InxGa1−xN refractive index calculations

The growth of In_xGa_1−xN Wurtzite structure is a well established fact. It permits to design optoelectronic devices such as laser diodes or LEDs, from the near ultraviolet to the infrared light spectrum. This sweeps indeed, the whole of the visible spectrum and, hence, appears to be very useful to the recent development of liquid crystal display screens, or designing photodiodes and perhaps solar cells (after studying their energetical efficiencies). Nevertheless, refractive indices of In_xGa_1−xN structure have not been studied.The refractive index of such structures is increasing from the GaN refractive index to the InN one, with therefore, a bowing of the curve due to the lattice mismatch between these two constituting binary alloys.The index is, in a certain range of the “n(x)” characteristic, less than the GaN one. This seems to be particularly interesting in the integrated optics domain or optical waveguides realization, because the growth of GaN is easier than the growth of In_xGa_1−xN.     

Low sensitivity to temperature compressive-strained structure quantum well laser Ga1−xInxAs1−yNy/GaAs

The objective is to exploit the properties of the GaInNAs/GaAs alloy compressive strain structure to design a laser diode likely to meet the needs of optical communications. Modelling concerns mainly the study of the potentialities of thermal stability and dynamic response offered by these new techniques of electric and optical confinement. Band structure is modelled and typical quantum well properties are illustrated. A thorough study of the structural parameters is undertaken to take into account from the design criteria the temperature sensitivity. Minimising the Auger coefficient in the order of 10⁻²⁹ cm⁶/s appears to allow achieving efficient laser diodes production.     

AlxGa1−xN/GaN multi-quantum-well ultraviolet detector based on p-i-n heterostructures

We report on characterization of a set of AlGaN/GaN multiple-quantum-well (MQW) photodetectors. The model structure used in the calculation is the p-i-n heterojunction with 20 AlGaN/GaN MQW structures in i-region. The MQW structures have 2 nm GaN quantum well width and 15 nm Al_xGa_1−xN barrier width. The cutoff wavelength of the MQW photodetectors can be tuned by adjusting the well width and barrier height. Including the polarization field effects, on increasing Al mole fraction, the transition energy decreases, the total noise increases, and the responsivity has a red shift, and so the detectivity decreases and has a red shift.     

Determination of boron concentration in heavily doped p-type Si1−xGex/Si heterostructure by infrared ellipsometric spectroscopy

Si_1−xGe_x/Si heterostructures play a primary role in the Si-based fast electronics developments of today. In this work, we will present the experimental results of infrared spectroscopic ellipsometry (IRSE) for structural determination of the boron heavily doped SiGe/Si sample grown by ultra-high vacuum chemical vapor deposition (UHVCVD) (the Ge atomic percent, the thickness of SiGe film and boron concentration). Especially, the principle of boron concentration in p-type SiGe film layer determined by IRSE was elucidated in detail. In addition, in order to corroborate the validity of IRSE for determining dopant concentration, secondary ion mass spectroscopy (SIMS) experiment has also been carried out. The close experimental agreement between IRSE and SIMS demonstrate that IRSE as a contactless, and non-destructive technology can be used in-line tools in production used for measuring the Ge content, the thickness of SiGe layer and boron concentration in p-type dopant SiGe/Si heterostructure, which often used the base layer of SiGe hetero-junction bipolar transistor (HBT) devices.     

Fabrication and electrical characteristics of ultrathin (HfO2)x(SiO2)1−x films by surface sol-gel method and reaction-anneal treatment

Hafnium oxide (HfO₂) films were deposited on Si substrates with a pre-grown oxide layer using hafnium chloride (HfCl₄) source by surface sol-gel process, then ultrathin (HfO₂)_x(SiO₂)_1−x films were fabricated due to the reaction of SiO₂ layer with HfO₂ under the appropriate reaction-anneal treatment. The observation of high-resolution transmission electron microscopy indicates that the ultrathin films show amorphous nature. X-ray photoelectron spectroscopy analyses reveal that surface sol-gel derived ultrathin films are Hf-Si-O alloy instead of HfO₂ and pre-grown SiO₂ layer, and the composition was Hf_0.52Si_0.48O₂ under 500 °C reaction-anneal. The lowest equivalent oxide thickness (EOT) value of 0.9 nm of film annealed at 500 °C has been obtained with small flatband voltage of −0.31 V. The experimental results indicate that a simple and feasible solution route to fabricate (HfO₂)_x(SiO₂)_1−x composite films has been developed by means of combination of surface sol-gel and reaction-anneal treatment.     

