Contribution to the study of impurity levels in CdTe films: A material for λ-ray detectors and solar cells

From a study of space-charge-limited conduction and transient photoconductivity effects we propose two theoretical models and we apply them to n-type CdTe films with a uniform distribution of traps in the band gap. The essential characteristics of the phenomena and the corresponding values of the position and width of the distribution, the density and capture cross section of the trapping and recombination centres and the lifetime of the carriers are determined.     

Trapping lifetime and carrier mobility measurements in CuInSe(2 ) using surface-acoustic-wave technique

A novel technique based on the measurement of the frequency spectrum of the acoustoelectric current is used to determine the trapping time associated with dominant traps in p-type CuInSe(2). At room temperature, two trap levels with trapping time constant of 2x10(-4) and 6.7x10(-5) s are detected. Under white incandescent light, two more traps with trapping time constants of 1.4x10(-3) and 6x10(-4) s are detected. The minority (electron) and majority (hole) carrier mobilities in this material are also measured using the acoustoelectric technique, and they are 6+/-3 and 3.1+/-0.15 cm(2)/V-s, respectively. The hole carrier concentration was estimated to be around 5x10(15) cm(-3), and the surface of the sample was depleted.     

Parameter evaluation in automated digital deep-level transientspectroscopy (DLTS)

A method of directly evaluating the activation energy ΔE, capture cross section σ, and density N_T, of deep-level traps from the pulsed reverse bias capacitance transient is described. The main advantages of this technique are that it requires only a single temperature scan, and it can resolve nonexponential transients due to closely-spaced energy levels. The test samples used for this paper consisted of Schottky diodes fabricated on nonirradiated and 1-MeV electron-irradiated n-type VPE (vapor-phase epitaxy) GaAs wafers. The well known EL2 trap was identified with ΔE of 0.81 eV, and σ _n of 1.0×10^-13 cm² for the nonirradiated sample. These values were found to be in good agreement with published data using established, conventional DLTS techniques. For the irradiated samples a nonexponential capacitance transient was found in the EL2 range of temperatures. The discussed technique was able to resolve two closely spaced deep levels lying at E_c-0.81 eV and E_c-0.84 eV, and with capture cross sections of 1.5×10^-13 cm² and 2.5×10^-12 cm², respectively     

Effect of deep-trap level on transverse acoustoelectric voltage measurements

The effect of trapped charges on the transverse acoustoelectric voltage (TAV) is investigated with the aim of extending the use of TAV measurements to the study of semiconductors with high defect density. Even if surface acoustic wave frequencies are as high as 100 MHz, charge trapping can influence the TAV. This has been verified by two particular experiments performed on Si/SiO(2) structures with high density of interface states. A theoretical model is proposed to explain the effects of the presence of deep-trap levels on the TAV. Novel boundary conditions for the acoustoelectric equations are introduced and an approximate solution for the TAV amplitude is presented. The model is used to define a novel procedure for the determination of interface-states' density using TAV versus bias voltage measurements.     

Minority and majority carrier traps associated with oxidation induced stacking faults in silicon

Abstract

Majority and minority traps associated with oxidation induced stacking faults (OISFs) have been investigated by deep level transient spectroscopy and minority carrier transient spectroscopy. Electron and hole traps have been characterised in nand p type Si, and the activation energies of all extended defect related traps are found to be dependent on the occupancy of the state associated with the extended defect. Majority and minority carrier traps in n type Si exhibit non-exponential trap filling, which indicates the presence of a significant electrostatic barrier around the OISF. The electrical properties of hole (minority) traps measured by minority carrier transient spectroscopy in n type Si are found to be different from the deep level transient spectroscopy signature of hole (majority) traps in p type Si, and this is explained by examining differences between conditions during the measurements. By examining separately the electron and hole capture properties of OISF related traps, one particular trap can be identified as a recombination centre. The capture cross-section of the OISF related hole trap in n type Si has been measured and it was found that, at low occupancy, the trap capture cross-section is 7 × 10^-14 cm².     

Deep-trap discharge in semiconductors in response to concurrent heat and light flows

We have numerically solved differential equations describing the detrapping kinetics of impurity traps during heating (thermally stimulated conductivity) under illumination (induced impurity photoconductivity) with arbitrary trap parameters. An algorithm has been formulated for determining parameters of impurity traps in semiconductors (ionization energy, photon capture cross section, and concentration) from isothermal detrapping curves measured under illumination.     

