Carrier Drift Length Research Articles

Band Gap Engineering in β-Ga2O3 for a High-Performance X-ray Detector

Gallium oxide (Ga2O3) attracts great attention in the field of X-ray detection because of its ultrawide band gap, high breakdown electric field, and high X-ray absorption coefficient. However, unintentionally doped Ga2O3 tends to have low resistivity because of the shallow donors provided by unintentionally doped impurity elements or intrinsic defects. Iron and magnesium ion doping can increase the resistivity of β-Ga2O3, but the carrier drift length and carrier collection efficiency are greatly reduced because of the introduction of deep-level impurities. Here, Al-doped β-Ga2O3 (β-Ga2O3:Al) single crystals with high resistivity are obtained through band gap engineering. The mechanism by which Al3+ doping can increase the resistivity of the β-Ga2O3 crystal is discussed. A β-Ga2O3:15%Al-based X-ray detector with high resistivity and quality is prepared. The detector demonstrates a high sensitivity of 851.6 μC Gyair–1 cm–2, which is 42 times higher than that of the commercial amorphous Se X-ray detector. Furthermore, the detector exhibits a fast response speed and both rise time and decay time of less than 0.05 s. The high performance of the detector is attributed to the high resistivity and high quality of the crystal. This work presents a method to obtain high-performance X-ray detectors based on β-Ga2O3 single crystals through band gap engineering.

Explaining the Fill‐Factor and Photocurrent Losses of Nonfullerene Acceptor‐Based Solar Cells by Probing the Long‐Range Charge Carrier Diffusion and Drift Lengths

AbstractOrganic solar cells (OSC) nowadays match their inorganic competitors in terms of current production but lag behind with regards to their open‐circuit voltage loss and fill‐factor, with state‐of‐the‐art OSCs rarely displaying fill‐factor of 80% and above. The fill‐factor of transport‐limited solar cells, including organic photovoltaic devices, is affected by material and device‐specific parameters, whose combination is represented in terms of the established figures of merit, such as θ and α. Herein, it is demonstrated that these figures of merit are closely related to the long‐range carrier drift and diffusion lengths. Further, a simple approach is presented to devise these characteristic lengths using steady‐state photoconductance measurements. This yields a straightforward way of determining θ and α in complete cells and under operating conditions. This approach is applied to a variety of photovoltaic devices—including the high efficiency nonfullerene acceptor blends—and show that the diffusion length of the free carriers provides a good correlation with the fill‐factor. It is, finally, concluded that most state‐of‐the‐art organic solar cells exhibit a sufficiently large drift length to guarantee efficient charge extraction at short circuit, but that they still suffer from too small diffusion lengths of photogenerated carriers limiting their fill factor.

Spectral signature of near-surface damage in CdTe X-ray detectors

Knowledge of the response matrix of an X-ray detector is an important prerequisite for efficient use. This matrix is usually complicated due to the multitude of effects that determine the photon interaction and energy detection processes. Consequently, assembly of a response matrix often proceeds via simulation, modelling features of the detector and using experimental data as input.This paper is concerned with one such feature: the spectral signature of a near-surface damage layer in cadmium telluride semiconductor detectors. An empirical modification to the well-known Hecht relation is proposed that allows an improved description of the spectral shape, and an interpretation in terms of a depth-dependent carrier drift length (lifetime) near the surface is developed. Applying this to both the front and the rear surface, good agreement between measurement and simulation is obtained for the 31 keV and the 81 keV photon lines of barium-133. The near-surface modification reaches to a depth of 100–200 μm.

