Open Access
Add to BibSonomy
Abstract
A novel approach to fabricate efficient nitride light-emitting diodes (LEDs) grown on gallium polar surface operating at cryogenic temperatures is presented. We investigate and compare LEDs with standard construction with structures where p-n junction field is inverted through the use of bottom tunnel junction (BTJ). BTJ LEDs show improved turn on voltage, reduced parasitic recombination and increased quantum efficiency at cryogenic temperatures. This is achieved by moving to low resistivity n-type contacts and nitrogen polar-like built-in field with respect to current flow. It inhibits the electron overflow past quantum wells and improves hole injection even at T=12K. Therefore, as cryogenic light sources, BTJ LEDs offer significantly enhanced performance over standard LEDs.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Unique properties of group III-V nitride semiconductors led to variety of applications in high power and high efficiency optoelectronic and high power and frequency electronic devices [1,2]. But unique versatility of this material group allows to further expand its applications into new fields of photonic and electronic devices based on quantum effects [3]. To fully use this potential, a need for reliable cryogenic light sources emerges. Such extremely low temperature puts strong emphasis on a well-known challenges related to high ionization energy of Mg acceptor in GaN [4].
Typically, GaN based light-emitting diodes (LEDs) are grown using metal organic vapor phase epitaxy (MOVPE). In this growth technique, as-grown Mg doped layers are inactive due to the passivation by hydrogen atoms introduced during growth. In order to activate p-type conductivity Mg-H bonds need to be broken by thermal annealing [5] or electron irradiation [6]. Unfortunately, the hydrogen can be removed from the structure only through the exposed surfaces. For p-type layers located below n-type, this lead to only partial and ununiform activation through side walls, resulting in current spreading issues [7]. The other epitaxy technology - plasma assisted molecular beam epitaxy (PAMBE) is the hydrogen free technique, giving possibility to obtain high hole concentration in caped p-type layers without any annealing [8,9]. PAMBE was used before to construct efficient light emitters [10,11] and stacks of laser diodes interconnected by tunnel junctions (TJs) [9].
Even for hydrogen-free layers p-type conductivity is still low due to the activation energy for Mg acceptor reaching 170 meV, resulting in low holes concentration [4]. The low p-type conductivity is especially noticeable at cryogenic temperatures where carrier freezeout in the p-type layers leads to: increased diode operation voltage [12,13], low injection efficiency (IE) [14] enabling carriers overshot past quantum well (QW) [15] and uniformity of emitted wavelength associated with radiative recombination outside of QW decreasing light intensity [16,17].
In this study we address mentioned challenges by using highly doped TJ contact, which conductivity weakly depends on temperature [18,19]. In first approach we integrate LED with TJ in top-TJ (TTJ) arrangement, where the p-type top contact layers of LED are replaced by conductive n-type layer enhancing current spreading and allowing for smaller metal footprint. This type of devices was previously described in literature [20–22] and even allows for further stacking of devices [9,23,24]. Lower contact resistance and thinner resistive p-type layers should allow to mitigate the increase in operation voltage with decreasing temperature. On the other hand, structure would still suffer from decreased IE and resulting parasitic recombination outside the active region at low temperatures caused by severe carrier freezeout in electron blocking layer (EBL).
In the second approach, we integrate LED with TJ in bottom-TJ (BTJ) design. For this construction the order of p and n type layers in LED (and forward bias direction) is inversed comparing to standard LED to inverse built-in polarization direction with respect to the current flow. This built-in, nitrogen-polar-like polarization creates barriers for carrier overflow. Such barriers at QW interfaces act similarly to an EBL by limiting electron (and hole) overflow leading to increased recombination in the QW (and reducing any parasitic recombination elsewhere) [25]. This type of current overflow mechanism does not rely on temperature, contradictory to standard p-type doped EBLs, what is crucial for low temperature performance. To allow for straight forward comparison of investigated structures PAMBE was used.
