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

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 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.This work was partly supported by Conselho Nacional de Pesquisas (CNPq, Brazil) under Contract No. 200541/ 99-4

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 ≲1at.% of Er. Large redistribution of Er, with segregation at the amorphous/crystalline interface, starts at ≳500°C, while lower temperatures are required for material recrystallization. However, even at 400°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 GaN under ion bombardment

The damage buildup until amorphization in wurtzite GaN films under keV Light(C-12) and heavy (Au-197) ion bombardment at room and liquid nitrogen (LN2) temperatures is studied by Rutherford backscattering/channeling (RBS/C) spectrometry and transmission electron microscopy (TEM). The effect of beam flux on implantation damage in GaN is reported. A marked similarity between damage buildup for Light and heavy ion bombardment regimes is observed. The results point to substantial dynamic annealing of irradiation defects even during heavy ion bombardment at LN2 temperature. Amorphization starts from the GaN surface with increasing ion dose for both LN2 and room-temperature bombardment with light or heavy ions. A strong surface defect peak, seen by RBS/C, arises from an amorphous layer at the GaN surface, as indicated by TEM. The origin of such an amorphous layer is attributed to the trapping of mobile point defects by the GaN surface, as suggested by the flux behavior. However, in the samples implanted with light ions to low doses (1 X 10(15) cm(-2)), no amorphous layer on the GaN surface is revealed by TEM. Damage buildup is highly sig-modal for LN: temperature irradiation with light or heavy ions. Formation of planar defects in the crystal bulk is assumed to provide a "nucleation site" for amorphization with increasing ion dose during irradiation at LN2 temperature. For room-temperature bombardment with heavy ions. the damage in the GaN bulk region saturates at a level lower than that of the amorphous phase, as measured by RBS/C, and amorphization proceeds From the GaN surface with increasing ion dose. For such a saturation regime at room temperature, implantation damage in the bulk consists of point-defect clusters and planar defects which are parallel to the basal plane of the GaN film. Various defect interaction processes in GaN during ion bombardment are proposed to explain the observed somewhat unexpected behavior of disorder buildup

Deformation behavior of ion-irradiated polyimide

We study nanoindentationhardness, Young’s 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 Young’s modulus and decreases the tensile strength. These changes depend close to linearly on ion fluence and superlinearly (with a power-law exponent factor of ∼1.5) 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

Effect of ion species on the accumulation of ion-beam damage in GaN

Wurtzite GaN epilayers bombarded with a wide range of ion species (10 keV H-1, 40 keV C-12, 50 keV O-16, 600 keV Si-28, 130 keV Cu-63, 200 keV Ag-107, 300 keV Au-197, and 500 keV Bi-209) are studied by a combination of Rutherford backscattering/channeling (RBS/C) spectrometry and cross-sectional transmission electron microscopy. Results show that strong dynamic annealing processes lead to a complex dependence of the damage-buildup behavior in GaN on ion species. For room-temperature bombardment with different ion species, bulk disorder, as measured by RBS/C, saturates at some level that is below the random level, and amorphization proceeds layer-by-layer from the GaN surface with increasing ion dose. The saturation level of bulk disorder depends on implant conditions and is much higher for light-ion bombardment than for the heavy-ion irradiation regime. In the case of light ions, when ion doses needed to observe significant lattice disorder in GaN are large (greater than or similar to 10(16) cm(-2)), chemical effects of implanted species dominate. Such implanted atoms appear to stabilize an amorphous phase in GaN and/or to act as effective traps for ion-beam-generated mobile point defects and enhance damage buildup. In particular, the presence of a large conce ntration of carbon in GaN strongly enhances the accumulation of implantation-produced disorder. For heavier ions, where chemical effects of implanted species seem to be negligible, an increase in the density of collision cascades strongly increases the level of implantation-produced lattice disorder in the bulk as well as the rate of layer-by-layer amorphization proceeding from the surface. Such an increase in stable damage and the rate of planar amorphization is attributed to (i) an increase in the defect clustering efficiency with increasing density of ion-beam-generated defects and/or (ii) a superlinear dependence of ion-beam-generated defects, which survive cascade quenching, on the density of collision cascades. Physical mechanisms responsible for such a superlinear dependence of ion-beam-generated defects on collision cascade density are considered. Mechanisms of surface and bulk amorphization in GaN are also discussed

Damage buildup in Si under bombardment with MeV heavy atomic and molecular ions

Accumulation of structural disorder in Si bombarded at −196 °C with 0.5 MeV ²⁰⁹Bi₁ and 1 MeV ²⁰⁹Bi₂ 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 Bi₂ ions produce more lattice damage than Bi₁ ions implanted to the same dose. The ratio of lattice disorder produced by Bi₂ and Bi₁ 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

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 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

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 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-temperatureeffects 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