Jingyu Lin - Academia.edu (original) (raw)

Papers by Jingyu Lin

Research paper thumbnail of Hyperspectral Nonlinear Optical Light Generation from a Monolithic GaN Microcavity

Advanced Optical Materials, 2017

Localized spatial excitation of a single hexagonal GaN micropyramid with (1 101) facets formed by... more Localized spatial excitation of a single hexagonal GaN micropyramid with (1 101) facets formed by selective area growth is optimized for nonlinear optical light (NLO) generation due to second harmonic generation (SHG) and multiphoton luminescence (MPL). Multiphoton transition induced ultraviolet and yellow luminescence is observed for excitations above and below half bandgap energy. SHG and MPL observed for excitations below half the bandgap energy are superimposed to realize broadband emission in the UV-visible range. The light generation is optimized by controlling the cavity modes formed by the hexagonal facets and the tip enhanced effects from the pyramid. The MPL is optimum at the apex of the pyramid. The SHG is most efficient within the pyramid (≈4 µm above the base) due to the formation of spatially stable cavity modes within the cavity. The NLO interactions within the pyramid are optimized to realize microphotonic white light sources and coherent tunable UV-visible sources using spatially controlled excitation without any change in material parameters. At the bandgap of GaN, the resonant two-photon emission dominates the nonlinear light generation process compared to the coherent SHG light generated within the cavity.

Research paper thumbnail of Annealing of dry etch damage in metallized and bare (-201) Ga2O3

Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena, 2017

The surface of single-crystal (-201) oriented β-Ga2O3 was etched in BCl3/Ar inductively coupled p... more The surface of single-crystal (-201) oriented β-Ga2O3 was etched in BCl3/Ar inductively coupled plasmas under conditions (an excitation frequency of 13.56 MHz, a source power of 400 W, and a dc self-bias of −450 V) that produce removal rates of ∼700 Å min−1. Annealing at 400 and 450 °C was carried out after etching on Ni/Au Schottky diodes formed on the surface either before or after the annealing step. Current–voltage (I–V) measurements were used to extract the Schottky barrier height (Φ), diode ideality factor (n), and reverse breakdown voltage (VRB) for plasma damaged diodes after annealing. Annealing at 450 °C was found to essentially restore the values of Φ, n, and VRB to their reference (unetched) values on samples metallized after etching and annealing. Thermal annealing at either temperature of metallized diodes degraded their reverse breakdown voltage, showing that Ni/Au is not stable on β-Ga2O3 at these temperatures. Photoluminescence revealed a decrease in total emission ...

Research paper thumbnail of Current injection 154 µm light-emitting devices based on Er-doped GaN/AlGaN multiple quantum wells

Optical Materials Express, 2016

We report on the growth, fabrication and electroluminescence (EL) characteristics of light-emitti... more We report on the growth, fabrication and electroluminescence (EL) characteristics of light-emitting diodes (LEDs) based on Er-doped GaN (GaN:Er) and GaN/AlGaN multiple quantum well (MQW:Er) active layers. The LED structures were grown using metal organic chemical vapor deposition and processed into 300x300 µm 2 mesa devices. The LEDs exhibit emission at 1.54 µm, due to Er intra-4f transitions, under forward bias conditions. The 1.54 µm emission properties from LEDs with MQWs:Er and GaN:Er active layers were probed. The LEDs fabricated using MQWs:Er exhibited improved performance as evidenced by a factor of 4 enhancement in the optical power output as compared to conventional GaN:Er based LEDs. The results demonstrate a significant advance in the development of current injected, chip-scale emitters and waveguide amplifiers based on Er doped semiconductors.

Research paper thumbnail of Growth and optoelectronic properties of III-nitride quaternary alloys

SPIE Proceedings, 2001

InA1GaiN quaternary alloys with different In and Al compositions were grown on sapphire substrate... more InA1GaiN quaternary alloys with different In and Al compositions were grown on sapphire substrates with GaN buffer by metal-organic chemical vapor deposition (MOCVD). Optical properties of these quaternary alloys were studied by picosecond time-resolved photoluminescence. Our studies have revealed that InAlGaiN quaternary alloys with lattice matched with GaN (y-4.7x) have the highest optical quality. More importantly, we can achieve not only higher emission energies but also higher emission intensities (or quantum efficiencies) in JnMGa1N quaternary alloys than that of GaN. The quantum efficiency of InALGaiN quaternary alloys was also enhanced significantly over A1GaN alloys with a comparable Al content. We have also fabricated ultraviolet (UV) photoconductive detectors based on InAlGai.N/GaN qUaternary alloy heterostructures. We found that with varying In and Al compositions, the cutoff wavelength of the InAlGai.N detectors could be varied to the deep Uv range tnd that the responsivity of the InAlGaiN quaternary alloys exceeded that of A1GaN alloys with comparable cutoff wavelengths by a factor of five. This showed that InAlGai.N quaternary alloys is a very important material for solar-blind UV detector applications particularly in the deep UV range where Al rich A1GaN alloys have problems with low quantum efficiency and cracks due in part to lattice mismatch with GaN. Our results strongly suggested that InAlGai.N quatemary alloys open a new avenue for the fabrication of many novel optoelectronic devices such as high efficient light emitters and detectors, particularly in the UV region.

