Deep Impurities In Semiconductors Research Articles

Tunneling ionization of deep centers in high-frequency electric fields

A theoretical and experimental study of the tunneling ionization of deep impurities in semiconductors has been carried out for high-frequency alternating electric fields. It is shown that tunneling ionization occurs by phonon-assisted tunneling which proceeds at high electric field strengths into direct tunneling without involving phonons. In the quasistatic regime of low frequencies the tunneling probability is independent of frequency. Raising the frequency leads to an enhancement of the tunneling ionization. The transition from the quasistatic limit to frequency-dependent tunneling is determined by the tunneling time which, in the case of impurities interacting with thermal phonons, is controlled by the temperature. In both the quasistatic and high-frequency limits, the application of an external magnetic field perpendicular to the electric field reduces the ionization probability when the cyclotron frequency becomes larger than the reciprocal tunneling time.

Characterization of deep impurities in semiconductors by terahertz tunneling ionization

Tunneling ionization in high-frequency fields as well as in static fields is suggested as a method for the characterization of deep impurities in semiconductors. It is shown that an analysis of the field and temperature dependences of the ionization probability allows to obtain defect parameters like the charge of the impurity, tunneling times, the Huang–Rhys parameter, the difference between optical and thermal binding energy and the basic structure of the defect adiabatic potentials. Compared to static fields, high-frequency electric fields in the terahertz-range offer various advantages, as they can be applied contactlessly and homogeneously even to bulk samples using the intense radiation of a high-power pulsed far-infrared laser.

Characterization of deep impurities in semiconductors by terahertz tunneling ionization

Tunneling ionization in high frequency fields as well as in static fields is suggested as a method for the characterization of deep impurities in semiconductors. It is shown that an analysis of the field and temperature dependences of the ionization probability allows to obtain defect parameters like the charge of the impurity, tunneling times, the Huang–Rhys parameter, the difference between optical and thermal binding energy, and the basic structure of the defect adiabatic potentials. Compared to static fields, high frequency electric fields in the terahertz-range offer various advantages, as they can be applied contactlessly and homogeneously even to bulk samples using the intense radiation of a high power pulsed far-infrared laser. Furthermore, impurity ionization with terahertz radiation can be detected as photoconductive signal with a very high sensitivity in a wide range of electric field strengths.

π-electronic structures of ‘doped molecules’ predicted from those of ‘host molecules’

Abstract Interrelationships exist among the π-electronic structures of anthracene (which can be regarded as a ‘host molecule’) and benzo[g]quinoline, benz[g]isoquinoline and acridine, molecules which can be regarded as anthracene ‘doped’ with N replacing C at one of its sites. Similar interrelationships generally exist for any set of macromolecules that can be regarded as ‘doped’ versions of a ‘host’ macromolecule and can be derived and exploited using a combination of the Pullman theory of π-electronic conjugated structures and the Green's function theory of deep impurities in semiconductors.

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.

High Field Limitation of Poole-Frenkel Emission Caused by Tunneling

ABSTRACTThe electric field dependence of emission of carriers from deep impurities in semiconductors has been investigated applying static and terahertz electric fields. It is shown that at high electric field strengths carrier emission is dominated by phonon assisted tunneling which may easily be recognized by plotting the emission rate as a function of the square of the electric field strength in a log-lin plot giving a straight line. For charged impurities the transition from phonon assisted tunneling to Poole-Frenkel effect at low fields can be traced back.

Carrier Tunneling in High-Frequency Electric Fields

An enhancement of tunnel ionization of deep impurities in semiconductors in an alternating field as compared to static fields has been observed. The transition between the quasistatic 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. A theory of tunnel ionization of deep impurities by high-frequency fields has been developed.

Characterization of Deep Impurities in Semiconductors by Terahertz Tunnel Ionization

AbstractThe ionization of deep impurity centers by high intensity far-infrared radiation, with photon energies tens of times lower than the impurity binding energy, has been investigated. It is shown that the ionization is caused by phonon-assisted tunneling in the electric field of radiation, in which carrier emission is accompanied by electron tunneling and defect tunneling in the configuration space. The field and temperature dependencies of the ionization probability allow to obtain defect parameters like tunneling times and the Huang-Rhys factor as well as the basic structure of the defect adiabatic potentials. In very strong fields, the ionization changes to direct tunneling without assistance of phonons. It is shown that for the case of charged impurities and in a relatively low electric field strength the Poole-Frenkel effect plays a dominant role in ionization, which allows to identify the charge state of the center. Within a broad range of intensities, wavelengths and temperature, the terahertz electric field of the radiation acts as a static field. An enhancement of tunneling with increase of frequency has been observed for very high frequencies or low temperatures.

Thermal excitation rates and capture cross sections associated with deep impurities in semiconductors

The thermal excitation rates of carriers and radiative capture cross sections associated with deep impurities in semiconductors are calculated. It is shown that the lifetime for the carriers in deep traps is large and that for moderate and deep levels the magnitude of the cross sections is not negligible.

Quantum Calculation of Multiphonon Capture Cross‐Sections in the Model of Tangent Adiabatic Potentials

AbstractThe multiphonon capture cross‐section of deep impurities in semiconductors is considered taking into account the tangency of the adiabatic potential. The calculation is based on the quantum description of the one‐coordinate vibrational model. A comparison with the quasiclassical approach is given.

