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EL spectra at 100 K of (a) SQW-LED, (b) VWT-LED and (c) MQW-LED. Different wave peaks dominate in emission spectrum with increasing current density in region W1, W2 and W3.
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Temperature-dependent electroluminescence from InGaN/GaN light-emitting diodes (LEDs) grown on Si (111) are investigated. With the increase of current density, internal quantum efficiencies (IQEs) firstly rise accompanied by full width at half maximum (FWHM) shrinkage and then IQEs droop combined with FWHM broadening are presented. With the decline...
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... EL spectra test, a roughening process on the n-GaN surface was adopted to eliminate the Fabry-Perot interference oscillations [29]. EL spectra at 100 K of three samples are presented in figure 3. Inspection of the figure reveals that the spectra of SQW LED consist of one Gauss-like wave peak. ...
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... As the current injection increases, the FWHM of both devices also increases. The changes in FWHM may be ascribed to the internal quantum efficiency (IQE) droop [29] or the increase of carrier density (N) and the temperature (T) in QWs. Changing N and T may result in variation of carrier distribution in the allowed bands, and all result in increasing FWHM. ...
Micro-nano optomechanical accelerometers are widely used in automobile, aerospace, and other industrial applications. Here, we fabricate mechanical sensing components based on an electrically pumped GaN light-emitting diode (LED) with a beam structure. The relationship between the blueshift of the electroluminescence (EL) spectra and the deformation of the GaN beam structure based on the quantum-confined Stark effect (QCSE) of the InGaN quantum well (QW) structure is studied by introducing an extra mass block. Under the equivalent acceleration condition, in addition to the elastic deformation of GaN-LED, a direct relationship exists between the LED’s spectral shift and the acceleration’s magnitude. The extra mass block (gravitational force: 7.55×10 ⁻¹¹ N) induced blueshift of the EL spectra is obtained and shows driven current dependency. A polymer sphere (PS; gravitational force: 3.427×10 ⁻¹² N) is placed at the center of the beam GaN-LED, and a blueshift of 0.061 nm is observed in the EL spectrum under the injection current of 0.5 mA. The maximum sensitivity of the acceleration is measured to be 0.02 m/s ² , and the maximum measurable acceleration is calculated to be 1.8×10 ⁶ m/s ² . It indicates the simultaneous realization of high sensitivity and a broad acceleration measurement range. This work is significant for several applications, including light force measurement and inertial navigation systems with high integration ability.
... However, for the measurements of micro-LEDs' optoelectronic properties under the stress tests, previous works mainly focused on the optical power by using an integrating sphere under room temperature and studied the degradation behaviors [13,14]. In addition, the temperature and injection current dependence of electroluminescence just have been utilized to study the optical properties of InGaN quantum-well structure [15,16]. But till now, using the injection current-and temperature-dependent EL spectra of the InGaN/GaN MQWs in aging study to investigate the degradation mechanisms have been less reported. ...
... The radiative recombination of higher-energy-level electrons and the holes in the valence band would release higher energy photon, causing the appearance of the shorter-wavelength emission peak 1 in the EL spectra, as shown in Fig. 3(a) and 3(c). In addition, the defects formed by current stress in quantum barriers would capture a part of the carriers injected into the active region, but these defects can be frozen at low temperature and lose the binding effect on the carrier [16]. The radiative recombination of the carriers in quantum barrier under low temperature and injection current can result in another emission peak at 390 nm (peak 2) in the EL spectra. ...
... Under low injection current, the carrier band filling effect would increase with the increase of injection current density. The filling carriers of the low-energy localized states in quantum well dominate the radiative recombination process of the MQWs, leading to an initial steep broadening of FWHM, especially at low temperature [16]. Under temperature 5 K, the recombination process of electron-hole pairs in the QW region is dominated by low-energy state filling. ...
InGaN-based micro-LEDs are promising for many applications, including visible light communication (VLC), micro-display, etc. However, to realize the above full potential, it is important to understand the degradation behaviors and physical mechanisms of micro-LEDs during operation. Here, the optoelectronic properties of InGaN-based blue micro-LEDs were investigated over a wide range of injection currents (1-100 mA) and temperatures (5-300 K) before and after stress. The results show that the optical power of the micro-LED degrades after stress, especially at lower current density, indicating that the Shockley-Read-Hall (SRH) nonradiative recombination increased for the stressed device. In addition, the slopes of log L-log I curves changes from 1.0 to 2.1 at low current density, and the ideality factor extracted from the I-V curves change from 1.9 to 3.4 after current stress, indicating there is an increase of the defects in the active layer after stress. The activation energy of defects evaluated from the temperature-dependent electroluminescence (EL) spectra is about 200 meV, which could be related to the N-vacancy related defects. Besides, the peak wavelength, peak energy and the full width at half maximum of the injection current- and temperature-dependent EL spectra were discussed. The electron-hole pair combines in the form of SRH nonradiative recombination, causing some carriers to redistribute and a state-filling effect in higher-energy states in multiple quantum wells (MQWs), resulting in the appearance of the shorter-wavelength luminescence in the EL spectra. These findings can help to further understand the degradation mechanisms of InGaN micro-LEDs operated under high current density.
