Ferdinando Iucolano

Temperature dependence of the specific resistance in Ti∕Al∕Ni∕Au contacts on n-type GaN

The temperature dependence of the specific resistance ρc in annealed Ti∕Al∕Ni∕Au contacts on n-type GaN was monitored, obtaining information on the current transport mechanisms. After annealing at 600°C, the contacts exhibited a rectifying behavior and became Ohmic only after high temperature processes (>700°C), with ρc in the low 10−5Ωcm2 range. The results demonstrated that the current transport is ruled by two different mechanisms: thermoionic field emission occurs in the contacts annealed at 600°C, whereas field emission dominates after higher temperature annealing. The significant physical parameters related to the current transport, i.e., the Schottky barrier height and the carrier concentration under the contact, could be determined. In particular, a reduction of the Schottky barrier from 1.21eV after annealing at 600°Cto0.81eV at 800°C was determined, accompanied by a strong increase of the carrier concentration, i.e., from 2×1018cm−3 in the as-prepared sample to 4.6×1019cm−3 in the annealed conta...

Temperature behavior of inhomogeneous Pt∕GaN Schottky contacts

In this letter, a correlation between the nanoscale localized electrical properties of the Pt∕GaN Schottky barrier and the temperature behavior of macroscopic Schottky diodes is demonstrated. Although a significant improvement of the ideality factor of the diodes is achieved after annealing at 400°C, local current-voltage measurements, performed with a biased tip of a conductive atomic force microscope, revealed the inhomogeneous nature of the barrier. Its nanoscale degree of homogeneity was quantitatively described by means of Tung’s model [Phys. Rev. B 45, 13509 (1992)], allowing the authors to explain the temperature dependence of the electrical characteristics of the macroscopic diodes.

Nanoscale carrier transport in Ti∕Al∕Ni∕Au Ohmic contacts on AlGaN epilayers grown on Si(111)

In this letter, a correlation between nanostructure and current flow in Ti∕Al∕Ni∕Au Ohmic contacts on AlGaN films grown on Si(111) is reported. A cross correlation between conductive-atomic force microscopy and structural analyses (x-ray diffraction, transmission electron microscopy) demonstrates that the structure and the electrical properties of the different phases formed inside the reacted layer upon annealing are crucial for the nanoscale current transport. The experimental measurement of the resistivity of the main phases formed upon annealing (AlNi, AlAu4, and Al2Au) indicated that the low resistivity Al2Au phase provides preferential conductive paths for the current flow through the contact.

Correlation between microstructure and temperature dependent electrical behavior of annealed Ti/Al/Ni/Au Ohmic contacts to AlGaN/GaN heterostructures

This letter reports on the temperature behavior of the structural and electrical properties of Ti/Al/Ni/Au contacts to AlGaN/GaN heterostructures. While Ohmic contacts formed at 750 °C showed a decreasing temperature behavior of the specific contact resistance ρC, which was explained by a thermionic field emission mechanism, an increasing trend is observed in the contacts formed at 850 °C. In this case, ρC exhibits a “metal-like” behavior, i.e., describable by a T1.8 dependence. The microstructural analysis of the interfacial region allowed to explain the results with the formation of metallic intrusions contacting directly the two dimensional electron gas.

Influence of high-temperature GaN annealed surface on the electrical properties of Ni/GaN Schottky contacts

In this work, the electrical properties of Ni/GaN Schottky contacts formed on high-temperature annealed (1100–1200 °C) GaN surfaces were studied. Although the morphology of the GaN surface was not changing after annealing, a worsening of the electrical behavior of the Schottky contact occurred, with a reduction in the barrier height and an increase in the leakage current. Moreover, a different temperature dependence of the reverse electrical characteristics of the Schottky diodes was observed. In particular, for the sample annealed at 1150 °C for 5 min, one-dimensional variable-range-hopping conduction was one of the dominant carrier transport mechanisms. The presence of a high density of interface states was indicated as a possible reason of this electrical behavior.

