Electrical and microstructural investigation of Au/Pd/ri ohmic contacts for AIGaAs/GaAs heterojunction bipolar transistors

Abstract

The microstructure and electrical properties of as deposited and annealed Au (400 nm)/Pd (75 nm)/Ti (10 nm) contact structures to p type GaAs, C doped with a concentration of 5 × 10¹⁸ and 5 × 10¹⁹ cm^?3, have been investigated using transmission electron microscopy, and current-voltage measurements as afunction of temperature in the range 198–348 K. The specific contact resistivities have also been measured using the transmission line method. It was found that increasing the epilayer doping level by an order of magnitude, from 5 × 10¹⁸ to 5 × 10¹⁹ cm^?3, caused the dominant current transport mechanism to change from thermionic field emission to field emission. For the lower level doped epilayers generationrecombination within the depletion region was found to be the dominant current transport mechanism for temperatures below 289 K. The contacts to the more highly doped epilayers (C doped, 5 × 10¹⁹ cm^?3) had specific contact resistivities of 0·08 ± 0·03 Ωmm and 0·05 ± 0·06 Ωmm, respectively. These values, together with a minimal metal penetration in the semiconductor of <15 nm, indicate that these contacts are suitable for heterojunction bipolar device applications.

MST/3325     

Transmission electron microscopy of Li+ implanted Al films

Aluminium films implanted with 30 keV lithium ions of doses from 2.2 × 10¹⁶ to 1 × 10¹⁷ ions cm^?2 were studied by transmission electron microscopy. Samples implanted with 6 × 10¹⁶ and 1 × 10¹⁷ ions cm^?2 showed the presence of the Al-Li phase, while no change was observed in specimens implanted with 2.2 × 10¹⁶ ions cm^?. Microdiffraction patterns obtained from different areas of an aluminium film implanted with 2.0 × 10¹⁷ Li⁺ ions cm^?2 at 35 keV, revealed the presence of a stable Al-Li phase along with a metastable phase. Furthermore, on annealing the implanted samples at 200°C for 20 min an increase in ‘d’ values was noticed when compared with non-annealed specimens. However the annealed samples showed a very small lattice mismatch between the metastable phase and the Al matrix, thus indicating the presence of a metastable Al₃Li phase, responsible for hardening in binary Al-Li alloy.     

Multiple implantation of29Si+ in semi-insulating GaAs and its characterisation

Formation of a uniformn-layer by multiple²⁹Si⁺ implantation on LEC grown semi-insulating GaAs 〈100〉 substrate and its characterisation by differential Hall measurement at room temperature is reported. The implantation energies are 60, 160 and 260 keV with corresponding doses of 1 × 10¹², 2·55 × 10¹² and 3 × 10¹² cm⁻². Asimplanted, uncapped substrates were furnace-annealed with face-to-face configuration in an N₂ ambient at 850°C with arsenic overpressure. After annealing, the samples were subjected to Hall measurements using Van der Pauw configuration. Experimental and theoretical (LSS) profiles are compared. Electrical activation of the dopant atoms was found to range from 65–90% with average mobility values lying between 2000–2300 cm² V⁻¹ s⁻¹. Uniform concentration of then-layer ∼ 10¹⁷ cm⁻³ up to a depth of 0·3 μm has been achieved. These layers are used for the fabrication of power MESFETs.     

Preparation of Sb:SnO2 thin films and its effect on opto-electrical properties

The present study focuses on pure and antimony (Sb)-doped tin oxide thin film and its influence on their structural, optical, and electrical properties. Both undoped and Sb-doped SnO₂ thin films have been grown by using simple, inexpensive pyrolysis spray technique. The deposition temperature was optimized to 450 °C. X-ray diffractions pattern have revealed that the films are polycrystalline and have tetragonal rutile-type crystal structure. Undoped SnO₂ films grow along (110) preferred orientation, while the Sb-doped SnO₂ films grow along (200) direction. The size of Sb-doped tin oxide crystals changes from 26.3 to 58.0 nm when dopant concentration is changed from 5 to 25 wt%. The transmission spectra revealed that all the samples are transparent in the visible region, and the optical bandgap varies between 3.92 and 3.98 eV. SEM analysis shows that the surface morphology and grain size are affected by the doping rate. All the films exhibit a high transmittance in the visible region and show a sharp fundamental absorption edge at about 0.38–0.40 nm. The maximum electrical conductivity of 362.5 S/cm was obtained for the film doped with 5 wt% Sb. However, the carrier concentration is increased from 0.708?×?10¹⁸ to 4.058?×?10²⁰ cm³. The electrical study reveals that the films have n-type electrical conductivity and depend on Sb concentration. We observed a decrease in sheet resistance and resistivity with the increase in Sb dopant concentration. For the dopant concentration of 5 wt% of Sb in SnO₂, the Rs and ρ were found minimum with the values of 88.55 (Ω cm^?2) and 2.75 (Ω cm), respectively. We observed an increase in carrier concentration and a decrease in mobility with the addition of Sb up to 25 wt%. The highest figure of merit values 2.5?×?10^–3 Ω^?1 is obtained for the 5wt% Sb, which may be considered potential materials for solar cells' transparent windows.

