C-V analysis of rapidly thermal annealed SF6 plasma treated AlGaN/GaN heterostructures

We studied influence of rapid thermal annealing on electrical parameters of SF₆ plasma treated AlGaN/GaN heterostructures. The main emphasis by the evaluation was laid on C-V measurement and simulation, but also I-V and SIMS measurement were used. It was found that the diminished sheet carrier concentration of a two-dimensional electron gas after plasma treatment recovered significantly at the temperature of 500 °C. By using C-V measurement, it was possible to assess besides the changes of the two-dimensional electron gas concentration after annealing also the changes in the Schottky barrier heights and to find out the doping concentration in AlGaN barrier and GaN channel layer. The trend in Schottky barrier height changes after annealing was confirmed also by I-V measurement.     

Thermal stability of W2B and W2B5 contacts on ZnO

Rectifying contact formation on n-type bulk single crystal ZnO using novel W₂B or W₂B₅ metallization schemes was studied using current-voltage, scanning electron microscopy and Auger electron spectroscopy (AES) measurements. When a single Au overlayer was used to reduce the metal sheet resistance, the contacts were ohmic for all annealing conditions due to outdiffusion of Zn through the metal. By sharp contrast, when a bilayer of Pt/Au was used on top of the boride layers, rectifying contacts with barrier heights of ∼0.4 eV for W₂B were obtained. The highest barrier height of 0.66 eV was achieved for W₂B₅ annealed at 600 °C, although at this condition the contact showed a reacted appearance and AES showed almost complete intermixing of the metallization.     

The effects of annealing on Au/pyronine-B/MD n-InP Schottky structure

Thin film of non-polymeric organic compound pyronine-B has been fabricated on moderately doped (MD) n-InP substrate as an interfacial layer using spin coating technique for the electronic modification of Au/MD n-InP Schottky contact. The electrical characteristics have been determined at room temperature. The barrier height and the ideality factor values for Au/pyronine-B/MD n-InP Schottky diode have been obtained from the forward bias I-V characteristics at room temperature as 0.60 eV and 1.041; 0.571 and 1.253 eV after annealing at 100 and 250 °C, respectively. An increase in annealing temperature at the Au/n-InP Schottky junction is shown to increase the reverse bias leakage current by about one order of magnitude and decrease the Schottky barrier height by 0.027 eV. Furthermore, the barrier height values for the Au/pyronine-B/MD n-InP Schottky diode have also been obtained from the C-V characteristics at room temperature as 1.001 and 0.709 eV after annealing at 100 and 250 °C, respectively. Finally, it was seen that the diode parameters changed with increase in the annealing temperature.     

W2B-based ohmic contacts to n-GaN

Ohmic contact formation on n-GaN using a novel Ti/Al/W₂B/Ti/Au metallization scheme was studied using contact resistance, scanning electron microscopy and Auger electron spectroscopy measurements. A minimum specific contact resistivity of 7 × 10⁻⁶ Ω cm² was achieved at an annealing temperature of 800 °C. The contact resistance was essentially independent of measurement temperature, indicating that field emission plays a dominant role in the current transport .The Ti began to outdiffuse to the surface at temperatures of ∼500 °C, while at 800 °C the Al also began to intermix within the contact. By 1000 °C, the contact showed a reacted appearance and AES showed almost complete intermixing of the metallization. The contact resistance showed excellent stability for extended periods at 200 °C, which simulates the type of device operating temperature that might be expected for operation of GaN-based power electronic devices.     

Current transport mechanism in Al/Si3N4/p-Si (MIS) Schottky barrier diodes at low temperatures

