Band-gap Renormalization Effects Research Articles

Giant gate-tunable bandgap renormalization and excitonic effects in a 2D semiconductor.

Understanding the remarkable excitonic effects and controlling the exciton binding energies in two-dimensional (2D) semiconductors are crucial in unlocking their full potential for use in future photonic and optoelectronic devices. Here, we demonstrate large excitonic effects and gate-tunable exciton binding energies in single-layer rhenium diselenide (ReSe2) on a back-gated graphene device. We used scanning tunneling spectroscopy and differential reflectance spectroscopy to measure the quasiparticle electronic and optical bandgap of single-layer ReSe2, respectively, yielding a large exciton binding energy of 520 meV. Further, we achieved continuous tuning of the electronic bandgap and exciton binding energy of monolayer ReSe2 by hundreds of milli-electron volts through electrostatic gating, attributed to tunable Coulomb interactions arising from the gate-controlled free carriers in graphene. Our findings open a new avenue for controlling the bandgap renormalization and exciton binding energies in 2D semiconductors for a wide range of technological applications.

Recombination and diffusion processes in electronic grade 4H silicon carbide

Carrier dynamics in n-type 4H-SiC epilayers of varying thicknesses and low Z1/2 defect concentrations are investigated here in wide ranges of excess carrier density and temperature. Several techniques are employed to monitor carrier diffusion and recombination processes, including light induced transient grating (LITG), microwave photoconductance decay (MPCD) and free carrier absorption (FCA) using ps-laser pulses at 355 nm. The observed increase of the diffusion coefficient with the increasing excitation level is explained by the transition from the minority to the bipolar transport regime. Its subsequent decrease with even higher excitations is found to be governed by band-gap renormalization and degeneracy effects. The bulk lifetime, limited by hole traps at 0.19–0.24 eV above the valence band at lower excitations, was found to decrease from few microseconds to hundreds of nanoseconds during the transition regime from the minority to the bipolar transport. Our temperature-dependent measurements confirmed the trap activation energy and provided the approximate functional form of electron and hole lifetimes as τe = 340 × (T/300 K)3/2 ns and τh = 100 × (T/300 K)–1/2 ns, for the temperature T range 80–800 K. It was found to hold for 65 and 120 μm sample thicknesses, while the lifetimes were found to be twice shorter for the sample 35 μm thick.

Bandgap narrowing and Mott transition in Si-doped Al0.7Ga0.3N

Deep ultraviolet light-emitting diodes (LEDs) composed of III-Nitride semiconductors need layers of heavy doping (>1 × 1019 cm−3) to overcome large dopant activation energies and maintain high electrical conductivity. This work reports that at doping densities of [Si] ∼1.5 × 1019/cm3 for n-Al0.7Ga0.3N, Burstein-Moss and bandgap renormalization effects result in a net reduction of the bandgap of ∼70 meV. At these doping levels, a transition to a metallic conductivity state is observed, with a vanishing of the effective dopant activation energy. The sheet and contact resistivities of Rsh,n = 0.045 Ω cm and ρc,n = 1.13 × 10−6 Ω cm2 are achieved, with uniform conductivity in the vertical direction. The results show that when heavily doped n-AlGaN cladding regions are used for high efficiency deep-UV LEDs or lasers, the accompanying bandgap narrowing reduces the window of optical transparency at the lowest wavelengths that can take advantage of high conductivity.

Electronic structures and optical properties of Si- and Sn-doped β-Ga2O3: A GGA+U study**Project supported by the Science and Technology Program of Guangdong Province, China (Grant No. 2015B010112002) and the Science and Technology Project of Guangzhou City, China (Grant No. 201607010250).

The electronic structures and optical properties of β-Ga2O3 and Si- and Sn-doped β-Ga2O3 are studied using the GGA + U method based on density functional theory. The calculated bandgap and Ga 3d-state peak of β-Ga2O3 are in good agreement with experimental results. Si- and Sn-doped β-Ga2O3 tend to form under O-poor conditions, and the formation energy of Si-doped β-Ga2O3 is larger than that of Sn-doped β-Ga2O3 because of the large bond length variation between Ga–O and Si–O. Si- and Sn-doped β-Ga2O3 have wider optical gaps than β-Ga2O3, due to the Burstein–Moss effect and the bandgap renormalization effect. Si-doped β-Ga2O3 shows better electron conductivity and a higher optical absorption edge than Sn-doped β-Ga2O3, so Si is more suitable as a dopant of n-type β-Ga2O3, which can be applied in deep-UV photoelectric devices.

