Minority Hole Research Articles

Time-resolved cathodoluminescence measurement of the effects of α -particle-related damage on minority hole lifetime in free-standing n-GaN

Time-resolved cathodoluminescence using 30 keV ultrafast electron pulses has been used to perform direct measurements of the minority hole lifetime τh as a function of 3.7 MeV α-particle fluence in high-quality free-standing n-type GaN substrates. The lifetime damage factor K calculated from these measurements was found to monotonically decrease from 6.9 × 10−2 to 6.4 × 10−4 cm2 s−1 ion−1 with increasing α-fluence from 108 to 1012 cm−2, implying a reduction in trap cross section and/or an aggregation of α-induced traps. The small, ∼200–300 nm, hole diffusion length estimated from the minority hole lifetime for the highest α-fluence necessitates the deployment of α-voltaic device strategies and architectures that emphasize depletion and drift over diffusion for effective charge collection and optimal power conversion efficiency.

Self-formed interfacial oxide layer minimizes reverse bias dark current in PbS colloidal quantum dot photodiodes

PbS colloidal quantum dot (CQD) photodiodes possess high photoresponsivity and broad spectral sensitivity, making them tremendously attractive for low-cost short-wave infrared (SWIR) sensing. A critical challenge in achieving high detectivity in SWIR photodiodes is to minimize their reverse bias dark current density (Jdark) and elucidate the origin of Jdark. We demonstrate in this work how the self-formed AlOx at the ZnO/Al interface dramatically reduced Jdark of the PbS CQD SWIR photodiodes. In such a simple and controlled way, the interfacial states were thoroughly passivated by post-exposure to dry air, which could block the dominant pathways for minority hole injection from Al electrode. The treated photodiode showed low Jdark down to 1.58 × 10−7 A/cm2 at –0.5 V, high photoresponsivity of 0.62 A/W, and high specific detectivity of 2.05 × 1012 Jones at 1413 nm, establishing a readily approach to optimizing detectivity in PbS CQD SWIR photodetectors.

The thermo-E.M.F. of an n-type silicon: assessment of the contribution due to the presence of minority carriers

In the common thermoelectric theory, minority charge carriers are assumed to be absent in n- or p-type thermoelectric materials. This study considers their presence and evaluates the effects of that presence on the thermo-electromotive force (Thermo-E.M.F.) of a non-degenerate n-type semiconductor. The calculations are done in the case of silicon. The contribution due to the presence of the minority holes to the total Thermo-E.M.F. depends on the thermopower of minority carriers, their electrical and thermal conductivities. It also depends on their bulk and surface recombinations and depends on the majority carriers only through their thermal and electrical conductivities. In the case of silicon, that contribution remains generally very low although it can increase or decrease the total Thermo-E.M.F. depending on the concentration of the doping elements, the bulk and surface recombination rates, and the length of the sample.

Trap-assisted tunneling current and quantum efficiency loss in InGaAsSb short wavelength infrared photo detectors

We report the trap-assisted tunneling current and quantum efficiency (QE) loss in short wavelength infrared In0.22Ga0.78As0.2Sb0.8 photo detectors. Combining experiment data with a current–voltage model, we found that the trap-assisted tunneling current was boosted by increasing Beryllium (Be) doping in active region at room temperature. However higher Be doping level imposes no negative impacts on QE. Four traps with energy levels located at 49 , 60 , 155 and 199 below conduction band minimum in a In0.22Ga0.78As0.2Sb0.8 alloy are extracted from the fitting of I–V curves. Transparency measurement of an un-intentional doped In0.22Ga0.78As0.2Sb0.8 sample yields an absorption coefficient of 5191 cm−1 at 2.25 . Combining with the measured value of absorption coefficient, the QE dependence on diode length of In0.22Ga0.78As0.2Sb0.8 photo detectors is presented. Finally, by fitting quantum efficiencies of In0.22Ga0.78As0.2Sb0.8 photo detectors, we obtained that the minority electrons diffusion length is larger than 4 and the minority holes diffusion length is 0.2 . QE loss occurs at top N region of In0.22Ga0.78As0.2Sb0.8 photo detectors due to a short holes diffusion length.