Electrical and optical properties of low-bandgap oxide Zn2Mo3O8 for optoelectronic applications

Semiconducting metal oxides are attractive for various applications since most oxides are non-toxic, stable, and easy to deposit. Wide band-gap materials have been studied more extensively, compared to low bandgap materials, which introduces limitations to the applications of oxides. Study of low band-gap semiconducting oxides can propel the usage of oxides in a wider range of applications. Here, the electronic, structural, and optical properties of Zn2Mo3O8 (ZMO) are investigated. Stoichiometric polycrystalline films of ZMO are deposited using pulsed laser deposition system at room temperature. The unintentionally n-doped films show a hall electron mobility of 0.7 cm2 V−1 s−1 and have a bandgap of 2.1 eV. The photoelectron spectra contain complex peak profiles which are explained to be a manifestation of final state effects. The orbital contribution to the valence band of ZMO is probed using resonant photoelectron spectroscopy, which confirms that the valence band is composed of Mo 4d levels. The conduction and valence band edges are predicted to be at 4.2 eV and 6.3 eV, so most of the conventional wide band-gap oxides can be used as hole-blocking layers with ZMO. Under A.M. 1.5 illumination, single-sided Schottky diode with Fluorine-doped tin oxide/TiO2/ZMO/Au structure shows no photovoltaic action, possibly due to high exciton binding energy and low carrier drift lengths. However, the Schottky device shows a higher current under illumination, which suggests that with improvement in carrier drift lengths, ZMO can find applications in low-cost optoelectronic devices on flexible substrates like plastic or Polyethylene terephthalate.

The origin of performance limitations in miniemulsion nanoparticulate organic photovoltaic devices

Nanoparticulate organic films are an attractive area of organic photovoltaic (OPV) research given their potential for controlling active layer morphology on the nanoscale. However, the power conversion efficiency of these devices remains limited in comparison to the analogous bulk heterojunction technology. Here, we report a systematic characterisation of charge carrier loss pathways in nanoparticulate OPVs prepared using poly(3-hexylthiophene) as a donor and phenyl-C61-butyric acid methyl ester as an acceptor material. Optical modelling of the nanoparticle active layer morphology indicates minimal losses from scattering and negligible plasmon effects from the discrete 40nm particles. A comparison of the modelled internal absorption for the nanoparticle films confirms negligible differences in comparison to a standard bulk heterojunction active layer structure. By contrast, the internal quantum efficiency (IQE), determined with the aid of optically modelled internal active layer and parasitic non-active layer absorption, exhibited values of 24% for the nanoparticle device whilst the bulk heterojunction showed a value of 76%. Subsequent modelling of the EQE and IQE (supported by photoluminescence quenching measurements) indicated an exciton dissociation yield of 24% for nanoparticulate devices in comparison to 81% for the corresponding bulk heterojunction, in excellent agreement with the device internal quantum efficiencies. Transient measurements of charge transport and bimolecular recombination lifetime revealed a charge carrier drift length longer than the film thickness at short circuit conditions for both device structures. Impedance spectroscopy measurements confirmed very low photoinduced chemical capacitances at short circuit and returned charge collection efficiencies in excess of 80% for both devices, suggesting only minor charge collection losses in either device at short circuit. Collectively, these results demonstrate that the dominant photocurrent loss mechanism in nanoparticulate OPVs is a poor charge generation yield rather than reduced light absorption, increased bimolecular recombination or charge extraction barriers.

Colloidal quantum dot solar cell electrical parameter non-destructive quantitative imaging using high-frequency heterodyne lock-in carrierography and photocarrier radiometry

Colloidal quantum dot (CQD) solar cells with a certified power conversion efficiency of 11.28% were characterized using camera-based heterodyne lock-in carrierography (HeLIC) and photocarrier radiometry (PCR). Carrier lifetime, diffusivity, and diffusion and drift length of a CQD solar cell were imaged in order to investigate carrier transport dynamics, as well as solar cell homogeneity and the effects of Au contacts on carrier transport dynamics. Using room temperature HeLIC imaging which has also been demonstrated using PCR measurements, shorter carrier lifetimes (ca. 0.5μs) were found in Au contact regions that can be attributed to enhanced non-radiative recombinations through trap states at Au/CQD interfaces. This imaging methodology shows strong potential for elucidating the energy loss physics of CQD solar cells and for industrial non-destructive large-area photovoltaic device characterization.

