Carrier Compensation Research Articles

Colossal grain growth in Cd(Se,Te) thin films and their subsequent use in CdTe epitaxy by close-spaced sublimation

Many technologies deposit thin films on inexpensive substrates, resulting in small grains due to classic nucleation and grain growth theory. For example, state-of-the-art solar cells are made by depositing CdSeTe and CdTe layers on inexpensive glass coated with nanocrystalline transparent conducting oxides (TCOs), like SnO2. Characteristically, the grain size of these films is on the order of the film thickness, i.e. a few microns. CdTe small-grain films have poor electro-optical properties and require CdCl2 passivation which fails to fully passivate grain boundaries, causes carrier compensation, and prevents implementing other II–VI alloys and materials to improve performance. Here, we present a method to increase grain size to 1 mm in CdSexTe1−x thin films deposited on glass/TCO substrates without CdCl2 treatment. The colossal grain growth is driven by mechanisms distinct from classic nucleation, grain growth, and Ostwald ripening and only occurs at low selenium content (x ∼ 0.1). We also demonstrate how these films can serve as templates for subsequent large-grain epitaxy of other compositions like CdTe, again without exposure to CdCl2. The results open new paths for thin film solar cell technology, and thin film devices in general.

Electronic degeneracy conduction in highly Si-doped Al0.6Ga0.4N layers based on the carrier compensation effect

Electronic degeneracy to express metallic conduction in Al-rich AlGaN for the electron injection layer enhances the efficiencies of deep ultraviolet light emitters. This study systematically demonstrates the Si doping range and conditions to realize degenerate n-type Al0.6Ga0.4N layers based on the electron compensation effect. The temperature-independent electron concentrations resulting from the degenerate band appear in high Si doping conditions to overcome the electron compensation due to carbon on nitrogen sites (CN). However, excessive Si doping of over 4.0 × 1019 cm−3 leads to the collapse of the electronic degeneracy and a switch to the temperature-dependent electron transport via the impurity bands, where the luminescence bands originating from III vacancy-Si complexes (VIII-nSi) are dominant. The key parameter is the effective donor concentration, Nd − Na, based on the reduction in electron concentrations via acceptor-like deep levels such as CN and VIII-nSi. The Hall-effect analyses for n-type Al0.6Ga0.4N layers with various Si concentrations yielded an Nd − Na value of (9.5 ± 2.9) × 1018 cm−3 to vanish the ionization energy of Si donors, which is approximately six times higher than that in GaN. The results suggest not only the optimal doping range to obtain an Al-rich AlGaN layer with metallic conduction but also the necessity of the growth condition to minimize electron compensation.

Electrical properties of SiC-AlN ceramics pressureless sintered under N2 atmosphere

SiC-AlN ceramics were pressureless sintered at 2150 °C under N2 atmosphere, while B4C and C were added to promote the sintering process. The microstructure, electrical and mechanical properties of the as-sintered ceramics prepared by different AlN addition are discussed. More AlN addition tends to lead a smaller grain size. As AlN contents go up, the varistor voltage of the samples increases sharply from 17.304v·mm−1 to more than 100 v·mm−1 because of the improvement of grain boundary resistance. Correspondingly, the reasons for the increase of grain boundary resistance are: (1) the improvement of carrier compensation increases the Schottky barrier height and width; (2) more grain boundaries increase the number of Schottky barriers. In addition, the electrical properties of the same components sintered under N2 and Ar have been compared, that the varistor voltage of the same component SiC-AlN ceramics sintered under N2 are much higher than those under Ar (about 2 orders of magnitude). With the increase of AlN contents, the mechanical properties remain unchanged except the little reduction of Vickers hardness.

Extremely large magnetoresistance, anisotropic Hall effect, and Fermi surface topology in single-crystalline WSi2

We report on the observation of a nonsaturating, extremely large magnetoresistance (XMR) and the Fermi surface topology of a high quality $\mathrm{W}{\mathrm{Si}}_{2}$ single crystal grown by the Czochralski method. The magnetoresistance at $T=2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ reaches a value $\ensuremath{\approx}{10}^{5}%$ in 14 T magnetic field with no sign of saturation. The Hall resistivity data of $\mathrm{W}{\mathrm{Si}}_{2}$ was found to be highly anisotropic. The analysis of magnetoconductivity data of $\mathrm{W}{\mathrm{Si}}_{2}$ revealed a near compensation of charge carrier with relatively low carrier density as compared to that of a normal metal. The observed anisotropic Hall resistivity in $\mathrm{W}{\mathrm{Si}}_{2}$ is due to the presence of multiple bands and Fermi pockets responsible for the transport phenomena in it. The extremely large carrier mobility and near compensation of charge carriers are responsible for the nonsaturating XMR behavior in $\mathrm{W}{\mathrm{Si}}_{2}$ crystal. The band structure calculation and de Haas-van Alphen effect measurement depict a cylindrical Fermi surface from which the associated quantum parameters have been obtained. The magnetotransport data of $\mathrm{W}{\mathrm{Si}}_{2}$ along both the crystallographic directions follows the universal temperature-field triangular phase diagram as observed in other materials exhibiting XMR behavior.