2. Experimental
Investigated samples are schematically shown in Fig. 1. Structures were grown by PAMBE under metal-rich conditions on bulk n-type GaN substrate along [0001] direction. A reference, typical p-type up LED [Fig. 1(a)] and LED with TTJ [(Fig. 1(b)] shared the same blue LED part consisting of 2.6 nm In0.17Ga0.83N QW sandwiched between 20 nm In0.04Ga0.96N UID barriers followed by 20 nm p-type Al0.13Ga0.87N:Mg EBL and GaN:Mg cladding doped with Mg at concentrations 3·1019 cm-3and 1·1018 cm-3, respectively. The reference LED was capped with p-type 20 nm In0.02Ga0.98N:Mg and 5 nm In0.14Ga0.86N:Mg contact layer. Whereas, for TTJ LED cladding was followed by In0.02Ga0.98N /In0.17Ga0.83N/In0.02Ga0.98N TJ doped with Mg and Si concentration 3·1019cm-3and 1·1020 cm-3, respectively. Structure was capped with n-type GaN:Si contact layer. The third sample, presented in Fig. 1(c), was grown starting with TJ (similar to TTJ LED but with a reversed layers ordering) followed by the same 120 nm thick GaN:Mg layer with last 20 nm doped at higher level of 2·1019 cm-3 to improve hole injection. After p-type layers there was 2.6 nm In0.17Ga0.83N QW sandwiched between 30 nm In0.05Ga0.96N UID barriers capped by 100 nm n-type GaN:Si doped at the level 5·1018 cm-3. As previously described, the beneficial alignment of built-in fields in case of BTJ LED allows for no EBL, meant as no extra conduction band offset to minimize electron overflow in the structure [26]. Order of TJ’s and LED’s p-n layers for BTJ LED is inverted with respect to the reference LED and TTJ LED. This results in inversion of current flow direction marked by an arrow denoted J on Fig. 1. To make the comparison more clear, the positive bias direction is always assumed to be the forward direction of each LED.
Fig. 1. Schematic image indicating differences in compared heterostructures together with corresponding photos of processed devices under bias illustrating current spreading in: (a) reference LED, (b) LED with top tunnel junction (TTJ) construction, and (c) LED with bottom tunnel junction (BTJ) construction.
The structures were processed using optical lithography to define devices and e-beam evaporation to deposit metal contacts. The metal stack of Ti/Al/Ni/Au was used as a bottom contact to the n-type substrate for all three samples. A stack of Ni/Au was deposited on top of the reference sample to the contact p-type layers. For structures in Figs. 1(b), 1(c) utilization of TJ as a hole injection layer allowed for the use of the same n-type metal stack as for the bottom contact.
The structures were processed into LED devices with full square metalization covering the whole mesa ranging from 100 × 100 µm2 to 300 × 300 µm2 and with grid metalization deposited on the biggest mesa size 300 × 300 µm2 to observe current spreading. No extra thin metal or semitransparent metarial for better current spreading was deposited on the surface. The picturesof grid metalization devices under the same current density (calculated for total mesa size) are shown below corresponding structures in Fig. 1. The high lateral resistivity of p-type layers for reference LED limits the current spreading, resulting in ununiform light emission. In comparison, devices which feature the TJ contact profit from low resistivity n-type layer leading to good current spreading [27]. The dark spots visible in the same figure may orginate form not fully optimazied metal covrage during MBE growth, also reported elsewhere [28]. To allow more direct comparison between devicesand achive uniform current spreading, for further measurments only devices with full metalization were considered. For the reference LED a 200 × 200 µm2 was chosen, whereas, for TTJ and BTJ LEDs, a 100 × 100 µm2 was used.
Separated chips were mounted in TO5.6 cases and were braced by copper plate in helium closed cycle Oxford Optistat cryostat sample holder. Electrical measurments were done using Keithley 2614B Sourcemeter. Electroluminescenc (EL) spectra were collected by SPEX500M spectrometer, while total optical power was measured away from the cryostat using Thorlabs S130C adjusted for QW emission wavelength. Only small part of emitted power was collected so EQE and Optical power values have only relative meaning.
3. Results and discussion
I-V characteristics in full accessible temperature range is presented in Fig. 2. The data were obtained using a current source for temperatures between 300 K and 12 K for all three devices. At 300 K current values for all samples are comparable, with slightly higher on-resistance obtained for TTJ and BTJ LEDs. This effect is caused by extra series resistance of TJ in case of TTJ LED and further enhanced for BTJ LED by undesirable built-in field in TJ. Moreover, nitrogen polar-like heterostructures are characterized by slightly higher on-resistance, as discussed in detail in [26]. For measurements at lower temperature all devices suffer from a typical diode turn on voltage increase [12]. In case of the reference LED, significantly higher operation voltages are observed due to the increase in contact resistivity [13]. Samples with TTJ and BTJ construction perform better in compression to the reference LED with a p-type top layer. This can be explained by high activation energy of Mg dopant (∼170 meV) in comparison to Si dopant (∼13 meV) [4], making p-type contact layer more susceptible to carrier freezeout with respect to the n-type contact layers in both TTJ and BTJ LEDs.
