Distribution Of Gap States Research Articles

Photo-induced changes of thermally stimulated conductivity spectra in undoped hydrogenated amorphous silicon

The changes in the gap state distributions produced by light soaking in undoped a-Si:H prepared by remote-plasma chemical vapour deposition were determined from the simultaneous measurement of thermally stimulated conductivity and low-intensity photoconductivity. The temperature dependence and the changes of the product of the microscopic mobility and the recombination lifetime, μ m τ r, produced by light soaking were also determined. In the gap profiles in the energy interval between 0.2 and 0.8 eV below the conduction band edge, at least two electron traps are found at 0.30 ± 0.02 and 0.54 ± 0.02 eV. The density of the trap located at 0.54 eV due to the doubly occupied dangling bond, D −, is increased by light soaking together with a simultaneous decrease of traps located at 0.30 eV due to the weak bonds. These effects are interpreted as defect creation and annealing, respectively. The temperature dependence of the μ m τ r product is found to be due to that of μ m, and thermal quenching and light quenching of μ m τ r are mainly due to those of τ r.

Analysis of Non-Uniform Creation of Light-Induced Defects in Schottky Barrier Solar Cell Structures

ABSTRACTHydrogenated amorphous silicon (a-Si:H) TCO/n+/i/Ni Schottky barrier solar cells were degraded with illuminations of white and red light through both sides of the structure. Because the forward dark I-V's are sensitive to the distribution and any spatial variation of defects in the i-layer, these measurements were used to characterize the degraded cell structures. These characteristics were analyzed using a charged defect distribution of gap states consisting of D+, D°, and D states derived from corresponding film analysis. It was found that the non-uniformities of light induced defects created in the i-layer can be represented by two regions of different defect densities. The relative densities depend on the direction of illumination and their ratio even with white light is about a factor of four. This non-uniformity is expected to be less for p-i-n cells which have higher built-in potential and hence electric fields.

Schottky-barrier height and electronic structure of the Si interface with metal silicides: CoSi2, NiSi2, and YSi2.

Using the linear muffin-tin orbitals in the atomic-sphere approximation with the local-density approximation (LDA), we studied the electronic structure and Schottky-barrier height (SBH) of the Si interface with metal silicides: ${\mathrm{CoSi}}_{2}$, ${\mathrm{NiSi}}_{2}$, and ${\mathrm{YSi}}_{2}$. We used large supercells with 9 ${\mathrm{Si}}_{2}$ and 8--10 silicide (${\mathrm{CoSi}}_{2}$, ${\mathrm{YSi}}_{2}$) layers for the (111) interface and with 11 ${\mathrm{Si}}_{2}$ and 11 silicide (${\mathrm{NiSi}}_{2}$, ${\mathrm{CoSi}}_{2}$) layers for the (001) interface. Together with our previous calculations on the two types of ${\mathrm{NiSi}}_{2}$/Si(111) interfaces, we demonstrate that the LDA calculation with a large supercell correctly reproduces the dependence of experimental SBH's on the interface structure and type of metal silicide. Electron transfer at the silicide-Si interface depends significantly on the atomic structure of the interface, especially the interfacial space, whether atoms are crowded or not. The energy distribution of interfacial gap states varies significantly with interface atomic structures. These electronic structures directly depend on the interface atomic structure; in contrast the calculated SBH's do not always depend on the interface structure. We discuss the underlying mechanisms for the formation of Schottky barriers.

Charged defect states in intrinsic hydrogenated amorphous silicon films

Steady-state photoconductivity, sub-band-gap absorption and electron spin resonance (ESR) measurements were carried out on annealed and light soaked intrinsic hydrogenated amorphous silicon (a-Si:H) films. The experimental results were modeled using detailed numerical analysis. The defect densities derived from the sub-band-gap absorption in the light soaked films were correlated with the ESR spin densities. Self-consistent fitting of the data was obtained using a gap state distribution which consists of positively charged defect states above, negatively charged defect states below and neutral defect states at about the midgap. Both the annealed and the light degraded states are modeled using the same gap states which increase upon light soaking and have a slight increase in the ratio of the neutral to charged defect densities. These results on intrinsic a-Si:H are consistent with proposed charged defect models and clearly indicate the importance of charged defect states in determining sub-band-gap absorption as well as its correlation with neutral dangling bonds.

