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Observation of GaN-based islands surrounded by V-defects in the barrier layer of green LED is reported for InGaN MQWs deposited under no hydrogen or at growth temperatures of less than 800°C. Nanoscale mechanical properties of the areas enclosed and outside of the ring defects does not show any appreciable variation as measured by UFM. Chemical etching of the MQW structure in addition to cross-sectional TEM analysis ruled out the possibility of growth of inversion domains of N-polar GaN in a Ga-polar GaN matrix.
We have found that near-band-edge cathodoluminescence (CL) emission decreases with time for some nominally undoped GaN samples. The rate of intensity decrease depends on the incident beam current. It also depends on the size of the area being scanned, which is determined by the magnification used, although the electron beam voltage and current are held constant and similar regions of GaN are being examined. Faster decrease with time occurs with higher beam currents and higher magnifications. The reduced luminescence efficiency persists over at least 24 hour beam-off periods. The dependence of CL intensity on beam current and on scanned area may be a result of different levels of local charging, and structural modifications caused by this charging, for different beam current and magnification settings.
Long term control of the substrate temperature in production scale MOVPE reactors is the most significant issue effecting yields in highly temperature sensitive epitaxial growth processes. Recent advances in non-contact emissivity compensated pyrometry wafer temperature measurements have allowed the development of a novel multi-wafer (6×2”) rotating disc MOVPE reactor with real time substrate temperature control. With this system, the substrate temperature is a directly controlled process variable, in contrast to some conventional MOVPE systems which use thermocouples for process temperature control. In addition to controlling the absolute temperature of the substrates, the temperature uniformity across the substrates is also controlled by pyrometry. This provides for a uniform temperature (+/- 1.5 °C) across the substrates independent of the flow conditions within the reactor. Thermal uniformity is also automatically maintained during temperature ramping. The highly temperature sensitive quaternary InGaAsP is used as an epitaxial metric for this novel control system, to demonstrate the advantages of pyrometry controlled substrate temperature. These advantages include: excellent long term substrate temperature reproducibility; invariance of substrate temperature to substrate doping level; the ability to transfer processes from one type of wafer carrier or reactor to another with minimal adjustment.
Near-band-edge cathodoluminescence emission for some nominally undoped GaN samples decreases with time during electron bombardment in a scanning electron microscope. The rate of decrease depends on the incident beam voltage and current and on the size of the area being scanned, which is determined by the magnification used for the scanning electron microscope. Faster intensity decreases occur with lower beam voltages (10kV versus 20kV), higher beam currents (600nA or 200nA versus 35nA) and smaller scanned areas (120K magnification versus 15K magnification). For larger scanned areas, the maximum luminescence intensity occurs 10's or 100's of seconds after the start of electron bombardment, and the intensity then decreases with further bombardment. In some cases the size of the degraded region is larger than the bombarded area. These observations suggest that the degradation is caused by electromigration of impurities or other charged defects within the GaN resulting from local charging by electron bombardment.
The hypothesis of an Altaic language family, comprising the Turkic, Mongolic,
Tungusic, Korean and, in most recent versions, Japanese languages continues to be
a viable linguistic proposal, despite various published claims that it is no longer
accepted. A strong body of research continues to appear, developing and refining the
hypothesis, along with publications that argue against a demonstrated relationship
among these languages. This paper shows that many of the arguments against a
genetic relationship fail to address the criteria demanded in modern historical
linguistics, while many of the responses from proponents of the Altaic theory have
failed to address the criticisms raised. We hope that arguments focusing on the real
issues of phonological correspondences and morphological systems will shed greater
light on the relationship among these languages.
In order to overcome some of the inherent difficulties with applying conventional pyrometry to production scale rotating disc MOVPE reactors, the in-situ technique of emissivity compensated pyrometry has been applied. Due to the extreme temperature sensitivity of epitaxial processes involving InxGa1−xAs1−yPy, this material system provides the optimum proving grounds for this technology. With this temperature measurement technique, we have developed a method for measuring the temperature of the substrate during MOVPE growth in high speed rotating disc reactors that does not require any modification to the reactor hardware. Real world testing, during the growth of complex epitaxial structures involving the highly temperature sensitive InxGa1−xAs1−yPy quaternary, indicates a very strong correlation between the measured temperature of the substrate during growth, and actual epitaxial wafer measurements after growth. This temperature measurement system can be used to maintain the wavelength of the InxGa1−xAs1−yPy quaternary materials within a range of 2 nm.
