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Dive into the research topics where J. C. Zolper is active.

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Featured researches published by J. C. Zolper.


Applied Physics Letters | 1995

Ion implantation doping and isolation of GaN

S. J. Pearton; C. B. Vartuli; J. C. Zolper; C. Yuan; R. A. Stall

N‐ and p‐type regions have been produced in GaN using Si+ and Mg+/P+ implantation, respectively, and subsequent annealing at ∼1100 °C. Carrier activation percentages of 93% for Si and 62% for Mg were obtained for implant doses of 5×1014 cm−2 of each element. Conversely, highly resistive regions (≳5×109 Ω/⧠) can be produced in initially n‐ or p‐ type GaN by N+ implantation and subsequent annealing at ∼750 °C. The activation energy of the deep states controlling the resistivity of these implant‐isolated materials is in the range 0.8–0.9 eV. These process modules are applicable to the fabrication of a variety of different GaN‐based electronic and photonic devices.


Applied Physics Letters | 1996

Ca and O ion implantation doping of GaN

J. C. Zolper; R. G. Wilson; S. J. Pearton; R. A. Stall

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.


Thin Solid Films | 1997

A survey of ohmic contacts to III-V compound semiconductors

Albert G. Baca; J. C. Zolper; R.D. Briggs; F. Ren; S. J. Pearton

A survey of ohmic contact materials and properties to GaAs, InP, GaN will be presented along with critical issues pertaining to each semiconductor material. Au-based alloys (e.g., GeAuNi for n-type GaAs) are the most commonly used contacts for GaAs and InP materials for both n- and p-type contacts due to the excellent contact resistivity, reliability, and usefulness over a wide range of doping levels. Research into new contacting schemes for these materials has focused on addressing limitations of the conventional Au-alloys in thermal stability, propensity for spiking, poor edge definition, and new approaches for a non-alloyed contact. The alternative contacts to GaAs and InP include alloys with higher temperature stability, contacts based on solid phase regrowth, and contacts that react with the substrate to form lower bandgap semiconductors alloys at the interface. A new area of contact studies is for the wide bandgap group III-Nitride materials. At present, low resistivity ohmic contact to p-type GaN has not been obtained primarily due to the large acceptor ionization energy and the resultant difficulty in achieving high free hole concentrations at room temperature. For n-type GaN, however, significant progress has been reported with reactive Ti-based metalization schemes or the use of graded InGaN layers. The present status of these approaches will be reviewed.


Applied Physics Letters | 1996

Ion‐implanted GaN junction field effect transistor

J. C. Zolper; R. J. Shul; Albert G. Baca; R. G. Wilson; S. J. Pearton; R. A. Stall

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.


Applied Physics Letters | 1997

Electrical and structural analysis of high-dose Si implantation in GaN

J. C. Zolper; H.H. Tan; James Williams; Jin Zou; D. J. H. Cockayne; S. J. Pearton; Mary H. Crawford; R. F. Karlicek

For the development of ion implantation processes for GaN to advanced devices, it is important to understand the dose dependence of impurity activation along with implantation-induced damage generation and removal. We find that Si implantation in GaN can achieve 50% activation at a dose of 1×1016 cm-2, despite significant residual damage after the 1100 °C activation anneal. The possibility that the generated free carriers are due to implantation damage alone and not Si-donor activation is ruled out by comparing the Si results to those for implantation of the neutral species Ar. Ion channeling and cross-sectional transmission electron microscopy are used to characterize the implantation-induced damage both as implanted and after a 1100 °C anneal. Both techniques confirm that significant damage remains after the anneal, which suggests that activation of implanted Si donors in GaN doses not require complete damage removal. However, an improved annealing process may be needed to further optimize the transport properties of implanted regions in GaN.


Applied Physics Letters | 1996

Nonalloyed Ti/Al Ohmic contacts to n‐type GaN using high‐temperature premetallization anneal

Luke F. Lester; J. M. Brown; J. Ramer; L. Zhang; S. D. Hersee; J. C. Zolper

On Si‐implanted n‐type GaN, a nonalloyed Ti/Al metallization has been found to form an Ohmic contact that has a specific contact resistance as low as 1.0×10−5 Ω cm2. The Ohmic character is believed to be caused by the 1120 °C implant activation anneal which generates nitrogen vacancies that leave the surface heavily n type. This theory is indirectly confirmed on unimplanted n‐type GaN by comparing the rc of nonalloyed Ti/Al on unannealed GaN with that of nonalloyed Ti/Al on 1120 °C annealed GaN. The former has rectifying electrical characteristics, while the latter forms an Ohmic contact with an rc=1.3×10−3 Ω cm2.


