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(a) Schematic structure of grown and simulated LEDs. (b) Example photographs of measured devices operating on intermediate single QW in low and high current density.
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The design of the active region is one of the most crucial problems to address in light emitting devices (LEDs) based on III-nitride, due to the spatial separation of carriers by the built-in polarization. Here, we studied radiative transitions in InGaN-based LEDs with various quantum well (QW) thicknesses—2.6, 6.5, 7.8, 12, and 15 nm. In the case...
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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). Th...
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... However, at higher carrier densities this is no longer the case as the electrostatic built-in field will be screened. Previous theoretical and experimental works indicate that for the QW systems targeted here (see discussion below on the QW model system), the screening effect becomes important at carrier densities on the order of n = 10 19 cm −3 and above [6,26,27]. Thus, a fully self-consistent approach is required. ...
... Overall, and based on the method described above, we find that the screening of the built-in field becomes noticeable (i.e. reduction of the electrostatic built-in field by, for example, more than 10%) for carrier densities exceeding n = 5 × 10 18 cm −3 ; this finding is in line with recent literature data [6,27]. ...
Understanding Auger recombination in (In,Ga)N-based quantum wells is of central importance to unravelling the experimentally observed efficiency “droop” in modern (In,Ga)N light emitting diodes (LEDs). While there have been conflicting results in the literature about the importance of non-radiative Auger recombination processes for the droop phenomenon, it has been discussed that alloy fluctuations strongly enhance the Auger rate. However, these studies were often focused on bulk systems, not quantum wells, which lie at the heart of (In,Ga)N-based LEDs. In this study, we present an atomistic analysis of the carrier density dependence of the Auger recombination coefficients in (In,Ga)N/GaN quantum wells. The model accounts for random alloy fluctuations, the connected carrier localization effects, and carrier density dependent screening of the built-in polarization fields. Our studies reveal that at low temperatures and low carrier densities the calculated Auger coefficients are strongly dependent on the alloy microstructure. However, at elevated temperatures and carrier densities, where the localized states are saturated, the different alloy configurations studied give (very) similar Auger coefficients. We find that over the range of carrier densities investigated, the contribution of the electron-electron-hole related Auger process is of secondary importance compared to the hole-hole-electron process. Overall, for higher temperatures and carrier densities, our calculated total Auger coefficients are in excess of 10 ⁻³¹ cm ⁶ s ⁻¹ , which, based on current understanding in the literature, is sufficient to result in a significant efficiency droop. Thus, our results are indicative of Auger recombination being a significant contributor to the efficiency droop in (In,Ga)N-based light emitters even without defect assisted processes
... reasons for the high efficiency of wide QWs are the screening of the electric field due to high carrier density as well as the involvement of excited states, leading to a high wave-function overlap. In various papers [16][17][18][19][20][21][22], experimental evidence for transitions between excited states has been reported and simulations of wide QWs with screened electric field show high overlap of the first excited states e 2 h 2 , but even higher states have rarely been considered. In asymmetric QWs, the selection rules are relaxed, allowing all transitions between sublevels with their strength depending on the screening of the field. ...
... Now, how can we understand this unusual behavior of below-threshold emission in wide-well laser diodes? Conventional devices with a QW thickness of 2-3 nm usually exhibit only one peak featuring a moderate blue-shift for increasing current [21]. This corresponds to the ground-state transition e 1 → h 1 and excited states are not involved here, because of their vanishing population due to the large energy difference between ground and excited states in thin QWs. ...
The properties of III-nitride quantum wells (QWs) are significantly affected by polarization fields. We show that, in the case of wide QWs, there exist two fundamentally different transition types characterized by quantum-confinement effects or bulklike behavior—depending on the carrier density in the structure. At low carrier density, spontaneous and piezoelectric polarization lead to tight confinement and low spatial overlap between the electron and hole wave functions. However, if the carrier density is sufficient for screening of the polarization fields, the wave functions spread out, reach high overlap, and the energy states come close to each other, similar to a bulk semiconductor. Streak camera measurements in the subthreshold regime of nitride laser diodes and numerical simulations show consistent results and let us clearly differentiate between contributions of individual states for low carrier density and the emergence of a high density of energetically close states at high carrier density.
