High-resolution Mobility Spectrum Research Articles

Inversion layer electron mobility distribution in fully-depleted silicon-on-insulator MOSFETs

Magnetic-field dependent geometrical magnetoresistance measurements and high-resolution mobility spectrum analysis have been employed to characterise the inversion-layer electron mobility distribution in fully-depleted silicon-on-insulator MOS transistors. The results reveal that the room-temperature electron population in the 12 nm thick Si channel layer is characterised by a broad electron mobility distribution that significantly departs from the ideal single-value discrete carrier approximation. At temperatures below 60 K, the linewidth of the distribution is shown to become significantly narrower, approaching a delta-like function at 30 K. Self-consistent Schrödinger-Poisson numerical calculations suggest that, since the total carrier population at 295 K is comprised of electrons in the ground-state level of the two subband ladders, intervalley scattering and phonon scattering will significantly influence the linewidth of the mobility distribution in the transistor channel. To the best of our knowledge, electron mobility distributions in Si-based devices have not been previously studied nor resolved in such detail.

Instant acquisition of high resolution mobility spectra in a differential mobility analyzer with 100 independent ion collectors: Instrument calibration

A parallel plate differential mobility analyzer (DMA) having 100 independent current collectors is calibrated to relate the axial distances Ln between the inlet slit and the detector position to th...

Vertical carrier transport in strain-balanced InAs/InAsSb type-II superlattice material

Anisotropic carrier transport properties of unintentionally doped InAs/InAs0.65Sb0.35 type-II strain-balanced superlattice material are evaluated using temperature- and field-dependent magnetotransport measurements performed in the vertical direction on a substrate-removed metal-semiconductor-metal device structure. To best isolate the measured transport to the superlattice, device fabrication entails flip-chip bonding and backside device processing to remove the substrate material and deposit contact metal directly to the bottom of an etched mesa. High-resolution mobility spectrum analysis is used to calculate the conductance contribution and corrected mixed vertical-lateral mobility of the two carrier species present. Combining the latter with lateral mobility results from in-plane magnetotransport measurements on identical superlattice material allows for the calculation of the true vertical majority electron and minority hole mobilities; amplitudes of 4.7 ×103 cm2/V s and 1.60 cm2/V s are determined at 77 K, respectively. The temperature-dependent results show that vertical hole mobility rapidly decreases with decreasing temperature due to trap-induced localization and then hopping transport, whereas vertical electron mobility appears phonon scattering-limited at high temperature, giving way to interface roughness scattering at low temperatures, analogous to the lateral electron mobility but with a lower overall magnitude.

Fast high-resolution nanoDMA measurements with a 25 ms response time electrometer

Two fast electrometer circuits (1011 and 1012 V/A) are installed in a Faraday cage having a relatively small residence time. Removing readily distinguishable occasional spikes, the root mean square (r.m.s.) noise level at 1012 V/A is 0.11 fA when acquiring data at 1 Hz. This value is close to the expected thermal resistor noise at room temperature (0.09 mV). Both electrometers exhibit a 20 ms flow-related delay, followed by respective half-height rise-times of ∼4 and 25 ms. Fast high-resolution mobility spectra in the 1–2 nm size range are acquired with electrosprayed tetraheptylammonium ions by combining these electrometers with a high-speed DMA. At 1012 V/A, there is no ion mobility peak distortion when acquiring data with discrete voltage steps and dwelling 100 ms at each voltage. With the 1011 V/A electrometer, the DMA voltage VDMA is continuously swept up and down over 600 V in a triangular wave, at up to 1200 V/s. A shift ΔVDMA in the peak center is apparent, with little peak shape distortion. ΔVDMA is symmetric with respect to up or down sweep, and linear with sweep frequency, corresponding approximately to a pure delay Δt = 25 ms. This peak displacement may be offset by adding the correction ΔVDMA = Δt (dVDMA/dt) to the measured peak voltage. Extrapolating the measurements made here over a mobility range Zmax/Zmin of 4 to a much wider mobility range of 300 typical of aerosol studies, we conclude that almost undistorted high-resolution mobility spectra may be acquired in 1.3 s.Copyright © 2017 American Association for Aerosol Research

