Shallow Acceptor States Research Articles

Long‐Range Interaction Between NO2 Molecules, Impurity Boron Atoms and Si Atoms with Dangling Bonds in Porous Silicon

Density functional theory (DFT) simulation show that in p‐doped porous silicon (PS), boron (B) atoms are passivated by surface dangling bonds of Si atoms (pb‐centers) at distances up to 25 Å. A positive charge of the pb‐centers leads to the appearance of an increased potential region and a Coulomb blockade (CB) of free holes. When a NO2 molecule is adsorbed on a surface OH group near the pb‐center, a shallow acceptor state arises. This state is related to the NO2 molecule and not to the B atom. When an electron is captured from the valence band by the NO2 molecule, the potential around it and the pb‐center decreases, which removes CB.

Structure and properties of intrinsic and extrinsic defects in black phosphorus.

The electronic and geometric structures of a range of intrinsic and extrinsic defects in black phosphorus (BP) are calculated using Density Functional Theory (DFT) and a hybrid density functional. The results demonstrate that energy barriers to form intrinsic defects, such as Frenkel pairs and Stone-Wales type defects, exceed 3.0 eV and their equilibrium concentrations are likely to be low. Therefore, growth conditions and sample preparation play a crucial role in defect chemistry of black phosphorus. Mono-vacancies (MV) are shown to introduce a shallow acceptor state in the bandgap of BP, but exhibit fast hopping rates at room temperature. Coalescence of MVs into di-vacancies (DV) is energetically favourable and eliminates the band gap states. Thus MVs are not likely to be the main contributor to p-doping in BP. Extrinsic defects are a plausible alternative, with SnP found to be the most promising candidate. Other defects considered include I, O, Fe, Cu, Zn and Ni in surface adsorbed, intercalated and substitutional geometries, respectively. Furthermore, BP was found to be magnetic for isolated MVs and Fe doping, motivating further research in the area of magnetic functionalisation.

Morphology and low-temperature photoluminescence investigation of bat-like Li-doped ZnO nanostructures

The bat-like ZnO: Li multi-branched nanostructures grown on Si substrates have been successfully fabricated by simple thermal evaporation method. The morphology of the as-prepared sample was characterized by scanning electron microscopy. The results show that each individual product looks like a bat of baseball composed of a top of hexagonal prism, a continuous round table, and a homogeneous bat. The room temperature Raman spectrum indicate that the Li ions have been doped into the lattice position of Zn ions and increase the free carrier concentration. The low-temperature photoluminescence spectra show that the existence of a shallow acceptor at 138meV above the VBM in the Li-doped samples from the DAP luminescence at 3.235eV. A Li-related defect complex is made responsible for this shallow acceptor state. With the increase of temperature, the phenomenon of red-shift for the emission peaks is shown.

Charge-based deep level transient spectroscopy of B-doped and undoped polycrystalline diamond films

The undoped and B-doped polycrystalline diamond thin film was synthesized by hot filament chemical vapor deposition and microwave plasma, respectively. The structural characterization was performed by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The electrical properties of synthesized diamond layer were characterized by dc-conductivity method and charge deep level transient spectroscopy. The B-doped diamond layers show higher sp2/sp3 ratios in comparison with that of undoped layers what can have an essential influence on the localized density of states associated with shallow hydrogen acceptor states what is reflected in the values of activation energies which reached the values of 38 meV for B-doped and 55 meV for undoped diamond layers, respectively. The existence of deep level traps, as, for example, associated with B-related acceptors, was not observed.

Shallow Acceptor State in Mg-Doped CuAlO2 and Its Effect on Electrical and Optical Properties: An Experimental and First-Principles Study.

