Search Results - (Author, Cooperation:S. O. Kucheyev)

2018-05-26

American Physical Society (APS)

0031-9007

1079-7114

Physics

Condensed Matter: Structure, etc.

2014-06-21

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

1089-7550

AIP Digital Archive

Physics

The influence of ion-beam-produced lattice defects as well as H, B, C, N, O, and Si, introduced by ion implantation, on the luminescence properties of wurtzite GaN is studied by cathodoluminescence spectroscopy. Results indicate that intrinsic lattice defects produced by ion bombardment mainly act as nonradiative recombination centers and do not give rise to the yellow luminescence (YL) of GaN. Experimental data unequivocally shows that C is involved in the defect-impurity complex responsible for YL. In addition, C-related complexes appear to act as efficient nonradiative recombination centers. Implantation of H produces a broad luminescent peak which is slightly blueshifted with respect to the C-related YL band in the case of high excitation densities. The position of this H-related YL peak exhibits a blueshift with increasing excitation density. Based on this experimental data and results reported previously, the chemical origin of the YL band is discussed. © 2002 American Institute of Physics.

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1089-7550

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Physics

Electrical isolation of n-type GaN epilayers bombarded with MeV light ions is studied by energy dispersive x-ray spectrometry (EDS). We show that the maximum bremsstrahlung x-ray energy (the Duane–Hunt limit) can be used to monitor the isolation process in GaN. This method allows the dose region above the threshold dose for isolation to be conveniently studied, whereas the application of conventional (low-voltage) electrical techniques in this dose range with large sheet resistances of the material ((approximately-greater-than)1011 Ω/sq) is often impossible due to comparable parasitic resistances of the experimental setup. A correlation of EDS and resistance measurements of GaN strongly suggests that the magnitude of sample charging scales with the number of ion-beam-produced deep electron traps which are empty at equilibrium. The results presented demonstrate the utility of EDS as a powerful and simple technique to study electrical isolation in wide band-gap semiconductors. © 2002 American Institute of Physics.

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1089-7550

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Physics

We study the evolution of sheet resistance of n-type GaN epilayers irradiated with MeV 1H and 12C ions. Results show that both implantation temperature (varied from 77 up to 423 K) and ion beam flux affect the process of electrical isolation in the case of irradiation with 12C ions. This behavior is consistent with significant dynamic annealing occurring in GaN during MeV light-ion bombardment, which suggests a scenario where the centers responsible for electrical isolation are defect clusters or anti-site-related defects. Dynamic annealing causes simple ion-beam-generated Frenkel pairs to annihilate (or cluster) during irradiation at liquid nitrogen temperature and above. These beam-flux and irradiation-temperature effects are not observed during bombardment with lighter 1H ions, which produce very dilute collision cascades. A qualitative model is proposed to explain temperature and flux effects in GaN in the MeV light-ion bombardment regime used for electrical isolation. © 2002 American Institute of Physics.

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1089-7550

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Physics

Mechanisms of blistering of wurtzite GaN films implanted with H ions are studied. In particular, we report on the influence of the following parameters on the blistering process: (i) ion energy (from 20 to 150 keV), (ii) ion dose (up to 1.2×1018 cm−2), (iii) implantation temperature (from −196 to 250 °C), and (iv) annealing temperature (up to 900 °C). Results show that both the onset of blistering and blistering surface patterns strongly depend on implant conditions. This study may have significant technological implications for ion slicing and "etching" of GaN using high-dose implantation with H ions. © 2002 American Institute of Physics.

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1089-7550

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Physics

The effects of elevated-temperature ion bombardment of wurtzite GaN films preamorphized by ion implantation are studied by Rutherford backscattering/channeling spectrometry and transmission electron microscopy. Amorphous layers annealed in vacuum at 500 °C exhibit polycrystallization. Bombardment of amorphous layers with 2 MeV 63Cu+ ions at elevated temperatures leads to anomalous erosion of GaN (with a sputtering yield of ∼102 at 500 °C), rather than to ion-beam-induced epitaxial crystallization. Temperature dependence of the erosion rate suggests that such a large sputtering yield results from a two-step process of (i) thermally- and ion- beam-induced material decomposition and (ii) ion beam erosion of a highly N-deficient near-surface layer of GaN. This study shows that amorphization during ion implantation should be avoided due to the present inability to epitaxially recrystallize amorphous layers in GaN. © 2000 American Institute of Physics.

