Band-gap Renormalization Effects Research Articles

Electron concentration dependence of optical band gap shift in Ga-doped ZnO thin films by magnetron sputtering

Ga-doped ZnO (GZO) thin films were deposited on glass substrates by a radio frequency magnetron sputtering technique. The optical properties of the deposited GZO films were evaluated using an optical transmission measurement. The optical band gap increased from 3.32eV to 3.45eV with the increasing carrier density from 2.0×1020cm−3 to 3.24×1020cm−3. Based on the experimental results, the optical band gap as a function of carrier density is systematically investigated with four available theoretical models taken into consideration. The blueshift of the optical band gap in GZO films can be well interpreted with a complex model which combines the Burstein–Moss effect, the band gap renormalization effect and the nonparabolic nature of conduction band. In addition, the BM contribution is almost offset by the BGR effect in both conduction band and valence band due to the approximate equality between electron and hole effective masses in GZO films with a nonparabolic conduction band. The tunability of optical band gap in GZO thin films by carrier density offers a number of potential advantages in the development of semiconductor optoelectronic devices.

Electronic structures and optical properties of IIIA-doped wurtzite Mg0.25Zn0.75O

The energy band structures, density of states, and optical properties of IIIA-doped wurtzite Mg0.25Zn0.75O (IIIA = Al, Ga, In) are investigated by a first-principles method based on the density functional theory. The calculated results show that the optical bandgaps of Mg0.25Zn0.75O:IIIA are larger than those of Mg0.25Zn0.75O because of the Burstein—Moss effect and the bandgap renormalization effect. The electron effective mass values of Mg0.25Zn0.75O:IIIA are heavier than those of Mg0.25Zn0.75O, which is in agreement with the previous experimental result. The formation energies of MgZnO:Al and MgZnO:Ga are smaller than that of MgZnO:In, while their optical bandgaps are larger, so MgZnO:Al and MgZnO:Ga are suitable to be fabricated and used as transparent conductive oxide films in the ultra-violet (UV) and deep UV optoelectronic devices.

Transparent conductive Mg and Ga co-doped ZnO thin films for solar cells grown by magnetron sputtering: H2 induced changes

Transparent conductive hydrogenated Mg and Ga co-doped ZnO (HMGZO) thin films for solar cells were deposited via pulsed direct current (DC) magnetron sputtering on glass substrates at a substrate temperature of 553K. The structural, morphological, electrical, and optical properties of HMGZO thin films were investigated with various H2 flow rates. The experiment results showed that the HMGZO thin films are polycrystalline with a hexagonal wurtzite structure exhibiting a preferred (002) crystal plane orientation and typical HMGZO thin films present cone-like texture surface. The carrier concentration rapidly increases from 7.57×1019cm−3 to 5.25×1020cm−3 with increasing the H2 flow rate from 0sccm to 4.0sccm. Optical measurements indicated that the optical band-gap (Eg) of HMGZO thin films varies from 3.43eV to 3.66eV with adjusting H2 flow rate from 0sccm to 4.0sccm. The glass/HMGZO thin film deposited at the H2 flow rate of 4.0sccm exhibits the lowest resistivity of 6.24×10−4Ωcm (sheet resistance Rs~8.68Ω) and an average transmittance (Ta) of 78.8% in the wavelength range from 340nm to 1100nm. Burstein–Moss band-filling, band gap renormalization effects determined by carrier concentrations and the incorporation of Mg atoms together contribute to an Eg widening phenomenon. The electron mobility of HMGZO thin films can be effectively improved through thermal annealing process. Finally, the HMGZO thin film was preliminarily applied in pin a-Si:H thin film solar cell with an efficiency of 8.20% (Voc=0.904V, Jsc=12.906mA/cm2 and FF=0.702).

