Fabrication Of Ohmic Contacts Research Articles

Efficient recovery of gold from low concentration Au(S2O3)23– solution by fabricating Ag/MoS2 Ohmic contact

The surface decoration of precious metal particles is one of the most effective strategies for improving the photocatalytic activity of semiconductors. In this work, a MoS2-based Ohmic contact was fabricated by depositing Ag nanoparticles on the surface of MoS2 nanosheets (MoS2 NSs) to enhance the photoreduction of Au(Ⅰ) from a low concentration Au(S2O3)23– solution. After the fabrication of Ohmic contact, more efficient interface charge transfer can be achieved under the drive of built-in electric field. As a result, the photogenerated singlet excitons in MoS2 NSs are readily dissociated into free charges with the electrons drifting to Ag NPs, leading to a higher electron concentration on the surface of Ag/MoS2 NSs. The gold recovery of Ag/MoS2 NSs-4.15% reaches up to 96.67% for the low concentration Au(S2O3)23– solution, which is about 3.9 times higher than that of pristine MoS2 NSs (24.82%). The results of this work are encouraging for further development in the surface science of photocatalysis and demonstrate a pathway for efficient gold recovery via photoreduction.

Fabrication of Ohmic Contact on N-Type SiC by Laser Annealed Process: A Review

In recent years, because of stringent needs in the fabrication of silicon carbide (SiC) power devices, laser annealing has been introduced to achieve local ohmic contact. In this paper, the laser annealing research for the ohmic contact process of SiC power devices is reviewed, which is mainly divided into four aspects: laser process mechanism, ohmic contact electrode materials, and substrate materials. The effect of laser parameters on ohmic contact and the annealing process on SiC diode devices is also reviewed. Progress of other substrate materials, namely 6H-SiC and semi-insulating 4H-SiC-based devices with laser annealed ohmic contacts, is also briefly discussed, in which formation of semi-insulating SiC ohmic contacts is derived from laser irradiation at the interface to produce 3C-SiC. Some experiment results have been shown in the passage, such as XRD, SEM, TEM, etc. In the review, it points out that the direction of application and development of the laser annealing process for improving the ohmic contact of SiC power devices is highly encouraging.

Dopant Activation Depth Profiling for Highly Doped Si:P By Scanning Spreading Resistance Microscopy (SSRM) and Differential Hall Effect Metrology (DHEM)

Heavily n-doped epitaxially grown Si layers are of great importance for source/drain (S/D) application in advanced node nMOS devices. For contact resistivity reduction, the dopant activation level is very important. Various techniques are being used to evaluate dopant activation in Si:P layers. Among these two methods are Scanning Spreading Resistance Microscopy (SSRM) and Differential Hall Effect Metrology (DHEM). SSRM uses an atomic force microscope equipped with a hard conductive probe that is scanned in contact mode on the cross-sectioned sample’s surface and measures spreading resistance. Measured resistance values as a function of depth are converted into resistivity and carrier concentration depth profiles using calibration measurements and conversion relationships. DHEM provides depth profiles of mobility, resistivity and carrier concentration through a semiconductor layer by making successive sheet resistance (Rs) and Hall voltage measurements using Hall effect/Van der Pauw techniques, as the electrically active thickness of the layer is reduced through successive oxidation steps. Controlled oxidation is achieved by electrochemical anodization. Data collected can then be processed to yield the depth profiles. In this contribution we have carried out SSRM and DHEM measurements on Si:P epi layers subjected to different processing conditions including annealing and ohmic contact fabrication. Secondary Ion Mass Spectrometry (SIMS) was used to measure the total (active + inactive) dopant profiles through the films. Effects of these processes on dopant diffusion and activation were studied and the results from DHEM and SSRM were compared.In-situ phosphorus (P) doped Si epi-layers were grown over 300mm diameter boron doped monitor wafers. While one set was kept as the reference, a second set was treated by a spike-annealing process at 1000 °C. In a third set a Ti/TiN contact fabrication process was carried out and the contact was removed before analysis. In the fourth set contact process was applied to the spike annealed wafer before removal of the Ti/TiN layers. Bulk sheet resistance measurements were made using 4-point probe (4PP) for all the samples. SSRM measurements were carried out at IMEC. Cross-shaped Van der Pauw test-patterns were formed on 8mmx8mm areas on the samples and DHEM measurements were performed at ALP. Figure below shows the dopant depth profiles obtained by SIMS and the carrier concentration profiles from DHEM and SSRM techniques for samples D02 (as deposited Si:P) and D03 (spike annealed). One can make some general observations from the data in this figure. The total dopant concentration as measured by SIMS is ~ 1.4E21/cm3. There is only a small difference in the total dopant distribution profiles (SIMS) between the as deposited sample and the spike annealed sample. However, the spike annealed sample shows much higher dopant activation as measured by DHEM. Carrier concentration is ~2.5x higher in sample D03 (~5E20/cm3) compared to sample D02 (~2E20/cm3). Activation levels measured by SSRM, however, are lower for both samples, and the peak carrier concentration value increases only slightly upon spike annealing, going from ~2E20/cm3 in sample D02 to ~2.2E20/cm3 in sample D03. DHEM clearly indicates the sharp interface between the p-type substrate and the n-type epi-layer and its depth calibration agrees well with the expected thicknesses of the epi layers. The tail of the SSRM data is much more graded. Data from other samples will be presented and discussed in the final manuscript. Figure 1