Fig. 2. I–V characteristics at temperatures from 300 K to 12 K obtained using current source for: (a) reference LED, (b) LED with TTJ, (c) LED with BTJ.
Moreover, a negative differential resistance (NDR) was observed for reference LED and TTJ LED below 100 K and 70 K, respectively, resulting in sudden change of devices’ resistance. We distinguish two regions: first at low current and second at high current - after resistance change. Similar effect was previously reported for nitride LED operating at room temperature and was attributed to the presence of undoped GaN layer at heterojunction [29].
Figures 3(a) and 3(d) show EL spectra at 300K. Wavelength blueshift with increasing current is observed for both reference and BTJ LED. This is expected effect for GaN-InGaN LEDs and is attributed to the screening of built-in fields [30,31]. A higher blueshift for BTJ LED indicates higher injection efficiency of this design. A more detailed comparison for this type of devices at room temperature can be found in [25,26,28]. The difference between samples is more pronounced at cryogenic temperatures. Figure 3(b) presents zoomed in I-V characteristic for reference LED at 12 K. Corresponding spectra collected at indicated current densities are presented in Fig. 3(c). At very low current regime the main InGaN QW peak at 454 nm dominates. Two side peaks at lower energies than QW emission by 90 and 180 meV are first and second longitudinal-optical (1,2 LO) phonon replicas, respectively [12,32]. In collected spectra, two extra higher energy peaks are present: one wider peak ranging from 380 nm to 440 nm, originating in p-type layers [33] and second at 373 nm related to InGaN barrier band gap, as calculated using Vegard’s law and Varnish formula [34]. With increasing current density, a new peak at 360 nm related to GaN band gap at 12 K emerges. Whereas, in case of BTJ LED both I-V characteristic and EL spectra, presented in Figs. 3(d), 3(e), show no NDR nor parasitic recombination outside QW with increasing current at 12K. This allows for the use of BTJ LED at low temperature and high injection currents. A similar blueshift with increasing current as at 300K is observed indicating higher carrier number in QW in comparison with reference LED, in Fig. 3(c), where the emitted wavelength from QW do not shifts with increasing current..
Fig. 3. (a-c) results for reference LED: (a) EL spectra at 300 K at various current densities, (b) I-V characteristic at 12 K in semilogarithmic scale presenting the NDR region with (c) corresponding EL spectra at selected current densities (d-f) results for BTJ LED: (d) EL spectra at 300 K at various current densities, (e) I-V characteristic at 12 K in semilogarithmic scale with (f) corresponding EL spectra at selected current densities. Dashed lines in (b) and (e) mark current densities for spectra presented in (c) and (f) respectively.
External quantum efficiency (EQE) for 300K, 150K and 12K as a function of current density for reference LED and BTJ LED is presented in Fig. 4(a). Efficiency of BTJ LED dominates over reference LED in the entire measured temperature range. At high current regime, above 2.6 A/cm2 at 12 K, together with sudden change of resistivity, an abrupt decrease in EQE and optical power [Fig. 4(b)] was observed. EL spectra in Fig. 3(b) establishes, as it was reported before [16], that in this current region the parasitic recombination outside the QW starts to dominate the emission what is further explained in later part of the paper. With increasing current density, InGaN barrier-related peak decreases indicating that number of holes leaving p-type layers is smaller and majority of radiative recombination takes place in GaN p-type layers, outside the InGaN QW. Qualitatively the same results (not presented here) were obtained for TTJ LED. In case of BTJ LED situation is different: no drop in EQE was observed in both I-V [Fig. 2(c)] and optical power [Fig. 4(b)] measurements. This allows to use BTJ LED also at high current densities where its EQE and optical power is incommensurably higher than that of standard LED. Therefore, the BTJ LED is a valuable candidate for high power light source at cryogenic temperatures for example for on-chip electrical pumping.
