Acceptor-type States Research Articles

The nature of the DX state in Ge-doped AlGaN

Electrical conductivity in high Al-content AlGaN has been severely limited, presumably due to a DX transition forming an acceptor state and subsequent self-compensation, which imposed an upper limit on the achievable free carrier concentration. To elucidate this idea, this paper examines Ge doping as a function of Al-content in AlGaN and finds a different behavior: for Al compositions below 40%, Ge behaved as a shallow donor with an ionization energy below 20 meV, while for Al compositions above 40%, above DX transition, it emerged as a deep donor. The ionization energy of this deep state increased with increasing Al content and reached 150 meV for 60% AlGaN. Around the DX transition, a continuous change from the shallow to deep donor was observed. In contrast to the density functional theory predictions, acceptor-type states corresponding to a DX-type transition were not observed. This finding may have profound technological consequences for the development of AlGaN- and AlN-based devices as it offers a feasible pathway to high n-conductivity in these compounds.

Photon Recycling by Energy Transfer in Piezochemically Synthesized and Close-Packed Methylammonium Lead Halide Perovskites

Photon recycling by multiple reabsorption–emission is responsible for the long-range energy transport in large crystals and thick films of lead halide perovskites, resulting in red-shifted and delayed emission. Apart from such a radiative process, nonradiative energy transfer influences photon recycling in perovskites with close-packed donor–acceptor-type states. In this study, we report the role of nonradiative energy transfer on photon recycling in piezochemically synthesized and close-packed pure and mixed halide methylammonium lead perovskites. Here, the pressure applied to precursors of perovskites helps us to synthesize and close-pack perovskite crystallites into pellets. Nonetheless, interestingly, we find that the applied pressure redistributes the emission maxima or band-gap of these perovskites. The temporally and spectrally resolved photoluminescence from the mixed halide sample unveils nonradiative energy transfer from a higher (bromide) to a lower (iodide) band-gap domain, where the rate of relaxation of the bromide domain is higher than that of the pure bromide perovskite. These results help us to confirm the role of nonradiative energy transfer on photon recycling in perovskites.

Influence of deep levels on capacitance-voltage characteristics of AlGaN/GaN heterostructures

We studied the influence of deep levels in GaN buffer and AlGaN barrier layer on capacitance–voltage characteristics of the AlGaN/GaN structure. Deep level states were assumed to be both donor and acceptor type and were modeled with discrete peak distribution in energy with certain energy distance from the conduction-band minimum for acceptor-type states or valence-band maximum for donor states. For all the cases studied, the presence of the deep levels in the structure influenced mainly the capacitance plateau of the curves and caused the appearance of the capacitance valley. In addition, the deep levels in AlGaN shifted the capacitance curves to more negative voltages, and the deep levels in GaN changed the slope of capacitance decrease after two-dimensional electron-gas depletion.

Effects of Si deposition on the properties of Ga-rich (4×6) GaAs (001) surfaces

In order to achieve better surface passivation on technologically important (001) GaAs surfaces, microscopic and macroscopic properties of the clean (001) surface with Ga-rich (4×6) reconstruction, and effects of monolayer level Si deposition on this surface, were investigated. According to scanning tunneling spectroscopy and contactless capacitance–voltage (C–V) measurements, the Ga-rich genuine (G-)(4×6) surface had high-density acceptor-type states above EC−0.5 eV. However, the surface exhibited a much lower and wider surface state density (NSS) distribution as well as a much stronger band-edge photoluminescence (PL) intensity than the conventional As-rich surfaces. Irradiation of Si molecular beam on the (4×6) surface resulted in epitaxial growth of Si. This completely removed the acceptor-type surface states from the energy gap, and further enhanced the PL intensity. The result indicates that the Ga-rich G-(4×6) surface is more ordered and more stable against processing than conventional As-rich surfaces. Metal–insulator–semiconductor capacitors fabricated by further depositing a thick SiO2 layer indicated that the Fermi level was completely unpinned over the entire band gap.

Si(110)+Ni system: Structural, vibrational and electronic properties

Vibrational properties (Debye temperature and thermal expansion coefficients) as well as electronic ones (work function, electron affinity, spectra of surface electron states in the gap) were investigated on the clean “16x2” and Ni-induced (4x5), (5x1) and (2x1) structures of the Si(110) face. Atomic structure was monitored with LEED. Special attention was given to (4x5)Ni, (2x1)Ni and (5x1)Ni phases which showed reversible structural phase transitions. All parameters mentioned depend on the surface atomic structure. The elastic forces increase in strength and become more anharmonic upon the diminution of the elementary cell's size. Harmonic and anharmonic force constants have been obtained. The surface state spectra have a gap between donor- and acceptor-type states which varies from 0.2 to 0.4 eV depending upon structural phase transitions. The band bending of all the surface lattices indicates the presence of a depletion layer. Commonly speaking all the characteristics of long period (4x5)Ni and (5x1)Ni structures have been found to be similar but to differ remarkably from the (2x1)Ni one. Surface dimers are suggested to be the structural elements of all the surface phases. Dimers are formed during mutual saturation of dangling bonds on the Si(110) face.

Electrical properties of Si−SiO 2 structures treated in helium plasma

Experimental data are presented for the influence of helium plasma on the electrical properties of Si−SiO 2 structures with dry thermal oxide (d=16−72 nm). It is found that for strongly damaged structures low temperature helium plasma introduces acceptor-type interface states near the conduction band edge and has an annealing effect on the deep acceptor-type states. It has also shown that the initial characteristics of the Si−SiO 2 structures and the gas used are critical for the plasma-forming of both the interface and inversion channel properties.

Effects of electron-beam radiation on MOS structures as influenced by the silicon dopant

The flatband shift induced by 25-keV electron-beam radiation in unbiased MOS structures depends in great part on the type and concentration of the dopant in the silicon. The flatband shifts for n-type samples are less pronounced than those for p-type samples, and in general are in the opposite direction. The radiation-induced interface states, however, are independent of the dopant. The difference between the flatband shifts is shown to be best explained by a model based on the assumption that the interface states above midgap are acceptor-type states, and those below are donor-type states. A quantitative analysis made with this model indicates that the radiation-induced fixed oxide charge also is independent of the dopant. The dopant dependence of the flatband shifts can thus be attributed entirely to the charging effects of the interface states. By use of the same model, the apparent discrepancy between the shifts in the threshold voltage and in the flatband voltage of a MOSFET can also be readily resolved. A simple midgap method is implemented to obtain the fixed oxide charge without using interface-state information; the results agree well with those obtained by the more conventional flatband method, which must take the interface-state effects into account.

Relation between interface states and temperature behavior of the barrier height of silver contacts on clean cleaved n-type silicon

The properties of silver-silicon interfaces formed by cleaving n-type silicon in ultra high vacuum (UHV) in a stream of evaporating silver atoms were studied. The barrier heights of these contacts were measured at different temperatures by using C- V techniques. All measurements were performed in UHV. The dependence of the barrier height upon temperature did not follow the temperature dependence of the Si band gap as it is usually found. The measured temperature behavior depended on the roughness of the Si surface. The temperature behavior can be explained by assuming a specific band structure of the interface states. For Ag contacts on atomically smooth n-type Si, the interface states were found to be arranged in two bands, one band 4 × 10 −3 eV wide with acceptor type states 0.18 eV below the intrinsic level E i and a density of 10 17 states/cm 2 eV, and the other 1 eV wide with donor type states with its upper edge 0.28 eV below E i, and a density of 4 × 10 14 states/cm 2eV.