Impurity Ionization Energy Research Articles

Simulation of the Incomplete Ionization of the <i>n</i>-Type Dopant Phosphorus in 4H-SiC, Including Screening by Free Carriers

The simulation of the incomplete ionization of substitutional dopants in Silicon Carbide (SiC) is often performed using Boltzmann statistics and ionization energy values that do not depend on free carrier concentrations. But in the case of heavy doping Fermi-Dirac statistics is needed, while the case of an inhomogeneous dopants distribution or that of an excess carrier injection requires local free carrier concentration-dependent impurity ionization energies. Here a model for describing partial ionization from diluted to high homogeneous doping densities in SiC and in thermal equilibrium is presented and compared with results on Phosphorus doped 4H-SiC.

First-principles investigation of electronic structure and optical properties in N-F codoped ZnO with wurtzite structure

The electronic structures and optical properties of pure, N-doped and N-F codoped ZnO are investigated based on the density-functional theory. The calculations of the impurity formation energies and ionization energies for these systems indicate that incorporating the reactive donor F into N doped ZnO systems, not only enhances the N acceptor solubility, but also leads to a shallower N acceptor energy level in the band gap in p-type codoped ZnO. In addition, we analyze the imaginary part of the dielectric functions, and reflectivities for pure and N-F codoped ZnO. Compared with the pure ZnO, the remarkable feature in the dielectric function for N-F codoped ZnO is that there is a sharp peak in the low-energy region.

Analysis of sunlight loss for femtosecond laser microstructed silicon and its solar cell efficiency

Black silicon, which is obtained by irradiating the surface of a Si wafer with femtosecond laser pulses in the presence of a sulfur-bearing gas, holds great promise in the preparation of high-performance intermediate band silicon solar cells. Using a three-level model, the enhanced usefulness of sunlight of the microstructured silicon was firstly analyzed. A detailed study on the relationship between the light loss, the ionization energy of doped impurities in silicon and the impurity band width were given. Then the effect of the position of intermediate band within the forbidden gap of silicon on the theoretical conversion efficiency for the corresponding solar cell is discussed using the Detailed Balance Theory. Finally problems need to be resolved in making intermediate band solar cells based on femtosecond laser microstructured silicon are pointed out with great emphasis.

Gate-Controlled Donor Activation in Silicon Nanowires

Due to the proximity to an embedding medium with low dielectric constant (e.g., oxides), semiconductor nanowires have higher impurity ionization energy than their bulk counterparts, resulting lower free carrier density. Using ab initio calculations within density functional theory, we propose a way to reduce the ionization energy in nanowires by fabricating a special cross section with appropriate engineering of doping and an applied gate voltage. We demonstrate on a phosphorus-doped silicon nanowire that the ionization energy can be effectively tuned and the impurity backscattering can also be reduced. For instance, even without special engineering of doping, the free carrier density may increase by 40% in a silicon nanowire with 15 nm diameter and special cross section. Our proposal has profound implications to fabricate nanowire devices with high carrier density.

Screening and polaronic effects induced by a metallic gate and a surrounding oxide on donor and acceptor impurities in silicon nanowires

We present self-consistent tight binding calculations of the electronic structure of donor and acceptor impurities in silicon nanowires surrounded by a gate oxide (SiO2 or HfO2) and a metallic gate. These environments efficiently screen the potential of the impurities so that their ionization energy strongly decreases with respect to the case of freestanding nanowires. It is also shown that the carriers trapped by the impurities form a polaron due to the response of the ions in the surrounding oxide layer. We predict that the polaron shift represents a large part of the impurity ionization energy, in particular, in HfO2. Our work demonstrates the importance of screening and polaronic effects on the transport properties in nanoscale devices based on Si nanowires.

