GaN Multiple Quantum Wells Research Articles

Highly efficient near ultraviolet LEDs using InGaN/GaN/AlxGa1-xN/GaN multiple quantum wells at high temperature on sapphire substrate

This work demonstrated the highly efficient near-ultraviolet (UV) light-emitting diodes (LEDs) grown on sapphire substrates with InGaN/GaN/AlxGa1-xN/GaN multiple-quantum wells (MQWs), where x is the number of Al composition. The proposed GaN/AlxGa1-xN/GaN quantum barrier and InGaN quantum well of UV-GaN based LED has successfully grown using metal-organic chemical vapor deposition. The result reveals that the GaN/AlxGa1-xN/GaN barrier has higher light output power and lower current leakage than the GaN barrier. A significant increase in wall-plug efficiency was observed at the same wavelength range. The proposed structure in this study serves two functions: it increases the barrier bandgap to enhance carrier confinement. Second, promoting v-pit in the MQW layer may reduce the current resistance while increasing hole mobility. As a result, the internal quantum efficiency (IQE) may increase and lower electrical power consumption while enhancing performance.

High External Quantum Efficiency Green Light Emitting Diodes on Stress-Manipulated AlNO Buffer Layers

We demonstrated high-brightness InGaN/GaN green light emitting diodes (LEDs) with ex-situ sputtered stress-manipulated AlNO buffer on 4-inch patterned sapphire substrates. The lattice constant of the AlNO buffer was adjusted by oxygen flow. As a result, the dislocation density and the in-plane compressive stress caused by lattice mismatch were greatly reduced, while the interface quality of the InGaN/GaN multiple quantum wells and the uniformity of the indium composition were greatly improved. At 20A/cm², the external quantum efficiency and wall plug efficiency of the 526.4-nm-green LEDs grown on the sputtered AlNO buffer reached 46.1% and 41.9%, which were both higher than reported values.

The Investigation of Carrier Leakage Mechanism Based on ABC-Models in InGaN/GaN MQW and Its Effect on Internal Quantum Efficiency under Optical Excitation

In this work, a GaN-based multiple quantum well (MQW) sample has a much higher IQE although it has a stronger non-radiative recombination. Through experimental verification, the higher IQE is attributed to the suppressed carrier leakage mechanism, which is normally neglected under optical excitation. To achieve a more reasonable IQE expression in a GaN MQW structure, leakage factor m is introduced into the ABC-models. Meanwhile, by analyzing the Arrhenius fitting of the plot of IQE-temperature and leakage factor m, the key temperature and excitation power turning on the carrier leakage mechanism was roughly determined to be below 220 K and 10 mW, respectively. Such a low turn-on temperature and excitation power indicates a much easier carrier leakage mechanism in GaN-based MQW, which may be caused by the small effective electron mass of InGaN (0.11–0.22 m*) and the narrow thickness of quantum well via the model calculation of energy band structure via simulation software LASTIP. Moreover, higher IQE can be achieved by suppressing the carrier leakage mechanism via structural optimization (such as electron block layer) in GaN-based MQW.

Anomalous Temperature Dependence of Photoluminescence Caused by Non-Equilibrium Distributed Carriers in InGaN/(In)GaN Multiple Quantum Wells.

An increase of integrated photoluminescence (PL) intensity has been observed in a GaN-based multiple quantum wells (MQWs) sample. The integrated intensity of TDPL spectra forms an anomalous variation: it decreases from 30 to 100 K, then increases abnormally from 100 to 140 K and decreases again when temperature is beyond 140 K. The increased intensity is attributed to the electrons and holes whose distribution are spatial non-equilibrium distributed participated in the radiative recombination process and the quantum barrier layers are demonstrated to be the source of non-equilibrium distributed carriers. The temperature dependence of this kind of spatial non-equilibrium carriers’ dynamics is very different from that of equilibrium carriers, resulting in the increased emission efficiency which only occurs from 100 to 140 K. Moreover, the luminescence efficiency of MQWs with non-equilibrium carriers is much higher than that without non-equilibrium carriers, indicating the high luminescence efficiency of GaN-based LEDs may be caused by the non-equilibrium distributed carriers. Furthermore, a comparison analysis of MQWs sample with and without hydrogen treatment further demonstrates that the better quantum well is one of the key factors of this anomalous phenomenon.

