Identification of the Dominant Electron Deep Trap in a-Si:H: A Critical Test of the Defect Pool vs. Defect Relaxation Pictures

1997 ◽  
Vol 467 ◽  
Author(s):  
Daewon Kwon ◽  
J. David Cohen
Keyword(s):  
Fermi Level ◽  
Thermal Emission ◽  
Deep Trap ◽  
Esr Spectra ◽  
Defect Band ◽  
Large Populations ◽  
Pool Model ◽  
Charged Defects ◽  
Defect Relaxation ◽  
Rule Out

ABSTRACTModulated photocurrent (MPC) measurements in intrinsic a-Si:H reveal a prominent band of electron deep traps with a thermal emission energy near 0.6eV. We have identified this defect band by directly comparing MPC and ESR spectra for both an intrinsic and a lightly n-type doped sample for a various metastable states such that the Fermi level, EF, ranges from less than 0.5eV to more than 0.7eV below Ec. This comparsion unambiguously demonstrates that the MPC band arises from the Do charge state of the defects (specifically, the D−&Do transition). This identification is also confirmed when the quasi-Fermi level is varied by the application of light bias even though the peak emission rate from the MPC defect band is changed by more than a factor of 100. These observations specifically rule out the possibility of large populations of charged defects in intrinsic samples predicted by proponents of the defect pool model. Instead, observed behaviors have a natural explanation in terms of a defect relaxation process.

Deep Defect Relaxation in Hydrogenated Amorphous Silicon: New Experimental Evidence and Implications

1998 ◽  
Vol 507 ◽  
Author(s):  
J. David Cohen ◽  
Fan Zhong ◽  
Daewon Kwon ◽  
C.-C. Chen
Keyword(s):  
Fermi Level ◽  
Emission Rate ◽  
Thermal Emission ◽  
Hydrogenated Amorphous Silicon ◽  
Depletion Region ◽  
Time Resolved ◽  
Apparent Shift ◽  
Hydrogenated Amorphous ◽  
Defect Relaxation ◽  
Thermal Emission Rate

ABSTRACTWe review modulated photocurrent experiments which indicate that thermal emission rate for Do defects in intrinsic samples varies in response to changes in the Fermi-level or quasi-Fermi position. This apparent shift in energy threshold is confirmed using time resolved sub-band-gap spectroscopy. We also demonstrate that such a variation of emission rate with changes in the Fermi-level position, if present within the depletion region near a barrier junction, is consistent with the details of the temperature dependence of the junction capacitance in intrinsic samples.

scholarly journals Solution to the debris disc mass problem: planetesimals are born small?

2020 ◽  
Vol 500 (1) ◽  
pp. 718-735
Author(s):  
Alexander V Krivov ◽  
Mark C Wyatt
Keyword(s):  
Total Mass ◽  
Thermal Emission ◽  
Kuiper Belt ◽  
Dust Production ◽  
Mass Problem ◽  
Planetary Orbits ◽  
Cascade Models ◽  
Giant Impacts ◽  
Rule Out

ABSTRACT Debris belts on the periphery of planetary systems, encompassing the region occupied by planetary orbits, are massive analogues of the Solar system’s Kuiper belt. They are detected by thermal emission of dust released in collisions amongst directly unobservable larger bodies that carry most of the debris disc mass. We estimate the total mass of the discs by extrapolating up the mass of emitting dust with the help of collisional cascade models. The resulting mass of bright debris discs appears to be unrealistically large, exceeding the mass of solids available in the systems at the preceding protoplanetary stage. We discuss this ‘mass problem’ in detail and investigate possible solutions to it. These include uncertainties in the dust opacity and planetesimal strength, variation of the bulk density with size, steepening of the size distribution by damping processes, the role of the unknown ‘collisional age’ of the discs, and dust production in recent giant impacts. While we cannot rule out the possibility that a combination of these might help, we argue that the easiest solution would be to assume that planetesimals in systems with bright debris discs were ‘born small’, with sizes in the kilometre range, especially at large distances from the stars. This conclusion would necessitate revisions to the existing planetesimal formation models, and may have a range of implications for planet formation. We also discuss potential tests to constrain the largest planetesimal sizes and debris disc masses.

The Role of Deep Defect Relaxation Dynamics in Optical Processes in Hydrogenated Amorphous Silicon

1995 ◽  
Vol 377 ◽  
Author(s):  
Fan Zhong ◽  
J. David Cohen
Keyword(s):  
Relaxation Dynamics ◽  
Energy Position ◽  
Defect Band ◽  
Transient Photoconductivity ◽  
Photoconductivity Decay ◽  
Long Time ◽  
Intimate Relation ◽  
Hydrogenated Amorphous ◽  
Defect Relaxation

ABSTRACTWe report results of a transient modulated photocurrent technique which allows us to observe the time evolution of the D0 sub-band under the application of optical bias light and after turning off this bias light Our measurements show that the D0 band shifts monotonically to shallower thermal energies after the bias light is applied, with roughly 10 seconds to saturation at 300K and to deeper thermal energies after removing the bias light, with a decay time of over 1000 seconds. We have also found there exists an intimate relation between the motion of the D0 band and that of the quasi Fermi level as deduced from the transient photoconductivity and therefore, in particular, to the long time photoconductivity decay. This relation is exactly reproduced by the assumption of a D0 band whose energy position evolves in time, together with a recombination process dominated by changes in the charge state of a deeper defect band under light bias.

