of the InGaN/GaN MQW active region grown above them. Understanding the mechanism why the luminescence efficiency is suppressed in deeper QWs may be very important for LED community and can help to develop new improved technologies for the growth of InGaN/GaN MQW active region.

Redefinition the quasi-Fermi energy levels separation of electrons and holes inside and outside quantum wells of GaN based multi-quantum-well semiconductor laser diodes

Both accurate experiments and a self-consistent theoretical simulation using the near-ABC or ab initio method have uncovered the anomalous changes in the junction voltage (Vj ) of GaN-based lasers (Li et al 2013 Appl. Phys. Lett. 102 123501, Feng et al 2018 J. Phys. D: Appl. Phys. 51 095102 and Feng et al Appl. Phys. B 124 39). Here, further accurate emission spectral characteristics and band-gap simulation of a GaN based laser diodes (LDs) confirmed that previous researchers may have confused the junction voltage (Vj ) and the separation between the two Fermi levels of electrons and holes in the active region divided by the electronic charge (ΔEf /e). For the multiple-quantum wells (MQWs), there are obvious differences between them. For an ideal GaN-based LDs, despite the increase in junction voltage, physical quantities including the separation between the two Fermi levels of electrons and holes in the MQW region, and the optical net gain remain unchanged beyond the threshold region after an unpredictable sharp increase in the threshold region.

Modeling of InGaP/InGaAs-GaAsP/Ge multiple quantum well solar cell to improve efficiency for space applications

Multiple quantum wells (MQWs) are incorporated in the current limiting middle subcell of standard InGaP/InGaAs/Ge multi-junction solar cell (MJSC). Considering a realistic radiation dose in geo-stationary orbits (1 MeV electron irradiation-1 × 1016 cm−2 fluence), carrier lifetime is calculated and defined in the top cell and middle subcell region. The thermionic emission and recombination lifetimes are taken into consideration while defining lifetime in the middle MQW region. The physical thickness of quantum well (QW) and quantum barrier (QB) are slightly adjusted to numerically satisfy the strain balancing condition and to achieve higher efficiency at 1-sun (AM0 spectrum). In addition to interface recombination, carrier removal effects were taken into consideration by changing the intrinsic region in the PiN middle subcell into n-type. With the nonidealities taken into consideration, the proposed device exhibits an efficiency of 32% at 1-sun AM0 radiation and a peak efficiency of 37.63% at concentrated sunlight. Trap concentration of 1 × 1017 cm−3 in top cell with 10 ns lifetime defined in the middle cell emitter, base and MQW region is obtained as threshold point beyond which the device exhibits significant degradation.

Enhanced localisation effect and reduced quantum-confined Stark effect of carriers in InGaN/GaN multiple quantum wells embedded in nanopillars

A conventional planar InGaN/GaN multiple quantum well (MQW)-based light emitting diode (LED) producing a blue-green dual-wavelength spectrum, due to phase separation in the InGaN well layers, is grown, and fabricated into a nanopillar (NP) LED array structure with an etching depth penetrating through the MQW region using a top-down dry etching process. Excitation power and temperature dependences of the photoluminescence (PL) spectra of the planar and NP samples have shown that the fabrication of the nanotexture not only decreases the quantum-confined Stark effect (QCSE), but also enhances the carrier localisation effect, both in the InGaN matrix region and in the In-rich quasi-quantum dot (QD) region, due to the reduction of the strain relaxation-induced piezoelectric field in the MQWs embedded in the nanopillars. Moreover, the fabrication of the nanotexture also improves the light extraction efficiency (LEE), due to the increased light-extracting surface area and light-guiding effect. These results are also consistent with the measured quantum efficiencies of the two samples.

InGaN stress compensation layers in InGaN/GaN blue LEDs with step graded electron injectors

We investigate the effect of InGaN stress compensation layer on the properties of light emitting diodes based on InGaN/GaN multiple quantum well (MQW) structures with step-graded electron injectors. Insertion of an InGaN stress compensation layer between n-GaN and the step graded electron injector provides, among others, strain reduction in the MQW region and as a result improves epitaxial quality that can be observed by 15-fold decrease of V-pit density. We observed more uniform distribution of In between quantum wells in MQW region from results of electro- and photoluminescence measurement. These structural improvements lead to increasing of radiant intensity by a factor of 1.7–2.0 and enhancement of LED efficiency by 40%.

