Sub-bandgap Optical Absorption Research Articles

Characterizations of Subbandgap Optical Absorption in Undoped‐GaN and 90 nm‐Thick Al1−xInxN Thin Film on Sapphire Substrates Grown by Metal–Organic Chemical Vapor Deposition

Subbandgap optical absorption (SOA) in undoped GaN and 90 nm‐thick Al1−xInxN thin films grown on sapphire substrates is investigated using photothermal deflection spectroscopy (PDS) and photoluminescence (PL). An Al1−xInxN alloy (x = 0.17) is grown on a GaN/sapphire template by metal–organic chemical vapor deposition (MOCVD), and the SOA is observed using PDS. To develop an estimation method for the absorption coefficient (α) of SOA in GaN and Al1−xInxN thin films, the use of a thick GaN substrate is proposed, which is grown by hydride vapor‐phase epitaxy, as a converter of the PDS signal intensity to α, and the accuracies of the estimated α are discussed. Comparing the PDS and PL results, it is revealed that nonradiative recombination centers leading to the reduction of the near‐band‐edge PL intensity are not the dominant sources of SOAs in GaN. Other in‐gap states formed by impurities and/or their complexes with vacancy‐type defects are possible sources of a large SOA in the MOCVD‐grown GaN template. Considering the PDS results and reported peak reflectivity of Al1−xInxN/GaN distributed Bragg reflectors, the α value of sub‐100 nm‐thick Al1−xInxN alloy grown on GaN/sapphire template is expected to be ≈100 cm−1 or less below 3.0 eV.

Suppressing substrate oxidation during plasma-enhanced atomic layer deposition on semiconductor surfaces

Plasma-enhanced atomic layer deposition (PE-ALD) is widely employed in microelectronics, energy, and sensing applications. Typically, PE-ALD processes for metal oxides utilize remote inductively coupled plasmas operated at powers of >200 W, ensuring a sufficient flux of oxygen radicals to the growth surface. However, this approach often leads to significant oxidation of chemically sensitive substrates, including most technological semiconductors. Here, we demonstrate that plasma powers as low as 5 W can effectively suppress substrate oxidation while maintaining the structural, optical, and electronic quality of the films. Specifically, we investigate the growth of titanium oxide (TiOx) using two commonly used metalorganic precursors, titanium isopropoxide and tetrakis(dimethylamino)titanium. Films deposited with 5 and 300 W oxygen plasma power are nearly indiscernible from one another, exhibiting significantly lower defect concentrations than those obtained from thermal ALD with H2O. The low plasma power process preserves desired physical characteristics of PE-ALD films, including large optical constants (n > 2.45 at 589 nm), negligible defect-induced sub-bandgap optical absorption (α < 102 cm−1), and high electrical resistivity (>105 Ω cm). Similar behavior, including suppressed interface oxidation and low defect content, is observed on both Si and InP substrates. As an example application of this approach, the assessment of InP/TiOx photocathodes and Si/TiOx photoanodes reveals a significant improvement in the photocurrent onset potential in both cases, enabled by suppressed substrate oxidation during low power PE-ALD. Overall, low power PE-ALD represents a generally applicable strategy for producing high quality metal oxide thin films while minimizing detrimental substrate reactions.

Sub-bandgap optical absorption processes in 300-nm-thick Al1−xInxN alloys grown on a c-plane GaN/sapphire template

We investigate the sub-bandgap optical absorption (SOA) in 300-nm-thick Al1−xInxN alloys used in cladding layers of edge-emitting laser diodes and distributed Bragg reflectors of vertical-cavity surface-emitting lasers. Al1−xInxN alloys, with indium content x ranging from 0.114 to 0.185, were grown by metal-organic chemical vapor deposition on a c-plane GaN/sapphire template. SOAs on 300-nm-thick thin films were characterized using photothermal deflection spectroscopy (PDS). Thermal emission, such as nonradiative recombination with phonon emission, is the dominant energy relaxation process occurring after SOA in Al1−xInxN alloys. The absorption coefficient of the SOA was estimated to be 0.6–7.0 × 103 cm−1 in these samples by combining PDS and spectroscopic ellipsometry. The drastic increase in the SOA, when x exceeded the lattice-matched composition of the GaN/sapphire template, indicates that impurities, vacancy-type defects, and their complexes with increasing x are possible candidates that result in SOA in Al1−xInxN alloys.