Band offset and electrical properties of ErZO/β-Ga2O3 and GZO/β-Ga2O3 heterojunctions

The fabrication of ohmic contacts on wide bandgap semiconductors, such as β-Ga2O3, is difficult as the scarcity of metals with low-enough work function. The insertion of n+-doped metal oxide layer between metal and the β-Ga2O3 layers are effective for improving the ohmic behaviour. β-Ga2O3, Erbium doped ZnO (ErZO), and Gallium doped ZnO (GZO) films with high crystallinity were deposited by pulsed laser deposition. The band offsets and electrical properties of ErZO/β-Ga2O3 and GZO/β-Ga2O3 heterojunctions were investigated, which revealed the nested gap (type I) structures. The conduction band offsets were determined to be 0.88 eV for ErZO and 0.96 eV for GZO, and their corresponding valance band offsets were found to be 0.46 eV and 0.16 eV. In addition, the difference of the work functions between ErZO and β-Ga2O3 was 0.05 eV; and 0.16 eV between GZO and β-Ga2O3. Solar blind UV detector having fork electrode structure of as-grown β-Ga2O3/ErZO/Ti/Au structure showed good ohmic performance and have superior detector performance comparing with the other two electrode structures, which was ascribed to ErZO’s low band offset and small work function difference with β-Ga2O3. The present result provides an option for fabricating ohmic contact on β-Ga2O3 without the need for post-growth annealing.

Room-Temperature Mid-Infrared Detection via Resonant Microwave Circuits

We demonstrate a room-temperature mid-wave infrared ( $\text {MWIR} ) photoconductive detection architecture utilizing a compact, Ku -band microwave split-ring resonator (SRR) loaded with a micrometer-scale indium arsenide antimonide (InAsSb) pixel. The resonant microwave photoconductor (RMPC) does not require the fabrication of ohmic contacts or the need for dc current collection. We compare the RMPC architecture with a traditional dc photoconductor (DCPC) and evaluate the conditions in which the overall sensitivity of the RMPC exceeds that of the DCPC. Time-domain simulations suggest that the resonator can shape and suppress the fundamental Johnson thermal noise of the semiconductor photon-absorbing element, resulting in decreased total noise and thus increased detectivity, with the SRR circuit itself being the primary generator of thermal noise. For this material system, we calculate that the RMPC has a factor of three improvement in the overall Johnson-limited detectivity ( ${D}^\ast $ ) when compared to the standard DCPC and discuss both the situations for which this improvement could be significantly larger as well as practical system noise considerations.

Low-Temperature Ohmic Contacts to n-ZnSe for all-Electrical Quantum Devices

The II/VI semiconductor ZnSe is an ideal host for novel devices for quantum computation and communication as it can be made nuclear-spin free to obtain long electron spin coherence times, exhibits no electron valley-degeneracy, and allows optical access. A prerequisite to electrical quantum devices is low-resistive Ohmic contacts operating at temperatures below 10 K, which have not been achieved in ZnSe yet. Here, we present a comprehensive study on the realization of Ohmic contacts to ZnSe by three different technological approaches, ion implantation of halogen donors, epitaxial doping with in situ contact processing, and finally, a unique ZnSe regrowth technique. The latter allows fabrication of Ohmic contacts with local doping that can be used to connect to a buried conducting channel such as those used in unipolar devices. DC measurements revealed high contact resistivity for Ohmic contacts to ZnSe doped via halogene ion implantation, while in situ aluminum (Al) contacts on epitaxially chlorine-doped ZnSe yield record low contact resistivities in the order of 10–5 Ω cm2 even at cryogenic temperatures. Finally, making use of the regrowth technique, local Ohmic contacts to ZnSe are demonstrated, which still feature low contact resistivities of (1.4 ± 0.4) × 10–3 Ω cm2 at 4 K. These findings pave the way for new electrical devices in the ZnSe material system such as field-effect transistors, electrostatically defined qubits, or quantum repeaters operating at cryogenic temperatures.