Native point defects and low p-doping efficiency in Mg2(Si,Sn) solid solutions: A hybrid-density functional study

We perform hybrid-density functional calculations to investigate the charged defect formation energy of native point defects in Mg2Si, Mg2Sn, and their solid solutions. The band gap correction by hybrid-density functional is found to be critical to determine the charged defect density in these materials. For Mg2Si, Mg interstitials are dominant and provide unintentional n-type conductivity. Additionally, as the Mg vacancies can dominate in Mg-poor Mg2Sn, p-type conductivity is possible for Mg2Sn. However, the existence of low formation energy defects such as MgSn1+ and IMg2+ in Mg2Sn and their diffusion can cause severe charge compensation of hole carriers resulting in low p-type doping efficiency and thermal degradation. Our results indicate that, in addition to the extrinsic doping strategy, alloying of Mg2Sn with Mg2Si under Mg-poor conditions would be necessary to enhance the p-type conductivity with less charge compensation.

Investigation of carrier compensation traps in n−-GaN drift layer by high-temperature deep-level transient spectroscopy

Carrier compensation traps in n−-GaN drift layers grown on Si substrates were investigated using high-temperature deep-level transient spectroscopy (DLTS). The upper limit of the temperature range (700 K) allows for the study of deeper levels in the bandgap than those previously reported by conventional DLTS. Three trap states were revealed to be responsible for carrier compensation. Besides the residual carbon (C) acceptor, two deep electron traps detected in the DLTS high-temperature range, labeled E2 and E3 with energies EC of 0.98 and 1.38 eV, respectively, were also found to have contributions to the carrier compensation. A comprehensive investigation combining with positron annihilation spectroscopy measurements revealed that E2 and E3 are related to the (–/2–) and (0/–) acceptor levels of the VGa–ON complex, respectively. The relatively high concentrations of E2 and E3 imply that the VGa–ON complex is an essential carrier compensation source in the drift layer and plays a crucial role in developing kV-class vertical GaN power devices.

Impact of dopant-induced optoelectronic tails on open-circuit voltage in arsenic-doped Cd(Se)Te solar cells

Fluctuations refer to inhomogeneity in the distribution of donors and acceptors at the nanometer scale and occur in many compound solar cell materials such as Cu(In,Ga)Se2, Cu2ZnSn(S,Se)4, and CdSexTe1−x. In this work, numerical simulations show that these fluctuations produce not only electrostatic potential variation, but also, local changes in the carrier density and effective bandgap. For a CdSexTe1−x absorber doped with arsenic, simulations and cathodoluminescence data within single grains demonstrate how donor and acceptor densities—consistent with capacitance-voltage and secondary-ion mass-spectrometry data—produce tails in photoluminescence, quantum efficiency, and absorption measurements. Using multiple theoretical approaches, we demonstrate that the fluctuations can hinder expected performance gains from increased carrier density, and we describe the significant open-circuit voltage deficit observed in the CdSexTe1−x:As solar technology. Our results demonstrate that it is critical to characterize and reduce carrier compensation to realize a higher efficiency.

Opportunity to improve thin film solar cell performance identified

Resolving carrier compensation and fluctuating potentials can further increase the performance of cadmium-telluride thin film solar cells.

Nontrivial topological states in the tantalum dipnictides TaX2 (X = As, P)

The intriguing quasiparticles in solids, known as Dirac, Weyl, and Majorana fermions, etc. with properties akin to those theoretically predicted but never realized in high-energy physics, have excited intensive research activities in recent years. The transition metal dipnictides are attractive because of their extremely large magnetoresistance that is usually attributed to the carrier compensation effect. We briefly review herein the crystal growth, magneto-transport measurements, and theoretical studies of tantalum dipnictides TaAs2 and TaP2, with the emphasis being put on discussion of the nontrivial topological states in both materials. The analysis of quantum oscillations of the magnetoresistance unveils nonzero Berry phase. The chiral negative longitudinal magnetoresistance further points to possible Weyl state, which was recently theoretically suggested as the result of the magnetic field-induced Zeeman splitting effect no matter how large the field is. The analysis of these intriguing behaviors could brush up our understanding about the family of materials and would be valuable for future work on investigating the magnetic field-induced topological phase transition in such semimetals.

