Capacitance-voltage Profiling Research Articles

From 20.9 to 22.3% Cu(In,Ga)(S,Se)2 solar cell: Reduced recombination rate at the heterojunction and the depletion region due to K-treatment

Certified efficiency of 22.3% has been achieved for Cu(In,Ga)(Se,S)2 solar cell. Compared to our previous record cell with 20.9% efficiency, the major breakthrough is due to the increased Voc, benefited from potassium treatment. A lower reverse saturation current and a longer carrier collection length deduced from electron-beam induced current indicate that the degree of carrier recombination at the heterojunction and depletion region for the 22.3% cell is lower. Further characterizations (capacitance–voltage profiling, temperature-dependent Voc, Suns–Voc) and analysis indicate that the recombination coefficients at all regions were reduced, especially for the interface and depletion regions. Device simulation was performed assuming varying defect densities to model the current–voltage curve for the 22.3% cell. The best model was also used to estimate the achievable Voc if defect densities were further reduced. Furthermore, by using higher bandgap Cd-free buffer layers, a higher Jsc was achieved which gives an in-house solar cell efficiency of 22.8%. Recombination analysis on the 22.8% cell indicates that the interface recombination is further reduced, but the recombination coefficients at the depletion region was higher, pointing out that further improvement on the depletion region recombination could help to achieve a higher Voc and therefore an efficiency beyond 23%.

Electrochemical capacitance–voltage measurements and modeling of GaAs nanostructures with delta-doped layers

The paper presents the results of electrochemical capacitance-voltage profiling and simulation of quantum-sized semiconductor structures with quantum wells and delta-doped layers based on gallium arsenide. The experimental ECV data were obtained by superposition of measured capacitance-voltage characteristics during the gradual etching of the nanostructure. As a result of simulation, the concentration distribution and energy lineups for structures with delta-layers and quantum wells in gallium arsenide were calculated. The results of simulation are in qualitative agreement with the experimental results and data found in literature.

Light-induced changes in the minority carrier diffusion length of Cu(In,Ga)Se2 absorber material

In this study strong evidence for an illumination-induced change in minority charge carrier diffusion length is given for Cu(In,Ga)Se2 solar cells. After annealing under illumination (light soaking) the cells show the well-known metastable increase in open circuit voltage, but also a metastable reduction in current collection efficiency (which can be reversed by annealing in the dark). Partly, this can be attributed to an increase in doping density causing a reduced space charge region width as verified by capacitance-voltage profiling. Nevertheless, by using time-resolved photoluminescence and electron-beam-induced current measurements we found that the changes in doping density and space charge region width are not sufficient to describe the modification in current collection efficiency. Additionally there seems to be a reduction in minority carrier diffusion length and lifetime after white light soaking. This can be explained by a metastable change of electronic defects as found in temperature-dependent admittance spectroscopy. Device simulations confirm the impact of the found defects on the photocurrent.

Characterization of electronic properties of natural type IIb diamonds

Precision admittance spectroscopy measurements were carried out over wide temperature and frequency ranges for a set of natural single crystal type IIb diamond samples. Peaks of conductance spectra vs. temperature and frequency were used to compute the Arrhenius plots, and activation energies were derived from these plots. The capacitance-voltage profiling was used to estimate the majority charge carrier concentration and its distribution into depth of the samples. Apparent activation energies between 315 and 325meV and the capture cross section of about 10−13cm2 were found for samples with uncompensated boron concentrations in the range of 1 to 5×1016cm−3 (0.06–0.3ppm). The obtained boron concentrations are in good coincidence with FTIR results for the samples. Also, a reason for the difference between the observed admittance activation energy and the previously reported ionization energy for the acceptor boron in diamond (0.37eV) is proposed.

Compositional dependence of charge carrier transport in kesterite Cu2ZnSnS4 solar cells

Cu2ZnSnS4 solar cells deposited by thermal co-evaporation have been characterized structurally and electronically to determine the dependence of the electronic properties on the elemental composition of the kesterite phase, which can significantly deviate from the total sample composition. To this end, the kesterite phase content and composition were determined by a combination of X-ray fluorescence and X-ray absorption measurements. The electronic properties, such as carrier density and minority carrier diffusion length, were determined by electron beam induced current measurements and capacitance-voltage profiling. The charge-carrier transport properties are found to strongly depend on the Cu/(Sn+Zn) ratio of the kesterite phase. For the Cu-poor sample, a minority carrier diffusion length of 270 nm and a total collection length of approx. 500 nm are deduced, indicating that current collection should not be an issue in thin devices.

