Shallow Donor Levels Research Articles

Origin and anomalous behavior of dominant defects in 4H-SiC studied by conventional and Laplace deep level transient spectroscopy

Several deep level defects were observed by conventional deep level transient spectroscopy (DLTS) and high-resolution Laplace DLTS (LDLTS) in n-type 4H-SiC junction barrier Schottky diodes. We have shown that the broad DLTS peak labeled Z1/2 has, in fact, two components, Z1 and Z2, with activation enthalpies for electron emission of 0.63 eV and 0.68 eV, respectively. The reorientation process between these two components was observed. A combination of double-correlated DLTS and LDLTS demonstrated an anomalous reduction of the emission rate and an increase of the activation enthalpy of Z2 with an increase of the reverse bias applied to the diode. The possible explanation of this phenomenon could be correlated with a tensile stress in epitaxial SiC layers. The results observed are discussed in the frame of the model that correlates Z1 and Z2 with carbon vacancies (VC), located at hexagonal (h) and cubic (k) lattice sites, respectively. We also discussed the origin of other traps E0–E5 with particular emphasis on a N-related shallow donor level located at 0.04 eV below the conduction band, which has never been previously reported by DLTS studies.

Atomic layer deposition of nitrogen incorporated molybdenum oxide: Unveiling carrier transport mechanism and its application in Li-ion battery

Nitrogen incorporated molybdenum oxide (MoOx:N) thin films are obtained by stacking of atomic layer deposited grown MoOx and MoNx in a nanolaminate architecture without any postdeposition processing. Evolution of stoichiometrically dependent electron conductivity is obtained that is efficiently controlled by varying the ratio of the number of layers (MoOx to MoNx) in the nanolaminate structure. The carrier type, the free carrier concentration, and the mobility of the as-grown materials are studied by temperature-dependent Hall-effect measurement. The authors hypothesize that the addition of N-atom creates oxygen vacancies in the films that in turn act as a shallow donor level that results in improved conductivity in the ternary material. As-grown materials, both in the form of planar architecture and coaxially grown on multiwalled carbon nanotubes are investigated as a prospective anode material in the Li-ion battery. The comparison reveals a substantially improved performance of N-incorporated samples than that of the pristine materials. The highest discharge capacity of 1287 mA h g−1 was achieved in the first cycle at a current rate of 0.1 A g−1, and a stable capacity of 974 and 610 mA h g−1 is achieved when discharged at 0.1 and 2 A g−1, respectively, from the core-shell structures.

Realization of BaZrS3 chalcogenide perovskite thin films for optoelectronics

BaZrS3 is a prototypical chalcogenide perovskite, an emerging class of unconventional semiconductor. Recent results on powder samples reveal that it is a material with a direct band gap of 1.7–1.8 eV, a very strong light-matter interaction, and a high chemical stability. Due to the lack of quality thin films, however, many fundamental properties of chalcogenide perovskites remain unknown, hindering their applications in optoelectronics. Here we report the fabrication of BaZrS3 thin films, by sulfurization of oxide films deposited by pulsed laser deposition. We show that these films are n-type with carrier densities in the range of 1019-1020 cm−3. Depending on the processing temperature, the Hall mobility ranges from 2.1 to 13.7 cm2/Vs. The absorption coefficient is > 105 cm−1 at photon energy >1.97 eV. Temperature dependent conductivity measurements suggest shallow donor levels. By assuring that BaZrS3 is a promising candidate, these results potentially unleash the family of chalcogenide perovskites for optoelectronics such as photodetectors, photovoltaics, and light emitting diodes.

