Showing papers by "Stephen J. Pearton published in 2002"

Magnetic properties of n-GaMnN thin films

Abstract: GaMnN thin films were synthesized using gas-source molecular-beam epitaxy. Mn concentrations between 3 and 12 at. % were investigated. No evidence of second-phase formation was observed by powder x-ray diffraction or high-resolution cross section transmission electron microscopy in films with 9% or less Mn. The films were n type as determined by capacitance–voltage or Hall analysis. Magnetic characterization performed using a squid magnetometer showed evidence of ferromagnetic ordering at room temperature for all samples. In agreement with theoretical predictions, material with 3% Mn showed the highest degree of ordering per Mn atom. At 320 K, the samples show a nonzero magnetization indicating a TC above room temperature.

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

Abstract: 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 increasedor 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.

Breakdown voltage and reverse recovery characteristics of free-standing GaN Schottky rectifiers

Abstract: Schottky rectifiers with implanted p/sup +/ guard ring edge termination fabricated on free-standing GaN substrates show reverse breakdown voltages up to 160 V in vertical geometry devices. The specific on-state resistance was in the range 1.7-3.0 /spl Omega//spl middot/cm/sup 2/, while the turn-on voltage was /spl sim/1.8 V. The switching performance was analyzed using the reverse recovery current transient waveform, producing an approximate high-injection, level hole lifetime of /spl sim/15 ns. The bulk GaN rectifiers show significant improvement in forward current density and on-state resistance over previous heteroepitaxial devices.

Thermal Stability of Ion-implanted Hydrogen in ZnO

Abstract: The evolution of implanted 2H profiles in single-crystal ZnO was examined as a function of annealing temperature (500–700 °C) by secondary ion mass spectrometry. The as-implanted profiles show a peak concentration of ∼2.7×1019 cm−3 at a depth of ∼0.96 μm for a dose of 1015 cm−2. Subsequent annealing causes outdiffusion of 2H from the ZnO, with the remaining hydrogen decorating the residual implant damage. Only 0.2% of the original dose is retained after annealing at 600 °C. Rutherford backscattering/channeling of samples implanted with 1H at a dose of 1016 cm−2 showed no change in backscattering yield near the ZnO surface, but did result in an increase near the end-of-range from 6.5% of the random level before 1H implantation to ∼7.8% after implantation. Results of both cathodoluminescence and photoluminescence studies show that even for a 1H dose of 1015 cm−2, the intensity of the near gap emission from ZnO is reduced more than 2 orders of magnitude from the values in unimplanted samples. This is due to ...

Characteristics of MgO/GaN gate-controlled metal–oxide– semiconductor diodes

Abstract: Gate-controlled n+p metal–oxide–semiconductor diodes were fabricated in p-GaN using MgO as a gate dielectric and Si+ implantation to create the n+ regions. This structure overcomes the low minority carrier generation rate in GaN and allowed observation of clear inversion behavior in the dark at room temperature. By contrast, diodes without the n+ regions to act as an external source of minority carriers did not show inversion even at measurement temperatures of 300 °C. The gated diodes showed the expected shape of the current–voltage characteristics, with clear regions corresponding to depletion and inversion under the gate. The MgO was deposited prior to the Si implantation and was stable during the activation annealing for the Si-implanted n+ regions.

Magnetic and structural properties of Fe, Ni, and Mn-implanted SiC

Abstract: Direct implantation of Fe, Ni or Mn at doses of 3–5×1016 cm−2 into p-type 6H-SiC substrates was carried out at a sample temperature of ∼350 °C. Subsequent annealing was performed at 700–1000 °C for 5 mins. Residual damage in the form of end-of-range defects and dislocation loops in the region from the surface to a depth of ∼0.20 μm were examined by transmission electron microscopy. To the sensitivity of both x-ray diffraction and selected area diffraction pattern analysis, no secondary phases could be detected. Signatures of ferromagnetism were observed in all the highest dose samples, with apparent Curie temperatures of 50 K (Ni), 250 K (Mn), and 270 K (Fe).

