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Phase-equilibrium relations have been determined at 1000 kg/cm2 water pressure for compositions within the system NaAlSi3O8-KalSi3O8-NaAlSiO4-KAlSiO4 in the area adjacent to the temperature minimum. The composition and temperature of the minimum are Ne50Ks19Qz31 and 750° ± 7° C respectively. The compositions of 102 plutonic rocks and 122 extrusive rocks, from Washington's tables, that carry 80% or more of normative Ab + Or + Ne have been plotted; the areas of high density show a marked similarity to the positions of the low-temperature regions of the synthetic system and suggest that many undersaturated rocks are derived by fractional crystallization from a trachytic magma.
Deuteron magnetic resonance line shapes and spin lattice relaxation times are presented for a-Si:D, F and a-SiGe:D, F. These parameters differ from those for typical a-Si:D, H samples, but in some respects are similar to those for an annealed a-Si:D, H sample. The a-SiGe:D, F spectra display an unusually large broad central weakly bound D resonance component and a barely-resolved Ge-D quadrupolar doublet. Comparisons indicate substantial differences in void morphology between the a-Si:D, F and a-SiGe:D, F.
The structural, electrical and optical properties of amorphous Si-Ge alloys prepared either from mixtures of hydrides or of fluorides will be compared. It will be shown that the majority of many properties studied are essentially the same, the major exceptions being the structure of thin films used for TEM investigation, and the photocurrents developed between contacts in a coplanar configuration. Possible explanations for the result of including fluorine in the preparation plasma will be discussed.
Atom-probe techniques have been used to characterise nanostructured metallic materials prepared by thermal evaporation and by sputtering. Multilayer samples of Fe-Cr have been prepared by sputter deposition and analysed using the Oxford position-sensitive atom probe. This has made it possible to observe the quality of interfaces in the material, and also accurately determine local compositions at each layer within the multilayer stack. Preliminary experiments aimed at producing dual phase nanocrystalline films by thermal evaporator deposition are also reported.
The preparation of homogeneous gels from liquid solutions necessitates the prevention of precipitation. This becomes increasingly difficult in multicomponent systems. Currently, there are no systematic considerations on precipitation in sol-gel solutions. This paper is concerned with the methods to control precipitation in alkoxide solutions. Each method is applied to specific systems, containing alkoxides of various metals, such as yttrium, lanthanum, barium, and titanium, in alcoholic solutions.
The protection of glass against water-initiated chemical corrosion is a significant problem. Presently, organic polymers and inorganically modified polymers are being used to protect art glass. However, the service life and durability of these coatings are not entirely satisfactory. Oxide coatings would offer much greater resistance to water penetration. Water permeability through a polymer coating is some ten orders of magnitude larger than that through an oxide coating. This, coupled with a higher scratch resistance, makes oxide coatings promising candidates for the protection of stained glass. Methods to deposit an oxide on glass at relatively low temperatures include sol-gel processing and vapor deposition. This paper compares the performance of different oxide and non-oxide coatings and presents methods for the deposition of oxide coatings at low temperatures.
The sol-gel process can be used to produce porous inorganic matrices that
are doped with organic molecules. These doped gels can be used as a
quantitative method for the spectrophotometric determination of trace
concentrations of metallic ions. For the detection of hexavalent chromium,
malachite green was used as the dopant. Preliminary results indicate
concentrations on the order of 5 ppb are detectable using this method.
The light ion impurities C, O and H have been implanted or diffused into GaN and related compounds and their effect on the electrical properties of these materials measured by Hall, C-V and SIMS as a function of annealing temperatures from 300-1100ºC. While C in as-grown GaN appears to create an acceptor under MOMBE conditions, implanted C shows no measurable activity. Similarly, implanted O does not show any shallow donor activity after annealing at ≤ 700°C, but can create high resistivity regions (106 Ω/□) in GaN, AlInN and InGaN for device isolation when annealed at 500–700°C. Finally, hydrogen is found to passivate shallow donor and acceptor states in GaN, InN, InAIN and InGaN, with dissociation of the neutral complexes at >450°C. The liberated hydrogen does not leave the nitride films until much higher annealing temperatures (>800°C). Typical reactivation energies are ∼2.0eV for impurity-hydrogen complexes.
