High brightness InGaN green LEDs with an ITO on n/sup ++/-SPS upper contact

Indium tin oxide (ITO) (260 nm) and Ni (5 nm)/Au (10 nm) films were deposited onto glass substrates, p-GaN layers, n/sup +/-InGaN/GaN short-period-superlattice (SPS), n/sup ++/-SPS and nitride-based green light-emitting diodes (LEDs). It was found that ITO could provide us an extremely high transparency (i.e., 95% at 520 nm). It was also found that the 1.03/spl times/10/sup -3/ /spl Omega/cm/sup 2/ specific contact resistance of ITO on n/sup ++/-SPS was reasonably small. Although the forward voltage of the LED with ITO on n/sup ++/-SPS upper contacts was slightly higher than that of the LED with Ni/Au on n/sup ++/-SPS upper contacts, the 20 mA output power and external quantum efficiency of the former could reach 4.98 mW and 8.2%, respectively, which were much larger than the values observed from the latter. The reliability of ITO on n/sup ++/-SPS upper contacts was also found to be reasonably good.     

Nitride-based light-emitting diodes with Ni/ITO p-type ohmic contacts

The optical and electrical properties of Ni(5 nm)-Au(5 nm) and Ni(3.5 nm)-indium tin oxide (ITO) (60 nm) films were studied. It was found that the normalized transmittance of Ni/ITO film could reach 87% at 470 nm, which was much larger than that of the Ni-Au film. It was also found that the specific contact resistance was 5 /spl times/ 10/sup -4/ /spl Omega/ /spl middot/ cm/sup 2/ and 1 /spl times/ 10/sup -3/ /spl Omega/ /spl middot/ cm/sup 2/, respectively, for Ni-Au and Ni/ITO on p-GaN. Furthermore, it was found that the 20 mA output power of light-emitting diode (LED) with Ni-Au p-contact layer was 5.26 mW. In contrast, the output power could reach 6.59 mW for the LED with Ni/ITO p-contact layer.     

Improvement of the luminous intensity of light-emitting diodes by using highly transparent Ag-indium tin oxide p-type ohmic contacts

We have investigated Ag-indium tin oxide (ITO) scheme for obtaining high-quality p-type ohmic contacts for GaN-based light-emitting diodes (LEDs). The Ag(1 nm)-ITO(200 nm) contacts exhibit greatly improved electrical characteristics when annealed at temperatures in the range 400/spl deg/C-600/spl deg/C for 1 min in air, yielding specific contact resistances of /spl sim/10/sup -4/ /spl Omega//spl middot/cm/sup 2/. In addition, the contacts give transmittance of about 96% at 460 nm, which is far better than that of the conventionally used oxidized Ni-Au contacts. It is shown that the luminous intensity of blue LEDs fabricated with the Ag-ITO contacts is about three times higher than that of LEDs with oxidized Ni-Au contacts. This result strongly indicates that the Ag-ITO scheme can serve as a highly promising p-type ohmic contact for the realization of high brightness near ultraviolet LEDs.     

Highly reliable GaN-based light-emitting diodes formed by p-In/sub 0.1/Ga/sub 0.9/N-ITO structure

Indium-tin-oxide (ITO) is deposited as a transparent current spreading layer of GaN-based light-emitting diodes (LEDs). To reduce the interfacial Schottky barrier height, a thin p-In/sub 0.1/Ga/sub 0.9/N layer is grown as an intermediate between ITO and p-GaN. The contact resistivity around 2.6/spl times/10/sup -2/ /spl Omega//spl middot/cm/sup 2/ results in a moderately high forward voltage LED of 3.43 V operated at 20 mA. However, the external quantum efficiency and power efficiency are enhanced by 46% and 36%, respectively, in comparison with the conventional Ni-Au contact LEDs. In the life test, the power degradation of the p-In/sub 0.1/Ga/sub 0.9/N-ITO contact samples also exhibits a lower value than that of the conventional ones.     

