Capacitance-voltage Profiling Research Articles

Carrier density distribution in AlGaAs/GaAs superlattices with different numbers of quantum wells determined by capacitance-voltage profiling

The charge carrier density distribution in uniformly doped AlGaAs/GaAs superlattices with layer thicknesses of 1.5/10 nm and different numbers of quantum wells was studied. Both energy band theory and capacitance–voltage profiling were used to determine the carrier concentration profiles of the structures. During the analysis of the experimental and simulated capacitance–voltage characteristics, it was found that the maximum electron concentrations increase with an increase in the number of quantum wells from ∼ 7.1⋅1016cm−3 for three wells to ∼ 9.3⋅1016cm−3 for 25 wells with an overall superlattice doping level of ∼ 1017cm−3.

Editors’ Choice—Structural, Electrical, and Luminescent Properties of Orthorhombic κ-Ga2O3 Grown by Epitaxial Lateral Overgrowth

The properties of orthorhombic κ-Ga2O3 films grown by Epitaxial Lateral Overgrowth (ELOG) were studied by Scanning Transmission Electron Microscopy (STEM), X-ray diffraction, capacitance-voltage profiling, Microcathodoluminescence (MCL) spectroscopy and imaging. ELOG mask was formed by deposition of SiO2 stripes on TiO2 buffer prepared on basal plane sapphire, with the stripes going along the [110] direction of sapphire. κ-Ga2O3 ELOG growth was performed using Halide Vapor Phase Epitaxy (HVPE), with ELOG wing of the structure formed by lateral overgrowth over the 20 μm-wide SiO2 stripes, while growth in between the stripes proceeded initially by vertical growth in the 5-μm-wide windows. TEM analysis showed that the material in the windows comprised 120o rotational nanodomains typical of κ-Ga2O3, while, in the wing regions, the material was single-domain monocrystalline. The films were conducting, with the net donor density close to 1013 cm−3. The data suggested the material in the windows have much higher resistance than in the wings. MCL spectra and imaging revealed much higher density of nonradiative recombination centers in the windows than in the wings.

Multicrystalline, Highly Oriented Thick‐Film Silicon from Reduction of Soda‐Lime Glass

AbstractThe study describes synthesis and characterization of > 10 µm thick multicrystalline (mc), highly oriented, p‐doped silicon layers by aluminothermic reduction of low‐cost soda‐lime glass. X‐ray diffraction shows a highly preferred (111)‐orientation and excellent crystallinity. Low compressive stress and very good crystallinity are confirmed by the peak position and width of the Raman LO‐phonon line, approaching the one of bulk single‐crystalline wafer material. Due to strong bonding to the glass substrate layer, spalling is not observed. A conductive aluminum‐rich oxide layer is formed underneath the silicon, serving as an electrical back‐contact for electronic devices. Using secondary ion mass spectrometry very low concentrations of 1014–1015 cm−3 of impurities are found originating from the soda‐lime glass with an iron content below the detection limit. Furthermore, a plateau‐like, very homogenous Al concentration of ≈4 × 1018 cm−3 over a thickness of ≈10 µm is found, which corresponds to the solubility of Al in Si at the process temperature. Complete electronic activation within the plateau region is confirmed by carrier concentration measurements using electrochemical capacitance–voltage profiling and Raman spectroscopy. Hole concentrations in the range of few 1018 cm−3 are beneficial for the p‐type base material of full‐emitter cell mc‐silicon photovoltaic devices.

Unintentional p-type conductivity in intrinsic Ge-rich SiGe/Ge heterostructures grown on Si(001)

In this work, we investigate the effective background charge density in intrinsic Si0.06Ge0.94/Ge plastically relaxed heterostructures deposited on Si(001). Hall effect measurements and capacitance–voltage profiling reveal a p-type conductivity in the nominally intrinsic layer with a hole concentration in the mid 1015 cm−3 range at temperatures between 50 and 200 K. In view of the carrier freeze out that we observe below 50 K, we attribute the origin of these carriers to the ionization of shallow acceptor-like defect states above the valence band. In addition, one dominant hole trap located at mid-gap position is found by deep level transient spectroscopy. Carrier trapping kinetics measurements can be interpreted as due to a combination of point defects, likely trapped in the strain field of extended defects, i.e., the threading dislocation.