Minority-carrier dynamics in β-gallium oxide probed by depth-resolved cathodoluminescence

The behavior of hole polarons in β-gallium oxide (Ga2O3) has attracted significant attention. Depth-resolved cathodoluminescence (CL) was used to investigate the minority carrier dynamics in β-Ga2O3. First, a model describing CL intensity was proposed by considering the depth-dose function and surface recombination. A universal depth-dose function for β-Ga2O3, which has the form of a third-degree polynomial, was presented based on Monte Carlo simulation by introducing a normalized depth, which is the depth normalized by the electron beam range. Second, two experimental approaches, plan-view and cross-sectional CL measurements, were applied to unintentionally doped β-Ga2O3 (−201) wafers, and the experimental results were compared with those of the proposed model. The hole diffusion length was estimated to be within the range of 200–400 nm through the plan-view measurement, whereas a hole diffusion length of 250 nm was obtained through the cross-sectional measurement. The values were consistent with each other, and the model reproduced the experimental results well. This indicates that the nonequilibrium minority hole in the unintentionally doped β-Ga2O3 is mobile and forms a ‘weak’ polaron. The reduced recombination velocity of the (−201) face was estimated to be approximately ten for the plan-view measurement, whereas that of ten or more was assumed for the cross-sectional measurement. No inconsistency was observed, but the low-energy plan-view measurement is considered more suitable for investigating the surface recombination velocity.

Experimental Verification of Single‐Type Electron Population in Indium Tin Oxide Layers

Accurate determination of electronic transport properties of individual transparent conductive oxide layers, namely indium tin oxide (ITO), is essential for further development and design of photonic devices with ITO layer as a tunable ultrafast optoelectronic component. Precise magnetotransport measurements are here implemented to achieve carrier mobility distribution that gives insight into types and characteristics of carrier species. ITO thin films with various sheet resistance of ≈10, 75, and 350 Ω sq−1, respectively, are examined at near‐room temperature. Unimodal mobility distribution is revealed in ITO films, independently on their resistivity, with no evidence of unseparated contributions from surface or interface states. The electron mobility varies depending on ITO's resistivity, ranging from 36.8 to 47.2 cm2 V−1 s−1 at 300 K. Importantly, no minority hole conduction is present. The ITO thin films exhibit solely bulk‐like conduction with an absence of parallel conductions. In addition, the existence of single‐type electron population in ITO that can be viewed as an important validation of exclusively donor‐type defects and/or impurities contributing to total ITO conductivity is experimentally confirmed. These results indicate that ITO can be viewed as an integrated counterpart for photonic metadevices.

Cold Carrier Injection Mechanism for Gate Oxide Integrity of High Voltage NDMOS

This article studies a reliability mechanism for field-plate-assisted reduced surface field (RESURF) effect 225-V NDMOS devices based on anode hole oxide injection for carriers that are in thermal equilibrium with the surrounding lattice. The injection mechanism is facilitated by a unique combination of layout and application biases that result in electric field vectors pulling low-energy minority hole carriers into oxide traps that overlap the drain potential. The effect of this positive charge trapping along the field oxide is to inhibit the RESURF mechanism, while also weakening the gate oxide where it overlaps the drain that ultimately results in a rupture. To quantify the process, a new accelerated aging technique is described that uses the parasitic n-p-n bipolar parallel of the nMOS channel to significantly increase the number of holes while still maintaining the MOS application voltages needed to enable the mechanism. This provides a cost-effective way to accurately accelerate this reliability mechanism with significantly smaller lead times relative to using higher biases and temperatures. This technique is then used with technical computer-aided design (TCAD) analysis to determine the impact of different manufacturing variables, where process controls are introduced for targeted manufacturing limits that can eliminate this mechanism from occurring.

