Crack-free GaN-based light-emitting diodes (LEDs) were grown on 150-mm-diameter Si substrates by using low-pressure metal-organic chemical vapor deposition. The relationship between the LED devices and the thickness of quantum barriers (QBs) was investigated. The crystal quality and surface cracking of GaN-on-Si were greatly improved by an AlxGa1−xN buffer layer composed of graded Al. The threading dislocation density of the GaN-on-Si LEDs was reduced to <7×108 cm−2, yielding LEDs with high internal quantum efficiency. Simulation results indicated that reducing the QB thickness improved the carrier injection rate and distribution, thereby improving the droop behavior of the LEDs. LEDs featuring 6-nm-thick QBs exhibited the lowest droop behavior. However, the experimental results showed an unanticipated phenomenon, namely that the peak external quantum efficiency (EQE) and light output power (LOP) gradually decreased with a decreasing QB thickness. In other words, the GaN-on-Si LEDs with 8-nm-thick QBs exhibited low droop behavior and yielded a good peak EQE and LOP, achieving a 22.9% efficiency droop and 54.6% EQE.
We investigated the influence of free-standing GaN (FS-GaN) substrates on the performance of ultraviolet light-emitting-diodes (UV-LEDs) grown on top by atmospheric pressure metal-organic chemical vapor deposition. High-resolution double-crystal x-ray diffraction (HRDCXD) analysis demonstrated high-order satellite peaks and clear fringes between them for UV-LEDs grown on the FS-GaN substrate, from which the interface roughness was estimated. In addition, the full width at half maximum of the HRDCXD rocking curve in the (0002) and the (101¯2) reflections were reduced to below 90 arc sec. The Raman results indicated that the GaN-based epilayer of strain free was grown. Additionally, the effect of the FS-GaN substrate on the crystal quality of the UV-LEDs was examined in detail by transmission electron microscopy (TEM). The TEM characterizations revealed no defects and V-pits were found in the scanned area. Based on the results mentioned above, the light output power of UV-LEDs on the FS-GaN substrate can be enhanced drastically by 80% and 90% at 20 and 100 mA, respectively. Furthermore, an ultralow efficiency degradation of about 3% can be obtained for the UV-LEDs on the FS-GaN substrate at a high injection current. The use of an FS-GaN substrate is suggested to be effective for improving the emission efficiency and droop of UV-LEDs grown thereon.
We reported the defects and optical characterizations of the ultraviolet light-emitting diodes grown on free-standing GaN
substrate (FS-GaN) and sapphire. Cross-sectional transmission electron microscopy (TEM) images showed that the total
defect densities of grown UV LEDs on FS-GaN and sapphire including edge, screw and mixed type were 3.6×106 cm-2 and 5.5×108 cm-2. When substrate of UV LEDs was changed from sapphire to FS-GaN, it can be clearly found that the crystallography of GaN epilayers was drastically different from that GaN epilayers on sapphire. Besides, the microstructures or indium clustering can be not observed at UV LEDs on FS-GaN from TEM measurement. The internal quantum efficiency of UVLEDs on FS-GaN and sapphire were 34.8 % and 39.4 % respectively, which attributed to
indium clustering in multi-layers quantum wells (MQWs) of UV LEDs on sapphire. The relationship between indiumclustering
and efficiency droop were investigated by temperature-dependent electroluminescence (TDEL) measurements.
We reported the influence of free-standing (FS) GaN substrate on ultraviolet light-emitting-diodes (UV LEDs) by atmospheric-pressure metal-organic chemical vapor deposition (APMOCVD). The Raman spectrum shows the in-plane compressive stress of the GaN epitaxial structures grown on FS GaN substrate. Besides, the Raman spectrum reveals the relation between the crystal quality and the carrier localization degree in multi-quantum wells (MQWs). High resolution X-ray diffraction (HRXRD) analysis results show that the In0.025Ga0.975N/Al0.08Ga0.92N MQWs grown on FS GaN substrate has higher indium mole fraction than sapphire at the same growth conditions. The higher indium incorporation is corresponding with the red-shift 6 nm (387 nm) of the room temperature photoluminescence (PL) peak. The full widths at half maximum (FWHM) of omega-scan rocking curve in (002) and (102) reflectance on FS GaN substrate (83 arcsec and 77 arcsec) are narrower than UV LEDs grown on sapphire (288 arcsec and 446 arcsec). This superior quality may attribute to homoepitaxial growth structure and better strain relaxation in the FS GaN substrate. An anomalous temperature behavior of PL in UV LEDs designated as an S-shaped peak position dependence and W-shaped linewidth dependence indicate that exciton/carrier motion occurs via photon-assisted tunneling through localized states, what results in incomplete thermalization of localized excitons at low temperature. The Gaussian broadening parameters of carrier localization is about 16.98 meV from the temperature dependent photoluminescence (TDPL) measurement. The saturation temperature from the TDPL linewidth of UV LEDs on FS GaN substrate at about 175 K represents a crossover from a nonthermalized to thermalized energy distribution of excitons.
