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1. RTD Light Emission around 1550 nm with IQE up to 6% at 300 K

   https://doi.org/10.1109/DRC50226.2020.9135175  

   Brown, E. R. ; Zhang, W-D. ; Fakhimi, P. ; Growden, T. A. ; Berger, P.R. ( June 2020 , IEEE Device Research Conference (DRC), Columbus, OH (June 22-24, 2020))

   Resonant tunneling diodes (RTDs) have come full-circle in the past 10 years after their demonstration in the early 1990s as the fastest room-temperature semiconductor oscillator, displaying experimental results up to 712 GHz and fmax values exceeding 1.0 THz [1]. Now the RTD is once again the preeminent electronic oscillator above 1.0 THz and is being implemented as a coherent source [2] and a self-oscillating mixer [3], amongst other applications. This paper concerns RTD electroluminescence – an effect that has been studied very little in the past 30+ years of RTD development, and not at room temperature. We present experiments and modeling of an n-type In0.53Ga0.47As/AlAs double-barrier RTD operating as a cross-gap light emitter at ~300K. The MBE-growth stack is shown in Fig. 1(a). A 15-μm-diam-mesa device was defined by standard planar processing including a top annular ohmic contact with a 5-μm-diam pinhole in the center to couple out enough of the internal emission for accurate free-space power measurements [4]. The emission spectra have the behavior displayed in Fig. 1(b), parameterized by bias voltage (VB). The long wavelength emission edge is at  = 1684 nm - close to the In0.53Ga0.47As bandgap energy of Ug ≈ 0.75 eV at 300 K.more »The spectral peaks for VB = 2.8 and 3.0 V both occur around  = 1550 nm (h = 0.75 eV), so blue-shifted relative to the peak of the “ideal”, bulk InGaAs emission spectrum shown in Fig. 1(b) [5]. These results are consistent with the model displayed in Fig. 1(c), whereby the broad emission peak is attributed to the radiative recombination between electrons accumulated on the emitter side, and holes generated on the emitter side by interband tunneling with current density Jinter. The blue-shifted main peak is attributed to the quantum-size effect on the emitter side, which creates a radiative recombination rate RN,2 comparable to the band-edge cross-gap rate RN,1. Further support for this model is provided by the shorter wavelength and weaker emission peak shown in Fig. 1(b) around = 1148 nm. Our quantum mechanical calculations attribute this to radiative recombination RR,3 in the RTD quantum well between the electron ground-state level E1,e, and the hole level E1,h. To further test the model and estimate quantum efficiencies, we conducted optical power measurements using a large-area Ge photodiode located ≈3 mm away from the RTD pinhole, and having spectral response between 800 and 1800 nm with a peak responsivity of ≈0.85 A/W at  =1550 nm. Simultaneous I-V and L-V plots were obtained and are plotted in Fig. 2(a) with positive bias on the top contact (emitter on the bottom). The I-V curve displays a pronounced NDR region having a current peak-to-valley current ratio of 10.7 (typical for In0.53Ga0.47As RTDs). The external quantum efficiency (EQE) was calculated from EQE = e∙IP/(∙IE∙h) where IP is the photodiode dc current and IE the RTD current. The plot of EQE is shown in Fig. 2(b) where we see a very rapid rise with VB, but a maximum value (at VB= 3.0 V) of only ≈2×10-5. To extract the internal quantum efficiency (IQE), we use the expression EQE= c ∙i ∙r ≡ c∙IQE where ci, and r are the optical-coupling, electrical-injection, and radiative recombination efficiencies, respectively [6]. Our separate optical calculations yield c≈3.4×10-4 (limited primarily by the small pinhole) from which we obtain the curve of IQE plotted in Fig. 2(b) (right-hand scale). The maximum value of IQE (again at VB = 3.0 V) is 6.0%. From the implicit definition of IQE in terms of i and r given above, and the fact that the recombination efficiency in In0.53Ga0.47As is likely limited by Auger scattering, this result for IQE suggests that i might be significantly high. To estimate i, we have used the experimental total current of Fig. 2(a), the Kane two-band model of interband tunneling [7] computed in conjunction with a solution to Poisson’s equation across the entire structure, and a rate-equation model of Auger recombination on the emitter side [6] assuming a free-electron density of 2×1018 cm3. We focus on the high-bias regime above VB = 2.5 V of Fig. 2(a) where most of the interband tunneling should occur in the depletion region on the collector side [Jinter,2 in Fig. 1(c)]. And because of the high-quality of the InGaAs/AlAs heterostructure (very few traps or deep levels), most of the holes should reach the emitter side by some combination of drift, diffusion, and tunneling through the valence-band double barriers (Type-I offset) between InGaAs and AlAs. The computed interband current density Jinter is shown in Fig. 3(a) along with the total current density Jtot. At the maximum Jinter (at VB=3.0 V) of 7.4×102 A/cm2, we get i = Jinter/Jtot = 0.18, which is surprisingly high considering there is no p-type doping in the device. When combined with the Auger-limited r of 0.41 and c ≈ 3.4×10-4, we find a model value of IQE = 7.4% in good agreement with experiment. This leads to the model values for EQE plotted in Fig. 2(b) - also in good agreement with experiment. Finally, we address the high Jinter and consider a possible universal nature of the light-emission mechanism. Fig. 3(b) shows the tunneling probability T according to the Kane two-band model in the three materials, In0.53Ga0.47As, GaAs, and GaN, following our observation of a similar electroluminescence mechanism in GaN/AlN RTDs (due to strong polarization field of wurtzite structures) [8]. The expression is Tinter = (2/9)∙exp[(-2 ∙Ug 2 ∙me)/(2h∙P∙E)], where Ug is the bandgap energy, P is the valence-to-conduction-band momentum matrix element, and E is the electric field. Values for the highest calculated internal E fields for the InGaAs and GaN are also shown, indicating that Tinter in those structures approaches values of ~10-5. As shown, a GaAs RTD would require an internal field of ~6×105 V/cm, which is rarely realized in standard GaAs RTDs, perhaps explaining why there have been few if any reports of room-temperature electroluminescence in the GaAs devices. [1] E.R. Brown,et al., Appl. Phys. Lett., vol. 58, 2291, 1991. [5] S. Sze, Physics of Semiconductor Devices, 2nd Ed. 12.2.1 (Wiley, 1981). [2] M. Feiginov et al., Appl. Phys. Lett., 99, 233506, 2011. [6] L. Coldren, Diode Lasers and Photonic Integrated Circuits, (Wiley, 1995). [3] Y. Nishida et al., Nature Sci. Reports, 9, 18125, 2019. [7] E.O. Kane, J. of Appl. Phy 32, 83 (1961). [4] P. Fakhimi, et al., 2019 DRC Conference Digest. [8] T. Growden, et al., Nature Light: Science & Applications 7, 17150 (2018). [5] S. Sze, Physics of Semiconductor Devices, 2nd Ed. 12.2.1 (Wiley, 1981). [6] L. Coldren, Diode Lasers and Photonic Integrated Circuits, (Wiley, 1995). [7] E.O. Kane, J. of Appl. Phy 32, 83 (1961). [8] T. Growden, et al., Nature Light: Science & Applications 7, 17150 (2018).« less
2. Distributed polarization-doped GaN p–n diodes with near-unity ideality factor and avalanche breakdown voltage of 1.25 kV

