Photoluminescence as a tool for the study of the electronic surface properties of gallium arsenide

Electronic surface parameters of GaAs have been determined from a qualitative and quantitative analysis of the relative photoluminescence intensity at 300 K. Characteristics of etched (100) surfaces ofn- andp-type liquid phase epitaxial GaAs have been found to be governed by negative surface charges. A density of charged surface states of about 10¹² cm⁻² and a band bending of 0.59 eV have been found forn-type material with an electron concentration of 1.1×10¹⁷ cm⁻³. Forp-type samples with hole densities ranging from 6×10¹⁵ to 4.3×10¹⁸ cm⁻³ the estimated density of negatively charged surface states was below 2×10¹¹ cm⁻², and the band bending was not more than a few kT.     

Influence of high-energy electron irradiation on field emission properties of multi-walled carbon nanotubes (MWCNTs) films

The effect of very high energy electron beam irradiation on the field emission characteristics of multi-walled carbon nanotubes (MWCNTs) has been investigated. The MWCNTs films deposited on silicon (Si) substrates were irradiated with 6 MeV electron beam at different fluence of 1×10¹⁵, 2×10¹⁵ and 3×10¹⁵ electrons/cm². The irradiated films were characterized using scanning electron microscope (SEM) and micro-Raman spectrometer. The SEM analysis clearly revealed a change in surface morphology of the films upon irradiation. The Raman spectra of the irradiated films show structural damage caused by the interaction of high-energy electrons. The field emission studies were carried out in a planar diode configuration at the base pressure of ∼1×10⁻⁸ mbar. The values of the threshold field, required to draw an emission current density of ∼1 μA/cm², are found to be ∼0.52, 1.9, 1.3 and 0.8 V/μm for untreated, irradiated with fluence of 1×10¹⁵, 2×10¹⁵ and 3×10¹⁵ electrons/cm². The irradiated films exhibit better emission current stability as compared to the untreated film. The improved field emission properties of the irradiated films have been attributed to the structural damage as revealed from the Raman studies.     

Interface of anodic sulfide-oxide on n-type InSb

A novel anodic sulfidization process for forming native sulfide-oxide films on n-type InSb is described. The results of Auger electron spectroscopy analysis indicate that native sulfide-oxide films are formed from aqueous sulfide solutions. The measured capacitance-voltage characteristics of metal-insulator-semiconductor devices indicate that the films have a low fixed surface charge density of the order of 3×10¹⁰ cm^–2 and small hysteresis. The native sulfide-oxide films leave the surface of n-type InSb practically at flatband and in this respect are superior to the passivation layers of anodic oxide and direct plasma SiN _x . The interface between InSb and its native sulfide-oxide in combination with plasma CVD SiN _x has excellent electrical properties.     

Nonlinear optical properties of nanometer-size silver composite azobenzene containing polydiacetylene film

Diacetylene monomer containing p-nitrophenyl azobenzene moiety (NADA) was synthesized. Silver nanoparticles with different concentrations were adulterated in the above polymerized NADA (PNADA) films and the third-order nonlinear optical properties were investigated in detail. UV–vis spectra and transmission electron microscopy were used to confirm the formation of PNADA/Ag nanocomposite films. The silver nanoparticles (average size of 10 nm) were well dispersed in the polymer films. The value of the nonlinear refractive index n ₂ for PNADA films (8.48×10⁻¹⁵ cm²/W) was much higher than that of pure polydiacetylene films. Further, the introduction of silver nanoparticles into the PNADA polymer films led to the further enhancement of nonlinear optical properties. The maximum value of n ₂ for PNADA/Ag nanocomposite films could be 11.6×10⁻¹⁵ cm²/W. This enhancement should be ascribed to the surface plasmon resonance of silver nanoparticles.     