... However, a technologically much simpler way of eliminating the built-in field using the well-established growth on c-plane is to move to wider wells (10-25 nm). There, the observed radiative recombination significantly increases compared to the conventional wells [10][11][12][13][14]. The reason for this is the efficient screening of built-in field by electrons and holes, which occupy the widely separated ground states E 1 and H 1 (localized at the opposite edges of the well). ...
... There is almost no wavelength shift for the wide wells. Similar results have been obtained in previous papers [12][13][14]. This suggests that as soon as the wide well starts to emit light, the electric field in the well is almost fully screened (see Fig. 1); the emission intensity changes with excitation power but the emission wavelength remains stable, which implies a stable electric field in the well. ...
Wide (15-25 nm) InGaN/GaN quantum wells in LED structures were studied by time-resolved photoluminescence (PL) spectroscopy and compared with narrow (2.6 nm) wells in similar LED structures. Using below-barrier pulsed excitation in the microsecond range, we measured increase and decay of PL pulses. These pulses in wide wells at low-intensity excitation show very slow increase and fast decay. Moreover, the shape of the pulses changes when we vary the separation between them. None of these effects occurs for samples with narrow wells. The unusual properties of wide wells are attributed to the presence of “dark charge” i.e., electrons and holes in the ground states. Their wave functions are spatially separated and due to negligible overlap they do not contribute to emission. However, they screen the built-in field in the well very effectively so that excited states appear with significant overlap and give rise to PL. A simple model of recombination kinetics including “dark charge” explains the observations qualitatively.
... A schematic drawing showing the final structure is shown in Fig. 4. The groove needed for ECE is positioned next to the laser ridge and p-type metallization covers the opposite side of the device. P-type contact metallization consists of two parts, thin and thick, deposited in the first and last processing step, respectively, as previously reported [35,37]. After the deposition of the thin p-type contact metallization (denoted in orange color in Fig. 4), 5-µm-wide laser mesas were formed by reactive ion etching (RIE). ...
We demonstrate electrically pumped III-nitride edge-emitting laser diodes (LDs) with nanoporous bottom cladding grown by plasma-assisted molecular beam epitaxy on c-plane (0001) GaN. After the epitaxy of the LD structure, highly doped 350 nm thick GaN:Si cladding layer with Si concentration of 6·10¹⁹ cm⁻³ was electrochemically etched to obtain porosity of 15 ± 3% with pore size of 20 ± 9 nm. The devices with nanoporous bottom cladding are compared to the reference structures. The pulse mode operation was obtained at 448.7 nm with a slope efficiency (SE) of 0.2 W/A while the reference device without etched cladding layer was lasing at 457 nm with SE of 0.56 W/A. The design of the LDs with porous bottom cladding was modelled theoretically. Performed calculations allowed to choose the optimum porosity and thickness of the cladding needed for the desired optical mode confinement and reduced the risk of light leakage to the substrate and to the top-metal contact. This demonstration opens new possibilities for the fabrication of III-nitride LDs.
... The groove needed for ECE is positioned next to the laser ridge and p-type metallization covers the opposite side of the device. P-type contact metallization consists of two parts, thin and thick, deposited in the first and last processing step, respectively, as previously reported 34,36 . After the deposition of the thin p-type contact metallization (denoted in orange color in Figure 4), 5-µm-wide laser mesas were formed by reactive ion etching (RIE). ...
We demonstrate electrically pumped III-nitride edge-emitting laser diodes (LDs) with nanoporous bottom cladding. The LD structure was grown by plasma-assisted molecular beam epitaxy. Highly doped 350 nm thick GaN:Si cladding layer with Si concentration of 6 x 1019 cm-3 was electrochemically etched to obtain porosity of 15 +/- 3% with pore size of 20 +/- 9 nm. The devices with nanoporous bottom cladding are compared to the refer-ence structures. The pulse mode operation was obtained at 448.7 nm with a slope efficiency (SE) of 0.2 W/A while the reference device without etched cladding layer was lasing at 457 nm with SE of 0.56 W/A. The de-sign of the LDs with porous bottom cladding was modelled theoretically. Performed calculations allowed to choose the optimum porosity and thickness of the cladding needed for the desired optical mode confinement and reduced the risk of light leakage to the substrate and to the top-metal contact. This demonstration opens new possibilities for the fabrication of III-nitride LDs.