Research paper thumbnail of Effects of growth pressure on erbium doped GaN infrared emitters synthesized by metal organic chemical vapor deposition

Optical Materials Express, 2012

Er doped GaN (GaN:Er) p-in structures were prepared by metal organic chemical vapor deposition. E... more Er doped GaN (GaN:Er) p-in structures were prepared by metal organic chemical vapor deposition. Effects of growth pressure on the optical performance of GaN:Er p-in structures have been investigated. Electroluminescence measurements revealed that the optimal growth pressure window for obtaining strong infrared emission intensity at 1.54 µm is around 20 torr, while the greater amount of Ga vacancies or non-raditive transitions were observed from the ones grown at lower or higher pressure. Our results point to possible applications in optical communications using current injected optical amplifiers based on GaN:Er p-in structures.

Research paper thumbnail of A Simplified Method of Making Flexible Blue LEDs on a Plastic Substrate

IEEE Photonics Journal, 2015

A much-simplified method of making flexible GaN blue light-emitting diode (LED) array on a plasti... more A much-simplified method of making flexible GaN blue light-emitting diode (LED) array on a plastic substrate was demonstrated. A sticky elastomeric stamp was first brought into contact with prefabricated GaN LED array on a sapphire substrate. Laser liftoff was applied by shining laser light through the sapphire substrate. The released LED array sitting on the stamp was transferred to a polyethylene terephthalate substrate that was coated with an adhesive layer to finish the fabrication process. Careful investigation of the built-in stress in the GaN LED layer using Raman spectroscopy revealed that the maximum stress that allows for intact GaN LED layer release and transfer was 0.7 GPa. The method drastically simplifies the cumbersome conventional GaN layer transferring method while preserving the original layout of the GaN LED array. Due to its simple and practical characteristics, the method is expected to greatly facilitate the development of versatile transferrable GaN LED applications on various substrates at a much-reduced cost.

Research paper thumbnail of Erbium-doped a-plane GaN epilayers synthesized by metal-organic chemical vapor deposition

Optical Materials Express, 2015

Research paper thumbnail of Optical properties of strain-free AlN nanowires grown by molecular beam epitaxy on Si substrates

Applied Physics Letters, 2014

The optical properties of catalyst-free AlN nanowires grown on Si substrates by molecular beam ep... more The optical properties of catalyst-free AlN nanowires grown on Si substrates by molecular beam epitaxy were investigated. Such nanowires are nearly free of strain, with strong free exciton emission measured at room temperature. The photoluminescence intensity is significantly enhanced, compared to previously reported AlN epilayer. Moreover, the presence of phonon replicas with an energy separation of ∼100 meV was identified to be associated with the surface-optical phonon rather than the commonly reported longitudinal-optical phonon, which is further supported by the micro-Raman scattering experiments.

Research paper thumbnail of Dynamics of localized excitons in InGaN/GaN quantum wells

Journal of Vacuum Science & Technology B, 1998

By using a ''nanoscale aperture'' method, we studied the dynamics of localized excitons in active... more By using a ''nanoscale aperture'' method, we studied the dynamics of localized excitons in active InGaN layers in an InGaN/GaN multiple quantum well ͑MQW͒. Local photoluminescence ͑PL͒ from local excitation shows very different characteristics from far-field luminescence. PL spectra from these nanoapertures are, in general, blueshifted relative to the spectra acquired in the unmasked regions. The spectra vary from aperture to aperture, reflecting the spatial fluctuation of the In composition in the MQW. In addition, one typically observes spectra with clearly resolved multiple peaks from the small apertures, which may be attributed to compositional and size fluctuations over a length scale longer than the exciton diameter but smaller than the aperture size. The excitation intensity dependence of PL indicates a band-filling effect in the multiple-peak structure. Finally, time-resolved studies reveal a very interesting spectral weight shifting between the higher-energy peaks and the lower-energy peaks.

Research paper thumbnail of Dry etching techniques for active devices based on hexagonal boron nitride epilayers

Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2013

Hexagonal boron nitride (hBN) has emerged as a fundamentally and technologically important materi... more Hexagonal boron nitride (hBN) has emerged as a fundamentally and technologically important material system owing to its unique physical properties including layered structure, wide energy bandgap, large optical absorption, and neutron capture cross section. As for any materials under development, it is necessary to establish device processing techniques to realize active devices based on hBN. The authors report on the advancements in dry etching techniques for active devices based on hBN epilayers via inductively coupled plasma (ICP). The effect of ICP radio frequency (RF) power on the etch rate and vertical side wall profile was studied. The etching depth and angle with respect to the surface were measured using atomic force microscopy showing that an etching rate ∼1.25 μm/min and etching angles >80° were obtained. Profilometer data and scanning electron microscope images confirmed these results. This work demonstrates that SF6 is very suitable for etching hBN epilayers in RF pl...