Identification of the primary compensating defect level responsible for determining blocking voltage of vertical GaN power diodes

Electrical performance and characterization of deep levels in vertical GaN P-i-N diodes grown on low threading dislocation density (∼104 - 106 cm−2) bulk GaN substrates are investigated. The lightly doped n drift region of these devices is observed to be highly compensated by several prominent deep levels detected using deep level optical spectroscopy at Ec-2.13, 2.92, and 3.2 eV. A combination of steady-state photocapacitance and lighted capacitance-voltage profiling indicates the concentrations of these deep levels to be Nt = 3 × 1012, 2 × 1015, and 5 × 1014 cm−3, respectively. The Ec-2.92 eV level is observed to be the primary compensating defect in as-grown n-type metal-organic chemical vapor deposition GaN, indicating this level acts as a limiting factor for achieving controllably low doping. The device blocking voltage should increase if compensating defects reduce the free carrier concentration of the n drift region. Understanding the incorporation of as-grown and native defects in thick n-GaN is essential for enabling large VBD in the next-generation wide-bandgap power semiconductor devices. Thus, controlling the as-grown defects induced by epitaxial growth conditions is critical to achieve blocking voltage capability above 5 kV.

Trapped charge densities in Al2O3-based silicon surface passivation layers

In Al2O3-based passivation layers, the formation of fixed charges and trap sites can be strongly influenced by small modifications in the stack layout. Fixed and trapped charge densities are characterized with capacitance voltage profiling and trap spectroscopy by charge injection and sensing, respectively. Al2O3 layers are grown by atomic layer deposition with very thin (∼1 nm) SiO2 or HfO2 interlayers or interface layers. In SiO2/Al2O3 and HfO2/Al2O3 stacks, both fixed charges and trap sites are reduced by at least a factor of 5 compared with the value measured in pure Al2O3. In Al2O3/SiO2/Al2O3 or Al2O3/HfO2/Al2O3 stacks, very high total charge densities of up to 9 × 1012 cm−2 are achieved. These charge densities are described as functions of electrical stress voltage, time, and the Al2O3 layer thickness between silicon and the HfO2 or the SiO2 interlayer. Despite the strong variation of trap sites, all stacks reach very good effective carrier lifetimes of up to 8 and 20 ms on p- and n-type silicon substrates, respectively. Controlling the trap sites in Al2O3 layers opens the possibility to engineer the field-effect passivation in the solar cells.

Influence of relaxation processes on the evaluation of the metastable defect density in Cu(In,Ga)Se2

In this contribution, we investigated by means of numerical simulations the influence of relaxation processes related to metastable defects on electrical characteristics of Cu(In,Ga)Se2. In particular, we analyzed the relaxation of a metastable state induced by illumination at a fixed temperature as well as the dependence of the hole concentration on the temperature during cooling. The knowledge of these two relaxation processes is crucial in the evaluation of the hole concentration in the relaxed state and after light soaking. We have shown that the distribution of the metastable defects can be considered frozen below 200 K. The hole capture cross section was estimated as ∼3 × 10−15 cm2. It was shown that the usually used cooling rates may lead to relevant changes of the hole concentration. We calculated the lower limit of the hole concentration after cooling, and we presented how it depends on densities of shallow acceptors and metastable defects. Moreover, we proposed a method which allows for the evaluation of shallow acceptor and metastable defect densities from two capacitance-voltage profiles measured in the relaxed and light soaking states. Finally, we indicated experimental conditions in which the influence of relaxation processes on the accuracy of this method is the smallest.

