The first continuous-wave InGaN multiple quantum well laser diodes grown by molecular beam epitax... more The first continuous-wave InGaN multiple quantum well laser diodes grown by molecular beam epitaxy are reported. Ridge waveguide lasers are fabricated on free-standing GaN substrates and operated at room temperature under continuous-wave current injection. For 2.2/spl times/1000 /spl mu/m lasers with highly reflective facet coatings a continuous-wave threshold current of 125 mA is obtained, corresponding to a threshold current density of 5.7 kA cm/sup -2/. The lasers have a threshold voltage of 8.6 V and lase at a wavelength of /spl sim/405 nm for >3 min at 20/spl deg/C.
The performance of vertical-cavity surface-emitting lasers (VCSELs) is strongly dependent on the ... more The performance of vertical-cavity surface-emitting lasers (VCSELs) is strongly dependent on the spectral detuning between the gain peak and the resonance wavelength. Here, we use angle-resolved photoluminescence spectroscopy to investigate the emission properties of AlGaN-based VCSELs emitting in the ultraviolet-B spectral range with different detuning between the photoluminescence peak of the quantum-wells and the resonance wavelength. Accurate setting of the cavity length, and thereby the resonance wavelength, is accomplished by using doping-selective electrochemical etching of AlGaN sacrificial layers for substrate removal combined with deposition of dielectric spacer layers. By matching the resonance wavelength to the quantum-wells photoluminescence peak, a threshold power density of 0.4 MW/cm2 was achieved, and this was possible only for smooth etched surfaces with a root mean square roughness below 2 nm. These results demonstrate the importance of accurate cavity length contr...
Wavelength-dispersive X-ray (WDX) spectroscopy was used to measure silicon atom concentrations in... more Wavelength-dispersive X-ray (WDX) spectroscopy was used to measure silicon atom concentrations in the range 35–100 ppm [corresponding to (3–9) × 1018 cm−3] in doped AlxGa1–xN films using an electron probe microanalyser also equipped with a cathodoluminescence (CL) spectrometer. Doping with Si is the usual way to produce the n-type conducting layers that are critical in GaN- and AlxGa1–xN-based devices such as LEDs and laser diodes. Previously, we have shown excellent agreement for Mg dopant concentrations in p-GaN measured by WDX with values from the more widely used technique of secondary ion mass spectrometry (SIMS). However, a discrepancy between these methods has been reported when quantifying the n-type dopant, silicon. We identify the cause of discrepancy as inherent sample contamination and propose a way to correct this using a calibration relation. This new approach, using a method combining data derived from SIMS measurements on both GaN and AlxGa1–xN samples, provides the ...
We demonstrate a thin-film flip-chip (TFFC) light-emitting diode (LED) emitting in the ultraviole... more We demonstrate a thin-film flip-chip (TFFC) light-emitting diode (LED) emitting in the ultraviolet B (UVB) at 311 nm, where substrate removal has been achieved by electrochemical etching of a sacrificial Al0.37Ga0.63N layer. The electroluminescence spectrum of the TFFC LED corresponds well to the as-grown LED structure, showing no sign of degradation of structural and optical properties by electrochemical etching. This is achieved by a proper epitaxial design of the sacrificial layer and the etch stop layers in relation to the LED structure and the electrochemical etch conditions. Enabling a TFFC UV LED is an important step toward improving the light extraction efficiency that limits the power conversion efficiency in AlGaN-based LEDs.
Stimulated emission from AlGaN based quantum wells (QWs) emitting at ultraviolet wavelengths is i... more Stimulated emission from AlGaN based quantum wells (QWs) emitting at ultraviolet wavelengths is investigated theoretically. Maxwell–Bloch equations in the second Born approximation are solved self-consistently with the Poisson equation. The valence band dispersion is obtained from a 6-band kp-model. For a QW emitting at around 270 nm with a thickness of 2.2 nm, an estimated FWHM of 10 meV for homogeneous broadening and an excitonic red shift of 100 meV are extracted under typical laser conditions. From a comparison to experimental data of stimulated emission, an inhomogeneous broadening energy of 39 meV FWHM is evaluated. Calculations show that high TE gain can be achieved for thin QWs around 2 nm thickness in a multiple QW arrangement or for single QWs thicker than 6 nm.