Crystalline quality improvement of GaN coalesced using nano‐PSS with void‐embedded nanostructure by MOVPE

AbstractThe nanoscale patterned sapphire substrates (NPSS) were prepared combining nanoimprint lithography and dry/wet etching technology. The NPSS was patterned with 300 nm diameter, 330 nm depth, 550 nm periodicity hexagonal close packed pillars. Coalescence of GaN has been achieved on the NPSS using MOVPE. The growth mode and conditions were discussed in detail, which was different from that of GaN epitaxial on planar sapphire. The AFM images showed the GaN on NPSS had a smoother surface, and the XRD results showed that the crystal quality had been improved by the NPSS. The threading dislocation density (TDD) suppression was observed in transmission electron microscopy. Depth‐resolved CL measurements were used to quantify the redshift in terms of bandgap renormalization and strain effects. It was supposed that the compressive stress in the GaN layers was changed gradually towards the growth surface. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin dynamics of electron with high excess energy in GaAs investigated by the simplified circularly dichromatic pump–probe model

Abstracts Time-resolved circularly polarized pump–probe spectroscopy is used to study the spin dynamics of electron with high excess energy in bulk GaAs at room temperature, an abnormal phenomenon that the curve taken from co-helicity circularly polarized pump–probe beam drops below the curve taken from cross-helicity circularly polarized pump–probe beam is observed. With both the spin-dependent band-filling and band-gap renormalization effects being taken into account, a simplified circularly dichromatic pump–probe model is further developed, the model can be used to fit the scanning experimental traces to retrieve the spin relaxation time, carrier recombination time, intensity coefficient of spin-dependent band-filling and band-gap renormalization effects, etc. With the parameters retrieved from the experimental data, the physical origin of the spectrum reversal can be explained in detail, both of the spin-dependent band-filling and band-gap renormalization effects contribute to the reversal spectrum.

Negative Slope for Second Harmonic Generation Observed at High Excitation Intensities in ZnO Nanorods

The nonlinear optical properties of ZnO nanorods (NRs) synthesized by coprecipitation method were investigated using a focused femtosecond laser light. The excitation wavelength was tuned from 750 to 795 nm so that excitons could be selectively generated via two-photon absorption above or below the exciton ground state of ZnO NRs. Both second harmonic generation (SHG) and two-photon-induced luminescence (TPL) were observed in the nonlinear response spectrum of ZnO NRs. The relative intensities of SHG and TPL were found to depend not only on excitation wavelength, but also on excitation intensity. At high excitation intensities, the nonlinear response spectrum became dominated by TPL for excitation wavelengths shorter than 770 nm, whereas it was still governed by SHG for excitation wavelengths longer than 770 nm. In addition, the intensities of SHG and TPL did not scale quadratically with excitation intensity but exhibited different slopes in different excitation intensity regimes, implying the existence of competition between them. More interestingly, a negative slope, which indicates a reduction of SHG with increasing excitation intensity, was observed at high excitation intensities for excitation wavelengths longer than 770 nm, implying the energy redistribution or energy transfer between SHG and TPL. Meanwhile, a slope much was identified for TPL at high excitation intensities. It is suggested that the reduction in the bandgap energy resulting from the effects of bandgap renormalization and temperature rise were responsible for the rapid increase of TPL. A weak exciton emission was resolved for excitation wavelengths longer than 770 nm and it was explained by the existence of Rabi oscillation.

Electronic transition and electrical transport properties of delafossite CuCr1−xMgxO2 (0 ≤ x ≤ 12%) films prepared by the sol-gel method: A composition dependence study

