Donor Activation Energy Research Articles

Характеристики электрических свойств нанокристаллических систем селенидов цинка и кадмия

The physical properties of nanocrystalline systems of cadmium and zinc selenides have been studied experimentally and theoretically. The effect of temperature and various gaseous media on the specific conductivity of films of cadmium and zinc selenides has been experimentally studied. The values of the Fermi energy, the concentration of donor charge carriers in the conduction band, the activation energy of donors, and the temperature coefficient of resistance are theoretically calculated. It is shown that the calculated values correlate with the experimental ones, which will make it possible to predict the stability of the operation of semiconductor devices.

On the preparation and electrical properties of thallium bismuth diselenide crystals

Thallium bismuth diselenide (TlBiSe2) crystals were grown by an optimized process based on Bridgman technique. The material has been characterized by X-ray powder diffraction. An investigation was done on the influence of temperature on the electrical conductivity and Hall effect. The energy gap as well as the ionization energy was calculated. The electrical conductivity temperature dependence was tested to determine the dominant mechanism of conductivity in a temperature range of 203 K and 338 K. Our outcomes show an n-type semiconducting behaviour due to a degenerate state. The scattering character of the free carriers was defined by determination Hall mobilities, the concentration of carriers, activation energies of donors, the diffusion coefficient, and diffusion length of majority carriers. The performed characterization confirms that the grown TlBiSe2 crystals are narrow gap semiconductors with promising properties for practical application.

Hydride vapor phase epitaxy of Si-doped AlN layers using SiCl4 as a doping gas

Growth of Si-doped AlN layers by hydride vapor phase epitaxy on AlN/sapphire templates prepared by metalorganic vapor phase epitaxy and bulk AlN substrates prepared by physical vapor transport was investigated using silicon tetrachloride (SiCl4) as a doping gas. On the AlN/sapphire templates, when the SiCl4 supply was low, the incorporation ratio of Si decreased due to the influence of O impurities resulting from the decomposition of the sapphire. In addition, pits were formed on the surfaces, and the pit density increased significantly at Si concentrations exceeding 2 × 1019 cm−3. In contrast, on the bulk AlN substrates, the Si concentration increased linearly with increasing SiCl4 supply, and a pit- and stress-free layer could be grown with a Si concentration as high as 6.5 × 1019 cm−3. Hall effect measurements revealed that this layer exhibited n-type conductivity with a donor activation energy of 253 meV. However, the carrier density at room temperature (RT) was as low as 3.6 × 1013 cm−3 because of the high compensation ratio due to the presence of acceptors. A broad peak centered at 3.3 eV was observed in the RT photoluminescence spectra of the Si-doped AlN layers grown on the bulk AlN substrates, indicating that Al vacancies formed by the Fermi level effect due to Si doping acted as acceptors that compensated for carriers.

Superinjection in Diamond p - i - n Diodes: Bright Single-Photon Electroluminescence of Color Centers Beyond the Doping Limit

Color centers in diamond are considered one of the most promising platforms for electrically driven single-photon sources operating under ambient conditions. Efficient electrical excitation of these emitters is challenging, due to the extremely high activation energy of donors in diamond, which limits the density of free electrons. It may possible to overcome this doping limit, though. The authors predict and numerically demonstrate a superinjection effect in homojunction diamond $p\ensuremath{-}i\ensuremath{-}n$ diodes that allows a 3000-fold increase in free-electron density, thereby enhancing the brightness of single-photon electroluminescence by three orders of magnitude.

PL properties and defects of SnS layers based on n-type buffer layers/p-type SnS structures

The defect-related levels of n-type CdS/p-type SnS layers with thermal annealing were investigated through photoluminescence (PL) measurement. The SnS layer exhibited PL band at 1.13 eV. Based on the PL spectra observed at various excitation intensities and temperatures, PL peaks were attributed to donor to valence band transitions. Activation energy of donors in SnS was estimated to be 46 meV. The donor corresponding to the activation energy was due to interstitial Cadmium (Cdi). Based on these data, a schematic diagram of several defects was created for post-annealed CdS/SnS.

Superinjection in diamond homojunction P-I-N diodes

Diamond and many newly emerged wide bandgap semiconductor materials show outstanding optical and magnetic properties. However, they cannot be as efficiently doped as silicon or gallium arsenide, which limits their practical applicability. Here, we theoretically predict a superinjection effect in diamond p-i-n diodes, which allows for the injection of orders of magnitude more electrons into the i-region of the diode than doping of the n-type injection layer allows. Moreover, we show that the efficiency of electron injection can be further improved using an i-p grating implemented in the i-region of the diode. The predicted superinjection effect enables us to overcome fundamental limitations related to the high activation energy of donors in diamond and provides the opportunity to design high-performance devices.