In this work, the growth and conductivity of semipolar AlxGa1−xN:Si with (11-22) orientation are ... more In this work, the growth and conductivity of semipolar AlxGa1−xN:Si with (11-22) orientation are investigated. AlxGa1−xN:Si (x = 0.60 ± 0.03 and x = 0.80 ± 0.02) layers were grown with different SiH4 partial pressures, and the electrical properties were determined using Hall measurements at room temperature. The aluminum mole fraction was measured by wavelength dispersive x-ray spectroscopy and x-ray diffraction, and the Si-concentration was measured by wavelength dispersive x-ray spectroscopy and secondary ion mass spectroscopy. Layer resistivities as low as 0.024 Ω cm for x = 0.6 and 0.042 Ω cm for x = 0.8 were achieved. For both aluminum mole fractions, the resistivity exhibits a minimum with the increasing Si concentration, which can be explained by compensation due to the formation of cation vacancy complexes at high doping levels. The onset of self-compensation occurs at larger estimated Si concentrations for larger Al contents.
The impact of plasma etching on the formation of low-resistance n-contacts on the AlGaN:Si curren... more The impact of plasma etching on the formation of low-resistance n-contacts on the AlGaN:Si current spreading layer during the chip fabrication of ultraviolet light-emitting diodes (UV LEDs) emitting at 265 nm is investigated. A two-step plasma etching process with a first rapid etching using BCl3/Cl2 gas mixture and a second slow etching step using pure Cl2 gas has been developed. The etching sequence provides smooth mesa side-walls and an n-AlGaN surface with reduced surface damage. Ohmic n-contacts with a contact resistivity of 3.5 × 10−4 Ωcm2 are obtained on Si-doped Al0.65Ga0.35N layers and the operating voltages of the UVC LEDs were reduced by 2 V for a current of 20 mA.
The scanning electron microscopy techniques of electron backscatter diffraction (EBSD), electron ... more The scanning electron microscopy techniques of electron backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI) and cathodoluminescence (CL) hyperspectral imaging provide complementary information on the structural and luminescence properties of materials rapidly and non-destructively, with a spatial resolution of tens of nanometres. EBSD provides crystal orientation, crystal phase and strain analysis, whilst ECCI is used to determine the planar distribution of extended defects over a large area of a given sample. CL reveals the influence of crystal structure, composition and strain on intrinsic luminescence and/or reveals defect-related luminescence. Dark features are also observed in CL images where carrier recombination at defects is non-radiative. The combination of these techniques is a powerful approach to clarifying the role of crystallography and extended defects on a material’s light emission properties. Here we describe the EBSD, ECCI and CL techniques...
Nano-engineering III-nitride semiconductors offers a route to further control the optoelectronic ... more Nano-engineering III-nitride semiconductors offers a route to further control the optoelectronic properties, enabling novel functionalities and applications. Although a variety of lithography techniques are currently employed to nano-engineer these materials, the scalability and cost of the fabrication process can be an obstacle for large-scale manufacturing. In this paper, we report on the use of a fast, robust and flexible emerging patterning technique called Displacement Talbot lithography (DTL), to successfully nano-engineer III-nitride materials. DTL, along with its novel and unique combination with a lateral planar displacement (D2TL), allow the fabrication of a variety of periodic nanopatterns with a broad range of filling factors such as nanoholes, nanodots, nanorings and nanolines; all these features being achievable from one single mask. To illustrate the enormous possibilities opened by DTL/D2TL, dielectric and metal masks with a number of nanopatterns have been generated...