Fig. 4. (a) Normalized external quantum efficiency (EQE) at: 300 K, 150 K and 12 K and (b) output power vs current density at 12 K for reference sample and BTJ LED.
Figure 5 displays normalized EL spectra for temperature ranging from 300K to 12K for both reference and BTJ LED at high current region. The main emission peak from the QW occurs at 451 nm and 467 nm for reference LED and BTJ LED, respectively. The difference in emission wavelength is due to small compositional difference in QWs for both structures. For all samples, with decreasing temperature, the main QW peak becomes narrower and lower energy subpeaks of LO phonon replicas emerge. For the reference LED in Fig. 6(a), an increase in parasitic recombination in p-type, GaN, and InGaN barrier can be seen below 100 K. The InGaN barrier peak is also visible at room temperature at high current densities for similar Ga polar devices [26]. On the other hand, in BTJ sample, as presented in Fig. 5(b), no measurable recombination in p-type layers nor any NDR related change at 25 A/cm2 was observed. Only emission related to recombination in QW and to phonon replicas was obtained.
Fig. 5. Logarithm of normalized EL spectra obtained between 300 K and 12 K at 25 A/cm2 current density for: (a) reference LED and (b) LED with BTJ. Peaks marked with triangles correspond to first and second longitudinal-optical (1,2 LO) phonon replicas of the main QW peak.
Fig. 6. Band profile at ∼5 A/cm2 for: (a) reference LED at 300 K and 100 K with vertical axis shifted for better comparison and (b) BTJ LED with common vertical axis. Dashed lines illustrate quasi Fermi levels for electrons and holes. Arrows depict carriers’ injection direction and observed radiative recombination regions at 100 K for both samples. Radiative and nonradiative recombination rates for (c) reference LED at 300 K and 100 K (d) BTJ LED.
To explain the difference in parasitic recombination between reference and BTJ LEDs together with peculiar observation of NDR, a set of band profiles was calculated using Drift-Diffusion Poisson Schrodinger Solver [35]. With decreasing temperature concentration of ionized dopants is decreased what influences band profiles presented in Fig. 6. The difference in activation energy between Si dopant (13 meV) and Mg dopant (∼170 meV and ∼220 meV for GaN and AlGaN EBL [36], respectively) leads to asymmetry in carrier freezeout in n-type and p-type layers and even inside p-type between GaN and AlGaN layers. More importantly hole freezeout in the EBL changes the position of Fermi level within p-type, shifting effective band offset from conduction into valence band decreasing EBL effectiveness. Together with unfavorable piezoelectric fields alignment it promotes overshot above QW and EBL, leading to two new radiative recombination peaks: in p-type and GaN layer. In Fig. 6(a) calculated band profiles for reference LED under forward current of 5 A/cm2 at 300K and 100K are compared. To visualize EBL effectiveness, both diagrams are shifted so that electron quasi Fermi level in the InGaN barrier proceeding EBL is the same for both conditions. Bellow at Fig. 6(c), (a) comparison of recombination rates at 300K and 100K is shown. At lower temperature an overall decrees in recombination is observed. As indicated by measurements, recombination originating outside QW dominates.
Sample with BTJ does not exhibit both NDR effect and parasitic recombination. We attribute this is to built-in nitrogen-polar-like polarization. Figure 6(b) shows a schematic band structure illustrating its influence on carriers flow for BTJ LED. For BTJ LED [in contrast to reference LED - presented in Fig. 6(a)], built-in polarization in InGaN acts as carrier blocking layer at each heterointerface preventing overflow. This in turn prevents electron and hole concentrations from overlapping outside the QW and suppresses undesired recombination in InGaN barrier and subsequent GaN and p-type layers. Higher injection efficiency at low temperatures is also responsible for pronounced built-in field screening, which is indicated by blueshift in emission spectra presented in Fig. 3(d). Additionally, built-in fields attract holes at InGaN barrier-EBL interface leading to polarization induced hole doping, which is temperature independent, and improve cryogenic temperature operation. Figure 6(d) shows that even at 300K a parasitic recombination in BTJ LED is lower in contrast to reference LED in Fig. 6(c) (what is in agreement with [25]). At lower temperature there is no decline in recombination in QW and opposite to reference LED parasitic recombination is reduced.
