Bandgap Narrowing Effect Research Articles

Zinc interstitial threshold in Al-doped ZnO film: Effect on microstructure and optoelectronic properties

In order to know the threshold quantity of the zinc interstitials that contributes to an increase in carrier concentration in the Al-doped ZnO (AZO) films and their effect on the overall microstructure and optoelectronic properties of these films, in this work, Zn-rich-AZO and ZnO thin films are fabricated by adding excess zinc (from a zinc metallic target) during their deposition in RF magnetron sputtering and are then investigated using a wide range of experimental techniques. All these films are found to grow in a ZnO hexagonal wurtzite crystal structure with strong (002) orientation of the crystallites, with no indication of Al2O3, metallic Zn, and Al. The excessively introduced zinc in these AZO and/or ZnO films is found to increase the shallow donor level defects (i.e., zinc interstitials and oxygen-related electronic defect states), which is found to significantly increase the carrier concentration in these films. Additionally, aluminum is seen to enhance the creation of these electronic defect states in these films, thereby contributing more to the overall carrier concentration of these films. However, carrier mobility is found to decrease when the carrier concentration values are higher than 4 × 1020 cm−3, because of the electron-electron scattering. Whereas the optical band gap of the ZnO films is found to increase with increasing carrier concentration because of the Burstein-Moss shift, these decrease for the AZO films due to the band gap narrowing effect caused by excess carrier concentration.

Side effects of the strain-doping approach to develop optical materials based on Ge

Following the strain-doping approach for development of Ge based optical gain material, we have studied the impact of doping and strain on the optical properties of Germanium. Emphasizing the importance of the bandgap narrowing effect due to doping on the emission wavelength, we have computed a strain-doping-energy maps, which provide the strain and doping windows that can be considered in order to achieve a specific value of the Γ bandgap. Finally, we discuss the polarization of the emitted light, and its dependence on strains.

InGaN/GaN micro mirror with electrostatic comb drive actuation integrated on a patterned silicon-on-insulator wafer.

A double-sided mask process is used to define the micro actuators on a silicon-on-insulator (SOI) wafer. Then, an InGaN/GaN multilayer film was deposited on the surface of the above patterned SOI substrate by employed MBE technology. Thus, the final device consisting of comb-drive actuators, springs and micro-mirror is implemented to obtain the two-dimensional (2D) tuning effect of the mirror. The displacement response is characterized by applying voltage to the micro actuators. It shows an approximation linear relationship between the displacement and the square of applied voltage. And the influences of the cross-axis coupling effect are also measured and evaluated. Moreover, the luminous performances of the InGaN/GaN multiquantum well films were also studied by analyzing temperature-dependent and laser intensity-dependent PL spectra. The authors observed the localized state effect and/or band-gap narrowing effect which results in an S-shaped behavior with increasing temperature. The PL intensity and wavelength are also significantly affected by the excitation energy of laser.

Large bandgap narrowing in rutile TiO2 aimed towards visible light applications and its correlation with vacancy-type defects history and transformation

Extension of photoactivity of TiO2 to the visible region is achievable via effective control over the intrinsic defects such as oxygen and Ti vacancies, which has several applications in visible photocatalysis and sensing. We present here the first observation of an apparent bandgap narrowing and bandgap tuning effect due to vacancy cluster transformation in rutile TiO2 structures to 1.84 eV from the bulk bandgap of 3 eV. A gradual transformation of divacancies (VTi–O) to tri vacancies () achieved through a controlled solvothermal scheme appears to result in an apparent narrowing bandgap and tunability, as supported by positron annihilation lifetime and electron paramagnetic resonance spectroscopy measurements. Visible photocatalytic activity of the samples is demonstrated in terms of photodegradation of rhodamine B dye molecules.

Effective bandgap narrowing of Cu–In–Zn–S quantum dots for photocatalytic H2 production via cocatalyst-alleviated charge recombination

Effective bandgap narrowing of Cu–In–Zn–S quantum dots is achieved with increased tolerance of Cu from the cocatalyst-alleviated charge recombination.