Conduction band electron relaxation and spin relaxation dynamics in CdZnTe alloy

Conduction band electron relaxation and spin relaxation dynamics in Cd0.96Zn0.04Te alloy are investigated using time-resolved pump-probe spectroscopy. The measured linearly polarization pump-probe spectroscopy demonstrates the presence of a fast process and a slow process, both of which indicate electron trapping and recombination at the defect/impurity level. The time constants of the fast process are within the range of 3.1 to 4.9 ps, and those of the slow process are within the range of 16.1 to 18.1 ps. During the ultrafast thermalization process in the first picosecond, an oscillating signal that results from the band gap renormalization (BGR) effect is found. The D’yakonov-Perel’ (DP) mechanism dominates the spin relaxation process, and the spin relaxation slows down with the incorporation of Zn, because of the decrease of spin-orbit splitting energy and enhancement of electron-impurity scattering.

Hot phonon and carrier relaxation in Si(100) determined by transient extreme ultraviolet spectroscopy.

The thermalization of hot carriers and phonons gives direct insight into the scattering processes that mediate electrical and thermal transport. Obtaining the scattering rates for both hot carriers and phonons currently requires multiple measurements with incommensurate timescales. Here, transient extreme-ultraviolet (XUV) spectroscopy on the silicon 2p core level at 100 eV is used to measure hot carrier and phonon thermalization in Si(100) from tens of femtoseconds to 200 ps, following photoexcitation of the indirect transition to the Δ valley at 800 nm. The ground state XUV spectrum is first theoretically predicted using a combination of a single plasmon pole model and the Bethe-Salpeter equation with density functional theory. The excited state spectrum is predicted by incorporating the electronic effects of photo-induced state-filling, broadening, and band-gap renormalization into the ground state XUV spectrum. A time-dependent lattice deformation and expansion is also required to describe the excited state spectrum. The kinetics of these structural components match the kinetics of phonons excited from the electron-phonon and phonon-phonon scattering processes following photoexcitation. Separating the contributions of electronic and structural effects on the transient XUV spectra allows the carrier population, the population of phonons involved in inter- and intra-valley electron-phonon scattering, and the population of phonons involved in phonon-phonon scattering to be quantified as a function of delay time.

Mode-Hopping Phenomena in the InGaN-Based Core–Shell Nanorod Array Collective Lasing

A two-dimensional monolithic InGaN-based core–shell nanorod array was excited under a varying temperature. We observed an abrupt dominant mode hopping from 3.29 to 3.41 eV in the collective lasing as the temperature decreased from T = 240 to 225 K. Photoluminescence under a near-threshold pumping density revealed the splitting of spontaneous emission between the core and the shell with a decreasing temperature. The bandgap evolution of GaN and InGaN showed opposite trends due to the interaction of temperature effects and bandgap renormalization effects. Theoretical simulation revealed the differences of gain spectra evolution between room temperature and low temperature due to the difference in the carrier dynamics. In the optically coupled nanorod array, the dominant lasing mode was not only determined by its structure, but also strongly influenced by external operating conditions. Rather than singular microcavity lasers, the coupled nanorod array demonstrated a broader parameter space for on-chip wavele...

Preparation and Some Properties of Metal Organic Chemical Vapour Deposited Al-Doped ZnO Thin Films Using Single Solid Precursors

Zinc Oxide (ZnO) and Aluminium doped ZnO (AZO) thin films were deposited on soda lime glass by Metal Organic Chemical Vapour deposition technique (MOCVD), using prepared compound mixtures of Zinc Acetate di-hydrate (Zn(CH3COO)2⋅2H2O; ZAD) and Aluminium Acetyl-Acetonate (Al(C5H702)3; AAA) precursors at a temperature of 420°C. Effects of the varying mole percent concentrations of AAA precursor additives on the Al dopant concentrations in ZnO were systematically studied. The observations were made via investigations carried out on the morphological, optical, electrical and compositional properties of the deposited thin films. The thin films morphology was found to be strongly dependent on the varying concentration of AAA in the precursor mixtures. The average optical transmittance of the thin films in the uv-visible region was over 85% except 5 mol.% Al. While the energy band gaps were found to be in range of 3.27 - 3.36 eV. There is a blue-shift of the energy band edge observed between 0 and 5 mol.% AAA, which may be due to Burstein-Moss’ band gap widening effect and an opposing band gap renormalization effect at 10 mol.% AAA along with an extra band gap stabilization effect (Roth’s effect) at 15 mol.% AAA in rather quasi-sinusoidal or anomalous behaviour. The optical transmittance and electrical conductivity of ZnO were enhanced with addition of Al dopants. The RBS confirm the presence of Al, Zn and O, and evidence that Al dopants were successfully incorporated into the ZnO.

