Ricky W. Chuang

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.

ZnO-on-GaN heterojunction light-emitting diode grown by vapor cooling condensation technique

The growth of ZnO-on-GaN heterostructures was implemented using the vapor cooling condensation system. The technique thus developed was employed to fabricate both the p-GaN∕n-ZnO:In (p-n) and p-GaN∕i-ZnO∕n-ZnO:In (p-i-n) heterojunction light-emitting diodes (LEDs). A rectifying diodelike behavior was clearly observed from both the p-n and p-i-n heterojunction LEDs, with the forward turn-on voltage of 3V and the reverse breakdown voltage of −15V determined for the p-n heterojunction LEDs, compared to 7 and −23V, respectively, for the p-i-n heterojunction LEDs. Based on the results of photoluminescence and electroluminescence studies conducted on these LED structures, the ZnO layer responsible for the peak emission wavelength of 385nm were also verified experimentally.

Silver-indium joints produced at low temperature for high temperature devices

A two-step fluxless bonding process adopted to produce high temperature silver-indium joints (80 wt% silver and 20 wt% indium) at relatively low process temperature of 206/spl deg/C has been developed. After annealing the joint continuously for 26 h at 145/spl deg/C, its melting temperature increases to 765-780/spl deg/C, as confirmed by a de-bonding test. The technique thus developed provides a viable alternative to packaging many high temperature devices running at 350/spl deg/C and above. The bonding quality of the Ag-In joints produced was examined using scanning acoustic microscopy. The joint cross-section was also studied using a scanning electron microscope equipped with an energy dispersive X-ray (EDX) spectroscope to find the local microstructure and composition. The results have shown that the joint is nearly void-free and uniform in thickness ranging from 7.2 to 7.8 /spl mu/m. The annealed sample joint, as determined by EDX, is mainly composed of AgIn/sub 2/, Ag/sub 2/In, and AuIn/sub 2/ grains embedded in an Ag-rich Ag-In alloy matrix. During joint formation, the intermetallic compound AgIn/sub 2/, in particular, prevents the indium layer from oxidation, and therefore, no flux is needed. In addition, low process temperatures help to reduce the thermal stresses developed in the bonded structure due to thermal expansion mismatch. Finally, reliability tests were conducted on three sets of annealed joints using a high temperature oven running continuously at 500/spl deg/C for 10, 100, and 1000 h each. Scanning acoustic microscopy (SAM) images on these samples confirmed that the joints have an excellent survivability in a high temperature environment.

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.

In0.23Ga0.77N/GaN MQW LEDs with a low temperature GaN cap layer

Abstract Mg-doped p-GaN epitaxial layers prepared at different temperatures were prepared and characterized. It was found that we could achieve a higher hole concentration and a rough surface by reducing the growth temperature down to 800 °C. In 0.23 Ga 0.77 N/GaN multiquantum well (MQW) light emitting diodes (LEDs) with such a low 800 °C-grown p-GaN cap layer were also fabricated. It was found that we could enhance the LED output intensity by more than 90% with the low 800 °C-grown p-GaN cap layer, as compared to the conventional high 1000 °C-grown p-GaN cap layer.

GaN-Based LEDs With a Chirped Multiquantum Barrier Structure

We report the fabrication of GaN-based blue light-emitting diodes (LEDs), which separately incorporate the three different electron blocking layers (EBLs), namely, a conventional AlGaN, a uniform multiquantum barrier (UMQB), and a chirped multiquantum barrier (CMQB). On the administration of 20 mA injection current, the corresponding LED output powers measured were 27.5, 27.2, and 25.4 mW for CMQB LED, UMQB LED, and LED, respectively, with a conventional AlGaN EBL. It was also found that the LED with CMQB EBL exhibited a significantly lower drooping effect and a smaller forward bias as compared with LEDs with a conventional AlGaN EBL and UMQB EBL.

Gallium nitride metal-semiconductor-metal photodetectors prepared on silicon substrates

Gallium nitride (GaN) ultraviolet metal-semiconductor-metal photodetectors (PDs) grown on Si substrates were demonstrated. The dark current of PDs fabricated on Si substrates was substantially smaller in magnitude compared to identical devices prepared on sapphire substrates. With an incident wavelength of 359nm, the maximum responsivities of the n−‐GaN MSM photodetectors with TiW and Ni∕Au contact electrodes were 0.187 and 0.0792A∕W, corresponding to quantum efficiencies of 64.7% and 27.4%, respectively. For a given bandwidth of 1kHz and a given bias of 5V, the corresponding noise equivalent powers of our n−‐GaN MSM photodetectors with TiW and Ni∕Au electrodes were 1.525×10−12 and 5.119×10−12W, respectively. Consequently, the values of detectivity (D*) determined for devices with TiW and Ni∕Au electrodes were then calculated to be 1.313×1012 and 3.914×1011cmHz0.5W−1, respectively.

