InGaN/GaN结构长金属电极的欧姆接触材料选择

绿光光源可广泛应用于固态照明、可见光通信、电子显示、光遗传学等领域。相比于蓝光LED，高性能低维绿色发光器件的设计与制备受限于绿光效率低（Green gap）和高注入电流下效率下降（Efficiency droop）两个主要问题的困扰。本文采用化学气相沉积方法（CVD）生长镓掺杂的氧化锌微米线（ZnO∶Ga MW），结合p型InGaN衬底制备了n‐ZnO∶Ga MW/p‐InGaN异质结发光二极管。该器件的输出波长为540 nm，半峰宽约为32 nm，在相对较大的注入电流下，器件发光峰位、半峰宽等发光特征参数没有明显的变化，且相对外量子效率（REQE）在较大电流下呈现出相对较小的下降，体现了较高的发光稳定性。此外，利用金纳米薄膜改善了ZnO∶Ga微米线与InGaN衬底间的接触，实现了结区界面的优化，成功提高了发光二极管的发光强度。实验结果表明，采用n‐ZnO∶Ga微米线结合p‐InGaN衬底构筑的异质结可用于制备高稳定性高亮度的微型绿光发光二极管。

Ultralow Ohmic contact resistance and a self-aligned device structure are necessary to reduce the effect of parasitic elements and obtain higher ft and fmax in high electron mobility transistors (HEMTs). N-polar (0001¯) GaN HEMTs, offer a natural advantage over Ga-polar HEMTs, in terms of contact resistance since the contact is not made through a high band gap material [Al(Ga)N]. In this work, we extend the advantage by making use of polarization induced three-dimensional electron-gas through regrowth of graded InGaN and thin InN cap in the contact regions by plasma (molecular beam epitaxy), to obtain an ultralow Ohmic contact resistance of 27 Ω μm to a GaN 2DEG.

We propose indium (In), a low work function and nitride-forming element, as an efficient ohmic contact layer to N-face n-GaN. While conventional Al-based ohmic contacts show severe degradation after annealing at 300 °C, In-based ohmic contacts display considerable improvement in contact resistivity. The annealing-induced enhancement of ohmic behavior in In-based contacts is attributed to the formation of an InN interfacial layer, which is supported by x-ray photoemission spectroscopy measurements. These results suggest that In is of particular importance for application as reliable ohmic contacts to n-GaN of GaN-based vertical light-emitting diodes.

A strained InGaN contact layer inserted between Pd/Au and p-type GaN resulted in low ohmic contact resistance without any special treatments. The thickness and In mole fraction of the p-type InGaN varied from 2 nm to 15 nm and from 0.14 to 0.23, respectively. Strained InGaN layers are effective in reducing the contact resistance. A contact layer of 2 nm thick strained In0.19Ga0.81N showed the lowest specific contact resistance of 1.1×10−6 Ω cm2. The mechanism for the lower contact resistance is ascribed to enhanced tunneling transport due to large polarization-induced band bending at the surface as well as to the high hole concentration in p-type InGaN.

… In this paper, we report on the formation of Ohmic contacts on graded polarization doped n-InGaN. Here, the n-type polarization doping in the graded InGaN is discussed in relation to …

In an interconnected high-vacuum illustration system, the surfaces of p-InGaN/GaN heterostructures grown with integrated metalorganic chemical vapor deposition were treated in O2 gas for different times at room temperature and then transferred to another chamber for fabricating ohmic contacts via sputtering Pd/Pt/Au multi-layers. X-ray photoelectron spectroscopy measurements unveil that the proportions of Ga–O of the treated samples significantly increase compared with that of the as-grown sample, indicating that a thin layer of GaOx forms on the p-InGaN/GaN surface after exposing to oxygen or air atmospheres. Meanwhile, the samples exposed to O2 or air were found to have much higher specific contact resistance, i.e., higher by two orders of magnitude than that of the as-grown sample. The specific contact resistance of the as-grown sample was derived as 9.3 × 10−5 Ω cm2 using the circular transmission line measurement. Furthermore, the Schottky barrier height of the samples was determined from the measured I–V curves with the thermionic field emission model and was revealed to be closely related to surface treatments. Despite the fact that the insulating layer of GaOx was as thin as 1.2–1.4 Å for the samples exposed to oxygen at room temperature, it can act as an extra barrier layer causing significant increase in the specific contact resistance via blocking the tunneling of carriers. Therefore, the effective removal of p-InGaN surface oxide plays a vital role in preparing good ohmic contacts.

… It was found that in InGaN/GaN superlattices, the hole concentration … GaN and InGaN. [ 30 ] Kumakura and coworkers investigated Pt/Au contacts on p-GaN with a strained p-InGaN layer …

… on ohmic contacts to GaN-based materials. In the first part, the basic principle and concepts of semiconductor-metal contacts … of InGaN/AlGaN superlattice can realize the positive ohmic …

… Ohmic contacts to -type GaN have been studied [11]. In this study, we have demonstrated a highly transparent Ohmic contact … -GaN before the ITO film without postdeposition annealing. …

… destroying the p-GaN. The surface-… in Ohmic contact with the p-GaN. The experimental results indicate that the surface-textured ITO layer is suitable for fabricating high-brightness GaN-…

… ohmic contacts to p-GaN using a strained p-InGaN contact layer, and achieved the lowest contact … of the ohmic contacts to p-GaN using a strained p-InGaN contact layer up to 400 C. …

… We report on the metal work function dependence on Schottky barrier characteristics and … contact resistance. Possible carrier transport mechanisms of ohmic contacts to the InGaN/GaN …

