Ohmic Contact Characteristics Research Articles

Effect of Sulfurization Temperature on Properties of Cu2ZnSnS4 Thin Films and Diffusion of Ti Substrate Elements

The addition of flexible Cu2ZnSnS4 (CZTS) thin film solar cells to titanium (Ti) substrates is an attractive way to achieve the low-cost manufacturing of photovoltaics. Prior research has indicated that the appropriate diffusion of Ti elements can enhance the crystalline growth of CZTS films. However, the excessive diffusion of Ti has been shown to adversely affect the photovoltaic performance of CZTS photovoltaic devices. Therefore, it is essential to regulate the diffusion of Ti elements within CZTS thin films to optimize their photovoltaic properties. The tendency for Ti substrate elements to diffuse into CZTS films is also influenced by the activation energy associated with these Ti elements. The sulfurization temperature is posited to be a critical factor in modulating the diffusion and activation energy of Ti elements within CZTS thin films. Consequently, this research investigates the alteration of the sulfurization temperature of CZTS thin films in order to enhance the properties of these thin films and to examine the diffusion behavior of titanium elements. The results reveal that as the sulfurization temperature increases, the diffusion of Ti elements within the CZTS thin films initially increases, then decreases, and subsequently increases again. This pattern suggests that the diffusion of Ti elements is affected not only by the activation energy of the Ti elements but also by the defect hopping distance within the CZTS thin films. Notably, at a sulfurization temperature of 550 °C, the grains at the base of the CZTS thin film demonstrate an increased density, which is associated with a reduced defect hopping distance, thereby hindering the diffusion of Ti elements within the CZTS thin films. Furthermore, at this specific sulfurization temperature, the slope of the current–voltage (I–V) curve for the CZTS/Ti structure reaches its maximum, indicating optimal ohmic contact characteristics.

Schottky-contact intrinsic current blocking layer for high efficiency AlGaInP-based red mini-LEDs.

AlGaInP-based red light emitting diodes (LEDs) are considered as promising light sources in future full-color displays. At present, vertical chip configuration is still the mainstream device structure of AlGaInP-based red LEDs. However, current crowding around p-electrode severely hinders an efficient improvement. Here, we propose a Schottky-contact current blocking layer (SCBL) to enhance current spreading and to improve light extraction efficiency of AlGaInP-based red vertical miniaturized LEDs (mini-LEDs). By utilizing the Schottky contact between ITO and p-GaP, the SCBL can hinder current crowding around the p-electrode. The current is forced to inject into an active region through a p-GaP+ ohmic contact layer, avoiding light absorption by p-electrode. Through the transfer length method, the Schottky contact characteristics between the ITO and p-GaP as well as the ohmic contact characteristics between ITO and p-GaP+ are demonstrated. Benefiting from superior current spreading and improved light extraction, a mini-LED with SCBL realizes an enhancement of 31.8% in external quantum efficiency (EQE) at 20 mA in comparison with a mini-LED without SCBL.

Exploring the Odd–Even Effect, Current Stabilization, and Negative Differential Resistance in Carbon-Chain-Based Molecular Devices

The transport properties of molecular devices based on carbon chains are systematically investigated using a combination of non-equilibrium Green’s function (NEGF) and density functional theory (DFT) first-principle methods. In single-carbon-chain molecular devices, a distinct even–odd behavior of the current emerges, primarily influenced by the density of states (DOS) within the chain channel. Additionally, linear, monotonic currents exhibit Ohmic contact characteristics. In ladder-shaped carbon-chain molecular devices, a notable current stabilization behavior is observed, suggesting their potential utility as current stabilizers within circuits. We provide a comprehensive analysis of the transport properties of molecular devices featuring ladder-shaped carbon chains connecting benzene-ring molecules. The occurrence of negative differential resistance (NDR) in the low-bias voltage region is noted, with the possibility of manipulation by adjusting the position of the benzene-ring molecule. These findings offer a novel perspective on the potential applications of atom chains.

ZrTe2 Compound Dirac Semimetal Contacts for High-Performance MoS2 Transistors.

