P-type Conditions Research Articles

Complex defects in Al0.5Ga0.5N by first principles calculations

The complex native point defects in Al0.5Ga0.5N are studied by density functional theory (DFT) and Heyd, Scuseria and Ernzerhof (HSE) hybrid functional. The lower formation energy as well as the donor and acceptor properties of Al0.5Ga0.5N with different complex native point defects are obtained. It is found that VGa-GaN exhibits donor property under p-type conditions while VAl-VN and VGa -VN exhibit acceptor properties under n-type conditions. Then, the density of states studies indicate that the defect peaks in the Al0.5Ga0.5N bandgap are all contributed by the defect atoms or atoms near the defects. Moreover, the charge distribution and bonding states analyses show that the Ali atom in VAl -Ali forms ionic bonds with the N atoms in Al0.5Ga0.5N and the antisite Ga atom in VGa-GaN forms ionic bonds with the N atoms in Al0.5Ga0.5N. Furthermore, the thermodynamic transition energy levels exploration reveals that VGa-GaN is most likely to undergo thermodynamic transitions. Meanwhile, the binding energies analyses elucidate that VGa-GaN is the most stable in Al0.5Ga0.5N. The formation mechanism of complex native point defects in Al0.5Ga0.5N has been revealed, which helps to get a deeper insight to the growth and doping of AlGaN and expands its application in high-power and high-frequency optoelectronic devices.

Role of intrinsic defects in cubic NaNbO3: A computational study based on hybrid density-functional theory

Antiferroelectric NaNbO3 is a candidate material for application in high-energy density dielectric capacitors. In this context, various doping strategies have been used for installing the desired narrow double P–E loop behavior in this lead-free material. However, controlled doping requires a detailed understanding of the type and population of intrinsic defects, which have not been studied so far. In this study, we, therefore, calculate formation energies, electronic transition levels, and doping behavior of intrinsic defects in cubic NaNbO3 by means of electronic structure calculations based on density functional theory using a hybrid exchange-correlation functional (HSE06) and finite-size correction. The results show that the dominant defects are Na and O vacancies, and that the material is an n-type semiconductor for almost all oxygen partial pressures. Additionally, we predict the presence of a defect complex (VNa– VO– VNa) consisting of two Na vacancies and one O vacancy in two possible structures, which is stable for n- or p-type doping conditions.

Hydrogen states in mixed-cation CuIn(1-x)GaxSe2 chalcopyrite alloys: a combined study by first-principles density-functional calculations and muon-spin spectroscopy

ABSTRACT First-principles calculations were performed jointly with muon-spin (μSR) spectroscopy experiments in order to examine the electrical activity of hydrogen in mixed-cation chalcopyrite Cu(In,Ga)Se (CIGS) alloys and other related compounds commonly used as absorbers in solar-cell technology. The study targeted the range of Ga concentrations most relevant in typical solar cells. By means of a hybrid-functional approach the charge-transition levels of hydrogen were determined and the evolution of the defect pinning level, E(+/–), was monitored as a function of the Ga content. The use of E(+/–) as a metric of the charge-neutrality level allowed the alignment of band structures, thus providing the band offsets between the CuInSe compound and the CIGS alloys. The μSR measurements in both thin-film and bulk CIGS materials confirmed that the positively charged state is the thermodynamically stable configuration of hydrogen for p-type conditions. The interpretation of the μSR data further addressed the existence of a metastable quasi-atomic neutral configuration that was resolved from the calculations and led to a formation model for muon implantation.

Electrode-induced impurities in tin halide perovskite solar cell material CsSnBr3 from first principles

All-inorganic lead-free CsSnBr3 is attractive for applications in solar cells due to its nontoxicity and stability, but the device performance to date has been poor. Besides the intrinsic properties, impurities induced from electrodes may significantly influence the device performance. Here, we systematically studied the stability, transition energy levels, and diffusion of impurities from the most commonly used electrodes (Au, Ag, Cu, graphite, and graphene) in CsSnBr3 based on density functional theory calculations. Our results reveal that, whereas graphite and graphene electrodes exhibit negligible influence on CsSnBr3 due to the relatively high formation energies for carbon impurities in CsSnBr3, atoms from the metal electrodes can effectively diffuse into CsSnBr3 along interstice and form electrically active impurities in CsSnBr3. In this case, a significant amount of donor interstitial impurities, such as Ag_i^ +, Cu_i^ +, and Au_i^ +, will be formed under p-type conditions, whereas the Sn-site substitutional acceptor impurities, namely Au_{Sn}^{2 - }, Ag_{Sn}^{2 - }, and Cu_{Sn}^{2 - }, are the dominant impurities, especially under n-type conditions. In particular, except for Au_i^ +, all these major impurities from the metal electrodes act as nonradiative recombination centers in CsSnBr3 and significantly degrade the device performance. Our work highlights the distinct behaviors of the electrode impurities in CsSnBr3 and their influence on the related devices and provides valuable information for identifying suitable electrodes for optoelectronic applications.