Study of thick film Ni(1−x)CoxMn2O4,(0?x?1) using overlay technique on thick film microstrip ring resonator

The effect of thick film Ni_(1−x)Co_xMn₂O₄ in-touch overlay on the X band resonance characteristics of thick film microstrip ring resonator is studied. The thick film overlay decreases the resonance frequency and increases the peak output. From the frequency shift the dielectric constant of the thick film Ni_(1−x)Co_xMn₂O₄ has been calculated. For the first time Ag thick film microstrip ring resonator has been used to study thick film Ni_(1−x)Co_xMn₂O₄ in the X band.     

Changes of electrical properties by Al content of AlxGa1−xAs in pHEMT structures as observed using HRXRD

Characterizations on the pseudomorphic High Electron Mobility Transistor structure under High-Resolution X-ray Diffraction (HRXRD) have been carried out at room temperature. Variation of Al contents in Al_xGa_1−xAs alloys has been found to show a shift of diffraction peaks. This variation is also found to show the change of lattice constant of crystal and also sheet carrier concentration as obtained from a Hall effect measurement. The latter phenomenon is considerably interesting to study in the early stage of the electrical properties of device based on the crystal structure.     

Impact of strain relaxation of AlmGa1−mN layer on 2-DEG sheet charge density and current voltage characteristics of lattice mismatched AlmGa1−mN/GaN HEMTs

An accurate charge control model to investigate the effect of aluminum composition, strain relaxation, thickness and doping of the Al_mGa_1−mN barrier layer on the piezoelectric and spontaneous polarization induced 2-DEG sheet charge density, threshold voltage and output characteristics of partially relaxed Al_mGa_1−mN/GaN HEMTs is proposed. The strain relaxation of the barrier imposes an upper limit on the maximum 2-DEG density achievable in high Al content structures and is critical in determining the performance of lattice mismatched Al_mGa_1−mN/GaN HEMTs. The model incorporates the effects of field dependent mobility, parasitic source/drain resistance and velocity saturation to evaluate the output characteristics of Al_mGa_1−mN/GaN HEMTs. Close proximity with published results confirms the validity of the proposed model.     

Hybrid functional calculations on the band gap bowing parameters of InxGa1-xN

The electronic band structures and band gap bowing parameters of In_xGa_1-xN are studied by the firstprinciples method based on the density functional theory. Calculations by employing both the Heyd-ScuseriaErnzerh of hybrid functional（HSE06） and the Perdew-Burke-Ernzerhof（PBE） one are performed. We found that the theoretical band gap bowing parameter is dependent significantly on the calculation method, especially on the exchange-correlation functional employed in the DFT calculations. The band gap of In_xGa_1-xN alloy decreases considerably when the In constituent x increases. It is the interactions of s–s and p–p orbitals between anions and cations that play significant roles in formatting the band gaps bowing. In general, the HSE06 hybrid functional could provide a good alternative to the PBE functional in calculating the band gap bowing parameters.     

Changes in effective work function of HfxRu1−x alloy gate electrode

Effective work function (?_m,eff) values of Hf_x Ru_1−x alloy gate electrodes on SiO₂ metal-oxide-semiconductor (MOS) capacitors were carefully examined to assess whether the ?_m,eff was determined by the crystalline structure or the composition of the Hf_xRu_1−x alloy. X-ray diffraction results indicated that the crystalline structures of Hf_xRu_1−x alloy were divided into hexagonal-Ru, cubic-HfRu or hexagonal-Hf with the increase of Hf content. The ?_m,eff values could be controlled continuously from 4.6 to 4.0 eV by changing the Hf content. The experimental ?_m,eff value showed a good agreement with theoretical results considering the compositional ratio of pure Hf and Ru. These results suggest that the ?_m,eff of Hf_xRu_1−x alloy gates on SiO₂ MOS capacitors is dominantly determined by the Hf_xRu_1−x composition rather than the crystalline structure.     

Electrical properties of metal-ferroelectric-insulator-semiconductor structure using BaxSr1−xTiO3 for ferroelectric-gate field effect transistor

Perovskite ferroelectric Ba_xSr_1−xTiO₃ (x = 0.5, 0.6, 0.7 and 0.8) thin films have been fabricated as metal-ferroelectric-insulator-semiconductor (MFIS) configurations using a sol-gel technique. The C-V characteristics for different Ba-Sr ratios and different film thicknesses have been measured in order to investigate the ferroelectric memory window effect. The results show that the memory window width increases with the increase both of Ba content and film thickness. This behavior is attributed to the grain size and dipole dynamics effect. It is found also that the memory window increases as the applied voltage increases. In addition, the leakage current density for the films is measured and it is found to be of the order of 10⁻⁸ A/cm² for all tested samples, indicating that the films have good insulating characteristics.     

