High Hole Concentration Research Articles

Electronic Transport and Thermoelectric Properties of Te-Doped Tetrahedrites Cu12Sb4-yTeyS13

Tetrahedrite is a promising thermoelectric material mainly due to its low thermal conductivity, a consequence of its complicated crystal structure. However, tetrahedrite has a high hole concentration; therefore, optimizing carrier concentration through doping is required to maximize the power factor. In this study, Te-doped tetrahedrites Cu12Sb4-yTeyS13 (0.1 ≤ y ≤ 0.4) were prepared using mechanical alloying and hot pressing. The mechanical alloying successfully prepared the tetrahedrites doped with Te at the Sb sites without secondary phases, and the hot pressing produced densely sintered bodies with a relative density >99.7%. As the Te content increased, the lattice constant increased from 1.0334 to 1.0346 nm, confirming the successful substitution of Te at the Sb sites. Te-doped tetrahedrites exhibited p-type characteristics, which were confirmed by the positive signs of the Hall and Seebeck coefficients. The carrier concentration decreased but the mobility increased with Te content. The electrical conductivity was relatively constant at 323–723 K, and decreased with Te substitution from 2.6 × 104 to 1.6 × 104 Sm-1 at 723 K. The Seebeck coefficient increased with temperature and Te content, achieving values of 184–204 μVK-1 at 723 K. The thermal conductivity was <1.0 Wm-1K-1, and decreased with increasing Te content. Cu12Sb3.9Te0.1S13 exhibited the highest dimensionless figure of merit (ZT = 0.80) at 723 K, achieving a high power factor (0.91 mWm-1K-2) and a low thermal conductivity (0.80 Wm-1K-1).

High Hole Concentration and Diffusion Suppression of Heavily Mg-Doped p-GaN for Application in Enhanced-Mode GaN HEMT

The effect of Mg doping on the electrical and optical properties of the p-GaN/AlGaN structures on a Si substrate grown by metal organic chemical vapor deposition was investigated. The Hall measurement showed that the activation efficiency of the sample with a 450 sccm Cp2Mg flow rate reached a maximum value of 2.22%. No reversion of the hole concentration was observed due to the existence of stress in the designed sample structures. This is attributed to the higher Mg-to-Ga incorporation rate resulting from the restriction of self-compensation under compressive strain. In addition, by using an AlN interlayer (IL) at the interface of p-GaN/AlGaN, the activation rate can be further improved after the doping concentration reaches saturation, and the diffusion of Mg atoms can also be effectively suppressed. A high hole concentration of about 1.3 × 1018 cm−3 can be achieved in the p-GaN/AlN-IL/AlGaN structure.

The Effect of RF Sputtering Temperature Conditions on the Structural and Physical Properties of Grown SbGaN Thin Film

By using a single ceramic SbGaN target containing a 14% Sb dopant, Sb0.14GaN films were successfully grown on n-Si(100), SiO2/Si(100), and quartz substrates by an RF reactive sputtering technology at different growth temperatures, ranging from 100 to 400 °C. As a result, the structural characteristics, and optical and electrical properties of the deposited Sb0.14GaN films were affected by the various substrate temperature conditions. By heating the temperature deposition differently, the sputtered Sb0.14GaN films had a wurtzite crystal structure with a preferential (101¯0) plane, and these Sb0.14GaN films experienced a structural distortion and exhibited p-type layers. At the highest depositing temperature of 400 °C, the Sb0.14GaN film had the smallest bandgap energy of 2.78 eV, and the highest hole concentration of 8.97 × 1016 cm−3, a conductivity of 2.1 Scm−1, and a high electrical mobility of 146 cm2V−1s−1. The p-Sb0.14GaN/n-Si heterojunction diode was tested at different temperatures, ranging from 25 to 150 °C. The testing data showed that the change of testing temperature affected the electrical characteristics of the diode.