Silicon four-quadrant photodetector working at the 1550-nm telecommunication wavelength

In this Letter, we demonstrate a silicon four-quadrant photodetector working at the 1550-nm telecommunication wavelength and apply it to the measurements of the light-beam positions and deflection angles. Incident light changes the admittance of each quadrant photodetector through subbandgap optical absorption, and this change of admittance is read out through transimpedance amplification and lock-in readout circuitry. By monitoring the optical power received by the four quadrant photodetectors, we measure and track the position of the optical beam. Without any modification, the same device and associated circuit can also work at wavelengths shorter than the long-wavelength limit of silicon, for example, at 780 nm, as we demonstrate.

Design, photoelectric properties and electron transition mechanism of Cr doped p-CuGaS2 compound based on intermediate band effect

The p-CuGaS2 and Cr doped p-CuGaS2 compounds are first fabricated by the solvothermal method using CuI, C12H26S, C15H21GaO6, and C15H21CrO6 as starting materials. The Cr3+ is successfully doped into the CuGaS2 compounds with a single chalcopyrite structure and partially replaced Ga3+. The two additional sub-bandgap optical absorption edges (‘B’ and ‘C’) are observed in the UV–Vis–NIR absorption spectra of the sample with a Cr doping content of 0.99%. ‘C’ and ‘B’ correspond to the absorption extra photons of hν1 (valence band maxima intermediate band, 0.8 eV) and hν2 (intermediate band conduction band minimum, 1.2 eV), which proves the formation of the intermediate band in the Cr doped CuGaS2. The Hall effect measurement shows that the resistivity of Cr doped CuGaS2 compound decreases with the increase of Cr doping content, while the carrier concentration and mobility are increased. The half-filled state of intermediate band is confirmed according to Fermi level tested by the ultraviolet photoelectron spectroscopy. The photocurrent responses analysis suggests that the intermediate band leads to the increase of photo-induced carriers. The band structure and electron transition mechanism of Cr doped CuGaS2 compounds are discussed in details.

Chalcogen-hyperdoped germanium for short-wavelength infrared photodetection

Obtaining short-wavelength-infrared (SWIR; 1.4 μm–3.0 μm) room-temperature photodetection in a low-cost, group IV semiconductor is desirable for numerous applications. We demonstrate a non-equilibrium method for hyperdoping germanium with selenium or tellurium for dopant-mediated SWIR photodetection. By ion-implanting Se or Te into Ge wafers and restoring crystallinity with pulsed laser melting induced rapid solidification, we obtain single crystalline materials with peak Se and Te concentrations of 1020 cm−3 (104 times the solubility limits). These hyperdoped materials exhibit sub-bandgap absorption of light up to wavelengths of at least 3.0 μm, with their sub-bandgap optical absorption coefficients comparable to those of commercial SWIR photodetection materials. Although previous studies of Ge-based photodetectors have reported a sub-bandgap optoelectronic response only at low temperature, we report room-temperature sub-bandgap SWIR photodetection at wavelengths as long as 3.0 μm from rudimentary hyperdoped Ge:Se and Ge:Te photodetectors.

Mixed cobalt-nitrides CoxN and Ta2N bifunction-modified Ta3N5 nanosheets for enhanced photocatalytic water-splitting into hydrogen

A novel mixed cobalt nitrides CoxN (x = 1, 2, and 5.47) and Ta2N bifunction-modified Ta3N5 nanosheet photocatalyst was successfully synthesized through a high-temperature ammonolysis of Co2+/Co3+-adsorbed Ta3N5@Ta2O5 nanoparticles. Effects of Co2+ or/and Co3+ modifying on the structural, electronic, optical, and photoelectrochemical performances of Ta3N5 were investigated systematically. Co-modifying extended the absorption over the whole visible region due to the sub-band gap optical absorption from the increased heterojunction interfaces of CoxN/Ta3N5 or/and Ta2N/Ta3N5. Co2+/Co3+ dual-modifying inhibited the crystallite growth of Ta3N5. Uniformly dispersed Co elements on the Ta3N5 surface provided the more surface active sites. Co3+ played a key role in controlling the modifying amounts of both Ta2N and Co5.47N. Co2+/Co3+ dual-modifying produced a synergetic effect on enhancing the charge carrier separation, decreasing the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) overpotentials of Ta3N5, and then efficiently increasing the photocatalytic H2-evolution activity. 1.5%Co2+/1.5%Co3+-TN possessed an excellent photocatalytic H2-evolution activity (75.69 μmol g−1 h−1) under visible light irradiation, 3.48 times higher than Ta3N5@Ta2O5 nanoparticles. Co5.47N and Ta2N as HER cocatalysts, and CoN and Co2N as OER cocatalysts, decreased the HER and OER overpotentials of Ta3N5, accelerating the kinetics of surface catalytic reaction. A synergetic mechanism for enhanced photocatalytic H2-evolution performance was discussed in detail. Moreover, self-oxidation of Ta3N5 can be suppressed by surface-modified cobalt nitrides and Ta2N. This work provides a promising photocatalyst and a new strategy for develop efficient and stable Ta3N5-based photocatalysts for solar energy conversion.

