Photoinduced Current Transient Spectroscopy Research Articles

Characterization of deep level defects in Tl6I4S single crystals by photo-induced current transient spectroscopy

Defect levels in semi-insulating Tl6I4S single crystals grown by the horizontal Bridgman technique have been characterized using photo-induced current transient spectroscopy (PICTS). These measurements revealed six electron traps located at (0.059 ± 0.007), (0.13 ± 0.012), (0.31 ± 0.074), (0.39 ± 0.019), (0.62 ± 0.110), and (0.597 ± 0.105). These defect levels are attributed to vacancies (VI, VS) and antisite defects (IS, TlS, TlI) upon comparison to calculations of native defect energy levels using density functional theory and defects recently reported from photoluminescence and photoconductivity measurements.

Effects of surface ligands on energetic disorder and charge transport of P3HT:CdSe hybrid solar cells

In this work, the effects of surface ligands of cadmium selenide (CdSe) nanocrystals on energetic disorder and charge transport of poly (3‐hexylthiophene‐2,5‐diyl):cadmium selenide (P3HT:CdSe) hybrid bulk heterojunction solar cells were studied. Photo‐induced current transient spectroscopy (PICTS) was employed to quantitatively characterize the trap state energies in P3HT:CdSe hybrid solar cells from pyridine (Py) and tert‐butylthiol (tBT) treated CdSe nanocrystals. A data processing workflow to extract the trap emission spectra was implemented in the form of solving an inverse problem with regularization techniques. We observed significant differences in the trap state distribution in P3HT:CdSe(tBT) compared to P3HT:CdSe(Py) solar cells, and we found that devices based on P3HT:CdSe(Py) had deeper level trap states. Additionally, transient photoconductivity measurements were performed on P3HT:CdSe solar cells for the purpose of examining the effects of ligands on charge transport. The results were self‐consistent with PICTS analysis in that P3HT:CdSe(tBT) devices exhibited a greater than three‐fold enhancement in carrier mobility relative to P3HT:CdSe(Py) devices, which we attributed to a decreased depth of trap states.

Detection of dislocation‐related midgap levels in pulsed laser deposited GaN by photo‐induced current transient spectroscopy

Deep levels in unintentionally doped GaN epilayers grown on sapphire (0001) by ultra high vacuum pulsed laser deposition at 600 and 700 °C have been studied using photo‐induced current transient spectroscopy (PICTS). The GaN epitaxial layers were deposited by laser ablating a hydride vapor phase epitaxy grown bulk GaN polycrystalline target in the ambient of r.f. nitrogen radicals. The activation energy of dark conductivity for these layers lies in the range 0.17–0.25 eV. An acceptor‐like deep trap level is detected in the PICTS spectrum with activation energy of 1.32 eV. For the 700 °C grown GaN layer, an additional well‐pronounced acceptor‐like defect level near mid‐gap at 1.52 eV has been observed. From the analysis of trap capture kinetics, both defect levels are identified to be extended ‘bandlike’ electronic states associated with extended defects such as dislocations present in GaN layers.

Behavior of deep level defects on voltage-induced stress of Cu(In,Ga)Se 2 solar cells

Abstract The behavior of deep level defects by a voltage-induced stress for CuInGaSe 2 (CIGS) solar cells has been investigated. CIGS solar cells were used with standard structures which are Al-doped ZnO/i-ZnO/CdS/CIGSe 2 /Mo on soda lime glass, and that resulted in conversion efficiencies as high as 16%. The samples with the same structure were isothermally stressed at 100 °C under the reverse voltages. The voltage-induced stressing in CIGS samples causes a decrease in the carrier density and conversion efficiency. To investigate the behavior of deep level defects in the stressed CIGS cells, photo-induced current transient spectroscopy was utilized, and normally 3 deep level defects (including 2 hole traps and 1 electron trap) were found to be located at 0.18 eV and 0.29 eV above the valence band maximum (and 0.36 eV below the conduction band). In voltage-induced cells, especially, it was found that the decrease of the hole carrier density could be responsible for the increase of the 0.29 eV defect, which is known to be observed in less efficient CIGS solar cells. And the carrier density and the defects are reversible at least to a large extent by resting at room-temperature without the bias voltage. From optical capture kinetics in photo-induced current transient spectroscopy measurement, the types of defects could be distinguished into the isolated point defect and the extended defect. In this work, it is suggested that the increase of the 0.29 eV defect by voltage-induced stress could be due to electrical activation accompanied by a loss of positive ion species and the activated defect gives rise to reduction of the carrier density.

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.

Pyroelectric properties and structural defects of a layered TlInS2 crystal doped with lanthanum

The results of investigations of the pyroelectric current in a layered TlInS2 crystal doped with lanthanum are presented. A comparative analysis of the obtained results and data of photo-induced current transient spectroscopy has been performed. The revealed features of the contribution from electrically active defects to pyroelectric properties of the crystal have been discussed.