Low-field transport in SiO2

Low-field electronic transport trends in silicon dioxide (SiO2) are usually rationalized in terms of a mobility that is established by the rate of carrier (electron or hole) scattering. However, this perspective leads to a dilemma in which the hole mean free path in SiO2 is inferred to be much less than the interatomic spacing. An alternative picture is proposed herein in which electronic transport in an amorphous insulator such as SiO2 is diffusive (as exemplified by Brownian motion) and primarily involves carrier trapping, rather than scattering. Within this framework, electron transport in SiO2 is found to be a non-equilibrium, diffusive process with negligible trapping, whereas hole transport in SiO2 is dominated by quasi-equilibrium trapping. Non- or quasi-equilibrium behavior are distinguished by the relative magnitude of the carrier drift length compared to the SiO2 thickness, or equivalently, by the transit time compared to the carrier capture (trapping) time.

Enhancement of short-circuit current density in Cu2O/ZnO heterojunction solar cells

This study reports the achievement of a high short-circuit current density (Jsc) of 9.53 mA/cm2 for low-cost electrodeposited (ED) semi-transparent Cu2O/ZnO nanorod (NR) solar cells. High-quality chemical-bath-deposited ZnO NRs that align with the carrier collection path were used to replace the traditional sputtered ZnO film. An almost four-fold increase (from 1.63 to 6.41 mA/cm2) in Jsc was obtained with the NRs compared to the level obtained with a sputtered ZnO thin film cell. Decreased the ED Cu2O absorber film thickness is able to compensate for the recombination loss that results from Cu2O's short minority carrier drift and diffusion length (both on the order of 100 nm), further boosting Jsc to 7.77 mA/cm2. Additional photo-generated carriers were created for the semi-transparent solar cells when a silver mirror was deposited on the backside of the glass; this further enhanced absorption and improved Jsc to 9.53 mA/cm2. This is the highest Jsc value reported to date for a low-cost ED Cu2O/ZnO solar cell.

An upgraded drift–diffusion model for evaluating the carrier lifetimes in radiation-damaged semiconductor detectors

The transport properties of a series of n- and p-type Si diodes have been studied by the ion beam induced charge (IBIC) technique using a 4MeV proton microbeam. The samples were irradiated with 17MeV protons at fluences ranging from 1×1012 to 1×1013p/cm2 in order to produce a uniform profile of defects with depth. The analysis of the charge collection efficiency (CCE) as a function of the reverse bias voltage has been carried out using an upgraded drift–diffusion (D–D) model which takes into account the possibility of carrier recombination not only in the neutral substrate, as the simple D–D model assumes, but also within the depletion region. This new approach for calculating the CCE is fundamental when the drift length of carriers cannot be considered as much greater that the thickness of the detector due to the ion induced damage. From our simulations, we have obtained the values of the carrier lifetimes for the pristine and irradiated diodes, which have allowed us to calculate the effective trapping cross sections using the one dimension Shockley–Read–Hall model. The results of our calculations have been compared to the data obtained using a recently developed Monte Carlo code for the simulation of IBIC analysis, based on the probabilistic interpretation of the excess carrier continuity equations.

Analysis of bulk heterojunction material parameters using lateral device structures

We review the key optoelectronic properties of lateral organic bulk heterojunction (BHJ) device structures with asymmetric contacts. These structures are used to develop a detailed model of charge transport and recombination properties within materials used for organic photovoltaics. They permit a variety of direct measurement techniques, such as nonlinear optical microscopy and in situ potentiometry, as well as photoconductive gain and carrier drift length studies from photocurrent measurements. We present a theoretical framework that describes the charge transport physics within these devices. The experimental results presented are in agreement with this framework and can be used to measure carrier concentrations, recombination coefficients, and carrier mobilities within BHJ materials. Lateral device structures offer a useful complement to measurements on vertical photovoltaic structures and provide a more complete and detailed picture of organic BHJ materials.