Defect characterization of unannealed neutron transmutation doped silicon by means of deep temperature microwave detected photo induced current transient spectroscopy

Unannealed neutron transmutation doped silicon substrates with a target resistivity of approximately 1000Ωcm are characterized for radiation induced defects by means of microwave detected photoinduced current transient spectroscopy (MD-PICTS). This technique is a contactless advancement of conventional PICTS and does not require the fabrication of ohmic contacts. Defect spectroscopy by means of MD-PICTS is conducted in a broad temperature range between 30 K and 293 K, which makes it possible to identify energetically shallow as well as deep traps. In addition, a wavelength dependent analysis is performed to determine whether a defect is located at the surface or in the bulk material. Three traps with an average activation energy of 68meV, 85meV, and 150meV are observed. In addition, an indication for deep defect states with activation energies between 320meV and 480meV is found. According to the wavelength dependent analysis, it is assumed that all observed traps are bulk defects. Finally, a minority carrier lifetime of approximately 0.7 μs is determined, suggesting that the crystal is heavily damaged by neutron radiation.

Comparison between Ni-SALICIDE and Self-Aligned Lift-Off Used in Fabrication of Ohmic Contacts for SiC Power MOSFET

A comparison between self-aligned process (using lift-off) and Ni-SALICIDE used in fabrication of ohmic contacts for SiC Power MOSFET is done. Both processes are demonstrated for 3.3 kV SiC VDMOS transistors fabricated on 100 mm substrates. It is shown that the Ni-SALICIDE process with first silicidation at 500 °C does not degrade the electrical properties of silicon dioxide; particularly, a degradation of the interlayer dielectric between source and gate is not evident. Additionally, this first silicidation is found to have a positive impact on the specific resistance of contacts formed on p-type SiC using NiAl2.6% as an ohmic metal.

Measurement of carrier lifetime in micron-scaled materials using resonant microwave circuits

The measurement of minority carrier lifetimes is vital to determining the material quality and operational bandwidth of a broad range of optoelectronic devices. Typically, these measurements are made by recording the temporal decay of a carrier-concentration-dependent material property following pulsed optical excitation. Such approaches require some combination of efficient emission from the material under test, specialized collection optics, large sample areas, spatially uniform excitation, and/or the fabrication of ohmic contacts, depending on the technique used. In contrast, here we introduce a technique that provides electrical readout of minority carrier lifetimes using a passive microwave resonator circuit. We demonstrate >105 improvement in sensitivity, compared with traditional photoemission decay experiments and the ability to measure carrier dynamics in micron-scale volumes, much smaller than is possible with other techniques. The approach presented is applicable to a wide range of 2D, micro-, or nano-scaled materials, as well as weak emitters or non-radiative materials.

Characteristics of He Ion Implanted Layers on Single‐Crystal Diamond

Ohmic contact formation on lightly doped or intrinsic diamond is highly desirable particularly for detection application. Nevertheless, good quality of such contacts is difficult to obtain and the production of a thin graphitic layer underneath diamond surface can be an interesting approach. In this paper, intrinsic freestanding single‐crystal CVD samples are implanted with 10 keV He ions at different fluences. Simulations are performed to show that the used fluences are high enough to ensure the creation of an amorphous layer. The implanted layers are characterized before and after annealing by a combination of photoluminescence imaging, confocal microscopy, transmission electron microscopy (TEM), resistivity measurements, and Raman spectroscopy. After annealing, the layer resistivity and the layer density decrease. The values measured after annealing are close to the values measured on graphite. Raman spectroscopy results show that the implantation leads to the creation of a highly disordered graphite layer which quality improves after annealing to reach that of nanocrystalline graphite. It also shows that the use of light ions at relatively low energy makes possible the formation of this graphitic layer at the surface of the sample. These promising results pave the way for ohmic contact fabrication on lightly doped diamond.

