Ohmic Contact Metallization Research Articles

Improved Thermal Stability Observed in Ni-Based Ohmic Contacts to n-Type SiC for High-Temperature Applications

The high-temperature stability of a Pt/TaSi2/Ni/SiC ohmic contact metallization scheme was characterized using a combination of current–voltage measurements, Auger electron spectroscopy, and transmission electron microscopy imaging and associated analytical techniques. Increasing the thicknesses of the Pt and TaSi2 layers promoted electrical stability of the contacts, which remained ohmic at 600°C in air for the extent of heat treatment; the specific contact resistance showed only a gradual increase from an initial value of 5.2 × 10−5 Ω cm2. We observed a continuous silicon oxide layer in the thinner contact structures, which failed after 36 h of heating. Meanwhile, thicker contacts with enhanced stability contained a much lower oxygen concentration that was distributed across the contact layers, precluding the formation of an electrically insulating contact structure.

Study of the Formation Mechanism of Cu/Ge/Pd Ohmic Contact to n-Type InGaAs

This study investigates electrical characteristics and the formation mechanism of the Cu/Ge/Pd Ohmic contact to n-type InGaAs. After annealing the contact at 250°C for 20 min, Cu 3 Ge and Pd 12 Ga 5 As 2 compounds formed and Ge diffused into the InGaAs layer, achieving a heavily doped InGaAs layer with a low contact resistivity of 1 × 10 −6 Ω cm 2 . Thermal stability tests were performed on the Cu/Ge/Pd Ohmic contact to InGaAs after Ohmic contact formation, showing no obvious degradation after a 72 h reliability test at 250°C. The results indicate excellent electrical characteristics and thermal stability using Cu/Ge/Pd as an Ohmic contact metal to an n-InGaAs layer.

Contact mechanisms and design principles for (Schottky and Ohmic) metal contacts to semiconductor nanowires

Contact mechanisms and design principles for (Ohmic and Schottky) metal (M) contacts to semiconductor nanowires (NWs) have been studied. The NWs have been assumed to be cylindrical. A unified model has been developed for the contacts. The model takes into consideration the amorphicity of the M/NW interface structure, the diameter dependence of the energy band gap, the barrier height modulation, and the fluctuations in both the barrier height and the applied bias. While the fluctuations in the barrier height are assumed to involve band tails, the fluctuations in applied bias are assumed to involve tiny Gaussian peaks. Several different features of the Ohmic and the Schottky contacts have been addressed. These include temperature and dimension dependencies of the current-voltage characteristics, the influence of the M/NW interface on the contact characteristics, the relationship between the barrier height and the ideality factor, and the barrier height reduction as a function of temperature. The model appears to be very general. It seems to explain all experimental results available to date in the literature. The calculated results are almost always in good correspondence with the experimental results. The study seemingly demonstrates an alternative to the doping dependence of the Ohmic contacts. It elucidates the fundamental physics underlying M/NW contacts. It highlights means to yield low-resistivity Ohmic contacts by thermionic emission. It describes design criteria for both Ohmic and Schottky contacts. The design criteria for Ohmic contacts tend to address the long-term reliability concerns for devices. They explain also the behavior of both good and bad Ohmic contacts. All these may be the most striking attributes of the study. These attributes explain why Schottky contacts to NWs, with proper gate modulation, may act also as Schottky barrier transistors.

A Novel Tungsten–Nickel Alloy Ohmic Contact to SiC at 900 $^{\circ}\hbox{C}$

A novel tungsten-nickel ohmic contact metallization on 4H-SiC and 6H-SiC capable of surviving temperatures as high as 900°C is reported. Preliminary results revealed the following: (1) ohmic contact on n-type 4H-SiC having net doping levels (N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> 's) of 1.4 and 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , with specific contact resistances ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sNd</sub> 's of 7.69 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> and 5.81 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> Ω · cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively, after rapid thermal annealing (RTA), and 5.9 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> and 2.51 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> Ω · cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively, after subsequent soak at 900°C for 1 h in argon, and (2) ohmic contact on n- and p-type 6H-SiC having N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> > 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> and N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">α</sub> > 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , with ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sNd</sub> = 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> and ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sNa</sub> = 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> Ω · cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively, after RTA, and ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SNd</sub> = 2.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> and ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SNa</sub> = 1.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> Ω · cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> after subsequent treatment at 900°C for 1 h in argon, respectively.