Deep levels in GaInAs grown by molecular beam epitaxy and their concentration reduction with annealing treatment

X-ray diffraction (XRD), current–voltage (IV), capacitance–voltage (CV), deep-level transient Fourier spectroscopy (DLTFS) and isothermal transient spectroscopy (ITS) techniques are used to investigate the thermal annealing behaviour of three deep levels in Ga_0.986In_0.014As heavily doped with Si (6.8 × 10¹⁷ cm⁻³) grown by molecular beam epitaxy (MBE). The thermal annealing was performed at 625 °C, 650 °C, 675 °C, 700 °C and 750 °C for 5 min. XRD study shows good structural quality of the samples and yields an In composition of 1.4%. Two main electron traps are detected by DLTFS and ITS around 280 K, with activation energies of 0.58 eV and 0.57 eV, capture cross sections of 9 × 10⁻¹⁵ cm² and 8.6 × 10⁻¹⁴ cm² and densities of 2.8 × 10¹⁶ cm⁻³ and 9.6 × 10¹⁵ cm⁻³, respectively. They appear overlapped and as a single peak, which divides into two smaller peaks after annealing at 625 °C for 5 min.

Annealing at higher temperatures further reduces the trap concentrations. A secondary electron trap is found at 150 K with an activation energy of 0.274 eV, a capture cross section of 8.64 × 10⁻¹⁵ cm² and a density of 1.38 × 10¹⁵ cm⁻³. The concentration of this trap level is also decreased by thermal annealing.     

A study on thermal emission of charges at Si3N4—GaAs interfaces after annealing in N2 and N2 + H2 mixtures

Si₃N₄—GaAs interfaces subjected to annealing in N₂ + H₂ mixture or pure N₂ atmosphere were investigated by a small-signal charge deep-level transient spectroscopy (Q-DLTS) method. The method measures the physical parameters of selective populations of the interface traps continuum. A dependence of the capture cross-section on activation energy was constructed for the continuum of interface states at the Si₃N₄—GaAs interface. The dependence shows an exponential character in the part of the gap ranging from 0.3 to 1.0 eV below the conduction band minimum. It was found that annealing in the temperature interval 400–450 °C reduces the zero-bias band bending by about 0.1 eV. At temperatures of 500 °C and more, degradation of the interface started; compared with annealing in pure N₂ ambient, annealing in an N₂ + H₂ mixture degraded the interface slightly more.     

Investigation of defects in polyhedral oligomeric silsesquioxanes based organic light emitting diodes

We have investigated novel poly(p-phenylene vinylene) (PPV) derivative based organic light emitting diodes (OLEDs). We have used poly(2,3-diphenyl-1,4-phenylenevinylene) (DP-PPV) as an emitter in which an inorganic core of polyhedral oligomeric silsesquioxanes (POSS) have been incorporated. The hybrid structure obtained shows an improvement on the stability and an enhancement of electroluminescence properties. Charge-based deep level transient spectroscopy (Q-DLTS) has been used to study the defect states in indium-tin-oxide (ITO)/polyethylene dioxythiophene:polystyrene sulfonate (PEDOT:PSS)/POSS-DP-PPV/CaAl light emitting devices. Analysis of the Q-DLTS spectra obtained in devices, reveal at least six trap levels. The mean activation energies of traps are distributed in the range 0.3–0.5 eV within the band gap of the hybrid polymer and capture cross sections are of the order of 10¹⁶–10²⁰ cm². The trap densities are in the range of 10¹⁶–10¹⁷ cm^?3. Although the origin of these traps remains not clearly established, we suggest that the trap states with a large capture cross-section would likely to be originated from the inorganic part of hybrid material while those with lower capture cross-section would be related to its organic part.     

Linear modulation optically stimulated luminescence and thermoluminescence techniques in Al2O3:C

An analysis of the linear modulation-OSL (LM-OSL) peak and the main TL peak from irradiated Al2O3:C is presented. Data are presented indicating that the LM-OSL peak is composed of three overlapping components originating from populated traps with optical cross sections of 10(-18) - 10(-20) cm2. Studies of the main TL peak before and after LM-OSL measurement indicate that the first two LM-OSL components, corresponding to traps with the largest optical cross sections, originate from traps which also contribute to the low temperature side of the TL peak and that the third component, corresponding to the traps with smallest optical cross section, are due to traps that contribute to the high temperature side of the TL peak. Some consequences for dosimetry are discussed.     

