Net Carrier Density Research Articles

Low contact resistivity at the 10−4 Ω cm2 level fabricated directly on n-type AlN

Ultrawide bandgap aluminum nitride (AlN) stands out as a highly attractive material for high-power electronics. However, AlN power devices face performance challenges due to high contact resistivity exceeding 10−1 Ω cm2. In this Letter, we demonstrate achieving a low contact resistivity at the 10−4 Ω cm2 level via refined metallization processes applied directly to n-AlN. The minimum contact resistivity reached 5.82 × 10−4 Ω cm2. Our analysis reveals that the low contact resistance primarily results from the stable TiAlTi/AlN interface, resilient even under rigorous annealing conditions, which beneficially forms a thin Al–Ti–N interlayer, promotes substantial nitrogen vacancies, enhances the net carrier density at the interface, and lowers the contact barrier. This work marks a significant milestone in realizing superior Ohmic contacts for n-type AlN, paving the way for more efficient power electronic and optoelectronic devices.

Radiation effects of high-fluence reactor neutron on Ni/ β -Ga2O3 Schottky barrier diodes

This study investigates the broad-energy-spectrum reactor-neutron irradiation effects on the electrical characteristics of Ni/β-Ga2O3 Schottky barrier diodes (SBDs), where the irradiated neutron fluence was up to 1 × 1016 cm−2. On the one hand, the high neutron fluence of 1016 cm−2 resulted in a reduction in forward current density by two orders of magnitude and an extremely high on-resistance property due to the radiation-generated considerable series resistance in the SBD. On the other hand, the irradiation brought little influence on the Ni/β-Ga2O3 Schottky contact, since the extracted ideality factor and barrier height from temperature-dependent current–voltage (I–V–T) characteristics showed no significant changes after the radiation. Moreover, the capacitance–voltage (C–V) characterization revealed that the net carrier density in the β-Ga2O3 material was only reduced by 25% at the neutron fluence of 1015 cm−2 but a significant reduction by 2–3 orders at 1016 cm−2. Within the neutron fluence range of 2 × 1014 cm−2 up to 1016 cm−2, the carrier removal rates trended to be saturated with the increased fluences, following an exponential regular. In addition, the C–V measurement on the 1016 cm−2 irradiated sample exhibited an obvious frequency dispersion, and the extracted carrier distribution was not uniform.

Degradation behavior of CIGS solar Cells: A parametric analysis

A parametric study of the effect of bulk and interface properties on device characteristics as a function of heat and light stress under open-circuit and short-circuit conditions is developed for Cu(In,Ga)(S,Se)2 solar cells using SCAPS-1D simulator. The variables and interdependencies modeled include: (1) conduction band offset (CBO) between buffer and absorber; (2) buffer ionized donor density; (3) CIGS shallow acceptor density; (4) CIGS deep acceptor density; (5) CIGS shallow donor density; (6) ionized acceptor concentration in the ordered vacancy compound (OVC) CIGS/buffer interface; and (7) back contact work-function. An increase in absorber shallow acceptor or a decrease in shallow donor density results in an increase in net carrier density and open-circuit voltage (VOC). A decrease in VOC is observed with an increase in the deep acceptors or neutral midgap defects in the CIGS due to higher charge carrier recombination. A change of CBO from spike to cliff condition is a dominant mechanism of decreasing VOC with an interface defect density of 1012 cm−2. The minimum depletion width is found to be specifically sensitive to CBO and interface defect density. An inflection in the capacitance–voltage curves (in reverse bias) is observed with simultaneous increase in bulk deep acceptor density and shallow donor density near the back contact.