Reduced defect density at the CZTSSe/CdS interface by atomic layer deposition of Al2O3

The greatest challenge for improving the power conversion efficiency of Cu2ZnSn(S,Se)4 (CZTSSe)/CdS/ZnO thin film solar cells is increasing the open circuit voltage (VOC). Probable leading causes of the VOC deficit in state-of-the-art CZTSSe devices have been identified as bulk recombination, band tails, and the intertwined effects of CZTSSe/CdS band offset, interface defects, and interface recombination. In this work, we demonstrate the modification of the CZTSSe absorber/CdS buffer interface following the deposition of 1 nm-thick Al2O3 layers by atomic layer deposition (ALD) near room temperature. Capacitance-voltage profiling and quantum efficiency measurements reveal that ALD-Al2O3 interface modification reduces the density of acceptor-like states at the heterojunction resulting in reduced interface recombination and wider depletion width. Indications of increased VOC resulting from the modification of the heterojunction interface as a result of ALD-Al2O3 treatment are presented. These results, while not conclusive for application to state-of-the-art high efficiency CZTSSe devices, suggest the need for further studies as it is probable that interface recombination contributes to reduced VOC even in such devices.

P-Channel InGaN/GaN heterostructure metal-oxide-semiconductor field effect transistor based on polarization-induced two-dimensional hole gas.

The concept of p-channel InGaN/GaN heterostructure field effect transistor (FET) using a two-dimensional hole gas (2DHG) induced by polarization effect is demonstrated. The existence of 2DHG near the lower interface of InGaN/GaN heterostructure is verified by theoretical simulation and capacitance-voltage profiling. The metal-oxide-semiconductor FET (MOSFET) with Al2O3 gate dielectric shows a drain-source current density of 0.51 mA/mm at the gate voltage of −2 V and drain bias of −15 V, an ON/OFF ratio of two orders of magnitude and effective hole mobility of 10 cm2/Vs at room temperature. The normal operation of MOSFET without freeze-out at 8 K further proves that the p-channel behavior is originated from the polarization-induced 2DHG.

Investigation of Ion-Implanted Photosensitive Silicon Structures by Electrochemical Capacitance–Voltage Profiling

The method of electrochemical capacitance–voltage profiling is used to study boron-implanted silicon structures for CCD matrices with backside illumination. A series of specially prepared structures with different energies and doses of ion implantation and also with various materials used for the coating layers (aluminum, silicon oxide, and their combinations) is studied. The profiles of the depth distribution of majority charge carriers of the studied structures are obtained experimentally. Also, using the Poisson equation and the Fredholm equation of the first kind, the distributions of the charge-carrier concentration and of the electric field in the structures are calculated. On the basis of the analysis and comparison of theoretical and experimental concentration profiles, recommendations concerning optimization of the structures’ parameters in order to increase the value of the pulling field and decrease the effect of the surface potential on the transport of charge carriers are suggested.

Electrical mechanisms for carrier compensation in homoepitaxial nonpolar m-ZnO doped with nitrogen

By combining photoluminescence, capacitance-voltage profiling and deep level optical spectroscopy, the optical and electrical signatures of the deep levels induced by N in MBE-grown homoepitaxial m-ZnO layers are identified and correlated to different physical origins. The films are electrically compensated, with carrier concentrations that decrease from ∼1 · 1016 cm−3 to ∼2 · 1015 cm−3 as a result of increasing N incorporation. Regardless of the presence of N, an intrinsic trap is found in all films at EV + 0.25 eV, most likely related to VZn defects. More interestingly, N induces three new deep levels close to the valence band whose bandgap position is electrically observed to be at EV + 0.48 eV, EV + 0.17 eV and EV + 0.12 eV. The deepest trap at EV + 0.48 eV correlates well with a N-induced level observed in previous studies on both ZnMgO and ZnO films. The EV + 0.17 eV trap behaves as a minority (hole) carrier trap, and can be uniquely correlated with the acceptor level involved in the N-induced DAP emission observed in the photoluminescence spectra. Finally, the shallowest level at EV + 0.12 eV shows an electrical signature completely distinguishable from the EV + 0.17 eV level, and dominates the deep level spectra after N-incorporation with a trap concentration of ∼1.2 · 1015 cm−3.

Defects in solution-processed dithienylsilole-based small-molecule photovoltaic thin-films

DTS-(FBTTh2)2 is a prominent solution-processable small-molecule donor for donor-acceptor bulk-heterojunction organic photovoltaics. Power conversion efficiency of DTS-(FBTTh2)2 based photovoltaic devices exceeds 8%. This paper reports on the distribution of sub-bandgap trap states in DTS-(FBTTh2)2. Trap states were probed using admittance spectroscopy and low-frequency capacitance-voltage profiling and analyzed using established theoretical models. Three distributions were revealed in the trap density of states energy spectra. Key observations were (1) thicker solution-processed films with higher drying time had 55% less traps than thinner films that dried relative faster (2) blending of DTS-(FBTTh2)2 with the acceptor PC70BM introduced traps at the center of the donor-acceptor interfacial bandgap. Charge carrier dynamics in DTS-(FBTTh2)2 based thin-films was also characterized using impedance spectroscopy.

