United States

Nonradiative recombination centers (NRCs) are known to negatively affect the efficiency of light-emitting diodes (LEDs). NRCs contribute not only to Shockley–Read–Hall (SRH) recombination, but also to trap-assisted Auger recombination (TAAR). The relationship between NRCs and the TAAR processes has been predicted theoretically and observed experimentally to a limited degree. However, some details about TAAR are still under debate, including its magnitude relative to intrinsic Auger and SRH recombination, its dependence on carrier density (e.g., ∝n2 or ∝n3), and its role in the green gap, particularly in high-quality commercial-grade LEDs. TAAR is typically assumed to be linearly dependent on trap density, directly tying the magnitude of TAAR to the quality of the sample. Previous studies have examined TAAR using time-resolved photoluminescence and optical differential lifetime measurements, which do not capture certain effects related to electrical injection. In this work, we report a quantitative study of TAAR in c-plane commercial-grade green LEDs grown by metal-organic chemical vapor deposition (MOCVD). We investigate a series of three wafers with identical active regions (3X, 3-nm-thick quantum wells) but different NRC trap densities (4.4e14 cm-3 &lt; Nt &lt; 1.5e15 cm-3). We use small-signal electroluminescence (SSEL) and deep-level optical spectroscopy (DLOS) to study the carrier dynamics and density of NRCs in the samples, respectively. SSEL is used to determine the carrier lifetime, which is used in conjunction with the internal quantum efficiency (IQE) to decouple the radiative and nonradiative recombination rates. The total recombination lifetime in higher quality growth is found to be longer than that in lower quality growth and is attributed to the longer nonradiative recombination time in higher quality samples. On the other hand, the radiative recombination rates for different growth qualities at a given carrier density (n) are identical, as expected from samples with the same active region design. We extract the relationship between the density of NRCs and the corresponding nonradiative recombination rates at a given carrier density (n). The difference in nonradiative recombination between the samples offers information about both SRH and TAAR with respect to growth quality. The linear dependence between the SRH recombination rate and the density of NRCs at low n was confirmed. However, TAAR was found to be sublinear with the density of NRCs, possibly due to the relatively small changes (~3X) in defect density (Nt) in these samples. We find that TAAR is best fit to an n2 dependence on carrier density. This is supported by the observation that all three samples have similar IQE at high carrier density, which would not be consistent with an n3 process. We also show that the TAAR recombination coefficients (D) are on the order of ~1e13 cm3 s-1, which is similar to that reported for violet LEDs and shows the weak dependence of TAAR on bandgap. Using the extracted TAAR coefficients, we calculate that TAAR accounts for about 10% of the total nonradiative recombination rate at moderate carrier densities. Thus, intrinsic Auger dominates the nonradiative recombination and TAAR is not a major contributor to the green gap in these commercial LEDs. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

EMC 2023

Trap-Assisted Auger Recombination in Commercial Green InGaN/GaN LEDs

deep-level defects

ingan/gan leds

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. This year’s event will feature a plenary session, topical sessions, a poster session and more.

EMC is coordinated with the Device Research Conference (DRC). This recognizes the strong interaction between device and electronic materials research and provides fruitful exchange of information between attendees of both Conferences.

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. 

Nonradiative recombination centers (NRCs) are known to negatively affect the efficiency of light-emitting diodes (LEDs). NRCs contribute not only to Shockley–Read–Hall (SRH) recombination, but also to trap-assisted Auger recombination (TAAR). The relationship between NRCs and the TAAR processes has been predicted theoretically and observed experimentally to a limited degree. However, some details about TAAR are still under debate, including its magnitude relative to intrinsic Auger and SRH recombination, its dependence on carrier density (e.g., ∝n2 or ∝n3), and its role in the green gap, particularly in high-quality commercial-grade LEDs. TAAR is typically assumed to be linearly dependent on trap density, directly tying the magnitude of TAAR to the quality of the sample. Previous studies have examined TAAR using time-resolved photoluminescence and optical differential lifetime measurements, which do not capture certain effects related to electrical injection. In this work, we report a quantitative study of TAAR in c-plane commercial-grade green LEDs grown by metal-organic chemical vapor deposition (MOCVD). We investigate a series of three wafers with identical active regions (3X, 3-nm-thick quantum wells) but different NRC trap densities (4.4e14 cm-3 < Nt < 1.5e15 cm-3). We use small-signal electroluminescence (SSEL) and deep-level optical spectroscopy (DLOS) to study the carrier dynamics and density of NRCs in the samples, respectively. SSEL is used to determine the carrier lifetime, which is used in conjunction with the internal quantum efficiency (IQE) to decouple the radiative and nonradiative recombination rates. The total recombination lifetime in higher quality growth is found to be longer than that in lower quality growth and is attributed to the longer nonradiative recombination time in higher quality samples. On the other hand, the radiative recombination rates for different growth qualities at a given carrier density (n) are identical, as expected from samples with the same active region design. We extract the relationship between the density of NRCs and the corresponding nonradiative recombination rates at a given carrier density (n). The difference in nonradiative recombination between the samples offers information about both SRH and TAAR with respect to growth quality. The linear dependence between the SRH recombination rate and the density of NRCs at low n was confirmed. However, TAAR was found to be sublinear with the density of NRCs, possibly due to the relatively small changes (~3X) in defect density (Nt) in these samples. We find that TAAR is best fit to an n2 dependence on carrier density. This is supported by the observation that all three samples have similar IQE at high carrier density, which would not be consistent with an n3 process. We also show that the TAAR recombination coefficients (D) are on the order of ~1e13 cm3 s-1, which is similar to that reported for violet LEDs and shows the weak dependence of TAAR on bandgap. Using the extracted TAAR coefficients, we calculate that TAAR accounts for about 10% of the total nonradiative recombination rate at moderate carrier densities. Thus, intrinsic Auger dominates the nonradiative recombination and TAAR is not a major contributor to the green gap in these commercial LEDs. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

