Free Hole Density Research Articles

Growth and Characterization of Lanthanum Germanide Thin Films by the Thermal Evaporation Technique

AbstractLanthanum germanide (La6Ge) thin films are successfully fabricated using the thermal evaporation technique under a vacuum pressure of 10−5 mbar. The resulting films display an orthorhombic structure, characterized by lattice parameters of a = 8.725 Å, b = 8.063 Å, and c = 5.569 Å. Optical analysis of the La6Ge thin films reveal their high transparency, with an energy bandgap of 3.75±0.04 eV. The bandgap exhibits indirect allowed transitions and featured energy band tails with widths measuring (1.64±0.14) eV. In addition, dielectric dispersion analyses indicate the presence of two dominant dielectric resonance peaks centered at 3.15 and 2.08 eV. Moreover, the La6Ge films demonstrate a terahertz (THz) cutoff frequency of 1.0 THz when illuminated with infrared and visible light. This cutoff frequency increases to 45.6 THz in the ultraviolet range. Furthermore, by utilizing the Drude–Lorentz method, the investigation of optical conductivity parameters reveals that the lanthanum germanide optical filters can achieve free hole density and drift mobility values of 14.44 cm2 V−1 s−1 and 2.8×1018 cm−3, respectively, under infrared light irradiation. The outstanding optical and dielectric properties exhibited by the La6Ge thin films make them excellent candidates for highly transparent optical filters suitable for terahertz technology.

Dissociative Host-Dopant Bonding Facilitates Molecular Doping in Halide Perovskites

Molecular doping is a promising strategy to fine-tune the electronic properties of halide perovskites and accelerate their implementation in next-generation optoelectronics. However, a deeper understanding of the role of host-dopant interactions in these systems is needed to fully exploit the potential of this avenue. Herein, we demonstrate a surface post-treatment strategy employing n-type molecular dopant n-DMBI-H to modulate free hole density in p-type CH3NH3Sn0.75Pb0.25I3 films. We show that the adsorption of n-DMBI-H on surface Sn atoms, followed by the dissociation of an electron-donating hydride from the dopant, facilitates charge transfer to the perovskite and hole trapping at the dissociated hydride. We identify this mechanism as a key factor dictating doping compensation in perovskites, allowing carrier density control within nearly 1 order of magnitude via the dissociated molecular dopant located at film surfaces and grain boundaries. We then exploit n-DMBI-H in perovskite/transport layer junctions, achieving reduced carrier losses and improved contact selectivity and performance in p-i-n, Sn-rich perovskite solar cells. We expect this work to provide carrier density tuning guidelines for a broad range of tin-based perovskite applications.

Carrier density tuning in CuS nanoparticles and thin films by Zn doping via ion exchange.

Copper sulphide (covellite) nanoplatelets have recently emerged as a plasmonic platform in the near-infrared with ultrafast nonlinear optical properties. Here we demonstrate that the free-carrier density in CuS, which is an order of magnitude lower than in traditional plasmonic metals, can be further tuned by chemical doping. Using ion exchange to replace Cu with an increasing content of Zn in the nanoparticles, the free-hole density can be lowered, resulting in a long-wavelength shift of the localised plasmon resonances from 1250 nm to 1750 nm. The proposed approach provides new opportunities for tuning the plasmonic response of covellite nanocrystals as well as the carrier relaxation time which decreases for lower free-carrier densities.

Mg-doping and free-hole properties of hot-wall MOCVD GaN

The hot-wall metal-organic chemical vapor deposition (MOCVD), previously shown to enable superior III-nitride material quality and high performance devices, has been explored for Mg doping of GaN. We have investigated the Mg incorporation in a wide doping range (2.45×1018 cm−3 up to 1.10×1020 cm−3) and demonstrate GaN:Mg with low background impurity concentrations under optimized growth conditions. Dopant and impurity levels are discussed in view of Ga supersaturation, which provides a unified concept to explain the complexity of growth conditions impact on Mg acceptor incorporation and compensation. The results are analyzed in relation to the extended defects, revealed by scanning transmission electron microscopy, x-ray diffraction, and surface morphology, and in correlation with the electrical properties obtained by Hall effect and capacitance–voltage (C–V) measurements. This allows to establish a comprehensive picture of GaN:Mg growth by hot-wall MOCVD providing guidance for growth parameters optimization depending on the targeted application. We show that substantially lower H concentration as compared to Mg acceptors can be achieved in GaN:Mg without any in situ or post-growth annealing resulting in p-type conductivity in as-grown material. State-of-the-art p-GaN layers with a low resistivity and a high free-hole density (0.77 Ω cm and 8.4×1017 cm−3, respectively) are obtained after post-growth annealing demonstrating the viability of hot-wall MOCVD for growth of power electronic device structures.