Recent investigations reveal elemental semimetal (Bi and Sb) contacts fabricated with conventional deposition processes exhibit a remarkable capacity of approaching the quantum limit in two-dimensional (2D) semiconductor contacts, implying it might be an optimal option to solve the contact issue of 2D semiconductor electronics. Here, we demonstrate novel compound Dirac semimetal ZrTe2 contacts to MoS2 constructed by a nondestructive van der Waals (vdW) transfer process, exhibiting excellent ohmic contact characteristics with a negligible Schottky barrier. The band hybridization between ZrTe2 and MoS2 was verified. The bilayer MoS2 transistor with a 250 nm channel length on a 20 nm thick hexagonal boron nitride (h-BN) exhibits an ION/IOFF current ratio over 105 and an on-state current of 259 μA μm-1. The current results reveal that 2D compound semimetals with vdW contacts can offer a diverse selection of proper semimetals with adjustable work functions for the next-generation 2D-based beyond-silicon electronics.

First-principles study of the rectifying properties of Au/SnO2 interface

Based on the first-principles calculations, we investigate the properties of Au contacts, surface structure, and band bending for Au/SnO2(101) and Au/SnO2(100) contacts under different conditions. Our results show that the Schottky barrier height (SBH) strongly depends on the interface structure. A stoichiometric Au/SnO2(101) interface forms a Schottky contact, while Au/SnO2(100) exhibits ohmic contact characteristics. When oxygen vacancy (VO) is present in the interfacial SnO2 layer, the Fermi level shifts up to the conduction band minimum, resulting in a smaller SBH. Although Au contact on SnO2(101) with VO has non-Schottky properties, it gains Schottky properties after doping with a low-valence element. In addition, the SBHs of each interface structure are different, indicating that the modulation effect of SBH mainly depends on the rearrangement of interfacial potentials caused by the different interface structures. Such a rearrangement causes corresponding changes in the energy band at the interface, resulting in different SBHs.

Study on P-AlGaAs/Al/Au Ohmic Contact Characteristics for Improving Optoelectronic Response of Infrared Light-Emitting Device.

The Al/Au alloy was investigated to improve the ohmic characteristic and light efficiency of reflective infrared light-emitting diodes (IR-LEDs). The Al/Au alloy, which was fabricated by combining 10% aluminum and 90% gold, led to considerably improved conductivity on the top layer of p-AlGaAs of the reflective IR-LEDs. In the wafer bond process required for fabricating the reflective IR-LED, the Al/Au alloy, which has filled the hole patterns in Si3N4 film, was used for improving the reflectivity of the Ag reflector and was bonded directly to the top layer of p-AlGaAs on the epitaxial wafer. Based on current-voltage measurements, it was found that the Al/Au alloyed material has a distinct ohmic characteristic pertaining to the p-AlGaAs layer compared with those of the Au/Be alloy material. Therefore, the Al/Au alloy may constitute one of the favored approaches for overcoming the insulative reflective structures of reflective IR-LEDs. For a current density of 200 mA, a lower forward voltage (1.56 V) was observed from the wafer bond IR-LED chip made with the Al/Au alloy; this voltage was remarkably lower in value than that of the conventional chip made with the Au/Be metal (2.29 V). A higher output power (182 mW) was observed from the reflective IR-LEDs made with the Al/Au alloy, thus displaying an increase of 64% compared with those made with the Au/Be alloy (111 mW).

Analytical and Physical Investigation on Source Resistance in InxGa1−xAs Quantum-Well High-Electron-Mobility Transistors

We present a fully analytical model and physical investigation on the source resistance (RS) in InxGa1-xAs quantum-well high-electron mobility transistors based on a three-layer TLM system. The RS model in this work was derived by solving the coupled quadratic differential equations for each current component with appropriate boundary conditions, requiring only six physical and geometrical parameters, including ohmic contact resistivity (ρc), barrier tunneling resistivity (ρbarrier), sheet resistances of the cap and channel regions (Rsh_cap and Rsh_ch), side-recessed length (Lside) and gate-to-source length (Lgs). To extract each model parameter, we fabricated two different TLM structures, such as cap-TLM and recessed-TLM. The developed RS model in this work was in excellent agreement with the RS values measured from the two TLM devices and previously reported short-Lg HEMT devices. The findings in this work revealed that barrier tunneling resistivity already played a critical role in reducing the value of RS in state-of-the-art HEMTs. Unless the barrier tunneling resistivity is reduced considerably, innovative engineering on the ohmic contact characteristics and gate-to-source spacing would only marginally improve the device performance.