Pulsed laser deposition of Zr–N codoped <Emphasis Type="Italic">p</Emphasis>-type ZnO thin films

Present p-type ZnO films tend to exhibit high resistivity and low carrier concentration, and they revert to their natural n-type state within days after deposition. One approach to grow higher quality p-type ZnO is by codoping the ZnO during growth. This article describes recent results from the growth and characterization of Zr–N codoped p-type ZnO thin films by pulsed laser deposition (PLD) on (0001) sapphire substrates. For this work, both N-doped and Zr–N codoped p-type ZnO films were grown for comparison purposes at substrate temperatures ranging between 400 to 700 °C and N₂O background pressures between 10⁻⁵ to 10⁻² Torr. The carrier type and conduction were found to be very sensitive to substrate temperature and N₂O deposition pressure. P-type conduction was observed for films grown at pressures between 10⁻³ to 10⁻² Torr. The Zr–N codoped ZnO films grown at 550 °C in 1×10⁻³ Torr of N₂O show p-type conduction behavior with a very low resistivity of 0.89 Ω-cm, a carrier concentration of 5.0×10¹⁸ cm⁻³, and a Hall mobility of 1.4 cm² V⁻¹ s⁻¹. The structure, morphology and optical properties were also evaluated for both N-doped and Zr–N codoped ZnO films.     

Magnetoplasticity and diffusion in silicon single crystals

The effect of static magnetic fields on the dynamics of surface dislocation segments, as well as the diffusion mobility of a dopant in silicon single crystals, has been analyzed. It has been experimentally found that the preliminary treatment of p-type silicon plates (the dopant is boron with a concentration of 10¹⁶ cm⁻³) in the static magnetic field (B = 1 T, a treatment time of 30 min) leads to an increase in the mobility of surface dislocation segments. The characteristic times of observed changes (about 80 h) and the threshold dopant concentration (10₁₅ cm⁻³) below which the magneto-optical effect in silicon is not fixed have been determined. It has been found that diffusion processes in dislocation-free silicon are magnetically sensitive: the phosphorus diffusion depth in p-type silicon that is preliminarily aged in the static magnetic field increases (by approximately 20%) compared to the reference samples.     

Electron traps in wide-gap n-Hg0.3Cd0.7Te characterized by time-resolved photoconductivity

Time-resolved photoconductivity measurements have been used to characterize electron traps in wide-gap n-HgO_0.3Cd_0.7Te for the first time. The characterization was made possible by combining the time-resolved photoconductive data with the analytical method conventionally used in DLTS spectroscopy. Two electron traps were found in the band gap with 61 meV and 79 meV below the conduction band edge, their concentrations are 1.1×10¹³ cm^–3 and 5.8×10¹¹ cm^–3, respectively. Compared with DLTS spectroscopy, this characterization method markedly simplifies sample preparation and experimental procedure.     

Optical emission diagnostics of laser-induced plasma for diamond-like film deposition

Optical emission from a laser-induced plasma plume is recorded during KrF excimer laser ablation of graphite in a gas mixture of Ar and H₂ (3%) for deposition of diamond-like thin films. At sub-GW/cm² laser intensities the spectrum is dominated by the bands of C₂ and CN. From the band intensities, the vibrational temperatures of both radicals are calculated to be 12–15×10³ K, and their concentrations are estimated to be 5×10¹⁴ cm^–3 and 2×10¹⁴ cm^–3, respectively.     

Formation of p-type ZnMgO thin films by In-N codoping method

In-N codoped ZnMgO films have been prepared on glass substrates by direct current reactive magnetron sputtering. The p-type conduction could be obtained in ZnMgO films by adjusting the N₂O partial pressures. The lowest resistivity was found to be 4.6 Ω cm for the p-type ZnMgO film deposited under an optimized N₂O partial pressure of 2.3 mTorr, with a Hall mobility of 1.4 cm²/V s and a hole concentration of 9.6 × 10¹⁷ cm⁻³ at room temperature. The films were of good crystal quality with a high c-axis orientation of wurtzite ZnO structure. The presence of In-N bonds was identified by X-ray photoelectron spectroscopy, which may enhance the nitrogen incorporation and respond for the realization of good p-type behavior in In-N codoped ZnMgO films. Furthermore, the ZnMgO-based p-n homojunction was fabricated by deposition of an In-doped n-type ZnMgO layer on an In-N codoped p-type ZnMgO layer. The p-n homostructural diode exhibits electrical rectification behavior of a typical p-n junction.     

