Junji Kotani

Leakage mechanism in GaN and AlGaN Schottky interfaces

Based on detailed temperature-dependent current–voltage (I–V–T) measurements the mechanism of leakage currents through GaN and AlGaN Schottky interfaces is discussed. The experiments were compared to calculations based on thin surface barrier model in which the effects of surface defects were taken into account. Our simulation method reproduced the experimental I–V–T characteristics of the GaN and AlGaN Schottky diodes, and gave excellent fitting results to the reported Schottky I–V curves in GaN for both forward and reverse biases at different temperatures. The present results indicate that the barrier thinning caused by unintentional surface-defect donors enhances the tunneling transport processes, leading to large leakage currents through GaN and AlGaN Schottky interfaces.

Analysis and control of excess leakage currents in nitride-based Schottky diodes based on thin surface barrier model

Using a rigorous computer simulation program for current transport through a Schottky barrier with an arbitrary potential profile, the leakage current mechanism in GaN and AlGaN Schottky diodes was investigated on the basis of the thin surface barrier (TSB) model recently proposed by the authors’ group. Computer simulation assuming various possible defect density distributions was carried out to reproduce the measured temperature dependent current voltage (I–V)-temperature characteristics of the GaN and AlGaN Schottky diodes which showed excessive reverse leakage. By assuming exponentially decaying distributions from surface for defect donors with energy depth of 0.25 eV for GaN and 0.37 eV for Al0.15Ga0.85N, I–V curves measured by our group as well as reported in the literatures were almost completely reproduced both in forward and reverse direction over a wide temperature range. The defect donors are proposed to be N vacancies or their related complexes that are formed during metal deposition. The resul...

Mechanism of surface conduction in the vicinity of Schottky gates on AlGaN∕GaN heterostructures

Lateral surface leakage current (Is) on an AlGaN∕GaN heterostructure was systematically investigated by using a two-parallel gate structure with a gap distance (LGG) of 200nm–5μm. The surface current Is systematically increased as LGG decreased. A simple resistive layer conduction that should show 1∕LGG dependence failed to account for the drastic increase in Is when LGG was reduced to less than 1μm. However, no dependence on LGG was seen in vertical current that flows in the Schottky interface. The Is showed a clear temperature dependence proportional to exp(−T−1∕3), indicating two-dimensional variable-range hopping through high-density surface electronic states in AlGaN. A pronounced reduction in surface current of almost four orders of magnitude was observed in a sample with SiNx passivation.

Large reduction of leakage currents in AlGaN Schottky diodes by a surface control process and its mechanism

Leakage currents in AlGaN Schottky diodes were investigated systematically by using a rigorous computer simulation based on the thin surface barrier model taking account of unintentionally doped surface donors. The leakage currents in AlGaN Schottky diodes have stronger bias dependence and smaller temperature dependences as compared with those of GaN diodes. It was shown that these features were associated with shallow oxygen donors located near the AlGaN surface. Then, an attempt was made to remove oxygen and suppress leakage currents by a surface control process using an ultrathin Al layer and subsequent annealing. An in situ x-ray photoelectron spectroscopy analysis indicated the formation of Al2O3 layer during the surface control process, suggesting efficient gettering of oxygen from the surface. C-V analysis directly indicated the reduction of shallow donors by the surface control process. A remarkable reduction of reverse leakage currents of four to five orders of magnitude took place in large area ...

Effects of Surface Oxidation of AlGaN on DC Characteristics of AlGaN/GaN High-Electron-Mobility Transistors

We have investigated the effects of thin native oxide layers on the AlGaN surface on the DC characteristics of AlGaN/GaN high electron mobility transistors (HEMTs). After HEMT fabrication, the AlGaN surface between the electrodes was intentionally oxidized using either O2 or N2O plasma. X-ray photoelectron spectroscopy (XPS) analysis showed that both methods produced a native oxide of AlGaN with a thickness of about 1 nm and that an N–O chemical bond was present in the N2O plasma oxide. We observed pronounced degradation in the DC characteristics and current collapse in the HEMT with O2 plasma oxidation. In contrast, the formation of native oxide by the N2O plasma had no effect on the DC characteristics or current stability of the AlGaN/GaN HEMT. Possible mechanisms for device degradation were discussed in terms of stress and deep levels in the AlGaN layer induced by oxide formation or oxygen incorporation.

