Electrical and thermal stability of Ag ohmic contacts for GaN-based flip-chip light-emitting diodes by using an AgAl alloy capping layer

We have investigated Ag(200 nm)/AgAl(100 nm) ohmic contacts to p-type GaN for near-UV (405 nm) flip-chip light-emitting diodes (LEDs). It is shown that the use of an AgAl alloy capping layer (with 8 at% Al) results in better electrical and optical properties as compared to single Ag contacts when annealed at 430 °C. For example, Ag/AgAl (8 at% Al) contacts give specific contact resistance of 4.6×10^–4 Ω cm² and reflectance of 90% at a wavelength of 405 nm. However, use of an AgAl (with 50 at% Al) layer is not effective. LEDs fabricated with the Ag/AgAl (8 at% Al) reflectors produce higher light output as compared with the ones with single Ag reflectors. Ohmic mechanisms of the Ag/AgAl (8 at% Al) contacts are described and discussed.     

Sn addition on the tensile properties of high temperature Zn–4Al–3Mg solder alloys

The Zn–4Al–3Mg based solder alloy is a promising candidate to replace the conventional Pb–5Sn alloy in high-temperature electronic packaging. In this study, the tensile properties of Zn–4Al–3Mg–xSn alloys (x = 0, 6.8 and 13.2 wt.%) at high temperatures (e.g., 100 °C, and 200 °C) were investigated. It was found that the uniaxial tensile strength (UTS) of Zn–4Al–3Mg–xSn solder alloys all decrease monotonously with the increment of temperature. The elongation ratio at 100 °C is superior to that at room temperature whereas follows a significant drop at 200 °C. The microstructure observations show that a typical brittle fracture of Zn–4Al–3Mg alloy occurs at room temperature and 200 °C under normal tension, whereas a ductile fracture is found at 100 °C. The 6.8 wt.% Sn addition in Zn–4Al–3Mg alloy causes a dramatic decrease of yield strength, and a slight deterioration of the ductility.     

Iridium-based semi-transparent current spreading layer on short-period-superlattice (SPS) tunneling contact of InGaN/GaN LEDs

Iridium-containing and Ni(4 nm)/Au(6 nm) films were evaporated separately on the n⁺-InGaN–GaN short-period-superlattice (SPS) structure of light-emitting diodes (LEDs). The collective deposition of iridium and other metals as an ohmic contact induces the formation of highly transparent IrO₂, which helps to enhance the light output and decrease the series resistance of LEDs. By comparing different metal films used as current spreading contact layer, Ir/Ni film annealed at 500 °C for 20 min in O₂ ambient renders devices with lowest turn-on voltage at 20 mA and highest luminous intensity. Moreover, we also analyzed films using atomic force microscopy (AFM) with an emphasis on studying how the surface quality of Ir/Ni and Ni/Au films influences the current spreading and luminosity of LEDs.     

Study on the thermal stability of Ga-doped ZnO thin film: A transparent conductive layer for dye-sensitized TiO2 nanoparticles based solar cells

Generally, optoelectronic devices are fabricated at a high temperature. So the stability of properties for transparent conductive oxide (TCO) films at such a high temperature must be excellent. In the paper, we investigated the thermal stability of Ga-doped ZnO (GZO) transparent conductive films which were heated in air at a high temperature up to 500 °C for 30 min. After heating in air at 500 °C for 30 min, the lowest sheet resistance value for the GZO film grown at 300 °C increased from 5.5 Ω/sq to 8.3 Ω/sq, which is lower than 10 Ω/sq. The average transmittance in the visible light of all the GZO films is over 90%, and the highest transmittance is as high as 96%, which is not influenced by heating. However, the transmittance in the near-infrared (NIR) region for the GZO film grown at 350 °C increases significantly after heating. And the grain size of the GZO film grown at 350 °C after annealing at 500 °C for 30 min is the biggest. Then dye-sensitized TiO2 NPs based solar cells were fabricated on the GZO film grown at 350 °C (which exhibits the highest transmittance in NIR region after heating at 500 °C for 30 min) and 300 °C (which exhibits the lowest sheet resistance after heating at 500 °C for 30 min). The dye-sensitized solar cell (DSSC) fabricated on the GZO film grown at 350 °C exhibits superior conversion efficiency. Therefore, transparent conductive glass applying in DSSCs must have a low sheet resistance, a high transmittance in the ultraviolet–visible–infrared region and an excellent surface microstructure.     

