N. M. Johnson

Activation of acceptors in Mg‐doped GaN grown by metalorganic chemical vapor deposition

The activation kinetics of acceptors was investigated for heteroepitaxial layers of GaN, doped with Mg. After growth, the samples were exposed to isochronal rapid thermal anneals in the temperature range from 500 to 775 °C. The samples were studied by variable temperature Hall effect measurements and photoluminescence (PL) spectroscopy in the as‐grown condition and after each temperature step. The thermal treatment reduced the resistivity by six orders of magnitude and the p‐type conductivity was found to be dominated by an acceptor with an activation energy of ∼170 meV. This acceptor is attributed to Mg atoms substituting for Ga in the GaN lattice and the activation process is consistent with dissociation of electrically inactive Mg–H complexes. It is shown that the appearance of a blue emission band in the PL spectrum of Mg‐doped GaN does not directly correlate with the increase in p‐type conductivity.

Activation energies of Si donors in GaN

The electronic properties of Si donors in heteroepitaxial layers of GaN were investigated. The n‐type GaN layers were grown by metalorganic chemical vapor deposition and either intentionally doped with Si or unintentionally doped. The samples were evaluated by variable temperature Hall effect measurements and photoluminescence (PL) spectroscopy. For both types of samples the n‐type conductivity was found to be dominated by a donor with an activation energy between 12 and 17 meV. This donor is attributed to Si atoms substituting for Ga in the GaN lattice (SiGa). The range of activation energies is due to different levels of donor concentrations and acceptor compensation in our samples. The assignment of a PL signature to a donor–acceptor pair recombination involving the Si donor level as the initial state of the radiative transition yields the position of the optical Si donor level in the GaN bandgap at ∼Ec–(22±4) meV. A deeper donor level is also present in our GaN material with an activation energy of ∼3...

Electronic traps and Pb centers at the Si/SiO2 interface: Band‐gap energy distribution

Energy distribution of Pb centers (⋅Si≡Si3) and electronic traps (Dit) at the Si/SiO2 interface in metal‐oxide‐silicon (MOS) structures was examined by electric‐field‐controlled electron paramagnetic resonance (EPR) and capacitance‐voltage (C‐V) analysis on the same samples. Chips of (111)‐oriented silicon were dry‐oxidized for maximum Pb and trap density, and metallized with a large MOS capacitor for EPR and adjacent small dots for C‐V measurements. Analysis of C‐V data shows two Dit peaks of amplitude 2×1013 eV−1 cm−2 at Ev+0.26 eV and Ev+0.84 eV. The EPR spin density reflects addition or subtraction of an electron from the singly occupied paramagnetic state and shows transitions of amplitude 1.5×1013 eV−1 cm−2 at Ev+0.31 eV and Ev+0.80 eV. This correlation of electrical and EPR responses and their identical chemical and physical behavior are strong evidence that ⋅Si≡Si3 is a major source of interface electronic traps in the 0.15–0.95 eV region of the Si band gap in unpassivated material.

Growth of gallium nitride by hydride vapor-phase epitaxy

Abstract This paper reviews the growth of GaN thick films by hydride vapor-phase epitaxy (HVPE). Emphasis is placed on recent developments, including the growth of nondegenerate material, characterization of film properties and suitability of such films for epitaxial device overgrowths. Films up to 74 μm thick have been deposited on sapphire substrates with no evidence of thermally induced cracking and a room-temperature Hall mobility of 880 cm 2 /V s at 293 K. Dislocation densities have been found to decrease with film thickness to 5 × 10 7 cm −2 for a 40 μm thick sample. Epitaxial films overgrown on these HVPE GaN buffers, both by organometallic vapor-phase epitaxy and molecular-beam epitaxy, replicate the defect structure of the HVPE buffer, resulting in dislocation densities no higher than the HVPE buffer and lower than are typically observed for nitride epilayers grown on other substrate materials. Efforts towards film/substrate separation will be discussed.