   https://doi.org/10.1063/5.0083302  

   Nomoto, Kazuki ; Li, Wenshen ; Song, Bo ; Hu, Zongyang ; Zhu, Mingda ; Qi, Meng ; Protasenko, Vladimir ; Zhang, Zexuan ; Pan, Ming ; Gao, Xiang ; et al ( March 2022 , Applied Physics Letters)

   Polarization-induced (Pi) distributed or bulk doping in GaN, with a zero dopant ionization energy, can reduce temperature or frequency dispersions in impurity-doped p–n junctions caused by the deep-acceptor-nature of Mg, thus offering GaN power devices promising prospects. Before comprehensively assessing the benefits of Pi-doping, ideal junction behaviors and high-voltage capabilities should be confirmed. In this work, we demonstrate near-ideal forward and reverse I–V characteristics in Pi-doped GaN power p–n diodes, which incorporates linearly graded, coherently strained AlGaN layers. Hall measurements show a net increase in the hole concentration of 8.9 × 10¹⁶ cm⁻³in the p-layer as a result of the polarization charge. In the Pi-doped n-layer, a record-low electron concentration of 2.5 × 10¹⁶ cm⁻³is realized due to the gradual grading of Al_0-0.72GaN over 1  μm. The Pi-doped p–n diodes have an ideality factor as low as 1.1 and a 0.10 V higher turn-on voltage than the impurity-doped p–n diodes due to the increase in the bandgap at the junction edge. A differential specific on-resistance of 0.1 mΩ cm²is extracted from the Pi-doped p–n diodes, similar with the impurity-doped counterpart. The Pi-doped diodes show an avalanche breakdown voltage of ∼1.25 kV, indicating a high reverse blocking capability even without an ideal edge-termination. This work confirms that distributed Pi-doping can bemore »incorporated in high-voltage GaN power devices to increase hole concentrations while maintaining excellent junction properties.

