Dr. Steven P. DenBaars is a Professor of Materials and Co-Director of the Solid-State Lighting and Energy Electronics Center (SSLEEC) at the University of California Santa Barbara. In 2005 he was appointed the Mitsubishi Chemical Chair in Solid State Lighting and Displays. Specific research interests include growth of wide-bandgap semiconductors (GaN based), and their application to Blue LEDs and lasers and high power electronic devices. Prof. DenBaars has been an active in entrepreneurship, having helped co-found 3 university start-ups in the field of optoelectronics. He received the IEEE Fellow award in 2005, and elected member of the National Academy if Engineers 2012, and National Academy of Inventors in 2014. He has authored or co-authored over 880 technical publications, 250 conference presentations, and over 150 patents. Supervisors: Professor P. Daniel Dapkus
The V-defect is a naturally occurring inverted hexagonal pyramid structure that has been studied ... more The V-defect is a naturally occurring inverted hexagonal pyramid structure that has been studied in GaN and InGaN growth since the 1990's. Strategic use of V-defects in pre-quantum well superlattices, or equivalent preparation layers, have enabled record breaking efficiencies for green, yellow, and red InGaN light emitting diodes (LEDs) utilizing lateral injection of holes through the semipolar sidewalls of the V-defects. In this article, we use advanced characterization techniques such as scattering contrast transmission electron microscopy, high angle annular dark field scanning transmission electron microscopy, x-ray fluorescence maps, and atom probe tomography to study the active region compositions, V-defect formation, and V-defect structure in green and red LEDs grown on (0001) patterned sapphire and (111) Si substrates. We identify two distinct types of V-defects. The 'large' V-defects are those that form in the pre-well superlattice and promote hole injection, usually nucleating on mixed (Burgers vector 𝐛 = ±𝐚 ± 𝐜) character threading dislocations. In addition, 'small' V-defects often form in the multi-quantum well region and are believed to be deleterious to high efficiency LEDs by providing non-radiative pathways. The small V-defects are often associated with basal plane stacking faults or stacking fault boxes. Furthermore, we show through scattering contrast TEM that during V-defect filling the threading dislocation, which runs up the center of the V-defect, will 'bend' onto one of the six {101 ̅ 1} semipolar planes. This result is essential to understanding non-radiative recombination in V-defect engineered LEDs.
AlGaN-based deep ultraviolet (DUV) micro-light-emitting diodes (μLEDs) with emission wavelengths ... more AlGaN-based deep ultraviolet (DUV) micro-light-emitting diodes (μLEDs) with emission wavelengths between 277 and 304 nm with mesa dimensions down to 20 μm were fabricated. Their size-dependent electrical and optical characteristics were analyzed. At 20 A cm−2, the external quantum efficiency (EQE) increased from 2.0% to 2.3% mainly due to the improved light extraction efficiency; the forward voltage was 7.6 V in 20 μm sized μLEDs in comparison to 9.1 V in 300 μm LEDs due to better current spreading in the smaller devices. The peak EQEs of the 20 μm μLEDs were 2.5% and 4.0% for 277 and 304 nm, among the highest reported for DUV μLEDs.
In this work, we present fully transparent metal organic chemical vapor deposition (MOCVD)-grown ... more In this work, we present fully transparent metal organic chemical vapor deposition (MOCVD)-grown InGaN cascaded micro-light-emitting diodes (µLEDs) with independent junction control. The cascaded µLEDs consisted of a blue emitting diode, a tunnel junction (TJ), a green emitting diode, and a TJ, without using any conductive oxide layer. We can control the injection of carriers into blue, green, and blue/green junctions in the same device independently, which show high optical and electrical performance. The forward voltage (Vf) at 20 A/cm2 for the TJ blue µLEDs and TJ green µLEDs is 4.06 and 3.13 V, respectively. These results demonstrate the efficient TJs and fully activated p-type GaN in the cascaded µLEDs. Such demonstration shows the important application of TJs for the integration of µLEDs with multiple color emissions.