High-resolution mobility spectrum analysis of magnetoresistance in fully-depleted silicon-on-insulator MOSFETs

Multi-carrier transport in planar fully-depleted silicon-on-insulator (FD-SOI) MOSFETs has been investigated employing magnetic-field dependent geometrical magnetoresistance measurements and high-resolution mobility spectrum analysis. The results indicate that electronic transport in the 10nm thick Si channel layer is due to two distinct and well-defined electron species. Although self-consistent Schrödinger–Poisson numerical calculations indicate significant localization of the total electron population near the back and front interfaces, the results of mobility spectrum analysis suggest that the mobility distributions associated with these spatially localized populations are strongly coupled through carrier scattering processes, and do not have independent and distinguishable mobility distributions. The two detected electron mobility distributions are thus evidence of sub-band modulated transport in 10-nm thick Si planar FD-SOI MOSFETs. The mobility maximum of the dominant carrier was found to occur under gate bias conditions that result in a minimum perpendicular effective electric field.

Hall-effect characterization of electron transport at SiO2/4H-SiC MOS interfaces

Magnetic-field dependent resistivity and Hall-effect measurements combined with high resolution mobility spectrum analysis (HR-MSA) were employed to study room-temperature electronic transport in 4H-SiC metal–oxide–semiconductor field-effect transistor (MOSFET) structures. It is shown that the mobility distribution for electrons at the SiO2/SiC interface is significantly broader than expected for quantum confined carriers, exhibiting Hall scattering factors significantly greater than the generally assumed unity value. The interfacial electron mobility and Hall scattering factor are likely to be determined by potential fluctuations arising from a disordered transition layer on the SiC side of the SiO2/SiC interface. For the MOSFET structures studied, charge trapping at the SiO2/SiC interface was found to determine the interfacial free electron sheet density, in agreement with prior studies on similar device structures. HR-MSA has enabled unambiguous discrimination between electrons in the ion-implanted buried channel layer and at SiO2/SiC interface in a depletion-mode MOSFET structure.

Heavy and light hole transport in nominally undoped GaSb substrates

In this work, we report results of a study of electronic transport in nominally undoped p-type GaSb wafers typically employed as substrate material for the epitaxial growth of InAs/GaInSb type-II superlattices. Magnetic field dependent Hall-effect measurements and high-resolution mobility spectrum analysis clearly indicate p-type conductivity due to carriers in both the heavy and light hole bands. The extracted hole concentrations indicate a thermal activation energy of 17.8 meV for the dominant native acceptor-like defects. A temperature-independent effective mass ratio of 9.0 ± 0.8 was determined from the ratio of measured heavy and light hole concentrations. Over the 56 K–300 K temperature range, the light hole mobility was found to be 4.7 ± 0.7 times higher than the heavy hole mobility. The measured room temperature mobilities for the light and heavy holes were 2550 cm2/Vs and 520 cm2/Vs, respectively.

Evidence of Sub-Band Modulated Transport in Planar Fully Depleted Silicon-on-Insulator MOSFETs

Modulation of the sub-band electron population in the inversion channel of 10-nm planar fully depleted silicon-on-insulator MOSFETs is evidenced by the bias dependence of inversion layer transport parameters. Two distinct inversion-layer electron species were detected by magnetic-field-dependent magnetoresistance measurements and high-resolution mobility spectrum analysis. According to self-consistent Poisson-Schrodinger calculations, these species correspond to carriers in distinct sub-bands within the Si channel region. The mobility peak of the carrier with the highest sheet density occurs under gate bias conditions that result in a minimum perpendicular effective electric field.