Rapid thermal annealed InGaN/GaN flip-chip LEDs

Nitride-based flip-chip (FC) light-emitting diodes (LEDs) emitting at 465 nm with Ni transparent ohmic contact layers and Ag reflective mirrors were fabricated. With an incident light wavelength of 465 nm, it was found that transmittance of normalized 300/spl deg/C rapid thermal annealed (RTA) Ni(2.5 nm) was 93% while normalized reflectance of 300/spl deg/C RTA Ni(2.5 nm)/Ag(200 nm) was 92%. It was also found that 300/spl deg/C RTA Ni(2.5 nm) formed good ohmic contact on n/sup +/ short-period-superlattice structure with specific contact resistance of 7.8/spl times/10/sup -4/ /spl Omega//spl middot/cm/sup 2/. With 20-mA current injection, it was found that forward voltage and output power were 3.15 V and 16.2 mW for FC LED with 300/spl deg/C RTA Ni(2.5 nm)/Ag(200 nm). Furthermore, it was found that reliabilities of FC LEDs were good.     

High-reflectivity Al-Pt nanostructured Ohmic contact to p-GaN

The effect of nanoscale Pt islands on the electrical characteristics of contacts to p-type gallium nitride (GaN) has been investigated to explore the feasibility for the flip-chip configuration light-emitting diodes (LEDs) using an Al-based reflector. An as-deposited Al contact to p-GaN with a net hole concentration of 3/spl times/10/sup 17/cm/sup -3/ was rectifying. However, an Al contact with nanoscale Pt islands at the interface exhibited ohmic behavior. A specific contact resistivity of 2.1/spl times/10/sup -3//spl Omega//spl middot/cm/sup 2/ and a reflectance of 84% at 460 nm were measured for the Al contact with nanoscale Pt islands. Current-voltage temperature measurements revealed a Schottky barrier height reduction from 0.80 eV for the Al contact to 0.58 eV for the Al contact with nanoscale Pt islands. The barrier height reduction may be attributed to electric field enhancement and the enhanced tunneling due to the presence of the nanoscale Pt islands. This will offer an additional silver-free option for the p-type ohmic contact in flip-chip configuration LEDs. Theory suggests that the ohmic contact characteristics may be improved further with smaller Pt islands that will enhance tunneling across the interface with the GaN and in the vicinity of the Pt-Al interface.     

Highly reliable nitride-based LEDs with SPS+ITO upper contacts

Nitride-based blue light emitting diodes (LEDs) with an n/sup +/-short period superlattice (SPS) tunnel contact layer and an indium tin oxide (ITO) transparent contact were fabricated. Compared with conventional nitride-based LEDs with Ni/Au upper contacts, it was found that we could achieve a 60% increase in electroluminescence (EL) intensity by using ITO upper contacts. However, it was also found that the lifetime of ITO LEDs were much shorter. Furthermore, it was found that we could achieve a longer lifetime and a smaller reverse leakage current (I/sub R/) by the deposition of a SiO/sub 2/ layer on top of the ITO LEDs.     

Low resistance and reflective Mg-doped indium oxide-Ag ohmic contacts for flip-chip light-emitting diodes

We have investigated an Mg-doped In/sub x/O/sub y/(MIO)-Ag scheme for the formation of high-quality ohmic contacts to p-type GaN for flip-chip light-emitting diodes (LEDs). The as-deposited sample shows nonlinear current-voltage (I--V) characteristics. However, annealing the contacts at temperatures of 330/spl deg/C-530/spl deg/C for 1 min in air ambient results in linear I--V behaviors, producing specific contact resistances of 10/sup -4/--10/sup -5/ /spl Omega//spl middot/cm/sup 2/. In addition, blue LEDs fabricated with the MIO-Ag contact layers give forward-bias voltages of 3.13-3.15 V at an injection current of 20 mA. It is further shown that LEDs made with the MIO-Ag contact layers give higher output power compared with that with the Ag contact layer. This result strongly indicates that the MIO-Ag can be a promising scheme for the realization of high brightness LEDs for solid-state lighting application.     