Low-resistance ohmic contacts in AlGaN/GaN high-electron-mobility transistor (HEMT) devices require high-temperature (HT) annealing (> 800 °C) which can deteriorate material quality, surface morphology, and edge acuity of the metal stacks. This article demonstrates the high-frequency and high-power performance of the AlGaN/GaN HEMT devices with low-temperature metal-organic chemical vapor deposition (MOCVD) regrown degenerately doped InGaN ohmic contacts compared with GaN-based regrown contacts. Ohmic contacts fabricated by regrowth methods could be a valuable alternative for both metal-based alloyed ohmic contacts and implantation-based ohmic contacts. Using a T-gate and MOCVD regrown InGaN ohmic contacts, the AlGaN/GaN HEMT with an <inline-formula> <tex-math notation="LaTeX">${L}_{g}$ </tex-math></inline-formula> of 150 nm and S-D spacing of <inline-formula> <tex-math notation="LaTeX">$2.5~\mu \text{m}$ </tex-math></inline-formula> demonstrated a maximum drain current of 0.94 A/mm and a peak transconductance of 337 mS/mm. The same device exhibited a forward current gain frequency <inline-formula> <tex-math notation="LaTeX">${f}_{t}$ </tex-math></inline-formula> of 36.8 GHz and a maximum frequency of oscillation <inline-formula> <tex-math notation="LaTeX">${f}_{\text {max}}$ </tex-math></inline-formula> of 75.0 GHz. A power density of 3.07 W/mm with a 60% drain efficiency was measured at 35 GHz with a <inline-formula> <tex-math notation="LaTeX">${V}_{\text {ds}}$ </tex-math></inline-formula> of 20 V and a quiescent current of 100 mA/mm.

… Low-resistance Ni/Au ohmic contacts were formed on p-GaN and p-InGaN/GaN … The p-InGaN/GaN contact exhibited better electrical behaviors than the p-GaN contact. Annealing …

… on transparent ohmic contacts to p-InGaN/p-GaN. In this work, … Ni/Au ohmic contacts to p-GaN using strained p-type InGaN … strained InGaN layer results in a large reduction in the contact …

… /GaN based … GaN nonalloyed ohmic contacts on different epitaxial structures were investigated. Once the optimal recess etch depth and regrowth thickness for drain and source contacts …

… stress is the worsening of the properties of the ohmic contacts. … ohmic contacts of the devices during thermal stress, we have carried out a similar stress test on TLM structures on p-GaN. …

A GaN-based light-emitting diode (LED) with a direct-Ohmic contact structure, formed by an indium-tin-oxide (ITO) transparent film and Au thermal-diffused and removed layer, is studied. By depositing an Au metallic film on the Mg-doped GaN layer followed by thermal annealing and removed processes, an ITO direct-Ohmic contact at p-GaN/ITO interface is formed. An enhanced light output power of 18.0% is also found at this condition. This is mainly attributed to the larger and more uniform light-emission area resulted from the improved current spreading capability by the use of an ITO direct-Ohmic contact structure.

… n-GaN, 0.1-μm-thick InGaN–GaN (MQWs) active layer, 0.2-μm-thick p-… of ohmic contacts at the interface of ITO/p-GaN and exhibited the excellent performance of LEDs with ITO contacts. …

… In conclusion, an InGaN/GaN multiple-quantum-well (MQW) light-emitting diode (LED) with an ITO direct-Ohmic contact property to the p-GaN layer is achieved by means of Au thin film …

… The wide-bandgap GaN and related materials have been extensively investigated in recent … in making high-performance metal contacts to GaN-based materials, which is crucial for …

… contact characteristics of 17 metals (Sc, Hf, Zr, Ag, Al, V, Nb, Ti, Cr, W, Mo, Cu, Co, Au, Pd, Ni, and Pt) to n-type GaN … ) characteristics of all these metal contacts are presented. For those …

Contact mechanisms and design principles of alloyed ohmic contacts to n-GaN are investigated. For the investigation, both tunnel contacts and thermionic contacts are considered. While the tunnel contacts include the Ti/Al/Ti/Au, Ti/Al/Ni/Au, Ti/Al/Pd/Au, (Ta,Ti)/Ni/Au, Ti/Al/Mo/Au, and Ti/Au/Pd/Au contacts, the thermionic contacts include the Ni/Au contacts. The proposed design principles correctly dictate the characteristics of all these contacts. At present, tunneling is believed to be the primary mechanism for low resistivity of the tunnel contacts. The present study demonstrates that both tunneling and thermionic emission are equally important for the low resistivity of these contacts. Band-gap narrowing and/or image force lowering due to heavy doping also contribute to the resistivity reduction of these contacts. An exciting feature of the present study is the observation of a very low work function intermetallic alloy formed during annealing of an appropriate combination of large work function metals. If the annealing conditions are optimized, the contacts become very robust, thermally stable, and lowly resistive with thermionic emission as the primary mechanism for electron transport. The observation is very promising and has potential to open up avenues for different types of thermionic contacts. The fundamental physics underlying the design principles are discussed. These principles are general enough to be applicable to other III–V nitrides, at the least.

We report measurements on the nature of aluminum and gold contacts to GaN. The GaN films were deposited onto the R-plane of sapphire substrates by molecular beam epitaxy and are autodoped n-type. Metal contacts were deposited by evaporation and were patterned photolithographically. Current-voltage characterization shows that the as-deposited aluminum contacts are ohmic while the as-deposited gold contacts are rectifying. The gold contacts become ohmic after annealing at 575 °C, a result attributed to gold diffusion. The specific contact resistivity of the ohmic aluminum and gold contacts were found by transfer length measurements to be of device quality (10−7–10−8 Ω m2). The results of these studies suggest a direct correlation between barrier height and work function of the metal, consistent with the strong ionic character of GaN.