Indoor and outdoor atmospheric ion mobility spectra, diurnal variation, and relationship with meteorological parameters

We report measurements of small ion mobility spectra and concentrations made over a 5 month period using a Gerdien-type ion mobility spectrometer at one indoor site in central Bristol, UK, and one indoor and one outdoor site in a semirural area. We assess the diurnal variation in ion concentration and mobility, variability with meteorological parameters, and features within the ion mobility spectra. Ion concentration (typically in the range 100–1000 cm−3) was highest overnight and lowest in late morning and afternoon. Total conductivity also followed this trend suggesting that small ions contribute more to conductivity than larger charged particles, although at the semirural site, indoor activity produced charged aerosol intermittently, while infiltration of outdoor aerosol contributed at the urban site. Negative ion mobility (typically 1.6–1.9 cm2 V−1 s−1) was higher overnight than at other times, while positive ion mobility (typically 1.2–1.4 cm2 V−1 s−1) showed no diurnal variability. In general, temperature, pressure, wind speed, and water vapor pressure were inversely related to both concentration and mobility, except outdoors when (negative) mobility increased with increasing temperature and pressure, while increased relative humidity was associated with higher ion mobilities and concentrations. Analysis of high-resolution mobility spectra revealed several mobilities for each polarity at which peaks were identified across all sites, suggesting similarity in composition but with varying importance at each site. Intermediate ions (mobilities 0.2–0.3 cm2 V−1 s−1) were observed at the outdoor site, but individual nucleation events which may be associated with them were not identifiable.

Mobility spectrum analysis of carrier transport at insulator/semiconductor interfaces

Display Omitted HighlightsMobility spectrum analysis (MSA) is a powerful tool to determine accurate transport parameters.MSA provides information about carrier mobility distributions.High resolution MSA stimulating progress into the modelling carrier mobility distributions. In this work, we review the results of mobility spectrum analysis (MSA) based studies of electronic transport in GaN-based high electron mobility field effect transistor (HEMT) structures, including recent progress in the modelling of mobility distributions and carrier scattering in two-dimensional electron gases in AlGaN/GaN HEMTs. It is shown that the MSA approach can provide crucial information for epitaxial growth process optimisation, since it allows identification of parasitic conduction channels, and can also yield greater insight into the fundamental scattering mechanism and electronic transport phenomena in two-dimensional inversion and accumulation layers. Furthermore, the availability of high quality insulator/semiconductor-like heterostructures, wherefrom high resolution mobility spectra can be obtained, is stimulating progress into the modelling and simulation of carrier mobility spectra and mobility distributions.

Electron Transport in InAsSb-Based nBn Photodetector Structures

Magnetic-field-dependent Hall-effect measurements and high-resolution mobility spectrum analysis have been employed to determine electronic transport parameters in InAsSb-based nBn structures. Three samples were studied, with nominally identical epitaxial layer structure but with barrier layers of different compositions. Two separate well-defined electron species, associated with the two distinct doping regions, were identified. The extracted electron concentrations were found to be in excellent agreement with the nominal doping density of the absorber and the back contact layers for all samples studied. For all samples, electron mobility appears to be limited by impurity scattering, whereas the relatively small differences in extracted mobility values between samples are likely to be due to unintentional variations in ionized impurity and/or defect concentration in the samples.

High-Resolution Mobility Spectrum Analysis of Multicarrier Transport in Advanced Infrared Materials

In this paper the recently developed high-resolution mobility spectrum analysis is demonstrated. In a number of simulations the high resolution of the algorithm is demonstrated in the high and low mobility ranges. The effect of random noise, maximum magnetic field limit, and the number of magnetic field points used in the experiment is also demonstrated. Also discussed are requirements critical for obtaining high-quality experimental data. The application of this new algorithm to complex semiconductor structures to study lateral and vertical transport is also demonstrated, resulting in insight into previously unavailable details.