technical paper

GaN-based light emitting diodes (LEDs) for white light illumination are one of the biggest technological achievements in energy efficiency. However, challenges remain with GaN-based LEDs such as efficiency droop and low efficiency for longer wavelengths. Auger-Meitner recombination, a three-carrier process, has been identified as the dominant mechanism for efficiency droop via electron emission spectroscopy (EES). In EES, LEDs were activated to negative electron affinity (NEA) to allow for the emission of hot electrons from the surface which were quantitatively correlated to Auger-Meitner recombination processes. In recent years, V-defects have been proposed as the solution to the excess driving voltage of long wavelength nitride-based LEDs, one of the sources of their diminished efficiency. One way to image the effects of V-defects is using electron emission microscopy (EEM). EEM is a newly developed technique analogous to EES that allows for spatially resolved microscopy of hot electrons. EEM uses a low energy electron microscope with its illumination column turned off, such that only internally generated electrons escaping out the surface during electrical driving of the device are observed. Commercial blue (λ ≈ 460 nm) and green (λ ≈ 525 nm) LED wafers from Seoul Viosys were acquired and processed into LEDs. The diodes were formed by Pd/Au (20/30 nm) p-contacts with open circular and hexagonal apertures lithographically defined with known dimensions. A C-shaped n-contact partially surrounding the p-contact was formed by reactive ion etching into the n-region and contacted with Ti/Au (30/300 nm). EEM was performed in an ELMITEC LEEM III system modified to allow for electrical biasing of devices from the sample stage, where Cs was deposited onto the sample using a SAES Getters Cs source to activate the sample to NEA. For LEDs, EEM images the non-radiatively-recombined electrons that cross the diode junction and diffuse through the p-GaN and escape out the surface, which can include overflow, Auger-Meitner, and trap-assisted Auger-Meitner electrons. EEM images of blue and green LEDs show non-electron emitting regions (dark spots) with densities of ~2 × 107 cm-2 and ~3 × 108 cm-2, respectively, which we identify as V-defects. This is confirmed through the corresponding dark spots of panchromatic CL measurements of the same samples, as well as SEM and plan-view TEM for V-defect densities. Since the density and dimensions of the non-electro-emitting and non-illuminating regions are identical, we attribute our measured lack of electron emission directly from the V-defect areas to electrons fast traveling through the sidewalls of the V-defects and being injected laterally and efficiently into planar quantum wells without any Auger-Meitner recombination or overflow within the sidewalls. This is supported by the stronger electron emission observed at the ridge of the V-defects due to larger carrier densities there and thus increased hot electrons generation. Our results agree with simulations that electrons travel up the sidewalls of V-defects that have been nucleated prior to the active region and directly inject themselves into the quantum wells in the active region, so the carrier density in these sidewalls is negligible [1]. Support at UCSB was provided by U.S. Department of Energy under the Office of Energy Efficiency & Renewable Energy (EERE) Award No. DE-EE0009691. [1] C.-H. Ho, J. S. Speck, C. Weisbuch, and Y.-R. Wu. Phys. Rev. Appl. 17, 014033 (2022).