Quantification of Sheet Resistance in Boron‐Diffused Silicon Using Micro‐Photoluminescence Spectroscopy at Room Temperature

A micro‐photoluminescence‐based technique is presented, to quantify and map sheet resistances of boron‐diffused layers in silicon solar cell precursors with micron‐scale spatial resolution at room temperature. The technique utilizes bandgap narrowing effects in the heavily‐doped layers, yielding a broader photoluminescence spectrum at the long‐wavelength side compared to the spectrum emitted from lightly doped silicon. By choosing an appropriate spectral range as a metric to assess the doping density, the impacts of photon reabsorption on the analysis can be avoided; thus, an accurate characterization of the sheet resistance can be made. This metric is demonstrated to be better representative of the sheet resistance than the surface doping density or the total dopant concentration of the diffused layer. The technique is applied to quantify sheet resistances of 12‐μm‐wide diffused fingers in interdigitated back‐contact solar cell precursors and large diffused areas. The results are confirmed by both 4‐point probe and time‐of‐flight secondary‐ion mass spectrometry measurements. Finally, the practical limitations associated with extending the proposed technique into an imaging mode are presented and explained.

Self-Assembled Heteroepitaxial AuNPs/SrTiO3: Influence of AuNPs Size on SrTiO3 Band Gap Tuning for Visible Light-Driven Photocatalyst

Self-assembled heteroepitaxial offers tremendous opportunity to tailor optical and charge transport properties in noble metal–semiconductor interface. Here, we incorporated gold nanoparticles (AuNPs) onto the {001} facets of semiconductor strontium titanate, SrTiO3 (STO), by means of heteroepitaxial approach to investigate the band gap tuning and its effect of photoresponse. We demonstrate that the Fermi energy level of the system can be tuned by controlling the AuNPs size. X-ray photoelectron spectroscopy (XPS) shows that the energy difference between Sr3d and Au4f core levels measured in the AuNPs/STO (100) heterojunction increases from 47.90 to 49.26 eV with decreasing AuNPs size from 65 to 16 nm, respectively. Hence, the Fermi energy level was shifted toward the conductive band of STO (100), and the system charge transfer efficiency was improved. It was also found that smaller AuNPs sizes exhibited a higher photoactivity as the result of the band gap narrowing effect. Photoactivity was improved by bro...

Direct bandgap photoluminescence from n-type indirect GaInP alloys

This work studies Te doping effects on the direct bandgap photoluminescence (PL) of indirect GaxIn1−xP alloys (0.72≤x≤0.74). The temperature-dependent PL shows that the energy difference between direct Γ valley and indirect X valleys is reduced due to the bandgap narrowing (BGN) effect, and the direct band transition gradually dominates the PL spectra as temperature increases. Carrier thermalization has been observed for Te-doped GaxIn1−xP samples, as integrated PL intensity increases with increasing temperature from 175 to 300 K. The activation energy for carrier thermalization is reduced as doping concentration increases. Both BGN effect and carrier thermalization contribute to the carrier injection into the Γ valley. As a result, the direct band transition is enhanced in the Te-doped indirect GaxIn1−xP alloys. Therefore, the PL intensity of the Ga0.74In0.26P sample with active doping concentration of 9×1017 cm−3 is increased by five times compared with that of a nominally undoped sample. It is also found that the PL intensity is degraded significantly when the doping concentration is increased to 5×1018 cm−3. From cross-section transmission electron microscopy, no large dopant clusters or other extended defects were found contributing to this degradation.