Design and growth of GaN-based blue and green laser diodes

GaN-based laser diodes (LDs) extend the wavelength of semiconductor LDs into the visible and ultraviolet spectrum ranges, and are therefore expected to be widely used in quantum technology, bio & medical instruments, laser displays, lighting and materials processing. The development of blue and green LDs is still challenging, even though they are based on the same III-nitride materials as GaN-based light-emitting diodes. The challenges and progress of GaN-based blue and green LDs are reviewed from the aspects of epitaxial growth and layer structure design. Due to large differences in lattice constants and growth conditions for InN, GaN, and AlN, considerable effort is required to improve the quality of InGaN multiple quantum well (MQW) gain medium for blue and especially green LDs. p-type doping profiles, conditions and layer structures are critical to reduce the internal losses and to mitigate the degradation of InGaN MQWs. Hole injection is also a key issue for GaN-based LDs.

First-principles study of electronic and diffusion properties of intrinsic defects in 4H-SiC

As a wide bandgap semiconductor, SiC holds great importance for high temperature and high power devices. It is known that the intrinsic defects play key roles in determining the overall electronic properties of semiconductors; however, a comprehensive understanding of the intrinsic defect properties in the prototype 4H-SiC is still lacking. In this study, we have systematically investigated the electronic properties and kinetic behaviors of intrinsic point defects and defect complexes in 4H-SiC using advanced hybrid functional calculations. Our results show that all the point defects in 4H-SiC have relatively high formation energies, i.e., low defect concentrations even at high growth temperatures. Interestingly, it is found that the migration barriers are very high for vacancies (>3 eV) but relatively low for interstitial defects (∼1 eV) in SiC. Meanwhile, the diffusion energy barriers of defects strongly depend on their charge states due to the charge-state-dependent local environments. Furthermore, we find that VSi in SiC, a key defect for quantum spin manipulation, is unstable compared to the spin-unpolarized VC–CSi complex in terms of the total energy (under p-type conditions). Fortunately, the transformation barrier from VSi to VC–CSi is as high as 4 eV, which indicates that VSi could be stable at room (or not very high) temperature.

Native point defects and carbon clusters in 4H-SiC: A hybrid functional study

We report first-principles calculations that clarify the formation energies and charge transition levels of native point defects and carbon clusters in the 4H polytype of silicon carbide (4H-SiC) under a carbon-rich condition. We applied a hybrid functional that reproduces the experimental bandgap of SiC well and offers reliable defect properties. For point defects, we investigated single vacancies, antisites, and interstitials of Si and C on relevant sites. For carbon clusters, we systematically introduced two additional C atoms into the perfect 4H-SiC lattice with and without removing Si atoms and performed structural optimization to identify stable defect configurations. We found that neutral Si antisites are energetically favorable among Si-point defects in a wide range of the Fermi level position around the intrinsic regime, whereas negatively-charged Si vacancies and a positively-charged Si interstitial on a site surrounded by six Si and four C atoms become favorable under n- and p-type conditions, respectively. For C-point defects, neutral C antisites are favorable under intrinsic and n-type conditions, whereas positively-charged C vacancies become favorable under p-type conditions. We also found that a di-carbon antisite is more favorable than a C-split interstitial, which is the most stable form of single C interstitials.

The influence of irradiation induced vacancies on the mobility of helium in boron carbide

Boron carbide, used as a neutron absorber, undergoes nuclear reactions producing relevant quantities of He. The understanding of He kinetics at the atomic scale in the material is still in its infancy, in spite of decades of experimental work devoted to the characterization of He containing, irradiated, boron carbide samples. The interplay of He itself with intrinsic defects created by irradiation on kinetics is still almost completely unknown.In this paper we present an exhaustive study of vacancies and substitutional helium impurities in boron carbide using Density Functional Theory. Analyzing the stability and mobility of these defects allows us to consider diffusion mechanisms other than the known interstitial mechanisms.We find that vacancies trap He interstitials, raising the activation energy of 2D diffusion to approximately 2 eV. The trapping mechanism is different according to the charge state of the vacancy: in p-type conditions, when vacancies are neutral or positive, He diffuses via a dissociative mechanism and is trapped in a substitutional position; in n-type conditions, negative vacancies trap He atoms traveling in an adjacent {111} plane by a charge transfer driven distorsion.No favorable vacancy assisted diffusion mechanism was identified for substitutional He atoms, except the dissociative one previously mentioned. Other possible vacancy diffusion mechanisms, which we also analyzed, are hindered by the high activation energy of vacancy self-diffusion.

