Paramagnetic Ge dangling bond type defects at (1 0 0)Si1−xGex/SiO2 interfaces (Invited Paper)

The work addresses the occurrence of Ge dangling bond type point defects at Ge_xSi_1?x/insulator interfaces as evidenced by conventional electron spin resonance (ESR) spectroscopy. Using multifrequency ESR, we report on the observation and characterization of a first nontrigonal Ge dangling bond (DB)-type interface defect in SiO₂/(1 0 0)Ge_xSi_1?x/SiO₂/(1 0 0)Si heterostructures (0.27 ? x ? 0.93) manufactured by the condensation technique, a selective oxidation method enabling Ge enrichment of a buried epitaxial Si-rich SiGe layer. The center, exhibiting monoclinic-I (C_2v) symmetry is observed in highest densities of ～7 × 10¹² cm^?2 of Ge_xSi_1?x/SiO₂ interface for x ～ 0.7, to disappear for x outside the ]0.45–0.87[ interval, with remarkably no copresence of Si P_b-type centers. Neither are trigonal Ge DB centers observed, enabling unequivocal spectral analysis. Initial study of the defect passivation under annealing in molecular H₂ has been carried out. On the basis of all data the defect is depicted as a Ge P_b1-type center, i.e., distinct from a trigonal basic Ge P_b(0)-type center (Ge₃Ge). The modalities of the defect’s occurrence as unique interface mismatch healing defect is discussed, which may widen our understanding of interfacial DB centers in general.     

Room temperature oxidation of Cu3Ge and Cu3(Si1−xGex) on Si1−xGex

Room temperature oxidation of Cu₃(Si_1−xGe_x) and Cu₃Ge films grown on Si_1−xGe_x at a temperature of 200–400°C was studied using transmission electron microscopy (TEM) in conjunction with energy dispersive spectrometry (EDS). For Cu₃(Si_1−xGe_x) and Cu₃Ge films grown at 200°C and then exposed to air, room temperature oxidation occurred. The Cu₃Ge film was superior to the Cu₃(Si_1−xGe_x) film in reducing the oxidation rate because of its higher Ge concentration. Annealing at higher temperatures such as 400°C resulted in Ge segregation out of the Cu₃(Si_1−xGe_x) film or Si diffusion from the Si_1−xGe_x substrate into the Cu₃Ge overlayer, and hence enhanced the oxidation rate of Cu₃(Si_1−xGe_x) and Cu₃Ge films. The present study shows that upon exposure to air even the Cu₃Ge film grown on Si_1−xGe_x is subject to room-temperature oxidation, revealing that the use of Cu₃Ge contacts on Si_1−xGe_x may be limited by some strict conditions.     

Shallow acceptors in strained Ge/Ge1−x Six heterostructures with quantum wells

The energies of localized acceptor states in quantum wells (strained Ge layers in Ge/Ge_1?x Si_x heterostructures) were analyzed theoretically in relation to the quantum well width and the impurity position in the well. The impurity absorption spectrum in the far IR range is calculated. Comparison of the results of the calculation with experimental photoconductivity spectra allows an estimation of the acceptor distribution in the quantum well to be made. In particular, it was concluded that acceptors may largely concentrate near the heterointerfaces. The absorption spectrum is calculated taking into account the resonance impurity states. This allows the features observed in the short-wavelength region of the spectrum to be interpreted as being due to transitions into the resonance energy levels “linked” to the upper size-quantization subbands.     

Features of electron transport in relaxed Si/Si1 − x Ge x transistor heterostructures with a high doping level

