Photoinduced Current Transient Spectroscopy Measurements Research Articles

Electronic Structure and Optoelectronic Properties of Bismuth Oxyiodide Robust against Percent‐Level Iodine‐, Oxygen‐, and Bismuth‐Related Surface Defects

AbstractIn the search for nontoxic alternatives to lead‐halide perovskites, bismuth oxyiodide (BiOI) has emerged as a promising contender. BiOI is air‐stable for over three months, demonstrates promising early‐stage photovoltaic performance and, importantly, is predicted from calculations to tolerate vacancy and antisite defects. Here, whether BiOI tolerates point defects is experimentally investigated. BiOI thin films are annealed at a low temperature of 100 °C under vacuum (25 Pa absolute pressure). There is a relative reduction in the surface atomic fraction of iodine by over 40%, reduction in the surface bismuth fraction by over 5%, and an increase in the surface oxygen fraction by over 45%. Unexpectedly, the Bi 4f7/2 core level position, Fermi level position, and valence band density of states of BiOI are not significantly changed. Further, the charge‐carrier lifetime, photoluminescence intensity, and the performance of the vacuum‐annealed BiOI films in solar cells remain unchanged. The results show BiOI to be electronically and optoelectronically robust to percent‐level changes in surface composition. However, from photoinduced current transient spectroscopy measurements, it is found that the as‐grown BiOI films have deep traps located ≈0.3 and 0.6 eV from the band edge. These traps limit the charge‐carrier lifetimes of BiOI, and future improvements in the performance of BiOI photovoltaics will need to focus on identifying their origin. Nevertheless, these deep traps are three to four orders of magnitude less concentrated than the surface point defects induced through vacuum annealing. The charge‐carrier lifetimes of the BiOI films are also orders of magnitude longer than if these surface defects were recombination active. This work therefore shows BiOI to be robust against processing conditions that lead to percent‐level iodine‐, bismuth‐, and oxygen‐related surface defects. This will simplify and reduce the cost of fabricating BiOI‐based electronic devices, and stands in contrast to the defect‐sensitivity of traditional covalent semiconductors.

Hydrogen plasma treatment of β -Ga2O3: Changes in electrical properties and deep trap spectra

The effects of hydrogen plasma treatment of β-Ga2O3 grown by halide vapor phase epitaxy and doped with Si are reported. Samples subjected to H plasma exposure at 330 °C developed a wide (∼2.5 μm-thick) region near the surface, depleted of electrons at room temperature. The thickness of the layer is in reasonable agreement with the estimated hydrogen penetration depth in β-Ga2O3 based on previous deuterium profiling experiments. Admittance spectroscopy and photoinduced current transient spectroscopy measurements place the Fermi level pinning position in the H treated film near Ec-1.05 eV. Annealing at 450 °C decreased the thickness of the depletion layer to 1.3 μm at room temperature and moved the Fermi level pinning position to Ec-0.8 eV. Further annealing at 550 °C almost restored the starting shallow donor concentration and the spectra of deep traps dominated by Ec-0.8 eV and Ec-1.05 eV observed before hydrogen treatment. It is suggested that hydrogen plasma exposure produces surface damage in the near-surface region and passivates or compensates shallow donors.

Electrical properties of bulk semi-insulating β-Ga2O3 (Fe)

The Fermi level in bulk semi-insulating β-Ga2O3 doped with Fe (∼5 × 1018 cm−3) is found to be pinned near Ec − 0.85 eV. At temperatures ≥400 K, Ni Schottky diodes showed good rectification and measurable low frequency capacitance, allowing the measurement of capacitance-frequency (C-f), capacitance-voltage (C-V), and capacitance-temperature (C-T) characteristics. The activation energy and the electron capture cross section obtained were (0.75–0.82) eV and (2–5) × 10−15 cm2, in good agreement with the reported signature of the E2 electron trap assigned to Fe. The concentration of the filled centers determined from C-V was close to the concentration of residual shallow donors in undoped materials. Photoinduced current transient spectroscopy measurements showed that Fe doping does not promote the generation of high densities of deep traps other than those related to Fe.