The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of metal-insulator-semiconductor (Al/Si₃N₄/p-Si) Schottky barrier diodes (SBDs) were measured in the temperature range of 80-300 K. By using the thermionic emission (TE) theory, the zero-bias barrier height Φ_B0 calculated from I-V characteristics was found to increase with increasing temperature. Such temperature dependence is an obvious disagreement with the negative temperature coefficient of the barrier height calculated from C-V characteristics. Also, the ideality factor decreases with increasing temperature, and especially the activation energy plot is nonlinear at low temperatures. Such behaviour is attributed to Schottky barrier inhomogeneties by assuming a Gaussian distribution of barrier heights (BHs) at interface. We attempted to draw a Φ_B0 versus q/2kT plot to obtain evidence of a Gaussian distribution of the BHs, and the values of Φ_Bo = 0.826 eV and α_o = 0.091 V for the mean barrier height and standard deviation at zero-bias, respectively, have been obtained from this plot. Thus, a modified ln(I_o/T²) − q²σ_o²/2(kT)² versus q/kT plot gives Φ_B0 and Richardson constant A^* as 0.820 eV and 30.273 A/cm² K², respectively, without using the temperature coefficient of the barrier height. This value of the Richardson constant 30.273 A/cm² K² is very close to the theoretical value of 32 A/cm² K² for p-type Si. Hence, it has been concluded that the temperature dependence of the forward I-V characteristics of the Al/Si₃N₄/p-Si Schottky barrier diodes can be successfully explained on the basis of TE mechanism with a Gaussian distribution of the barrier heights. In addition, the temperature dependence of energy distribution of interface state density (N_SS) profiles was determined from the forward I-V measurements by taking into account the bias dependence of the effective barrier height and ideality factor.     

Use of TiB2 diffusion barriers for Ni/Au ohmic contacts on p-GaN

The use of a TiB₂ diffusion barrier for Ni/Au contacts on p-GaN is reported. The annealing temperature (25-950 °C) dependence of ohmic contact characteristics using a Ni/Au/TiB₂/Ti/Au metallization scheme deposited by sputtering were investigated by contact resistance measurements and auger electron spectroscopy (AES). The as-deposited contacts are rectifying and transition to ohmic behavior for annealing at ≥500 °C . A minimum specific contact resistivity of ∼3 × 10⁻⁴ Ω cm⁻² was obtained after annealing over a broad range of temperatures (800-950 °C for 60 s). The contact morphology became considerably rougher at the higher end of this temperature range. AES profiling showed significant Ti and Ni outdiffusion through the TiB₂ at 800 °C. By 900 °C the Ti was almost completely removed to the surface, where it became oxidized. Use of the TiB₂ diffusion barrier produces superior thermal stability compared to the more common Ni/Au, whose morphology degrades significantly above 500 °C.     

Temperature dependent electrical characteristics of Sn/p-Si Schottky diodes

Current-voltage and capacitance-voltage characteristics of Sn/p-Si Schottky diodes measured in the temperature range 80-320 K are presented and analysed. Anomalous strong temperature dependencies of the ideality factor and apparent barrier height were obtained. There was also a considerable difference between the apparent barrier heights obtained from current-voltage and capacitance-voltage characteristics. These anomalies are explained by the domination of the current by a high level of thermionic-field emission, and by the presence of deep levels near the Sn/Si interface, which yield a reduction of free hole concentration and a significant temperature dependence of the charge stored near the metal-semiconductor (MS) interface. The evaluation of temperature dependence of forward current for thermionic-field emission resulted in the following parameters: characteristic energy E₀₀ = 9.8 meV, Schottky barrier height at zero bias Φ_b0 = 0.802 eV, bias coefficient of barrier height β = 0, and effective Richardson constant A* = 37.32 A cm⁻² K⁻².     

Structure and dielectric studies of nano-composite Fe2O3: BaTiO3 prepared by sol-gel method

Nano-structure pure barium titanate (BaTiO₃) and that was doped with iron oxide (Fe₂O₃), have been prepared by sol-gel method, using barium acetate (Ba(Ac)₂) and titanium butoxide (Ti(C₄H₉O)₄), as precursors. The as-grown prepared samples by sol-gel technique were found to be amorphous, which crystallized to the tetragonal phase after synthesized at 750 °C in air for 1 h as detected from the XRD patterns. The XRD data were confirmed by transmission electron microscope (TEM). The dielectric properties namely; dielectric constant (ε′) and loss tangent (tan δ) in the frequency range between 42 Hz and 1 MHz, at range of temperature 25-250 °C were investigated. The temperature dependence of ε′ and tan δ for the undoped and doped materials, at 1 kHz, was also investigated. As a result, tan δ increased rapidly with decreasing temperature below 125 °C (Curie temperature) while above this temperature, tan δ shows temperature independent. As a result, below and above Curie temperature, ferroelectric phase and paraelectric phase of BaTiO₃ can be obtained, respectively.     