High-efficiency near-UV light-emitting diodes on Si substrates with InGaN/GaN/AlGaN/GaN multiple quantum wells

High quantum efficiency LEDs with InGaN/GaN/AlGaN/GaN MQWs have been demonstrated. The proposed GaN interlayer barrier can not only increase the concentration and the spatial overlap of carriers, but also improve the quality of the MQWs.

Intersubband Transitions in Nonpolar GaN-based Resonant Phonon Depopulation Multiple-Quantum Wells for Terahertz Emissions

We investigate the polarization effect in intersubband transitions in polar and nonpolar GaN-based multiple-quantum well (MQW) structures for terahertz (THz) emissions by using systematic comparisons and design a nonpolar GaN/Al0.2Ga0.8N two-well-based MQW structure with an emitting photon of 7.27 THz (30.07 meV). Its lower energy separation (92.7 meV) matches the resonant phonon depopulation condition for better population inversion. It shows a lower threshold current density Jth at all temperatures (1.548 kA/cm2 at 90 K) and a higher output power of up to 86.1 mW at 5.8 K and 33.6 mW at 100 K. Our results for the polar GaN MQW are very close to the experimental data in the literature. We find that the Jth of the nonpolar GaN MQW increases more slowly than that of the polar GaN MQW as temperature increases, indicating the nonpolar GaN MQW may be a worth-trying direction for improving the operation temperature. These results can provide meaningful references for the design and fabrication of nonpolar GaN-based THz MQW or quantum cascade structures.

Polarization relaxation in InGaN/(In)GaN multiple quantum wells

In this paper, we report the polarization reduction of polarization field strength in InGaN/(In)GaN multiple quantum wells (MQWs), which is called polarization relaxation because its behavior is somehow similar to the strain relaxation in MQWs. It is found that the polarization is dependent not only on the MQWs structure, i.e. indium content in the well and barriers and the well thickness, but also on the growth conditions such as growth temperature. The threshold of polarization relaxation is related to the indium content in the quantum barriers. Further analysis shows that the strain energy in MQWs has significant influence on the degree of polarization relaxation. The critical layer thickness for the polarization relaxation is lower than that for the lattice relaxation, indicating that the polarization relaxation happens prior to the lattice relaxation. Moreover, the degree of polarization relaxation has a dependence on the growth temperature, indicating that the polarization relaxation can be enhanced by the thermal energy during epi-growth. The mechanisms behind this phenomenon is discussed.

Crystal Size and Stress Account in Reciprocal Space Map

In this study, five periodic InGaN / GaN LED (light emitting diode) structures grown by the Metal Organic Vapor Deposition (MOCVD) at different active layer growth temperatures were studied. These structures were grown as InGaN / GaN multiple quantum wells (MQW) between c-oriented sapphire substrate and n-GaN and p-AlGaN + GaN contacts. These constructions were characterized by the high-resolution X-ray diffraction (HR-XRD) system. HRXRD patterns obtained by X-ray diffraction and Reciprocal space maps were performed from the same data. One of the most effective ways of studying the crystal lattice is reciprocal space mapping with HR-XRD technique. This technique does not damage the sample. Information can be obtained from the internal system of the sample or from the intermediate layer including substrate. Using the FWHM (β hkl ) values and the elastic coefficients of the structures obtained for each of the three samples separately with the inverse mesh technique, D (nm) particle size, σ (GPa) uniform stress, e strain, u (kJm -3 ) anisotropic energy density parameters were calculated. These calculations were done in a Scherrer method and Uniform Deformation Model (UDM) which is the Williamson Hall method, modified uniform Williamson stresses model (USDM) and Uniform Deformation Energy Density Model (UDEDM). The results show that the stretching in the crystal size is very little. Line expansion in HR-XRD is due to small crystal size and lattice strain. UDEDM, one of the W-H methods, has emerged as the most suitable model for stretching..

