Investigation of Heusler alloy-semiconductor interfaces

Using the electron density functional theory, the electronic structure and magnetic properties of possible contacts on the (001) interface between XYZ and X ₂ YZ Heusler alloys (NiMnSb, Co₂ MnSi) and III–V semiconductors (InP, GaAs) are studied. It is demonstrated that, in both cases, the high degree of spin polarization is achieved in Ni/P(As) or Co/As contacts. The influence of structure defects located on the surface and interfaces on the spin polarization at the Fermi level is studied. The nature of surface states at the Heusler alloy-semiconductor interface and electron factors that favor preservation or loss of the half-metallic behavior in the contacts are analyzed. Calculations of the local magnetic moments show that the magnetic properties of atoms in the contact are not changed significantly at the interface because of the partial compensation of their coordination by atoms of the semiconductor. The spin polarization can be increased by doping of the X element sublattice.     

Electrical metal contacts to atomically thin 2H-phase MoTe2 grown by metal–organic chemical vapor deposition

We grow atomically thin molybdenum ditelluride (MoTe₂) films on a SiO₂/Si substrate by means of metal–organic chemical vapor deposition (MOCVD). Our Raman spectroscopy measurements reveal the formation of 2H-phase MoTe₂ films. Further, transmission electron microscopy and X-ray photoelectron spectroscopy studies indicate a three-atomic-layer structure and the surface element composition of MoTe₂ films. In this study, we mainly focus on the influence of metal contacts attached to the films on their electrical performance. We fabricate 2H-phase-MoTe₂-based field-effect transistors (FETs) with various metal contacts such as titanium/gold, nickel and palladium, which present p-type semiconductor properties. We also examine the influence of the work functions of the contact metals on the electrical properties of three-atomic-layer-MoTe₂-based FET devices. For a p-type MoTe₂ semiconductor, higher work functions of the contact metals afford narrower Schottky barrier heights (SBHs) and eventually highly efficient carrier injection through the contacts.     

Structure and properties of interphase boundaries of gallium arsenide-metal (dielectric)

To study the nature and properties of potential barriers in gallium arsenide devices, we have investigated structural phase transitions in GaAs contacts with multilayer films containing refractory transition-metal borides (TiB₂, LaB₆). We verified the important role in degrading Schottky barrier device performance played by local mechanical stresses introduced at the interface by lateral nonuniformities in interphase interactions. We examine the electrical properties of MIS gallium arsenide devices, taking into account the high density of electronic surface states (ESS). We show it is possible to control the density of ESS by selecting the dielectric, and we discuss its deposition and annealing with a pulsed laser. We discuss the nature of potential barriers in gallium arsenide devices, drawing upon our data and previously published data and modern theoretical models.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, No. 10, pp. 52–62, October, 1993.     

Electronic properties of SiC surfaces and interfaces: some fundamental and technological aspects

The wide band gap semiconductor silicon carbide (SiC) is the first-choice material for power electronic devices operating at high voltages, high temperatures, and high switching frequencies. Due to their importance for crystal growth, processing, and device fabrication, the electronic properties of SiC surfaces and interfaces to other materials such as metals and dielectrics are of particular interest. Unreconstructed, H-terminated SiC surfaces which are passivated in a chemical as well as an electronic sense are obtained in a thermal hydrogenation process. It is demonstrated that deposition of Al₂O₃ on H-terminated SiC(0001) leads to an interface which is lower in defects than the thermally grown SiO₂/SiC interface. Furthermore, starting from hydrogenated SiC{0001} surfaces it is possible to prepare unreconstructed (1×1) surfaces with one dangling bond per unit cell. These surfaces show indications for strong electron correlation effects. PACS 68.47.Fg; 73.20.At; 79.60.Bm; 68.35.Bs; 68.35.Dv     

Schottky contacts in germanium nanowire network devices synthesized from nickel seeds

This paper presents reliable process to the synthesis of germanium nanowires by the vapor–liquid–solid method using nickel as an alternative catalyst to gold, the most commonly used metal, without toxic gas precursors. The structural study showed single-crystalline germanium nanowires with diamond structure, lengths of tens of microns and diameters smaller than 40 nm. The reduced dimensions of the nanowires led to phonons localization effect, with correlation lengths of the same order of the nanowires diameters. Additionally, the analysis of electronic properties of metal-nanowire-metal devices indicated the presence of Schottky barriers, whose values depend linearly on temperature. This linear dependence was assigned to the tunneling process through an insulator layer (mostly GeO_x) at the metal-semiconductor interface. These results point to the existence of another channel for electrons transference from metal to semiconductor being very significant to electronic devices fabrication.     

