Min-Koo Han

Effects of SiO2 Passivation on Oxygen Annealed AlGaN/GaN HEMTs

− The FTS(Fischer-Tropsch synthesis) was carried out over precipitated iron-based catalysts with or without SiO 2 in a fixed-bed reactor at 250 C and 1.5 MPa. The catalysts with SiO 2 showed much higher catalytic activity for the FTS than those without SiO 2 , displaying excellent stability during 144 h of reaction. The X-ray diffraction and N 2 physisorption revealed that the catalysts with SiO 2 showed enhanced dispersion of Fe 2 O 3 compared with those without SiO 2 . Also, the results of temperature-programmed reduction by H 2 showed that the addition of SiO 2 markedly promoted the reduction of Fe 2 O 3 into Fe 3 O 4 and FeO at low temperatures below 260 C. In contrast, surface basicity of the catalysts, which was analyzed by temperature-programmed desorption of CO 2 , decreased as a result of SiO 2 addition. We attribute the high and stable performance of the catalysts with SiO 2 to the improved dispersion and reducibility by the SiO 2 addition.

Highly Stable Bottom-Gate Nanocrystalline Silicon Thin Film Transistor Fabricated Employing ICP-CVD

Bottom-gate nanocrystalline silicon (nc-Si) thin film transistors (TFTs) were fabricated and evaluated their characteristics and electrical stability under various stress condition. nc-Si with high crystallinity was deposited employing Inductively coupled plasma chemical vapor deposition(ICP-CVD) system. We employed helium gas diluted deposition and all the process temperature was kept under 350C. We fabricated conventional inverted-staggered nc-Si TFTs. Fabricated nc-Si TFTs showed fine electrical characteristics, such as electrical mobility of 0.64~0.77 cm/V·sec. We investigated its stability through constant-voltage stress and constant-current stress. The threshold voltage shift after 30,000 seconds gate bias (10V) stress was only 0.098V, which is considerably less compared to a-Si:H TFT. Under the static current stress condition, the threshold voltage of the nc-Si TFT was shifted less than that of a-Si:H TFT. It demonstrates that nc-Si TFT exhibit better stability than conventional a-Si:H TFT.

Enhancement of Interface Properties between Passivation Layers and InSb by Using Remote PECVD

We report the enhanced interface properties between passivation layers and InSb by using remote PECVD system. SiO2 and Si3N4 layers deposited by remote PECVD showed lower interface trap densities than layers deposited by normal PECVD. SiO2 layers deposited by remote PECVD showed 7.1×1011u2009cm−2u2009eV−1 of interface trap density at midgap which is slightly lower than SiO2 layers deposited by PECVD. Si3N4 layers deposited by remote PECVD showed 1.6∼1.7×1012u2009cm−2u2009eV−1 at midgap which is 3 times lower than Si3N4 layers deposited by PECVD. Interface properties of SiO2 are superior to that of Si3N4 in both case of PECVD and remote PECVD. To investigate the interface properties between SiO2 and InSb, X‐ray photoelectron spectroscopy was conducted. Indium and antimony oxide phases were found at the interface and these oxide phases could act as the origin of interface traps.

Low leakage current circular AlGaN/GaN Schottky barrier diode

We proposed circular AlGaN/GaN Schottky barrier diode, which has no mesa structure near the current path. Proposed device showed low leakage current of 10 nA/mm at -100 V while that of the rectangular device was 34 nA/mm at the same condition. Proposed circular AlGaN/GaN SBD showed high forward current of 88.61 mA at 3.5 V while that of the conventional device was 14.1 mA at the same condition.

Hot Carrier Stress Effects of SiO2 Passivated AlGaN/GaN High Electron Mobility Transistors