Development of ZnTe film with high copper doping efficiency for solar cells

Since a hole barrier was formed in back contact due to mismatch of work function, the back contact material for CdTe cell has been a significant research direction. The ZnTe:Cu is an ideal back contact material, which reduces the valence band discontinuity and can be used as the electron back reflection layer to inhibit interface recombination. The conductivity of ZnTe:Cu film is improved by applying RF-coupled DC sputtering and post-deposition heat treatment. The doping efficiency is computed as the ratio of free hole density and copper concentration, which can be correlated with performance for CdTe-based solar cell. The higher doping efficiency means that more copper atoms substitute for Zn sites in ZnTe lattices and less mobilized copper atoms remain which can enter into the CdTe absorber layer. Copper atoms are suspected as dominant element for CdTe-based cell degradation. After optimizing the ZnTe:Cu films, a systematic study is carried out to incorporate ZnTe:Cu film into CdTe solar cell. The EQE spectrum is kept relatively stable over the long wavelength range without decreasing. It is proved that the conduction band barrier of device with ZnTe:Cu/Au contact material has an effect on the EQE response, which works as free electron barrier and reduces the recombination rate of free carrier. According to the dark J–V data or the light J–V data in the linear region, the current indicates that the intercept gives the diode reverse saturation current. The results of ideality factor indicate that the dominant recombination occurs in the space charge region. In addition, the space charge density and depletion width of solar cell can be estimated by C–V profiling.

High-densities of free holes in homoepitaxial n-GaN induced by fluorine-plasma ion implantation

In this work, fluorine plasma ion implantation is used to induce high densities of free holes in homoepitaxial n-GaN for the first time, as evidenced by electroluminescence and p-n junction rectification. F+ ions implanted into the near-surface layer capture electrons from shallow bulk donors, bending the band upward until a high density of free holes is induced at the valence band maximum, and surface donors, depinning the Fermi level to form p-type ohmic contact. Free holes tunnel from the metal into states at the valence band maximum, where they form a conductive p-type channel, benefitting the development of GaN-based junction devices.

Effect of Inhomogeneous Broadening in Ultraviolet III-Nitride Light-Emitting Diodes.

In the past years, light-emitting diodes (LED) made of GaN and its related ternary compounds with indium and aluminium have become an enabling technology in all areas of lighting. Visible LEDs have yet matured, but research on deep ultraviolet (UV) LEDs is still in progress. The polarisation in the anisotropic wurtzite lattice and the low free hole density in p-doped III-nitride compounds with high aluminium content make the design for high efficiency a critical step. The growth kinetics of the rather thin active quantum wells in III-nitride LEDs makes them prone to inhomogeneous broadening (IHB). Physical modelling of the active region of III-nitride LEDs supports the optimisation by revealing the opaque active region physics. In this work, we analyse the impact of the IHB on the luminescence and carrier transport III-nitride LEDs with multi-quantum well (MQW) active regions by numerical simulations comparing them to experimental results. The IHB is modelled with a statistical model that enables efficient and deterministic simulations. We analyse how the lumped electronic characteristics including the quantum efficiency and the diode ideality factor are related to the IHB and discuss how they can be used in the optimisation process.

Dynamics of exciton–polariton emission in CuI

We report on temperature-dependent (10 K – 250 K) spectral and dynamical properties of free exciton–polariton and bound exciton emission in copper iodide (CuI) bulk single crystals analyzed by means of time-resolved photoluminescence spectroscopy. The characteristic line shape of the polariton emission at low temperatures is interpreted in terms of the “k-linear term effect” on the degenerate Z1,2 excitons in CuI. For free exciton–polaritons, an increase in the decay time with increasing temperature up to 360 ps at 160 K is observed. For bound exciton emission, decay times between 180 ps and 380 ps are observed at low temperatures, revealing the expected EB3/2 dependence of radiative lifetime on the localization energy. Based on the observed rise times of bound excitons at low temperatures, a defect density of shallow acceptors of 1 × 1017 cm−3 was estimated, in agreement with measured room temperature free hole density.