Abstract Low-Schottky barrier height (SBH) metal contacts to 2D materials is indispensable for achieving high performance in atomic layer 2D materials channel based optoelectronic devices. In this study, we systematically investigate the detailed face contact properties of monolayer (ML) hexagonal gallium nitride (GaN) with six different commonly used metals (Au, Ag, Pd, Pt, Ti, and Ni) in field-effect transistors (FETs) utilizing the first principles electronic structure calculations based on density functional theory (DFT). It is found that no tunnelling barriers (TB) exist in all the ML GaN-metal face contact systems by calculating and analyzing the average effective potentials (Veff). Moreover, ML GaN undergoes a vanishing of Schottky barriers in the vertical interfaces by analyzing the binding energy, electron localization function (ELF), and projected state density (PDOS) of six contact combinations. In terms of the energy band calculation, ML GaN forms n-type Schottky contacts with Au, Pd, Pt and Ti electrodes, and an n-type ohmic contacts with Ag electrode, while a p-type ohmic contact with Ni electrode is obtained in the lateral interfaces. The results would provide an insight into the ML GaN-metal interfaces, and be beneficial for developing low dimensional GaN-based devices with high performance.

… contacts to n-GaN using Al, Ti/Al and Ti/Au has been investigated. n-GaN Ti/Al contacts showed the lowest specific contact … [9] on molecular beam epitaxy grown GaN materials. Si was …

… as contact material on p-GaN. None of the contacts prepared with platinum as contact metal … values of the resistance are much higher compared to the other contact metals in this study. …

In this paper, the electrical properties of different metal/p-GaN contacts (Ti/Al, TiN/Ti/Al and Ni/Au) have been investigated to get a deeper understanding of the behavior of p-GaN/AlGaN/GaN heterostructures for normally-off HEMTs. In particular, the study of the temperature dependent current-voltage characteristics allowed to identify the dominant carrier transport mechanism at the metal/p-GaN interface (Thermionic Field Emission). From the fit of the experimental current-voltage data it was possible to determine the Schottky barrier height values for the three systems, 2.08 eV (Ti/Al), 1.57 eV (TiN/Ti/Al) and 1.89 eV (Ni/Au). Hence, choosing the highest barrier height contact (Ti/Al) as gate electrode on a p-GaN/AlGaN/GaN heterostructure, optimized based on simulations, allowed to obtain devices with a normally-off behavior and a positive Vth of +1.3 V.

… contacts with low contact resistances are crucial for realization of the GaN-based electronic devices such as metal … reported to be a good ohmic contact material to n-type GaN [4], and …

… GaN In p-GaN material, the fabrication of Ohmic contacts is quite difficult [35]. For this reason, … Ti/Al-based metal contacts to p-type 4H-SiC and p-type GaN is reported. Ti/Al-based Ohmic …

For the application of third-generation compound semiconductors based on high-voltage, high-power, and high-frequency devices, particularly in photoconductive semiconductor switches (PCSSs), the ohmic contact preparation technology on semi-insulating substrates is crucial. However, iron-doped semi-insulating gallium nitride (SI-GaN:Fe) with low carrier concentration and high bulk resistivity has a very difficult time achieving good ohmic contact characteristics. Here, we have demonstrated preliminary results on an interesting class of good ohmic contact of Ti/Al/Ni/Au on Ga-face of SI-GaN:Fe. The results indicated that the Ti and Al thickness ratio is a critical parameter when determining the optimum annealing temperature and time for ohmic contact. With alloyed Ti/Al/Ni/Au stacks of 20 nm/120 nm/55 nm/45 nm on SI-GaN:Fe after annealing at 850 °C for 35 s in N2 atmosphere, a low specific contact resistivity of $10^{-3}\,\,\Omega \cdot $ cm2 is achieved. At a future date, such excellent ohmic contact characteristics can be applied to the SI-GaN:Fe-based high-power PCSS.

… In such a GaN material system very high annealing temperatures of up to 900 C are … metal–GaN/AlGaN interface and a different metal semiconductor alloying. The diffusion of metals …

… In fact, present limitations in GaNbased HEMTs are related to … the deficiencies of widely used contact metals; or collapse in … on GaN and AlGaN Schottky (Pt/Ti/Au) diodes and metal-…

Here we present a method to model the metal–semiconductor (M–S) band structure to an implanted ohmic contact to a wide band gap semiconductor (WBG) such as GaN and SiC. The …

… specific contact resistance in the Mg electrode for m-plane GaN, we investigated the profiles of the Mg, Ga, and N atoms in the vicinity of the interface between the electrode and GaN …

Graphene combines high conductivity (sheet resistance down to a few hundred Ω/sq and even less) with high transparency (>90%) and thus exhibits a huge application potential as a transparent conductive electrode in gallium nitride (GaN)-based light-emitting diodes (LEDs), being an economical alternative to common indium-based solutions. Here, we present an overview of the state-of-the-art graphene-based transparent conductive electrodes in GaN-based LEDs. The focus is placed on the manufacturing progress and the resulting properties of the fabricated devices. Transferred as well as directly grown graphene layers are considered. We discuss the impact of graphene-based transparent conductive electrodes on current spreading and contact resistance, and reveal future challenges and perspectives on the use of graphene in GaN-based LEDs.

… in the ohmic behavior of the electrode in terms of contact resistance, we performed XPS analysis at the interface between the GaOx electrode and the p-GaN layer. The XPS results …

We demonstrate the effect of Ti thickness on the contact resistance between GaN nanowires and Ti∕Au electrodes. We have carried out systematic characterization of many GaN nanowires contacted by various Ti∕Au electrodes. We conclude that the average resistance is reduced by almost six orders of magnitude as Ti thickness increases from 0 to 20nm, and the resistance value then saturates when the Ti thickness further increases. Our observation can be explained by the formation of TiOx through the reaction of surface oxide and the Ti layer. Scanning Auger microscopy of the Au∕Ti∕SiO2 interface also supports this explanation.

The authors report the improvement of GaN light-emitting diodes (LEDs) by applying a ZnNi/indium tin oxide (ITO) (5nm∕380nm) electrode with high transparency and low resistance to p-GaN. The Pt/ITO (5nm∕380nm), Ni∕Au∕ITO (2.5nm∕5nm∕380nm), and Ni∕Au (2.5nm∕5nm) electrodes were prepared and annealed at 400, 500, and 600°C for 1min in air. The ZnNi/ITO contacts showed the lowest specific contact resistance of ∼1.27×10−4Ωcm2 and the highest transmittance of ∼90% at 460nm. LEDs fabricated with ZnNi/ITO p electrodes showed the best performance with a forward voltage of 3.28V and a typical brightness of 11. 7mcd at 20mA.