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1077-3118

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Physics

The deformation behavior of bulk ZnO single crystals is studied by a combination of spherical nanoindentation and atomic force microscopy. Results show that ZnO exhibits plastic deformation for relatively low loads ((approximately-greater-than)4–13 mN with an ∼4.2 μm radius spherical indenter). Interestingly, the elastic–plastic deformation transition threshold depends on the loading rate, with faster loading resulting, on average, in larger threshold values. Multiple discontinuities (so called "pop-in" events) in force–displacement curves are observed during indentation loading. No discontinuities are observed on unloading. Slip is identified as the major mode of plastic deformation in ZnO, and pop-in events are attributed to the initiation of slip. An analysis of partial load–unload data reveals values of the hardness and Young's modulus of 5.0±0.1 and 111.2±4.7 GPa, respectively, for a plastic penetration depth of 300 nm. Physical processes determining deformation behavior of ZnO are discussed. © 2002 American Institute of Physics.

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1077-3118

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Physics

The influence of In content on the accumulation of structural damage in InxGa1−xN films (with x=0.0–0.2) under heavy-ion bombardment is studied by a combination of Rutherford backscattering/channeling spectrometry and transmission electron microscopy. Results show that an increase in In concentration strongly suppresses dynamic annealing processes and, hence, enhances the buildup of stable lattice disorder in InGaN under ion bombardment, A comparison of the damage buildup behavior and defect microstructure in InGaN with those in GaN is presented. Results of this study may have significant technological implications for estimation and control of implantation-produced damage in InGaN/GaN heterostructures. © 2001 American Institute of Physics.

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1077-3118

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Physics

The evolution of sheet resistance of n-type GaN epilayers exposed to irradiation with MeV H, Li, C, and O ions is studied in situ. Results show that the threshold dose necessary for complete isolation linearly depends on the original free electron concentration and reciprocally depends on the number of atomic displacements produced by ion irradiation. Furthermore, such isolation is stable to rapid thermal annealing at temperatures up to 900 °C. In addition to providing a better understanding of the physical mechanisms responsible for electrical isolation, these results can be used for choosing implant conditions necessary for an effective electrical isolation of GaN-based devices. © 2001 American Institute of Physics.

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1077-3118

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Physics

The accumulation of structural damage in AlxGa1−xN films (with x=0.05–0.60) under heavy-ion bombardment at room temperature is studied by a combination of Rutherford backscattering/channeling spectrometry and cross-sectional transmission electron microscopy (XTEM). Results show that an increase in Al concentration strongly enhances dynamic annealing processes in AlGaN and suppresses ion-beam-induced amorphization. All AlGaN wafers studied show damage saturation in the bulk for high ion doses. Interestingly, the disorder level in the saturation regime is essentially independent of Al content. In contrast to the case of GaN, no preferential surface disordering is observed in AlGaN during heavy-ion bombardment. XTEM reveals similar implantation-produced defect structures in both GaN and AlGaN. © 2002 American Institute of Physics.

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1077-3118

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Physics

The mechanical deformation of wurtzite GaN epilayers grown on sapphire substrates is studied by spherical indentation, cross-sectional transmission electron microscopy (XTEM), and scanning cathodoluminescence (CL) monochromatic imaging. CL imaging of indents which exhibit plastic deformation (based on indentation data) shows an observable "footprint" of deformation-produced defects that result in a strong reduction in the intensity of CL emission. Multiple discontinuities are observed during loading when the maximum load is above the elastic-plastic threshold, and such a behavior can be correlated with multiple slip bands revealed by XTEM. No evidence of pressure-induced phase transformations is found from within the mechanically damaged regions using selected-area diffraction patterns. The main deformation mechanism appears to be the nucleation of slip on the basal planes, with dislocations being nucleated on additional planes on further loading. XTEM reveals no cracking or delamination in any of the samples studied for loads of up to 250 mN. © 2002 American Institute of Physics.

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1077-3118

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Physics

Cathodoluminescence (CL) spectroscopy shows that even relatively low-dose keV light-ion bombardment (corresponding to the generation of ∼5×1019 vacancies/cm3) of wurtzite GaN results in a dramatic quenching of visible CL emission. Postimplantation annealing at temperatures up to 1050 °C generally causes a partial recovery of measured CL intensities. However, CL depth profiles indicate that, in most cases, such a recovery results from CL emission from virgin GaN, beyond the implanted layer due to a reduction in the extent of light absorption within the implanted layer. In this case, CL emission from the implanted layer remains completely quenched even after such an annealing. These results show that an understanding of the effects of ion bombardment and postimplantation annealing on luminescence generation and light absorption is required for a correct interpretation of luminescence spectra of GaN optically doped by keV ion implantation. © 2001 American Institute of Physics.