The electric field at the hole-injecting metal/organic interface controls the bias dependence of the current–voltage hole mobility

Based upon the room temperature current–voltage data of some published organic diode structures the unique phenomenon of the decreasing hole mobility, μ, with the increasing applied electric field, E a, is interpreted. The measurable quantity, the hole drift mobility μ d is formulated in terms of E a and the electric field at the hole injecting metal/organic interface, E int, dependent algebraic function multiplied by the intrinsic hole mobility, μ max that is organic morphology dependent but E a independent scaling factor. On account that the intrinsic mobility, μ max, is uncoupled from both E a and E int it is shown that the origin of the negative field hole mobility effect occurs due to E int, that is a linear function of E a. The bias and the space distribution of the internal organic electric field, E, as well as the free hole density, p, for poly(3-hexylthiophene) is calculated in detail. Depending on the organic layer morphology the internal electric field may exhibit, at the particular value of E a, a deep well in the vicinity of the hole injecting metal/organic interface. Then the strong peak of the free hole density exists there the effect of which is spreading some 10 nm into the organic. If E int happens to be E a independent constant, then from the resulting space charge limited current density, the increasing hole drift mobility, μ d, with the increasing applied electric field, E a, is deduced. The published current–voltage data of two distinct metal-substituted phthalocyanine thin films provide an additional confirmation of the described formalism.

Intracavity third-harmonic generation in Si:B pumped by intense terahertz pulses

We observe third-harmonic generation (THG) in boron-doped silicon (Si:B) upon pumping with picosecond 1.56-THz pulses from a free-electron laser with a peak electric field strength of up to 12 kV/cm. The measurements are performed at cryogenic temperatures where the majority of holes are bound to the acceptor dopants. The dependence of the THG on the pump intensity exhibits a threshold-free power-law behavior with an exponent close to 4. The observations can be explained by THz emission by free holes accelerated in the nonparabolic valence band, under the assumption that the density of free holes increases with the pump intensity. A quantitative treatment supports that these carriers are generated by impact ionization, initiated by the population of thermally ionized carriers, as opposed to direct tunneling ionization. In addition, we also observe intracavity THG by embedding the Si:B in a one-dimensional photonic crystal cavity. The THG efficiency is increased by a factor of eight due to the field enhancement in the cavity, with the potential to reach a factor of more than 100 for pump pulses with a spectrum narrower than the linewidth of the cavity resonance.

On the Origin of Charge Compensation in Aluminum-Implanted n-Type 4H-SiC by Analysis of Hall Effect Measurements

Aluminum implanted 4H-SiC often shows an unexpected increase of the free hole density at elevated temperatures in Hall Effect measurements. Here we show that this phenomenon cannot solely be traced down to the Hall scattering factor and the presence of excited acceptor states. It is necessary to assume an additional defect center in the lower half of the band gap with ionization energies higher than that of aluminum to explain this behavior. Therefore, we investigated ion-implanted square van-der-Pauw samples with Hall Effect and complementary SIMS measurements. An analysis of the data using the neutrality equation reveals compensation ratios of 20 % to 90 %, depending on the aluminum concentration and the concentration of the deep defect center of up to 50 % of the doping.

Molecular beam epitaxy growth and temperature-dependent electrical characterization of carbon-doped GaAs on GaAs(1 1 1)B

Here we present a study of the solid source molecular beam epitaxy growth of carbon doped GaAs(1 1 1)B oriented layers in a wide doping concentration range from 1016 to 1020cm-3. The layers have been fabricated on GaAs(1 1 1)B substrates with a 1° miscut towards (2 1 1) and carbon was supplied by a heated graphite filament source. Atomic force microscopy revealed smooth surfaces (root mean square roughness 0.3 nm) up to the highest carbon concentration. The carbon concentration was determined by secondary ion mass spectroscopy. All samples exhibit p-type conductivity and temperature-dependent Hall- and resistivity measurements show carrier freeze out for doping concentration in the 1016 cm−3 and 1017 cm−3 range, whereas we observe degenerated behavior for carbon concentrations above ca. 1×1019 cm−3. The hole mobility decreases with increasing carbon concentration. The acceptor ionization energy for carbon was determined via low-temperature photoluminescence measurements to 26.3 meV. From the free hole density determined from Hall-measurements at 300 K and the overall carbon concentration, we estimate that approximately 100% of the carbon atoms are electrically active acceptors for carbon concentrations below 1×1018 cm−3. For concentrations above 1×1019 cm−3 compensation sets in and the doping efficiency is well below 100%. For comparison, samples in the standard (1 0 0)-orientation have been produced and carbon incorporation behavior during growth is very similar.