Electron Emission Microscopy of an Electrically Driven III-Nitride-Based LED: Evidence of Lateral Electron Injection at V-Defect Sidewalls

Rapid progress has been achieved in perovskite solar cells, improving the efficiency from 3.8 % to 25.7 % in less than a decade. However, the stability of perovskites still need to be improved before commercialization. This study reports the thermal stability of perovskites exposed to an ion beam irradiation. Such combined stressors are seen in atomic/nanoscale microscopy, where a perovskite lamella is characterized using a controlled heating/cooling stage. Focused ion beams (FIBs) are frequently used to section perovskites of interest. Previous studies proposed that high-energy electron beams could cause unexpectedly fast thermal degradation. Alternatively, the perovskite surface may be already altered during FIB processes, accelerating the deterioration. We use a grazing angle argon ion (Ar+) beam directly irradiated on methyl-ammonium lead iodide (MAPbI3) to test the impact of ion beams to degradation mechanisms. This perovskite film was spin-coated on a stack of glass / FTO (fluorine-doped tin oxide; 200 nm) / SnO2 (35 nm). A 4 kV Ar+ ion beam was injected into a part of the MAPbI3 sample at an angle of 3° for 10 minutes. A strong photoluminescence (PL) peak was observed near 765 nm (1.62 eV) for both samples, attributed to the bandgap emission. However, the PL intensity decreased by 40 % after the Ar+ polishing. A finite difference time domain (FDTD) simulation estimates an approximately 40 % of the PL excitation (244 nm laser) can be absorbed within ≈15 nm of the MAPbI3 subsurface. This result agrees with the ion-beam penetration depth, calculated by Monte Carlo simulations. The PL analysis and the numerical simulations indicate that the 4 kV ion beam irradiation at an incident angle of 3° introduces an optically inactive “dead layer (≈15 nm)” on MAPbI3. We performed x-ray photoemission spectroscopy (XPS) to investigate possible surface compositional changes of MAPbI3 under Ar+ polishing. Quantitative XPS analysis shows an apparent reduction of the organic components (e.g., C, O, and N) after the ion beam irradiation. In contrast, XPS signals of the inorganic species (e.g., Pb, I) significantly increase with introducing the doublet metallic Pb (0) near the bonding energy of 137 eV and 142 eV. Overall, the I/Pb ratio decreases from 2.6 (pristine) to 2.1 (polished), suggesting the ion beam irradiation modifies the subsurface of MAPbI3 to be a lead-rich, iodine-deficient layer (< 10 nm). The impact of the modified subsurface of MAPbI3 on thermal deterioration was qualitatively investigated using a heat lamp at 150 °C. Owing to the partial surface decomposition, we observed the ion-beam exposed MAPbI3 shows a faster decay than the control under the heating. Apparent deterioration of MAPbI3 (yellow color) on the Ar+ polished section after applying the heating stressor for 20 minutes. At this stage, the color of the pristine area remains unchanged (dark-brown color). The deterioration of the pristine area began at around 40 minutes. Both sample sections fully deteriorated at 70 minutes. Our experiment confirms that subsurface modification (≈15 nm) can significantly influence the deterioration mechanisms of MAPbI3. This talk will discuss a new method of monitoring the deterioration of perovskites in real-time. A prototype design consists of a patch antenna (≈ 3 cm × 4 cm) and an array of four MAPbI3 solar cells (1 cm × 1 cm). The distance of the adjacent solar cells is set to 1.4 cm in width and 0.6 cm in length. Computer Simulation Technology (CST) simulations display the resonance frequency of the patch antenna changes from 7.13 GHz to 7.07 GHz as the MAPbI3 film decomposes to volatile species and PbI2. While the shift is small, our pilot design demonstrates the feasibility of wireless monitoring the deterioration of perovskite solar cells. Tailoring antenna design and experimental validations are in progress.

Monitoring Stability of Metal-Halide Perovskites Under Combined Stressors of Ion-Beam and Heat

