Low Resistance Ohmic Contacts Research Articles

The Effect of the Original Thickness of Ag in the Graphene–Ag Nanodots Transparent Conductive Layer on the Electrical and Optical Properties of GaN-Based UV-LEDs

We investigate the electrical and optical properties of graphene–Ag nanodot compounds as transparent conductive layers (TCLs) for gallium nitride (GaN)-based ultraviolet light-emitting diodes (UV-LEDs) with different Ag thicknesses. As the thickness of Ag increases from 2 to 5 nm on the 365-nm UV-LEDs, the transmittance decreases from 82.5% to 65.2% and ohmic contact resistance decreases from 0.2 to ${1.3}{\times} {10}^{-5} \Omega \text {cm}^{2}$ . At a wavelength of about 320 nm, the TCLs exhibit high transparency, which is also confirmed by the simulation data of the finite-difference time-domain solution. The simulation and experimental results suggest that Ag with a thickness of 5 nm can potentially obtain both low ohmic contact resistance and high transmittance at the wavelength of about 320 nm, which is important for medical applications. We also study the nanodot migration and coalescence mechanism of 2- and 5-nm Ag samples by scanning electron microscopy, as well as the influence of the existence of graphene. The 5-nm Ag sample has smaller and denser Ag nanodots and thus exhibits better electrical properties. In addition, the Ag nanodots after the second annealing procedure with graphene covering are larger than those without graphene covering.

A High Frequency Hydrogen-Terminated Diamond MISFET With ${f}_{{\text{T}}}/{f}_{\max}$ of 70/80 GHz

A high frequency hydrogen-terminated diamond metal-insulator-semiconductor field-effect transistor (MISFET) with extremely small source-drain distance of about 350 nm was realized by self-aligned process on (001)-oriented single crystal diamond substrate. To suppress the gate leakage current, a low temperature Al2O3 film was deposited as the gate insulator by inductance coupled plasma enhanced atomic layer deposition. A low ohmic contact resistance of $1.84~\Omega \cdot $ mm was measured by using transmission line method. The fabricated 100 nm gate length MISFET shows a high current density of 585 mA/mm and a high transconductance of 206 mS/mm. By minimizing the parasitic parameters of the device, a high current gain cut-off frequency ${f} _{\textsf {T}}$ of 70 GHz and maximum frequency of oscillation ${f} _{\textsf {max}}$ of 80 GHz have been realized.

Characterization of Metal Contacts to Two-Dimensional MoTe2

Transition metal dichalcogenide (TMD) atomic layers are an atomically thin material in the form of MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se, or Te). Among them, MoTe2 is attractive because of its narrow band gap (i.e., ~ 1 eV), leading to optical and electrical applications such as field-effect transistors, photodetectors, lightemitting diodes, and photovoltaics. The TMD atomic layers, however, suffer from the extremely high contact resistance of the metal electrodes. The formation of a low-resistance ohmic contact is essential to achieving good device performance. Here, we examined the contact resistance of the two-dimensional MoTe2 atomic tri-layers from transmission line model (TLM) measurements. 2H-phase MoTe2 atomic tri-layers were synthesized on a silicon dioxide/silicon substrate by using metal-organic chemical vapor deposition. The TLM pattern was fabricated on the tri-layers to examine the specific contact resistance of metals. This method is highly effective for minimizing the contact resistance of TMD atomic layers.

A versatile low-resistance ohmic contact process with ohmic recess and low-temperature annealing for GaN HEMTs

Deeply recessed ohmic contacts for GaN-based high electron mobility transistors (HEMTs) are demonstrated. It is shown that low-resistance ohmic contacts can be achieved with recessing beyond the AlGaN Schottky barrier where the ohmic contacts are formed on the sidewall of the recess. This makes the process versatile and relatively insensitive to the exact recess depth. The ohmic contact is based on a gold-free metallization scheme consisting of a Ta/Al/Ta metal stack requiring a low-temperature annealing. Important parameters for this type of ohmic contact process include the metal coverage, slope of the etched sidewall, bottom Ta-layer thickness, as well as annealing temperature and duration. The optimized contact resistance is as low as 0.24 Ω mm after annealing at 575 °C. Moreover, this sidewall contact approach was successfully implemented on different epitaxial heterostructures with different AlGaN barrier thickness as well as with and without AlN exclusion layer. All the samples exhibited excellent contact resistances in a wide range of recess depths. The Ta-based, sidewall ohmic contact process is a promising method for forming an ohmic contact on a wide range of GaN HEMT epitaxial designs.

