Minority Carrier Trapping Research Articles

Determination of Traps in Poly(p-phenylene vinylene) Light Emitting Diodes by Chargebased Deep Level Transient Spectroscopy

AbstractCharge-based deep level transient spectroscopy (Q-DLTS) has been used to study the defect states that exist within poly(p-phenylene vinylene) (PPV), a semiconducting polymer with a band gap of about 2.4 eV. The technique allows the determination of activation energies, capture cross-sections and trap concentrations. In some circumstances, it is also possible to distinguish between minority and majority carrier traps. The structures investigated here consisted of ITO/PPV/MgAg light emitting diode (LED) devices. Two types of trapping centres were found. The first type has activation energies in the range 0.49 – 0.53 eV and capture cross-sections of the order of 10-16 – 10-18 cm2. It shows a Poole-Frenkel, field assisted-emission process. This level has been identified as a bulk acceptor-like majority carrier (i.e., hole) trap. The second type has activation energies in the range 0.40 – 0.42 eV and capture cross-sections of the order of 10-19 cm2. This level has been identified as a minority carrier (i.e., electron) trap. This second trap type is therefore expected to limit minority carrier injection into the PPV layer within the LED, and hence reduce electroluminescence under forward bias conditions.

Study of electrically active defects in n-GaN layer

Deep level defects in both p +/n junctions and n-type Schottky GaN diodes are studied using the Fourier transform deep level transient spectroscopy. An electron trap level was detected in the range of energies at E c− E t=0.23–0.27 eV with a capture cross-section of the order of 10 −19–10 −16 cm 2 for both the p +/n and n-type Schottky GaN diodes. For one set of p +/n diodes with a structure of Au/Pt/p +–GaN/n–GaN/n + –GaN/Ti/Al/Pd/Au and the n-type Schottky diodes, two other common electron traps are found at energy positions, E c− E t=0.53–0.56 eV and 0.79–0.82 eV. In addition, an electron trap level with energy position at E c− E t=1.07 eV and a capture cross-section of σ n=1.6×10 −13 cm 2 are detected for the n-type Schottky diodes. This trap level has not been previously reported in the literature. For the other set of p +/n diodes with a structure of Au/Ni/p +–GaN/n–GaN/n +–GaN/Ti/Al/Pd/Au, a prominent minority carrier (hole) trap level was also identified with an energy position at E t− E v=0.85 eV and a capture cross-section of σ n=8.1×10 −14 cm 2. The 0.56 eV electron trap level observed in n-type Schottky diode and the 0.23 eV electron trap level detected in the p +/n diode with Ni/Au contact are attributed to the extended defects based on the observation of logarithmic capture kinetics.

Vertical versus Lateral GaN Schottky Ultraviolet Detectors and Their Gain Mechanism

We have implemented Schottky barrier GaN ultraviolet detectors, both in vertical and in lateral configuration. The devices exhibit a high gain at both reverse and forward bias. The gain mechanism is attributed to trapping of minority carriers at the semiconductor–metal interface, with excellent agreement between the calculated responsivity and the experiment. However, the vertical detector zero-bias responsivity (without gain) is two orders of magnitude higher. This is attributed to improved ohmic back contacts, due to the highly doped buried layer.

Determination of traps in poly(p-phenylene vinylene) light emitting diodes by charge-based deep level transient spectroscopy

Charge-based deep level transient spectroscopy has been used to study the defect states that exist within poly(p-phenylene vinylene) (PPV), a semiconducting polymer with a band gap of about 2.4 eV. The technique allows the determination of activation energies, capture cross sections, and trap concentrations. In some circumstances, it is also possible to distinguish between minority and majority carrier traps. The structures investigated here consisted of indium–tin–oxide (ITO)/PPV/MgAg light emitting diode (LED) devices. Two types of trapping centers were found. The first type has activation energies in the range 0.49–0.53 eV and capture cross sections on the order of 10−16–10−18 cm2. It shows a Poole–Frenkel, field assisted–emission process. This level has been identified as a bulk acceptor-like majority carrier (i.e., hole) trap. The second type has activation energies in the range 0.40–0.42 eV and capture cross sections on the order of 10−19 cm2. This level has been identified as a minority carrier (i.e., electron) trap. This second trap type is therefore expected to limit minority carrier injection into the PPV layer within the LED, and hence reduce electroluminescence under forward bias conditions.