Applied Physics Letters | 1996

Sputtered AlN encapsulant for high‐temperature annealing of GaN

J. C. Zolper; D. J. Rieger; Albert G. Baca; S. J. Pearton; J. W. Lee; R. A. Stall

Reactively sputtered AlN is shown by electrical characterization of Pt/Au Schottky diodes to be an effect encapsulant for GaN annealed at 1100 °C. Schottky diodes formed on GaN encapsulated with AlN during the anneal had low reverse leakage currents with breakdown voltages in excess of 40 V. In contrast, samples annealed without the AlN layer had 3–4 orders‐of‐magnitude higher reverse leakage currents. Atomic force microscopy images of as‐grown and annealed samples also demonstrate an increase in surface roughness and a change in morphology of the uncapped samples following annealing. Auger electron spectroscopy supports the hypothesis that the AlN encapsulant is reducing N loss from the GaN substrate. N loss in the uncapped samples is expected to create an n+‐region at the surface that accounts for the high reverse leakage current and improved Ohmic behavior for the uncapped samples. The use of AlN encapsulation will enable the realization of all ion implanted GaN metal semiconductor field effect transis...


Applied Physics Letters | 1996

Morphology and photoluminescence improvements from high-temperature rapid thermal annealing of GaN

J. C. Zolper; M. Hagerott Crawford; A. J. Howard; J. Ramer; S. D. Hersee

Rapid thermal annealing of GaN in an Ar or N2 ambient up to 1100 °C is shown to improve surface morphology and photoluminescence intensity. For both ambients the average rms surface roughness as determined by atomic force microscopy decreases from ∼4 nm on the as‐grown material to ∼1 nm after a 1100 °C anneal. The band‐edge luminescence intensity was increased by a factor of 4 after a 1100 °C anneal in a N2 ambient and a factor of 2 for annealing at 1100 °C in an Ar ambient as compared to as‐grown material. The 1100 °C anneal improves the ratio of band edge to deep‐level luminescence and also reduces the electron concentration and mobility. The reduction in mobility can be explained in terms of a two‐band conduction mechanism where defect band conduction dominates at the lower carrier densities or an increase in the free‐carrier compensation ratio.


IEEE Photonics Technology Letters | 1994

Vertical cavity surface emitting lasers with 21% efficiency by metalorganic vapor phase epitaxy

Kevin L. Lear; R.P. Schneider; Kent D. Choquette; S.P. Kilcoyne; Jeffrey J. Figiel; J. C. Zolper

Proton implanted, vertical cavity top-surface emitting lasers exhibit the highest single-mode and multi-mode output powers, highest power conversion efficiency, and lowest threshold voltage for such devices reported to date. These lasers use new mirror grading designs that are enabled by metalorganic vapor phase epitaxys capabilities of alloy grading and carbon doping. The results validate this growth technology by exceeding the previous best results which were based on molecular beam epitaxy.<<ETX>>


Applied Physics Letters | 1998

Annealing of ion implanted gallium nitride

H.H. Tan; James Williams; Jin Zou; D. J. H. Cockayne; S. J. Pearton; J. C. Zolper; R. A. Stall

In this paper, we examine Si and Te ion implant damage removal in GaN as a function of implantation dose, and implantation and annealing temperature. Transmission electron microscopy shows that amorphous layers, which can result from high-dose implantation, recrystallize between 800 and 1100 °C to very defective polycrystalline material. Lower-dose implants (down to 5×1013 cm−2), which are not amorphous but defective after implantation, also anneal poorly up to 1100 °C, leaving a coarse network of extended defects. Despite such disorder, a high fraction of Te is found to be substitutional in GaN both following implantation and after annealing. Furthermore, although elevated-temperature implants result in less disorder after implantation, this damage is also impossible to anneal out completely by 1100 °C. The implications of this study are that considerably higher annealing temperatures will be needed to remove damage for optimum electrical properties.

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R. J. Shul

Sandia National Laboratories

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Albert G. Baca

Sandia National Laboratories

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Kevin L. Lear

Colorado State University

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J. F. Klem

Sandia National Laboratories

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Julian Cheng

University of New Mexico

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R.P. Schneider

Sandia National Laboratories

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