4. Conclusions
Superior qualitative and quantitative operation of BTJ LED over standard LED at low temperature was presented. It was shown that, comparing to standard LED, incorporation of TJ decreases the turn on voltage at cryogenic temperatures for both TTJ and BTJ alignments. Reference and TTJ LEDs suffers from carrier freezeout and high built-in piezoelectric fields causing NDR, carriers overflow and subsequent parasitic recombination outside the QW. To improve wavelength uniformity a structure with BTJ was proposed where nitrogen-like built-in fields help induce hole doping at InGaN barrier-EBL interface to maintaining high injection efficiency. This reduces electron overflow at cryogenic temperatures. Electroluminescence spectra proved that BTJ LED do not exhibits any parasitic recombination in GaN, InGaN barriers or p-type layers. The BTJ LED grown by PAMBE can be a promising candidate for cryogenic light source in wide variety of applications.
Funding
Foundation for Polish Science co-financed by the European Union under the European Regional Development Fund (POIR.04.04.00-00- 210C/16-00, POIR.04.04.00-00-5D5B/18-00); Polish National Centre for Research and Development (LIDER/29/0185/L-7/15/NCBR/2016); Narodowe Centrum Nauki (2015/17/B/ST7/04091, UMO-2018/31/B/ST5/03719, UMO-2019/35/N/ST7/04182).
Acknowledgments
The authors would like to thank Alexandr Khachapuridze for his help with samples characterization.
Disclosures
The authors declare no conflicts of interest.
References
1. S. Nakamura, S. Pearton, and G. Fasol, The Blue Laser Diode, 2sd ed. (Springer, 2000).
2. U. K. Mishra, P. Parikh, and Y. F. Wu, “AlGaN/GaN HEMTs - An overview of device operation and applications,” Proc. IEEE 90(6), 1022–1031 (2002). [CrossRef]
3. D. Jena, R. Page, J. Casamento, P. Dang, J. Singhal, Z. Zhang, J. Wright, G. Khalsa, Y. Cho, and H. G. Xing, “The new nitrides: Layered, ferroelectric, magnetic, metallic and superconducting nitrides to boost the GaN photonics and electronics eco-system,” Jpn. J. Appl. Phys. 58(SC), SC0801 (2019). [CrossRef]
4. J. Piprek, Nitride Semiconductor Device, 1st ed. (Wiley-VCH, 2007).
5. S. Nakamura, T. Mukai, M. Senoh, and N. Iwasa, “Thermal annealing effects on P-type Mg-doped GaN films,” Jpn. J. Appl. Phys. 31(Part 2, No. 2B), L139–L142 (1992). [CrossRef]
6. H. Amano, M. Kito, K. Hiramatsu, and I. Akasaki, “P-type conduction in Mg-doped GaN treated with low-energy electron beam irradiation (LEEBI),” Jpn. J. Appl. Phys. 28(Part 2, No. 12), L2112–L2114 (1989). [CrossRef]
7. Y. Kuwano, M. Kaga, T. Morita, K. Yamashita, K. Yagi, M. Iwaya, T. Takeuchi, S. Kamiyama, and I. Akasaki, “Lateral hydrogen diffusion at p-GaN layers in nitride-based light emitting diodes with tunnel junctions,” Jpn. J. Appl. Phys. 52(8S), 08JK12 (2013). [CrossRef]
8. A. J. Ptak, T. H. Myers, L. T. Romano, C. G. Van De Walle, and J. E. Northrup, “Magnesium incorporation in GaN grown by molecular-beam epitaxy,” Appl. Phys. Lett. 78(3), 285–287 (2001). [CrossRef]
9. M. Siekacz, G. Muziol, M. Hajdel, M. Żak, K. Nowakowski-Szkudlarek, H. Turski, M. Sawicka, P. Wolny, A. Feduniewicz-Żmuda, S. Stanczyk, J. Moneta, and C. Skierbiszewski, “Stack of two III-nitride laser diodes interconnected by a tunnel junction,” Opt. Express 27(4), 5784 (2019). [CrossRef]