... The FWHM variation could be divided into three regions. The first region was from 1 to 20 A cm −2 , where the FWHM shrank due to the screen effect [46], and reached an extreme value of 28.95 nm, coinciding with the peak position of EQE. In the second region from 20 to 5 kA cm −2 , the FWHM kept elevating owing to the carrier band filling effect. ...
In this paper, the GaN-based green Micro-LEDs with various sizes (from 3 to 100 μm) were fabricated and electro-optically characterized. Atom layer deposition (ALD) passivation and potassium hydroxide (KOH) treatment were applied to eliminate the sidewall damage. The size dependence of Micro-LED was systematically analyzed with current-versus-voltage and current density-versus-voltage relationship. According to the favorable ideality factor results (<1.5), the optimized sidewall treatment was achieved when the device size shrank down to <10 μm. In addition, the EQE droop phenomenon, luminance and output power density characteristics were depicted up to the highest current density injection condition to date (120 kA/cm2), and 6 μm device exhibited an improved EQE performance with the peak EQE value of 16.59% at 20 A/cm2 and over 600k and 6M cd/cm2 at 1 and 10 A/cm2, indicating a greater brightness quality for over 3000 PPI multiple display application. Lastly, the blue shift of 6 μm device with elevating current density was observed in electroluminescence (EL) spectra and converted to CIE 1931 color space. The whole shifting track and color variation from 1 A/cm2 to 120 kA/cm2 were demonstrated by color coordinates.
... In other words, the NBE to the YL intensity ratio increased. By contrast, as shown in Figure 2b, compared to the PL-325 spectra, the PL-405 spectra at low and high excitation powers were dominated by green emission at approximately 2.41 eV, which is similar to that obtained from the green InGaN/GaN MQWs-based LEDs [19,20]. In addition, a band-tail state was observed on the low-energy side of the PL-405 spectrum at Crystals 2021, 11, 1061 4 of 10 low excitation power. ...
... In other words, the NBE to the YL intensity ratio increased. By contrast, as s Figure 2b, compared to the PL-325 spectra, the PL-405 spectra at low and high e powers were dominated by green emission at approximately 2.41 eV, which is s that obtained from the green InGaN/GaN MQWs-based LEDs [19,20]. In addition tail state was observed on the low-energy side of the PL-405 spectrum at low e power. ...
The excitation power and temperature dependence of the photoluminescence (PL) and electroluminescence (EL) spectra were studied in green InGaN/GaN multiple quantum well (MQW)-based light-emitting diodes (LED). An examination of the PL-325, PL-405, and EL spectra at identical optical or electrical generation rates at room temperature showed that the normalized spectra exhibited different characteristic peaks. In addition, the temperature behavior of the peak energy was S-shaped for the PL-405 spectrum, while it was V-shaped for the EL spectrum. These measurement results demonstrate that the excitation source can affect the carrier dynamics about the generation (injection), transfer, and distribution of carriers.
... The obtained values of FWHM at 80 mA are 0.361, 0.381 and 0.382 eV for the EL spectra measured before, after 50 h and 100 h stress, respectively. The slight broadening of FWHM is associated with the band filling effect at 80 mA [32,33] which may enhance the nonradiative recombination rate, such as, Auger recombination [32]. Moreover, the broadening of FWHM and the maximum peak position shift during the treatment lead for the increase of device junction temperature, self-heating effect and compositional inhomogeneities in the active region [30,[34][35][36][37][38]. ...
... The obtained values of FWHM at 80 mA are 0.361, 0.381 and 0.382 eV for the EL spectra measured before, after 50 h and 100 h stress, respectively. The slight broadening of FWHM is associated with the band filling effect at 80 mA [32,33] which may enhance the nonradiative recombination rate, such as, Auger recombination [32]. Moreover, the broadening of FWHM and the maximum peak position shift during the treatment lead for the increase of device junction temperature, self-heating effect and compositional inhomogeneities in the active region [30,[34][35][36][37][38]. ...
