Variable Temperature Hall Effect Measurements Research Articles

Evaluation of the Near-Zero Temperature Coefficient of Resistivity (NZ-TCR) of ALD TiSixn Films

Atomic Layer Deposition (ALD) of ternary TiSixN leads to nanocomposites of metallic TiN atomically mixed with insulating Si3N4. Formulating TiSixN films with various Ti:Si ratios lead to the emergence of a temperature regime where resistivity is independent of thermal drift, denoted as near-zero temperature coefficient of resistivity (nz-TCR). Further, the ease with which nanocomposites of TiSixN can be deposited using ALD offer precise tunability in Ti:Si ratio, thickness, mass density, crystallinity and electrical properties.Recently, our group explored TiSixN films deposited using a Eugenus® 300 mm commercial QXP mini-batch system by modulating the ratio of Ti and Si precursors with NH3 as a co-reactant. Si-content was varied from 0 at % (pure TiN) to 24.2 at % Si while maintaining thickness ~ 140 nm. The X-ray reflectivity and grazing incidence X-ray diffraction measurements showed a reduction in film density and transition from nano-crystalline to pure amorphous phase with increase in Si-fraction. Spectroscopic ellipsometry revealed the optical constants, composition, and electrical resistivities and were supported by X-ray photoelectron spectroscopy and electrical measurements. Room-temperature resistivity measurements show an increase in film resistivity with increasing at % Si. Temperature-dependent Van der Pauw measurements found a nz-TCR of -23 ppm K-1 in the temperature range of 298 K – 398 K and at 3.4 at % Si content.We have now discovered that an at % Si = 3.0% induces a nz-TCR of -5.7 ppm K-1 from 80 K – 420 K – one of the best reported nz-TCR values for ALD thin films. Fine tuning the at % Si in TiSixN films, possible only via ALD, significantly elongated the temperature window of nz-TCR behavior. Mapping the local conductivity of individual grains through conductive atomic force microscopy (c-AFM) indicated higher resistance at the grain boundaries. The local composition at the grain boundaries may play a major role in determining the nz-TCR behavior of TiSixN films. In addition, variable temperature Hall effect measurements were performed to provide deeper insights into the nz-TCR mechanism, decoupling carrier concentration from carrier mobility effects while determining film resistivity.Compared to other nz-TCR films, which are deposited using physical vapor deposition techniques, ALD based nz-TCR films presents a unique synthesis platform for interconnect technology in topologically complex, 3D devices, circuits and sensors that undergo large temperature variation during operation but need to maintain stability in their electrical characteristics.

Low-Pressure CVD of GeE (E = Te, Se, S) Thin Films from Alkylgermanium Chalcogenolate Precursors and Effect of Deposition Temperature on the Thermoelectric Performance of GeTe.

The homologous series [GenBu3(EnBu)] (E = Te, Se, S; (1), (3) and (4)) and [GenBu2(TenBu)2] (2) have been synthesized as mobile oils in excellent yield (72-93%) and evaluated as single-source precursors for the low-pressure chemical vapor deposition (LPCVD) of GeE thin films on silica. Compositional and structural characterizations of the deposits have been performed by grazing-incidence X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray analysis, and Raman spectroscopy, confirming the phase purity and stoichiometry. Electrical characterization via variable-temperature Hall effect measurements is also reported. Given the strong interest in GeTe and its alloys for thermoelectric applications, variable-temperature Seebeck data were also investigated for a series of p-type GeTe films. The data show that it is possible to tune the thermoelectric response through intrinsic Ge vacancy regulation by varying the deposition temperature, with the highest power factor (40 μW/K2cm@629 K) and effective ZT values observed for the films deposited at higher temperatures.