Comparative Study of Ohmic Contact Metallizations to Nanocrystalline Diamond Films

Nanocrystalline diamond (NCD) films were deposited using plasma-enhanced chemical vapor deposition. The NCD films were Boron-doped for p-type conductivity, yielding sheet resistances from 6.17x1011 to 522.5 /. Four different metals were deposited as Ohmic contacts and investigated for contact resistance and thermal stability. Contact and film annealing was performed under different atmospheric conditions with variable N2 content.

Dependence of ohmic contact resistance on barrier thickness of AlN/GaN HEMT structures

Abstract A multi-faceted study on the reduction of ohmic contact resistance to AlN/GaN-based heterostructures is presented. Minimum contact resistance of 0.5 Ω mm has been achieved by partially etching the AlN barrier layer using a chlorine-based plasma dry-etch prior to ohmic contact metallization. For thin GaN-capped AlN/GaN heterostructures, we find it is necessary to remove the GaN cap in the vicinity of the contact metal in order to obtain a linear current–voltage relationship. We compare our results of the pre-metallization etched contacts to those without an etch as well as to results reported in the literature.

Direct contact mechanism of Ohmic metallization to AlGaN/GaN heterostructures via Ohmic area recess etching

The effects of recess etching of the Ohmic contact area on the performance of Ti/Al/Mo/Au metallization and its interfacial reactions with AlGaN/AlN/GaN epilayers were investigated. The best Ohmic contact performances of 0.31, 0.41, and 0.26 Ω mm for three epilayers from two different sources were obtained only when the two-dimensional electron gas (2DEG) channels were completely removed under the Ohmic contact metallization. This is due to the direct sideway contact made by the electrode to the 2DEG around the edges of the active-layer mesas or pads; this is believed to be a more efficient carrier transport mechanism than tunneling through the AlGaN barrier.

Influence of annealed ohmic contact metals on electron mobility of strained AlGaN/GaN heterostructures

The influence of annealed ohmic contact metals on the electron mobility of a two dimensional electron gas (2DEG) is investigated on ungated AlGaN/GaN heterostructures and AlGaN/GaN heterostructure field effect transistors (AlGaN/GaN HFETs). Current-voltage (I-V) characteristics for ungated AlGaN/GaN heterostructures and capacitance-voltage (C-V) characteristics for AlGaN/GaN HFETs are obtained, and the electron mobility for the ungated AlGaN/GaN heterostructure is calculated. It is found that the electron mobility of the 2DEG for the ungated AlGaN/GaN heterostructure is decreased by more than 50% compared with the electron mobility of Hall measurements. We propose that defects are introduced into the AlGaN barrier layer and the strain of the AlGaN barrier layer is changed during the annealing process of the source and drain, causing the decrease in the electron mobility.

Degradation of TiAlNiAu as ohmic contact metal for GaN HEMTs

Ti/Al/Ni/Au stack is widely used to form ohmic contact on GaN based semiconductor material. Long term thermal storage tests conducted to assess AlGaN/GaN HEMT technology have shown a dramatic degradation of this metal when stored more than 100 h at temperatures above 340 °C. The first evidence of degradation is the increase of resistance and the surface morphology evolution leading passivation film to crack. AES and EDS analyses have demonstrated that Ga out-diffusion and Au inter-diffusion are the root causes of this degradation. Cross sections outlined voids occurrence.

Impact of the contact metallization on the performance of photoconductive THz antennas

Both AuGe based alloys and Ti/Au metal layer stacks are widely used as ohmic metal contacts for photoconductive THz antennas made of low temperature grown GaAs. Here, we present the first systematic comparison between these two metallization types. A series of antennas of both kinds is excited by femtosecond laser pulses and by the emission from two diode lasers, i.e. we test the structures as pulsed THz emitters and as photomixers. In both cases, coherent and incoherent detection schemes are employed. We find that the power emitted from the antennas with AuGe metallization is 50% higher than that of antennas with a Ti/Au metal layer. From a comparison with a photomixer model we conclude that the higher output power results from a lower contact resistance of the AuGe contacts leading to an increased current flow. However, Ti/Au contacts have a higher thermal stability which might be advantageous if high system stability is called for.