Maximizing the electronic charge carriers in donor-doped hematite under oxygen-rich conditions via doping and co-doping strategies revealed by density functional theory calculations

The low electronic conductivity of hematite (α-Fe2O3) limits its best performance in many applications. Though highly reducing conditions induce an intrinsic n-type behavior, reaching extremely low oxygen partial pressure (pO2) values is not practical. Alternatively, certain dopants provide hematite with excess electrons at practical pO2 values. This study employs density functional theory with thermodynamic analysis to compute the concentration of electronic defects in hematite as a function of pO2, upon doping with 1% of 3d, 4d, and 5d transition metals. Isothermal Kröger–Vink diagrams at 1100 K are plotted to reveal the charge compensation mechanism controlling the electronic carriers in doped hematite and the maximum attainable pO2 value, which achieves approximately one electron per dopant. A higher pO2 value is a metric for an effective donor. Ti, Zr, Hf, Nb, Ta, Mo, and W are shown to be effective donors, especially Nb, Ta, and W, which achieve a 1:1 electron/dopant ratio around atmospheric pressure and a maximum electron/dopant ratio greater than one. The latter is a new metric introduced in this study to quantify the doping efficacy of a donor. Moreover, our study shows that W, Ta, and Nb co-doping in specific percentages with any of the other investigated dopants ensures the n-type behavior of the co-doped hematite while opening the possibility of improving other properties via the other dopant. The other dopant can be Ni or Co to enhance the surface catalytic properties or Zn to increase the minority hole carriers. Both properties are desirable in applications such as photoelectrochemical cells.

Perspective on III–V barrier detectors

In a photodiode made from a narrow bandgap III–V material such as InSb, the dark current is usually dominated by thermal generation-recombination (GR) in the depletion region. In an XBn or XBp barrier detector, the GR current is suppressed by confining the depletion region to a wide bandgap barrier material with a band alignment that blocks majority carriers. Diffusion limited barrier detectors are essentially unipolar and represent a device architecture with unity gain that is fundamentally different from that of the traditional photodiode. High performance barrier detector arrays spanning the mid- and long-wave infrared atmospheric transparency windows are currently being produced with both bulk alloy and type II superlattice (T2SL) absorbers several micrometers thick. In T2SLs, 5–10 μm diffusion lengths have been demonstrated for both InAs/GaSb XBp and InAs/InAsSb XBn devices. The former exhibit minority electrons with a short lifetime and a high mobility, while the latter exhibit minority holes with a long lifetime and a low mobility. The contrasting behavior is understood in terms of competing GR and Auger recombination mechanisms, and a transition between metallic and nonmetallic conduction. These properties present unique challenges for the future design of monolithic dual band photodetectors.

High-resolution radiation detection using Ni/SiO2/n-4H-SiC vertical metal-oxide-semiconductor capacitor

In this article, we demonstrate the radiation detection performance of vertical metal-oxide-semiconductor (MOS) capacitors fabricated on 20 μm thick n-4H-SiC epitaxial layers with the highest energy resolution ever reported. The 100 nm SiO2 layer was achieved on the Si face of n-4H-SiC epilayers using dry oxidation in air. The Ni/SiO2/n-4H-SiC MOS detectors not only demonstrated an excellent energy resolution of 0.42% (ΔE/E×100) for 5.48 MeV alpha particles but also caused a lower enhancement in the electronic noise components of the spectrometer compared with that observed for the best high-resolution Schottky barrier detectors. The MOS detectors also exhibited a high charge collection efficiency (CCE) of 96% at the optimized operating bias despite the presence of the oxide layer. A drift-diffusion model applied to the CCE vs gate bias voltage data revealed a minority (hole) carrier diffusion length of 24 μm. Capacitance mode deep level transient spectroscopy (C-DLTS) scans in the temperature range 84–800 K were carried out to identify the resolution limiting electrically active defects. Interestingly, the C-DLTS spectra revealed both positive and negative peaks, indicating the simultaneous presence of electron (majority) and hole (minority) trap centers. It has been inferred that at the steady-state bias for the C-DLTS measurement, the MOS detector operates in the inversion mode at certain device temperatures, causing holes to populate the minority trap centers and, hence, manifests minority carrier peaks as well.