Tuning the electronic band structure in a kagome ferromagnetic metal via magnetization

Materials with zero energy band gap display intriguing properties including high sensitivity of the electronic band structure to external stimulus such as pressure or magnetic field. An interesting candidate for zero energy band gap is Weyl nodes at the Fermi level ${E}_{\mathrm{F}}$. A prerequisite for the existence of Weyl nodes is to either have inversion or time reversal symmetry broken. Weyl nodes in systems with broken time reversal symmetry are ideal to realize the tunability of the electronic band structure by magnetic field. Theoretically, it has been shown that in ferromagnetic Weyl materials, the band structure is dependent upon the magnetization direction and thus the electronic bands can be tuned by controlling the magnetization direction. Here we demonstrate, by analysis of the Hall resistivity, tuning of the band structure in a kagome Weyl ferromagnetic metal ${\mathrm{Fe}}_{3}{\mathrm{Sn}}_{2}$ with magnetization and magnetic field. Owing to spin-orbit coupling, we observe changes in the band structure depending on the magnetization direction that amount to a decrease in the net carrier density by a factor of 4 when the magnetization lies in the kagome plane as compared to when the magnetization is along the $c$ axis. Our discovery opens the way for tuning the carrier density in ferromagnetic materials.

Effects of the addition of graphene on the defect-related photoluminescent and electrical properties of n-type ZnO thin films

This study determines the effect of the addition of graphene on the photoluminescent and electrical properties of ZnO films that are prepared using the sol-gel method. ZnO and graphene-doped ZnO thin films that are annealed at 500 °C for 30 min in nitrogen exhibit n-type conductivity. The correlation between photoluminescence (PL) spectral features and defects of ZnO means that there are oxygen vacancies (VO), zinc vacancies (VZn) and oxygen interstitials (Oi) in a ZnO thin film and there are VO, VZn, Oi in a graphene-doped ZnO thin film. The PL result shows that the incorporation of graphene into ZnO leads to a decrease in the number of VO, VZn and Oi and a transition from VZn to the substitution of carbon for zinc (CZn). The decreased number of VO and Oi is attributed to the substitution of VO by Oi, so equal numbers of acceptors and donors are annihilated and there is no change in the net electron carrier density. The transition from VZn to CZn increases the electron carrier concentration, which is opposite to the electrical observation. The results from PL, X-ray diffraction and X-ray photoelectron spectroscopy measurements demonstrate that a CZn+2Oi complex is formed in a graphene-doped ZnO thin film. The decrease in the electron concentration in a graphene-doped ZnO thin film is associated with the formation of the acceptor-like CZn+2Oi complex.

Cathode luminescence analysis of Cu(In,Ga)Se2 solar cells treated with thiourea solution

Cross-sectional cathode luminescence (CL) was performed on working Cu(In,Ga)Se2 (CIGS) solar cells to clarify the mechanism behind the performance improvement of solar cells subjected to a thiourea treatment. The low panchromatic CL intensity at the depletion zone demonstrates that the existent electric field sweeps the carriers nearby, lowering their possibility of non-radiative and radiative recombination. Accordingly, the low CL intensity at grain boundaries are not necessarily caused by high non-radiative recombination rate if band bending is present. The photon energy mapping showed an emission distribution according to the double-graded bandgap profile. A blue shift in photon emission at the minimum bandgap area was observed for the thiourea-treated sample. We presume that S ions provided by the thiourea solution passivate subgap defects such as donor-type Se vacancies, resulting in radiative transitions with higher energies and higher net carrier density that eventually contributed to the higher fill-factor and higher open-circuit voltage.

Boosting Carrier Mobility in Zinc Oxynitride Thin-Film Transistors via Tantalum Oxide Encapsulation.

