Above-bandgap Illumination Research Articles

Comparative study of the photodarkening relaxation-kinetics in amorphous selenium for above-bandgap and sub-bandgap illumination

The relaxation of photodarkening (PD) brought about by above-bandgap and sub-bandgap excitations is studied in a wide temperature range in amorphous selenium (a-Se) films designed for avalanche photodetectors. The experimental results suggest that in contrast to sub-bandgap excitation, above-bandgap excitation does not cause the formation of self-trapped excitons, viz., photoinduced transformation of the a-Se ground state configuration into a metastable higher-energy configuration. For above-bandgap excitation only transient PD is observed, and its subsequent relaxation can be explained by thermalization and energy relaxation of uncorrelated carriers through the band-tails in order to restore the ground state configuration. In contrast, sub-bandgap excitation causes both transient and reversible PD, with the latter controlled by the formation of self-trapped excitons, whose relaxation requires overcoming an energy barrier of 0.78 ± 0.02 eV either by thermal activation (at elevated temperatures) or by configurational tunneling (for temperatures below room temperature).

Fermi Level Control of Point Defects During Growth of Mg-Doped GaN

In this study, Fermi level control of point defects during metalorganic chemical vapor deposition (MOCVD) of Mg-doped GaN has been demonstrated by above-bandgap illumination. Resistivity and photoluminescence (PL) measurements are used to investigate the Mg dopant activation of samples with Mg concentration of 2 × 1019 cm−3 grown with and without exposure to ultraviolet (UV) illumination. Samples grown under UV illumination have five orders of magnitude lower resistivity values compared with typical unannealed GaN:Mg samples. The PL spectra of samples grown with UV exposure are similar to the spectra of those grown without UV exposure that were subsequently annealed, indicating a different incorporation of compensating defects during growth. Based on PL and resistivity measurements we show that Fermi level control of point defects during growth of III-nitrides is feasible.

Charge Transfer across the n-Type GaN–Electrolyte Interface

The interfacial charge transfer characteristics of n-type GaN are investigated in pure phosphate buffered saline, as well as in solutions containing I–/I3– or hydroquinone/benzoquinone redox couples. Cyclic voltammetry and transient photoresponse measurements in the presence of above-bandgap illumination reveal that hole transfer to the solution is mediated by surface states in all cases. For measurements in pure PBS, a modification of the surface during cyclic potential sweeps is observed. In contrast, the presence of the redox species used in this work efficiently suppresses the oxygen evolution reaction and the associated surface modification. Furthermore, charge transfer to the redox couple is fully reversible using GaN as a dark cathode and photoanode, respectively. The presented study is of significant importance for applications of GaN in photocatalysis and biosensing, where the stability of (bio)functionalized surfaces is an essential requirement.

The over-saturation phenomenon of a Hg0.46Cd0.54Te photovoltaic detector irradiated by a CW laser

A Hg0.46Cd0.54Te (∼0.91 eV bandgap) photovoltaic detector is exposed to 10.6 µm and 1319 nm intense laser radiations. The open-circle voltage (Voc) of the detector is measured and found to decrease with laser power density above the saturation threshold. The characteristics of over-saturation phenomena are quite sensitive to the temperature. For the Hg0.46Cd0.54Te detector, the photon energy of a 10.6 µm laser (sub-bandgap laser) is less than the bandgap, and the photon energy of a 1319 nm laser (above-bandgap laser) is above the bandgap. Both lasers can induce an over-saturation phenomenon, but the dominant mechanism is different. The results indicate that photovoltage and thermoelectric voltage are the dominant mechanisms for above-bandgap illumination, and thermovoltage and thermoelectric voltage are the dominant mechanisms for sub-bandgap illumination.

