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
We demonstrate AlGaN-cladding-free (ACF) m-plane InGaN/GaN laser diodes (LDs) with peak output po... more We demonstrate AlGaN-cladding-free (ACF) m-plane InGaN/GaN laser diodes (LDs) with peak output powers and estimated front facet optical power densities that are comparable to the state-of-the-art single-stripe emitter c-plane LDs. The threshold current density, slope efficiency, and peak output power were 4.66 kA/cm2, 1.29 W/A, and 1.6 W, respectively, after facet coating and under pulsed conditions. Catastrophic optical mirror damage was not observed up to an estimated optical power density of 51.2 MW/cm2 at the front mirror facet, indicating the potential for using ACF m-plane InGaN/GaN LDs in high-power LD applications.
We have previously demonstrated the AlGaN-cladding-free (ACF) laser diode (LD) concept over a wid... more We have previously demonstrated the AlGaN-cladding-free (ACF) laser diode (LD) concept over a wide visible spectral range and various nonpolar/semipolar crystallographic orientations. The benefits of this ACF epitaxial design include lower operating voltages and higher production yields. Nonpolar LDs have been demonstrated out to 500nm1 with semipolar reaching as long as 534nm2. No results have been published on nonpolar/semipolar orientations for short wavelength LDs. In this paper we report on the short wavelength limits of this epitaxial design on nonpolar bulk m-plane GaN substrates.
The dependence of device characteristics on cavity length is used to determine the injection effi... more The dependence of device characteristics on cavity length is used to determine the injection efficiency, internal loss, and material gain of electrically injected AlGaN-cladding-free m-plane InGaN/GaN laser diodes. Estimates for the transparency carrier density are discussed in the context of recombination coefficients that have been reported for c-plane InGaN-based light-emitting devices.
Different from the case for polar (0001) InxGa1−xN multiple quantum wells (MQWs), the effective r... more Different from the case for polar (0001) InxGa1−xN multiple quantum wells (MQWs), the effective radiative lifetimes (τR,eff) at 8K of violet (V: 3.15eV), purple (P: 3.00eV), and blue (B: 2.83eV) emission peaks in nonpolar (11¯00) InxGa1−xN∕GaN MQWs fabricated on various GaN templates were found to be nearly independent of InN molar fraction x being approximately 1ns. The result indicates the absence of polarization fields parallel to the MQW normal. For each luminescence peak, the effective nonradiative lifetime (τNR,eff) at room temperature of the MQWs grown on “Ga-polar” wings of the GaN template prepared by lateral epitaxial overgrowth (LEO) was longer than that for the MQWs grown on “N-polar” wings, windows, or on conventional GaN templates, which had high density basal plane stacking faults and threading dislocations. Since τR,eff was little affected by the presence of defects, the increase in τNR,eff brought fivefold improvement in the equivalent internal quantum efficiency (ηinteq) of V peak. Because B peak was mostly generated from defective areas, the increase in ηinteq was not so remarkable (29%–36%). However, these values are approximately twice that reported for (112¯0) InGaN∕GaN MQWs grown on LEO GaN templates [Onuma et al., Appl. Phys. Lett. 86, 151918 (2005)].
ABSTRACT We compare thermal performance of violet and blue single-quantum-well nonpolar m-plane I... more ABSTRACT We compare thermal performance of violet and blue single-quantum-well nonpolar m-plane InGaN/GaN light-emitting diodes (LEDs) over a temperature range from 25 to 140 degrees C. The hot/cold factor, defined as the ratio between the output power at high and low temperatures, improved as the quantum-well thickness increased from 3 to 10 nm. A hot/cold factor of 93% was measured under high injection current for the 10-nm-thick blue-emitting quantum well LED, indicating high thermal stability. The increase in hot/cold factor with current density for blue samples indicates possible contributions of Shockley-Read-Hall recombination for quantum wells of increased indium composition. (c) 2013 The Japan Society of Applied Physics
Abstract We demonstrate the first blue AlInGaN-based laser diodes (LDs) grown on semipolar (33 eq... more Abstract We demonstrate the first blue AlInGaN-based laser diodes (LDs) grown on semipolar (33 equation image) free-standing GaN substrates. Etched facet ridge waveguide LDs were fabricated and tested under pulsed operation. Lasing was achieved at 477.5 nm ...
