Optical and electrical properties of gallium nitride (GaN)-based blue light-emitting diodes (LEDs) with various indium gallium nitride (InGaN) quantum well (QW) thicknesses were investigated. As the QW thickness was increased, the light output power of GaN-based LEDs also increased. The increase can be attributed to the increase in the carrier radiative recombination rate in the active region. However, the turn-on voltages of these fabricated LEDs are different. This was attributed to the increase in the polarization field with increasing QW thickness. In regard to the hot/cold factor, LEDs with a thicker QW achieved better performance at a low-injection current owing to the lower defect density. The hot/cold factor at a high-injection current would be mainly influenced by the efficiency droop mechanism.
We report the effects of microcell layout on the performances of GaN-based high-voltage light-emitting diodes (HV-LEDs). Compared with samples with an S-type layout pattern, it was found that the samples with an I-type layout pattern exhibit smaller forward voltage, larger light output power, lower thermal temperature, and better wall-plug efficiency (WPE). It was also found that we could further improve the performances of HV-LED chips by introducing extra metal fingers to enhance current spreading. Compared with the S-type sample without metal fingers, we could reduce the efficiency droop from 38.6% to 14.8% by using an I-type sample with metal fingers. Furthermore, it was found that WPE reduced by around 40% after a 1000 h aging test for the S-type sample without metal fingers. In contrast, almost no decrease in WPE could be observed from the I-type sample with metal fingers after the same aging time.
The study of thermal properties for GaN-based light-emitting diodes (LEDs) with various last barrier thicknesses was reported. It was found that the LED output power decreased as we increased the thickness of the last barrier. It was also found that the LED output powers decrease with the increase of temperature for the LEDs with 12-, 24-, and 48-nm-thick last barriers. However, it was found that the LED output power increases with the increase of temperature for the LED with a 72-nm-thick last barrier due to the fact that more holes could enter the multiquantum well active region at elevated temperatures. Furthermore, it was found that the output power decreased by 0.15%/°C, 0.15%/°C, and 0.11%/°C for the LEDs with 12-, 24-, and 48-nm-thick last barriers, respectively, but increased by 0.09%/°C for the LED with a 72-nm-thick last barrier.
KEYWORDS: Ions, Energy transfer, Solid state physics, Luminescence, Terbium, Scanning electron microscopy, Quenching (fluorescence), Crystals, X-ray diffraction, Solid state electronics
Green emitting LiBa1-xPO4:xTb3+ up-conversion phosphors with various concentrations (x=0.1, 0.2, 0.3, 0.4, 0.5) of Tb3+ ions were synthesized by solid state reaction method at 1300 □ for 3 hour in air. The impure phases were appeared as Tb3+ ion concentration was further increased (x more than 0.2). The luminescence intensity reached a maximum when the concentration of Tb3+ ion was x = 0.2, and then decreased with the increases of the Tb3+ concentration due to concentration quenching effect. In addition, it is identified that the d-d interaction plays a major role in the mechanism of concentration quenching of LiBaPO4:Tb3+ and all the chromaticity (x, y) of LiBa1-xPO4:xTb3+ phosphors are located in the green region (0.33, 0.56 ).
The authors report the formation of air-voids at GaN/cone-shaped-patterned-sapphire-substrate interface by laser
scribing and lateral etching with one-step growth. With 5 and 20 min lateral etching, it was found that pyramid-like airvoid
was formed with an average height of 0.98 and 1.9 μm, respectively, on top of each corn of the substrate. It was
also found that we can enhance output power of GaN-based light-emitting diodes by 6.6 and 11.5%, respectively, by
immersing the wafer in a mixture of H3PO4 and H2SO4 solution at 220°C for 5 and 20 min, respectively.
We demonstrated very thick (~400 nm) AlInGaN quaternary alloy grown on GaN epilayer by MOCVD. The Optical,
electronic, crystalline quality and surface morphology of AlInGaN/GaN hetero structures with various TMI molar flow
rate were be extensively discussed. With Al0.89In0.02GaN/GaN fully lattice matched structure, less and small V-defect
pits and good crystal quality comparing with the other lattice mismatched AlInGaN/GaN hetero-structures were
discovered. Finally, the difference AlInGaN quaternary epilayers could be directly applied to high power LED structure
in the future by the same barrier growth conditions.
The processing technology of 1.3&mgr;m InAs-InGaAs quantum-dot VCSELs with fully doped DBRs grown by MBE will be
demonstrated. The threshold currents of the fabricated devices with 10 &mgr;m oxide-confined aperture are 0.7mA, which
correspond to 890A/cm2 threshold current density. And the threshold voltage of the device is 1.03V and maximum
output power is 33 &mgr;W. The series resistance is 85 &OHgr; which is 10 times lower then our preliminary work and 3 times
lower then intracavity contacted InAs-InGaAs quantum-dot VCSEL. This relatively lower resistance can even comparable with the best result reported in InGaAs oxide-confined VCSELs with intracavity contact.
In our previous work, we first propose the Au/AuBe is a good ohmic contact material to p-type ZnTe. After apply the material to II-VI light emitting diode, the operation voltage as low as 2.4V can be observed. The II-VI blue light emitting diode (LED) were grown in a RIBER 32P system with Zn(6N), Se(6N), Te(6N), Cd(6N) on (001) GaAs+ substrate. The structure consisted of a GaAs:Si(n+) buffer layer, ZnSe:C1 (0.5um), ZnSe(200A)/ZnCdSe (100A) multiple quantum well, ZnSe(200A), ZnSe:N(0.3um), ZnSe:N/ZnTe:N multilayer and ZnTe:N (300A). Standard photolithography technology was doen to fabricate the diode. The mesa etch was done by (formula available in paper) etching solution. The p-type ohmic was done by AuBe/Au metal. The emission wavelength was 530nm (room temperature) and 495 (30K) with 2.4V under CW operation. Since low operation voltage introduced less heat in the device, better thermal behavior can be expected with this low operation voltage.