… a p-type GaN contact layer, a p-type AlGaN cladding layer, a p-type GaN waveguide layer, an … ) active layer, an n-type GaN waveguide layer, n-type AlGaN cladding layer and an n-type …

In this article, we propose a high-frequency model taking into account the series resistance of pseudo-vertical gallium nitride (GaN)-based p-i-n diodes. This model relies on the specific contact resistance on p-type GaN and the sheet resistance of the bottom n-type GaN. Those two quantities are obtained while fitting the RF experimental data and are slightly different from the dc values. The interest of this model is that the effective values, at the working frequency instead of “only” the dc values, are extracted using only a few devices. The novelty of the work resides in the consideration of the geometrical dimensions of the diode (anode radius and distance between anode and cathode) in the model. Furthermore, the model allows the determination of the most limiting geometrical parameters and can predict the series resistance of other topologies. In the present case, we show the most limiting factor is the radius of the anode due to the difficulties of achieving low specific contact resistance on p-type GaN. The parasitic capacitance is also extracted using the measured devices, allowing the modeling of the effective capacitance as a function of the frequency.

Abstract In order to alleviate the negative impacts of high temperature (850 °C) annealing on high electron mobility transistor (HEMT) technology, a process required only low temperature annealing (less than 600 °C) was introduced to form ohmic contact to HEMT. By the process, the electrode was in contact with the AlGaN/GaN interface. The value of specific contact resistivity (ρc) of the ohmic contact was 6.29 × 10−5 Ω cm2. An improved HEMT power device was presented. For comparison, a conventional device with Ti/Al/Ni/Au ohmic contact annealed at 850 °C was prepared. It was found that after annealing at 600 °C, the sheet resistance (Rsh) of the improved device decreased to 313 Ω/□ (74.5% of that of the conventional device), and the advantage of our process on device performance was greater as the distance between ohmic contacts increased. The saturation current of the improved device was increased by approximately 23% when the distance between source and drain was 40 μm.

… Z D where D is the width of the electrodes, and Z is the distance between the two electrodes. The total resistance correlates with contact resistance and sheet resistance. R c denotes the …

Abstract We investigated the conditions of sample preparation for obtaining low contact resistance to Al/Ti electrode on n-GaN fabricated by Si implantation and analyzed its interface structure. In conditions of sample preparation showing the lowest resistance, we found that there is an AlN layer and not the generally believed TiN layer on the interface between the electrode and the GaN substrate. Silicon was implanted into GaN followed by annealing at 1200 °C to investigate the effects of surface carrier concentration on ohmic contact. Layers of Ti and Al were then deposited and annealed at 550–800 °C. In the sample annealed at 600 °C with a relatively high fluence of 5 × 1014 atoms/cm2, the sample displayed the lowest contact resistance of 1.2 × 10−7 Ωcm2. This indicates that the formation of a high concentration of impurities on the GaN surface, resulting from Si implantation effectively reduced the contact resistance of the Al/Ti electrode. Ti and Al were also deposited on un-implanted GaN substrates, and the resulting multilayer electrodes with different Al/Ti ratios were annealed at 550–900 °C to investigate the electrode structure and the detail structure of metal/semiconductor interface. The film compositions and the interfacial microstructures of the samples were analyzed by the Rutherford backscattering spectroscopy method, optical microscopy, high-resolution scanning electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDX), and electron energy loss spectroscopy (EELS). For samples with Al/Ti ratios of ∼4.2, the under part of electrode became rich in Al at annealing temperatures of 550–600 °C. On the basis of the STEM and EELS analysis, Ti was not present at immediately above the GaN substrate interface, whereas AlN (thickness = 1–2 nm) was present. The RBS-determined total amount of Ga was 5 × 1015 atoms/cm2, which corresponds to the amount of Ga in 1.1 nm of the GaN layer that is probably replaced by the AlN layer.

… Contact resistance from diode measurements … method where the contact resistance dominates over other resistances. This can be the case by measuring the series resistance of current …

… These conducting contact electrodes were also deposited onto the n-GaN epitaxial layers for the fabrication of GaN MSM UV sensors. Fig. 1(a) shows the schematic structure of the GaN …

… tunneling conduction and the lower contact resistance (anode electrode) can be achieved due to the n -In … N/GaN SPS tunneling top contact layer, compared to conventional devices. …

… The p-GaN and InGaN/GaN LED samples used in this study were all grown on c-face (0 0 … contact was subsequently evaporated onto the p-type GaN surface to serve as the p-electrode…

… We show that an inter-digitated electrode configuration with distance between the contact pad and the edge of pn junction equal to transfer length in the current spreading ITO layer …

… In LEDs, photons generated at the InGaN/GaN MQW should pass through a transparent p-… the high contact resistance still needs to be improved in order to apply to a p-electrode of GaN-…

… Ni/Au contact on the electrical properties and optical performance of GaN/InGaN multiple … the p- and n-ohmic metals to achieve bond-pad electrodes. To measure the specific ohmic …

… InGaN-based LEDs, semitransparent electrodes are widely used. These semitransparent electrodes … ITO/GaN interface and showed that the samples give contact resistivity comparable …

Indium tin oxide (ITO), with its transparency and strong adhesion to GaN, has been used as a replacement for Ni∕Au as a contact on p-GaN. However, ITO suffers from high contact resistance on p-GaN. In this work, low contact resistance between ITO and the p-GaN layer was consistently achieved using various strained InGaN layers as the interface layers between ITO and p-GaN layer. The doping of InGaN, whether n type or p type, has a relatively weak effect on the contact resistance as long as the thickness of the InGaN layer is adequately controlled. The secondary-ion-mass spectroscopy depth profile reveals that the n-type InGaN strained contact layer was also heavily doped with Mg. Results of this study demonstrate that the piezoelectric field between InGaN and p-GaN is important in reducing the barrier height of Ohmic contact.