The role of extended defects in determining the atomic scale surface morphology of nonpolar {11¯0... more The role of extended defects in determining the atomic scale surface morphology of nonpolar {11¯00} m-plane gallium nitride has been elucidated. The heteroepitaxially grown m-GaN films are commonly reported to yield striated surface morphologies (slate morphology) correlated with their high densities of basal plane stacking faults. Here, the growth window was explored to allow nonslate morphologies for hydride vapor phase epitaxy. Lateral epitaxial overgrowth was then utilized to produce m-GaN films with three regimes of different extended defect contents. Elimination of stacking faults from the m-GaN yielded step-flow features with an average step height of 4–7 ML even for slate morphology growth conditions.
Prospective equivalent internal quantum efficiency ͑ int ͒ of approximately 34% at 300 K was demo... more Prospective equivalent internal quantum efficiency ͑ int ͒ of approximately 34% at 300 K was demonstrated for the blue emission peak of nonpolar m-plane ͑1100͒ In x Ga 1−x N / GaN multiple quantum well light emitting diodes ͑LEDs͒ fabricated on freestanding m-plane GaN substrates. Although the int value is yet lower than that of conventional c-plane blue LEDs ͑Ͼ70% ͒, the
The emission mechanisms of strained InGaN quantum wells (QWs) were shown to vary depending on the... more The emission mechanisms of strained InGaN quantum wells (QWs) were shown to vary depending on the well thickness L and InN molar fraction x. The QW resonance energy was shifted to lower energy by the quantum confined Stark effect (QCSE) due to the internal piezoelectric field, F PZ. The absorption spectrum was modulated by QCSE and quantum-confined Franz-Keldysh effect (QCFK) for the wells, in which, for the first approximation, the product of F PZ and L (potential drop across the well) exceeds the valence band discontinuity, DE V. In this case, dressed holes are confined in the triangular potential well formed at one side of the well. This produces apparent Stokes-like shift (vertical component). The QCFK further modulated the absorption energy for the wells with L greater than the three dimensional free exciton Bohr radius, a B. For the wells having high InN content (F PZ ×L\ DE V , DE C), electron and hole confined levels drop into the triangular potential wells formed at opposite sides of the wells, which reduces the wavefunction overlap. Doping of Si in the barriers partially screens F PZ resulting in a smaller Stokes-like shift, shorter recombination decay time, and higher emission efficiency. Si-doping was found to improve the interface quality and surface morphology, resulting in an efficient carrier transfer from high to low bandgap energy portions of the well. Effective in-plane localization of carriers in quantum disk size potential minima, which are produced by nonrandom alloy potential fluctuations enhanced by the large bowing parameter and F PZ , produces confined e-h pair whose wavefunctions are still overlapped. Their excitonic features are pronounced provided that LBa B and F PZ × LBE V (quantized exciton). Several cw laser wafers exhibit stimulated emission from these energy tail states even at room temperature.
State-of-the-art continuous-wave InGaN laser diodes in the violet, blue, and green wavelength reg... more State-of-the-art continuous-wave InGaN laser diodes in the violet, blue, and green wavelength regimes. [Proceedings of SPIE 7686, 76860L (2010)]. James W. Raring, Eric M. Hall, Matthew C. Schmidt, Christiane Poblenz, Ben ...
We present new advances in green, blue, and violet InGaN-based laser diodes fabricated on nonpola... more We present new advances in green, blue, and violet InGaN-based laser diodes fabricated on nonpolar and semipolar GaN substrates. Using these novel crystal orientations, we report high power, high efficiency, continuous-wave operation from single-lateral-mode ...
