R. G. Wilson

Ferromagnetism in Mn-implanted ZnO:Sn single crystals

We have investigated the magnetic properties of Mn-implanted n-type ZnO single crystals that are codoped with Sn. Theory predicts that room-temperature carrier-mediated ferromagnetism should be possible in manganese-doped p-type ZnO, although Mn-doped n-type ZnO should not be ferromagnetic. While previous efforts report only low-temperature ferromagnetism in Mn-doped ZnO that is n type via shallow donors, we find evidence for ferromagnetism with a Curie temperature of ∼250 K in ZnO that is codoped with Mn and Sn. As a 4+ valence cation, Sn should behave as a doubly ionized donor, thus introducing states deep in the gap. Hysteresis is clearly observed in magnetization versus field curves. Differences in zero-field-cooled and field-cooled magnetization persists up to ∼250 K for Sn-doped ZnO crystals implanted with 3 at. % Mn. Increasing the Mn concentration to 5 at. % significantly reduces the magnetic hysteresis. This latter observation is inconsistent with the origin for ferromagnetism being due to segreg...

Magnetic and structural properties of Mn-implanted GaN

High doses (1015–5×1016 cm−2) of Mn+ ions were implanted into p-GaN at ∼350 °C and annealed at 700–1000 °C. At the high end of this dose range, platelet structures of GaxMn1−xN were formed. The presence of these regions correlated with ferromagnetic behavior in the samples up to ∼250 K. At low doses, the implanted led to a buried band of defects at the end of the ion range.

Ferromagnetism in cobalt-implanted ZnO

The magnetic and structural properties of cobalt-implanted ZnO single crystals are reported. High-quality, (110)-oriented single-crystal Sn-doped ZnO substrates were implanted at ∼350 °C with Co to yield transition metal concentrations of 3–5 at. % in the near-surface (∼2000 A) region. After implantation, the samples were subject to a 5 min rapid thermal annealing at 700 °C. Magnetization measurements indicate ferromagnetic behavior, with hysteresis observed in the M vs H behavior at T=5 K. Coercive fields were ⩽100 Oe at this measurement temperature. Temperature-dependent magnetization measurements showed evidence for ordering temperatures of >300 K, although hysteresis in the M vs H behavior was not observed at room temperature. Four-circle x-ray diffraction results indicate the presence of (110)-oriented hexagonal phase Co in the ZnO matrix. From the 2θ full width at half maximum (FWHM) of the Co (110) peak, the nanocrystal size is estimated to be ∼3.5 nm, which is below the superparamagnetic limit at ...

1.54‐μm photoluminescence from Er‐implanted GaN and AlN

We report the observation of the 1.54‐μm luminescence of optically excited Er3+ in ion‐implanted epitaxially grown GaN and AlN films using below band‐gap excitation. The Er‐implanted layers were co‐implanted with oxygen. At room temperature, this luminescence for GaN grown on sapphire is nearly as intense as it is at 6 or 77 K and exhibits many resolved transitions between crystal‐field levels of the 4I13/2 first excited multiplet and the 4I15/2 ground multiplet.

Unconventional carrier-mediated ferromagnetism above room temperature in ion-implanted (Ga, Mn)P:C.

Ion implantation of Mn ions into hole-doped GaP has been used to induce ferromagnetic behavior above room temperature for optimized Mn concentrations near 3 at. %. The magnetism is suppressed when the Mn dose is increased or decreased away from the 3 at. % value, or when n-type GaP substrates are used. At low temperatures the saturated moment is on the order of 1 Bohr magneton, and the spin wave stiffness inferred from the Bloch-law T(3/2) dependence of the magnetization provides an estimate T(c)=385 K of the Curie temperature that exceeds the experimental value, T(c)=270 K. The presence of ferromagnetic clusters and hysteresis to temperatures of at least 330 K is attributed to disorder and proximity to a metal-insulating transition.