The reliable n+-ZnSSe metal-semiconductor-metal (MSM) blue-green light emitting diodes (LEDs) have been fabricated. The contact metal was CuGe/Pt/Au. The current transport mechanism agree very well with the back-to-back tunneling diodes. The kink phenomena were observed in the MSM current- voltage curves. In the metal-semiconductor interface, the element Zn in ZnSSe can be replaced by Cu results in some acceptor levels as radiative recombination centers in the MS interface. The peak wavelength in the LED electroluminescent (EL) spectra was strongly dependent on the injection currents from 5 to 40 mA. The peak wavelength and full width at half maximum are 510 and 10 nm, respectively, at 10 mA injection current. When the injection current increases to 15 mA, the peak wavelength shifted to 530 nm due to different recombination centers. Further increasing the injection currents, the peak wavelength shifted slightly to the long wavelength side.
This research attempted to use metals with lower work functions, such as Ti, Al, to form ohmic contacts to n-GaN. Then we used metals with higher work functions, such as Ni, Pd, Pt, and Au to form ohmic contacts to p-GaN. The work functions of these metals indeed influence the performance of ohmic contacts, indicating that the Fermi level of GaN is unpinned. The specific contact resistance measured and calculated by TCL model, was 2.35 X 10-3 (Omega) cm2 for as-deposited Ti/Al on GaN. After RTA processes at different temperatures in the range of 400 to approximately 900 degrees Celsius, the minimum (rho) c of 7.4 X 10-5 (Omega) cm2 can be obtained for RTA temperature of 600 degrees Celsius. The oxidized Ni/Au contact exhibited the lowest contact resistance of 1.02 X 10-2 (Omega) cm2, among Ni/Au, Pd/Au, Pt/Au contact schemes on p-GaN. It was also observed that the I-V curves of the triple-layer contact, Pt/Ni/Au, was near-linear while the others were rectifying even after annealing.
We have found that the electrical and optical properties of GaN epilayers strongly depend on buffer TMGa flow rates and NH3 flow rates. At low flow rate of 4 sccm, the buffer layer quality was good so the concentrations of undoped GaN epilayers decreased and a stronger band-edge emission of 362 nm can be observed in photoluminescence spectra. The carrier density of the films can be reduced from 1018 to 1017 cm-3 by increasing the NH3 flow rate from 0.5 up to 1 SLM and comparatively increased the near-band-edge emission. So, with a NH3 flow rate as high as 1 SLM, the GaN epilayers with good optical quality can be obtained. The excitation power density of He-Cd laser influences the photoluminescence property of GaN epilayers. At high excitation power density of 637 W/cm2, the near-band-edge luminescence (362 nm) is dominant and the deep level luminescence (near 550 nm) appears very weak. But at low excitation power density, the luminescence from defect yellow band levels is stronger than the near-band-edge transitions.
A study of near-band-edge optical properties on ZnSe epilayers grown on GaAs substrates using various modulation techniques is presented. We compare the contactless electroreflectance (CER) and piezoereflectance (PzR) spectra to ascertain that our ZnSe epilayers of 1.2 micrometers in thickness grown on GaAs substrates are under a biaxial tensile strain. The defect-related transitions near the ZnSe/GaAs interface are also compared by identifying the photoreflectance (PR) and other spectra. In addition, in order to observe the temperature-dependent energy splitting and strains, we present a detailed investigation of the heavy and light-hole related transition energies as a function of temperature in the 15-200 K range by identifying the excitonic signatures in the CER spectra. We have also calculated the energy splitting between heavy and light-hole valence bands by utilizing the temperature-dependent elastic constants for ZnSe and the thermal-expansion coefficients for ZnSe and GaAs. Both the experimental result and the theoretical calculation have shown a similar trend that the biaxial tensile strains decrease in magnitude with increasing temperatures in the 1.2 micrometers ZnSe epilayer grown on a GaAs substrate.
Experimental and theoretical results are presented for current-voltage and dynamic resistance-voltage characteristics of Hg1-xCdxTe ion-implanted p-n junction photodiodes with x approximately equals 0.22. By measuring the temperature dependence of the dc characteristics in the temperature range 25-140K, the dark current mechanism are studied. At high temperature and in low reverse bias region, the diffusion current dominates. On the other hand, at medium temperature and medium reverse bias, trap-assisted tunneling plays an important role. At low temperature and in the medium reverse bias region, band-to-band tunneling is the key leakage current source.However, when the temperature is further lowed down to 25K and the applied reverse bias is very small, the band-to-band tunneling current will be ruled out and the trap-assisted tunneling mechanism dominates again. We have measured 1/f noise in HgCdTe photodiode as a function of temperature, diode bias, dark current. The dependence of 1/f noise on dark current was measured over a wide temperature range on devices. The temperature dependence of the 1/f noise was found to be the same as the temperature dependence of the surface generation and leakage currents. We obtained the maximum specific detectivity value and the maximum signal-to-noise ratio are about 3.51 X 1010 cm Hz1/2/W and 5096 respectively.
High quality SiO2 layers were deposited on top of strained SiGe by direct photochemical vapor deposition with a deuterium lamp as the excitation source. It was found that the deposition rate increase linearly with the chamber pressure. Auger Electron spectroscopy profile shows that these is no Ge rejected and no Ge-rich layer formed after devices were fabricated. At room temperature, the leakage current is about 3 X 10-9 A/cm2 under a 2 X 106 V/cm electric field. The breakdown field can reach over 16 MV/cm for these SiGe MOS diodes.
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