An Experimental Evaluation of Modulated Photocurrent Spectroscopy as A Density of States Probe

1999 ◽  
Vol 557 ◽  
Author(s):  
S. Reynolds ◽  
C. Main ◽  
D.P. Webb ◽  
M.J. Rose
Keyword(s):  
Thermal Activation ◽  
Optical Excitation ◽  
Band Tail ◽  
Characteristic Energy ◽  
Tail State ◽  
Transient Photocurrent ◽  
Photocurrent Spectroscopy ◽  
A Value ◽  
Quasi Fermi Level ◽  
Absolute Density

AbstractModulated and Fourier-transformed transient photocurrent (MPC and TPC-FT) spectroscopies have been evaluated through a study of the density and capture properties of localised states in as-prepared and light-soaked PECVD a-Si:H samples over a range of temperatures and optical excitations. Both techniques return a conduction band tail state characteristic energy of approximately 22 meV. However, defect state spectra differ in detail and are strongly influenced by dc optical excitation. A feature correlating with the quasi-Fermi level position is observed, but the capture coefficient implied (of order 10-6 cm3 s-1) is some two orders greater than that calculated from thermal activation of emission frequencies. Such a value would suggest an implausibly low absolute density of defects. Possible explanations are briefly discussed and additional investigations proposed.

Electron Transport and Conduction-Band-Tail States in a-Si:H Deposited with a Remote Hydrogen Plasma

1993 ◽  
Vol 297 ◽  
Author(s):  
C.E. Nebel ◽  
R.A. Street ◽  
N.M. Johnson ◽  
J. Walker
Keyword(s):  
Electron Transport ◽  
Conduction Band ◽  
Defect Density ◽  
Plasma Deposition ◽  
Hydrogen Plasma ◽  
Band Tail ◽  
Mobility Edge ◽  
Characteristic Energy ◽  
Tail State ◽  
Electron Transport Properties

Electron transport properties of a-Si:H prepared in a remote hydrogen plasma deposition reactor (RHPD) at TD = 400°C were investigated in the temperature regime 110 K ≤ T ≤ 300 K by time-of-flight and post-transit spectroscopy experiments. Based on these data the conduction-band-tail state distribution was calculated. In the energy range 85 meV ≤ Ec- E ≤ 350 meV below the mobility edge Ec the tail is well described by an exponential distribution with a characteristic energy of ≃ 21 meV. Deeper in the mobility gap (Ec-E > 350 meV) the tail smoothly passes over into the defect density which is approximately six orders of magnitude smaller than at the mobility edge. Comparisons with data deduced on conventionally prepared a-Si:H (RF-, DC-glow discharge) at TD = 230 °C show that electron transport and the conduction band tail of the RHPD material are comparable.

Relationship between Phase Shift, Square-Wave Response and Density of States in Modulated Photocurrent Spectroscopy

2006 ◽  
Vol 910 ◽  
Author(s):  
Steve Reynolds ◽  
Charlie Main
Keyword(s):  
Silicon Films ◽  
Complex Frequency ◽  
Wave Excitation ◽  
Band Tail ◽  
Tail State ◽  
Square Wave ◽  
Photocurrent Spectroscopy ◽  
State Capture ◽  
And Behavior ◽  
Defect Densities

AbstractLow- and high-frequency modulated photoconductivity measurements (LF and HF MPC) have been made on amorphous silicon films prepared by the expanding thermal plasma (ETP) and RF PECVD techniques. Time constants have been measured by decay of square wave excitation and behavior of complex frequency response. The influence of quasi-Fermi level position has been examined. Band tail slopes of 32 meV (ETP) and 37 meV (PECVD), and defect densities of order 1018 and 1017 cm-3 eV-1 respectively are found. Tail state capture coefficients of order 3×10-8 cm3 s-1 are calculated from overlapping LF and HF regimes. Defect state values for ETP (< 10-8 cm3 s-1) are smaller than for PECVD silicon films (> 10-7 cm-3 s-1).