Mobility spectrum analysis of anisotropic electron transport in N-polar GaN/AlGaN heterostructures on vicinal sapphire substrates

In this work, we present results of a study of anisotropic two-dimensional electron gas (2DEG) transport in N-polar GaN/AlGaN heterostructures grown on slightly mis-oriented sapphire substrates. High-resolution mobility spectrum analysis of magnetic-field dependent Hall-effect and resistivity indicate an isotropic 2DEG sheet carrier density, yet significant anisotropy was observed in carrier mobility. A single electron species with a narrow mobility distribution was found to be responsible for conduction parallel to the multi-atomic steps resulting from growth on the vicinal substrates; whilst in the perpendicular direction two distinct electrons peaks are evident at T ⩽ 150 K, which merge near room temperature. The linewidth of the mobility distributions for transport in the perpendicular direction was found to be significantly broader than that of the single electron in the parallel direction. The broader mobility distribution and the lower average mobility for the 2DEG in the perpendicular direction are attributable to interface roughness scattering associated with the GaN/AlGaN interfacial steps.

Measurements of corona ion induced atmospheric electricity modification near to HV power lines

Corona ions emitted from high voltage power lines and propagated by the wind can attach at a given rate to atmospheric aerosols and will create disturbances in the Earth's DC electric field. Local electric field variations have been measured using field meters upwind and downwind of power lines and a fixed site monitoring station has been built to enable space charge near a power line to be continuously recorded. At similar sites, the high resolution mobility spectra of atmospheric ions and charged small aerosols have been ascertained using two custom-built mobility spectrometers. These measurement techniques have indicated an increase in the charge state of small aerosols at ground level and an increase in space charge with high variability downwind of high voltage power lines.

An experimental study of ion-induced nucleation using a drift tube ion mobility spectrometer/mass spectrometer and a cluster-differential mobility analyzer/Faraday cup electrometer

An experimental system employing a drift tube ion mobility spectrometer/mass spectrometer (DT-IMS/MS) and a cluster-differential mobility analyzer with Faraday cup electrometer (C-DMA/FCE) was developed for the study of ion-induced nucleation. Using this, both the time scales and size scales of ions to be measured can be extended significantly. The system offers the potential for tracing the entire process of ion-induced nucleation initiated by the radiative ionization of air. The main processes are the formation of stable cluster ions and their growth to nanometer-sized ultrafine particles. Negative ions and negatively charged nano-particles generated by α -ray radiolysis in SO 2 / H 2 O / air mixtures were examined. High-resolution mobility spectra in which ions are distributed in the relatively high mobility range of 2.1– 1.5 cm 2 V - 1 s - 1 were obtained by DT-IMS/MS. Alternatively, more aged ions with mobilities as low as 0.05 cm 2 V - 1 s - 1 were observed in the C-DMA spectra. The differences between both spectra reflect the nature by which cluster ions grow into nano-particles. In addition to mobility measurements, mass spectra of ions can be obtained by DT-IMS/MS, which allows the identification of nucleating ions and relevant ion chemistry to be investigated. In the mass spectra at low SO 2 and H 2 O concentrations, SO 2 - , SO 3 - , SO 4 - , and SO 5 - were observed as major ions, indicating that SO 2 is preferentially ionized by ion–molecule reactions with primary negative ions. When the concentrations of SO 2 and H 2 O were increased, HSO 4 - and HSO 4 - · H 2 SO 4 ions became prominent, suggesting that the oxidization of SO 2 into H 2 SO 4 is enhanced, probably due to the enhanced production of OH radicals by ion–molecule reactions involving H 2 O . This suggests that the role of SO 2 in particle formation differs depending on the concentrations of SO 2 and H 2 O . At low SO 2 and H 2 O concentrations, SO 2 tends to form core ions that lead to ion-induced particle formation, whereas more SO 2 molecules are transformed into gaseous H 2 SO 4 and homogenous nucleation involving H 2 SO 4 and H 2 O is enhanced at higher concentrations of SO 2 and H 2 O .