The present study investigated the structural and optical characterizations of the growth of GaN-based green lightemitting diodes using a TiN buffer layer. The purpose of growing GaN-based green LEDs on the TiN interlayer was to produce the naturally occurring hexagonal pattern structure on the surface of undoped-GaN. Then dislocations of the grown InGaN/GaN MQWs green LEDs structure on the uGaN template with the TiN interlayer produced base plane staking faults through epitaxial lateral overgrowth. Cross-section transmission electron microscope images showed that the dislocation density of green LEDs was decreased from 5 × 108 cm-2 to 7 × 107 cm-2, and that the dislocations in the green LEDs structure were reproduced. The full widths at half maximum of the omega-scan rocking curves in (002) and (102) reflectance on the GaN-based green LEDs were 334 and 488 arcsec, respectively. As the injection current was increased from 5 mA to 40 mA, the electroluminescence peak wavelength of the GaN-based green LEDs was shifted from 508 nm to 481 nm, a blue-shift of 27 nm. The forward voltage measured at an injection current of 20 mA was 4.9 V for the GaN-based green LEDs according to the current-voltage characteristics. Due to an increase in the In mole fraction of the GaN-based green LEDs on the uGaN template with the TiN interlayer, the strain and phase separation were increased, and the multiple quantum wells structural quality and device performances of the GaN-based green LEDs were decayed. A yellow band with a wavelength of 551nm was thereby produced according to room temperature photoluminescence measurement. Meanwhile, cross-section transmission electron microscope images indicated V-defects in multiple quantum wells structures of the green LEDs.
Triple-junction solar cells offer extremely high power conversion efficiency with minimal semiconductor material usage, and hence are promising for large-scale electricity generation. To fully exploit the broad absorption range, antireflective schemes based on biomimetic nanostructures become very appealing due to sub-wavelength scale features that can collectively function as a graded refractive index (GRIN) medium to photons. The structures are generally fabricated with a single-type dielectric material which guarantees both optical design robustness and mechanical durability under concentrated illumination. However, surface recombination and current matching issues arising from patterning still challenge the realization of biomimetic nanostructures on a few micrometer thick epitaxial layers for MJSCs. In this presentation, bio-inspired antireflective structures based on silicon nitride (SiNx) and titanium dioxide (TiO2) materials are demonstrated on monolithically grown Ga0.5In0.5P/In0.01Ga0.99As/Ge triple-junction solar cells. The nano-fabrication employs scalable polystyrene nanosphere lithography, followed by inductively-coupled-plasma reactive-ion-etching (ICP-RIE). We show that the fabricated devices exhibit omni-directional enhancement of photocurrent and power conversion efficiency, offering a viable solution to concentrated illumination with large angles of incidence. Moreover, a comprehensive design scheme is also presented to tailor the reflectance spectrum of sub-wavelength structures for maximum photocurrent output of tandem cells.
The electrical resistivity of monolayer graphene exhibit significant changes upon
expose to different concentration of oxygen (O2) at room temperature. The monolayer
graphene, grown by chemical vapor deposition (CVD) with perfect uniformity within
1cm×1cm will attach O2 molecules which will act as a p-type dopant and enhance the
hole conductivity, make a change of resistivity of graphene thin film. We quantified
the change of resistivity of graphene versus different O2 concentration and the
detection limit of the simple O2 sensor was 1.25% in volume ratio.