The growth mechanisms during metalorganic vapor phase epitaxy (11-22) oriented Al x Ga1-x N with ... more The growth mechanisms during metalorganic vapor phase epitaxy (11-22) oriented Al x Ga1-x N with x ∼ 0.80 on m-plane sapphire are studied with the intention of mitigating the expansion of misoriented grains, composed of the (1-10-3) crystal orientation and achieving a flat surface with only the (11-22) orientation. An increase in reactor pressure, metalorganic supply, and V/III ratio led to a decrease in the grain density from 1.0 × 109 cm−2 to 1.5 × 107 cm−2. By comparing different growth regimes, we found that the main factor suppressing the growth of the (1-10-3) orientation and decreasing the grain density in the AlGaN layers is the growth rate, which decreased with increasing reactor pressure, MO supply, and V/III ratio due to increasing pre-reactions in the gas phase. To mitigate pre-reactions even with lower growth rates, growth conditions with higher total flow and lower TMAl flow were chosen, yielding low growth rates of 0.13 μm h−1 and a grain density of 3.0 × 107 cm−2 at ...
The application of quaternary In x Al y Ga1−x− y N active regions is a promising path towards hig... more The application of quaternary In x Al y Ga1−x− y N active regions is a promising path towards high efficiency UVB-light emitting diodes (LEDs). For the utilization of In x Al y Ga1−x−y N, detailed knowledge of the interplay between growth parameters, adatom incorporation, optical and structural properties is crucial. We investigated the influence of the trimethylaluminium (TMAl) and trimethylindium (TMIn) flux on the composition and luminescence properties of In x Al y Ga1−x−y N layers by multi-mode scanning electron microscopy. We found that varying the molar TMIn flow from 0 to 17.3 μmol min−1 led to an InN concentration between 0% and 3.2% and an emission energy between 4.17 and 3.75 eV. The variation of the molar TMAl flow from 3.5 to 35.4 μmol min−1 resulted in a AlN composition between 7.8% and 30.7% with an emission energy variation between 3.6 and 4.1 eV. Cathodoluminescence hyperspectral imaging provided evidence for the formation of nanoscale InN-rich regions. Analyzing th...
An extensive analysis of the degradation characteristics of AlGaN-based ultraviolet light-emittin... more An extensive analysis of the degradation characteristics of AlGaN-based ultraviolet light-emitting diodes emitting around 265 nm is presented. The optical power of LEDs stressed at a constant dc current of 100 mA (current density = 67 A/cm2 and heatsink temperature = 20 °C) decreased to about 58% of its initial value after 250 h of operation. The origin of this degradation effect has been studied using capacitance-voltage and photocurrent spectroscopy measurements conducted before and after aging. The overall device capacitance decreased, which indicates a reduction of the net charges within the space-charge region of the pn-junction during operation. In parallel, the photocurrent at excitation energies between 3.8 eV and 4.5 eV and the photocurrent induced by band-to-band absorption in the quantum barriers at 5.25 eV increased during operation. The latter effect can be explained by a reduction of the donor concentration in the active region of the device. This effect could be attri...
The performance characteristics of AlGaN-based deep ultraviolet light emitting diodes (UV-LEDs) g... more The performance characteristics of AlGaN-based deep ultraviolet light emitting diodes (UV-LEDs) grown by metalorganic vapor phase epitaxy on sputtered and high temperature annealed AlN/sapphire templates are investigated and compared with LEDs grown on epitaxially laterally overgrown (ELO) AlN/sapphire. The structural and electro-optical properties of the devices on 350 nm sputtered and high temperature annealed AlN/sapphire show similar defect densities and output power levels as LEDs grown on low defect density ELO AlN/sapphire templates. After high temperature annealing of the 350 nm sputtered AlN, the full widths at half maximum of the (0002) and (101¯2) reflections of the high resolution x-ray diffraction rocking curves decrease by one order of magnitude to 65 arc sec and 240 arc sec, respectively. The curvature of the sputtered and HTA AlN/sapphire templates after regrowth with 400 nm MOVPE AlN is with −80 km−1 much lower than the curvature of the ELO AlN/sapphire template of ...