Structural Characterization, Optical Absorption and Electrical Conduction in Ordered Defect Compound Cu3In5Se9 of the Ternary Cu-In-Se Semiconductor System

The optical absorption coefficient α and electrical conduction as a function of temperature of the semiconductor Cu3In5Se9, an ordered defect compound which crystallizes in a tetragonal structure with space group P$$ \bar{4} $$2c, have been studied. The band gap energy EG varies between 0.994 eV and 0.983 eV in the temperature range between 25 and 300 K. The exponential variation of α with photon energy, observed just below the fundamental absorption edge, confirms the existence in Cu3In5Se9 of the Urbach’s tail. The phonon energy hνp associated with this tail is 101 meV. This is about three times higher than the highest optical phonon mode reported for Cu3In5Se9 from infrared reflectivity spectra. The origin of this high energy is attributed due to the contribution of localized modes produced by structural disorders due to deviation from ideal stoichiometry and donor–acceptor defects pairs. From the analysis of electrical data of n-type Cu3In5Se9 in the temperature range from 80 K to 300 K, it was found that above 100 K the electrical conduction is due to the activation of two shallow donor levels of about 40 meV and 80 meV, probably due to selenium vacancies.

Effect of defect states in the optical and magnetic properties of nanocrystalline NiO synthesised in a single step by an aerosol process

Nanocrystalline nickel oxide (NiO) was successfully synthesised as a phase-pure product from an inorganic precursor through a “single-pot”, gas-phase synthesis method, nebulised spray pyrolysis (NSP). Functional properties such as magnetic behaviour, luminescence and band gap were studied and compared with the properties of NiO powder synthesised by a conventional reverse co-precipitation process (RCP) which also needed an additional calcination step. Although NiO crystallised in the cubic (rocksalt) form, Rietveld analysis of X-ray diffraction data revealed the synthesised nanopowders to be in a distorted cubic (rocksalt) or monoclinic form. This distortion resulted from the antiferromagnetic exchange interaction energy and increased magnetic surface anisotropy, as seen from the bond lengths determined from Fourier transform infra-red (FTIR) studies. Raman spectroscopy also confirmed the distortion in the structure indicated by the presence of a first order phonon peak. The band gap decreased in case of both the powders and was attributed to the shallow donor or acceptor levels created due to the presence of defect states as determined from photoluminescence studies. The NSP powders also exhibited a dependence on the excitation wavelength during emission luminescence. A mixed antiferromagnetic and ferromagnetic behaviour was observed in the NSP powders and the susceptibility of each was deconvoluted through low and high field magnetometry studies. Further, the nanopowders from both the processes showed “core-shell” magnetism, with paramagnetic behaviour in the shell and antiferromagnetism in the core. The thicknesses of the core and shell were also determined.

Improved Detectivity of Flexible a-InGaZnO UV Photodetector via Surface Fluorine Plasma Treatment

Amorphous InGaZnO (a-IGZO) UV photodetectors with a metal-semiconductor-metal structure were fabricated on polyethylene terephthalate (PET) substrates, and the effects of surface fluorine plasma treatment on a-IGZO were investigated. With this treatment, the oxygen vacancies (Ovac’s) at the a-IGZO surface were effectively passivated, thus reducing the shallow-level donor states and electron traps. Accordingly, the direct and/or trap-assisted tunnelings through the metal/a-IGZO Schottky barrier were suppressed, leading to a significant reduction in the dark current. On the other hand, the photo current was nearly unchanged if the treatment time was less than 20min. It is due to the simultaneous reductions in hole trap and recombination center, both of which are related to the Ovac’s at deep levels. As a result, a high detectivity of ${4.36} \times {10}^{{13}}$ Jones was realized with an increase of nearly 10 times. Moreover, a highly uniform trap distribution was achieved, and it could be beneficial to the photoconductive reliability of the device.

A New Class of Bifunctional Perovskites BaMX4 (M = Co, Ni, Fe, Mn; X = F, Cl, Br, I): An n-Type Semiconductor with Combined Multiferroic and Photovoltaic Properties

We design a new class of bifunctional materials, namely, BaMX4 (M = Co, Ni, Fe, Mn; X = F, Cl, Br, I), with corner-shared octahedral layered perovskite structures. Our joint experimental and theoretical study demonstrates that BaCoF4, a prototype in this new class, possesses a unique combination of multiferroic and photovoltaic properties. Moreover, BaCoF4 exhibits both an antiferromagnetic spin structure and ferroelectric polarization along the (001) direction. These combined electronic and optical features render BaCoF4 a promising bifunctional material for application in spintronic or photovoltaic devices. Density functional theory calculations suggest that the dominant point defects in BaCoF4 are mostly shallow-level donor defects, leading to fascinating n-type self-doping. As such, a BaCoF4 layer may be exploited for electron transport and light absorption altogether, which may enhance the photovoltaic performance in solar cells. To confirm this predicted feature, we incorporate BaCoF4 as an electron...