Nitride-based MQW LEDs with multiple GaN-SiN nucleation layers

Nitride-based light emitting diodes (LEDs) separately prepared with a conventional single low-temperature (LT) GaN nucleation layer and multiple GaN-SiN nucleation layers were both prepared. It was found that we could reduce defect density and thus improve crystal quality of the GaN-based LEDs by using multiple GaN-SiN nucleation layers. With a 20-V applied reverse bias, it was found that the reverse leakage currents measured from the LED with a single LT GaN nucleation layer and the one with 10-pair GaN-SiN nucleation layers were 1.5/spl times/10/sup -4/ and 2.5/spl times/10/sup -6/ A, respectively. It was also determined that we could use the multiple GaN-SiN nucleation layers to enhance the output intensity of near ultraviolet (UV) LEDs and to improve the reliability of nitride-based LEDs.     

Full characterization of packaged Er-Yb-codoped phosphate glass waveguides

We present a procedure for the characterization of packaged Er-Yb-codoped phosphate glass waveguides. The procedure is based on precise measurements of the output optical powers when the waveguide is diode-laser pumped at 980 nm. The dependence of these optical powers on the input pump power is then fitted to the results from a numerical model that describes in detail the propagation of the optical powers inside the waveguide. The best fit is obtained for the following parameters: the signal wavelength scattering losses are /spl alpha/(1534)=8.3/spl times/10/sup -2/ dB/cm, the Yb/sup 3+/ absorption and emission cross sections (/spl ap/980 nm) are 5.4/spl times/10/sup -25/ m/sup 2/ and 7.0/spl times/10/sup -25/ m/sup 2/, the Er/sup 3+/ absorption and emission cross sections (/spl ap/980 nm) are 1.6/spl times/10/sup -25/ m/sup 2/ and 1.2/spl times/10/sup -25/ m/sup 2/, the Yb/sup 3+/--Er/sup 3+/ energy-transfer coefficient is 1.8/spl times/10/sup -23/ m/sup 3//s and the cooperative-upconversion coefficient is 8/spl times/10/sup -25/ m/sup 3//s. An approximate method is introduced that allows the determination of the absorption and emission cross section distributions for the erbium /sup 4/I/sub 13/2//spl hArr//sup 4/I/sub 15/2/ transition from the amplified spontaneous emission power spectrum.     

Indium tin oxide transparent electrodes for broad-area top-emitting vertical-cavity lasers fabricated using a single lithography step

We report for the first time top-emitting buried oxide vertical cavity lasers using transparent indium tin oxide electrodes. Our process enables broad-area InAlGaAs VCSELs to be fabricated in a single lithography step, thus allowing the fast turn-around time necessary for evaluating VCSEL epitaxial materials. The ITO contacts attain a peak transmission of 96%, a specific contact resistance of 10/sup -5/ /spl Omega//spl middot/cm/sup 2/, and a sheet resistivity of 2.5/spl times/10/sup -4/ /spl Omega//spl middot/cm. Under room temperature CW pumping, the devices exhibit a minimum threshold current density of 1.2 kA/cm/sup 2/ at a wavelength of 801 mm, and have a maximum light output power of 5.2 mW.     

Highly Reliable High-Brightness GaN-Based Flip Chip LEDs

The properties of indium-tin-oxide (ITO)/Ni films as transparent ohmic contacts of nitride-based flip chip (FC) light emitting diodes (LEDs) were studied. It was found that 300degC rapid thermal annealed (RTA) ITO(15 nm)/Ni(1 nm) could provide good electrical and optical properties for FC LED applications. It was also found that 20-mA operation voltage and output power of the 465-nm FC LEDs with ITO/Ni/Ag reflective mirror were 3.16 V and 21 mW, respectively. Furthermore, it was found that output intensity of the proposed LED only decayed by 5% after 1200 h under 30-mA current injection at room temperature.     