The low-temperature electrical and magnetotransport characteristics of partially relaxed Si/Si_{1 ? x} Ge _x heterostructures with an electron conduction channel in an elastically strained nanoscale silicon layer are investigated. It is demonstrated that the electron gas in the system exhibits 2D properties. A dependence of the conductivity along layers in the system on the degree of elastic-stress relaxation in it is observed. To understand the observed regularities, the potential and the electron distribution over the structure layers are calculated in detail for samples with different layer strains and doping levels. For the structure with x = 0.25, the parameters of the potential barrier and characteristics of the quantum well formed in the Si layer are estimated. It is established that the characteristics of the potential formed near interfaces strongly depend on the initial parameters of the system, in particular, on the degree of the plastic relaxation of elastic stresses and on the doping level. The formation of a thin tunneling-transparent barrier near the upper interface can lead to the redistribution of electrons between the 2D and 3D conduction channels in the structure, which ensures the spread of the measured transport characteristics of the samples during the measurements. The interlayer tunneling transitions of carriers from the 2D state in the Si transport channel to the 3D state of the Si_{1 ? x} Ge _x crystal matrix, which are separated by a tunneling-transparent potential barrier near the heterointerface, were observed for the first time during transport in the direction transverse to the layer plane.     

Band offsets at interfaces of (1 0 0)InxGa1−xAs (0 ⩽ x ⩽ 0.53) with Al2O3 and HfO2

The electron energy band alignment at interfaces of In_xGa_1?xAs (0 ? x ? 0.53) with atomic-layer deposited insulators Al₂O₃ and HfO₂ is characterized using combined measurements of internal photoemission of electrons and photoconductivity. The measured energy of the In_xGa_1?xAs valence band top is found to be only marginally influenced by the semiconductor composition. This result suggests that the observed bandgap narrowing from 1.42 to 0.75 eV when the In content increases from 0 to 0.53 occurs mostly through downshift of the semiconductor conduction band bottom. Electron states originating from the interfacial oxidation of In_xGa_1?xAs lead to reduction of the electron barrier at the semiconductor/oxide interface.     

Device characteristics of AlGaN/GaN MIS–HEMTs with high-k HfxZr1−xO2 (x = 0.66, 0.47, 0.15) insulator layer

This study investigates the characteristics of AlGaN/GaN MIS–HEMTs with Hf_xZr_1−xO₂ (x = 0.66, 0.47, and 0.15) high-k films as gate dielectrics. Sputtered Hf_xZr_1−xO₂ with a dielectric constant of 20–30 and a bandgap of 5.2–5.71 eV was produced. By increasing the Zr content of HfZrO₂, the V_TH shifted from −1.8 V to −1.1 V. The highest Hf content at this study reduced the gate leakage by approximately one order of magnitude below that of those Zr-dominated HFETs. The maximum I_DS currents were 474 mA/mm, 542 mA/mm, and 330 mA/mm for Hf content of 66%, 47%, 15% at V_GS = 3 V, respectively.     

AlxGa1−xN/GaN/AlN heterostructures grown on Si(1 1 1) substrates by MBE for MSM UV photodetector applications

Al_xGa_1−xN/GaN/AlN heterostructures on silicon (Si) substrate was developed by nitrogen plasma-assisted molecular beam epitaxy (MBE) and their properties were investigated by scanning electron microscopy (SEM), electron dispersive X-ray (EDX), atomic force microscopy (AFM), high resolution X-ray diffraction (XRD), Raman spectroscopy and Hall effect measurements. High purity gallium (7N) and aluminum (6N5) were used in the Knudsen cells. High purity nitrogen with 7N purity was supplied to radio frequency (RF) source to generate reactive nitrogen species. The nitrogen pressure and a discharge power were kept at 1.5×10⁻⁵ Torr and 300 W, respectively. From SEM measurements, the surface morphology of samples presented 2- and 3-dimensional growth modes. The EDX measurements showed that there were no foreign elements in the grown samples. The HR-XRD measurement has confirmed that the Al_xGa_1−xN/GaN/AlN heterostructures samples were epitaxially grown on Si substrate. All the dominant E₂ phonon modes were found in Raman spectra results. Lastly, Al_xGa_1−xN/GaN/AlN heterostructures based metal–semiconductor–metal (MSM) UV photodetectors were fabricated and the electrical characteristics of the devices were investigated by using current–voltage (I–V) and photo-conductivity measurements. The devices presented good I–V and photoconductivity characteristics.     

