Optoelectronic properties of Zn0.52Se0.48/Si Schottky diodes

Zn_0.52Se_0.48/Si Schottky diodes are fabricated by depositing zinc selenide (Zn_0.52Se_0.48) thin films onto Si(1 0 0) substrates by vacuum evaporation technique. Rutherford backscattering spectrometry (RBS) analysis shows that the deposited films are nearly stoichiometric in nature. X-ray diffractogram of the films reveals the preferential orientation of the films along (1 1 1) direction. Structural parameters such as crystallite size (D), dislocation density (δ), strain (ε), and the lattice parameter are calculated as 29.13 nm, 1.187 × 10⁻¹⁵ lin/m², 1.354 × 10⁻³ lin⁻² m⁻⁴ and 5.676 × 10⁻¹⁰ m respectively. From the I–V measurements on the Zn_0.52Se_0.48/p-Si Schottky diodes, ideality and diode rectification factors are evaluated, as 1.749 (305 K) and 1.04 × 10⁴ (305 K) respectively. The built-in potential, effective carrier concentration (N_A) and barrier height were also evaluated from C–V measurement, which are found to be 1.02 V, 5.907 × 10¹⁵ cm⁻³ and 1.359 eV respectively.     

Optimization of the electrical properties of Al/a-SiC:H Schottky diodes by means of thermal annealing of a-SiC:H thin films

The present work reports on the optimization of the electrical properties of Al/a-SiC:H Schottky diodes by means of thermal annealing of a-SiC:H thin films. Optical transmission experiments have shown that the optical properties of the films are affected by thermal annealing when T_a>600 °C, due to emission of hydrogen bonded to silicon. Although the electrical properties of Al/a-SiC:H Schottky diodes are invariant for T_a?400 °C, for higher T_a these properties are improved with the optimum result achieved at . At this annealing temperature the linear log I-V characteristics span about eight orders of magnitude and the ideality factor is 1.09±0.04, making these diodes very interesting for many potential applications. For higher T_a (>600 °C) the electrical properties of Al/a-SiC:H Schottky diodes deteriorate with complete degradation at . For temperatures up to 600 °C this behavior is attributed to relaxation of the strain in the amorphous network which is possibly combined with weak hydrogen emission for temperatures up to 600 °C, leading to an optimum material quality. For further increase of T_a (>600 °C) the observed deterioration of the electrical properties of Al/a-SiC:H Schottky diodes is due to the intensive emission of hydrogen atoms bonded to silicon that cause voids in the amorphous network. These results are also supported by the experimental values of the room temperature apparent barrier height of the Al/a-SiC:H junction ?_bRT and its temperature coefficient γ.     

An amorphous SiC/Si heterojunction p-i-n diode for low-noise andhigh-sensitivity UV detector

The authors report the UV photoresponse of an a-SiC/a-Si heterojunction p-i-n diode with the structure of glass/TCO (transparent conducting oxide, SnO₂:F)/p-a-SiC:H/i-a-Si:H/n-a-Si:H/Al. The diode has been designed for a high-sensitivity and low-noise UV detector. The diode has its peak responsivity (0.254 A/W) and quantum efficiency (81.5%) at 385 nm. This structure possesses (1) the window effect by using the wide-bandgap a-SiC:H as the front layer (p-layer) and (2) the carrier confinement effect at the p-SiC:H/i-a-Si:H interface. Enhancements are proposed to raise UV response and suppress long-wave responsivity. The diode was designed to be operated under zero external bias to suppress the dark-current-induced noise. Results show a 200% higher UV sensitivity than a GaAsP Schottky photodiode in the 200-400-nm wavelength region     

Electroluminescence characteristics and current-conductionmechanism of a-SiC:H p-i-n thin-film light-emitting diodes with barrierlayer inserted at p-i interface

