Photocapacitance Measurements Research Articles

Influence of Se beam pressure on deep-level defects in Cu(In,Ga)Se_2 thin films studied by photocapacitance and time-resolved photoluminescence measurements

The properties of deep-level defects in Cu(In,Ga)Se2 thin films grown under different Se beam pressure were investigated by photocapacitance (PC) and time-resolved photoluminescence (TRPL) measurements. A generally known deep-level defect located at around 0.8 eV above the valence band was identified for all samples by transient photocapacitance measurement, the position of which was unvaried with the Se beam pressure. The defect concentration and the capture cross section of the minority carrier were obtained from the steady-state photocapacitance and TRPL measurements, which were evaluated to be on the order of 1013 cm−3 and 10−14 cm2, respectively, in the Se beam pressure range from 1.3×10−3 to 4.4×10−3 Pa. The relatively higher defect concentration and larger minority carrier capture cross section were considered as possible causes of the degradation of the conversion efficiency of CIGS solar cells grown under low Se beam pressure.

Characterization of deep defects in boron-doped CVD diamond films using transient photocapacitance method

We have developed a highly-sensitive transient photocapacitance measurement (TPM) system for deep defects in wide bandgap materials, and applied it to characterize the boron-doped diamond films grown on a high-pressure/high-temperature-synthesized Ib diamond substrate using high-power-density microwave-plasma chemical vapor deposition method. The developed TPM system has both a low detection limit of less than 0.5 fF for changes in the photocapacitance and a low measurement temperature drift of less than 0.03K in 12h. By using the TPM system, we have successfully found an acceptor-type defect around 1.2eV above the valence-band maximum for the B-doped diamond film with a considerably high crystalline quality that had some strong exciton emission peaks in the cathodoluminescence spectra taken at ≈80K. The photoionization cross section and the defect density estimated for the observed defect were 3.1×10–15cm2 and 2.8×1016cm−3, respectively.

Deep levels in low resistive Zn3P2

The low resistivity p‐type polycrystalline Zn3P2 grown by means of closed tube vapor transport was investigated. Deep carrier traps have been studied by means of deep level transient spectroscopy (DLTS) and by photocapacitance transient measurements. From DLTS measurements parameters of four main hole traps were determined. The first with activation energy 0.17 eV above the top of the valence band, probably connected with zinc vacancy, the defect with activation energy 0.34 eV which origin is unknown, the defect with activation energy 0.46 eV, probably oxygen related and the defect with activation energy 0.77 eV with unknown origin. For the defects with energies 0.17 and 0.46 eV nonzero values of capture energies were found: 0.03 and 0.16 eV, respectively. From photocapacitance measurements three optical activation energies were determined as steps on the curve: 0.83, 0.99, and 1.25 eV. Defects with optical activation energies of 0.83 eV is probably the same defect as defect with activation energy 0.46 eV and energy barrier for capture 0.16 eV obtained from DLTS measurements, and may be responsible for persistent photoconductivity (PPC) efect in high resistive samples of Zn3P2.

A new shape memory alloy film/p-Si solar light four quadrant detector for solar tracking applications

Shape memory alloys (SMAs) are promising materials for industrial and electronic actuators applications. The Cu-Al-Mn-Ni shape memory alloy was coated as a Schottky contact on p-Si substrate to fabricate a four quadrant sensor using a vacuum coating system. A front illuminated shape memory film-p-type silicon -based four-quadrant solar light Schottky sensor was fabricated using the various metal contact masks. The electrical and photoresponse properties of Cu-Al-Mn-Ni/p-Si diode were investigated. These properties of the diode were analyzed under in a wide range of frequencies and illumination intensities. The reverse current of the diode increases with increasing illumination intensities. This result confirms that the diode exhibits both photoconducting and photovoltaic behavior. The forward bias current of the diode increases exponentially with voltage confirming rectification behavior. Electronic parameters such as ideality factor, barrier height and series resistance of the diode were determined from I-V characteristics and Cheung’s functions. The transient photocurrent, photocapacitance and photoconductance measurements indicate that the diode is very sensitive to illumination. Also, the admittance measurements of the diode exhibited a decrease in capacitance and increase in conductance with increasing frequency. The frequency dependence of capacitance and conductance is attributed to the presence of interface states. It is evaluated that Cu-Al-Mn-Ni/p-Si shape memory alloy film/p-Si based four quadrant detector can be used in solar tracking applications.