Assessing the role of hydrogen in Fermi-level pinning in chalcopyrite and kesterite solar absorbers from first-principles calculations

Understanding the impact of impurities in solar absorbers is critical to engineering high-performance in devices, particularly over extended periods of time. Here, we use hybrid functional calculations to explore the role of hydrogen interstitial (Hi) defects in the electronic properties of a number of attractive solar absorbers within the chalcopyrite and kesterite families to identify how this common impurity may influence device performance. Our results identify that Hi can inhibit the highly p-type conditions desirable for several higher-band gap absorbers and that H incorporation could detrimentally affect the open-circuit voltage (Voc) and limit device efficiencies. Additionally, we find that Hi can drive the Fermi level away from the valence band edge enough to lead to n-type conductivity in a number of chalcopyrite and kesterite absorbers, particularly those containing Ag rather than Cu. We find that these effects can lead to interfacial Fermi-level pinning that can qualitatively explain the observed performance in high-Ga content CIGSe solar cells that exhibit saturation in the Voc with increasing band gap. Our results suggest that compositional grading rather than bulk alloying, such as by creating In-rich surfaces, may be a better strategy to favorably engineering improved thin-film photovoltaics with larger-band gap absorbers.

Structural, electronic, and thermoelectric properties of La2CuBiS5

The basic physical properties of La2CuBiS5 are studied by the first-principle calculations and the semiclassical Boltzmann theory. Charge density difference calculations show that electrons accumulate between Bi–S atoms, indicating considerable covalent bonding of Bi and S atoms. A similar charge density difference indicates that the Cu–S bonds also exhibit covalent character. The calculated minimum thermal conductivity of La2CuBiS5 is low, which is conducive to its use as a thermoelectric material. Owing to a bipolar effect, induced by thermal excitation, the material’s Seebeck coefficient decreases sharply at T = 800 K. For the n-type and p-type doping conditions, the largest values of S 2 σ / τ were calculated as −1.71×1011 and 1.837×1011 W K−2 ms−1, respectively. The combination of a large dispersion and a high band degeneracy along the Γ - Y direction in the band structure simultaneously induces the highest S y value and a high σ / τ y value. Thus, the thermoelectric performance of La2CuBiS5 is anisotropic and most favorable along the y direction.

Tailoring lanthanide doping in perovskite CaTiO3 for luminescence applications.

Perovskite oxide materials have been attracting significant attention due to their rich physical and chemical properties. With its proven stability and bio-compatibility, we suggest the lanthanide-doped perovskite to be a promising material for biological luminescence applications. Here, taking CaTiO3 as a concrete example, we systematically investigate its doping properties using first-principles computational methods. We determine the conditions allowing the growth of CaTiO3 against various competing phases. We obtain the formation energies of various intrinsic point defects in the material. The doping configuration and the charge state of the lanthanide dopants are determined. We find that for heavier elements in the lanthanide family, the substitution at the Ca site is favored under p-type growth conditions and tends to be trivalent, whereas the substitution at the Ti site is favored under n-type growth conditions and tends to be divalent. And for lighter elements in the family, the substitution at the Ca site is more favored for most cases and the dopant is more likely to be trivalent. By tuning the growth conditions, one could control the valence state of the lanthanide dopant, which in turn controls the luminescence spectra. We collect and identify the emission peaks in the infrared biological window, based on which possible doping schemes are suggested for bio-labeling and imaging applications.

First-principles study on native point defects of cubic cuprite Ag2O

Using the first-principles calculations, we have systematically investigated the atomic configurations, electronic structures, formation energies and transition energies of native point defects in cuprite Ag2O. Under the conditions of Ag-rich, we find that the oxygen vacancy (VO) and the oxygen interstitial (Oi) have the lowest formation energies in p-type and n-type conditions, respectively. Silver vacancy (VAg) acts as a shallow acceptor, which has high formation energy in p-type sample. Oxygen anti-site (OAg) is the most stable state and acts as an acceptor-type point defect in the O-rich conditions. Ag interstitial (Agi) is a shallow donor, which can be formed easily in the Ag-rich conditions. Moreover, we study the band offsets of heterojunction between Ag2O in cuprite structure and ZnO, GaN, and AlN in the wurtzite structure. These results would provide guidance for the experimental studies of point defects in cuprite Ag2O.