The first continuous-wave InGaN multiple quantum well laser diodes grown by molecular beam epitax... more The first continuous-wave InGaN multiple quantum well laser diodes grown by molecular beam epitaxy are reported. Ridge waveguide lasers are fabricated on free-standing GaN substrates and operated at room temperature under continuous-wave current injection. For 2.2/spl times/1000 /spl mu/m lasers with highly reflective facet coatings a continuous-wave threshold current of 125 mA is obtained, corresponding to a threshold current density of 5.7 kA cm/sup -2/. The lasers have a threshold voltage of 8.6 V and lase at a wavelength of /spl sim/405 nm for >3 min at 20/spl deg/C.
The performance of vertical-cavity surface-emitting lasers (VCSELs) is strongly dependent on the ... more The performance of vertical-cavity surface-emitting lasers (VCSELs) is strongly dependent on the spectral detuning between the gain peak and the resonance wavelength. Here, we use angle-resolved photoluminescence spectroscopy to investigate the emission properties of AlGaN-based VCSELs emitting in the ultraviolet-B spectral range with different detuning between the photoluminescence peak of the quantum-wells and the resonance wavelength. Accurate setting of the cavity length, and thereby the resonance wavelength, is accomplished by using doping-selective electrochemical etching of AlGaN sacrificial layers for substrate removal combined with deposition of dielectric spacer layers. By matching the resonance wavelength to the quantum-wells photoluminescence peak, a threshold power density of 0.4 MW/cm2 was achieved, and this was possible only for smooth etched surfaces with a root mean square roughness below 2 nm. These results demonstrate the importance of accurate cavity length contr...
Wavelength-dispersive X-ray (WDX) spectroscopy was used to measure silicon atom concentrations in... more Wavelength-dispersive X-ray (WDX) spectroscopy was used to measure silicon atom concentrations in the range 35–100 ppm [corresponding to (3–9) × 1018 cm−3] in doped AlxGa1–xN films using an electron probe microanalyser also equipped with a cathodoluminescence (CL) spectrometer. Doping with Si is the usual way to produce the n-type conducting layers that are critical in GaN- and AlxGa1–xN-based devices such as LEDs and laser diodes. Previously, we have shown excellent agreement for Mg dopant concentrations in p-GaN measured by WDX with values from the more widely used technique of secondary ion mass spectrometry (SIMS). However, a discrepancy between these methods has been reported when quantifying the n-type dopant, silicon. We identify the cause of discrepancy as inherent sample contamination and propose a way to correct this using a calibration relation. This new approach, using a method combining data derived from SIMS measurements on both GaN and AlxGa1–xN samples, provides the ...
We demonstrate a thin-film flip-chip (TFFC) light-emitting diode (LED) emitting in the ultraviole... more We demonstrate a thin-film flip-chip (TFFC) light-emitting diode (LED) emitting in the ultraviolet B (UVB) at 311 nm, where substrate removal has been achieved by electrochemical etching of a sacrificial Al0.37Ga0.63N layer. The electroluminescence spectrum of the TFFC LED corresponds well to the as-grown LED structure, showing no sign of degradation of structural and optical properties by electrochemical etching. This is achieved by a proper epitaxial design of the sacrificial layer and the etch stop layers in relation to the LED structure and the electrochemical etch conditions. Enabling a TFFC UV LED is an important step toward improving the light extraction efficiency that limits the power conversion efficiency in AlGaN-based LEDs.
Stimulated emission from AlGaN based quantum wells (QWs) emitting at ultraviolet wavelengths is i... more Stimulated emission from AlGaN based quantum wells (QWs) emitting at ultraviolet wavelengths is investigated theoretically. Maxwell–Bloch equations in the second Born approximation are solved self-consistently with the Poisson equation. The valence band dispersion is obtained from a 6-band kp-model. For a QW emitting at around 270 nm with a thickness of 2.2 nm, an estimated FWHM of 10 meV for homogeneous broadening and an excitonic red shift of 100 meV are extracted under typical laser conditions. From a comparison to experimental data of stimulated emission, an inhomogeneous broadening energy of 39 meV FWHM is evaluated. Calculations show that high TE gain can be achieved for thin QWs around 2 nm thickness in a multiple QW arrangement or for single QWs thicker than 6 nm.