Electrical properties and phase transition of CdTe under high pressure

In situ electrical resistivity measurement of powdered CdTe has been performed under highpressure using a diamond anvil cell equipped with a microcircuit. With the pressureincreasing from 2.7 to 3.8 GPa, a sharp decrease in resistivity of over three orders ofmagnitude is observed. This is due to the appearance of rock-salt CdTe. At about6.5 GPa a resistivity inflexion appears which has not been reported before. It iscaused by the obvious decrease of band gap in certain symmetry directions of theBrillouin zone of rock-salt CdTe. From 6.5 to 10 GPa, the descending trend of theresistivity turns gently. At about 10 GPa, a cusp corresponding to the transition to theCmcm phase is distinguished. Between 10 and 38 GPa, three leap points have been detected at15.5 GPa, 22.2 GPa and 30 GPa, which imply abundant electronic phase transitionsof CdTe. Resistivity measurements were also performed in a wide temperaturerange from liquid nitrogen temperature (77 K) to 450 K. It is proved that rock-saltCdTe does not show a typical metallic transport character. In the pressure rangeof 6.0–7.0 GPa, CdTe has a band gap of about 445 meV. The relationship oflnρ to1/T is also fitted linearly to yield the ionization energy of impurities at different pressures.

Ionization energy of donor and acceptor impurities in semiconductor nanowires: Importance of dielectric confinement

Calculations of the electronic states of donor and acceptor impurities in nanowires show that the ionization energy of the impurities is strongly enhanced with respect to the bulk, above all when the wires are embedded in a material with a low dielectric constant. In free-standing nanowires with diameter below $10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, the ionization of the impurities at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is strongly reduced and heavy doping is necessary to obtain conductive systems. These results imply that the critical density for metal-nonmetal transitions is not the same as in the bulk. Experiments are proposed to test the predictions.

Impurity absorption and luminescence of CuGaSe2 crystals

A comparison of the experimental and calculated absorption spectra of CuGaSe2 crystals revealed the existence of two acceptor levels with ionization energies of 66 and 167 meV in the samples under study. It was found that the luminescence spectra of CuGaSe2 measured at 77 K exhibit four impurity-band transitions with impurity ionization energies of 66, 99, 136, and 190 meV. An analysis of the temperature dependence of the luminescence intensity in the temperature range 11–77 K revealed a band peaking at 1.671 eV due to the radiation of donor-acceptor pairs. The calculated sum of the ionization energies of the impurities responsible for the formation of donor-acceptor pairs and the temperature dependence of the relative intensities of impurity-band emission were used to determine the ionization energies of the corresponding donor and acceptor.

Reflectivity spectrum and electrical properties of TlIn0.98Ce0.02Se2 single crystals

The room-temperature reflectivity spectrum of TlIn0.98Ce0.02Se2 crystals is measured at photon energies from 1 to 6 eV, and their conductivity, Hall coefficient, and thermoelectric power are determined in the temperature range 80–460 K. The results are used to evaluate the ionization energy of impurities (ΔE a = 0.09 eV), the band gap of the crystals (E g = 1.30 eV), and their refractive index (n = 2.5).

Dynamic Chaos in a Partially Illuminated Compensated Semiconductor Under the Conditions of Impurity-Related Breakdown

The nonlinear dynamics of a compensated semiconductor in the case of an impurity-related electrical breakdown in a classically strong magnetic field under the effect of shortened Hall contacts and resonance radiation, the frequency of which corresponds to the ionization energy of hydrogen-like donor impurities, is evaluated. As a result, both regular and chaotic self-sustained oscillations are obtained. It is found that all three scenarios for the origination of chaos are realized for corresponding values of the bifurcation parameters. The results obtained have the potential become the theoretical foundation for the operation of an easily controlled high-frequency oscillator whose modes of operation (the switched-on mode and the transition from the regular mode to a chaotic mode and vice versa) could be changed by varying the illumination intensity.