Current–voltage temperature characteristics of Au/n-Ge (1 0 0) Schottky diodes

The variation in electrical characteristics of Au/n-Ge (1 0 0) Schottky contacts have been systematically investigated as a function of temperature using current–voltage (I−V) measurements in the temperature range 140–300 K. The I–V characteristics of the diodes indicate very strong temperature dependence. While the ideality factor n decreases, the zero-bias Schottky barrier height (SBH) (Φ_B) increases with the increasing temperature. The I–V characteristics are analyzed using the thermionic emission (TE) model and the assumption of a Gaussian distribution of the barrier heights due to barrier inhomogeneities at the metal–semiconductor interface. The zero-bias barrier height Φ_B vs. 1/2 kT plot has been used to show the evidence of a Gaussian distribution of barrier heights and values of Φ_B=0.615 eV and standard deviation σ_s0=0.0858 eV for the mean barrier height and zero-bias standard deviation have been obtained from this plot, respectively. The Richardson constant and the mean barrier height from the modified Richardson plot were obtained as 1.37 A cm⁻² K⁻² and 0.639 eV, respectively. This Richardson constant is much smaller than the reported of 50 A cm⁻² K⁻². This may be due to greater inhomogeneities at the interface.     

EDXRF measurements on gold diffusion-doped Bi1.8Pb0.35Sr1.9Ca2.1Cu3Oy

Gold (Au) diffusion in superconducting Bi_1.8Pb_0.35Sr_1.9Ca_2.1Cu₃O_y was investigated over the temperature range 500-800 °C by the energy dispersive X-ray fluorescence (EDXRF) technique. It is found that the Au diffusion coefficient decreases as the diffusion-annealing temperature decreases. The temperature dependences of Au diffusion coefficient in grains and over grain boundaries are described by the relations D₁=6.7×10⁻⁵exp(−1.19 eV/k_BT) and D₂=9.7×10⁻⁴exp(−1.09 eV/k_BT), respectively. The diffusion doping of Bi-2223 by Au causes a significant increase of the lattice parameter c by about 0.19%. For the Au-diffused samples, dc electrical resistivity and transport critical current density measurements indicated the critical transition temperature increased from 100 to 104 K and the critical current density increased from 40 to 125 A cm⁻², in comparison with those of undoped samples. From scanning electron microscope (SEM) and X-ray diffraction (XRD) measurements it is observed that Au doping of the sample also improved the surface morphology and increased the ratio of the high-T_c phase to the low-T_c phase. The possible reasons for the observed improvement in microstructure and superconducting properties of the samples due to Au diffusion are also discussed.     

Investigation on the barrier height and inhomogeneity of nickel silicide Schottky

The apparent Schottky barrier height (SBH) of the nickel silicide Schottky contacts annealed at different temperatures was investigated based on temperature dependence of I-V characteristic. Thermionic emission-diffusion (TED) theory, single Gaussian and double Gaussian models were employed to fit I-V experimental data. It is found the single Gaussian and double Gaussian SB distribution model can give a very good fit to the I-V characteristic of apparent SBH for different annealing temperatures. Also, the apparent SBH and the leakage current increase with annealing temperatures under reverse voltage. In addition, the homogeneity of interfaces for the samples annealed at temperatures of 500 and 600 °C is much better than that of the samples annealed at temperatures of 400, 700, and 800 °C. This may result from the phase transformation of nickel silicide due to the different annealing temperatures and from the low Schottky barrier (SB) patches.     

Low-temperature sintering and microwave dielectric properties of Ca2Zn4Ti15O36 ceramics

The sintering behavior, microstructures, and microwave dielectric properties of Ca₂Zn₄Ti₁₅O₃₆ ceramics with B₂O₃ addition were investigated. The crystalline phases and microstructures of Ca₂Zn₄Ti₁₅O₃₆ ceramics with 0-10 wt% B₂O₃ addition were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). The sintering temperature of Ca₂Zn₄Ti₁₅O₃₆ ceramic was lowered from 1170 to 930 °C by 10 wt% B₂O₃ addition. Ca₂Zn₄Ti₁₅O₃₆ ceramics with 8 wt% B₂O₃ addition sintered at 990 °C for 2 h exhibited good microwave dielectric properties, i.e., a quality factor (Qf) 11,400 GHz, a relative dielectric constant (ε_r) 41.5, and a temperature coefficient of resonant frequency (τ_f) 94.4 ppm/°C.     