Ni/W/Ni ohmic contacts for both n- and p-type 4H-SiC

In this study, we used a Ni/W/Ni-layered structure to provide low-resistive ohmic contacts with good thermal stability for both n-type and p-type 4H-SiC. As reference, we used Ni and Ni/Ti/Ni as control groups with specific contact resistivities, and we verified the thermal stability of the structures by specific contact resistivity measurements and thermal duration tests. We found that for both n-type and p-type semiconductors, Ni/W/Ni is superior in terms of thermal stability and specific contact resistivity. Using XRD, we also analyzed the components involved in ohmic contact and thermal stability tests.

One-step synthesis and photoelectrical performance of feather-like tellurium on nickel foil

Feather-like tellurium (Te) was synthesized on Ni substrates by the one-step electrodeposition method no external agents. The morphology, structure, optical and photoelectricity properties of the as-prepared Te film were investigated in detail. The results revealed that the feather-like Te film with a preferred (110) orientation demonstrated hexagonal structure and strong light absorption in the visible range. Te and Ni formed low resistance Ohmic contact; the conductivity of feather-like Te on Ni foil (S3) was measured to be 20.63 × 10−5 s/cm. In addition, the photocurrent density of as-prepared feather-like Te (S3) measured to be 0.0170 mA/cm2, nearly increased by 77% than lowest S4 (0.0039 mA/cm2). This indicates feather-like Te has potential desirable applications in photoelectric devices.

Low Resistance Ti<sub>5</sub>Si<sub>3</sub>/TiC Ohmic contact on Ion-Implanted n-Type 4H-SiC C Face

Low-resistance Ohmic contact on n+ 4H-SiC C-face with Titanium was demonstrated. In a conventional NiSi Ohmic contat on n-type 4H-SiC, a carbon agglomeration at the silicide/SiC interface occurs, and contact resistance becomes larger. For suppressing the carbon agglomeration, laser annealing and Ti metal were introduced to form both silicide and carbide. Ti (75 nm)/SiC and Ni (75 nm)/SiC Ohmic contacts were formed on backside C-face of high concentration impurity doped 4H-SiC substrates with and without activation annealing. Electrical properties were investigated after 40 nanoseconds pulse laser annealing in Ar ambient. As the result, the lowest specific contact resistance of 7.9×10-5 Ωcm2 was obtained in Ti (75 nm)/SiC sample in the case of ion implanted sample at 500°C and with activation annealing at a laser power of 2.2 J/cm2.

Low Temperature Ni-Al Ohmic Contacts to p-Type 4H-SiC Using Semi-Salicide Processing

Most semiconductor devices require low-resistance ohmic contact to p-type doped regions. In this work, we present a semi-salicide process that forms low-resistance contacts (~10-4 Ω cm2) to epitaxially grown p-type (>5×1018 cm-3) 4H-SiC at temperatures as low as 600 °C using rapid thermal processing (RTP). The first step is to self-align the nickel silicide (Ni2Si) at 600 °C. The second step is to deposit aluminium on top of the silicide, pattern it and then perform a second annealing step in the range 500 °C to 700 °C.