Band Tailing and Transport in a-SiGe:H-Alloys

1989 ◽  
Vol 149 ◽  
Author(s):  
G. H. Bauer ◽  
C. E. Nebel ◽  
M. B. Schubert ◽  
G. Schumm
Keyword(s):  
Raman Scattering ◽  
Valence Band ◽  
State Density ◽  
Conduction Band Edge ◽  
Compound Structure ◽  
Tail State ◽  
Photocurrent Spectroscopy ◽  
Transport Studies ◽  
Small X ◽  
Band Tailing

ABSTRACTOptical and transport studies of both cb- and vb-tail states in a-Si1−xGex:H such as subband absorption (PDS), instationary photocurrent experiments (TOF, PTS) for electrons and holes, Modulated Photocurrent Spectroscopy (MPS), and Raman scattering have been performed. The main consequences of Ge-alloying into the a-Si:H network are i) an increase in cb-tail state density at the conduction band edge and in the exponential cb- tail even for small x (O<x<0.3), accompanied by ii) a rise in deep cb-tail and midgap states which to some extent can be reduced by appropriate deposition methods; iii) at the valence band side up to x≈0.3 the tail seems not to be affected at all and for 0.3<x<0.9 the vb-tail obviously can be kept similar to that of a-Si:H (Evo≈(50–60) meV). Halfwidths of Raman TO-like modes point to the existence of a rigid Si-network in O<x<0.3 in which Ge is incorporated and to a transition at x>0.35 into a Si-Ge compound structure with maximum disorder at x≈0.5.

Rocket observations of ultraviolet and optical emissions in the dayside aurora

1986 ◽  
Vol 64 (10) ◽  
pp. 1401-1411
Author(s):  
A. B. Christensen ◽  
R. W. Eastes ◽  
P. D. Feldman ◽  
E. P. Gentieu
Keyword(s):  
Energy Flux ◽  
Near Infrared ◽  
Extreme Ultraviolet ◽  
Infrared Region ◽  
Precipitation Event ◽  
Emission Rates ◽  
Emission Layer ◽  
Characteristic Energy ◽  
A Value ◽  
Spatial Fluctuations

Observations of emissions extending from the extreme ultraviolet to the near-infrared region of the spectrum were obtained from a rocket payload flown from Cape Parry, N.W.T., Canada, during the CENTAUR campaign December 7, 1981. The flight, NASA 29.017CE (Terrier-Malemute), was launched near local magnetic noon during an electron-precipitation event in the dayside cleft region. Emission rates versus altitude and zenith angle were obtained for the principal atomic-oxygen emission features OI (989 Å), OI (1304 Å), OI (1356 Å), OI (6300 Å), OI (7774 Å), and OI (8446 Å), as well as HI (1216 Å) and OII (834 Å). Some data were also obtained for the [Formula: see text] (first negative) bands. The results suggest an emission layer peaked in the vicinity of 250 to 300 km. Temporal and spatial fluctuations were most pronounced at apogee and on the downleg portion of the flight. Emissions strongly affected by multiple scattering fluctuated less than optically thin emissions.In this paper, estimates of the input energy flux and characteristic energy are made based on the intensity of the OI (6300 Å) and OI (1356 Å) emissions and the altitude of the emission layer. A value of 0.3 to 0.4 erg/cm2∙s for the energy flux and a characteristic energy of approximately 200 eV are consistent with the observations (1 erg = 0.1 μJ).

Comments on Electron and Hole Transport in the Band Tail States of Amorphous Silicon at Low Temperatures

1989 ◽  
Vol 149 ◽  
Author(s):  
M. Silver ◽  
W. E. Spear
Keyword(s):  
Amorphous Silicon ◽  
Drift Mobility ◽  
Hole Transport ◽  
Experimental Results ◽  
Exponential Tail ◽  
Band Tail ◽  
State Distribution ◽  
Temperature Drift ◽  
Model Calculations ◽  
Tail State

ABSTRACTRecent experimental results on the low temperature drift mobility in amorphous silicon are examined on the basis of the approach to hopping transport developed by Silver and Bässler. It is shown on general grounds that the main features of the experimental results cannot be explained by a purely exponential tail state distribution, but are consistent with the distribution used by Spear and Cloude (1988) in model calculations.

