Doping Superlattices Research Articles

Solving the Asymmetric Doping for Wide-Gap Semiconductors by Host Functionalization: Quantum Engineering Strategy.

Asymmetric doping of wide-gap semiconductors has long been a major challenge, hindering their wider applications. Despite numerous attempts to address this issue through engineering doping levels, the results were still inconclusive. In this work, we propose a quantum engineering strategy based on the state-of-the-art spin-polarized HSE06 hybrid functional method. The local band offset between the host and quantum structures can considerably compensate for the large carrier activation energy (Ea). We chose the system of the AlN host embedded by GaN quantum dots as an example to validate the feasibility of this strategy. The Ea of Si (n-type) and Be (p-type) dopants can be reduced from 222 and 404 meV to negative values and 2 meV, respectively. Therefore, electron and hole density can be increased to more than 1019 and 1020 cm-3, respectively. We also tested potential dopants (C and Ge for the n-type, Mg and Ca for the p-type), and the technique is equally effective. This mechanism can also be used to understand the experimental observations of the superlattice doping strategy. Overall, our study demonstrates that the quantum engineering strategy provides a potential solution to overcome the asymmetric doping problem for universal wide-gap semiconductors and supports a feasible pathway for more efficient devices in the future.

Influence of confined acoustic phonons on the acousto-electric field in doped semiconductor superlattices

By using a quantum kinetic equation for electrons, the expression of the acoustoelectric field under the influence of confined acoustic phonons in doped semiconductor superlattices (DSSL) is obtained. From these expressions, the acousto-electric field depends on temperature, acoustic wave frequency, Fermi energy level, doped concentration, and quantum number m characterizing the phonons confinement. The results are numerically calculated for the GaAs:Be/GaAs:Si DSSL and show that the appearance of phonons confinement makes the acousto-electric field value become different than the cases of unconfined phonons.

Enhanced hole transport of nonpolar InGaN-based light-emitting diodes with lateral p-type superlattice doping structure

In this work, we propose a novel structure for nonpolar (10–10)-plane InGaN-based light-emitting diode (LED) using a lateral p-type Al0.2Ga0.8N/GaN superlattice structure as the hole injection layer. The main objective is to increase the hole concentration and facilitate vertical hole injection. The nonpolar InGaN-based LED lacks polarization along the growth plane (10–10), but the lateral direction along [0001] exhibits strong polarization. Therefore, the Al0.2Ga0.8N/GaN superlattice structure, which is periodic along the [0001] direction, induces net polarization charges at the GaN/Al0.2Ga0.8N interface, resulting in increased ionization rates of the acceptors. Additionally, the high-density two-dimensional hole gases formed at the Al0.2Ga0.8N/GaN interfaces along the [0001] direction can efficiently inject vertically into the quantum wells. Based on the numerical simulation results, the proposed LED structure offers improved electrical characteristics, effective hole injection, and enhanced optical performance compared to the nonpolar LED with conventional p-type doping structure.

Carrier density distribution in AlGaAs/GaAs superlattices with different numbers of quantum wells determined by capacitance-voltage profiling

The charge carrier density distribution in uniformly doped AlGaAs/GaAs superlattices with layer thicknesses of 1.5/10 nm and different numbers of quantum wells was studied. Both energy band theory and capacitance–voltage profiling were used to determine the carrier concentration profiles of the structures. During the analysis of the experimental and simulated capacitance–voltage characteristics, it was found that the maximum electron concentrations increase with an increase in the number of quantum wells from ∼ 7.1⋅1016cm−3 for three wells to ∼ 9.3⋅1016cm−3 for 25 wells with an overall superlattice doping level of ∼ 1017cm−3.

The methods of Mg-doped GaN-based p-type semiconductors

This article mainly discusses the disadvantages of the third-generation semiconductor material based on GaN and points the ways of optimization and development. The P-type doping optimization method is especially focused. As the core element of P-type doping, Mg element has its advantages and disadvantages in research and use, and some improvement methods are pointed out. In addition, the principles of several specific P-type doping methods, mainly including polarization-induced doping, Mg-delta doping, superlattice doping, and Mg-In co-doping are discussed.

