Carrier Transit Research Articles

Low-temperature processed ZnO and CdS photodetectors deposited by pulsed laser deposition

UV-VIS photodetectors using an interdigital configuration, with zinc oxide (ZnO) and cadmium sulfide (CdS) semiconductors deposited by pulsed laser deposition, were fabricated with a maximum processing temperature of 100 °C. Without any further post-growth annealing, the photodetectors are compatible with flexible and transparent substrates. Aluminum (Al) and indium tin oxide (ITO) were investigated as contacts. Focusing on underwater communications, the impact of metal contact (ITO versus Al) was investigated to determine the maximum responsivity using a laser with a 405 nm wavelength. As expected, the responsivity increases for reduced metal finger separation. This is a consequence of reduced carrier transit time for shorter finger separation. For ITO, the highest responsivities for both films (ZnO and CdS) were ∼3 A W−1 at 5 V. On the other hand, for Al contacts, the maximum responsivities at 5 V were ∼0.1 A W−1 and 0.7 A W−1 for CdS and ZnO, respectively.

Enhanced photocurrent and dynamic response in vertically aligned In₂S₃/Ag core/shell nanorod array photoconductive devices.

Enhanced photocurrent values were achieved through a semiconductor-core/metal-shell nanorod array photoconductive device geometry. Vertically aligned indium sulfide (In2S3) nanorods were formed as the core by using glancing angle deposition technique (GLAD). A thin silver (Ag) layer is conformally coated around nanorods as the metallic shell through a high pressure sputter deposition method. This was followed by capping the nanorods with a metallic blanket layer of Ag film by utilizing a new small angle deposition technique combined with GLAD. Radial interface that was formed by the core/shell geometry provided an efficient charge carrier collection by shortening carrier transit times, which led to a superior photocurrent and gain. Thin metal shells around nanorods acted as a passivation layer to decrease surface states that cause prolonged carrier lifetimes and slow recovery of the photocurrent in nanorods. A combination of efficient carrier collection with surface passivation resulted in enhanced photocurrent and dynamic response at the same time in one device structure. In2S3 nanorod devices without the metal shell and with relatively thicker metal shell were also fabricated and characterized for comparison. In2S3 nanorods with thin metal shell showed the highest photosensitivity (photocurrent/dark current) response compared to two other designs. Microstructural, morphological, and electronic properties of the core/shell nanorods were used to explain the results observed.

Modeling of Multiple-Quantum-Well p-i-n Photodiodes

A model for multiple-quantum well (MQW) p-i-n photodiode (PD) is presented. The model accounts for the responsivity spectrum and the polarization-dependent loss. The important physical effects for modeling MQW PDs are the carrier transit time through the MQW layers and the free carrier density in the quantum wells. Furthermore, both effects influence the internal quantum efficiency and saturation of the optical absorption. The model is verified by comparing with the measurement results of a waveguide integrated MQW p-i-n PD. It is suitable for optimizing the design and describing the properties of MQW p-i-n PDs.

Effects of InAlAs strain reducing layer on the photoluminescence properties of InAs quantum dots embedded in InGaAs/GaAs quantum wells

Two kinds of self-assembled quantum dots (QDs) embedded within InGaAs/GaAs quantum wells were grown by molecular beam epitaxy: one was capped with an InAlAs strain reducing (SR) layer, while the other was not. Their emission dynamics was investigated by time-resolved and temperature dependent (TD) photoluminescence (PL) measurements. A significant redshift can be observed in the emission peak position of InAs QDs with thin InAlAs SR cap layer, which results from SR effects. Different behaviors of the integrated PL intensity for the samples with or without InAlAs layer may be ascribed to the reduced carrier transition at higher temperature for the higher energy barrier of the InAlAs layer, and the TD mode of carrier migration. The PL decay time of quantum dots grown with InAlAs layer was much longer than that without the layer, which implies that the InAlAs layer with higher energy barrier may enhance the quantum restriction of carriers in InAs QDs. These observations are discussed from the viewpoint of strain compensation and potential barrier variation with SR layers. Our experiments also demonstrate that the main mode of carrier migration is quantum tunneling effect at lower temperature, while it is quantum transition at higher temperature. The results demonstrate the importance of InAlAs SR layer for the optical quality of InAs QDs.