Electric field effect on photoconductivity decay in <Emphasis Type="Italic">n</Emphasis>-InSe single crystals

We have studied the effect of external electric field on the transient response of the intrinsic photoconductivity of n-InSe single crystals with various initial 77-K dark resistivities (ρ_d). The results demonstrate that the behavior of the intrinsic photoconductivity of the crystals depends on both ρ_d and applied voltage. In low electric fields, the predominant process in both the low-resistivity and high-resistivity crystals is the capture of current carriers at stoichiometric point defects in low and higher electric fields, respectively. At the same time, in the high-resistivity crystals in high electric fields the influence of partial disorder related to the layered crystal structure of the material should be taken into account.     

Direct observation of transient fluorine atoms with 25-µm wavelength-stabilized diode laser absorption

Through the use of continuous diode laser absorption, detection of transient fluorine atoms with an initial number density in the range of 10(14) cm(-3) has been demonstrated. A crucial part of the continuous-detection technique was laser frequency stabilization with a reference cell of atomic fluorine with Zeeman modulation of the absorption lines to generate a feedback signal. Long-term wavelength stability was demonstrated with second-harmonic phase-sensitive detection of the second-derivative signal for periods up to several hours. For determination of the short-term wavelength stability in the range of microseconds to seconds, a transient signal was generated by photolysis of F(2) with an excimer laser at 308 nm. The initial diode laser absorption was compared to a calculated value obtained from the measured excimer laser fluence, the known dissociation cross section of F(2), and the atomic fluorine absorption cross section, which included a statistical population distribution, the finite bandwidth of the laser dode, and the effects of pressure broadening. The observed absorption was approximately 33% less than the calculated value, possibly because of the diode laser's wavelength instability on the time scale of a few seconds, which is consistent with an observed amplitude instability from pulse to pulse when pulsed at 1-10 Hz.     

A Simple and Low-Cost Personal Computer-Based Automatic Deep-Level Transient Spectroscopy System for Semiconductor Devices Analysis

A simple, low-cost, and flexible, automatic, deep-level transient spectroscopy (DLTS) system with an Apple II microcomputer for semiconductor devices analysis is described. By using an interactive Basic software program, all the instrument parameters including sampling aperture locations, temperatures, excitation pulse frequency, width and amplitude can be controlled automatically. The transient signals are also averaged by the software program. Furthermore, a linear square regression method is used to calculate the activation energy. The trap density, capture cross section, activation energy of n-type Sidoped GaAs, grown by molecular beam epitaxy (MBE) system, was measured and compared with conventional boxcar DLTS sytem. In addition, the DLTS signal spectra and the Arrhenius plots can be plotted by a line printer and displayed on a CRT monitor during one thermal scan. An IEEE-488 interface bus was designed for communication between personal computers and instruments. This configuration provides the advantages of ease of operation and rapid set up, especially for the purpose of data acquisition and processing.     

Investigation of electrical conduction in polyvinyl formal

Current-voltage (I–V) characteristics of pure polyvinyl formal (PVF) were investigated at different fields, range 5–100 kV/cm, as a function of temperature, range 313–363 K. It was observed that while at low fields (up to 25 kV/cm), the conduction was Ohm’s law-dependent at high fields (beyond 25 kV/cm), the conduction was Poole-Frenkel (P-F) mechanism-dependent. An attempt was made to identify the nature of the current by comparing its observed dependence on temperature, electric field and electrode materials with their respective characteristic features of the existing theories of electrical conduction. The current showed a strong dependence on temperature. To identify the possible mechanism of conduction, current versus square root of field characteristics were drawn with aluminium, silver, copper and gold as upper electrodes and Al as the lower electrodes. The observed characteristic suggested that the charge carriers were generated by the field-assisted lowering of coulombic barriers at the traps, and were subsequently conducted through the bulk of the material by a hopping process between the localized states by a Jonscher-Ansari-modified P-F mechanism. The calculated value of the modified P-F barrier was ⋍ 1·94×10⁻¹⁹ J (1·21 eV).     

Dependence of the surface generation velocity at silicon-(lead borosilicate) glass interface on conditions of nonequilibrium depletion region formation

The generation characteristics of silicon-(lead borosilicate) glass interface have been studied by the method of isothermal capacitance transient spectroscopy in the metal-insulator-semiconductor (MIS) structure. It is established that the effective surface generation velocity depends on the amplitude of the inverting voltage pulse that drives the MIS structure out of the equilibrium state. The observed dependence may be caused by the injection of charge carriers through the silicon-glass interface followed by their localization on traps in the region of glass near the interface.     