Li-based selenized Cu2ZnSnS4 surface: Possible route to overcoming voc-deficit of kesterite solar cells

Usually, open-circuit voltage (Voc) of thin film solar cells significantly depends on the band bending at the front interface of an absorber/buffer. The failed band bending at a Cu2ZnSnS4/CdS (CZTS/CdS) interface severely hinders the increase in Voc due to the excessively high concentration of holes at the CZTS side. Alleviating the strong p-type or converting it to weak n-type at the CZTS surface is a credible idea to overcome the Voc− deficit. First-principles calculations show that the Li-based selenized CZTS surface presents the desired property with excellent advantages: (i) The greatly improved defects and band offset suppress carrier nonradiative recombination and facilitate electrons transmission, respectively. (ii) Its intrinsic weak n-type characteristic effectively promotes the large band bending at the interface.

Electrical properties and structural defects of p-type GaN layers grown by halide vapor phase epitaxy

The electrical properties and structural defects of p-type GaN layers with Mg concentrations from 8.0 × 1018 to 8.3 × 1019 cm−3 grown by halide vapor phase epitaxy (HVPE) were investigated. In all samples, p-type conduction was confirmed at room temperature. The hole concentration at room temperature decreased in a heavily Mg-doped sample. By analyzing the results of Hall-effect measurements at various temperatures, the acceptor concentration decreased in a heavily Mg-doped sample, whereas the compensating donor concentration increased. These results affect the decrease in the hole concentration. The hole mobility decreased with increasing acceptor concentration. In the heavily Mg-doped sample, pyramidal inversion domains (PIDs) were formed. The size of each PID in an HVPE-grown sample is in good agreement with that Mg-doped GaN layers grown by metalorganic vapor phase epitaxy (MOVPE). Thus, the formation mechanism of PIDs in HVPE-grown samples is possibly the same as that in MOVPE-grown samples. Energy-dispersive X-ray spectroscopy shows that Mg atoms accumulate in PIDs, which suggests that Mg atoms in PIDs are electrically inactive, inhibiting the increase in the acceptor concentration. These results are useful guidelines for fabricating p-type GaN layers with higher hole concentrations by HVPE.

Electrodeposition of cuprous oxide on a porous copper framework for an improved photoelectrochemical performance

Photoelectrochemical (PEC) water splitting can be an efficient and economically feasible alternative for hydrogen production if easily processed photoelectrodes made of inexpensive and abundant materials are employed. Here, we present the preparation of porous Cu2O photocathodes with good PEC performance using solely inexpensive electrodeposition methods. Firstly, porous Cu structures with delicate pore networks were deposited on flat Cu substrates employing hydrogen-bubble-assisted Cu deposition. In a second electrodeposition step, the porous Cu structures were mechanically reinforced and subsequently detached from the substrates to obtain free-standing porous frameworks. In a third and final step, photoactive Cu2O films were electrodeposited. The PEC water splitting performance in 0.5 M Na2SO4 (pH ∼6) shows that these photocathodes have photocurrents of up to −2.25 mA cm−2 at 0 V versus RHE while maintaining a low dark current. In contrast, the Cu2O deposited on a flat Cu sample showed photocurrents only up to −1.25 mA cm−2. This performance increase results from the significantly higher reactive surface area while maintaining a thin and homogeneous Cu2O layer with small grain sizes and therefore higher hole concentrations as determined by Mott-Schottky analysis. The free-standing porous Cu2O samples show a direct optical transmittance of 23% (λ = 400–800 nm) and can therefore be used in tandem structures with a photoanode in full PEC cells.Graphical abstract

Highly-doped p-type few-layer graphene on UID off-axis homoepitaxial 4H–SiC

In this report we investigate structural and electrical properties of epitaxial Chemical Vapor Deposition quasi-free-standing graphene on an unintentionally-doped homoepitaxial layer grown on a conducting 4H–SiC substrate 4° off-axis from the basal [0001] direction towards [11-20]. Due to high density of SiC vicinal surfaces the deposited graphene is densely stepped and gains unique characteristics. Its morphology is studied with atomic force and scanning electron microscopy. Its few-layer character and p-type conductance are deduced from a Raman map and its layers structure determined from a high-resolution X-ray diffraction pattern. Transport properties of the graphene are estimated through Hall effect measurements between 100 and 350 K. The results reveal an unusually low sheet resistance below 100 Ω/sq and high hole concentration of the order of 1015 cm−2. We find that the material's electrical properties resemble those of an epitaxially-grown SiC PIN diode, making it an attractive platform for the semiconductor devices technology.