Use of ion implantation to facilitate the discovery and characterization of ferromagnetic semiconductors

Abstract: The discovery of epitaxially grown ferromagnetic, type III–V semiconductors (Ga,Mn)As (Tc=110 K) and (In,Mn)As (Tc=35 K) holds promise for developing semiconductor electronics that utilize the electron’s spin degree of freedom in addition to its charge. It has been theoretically predicted that some semiconducting systems could be ferromagnetic above room temperature, when optimally doped (p-GaN with 5% Mn). We report here on the use of ion implantation to incorporate magnetic ions into a variety of semiconducting substrates, thereby facilitating investigation of the nature of ferromagnetism in semiconducting systems that are difficult to grow with other methods. The magnetic ions, Mn, Fe, and Ni, were implanted into each of the epitaxially grown semiconductors GaN, GaP, and SiC to achieve volume concentrations between 1 and 5 at. %. The implanted samples were subsequently annealed at 700–1000 °C to recrystallize the samples and remove implant damage. The implanted samples were examined with both x-ray dif...

Electrical Characterization of GaN Metal Oxide Semiconductor Diodes Using MgO as the Gate Oxide

Abstract: GaN metal oxide semiconductor diodes were demonstrated utilizing MgO as the gate oxide. MgO was grown at 100°C on metal oxide chemical vapor deposition grown n-GaN in a molecular beam epitaxy system using a Mg elemental source and an electron cyclotron resonance oxygen plasma. H 3 PO 4 -based wet-chemical etchant was used to remove MgO to expose the underlying n-GaN for ohmic metal deposition. Electron deposited Ti/Al/Pt/Au and Pt/Au were utilized as ohmic and gate metallization, respectively. An interface trap density of low-to-mid-10 11 eV -1 cm -2 was obtained from temperature conductance-voltage measurements. Terman method was also used to estimate the interface trap density, and a slightly lower number was obtained as compared to the conductance method. Results from elevated temperature (up to 300°C) conductance measurements showed an interface state density roughly three times higher (6 × 10 11 eV -1 cm -2 ) than at 25°C.

Room temperature magnetism in GaMnP produced by both ion implantation and molecular-beam epitaxy

Abstract: The magnetization of the dilute magnetic alloy GaMnP:C prepared by the implantation of Mn into p-GaP:C or by direct molecular-beam epitaxy is reported. The material implanted to produce a Mn level of 3% produces ferromagnetic behavior that persists up to a temperature of 330 K, while the epitaxially derived material shows evidence of ferromagnetism at a temperature of 300 K. In both cases, no second phases were observed by x-ray diffraction, transmission electron microscopy, or selected area diffraction pattern analysis. A phase diagram of the GaMnP:C system, determined by epitaxial growth, is also reported.

Effect of high-density plasma etching on the optical properties and surface stoichiometry of ZnO

Abstract: Bulk, single-crystal ZnO was etched in Cl2/Ar and CH4/H2/Ar inductively coupled plasmas as a function of ion impact energy. For CH4/H2/Ar, the etch rate (R) increases with ion energy (E) as predicted from a model of ion enhanced sputtering by a collision–cascade process, R∝(E0.5−ETH0.5), where the threshold energy, ETH, is ∼96 eV. Band edge photoluminescence intensity decreases with incident ion energy in both chemistries, with a 70% decrease even for low energies (∼116 eV). Surface roughness is also a function of ion energy with a minimum at ∼250 eV, where Auger electron spectroscopy shows there is no measurable change in near-surface stoichiometry from that of unetched control samples.

Optical and electrical properties of GaMnN films grown by molecular-beam epitaxy

Abstract: Optical absorption spectra, microcathodoluminescence (MCL) spectra, and electrical properties of GaMnN films grown by molecular-beam epitaxy with Mn concentration in the range of 3 to 10 at. % were studied. Optical absorption and MCL spectra show the presence of strong bands corresponding to the transition from the Mn acceptors near Ec−2 eV to the conduction band. The other strong band observed in MCL measurements was the blue band peaked near 2.9 eV and associated with the transition from the valence band to deep donors with a level near Ec−0.5 eV. All GaMnN samples were shown to be lightly n-type which suggests close self-compensation of the Mn acceptors by some native defect donors. A plausible scenario is that such compensating donors could be due to nitrogen vacancies and that the Ec−0.5 eV donor defects are complexes between the Mn acceptors and the nitrogen vacancy donors.

Effects of Sc 2 O 3 and MgO passivation layers on the output power of AlGaN/GaN HEMTs

Abstract: The low temperature (100/spl deg/C) deposition of Sc/sub 2/O/sub 3/ or MgO layers is found to significantly increase the output power of AlGaN/GaN HEMTs. At 4 GHz, there was a better than 3 dB increase in output power of 0.5/spl times/100 /spl mu/m/sup 2/ HEMTs for both types of oxide passivation layers. Both Sc/sub 2/O/sub 3/ and MgO produced larger output power increases at 4 GHz than conventional plasma-enhanced chemical vapor deposited (PECVD) SiN/sub x/ passivation which typically showed /spl les/2 dB increase on the same types of devices. The HEMT gain also in general remained linear over a wider input power range with the Sc/sub 2/O/sub 3/ or MgO passivation. These films appear promising for reducing the effects of surface states on the DC and RF performance of AlGaN/GaN HEMTs.