Research paper thumbnail of Dependence of Ni/AlGaN Schottky barrier height on Al mole fraction

Journal of Applied Physics, 2000

The dependence of the Schottky barrier height of Ni/AlxGa1−xN contact on the Al mole fraction up ... more The dependence of the Schottky barrier height of Ni/AlxGa1−xN contact on the Al mole fraction up to x=0.23 was studied. The barrier heights were measured by I–V, capacitance–voltage, and the internal photoemission method. The Al mole fractions were estimated from the AlGaN band gap energies measured by photoluminescence. In the range of x<0.2 a linear relationship between the barrier height and Al mole fraction was obtained. This was consistent with the slope predicted by the Schottky rule. For x=0.23, the measured barrier height was lower than predicted. We believed this was due to crystalline defects at the Ni/AlGaN interface.

Research paper thumbnail of III-nitride-based planar lightwave circuits for long wavelength optical communications

IEEE Journal of Quantum Electronics, 2005

Planar lightwave circuits based on III-nitride widebandgap semiconductors are proposed and the fe... more Planar lightwave circuits based on III-nitride widebandgap semiconductors are proposed and the feasibility of developing III-nitride-based novel photonic integrated circuits for applications in fiber-optical communications is discussed. III-nitrides have low attenuation in the near-infrared wavelength region because of their wide bandgaps, while as semiconductors their refractive indexes can be modulated by carrier injection. III-nitrides are also well known for their ability to operate at high temperatures, high power levels and in harsh environments. These characteristics make III-nitrides ideal candidates for tunable optical phased-array (PHASAR) devices for optical communications.We have characterized the optical properties of Al Ga 1 N epilayers in the 1550-nm wavelength region, including the refractive indexes and the impact of Al concentrations. Single-mode ridged optical waveguide devices using GaN-AlGaN heterostructures have been designed, fabricated and characterized for operation in the 1550-nm wavelength window. The birefringence of wurtzite GaN grown on sapphire substrate has been observed. Refractive indexes were found to be different for signal optical field perpendicular and parallel to the crystal axis (=). More importantly, we found an approximately 10% change in the index difference 1 = with varying the waveguide orientation within the plane, and a 60 periodicity was clearly observed. This is attributed to the hexagonal structure of the nitride materials.Various functional waveguide devices have been realized, including 2 2 directional couplers and eight-wavelength array-waveguide gratings. Theoretical predictions of temperature sensitivity and the efficiency of carrier-induced refractive change are provided.

Research paper thumbnail of Enhancing erbium emission by strain engineering in GaN heteroepitaxial layers

Applied Physics Letters, 2010

Much research has been devoted to the incorporation of erbium (Er) into semiconductors aimed at a... more Much research has been devoted to the incorporation of erbium (Er) into semiconductors aimed at achieving photonic integrated circuits with multiple functionalities. GaN appears to be an excellent host material for Er ions due to its structural and thermal stability. Er-doped GaN (GaN:Er) epilayers were grown on different templates, GaN/Al2O3, AlN/Al2O3, GaN/Si (111), and c-GaN bulk. The effects of stress on 1.54 μm emission intensity, caused by lattice mismatch between the GaN:Er epilayer and the substrate, were probed. The emission intensity at 1.54 μm increased with greater tensile stress in the c-direction of the GaN:Er epilayers. These results indicate that the characteristics of photonic devices based on GaN:Er can be optimized through strain engineering.

Research paper thumbnail of The origin of 2.78 eV emission and yellow coloration in bulk AlN substrates

Applied Physics Letters, 2009

The yellow color of bulk AlN crystals was found to be caused by the optical absorption of light w... more The yellow color of bulk AlN crystals was found to be caused by the optical absorption of light with wavelengths shorter than that of yellow. This yellow impurity limits UV transparency and hence restricts the applications of AlN substrates for deep UV optoelectronic devices. Here, the optical properties of AlN epilayers, polycrystalline AlN, and bulk AlN single crystals have been investigated using photoluminescence (PL) spectroscopy to address the origin of this yellow appearance. An emission band with a linewidth of ∼0.3 eV (at 10 K) was observed at ∼2.78 eV. We propose that the origin of the yellow color in bulk AlN is due to a band-to-impurity absorption involving the excitation of electrons from the valence band to the doubly negative charged state, (VAl2−), of isolated aluminum vacancies, (VAl)3−/2− described by VAl2−+hν=VAl3−+h+. In such a context, the reverse process is responsible for the 2.78 eV PL emission.

Research paper thumbnail of Mg acceptor level in AlN probed by deep ultraviolet photoluminescence

Applied Physics Letters, 2003

Mg-doped AlN epilayers were grown by metalorganic chemical vapor deposition on sapphire substrate... more Mg-doped AlN epilayers were grown by metalorganic chemical vapor deposition on sapphire substrates. Deep UV picosecond time-resolved photoluminescence (PL) spectroscopy has been employed to study the optical transitions in Mg-doped AlN epilayers. From PL emission spectra and the temperature dependence of the PL emission intensity, a binding energy of 0.51 eV for Mg acceptor in AlN was determined. Together with previous experimental results, the Mg acceptor activation energy in AlxGa1−xN as a function of the Al content (x) was extrapolated for the entire AlN composition range. The average hole effective mass in AlN was also deduced to be about 2.7 m0 from the experimental value of the Mg binding energy together with the use of the effective mass theory.