... Origin of localized excitons in In-containing three-dimensional bulk (Al,In,Ga)N alloy films ... more ... Origin of localized excitons in In-containing three-dimensional bulk (Al,In,Ga)N alloy films probed by time-resolved photoluminescence and monoenergetic positron annihilation techniques. ... 3-D) bulk (Al,In,Ga)N alloy films, such as InGaN, AlInN and AlInGaN, is proposed. ...
This talk will summarize the important materials and device results in gallium nitride based ligh... more This talk will summarize the important materials and device results in gallium nitride based light emitter technology. GaN has emerged as the most promising material for high brightness LEDs with colors ranging from W, blue, green, and white. Recent progress on ultraviolet 0 emitting LEDs using AlGaN single quantum wells indicates wavelengths as short as 2 9 2 m are achievable. UV LEDs are of great interest for solid state white lighting due to the high conversion efficiencies of typical phosphors in the UV specbum. This paper will focus on recent progress in improving the properties of W LEDs.
The impact of AlGaN growth conditions on AlGaN:Si resistivity and surface morphology has been inv... more The impact of AlGaN growth conditions on AlGaN:Si resistivity and surface morphology has been investigated using metalorganic chemical vapor deposition. Growth parameters including growth temperature, growth rate, and trimethylindium (TMI) flow have been systematically studied to minimize the resistivity of AlGaN:Si. We observed a strong anticorrelation between AlGaN:Si conductivity and growth temperature, suggesting increased silicon donor compensation at elevated temperatures. Secondary ion mass spectrometry and positron annihilation spectroscopy ruled out compensation by common impurities or group-III monovacancies as a reason for the observed phenomenon, in contrast to theoretical predictions. The underlying reason for AlGaN:Si resistivity dependence on growth temperature is discussed based on the possibility of silicon acting as a DX center in Al0.65Ga0.35N at high growth temperatures. We also show remarkable enhancement of AlGaN:Si conductivity by introducing TMI flow during g...
The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes (LEDs) is limited due to ... more The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes (LEDs) is limited due to their poor light extraction efficiency (LEE). To improve the LEE of AlGaN LEDs, we developed a fabrication technology to process AlGaN LEDs grown on SiC into thinfilm flip-chip LEDs (TFFC LEDs) with high LEE. This process transfers the AlGaN LED epi onto a new substrate by wafer-to-wafer bonding, and by removing the absorbing SiC substrate with a highly selective SF6 plasma etch that stops at the AlN buffer layer. We optimized the inductively coupled plasma (ICP) SF6 etch parameters to develop a substrateremoval process with high reliability and precise epitaxial control, without creating micromasking defects or degrading the health of the plasma etching system. The SiC etch rate by SF6 plasma was ~46 µm/hr at a high RF bias (400 W), and ~7 µm/hr at a low RF bias (49 W) with very high etch selectivity between SiC and AlN. The high SF6 etch selectivity between SiC and AlN was essential for removing the SiC substrate and exposing a pristine, smooth AlN surface. We demonstrated the epi-transfer process by fabricating high light extraction TFFC LEDs from AlGaN LEDs grown on SiC. To further enhance the light extraction, the exposed N-face AlN was anisotropically etched in dilute KOH. The LEE of the AlGaN LED improved by ~3X after KOH roughening at room temperature. This AlGaN TFFC LED process establishes a viable path to high external quantum efficiency (EQE) and power conversion efficiency (PCE) UV-C LEDs.
A compositionally step-graded (CSG) InGaN barrier is designed for the active region of c-plane bl... more A compositionally step-graded (CSG) InGaN barrier is designed for the active region of c-plane blue light-emitting diodes (LEDs). High external quantum efficiencies of 45, 42, 39 and 36% are achieved at current densities of 100, 200, 300 and 400 A/cm 2 , respectively. Compared with GaN barrier devices, LEDs with CSG InGaN barriers showed higher power, lower operating voltage and smaller wavelength blueshift, compared with GaN barrier LEDs. Owing to the low-voltage performance, higher wall-plug efficiency can be achieved for blue LEDs with CSG InGaN barriers.