     

Carrier properties of B atomic-layer-doped Si films grown by ECR Ar plasma-enhanced CVD without substrate heating

Abstract

The atomic-layer (AL) doping technique in epitaxy has attracted attention as a low-resistive ultrathin semiconductor film as well as a two-dimensional (2-D) carrier transport system. In this paper, we report carrier properties for B AL-doped Si films with suppressed thermal diffusion. B AL-doped Si films were formed on Si(100) by B AL formation followed by Si cap layer deposition in low-energy Ar plasma-enhanced chemical-vapor deposition without substrate heating. After fabrication of Hall-effect devices with the B AL-doped Si films on unstrained and 0.8%-tensile-strained Si(100)-on-insulator substrates (maximum process temperature 350°C), carrier properties were electrically measured at room temperature. Typically for the initial B amount of 2?×?10¹⁴ cm^?2 and 7?×?10¹⁴ cm^?2, B concentration depth profiles showed a clear decay slope as steep as 1.3 nm/decade. Dominant carrier was a hole and the maximum sheet carrier densities as high as 4?×?10¹³ cm^?2 and 2?×?10¹³ cm^?2 (electrical activity ratio of about 7% and 3.5%) were measured respectively for the unstrained and 0.8%-tensile-strained Si with Hall mobility around 10–13 cm² V^?1 s^?1. Moreover, mobility degradation was not observed even when sheet carrier density was increased by heat treatment at 500–700 °C. There is a possibility that the local carrier (ionized B atom) concentration around the B AL in Si reaches around 10²¹ cm^?3 and 2-D impurity-band formation with strong Coulomb interaction is expected. The behavior of carrier properties for heat treatment at 500–700 °C implies that thermal diffusion causes broadening of the B AL in Si and decrease of local B concentration.     

Studies of hydrogen doped lithium nitride

Single crystals of hydrogen-doped lithium nitride have been grown and relative estimates of dopant concentrations obtained from infra-red transmission studies. This technique together with nuclear reaction analyses has also been used to characterize surface layers of lithium hydroxide/lithium carbonate which form readily on the crystals when exposed to the atmosphere. Complex plane analysis of a.c. conductivity data in the temperature range 25–200°C has enabled the separation of bulk crystal and surface layer properties. A value of 2.8 10^?3 Ω^?1 cm^?1 at 25°C, with an activation energy of 0.23 ev, for ionic transport perpendicular to the c-axis, within the bulk of the crystal, has been achieved.     

Characterization of ohmic contacts to InP

The results of a study of the electrical and metallurgical properties of thin metallic layers deposited on InP for use as ohmic contacts are presented. The layers were heat treated at temperatures up to 550°C and were examined with Auger electron spectroscopy. For contact to n-type InP three thin film systems were investigated: gold, nickel and a composite Ni/Au/Ge layer. Nickel was found to produce ohmic behavior in the Ni/Au/Ge/InP system with a minimum specific contact resistance r_c of 3×10^?5 Ω cm² for a net doping of 3×10¹⁶ cm^?3. For contact to p-type InP a film consisting of Au/Mg was investigated. For heat treatment of the Au/Mg/InP system above 350°C, r_c decreased as the temperature of the heat treatment increased and the surface morphology exhibited increasing signs of alloying at higher temperatures. The smoothest surface was obtained at 446°C for 50 min with r_c≈1×10^?4Ω cm² for a net doping of 6×10¹⁷ cm^?3.     

Preparation and characterization of Cd.95Mn.05Se single crystals

Homogeneous crystals of Cd_.95Mn_.05Se of high optical quality have been grown by a modified Bridgman method. Magnetic susceptibility measurements verify the uniform distribution of Mn(II) obtained after annealing at 600°C.Crystals grown in the presence of 5 atomic percent excess selenium showed high resistivity; the addition of 1 mg iodine to a 10 g charge resulted in n-type conductivity and a room-temperature carrier concentration of 2.9 × 10¹⁶ cm^?3. The Hall mobility of these crystals was approximately 290 cm² V^?1 sec^?1.     

