Electrical properties of ZnSe crystals doped with transition elements

The conductivity and photoconductivity of ZnSe crystals doped with transition elements are studied. It is shown that the doping of ZnSe crystals with 3d impurity elements is not accompanied by the appearance of electrically active levels of these impurities. At the same time, the introduction of these impurities into the cation sublattice brings about the formation of electrically active intrinsic defects. It is established that ZnSe crystals doped with Ti, V, Cr, Fe, Co, or Ni exhibit high-temperature impurity photoconductivity. Photoconductivity mechanisms in the crystals are proposed. From the position of the first ionization photoconductivity band, the energies of ground states of 3d ²⁺ ions in ZnSe crystals are determined.     

The role of trapping levels of nonequilibrium electrons during the formation of pinning centers for domain walls in the magnetic semiconductor CdCr<Subscript>2</Subscript>Se<Subscript>4</Subscript>

A complex study of electrical conductivity and photoconductivity in constant and variable electric fields, thermally stimulated conductivity, and the photoferromagnetic effect are carried out in the temperature range 10–300 K for a CdCr₂Se₄ magnetic semiconductor with various concentrations of Ga impurity and Se vacancies (V_Se). The phenomenon of hopping photoinduced conduction is observed for the first time. It is shown that shallow donor levels that exchange photoelectrons trapped by them with Cr³⁺ magnetic ions may be responsible for the photoferromagnetic effect. As a result of this exchange, the Cr³⁺ ions acquire valence and spin instabilities, which lead to nonequilibrium pinning of the domain wall by these ions.     

Study of the impurity photoconductivity and luminescence in ZnSe:Ni crystals in the visible spectral region

The photoconductivity and photoluminescence spectra of ZnSe:Ni crystals in the visible spectral region are studied. It is established that the high-temperature impurity photoconductivity of ZnSe:Ni crystals is controlled by the optical transitions of electrons from the ground state ³ T ₁(F) to high-energy excited states, with subsequent thermally activated transitions of electrons to the conduction band. A photoconductivity band associated with the photoionization of Ni impurities is revealed. The intracenter luminescence of ZnSe:Ni crystals is efficiently excited with light corresponding to the intrinsic absorption region of Ni²⁺ ions.     

Role of charged defects in the photoconductivity of Se95As5 chalcogenide glassy semiconductor with the EuF3 impurity

The energy spectrum of local states associated with charged defects D ^? and D ⁺ playing a significant role in carrier generation and recombination in the chalcogenide glassy semiconductor system Se₉₅As₅ containing EuF₃ impurities is proposed based on the study of the temperature dependence of the dark conductivity and steady-state photoconductivity, current-luminance characteristic, and the spectral distribution of the photocurrent. It is shown that EuF₃ impurities nonmonotonically change the concentrations of these states. Low concentrations form chemical compounds with selenium and arsenic due to the chemical activity of the rare-earth element and fluorine ions, which result in a decrease in the concentration of initial intrinsic defects. High concentrations, according to the charged-defect model, lead to a decrease in the concentration of D ⁺ centers and an increase in the concentration of D ^? centers due to the presence of Eu³⁺ ions. Some parameters of the charged-defect model are estimated, in particular the effective correlation energy U _eff (0.6 eV) and the polaron relaxation energy (W ⁺ = 0.4 eV, W ^? = 0.45 eV).     

Dislocation–impurity interaction in Si

The dynamic interaction between dislocations and impurities B, P and Ge in Si with concentrations up to 2.5×10²⁰ cm⁻³ is investigated by the etch-pit technique, in comparison with that of O impurity in Si. Dislocation generation from a surface scratch is strongly suppressed when the concentration of B and P impurities exceeds 1×10¹⁹ cm⁻³, originating from the immobilization by preferential impurity segregation. Dislocation velocity in motion enhances on increasing the concentration in B and P impurities. Neutral impurity Ge has weak effect on dislocation generation and velocity enhancement.     