Ultrapure gallium up to 99.9999%/ 99.99999% (6N/7N) purity level is a highly demanding material needed for the growth of gallium‐based group III–V semiconductor compounds and optoelectronic devices. However, general extraction of gallium from Bayer liquor contains high impurity content and ultra‐purification of the same cannot be accomplished by a single step. Thus, the purpose of this review is to assess various purification processes for the production of ultra‐pure gallium and to critically examine its applications in the optoelectronics industry. Through this research survey, it is found that zone refining of the zone melting process stands tall over other methods in purifying materials even up to 13N. Hence, scientists are adopting detailed mathematical models and simulation tools for designing unique zone refining systems for material purification. Current‐day technology even adopts intelligence methods such as machine learning, which sheds light on the importance of different zone refining parameters that influence the purification process. Here, the practical aspects of zone refining and how the feedback from the theoretical models or performance prediction through intelligence methods can be effectively incorporated into practice have also been emphasized
This study examines the effects of negative voltage pulses (NVPs) on emission from (In,Ga)N/GaN light-emitting diodes (LEDs) with a wide quantum well in the active layer. For low dc forward voltages (1–3 V), the diode emits light only during the periodic application of the NVPs, provided that the separation of NVPs is sufficiently large (from a few microseconds to milliseconds). These effects are due to a slow buildup of “dark charge,” i.e., charge in the ground state of the wide well. The NVPs reduce the electric field in the well and deplete dark charge from the well (through radiative and nonradiative recombination). After the NVP, the concentration of dark charge once again slowly increases and saturates. For low forward voltages (or currents), the saturation occurs after milliseconds, which is caused by the long buildup times of dark charge. By varying the duration of NVPs, we can estimate the decay time of dark charge for a given negative voltage. This decay time decreases rapidly with increasing (absolute value of) negative voltage. We present a numerical model of LED emission, which is in qualitative agreement with experimental results. Short pulses of light generated by NVPs may create new applications for wide-well LEDs and laser diodes.
III-nitrides possess several unique qualities, which allow them to make the world brighter, but their uniqueness is not always beneficial. The uniaxial nature of the wurtzite crystal leads to strikingly large electric polarization fields, which along with the high acceptor ionization energy cause low injection efficiency and uneven carrier distribution for multiple quantum well (QW) light emitting devices. In this work, we explore the carrier distribution in Ga-polar LED in two configurations: standard “p-up” and “p-down”, which is accomplished by utilizing a bottom-tunnel junction. This enables the inversion of the sequence of the p and n layers while altering the direction of the current flow with respect to the inherent polarization. To probe the carrier distribution two, color-coded QWs are used in alternating sequences. Our study reveals that for “p-down” devices carrier transport through multiple QWs is limited by the potential barrier at the QW interface, which is in contrast to results for “p-up” structures, where hole mobility is the bottleneck. Moreover, investigated “p-down” LEDs exhibit an extremely low turn-on voltage.
Herein, the lasing threshold and gain characteristics of ultraviolet‐C optically pumped edge‐emitting lasers with thick single‐quantum‐well (SQW) active regions are investigated by the variable‐stripe length method. Positive net modal gain is observed in lasers with AlGaN‐based SQWs with thicknesses up to 12 nm. The lasers show a reduction of the threshold power density with increasing SQW thickness, with the lowest threshold of 1.3 MW cm ⁻² achieved for a 9 nm SQW laser. The high gain and low threshold in lasers with thick quantum wells are attributed to lasing from excited states, where the polarization fields are screened by the carriers in the fundamental state and the barriers, thus recovering larger electron and hole wavefunctions overlap. These findings are supported by k · p simulations, and for a 9 nm SQW, the calculation predicts a contribution of the fundamental transition to the gain of 75% for non‐resonant optical excitation and below 1% for resonant optical or electrical excitation.