10. C. Skierbiszewski, H. Turski, G. Muziol, M. Siekacz, M. Sawicka, G. Cywiński, Z. R. Wasilewski, and S. Porowski, “Nitride-based laser diodes grown by plasma-assisted molecular beam epitaxy,” J. Phys. D: Appl. Phys. 47(7), 073001 (2014). [CrossRef]
11. G. Muziol, M. Siekacz, K. Nowakowski-Szkudlarek, M. Hajdel, J. Smalc-Koziorowska, A. Feduniewicz-Żmuda, E. Grzanka, P. Wolny, H. Turski, P. Wiśniewski, P. Perlin, and C. Skierbiszewski, “Extremely long lifetime of III-nitride laser diodes grown by plasma assisted molecular beam epitaxy,” Mater. Sci. Semicond. Process. 91, 387–391 (2019). [CrossRef]
12. I. E. Titkov, S. Y. Karpov, A. Yadav, V. L. Zerova, M. Zulonas, B. Galler, M. Strassburg, I. Pietzonka, H. J. Lugauer, and E. U. Rafailov, “Temperature-Dependent internal quantum efficiency of blue high-brightness light-emitting diodes,” IEEE J. Quantum Electron. 50(11), 911–920 (2014). [CrossRef]
13. D. S. Meyaard, J. Cho, E. Fred Schubert, S. H. Han, M. H. Kim, and C. Sone, “Analysis of the temperature dependence of the forward voltage characteristics of GaInN light-emitting diodes,” Appl. Phys. Lett. 103(12), 121103 (2013). [CrossRef]
14. C. H. Wang, J. R. Chen, C. H. Chiu, H. C. Kuo, Y. L. Li, T. C. Lu, and S. C. Wang, “Temperature-dependent electroluminescence efficiency in blue InGaN-GaN light-emitting diodes with different well widths,” IEEE Photonics Technol. Lett. 22(4), 236–238 (2010). [CrossRef]
15. D. P. Han, M. G. Kang, C. H. Oh, H. Kim, K. S. Kim, D. S. Shin, and J. I. Shim, “Investigation of carrier spill-over in InGaN-based light-emitting diodes by temperature dependences of resonant photoluminescence and open-circuit voltage,” Phys. Status Solidi A 210(10), 2204–2208 (2013). [CrossRef]
16. S. Grzanka, G. Franssen, G. Targowski, K. Krowicki, T. Suski, R. Czernecki, P. Perlin, and M. Leszczyński, “Role of the electron blocking layer in the low-temperature collapse of electroluminescence in nitride light-emitting diodes,” Appl. Phys. Lett. 90(10), 103507 (2007). [CrossRef]
17. A. Hori, D. Yasunaga, A. Satake, and K. Fujiwara, “Temperature dependence of electroluminescence intensity of green and blue InGaN single-quantum-well light-emitting diodes,” Appl. Phys. Lett. 79(22), 3723–3725 (2001). [CrossRef]
18. S. Krishnamoorthy, F. Akyol, P. S. Park, and S. Rajan, “Low resistance GaN/InGaN/GaN tunnel junctions,” Appl. Phys. Lett. 102(11), 113503 (2013). [CrossRef]
19. E. A. Clinton, E. Vadiee, S. C. Shen, K. Mehta, P. D. Yoder, and W. A. Doolittle, “Negative differential resistance in GaN homojunction tunnel diodes and low voltage loss tunnel contacts,” Appl. Phys. Lett. 112(25), 252103 (2018). [CrossRef]
20. M. Malinverni, C. Tardy, M. Rossetti, A. Castiglia, M. Duelk, C. Vélez, D. Martin, and N. Grandjean, “InGaN laser diode with metal-free laser ridge using n+-GaN contact layers,” Appl. Phys. Express 9(6), 061004 (2016). [CrossRef]
21. E. C. Young, B. P. Yonkee, F. Wu, S. H. Oh, S. P. DenBaars, S. Nakamura, and J. S. Speck, “Hybrid tunnel junction contacts to III-nitride light-emitting diodes,” Appl. Phys. Express 9(2), 022102 (2016). [CrossRef]
22. C. Skierbiszewski, G. Muziol, K. Nowakowski-Szkudlarek, H. Turski, M. Siekacz, A. Feduniewicz-Zmuda, A. Nowakowska-Szkudlarek, M. Sawicka, and P. Perlin, “True-blue laser diodes with tunnel junctions grown monolithically by plasma-assisted molecular beam epitaxy,” Appl. Phys. Express 11(3), 034103 (2018). [CrossRef]