Ohmic contacts to Mg-doped p-GaN grown by MOCVD  are studied using a circular transmission line model (TLM) to avoid the need for isolation. For samples which use a p-dopant activation anneal before metallization, no appreciable difference in the specific contact resistance, rc, as a function of different capping options is observed. However, a lower rc is obtained when no pre-metallization anneal is employed, and the post-metallization anneal simultaneously activates the p-dopant and anneals the contact. This trend is shown for Pt/Au, Pt, Pd/Pt/Au, and Ni/Au contacts to p-GaN. The rc 's for these metal contacts are in the range of 1.4–7.6 × 10-3 ohm-cm2 at room temperature at a bias of 10mA. No particular metallization formula clearly yields a consistently superior contact. Instead, the temperature of the contact has the strongest influence.
Detailed studies of the electrical properties of the Pt/Au contacts reveal that the I-V linearity improves significantly with increasing temperature. At room temperature, a slightly rectified I-V characteristic curve is obtained, while at 200°C and above, the I-V curve is linear. For all the p-GaN samples, it is also found that the sheet resistance decreases by an order of magnitude with increasing temperature from 25°C to 350°C. The specific contact resistance is also found to decrease by nearly an order of magnitude for a temperature increase of the same range. A minimum rc of 4.2 × 10-4 ohm-cm2 was obtained at a temperature of 350°C for a Pt/Au contact. This result is the lowest reported rc for ohmic contacts to p-GaN.
Our purpose is simple: to apply a rudimentary kind of logical analysis to the problem, which sometimes arises in linguistic typology and in comparative linguistics, of whether there are any languages with fewer than two vowels. The principal reason why this question has occupied the attention of linguists is probably the fact that Proto-Indo-European is sometimes said to have had only one vowel phoneme (see especially Hjelmslev (1936–7), Borgstrøm (1949, 1954) and Lehmann (1952), following Saussure (1879). Yet this was precisely one of the features of PIE reconstruction that Jakobson selected for attack on typological grounds, reasoning that ‘a conflict between the reconstructed state of a language and the general laws which typology discovers makes the reconstruction questionable’ (1958: 23).
Electron cyclotron resonance (ECR) plasma etching characteristics of gallium nitride (GaN) are investigated using low pressure (4-10 mTorr) SiCl4/Ar and Cl2/H2/Ar ECR discharges. The purpose of this effort is to develop a dry etching process for making laser mirrors on GaN and to examine dry etching processes of GaN that do not require hydrogen, which is known to cause carrier compensation in GaN. The etch rate is found to increase near-linearly with increasing DC bias, and a minimum DC bias of 100V is required to initiate etching in SiCl4/Ar. We have also found that the material quality significantly affects the etch rate. The latter decreases with x-ray rocking curve half-width and increases with defect density. A reasonable etch rate of 660Ǻ/min and good surface morphologies obtained in SiCl/Ar ECR etching make this process suitable for gate recess of an FET. An etch rate of 5270Ǻ/min has been achieved in Cl2/H2/Ar plasmas. This is the highest reported etch rate of GaN so far. The smooth and vertical etch sidewalls (etch to mask selectivity of 16 is obtained) make this process promising for dry-etched laser mirrors on GaN.
We report on ohmic contacts to Si-implanted and un-implanted n-type GaN on sapphire. A ring shaped contact design avoids the need to isolate the contact structures by additional implantation or etching. Metal layers of Al and Ti/Al were investigated. On un-implanted GaN, post metalization annealing was performed in an RTA for 30 seconds in N2 at temperatures of 700, 800, and 900°C, A minimum specific contact resistance (rc) of 1.4×10−5 Ω-cm2 was measured for Ti/Al at an annealing temperature of 800°C. Although these values are reasonably low, variations of 95% in specific contact resistance were measured within a 500 µm distance on the wafer. These results are most likely caused by the presence of compensating hydrogen. Specific contact resistance variation was reduced from 95% to 10% by annealing at 900°C prior to metalization. On Si-implanted GaN, un-annealed ohmic contacts were formed with Ti/Al metalization. The implant activation anneal of 1120°C generates nitrogen vacancies that leave the surface heavily n-type, which makes un-annealed ohmic contacts with low contact resistivity possible.
The III-V nitride-containing semiconductors InN, GaN, and AIN and their ternary alloys are the focus of extensive research for application to visible light emitters and as the basis for high temperature electronics. Recent advances in ion implantation doping of GaN and studies of the effect of rapid thermal annealing up to 1100 °C are making new device structures possible. Both p- and n-type implantation doping of GaN has been achieved using Mg co-implanted with P for p-type and Si-implantation for n-type. Electrical activation was achieved by rapid thermal anneals in excess of 1000 °C. Atomic force microscopy studies of the surface of GaN after a series of anneals from 750 to 1100 °C shows that the surface morphology gets smoother following anneals in Ar or N2. The photoluminescence of the annealed samples also shows enhanced bandedge emission for both annealing ambients. For the deep level emission near 2.2 eV, the sample annealed in N2 shows slightly reduced emission while the sample annealed in Ar shows increased emission. These annealing results suggest a combination of defect interactions occur during the high temperature processing.
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