In order to improve the electroluminescence (EL) characteristics of the hydrogenated amorphous silicon carbide (a-SiC:H) p-i-n thin-film light-emitting diode (TFLED), a barrier layer (BL) was inserted at its p-i interface to enhance the hole injection efficiency under forward-bias operation. The a-SiC:H TFLED's with various optical gaps of BL had been fabricated and characterized. In addition, a composition-graded n⁺-layer was used to reduce its series and contact resistances to the Al electrode and hence the EL threshold voltage (V_th) of an a-SiC:H BL TFLED. The highest obtainable brightness of an a-SiC:H BL TFLED was 342 cd/m² at an injection current density of 600 mA/cm² and the lowest EL V _th achievable was 6.0 V. The current-conduction mechanism of an a-SiC:H BL TFLED had also been investigated. Within the lower applied-bias region, it showed an ohmic current, while within the higher applied-bias region, a space-charge-limited current (SCLC) was observed     

Schottky barrier height of a new ohmic contact NiSi2 to n-type 6H-SiC

We present a new ohmic contact material NiSi₂ to n-type 6H-SiC with a low specific contact resistance. NiSi₂ films are prepared by annealing the Ni and Si films separately deposited on (0 0 0 1)-oriented 6H-SiC substrates with carrier concentrations (n) ranging from 5.8×10¹⁶ to 2.5×10¹⁹ cm⁻³. The deposited films are annealed at 900 °C for 10 min in a flow of Ar gas containing 5 vol.% H₂ gas. The specific contact resistance of NiSi₂ contact exponentially decreases with increasing carrier concentrations of substrates. NiSi₂ contacts formed on the substrates with n=2.5×10¹⁹ cm⁻³ show a relatively low specific contact resistance with 3.6×10⁻⁶ Ω cm². Schottky barrier height of NiSi₂ to n-type 6H-SiC is estimated to be 0.40±0.02 eV using a theoretical relationship for the carrier concentration dependence of the specific contact resistance.     

GaN p–n junction diode formed by Si ion implantation into p-GaN

²⁸Si⁺ implantation into Mg-doped GaN, followed by thermal annealing in N₂ was performed to achieve n⁺-GaN layers. The carrier concentrations of the films changed from 3×10¹⁷ (p-type) to 5×10¹⁹ cm⁻³ (n-type) when the Si-implanted p-type GaN was properly annealed. Specific contact resistance (ρ_c) of Ti/Al/Pt/Au Ohmic contact to n-GaN, formed by ²⁸Si⁺ implantation into p-type GaN, was also evaluated by transmission line model. It was found that we could achieve a ρ_c value as low as 1.5×10⁻⁶ Ω cm² when the metal contact was alloyed in N₂ ambience at 600 °C. Si-implanted GaN p–n junction light-emitting diodes were also fabricated. Electroluminescence measurements showed that two emission peaks at around 385 and 420 nm were observed, which could be attributed to the near band-edge transition and donor-to-acceptor transition, respectively.     

High temperature ohmic contacts to 3C–silicon carbide films

Currently, large-area 3C–SiC films are available from a number of sources and it is imperative that stable high temperature contacts be developed for high power devices on these films. By comparing the existing data in the literature, we demonstrate that the contact behavior on each of the different polytypes of SiC will vary significantly. In particular, we demonstrate this for 6H–SiC and 3C–SiC. The interface slope parameter, S, which is a measure of the Fermi-level pinning in each system varies between 0.4–0.5 on 6H–SiC, while it is 0.6 on 3C–SiC. This implies that the barrier heights of contacts to 3C–SiC will vary more significantly with the choice of metal than for 6H–SiC. Aluminum, nickel and tungsten were deposited on 3C–SiC films and their specific contact resistance measured using the circular TLM method. High temperature measurements (up to 400°C) were performed to determine the behavior of these contacts at operational temperatures. Aluminum was used primarily as a baseline for comparison since it melts at 660°C and cannot be used for very high temperature contacts. The specific contact resistance (ρ_c) for nickel at room temperature was 5×10⁻⁴ Ω cm², but increased with temperature to a value of 1.5×10⁻³ Ω cm² at 400°C. Tungsten had a higher room temperature ρ_c of 2×10⁻³ Ω cm², which remained relatively constant with increasing temperature up to 400°C. This is related to the fact that there is hardly any reaction between tungsten and silicon carbide even up to 900°C, whereas nickel almost completely reacts with SiC by that temperature. Contact resistance measurements were also performed on samples that were annealed at 500°C.     