Charged Defect State Distributions Obtained from the Analysis of Photoconductivities in Intrinsic a-Si:H Films

ABSTRACTSteady-state photoconductivity, sub-bandgap absorption and electron spin resonance (ESR) Measurements were carried out on annealed and light soaked intrinsic hydrogenated Amorphous silicon (a-Si:H) films. The experimental results were modeled using detailed numerical Model. The defect densities derived from the sub-bandgap absorption in the light soaked films were correlated with the ESR spin densities. Selfconsistent fitting of the data was obtained using a gap state distribution which consists of positively charged defect states above, negatively charged defect states below and neutral defect states at about Midgap. Both the annealed and the light degraded states are modeled using the same distribution of gap states whose densities increase upon light soaking with a slight increase in the ratio of the neutral to charged defect densities. These results on intrinsic a-Si:H are consistent with those of charged defect Models.

Recombination Processes of the Broadband And 1.681 Ev Optical Centers in Diamond Films

ABSTRACTMicro- and Macro-photoluminescence techniques were employed in this research to investigate the role of nitrogen-doping on the optical spectra of chemical vapor deposited diamond films and to determine whether the origin of the broadband luminescence is due to vibronic interaction of the nitrogen centers. The temperature behavior of the broadband PL and of the 1.681 eV silicon related optical center were analyzed. The intensity of the broadband was found to exhibit a temperature dependence characteristic of optical emission from a continuous distribution of gap states while the temperature dependence of the 1.681 eV band followed the Boltzmann quenching process.

Time and frequency domain studies of photoconductivity in amorphous semiconductors

We present a general spectroscopic technique for the computation of the distribution of gap-states (DOS) in amorphous semiconductors from transient photocurrent decay (TPC). The technique assumes trap-limited and is otherwise model-independent. It is valid whether the TPC exhibits anomalous or conventional dispersion, and also works without modification for pre- and post-recombination regions of the decay. A numerical Fourier integral procedure is used to convert the TPC i( t) data to frequency domain spectra I( ω). The DOS is then computed using a procedure developed by the authors [1] for analysis of modulated photocurrent (MPC) data. The method avoids distortions and computational difficulties associated with other TPC analytical techniques. We report on the application of the method to experimental data on a-Si:H, demonstrating the wide energy range of states accessed, and highlighting the observation that the observed long-time power-law TPC decay, normally associated with a featureless exponential state distribution is consistent with structure in the DOS.

Time-of-flight and constant-photocurrent measurements in a-Si:S:H alloys

Several sets of a-Si:S:H samples prepared by the plasma enhanced chemical vapour deposition method, have been examined by means of time-of-flight (TOF) and constant-photocurrent measurements (CPM), by optical spectroscopy and with Rutherford backscattering. With CPM it was found that an increase in sulfur content corresponds to an increase in the Urbach parameter and of the absorption tail. For one sample with a sulfur content of 1.5%, a TOF transit could be resolved; by the other experiments this was confirmed to be the best-quality sample. A post-transit-time analysis, giving the distribution of gap states, could be carried out successfully only for this sample.

Charge redistribution process on gap states in hydrogenated amorphous silicon.