Investigation of n-ohmic contact of vertical GaN-based light-emitting diodes on graphite substrate with Ag-In bonding

Vertical light-emitting diodes (VLEDs) were successfully transferred from a GaN-based sapphire substrate to a graphite substrate by using low-temperature and cost-effective Ag-In bonding, followed by the removal of the sapphire substrate using a laser lift-off (LLO) technique. One reason for the high thermal stability of the AgIn bonding compounds is that both the bonding metals and Cr/Au n-ohmic contact metal are capable of surviving annealing temperatures in excess of 600 °C. Therefore, the annealing of n-ohmic contact was performed at temperatures of 400 °C and 500 °C for 1 min in ambient air by using the rapid thermal annealing (RTA) process. The performance of the n-ohmic contact metal in VLEDs on a graphite substrate was investigated in this study. As a result, the final fabricated VLEDs (chip size: 1000 µm×1000 µm) demonstrated excellent performance with an average output power of 538.64 mW and a low operating voltage of 3.21 V at 350 mA, which corresponds to an enhancement of 9.3% in the light output power and a reduction of 1.8% in the forward voltage compared to that without any n-ohmic contact treatment. This points to a high level of thermal stability and cost-effective Ag-In bonding, which is promising for application to VLED fabrication.     

Influence of electrical field dependent depletion at metal–polymer junctions on resistive switching of poly(N-vinylcarbazole) (PVK)-based memory devices

Polymer memory devices using Au nanoparticles (Au NPs) incorporated poly(N-vinylcarbazole) (PVK) as the active layer and Al films as the electrodes are investigated. The Al/PVK:Au NPs/Al devices exhibit electrical bistability in the I–V characteristics and show a conductance difference ratio between the high-resistance state (HRS) and low-resistance state (LRS) by a factor of 10⁵. Furthermore, the Au nanoparticle/PVK hybrid memory device can be programmed and exhibits excellent thermal stability up to 154 °C in ambient atmosphere. The current conduction is dominated by Schottky emission at HRS and exhibits Ohmic behavior at LRS. The dependence of the current conduction on temperature reveals the connection between the conduction character and the energy-band offsets at the metal (Al or Au)–PVK junctions. In addition, the resistive switching is correlated with the width of depletion region in PVK, which varies with the change of hole carrier concentration upon applying electrical field.     

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.     

Effect of annealing on the microstructure and bonding interface properties of Ag–2Pd alloy wire

This study investigated the mechanical and electrical properties of Ag–2Pd wire after thermal annealing. The thermal stability of the tested wire was examined by separately imposing static annealing at 275 °C, 325 °C and 375 °C in a vacuum environment. It was found that annealing the Ag–2Pd wire at 275 °C promoted the formation of a fully annealed structure with equiaxed grains. Annealing Ag–2Pd wire had a shorter heat affect zone (HAZ) length than those of conventional wire, and offered outstanding mechanical properties. A long-term electrical test found Ag₃(Pd)Al and Ag₂(Pd)Al compounds between the Ag–Pd ball and Al pad. These results confirmed the high-reliability properties of annealed Ag–2Pd wires for the wire bonding process.     

Thermal stability of back side metallization multilayer for power device application

Metallization multilayers on the back side of a power device were focused in this study. Si wafers coated with high melting point metals were exposed at 300 °C for 300 h to investigate diffusion condition of the metallization layer. We developed and examined the thermal stability of die bonding material (Au paste) including sub–micrometer–sized Au particles. Auger electron spectroscopy was applied to observe the atomic composition of the multilayers in depth direction after the high temperature aging. Surface morphology was observed using optical microscope and scanning electron microscope. While atomic composition on Ti/Au changed drastically after the high temperature aging, other multilayers maintained their metallization composition. However, the surface morphology was slightly changed on Ti/Ru/Au, W/Au, and Ta/Au. Bond strength on the Ti/Pt/Au kept over 40 MPa with unified bonding layer after exposing at 300 °C for 1000 h.     