Capacitance and Transient Photocapacitance Studies of Tetrahedral Amorphous Carbon

2000 ◽  
Vol 621 ◽  
Author(s):  
Kimon C. Palinginis ◽  
A. Ilie ◽  
W.I. Milne ◽  
J. David Cohen
Keyword(s):  
Amorphous Carbon ◽  
Urbach Energy ◽  
Defect Density ◽  
Intrinsic Property ◽  
Band Tail ◽  
Junction Capacitance ◽  
Tetrahedral Amorphous Carbon ◽  
Thermally Activated ◽  
A Value ◽  
Defect Densities

ABSTRACTWe have applied junction capacitance and transient photocapacitance measurements to undoped tetrahedral amorphous carbon (ta-C)/silicon carbide (SiC) heterostructures to deduce defect densities and defect distributions in ta-C. The junction capacitance measurements show two thermally activated processes. One can be related to the activation of carriers out of defects at the ta-C/SiC interface while the other one with an activation energy of 0.36eV is an intrinsic property of the ta-C. The defect density at the ta-C/SiC interface is estimated to be roughly 9 ± 2 × 109 cm−2. The transient photocapacitance measurements have allowed us to observe the broader band tail of ta-C, giving a value (Urbach energy) of 230meV.

scholarly journals Tail state mediated conduction in zinc tin oxide thinfilm phototransistors under below bandgap optical excitation

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Soumen Dhara ◽  
Kham M. Niang ◽  
Andrew J. Flewitt ◽  
Arokia Nathan ◽  
Stephen A. Lynch
Keyword(s):  
Tin Oxide ◽  
Direct Evidence ◽  
Uv Light ◽  
Optical Excitation ◽  
Amorphous Structure ◽  
Persistent Photoconductivity ◽  
Current Analysis ◽  
Current Ratio ◽  
Tail State ◽  
Zinc Tin Oxide

AbstractWe report on the appearance of a strong persistent photoconductivity (PPC) and conductor-like behaviour in zinc tin oxide (ZTO) thinfilm phototransistors. The active ZTO channel layer was prepared by remote plasma reactive sputtering and possesses an amorphous structure. Under sub-bandgap excitation of ZTO with UV light, the photocurrent reaches as high as ~ 10−4 A (a photo-to-dark current ratio of ~ 107) and remains close to this high value after switching off the light. During this time, the ZTO TFT exhibits strong PPC with long-lasting recovery time, which leads the appearance of the conductor-like behaviour in ZTO semiconductor. In the present case, the conductivity changes over six orders of magnitude, from ~ 10−7 to 0.92/Ω/cm. After UV exposure, the ZTO compound can potentially remain in the conducting state for up to a month. The underlying physics of the observed PPC effect is investigated by studying defects (deep states and tail states) by employing a discharge current analysis (DCA) technique. Findings from the DCA study reveal direct evidence for the involvement of sub-bandgap tail states of the ZTO in the strong PPC, while deep states contribute to mild PPC.

Transient Photocurrent Spectroscopy of Trap Levels in Ultra-Thin SiO2 Films

1996 ◽  
Vol 428 ◽  
Author(s):  
Y. Miura ◽  
S. Fujieda
Keyword(s):  
Cross Section ◽  
Monochromatic Light ◽  
Electron Trap ◽  
External Circuit ◽  
Broad Distribution ◽  
Transient Photocurrent ◽  
Photocurrent Spectroscopy ◽  
Optical Cross Section ◽  
Tunneling Injection ◽  
Trap Levels

AbstractStress-induced trap levels near Si/SiO2 interfaces for MOS diodes with 10 imi-thick oxides are investigated by measuring the transient photocurrent, which depends on the incident photon energy. The electron trap levels are filled by tunneling injection, and the electrons are depopulated by monochromatic light irradiation. The transient photocurrent, which is measured as an external circuit current, decays exponentially with time. Based on a proposed detrapping model, the optical cross section is estimated to be about 1×10−17 cm2 for hv=2−3 eV. The obtained photo-accessible trap density has a broad distribution at around hv=2.5 eV.

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