The charge relaxation in an electron capture process on the continuously distributed gap states in a-Si:H has been analyzed. It is shown that a process of charge redistribution among the gap states occurs during a filling pulse, causing the thermal emission energy of the trapped electrons to shift deeper into the mobility gap. For an exponential distribution of gap states, an electron's thermal emission time is proportional to the filling pulse width. These results provide a natural explanation for the recently reported results on a junction capacitance transient experiment, which were attributed to a novel defect relaxation process

Conduction mechanisms in undoped thin films of C 60 and C 60/70

Undoped thin films of C 60 and C 60/70 mixture have been electrically and optically characterized over a wide temperature range T=10…475 K. In this paper the results of the field-effect conductance, conductivity, thermoelectric power, thermoabsorption and electroabsorption measurements will be presented. The transittor studies gave an n-type behaviour, and the thermally activated field-effect conductance obeyed the Meyer-Neldel rule. The conductivity showed an activated temperature dependence above 250 K, too, but turned into a nonactivated dependence below 200 K. The results may be explained by assuming a distribution of gap states. Furthermore, we have deposited and characterized Langmuir-Blodgett films of C 60/70 and its heterostructures with poly(3-hexylthiophene).

Optical absorption and photoluminescence of unhydrogenated amorphous carbon films

Unhydrogenated amorphous carbon (a-C) films were prepared by a laser beam technique. Optical absorption, photoluminescence (PL) and the dependence of PL spectra on the incident laser intensity were studied. The optical gap, peak energy of the PL spectrum and band tail width change with preparation bias voltage. At room temperature PL spectra exhibit a superlinear behaviour at lower incident intensity, linear behaviour at higher intensities, and a large blue shift of peak energy with increasing excitation intensity. These results can be qualitatively explained in the light of the distribution of gap states and tail state population.

Gap states in hydrogenated microcrystalline silicon grown by glow discharge technique

The gap state distribution in n-type hydrogenated microcrystalline silicon (μc-Si:H) films deposited by the glow discharge method is determined using deep level transient spectroscopy. The spectra obtained for μc-Si:H films, when compared with those observed in n-type a-Si:H deposited by the same technique, appear to be vastly different and clearly indicate four distinct defect levels. On the other hand the peaks obtained are also not as sharp as those normally observed in crystalline silicon. These observations are important to understand the nature of defects in such material systems with very small grain size.

Gap-states distribution of ion-implanted Si and GaAs from subgap absorption measurements

The distribution of gap states has been obtained from subgap absorption measurements in ion-implanted Si and GaAs layers and the changes induced by increasing the implantation ion dose have been monitored. The variations in slope which are observed with increasing dose in the spectral region at energies lower than the band-edge region are shown to be mainly affected by a shift of the energy position of the peak of the calculated distributions.

Determination of gap-state distributions in amorphous semiconductors from transient photocurrents using a fourier transform technique

Abstract We present a novel spectroscopic technique for the computation of the distribution of gap-states (DOS) in amorphous semiconductors from transient photocurrent decay exhibiting either anomalous or conventional dispersion. A numerical Fourier Transform procedure is used to convert the impulse response time decay data to phase and a]plitude frequency spectra. The DOS is then computed using a procedure developed for analysis of modulated photocurrent (MPC) data in a multiple-trapping context. The method avoids many of the distortions and practical difficulties associated with other TPC analytical techniques. Computer generated TPC and MPC data are used to evaluate the transform technique in comparison with a ‘direct’ interpretive method for model systems of discrete and distributed states.

Gap-states distribution in amorphous-silicon films as obtained by photothermal deflection spectroscopy.

The paper reports results on intrinsic, device-quality amorphous silicon obtained with use of photothermal deflection spectroscopy. The gap-states distribution is obtained by means of a simple and reliable derivative procedure on the absorption-coefficient spectra. A comparison with other models is made. Finally, it is shown that the peak energy of the defects can be different for surface and bulk states, and so a shift of the occupied defect peak is observed if a considerable number of surface states are introduced. This explains the higher values for the dangling-bond correlation energy obtained by means of optical methods and agrees with other experimental evaluations of the distribution of surface states such as those carried out by means of total-yield spectroscopy.

