Energy Distribution Of Defects Research Articles

MECHANISM OF ELECTROLUMINESCENCE FROM a-Si:H AND STUDIES OF DEFECT ENERGY DISTRIBUTION IN INTRINSIC LAYER OF a-Si:H SOLAR CELLS BY ELECTROLUMINESCENCE SPECTRA

The features of the electroluminescence spectra in amorphous silicon p-i-n diodes were ecxellently explained by dispersive-transport-controlled recombination. The misunderstanding to the efficiency and peak position of the electroluminescence for many yera was thereby clarified by the theory. The experimental methods of local states in the intrinsic layer of a-Si-H solar cells studied by the electro-luminescence were described in detail. The studied results showed that the defect energy distribution exhibited a single narrow peak when the samples were prepared by diluted silane using hydrogen, while the defect energy distribution was wider with double peaks in the samples deposited by pure silane.

Defects genesis in a-Si: H p-i-n solar cells

In this paper we address the problem of monitoring the amorphous silicon intrinsic material inside p-i-n thin film structures. Most of the techniques adopted for characterisation of single thick films cannot be used in this case; available techniques indeed lack precision, mainly due to spatial and energy distributions of defects. We achieve information on material properties by comparison of several different measurements performed on the samples. Capacitance measurement under forward bias is included in this set, together with I-V measurements in light and dark conditions, quantum yield, and temperature dependence of the dark current. A theoretical analysis of the different measurements shows how each result can be combined with the others to obtain a unified picture. Measurements were performed on samples with different p-i interface obtained by changing the structure of the amorphous silicon-carbide buffer layer. We show that intrinsic amorphous silicon grown at the interface is subject to an increase of quasi-shallow defects levels and to a decrease of deep defects with increasing buffer deposition time. This behaviour configures a balance effect between deep defects and quasi-shallow defects, ruled by the interface status.

Determination of defect distributions from admittance measurements and application to Cu(In,Ga)Se2 based heterojunctions

A method to deduce energy distributions of defects in the band gap of a semiconductor by measuring the complex admittance of a junction is proposed. It consists of calculating the derivative of the junction capacitance with respect to the angular frequency of the ac signal corrected by a factor taking into account the band bending and the drop of the ac signal over the space charge region of the junction. Numerical modeling demonstrates that defect distributions in energy can be reconstructed by this method with high accuracy. Defect distributions of polycrystalline Cu(In,Ga)Se2 thin films are determined by this method from temperature dependent admittance measurements on heterojunctions of Cu(In,Ga)Se2 with ZnO that are used as efficient thin film solar cells.

Field-Effect Conductance Activation Energy in Polycrystalline Silicon Thin-Film Transistors after Bias Stress

We have bias-stressed hydrogen-passivated polycrystalline silicon thin-film transistors by varying the bias condition and have measured the change in the field-effect conductance activation energy as a result of the bias stress. For stress by a high gate bias voltage, a slight broadening occurs in the activation energy curve, while stress by a low gate bias voltage gives rise to a shoulder. Numerical calculation shows that the change in activation energy is due to defect generation in the poly-Si layer and for stress by low gate voltage, defects are localized near the drain region. The energy distribution of defects cannot be obtained accurately, but its position is higher than 0.2 eV above midgap.

Nonlinear Dependence of μeτe on Subgap Absorption in Pulse and cw Light-Soaked a-Si:H

AbstractWe report new results from subgap absorption and transport measurements on pulse and continuous (cw) light-soaked a-Si:H which include that (1) the mobility-lifetime product of electrons (μeτe) shows the same dependence on defect subgap absorption irrespective of whether the sample was soaked by pulse or continuous lights, but that (2) the dependence of μeτe on defect subgap absorption is not linear. These results are compared with annealing energy distribution of light-induced defects, and discussed in terms of (1) annealing energy (Eann) distribution of light-induced defects (LIDs), and of (2) photo-induced structural change.

