Localization of holes in an InAs/GaAs quantum-dot molecule

Deep-level transient spectroscopy is used to study the emission of holes from the states of a vertically coupled system of InAs quantum dots in p-n InAs/GaAs heterostructures. This emission was considered in relation to the thickness of a GaAs interlayer between two layers of InAs quantum dots and to the reversebias voltage U_r. It is established that hole localization at one of the quantum dots is observed for a quantum-dot molecule composed of two vertically coupled self-organized quantum dots in an InAS/GaAs heterostructure that has a 20-Å-thick or 40-Å-thick GaAs interlayer between two layers of InAs quantum dots. For a thickness of the GaAs interlayer equal to 100 Å, it is found that the two layers of quantum dots are incompletely coupled, which results in a redistribution of the hole localization between the upper and lower quantum dots as the voltage U_r applied to the structure is varied. The studied structures with vertically coupled quantum dots were grown by molecular-beam epitaxy using self-organization effects.     

Model Development for Current–Voltage and Transconductance Characteristics of Normally-off AlN/GaN MOSHEMT

In this paper, an AlN/GaN-based MOSHEMT is proposed, in accordance to this, a charge control model has been developed analytically and simulated with MATLAB to predict the characteristics of threshold voltage, drain currents and transconductance. The physics based models for 2DEG density, threshold voltage and quantum capacitance in the channel has been put forward. By using these developed models, the drain current for both linear and saturation models is derived. The predicted threshold voltage with the variation of barrier thickness has been plotted. A positive threshold voltage can be obtained by decreasing the barrier thickness which builds up the foundation for enhancement mode MOSHEMT devices. The predicted I_d–V_gs, I_d–V_ds and transconductance characteristics show an excellent agreement with the experimental results and hence validate the model.     

Analog/RF Study of Self-aligned In<Subscript>0.53</Subscript>Ga<Subscript>0.47</Subscript>As MOSFET with Scaled Gate Length

This study presents the impact of gate length scaling on analog and radio frequency (RF) performance of a self- aligned multi-gate n-type In_0.53Ga_0.47As metal oxide semiconductor field effect transistor. The device is fabricated using a self-aligned method, air-bridge technology, and 8 nm thickness of the Al₂O₃ oxide layer with different gate lengths. The transconductance-to-normalized drain current ratio (g _m/I _D) method is implemented to investigate analog parameters. Moreover, g _m and drain conductance (g _D) as key parameters in analog performance of the device are evaluated with g _m/I _D and gate length variation, where g _m and g _D are both showing enhancement due to scaling of the gate length. Early voltage (V _EA) and intrinsic voltage gain (A _V) value presents a decreasing trend by shrinking the gate length. In addition, the results of RF measurement for cut-off and maximum oscillation frequency for devices with different gate lengths are compared.     

The electrical and optical properties of InAs irradiated with electrons (∼2 MeV): The energy structure of intrinsic point defects

The results of experimental studies of the electrical and optical properties of n-and p-InAs crystals irradiated with electrons at an energy of ～2 MeV and doses as high as D = 1 × 10¹⁹ cm^?2 are reported. Specific features of the annealing (at temperatures as high as 800°C) of radiation defects are also reported. The electronic structure of nonrelaxed V_As, V_In, As_In, In_As defects is calculated. A relation between the electrical properties and the Fermi level position in irradiated InAs on the one hand and the electronic structure of the intrinsic defects and special features of the energy-band spectrum of InAs on the other hand is discussed.     

Growth,structure, and properties of GaAs-based (GaAs)1–x–y (Ge2) x (ZnSe) y epitaxial films

The possibility of growing the (GaAs)_1–x–y (Ge₂) _x (ZnSe) _y alloy on GaAs substrates by the method of liquid-phase epitaxy from a tin solution–melt is shown. X-ray diffraction shows that the grown film is single-crystal with the (100) orientation and has the sphalerite structure. The crystal-lattice parameter of the film is a _f = 0.56697 nm. The features of the spectral dependence of the photosensitivity are caused by the formation of various complexes of charged components. It is established that the I–V characteristic of such structures is described by the exponential dependence I = I ₀exp(qV/ckT) at low voltages (no higher than 0.4 V) and by the power dependence J ~ V ^α, where the exponent α varies with increasing voltage at high voltages (V > 0.5 V). The results are treated within the framework of the theory of the drift mechanism of current transfer taking into account the possibility of the exchange of free carriers within the recombination complex.     

