Gary C. Farlow

Ion implantation and annealing of crystalline oxides

The technique of ion implantation is being investigated as a general method for altering the near-surface properties of insulating materials. The primary motivation behind these investigations is to develop ion implantation as a practical means of controlling and improving the near-surface mechanical, optical, or electronic properties of insulators. Changes in these properties depend on the microstructures and compositions developed in the material during the ion implantation process and subsequent thermal treatments. In many cases, structures and compositions can be produced by implantation and thermal annealing that cannot be achieved by conventional techniques. In this work, the response of a wide range of crystalline oxides to ion implantation and subsequent thermal processing will be reviewed. The materials treated here include Al 2 O 3 , LiNbO 3 , CaTiO 3 , SrTiO 3 , ZnO, and MgO, as well as the non-oxide materials Si 3 N 4 and SiC. The response of these insulators to ion implantation varies widely and depends on the specific material, the implantation species and dose, and the implantation temperature. Ion implantation produces displacement and other damage in the near-surface region, and in many cases, the surfaces of originally crystalline insulators are turned amorphous. Thermal annealing can often be used to restore crystallinity to the damaged near-surface region, and additionally, metastable solid solutions can be produced. For a number of oxide materials, the annealing behavior has been studied in detail using both Rutherford backscattering-ion channeling techniques and transmission electron microscopy. These studies show that, in some materials, the annealing behavior is quite simple and takes place by solid-phase epitaxial crystallization where the amorphous-to-crystalline transformation occurs at an interface that moves toward the free surface during the annealing process. In such materials, the regrowth kinetics have been measured, and the associated activation energies for crystallization have been determined. The formation of metastable solid solutions during crystallization of the amorphous phase will also be discussed.

Radiation Hardness of ZnO at Low Temperatures

In situ Hall-effect measurements have been carried out on vapour-phase-grown, n-type ZnO irradiated with 1.0 and 1.5 MeV electrons in the [000-1] direction. The electrical properties change very little during irradiation at temperatures as low as 130 K, the lowest temperature presently attainable under 1 MeV, 0.3 µA cm−2 irradiation. It is concluded that long-term damage in ZnO is limited by defect annihilations that are rapid on the time scale of the experiment (<1 min), even at 130 K.

On the nitrogen vacancy in GaN

The dominant electrically active defect produced by 0.42 MeV electron irradiation in GaN is a 70 meV donor. Since only N-sublattice displacements can be produced at this energy, and since theory predicts that the N interstitial is a deep acceptor in n-type GaN, we argue that the 70 meV donor is most likely the isolated N vacancy. The background shallow donors, in the 24–26 meV range, actually decrease in concentration, probably due to interactions with mobile N interstitials that are produced by the irradiation. Thus, the recent assignment of a photoluminescence (PL) line as an exciton bound to a 25 meV N-vacancy donor is incompatible with our results. Moreover, we do not observe that PL line in our sample.

Electron irradiation induced deep centers in hydrothermally grown ZnO

An n-type hydrothermally grown ZnO sample becomes semi-insulating (ρ~108 Ω cm) after 1-MeV electron-irradiation. Deep traps produced by the irradiation were studied by thermally stimulated current spectroscopy. The dominant trap in the as-grown sample has an activation energy of 0.24 eV and is possibly related to LiZn acceptors. However, the electron irradiation introduces a new trap with an activation energy of 0.15 eV, and other traps of energy 0.30 and 0.80 eV, respectively. From a comparison of these results with positron annihilation experiments and density functional theory, we conclude that the 0.15-eV trap may be related to VZn.