Electronic structure and defect properties of Tl6SeI4: Density functional calculations

We report density functional calculations of electronic structure, phase diagram, and dielectric, optical, and defect properties of Tl${}_{6}$SeI${}_{4}$. We discuss how electronic structure and defect properties affect resistivity and carrier mobility-lifetime (\ensuremath{\mu}\ensuremath{\tau}) products in Tl${}_{6}$SeI${}_{4}$. We find large Born effective charges due to covalency involving Tl-6$p$ states. High Born charges generally enhance the static dielectric constant. This provides a mechanism for effective screening of charged defects and impurities. We find that high resistivity can be obtained under near-stoichiometric growth conditions via Fermi level pinning near the middle of the band gap by shallow donors and acceptors, as opposed to deep traps that can give high resistivity, but at the expense of short carrier drift lengths. Defect calculations also reveal the presence of deep native donors that may cause electron trapping. The experimentally observed good \ensuremath{\mu}\ensuremath{\tau} products may be explained by a combination of small effective masses and effective screening of charged defects. High resistivity and good \ensuremath{\mu}\ensuremath{\tau} products make Tl${}_{6}$SeI${}_{4}$ a promising room-temperature radiation detector material. We also show the calculated defect diffusion barriers, which affect defect migration under external bias in a detector.

Electrical characteristics of lateral organic bulk heterojunction device structures

Lateral structures have been used to characterize charge transport phenomena in organic bulk heterojunctions. Through the analysis of the current vs. voltage relationships and their light intensity dependence, space charge limited extraction currents and injection currents have been observed and characterized. Additionally, the drift length of charge carriers has been estimated by characterizing devices of varying lengths. These studies show that lateral structures are a promising way to study the basic physics of organic bulk heterojunction materials as they offer degrees of freedom unavailable in sandwich structures and such studies complement what can be learned from conventional sandwich structures.

Built-in electric field thickness design for betavoltaic batteries

Isotope source energy deposition along the thickness direction of a semiconductor is calculated, based upon which an ideal short current is evaluated for betavoltaic batteries. Electron-hole pair recombination and drifting length in a PN junction built-in electric field are extracted by comparing the measured short currents with the ideal short currents. A built-in electric field thickness design principle is proposed for betavoltaic batteries: after measuring the energy deposition depth and the carrier drift length, the shorter one should then be chosen as the built-in electric field thickness. If the energy deposition depth is much larger than the carrier drift length, a multi-junction is preferred in betavoltaic batteries and the number of the junctions should be the value of the deposition depth divided by the drift length.

Model for intermediate band solar cells incorporating carrier transport and recombination

A model for intermediate band solar cells is presented to assess the effect of carrier transport and recombination (CTR) on the efficiency of these devices. The model includes dependencies of physical parameters including optical absorption, carrier lifetime, and carrier transport on the density of intermediate band electronic states. Simulation results using this model indicate that conversion efficiency degrades when the net carrier recombination lifetime is small (range of nanoseconds) or when the device length is long relative to carrier drift length. The intermediate band solar cell model provides a method of determining realistic conversion efficiencies based on experimentally measurable input parameters for CTR. The incorporation of CTR provides insight on the dependence of optimal density of states and energetic position of the intermediate band based on carrier lifetime and mobility. The material ZnTeO (EG=2.3 eV, EI=1.8 eV) is used as a numerical example for the intermediate band solar cell model, where conversion efficiency drops from 30.36% to 19.4% for a 10 μm long device for a recombination lifetime decrease from 1 μs to 5 ns. The optimal impurity concentration is determined to be 1018 cm−3 for an optical absorption cross section of 10−14 cm2. The conversion efficiency of a ZnTe solar cell with a total recombination lifetime of 10 ns is calculated to increase from 14.39% to 26.87% with the incorporation of oxygen.

Effect of dislocations on charge carrier mobility–lifetime product in synthetic single crystal diamond

The authors report correlations between variations in charge transport of electrons and holes in synthetic single crystal diamond and the presence of nitrogen impurities and dislocations. The spatial distribution of these defects was imaged using their characteristic luminescence emission and compared with maps of carrier drift length measured by ion beam induced charge imaging. The images indicate a reduction of electron and hole mobility–lifetime product due to nitrogen impurities and dislocations. Very good charge transport is achieved in selected regions where the dislocation density is minimal.