Integrated Tunable Quantum-Dot Laser for Optical Coherence Tomography in the 1.7

In this paper, we present the design and characterization of a monolithically integrated tunable laser for optical coherence tomography in medicine. This laser is the first monolithic photonic integrated circuit containing quantum-dot amplifiers, phase modulators, and passive components. We demonstrate electro-optical tuning capabilities over 60 nm between 1685 and 1745 nm, which is the largest tuning range demonstrated for an arrayed waveguide grating controlled tunable laser. Furthermore, it demonstrates that the active-passive integration technology designed for the 1550 nm telecom wavelength region can also be used in the 1600-1800 nm region. The tunable laser has a 0.11 nm effective linewidth and an approximately 0.1 mW output power. Scanning capabilities of the laser are demonstrated in a free space Michelson interferometer setup where the laser is scanned over the 60 nm in 4000 steps with a 500 Hz scan frequency. Switching between two wavelengths within this 60 nm range is demonstrated to be possible within 500 ns.

\mu{\rm m}

We proposed a surface control process for suppressing the tunneling leakage of Schottky gates on AlGaN/GaN heterostructures. For the recovery of nitrogen-vacancy-related defects and reduction in the amount of oxygen impurities at the AlGaN surface, the process consisted of nitrogen radical treatment, the deposition of an ultrathin Al layer, UHV annealing and finally the removal of the Al layer. Ni/Au Schottky gates fabricated on processed AlGaN surfaces showed pronounced reduction in leakage current and a clear temperature dependence of I–V characteristics, indicating the effective suppression of tunneling leakage in current transport through AlGaN Schottky interfaces.

Wavelength Region

We fabricated a multi-mesa-channel (MMC) structure by forming a periodic trench just under a gate electrode to improve the uniformity of effective electric field in the channel in an AlGaN/GaN high electron mobility transistor (HEMT). A unique performance, i.e., a nearly temperature-independent saturation drain current, was observed in the MMC device in a wide temperature range. A two-dimensional (2D) potential calculation indicates that the mesa-side gate effectively modulates the potential, resulting in a field surrounding 2D electron gas. Such a surrounding-field effect and a relatively lower source access resistance may be related to a unique current behavior in the MMC HEMT.

Surface Control Process of AlGaN for Suppression of Gate Leakage Currents in AlGaN/GaN Heterostructure Field Effect Transistors

The gate leakage and gate control characteristics of AlGaN/GaN heterostructure field effect transistors (HFETs) were systematically investigated in an attempt to clarify possible effects of surface states. The experiments were compared to rigorous computer simulations. We observed large amounts of leakage currents in the Schottky diodes fabricated on the AlGaN epitaxial layers. By the calculation based on a thin surface barrier model in which the effects of surface defect donor were taken into account, this large leakage was well explained by enhancement of tunneling transport processes due to the barrier thinning associated with ionization of surface-defect donor. On the other hand, the analysis on the current-voltage characteristics for the nanometer-scale Schottky contacts on AlGaN/GaN HFETs, indicated additional lateral leakage components. The comparison of the gate control characteristics between experiment and calculation clearly showed that the effective lateral expansion of gate length significant...

Nearly Temperature-Independent Saturation Drain Current in a Multi-Mesa-Channel AlGaN/GaN High Electron Mobility Transistor

A deep level with an activation energy of 1.0 eV in n-type Al0.26Ga0.74N grown by metal-organic chemical vapor deposition was detected by deep-level transient spectroscopy (DLTS) with a sampling time window of several seconds. The deep-level density was 6×1015 cm−3. At the temperatures around which the DLTS peaks were observed, capacitance transient was measured. Under the dark condition, a capacitance increase was observed, corresponding to the thermal emission of electrons from the level with 1.0 eV activation energy. After that, we observed a large capacitance increase under illumination with 2.3 eV photon energy. On the basis of potential simulation taking account of deep levels, we found that the photoinduced capacitance change arose from electron emission from additional near-midgap levels in energy ranging from EC−1.5 to EC−2.3 eV.