Determination of dislocation density of a class of n-GaN based on the variable temperature Hall-effect method

An analytical model for electron mobility in a class of wurtzite n-GaN, whose carrier concentration is over 1018 cm-3 (Mott's critical limit), is developed. With the dislocation density and two donor levels serving as the important parameters, the proposed model can accurately predict the electron mobility as a function of temperature. The edge and screw dislocation densities in two samples, which are respectively grown on sapphire (001) by metal organic chemical vapor deposition and hydride vapor phase epitaxy, are determined by using this model which is discussed in detail. It is shown that the data-fitting of H-T characteristic curve is a highly suitable technique for accurately determining the edge and screw dislocation densities in n-GaN films. Quantitative analyses of donor concentration and donor activation energy indicate that the impurity band occurs when the carrier concentration is under 1017 cm-3, much lower than the critical carrier concentration of Mott transition (1018 cm-3). Such a behavior can also be confirmed by the results from solving the Boltzmann transport equation by using the Rode iterative method. Another anomaly is that the dislocation density in Mott transition material perhaps is lower than that of material with carrier concentration under 1018 cm-3. This fact indicates that the cause of Mott transition should not be the shallow donor impurities around dislocation lines, but perhaps the deeper donor impurities or other defects. In the theoretical model calculation, two transition characteristics together with the donor distribution and its energy equilibrium are taken into account. Based both on the Mott transition and the H-like electron state model, the relaxation energies for the shallow-donor defects along the screw and edge dislocation lines are calculated by using an electrical ensemble average method. Besides, an assumption that should be made is that there are 6 shallow-donor defect lines around one dislocation line. The research results show that the Hall mobility should be taken as the live degree of the ionizing energy for the shallow-donor defects along the dislocation line. The experimental results indicate that our calculation function can be best fit by the experimental curve, with the values of dislocation density being between our model and others determined by X-ray diffraction or by chemical etching method, which are all in good agreement with each other. The method reported can be applied to the wurtzite n-GaN films grown by various preparation technologies under any condition, with the peak-mobility temperature about or over 300 K, whose Hall mobility near 0 K perhaps is over 10 cm2/(Vs) and even 100 cm2/(Vs).

Electronic properties of the residual donor in unintentionally doped β-Ga2O3

Electron paramagnetic resonance was used to study the donor that is responsible for the n-type conductivity in unintentionally doped (UID) β-Ga2O3 substrates. We show that in as-grown materials, the donor requires high temperature annealing to be activated. In partly activated materials with the donor concentration in the 1016 cm−3 range or lower, the donor is found to behave as a negative-U center (often called a DX center) with the negative charge state DX− lying ∼16–20 meV below the neutral charge state d0 (or Ed), which is estimated to be ∼28–29 meV below the conduction band minimum. This corresponds to a donor activation energy of Ea∼44–49 meV. In fully activated materials with the donor spin density close to ∼1 × 1018 cm−3, donor electrons become delocalized, leading to the formation of impurity bands, which reduces the donor activation energy to Ea∼15–17 meV. The results clarify the electronic structure of the dominant donor in UID β-Ga2O3 and explain the large variation in the previously reported donor activation energy.

Post-deposition treatment of Cu2ZnSnSe4 with alkalis

Low temperature post-deposition treatment of Cu2ZnSnSe4 with NaF and KF significantly improved the solar cell efficiency (from 6.4% to 7.8% and 7.7% on average, respectively) due to enhanced fill factor (from 0.58 to 0.61 and 0.62), open-circuit voltage (Voc) (from 314mV to 337mV and 325mV) and short-circuit current density (from 35.3mA⋅cm−2 to 38.3mA⋅cm−2 and 38.6mA⋅cm−2). Voc improvement was higher for solar cells with NaF treatment due to an increase in radiative efficiency at room temperature and shallower defect activation energy as determined by photoluminescence (PL) and temperature dependent admittance spectroscopy, respectively. In the case of KF treatment, red-shift of the PL, higher band tail density of state and donor activation energy deeper in the band gap were limiting further improvement of the Voc compared to NaF treatment.

Conducting Si-doped γ-Ga2O3 epitaxial films grown by pulsed-laser deposition

We report structural, electrical, and optical properties of Si-doped γ-Ga2O3 films epitaxially grown on (100) MgAl2O4 substrate by pulsed-laser deposition. The γ-Ga2O3:Si films of a metastable spinel phase had neither secondary phase nor rotation domain. A highly doped film exhibited n-type conductivity with a carrier concentration of 1.8×1019cm−3 and a Hall mobility of 1.6cm2V−1s−1 at 300K. Donor activation energy was estimated to be less than 7meV from nearly temperature-independent transport properties down to 77K. The successful impurity doping indicates that γ-Ga2O3 can be used as an n-type wide-band-gap semiconductor.