Highly transparent CuCr1−xMgxO2 (0 ≤ x ≤ 12%) films were prepared on (001) sapphire substrates by sol-gel method. The microstructure, phonon modes, optical band gap, and electrical transport properties have been systematically discussed. It was found that Mg-doping improved the crystal quality and enhanced the (00l) preferred orientation. The spectral transmittance of films approaches about 70%–75% in the visible-near-infrared wavelength region. With increasing Mg-composition, the optical band gap first declines and climbs up due to the band gap renormalization and Burstein-Moss effect. The direct and indirect band gaps of CuCr0.94Mg0.06O2 film are 3.00 and 2.56 eV, respectively. In addition, it shows a crossover from the thermal activation behavior to that of three-dimensional variable range hopping from temperature-dependent electrical conductivity. The crossover temperature decreases with increasing Mg-doping composition, which can be ascribed to the change of spin-charge coupling between the hole and the local spin at Cr site. It should be noted that the electrical conductivity of CuCr1−xMgxO2 films becomes larger with increasing x value. The highest electrical conductivity of 3.85 S cm−1 at room temperature for x = 12% is four-order magnitude larger than that (8.81 × 10−4 S cm−1) for pure CuCrO2 film. The high spectral transmittance and larger conductivity indicate that Mg-doped CuCrO2 films are promising for optoelectronic device applications.

Spin Dependent Pump Probe Reflection Spectroscopy in Bulk InP Crystal

The change of the reflectivity signal is modeled by taking into account of the spin dependent band filling and band gap renormalization effects with the carriers density from 5×1016/cm3to 2×1017/cm3. The calculated results show the reflectivity signal is a very sensitive function of the carrier density. The most interesting feature is that the reflectivity change from photo-bleaching into photo-absorption enhancement is observed with increasing pump photon energy. The theoretical predictions obtained in this work are consistent with the corresponding experiments.

Temperature-dependent exciton-related transition energies mediated by carrier concentrations in unintentionally Al-doped ZnO films

The authors reported on a carrier-concentration mediation of exciton-related radiative transition energies in Al-doped ZnO films utilizing temperature-dependent (TD) photoluminescence and TD Hall-effect characterizations. The transition energies of free and donor bound excitons consistently change with the measured TD carrier concentrations. Such a carrier-concentration mediation effect can be well described from the view of heavy-doping-induced free-carrier screening and band gap renormalization effects. This study gives an important development to the currently known optical properties of ZnO materials.

Large photoluminescence redshift of ZnTe nanostructures: The effect of twin structures

In this work, ZnTe single crystalline nanowires and twinning nanobelts were synthesized to investigate the relationship between microstructure and optical properties. Power-dependent photoluminescence measurements reveal that the emission peaks of the single crystalline nanowires redshift with increasing the excitation power as a result of the band-gap renormalization effect. However, the photoluminescence redshift of the twinning nanobelts is larger than that of the nanowires, which is caused by both band-gap renormalization effect and electron–hole plasma effect induced by the increase of carrier density in the twin structure at high excitation power.

Unusual near-band-edge photoluminescence at room temperature in heavily-doped ZnO:Al thin films prepared by pulsed laser deposition

Room temperature photoluminescence (PL) properties of heavily-doped ZnO:Al thin films (with carrier concentration n in the range of 5–20 × 1020 cm−3) prepared by pulsed laser deposition have been investigated. Despite their high carrier concentration, the films exhibited strong room temperature near-band-edge bound excitons at ∼3.34 eV and an unusual peak at ∼3.16 eV, and negligible deep-level emission even for the films deposited at a temperature as low as 25 °C. The radiative efficiency of the films increased with growth temperature as a result of increased n and improved crystallinity. A large blue shift of optical band gap was observed, which is consistent with the n-dependent Burstein–Moss and band gap-renormalization effects. Comparison of the results of the PL and optical measurements revealed a large Stokes shift that increased with increase in n. It has been explained by a model based on local potential fluctuations caused by randomly-distributed doping impurities.

Dopant-induced band filling and bandgap renormalization in CdO : In films

The effect of carrier concentration on the Fermi level and bandgap renormalization in over 30 indium-doped cadmium oxide (CdO : In) films with carrier concentrations ranging from 1 to 15 × 1020 cm−3 was studied using the two-band k · p model with electron–electron and electron–ion interactions. It is shown that the Tauc relation, which is based on parabolic valence and conduction bands, overestimates the optical bandgap in the CdO films. Theoretical calculations of the optical bandgap give good agreement with experiments by taking into account the Burstein–Moss effect for a nonparabolic conduction band and bandgap renormalization effects. The band filling and bandgap renormalization in these CdO : In films are about 0.5–1.2 eV and 0.1–0.3 eV, respectively.