Effect of carrier transfer process between two kinds of localized potential traps on the spectral properties of InGaN/GaN multiple quantum wells

Two InGaN/GaN multiple-quantum-well (MQW) samples with identical epitaxial structures are grown at different growth rates via metal-organic chemical vapor deposition system. The room temperature photoluminescence intensity of the fast-grown sample is much stronger than that of the slow-grown one. In addition, the fast-grown sample has two luminescence peaks at low temperatures, and the height of main peak anomalously increases with increasing temperature below 100 K. Such improved emission efficiency and the untypical temperature-induced increase of peak height can be attributed to the carrier's transferring between two kinds of localized traps with different potential depth in the fast-grown sample, where the distribution of indium is seriously inhomogeneous. The enhanced fluctuation of indium is caused by the reduced migration time of adsorbed atoms due to the increased growth rate during the epitaxial growth of MQW region.

Carrier accumulation in the active region and its impact on the device performance of InGaN-based light-emitting diodes

Carrier recombination and transport processes play key roles in determining optoelectronic performance characteristics such as the efficiency droop and forward voltage in InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs). In this work, we investigate the dominant carrier transport and recombination processes inside and outside the MQW region as functions of injection current from the viewpoint of carrier-energy-loss mechanisms. It is experimentally shown that carrier accumulation and subsequent spill-over at MQW active layers due to the insufficient carrier recombination rate, mainly the radiative recombination rate, explain the dependences of both the efficiency droop and the forward voltage on the injection current.

UVA light-emitting diode grown on Si substrate with enhanced electron and hole injections.

In this work, III-nitride based ∼370 nm UVA light-emitting diodes (LEDs) grown on Si substrates are demonstrated. We also reveal the impact of the AlN composition in the AlGaN quantum barrier on the carrier injection for the studied LEDs. We find that, by properly increasing the AlN composition, both the electron and hole concentrations in the multiple quantum wells (MQWs) are enhanced. We attribute the increased electron concentration to the better electron confinement within the MQW region when increasing the AlN composition for the AlGaN barrier. The improved hole concentration in the MQW region is ascribed to the reduced hole blocking effect by the p-type electron blocking layer (p-EBL). This is enabled by the reduced density of the polarization-induced positive charges at the AlGaN last quantum barrier (LB)/p-EBL interface, which correspondingly suppresses the hole depletion at the AlGaN LB/p-EBL interface and decreases the valence band barrier height for the p-EBL. As a result, the optical power is improved.

GaAsBi/GaAs multi-quantum well LED grown by molecular beam epitaxy using a two-substrate-temperature technique

We report a GaAs0.96Bi0.04/GaAs multiple quantum well (MQW) light emitting diode (LED) grown by molecular beam epitaxy using a two-substrate-temperature (TST) technique. In particular, the QWs and the barriers in the intrinsic region were grown at the different temperatures of = 350 °C and respectively. Investigations of the microstructure using transmission electron microscopy (TEM) reveal homogeneous MQWs free of extended defects. Furthermore, the local determination of the Bi distribution profile across the MQWs region using TEM techniques confirm the uniform Bi distribution, while revealing a slightly chemically graded GaAs-on-GaAsBi interface due to Bi surface segregation. Despite this small broadening, we found that Bi segregation is significantly reduced (up to 18% reduction) compared to previous reports on Bi segregation in GaAsBi/GaAs MQWs. Hence, the TST procedure proves as a very efficient method to reduce Bi segregation and thus increase the quality of the layers and interfaces. These improvements positively reflect in the optical properties. Room temperature photoluminescence and electroluminescence (EL) at 1.23 μm emission wavelength are successfully demonstrated using TST MQWs containing less Bi content than in previous reports. Finally, LED fabricated using the present TST technique show current–voltage (I–V) curves with a forward voltage of 3.3 V at an injection current of 130 mA under 1.0 kA cm−2 current excitation. These results not only demonstrate that TST technique provides optical device quality GaAsBi/GaAs MQWs but highlight the relevance of TST-based growth techniques on the fabrication of future heterostructure devices based on dilute bismides.

Investigation on the performance and efficiency droop behaviors of InGaN/GaN multiple quantum well green LEDs with various GaN cap layer thicknesses

The performance and efficiency droop behaviors of InGaN/GaN multiple quantum well (MQW) green LEDs with various low temperature grown GaN cap (LT-cap) layer are investigated. It is found that the output power increases when a thin LT-cap layer (0.37 nm) is inserted and it decreases as the LT-cap layer thickness increases up to 1.5 nm. In addition, the apparent efficiency droop decreases when the thickness of LT-cap layer increases from 0 to 1.5 nm. However, the related physical mechanisms are different. When the relatively thin LT-cap layers are inserted, the carrier confinement effect of QWs enhances due to the increased In content of InGaN QWs. In this case, electrons are hardly to escape from MQW region to p-GaN region, which results in a relatively high output power at high injection current, thus a small efficiency droop is obtained. However, when the LT-cap layer thickness is relatively thick, due to the large defect density of InGaN/GaN MQWs, although the small efficiency droop also can be observed, the relatively low output power is obtained at all injection currents.