Shallow acceptor states in Mg-doped CuAlO2 and their effect on structural, electrical, and optical properties are investigated by combining first-principles calculations and experiments. First-principles calculations demonstrate that Mg substituting at the Al site in CuAlO2 plays the role of shallow acceptor and has a low formation energy, suggesting that Mg doping can increase hole concentration and improve the conductivity of CuAlO2. Hall effect measurements indicate that the hole concentration of the Mg-doped CuAlO2 thin film is 2 orders of magnitude higher than that of undoped CuAlO2. The best room temperature conductivity of 8.0 × 10-2 S/cm is obtained. A band gap widening is observed in the optical absorption spectra of Mg-doped CuAlO2, which is well supported by the results from first-principles electronic structure calculations.

Shallow acceptor state in ZnO realized by ion irradiation and annealing route

Activation of shallow acceptor state has been observed in ion irradiated and subsequently air annealed polycrystalline ZnO material. Low temperature photoluminescence (PL) spectrum of the sample exhibits clear signature of acceptor bound exciton (ABX) emission at 3.360 eV. The other two samples, pristine and ion irradiated (without annealing), however, do not show acceptor related PL emission in the nearby energy region. Electron transition from shallow donor (most probable site is interstitial zinc for undoped ZnO) to such newly formed shallow acceptor state creates new donor-acceptor pair (DAP) luminescence peak ∼ 3.229 eV. ABX and DAP peak energy positions confirm that the acceptor is N related. The acceptor exciton binding energy has been estimated to be 180 ± 15 meV which is in conformity with earlier reports. The activation of shallow acceptors without any source of atomic nitrogen can only be possible through diffusion of molecular nitrogen inside the sample during annealing. The N2 molecules get trapped at bulk defect sites incorporated by ion irradiation and subsequent annealing. X-ray diffraction (XRD) and Raman spectroscopic (RS) investigation have been employed to probe the changing defective nature of the ZnO samples. Irradiation induced increased disorder has been detected (both by XRD and RS) which is partially removed/modified by annealing at 300 °C. Simultaneous activation of molecular nitrogen acceptor in purposefully defective ZnO is the key finding of this work. Results presented here provide a simple but controlled way of producing shallow acceptor state in ZnO. If optimized through suitable choice of ion, its energy and fluence as well as the annealing temperature, this methodology can trigger further scope to fabricate devices using ZnO epitaxial thin films or nanowires.

The Luminescent Inhomogeneity and the Distribution of Zinc Vacancy-Related Acceptor-Like Defects in N-Doped ZnO Microrods.

Vertically aligned N-doped ZnO microrods with a hexagonal symmetry were fabricated via the chemical vapor transport with abundant N2O as both O and N precursors. We have demonstrated the suppression of the zinc interstitial-related shallow donor defects and have identified the zinc vacancy-related shallow and deep acceptor states by temperature variable photoluminescence in O-rich growth environment. Through spatially resolved cathodoluminescence spectra, we found the luminescent inhomogeneity in the sample with a core-shell structure. The deep acceptor-isolated VZn and the shallow acceptor VZn-related complex or clusters mainly distribute in the shell region.

Experimental investigation on nitrogen related complex acceptors in nitrogen-doped ZnO films

In this article, experiments have been designed to investigate the nature, introduction, and control of the shallow acceptor states in nitrogen-doped ZnO material. With the help of a batch of characterizing experiments, we have confirmed shallow acceptor states in the form of (NH4)Zn and (N2)Zn. The content of the acceptor-like nitrogen complexes could be tuned by varying the VI/II ratio in gas phase during growth. The formation mechanism of the complexes has also been discussed. The formation energy of the (NH4)Zn complex could be lower than that of VZn at O-rich condition, making its formation at zinc site energetically favorable. However, although the formation energy of the (N2)Zn complex is slightly higher than that of VZn, the complex could be stabilized by binding to adjacent lattice oxygen or additional hydrogen, which could lower the total energy through transferring electrons to the high energy states. Furthermore, direct introduction of (N2)Zn via growth has been suggested to be more efficient as compared to that via post-growth process demonstrated previously. All these mechanisms lead to a reasonable co-existence of the (NH4)Zn and (N2)Zn acceptor-like complexes in ZnO lattice, which is responsible for the experimentally observed shallow acceptor state in nitrogen-doped ZnO material.