Novel TaO x encapsulation was presented to enhance the field-effect mobility (μFE) of ZnON thin-film transistors (TFTs) consisting of a metallic Ta film deposited onto the ZnON surface followed by a modest annealing process. The resulting TaO x/ZnON film stack exhibited a more uniform distribution of nanoscale ZnON crystallites with increased stoichiometric anion lattices compared to the control ZnON film. The control ZnON TFTs exhibited a reasonable μFE, subthreshold gate swing (SS), and ION/OFF ratio of 36.2 cm2/V·s, 0.28 V/decade, and 2.9 × 108, respectively. A significantly enhanced μFE value of 89.4 cm2/V·s was achieved for ZnON TFTs with a TaO x encapsulation layer, whereas the SS of 0.33 V/decade and ION/OFF ratio of 8.6 × 108 were comparable to those of the control device. This improvement could be explained by scavenging and passivation effects of the TaO x film on the ZnON channel layer. Density of states (DOS)-based modeling and simulation were performed to obtain greater insight with regard to increasing the performance of the ZnON TFTs with a TaO x encapsulation layer. A smaller number of subgap states near the conduction band (CB) minimum and a higher net carrier density for the TaO x-capped device increased the Fermi energy level toward the CB edge under thermal equilibrium conditions, leading to efficient band conduction and fast carrier transport under the on-state condition.

Effects of proton radiation on field limiting ring edge terminations in 4H–SiC junction barrier Schottky diodes

In this study, the effects of high-energy proton radiation on the effectiveness of edge terminations using field limiting rings (FLRs) in 4H–SiC junction barrier Schottky (JBS) diodes were examined in detail. The devices were irradiated using 5-MeV protons at fluences ranging from 5× 1012 cm−2 to 5× 1014 cm−2. Further, the reverse breakdown performances of the investigated devices were measured both before and after irradiation. Proton irradiation initially decreased the breakdown voltage (BV); subsequently, the BV was increased as the proton fluence increased. At a fluence of 5× 1013 cm−2, the BV was reduced by approximately 18%, whereas it was reduced by approximately 5% at a higher proton fluence of 5× 1014 cm−2. The related degradation mechanism that was associated with this phenomenon was also investigated using the numerical simulations of the current-voltage ( I - V ) and capacitance-voltage ( C - V ) characteristics of the device. The main contribution to the radiation-induced changes in BV originates from the variations of charge distribution at the SiO2/4H–SiC interface and the reduction of the net carrier density in the drift region. Both the aforementioned variations affect the spread of the electric field in the FLR edge termination regions.

Effects of fluorine incorporation into β-Ga2O3

β-Ga2O3 rectifiers fabricated on lightly doped epitaxial layers on bulk substrates were exposed to CF4 plasmas. This produced a significant decrease in Schottky barrier height relative to unexposed control diodes (0.68 eV compared to 1.22 eV) and degradation in ideality factor (2.95 versus 1.01 for the control diodes). High levels of F (>1022 cm−3) were detected in the near-surface region by Secondary Ion Mass Spectrometry. The diffusion of fluorine into the Ga2O3 was thermally activated with an activation energy of 1.24 eV. Subsequent annealing in the range 350–400 °C brought recovery of the diode characteristics and an increase in barrier height to a value larger than in the unexposed control diodes (1.36 eV). Approximately 70% of the initial F was removed from the Ga2O3 by 400 °C, with the surface outgas rate also being thermally activated with an activation energy of 1.23 eV. Very good fits to the experimental data were obtained by integrating physics of the outdiffusion mechanisms into the Florida Object Oriented Process Simulator code and assuming that the outgas rate from the surface was mediated through fluorine molecule formation. The fluorine molecule forward reaction rate had an activation energy of 1.24 eV, while the reversal rate of this reaction had an activation energy of 0.34 eV. The net carrier density in the drift region of the rectifiers decreased after CF4 exposure and annealing at 400 °C. The data are consistent with a model in which near-surface plasma-induced damage creates degraded Schottky barrier characteristics, but as the samples are annealed, this damage is removed, leaving the compensation effect of Si donors by F− ions. The barrier lowering and then enhancement are due to the interplay between surface defects and the chemical effects of the fluorine.