Deep levels and their impact on generation current in Sn-doped InGaAsN

We have investigated deep levels in 1.05 eV, Sn-doped, n-type In0.075Ga0.925As0.975N0.025 lattice-matched to GaAs. The samples were grown by metal–organic chemical vapor deposition. Capacitance–voltage measurements were used to determine the electron concentration in both as-grown and postgrowth annealed samples, and a decrease in net electron concentration of about 1.5×1017 cm−3 was observed following annealing. Deep level transient spectroscopy measurements are consistent with the presence of four majority-carrier electron traps, E1 (a broad distribution extending from the conduction band edge to approximately EC−0.2 eV), E2(0.36 eV), E3(0.34 eV), and E4(0.82 eV), as well as one minority-carrier hole trap, H1(0.71 eV), in our material. It is shown that E2 and H1, both of which are present in the annealed material only, are likely the same defect observed under conditions of electron and hole emission, respectively. Current–voltage-temperature measurements indicate a thermal activation energy of 0.35 eV for reverse bias current transport, which is in close agreement with the activation energy of E2. It is thus demonstrated that the E2/H1 defect is a recombination–generation center which contributes to current transport in our InGaAsN-based test diode.

Gain mechanism in GaN Schottky ultraviolet detectors

Schottky barrier GaN ultraviolet detectors, both in vertical and in lateral configuration, as well as in a metal–semiconductor–metal geometry were implemented. All devices exhibit a high gain at both reverse and forward bias. The photoresponse in the forward bias is in the positive current direction. We attribute the gain to trapping of minority carriers at the semiconductor–metal interface. The excellent agreement between the calculated responsivity and the experiment indicates that the model is valid for all device structures under study, and represents a unified description of gain mechanism in GaN Schottky detectors.

DLTS measurements on CuInS 2 solar cells

Deep-level transient spectroscopy (DLTS) was used in this work to reveal information about deep defect levels in CuInS 2 based solar cells. A strong variation of the defect spectrum from sample to sample was found. A minority carrier trap at 0.25 eV and a majority carrier trap at 0.35 eV were identified as bulk traps by variation of pulse height. Two majority carrier traps at 0.24 and 0.8 eV were confirmed as interface traps.

Deep-level transient spectroscopy (DLTS) of CdS/CuIn 1− xGa xSe 2-based solar cells prepared from electroplated and auto-plated precursors, and by physical vapor deposition

We fabricated high-efficiency thin-film CuIn 1− x Ga x Se 2-based solar cells from solution-based electroplated, auto-plated precursors and by physical vapor deposition. The devices were characterized by deep-level transient spectroscopy (DLTS). DLTS recorded a new electron trap level in addition to two hole (majority-carrier) trap levels and one electron (minority-carrier) trap level for low-efficiency devices. We believe that the additional electron trap level is an effective recombination center, which leads to the poor performance of the device.

The influence of doping on charge carrier transport in a-Si : H

The influence of doping on the (opto)electronic properties of a-Si : H films is investigated by transient photoconductivity and photo-induced absorption measurements. The decay rate of the majority charge carrier decreases with doping due to fast minority carrier trapping already active at low doping levels. The transition from electron to hole conduction is observed at 3 ppm B 2H 6. At high doping levels the recombination rate becomes faster by recombination via doping-induced defects.

Irradiation-induced defects in Ge studied by transient spectroscopies

Irradiation-induced impurity-point defect complexes have been investigated in n-type germanium crystals that were doped with either antimony or oxygen. Several majority-carrier traps and one minority-carrier trap are characterized by means of deep-level transient spectroscopy and minority-carrier transient spectroscopy. The antimony-vacancy complex $(E$ center) ${E}_{0.37}$ is found to anneal in a way that is fundamentally different from that in silicon, since it is retarded under reverse bias. Temperature-dependent carrier capture cross sections of the E center are an order of magnitude lower than those of the oxygen-vacancy complex $(A$ center) ${E}_{0.27}$ $({\ensuremath{\sigma}}_{n}\ensuremath{\sim}1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}$ and $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17} {\mathrm{cm}}^{2},$ respectively). A trap ${E}_{0.23}$ which is antimony related grows in at room temperature, seemingly by interstitial capture. A trap ${E}_{0.29}$ is assigned to the divacancy, since it is observed after proton irradiation but not after electron irradiation. A minority-carrier trap ${H}_{0.30},$ displaying a strong Poole-Frenkel effect, is Sb related and possibly related to the E center. In view of the experiments, we comment on a range of diverging results in the literature.