MoO3 induces p-type surface conductivity by surface transfer doping in diamond

Surface transfer doping of diamond using high electron affinity transition metal oxides (TMOs), such as MoO3, has emerged as a key enabling technology for the development of diamond-based surface two-dimensional (2D) electronics. However, the omission of a critical pre-annealing step in the device fabrication process to remove atmospheric adsorbates prior to TMO deposition, as seen in numerous studies, makes the role of TMOs ambiguous in light of air-induced surface conductivity, preventing a full understanding of the intrinsic surface transfer doping behavior of TMO-based surface acceptors. Here, using in-situ four-probe electrical measurements we explicitly show the insulating-to-conducting transition in diamond surface driven by MoO3 induced surface transfer doping. Variable-temperature Hall-effect measurements reveal weak temperature-dependence down to 250 mK, evidencing the 2D Fermi liquid nature of the resulting surface conducting channel on diamond. Using first-principles calculations, we confirm the interfacial charge exchange upon MoO3 adsorption leading to a degenerate hole conducting layer on diamond. This work provides significant insights into understanding the surface transfer doping of diamond induced by TMOs, and paves the way for the investigations of many interesting quantum transport properties in the resulting 2D hole conducting layer, such as phase-coherent magnetotransport, on diamond surface.

Modeling of hall mobility for In2O3 thin film by metal organic chemical vapor deposition

In the light of variable temperature (4.2–300 K) Hall-effect measurements a physics-based model for Hall mobility of indium oxide (In2O3), thin film processed by metal organic chemical vapor deposition (MOCVD) has been established. It illustrates the relation among Hall mobility, scattering mechanisms and carrier concentrations exhaustively. Dependence of the potential barrier between grain boundaries on the carrier concentration has been factored in. Concomitantly, account have been taken of exponential tails and the degeneracy in In2O3 film. The proposed model reassured by a comparison of the experimental and theoretical calculated data is feasible and reliable. Results demonstrate that under low carrier densities, the prevailing scattering mechanism would be grain boundary scattering, nevertheless, upon exceeding the concentration of 1019cm−3, Hall mobility is chiefly confined to scattering by ionized impurities.

Polarization modulation effect of BeO on AlGaN/GaN high-electron-mobility transistors

We investigate the polarization modulation effect of a single-crystalline BeO layer on AlGaN/GaN high-electron-mobility transistors (HEMTs). The BeO layer with macroscopic polarization on top of the AlGaN barrier layer increases the 2-dimensional electron gas density in the triangular quantum well (QW) at the interface of the AlGaN/GaN heterostructure. Electronic band bending of BeO and a deeper triangular QW observed from the simulated conduction band profile indicate that the BeO layer can modify the polarization field at the AlGaN/GaN interface. A ∼20-nm-thick single-crystalline BeO thin film is grown on AlGaN/GaN HEMTs by atomic-layer deposition. Room-temperature and variable-temperature Hall-effect measurements confirm that the HEMT with BeO forms a channel with a 14% increase of the sheet carrier concentration as compared with a conventional HEMT. An improved output performance is also observed in the I-V characteristics which confirms the polarization modulation effect of the BeO layer.

Analyses of determination conditions of n-GaN dislocation density by triple-axis X-ray diffraction

