Graduate Thesis Or Dissertation
 

Electronic and Optical Properties of Defects in Amorphous Oxide and Transition Metal Dichalcogenide Semiconductors

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  • The optical and electronic properties of amorphous oxide thin films depend crucially on chemical composition, and deposition process variations which give rise to sub-gap defect states. Consequently, there is a need for a reliable, high-throughput method to extract sub-gap defect densities of states in amorphous oxide thin films. We present a novel in-situ method which uses ultrabroadband photoconduction (UBPC) measurements on TFT devices to extract the density of defect states in the sub-gap of a-IGZO. Both the TFT photoconduction response and geometric properties are used to isolate the absolute defect concentration in the sub-gap. The measured defect concentration increases from ~10^16 cm^-3 - 10^20 cm^-3 as laser photon energy is tuned from near conduction band state excitation to the valence band. The density of states (DoS) is calculated from the derivative of the defect concentration with respect to energy. The resulting fully-experimentally derived DoS reveals a series of broad defect peaks in the sub-gap and an exponential valance band Urbach tail. Density functional theory simulations classify the origin of the measured sub-gap density of states peaks as a series of donor-like oxygen vacancy states and acceptor-like metal vacancy states. Donor peaks are found both near the conduction band and deep in the sub-gap, with measured peak densities in the range of 10^17-10^18 cm^-3 eV^-1. Two deep acceptor-like metal vacancy peaks lie adjacent to the valance band Urbach tail region at 2.0 to 2.5 eV below the conduction band edge, with measured peak densities in the range of 10^18 cm^-3 eV^-1. By applying detailed charge balance, we show that increasing the density of metal vacancy deep acceptors shifts the a-IGZO TFT threshold voltage to more positive values. The DFT sub-gap identification is confirmed by showing the measured TFT electron capture times for metal vacancy acceptors are twice as long as that of oxygen vacancy donors. We found long recombination lifetimes of photoionized electrons into acceptor-like vacancies is one cause of TFT transfer curve hysteresis for photoexcitation wavelength, hv > 2.0 eV. To conclude the discussion of defects in a-IGZO, UBPC is used to directly measure the effect of hydrogen incorporated into a-IGZO TFTs. After hydrogen incorporation, oxygen vacancy DoS peaks are partially suppressed and the DoS near the valence band increases. This suggests that hydrogen hybridizes with oxygen vacancy sites resulting in metal-hydrogen (M-H) bonds, which form a new state at ~ 0.4 eV above the valence band. TFT transfer curves and spectrally resolved transient photocurrent lifetimes suggest that hydrogen can also replace acceptor-like metal vacancy states with donor-like O-H bonds. This had the effect of increasing the free electron concentration, decreasing the TFT threshold voltage by ~ 5 V and decreasing the long recombination time associated with metal-vacancy acceptor-like states by a factor of two. We then take up a discussion on the effects of defects on the photocarrier extraction efficiency and interlayer mobility in transition metal dichalcogenides (TMDs). By combining ultrafast photocurrent and transient absorption microscopy techniques, the complex dynamics of TMDs are distilled into a timeline of the efficiency-limiting steps. These steps are well described by a simple kinetic rate law that accounts for nonlinear exciton-exciton annihilation (~ 5 ps), linear depletion of carriers (~ 50 ps), and nonlinear defect-assisted Auger recombination (~ 1 ns) in WSe2. The rate law also predicts a nonlinear power dependence on incident photon-flux for transient absorption and AC-photocarrier extraction, which is verified in both WSe2 and MoS2. Experiments were performed on photodetectors made of few-layer 2D WSe2, which achieved both fast (down to ~ 80 ps) and efficient (up to ε ∼ 45%) photocurrent response despite defect-assisted carrier recombination competing with escape rates. Both the ultrafast photocurrent and transient absorption decay signals accelerated markedly due to a decrease in the electron escape time, τe, from 1.6 ns to 82 ps with increasing interlayer E-field. These escape rates suggest WSe2 has an interlayer electron (hole) mobility of only 0.129 (0.031) cm^2 V^-1 s^-1; nonetheless, efficient photocarrier extraction is still achieved as direct recombination becomes unlikely after electron-hole pairs are separated and localized on differing layers by the built-in or applied field. Spectrally resolved transient absorption and photocurrent each identify a photocarrier-trap-site with nanosecond lifetimes at ~ 1.25 eV below the conduction band, suggesting W-vacancies are the dominant sub-gap defects assisting in Auger-processes for incident photon fluxes as low as 10^12 photon cm^-2.
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  • Intellectual Property (patent, etc.)
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  • 2020-02-21 to 2022-03-22

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