Numerical simulation of the temperature dependence of the ionization energy of hydrogen-like impurities in semiconductors: Application to transmutation-doped Ge: Ga

An electrostatic model describing the dependence of the thermal ionization energy of impurities on their concentration, compensation factor, and temperature is developed. The model takes into account the screening of impurity ions by holes (electrons) hopping from impurity to impurity, the change in the impurity-band width, and its displacement with respect to the edge of the valence band for acceptors (conduction band for donors). The displacement of the impurity band is due to the functional dependence of the hole (electron) affinity of the ionized acceptor (donor) on the screening of the Coulomb field of the ions. The spatial distribution of the impurity ions over the crystal was assumed to be Poisson-like, and the energy distribution was assumed to be normal (Gaussian). For the relatively low doping levels under investigation, the behavior of the density of states at the edges of the valence and conduction bands was assumed to be the same as for the undoped crystal. The results of the numerical study are in agreement with the decrease in the ionization energy that is experimentally observed for moderately compensated Ge: Ga as the temperature and the doping level are decreased. It is predicted that the temperature dependence of the thermal ionization energy has a small anomalous maximum at small compensation factors.

Transport Properties of TlInSe2〈Ln〉 (Ln = Eu, Sm, Yb)

The electrical conductivity and Hall coefficient of TlInSe2〈Ln〉 (Ln = Eu, Yb, Sm) were measured between 300 and 900 K. The results were used to determine the band gap, ionization energy of impurities, conductivity type, and Hall mobility of charge carriers in TlInSe2〈Ln〉. The temperature at which the Hall coefficient changes sign is shown to increase with doping level.

Recombination processes in unintentionally doped GaTe single crystals

Emission spectra of GaTe single crystals in the range of 1.90–1.38 eV have been analyzed at different temperatures and excitation intensities by photoluminescence, photoluminescence excitation, and selective photoluminescence. A decrease in band gap energy with an increase in temperature was obtained from the redshift of the free exciton recombination peak. The energy of longitudinal optical phonons was found to be 14±1 meV. A value of 1.796±0.001 eV for the band gap at 10 K was determined, and the bound exciton energy was found to be 18±0.3 meV. The activation energy of the thermal quenching of the main recombination peaks and of the ones relating to the ionization energy of impurities and defects was analyzed. The results obtained show the existence of two acceptor levels with ionization energies of 110±5 and 150±5 meV, respectively, and one donor level with an ionization energy of 75±5 meV. The study of chemical composition by inductively coupled plasma-optical emission spectroscopy and x-ray energy dispersion spectroscopy shows the existence of Na, Li, and Si. Sodium and lithium impurities could be associated with acceptor levels at gallium substitutional sites, and silicon ones with a donor level at Ga sites, whose vacancies can also be involved in these electronic levels.

N - and p-type dopants for cubic silicon nitride

The formation and ionization energies of impurities in cubic silicon nitride are investigated through first-principles calculations. Among the elements in the groups III to VI, P and O are preferable for n-type doping, while Al is favorable for p-type doping in terms of the formation and ionization energies. The compensation of doped carriers associated with the incorporation of these impurities into anti and interstitial sites can be suppressed if appropriate growth conditions are chosen.

Ionization energy of copper in Hg0.8Cd0.2Te crystals under conditions of light and intermediate doping

Resistivity and the Hall effect were studied in copper-doped p-Hg 0.8 Cd 0.2 Te crystals in the temperature range of 4.2–125 K and the range of Cu concentrations from 2.6×1015 to 2×1018 cm−3. It is shown that the conventional method for determining the ionization energy of impurities from the slope of the dependence R H (T) is inapplicable in this case. In order to obtain the correct results, it is necessary to take into account the structure of the impurity band and the screening of the impurity charge with free charge carriers. A simplified model of the impurity band is suggested; this model makes it possible to calculate the ionization energy of acceptors under conditions of light doping and a small degree of compensation. This approach is used to find that ionization energy of copper depends only slightly on the copper concentration at T=0 and is equal to E A =7.6 meV for an isolated acceptor, which coincides with the theoretical value. At finite temperatures, the ionization energy of acceptors decreases appreciably as a result of screening.