A large portion of energy consumption (50%) in buildings arises from heat transfer to buildings, specifically through windows due to solar illumination. Energy-efficient thermochromic smart windows, which are composed of special materials that enable them to adjust light transmission dynamically based on the ambient temperature, can allow energy demand for building cooling to be reduced. Hybrid halide perovskites are promising candidates for thermochromic smart windows owing to their capability to undergo reversible color and phase transitions between low and high temperatures. Furthermore, a new concept for smart photovoltaic windows has been realized based on inorganic perovskite CsPbI3-xBrx thin films, which possess both photovoltaic and thermochromic features. However, the perovskite-based thermochromic smart windows require relatively high transition temperatures (>50°C), and this hinders their practical application in buildings since in most places the window temperatures do not reach 50°C in direct sunlight. Therefore, a configuration based on a thermochromic perovskite layer integrated with an external heating source is needed for the activation of thermochromism. Transparent heaters are a key component in thermochromic smart window devices that employ transparent conductive electrodes (TCEs) to provide electric heating. Low resistance and high optical transparency of TCEs are two essential prerequisites for the fabrication of thermochromic smart windows. Among various TCEs, indium tin oxide (ITO) is most commonly used; but the brittle nature limits its application in flexible optoelectronics, and the scarcity of indium results in high material cost. Transparent and conductive metal nanomesh (NM) films, which are composed of periodic nanostructured metal networks, have gained considerable attention as high-performance TCEs offering several advantages, including excellent mechanical flexibility, high conductivity, tunable transmittance, and low fabrication cost. In this study, we introduce a new class of dielectric/metal/dielectric nanomesh films based on ZnO/Au/Al2O3 with periodicities of 200 nm and 370 nm which are fabricated using nanosphere lithography. For the metal mesh film design, an Au (15nm) thin film is chosen as the electrode material due to its malleability and high electrical conductivity. Thin layers of Al2O3 film (20 nm) and ZnO film (20 nm) are used as high refractive index dielectric layers to induce antireflective effect and suppress surface plasmons. We use relatively smaller metal mesh periodicities of 200 nm and 370 nm as compared to previously reported nanomesh films to create a potential pathway to optoelectronic applications, such as smart windows. As a result, the ZnO/Au/Al2O3 NM film with a 370-nm periodicity exhibits a transmittance over 90% at 550 nm and sheet resistance lower than 20 Ω/sq. Based on a standard figure of merit, this structure outperforms current state-of-the-art nanomesh films. Moreover, we integrate the ZnO/Au/Al2O3 NM film into thermochromic perovskite smart windows that operate by inducing a thermochromic transition from transparent to opaque state via electric heating of ZnO/Au/Al2O3 NM film at 50°C, which can be then maintained solely by 1-sun AM 1.5G solar illumination. This climate-adaptive, low power-activated, and fast-switching smart window structure opens new pathways toward its practical application in the real world.

Climate-Adaptive Thermochromic Perovskite Smart Window Structures

Over the last fifty years, miniaturization has been the primary driver in improving efficiency, facilitating higher speed and reduced cost in MOSFETs while enabling an exponential increase in computational and storage capabilities. The discovery of stable ultra-thin ferroelectric and other high K dielectric gate oxides have provided an additional impetus in lowering the threshold voltage and the power dissipation in FETs [1] . However, despite the scaling down in physical dimensions of the gate oxide, the power supply voltage or the physical voltage has been stuck around 1 V or above for the last 15 years [2]. Even though fluorite oxides like Hafnia and Zirconia show ferroelectricity at a few unit cells of thickness, their extremely high coercive fields (>1 MV/cm) impede the reduction in switching voltage. It has thus become imperative to develop and investigate leakage free, low voltage technological platforms. The more conventional ferroelectric systems like perovskite oxides have recently shown much lower switching voltages (<0.1 V) [3]. But they have also been simultaneously plagued by high leakage currents and depolarizing fields. In this work, we have achieved ultra-low coercive voltages (<50mV) [4] in ferroelectric thin films based on epitaxial oxide perovskites in the Metal-Insulator-Metal geometry. More specifically we have deposited epitaxial BaTiO3 (BTO) thin films on GdScO3 substrates using Pulsed Laser Deposition (PLD). We have minimised depolarization effects in our thin films by sandwiching the BTO with an epitaxial conductive oxide perovskite (SrRuO3) on either sides. Our BTO films have also shown leakage currents at least 3 orders of magnitude lower than HfO2 and SiO2 at sub 1 nm equivalent oxide thicknesses [4]. In order to satisfy a broader set of applications, it is essential to achieve a similar or better set of ferroelectric/leakage properties on Metal-Insulator-Semiconductor (MOS) structures. Even though Si has been the workhorse of the semiconductor industry for at least half a century, the scaling requirements have increasing eaten into its primary advantages, namely its thermally grown dielectric native oxide. There is an urgent need to replace SiO2 with other high k oxides and hence there is an impetus towards replacing Si with other commercially viable semiconductors. Ge in particular has many advantages as the channel material. It has higher electron and hole mobilities than Si. There is also an additional advantage in that the native germanium oxides are much less stable and hence are easier to remove, thereby rendering a low switching voltage platform. Moreover, Ge provides an excellent epitaxial template for BTO (<0.5 % strain). The primary challenge in a MOS structure would be achieving a defect free and abrupt semiconductor-oxide interface. On that account, all oxides of Ge have to be removed from the surface with little or no change in roughness. There could also be trap states and dead layers at the interface due to the structural dissimilarity of BTO and Ge. In addition, although BTO has a reasonable valence band offset with Ge, the conduction band offset is negligible. In order to minimise leakage currents and trap states, sufficient band and interface engineering is necessary. We have employed thin blocking layers of epitaxially compatible perovskite dielectric oxides such as BaZrO3 and CaZrO3 with high enough valence and conduction band offsets at the Ge interface to help on that front. REFERENCES : 1. Böscke, T. S., J. Müller, D. Bräuhaus, U. Schröder, and U. Böttger. "Ferroelectricity in hafnium oxide thin films." Applied Physics Letters 99, no. 10 (2011): 102903. 2. Andrew Danowitz et al. “CPU DB: recording microprocessor history”. In: Communi- cations of the ACM 55.4 (2012), pp. 55–63. 3. Jiang, Y., E. Parsonnet, A. Qualls, W. Zhao, S. Susarla, D. Pesquera, A. Dasgupta et al. "Enabling ultra-low-voltage switching in BaTiO3." Nature Materials (2022): 1-7 4. Extended Abstract figure 2

