T. N. Theis

Effect of local alloy disorder on emission kinetics of deep donors (DX centers) in AlxGa1−xAs of low Al content

We report measurements by deep level transient spectroscopy of electron emission from the deep donor level (DX center) in Si‐doped GaAs and AlxGa1−xAs of very low Al content. For the first time, discrete emission rates corresponding to different local configurations of Ga and Al atoms around the Si donor are resolved. The large change in emission kinetics previously observed between GaAs and AlxGa1−xAs (x≥0.14) is thus shown to arise from the local alloy disorder which is absent in GaAs.

Noise spectroscopy of deep level (DX) centers in GaAs-AlxGa1−xAs heterostructures

We have measured the generation‐recombination noise from the donor‐related DX centers in current biased GaAs/AlxGa1−xAs heterostructures from 1 Hz to 25 kHz and from 77 to 330 K. A significant noise contribution from these traps is observed even at Al mole fractions below 0.2, where the trap level is resonant with the conduction band. The activated behavior of the noise spectrum from this resonant level is very similar to that observed at higher Al mole fractions, when the level lies deep in the fundamental gap. This result can be predicted, based on the recently elucidated relationship of the trap level to the band structure of AlxGa1−xAs. In accordance with other experimental results, the noise spectra demonstrate that the emission and capture kinetics of the level are unperturbed by its resonance with the conduction band. We briefly discuss some implications of these results for heterostructure transistor design.

Surface plasmon polariton contributions to strokes emission from molecular monolayers on periodic Ag surfaces

Abstract Surface enhanced Raman scattering from molecular monolayers adsorbed on a holographic grating and covered by Ag has been studied. The coupling of surface plasmon polaritons to light through the grating produces emission similar to the continuum observed in other surface Raman experiments. The angular dependence of the surface Raman scattering shows that enhanced Raman cross-sections can arise from scattering of surface plasmon polaritons into light by the localized molecular vibrations.

Diffraction‐grating‐enhanced light emission from tunnel junctions

We report the observation of highly polarized, angle‐tunable visible light emission from Al‐AlOx‐Ag tunnel junctions fabricated on holographic gratings. From the angular dependence of the emission frequency we identify the electromagnetic mode responsible for the emission. The mode is surface plasmonlike with a dispersion that closely follows the light line. This is in contrast to the much lower phase velocity ’’junction’’ mode believed responsible for the light emission on surface‐roughened tunnel junctions. We argue that both modes should be considered in describing the emission from these devices.

Hot‐electron capture to DX centers in AlxGa1−xAs at low Al mole fractions (x<0.2)

We report quantitative studies of hot‐electron trapping by DX centers in short channel GaAs/n‐AlxGa1−x As field‐effect transistors. A remarkable result is that persistent hot‐electron capture occurs even at very low values of the Al mole fraction, x≲0.2, where thermal capture is not observed. Thermal emission studies confirm that the trap state is associated with the DX center. Thus, for x≲0.2 the trap state is metastable, but can be persistently populated by heating the free electrons with an electric field. This and other features of the capture process support a large lattice relaxation model of the center.

Bistability of the DX center in GaAs and Al x Ga 1-x As, and experimental tests for negative U of the DX level

We summarize a large body of experimental and theoretical work, especially in Si-doped GaAs and AlxGa1-xAs, regarding the bistability of theDX center. There is good evidence that theDX center is just the simple donor, and that each donor can exist in either of two distinct lattice configurations, each with its own spectrum of bound electronic states. Generally, the substitutional configuration binds electrons in shallow hydrogenic states, but many observations also indicate a deep (highly localized) state ofA1 symmetry. These states are to be distinguished from bound states of a lattice-distorted configuration, the lowest-lying of which is the deepDX level. The occupation of theDX level in thermal equilibrium with the states of the conduction band can be reasonably well modeled by assuming thatDX is either a one-electron or a two-electron state, and we discuss the reasons for this ambiguity. However, we then show that such thermal equilibrium results are consistent with thermal capture and emission kineticsonly if we assume thatDX is a two-electron state. Our results thus support the model of Chadi and Chang in which the distorted configuration is stabilized by capture of two electrons. In other words, the defect exhibits negative effective correlation energy (negativeU).

Origin of ‘‘residual’’ persistent photoconductivity in selectively doped GaAs/AlxGa1−xAs heterojunctions

We present experimental results which clearly separate the various physical mechanisms which cause persistent photoconductivity in GaAs/AlxGa1−xAs heterojunctions. For high Al mole fraction the major contribution is from the donor‐related DX center. This contribution is eliminated by reducing the Al mole fraction x, but we observe a ‘‘residual’’ effect for x≲0.2. We show that this is due to the persistent photovoltage developed between channel and semi‐insulating substrate. Charge trapping in the epitaxial GaAs buffer layer contributes negligibly, contrary to the assumptions of other workers. This is demonstrated by fabricating modulation‐doped field‐effect transistors of low Al mole fraction on conductive substrates. In these devices persistent photoconductivity is eliminated as long as the substrate (back gate) potential is fixed with respect to the channel.

Effect of local alloy disorder on the emission kinetics of deep donors ( DX centers) in Al x Ga 1-x As

In this paper we summarize our recent studies of the effects of local alloy disorder on the properties ofDX levels. A single emission rate is observed in GaAs where all Si-donors have identical local environments. In contrast, three discrete emission rates are observed in dilute AlGaAs alloys, suggesting that the group IV donor moves towards the interstitial site, thereby “selecting” three of the twelve surrounding group III atoms. We present evidence for an ordering of theDX levels consistent with Morgan’s model of a deepening potential well for theDX level as Al atoms are subsequently substituted for Ga atoms near the relaxed donor. These conclusions are consistent with earlier calculations of Chadi and Chang.

Long and short range effects in surface enhanced Raman scattering

Surface plasmon polariton enhanced Raman scattering, from molecular monolayers adsorbed onto continuous Ag surfaces at 300 K and low temperatures, has been studied. The observed spectra, at the multilayer level, are similar to the Raman spectra of the condensed bulk. The first layer scattering shows changes in both the scattering intensity and the spectral features similar to those seen in surface enhanced Raman experiments using substrates prepared in other ways.

Exponential thermal emission transients from DX centers in heavily Si‐doped GaAs

The kinetics for the thermal emission of electrons from DX levels are shown to be exponential in heavily Si‐doped GaAs. Isothermal voltage transients, obtained at constant capacitance, show a perfect exponential behavior. In contrast, a clear deviation from a single exponential function is observed when the transients are recorded at constant voltage, due to the nonuniform doping profile in these structures. The exponential emission kinetics seen in GaAs support the proposal that nonexponential emission kinetics observed at constant capacitance in AlxGa1−xAs are due to different emission rates for DX levels having different local atomic configurations in the alloy.