Gap Photoresponse Research Articles

Engineering Vacancies in Bi2S3yielding Sub‐Bandgap Photoresponse and Highly Sensitive Short‐Wave Infrared Photodetectors

AbstractDefects play an important role in tailoring the optoelectronic properties of materials. Supported by density functional theory (DFT) calculations, herein it is demonstrated that sulphur vacancies are able to engineer sub‐band gap photoresponse in the short‐wave infrared range due to formation of in‐gap states in Bi2S3single crystals. Sulfurization and subsequent refill of the vacancies result in faster response but limit the spectral range to the near infrared as determined by the bandgap of Bi2S3. A facile chemical treatment is then explored to accelerate the speed of sulphur deficient (SD)‐based detectors on the order of 10 ms without sacrificing its spectral coverage of the infrared, while holding a high specific detectivity (D*) close to 1015Jones in the visible–near infrared range and 1012Jones at 1.6 µm. This work also provides new insights into the role sulphur vacancies play in the electronic structure and, as a result, in sub‐bandgap photoresponse enabling ultrasensitive, fast, and broadband photodetectors.

Ultrasensitive Mid-wavelength Infrared Photodetection Based on a Single InAs Nanowire.

One-dimensional InAs nanowire (NW)-based photodetectors have been widely studied due to their potential application in mid-wavelength infrared (MWIR) photon detection. However, the limited performance and complicated photoresponse mechanism of InAs NW-based photodetectors have held back their true potential for real application. In this study, we developed ferroelectric polymer P(VDF-TrFE)-coated InAs NW-based photodetectors and demonstrated that the electrostatic field caused by polarized ferroelectric materials modifies the surface electron-hole distribution as well as the band structure of InAs NWs, resulting in ultrasensitive photoresponse and a wide photodetection spectral range. Our single InAs NW photodetectors exhibit a high responsivity ( R) of 1.6 × 104 A W-1 as well as a corresponding detectivity ( D*) of 1.4 × 1012 cm·Hz1/2 W-1 at a light wavelength of 3.5 μm without an applied gate voltage, ∼3-4 orders higher than the maximum value of photoresponsivity reported or commercially used MWIR photodetectors. Moreover, our device shows below band gap photoresponse for 4.3 μm MWIR light with R of 9.6 × 102 A W-1 as well as a corresponding D* of ∼8.5 × 1010 cm·Hz1/2 W-1 at 77 K. Our study shows that this approach is promising for fabrication of high-performance NW-based photodetectors for MWIR photon detection.

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.

Effect of layer thickness on device response of silicon heavily supersaturated with sulfur

We report on a simple experiment in which the thickness of a hyperdoped silicon layer, supersaturated with sulfur by ion implantation followed by pulsed laser melting and rapid solidification, is systematically varied at constant average sulfur concentration, by varying the implantation energy, dose, and laser fluence. Contacts are deposited and the external quantum efficiency (EQE) is measured for visible wavelengths. We posit that the sulfur layer primarily absorbs light but contributes negligible photocurrent, and we seek to support this by analyzing the EQE data for the different layer thicknesses in two interlocking ways. In the first, we use the measured concentration depth profiles to obtain the approximate layer thicknesses, and, for each wavelength, fit the EQE vs. layer thickness curve to obtain the absorption coefficient of hyperdoped silicon for that wavelength. Comparison to literature values for the hyperdoped silicon absorption coefficients [S.H. Pan et al. Applied Physics Letters 98, 121913 (2011)] shows good agreement. Next, we essentially run this process in reverse; we fit with Beer’s law the curves of EQE vs. hyperdoped silicon absorption coefficient for those wavelengths that are primarily absorbed in the hyperdoped silicon layer, and find that the layer thicknesses obtained from the fit are in good agreement with the original values obtained from the depth profiles. We conclude that the data support our interpretation of the hyperdoped silicon layer as providing negligible photocurrent at high S concentrations. This work validates the absorption data of Pan et al. [Applied Physics Letters 98, 121913 (2011)], and is consistent with reports of short mobility-lifetime products in hyperdoped layers. It suggests that for optoelectronic devices containing hyperdoped layers, the most important contribution to the above band gap photoresponse may be due to photons absorbed below the hyperdoped layer.