Effects of Electron-Irradiation on Electrical Properties of AlGaN/GaN Schottky Barrier Diodes

Effects of 1 MeV electron-irradiation at room temperature on the electrical properties of AlGaN/GaN heterostructures, including leakage currents, threshold voltages, and electron traps, have been investigated using Schottky barrier diodes (SBDs) fabricated on the AlGaN. The SBDs, before and after the irradiation with a dose of 5×1015 cm−2, were characterized by temperature dependent current-voltage and capacitance-voltage measurements and deep level transient spectroscopy. It is found that the irradiation causes (i) significant increase in leakage currents, dominated by tunneling conduction, at both reverse and low-forward biases; (ii) a clear negative shift in threshold voltage in the pinch-off region; and (iii) creation of traps Ae(∼1.1 eV) or A2(1.2 eV) and Ee(0.09 eV) in the GaN buffer and AlGaN regions. The irradiation-induced traps can be used to account for the increase in leakage currents and shift in threshold voltage. However, as compared to traps A2(1.2 eV) and E(0.13 eV) induced in thick GaN l...

Defect segregation and optical emission in ZnO nano- and microwires

The spatial distribution of defect related deep band emission has been studied in zinc oxide (ZnO) nano- and microwires using depth resolved cathodoluminescence spectroscopy (DRCLS) in a hyperspectral imaging (HSI) mode within a UHV scanning electron microscope (SEM). Three sets of wires were examined that had been grown by pulsed laser deposition or vapor transport methods and ranged in diameter from 200 nm-2.7 μm. This data was analyzed by developing a 3D DRCLS simulation and using it to estimate the segregation depth and decay profile of the near surface defects. We observed different dominant defects from each growth process as well as diameter-dependent defect segregation behavior.

Thermodynamic constraints on ion beam mixing of metals on insulators

Several different insulating substrates were coated with various metal films and ion beam irradiated using either Xe or Kr ions. These were then examined by Rutherford backscattering spectroscopy and scanning electron microscopy to determine if interfacial mixing had taken place. These results were compared with the sign of the reaction enthalpy of the metal and substrate to test the proposition that metals mix on insulators if the reaction enthalpy is negative and do not mix if it is positive. The enthalpy rule is, in general, valid. Two exceptions were found: Cr on SiO/sub 2/ and Zr on Al/sub 2/O/sub 3/; however, these exceptions contain ambiguous features. Irradiation with a light, reactive ion was found to produce no mixing.

The Effects of Temperature and Electron Radiation on the Electrical Properties of AlGaN/GaN HFETs

Al0.27Ga0.73N/GaN HFETs were electron irradiated at ~ 80 K. The gate leakage and transistor current were measured and compared to theoretical tunneling models. The results are consistent with previous work but explicitly show that radiation produces point defects in the AlGaN that are positively charged at low temperature.

Ion Implantation and Annealing of Crystalline Oxides

The response of several crystalline oxides or ceramic materials to ion implantation and subsequent thermal annealing is described. For both SrTiO, and CaTiOs single crystals, the near-surface region can be turned amorphous by relatively low doses of heavy ions (Pb, 10’5/cm2, 540 keV). During annealing, the amorphous region recrystallizes epitaxlally with the underlying substrate by simple solid-phase epitaxy, and the crystallization kinetics have been determined for both of these materials. In Also,, the amorphous phase of the pure material is produced by a stoichiometric implant at liquid nitrogen temperature. During annealing, the amorphous film crystallizes in the (crystalline) y phase, followed by the transformation of the y to the a phase at a well-defined interface. The kinetics characterizing the growth of a-Also, have been determined. Preliminary results are presented on the effect of impurities (Fe) on the nature and kinetics of the crystallization of amorphous AlaOs.

High-dose implantation of Si in SiO2: formation of Si crystallites after annealing

Abstract The behavior of high-dose (8 × 10 17 /cm 2 ) silicon implanted in X-cut, α-quartz at four different temperatures has been investigated. The formation of buried crystallites of silicon under substantial compressive stress and outdiffused silicon under tensile stress on the surface is observed after rapid thermal annealing. In samples annealed at 1000, 1100 or 1150°C the implanted silicon formed buried crystallites, with no evidence of outdiffusion. Differential interference microscopy, Rutherford backscattering spectroscopy and Raman microprobe spectroscopy provided information on the structure of the silicon within and on the surface of the implanted quartz substrate.