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ABSTRACT: The effects of recess etching of the Ohmic contact area on the performance of Ti/Al/Mo/Au metallization and its interfacial reactions with AlGaN/AlN/GaN epilayers were investigated. The best Ohmic contact performances of 0.31, 0.41, and 0.26 Ω mm for three epilayers from two different sources were obtained only when the two-dimensional electron gas (2DEG) channels were completely removed under the Ohmic contact metallization. This is due to the direct sideway contact made by the electrode to the 2DEG around the edges of the active-layer mesas or pads; this is believed to be a more efficient carrier transport mechanism than tunneling through the AlGaN barrier.
Applied Physics Letters 11/2009; · 3.84 Impact Factor
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ABSTRACT: The impact of C F <sub>4</sub> plasma treatment on the transport properties of the two dimensional electron gas (2DEG) in Al Ga N / Ga N heterostrustures has been studied. Systematic Hall measurements of the plasma-treated samples show a large degradation in mobility and sheet concentration, which can be partially recovered with short-duration rapid thermal annealing. Further annealing progressively degrades both mobility and sheet concentration. Secondary ion mass spectrometry of the heterostructure reveals accumulation of fluorine at the Al Ga N / Ga N interface close to the 2DEG channel as a result of annealing. Following our systematic electrical and analytical studies of the behavior of fluorine incorporated into the heterostructure epilayer due to bombardment, a vacancy-mediated postannealing redistribution of fluorine has been proposed.
Journal of Applied Physics 03/2009; · 2.17 Impact Factor
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ABSTRACT: Electrical and microstructural characterizations of the Ti/Al/Mo/Au Ohmic contacts to ultrathin AlN/GaN heterostructures were carried out. It was found that as-deposited contacts had linear I-V behavior due to high tunneling current across the thin AlN barrier. A contact resistance of 0.455 Ω mm was obtained for samples annealed at 800 °C without any premetallization plasma treatment. Transmission electron microscopy studies showed that despite the use of Ti, the AlN layer remained intact. Mushroom-shaped TiN protrusions were formed only along threading dislocations, which terminated in the AlN layer. The TiN protrusions acted as metal plugs/spikes thereby aiding carrier transport.
Applied Physics Letters 07/2008; 93(3):032109-032109-3. · 3.84 Impact Factor
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ABSTRACT: Investigations of Ti / Al / Mo / Au Ohmic contact formation, premetallization plasma treatment effects, and interfacial reactions for n<sup>+</sup>- Ga N capped Al Ga N / Al N / Ga N heterostructures are presented. Ti thickness played an important role in determining contact performance. Transmission electron microscopy studies confirmed that thick Ti layer was necessary to fully consume the GaN cap and the top of AlGaN to enable a higher tunneling current flow. A direct correlation of plasma treatment conditions with I-V linearity, current level, and contact performance was established. The plasma-affected region is believed to extend over 20 nm into the AlGaN and GaN.
Applied Physics Letters 08/2007; · 3.84 Impact Factor
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ABSTRACT: Schottky barrier heights (ϕB) and ideality factors (n) of Ti/Pt/Au diodes on n-InAlAs were characterized. Transmission electron microscopy (TEM) investigations were utilized to correlate the electrical performance with interfacial reactions. The enhancement of ϕB and increase in n were obtained with increasing annealing temperatures. TEM studies confirmed that amorphous layers were formed at the Ti/InAlAs interface at short annealing times, while prolonged annealing resulted in the crystallization of TiAs, defective layer formation, and Kirkendall void formation. Such aggressive reactions after prolonged annealing extended deep into the InAlAs and may affect the active region of InAlAs/InGaAs-based transistors. The activation energy for this reaction was calculated to be 1.5±0.1 eV.