Current-voltage characteristics of Si:B blocked impurity-band structures under conditions of hopping-transport-limited photoresponse

The photoconductivity of Si:B blocked-impurity-band (BIB) structures with boron concentration in the active layer ∼10¹⁸ cm⁻³ has been studied. Measurements were performed in the temperature range 4.2–10 K at different intensities of the exciting radiation 10¹⁰–10¹⁵ photons/cm²·s. Photoexcitation at 5.5 μm was realized using a semiconductor laser. At temperatures below 6 K and low bias voltages (<0.5 V) the current-voltage characteristics were found to have a threshold-like character. The threshold voltage rises as the temperature is lowered and the radiation intensity is increased. A model based on the Frenkel’-Poole effect in the impurity band has been developed. This model can be used to numerically describe the current-voltage characteristics with accuracy better than 5%. As a result, it is found that the photoconductivity rises and then reaches a plateau as the radiation intensity increases. Under these conditions, as under equilibrium conditions (in darkness), the hopping conductivity also depends exponentially on the electric field. This fact is explained in terms of the destruction by the electric field of (A ⁺-A ⁻) impurity complexes which appear under nonequilibrium conditions. Fiz. Tekh. Poluprovodn. 32, 192–199 (February 1998)     

Kinetics of terahertz photoconductivity in <Emphasis Type="Italic">p</Emphasis>-Ge under impurity breakdown conditions

Relaxation times of impurity photoconductivity in p-Ge samples excited by a nanosecond narrow-band source of terahertz radiation are studied at various bias voltages. It is shown that the relaxation time in prebreakdown fields increases with the applied electric field and decreases as the impurity breakdown field is exceeded. Nonmonotonic photoconductivity kinetics is observed in the studied samples differing by acceptor concentrations and degrees of compensation when approaching the impurity breakdown field.     

Specific features of the photoexcitation spectra of epitaxial InN layers grown by molecular-beam epitaxy with the plasma activation of nitrogen

The results of studies of the photoexcitation spectra of epitaxial InN layers formed by molecular-beam epitaxy with the plasma activation of nitrogen are reported. The concentration of free charge carriers in the layers is 10¹⁸–10¹⁹ cm^–3. The photoconductivity, photoluminescence, and absorption spectra exhibit a shift of the long-wavelength threshold of interband transitions in accordance with the Burstein–Moss effect for n-InN with different concentrations of equilibrium electrons. In the samples, absolute negative photoconductivity with a nanosecond relaxation time is observed. The results of photoelectric, absorption, and luminescence spectroscopy experiments are correlated with the technological parameters and electron microscopy data.     

On the cascade capture of electrons at charged dipoles in weakly compensated semiconductors

The times of the cascade capture of electrons at a donor–acceptor charged dipole for the case of the pulsed and steady-state excitation of impurity photoconductivity in GaAs, Ge, and Si are calculated. It is shown that the dependence of the frequency of the cascade capture on the concentration of the charged impurity becomes sublinear in semiconductors under consideration at a concentration of the charged impurity higher than 10¹³ cm^–3.     

Carrier recombination and deep levels in PbTe

The results of transient photoconductivity lifetime measurements and of impurity photoconductivity as well as Hall effect measurements obtained on PbTe films show that deep electron trapping levels exist in monocrystalline epitaxially grown PbTe. To our knowledge this is the first direct observation of deep levels in the gap of PbTe. The energetic position of the levels and their concentration were determined. The observed levels profoundly influenced the efficiency of PbTe infrared emitter diodes. This was shown experimentally by comparing the luminescence intensity of various diodes with different carrier concentration in the active zone. A strong decrease of luminescence intensity was observed for carrier concentrations greater than 10¹⁷ cm⁻³. This decrease could be explained by recombination via the observed levels in the gap.     

Some aspects of selection of impurities improving photoelectric characteristics of chalcogenide vitreous semiconductors

The effect of low concentrations of two groups of doping impurities (one group includes Sn, Pb, Dy, Ho, Y and the other group includes In, Cs, and Al) on photoelectric properties of As₂Se₃ and (As₂S₃)_0.3(As₂Se₃)_0.7 was investigated. Studies of spectral distributions of photoconductivity and optical absorption showed that there is a distinct increase in photoconductivity if the level of doping with elements of the first group is as high as 0.015%. Elements of the second group of impurities did not exert any significant effect on photoconductivity. The effect of impurities of the first group is attributed to occupation of vacant sites (formed as a result of removal of highly volatile Se and S atoms) by low concentrations of impurity atoms. The observed effect is related to the fact that substitutional atoms can preserve covalent bonds and to the small difference between atomic sizes and electron affinity for doping impurities and S and Se. Atoms of the second group of impurities lack these features.     

