Alexander Azarov

Probing Defects in Nitrogen-Doped Cu2O

Nitrogen doping is a promising method of engineering the electronic structure of a metal oxide to modify its optical and electrical properties; however, the doping effect strongly depends on the types of defects introduced. Herein, we report a comparative study of nitrogen-doping-induced defects in Cu2O. Even in the lightly doped samples, a considerable number of nitrogen interstitials (Ni) formed, accompanied by nitrogen substitutions (NO) and oxygen vacancies (VO). In the course of high-temperature annealing, these Ni atoms interacted with VO, resulting in an increase in NO and decreases in Ni and VO. The properties of the annealed sample were significantly modified as a result. Our results suggest that Ni is a significant defect type in nitrogen-doped Cu2O.

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.

Optical activity and defect/dopant evolution in ZnO implanted with Er

The effects of annealing on the optical properties and defect/dopant evolution in wurtzite (0001) ZnO single crystals implanted with Er ions are studied using a combination of Rutherford backscattering/channeling spectrometry and photoluminescence measurements. The results suggest a lattice recovery behavior dependent on ion dose and involving formation/evolution of an anomalous multipeak defect distribution, thermal stability of optically active Er complexes, and Er outdiffusion. An intermediate defect band occurring between the surface and ion-induced defects in the bulk is stable up to 900 °C and has a photoluminescence signature around 420 nm well corresponding to Zn interstitials. The optical activity of the Er atoms reaches a maximum after annealing at 700 °C but is not directly associated to the ideal Zn site configuration, since the Er substitutional fraction is maximal already in the as-implanted state. In its turn, annealing at temperatures above 700 °C leads to dissociation of the optically active Er complexes with subsequent outdiffusion of Er accompanied by the efficient lattice recovery.

Beryllium sites in MBE-grown BeZnO alloy films

We report the possible sites that beryllium atoms can occupy in BeZnO alloy films by a comparative study. The epitaxial BexZn1?xO (x?=?0, 0.06, 0.1, 1) films were prepared by a molecular beam epitaxy technique, and their structural properties were characterized with x-ray diffraction, Raman spectroscopy and x-ray photoelectron spectroscopy, while the Be composition, x, was measured with time of flight elastic recoil detection analysis. When x is 6%, substitutional and interstitial Be atoms are revealed to be co-existing in BexZn1?xO films. Furthermore, phase segregation of BeO is observed when x increases to 10% due to the huge difference of the bond length between Be?O and Zn?O. In this case, most of the Be atoms are found to form BeO particles, rather than a BeZnO alloy, so as to reduce the formation energy. The incorporation of Be atoms in the ZnO lattice consequently decreases in Be0.1Zn0.9O, resulting in a smaller bandgap than that of Be0.06Zn0.94O.

Fluorine doping: a feasible solution to enhancing the conductivity of high-resistance wide bandgap Mg0.51Zn0.49O active components.

N-type doping of high-resistance wide bandgap semiconductors, wurtzite high-Mg-content MgxZn1–xO for instance, has always been a fundamental application-motivated research issue. Herein, we report a solution to enhancing the conductivity of high-resistance Mg0.51Zn0.49O active components, which has been reliably achieved by fluorine doping via radio-frequency plasma assisted molecular beam epitaxial growth. Fluorine dopants were demonstrated to be effective donors in Mg0.51Zn0.49O single crystal film having a solar-blind 4.43 eV bandgap, with an average concentration of 1.0 × 1019 F/cm3.The dramatically increased carrier concentration (2.85 × 1017 cm−3 vs ~1014 cm−3) and decreased resistivity (129 Ω · cm vs ~106 Ω cm) indicate that the electrical properties of semi-insulating Mg0.51Zn0.49O film can be delicately regulated by F doping. Interestingly, two donor levels (17 meV and 74 meV) associated with F were revealed by temperature-dependent Hall measurements. A Schottky type metal-semiconductor-metal ultraviolet photodetector manifests a remarkably enhanced photocurrent, two orders of magnitude higher than that of the undoped counterpart. The responsivity is greatly enhanced from 0.34 mA/W to 52 mA/W under 10 V bias. The detectivity increases from 1.89 × 109 cm Hz1/2/W to 3.58 × 1010 cm Hz1/2/W under 10 V bias at room temperature.These results exhibit F doping serves as a promising pathway for improving the performance of high-Mg-content MgxZn1-xO-based devices.