Enhanced tunneling in GaN/InGaN multi-quantum-well heterojunction diodes after short-term injection annealing

Abstract: Multi-quantum-well GaN/InGaN heterojunction diodes prepared by metalorganic chemical vapor deposition on sapphire showed effects of strong tunneling in their I–V characteristics. The space charge region was shown to be located in the GaN/InGaN superlattice (SL). The injection of moderately high forward currents through the structure for several hours enhanced the overall tunneling through the structure and facilitated faster tunneling between the layers in the GaN/InGaN SL. These results may have relevance to the aging characteristics of light-emitting diodes under bias.

High-energy proton irradiation effects on AlGaN/GaN high-electron mobility transistors

Abstract: AlGaN/GaN high-electron mobility transistors (HEMTs) show decreases in extrinsic transconductance, drain-source current threshold voltage, and gate current as a result of irradiation with 40 MeV protons at doses equivalent to decades in low-earth orbit, The data are consistent with the protons creating deep electron traps that increase the HEMT channel resistance. Postirradiation annealing at 300°C was able to restore ∼ 70% of the initial gm and IDS values in HEMTs receiving proton doses of 5 × 1010 cm-2.

High energy proton irradiation effects on SiC Schottky rectifiers

Abstract: 4H-SiC Schottky rectifiers with dielectric overlap edge termination were exposed to 40 MeV protons at fluences from 5×107–5×109 cm−2. The reverse breakdown voltage decreased from ∼500 V in unirradiated devices to ∼−450 V after the highest proton dose. The reverse leakage current at −250 V was approximately doubled under these conditions. The forward current at −2 V decreased by ∼1% (fluence of 5×107 cm−2) to ∼42% (fluence of 5×109 cm−2), while the current at lower biases was increased due to the introduction of defect centers. The ideality factor, on-state resistance, and forward turn-on voltage showed modest increases for fluences of ⩽5×108 cm−2, but were more strongly affected (increase of 40%–75%) at the highest dose employed.

Electrical and optical properties of GaN films implanted with Mn and Co

Abstract: Optical transmission spectra, microcathodoluminescence spectra, capacitance–voltage and capacitance–frequency curves, temperature dependence of resistivity and deep level spectra with both electrical and optical injection were measured on n-GaN samples implanted with high doses of Mn (3×1016 and 4×1016 cm−2) and Co (4×1016 cm−2). From optical transmission it was found that Mn forms a deep acceptor near Ev+1.8 eV while the Co acceptor is about 0.1 eV deeper. In addition, Mn and Co form complexes with native defects and these complexes are deep electron traps with a level near Ec−0.5 eV. Such complexes are most likely responsible for a strong blue luminescence band with energy near 2.9 eV. Adjacent to the implanted region a defect region about 1 μm deep is formed, most likely by out-diffusion of point defects from the implanted zone during the 700 °C annealing used to partially remove the radiation damage. This region is characterized by a high density of electron traps at Ec−0.25 eV and Ec−0.7 eV and hole ...

Comparison of Surface Passivation Films for Reduction of Current Collapse in AlGaN/GaN High Electron Mobility Transistors

Abstract: Three different passivation layers (SiN x , MgO, and Sc 2 O 3 ) were examined for their effectiveness in mitigating surface-state-induced current collapse in AlGaN/GaN high electron mobility transistors (HEMTs). The plasma-enhanced chemical vapor deposited SiN x produced ∼70-75% recovery of the drain-source current, independent of whether SIH 4 /NH 3 or SiD 4 /ND 3 plasma chemistries were employed. Both the Sc 2 O 3 and MgO produced essentially complete recovery of the current in GaN-cap HEMT structures and ∼80-90% recovery in AlGaN-cap structures. The Sc 2 O 3 had superior long-term stability, with no change in HEMT behavior over 5 months aging.