Role of self-trapped holes in the photoconductive gain of β-gallium oxide Schottky diodes

Solar-blind photodetection and photoconductive gain >50 corresponding to a responsivity >8 A/W were observed for β-Ga2O3 Schottky photodiodes. The origin of photoconductive gain was investigated. Current-voltage characteristics of the diodes did not indicate avalanche breakdown, which excludes carrier multiplication by impact ionization as the source for gain. However, photocapacitance measurements indicated a mechanism for hole localization for above-band gap illumination, suggesting self-trapped hole formation. Comparison of photoconductivity and photocapacitance spectra indicated that self-trapped hole formation coincides with the strong photoconductive gain. It is concluded that self-trapped hole formation near the Schottky diode lowers the effective Schottky barrier in reverse bias, producing photoconductive gain. Ascribing photoconductive gain to an inherent property like self-trapping of holes can explain the operation of a variety of β-Ga2O3 photodetectors.

Electrical and photoresponse properties of Al/graphene oxide doped NiO nanocomposite/p-Si/Al photodiodes

The electrical and photoconducting properties of Al/graphene oxide doped NiO nanocomposite/p-Si/Al photodiodes with various graphene oxide contents were investigated by using current–voltage, transient photocurrent, photocapacitance and photoconductance measurements at various illumination intensities in the range of 10–100 mW/cm2. Graphene oxide doped NiO nanocomposite thin films were prepared by sol–gel spin method. Experimental results indicate that the reverse current of the photodiodes increases with the increasing illumination intensity. The value of transient photocurrent, photocapacitance and photoconductance measured as a function of time increases after illuminating and returns to original value after turning off the illumination. In addition, the frequency-dependent capacitance and conductance measurements was performed to indicate the existence of interface states. The obtained results suggest that the fabricated diode can be used as a photodiode in optoelectronic applications.

Efficient photon management with Ag nanoparticles coated TiO2 nanowire clusters for photodetector application

TiO2 nanowire clusters (NWCs) covered with Ag nanoparticles (NPs) were synthesized on Si substrates using GLAD technique. Ag NPs/TiO2 NWCs showed enhanced optical absorption all over the spectrum compared to bare TiO2 NWCs. The leakage current of the TiO2 NWCs device (−10.8 μA) was reduced by ∼9 × 103 times for Ag NPs/TiO2 NWCs detector (−1.2 nA) at −1 V applied bias. Ag NPs/TiO2 NWCs detector exhibited maximum photoresponsivity ∼2.4 times (−2 V) as compared to the bare TiO2 NWCs detector. Using 10 K photocapacitance measurement, a sharp peak at 355 nm (∼3.5 eV) was detected due to the main band transition. The other sub band gap defects were observed within 410–490 nm along with a broad hump between 595–700 nm. The light dependent capacitance-time (I-T) characteristic of the Ag/TiO2 NWCs detector shows rise-time (T r ) of 9.6 s and decay time (T d ) of 10 s.

Deep-level defects and turn-on capacitance recovery characteristics in AlGaN/GaN heterostructures

We report on turn-on capacitance recovery measurements as a simple short-time method of evaluating carrier-trapping phenomena in a two-dimensional electron gas (2DEG) in the bulk region of AlGaN/GaN heterostructures, employing their Schottky barrier diodes. Using this technique, we have investigated an in-depth relation between deep-level defects and 2DEG carrier trapping in an AlGaN/GaN heterostructure with a GaN buffer layer containing a high C concentration. Steady-state photo-capacitance spectroscopy measurements revealed three C-related deep-level defects located at ~2.07, ~2.80 and ~3.23 eV below the conduction band in the GaN buffer layer. Additionally, turn-on capacitance recovery measurements showed a large decrease in recovery time under white-light optical illuminations with long-pass filters between 370 and 390 nm. It is concluded that the ~3.23 eV level is mainly responsible for the 2DEG carrier-trapping phenomena in the GaN buffer layer of the AlGaN/GaN heterostructure.

The effect of copper on the sub-bandgap density of states of CdTe solar cells

Two optical sub-bandgap transitions in CdTe thin-film solar cells have been identified using detailed transient photocapacitance and transient photocurrent spectroscopy measurements. A broad response centered at EV + 0.9 eV directly correlates with the quantity of Cu present in the absorber layer, while a second response at EV + 1.2 eV does not depend on Cu or Zn and may be an intrinsic defect. These results demonstrate the influence of Cu on the sub-bandgap density of states of CdTe, and they are critical to understanding, modeling, and improving its optoelectronic properties.

Graphene–cobalt phthalocyanine based on optoelectronic device for solar panel tracking systems

The photoconducting properties of graphene–cobalt phthalocyanine organic semiconductor photodiodes were investigated. GO doped cobalt phthalocyanine (CoPc) thin films were prepared with various graphene oxide (GO) contents. The electrical properties of the diodes were analyzed by using current–voltage and capacitance/conductance–voltage measurements. The reverse current of the diodes increases with the increasing illumination intensity. The transient photocurrent, photocapacitance, and photoconductance measurements of the diodes were also investigated. The photocurrent, capacitance and conductance increase after illuminating and returns to original value after turning off the illumination. Graphene–cobalt phthalocyanine based on optoelectronic device exhibited both a photodiode and photocapacitor behavior under solar light illuminaton.