Boron–oxygen complex yields n-type surface layer in semiconducting diamond

Diamond is a wide-bandgap semiconductor possessing exceptional physical and chemical properties with the potential to miniaturize high-power electronics. Whereas boron-doped diamond (BDD) is a well-known p-type semiconductor, fabrication of practical diamond-based electronic devices awaits development of an effective n-type dopant with satisfactory electrical properties. Here we report the synthesis of n-type diamond, containing boron (B) and oxygen (O) complex defects. We obtain high carrier concentration (∼0.778 × 1021 cm-3) several orders of magnitude greater than previously obtained with sulfur or phosphorous, accompanied by high electrical conductivity. In high-pressure high-temperature (HPHT) boron-doped diamond single crystal we formed a boron-rich layer ∼1-1.5 μm thick in the {111} surface containing up to 1.4 atomic % B. We show that under certain HPHT conditions the boron dopants combine with oxygen defects to form B-O complexes that can be tuned by controlling the experimental parameters for diamond crystallization, thus giving rise to n-type conduction. First-principles calculations indicate that B3O and B4O complexes with low formation energies exhibit shallow donor levels, elucidating the mechanism of the n-type semiconducting behavior.

IR photoresponse characteristics of Mg2Ge pn-junction photodiodes fabricated by rapid thermal annealing

In this study, high purity n-type Mg2Ge single crystals are grown in graphite crucibles under nitrogen atmosphere using a facile growth process. X-ray diffraction proves that the grown crystal consists of only Mg2Ge single phase without any other additional impurity peaks. The clear symmetrical Laue diffraction pattern of the cleaved substrate demonstrates the single crystalline nature of the grown Mg2Ge crystal ingot. The Hall Effect measurement of the polished Mg2Ge wafer shows that the latter had a moderate carrier density (5.66 * 1016 cm−3), mobility (253 cm2/V) and electrical resistivity (0.5 Ω. cm) at room temperature. Detailed analysis of the temperature dependence of the carrier density demonstrates that the shallow donor level, which gives rise to the n-type conduction of grown Mg2Ge crystals is due to the unintentional Al impurities from Mg source material. Mg2Ge pn-junction photodiodes are also manufactured for the first time by thermal diffusion of silver thin layer into n-Mg2Ge at 550 °C for 5 min in an argon atmosphere. The results reveal that the fabricated p-n photodiodes had a clear rectifying behavior and a remarkable spectral response in the wavelength range from 0.8 to 1.7 μm with a maximum room temperature zero-biased photoresponse of 7 mA/W at 1.2 μm. These outcomes indicate that the fabricated Mg2Ge photodiodes are promising to detect short-wavelength infrared (SWIR) light in the above-mentioned wavelength domain.

Defect levels induced by double substitution of B and N in 4H-SiC

Studies of substitutional impurities in 4H-SiC play a major role in identifying the most enhanced defect levels induced. N and B substitution in 4H-SiC on separate studies reveal that the NSi induced negative-U charge state ordering while the BSi induced shallow acceptor level. In this report, in order to predict the most stable defect and its electrically active defect levels, the Heyd, Scuseria and Ernzerhof (HSE06) hybrid functional was used to model and predict the energetics and defect levels induced by double substitution of B and N pair (NSiBC, NCBC, NSiBSi and NCBSi) in a 4H-SiC. While both B and N substituted for C under equilibrium conditions is the most energetically favourable pair, the N and B both substituted for Si has a relatively high formation energy. The energies of the various configurations suggest that the substitution of B and N pair in 4H-SiC is always stable, and the NCBC is the most energetically favourable defect. While all the defects studied induced acceptor levels, only the NCBC and NSiBC induced shallow donor levels. The (0/−1) acceptor level induced by the defects is close to the conduction band minimum for the case of NCBC and NCBSi and deep lying middle of the band gap of 4H-SiC for the NSiBSi and NSiBC.