A comprehensive analysis of the degradation behavior of (In)AlGaN-based deep-ultraviolet light-emitting diode (DUV LED) stressed at a constant dc current of 60 mA has been presented. This work is based on combined X-ray diffraction (XRD), atomic force microscopy (AFM), electroluminescence (EL), cathodoluminescence (CL), current-voltage (I–V) and capacitance-voltage (C–V) characteristics. We used XRD and AFM data to investigate the structural properties of the epilayer. The EL spectrum shows two clear emission peaks at around 4.71 and 3.77 eV and a plateau region within 4.24 to 3.82 eV. To know the origin of the plateau region and the peak at 3.77 eV, we performed CL measurement. As a consequence of constant current stress, we obtained the following
results: (1) a gradual degradation in EL power due to increase of Shockley–Read–Hall (SRH) recombination and the generation of new point defects in the active region; (2) a significant improvement in the leakage current, that is attributed to the enhance in trap assisted tunneling; and (3) a strong increment in the device capacitance in both reverse and forward bias regions, that is ascribed to the improvement in the net charge concentration and
their redistribution in the active region of the LED.
... beside this, other growth conditions are the same as those of sample B. Fig. 1 shows the epitaxial structure diagram of the sample A, B and C. Vertical thin film LED chips with size of 1 × 1 mm 2 were fabricated with top surface (n-side) roughened and the p-GaN surface coated with Ag reflector. The detailed chip fabrication process has been reported [20]. The dominant wavelength of three samples is 551 nm under 35 A/cm 2 at room temperature. ...
The effects of the preparation layer grown under different conditions, which is an InGaN/GaN SLs structure inserted between the n-GaN layer and the MQWs, on the performance of green-yellow LED have been investigated. In this paper, the quality of InGaN/GaN multiple quantum wells (MQWs) and the optoelectronic properties of LED have been focused on. According to the experimental results, when the growth temperature of GaN barriers in SLs was increased, the interface between InGaN layers and GaN layers became more abrupt, and the indium distribution in the quantum wells became more uniform. In consequence, the forward voltage was reduced, the external quantum efficiency (EQE) was improved. By decreasing the growth rate of the high temperature barriers, the indium uniformity became even better, forward voltage was reduced again and EQE was further improved.
... The details of the chip fabrication process have been reported. 23,24 After processing, the LED chips were packaged as the LUXEON structure with silicone for LED encapsulation. [25][26][27] The electroluminescence (EL) measurements for packaged LEDs were performed by using the instrument, which contains a Keithley Instruments 2635A sourcemeter manufactured by Keithley Instruments, Inc. and a Instrument Systems CAS140CT spectrometer produced by MMR Technologies, Inc. ...
InGaN-based multiple quantum well (MQW) green light-emitting diodes with a InGaN/GaN superlattice as a strain relief layer (SSRL) were grown on Si(111) substrates by metal organic chemical vapor deposition. The influences of the thickness ratio of InGaN to GaN in SSRL on the optoelectrical properties have been investigated. Electrical measurements show that the sample with a higher thickness ratio has a lower series resistance. This is mainly ascribed to the improvement of carrier vertical transport due to the thinner GaN in SSRL. However, it is found that the leakage current increases with the thickness ratio from 1:1 to 2.5:1, which could be attributed to the larger density of small size V-pits forming at the first few QW pairs. Compared with the smaller thickness ratio, the sample with a higher thickness ratio of InGaN to GaN in SSRL is found to exhibit larger strain relaxation (about 33.7%), but the electroluminescence measurement exhibits inferior emission efficiency. Carrier leakage via the small V-pits and the rougher interface of MQW are believed to be responsible for the reduction of emission efficiency.
... These include the relaxation and thermal expansion of localized carriers induced by increasing temperature [10,11], the band-filling of localized states of injected carriers [11][12][13], and the enhanced scattering-induced carrier transfer from the shallower localized states down to deeper localized states by tunneling [14], due to the marked potential inhomogeneity in the InGaN well layers. Moreover, some optical features are also often explained by the Coulomb screening effect of QCSE and electron leakage or electron overflow [15][16][17]; however, to our knowledge, although the In-rich related photoluminescence (PL) properties have been widely investigated, the In-rich related electroluminescence (EL) properties are still not clearly understood and remain to be studied further [18]. Therefore, further investigation of the emission mechanisms in InGaN/GaN MQW-based LED structures with high In-contents, is deemed essential for further developing high-performance optoelectronics devices that emit photons at long wavelengths. ...
