Burstein-Moss Shift Research Articles

Temperature Dependent Structural and Optical Properties of Nanostructured Cr Doped CdO Thin Films Prepared by DC Reactive Magnetron Sputtering

Nanostructured Cr doped CdO thin films were deposited by DC reactive magnetron sputtering technique on glass substrates at different substrate temperatures. The structural, electrical and optical properties of Cr doped CdO films were investigated. From XRD data, all the films are highly oriented in the (2 0 0) direction. Surface morphology of the samples has been studied using FESEM and the elemental composition of the films is determined from EDS spectra. The minimum resistivity of 1.92 × 10-4Ω.cm is obtained for the thin film deposited at substrate temperature of 350°C. The optical transmittance of Cr doped CdO thin films increases with the increase of substrate temperature from room temperature to 350°C. The optical absorption edge of the films is shifted towards lower wavelengths with increase in the substrate temperature. The optical direct band gap values of Cr doped CdO films increased with the increase of substrate temperature. The increase in optical band gap may be due to improvement in crystallinity of the films and Burstein-Moss shift.

Mn-Doped ZnO Nanoparticles Prepared by Solution Combustion Method

In this paper, we have reported the structural, morphological and optical studies of pure and Mn-doped ZnO nanoparticles synthesized by the simple solution combustion method. The structural, morphological and optical studies are carried out by powder XRD, FE-SEM, HR-TEM, Raman, UV-vis absorption and PL spectra. The XRD pattern indicates that the prepared particles are in hexagonal wurtzite structure with the crystalline size of around 30-60 nm. The FE-SEM and HR-TM images are coincide with each other for aggregation of particles nature. The elemental analyses of pure and doped samples are evaluated by EDX. Raman spectra of the samples show remarkable result in the polar and non-polar branches. Room temperature PL shows the near band edge related emission and the results are related several intrinsic defects in the Mn-doped ZnO nanoparticles. Blue shift of the UV emission has occurred in all the doped samples attributable to Burstein-Moss shift. [DOI: 10.1380/ejssnt.2014.283]

Effect on the electronic structures and optical bandgaps of Ga-doped wurtzite TM0.125Zn0.875O(TM=Be, Mg)

The optimized structure parameters, electron density of states, energy band structures and optical bandgaps of the TM0.125Zn0.875O (TM=Be, Mg) alloys and Ga-doped TM0.125Zn0.875O are calculated and analyzed by using the ultra-soft pseudopotential approach of the plane-wave based upon density functional theory. The theoretical results show the Ga-doped TM0.125Zn0.875O materials are easily obtained and their structures are more stable. The Ga-doped TM0.125Zn0.875O are good n-type materials and their energy bandgaps are determined by Ga 4s states of the conduction band minimum and O 2p states of the valence band maximum. Compared with the TM0.125Zn0.875O alloys, the optical bandgaps of Ga-doped TM0.125Zn0.875O become wider due to the Burstein-Moss shift and many-body effects, which is consistent with previous experimental data. The Ga-doped TM0.125Zn0.875O materials are suitable as TCO films for the UV and deep UV optoelectronic device.

The analysis of magnetic controlled DC sputtering AZO films

The Al-doped zinc oxide (AZO) transparent conducting thin films were deposited by the use of magnetic controlled DC sputtering method. To obtain best process parameters, we compare and analyse different process parameters like working pressure and substrate temperature, and properties of AZO films with DC sputtering. The transmittance of AZO film is over 85% at visible region (380~800 nm). Blue shift effect was observed clearly within 350~390 nm, as the substrate temperature increases from 225oC to 325oC. This is primarily due to the Burstein-Moss shift effect. The FWHM of (002) orientation peak of the XRD decreased with increasing substrate temperature while the grain size, carrier concentration and carrier mobility increased with increasing substrate temperature. The resistivity decreases with the operating temperature due to the combined action of increasing carrier concentration and carrier mobility, and has a minimum resistivity of 0.34 mΩ-cm. The X-ray diffraction peak intensity increases with the increasing substrate temperature. As the substrate temperature increases from 225oC to 325oC, the increasing X-ray diffraction peak intensity of (002) is primarily due to the crystallisation of the AZO film. The crystalline size of various AZO films, as can be evaluated by Scherrer's Formula, is around 28–43 nm.