The localization and crystallographic dependence of Si suboxide species at the SiO2/Si interface

X‐ray photoemission spectroscopy has been used to examine the localization and crystallographic dependence of Si+1, Si+2, and Si+3 suboxide states at the SiO2/Si interface for (100)‐ and (111)‐oriented substrates with gate oxide quality thermal oxides. The Si+1 and Si+2 states are localized within 6–10 A of the interface while the Si+3 state extends ∼30 A into the bulk SiO2. The distribution of Si+1 and Si+2 states shows a strong crystallographic dependence with Si+2 dominating on (100) substrates and Si+1 dominating on (111) substrates. This crystallographic dependence is anticipated from consideration of ideal unreconstructed (100) and (111) Si surfaces, suggesting that (1) the Si+1 and Si+2 states are localized immediately within the first monolayer at the interface and (2) the first few monolayers of substrate Si atoms are not significantly displaced from the bulk. The total number of suboxide states observed at the SiO2/Si interface corresponds to 94% and 83% of a monolayer for these (100) and (111) ...

Density of gap states of silicon grain boundaries determined by optical absorption

The results of optical absorption measurements on fine‐grain polycrystalline‐silicon thin films indicate that the singly occupied dangling silicon bond lies 0.65±0.15 eV below the conduction‐band minimum in the grain boundary. The grain boundary band gap is ∼1.0 eV and there is evidence for exponential tailing of the band edges. The optical absorption was determined by photothermal deflection spectroscopy. The dangling silicon bond density has been measured on polycrystalline‐silicon thin films as a function of hydrogen passivation of the grain boundaries and on silicon‐on‐saphhire films. The optical absorption exhibits a defect shoulder which varies as the dangling bond density.

Large band gap bowing of InxGa1−xN alloys

Band gap measurements have been performed on strained InxGa1−xN epilayers with x⩽0.12. The experimental data indicate that the bowing of the band gap is much larger than commonly assumed. We have performed first-principles calculations for the band gap as a function of alloy composition and find that the bowing is strongly composition dependent. At x=0.125 the calculated bowing parameter is b=3.5 eV, in good agreement with the experimental values.

Characteristic electronic defects at the Si‐SiO2 interface

On unannealed, thermally oxidized silicon, electron spin resonance reveals an oriented interface defect which is termed the Pb center and identified as the trivalent silicon defect. Deep level transient spectroscopy (DLTS) reveals two broad characteristic peaks in the interface‐state distribution: one ∼0.3 eV above the silicon valence‐band maximum and a second ∼0.25 eV below the conduction band. Isochronal anneals of oxidized silicon, coated with aluminum, show that the spin density and the densities of the two DLTS peaks have the same annealing kinetics. On large‐area, Al‐gated capacitors the spin density can be modulated with an applied voltage; sweeping the silicon band gap at the interface through the Fermi level reveals that the spin density is approximately constant over the central region of the band gap but decreases near the band edges. The variation of the spin density with gate voltage identifies an amphoteric center with both electronic transitions in the band gap. Both the annealing behavior ...

Local vibrational modes of the Mg–H acceptor complex in GaN

Local vibrational modes (LVMs) are reported for Mg‐doped GaN grown by metalorganic chemical vapor deposition. Hetero‐epitaxial layers of GaN:Mg, either as‐grown, thermally activated, or deuterated, were investigated with low‐temperature, Fourier‐transform infrared absorption spectroscopy. The as‐grown material, which was semi‐insulating, exhibits a LVM at 3125 cm−1. Thermal annealing increases the p‐type conductivity, as established with Hall effect measurements, and proportionally reduces the intensity of this LVM. Deuteration of the activated material creates a LVM at 2321 cm−1. The isotopic shift establishes the presence of hydrogen in the vibrating complex. The new LVMs are assigned to the stretch modes of the Mg–H and Mg–D complexes in GaN, with the vibrational frequencies indicative of a strong N–H bond as recently proposed from total‐energy calculations.

Capture and tunnel emission of electrons by deep levels in ultrathin nitrided oxides on silicon

Electron injection into ultrathin nitrided oxides on silicon reveals both high densities of electronic defects, which readily capture electrons, and efficient tunnel emission of trapped charge. High‐temperature nitridation of thermally grown oxides was verified with Auger depth profiling. In 11–17‐nm‐thick nitrided oxides, the electron trap density is ≥1×1019 cm−3 as determined from saturated charge accumulation, the majority of the traps are energetically situated more than 2 eV below the conduction band as determined by post‐injection anneals up to 300 °C, and the capture cross section is of the order of 10−14 cm2 as estimated from the trapping kinetics. Complete extraction of trapped charge is achieved in the thinnest films (e.g., ≤11 nm thick), and the tunnel emission mechanism is evidenced by the independence of the discharge time on temperature. Implications of the above findings for applications of ultrathin nitrided oxides in very large scale integration and for their low sensitivity to ionizing r...