Optical and electrical properties of MoS2 and Fe-doped MoS2

We prepared undoped and Fe-doped MoS2 layered crystals using a chemical vapor transport method to compare their optical and electrical properties. Optical behaviors of carrier transitions were observed successfully in both undoped and Fe-doped MoS2 samples using reflectance and piezoreflectance. Frequency-dependent photoconductivity (PC) measurements reveal an additional deep Fe doping level for the Fe-doped MoS2 sample. In addition, a longer carrier lifetime was calculated for the Fe-doped MoS2 sample than for the undoped MoS2 sample through PC analysis. Hall measurements were also performed for both samples and indicated that the Fe-doped MoS2 sample exhibited a higher carrier concentration and a lower mobility owing to the effect of Fe dopants. Furthermore, both samples were confirmed to have n-type carriers.

Physics of fully depleted CCDs

In this work we present simple, physics-based modelsfor two effects that have been noted in the fully depleted CCDsthat are presently used in the Dark Energy Survey Camera. Thefirst effect is the observation that the point-spread functionincreases slightly with the signal level. This is explainedby considering the effect on charge-carrier diffusion due to the reduction in the magnitude of the channel potential as collected signal charge acts to partially neutralize the fixed charge in the depleted channel. The resulting reduced voltage drop across thecarrier drift region decreases the vertical electric field andincreases the carrier transit time. The second effect is the observation of low-level, concentric ring patterns seen in uniformly illuminated images. This effect is shown to be most likely due to lateral deflection of charge during the transit of the photo-generated carriers to the potential wells as a result of lateral electric fields. The lateral fields are a result of space charge in the fully depleted substrates arising from resistivity variations inherent to the growth of the high-resistivity silicon used to fabricate the CCDs.

Extracting the charge carrier mobility from the nanosecond photocurrent response of organic solar cells and photodiodes

We present two simple methods to estimate the effective mobility of the faster charge carrier species from the transient nanosecond photoresponse of an organic solar cell or photodiode. In combination with detailed numerical drift-diffusion simulations in the framework of the multiple-trapping model, we identify the energetic relaxation of the charge carriers and hence a decrease of the effective charge carrier mobility while drifting towards the electrodes. From the characteristic shape of the transient current density, the temperature as well as the nonlinear voltage dependence of the charge carrier transit time, we can quantify an exponential trap distribution. In addition, the nonlinearity of the transit time, as also known from comparable time-of-flight measurements, can be explained by charge carrier relaxation processes in the presence of trap states. The effective charge carrier mobility is shown to be field independent but highly temperature dependent.

Time‐resolved electroabsorption measurement of carrier velocity in inverted polarity In1‐xGax N/GaN heterostructures due to internal electric fields

AbstractCarrier transport measurements were made in a c ‐plane inverted‐polarity n ‐GaN/i ‐In1–xGax N/p ‐GaN heterostructure with a 200 nm thick In1–xGax N layer using ultrafast spectroscopy techniques. Time‐domain THz measurements indicate that the direction of electron transport is dominated by drift towards the n ‐GaN layer. Time‐resolved electroabsorption measurements were used to determine carrier velocities by monitoring the change in transmission of a probe beam tuned to the In1‐xGax N absorption edge due to the transport of photogenerated carriers under the built‐in internal electric field. Carrier transit times associated with screening of the electroabsorption are resolved at the lowest injection level. The signal rise time shows two distinct time scales, which correspond to an electron velocity of 3.3 ± 0.5 x 107 cm/s and a hole velocity of 6.7 ± 0.3 x 106 cm/s in an internal electric field of ∼150 kV/cm. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Application of capacitance spectrum and the imaginary part of impedance spectrum to study carrier dynamics of N,N′-diphenyl-N,N′bis(1,1′-biphenyl)-4,4′-diamine

In this report, the hole-only device was fabricated in order to study the carrier mobility of N,N′-diphenyl-N,N′bis(1,1′-biphenyl)-4,4′-diamine through the space charge limited current model. Then the capacitance spectrum and the corresponding imaginary part of impedance spectrum were measured. When the frequency-dependent mathematical physics equations were directly used to fit the capacitance spectrum, it was found that there was obviously error in the curve between the range of ω~τt−1. Considering the distribution of the carrier transit time, two models, the exponential distribution and the Gaussian distribution, were added to fit the experimental curve. Through such distribution functions, the curve could be more satisfactorily to be fitted. And from the logic thinking, we think the Gaussian distribution is more reasonable and more close to the real transit time distribution. Further, we used the imaginary part of the impedance spectra for research and also achieved satisfactory results. Finally, the four kinds of carrier mobility tested by the above four research methods were all found to be in accordance with the Poole–Frenkel model, with only small difference in the values.