23. S. J. Kowsz, E. C. Young, B. P. Yonkee, C. D. Pynn, R. M. Farrell, J. S. Speck, S. P. DenBaars, and S. Nakamura, “Using tunnel junctions to grow monolithically integrated optically pumped semipolar III-nitride yellow quantum wells on top of electrically injected blue quantum wells,” Opt. Express 25(4), 3841 (2017). [CrossRef]
24. M. Saha, A. Biswas, and H. Karan, “Monolithic high performance InGaN/GaN white LEDs with a tunnel junction cascaded yellow and blue light-emitting structures,” Opt. Mater. 77, 104–110 (2018). [CrossRef]
25. H. Turski, S. Bharadwaj, H. Xing, and D. Jena, “Polarization control in nitride quantum well light emitters enabled by bottom tunnel-junctions,” J. Appl. Phys. 125(20), 203104 (2019). [CrossRef]
26. S. Bharadwaj, J. Miller, K. Lee, J. Lederman, M. Siekacz, H. Grace Xing, D. Jena, C. Skierbiszewski, and H. Turski, “Enhanced injection efficiency and light output in bottom tunnel-junction light-emitting diodes,” Opt. Express 28(4), 4489 (2020). [CrossRef]
27. B. P. Yonkee, E. C. Young, S. P. DenBaars, S. Nakamura, and J. S. Speck, “Silver free III-nitride flip chip light-emitting-diode with wall plug efficiency over 70% utilizing a GaN tunnel junction,” Appl. Phys. Lett. 109(19), 191104 (2016). [CrossRef]
28. K. Lee, S. Bharadwaj, Y.-T. Shao, L. van Deurzen, V. Protasenko, D. A. Muller, H. G. Xing, and D. Jena, “Light-emitting diodes with AlN polarization-induced buried tunnel junctions: A second look,” Appl. Phys. Lett. 117(6), 061104 (2020). [CrossRef]
29. H. Turski, R. Yan, S. J. Bader, G. Muziol, C. Skierbiszewski, H. G. Xing, and D. Jena, “S-shaped negative differential resistance in III-Nitride blue quantum-well laser diodes grown by plasma-assisted MBE,” in Device Research Conference - Conference Digest, DRC (Institute of Electrical and Electronics Engineers Inc., 2017).
30. G. Muziol, H. Turski, M. Siekacz, K. Szkudlarek, L. Janicki, S. Zolud, R. Kudrawiec, T. Suski, and C. Skierbiszewski, “Beyond quantum efficiency limitations originating from the piezoelectric polarization in light-emitting devices,” ACS Photonics 6, 1963–1971 (2019). [CrossRef]
31. T. Takeuchi, S. Sota, M. Katsuragawa, M. Komori, H. Takeuchi, H. Amano, and I. Akasaki, “Quantum-Confined Stark Effect due to Piezoelectric Fields in GaInN Strained Quantum Wells,” Jpn. J. Appl. Phys. 36(Part 2, No. 4A), L382–L385 (1997). [CrossRef]
32. X. A. Cao, S. F. LeBoeuf, L. B. Rowland, C. H. Yan, and H. Liu, “Temperature-dependent emission intensity and energy shift in InGaN/GaN multiple-quantum-well light-emitting diodes,” Appl. Phys. Lett. 82(21), 3614–3616 (2003). [CrossRef]
33. L. Eckey, U. Von Gfug, J. Holst, A. Hoffmann, A. Kaschner, H. Siegle, C. Thomsen, B. Schineller, K. Heime, M. Heuken, O. Schön, and R. Beccard, “Photoluminescence and Raman study of compensation effects in Mg-doped GaN epilayers,” J. Appl. Phys. 84(10), 5828–5830 (1998). [CrossRef]
34. S. A. Gaikwad, E. P. Samuel, D. S. Patil, and D. K. Gautam, “Temperature dependent analysis of refractive index, band gap and recombination coefficient in nitride semiconductor lasers,” Indian J. Pure Appl. Phys. 45, 238–242 (2007).
35. Y. R. Wu, M. Singh, and J. Singh, “Gate leakage suppression and contact engineering in nitride heterostructures,” Mater. Res. Soc. Symp. Proc. 798, Y11.1 (2003). [CrossRef]
36. M. Suzuki, J. Nishio, M. Onomura, and C. Hongo, “Doping characteristics and electrical properties of Mg-doped AlGaN grown by atmospheric-pressure MOCVD,” J. Cryst. Growth 189-190, 511–515 (1998). [CrossRef]