Applied Physics Letters 07/2007; 91(2):022110-022110-3. · 3.84 Impact Factor
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ABSTRACT: The characteristics of Ir on InAlAs and on InAlAs/InGaAs/InP high electron mobility transistor (HEMT) heterostructures were characterized. A maximum Schottky barrier height (φB) of 825 meV was achieved for Ir/InAlAs after annealing at 400 °C. Transmission electron microscopy investigations confirmed that an amorphous layer (a layer) exists at the Ir/InAlAs interface at that temperature. Results indicate that enhancement of φB is associated with the a layer, while beyond 400 °C, the decrease of φB is due to the crystallization of the a layer and the formation of IrAs2. The enhancement of φB for Ir/InAlAs and the slow diffusion of Ir in IrAlAs make it a superior thermally stable gate metal for InAlAs/InGaAs HEMTs.
Applied Physics Letters 11/2006; 89(21):211910-211910-3. · 3.84 Impact Factor
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ABSTRACT: We report on the electrical and microstructural characterization of Si-containing Ti/Al/Mo/Au contacts for AlGaN/GaN heterostructures. Excellent Ohmic contact formation with contact resistance and specific contact resistivity as low as 0.12 Ω mm and 3.8×10−7 Ω cm2, respectively, have been obtained by the optimization of Si distribution in the metallization. The presence of Si strongly affects the Ohmic performance and microstructural makeup of the annealed contacts. Greater enhancement in Ohmic performance is achieved when optimized amount of Si is dispersed throughout the metallization.
Applied Physics Letters 05/2006; 88(21):212107-212107-3. · 3.84 Impact Factor
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ABSTRACT: A comparative study of Ta / Al / Mo / Au and Ti / Al / Mo / Au metallizations for AlGaN / GaN high electron mobility transistors is presented. By the optimization of surface treatment schemes and annealing temperature, contact resistances of 0.172 and 0.228 Ω mm , and specific contact resistivities of 2.96×10<sup>-7</sup> and 1.09×10<sup>-6</sup> Ω cm <sup>2</sup> were obtained for Ti / Al / Mo / Au and Ta / Al / Mo / Au , respectively. Auger electron spectroscopy (AES), x-ray diffraction (XRD), and transmission electron microscopy (TEM) were utilized to study microstructural changes occurring in the metallization layers as a result of heat treatment. Results indicate dynamic systems of severe intermixing between atoms from the metallization layers and epilayers resulting in changes in surface morphology, as determined by atomic force microscopy (AFM). The formation of intermetallics and interfacial compounds was observed. The combined effect of surface treatment and annealing-induced reaction is proposed as the mechanisms for low-resistance Ohmic contact formation.
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 12/2005; · 1.34 Impact Factor
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ABSTRACT: The effects of Si Cl <sub>4</sub> reactive-ion-etching (RIE) plasma treatment on n- Ga N and n- Al <sub>0.20</sub> Ga <sub>0.80</sub> N surfaces, and the subsequent cleaning of the surfaces using ammonium hydroxide ( N H <sub>4</sub> O H ) , hydrochloric acid (HCl), and buffered oxide etch (BOE) solutions, have been investigated using x-ray photoelectron spectroscopy and Auger electron spectroscopy measurements. Of these cleaning schemes, BOE was found to be the most effective treatment to remove oxides from the surfaces of the Si Cl <sub>4</sub> plasma treated samples. The Si Cl <sub>4</sub> plasma treatment of GaN and AlGaN resulted in the blueshift of Ga–N ( Ga 3d) peaks to higher binding energies corresponding to a shift of the Fermi level (E<sub> F </sub>) toward the conduction band edge at the surface. It has been reported that this type of shift is caused by the creation of N vacancies, which act as n -type dopant [D. W. Jenkins and J. D. Dow, Phys. Rev. B. 39, 3317 (1989); M. E. Lin, Z. F. Fan, Z. Ma, L. H. Allen, and H. Morkoç, Appl. Phys. Lett. 64, 887 (1994); A. T. Ping, Q. Chen, J. W. Yang, M. A. Khan, and I. Adesida, J. Electron. Mater. 27, 261 (1998)] on the surface due to Si Cl <sub>4</sub> plasma treatment. This corresponds to an increase in n -type dopant density on the surface. Thus, Si Cl <sub>4</sub> plasma treatment in a RIE system thins the Schottky barrier heights of n- Ga N and <-
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formula>n- Al Ga N and aids in the formation of ohmic contacts on such surfaces.
