Quantitative Mobility Spectrum Analysis Research Articles

A simple parallel conduction extraction method (SPCEM) for MODFETs and undoped GaN-based HEMTs

We report a simple method to extract the mobility and sheet carrier densities of conduction channels in conventional modulation doped field-effect transistor (MODFET) structures and unintentionally doped GaN-based high-electron mobility transistor (HEMT) structures for a special case. Extraction of the conduction channels from the magnetic field-dependent data can present number of problems; even the most recent methods encounter great difficulties. For the GaN-based HEMT structures which have lower mobilities and larger effective masses than that of GaAs-based counterparts, these difficulties become more prominent. In this study, we describe a simple method for magnetotransport analysis to extract conduction channels successfully for a special case that is commonly encountered: one bulk channel and one two-dimensional electron gas (2DEG) channel. Advantage of this method is mainly its simplicity. The analysis can be done with only two magnetic field-dependent measurements per temperature step. The method is applied to the magnetotransport results of an unintentionally doped AlGaN/AlN/GaN/AlN heterostructure over a temperature range of 29–350K and in a magnetic field range of 0–1.5T (μB<1). The results are then compared with those obtained using a commercial package for these calculations namely: quantitative mobility spectrum analysis (QMSA).

Multiple carrier transport in N‐face indium nitride

AbstractWe present temperature (20–300 K) dependent multi‐carrier measurements of electron species in N‐face indium nitride. N‐face InN samples were grown to different thicknesses (500–2000 nm) via plasma‐assisted molecular beam epitaxy on C‐face SiC substrates. Surface and bulk electron transport properties were extracted using a quantitative mobility spectrum analysis. Mobility of both bulk and surface electron species increase with film thickness. The temperature dependence of the mobility of both species differs to that of In‐polar samples studied previously, while the mobility of surface electrons is more than twice that of In‐polar samples with only a slight corresponding reduction in sheet concentration. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Arsenic δ-doped HgTe∕HgCdTe superlattices grown by molecular beam epitaxy

Arsenic incorporation in HgTe∕Hg0.05Cd0.95Te superlattices grown by molecular beam epitaxy (MBE) is reported. The incorporation was carried out by a δ-doping approach where arsenic was incorporated during MBE growth as acceptors. The superlattices were characterized via high resolution x-ray diffraction, Fourier transform infrared spectroscopy, secondary ion mass spectrometry, and magnetotransport Hall measurements coupled with the quantitative mobility spectrum analysis algorithm.

Self-consistent scattering analysis ofAl0.2Ga0.8N/AlN/GaN/AlN heterostructures grown on 6H-SiC substrates using photo-Hall effect measurements

Hall effect measurements on undopedAl0.2Ga0.8N/AlN/GaN/AlN heterostructures grown on 6H-SiC substrates were carried out as a function of thetemperature (30–300 K) and magnetic field (0–1.4 T). Measurements were carried outunder dark and after-illumination conditions. After the dark measurements, thesamples were illuminated with a blue light emitting diode for 30 min, and then thesame measurements were carried out for the after-illumination condition. Themagnetic field dependent Hall results were analyzed and the 2DEG contribution wasfound using the quantitative mobility spectrum analysis (QMSA) technique. Aself-consistent scattering analysis between the dark and illuminated conditions wasimplemented. The importance of this implementation was to find an indirect way tolocate more certain fit parameters, such as interface roughness parameters and thebackground impurity value, which cannot be found using dark measurement dataalone.

Quantitative mobility spectrum analysis of carriers in GaSb/InAs/GaSb superlattice

The authors have investigated electrical transport in a type II GaSb/InAs superlattice grown on GaSb using “quantitative mobility spectrum analysis.” Their results indicate that the superlattice contributes a lone electron specie with an ambient temperature mobility of ∼104 cm2/V s. Variable temperature studies in the range 50–300 K show that the carrier is associated with an activation energy of 0.27 eV, which is very close to the superlattice band gap.