Investigation of the properties of deep-level defect in Cu(In,Ga)Se2 thin films by steady-state photocapacitance and time-resolved photoluminescence measurements

The quantitative characterization of the deep-level defect located at around 0.8 eV above the valence band in Cu(In1−x,Gax)Se2 (CIGS) thin films was performed by steady-state photocapacitance (SSPC) and time-resolved photoluminescence (TRPL) measurements. The defect concentration, photoionization cross section and electron capture cross section were evaluated to be on the order of 1013–1014 cm−3, 10−17–10−16 cm2, and 10−14 cm2, respectively, in the Ga content x range from 0.30 to 0.80; the first two values increased with increasing Ga content while the last one remained almost constant. By comparing the obtained values with the actual device performance, the cause of the degradation of the conversion efficiency of CIGS solar cells with higher Ga content was discussed in relation to the 0.8 eV defect.

Photocapacitance study of GaSb: In, As for defect analysis in InAs/GaSb type-II strained layer superlattices

Steady-state photocapacitance measurements were used to characterize GaSb incorporated with In, As, and a control sample. Evidence of a trap level at 0.55eV was observed for all samples. The change in the capacitance for the sample with indium was about half the change for the other samples, indicating that the addition of indium modified the near-mid-gap trap levels. Another change in capacitance starting at 0.71eV was attributed to electrons from the valence band filling levels close to the conduction band.

Fill factor related issues in hydrogenated amorphous Si solar cells

It is a fact that the fill factor (FF) decreases with decreasing μτ product, increasing illumination intensity, increasing absorber thickness, and long wavelength illumination in well-engineered hydrogenated amorphous silicon (a-Si:H) solar cells. The FF issues in the a-Si:H solar cell are not just a contact issue as in crystalline Si solar cell, but are more interesting and complicated issues that relate to the quality of materials and solar cell operating conditions. In past, a simple parameter of “collection length” or drift length, which is defined as μτE, was quite successful to explain the fact. In this article, we extensively study the collection length in terms of the internal electric field, which changes with the quality of material and operating condition. The Analysis of Microelectronic and Photonic Structures (AMPS) simulation shows that the decrease in fill factor is caused by photogenerated space charge trapped in the band-tail states rather than in defects when the defects are relatively small. This charge screens the applied field, reducing the internal field and the collection length, which reduce the FF. A voltage Vs from the photogenerated space charge was introduced to describe the behavior. This voltage varies with light intensity for large valance band-tail width and low hole mobility a-Si:H absorbers. The simulation also found that the space charge in mid-gap states is small compared with that in the tails and can be ignored under normal solar-cell operating conditions. Experimentally, the photocapacitance measurement was used as a means to probe the space charge. The decrease of FF with the increasing light intensity can be qualitatively explained by the light intensity dependence of photocapacitance, the declining electric field, and the decreasing collection length.

Leakage mechanisms in InAlN based heterostructures

We propose a model for leakage currents in Schottky contacts on InAlN/GaN heterostructures based on two distinct tunneling mechanisms. Our modeling relies on structural parameters, in particular, InAlN dielectric constant, interface polarization charges and Schottky barrier height, which are experimentally determined in the first part of our work. The first leakage mechanism is dominant in heterostructures with very thin (≤7 nm) InAlN barriers and consists in tunneling assisted by a deep level located 1.7 eV below the InAlN conduction band edge. We provide experimental evidence for this level through photocapacitance measurements. The second mechanism is on the other hand dominant in thicker InAlN layers and is linked to the appearance of highly doped regions where direct tunneling through the whole InAlN barrier is significantly enhanced. We also show that the two mechanisms may coexist for InAlN layers of intermediate thickness. Our findings confirm a progressive degradation of the InAlN material quality as the layer is grown thicker.

Photocapacitance study of type-II GaSb/GaAs quantum ring solar cells

In this study, the density of states associated with the localization of holes in GaSb/GaAs quantum rings are determined by the energy selective charging of the quantum ring distribution. The authors show, using conventional photocapacitance measurements, that the excess charge accumulated within the type-II nanostructures increases with increasing excitation energies for photon energies above 0.9 eV. Optical excitation between the localized hole states and the conduction band is therefore not limited to the Γ(k = 0) point, with pseudo-monochromatic light charging all states lying within the photon energy selected. The energy distribution of the quantum ring states could consequently be accurately related from the excitation dependence of the integrated photocapacitance. The resulting band of localized hole states is shown to be well described by a narrow distribution centered 407 meV above the GaAs valence band maximum.