In this paper, we report the successful demonstration of bright InGaN-based microLED devices emit... more In this paper, we report the successful demonstration of bright InGaN-based microLED devices emitting in the red spectral regime grown by metal organic chemical vapor deposition (MOCVD) on c-plane semi-relaxed InGaN substrates on sapphire. Through application of an InGaN/GaN base layer scheme to ameliorate high defect density and maintain appropriate lattice constant throughout the growth, high-In quantum wells (QWs) can be grown with improved crystal quality. Improvement to the design of the growth scheme also yields higher power output resulting in an increase to the external quantum efficiency (EQE). Combined, these two improvements allow for an 80 × 80 µm 2 microLED device emitting at 609 nm to achieve 0.83% EQE. Furthermore, the true In content of the QW is measured using atomic probe tomography (APT) to confirm the improved In incorporation during high temperature active region growth. These developments represent advancement toward the realization of bright, highly efficient red III-nitride LEDs to be used in RGB applications under one material system.
Bulletin of the American Physical Society, Mar 5, 2007
AlN is an important material for AlGaN-based electronic and optoelectronic devices such as UV Lig... more AlN is an important material for AlGaN-based electronic and optoelectronic devices such as UV Light Emitting Diodes and High Electron Mobility Transistors. We have grown AlN films with reduced Threading Dislocation (TD) densities using Cantilever Epitaxy with the Hydride Vapor Phase Epitaxy growth method. Prior to growth, 6H-SiC substrates were processed using standard lithography and ICP etching to form periodic
Unintentionally doped (UID) and Si doped Al0.82In0.18N samples were grown on Si-doped GaN by meta... more Unintentionally doped (UID) and Si doped Al0.82In0.18N samples were grown on Si-doped GaN by metalorganic chemical vapor deposition. The high structural quality of the Al0.82In0.18N layers was confirmed by high resolution X-ray diffraction and transmission electron microscopy. Secondary ion mass spectrometry measurement showed oxygen levels of (2–6)×1018 cm-3 regardless of the explored growth conditions. Vertical Schottky diodes were fabricated with a Pd-based Schottky contact on Al0.82In0.18N and Ti/Al/Ni/Au-based ohmic contact on n-type GaN. Capacitance–voltage (C–V) analysis showed that UID Al0.82In0.18N was an n-type with a carrier density of about 3×1017 cm-3 in the bulk region at 300 K. Based on the temperature dependent C–V analysis from 100 to 300 K, the donor activation energy was found to be 4 meV, showing very weak temperature dependence despite the large bandgap of Al0.82In0.18N. Si-doped Al0.82In0.18N ([Si] ≃2×1018 cm-3) showed almost no carrier freeze-out at carrier density of 1.0×1018 cm-3.
Semiconductor Science and Technology, Feb 17, 2021
In this work, we demonstrate the detailed optimization of metalorganic chemical vapor deposition ... more In this work, we demonstrate the detailed optimization of metalorganic chemical vapor deposition (MOCVD)-grown tunnel junctions (TJs) utilizing selective area growth (SAG) for regular size (0.1 mm2) and micro-size InGaN light-emitting diodes (LEDs and µLEDs). Finite-difference time-domain simulations show that the SAG apertures result in a more directional light emission of far-field radiation pattern for the SAG TJ LEDs grown on patterned sapphire substrate. Moreover, it is noted that the n-InGaN insertion layer and Si-doped concentration in the n+GaN TJs layer is essential to realize a low forward voltage (V f) in TJs LEDs. For both 0.1 mm2 LEDs and µLEDs, the V f is independent on the SAG aperture space varied from 3 to 8 µm when the Si-doping level in the n+GaN layer is as high as 1.7 × 1020 cm−3. The optimized TJ LEDs exhibit a comparable differential resistance of 1.0 × 10−2 Ω cm2 at 100 A cm−2 and a very small voltage penalty of 0.2–0.3 V compared to the conventional indium tin oxide contact LEDs. The low V f penalty is caused by a higher turn on voltage, which is the smallest one among the MOCVD-grown TJs LEDs and comparable to the best molecular beam epitaxy-grown TJs LEDs.
Semiconductor Science and Technology, Oct 27, 2020
High performance GaN-based micro-light-emitting diodes (µLEDs) with epitaxial n-InGaN/n-GaN tunne... more High performance GaN-based micro-light-emitting diodes (µLEDs) with epitaxial n-InGaN/n-GaN tunnel junctions (InGaN TJs) were grown by metalorganic chemical vapor deposition (MOCVD). The InGaN TJs µLEDs show a significant reduction of forward voltage (Vf) by ∼0.6 V compared to the common TJs µLEDs. The Vf at 20 A cm−2 is very low varied from 3.15 V to 3.19 V in small InGaN TJ µLEDs with a size less than 40 × 40 µm2, and then significantly increases in large LEDs. Selective area growth (SAG) of TJs can overcome such size limitation by vertical out diffusion of hydrogen through the apertures on top of p-GaN. The InGaN TJ µLEDs overgrown by SAG show a size-independent low Vf ranged from 3.08 V to 3.25 V. The external quantum efficiency (EQE) of the packaged TJ µLEDs was improved by 6% compared to the common µLEDs with indium tin oxide (ITO) contact. This work solves the key challenges of MOCVD-grown TJs.