Переход от сплавной к несплавной технологии омических контактов при росте диапазона рабочих частот СВЧ МИС на основе нитрида галлия

Переход от сплавной к несплавной технологии омических контактов при росте диапазона рабочих частот СВЧ МИС на основе нитрида галлия

Specifics of fabrication of Ohmic contacts to AlGaAs/GaAs heterostructures with a 2D electron gas

This paper reviews the literature concerning the specifics of creating Ohmic contacts to AlGaAs/GaAs heterostructures with a 2D electron gas with high electron mobility. The process of annealing the contacts based of the Ni/Au/Ge system is considered, and the recommended parameters of the metalization layers are borrowed from the literature. This process allows reproducible fabrication of Ohmic contacts with a low electrical resistance to temperatures below 4K. Several mechanisms are analyzed which could result in the experimentally observed dependence of the contact parameters on crystallographic orientation. A method of contact fabrication with Au/ Ge/Pd metallization is described for which the contact is formed by mutual diffusion and interaction of the metals and the semiconductor in the solid phase at temperatures below 200 °C. This provides for high composition homogeneity of the contacts, a smooth metal / semiconductor boundary and can reduce the effect of orientation on the electric characteristics of the contact.

A detailed study on the frequency-dependent electrical characteristics of Al/HfSiO4/p-Si MOS capacitors

The detailed electrical characterizations of Al/HfSiO4/p-Si (MOS) capacitors were investigated. HfSiO4 thin films were fabricated by RF sputtering system with the power of 300 W onto p-type (100) Si substrate and then annealed at 750 °C in Nitrogen environment for 40 min. After fabrication of ohmic contacts, the electrical characteristics of the capacitors were determined by C–V and G/ω–V measurements for several frequencies from 50 kHz to 1 MHz. It is observed that the measured capacitance and conductance curves are quite sensitive to applied voltage frequency due to time dependent interface states (Dit), border traps (Nbt), and series resistance (Rs). We have observed that the series resistance may significantly deviate from the MOS capacitor characteristics and the relevant correction must be performed. In addition, the calculated interface state density was found to be in the order of 1010 eV−1 cm−2 which is in good agreement with reported convenient dielectric layers for MOS based technology. Moreover flat band voltage variation also observed under applied voltage frequencies and this behavior were attributed the basically border states. On the other hand, barrier potentials varied from 0.615 to 0.559 eV with increasing in frequencies depending on the charge accumulations due to time dependent trap sites. Consequently, Dit, Nbt, Rs are important factors that can affect electrical characteristics of the MOS capacitors. Although the reported values vary under different applied frequencies, the fabricated HfSiO4 dielectric layer exhibits demanding electrical characteristics to be used in MOS-based technologies.

Fabrication of Ohmic contact on semi-insulating 4H-SiC substrate by laser thermal annealing

The Ni contact layer was deposited on semi-insulating 4H-SiC substrate by magnetron sputtering. The as-deposited samples were treated by rapid thermal annealing (RTA) and KrF excimer laser thermal annealing (LTA), respectively. The RTA annealed sample is rectifying while the LTA sample is Ohmic. The specific contact resistance (ρc) is 1.97 × 10−3 Ω·cm2, which was determined by the circular transmission line model. High resolution transmission electron microscopy morphologies and selected area electron diffraction patterns demonstrate that the 3C-SiC transition zone is formed in the near-interface region of the SiC after the as-deposited sample is treated by LTA, which is responsible for the Ohmic contact formation in the semi-insulating 4H-SiC.

Thermal stability of PdGe films on Ge(100) substrate

The thermal stability of PdGe on Ge(100) was investigated by scanning electron microscopy, X-ray diffraction and atom probe tomography. The initial PdGe film agglomerates when heated at high temperature (≥500°C), forming spherical islands. This degradation is highly detrimental for PdGe ohmic contact fabrication in the Ge-based nano-electronics technology. However, capping the Pd layer with W before Pd-Ge reaction increases the thermal stability of the PdGe film up to 600°C. In addition, the PdGe texture is modified if the sample is heated during Pd deposition, increasing the thermal stability of PdGe up to 700°C with the W cap.

