Showing papers by "Stephen J. Pearton published in 2001"

Ion implantation into GaN

Abstract: The current status of ion beam processing of GaN is reviewed. In particular, we discuss the following aspects of ion implantation into GaN: (i) damage build-up and amorphization, (ii) preferential surface disordering and loss of nitrogen during ion bombardment, (iii) ion-beam-induced porosity of amorphous GaN due to material dissociation, (iv) anomalous surface erosion during ion bombardment at elevated temperatures, (v) the effect of implantation disorder on mechanical properties, (vi) current progress on annealing of implantation disorder, (vii) electrical and optical doping, and (viii) electrical isolation. Emphasis is given to current problems which may hinder a successful application of ion implantation in the fabrication of GaN-based devices.

Magnetic and structural properties of Mn-implanted GaN

Abstract: 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.

Indication of ferromagnetism in molecular-beam-epitaxy-derived N-type GaMnN

Abstract: Growth by molecular-beam epitaxy of the dilute magnetic alloy GaMnN is reported. The GaMnN contains 7.0% Mn as determined by Auger electron spectroscopy, and is single phase as determined by x-ray diffraction and reflection high-energy electron diffraction. Both magnetic and magnetotransport data are reported. The results show the anomalous Hall effect, negative magnetoresistance, and magnetic hysteresis at 10 K, indicating that Mn is incorporating into the GaN and forming the ferromagnetic semiconductor GaMnN. At 25 K the anomalous Hall term vanishes, indicating a Curie temperature between 10 and 25 K.

dc and rf performance of proton-irradiated AlGaN/GaN high electron mobility transistors

Abstract: AlGaN/GaN high electron mobility transistors (HEMTs) with a range of gate lengths (0.8–1.2 μm) and widths (100–200 μm) were exposed to 40 MeV protons at fluences of 5×109 or 5×1010 cm−2. The drain–source currents in the devices decreased by 15%–20% at the higher fluence, while the extrinsic transconductance decreased by ∼30% under the same conditions. Based on the increases in the reverse breakdown voltage and the channel resistance, the main degradation mechanism is believed to be creation of deep trap states in the band gap which remove electrons from the channel. The maximum frequency of oscillation, fMAX, also decreased as a result of the proton-induced damage, with a change of −20% at the shorter gate widths and −50% at the largest widths. The reverse recovery switching time was essentially unaffected by the irradiation, remaining at ∼1.6×10−8 s. Postradiation annealing at 800 °C was successful in restoring the dc and rf performance parameters to ⩾90% of their original values. The AlGaN/GaN HEMTs are much more robust than their AlGaAs/GaAs counterparts to displacement damage and appear well-suited to radiation environment applications.

Comparison of GaN p-i-n and Schottky rectifier performance

Abstract: The performance of GaN p-i-n and Schottky rectifiers fabricated on the same wafer was investigated as a function of device size and operating temperature. There was a significant difference in reverse breakdown voltage (490 V for p-i-n diodes; 347 V for the Schottky diodes) and forward turn-on voltage (/spl sim/5 V for the p-i-n diodes; /spl sim/3.5 V for the Schottky diodes). Both types of device showed a negative temperature coefficient for reverse breakdown, with value -0.34/spl plusmn/0.05 V/spl middot/K/sup -1/.

Gadolinium Oxide and Scandium Oxide: Gate Dielectrics for GaN MOSFETs

Abstract: Scandium oxide was deposited epitaxially on (0001) GaN via gas-source molecular beam epitaxy (GSMBE) using elemental Sc and an electron cyclotron resonance (ECR) oxygen plasma. HXRD indicated that the Sc 2 O 3 has a lower defect density than similarly prepared films of Gd 2 O 3 . The scandium oxide was atomically smooth, with a rms roughness of 0.5-0.8 nm, and was uniform in stoichiometry as measured by Auger electron spectroscopy (AES) depth profiling. An interface state density of mid 10 11 eV -1 cm -2 was determined from capacitance-voltage profiling using the Terman method. This interface state density was roughly a factor of five lower than that obtained from similar diodes made from SiO 2 on GaN.

GaN electronics for high power, high temperature applications

Abstract: A brief review is given of recent progress in fabrication of high voltage GaN and AlGaN rectifiers, GaN/AlGaN heterojunction bipolar transistors and GaN metal-oxide semiconductor field effect transistors. Improvements in epitaxial layer quality and in fabrication techniques have led to significant advances in device performance.