Ultrafast terahertz response of multilayer graphene in the nonperturbative regime

The nonlinear dynamics of electrons in multilayer epitaxial graphene is investigated by time-resolved terahertz (THz) spectroscopy in a regime where the interaction of electrons with the external field dominates over scattering processes. The predominantly coherent electron response to the THz field involves both intra- and interband currents, leading to coherently driven interband transitions of carriers and to the generation of higher harmonics of the THz carrier frequency. The overall behavior of the graphene layers is always absorptive, even after generation of an initial electron-hole distribution by femtosecond midinfrared excitation. The results are in agreement with theoretical calculations of the nonperturbative THz response.

Impurity-related terahertz emission from quantum well nanostructures

Terahertz emission in GaAs/AlGaAs quantum well structures doped with shallow impurities was studied in conditions of interband optical excitation for n-doped structures and impurity breakdown in the lateral electric field for p-doped structures. Emission spectra were obtained. It was shown that the observed emission is related to optical transitions of charge carriers between impurity levels and to impurity-band transitions. The depopulation of the final states under interband optical pumping was realized with recombination of non-equilibrium holes and electrons localized at neutral donors.

Impurity-related terahertz emission from quantum well nanostructures

Terahertz emission in GaAs/AlGaAs quantum well structures doped with shallow impurities was studied in conditions of interband optical excitation for n-doped structures and impurity breakdown in the lateral electric field for p-doped structures. Emission spectra were obtained. It was shown that the observed emission is related to optical transitions of charge carriers between impurity levels and to impurity-band transitions. The depopulation of the final states under interband optical pumping was realized with recombination of non-equilibrium holes and electrons localized at neutral donors.

Selectively enhanced UV and NIR photoluminescence from a degenerate ZnO nanorod array film

Energy band engineering is a promising method to tune the photoelectric properties of semiconductors. In this paper, we report an un-element-doped ZnO nanorod array film with a degenerate energy band via annealing in a hydrogen atmosphere. Due to the energy band modification, the photogenerated carrier transitions in the degenerate energy band, involving the valance band, the defect bands and the degenerate conduction band, cause unique photoelectric properties in the degenerate ZnO film. The degenerate ZnO film performs with outstanding conductivity and exhibits an obvious optical absorption in the visible region. Its photoluminescence and photoresponse properties are investigated to understand the fundamentals of photogenerated carrier excitation and recombination in the degenerate ZnO film. Interestingly, the degenerate ZnO film performs with poor photoresponse and has poor photocurrent efficiencies for ultraviolet, blue, and green illuminations, but the photoresponse for near-infrared illumination is attractive. Our results also demonstrate the selectively enhanced down-conversion photoluminescence both at the UV band and at the near-infrared band from the degenerate ZnO film after being excited by an ultraviolet laser source.

Physical modeling and simulation of a high-performance charge sensitive infrared phototransistor

Charge sensitive infrared phototransistors are well known for their capability for response spectrum tuning and single photon detection. In this paper, we established a physical model for a charge sensitive infrared phototransistor operating at \(45\,\upmu \hbox {m}\) wavelength using Crosslight Apsys software. Several key physical mechanisms involved such as inter-subband optical transition and resonant tunneling of carriers were applied and fine tuned to obtain a better simulation result. The calculated absorption spectrum and the simulated data graphs demonstrate that this cell can be used for long wavelength detection with relatively high sensitivity.

Physical prerequisites for formation of active centers of nanosize particles

The features of the energy state of the surface of the crystal and the electronic states in the surface layers of conjugated metal particles are analyzed. Fermi levels’ changes in the contacts metal surfaces are given. Calculations of the statistical distribution of electrical charges on nanoparticles are performed. It is shown that in the presence of excess charge, even equal to elementary one, the modifying activity on modifier nanoparticle is quite high. The emergence of such charges in metals is conditioned by the difference of Fermi levels in the contact faces of the crystallites. Electric charges in the dielectric nanoparticles are present due to the barrier effect which is caused by the uneven transitions of charge carriers at the surface of the juvenile. The emergence of electrically charged particles is explained by the statistical distribution of charges while obtaining nanoparticles both “from above” (by dispersion) and “from below” (by growing). Obviously, in real situations, the emergence of the electric charge of nanoparticles may be caused by the combination of the mentioned physical mechanisms.

Carrier emission from defects in intrinsic hydrogenated amorphous silicon studied by junction-capacitance methods

Capacitance transient measurements have been performed on undoped hydrogenated amorphous silicon samples. The transient decays measured at wide range of temperatures and pulse durations cannot be fitted by power-law (as in the case of n-type) or by a sum of power-law and stretched-exponentials (as in the case of p-type) but with the sum of stretched and squeezed exponentials. The observed decays were attributed to carrier transition from localized states situated near the extreme edges of both band tails. These localized states are due to filamentary structure formed at void surfaces.