This study investigates the physical properties of Al-doped MgxZn1-xO (AMZO) films. Al-doped MgxZn1-xO films were deposited by radio-frequency (RF) magnetron sputtering system using a 4 inch ZnO/MgO/Al2O3 (76/19/5 wt%) target. This study determined the resulting x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), Hall measurement, and transparent performance of the films. XRD results indicate that the diffraction angles of the annealed AMZO film shifted toward the high-angle side, indicating that thermal annealing could relax the compressive strain components in the as-deposited samples. XPS results reveal a high carbon content on the surface of MgxZn1-xO. This may be due to contamination. The average Mg content of the as-grown AMZO is about 19.23 at. % at a depth of 40 nm. The Al-doped MgxZn1-xO film in this study shows high transparency with transmittances over 95 % in the visible region (400 ~ 800nm), and a sharp absorption edge is visible in the UV region due to the Mg content. The Hall measurement of Al-doped MgxZn1-xO films deposited at lower RF power show higher doping concentrations, lower resistivity and higher mobility as a function of the annealing temperature. Experimental results indicate that Al-doped MgxZn1-xO film with 1000 °C annealing contains more oxygen vacancies, which play the role of donor. Oxygen vacancies generate states in the band gap and increase conductivity.
AlGaN/GaN high electron mobility transistors (HEMTs) with polar and nonpolar ZnO nanowires
modified gate exhibit significant changes in channel conductance upon expose to different
concentration of carbon monoxide (CO) at room temperature. The ZnO nanowires, grown by chemical
vapor deposition (CVD), with perfect crystal quality will attach CO molecule and release electrons,
which will lead to a change of surface charge in the gate region of the HEMTs, inducing a higher
positive charge on the AlGaN surface, and increasing the piezoinduced charge density in the HEMTs
channel. These electrons create an image positive charge on the gate region for the required neutrality,
thus increasing the drain current of the HEMTs. The HEMTs source-drain current was highly
dependent on the CO concentration. The limit of detection achieved was 400 ppm and 3200ppm in the
open cavity with continuous gas flow using a 50x50μm2 gate sensing area for polar and nonpolar ZnO
nanowire gated HEMTs sensor.
We have prepared bulk p-AlInGaN layers and light emitting diodes (LEDs) with p-AlInGaN surface layers by
metalorganic chemical vapor deposition (MOCVD). It was found that surfaces of the LEDs with p-AlInGaN layers were
rough with high density of hexagonal pits. It was also found that pit width and pit density depend on the growth
temperature of the p-AlInGaN layer. Furthermore, it was found that we can achieve 62% enhancement in output
intensity from the LED with 820°C p-AlInGaN cap layer without increasing the LED operation voltage.
Coupling of a InGaN/GaN multi-quantum well (MQW) and semitransparent metal layer is shown to result in dramatic
enhancement of spontaneous emission rate by the surface plasmon effect in the optical spectral range. A five-pairs
18.5nm InGaN/GaN MQW is positioned 175nm, form various thickness (t=5~50nm) silver layer. And periodic patterns
(p=0.25~0.8μm) are defined in the top semitransparent metal layer by e-beam lithography, which are grating structures
can be incorporated into the metal film to excite surface plasmon between the interference of the metal film and
semiconductor. We have experimentally measured photoluminescence intensity and peak position of spontaneous
emission of the fabricated structures and compared with the unprocessed samples, whilst still ensuring that most of the
emission takes place into the surface plasmon (SP) mode. And the implication of these results for extracting light by
reducing total internal reflection (TIR) from light emission diode is discussed.
We develop a miniaturized optical signal pickup module, with a working wavelength of 650 nm, and an image numerical aperture (NA) of 0.6, comprised of several SiNX optical phase elements on stacked Si substrates, for use in optical storage systems. The optical module, which is optical-on-axis and transmissible in both visible and infrared ranges, is designed to include not only a light source, but also diffractive optical elements (DOEs), which can be made with micro-optoelectromechanical systems (MOEMS) technology. Its optical operation is simulated by ray tracing to optimize the spot size (~0.6 µm) focused on the disk by adjusting the tolerance of each element in the alignment. All the Si-based transmission optical elements are fabricated and stacked by self-alignment bonding to reduce the tolerance of the assembled system. We obtain a circular focused spot when the full-width at half maximum (FWHM) of the zero-order beam is 3.1 µm; the diffraction limited spot size on the optical disk is 0.7 µm.