A study on multiple defect states in low-carbon doped GaN layers and its correlation with AlGaN/GaN high electron mobility transistor operation

A study on defect states in relatively low-carbon doped GaN is presented. A large current collapse was observed in AlGaN/GaN high electron mobility transistor (HEMT) operation when the device channel was doped with carbon of 1×1017cm−3. Deep level transient spectroscopy measurements showed a positive and even negative correlation between the densities of carbon and those of shallow trap states. Along with their small concentrations, shallow traps could not be associated with the collapse of the HEMT. Photo capacitance measurements yielded large signal at very deep levels of 1.6 and 2.4eV in carbon doped GaN. Especially, the 2.4eV deep trap was estimated to be acceptor type and related to some indirect states that the minority carrier transient spectroscopy could not characterize. A 20% of doped carbon was allocated to the very deep traps, and the large current collapse was attributed to these carbon-related states.

Photovoltaic and phototransient interface states properties of nanocomposite Fe3O4–graphene/n-Si/Al photodiode

A photodiode based on Fe3O4–graphene/n-Si/Al diode was fabricated. The electrical and capacitance properties of the diode were investigated by transient current and capacitance measurements. Ag/Fe3O4–graphene/n-Si photovoltaic device gives Isc of 0.111mA and Voc of 375mV under 1.5A.M. The maximum electrical power Pmax value of the device was found to be 7.567μV. This suggests that the studied photovoltaic device can be used as a microvoltage generator. The capacitance of the device increases with illumination. This indicates that the device exhibits a photocapacitance behavior. This behavior was analyzed by transient photocapacitance measurements. The transient interface states density plots of the diode were obtained. The photocapacitance mechanism was explained with the change in the interface states.

Correlation between deep-level defects and turn-on recovery characteristics in AlGaN/GaN hetero-structures

We report on a correlation between deep-level defects and turn-on recovery characteristics in AlGaN/GaN hetero-structures, employing Schottky barrier diodes. Photo-capacitance spectroscopy measurements reveal three specific deep levels located at ∼2.07, ∼2.80, and ∼3.23 eV below the conduction band, presumably attributable to Ga vacancies and/or impurity C present in the GaN buffer layer. Additionally, from photo-assisted turn-on current recovery measurements, by using 390 and 370 nm long-pass filters, the recovery time becomes significantly faster due to inactivation of their corresponding deep-level traps. Therefore, the ∼2.80 and ∼3.23 eV levels are probably responsible for the carrier-trapping phenomena in the bulk region.

Plasma-Induced Damage and Recovery on Au/n-GaN Schottky Diode in Different Processes

The effects of plasma-induced damage on deep traps in n-GaN have been investigated using current–voltage (I–V), capacitance–voltage (C–V), and photocapacitance (PHCAP) measurements. The Au/n-GaN Schottky barrier diodes were fabricated in an inductively coupled plasma ion etching (ICP-RIE) system. After mesa etching to achieve ohmic contact, the n-GaN surface, at which Schottky contacts are fabricated, is etched ∼100 nm by ICP-RIE with various Cl2/Ar ratios and RIE bias powers (PB), to introduce plasma damage. The electrical properties of the fabricated Shottky barrier diodes (SBDs) strongly dependent on the RIE gas composition and the bias power PB applied to the sample stage. In order to overcome the residue and plasma damage on the Schottky area, the samples were treated with HCl at 110 °C for 30 min. Several deep levels (1.8, 2.5, and 3.0 eV below the conduction band) were detected by PHCAP measurement. Improved electrical characteristics were achieved as a result of the HCl treatment and sintering process. The PHCAP measurement results also revealed the effectiveness of thermal and chemical treatments.

Plasma-Induced Damage and Recovery on Au/n-GaN Schottky Diode in Different Processes

The effects of plasma-induced damage on deep traps in n-GaN have been investigated using current–voltage (I–V), capacitance–voltage (C–V), and photocapacitance (PHCAP) measurements. The Au/n-GaN Schottky barrier diodes were fabricated in an inductively coupled plasma ion etching (ICP-RIE) system. After mesa etching to achieve ohmic contact, the n-GaN surface, at which Schottky contacts are fabricated, is etched ∼100 nm by ICP-RIE with various Cl2/Ar ratios and RIE bias powers (PB), to introduce plasma damage. The electrical properties of the fabricated Shottky barrier diodes (SBDs) strongly dependent on the RIE gas composition and the bias power PB applied to the sample stage. In order to overcome the residue and plasma damage on the Schottky area, the samples were treated with HCl at 110 °C for 30 min. Several deep levels (1.8, 2.5, and 3.0 eV below the conduction band) were detected by PHCAP measurement. Improved electrical characteristics were achieved as a result of the HCl treatment and sintering process. The PHCAP measurement results also revealed the effectiveness of thermal and chemical treatments.