Fine‐Tuning the Sn Content in CZTSSe Thin Films to Achieve 10.8% Solar Cell Efficiency from Spray‐Deposited Water–Ethanol‐Based Colloidal Inks

Thin film solar cells with Al/ITO/ZnO/CdS/CZTSSe/Mo‐glass structure are fabricated employing a fast and low‐cost preparation procedure using an aqueous ink deposited by nonpyrolytic spray, followed by high temperature crystallization and selenization steps. Capacitance–voltage measurements on previously reported devices with >8% efficiency under 1 sun irradiation show a charge carrier density of the order of 1017 cm−3. Moreover, admittance spectroscopy indicates the presence of mid‐bandgap defects that are tentatively attributed to a Sn deficit in the film. In order to reduce the number of these deep defects within the active layer of our solar cells, the Sn content is tuned in the precursor ink. Their morphology, elemental composition, crystal phases, capacitance–voltage profiling, admittance, photoluminescence, and photovoltaic performances are characterized. The results indicate that tuning the Sn content offers a strong leverage upon some key properties of the active layer, in particular the grain size, and the charge carrier and defect density. By employing this leverage to optimize the performance of our CZTSSe layers, the cell performances are increased to 10.0% without antireflection coating (ARC) and to 10.8% (on 0.25 cm2) with an ARC.

The Influence of the n-Side Doping on Metastable Defect Concentrations in Cu(In,Ga)Se<sub>2</sub> Evaluated From Space Charge Profiles

In this study, we investigate, through numerical simulations, the influence of the n-side of the Cu(In,Ga)Se2 (CIGS)-based photovoltaic devices on capacitance space charge profiles, with a special emphasis on evaluating metastable defect concentrations. We calculated that for plausible doping level of the CdS buffer ranging from 1016 to 1018 cm−3, the values of hole concentrations in the absorber delivered by capacitance can be underestimated, and the ZnO/CdS/CIGS devices should not be treated a priori as n+-p junctions. We showed how this effect can influence the values of metastable defect concentrations extracted from capacitance–voltage (CV) profiles. We indicated that the apparent nonuniformity of experimental CV profiles might be due to an incomplete depletion of the buffer layer. Based on our results, we proposed experiments that can help unveil the correct shallow acceptor and metastable defect concentration in CIGS.

Formation of GaAs and Ga1−xAlxAs (0 ≤ x ≤ 0.3) layers on GaAs (111)A substrate by organometallic vapor phase epitaxy

The GaAs and Ga1−xAlxAs (0 ≤ x ≤ 0.3) epitaxial growth on GaAs (111)A substrate was carried out by organometallic vapor phase epitaxy at total pressure ∼70 Torr. The regime of quantitative modulation of trimethyl gallium, Ga(CH3)3, gas flow was applied to optimize the atomic ratio between AIII and BV, while the tangential and normal parameters of the growth rate were comparable. Deposited GaAs and Ga1−xAlxAs (0 ≤ x ≤ 0.3) layers with crystallographic orientation of (111)A were obtained at ∼0.1–0.2 Torr of arsine, AsH3, pressure and low crystallization temperature ∼570–620 °C. Proposed optimization of the BV/AIII relationship and reaction conditions resulted in a smooth surface of the deposited layers. Using the technological approach, a si-GaAs/n-GaAs:Si/p-GaAlAs:Zn/p+–GaAs:Zn hetero-structure was successfully synthesized. Structural and electrical properties of the prepared epitaxial structure were studied by high-resolution x-ray diffraction and electrochemical capacitance-voltage profiling. Obtained experimental results confirmed excellent crystal and interfacial quality as well as steep transitions in charge carrier concentration through the deposited layers.