Charge and spin transport over record distances in GaAs metallic n -type nanowires

We have investigated charge and spin transport in $n$-type metallic GaAs nanowires ($\ensuremath{\approx}{10}^{17}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ doping level) grown by hydride vapor phase epitaxy (HVPE) on Si substrates. For this doping level, charge and spin transport might appear difficult because of the expected localization of minority holes in the valence band potential fluctuations generated by statistical fluctuations of the donor concentration. In contrast with these expectations, it is found, using spatially and spectrally resolved investigation of the luminescence intensity and circular polarization under laser excitation, that (i) establishment of a charge thermodynamic equilibrium between the photoelectrons and the Fermi sea occurs over a distance from the excitation spot of $2\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$. At this distance, the spin polarization is still observed, implying that photoelectrons have preserved their spin orientation and that the two spin reservoirs remain distinct. (ii) Charge can be transported over record distances larger than $20\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$ at 6 K. (iii) Spatially-resolved investigations show that a photoelectron spin polarization of $20%$ can be transported over a record distance of more than $20\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$. This long distance transport occurs because of the presence of large internal electric fields of ambipolar origin, further enhanced by the spatial redistribution of the Fermi sea. These findings have potential applications for long distance spin transport in $n$-type doped nanowires.

Electron beam probing of non-equilibrium carrier dynamics in 18 MeV alpha particle- and 10 MeV proton-irradiated Si-doped β -Ga2O3 Schottky rectifiers

Minority hole diffusion length and lifetime were measured in independent experiments by electron beam-induced current and time-resolved cathodoluminescence in Si-doped β-Ga2O3 Schottky rectifiers irradiated with 18 MeV alpha particles and 10 MeV protons. Both diffusion length and lifetime exhibited a decrease with increasing temperature. The non-equilibrium minority hole mobility was calculated from the independently measured diffusion length and lifetime, indicating that the so-called hole self-trapping is most likely irrelevant in the 77–295 K temperature range.

Picene and PTCDI based solution processable ambipolar OFETs

Facile and efficient solution-processed bottom gate top contact organic field-effect transistor was fabricated by employing the active layer of picene (donor, D) and N,N′-di(dodecyl)-perylene-3,4,9,10-tetracarboxylic diimide (acceptor, A). Balanced hole (0.12 cm2/Vs) and electron (0.10 cm2/Vs) mobility with Ion/off of 104 ratio were obtained for 1:1 ratio of D/A blend. On increasing the ratio of either D or A, the charge carrier mobility and Ion/off ratio improved than that of the pristine molecules. Maximum hole (µmax,h) and electron mobilities (µmax,e) were achieved up to 0.44 cm2/Vs for 3:1 and 0.25 cm2/Vs for 1:3, (D/A) respectively. This improvement is due to the donor phase function as the trap center for minority holes and decreased trap density of the dielectric layer, and vice versa. High ionization potential (− 5.71 eV) of 3:1 and lower electron affinity of (− 3.09 eV) of 1:3 supports the fine tuning of frontier molecular orbitals in the blend. The additional peak formed for the blends at high negative potential of − 1.3 V in cyclic voltammetry supports the molecular level electronic interactions of D and A. Thermal studies supported the high thermal stability of D/A blends and SEM analysis of thin films indicated their efficient molecular packing. Quasi-π–π stacking owing to the large π conjugated plane and the crystallinity of the films are well proved by GIXRD. DFT calculations also supported the electronic distribution of the molecules. The electron density of states (DOS) of pristine D and A molecules specifies the non-negligible interaction coupling among the molecules. This D/A pair has unlimited prospective for plentiful electronic applications in non-volatile memory devices, inverters and logic circuits.