Self-compensation due to point defects in Mg-doped GaN

Using hybrid density functional theory, we address point defects susceptible to cause charge compensation upon Mg doping of GaN. We determine the free energy of formation of the nitrogen vacancy and of several Mg-related defects. The entropic contribution as a function of temperature is determined within the quasiharmonic approximation. We find that the Mg interstitial shows a noticeably lower free energy of formation than the Mg substitutional to Ga in p-type conditions. Therefore, the Mg impurity is amphoteric behaving like an acceptor when substitutional to Ga and like a double donor when accommodated in an interstitial position. The hybrid-functional results are then linked to experimental observations by solving the charge neutrality equations for semiconductor dominated by impurities. We show that a thermodynamic equilibrium model is unable to account for the experimental hole concentration as a function of Mg doping density, due to nitrogen vacancies and Mg interstitials acting as compensating donors. To explain the experimental result, which includes a dropoff of the hole concentration at high Mg densities, we thus resort to nonequilibrium models. We show that either nitrogen vacancies or Mg interstitials could be at the origin of the self-compensation mechanism. However, only the model based on interstitial Mg donors provides a natural mechanism to account for the sudden appearance of self-compensation. Indeed, the amphoteric nature of the Mg impurity leads to Fermi-level pinning and accounts for the observed dropoff of the hole concentration of GaN samples at high Mg doping. Our work suggests that current limitations in p-type doping of GaN could be overcome by extrinsically controlling the Fermi energy during growth.

Modelling the chemistry of Mn-doped MgO for bulk and (100) surfaces.

We have investigated the energetic properties of Mn-doped MgO bulk and (100) surfaces using a QM/MM embedding computational method, calculating the formation energy for doped systems, as well as for surface defects, and the subsequent effect on chemical reactivity. Low-concentration Mn doping is endothermic for isovalent species in the bulk but exothermic for higher oxidation states under p-type conditions, and compensated by electrons going to the Fermi level rather than cation vacancies. The highest occupied dopant Mn 3d states are positioned in the MgO band gap, about 4.2 eV below the vacuum level. Surface Mn-doping is more favourable than subsurface doping, and marginally exothermic on a (100) surface at high O2 pressures. For both types of isovalent Mn-doped (100) surfaces, the formation energy for catalytically important oxygen defects is less than for pristine MgO, with F0 and F2+-centres favoured in n- and p-type conditions, respectively. In addition, F+-centres are stabilised by favourable exchange coupling between the Mn 3d states and the vacancy-localised electrons, as verified through calculation of the vertical ionisation potential. The adsorption of CO2 on to the pristine and defective (100) surface is used as a probe of chemical reactivity, with isovalent subsurface Mn dopants mildly affecting reactivity, whereas isovalent surface-positioned Mn strongly alters the chemical interactions between the substrate and adsorbate. The differing chemical reactivity, when compared to pristine MgO, justifies further detailed investigations for more varied oxidation states and dopant species.

Tuning the Ge(Sn) Tunneling FET: Influence of Drain Doping, Short Channel, and Sn Content

We present experimental results on the realization of p-channel mode Ge(Sn) heterojunction band-to-band tunneling field effect transistors. We investigate the influence of three device parameters (drain doping, channel length, and tunnel barrier height at source side) of the semiconductor body of the devices on the device performance. We achieve a complete suppression of the n-channel mode in p-type operating conditions by systematically reducing the p-type drain doping from 1·10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> to 2 · 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , examined in sample series A. In the second sample series B, we investigate the influence of a reduction of the channel length down to 15 nm on transistor performance. To improve the ON current I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> without degrading the OFF current I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OFF</sub> , we introduce a 10-nm delta layer of a Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> alloy at the source/channel junction in the third sample series C. We demonstrate an improved ON current I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> compared with the reference sample without a GeSn delta layer.