In this study, ZnO:Al(AZO) Ni/AZO and NiOx/AZO films were deposited on p-type GaN films followed by thermal
annealing to form Ohmic contacts. After thermal annealing, the resistivities reduced from 5×10-3 to 4.4×10-4 Ω-cm,
2.6×10-3Ω-cm, and 1.1×10-3Ω-cm for AZO, Ni/AZO, and NiOx/AZO films, respectively. The Ohmic characteristic
could be highly improved after inserting Ni and NiOx between AZO and p-GaN. Both the Ni/AZO and NiOx/AZO
contacts exhibit Ohmic characteristic after annealed at 800°C in N2 ambient. The light transmittance of Ni/AZO and
NiOx/AZO films were higher than 80% in the range of 380-700nm after the 800°C -annealing treatment. In addition,
we fabricated InGaN/GaN MQW LEDs with a dimension of 1×1mm2 using the transparent Ni/AZO and NiOx/AZO
Ohmic contact as a current spreading layer for p-GaN in order to increase the light extrication efficiency. For the LED
with Ni/AZO contact, the light output approach to saturation when the injection current was about 400mA. But the
light output still doesn't approach to saturation when the injection current was 500mA for the LED with NiOx/AZO
contact. This may be due to that the resistivity of Ni/AZO was higher than that of NiOx/AZO and exhibit more heavy
current clouding effect. The increasing of resistivity may be due to the interdiffusion of Ni into AZO. Comparing to
GaN LED with Ni/Au ohmic contact, the light output intensity of LEDs with Ni/AZO and NiOx/AZO contacts was
increased by 41% and 60% at 350mA, respectively.
Red and green emissions are observed from P ion implanted ZnO. Red emission at ~680 nm (1.82 eV) is associated with the donor-acceptor pair (DAP) transition, where the corresponding donor and acceptor are interstitial zinc (Zni) and interstitial oxygen (Oi), respectively. Green emission at ~ 516 nm (2.40 eV) is associated with the transition between the conduction band and antisite oxygen (OZn). Green emission at ~516nm (2.403 eV) was observed for ZnO annealed at 800 oC under ambient oxygen, whereas, it was not visible when it was annealed in ambient nitrogen. Hence, the green emission is most likely not related to oxygen vacancies on ZnO sample, which might be related to the cleanliness of ZnO surface, a detailed study is in progress. The observed micro-strain is larger for N ion implanted ZnO than that for P ion implanted ZnO. It is attributed to the larger straggle of N ion implanted ZnO than that of P ion implanted ZnO. Similar phenomenon is also observed in Be and Mg ion implanted GaN.
High-performance, blue micro-size InGaN light emitting diodes (LEDs) with diameters of 3 to 20 μm have been fabricated. An ion implantation technique and a 12 micron electro-ridge were used to simplify fabrication processes. The 3 to 20μm LEDs that exhibited a large emission photon blue shift (87.5meV ~52.9meV) were observed in electro-luminescence (EL) spectra. Under an increased injection current, the quantum wells become populated with charge carriers, which screened the internal piezoelectric field and caused the energy blue shift of EL eventually. A high injection current caused a high junction temperature that narrowed the band gap (red shift). The size dependent energy shift is largely owing to the competition between the blue and the red shifts. At a bias voltage of 8.96V (which is 140% of the turn on voltage, 6.4V), the 10 μm device exhibited an injection current of 7.9mA. This value exceeds that in literature, i.e., 4mA at a bias voltage of 14V (which is 140% of the turn on voltage, 10V). This phenomenon may be owing to that the ion implantation and electro-ridge designs herein involved a lower series resistance. The external quantum efficiencies (E.Q.E.) of the micro size LEDs herein were all 0.4%~3.3%, which is better than the values reported in literature, which were ranged between 0.004% and 1.29% for an individual LED and an array LED, respectively. The E.Q.E. of the 15μm device at maximum injection current had the optimum value yet obtained for micro-size LEDs. The dependence of the blue shift and the E.Q.E. on the size warrants further study.
The hydrogenated Silicon nitride film is well developed to form a passivation layer for non-volatile memory devices. It has many superior chemical, electrical, and mechanical properties. In addition, it also has excellent optical properties. It is transparent in UV and DUV range, with a high refractive index of about 1.7~2. Owing to its superior mechanical and optical properties, we used a hydrogenated silicon nitride (SiNXHY) membrane as an optical phase element. By using e-beam lithography, we demonstrate on feasibility for the fabrication of subwavelength optical elements, such as waveplate, polarizer, and polarized beam splitter on a silicon-based low stress SiNXHY membrane for the UV region applications. An SiNXHY film was deposited by plasma enhanced chemical vapor deposition (PECVD) and the free- standing membrane is formed by KOH silicon backside etching, from which substrate materials are removed. The membrane's morphology and geometries of subwavelength optical elements were verified by means of an scanning electron microscope (SEM), and the optical performance characteristics of these subwavelength optical elements are shown. The experimental datas agree well with theoretical predictions.