Fabrication of Germanosilicate glass optical fibers containing Tm/sup 2+/ ions and their nonlinear optical properties

Germanosilicate glass optical fibers incorporated with the Tm/sup 2+/ ions were fabricated to enhance optical nonlinearity by providing a strong reduction environment based on the solution doping technique in the modified chemical vapor deposition (MCVD) process. The incorporation of the Tm/sup 2+/ ions into the fiber core was identified by the electron paramagnetic resonance (EPR) spectrum in the fiber preform, and the absorption and emission properties between 350 and 1600 nm of the Tm/sup 2+/ ions in optical fibers and the fiber preform. A strong broad absorption band due to the Tm/sup 2+/ ions appeared from 350 to /spl sim/900 nm, and a broad emission from /spl sim/600 to /spl sim/1050 nm and the other emission from /spl sim/1050 to /spl sim/1300 nm, which were not shown in the Tm/sup 3+/ ions, were found upon Ar-ion laser pumping at 515 nm. Both absorption and emission results confirm that the Tm/sup 2+/ ions in the germanosilicate glass have the 4f-5d energy band from 350 to /spl sim/900 nm and the 4f-4f energy level at /spl sim/1115 nm. Also, the resonant nonlinearity at /spl sim/1310 and /spl sim/1530 nm due to the Tm/sup 2+/ ions in the fiber was measured upon the 515 nm optical pumping by using a long-period fiber grating (LPG) pair method. The nonlinear refractive index n/sub 2/ at /spl sim/1310 and /spl sim/1530 nm was found to be /spl sim/4/spl times/10/sup -15/ m/sup 2//W, where 70% and 30% of the n/sub 2/ are attributed to the nonradiative transitions and the radiative transitions, respectively.     

Enhanced output power of InGaN-GaN light-emitting diodes with high-transparency nickel-oxide-indium-tin-oxide Ohmic contacts

This study develops a highly transparent nickel-oxide (NiO/sub x/)-indium-tin-oxide (ITO) transparent Ohmic contact with excellent current spreading for p-GaN to increase the optical output power of nitride-based light-emitting diodes (LEDs). The NiO/sub x/-ITO transparent Ohmic contact layer was prepared by electron beam in-situ evaporation without postdeposition annealing. Notably, the transmittance of the NiO/sub x/-ITO exceeds 90% throughout the visible region of the spectrum and approaches 98% at 470 nm. Moreover, GaN LED chips with dimensions of 300 /spl times/ 300 /spl mu/m fabricated with the NiO/sub x/-ITO transparent Ohmic contact were developed and produced a low forward voltage of 3.4 V under a nominal forward current of 20 mA and a high optical output power of 6.6 mW. The experimental results indicate that NiO/sub x/-ITO bilayer Ohmic contact with excellent current spreading and high transparency is suitable for fabricating high-brightness GaN-based light-emitting devices.     

Improvement of InGaN-GaN light-emitting diodes with surface-textured indium-tin-oxide transparent ohmic contacts

Presents a surface-textured indium-tin-oxide (ITO) transparent ohmic contact layer on p-GaN to increase the optical output of nitride-based light-emitting diodes (LED) without destroying the p-GaN. The surface-textured ITO layer was prepared by lithography and dry etching, and dimensions of the regular pattern were approximately 3 /spl times/ 3 /spl mu/m. The operating voltage of the surface-textured LED was almost the same as that of the typical planar LED since the ITO layer was in ohmic contact with the p-GaN. The experimental results indicate that the surface-textured ITO layer is suitable for fabricating high-brightness GaN-based light emitting devices.     