Identification of intrinsic deep level defects responsible for electret behavior in TlGaSe2 layered semiconductor

Unusual behavior of pyroelectric current signal polarity near the Curie point (Tc) was observed for TlGaSe2 a ferroelectric-semiconductor. It has been revealed that the polarity of the spontaneous polarization near Tc depends on the sample poling prehistory. In particular, applying an external electric field only in the temperature range of the paraelectric state during cooling regime in darkness brought to the depolarization current at Tc with the sign opposite to the external field polarity. Otherwise, if the sample was poled in the temperature interval of the incommensurate phase, pyroelectric current exhibits a peak at Tc with the polarity that is the same as for the external poling electric field. These observations indicate that internal electric field is present in the bulk and near-surface layer regions of the electrically poled single crystal TlGaSe2. Possible mechanisms and origins responsible for the internal electric fields in TlGaSe2 are discussed. It is shown that the formation of internal electric fields in TlGaSe2 is due to charging of intrinsic native defects during the poling process.Characteristics of electrically active intrinsic defects in TlGaSe2 were investigated by using of Photo-Induced Current Transient Spectroscopy (PICTS) technique. Six deep defect levels in the band gap of TlGaSe2 were determined, which were localized both in the bulk and on the surface of the sample and could be electrically charged. The correlation between polarization effects and PICTS results has been established.It was shown that native deep defects (A3–A6) localized in the bulk of crystal are responsible for hetero-charge formation and negative sign of the pyroelectric current peak observed around the Curie temperature after poling the sample in the temperature intervals well above Tc. It was also shown that the positive sign pyrocurrent observed near the Curie point is attributed to the homo-charge formed by native A2-trapping centers which are localized near the surface region of TlGaSe2. Native deep level trap having an activation energy of 0.26eV and the capture cross section of 2.8×10−13cm2 were established for A2 from PICTS measurements.

Defect levels in Cu(In,Ga)Se2 polycrystalline layers by sub-bandgap photo-induced current transient spectroscopy

Photoinduced current transient spectroscopy (PICTS) employing sub-bandgap excitation pulses is used for studying defect levels in polycrystalline thin films of Cu(In,Ga)Se2 (CIGS). It is shown that the persistent photoconductivity effect accompanying photocurrent measurements distorts and often totally obscures PICTS spectra in the case of copper-poor polycrystalline layers. In order to overcome this difficulty, the use of sub-bandgap light in PICTS measurements is proposed. The results for both types of excitation – sub-bandgap (wavelength 1300nm) and above bandgap (wavelength 975nm) – are compared. We show that sub-bandgap light provides better-resolved PICTS spectra than in the case of standard measurements when contribution of photocurrent decay due to persistent photoconductivity is significant. The results for a set of CIGS polycrystalline layers fabricated using various preparation protocols are shown and discussed. Two most pronounced peaks are identified with transitions observed previously in the defect level spectra of the CuInSe2 and CuGaSe2 epitaxial layers.

Defect Characterization in Fully Encapsulated CdZnTe

Fully-encapsulated CZT crystals were grown by vertical Bridgman technique using boron oxide as encapsulant for preventing material decomposition. To detect possible effects of boron on the crystal microstructure, we performed current-voltage measurements and Photo-Induced Current Transient Spectroscopy measurements on samples grown by boron oxide encapsulated vertical Bridgman (set EVB) and standard vertical Bridgman (set VB). In both sets, the well-known A-center and a midgap trap dominate the PICTS spectra. However, significant differences are evident, related to the different defect contents. Our findings indicate that most of the boron atoms are electrically inactive and do not affect the transport properties of the material, confirming that boron oxide vertical Bridgman technique can be adopted for the growth of detector grade CZT crystals.