Incorporation and activation of arsenic in MBE-grown HgCdTe

Research into p-type doping of HgCdTe with arsenic has concentrated on the use of a conventional effusion cell and optimization of growth conditions to achieve an increase in incorporation efficiency. This study investigates the use of a cracker cell, which is now the preferred method of doping HgCdTe due to its higher arsenic incorporation efficiency under optimum growth conditions. A detailed investigation of a number of arsenic doped HgCdTe layers grown on CdZnTe substrates by molecular beam epitaxy using a cracker cell as a source of arsenic is presented. Growth parameters influencing the amount of arsenic incorporated, such as the cracker-cell bulk temperature and substrate temperature, were investigated. Arsenic depth profiles were obtained via detailed secondary ion mass spectrometry where all major constituents in the epilayers were analysed. Magneto-transport Hall measurements were performed on as-grown material and those that underwent high-temperature anneals typical for arsenic activation. Using the quantitative mobility spectrum analysis technique, contributions to total conductivity arising from various carriers present in the samples have been separated. As-grown samples were found to exhibit n-type behaviour consistent with arsenic incorporating on cation sublattice, while samples that underwent high-temperature annealing show partial activation of arsenic with electron compensation.

Effect of MBE Growth Conditions on Multiple Electron Transport in InN

A quantitative mobility spectrum analysis (QMSA) of multiple magnetic field data has been used to determine the transport properties of bulk and surface electron species in InN films, grown by plasma-assisted molecular beam epitaxy (PAMBE) with varying substrate temperatures and In/N flux ratios. While all films have similar bulk electron densities, ∼4 × 1017 cm−3, the highest mobility was obtained in the highest growth temperature film (3100 cm2/V s at 150 K), while In-rich growth also gave good mobility values even at a much lower growth temperature. The surface sheet electron concentration increased with surface roughness, which increased with N-flux during growth.

Electronic transport characterization of AlGaN∕GaN heterostructures using quantitative mobility spectrum analysis

Resistivity and Hall effect measurements in nominally undoped Al0.25Ga0.75N∕GaN heterostructures grown on sapphire substrate by metal-organic chemical vapor deposition are carried out as a function of temperature (20–350K) and magnetic field (0–1.5T). The measurement results are analyzed using the quantitative mobility spectrum analysis techniques. It is found that there is strong two-dimensional electron gas localization below 100K, while the thermally activated minority carriers with the activation energies of ∼58 and ∼218meV contribute to the electron transport at high temperatures.

Determination of two-dimensional electron and hole gas carriers in AlGaN/GaN/AlN heterostructures grown by Metal Organic Chemical Vapor Deposition

Resistivity and Hall effect measurements on nominally undoped Al 0.25Ga 0.75N/GaN/AlN heterostructures grown on sapphire substrates prepared by metal organic chemical vapor deposition have been carried out as a function of temperature (20–300 K) and magnetic field (0–1.4 T). Variable magnetic field Hall data have been analyzed using the improved quantitative mobility spectrum analysis technique. The mobility and density of the two-dimensional electron gas at the AlGaN/GaN interface and the two-dimensional hole gas at the GaN/AlN interface are separated by quantitative mobility spectrum analysis. The analysis shows that two-channel conduction is present in nominally undoped Al 0.25Ga 0.75N/GaN/AlN heterostructures grown on sapphire substrate.

Effect of High-Density Plasma Process Parameters on Carrier Transport Properties in p-to-n Type Converted Hg0.7Cd0.3Te Layer

Exposure of p-type HgCdTe material to Ar/H 2 -based plasma is known to result in p-to-n conductivity-type conversion. While this phenomenon is generally undesirable when aiming to perform physical etching for device delineation and electrical isolation, it can be used in a novel process for formation of n-on-p junctions. The properties of this n-type converted material are dependent on the condition of the plasma to which it is exposed. This paper investigates the effect of varying the plasma process parameters in an inductively coupled plasma reactive ion etching (ICPRIE) tool on the carrier transport properties of the p-to-n type converted material. Quantitative mobility spectrum analysis of variable-field Hall and resistivity data has been used to extract the carrier transport properties. In the parameter space investigated, the n-type converted layer carrier transport properties and depth have been found to be most sensitive to the plasma process pressure and temperature. The levels of both RIE and ICP power have also been found to have a significant influence.