Surface doping of ZnO nanowires with Bi: Density-functional supercell calculations of defect energetics

Defect calculations using the density and hybrid functionals in combination with the supercell approach are employed to characterize the electrical properties of a number of ZnO nanowires of various thicknesses doped with Bi atoms occupying surface sites. The variation of the differences between the total energies of charged and neutral supercells with the supercell size is studied, which led the authors to devise an extrapolation procedure to obtain reliable defect energetics in the dilute defect limit. The calculated defect formation energies indicate that although the substitution of Bi into Zn or O sites can take place spontaneously under suitable thermodynamic conditions, the substitution into Zn sites is generally more likely. The defect (charge-state) transition energies are computed and parameterized as a function of the nanowire thickness. It is revealed that the substitution of Bi into O (Zn) sites on the surface of ZnO nanowires yields deep acceptor (shallow donor) levels (except for extremely thin nanowires). It is therefore concluded that the incorporation of Bi into the surface of ZnO nanowires results in $n$-type doping.

A comparative first-principles study of point defect properties in the layered MX2 (M = Mo, W; X = S, Te): Substitution by the groups III, V and VII elements

Dopability in semiconductors plays a crucial role in device performance. Using the first-principles density-functional theory calculations, we investigate systematically the doping properties of layered MX2 (M = Mo, W; X = S, Te) by replacing M or X with the groups III, V and VII elements. It is found that the defect BM is hard to form in MX2 due to the large formation energy originating from the crystal distortion, while AlM is easy to realize compared to the former. In MoS2, WS2 and MoTe2, Al is the most desirable p-type dopant under anion-rich conditions among the group III components. With respect to the doping of the group V elements, it is found that the substitutions on the cation sites have deeper defect levels than those on the anion sites. AsTe and SbTe in MoTe2 and WTe2 are trend to form shallow acceptors under cation-rich conditions, indicating high hole-concentrations for p-type doping, whereas SbS in MoS2 and PTe in WTe2 are shown to be good p-type candidates under cation-rich conditions. In despite of that the substitutions of group VII on X site have low formation energies, the transition energies are too high to achieve n-type MoS2 and WS2. Nevertheless, for MoTe2, the substitutions with the group VII elements on the anion sites are suitable for n-type doping on account of the shallow donor levels and low formation energies under Mo-rich condition. As to WTe2, F is the only potential donor due to the shallow transition energy of FTe. Our findings of filtering out unfavorable and identifying favorable dopants in MX2 are very valuable for experimental implementations.

Effect of boron addition in modulating the optoelectronic properties of undoped and Al-doped ZnO thin films

Designing a transparent conductor for specific applications may require fundamental understanding of its microstructure and thus correlated optoelectronic properties with respect to a number of experimental parameters. To this end, the present study introduced varying concentrations of boron into the undoped and the Al-doped ZnO films by using co-deposition route in RF magnetron sputtering on soda lime glass substrates. A wide range of experimental techniques are then used to characterize these films. The films are found to demonstrate a high optical transparency of >83% and showed blue shift as a result of boron doping with Eg being as high as 3.98 eV. For the B3+-doped ZnO films, an increase in carrier concentration with increasing boron content is related to the cumulative effects of increased boron substitution in the zinc lattice positions and an increase in the intrinsic shallow donor level defects, caused by zinc interstitials and oxygen vacancies. An inverse effect is seen for the (B3+, Al3+)-doped ZnO films with increasing boron introduction, caused by reduced Al substitution in zinc lattice positions because of higher B–O bond strength as well as reduced doping efficiencies of B3+ and/or Al3+ ions due to decreasing crystallinity in these films. Finally, very high boron content in both these films is found to develop B2O3 and/or B7O phases, thereby not contributing entirely to the carrier generation and also reducing the carrier mobility of these films.