High-temperature non-eutectic indium-tin joints fabricated by a fluxless process

Abstract A new alternative solder joint made of a non-eutectic indium–tin (In–Sn) multilayer composite deposited in high vacuum is reported. The unique features of this design are that it is fluxless, oxidation-free, and more importantly the fabricated joint achieves a re-melting temperature significantly higher than the bonding temperature. The In–Sn non-eutectic multilayer structure with a thin gold film evaporated as a cap layer has a predominantly Sn-rich matrix with a composition of 6 wt.% Au, 14 wt.% In and 80 wt.% Sn. The quality of the joints was first evaluated using a combination of X-ray microfocus and scanning acoustic microscopy techniques, and results have shown that the joints are nearly void-free. In addition, analysis by scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray (EDX) spectroscope performed on the joint cross-section clearly indicated a uniform joint thickness of 7.6 μm, while AuIn2 grains embedded in a heavily Sn-rich matrix were clearly detected. Lastly, we set out to determine the re-melting temperatures of these fabricated joints. Temperature values ranging from 175 to 190 °C were found, higher than the bonding temperature of 150 °C. The results clearly show that the joint composition is heavily Sn-rich, leading to the high re-melting temperature The increase in re-melting temperature opens up the post-processing temperature window of devices in manufacturing processes. No flux is needed during the bonding process, making it particularly useful to packaging devices for which the use of flux is strictly prohibited.

Characterization of ZnO metal–semiconductor–metal ultraviolet photodiodes with palladium contact electrodes

ZnO metal–semiconductor–metal (MSM) photodiodes with palladium (Pd) contact electrodes were fabricated. It was found that the barrier height at the Pd/ZnO interface was 0.701 eV. With an incident wavelength of 370 nm and 1 V applied bias, it was found that the maximum responsivity of the Pd/ZnO/Pd MSM photodiodes was 0.051 A/W, which corresponds to a quantum efficiency of 11.4%. For a given bandwidth of 100 Hz and 1 V applied bias, we found that the noise equivalent power and the corresponding detectivity D ∗ were 1.13 × 10 −12 W and 6.25 × 10 11 cmHz 0.5 W −1 , respectively.

Fluxless non-eutectic joints fabricated using gold-tin multilayer composite

Fluxless bonding processes using two different noneutectic gold-tin (Au-Sn) multilayer composites to fabricate high quality solder joints have been successfully achieved. In contrast to the well-known eutectic solders of 80 wt. % Au and 20 wt. % Sn commonly selected by the packaging industry, we have adopted a substantially cost-effective strategy by purposely designing and constructing our solder joints to be predominantly tin-rich instead, rather than relying extensively on gold as a major ingredient. In research two designs were implemented; one with compositions of 80 at. % Sn (70.54 wt. % Sn) and 20 at. % Au (29.46 wt. % Au), and another with 95 at. % Sn (91.82 wt. % Sn) and 5 at. % Au (8.18 wt. % Au). The bonding process temperatures chosen for constructing 80Sn-20Au and 95Sn-5Au joints are 285/spl deg/C and 225/spl deg/C, respectively. Once produced, both types of joints were examined using the combination of X-ray microfocus imaging and scanning acoustic microscopy (SAM) to confirm the bonding quality and the results obtained are nearly void-free. To study the microstructure and composition of the samples the scanning electron microscopy (SEM) equipped with energy dispersive X-ray (EDX) detector were conducted on the joint cross sections and the solder thickness of 3.9 and 2.1/spl mu/m were uniformly identified throughout from 80Sn-20Au and 95Sn-5Au joints, respectively. Furthermore, the EDX data obtained have consistently shown that a mixture of AuSn, AuSn/sub 2/, and AuSn/sub 4/ intermetallics were spotted from the 80Sn-20Au sample joints, while AuSn/sub 2/ and AuSn/sub 4/ embedded in /spl beta/-Sn matrix were discovered from 95Sn-5Au specimens. In addition, the shear tests conducted on the samples unequivocally suggest the shear strength of each joint measured is actually greater than the die itself. Finally, the re-melting temperatures of 80Sn-20Au and 95Sn-5Au solder specimens ranging from 275 to 281/spl deg/C and 214 to 220/spl deg/C, respectively, were also experimentally determined.