Atomic scale origins of sub-band gap optical absorption in gold-hyperdoped silicon

Gold hyperdoped silicon exhibits room temperature sub band gap optical absorption, with potential applications as infrared absorbers/detectors and impurity band photovoltaics. We use first-principles density functional theory to establish the origins of the sub band gap response. Substitutional gold AuSi and substitutional dimers AuSi − AuSi are found to be the energetically preferred defect configurations, and AuSi gives rise to partially filled mid-gap defect bands well offset from the band edges. AuSi is predicted to offer substantial sub-band gap absorption, exceeding that measured in prior experiments by two orders of magnitude for similar Au concentration. This suggests that in experimentally realized systems, in addition to AuSi, the implanted gold is accommodated by the lattice in other ways, including other defect complexes and gold precipitates. We further identify that it is energetically favorable for isolated AuSi to form AuSi − AuSi, which by contrast do not exhibit mid-gap states. The formation of dimers and other complexes could serve as nuclei in the earliest stages of Au precipitation, which may be responsible for the observed rapid deactivation of sub-band gap response upon annealing.

On the insulator-to-metal transition in titanium-implanted silicon

Hyperdoped silicon with deep level impurities has attracted much research interest due to its promising optical and electrical properties. In this work, single crystalline silicon supersaturated with titanium is fabricated by ion implantation followed by both pulsed laser melting and flash lamp annealing. The decrease of sheet resistance with increasing Ti concentration is attributed to a surface morphology effect due to the formation of cellular breakdown at the surface and the percolation conduction at high Ti concentration is responsible for the metallic-like conductivity. The insulator-to-metal transition does not happen. However, the doping effect of Ti incorporation at low concentration is not excluded, which might be responsible for the sub-bandgap optical absorption reported in literature.

Au-rich filamentary behavior and associated subband gap optical absorption in hyperdoped Si

Au-hyperdoped Si, synthesized by ion implantation and pulsed laser melting, is known to exhibit a strong sub-band gap photoresponse that scales monotonically with the Au concentration. However, there is thought to be a limit to this behavior since ultrahigh Au concentrations ($g1\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$) are expected to induce cellular breakdown during the rapid resolidification of Si, a process that is associated with significant lateral impurity precipitation. This work shows that the cellular morphology observed in Au-hyperdoped Si differs from that in conventional, steady-state cellular breakdown. In particular, Rutherford backscattering spectrometry combined with channeling and transmission electron microscopy revealed an inhomogeneous Au distribution and a subsurface network of Au-rich filaments, within which the Au impurities largely reside on substitutional positions in the crystalline Si lattice, at concentrations as high as $\ensuremath{\sim}3$ at. %. The measured substitutional Au dose, regardless of the presence of Au-rich filaments, correlates strongly with the sub-band gap optical absorptance. Upon subsequent thermal treatment, the supersaturated Au forms precipitates, while the Au substitutionality and the sub-band gap optical absorption both decrease. These results offer insight into a metastable filamentary regime in Au-hyperdoped Si that has important implications for Si-based infrared optoelectronics.