Surface Characterization of Ga-doped ZnO layers

ABSTRACTEpitaxial ZnO layers heavily doped with Ga (GZO) were grown at 400 °C under metaland oxygen-rich conditions in terms of metal-to-reactive oxygen ratio by plasma-assisted molecular beam epitaxy (MBE). Several atomic force microscopy (AFM) techniques were used to characterize the surface morphology and electrical properties of these GZO films in ambient conditions. Local I-V spectra indicate that layers grown under both O-rich and metal-rich conditions are highly resistive until a relatively high voltage sweep (±12 V) is used. After removal of an insulating surface layer, conduction is possible at lower voltages, but eventually the film resistivity increases and it again becomes insulating. In addition to local I-V spectra, local charge injection and subsequent surface potential measurements were used to probe surface charging characteristics. For charge injection experiments, a reverse-bias voltage is applied to the sample while scanning in contact mode with a metallized tip. The resultant change in surface potential due to trapped charge is subsequently observed using scanning Kelvin probe microscopy (SKPM). The layers deposited in a metal-rich environment demonstrate the expected behavior, but the O-rich layers show anomalous negative and positive charging. Finally, surface photovoltage (SPV) measurements using above-bandgap UV illumination were performed. The GZO layers produce SPV values of 0.4 to 0.5 eV, where the films deposited in an O-rich environment have slightly higher SPV values and faster restoration.

Surface photovoltage in undoped n-type GaN

Steady-state and transient surface photovoltage (SPV) in undoped GaN is studied in vacuum and air ambient at room temperature and 400 K with a Kelvin probe. The results are explained within a phenomenological model accounting for the accumulation of photogenerated holes at the surface, capture of free electrons from the bulk over the near-surface potential barrier, and emission of electrons from surface states into the bulk. Simple analytical expressions are obtained and compared with experimental results. In particular, the proposed model explains the logarithmic decay of the SPV after stopping illumination. Internal and external mechanisms of the SPV are discussed in detail. It is shown that an internal mechanism dominates at low illumination intensity and/or small photon energies, while external mechanisms such as charging of a surface oxide layer and photoinduced processes play a significant role for above-bandgap illumination with sufficient intensity.

Accelerated Light-Induced Degradation (ALID) for Monitoring of Defects in PV Silicon Wafers and Solar Cells

In crystalline silicon, above-bandgap illumination can transform defects into strong recombination centers, degrading minority-carrier lifetime and solar cell efficiency. This light-induced degradation (LID) is due primarily to boron–oxygen and iron–boron defects, and can be reversed using thermal treatments that are distinctly different for each type of defect. Combining illumination and thermal treatment, we have designed an accelerated light-induced degradation (ALID) cycle that, within minutes, transforms defects into the distinct states needed to isolate individual contributions from boron–oxygen dimers (BO2i) and interstitial iron (Fei). In this cycle, the concentrations of BO2i and Fei are determined using surface photovoltage (SPV) diffusion length measurement. The ALID cycle uses reversible defect reactions and gives very good repeatability of wafer-scale mapping of BO2i and Fei in photovoltaic (PV) wafers and final solar cells.

Size-Dependent Optical Properties of Colloidal ZnO Nanoparticles Charged by Photoexcitation

The above-bandgap illumination of colloidal ZnO nanoparticles (NPs) in ethanol solutions is found to lead to reversible shifts of the absorption and photoluminescence (PL) excitation spectra, indicating charging of the nanoparticles with electrons. A rapid drop of deep-level PL intensity at the early stage of illumination is observed simultaneously with the splitting-off and growth of a new red-shifted near-band-edge PL band. Such a splitting of the near-bandgap PL band under illumination is observed for the first time and corroborates with the previous assumptions about the behavior of the NP ensemble emission upon gradual NPs’ charging with electrons. The possible relation between the new PL band and photoinduced charging of NPs with excess electrons is discussed on the basis of the dependence of the PL spectrum evolution and absorption band shift relaxation on the NP size and controllable access of oxygen during illumination.

Physical ageing in the above-bandgap photoexposured glassy arsenic selenides

Physical ageing induced by above-bandgap light illumination is studied in glassy As-Se using differential scanning calorimetry. It is shown that measurable effect like to known short-term physical ageing is observed only in Se-rich glasses. The kinetics of this effect is compared with that caused by natural storage in a dark.