… layer, five-periods of InGaN–GaN MQW as active layers, a 20 nm thick … -GaN layer as P and N electrodes, respectively. In the case of fabricating planar-surface LEDs, P and N electrodes …

… surface to serve as the p-electrode. On the other hand, Ti/Al/Ti/Au contact was deposited onto the exposed n-type GaN layer to serve as the n-type electrode. The size of our LEDs was …

We investigated the effect of Ti/Al and ITO/Ag n-type contacts on the emission uniformity and light output of different chip-size vertical-geometry light-emitting diodes (VLEDs) for vehicle headlamp application. The forward voltage of the Ti/Al-based reference VLEDs decreased from 3.38 to 3.20 V at 1500 mA with increasing chip size from (1280 × 1000 µm2) to (1700 × 1700 µm2), whereas that of the ITO/Ag-based samples changed from 3.37 to 3.15 V. Regardless of chip size, the ITO/Ag-based samples revealed higher light output power than the reference samples. For example, the ITO/Ag-based samples (chip size of 1700 × 1700 µm2) exhibited 3.4% higher light output power at 1500 mA than the reference samples. The ITO/Ag samples underwent less degradation in the Wall-plug efficiency (WPE) than the reference sample. For instance, the ITO/Ag-based samples (1700 × 1700 µm2) gave 4.8% higher WPE at 1500 mA than the reference samples. The ITO/Ag-based samples illustrated more uniform emission than the Ti/Al-based sample. Both the reference and ITO/Ag-based samples underwent no degradation when operated at 1500 mA for 1000 h.

We demonstrate a GaN-based light-emitting diode (LED) with nonalloyed metal contacts and textured Ga-doped ZnO (GZO) contact layer to serve as the n- and p-type electrode pads, respectively. Compared with the conventional LEDs with flat surface and Cr/Au metal contacts, the nonalloyed Ag/Cr/Au contacts used in the present experimental LEDs play the role of reflector to prevent the emitted light from absorption by the opaque electrode pads. Enhancement of light output power observed from the experimental LEDs is also due to the textured GZO layer that can disperse the angular distribution of photons at the GZO/air interface. With an injection current of 20 mA, the output power of experimental LEDs can be improved markedly by a magnitude of 30% compared with conventional GaN-based LEDs.

… contact properties were studied by depositing ITO on p-GaN to investigate a possibility of forming transparent ohmic contact on p-GaN. … 1 shows the circular electrodes patterns used in …

… in the performance of InGaN/GaN multi-quantum well-based … oxide (ITO) p-electrode and p-GaN. The solar cell fabricated … that the contact resistance at the p-GaN/p-electrode interface …

… The ohmic contact resistance and transparent electrodes are two of most critical issues that will ultimately determine the functional successes of wide bandgap III-nitrides based …

… and morphological characteristics of Ti/Al/Ti/Au ohmic contacts by optimizing metal layer … compositions of previously researched Ti/Al/Ti/Au contacts on the ternary Ti-Al-Au composition …

We investigate the effect of W diffusion barrier in Ti/W/Al ohmic contacts formed on N-face n-GaN. The contacts exhibit contact resistivity of as low as 2.3 × 10−4 Ω cm2 and better thermal stability than Ti/Al contacts. Cross-sectional transmission electron microscopy micrographs reveal that in-diffused Al atoms on the n-GaN surface react with N atoms to form an AlN layer in Ti/Al contacts, resulting in upward band bending, and consequently, a high contact resistivity. The use of a 10-nm-thick W layer suppresses the in-diffusion of Al atoms to n-GaN, thereby preventing the formation of AlN and enhancing the thermal stability of Ti/W/Al contacts.

… on the N-face n-type GaN surface on the electrical properties of … Ti (50 nm)/Al (35 nm) ohmic contact. The O2 plasma is first applied to the N-face n-type GaN surface so that Ga–N bond …

The fabrication and characterization of an Au-free Ti/Al/TiN (20/100/100 nm) contact stack to unintentionally doped n-GaN with TiN serving as the diffusion barrier is presented. Sputter deposition and lift-off in combination with post deposition annealing at 850 °C are used for contact formation. After annealing, contact shows ohmic behavior to n-GaN and a specific contact resistivity of 1.60 × 10−3 Ω cm2. To understand the contact formation on the microscopic scale, the contact was characterized by current–voltage measurements, linear transmission line method, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. The results show the formation of Ti-N bonds at the GaN/Ti interface in the as-deposited stack. Annealing leads to diffusion of Ti, Al, Ga, and N, and the remaining metallic Ti is fully consumed by the formation of the intermetallic tetragonal Al3Ti phase. Native oxide from the GaN surface is trapped during annealing and accumulated in the Al interlayer. The ...

… Kim, “TiN/Al Ohmic contacts to N-face n-type GaN for high-… as an efficient ohmic contact to N-face n-GaN of GaNbased … in Ti/W/Al ohmic contacts on N-face n-GaN,” Appl. Phys. Lett., vol. …

To investigate the possibility of preparing a nonalloyed ohmic contact to n-type GaN, the effects of GaN surface treatments on the metal/GaN interface were studied using x-ray photoelectron spectroscopy. A specific contact resistance of 5.0×10−5 Ω cm2 for the Ti/Al nonalloyed ohmic contacts to (NH4)2Sx-treated n-type GaN can be obtained. The nonalloyed ohmic contact formation would be impeded by the native oxide and the hydroxyl induced from the surface treatment of chemical solutions.

