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.     

Thermally stable Ge/Ag/Ni Ohmic contact for InAlAs/InGaAs/InP HEMTs

Excellent annealed ohmic contacts based on Ge/Ag/Ni metallization have been realized in a temperature range between 385 and 500/spl deg/C, with a minimum contact resistance of 0.06 /spl Omega//spl middot/mm and a specific contact resistivity of 2.62 /spl times/10/sup -7/ /spl Omega//spl middot/cm/sup 2/ obtained at an annealing temperature of 425/spl deg/C for 60 s in a rapid thermal annealing (RTA) system. Thermal storage tests at temperatures of 215 and 250/spl deg/C in a nitrogen ambient showed that the Ge/Ag/Ni based ohmic contacts with an overlay of Ti/Pt/Au had far superior thermal stabilities than the conventional annealed AuGe/Ni ohmic contacts for InAlAs/InGaAs high electron mobility transistors (HEMTs). During the storage test at 215/spl deg/C, the ohmic contacts showed no degradation after 200 h. At 250/spl deg/C, the contact resistance value of the Ge/Ag/Ni ohmic contact increased only to a value of 0.1 /spl Omega//spl middot/mm over a 250-h period. Depletion-mode HEMTs (D-HEMTs) with a gate length of 0.2 /spl mu/m fabricated using Ge/Ag/Ni ohmic contacts with an overlay of Ti/Pt/Au demonstrated excellent dc and RF characteristics.     

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.     

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 LEDs with an SPS tunneling contact Layer and an ITO transparent contact

The indium-tin-oxide [ITO(80 nm)] and Ni(5 nm)-Au(10 nm) films were separately deposited on glass substrates, p-GaN layers, n/sup +/-InGaN-GaN short-period-superlattice (SPS) structures, and nitride-based light-emitting diodes (LEDs). It was found that ITO on n/sup +/-SPS structure could provide us an extremely high transparency (i.e., 93.2% at 465 nm) and also a reasonably small specific contact resistance of 1.6/spl times/10/sup -3//spl Omega//spl middot/cm/sup 2/. Although the forward voltage which corresponds to 20-mA operating current for LED with ITO on n/sup +/-SPS upper contact was slightly higher than that of the LED with Ni-Au on n/sup +/-SPS upper contact, a 30% higher output intensity could still be achieved by using ITO on n/sup +/-SPS upper contact. Moreover, the output power of packaged LED with ITO was about twice as large as that of the other conventional Ni-Au LEDs.     

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.     

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.     

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.     

Suppression of high-resistance phases of Ni silicide by Se passivation of Si(100)

High-resistance phases of Ni-rich Ni silicide are formed on Si(100) below 400/spl deg/C, while high-resistance phases of Si-rich Ni silicide are formed above 600/spl deg/C. The desired low-resistance NiSi is formed between 400/spl deg/C and 600/spl deg/C. In this paper, the authors report the suppression of high-resistance phases of Ni silicide by passivating the Si(100) surface with a monolayer of Se. A 500-/spl Aring/ Ni on n-type low 10/sup 15/ cm/sup -3/ doped Si(100) wafers, passivated with Se, shows a sheet resistance of /spl sim/2.55 /spl Omega//square upon annealing between 200/spl deg/C and 500/spl deg/C, while the sheet resistance of the 500-/spl Aring/ Ni on identical wafers without Se-passivation jumps to /spl sim/7.92 /spl Omega//square between 300/spl deg/C and 350/spl deg/C. Between 600/spl deg/C and 700/spl deg/C, the sheet resistance of the Se-passivated samples is /spl sim/ 10% lower than that of the control samples. Transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy all confirm that the suppression of high-resistance Ni silicides below 500/spl deg/C is attributed to the suppression of silicidation and above 600/spl deg/C to the delay in Si-rich Ni silicide formation at the Ni/Se-passivated Si(100) interface.     

Improvement of the electrical performance of near UV GaN-based light-emitting diodes using Ni nanodots

We report on the improvement of the electrical characteristics of GaN-based light-emitting diodes (LEDs) fabricated with Ag (1 nm)/indium tin oxide (ITO) (250 nm) p-type contacts by using Ni nanodots. As-deposited Ag/ITO contacts with and without the Ni nanodots give non-linear electrical behaviour. However, annealing the samples at 530–630 °C for 1 min in air results in ohmic behaviour. The reverse leakage current characteristics of near UV LEDs are much improved when the Ni nanodots are used. The output power (at 20 mA) of LEDs fabricated with the Ni nanodots is enhanced by a factor of 1.34 as compared with that of LEDs made without the Ni nanodots.     