High RF Performance AlGaN/GaN HEMT Fabricated by Recess-Arrayed Ohmic Contact Technology

In this letter, a high-performance gate-recessed AlGaN/GaN high electron mobility transistor (HEMT) fabricated by the recess-arrayed ohmic contact technology is presented. An array of square columns was etched in the ohmic region, using Ti/Al/Ni/Au metal deposition and rapid annealing to fabricate a low-resistance ohmic contact. By using the transmission line method measurement, an ohmic contact resistance ( ${R}_{c}$ ) of 0.12 $\Omega$ • mm is measured, which is 70% lower than that of the reference device. The patterned ohmic contact HEMT (POC HEMT) exhibits the saturation current of 1130 mA/mm at ${V}_{ds}=10$ V and ${V}_{gs}=1$ V, with a shift of the knee voltage from 6 V to 3 V. The RF measurement shows the power-added efficiency (PAE) of 71.6% at 5 GHz. After the third harmonic tuning, the PAE of 85.2% and the corresponding power density of 11.2 W/mm and power gain of 16.9 dB are obtained. According to the achieved results, the recess-arrayed ohmic contact processed GaN HEMT has a great potential for high RF efficiency and power applications.

Preparation and characterization of pulsed laser deposited Sb2Te3 back contact for CdTe thin film solar cell

A low electric resistive and stable ohmic back contact are crucial to the commercial application of CdTe solar cells. In this study the Sb2Te3 thin film is employed as a buffer layer in the back contact of CdTe solar cells. The Sb2Te3 thin films are deposited at different substrate temperature by pulsed laser deposition and then characterized to investigate the thin film properties, firstly. Hall measurements shows the Sb2Te3 thin film presents strong p-type semiconductor features with high Hall mobility and carrier concentration. CdTe solar cells with Sb2Te3 buffer layer in different layer thickness and deposition temperature are fabricated to obtain the optimal experiment conditions for device performance. The dark J-V curve of CdTe solar cell with Sb2Te3 buffer layer exhibits a typical diode curve without roll-over phenomenon. An insertion of Sb2Te3 buffer layer eliminates the Schottky barrier resulting in remarkable enhancement of open circuit voltage, short-circuit current and fill factor. Besides, the external quantum efficiency gets improvement in almost all of the absorber wavelength compared to the cells with Au-only contact.

(Invited) Exploration of Process Techniques for Ga2O3 Based Electronics

Monoclinic β-phase Ga2O3 has outstanding potential for power electronics, and high quality, large diameter bulk crystals and epitaxial layers of Ga2O3 are already available with a range of controllable n-type doping levels by edge-defined film-fed (EFG) growth using iridium crucibles, by Czochralski or by float zone. The direct energy bandgap of Ga2O3, ∼4.9 eV, yields a very high theoretical breakdown electric field (∼8 MV/cm). For power electronics, the Baliga figure-of-merit proportional carrier mobility, critical electric field and breakdown voltage, is almost four times higher for Ga2O3 than for GaN. Currently, the major limitation for Ga2O3 based device fabrication is the lack of low resistance Ohmic contacts, low damage dry etching process, and thermally stable Schottky contacts. In this work, we report a technique by employing Aluminum Zinc Oxide (AZO) to improve Ohmic contacts on Ga2O3, studies of etch rates and etching induced damages with Cl2/Ar and BCl3/Ar based discharge as well as Ni/Au and Pt/Au Schottky contacts. High quality Ohmic contacts are a prerequisite for any device to deliver low contact resistance at moderate anneal temperatures. Additional contact resistance leads to slower device switching speeds as well as reliability issues due to local contact heating from high contact resistance during device operation. AZO thin films have been widely studied for transparent and flexible device applications such as liquid crystal displays, plasma display panels, electronic paper displays, organic light emitting diode, solar cells, touch panels, gas sensors and other optoelectronics devices. To be used as a contact layer on Ga2O3, a band alignment to ensure a less barrier to electron transport from the contact metal layer via AZO into the Ga2O3.is a prerequisite. The AZO/Ga2O3 heterojunction was realized with a nested gap alignment of band offsets with a valence band offset of 0.61eV ± 0.06eV and a conduction band offset of 0.79 eV ± 0.80 eV determined from x-RayPhotoelectron Spectroscopy (XPS) measurements. This is useful information for Ohmic contact schemes on n-type Ga2O3 devices. A minimum transfer resistance and specific contact resistance of 0.42 Ω-mm and 2.82 ´ 10-5 Ω-cm2 were achieved after a relative low annealing temperature of 400°C. It is essential to generate high resolution pattern transfer processes on semiconductor with wet chemical or dry etching. While numerous wet etchants been reported, these etching processes involve strong acids and elevated temperatures, and the standard positive photoresists will not be able to use as the etching mask. For dry etching, although, the etching rates are generally low, and the resultant oxygen deficient surfaces, standard device patterning and process are compatible with Ga2O3 based device fabrication. Both Cl2/Ar and BCl3/Ar discharges were used to etch high quality bulk Ga2O3 with a Plasma-Therm Inductively Coupled Plasma (ICP) system in this work. The etching is ion-enhanced and leads to a reduction of Schottky barrier height and diode reverse breakdown voltage in diodes fabricated on the etched surfaces depending on the rf and IPC powers. For low rf and ICP power conditions, the reverse breakdown voltage decreased around 6%, while barrier height reduced from 1.2 to 1.01 eV. Under higher power conditions, the reverse breakdown voltage decreased around 35%, and the barrier height was reduced to 0.86 eV. For Schottky diodes, Ni/Au or Pt/Au were used to form Schottky contact on bulk or epitaxial β- Ga2O3. The barrier heights were 1.07 and 1.04 eV for Ni/Au and Pt/Au, respectively, at 25°C. The diode on-state resistances (Ron) decreased with increasing temperature. The temperature coefficient of reverse breakdown voltage (VB), β, was −4 mV/K for Ni/Au and −0.1 mV/K for Pt/Au. The figure-of-merit (VB 2/RON) was above 3 MW/cm2 at 25°C for Ni/Au diodes and was still ∼1 MW/cm2 at 200°C. The reverse recovery times were also measured as a function of temperature and were of the order of 21–28 ns over the range 25–150°C.