Injection current, and temperature, dependences of the electroluminescence (EL) spectrum from green InGaN/GaN multiple quantum well (MQW)-based light-emitting diodes (LED) grown on a Si substrate, are investigated over a wide range of injection currents (0.5 µA–350 mA) and temperatures (6–350 K). The results show that an increasing temperature can result in the change of injection current-dependent behavior of the EL spectrum in initial current range. That is, with increasing the injection current in the low current range, the emission process of the MQWs is dominated by filling effect of low-energetic localized states at the low temperature range of around 6 K, and by Coulomb screening of the quantum confinement Stark effect followed by a filling effect of the higher levels of the low-energetic localized states at the intermediate temperature range of around 160 K. However, when the temperature is further raised to the higher temperature range of around 350 K, the emission process of the MQWs in the low current range is dominated by carrier-scattering effect followed by non-radiative recombination process. The aforementioned current-dependent behaviors of the EL spectrum are mainly attributed to the strong localized effect of the green LED, as confirmed by the anomalous temperature dependence of the EL spectrum measured at the low injection current of 5 µA. In addition, the injection current dependence of external quantum efficiency at different temperatures shows that, with increasing temperature from 6 to 350 K, in addition to the enhanced non-radiative recombination, electron overflow becomes more significant, especially in the higher temperature range above 300 K.
... Such current-dependent EQE droop in its turn augments the self-heating. As the temperature increases, the proportion of carriers that takes place in these nonradiative process increases [14]. Han et al. [15] deposited graphene-oxide-microscale patterns on a sapphire substrate to improve heat dissipation. ...
Inevitable self-heating effects have been investigated for LEDs to date. In this paper, we present the thermally-induced penalty (TIP), and derive an upper external quantum efficiency (EQE) limit for a given bare LED by adding TIP to the real EQE. We find that the red-AlGaInP LED exhibits no efficiency droop under room temperature and the TIP is responsible for its real EQE droop. Among the TIPs of red, green, and blue LEDs, the red-AlGaInP LED suffers the most severe penalty. This TIP can serve as a criterion for LED cooling design, with its lower values leading to higher cooling rates.
... [1,4] 的 Si 衬底上生长 LED 外延结构 [6] : 包含 [19,20] . 一般而言, N 极性 ...
Light extraction efficiency of thin-film GaN-based light-emitting-diode (LED) chip can be effectively improved by surface roughening. The film transfer is an indispensable process in the manufacture of thin-film LED chip, which means transferring the LED film from the growth substrate to a new substrate, and then removing the growth substrate. After the growth substrate is removed, the buffer layer is used to cushion the mismatch between the substrate and the n-GaN exposed, which has a significant influence on the roughening behavior of n-GaN. Unlike the GaN buffer layer grown on sapphire substrate, AlN buffer layer is usually used when n-GaN is grown on Si substrate. In this paper, the surface treatment of the AlN buffer layer by reactive ion etching (RIE) is used to improve the surface roughening effect of N-polar n-GaN grown on the silicon substrate in the hot alkali solution (85℃, 20% KOH mass concentration of solution), and the mechanism of the influence of the surface treatment on the roughening behavior is discussed by X-ray photoelectron spectroscopy (XPS) and other advanced methods. The degree of etching surface AlN buffer layer is detected by energy dispersive spectrometer (EDS), the sample surface state after RIE etching is analyzed by XPS, the morphology of the surface roughening is observed by scanning electron microscope (SEM) and the effect of surface roughening on the optical power of LED devices is verified by the photoelectric performance test. The EDS results show that the AlN buffer layer remains after RIE etching 10-30 min and the AlN disappears after RIE etching for 40 min. The SEM results show that surface states of AlN buffer layer have a great influence on the roughening behavior of n-GaN in KOH solution. The sample with part of AlN buffer layer has a good roughening effect and proper size hexagonal pyramid distributing uniformly. In addition, the rate of coarsening is too fast for the samples with AlN buffer layer completely removed, while the rate is too slow for the samples without any etching process. In summation, using RIE etching to remove a part of the AlN buffer layer can effectively improve the roughening effect of N-polar n-GaN in KOH solution. We believe that lots of N-vacancies are produced on the surface of the sample after RIE etching, which provides the electrons, thereby causing the surface Fermi level to be elevated. The XPS analysis shows that the RIE etching can improve the electronic binding energy of Al 2p of AlN buffer layer, resulting in a shift of the surface Fermi level near to the conduction band, and reducing the Schottky barrier between the KOH solution and the surface of the sample, which is beneficial to the surface roughening. To remove a part of the AlN buffer by using plasma etching layer can improve the roughening effect of N-polar n-GaN in KOH solution, resulting in the output power of the corresponding LED device being improved obviously.