Influence of Aluminum Compensation Effects in 4H-SiC on the Performance of VDMOS Transistors

The compensation of charge carriers is an important aspect to be considered in Aluminum doped areas in 4H-SiC. In this paper, a straightforward method has been found to implement compensation effects into a basic device simulation model and to improve the conformance of electrical measurement and simulation results. By implementing the compensation factors, which depend on Aluminum doping concentration, device simulation in combination with basic device cell structure can be used to create electrical characteristics that are in accordance with measured characteristics. This is a simple alternative for complex process simulation, taking into account physical effects like defects in the crystal structure. The method was used for simulation of lateral MOSFETS transfer characteristic as well as VDMOS blocking characteristic. Found compensation values were 80 % in the 1.5 ∙ 1017 cm-3 Al-doped channel region and 23% in the deep, 7.5 ∙ 1017 cm-3 Al-doped, shielding region.

Comparative Results of Low Temperature Annealing of Lightly Doped N-Layers of Silicon Carbide Irradiated by Protons and Electrons

The influence of low-temperature (up to 400°С) annealing on the current-voltage and capacitance–voltage characteristics of Schottky diodes irradiated with protons with an energy of 15 MeV compared with the results of annealing of structures after irradiation with electrons with an energy of 0.9 MeV has been studied. It was shown that in case of proton irradiation with a dose of 4E13 cm-2 and electron irradiation with a dose of 1E16 cm-2, only partial carrier compensation occurs and the differential resistance is completely determined by the number of carriers in the epitaxial layer. The irradiation method has almost no effect on the direct current dependence on the voltage in the exponential segment. The ideality coefficient remains almost unchanged within 1.03 ÷ 1.04. During annealing after proton irradiation, a large activation energy of the process is required. The temperature of the beginning of annealing process during proton irradiation shifts to a larger value, from 150 °C to 250 °C when compared with electron irradiation. It has been demonstrated that at low doses of proton irradiation, the low-temperature annealing leads to the return to the conduction band of up to 65% of the removed charge carriers. After electron irradiation, low-temperature annealing returns up to 90% of the removed charge carriers to the conduction band. This indicates that at room temperature both proton irradiation as well as electron irradiation introduce both stable and unstable defects but in different proportions.

Simultaneous optimization of power factor and thermal conductivity via Te and Se double substitution in Cu12Sb4S13 tetrahedrite

The present study reports the structural, electronic, elastic, and thermoelectric properties of Cu12Sb3.9Te0.1S13-xSex (x = 0, 0.1, 0.5, 0.75 and 1) tetrahedrite. The thermal conductivity was reduced due to charge carrier compensation (by Te), weakening of the Sb-S bond and local chemical disorder (induced by Se), resulting in a low mean sound velocity. Simultaneously, the power-factor was maintained high from an enhancement of the density of states near the Fermi level due to Se. Consequently, a maximum figure of merit ~0.84 at 673 K was achieved for the Cu12Sb3.9Te0.1S12.5Se0.5 sample due to concomitant optimisation of the power-factor and thermal conductivity.

Homo-epitaxial growth of n-GaN layers free from carbon-induced mobility collapse and off-angle-dependent doping variation by quartz-free hydride vapor phase epitaxy

Certain undesired phenomena are observed in n-GaN layers grown by metal–organic chemical vapor deposition (MOCVD) due to the unavoidable C-induced carrier compensation. They are a drastic reduction in carrier mobility, called mobility collapse, and significant non-uniformity in the carrier concentration due to the off-angle dependence of the C-incorporation efficiency of the process. These phenomena are particularly severe for low doping levels between 1015 and 1016/cm3, which are suitable for fabricating drift layers used in vertical-type GaN power devices that operate in the range of a few kilovolts to tens of kilovolts. However, the C-related undesired characteristics are absent in homo-epitaxial n-GaN layers grown by quartz-free hydride vapor phase epitaxy (QF-HVPE), recently developed by us. The utilization of C-free raw materials alongside quartz-free parts enables the growth of highly pure GaN crystals with negligible Si, C, and O incorporations. These crystals exhibited an electron concentration in the low-1015/cm3 range with the highest reported room temperature electron mobility, μ, of 1470 cm2/V s among GaN crystals, whereas n-GaN layers with similar carrier concentrations but containing C-compensation, as in the case of those grown by MOCVD, exhibited a severe mobility collapse (μ = 288 cm2/V s). High uniformity in the carrier concentration with a small standard deviation of 4.0% was observed in a 2-in. n-GaN wafer grown by QF-HVPE on a GaN substrate with an off-angle variation of 0.3°. On the other hand, the standard deviation of the carrier concentration in wafers grown by MOCVD was approximately 17% because of the off-angle-dependent C-incorporation.