Pt-based metallization of PMOS devices for a compatible monolithic integration of semiconducting/Yba2Cu3O7−δ superconducting devices on silicon

Mo, Pt, Pt/Mo and Pt/Ti thin films have been deposited onto Si and SiO₂ substrates by RF sputtering and annealed in the YBa₂Cu₃O_7−δ (YBCO) growth conditions. The effect of annealing on the sheet resistance of unpatterned layers was measured. A Pt-based multilayered metallization for the PMOS devices was proposed and tested for a compatible monolithic integration of semiconducting devices and YBCO sensors on the same silicon substrate. The best results were obtained with a Pt/Ti/Mo-silicide structure showing 0.472 Ω_□ interconnect sheet resistivity and 2×10⁻⁴ Ω cm² specific contact resistivity after annealing for 60 min at 700 °C in 0.5 mbar O₂ pressure.     

Silicon matrix disorder in amorphous hydrogenated silicon alloys

The authors report a detailed investigation of correlations between Urbach energies from photothermal deflection spectroscopy and Raman half-widths of transverse optic (TO)-like Si-Si modes as a measure of silicon matrix disorder in glow-discharge amorphous hydrogenated silicon (a-Si:H) and a-SiGe:H, as well as in glow-discharge and sputtered a-SiC:H and a-SiN:H. A corresponding decrease in TO full width at half-maximum (FWHM) and Urbach energy E₀ for soft deposition techniques yields bond angle distributions as narrow as 8.5° for the best a-Si:H films. Even at the lowest levels of nitrogen incorporation, simultaneous increases in E₀ and TO-like half-widths indicate that lattice distortions occur due to threefold coordination of nitrogen in the a-Si:H matrix. In contrast, no deviation of silicon TO-FWHM could be detected in a-SiC:H of up to 35 at.% of carbon content, whereas Urbach edges broaden in a well-known manner that is interpreted in terms of -CH₃ incorporation into the amorphous network. Diborane doping and sputter deposition, however, give rise to lattice distortions in a-SiC:H, which reflects changes in the carbon coordination     

Silicon Schottky barriers and p-n junctions with highly stablealuminum contact metallization

Reactively sputtered amorphous Ta₃₆Si₁₄N₅₀ thin films are investigated as diffusion barriers to improve the thermal stability of contacts to electronic devices, specifically between Al overlayers and Si substrates. Electrical measurements on Schottky diodes and on shallow n ⁺-p junction diodes are used to evaluate the thermal stability of the (Si)/W₄₈Si₂₀N₃₂/Ta₃₆Si ₁₄N₅₀/Al metallization. The W₄₈Si₂₀N₃₂ contacting layer is added to raise the Schottky barrier height on n-type Si. It is shown that a 100-nm-thick Ta₃₆Si₁₄N₅₀ layer effectively prevents the intermixing between Al and Si. With this barrier layer, both shallow junctions and Schottky diodes are electrically stable up to 700°C for 20 min (above the Al melting point of 660°C ), which makes this material the best thin-film diffusion barrier on record     

Preliminary study on pyroelectric lithium tantalite by a novel electrostatic spray pyrolysis technique

In this paper, we report the first results obtained for pyroelectric LiTaO₃ thin films prepared on silicon substrates by electrostatic spray pyrolysis (ESP), a simple, low cost and efficient technique not widely used for LiTaO₃ deposition, using lithium acetylacetonate and tantalum ethoxide in a mixture of methanol. The effect of the growth and annealing temperature on the structural and optical properties has been investigated. X-ray diffraction measurements have shown that LiTaO₃ thin films became preferentially oriented in the (0,1,2) plane after annealing treatment in an oxygen environment using the rapid thermal processing. On the other hand, a thermal stress's modeling is performed to observe the effect of growth temperature on the as-deposited films and the substrates. The SEM images have shown that the film heat-treated at 600 °C became more homogeneous and smoother than that before annealing. The optical phonon modes of the LiTaO₃ thin films have been also investigated using infrared reflectivity (FTIR) and Raman spectroscopy.     