Red micro-light-emitting diodes (μLEDs) have been generated significant interest for the next gen... more Red micro-light-emitting diodes (μLEDs) have been generated significant interest for the next generation μLEDs displays. It has been shown that the external quantum efficiency (EQE) of AlInGaP red μLEDs markedly decreases as the size goes to very small dimension. Here, we demonstrate size-independent peak EQE of 611 nm InGaN red μLEDs. Packaged μLEDs show a peak EQE varied from 2.4% to 2.6% as the device area reduces from 100 × 100 to 20 × 20 μm2. These results demonstrate the promising potential for realizing high efficiency red μLED with very small size using InGaN materials.
We demonstrate high-performance 10 × 10 μm2 InGaN amber micro-size LEDs (μLEDs). At 15 A cm−2, th... more We demonstrate high-performance 10 × 10 μm2 InGaN amber micro-size LEDs (μLEDs). At 15 A cm−2, the InGaN μLEDs show a single emission peak located at 601 nm. The peak external quantum efficiency (EQE) and wall-plug efficiency are 5.5% and 3.2%, respectively. Compared to the 100 × 100 μm2 μLEDs, the 10 × 10 μm2 InGaN red μLEDs maintain a similar EQE value with the same efficiency droop. These results point out that InGaN materials are much more promising for higher efficiency than the common AlInGaP materials for the ultra-small size red μLEDs required by augmented reality and virtual reality displays.
Growth of high quality AlGaN is essential to develop efficient UV-C LEDs. However, growth of AlGa... more Growth of high quality AlGaN is essential to develop efficient UV-C LEDs. However, growth of AlGaN is challenging due to the lack of native substrates, low surface mobility of Aluminum adatoms and low doping efficiency for p-AlGaN. In this work, we illustrate our approach to develop UV-C LED on SiC starting with MOCVD growth of high-quality AlN buffer layer with TDD $\sim 8\times 10^{8}/\text{cm}^{2}$ as measured by TEM, followed by growth of AlGaN UV-C LED structure and then fabricated into Thin-Film Flip-Chip (TFFC) devices via novel epi-transfer process and nondestructive removal of SiC by ICP-RIE. We investigated the effects of Indium, growth rate and Ammonia to improve the conductivity of n-AlGaN. As a result, the free electron concentration as measured by RT Hall effect increases to 3.2×1019/cm3, which facilitates a formation of a direct ohmic contact to 66% AlGaN using Vanadium-based metal stack V/Al/V/Au. Finally, we report on the development of AlGaN UV-C LED on SiC with high $\text{EQE}>2\%$ and output power of 2 mW at DC current of 20mA with an emission wavelength of 262 nm. Therefore, we conclude that SiC is well-suited to be utilized as a substrate for high-power AlGaN UV-C LEDs.
AlGaN-based UV-C LEDs (260-300 nm) remain inefficient compared to InGaN visible LEDs due to optic... more AlGaN-based UV-C LEDs (260-300 nm) remain inefficient compared to InGaN visible LEDs due to optically absorptive layers limiting light extraction, optical polarization, and poor material quality. Sapphire, the most popular substrate material, is transparent and inexpensive but has many disadvantages in material quality and device performance. In contrast, SiC has small lattice mismatch with AlN (~1%), similar crystal structure, more chemically stable and contains no oxygen, which degrades the IQE and compensates holes. We report low threading dislocations density (TDD) AlN on SiC (TDD < 7x108cm-2) by metalorganic chemical vapor deposition (MOCVD). We demonstrate innovative thin-film flipchip (TFFC) LEDs with 7.8 mW at 95 mA at 278.5 nm grown on AlN/SiC with TDD~1x109 cm-2. (Respectively, EQE and WPE are 1.8% and 0.6%.) We also demonstrate that KOH roughening does not impact the IV voltage of TFFC LED. KOH roughening enhanced the light extraction efficiency (LEE) by 100% and ~180% for UV LEDs with 10 nm p-GaN and 5 nm p-GaN, respectively.