Rate Equation Analysis of the Dynamics of First-order Exciton Mott Transition

We performed a rate equation analysis on the dynamics of exciton Mott transition (EMT) with assuming a detailed balance between excitons and unbound electron-hole (e-h) pairs. Based on the Saha equation with taking into account the empirical expression for the band-gap renormalization effect caused by the unbound e-h pairs, we show that the ionization ratio of excitons exhibits a bistability as a function of total e-h pair density at low temperatures. We demonstrate that an incubation time emerges in the dynamics of EMT from oversaturated exciton gas phase on the verge of the bistable region. The incubation time shows a slowing down behavior when the pair density approaches toward the saddle-node bifurcation of the hysteresis curve of the exciton ionization ratio.

Bandgap renormalization in single-wall carbon nanotubes

Single-wall carbon nanotubes (SWNTs) have been extensively explored as an ultrafast nonlinear optical material. However, due to the numerous electronic and morphological arrangements, a simple and self-contained physical model that can unambiguously account for the rich photocarrier dynamics in SWNTs is still absent. Here, by performing broadband degenerate and non-degenerate pump-probe experiments on SWNTs of different chiralities and morphologies, we reveal strong evidences for the existence of bandgap renormalization in SWNTs. In particularly, it is found that the broadband transient response of SWNTs can be well explained by the combined effects of Pauli blocking and bandgap renormalization, and the distinct dynamics is further influenced by the different sensitivity of degenerate and non-degenerate measurements to these two concurrent effects. Furthermore, we attribute optical-phonon bath thermalization as an underlying mechanism for the observed bandgap renormalization. Our findings provide new guidelines for interpreting the broadband optical response of carbon nanotubes.

Room temperature excitonic dynamics of non-polar a-plane ZnO epifilms

Pump polarization dependent carrier dynamics, particularly excitonic dynamics, of non-polar a-plane zinc oxide (ZnO) epifilms with two different thicknesses were investigated using time resolved measurements. Unlike the electron and hole dynamics through the above-bandgap excitation, transient differential reflectance (TDR) traces revealed similar trends under two orthogonal pump polarization conditions relative to the c-axis (Epu⊥c and Epu∥c) of a-ZnO around near-exciton-resonance excitation. By means of a band diagram, the bandgap renormalization (BGR) effect can be reasonably explained by the screening of the Coulomb potential energy due to the accumulation of relaxed free carriers that were initially excited through the absorption of two cascaded pump photons via the excitonic level, a process known as two photon absorption (TPA). Thus, the modulation depths of the TPA around zero time delay, due to simultaneous absorption of one pump and one probe photon via the excitonic level, increased linearly with the pump fluence, proportional to the modulation depth resulting from the BGR effects.

Characteristics of low-resistivity aluminum-doped zinc oxide films deposited at room temperature by off-axis radio-frequency sputtering on flexible plastic substrates

The crystalline structure, morphology, composition, electrical transport, and optical properties of aluminum-doped zinc oxide (AZO) films are studied for applications in transparent electronics and optoelectronic devices. AZO thin films of c-axis-oriented growth and with different thickness were deposited on PET flexible plastic substrates at room temperature by rf magnetron sputtering. A larger grain size with a decreased strain e value is observed in a thicker film, while changes in composition for films with different thicknesses are insignificant. Moreover, the resistivity of film decreases with increasing thickness, and the low-temperature electrical transport properties can be described by the scenario of quantum corrections to conductivity. With the room-temperature growth conditions, the resistivity of 4.5 × 10−4 Ω cm, carrier concentration of 6.4 × 1020 cm−3, and transmittance of 80 % for the 1100-nm-thick film are obtained. In addition, the optical bandgap energy decreases with increasing film thickness, which can be attributed to the bandgap renormalization and crystallite size effects.