Structural, optical, and electrical properties of indium-doped cadmium oxide films prepared by pulsed filtered cathodic arc deposition

Indium-doped cadmium oxide (CdO:In) films were prepared on glass and sapphire substrates by pulsed filtered cathodic arc deposition (PFCAD). The effects of substrate temperature, oxygen pressure, and an MgO template layer on film properties were systematically studied. The MgO template layers significantly influence the microstructure and the electrical properties of CdO:In films, but show different effects on glass and sapphire substrates. Under optimized conditions on glass substrates, CdO:In films with thickness of about 125 nm showed low resistivity of 5.9 × 10−5 Ωcm, mobility of 112 cm2/Vs, and transmittance over 80 % (including the glass substrate) from 500 to 1500 nm. The optical bandgap of the films was found to be in the range of 2.7 to 3.2 eV using both the Tauc relation and the derivative of transmittance. The observed widening of the optical bandgap with increasing carrier concentration can be described well only by considering bandgap renormalization effects along with the Burstein–Moss shift for a nonparabolic conduction band.

Investigation of large Stark shifts in InGaN/GaN multiple quantum wells

Photoluminescence (PL) spectra of InGaN/GaN multiple quantum wells excited by ultrafast laser pulses are investigated over broad ranges of excitation levels and temperatures. The PL peak energy undergoes blue, red, zero, and blue shifts with increasing the excitation fluence density. Such a peculiar behavior can be explained based on competing processes of screening of the built-in electric field by the photogenerated carriers, band-gap renormalization, and band-filling effect. We have also measured and analyzed the dependence of the PL energy and linewidth on the temperature. Due to the interplay between the band-gap renormalization and band-filling effect, the PL energy shifts to the highest value, whereas the PL linewidth reaches the minimum value at ≈60 K.

Cathodoluminescence studies of GaN coalesced from nanopyramids selectively grown by MOVPE

Coalescence of GaN over arrays of GaN nanopyramids has important device applications and has been achieved on nano-imprint lithographically patterned GaN/sapphire substrates using metal organic vapour phase epitaxy. Spatially and spectrally resolved cathdoluminescence (CL) from such coalesced layers are studied in detail. The observed redshift of the GaN band edge emission with increasing electron beam depth of maximum CL into the coalesced layer is discussed in relation to a carrier-induced peak shift, likely due to Si out-diffusion from the mask material into the GaN. Depth-resolved CL measurements are used to quantify the redshift in terms of bandgap renormalization and strain effects. CL maps showing the GaN near band edge peak energy distribution reveal micron-scale domain-like variations in peak energy and are attributed to the effects of local strain.

Effects of doping and indium inclusions on the structural and optical properties of InN thin films grown by MOCVD

High-resolution x-ray diffraction (HRXRD), Raman scattering and photoluminescence (PL) are used to investigate the structural and optical properties of InN thin films grown by metalorganic chemical vapour deposition (MOCVD). They reveal that the phonon frequency ϖ (in cm−1) of varies with residual biaxial strains εxx (in %) induced by Mg doping, Zn doping and In inclusions as ϖ = 488.9–43.9 × εxx, relating to a Raman factor of 10.8 cm−1 GPa−1. They also reveal a strong correlation between PL energies and electron concentrations due to bandgap renormalization and band-filling effects. The evolution of HRXRD, due to chemical etching, provides evidence that the In inclusions not only appear on the surface and/or at grain boundaries but are also embedded in the InN crystals. Optical absorptions and PL spectra exhibit no apparent changes upon chemical etching, confirming the absence or minor effect of coupling on In-related plasmonic resonance and/or interband transitions, especially for those In inclusions present on the surface and/or at the grain boundaries of InN.