In this work, the growth and conductivity of semipolar AlxGa1−xN:Si with (11-22) orientation are ... more In this work, the growth and conductivity of semipolar AlxGa1−xN:Si with (11-22) orientation are investigated. AlxGa1−xN:Si (x = 0.60 ± 0.03 and x = 0.80 ± 0.02) layers were grown with different SiH4 partial pressures, and the electrical properties were determined using Hall measurements at room temperature. The aluminum mole fraction was measured by wavelength dispersive x-ray spectroscopy and x-ray diffraction, and the Si-concentration was measured by wavelength dispersive x-ray spectroscopy and secondary ion mass spectroscopy. Layer resistivities as low as 0.024 Ω cm for x = 0.6 and 0.042 Ω cm for x = 0.8 were achieved. For both aluminum mole fractions, the resistivity exhibits a minimum with the increasing Si concentration, which can be explained by compensation due to the formation of cation vacancy complexes at high doping levels. The onset of self-compensation occurs at larger estimated Si concentrations for larger Al contents.
The impact of plasma etching on the formation of low-resistance n-contacts on the AlGaN:Si curren... more The impact of plasma etching on the formation of low-resistance n-contacts on the AlGaN:Si current spreading layer during the chip fabrication of ultraviolet light-emitting diodes (UV LEDs) emitting at 265 nm is investigated. A two-step plasma etching process with a first rapid etching using BCl3/Cl2 gas mixture and a second slow etching step using pure Cl2 gas has been developed. The etching sequence provides smooth mesa side-walls and an n-AlGaN surface with reduced surface damage. Ohmic n-contacts with a contact resistivity of 3.5 × 10−4 Ωcm2 are obtained on Si-doped Al0.65Ga0.35N layers and the operating voltages of the UVC LEDs were reduced by 2 V for a current of 20 mA.
The scanning electron microscopy techniques of electron backscatter diffraction (EBSD), electron ... more The scanning electron microscopy techniques of electron backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI) and cathodoluminescence (CL) hyperspectral imaging provide complementary information on the structural and luminescence properties of materials rapidly and non-destructively, with a spatial resolution of tens of nanometres. EBSD provides crystal orientation, crystal phase and strain analysis, whilst ECCI is used to determine the planar distribution of extended defects over a large area of a given sample. CL reveals the influence of crystal structure, composition and strain on intrinsic luminescence and/or reveals defect-related luminescence. Dark features are also observed in CL images where carrier recombination at defects is non-radiative. The combination of these techniques is a powerful approach to clarifying the role of crystallography and extended defects on a material’s light emission properties. Here we describe the EBSD, ECCI and CL techniques...
Nano-engineering III-nitride semiconductors offers a route to further control the optoelectronic ... more Nano-engineering III-nitride semiconductors offers a route to further control the optoelectronic properties, enabling novel functionalities and applications. Although a variety of lithography techniques are currently employed to nano-engineer these materials, the scalability and cost of the fabrication process can be an obstacle for large-scale manufacturing. In this paper, we report on the use of a fast, robust and flexible emerging patterning technique called Displacement Talbot lithography (DTL), to successfully nano-engineer III-nitride materials. DTL, along with its novel and unique combination with a lateral planar displacement (D2TL), allow the fabrication of a variety of periodic nanopatterns with a broad range of filling factors such as nanoholes, nanodots, nanorings and nanolines; all these features being achievable from one single mask. To illustrate the enormous possibilities opened by DTL/D2TL, dielectric and metal masks with a number of nanopatterns have been generated...