Coexistence of Bloch and Parametric Mechanisms of High-Frequency Gain in Doped Superlattices

The detailed theoretical study of high-frequency signal gain, when a probe microwave signal is comparable to the AC pump electric field in a semiconductor superlattice, is presented. We identified conditions under which a doped superlattice biased by both DC and AC fields can generate or amplify high-frequency radiation composed of harmonics, half-harmonics, and fractional harmonics. Physical mechanisms behind the effects are discussed. It is revealed that in a general case, the amplification mechanism in superlattices is determined by the coexistence of both the phase-independent Bloch and phase-dependent parametric gain mechanisms. The interplay and contribution of these gain mechanisms can be adjusted by the sweeping AC pump strength and leveraging a proper phase between the pump and strong probe electric fields. Notably, a transition from the Bloch gain to the parametric gain, often naturally occurring as the amplitude of the amplified signal field grows, can facilitate an effective method of fractional harmonic generation in DC–AC-driven superlattices. The study also uncovers that the pure parametric generation of the fractional harmonics can be initiated via their ignition by switching the DC pump electric field. The findings open a promising avenue for the advancement of new miniature GHz–THz frequency generators, amplifiers, and dividers operating at room temperature.

Impact of illumination on quantum lifetime in selectively doped GaAs single quantum wells with short-period AlAs/GaAs superlattice barriers

Impact of illumination on high-mobility dense 2D electron gas in selectively doped single GaAs quantum well with short-period AlAs/GaAs superlattice barriers at T=4.2 K in magnetic fields B<2 T has been studied. It was demonstrated that illumination at low temperatures gives rise to enhancement of electron density, mobility and quantum lifetime in studied heterostructures. The enhancement of quantum lifetime after illumination for single GaAs quantum well with modulated superlattice doping had been explained as consequence of decrease in effective concentration of remote ionized donors. Keywords: persistent photoconductivity, quantum lifetime, anisotropic mobility, superlattice barriers.

Влияние подсветки на квантовое время жизни в селективно-легированных одиночных GaAs квантовых ямах с короткопериодными AlAs/GaAs-сверхрешеточными барьерами

Impact of illumination on high-mobility dense 2D electron gas in selectively doped single GaAs quantum well with short-period AlAs/GaAs superlattice barriers at T = 4.2 K in magnetic fields B < 2 T has been studied. It was demonstrated that illumination at low temperatures gives rise to enhancement of electron density, mobility and quantum lifetime in studied heterostructures. The enhancement of quantum lifetime after illumination for single GaAs quantum well with modulated superlattice doping had been explained as consequence of decrease in effective concentration of remote ionized donors.

Determination of the electron distribution in thin barrier AlGaAs/GaAs superlattices by capacitance-voltage profiling

Electron density distribution in uniformly doped AlGaAs/GaAs superlattices with respective layer thicknesses 1.5/10 nm and a different number of quantum wells was investigated. Experimental samples containing 3, 5 and 25 periods with the same layer parameters were grown by molecular beam epitaxy. Capacitance-voltage profiling was used to determine the carrier concentration profiles in the structures both numerically and experimentally. During the analysis of experimental capacitance-voltage characteristics it was found that the maximum electron concentration increases with an increase in the number of quantum wells starting from 7,1∙1016 сm–3 for 3 wells up to 9,2∙1016 сm–3 for 25 wells with overall superlattice doping level of 1017 сm–3. In some samples saturation areas are observed on the concentration profiles, that are associated with the region of superlattice. Concentration values, obtained from computer modeling, correspond to the experimental data with an error of less than 10 %. Capacitance-voltage profiling is a suitable technique for determining the carrier concentration profiles in thin barrier superlattices. Despite the fact that the method provides distribution of the “apparent” carrier concentration profile, it can be used to estimate the dopant atoms distribution in the strongly coupled quantum well heterostructures.