Quantum well microdisk laser structures have been fabricated in the GaN/InGaN, GaAs/AlGaAs and GaAs/InGaP systems using a combination of ECR dry etching (Cl2/CH4/H2/Ar, BC13/Ar or CH4/H2/Ar plasma chemistries respectively) and subsequent wet chemical etching of a buffer layer underlying the quantum wells. While wet etchants such as HF/H2O and HCI/HNO3/H2O are employed for AlGaAs and InGaP, respectively, a new KOH-based solution has been developed for AlN which is completely selective over both GaN and InGaN. Typical mask materials include PR or SiNx, while the high surface recombination velocity of exposed AlGaAs (∼105cm·sec-1) requires encapsulation with ECR-CVD SiNx to stabilize the optical properties of the modulators.
Damage introduction in III-V nitrides during dry etching can be simulated by exposingthe samples to pure Ar plasmas for study of the physical (ion-bombardment) effects.Changes in conductivity of InN, In0.5Ga 0.5N and In0.5Al0.5N layers exposed to Ar plasmasunder both Electron Cyclotron Resonance and reactive ion etching conditions have beenmeasured as a function of rfpower, pressure and exposure time. The combination of highmicrowave and high rf powers produces large increases (10-_104 times) in sheet resistanceof the nitrides, but conditions more typical of real etching processes (rf power < 150W) donot change the bulk electrical properties. The nitrides are more resistant to damageintroduction than other III-V semiconductors. The removal of damage-related trapsoccurs with an activation energy of ∼2.7eV. High ion currents during ECR etching canproduce substantial conductivity changes, whereas the lower currents under RIEconditions do not affect the nitrides. It is difficult to avoid preferential loss of N in thenear-surface of these materials, which leads to leakage currents in rectifying metal contactsdeposited on these surfaces.
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.
Wet chemical etching of A1N and InxAl1-xN was investigated in KOH-based solutions as a function of etch temperature, and material quality. The etch rates for both materials increased with increasing etch temperatures, which was varied from 20 °C to 80 °C. The crystal quality of A1N prepared by reactive sputtering was improved by rapid thermal annealing at temperatures to 1100 °C with a decreased wet etch rate of the material measured with increasing anneal temperature. The etch rate decreased approximately an order of magnitude at 80 °C etch temperature after a 1100 °C anneal. The etch rate for In0.19Al0.81N grown by Metal Organic Molecular Beam Epitaxy was approximately three times higher for material on Si than on GaAs. This corresponds to the superior crystalline quality of the material grown on GaAs. Etching of InxAl1-xN was also examined as a function of In composition. The etch rate initially increased as the In composition changed from 0 to 36%, and then decreased to 0 Å/min for InN. The activation energy for these etches is very low, 2.0 ± 0.5 kcal•mol-1 for the sputtered A1N. The activation energies for InAIN were dependent on In composition and were in the range 2–6 kcal mol-1. GaN and InN layers did not show any etching in KOH at temperatures up to 80 °C.
ICl/Ar ECR discharges provide the fastest dry etch rates reported for GaN, 1.3 µm/min. These rates are much higher than with Cl2/Ar, CH4/H2/Ar or other plasma chemistries. InN etch rates up to 1.15 µm/min and 0.7 µm/min for In0.5Ga0.5N are obtained, with selectivities up to 5 with no preferential loss of N at low rf powers and no significant residues remaining. The rates are much lower with IBr/Ar, ranging from 0.15 µm/min for GaN to 0.3 µm/min for InN. There is little dependence on microwave power for either chemistry because of the weakly bound nature of IC1 and IBr. In all cases the etch rates are limited by the initial bond breaking that must precede etch product formation and there is a good correlation between materials bond energy and etch rate. The fact that low microwave power can be employed is beneficial from the viewpoint that photoresist masks are stable under these conditions, and there is no need for use of silicon nitride or silicon dioxide. Selectivities for GaN over A1N with IC1 and IBr are still lower than with Cl2- only.
Etch rates up to 7,000Å/min. for GaN are obtained in Cl2/H2/Ar or BCl3/Ar ECR discharges at 1–3mTorr and moderate dc biases. Typical rates with HI/H2 are about a factor of three lower under the same conditions, while CH4/H2 produces maximum rates of only ˜2000Å/min. The role of additives such as SF6, N2, H2 or Ar to the basic chlorine, bromine, iodine or methane-hydrogen plasma chemistries are discussed. Their effect can be either chemical ( in forming volatile products with N) or physical ( in breaking bonds or enhancing desorption of the etch products). The nitrides differ from conventional III-V's in that bondbreaking to allow formation of the etch products is a critical factor. Threshold ion energies for the onset of etching of GaN, InGaN and InAlN are ≥75eV.