Fabrication and characterization of continuous wave direct UV (λ=244 nm) written channel waveguides in chalcogenide (Ga:La:S) glass

Gallium lanthanum sulphide (Ga:La:S) optical glass is an interesting material for both fiber and planar technologies, as it offers possibilities for a wide array of devices suitable for use in both nonlinear applications and as IR lasers. Direct laser writing into this glass has yielded low-loss single-mode channel waveguides. Samples were exposed to above-bandgap illumination of focused UV (/spl lambda/=244 nm) light at varying intensities (I/sub UV/=1.5-90 kW/cm/sup 2/) and scan velocities (V/sub SCAN/=0.005-0.067 m/s). The exposed regions were evaluated through atomic force microscopy (AFM), and surface compaction (0.3-3.6 /spl mu/m) was observed. Sample topography was examined using a scanning electron microscope (SEM) with analysis of chemical changes within the exposed regions performed with energy-dispersive X-ray microscopy (EDAX). Waveguide attenuation was measured to be 0.2/spl plusmn/0.1 dB/cm at 1.3 /spl mu/m with a positive change in refractive index (/spl Delta/n=10/sup -3/). The chemical mechanism for these photo-induced changes with resulting photodensification has been correlated with a relative increase in the lanthanum content within the waveguide core.

Photoinduced changes in the linear and non-linear optical properties of chalcogenide glasses

The linear refractive index, n, and the position of the absorption edge for thin chalcogenide/chalcohalide films from the systems As–S, As–S–X (X=Bi, Tl or I), and Ge–S–Me (Me=Bi, Tl, As) have been measured before and after exposure to above-bandgap illumination. For compositions containing either As or Ge, the absorption edge was shifted to longer wavelengths (i.e. photodarkening occurred) for As-containing films, and to shorter wavelengths (i.e. photobleaching occurred) for Ge-containing films. As–S–Ge films exhibit either photodarkening or photobleaching, depending on Ge content. The largest values of these edge shifts ranged from 20 up to 60 nm, in the case of Ge 31S 63Tl 6. The conditions for synthesizing the glasses studied are reported. A formula for predicting the non-linear refractive index, n 2, for chalcogenide/chalcohalide glasses from the dispersion of n, and which enables n 2 to be related to structural parameters, has been developed and compared with existing formulae for other types of glass and crystals. Using the various formulae and the measured values of n, n 2 for these materials has been predicted. The results indicate that glasses with compositions near As 42S 58 or As 2S 3Tl 0.13 may, after UV exposure, exhibit significantly larger values of n 2 than as-deposited As 40S 60.

The non-radiative efficiency in hydrogenated amorphous silicon

Abstract The non-radiative efficiencyηnr has been directly measured in hydrogenated amorphous silicon using photo-pyroelectric spectroscopy, for photon energies ranging from 1·5 to 2·2 eV at room temperature. The non-radiative efficiency is fairly constant for above-bandgap illumination, but displays a sharp minimum for photon energies near the bandgap energy.ηnr increases with sub-bandgap illumination, saturating at a value roughly one half as large as its high-energy value.

Optical spectra, relaxation, and energy transfer of Eu 3+ and Cr 3− in an europium phosphate glass

Amorphous Cd-doped As2Se3 films with nominal Cd contents up to 12 at.% were prepared by thermal evaporation. Atomic force microscopy studies confirm the uniform film structure with surface roughnesses below 1 nm, independent of the Cd content. As shown by energy-dispersive X-ray spectroscopy, the Cd content in the film reveals a strong gradient and decreases with the film depth. For heavily Cd-doped (above 7 at.%) As2Se3 films, Raman features attributed to CdSe longitudinal optical (LO) phonon and its overtone (2LO) are revealed in the Raman spectra as an evidence for the formation of CdSe nanocrystallites in the As2Se3:Cd film under above-bandgap or below-bandgap laser illumination. The CdSe nanocrystallites undergo tensile strain due to a photoplastic effect in the As2Se3 film, i. e. partial removal of the material from the laser spot. The tensile strain value, estimated from the LO phonon frequency shift, is shown to reach nearly 2 GPa.