GaAs surface plasma treatments for Schottky contacts

The properties of different rectifying metallizations (Al, Ti/Pt, WN_x) on GaAs have been investigated for various surface preparation procedures. In particular, in situ hydrogen plasma treatments were used to remove residual surface contamination (mainly O and C) and a nitrogen plasma to grow a thin mixed nitride layer. Al and Ti/Pt Schottky diodes with an ideality factor very close to 1, but with reduced barrier height, were found after the H₂ plasma as a consequence of H diffusion into GaAs. The Schottky barrier height was further reduced if a H₂ + N₂ plasma was performed. The N content in the sputtering environment during the WN_x deposition affects the diode properties of plasma-treated WN_x contacts. By increasing the N₂ partial pressure, the diode barrier height is reduced, probably due to nitridization of the GaAs surface. Such differences disappear after annealing the diodes in arsine overpressure at 800°C. WN_x contacts deposited under different conditions on H₂ plasma treated substrates also show a similar Schottky barrier height after such annealing.     

n+-SnO2/a-SiC:H/metal thin-film photodiodeswith voltage-controlled spectral sensitivity

n⁺-SnO₂/a-SiC/metal photodiodes with voltage-controlled photosensitivity have been realized by using both carbon-rich and silicon-rich a-SiC alloys. Carbon-rich devices show a response peak located at 530 nm independent of the applied voltage, which in turn only affects the peak height. At variance, in silicon-rich structures the response peak is located at 480, 510, and 570 nm when the applied voltage is -4, 0, and +4 V, respectively, with corresponding quantum yield values of 17, 3, and 25%. For explaining the observed behavior we present a simple model of n⁺-SnO₂/a-SiC/metal diodes, which takes into account light-induced modulation of n⁺-SnO₂/a-SiC barrier height, primary photocurrent generation and photoconductivity effects     

Electrical properties of high permittivity ZrO2 gate dielectrics on strained-Si

Deposition and electrical properties of high dielectric constant (high-k) ultrathin ZrO₂ films on tensilely strained silicon (strained-Si) substrate are reported. ZrO₂ thin films have been deposited using a microwave plasma enhanced chemical vapor deposition technique at a low temperature (150 °C). Metal insulator semiconductor (MIS) structures are used for high frequency capacitance–voltage (C–V), current–voltage (I–V), and conductance–voltage (G–V) characterization. Using MIS capacitor structures, the reliability and the leakage current characteristics have been studied both at room and high temperature. Schottky conduction mechanism is found to dominate the current conduction at a high temperature. Observed good electrical and reliability properties suggest the suitability of deposited ZrO₂ thin films as an alternative as gate dielectrics. Compatibility of ZrO₂ as a gate dielectric on strained-Si is shown.     

Normally-off 4H-SiC trench-gate MOSFETs with high mobility

A new 4H-SiC trench-gate MOSFET structure with epitaxial buried channel for accumulation-mode operation, has been designed and fabricated, aiming at improving channel electron mobility. Coupled with improved fabrication processes, the MOSFET structure eliminates the need of high dose N⁺ source implantation. High dose N⁺ implantation requires high-temperature (1550 °C) activation annealing and tends to cause substantial surface roughness, which degrades MOSFET threshold voltage stability and gate oxide reliability. The buried channel is implemented without epitaxial regrowth or accumulation channel implantation. Fabricated MOSFETs subject to ohmic contact rapid thermal annealing at 850 °C for 5 min exhibit a high peak field-effect mobility (μ_FE) of 95 cm²/V s at room temperature (25 °C) and 255 cm²/V s at 200 °C with stable normally-off operation from 25 °C to 200 °C. The dependence of channel mobility and threshold voltage on the buried channel depth is investigated and the optimum range of channel depth is reported.     