The Electronic Properties of a-Si:H Deposited With Hydrogen or Helium Dilution

AbstractWe have applied the Drive-Level Capacitance Profiling (DLCP) method to n-i-p a-Si:H diodes to characterize the mid-gap defect densities in the i layer. Our results show that there are no significant changes in the drive-level densities, Ndl, in n-i-p diodes and p+-i-m as well as n-i-m Schottky diodes, which indicates that DLCP can directly provide reliable energy distribution and spatial distribution of the mid-gap defects in the n-i-p device. We have found that the ratio of Ndl to ND*, the ionized defect density determined by hole onset measurement, is changed with the deposition conditions; it is 3 for standard samples, 2 for helium diluted samples and 6 for hydrogen diluted samples. These results indicate that there may be different defect distributions in these materials, which suggest the ratio of charged (D-) density to the neutral defect (D0) density may be altered when growth conditions are varied.

Relation of the Electroluminescence Measurements to A-Si.H Solar Cell Parameters

ABSTRACTWe have measured the intensity of electroluminescence (EL) and its energy spectrum in a-Si:H solar cells having an initial energy conversion efficiency from 5.75 to 9.8 %, and open-circuit voltages (Voc) between 0.799 and 0.952 V. We found that (a) at room temperature, EL efficiency is proportional to the initial solar cell energy conversion efficiency; (b) the defect energy distribution in the i-layer can be detected by EL energy spectrum at room temperature; and (c) Voc is simply related to the quasi-Fermi level splitting obtainable in the i-layer and that the buffer layer serves to increase this splitting.

The Electronic Structure of a-Si,Ge:H Alloys

ABSTRACTWe have carried out a detailed study of the energy distribution of deep defects for high quality glow discharge a-Si,Ge:H alloys using both thermal emission and optical Methods: drive-level capacitance profiling, transient photocapacitance and photocurrent plus modulation photocurrent spectroscopy. Four distinct bands of transitions involving defect states have been identified: two associated with thermal transitions, and the other two related to optical transitions. We have, for the first time, observed a negative signal in the photocapacitance spectra at photon energies near 1.2eV. This striking aspect verifies the presence of a distinct defect band above Ep from which electron thermal emission is greatly suppressed. Our Measurements also disclose a fairly narrow defect band below the Fermi level which contrasts with the defect properties observed in a-Si:H. Time resolved photocapacitance spectra indicate that this defect band exhibits configuration relaxation.

Relaxation of the D center in amorphous silicon and how this accounts for the observed energy distributions of deep defects within the mobility gap

Transient capacitance measurements disclose that the mobility gap energy of the D defect in a-Si:H relaxes following a change in its charge state. Using the relaxation parameters thus determined, we solve the resulting dynamical equation to obtain the steady-state energy distributions of both the D − and D 0 sub-bands. These results agree with the experimental distributions determined by modulated photocurrent and steady-state junction capacitance measurements.

A comprehensive method combining deep-depletion profiling and capacitance transients to evaluate energy and depth distribution of MOS bulk defects

A measurement procedure is presented which combines deep-depletion profiling of MOS capacitors with transient analysis to simultaneously provide the doping profile and the bulk minority generation current. The evaluation of the transient yields the reverse current directly as a function of the depletion width, thus enabling depth profiling of the generation centers. The substrate temperature is an additional parameter that permits the activation energy of the generation mechanisms to be deduced and diffusion currents from the neutral bulk to be identified. Investigation of typical samples shows that the diffusion component is important in devices on lightly doped substrates, whereas it is not found in devices on heavily doped buried layers. >

New insights on the electronic properties of the trivalent silicon defects at oxidized 〈100〉 silicon surfaces

We perform a deep level transient spectroscopy (DLTS) measurement of the band-gap energy distribution of the trivalent silicon defects (Pb centers) on as-oxidized 〈100〉 silicon wafers. By comparison with the 〈111〉 silicon surface, we isolate the energy distribution of the Pb1 center. Its acceptor level is found at 0.42 ± 0.02 eV from the conduction band while the acceptor level for the 〈100〉 Pb0 center is found at 0.22 ± 0.01 eV, a value smaller than at the 〈111〉 surface (0.33 ± 0.01 eV). We obtain new results about the capture cross sections of the 〈100〉Pb centers by energy-resolved DLTS trap filling experiments. The electron capture cross section of 〈100〉Pb1 is determined for the first time (5×10−16 cm2), while the electron capture cross section for 〈100〉 Pb0 (8×10−15 cm2) is found to be in agreement with earlier results.

