Unintentional N-type Conductivity Research Articles

Atomistic Origins of Various Luminescent Centers and n-Type Conductivity in GaN: Exploring the Point Defects Induced by Cr, Mn, and O through an Ab Initio Thermodynamic Approach.

GaN is a technologically indispensable material for various optoelectronic properties, mainly due to the dopant-induced or native atomic-scale point defects that can create single photon emitters, a range of luminescence bands, and n- or p-type conductivities. Among the various dopants, chromium and manganese-induced defects have been of particular interest over the past few years, because some of them contribute to our present-day light-emitting diode (LED) and spintronic technologies. However, the nature of such atomistic centers in Cr and Mn-doped GaN is yet to be understood. A comprehensive defect thermodynamic analysis of Cr- and Mn-induced defects is essential for their engineering in GaN crystals because by mapping out the defect stabilities as a function of crystal growth parameters, we can maximize the concentration of the target point defects. We therefore investigate chromium and manganese-induced defects in GaN with ab initio methods using the highly accurate exchange-correlation hybrid functionals, and the phase transformations upon excess incorporation of these dopants using the CALPHAD method. We also investigate the impact of oxygen codoping that can be unintentionally incorporated during crystal growth. Our analysis sheds light on the atomistic cause of the unintentional n-type conductivity in GaN, being ON-related. In the case of Cr doping, the formation of CrGa defects is the most dominant, with an E +/0 charge transition at E VBM + 2.19 eV. Increasing nitrogen partial pressure tends to enhance the concentration of CrGa. However, in the case of doping with Mn, several different Mn-related centers can form depending on the growth conditions, with MnGa being the most dominant. MnGa possesses the E 2+/+, E +/0, and E 0/- charge transitions at 0.56, 1.04, and 2.10 eV above the VBM. The incorporation of oxygen tends to cause the formation of the MnGa-VGa center, which explains a series of prior experimental observations in Mn-doped GaN. We provide a powerful tool for point defect engineering in wide band gap binary semiconductors that can be readily used to design optimal crystal growth protocols.

The deep-acceptor nature of the chalcogen vacancies in 2D transition-metal dichalcogenides

Chalcogen vacancies in the semiconducting monolayer transition-metal dichalcogenides (TMDs) have frequently been invoked to explain a wide range of phenomena, including both unintentional p-type and n-type conductivity, as well as sub-band gap defect levels measured via tunneling or optical spectroscopy. These conflicting interpretations of the deep versus shallow nature of the chalcogen vacancies are due in part to shortcomings in prior first-principles calculations of defects in the semiconducting two-dimensional TMDs that have been used to explain experimental observations. Here we report results of hybrid density functional calculations for the chalcogen vacancy in a series of monolayer TMDs, correctly referencing the thermodynamic charge transition levels to the fundamental band gap (as opposed to the optical band gap). We find that the chalcogen vacancies are deep acceptors and cannot lead to n-type or p-type conductivity. Both the (0/−1) and (−1/−2) transition levels occur in the gap, leading to paramagnetic charge states S=1/2 and S = 1, respectively, in a collinear-spin representation. We discuss trends in terms of the band alignments between the TMDs, which can serve as a guide to future experimental studies of vacancy behavior.

Sources of n-Type Conductivity in Single Crystal β-Ga2O3 and Monoclinic WO3 Nanoparticles

The source of the commonly observed unintentional n-type conductivity in wide-band-gap oxides is controversial. For high-quality β-Ga2O3 single crystals, the physical characteristics of H-containing defects are studied by solid-state nuclear magnetic resonance (ssNMR) and Fourier transform infrared (FT-IR) spectroscopy. NMR observation demonstrated that the hydrogen atoms occur exclusively in the positive charge state, and three trapped protons are housed in two “half vacancies” left by the Ga vacancy to form a VGa–H2+1 complex. Protons trapped in the vacancies are hardly ever able to diffuse out due to the strong hydrogen bonds they establish, even at relatively high temperatures. The hydrogen atoms in one of the two “half vacancies” are the bridged protons in Ga–O(H2)–Ga bonds, the hydrogen atoms can readily donate electrons to the conduction band, and these hydrogen atoms become the shallow donors. The VGa–H2+1 complex provides high-quality β-Ga2O3 single crystals with the extra-stability and the n-type conductivity. For monoclinic WO3 nanoparticles, NMR, FT-IR, and EPR results show that the bridged proton in W–O(H2)–W groups was assumed to be responsible for the so-called “hidden” hydrogen, which can be converted into a shallow donor in the course of sample processing. Our results provide a guide to more refined experimental studies of point defects and impurities in wide-band-gap oxides and their influence on the control of n-type conductivity.