Photo-Induced Current Transient Spectroscopy of Semi-insulating Single Crystal Cs2Hg6S7

The ternary compound Cs2Hg6S7 has shown considerable promise as a wide gap semiconductor for hard radiation detection at room temperature. We report on the measurement of defect levels in Cs2Hg6S7 using photo-induced current transient spectroscopy. We observe a series of defect levels with mean activation energies of 0.053, 0.052, 0.34, 0.35, and 0.46 eV. The defects are attributed to Cs vacancies and Cs and Hg antisite defects. Defect capture cross-sections are in the range 10−20–10−15 cm2.

Photo-induced current transient spectroscopy of single crystal Tl6I4Se

The compound Tl6I4Se is a promising wide band gap semiconductor for hard radiation detection at room temperature. To further improve its detection efficiency, native defects have been investigated using photo-induced current transient spectroscopy (PICTS). We observe two shallow acceptor levels with mean activation energies of 76, 175 meV, and two shallow donor defects 62, and 96 meV, respectively. No deeper donor levels are observed. The levels are attributed to native point defects. Defect capture cross sections in the range 10−21 to 10−18 cm2 were measured. The small capture cross sections are attributed to the effective screening of the defects due to a large static dielectric constant.

Photoelectric activity of structural defects of a single crystal of the ferroelectric-semiconductor TlInS2: La

Photoinduced current transient spectroscopy has been used in studying deep-level defects of the ferroelectric-semiconductor TlInS2. The specific features of defect detection in the lanthanum-doped and undoped single crystals have been compared. The explanation of the decrease in the photoelectric activity of defects in the doped sample has been proposed.

Identification of Structures of the Deep Levels in 4H-SiC

We investigated annealing behavior of the carrier lifetime and the deep levels in electron irradiated n-type and semi-insulating 4H-SiCs. We observed two peaks for each sample by photo induced current transient spectroscopy (PICTS) measurements, and their heights depended on annealing temperature. By comparing the annealing behavior of the peak height with reported temperature dependence of concentrations of various defects, we speculated that the observed peaks originate from either VSiVC, CSiVC or EI4.

Electrical properties of deuteron irradiated high resistivity silicon

We have investigated resistivity changes introduced on the high-resistivity p-type silicon wafer by the irradiation with deuteron beam with an energy of 4.4GeV performed in the NUCLOTRON superconducting accelerator. Two contactless techniques were used for the measurements of resistivity changes: namely the microwave split post dielectric resonator (SPDR) technique and capacitance measurements in the frequency domain. The first technique allows resistivity measurements in the plane of the wafer, while the second one in the direction perpendicular to the wafer. The resistivity map obtained with the SPDR technique enabled us to obtain a permanent fingerprint of the accelerator beam intensity profile. It has been shown that after the irradiation, the material resistivity increased to ∼3.9×105Ωcm in the wafer region exposed to the maximum beam intensity. Complementary studies of the properties and concentrations of radiation deep-level defects were performed by the high-resolution photo-induced current transient spectroscopy (HRPITS). These studies have shown that the irradiation of the high resistivity silicon with 4.4-GeV deuterons results in the formation of several types of deep-level defects responsible for the charge compensation.

Study of the effect of the stress on CdTe nuclear detectors

CdTe detectors are commonly used for X and γ ray applications. The performance of these detectors is strongly affected by different types of mechanical stress; such as that caused by differential expansion between the semiconductor and its intimate metallic contacts and that caused by applied pressure during the bonding process. The aim of this work was to study the effects of stress on the performance of CdTe detectors. A difference in expansion coefficients induces transverse stress under the metallic contact, while contact pressure induces longitudinal stress. These stresses have been simulated by applying known static pressures. For the longitudinal case, the pressure was applied directly to the metallic contact; while in the transverse case, it was applied to the side. We have studied the effect of longitudinal and transverse stresses on the electrical characteristics including leakage current measurements and γ-ray detection performance. We have also investigated induced defects, their nature, activation energies, cross sections, and concentrations under the applied stress by using photo-induced current transient spectroscopy and thermoelectric effect spectroscopy techniques. The operational stress limit is also given.

Deep Level Transient Spectroscopy System Designed by LabVIEW

Deep level transient spectroscopy (DLTS) and photo induced current transient spectroscopy (PICTS) are commonly used methods for the identification semiconductor impurities and defects. In this paper, a measurement system of DLTS and PICTS has been developed by LabVIEW. A series of different instruments construct this systems hardware (signal generator; current amplifier; capacitance meter; oscilloscope,etc.) while software is also easy to program by LabVIEW. This system demonstrates high generality for both DLTS and PICTS, and data acquired can be stored or read in computer. By contrast, it is much lower cost comparative to commercial DLTS or PICTS system. Testing result of silicon P+N junction coincides with semiconductor theory research.

Defects in 6LiInSe2 neutron detector investigated by photo-induced current transient spectroscopy and photoluminescence

6LiInSe2 is a promising thermal neutron semiconductor detector material. The performance of the detector is affected by the carrier mobility-lifetime products. Therefore, defects that function as carrier recombination centers need to be identified. In this letter, characterization of defect levels in 6LiInSe2 by photo-induced current transient spectroscopy (PICTS) and photoluminescence is reported. PICTS measurements revealed electron-related defects located at 0.22, 0.36, and 0.55 eV and hole-related defects at 0.19, 0.30, and 0.73 eV. Free exciton and donor-acceptor pairs (DAP) emissions were observed. The PICTS defect level values are consistent with those extracted from DAP transitions.