Experimental study of carbon particle charging at shock-wave pyrolysis of C3O2

The temporal variation in electron and ion concentrations have been measured in shock-heated mixtures of Ar + (0-2)% C₃O₂ in the 2000-3600 K temperature and 15-30 bar pressure range. Experiments in pure argon proved that the observed free electrons and ions originate from inherent impurities of sodium. The equilibrium concentrations of free charges in argon were established during (1-3) × 10⁻⁵ s and varied from 4 × 10¹¹ cm⁻³ at T₅ = 2500 K to 5 × 10¹² cm⁻³ at 3500 K. In the reactive mixtures, containing C₃O₂, the time profiles of electron and ion concentrations showed a more complicate behavior—a fast rise to a maximum followed by a gradual decay. The maximum ion concentrations were much higher and electron concentrations were much lower than in similar conditions in argon. The extent of the subsequent decay of electron concentration increased proportionally to the square of the C₃O₂ concentration. In the mixture with 2% C₃O₂ the final electron concentration was about 100 times less than in pure argon. The characteristic decay time of free charges varied from 400 to 40 μs and decreased proportionally to the square root of the charge concentration. The data analysis is based on the assumption that the observed redistribution of electron and ion concentrations is caused by charging of the carbon particles formed during pyrolysis of C₃O₂. The kinetics of particle charging and the final distribution of charges were evaluated by the analysis of electron and ion fluxes to the particles in accordance with the electric potentials of charged particles and corresponding sodium ionization. A predominance of negatively charged particles, caused by the high electron mobility, resulted in their much higher concentration than the concentration of free electrons.     

Electrical characterization of defects introduced in n-GaAs by alpha and beta irradiation from radionuclides

We investigated defect production in n-type GaAs with two different free-carrier densities (4×10¹⁴ and 1×10¹⁶/cm³) by using particles liberated from radionuclides. ⁹⁰Sr and ²⁴¹Am were employed as beta and alpha sources, respectively. The results obtained for electron irradiation showed that the same set of primary defects can be produced by beta irradiation from the Sr source as by electrons produced in an accelerator. Similarly, the defects produced by alpha irradiation from the Am source closely resemble those introduced by alpha irradiation in a Van de Graaff accelerator. It was found that the relative concentrations of the primary defects in electron-irradiated GaAs are different to those in alpha-particle irradiated GaAs. Further, for the first time, an alpha irradiation induced defect which seems to be related to the doping concentration was observed in the 10¹⁶/cm³ Si doped GaAs. It is concluded that the use of radionuclides is an inexpensive and convenient method to introduce and to study radiation induced defects in semiconductors.     

Carrier concentration in modulation-doped AlGaAs-GaAs heterostructures

Single-interface modulation-doped AlGaAs-GaAs heterostructures have very high mobilities if thick undoped spacers are introduced between the Si donors and the twodimensional electron gas. Electron densities are limited to values below 10¹² cm^–2. Higher channel densities are desirable for device applications and can be obtained by confining the electrons in quantum wells doped from both sides. Single quantum-well structures have been grown with sheet carrier densities exceeding 3×10¹² cm^–2 at 300 K and 77 K mobilities of 54,000 cm²/Vs. Single quantum wells doped from one side only with low electron concentrations of 2×10¹¹ cm^–2 have 4.2 K mobilities of 200,000cm²/Vs.     