Microstructural, optical, and electrical properties of Ni–Al co-doped ZnO films prepared by DC magnetron sputtering

Ni–Al co-doped ZnO (NiAl:ZnO) films with fixed Al content at 2wt% and different Ni contents (2.5, 3, and 5wt%) were deposited by DC magnetron sputtering in an argon atmosphere at room temperature. X-ray diffraction revealed that all films showed a highly preferential (002) c-axis orientation. XPS revealed the presence of metallic Ni, NiO, and Ni2O3 states, and Ni atoms were successfully doped in NiAl:ZnO films, which did not result in a change in ZnO crystal structure and orientation. The electrical resistivity of NiAl:ZnO film was decreased to 2.59×10−3Ωcm at a Ni doping concentration of 3wt% compared with undoped Al-doped ZnO film (5.58×10−3Ωcm). The mean optical transmittance in the visible range was greater than 80% for all films. Band gap widening (4.18eV) was observed in the NiAl:ZnO films with 5wt% Ni, attributed to the Burstein–Moss shift due to the increase of carrier concentration.

Electrical and optical properties of transparent conducting InxGa1−xN alloy films deposited by reactive co-sputtering of GaAs and indium

Thin films of InxGa1–xN alloys were deposited by reactive sputtering using a GaAs target, covered partially with indium and co-sputtered with nitrogen. X-ray and electron diffraction studies indicate the formation of single phase InxGa1–xN films at ~500°C. Hall effect and resistivity measurements show that the alloy films with x≥0.5 have high carrier concentrations in the range of 1020–1021cm−3 and mobility of ~10cm2V−1s−1. Optical measurements of the alloy films show a strong dependence of the band gap on carrier concentration, which is attributed to the Burstein–Moss shift and free carrier effects in the near-infrared region. The values of electron effective mass obtained from plasma resonance data and the Burstein–Moss shift show good agreement. Over a limited composition window of x in the range of 0.5–0.6, the alloy films exhibit low electrical resistivity (≈10−3Ω-cm) and high transparency in part of the visible and near infrared regions, followed by high reflectance in the infrared region, which show their potential for applications as transparent electrodes in photovoltaic and photonic devices and as heat mirrors.

Ge doped GaN with controllable high carrier concentration for plasmonic applications

Controllable Ge doping in GaN is demonstrated for carrier concentrations of up to 2.4 × 1020 cm−3. Low temperature luminescence spectra from the highly doped samples reveal band gap renormalization and band filling (Burstein-Moss shift) in addition to a sharp transition. Infrared ellipsometry spectra demonstrate the existence of electron plasma with an energy around 3500 cm−1 and a surface plasma with an energy around 2000 cm−1. These findings open possibilities for the application of highly doped GaN for plasmonic devices.

Influence of Fe dopant concentration and annealing temperature on the structural and optical properties of ZnO thin films deposited by sol–gel method

Nanostructured Fe doped ZnO thin films were deposited onto glass substrates by sol–gel spin coating method. Influence of Fe doping concentration and annealing temperature on the structural, compositional, morphological and optical properties were investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV–Vis spectroscopy and photoluminescence (PL) measurements. XRD analysis showed that all the films prepared in this work possessed a hexagonal wurtzite structure and were preferentially oriented along the c-axis. Pure ZnO thin films possessed extensive strain, whereas Fe doped films possessed compressive strain. In the doped films, least value of stress and strain was observed in the 0.5 at.% Fe doped thin film, annealed at 873 K. Average crystallite size was not significantly affected by Fe doping, but it increased from 15.57 to 17.79 nm with increase in annealing temperature from 673 to 873 K. Fe ions are present in +3 oxidation state as revealed by XPS analysis of the 0.5 at.% Fe doped film. Surface morphology is greatly affected by changes in Fe doping concentration and annealing temperature which is evident in the SEM images. The increase in optical band gap from 3.21 to 3.25 eV, with increase in dopant concentration was attributed to Moss–Burstein shift. But increase in annealing temperature from 673 to 873 K caused a decrease in band gap from 3.22 to 3.20 eV. PL spectra showed emissions due to excitonic combinations in the UV region and defect related emissions in the visible region in all the investigated films.