Specific features of electron spin resonance in 4<Emphasis Type="Italic">H</Emphasis>-SiC in the vicinity of the insulator-metal phase transition: II. Analysis of the width and shape of lines

The variation in the width and shape of the ESR line of nitrogen in 4H-SiC in the concentration range corresponding to the insulator-metal phase transition was investigated. It is shown that the spin relaxation in the region of hopping and metal conduction occurs at electrical multipoles (clusters) whose sizes decrease from rather large to small (characteristic of interimpurity distances) as the concentration of impurity centers increases. Analysis of the temperature dependences of the resistance made it possible to estimate the critical concentration for the insulator-metal phase transition (N _D -N _A )_c≈1.5×10¹⁹ cm^?3. The values of other characteristic concentrations that determine the effects of electron-electron interaction in the system under study were also found.     

Effect of iron impurities on the photoluminescence and photoconductivity of ZnSe crystals in the visible spectral region

The photoconductivity and photoluminescence of ZnSe:Fe crystals in the visible spectra region are studied. The scheme of optical transitions within Fe²⁺ impurity centers is established. It is shown that the high-temperature photoconductivity of ZnSe:Fe crystals is controlled by optical transitions of electrons from the ⁵ E(F) ground state to the higher levels of excited states of Fe²⁺ ions, with subsequent thermal activation of the electrons to the conduction band. Efficient excitation of intracenter luminescence of ZnSe:Fe crystals is attained with light corresponding to the region of intrinsic absorption in Fe²⁺ ions.     

Mobility of holes in p-type silicon determined by the self-consistent method

The mobility of majority carriers is calculated in p-type silicon in the impurity concentration range from 10¹⁶ to 10²⁰ cm^?3. Taking into account the complexity of the band structure of holes and using the self-consistent model development earlier for electrons, the value of the Fermi energy and the screening length is determined. The following scattering types are considered: scattering by acoustical phonons, scattering by non-polar optical phonon and scattering by ionized impurities. The influence of interband transition of heavy and light holes on the total relaxation time is analysed. It is shown that it does not lead to essential changes in mobility. The obtained results are compared with experimental results and satisfactory agreement is found.     

Mechanisms of incorporation of an antimony impurity into cadmium telluride crystals

The results of electrical studies of CdTe crystals grown by the Bridgman-Stockbarger method and doped with Sb impurity to concentrations of 10¹⁷–3×10¹⁹ cm^?3 were considered. An analysis of the temperature dependences of the Hall coefficient, the charge-carrier mobility, and photoconductivity under the intrinsic-absorption excitation for various portions of the ingots made it possible to conclude that Sb_Te and Sb_Cd centers and Sb_TeSb_Cd associations are introduced upon doping CdTe crystals with Sb impurity. The hole conduction in the doped crystals is controlled by A ₃ (Sb_Te) acceptors whose concentration is no higher than 5×10¹⁶ cm^?3 and is much lower than the actual Sb concentration. The ionization energy of the A ₃ acceptors is 0.28±0.01 eV. Under nonequilibrium conditions, these acceptors act as attachment centers for holes (at high temperatures) and as slow-rate recombination centers for electrons (at low temperatures).     