Laser-induced pulse shapes in partially depleted epitaxial GaAs radiation detectors

Measurements of laser-induced pulse shapes are reported in partially-depleted 150μm thick epitaxial GaAs, carried out as a function of device temperature and bias voltage. Residual impurity concentrations of 1–4×1014cm−3 at room temperature drop to 7×1013cm−3 at 219K, resulting in a significant increase in the depth of the space charge region. A focussed pulsed laser beam was used to probe the cleaved edge of a device and so investigate the dependence of the signal pulse shape on the interaction distance from the Schottky contact. Carrier lifetimes of up to 500μs were observed consistent with charge diffusion in the low-field region of the device. Photo-induced current transient spectroscopy measurements provide further evidence of a number of mid bandgap traps with peak emission rates between 300–330K.

Defect topography on GaAs wafers by microwave‐detected photo‐induced current transient spectroscopy

Photo-generated charge carriers can be detected by microwave absorption. This enables non-destructive measurements of photoconductivity with a spatial resolution depending on the spot size of the light source. In addition, the photoconductivity can be measured as a function of time during light on and off with high time resolution. The resulting photoconductivity transients contain information about defect parameters like conventional methods such as deep level transient spectroscopy or photo-induced current transient spectroscopy, however, with the advantage of high spatial resolution. First results of microwave detected photo-induced current transient spectroscopy measurements on different semi-insulating gallium arsenide samples will be presented.

Heterojunction properties of electrodeposited CdTe/CdS solar cells

Solar cells of p-CdTe/n-CdS structure with a conversion efficiency of 10%±1% (AM1 spectrum, 127 mW/cm2 irradiance) were prepared by electrodeposition of CdTe on CdS-coated conducting glass. CdS was coated by chemical bath deposition. The tin–oxide conducting glass was prepared by the spray pyrolysis technique. The current–voltage, capacitance–voltage, photocurrent–voltage, photocurrent spectroscopy, and photoinduced current transient spectroscopy measurements proved to be powerful tools for the characterization of junction. The concentration of donors and acceptors in both sides of the junction are comparable, in the range of 1016 cm−3. The cell built-in potential is 1.20 V from which 0.65 V drops across the depletion width of CdTe. From the analysis of experimental data to construct the energy band diagram, it becomes evident that an electric dipole layer must exist at the CdTe/CdS interface. The presence of this layer is associated with a discontinuity of electrostatic potential at the interface by 0.28 V and a conduction-band spike of 64 meV. The conduction-band spike and an interfacial recombination center are accountable for the collection losses of photogenerated carriers. The recombination center that is likely related to the interstitial cadmium defect is energetically located 0.63 eV below the conduction-band edge and has a capture cross section of 8.8×10−12 cm2. © 2001 American Institute of Physics.

High Fe2+/3+ trap concentration in heavily compensated implanted InP

High Fe concentrations (up to 2×1019 cm-3) have been implanted in n-doped InP to compensate the substrate donors. The resulting semi-insulating layers have been investigated by current–voltage (I-V) measurements and photo-induced current transient spectroscopy (PICTS) analyses to characterise the Fe activation process and to study the Fe related deep levels. The activation of the Fe2+/3+ trap has been assessed by the identification of the deep level located at EC-0.64 eV. The outcomes of the PICTS measurements have been correlated with the electrically active Fe concentration calculated from a numerical simulation of the I-V characteristics. We observe an increasing linear relation between the electrically active Fe concentration and the substrate doping density, with a maximum active Fe concentration as high as 2×1018 cm-3, i.e. more than an order of magnitude above the equilibrium Fe solid solubility. These data are presented and their implications discussed.

Photoluminescence spectra of Zn1−xCdxAl2Se4-4xS4x single crystals

We investigated the photoluminescence as well as the crystal structure and optical energy gaps of the Zn1-xCdxAl2Se4-4xS4x solid solution system based on the Al-related compounds of ZnAl2Se4, ZnAl2S4, CdAl2Se4, and CdAl2S4. The single crystals of the system with 0.0 ≤ x ≤ 1.0 were grown by the chemical transport reaction technique. The Zn1-xCdxAl2Se4-4xS4x crystallizes in a defect chalcopyrite structure for a whole composition and has an optical energy gap ranging from 3.525 to 3.577 eV at 13 K. The photoluminescence spectra at 13 K showed a strong emission band in the blue spectral region and a weak broad emission band in the visible region due to donor–acceptor pair recombination. The composition and temperature dependence of these bands were examined in the investigated regions. The simple energy band scheme for the radiative mechanisms of the Zn1-xCdxAl2Se4-4xS4x is proposed on the basis of our experimental results along with photo-induced current transient spectroscopy measurements.