Mixage ionique sur des structures Au/InP

The reactions induced by Zn⁺ implantations near the interface of Au/InP contacts have been studied by using scanning electron microscopy, X-ray diffraction, He⁺ Rutherford backscattering, secondary ion mass spectrometry and current-voltage measurements. A 5 × 10¹⁴ Zn ions cm^-2 dose does not induce compound formation but accelerates the growth of Au₃In and Au₂P₃ patches during post-annealing treatment. After a 5 × 10¹⁵ Zn ions cm^-2 implantation, many compounds, different from those obtained by a thermal anneal, are detected. These compounds, which depend on the implantation temperature (25 or 200°C), have a layered structure. In this case no Au₂P₃ is observed. However, for the range of doses (from 10¹⁴ to 5 × 10¹⁵ Zn ions cm^-2), the temperatures of implantation (25 and 200°C) and the range of annealing temperatures (from 320 to 450°C) that were studied, no contact with a low resistivity is formed. The electrical properties are in fact limited by an InP layer damaged by the ion implantation in which the zinc atoms are trapped in an electrically inactive form.     

The synthesis and characterization of Cu<Subscript>2</Subscript>ZnSnS<Subscript>4</Subscript> thin films from melt reactions using xanthate precursors

Kesterite, Cu₂ZnSnS₄ (CZTS), is a promising absorber layer for use in photovoltaic cells. We report the use of copper, zinc and tin xanthates in melt reactions to produce Cu₂ZnSnS₄ (CZTS) thin films. The phase of the as-produced CZTS is dependent on decomposition temperature. X-ray diffraction patterns and Raman spectra show that films annealed between 375 and 475 °C are tetragonal, while at temperatures <375 °C hexagonal material was obtained. The electrical parameters of the CZTS films have also been determined. The conduction of all films was p-type, while the other parameters differ for the hexagonal and tetragonal materials: resistivity (27.1 vs 1.23 Ω cm), carrier concentration (2.65 × 10⁺¹⁵ vs 4.55 × 10⁺¹⁷ cm^?3) and mobility (87.1 vs 11.1 cm² V^?1 s^?1). The Hall coefficients were 2.36 × 10³ versus 13.7 cm³ C^?1.     

Editorial

Abstract

Silicon thin films, phosphorus doped in situ, have been deposited on to glass substrates using low pressure chemical vapour deposition at 550°C. The doping level is determined by adjusting the phosphine/silane molar ratio. Using this method a wide range of concentration is controllable. For a gas molar ratio varying between 4×10^?8 and 4 × 10^?4, phosphorus atomic incorporation is in the range 3 × 10¹⁶?3 × 10²⁰ cm^?3. The resistivity of the layers varies from 8·3 × 10⁵ to 1·5 × 10^?3 Ω cm. Lightly doped samples were passivated by hydrogenation, and Hall measurements were carried out, showing a marked improvement of the electrical properties. A lightly in situ doped drain thin film transistor suitable for active matrix applications was fabricated which exhibited good electrical properties.

MST/3335     

Thermal annealing behaviour of Pd Schottky contacts on melt-grown single crystal ZnO studied by IV and CV measurements

Current-voltage (IV) and capacitance-voltage (CV) measurement techniques have successfully been employed to study the effects of annealing highly rectifying Pd/ZnO Schottky contacts. IV results reveal a decrease in the contact quality with increasing annealing temperature as confirmed by a decrease in the zero bias barrier height and an increase in the reverse current measured at −1.5 V. An average barrier height of (0.77 ± 0.02) eV has been calculated by assuming pure thermionic emission for the as-deposited material and as (0.56 ± 0.03) eV after annealing at 550 °C. The reverse current has been measured as (2.10 ± 0.01) × 10⁻¹⁰ A for the as-deposited and increases by 5 orders of magnitude after annealing at 550 °C to (1.56 ± 0.01) × 10⁻⁵ A. The depletion layer width measured at −2.0 V has shown a strong dependence on thermal annealing as it decreases from 1.09 μm after annealing at 200 °C to 0.24 μm after annealing at 500 °C, resulting in the modification of the dopant concentration within the depletion region and hence the current flowing through the interface from pure thermionic emission to thermionic field emission with the donor concentrations increasing from 6.90 × 10¹⁵ cm⁻³ at 200 °C to 6.06 × 10¹⁶ cm⁻³ after annealing at 550 °C. This increase in the volume concentration has been explained as an effect of a conductive channel that shifts closer to the surface after sample annealing. The series resistance has been observed to decrease with increase in annealing temperature. The Pd contacts have shown high stability up to an annealing temperature of 250 °C as revealed by the IV and CV characteristics after which the quality of the contacts deteriorates with increase in annealing temperature.     