Ferromagnetism in self-assembled Ge quantum dots material followed by Mn-implantation and annealing

Ten and twenty layers of self-assembled Ge QDs with 44 and 59-nm-thick Si barrier were grown on high resistivity (1 0 0) p-type Si substrates by rapid thermal chemical vapor deposition followed by Mn ion implantation and post-annealing. A presence of ferromagnetic structure was confirmed in the insulating GeMn diluted magnetic quantum dots (DMQD) and semiconducting GeMn DMQD. The DMQD materials were found to be homogeneous, and to exhibit p-type conductivity and ferromagnetic ordering with a Curie temperature, T_C = 350 and 230 K. The X-ray diffraction (XRD) data show that there is a phase separation of Mn₅Ge₃ from MnGe nanostructure. Temperature dependent electrical resistivity in semiconducting DMQD material indicates that manganese introduces two acceptor levels in germanium at 0.14 eV from the valence band and 0.41 eV from the conduction band implying Mn substituting Ge. Therefore, it is likely that the ferromagnetic exchange coupling of DMQD material with T_C = 230 K is hole-mediated due to formation of polarons and the ferromagnetism in sample with T_C > 300 K is due to Mn₅Ge₃ phase.     

Microscopic properties of H2 diluted HWCVD deposited a-SiC:H film

Hydrogenated amorphous silicon carbon (a-SiC:H) films were deposited by hot wire chemical vapor deposition. The evolution of microscopic properties like network bonding, disorder, density and chemical composition are studied as a function of H₂ dilution. The sp² and sp³ carbon clusters, hydrogen content and the density of the material has significant effect on the dielectric properties like the leakage current of M/a-SiC:H/〈Si〉 MIS structures made with both Cu and Al metal electrode. A higher leakage current is observed in the case of Cu electrode. These changes are important for its applicability as a low dielectric constant barrier material in microelectronic devices.     

Demonstration of GaN MIS diodes by using AlN and Ga2O3(Gd2O3) as dielectrics

GaN MIS diodes were demonstrated utilizing AlN and Ga₂O₃(Gd₂O₃) as insulators. A 345 Å of AlN was grown on the MOCVD grown n-GaN in a MOMBE system using trimethylamine alane as Al precursor and nitrogen generated from a SVT RF N₂ plasma. For the Ga₂O₃(Gd₂O₃) growth, a multi-MBE chamber was used and a 195 Å oxide was E-beam evaporated from a single crystal source of Ga₅Gd₃O₁₂. The forward breakdown voltage of AlN and Ga₂O₃(Gd₂O₃) diodes are 5 and 6 V, respectively, which are significantly improved over 1.2 V from that of a Schottky contact. From the C–V measurements, both kinds of diodes showed good charge modulation from accumulation to depletion at different frequencies. The insulator/GaN interface roughness and the thickness of the insulator were measured with X-ray reflectivity.     

Acetone-assisted deposition of silver films in supercritical carbon dioxide

Growth of silver films was studied by supercritical fluid deposition (SCFD) using H₂ and acetone as reducing agents for (1,5-cyclooctadiene)(hexafluoroacetylacetonato)silver(I) in supercritical CO₂ (scCO₂). H₂ reduction did not yield continuous Ag films, whereas continuous films were deposited on Ru substrates by acetone-assisted reduction of 0.006–0.03 mol% precursor in the temperature range of 150–250 °C. Surface qualities of the Ag films were effectively improved by decreasing water content in acetone reagent, as well as reducing acetone and precursor concentrations in scCO₂. Ultimately, a 50 nm-thick film with shiny surface was obtained in optimized conditions. A possible mechanism for acetone reduction of Ag precursor on Ru surface was also proposed.