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 12/2005; · 1.34 Impact Factor
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ABSTRACT: The post-annealed interfacial microstructures of thin Ti/Al/Mo/Au metallization on n‐GaN were investigated using analytical transmission electron microscopy. A 5–20‐nm-thick continuous TiN layer was observed to have been formed due to the reaction between Ti and GaN. A continuous Au and Al segregation with a narrow thickness occurring exactly at the TiN/GaN interface was identified. Detailed structure and composition of the segregation layer were characterized. It is noted that despite the presence of Au at the interfacial region, an excellent Ohmic contact resistance was obtained. It suggests that the segregation of Au at the semiconductor∕metal interface does not necessarily have detrimental effects on contact performance.
Journal of Applied Physics 11/2005; 98(10):106105-106105-3. · 2.17 Impact Factor
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ABSTRACT: Transmission electron microscopy (TEM) is utilized to elucidate the postannealing interfacial microstructure of Ti/Al/Mo/Au metallization with AlGaN/GaN heterostructures to gain insight into the formation mechanism of low-resistance ohmic contacts. The reaction between the metal and the AlGaN layer does not proceed uniformly. Localized penetration through the AlGaN layer beyond the two-dimensional electron gas (2DEG) is observed while partial consumption of the AlGaN layer is noted in other areas. Analytical TEM analyses confirm that the main reaction product is TiN. A correlation between the appearance of TiN islands and threading dislocations is observed. Threading dislocations serve as short-circuit diffusion channels, and are responsible for the nonuniform reaction. TiN islands have a large total area of intimate contact with the 2DEG, and since no tunneling of electron through the AlGaN is required, a low-resistance ohmic contact is obtained.
Applied Physics Letters 09/2005; 87(14):141915-141915-3. · 3.84 Impact Factor
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ABSTRACT: A self-aligned process used to fabricate p -type SiGe metal–oxide–semiconductor modulation-doped field effect transistors (MOS-MODFET) is described. Self- and nonself-aligned p -type Si <sub> 0.2 </sub> Ge <sub> 0.8 </sub>/ Si <sub> 0.7 </sub> Ge <sub> 0.3 </sub> MOS-MODFETs with gate-lengths from 1 μm down to 100 nm were fabricated. The dc and microwave characteristics of these devices are presented. In comparison with nonself-aligned devices, self-aligned devices exhibited higher extrinsic transconductances, lower threshold voltages, higher unity current gain cutoff frequencies f<sub>T</sub>, and maximum oscillation frequencies f<sub> MAX </sub>. Self-aligned MOS-MODFETs with a gate length of 100 nm exhibited an extrinsic transconductance of 320 mS/mm, an f<sub>T</sub> of 64 GHz, and an f<sub> MAX </sub> of 77 GHz. To our knowledge, these are the highest data ever reported for any MOS-type p -FETs with a SiGe channel. All these excellent performances were measured at very low drain and gate biases. © 2000 American Vacuum Society.