We demonstrate AlGaN-cladding-free (ACF) m-plane InGaN/GaN laser diodes (LDs) with peak output po... more We demonstrate AlGaN-cladding-free (ACF) m-plane InGaN/GaN laser diodes (LDs) with peak output powers and estimated front facet optical power densities that are comparable to the state-of-the-art single-stripe emitter c-plane LDs. The threshold current density, slope efficiency, and peak output power were 4.66 kA/cm2, 1.29 W/A, and 1.6 W, respectively, after facet coating and under pulsed conditions. Catastrophic optical mirror damage was not observed up to an estimated optical power density of 51.2 MW/cm2 at the front mirror facet, indicating the potential for using ACF m-plane InGaN/GaN LDs in high-power LD applications.
We have previously demonstrated the AlGaN-cladding-free (ACF) laser diode (LD) concept over a wid... more We have previously demonstrated the AlGaN-cladding-free (ACF) laser diode (LD) concept over a wide visible spectral range and various nonpolar/semipolar crystallographic orientations. The benefits of this ACF epitaxial design include lower operating voltages and higher production yields. Nonpolar LDs have been demonstrated out to 500nm1 with semipolar reaching as long as 534nm2. No results have been published on nonpolar/semipolar orientations for short wavelength LDs. In this paper we report on the short wavelength limits of this epitaxial design on nonpolar bulk m-plane GaN substrates.
The dependence of device characteristics on cavity length is used to determine the injection effi... more The dependence of device characteristics on cavity length is used to determine the injection efficiency, internal loss, and material gain of electrically injected AlGaN-cladding-free m-plane InGaN/GaN laser diodes. Estimates for the transparency carrier density are discussed in the context of recombination coefficients that have been reported for c-plane InGaN-based light-emitting devices.
Different from the case for polar (0001) InxGa1−xN multiple quantum wells (MQWs), the effective r... more Different from the case for polar (0001) InxGa1−xN multiple quantum wells (MQWs), the effective radiative lifetimes (τR,eff) at 8K of violet (V: 3.15eV), purple (P: 3.00eV), and blue (B: 2.83eV) emission peaks in nonpolar (11¯00) InxGa1−xN∕GaN MQWs fabricated on various GaN templates were found to be nearly independent of InN molar fraction x being approximately 1ns. The result indicates the absence of polarization fields parallel to the MQW normal. For each luminescence peak, the effective nonradiative lifetime (τNR,eff) at room temperature of the MQWs grown on “Ga-polar” wings of the GaN template prepared by lateral epitaxial overgrowth (LEO) was longer than that for the MQWs grown on “N-polar” wings, windows, or on conventional GaN templates, which had high density basal plane stacking faults and threading dislocations. Since τR,eff was little affected by the presence of defects, the increase in τNR,eff brought fivefold improvement in the equivalent internal quantum efficiency (ηinteq) of V peak. Because B peak was mostly generated from defective areas, the increase in ηinteq was not so remarkable (29%–36%). However, these values are approximately twice that reported for (112¯0) InGaN∕GaN MQWs grown on LEO GaN templates [Onuma et al., Appl. Phys. Lett. 86, 151918 (2005)].
ABSTRACT We compare thermal performance of violet and blue single-quantum-well nonpolar m-plane I... more ABSTRACT We compare thermal performance of violet and blue single-quantum-well nonpolar m-plane InGaN/GaN light-emitting diodes (LEDs) over a temperature range from 25 to 140 degrees C. The hot/cold factor, defined as the ratio between the output power at high and low temperatures, improved as the quantum-well thickness increased from 3 to 10 nm. A hot/cold factor of 93% was measured under high injection current for the 10-nm-thick blue-emitting quantum well LED, indicating high thermal stability. The increase in hot/cold factor with current density for blue samples indicates possible contributions of Shockley-Read-Hall recombination for quantum wells of increased indium composition. (c) 2013 The Japan Society of Applied Physics
Abstract We demonstrate the first blue AlInGaN-based laser diodes (LDs) grown on semipolar (33 eq... more Abstract We demonstrate the first blue AlInGaN-based laser diodes (LDs) grown on semipolar (33 equation image) free-standing GaN substrates. Etched facet ridge waveguide LDs were fabricated and tested under pulsed operation. Lasing was achieved at 477.5 nm ...