Dislocation densities of two hydride vapor phase epitaxy-grown hexagonal GaN samples, which are Si doped and unintentionally doped respectively, are determined by triple-axis X-ray diffractometry and van der Pauw variable temperature Hall-effect measurement. The dislocation densities of these two samples should be at the same level from the X-ray testing, the -FWHM (full width at half maximum) values of all corresponding reflections for these two samples are almost the same. But from the Hall-effect measurements, the dislocation density values should be different from each other remarkably, because the unintentionally doped sample belongs to Mott transition material, while the Si-doped one does not. This fact indicates that the X-ray testing is perhaps inaccurate under some conditions, although the triple-axis X-ray diffractometry is a highly suitable technique for discriminating different kinds of structural defects such as edge and screw dislocations that lead to characteristic broadening of symmetric and asymmetric Bragg reflection. The experimental result obtained so far (say, for hot-electron bolometer) shows that the dislocation density value from mobility fitting model is in good accordance with that from -FWHM fitting using Srikant method. The anomaly that the dislocation density from -FWHM fitting is much lower than that from mobility fitting for the same sample (sample 59#), indicates that dislocations located in grain boundary may not be tested by triple-axis X-ray diffractometry. According to mosaic model, the layer is assumed to consist of single crystallites, called mosaic blocks, which are assumed to be slightly misoriented with respect to each other. The out-of-plane rotation of the block perpendicular to the surface normal is of the mosaic tilt, and the in-plane rotation around the surface normal is of the mosaic twist. The average absolute values of tilt and twist angles are directly related to the FWHM values of the corresponding distributions of crystallographic orientations. So, the X-ray testing can determine the average orientation of the grains with the same interplanar distance, excluding the information about the grain boundary at which X-ray cannot interfere because of disdortion of lattice. The experimental results and calculation analyses indicate that the dislocation density value from Srikant model is accurate when the ratio of twist angle to tilt angle exceeds 2.0, or the magnitude of the lateral coherence length is larger than 1.5 m.

Investigation of Isoelectronic Doping in p-GaN Based on the Thermal Quenching of UVL Band

In this paper, isoelectronic doping with indium (In) in p-GaN is investigated on the basis of the thermal quenching of ultraviolet luminescence (UVL) band. A phenomenological rate-equation model is proposed as a nondestructive diagnostic technique to acquire the dopant ionization energy of p-GaN. In terms of this model, we analyze the evolution of the ionization energy of p-GaN samples grown with different trimethyindium flow rate and find that isoelectronic doping with appropriate In incorporation is able to significantly reduce the ionization energy of the acceptor from 245 to 112 meV, which are consistent with the results of the variable temperature Hall-effect measurements. The comparison between the samples’ full width at half maximum of the UVL band indicates that excessive In incorporation will degenerate the conductivity of p-GaN.

Electrical properties of n-type AlGaN with high Si concentration

The electrical properties of Si-doped AlGaN layers (AlN molar fractions: 0.03–0.06) with the donor concentrations (ND) from 8.8 × 1017 to 4.5 × 1020 cm−3 were investigated by variable-temperature Hall effect measurement using the van der Pauw method. A minimum resistivity of 3.6 × 10−4 Ω cm was obtained for Si-doped AlGaN with a smooth surface at room temperature. We found that the activation energy of the Si donor is affected by the Coulomb interaction in the AlGaN layer with ND values from 8.8 × 1017 to 2.5 × 1020 cm−3. In several AlGaN layers, the free-electron concentration did not vary with sample temperature, as expected in the case of degeneracy. The localization of GaN in the AlGaN layer was speculated as a cause of degeneracy of samples.

A new method to determine the dislocation density in wurtzite n-GaN

We develop a new method to determine the edge and screw dislocation density in wurtzite n-GaN film. The method is to fit the van der Pauw variable temperature Hall-effect measurements with a analytic expression of low-field electron mobility in n-GaN. Our calculations take the comprehensive effect between the dislocation line and the shallow-donor defects as the main cause to depress the carrier mobility. Because of the crystal distortion near the dislocation line, the energy is so high that shallow-donor defects in the GaN crystal can be captured near the dislocation line. In other words, the shallow-donor defects distribute in lines along the dislocation line, but the shallow-donor defects along the screw and edge dislocation line have different energy levels. The shallow-donor defects take energy from lattice and the carrier, which is in relaxation process, then deliver the energy through ionizing. So, it is found that the following assumptions need to be made in order to obtain the model function for the mobility over a wide temperature range: i) there are 6 shallow-donor defect lines around one dislocation line; ii) two donor energy levels belonging to the screw and edge dislocation respectively must be taken into account; iii) the exchange energy between the carrier and the shallow-donor defect is ħωLO, the energy value of polar optical phonon. Under these assumptions, experiments indicate that our calculation function can fit the experimental curve best. The values of dislocation density from our model and others determined by x-ray diffraction or by chemical etching method are in good agreement, and the values of donor energy levels from our model and Rode iterative method to solve the Boltzmann equation are also in good accordance with each other. This method is applicable for the wurtzite n-GaN films grown by various preparation technologies under any condition, which is for the sample with the peak-mobility temperature about or under 200 K, not for the sample with the peak-mobility temperature about or above 300 K, which room-temperature mobility usually is about or less than 100 cm2/(V·s).