AlGaN/GaN high electron mobility transistors (HEMTs) have been reported for microwave applications and high power devices due to its wide bandgap, high output power and high breakdown voltage. The electron injections into the surface trap states decrease a forward drain current and breakdown voltage of AlGaN/GaN HEMTs. Various passivations such as Si3N4, SiO2, benzocyclobutene and Sc2O3 have been reported to suppress the surface states of AlGaN/GaN HEMTs so that passivation should be required in order to improve electric characteristics of AlGaN/GaN HEMTs. However, the electric reliability of AlGaN/GaN HEMTs before and after SiO2 passivation has been scarcely reported. The purpose of our work is to report hot carrier stress effects of AlGaN/GaN HEMTs before and after SiO2 passivation. AlGaN/GaN HEMTs with platinum Schottky gate were fabricated. A 400 nm-thick SiO2 passivation of AlGaN/GaN HEMTs was performed by using inductively coupled plasma-chemical vapor deposition at 300 C. Hot carrier stress with VDS=10 V, VGS=0 V and stress time of 50000 sec was applied to HEMTs before and after SiO2 passivation. Experimental results show that SiO2 passivation suppresses the injections of electrons into the surface states during the hot carrier stress. The SiO2 passivation successfully suppresses the degradation due to the hot carrier stress compared with unpassivated device. Fig. 1 shows measured transfer characteristics of SiO2 passivated and unpassivated device after the stress. After SiO2 passivation, the leakage current is decreased from 10 to 10 A due to the suppression of surface leakage current. After the stress, the positive shift of threshold voltage (VTH) is observed in both SiO2 passivated and unpassivated devices due to trapped electrons in AlGaN under Schottky gate. The ∆VTH of SiO2 passivated and unpassivated devices after the stress are 0.14 and 0.22 V respectively. It clearly shows that SiO2 passivation of AlGaN/GaN HEMTs is critical for the hot carrier stress. The SiO2 passivation suppresses VTH shift of AlGaN/GaN HEMT from the hot carrier stress. Hot carrier stress decreases the forward drain current of AlGaN/GaN HEMTs due to the increased VTH, electron injections into surface states and generated interface states. After applying hot carrier stress with VDS=10 V and VGS=0 V, the forward drain current of SiO2 passivated device is decreased by 17.9 mA/mm while that of unpassivated one is decreased by 22.4 mA/mm. This experimental result shows that SiO2 passivated device is more tolerable to the hot carrier stress compared with unpassivated one because SiO2 passivation suppresses the electron injections into the surface states during the hot carrier stress. That also decreases the channel depletion due to the hot carrier stress. The leakage currents of SiO2 passivated and unpassivated device are decreased after the hot carrier stress. In the reverse bias, stressed devices has more depletion regions due to the generated interface states and negatively charged surface states. The leakage current at VGS=-6 V of SiO2 passivated device after the stress is 2.09 nA while that before stress is 4.72 nA. Fig. 2 shows measured transconductance of SiO2 passivated and unpassivated device after the hot carrier stress. The transconductance is shifted positively due to the increase of VTH after the hot carrier stress. Measured gm.max of SiO2 passivated device decreases from 116.8 mS/mm to 114.2 mS/mm due to the decreased mobility and enhanced scattering in channel. The mechanism of the decreased forward drain current after the hot carrier stress is investigated. The difference between ∆IDS (decrease of drain current) and gm∆VTH (increase of VTH) would be attributed to generated states during the stress. Fig. 3 shows the measured -∆IDS and gm∆VTH of SiO2 passivated and unpassivated device before and after the stress. In both SiO2 passivated and unpassivated device, -∆IDS and gm∆VTH are not identical. It is noted that the interface states are generated and the electrons are injected into the surface states during the hot carrier stress. Measured ∆IDS and gm∆VTH of SiO2 passivated device are less than those of unpassivated one. Our experimental results show that SiO2 passivated AlGaN/GaN HEMT are more robust to the hot carrier stress compared with unpassivated one. We have reported the hot carrier stress effects of AlGaN/GaN HEMTs before and after SiO2 passivation. After the hot carrier stress with VDS=10 V and VGS=0 V, ∆VTH and ∆IDS of SiO2 passivated device are 0.14 V and 17.9 mA/mm while those of unpassivated one are 0.22 V and 22.4 mA/mm respectively. The SiO2 passivation suppresses the electron injections into the surface states during the hot carrier stress. The interface states between AlGaN and GaN are generated during the hot carrier stress and those are evidenced by measuring -∆IDS and gm∆VTH. The SiO2 passivation may be very effective to improve not only electric characteristics but also reliability of AlGaN/GaN HEMTs. [1] B. M. Green, IEEE Electron Device Lett., 21, pp. 268, 2000. [2] S. Arulkumaran, Appl. Phys. Lett., 84, pp. 613, 2004. [3] W.-K. Wang, IEEE Electron Device Lett., 25, pp.763, 2004. [4] J. K. Gillespie, IEEE Electron Device Lett., 23, pp.505, 2002. [5] G.Meneghesso, J. Appl. Phys., 82, pp. 5547, 1997.