Understanding carrier trapping in multicrystalline silicon

The physical origin of minority carrier trapping centers in multicrystalline silicon is explored in both gettered and non-gettered material. The experimental evidence suggests that there are two types of trap present. One species can be removed by gettering and is related to the presence of boron–impurity pairs or complexes. The other type is impervious to gettering and is correlated to the dislocation density. Annealing experiments reveal that the trapping centers caused by boron–impurity complexes can be dissociated, and that these trapping centers do not contribute to recombination. The effect of trapping centers on open-circuit voltage is shown to be negligible when the trap density is less than the dopant density.

Fabrication and characterization of metal–semiconductor–metal (MSM) ultraviolet photodetectors on undoped GaN/sapphire grown by MBE

Interdigital metal–semiconductor–metal (MSM) ultraviolet photoconductive detectors have been fabricated on undoped GaN films grown by molecular beam epitaxy (MBE). Response dependence on wavelength, applied current, excitation powers and chopper frequency has been extensively investigated. It is shown that the photodetector's spectral response remained nearly constant for wavelengths above the band gap and dropped sharply by almost three orders of magnitude for wavelengths longer than the band gap. It increases linearly with the applied constant current, but very nonlinearly with illuminating power. The photodetectors showed high photoconductor gains resulting from trapping of minority carriers (holes) at acceptor impurities or defects. The results demonstrated the high quality of the GaN crystal used to fabricate these devices.

Deep Level Transient Spectroscopic Analysis on Au/SiO2/InP MOS Structures

Deep Level Transient Spectroscopy (DLTS) studies have been carried out on Au/SiO2/InP Metal–Oxide–Semiconductor (MOS) diodes. Majority and minority carrier traps have been observed for the room temperature SiO2 deposited samples. The effect of substrate temperature during SiO2 deposition has been analysed using DLTS. It has been observed that the minority carrier trap is removed while SiO2 deposition is carried out at an elevated temperature. The detected traps have been analysed under different reverse bias values to distinguish between the bulk and interface traps.

Boron-related minority-carrier trapping centers in p-type silicon

Photoconductivity-based measurements of recombination lifetimes in multicrystalline silicon are often hampered by carrier trapping effects, which cause a characteristically large relative increase in the photoconductance. Single-crystal p-type float-zone wafers of varying resistivities were cross contaminated with multicrystalline wafers that exhibited such trapping. A proportion of the impurities present in the multicrystalline samples was found to effuse into the float-zone wafers, where they act as both recombination centers and trapping centers. By the application of a simple theoretical model, the trap density in the float-zone samples was determined, and found to be directly proportional to the boron-dopant concentration. These results suggest that the trapping centers are caused by boron-impurity pairs.

Deep level analysis of radiation-induced defects in Si crystals and solar cells

Deep level transient spectroscopy (DLTS) analysis of radiation-induced defects in p-type Si crystals and solar cells has been carried out to clarify the mechanism of the anomalous degradation of Si n+–p–p+ structure space cells induced by high-energy, high-fluence electron/proton irradiations. A large concentration of a minority-carrier trap with an activation energy of about 0.18 eV has been observed in irradiated p-Si using DLTS measurements, as well as the majority-carrier traps at around Ev+0.18 eV and Ev+0.36 eV, Correlations between DLTS data and solar-cell properties for irradiated and annealed Si diodes and solar cells have shown that type conversion of p-Si base layer from p-type to n-type is found to be mainly caused by introduction of the 0.18 eV minority-carrier trap center, that is, this center acts as a deep-donor center. The Ev+0.36 eV majority-carrier trap center is thought to also act as a recombination center that decreases minority-carrier lifetime (diffusion length). Moreover, origins of radiation-induced defects in heavily irradiated p-Si and generation of deep-donor defect has also been discussed.