Thermoelectric Performance Optimization and Phase Transition of GeTe by Alloying with Orthorhombic CuSbSe2

GeTe-based compounds are promising thermoelectric materials at medium temperature, while the performance of the GeTe matrix is at the mercy of its high hole concentration and high thermal conductiv...

Dilute nitrides heterostructures grown by liquid phase epitaxy for solar cells applications

In this paper, we present a study on liquid phase epitaxy (LPE) grown dilute nitride GaAsSbN layers and p-i-n heterostructures for use in multijunction solar cells. The composition of the layers and chemical bonding of Sb and N in the compounds were determined by energy- dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). The electrical properties of the grown samples were characterized by Hall effect measurements. Nominally undoped layers are n-type with Hall carrier concentration of 5 × 1016cm-3. Mg was successfully used as acceptor dopant for obtaining closely compensated layers with electron concentration of 1015 cm-3 as well as p-type layers with high free hole concentrations in the range (5-7) × 1018cm-3. Temperature-dependent photoluminescence spectra at low and high excitation were measured to evaluate the optical quality and identify localized states in the grown layers. Non-contact surface photovoltage method provided information about the absorption characteristics of the GaAsSbN layers. A series of GaAs/GaAsSbN/GaAs heterostructures based on closely compensated i-GaAsSbN have been also grown by LPE. The red limit of the structures determined from surface photovoltage measurements was extended down to 1.2 eV. Single junction p-i-n solar cells with area 0.16 cm2 were performed based on the grown structures. A power conversion efficiency of 4.1 % was measured for the fabricated cells under AM1.5 air global conditions.

High Mg activation in implanted GaN by high temperature and ultrahigh pressure annealing

We demonstrate high p-type conductivity and hole concentrations >1018 cm−3 in Mg-implanted GaN. The implantation was performed at room temperature and by post-implantation annealing at 1 GPa of N2 and in a temperature range of 1200–1400 °C. The high pressure thermodynamically stabilized the GaN surface without the need of a capping layer. We introduce a “diffusion budget,” related to the diffusion length, as a convenient engineering parameter for comparing samples annealed at different temperatures and for different times. Although damage recovery, as measured by XRD, was achieved at relatively low diffusion budgets, these samples did not show p-type conductivity. Further analyses showed heavy compensation by the implantation-induced defects. Higher diffusion budgets resulted in a low Mg ionization energy (∼115 meV) and almost complete Mg activation. For even higher diffusion budgets, we observed significant loss of Mg to the surface and a commensurate reduction in the hole conductivity. High compensation at low diffusion budgets and loss of Mg at high diffusion budgets present a unique challenge for shallow implants. A direct control of the formation of compensating defects arising from the implantation damage may be necessary to achieve both hole conductivity and low Mg diffusion.

Impact of Cu addition on the optoelectronic properties of Zn3N2 thin films: n to p-type transitions

Copper-doped Zinc Nitride (Cu:Zn3N2) films are grown on glass and Si substrates at various concentration of Cu (2.7, 4.9 and 5.7 at.%) by reactive RF magnetron co-sputtering. The impact of Cu atomic concentration on the structural and optoelectronic properties of Zn3N2 films is discussed in detail. Incorporation of Cu ions into Zn3N2 host lattice is confirmed by EDAX analysis. Hall Effect results indicate that the fabricated film exhibited p-type conductivity with high hole concentration of 1.78–1.36 × 1018 cm−3, resistivity of 0.48–0.56 Ω cm, and high hole mobility of ~8 cm2 V−1 s−1. The transmittance of Cu:Zn3N2 films are found to be in the range of 11–18% at the wavelength of 500 nm and hence the band gap decreases from 1.80 to 1.61 eV when Cu doping content is increased. These findings would assist Cu:Zn3N2 as a potential candidate for p-type layer in thin film solar cells.