Theoretical investigations of the (110) interface between the full Heusler alloys and GaAs

The ab initio calculations of the electronic structure and magnetic properties of the (110) interface between Co₂YZ (Y = Cr or Mn and Z = Al, Si, or Ge) and GaAs are carried out by means of the density functional theory depending on the contact configuration. It is revealed that two of four possible atomic interface configurations have high spin polarization. For Co₂MnSi/GaAs(110), one of the contacts has almost 100% spin polarization. Calculations of the adhesion energy on the interfaces allow the most stable contacts to be established.     

Effect of heat treatment on Ag/Y-Ba-Cu-O contact resistance

The contact resistance (R _c) of the metal/YBCO interface has been studied in pressed indium, painted colloidal silver and thermally-evaporated silver contact pads. Indium contacts always show the highest resistance amongst these three systems. In thermally-evaporated Ag contacts, post-deposition thermal treatments show favourable effects on the reduction ofR _c. Heat treatment in oxygen atmosphere in two steps is essential to reduce theR _c values. Significant improvement in obtaining low resistivity contacts has been attributed to the diffusion of silver atoms to grain boundaries at the surface of YBCO and to the enrichment of oxygen-deficient layer at the interface during thermal treatment.     

Quantum point contacts on InGaAs/InP heterostructures

For the first time we have observed quantized conductance in a split gate quantum point contact prepared in a strained In_0.77Ga_0.23As/InP two-dimensional electron gas (2DEG). Although quantization effects in gated two-dimensional semiconductor structures are theoretically well known and proven in various experiments on AlGaAs/GaAs and also on In_0.04Ga_0.96As/GaAs, no quantum point contact has been presented in the InGaAs/InP material with an indium fraction as high as 77% so far. The major problem is the comparatively low Schottky barrier of the InGaAs (φ_B≈ 0.2 eV) making leakage-free gate structures difficult to obtain. Nevertheless this heterostructure—especially with the highest possible indium content—has remarkable properties concerning quantum interference devices and semiconductor/superconductor hybrid devices because of its large phase coherence length and the small depletion zone, respectively. In order to produce leakage-free split gate point contacts the samples were covered with an insulating SiO₂layer prior to metal deposition. The gate geometry was defined by electron-beam lithography. In this paper we present first measurements of a point contact on an In_0.77Ga_0.23As/InP 2DEG clearly showing quantized conductance.     

Specific contact resistance of IGZO thin film transistors with metallic and transparent conductive oxides electrodes and XPS study of the contact/semiconductor interfaces

In this work, the specific contact resistance (ρ_c) between amorphous indium-gallium-zinc-oxide (IGZO) semiconductor and different contact electrodes was obtained from thin film transistors (TFTs). Ti/Au (10/100 nm), aluminum doped zinc oxide (AZO, 100 nm) and indium tin oxide (ITO, 100 nm) were used as source/drain electrodes to fabricate IGZO TFTs. Chemical states of the contacts/semiconductor interfaces were examined by depth profile X-ray photoelectron spectroscopy (XPS) analysis to explain the origin of the differences on specific contact resistance. The lowest ρ_c achieved using Ti/Au was related to the formation of a TiO_x interlayer due to oxygen atoms diffusing out from the semiconductor under layer, increasing the carrier concentration of IGZO at the interface and lowering the ρ_c. On the contrary, no interfacial reactions were observed between IGZO and AZO or ITO source/drain. However, IGZO resistivity increased with ITO contacts likely due to oxygen vacancies filling during ITO deposition. This fact seems to be the origin of the high contact resistance between IGZO and ITO, compared to IGZO-AZO and IGZO-Ti/Au interfaces.     