Analysis of defect structure in GaN epilayers doped with implanted Si+ by RBS/c method

In this work, the Si+ doping of undoped GaN (n ∼1 × 1016 cm−3) on sapphire by a single step implantation or sequential implantation/recrystallization processes, to a total fluence of 1 × 1016 at/cm2 and subsequent activation annealing at 1100 °C for 2 min, were demonstrated and analysed. We proved that the multicycle implantation/recrystallization process is better in terms of structural and electrical parameters of implanted region than conventional single step implantation. Rutherford backscattering spectrometry in channeling geometry (RBS/c), McChasy simulations and atomic force microscopy (AFM) studies showed, that the proposed process does not deteriorate surface morphoplogy and leads to a significantly lower defect concentrations. Low resistivity ohmic contacts, with RC < 0.1 Ω mm, to low sheet resistivity Si-implanted GaN region (RSH = (36.9 ± 0.7) Ω/sq.), were obtained with the use of sequential implantation with 30 s recrystallization annealing.

Chromium Trioxide Hole-Selective Heterocontacts for Silicon Solar Cells.

A high recombination rate and high thermal budget for aluminum (Al) back surface field are found in the industrial p-type silicon solar cells. Direct metallization on lightly doped p-type silicon, however, exhibits a large Schottky barrier for the holes on the silicon surface because of Fermi-level pinning effect. As a result, low-temperature-deposited, dopant-free chromium trioxide (CrO x, x < 3) with high stability and high performance is first applied in a p-type silicon solar cell as a hole-selective contact at the rear surface. By using 4 nm CrO x between the p-type silicon and Ag, we achieve a reduction of the contact resistivity for the contact of Ag directly on p-type silicon. For further improvement, we utilize a CrO x (2 nm)/Ag (30 nm)/CrO x (2 nm) multilayer film on the contact between Ag and p-type crystalline silicon (c-Si) to achieve a lower contact resistance (40 mΩ·cm2). The low-resistivity Ohmic contact is attributed to the high work function of the uniform CrO x film and the depinning of the Fermi level of the SiO x layer at the silicon interface. Implementing the advanced hole-selective contacts with CrO x/Ag/CrO x on the p-type silicon solar cell results in a power conversion efficiency of 20.3%, which is 0.1% higher than that of the cell utilizing 4 nm CrO x. Compared with the commercialized p-type solar cell, the novel CrO x-based hole-selective transport material opens up a new possibility for c-Si solar cells using high-efficiency, low-temperature, and dopant-free deposition techniques.