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.

Hybrid density functional studies of native defects and H impurities in wurtzite CdSe.

In this study, the formation energies and electronic properties of six native defects as well as H impurities in wurtzite (wz) CdSe are systematically investigated using hybrid density functional calculations. It is shown that native defects, including antisite CdSe and interstitial Cdi, may be sources of the unintentional n-type conductivity in CdSe under Se-poor conditions; meanwhile, the vacancy defect VSe is not a good donor. However, when the common H impurity is considered, it is suggested that both the substitutional impurity HSe and the interstitial impurity Hi are the dominant and effective origins of the unintentional n-type conductivity in Se-poor conditions. However, unintentional p-type conductivity in CdSe is challenging to form regardless of the growth conditions. Moreover, hybrid functional calculations of the electronic structures show that the six native point defects and the extrinsic impurities Hi and HSe will cause more or fewer changes in the band gap. Among all considered defects and impurities, it is found that only the interstitial defect Cdi introduces impurity levels into the band gap. In particular, the present hybrid functional calculations theoretically affirm that the vacancy defect VCd in CdSe can induce a 2 μB magnetic moment; however, other native defects will not introduce any magnetic moment.

Thermodynamic of intrinsic defects in β-Ga2O3

The stability of intrinsic point defects in β-Ga2O3 under varied temperature, Fermi energy and oxygen partial pressure has been studied using the hybrid functional (HSE06) and thermodynamic method. The results indicate that the contribution of vibrational entropy to formation energy should not be ignored, especially under high temperature. Gallium interstitials, oxygen vacancies, and gallium vacancies are the most stable intrinsic point defects in crystal when Fermi level varies from the valence-band maximum to the conduction-band minimum. Donors (oxygen vacancies and gallium interstitials) and acceptors (gallium vacancies and oxygen interstitials) have deep-level transitions (more than 1eV), so they don't contribute to the unintentional n-type or p-type conductivity. We propose that the high temperature and high oxygen partial pressure are in favor of p-type doping in β-Ga2O3 crystal by weakening the self-compensation effect.

Photoluminescence Study of Deep Level Defects in ZnO Thin Films

In the present work, zinc oxide (ZnO) thin films were prepared by heating, at 500 ∘C, metallic Zn films deposited onto Si (100) substrates by RF magnetron sputtering. According to the x-rays diffraction patterns, the polycrystalline hexagonal ZnO phase (wurtzite) was obtained with a preferential orientation along the (101) planes. The increase of both the crystallites size and the Zn-O bond length, as a function of the heating time, reflect the improvement of the crystalline quality of the investigated films. The investigated films emitted in ultraviolet, visible and infrared regions. The ultraviolet emission was linked to the crystalline quality of the films. All the visible emission related defects were identified. Their concentrations vary differently as a function of the heating time. The infrared emission originated from the oxygen anti-sites (OZn). The correlation between the decrease of the electrical resistivity and the increase of both 2 + charged oxygen vacancies ( $V_{O}^{++})$ and hydrogen impurities (H-I) defects suggested that the unintentional n-type conductivity in ZnO came from the collective contribution of $V_{O}^{++}$ and H-I defects.

Hybrid functional study of native point defects and impurities in ZnGeN2

Using hybrid density functional theory, we investigate the properties of native point defects and hydrogen and oxygen impurities in ZnGeN2, a wide-band-gap semiconductor that is promising for applications in electronic and optoelectronic devices. We find that cation antisites have the lowest formation energies amongst all of the native point defects for a wide range of chemical potential conditions. However, native point defects cannot act as sources of doping. Unintentional n-type conductivity in ZnGeN2 must be attributed to impurities: substitutional oxygen on a nitrogen site and interstitial hydrogen act as donors.

Oxygen vacancies: The origin ofn-type conductivity in ZnO

Oxygen vacancy (VO) is a common native point defects that plays crucial roles in determining the physical and chemical properties of metal oxides such as ZnO. However, fundamental understanding of VO is still very sparse. Specifically, whether VO is mainly responsible for the n-type conductivity in ZnO has been still unsettled in the past fifty years. Here we report on a study of oxygen self-diffusion by conceiving and growing oxygen-isotope ZnO heterostructures with delicately-controlled chemical potential and Fermi level. The diffusion process is found to be predominantly mediated by VO. We further demonstrate that, in contrast to the general belief of their neutral attribute, the oxygen vacancies in ZnO are actually +2 charged and thus responsible for the unintentional n-type conductivity as well as the non-stoichiometry of ZnO. The methodology can be extended to study oxygen-related point defects and their energetics in other technologically important oxide materials.