Surface-field induced interband tunneling in InAs

Metal/insulator/semiconductor junctions are prepared on degeneratep-type InAs substrates with hole concentrations ranging from 2.3×10¹⁷ cm^–3 to 2.7×10¹⁸ cm^–3. The low work function of the top metal Yb, Al, or Au and charged interface states influence a two-dimensional (2D) electron inversion layer at the InAs surface. The insulator barrier that is formed by thermal oxidation is designed sufficiently thin, so that the bias voltage applied at the metal electrode mainly drops across the depletion layer separating the electron channel from the bulk. The current-voltage (I–V) characteristics exhibit strong negative differential conductance due to interband, tunneling from the 2D subband into the 3D valence band with peak-to-valley current ratios up to 3.1, 18, and 32 at 300 K, 77 K, and 4.2 K, respectively. In agreement with a theoretical model based on coherentelastic tunneling, the form of the I–V curves resembles those of double-barrier resonant tunnel devices rather than those of 3D Esaki diodes. The series resistance is obtained from the saturation of the differential conductance dI/dV at high forward bias and from the shift of structures in d² I/dV ² arising from phonon assisted tunneling.Dedicated to G. Lautz on the occasion of his 65th birthday     

Hf-sputtered indium oxide films doped with tin

Indium oxide films doped with tin (ITO-films) have been hf-sputtered from an 80 at-%In₂O₃/20 at-%SnO₂ target onto glass substrates. The sputter atmosphere contained mainly argon (10⁻²Torr) with addition of oxygen (0≦p _{O 2}≦2·10⁻²Torr). The sputtered films aren-conductors. The conductivity and density of charge carriers depend on the oxygen content of the sputter gas. They could be varied by two orders of magnitude. In air or in oxygen atmosphere the films oxidize at the surface and for a certain depth beneath the surface, thus decreasing the conductivity. The Hall mobility of the sputtered films is smaller (≈10 cm²V⁻¹ s⁻¹) than one observes at ITO films produced by CVD sparaying or other methods. The conductivity of as sputtered films approached maximum values of about 1000Ώ⁻¹cm⁻¹.     

Magnetically stabilized electron-hole liquid in indium antimonide

Luminescence spectra of sufficiently pure n-type indium antimonide crystals (N _D−N _A=(1–22)·10¹⁴ cm⁻³) in a magnetic field of up to 56 kOe, at temperatures of 1.8–2 K, and high optical pumping densities (more than 100 W/cm²) have been studied. More evidence of the existence of electron-hole liquid stabilized by magnetic field has been obtained, and its basic thermodynamic parameters as functions of magnetic field have been measured. When the magnetic field increases from 23 to 55.2 kOe, the liquid density increases from 3.2·10¹⁵ to 6.7·10¹⁵ cm⁻³, the binding energy per electron-hole pair rises from 3.0 to 5.2 meV, and the binding energy with respect to the ground exciton level (work function of an exciton in the liquid) rises from 0.43 to 1.2 meV. Zh. éksp. Teor. Fiz. 111, 737–758 (February 1997)     

Structural modifications of diamond like carbon films induced by MeV nitrogen ion irradiation

Diamond-like carbon (DLC) films were deposited on Si(1 0 0) substrates using plasma deposition technique. The deposited films were irradiated using 2 MeV N⁺ ions at fluences of 1×10¹⁴, 1×10¹⁵ and 5×10¹⁵ ions/cm². Samples have been characterized by using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). Analysis of Raman spectra shows a gradual shift of both D and G band peaks towards higher frequencies along with an increase of the intensity ratio, I(D)/I(G), with increasing ion fluence in irradiation. These results are consistent with an increase of sp² bonding. XPS results also show a monotonic increase of sp²/sp³ hybridization ratio with increasing ion fluence. Plan view TEM images show the formation of clusters in the irradiated DLC films. HRTEM micrographs from the samples irradiated at a fluence of 5×10¹⁵ ions/cm² show the lattice image with an average interplanar spacing of 0.34 nm, revealing that the clusters are graphite clusters. The crystallographic planes in these clusters are somewhat distorted compared to the perfect graphite structure.     