Study on the measurement accuracy of circular transmission line model for low-resistance Ohmic contacts on III-V wide band-gap semiconductors

The accuracy and error propagation for determining the low specific contact resistance of Ohmic contacts on III-V wide band-gap semiconductors based on the circular transmission line model have been analyzed and the validity of this method is discussed in detail. The accuracy is more susceptible to the factors including data fitting method, electrical measurement technique and contact area correction. By using the equations of the original circular transmission line model to extract the fitting parameters, the calculation accuracy is much improved and the inapplicability of the linear least-square fitting is prevented. To further improve the accuracy, a four-probe current-voltage measurement technique was adopted to reduce the parasitic series resistances and the uncertainty bound, especially for the Ohmic contact with low sheet resistance of the semiconductor. Moreover, we have studied the size effect of contact pads of patterns and demonstrated that contact area correction is necessary for the semiconductor with high sheet resistance. A comprehensive error analysis is also performed to fully understand all the impact factors on this advanced method of specific contact resistance measurement, which is benefit for device performance evaluation and failure analysis.

Low resistance Ga-doped ZnO ohmic contact to p-GaN by reducing the sputtering power

Ga-doped ZnO (GZO) thin films were deposited on p-GaN as transparent conducting layers (TCLs) of GaN-based LEDs by radio-frequency (RF) magnetron sputtering. The sputtering power effects on the electrical properties of GZO contacts to p-GaN were studied. It was found that low resistance ohmic contact with a specific contact resistance of 6.0 × 10−4 Ω cm2 was achieved by reducing the sputtering power to 50 W. With increasing the sputtering power, these GZO contacts exhibited non-ohmic contact behavior. The XPS analysis indicated that the formation of gallium oxides phases at the interface and the diffusion of Zn atoms into p-GaN surface region were responsible for the degeneration of ohmic contact. In addition, due to the formation of good ohmic contact and high transmittance of GZO TCL deposited at 50 W, a low forward voltage of 3.64 V at 20 mA was realized, and the light output power was higher than that of LED with conventional ITO TCL.

Optimization of Au-Free Ohmic Contact Based on the Gate-First Double-Metal AlGaN/GaN MIS-HEMTs and SBDs Process

The compatibility of Au-free (Ti/Al/Ti/TiN) ohmic contacts in the gate-first double-metal (GFDM) process for AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) and Schottky barrier diodes (SBDs) on the same 150-mm wafer was investigated and discussed for the first time, including contact pretreatments, Al diffusion in dielectric layers, and vias (contact windows between two metal layers) etching conditions. All of these steps are crucial to ohmic contacts as well as overall AlGaN/GaN device fabrication process. With the optimized ohmic contacts steps, not only an extremely low ohmic contact resistance ( ${R}_{C}$ ) value of $1.07~\Omega \cdot \text {mm}$ but also an excellent uniformity on the 150-mm wafer was obtained. The performance and uniformity of the MIS-HEMTs and SBDs based on the optimized GFDM process were also discussed.

Enhancing ambipolar carrier transport of black phosphorus field-effect transistors with Ni-P alloy contacts.

Reducing power consumption and leakage current in complementary metal-oxide semiconductors (CMOSs) has gained importance for further increasing the transistor density. An effective strategy to achieve this is to use ambipolar carrier transport that can exploit both holes and electrons in a single transistor. We report the enhancement of ambipolar behavior in black phosphorus (BP) field-effect transistors (FET) by forming a low-resistance Ni2P alloy contact via low-vacuum annealing at 250 °C, where the transformation of BP into Ni2P alloy selectively occurred at the source/drain electrodes with the BP channel remaining pristine. The N-channel current on/off ratio and field-effect electron carrier mobility of BP FETs were improved by 98% and 1290%, respectively. Our results suggest that high-performance ambipolar BP FETs with low-resistance ohmic contacts can be achieved via low-temperature vacuum annealing for next-generation CMOS applications.