Ohmic Contact Characteristics of AlGaN-based Deep-ultraviolet Light-emitting-diodes with NiAu Transparent Electrode

在p⁃AlGaN表面沉积Ni/Au/Ni/Aué€æ˜Žç”µæžä½“ç³»ï¼Œé€šè¿‡ä¼ è¾“çº¿æ¨¡åž‹æµ‹è¯•ï¼Œç ”ç©¶äº†é€€ç«æ¸©åº¦å¯¹Ni/Au/Ni/Au与p⁃AlGaN材料接触特性的影响。结果表明，AlGaN基深紫外LED采用Ni/Au/Ni/Au金属体系，在600 ℃空气氛围下退火3 min形成p型半导体材料NiO。进一步优化Ni/Au/Ni/Au体系金属厚度，当Ni/Au/Ni/Au各层厚度由20/20/20/20 nm减薄至2/2/5/5 nm，并在600 ℃空气氛围退火3 minï¼Œå ¶ä¸Žp⁃AlGaN材料的接触电阻率从3.23×10-1 Ω·cm2降到2.58×10-4 Ω·cm2。采用上述优化的Ni/Au/Ni/Au体系制备的深紫外LEDå™¨ä»¶ï¼Œå™¨ä»¶å ‰ç”µç‰¹æ€§å¾—åˆ°äº†æ”¹å–„ã€‚åœ¨150 mA驱动下工作电压低至5.8 Vï¼›é€šè¿‡æå‡ç”µæžé€è¿‡çŽ‡ï¼Œå ‰è¾“å‡ºåŠŸçŽ‡æå‡18.9%。

Interlayer Doping of Cu on Bilayer Black Phosphorus for Enhanced Charge Transfer and Transport Properties.

Metal doping between black phosphorus (BP) layers has great advantages in modulating electronic properties. Here, the effects of Cu intercalation on charge transfer and carrier dynamics are investigated by theoretical calculations. Relative to the pristine bilayer BP, Cu suppresses the nonradiative electron-hole recombination, reducing the major pathways of energy and current loss. Furthermore, we investigate a novel pn homogeneous junction based on the Cu-doped bilayer BP, which shows enhanced transport properties and Ohmic contact characteristics. This is because doping leads to the transformation of BP from p-type to n-type, charge accumulation on conduction bands allows electrons to be easily transferred to the p-type bilayer BP, and associated electrical properties can be modulated by the doping concentration. This study has fundamental importance for understanding structure-property relationships in metal intercalation, which is an important guidance for integration and interlayer engineering for two-dimensional materials.

Low Specific Contact Resistivity of 10−3Ω·cm2 for Ti/Al/Ni/Au Multilayer Metals on SI-GaN:Fe Substrate

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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-3}\,\,\Omega \cdot $ </tex-math></inline-formula> cm2 is achieved. At a future date, such excellent ohmic contact characteristics can be applied to the SI-GaN:Fe-based high-power PCSS.

Electrical Conductance at Directly Bonded Si/Si Interfaces in Dependence on Oxygen Concentration in Bonding Ambient

Direct semiconductor wafer bonding is a versatile fabrication scheme for high-performance optoelectronic devices. In the present study, the influence of oxygen concentration in the bonding ambient on the electrical conductance at directly bonded Si/Si interfaces is experimentally investigated in relation to interfacial oxidation. The interfacial electrical conductivity is observed higher for lower oxygen concentration at each bonding temperature in the range of 200 °C–400 °C. Ohmic contact characteristics are found attainable in the bonded interfaces by proper choice of bonding conditions. To support the electrical conductance trend, an X-ray photoelectron spectroscopy analysis confirms the extent of interfacial oxidation to be higher for lower oxygen concentration and higher bonding temperature. In addition, solar cell fabrication and operation with a current path through the bonded interface are demonstrated by using the bonding method in a low oxygen concentration ambient. The energy conversion efficiency of the bonded cell is observed comparable to that of an unbonded reference, to thus verify the suitability of the bonding scheme for device applications.

Investigation on the Effect of Annealing Temperature on the Side Ohmic Contact Characteristics for Double Channel GaN/AlGaN Epitaxial Layer

A side ohmic contact mode for the double channel GaN/AlGaN epitaxial layer is proposed in this paper. Rectangle transmission line model (TLM) electrodes are prepared, and the specific contact resistance is tested at the annealing temperatures from 700 °C to 850 °C. The results show that the minimum specific contact resistance is 2.58 × 10−7 Ω·cm2 at the annealing temperature of 750 °C, which is three to four times lower than the surface contact mode. Scanning electron microscope (SEM), energy dispersive spectrometer (EDS), and atomic force microscope (AFM) were carried out for the analysis of the morphology, element composition, and the height fluctuation at the contact edge. With the increase in the annealing temperature, the specific contact resistance decreases due to the alloying of electrodes and the raised number of N vacancies. However, when the annealing temperature exceeds 800 °C, the state of the stress in the electrode films transforms from compressive stress to tensile stress. Besides, the volume expansion of metal electrode film and the increase in the roughness at the contact edge leads to the degradation of the side ohmic contact characteristics.