The performance of In x Ga 1-x N-based m-plane LEDs and laser diodes grown by metalorganic chemic... more The performance of In x Ga 1-x N-based m-plane LEDs and laser diodes grown by metalorganic chemical vapor deposition on bulk GaN substrates is currently limited by lower indium uptake and inhomogeneous linewidth broadening in the blue spectrum compared to semipolar planes and c-plane. Linewidth broadening is partially attributed to inhomogeneous indium composition that is associated with template morphology. We investigate the morphological evolution of homoepitaxial GaN growth on bulk m-plane substrates in three co-loaded miscut orientations: nominally on-axis, 1° in the -c-direction (-c-miscut), and 1° in the a-direction (a-miscut). Atomic force microscopy reveals four-sided pyramidal hillocks for on-axis growth with faces inclined toward the ሾ112 ത 0ሿ a-axis (a-faces) and the [0001] c-axis (c-faces). The a-faces exhibit steps oriented in an a+c direction with longer terrace widths than the c-face steps. The -cmiscut template growth sometimes forms diagonal striations, characterized by regions with stable a+c step direction. The a-miscut template growth exhibits meandering steps oriented in the a+/-c directions that bunch to form diagonal striations. These results reveal that c-direction steps are unstable compared to a+/-c directions. We further demonstrate that m-plane GaN substrates with combined a+c miscut lead to narrower In x Ga 1-x N photoluminescence emission spectra in blue with enhanced indium incorporation.
IEEE/OSA Journal of Display Technology, Apr 1, 2013
ABSTRACT This work examines the effects of polarization-related electric fields on the energy ban... more ABSTRACT This work examines the effects of polarization-related electric fields on the energy band diagrams, wavelength shift, wave function overlap, and efficiency droop for InGaN quantum wells on various crystal orientations, including polar (0001) (c-plane), semipolar (20 (2) over bar1), semipolar (20 (2) over bar(1) over bar), and non-polar (10 (1) over bar0) (m-plane). Based on simulations, we show that the semipolar (20 (2) over bar(1) over bar) orientation exhibits excellent potential for the development of high-efficiency, low-droop light-emitting diodes (LEDs). We then present recent advancements in crystal growth, optical performance, and thermal performance of semipolar (20 (2) over bar(1) over bar) LEDs. Finally, we demonstrate a low-droop, high-efficiency single-quantum-well blue semipolar (20 (2) over bar(1) over bar) LED with an external quantum efficiency of more than 50% at 100 A/cm(2).
The V-defect is a naturally occurring inverted hexagonal pyramid structure that has been studied ... more The V-defect is a naturally occurring inverted hexagonal pyramid structure that has been studied in GaN and InGaN growth since the 1990's. Strategic use of V-defects in pre-quantum well superlattices, or equivalent preparation layers, have enabled record breaking efficiencies for green, yellow, and red InGaN light emitting diodes (LEDs) utilizing lateral injection of holes through the semipolar sidewalls of the V-defects. In this article, we use advanced characterization techniques such as scattering contrast transmission electron microscopy, high angle annular dark field scanning transmission electron microscopy, x-ray fluorescence maps, and atom probe tomography to study the active region compositions, V-defect formation, and V-defect structure in green and red LEDs grown on (0001) patterned sapphire and (111) Si substrates. We identify two distinct types of V-defects. The 'large' V-defects are those that form in the pre-well superlattice and promote hole injection, usually nucleating on mixed (Burgers vector 𝐛 = ±𝐚 ± 𝐜) character threading dislocations. In addition, 'small' V-defects often form in the multi-quantum well region and are believed to be deleterious to high efficiency LEDs by providing non-radiative pathways. The small V-defects are often associated with basal plane stacking faults or stacking fault boxes. Furthermore, we show through scattering contrast TEM that during V-defect filling the threading dislocation, which runs up the center of the V-defect, will 'bend' onto one of the six {101 ̅ 1} semipolar planes. This result is essential to understanding non-radiative recombination in V-defect engineered LEDs.
