III-nitride nanowire (NW) LEDs have been intensively studied for several emerging applications. However, the performance of these LEDs is still limited due to many factors. A leakage current may cause idle power consumption and affect the reliability and luminescence efficiency of the devices. Hence, it is one of the most important limiting factors from an application point of view. In this context, we have experimentally observed temperature-dependent forward and reverse leakage current–voltage characteristics of InGaN/AlGaN NW-based red microLEDs. The characteristic curves are fitted using different constant parameters such as the space charge term, zero bias current, and the characteristic energy. They are found to have error bars of less than 10%. The extra space charge term is believed to be due to inherent space charges trapped with the NWs and presents at every instance of the operation of the diode. The characteristic energy and ideality factors are compared to the reported values. An Arrhenius plot is used to calculate the thermal activation energy in the high- and low-temperature regions for both bias conditions. Our results show that the voltage-dependent activation energy is found to be about double in the case of the forward bias compared to that of the reverse bias in all voltage ranges. However, in a high voltage regime, the magnitudes of these parameters are almost four and six times greater for the forward and reverse biases, respectively, compared to those in the lower voltage regions. This study presents vital insight into the design and fabrication of high-performance NW-based LEDs.
There are numerous applications for deep UV AlGaN Light-Emitting Diodes (LEDs) in virus inactivation, air and water purification, sterilization, bioagent detection and UV polymer curing. The long-term stability of these LEDs is also of interest for long-duration space missions such as the Laser Interferometer Space Antenna (LISA), the first gravitational wave detector in space. We review the literature on long-term aging of these devices as a function of drive current, temperature and dc versus pulsed operation. The LEDs typically show a gradual decline in output power (up to 50%) over extended operating times (>100 h) and the rate of decline is mainly driven by current and temperature. Experimentally, the degradation rate is dependent on the cube of drive current density and exponentially on temperature. The main mechanism for this decline appears to be creation/migration of point defects. Pre-screening by considering the ratio of band edge-to-midgap emission and LED ideality factor is effective in identifying populations of devices that show long lifetimes (>10,000 h), defined as output power falling to 70% of the initial value.
more » « less- Award ID(s):
- 1856662
- NSF-PAR ID:
- 10417545
- Publisher / Repository:
- The Electrochemical Society
- Date Published:
- Journal Name:
- ECS Journal of Solid State Science and Technology
- Volume:
- 12
- Issue:
- 6
- ISSN:
- 2162-8769
- Page Range / eLocation ID:
- Article No. 066002
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
-
more » « less
AlGaN-based germicidal UV LEDs show promise in fighting the COVID-19 pandemic through disinfection of air, water, and surfaces. We report UV LEDs grown by MOCVD on SiC substrates, fabricated into thin-film flip chip devices. Replacing the
uniform p-Alx Ga1-x N layer (x = 0.2) with ashort-period-superlattice of alternating (x = 0.1 and 0.8) Al-composition improved EQE from 1.3% to 2.7% (3.2% with encapsulation) at 20 A/cm2. Peak EQE and WPE values of 4.8% and 2.8% (287 nm) were measured at current densities below 2 A/cm2, and maximum output power of 7.4 mW (76 mW/mm2) was achieved at 284 nm. Further WPE improvements are expected with both superlattice and uniform layer optimization, improved p-contact metallization, and active region optimization. -
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. 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 ci, 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).more » « less
-
Morkoç, Hadis ; Fujioka, Hiroshi ; Schwarz, Ulrich T. (Ed.)Although AlGaN-based deep ultraviolet (UV) light-emitting diodes (LEDs) have been studied extensively, their quantum efficiency and optical output power still remain extremely low compared to the InGaN-based visible color LEDs. Electron leakage has been identified as one of the most possible reasons for the low internal quantum efficiency (IQE) in AlGaN based UV LEDs. The integration of a p-doped AlGaN electron blocking layer (EBL) or/and increasing the conduction band barrier heights with prompt utilization of higher Al composition quantum barriers (QBs) in the LED could mitigate the electron leakage problem to an extent, but not completely. In this context, we introduce a promising approach to alleviate the electron overflow without using EBL by utilizing graded concave QBs instead of conventional QBs in AlGaN UV LEDs. Overall, the carrier transportation, confinement capability and radiative recombination are significantly improved. As a result, the IQE, and output power of the proposed concave QB LED were enhanced by ~25.4% and ~25.6% compared to the conventional LED for emission at ~254 nm, under 60 mA injection current.more » « less
-
more » « less
Potassium hydroxide (KOH) and ammonium sulfide
have been used as a surface passivation treatment to improve the electrical and optical performance of AlGaN nanowire ultraviolet (UV) light-emitting diodes (LEDs). Enhancements in photoluminescence at 335 nm (49%), optical output power (65%), and electroluminescence (83%), with respect to the as-grown nanowire LED are recorded for the AlGaN nanowire UV LEDs with surface passivation. These enhancements are attributed to the reduced nonradiative recombination on the nanowire surfaces. This study provides a potential surface passivation approach to produce high-power AlGaN nanowire LEDs operating in the UV spectrum.