Manipulation of n and p type dope black phosphorene layer: A first principles study

We investigated the physical properties of the semiconducting impurity (Ga, Ge, As, and Se) doped phosphorene layer systems. The Ga and As doped layer showed a semiconducting behavior while the Ge doped system displayed a metallic state. The magnetic state appeared in the Se doped phosphorene. Deep donor or acceptor sates was found with doping of semiconducting impurities. To investigate an effective n and p type doped phosphorene, we explored the charge carrier doping effect. With an external electron carrier doping, the donor level induced by the Se impurity became an effective shallow donor level. Due to the Fermi level shifting, the metallic to semiconductor or vice versa was found in Ge and As. For external hole carrier doping, all the systems showed metallic band structures except for the Se doped structure. The Se impurity created a shallow acceptor state with an extra hole. Particularly, we propose that the most promising dopant is the Se because it creates a magnetic state in phosphorene and an extra electron or hole doping can induce an n-type or p-type phosphorene respectively. This suggests that the Se doped phosphorene has potential applications for both dilute magnetic semiconductor and p–n junction device applications.

Optical investigations of Be doped ZnO films grown by molecular beam epitaxy

In this article, the optical properties of ZnO:Be films grown by plasma-assisted molecular beam epitaxy were investigated by the excitation density-dependent and temperature-dependent photoluminescence measurements. The low temperature photoluminescence spectra showed a dominant excitons bound to neutral donors (D°X) emission centered at 3.3540eV and strong donor-acceptor pair (DAP) transitions at 3.3000eV. In addition, it showed that the intensity ratio of the DAP and D°X peaks changed with background electron concentration. Furthermore, a shallow acceptor state with ionization energy of 116meV was found and attributed to Zn vacancy. The present study further suggests that Be and N codoping ZnO might be suitable for fabricating reliable p-type ZnO materials.

An Sb-doped p-type ZnO nanowire based random laser diode

An electrically pumped Sb-doped ZnO nanowire/Ga-doped ZnO p–n homojunction random laser is demonstrated. Catalyst-free Sb-doped ZnO nanowires were grown on a Ga-doped ZnO thin film on a Si substrate by chemical vapor deposition. The morphology of the as-grown titled nanowires was observed by scanning electron microscopy. X-ray photoelectron spectroscopy results indicated the incorporation of Sb dopants. Shallow acceptor states of Sb-doped nanowires were confirmed by photoluminescence measurements. Current–voltage measurements of ZnO nanowire structures assembled from p- and n-type materials showed a typical p–n diode characteristic with a threshold voltage of about 7.5 V. Very good photoresponse was observed in the UV region operated at 0 V and different reverse biases. Random lasing behavior with a low-threshold current of around 10 mA was demonstrated at room temperature. The output power was 170 nW at 30 mA.

Disordered small defect clusters in silicon

The ionizing radiation induced disordered defect clusters and their relaxation in silicon were simulated by the density functional method. It was found that a non-relaxed disordered cluster gives rise to a great number of localized states having their energy levels within the semiconductor forbidden band gap. After the relaxation, however, the density of these states significantly decreases leaving only several relatively shallow donor and acceptor state levels that may contribute to trapping of free carriers and shrinkage of an effective band gap.

Vacancy and Doping States in Monolayer and bulk Black Phosphorus.

The atomic geometries and transition levels of point defects and substitutional dopants in few-layer and bulk black phosphorus are calculated. The vacancy is found to reconstruct in monolayer P to leave a single dangling bond, giving a negative U defect with a +/− transition level at 0.24 eV above the valence band edge. The V− state forms an unusual 4-fold coordinated site. In few-layer and bulk black P, the defect becomes a positive U site. The divacancy is much more stable than the monovacancy, and it reconstructs to give no deep gap states. Substitutional dopants such as C, Si, O or S do not give rise to shallow donor or acceptor states but instead reconstruct to form non-doping sites analogous to DX or AX centers in GaAs. Impurities on black P adopt the 8-N rule of bonding, as in amorphous semiconductors, rather than simple substitutional geometries seen in tetrahedral semiconductors.