Ge redistribution in SiO2/SiGe structures under thermal oxidation: Dynamics and predictions

(Received 29 August 2011; accepted 15 December 2011; published online 24 January 2012)Several fundamental aspects of the oxidation-induced redistribution of Ge in thin films of SiGe arestudied. This includes the incorporation of Ge into the oxide and the formation of what isalternatively referred to as pile-up, snow-plow, or a germanium-rich layer. Experimental data fromthe present work shows longer oxidation times leading to an increase of Ge content in the pile-upregion and eventually creating a single high Ge content pile-up layer by entirely consuming theinitial SiGe layer. The pile-up effect was shown to occur at the oxidation interface, with the highestGe content occurring at the same interface. For a given oxide thickness, the redistribution of Geand the formation of a pile-up region was shown experimentally to be independent of temperaturein the range between 800 C and 1000 C. Simulations using common models for the oxidation ofSi and diffusion of Si in SiGe indicate that temperature does have an influence on the compositionof the pile-up layer, though the range of achievable compositions is limited. The flux of Si due todiffusion of Si in SiGe relative to the oxidation-induced flux of Si out of the SiGe is integral to theformation and dimensions of a pile-up region. Two predictive relations were derived for describingthe dynamics of oxidation of SiGe. The first relation is given for determining the pile-up layerthickness as a function of oxide thickness and the composition of the pile-up layer. The secondrelation assumes a limited supply of Si and is for determination of the minimum initial thickness ofa SiGe layer to avoid oxidation of Ge. The validity of these equations was confirmedexperimentally by RBS and XPS data from the present work. The proposed models may be used innanostructuring of thin films of SiGe by oxidation and in the design of core-shell structures andtransistors. This is all done with a focus on oxidation of epitaxial thin films (< 100nm) ofSi

Extended defects in ZnO: Efficient sinks for point defects

Dopant-defect reactions dominate the defect formation in mono-crystalline ZnO samples implanted with Ag and B ions. This is in contrast to most other ion species studied and results in an enhanced concentration of extended defects, such as stacking faults and defect clusters. Using a combination of B and Ag implants and diffusion of residual Li atoms as a tracer, we demonstrate that extended defects in ZnO act as efficient traps for highly mobile Zn interstitials. The results imply that dynamic annealing involving interaction of point defects with extended ones can play a key role in the disorder saturation observed for ZnO and other radiation-hard semiconductors implanted with high doses.

Enhancement-mode ZnO/Mg0.5Zn0.5O HFET on Si

We report a bottom-gate and enhancement-mode ZnO/Mg0.5Zn0.5O heterojunction field effect transistor (HFET) on Si. This new heterostructure which is grown by using molecular beam epitaxy (MBE) reduces interface defects and traps. By tailoring Mg composition (x) in the MgxZn1−xO barrier layer up to 50%, the Mg0.5Zn0.5O exhibits insulating properties and the resultant HFET works in an enhancement mode with a field effective mobility of μFE = 21cm2 V−1 s−1, transconductance of gm = 44mSmm−1, on/off ratio of 1 × 105 and off current ~1.33 × 10−8 Amm−1. The device shows good ambient stability.

Formation of shallow boron emitters in crystalline silicon using flash lamp annealing: Role of excess silicon interstitials

Shallow, Boron (B)-doped p+ emitters have been realized using spin-on deposition and Flash Lamp Annealing (FLA) to diffuse B into monocrystalline float zone Silicon (Si). The emitters extend between 50 and 140 nm in depth below the surface, have peak concentrations between 9 × 1019 cm–3 and 3 × 1020 cm–3, and exhibit sheet resistances between 70 and 3000 Ω/□. An exceptionally large increase in B diffusion occurs for FLA energy densities exceeding ∼93 J/cm2 irrespective of 10 or 20 ms pulse duration. The effect is attributed to enhanced diffusion of B caused by Si interstitial injection following a thermally activated reaction between the spin-on diffusant film and the silicon wafer.

Effect of buried extended defects on the radiation tolerance of ZnO

Interaction between defects of different types often determines their evolution in semiconductors under ion irradiation. Here, we investigate the role of buried pre-existing extended defects in the damage accumulation in ZnO single crystals implanted at room temperature and at 15 K with 80 keV N ions. For the room temperature implants but not for the 15 K ones, the defect accumulation around the projected range is strongly suppressed while a pronounced build-up occurs in the region of the pre-existing defects. This is attributed to thermally activated migration of point defects from the implanted volume and subsequent trapping/annihilation by the pre-existing defects. The obtained results imply that a defect engineering scheme involving formation of a high density of extended defects outside the active volume of a ZnO-based device could be beneficial to enhance the radiation tolerance.