AlGaN-based deep ultraviolet (DUV) micro-light-emitting diodes (μLEDs) with emission wavelengths ... more AlGaN-based deep ultraviolet (DUV) micro-light-emitting diodes (μLEDs) with emission wavelengths between 277 and 304 nm with mesa dimensions down to 20 μm were fabricated. Their size-dependent electrical and optical characteristics were analyzed. At 20 A cm−2, the external quantum efficiency (EQE) increased from 2.0% to 2.3% mainly due to the improved light extraction efficiency; the forward voltage was 7.6 V in 20 μm sized μLEDs in comparison to 9.1 V in 300 μm LEDs due to better current spreading in the smaller devices. The peak EQEs of the 20 μm μLEDs were 2.5% and 4.0% for 277 and 304 nm, among the highest reported for DUV μLEDs.
In this work, we present fully transparent metal organic chemical vapor deposition (MOCVD)-grown ... more In this work, we present fully transparent metal organic chemical vapor deposition (MOCVD)-grown InGaN cascaded micro-light-emitting diodes (µLEDs) with independent junction control. The cascaded µLEDs consisted of a blue emitting diode, a tunnel junction (TJ), a green emitting diode, and a TJ, without using any conductive oxide layer. We can control the injection of carriers into blue, green, and blue/green junctions in the same device independently, which show high optical and electrical performance. The forward voltage (Vf) at 20 A/cm2 for the TJ blue µLEDs and TJ green µLEDs is 4.06 and 3.13 V, respectively. These results demonstrate the efficient TJs and fully activated p-type GaN in the cascaded µLEDs. Such demonstration shows the important application of TJs for the integration of µLEDs with multiple color emissions.
In this paper, we report the successful demonstration of bright InGaN-based microLED devices emit... more In this paper, we report the successful demonstration of bright InGaN-based microLED devices emitting in the red spectral regime grown by metal organic chemical vapor deposition (MOCVD) on c-plane semi-relaxed InGaN substrates on sapphire. Through application of an InGaN/GaN base layer scheme to ameliorate high defect density and maintain appropriate lattice constant throughout the growth, high-In quantum wells (QWs) can be grown with improved crystal quality. Improvement to the design of the growth scheme also yields higher power output resulting in an increase to the external quantum efficiency (EQE). Combined, these two improvements allow for an 80 × 80 µm 2 microLED device emitting at 609 nm to achieve 0.83% EQE. Furthermore, the true In content of the QW is measured using atomic probe tomography (APT) to confirm the improved In incorporation during high temperature active region growth. These developments represent advancement toward the realization of bright, highly efficient red III-nitride LEDs to be used in RGB applications under one material system.
Bulletin of the American Physical Society, Mar 5, 2007
AlN is an important material for AlGaN-based electronic and optoelectronic devices such as UV Lig... more AlN is an important material for AlGaN-based electronic and optoelectronic devices such as UV Light Emitting Diodes and High Electron Mobility Transistors. We have grown AlN films with reduced Threading Dislocation (TD) densities using Cantilever Epitaxy with the Hydride Vapor Phase Epitaxy growth method. Prior to growth, 6H-SiC substrates were processed using standard lithography and ICP etching to form periodic
Unintentionally doped (UID) and Si doped Al0.82In0.18N samples were grown on Si-doped GaN by meta... more Unintentionally doped (UID) and Si doped Al0.82In0.18N samples were grown on Si-doped GaN by metalorganic chemical vapor deposition. The high structural quality of the Al0.82In0.18N layers was confirmed by high resolution X-ray diffraction and transmission electron microscopy. Secondary ion mass spectrometry measurement showed oxygen levels of (2–6)×1018 cm-3 regardless of the explored growth conditions. Vertical Schottky diodes were fabricated with a Pd-based Schottky contact on Al0.82In0.18N and Ti/Al/Ni/Au-based ohmic contact on n-type GaN. Capacitance–voltage (C–V) analysis showed that UID Al0.82In0.18N was an n-type with a carrier density of about 3×1017 cm-3 in the bulk region at 300 K. Based on the temperature dependent C–V analysis from 100 to 300 K, the donor activation energy was found to be 4 meV, showing very weak temperature dependence despite the large bandgap of Al0.82In0.18N. Si-doped Al0.82In0.18N ([Si] ≃2×1018 cm-3) showed almost no carrier freeze-out at carrier density of 1.0×1018 cm-3.