Evidence for an iron-hydrogen complex in p-type silicon

Interactions of hydrogen with iron have been studied in Fe contaminated p-type Czochralski silicon using capacitance-voltage profiling and deep level transient spectroscopy (DLTS). Hydrogen has been introduced into the samples from a silicon nitride layer grown by plasma enhanced chemical vapor deposition. After annealing of the Schottky diodes on Si:Fe + H samples under reverse bias in the temperature range of 90–120 °C, a trap has been observed in the DLTS spectra which we have assigned to a Fe-H complex. The trap is only observed when a high concentration of hydrogen is present in the near surface region. The trap concentration is higher in samples with a higher concentration of single interstitial Fe atoms. The defect has a deep donor level at Ev + 0.31 eV. Direct measurements of capture cross section of holes have shown that the capture cross section is not temperature dependent and its value is 5.2 × 10−17 cm2. It is found from an isochronal annealing study that the Fe-H complex is not very stable and can be eliminated completely by annealing for 30 min at 125 °C.

Extremely Low-Threshold Current Density InGaAs/AlGaAs Quantum-Well Lasers on Silicon

An InGaAs/AlGaAs quantum-well laser structure was grown on a silicon substrate by adopting a three-step grown thin (1.8 μm) and simple buffer layer. The whole structure was grown by metalorganic chemical vapor deposition. The material quality was characterized by transmission electron microscopy, photoluminescence spectra, and electrochemical capacitance–voltage profiler. It shows that the threading dislocation density and interface roughness are effectively reduced, and the active region's optical properties grown on a silicon substrate are comparable to that grown on a GaAs substrate. Broad stripe lasers have also been fabricated. The cavity length and stripe width are 1 mm and 15 μm, respectively. An extremely low-threshold current density of 313 A/cm 2 has been achieved under pulsed condition at room temperature. Meanwhile, the laser can operate under continuous wave condition at the temperature of 240 K. The above results make us more confident for realizing better performance of Si-based III–V semiconductor lasers by further improvement of the material growth method.

Interfacial Charge Induced Magnetoelectric Coupling at BiFeO₃/BaTiO₃ Bilayer Interface.

Bilayer thin films of BiFeO3-BaTiO3 at different thicknesses of BiFeO3 were prepared by RF-magnetron sputtering technique. A pure phase polycrystalline growth of thin films was confirmed from XRD results. Significantly improved ferroelectric polarization (2Pr ∼ 30 μC/cm(2)) and magnetic moment (Ms ∼ 33 emu/cc) were observed at room temperature. Effect of ferroelectric polarization on current conduction across the interface has been explored. Accumulation and depletion of charges at the bilayer interface were analyzed by current-voltage measurements which were further confirmed from hysteretic dynamic resistance and capacitance voltage profiles. Magnetoelectric coupling due to induced charges at grain boundaries of bilayer interface was further investigated by room temperature magnetocapacitance analysis. A room temperature magnetocapacitance was found to originate from induced charge at the bilayer interface which can be manipulated by varying the thickness of BFO to obtain higher ME coupling coefficient. Dynamic magnetoelectric coupling was investigated, and maximum longitudinal magnetoelectric coupling was observed to be 61 mV/cm·Oe at 50 nm thickness of BiFeO3. The observed magnetoelectric properties are potentially useful for novel room temperature magnetoelectric and spintronic device applications.

Enhancement of two dimensional electron gas concentrations due to Si3N4 passivation on Al0.3Ga0.7N/GaN heterostructure: strain and interface capacitance analysis

Enhancement of two dimensional electron gas (2DEG) concentrations at Al0.3Ga0.7N/GaN hetero interface after a-Si3N4 (SiN) passivation has been investigated from non-destructive High Resolution X-ray Diffraction (HRXRD) analysis, depletion depth and capacitance-voltage (C-V) profile measurement. The crystalline quality and strained in-plane lattice parameters of Al0.3Ga0.7N and GaN were evaluated from double axis (002) symmetric (ω-2θ) diffraction scan and double axis (105) asymmetric reciprocal space mapping (DA RSM) which revealed that the tensile strain of the Al0.3Ga0.7N layer increased by 15.6% after SiN passivation. In accordance with the predictions from theoretical solution of Schrödinger-Poisson’s equations, both electrochemical capacitance voltage (ECV) depletion depth profile and C-V characteristics analyses were performed which implied effective 9.5% increase in 2DEG carrier density after passivation. The enhancement of polarization charges results from increased tensile strain in the Al0.3Ga0.7N layer and also due to the decreased surface states at the interface of SiN/Al0.3Ga0.7N layer, effectively improving the carrier confinement at the interface.