Charge transport in polycrystalline and single crystal synthetic diamond using ion beam induced charge imaging

Ion beam induced charge (IBIC) imaging has been used to investigate charge transport in high quality polycrystalline CVD diamond and single crystal synthetic diamond. In polycrystalline material the intra-crystallite charge transport within single crystals was studied. Micrometer resolution images of charge signal amplitude in different regions of a diamond crystallite are obtained as a function of the electric field strength. The charge signal amplitude, which is directly related to charge carrier drift length, is correlated to the distribution of radiative centres in the diamond identified using cathodoluminescence imaging. By comparison, IBIC imaging of single crystal synthetic diamond was also carried out. The sample, which contained deliberately introduced nitrogen lines, showed electron and hole CCE values close to 100%, in the low nitrogen regions. By comparison with CL and UV luminescence images, direct evidence is observed for the role of nitrogen and dislocations as deep trapping centres and their reduction of both electron and hole lifetimes.

Memory effects in CVD diamond

CVD diamond, as other wide energy gap semiconductors or insulators, is known to present polarization effects when used as a nuclear particle detector, which are due to the building up of an internal space-charge electric field and are responsible for “memory” effects if the polarization is not erased. Polarization effects are larger when the thickness of the sample is longer than the drift length of carriers and give rise to a shortening of the electric field region inside the sample. In this work, IBIC (Ion Beam Induced Charge) measurements with 1 and 2 MeV proton beams have been carried out on a relatively thin CVD diamond sample with the conclusion that no memory or hysteresis effects were present even by switching from one polarity to the opposite one in subsequent measurements. However, it resulted that the values of the charge collection efficiency (cce) did depend on counting rate and the lowering at large counting rates was attributed to a local polarization due to trapping. Finally, in some cases cce turned out to be larger than 1, a result which could be either due to local detrapping or to non-blocking contacts.

Electronic properties of CVD diamond

The electronic properties of chemical vapour deposited (CVD) diamond are reviewed based on data measured by transient and spectrally resolved photoconductivity experiments, photo-thermal deflection spectroscopy (PDS) and electron paramagnetic resonance (EPR) where substitutional nitrogen (P1-centre) and carbon defects (H1-centre) are detected. The results show that nominally undoped high quality polycrystalline CVD diamond is a n-type semiconductor due to the presence of substitutional nitrogen. The sub-band-gap optical absorption is governed by amorphous graphite present at grain boundaries. Spectrally resolved photoconductivity experiments measured in the same regime are partially dominated by diamond bulk properties which are comparable to single crystalline Ib and IIa diamond and partially by grain boundaries. Mobilities and drift length of carriers are discussed and compared to properties of single crystalline diamond.

Length scales of charge transport in organic photorefractive materials

The drift length of charge carriers has a significant influence on the dynamics of the space-charge field in organic photorefractive materials. This letter introduces a relatively simple method for the determination of the drift length, which takes into account that the charge carrier mobility depends on the sample thickness. By combining results of time-of-flight and holographic time-of-flight experiments using the stochastic transport model of Scher and Montroll, the effective drift length can be determined as 2.4 μm in the investigated photorefractive glass.

High collection efficiency CVD diamond alpha detectors

Advances in Chemical Vapour Deposited (CVD) diamond have enabled the routine use of this material for sensor device fabrication, allowing exploitation of its unique combination of physical properties (low temperature susceptibility (>500/spl deg/C), high resistance to radiation damage (>100 Mrad) and to corrosive media). A consequence of CVD diamond growth on silicon is the formation of polycrystalline films which has a profound influence on the physical and electronic properties with respect to those measured on monocrystalline diamond. We report the optimisation of physical and geometrical device parameters for radiation detection in the counting mode. Sandwich and co-planar electrode geometries are tested and their performances evaluated with regard to the nature of the field profile and drift distances inherent in such devices. The carrier drift length before trapping was measured under alpha particles and values as high as 40% of the overall film thickness are reported. Further, by optimising the device geometry, we show that a gain in collection efficiency, defined as the induced charge divided by the deposited charge within the material, can be achieved even though lower bias values are used.