Electrical properties at p-ZnSe/metal interfaces

In order to prepare low resistance ohmic contacts to p-ZnSn by the “deposition and annealing (DA)” technique which has been extensively used for GaAs and Si-based devices, formation of a heavily doped layer by the p-ZnSe/metal reaction is required. For p-ZnSe/Ni contacts, Ni and Se reacted preferentially at the ZnSe/Ni interface upon annealing at temperatures higher than 250°C. However, capacitance-voltage measurements showed that the net acceptor concentration (N_A-N_D) close to the p-ZnSe/Ni interface was reduced upon the Ni/ZnSe reaction, resulting in high contact resistance. For p-ZnSe/Au contacts, neither Au/ZnSe reaction nor reduction of the acceptor concentration were observed after annealing at temperatures lower than 300°C. This indicates that although the metal/p-ZnSe reaction is mandatory to prepare a heavily doped layer, the reaction induced an increase in the compensation donors in the p-ZnSe substrate. In order to increase the acceptor concentration in the vicinity of the p-ZnSe/metal interface through diffusion from the contact materials, Li or O which was reported to play the role of an acceptor in ZnSe was deposited with a contact metal and annealed at elevated temperatures. Ni or Ag was selected as the contact metal, because these metals were expected to enhance Li or O doping by reacting with ZnSe. However, the current density-voltage characteristics of the Li(N)/Ni and Ag(O) contacts exhibited rectifying behavior, and the contact resistances increased with increasing annealing temperature. The present results indicated that, even though the acceptor concentration in the p-ZnSe substrate increased by diffusion of the dopants from the contact elements, an increment of the compensation donors was larger than that of the acceptors. The present experiments indicated that preparation of low resistance ohmic contacts by forming a heavily doped intermediate layer between p-ZnSe and metal is extremely difficult by the DA technique.     

Reversible Degradation of Ohmic Contacts on p-GaN for Application in High-Brightness LEDs

This paper analyzes the high-temperature long-term stability of ohmic contacts on p-type gallium nitride (p-GaN). The contributions of the ohmic contacts and semiconductor material degradation are separated by adopting the transmission line method (TLM). Before stress, the current-voltage (I-V) curves measured at the pads of the TLMs showed a linear shape, indicating a good ohmic behavior of the contacts. Thermal treatment at 250degC was found to induce the worsening of the electrical characteristics of the contacts: identified degradation modes consist of a shift of the I-V curves toward higher voltages and strong nonlinearity of the characteristics around zero. This paper shows that the high-temperature instabilities of ohmic contacts on p-GaN are related to the interaction between the device surface and the plasma-enhanced chemical vapor deposition SiN passivation layer. Hydrogen contained in the passivation layer is supposed to play an important role in the degradation process: the interaction with the acceptor dopant at the metal/semiconductor interface induces the decrease of the effective acceptor concentration. As a consequence, both the ohmic contact characteristics and the semiconductor sheet resistance are worsened.     

Schottky barrier heights of contact metals to p-type ZnSe

Schottky barrier heights (SBHs) of a variety of metals (In, Cd, Nb, Ti, W, Cu, Ag, Au, Ni, Pt, and Se) contacting to p-ZnSe grown by a molecular beam epitaxy method were determined by analyzing capacitance-voltage (C-V) and/or current density-voltage (J-V) curves. The SBH values of the Au and Ni contacts were determined from intersections of straight lines of the C⁻²-V curves to be 1.23 and 1.13 eV, respectively. The J-V calculations provided a large SBH value of 1.2 ± 0.1 eV for a variety of metals, indicating that the Fermi-level could be pinned at the contact interface. Reduction of the SBH values to a level lower than 0.4 eV and/or increase of doping concentrations to a level higher than 10²⁰ cm⁻³ are essential to obtain an ohmic contact with contact resistivity of around 10⁻³ Ω·cm².     

Ni基n型SiC材料的欧姆接触机理及模型研究

研究了Ni基n型SiC材料的欧姆接触的形成机理,认为合金化退火过程中形成的C空位(Vc)而导致的高载流子浓度层对欧姆接触的形成起了关键作用。给出了欧姆接触的能带结构图,提出比接触电阻ρC由ρC1和ρC2两部分构成。ρC1是Ni硅化物与其下在合金化退火过程中形成的高载流子浓度层间的比接触电阻,ρC2则由高载流子浓度层与原来SiC有源层之间载流子浓度差形成的势垒引入。该模型较好地解释了n型SiC欧姆接触的实验结果,并从衬底的掺杂水平、接触金属的选择、合金化退火的温度、时间、氛围等方面给出了工艺条件的改进建议。     