Research paper thumbnail of Electrical and optical properties of Mg-doped Al0.7Ga0.3N alloys

Applied Physics Letters, 2005

Mg-doped Al0.7Ga0.3N epilayers (∼1μm) were grown on an AlN/sapphire template by metalorganic chem... more Mg-doped Al0.7Ga0.3N epilayers (∼1μm) were grown on an AlN/sapphire template by metalorganic chemical vapor deposition and the electrical and optical properties of these epilayers were studied. For optimized Mg-doped Al0.7Ga0.3N epilayers, we have obtained a resistivity around 105Ωcm at room temperature and confirmed p-type conduction at elevated temperatures(>700K) with a resistivity of about 40Ωcm at 800 K. From the temperature dependent Hall effect measurement, the activation energy of Mg acceptor is found to be around 400 meV for Al0.7Ga0.3N alloy. The optimized Mg-doped Al0.7Ga0.3N epilayers have been incorporated into the deep-ultraviolet (UV) (λ<300nm) light-emitting diode (LED) structures as an electron blocking layer. An enhancement in the performance of the UV LEDs was obtained. LEDs with peak emission wavelengths at 280 nm were fabricated with a circular geometry (300 μm disk diameter). Output power reached 0.35 mW at 20 mA and 1.1 mW at 150 mA dc current. The impor...

Research paper thumbnail of Enhanced light extraction in III-nitride ultraviolet photonic crystal light-emitting diodes

Applied Physics Letters, 2004

III-nitride photonic crystal (PC) ultraviolet (UV) light-emitting diodes (LEDs) were fabricated. ... more III-nitride photonic crystal (PC) ultraviolet (UV) light-emitting diodes (LEDs) were fabricated. Triangular arrays of the PCs with different diameters∕periodicities were patterned using electron-beam lithography and inductively coupled plasma dry etching. The optical power output of LEDs was enhanced by a factor of 2.5 due to PC formation. It was observed that the optical enhancement factor depends strongly on the lattice constant and hole size of the PCs. The achievement of nitride PCs is expected to benefit many applications of III-nitride optoelectronics, particularly for the improvement of extraction efficiency in III-nitride deep-UV emitters (λ<340nm), which are crucial for many important applications, but presently have a very low quantum efficiency.

Research paper thumbnail of Neutral-donor-bound exciton recombination dynamics in GaN grown by metalorganic chemical vapor deposition

Applied Physics Letters, 1995

Neutral-donor-bound exciton recombination (I2) dynamics have been studied by photoluminescence in... more Neutral-donor-bound exciton recombination (I2) dynamics have been studied by photoluminescence in an unintentionally doped n-type GaN epitaxial layer grown by metalorganic chemical vapor deposition. The luminescence emission line shape, peak position, and intensity as functions of temperature have been measured. In particular, time-resolved emission spectroscopy has been employed to study the dynamic processes of the bound exciton recombination, from which the temperature and the emission energy dependencies of the recombination lifetime of this transition have been obtained.

Research paper thumbnail of Structural phase behavior in II–VI semiconductor nanoparticles

Applied Physics Letters, 1995

Cubic structured CdS, CdSe, and CdTe, II–VI semiconductor nanoparticles have been synthesized usi... more Cubic structured CdS, CdSe, and CdTe, II–VI semiconductor nanoparticles have been synthesized using aqueous solution precipitation at room temperature. The ‘‘as-prepared’’ particles have a size of about 30 Å. Thermal annealing causes (a) an increase in particle size; (b) a structural transition from the cubic to the bulk, hexagonal structure for CdS and CdSe; and (c) no structural transition for CdTe. The unexpected cubic phase for small particles of CdS and CdSe may be due to either metastability or an equilibrium surface effect. The latter would imply a strong correlation between structure and the size and surface properties, and that there is a minimum size for hexagonal phase stability.

Research paper thumbnail of Acceptor-bound exciton recombination dynamics in p-type GaN

Applied Physics Letters, 1995

Dynamics of the neutral-acceptor-bound exciton transition (the I1 line) in a Mg doped p-type GaN ... more Dynamics of the neutral-acceptor-bound exciton transition (the I1 line) in a Mg doped p-type GaN epitaxial layer grown by metalorganic chemical vapor deposition (MOCVD) have been studied by time-resolved photoluminescence emission spectroscopy. Two emission lines in the I1 transition region have been resolved in the time-resolved spectra, possibly due to the existence of two energy states of the Mg impurities after postgrowth thermal annealing. The recombination lifetimes of the acceptor-bound exciton transition have been measured under different conditions including temperature, excitation intensity, and emission energy. From these measurements, a value of about 450 ps for the radiative recombination lifetime has been obtained, which is an important physical quantity for optoelectronic device applications based on GaN.