Depth profiles of the majority carrier concentration and the minority carrier effective lifetime in gettered GaAs

The first results are reported on the concentration profiles of majority carriers and the effective lifetime profiles of minority carriers in thick (1.6 mm) GaAs plates subjected to surface gettering. It was established that the one-and two-sided coating of the GaAs plates with Y films, followed by heat treatment at 700–800°C, allows a high-resistivity material to be obtained with homogeneous distributions of the electron concentration and the hole lifetime in depth of the semiconductor.     

Generalized far-infrared magneto-optic ellipsometry for semiconductor layer structures: determination of free-carrier effective-mass,mobility, and concentration parameters in n-type GaAs

We report for the first time on the application of generalized ellipsometry at far-infrared wavelengths (wave numbers from 150 cm(-1) to 600 cm(-1)) for measurement of the anisotropic dielectric response of doped polar semiconductors in layered structures within an external magnetic field. Upon determination of normalized Mueller matrix elements and subsequent derivation of the normalized complex Jones reflection matrix r of an n-type doped GaAs substrate covered by a highly resistive GaAs layer, the spectral dependence of the room-temperature magneto-optic dielectric function tensor of n-type GaAs with free-electron concentration of 1.6 x 10(18) cm(-3) at the magnetic field strength of 2.3 T is obtained on a wavelength-by-wavelength basis. These data are in excellent agreement with values predicted by the Drude model. From the magneto-optic generalized ellipsometry measurements of the layered structure, the free-carrier concentration, their optical mobility, the effective-mass parameters, and the sign of the charge carriers can be determined independently, which will be demonstrated. We propose magneto-optic generalized ellipsometry as a novel approach for exploration of free-carrier parameters in complex organic or inorganic semiconducting material heterostructures, regardless of the anisotropic properties of the individual constituents.     

Mobility activation in thermally deposited CdSe thin films

Effect of illumination on mobility has been studied from the photocurrent decay characteristics of thermally evaporated CdSe thin films deposited on suitably cleaned glass substrate held at elevated substrate temperatures. The study indicates that the mobilities of the carriers of different trap levels are activated due to the energy of incident illumination, which results in the existence of two distinct trap levels. In each trap depth the energy of the trap increases linearly. It infers that there is a linear distribution of traps of different energies below the conduction band.     

Development of current-based microscopic defect analysis methods and associated optical filling techniques for the investigation on highly irradiated high resistivity silicon detectors

Current based microscopic defect analysis methods such as current deep level transient spectroscopy (I-DLTS) and thermally stimulated current (TSC) have been further developed in accordance with the need for the defect analysis of highly irradiated (Φ_n > 10¹³ n/cm²) high resistivity silicon detectors. The new I-DLTS/TSC system has a temperature range of 8 K ≤ T ≤ 450 K and a high sensitivity that can detect a defect concentration of less than 10¹⁰/cm³ (background noise as low as 10 fA). A new filling method using different wavelength laser illumination has been applied, which is more efficient and suitable than the traditional voltage pulse filling. It has been found that the filling of a defect level depends on such factors as the total concentration of free carriers generated or injected, the penetration length of the laser (laser wavelength), the temperature at which the filling is taking place, as well as the decay time after the filling (but before the measurement). The mechanism of the defect filling can be explained by the competition between trapping and detrapping of defect levels, possible capture cross section temperature dependence, and interaction among various defect levels in terms of charge transferring. Optimum defect filling conditions have been suggested for highly irradiated high resistivity silicon detectors.     

The capture cross section of a dipole trap

An active distance of attraction model is developed for the capture cross section σ_s of a dipole trap, yielding a simple analytical form. A first order analysis of the field dependence shows that the result is valid up to moderately high field strengths, e.g. up to 5 × 10⁶ V m^-1 for the example quoted in the text. It should be borne in mind, however, that this marks the limit of the analysis rather than the actual onset of a pronounced field dependence of σ_s.The assumption of certainty of energy loss by the electron on entering the ?2kT equipotential surface limits the applicability of the result to low mobility solids and certain liquids; fortuitously these happen to encompass the substances predicted to be most suitable as bulk media in dipole capture investigations.