Alloyed Si/Al-based ohmic contacts to AlGaN/GaN nitride heterostructures

For the first time in Russia, the Si/Al/Ti/Au alloyed contact composition is investigated for the formation of ohmic contacts to AlGaN/GaN heterostructures using thermal annealing. The obtained results are compared with those for conventional Ti/Al/Ni/Au ohmic contacts. Use of the composition under investigation makes it possible to decrease the annealing temperature to 675–700°C, which results in improvement in the morphology of alloyed ohmic contacts in comparison with conventional contacts. The value of the contact resistance using the Si/Al-based composition to the AlGaN/GaN heterostructure is obtained in relation to the temperature and annealing duration. It is shown that no qualitative change in the resistance occurs at an annealing duration of several minutes in the temperature range of 700–750°C. In the temperature range of 675–700°C, there is an asymptotic decrease in the contact resistance with increasing annealing duration. The smallest value of the contact resistance amounts to 0.41 Ω mm.     

Effect of molybdenum electrode annealing on the interface properties between metal and pentacene

Contact resistance between molybdenum (Mo) electrode and pentacene was studied with transmission line method (TLM). The Mo electrodes were annealed at 200 °C, 400 °C, 600 °C and 800 °C for 1 h and pentacene layer of 300 Å thickness was vacuum deposited on patterned Mo to form Mo–pentacene contact. Current–voltage measurement for Mo–pentacene contact showed linear relationship and it was confirmed that ohmic contact was formed. XRD and AFM measurements showed that Mo could be crystallized at annealing temperatures above 600 °C. 800 °C annealed Mo showed larger grains and work function was increased from 4.60 eV to 4.80 eV due to the decrease in defect density. The contact resistance was reduced down to 11.2 MΩ cm from 37.8 MΩ cm of as-deposited Mo. Also the pentacene film deposited on annealed Mo was denser with better crystallinity. Bottom contact organic field-effect transistor made with 800 °C annealed Mo showed better performance than as deposited Mo.     

Development of chip-on-flex bonding using Sn-based bumps and non-conductive adhesive

We developed a reliable and low cost chip-on-flex (COF) bonding technique using Sn-based bumps and a non-conductive adhesive (NCA). Two types of bump materials were used for the bonding process: Sn bumps and Sn–Ag bumps. The bonding process was performed at 180 °C for 10 s using a thermo-compression bonder after dispensing the NCA. Sn-based bumps were easily deformed to contact Cu pads during the bonding process. A thin layer of Cu₆Sn₅ intermetallic compound was observed at the interface between Sn-based bumps and Cu pads. After bonding, electrical measurements showed that all COF joints had very low contact resistance, and there were no failed joints. To evaluate the reliability of COF joints, high temperature storage tests (150 °C, 1000 h), thermal cycling tests (−25 °C/+125 °C, 1000 cycles) and temperature and humidity tests (85 °C/85% RH, 1000 h) were performed. Although contact resistance was slightly increased after the reliability test, all COF joints passed failure criteria. Therefore, the metallurgical bond resulted in good contact and improved the reliability of the joints.     

Characterization of TiAl alloy films for potential application in MEMS bimorph actuators

In this letter, we demonstrate the feasibility of applying TiAl alloy film for the fabrication of bimorph actuators. The TiAl alloy films were prepared by thermal annealing at 400°C of Ti/Al multilayers, which were deposited by DC magnetron sputtering from Ti and Al targets. The microstructure and surface morphology of TiAl alloy films were analyzed by X-ray diffraction and scanning electron microscopy, which showed that TiAl alloy film is formed in the mixed phases of TiAl₃ and Ti₃₆Al₆₄, depending on the deposition conditions. The resistivity of TiAl film is about 9 μΩ cm, and the stress is about 200 MPa. Our nano-indentation measurements showed that the Young's modulus and hardness of TiAl alloy films are 175 and 6.5 GPa, respectively, which are larger than that of Al and comparable to Si. We have successfully fabricated the bimorph actuators based on the TiAl alloy films and our test cantilevers up to 500 μm long showed very straight with tip bending as small as ±5 μm, indicating negligible stress gradient in TiAl film. Our preliminary testing results indicated that TiAl alloy film has potential application for bimorph actuators.     