Local Electronic Structure Changes in Polycrystalline CdTe with CdCl2 Treatment and Air Exposure

Postdeposition CdCl2 treatment of polycrystalline CdTe is known to increase the photovoltaic device efficiency. However, the precise chemical, structural, and electronic changes that underpin this improvement are still debated. In this study, spectroscopic photoemission electron microscopy was used to spatially map the vacuum level and ionization energy of CdTe films, enabling the identification of electronic structure variations between grains and grain boundaries (GBs). In vacuo preparation and inert transfer of oxide-free CdTe surfaces isolated the separate effects of CdCl2 treatment and ambient oxygen exposure. Qualitatively, grain boundaries displayed lower work function and downward band bending relative to grain interiors, but only after air exposure of CdCl2-treated CdTe. Analysis of numerous space charge regions at grain boundaries showed an average depletion width of 290 nm and an average band bending magnitude of 70 meV, corresponding to a GB trap density of 1011 cm-2 and a net carrier density of 1015 cm-3. These results suggest that both CdCl2 treatment and oxygen exposure may be independently tuned to enhance the CdTe photovoltaic performance by engineering the interface and bulk electronic structure.

P-type doping efficiency in CdTe: Influence of second phase formation.

Cadmium telluride (CdTe) high purity, bulk, crystal ingots doped with phosphorus were grown by the vertical Bridgman melt growth technique to understand and improve dopant solubility and activation. Large net carrier densities have been reproducibly obtained from as-grown ingots, indicating successful incorporation of dopants into the lattice. However, net carrier density values are orders of magnitude lower than the solubility of P in CdTe as reported in literature, 1018/cm3 to 1019/cm3 [J. H. Greenberg, J. Cryst. Growth 161, 1-11 (1996) and R. B. Hall and H. H. Woodbury, J. Appl. Phys. 39(12), 5361-5365 (1968)], despite comparable starting charge dopant densities. Growth conditions, such as melt stoichiometry and post growth cooling, are shown to have significant impacts on dopant solubility. This study demonstrates that a significant portion of the dopant becomes incorporated into second phase defects as compounds of cadmium and phosphorous, such as cadmium phosphide, which inhibits dopant incorporation into the lattice and limits maximum attainable net carrier density in bulk crystals. Here, we present an extensive study on the characteristics of these second phase defects in relation to their composition and formation kinetics while providing a pathway to minimize their formation and enhance solubility.

Band Edge Positions and Their Impact on the Simulated Device Performance of ZnSnN2-Based Solar Cells

ZnSnN2 (ZTN) has been proposed as a new earth abundant absorber material for photovoltaic (PV) applications. While carrier concentration has been reduced to values suitable for device implementation, other properties such as ionization potential, electron affinity, and work function are not known. Here, we experimentally determine the value of ionization potential (5.6 eV), electron affinity (4.1 eV), and work function (4.4 eV) for ZTN thin film samples with Zn cation composition Zn/(Zn+Sn) = 0.56 and carrier concentration $n$ = 2 × 1019 cm−3 . Using both experimental and theoretical results, we build a model to simulate the device performance of a ZTN/Mg:CuCrO2 solar cell, showing a potential efficiency of 23% in the limit of no defects present. We also investigate the role of band tails and recombination centers on the cell performance. In particular, device simulations show that band tails are highly detrimental to the cell efficiency, and recombination centers are a major limitation if present in concentration comparable to the net carrier density. The effect of the position of the band edges of the p- type junction partner was assessed too. Through this study, we determine the major bottlenecks for the development of ZTN-based solar cell and identify avenues to mitigate them.

Measuring the carrier dynamics of photocatalyst micrograins using the Christiansen effect.