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 12/2000; · 1.34 Impact Factor
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ABSTRACT: High performance p-type modulation-doped field-ef-fect transistors (MODFET's) and metal-oxide semiconductor MODFET (MOS-MODFET) with 0.1 m gate-length have been fabricated on a high hole mobility SiGe/Si heterojunction grown by ultrahigh vacuum chemical vapor deposition. The MODFET devices exhibited an extrinsic transconductance () of 142 mS/mm, a unity current gain cut-off frequency () of 45 GHz and a maximum oscillation frequency (MAX) of 81 GHz. 5 nm-thick high quality jet-vapor-deposited (JVD) SiO 2 was utilized as gate dielectric for the MOS-MODFET's. The devices exhibited a lower gate leakage current (1 nA/ m at = 6 V) and a wider gate operating voltage swing in comparison to the MODFET's. However, due to the larger gate-to-channel distance and the existence of a parasitic surface channel, MOS-MODFET's demonstrated a smaller peak of 90 mS/mm, of 38 GHz, and MAX of 64 GHz. The threshold voltage shifted from 0.45 V for MODFET's to 1.33 V for MOS-MODFET's. A minimum noise figure (NF min) of 1.29 dB and an associated power gain () of 12.8 dB were measured at 2 GHz for MODFET's, while the MOS-MODFET's exhibited a NF min of 0.92 dB and a of 12 dB at 2 GHz. These dc, rf, and high frequency noise character-istics make SiGe/Si MODFET's and MOS-MODFET's excellent candidates for wireless communications.
IEEE Transactions on Electron Devices 01/2000; 47. · 2.32 Impact Factor
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ABSTRACT: A metal-semiconductor-metal photodetector (MSMPD) with a hybrid combination of transparent cadmium-tin-oxide and opaque Ti:Au electrodes is proposed and demonstrated. A significant decrease in dark current is obtained by independently engineering the Schottky barrier heights at the anode and cathode. In addition, the use of transparent electrodes enables more light to be coupled into the front-side illuminated MSMPDs thereby increasing the responsivity. For an applied bias of 5 V, these devices exhibited an extremely low dark current density of 21.2 fA/μm2 and a high responsivity of 0.56 A/W to perpendicularly incident 1.31 μm wavelength light. © 1997 American Institute of Physics.
Applied Physics Letters 06/1997; 70(22):3026-3028. · 3.84 Impact Factor
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ABSTRACT: An investigation of the effects of HBr reactive ion etching (RIE) processing for gate recessing in lattice‐matched InAlAs/InGaAs heterostructure field‐effect transistors (HFETs) has been conducted. The effect of varying the Schottky barrier layer thickness on device performance and the susceptibility of HFETs to RIE‐induced damage are presented for barrier layer thicknesses ranging from 10 to 25 nm. The effect of plasma self‐bias voltage during gate recess etching on overall device performance for a given layer structure is also examined for voltages ranging from -100 to -200 V. Device performance is assessed through direct current (dc) characterization of transconductance, threshold voltage, reverse gate leakage current, and gate‐drain breakdown voltage, and through microwave characterization of the devices. Devices with barrier layers less than 20 nm thick are found to suffer the most degradation due to RIE‐induced damage. For devices with sufficiently thick barrier layers, dc and microwave device parameters compare well with those of corresponding devices fabricated using a selective wet‐etch process.
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 12/1994; · 1.34 Impact Factor
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ABSTRACT: The depth distribution of defects induced by reactive ion etching in InAlAs/InGaAs heterostructures has been determined from the measurements of low‐temperature mobilities of samples with varying two‐dimensional electron gas (2DEG) depth. The sample with a 2DEG depth of 35 nm from the surface showed no change in mobilities or sheet carrier densities, whereas the one with a 2DEG depth of 12.5 nm, etched under the same conditions using HBr plasma at a self‐bias of -100 V, showed a large change in both mobilities and sheet carrier densities. The defect distribution, which was estimated using the reciprocal of mobilities, has been found to be exponential. A 1/e penetration depth of about 7.8 nm has been obtained for HBr plasma for self‐bias voltages between -100 and -200 V and has been found to be independent of etching time. The exponential distribution of defects suggests that either ion channeling or diffusion is the possible mechanism of defect production in regions deeper than the projected ion range.