Dependence of the photovoltaic performance of pseudomorphic InGaN/GaN multiple-quantum-well solar cells on the active region thickness

We investigate the photovoltaic performance of pseudomorphic In0.1Ga0.9N/GaN multiple-quantum well (MQW) solar cells as a function of the total active region thickness. An increase in the number of wells from 5 to 40 improves the short-circuit current and the open-circuit voltage, resulting in a 10-fold enhancement of the overall conversion efficiency. Further increasing the number of wells leads to carrier collection losses due to an incomplete depletion of the active region. Capacitance-voltage measurements point to a hole diffusion length of 48 nm in the MQW region.

Influences of the p-GaN Growth Temperature on the Optoelectronic Performances of GaN-Based Blue Light-Emitting Diodes

We investigate the influence of p-(Al)GaN growth temperature ( $T_{g}$ ) on the optoelectronic performances of InGaN/GaN multiple-quantum-well (MQW) blue light-emitting diodes (LEDs). We systematically combine various characterization techniques, such as current–voltage, current-light output power, capacitance–voltage ( $C$ – $V$ ) under forward and reverse biases, photocurrent and electroreflectance (ER) spectroscopies, temperature-dependent electroluminescence, and the internal quantum efficiency (IQE). From the experimental analyses, it is shown that increasing $T_{g}$ induces: 1) the reduced optical loss by the improved crystal quality of the p-GaN layer (improved light extraction efficiency) and 2) the decreased IQE due to the Mg diffusion from the p-(Al)GaN layer to the MQW region. This paper demonstrates that the fine control of the Mg diffusion from the p-GaN layer to the InGaN/GaN MQW region is the key factor for achieving highly efficient blue LEDs. Moreover, a method of estimating the Mg diffusion length is proposed for the first time by analyzing both the $C$ – $V$ curves and the ER spectra under reverse biases.

Electrical, Luminescent and Structural Properties of Nanopillar GaN/InGaN Multi-Quantum-Well Structures Prepared by Dry Etching

GaN/InGaN multiple quantum well (MQW) structures with undoped n-GaN cap imitating true light emitting diodes were grown on sapphire and converted to deep nanopillar (NP) structures by dry etching beyond the MQW region. Structural measurements, electrical measurements on Schottky diodes, microcathodoluminescence (MCL) spectra measurements indicate a strong relaxation of strain in NP MQWs manifested in the prominent increase of the bowing radius of the structures and in the blueshift of the MQW peak in MCL spectra. Various treatments of the as-etched NP MQWs (annealing at 700°C, etching in KOH, and soaking in (NH4)2S) progressively decreased the leakage current of Schottky diodes and increased the MQW MCL peak intensity due to the suppression of the dry-etching damage of the nanopillar sidewalls.

Influence of the InGaN/GaN quasi-superlattice underlying layer on photoluminescence in InGaN/GaN multiple quantum wells

Photoluminescence (PL) properties of two different InGaN/GaN multiple quantum well (MQW) structures, without and with an InGaN/GaN quasi-superlattice (QSL) underlying buffer layer, were investigated. The results show that inserting a QSL between the n-GaN and MQWs can release the strain in the MQW region, since the sample with a QSL shows a smaller excitation power-dependent blue-shift of its peak energy than that without. Meanwhile, inserting a QSL enhances the localization effect of the carriers inferred from an unusual red-shift of the peak energy with increasing excitation power in low excitation range, and from a more obvious “S-shaped” temperature-dependent behavior of the peak energy characteristic: the strain release facilitates the slight composition fluctuation or phase separation of the InGaN well layers. The reduction of the quantum-confined Stark effect and enhancement of the localization effect of the MQWs induced by the strain release, greatly enhance the radiative recombination rate of the MQWs.

Investigating the Effect of Piezoelectric Polarization on GaN-Based LEDs with Different Prestrain Layer by Temperature-Dependent Electroluminescence

The effect of piezoelectric polarization on GaN-based light emitting diodes (LEDs) with different kinds of prestrain layers between the multiple quantum wells (MQWs) and n-GaN layer is studied and demonstrated. Compared with the conventional LED, more than 10% enhancement in the output power of the LED with prestrain layer can be attributed to the reduction of polarization field within MQWs region. In this study, we reported a simple method to provide useful comparison of polarization fields within active region in GaN-based LEDs by using temperature-dependent electroluminescence (EL) measurement. The results pointed out that the polarization field of conventional LED was stronger than that of the others due to larger variation of the wavelength transition position (i.e., blue-shift change to red-shift) from 300 to 350 K, and thus the larger polarization field must be effectively screened by injecting more carriers into the MQWs region.