Vertical and lateral GaN rectifiers on free-standing GaN substrates

Abstract: Edge-terminated Schottky rectifiers fabricated on quasibulk GaN substrates showed a strong dependence of reverse breakdown voltage VB on contact dimension and on rectifier geometry (lateral versus vertical). For small diameter (75 μm) Schottky contacts, VB measured in the vertical geometry was ∼700 V, with an on-state resistance (RON) of 3 mΩ cm2, producing a figure-of-merit VB2/RON of 162.8 MW cm−2. Measured in the lateral geometry, these same rectifiers had VB of ∼250 V, RON of 1.7 mΩ cm2 and figure-of-merit 36.5 MW cm−2. The forward turn-on voltage (VF) was ∼1.8 V (defined at a current density of 100 A cm−2), producing VB/VF ratios of 139–389. In very large diameter (∼5 mm) rectifiers, VB dropped to ∼6 V, but forward currents up to 500 mA were obtained in dc measurements.

Characterization of high dose Fe implantation into p-GaN

Abstract: High concentrations (3–5 at. %) of Fe were incorporated into p-GaN by direct implantation at elevated substrate temperature (350 °C). Subsequent annealing at 700 °C produced apparent ferromagnetic behavior up to ∼250 K for the 3 at. % sample. Selected area diffraction patterns did not reveal the presence of any other phases in the Fe-implanted region. The direct implantation process appears promising for examining the properties of magnetic semiconductors with application to magnetotransport and magneto-optical devices.

High density plasma via hole etching in SiC

Abstract: Throughwafer vias up to 100 μm deep were formed in 4H-SiC substrates by inductively coupled plasma etching with SF6/O2 at a controlled rate of ∼0.6 μm min−1 and use of Al masks. Selectivities of >50 for SiC over Al were achieved. Electrical (capacitance–voltage: current–voltage) and chemical (Auger electron spectroscopy) analysis techniques showed that the etching produced only minor changes in reverse breakdown voltage, Schottky barrier height, and near surface stoichiometry of the SiC and had high selectivity over common frontside metallization. The SiC etch rate was a strong function of the incident ion energy during plasma exposure. This process is attractive for power SiC transistors intended for high current, high temperature applications and also for SiC micromachining.

Magnetic Properties of Fe- and Mn-Implanted SiC

Abstract: under conditions that avoided amor-phization ~substrate temperature ;350°C!. After annealing at 700°C, the magnetic properties of the samples were examined bysuperconducting quantum interference device magnetometry. Both the Fe- and Mn-implanted samples showed ferromagneticproperties up to approximately 250 K for the highest doses employed. The origin of the ferromagnetism is not the formation ofsecondary phases involving precipitation of Fe or Mn.© 2001 The Electrochemical Society. @DOI: 10.1149/1.1414945# All rights reserved.Manuscript submitted June 14, 2001; revised manuscript received August 24, 2001. Available electronically October 15, 2001.

Self-aligned process for emitter- and base-regrowth GaN HBTs and BJTs

Abstract: The development of a self-aligned fabrication process for small emitter contact area ( 2×4 μ m2) GaN/AlGaN heterojunction bipolar transistors and GaN bipolar junction transistors is described. The process features dielectric-spacer sidewalls, low damage dry etching and selected-area regrowth of p-GaAs(C) on the base contact or n-GaN/AlGaN on the emitter contact. Series resistance effects are still found to influence the device performance.

Magnetic properties of P-type GaMnP grown by molecular-beam epitaxy

Abstract: Growth by molecular-beam epitaxy of the dilute magnetic alloy GaMnP:C is reported. The GaMnP:C contains 9.4% Mn as determined by Auger electron spectroscopy, and is single phase as determined by x-ray diffraction, reflection high-energy electron diffraction, and transmission electron microscopy. Both magnetization and magnetotransport data are reported. The results show the anomalous Hall effect, negative magnetoresistance, and magnetic hysteresis at 10 K, indicating that Mn is incorporating into the GaP:C and forming the ferromagnetic semiconductor GaMnP:C. Temperature-dependent magnetization and anomalous Hall data show that magnetic behavior persists to at least 200 K, which is a very high value for a III–V based dilute magnetic semiconductor.