Room-Temperature Lasing Action in GaN Quantum Wells in the Infrared 1.5 μm Region

Large-scale optoelectronics integration is strongly limited by the lack of efficient light sources, which could be integrated with the silicon complementary metal-oxide-semiconductor (CMOS) technology. Persistent efforts continue to achieve efficient light emission from silicon in the extending the silicon technology into fully integrated optoelectronic circuits. Here, we report the realization of room-temperature stimulated emission in the technologically crucial 1.5 micron wavelength range from Er-doped GaN multiple-quantum wells on silicon and sapphire. Employing the well-acknowledged variable stripe technique, we have demonstrated an optical gain up to 170 cm-1 in the multiple-quantum well structures. The observation of the stimulated emission is accompanied by the characteristic threshold behavior of emission intensity as a function of pump fluence, spectral linewidth narrowing and excitation length. The demonstration of room-temperature lasing at the minimum loss window of optical fibers and in the eye-safe wavelength region of 1.5 micron are highly sought-after for use in many applications including defense, industrial processing, communication, medicine, spectroscopy and imaging. As the synthesis of Er-doped GaN epitaxial layers on silicon and sapphire has been successfully demonstrated, the results laid the foundation for achieving hybrid GaN-Si lasers providing a new pathway towards full photonic integration for silicon optoelectronics.

Reduction of Polarization Field Strength in Fully Strained c-Plane InGaN/(In)GaN Multiple Quantum Wells Grown by MOCVD.

The polarization fields in c-plane InGaN/(In)GaN multiple quantum well (MQW) structures grown on sapphire substrate by metal-organic chemical vapor deposition are investigated in this paper. The indium composition in the quantum wells varies from 14.8 to 26.5% for different samples. The photoluminescence wavelengths are calculated theoretically by fully considering the related effects and compared with the measured wavelengths. It is found that when the indium content is lower than 17.3%, the measured wavelengths agree well with the theoretical values. However, when the indium content is higher than 17.3%, the measured ones are much shorter than the calculation results. This discrepancy is attributed to the reduced polarization field in the MQWs. For the MQWs with lower indium content, 100% theoretical polarization can be maintained, while, when the indium content is higher, the polarization field decreases significantly. The polarization field can be weakened down to 23% of the theoretical value when the indium content is 26.5%. Strain relaxation is excluded as the origin of the polarization reduction because there is no sign of lattice relaxation in the structures, judging by the X-ray diffraction reciprocal space mapping. The possible causes of the polarization reduction are discussed.

Dislocation Reduction and Stress Relaxation of GaN and InGaN Multiple Quantum Wells with Improved Performance via Serpentine Channel Patterned Mask.

The existence of high threading dislocation density (TDD) in GaN-based epilayers is a long unsolved problem, which hinders further applications of defect-sensitive GaN-based devices. Multiple-modulation of epitaxial lateral overgrowth (ELOG) is used to achieve high-quality GaN template on a novel serpentine channel patterned sapphire substrate (SCPSS). The dislocation blocking brought by the serpentine channel patterned mask, coupled with repeated dislocation bending, can reduce the dislocation density to a yet-to-be-optimized level of ∼2 × 10(5) to 2 × 10(6) cm(-2). About 80% area utilization rate of GaN with low TDD and stress relaxation is obtained. The periodical variations of dislocation density, optical properties and residual stress in GaN-based epilayers on SCPSS are analyzed. The quantum efficiency of InGaN/GaN multiple quantum wells (MQWs) on it can be increased by 52% compared with the conventional sapphire substrate. The reduced nonradiative recombination centers, the enhanced carrier localization, and the suppressed quantum confined Stark effect, are the main determinants of improved luminous performance in MQWs on SCPSS. This developed ELOG on serpentine shaped mask needs no interruption and regrowth, which can be a promising candidate for the heteroepitaxy of semipolar/nonpolar GaN and GaAs with high quality.

Titanium oxide nanoparticles spin-coated onto r-plane sapphire substrate: Effects on structural and optical properties of nonpolar a-plane GaN and InGaN/GaN multiple quantum wells