Vertical carrier transport in strain-balanced InAs/InAsSb type-II superlattice material

Anisotropic carrier transport properties of unintentionally doped InAs/InAs0.65Sb0.35 type-II strain-balanced superlattice material are evaluated using temperature- and field-dependent magnetotransport measurements performed in the vertical direction on a substrate-removed metal-semiconductor-metal device structure. To best isolate the measured transport to the superlattice, device fabrication entails flip-chip bonding and backside device processing to remove the substrate material and deposit contact metal directly to the bottom of an etched mesa. High-resolution mobility spectrum analysis is used to calculate the conductance contribution and corrected mixed vertical-lateral mobility of the two carrier species present. Combining the latter with lateral mobility results from in-plane magnetotransport measurements on identical superlattice material allows for the calculation of the true vertical majority electron and minority hole mobilities; amplitudes of 4.7 ×103 cm2/V s and 1.60 cm2/V s are determined at 77 K, respectively. The temperature-dependent results show that vertical hole mobility rapidly decreases with decreasing temperature due to trap-induced localization and then hopping transport, whereas vertical electron mobility appears phonon scattering-limited at high temperature, giving way to interface roughness scattering at low temperatures, analogous to the lateral electron mobility but with a lower overall magnitude.

Roles of carbon impurities and intrinsic nonradiative recombination centers on the carrier recombination processes of GaN crystals

The relationship between the concentration of carbon (C) impurity, [C], and quantum efficiency (QE) of radiation in n-type GaN crystals was clarified under photo-excitation conditions at room temperature. Since C acts as a trap for minority holes as well as a compensator for majority electrons, the external QE (EQE) and internal QE (IQE) values for the near-band-edge (NBE) emission showed a monotonic increase with decreasing [C], where NBE EQE (IQE) values are 0.02% (0.70%) and 0.53% (14.2%) for [C] = 2.0 × 1016 cm−3 and [C] = 4.0 × 1014 cm−3, respectively, under a cw photo-pumping density of 140 W cm−2.

Minority Carrier Lifetime Extraction Methodology of SiC Epitaxial Layer

SiC has been focused as a strong candidate for high efficient power device semiconductors at high voltage regions1. Recent demonstrations of insulated gate bipolar transistors (IGBT) at a voltage over 15kV have proven the advantage of the use of SiC2. As the output characteristics are determined by the minority (hole) carrier lifetime (τp), measurement and control of the τp is utmost important. Optical methods are limited to measurements on separate wafers3, leading to differ from the real devices. In this presentation, we will propose an extraction methodology of minority carrier lifetime of an SiC epitaxial layer based on electrical measurement of pn diodes. The methodology is based on current measurements on parallel pn diodes with a spacing of W p under two different setups, shown in figure 1. The first one, named “single-setup”, is a setup measuring the center anode current while other anodes are kept open. The another one, named “multi-setup”, is a setup where all the anodes are grounded. The difference between the cathode voltage (V C) under same current, defined as ΔV, reflects the difference in the resistance of the n-base region. Based on device simulations with a 20μm-thick SiC epitaxial layer (N d=4.9×1015 /cm3) as an n-base region, one can find that ΔV is strongly dependent on the τp of the region (fig. 2(a) and (b)). Figure 3 shows the relationship between τp and ΔV at different W p. With this relationship one can obtain a τp precisely once the ΔV is experimentally obtained. In conclusions, we have proposed a methodology to extract the τp of an SiC epitaxial layer based on pn diodes with different setups. The proposed method allows us to estimate the τp with high precision. [1] M. K. Das, Mat. Sci. Forum, 600-603, pp. 1183 (2009). [2] W. Sung, ISPSD, pp. 271 (2009). [3] P. B. Klien, J. Appl. Phys., 103, 033702 (2008). Figure 1

Effectiveness of nanoinclusions for reducing bipolar effects in thermoelectric materials