Semiconductor Science and Technology, Feb 17, 2021
In this work, we demonstrate the detailed optimization of metalorganic chemical vapor deposition ... more In this work, we demonstrate the detailed optimization of metalorganic chemical vapor deposition (MOCVD)-grown tunnel junctions (TJs) utilizing selective area growth (SAG) for regular size (0.1 mm2) and micro-size InGaN light-emitting diodes (LEDs and µLEDs). Finite-difference time-domain simulations show that the SAG apertures result in a more directional light emission of far-field radiation pattern for the SAG TJ LEDs grown on patterned sapphire substrate. Moreover, it is noted that the n-InGaN insertion layer and Si-doped concentration in the n+GaN TJs layer is essential to realize a low forward voltage (V f) in TJs LEDs. For both 0.1 mm2 LEDs and µLEDs, the V f is independent on the SAG aperture space varied from 3 to 8 µm when the Si-doping level in the n+GaN layer is as high as 1.7 × 1020 cm−3. The optimized TJ LEDs exhibit a comparable differential resistance of 1.0 × 10−2 Ω cm2 at 100 A cm−2 and a very small voltage penalty of 0.2–0.3 V compared to the conventional indium tin oxide contact LEDs. The low V f penalty is caused by a higher turn on voltage, which is the smallest one among the MOCVD-grown TJs LEDs and comparable to the best molecular beam epitaxy-grown TJs LEDs.
Semiconductor Science and Technology, Oct 27, 2020
High performance GaN-based micro-light-emitting diodes (µLEDs) with epitaxial n-InGaN/n-GaN tunne... more High performance GaN-based micro-light-emitting diodes (µLEDs) with epitaxial n-InGaN/n-GaN tunnel junctions (InGaN TJs) were grown by metalorganic chemical vapor deposition (MOCVD). The InGaN TJs µLEDs show a significant reduction of forward voltage (Vf) by ∼0.6 V compared to the common TJs µLEDs. The Vf at 20 A cm−2 is very low varied from 3.15 V to 3.19 V in small InGaN TJ µLEDs with a size less than 40 × 40 µm2, and then significantly increases in large LEDs. Selective area growth (SAG) of TJs can overcome such size limitation by vertical out diffusion of hydrogen through the apertures on top of p-GaN. The InGaN TJ µLEDs overgrown by SAG show a size-independent low Vf ranged from 3.08 V to 3.25 V. The external quantum efficiency (EQE) of the packaged TJ µLEDs was improved by 6% compared to the common µLEDs with indium tin oxide (ITO) contact. This work solves the key challenges of MOCVD-grown TJs.
Red micro-light-emitting diodes (μLEDs) have been generated significant interest for the next gen... more Red micro-light-emitting diodes (μLEDs) have been generated significant interest for the next generation μLEDs displays. It has been shown that the external quantum efficiency (EQE) of AlInGaP red μLEDs markedly decreases as the size goes to very small dimension. Here, we demonstrate size-independent peak EQE of 611 nm InGaN red μLEDs. Packaged μLEDs show a peak EQE varied from 2.4% to 2.6% as the device area reduces from 100 × 100 to 20 × 20 μm2. These results demonstrate the promising potential for realizing high efficiency red μLED with very small size using InGaN materials.
We demonstrate high-performance 10 × 10 μm2 InGaN amber micro-size LEDs (μLEDs). At 15 A cm−2, th... more We demonstrate high-performance 10 × 10 μm2 InGaN amber micro-size LEDs (μLEDs). At 15 A cm−2, the InGaN μLEDs show a single emission peak located at 601 nm. The peak external quantum efficiency (EQE) and wall-plug efficiency are 5.5% and 3.2%, respectively. Compared to the 100 × 100 μm2 μLEDs, the 10 × 10 μm2 InGaN red μLEDs maintain a similar EQE value with the same efficiency droop. These results point out that InGaN materials are much more promising for higher efficiency than the common AlInGaP materials for the ultra-small size red μLEDs required by augmented reality and virtual reality displays.
Growth of high quality AlGaN is essential to develop efficient UV-C LEDs. However, growth of AlGa... more Growth of high quality AlGaN is essential to develop efficient UV-C LEDs. However, growth of AlGaN is challenging due to the lack of native substrates, low surface mobility of Aluminum adatoms and low doping efficiency for p-AlGaN. In this work, we illustrate our approach to develop UV-C LED on SiC starting with MOCVD growth of high-quality AlN buffer layer with TDD $\sim 8\times 10^{8}/\text{cm}^{2}$ as measured by TEM, followed by growth of AlGaN UV-C LED structure and then fabricated into Thin-Film Flip-Chip (TFFC) devices via novel epi-transfer process and nondestructive removal of SiC by ICP-RIE. We investigated the effects of Indium, growth rate and Ammonia to improve the conductivity of n-AlGaN. As a result, the free electron concentration as measured by RT Hall effect increases to 3.2×1019/cm3, which facilitates a formation of a direct ohmic contact to 66% AlGaN using Vanadium-based metal stack V/Al/V/Au. Finally, we report on the development of AlGaN UV-C LED on SiC with high $\text{EQE}>2\%$ and output power of 2 mW at DC current of 20mA with an emission wavelength of 262 nm. Therefore, we conclude that SiC is well-suited to be utilized as a substrate for high-power AlGaN UV-C LEDs.