Arsenic Doping Upon the Deposition of CdTe Layers from Dimethylcadmium and Diisopropyltellurium

The incorporation and activation of arsenic from tris(dimethylamino)arsine in CdTe layers grown by metal-organic chemical vapor deposition from dimethylcadmium and diisopropyltellurium on GaAs substrates are investigated. The incorporation of arsenic into CdTe depends on the crystallographic orientation of the layers and increases in the series (111)B → (100) → (310). The arsenic concentration in the CdTe layers is proportional to the tris(dimethylamino)arsine flow rate at a power of 1.4 and increases with decreasing diisopropyltellurium/dimethylcadmium ratio from 1.4 to 0.5. After deposition, the CdTe:As layers have p-type conductivity with an arsenic concentration of 1 × 1017–7 × 1018 cm–3 and a hole concentration of 2.7 × 1014–4.6 × 1015 cm–3, respectively; the fraction of electrically active arsenic does not exceed ~0.3%. After annealing in argon (250–450°C), the highest hole concentration is 1 × 1017 cm–3, and the arsenic activation efficiency is ~4.5%. The ionization energy of arsenic determined from the temperature dependence of the hole concentration is in the range of 98–124 meV. The low-temperature photoluminescence spectra of the layers have an emission peak with an energy of ~1.51 eV, which can be attributed to donor–acceptor recombination, where AsTe is an acceptor with an ionization energy of ~90 meV.

Z_{2} Parton Phases in the Mixed-Dimensional t-J_{z} Model.

We study the interplay of spin and charge degrees of freedom in a doped Ising antiferromagnet, where the motion of charges is restricted to one dimension. The phase diagram of this mixed-dimensional t-J_{z} model can be understood in terms of spinless chargons coupled to a Z_{2} lattice gauge field. The antiferromagnetic couplings give rise to interactions between Z_{2} electric field lines which, in turn, lead to a robust stripe phase at low temperatures. At higher temperatures, a confined meson-gas phase is found for low doping whereas at higher doping values, a robust deconfined chargon-gas phase is seen, which features hidden antiferromagnetic order. We confirm these phases in quantum MonteCarlo simulations. Our model can be implemented and its phases detected with existing technology in ultracold atom experiments. The critical temperature for stripe formation with a sufficiently high hole concentration is around the spin-exchange energy J_{z}, i.e., well within reach of current experiments.

Improvement of thermoelectric properties of SnTe by Mn Bi codoping

SnTe is an attracted lead-free thermoelectric material, but the thermoelectric performance is severely limited by the high hole concentration, undesirable valence band structure and high thermal conductivity. In this work, we study the multiple effects of MnBi codoping in SnTe to synergistically improve the overall power factor and ZT values. It is found that the MnBi codoping effectively reduces the hole concentration to an optimal range. As expected, Mn and Bi doping obviously decreases the energy separation between the two valence band maxima, enabling pronounced band convergence and improved Seebeck coefficient. Moreover, MnBi codoping introduces various phonon scattering centers to scatter a wide spectrum of phonons for a suppressed lattice thermal conductivity of 0.67 W m−1 K−1 at 850 K. The synergy of these effects yields a peak ZT of 1.3 at 850 K and an average ZT of 0.68 (300–850 K) in Sn0.90Mn0.07Bi0.03Te, suggesting SnTe-based material a promising candidate for medium-temperature thermoelectric applications.

Molybdenum Tungsten Disulfide with a Large Number of Sulfur Vacancies and Electronic Unoccupied States on Silicon Micropillars for Solar Hydrogen Evolution.