Effects of Annealing on Type Converted Si and Space Solar Cells Irradiated with Heavy Fluence 1 MeV Electrons

We present a detailed quantitative study of the thermal annealing characteristics of deep level defects in Si and space solar cells with boron-doped p-Si base layer, introduced by 1 MeV electrons irradiation. Present isochronal annealing provides an overall different annealing behavior of the defects in type converted (n-type) heavy dose electrons (φ=1×1017 e/cm2) irradiated samples, contrary to earlier low dose electrons studies. Isochronal annealing provides evidence that the minority carrier trap (EC-0.18 eV) and majority carrier trap (EC-0.71 eV) that appear after type conversion play a dominant role in carrier removal and are the major defects responsible for the type conversion as well as the severe degradation of space solar cells. This study also sheds light on the fact that heavy electron irradiation not only changes the structure of the device (p to n-type) but also makes the defect structure more complex as compared to simple defects structure in low dose samples.

Majority- and minority-carrier traps in Te-doped AlInP

The properties of deep levels found in Te-doped AlInP grown by metal–organic chemical vapor deposition have been studied. By using pn-junction structure, both minority- and majority-carrier traps can be observed. Two deep levels are found in Te-doped AlInP: one majority-carrier trap and one minority-carrier trap. The activation energies of majority- and minority-carrier traps are 0.24±0.05 and 0.25±0.03 eV, respectively. The majority-carrier trap is uniformly distributed, indicating that this level belongs to some kind of bulk defect.

Recombination and trapping in multicrystalline silicon

Minority carrier recombination and trapping frequently coexist in multicrystalline silicon (mc-Si), with the latter effect obscuring both transient and steady-state measurements of the photoconductance. In this paper, the injection dependence of the measured lifetime is studied to gain insight into these physical mechanisms. A theoretical model for minority carrier trapping is shown to explain the anomalous dependence of the apparent lifetime with injection level and allow the evaluation of the density of trapping centers. The main causes for volume recombination in mc-Si, impurities and crystallographic defects, are separately investigated by means of cross-contamination and gettering experiments. Metallic impurities produce a dependence of the bulk minority carrier lifetime with injection level that follows the Shockley-Read-Hall recombination theory. Modeling of this dependence gives information on the fundamental electron and hole lifetimes, with the former typically being considerably smaller than the latter, in p-type silicon, Phosphorus gettering is used to remove most of the impurities and reveal the crystallographic limits on the lifetime, which can reach 600 /spl mu/s for 1.5 /spl Omega/cm mc-Si. Measurements of the lifetime at very high injection levels show evidence of the Auger recombination mechanism in mc-Si. Finally, the surface recombination velocity of the interface between mc-Si and thermally grown SiO/sub 2/ is measured and found to be as low as 70 cm/s for 1.5 /spl Omega/cm material after a forming gas anneal and 40 cm/s after an anneal. These high bulk lifetimes and excellent surface passivation prove that mc-Si can have an electronic quality similar to that of single-crystalline silicon.

Photoconductivity of C<sub>60</sub>-H<sub>2</sub>Pc Coevaporated Thin Films

Changes in electronic and photoelectronic properties of fullerene (C60) and metal-free phthalocyanine (H2Pc) coevaporated thin films were investigated by changing their mixing ratio. The influences of thermal annealing and oxygen adsorption on the properties were also investigated. The electronic and photoelectronic properties changed by incororation of H2Pc into C60, and thermal annealing changed their properties dramatically. Thin films in which one component was doped lightly into the other showed high photoconductive sensitivity in comparison with that of C60 or H2Pc alone in over all visible region. Existence of sensitization center and process caused from minority carrier trapping was confirmed from thermal quenching of photoconductivity. Mechanisms for carrier generation and transport were discussed using a model in which photons of high and low energies were absorbed in C60 and H2Pc molecules, respectively, followed with majority carrier injection and trapping.

Surface Recombination Kinetics at the GaAs/Electrolyte Interface via Photoluminescence Efficiency Measurements

Photoluminescence from GaAs/electrolyte junctions can provide detailed information concerning the kinetics of surface recombination. Here we present the results of a study of picosecond laser excited photoluminescence from the n-GaAs/Na2S electrolyte junction. We have developed a model for the dependence of the photoluminescence efficiency (PLE) on the surface minority carrier trapping rate following photoexcitation with a short light pulse. The model indicates that the functional dependence of the PLE on the surface minority carrier trapping rate is identical to that under CW excitation. The PLE from the n-GaAs/Na2S junction is shown to depend strongly on the excitation intensity, which we attribute to surface trap state filling. We have used our model to measure the surface minority trapping velocity as a function of excitation power at the flat band potential. From these measurements we conclude that the residence time of minority carriers within the trap states is on the order of microseconds, which i...