Sub-bandgap Photoconductivity Research Articles

Enhancing performance of Au-hyperdoped Si photodetectors for infrared detection

Hyperdoping Si with transition metals to form intermediate bands for infrared absorption has attracted attention recently for producing sub-bandgap photoconductivity. In particular, Si hyperdoped with Au has been demonstrated to exhibit optoelectronic response at 1550 nm. However, the reported external quantum efficiencies (EQEs) are low, and the device fabrication processes had not been optimized. In this paper, we demonstrate a significant improvement in sub-bandgap EQE through modification of the material and device fabrication processes. By increasing the Si:Au layer thickness, modification of device design, and formation of Ohmic contacts, the EQE was measured to be as high as 0.44% at 1550 nm, nearly two orders of magnitude higher than previous reports from similar devices. Additionally, the EQE was measured to be in the 10-3 range for wavelengths as long as 2.4 μm. The EQE spectrum showed features that were attributed to defect levels from a substitutional Au acceptor defect. The above bandgap EQE showed gain in one device. Thermal annealing at 300 °C does not improve the efficiency of Si:Au photodiodes. These results demonstrate the viability of Au-hyperdoped Si for infrared detection below the bandgap of Si.

Strong subbandgap photoconductivity in GaP implanted with Ti

AbstractPhotovoltaic solar cells based on the intermediate band (IB) concept could greatly enhance the efficiency of future devices. We have analyzed the electrical and photoconductivity properties of GaP supersaturated with Ti to assess its suitability for IB solar cells. GaP:Ti was obtained by ion implantation followed by pulsed‐laser melting (PLM) using an ArF excimer laser. It was found that PLM energy densities between 0.35 and 0.55 J/cm2 produced a good recovery of the crystalline structure of GaP (both unimplanted and implanted with Ti), as evidenced by high mobility measured values (close to the reference GaP). Outside this energy density window, the PLM failed to recover the crystalline structure producing a low mobility layer that is electrically isolated from the substrate. Spectral photoconductivity measurements were performed by using the van der Pauw set up. For GaP:Ti a significant enhancement of the conductivity was observed when illuminating the sample with photon energies below 2.26 eV, suggesting that this photoconductivity is related to the presence of Ti in a concentration high enough to form an IB within the GaP bandgap. The position of the IB was estimated to be around 1.1 eV from the conduction band or the valence band of GaP, which would lead to maximum theoretical efficiencies of 25% to 35% for a selective absorption coefficients scenario and higher for an overlapping scenario.

Spectral measurements of picosecond photoconductivity in terahertz detectors made from low temperature grown GaAs and GaInAs

We report on the performance of photoconductive terahertz detectors made from low temperature grown GaAs (LTG GaAs) and Ga0.7In0.3As (LTG GaInAs) in the wavelength range from 620 to 1550 nm. These THz detectors exhibited a significant sub-bandgap photoconductivity due to the electron transitions from the defect band. A simple model evaluating picosecond photoconductivity was proposed to explain the experimental data. By comparing the modeling results with the experimental data, the defect band position was found to be at 0.77 and at 0.4 eV below the conduction band minimum for LTG GaAs and LTG GaInAs, respectively.

Method to probe the electrical activity of dislocations in non-intentionally doped n-GaN

Abstract Here is presented a method to probe the electrical activity of dislocations in non-intentionally doped n-GaN epitaxial layers based on the study of their sub-band gap photoconductivity, monitoring their electron concentration and mobility. Non-intentionally doped n-GaN layers bearing charged and thus highly dispersive and recombining dislocations when illuminated with sub-band gap photons show a strong increase on their conductivity, due to an equivalent increase on the electron mobility while the electron concentration remains unchanged. On the other side, non-intentionally doped n-GaN layers bearing electrically inactive dislocations display almost no photoconduction, as both; carrier concentration and their mobility remain unchanged under the same illumination conditions. The method, simultaneously assess the electrical activity of dislocations and the material quality, and can be applied to any other semiconducting material bearing high dislocations densities.