Technology and Investigation of Ohmic Contacts to Thermoelectric Materials

Technology is developed, materials and regimes of the fabrication of ohmic contacts to the effective thermoelectric materials Bi2Te2.8Se0.2 (n-type) and Bi0.5Sb1.5Te3 (p-type) are determined. Ohmic contacts were obtained by the vacuum deposition of nickel. Factors determining adhesion strength and resistivity of fabricated contacts are determined. Process of surface preparation of the thermoelectric materials before the ohmic contact deposition is optimized during the technology development. The use of electrochemical polishing, ultrasound treatment, finish cleaning in toluene and isopropyl alcohol vapor, and annealing in vacuum allowed achieving stable results in the formation of contacts. It was shown that contacts fabricated using of electron-beam evaporation of nickel possess maximum adhesion strength of 18–19 N/mm. It was found that high adhesion is caused by the existence of transition layer in the metal-thermoelectric material contact range, formed due to the interaction of metal with the components of thermoelectric material. Proposed technology allows obtaining ohmic contacts with the resistance of the unit area not exceeding 10−10 Ohmm.

Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor performance, which suggest a new direction for the development of next-generation ultrathin and flexible photonic and electronic devices. Enhanced luminescence quantum efficiency has been widely observed in these atomically thin 2D crystals. However, dimension effects beyond quantum confinement thicknesses or even at the micrometer scale are not expected and have rarely been observed. In this study, molybdenum diselenide (MoSe2) layer crystals with a thickness range of 6-2,700 nm were fabricated as two- or four-terminal devices. Ohmic contact formation was successfully achieved by the focused-ion beam (FIB) deposition method using platinum (Pt) as a contact metal. Layer crystals with various thicknesses were prepared through simple mechanical exfoliation by using dicing tape. Current-voltage curve measurements were performed to determine the conductivity value of the layer nanocrystals. In addition, high-resolution transmission electron microscopy, selected-area electron diffractometry, and energy-dispersive X-ray spectroscopy were used to characterize the interface of the metal-semiconductor contact of the FIB-fabricated MoSe2 devices. After applying the approaches, the substantial thickness-dependent electrical conductivity in a wide thickness range for the MoSe2-layer semiconductor was observed. The conductivity increased by over two orders of magnitude from 4.6 to 1,500 Ω(-) (1) cm(-) (1), with a decrease in the thickness from 2,700 to 6 nm. In addition, the temperature-dependent conductivity indicated that the thin MoSe2 multilayers exhibited considerably weak semiconducting behavior with activation energies of 3.5-8.5 meV, which are considerably smaller than those (36-38 meV) of the bulk. Probable surface-dominant transport properties and the presence of a high surface electron concentration in MoSe2 are proposed. Similar results can be obtained for other layer semiconductor materials such as MoS2 and WS2.

Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures

Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures

Thickness-dependent electrical conductivities and ohmic contacts in transition metal dichalcogenides multilayers

We report on the observation of the substantial thickness (t)-dependent electrical conductivity (σ) at a wide thickness range for an MoSe2 layer semiconductor. The conductivity increases for more than two orders of magnitude from 4.6 to 1500 Ω−1 cm−1 with a decrease in thickness from 2700 to 6 nm. The conductivity was found to follow a nearly linear relationship with the reciprocal thickness, i.e. σ ∝ 1/t. The temperature-dependent conductivity measurements also show that the MoSe2 multilayers have much lower activation energies at 3.5−8.5 meV than those (36−38 meV) of their bulk counterparts, indicating the different origins of the majority carrier. These results imply the presence of higher surface conductivity or carrier surface accumulation in this layer crystal. The fabrication of ohmic contacts for the MoSe2 layer nanocrystals using the focused-ion beam (FIB) technique was also demonstrated. This study provides a new understanding which is crucial for the development of flexible electronic devices and transparent conducting materials using ultrathin dichalcogenide layer materials.

Low resistance palladium/molybdenum based ohmic contacts to n-GaSb grown on GaAs

Low resistance ohmic contacts were fabricated on n-type GaSb grown by molecular beam epitaxy. N-type GaSb epilayers with different doping concentrations and thicknesses were fabricated and studied in order to investigate the current transport mechanism between the metal contacts and the semiconductor. Different metallization schemes were implemented to achieve the lowest possible contact resistance. Rapid thermal annealing was performed at various temperatures to achieve the optimal gold penetration into the GaSb epilayers for low resistance. Ohmic contact fabrication and electrical characterization are discussed in detail. The microstructure analysis of the semiconductor and metal contact interfaces was performed using cross-section transmission electron microscopy and energy dispersive spectroscopy. Specific contact resistances as low as 3 × 10−6 Ω cm2 were obtained.