Room-temperature sub-band gap optoelectronic response of hyperdoped silicon

Room-temperature infrared sub-band gap photoresponse in silicon is of interest for telecommunications, imaging and solid-state energy conversion. Attempts to induce infrared response in silicon largely centred on combining the modification of its electronic structure via controlled defect formation (for example, vacancies and dislocations) with waveguide coupling, or integration with foreign materials. Impurity-mediated sub-band gap photoresponse in silicon is an alternative to these methods but it has only been studied at low temperature. Here we demonstrate impurity-mediated room-temperature sub-band gap photoresponse in single-crystal silicon-based planar photodiodes. A rapid and repeatable laser-based hyperdoping method incorporates supersaturated gold dopant concentrations on the order of 10(20) cm(-3) into a single-crystal surface layer ~150 nm thin. We demonstrate room-temperature silicon spectral response extending to wavelengths as long as 2,200 nm, with response increasing monotonically with supersaturated gold dopant concentration. This hyperdoping approach offers a possible path to tunable, broadband infrared imaging using silicon at room temperature.

Subband gap photoresponse of nanocrystalline silicon in a metal-oxide-semiconductor device

In this paper we report on the photoconduction and photovoltaic properties of nanocrystalline silicon. Silicon nanocrystals (Si-ncs) have been prepared by using plasma-enhanced chemical vapor deposition on a p-type silicon substrate. The Si-ncs have been formed into the dielectric of a metal-oxide-semiconductor device. I-V characteristics of the devices have been studied under dark and illumination. Illumination was performed with light in the wavelength range of 350–1630 nm. A photovoltaic effect has been observed in the illuminated I-V characteristics in the range of 350–1100 nm. For longer wavelengths no measurable photovoltaic effect has been observed, but considerable photocurrent has been measured for 1300–1630 nm light under reverse bias condition. This photoresponse is attributed to absorption through subband gap states at the Si-nc and silicon oxynitride matrix interface.

Dye sensitization of synthetic p-type diamond electrode

A synthetic semiconductive p-type diamond electrode was photosensitized by the Ru(bpy)32+ molecule in an electrolyte solution. The photoexcited molecule injects a hole into the valence band. However, under a certain potential, charge recombination occurred through interband states; the hole on the top of the valence band recombined with the photoreduced Ru(bpy)31+. The mechanism of this charge recombination was studied by measuring the transient photopotential and the intrinsic sub-band gap photoresponse of the electrode.

A photoelectrochemical and ac impedance study of anodic titanium oxide films

Abstract The electrochemical behavior of sulphuric acid anodised titanium prepared galvanostatically under high field conditions over a wide range of maximum potentials (Vmax) was examined using photoelectrochemistry (PEC) and ac impedance spectroscopy (EIS). Film structure and morphology were examined by transmission electron microscopy (TEM). The variation in reciprocal capacitance against Vmax obtained from EIS was evaluated using a constant phase element model. For Vmax=0–10 V a straight line relationship was observed, enabling evaluation in terms of a parallel plate capacitor. This was followed by a region of decreasing reciprocal capacitance, indicating a substantial change in the nature of the material. Finally a slow rise occurred beyond 30 V. Large changes in the band structure of the anodic oxide were observed by PEC near the optical band edge for 0–10 V and in the sub-band gap region for 10–20 V. A defective TiO2 structure with impurity bands located deeply inside the band gap was still observed for 60 V and 84 V, where microcrystals of anatase are present. Strong correlation between the reciprocal capacitance, optical band gap, sub-band gap photoresponse and changes in crystallinity and degree of order within the anodic films were established.

Surface Treatment Induced Sub‐Band Gap Photoresponse of GaP Photoelectrodes

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