AlGaN-based UV-C LEDs (260-300 nm) remain inefficient compared to InGaN visible LEDs due to optic... more AlGaN-based UV-C LEDs (260-300 nm) remain inefficient compared to InGaN visible LEDs due to optically absorptive layers limiting light extraction, optical polarization, and poor material quality. Sapphire, the most popular substrate material, is transparent and inexpensive but has many disadvantages in material quality and device performance. In contrast, SiC has small lattice mismatch with AlN (~1%), similar crystal structure, more chemically stable and contains no oxygen, which degrades the IQE and compensates holes. We report low threading dislocations density (TDD) AlN on SiC (TDD < 7x108cm-2) by metalorganic chemical vapor deposition (MOCVD). We demonstrate innovative thin-film flipchip (TFFC) LEDs with 7.8 mW at 95 mA at 278.5 nm grown on AlN/SiC with TDD~1x109 cm-2. (Respectively, EQE and WPE are 1.8% and 0.6%.) We also demonstrate that KOH roughening does not impact the IV voltage of TFFC LED. KOH roughening enhanced the light extraction efficiency (LEE) by 100% and ~180% for UV LEDs with 10 nm p-GaN and 5 nm p-GaN, respectively.
The performance of In x Ga 1-x N-based m-plane LEDs and laser diodes grown by metalorganic chemic... more The performance of In x Ga 1-x N-based m-plane LEDs and laser diodes grown by metalorganic chemical vapor deposition on bulk GaN substrates is currently limited by lower indium uptake and inhomogeneous linewidth broadening in the blue spectrum compared to semipolar planes and c-plane. Linewidth broadening is partially attributed to inhomogeneous indium composition that is associated with template morphology. We investigate the morphological evolution of homoepitaxial GaN growth on bulk m-plane substrates in three co-loaded miscut orientations: nominally on-axis, 1° in the -c-direction (-c-miscut), and 1° in the a-direction (a-miscut). Atomic force microscopy reveals four-sided pyramidal hillocks for on-axis growth with faces inclined toward the ሾ112 ത 0ሿ a-axis (a-faces) and the [0001] c-axis (c-faces). The a-faces exhibit steps oriented in an a+c direction with longer terrace widths than the c-face steps. The -cmiscut template growth sometimes forms diagonal striations, characterized by regions with stable a+c step direction. The a-miscut template growth exhibits meandering steps oriented in the a+/-c directions that bunch to form diagonal striations. These results reveal that c-direction steps are unstable compared to a+/-c directions. We further demonstrate that m-plane GaN substrates with combined a+c miscut lead to narrower In x Ga 1-x N photoluminescence emission spectra in blue with enhanced indium incorporation.
IEEE/OSA Journal of Display Technology, Apr 1, 2013
ABSTRACT This work examines the effects of polarization-related electric fields on the energy ban... more ABSTRACT This work examines the effects of polarization-related electric fields on the energy band diagrams, wavelength shift, wave function overlap, and efficiency droop for InGaN quantum wells on various crystal orientations, including polar (0001) (c-plane), semipolar (20 (2) over bar1), semipolar (20 (2) over bar(1) over bar), and non-polar (10 (1) over bar0) (m-plane). Based on simulations, we show that the semipolar (20 (2) over bar(1) over bar) orientation exhibits excellent potential for the development of high-efficiency, low-droop light-emitting diodes (LEDs). We then present recent advancements in crystal growth, optical performance, and thermal performance of semipolar (20 (2) over bar(1) over bar) LEDs. Finally, we demonstrate a low-droop, high-efficiency single-quantum-well blue semipolar (20 (2) over bar(1) over bar) LED with an external quantum efficiency of more than 50% at 100 A/cm(2).
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Papers by Steven Denbaars