Formation of low-resistance and transparent indium tin oxide ohmic contact for high-brightness GaN-based light-emitting diodes using a Sn–Ag interlayer

We report on the formation of low-resistance and highly transparent indium tin oxide (ITO) ohmic contacts to p-GaN using a Sn–Ag alloy interlayer. Although the as-deposited Sn–Ag(6 nm)/ITO(200 nm) contacts show non-ohmic behaviors, the scheme becomes ohmic with specific contact resistance of 4.72×10⁻⁴ Ω cm² and produce transmittance of ∼91% at wavelength of 460 nm when annealed at 530 °C. Blue light-emitting diodes (LEDs) fabricated with the Sn–Ag/ITO contacts give forward-bias voltage of 3.31 V at injection current of 20 mA. LEDs with the Sn–Ag/ITO contacts show the improvement of the output power by 62% (at 20 mA) compared with LEDs with Ni/Au contacts.     

Temperature-dependent electroluminescence of AlGaN-based UV LEDs

The electrical and optical characteristics of AlGaN-based ultraviolet (UV) light-emitting diodes (LEDs) (265-365 nm) at elevated temperatures (25/spl deg/C-175/spl deg/C) were investigated, and compared to those of InGaN-based visible LEDs (400-465 nm). Strong carrier localization and localized-state emission were retained in the InGaN LEDs up to 175/spl deg/C, leading to temperature-independent emission intensity at low-energy tails. The deep-UV LEDs, however, showed dominant band-edge emission, much smaller alloy broadening, and weaker localization effects. The optical power of the UV LEDs decreased much more rapidly with increasing temperature. The characteristic temperature was in the range of 31-73 K, and decreased with increasing Al content in the active region. These findings implicate the lack of localization effects in AlGaN alloys as one of the causal factors in the poor thermal performance of the UV LEDs and suggest that increasing carrier-confining potentials will provide a critical means to improve their radiative efficiencies.     

Nitride-based flip-chip LEDs with transparent Ohmic contacts and reflective mirrors

Nitride-based flip-chip light-emitting diodes (LEDs) with various transparent ohmic contacts and reflective mirrors were fabricated. At 470 nm, it was found that Ni could provide 92% transmittance while Ag could provide 92.4% reflectively. It was also found that the 20-mA forward voltages measured from LEDs with Ni+Ag, Ni+Al, and Ni+Pt were 3.15, 3.29, and 3.18 V while the output powers were 16, 13.3, and 11.6 mW, respectively. Furthermore, it was found that lifetimes of the fabricated flip-chip LEDs were good.     

Hafnium silicate nanocrystal memory using sol-gel-spin-coating method

The authors fabricate the hafnium silicate nanocrystal memory for the first time using a very simple sol-gel-spin-coating method and 900 /spl deg/C 1-min rapid thermal annealing (RTA). From the TEM identification, the nanocrystals are formed as the charge trapping layer after 900 /spl deg/C 1-min RTA and the size is about 5 nm. They demonstrate the composition of nanocrystal is hafnium silicate from the X-ray-photoelectron-spectroscopy analysis. They verify the electric properties in terms of program/erase (P/E) speed, charge retention, and endurance. The sol-gel device exhibits the long charge retention time of 10/sup 4/ s with only 6% charge loss, and good endurance performance for P/E cycles up to 10/sup 5/.     

Optimization of AuGe-Ni-Au ohmic contacts for GaAs MOSFETs

GaAs-based metal-oxide-semiconductor field-effect transistors (MOSFETs) are promising devices for high-speed and high-power applications. One important factor influencing the performance of a GaAs MOSFET is the characteristics of ohmic contacts at the drain and source terminals. In this paper, AuGe-Ni-Au metal contacts fabricated on a thin (930 /spl Aring/) and lightly doped (4/spl times/10/sup 17/ cm/sup -3/) n-type GaAs MOSFET channel layer were studied. The effects of controllable processing factors such as the AuGe thickness, the Ni/AuGe thickness ratio, alloy temperature, and alloy time to the characteristics of the ohmic contacts were analyzed. Contact qualities including specific contact resistance, contact uniformity, and surface morphology were optimized by controlling these processing factors. Using the optimized process conditions, a specific contact resistance of 5.6/spl times/10/sup -6/ /spl Omega//spl middot/cm/sup 2/ was achieved. The deviation of contact resistance and surface roughness were improved to 1.5% and 84 /spl Aring/, respectively. Using the improved ohmic contacts, high-performance GaAs MOSFETs (2 /spl mu/m/spl times/100 /spl mu/m) with a large drain current density (350 mA/mm) and a high transconductance (90 mS/mm) were fabricated.     