Research paper thumbnail of Hyperspectral Nonlinear Optical Light Generation from a Monolithic GaN Microcavity

Advanced Optical Materials, 2017

Localized spatial excitation of a single hexagonal GaN micropyramid with (1 101) facets formed by... more Localized spatial excitation of a single hexagonal GaN micropyramid with (1 101) facets formed by selective area growth is optimized for nonlinear optical light (NLO) generation due to second harmonic generation (SHG) and multiphoton luminescence (MPL). Multiphoton transition induced ultraviolet and yellow luminescence is observed for excitations above and below half bandgap energy. SHG and MPL observed for excitations below half the bandgap energy are superimposed to realize broadband emission in the UV-visible range. The light generation is optimized by controlling the cavity modes formed by the hexagonal facets and the tip enhanced effects from the pyramid. The MPL is optimum at the apex of the pyramid. The SHG is most efficient within the pyramid (≈4 µm above the base) due to the formation of spatially stable cavity modes within the cavity. The NLO interactions within the pyramid are optimized to realize microphotonic white light sources and coherent tunable UV-visible sources using spatially controlled excitation without any change in material parameters. At the bandgap of GaN, the resonant two-photon emission dominates the nonlinear light generation process compared to the coherent SHG light generated within the cavity.

Research paper thumbnail of Annealing of dry etch damage in metallized and bare (-201) Ga2O3

Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena, 2017

The surface of single-crystal (-201) oriented β-Ga2O3 was etched in BCl3/Ar inductively coupled p... more The surface of single-crystal (-201) oriented β-Ga2O3 was etched in BCl3/Ar inductively coupled plasmas under conditions (an excitation frequency of 13.56 MHz, a source power of 400 W, and a dc self-bias of −450 V) that produce removal rates of ∼700 Å min−1. Annealing at 400 and 450 °C was carried out after etching on Ni/Au Schottky diodes formed on the surface either before or after the annealing step. Current–voltage (I–V) measurements were used to extract the Schottky barrier height (Φ), diode ideality factor (n), and reverse breakdown voltage (VRB) for plasma damaged diodes after annealing. Annealing at 450 °C was found to essentially restore the values of Φ, n, and VRB to their reference (unetched) values on samples metallized after etching and annealing. Thermal annealing at either temperature of metallized diodes degraded their reverse breakdown voltage, showing that Ni/Au is not stable on β-Ga2O3 at these temperatures. Photoluminescence revealed a decrease in total emission ...

Research paper thumbnail of Current injection 154 µm light-emitting devices based on Er-doped GaN/AlGaN multiple quantum wells

Optical Materials Express, 2016

We report on the growth, fabrication and electroluminescence (EL) characteristics of light-emitti... more We report on the growth, fabrication and electroluminescence (EL) characteristics of light-emitting diodes (LEDs) based on Er-doped GaN (GaN:Er) and GaN/AlGaN multiple quantum well (MQW:Er) active layers. The LED structures were grown using metal organic chemical vapor deposition and processed into 300x300 µm 2 mesa devices. The LEDs exhibit emission at 1.54 µm, due to Er intra-4f transitions, under forward bias conditions. The 1.54 µm emission properties from LEDs with MQWs:Er and GaN:Er active layers were probed. The LEDs fabricated using MQWs:Er exhibited improved performance as evidenced by a factor of 4 enhancement in the optical power output as compared to conventional GaN:Er based LEDs. The results demonstrate a significant advance in the development of current injected, chip-scale emitters and waveguide amplifiers based on Er doped semiconductors.

Research paper thumbnail of Growth and optoelectronic properties of III-nitride quaternary alloys

SPIE Proceedings, 2001

InA1GaiN quaternary alloys with different In and Al compositions were grown on sapphire substrate... more InA1GaiN quaternary alloys with different In and Al compositions were grown on sapphire substrates with GaN buffer by metal-organic chemical vapor deposition (MOCVD). Optical properties of these quaternary alloys were studied by picosecond time-resolved photoluminescence. Our studies have revealed that InAlGaiN quaternary alloys with lattice matched with GaN (y-4.7x) have the highest optical quality. More importantly, we can achieve not only higher emission energies but also higher emission intensities (or quantum efficiencies) in JnMGa1N quaternary alloys than that of GaN. The quantum efficiency of InALGaiN quaternary alloys was also enhanced significantly over A1GaN alloys with a comparable Al content. We have also fabricated ultraviolet (UV) photoconductive detectors based on InAlGai.N/GaN qUaternary alloy heterostructures. We found that with varying In and Al compositions, the cutoff wavelength of the InAlGai.N detectors could be varied to the deep Uv range tnd that the responsivity of the InAlGaiN quaternary alloys exceeded that of A1GaN alloys with comparable cutoff wavelengths by a factor of five. This showed that InAlGai.N quaternary alloys is a very important material for solar-blind UV detector applications particularly in the deep UV range where Al rich A1GaN alloys have problems with low quantum efficiency and cracks due in part to lattice mismatch with GaN. Our results strongly suggested that InAlGai.N quatemary alloys open a new avenue for the fabrication of many novel optoelectronic devices such as high efficient light emitters and detectors, particularly in the UV region.