The role of point defects in PbS, PbSe and PbTe: a first principles study

Intrinsic defects are of central importance to many physical and chemical processes taking place in compound nanomaterials, such as photoluminescence, accommodation of off-stoichiometry and cation exchange. Here, the role of intrinsic defects in the above mentioned processes inside rock salt (RS) lead chalcogenide systems PbS, PbSe and PbTe (PbX) was studied systematically using first principles density functional theory. Vacancy, interstitial, Schottky and Frenkel defects were considered. Rock salt PbO was included for comparison. The studied physical properties include defect formation energy, local geometry relaxation, Bader charge analysis, and electronic structure. The defect formation energies show that monovacancy defects and Schottky defects are favoured over interstitial and Frenkel defects. Schottky dimers, where the cation vacancy and anion vacancy are adjacent to each other, have the lowest defect formation energies at 1.27 eV, 1.29 eV and 1.21 eV for PbS, PbSe and PbTe, respectively. Our results predict that a Pb monovacancy gives rise to a shallow acceptor state, while an X vacancy generates a deep donor state, and Schottky defects create donor-acceptor pairs inside the band gap. The surprisingly low formation energy of Schottky dimers suggests that they may play an important role in cation exchange processes, in contrast to the current notion that only single point defects migrate during cation exchange.

Molecular nitrogen acceptors in ZnO nanowires induced by nitrogen plasma annealing

X-ray absorption near-edge spectroscopy, photoluminescence, cathodoluminescence, and Raman spectroscopy have been used to investigate the chemical states of nitrogen dopants in ZnO nanowires. It is found that nitrogen exists in multiple states: ${\mathrm{N}}_{\mathrm{O}},\phantom{\rule{0.16em}{0ex}}{\mathrm{N}}_{\mathrm{Zn}}$, and loosely bound ${\mathrm{N}}_{2}$ molecule. The results establish a direct link between a donor-acceptor pair emission at 3.232 eV and the concentration of loosely bound ${\mathrm{N}}_{2}$. This work confirms that ${\mathrm{N}}_{2}$ at Zn site is a potential candidate for producing a shallow acceptor state in N-doped ZnO as theoretically predicted by Lambrecht and Boonchun [Phys. Rev. B 87, 195207 (2013)]. Additionally, shallow acceptor states arising from ${\mathrm{N}}_{\mathrm{O}}$ complexes have been ruled out in this paper.

Role of phase composition for electronic states in CH3NH3PbI3 prepared from CH3NH3I/PbCl2 solution

Modulated surface photovoltage (SPV) spectra have been correlated with the phase composition in layers of CH3NH3PbI3 (MAPbI3) prepared from MAI and PbCl2 and annealed at 100 °C. Depending on the annealing time, different compositions of MAPbI3, MAPbCl3, MACl, PbI2, and an un-identified phase were found. It has been demonstrated that evaporation of MAI and HI is crucial for the development of electronic states in MAPbI3 and that only the appearance and evolution of the phase PbI2 has an influence on electronic states in MAPbI3. With ongoing annealing, (i) a transition from p- to n-type doping was observed with the appearance of PbI2, (ii) shallow acceptor states were distinguished and disappeared in n-type doped MAPbI3, and (iii) a minimum of the SPV response related to deep defect states was found at the transition from p- to n-type doping. The results are discussed with respect to the further development of highly efficient and stable MAPbI3 absorbers for solar cells.

Realization of Cu-Doped p-Type ZnO Thin Films by Molecular Beam Epitaxy.