Carbon-rich hexagonal (BN)C alloys

Thin films of hexagonal boron nitride carbon, h-(BN)1−x(C2)x, alloys in the C-rich side have been synthesized by metal-organic chemical vapor deposition (MOCVD) on c-plane sapphire substrates. X-ray diffraction measurements confirmed single hexagonal phase of h-(BN)1−x(C2)x epilayers. Electrical transport and Raman spectroscopy measurements results revealed evidences that homogenous h-(BN)1−x(C2)x alloys with x ≥ 95% can be synthesized by MOCVD at a growth temperature of 1300 °C. The variable temperature Hall-effect measurements suggested that a bandgap opening of about 93 meV with respect to graphite has been obtained for h-(BN)1−x(C2)x with x = 0.95, which is consistent with the expected value deduced from the alloy dependence of the energy gap of homogenous h-(BN)1−x(C2)x alloys. Atomic composition results obtained from x-ray photoelectron spectroscopy measurements revealed that the carrier type in C-rich h-(BN)1−x(C2)x alloys is controlled by the stoichiometry ratio between the B and N via changing the V/III ratio during the growth. The demonstration of bandgap opening and conductivity control in C-rich h-(BN)1−x(C2)x alloys provide feasibilities for realizing technologically significant devices including infrared (IR) emitters and detectors active from near to far IR and multi-spectral IR emitters and detectors.

Aluminium- and gallium-doped homoepitaxial ZnO thin films: Strain-engineering and electrical performance

Aluminium- and gallium-doped ZnO thin films with nominal source target compositions of and were grown homoepitaxially and in-plane lattice matched on c-plane ZnO single crystalline substrates. In dependence on dopant concentration and oxygen partial pressure during growth we found compressive or tensile out-of-plane epitaxial strain up to . Electrical investigations showed that the conductivity of the substrates alters transport properties at room temperature. Correct interpretation requires variable temperature Hall effect measurements. These reveal, that besides the contributions of the substrate and the deposited thin film an additional conduction channel, likely located at the surface of the samples must be considered to explain and model the temperature dependence of the free electron concentration and their mobility.

Evidence for a shallow Cu acceptor in Si from infrared spectroscopy and photoconductivity

Copper impurities affect optical and electronic properties in a diverse range of materials. Using IR spectroscopy, we provide evidence for a shallow Cu acceptor in Si. The observation of ${P}_{1/2}$ excitation lines indicates a Cu acceptor with a hole ionization energy of only 27 meV, which is shallower than any of the group-III acceptors. The observed center has an IR spectrum that is characteristic of a hydrogenic acceptor. Variable-temperature Hall-effect measurements confirm this result. The observation of shallow, hydrogenic behavior is consistent with a universal level model, in which the Cu acceptor level is fixed relative to vacuum, irrespective of the host.