Research paper thumbnail of Effects of growth pressure on erbium doped GaN infrared emitters synthesized by metal organic chemical vapor deposition

Optical Materials Express, 2012

Er doped GaN (GaN:Er) p-in structures were prepared by metal organic chemical vapor deposition. E... more Er doped GaN (GaN:Er) p-in structures were prepared by metal organic chemical vapor deposition. Effects of growth pressure on the optical performance of GaN:Er p-in structures have been investigated. Electroluminescence measurements revealed that the optimal growth pressure window for obtaining strong infrared emission intensity at 1.54 µm is around 20 torr, while the greater amount of Ga vacancies or non-raditive transitions were observed from the ones grown at lower or higher pressure. Our results point to possible applications in optical communications using current injected optical amplifiers based on GaN:Er p-in structures.

Research paper thumbnail of A Simplified Method of Making Flexible Blue LEDs on a Plastic Substrate

IEEE Photonics Journal, 2015

A much-simplified method of making flexible GaN blue light-emitting diode (LED) array on a plasti... more A much-simplified method of making flexible GaN blue light-emitting diode (LED) array on a plastic substrate was demonstrated. A sticky elastomeric stamp was first brought into contact with prefabricated GaN LED array on a sapphire substrate. Laser liftoff was applied by shining laser light through the sapphire substrate. The released LED array sitting on the stamp was transferred to a polyethylene terephthalate substrate that was coated with an adhesive layer to finish the fabrication process. Careful investigation of the built-in stress in the GaN LED layer using Raman spectroscopy revealed that the maximum stress that allows for intact GaN LED layer release and transfer was 0.7 GPa. The method drastically simplifies the cumbersome conventional GaN layer transferring method while preserving the original layout of the GaN LED array. Due to its simple and practical characteristics, the method is expected to greatly facilitate the development of versatile transferrable GaN LED applications on various substrates at a much-reduced cost.

Research paper thumbnail of Erbium-doped a-plane GaN epilayers synthesized by metal-organic chemical vapor deposition

Optical Materials Express, 2015

Research paper thumbnail of Optical properties of strain-free AlN nanowires grown by molecular beam epitaxy on Si substrates

Applied Physics Letters, 2014

The optical properties of catalyst-free AlN nanowires grown on Si substrates by molecular beam ep... more The optical properties of catalyst-free AlN nanowires grown on Si substrates by molecular beam epitaxy were investigated. Such nanowires are nearly free of strain, with strong free exciton emission measured at room temperature. The photoluminescence intensity is significantly enhanced, compared to previously reported AlN epilayer. Moreover, the presence of phonon replicas with an energy separation of ∼100 meV was identified to be associated with the surface-optical phonon rather than the commonly reported longitudinal-optical phonon, which is further supported by the micro-Raman scattering experiments.

Research paper thumbnail of Dynamics of localized excitons in InGaN/GaN quantum wells

Journal of Vacuum Science & Technology B, 1998

By using a ''nanoscale aperture'' method, we studied the dynamics of localized excitons in active... more By using a ''nanoscale aperture'' method, we studied the dynamics of localized excitons in active InGaN layers in an InGaN/GaN multiple quantum well ͑MQW͒. Local photoluminescence ͑PL͒ from local excitation shows very different characteristics from far-field luminescence. PL spectra from these nanoapertures are, in general, blueshifted relative to the spectra acquired in the unmasked regions. The spectra vary from aperture to aperture, reflecting the spatial fluctuation of the In composition in the MQW. In addition, one typically observes spectra with clearly resolved multiple peaks from the small apertures, which may be attributed to compositional and size fluctuations over a length scale longer than the exciton diameter but smaller than the aperture size. The excitation intensity dependence of PL indicates a band-filling effect in the multiple-peak structure. Finally, time-resolved studies reveal a very interesting spectral weight shifting between the higher-energy peaks and the lower-energy peaks.

Research paper thumbnail of Dry etching techniques for active devices based on hexagonal boron nitride epilayers

Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2013

Hexagonal boron nitride (hBN) has emerged as a fundamentally and technologically important materi... more Hexagonal boron nitride (hBN) has emerged as a fundamentally and technologically important material system owing to its unique physical properties including layered structure, wide energy bandgap, large optical absorption, and neutron capture cross section. As for any materials under development, it is necessary to establish device processing techniques to realize active devices based on hBN. The authors report on the advancements in dry etching techniques for active devices based on hBN epilayers via inductively coupled plasma (ICP). The effect of ICP radio frequency (RF) power on the etch rate and vertical side wall profile was studied. The etching depth and angle with respect to the surface were measured using atomic force microscopy showing that an etching rate ∼1.25 μm/min and etching angles >80° were obtained. Profilometer data and scanning electron microscope images confirmed these results. This work demonstrates that SF6 is very suitable for etching hBN epilayers in RF pl...