Subgap State Engineering Using Nitrogen Incorporation to Improve Reliability of Amorphous InGaZnO Thin-Film Transistors in Various Stressing Conditions

Instability of amorphous InGaZnO thin-film transistors (a-IGZO TFTs) remains an obstacle for commercialization. Here, we systematically discuss the effect of nitrogen incorporation on a-IGZO TFT stability and developed Ar/O2/N2 atmosphere to improve the stability under stressing in different conditions. Based on X-ray photoelectron spectrometer results, it is revealed that the positive gate bias stress (PGBS) stability is significantly improved due to microscopically passivated metal–oxygen bonds. Yet, the negative gate bias and light stress (NBLS) stability is seriously deteriorated with heavily nitrogen incorporation probably due to the bandgap narrowing effect. By optimizing a mixed O2/N2 atmosphere, the subgap states are finely tuned to afford optimal performance and stability. The developed IGZO TFTs exhibit mobility (12.67 cm2/Vs), small shift of threshold voltage under PGBS (reduced by 64% as compared with the pristine a-IGZO TFTs), and good negative gate bias stability and with NBLS stability as well.

Optoelectronic properties of Mg doping GaN nanowires

To investigate the influences of Mg doping and different doping sites on the electronic structures and optoelectronic properties of GaN nanowires, models of GaN, GaN(Mg)–Sur, GaN(Mg)–Sub and GaN(Mg)–Cor are built. Based on first-principles calculation, the formation energy, atomic structure, electronic structure, and optical properties of GaN nanowires are obtained. For the Mg doped nanowires, Mg atom is easier to substitute the Ga atom in the core of GaN nanowires, named as GaN(Mg)–Cor. Mg atom makes the lattice around the substituted atom distorted. In addition, the bandgap of Mg doped materials is smaller than the undoped, which results from the bandgap narrowing effect. After Mg impurity introducing, there turns up a new peak at 2.5 eV in the imaginary part of dielectric function curves, the absorption curves and the reflection curves. Besides, the other peaks of them shift to lower energy.

Effect of band gap narrowing on GaAs tunnel diode I-V characteristics

We report on experimental and theoretical study of current-voltage characteristics of C/Si-doped GaAs tunnel diode. For the investigation of the experimental data, we take into account the band-gap narrowing (BGN) effect due to heavily-doped sides of the tunnel diode. The BGN of the n- and p-sides of tunnel diode was measured by photoluminescence spectroscopy. The comparison between theoretical results and experimental data reveals that BGN effect enhances tunneling currents and hence should be considered to identify more accurately the different transport mechanisms in the junction. For C/Si-doped GaAs tunnel diode, we found that direct tunneling is the dominant transport mechanism at low voltages. At higher voltages, this mechanism is replaced by the rate-controlling tunneling via gap states in the forbidden gap.

The injected dark current of a p+n and a p+n n+ silicon solar cell taking into account the narrowing of band gap due to heavy doping

In addition to the photocurrent, the dark current of solar cell plays a vital role in determining the overall efficiency of a solar cell. In this paper, the injected dark current of a p + n junction solar cell and that of a p + n n + back-surface-field cell have been studied analytically, taking into account the band gap narrowing effect that exists at high doping concentrations. It is observed that there is a significant difference in the magnitude of the injected dark current obtained when the band gap narrowing effect is taken into consideration as compared to the case when it is not considered. Also, it is observed that the magnitude of the injected dark current decreases for smaller values of back surface recombination velocity, which corresponds to BSF structure. This is one of the reasons why the p + n n + BSF solar cell has better output than the conventional p + n cell. The results obtained here are similar to the case of an n + p p + solar cell reported earlier by researchers.

Narrowing of band gap and effective charge carrier separation in oxygen deficient TiO2 nanotubes with improved visible light photocatalytic activity