Effect of Mn doping on the structural, optical, and magnetic properties of In2O3 films

(In1−xMnx)2O3 films were grown by radio frequency-magnetron sputtering technique. Effect of Mn doping on the structural, optical, and magnetic properties of films is investigated systematically. The detailed structure analyses suggest that Mn ions substitute for In3+ sites of the In2O3 lattice in the valence of +2 states, and Mn-related secondary phases or clusters as the source of ferromagnetism is safely ruled out. All films show typical room temperature ferromagnetism. The saturation magnetization Ms increases first, and then decreases, while carrier concentration nc decreases monotonically with Mn doping, implying that the ferromagnetism is not directly induced by the mediated carriers. The optical bandgap Eg of films decreases monotonically with the increase of Mn concentration, and there exists a linear functional dependence between Eg and nc2/3, which is consistent with Burstein-Moss shift arguments. It can be concluded that the ferromagnetic order in Mn-doped In2O3 films is intrinsic, arising from Mn atoms substitution for the In sites of In2O3 lattice. The oxygen vacancies play a mediation role on the ferromagnetic couplings between the Mn ions.

Optical properties of Si-doped and Be-doped InAlAs lattice-matched to InP grown by molecular beam epitaxy

In this paper, we determine the optical constants and carrier mobilities of Si-doped and Be-doped InAlAs lattice matched to InP. The samples were grown using molecular beam epitaxy and characterized using Hall measurements, variable angle spectroscopic ellipsometry, and room temperature photoluminescence spectroscopy. A Moss-Burstein shift in the fundamental absorption edge was observed in both Si-doped and Be-doped materials. We fitted a multiple-oscillator, critical point model to the dielectric function of the materials extracted using the spectroscopic ellipsometry. The tabulated input parameters of this model allow for accurate calculations of the dielectric function of doped InAlAs to be made, which is useful information for simulating a variety of InP-based optoelectronic devices.

Germanium doping of self-assembled GaN nanowires grown by plasma-assisted molecular beam epitaxy

Germanium doping of GaN nanowires grown by plasma-assisted molecular beam epitaxy on Si(111) substrates is studied. Time of flight secondary ion mass spectrometry measurements reveal a constant Ge-concentration along the growth axis. A linear relationship between the applied Ge-flux and the resulting ensemble Ge-concentration with a maximum content of 3.3×1020 cm−3 is extracted from energy dispersive X-ray spectroscopy measurements and confirmed by a systematic increase of the conductivity with Ge-concentration in single nanowire measurements. Photoluminescence analysis of nanowire ensembles and single nanowires reveals an exciton localization energy of 9.5 meV at the neutral Ge-donor. A Ge-related emission band at energies above 3.475 eV is found that is assigned to a Burstein-Moss shift of the excitonic emission.

Effects of H<sub>2</sub> Flow Rate and H<sub>2</sub> Plasma Treatment on the Properties of AZO Films

In this study, a 1350oC-sintered 98 mol% ZnO-1 mol% Al2O3 (AZO, Zn:Al= 98:2) ceramic was used as a target and deposited on glass using a r.f. magnetron sputtering system at a deposition temperature of 200°C. The effects of different H2 flow rates (H2/(H2+Ar)=0% ~ 9.09%, abbreviated as H2-deposited AZO films) added during the deposition process on the crystallization, resistivity, and optical transmission spectrum of AZO films were investigated. The Burstein-Moss shift effects were measured and used to prove that the defects of AZO films decreased with increasing H2 flow rate. For comparison, the 2% H2-deposited AZO films were also treated by the H2 plasma at room temperature for 60 min (plasma-treated AZO films). The effects of H2 plasma on the properties of the H2-deposited AZO films were also studied. The value variations in the optical band gap (Eg) of the H2-deposited and plasma-treated AZO films were evaluated from the plots of (αhv)2 = c (hν-Eg).