Schottky rectifiers fabricated on free-standing GaN substrates

Abstract: GaN Schottky rectifiers have been fabricated on free-standing substrates and on epi/substrate structures. Forward turn-on voltages were as low as 3 V at 25°C. Reverse recovery was complete in Ω cm −2 for diodes with VB=450 V, producing a figure-of-merit (VB)2/RON of ∼10 MW cm−2.

Fermi level dependence of hydrogen diffusivity in GaN

Abstract: Hydrogen diffusion studies were performed in GaN samples with different Fermi level positions. It is shown that, at 350 °C, hydrogen diffusion is quite fast in heavily Mg doped p-type material with the Fermi level close to Ev+0.15 eV, considerably slower in high-resistivity p-GaN(Zn) with the Fermi level Ev+0.9 eV, while for conducting and semi-insulating n-GaN samples with the Fermi level in the upper half of the band gap no measurable hydrogen diffusion could be detected. For these latter samples it is shown that higher diffusion temperature of 500 °C and longer times (50 h) are necessary to incorporate hydrogen to appreciable depth. These findings are in line with previously published theoretical predictions of the dependence of hydrogen interstitials formation in GaN on the Fermi level position.

Effect of gate length on DC performance of AlGaN/GaN HEMTs grown by MBE

Abstract: The DC performance of AlGaN/GaN high electron mobility transistors grown by plasma-assisted molecular beam epitaxy was investigated for gate lengths in the range 0.1–1.2 μm. On 0.25 μm gate length devices we obtained 40 V DS operation with >50 mA peak I D . The peak drain current density was 0.44 A/mm for 100 μm gate width devices with 1.2 μm gate lengths. The extrinsic transconductance ( g m ) decreased with both gate length and gate width and was ⩾75 mS/mm for all gate widths for 0.25 μm devices. E-beam written gates typically produced a slightly lower Schottky barrier height than optically patterned gates.

Inductively coupled high-density plasma-induced etch damage of GaN MESFETs

Abstract: The fabrication of a wide variety of GaN-based photonic and electronic devices depends on dry etching, which typically requires ion-assisted removal of the substrate material. Under conditions of both high plasma flux and energetic ion bombardment, GaN etch rates greater than 0.5 μm min −1 and anisotropic etch profiles are readily achieved in inductively coupled plasma (ICP) etch systems. Unfortunately, under these conditions, plasma-induced damage often occurs. Attempts to minimize such damage by reducing the ion energy or increasing the chemical activity in the plasma often result in a loss of etch rate or profile control which can limit dimensional control and reduce the utility of the process for device applications requiring anisotropic etch profiles. It is therefore necessary to develop plasma etch processes which couple anisotropy for critical dimension and sidewall profile control and high etch rates with low-damage for optimum device performance. In this study, we report changes in source resistance, reverse breakdown voltage, transconductance, and drain saturation current for GaN MESFET structures exposed to an Ar ICP plasma. In general, plasma-induced damage was more sensitive to ion bombardment energies as compared to plasma flux.

Comparison of F 2 plasma chemistries for deep etching of SiC

Abstract: A number of F2-based plasma chemistries (NF3, SF6, PF5, and BF3) were investigated for high rate etching of SiC. The most advantageous of these is SF6, based on the high rate (0.6 μm·min−1) it achieves and its relatively low cost compared to NF3. The changes in electrical properties of the near-surface region are relatively minor when the incident ion energy is kept below approximately 75 eV. At a process pressure of 5 mtorr, the SiC etch rate falls-off by ∼15% in 30 μm diameter via holes compared to larger diameter holes (>60 μm diameter) or open areas on the mask. We also measured the effect of exposed SiC area on the etch rate of the material.

Electronic states in modulation doped p-AlGaN/GaN superlattices

Abstract: The properties of p-AlGaN/GaN modulation doped superlattices (SLs) prepared by molecular beam epitaxy were studied by means of conductivity versus temperature, admittance spectroscopy, photoinduced current spectroscopy, microcathodoluminescence (MCL) spectra measurements, and measurements of effective diffusion lengths. It is shown that in SLs grown on GaN underlayers the sheet resistivity is about two orders of magnitude lower than for reference p-GaN films and the resistivity of SLs remains lower up to temperatures of about 350 °C. For SLs grown on AlGaN underlayers the gain in resistivity is much more moderate and certain advantages in using such SLs are envisaged only for temperatures below room temperature. The reason for this lower gain is a considerable decrease in hole mobility compared to p-GaN. The effect is somewhat tentatively attributed to worse crystalline perfection of these SLs. It is also shown that such SLs are characterized by a strongly broadened MCL peak and the presence of additional...