Structural and optical properties of ZrB2 and HfxZr1−xB2 films grown by vicinal surface epitaxy on Si(1 1 1) substrates

ZrB₂ and Hf_xZr_1?xB₂ films were grown on 4° miscut Si(1 1 1) substrates by chemical vapor deposition of gaseous Hf(BH₄)₄ and Zr(BH₄)₄. The films display superior structural and optical properties when compared with ZrB₂ films grown on on-axis Si(1 1 1). The observed improvements include an optically featureless surface with rms roughness of ～2.5–3.5 nm, a ～50% reduction in the amount of residual strain, and a ～50% lower resistivity. These properties should promote the use of diboride films as buffer layers for nitride semiconductor epitaxy on large-area Si substrates.     

Ge/Si photodiodes with embedded arrays of Ge quantum dots forthe near infrared (1.3–1.5 µm) region

A method has been devised for MBE fabrication of p-i-n photodiodes for the spectral range of 1.3–1.5 µm, based on multilayer Ge/Si heterostructures with Ge quantum dots (QDs) on a Si substrate. The sheet density of QDs is 1.2×10¹² cm^?2, and their lateral size is ～8 nm. The lowest room-temperature dark current reported hitherto for Ge/Si photodetectors is achieved (2×10^?5 A/cm² at 1 V reverse bias). A quantum efficiency of 3% at 1.3 µm wavelength is obtained.     

Transport in GaAs/Al x Ga1−x As superlattices with narrow forbidden minibands: Low-frequency negative differential conductivity and current oscillations

Semiconductors - Current-voltage characteristics have been measured and low-frequency current instabilities have been studied for GaAs/Al x Ga1?x As superlattices with narrow forbidden...     

Evaluation of compositional intermixing at interfaces in Si(Ge)/Si1−x Gex heteroepitaxial structures grown by molecular beam epitaxy with combined sources of Si and GeH4

The main causes of the diffusion spreading of a solid-solution composition near the boundaries of the Si transport channel in a Si/Si_1?x Ge_x heterostructure grown by molecular-beam epitaxy combined with solid (Si) and gaseous (GeH₄) sources are considered. For the grown structures, the contributions from various mechanisms involved in forming the profile of the metallurgical boundary of the layer are compared and the effect of channel boundary spreading on the mobility of a two-dimensional electron gas in the channel is evaluated.     

Influence of the AlN molar fraction on the structural and optical properties of praseodymium-doped AlxGa1−xN (0?x?1) alloys

The effects of the AlN molar fraction on structural and optical properties of praseodymium implanted Al_xGa_1−xN (0?x?1) layers were investigated. Using photoluminescence and excitation luminescence techniques we are able to observe the intra-4f²³P₁→³H₅ (526 nm) and ³P₀→³F₂ (652 nm) transitions of the Pr³⁺ ion. The red emission peak position shifts to lower energies with increasing Al content in the alloys. The peak full-width at half-maximum increases with the Al content up to x=0.7 due to disorder effects. The implantation damage and Pr-incorporation was investigated by Rutherford backscattering spectrometry doing channelling measurements along the 〈0 0 0 1〉 direction. X-ray diffraction reciprocal space maps show an expansion of the c lattice parameter in the implanted region which is reversed after thermal annealing at 1300 °C.     

Role of intervalley scattering in the radiative recombination in Pb1 − x Eu x Te alloys (0 ≤ x ≤ 1)

Measurements of the photoluminescence from epitaxial layers of Pb_{1 ? x} Eu _x Te alloys with 0 ≤ x ≤ 0.32 are carried out. It is found that the luminescence intensity decreases with increasing Eu content and, already for x as low as about 0.1, drops by more than one order of magnitude. No luminescence is observed for 0.2 < x ≤ 0.32. This behavior is explained by the fact that, for x ≈ 0.1, the absolute minimum in the conduction band changes from the L to X point, which results in the scattering of nonequilibrium electrons to the X valley and, thus, causes a decrease in the quantum efficiency of the emission. According to published data, for x > 0.85, optical transitions also take place with the participation of the X valley; in this case, the emission is governed by the formation of magnetic polarons. The temperature dependences of the band gap are determined for 0 ≤ x ≤ 0.11. These dependences have a wide linear region characterized by a positive dE _g /dT coefficient, which decreases with the Eu content to become negative in pure EuTe.     