Photoluminescence characterization of ultrahigh purity silicon

When photoluminescence (PL) measurements on silicon are calibrated against an established measurement technique, they become a powerful characterization tool capable of quantitatively identifying impurities. PL is particularly useful for characterizing ultrahigh purity silicon, due to its ability to determine the concentrations of the shallow impurities down to ≈5 × 10¹⁰ atoms-cm⁻³. We have utilized variable temperature Halleffect measurements to calibrate PL measurements made at 4.2 K on ultrahigh purity float-zone silicon, establishing calibration relationships to determine boron, phosphorus, and gallium concentrations. The concentrations range from the low −10¹² atoms-cm^-3 for phosphorus and boron, to 1 × 10¹⁴ atoms-cm⁻³ for boron and 1 × 10¹⁶ atoms-cm⁻³ for phosphorus. The preliminary gallium calibration concentration range is from the high-10¹² to the mid-10¹⁰ atoms-cm³. Previous PL calibrations based on variable temperature Hall-effect measurements have not produced the theoretically expected proportionality between the impurity concentration and the PL line intensities of the transverse-optical phonon-assisted replicas of the impurity bound exciton normalized to the PL line intensity of the free-exciton. By performing the PL measurements at a fixed free-exciton density, we have achieved the theoretically expected proportionality. The experimental methods and an overview of the theory concerning fixed free-exciton con-ditions, the associated Hall-effect analyses, and the PL calibrations are presented.     

Photoelectric properties of ZnO films doped with Cu and Ag acceptor impurities

The influence exerted by doping with Cu and Ag acceptor impurities at a content of 1, 3, and 5 at. % on the luminescence and photoconductivity of zinc oxide films has been studied. Electron-beam evaporation in optimal modes has been used to obtain films with predominant luminescence in the UV spectral range. It has been shown that the incorporation of copper yields three types of point defects in ZnO: Cu_Zn (3d¹⁰), Cu_Zn (3d⁹), and Cu_i; and in silver, a single type: Ag_Zn (3d¹⁰). Precipitation of a silver oxide phase at the highest impurity concentration has been observed. Impurity incorporation leads to a pronounced increase in the resistance and photosensitivity of films.     

Specific features of transmutational doping of 30Si-enriched silicon crystals with phosphorus: Studies by the method of electron spin resonance

Electron spin resonance (ESR) is used to study the neutron transmutation doping of silicon crystals enriched with ³⁰Si isotope: phosphorus donors and radiation defects produced in the course of transmutational doping are observed. The ESR signals related to the phosphorus uncontrolled impurity in ³⁰Si before transmutational doping (the P concentration is ～10¹⁵ cm^?3) and phosphorus introduced by neutron irradiation with doses ～1 × 10¹⁹ cm^?2 and ～1 × 10²⁰ cm^?2 (the P concentrations are ～5 × 10¹⁶ and ～7 × 10¹⁷ cm^?3, respectively) are studied. As a result of drastic narrowing of the phosphorus ESR lines in ³⁰Si, the intensity of lines increased appreciably, which made it possible to measure the phosphorus concentration in the samples with a small volume (down to 10^?6 mm^?3). The methods for determining the concentration of P donors from hyperfine structure in the ESR spectra of isolated P atoms, exchange-related pairs, and clusters that consist of three, four, and more P donors are developed. In the region of high concentrations of P donors, in which case the hyperfine structure disappears, the concentration of P donors was estimated from the exchange-narrowed ESR line.     

Influence of the screening effect on passivation of <Emphasis Type="Italic">p</Emphasis>-type silicon by hydrogen

Passivation of p-Si by hydrogen through its diffusion was simulated by solving diffusion-kinetic equations with allowance made for hydrogen-acceptor-pair formation, internal electric field, and the screening effect. Screening of hydrogen and acceptor ions by free carriers leads to a decrease in the radius of interaction between the ions and to the weakening of the concentration dependence of hydrogen diffusivity in heavily doped Si. At a binding energy of the pairs of 0.70–0.79 eV, calculated and experimental concentration profiles of holes and the hydrogen-acceptor pairs are in agreement over a wide range of boron concentrations, from 4×10¹⁴ to 1.2×10²⁰ cm^?3. The radius of the Coulomb interaction of hydrogen and boron ions is 35 Å in lightly doped Si and decreases as the dopant concentration increases.     

Excited states of chalcogen ions in germanium

The energy spectra of chalcogen impurities in germanium are studied by photoconductivity, optical absorption, and photoluminescence. The previously found excited states of these impurities are identified. The observed excited states of tellurium (Te⁺) and selenium (Se⁺) ions are shown to be associated with the Γ and L points of the germanium Brillouin zone, respectively. Fiz. Tekh. Poluprovodn. 32, 155–158 (February 1998)