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3. High-performance electron-blocking-layer-free deep ultraviolet light-emitting diodes implementing a strip-in-a-barrier structure

   https://doi.org/10.1364/OL.400917  

   Velpula, Ravi Teja ; Jain, Barsha ; Velpula, Swetha ; Nguyen, Hoang-Duy ; Nguyen, Hieu Pham Trung ( September 2020 , Optics Letters)

   In this Letter, the electron-blocking-layer (EBL)-free AlGaN ultraviolet (UV) light-emitting diodes (LEDs) using a strip-in-a-barrier structure have been proposed. The quantum barrier (QB) structures are systematically engineered by integrating a 1 nm intrinsic${\mathrm{A}\mathrm{l}}_x{\mathrm{G}\mathrm{a}}_{(1-<\#comment/>x)}\mathrm{N}$strip into the middle of QBs. The resulted structures exhibit significantly reduced electron leakage and improved hole injection into the active region, thus generating higher carrier radiative recombination. Our study shows that the proposed structure improves radiative recombination by$\sim <\#comment/>220\mathrm{\%<\#comment/>}$, reduces electron leakage by$\sim <\#comment/>11$times, and enhances optical power by$\sim <\#comment/>225\mathrm{\%<\#comment/>}$at 60 mA current injection compared to a conventional AlGaN EBL LED structure. Moreover, the EBL-free strip-in-a-barrier UV LED records the maximum internal quantum efficiency (IQE) of$\sim <\#comment/>61.5\mathrm{\%<\#comment/>}$which is$\sim <\#comment/>72\mathrm{\%<\#comment/>}$higher, and IQE droop is$\sim <\#comment/>12.4\mathrm{\%<\#comment/>}$, which is$\sim <\#comment/>333\mathrm{\%<\#comment/>}$less compared to the conventional AlGaN EBL LED structure at$\sim <\#comment/>284.5\mathrm{n}\mathrm{m}$wavelength. Hence, the proposed EBL-free AlGaN LED is the potential solution to enhance the optical power and produce highly efficient UV emitters.
4. Improved Performance of Electron Blocking Layer Free AlGaN Deep Ultraviolet Light-Emitting Diodes Using Graded Staircase Barriers

   https://doi.org/10.3390/mi12030334  

   Jain, Barsha ; Velpula, Ravi Teja ; Patel, Moulik ; Sadaf, Sharif Md. ; Nguyen, Hieu Pham ( March 2021 , Micromachines)

   To prevent electron leakage in deep ultraviolet (UV) AlGaN light-emitting diodes (LEDs), Al-rich p-type AlxGa(1−x)N electron blocking layer (EBL) has been utilized. However, the conventional EBL can mitigate the electron overflow only up to some extent and adversely, holes are depleted in the EBL due to the formation of positive sheet polarization charges at the heterointerface of the last quantum barrier (QB)/EBL. Subsequently, the hole injection efficiency of the LED is severely limited. In this regard, we propose an EBL-free AlGaN deep UV LED structure using graded staircase quantum barriers (GSQBs) instead of conventional QBs without affecting the hole injection efficiency. The reported structure exhibits significantly reduced thermal velocity and mean free path of electrons in the active region, thus greatly confines the electrons over there and tremendously decreases the electron leakage into the p-region. Moreover, such specially designed QBs reduce the quantum-confined Stark effect in the active region, thereby improves the electron and hole wavefunctions overlap. As a result, both the internal quantum efficiency and output power of the GSQB structure are ~2.13 times higher than the conventional structure at 60 mA. Importantly, our proposed structure exhibits only ~20.68% efficiency droop during 0–60 mA injection current, which is significantlymore »lower compared to the regular structure.« less
5. Enhanced hole transport in AlGaN deep ultraviolet light-emitting diodes using a double-sided step graded superlattice electron blocking layer

   https://doi.org/10.1364/JOSAB.399773  

   Jain, Barsha ; Velpula, Ravi Teja ; Velpula, Swetha ; Nguyen, Hoang-Duy ; Nguyen, Hieu Pham Trung ( August 2020 , Journal of the Optical Society of America B)

   In this paper, deep ultraviolet AlGaN light-emitting diodes (LEDs) with a novel double-sided step graded superlattice (DSGS) electron blocking layer (EBL) instead of a conventional EBL have been proposed for$\sim <\#comment/>254\mathrm{n}\mathrm{m}$wavelength emission. The enhanced carrier transport in the DSGS structure results in reduced electron leakage into the$p$-region, improved hole activation and hole injection, and enhanced output power and external quantum efficiency. The calculations show that output power of the DSGS structure is$\sim <\#comment/>3.56$times higher and electron leakage is$\sim <\#comment/>12$times lower, compared to the conventional structure. Moreover, the efficiency droop at 60 mA in the DSGS LED was found to be$\sim <\#comment/>9.1\mathrm{\%<\#comment/>}$, which is$\sim <\#comment/>4.5$times lower than the regular LED structure.