Applied Physics Letters 06/1994; · 3.84 Impact Factor
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ABSTRACT: A highly selective reactive ion etching (RIE) process based on HBr plasma has been demonstrated for the removal of InGaAs on InAlAs over a wide range of plasma self‐bias voltages. Etch selectivities of 160 at -100 V and 50 at higher voltages up to -300 V have been obtained. It is shown with the aid of x‐ray photoelectron spectroscopy analysis that the surface residues on the etched structures can be removed by a simple treatment in dilute HCl. No incorporation of impurities from the plasma, such as hydrogen and bromine, was detected by secondary ion mass spectroscopy analysis in samples treated in RIE up to -150 V. No degradation in mobility and sheet carrier density of the two‐dimensional electron gas was observed in modulation‐doped InAlAs/InGaAs field‐effect transistor (FET) structures, at low self‐bias voltages. The dc and rf device parameters of FETs fabricated using RIE as the gate‐recess process compare favorably with those of corresponding devices fabricated using a selective wet‐etching process.
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 12/1993; · 1.34 Impact Factor
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ABSTRACT: A reactive ion etching process using HBr plasma has been developed for the selective removal of InGaAs on InAlAs. Etch rates of 110 and 0.67 Å/min are obtained for InGaAs and InAlAs, respectively, at a plasma self‐bias voltage of -100 V, a chamber pressure of 120 mTorr, and a HBr flow rate of 10 sccm. This translates to an etch selectivity of over 160 which is the highest that has been obtained for this material system. The etch stop mechanism on the InAlAs is deduced from x‐ray photoelectron spectroscopy to be due to involatile Al 2 O 3 that is formed by residual O 2 and/or H 2 O vapor in the chamber. It is shown that the surface of the etched InAlAs can be restored to the state prior to HBr etching by sequential rinsing in dilute HCl and H 2 O. This process should be useful for the fabrication of InAlAs/InGaAs heterostructure field‐effect transistors.
Applied Physics Letters 06/1993; · 3.84 Impact Factor
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ABSTRACT: The etching of GaAs and Al x Ga 1- x As have been characterized in (NH 4 ) 2 S x solution with various excess sulfur concentrations and for temperatures ranging from 20 to 60 °C. The etch rate varies with the concentration of excess sulfur and is highest at 60 °C using a 4% excess sulfur solution. The etch rate of Al x Ga 1- x As increases exponentially with increasing Al mole fraction. Activation energies of 19.8 and 15.9 kcal/mole are obtained for GaAs and Al 0.3 Ga 0.7 As in 4% (NH 4 ) 2 S x , respectively. These high values and the linear time dependence of etch rates signify that the etching process of Al x Ga 1- x As in (NH 4 ) 2 S x solutions is predominantly reaction‐rate limited. Possible chemical processes involved in the etching and the formation of a passivating sulfur layer are discussed.
Applied Physics Letters 04/1992; · 3.84 Impact Factor
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ABSTRACT: Monte Carlo simulations based on a continuous slowing‐down approximation (CSDA) model have been used along with experiment to study the alignment signals formed by electrons that are backscattered from anisotropically etched bare‐silicon tapered step marks. The CSDA model is demonstrated to be useful for the study of electron backscattering, particularly for materials with low atomic number. It is shown that a step angle of 54.7° gives a backscattered electron signal close to the maximum and that universal curves for the effects of step height and beam voltage on contrast and average signal slope can be obtained by normalizing to the Bethe range. Alignment signals in ternary takeoff angles and quadrantal azimuthal angles are studied. The tradeoff in expanded signal‐to‐noise ratios for various detector angular configurations and for processed signals is examined. The detector scheme using low takeoff angles in the azimuthal quadrant facing the step is found to be the best. Experimental results from an ETEC Autoscan Scanning Electron Microscope (SEM) confirm may theoretically predicted aspects of alignment signals. The crystallographic effect of electron channeling which is not considered in the Monte Carlo simulation is evaluated experimentally.
Journal of Applied Physics 03/1982; · 2.17 Impact Factor