Indium oxide—a transparent, wide-band gap semiconductor for (opto)electronic applications

The present review takes a semiconductor physics perspective to summarize the state-of-the art of In2O3 in relation to applications. After discussing conventional and novel applications, the crystal structure, synthesis of single-crystalline material, band-structure and optical transparency are briefly introduced before focussing on the charge carrier transport properties. The issues of unintentional n-type conductivity and its likely causes, the surface electron accumulation, and the lack of p-type conductivity will be presented. Intentional doping will be demonstrated to control the electron concentration and resistivity over a wide range, but is also subject to compensation. The control of the surface accumulation in relation to Schottky and ohmic contacts will be demonstrated. In the context of scattering mechanisms, the electron mobility and its limits will be discussed. Finally, the Seebeck coefficient and its significance will be shown, and ferromagnetic doping of In2O3 will be critically discussed. With this overview most if not all ingredients for the use of In2O3 as semiconductor material in novel or improved conventional devices will be given.

Electron transport in semiconducting SnO2: Intentional bulk donors and acceptors, the interface, and the surface

Abstract

Electron mobility limited by scattering from screened positively charged dislocation lines within indium nitride

In the present work, we address the open question of the contribution from threading dislocations to the problem of unintentional n-type conductivity exhibited by indium nitride through an examination of the effect that positively charged dislocation lines have on the transverse electron mobility within this material. Assuming that the threading dislocation lines within indium nitride act as a source for free electrons, the screening associated with the positively charged threading dislocation lines is evaluated. The impact this screening has on the dislocation limited electron mobility within this material is then considered. Our results indicate that one of the implications of attributing a donor character to the threading dislocation lines within indium nitride would be a strong non-uniformity in the free electron concentration in the plane of growth of this semiconductor. This contrasts dramatically with the case of gallium nitride.

Hydrogen in oxides and nitrides: unexpected physics and impact on devices

Controlling the conductivity of wide-band-gap semiconductors is key to enabling applications in electronics and optoelectronics. Many oxides exhibit unintentional n-type conductivity, and oxygen vacancies have been widely discussed as the source of this conductivity. Based on first-principles investigations we have shown that this cannot be true in ZnO and SnO2. We suggest that the conductivity is due to unintentional incorporation of donor impurities, with hydrogen being a likely candidate. Both interstitial and substitutional hydrogen act as shallow donors in a number of oxides. The atomic and electronic structures of these centers is discussed.

Oxygen vacancies and donor impurities in β-Ga2O3

Using hybrid functionals we have investigated the role of oxygen vacancies and various impurities in the electrical and optical properties of the transparent conducting oxide β-Ga2O3. We find that oxygen vacancies are deep donors, and thus cannot explain the unintentional n-type conductivity. Instead, we attribute the conductivity to common background impurities such as silicon and hydrogen. Monatomic hydrogen has low formation energies and acts as a shallow donor in both interstitial and substitutional configurations. We also explore other dopants, where substitutional forms of Si, Ge, Sn, F, and Cl are shown to behave as shallow donors.

Simulation of pure and defective wurtzite-type ZnO

Changes in the structural and electronic properties of zinc oxide (ZnO) due to the O vacancy and F-centre were studied using a semi-empirical quantum-chemical approach based on Hartree–Fock theory. A periodic supercell of 128 atoms was exploited throughout the study. The semi-empirical parameters for the Zn atom are obtained by reproducing the main properties of the ZnO crystal as well as the first three ionization potentials of the Zn atom. The perturbation imposed by the defect leads to atomic relaxation, which is computed and discussed in detail. It is found that electron density redistribution in the vicinity of defects plays an important role in the determination of atomic movements. The introduction of an oxygen vacancy generates a local one-electron energy level placed below the conduction band while the presence of an F-centre produces a local energy level just above the upper valence band of the material. The deep situation of the local energy level corresponding to the F-centre implies that the F-centre cannot serve as a source of unintentional n-type electrical conductivity in ZnO. Changes in the chemical bonding are observed, showing that it becomes slightly more covalent because of oxygen-vacancy-type defects.