Inhibiting the formation of MoS2 between Cu2ZnSnS4 thin film and Mo(211) foil substrate by inserting a Mo(110) intermediate layer

In Cu2ZnSnS4 (CZTS) thin film solar cells, a MoS2 layer is often found at the CZTS/Mo interface, which limits the photovoltaic performances of solar cells. In this work, a Mo (110) intermediate layer was inserted between the CZTS thin film and Mo (221) foil substrate to inhibit the formation of MoS2 layer. The experimental results showed that a MoS2 layer with a thickness of 2.0 μm was observed at the CZTS/Mo foil interface without Mo intermediate layer. After the insertion of a sputtered Mo (110) intermediate layer between CZTS and Mo foil substrate, the thickness of MoS2 reduced as the thickness of Mo intermediate layer increased. The MoS2 interface layer can not be detected from the cross-sectional SEM images when the thickness of Mo intermediate layer exceeded 277 nm. The inhibition of MoS2 was considered to be due to the chemical inertness of Mo (110) during sulfurization. The ohmic contact characteristic of CZTS/Mo foil was improved when the Mo layer had a suitable thickness. Therefore, the interface properties between CZTS and Mo (221) foil can be enhanced by using a Mo (110) intermediate layer.

Research on ohmic contact characteristics of single boron nitride nanoribbon

Research on ohmic contact characteristics of single boron nitride nanoribbon

Ternary Transition Metal Chalcogenide Nb2Pd3Se8: A New Candidate of 1D Van der Waals Materials for Field‐Effect Transistors

AbstractIn this work, high‐quality 1D van der Waals (vdW) Nb2Pd3Se8 is synthesized, showing an excellent scalability from bulk to single‐ribbon due to weakly bonded repeating unit ribbons. The calculation of electronic band structures confirmed that this novel Nb2Pd3Se8 is a semiconducting material, displaying indirect‐to‐direct bandgap transition with decreasing the number of unit‐ribbons from bulk to single. Field effect transistors (FETs) fabricated on the mechanically exfoliated Nb2Pd3Se8 nanowires exhibit n‐type transport characteristics at room temperature, resulting in the values for the electron mobility and Ion/Ioff ratio of 31 cm2 V−1 s−1 and ≈104, respectively. Through transport measurements at various temperatures from room temperature down to 90 K, it is confirmed that Nb2Pd3Se8 FETs can achieve negligible Schottky barrier height (SBH) for the Au contacts at the temperature range, displaying clear ohmic contact characteristics. Furthermore, top‐gated FETs fabricated with the Al2O3 dielectric layer are studied simultaneously with back‐gated FETs.

Substitutional doping at S site of MoS2/G heterostructure: The influence on voltage-current and electronic characteristics

The effect of substitutional doping at S site on the electronic properties of MoS 2 /Graphene (MoS 2 /G) was studied by First-principles calculations. Through the analysis on the energy band and density of states of MoS 2 /Graphene, it was found that the S vacancy in MoS 2 monolayer caused the Dirac point shifted, and produced the defect energy levels at the Dirac point, which hindered the carrier transmission. Periods II-IV elements were doped at the S vacancy to improve the electronic properties and performance of MoS 2 /G. The calculated results show that the charge transfer of MoS 2 /G was greatly enhanced by the doping of N, P and As atoms. The current-voltage characteristic of N, P and As-doped G/MoS 2 /MoS 2 /G structure has higher drive-current and stable OFF-state current which can reduce device current consumption. For O-doped and Se-doped MoS 2 /G heterostructures , there is no impurity level introduced, and the Schottky barrier height increase (decrease) with the increase (decrease) of the biaxial strain, meantime the p-type Schottky contact , n-type Schottky contact and Ohmic contact characteristic can be converted mutually by adjusting biaxial strain or vertical external force. Our findings will be of great guiding significance for the designing of interface devices and explorations on nano-electronics. • The passing carriers were trapped by S vacancy in MoS 2 /G heterostrcture. • The current-voltage characteristics of N-doped, P-doped and As-doped of G/MoS 2 /MoS 2 /G were improved. • The contact type of O-doped, Se-doped and undoped MoS 2 /G can be converted by adjusting external force.