The growth mechanisms during metalorganic vapor phase epitaxy (11-22) oriented Al x Ga1-x N with ... more The growth mechanisms during metalorganic vapor phase epitaxy (11-22) oriented Al x Ga1-x N with x ∼ 0.80 on m-plane sapphire are studied with the intention of mitigating the expansion of misoriented grains, composed of the (1-10-3) crystal orientation and achieving a flat surface with only the (11-22) orientation. An increase in reactor pressure, metalorganic supply, and V/III ratio led to a decrease in the grain density from 1.0 × 109 cm−2 to 1.5 × 107 cm−2. By comparing different growth regimes, we found that the main factor suppressing the growth of the (1-10-3) orientation and decreasing the grain density in the AlGaN layers is the growth rate, which decreased with increasing reactor pressure, MO supply, and V/III ratio due to increasing pre-reactions in the gas phase. To mitigate pre-reactions even with lower growth rates, growth conditions with higher total flow and lower TMAl flow were chosen, yielding low growth rates of 0.13 μm h−1 and a grain density of 3.0 × 107 cm−2 at ...
The application of quaternary In x Al y Ga1−x− y N active regions is a promising path towards hig... more The application of quaternary In x Al y Ga1−x− y N active regions is a promising path towards high efficiency UVB-light emitting diodes (LEDs). For the utilization of In x Al y Ga1−x−y N, detailed knowledge of the interplay between growth parameters, adatom incorporation, optical and structural properties is crucial. We investigated the influence of the trimethylaluminium (TMAl) and trimethylindium (TMIn) flux on the composition and luminescence properties of In x Al y Ga1−x−y N layers by multi-mode scanning electron microscopy. We found that varying the molar TMIn flow from 0 to 17.3 μmol min−1 led to an InN concentration between 0% and 3.2% and an emission energy between 4.17 and 3.75 eV. The variation of the molar TMAl flow from 3.5 to 35.4 μmol min−1 resulted in a AlN composition between 7.8% and 30.7% with an emission energy variation between 3.6 and 4.1 eV. Cathodoluminescence hyperspectral imaging provided evidence for the formation of nanoscale InN-rich regions. Analyzing th...
An extensive analysis of the degradation characteristics of AlGaN-based ultraviolet light-emittin... more An extensive analysis of the degradation characteristics of AlGaN-based ultraviolet light-emitting diodes emitting around 265 nm is presented. The optical power of LEDs stressed at a constant dc current of 100 mA (current density = 67 A/cm2 and heatsink temperature = 20 °C) decreased to about 58% of its initial value after 250 h of operation. The origin of this degradation effect has been studied using capacitance-voltage and photocurrent spectroscopy measurements conducted before and after aging. The overall device capacitance decreased, which indicates a reduction of the net charges within the space-charge region of the pn-junction during operation. In parallel, the photocurrent at excitation energies between 3.8 eV and 4.5 eV and the photocurrent induced by band-to-band absorption in the quantum barriers at 5.25 eV increased during operation. The latter effect can be explained by a reduction of the donor concentration in the active region of the device. This effect could be attri...
The performance characteristics of AlGaN-based deep ultraviolet light emitting diodes (UV-LEDs) g... more The performance characteristics of AlGaN-based deep ultraviolet light emitting diodes (UV-LEDs) grown by metalorganic vapor phase epitaxy on sputtered and high temperature annealed AlN/sapphire templates are investigated and compared with LEDs grown on epitaxially laterally overgrown (ELO) AlN/sapphire. The structural and electro-optical properties of the devices on 350 nm sputtered and high temperature annealed AlN/sapphire show similar defect densities and output power levels as LEDs grown on low defect density ELO AlN/sapphire templates. After high temperature annealing of the 350 nm sputtered AlN, the full widths at half maximum of the (0002) and (101¯2) reflections of the high resolution x-ray diffraction rocking curves decrease by one order of magnitude to 65 arc sec and 240 arc sec, respectively. The curvature of the sputtered and HTA AlN/sapphire templates after regrowth with 400 nm MOVPE AlN is with −80 km−1 much lower than the curvature of the ELO AlN/sapphire template of ...
Uploads