Band offsets and deep defect distribution in hydrogenated amorphous silicon-crystalline silicon heterostructures

Voltage filling pulse measurements taken on a-Si:H/c-Si heterostructure Schottky diode samples are used to examine the capture of electrons from the c-Si substrate into a-Si:H defect states. These measurements, along with the capacitance versus temperature spectra of these diodes, indicate a nearly zero conduction-band offset (50±50 meV). In addition, we have observed trapping of holes at the valence-band discontinuity ΔEv. A clear threshold for the subsequent optical release of these holes yields a value of ΔEv =0.58±0.02 eV. Our measurements also provide the energy and spatial distribution of deep defects within the a-Si:H layer and indicate a region of anomalously large defect density (1018 cm−3) within roughly 350 Å of the a-Si:H/c-Si interface.

Junction capacitance studies of deep defects in undoped hydrogenated amorphous silicon

Recent results obtained by applying two methods, drive-level capacitance profiling and transient photocapacitance spectroscopy, to the study of deep defects in undoped hydrogenated amorphous silicon (a-Si:H) are reviewed. From the first method, we illustrate how it is possible to obtain the density, energy distribution, electronic occupation and spatial distribution of light-induced metastable deep defects for a set of hydrogenated amorphous silicon samples with well-characterized impurity levels. These results strongly suggest an extrinsic carbon-related component to the degree of instability in some films. We also illustrate how a detailed comparison between transient photocapacitance and photocurrent spectra provides specific information about gap state dynamics and carrier-transport processes in a-Si:H films and, in particular, metastable changes in those properties.

Light-induced defect creation in hydrogenated amorphous silicon: A detailed examination using junction-capacitance methods.

From junction-capacitance measurements we have determined the density, energy distribution, and electronic occupation of light-induced metastable defects near the midgap for a set of nearly intrinsic hydrogenated-amorphous-silicon samples with well-characterized impurity levels. In particular, we have inferred the changes in both neutral and negative dangling bonds for a series of isochronal anneals between the light-soaked state and full-dark-annealed state of each sample. Our results indicate that the dominant defect-creation mechanism is not Si--Si bond breaking, and also indicate the existence of at least two quasi-independent metastable defect-creation processes.

Modified equations for the evaluation of energy distribution of defects in as-grown thin films by Vand’s theory

It is pointed out that the use of Vand’s original equations for the evaluation of defect density distribution in energy in as-grown thin films lead to results which are in error by at least 6% in the case of thin films vacuum deposited at room temperature. Modified equations that can be applied to thin films formed at any arbitrary temperature T0 K have been obtained. It is found that except for one equation, the remaining three equations are identical to those of Vand. It has also been shown that Vand’s original equations can be applied in the case of thin films formed at an arbitrary temperature if the actual time t in the original equation is replaced by a time t0 measured as if heating had started at 0 K. Experimental plots of decay energy distribution of defect cluster density [F0(E) vs E plots] obtained using the modified equations and Vand’s original equations in the case of bismuth thin films deposited at different substrate temperatures (300–450 K) are also presented to show the extent of the errors and their substrate temperature dependence. These errors are also compared with the errors inherent in the theory and the approximations made in the derivation.

Determination of the quasicontinuous defect-energy distribution for a disordered insulator by thermally stimulated depolarization

A method is described for determining the quasicontinuous energy distribution of the electrically active defects in a disordered insulator from a family of thermally stimulated depolarization-current curves as measured for various polarization temperatures. It is shown that the method is comparable in informativeness with the method of fractional thermally stimulated depolarization while it is technically simpler to operate. Results are presented from a study on the defect energy distributions in amorphous films of silicon monoxide.

Energy distribution of defects in thin films deposited at low temperature

We have measured changes in the resistance with time and temperature during the annealing of thin films of pure magnesium and of an Mg-Fe alloy deposited under high vacuum conditions on a substrate maintained at liquid helium temperature. Using a simple hypothesis concerning the kinetics of relaxation of the defects, the energy distribution of defects initially present in the film can be determined.