Effect of neutron irradiation on optical spectra of sapphire crystals

We have studied the effects of neutron irradiation (fluences from 5 × 10¹⁵ to 5 × 10¹⁹ cm^?2) and thermal annealing on the optical spectra of sapphire crystals and the generation and annealing of visible luminescence and absorption centers in the crystals. The radiation-induced color centers with λ = 460, 570, 620, and 780 nm have been shown to be annealed in steps at temperatures from 70 to 1150°C. The process can be represented by two exponentials with activation energies of 0.05 and 0.30 eV. We have revealed antiannealing of the 460-, 570-, and 620-nm color centers at 200–300 and 300–450°C, and evaluated the thermal ionization potential U of the 400-and 460-nm color centers. The results suggest the possibility of charge transfer from the former center (U = 0.5 eV) to the latter (U = 0.2 eV). Increasing the neutron fluence from 2 × 10¹⁸ to 5 × 10¹⁹ cm^?2 is shown to increase the color center concentration by one order of magnitude. We have analyzed the nature of the radiation-induced centers and have shown that the concentrations of 460-nm absorption and 540-nm emission centers are power law functions of neutron fluence, with exponents of 0.79 and 0.81, respectively. These findings strongly suggest that the same mechanism underlies the generation of these centers.     

Study on properties of forsterite/cordierite ceramic composites

The forsterite/cordierite ceramic composites are prepared by standard ceramic method, which properties and microstructures are characterized by X-ray diffraction and Scanning electron microscopy. It is found that the values of volume resistivities rapidly decline from 1 × 10¹³ to 10³ Ω cm as the testing temperatures increase from 20 to 600 °C and the resistivity transition temperatures of forsterite, cordierite and their composites are at about 300, 200 and 250 °C, respectively. The values of ε _r and tan δ are somewhat independent of the temperatures between 20 and 200 °C, but increase rapidly between 200 and 600 °C. The thermal expansion coefficient of ceramic composites decline with the cordierite content increasing and could change from about 2.5 × 10^?6–10.5 × 10^?6 °C^?1, in which the major phases are Mg₂SiO₄ and Mg₂Al₄Si₅O₁₈.     

Optical and hall properties of Al-doped ZnO thin films fabricated by pulsed laser deposition with various substrate temperatures

The 3 wt% Al-doped zinc oxide (AZO) thin films were fabricated on quartz substrates at a fixed oxygen pressure of 200 mTorr with various substrate temperatures (room temp. ～500 °C) by using pulsed laser deposition in order to investigate the microstructure, optical, and electrical properties of AZO thin films. All thin films were shown to be c-axis oriented, exhibiting only a (002) diffraction peak. The AZO thin film, fabricated at 200 mTorr and 400 °C, showed the highest (002) orientation and the full width at half maximum (FWHM) of the (002) diffraction peak was 0.42°. The c-axis lattice constant decreased with increasing substrate temperature. The electrical property indicated that the highest carrier concentration (1.27 × 10²¹ cm^?3) and the lowest resistivity (6.72 × 10^?4 Ωcm) were obtained in the AZO thin film fabricated at 200 mTorr and 400 °C. The optical transmittance in the visible region was higher than 80 %. The Burstein-Moss effect, which shifts to a high photon energy, was observed.     

Surface attachment of protonated polyimidazolium monolayer on titanate nanotubes as a novel proton conductor

A novel proton conductor has been designed by the surface immobilization of protonated polyimidazolium monolayer on titanate nanotubes (TiNTs) through a polymer brush strategy. 2,2′-Azobis(2-methylpropionitrile) (AIBN)-type initiators are first attached to TiNTs followed by a free radical polymerization of protonated 1-vinylimidazole (VyImBF₄) on the surface. The chemical structure of the resulting poly(VyImBF₄)-modified TiNTs is verified by Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). TGA curve indicates their good thermal stability. The maximum proton conductivity achieves 6.74?×?10^?4 S cm^?1 at 200 °C under dry condition and 3.60?×?10^?2 S cm^?1 at 120 °C under 100% humidity, respectively, when the polymerization is carried out under a polymerization time of 3 h and an immobilized initiator concentration of approximately 42.4 mmol L^?1. The proposed preparation of poly(VyImBF₄)-modified TiNTs would give a new idea for the design of other ion conductors.     