Probing unintentional Fe impurity incorporation in MOCVD homoepitaxy GaN: Toward GaN vertical power devices

Unintentional impurity incorporation in GaN drift layers represents a challenging issue that can limit their potential performance in vertical power devices. In this paper, we focus on studying the origins of Fe impurity incorporation in metal-organic chemical vapor deposition (MOCVD) grown GaN materials. Acting as a compensator in n-type GaN drift layers, Fe impurities can reduce the electron mobility in GaN and limit the lowest controllable doping level. Two sources, the sample cleaning process and growth susceptor, were identified as the main mechanisms of Fe incorporation in the MOCVD GaN growth process. It was found that solvent cleaning of the wafer can introduce significant Fe contamination at the growth interface, which would slowly be incorporated into the GaN epilayer, thus causing background Fe impurity as high as 1017 cm−3 level. Moreover, the Fe impurity in the coating material on the susceptor can introduce additional Fe impurity during the growth process. Our studies revealed that the Fe impurity level could be significantly suppressed by more than two orders when an alternative cleaning process was used and the susceptor surface was fully covered by substrates. Characterization of the Fe impurity concentrations was performed via secondary ion mass spectrometry. The trap level (EC − 0.57) eV from deep-level transient spectroscopy that had previously been attributed to Fe confirmed the carrier compensation effect from Fe. Room temperature Hall mobility as high as 1007 cm2/V s was achieved on the MOCVD grown low-Fe GaN. Results from this work will provide guidance for achieving high purity GaN toward high performance GaN vertical power devices.

Interplay between C-doping, threading dislocations, breakdown, and leakage in GaN on Si HEMT structures

This work describes electrical characteristics and the correlation to material properties of high electron mobility transistor structures with a C-doped GaN current blocking layer, grown either by an extrinsic or auto-doping process with different doping levels. Increasing degradation of crystalline quality in terms of threading dislocation density for increasing C-doping levels was observed for all samples. Different growth conditions used for the auto-doped samples played no role for overall degradation, but a higher fraction of threading screw dislocations was observed. Independent of the doping process, 90% of all TSDs were noted to act as strong leakage current paths through the AlGaN barrier. This was found statistically and was directly verified by conductive atomic force microscopy in direct correlation with defect selective etching. Vertical breakdown was observed to increase with increasing C-concentration and saturated for C-concentrations above around 1019 cm−3. This was attributed to an increasing compensation of free charge carriers until self-compensation takes place. A progressive influence of TDs for high C-concentrations might also play a role but could not be explicitly revealed for our material.

X-ray absorption spectroscopy study of Ga-doping in reactively sputtered ZnO films

Ga-doped ZnO (GZO) films were grown by reactive co-sputtering of a Zn-GaAs target (3% area coverage of Zn by GaAs) at 375 °C with different partial pressures of oxygen. The resistivity of GZO films increases drastically from ≲10−3 Ω-cm to ~ 0.2 Ω-cm as the O2 in the sputtering atmosphere is changed from 5 % to 6 %. The films grown at 5% or lower O2 have the Ga/Zn ratio of ~ 0.1 and display low resistivity with carrier concentration ~ 1021 cm−3, while the films grown at 6% or higher O2 have lower Ga/Zn ratio of ~ 0.01 and display high resistivity with carrier concentration ~ 1019 cm−3. Extended X-ray absorption fine structure (EXAFS) measurements at Zn and Ga K-edges reveal substitutional incorporation of Ga in low resistivity films. X ray photoelectron spectroscopy (XPS) studies reveal a substantial decrease in oxygen vacancies together with increase in oxygen interstitials, in high resistivity GZO films. EXAFS measurements of the high resistivity films show a substantial increase in Ga-O and Ga-Zn bond distances and Ga-O coordination, indicating incorporation of oxygen interstitials in the vicinity of Ga. The comparison of measured and simulated X–ray absorption near edge spectra at O K-edge, together with XPS and EXAFS results, provides adequate experimental evidence of the presence of acceptor type (GaZn+Oi) defect complex, which cause carrier compensation in high resistivity films.

Anisotropic Picosecond Spin-Photocurrent from Weyl Semimetal WTe2.