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.     

Improving the electrical integrity of Cu-CoSi/sub 2/ contacted n/sup +/p junction diodes using nitrogen-incorporated Ta films as a diffusion barrier

The study on improving the electrical integrity of Cu-CoSi/sub 2/ contacted-junction diodes by using the reactively sputtered TaN/sub x/ as a diffusion barrier is presented in this paper. In this study, the Cu (300 nm)-CoSi/sub 2/ (50 nm)/n/sup +/p junction diodes were intact with respect to metallurgical reaction up to a 350/spl deg/C thermal annealing while the electrical characteristics started to degrade after annealing at 300/spl deg/C in N/sub 2/ ambient for 30 min. With the addition of a 50-nm-thick TaN/sub x/ diffusion barrier between Cu and CoSi/sub 2/, the junction diodes were able to sustain annealing up to 600/spl deg/C without losing the basic integrity of the device characteristics, and no metallurgical reaction could be observed even after a 750/spl deg/C annealing in a furnace. In addition, the structure of TaN/sub x/ layers deposited on CoSi/sub 2/ at various nitrogen flow rates has been investigated. The TaN/sub x/ film with small grain sizes deposited at nitrogen flow ratios exceeding 10% shows better barrier capability against Cu diffusion than the others.     

Nonalloyed ohmic contacts in AlGaN/GaN HEMTs by ion implantation with reduced activation annealing temperature

This letter reports AlGaN/GaN high-electron mobility transistors with capless activation annealing of implanted Si for nonalloyed ohmic contacts. Source and drain areas were implanted with an Si dose of 1/spl times/10/sup 16/ cm/sup -2/ and were activated at /spl sim/1260/spl deg/C in a metal-organic chemical vapor deposition system in ammonia and nitrogen at atmospheric pressure. Nonalloyed ohmic contacts to ion-implanted devices showed a contact resistance of 0.96 /spl Omega//spl middot/mm to the channel. An output power density of 5 W/mm was measured at 4 GHz, with 58% power-added efficiency and a gain of 11.7 dB at a drain bias of 30 V.     

The origins of leaky characteristics of Schottky diodes on p-GaN

The possible origins of the leaky characteristics of a Schottky barrier on p-GaN have been investigated. The as-grown samples did not show any electrical activity using Hall measurements. Ni diodes made on as-activated samples, either at 950/spl deg/C for 5 s or at 750/spl deg/C for 5 min exhibited quasiohmic behavior. Upon sequential etching of the sample to remove a surface layer of 150 /spl Aring/, 1200 /spl Aring/, and 5000 /spl Aring/ from the sample, the I-V behavior became rectifying. I-V-T measurements showed that the slopes of the lnI-V curves were independent of the temperature, indicative of a prominent component of carrier tunneling across the Schottky junction. C-V measurements at each etch-depth indicated a decreasing acceptor concentration from the surface. The highly doped (>1.7 /spl times/ 10/sup 19/ cm/sup -3/) and defective surface region (within the top 150 /spl Aring/ from surface) rendered the as-activated Schottky diodes quasiohmic in their I-V characteristics. The leaky I-V characteristics, often reported in the literature, were likely to originated from the surface layer, which gives rise to carrier tunneling across the Schottky barrier. This highly doped/defective surface region, however, can play an important role in ohmic contact formation on p-GaN.     

GaN MSM UV photodetectors with titanium tungsten transparent electrodes

GaN metal-semiconductor-metal (MSM) ultraviolet photodetectors with titanium tungsten (TiW) transparent electrodes were fabricated and characterized. It was found that the 10-nm-thick TiW film deposited with a 300-W RF power can still provide a reasonably high transmittance of 75.1% at 300 nm, a low resistivity of 1.7/spl times/10/sup -3/ /spl Omega//spl middot/cm and an effective Schottky barrier height of 0.773 eV on u-GaN. We also achieved a peak responsivity of 0.192 A/W and a quantum efficiency of 66.4% from the GaN ultraviolet MSM photodetector with TiW electrodes. With a 3-V applied bias, it was found that minimum noise equivalent power and maximum D/sup */ of our detector were 1.987/spl times/10/sup -10/ W and 6.365/spl times/10/sup 9/ cmHz/sup 0.5/W/sup -1/, respectively.