The effect of strain relaxation on electron transport in undoped Al 0.25Ga 0.75N/GaN heterostructures

The two-dimensional electron gas (2DEG) transport properties of two-step growth undoped Al 0.25Ga 0.75N/GaN heterostructures with semi-insulating buffer, grown by MOCVD, were investigated in a temperature range of 20–350 K. Using the quantitative mobility spectrum analysis (QMSA) method, it was shown that significant parallel conduction does not occur in worked structures. In-plain growth axis strains are calculated using the total polarization-induced charge density taken as the sheet carrier density measured from the Hall effect. It was found that the calculated strain values are in good agreement with those reported. Influences of the two-step growth parameters such as growth ramp time, the annealing temperature of the GaN nucleation layer on the mobility, and density of the 2DEG are also discussed.

Magnetotransport properties of AlxGa1−xN∕AlN∕GaN heterostructures grown on epitaxial lateral overgrown GaN templates

We studied the low-temperature magnetotransport properties of AlxGa1−xN∕AlN∕GaN heterostructures with a two-dimensional electron gas (2DEG). Structures with different Al compositions were grown by metal-organic vapor-phase epitaxy on three types of templates: conventional undoped GaN, in situ epitaxial lateral overgrown GaN using a SiNx nanomask layer, and ex situ epitaxial lateral overgrown GaN (ELO-GaN) using a stripe-patterned SiO2 mask. All of the samples display Shubnikov–de Haas (SdH) oscillations that confirm the existence of 2DEGs. Field-dependent magnetoresistance and Hall measurements further indicate that the overgrown heterostructures have a parallel conducting layer in addition to the 2DEG. To characterize the parallel channel, we repeated the measurements after the 2DEG was etched away. 2DEG carrier density values were then extracted from the SdH data, whereas the zero-field 2DEG conductivity was determined by subtracting the parallel channel conductivity from the total. The quantitative mobility spectrum analysis could not be applied in some cases, due to a large contact resistance between the parallel channels. The resulting 2DEG mobility is about a factor of 2 higher in the ELO-GaN and SiN–GaN samples as compared to the standard control samples. The mobility enhancement is attributed to a reduction of threading dislocations by the two ELO techniques employed.

Scattering analysis of 2DEG carrier extracted by QMSA in undoped Al0.25Ga0.75N/GaN heterostructures

Hall effect measurements on undoped Al0.25Ga0.75N/GaN heterostructures grown by a metalorganic chemical vapour deposition (MOCVD) technique have been carried out as a function of temperature (20–350 K) and magnetic field (0–1.5 T). Magnetic field dependent Hall data were analysed using the quantitative mobility spectrum analysis (QMSA) technique. The mobility and density within the two-dimensional electron gas (2DEG) at the Al0.25Ga0.75N/GaN interface and within the underlying GaN layer were successfully separated by QMSA. Mobility analysis has been carried out using both the measured Hall data at a single field and the extracted data from QMSA. Analysis of the temperature-dependent mobility of 2DEG extracted from QMSA indicates that the interface roughness and alloy disorder scattering mechanisms are the dominant scattering mechanisms at low temperatures while at high temperatures only polar optical phonon scattering is the dominant mechanism. Al0.25Ga0.75N/GaN interface related parameters such as well width, deformation potential constant and correlation length were also accurately obtained from the fits of the simple analytical expressions of scattering mechanisms to the 2DEG mobility.

Carrier compensation and scattering mechanisms in Si-doped InAsyP1−y layers grown on InP substrates using intermediate InAsyP1−y step-graded buffers

Electronic transport properties of strain-relaxed Si-doped InAsyP1−y layers with arsenic mole fractions between y=0.05 and y=0.50 were studied. All layers were grown on semi-insulating InP substrates by solid source molecular beam epitaxy using intermediate InAsyP1−y step-graded buffers to reduce dislocation density. Variable magnetic field (0–8.5T) Hall effect measurements in conjunction with quantitative mobility spectrum analysis in the temperature range of 25–300K were used to extract individual carrier mobilities, densities, and donor ionization energy as a function of temperature and alloy composition. The low field mobility is calculated by taking into account various scattering mechanisms, and these results are compared with the experimental results. At a constant electron carrier concentration of ∼2×1016cm−3, the 300K carrier mobility increases from 2856to5507cm2∕Vs with increasing arsenic mole fraction from 0.05 to 0.50. The experimental mobilities are in close agreement with the theoretical results using various scattering mechanisms. Both optical polar phonon scattering and ionized impurity scattering are important at 300K while at 100K, ionized impurity scattering is the limiting process. Alloy scattering is found to be only of second order importance. The Si donor ionization energy was determined to be ∼2–4meV for all alloy compositions.