… Interfacial reactions on n-GaN have led to the formation … ohmic contact formation (n-GaN/Ti/Al annealed at 700 C). XRD analysis indicated that Ti 2 N was formed in n-GaN/Ti/Al contacts …

The intermetallic TiAl3 has been used as a thermally stable cap for Ti/Al ohmic contacts to n-GaN. The electrical performance of the TiAl3-capped contact is nearly the same as that of a standard Ti/Al/Ni/Au contact processed on the same substrate, but the Ti/Al/TiAl3 contact’s performance is optimized at a much lower temperature. The Ti/Al/TiAl3 contact achieved a lowest specific contact resistance (ρc) of 2.1×10−5 Ω cm2 following 1 min at 700 °C in flowing, oxygen-gettered ultrahigh purity (UHP) Ar. The Ti/Al/Ni/Au contact standard achieved a ρc of 1.8×10−5 Ω cm2 following a 15 s anneal at 900 °C in flowing, oxygen-gettered UHP Ar. The TiAl3-capped contact structure shows little sensitivity to the amount of oxygen in the annealing ambient for optimization, and we found that it could achieve a ρc of 1.1×10−5 Ω cm2 following 5 min at 600 °C in air. This performance is almost identical to that attained when the contact was annealed in oxygen-gettered UHP Ar and ordinary Ar. Anneals were extended to a total time of 20 min in the three ambient atmospheres, and the Ti/Al/TiAl3 contact showed no significant difference in its performance. The fact that this contact structure can withstand optimization anneals in air suggests that it could be annealed alongside a Ni/Au contact to p-GaN in air and still achieve a low contact resistance. The performance of the TiAl3-capped bilayer was found to be stable following thermal aging for more than 100 h at 350 °C in air, which was also comparable to an optimally annealed Ti/Al/Ni/Au contact aged at the same time. The TiAl3 material should be an oxidation cap solution for many other Ti/Al contact structures, almost regardless of the Ti:Al layer thickness ratio, since the TiAl3 will be stable on the upper Al layer. Use of this cap eliminates the need to alter a previously optimized bilayer, thus it is a means of enhancing any existing Ti/Al bilayer contact’s performance without necessitating the reoptimization of the layers to accommodate the cap.

The relationship between the Schottky barrier height and nitrogen–vacancy-related defects in Ti alloyed Ohmic contacts to n-type GaN (n-GaN) has been investigated using x-ray photoelectron spectroscopy measurements. It is shown that the alloyed Ohmic behavior of the contacts can be attributed to the presence of a large number of nitrogen–vacancy-related defects and not to the formation of lower barriers at the annealed Ti/n-GaN interface. The large number of interface states, related to the nitrogen–vacancy defects has led to the pinning of the Fermi level at 0.5 eV below the conduction-band edge, and has left the GaN surface very heavily n type, thereby forming a tunneling junction.

Silver is studied as a replacement for Au in V- and Ti-based Ohmic contacts to GaN-capped n-Al0.27Ga0.73N∕GaN heterostructures for high electron mobility transistors. An optimized V∕Al∕V∕Ag contact provided a low contact resistance of 0.27Ωmm and specific contact resistance of 1.7×10−6Ωcm2, and was much smoother than analogous Au-bearing metallizations. Transmission electron microscopy reveals a very limited reaction of the annealed metallization and semiconductor, leaving the Al0.27Ga0.73N layer intact. The majority of the Al0.27Ga0.73N interface is contacted by Ag-bearing phases. Silver has a lower work function than Au and facilitates the formation of a low-resistance Ohmic contact.

This paper compares the metal/semiconductor barrier height properties of non-recessed Ti/Al/Ti and Ta/Al/Ta contacts on AlGaN/GaN heterostructures. Both contacts exhibited a rectifying behavior after deposition and after annealing at temperatures up to 550 °C. The ohmic behavior was reached after annealing at 600 °C. High-resolution morphological and electrical mapping by conductive atomic force microscopy showed a flat surface for both contacts, with the presence of isolated hillocks, which had no significant impact on the contact resistance. Structural analyses indicated the formation of the Al3Ti and Al3Ta phases upon annealing. Furthermore, a thin interfacial TiN layer was observed in the Ti/Al/Ti samples, which is likely responsible for a lower barrier and a better specific contact resistance (c = 1.6 10−4 Ωcm2) with respect to the Ta/Al/Ta samples (c = 4.0 10−4 Ωcm2). The temperature dependence of the specific contact resistance was described by a thermionic field emission mechanism, determining barrier height values in the range of 0.58–0.63 eV. These results were discussed in terms of the different microstructures of the interfaces in the two systems.

… to construct a compelling Cu-based ohmic contact stack [28]. … of Ti/TiN/Cu trilayers as an ohmic contact to AlGaN/GaN HEMT devices fabricated on silicon substrates. Devices with the Ti/…

… schemes have been developed for a low barrier ohmic contact to n-GaN. … scheme AuTiSi/n-GaN, in order to achieve a low ohmic contact system for the GaN applications. The specific …

… Ni/Au films on p-GaN was examined to elucidate the formation of a low resistance ohmic contact to p-GaN … The p-GaN/Ni/Au samples were heat treated at 500 C in air mainly composed …

A contact has been developed to achieve a low specific contact resistance to p-type GaN. The contact consisted of a bi-layer Ni/Au film deposited on p-type GaN followed by heat treatment in air to transform the metallic Ni into NiO along with an amorphous Ni–Ga–O phase and large Au grains. A specific contact resistance as low as 4.0×10−6 Ω cm2 was obtained at 500 °C. This low value was obtained by the optimization of Ni/Au film thickness and heat treatment temperatures. Below about 400 °C, Ni was not completely oxidized. On the other hand, at temperatures higher than about 600 °C, the specific contact resistance increased because the NiO detached from p-GaN and the amount of amorphous Ni–Ga–O phase formed was more than that of the sample annealed at 500 °C. The mechanism of obtaining low-resistance ohmic contacts for the oxidized Ni/Au films was explained with a model using energy band diagrams of the Au/p-NiO/p-GaN structure.