Optimizing the fabrication process for high performance graphene field effect transistors

As an emerging material, graphene has attracted vast interest in solid-state physics, materials science, nanoelectronics and bioscience. Graphene has zero bandgap with its valence and conduction bands are cone-shaped and meet at the K points of the Brillouin zone. Due to its high intrinsic carrier mobility, large saturation velocity, and high on state current density, graphene is also considered as a promising candidate for high-frequency devices. To improve the reliability of graphene FETs, which include shifting the Dirac point voltage toward zero, increasing the channel mobility and decreasing the source/drain contact resistance, we optimized the device fabrication process. For CVD grown graphene, the film transfer and the device fabrication processes may produce interfacial states between graphene and the substrate and make graphene p or n-type, which shift the fermi level far away from the Dirac point. We have found that after graphene film transfer, an annealing process at 400 °C under N₂ ambient will shift Dirac point toward zero gate voltage. Ti/Au, Ni, and Ti/Pd/Au source/drain structures have been studied to minimize the contact resistance. According to the measured data, Ti/Pd/Au structure gives the lowest contact resistance (～500 ohm μm). By controlling the process of graphene growth, transfer and device fabrication, we have achieved graphene FETs with a field effective mobility of 16,000 cm²/V s after subtraction of contact resistance. The contact resistivity was estimated in the range of 1.1 × 10^?6 Ω cm² to 8.8 × 10^?6 Ω cm², which is close to state of the art III–V technology. The maximum transconductance was found to be 280 mS/mm at V_D = 0.5 V, which is the highest value among CVD graphene FETs published to date.     

Growth and characterization of indium oxide diodes prepared by reactive magnetron sputtering

The electrical properties of Cr/Pt/Au and Ni/Au ohmic contacts with unintentionally doped In₂O₃ (U-In₂O₃) film and zinc-doped In₂O₃ (In₂O₃:Zn) prepared by reactive magnetron sputtering deposition are described. The lowest specific contact resistance of Cr/Pt/Au and Ni/Au is 2.94 × 10⁻⁶ and 1.49 × 10⁻² Ω-cm², respectively, as determined by the transmission line model (TLM) after heat treatment at 300 °C by thermal annealing for 10 min in nitrogen ambient. The indium oxide diodes have an ideality factor of 1.1 and a soft breakdown voltage of 5 V. The reverse leakage current prior to breakdown is around 10⁻⁵ A.     

Microstructure,electric flame-off (EFO) characteristics and tensile properties of silver–lanthanum alloy wire

Silver has potential for application in the electronic packaging industry because of its great electrical and thermal properties and lower price compared to that of gold. Silver oxidizes easily, so doping lanthanum to form Ag–La alloy improves its anti-oxidation capacity. In this study, the microstructure, tensile properties, electronic flame-off (EFO) characteristics, and fusing current of Ag–La alloy wire (φ = 20 μm) are studied. Samples annealed at three temperatures (325 °C, 375 °C, and 425 °C) are analyzed. According to the experimental results, after annealing at 425 °C, Ag–La alloy wire recrystallized, giving it a tensile strength similar to that of pure silver wire and a uniform structure. Doping lanthanum reduced the diameter of free air balls (FABs) in the EFO process. The fusing current of Ag–La wire was about 0.45 A, and the grains of Ag–La wire grew to the size of the wire diameter when a 0.4 A current (90% fusing current) was applied for a long time. Ag–La alloy wire can be used in the electronic packaging industry.     

GaN-based HEMTs tested under high temperature storage test

In this work, the impact of 1000 h thermal storage test at 325 °C on the performance of gallium nitride high electron mobility transistors grown on Si substrates (GaN-on-Si HEMTs) is investigated. The extensive DC- and pulse-characterization performed before, during and after the stress did not reveal degradation on the channel conduction properties as well as formation of additional trapping states. The failure investigation has shown that only the gate and drain leakage currents were strongly affected by the high temperature storage test. The physical failure analysis revealed a Au inter-diffusion phenomenon with Ni at the gate level, resulting in a worsening of the gate–AlGaN interface. It is speculated that this phenomenon is at the origin of the gate and drain leakage current increasing.     

Study on thermal performance of high power LED employing aluminum filled epoxy composite as thermal interface material

This paper elucidates the thermal behavior of an LED employing metal filled polymer matrix as thermal interface material (TIM) for an enhanced heat dissipation characteristic. Highly thermal conductive aluminum (Al) particles were incorporated in bisphenol A diglycidylether (DGEBA) epoxy matrix to study the effect of filler to polymer ratio on the thermal performance of high power LEDs. The curing behavior of DGEBA was studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The dispersion nature of the Al fillers in polymer matrix was verified with Field Emission Scanning Electron Microscope (FESEM). The thermal performance of synthesized Al filled polymer composite as TIM was tested with an LED employing thermal transient measurement technique. Comparing the filler to polymer ratio, the rise in junction temperature for 60 wt% Al filled composite was higher by 11.1 °C than 50 wt% Al filled composite at cured state. Observed also from the structure function analysis that the total thermal resistance was 10.96 K/W higher for 60 wt% Al filled composite compared to 50 wt% Al filled composite. On the other hand, a significant rise of 9.5 °C in the junction temperature between cured and uncured samples of 50 wt% Al filled polymer TIM was observed and hence the importance of curing process of metal filled polymer composite for effective heat dissipation is discussed extensively in this work.     