Metal-N-type semiconductor ohmic contact with a shallow N+ surface layer

Most papers covering metal-semiconductor ohmic contact theory which have been published up to date consider systems with homogeneous impurity concentration in the semiconductor. However, there are techniques of ohmic contact formation on nondegenerate semiconductor where only a very shallow surface layer is impurity enriched. In this paper a model of such contacts is proposed and a simple approximate analytical expression for the specific resistivity is derived. If the impurity concentration in the surface layer is very high, the contact specific resistivity is essentially proportional to N_B^?1, N_B being the semiconductor substrate impurity concentration. To make a good ohmic contact, it is sufficient that the width of the heavily doped surface layer be equal to the equilibrium contact depletion region width. Any further enlargement of the enriched layer practically does not influence the total sample resistance due to the dominant share of the semiconductor body resistance. Experimental results confirm these conclusions qualitatively.     

Universal error corrections for finite semiconductor resistivity in cross-Kelvin resistor test structures

The Cross-Kelvin Resistor test structure is commonly used for the extraction of the specific contact resistance of ohmic contacts. Analysis using this structure are generally based on a two-dimensional model that assumes zero voltage drop in the semiconductor layer in the direction normal to the plane of the contact. This paper uses a three-dimensional (3-D) analysis to show the magnitude of the errors introduced by this assumption, and illustrates the conditions under which a 3-D analysis should be used. This paper presents for the first time 3-D universal error correction curves that account for the vertical voltage drop due to the finite depth of the semiconductor layer.     

Formation mechanism of InxGa1−xAs ohmic contacts to n-type GaAs prepared by radio frequency sputtering

The formation mechanisms of InAs/Ni/W ohmic contacts to n-type GaAs prepared by radio-frequency (rf) sputtering were studied by measuring contact resistances (R_c) using a transmission line method and by analyzing the interfacial structure mainly by x-ray diffraction and transmission electron microscopy. Current-voltage characteristics of the InAs/Ni/W contacts after annealing at temperatures above 600°C showed “ohmic-like behavior.” In order to obtain the “ohmic” behavior in the contacts, pre-heating at 300°C prior to high temperature annealing was found to be essential. The contacts showed ohmic behavior after annealing at temperatures in the range of 500∼850°C and contact resistance values of as low as ∼0.3Ω-mm were obtained. By analyzing the interfacial structures of these contacts, In_xGa_1−xAs layers with low density of misfit dislocations at the In_xGa_1−xAs and GaAs interface were observed to grow epitaxially on the GaAs substrate upon heating at high temperatures. This intermediate In_xGa_1−xAs layer is believed to divide the high energy barrier at the contact metal and GaAs interface into two low barriers, resulting in reduction of the contact resistance. In addition, Ni was found to play a key role to relax a strain in the In_xGa_1−xAs layer (introduced due to lattice mismatch between the In_xGa_1−xAs and GaAs) by forming an intermediate Ni_xGaAs layer on the GaAs surface prior to formation of the In_xGa_1−xxAs layer.     

Very low resistance nonalloyed ohmic contacts to Sn-doped molecular-beam epitaxial GaAs

This paper presents the results of an investigation conducted to understand the formation of very low resistance nonalloyed ohmic contacts to Sn-doped nGaAs grown by molecular-beam, epitaxy (MBE). The measured specific contact resistance is in good agreement with a new thermionic field-emission model presented in this paper that accounts for a number of physical phenomena that are critical in determining the carrier transport mechanisms across the metal-semiconductor interfaces. The experimental and theoretical results are more than two orders of magnitude smaller than previous contact resistance calculations. This discrepancy predominantly occurs because previous contact theories do not account for metal wave functions and include only conduction band effects when calculating the electron wave vector in the semiconductor energy gap. A specific contact resistance as low asR_{c} approx 2 times 10^{-7} Omega.cm²at T = 300 K is obtained for the nonalloyed contacts fabricated in this work.     

Ohmic contacts to low-resistivity ZnS: Preparation,noise properties and nature of contact resistance

The results of simultaneous investigations of noise characteristics and contact resistance of different contacts to low-resistivity (ρ_b = 1–10 ohm cm) ZnS single crystals including so-called ohmic contacts with a contact resistance ρ_c = 0.1–0.5 ohm cm² are described and analyzed. A conclusion is drawn that the difficulties in obtaining ohmic contacts to low-resistivity ZnS are connected with the presence on the semiconductor contact surface of acceptor centers filled by electrons which are responsible for the increase of band bending at the semiconductor surface. In the case of ohmic contacts the contact barrier is lowered significantly owing to formation of the ionized donor centers in the contact region; electrons penetrate such barriers by tunneling.     