The optical measurement of photocatalyst materials is subject to Mie scattering when the particle size is comparable to the wavelength of the probe light. A novel approach was developed to deal with this scattering problem in the transient spectroscopy of photocatalyst micrograins using the Christiansen effect because the probe light in the vicinity of the Christiansen frequency can be transmitted. Scattering theory was used to analyze the transient spectra of micrograins and estimate the extinction coefficient at the Christiansen frequency. The Drude-Lorentz model was used to calculate the complex refractive index considering the contributions from both phonons and free carriers. We found that the net photogenerated carrier density was linearly correlated with the absorbance at the Christiansen frequency. With the parameters obtained from Raman scattering measurements, the absolute net carrier density was also determined. We further demonstrated the versatility of this method by applying it to the photogenerated carrier dynamics of intrinsic 6H-SiC grains. The transient broadband mid-IR spectra were measured by the pump-probe technique, and the transient absolute net carrier density was estimated. The carrier relaxation dynamics was fitted with three components with lifetime constants that agreed well with those obtained for SiC by transient broadband THz conductivity spectroscopy. We predict that this method could be extended to other photocatalytic materials with suitable probe frequencies.

Piezoelectric Effects on the Exciton Dissociation Rate in Organic-Inorganic Hybrid Systems

In this study, we perform a numerical investigation on the piezoelectric effects on the exciton dissociation (i.e., Onsager dissociation rate) on the interface in an organic (P3HT)-inorganic (n-ZnO) hybrid system using a Finite Element Method (FEM). The results show that the maximum dissociation rate on the interface is determined by the positive piezoelectric potential, which is associated with the piezoelectric effects that assist exciton dissociation by the collection of free carriers inside the hybrid system. We numerically prove this effect by calculating the piezoelectric charge, the number of holes, and the electron density on the interface under elongated hybrid systems. The results obtained are expected to give a better understanding of piezoelectric effects on the exciton dissociation and net carrier density.

Time-delayed behaviors of transient four-wave mixing signal intensity in inverted semiconductor with carrier-injection pumping

An analytical expression of transient four-wave mixing (TFWM) in inverted semiconductor with carrier-injection pumping was derived from both the density matrix equation and the complex stochastic stationary statistical method of incoherent light. Numerical analysis showed that the TFWM decayed decay is towards the limit of extreme homogeneous and inhomogeneous broadenings in atoms and the decaying time is inversely proportional to half the power of the net carrier densities for a low carrier-density injection and other high carrier-density injection, while it obeys an usual exponential decay with other decaying time that is inversely proportional to half the power of the net carrier density or it obeys an unusual exponential decay with the decaying time that is inversely proportional to a third power of the net carrier density for a moderate carrier-density injection. The results can be applied to studying ultrafast carrier dephasing in the inverted semiconductors such as semiconductor laser amplifier and semiconductor optical amplifier.

Polarization induced self-doping in epitaxial Pb(Zr0.20Ti0.80)O3 thin films.

The compensation of the depolarization field in ferroelectric layers requires the presence of a suitable amount of charges able to follow any variation of the ferroelectric polarization. These can be free carriers or charged defects located in the ferroelectric material or free carriers coming from the electrodes. Here we show that a self-doping phenomenon occurs in epitaxial, tetragonal ferroelectric films of Pb(Zr0.2Ti0.8)O3, consisting in generation of point defects (vacancies) acting as donors/acceptors. These are introducing free carriers that partly compensate the depolarization field occurring in the film. It is found that the concentration of the free carriers introduced by self-doping increases with decreasing the thickness of the ferroelectric layer, reaching values of the order of 1026 m−3 for 10 nm thick films. One the other hand, microscopic investigations show that, for thicknesses higher than 50 nm, the 2O/(Ti+Zr+Pb) atomic ratio increases with the thickness of the layers. These results suggest that the ratio between the oxygen and cation vacancies varies with the thickness of the layer in such a way that the net free carrier density is sufficient to efficiently compensate the depolarization field and to preserve the outward direction of the polarization.