Ion energy dependence of dry etch damage depth in Ga2O3 Schottky rectifiers

The β-polytype of Ga2O3 has a high critical electric field strength (∼8 MV.cm−1) and can be grown as large diameter (6 in.) single crystal boules by inexpensive melt-growth techniques. This makes it promising for next generation power electronics and solar-blind UV photodetectors. Since wet etching is not feasible for patterning such devices due to the chemical inertness of Ga2O3, attention is focused on dry etching, with the attendant issue of surface modification due to chemical or ion-induced changes. In this work, we demonstrate that dry etch damage in β-Ga2O3 is manifested by a reduction in near-surface carrier concentration, possibly due to the introduction of Gav acceptor states. The depth to which the carrier density is affected depends on the interplay between ion energy and etch rate, with faster etch rates reducing the depth of the remaining damaged region. The maximum damage depth observed experimentally from capacitance-voltage profiling on Schottky rectifiers was ∼ 110 nm, well beyond the range of the ions in the Inductively Coupled Plasmas and emphasizing that rapid diffusion of point defects occurs into the sample during the etch step. Using Schottky rectifier structures as a platform to understand the effect of dry etch damage on device parameters, we found that on-state resistance was more affected than ideality factor or breakdown voltage, with a large increase (5x) for ion energies ≥ 325 eV. Reverse current density characteristics showed the presence of trap-assisted space-charge-limited conduction in the damaged layers. The implications for high power rectifier fabrication are discussed.

Transport and trap states in proton irradiated ultra-thick κ-Ga2O3

Changes induced by irradiation with 1.1 MeV protons in the transport properties and deep trap spectra of thick (>80 μm) undoped κ-Ga2O3 layers grown on sapphire are reported. Prior to irradiation, the films had a donor concentration of ∼1015 cm−3, with the two dominant donors having ionization energies of 0.25 and 0.15 eV, respectively. The main electron traps were located at Ec−0.7 eV. Deep acceptor spectra measured by capacitance-voltage profiling under illumination showed optical ionization thresholds near 2, 2.8, and 3.4 eV. The diffusion length of nonequilibrium charge carriers for ɛ-Ga2O3 was 70 ± 5 nm prior to irradiation. After irradiation with 1.1 MeV protons to a fluence of 1014 cm−2, there was total depletion of mobile charge carriers in the top 4.5 μm of the film, close to the estimated proton range. The carrier removal rate was 10–20 cm−1, a factor of 5–10 lower than in β-Ga2O3, while the concentration of deep acceptors in the lower half of the bandgap and the diffusion length showed no significant change.

Enhancing the electron mobility in Si-doped (010) β-Ga2O3 films with low-temperature buffer layers

We demonstrate a new substrate cleaning and buffer growth scheme in β-Ga2O3 epitaxial thin films using metal–organic vapor phase epitaxy (MOVPE). For the channel structure, a low-temperature (LT, 600 °C) un-doped Ga2O3 buffer was grown, followed by a transition layer to a high-temperature (HT, 810 °C) Si-doped Ga2O3 channel layers without growth interruption. The (010) Ga2O3 Fe-doped substrate cleaning uses solvent cleaning, followed by additional hydrofluoric acid (49% in water) treatment for 30 min before the epilayer growth. This step is shown to compensate the parasitic Si channel at the epilayer–substrate interface that originates from the substrate polishing process or contamination from the ambient. From secondary ion mass spectroscopy (SIMS) analysis, the Si peak atomic density at the substrate interface is found to be several times lower than the Fe atomic density in the substrate—indicating full compensation. The elimination of the parasitic electron channel at the epi–substrate interface was also verified by electrical (capacitance–voltage profiling) measurements. In the LT-grown (600 °C) buffer layers, it is seen that the Fe forward decay tail from the substrate is very sharp, with a decay rate of ∼9 nm/dec. X-ray off-axis rocking curve ω-scans show very narrow full width at half maximum (FWHM) values, similar to the as-received substrates. These channels show record high electron mobility in the range of 196–85 cm2/V⋅s in unintentionally doped and Si-doped films in the doping range of 2 × 1016–1 × 1020 cm−3. Si delta-doped channels were also grown utilizing this substrate cleaning and the hybrid LT buffers. Record high electron Hall mobility of 110 cm2/V⋅s was measured for sheet charge density of 9.2 × 1012 cm−2. This substrate cleaning, combined with the LT buffer scheme, shows the potential of designing Si-doped β-Ga2O3 channels with exceptional transport properties for high-performance Ga2O3-based electron devices.