台面型InP/InGaAs PIN光伏探测器的p-InP低阻欧姆接触

研究了电子束蒸发淀积的非合金膜系Au/Pt/Ti/p-InP(2×1018cm-3)接触的物理特性,通过450℃、4 min的快速退火,获得了欧姆接触,其比接触电阻为7.3×10-5 Wcm2.接触电极退火后,采用离子溅射法淀积加厚电极Cr/Au.利用俄歇电子能谱(AES)进行深度剖面分析,表明Pt层能够相对有效地阻挡...     

Formation peculiarities and properties of ohmic contacts to n-GaN(AlN) and artificial diamond

The paper considers ohmic contacts of Au-TiB _x -Al-Ti-n-GaN, Au-Pd-Ti-Pd-n-AlN and Au-Pd-Ti-n-C to the promising for use in microelectronics wide-gap semiconductors. Ohmic contact formation takes place after sequential layering of metal with further fast thermal processing, which leads to solid-phase reactions between the semiconductor and metal. It is shown that the use of X-ray amorphous TiB _x layer in ohmic contact as the diffusion barrier allows for creating thermal stability contacts up to T = 900 °C. Current flow in the considered ohmic contacts is described using a model with current flow along metal shunts considering diffusion limitation on the charge carrier supply.     

Flow of the current along metallic shunts in ohmic contacts to wide-gap III–V semiconductors

It is established experimentally that the contact metal—wide-gap semiconductor (GaAs, GaP, GaN) with the Schottky barrier transforms into the ohmic contact either in the process of continuous heating or in the process of holding at an elevated temperature before the formation of any recrystallized layers. In this case, resistance of the contact reduced to the unit area increases as the temperature increases for semiconductors with a high dislocation density (GaP, GaN). It is assumed that in such contacts, the current flows along the metallic shunts, which shorten the layer of space charge and are formed by metal atoms diffused along the dislocation lines or other imperfections of the semiconductor. In semiconductors with a low dislocation density (GaAs), resistance of the ohmic contact per unit area decreases with increasing the temperature as it was expected for the thermionic mechanism of current flowing.     

A new silicon vidicon with a CdTe—Si target

A new type of a silicon vidicon target which utilizes blocking contacts between bulk semiconductor and a high resistive layer has been developed. The high resistive layer forms a blocking contact to the bulk and works also as a "resistive sea" layer, reducing complex fabrication processes. Structure of the target, tube characteristics, and video patterns are presented.     

Use of Sn-doped GaAs for non-alloyed ohmic contacts to HEMTs

Segregation of Sn to the surface of MBE grown n⁺-GaAs layers allows fabrication of non-alloyed Ti/Pt/Au, Al or Ti/W ohmic contacts with low specific contact resistivities (1.1×10^-6 Ω·cm^-2). These contacts were used to realise high performance HEMTs (g_m=230 mS/mm for 1.0 μm gate length) in which Si is used as the dopant in the donor AlGaAs layer and Sn is employed in the GaAs contact layer     

Low Resistance Contacts to p-CulnSe2 and p-CdTe Crystals

Room temperature formation of ohmic contacts by electroplating gold on chemically treated surfaces of p-CuInSe₂ and p-CdTe single crystals is reported. The effect of Br₂/methanol and KOH+KCN+H₂O treatments prior to plating was analyzed in the case of CuInSe₂. It is shown that the former treatment yields better ohmic contacts, with lower contact resistance, than the latter. While annealing these contacts made them highly non-ohmic, the method gives reasonably ohmic contacts on surfaces, that were purposely oxidized prior to contact preparation. In the case of p-CdTe stable, low resistance ohmic contacts were obtained at room temperature by electrochemical diffusion of Hg from solution, prior to gold plating. The treatment forms a highly degenerated p^+- HgCdTe layer. The contacts, which have a very low contact to bulk resistivity ratio, were further improved by vacuum annealing.     

Al ohmic contacts to HCI-treated Mg<Subscript>x</Subscript>Zn<Subscript>1−x</Subscript>O

The Al nonalloyed ohmic contacts were fabricated on Mg_xZn_1−xO (0≤x≤0.2) thin films. HCl surface treatment significantly reduced the specific contact resistances to value around 10⁻⁴ Ω cm². X-ray photoelectron spectroscopy (XPS) analysis revealed that the HCl treatment increased the oxygen vacancy density and introduced chlorine to the semiconductor surface, resulting in a thin conductive layer and thus reduced specific contact resistance. A subsequent oxygen plasma treatment reduced the oxygen vacancy density, and correspondingly increased the specific contact resistance. Al-ZnO contacts were insensitive to the HCl treatment, due to the formation of a highly conductive Al-doped thin interface layer.     