Pushing the Limits of Switching Voltage, Leakage in Ultrathin BaTiO3 Thin Films

The Magnetic Tunnel Junction (MTJ) was first successfully fabricated and tested in the 90s. it consist of a Tri-layer of two ferromagnet, sandwiching a 2nm ultra-thin layer of dielectric or insulator called the tunneling barrier. If a voltage is applied to the ferromagnets, electrons can tunnel across the barrier in the process called quantum mechanical tunneling. The tunnelling current in this case is maximum if the direction of magnetization of the two electrodes are parallel to each, and minimum when the two electrodes are anti parallel to the direction of magnetization. The tunnelling current and resistance of this device will therefore change in response to changes in the magnetic orientation of the two ferromagnetic electrodes. When we introduce an organo-metallic molecule to the exposed edges of the ferromagnetic electrodes, the molecule covalently bonds to two ferromagnetic electrodes creating a molecular channel through which electrons can spin through. The Magnetic Tunnel Junction Molecular spintronic device (MTJMSD) can transform ferromagnetic and materials and produce intriguing spin transport characteristics with high sensitivity, low power consumption, very small in size and less expensive. This disclosure is directed toward a new MTJ based spintronics device for converting chemical absorption into a change in magnetoresistance. The magnetic material used is dependent on the organic structure of the thin film electrodes for its absorption at cross junction of the MTJMSD and thus modify its magnetic properties. A change in the magnetic property will lead to a change in the resistance of a magneto resistive structure allowing a standard electrical detection of the chemical change in the sensor surface. Due to its high sensitivity, excellent scalability, and low power consumption, MTJMSD based sensors can be widely implemented in different applications. In our investigation, we tested some commonly used chemical in the lab on our device, both at the excited state (with light) and at the ground state (in the dark) We tested Acetone (CH3COCH3), Ethyl Alcohol (CH2CHOH) and Dichloromethane (CH2Cl2) and studied the trend. The IV measurements were conducted before and after molecular attachment. There was no clear distinction in the detectivity between the measurement done under the light and in the dark, however, a general observation before and after molecular attachment showed a significant increase in the current. In the present work, we fabricated a metal/insulator/semiconductor MTJ as a testbed for our MTJMSD based chemical sensing device. This paper is part of an ongoing investigation with the aim of fabricating a MTJMSD based sensor for detection dopamine concentration and other chemicals.

Magnetic Tunnel Junction Molecular Spintronics Based Chemical Sensing Device

Steady-state current induced degradation mechanisms of 275-nm-band AlGaN quantum well (QW) deep-ultraviolet (DUV) light-emitting diodes (LEDs) were investigated. The optical output power (Po) showed an initial rapid decrease by 20-30 % within the aging time (tage) of 100 h and then gradually decreased to about 60 % by tage of 1000 h. For elucidating the causes for the initial and gradual degradations, complementary electrical, time-resolved photoluminescence (TRPL), and impurity characterizations were carried out, making a connection with the junction band profiles. Because the initial degradation of the wells was less significant than the Po, the short-term degradation is attributed to the reduction in carrier injection efficiency (CIE), not in internal quantum efficiency (IQE) of the wells, most probably due to de-passivation of initially H-passivated vacancy clusters in Mg-doped AlGaN electron blocking layer (EBL) especially close to the multiple QWs. The principal cause for the gradual decrease in Po is tentatively attributed to a gradual decrease in IQE of the QWs. (Contents) Recently, an increasing demand for antivirus has attracted attention for DUV LEDs operating between 200 and 280 nm, because such radiation is known to inactivate bacteria or viruses, including SARS-CoV-2. AlGaN-based 275-nm-band DUV LEDs are suitable for such purposes, and Po of several manufacturers are increasing. However, simultaneous realization of high power and high quantum efficiency is still difficult and the device lifetime is far less than that of visible and near UV LEDs. With respect to the device lifetime, there is a consensus that the degradation can be divided into two groups: a rapid reduction in Po within tage below about 100 h and a gradual reduction for tage ≥ 100 h. The results have been explained to be due to the increase in the number of point defects acting as nonradiative recombination centers (NRCs) in the QWs. However, direct evidence for increased concentration of NRCs has not been given yet. In this presentation, the results of electrical, TRPL, and secondary-ion mass spectrometry (SIMS) measurements carried out on 275-nm-band AlGaN LEDs are shown to define the locations of major degradations. Two sets (lot A and B) of AlGaN DUV LEDs grown on c-plane sapphire have been stressed under hard conditions: forward current IF of 350 mA and junction temperature Tj of 105 °C. Consistent with previous literatures, Po for the near-band-edge (NBE) emission showed an initial rapid decrease by 20-30 % (tage < 100 h) and a gradual decrease to about 60 % by tage of 1000 h. The initial decrease was accompanied by the increases in both IF below the diffusion voltage (VD) and reverse leakage current (IR). For elucidating dominant causes for the degradations, TRPL measurements were carried out using the excitation photon energy below the barrier and above the well to extract pure photoluminescence (PL) lifetimes of the wells. Because the decrease in the room-temperature PL lifetime that represents the nonradiative lifetime was less significant than the Po, the initial degradation is attributed to the decrease in CIE, not in IQE of the wells. From the energy band diagrams calculated for biases V = -5, 0, 4, and 7 V, both the below VD and reverse leakages appear to take place in the AlGaN EBL, especially at the interface close to the active region. Because the decrease in H concentration in the EBL to multiple QW region was confirmed by SIMS measurement, the initial degradation is most probably due to de-passivation of initially H-passivated vacancy clusters (NRCs) in the EBL close to the multiple QWs. The principal cause for the gradual decrease in Po is tentatively attributed to a gradual decrease in IQE of the QWs. This work was supported by MOE program for implementation of innovative infection-control and digital technologies with low CO2 emissions, MEXT Crossover Alliance, an JST SPRING (#JPMJFS2102), Japan.