11.9 W Output Power at S-band from 1 mm AlGaN/GaN HEMTs

We present radio-frequency (RF) power results of GaN-based high electron mobility transistors (HEMTs) with total gate widths (Wg) up to 1 mm. The AlGaN/GaN epi-structures are MOVPE-grown on 2-inches semi-insulating (s.i.) 4H-silicon carbide substrates. The HEMTs have been fabricated using an optimized process flow comprising a low-power Ar-based plasma after ohmic contact metallization, cleaning of the AlGaN surface prior to the Schottky gate metallization using a diluted ammonia (NH4OH) solution, and passivation of the AlGaN surface using a silicon nitride layer deposited by plasma enhanced chemical vapor deposition. We will show that the best RF power performance has been achieved by HEMTs with iron-doped GaN buffer layers (GaN:Fe). Devices with a total gate width of 1 mm yielded a maximum output power of 11.9 W at S-band (2 - 4 GHz) under class AB bias conditions (VDS = 40 - 60 V, and VGS = -4.65 - - 4.0 V).

Synthesis, contact printing, and device characterization of Ni-catalyzed, crystalline InAs nanowires

InAs nanowires have been actively explored as the channel material for high performance transistors owing to their high electron mobility and ease of ohmic metal contact formation. The catalytic growth of nonepitaxial InAs nanowires, however, has often relied on the use of Au colloids which is non-CMOS compatible. Here, we demonstrate the successful synthesis of crystalline InAs nanowires with high yield and tunable diameters by using Ni nanoparticles as the catalyst material on amorphous SiO2 substrates. The nanowires show superb electrical properties with field-effect electron mobility ~2700 cm2/Vs and ION/IOFF >103. The uniformity and purity of the grown InAs nanowires are further demonstrated by large-scale assembly of parallel arrays of nanowires on substrates via the contact printing process that enables high performance, “printable” transistors, capable of delivering 5 10 mA ON currents (~400 nanowires).

Design and Fabrication of Planar GaAs Gunn Diodes

We studied planar graded-gap injector GaAs Gunn diodes designed for operation at 94GHz. Two types of planar Gunn diodes were designed and fabricated. In the first diode, a cathode was situated inside a circular anode with a diameter of 190μm. The distance between the anode and cathode varied from 60μm to 68μm depending on the cathode size. Also, we designed a structure with a constant distance between the anode and cathode of 10μm. In the second diode, the anode was situated inside the cathode for the flip-chip mounting on the oscillator circuits. The fabrication of the Gunn diode was based on ohmic contact metallization, mesa etching, and air-bridge and overlay metallization. DC measurements were carried out, and the nature of the negative differential resistance, the operating voltage, and the peak current in the graded-gap injector GaAs Gunn diodes are discussed for different device structures. It is shown that the structure with the shorter distance between the cathode and anode has a higher peak current, higher breakdown voltage, and lower threshold voltage than those of the structure with the larger distance between the cathode and anode.

Ohmic contact formation on n-type Ge

Severe Fermi level pinning at the interface between n-Ge and a metal leads to the formation of a Schottky barrier, almost independent on the metal work function. Therefore, it seems impossible to form metal Ohmic contacts on moderately, n-type doped Ge layers. For p-type Ge, the Fermi level pinning works opposite: all metal contacts show Ohmic behavior. This fixed behavior can be altered by the introduction of a thin Ge3N4 layer. Ge3N4 seems effective in reducing Fermi level pinning and, therefore, allows the formation of Ohmic contacts on n-type Ge and a rectifying contact on p-type Ge.

Surface-channel MESFET with boron-doped contact layer

In this investigation an attempt has been made to incorporate a high temperature stable refractory metal ohmic contact deposited onto an oxygen terminated contact area into the surface channel field effect transistor concept based on a H-terminated surface in the channel area. First transistors were fabricated. A drain current density of 75 mA/mm and a threshold voltage of V th = −1.5 V was obtained for 1 µm gate length.