… to form a high quality ohmic contact on p-type GaN, using nickel (Ni) and gold (Au) thin layer … ohmic contacts to p-GaN, comparing the standard GaN/Ni/Au contact and an GaN/Au/Ni …

Effects of a NiO layer on the electrical properties of NiAu-based ohmic contacts for p-GaN were studied by depositing a p-type NiO layer on the p-GaN using a sputter-deposition technique. NiO layers doped with Li[NiO(Li)] had a p-type conduction with sheet resistivity of around 1 Ω cm after annealing at temperatures lower than 500 °C. A variety of the NiAu-based contacts with the NiO layers such as NiO/Au, NiO(Li)/Au, Ni/NiO(Li)/Au, Ni/Li2O/NiO/Au, and Ni/Li2O/Ni/NiO/Au contacts were prepared by depositing on the p-GaN, where a slash “/” indicates the deposition sequence. However, these contacts did not provide specific contact resistances (ρc) lower than that (ρc∼10−2 Ω cm2) of the conventional Ni/Au contacts prepared by annealing in N2 ambient. From the present results, it was believed that the p-NiO layer did not act as an intermediate semiconductor layer to reduce the Schottky barrier height at the p-GaN/Au interface.

We report a low-resistant, thermally stable, and transparent ohmic contact on p-type GaN using an indium-tin-oxide (ITO) overlayer on Ni/Au contact. Ni (20 A)/Au (30 A)/ITO (600 A) contact with preannealing at 500 °C before ITO deposition showed lower contact resistivity by one order of magnitude than the contact without the preannealing. The preannealing produced NiO, acting in the role of diffusion barrier for outdiffusion of N and Ga atoms and indiffusion of In during the subsequent post-annealing. Thus, the formation of Au–In solid solution was effectively suppressed, resulting in the decrease of contact resistivity and enhancement in thermal stability.

The effect of thermal annealing on current–voltage properties of GaN light emitting diodes (LEDs) has been studied. At annealing temperatures above 700 °C, the p–n junction of the diodes became very leaky and Ga-contained metallic bubbles were observed on the surface of Ni/Au p-ohmic contact. Transmission electron microscopy and energy dispersive x-ray spectrometer studies revealed that these metallic bubbles resided directly on top of the threading dislocations in GaN and both Ni and Au were indiffused into the LED structure along the cores of the TDs. The conducting paths formed by the metal containing dislocation cores are believed to be the cause for the observed short circuit behavior of p–n junctions at high annealing temperatures.

To enhance the efficiency and performance of p-type GaN-based power devices (diode, MOSFET) and p-channel transistors (p-FET), forming an Ohmic contact with low specific contact resistance (ρc) at p-GaN can be an effective way. However, the specific contact resistance values of p-GaN remain limited to the range of mid-10−2Ω cm2 due to the low activation ratio of Mg dopants and high work function. Here, we propose an Ohmic contact method using a Pd-based metal stack to achieve lower specific contact resistance compared to conventional p-GaN contact metals such as Ni/Au or Pt/Au. A low specific contact resistance of 1.08 × 10−5Ω cm2 was demonstrated for p-GaN by annealing a Pd-based tri-layer metal contact in an oxygen-rich ambient with an optimized Pd thickness. The mechanism driving this low specific contact resistance was investigated using secondary ion mass spectroscopy, transmission electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy, which indicated the mutual diffusion of Ni, Pd, and Ga atoms within the metal alloy. A NiO layer was formed on the top of the metal alloy, and Pd-Ga compounds were formed at the metal/p-GaN interface through mutual diffusion of atoms. This process increased the number of Ga vacancies in p-GaN, playing a crucial role in reducing its contact resistance.

… in such low contact resistance ranging … low contact-resistance ohmic contact to p-type GaN is harder to achieve because of the difficulty to obtain high hole concentration for p-type GaN. …

… offset between Ni and p-GaN surfaces. In this paper, we report the results of an ohmic contact related to moderately doped p-GaN (44x10" cm"). The formation of a GaN:Ni alloy and Zn-…

We report on a promising Ni (5 nm)/Al-doped ZnO (AZO) (450 nm) metallization scheme for low resistance and transparent ohmic contacts to p-GaN (5×1017 cm−3). It is shown that the as-deposited Ni/AZO contact shows a nonohmic characteristic due to the insulating nature of the as-deposited AZO. However, annealing the contacts at 450 and 550 °C for 2 min in air ambient results in linear current–voltage characteristics, giving a specific contact resistance of 1.01×10−5 and 8.46×10−6 Ω cm2, respectively. It is further shown that annealing the contact at 550 °C for 5 min produces a specific contact resistance of 6.23×10−6 Ω cm2. The light transmittance of the contacts annealed at 550 °C for 2 min is measured to be higher than 76% at wavelengths in the range of 400–550 nm. It is shown that the Ni/AZO contact could be a suitable scheme for high-performance optical devices.

In this article we proposed a modified transmission line model method to analyze non-Ohmic contact. It can differentiate the contact resistance while the current flows into and out of the contact. When the earlier two factors are equal, this method degenerate to traditional method. By this method, we obtained contact parameters for both p-GaN and n-GaN and revealed the validity of this method. To further study the contact characteristics and transportation mechanism between alloyed Ni/Au and p-GaN, we also measured barrier information at different temperature and current. The result indicates the hole passes through the barrier by thermionic emission and the barrier may lowered by image force.

… (p-GaN). The contributions of the ohmic contacts and semiconductor material degradation are separated by adopting the transmission line method (… contacts on p-GaN are related to the …

… ABSTRACT ThecontactsofTi/Au,Ti/Al/Au,andTi/Al/Ni/Au films deposited on n-GaN were studied by current–voltage (I–V) and transmission-line-method measurements. The effect of …

… ) regrowth of n + -GaN. The fabricated regrown … by transmission line method (TLM). This work provides a robust and promising pathway to surmount the ohmic contact bottleneck of GaN …

We investigate the thermal stability of InGaN solar cells under thermal stress at elevated temperatures from 400 °C to 500 °C. High Resolution X-Ray Diffraction analysis reveals that material quality of InGaN/GaN did not degrade after thermal stress. The external quantum efficiency characteristics of solar cells were well-maintained at all temperatures, which demonstrates the thermal robustness of InGaN materials. Analysis of current density–voltage (J–V) curves shows that the degradation of conversion efficiency of solar cells is mainly caused by the decrease in open-circuit voltage (Voc), while short-circuit current (Jsc) and fill factor remain almost constant. The decrease in Voc after thermal stress is attributed to the compromised metal contacts. Transmission line method results further confirmed that p-type contacts became Schottky-like after thermal stress. The Arrhenius model was employed to estimate the failure lifetime of InGaN solar cells at different temperatures. These results suggest that while InGaN solar cells have high thermal stability, the degradation in the metal contact could be the major limiting factor for these devices under high temperature operation.

