GaAs-based devices are currently widely used in wireless applications owing to the advantages of high electron mobility and the availability of the semi-insulating substrate. Many efforts have been put into epitaxial growth and developments of device structures to improve the performance of GaAs based devices. 1,2 To further improve device performance, stability, as well as long-term reliability, device passivation becomes one of the most critical issues. Currently, plasma enhanced chemical vapor deposition (PECVD) with SiH 4 /NH 3 plasma chemistries is used for III-V device passivation. This process involves relatively high ion energies and low plasma density, and operates at a relatively high temperature (250-300ЊC). 3,4 The high energy ion bombardment leads to semiconductor damage. Furthermore, significant amounts of hydrogen and oxygen can be incorporated in the dielectric film and thus cause device instability.There has been extensive interest in the use of high density plasma sources as a replacement for conventional PECVD. Most of the work to date has been performed with electron cyclotron resonance (ECR) sources, which provide high ion density and low ion energy. 5,6 Therefore low temperature deposition can be realized. While these systems work very well, there is some reluctance to introduce them into manufacturing lines because the automatic plasma tuning networks for microwave discharges are less mature than their radio frequency (rf) counterparts, and the uniformity across large diameter wafers has not been fully realized. Recently inductively coupled plasma (ICP) sources powered with radio frequency have been successfully used for etching III-V based semiconductors. 7,8 However, depositions with ICP sources have not been widely investigated. 9,10 In this paper we report on the results of a parametric study of the influences of rf chuck power, ICP source power, chuck temperature, and operating pressure on the GaAs Schottky diodes. The NH 3 discharges were used for the damage study. There was no actual deposition during the experiments, therefore the results shown here can be extremes of the ion bombardment damage and H-passivation in the real deposition. ExperimentalThe layer structure of the Schottky study consists of a 6000 Å n-type Si-doped GaAs layer (2 ϫ 10 17 cm Ϫ3 ) epitaxially grown on n ϩ -GaAs substrates with metall organic molecular beam epitaxy (MOMBE). An ohmic contact was placed on the back side of the wafer using electron-beam evaporation. The contact metallization comprised of Ge/Ni/Au-Ge/Ag/Au (50 Å/50 Å/240 Å-168 Å/500 Å/ 2000 Å) and the metallization was then alloyed at 400ЊC for 20 s. Prior to the plasma treatment, the GaAs native oxides were removed with a solution of HCl:H 2 O (1:20 mixture). The samples were then exposed to the NH 3 discharges in a Plasma Therm 790 inductively coupled plasma (ICP) system. The ICP source operates at 2 MHz with powers of 0-950 W. The sample chuck was biased from 0-100 W at 13.56 MHz. Substrate temperature was also varied from 0-260 ЊC and pressure...
The purpose of this study was to investigate the toxicological response of nitrifying and heterotrophic populations in activate sludge to different copper concentrations. The changes in the active fraction of nitrifying and heterotrophic communities were quantitatively estimated by the respirometric analysis. Furthermore, the shift in community structure of mixed culture receiving continuous dose of copper was qualitatively determined by the fatty acid methyl esters (FAMEs) analysis and principal component analysis (PCA).Four bench-scale activated sludge units were fed with synthetic wastewater and operated at a mean cell residence time (MCRT) of 20 days. Copper concentrations in the influent were 0, 1, 3 and 6 mg/L. Toxicological responses of the nitrifying and heterotrophic populations were evaluated by the normalized ammonia uptake rate (AUR) and total organic carbon (TOC) removal rate, respectively. Substantial decrease in the performance of both nitrifiers and heterotrophs were observed at the copper concentrations of 3 and 6mg/L.In order to examine the effect of continuous dose of copper on viable population of nitrifiers and heterotrophs, the active fraction of both populations were estimated by a respirometric technique. Active fractions of the nitrifiers and hetertrophs in the noninhibited control unit were 1.0 and 30.1% of total biomass, respectively. The active fractions both populations decreased sharply to 0.1 and 0.5% (for nitrifier and heterotrophs, respectively) under the continuous dose of 3 mg/L, suggesting that the continuous loading of copper as low as 3 mg/L is enough to have detrimental effects on the viable communities in nitrifying activated sludge.Effect of copper on microbial community structure was evaluated by FAME analysis. Nineteen fatty acids were extracted from the samples harvested from the bench-scale activated sludge units. Of those fatty acids, seven were previously identified as fatty acids typically present in pure cultures of Nitrosomonas spp. and Nitrobacter spp. When fatty acid compositions of both nitrifying FAMEs and all FAMEs were examined by PCA, the resulting plots showed clear distinctions between populations in control units and those that exhibited decreased levels of nitrification. The combination of FAME analysis with PCA illustrated the significant shift in the nitrifying and heterotrophic community structure as a result of continuous copper dosage.
Hydrogen is an important component of the gas phase growth chemistry for GaN (eg. NH3, (CH 3)3 Ga)and the processing environment for subsequent device fabrication (eg. SiH4 for dielectric deposition, NH3 or H2 annealing ambients), and is found to readily permeate into heteroepitaxial material at temperatures ≤200°C. Its main effect has been the passivation of Mg acceptors in p-GaN through formation of neutral Mg-H complexes, which can be dissociated through minority-carrier (electron) injection or simple thermal annealing. Atomic hydrogen is also found to passivate a variety of other species in GaN, as detected by a change in the electrical or optical properties of the material. An example is the increase in luminescence efficiency of Er3+ ions in AIN after hydrogenation, through passivation of non-radiative states that would be an alternative path for de-excitation. The injection of hydrogen during a large variety of device fabrication steps has been detected by SIMS profiling using 2H isotopic labeling. Basically all of the acceptor species in GaN, namely Mg, C, Ca and Cd are found to form complexes with hydrogen.
We demonstrated an in-situ dielectric film passivation technique by dividing a thick film deposition into many thin film (<40Å) depositions and incorporating the ion bombardment between the depositions. N2 was used for the plasma treatment to passivate the SiNx film and a well passivated and thermally stable SiNx was achieved with this process. The refractive index of N2 treated SiNx film only changed 0.3% when the SiNx film was heated up to 1000 °C and the film with a continuous deposition showed a 2.5% change. From the results of SIMS analysis, the N2 treated SiNx film showed a excellent thermal stability after heat up to 1000 °C. The etch rates of passivated SiNx film in BOE and diluted HF are ≤40 Å/min which is much slower than that of un-treated SiNx (135 Å/min).
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