Oxygen vacancies are introduced into hydrothermally processed TiO2 nanotube by vacuum calcination. Formation of oxygen vacancies modifies the local coordination in TiO2 as evident from Raman spectroscopy and secondary ion mass spectrometry (SIMS) results. The surface area is increased from 172.5m2/g in pure to 405.1m2/g in defective TiO2 nanotube. The mid-band gap electronic states created by oxygen vacancies are mostly responsible for the effective narrowing of band gap. Charge carrier separation is sufficiently prolonged as the charged oxygen defect states inhibit facile carrier recombination. With high surface area, narrowed band gap and separated charge carriers defective TiO2 nanotube is a suitable candidate in the photodegradation of methylene blue (MB) and phenol under visible light illumination. Photosensitized electron transfer from MB to the conduction band of TiO2 and the photodegradation of MB is facilitated in presence of high density of oxygen vacancies. Unlike MB, phenol absorbs in the UV region and does not easily excited under visible light. Phenol shows activity under visible light by forming charge transfer complex with TiO2. Defect trapped carriers become available at the phenol–TiO2 interface and finally interact with phenol molecule and degrade it.

Characterizing amorphous silicon, silicon nitride, and diffused layers in crystalline siliconsolarcellsusingmicro-photoluminescence spectroscopy

We report and explain the photoluminescence (PL) spectra from crystalline silicon (c-Si) wafers passivated by hydrogenated amorphous silicon (a-Si:H) films under various measurement conditions, utilizing the different absorption coefficients and radiative recombination mechanisms in c-Si and a-Si:H. By comparison with the luminescence properties of a-Si:H, we also demonstrate that SiNx films deposited under certain silicon-rich conditions yield luminescence spectra similar to those of a-Si:H, indicating the presence of an a-Si:H-like phase in the SiNx films. This causes a reduction in the blue response of the solar cells via parasitic absorption. In addition, with the ability to detect the specific emission from heavily-doped silicon via band-gap narrowing effects, we can unambiguously separate individual spectral PL signatures of three different layers in a single substrate: the SiNx passivation films, the diffused layers, and the underlying c-Si substrate. Finally, we apply this technique to evaluate parasitic absorption in the passivation films, and the doping density of the diffused layers on different finished solar cells, highlighting the value of this nondestructive contactless, micron-scale technique for photovoltaic applications.

Unconventional interplay between heterovalent dopant elements: Switch-and-modulator band-gap engineering in (Y, Co)-Codoped CeO2 nanocrystals.

We report the experimental observation and theoretical explanation of an unconventional interplay between divalent Co and trivalent Y dopants, both of which incur oxygen vacancies in the CeO2 host that has predominantly tetravalent Ce cations. The Co dopant atoms were experimentally found to act as a switch that turns on the dormant effect of Y-modulated band-gap reduction. As revealed by density functional theory (DFT) calculations with structures verified by synchrotron-radiation x-ray measurements, a Co 3d band that hybridizes with Ce 4f band was lowered due to reduced O 2p repulsion arising from oxygen vacancies incurred by Y doping and therefore gave rise to the observed band-gap narrowing effect. Such switch-and-modulator scheme for band-gap engineering in nanocrystal materials can lead to important applications in environmental protection and solar energy harvesting technologies.

Band-gap narrowing and magnetic behavior of Ni-doped Ba(Ti0.875Ce0.125)O3 thin films

Band-gap narrowing and magnetic effects have been observed in a Ni-doped Ba(Ti0.875Ce0.125)O3 (BTC) thin film. Structural characterizations and microstructural analysis show that the as-prepared Ba(Ti0.75Ce0.125Ni0.125)O3−δ (BTCN) thin film exhibits a cubic perovskite structure with an average grain size of 25 nm. The Ce doping at the Ti-site results in an increasing perovskite volume to favour an O-vacancy-stabilized Ni2+ substitution. Raman spectroscopy, however, shows the cubic symmetry of crystalline structures is locally lowered by the presence of dopants, significantly deviating from the ideal Pm3m space group. Moreover, BTCN presents a narrowed band-gap, much smaller than that of BaTiO3 and BTC, due to new states of both the highest occupied molecular orbital and the lowest unoccupied molecular orbital in an electronic structure with the presence of Ni. Also, magnetic enhancement driven by co-doping has been confirmed in the films, which mainly stems from the exchange interaction of Ni2+ ions via an electron trapped in a bridging oxygen vacancy. These findings may open an avenue to discover and design optimal perovskite compounds for solar-energy devices and information storage.