Work Function Tuning and Doping Optimization of 22-nm HKMG Raised SiGe/SiC Source–Drain FinFETs

The basic requirements on process design of extremely scaled devices involve appropriate work function and tight doping control due to their significant effect on the threshold voltage as well as other critical electrical parameters such as drive current and leakage. This paper presents a simulation study of 22-nm fin field-effect transistor (FinFET) performance based on various process design considerations including metal gate work function (WF), halo doping (N _halo), source/drain doping (N _sd), and substrate doping (N _sub). The simulations suggest that the n-type FinFET (nFinFET) operates effectively with lower metal gate WF while the p-type FinFET (pFinFET) operates effectively with high metal gate WF in 22-nm strained technology. Further investigation shows that the leakage reduces with increasing N _halo, decreasing N _sd, and increasing N _sub. Taguchi and Pareto analysis-of-variance approaches are applied using an L₂₇ orthogonal array combined with signal-to-noise ratio analysis to determine the best doping concentration combination for 22-nm FinFETs in terms of threshold voltage (V _t), saturation current (I _on), and off-state current (I _off). Since there is a tradeoff between I _on and I _off, the design with the nominal-is-best V _t characteristic is proposed, achieving nominal V _t of 0.259 V for the nFinFET and ?0.528 V for the pFinFET. Pareto analysis revealed N _halo and N _sub to be the dominant factor for nFinFET and pFinFET performance, respectively.     

Temperature dependence of the optical energy gap for the CdS<Subscript>x</Subscript>Se<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> quantum dots

A temperature dependence of the optical energy gap E _g (T) for the CdS_xSe_1?x quantum dots synthesized in a borosilicate glass matrix was investigated in the range of 4.2–500 K. It was demonstrated that this dependence reproduced the dependence E _g (T) for bulk crystals and is described by the Varshni formula for \(\bar r > a_B \) over the entire temperature range. Here, \(\bar r\) is the average dot radius, and a_B is the Bohr radius for the exciton in a bulk crystal. With the transition to quantum dots with \(\bar r > a_B \), a decrease in the thermal coefficient of the band gap and a deviation from the Varshni dependence were observed in the temperature range of 4.2–100 K. The specific features observed are explainable by a decrease in the resulting macroscopic potential of the electron-phonon interaction and by modification of the vibration spectrum for dots as their volume decreases.     

The resonant tunneling of holes through double-barrier structures with InAs QDs at the center of a GaAs quantum well

The effect of InAs quantum dots (QDs) grown in the center of a GaAs quantum well on the tunneling characteristics of resonant-tunneling diodes based on p-AlAs/GaAs/AlAs heterostructures is studied. The introduction of QDs results in a shift and broadening of resonance peaks in the current-voltage characteristics of the diodes; however, this effect is found to be strongly dependent on the number of the 2D subband involved in the tunneling. The obtained dependence is attributed to origination of the fluctuation potential in the vicinity of the QD layer.     

Study of the photoelectric properties of Ge quantum dots in a ZnSe matrix on GaAs

The current-voltage (I–V) and spectral characteristics of a photocurrent are studied at T=4.2 and 300 K for an unstrained GaAs/ZnSe/QD-Ge/ZnSe/Al structure with tunneling-transparent ZnSe layers and Ge quantum dots (QDs). Features such as the Coulomb staircase were observed in I–Vcharacteristics at room temperature and in the absence of illumination. An energy-band diagram of the structure is constructed based on an analysis of the experimental data. In the GaAs/ZnSe/QD-Ge/ZnSe/p-Ge transistor structure with a p-Ge channel and Ge-QD floating gate, the total current of the channel both increased and decreased under exposure to light with various spectra. These variations in channel current are associated with the capture of a positive and negative charge at QDs during different optical transitions. The charge accumulation changes the state of a channel at the heterointerface from depletion to inversion and either decreases or increases the total current.     

Lateral electronic transport in short-period InAs/GaAs superlattices at the threshold of quantum dot formation

Temperature dependences of resistance at 0.7 K<T<300 K, the Hall and Shubnikov-de Haas effects in magnetic fields of up to 40 T, photoluminescence (PL), and morphology of a heterointerface (using an atomicforce microscope) of short-period InAs/GaAs superlattices were investigated. The investigations were carried out for a region of subcritical and critical thickness Q=2.7 monolayers (ML) of InAs. Upon exceeding the critical thickness, the self-organized growth of InAs quantum dots (QDs) set in. The formation of QD layers upon exceeding the critical thickness of InAs Q=2.7 ML is accompanied by a transition of conductivity from metallic to hopping. It is found that at InAs layer thicknesses of Q=0.33 ML and Q=2.0 ML, the PL intensities and electron mobilities in the structures have clearly pronounced maxima. Anisotropy of conductivity, which depends on the thickness of the deposited InAs layers, was observed.     