Identification of oxygen defects in CdTe revisited: First-principles study

By using first-principles calculations, several SO2 complexes in CdTe were studied. Based on experimental observation, SO2 complexes have been recently proposed by Lavrov et al. [Phys. Rev. B. 84, 233201 (2011)] to be the cause of the observed IR absorption peaks at 1096.8 and 1108.4 cm−1 in O-doped CdTe. Chen et al. [Phys. Rev. Lett. 96, 035508 (2006)] were originally proposed that the peaks come from OTe-VCd complex. Our calculations indicate that the SO2 molecule on the Te site [(SO2)Te] has a low formation energy but its calculated vibration frequencies (∼900 cm−1) are lower than the observed IR modes. However, (SO2)Te can form a complex with VCd with two possible configurations that give the vibration frequencies in a good agreement with the two observed IR peaks. The binding energies of the complex in these two configurations are about 1 eV under p-type conditions; indicating that the complex is quite stable. The two configurations are related to each other by a rotation of the SO2 molecule with an energy barrier of ∼0.4 eV. Therefore, the two configurations can co-exist at a low temperature and the high energy one gradually transforms to the low energy one as temperature increases. This agrees with the experimental observation that, at a high temperature, the two IR modes merged into one.

Formation energy of native point defects in LaBr3

Based on density function theory (DFT) and the local density approximation (LDA), the formation energy and transition levels of native point defects in LaBr3 were calculated under Br-rich conditions. From the calculated results, the following conclusions have been obtained: ① The dominant defect type is the triply positive lanthanum interstitial under p-type conditions. ② The triply negative lanthanum vacancy plays the most important role in n-type LaBr3. ③ Neutral and singly positive bromine antisites are more stable in the middle of the band gap. ④ The singly positive (negative) bromine antisite can be a potential compensation source in n-type (p-type) LaBr3. ⑤ All the transition levels in LaBr3 belong to deep levels. The optimized geometric structures of bromine interstitials and antisites show that there is no formation of Br-Br covalent bond.

Theoretical study of thermoelectric properties of few-layer MoS2 and WSe2

Molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) are prototypical layered two-dimensional transition metal dichalcogenide materials, with each layer consisting of three atomic planes. We refer to each layer as a trilayer (TL). We study the thermoelectric properties of 1-4TL MoS2 and WSe2 using a ballistic transport approach based on the electronic band structures and phonon dispersions obtained from first-principles calculations. Our results show that the thickness dependence of the thermoelectric properties is different under n-type and p-type doping conditions. Defining ZT1st peak as the first peak in the thermoelectric figure of merit ZT as doping levels increase from zero at 300 K, we found that ZT1st peak decreases as the number of layers increases for MoS2, with the exception of 2TL in n-type doping, which has a slightly higher value than 1TL. However, for WSe2, 2TL has the largest ZT1st peak in both n-type and p-type doping, with a ZT1st peak value larger than 1 for n-type WSe2. At high temperatures (T > 300 K), ZT1st peak dramatically increases when the temperature increases, especially for n-type doping. The ZT1st peak of n-type 1TL-MoS2 and 2TL-WSe2 can reach 1.6 and 2.1, respectively.

Theoretical Study on the Diffusion Mechanism of Cd in the Cu-Poor Phase of CuInSe2 Solar Cell Material

We have employed first-principles static and molecular dynamics (MD) calculations with semilocal and screened-exchange hybrid density functionals to study the diffusion of Cd in bulk CuIn5Se8, a copper-poor ordered vacancy compound of CuInSe2. The diffusion mechanism and the underlying kinetics/energetics were investigated by combining ab initio metadynamics simulations and nudged elastic band (NEB) calculations. We found that the migration of Cd occurs via a kick-out of Cu atoms, assisted by the pristine vacancies that are constitutive of this compound, and follows a double-hump energy profile. The rate-limiting step has a barrier of about 1 eV at 0 K but reduces to 0.3 eV at 850 K, pointing out non-negligible dynamical effects. Hybrid functional calculations reveal that Cd impurities are doubly positively charged (Cd2+) in p-type and intrinsic conditions. The position of the 0/2+ charge transition level explains why Cd impurities do not constitute deep traps for carriers, making them not harmful for the...

Hybrid functional with semi-empirical van der Waals study of native defects in hexagonal BN

The formation energies and transition energy levels of native defects in hexagonal BN have been studied by first-principles calculations based on hybrid density functional theory (DFT) together with an empirical dispersion correction of Grimme's DFT-D2 method. Our calculated results predict that the interstitial B is the most stable defect under N-rich and p-type conditions. While the B vacancy and interstitial N become the dominate defects when the electron chemical potential is near the conduction band maximum of host. Nevertheless, these compensating defects would be inactive due to their ultra deep ionization levels under both p- and n-type conditions.