Nitride-based LEDs with n/sup -/-GaN current spreading layers

Nitride-based light-emitting diodes (LEDs) with n/sup -/-GaN current spreading layers were proposed and fabricated. With a 0.1-/spl mu/m-thick n/sup -/-GaN current spreading layer, it was found that the output power could be enhanced by 35% without increasing the operation voltage of the LEDs at 20 mA. In addition, implementing the n/sup -/-GaN current spreading layer also significantly improved the electrostatic discharge characteristics of nitride-based LEDs.     

High-power broadly tunable Ho/sup 3+/-doped silica fibre laser

Tuning of the 2.1 /spl mu/m Ho/sup 3+/-doped silica fibre laser is demonstrated for the first time. The /sup 5/I/sub 7//spl rarr//sup 5/I/sub 8/ transition provides tuning over 144 nm, from 2019 to 2163 nm, and a maximum pump-limited output power of 1.58 W at 2100 nm was produced.     

Collector-pedestal InGaAs/InP DHBTs fabricated in a single-growth, triple-implant process

This letter reports InP/In/sub 0.53/Ga/sub 0.47/As/InP double heterojunction bipolar transistors (DHBTs) employing an N/sup +/ subcollector and N/sup +/ collector pedestal-formed by blanket Fe and patterned Si ion implants, intended to reduce the extrinsic collector-base capacitance C/sub cb/ associated with the device footprint. The Fe implant is used to compensate Si within the upper 130 nm of the N/sup +/ subcollector that lies underneath the base ohmic contact, as well as compensate the /spl sim/1-7/spl times/10/sup -7/ C/cm/sup 2/ surface charge at the interface between the indium phosphide (InP) substrate and the N/sup $/collector drift layer. By implanting the subcollector, C/sub cb/ associated with the base interconnect pad is eliminated, and when combined with the Fe implant and selective Si pedestal implant, further reduces C/sub cb/ by creating a thick extrinsic collector region underneath the base contact. Unlike previous InP heterojunction bipolar transistor collector pedestal processes, multiple epitaxial growths are not required. The InP DHBTs here have simultaneous 352-GHz f/sub /spl tau// and 403-GHz f/sub max/. The dc current gain /spl beta//spl ap/38, BV/sub ceo/=6.0 V, BV/sub cbo/=5.4 V, and I/sub cbo/<50 pA at V/sub cb/=0.3 V.     

Wideband DHBTs using a graded carbon-doped InGaAs base

We report an InP/InGaAs/InP double heterojunction bipolar transistor (DHBT), fabricated using a mesa structure, exhibiting 282 GHz f/sub /spl tau// and 400 GHz f/sub max/. The DHBT employs a 30 nm InGaAs base with carbon doping graded from 8/spl middot/10/sup 19//cm/sup 3/ to 5/spl middot/10/sup 19//cm/sup 3/, an InP collector, and an InGaAs/InAlAs base-collector superlattice grade, with a total 217 nm collector depletion layer thickness. The low base sheet (580 /spl Omega/) and contact (<10 /spl Omega/-/spl mu/m/sup 2/) resistivities are in part responsible for the high f/sub max/ observed.     

Multi-watt, high efficiency, diffraction-limited Nd:YAG planar waveguide laser

An efficient, longitudinally diode-pumped, diffraction-limited, Nd:YAG double-clad planar waveguide laser was operated on four transitions of the Nd/sup 3+/ ion. Optimized output powers of 4.3, 3.5, and 2.7 W were obtained for absorbed pump powers of /spl sim/7 W, for the transitions at the lasing wavelengths of 1.064 /spl mu/m, 946 nm, and 1.3 /spl mu/m, respectively. Operation of the weak /sup 4/F/sub 3/2//spl rarr//sup 4/I/sub 5/2/ transition, lasing at 1.833 /spl mu/m, was demonstrated at an absorbed pump power threshold of 300 mW and an output power of 400 mW, with a nonoptimized output coupling. Diffraction-limited performance was obtained in both the guided and nonguided axes.