Ambipolar organic field-effect transistors on unconventional substrates

In this paper we report on the realization of flexible all-organic ambipolar field-effect transistors (FETs) realized on unconventional substrates, such as plastic films and textile yarns. A double layer pentacene-C₆₀ heterojunction was used as the semiconductor layer. The contacts were made with poly(ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) and patterned by means of soft lithography microcontact printing (μCP). Very interestingly growing C₆₀ on a predeposited pentacene buffer layer leads to a clear improvement in the morphology and crystallinity of the film so it obtains n-type conduction despite the very high electron injection barrier at the interface between PEDOT:PSS and C₆₀. As a result, it was possible to obtain all-organic ambipolar FETs and to optimize their electrical properties by tuning the thicknesses of the two employed active layers. Moreover, it will be shown that modifying the triple interface between dielectric/semiconductor/electrodes is a crucial point for optimizing and balancing injection and transport of both kinds of charge carriers. In particular, we demonstrate that using a middle contact configuration in which source and drain electrodes are sandwiched between pentacene and C₆₀ layers allows significantly improving the electrical performance in planar ambipolar devices. These findings are very important because they pave the way for the realization of low-cost, fully flexible and stretchable organic complementary circuits for smart wearable and textile electronics applications.     

On interface properties of ultra-thin and very-thin oxide/a-Si:H structures prepared by oxygen based plasmas and chemical oxidation

Amorphous hydrogenated silicon (a-Si:H) belongs still to most promising types of semiconductors for its utilization in fabrication of TFTs and thin film solar cell technology due to corresponding cheap a-Si:H-based device production in comparison with, e.g. crystalline silicon (c-Si) technologies. The contribution deals with both two important modes of preparation of very-thin and ultra-thin silicon dioxide films in the surface region of a-Si:H semiconductor (oxygen plasma sources and liquid chemical methods) and electrical, optical and structural properties of produced oxide/semiconductor structures, respectively. Dominant aim is focused on investigation of oxide/semiconductor interface properties and their comparison and evaluation from view of utilization of used technological modes in the nanotechnological industry. Following three basic types of oxygen plasma sources were used for the first time in our laboratories for treatments of surfaces of a-Si:H substrates: (i) inductively coupled plasma in connection with its applying at plasma anodic oxidation; (ii) rf plasma as the source of positive oxygen ions for plasma immersion ion implantation process; (iii) dielectric barrier discharge ignited at high pressures.The liquid chemical manner of formation SiO₂/a-Si:H structures uses 68 wt% nitric acid aqueous solutions (i.e., azeotropic mixture with water). Their application in crystalline Si technologies has been presented with excellent results in the formation of ultra-thin SiO₂/c-Si structures [H. Kobayashi, M. Asuha, H.I. Takahashi, J. Appl. Phys. 94 (2003) 7328].Passivation of surface and interface states by liquid cyanide treatment is additional original technique applied after (or before) formation of almost all formed thin film/a-Si:H structures. Passivation process should be used if high-quality electronical parameters of devices can be reached.     

Electronic Properties of Metallic Nanoclusters on Semiconductor Surfaces: Implications for Nanoelectronic Device Applications

We review current research on the electronic properties of nanoscale metallic islands and clusters deposited on semiconductor substrates. Reported results for a number of nanoscale metal-semiconductor systems are summarized in terms of their fabrication and characterization. In addition to the issues faced in large-area metal-semiconductor systems, nano-systems present unique challenges in both the realization of well-controlled interfaces at the nanoscale and the ability to adequately characterize their electrical properties. Imaging by scanning tunneling microscopy as well as electrical characterization by current-voltage spectroscopy enable the study of the electrical properties of nanoclusters/semiconductor systems at the nanoscale. As an example of the low-resistance interfaces that can be realized, low-resistance nanocontacts consisting of metal nanoclusters deposited on specially designed ohmic contact structures are described. To illustrate a possible path to employing metal/semiconductor nanostructures in nanoelectronic applications, we also describe the fabrication and performance of uniform 2-D arrays of such metallic clusters on semiconductor substrates. Using self-assembly techniques involving conjugated organic tether molecules, arrays of nanoclusters have been formed in both unpatterned and patterned regions on semiconductor surfaces. Imaging and electrical characterization via scanning tunneling microscopy/spectroscopy indicate that high quality local ordering has been achieved within the arrays and that the clusters are electronically coupled to the semiconductor substrate via the low-resistance metal/semiconductor interface.     