Short-Term Degradation Mechanisms of 275-nm-Band AlGaN-Based Deep-Ultraviolet Light Emitting Diodes Fabricated on a Sapphire Substrate

Ultra-violet light emitting diodes (UV-LEDs) and lasers based on the III-Nitride material system are very promising since they enable compact, safe, and efficient solid-state sources of UV light for a range of applications. The primary challenges for UV LEDs are related to the poor conductivity of p-AlGaN layers and the low light extraction efficiency of LED structures. Tunnel junction-based UV LEDs provide a distinct and unique pathway to eliminate several challenges associated with UV LEDs[1-4]. In this work, we present for the first time, a reversed-polarization (p-down) AlGaN based UV-LED utilizing bottom tunnel junction (BTJ) design. We show that compositional grading enables us to achieve the lowest reported voltage drop of 1.1 V at 20 A/cm2 among transparent AlGaN based tunnel junctions at this Al-composition. Compared to conventional LED design, a p-down structure offers lower voltage drop because the depletion barrier for both holes and electrons is lower due to polarization fields aligning with the depletion field. Furthermore, the bottom tunnel junction also allows us to use polarization grading to realize better p- and n-type doping to improve tunneling transport. The epitaxial structure of the UV-LED was grown by plasma-assisted molecular beam epitaxy (PAMBE) on metal-organic chemical vapor deposition (MOCVD)-grown n-type Al0.3Ga0.7N templates. The transparent TJ was grown using graded n++-Al0.3Ga0.7N→ n++-Al0.4Ga0.6N (Si=3×1020 cm-3) and graded p++-Al0.4Ga0.6N →p++-Al0.3Ga0.7N (Mg=1×1020 cm-3) to take advantage of induced 3D polarization charges. The high number of charges at the tunnel junction region leads to lower depletion width and efficient hole injection to the p-type layer. The UV LED active region consists of three 2.5 nm Al0.2Ga0.8N quantum wells and 7 nm Al0.3Ga0.6N quantum barriers followed by 12 nm of p- Al0.46Ga0.64N electron blocking layer (EBL). The active region was grown on top of the tunnel junction. A similar LED with p-up configuration was also grown to compare the electrical performance. The surface morphology examined by atomic force microscopy (AFM) shows smooth growth features with a surface roughness of 1.9 nm. The dendritic features on the surface are characteristic of high Si doping on the surface. The composition of each layer was extracted from the scan by high resolution x-ray diffraction (HR-XRD). The electrical characteristics of a device show a voltage drop of 4.9 V at 20 A/cm2, which corresponds to a tunnel junction voltage drop of ~ 1.1 V. This is the best lowest voltage for transparent 30% AlGaN tunnel junctions to-date and is comparable with the lowest voltage drop reported previously on non-transparent (InGaN-based) tunnel junctions at similar Al mole fraction AlGaN. On-wafer electroluminescence measurements on patterned light-emitting diodes showed single peak emission wavelength of 325 nm at 100 A/cm2 which corresponds to Al0.2Ga0.8N, confirming that efficient hole injection was achieved within the structure. The device exhibits a wavelength shift from 330 nm to 325 nm with increasing current densities from 10A/cm2 to 100A/cm2. In summary, we have demonstrated a fully transparent bottom AlGaN homojunction tunnel junction that enables p-down reversed polarization ultraviolet light emitting diodes, and has very low voltage drop at the tunnel junction. This work could enable new flexibility in the design of future III-Nitride ultraviolet LEDs and lasers. Funding from National Science Foundation under grant number 2034140 is gratefully acknowledged. References: [1] Y. Zhang et al., Applied Physics Letters, vol. 109, no. 12, 2016 [2] A. Pandey, W. J. Shin, J. Gim, R. Hovden, and Z. Mi, Photonics Research, vol. 8, no. 3, 2020 [3] V. Fan Arcara et al., Journal of Applied Physics, vol. 126, no. 22, p. 224503, 2019. [4] K. Nagata et al., Applied Physics Express, vol. 14, no. 8, p. 084001, 2021.