An Assessment of the Light Modulated Photocurrent Method in the Study of the Density of Gap States in Hydrogenated Amorphous Silicon

ABSTRACTWe have applied the modulated photocurrent (MPC) method over a wide range of frequencies (5Hz-100kHz) and temperatures (120K-380K) to assess its ability to accurately deduce the mobility gap distribution in a-Si:H. We have also investigated the effects of moving both the Fermi level within some samples (by light soaking and partial annealing) and the quasi-Fermi level (by applying the bias light) to observe how such changes influence the deduced density of states (DOS). We then compared the MPC results directly with the DOS determined by junction capacitance measurements in the same sample devices. We have determined general conditions under which, we believe, the MPC results provide an accurate picture of the gap state distribution. However, we found that under other conditions, the appearance of the deep defect peaks and other features do not represent the actual defect distribution but, rather, are artifacts due to recombination processes.

Effect of high-temperature annealing on the distribution of GAP states in aSi:C:H

ESR measurements on amorphous hydrogenated silicon-carbon alloys as a function of annealing temperature T A showed that the spin density decreases sharply at T A = 400°C. In order to obtain more information about the distribution of gap states we have studied photoluminescence, optical absorption and photo-induced absorption in samples annealed at 415°C and we have compared the results with data of co-deposited, not-annealed samples. Samples were prepared with [C 2H 2]/[C 2H 2]+[SiH 4] gas flow ratios r = 0, r = 0.1, r = 0.2 and r = 0.5. All the results point to a drastic decrease of dangling bond concentration in the annealed material.

Thermodynamical equilibrium gap-state distribution in undoped a-Si: H

Abstract Based on weak-bond to dangling-bond conversion and including amorphous broadening of available defect energy levels, we have derived a quantitative thermodynamical equilibrium model for the gap-state distribution in a-Si:H. Results from the conventional defect pool model are compared with a generalized approach requiring both structural and electronic equilibrium of the formed defects. Independent of the microscopic defect reactions, both models predict for undoped a-Si: H splitting of the available defect band into three components: a band of D+ states above and bands of D − and D° states below the Fermi level, with a minimum of the gap-state density at the Fermi level. For typical values of the effective correlation energy (∼0·2eV) and r.m.s. defect bandwidth (0·1-0·15 eV) the models predict a 2-10 times higher density of charged compared with neutral defects in thermal equilibrium which strongly suggests a reinterpretation of experimental data with respect to the gap-state distribution in undo...

Photoelectrical properties of AgInTe2

Abstract This paper reports on the photoelectrical properties of AgInTe2 through measurements of the ‘transient’ and ‘steady-state’ photocurrents. The material displayed great sensitivity to optical and thermal stresses. The ac. response of the material under dark and illuminated conditions was also investigated through measurements of the capacitance and a.c. conductance over a frequency range 5 Hz-1 MHz at different temperatures and illumination levels. An analysis of experimental results indicated the existence of a wide distribution of gap states. Under all conditions of a.c. and d.c. operation, the photo-induced responses were fast at the beginning and, thereafter, the rise and decay processes became slow with respect to time. The present authors propose a simple analysis to study the initial fast response in terms of the RC time constant of the sample system. The versatility of Elliott's correlated barrier-hopping model is also demonstrated through measurements of light-induced variations in capacit...

Comparison of defects produced by 14-MeV neutrons and 1-MeV electrons in n-type silicon

Defects produced in lightly doped crystalline silicon by 1-MeV electrons and 14-MeV neutrons are studied. Deep level transient spectroscopy (DLTS) and Hall voltage measurements have been used for investigating the energy depth, nature, and density of radiation-induced defects. The extent of damage caused by 14-MeV neutrons is found to be comparatively high as inferred from the continuous distribution of gap states in the DLTS measurement. Identical defects were observed in both cases where the A-center (Ec−0.17 eV), E-center (Ec−0.39 eV), and divacancy (Ec−0.23 eV) were prominent. An additional defect level (Ec−0.86 eV) was observed from DLTS for neutron-irradiated samples.