Cu-doped p-type ZnO films are grown on c-sapphire substrates by plasma-assisted molecular beam epitaxy. Photoluminescence (PL) experiments reveal a shallow acceptor state at 0.15 eV above the valence band edge. Hall effect results indicate that a growth condition window is found for the formation of p-type ZnO thin films, and the best conductivity is achieved with a high hole concentration of 1.54 × 10(18) cm(-3), a low resistivity of 0.6 Ω cm, and a moderate mobility of 6.65 cm(2) V(-1) s(-1) at room temperature. Metal oxide semiconductor capacitor devices have been fabricated on the Cu-doped ZnO films, and the characteristics of capacitance-voltage measurements demonstrate that the Cu-doped ZnO thin films under proper growth conditions are p-type. Seebeck measurements on these Cu-doped ZnO samples lead to positive Seebeck coefficients and further confirm the p-type conductivity. Other measurements such as X-ray diffraction, X-ray photoelectron, Raman, and absorption spectroscopies are also performed to elucidate the structural and optical characteristics of the Cu-doped p-type ZnO films. The p-type conductivity is explained to originate from Cu substitution of Zn with a valency of +1 state. However, all p-type samples are converted to n-type over time, which is mostly due to the carrier compensation from extrinsic defects of ZnO.

A First-Principles Study on Electron Donor and Acceptor Molecules Adsorbed on Phosphorene

Density functional theory calculations have been carried out to investigate single-layer phosphorene functionalized with two kinds of organic molecules, i.e., an electrophilic molecule tetracyano-p-quinodimethane (TCNQ) as electron acceptor and a nucleophilic molecule tetrathiafulvalene (TTF) as electron donor. The TCNQ molecule introduces shallow acceptor states in the gap of phosphorene close to the valence band edge, which makes the doped system become a p-type semiconductor. However, when the TTF molecule is adsorbed on the phosphorene, the occupied molecular states introduced into the gap are of deep donor states so that effective n-doping for transport cannot be realized. This disadvantageous situation can be amended by applying an external out-of-plane electric field with direction from phosphorene to TTF, or an in-plane tensile strain, or their combination, under which the conduction band edge of the phosphorene moves closer to the TTF-derived donor states, and then the TTF-adsorbed phosphorene system becomes an n-type semiconductor. It is also noted that the out-of-plane electric field and in-plane strain can modulate the band gap of the TTF-adsorbed phosphorene markedly. The effective bipolar doping of single-layer phosphorene via molecular adsorption, especially n-doping against its native p-doping propensity, and the good response of band gap in the infrared waveband of the TTF-adsorbed phosphorene to the out-of-plane electric field and in-plane strain would broaden the way to the application of this new type of two-dimensional material in nanoelectronic and optoelectronic devices.

NMR study of native defects in PbSe

While each atom species in PbSe corresponds to a single crystallographic site and transport measurements reveal a single carrier density, $^{207}\mathrm{Pb}$ NMR reveals a more complicated picture than previously thought comprising three discrete homogeneous carrier components, each associated with $n$- or $p$-type carrier fractions. The origins of these fractions are discussed in terms of electronic heterogeneity of the native semiconductor. The interaction mechanism between nuclear spins and lattice vibrations via fluctuating spin-rotation interaction, applicable to heavy spin-1/2 nuclei [Phys. Rev. B 74, 214420 (2006)], does not hold. Instead, a higher-order temperature dependence dominates the relaxation pathway. Shallow acceptor states and deep level defects in the midgap explain the complex temperature dependence of the direct band gap.

Optical signatures of photoinduced Zn vacancies in ZnO single crystal

193-nm excimer laser interaction with ZnO single-crystal at 0.05–500 KW/cm2 intensities is investigated under ultra high vacuum conditions by time resolved photoluminescence (PL) spectroscopy. A dominant 3.18 eV PL emission band at 295 K is observed. This band shows unusually long 0.52 ± 0.01 μs lifetime, indicating a defect mediated emission mechanism. The demonstrated negative thermal quenching for this band confirms its free electron to acceptor type transition. The involved acceptor is attributed to zinc vacancy with ∼100 meV shallow acceptor state. This study finds that 193-nm interactions produce Zn vacancies in transient states at 0.05–50 KW/cm2 excitation intensities and in stable state at 500 KW/cm2 or above intensities. The transient zinc vacancy production at such low intensities further validates the Frenkel pair creation as mechanism for creating these defects in ZnO single-crystal.