Low temperature Ni/Si/Al ohmic contacts to p-type 4H-SiC

In this paper, we studied the electrical and structural properties of Ni/Si/Al Ohmic contacts to p-type 4H-SiC. The p-type epilayer (NA = 1 × 1019 cm−3) was grown on the N-type epilayer with Al dopants. The properties of Ni/Si/Al contacts were analyzed by changing the thickness of Si film and the annealing temperature. After annealing at 700 °C, the Ni/Si/Al contact forms ohmic contact. Compared to Ni/Al contacts, Ni/Si/Al ohmic contacts are formed at lower annealing temperature. The contact resistivity of Ni/Si/Al (80/20/100 nm) contact is 1.3 × 10−4 Ω·cm2 after annealing at 800 °C. X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) are used to analyze the microstructure and observe the surface topography. The results show that Ni and Si react at low temperature to form Ni2Si, which is the reason for the formation of Ni-based ohmic contacts. At the same time, the addition of the Si layer weakens the reaction between Ni and SiC, which reduces the generation of free C elements. The Ni/Si/Al contact shows better ohmic contact characteristics.

Reliable Ohmic Contact Properties for Ni/Hydrogen-Terminated Diamond at Annealing Temperature up to 900 °C

Ohmic contact with high thermal stability is essential to promote hydrogen-terminated diamond (H-diamond) electronic devices for high-temperature applications. Here, the ohmic contact characteristics of Ni/H-diamond at annealing temperatures up to 900 °C are investigated. The measured current–voltage curves and deduced specific contact resistance (ρC) are used to evaluate the quality of the contact properties. Schottky contacts are formed for the as-received and 300 °C-annealed Ni/H-diamonds. When the annealing temperature is increased to 500 °C, the ohmic contact properties are formed with the ρC of 1.5 × 10−3 Ω·cm2 for the Ni/H-diamond. As the annealing temperature rises to 900 °C, the ρC is determined to be as low as 6.0 × 10−5 Ω·cm2. It is believed that the formation of Ni-related carbides at the Ni/H-diamond interface promotes the decrease in ρC. The Ni metal is extremely promising to be used as the ohmic contact electrode for the H-diamond-based electronic devices at temperature up to 900 °C.

Characteristics of Ni-based ohmic contacts on n-type 4H-SiC using different annealing methods

Nickel is an excellent ohmic-contact metal on 4H-SiC. This paper discusses the formation mechanism of nickel ohmic contact on 4H-SiC by assessing the electrical properties and microstructural change. Under high-temperature annealing, the phase of nickel-silicon compound can be observed with X-ray diffraction, and the contact resistance also changes. A comparative experiment was designed to use X-ray diffraction and energy-dispersive spectroscopy to clarify the difference of ohmic-contact material composition and elemental analysis between samples prepared using pulsed laser annealing and rapid thermal annealing. It is found that more Ni2Si and carbon vacancies formed at the interface in the sample prepared using pulsed laser annealing, resulting in a better ohmic-contact characteristic.

The Significant Effect of Carbon and Oxygen Contaminants at Pd/p‐GaN Interface on Its Ohmic Contact Characteristics

A low‐resistance Ohmic contact of Pd/Pt/Au metallization scheme is formed on a p‐type GaN film grown by metal‐organic chemical vapor deposition (MOCVD). Two samples are prepared to explore the effect of contaminations at the metal and GaN interface on the properties of the Ohmic contact. One directly transfers the fresh GaN film from MOCVD to the metal deposition chamber through an ultrahigh vacuum (UHV) tube, without exposing the GaN surface to air; the other transfers the GaN film from MOCVD to metal deposition through the atmosphere. The results show a lower specific contact resistance (6.2 ± 0.9) × 10−5 Ω cm2 for the directly UHV‐transferred sample than the air‐transferred sample (2.1 ± 0.9) × 10−4 Ω cm2. Once exposed to air, carbonaceous particles and an amorphous oxide layer easily adhere to the p‐type GaN surface, which not only increase the Schottky barrier height but also generate more grain boundaries in the Pd/Pt/Au stacks. The grain boundaries, acting as diffusion channels, make Au atoms diffuse easily into the Pd layer and propagate to the p‐GaN surface layer after annealing. Therefore, the Ohmic contact fabricated in UHV ambience with lower oxygen and carbon contaminations at the interface shows superior characteristics than the samples prepared by the traditional method.