Photocurrent Transients in Presence of a Double Impurity in Semi-Insulating Semiconductors

A phenomenological model is developed to study the transient behaviour of the photocurrent due to detrapping of carriers from a double impurity in semi-insulating semiconductors. The model predicts a peak in the transient curve at constant temperature. An Arrhenius plot of tp (the time at the peak) yields the second ionization energy of the double impurity. The activation energy determined from the photoinduced current transient spectroscopy (PICTS) analysis corresponds to the first ionization energy of the impurity. The model explains the appearance of a negative peak in the PICTS spectrum which was reported by previous authors on a number of semiconductors. Experimental investigation of the current decay curves on MgxZn1—xTe (x = 0.10, 0.14, 0.30) showed that for x = 0.10 the behaviour of the photocurrent decay at different temperatures and the PICTS spectrum can be interpreted in terms of the double impurity model. The ionization energies of the impurity were determined to be 0.082 ± 0.007 eV and 0.12 to 0.13 eV.

Universal impurity ionization parameters in MIS C-V freeze-out characteristics and direct extraction of surface doping concentration

The carrier freeze-out is responsible for a kink-effect near the flat-band voltage in MIS capacitance-voltage (C-V) characteristics. By studying this phenomenon in depth, it has been demonstrated that the flat kink situation is governed by only two universal and constant parameters. Taking advantage of this particularity, a new method has been proposed and experimentally validated, aimed at extracting directly surface doping concentrations and impurity ionization energies.

Tunnel ionization of deep impurities in semiconductors induced by terahertz electric fields

An analysis is made of the ionization of deep impurity centers induced by high-intensity terahertz radiation whose photon energies are tens of times lower than the impurity ionization energy. Under these conditions, ionization can be described as phonon-assisted tunneling in which carrier emission is accompanied by defect tunneling in configuration space and electron tunneling in the electric field of the radiation. At high intensities the ionization is caused by direct tunneling. Within a broad range of intensity, frequency and temperature, the terahertz electric field of the radiation acts like a static field. For very high frequencies and low temperatures an enhancement in tunneling as compared to static fields takes place.

Excitation of Deep Defects by Intense Terahertz Radiation

An analysis is done of the ionization of deep impurity centers by high-intensity terahertz radiation, with photon energies tens of times lower than the impurity ionization energy. Under these conditions, ionization can be described as direct tunneling and phonon-assisted tunneling in which carrier emission is accompanied by defect tunneling in configuration space and electron tunneling in the electric field of the radiation. Within a broad range of intensity, frequency, and temperature, the terahertz electric field of the radiation acts like a static field. For very high frequencies and low temperatures an enhancement of tunneling as compared to static fields was observed. The transition between the quasi-static and the high frequency regime is determined by the tunneling time. For the case of deep impurities this is the time of redistribution of the defect vibrational system which depends strongly on temperature and the impurity structure. PACS numbers: 71.55.—i, 72.20.Ht, 72.40.1w , 72.30.+q

A model of how the thermal ionization energy of impurities in semiconductors depends on their concentration and compensation

An electrostatic model is derived for the dependence of the thermal ionization energy of hydrogenic impurities E1 on their concentration N and degree of compensation K, with allowance for the screening of ions by electrons (holes) that hop from impurity to impurity. It is shown that the change in E1 with increasing N and K is connected with broadening of the impurity band and its shift toward the valence (v) band for acceptors and toward the conduction band (c) for donors. The shift in the impurity band is explained by a decrease in the affinity energy of an ionized acceptor for a hole (or a donor for an electron) due to screening of the ions. The impurity ion distribution density over the crystal is assumed to be Poisson-like, while its energy distribution is normal. The electron densities of states in the v-and c-bands are assumed to be those of the undoped crystal for the temperature interval in which E1 is determined. The values of E1(N,K) calculated using the expressions given here coincide with known experimental data for transmutation-doped Ge crystals. A description is given of the dependence on N and K of the thermal ionization energy of Zn atoms in p-type Ge as they change from a charge state (−1) to (−2).