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.

Photo-induced current transient spectroscopy and photoluminescence studies of defects in AgGa0.6In0.4Se2

Defect levels in semi-insulating AgGa0.6In0.4Se2 crystals grown by the horizontal Bridgman technique and annealed in vacuum at 750 °C for 20 days have been characterized using photo-induced current transient spectroscopy (PICTS) and low-temperature photoluminescence (PL). PICTS measurements revealed four electron-related defects located at (0.027 ± 0.008), (0.051 ± 0.005), (0.31 ± 0.01) and (0.98 ± 0.01) eV and two hole-related defects at (0.040 ± 0.012) and (0.73 ± 0.02) eV. Free exciton (FE) and donor–acceptor pairs emissions were observed in low-temperature PL spectra. The defect levels measured from PICTS are consistent with those from PL. The compositional uniformity of the crystals was investigated by employing micro-PL mapping of FE at 77 K. The temperature coefficients of the band-gap energies were estimated and discussed.

Study of asymmetries of Cd(Zn)Te devices investigated using photo-induced current transient spectroscopy, Rutherford backscattering, surface photo-voltage spectroscopy, and gamma ray spectroscopies

Despite these recent advancements in preparing the surface of Cd(Zn)Te devices for detector applications, large asymmetries in the electronic properties of planar Cd(Zn)Te detectors are common. Furthermore, for the development of patterned electrode geometries, selection of each electrode surface is crucial for minimizing dark current in the device. This investigation presented here has been carried out with three objectives. Each objective is oriented towards establishing reliable methods for the selection of the anode and cathode surfaces independent of the crystallographic orientation. The objectives of this study are (i) investigate how the asymmetry in I-V characteristics of Cd(Zn)Te devices may be associated with the TeO2 interfacial layer using Rutherford backscattering to study the structure at the Au-Cd(Zn)Te interface, (ii) develop an understanding of how the concentration of the active traps in Cd(Zn)Te varies with the external bias, and (iii) propose non-destructive methods for selection of the anode and cathode which are independent of crystallographic orientation. The spectroscopic methods employed in this investigation include Rutherford backscattering spectroscopy, photo-induced current transient spectroscopy, and surface photo-voltage spectroscopy, as well as gamma ray spectroscopy to demonstrate the influence on detector properties.

Theory of Deep Level Spectroscopy in Semi-Insulating CdTe

Numerical simulations of thermoelectric effect spectroscopy (TEES) and photo-induced transient current spectroscopy (PICTS) in CdTe were performed with the aim to interpret experimental results and to look for possibilities to obtain credible information about trap capture cross-sections. The TEES and PICTS spectra were calculated by solving kinetic equations. Trap energy and capture cross section were determined by variable heating rate and double gate methods and compared with input data. Deviation of the established capture cross-section from the input value was demonstrated for the strong retrapping. It was concluded that the trap capture cross-section, especially if obtained anomalously low, might be incorrect due to invalid premises of models, which neglect retrapping. The possibility to acquire an information about trap properties from the shape or magnitude of glow curves was checked. It was demonstrated that the magnitude of TEES signal corresponding to slow or fast retrapping depends unequally on the initial optical excitation.

Carrier trapping and recombination in TlGaSe2 layered crystals

AbstractIn nominally undoped layered TlGaSe2 crystals the trapping centers have been investigated by photo‐induced current transient spectroscopy (PICTS). Five acceptor and donor traps have been detected. Quite large magnitudes of capture cross‐sections for donor traps at 0.23 and 0.45 eV have been determined. The depth‐resolved free‐carrier absorption (FCA) technique has been applied for the investigation of the majority hole lifetime, τR, at 295 and 77 K. The pronounced τR reduction with increasing injection level is attributed to the trapping effect of the minority electrons which provides a way of estimating trap centers concentration. A moderate τR variation across layer planes is observed but no carrier diffusion related to the recombination on the external crystal surfaces is detected. Moreover, abnormally sharp τR drop is found for carrier concentration above 1017 cm−3. The possibility of excess hole and electron special separation due to compositional stacking faults and the successive their enhanced recombination at high excess carrier concentrations is discussed.

Photoinduced current transient spectroscopy of defect levels in CuInSe2 and CuGaSe2 epitaxial and polycrystalline layers

Photoinduced current transient spectroscopy (PITS) was used to study defect level spectra in Cu(In,Ga)Se2 chalcopyrite compounds used as absorbers in solar cells. We show that for CuGaSe2 PITS provides similar results as modulated photocurrent (MPC) spectroscopy, while in the case of CuInSe2 much better results are achieved with PITS due to the lack of background illumination necessary in the MPC. We found four defect levels common for both CuInSe2 and CuGaSe2 (three electron traps with one of them acting as a recombination centre and one very deep hole trap). All those deep levels were observed both in the epitaxial and polycrystalline samples so we attributed them to bulk structural defects.