172 research outputs found
Electrical isolation of GaN by MeV ion irradiation
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 900āĀ°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.This work was partly supported by Conselho Nacional
de Pesquisas (CNPq, Brazil) under Contract No. 200541/
99-4
Ion-beam-induced reconstruction of amorphous GaN
Wurtzite GaN can be rendered amorphous by high-dose heavy-ion bombardment. We show here that relatively low-dose reirradiation of such amorphous GaN (a-GaN) with MeV light ions can significantly change some of the physical properties of a-GaN. In particular, light-ion reirradiation of a-GaN results in (i) an increase in material density, (ii) the suppression of complete decomposition during postimplantation annealing, (iii) a significant increase in the values of hardness and Young's modulus, and (iv) an apparent decrease in the absorption of visible light. Transmission electronmicroscopy shows that a-GaN remains completely amorphous after light-ion reirradiation. Therefore, we attribute the above effects of light-ion reirradiation to an ion-beam-induced atomic-level reconstruction of the amorphous phase. Results indicate that electronic energy loss of light ions is responsible for the changes in the mechanical properties and for the suppression of thermally induced decomposition of a-GaN. However, the changes in the density of a-GaN appear to be controlled by the nuclear energy loss of light ions
Segregation and precipitation of Er in Ge
Although Er-doped Genanomaterials are attractive for photonic applications, very little is known about the basic properties of Er in Ge. Here, the authors study the annealing behavior of Geimplanted with keV Er ions to doses resulting in ā²1at.% of Er. Large redistribution of Er, with segregation at the amorphous/crystalline interface, starts at ā³500Ā°C, while lower temperatures are required for material recrystallization. However, even at 400Ā°C, Er forms precipitates. The concentration of Er trapped in the bulk after recrystallization decreases with increasing temperature but is independent of the initial bulk Er concentration for the range of ion doses studied here.Work at the ANU was supported
by the ARC
Damage buildup in Si under bombardment with MeV heavy atomic and molecular ions
Accumulation of structural disorder in Si bombarded at ā196āĀ°C with 0.5 MeV Ā²ā°ā¹Biā and 1 MeV Ā²ā°ā¹Biā ions (the so-called molecular effect) is studied by Rutherford backscattering/channeling spectrometry. Results show that the damage buildup is sigmodal even for such heavy-ion bombardment at liquid nitrogen temperature. This strongly suggests that, for the implant conditions of this study, the buildup of lattice damage cannot be considered as an accumulation of completely disordered regions. Instead, damage-dose curves are well described by a cascade-overlap model modified to take into account a catastrophic collapse of incompletely disordered regions into an amorphous phase after damage reaches some critical level. Results also show that Biā ions produce more lattice damage than Biā ions implanted to the same dose. The ratio of lattice disorder produced by Biā and Biā ions is 1.7 near the surface, decreases with depth, and finally becomes close to unity in the bulk defect peak region. Parameters of collision cascades obtained using ballistic calculations are in good agreement with experimental data. The molecular effect is attributed to a spatial overlap of (relatively dense) collision subcascades, which gives rise to (i) nonlinear energy spike processes and/or (ii) an increase in the defect clustering efficiency with an effective increase in the density of ion-beam-generated defects.Research at StPSTU was supported in part by the Ministry
for General and Professional Education of the Russian
Federation
Effect of irradiation temperature and ion flux on electrical isolation of GaN
We study the evolution of sheet resistance of n-type GaN epilayers irradiated with MeV Ā¹H and Ā¹Ā²C 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 Ā¹Ā²C 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-temperatureeffects are not observed during bombardment with lighter Ā¹H 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
Deformation behavior of ion-irradiated polyimide
We study nanoindentationhardness, Youngās modulus, and tensile strength of polyimide (Kapton H) films bombarded with MeV light ions in the predominantly electronic stopping power regime. Results show that, for all the ion irradiation conditions studied, bombardment increases the hardness and Youngās modulus and decreases the tensile strength. These changes depend close to linearly on ion fluence and superlinearly (with a power-law exponent factor of ā¼1.5) on electronic energy loss. Physical mechanisms of radiation-induced changes to mechanical properties of polyimide are discussed.This work was performed under the auspices of the U. S.
Department of Energy by the University of California, LLNL
under Contract No. W-7405-ENG-48. The project (03-FS-
027) was funded by the Laboratory Directed Research and
Development Program at LLNL
Effects of excitation density on cathodoluminescence from GaN
Wurtzite GaN epilayers are studied by cathodoluminescence(CL)spectroscopy. Results show that the intensities of donorāacceptor 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
Effect of the density of collision cascades on implantation damage in GaN
Damage accumulation in wurtzite GaN films bombarded with 0.5 MeV Biā and 1 MeV Biā 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
Mechanical deformation of single-crystal ZnO
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 (>~4ā13 mN with an ~4.2 mm radius spherical indenter). Interestingly, the
elasticāplastic deformation transition threshold depends on the loading rate, with faster loading
resulting, on average, in larger threshold values. Multiple discontinuities (so called āāpop-ināā events)
in forceādisplacement 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 loadāunload data reveals
values of the hardness and Youngās modulus of 5.060.1 and 111.264.7 GPa, respectively, for a
plastic penetration depth of 300 nm. Physical processes determining deformation behavior of ZnO
are discussed
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