High-Power GaN Electronic Devices

Abstract: Gallium Nitride (GaN) and related materials (especially AlGaN) recently have attracted a lot of interest for applications in high-power electronics capable of operation at elevated temperatures and high frequencies. The AlGaInN system offers numerous advantages. These include wide bandgaps, good transport properties, the availability of heterostructures (particularly AlGaN/GaN), the experience base gained by the commercialization of GaN-based laser and light-emitting diodes and the existence of a high growth rate epitaxial method (hydride vapor phase epitaxy, HVPE) for producing very thick layers or even quasisubstrates. These attributes have led to rapid progress in the realization of a broad range of GaN electronic devices. AlxGa1-xN (x=0 ∼.25) Schottky rectifiers were fabricated in a lateral geometry employing p+-implanted guard rings and rectifying contact overlap onto an SiO2 passivation layer. The reverse breakdown voltage (VB) increased with the spacing between Schottky and ohmic metal contacts, re...

Characterization of High Dose Mn, Fe, and Ni implantation into p-GaN

Abstract: The magnetization of p-GaN or p-AlGaN/GaN superlattices was measured after implantation with high doses (3–5×1016 cm−2) of Mn, Fe, or Ni and subsequent annealing at 700–1000 °C. The samples showed ferromagnetic contributions below temperatures ranging from 190–250 K for Mn to 45–185 K for Ni and 80–250 K for Fe. The use of superlattices to enhance the hole concentration did not produce any change in ferromagnetic ordering temperature. No secondary phase formation was observed by x-ray diffraction, transmission electron microscopy, or selected area diffraction pattern analysis for the doses we employed.

Electrical characterization of gan metal oxide semiconductor diode using Sc2O3 as the gate oxide

Abstract: GaN metal oxide semiconductor diodes were demonstrated utilizing Sc 2 O 3 as the gate oxide. Sc 2 O 3 was grown at 100°C on MOCVD grown n-GaN layers in a molecular beam epitaxy system, using a scandium elemental source and an electron cyclotron resonance oxygen plasma. Ar/Cl 2 based discharges were used to remove Sc 2 O 3 , to expose the underlying n-GaN for ohmic metal deposition in an inductively coupled plasma system. Electron beam deposited Ti/Al/Pt/Au and Pt/Au were utilized as ohmic and gate metallizations, respectively. An interface trap density of 5 X 10 1 1 eV - 1 cm - 2 was obtained with the Terman method. Conductance-voltage measurements were also used to estimate the interface trap density and a slightly higher value was obtained as compared to the Terman method. Results of capacitance measurements at elevated temperature (up to 300°C) indicated the presence of deep states near the interface.

High current bulk GaN Schottky rectifiers

Abstract: GaN Schottky rectifiers employing guard-ring and SiO2 edge termination show almost ideal forward current characteristics, with ideality factor 1.08 and specific on-state resistance as low as 2.6×10−3 Ω cm2. A maximum forward current of 1.72 A at 6.28 V was achieved under pulsed (10% duty cycle) conditions. The reverse breakdown voltage was inversely dependent on rectifier area. The presence of defects in the GaN still dominates the reverse leakage, with both field emission and thermionic field emission contributions present. The parallel-plane breakdown voltage is never reached, even with the use of multiple edge termination methods, but the results show the promise of GaN rectifiers for power conditioning and electric utility applications.

Comparison of AlGaN/GaN high electron mobility transistors grown on AlN/SiC templates or sapphire

Abstract: The temperature and gate length effects on dc performance of AlGaN/GaN high-electron mobility transistors (HEMTs) grown on AlN/SiC templates or sapphire substrates are reported. The defect density in the structures grown on the AlN/SiC template is significantly lower than those grown on sapphire, as measured by transmission electron microscopy. Reverse breakdown voltages above 40 V were obtained for 0.25 μm gate length devices on both types of substrate. Extrinsic transconductances of ∼200 mS/mm for HEMTs on sapphire and ∼125 mS/mm for devices on AlN/SiC were achieved, with the latter devices showing significantly lower self-heating effects. Both types of HEMTs showed similar trends of drain current and transconductance with increasing temperature. There was a clear signature of optical phonon scattering as the dominant scattering mechanism from room temperature to 300 °C.

High breakdown M–I–M structures on bulk AlN

Abstract: The breakdown characteristics of metal-AlN-metal structures are reported as a function of contact diameter. The bulk AlN was grown by a HVPE method, resulting in a resistivity of 4×10 8 Ω cm. Front-side contact diameters of 175–600 μm were fabricated, displaying breakdown voltages up to ∼6300 V at 25 °C. Breakdown appeared to initiate at internal surfaces related to grain boundaries or cracks in the material. The results indicate the great promise of the Al(Ga)N system for high power rectifiers.