Nanoscale current transport through Schottky contacts on wide bandgap semiconductors

The current transport through Schottky contacts on wide band gap semiconductors (GaN and SiC) was studied on nanoscale by conductive atomic force microscopy. Two very different metal-semiconductor systems were investigated: (i) a uniform (∼5 nm thick) Pt contact on GaN, and (ii) a discontinuous contact formed by self-assembled Au nanoclusters on SiC. The local current-voltage (I-V) measurements allowed to demonstrate the “laterally inhomogeneous” electrical behavior of the Pt/GaN contact, which was formed by a distribution of nanoscale patches with different barrier heights. This behavior was explained in terms of the inhomogeneities of the Pt/GaN interface and/or of the electrically active defects present in the GaN epilayer. The standard deviation of the local barrier height histogram (σΦ) was correlated with the dependence of the ideality factor (n) on temperature, deduced from conventional I-V measurements at variable temperatures on macroscopic Pt/GaN diodes. The local Schottky barrier height at the ...

Electrical behavior of AlGaN/GaN heterostuctures upon high-temperature selective oxidation

In this paper, the influence of a high-temperature (900 °C) selective oxidation process on the electrical properties of AlGaN/GaN heterostructures was investigated. In particular, electrical measurements performed on appropriate devices and test patterns demonstrated that the current flow through the two-dimensional electron gas (2DEG) was suppressed, even if the thickness of the local oxide did not reach the AlGaN/GaN interface. The combination of macroscopic current-voltage and capacitance-voltage measurements with depth-resolved scanning capacitance microscopy elucidated the doping dependence and the compositional stability of the material during high-temperature oxidation. The reduction in the 2DEG sheet carrier density and the variation of the threshold voltage of simple high electron mobility transistor structures upon high-temperature annealing were also discussed.

Nanoscale structural and electrical evolution of Ta- and Ti-based contacts on AlGaN/GaN heterostructures

In this paper, the nanoscale structural and electrical evolution of Ta- and Ti-based contacts was investigated employing several analytical techniques. A correlation between the improvement of the electrical quality of the contacts and the formation of Al-alloyed phases (TaAl3 or TiAl3) during annealing was observed. However, while for the Ti/Al contacts an Ohmic behavior with a contact resistance Rc ∼ 1.8 Ω mm has been achieved after annealing at 500 °C, Ta/Al contacts exhibited a higher contact resistance (Rc ∼ 36.3 Ω·mm) even after annealing at 700 °C. The different electrical behaviour has been explained considering the different interface and the homogeneity of the current transport at a nanoscale level.

Slow and fast traps in metal-oxide-semiconductor capacitors fabricated on recessed AlGaN/GaN heterostructures

In this letter, slow and fast trap states in metal-oxide-semiconductor (MOS) capacitors fabricated on recessed AlGaN/GaN heterostructures were studied by frequency dependent conductance measurements. In particular, the comparison of devices before and after annealing in forming gas allowed to ascribe the fast states (with characteristic response time in the range of 5–50 μs) to SiO2/GaN “interface traps,” and the slow states (50–100 μs) to “border traps” located few nanometers inside the SiO2 layer. These results can be important to predict and optimize the threshold voltage stability of hybrid MOS-based transistors on GaN.

Effects of Annealing Treatments on the Properties of Al/Ti/p-GaN Interfaces for Normally OFF p-GaN HEMTs

This paper reports on the behavior of Al/Ti/ p-GaN interfaces as gate contacts for p-GaN/AlGaN/GaN normally off high electron mobility transistor (HEMTs), highlighting the impact of the thermal budget on the metal gate on the device characteristics. In fact, while the devices subjected to an annealing at 800°C show a considerable high leakage current, those with nonannealed Al/Ti gate contacts exhibit a normally off behavior, with a pinch-off voltage Vpo = +1.1 V and an on/off current ratio of 3 × 108. Temperature-dependent electrical measurements on back-to-back Schottky diodes allowed to determine a Schottky barrier height ΦB of 2.08 and 1.60 eV, for the nonannealed and 800°C annealed gate contacts, respectively. Hence, the increase in the leakage current observed upon annealing at 800°C was attributed to the lowering of the Schottky barrier height ΦB of the metal gate. The interfacial structural characterization explained the barrier lowering induced by the annealing. This scenario was discussed through the simulated band diagram of the heterostructures, considering the experimental values of ΦB. These results provide useful information for the device makers to optimize the fabrication flow of normally off HEMTs with p-GaN gate.