Boron, fluorine, and carrier profiles for B and BF2 implants into crystalline and amorphous Si

Depth distributions, measured by secondary ion mass spectrometry (SIMS), and carrier profiles, measured by differential capacitance‐voltage (C‐V) profiling, of boron and fluorine implanted as B, F, BF, or BF2 ions into random and channeling orientations of crystalline silicon, and into silicon amorphized by silicon ion implantation are reported. Low boron energies of 8 and 10 keV and the corresponding energies of 36 and 45 keV for BF2 ions are emphasized because of their use for high resolution device and circuit applications in silicon and silicon‐on‐sapphire. Amorphizing crystalline silicon prior to boron implantation eliminates the significant channeling tails on 8‐ or 10‐keV boron profiles. Fluorine penetrates more deeply into crystalline silicon than boron does. Both boron and fluorine redistribute during annealing at 925 °C/20 min for B, F, BF, or BF2 implants, but with quite different characteristics as illustrated, and depend on the implantation fluence (5×1014 and 2×1015 cm−2 reported here). The ...

Ca and O ion implantation doping of GaN

p‐ and n‐type doping of GaN have been realized by ion implantation of Ca and O, respectively. Rapid thermal annealing at 1100 °C or higher is required to achieve p‐type conduction in Ca or Ca+P implanted samples with an estimated ionization level of 169 meV and a corresponding activation efficiency of ∼100%. This is the first experimental report of an acceptor species in GaN, other than Mg, with an ionization energy level less than 180 meV. O‐implanted GaN displays an ionization level of ∼29 meV but with an activation efficiency of only 3.6% after a 1050 °C anneal that may result from insufficient vacancy generation for the lighter O ion or from the existence of a second, deeper O energy level. Neither Ca or O displayed measurable redistribution, based on secondary ion mass spectrometry measurements, even after a 1125 °C anneal.

Ion‐implanted GaN junction field effect transistor

Selective area ion implantation doping has been used to fabricate GaN junction field effect transistors (JFETs). p‐type and n‐type doping was achieved with Ca and Si implantation, respectively, followed by a 1150 °C rapid thermal anneal. A refractory W gate contact was employed that allows the p‐gate region to be self‐aligned to the gate contact. A gate turn‐on voltage of 1.84 V at 1 mA/mm of gate current was achieved. For a ∼1.7 μm×50 μm JFET with a −6 V threshold voltage, a maximum transconductance of 7 mS/mm at VGS=− 2V and saturation current of 33 mA/mm at VGS=0 V were measured. These results were limited by excess access resistance and can be expected to be improved with optimized n+ implants in the source and drain regions.

Photoluminescence spectroscopy of erbium implanted gallium nitride

Results of a photoluminescence (PL) and photoluminescence excitation (PLE) study of Er implanted GaN are presented. Upon optical excitation at 325 and 488 nm, we observed strong 1.54 μm Er3+ PL which remained temperature stable from 15 to 550 K. At 550 K, the integrated PL intensity decreased by ∼10% for above gap excitation (λex=325 nm) and ∼50% for below gap excitation (λex=488 nm) relative to its value at 15 K. The excellent temperature stability makes GaN:Er very attractive for high temperature optoelectronic device applications. PLE measurements were conducted to gain insight into the Er3+ excitation mechanisms in the GaN host. The PLE results show that Er3+ can be excited continuously over a broad wavelength region spanning from 425 to 680 nm. In addition, sharp PLE features were observed at approximately 495, 525, 553, 651, and 980 nm. The PLE spectrum suggests that optically active Er3+ ions can be excited either through carrier-mediated processes involving defects in the host or through resonant ...

Growth and Fabrication of GaN/AlGaN Heterojunction Bipolar Transistor

A GaN/AlGaN heterojunction bipolar transistor structure with Mg doping in the base and Si Doping in the emitter and collector regions was grown by Metal Organic Chemical Vapor Deposition in c-axis Al(2)O(3). Secondary Ion Mass Spectrometry measurements showed no increase in the O concentration (2-3x10(18) cm(-3)) in the AlGaN emitter and fairly low levels of C (~4-5x10(17) cm (-3)) throughout the structure. Due to the non-ohmic behavior of the base contact at room temperature, the current gain of large area (~90 um diameter) devices was <3. Increasing the device operating temperature led to higher ionization fractions of the mg acceptors in the base, and current gains of ~10 were obtained at 300 degree C.