Role of Firing Temperature, Sheet Resistance, and Contact Area in Contact Formation on Screen-Printed Metal Contact of Silicon Solar Cell

Formation of an Ohmic contact requires a suitable firing temperature, appropriate doping profile, and contact dimensions within resolution limits of the screen-printing process. In this study, the role of the peak firing temperature in standard rapid thermal annealing (RTA) six-zone conveyor belt furnace (CBF) and two inexpensive alternate RTA systems [a custom-designed, three-zone, 5″-diameter quartz tube furnace (QTF) and a tabletop, 3″-diameter rapid thermal processing (RTP)] has been investigated. In addition, the role of sheet resistance and contact area in achieving low-resistance ohmic contacts has been examined. Electrical measurements of ohmic contacts between silver paste/n +-emitter layer with varying sheet resistances and aluminum paste/p-doped wafer were carried out in transmission line method configuration. Experimental measurements of the contact resistivity (ρ c) exhibited the lowest values for CBF at 0.14 mΩ cm2 for Ag and 100 mΩ cm2 for Al at a peak firing temperature of 870°C. For the QTF configuration, lowest measured contact resistivities were 3.1 mΩ cm2 for Ag and 74.1 mΩ cm2 for Al at a peak firing temperature of 925°C. Finally, for the RTP configuration, lowest measured contact resistivities were 1.2 mΩ cm2 for Ag and 68.5 mΩ cm2 for Al at a peak firing temperature of 780°C. The measured contact resistivity exhibits strong linear dependence on sheet resistance. The contact resistivity for Ag decreases with contact area, while for Al the opposite behavior is observed.

Substrate-induced semiconductor-to-metal transition in monolayer WS2

Low-resistance ohmic contacts are a challenge for electronic devices based on two-dimensional materials. We show that an atomically precise junction between a two-dimensional semiconductor and a metallic contact can lead to a semiconductor-to-metal transition in the two-dimensional material--a finding which points the way to a possible method of achieving low-resistance junctions. Specifically, single-layer WS$_2$ undergoes a semiconductor-to-metal transition when epitaxially grown on Ag(111), while it remains a direct band gap semiconductor on Au(111). The metallicity of the single layer on Ag(111) is established by lineshape analysis of core level photoemission spectra. Angle-resolved photoemission spectroscopy locates the metallic states near the Q point of the WS$_2$ Brillouin zone. Density functional theory calculations show that the metallic states arise from hybridization between Ag bulk bands and the local conduction band minimum of WS$_2$ near the Q point.

Field-assisted diffusion of silver in SnO2 thin films

Tin oxide films are known for their wide spread applications in the field of optoelectronics as transparent conductors and as atmosphere sensitive ceramic pellets for chemical sensor fabrication. Forming durable ohmic contacts to these films is vitally necessary for the effective performance and long life of almost all such devices. Silver electrodes are commonly considered as low resistance ohmic contacts to many metal-oxide semiconductors. Particularly in devices operating at elevated temperatures, silver migration can considerably decrease useful device lifetime. Here, we study the performance of silver electrodes paste printed on SnO2 thin films and show that the I-V characteristics of the silver/SnO2 contact changes with operating time at elevated temperatures. SnO2 films are grown on alumina substrates by ultrasonic spray pyrolysis technique. A pair of Ag electrodes are paste-printed on the surface of the grown films followed by a heat-treatment step to stabilize the deposits. The as-fabricated Ag/SnO2 contacts present ohmic characteristics. Continuous application of DC voltages to the samples operating at 350 °C results in a significant drop in the devices’ resistance. The results originate from field-assisted migration of silver ions from the anode to the cathode electrode and formation of Ag micro-filaments in the polycrystalline SnO2 layer.