A resonant half bridge driver with novel common mode rejection technique implemented in 1.0 μm high voltage (650 V) DIMOS process

In this paper, a half bridge convert driver IC with novel common mode rejection technique is designed and implemented in 1.0 μm high voltage (650 V) Dielectric Isolation MOS (DIMOS) process. The Designed IC is suitable for medium power (under 500 W) applications such as consumer electronics. Half bridge converter driver IC with a novel common mode rejection technique, which is composed of noise filter and set inhibitor, shows high dv/dt noise immunity up to 66.67 V/ns. Spectre simulation was performed to verify the electrical characteristics of the designed IC.     

Formation and resettlement of (AuxNi1−x)Sn4 in solder joints of ball-grid-array packages with the Au/Ni surface finish

The interfacial reactions between eutectic PbSn solder and the solder ball pads with the Au/Ni surface finish were studied. Solder joints subjected to up to three repeated reflow-and-aging treatments were examined. For the reflow, the peak reflow temperature was 225°C, and the reflow time was 115 s. Each aging process was performed at 160°C for 500 h. After the first reflow, all the Au would disappear from the interface, and formed many (Au_xNi_1−x)Sn₄ particles inside the solder joints. The value of x was between 0.99 and 0.75. In addition, there was a thin layer of Ni₃Sn₄ (1.4 μm) at the interface. After one reflow and one subsequent aging, most of the (Au_xNi_1−x)Sn₄ would relocate from inside the solder joint to the interface, and the value of x for (Au_xNi_1−x)Sn₄ at the interface decreased to 0.45. This (Au_xNi_1−x)Sn₄ resettlement process repeated itself for additional reflow-aging cycles. More reflow-aging treatments, however, made the microstructure of (Au_0.45Ni_0.55)Sn₄ at the interface become more non-planar. It was shown that gravitational effect was not the driving force for the resettlement of (Au_xNi_1−x)Sn₄. It is proposed that the driving force is for (Au_xNi_1−x)Sn₄ to seek Ni at the interface so that it can become more Ni-rich. In other words, the driving force is lowering the Gibbs energy of (Au_xNi_1−x)Sn₄ by dissolving more Ni. A decomposition-diffusion mechanism is proposed to explain what happened. Kinetic rationales for this rapid resettlement of (Au_xNi_1−x)Sn₄ at such a low temperature were also discussed.     

Magnetic susceptibility of Bi2 − x Sb x Te3 (0 < x ≤ 1) crystals in the intrinsic conductivity region

Temperature dependences of the magnetic susceptibility of Bi₂Te₃-Sb₂Te₃ with 10, 25, and 50% of Sb₂Te₃ are experimentally studied using an MPMS MultiVu SQUID magnetometer (Quantum Design, USA) in the temperature range from 2 to 400 K with regard to the anisotropy of the magnetic susceptibility. The behavior of the magnetic susceptibility at temperatures above 250 K is analyzed where the electron system of the crystals passes to the nondegenerate state. It is shown that the temperature dependence of the magnetic susceptibility in the intrinsic conductivity region can be described within the framework of the Pauli and Landau-Peierls approaches.     

Study of optical characteristics of structures with strongly strained In<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>As quantum wells

Results of photoluminescence (PL) studies of heterostructures with strongly strained InxGa_{1 ? x} As quantum wells (QWs) are presented. It is shown that the dependence of the PL intensity on the QW thickness has a maximum whose position depends on the composition of the In _x Ga_{1 ? x} As solid solution. The PL wavelength at the maximum intensity is 1.13 µm at a QW thickness of 60 µm at a QW thickness of 50 Å for x = 0.39 and 0.42, respectively.     