The formation and thermal stability of multilayer ohmic contacts to n-GaAs with TiBx and Mo diffusion barriers

Thermal degradation of Au/Mo/TiB_x/AuGe multilayer ohmic contacts with Mo and TiB _x diffusion barriers was studied. The contacts were employed in Gunn-effect diodes. Depth profiling of the components in the contacts was performed using Auger electron spectroscopy. The microrelief of the metal/semiconductor interface and contact surface morphology were examined with atomic force microscopy and scanning electron microscopy, respectively. The measurements were taken before and after argon annealing at T=400, 600, or 800°C for 60 s. The resistance of the Gunn diode mesa was also measured. Annealing at 400°C is shown not to affect the sandwich structure of the contacts. Annealing at 600°C causes structure rearrangement in the layers up to cracking. It is found that the thermal threshold of degradation of the Au/Mo/TiB_x /AuGe/GaAs structure depends on the resistance of the TiB_x layer to thermal effects. Reasons for the degradation of Mo and TiB_x antidiffusion properties are discussed.     

First-principles study of metal-semiconductor contact between MX2 (M = Nb,Pt; X = S,Se) monolayers

First principles calculations are performed to investigate the structural and electronic properties of MX₂ (M = Nb, Pt; X = S, Se) monolayers and their van der Waals (vdW) heterostructures. The dynamical stability of monolayers and vdW heterostructures is confirmed by binding energy and phonon spectra. An indirect band gap nature is found for PtS₂ and PtSe₂ monolayers while NbS₂, NbSe₂ and all vdW heterostructures are metals. The intrinsic electronic properties of both NbX₂ and PtX₂ are well preserved due to weak vdW contact. It is demonstrated that a p-type Schottky contact with a small barrier height is formed at NbX₂-PtX₂ interface. The zero tunnel barrier and higher potential drop across the interface in these contacts imply large transfer of charge carriers across the interface, making them potential candidates in nanoelectronic device applications.     

Tunable surface and/or interface ferromagnetism of ZnO nanoparticles

The ZnO-based diluted magnetic semiconductor materials have been widely investigated since the room temperature ferromagnetism (FM) was predicted in the p$p$-type Mn doped ZnO. However, it is now clear that magnetic dopants or impurities are not necessary for introducing FM into ZnO. As confirmed by numerous theoretical and experimental works, tunable FM can be effectively introduced into ZnO nanoparticles (NPs) by controlling the surface and/or interface nanostructures. This review describes the recent advances in the surface and/or interface FM of ZnO NPs without any magnetic impurities. On the basis of the previous reports including our recent works, the origin of FM of ZnO NPs has been overviewed and discussed in terms of defects, complex reactions or compounds, and electron transfer at the NP surface or interface.     

Cobalt metal nanoparticles embedded in SiO2 dielectric layer for the application of nonvolatile memory

Metal–oxide–semiconductor structures (MOS) with the embedded Co nanoparticles (NPs) were efficiently fabricated by utilizing an external laser irradiation technique for the application of nonvolatile memory. Images of high resolution transmission electron microscopy measurements exhibited that the Co NPs of 5 nm in diameter were clearly embedded in SiO₂ gate oxide. Capacitance–voltage measurements certainly exhibited flat-band voltage shift of 2.2 V from 2 V to −8 V in sweeping range. The retention characteristics of MOS capacitors with the embedded Co NPs were also studied as a function of tunnel oxide thickness to confirm the suitability of nonvolatile memory devices with metal NPs. The experimental results reveal that our unique laser process will give possible promise for experimental efficient formation or insertion of metal NPs inside the gate oxide.     

Preparation and characterization of semiconductor GNR-CNT nanocomposite and its application in FET

So far, little is known about the experimental potential of graphene nanoribbon-carbon nanotube (GNR-CNT) heterostructure as a semiconductor nanocomposite. The present work examined the structural features, topography and electronic properties of GNR-CNT nanocomposite by using Raman spectroscopy, transmission electron microscopy, scanning tunneling microscopy and spectroscopy (STS). The homogenous semiconductor GNR-CNT nanocomposites were produced under optimized synthesis conditions. The narrow band gap was exhibited by optimization of the reduction step. The STS of the micro-scale surface of the nanocomposite shows local density of state in selected areas that represent the 0.08 eV band gap of a homogenous nanocomposite. The potential of the semiconductor nanocomposite was considered for application in stacked graphene nanoribbon-field effect transistors (SGNR-FETs). A simple method of device fabrication is proposed based on a semiconductor stacked GNR nanocomposite. The high hole mobility and rectifying effect of the p–n junction of the SGNR nanocomposite on TiO₂ are demonstrated. The optimal thickness for the back gate TiO₂ dielectric for the tested devices was 40 nm. This thickness decreased leakage current at the p–n junction of the SGNR/TiO₂ interface, which is promising heterojunction for optoelectronics. The thickness of gate dielectric and quantum capacitance of the gate was investigated at the low 40 nm thickness by calculating the mobility. In the proposed SGNR-FET, holes dominate electrical transport with a high mobility of about 1030 cm²/V s.     