Effects of Ge substrate on the structural and optical conductivity parameters of Bi2O3 thin films

In this article the structural, optical and dielectric properties of a 200 nm thick Bi2O3 thin films which are deposited onto amorphous germanium substrate are reported. Both of the Ge and Bi2O3 thin films are prepared by the thermal evaporation technique under vacuum pressure of 10−5 mbar. Bi2O3 thin films are found to prefer the monoclinic nature of structure with larger values of microstrain, dislocation density, stacking faults and smaller grain sizes upon replacement of the glass substrate by germanium. Optically, significant redshift in the energy band gap is observed when the films are grown onto Ge. The Ge/Bi2O3 heterojunctions exhibit a conduction and valence band offsets of value of 0.81 and 1.38 eV, respectively. In addition to the enhancement in the dielectric constant near the IR region, the Drude-Lorentz modeling of the Ge/Bi2O3 heterojunctions has shown remarkable effect of the Ge substrate on the optical conductivity parameters of Bi2O3. Particularly, the drift mobility increased by about one order of magnitude, the free hole density decreased by ˜24 times and the plasmon frequency ranges extended from 5.21 to 11.0 GHz to 2.59–12.80 GHz when germanium substrate is used. The optical features of the heterojunction nominate it for visible light communication technology.

Structural and optical properties of Cu2Se/Yb/Cu2Se thin films

Abstract In this study, two stacked layers of Cu2Se are nanosandwiched with 50 nm thick Yb slab. The films which are prepared by the thermal evaporation technique under vacuum pressure of 10–5 mbar are examined by the X-ray diffraction, scanning electron microscopy, energy dispersion spectroscopy and optical spectrophotometry techniques. The cubic phase of Cu2Se exhibits lattice contraction which results in strained structure and reduction in grain size upon insertion of Yb between two layers of Cu2Se. The nanosandwiching of Yb layer does not influence the optical energy band gap but significantly altered the optical conductivity parameters. The optical conductivity parameters are determined by the help of Drude-Lorentz approach for the dielectric dispersion. Particularly, while the scattering time at femtosecond levels and the drift mobility values increases, the free hole density decreases via the nanosandwiching procedure. The drift mobility and plasmon frequency of Cu2Se reached values of 35.2 cm2/Vs and 9.85 GHz, respectively, by the Yb nanosandwiching. The enhancements in mobility and free carrier density values make the Cu2Se thin films more appropriate for optoelectronic applications.

Physical Origins of the Transient Absorption Spectra and Dynamics in Thin-Film Semiconductors: The Case of BiVO4

Transient absorption (TA) spectroscopy is uniquely suited for understanding kinetic processes initiated by light over vast ranges of time. In combination with white light probes, the recorded differential absorption spectra can contain spectroscopic signatures characteristic of specific charge carrier population densities. However, disentangling the often-complex and convoluted spectra is made challenging without robust analysis methods relating the underlying physical mechanisms to the spectral components. In this work, we address the origin of the transient spectra of a model system of emerging solar energy harvesting materials using a monoclinic BiVO4 thin film. Using ground-state optical properties of the semiconductor, we find the main derivative-like spectral response to be related to shifting and broadening of oscillators, rather than specific carrier-related transitions. However, by using the Drude optical model of free carriers, we also identify the transient response related to free-hole density...

Optimal loading of quaternized chitosan nanofillers in functionalized polyvinyl alcohol polymer membrane for effective hydroxide ion conduction and suppressed alcohol transport