Sub-bandgap photoconductivity and photocapacitance in CIGS thin films and devices

Photoconductivity and photocapacitance of Cu(In,Ga)Se2 and CuGaSe2 thin films and devices induced by sub-bandgap illumination are investigated. Both effects have been attributed to the optical transition from valence band to the same empty levels situated around 0.8–0.9 eV above the valence band. The influence of the metastable states created by illumination and voltage bias on the sub-bandgap response has been studied. The experimental results are discussed in the framework of a model based on negative-U property of a native defect in chalcopyrites, i.e. V Se–V Cu divacancy. The arguments are presented that the levels involved in the optical transition observed in photoconductivity and photocapacitance might be antibonding levels of the acceptor configuration of this defect.

Sub-bandgap photoconductivity in ZnO epilayers and extraction of trap density spectra

Photoconductivity is observed in ZnO epilayers due to photoexcitation in the visible spectral region of 400–700 nm, below the ZnO bandgap energy of 3.4 eV. Photoconductive transients due to visible photoexcitation have time constants in the order of minutes. Treatment of the ZnO surface with SiO2 passivation layers results in a significant reduction in the photoconductive signal and photoconductive time constant. The photoconductive response is attributed to hole traps in ZnO, where a rate equation model is presented to describe the photoconductive characteristics. A method of extracting the hole trap density spectrum is presented on the basis of the rate equation model and assumptions for hole capture lifetime and carrier recombination lifetime that are validated by experimental time-resolved photoluminescence measurements of the material under study. Traps are found to be distributed near 0.75 eV and 0.9 eV from the valence band edge for SiO2 passivated and unpassivated ZnO epilayers, respectively.

Thin polycrystalline diamond for low-energy x-ray detection

The response of polycrystalline diamond detectors to low-energy x-ray beams is presented. Diamond metal∕semiconductor∕metal vertical structures have been realized to investigate the influence of the electric field in the dark and under x-ray irradiation. Nonlinear contributions to the current, compatible with field-assisted thermal ionization of trapped charges, have been evidenced, as well as the presence of defect-related electronic states acting as traps has been confirmed by subband-gap photoconductivity measurements. Moreover, correlations have been evidenced among irradiation time, variation of the current in the dark, and photoconductive response of the devices to the x-ray beam. An evaluation of the minority-carrier mobility-lifetime μτ product, at the end of the activation process, has led to a value of 9.2×10−7cm2∕V. Two orders of magnitude linear response to the beam intensity variations have been evaluated after removal of the bremsstrahlung radiation contribution on a 43-μm-thick device.

Urbach Tail in InP with Nanometer Metallic Precipitates

Subband-gap photoconductivity of InP:Cu has been investigated. An exponential dependence versus photon energy has been found. Our findings are in contrast to Fowler plots measured in GaAs:As. In our opinion, the exponential dependence is determined by photogeneration from the semiconductor. This mechanism of carrier generation is stronger than the photoemission from metal precipitates.

An Investigation of the Influence of Plasma Characteristics on the Electronic and Optical Properties of Device Quality a-Si:H Grown by Electron Cyclotron Resonance Plasma Deposition

ABSTRACTThe electronic and optical properties of device quality hydrogenated amorphous silicon (a-Si:H) films grown by electron cyclotron resonance (ECR) plasma deposition were studied together with in-situ plasma characteristics. Hydrogen and helium plasmas, excited by 50–250 watts of 2.45 GHz microwave power under ECR conditions, were used to decompose silane at 6 to 20 mtorr pressures during the deposition of a-Si:H films at a 297 C substrate temperature. Both the electron temperature and density, and ion flux are measured near the deposition surface using plane and cylindrical Langmuir probes. An attempt is made to correlate these plasma properties with the light and dark photoconductivity, optical gap, refractive index, and subband gap photoconductivity.