Interaction effects of pseudospin-based magnetic monopoles and kinks in a doped dipolar superlattice gas

Magnetic monopoles and kinks are topological excitations that have been extensively investigated in quantum spin systems, but usually, they are studied in different setups. We explore the conditions for the coexistence and interaction effects of these quasiparticles in the pseudospin chain of an atomic dipolar superlattice gas. In this chain, the magnetic kink is the intrinsic quasiparticle, and the particle (hole) defect takes over the role of the north (south) magnetic monopole, exerting monopolar magnetic fields on neighboring spins. A binding effect between the monopole and kink is revealed, which renormalizes the dispersion of the kink. The corresponding dynamical antibinding process is observed and arises due to the kink-antikink annihilation. The rich interaction effects of the two quasiparticles could stimulate corresponding investigations in bulk spin systems.

Control of upconversion luminescence by tailoring energy migration in doped perovskite superlattices.

We describe an experimental investigation of photon upconversion (UC) in a series of perovskite BaTiO3/SrTiO3 superlattices doped with different lanthanide compositions. We show that UC emission can be effectively enhanced by precisely incorporating a set of lanthanide ions into separated layers rather than homogeneously distributing the dopant ions in the host lattice. The use of an inert layer in the superlattice can suppress deleterious energy cross-relaxation. Furthermore, UC emission can be rendered by controlling the energy migration mediated by the Yb-doped sublattice. These results demonstrate the opportunity to modulate energy migration and transfer processes through the rational design of superlattice structures.

Evaluation of structural, electronic, optical, elastic, and mechanical properties of triclinic Sn-doped Ga2O3 using density functional theory

The electronic bandgap, density of states, optical coefficients, elastic constants, and bulk-to-shear ratio of Sn-doped Ga2O3 were calculated using density functional theory. The result of our study shows that a wide bandgap of 3.761 eV has been reported, which is found larger than the band energy Eg of 2.28 eV from our previous study in bulk Ga2O3. We also discovered the bandgap energy Eg reduces with the Sn chemical decompositions inside superlattice cell, and bandgap energy become direct when there are three unit-cells inside the doped superlattice. The bulk-to-shear ratio of 1.79 confirming the presence of the material ductility. Surprisingly, the Poisson's ratio of the Sn-doped Ga2O3 is in close match to the bulk Ga2O3 lattice bulk, which are both found to be ionic-covalent. We showed that, with our doped structure model, our computed elastic coefficients, bulk (shear) modulus, Poisson's ratio, and bulk-to-shear ratio provide different values (though not significant) to the bulk-lattice Ga2O3. This work aims to stimulate more research studies towards devices such as MOSFETs.

The influence of confined acoustic phonon on the quantum Peltier effect in doped semiconductor superlattice in the presence of electromagnetic wave

Based on the kinetic equation method, the quantum Peltier effect has been theoretically studied under the influence of confined acoustic phonon in doped semiconductor superlattice in the presence of the electromagnetic wave (laser radiation). There were complicated dependences of the analytical expression of the Peltier coefficient (PC) on quantities such as doped concentration of the superlattice, amplitude of the laser radiation, the cyclotron frequency of electrons and temperature of the system. In detailed consideration, the quantum number m was changed in order to characterize the influence of confined acoustic phonon. When setting the m to zero, we obtained the results that corresponded to the case of unconfined phonon. The theoretical results have been numerically evaluated and discussed for the GaAs:Si/GaAS:Be doped semiconductor superlattice (DSS). We found the oscillation of the PC according to enhancement of cyclotron frequency of electrons. Moreover, position of resonant peaks has shifted under the influence of phonon confinement. In the case of low doped concentration (n D), the PC decreased in non-linear way; then it reached a negative constant in high n D value. The non-linear change of the PC also has been detected when investigating its dependence on the laser amplitude. All numerical results have shown that the magnitude of the PC decreased due to the increase of phonon confinement effect. In short, the confinement of acoustic phonon caused the change of the quantum Peltier effect in DSS of GaAs:Si/GaAS:Be in quantitatively as well as qualitatively.