A polymer composite was prepared by incorporating 5-20 wt% quaternized chitosan (Q-chitosan) nanoparticles into quaternized polyvinyl alcohol (Q-PVA) to enhance the dimensional stability and conductive properties. Compared with the pristine Q-PVA membrane, the ionic conductivity of the Q-PVA composites with 5–10 wt% Q-chitosan was improved due to increased polymeric free volume hole density. The methanol permeability is suppressed in these Q-PVA/Q-chitosan composites because of the confined swelling in the presence of the fillers, shrinking the free volume size. The Q-PVA/5%Q-chitosan composite exhibits higher peak power density (Pmax) than other Q-PVA membranes in direct methanol fuel cells. A Pmax of 92 mW cm−2 is attained using the Q-PVA/5%Q-chitosan composite at 90 °C with 5–6 mg cm−2 catalyst loads on micro-porous layer-containing carbon cloth. The high load of 20 wt% Q-chitosan does not benefit ionic conduction and power generation, probably due to interfacial resistance at rough surfaces. Using the Q-PVA/5%Q-chitosan composite and reduced catalyst loads of 1–2 mg cm−2 on pristine carbon cloth, Pmax of 90 mW cm−2 can be achieved at 60 °C for both methanol and ethanol fuel cells, but declined at 90 °C due to insufficient catalysts. Overall the Q-PVA/Q-chitosan composite offers a potential electrolyte for energy devices operating in an alkaline environment.

Charge Trapping Analysis of High Speed Diamond FETs

Charge carrier trapping in diamond surface conduction field effect transistors (FETs) has been analyzed. For these devices two methods were used to obtain a negative electron affinity diamond surface; either plasma hydrogenation or annealing in an H2 environment. In both cases the Al2O3 gate dielectric can trap both electrons and holes in deep energy levels with emission timescales of seconds, while the diamond – Al2O3 interface traps exhibit much shorter time scales in the microsecond range. Capacitance-Voltage (CV) analysis indicates that these interface traps exhibit acceptor-like characteristics. Correlation with CV based free hole density measurements indicates that the conductance based interface trap analysis provides a method to quantify surface characteristics that lead to surface conduction in hydrogenated diamond where atmospheric adsorbates provide the acceptor states for transfer doping of the surface.

Spin and valley control of free carriers in single-layer WS2

The semiconducting single-layer transition metal dichalcogenides have been identified as ideal materials for accessing and manipulating spin- and valley-quantum numbers due to a set of favorable optical selection rules in these materials. Here, we apply time- and angle-resolved photoemission spectroscopy to directly probe optically excited free carriers in the electronic band structure of a high quality single layer of WS$_2$. We observe that the optically generated free hole density in a single valley can be increased by a factor of 2 using a circularly polarized optical excitation. Moreover, we find that by varying the photon energy of the excitation we can tune the free carrier density in a given spin-split state around the valence band maximum of the material. The control of the photon energy and polarization of the excitation thus permits us to selectively excite free electron-hole pairs with a given spin and within a single valley.

Impact of sulfur concentration on morphological, optical, electrical and thermoelectrical properties of nanostructured MoO3 thin films

We have investigated the effect of sulfur concentration ([S]/[Mo] = 0–10 at.%) on nanostructural, optical and electrical properties of MoO3 thin films grown on glass substrate by spray pyrolysis technique. X-ray diffraction analysis showed that the films were crystallized with mixed structures of MoO3 orthorhombic and MoS2 hexagonal structure. According to FESEM images, the studied films have a sponge-type structure on the order of nanometers. Optical measurements revealed that two distinct inflexions indicative two transitions correspond to MoO3/MoS2 phases in agreement with the XRD analysis. The Hall effect and thermoelectric measurements have shown p-type conductivity, and the free hole density increases with increasing sulfur to molybdenum ratio, in agreement with reflectance spectra of the layers. The obtained value of Seebeck coefficient for MoO3 film with [S]/[Mo] ratio of 8 at.% was as high as 169 μV/K.

Nanostructured Cu2ZnSnS4 thin films: influence of substrate temperature on structural, morphological, optical and electrical properties

Photovoltaic Cu2ZnSnS4 (CZTS) thin films have been deposited on the glass substrate by a simple and low-cost spray pyrolysis technique without sulfurization treatment in a toxic atmosphere. The influence of the substrate temperatures on the structural, compositional, morphological, optical and electrical properties of the CZTS films was investigated. The formation of kesterite structure in the films was confirmed using X-ray diffraction measurements. The improved crystallinity of the CZTS with a (112) orientation was observed with increasing the substrate temperature. The band gap of all films was found to be in the range from 1.44 to 1.56 eV which is close to the ideal band gap for the highest theoretical conversion efficiency of solar cell. AFM analysis revealed a smooth, compact and crack-free morphology. The electrical studies showed that all these samples had a p-type conductivity, and the free hole density and mobility increased with increasing the substrate temperature.