Interpretation of current transport properties at Ni/n–GaN Schottky interfaces

Abstract

Current transport properties at Ni/n–GaN Schottky interfaces formed on oxide-etched or thermally oxidized surfaces are studied by current–voltage–temperature (I–V–T) and capacitance–voltage measurements. The results support existence of surface patches with low Schottky barrier height (SBH), which cause a leakage current of the Schottky structures. Based on “surface patch” model, the fraction of the total patch area of 10−4–10−5 and the SBH lowering of 0.4 eV within patches are deduced for the oxide-etched Ni/n–GaN metalorganic chemical vapor deposition (MOCVD) samples. The influence of surface patches was found much larger for the present Ni/n–GaN molecular beam epitaxy (MBE) samples. The inclusion of N-polarity domains in the Ga-polarity layer seems to be main origin of the patches for the MBE-grown samples. Ni/n–GaN Schottky samples formed on dry-oxidized or wet-oxidized MOCVD layers represented reduction of the effective SBH with negligible increase of the n value, indicating that an intermediary native oxide between Ni and n-GaN degrades the SBH. I–V–T characteristics showed that the leakage current due to surface patches is comparable with that of the oxide-etched sample. Concerning thermal stability of the bulk GaN (MOCVD), the impurity concentration in the layer increased after wet oxidation above 500 °C, although little change was observed after wet oxidation at temperatures below 400 °C. Finally, a postfabrication annealing in nitrogen at 400 °C led to increase of the effective SBH even for the oxidized Ni/n–GaN Schottky samples.

Influence of surface preparation and i‐AlGaN thickness on electrical properties of i‐AlGaN/GaN heterostructures

“…As concerns the influence of thermal annealing, the annealing in O 2 led to considerable increase of the sheet carrier density, as clearly shown in Fig.1. The phenomenon seems consistent with the lowering of the effective Schottky barrier height (SBH) observed in Ni/n-(Al)GaN samples with oxidized surfaces [4]. That is, the increase of s n can be primarily explained by lowering of the surface barrier.…”

“…Since those troublesome problems tend to become larger for E-HEMTs, influence of the surface preparation and the AlGaN thickness on electrical properties of 2DEG and Schottky gate leakage current has to be fully investigated. Previously, we have reported the influence of surface native oxides and thermal annealing on electrical properties of Ni/AlGaN/GaN heterostructures with a fixed AlGaN layer thickness [4]. In the present work, electrical properties of bare i-AlGaN/GaN and Ni/i-AlGaN/GaN Schottky samples with different surface treatments and different thickness of the AlGaN layer, which is prepared by a wet-etching process, were systematically investigated by I-V, C-V and Hall effect measurements.…”

Suppression of gate leakage current in i‐AlGaN/GaN heterostructures by insertion of anodic Al2O3 layer and influence of thermal annealing on channel electrons

“…The result seems to indicate that 2DEG density is quite affected by the surface condition. Previously, we have shown that the native oxide degrades the effective SBH of Ni/n-(Al)GaN samples [6]. The decrease of the 2DEG density can be explained by the downward shift of the surface Fermi level after etching of the oxide, which enhance the electric field in the AlGaN layer and thus decrease 2DEG density.…”

Electrical properties of Ni/i‐AlGaN/GaN structures and influence of thermal annealing

“…Similar degradation has been observed for Ni/low-doped n-GaN samples, indicating the true SBH is lowered owing to the native oxide. The increased n-value can be explained by enhanced leakage current passing through surface patches with low SBH [7]. The strong bias dependence of the reverse current, which continued until pinch-off of 2DEG occurred, seems due to combination effect of tunnelling mechanism and surface patches.…”

“…For these reasons, characterizations of AlGaN/GaN systems are still necessary. Previously, we have measured electrical properties at metal/n-(Al)GaN Schottky interfaces and insulator/n-(Al)GaN interfaces formed on semiconductor single layers [7,8]. In the present work, electrical properties of Ni/i-AlGaN/GaN Schottky gate structures prepared by different surface preparation conditions were studied by I-V and C-V measurements in order to clarify the influence of surface native oxides and post-fabrication annealing in nitrogen, and the effect of insertion of a thin anodic Al 2 O 3 layer www.pss-c.com between Ni and AlGaN.…”