Study of InP/GaP quantum wells grown by vapor phase epitaxy

We consider structure with single quantum well InP 5 nm thick grown by vapor phase epitaxy on n-GaP wafer. By classical capacitance-voltage profiling of Schottky diode on sample and electrochemical profiling, electron accumulation were detected in InP layer so existence of quantum well is confirmed. Results of admittance spectroscopy and deep-level transient spectroscopy showed defect formation in GaP layers above InP with energy position of Ec-0.21 eV, 0.30 eV and 0.93 eV. Keywords: quantum well, capacitance-voltage profiling, electrochemical profiling.

Исследование квантовых ям InP/GaP, полученных методом газофазной эпитаксии

We consider structure with single quantum well InP 5 nm thick grown by vapor phase epitaxy on n-GaP wafer. By classical capacitance-voltage profiling of Schottky diode on sample and electrochemical profiling, electron accumulation were detected in InP layer so existence of quantum well is confirmed. Results of admittance spectroscopy and deep-level transient spectroscopy showed defect formation in GaP layers above InP with energy position of Ec-0.21 eV, 0.30 eV и 0.93 eV.

Determination of the electron distribution in thin barrier AlGaAs/GaAs superlattices by capacitance-voltage profiling

Electron density distribution in uniformly doped AlGaAs/GaAs superlattices with respective layer thicknesses 1.5/10 nm and a different number of quantum wells was investigated. Experimental samples containing 3, 5 and 25 periods with the same layer parameters were grown by molecular beam epitaxy. Capacitance-voltage profiling was used to determine the carrier concentration profiles in the structures both numerically and experimentally. During the analysis of experimental capacitance-voltage characteristics it was found that the maximum electron concentration increases with an increase in the number of quantum wells starting from 7,1∙1016 сm–3 for 3 wells up to 9,2∙1016 сm–3 for 25 wells with overall superlattice doping level of 1017 сm–3. In some samples saturation areas are observed on the concentration profiles, that are associated with the region of superlattice. Concentration values, obtained from computer modeling, correspond to the experimental data with an error of less than 10 %. Capacitance-voltage profiling is a suitable technique for determining the carrier concentration profiles in thin barrier superlattices. Despite the fact that the method provides distribution of the “apparent” carrier concentration profile, it can be used to estimate the dopant atoms distribution in the strongly coupled quantum well heterostructures.

Effects of Different Particle-Sized Al Powders on Sintering Properties of Aluminum Paste in Crystalline Silicon Solar Cell

In this paper, the effects of particle size difference of aluminum (Al) powder on physical properties of Al powder and sintering properties of Al paste were investigated. For this purpose, respectively, Al powders with four mean particle sizes were used, which were obtained by two classifications using the same nitrogen atomization process. Thermal analysis results showed that the smaller the Al particle size, the lower the oxidation temperature and the higher the reaction enthalpy, indicating higher reactivity of the Al powder. Four Al powders were prepared into Al paste by the same process, and the resistances of the paste increased with the decrease of the particle size of the aluminum powder. Electrochemical capacitance-voltage profiler (ECV) and scanning electron microscope (SEM) analysis showed that the smaller the Al particle size, the thicker the back surface field (BSF) and the higher the doping concentration of BSF. It was found that the smaller size of Al powder limited the migration of silicon, which resulted in higher concentration of silicon (Si) in the Al-Si liquid phase. This leads to reaching a balance at a higher temperature between the recrystallization of silicon from the alloy liquid and the dissolution of silicon in the liquid alloy and a higher doping concentration and a higher effective BSF doping thickness. Results of the study will provide reference for further study and application of Al paste.

Evidence of reduced interface states in Al2O3/AlGaN MIS structures via insertion of ex situ regrown AlGaN layer

We report on the impact of the 3 nm thick ex situ AlGaN regrown layer prior to insulator deposition on the interfacial properties of Al2O3/AlGaN/GaN metal–insulator–semiconductor (MIS) structures. MIS-capacitors (MIScaps) with regrown AlGaN layer exhibited anomalously excessive threshold voltage shift compared to reference sample without regrown AlGaN, suggesting highly reduced interface states density (D it). Moreover, MIScaps with regrown AlGaN layer exhibited “spill-over” in the capacitance–voltage profiles, further evidencing the improved Al2O3/AlGaN interfaces. Fabricated three-terminal MIS-HEMTs with regrown AlGaN showed less hysteresis in transfer curves, enhanced maximum drain current, and increased linearity over the reference device.