Local electronic structure of interstitial hydrogen in iron disulfide

The electronic structure of interstitial hydrogen in a compound semiconductor FeS$_2$ (naturally $n$-type) is inferred from a muon study. An implanted muon (Mu, a pseudo-hydrogen) forms electronically different defect centers discerned by the hyperfine parameter ($\omega_{\rm hf}$). A body of evidence indicates that one muon is situated at the center of an iron-cornered tetrahedron with nearly isotropic $\omega_{\rm hf}$ (Mu$_{\rm p}$), and that the other exists as a diamagnetic state (Mu$_{\rm d}$, $\omega_{\rm hf}\simeq 0$). Their response to thermal agitation indicates that the Mu$_{\rm d}$ center accompanies a shallow level (donor or acceptor) understood by effective mass model while the electronic structure of Mu$_{\rm p}$ center is more isolated from host than Mu$_{\rm d}$ to form a deeper donor level. These observations suggest that interstitial hydrogen also serves as an electronically active impurity in FeS$_2$. Based on earlier reports on the hydrogen diffusion in FeS$_2$, possibility of fast diffusion for Mu$_{\rm p}$ leading to formation of a complex defect state (Mu$^*_{\rm d}$, $T\le 100$ K) or to motional narrowing state (Mu$^*_{\rm p}$, $T\ge 150$ K) is also discussed.

Thermally activated double-carrier transport in epitaxial graphene on vanadium-compensated 6H-SiC as revealed by Hall effect measurements

In this report we demonstrate the results of charge carriers transport studies in graphene using a Hall effect sensor fabricated on quasi-free-standing monolayer graphene grown on a semi-insulating on-axis vanadium-compensated 6H-SiC(0001) substrate in an epitaxial Chemical Vapor Deposition process. The sensor is passivated with aluminum oxide through atomic layer deposition and offers current-mode sensitivity of 140 V/AT with thermal stability of − 0.02%/K within the range between 80 and 573 K. The electrical properties of the graphene layer are determined as a function of temperature ranging from 300 to 770 K. High-temperature characteristics of passivated and not passivated graphene are compared and their profiles explained through a double-carrier transport involving the spontaneous-polarization-induced holes in the graphene layer and the thermally activated electrons from a shallow donor level of nitrogen in the quasi-cubic (k1) site and a deep acceptor level of vanadium in the hexagonal (h) site both present in the bulk of the vanadium-compensated SiC substrate. Finally, we conclude that this mechanism is directly responsible for the limitation of the thermal stability of the sensor's current-mode sensitivity.

Composition-tuned optical absorption and luminescence of TlGaxIn1-xS2 layered mixed crystals at T = 10 K

Transmittance on TlGaxIn1-xS2 mixed crystals (0 ≤ x ≤ 1) was carried out in the 2.35–2.75 eV photon energy range at T = 10 K. Band gap energies of crystals were obtained utilizing the derivative spectra of transmittance and photon energy dependence of absorption coefficient. The dependence of direct band gap energy on composition at T = 10 K revealed that as gallium composition is elevated in the mixed crystals, the direct band gap energy increases from 2.56 to 2.67 eV.Photoluminescence spectra of TlGaxIn1-xS2 crystals were studied in the photon energy region of 2.30–2.95 eV at T = 10 K. The observed bands were attributed to transitions of electrons from shallow donor levels to valence band. The shift of PL bands to higher energies with increasing gallium content was revealed. Furthermore, composition ratio of crystals was determined from energy dispersive spectroscopy experiments.

The Role of Dopant Ions on Charge Injection and Transport in Electrochemically Doped Quantum Dot Films

Control over the charge density is very important for implementation of colloidal semiconductor nanocrystals into various optoelectronic applications. A promising approach to dope nanocrystal assemblies is charge injection by electrochemistry, in which the charge compensating electrolyte ions can be regarded as external dopant ions. To gain insight into the doping mechanism and the role of the external dopant ions, we investigate charge injection in ZnO nanocrystal assemblies for a large series of charge compensating electrolyte ions with spectroelectrochemical and electrochemical transistor measurements. We show that charge injection is limited by the diffusion of cations in the nanocrystal films as their diffusion coefficient are found to be ∼7 orders of magnitude lower than those of electrons. We further show that the rate of charge injection depends strongly on the cation size and cation concentration. Strikingly, the onset of electron injection varies up to 0.4 V, depending on the size of the electrolyte cation. For the small ions Li+ and Na+ the onset is at significantly less negative potentials. For larger ions (K+, quaternary ammonium ions) the onset is always at the same, more negative potential, suggesting that intercalation may take place for Li+ and Na+. Finally, we show that the nature of the charge compensating cation does not affect the source-drain electronic conductivity and mobility, indicating that shallow donor levels from intercalating ions fully hybridize with the quantum confined energy levels and that the reorganization energy due to intercalating ions does not strongly affect electron transport in these nanocrystal assemblies.