Quantitative mobility spectrum analysis of AlGaN∕GaN heterostructures using variable-field hall measurements

Carrier transport properties of AlGaN∕GaN heterostructures have been analyzed with the quantitative mobility spectrum analysis (QMSA) technique. The nominally undoped Al0.08Ga0.92N∕GaN sample was grown by plasma-assisted molecular beam epitaxy on a GaN/sapphire template prepared with hydride vapor phase epitaxy. Variable-magnetic-field Hall measurements were carried out in the temperature range of 5–300K and magnetic field range of 0.01–7T. QMSA was applied to the experimental variable-field data to extract the concentrations and mobilities associated with the high-mobility two-dimensional electron gas and the relatively low-mobility bulk electrons for the temperature range investigated. The mobilities at T=80K are found to be 7100 and 880cm2∕Vs, respectively, while the corresponding carrier densities are 7.0×1011 and 8×1014cm−3. Any conclusions drawn from conventional Hall measurements at a single magnetic field would have been highly misleading.

The temperature dependence of electron and magneto transport properties in Te-doped InSb

Resistivity, magnetoresistivity and Hall effect measurements in n-type Te-doped InSb grown by the LEC technique were carried out as a function of temperature (14–350 K) and magnetic field (0–1.35 T). The measurement results are analyzed using the conventional and quantitative mobility spectrum analysis methods. It is found that the impurity levels arising from LEC technique and intentionally dopants have a strong influence on both the electron and magneto transport properties. The temperature dependence of the magnetoresistivity coefficient shows an overall increase by representing two minima as the temperature decreases. These minima are associated with two types of impurities. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigation of multiple carrier effects in InN epilayers using variable magnetic field Hall measurements

Variable magnetic field Hall effect measurements were performed on InN samples grown by molecular beam epitaxy. Since mixed conduction effects due to both the presence of multiple conducting layers and sample inhomogeneity were present, standard measurements yielded only averaged results. The variable field approach allowed direct measurement of interfacial/surface conduction while also allowing determination of the bulk electrical properties. Analysis always indicated electrons with more than one value of mobility, often with a significant spread in the mobility. This suggests multiple conduction layers in the sample as well as sample inhomogeneity. Variable magnetic field analysis of a 7.5 μm thick InN sample suggests maximum mobilities greater than 4000 cm 2/Vs in “bulk” InN. The quantitative mobility spectrum analysis technique, reported here for the first time on InN, indicated a continuous and significant spread in mobility for the bulk electron, likely with sample thickness.

Application of quantitative mobility-spectrum analysis to multilayer HgCdTe structures

For modern semiconductor heterostructures containing multiple populations of distinct carrier species, conventional Hall and resistivity data acquired at a single magnetic field provide far less information than measurements as a function of magnetic field. However, the extraction of reliable and accurate carrier densities and mobilities from the field-dependent data can present a number of difficult challenges, which were never fully overcome by earlier methods, such as the multicarrier fit, the mobility-spectrum analysis of Beck and Anderson, and the hybrid mixed-conduction analysis. More recently, to overcome the limitations of those methods, several research groups have contributed to development of the quantitative mobility-spectrum analysis (QMSA), which is now available as a commercial product. The algorithm is analogous to a fast Fourier transform in that it transforms from the magnetic-field (B) domain to the mobility (μ) domain. The QMSA converts the field-dependent Hall and resistivity data into a visually meaningful transformed output, comprising the conductivity density of electrons and holes in the mobility domain. In this article, we apply QMSA to both synthetic and real experimental data that are representative of modern multilayer HgCdTe structures. We discuss such features as the accuracy of the extraction of individual layer conductivities and average mobilities, reconstruction of the carrier mobility distribution within a particular layer, the resolution of two carriers with similar mobilities, and limits of the sensitivity.