Edge termination design and simulation for bulk GaN rectifiers

Abstract: GaN bulk rectifiers show excellent on-state resistances (in the mΩ cm−2 range), forward turn-on voltages of ∼1.8 V and reverse-recovery times of <50 ns. A key requirement is to develop effective edge termination techniques in order to prevent premature surface-induced breakdown. We have performed a simulation study of the effects of varying the dielectric passivation material (SiO2, SiNx, AlN, Sc2O3, or MgO), the thickness of this material, the extent of metal overlap onto the dielectric and the ramp oxide angle on the resulting reverse breakdown voltage (VB) of bulk rectifiers. We find that SiO2 produces the highest VB of the materials investigated, that there is an optimum metal overlap distance for a given oxide thickness and small oxide ramp angles produce the highest VB.

1.6 A GaN Schottky rectifiers on bulk GaN substrates

Abstract: Large area bulk GaN rectifiers with implanted p+ guard rings were fabricated using additional dielectric overlap passivation. The devices were packaged to avoid self-heating at large operating currents. A forward current of 1.65 A was achieved in pulsed voltage mode, a record for GaN rectifiers. The on-state resistance was 3.7 m Ω cm2.

Band line-up and mechanisms of current flow in n-GaN/p-SiC and n-AlGaN/p-SiC heterojunctions

Abstract: The properties of n-GaN/p-SiC and n-AlGaN/p-SiC heterojunctions (HJ) prepared by hydride vapor phase epitaxy (HVPE) on 4H SiC substrates are reported. It is shown that the GaN/p-SiC HJ is staggered type II with the conduction bandoffset and the valence bandoffset values, respectively, ΔEc=−0.49 eV and ΔEv=0.65 eV. When changing GaN for AlGaN with Al mole fraction of x=0.25–0.3 the band alignment becomes normal type I with ΔEc=0.2 eV and ΔEv=0.6 eV. Current–voltage characteristics of both heterojunctions bear evidence of strong tunneling via defect states. The tunneling was found to be more pronounced in the AlGaN/SiC HJs even though these showed no evidence of formation of dark line defects at the interface, in contrast to GaN/SiC.

Finite difference analysis of thermal characteristics of CW operation 850 nm lateral current injection and implant-apertured VCSEL with flip-chip bond design

Abstract: The finite difference method was used to analyze the thermal characteristics of continuous wave 850 nm AlGaAs/GaAs implant-apertured vertical-cavity surface-emitting lasers (VCSELs). A novel flip-chip design was used to enhance the heat dissipation. The temperature rise in the active can be maintained below 40 °C at 4 mW output power with 10 mA current bias. By contrast, the temperature rise reaches above 60 °C without flip-chip bonding. The transient temperature during turn-on of a VCSEL was also investigated. The time needed for the device to reach the steady-state temperature was in the range of a few tenths of a millisecond, which is orders of magnitude larger than the electrical or optical switch time. Flip-chip bonding will reduce the shift of the wavelength, peak power, threshold current, and slope efficiency during VCSEL operation.

Magnetism in SiC implanted with high doses of Fe and Mn

Abstract: High concentrations (0.1-5 at.%) of Mn or Fe were introduced into the near-surface region (≤2000 A) of 6H-SiC substrates by direct implantation at ˜300°C. After annealing at temperatures up to 1000°C, the structural properties were examined by transmission electron microscopy (TEM) and selected-area diffraction pattern (SADP) analysis. The magnetic properties were examined by SQUID magnetometry. While the Mn-implanted samples were paramagnetic over the entire dose range investigated, the Fe-implanted material displayed a ferromagnetic contribution present at >175 K for the highest dose conditions. No secondary phases were detected, at least not to the sensitivity of TEM or SADP.

Optimization of gas flow and etch depth uniformity for plasma etching of large area GaAs wafers

Abstract: We demonstrated advanced techniques for control of gas flow in a reactor in order to achieve good etch depth uniformity for large area GaAs etching. It was found that a finite difference numerical method was quite useful for simulation of gas flow distribution in the reactor for dry etching of GaAs. The experimental results in BCl3/N2/SF6/He ICP plasmas confirmed that the simulated data fitted very well with real data. It is noted that a focus ring could help improve both gas flow and etch uniformity for large area GaAs plasma etch processing. The simulation results showed that optimization of clamp configuration could decrease gas flow uniformity as low as ±1.5% on a 100 mm (4 in.) GaAs wafer and ±3% for a 150 mm (6 in.) wafer with the fixed reactor and electrode, respectively. Comparison between simulated gas flow uniformity and real etch depth distribution data confirmed that control of gas flow distribution in the chamber would be significantly important in order to achieve excellent dry etch uniformity of large area GaAs wafers.