Zinc interstitial threshold in Al-doped ZnO film: Effect on microstructure and optoelectronic properties

In order to know the threshold quantity of the zinc interstitials that contributes to an increase in carrier concentration in the Al-doped ZnO (AZO) films and their effect on the overall microstructure and optoelectronic properties of these films, in this work, Zn-rich-AZO and ZnO thin films are fabricated by adding excess zinc (from a zinc metallic target) during their deposition in RF magnetron sputtering and are then investigated using a wide range of experimental techniques. All these films are found to grow in a ZnO hexagonal wurtzite crystal structure with strong (002) orientation of the crystallites, with no indication of Al2O3, metallic Zn, and Al. The excessively introduced zinc in these AZO and/or ZnO films is found to increase the shallow donor level defects (i.e., zinc interstitials and oxygen-related electronic defect states), which is found to significantly increase the carrier concentration in these films. Additionally, aluminum is seen to enhance the creation of these electronic defect states in these films, thereby contributing more to the overall carrier concentration of these films. However, carrier mobility is found to decrease when the carrier concentration values are higher than 4 × 1020 cm−3, because of the electron-electron scattering. Whereas the optical band gap of the ZnO films is found to increase with increasing carrier concentration because of the Burstein-Moss shift, these decrease for the AZO films due to the band gap narrowing effect caused by excess carrier concentration.

Oxygen-assisted preparation of mechanoluminescent ZnS:Mn for dynamic pressure mapping

Mechanoluminescent materials that convert mechanical stimuli to light emission have attracted extensive attention for potential applications in human-machine interactions. Here, we report a simple and available novel approach for the oxygen-assisted preparation of ZnS:Mn particles by solid-state reaction at atmospheric pressure without the formation of the corresponding oxides. The existence of O2 has a positive impact on the formation of S vacancies in wurtzite-phase ZnS, leading to the introduction of Mn2+ ion luminescent centers and shallow donor levels, which can improve the electron-hole recombination rate. The O2 ratio and Mn2+ ion doping concentration have significant effects on the luminous efficiency, which is optimal at 1%–20% and 1 at.%–2 at.% respectively. In addition, a device based on the piezo-photonic effect with excellent pressure sensitivity of 0.032 MPa−1 was fabricated, which can map the two-dimensional pressure distribution ranging from 2.2 to 40.6 MPa in situ. This device can be applied to real-time pressure mapping, smart sensor networks, high-level security systems, human-machine interfaces, and artificial skins.

Defect and interface analyses of non-stoichiometric n-type GaSb thin films grown on Ge(100) substrates by rapid thermal annealing

In this study, Ga0.6Sb0.4 thin films were grown on quartz and Ge(100) 1° off-axis substrates by RF magnetron sputtering at 500 °C. Ga0.6Sb0.4/Ge(100) shows n-type conductivity at room temperature (RT) and p-type conductivity at low temperatures, whereas undoped GaSb thin films exhibit p-type conductivity, irrespective of their growth methods and conditions. Their electrical properties were determined by rapid thermal annealing, which revealed that Ga0.6Sb0.4/Ge(100) contains two types of acceptors and two types of donors. The acceptors are considered to be GaSb and electrically active sites on dislocations originating at the Ga0.6Sb0.4/Ge(100) interface, while donors are believed to be Gai and electrically active sites originating at the Ga0.6Sb0.4/Ge(100) interface. In these acceptors and donors, the shallow donor concentration is higher than the shallow acceptor concentration, and the shallow donor level is deeper than the shallow acceptor level. Thus, we concluded that Ga0.6Sb0.4/Ge(100) shows n-type conductivity at RT due to electrically active sites originating at the Ga0.6Sb0.4/Ge(100) interface and native defects originating from excess Ga.