Screening Effects of Superlattice Doping on the Mobility of GaAs Two-Dimensional Electron System Revealed by in situ Gate Control

We investigate the screening effects of excess electrons in the doped layer on the mobility of a GaAs two-dimensional electron system (2DES) with a modern architecture using short-period superlattice (SL) doping. By controlling the density of excess electrons in the SL with a top gate while keeping the 2DES density constant with a back gate, we are able to compare 2DESs with the same density but different degrees of screening using one sample. Using a field-penetration technique and circuit-model analysis, we determine the density of states and excess electron density in the SL, quantities directly linked to the screening capability. The obtained relation between mobility and excess electron density is consistent with the theory taking into account the screening by the excess electrons in the SL. The quantum lifetime determined from Shubnikov-de Haas oscillations is much lower than expected from theory and did not show a discernible change with excess electron density.

The Ettingshausen effect in doped semiconductor superlattice under the influence of confined optical phonon

The electron – optical phonon scattering is considered in detail to studying the Ettingshausen effect in doped semiconductor superlattice under the influence of phonon confinement and laser radiation. The analytical expressions for tensors and the Ettingshausen coefficient are obtained by using the kinetic equation method. The Ettingshausen coefficient depends on temperature of the sample, amplitude and frequency of laser radiation, magnetic field and the quantum number m specific for the confinement of phonon. The dependences are clearly displayed in the numerical results for GaAs:Be/GaAs:Si doped semiconductor superlattice. The magnetic field makes the Ettingshausen coefficient change in quantitative under the influence of temperature or laser amplitude and change the resonance condition. The numerical results show that both resonance condition and resonance peaks position are affected by the increase of quantum number m. We also get the result corresponding to the unconfined optical phonon case when m is set to zero. Due to the change of the wave function and energy spectrum of electrons, most of results for the Ettingshausen effect in doped semiconductor superlattice obtained are different from the case of bulk semiconductor. Moreover, in comparison with the case of unconfined optical phonon, under the influence of phonon confinement effect, the Ettingshausen coefficient changes in magnitude, the number and position of resonance peaks.

Photoluminescence study of non-polar m-plane InGaN and nearly strain-balanced InGaN/AlGaN superlattices

Photoluminescence (PL) spectroscopy of nonpolar m-plane InGaN thin films with indium composition up to 21% and nearly strain-balanced In0.09Ga0.91N/Al0.19Ga0.81N superlattices grown by plasma-assisted molecular beam epitaxy was performed as a function of temperature. The experimental transition energies are consistently lower than the calculation based on structural parameters extracted from x-ray diffraction measurements. This indicates the presence of indium composition fluctuations in InGaN and hence local bandgap reduction that produces charge localization centers. The spectral width of the low-temperature PL of our m-plane InGaN/AlGaN superlattices is narrower than previously reported for m-plane InGaN/GaN quantum wells grown by MOCVD. The PL integrated intensity drops rapidly, though, as the temperature is increased to 300 K, indicating strong non-radiative recombination at room temperature. Time-resolved PL at low temperatures was performed to characterize the relaxation time scales in an undoped and a doped superlattice.

Phonon thermal conductivity reduction in silicene nanotubes with isotope substitution.