Bipolar carrier transport is often a limiting factor in the thermoelectric efficiency of narrow bandgap materials (such as Bi2Te3 and PbTe) at high temperatures due to the introduction of an additional term to the thermal conductivity and a reduction in the Seebeck coefficient. In this work, we present a theoretical investigation into the ability of nanoinclusions to reduce the detrimental effect of bipolar transport. Using the quantum mechanical non-equilibrium Green’s function (NEGF) transport formalism, we simulate electronic transport through two-dimensional systems containing densely packed nanoinclusions, separated by distances similar to the electron mean-free-path. Specifically, considering an n-type material, where the bipolar effect comes from the valence band, we insert nanoinclusions that impose potential barriers only for the minority holes. We then extract the material’s electrical conductivity, Seebeck coefficient, and electronic thermal conductivity including its bipolar contribution. We show that nanoinclusions can indeed have some success in reducing the minority carrier transport and the bipolar effect on both the electronic thermal conductivity and the Seebeck coefficient. The benefits from reducing the bipolar conductivity are larger the more conductive the minority band is to begin with (larger hole mean-free-path in particular), as expected. Interestingly, however, the benefits on the Seebeck coefficient and the power factor are even more pronounced not only when the minority mean-free-path is large, but when it is larger compared to the majority conduction band mean-free-path. Finally, we extract an overall estimate for the benefits that nanoinclusions can have on the ZT figure of merit.

24.5% efficient GaAs p-on-n solar cells with 120 µm h−1 MOVPE growth

By enhancing the transport of gas precursors though the chemical boundary layer using a narrow-channel reactor, the growth rate (GR) of GaAs by metal-organic vapor-phase epitaxy has been increased to 120 µm h−1 without saturation with an increase in TMGa supply. The minority hole lifetime in n-GaAs was clearly shortened with an increase in GR from 20 to 120 µm h−1. Consequently, the short-circuit current density as well as conversion efficiency slightly decreased with an increase in GR. Nevertheless, a conversion efficiency of 24.48% could be realized for a GaAs cell with the n-type base layer grown at 120 µm h−1.

Electron injection-induced effects in Si-doped β-Ga2O3

The impact of electron injection, using 10 keV beam of a Scanning Electron Microscope, on minority carrier transport in Si-doped β-Ga2O3 was studied for temperatures ranging from room to 120°C. In-situ Electron Beam-Induced Current technique was employed to determine the diffusion length of minority holes as a function of temperature and duration of electron injection. The experiments revealed a pronounced elongation of hole diffusion length with increasing duration of injection. The activation energy, associated with the electron injection-induced elongation of the diffusion length, was determined at ∼ 74 meV and matches the previous independent studies. It was additionally discovered that an increase of the diffusion length in the regions affected by electron injection is accompanied by a simultaneous decrease of cathodoluminescence intensity. Both effects were attributed to increasing non-equilibrium hole lifetime in the valence band of β-Ga2O3 semiconductor.

Ferrihydrite-Modified Ti-Fe2 O3 as an Effective Photoanode: The Role of Interface Interactions in Enhancing the Photocatalytic Activity of Water Oxidation.

Semiconductor electrodes integrated with cocatalysts are key components of photoelectrochemistry (PEC)-based solar-energy conversion. However, efforts to optimize the PEC device have been limited by an inadequate understanding of the interface interactions between the semiconductor-cocatalyst (sem|cat) and cocatalyst-electrolyte (cat|ele) interface. In our work, we used ferrihydrite (Fh)-modified Ti-Fe2 O3 as a model to explore the transfer process of photogenerated charge carriers between the Ti-Fe2 O3 -Fh (Ti-Fe2 O3 |Fh) interface and Fh-electrolyte (Fh|ele) interface. The results demonstrate that the biphasic structure (Fh/Ti-Fe2 O3 ) possesses the advantage that the minority hole transfer from Ti-Fe2 O3 to Fh is driven by the interfacial electric field at the Ti-Fe2 O3 |Fh interface; meanwhile, the holes reached at the surface of Fh can rapidly inject into the electrolyte across the Fh|ele interface. As a benefit from the improved charge transfer at the Ti-Fe2 O3 |Fh and Fh|ele interface, the photocurrent density obtained by Fh/Ti-Fe2 O3 can reach 2.32 mA cm-2 at 1.23 V versus RHE, which is three times higher than that of Ti-Fe2 O3 .