First-principles calculations of multiplet structures of transition metal deep impurities in II–VI and III–V semiconductors

A first-principles theory to calculate multiplet structures of transition metal (TM) deep impurities in semiconductors is developed. In the present theory the one-electron energy level structures calculated by the local density functional approach are combined with the ligand field theory from first principles. This method is applied to various 3d TM impurities in ZnS, ZnSe, GaP and GaAs. It is shown that the characteristic features of the observed optical spectra of TM impurities and the observed chemical trends of the multiplet structures by varying the host materials are reproduced satisfactorily by the present calculations. Furthermore, the crystal field splitting defined in the present theory is found to be quite different from that used in the empirical ligand field theory, and the present one is shown to be more suitable as the quantity defining the crystal field splitting in the light of the physical meaning.

Optical Absorption by Surface States and Atomic Reorganization Effects at the Semiconductor/Electrolyte Interface

We have studied the electromodulated infrared absorption of surface states at the n‐Si/acetonitrile electrolyte interface and discuss the results in connection with the standard theory of Harrick. Gold‐atom‐induced surface states and oxidation‐induced surface states have been separately investigated. The optical absorption of gold‐atom‐induced surface states is well accounted for in the framework of Harrick's theory by using for the optical cross sections of the surface states a model derived from Lucovsky's treatment of absorption by deep impurities in semiconductors. However, it appears that the optical absorption of oxidation‐induced surface states can be understood only by taking into account atomic reorganization effects upon filling or emptying the surface states.

Calculation of the spin-orbit-coupling parameter of deep impurities in semiconductors in the complete neglect of differential overlap approximation.

A complete neglect of differential overlap (CNDO) self-consistent-field calculation of the crystal-field-splitting parameter 10Dq is used to find the spin-orbit-coupling parameter and the orbital reduction factor. The general spin-orbit-coupling theory has been rewritten in terms of the CNDO approximation and applied to the systems ZnS:${\mathrm{Cu}}^{2+}$ and CdS:${\mathrm{Cu}}^{2+}$. The examination of 10Dq and the spin-orbit-coupling parameter \ensuremath{\zeta} in terms of the wave functions and some parameters involved shows that it is important to consider not only the effective charges but also the distribution of charge among the different atomic orbitals. The calculated spin-orbit-coupling parameters 600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for ZnS:${\mathrm{Cu}}^{2+}$ and 620 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for CdS:${\mathrm{Cu}}^{2+}$ agree quite well with the experimental values of 600 and 565 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively. The orbital reduction factors also agree with experiment. It must be emphasized, however, that our method does not include additional quenching of the spin-orbit-coupling parameter and the orbital reduction factor due to the Jahn-Teller effect. The method presented here can be used to find the quenching of various electronic properties due to covalency and is a uniform procedure for calculating the crystal-field and fine-structure parameters in the framework of the CNDO approximation.

Final state effects of deep impurities in semiconductors: I.L. Jones and J.C. Inkson Solid St. Commun.51, 59 (1984)

Final state effects of deep impurities in semiconductors: I.L. Jones and J.C. Inkson Solid St. Commun.51, 59 (1984)

Photoionization Cross‐Section for δ‐Shell Deep Levels

AbstractA δ‐shell model is presented of deep impurities in semiconductors as a simple but effective extension of the well‐known δ‐function model of Lucovsky, in order to address the problem of fairly sharp photo‐ionization spectra. Whereas Lucovsky treated only the photo‐ionization of electrons from s‐like impurity states to the conduction band (forbidden transitions), this paper considers also the photo‐ionization of holes to the valence band (allowed transitions). The new δ‐shell model gives an improved fit to experimental photo‐ionization data. It can be used to determine the range of the deep impurity potential as well as the binding energy, and can help to determine the nature of the photoionized charge (electron/hole).

Final state effects of deep impurities in semiconductors : I. L. Jones and J. C. Inkson. Solid St. Commun.51, 59 (1984)

Final state effects of deep impurities in semiconductors : I. L. Jones and J. C. Inkson. Solid St. Commun.51, 59 (1984)

A universal trend in the binding energies of deep impurities in semiconductors

Whereas the conventional practice of referring binding energies of deep donors and acceptors to the band edges of the host semiconductor does not produce transparent chemical trends when the same impurity is compared in different crystals, referring them to the vacuum level through the use of the photothreshold reveals a remarkable material invariance of the levels in III-V and II-VI semiconductors. It is shown that this is a consequence of the antibonding nature of the deep gap level with respect to the impurity atom-host orbital combinations.

Final state effects of deep impurities in semiconductors

Current theoretical and computational models for optical excitation processes of deep level impurities in semiconductors tend to concentrate on the impurity itself, largely ignoring the effects the impurity has no final states. Using a spherical band model, we show these effects can be included in calculations on optical absorption processes. The band state is modified by the presence of the impurity and this modified state is used in the calculation of optical matrix elements. We show that final state effects can cause significant changes in the local density of states and optical matrix elements.

Nonradiative Transitions in Deep Impurities in Semiconductors–Study in a Semiclassical Model

Nonradiative transitions in the deep impurity states in semiconductors are studied by a semiclassical model, in which the lattice distortion around the impurity is approximated as a classical harmonic oscillator modulating the potential depth for the electron. An integral equation is derived and solved numerically describing the nonadiabatic transition from the bound state to the scattering states which corresponds to the thermal ionization. Peculiar features in the transition dynamics are clarified in contrast with the transitions between discrete levels. In the adiabatic limit, which covers the situation at high temperatures in real semiconductors, the transition rate obeys the Arrhenius law with the pre-exponential factor given by the phonon frequency as pointed out by Sumi.

High magnetic field effects on shallow and deep impurities in semiconductors

It is shown that high magnetic fields provide an efficient means to distinguish strongly localized impurity states from shallow, effective-mass-like states, irrespective of the binding energies of these states. The ground and excited states of a simple model Hamiltonian for shallow and deep impurities are analyzed for arbitrary magnetic field strengths.