The role of extended defects in determining the atomic scale surface morphology of nonpolar {11¯0... more The role of extended defects in determining the atomic scale surface morphology of nonpolar {11¯00} m-plane gallium nitride has been elucidated. The heteroepitaxially grown m-GaN films are commonly reported to yield striated surface morphologies (slate morphology) correlated with their high densities of basal plane stacking faults. Here, the growth window was explored to allow nonslate morphologies for hydride vapor phase epitaxy. Lateral epitaxial overgrowth was then utilized to produce m-GaN films with three regimes of different extended defect contents. Elimination of stacking faults from the m-GaN yielded step-flow features with an average step height of 4–7 ML even for slate morphology growth conditions.
Prospective equivalent internal quantum efficiency ͑ int ͒ of approximately 34% at 300 K was demo... more Prospective equivalent internal quantum efficiency ͑ int ͒ of approximately 34% at 300 K was demonstrated for the blue emission peak of nonpolar m-plane ͑1100͒ In x Ga 1−x N / GaN multiple quantum well light emitting diodes ͑LEDs͒ fabricated on freestanding m-plane GaN substrates. Although the int value is yet lower than that of conventional c-plane blue LEDs ͑Ͼ70% ͒, the
The emission mechanisms of strained InGaN quantum wells (QWs) were shown to vary depending on the... more The emission mechanisms of strained InGaN quantum wells (QWs) were shown to vary depending on the well thickness L and InN molar fraction x. The QW resonance energy was shifted to lower energy by the quantum confined Stark effect (QCSE) due to the internal piezoelectric field, F PZ. The absorption spectrum was modulated by QCSE and quantum-confined Franz-Keldysh effect (QCFK) for the wells, in which, for the first approximation, the product of F PZ and L (potential drop across the well) exceeds the valence band discontinuity, DE V. In this case, dressed holes are confined in the triangular potential well formed at one side of the well. This produces apparent Stokes-like shift (vertical component). The QCFK further modulated the absorption energy for the wells with L greater than the three dimensional free exciton Bohr radius, a B. For the wells having high InN content (F PZ ×L\ DE V , DE C), electron and hole confined levels drop into the triangular potential wells formed at opposite sides of the wells, which reduces the wavefunction overlap. Doping of Si in the barriers partially screens F PZ resulting in a smaller Stokes-like shift, shorter recombination decay time, and higher emission efficiency. Si-doping was found to improve the interface quality and surface morphology, resulting in an efficient carrier transfer from high to low bandgap energy portions of the well. Effective in-plane localization of carriers in quantum disk size potential minima, which are produced by nonrandom alloy potential fluctuations enhanced by the large bowing parameter and F PZ , produces confined e-h pair whose wavefunctions are still overlapped. Their excitonic features are pronounced provided that LBa B and F PZ × LBE V (quantized exciton). Several cw laser wafers exhibit stimulated emission from these energy tail states even at room temperature.
State-of-the-art continuous-wave InGaN laser diodes in the violet, blue, and green wavelength reg... more State-of-the-art continuous-wave InGaN laser diodes in the violet, blue, and green wavelength regimes. [Proceedings of SPIE 7686, 76860L (2010)]. James W. Raring, Eric M. Hall, Matthew C. Schmidt, Christiane Poblenz, Ben ...
We present new advances in green, blue, and violet InGaN-based laser diodes fabricated on nonpola... more We present new advances in green, blue, and violet InGaN-based laser diodes fabricated on nonpolar and semipolar GaN substrates. Using these novel crystal orientations, we report high power, high efficiency, continuous-wave operation from single-lateral-mode ...