Nanoscale magnetic regions formed in GaN implanted with Mn.

Abstract: Platelet structures with diameters less than 250 A and hexagonal symmetry were formed in GaN by high dose Mn+ ion implantation and annealing at 700-1000 degrees C. Selected-area diffraction pattern analysis indicates that these regions are GaxMn1-xN with a different lattice constant to the host GaN. The presence of the GaMnN corresponds to ferromagnetic behavior of the samples with a Curie temperature of approximately 250 K.

Electrical and optical properties of modulation-doped p-AlGaN/GaN superlattices

Abstract: Electrical and optical properties of modulation-doped p-AlGaN/GaN superlattices are compared to those of similarly doped p-GaN films It is shown that modulation doping increases the sheet hole concentration by several times In p-AlGaN/GaN superlattices grown on GaN underlayers, this increase is accompanied by a significant increase in hole mobility which results in a remarkable decrease in sheet resistivity of the structure compared to p-GaN films and this decrease in sheet resistivity should hold up to temperatures exceeding 350 °C For superlattices prepared on AlGaN underlayers, the mobility decreases compared to p-GaN In such superlattices, one also observes a strong redshift and a strong broadening of the band edge luminescence peak coming most probably from increased mosaicity and strain which would also explain the observed deterioration of mobility The magnitude of the redshift in the position of the band edge luminescence band slightly increased upon application of reverse bias which is inter

Magnetic Properties of Mn and Fe-Implanted p-GaN

Abstract: The structural and magnetic properties of p-GaN implanted with high doses of Mn + or Fe + (0.1-5 at%) and subsequently annealed at 700-1000 °C were examined by transmission electron microscopy, selected-area diffraction patterns, X-ray diffraction and SQUID magnetometry. The implanted samples showed paramagnetic behavior on a large diamagnetic background signal for implantation doses below 3 at% Mn or Fe. At higher doses the samples showed signatures of ferromagnetism with Curie temperatures <250 K for Mn and <150 K for Fe implantation. The structural analysis of the Mn-implanted GaN showed regions consistent with the formation of Ga x Mn 1-x N platelets occupying ∼5% of the implanted volume. An estimate of ∼(5.5 ± 1.9) μ B per Mn was obtained, consistent with the expected value (5.0) for a half-filled shell. The formation of secondary phases such as Mn x Ga y or Mn x N y was excluded by careful diffraction analysis. The implantation process may have application in forming selected-area contact regions for spin-polarized carrier injection in device structures and in enabling a quick determination of the Curie temperatures in dilute magnetic semiconductor host materials.

High-speed modulation of 850-nm intracavity contacted shallow implant-apertured vertical-cavity surface-emitting lasers

Abstract: GaAs-AlGaAs quantum-well (850 nn) vertical-cavity surface-emitting lasers, with lateral current injection and shallow implanted apertures, show small signal modulation bandwidths of at least 11 GHz and large signal data rates of at least 10 Gb/s. The devices achieved a maximum output power of 2.1 mW, with a threshold current and voltage of 1 mA and 1.71 V, respectively. The shallow implantation step provides photolithographically precise aperture formation (using O/sup +/ ions), for efficient lateral current injection into the quantum-well active region of the laser, from intracavity contacts. The device aperture was 7 /spl mu/m in diameter, and the opening in the annular top contact was 13 /spl mu/m in diameter. The optical spectrum showed several transverse modes.

Dry etching mechanism of copper and magnetic materials with UV illumination

Abstract: To better understand the dry etching of copper and magnetic materials with UV illumination an etch mechanism is proposed based on the subprocesses occurring in an inductively coupled plasma (ICP) reactor. The photo-dissociation of Cl 2 provides chlorine-enriched environment near the surface, and UV photons enhance the surface reaction, leading to fast formation of metal chlorides on the surface with low activation energy. The overall etching process of metals with UV illumination is limited by absorption capacity of UV radiation by reaction products. The proposed etch mechanism also showed the potential gaseous etch products are CuCl, CuCl 2 , Cu 2 Cl, Cu 2 Cl 2 , Cu 2 Cl 3 , Cu 3 Cl 2 and Cu 3 Cl 3 , while the dominant gas species are CuCl 2 and Cu 2 Cl 3 with UV illumination. The ICP etching of magnetic materials in Cl 2 /Ar discharges with UV illumination showed no enhancement in etch rate for NiFe, but a substantial enhancement for NiFeCo mainly due to the lower binding energy of NiFeCo relative to NiFe.