Electrodepositing ZnxMnyOz alloys from zinc oxide to manganese oxide

Electrodeposition has emerged as a practical and simple method to synthesise semiconductor materials under different forms, thin films or nanostructured layers. This work reports on the cathodic electrodeposition of ZnMnO thin layers using both zinc and manganese chlorides as precursors. The composition of thin films can be varied from binary zinc oxide to manganese oxide varying the Mn/(Mn+Zn) ratio between 0 and 1. The composition of Zn_xMn_yO_z films was obtained by energy dispersive spectroscopy. Zn_1−xMn_xO films with Mn/Zn ratio less than 10% exhibit a crystalline wurtzite structure typical of ZnO fully oriented in the (0 0 2) direction. Higher Mn content leads to deformation of the ZnO lattice and the wurtzite structure is no longer maintained. X-ray photoelectron spectroscopy points out that Mn₃O₄ tends to be deposited when a high Mn/Zn ratio is used in the starting solution. Magnetic measurements on films with Mn/(Zn+Mn) ratio near 1 reveal magnetic characteristics similar to Mn₃O₄ compounds. The transmission spectra of Zn_xMn_yO_z show the typical absorption edge of crystalline ZnO while the wurtzite structure is maintained and it shifts to higher wavelengths when Mn content increases.     

Temperature dependent current-voltage characteristics of n-MgxZn1−xO/p-GaN junction diodes

This study investigates the temperature dependence of the current-voltage (I-V) characteristics of n-Mg_xZn_1−xO/p-GaN junction diodes. The n-Mg_xZn_1−xO films were deposited on p-GaN using a radio-frequency (rf) magnetron sputtering system followed by annealing at 500, 600, 700, and 800 °C in nitrogen ambient for 60 s, respectively. The n-Mg_xZn_1−xO/p-GaN diode at a substrate temperature of 25 °C had the lowest leakage current in reverse bias. However, the leakage current of the diodes increased with an increase in annealing temperatures. The temperature sensitivity coefficients of the I-V characterizations were obtained at different substrate temperatures (25, 50, 75 100, and 125 °C) providing extracted values of 26.4, 27.2, 17.9, and 0.0 mV/°C in forward bias and 168.8, 143.4, 84.6, and 6.4 mV/°C in reverse bias, respectively. The n-Mg_xZn_1−xO/p-GaN junction diode fabricated with Mg_xZn_1−xO annealed at 800 °C demonstrated the lowest temperature dependence. Based on these findings, the n-Mg_xZn_1−xO/p-GaN junction diode is feasible for GaN-based heterojunction bipolar transistors (HBTs).     

Electron transport in two dimensional electron gas formed at the heterojunction of AlxGa(1−x)N/GaN at microwave frequencies

Transport properties of the two dimensional electron gas (2DEG) formed at the interface of AlGaN/GaN heterostructure ranging from 20 K to 300 K has been investigated theoretically considering the various scattering mechanisms like the acoustic, the piezo, the surface roughness, the alloy, the polar optical phonon and the dislocation scattering. The dc mobility is found to remain constant up to 150 K and then it decrease sharply with further increase in temperature. The ac mobility is also found to decrease with increase in the temperature. The real part of ac mobility, i.e. μ_r decreases with the increase in the frequency very fast initially and then gradually attains a steady value. The imaginary part of the ac mobility μ_im initially increases with the increase in the frequency and then decreases after reaching the maximum value. The value of the ac mobility reduces quite reasonably as the 2D carrier concentration increases at lower range of the frequency. At the carrier concentration of 5 × 10¹⁷ m⁻², the ac mobility remains constant through a wide range of frequencies. With the increase in the dislocation densities, the values of the ac mobility are found to decrease at the lower range of frequencies. The thermo electric power is positive at the 2D carrier concentration of 5 × 10¹⁶ m⁻², the value of which increases with the increase in the temperature and gradually attains a steady value. But the thermoelectric power at n_2D of 10¹⁷ m⁻² is found to be negative in the value. The value of ZT is found to increase with the temperature and attains the maximum value of 0.007 at 150 K and the value of ZT then decreases with increase in the temperature.     

High-mobility pentacene thin-film transistor by using LaxTa(1−x)Oy as gate dielectric

Pentacene organic thin-film transistors (OTFTs) using La_xTa_(1−x)O_y as gate dielectric with different La contents (x = 0.227, 0.562, 0.764, 0.883) have been fabricated and compared with those using Ta oxide or La oxide. The OTFT with La_0.764Ta_0.236O_y can achieve a carrier mobility of 1.21 cm² V⁻¹s⁻¹s, which is about 40 times and two times higher than those of the devices using Ta oxide and La oxide, respectively. As supported by XPS, AFM and noise measurement, the reasons lie in that La incorporation can suppress the formation of oxygen vacancies in Ta oxide, and Ta content can alleviate the hygroscopicity of La oxide, resulting in more passivated and smoother dielectric surface and thus larger pentacene grains, which lead to higher carrier mobility.     