Degradation by sidewall recombination centers in GaN blue micro-LEDs at diameters

A matrix of blue GaN/InGaN multi-quantum-well (MQW) micro-Light-Emitting Diodes (micro-LEDs) with diode dimensions ranging from 2 to 100 µm was prepared by masked dry etching and characterized by Photoluminescence (PL), Microcathodoluminescence (MCL), capacitance-voltage profiling in the dark and under monochromatic illumination, current-voltage measurements, admittance spectra, Deep Level Transient Spectroscopy with electrical (DLTS) and optical (ODLTS) injection. The changes observed in the PL, MCL spectra are due to the formation of deep hole traps at Ev+ 0.75 eV and electron traps at Ec-1 eV in the sidewalls of the micro-LEDs. The former give rise to the red defect band peaked near 600 nm and contribute to the increase of tunneling and leakage current with decreasing the diode diameter. The latter are prominent centers of nonradiative recombination.

Local capacitance-voltage profiling and deep level transient spectroscopy of SiO2/SiC interfaces by scanning nonlinear dielectric microscopy

We extend our local capacitance-voltage profiling method for the real-space nanoscale investigation of the SiO2/SiC interface, which is still an issue in the application of SiC to power electronics devices. Our technique is based on time-resolved scanning nonlinear dielectric microscopy and can be used for collecting local capacitance-voltage and its voltage-derivative profiles at each measurement point. This technique can also be combined with local deep level transient spectroscopy for simultaneously imaging the spatial fluctuations of interface defect density in real-space. Here we use the proposed technique to measure SiC wafers with a thermal oxide layer and investigate the effect of interface nitridation. From the data volume obtained by our technique, various images are extracted and their correlation is analyzed to characterize and discuss the spatial fluctuations and hysteresis of the local capacitance-voltage profiles. As expected, the nitridation of the SiO2/SiC interface after thermal oxidation is effective for the reduction of hysteresis and fluctuations in local capacitance-voltage profiles. However, significant spatial fluctuations remain even after the nitridation treatment, which suggests the SiO2/SiC interface has intrinsic non-uniformity generating high surface potential fluctuations.

Surface Potential Fluctuations of SiO<sub>2</sub>/SiC Interfaces Investigated by Local Capacitance-Voltage Profiling Based on Time-Resolved Scanning Nonlinear Dielectric Microscopy

We investigate surface potential fluctuations on SiO2/SiC interfaces by local capacitance-voltage profiling based on time-resolved scanning nonlinear dielectric microscopy. As experimental indicators of surface potential fluctuations, we measured the spatial fluctuations of local capacitance-voltage and its first derivative profiles through the detection of the voltages at the infection points of the profiles. We show that, even for a sample with a nitrided interface with low interface defect density, the fluctuations of the measured voltages are much higher than the thermal energy at room temperature. This indicates the existence of high potential fluctuations, which can give the significant impacts on the carrier transport at the SiO2/SiC interface of SiC metal-oxide-semiconductor field effect transistors.

1 GeV proton damage in β-Ga2O3

The changes of electrical properties and deep trap spectra induced in n-type β-Ga2O3 by 1 GeV protons with a fluence of 4 × 1013 cm−2 were studied. The carrier removal rates were ∼100 cm−1 at this energy. For comparison, for 20 MeV proton irradiation at comparable fluences (5–10 × 1014 cm−2), the removal rate was ∼400 cm−1 for donor concentrations of 3 × 1016 cm−3 and ∼100 cm−1 for concentrations of <1016 cm−3. These removal rates were in stark contrast with modeling results that predicted the introduction rates of vacancies to be two orders of magnitude higher for 20 MeV protons. Measurements of deep electron and hole traps densities by deep level transient spectroscopy with electrical or optical injection (DLTS or ODLTS), and capacitance–voltage profiling under monochromatic light illumination showed that the 1 GeV proton irradiation resulted in the introduction of deep donors E2*(Ec-0.75 eV) and E3 (Ec-1 eV) and deep acceptors with optical ionization threshold near 2.3 eV producing a feature near 250 K in ODLTS and 3.1 eV with related ODLTS feature near 450 K. The total concentration of all deep traps was much lower than that necessary to explain the observed decrease in net donor density upon irradiation. The donor densities showed a nonuniform distribution in proton irradiated films with the concentration greatly decreased toward the surface. Possible reasons for the observed performance are discussed.