Excition states in semiconductor quantum dots in the modified effective mass approximation

A new modified effective mass approximation is suggested to describe the excitonic energy spectrum of quantum dots of radii a comparable to the exciton Bohr radius a _ex ⁰ . It is shown that, for quantum dots simulated by infinitely deep potential wells, the effective mass approximation is appropriate for describing excitons in quantum dots of radii a ≈ a _ex ⁰ , if the reduced effective mass of the excitons, μ, is considered as a function of the radius of the quantum dot a, μ = μ(a).     

Tuning the energy spectrum of InAs/GaAs quantum dots by varying the thickness and composition of the thin double GaAs/InGaAs cladding layer

It is shown that the ground state transition energy in quantum dots in heterostructures grown by atmospheric-pressure MOCVD can be tuned in the range covering both transparence windows of the optical fiber at wavelengths of 1.3 and 1.55 μm by varying the thickness and composition of the thin GaAs/In_xGa_1−x As double cladding layer. These structures also exhibit a red shift of the ground state transition energy of the In_xGa_1−x As quantum well (QW) as a result of the formation of a hybrid QW In_xGa_1−x As/InAs (wetting layer) between the quantum dots (QDs). The Schottky diodes based on these structures are characterized by an increased reverse current, which is attributed to thermally activated tunneling of electrons from the metal contact to QD levels. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 4, 2004, pp. 448–454. Original Russian Text Copyright ? 2004 by Karpovich, Zvonkov, Levichev, Baidus, Tikhov, Filatov, Gorshkov, Ermakov.     

On the size and temperature dependence of the energy gap in cadmium-selenide quantum dots embedded in fluorophosphate glasses

The temperature and size dependences of the energy gap in CdSe quantum dots with diameters of 2.4, 4.0, and 5.2 nm embedded in fluorophosphate glasses are investigated. It is shown that the temperature coefficient of the band gap dE _g /dT in the quantum dots differs from the bulk value and depends strictly on the dot size. It is found that, furthermore, the energy of each transition in these quantum dots is characterized by an individual temperature coefficient dE/dT.     

Electrical properties and luminescence spectra of light-emitting diodes based on InGaN/GaN heterostructures with modulation-doped quantum wells

The distribution of charged centers N(w), quantum efficiency, and electroluminescence spectra of blue and green light-emitting diodes (LED) based on InGaN/AlGaN/GaN p-n heterostructures were investigated. Multiple InGaN/GaN quantum wells (QW) were modulation-doped with Si donors in GaN barriers. Acceptor and donor concentrations near the p-n junction were determined by the heterodyne method of dynamic capacitance to be about N _A ≥ 1 × 10¹⁹ cm^?3 ? N _D ≥ 1 × 10¹⁸ cm^?3. The N(w) functions exhibited maxima and minima with a period of 11–18 (±2–3 nm) nm. The energy diagram of the structures has been constructed. The shifts of spectral peaks with variation of current (J=10^?6–3×10^?2 A) are smaller (13–12 meV for blue and 20–50 meV for green LEDs) than the corresponding values for the diodes with undoped barriers (up to 150 meV). This effect is due to the screening of piezoelectric fields in QWs by electrons. The dependence of quantum efficiency on current correlates with the charge distribution and specific features in the current-voltage characteristics.     

Structural and Electrical Properties of Ag/<Emphasis Type="Italic">n</Emphasis>-TiO<Subscript>2</Subscript>/<Emphasis Type="Italic">p</Emphasis>-Si/Al Heterostructure Fabricated by Pulsed Laser Deposition Technique