Hydrogen energy is a promising alternative for fossil fuels because of its high energy density and carbon-free emission. Si is an ideal light absorber used in solar water splitting to produce H2 gas because of its small band gap, appropriate conduction band position, and high theoretical photocurrent. However, the overpotential required to drive the photoelectrochemical (PEC) hydrogen evolution reaction (HER) on bare Si electrodes is severely high owing to its sluggish kinetics. Herein, a molybdenum tungsten disulfide (MoS2-WS2) composite decorated on a Si photoabsorber is used as a cocatalyst to accelerate HER kinetics and enhance PEC performance. This MoS2-WS2 hybrid showed superior catalytic activity compared with pristine MoS2 or WS2. The optimal MoS2-WS2/Si electrode delivered a photocurrent of -25.9 mA/cm2 at 0 V (vs reversible hydrogen electrode). X-ray absorption spectroscopy demonstrated that MoS2-WS2 possessed a high hole concentration of unoccupied electronic states in the MoS2 component, which could promote to accept large amounts of carriers from the Si photoabsorber. Moreover, a large number of sulfur vacancies are generated in the MoS2 constituent of this hybrid cocatalyst. These sulfur defects served as HER active sites to boost the catalytic efficiency. Besides, the TiO2-protective MoS2-WS2/Si photocathode maintained a current density of -15.0 mA/cm2 after 16 h of the photocatalytic stability measurement.

Акцепторное легирование мышьяком при осаждении слоев CdTe из диметилкадмия и диизопропилтеллура

The incorporation and activation of arsenic from tris(dimethylamino)arsine in CdTe layers grown by metalorganic chemical vapor deposition with dimethylcadmium and diisopropyltellurium on GaAs substrates are investigated. Arsenic incorporation into CdTe to depend on the crystallographic orientation of the layers and increases in the order (111)B<(100)<(310). Arsenic concentration in the CdTe layers is proportional to the tris(dimethylamino)arsine flow rate to the power of 1.4 and an increase with decrease of the diisopropyltellurium/dimethylcadmium ratio from 1.4 to 0.5. The as-grown CdTe:As layers had p-type conductivity with arsenic and hole concentrations of 1·1017–7·1018 and 2.7·1014–4.6·1015 cm–3, respectively, but the arsenic activation efficiency not exceeding 0.3%. After annealing in argon flow (250–450 ° C) the highest hole concentration and arsenic activation efficiency were 1·1017 cm–3 and ~4.5 % respectively. The ionization energy of arsenic determined from the temperature dependence of the hole concentration was in the range of 98–124 meV. Low-temperature photoluminescence spectra of the layers showed an emission peak with energy of 1.51 eV, which can be attributed to donor-acceptor recombination, where the acceptor is AsTe with ionization energy about 90 meV.

Enhancement of current injection efficiency of AlGaN-based deep-ultraviolet light-emitting diodes by controlling strain relaxation

The external quantum efficiency (EQE) in electrically injected AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) is severely limited by their poor current injection efficiency (CIE). We report improvement in the CIE via controlling the relaxation of strains in the p-AlGaN hole injection layer (HIL) and the electron blocking layer (EBL). Simulation results show that an unrelaxed strain in the HIL associated with a relaxed strain in EBL can significantly enhance CIE. Deeper analysis indicates that high hole concentrations can be generated at HIL/EBL interface by strain-induced piezoelectric fields, which can then provide abundant numbers of holes for injection into quantum wells. Two sub-280 nm DUV-LEDs were fabricated with specific designs for different strain relaxations in the p-AlGaN HIL by changing the HIL thickness from 200 to 20 nm. The strain difference was identified using Raman spectroscopy. Electroluminescence measurements demonstrated much higher EQE in the strained-HIL DUV-LEDs. By separating the EQE contributions of three efficiencies, i.e. the CIE, the radiative recombination efficiency and the light extraction efficiency, we found that the EQE enhancement could mainly be attributed to the improved CIE, which agreed well with the simulation results.