Demonstration of AlGaN Tunnel Junction p-Down UV Light Emitting Diodes

Injection laser diodes (LDs) in the III-N alloy system require the growth of heterostructures. Influenced by both lattice mismatch and thermal expansion coefficient mismatch in such structures, the alloy system is restrained from freely allowing full alloy selection for the best device performance. In the case of LDs emitting in the UV-A range, tensile strain limits the growth of adequately thick and/or high Al-mole-fraction AlGaN epitaxial layers on GaN substrates, and the growth of layers having thickness beyond the critical layer thickness results in cracking of the film. The inadequate thickness or index contrast of cladding layer (CL) to the active layer limits the LED or LD device performance due to low optical confinement, while the yield of the devices fabricated on the wafer is significantly reduced by the cracks. In this work, we describe a novel approach employing non-planar growth (NPG) that enables the deposition of crack-free AlGaN layers with relatively thick and high Al-mole-fraction on patterned GaN substrates with rectangular mesas with adequately large area for the fabrication of UV LEDs and injection laser devices. Cracking of AlGaN layers grown by MOCVD on (0001) GaN substrates with NPG mesas was evaluated. Stripe-shaped NPG mesas were formed on the GaN substrate with conventional dry etching. The direction of the stripe mesas was aligned to be parallel to the m axis of GaN. The widths of the NPG mesa were set to be 25 and 100 µm. AlxGa1-xN/AlyGa1-yN short period super lattices (SPSLs) were grown on the patterned GaN. The average Al mole fraction and total thickness of the AlGaN SPSL were set to be 0.16 and 1500 nm, respectively. Surface cracking after the growth was not observed on the AlGaN SPSL grown on the top of mesa, while the critical thickness for planar sample without mesa formation was 200 nm. Investigations with AlGaN with different Al mole fractions and thicknesses show that the surface cracking depends on the mesa width and depth. Mesas with narrower widths and/or deeper etching depth are more effective to prevent the cracking than others with either wider widths or shallower depths.[1,2] Additionally, the suppression of the formation of the crack is attributed to an anisotropic relaxation of the strain in the AlGaN SPSL which was enabled by the elastic deformation of GaN layer along the direction perpendicular to the stripe mesa. Our results clearly show that the NPG approach enables us to achieve thick AlGaN layers with high composition contrast to the substrate material with a thickness beyond critical layer thickness by relaxing the tensile stress in the AlGaN layers on the top of mesa. This is especially advantageous for fabrication of UV LDs as these devices require thick CLs with high refractive index contrast to the active layer to lower the lasing threshold. Using this approach, we have designed and grown a UV-A multiple-quantum-well LD heterostructure. The CL in upper and lower side of the active layers are composed of doped AlGaN SPSLs with average Al mole fraction and combined thickness of 0.16 and 1100 nm, respectively. The surface of the planar reference sample has a high density of cracks along the <11-20> directions while the NPG sample has no cracks on the top surface of several-mm-long NPG mesas. Test diodes device fabricated on the NPG sample before formation of the laser cavity structure show rectification with no leakage in a DC current-voltage measurement at 300K. Spontaneous light emission in UV-A range was successfully demonstrated by pulsed current injection at 300K. From these results, the NPG approach is proven to effectively suppress the cracking of relatively thick AlGaN layers with high Al mole fraction and thicknesses beyond the critical layer thickness and thus this approach is promising to achieve a high-performance UV-A LD and LEDs. [1] F. Mehnke et al., J. Appl. Phys. 131, 073103 (2022). [2] Y. Ando et al., J. Cryst. Growth 607, 127100 (2023).