Pnp InGaAsN-based HBT with graded base doping

Abstract: A grading of the base doping concentration is used to establish an electric field in the base of a Pnp AlGaAs/InGaAsN heterojunction bipolar transistor (HBT). An improved peak current gain value (β ≃ 60) is observed over a device with uniform base doping, due to better hole transport in the base. A cut-off frequency of 15 GHz measured from a 3 × 12 µm2 non-self aligned HBT demonstrates the practical impact the electric field has on the transit time across the base.

Co-implantation of Be+O and Mg+O into GaN

Abstract: Beryllium and magnesium were co-implanted with oxygen into GaN at precise donor-to-acceptor ratios of 0.5–2. High activation efficiency was achieved for Be+O co-implants at a donor–acceptor ratio of 2 and anneal temperatures of 1050–1100 °C. The activation process was not a simple, single-step mechanism. Hall data indicated a relatively low (⩽100 meV) ionization energy for Be in the co-implanted samples. This process may have application in improving p-ohmic contact resistance by selective area Be+O implantation. By sharp contrast Mg+O co-implants produced n-type conductivity under all conditions.

Device characteristics of the GaAs/InGaAsN/GaAs p-n-p double heterojunction bipolar transistor

Abstract: We have demonstrated the dc and rf characteristics of a novel p-n-p GaAs/InGaAsN/GaAs double heterojunction bipolar transistor. This device has near ideal current-voltage (I-V) characteristics with a current gain greater than 45. The smaller bandgap energy of the InGaAsN base has led to a device turn-on voltage that is 0.27 V lower than in a comparable p-n-p AlGaAs/GaAs heterojunction bipolar transistor. This device has shown f/sub T/ and f/sub MAX/ values of 12 GHz. In addition, the aluminum-free emitter structure eliminates issues typically associated with AlGaAs.

Simulation of npn and pnp AlGaN/GaN heterojunction bipolar transistors performances: limiting factors and optimum design

Abstract: The performance capabilities of npn and pnp AlGaN/GaN heterojunction bipolar transistors have been investigated by using a drift-diffusion transport model. Numerical results have been employed to study the effect of the p-type Mg doping and its incomplete ionization on device performance. The high base resistance induced by the deep acceptor level is found to be one of the causes of limited current gain values for npn devices. Reasonable improvements of the dc current gain /spl beta/ are observed by realistically reducing the base thickness and consequently the transit time, in accordance with processing limitations. Base transport enhancement is predicted by the introduction of a quasi-electric field in the base. The impact of the base resistivity on high-frequency characteristics is investigated for npn AlGaN/GaN devices. Simulation results reveal the difficulty to achieve decent current gain values at high current density for pnp HBTs in common emitter configuration. Despite the high electron mobility in the n-type base that aids in reducing the base resistance, a preliminary analysis for pnp devices indicates limited rf performances caused by the reduced minority hole transport across the base.

Level set approach to simulation of feature profile evolution in a high-density plasma-etching system

Abstract: The simulation of feature profile evolution in high-density plasma-etching processes has been carried out using a level-set technique. The main feature of this work is the inclusion of sheath dynamics, angular distribution of ions and reemission of neutrals in the trench, etch kinetics, and a level set equation for tracking a moving front of the feature profile. Sheath dynamics showed that the damped potential was somewhat shifted to the right and smaller than the applied potential. Etch profile simulations were performed for etching of silicon in inductively coupled plasmas of Cl2 and CF4 under various conditions. In dry etching of Si with CF4 discharges, polymer deposition was dominant at pCFx>10 mTorr, while surface fluorination (or ion-enhanced etching) was a main mechanism at pCFx<10 mTorr. The predicted etch profiles showed a slight bowing on the sidewalls and substantial tapering near the bottom, depending on the plasma parameters.