Daewon Kwon

Electrode interdependence and hole capacitance in capacitance–voltage characteristics of hydrogenated amorphous silicon thin-film transistor

The interelectrode capacitance–voltage (C–V) characteristics of back-channel-etched inverted-staggered hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) were investigated. It is demonstrated that this simple measurement can be used to diagnose TFT parameters such as the fabricated channel length, the channel resistance, and the error in the mask alignment of the source and drain overlap lengths. The C–V characteristics associated with the hole accumulation in a-Si:H TFTs with n+-type source/drain contacts were also examined. We observed that the ac capacitance increases for low frequencies and/or moderately high measurement temperatures provided the gate voltage is sufficiently negative. One possible mechanism for this hole capacitance is proposed that accounts for the observed frequency and temperature dependence.

Effect of Light Soaking on Hot Wire Deposited a-Si:H Films

We present the results of studies on the defect properties and the effect of light soaking for various hot wire deposited (HW) films. We employ junction capacitance measurements together with the transient photocapacitance spectroscopy to measure the deep defect densities in as-grown state (state A) and in light soaked state (state B). Good agreement is found between the defect densities measured from both measurements. The HW film with a hydrogen content of 10 – 12 at.% shows physical characteristics and defect densities similar to conventional PECVD films. The HW films with hydrogen content, C H , in the range 2 – 9 at.% show a smaller defect density in state B than the defect density of the film with higher C H . However, the film with a hydrogen level of less than 1 at.% exhibits markedly inferior physical properties.

Identification of the dominant electron deep trap in amorphous silicon from ESR and modulated photocurrent measurements : implications for defect models

Modulated photocurrent (MPC) measurements in intrinsic a-Si:H reveal a prominent band of electron traps with a thermal emission energy near 0.6 eV. We have identified this defect band by comparing MPC and electron paramagnetic resonance spectra for intrinsic and lightly n-type doped samples over a range of metastable states. These data directly show that the MPC band arises from the neutral charge state of the defects. This identification is also confirmed when the quasi-Fermi level is varied by the application of light bias. Such observations are totally inconsistent with a large population of charged defects in intrinsic samples predicted by recent versions of the defect pool model. Rather, our observations have a natural explanation in terms of a defect relaxation process.

Optical spectra of crystalline silicon particles embedded in an amorphous silicon matrix

Abstract Deuterated amorphous silicon films deposited by DC reactive magnetron sputtering were measured by Raman spectroscopy and TEM imaging. The films were found to consist of silicon crystallites embedded in an amorphous silicon matrix. The sub-band-gap optical spectra of these films were recorded using photocapacitance and transient photocurrent spectroscopy. These spectra reveal an amorphous silicon sub-band-gap spectrum together with a unique optical transition in the embedded c-Si particles. This transition corresponds to valence band electrons being optically inserted into empty levels lying within the amorphous silicon mobility gap. We believe these empty levels are associated either with the conduction band of the c-Si particles or with defect states at the crystalline–amorphous boundary.

Visible electroluminescence from porous silicon/hydrogenated amorphous silicon pn‐heterojunction devices

We report the fabrication and characterization of a p‐type porous silicon/ n‐type hydrogenated amorphous silicon (a‐Si:H) pn‐heterojunction electroluminescent device structure. The devices exhibit electroluminescence in forward bias, demonstrating minority carrier injection from n‐type a‐Si:H into p‐type porous silicon.

New results using capacitance transient studies to investigate deep defect relaxation in hydrogenated amorphous silicon

Abstract Recent studies of deep defect relaxation processes in a-Si:H using capacitance transient measurements are reported. First, to disprove any significant role of from contact effects, nearly identical transients for samples deposited on p+ crystalline Si with a blocking back contact or on n+ crystalline Si substrates with single junction characteristics have been obtained. Similar transients are also obtained for a film deposited on a thin n+ a-Si:H layer over a Cr coated substrate. Second, the effects of incorporating temperature steps during the time evolution of the transients have been investigated. Such transients are compared to the case where the emission of charge is allowed to proceed isothermally. These results verify the existence of a defect distribution whose thermal emission properties evolve in time.

Electronic Transitions in Mixed Phase Crystalline/Amorphous Silicon in the Low Crystalline Fraction Regime

Amorphous silicon films were prepared by dc reactive magnetron sputtering under conditions approaching the phase transition to microcrystallinity. using TEM imaging these films were found to contain clusters of 5 to 50 nm sized Si crystallites embedded in an amorphous silicon matrix. Photocapacitance and transient photocurrent sub-band-gap optical spectra of this material appear to consist of a superposition of a spectrum typical of amorphous silicon together with an optical transition, with a threshold near 1.1 eV, that exhibits a very large optical cross section. This transition arises from valence band electrons being optically inserted into empty levels lying within the amorphous silicon mobility gap. Using modulated photocurrent methods they have determined that these states also dominate the electron deep trapping in this material. They argue that these states arise from defects at the crystalline-amorphous boundary.

Identification of the dominant electron deep trap in a-Si:H: A critical test of the defect pool vs. defect relaxation pictures

Modulated photocurrent (MPC) measurements in intrinsic a-Si:H reveal a prominent band of electron deep traps with a thermal emission energy near 0.6 eV. The authors 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, E{sub F}, ranges from less than 0.5 eV to more than 0.7 eV below E{sub C}. This comparison unambiguously demonstrates that the MPC band arises from the D{sup 0} charge state of the defects (specifically, the D{sup {minus}}/D{sup 0} 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.

New evidence for deep defect relaxation in hydrogenated amorphous silicon from junction capacitance methods

The authors have carried out capacitance transient measurements on lightly PH{sub 3} doped a-Si:H films incorporating both ohmic and blocking contacts. They find that the previously reported anomalous variation of emission time with filling pulse duration is completely independent of whether one uses an n{sup +} c-Si substrate (providing a reasonably ohmic back contact) or a p{sup +} c-Si substrate (giving a blocking back contact). Quite similar behavior is observed on a sample which includes an n{sup +} a-Si:H layer at the back contact, exhibiting a variation of more than 2 orders of magnitude change in emission time when the filling pulse duration is decreased from 1s to 100{micro}s. Computer simulations are used to demonstrate that for a fixed (non-relaxing) deep state distribution, a filling problem in general cannot account for the observed experimental behavior. Finally, the authors report measurements that demonstrate that the filling of defects too close to the barrier interface can create difficulties observing the relaxation effects when an n{sup +} a-Si:H contacting layer is used.