… model consisting of resistances and intrinsic diodes to analyze the current spreading effect in an InGaN/GaN multiple-quantum-well light-emitting diode. Each circuit element was …

InGaN/GaN multiple-quantum-well light-emitting-diode structures utilizing tunnel contact junctions grown by metalorganic chemical vapor deposition have been demonstrated. The p+/n+ GaN tunnel junctions are located in the upper cladding layers of conventional devices, allowing n-type GaN instead of p-type GaN as a top contact layer. Thus, metal ohmic contacts are done at the same time on the top and the lower contact layers. The reverse-biased tunnel contact junction provides lateral current spreading without semitransparent electrode and spatially uniform luminescence exhibiting an improved radiative efficiency. The tunnel contact junction is shown to be an effective method to make possible hole injection via a lateral electron current, with only a small penalty in voltage drop compared to conventional devices.

… and current spreading electrode (TCSE) for thin film solar cells. TH Seo et al. [11] and M. … and current spreading electrode for InGaN/GaN LEDs, whereas the InGaN/GaN solar cell …

… n-electrode patterning on the current spreading and driving voltage of a side-view GaN∕sapphire … The numerical results (current distributions) for the striped pattern p-electrode are well …

… This study uses different electrode configurations of vertical LEDs to analyze the current spreading effect and LEE to optimize efficiency. This study investigates wall-plug efficiency (WPE…

A nonuniform current spreading in the current spreader layer greatly reduced the internal quantum efficiency (IQE) of the light‐emitting diodes (LED). The effects of the current spreading layer on the electrode contact area toward the IQE in a vertical design of gallium nitride (GaN)‐based LED chip is analytically analyzed. The contact area was varied by changing the value of the electrode's width from 2 to 12 μm. Efficiency droop and current density at peak IQE are analyzed based on contact area. The width of 2 μm requires 1.6 μAm−2 current density to achieve peak efficiency and produces a droop of 0.2150. The width of 12 μm requires 9.6 μAm−2 current density to achieve peak efficiency and produces 0.0557 droop. The increase in contact area increases the current density needed to achieve peak IQE while decreases efficiency droop. The optimal spreader contact width of this vertical LED design is 6 μm.

… light extraction with corresponding current spreading effect, which assumes … electrode designs could affect the far field pattern, the main focus of this paper is on the current spreading …

… current-spreading layer that spreads the current under the top electrode to regions not covered by the opaque top electrode… the p-type current-spreading layer above the p-GaN layer. [6,…

This work reports both experimental and theoretical studies on the InGaN/GaN light-emitting diodes (LEDs) with optical output power and external quantum efficiency (EQE) levels substantially enhanced by incorporating p-GaN/n-GaN/p-GaN/n-GaN/p-GaN (PNPNP-GaN) current spreading layers in p-GaN. Each thin n-GaN layer sandwiched in the PNPNP-GaN structure is completely depleted due to the built-in electric field in the PNPNP-GaN junctions, and the ionized donors in these n-GaN layers serve as the hole spreaders. As a result, the electrical performance of the proposed device is improved and the optical output power and EQE are enhanced.

… Comparing the structure of EIS-LED with that of V-LED, we can easily find that this result might be due to the effective current spreading from the electrode-insulator-semiconductor (EIS) …

A high-performance flip-chip light-emitting diode (FCLED) with a Ni/Ag metallic film as high reflectivity mirror (92.67%) of p-type electrode was successfully fabricated. The effect of geometric electrode patterns on the blue InGaN/GaN LEDs was investigated and analyzed qualitatively its current spreading in the active region. With different electrode patterns, these devices were experimented and simulated by simple electrical circuits in order to confirm its current-voltage characteristics and light emission pattern. It was found that the forward voltages of these FCLEDs were about 3.6 V (@350 mA). The light output power of FCLEDs with circle-round type electrode was 368 mW at an injection current of 700 mA. From these optoelectronic measurement and thermal infrared images, we proposed some design methodologies for improved current spreading, light output power, droop efficiency and thermal performance.

… In this study, effects of n-electrode patterns to the current spreading in the active region … (VLED) with GaN/InGaN multi quantum well (MQW). Several n-electrode patterns of the VLED are …

… Uniform current spreading is crucial for the performance of light-emitting diodes (LEDs). It … non-uniform current spreading. In this paper, the impact of different electrode patterns on the …

… of current spreading based on 3-… current spreading in the lateral-electrode type blue LEDs of 320× 320 µm2 size theoretically and experimentally. It is known that the current spreading …

… current spreading by introducing interdigitated electrode patterns to reduce the dimensions of the electrodes to less than the current spreading … of electrode geometry on current unifor…

… lateral conducting GaN-based LEDs (abbreviated as regular LEDs, which has a conventional p-side up lateral structure with both anode and cathode electrodes fabricated on the same …

… considerations regarding current spreading in GaN-based … resistances of both top transparent electrodes and n-layers are … transparent electrode and n-layer) on current spreading and …

Based on the proposed experimental method, the current spreading length of GaN-based light-… the measured current spreading length strongly depends on the injected current density. …

… Based on their model results they determined the critical transparent-electrode thickness as 3/3 nm (Ni/Au). Cao et al. [6] experimentally determined the optimum transparent metal layer …