The structural and optical properties of nonpolar a-plane (11−20) GaN and In0.18GaN0.82/GaN multiple quantum wells (MQWs) grown on TiO2 nanoparticle (NP)-coated r-plane (1−102) sapphire substrate have been investigated. A smooth surface morphology was observed by atomic force microscopy (AFM), indicating a good lateral overgrowth on TiO2 NPs. Transmission electron microscopy (TEM) images revealed a reduction in the density of basal stacking faults (BSFs) and threading dislocations (TDs) in the TiO2 NP-coated sample. From X-ray diffraction measurements, the broadening of ω-scan full width at half maximum (FWHM) was influenced by the tilt of the mosaic due to TiO2 NPs. A reduced PL-integrated intensity ratio (IBSF/INBE) in low-temperature photoluminescence (PL) spectra of the a-plane GaN film grown over TiO2 NPs was consistent with TEM results. The improved room temperature-PL intensity of TiO2-embedded MQWs is associated with the increase in light extraction efficiency, due to the TiO2 NPs and air voids. Temperature-dependent PL spectra revealed that the dominant PL emission was attributed to localization of carriers in quantum wire-like regions, where the BSFs intersect the QWs. The observed quantum efficiency at 300K also showed a good quantum confinement, even though alloy fluctuations might be expected in the QWs.

Magnitude-tunable sub-THz shear phonons in a non-polar GaN multiple-quantum-well p-i-n diode

Coherent transverse acoustic phonons are optically generated and detected through the piezoelectric coupling between the build-in electric fields and shear strains of a non-polar GaN multiple quantum wells embedded in a p-n junction. By optical transient transmission change measurement, the phonon frequency is observed to be 0.4 THz which corresponds to a wavelength of 12.5 nm, the periodicity of the multiple quantum wells, and the estimated phonon velocity corresponds to the transverse acoustic phonon velocity in GaN. Moreover, we can magnify the driving amplitude of the generated shear phonons by increasing the reverse bias of the p-i-n diode.

Investigation of Emission Polarization and Strain in InGaN–GaN Multiple Quantum Wells on Nanorod Epitaxially Lateral Overgrowth Templates

Non-polar (a-plane) InGaN-GaN multiple quantum wells (MQWs) on the GaN nanorod epitaxially lateral overgrowth templates with different nanorod height have been fabricated. The average in-plane strain in the InGaN MQWs has been determined from 2.73 × 10-2 to 2.58 × 10-2 while the nanorod height in templates increases from 0 to 1.7 μm. The polarization ratio of the emission from InGaN MQWs varies from 85 % to 53 % along with the increase of the GaN nanorod height. The reduction of polarization ratio has been attributed to the partial strain relaxation within the epitaxial structures as a result of growth on the GaN nanorod templates and the micro-size air-voids observed in the nanorod templates.

Strain relaxation effect by nanotexturing InGaN/GaN multiple quantum well

The relaxation of lattice-mismatched strain by deep postetching was systematically investigated for InGaN/GaN multiple quantum wells (MQWs). A planar heterojunction wafer, which included an In0.21Ga0.79N (3.2 nm)/GaN (14.8 nm) MQW, was etched by inductively coupled plasma dry etching, to fabricate high-density nanopillar, nanostripe, and nanohole arrays. The etching depth was 570 nm for all nanostructures. The diameter of the nanopillars was varied from 50 to 300 nm, then the mesa stripe width of the nanostripes and the diameter of the nanoholes were varied from 100 nm to 440 nm and 50 nm to 310 nm, respectively. The effect of strain relaxation on various optical properties was investigated. For example, in an array of nanopillars with diameter 130 nm and interval 250 nm, a large blueshift in the photoluminescence (PL) emission peak from 510 nm (as-grown) to 459 nm occurred at room temperature (RT). PL internal quantum efficiency (defined by the ratio of PL integral intensity at 300 K to that at 4.2 K) was enhanced from 34% (as-grown) to 60%, and the PL decay time at 4.2 K was reduced from 22 ns (as-grown) to 4.2 ns. These results clearly indicate the reduction of lattice-mismatched strain by postetching, which enhanced strain reduction with decreasing nanopillar diameter down to a diameter of 130 nm, where the strain reduction became saturated. The dependence of RT-PL decay time on nanopillar diameter was measured, and the surface nonradiative recombination velocity was estimated to be 5.8×102 cm/s. This relatively slow rate indicates a little etching damage.