Simultaneous Formation of Ni/Al Ohmic Contacts to Both n- and p-Type 4H-SiC

The fabrication procedure for silicon carbide power metal oxide semiconductor field-effect transistors can be improved through simultaneous formation (i.e., using the same contact materials and a one-step annealing process) of ohmic contacts on both the n-source and p-well regions. We have succeeded in the simultaneous formation of Ni/Al ohmic contacts to n- and p-type SiC after annealing at 1000°C for 5 min in an ultrahigh vacuum. Ohmic contacts to n-type SiC were found when the Al-layer thickness was less than about 6 nm, while ohmic contacts to p-type SiC were observed for an Al-layer thickness greater than about 5 nm. Only the contacts with an Al-layer thickness in the range of 5 nm to 6 nm exhibited ohmic behavior to both n- and p-type SiC, with a specific contact resistance of 1.8 × 10⁻⁴ Ω cm² and 1.2 × 10⁻² Ω cm² for n- and p-type SiC, respectively. An about 100-nm-thick contact layer was uniformly formed on the SiC substrate, and polycrystalline δ-Ni₂Si(Al) grains were formed at the contact/SiC interface. In the samples that exhibited ohmic behavior to both n- and p-type SiC, the distribution of the Al/Ni ratios in the δ-Ni₂Si(Al) grains was larger than that observed for any of the samples that showed ohmic behavior to either n- or p-type SiC. Furthermore, the grain size of the δ-Ni₂Si(Al) grains in the samples showing ohmic behavior to both n- and p-type SiC was smaller than the grains in any of the samples that showed ohmic behavior to either n- or p-type SiC. Thus, the large distribution in the Al/Ni ratios and a fine microstructure were found to be characteristic of the ohmic contacts to both n- and p-type SiC. Grains with a low Al concentration correspond to ohmic contacts to n-type SiC, while grains with a high Al concentration correspond to ohmic contacts to p-type SiC.     

Improvedn-type GaAs ohmic contacts compatible with a chlorine-based dry-etch process

We report the use of a Mo barrier layer within Ni/Au-Ge based ohmic contacts to GaAs for eliminating an etch stop reaction that occurs during Cl-based dry etching of heterojunction bipolar transistors. With conventional Ni/Au-Ge/Ag ohmic contacts, chlorinecontaining discharges produce a passivating layer of AgCl on the semiconductor surface, preventing further etching. This layer is absent when the Ag in the contact is replaced with Mo. The Mo has several advantages over other diffusion barrier layers and yields contacts with excellent adhesion, smooth morphology, and sharp edge definition. The average contact resistivity of these contacts ton ⁺-GaAs(n = 6 × 10¹⁸ cm^-3) was 0.074 ohm-mm, which is lower than the typical contact resistivity of conventional Ni/Au-Ge/ Ag metallization (0.11 ohm-mm).     

RF-Enhanced Contacts to Wide-Bandgap Devices

This letter proposes a novel approach to fabricate high-performance heterostructure microwave devices with nonohmic contacts. The contact can be as-deposited or made by "shallow" low-temperature annealing to form a low-height Schottky barrier while preserving the two-dimensional electron-gas layer (2DEG) at the heterointerface. Coupling between the metal and the 2DEG occurs via two paths: dc current injects through the barrier leakage current and ac-current component injects through capacitive coupling. Contacts with resistive/capacitive coupling have low microwave impedance and enhance the heterostructure field-effect transistor's maximum oscillation frequency, output power, and power-added efficiency as compared to resistive ohmic contacts     

Defect state assisted tunneling in intermediate temperature molecular beam epitaxy grown GaAs

Current transport in molecular beam epitaxy (MBE) GaAs grown at low and intermediate growth temperatures is strongly affected by defects. A model is developed here that shows that tunneling assisted by defect states can dominate, at some bias ranges, current transport in Schottky contacts to unannealed GaAs material grown at the intermediate temperature range of about 400°C. The deep defect states are modeled by quantum wells which trap electrons emitted from the cathode before re-emission to semiconductor. Comparison of theory with experimental data shows defect states of energies about 0.5 eVbelow conduction band to provide the best fit to data. This suggests that arsenic interstitials are likely to mediate this conduction. Comparison is also made between as-grown material and GaAs grown at the same temperature but annealed at 600°C. It is suggested that reduction of these defects by thermal annealing can explain lower current conduction at high biases in the annealed device as well as higher current conduction at low biases due to higher lifetime. Quenching of current by light in the as-grown material can also be explained based on occupancy of trap states. Identification of this mechanism can lead to its utilization in making ohmic contacts, or its elimination by growing tunneling barrier layers.