Yellow Band and Deep levels in Undoped MOVPE GaN.

Undoped layers of GaN grown by MOVPE on sapphire substrates have been characterized by photoluminescence, photocapacitance and photoinduced current transient spectroscopy (PICTS). Photocapacitance reveals in all samples two specific signatures at photon energies of 1 eV and 2.5 eV. The photocapacitance decrease observed at 1 eV seems to be due to an electron capture process from the valence band, whereas the capacitance increase at 2.5 eV is related to an electron emission process. The fact that the capacitance step at 1 eV is only seen after photoionization at energies above 2.5 eV, and the observed correlation between its amplitude and the photoluminescence intensity of the “yellow band”, lead us to conclude that both transitions are linked to the same trap, which is also suggested to be responsible for the yellow band. The position of this trap, at 2.5 eV below the conduction band, is confirmed by PICTS measurements, that show a hole thermal emission activation energy of 0.9 eV at 350 K.

Optical and electrical properties of rare earth (Yb,Er) doped GaAs grown by molecular beam epitaxy

Electrical and optical properties of erbium-doped and ytterbium-doped GaAs are reported in this article. The studied samples are semi-insulating and have been grown by molecular beam epitaxy. The Yb and Er concentrations in the GaAs epitaxial layers measured by secondary ion mass spectroscopy are 2–3×1017 cm−3. The photoluminescence of Yb intra-4f shell has not been observed, while that of Er has been widely reported. Photoinduced current transient spectroscopy measurements (PICTS) reveal that the Yb doping (Er doping) creates a level in the gap with an activation energy of 0.65 eV (0.67 eV). The depth of such levels may be responsible for the absence of Yb 4f photoluminescence in GaAs:Yb. Moreover, photoconductivity experiments show the presence of rare earth related traps. The energies of these traps correspond exactly to the difference between the gap energy and the corresponding activation energy found by PICTS. These observations confirm the excitation model based on the energy transfer from recombination to the rare earth transitions.

Comparison of PICTS measurements on Si-GaAs interpreted within generalized physical models

Abstract Experimental data measured by the Photo-Induced Current Transient Spectroscopy (PICTS), i.e. the temperature positions of maxima in PICTS spectra versus the time positions of boxcar gates, have been fitted within physical models with different stage of generalization. This procedure has been applied to several important deep levels observed in semiinsulating undoped and chromium doped GaAs crystals and the obtained results have been compared with each other.

Optical and Electrical Measurements of Low-Temperature InAlAs

Photoluminescence, photocurrent, thermally stimulated current and photoinduced current transient spectroscopy measurements done on molecular beam epitaxy In0.52Al0.48As layer, lattice matched to InP are reported. The investigated 1ayers were grown on semi-insulating InP wafers, at temperature range from 215 to 450°C. It was found that the Fermi leve1 was pinned to a dkinant midgap center (most likely similar to EL2 center). Moreover, there were at least 7 other defects but with much smaller concentrations. Their activation energies were equal to 0.076, 0.11, 0.185, 0.295, 0.32 and 0.40 eV. The layers exhibited a very low luminescence and a small photocurrent. PACS numbers: 71.55.Eq, 73.60.Br, 78.55.Cr

Deep level spectroscopy in p-GaSe single crystals

Deep levels have been examined in nonintentionally doped p-type gallium selenide single crystals using photoinduced current transient spectroscopy measurements. Two hole trapping levels at 0.20 and 0.80 eV above the valence band have been observed and the corresponding thermal capture cross section evaluated. The possible origin and nature of these centers are also discussed.