Local capacitance-voltage profiling and high voltage stress effect study of SiO2/SiC structures by time-resolved scanning nonlinear dielectric microscopy

We investigate SiO2/SiC structures by a nanoscale capacitance-voltage (CV) profiling method based on time-resolved scanning nonlinear dielectric microscopy (tr-SNDM). Owing to the high sensitivity to the variation in the tip-sample capacitance, wide bandwidth of the measurement, and high flexibility of offline processing, tr-SNDM permits the more accurate acquisition of the microscopic CV characteristics localized below the scanning tip. We demonstrate that tr-SNDM based local CV profiling is able to distinguish the different features of untreated and nitrided SiO2/SiC structures and image intrinsic spatial non-uniformity of the interfaces. We also investigate the nanoscale impacts of high voltage stress on the SiO2/SiC structures. By inspecting the local CV characteristics under high voltage stress, we suggest that the interface defect density locally increases during the stress application.

Nanoscale capacitance-voltage profiling of DC bias induced stress on a high-κ/SiO2/Si gate stack

We investigate DC bias stress effects on a HfOx/SiO2/p-Si gate stack using time-resolved scanning nonlinear dielectric microscopy. Our microscopy permits localized capacitance-voltage profiling and deep level transient spectroscopy on a particular area stressed prior to the measurement. We observed the increase of interface defect density and the drastic changes of local capacitance-voltage characteristics on the stressed area. Our numerical simulations explain that both fixed charges and interface defect density increased by the DC bias stress and caused the change of local characteristics. Because local capacitance-voltage characteristics is particularly important in device performance, time-resolved scanning nonlinear dielectric microscopy will be useful for the evaluation of local stress effects on miniaturized Si devices such as bias temperature instability.

Analysis of grain orientation and defects in Sb2Se3 solar cells fabricated by close-spaced sublimation

The performance of a superstrate TiO2/Sb2Se3 solar cell, fabricated by close-spaced sublimation technique (CSS), was improved after the deployment of a seed layer. The seed layer caused columnar Sb2Se3 film growth with texture coefficient analysis (TC) showing increased presence of crystal planes, which are inclined towards the [001] crystal direction. Given the highly anisotropic properties of Sb2Se3, preferential growth of (Sb4Se6)n ribbons along the [001] direction is best suited for effective charge collection. Hence, grain orientation of Sb2Se3 films was studied more closely via measurement of pole figures by XRD and orientation distribution maps by electron backscatter diffraction (EBSD). Although the measurements did not reveal strong preferred orientation, it was observed that the columnar Sb2Se3 growth enhanced texture along the [001] direction. Temperature-dependent admittance spectroscopy (TAS) and capacitance–voltage (CV) profiling were performed on the seed-assisted TiO2/Sb2Se3 solar cell to evaluate carrier density and deep defects in the Sb2Se3 absorber. TAS study revealed a deep defect with activation energy of 0.39 eV. CV profiles indicated that the density of defects could be as high as 1017 cm−3, which needs to be addressed by post-deposition treatments.

Enabling Solar Hydrogen Production over Selenium: Surface State Passivation and Cocatalyst Decoration

As the first material ever used in a solid-state solar cell, selenium (Se) has outstanding merits of light absorption, carrier mobility, intrinsic environmental stability, and straightforward film preparation, making it an attractive light-absorbing semiconductor for photovoltaic applications. However, the exploration of selenium for photoelectrochemical (PEC) cells remains vastly unreported. Here, we successfully enable selenium as an absorber in the photocathode for solar hydrogen production. A rapid thermal annealing process is adopted to prepare a tetragonal Se film with a preferred [100] orientation. Using a thin layer of TiO2 buffer and platinum decoration, a photocurrent density up to −7.2 mA/cm2 is achieved over the optimal Se/TiO2/Pt photocathode at 0 VRHE under simulated one sun illumination. The roles of surface state passivation by TiO2 and cocatalyst decoration by Pt in charge separation and transfer were thoroughly investigated by capacitance–voltage profiling, photoassisted Kelvin probe force microscopy, photoelectrochemical impedance spectroscopy, and also transient photovoltage decay. Our work provides Se as a promising candidate for solar hydrogen production and also highlights the effectiveness of surface state passivation and cocatalyst decoration in the fabrication of efficient PEC devices.