CdMnS thin films grown on indium tin oxide substrate exhibit a ferroelectric property. The remnant polarization of CdMnS films was around 0.025μC∕cm2 for 10% of manganese with a sustained polarization endurance for several cycles. Persisting and highly reproducible bistable switching of about five orders in magnitude between low and high impedance states was observed in current voltage measurements. Nondestructive readout measurement with a short pulse width of 20μs resulted in a resistance difference of two orders between two read levels with a good retention time. Possible use of CdMnS for nonvolatile memory applications is explored.
In this study, Cl2/Ar and Cl2/BCl3 inductively coupled plasmas were used to etch GaN and the effects of etch parameters such as gas combination and operation pressure on the characteristics of the plasmas and etch properties of GaN were investigated. The characteristics of the plasmas were estimated using a Langmuir probe and optical emission spectroscopy. Surface residue remaining after the etch was investigated using x-ray photoelectron spectroscopy (XPS). The increase of Ar and BCl3 in Cl2 generally reduced GaN etch rates except for the small addition of Ar or BCl3. With the addition of 10% Ar or 10% BCl3 to Cl2, the GaN etch rates showed the maximum etch rates. Also, the increase of operational pressure up to 30 mTorr increased the GaN etch rates. By optimizing etch process parameters, etch conditions having smooth and nearly vertical etch profiles with the etch rates close to 8500 A/min and the selectivity over SiO2 higher than 3.5 could be obtained with Cl2-rich Cl2/BCl3 gas combinations. The change of Cl radical density measured by optical emission spectroscopy as a function of gas combination showed the same trend as the change of GaN etch rates, therefore, chemical reactions between Ga in GaN and Cl from Cl2 appear to be one of the most important factors controlling the GaN etch rates. Ga/N ratios of the etched GaN surfaces measured by angle resolved XPS were less than 1 for all of the etch conditions used in the experiment. However, when Ar was added to Cl2, Ga/N ratio increased and, when BCl3 was added, the Ga/N ratio decreased from that of the GaN surface etched using pure Cl2.
The electrical properties of the light ion impurities H, O and C in GaN have been examined in both as-grown and implanted material. H is found to efficiently passivate acceptors such as Mg, Ca and C. Reactivation occurs at ≥450°C and is enhanced by minority carrier injection. The hydrogen does not leave the GaN crystal until >800°C, and its diffusivity is relatively high (˜10−11cm2/s) even at low temperatures (<200°C) during injection by wet etching, boiling in water or plasma exposure. Oxygen shows a low donor activation efficiency when implanted into GaN, with an ionization level of 30 - 40 meV. It is essentially immobile up to 1100°C. Carbon can produce low p-type levels (3×1017cm−3) in GaN during MOMBE, although there is some evidence it may also create n-type conduction in other nitrides.
LiGaO2 and LiAlO2 have similar lattice constants to GaN, and may prove useful as substrates for III-nitride epitaxy. We have found that these materials may be wet chemically etched in a number of acid solutions, including HF, at rates between 150–40,000 Å/min. Dry etching with SF6/Ar plasmas provides faster rates than Cl2/Ar or CH4/H2/Ar under Electron Cyclotron Resonance conditions, indicating the fluoride etch products are more volatile that their chloride or metalorganic/hydride counterparts. Dry etch rates are low ( < 2, 000 Å/min), providing high selectivity (>5) over the nitrides. The incorporation of hydrogen in these materials is also of interest because this could provide a reservoir of hydrogen that may passivate dopants in overlying nitride films. In 2H implanted samples, 50 % of the deuterium is lost by evolution from the surface by annealing at 400 °C for 20 min and all of the deuterium is gone at 700°C. The diffusivity of 2H is ∼10-13 cm2/s at 250°C in LiA1O2, approximately two orders of magnitude higher than in LiGaO2.
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