Magnetoplastic effect in silicon: A search for new methods of controlling structure-sensitive properties of elemental semiconductors

The effect of static magnetic fields up to 1 T on the state of impurity point defects and the mobility of surface dislocation segments in doped (0.01–1.00 Ω cm) silicon has been considered. Long-lived (∼100 h) changes in the state of point defects have been revealed from the mobility of dislocations introduced after the magnetic-field treatment. The concentration dependence of the magnetoplastic effect in p-type silicon has been studied. A threshold impurity concentration of 10¹⁵ cm⁻³ has been found, below which the magneto-plastic effect has not been observed. The influence of magnetic-field pretreatment on the expectation times and activation barriers for dislocation depinning from stoppers and the effect of thermal preannealing on the magnetoplasticity in Si have been considered.     

Structural and optical studies of the discontinuous behaviour of Te thin films subjected to He implantation

Electron microscopy investigations of tellurium thin films implanted with singly ionized He ions have revealed the appearance of a large number of surface structures when N _s, the number of implanted ions per unit area within the films, exceeds 0.2×10¹⁵ ions/cm² at a beam energy of 32 keV. This coincides with an observed discontinuity in the optical properties of the Te thin films and with a sudden decrease in the degree of orientation of the thin films as measured by X-ray diffraction. The same type of behaviour is observed for implantations with variable ion-beam energies and a fixed N _s of 0.5×10¹⁵ He ions/cm², with a discontinuity apparent near a value of 30 keV.     

Hole mobility and trapping in PVK films doped with CdSe/CdS and CdSe quantum dots

Experimental study of the hole mobility in polyvinylcarbazole (PVK) films doped with two kinds of nanocrystals, on bare core CdSe and core-shell CdSe/CdS quantum dots, with concentrations ranging from 3 · 10¹⁰ to 3 · 10¹⁵ cm⁻³, is presented. The quantum dots investigated were made using colloidal chemistry. The hole mobility was measured using the time-of-flight technique as a function of the applied electrical field in the range 10⁵–10⁶ V/cm and for temperatures from 20°C to 50°C. The transient curves, being featureless on a linear plot, show on a double logarithmic scale a sharp inflection point indicating a dispersive carrier drift process. The recovered values of the mobility are in the range 3 · 10⁻⁸–10⁻⁶ cm²·V⁻¹·s⁻¹ and their field and temperature dependences can be analyzed formally within the framework of the Gaussian disorder model proposed by B?ssler. The energetic disorder is, within the experimental accuracy, independent of the concentration and type of quantum dots for the CdSe quantum dots at all concentrations and for the CdS/CdSe quantum dots up to 10¹⁴ cm⁻³. The spatial disorder factors are very large (from 5.3 to 8.7) and do not depend in a systematic way upon the type and concentration of quantum dots (QDs). The experiments show that the apparent mobility does not change considerably with concentration, but it was found that the samples with CdSe/CdS quantum dots at concentrations from 10¹⁵ to 3 · 10¹⁵ cm⁻³ show a decreased photocurrent response. The dependence of the time-integrated transients (corresponding to the full charge value) upon the quantum-dot concentration has been determined. Differences in total photogenerated charge for pure and doped polymer films imply that the quantum dots of that type are the hole traps with capture times much more smaller than the transit time and with emission times a few orders longer than the transit time. CdSe quantum dots without a shell do not seem to exhibit the same properties as core shells and do not produce considerable changes in the charge transfer, even at a density of 10¹⁵ cm⁻³.     

Influence of growth conditions on the structural,optical, and electrical properties of ZnSe films

ZnSe films were grown by chemical vapour deposition on GaAs substrates. The influence of the source temperature (between 820 and 900° C) and the substrate temperature (between 620 and 790° C) on the film properties were investigated by Hall measurements, X-ray diffraction, and photoluminescence. With respect to blue luminescent devices the ratio of excitonic to deep level transitions was found to be optimum at low growth rates when the source temperatures were kept below 840° C. P-type conduction up to a net carrier concentration of 8×10¹⁸ cm^–3 could be obtained by substrate temperatures above 700° C. Lattice contraction versus substrate temperature pointed to a reduced incorporation of donors at higher growth temperatures.