Effect of Quasi-Fermi Level on the Degree of Electron Spin Polarization in GaAs**Supported by the National Natural Science Foundation of China under Grant Nos 11504194 and 11274189, the Project of Shandong-Provincial Higher Educational Science and Technology Program under Grant No J14LJ06, and the Application Foundation Research Program of Qingdao under Grant No 14-2-4-101-jch.

With spin-polarized-dependent band gap renormalization effect taken into account, the energy-dependent evolution of electron spin polarization in GaAs is calculated at room temperature and at a low temperature of 10 K. We consider the exciting light with right-handed circular polarization, and the calculation results show that the degree of electron spin polarization is dependent strongly on the quasi-Fermi levels of |1/2〉 and | − 1/2〉 spin conduction bands. At room temperature, the degree of electron spin polarization decreases sharply from 1 near the bottom of the conduction band, and then increases to a stable value above the quasi-Fermi level of the | − 1/2〉 band. The greater the quasi-Fermi level is, the higher the degree of electron spin polarization with excessive energy above the quasi-Fermi level of the | − 1/2〉 band can be achieved. At low temperature, the degree of electron spin polarization decreases from 1 sharply near the bottom of the conduction band, and then increases with the excessive energy, and in particular, up to a maximum of 1 above the quasi-Fermi level of the |1/2〉 band.

Charge Trapping and Exciton Dynamics in Large-Area CVD Grown MoS2

There is keen interest in monolayer transition metal dichalcogenide films for a variety of optoelectronic applications due to their direct band gap and fast carrier dynamics. However, the mechanisms dominating their carrier dynamics are poorly understood. By combining time-resolved terahertz (THz) spectroscopy and transient absorption, we are able to shed light on the optoelectronic properties of large area CVD grown mono- and multilayer MoS2 films and determine the origins of the characteristic two-component excited state dynamics. The photoinduced conductivity shows that charge carriers, and not excitons, are responsible for the subpicosecond dynamics. Identical dynamics resulting from sub-optical gap excitation suggest that charge carriers are rapidly trapped by midgap states within 600 fs. This process complicates the excited state spectrum with rapid changes in line-width broadening in addition to a red-shift due to band gap renormalization and simple state-filling effects. These dynamics are insensi...

Pump polarization dependent ultrafast carrier dynamics and two-photon absorption in an a-plane ZnO epitaxial film

The ultrafast carrier dynamics of non-polar a-plane ZnO epi-film, with the energy difference between the A- and C-valence bands of about 23 meV, grown on r-plane sapphire were investigated using the reflection type pump-probe technique under two perpendicular polarized pumps. By exciting the electron from A-valence band through pump polarization perpendicular to the c-axis of a-ZnO (Epu ⊥ c), the TDR trace revealed two photon absorption (TPA), band filling (BF) and bandgap renormalization (BGR) effects that can be reasonably explained by the electron dynamics in the conduction band. By exciting the electron from C-valence band through parallel pump polarization (Epu∥c), only the BF effect was observed in the TDR trace owing to the hole dynamics in the valence bands. The occurrence of TPA was determined by the pump efficiency depending on the energy difference between the pump photon and the intermediate exciton resonance state.

Effects of indium doping on the crystallographic, morphological, electrical, and optical properties of highly crystalline ZnO films

In this article, the authors have reported the growth and characterization of a batch of indium doped zinc oxide films to explore the controllable n-type doping in ZnO with high crystalline quality for the optoelectronic device application. The effects of indium doping on the crystalline structure, surface morphology, electrical, and optical properties have been shown. The mechanisms regarding the grain growth, the electron scattering, and the optical band gap have been investigated and discussed. The crystallite size is constrained by the competing effect between surface energy and lattice strain, and thus maintains invariable with increasing indium doping level. The coalescence of nearby crystallites leads to the growth of grains. The grain size has been found to increase with indium incorporation, suggesting that the surface diffusion rate of adatoms is enhanced by more indium. The increased Hall mobility can be explained by the reduced boundary effect, and can be also understood in the view of ion orbit radius and percolation conduction model. The band gap has been modified by the Burstein-Moss effect and the band gap renormalization effect. The band gap values of the samples with relatively higher indium doping level are dominated by the dopant-dependent band gap renormalization effect. This research provides a comprehensive understanding of the material properties of indium doped ZnO films grown by metal-organic chemical vapor deposition.