Graphene‐Analogues Boron Nitride Nanosheets Confining Ionic Liquids: A High‐Performance Quasi‐Liquid Solid Electrolyte

Solid electrolytes are one of the most promising electrolyte systems for safe lithium batteries, but the low ionic conductivity of these electrolytes seriously hinders the development of efficient lithium batteries. Here, a novel class of graphene‐analogues boron nitride (g‐BN) nanosheets confining an ultrahigh concentration of ionic liquids (ILs) in an interlayer and out‐of‐layer chamber to give rise to a quasi‐liquid solid electrolyte (QLSE) is reported. The electron‐insulated g‐BN nanosheet host with a large specific surface area can confine ILs as much as 10 times of the host's weight to afford high ionic conductivity (3.85 × 10^?3 S cm^?1 at 25 °C, even 2.32 × 10^?4 S cm^?1 at ?20 °C), which is close to that of the corresponding bulk IL electrolytes. The high ionic conductivity of QLSE is attributed to the enormous absorption for ILs and the confining effect of g‐BN to form the ordered lithium ion transport channels in an interlayer and out‐of‐layer of g‐BN. Furthermore, the electrolyte displays outstanding electrochemical properties and battery performance. In principle, this work enables a wider tunability, further opening up a new field for the fabrication of the next‐generation QLSE based on layered nanomaterials in energy conversion devices.     

Preparation and characterization of PAN–KI complexed gel polymer electrolytes for solid-state battery applications

The free standing and dimensionally stable gel polymer electrolyte films of polyacrylonitrile (PAN): potassium iodide (KI) of different compositions, using ethylene carbonate as a plasticizer and dimethyl formamide as solvent, are prepared by adopting ‘solution casting technique’ and these films are examined for their conductivities. The structural, miscibility and the chemical rapport between PAN and KI are investigated using X-ray diffraction, Fourier transform infrared spectroscopy and differential scanning calorimetry methods. The conductivity is enhanced with the increase in KI concentration and temperature. The maximum conductivity at 30°C is found to be 2.089 × 10^?5 S cm^?1 for PAN:KI (70:30) wt%, which is nine orders greater than that of pure PAN (< 10^?14 S cm^?1). The conductivity-temperature dependence of these polymer electrolyte films obeys Arrhenius behaviour with activation energy ranging from 0.358 to 0.478 eV. The conducting carriers of charge transport in these polymer electrolyte films are identified by Wagner’s polarization technique and it is found that the charge transport is predominantly due to ions. The better conducting sample is used to fabricate the battery with configuration K/PAN + KI/I₂+ C + electrolyte and good discharge characteristics of battery are observed.     

Single crystals of In2Te5

Single crystals of semiconducting compound In₂Te₅ were grown by chemical transport employing iodine as a transport agent. The crystals had a plate-like habit with the [100] direction perpendicular to the flat surface of the platelets. Nominal dimensions are 10 × 1 × 0.05 mm. In₂Te₅ has a monoclinic structure with dimensions of the base centered cell: $\text{a}\text{= 13.47}\text{A}\text{?}$, $\text{b}\text{= 16.51}\text{A}\text{?}$, $\text{c}\text{= 4.365}\text{A}\text{?}$, β = 92°5′. The space group is $\text{C}{}_{\mathrm{<mtext>2</mtext><mtext>c</mtext>}}$. Pycnometric density is 5.96 g/cm³. The single crystals were all p-type. The conductivity, thermoelectric power and hardness were about 10^?5Ω^?1cm^?1, 650 mkV/°C, and 30 kg/mm², respectively. The minimum energy gap is 1.26 eV.     

The chemical vapour deposition and characterization of ZrO2 films from organometallic compounds

ZrO₂ films were deposited on silicon substrates by oxygen-assisted decomposition of zirconium-β-diketonates at temperatures of 400–550°C. The deposits, fine-grained nearly stoichiometric monoclinic ZrO₂, were hard and showed strong adherence to the substrate. The films were characterized by transmission electron microscopy, X-ray diffraction and electron microprobe analysis and by measuring their dielectric and optical properties. The index of refraction was found to be 2.18, and the optical energy band gap was found to be 5.16 eV. The dielectric constant at 1 MHz was 17–18, and the dielectric strength varied between 1 × 10⁶ and 2.0 × 10⁶ V cm^?1. Capacitance-voltage measurements at 1 MHz indicated the presence of effective surface states with a concentration in the range (1.0?6.0) × 10¹¹cm^?2 for films deposited at temperatures above 500°C or for films deposited at 400–450°C and annealed at above 750°C. The flat-band voltages were between ?0.6 and + 0.2 V. The films showed satisfactory bias-temperature stability. The current-voltage characteristic followed an I ∝ V² dependence for negative bias and an I ∝ V^2.6 to I ∝ V^3.4 dependence for positive bias.