Vanadium supersaturated silicon system: a theoretical and experimental approach

The effect of high dose vanadium ion implantation and pulsed laser annealing on the crystal structure and sub-bandgap optical absorption features of V-supersaturated silicon samples has been studied through the combination of experimental and theoretical approaches. Interest in V-supersaturated Si focusses on its potential as a material having a new band within the Si bandgap. Rutherford backscattering spectrometry measurements and formation energies computed through quantum calculations provide evidence that V atoms are mainly located at interstitial positions. The response of sub-bandgap spectral photoconductance is extended far into the infrared region of the spectrum. Theoretical simulations (based on density functional theory and many-body perturbation in GW approximation) bring to light that, in addition to V atoms at interstitial positions, Si defects should also be taken into account in explaining the experimental profile of the spectral photoconductance. The combination of experimental and theoretical methods provides evidence that the improved spectral photoconductance up to 6.2 µm (0.2 eV) is due to new sub-bandgap transitions, for which the new band due to V atoms within the Si bandgap plays an essential role. This enables the use of V-supersaturated silicon in the third generation of photovoltaic devices.

Monovalent Cation Doping of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> for Efficient Perovskite Solar Cells

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical properties. The possibility of doping was investigated by adding monovalent cation halides with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+. A shift in the Fermi level and a remarkable decrease of sub-bandgap optical absorption, along with a lower energetic disorder in the perovskite, was achieved. An order-of-magnitude enhancement in the bulk hole mobility and a significant reduction of transport activation energy within an additive-based perovskite device was attained. The confluence of the aforementioned improved properties in the presence of these cations led to an enhancement in the photovoltaic parameters of the perovskite solar cell. An increase of 70 mV in open circuit voltage for AgI and a 2 mA/cm2 improvement in photocurrent density for NaI- and CuBr-based solar cells were achieved compared to the pristine device. Our work paves the way for further improvements in the optoelectronic quality of CH3NH3PbI3 perovskite and subsequent devices. It highlights a new avenue for investigations on the role of dopant impurities in crystallization and controls the electronic defect density in perovskite structures.

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells.

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical properties. The possibility of doping was investigated by adding monovalent cation halides with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+. A shift in the Fermi level and a remarkable decrease of sub-bandgap optical absorption, along with a lower energetic disorder in the perovskite, was achieved. An order-of-magnitude enhancement in the bulk hole mobility and a significant reduction of transport activation energy within an additive-based perovskite device was attained. The confluence of the aforementioned improved properties in the presence of these cations led to an enhancement in the photovoltaic parameters of the perovskite solar cell. An increase of 70 mV in open circuit voltage for AgI and a 2 mA/cm2 improvement in photocurrent density for NaI- and CuBr-based solar cells were achieved compared to the pristine device. Our work paves the way for further improvements in the optoelectronic quality of CH3NH3PbI3 perovskite and subsequent devices. It highlights a new avenue for investigations on the role of dopant impurities in crystallization and controls the electronic defect density in perovskite structures.

Theory of edge-state optical absorption in two-dimensional transition metal dichalcogenide flakes

We develop an analytical model to describe sub-bandgap optical absorption in two-dimensional semiconducting transition metal dichalcogenide (s-TMD) nanoflakes. The material system represents an array of few-layer molybdenum disulfide crystals, randomly orientated in a polymer matrix. We propose that optical absorption involves direct transitions between electronic edge-states and bulk-bands, depends strongly on the carrier population, and is saturable with sufficient fluence. For excitation energies above half the bandgap, the excess energy is absorbed by the edge-state electrons, elevating their effective temperature. Our analytical expressions for the linear and nonlinear absorption could prove useful tools in the design of practical photonic devices based on s-TMDs.

Franz–Keldysh effect in n-type GaN Schottky barrier diode under high reverse bias voltage

The photocurrent of GaN vertical Schottky barrier diodes was investigated under sub-bandgap wavelength light irradiation. Under a low reverse bias voltage, the photocurrent is induced by internal photoemission, while under a high reverse bias voltage, the photocurrent increases significantly with the bias voltage. This is due to sub-bandgap optical absorption in a depletion region due to the Franz–Keldysh effect. The voltage and wavelength dependences of the photocurrent are successfully explained quantitatively.

Approaching disorder-free transport in high-mobility conjugated polymers.

Conjugated polymers enable the production of flexible semiconductor devices that can be processed from solution at low temperatures. Over the past 25years, device performance has improved greatly as a wide variety of molecular structures have been studied. However, one major limitation has not been overcome; transport properties in polymer films are still limited by pervasive conformational and energetic disorder. This not only limits the rational design of materials with higher performance, but also prevents the study of physical phenomena associated with an extended π-electron delocalization along the polymer backbone. Here we report a comparative transport study of several high-mobility conjugated polymers by field-effect-modulated Seebeck, transistor and sub-bandgap optical absorption measurements. We show that in several of these polymers, most notably in a recently reported, indacenodithiophene-based donor-acceptor copolymer with a near-amorphous microstructure, the charge transport properties approach intrinsic disorder-free limits at which all molecular sites are thermally accessible. Molecular dynamics simulations identify the origin of this long sought-after regime as a planar, torsion-free backbone conformation that is surprisingly resilient to side-chain disorder. Our results provide molecular-design guidelines for 'disorder-free' conjugated polymers.