Role of impurities and dislocations for the unintentional n-type conductivity in InN

We present a study on the role of dislocations and impurities for the unintentional n-type conductivity in high-quality InN grown by molecular beam epitaxy. The dislocation densities and H profiles in films with free electron concentrations in the low 1017cm-3 and mid 1018cm-3 range are measured, and analyzed in a comparative manner. It is shown that dislocations alone could not account for the free electron behavior in the InN films. On the other hand, large concentrations of H sufficient to explain, but exceeding substantially, the observed free electron densities are found. Furthermore, enhanced concentrations of H are revealed at the film surfaces, resembling the free electron behavior with surface electron accumulation. The low-conductive film was found to contain C and it is suggested that C passivates the H donors or acts as an acceptor, producing compensated material in this case. Therefore, it is concluded that the unintentional impurities play an important role for the unintentional n-type conductivity in InN. We suggest a scenario of H incorporation in InN that may reconcile the previously reported observations for the different role of impurities and dislocations for the unintentional n-type conductivity in InN.

Sources of unintentional conductivity in InN

Using first-principles methods, we investigate the effects of monatomic hydrogen in InN. We find that hydrogen can occupy interstitial and substitutional sites. Interstitial hydrogen is stable in the bond-center configuration and acts exclusively as a shallow donor, with a H–N stretching vibration at 3050cm−1. Hydrogen can also substitute for nitrogen in InN, bonding equally to the four In nearest neighbors in a multicenter-bond configuration. Substitutional hydrogen has low formation energy and, counterintuitively, is a double donor. Our results suggest that monatomic hydrogen is a plausible cause of the unintentional n-type conductivity that is often observed in as-grown InN.

Hydrogen multicentre bonds

The concept of a chemical bond stands out as a major development in the process of understanding how atoms are held together in molecules and solids. Lewis' classical picture of chemical bonds as shared-electron pairs evolved to the quantum-mechanical valence-bond and molecular-orbital theories, and the classification of molecules and solids in terms of their bonding type: covalent, ionic, van der Waals and metallic. Along with the more complex hydrogen bonds and three-centre bonds, they form a paradigm within which the structure of almost all molecules and solids can be understood. Here, we present evidence for hydrogen multicentre bonds-a generalization of three-centre bonds-in which a hydrogen atom equally bonds to four or more other atoms. When substituting for oxygen in metal oxides, hydrogen bonds equally to all the surrounding metal atoms, becoming fourfold coordinated in ZnO, and sixfold coordinated in MgO. These multicentre bonds are remarkably strong despite their large hydrogen-metal distances. The calculated local vibration mode frequency in MgO agrees with infrared spectroscopy measurements. Multicoordinated hydrogen also explains the dependence of electrical conductivity on oxygen partial pressure, resolving a long-standing controversy on the role of point defects in unintentional n-type conductivity of ZnO (refs 8-10).

Origin of the n-type conductivity of InN: The role of positively charged dislocations

As-grown InN is known to exhibit high unintentional n-type conductivity. Hall measurements from a range of high-quality single-crystalline epitaxially grown InN films reveal a dramatic reduction in the electron density (from low 1019 to low 1017cm−3) with increasing film thickness (from 50to12000nm). The combination of background donors from impurities and the extreme electron accumulation at InN surfaces is shown to be insufficient to reproduce the measured film thickness dependence of the free-electron density. When positively charged nitrogen vacancies (VN+) along dislocations are also included, agreement is obtained between the calculated and experimental thickness dependence of the free-electron concentration.

New insights into the role of native point defects in ZnO

Using first-principles methods based on density functional theory and pseudopotentials, we have performed a detailed study of native point defects in ZnO. Contrary to the conventional wisdom, we find that native point defects are unlikely to be the cause of the frequently observed unintentional n-type conductivity. Oxygen vacancies, which have most often been invoked as shallow donors, have high formation energies in n-type ZnO, and are actually deep donors with a very high ionization energy. Zinc interstitials are shallow donors, but have high formation energies in n-type ZnO; in addition, they are fast diffusers, and thus unlikely to be stable in n-type ZnO. Zn antisites are also shallow donors, and have even higher formation energies than zinc interstitials. They may play a role under non-equilibrium conditions such as in irradiation experiments. Zinc vacancies are deep acceptors and may act as compensating centers in n-type ZnO. Oxygen interstitials are stable in the form of electrically inactive split interstitials as well as deep acceptors at the octahedral interstitial site under n-type conditions. Our results may provide a guide to more in-depth experimental studies of point defects in ZnO and their influence on the control of doping.