Effect of Intercalated LiF on Interface Contact Characteristics of MoS2 Field Effect Transistor: A First-Principles Study

Based on the first-principles calculation, the effect of intercalated LiF on the contact characteristics of the interface between Au electrode and MoS₂ layer is studied. It is found that adding LiF film can change the contact type between metal electrode Au and MoS₂ layer from Schottky contact to ohmic contact, which is accompanied by interfacial charge transfer from LiF layer to MoS₂ layer and the downward movement of d (d_xy and d_z2) orbital of Mo atom and p (p_x and p_y) orbital of S atom to Fermi level. And the interlayer spacing between LiF layer and Au electrode has a great impact on the interface contact characteristics. The electric field effect and stress effect of interface contact of Au, LiF and MoS₂ (Au/LiF/MoS₂)is more obvious than that of interface contact of Au and MoS₂ (Au/MoS₂). Au/LiF/MoS₂ shows ohmic contact with the interlayer spacing between Au layer and LiF layer less than 3.05 Å and with the electric field less than 0.15 VÅ⁻¹, respectively, while Au/MoS₂ still shows N-type Schottky contact. These findings are helpful to control the contact resistance and have guiding significance for high performance MoS₂ field effect transistor and other electronic components.     

Low temperature reactions at Si/metal interfaces; What is going on at the interfaces?

Si-metal contacts play one of the most important roles in Si-device or integrated circuit technology represented by IC, LSI and VLSI. The role of the contacts is to interconnect individual devices (transistors, diodes and so on) in a Si chip and connect them as a whole to the outer circuit. These contacts are numerous in the chip and can easily total more than one million. Therefore, the establishment of criteria for providing stable and reproducible electrical (ohmic or Schottky barrier) contacts has been a key problem in Si-LSI technology. Si is a typical covalent semiconductor with a large bond energy (≈eV/ bond) and consequently its melting point is high (≈1440 °C). However,crucial to the Si-metal contact problem is that when Si is in contact with metal it readily reacts with it at a temperature as low as ≈100 °C. This interfacial reaction induces large scale transport of materials across the Si/metal interface to give rise to several interesting phenomena. Examples of this are thick (≈1000 Å) SiO₂ growth for a short time (≈10 min) due to Si-Au reaction and uniform silicide layer formation at Si/Pd, Pt, Ni interfaces. Interesting point is that since the Si-Si covalent bonding is very strong without the presence of such effect of metal to weaken the Si bond adjacent to the metal, the above contact or interfacial reactions can rarely occur. As possible mechanism of this bond-weakening, several models have been proposed. One of them is by the present author postulating electronic screening of Coulomb interaction responsible for the covalent bonding due to mobile electrons in the metal films. In the present article, this “screening model” is critically discussed and compared with other models on the basis of experiments done by several laboratories on microscopic or atomic-scale observation of initial stages of the Si-Au and Si-Pd reactions by both electron and ion scattering spectroscopies. In addition new usage of the channeling effect of MeV He⁺ ions is demonstrated to be a powerful tool for interface and surface studies.     

Method to determine the interface’s fractal dimensions of metal-semiconductor electric contacts from their static instrumental characteristics

With the concept of fractal surfaces, the influence of the relief of the interface (roughness) and the heterogeneity of the potential barrier on the behavior of the current–voltage and capacity–voltage characteristics of the metal–semiconductor electric contacts with the Schottky barrier has been studied. The necessary and satisfactory conditions for the accurate relative measurements of the surface relief have been found with the mathematical apparatus of the Hausdorff–Besikovitsch fractional dimensionality 2 D _f3. It is shown that due to the fractal geometry the relative change of the real area of the interface of the metal–semiconductor contacts with the Schottky barrier is proportional to the ratio of their linear dimensions in the 4-D_f power. This is considerably slower than the change of the dimensions of the topological areas of their contact windows. The method to determine the fractal dimensionality of the real interface of the metal–semiconductor electric contacts with the Schottky barrier from the current–voltage and capacity–voltage characteristics has been developed.