Non-Planar Growth of Crack-Free High Al-Mole-Fraction AlGaN Heterostructures on Patterned GaN Substrates for Ultraviolet Light Emitting Diodes and Laser Diodes

A possible way to reduce the production cost of GaN-based LEDs is to serially grow several InGaN-GaN multiple quantum well (MQW) structures on the same sapphire substrate and connect these MQW structures with a tunnel junction (TJ). This approach can realize multiple wavelength LEDs covering the same chip area. Furthermore, cascaded LEDs can effectively increase the optical output power per chip area when each MQW section is driven in the low efficiency-droop regime by applying high voltages but low current densities to the whole LED. The complex nature of the growth of cascaded LED requires vertically-resolved structural and optical analytical tools to evaluate the different regions of the stack. Using nano-cathodoluminescence performed in scanning transmission electron microscope (STEM-CL) as well as electroluminescence (EL) microscopy, we have investigated a cascaded InGaN/GaN LED. The structure has been grown by metal-organic vapor phase epitaxy (MOVPE) in a single growth process on top of a GaN/sapphire template. Three 5-fold InGaN/GaN MQWs are separated by a heavily doped GaN:Mg/GaN:Ge TJ. On top, a GaN:Mg/GaN:Ge TJ was grown as contact scheme completing the cascaded LED structure. The Mg-doped GaN layers were thermally activated by a combination of in- and ex-situ activation steps. Additionally, the cascaded LED was processed by standard UV-lithography, mesa dry-etching and metal deposition. In the spatially averaged EL spectrum of the processed LED structure, the emission peak is centered at 455 nm modulated by Fabry-Pérot oscillations. The competition between the Fabry-Pérot modes leads to a lateral variation of the EL peak wavelength. Structural and optical properties have been investigated by highly spatially resolved STEM-CL in cross section. STEM high-angle annular dark field (HAADF) contrast resolves the full structure and the quantum wells with sharp interfaces. The panchromatic CL intensity exhibits highest intensity around 435 nm wavelength at each InGaN/GaN MQW section. Each individual layer of the structure reveals its own characteristic emission in local spectra acting as fingerprint luminescence. In particular, a cross-sectional CL linescan performed at 17 K across the MQW stack reveals the emission evolution in growth direction: all 3 MQW luminescence contributions are spectrally separated shifting from bottom to top to longer wavelengths. In more detail, each individual MQW shows a peak wavelength change in growth direction indicating different structural and/or chemical properties of the individual 5-fold quantum wells.

Advanced Cathodoluminescence Microscopy of a Cascaded InGaN/GaN LED

III-nitride LEDs have revolutionized modern lighting through highly efficient blue and white LEDs based on GaN. However, UV LEDs based on high Al-content AlGaN still have relatively low efficiencies due to many compounding factors. One of the largest components is the low light extraction efficiency (LEE), which currently limits the overall efficiency of devices to single digit percentages. The high index of refraction of the LED layers causes total internal reflection (TIR), where generated photons are reflected back into the LED layers where they are ultimately absorbed. Ever more complex surface modifications are proposed to reduce these losses, with limited success. In this study, it is shown that the index of refraction and TIR is not the limiting factor for light extraction but is instead bulk absorption in LED layers and in the substrate. Ray tracing simulations were developed in MatLab to investigate the effect of several factors on LEE, including the absorption coefficient and index of refraction of LED layers and the substrate, contact reflectivity, and the inclusion or exclusion of a GaN contact layer. These factors were studied in conjunction with the effect of roughening the extraction surface to reduce TIR. The simulation predicts that roughening the surface of an LED with a GaN contact layer will increase the LEE by 40 percent. Further, it was found that through removing the absorptive GaN contact layer and properly minimizing bulk absorption in the substrate, the light extraction efficiency can be improved by more than 10x. These simulations were verified experimentally for a UV LED grown on a native AlN substrate. A simple wet etch procedure using a KOH solution to roughen the extraction surface is proposed. Since N-polar AlN is selectively etched by KOH with a 900:1 ratio (1), this procedure effectively ensures no damage to the epitaxial surface. Pyramidal structures with dimensions on the order of 1 μm are formed on the extraction surface of the LED to increase photon scattering events and reduce losses due to TIR. After ten minutes of etching, the LEE of the device increased by 50 percent, validating the simulation results for devices with a GaN contact layer. This work points the way to overall efficiencies of greater than 40% in UVC LEDs and definitively shows that low LEE is due to absorption and not the high index of refraction in AlGaN based LEDs. 1. Guo, W. et al. Comparative study of etching high crystalline quality AlN and GaN. Journal of Crystal Growth 366, 20–25 (2013).

Premium content

Downloads

Next from EMC 2023

Electron Emission Microscopy of an Electrically Driven III-Nitride-Based LED: Evidence of Lateral Electron Injection at V-Defect Sidewalls

Similar lecture

Carrier Dynamics in Blue, Cyan and Green Commercial InGaN/GaN LEDs Measured by Small-Signal Electroluminescence