Sub-micron periodic gratings with pitch ∼3,000Å were formed in GaN and InGaN using holographic lithography and room temperature ECR BCl3/N2 dry etching at moderate microwave (500W) and rf (100W) powers. The process produces uniform gratings without the need for elevated sample temperatures during the etch step.
High-density plasma etching has been an effective patterning technique for the group-III nitrides due to ion fluxes which are 2 to 4 orders of magnitude higher than more conventional reactive ion etch (RIE) systems. GaN etch rates exceeding 0.68 μm/min have been reported in Cl2/H2/Ar inductively coupled plasmas (ICP) at -280 V dc-bias. Under these conditions, the etch mechanism is dominated by ion bombardment energies which can induce damage and minimize etch selectivity. High selectivity etch processes are often necessary for heterostructure devices which are becoming more prominent as growth techniques improve. In this study, we will report high-density ICP etch rates and selectivities for GaN, AIN, and InN as a function of cathode power, ICP-source power, and chamber pressure. GaN:AIN selectivities > 8:1 were observed in a Cl2/Ar plasma at 10 mTorr pressure, 500 W ICP-source power, and 130 W cathode rf-power, while the GaN:InN selectivity was optimized at ∼ 6.5:1 at 5 mTorr, 500 W ICP-source power, and 130 W cathode rf-power.
Transient thermal processing is employed for implant activation, contact alloying, implant isolation and dehydrogenation during III-nitride device fabrication. We have compared use of InN, AlN and GaN powder as methods for providing a N2 overpressure within a graphite susceptor for high temperature annealing of GaN, InN, A1N and InAlN. The AlN powder provides adequate surface protection to temperatures of ∼1100°C for AlN, > 1050°C for GaN, ∼600°C for InN and ∼800°C for the ternary alloy. While the InN powder provides a higher N2 partial pressure than AlN powder, at temperatures above ∼750°C the evaporation of In is sufficiently high to produce condensation of In droplets on the surfaces of the annealed samples. GaN powder achieved better surface protection than the other two cases.
High-density plasma technology is becoming increasingly attractive for the deposition of dielectric films such as silicon nitride and silicon dioxide. In particular, inductively-coupled plasma chemical vapor deposition (ICPCVD) offers a great advantage for low temperature processing over plasma-enhanced chemical vapor deposition (PECVD) for a range of devices including compound semiconductors. In this paper, the development of low temperature (< 200°C) silicon nitride and silicon dioxide films utilizing ICP technology will be discussed. The material properties of these films have been investigated as a function of ICP source power, rf chuck power, chamber pressure, gas chemistry, and temperature. The ICPCVD films will be compared to PECVD films in terms of wet etch rate, stress, and other film characteristics. Two different gas chemistries, SiH4/N2/Ar and SiH4/NH3/He, were explored for the deposition of ICPCVD silicon nitride. The ICPCVD silicon dioxide films were prepared from SiH4/O2/Ar. The wet etch rates of both silicon nitride and silicon dioxide films are significantly lower than films prepared by conventional PECVD. This implies that ICPCVD films prepared at these low temperatures are of higher quality. The advanced ICPCVD technology can also be used for efficient void-free filling of high aspect ratio (3:1) sub-micron trenches.
During gate mesa plasma etching of InN/InAlN field effect transistors the apparent conductivity in the channel can be either increased through three different mechanisms. If hydrogen is part of the plasma chemistry, hydrogen passivation of the shallow donors in the InAlN can occur, we find diffusion depths for 2H of ≥ 0.5 micron in 30 mins at 200°C. The hydrogen remains in the material until temperatures ≥ 700°C Energetic ion bombardment in SF6/O2 or BCl/Ar plasmas also compensates the doping in the InAlN by creation of deep acceptor states. Finally the conductivity of the immediate InAlN surface can be increased by preferential loss of N during BCl3 plasma etching, leading to poor rectifying contact characteristics when the gate metal is deposited on this etched surface. Careful control of plasma chemistry, ion energy and stoichiometry of the etched surface are necessary for acceptable pinch-off characteristics.