... Origin of localized excitons in In-containing three-dimensional bulk (Al,In,Ga)N alloy films ... more ... Origin of localized excitons in In-containing three-dimensional bulk (Al,In,Ga)N alloy films probed by time-resolved photoluminescence and monoenergetic positron annihilation techniques. ... 3-D) bulk (Al,In,Ga)N alloy films, such as InGaN, AlInN and AlInGaN, is proposed. ...
This talk will summarize the important materials and device results in gallium nitride based ligh... more This talk will summarize the important materials and device results in gallium nitride based light emitter technology. GaN has emerged as the most promising material for high brightness LEDs with colors ranging from W, blue, green, and white. Recent progress on ultraviolet 0 emitting LEDs using AlGaN single quantum wells indicates wavelengths as short as 2 9 2 m are achievable. UV LEDs are of great interest for solid state white lighting due to the high conversion efficiencies of typical phosphors in the UV specbum. This paper will focus on recent progress in improving the properties of W LEDs.
The impact of AlGaN growth conditions on AlGaN:Si resistivity and surface morphology has been inv... more The impact of AlGaN growth conditions on AlGaN:Si resistivity and surface morphology has been investigated using metalorganic chemical vapor deposition. Growth parameters including growth temperature, growth rate, and trimethylindium (TMI) flow have been systematically studied to minimize the resistivity of AlGaN:Si. We observed a strong anticorrelation between AlGaN:Si conductivity and growth temperature, suggesting increased silicon donor compensation at elevated temperatures. Secondary ion mass spectrometry and positron annihilation spectroscopy ruled out compensation by common impurities or group-III monovacancies as a reason for the observed phenomenon, in contrast to theoretical predictions. The underlying reason for AlGaN:Si resistivity dependence on growth temperature is discussed based on the possibility of silicon acting as a DX center in Al0.65Ga0.35N at high growth temperatures. We also show remarkable enhancement of AlGaN:Si conductivity by introducing TMI flow during g...
The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes (LEDs) is limited due to ... more The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes (LEDs) is limited due to their poor light extraction efficiency (LEE). To improve the LEE of AlGaN LEDs, we developed a fabrication technology to process AlGaN LEDs grown on SiC into thinfilm flip-chip LEDs (TFFC LEDs) with high LEE. This process transfers the AlGaN LED epi onto a new substrate by wafer-to-wafer bonding, and by removing the absorbing SiC substrate with a highly selective SF6 plasma etch that stops at the AlN buffer layer. We optimized the inductively coupled plasma (ICP) SF6 etch parameters to develop a substrateremoval process with high reliability and precise epitaxial control, without creating micromasking defects or degrading the health of the plasma etching system. The SiC etch rate by SF6 plasma was ~46 µm/hr at a high RF bias (400 W), and ~7 µm/hr at a low RF bias (49 W) with very high etch selectivity between SiC and AlN. The high SF6 etch selectivity between SiC and AlN was essential for removing the SiC substrate and exposing a pristine, smooth AlN surface. We demonstrated the epi-transfer process by fabricating high light extraction TFFC LEDs from AlGaN LEDs grown on SiC. To further enhance the light extraction, the exposed N-face AlN was anisotropically etched in dilute KOH. The LEE of the AlGaN LED improved by ~3X after KOH roughening at room temperature. This AlGaN TFFC LED process establishes a viable path to high external quantum efficiency (EQE) and power conversion efficiency (PCE) UV-C LEDs.
A compositionally step-graded (CSG) InGaN barrier is designed for the active region of c-plane bl... more A compositionally step-graded (CSG) InGaN barrier is designed for the active region of c-plane blue light-emitting diodes (LEDs). High external quantum efficiencies of 45, 42, 39 and 36% are achieved at current densities of 100, 200, 300 and 400 A/cm 2 , respectively. Compared with GaN barrier devices, LEDs with CSG InGaN barriers showed higher power, lower operating voltage and smaller wavelength blueshift, compared with GaN barrier LEDs. Owing to the low-voltage performance, higher wall-plug efficiency can be achieved for blue LEDs with CSG InGaN barriers.
Uploads
Papers by Steven Denbaars