Laser-beam-induced current mapping of spatial nonuniformities in molecular beam epitaxy As-grown HgCdTe

The formation of dislocations and corresponding built-in electric fields in molecular beam epitaxy (MBE)-grown HgCdTe can have a major impact on the performance and yield of photodetectors fabricated from this material. This paper investigates the presence of such built-in electric fields arising from dislocation segregation in MBE as-grown HgCdTe, and their subsequent removal via a low-temperature Hg-saturated anneal. The electrical properties and surface morphology of an HgCdTe layer grown on a thin CdTe buffer layer are compared with those of an HgCdTe layer grown directly on the CdZnTe substrate. Laser-beam-induced current (LBIC) imaging is a nondestructive technique capable of mapping built-in electric fields present in a semiconductor material, which, in the present case, has been used to reveal dislocation distributions present in as-grown, unintentionally doped, MBE-grown Hg0.71Cd0.29Te. Two-dimensional scanning LBIC measurements at 160 K allow spatial mapping of electric fields across the HgCdTe wafer. Subsequent isothermal annealing of the wafer in an Hg atmosphere has been found to decrease the magnitude of the built-in electric fields to below the LBIC detection limit. However, of particular note, is that before and after annealing, crosshatch patterns can be seen using Nomarski microscopy, with the crosshatching being predominantly in the [01\(\bar 1\)] direction and, to a lesser extent, in the [\(\bar 2\)31] and [\(\bar 2\)13] directions. Defect-decoration etching of the annealed wafer reveals dislocation banding parallel to the [01\(\bar 1\)] direction, which closely resembles the contrast observed in the LBIC image of the wafer before annealing. These Nomarski and LBIC images are compared with those of a second wafer, which incorporates a 40-nm CdTe buffer layer. The second wafer does not show significant Nomarski or LBIC contrast, indicating a flat, electrically uniform as-grown layer. Variable magnetic-field Hall measurements at 77 K and quantitative mobility-spectrum analysis (QMSA) indicate predominately p-type conduction with a doping density of 2×1015 cm−3 in the as-grown layer. After Hg annealing at 240°C, no LBIC signals are observed at 160 K, and Hall measurements at 77 K indicate the presence of two n-type carriers, with a combined doping density of 2×1015 cm−3. Double-crystal x-ray diffraction measurements show no evidence of twinned crystal volumes in the layers before or after annealing, or any change in the full-width at half-maximum (FWHM) (41 arcsec) of the (422) reflection. The similarity between the dislocation density distribution, as revealed by defect decoration, and the LBIC image suggests that Hg out-diffusion during growth is expedited in regions of high dislocation concentration, thus creating a nonuniform Hg vacancy-acceptor concentration. The as-grown acceptor concentration, in turn, modulates the hole concentration, creating p+/p− junctions and built-in electric fields in the material. Low-temperature annealing in a saturated-Hg atmosphere does not remove the crosshatch patterns or dislocation banding, but it fills the Hg vacancies, revealing the uniformly distributed n-type background, thus reducing the magnitude of any built-in electric fields. The LBIC mapping of MBE as-grown HgCdTe samples is, thus, capable of revealing defect distributions that would otherwise require a destructive technique, such as defect-decoration etching, to determine.

Fundamental materials studies of undoped, In-doped, and As-doped Hg1−xCdxTe

Variable magnetic-field Hall and transient photoconductance-lifetime measurements were performed on a series of undoped, In-doped, and As-doped HgCdTe samples grown by molecular beam epitaxy (MBE). Use of quantitative mobility-spectrum analysis (QMSA) combined with multiple carrier-fitting (MCF) techniques indicates that the majority of samples contain an interfacial n-type layer that significantly influences the interpretation of the electrical measurements. This n-type layer completely masks the high-quality electrical properties of undoped or low n-type In-doped HgCdTe, as well as complicating the interpretation of activation in As-doped p-type HgCdTe. Introduction of an intentional n-type background, typically created through doping with In to “recover” high mobility, is actually shown to increase the “bulk” layer conductivity to a level comparable to the interface layer conductivity. Photoconductance-lifetime measurements suggest that In-doping may introduce Shockley-Read-Hall (SRH) recombination centers. Variable-field Hall analysis is shown to be essential for characterizing p-type material. Photoconductance-lifetime measurements suggest that trapping states may be introduced during the incorporation and activation of As. Two distinctly different types of temperature dependencies were observed for the lifetimes of As-doped samples.