Optical and Structural Properties of In0.08GaN/In0.02GaN Multiple Quantum Wells Grown at Different Temperatures and with Different Indium Supplies

The optical and structural properties of In0.08Ga0.92N/In0.02Ga0.98N multiple quantum wells (MQWs) grown at different temperatures and with different supplies of indium were analyzed by atomic force microscopy and spectrally resolved cathodoluminescence (CL). By comparing the contrasts of monochromatic CL images with high-resolution secondary-electron images of the sample surface, it is shown that almost all contrasts of the CL images can be explained by lateral inhomogeneities of both the thickness and the InN mole fraction of the InGaN layers. Dark contrasts in the CL images solely related to dislocations were not observed, indicating very weak nonradiative recombination correlated with threading dislocations in the InGaN quantum wells. The lateral inhomogeneities of layer thickness and indium incorporation depend strongly on the growth conditions.

The Structure of GaN-Based Transverse Junction Blue LED Array for Uniform Distribution of Injected Current/Carriers

In this study, we demonstrate a GaN-based transverse junction blue LED array. This device was realized by the regrowth of n-type GaN layers on the sidewall of p-type GaN and undoped multiple quantum wells (MQWs). Due to the transverse flow of injection carriers, problems related to nonuniform current distribution, nonuniform carrier distribution among different MQWs, and bias-dependent shape of the electroluminescence spectra such as that occurring in traditional GaN-based blue LEDs with vertical p-n junctions and large active area (>1 mm2) are all greatly minimized in our structure.

Optimization of InGaN–GaN MQW Photodetector Structures for High-Responsivity Performance

InGaN-GaN multiple-quantum-well (MQW)-based photodetectors, with a detection edge at 450 nm and a high responsivity, have been fabricated and characterized. We show that the performance of MQW-based photodetectors strongly depends on a proper device design, i.e., number of QWs, and barrier and blocking layer thickness and doping level. Namely, the responsivity can be varied in the ~ 1 to ~ 100 mA/W range in similar structures and with the same number of QWs. These results support a model where the photocurrent increase is due to the improvement of collection efficiency caused by a change in transport mechanism for carriers photogenerated in the QWs. The transport mechanism depends on the location of the QWs in relation to the depletion region.

In Ga N ∕ Ga N multiple quantum wells grown on microfacets for white-light generation

We report the white color electroluminescence (EL) emission from InGaN∕GaN multiple quantum wells (MQWs) grown on GaN microfacets. The white color was realized by combining EL emission from InGaN∕GaN MQWs on c-plane (0001), semipolar {11−22}, and {1−101} microfacets of trapezoidal n-GaN arrays. The color of EL emission was changed from reddish to bluish color with injection current and showed a white color in the current range of 180–230mA. The variation in the color of EL emission was attributed to differences in current injection and quantum efficiency of MQWs grown on c-plane (0001) and semipolar GaN microfacets.

Enhancing the Emission Efficiency of In0.2Ga0.8N ∕ GaN MQW Blue LED by Using Appropriately Misoriented Sapphire Substrates

In this paper, we report the fabrication of an multiple-quantum-well (MQW) structure on vicinal sapphire substrates with a very small offset angle of 0–1° by low-pressure metallorganic chemical vapor deposition. Our study demonstrates that the quality of the MQW structure is very sensitive to the offset angle of the vicinal substrate. High-resolution X-ray diffraction analyses demonstrated high-order satellite peaks and clear fringes between them for all MQW structures fabricated, from which the interface roughness (IRN) was estimated. The IRN of the MQW structure fabricated on 0.2° off sapphire substrate was determined as 1.36% of the quantum well layer period. Besides, reciprocal lattice mapping was employed to examine the strain status of the MQW. The lattice relaxation of the same specimen mentioned above was estimated to be . It is therefore manifested that an MQW structure with abrupt interfaces and good layer periodicity was grown. From it, a shortest radiative lifetime of and a lowest fluctuation of in the emission energy of micro-photoluminescence mapping were achieved. In addition, superior material qualities of the whole film fabricated on 0.2° off substrate were recognized by cross-sectional transmission electron microscopy. Based on the results mentioned above, a high-quality MQW blue light, emitting diode (LED) has been fabricated on 0.2° off substrate, which demonstrated a strong room-temperature electroluminescence emission at the wavelength of and with the full width at half maximum of only . The same device also showed an output power of and an external quantum efficiency of 19.2%. Both these characteristics are improved drastically compared with the devices fabricated on the substrates with other offset angles. Conclusively, the use of an appropriately misoriented sapphire substrate is suggested to be effective for elevating the emission efficiency of MQW blue LED fabricated thereon.