Implementation of complex nanosystems using a versatile ultrahigh vacuum nonlithographictechnique

We have developed an ultrahigh vacuum technique for the implementation of complexnanosystems incorporating nonlithographic nanoparticles, ohmic contact metals andisolation dielectrics. The technique is compatible with the silicon integrated circuitmanufacturing process and is versatile, allowing the deposition of nanoparticles of anymetal, semiconductor or insulator with diameters as small as 2 nm with less than 5%size variation. In addition, the technique allows the creation of multi-layeredstructures of nanoparticles of different dimensions. The flexibility and the versatility ofthe technique have been demonstrated by depositing nanoparticles of variousmaterials as well as fabricating multi-layered structures incorporating nanoparticles.

Correlation between the leakage current and the thickness of GaN‐layer of AlGaN/GaN‐HFET

For further improvements in AlGaN/GaN heterojunction field-effect transistor performance, it is necessary to reduce the buffer leakage current of the GaN layer. We studied the correlation between the buffer leakage current and the thickness of GaN layers. Unintentionally doped GaN layers with different thicknesses from 0.2 μm to 2 μm were grown by metal organic chemical vapor deposition on SiC substrates. The buffer leakage current of the GaN layer was measured after deposition of ohmic metal contact and the dislocations were observed using a transmission electron microscopy. The thinner samples had a smaller buffer leakage current although a higher density of threading dislocations. We discussed the origin for the reduction in the buffer leakage current for the thinner GaN layers by assuming that the residual carrier in the GaN layer was compensated by dislocations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electroless deposition and silicidation of Ni contacts into p-type Porous Silicon

Porous Silicon (PS) has attracted much attention since the discovery of its photo luminescent behavior. It has also been used for various other applications such as electroluminescent light emitting-diodes (LEDs), photodetectors and solar cells. For such devices, it is important to make good metallic Ohmic contacts to the PS in order to maximize the efficiency. In order to produce buried contacts, barrier layers, Schottky devices, etc. in PS, it is advantageous to deposit metal that covers not only the surface of the porous layer, but also the inside walls and the bottom of the pores. In this work experiments were performed to examine the morphology and properties of electroless deposition of Nickel into p-type PS and subsequent formation of Nickel silicide after heat treatment. Circular PS samples of 6 mm diameter were produced by anodizing p-type Silicon wafers for 15 min and were subsequently plated with Ni using three different plating baths. The pores are on average 20 µm deep and 4 µm wide. Two samples of each type were heat treated in an nitrogen atmosphere for one hour at 400 and 600°C respectively to produce Nickel silicide. Reference samples were made by means of electron beam evaporation of Ni. SEM micrographs show that the best pore coverage was achieved using the Ni plating bath containing hypophosphite. I–V characterization shows that different rectifying and Ohmic contacts can be formed between electroless deposited Ni and PS depending on the conditions of the heat treatment. XRD and EDX characterizations show that both the NiSi and Ni2Si phases exist in the sample at the same time.

Role of electrode metallization in performance of semi-insulating GaAs radiation detectors

In the present work, a comparative study of semi-insulating (SI) GaAs radiation detectors with different blocking (Schottky) and ohmic contact metallization is presented. The detectors fabricated from “detector-grade” bulk SI GaAs are characterized by current–voltage measurements and their detection performance is evaluated from pulse-height spectra of 241Am and 57Co γ-sources. Observed results are evaluated and discussed. Importance of the optimized electrodes technology of SI GaAs detector with good performance is demonstrated.

The influence of ohmic back contacts on the properties of a-Si:H Schottky diodes

We have investigated the temperature dependence of the current–voltage ( I– V) characteristics of Au/a-Si:H Schottky structures using different back metal ohmic contacts (Al, Cr, Mg or combination of Mg/Sb). I– V characteristics were measured in a wide temperature range (from 145 to 425 K) to find out the temperature dependence of the ideality factor, barrier height and series resistance for the different metal contacts used. We have also calculated the modified Richardson constant and barrier height from the plot of thermionic emission saturation current as a function of temperature.