We have investigated the structural and electrical characteristics of the Ag/n-TiO₂/p-Si/Al heterostructure. Thin films of pure TiO₂ were deposited on p-type silicon (100) by optimized pulsed laser ablation with a KrF-excimer laser in an oxygen-controlled environment. X-ray diffraction analysis showed the formation of crystalline TiO₂ film having a tetragonal texture with a strong (210) plane as the preferred direction. High purity aluminium and silver metals were deposited to obtain ohmic contacts on p-Si and n-TiO₂, respectively. The current–voltage (I–V) characteristics of the fabricated heterostructure were studied by using thermionic emission diffusion mechanism over the temperature range of 80–300 K. Parameters such as barrier height and ideality factor were derived from the measured I–V data of the heterostructure. The detailed analysis of I–V measurements revealed good rectifying behavior in the inhomogeneous Ag/n-TiO₂/p-Si(100)/Al heterostructure. The variations of barrier height and ideality factor with temperature and the non-linearity of the activation energy plot confirmed that barrier heights at the interface follow Gaussian distributions. The value of Richardson’s constant was found to be 6.73 × 10⁵ Am^?2 K^?2, which is of the order of the theoretical value 3.2 × 10⁵ Am^?2 K^?2. The capacitance–voltage (C–V) measurements of the heterostructure were investigated as a function of temperature. The frequency dependence (Mott–Schottky plot) of the C–V characteristics was also studied. These measurements indicate the occurrence of a built-in barrier and impurity concentration in TiO₂ film. The optical studies were also performed using a UV–Vis spectrophotometer. The optical band gap energy of TiO₂ films was found to be 3.60 eV.     

The Stark shift of the hole states in separate InAs/GaAs quantum dots grown on (100) and (311)A GaAs substrates

Deep-level transient spectroscopy is used to study charge-carrier emission from the states of separate quantum dots in InAs/GaAs p-n heterostructures grown on (100)-and (311)A-oriented GaAs substrates in relation to the reverse-bias voltage U. It is established that the structures under consideration exhibit different bias-voltage dependences of the Stark shift for the energy levels of the quantum-dot states on the value of U.     

Resonance modulation of the intersubband electron-electron interaction in an AlSb(δ-Te)/InAs/AsSb(δ-Te) quantum well by magnetic field

The amplitude-frequency modulation of oscillations of the magnetoresistance of 2D electrons in an AlSb(δ-Te)/InAs/AsSb(δ-Te) quantum well is studied. In the dependence of the amplitude of the oscillations δ(1/B) _{T = const}, regions of negative Dingle temperature are observed. The anomalies in the dependence δ(1/B) _{T = const} are attributed to the fact that the quantizing magnetic field resonantly induces intersubband electron-electron interaction between the 2D electrons of the ground size-quantization subband and excited subband. The resonance fields B and the times corresponding to the collision-related broadening of the Landau levels are estimated. The concentration threshold of filling of the excited size-quantization subband is established at a level of n _s ≈ 8 × 10¹¹ cm^?2.     

Quantum dots grown in the InSb/GaSb system by liquid-phase epitaxy

The first results of the liquid-phase epitaxial growth of quantum dots in the InSb/GaSb system and atomic-force microscopy data on the structural characteristics of the quantum dots are reported. It is shown that the surface density, shape, and size of nanoislands depend on the deposition temperature and the chemical properties of the matrix surface. Arrays of InSb quantum dots on GaSb (001) substrates are produced in the temperature range T = 450–465°C. The average dimensions of the quantum dots correspond to a height of h = 3 nm and a base dimension of D = 30 nm; the surface density is 3 × 10⁹ cm^–2.     

Radiation-stimulated processes in transistor temperature sensors

The features of the radiation-stimulated changes in the I–V and C–V characteristics of the emitter–base junction in KT3117 transistors are considered. It is shown that an increase in the current through the emitter junction is observed at the initial stage of irradiation (at doses of D < 4000 Gy for the “passive” irradiation mode and D < 5200 Gy for the “active” mode), which is caused by the effect of radiation-stimulated ordering of the defect-containing structure of the p–n junction. It is also shown that the X-ray irradiation (D < 14000 Gy), the subsequent relaxation (96 h), and thermal annealing (2 h at 400 K) of the transistor temperature sensors under investigation result in an increase in their radiation resistance.     

Physical properties of metal–insulator–semiconductor structures based on <Emphasis Type="Italic">n</Emphasis>-GaAs with InAs quantum dots deposited onto the surface of an <Emphasis Type="Italic">n</Emphasis>-GaAs layer

The properties of metal–insulator–semiconductor (MIS) structures based on n-GaAs in which silicon oxide and yttria-stabilized zirconia and hafnia are used as the insulator containing InAs quantum dots, which are embedded at the insulator/n-GaAs interface, are investigated. The structures manifest the resistive switching and synaptic behavior.