Homoepitaxial and heteroepitaxial ZnO films were grown by plasma-assisted molecular beam epitaxy (P-MBE). Homoepitaxial ZnO layers were grown on an O-face melt-grown ZnO (0001) substrate. Heteroepitaxial ZnO layers were grown on an epitaxial GaN template predeposited by metalorganic chemical vapor deposition on a c-plane sapphire substrate. There exists a residual strain in the heteroepitaxial ZnO, which is ε = -0.25%. Low-intensity excitation PL spectra of ZnO epilayers excited by a He-Cd laser exhibit only bound-exciton emission with phonon replicas. The quality of ZnO epilayers is better than that of ZnO substrate. However, under high-intensity excitation by a N2 laser, the emission due to exciton-exciton collisions dominates the PL spectrum from heteroepitaxial ZnO layer but is not observed from homoepitaxial ZnO layer.
We have proposed a miniaturized optical signal pickup module comprised of several SiNX membrane devices on stacked Si substrates for use in optical storage system. The optical module was designed to include not only light sources and detectors, but also the diffractive optical elements (DOEs), which can be made with microoptoelectromechanical systems (MOEMS) technology. Its optical operation was simulated by ray-tracing to have an optimized spot size (~0.6μm) focused on the disk with setting the tolerance of each element for the alignment. All these Si-based transmission optical elements were fabricated and can be stacked by self-alignment bonding for system assembly.
Silicon nitride (SiNX) film is a commonly used material in silicon technology. In addition, it has excellent optical properties. It is transparent in both the UV and visible range, with a high refractive index of about 1.7~2. Owing to its superior mechanical and optical properties, we used a silicon nitride membrane as an optical phase element. We will fabricate nano-structured diffractive optical elements, such as wave-plate, polarizer, and polarized beam splitter on SiNXHY membrane by e-beam lithography for the UV-visible regime applications. The SiNXHY membranes were made from SiNXHY films deposited by an plasma enhanced chemical vapor deposition (PECVD) as an alternative method for low stress membrane fabrication used in UV-visible transmittance. The stress of silicon nitride film showed a change from compressive to tensile with increasing working pressure during film deposition. The UV-visible transmittance of the free standing membrane was measured, which showed that UV light is transparent at wavelength as short as 240nm. We will show the feasibility to fabricate nano-structured diffractive optical elements on the SiNXHY membrane combined with microoptoelectromechanical systems (MOEMS) technology for the application in the UV-visible regimes.
Photoluminescence (PL) measurement and optical pumping at 25K were studied for high-indium-composition InGaN/GaN multiple quantum well (MQW) structures grown by low-pressure metalorganic chemical vapor deposition. The result show that thermal annealing can reduce the compositional fluctuation of indium content. The optical pumping spectra show 5 stimulated emission (SE) peaks. This phenomenon might be attributed to intersubband transition. The transition between quantized levels for each peak was precisely identified by solving the time-independent Schrodinger equation and finite-difference method. The ratio of conduction- band discontinuities to the valence-band discontinuities of InGaN/GaN QW, (delta) Ec:(delta) Ev=38:62, can be obtained.
InGaN/GaN double heterostructure and multiple quantum wells (MQW) light-emitting diodes were grown by metalorganic vapor phase epitaxy. Band gap narrowing of the PL spectra for the InGaN/GaN MQW LEDs can be observed at room temperature. In addition, the emission wavelength of EL and PL spectra for the MQW blue LEDs exhibit a blue-shift phenomenon when increasing the injection current and laser power, respectively. This luminescence behavior can tentatively be understood as a competition between a spectral red-shift mechanism of piezoelectrically-induced quantum-confined Stark effect and a blue-shift mechanism of band-filling and charge screening effects.
Optoelectronics research has been established at OES/ITRI since 1981. The R&D program started from compound semiconductor crystal growth. The major contribution is the development of a modified horizontal Bridgman method for growing GaAs single crystals. In the area of optoelectronic devices, both liquid phase epitaxy (LPE) and the metalorganic vapor phase epitaxy (MOVPE) are used to fabricate most of the light emitting diodes (LED), laser diodes (LD), and photodetectors (PD). The light sources with emitting light wavelength from visible 0.65 micrometers to 1.3 micrometers were made. Detectors, such as Si PIN and InGaAs PIN were developed. We are focusing on a new material system, i.e., AlGaInP. The LED and LD at the wavelength of 0.67 micrometers were grown by MOVPE. The threshold current is about 60 mA for a ridge-waveguide laser.
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