Electronic- and band-structure evolution in low-doped (Ga,Mn)As

Modulation photoreflectance spectroscopy and Raman spectroscopy have been applied to study the electronic- and band-structure evolution in (Ga,Mn)As epitaxial layers with increasing Mn doping in the range of low Mn content, up to 1.2%. Structural and magnetic properties of the layers were characterized with high-resolution X-ray diffractometry and SQUID magnetometery, respectively. The revealed results of decrease in the band-gap-transition energy with increasing Mn content in very low-doped (Ga,Mn)As layers with n-type conductivity are interpreted as a result of merging the Mn-related impurity band with the host GaAs valence band. On the other hand, an increase in the band-gap-transition energy with increasing Mn content in (Ga,Mn)As layers with higher Mn content and p-type conductivity indicates the Moss-Burstein shift of the absorption edge due to the Fermi level location within the valence band, determined by the free-hole concentration. The experimental results are consistent with the valence-band origin of mobile holes mediating ferromagnetic ordering in the (Ga,Mn)As diluted ferromagnetic semiconductor.

Controlling Localized Surface Plasmon Resonances in GeTe Nanoparticles Using an Amorphous-to-Crystalline Phase Transition

Infrared absorption measurements of amorphous and crystalline nanoparticles of GeTe reveal a localized surface plasmon resonance (LSPR) mode in the crystalline phase that is absent in the amorphous phase. The LSPR mode emerges upon crystallization of amorphous nanoparticles. The contrasting plasmonic properties are elucidated with scanning tunneling spectroscopy measurements indicating a Burstein-Moss shift of the band gap in the crystalline phase and a finite density of electronic states throughout the band gap in the amorphous phase that limits the effective free carrier density.

Effects of deposition temperature and hydrogen flow rate on the properties of the Al-doped ZnO thin films and amorphous silicon thin-film solar cells

A compound of 98 mol% ZnO and 1 mol% Al2O3 (AZO, Al:Zn = 98:2) was sintered at 1350 °C as a target and the AZO thin films were deposited on glass using a radio frequency magnetron sputtering system. The effects of deposition temperature (from room temperature to ∼300 °C) on the optical transmission spectrum of the AZO thin films were studied. The Burstein–Moss shift was observed and used to prove that defects in the AZO thin films decreased with increasing deposition temperature. The variations in the optical band gap (Eg) values of the AZO thin films were evaluated from plots of (αhv)2=c(hν−Eg), revealing that the measured Eg values increased with increasing deposition temperature. The effects of the H2 flow rate during deposition (0 %∼11.76 %, deposition temperature of 200 °C) on the crystallization, morphology, resistivity, carrier concentration, carrier mobility, and optical transmission spectrum of the AZO thin films were measured. The chemical structures of the Ar-deposited and 2 % H2-flow rate-deposited AZO thin films (both were deposited at 200 °C) were investigated by XPS to clarify the mechanism of improvement in resistivity. The prepared AZO thin films were also used as transparent electrodes to fabricate amorphous silicon thin-film solar cells, and their properties were also measured.

Optical band gap and the Burstein–Moss effect in iodine doped PbTe using diffuse reflectance infrared Fourier transform spectroscopy

Optical absorption edge measurements are performed on I doped PbTe using diffuse reflectance infrared Fourier transform spectroscopy. The Burstein–Moss shift, an increase in the absorption edge (optical band gap) with increasing doping level, is explored. The optical gap increases on the order of 0.1 eV for doping levels ranging from 3 × 1018 to 2 × 1020 cm−3, relevant doping levels for good thermoelectric materials. Chemical potential is estimated from transport measurements—specifically, Hall effect and Seebeck coefficient—using a single band Kane model. In heavily doped semiconductors, it is well-known that the band gap shrinks with increasing doping level. This effect, known as band gap renormalization, is fit here using an n1/3 scaling law which reflects an electron–electron exchange interaction. The renormalization effect in these samples is shown to be more than 0.1 eV, on the same order of magnitude as the band gap itself. Existing models do not explain such large relative changes in band gap and are not entirely self-consistent. An improved theory for the renormalization in narrow gap semiconductors is required.