Integrity of copper–hafnium,hafnium nitride and multilayered amorphous-like hafnium nitride metallization under various thickness

The barrier properties and failure mechanism of sputtered Hf, HfN and multilayered HfN/HfN thin films were studied for the application as a Cu diffusion barrier in metallization schemes. The barrier capability and thermal stability of Hf, HfN and HfN/HfN films were determined using X-ray diffraction (XRD), leakage current density, sheet resistance (R_s) and cross-sectional transmission electron microscopy (XTEM). The thin multi-amorphous-like HfN thin film (10 nm) possesses the best barrier capability than Hf (50 nm) and amorphous-like HfN (50 nm). Nitrogen incorporated Hf films possess better barrier performance than sputtered Hf films. The Cu/Hf/n⁺–p junction diodes with the Hf barrier of 50 nm thickness were able to sustain a 30-min thermal annealing at temperature up to 500 °C. Copper silicide forms after annealing. The Hf barrier fails due to the reaction of Cu and the Hf barrier, in which Cu atoms penetrate into the Si substrate after annealing at high temperature. The thermal stabilities of Cu/Hf/n⁺–p junction diodes are enhanced by nitrogen incorporation. Nitrogen incorporated Hf (HfN, 50 nm) diffusion barriers retained the integrity of junction diodes up to 550 °C with lower leakage current densities. Multilayered amorphous-like HfN (10 nm) barriers also retained the integrity of junction diodes up to 550 °C even if the thickness is thin. No copper–hafnium and copper silicide compounds are found. Nitrogen incorporated hafnium diffusion barrier can suppress the formation of copper–hafnium compounds and copper penetration, and thus improve the thermal stability of barrier layer. Diffusion resistance of nitrogen-incorporated Hf barrier is more effective. In all characterization techniques, nitrogen in the film, inducing the microstructure variation appears to play an important role in thermal stability and resistance against Cu diffusion. Amorphousization effects of nitrogen variation are believed to be capable of lengthening grain structures to alleviate Cu diffusion effectively. In addition, a thin multilayered amorphous-like HfN film not only has lengthening grain structures to alleviate Cu diffusion, but block and discontinue fast diffusion paths as well. Hence, a thin multilayered amorphous-like HfN/HfN barrier shows the excellent barrier property to suppress the formation of high resistance η′-(Cu,Si) compound phase to 700 °C.     

Source/drain metallization effects on the specific contact resistance of indium tin zinc oxide thin film transistors

We report on the specific contact resistance of interfaces between thin amorphous semiconductor Indium Tin Zinc Oxide (ITZO) channel layers and different source/drain (S/D) electrodes (Al, ITO, and Ni) in amorphous oxide thin film transistors (TFTs) at different channel lengths using a transmission line model. All the contacts showed linear current–voltage characteristics. The effects of different channel lengths (200–800 μm, step 200 μm) and the contact resistance on the performance of TFT devices are discussed in this work. The Al/ITZO TFT samples with the channel length of 200 μm showed metallic behavior with a linear drain current-gate voltage (I_D–V_G) curve due to the formation of a conducting channel layer. The specific contact resistance (ρ_C) at the source or drain contact decreases as the gate voltage is increased from 0 to 10 V. The devices fabricated with Ni S/D electrodes show the best TFT characteristics such as highest field effect mobility (16.09 cm²/V·s), ON/OFF current ratio (3.27×10⁶), lowest sub-threshold slope (0.10 V/dec) and specific contact resistance (8.62 Ω·cm² at V_G=0 V). This is found that the interfacial reaction between Al and a-ITZO semiconducting layer lead to the negative shift of threshold voltage. There is a trend that the specific contact resistance decreases with increasing the work function of S/D electrode. This result can be partially ascribed to better band alignment in the Ni/ITZO interface due to the work function of Ni (5.04–5.35 eV) and ITZO (5.00–6.10 eV) being somewhat similar.