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1077-3118

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Physics

Structural studies reveal that heavy ion bombardment of GaN causes amorphization and anomalous swelling of the implanted region as a result of the formation of a porous structure. Results strongly suggest that such a porous structure consists of N2 gas bubbles embedded into a highly N-deficient amorphous GaN matrix. The evolution of the porous structure in amorphous GaN appears to be a result of stoichiometric imbalance where N- and Ga-rich regions are produced by ion bombardment. Prior to amorphization, ion bombardment does not produce a porous structure due to very efficient dynamic annealing processes in the crystalline phase. © 2000 American Institute of Physics.

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1077-3118

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Physics

Wurtzite GaN films grown on sapphire substrates are studied by nanoindentation with a spherical indenter. No systematic dependence of the mechanical properties of GaN epilayers on the film thickness (at least for thicknesses from 1.8 to 4 μm) as well as on doping type is observed. Slip is identified as one of the physical mechanisms responsible for plastic deformation of GaN and may also contribute to the "pop-in" events observed during loading. No visible material cracking is found even after indentations at high loads (900 mN), but a pronounced elevation of the material surrounding the impression is observed. © 2000 American Institute of Physics.

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1077-3118

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Physics

Wurtzite GaN films bombarded with heavy ions (197Au+) show anomalous swelling of the implanted region with corresponding volume expansion up to ∼50%. Results show that this phenomenon is due to the formation of a porous layer of amorphous GaN. An important implication of this study for the fabrication of GaN-based devices is that amorphization of GaN should be avoided during ion implantation. © 2000 American Institute of Physics.

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1077-3118

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Physics

The damage buildup in wurtzite GaN films under light (12C) and heavy (197Au) ion bombardment at temperatures from −196 to 550 °C is studied by Rutherford backscattering/channeling spectrometry. A strong surface peak of lattice disorder in addition to the expected damage peak in the region of the maximum of nuclear energy loss has been observed for all implant conditions of this study. Capping of GaN with SiOx and SixNy layers prior to implantation somewhat reduces but does not eliminate surface disordering. This suggests that nitrogen loss is not the main reason for the observed enhanced surface disorder, but, rather, the GaN surface acts as a strong sink for migrating point defects. However, pronounced loss of N during ion bombardment is observed for high dose implantation when the near-surface region is amorphized. Moreover, after amorphization, annealing at temperatures above about 400 °C leads to complete decomposition of the near-surface layer. © 2000 American Institute of Physics.

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1077-3118

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Physics

Wurtzite GaN epilayers are studied by cathodoluminescence (CL) spectroscopy. Results show that the intensities of donor–acceptor pair (DAP) and yellow luminescence (YL) peaks sublinearly depend on excitation density, presumably, due to saturation effects. The intensity of near-gap emission, however, exhibits a superlinear dependence on electron-beam excitation. In contrast to photoluminescence measurements, CL studies of GaN are usually performed in a regime with a strongly nonlinear dependence of luminescence intensities on excitation due to a large difference in carrier generation rates for these two techniques. As a result, the ratios of near-gap to YL and DAP emission intensities strongly depend on electron-beam current. Moreover, electron-beam spot size (i.e., beam focusing) dramatically affects CL intensity. An understanding of such saturation effects is necessary for a correct interpretation of CL spectra from GaN. © 2001 American Institute of Physics.

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1077-3118

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Physics

Damage accumulation in wurtzite GaN films bombarded with 0.5 MeV Bi1 and 1 MeV Bi2 ions (the so-called molecular effect) is studied by Rutherford backscattering/channeling spectrometry. Results show that an increase in the density of collision cascades dramatically enhances the level of implantation-produced lattice disorder in GaN. This effect is attributed to (i) an increase in the defect clustering efficiency with increasing density of ion-beam-generated point defects and/or (ii) to collective nonlinear energy spike processes. Such a strong influence of the density of collision cascades is important to take into account for a correct estimation of implantation-produced lattice disorder in GaN. © 2001 American Institute of Physics.

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1077-3118

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Physics

The deformation behavior of wurtzite GaN films modified by ion bombardment is studied by nanoindentation with a spherical indenter. Results show that implantation disorder significantly changes the mechanical properties of GaN. In particular, GaN amorphized by ion bombardment exhibits plastic deformation even for very low loads with dramatically reduced values of hardness and Young's modulus compared to the values of as-grown GaN. Implantation-produced defects in crystalline GaN suppress the plastic component of deformation and significantly change the values of hardness and Young's modulus. In addition, implantation disorder in crystalline GaN suppresses both "pop-in" events during loading and the appearance of slip traces on the sample surface as a result of indentation. This strongly suggests that slip nucleation (rather than a phase transformation) is the physical mechanism responsible for the pop-in events observed during loading of as-grown crystalline GaN. © 2001 American Institute of Physics.

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