Charge carrier transport properties in layer structured hexagonal boron nitride

Due to its large in-plane thermal conductivity, high temperature and chemical stability, large energy band gap (˜ 6.4 eV), hexagonal boron nitride (hBN) has emerged as an important material for applications in deep ultraviolet photonic devices. Among the members of the III-nitride material system, hBN is the least studied and understood. The study of the electrical transport properties of hBN is of utmost importance with a view to realizing practical device applications. Wafer-scale hBN epilayers have been successfully synthesized by metal organic chemical deposition and their electrical transport properties have been probed by variable temperature Hall effect measurements. The results demonstrate that undoped hBN is a semiconductor exhibiting weak p-type at high temperatures (> 700 °K). The measured acceptor energy level is about 0.68 eV above the valence band. In contrast to the electrical transport properties of traditional III-nitride wide bandgap semiconductors, the temperature dependence of the hole mobility in hBN can be described by the form of μ ∝ (T/T0)−α with α = 3.02, satisfying the two-dimensional (2D) carrier transport limit dominated by the polar optical phonon scattering. This behavior is a direct consequence of the fact that hBN is a layer structured material. The optical phonon energy deduced from the temperature dependence of the hole mobility is ħω = 192 meV (or 1546 cm-1), which is consistent with values previously obtained using other techniques. The present results extend our understanding of the charge carrier transport properties beyond the traditional III-nitride semiconductors.

Elevated temperature dependent transport properties of phosphorus and arsenic doped zinc oxide thin films

Elevated temperature dependent Hall effect measurements were performed in a wide temperature range from 80 to 800 K to study transport properties of zinc oxide (ZnO) thin films heavily doped with phosphorus (P) and arsenic (As), and grown on sapphire substrates by RF magnetron sputtering. Double thermal activation processes in both P- and As-doped ZnO thin films with small activation energy of ∼0.04 eV and large activation energy of ∼0.8 eV were observed from variable temperature Hall effect measurements. The samples exhibited n-type conductivities throughout the temperature range. Based on photoluminescence measurements at 11 K and theoretical results, the large activation energy observed in the temperature dependent Hall effect measurement has been assigned to a deep donor level, which could be related to oxygen vacancy (VO) in the doped ZnO thin films.

Temperature dependence of electrical characteristics of Pt/GaN Schottky diode fabricated by UHV e-beam evaporation

Temperature-dependent electrical characterization of Pt/n-GaN Schottky barrier diodes prepared by ultra high vacuum evaporation has been done. Analysis has been made to determine the origin of the anomalous temperature dependence of the Schottky barrier height, the ideality factor, and the Richardson constant calculated from the I-V-T characteristics. Variable-temperature Hall effect measurements have been carried out to understand charge transport at low temperature. The modified activation energy plot from the barrier inhomogeneity model has given the value of 32.2 A/(cm2 K2) for the Richardson constant A** in the temperature range 200 to 380 K which is close to the known value of 26.4A/(cm2 K2) for n-type GaN.

Micro-structural and temperature dependent electrical characterization of Ni/GaN Schottky barrier diodes

Micro-structural investigation of Ni/GaN Schottky barrier diodes has been carried out using high-resolution transmission electron microscopy and electron diffraction spectrum in order to emphasize the role of Ni/GaN interface in controlling the Schottky diode behavior. Variable temperature Hall effect measurement of GaN samples along with the current–voltage (I–V) characteristics of Ni/n-GaN Schottky barrier diodes have been measured in 100–380 K temperature range. Results are analyzed in terms of thermionic emission theory by incorporating the concept of barrier inhomogeneity at the metal/semiconductor interface. The observed anomaly of temperature dependence of Schottky barrier height and ideality factor are explained by invoking two sets of Gaussian distribution of SBH in the temperature ranges of 100–180 K and 220–380 K, respectively. The value of A** (effective Richardson constant) as determined from the modified Richardson plot is 29.2 A/(cm2 K2), which shows an excellent agreement with the theoretical value (26.4 A/(cm2 K2)) in the temperature range of 220–380 K.