Effects of annealing and structural phase transformation on the Urbach absorption in thin silver sulphide films

The sub-band gap optical absorption in silver sulphide thin films prepared by chemical conversion technique has been studied in the light of Urbach absorption law at different annealing temperatures. The study reveals the Urbach absorption parameters and band gap of the material are sensitive to annealing temperature. Interestingly, these parameters undergo a sudden change as the annealing temperature exceeds a certain critical value. The observed changes in the values of absorption parameters have been attributed to the structural phase transition and the modification of grain boundary interfaces and/or changes in the crystallographic orientation due to annealing.

On the sub-band gap optical absorption in heat treated cadmium sulphide thin film deposited on glass by chemical bath deposition technique

The sub-band gap optical absorption in chemical bath deposited cadmium sulphide thin films annealed at different temperatures has been critically analyzed with special reference to Urbach relation. It has been found that the absorption co-efficient of the material in the sub-band gap region is nearly constant up to a certain critical value of the photon energy. However, as the photon energy exceeds the critical value, the absorption coefficient increases exponentially indicating the dominance of Urbach rule. The absorption coefficients in the constant absorption region and the Urbach region have been found to be sensitive to annealing temperature. A critical examination of the temperature dependence of the absorption coefficient indicates two different kinds of optical transitions to be operative in the sub-band gap region. After a careful analyses of SEM images, energy dispersive x-ray spectra, and the dc current-voltage characteristics, we conclude that the absorption spectra in the sub-band gap domain is possibly associated with optical transition processes involving deep levels and the grain boundary states of the material.

In situmeasurements of the optical absorption of dioxythiophene-based conjugated polymers

Conjugated polymers can be reversibly doped by electrochemical means. This doping introduces new subband-gap optical absorption bands in the polymer while decreasing the band-gap absorption. To study this behavior, we have prepared an electrochemical cell allowing in situ measurements of the optical properties of the polymer. The cell consists of a thin polymer film deposited on gold-coated Mylar behind which is another polymer that serves as a counterelectrode. An infrared transparent window protects the upper polymer from ambient air. By adding a gel electrolyte and making electrical connections to the polymer-on-gold films, one may study electrochromism in a wide spectral range. As the cell voltage (the potential difference between the two electrodes) changes, the doping level of the conjugated polymer films is changed reversibly. Our experiments address electrochromism in poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3,4-dimethylpropylenedioxythiophene) (PProDOT-Me${}_{2}$). This closed electrochemical cell allows the study of the doping induced subband-gap features (polaronic and bipolaronic modes) in these easily oxidized and highly redox switchable polymers. We also study the changes in cell spectra as a function of polymer thickness and investigate strategies to obtain cleaner spectra, minimizing the contributions of water and gel electrolyte features.

Analysis of the shape of spectral dependence of absorption coefficient and stationary photoconductivity spectral response in nanocrystalline bismuth(III) sulfide thin films

Some aspects regarding the optical and photoelectrical properties of bismuth(III) sulfide nanocrystals deposited in thin film form were studied. The influence of electrostatic interaction between photogenerated charge carriers on the shape of the spectral dependence of absorption coefficient for as-deposited and thermally treated nanocrystalline Bi 2S 3 thin films was investigated. The experimentally obtained spectral dependencies of the optical absorption coefficient were analyzed using the models of Elliott and Toyozawa. It was shown that the sub-band gap optical absorption of the studied films is of exponential type. A modified Urbach rule function was employed to calculate the relevant parameters which determine the value of critical energy at which a transition between the two main absorption regimes occurs in studied semiconductor. Experimentally obtained changes of Urbach energies ( i.e. tailing parameters) upon thermal treatment of the films were explained in terms of the differences in the degree of structural disorder of the as-deposited and annealed films according to the model of Cody. The experimental optical absorption data were also used to model the photoconductivity spectral response of thermally annealed films using various approaches.