Investigation of diode parameters using I-V and C-V characteristics of In/SiO2/p-Si (MIS) Schottky diodes

A study on interface states density distribution and characteristic parameters of the In/SiO₂/p-Si (MIS) capacitor has been made. The thickness of the SiO₂ film obtained from the measurement of the corrected capacitance in the strong accumulation region for MIS Schottky diodes was 220 Å. The diode parameters from the forward bias I-V characteristics such as ideality factor, series resistance and barrier heights were found to be 1.75, 106-112 Ω and 0.592 eV, respectively. The energy distribution of the interface state density D_it was determined from the forward bias I-V characteristics by taking into account the bias dependence of the effective barrier height. The interface state density obtained using the I-V characteristics had an exponential growth, with bias towards the top of the valance band, from 9.44×10¹³ eV⁻¹ cm⁻² in 0.329-E_v eV to 1.11×10¹³ eV⁻¹ cm⁻² in 0.527-E_v eV at room temperature. Furthermore, the values of interface state density D_it obtained by the Hill-Coleman method from the C-V characteristics range from 52.9×10¹³ to 1.11×10¹³ eV⁻¹ cm⁻² at a frequency range of 30kHz-1 MHz. These values of D_it and R_s were responsible for the non-ideal behaviour of I-V and C-V characteristics.     

Optoelectronic characterization of Au/Ni/n-AlGaN photodiodes after annealing at different temperatures

The optoelectronic characteristics of Ni/Au Schottky photodiodes based on Al_0.35Ga_0.65N were investigated. The transmission of the Ni (50 Å)/Au (50 Å) layer was determined by evaporating it on a quartz substrate. As evaporated, the transmission coefficient in the 200–350 nm wavelength range was found to be 43 to 48%. Annealing at temperatures of up to 400 °C did not influence the transmission coefficient. After annealing at 500 °C, the transmission coefficient increased from 50 to 68% over the 200–350 nm range. The reverse bias current was optimised in terms of annealing temperature and was found to be as low as 1.94×10⁻¹³ A after annealing at 400 °C for a 0.6 mm diameter contact. The Schottky barrier heights increased with annealing temperature reaching as high as 1.46 and 1.89 eV for I–V and C–V measurements, respectively. The quantum efficiency was measured to be 20.5% and the responsivity reached its peak of 0.046 A/W at 275 nm. The cut-off wavelength was 292 nm.     

Electrical and structural properties of a stacked metal layer contact to n-InP

In this study, we found that the double metal contact structure in Pt/Al/n-InP diodes provides better rectification characteristics than conventional single-metal/n-InP Schottky diodes. The effective barrier height was measured to be 0.67 eV for a 400 °C-annealed Pt/Al/n-InP diode sample. The increase in the barrier height is attributed to the formation of Al₂O₃ at the metal/n-InP contact interface during thermal annealing. The formation of the phase Al₂O₃ phase was monitored by X-ray diffraction (XRD) analysis. The corresponding element profiles of Al and O were also confirmed at the metal/n-InP contact interface using secondary ion mass spectrum (SIMS) analysis. The lowering of the Schottky barrier height due to the inhomogeneity at the metal/n-InP junction is also discussed on the basis of the TE theory. The distribution of local effective Schottky barrier heights was explained by a model incorporating the existence of double Gaussian barrier heights, which represent the high barrier and low barrier of the full distribution in the temperature ranges of 83-198 and 198-300 K.     

Microstructure and interfacial properties of HfO2-Al2O3 nanolaminate films

High-k HfO₂-Al₂O₃ composite gate dielectric thin films on Si(1 0 0) have been deposited by means of magnetron sputtering. The microstructure and interfacial characteristics of the HfO₂-Al₂O₃ films have been investigated by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and spectroscopic ellipsometry (SE). Analysis by XRD has confirmed that an amorphous structure of the HfO₂-Al₂O₃ composite films is maintained up to an annealing temperature of 800 °C, which is much higher than that of pure HfO₂ thin films. FTIR characterization indicates that the growth of the interfacial SiO₂ layer is effectively suppressed when the annealing temperature is as low as 800 °C, which is also confirmed by spectroscopy ellipsometry measurement. These results clearly show that the crystallization temperature of the nanolaminate HfO₂-Al₂O₃ composite films has been increased compared to pure HfO₂ films. Al₂O₃ as a passivation barrier for HfO₂ high-k dielectrics prevents oxygen diffusion and the interfacial layer growth effectively.     