Effect of Gd3+ doping on structural, optical and frequency-dependent dielectric response properties of pseudo-cubic BaTiO3 nanostructures

We report on the structural, optical and dielectric characterization of solid state derived, pseudo-cubic nanoscale barium titanates (BTs) with gadolinium (Gd3+) as substitutional dopant. Referring to X-ray diffractograms, apart from the BT peaks related to perovskite structure, the non-existence of any additional peaks due to byproducts has revealed that Gd3+ has undergone substitutional doping into the BT host lattice. The well-separated BT nanoparticles of typical size ∼10–15 nm were observed through electron microscopy studies. Following a direct, allowed type carrier transition (n=1/2), a reduction in the optical band gap value (from 3.28 to 3.255 eV) was observed when the Gd-doping level was varied within 0–7 %. Conversely, the Urbach energy followed an increasing trend, from a value of 0.741 to 1.879 eV. Furthermore, the dielectric constant showed a decreasing tendency with doping content and with increasing frequency. However, in the low-frequency region, the loss tangent (tanδ), which is the combined result of orientational polarization and electrical conduction, was found to be quite high in the doped samples as compared to their un-doped counterpart. The frequency-dependent electrical data were also analyzed in the framework of conductivity and impedance formalisms. In particular, the ac conductivity which varies as ∼ω s approaches ideal Debye behavior (s→1) for a low Gd level and a higher doping concentration did not show improved dielectric feature of the host. The incorporation of rare-earth (Gd3+) ions into the BT host system could greatly manifest dielectric relaxation and carrier conduction mechanisms, in a given frequency range, and thus can find immense scope in miniaturized nanoelectronic elements including ceramic capacitors and transducers.

A phenomenological model for the photocurrent transient relaxation observed in ZnO-based photodetector devices.

We present a phenomenological model for the photocurrent transient relaxation observed in ZnO-based metal-semiconductor-metal (MSM) planar photodetector devices based on time-resolved surface band bending. Surface band bending decreases during illumination, due to migration of photogenerated holes to the surface. Immediately after turning off illumination, conduction-band electrons must overcome a relatively low energy barrier to recombine with photogenerated holes at the surface; however, with increasing time, the adsorption of oxygen at the surface or electron trapping in the depletion region increases band bending, resulting in an increased bulk/surface energy barrier that slows the transport of photogenerated electrons. We present a complex rate equation based on thermionic transition of charge carriers to and from the surface and numerically fit this model to transient photocurrent measurements of several MSM planar ZnO photodetectors at variable temperature. Fitting parameters are found to be consistent with measured values in the literature. An understanding of the mechanism for persistent photoconductivity could lead to mitigation in future device applications.

Hysteretic response of chemical vapor deposition graphene field effect transistors on SiC substrates

Graphene field effect transistors (GFETs) fabricated by chemical vapor deposition graphene deposited onto SiC substrates exhibit sensitivity to broadband visible light. The hysteretic nature of this GFET type was studied utilizing a new current-voltage measurement technique in conjunction with current-time measurements. This measurement method accounts for hysteretic changes in graphene response and enables transfer measurements that can be attributed to fixed gate voltages. Graphene hysteresis is shown to be consistent with electrochemical p-type doping, and current-time measurements clearly resolve a hole to electron to hole carrier transition in graphene with a single large change in gate voltage.

Observation of Higher-Order Sideband Transitions and First-Order Sideband Rabi Oscillations in a Superconducting Flux Qubit Coupled to a SQUID Plasma Mode

We report results of spectroscopic measurements and time-domain measurements of a superconducting flux qubit. The dc superconducting quantum interference device (SQUID), used for readout of the qubit, and a shunt capacitor formed an LC resonator generating a SQUID plasma mode. Higher-order red and blue sidebands were observed in a simple measurement scheme because the resonant energy of the resonator, 600 MHz, was comparable to the thermal energy. We also observed Rabi oscillations on the carrier transition and the first-order sideband transitions. Because the qubit was coupled to a single arm of the dc SQUID, the qubit-SQUID coupling was significant at zero bias current, where these phenomena were observed. The ratios between the Rabi periods for the carrier transition and the sideband transitions are compared with those estimated from the coupling constant, which was separately determined. The result may be explained by assuming initial excitation of the resonator.