Structural and optical properties of pure and Al doped ZnO nanocrystals

Pure and Al doped zinc oxide (ZnO) were prepared by co-precipitation method. The dopant concentration [Al/Zn in atomic percentage (wt%)] was varied from 0 to 3 wt%. Structural characterisation of the samples performed with XRD and SEM–EDAX confirmed that polycrystalline nature of samples containing ZnO nanoparticles of size in the range of 97–47 nm. UV–Vis studies showed that the absorbance peaks, observed in the wavelength range of 800–250 nm, decreased with the increase in dopant concentration indicating widening of the band gap. The calculations of band gap (analyzed in terms of Burstein–Moss shift) from the reflectance showed an increase from 3.37 to 3.49 eV with increasing Al concentration.

Impact of laser excitation variations on the photoluminescence of PbS quantum dots on GaAs

Gaussian shaped photoluminescence of colloidal 2nm PbS quantum dots on GaAs were measured in the temperature range of 5K to 300K by exciting the dots with varying continuous laser intensities (6W/cm2, 18W/cm2, and 60W/cm2) at 532nm. The work reveals the sensitivity of quantum dot emissions to optical pumping variations. The classical Fan theory describes the emission shift vs. temperature at the lowest excitation intensity, while the clearly altered extrinsic behavior at elevated excitation intensities was fitted with an empirical expression. This extrinsic character is attributed to electronic alterations (Burstein–Moss shift and many body effects) of the absorptive and radiative transition energies.

Effects of hydrogen flow on properties of hydrogen doped ZnO thin films prepared by RF magnetron sputtering

The hydrogen doped ZnO (ZnO:H) thin films were deposited on quartz glass substrates by radio frequency magnetron sputtering. The doping characteristics of ZnO:H thin films with varied hydrogen flow ratio were investigated. At low hydrogen flow ratio (H2/(H2+Ar)≤0.02), the ZnO:H thin films exhibited dominant (002) peaks from X-ray diffraction and the lattice constants became smaller. The particles were mainly a columnar structure. The particles’ size became smaller, and the island-like structure appeared on the thin films surface. In addition, the low resistivity properties of ZnO:H thin films was ascribed to the increase of the carriers concentration and carriers mobility; When the hydrogen flow ratio was more than 0.02 (M≥0.02), two absorption bands at 1400–1800 cm−1 and 3200–3900 cm−1 were observed from the FT-IR spectra, which indicated that the ZnO:H thin films had typical Zn–H bonding, O–H bonding (hydroxyl), and Zn–H–O bonding (like-hydroxyl). The scanning electron microscope (SEM) results show that a large number of hydroxyl agglomeration formed an island-like structure on the thin films surface. The absorption peak at about 575 cm−1 in the Raman spectra indicated that oxygen vacancies (VO) defects were produced in the process of high hydrogen doping. In this condition, the low resistivity properties of ZnO:H thin films were mainly due to the increasing electron concentration resulted from VO. Meanwhile, the Raman absorption peaks at approximately 98 cm−1 and 436 cm−1 became weaker, and the (002) XRD diffraction peak quenched and the lattice constants increased, which shows that the ZnO:H thin films no longer presented a typical ZnO hexagonal wurtzite structure. With the increasing of hydrogen flow ratio, the optical transmittance of ZnO:H thin films in the ultraviolet band show a clear Burstein–Moss shift effect, which further explained that electron concentration was increased due to the increasing VO with high hydrogen doping concentration. Moreover, the optical reflectance of the thin films decreased, indicating the higher roughness of the films surface. It was noteworthy that etching effect of H plasma was obvious in the process of heavy hydrogen doping.

Influence of doping level on shift of the absorption edge of gallium nitride films (Burstein-Moss effect)

In this ellipsometric study it was revealed possibility of observation of the absorption edge shift and changing of its slope for gallium nitride films doped by silicon. It seems to be attractive to connect specific features of Burstein-Moss shift with structural and electrical properties of films of gallium nitride in future. As well known the Burstein-Moss effect occurs when the carrier concentration exceeds conduction band edge density of states, which corresponds to degenerate doping in semiconductors.