Electron transport behaviors through donor-induced quantum dot array in heavily n-doped junctionless nanowire transistors

We investigated single electron tunneling through a phosphorus donor-induced quantum dot array in heavily n-doped junctionless nanowire transistor. Seven subpeaks splitting in current oscillations are clearly observed due to the coupling of quantum dot array under the bias voltage below 1.0 mV at the temperature of 6 K. The conduction system can be well described by a two-band Hubbard model. The activation energy of phosphorus donors is tuned by the gate voltage to be 7.0 meV for the lower Hubbard band and 4.4 meV for the upper Hubbard band due to the localization effects below threshold voltage. The evolution of electron behaviors in the quantum dots is identified by adjusting the gate voltage from quantum-dot regime to one-dimensional regime.

Growth and Analysis of Nanostructured CuInSe2-ZnIn2Se4 Solid Solutions: Their Electrical and Photoelectrical Properties

We report growth and investigation of structural and (photo−)electrical properties of CuInSe2– ZnIn2Se4 semiconductor crystals, grown by the horizontal Bridgman method. Their optical bandgap was evaluated from the absorption spectra. It increased monotonously from 1eV up to 1.08 eV at 300 K with increasing amount of ZnIn2Se4 in the range of 5–20 mol%. All crystals had n-type electrical conductivity. The solid solutions containing 5–10 mol% ZnIn2Se4 had demonstrated weak temperature dependence of the electrical conductivity, high electron density and low photoconductivity. This evidences their nearly degenerate state. In the crystals with 15 and 20 mol% ZnIn2Se4 a variable range hopping conduction mechanism was found to dominate between ∼30K and 110K. Above 130K it was changed by a thermally activated conductivity. The donor activation energy for the samples with 15 and 20 mol% ZnIn2Se4 were 0.018 eV and 0.04 eV, respectively. The characteristic feature of the spectral distribution of their photoconductivity at 30-70K is presence of a narrow peak at 1160 – 1190nm.

Stable and metastable Si negative-U centers in AlGaN and AlN

Electron paramagnetic resonance studies of Si-doped AlxGa1−xN (0.79 ≤ x ≤ 1.0) reveal two Si negative-U (or DX) centers, which can be separately observed for x ≥ 0.84. We found that for the stable DX center, the energy |EDX| of the negatively charged state DX−, which is also considered as the donor activation energy, abruptly increases with Al content for x ∼ 0.83–1.0 approaching ∼240 meV in AlN, whereas EDX remains to be close to the neutral charge state Ed for the metastable DX center (∼11 meV below Ed in AlN).

Influence of cation-vacancy defects on the properties of CuInSe2–ZnIn2Se4 solid solutions

CuInSe2–ZnIn2Se4n-type semiconductor crystals were grown by the horizontal variant of the Bridgman method. Their band gap was estimated from the position of the intrinsic absorption band edge. The obtained values of the band gap vary monotonically with the composition of the compounds: at 300K they increased from 1eV up to 1.08eV with increasing amount of ZnIn2Se4 in the range of 5–20mol%. The solid solutions containing 5–10mol% ZnIn2Se4 had demonstrated weak temperature dependence of the electrical conductivity, high electron density and low photoconductivity. This evidences their nearly degenerate state.In the crystals with 15 and 20mol% ZnIn2Se4 a variable range hopping conduction mechanism was found to dominate in the range ∼30–110K and a thermally activated conductivity at T⩾130K was observed. The determined values of the donor activation energy for the samples with 15 and 20mol% ZnIn2Se4 were 0.018eV and 0.04eV, respectively. The characteristic feature of the spectral distribution of their photoconductivity at 30–70K is presence of a narrow peak at 1160–1190nm.