Investigation of AlInN HEMT structures with different AlGaN buffer layers grown on sapphire substrates by MOCVD

We investigate the structural and electrical properties of AlxIn1–xN/AlN/GaN heterostructures with AlGaN buffers grown by MOCVD, which can be used as an alternative to AlInN HEMT structures with GaN buffer. The effects of the GaN channel thickness and the addition of a content graded AlGaN layer to the structural and electrical characteristics were studied through variable temperature Hall effect measurements, high resolution XRD, and AFM measurements. Enhancement in electron mobility was observed in two of the suggested AlxIn1−xN/AlN/GaN/Al0.04Ga0.96N heterostructures when compared to the standard AlxIn1–xN/AlN/GaN heterostructure. This improvement was attributed to better electron confinement in the channel due to electric field arising from piezoelectric polarization charge at the Al0.04Ga0.96N/GaN heterointerface and by the conduction band discontinuity formed at the same interface. If the growth conditions and design parameters of the AlxIn1−xN HEMT structures with AlGaN buffers can be modified further, the electron spillover from the GaN channel can be significantly limited and even higher electron mobilities, which result in lower two-dimensional sheet resistances, would be possible.

Structural and electrical properties of epitaxial Bi2Se3 thin films grown by hybrid physical-chemical vapor deposition

We report the epitaxial growth of Bi2Se3 thin films on (0001) Al2O3 substrates by hybrid physical-chemical vapor deposition (HPCVD). The HPCVD technique combines the thermal decomposition of trimethylbismuth with the thermal evaporation of Se and leads to a high Se partial pressure in the growth ambient. The Bi2Se3 films are highly c-axis oriented on sapphire but contain planar defects including stacking faults and twin boundaries. Variable-temperature Hall-effect measurements demonstrate a carrier concentration of 5.8 × 1018 cm−3 and a mobility of 900 cm2/Vs at 4.2 K. These results demonstrate the potential of HPCVD for producing high quality Bi2Se3 films for topological insulator studies.

Thermal stability of sol–gel p-type Al–N codoped ZnO films and electric properties of nanostructured ZnO homojunctions fabricated by spin-coating them on ZnO nanorods

The thermal stability of sol–gel p-type Al–N codoped ZnO films was investigated by high-resolution X-ray photoelectron spectroscopy (XPS). XPS revealed the chemical bonding states and solubility of N-related complex defects in the ZnO films. The concentrations of NO and (NC)O varied with annealing temperature, which led to the change in conduction between p-type and n-type. Variable-temperature Hall-effect measurement showed that NO acted as a shallow acceptor, with its energy level locating at ∼114meV above the valance band maximum. Transmission electron microscopy confirmed the presence of undesired carbon clusters as a graphite state in the ZnO films. In order for Al–N codoped ZnO films to exhibit p-type conductivity, samples could only be annealed in a certain range of temperatures. A hybrid structure with nanostructured ZnO homojunctions was fabricated by spin-coating the p-type Al–N codoped ZnO film on an n-type ZnO nanorod array (ZNA). The hybrid nanostructure was demonstrated to possess rectification behavior characteristic of a p–n junction. The leakage current of the nanostructured ZnO homojunctions was smaller by a factor of 2 than that of the film-based ZnO homojunction at a reverse bias of 5V. The p-type ZnO film/n-type ZNA structure can be applied as a versatile p–n optoelectronic device.

Electron transport properties in Al0.25Ga0.75N/AlN/GaN heterostructures with different InGaN back barrier layers and GaN channel thicknesses grown by MOCVD

AbstractThe electron transport properties in Al0.25Ga0.75N/AlN/GaN/InxGa1−xN/GaN double heterostructures with various indium compositions and GaN channel thicknesses were investigated. Samples were grown on c‐plane sapphire substrates by MOCVD and evaluated using variable temperature Hall effect measurements. In order to understand the observed transport properties, various scattering mechanisms, such as acoustic phonon, optical phonon, interface roughness, background impurity, and alloy disorder, were included in the theoretical model that was applied to the temperature‐dependent mobility data. It was found that low temperature (T < 160 K) mobility is limited only by the interface roughness scattering mechanism, while at high temperatures (T > 160 K), optical phonon scattering is the dominant scattering mechanism for AlGaN/AlN/GaN/InGaN/GaN heterostructures. The higher mobility of the structures with InGaN back barriers was attributed to the large conduction band discontinuity obtained at the channel/buffer interface, which leads to better electron confinement.