Growth and Characterization of In x Ga1−x As/GaAs1−y P y Strained-Layer Superlattices with High Values of y (~80%)

Strained-layer superlattice (SLS) structures, such as InGaAs/GaAsP lattice matched to GaAs, have shown great potential in absorption devices such as photodetectors and triple-junction photovoltaic cells. However, until recently they have been somewhat hindered by their usage of low-phosphorus GaAsP barriers. High-P-composition GaAsP was developed as the barrier for InGaAs/GaAsP strained-layer superlattice (SLS) structures, and the merits of using such a high composition of phosphorus are discussed. It is believed that these barriers represent the highest phosphorus content to date in such a structure. By using high-composition GaAsP the carriers are collected via tunneling (for barriers ≤30 Å) as opposed to thermionic emission. Thus, by utilizing thin, high-content GaAsP barriers one can increase the percentage of the intrinsic in a p-i-n structure that is composed of InGaAs wells in addition to increasing the number of periods that can be grown for given depletion width. However, standard SLSs of this type inherently possess undesirable compressive strain and quantum size effects (QSEs) that cause the optical absorption of the thin InGaAs SLS wells to shift to higher energies relative to that of bulk InGaAs of the same composition. To circumvent these deleterious QSEs, stress-balanced, pseudomorphic InGaAs/GaAsP staggered SLSs were grown. Staggering was achieved by removing a portion of one well and adding it to an adjacent well. The spectral response obtained from device characterization indicated that staggering resulted in thicker InGaAs films with reduced cutoff energy. Additionally, these data confirm that tunneling is a very effective means for carrier transport in the SLS.     

A compare of electrical characteristics in Al/p-Si (MS) and Al/C20H12/p-Si (MPS) type diodes using current–voltage (I–V) and capacitance–voltage (C–V) measurements

In this study, both the metal-semiconductor (MS) and metal-polymer-semiconductor (MPS), (Al/C₂₀H₁₂/p-Si), type Schottky barrier diodes (SBDs) were fabricated using spin coating method and they were called as D₁ and D₂ diodes, respectively. Their electrical characterization have been investigated and compared using the forward and reverse bias I–V and C–V measurements at room temperature. The main electrical parameters such as ideality factor (n), reverse saturation current (I_o), zero-bias barrier height (Φ_Bo), series (R_s) and shunt (R_sh) resistances, energy dependent profile of interface states (N_ss), the doping concentration of acceptor atoms (N_A) and depletion layer width (W_D) were determined and compared each other and literature. The rectifying ratio (RR) and leakage current (I_R) at ±3 V were found as 2.06×10³, 1.61×10⁻⁶ A and 15.7×10³, 2.75×10⁻⁷ A for D₁ and D₂, respectively. Similarly, the R_s and R_sh values of these diodes were found as 544 Ω, 10.7 MΩ and 716 Ω and 1.83 MΩ using Ohm’s Law, respectively. In addition, energy and voltage dependent profiles of N_ss were obtained using the forward bias I–V data by taking into account voltage dependent effective barrier height (Φ_e) and n and low-high frequency capacitance (C_LF–C_HF) methods, respectively. The obtained value of N_ss for D₂ (MPS) diode at about the mid-gap of Si is about two times lower than D₁ (MS) type diode. Experimental results confirmed that the performance in MPS type SBD is considerably high according to MS diode in the respect of lower values of N_ss, R_s and I_o and higher values of RR and R_sh.     

Low-temperature (77–300 K) current-voltage characteristics of 4H-SiC p+-p-n+ diodes: Effect of impurity breakdown in the p-type base

The effect of impurity breakdown on the low-temperature (77–300 K) current-voltage (I–V) characteristics of 4H-SiC diodes with a p-type base has been studied. Experimental samples were fabricated from CVD-grown (chemical vapor deposition) commercial p ⁺-p-n ⁺ 4H-SiC structures. A high electric field in the p-type base was created by applying a forward bias to the diodes. It was found that, at temperatures of 136, 89, and 81 K, the commonly observed “diode” portion of the I–V characteristics is followed by a portion in which the current grows more rapidly due to the impact ionization of frozen-out Al acceptor atoms in the ground (unexcited) state. At temperatures of 81 and 77 K, this portion is followed by one with a negative differential resistance due to the regenerative dynistor-like switching of the diode, caused by impact ionization of aluminum atoms in the excited state.