Research paper thumbnail of Dependence of Ni/AlGaN Schottky barrier height on Al mole fraction

Journal of Applied Physics, 2000

The dependence of the Schottky barrier height of Ni/AlxGa1−xN contact on the Al mole fraction up ... more The dependence of the Schottky barrier height of Ni/AlxGa1−xN contact on the Al mole fraction up to x=0.23 was studied. The barrier heights were measured by I–V, capacitance–voltage, and the internal photoemission method. The Al mole fractions were estimated from the AlGaN band gap energies measured by photoluminescence. In the range of x<0.2 a linear relationship between the barrier height and Al mole fraction was obtained. This was consistent with the slope predicted by the Schottky rule. For x=0.23, the measured barrier height was lower than predicted. We believed this was due to crystalline defects at the Ni/AlGaN interface.

Research paper thumbnail of III-nitride-based planar lightwave circuits for long wavelength optical communications

IEEE Journal of Quantum Electronics, 2005

Planar lightwave circuits based on III-nitride widebandgap semiconductors are proposed and the fe... more Planar lightwave circuits based on III-nitride widebandgap semiconductors are proposed and the feasibility of developing III-nitride-based novel photonic integrated circuits for applications in fiber-optical communications is discussed. III-nitrides have low attenuation in the near-infrared wavelength region because of their wide bandgaps, while as semiconductors their refractive indexes can be modulated by carrier injection. III-nitrides are also well known for their ability to operate at high temperatures, high power levels and in harsh environments. These characteristics make III-nitrides ideal candidates for tunable optical phased-array (PHASAR) devices for optical communications.We have characterized the optical properties of Al Ga 1 N epilayers in the 1550-nm wavelength region, including the refractive indexes and the impact of Al concentrations. Single-mode ridged optical waveguide devices using GaN-AlGaN heterostructures have been designed, fabricated and characterized for operation in the 1550-nm wavelength window. The birefringence of wurtzite GaN grown on sapphire substrate has been observed. Refractive indexes were found to be different for signal optical field perpendicular and parallel to the crystal axis (=). More importantly, we found an approximately 10% change in the index difference 1 = with varying the waveguide orientation within the plane, and a 60 periodicity was clearly observed. This is attributed to the hexagonal structure of the nitride materials.Various functional waveguide devices have been realized, including 2 2 directional couplers and eight-wavelength array-waveguide gratings. Theoretical predictions of temperature sensitivity and the efficiency of carrier-induced refractive change are provided.

Research paper thumbnail of Enhancing erbium emission by strain engineering in GaN heteroepitaxial layers

Applied Physics Letters, 2010

Much research has been devoted to the incorporation of erbium (Er) into semiconductors aimed at a... more Much research has been devoted to the incorporation of erbium (Er) into semiconductors aimed at achieving photonic integrated circuits with multiple functionalities. GaN appears to be an excellent host material for Er ions due to its structural and thermal stability. Er-doped GaN (GaN:Er) epilayers were grown on different templates, GaN/Al2O3, AlN/Al2O3, GaN/Si (111), and c-GaN bulk. The effects of stress on 1.54 μm emission intensity, caused by lattice mismatch between the GaN:Er epilayer and the substrate, were probed. The emission intensity at 1.54 μm increased with greater tensile stress in the c-direction of the GaN:Er epilayers. These results indicate that the characteristics of photonic devices based on GaN:Er can be optimized through strain engineering.

Research paper thumbnail of The origin of 2.78 eV emission and yellow coloration in bulk AlN substrates

Applied Physics Letters, 2009

The yellow color of bulk AlN crystals was found to be caused by the optical absorption of light w... more The yellow color of bulk AlN crystals was found to be caused by the optical absorption of light with wavelengths shorter than that of yellow. This yellow impurity limits UV transparency and hence restricts the applications of AlN substrates for deep UV optoelectronic devices. Here, the optical properties of AlN epilayers, polycrystalline AlN, and bulk AlN single crystals have been investigated using photoluminescence (PL) spectroscopy to address the origin of this yellow appearance. An emission band with a linewidth of ∼0.3 eV (at 10 K) was observed at ∼2.78 eV. We propose that the origin of the yellow color in bulk AlN is due to a band-to-impurity absorption involving the excitation of electrons from the valence band to the doubly negative charged state, (VAl2−), of isolated aluminum vacancies, (VAl)3−/2− described by VAl2−+hν=VAl3−+h+. In such a context, the reverse process is responsible for the 2.78 eV PL emission.

Research paper thumbnail of Mg acceptor level in AlN probed by deep ultraviolet photoluminescence

Applied Physics Letters, 2003

Mg-doped AlN epilayers were grown by metalorganic chemical vapor deposition on sapphire substrate... more Mg-doped AlN epilayers were grown by metalorganic chemical vapor deposition on sapphire substrates. Deep UV picosecond time-resolved photoluminescence (PL) spectroscopy has been employed to study the optical transitions in Mg-doped AlN epilayers. From PL emission spectra and the temperature dependence of the PL emission intensity, a binding energy of 0.51 eV for Mg acceptor in AlN was determined. Together with previous experimental results, the Mg acceptor activation energy in AlxGa1−xN as a function of the Al content (x) was extrapolated for the entire AlN composition range. The average hole effective mass in AlN was also deduced to be about 2.7 m0 from the experimental value of the Mg binding energy together with the use of the effective mass theory.