Analysis of confinement potential fluctuation and band-gap renormalization effects on excitonic transition in GaAs/AlGaAs multiquantum wells grown on (1 0 0) and (3 1 1)A GaAs surfaces

The competition between confinement potential fluctuations and band-gap renormalization (BGR) in GaAs/AlxGa1−xAs quantum wells grown on [100] and [311]A GaAs substrates is evaluated. The results clearly demonstrate the coexistence of the band-tail states filling related to potential fluctuations and the band-gap renormalization caused by an increase in the density of photogenerated carriers during the photoluminescence (PL) experiments. Both phenomena have strong influence on temperature dependence of the PL-peak energy (EPL(T)). As the photon density increases, the EPL can shift to either higher or lower energies, depending on the sample temperature. The temperature at which the displacement changes from a blueshift to a redshift is governed by the magnitude of the potential fluctuations and by the variation of BGR with excitation density. A simple band-tail model with a Gaussian-like distribution of the density of state was used to describe the competition between the band-tail filling and the BGR effects on EPL(T).

Optical and Electrical Properties of Ga-Doped ZnO Nanorod Arrays Fabricated by Thermal Evaporation

Well-aligned Ga-doped ZnO nanorod arrays with high optical and electrical property were fabricated by catalyst-free thermal evaporation on p-silicon substrate. As the Ga/Zn atom ratio in the source material was tuned from 0 to 0.2, wurtzite structure ZnO nanorod arrays were realized with length of -6 microm and growth direction along c-axis. With the addition of Ga, the intensity of the near-band-edge emission was enhanced and the deep-level emissions maintained neglectable. As the Ga/Zn atom ratio increased from 0 to 0.1, the red shift of the near-band-edge emission occurred due to Ga-doping induced band gap renormalization effect related with the enhancement of the carrier density, while the blue shifts of the emission were found once the Ga/Zn ratio is higher than 0.1 resulting from Burstein-Moss effect. The configuration of the vertical-aligned Ga-doped ZnO nanorod arrays on p-Si substrate makes it straightforward for the fabrication of p-n nanodiode, which shows an excellent rectifying characteristic with threshold voltage as low as -4.7 V with the Ga/Zn atomic ratio of 0.2.

Spectral dependence of transient reflectance in a ZnO epitaxial film at room temperature

We demonstrated the spectral dependence of time-resolved reflectance in a ZnO epitaxial film at room temperature. The ultrafast thermalization time increases as increasing the excitation photon energy accompanied with the hot phonon effect. The obtained recovery time of renormalized bandgap, associated with the nonradiative decay or the free exciton formation, is independent of photon energy. Based on a theoretical model to calculate the carrier-induced change of refractive index in ZnO, the spectral dependent transient reflectance can be successfully analyzed as a result of combined effects of band-filling (BF) and bandgap renormalization (BGR). The measured transient reflectance decreases with decreasing the photon energy for excitation above the bandgap states revealing the significance of competition of the BF and BGR effects.

Electron spin coherence and effect of spin polarization on electron relaxation dynamics in GaAs

Time-resolved circularly and linearly polarized pump-probe spectroscopy is used to study the evolution of the electron spin coherence and electron relaxation dynamics in bulk GaAs at 9.6 K. In particular, their dependence on photon energy (or electron excess energy) is carefully investigated. The absorption quantum beats which are observed in circularly polarized pump-probe spectroscopy are obtained, reflecting the dephasing of the electron spin coherence. A circularly dichromatic pump-probe model is developed with both the spin-polarization-dependent band-filling and band-gap renormalization effects being taken into account. The model is used to simulate the differential transmission spectra for the collinearly polarized, co-helicity circularly polarized and cross-helicity circularly polarized pump-probe configurations, respectively. It is found that the model simulates well the features of the absorption quantum beats for a spin-dependent thermalized distribution of the photocreated carriers by a circularly polarized pump pulse, such as the variation of the oscillatory amplitude and phase reversal of the absorption quantum beats with photon energy increase. The simulation is in good agreement with our experimental results and reveals the effect of spin polarization on electron relaxation dynamics.