采用杂化函数方法研究BxGa1-xN合金中的能带弯曲和交叉

采用Perdewe-Burkee–Ernzerhof的广义梯度近似（GGA-PBE）及Heyd-Scuseria-Ernzerhof屏蔽杂化函数方法(HSE06)对比研究了闪锌矿结构BxGa1-xN合金的电子结构性质。HSE06计算方法给出更接近实验结果的基态性质。采用两种计算方法得到的直接带隙的能带弯曲系数b? 都很大且依赖于成分;由两种方法得到的由直接带隙向间接带隙转变的合金成分非常相近。当向GaN中掺入小于55.7%的硼时,可以得到带隙值比GaN大的直接带隙BxGa1-xN 合金。     

Annealing effect on physical and electrical characteristics of thin HfO2, HfSixOy and HfOyNz films on Si

We report the effect of annealing on electrical and physical characteristics of HfO₂, HfSi_xO_y and HfO_yN_z gate oxide films on Si. Having the largest thickness change of 0.3 nm after post deposition annealing (PDA), HfO_yN_z shows the lowest leakage current. It was found for both as-grown and annealed structures that Poole-Frenkel conduction is dominant at low field while Fowler-Nordheim tunneling in high field. Spectroscopic ellipsometry measurement revealed that the PDA process decreases the bandgap of the dielectric layers. We found that a decreasing of peak intensity in the middle HfO_yN_z layer as measured by Tof-SIMS may suggest the movement of N toward the interface region between the HfO_yN_z layer and the Si substrate during the annealing process.     

Frequency and temperature dependence of the dielectric and AC electrical conductivity in (Ni/Au)/AlGaN/AlN/GaN heterostructures

The dielectric properties and AC electrical conductivity (σ_ac)of the (Ni/Au)/Al_0.22Ga_0.78N/AlN/GaN heterostructures, with and without the SiN_x passivation, have been investigated by capacitance-voltage and conductance-voltage measurements in the wide frequency (5kHz-5 MHz) and temperature (80-400 K) range. The experimental values of the dielectric constant (ε′), dielectric loss (ε′′), loss tangent (tanδ), σ_ac and the real and imaginary part of the electric modulus (M′ and M′′) were found to be a strong function of frequency and temperature. A decrease in the values of ε′ and ε′′ was observed, in which they both showed an increase in frequency and temperature. The values of M′ and M′′ increase with increasing frequency and temperature. The σ_ac increases with increasing frequency, while it decreases with increasing temperature. It can be concluded, therefore, that the interfacial polarization can occur more easily at low frequencies and temperatures with the number of interface states density located at the metal/semiconductor interface. It contributes to the ε′ and σ_ac.     

Fabrication of high-Ge-fraction strained Si1−xGex/Si hole resonant tunneling diode using low-temperature Si2H6 reaction for nanometer-order ultrathin Si barriers

Low-temperature Si barrier growth with atomically flat heterointerfaces was investigated in order to improve negative differential conductance (NDC) characteristics of high-Ge-fraction strained Si_1−xGe_x/Si hole resonant tunneling diode with nanometer-order thick strained Si_1−xGe_x and unstrained Si layers. Especially to suppress the roughness generation at heterointerfaces for higher Ge fraction, Si barriers were deposited using Si₂H₆ reaction at a lower temperature of 400 °C instead of SiH₄ reaction at 500 °C after the Si_0.42Ge_0.58 growth. NDC characteristics show that difference between peak and valley currents is effectively enhanced at 11-295 K by using Si₂H₆ at 400 °C, compared with that using SiH₄ at 500 °C. Non-thermal leakage current at lower temperatures below 100 K tends to increase with decrease of Si barrier thickness. Additionally, thermionic-emission dominant characteristics at higher temperatures above 100 K suggests a possibility that introduction of larger barrier height (i.e. larger band discontinuity) enhances the NDC at room temperature by suppression of thermionic-emission current.     

Etch damage evaluation on (Bi4−xLax)Ti3O12 thin films during the etch process using inductively coupled plasma sources

The etching mechanism of (Bi_4−xLa_x)Ti₃O₁₂ (BLT) thin films in Ar/Cl₂ inductively coupled plasma (ICP) and plasma-induced damages at the etched surfaces were investigated as a function of gas-mixing ratios. The maximum etch rate of BLT thin films was 50.8 nm/min of 80% Ar/20% Cl₂. From various experimental data, amorphous phases on the etched surface existed on both chemically and physically etched films, but the amorphous phase was thicker after the 80% Ar/20% Cl₂ process. Moreover, crystalline “breaking” appeared during the etching in Cl₂-containing plasma. Also the remnant polarization and fatigue resistances decreased more for the 80% Ar/20% Cl₂ etch than for pure Ar plasma etch.