A Precision CMOS Voltage Reference Exploiting Silicon Bandgap Narrowing Effect

A compact voltage-mode bandgap voltage reference (BGR) is presented. Instead of using overhead circuits, the silicon bandgap narrowing effect is exploited for bipolar junction transistor's (BJT) curvature reduction and residual curvature correction. Prototype measurements in a 0.18-μm standard CMOS process show that the curvature of the BJT is effectively reduced from its inherent 3.6 mV to 1.4 mV. The proposed BGR measures a minimum temperature coefficient of 8.7 ppm/°C from -55°C to 125°C by batch trimming one resistor. After a curvature trimming, it further improves to 4.1 ppm/°C. The BGR has a minimum supply voltage of 1.3 V, 4.3 μA nominal current consumption, 0.03%/V line sensitivity, and 2 mV/mA load sensitivity at 25°. The output rms noise in the 0.1~10 -Hz band measures 10.23 μV.

Niobium pentoxide (Nb2O5) thin films: rf Power and substrate temperature induced changes in physical properties

In the present work, thin Nb2O5 films were grown on glass substrates using rf magnetron sputtering technique, in a pure argon atmosphere at different rf power and various substrate temperatures. X-ray diffraction result revealed the amorphous nature of deposited films. The maximum optical transmittance of about 92% with clear interference pattern was realized from the optical study. A systematic reduction in energy band gap values with increasing substrate temperature and rf power could be attributed to the localized state near the band edges. This hypothesis is consistent with our photoluminescence study where we observe the slight shift in emission peak towards higher wavelength region due to the effect of optical band gap narrowing. The X-ray photoelectron spectroscopic study confirms the film purity and the presence of five-valent niobium (Nb5+) in Nb2O5 films. Surface morphological studies revealed the formation of Nb2O5 films with smooth, uniform and pinhole free in nature. Raman spectra show the NbO stretching mode of the deposited films.

Magnetism switching and band-gap narrowing in Ni-doped PbTiO3 thin films

Ions doping-driven structural phase transition accompanied by magnetism switching and band-gap narrowing effects has been observed in PbTi1−xNixO3−δ (xPTNO, x = 0.00, 0.06, and 0.33) thin films. With the increase of x, the xPTNO thin films exhibit not only a phase transition from the pseudotetragonal structure to a centrosymmetric cubic structure but also a drastic decrease of grain size. Moreover, the as-grown Ni-doped PbTiO3 (PTO) thin films show obvious room-temperature ferromagnetism and an increased saturation magnetization with increasing the Ni content, in contrast to undoped PTO, which shows diamagnetism. A bound magnetic polaron model was proposed to understand the observed ferromagnetic behavior of PTO-derived perovskite thin films. Furthermore, the 0.33PTNO thin film presents a narrowed band-gap, much smaller than that of PTO, which is attributed to new states of both the highest occupied molecular orbital and the lowest unoccupied molecular orbital in an electronic structure with the presence of Ni. These findings may open up a route to explore promising perovskite oxides as candidate materials for use in multiferroics and solar-energy devices.

Structural and optical properties of ZnO thin films with heavy Cu-doping prepared by magnetron co-sputtering

Cu-doping can modulate the properties of ZnO thin films. Most previous researchers studied ZnO:Cu films with moderate doping level, while this work was focused on heavy Cu-doping (13–23.8%) effect on ZnO thin films prepared by magnetron co-sputtering. The microstructure and optical properties of the films were systematically investigated. The results show that moderate Cu-doping enhances the crystal grain size of the films, whereas heavy doping reduces it. The fluorescence quenching effect, the red shift phenomenon and the band gap narrowing effect in emission and absorption characteristics resulting from Cu-doping are more significant at heavy doping level. A promising band gap reduction of 0.64eV was achieved at 23.8% doping level.