Electrical transport, magnetic properties of the half-metallic Fe3O4-based Schottky diode

Fe₃O₄ thin films were prepared successfully by using the rf-sputtering technique with Fe₂O₃ target. The inverse spinel structure of the film was determined by X-ray diffraction (XRD) and the single phase of the Fe₃O₄ was confirmed by the XPS measurements. The surface roughness increases with the increase of the partial pressure of hydrogen. A high saturated magnetic field, 5000 Oe, implies that there exist the antiphase boundaries (APBs) in the film. The higher coercive filed below T_V is ascribed to the lower symmetry of the monoclinic structure. The temperature dependence of resistance shows a very clear Verwey transition and it is implied that the electrical transport behavior follows the variable-range hopping (VRH) mechanism from 40 to 300 K. The current vs. voltage curves of Fe₃O₄/Si Schottky heterojunction exhibits good rectifying property. The ideality factor and Schottky barrier height were obtained from the fitting curves calculated by the standard thermionic emission/diffusion model.     

Design and characterization of TlInSe2 varactor devices

TlInSe₂ single crystal has been successfully prepared by the Bridgman crystal growth technique. The crystal, which exhibits compositional atomic percentages of 25.4%, 25.2% and 49.4% for Tl, In and Se, respectively, is found to be of tetragonal structure with lattice parameters of a=0.8035 and c=0.6883 nm. The crystals were used to design radio frequency sensitive varactor device. The temperature dependence of the current-voltage characteristics of the device allowed the calculation of the room temperature barrier height and ideality factor as 0.87 eV and as 3.2, respectively. Rising the device temperature increased the barrier height and decreased the ideality factor. This behavior was attributed to the current transport across the metal-semiconductor interface. The capacitance of the device is observed to increase with increasing voltage and increasing temperature as well. The temperature activation of the capacitance starts above 82 °C with a temperature coefficient of capacitance being 1.08×10⁻³ K⁻¹. Furthermore, the capacitance of the device was observed to increase with increasing frequency up to a maximum critical frequency of 4.0 kHz, after which the capacitance decreased with increasing frequency. The behavior reflected the ability of maximum amount of charge holding being at a 4.0 kHz. The analysis of the capacitance-voltage characteristics at fixed frequencies reflected a frequency dependent barrier height and acceptors density. The decrease in the barrier height and acceptors density with increasing frequency is mainly due to the inability of the free charge to follow the ac signal.     

Electrical properties of SrBi2(Nb0.5Ta0.5)2O9 ceramics

Aurivillius SrBi₂(Nb_0.5Ta_0.5)₂O₉ (SBNT 50/50) ceramics were prepared using the conventional solid-state reaction method. Scanning electron microscopy was applied to investigate the grain structure. The XRD studies revealed an orthorhombic structure in the SBNT 50/50 with lattice parameters a=5.522 Å, b=5.511 Å and c=25.114 Å. The dielectric properties were determined by impedance spectroscopy measurements. A strong low frequency dielectric dispersion was found to exist in this material. Its occurrence was ascribed to the presence of ionized space charge carriers such as oxygen vacancies. The dielectric relaxation was defined on the basis of an equivalent circuit. The temperature dependence of various electrical properties was determined and discussed. The thermal activation energy for the grain electric conductivity was lower in the high temperature region (T>303.6 °C, E_a−ht=0.47 eV) and higher in the low temperature region (T<303.6 °C, E_a−lt=1.18 eV).     

Improved GaSb surfaces using a (NH4)2S/(NH4)2S04 solution

Bulk (1 0 0) n-GaSb surfaces have been treated with a sulphur based solution ((NH₄)₂S/(NH₄)₂SO₄) to which sulphur has been added, not previously reported for the passivation of GaSb surfaces. Au/n-GaSb Schottky barrier diodes (SBDs) fabricated on the treated material show significant improvement compared to that of the similar SBDs on the as-received material as evidenced by the lower ideality factor (n), higher barrier height (?_b) and lower contact resistance obtained. Additionally, the reverse leakage current, although not saturating, has been reduced by almost an order of magnitude at −0.2 V. The sample surfaces were studied by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The native oxide, Sb–O, present on the as-received material is effectively removed on treating with ([(NH₄)₂S/(NH₄)₂SO₄]+S) and (NH₄)₂S. Analysis of the as-received surface by XPS, prior to and after argon sputtering, suggests that the native oxide layer is ≤8.5 nm.