We used molecular dynamics simulations to study the isotopic doping effects on phonon thermal conductivity in armchair silicene nanotubes (SNTs). The phonon thermal conductivity of armchair SNTs can be effectively tuned with isotope substitution. Randomly and superlattice-structured isotopic doping can significantly reduce thermal conductivity. By analyzing the phonon vibrational spectrum, we reveal the underlying physical insights into the relationship between randomly isotopic doping concentration and thermal conductivity. Given the same doping concentration, the superlattice-structured doping method can reduce thermal conductivity more significantly than the disordered doping. For the isotopic superlattice doping method, the completion between the phonon interfacial scattering and phonon tunneling may cause minimum thermal conductivity at the critical period length. This study provides a possible means to effectively reduce the thermal conductivity of thermoelectric SNTs through isotopic doping engineering.

Influence of Confined Acoustic Phonons on the Nonlinear Acousto-electric Effect in Doped Semiconductor Superlattices

Abstract: By using a quantum kinetic equation for electrons, we have studied the Acousto-electric effects in doped semiconductor superlattice (DSSL) under the influence of confined phonon. Considering the case of the electron - acoustic phonon interaction, we have found the expressions of the nonlinear quantum acousto-electric current. From these expression, the acousto-electric current (AEC) depends nonlinearly on temperature, acoustic wave frequency and the characteristic parameters of DSSL (For example: the doped concentration ). Moreover, the expression of the AEC under the influence of confined phonons fairly different from the case of unconfined phonons.
 The results are numerically calculated for the GaAs:Be/ GaAs:Si DSSL; therefore, it can be easily seen that the dependence of the acousto-electric current on the characteristic parameters of the acoustic wave, temperature, the characteristic parameters of DSSL and the quantum number m characterizing the phonons confinement. The results have showed that the appearance of phonons confinement make the AEC value changes remarkably. The AEC is almost stable in low acoustic wave frequency condition and changes as a parabolic curve when move up. On the other hand, in case of low doped concentration number the AEC surges as a parabolic function in the dependence on , then it remains stability at just below zero in high value.
 Keywords: Acousto-electric field, Quantum kinetic equation, Doped superlattices, Electron - phonon interaction.

Evidence for terahertz acoustic phonon parametric oscillator based on acousto-optic degenerate four-wave mixing in a silicon doping superlattice

We report first evidence for a 1.0 terahertz (THz) self-starting mirrorless acoustic phonon parametric oscillator (MAPPO) produced from acousto-optic phase-conjugate degenerate four-wave (D4WM) mixing in a THz laser-pumped silicon doping superlattice (DSL). The DSL was grown by molecular beam epitaxy on a (100) boron-doped silicon substrate. A superconducting NbTiN subwavelength grating was used to couple the THz laser radiation into the DSL. Superconducting granular aluminum bolometric detection, coupled with Si:B piezophonon spectroscopy, revealed excitation of THz coherent compressional and shear waves, along the <111> direction only. The Bragg scattering condition for distributed feedback, and the energy conservation requirement for the D4WM process, were both verified. A THz MAPPO could provide a testbed for studies of non-classical acoustic phonon fields.

Low-temperature homoepitaxial growth of two-dimensional antimony superlattices in silicon

The authors present a low-temperature process for the homoepitaxial growth of antimony superlattices in silicon. The all low-temperature superlattice doping process is compatible as a postfabrication step for device passivation. The authors have used low-temperature molecular beam epitaxy to embed atomically thin (2D), highly concentrated layers of dopant atoms within nanometers of the surface. This process allows for dopant densities on the order of 1013–1014 cm−2 (1020–1021 cm−3); higher than can be achieved with three-dimensional doping techniques. This effort builds on prior work with n-type delta doping; the authors have optimized the growth processes to achieve delta layers with sharp dopant profiles. By transitioning from a standard effusion cell to a valved cracker cell for antimony evaporation, the authors have achieved carrier densities approaching 1021 cm−3 with peak distribution at ∼10 Å FWHM for single delta layers. Even at the highest dopant concentrations studied, no deterioration in carrier mobility is observed, suggesting the upper limit for dopant incorporation and activation has not yet been met. The authors will discuss the details related to growth optimization and show results from in situ monitoring by electron diffraction. They will also report on elemental and electrical characterization of the films.