Screening-induced surface polar optical phonon scattering in dual-gated graphene field effect transistors

The effect of surface polar optical phonons (SOs) from the dielectric layers on electron mobility in dual-gated graphene field effect transistors (GFETs) is studied theoretically. By taking into account SO scattering of electron as a main scattering mechanism, the electron mobility is calculated by the iterative solution of Boltzmann transport equation. In treating scattering with the SO modes, the dynamic dielectric screening is included and compared to the static dielectric screening and the dielectric screening in the static limit. It is found that the dynamic dielectric screening effect plays an important role in the range of low net carrier density. More importantly, in-plane acoustic phonon scattering and charged impurity scattering are also included in the total mobility for SiO2-supported GFETs with various high-κ top-gate dielectric layers considered. The calculated total mobility results suggest both Al2O3 and AlN are the promising candidate dielectric layers for the enhancement in room temperature mobility of graphene in the future.

Effect of sodium on material and device quality in low temperature deposited Cu(In,Ga)Se2

Abstract Sodium is known to play an important role for the performance optimization of Cu(In,Ga)Se 2 (CIGSe) thin film solar cells, i.e. it is found to significantly increase the open-circuit voltage ( V OC ). For this work CIGSe absorbers were produced in a low temperature deposition process on polyimide substrates with varying sodium content. In agreement with previous studies we find an increase of V OC and overall solar cell performance with increasing sodium content and a decrease of the overall performance for very high sodium content. At the same time time-resolved photoluminescence measurements indicate that the minority carrier lifetime decreases from 50 ns to about 5 ns for increasing sodium content in the absorber. This correlation was found to be valid for three different types of low-temperature CIGSe deposition processes. The results can be explained by an increase in deep defect density with increasing Na content, proportional to the increased net charge carrier density as measured by capacitance–voltage profiling. For very high sodium content this leads to an actual decrease in the overall performance of the solar cells.

Efficiency enhancement of Cu(In,Ga)Se2 thin‐film solar cells by a post‐deposition treatment with potassium fluoride

AbstractAlkali‐free Cu(In,Ga)Se2(CIGS) absorbers grown on Mo‐coated alumina (Al2O3) substrates were doped with potassium (K) after CIGS growth by a potassium fluoride (KF) post‐deposition treatment (PDT). The addition of K to the absorber leads to a strong increase in cell efficiency from 10.0% for the K‐free cell to 14.2% for the K‐doped cell, mainly driven by an increase in the open‐circuit voltage Voc and the fill factor FF, and to an increase in the net charge carrier density. Hence K doping by KF‐PDT is comparable to doping with Na.magnified imageJ –V characteristics of a CIGS solar cell with KF‐PDT (red solid line) and of a K‐free reference cell without treatment (black dashed line). The solar cell treated with K shows a strong increase in Voc and FF compared to the solar cell without K.(© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Space charge, plasma potential and electric field distributions in HiPIMS discharges of varying configuration

An electron-emitting (emissive) probe has been used to study the temporal and spatial distribution of the plasma potential during high-power impulse magnetron sputtering (HiPIMS) discharges with various substrate and magnetic field configurations. The average power was 700 W, with a repetition frequency of 100 Hz and pulse duration of 100 µs. Strongly negative plasma potentials exceeding −300 V and electric fields up to 10 kV m−1, caused by strong separation of charges with net charge carrier densities Δn of about 1014 m−3, were observed during the ignition of the discharge. The spatial distribution of the plasma potential in the stable stage of the discharge showed values consistently 5 V more negative for a floating substrate compared with a grounded one, so enhancing electron transport around the insulated substrate to grounded walls. However, this change in the electrical configuration of the plasma does not alter significantly the fraction of ionized sputtered particles (of about 30%) that can potentially reach the substrate. By changing the degree of unbalance of the sputtering source, we find a strong correlation between the electric field strength in the magnetic trap (created through charge separation) and the absolute value (and shape) of the magnetic field. For the more unbalanced magnetron, a flattening of the plasma potential structure (decrease in the axial electric field) was observed close to the target. Our findings show in principle that manipulation of the potential barrier close to the target through changing the magnetic field can regulate the proportion of sputtered and ionized species reaching the substrate.