Photoreflectance investigation of band gap renormalization and the Burstein–Moss effect in Si doped GaN grown by MOVPE

This work reports on the photoreflectance (PR) study of Si doped GaN layers (GaN:Si) with different doping levels (2.1×1017–2.8×1019cm−3). GaN:Si samples have been successfully grown on sapphire substrate by metalorganic vapor phase epitaxy (MOVPE). Room temperature PR spectra showed Franz–Keldysh oscillation (FKO) features, reflecting the existence of a built-in electric field in the epilayers. The band gap energy determined using the FKO theory, is observed first to decrease and then to increase with increasing electron concentration. This result is explained by the competing effects of band gap renormalization and Burstein-–Moss band filling. Based on theoretical approach and experimental results, the interplay between these phenomena is systematically studied.

Capability of GaN based micro-light emitting diodes operated at an injection level of kA/cm².

Different size InGaN/GaN based micro-LEDs (μLEDs) are fabricated. An extremely high injection level above 16 kA/cm2 is achieved for 10 μm-diameter LED. The lateral current density and carrier distributions of the μLEDs are simulated by APSYS software. Streak camera time resolved photoluminescence (TRPL) results show clear evidence that the band-gap renormalization (BGR) effect is weakened by strain relaxation in smaller size μLEDs. BGR affects the relaxation of free carriers on the conduction band bottom in multiple quantum wells (MQWs), and then indirectly affects the recombination rate of carriers. An energy band model based on BGR effect is made to explain the high-injection-level phenomenon for μLEDs.

NixCd1−xO: Semiconducting alloys with extreme type III band offsets

We have synthesized alloys of NiO and CdO that exhibit an extreme type III band offset and have studied the structural, electrical, and optical properties of NixCd1−xO over the entire composition range. The alloys are rocksalt structured and exhibit a monotonic shift of the (220) diffraction peak to higher 2θ angles with increasing Ni concentration. The electron mobility and electron concentration decrease with increasing x, and samples become insulating for Ni content x > 0.44. This decrease in n-type conductivity is consistent with the movement of the conduction band minimum from below to above the Fermi stabilization energy with increasing Ni content. The optical absorption edge of the alloys can be tuned continuously from CdO to NiO. The intrinsic gap of the alloys was calculated with the electrical and optical measurements and accounting for Burstein-Moss carrier filling and carrier-induced bandgap renormalization effects. We observe an uncommon composition dependence of the intrinsic bandgap on the alloy composition. The effect is tentatively attributed to an interaction between extended states of the conduction band and localized d-states of Ni.

Transient reflectivity as a probe of ultrafast carrier dynamics in semiconductors: A revised model for low-temperature grown GaAs

We revisit pump-probe transient reflectivity (PPTR) as a probe of ultrafast carrier dynamics in photoconductive materials, using low-temperature grown GaAs (LT-GaAs) as an exemplar. The carrier dynamics in a series of annealed LT-GaAs wafers were measured by PPTR. The wafer growth and anneal conditions were tailored to produce a material system with a sub-picosecond carrier lifetime. The PPTR signals from these wafers are bipolar with time constants on the order of 100 fs and 1 ps, consistent with previous literature reports on LT-GaAs. We examined the utility of numerical simulations of the pump-probe transients described in [V. Ortiz et al., J. Appl. Phys. 102, 043515 (2007)] to model our experimental results. We discovered a discrepancy between the model's predictions and experiment with respect to the scaling of the PPTR response with injected carrier density, and show that this discrepancy is rooted in how the model accounts for the index of refraction change due to band filling (BF) and band gap renormalization (BGR). We demonstrate that any model that includes a non-negligible BGR effect is inconsistent with our experimental observations of LT-GaAs. We present a revised model to simulate PPTR signals that account for BF and incorporate optical absorption from mid-gap states. This model can reproduce the experimental results on LT-GaAs and enables comparative assessments of alternate trapping and recombination hypotheses. For LT-GaAs, we compared point defects and nanoparticles as sites for Shockley-Read-Hall recombination, with the result that nanoparticle trapping and recombination centers most accurately reproduce the PPTR probe of carrier dynamics in LT-GaAs.