The generation and detection of ultrafast spin current, preferably reaching a frequency up to terahertz, is the core of spintronics. Studies have shown that the Weyl semimetal WTe2 is of great potential in generating spin currents. However, the prior studies have been limited to the static measurements with the in-plane spin orientation. In this work, we demonstrate a picosecond spin-photocurrent in a Td-WTe2 thin film via a terahertz time domain spectroscopy with a circularly polarized laser excitation. The anisotropic dependence of the circular photogalvanic effect (CPGE) in the terahertz emission reveals that the picosecond spin-photocurrent is generated along the rotational asymmetry a-axis. Notably, the generated spins are aligned along the out-of-plane direction under the light normally incident to the film surface, which provides an efficient means to manipulate magnetic devices with perpendicular magnetic anisotropy. A spin-splitting band induced by intrinsic inversion symmetry breaking enables the manipulation of a spin current by modulating the helicity of the laser excitation. Moreover, CPGE nearly vanishes at a transition temperature of ∼175 K due to the carrier compensation. Our work provides an insight into the dynamic behavior of the anisotropic spin-photocurrent of Td-WTe2 in terahertz frequencies and shows a great potential for the future development of terahertz-spintronic devices with Weyl semimetals.

Effect of Sb doping in pure phase SnS thin films

SnS thin films were fabricated by closed-tube sulfurization of sputtered Sn precursors at temperature range of 200 °C–400 °C. Pure orthorhombic SnS phase with stochiometric composition (S/Sn = 1.03) was obtained at the optimum growth temperature of 250 °C. Hole concentration, resistivity and hole mobility of the as-grown films were found to be 1.3 × 1016 cm−3, 95.5 Ω cm, and 5 cm2 V−1 s−1, respectively. The as-grown film was then Sb doped by thermal diffusion method under controlled Sb vapor pressure. Thermal annealing with Sb improved the crystalline quality of the SnS films. The effects of carrier compensation were observed for Sb concentration greater than 0.55%. Addition of 1.38% Sb results in the electrical resistivity of 5.65 × 104 Ω cm and the carrier concentrations of 1.2 × 1014 cm−3. Activation energy of as-grown sample was 0.20 eV, while that of the sample doped with Sb (1.08%) was 30 meV larger. An increase in mobility with maximum value of 12 cm2 V−1 s−1 was observed for all doped SnS films. However, all doped films were found to keep their p-type conductivity.

Effect of Very High-Fluence Proton Radiation on 6H-SiC Photoconductive Proton Detectors

In this work, the effect of very high-fluence 100 MeV proton radiation on the performance of 6H-SiC photoconductive proton detectors is studied. The irradiation fluence is up to ${1.6}\times {10}^{{{17}}}{\mathrm {cm}}^{{\text {-2}}}$ and the degradation process of the SiC detector is continuously monitored for the first time. Before proton irradiation, the detector shows a low dark current of ~0.8nA/cm2 at 1000 V bias. As the irradiation fluence increases, the dark current continuously drops, which should be caused by irradiation damage induced carrier compensation defects. Meanwhile, the output current of the SiC detector shows an exponential decay behavior and tends to saturate at irradiation fluence up to ${5}\times {10}^{{{16}}}{\mathrm {cm}}^{{\text {-2}}}$ . At the end of the very high fluence irradiation test, the detector still exhibits ~20% of its original output current value, suggesting the excellent radiation hardness of SiC for proton detection.

Influence of neutron irradiation on deep levels in Ge-doped (010) β-Ga2O3 layers grown by plasma-assisted molecular beam epitaxy

The impact of high energy neutron irradiation on the creation of specific radiation-induced deep level defect states and the ensuing influence of these defects on the electronic properties of (010) β-Ga2O3, doped with Ge and grown by plasma-assisted molecular beam epitaxy, were explored. A significant amount of carrier removal was observed in the irradiated samples exposed to 1 MeV equivalent neutron fluences of 8.5 × 1014 cm−2 and 1.7 × 1015 cm−2, which suggests the formation of compensating defects by neutron irradiation. Using a combination of deep level transient/optical spectroscopy (DLTS/DLOS) techniques to probe the entire ∼4.8 eV bandgap with high energy resolution, three specific trap states were introduced by neutron irradiation at EC-1.22 eV, EC-2.00 eV, and EC-0.78 eV. Of these, the former two states, observed by DLOS, were also present prior to irradiation, whereas the trap at EC-0.78 eV, observed by DLTS, was not evident prior to neutron irradiation. The radiation dependence suggests that intrinsic point defects are the likely physical sources for these states. Subsequent lighted capacitance-voltage measurements further revealed that these three states are the source for the observed strong carrier compensation, with the trap at EC-2.00 eV appearing as the strongest compensating defect for the neutron-irradiated β-Ga2O3.