Ba-acceptor doping in ZnSnN2 by reactive RF magnetron sputtering: (002) faceted Ba–ZnSnN2 films

Zn-IV-N2 semiconductors display outstanding optoelectronic behavior and hence developed as an effective thin film absorber materials in photovoltaic devices. However, alkaline-earth metallic dopants can improve the performance of Zn-IV-N2 semiconductors. In view of this, Barium (Ba) acceptor was successfully doped into ZnSnN2 crystal lattice with various dopant concentrations (∼0–7%) by reactive RF magnetron co-sputtering at 450 °C. The orthorhombic structured Ba doped ZnSnN2 (Ba:ZTN) films with preferential orientation along (002) plane were obtained. Hall measurement studies revealed that the type of conduction is changed from the conventional n-type to p-type via Ba doping. The deposited Ba:ZTN films exhibited high hole concentration in the range of 1018-1019 cm−3 with low resistivity of ∼10−1 Ω cm and a reasonable mobility of ∼0.5–5 cm2 V−1 s−1. The doping of Ba at either Zn or Sn site would be responsible for the p-type conductivity in Ba:ZTN films. The variations in the surface properties such as morphology and topography of ZnSnN2 on Ba doping were described by FE-SEM and AFM analysis. Further analysis by UV–Vis–NIR and X-ray photoelectron spectroscopies were also performed in order to reveal the optical performance and chemical bonding, composition of Ba:ZTN thin films. The existence of Ba2+ ions in Ba:ZTN was confirmed by the obtained binding energies from XPS analysis. From the collective results, it is suggested that Ba:ZTN could be employed as effective p-type layer in thin film solar cells.

Controlling Defect Formation of Nanoscale AlN: Toward Efficient Current Conduction of Ultrawide‐Bandgap Semiconductors

AbstractUltrawide‐bandgap semiconductors such as AlN, BN, and diamond hold tremendous promise for high‐efficiency deep‐ultraviolet optoelectronics and high‐power/frequency electronics, but their practical application has been limited by poor current conduction. Through a combined theoretical and experimental study, it is shown that a critical challenge can be addressed for AlN nanostructures by using N‐rich epitaxy. Under N‐rich conditions, the p‐type Al‐substitutional Mg‐dopant formation energy is significantly reduced by 2 eV, whereas the formation energy for N‐vacancy related compensating defects is increased by ≈3 eV, both of which are essential to achieve high hole concentrations of AlN. Detailed analysis of the current−voltage characteristics of AlN p‐i‐n diodes suggests that current conduction is dominated by hole‐carrier tunneling at room temperature, which is directly related to the activation energy of Mg dopants. At high Mg concentrations, the dispersion of Mg acceptor energy levels leads to drastically reduced activation energy for a portion of Mg dopants, evidenced by the small tunneling energy of 67 meV, which explains the efficient current conduction and the very small turn‐on voltage (≈5 V) for the diodes made of nanoscale AlN. This work shows that nanostructures can overcome the dopability challenges of ultrawide‐bandgap semiconductors and significantly increase the efficiency of devices.

Fabrication of a polycrystalline SiGe- and Ge-on-insulator by Ge condensation of amorphous SiGe on a SiO2/Si substrate

In this work, the Ge condensation effect of amorphous SiGe on a SiO2/Si substrate is systematically investigated. As Ge condensation proceeds, the Ge content gradually enriches from an initial 0.24–1.0 with improving crystal quality. The enlargement of the grain size results in gradual roughening of the surface roughness. As the Ge content reaches 0.36, a high hole mobility of ∼211 cm2 · V−1 · s−1 is achieved with a hole concentration of ∼3.7 × 1015 cm−3. As the Ge content further accumulates, the grain number increases resulting in a higher hole concentration. The film mobility gradually deteriorates probably due to the following factors: strong impurity scattering at high hole concentration, increase of grain boundaries, decrease of SiGe thickness, and increase of surface roughness. A polycrystalline Ge-on-insulator with a hole concentration of ∼5.1 × 1018 cm−3 and mobility of ∼15 cm2 · V−1 · s−1 is ultimately fabricated. The investigation provides a promising method to fabricate a high hole mobility SiGe-on-insulator platform from low-cost amorphous SiGe.