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

1994 ◽  
Vol 336 ◽  
Author(s):  
F. Zhong ◽  
J.D. Cohen ◽  
J. Yang ◽  
S. Guha
Keyword(s):  
Thermal Emission ◽  
Drive Level ◽  
Optical Methods ◽  
Defect States ◽  
Thermal Transitions ◽  
Defect Band ◽  
Time Resolved ◽  
Photocurrent Spectroscopy ◽  
First Time ◽  
Negative Signal

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.

Observation of Configurational Switching of Deep Defects in a-Si:H Using Thermal Step Insertion During Capacitance Transient Measurements

1996 ◽  
Vol 420 ◽  
Author(s):  
Adam D. Gardner ◽  
J. David Cohen
Keyword(s):  
Thermal Emission ◽  
Full Range ◽  
Type A ◽  
Defect States ◽  
The Many ◽  
Capacitance Transient ◽  
Defect Relaxation ◽  
Transient Measurements ◽  
Temperature Switch ◽  
Emission Phase

AbstractIn standard, isothermal capacitance transient measurements, configurational changes in defect states are normally very difficult to identify because, depending on the relative rates of thermal emission to the configurational relaxation, charge will be emitted predominantly from only one configuration. We have found, however, that employing a thermal step during the emission phase of the transient enhances the effect of defect relaxation; one can then observe the resultant switch between distinct configurations. We have applied this method quite successfully to lightly doped n-type a-Si:H samples by varying the overall temperature (between 270K and 350K) and the magnitude of the temperature switch (from 20K to 35K). For the smallest temperature steps, the resultant transients suggest two distinct configurations that, we believe, reflect only a fraction of the many latent configurations that account for the full range of relaxation possible.

Fermi-level pinning by carrier com- pensating midgap donor defect band in homoepitaxially grown p-type ZnO by MBE

2014 ◽  
Vol 11 (7-8) ◽  
pp. 1353-1356 ◽  
Author(s):  
T. Masamoto ◽  
K. Noda ◽  
T. Maejima ◽  
R. Natsume ◽  
T. Matsuo ◽  
...  
Keyword(s):  
Fermi Level ◽  
Defect Band ◽  
Fermi Level Pinning ◽  
P Type ◽  
Donor Defect

The Influence of Interfaces on the Gap State Distribution of Undoped a-Si:H

1990 ◽  
Vol 192 ◽  
Author(s):  
G. Schumm ◽  
G. H. Bauer
Keyword(s):  
Fermi Level ◽  
Transit Time ◽  
Reverse Bias ◽  
State Distribution ◽  
Band Bending ◽  
Time Regime ◽  
Level Shifts ◽  
Pool Model ◽  
Annealing Effects ◽  
Gap State

ABSTRACTModulated primary photocurrent (MPC) studies on pin structures show spatial variations of the gap state distribution across the i-layer that can be correlated with Fermi level shifts by band bending towards interfaces. These results as well as reverse bias annealing effects are explained in terms of the defect pool model. It is demonstrated that MPC measurements are basically identical to TOF measurements with clear advantages in the post-transit time regime.

Synthesis and Characterization of Mn Doped CdS Quantum Dots from a Single Source Precursor

1999 ◽  
Vol 581 ◽  
Author(s):  
M. Azad Malik ◽  
Paul O'Brien ◽  
N. Revaprasadu
Keyword(s):  
Hexagonal Phase ◽  
Tetrahedral Site ◽  
Deep Trap ◽  
Esr Spectra ◽  
Cds Nanoparticles ◽  
Near Band Edge ◽  
Single Source ◽  
High Resolution Tem ◽  
Pl Spectrum ◽  
Mn Doped

ABSTRACTCdS and Mn-doped CdS capped with TOPO (tri-n-octylphosphineoxide) have been prepared by a single source route using bis(methylhexyldithiocarbamato)cadmium(II) and manganese dichloride as precursors. The nanoparticles obtained show quantum size effects in their optical spectra with the CdS nanoparticles exhibiting near band-edge luminescence. The PL spectrum of the doped CdS nanoparticles have an emission maximum at 585 nm attributed to the 4T1-6A1 electronic transition of Mn in a tetrahedral site. However the PL spectrum changes over time (weeks) and gave a deep trap emission. The Selected Area Electron Diffraction (SAED) and X-ray diffraction (XRD) pattern show both CdS and the Mn doped CdS particles to be of the hexagonal phase. Transmission Electron Microscopy (TEM) and High Resolution TEM show well-defined images of nanosize particles with clear lattice fringes. ESR spectra and ICP results confirm the presence of Mn in the CdS nanoparticles.

Improved defect-pool model for charged defects in amorphous silicon

1993 ◽  
Vol 48 (15) ◽  
pp. 10815-10827 ◽  
Author(s):  
M. J. Powell ◽  
S. C. Deane
Keyword(s):  
Amorphous Silicon ◽  
Pool Model ◽  
Charged Defects
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