Correlation between native defects and dopants in colloidal lanthanide-doped Ga2O3nanocrystals: a path to enhance functionality and control optical properties

We report the synthesis and study of the photoluminescence properties of colloidal lanthanide(III)-doped Ga2O3 nanocrystals. The Ga2O3 nanocrystal host lattice acts as a sensitizer of the Eu3+ dopant red emission arising from intra-4f orbital transitions, and concurrently exhibits a strong blue photoluminescence originating from defect-based donor–acceptor pair (DAP) recombination. The Eu3+ sensitization, enabled by the energy transfer from the nanocrystal host lattice to the dopant centers, allows for the generation of dual blue-red emission. Increasing doping concentration leads to a decrease in the donor activation energy allowing for simultaneous control of the optical and electrical properties of these multifunctional nanocrystals. Analyses of the steady-state and time-resolved photoluminescence spectra suggest that Eu3+ ions occupy at least two different sites, which were tentatively designated as the six-coordinate internal and surface-related dopants. Uniquely, both DAP and Eu3+ emissions have long lifetimes (in milliseconds), although Eu3+ luminescence has a slower decay rate. These phenomena enable a temporal modulation of the dual emission and photoluminescence chromaticity on the millisecond timescale. The generality of these findings was demonstrated by preparing Tb3+-doped Ga2O3 nanocrystals, as a blue-green dual emitter. Owing to their optical transparency, electrical properties, emission color versatility, robustness, and fabricability, colloidal lanthanide(III)-doped Ga2O3 nanocrystals are a promising class of multifunctional materials and complex phosphors.

Dominant shallow donors in zinc oxide layers obtained by low-temperature atomic layer deposition: Electrical and optical investigations

This work is focused on the electrical and optical analyses used to estimate the activation energy of the dominant shallow donor in thin ZnO films obtained at low temperature by the atomic layer deposition process. These two approaches, based on the temperature-dependent classical Hall effect and photoluminescence investigations, yielded a donor activation energy ED in the range of 30–40meV, including the estimated error margins. This value, as confirmed by layer composition studies, is attributed to the presence of zinc atoms in the interstitial positions of the ZnO lattice.

Enhancement of p-type conduction in Ag-doped ZnO thin films via Mg alloying: The role of oxygen vacancy

ZnO, ZnMgO, Ag-doped ZnO (ZnO:Ag), and Ag-doped ZnMgO (ZnMgO:Ag) thin films have been prepared by pulsed laser deposition. All the films have a preferred orientation with the c-axis perpendicular to the substrates. Hall-effect measurements indicate that the ZnO:Ag film exhibits p-type conduction, but obviously worse than that of the ZnMgO:Ag film. A comparative study of p-type ZnO:Ag and ZnMgO:Ag films using photoluminescence and x-ray photoelectron spectroscopy measurements shows that the enhanced p-type conduction in ZnMgO:Ag film is closely related to the increase of the activation energy of the intrinsic donors and the suppression of charge-compensating oxygen-related defects after Mg incorporation.

Temperature-dependent electron transport in ZnO micro/nanowires

Temperature dependent electron transport properties of individual ZnO mirco/nanowires from 293 K to 473 K were investigated by a four-probe method, and the size dependence of activation energy was studied. The conductivity increased with the increasing temperature, which was attributed to the thermal activations of donors: shallow donors and deep donors. The activation energy related to shallow donors was effectively independent of the radius, suggesting that it was associated with the surface conduction channel of ZnO wire, while a nearly inverse dependence of the activation energy of deep donors on the wire diameter may result from the dielectric confinement effect.

Carrier concentration dependence of donor activation energy in n-type GaN epilayers grown on Si (1 1 1) by plasma-assisted MBE

The n-type GaN layers were grown by plasma-assisted MBE and either intentionally doped with Si or unintentionally doped. The optical characteristics of a donor level in Si-doped, GaN were studied in terms of photoluminescence (PL) spectroscopy as a function of electron concentration. Temperature dependent PL measurements allowed us to estimate the activation energy of a Si-related donor from temperature-induced decay of PL intensity. PL peak positions, full width at half maximum of PL and activation energies are found to be proportional to the cube root of carrier density. The involvement of donor levels is supported by the temperature-dependent electron concentration measurements.

Hafnium-doped GaN with n-type electrical resistivity in the 10−4 Ω cm range

Hafnium (Hf) as an n-type dopant for group III nitride is proposed and investigated. Uniform doping of Hf into GaN realized an electrical resistivity as small as 7.89 × 10−4 Ω cm, which approaches that of a typical electron-beam deposited indium-tin oxide (ITO) thin film (∼3.75 × 10−4 Ω cm). Depending on doping level, the room-temperature (RT) donor activation energy for Hf in GaN was estimated to be in the range of 85-132.5 meV. Highly conductive Hf-doped GaN thin films with atomically smooth surface feature were achieved via Hf-delta doping, with RT electrical resistivity as small as 1.24 × 10−3 Ω cm. These highly conductive Hf-doped GaN films can be used as n-type transparent current-spreading layer (TCL) for large area light-emitting diodes (LEDs). With further improvement, highly conductive Hf-doped GaN has potential to be used as p-type TCL replacing the traditional indium-tin oxide for GaN-based LEDs.