Research paper thumbnail of Electrical and optical properties of Mg-doped Al0.7Ga0.3N alloys

Applied Physics Letters, 2005

Mg-doped Al0.7Ga0.3N epilayers (∼1μm) were grown on an AlN/sapphire template by metalorganic chem... more Mg-doped Al0.7Ga0.3N epilayers (∼1μm) were grown on an AlN/sapphire template by metalorganic chemical vapor deposition and the electrical and optical properties of these epilayers were studied. For optimized Mg-doped Al0.7Ga0.3N epilayers, we have obtained a resistivity around 105Ωcm at room temperature and confirmed p-type conduction at elevated temperatures(>700K) with a resistivity of about 40Ωcm at 800 K. From the temperature dependent Hall effect measurement, the activation energy of Mg acceptor is found to be around 400 meV for Al0.7Ga0.3N alloy. The optimized Mg-doped Al0.7Ga0.3N epilayers have been incorporated into the deep-ultraviolet (UV) (λ<300nm) light-emitting diode (LED) structures as an electron blocking layer. An enhancement in the performance of the UV LEDs was obtained. LEDs with peak emission wavelengths at 280 nm were fabricated with a circular geometry (300 μm disk diameter). Output power reached 0.35 mW at 20 mA and 1.1 mW at 150 mA dc current. The impor...

Research paper thumbnail of Enhanced light extraction in III-nitride ultraviolet photonic crystal light-emitting diodes

Applied Physics Letters, 2004

III-nitride photonic crystal (PC) ultraviolet (UV) light-emitting diodes (LEDs) were fabricated. ... more III-nitride photonic crystal (PC) ultraviolet (UV) light-emitting diodes (LEDs) were fabricated. Triangular arrays of the PCs with different diameters∕periodicities were patterned using electron-beam lithography and inductively coupled plasma dry etching. The optical power output of LEDs was enhanced by a factor of 2.5 due to PC formation. It was observed that the optical enhancement factor depends strongly on the lattice constant and hole size of the PCs. The achievement of nitride PCs is expected to benefit many applications of III-nitride optoelectronics, particularly for the improvement of extraction efficiency in III-nitride deep-UV emitters (λ<340nm), which are crucial for many important applications, but presently have a very low quantum efficiency.

Research paper thumbnail of Neutral-donor-bound exciton recombination dynamics in GaN grown by metalorganic chemical vapor deposition

Applied Physics Letters, 1995

Neutral-donor-bound exciton recombination (I2) dynamics have been studied by photoluminescence in... more Neutral-donor-bound exciton recombination (I2) dynamics have been studied by photoluminescence in an unintentionally doped n-type GaN epitaxial layer grown by metalorganic chemical vapor deposition. The luminescence emission line shape, peak position, and intensity as functions of temperature have been measured. In particular, time-resolved emission spectroscopy has been employed to study the dynamic processes of the bound exciton recombination, from which the temperature and the emission energy dependencies of the recombination lifetime of this transition have been obtained.

Research paper thumbnail of Structural phase behavior in II–VI semiconductor nanoparticles

Applied Physics Letters, 1995

Cubic structured CdS, CdSe, and CdTe, II–VI semiconductor nanoparticles have been synthesized usi... more Cubic structured CdS, CdSe, and CdTe, II–VI semiconductor nanoparticles have been synthesized using aqueous solution precipitation at room temperature. The ‘‘as-prepared’’ particles have a size of about 30 Å. Thermal annealing causes (a) an increase in particle size; (b) a structural transition from the cubic to the bulk, hexagonal structure for CdS and CdSe; and (c) no structural transition for CdTe. The unexpected cubic phase for small particles of CdS and CdSe may be due to either metastability or an equilibrium surface effect. The latter would imply a strong correlation between structure and the size and surface properties, and that there is a minimum size for hexagonal phase stability.

Research paper thumbnail of Acceptor-bound exciton recombination dynamics in p-type GaN

Applied Physics Letters, 1995

Dynamics of the neutral-acceptor-bound exciton transition (the I1 line) in a Mg doped p-type GaN ... more Dynamics of the neutral-acceptor-bound exciton transition (the I1 line) in a Mg doped p-type GaN epitaxial layer grown by metalorganic chemical vapor deposition (MOCVD) have been studied by time-resolved photoluminescence emission spectroscopy. Two emission lines in the I1 transition region have been resolved in the time-resolved spectra, possibly due to the existence of two energy states of the Mg impurities after postgrowth thermal annealing. The recombination lifetimes of the acceptor-bound exciton transition have been measured under different conditions including temperature, excitation intensity, and emission energy. From these measurements, a value of about 450 ps for the radiative recombination lifetime has been obtained, which is an important physical quantity for optoelectronic device applications based on GaN.