Herbert Goronkin

Submicron spin valve magnetoresistive random access memory cell

Spin valve magnetoresistive random access memory cells with widths varying from 1.5 to 0.25 μm and an aspect ratio of length/width more than 10 were fabricated and tested. In general, the switching field of the free magnetic layer was found to be inversely proportional to the width of the cell. Adequate pinning was shown for cell width down to 0.75 μm. For 0.5 and 0.25 μm wide cells, the switching field of the free magnetic layer is comparable to the pinning field of the other magnetic layer. So the pinned magnetic layer rotates with the free magnetic layer. The giant magnetoresistive ratio of the cell drops dramatically. Potentially, this may be a fundamental problem for this memory mode. Solutions are proposed.

Switching characteristics and magnetization vortices of thin-film cobalt in nanometer-scale patterned arrays

The switching characteristics and magnetization vortices of 15 nm thick cobalt structures patterned to different widths of 100, 200, and 600 nm were investigated. The effects of linewidth and aspect ratio (length/width) were systematically studied using an alternating gradient magnetometer (AGM), an atomic force microscope/magnetic force microscope (MFM) and superconducting quantum interference device. The AGM and MFM show that trapped magnetization vortices appear in structures with low aspect ratios (length/width)=1.5, 2, but not in structures with high aspect ratio (length/width)=3,4. It is found that the magnetization vortices of these patterned elements are strongly dependent on the width of the element with narrower linewidth more clearly showing the presence of magnetization vortices.

High density nonvolatile magnetoresistive RAM

Non-volatile memory cells based on ferromagnetically coupled giant magneto-resistive (GMR) material were patterned into submicron feature sizes. Switching characteristics of such cells allow for bipolar signal reading which is twice the intrinsic magnetoresistance change of the material. Excellent thermal stability is reported for deep submicron memory cells.

The effect of backgating on the design and performance of GaAs digital integrated circuits

This paper presents an analysis of the effect of backgating on GaAs digital integrated circuits. Examples of backgating by negatively biased ohmic contacts located at various distances from a single transistor are given. The effect on individual devices within a generalized logic gate is considered and the resulting circuit performance dependence on duty cycle and power supply levels is illustrated. The relative susceptibility of various circuit configurations, including enhancement, and depletion mode technologies is discussed. Finally, mask layout considerations are presented.

Backgating and light sensitivity in ion-implanted GaAs integrated circuits

Ion-implanted GaAs integrated circuits have been characterized under dark and illuminated conditions to determine the sensitivity of ac parameters to light. Auxiliary experiments were performed on discrete IC elements using white and monochromatic illumination and backgating. Discrete MESFETs were characterized under the various conditions to determine corresponding variations in the magnitudes of Sehottky-barrier height and parasitic channel resistances. We have determined that light sensitivity and backgating are strongly interrelated and arise from a depletion layer at the substrate active-layer interface. We will describe the effect of trap filling on channel resistance and the resulting speed-power variations arising from illumination and backgating.

Ohmic contact penetration and encroachment in GaAs/AlGaAs and GaAs FETs

Hypoeutectic AuGe/Ni ohmic contacts on GaAs and GaAs/AlGaAs were characterized by SEM, TEM, and energy dispersive X-ray analysis. The two main components of the contact were confirmed to be NiGeAs islands distributed in an Au-Ga alloy. The NiGeAs islands were larger and more evenly distributed on GaAs MESFETs than on GaAs/AlGaAs MODFETs. Vertical penetration was impeded but not halted by AlGaAs. Since the contacts did not melt during the alloying cycle, there was no lateral encroachment observed. >

Selective formation and alignment of InAs quantum dots over mesa stripes along the [011] and [001] directions on GaAs (100) substrates

We have studied the selective formation of InAs self-organized quantum dots on top of [001]- and [011]-oriented mesa stripes on patterned GaAs (100) substrates. The GaAs stripes are also grown by selective area epitaxy. The dot density and spatial distribution depend on both the stripe orientation and the width of the (100) top facet of the stripe. The density is higher for stripes aligned in the [001] direction, and lower for those aligned in the [011] direction, respectively, when compared to that obtained on a planar substrate under the same growth conditions. In addition, the dot uniformity is improved by reducing the top facet width below 200 nm in the growth of the mesa stripes, and well-aligned rows of dots are obtained for sub-100-nm widths.We have studied the selective formation of InAs self-organized quantum dots on top of [001]- and [011]-oriented mesa stripes on patterned GaAs (100) substrates. The GaAs stripes are also grown by selective area epitaxy. The dot density and spatial distribution depend on both the stripe orientation and the width of the (100) top facet of the stripe. The density is higher for stripes aligned in the [001] direction, and lower for those aligned in the [011] direction, respectively, when compared to that obtained on a planar substrate under the same growth conditions. In addition, the dot uniformity is improved by reducing the top facet width below 200 nm in the growth of the mesa stripes, and well-aligned rows of dots are obtained for sub-100-nm widths.

Transition from longitudinal‐optical phonon scattering to surface‐optical phonon scattering in polar semiconductor superlattices

Dielectric continuum models of optical‐phonon modes predict an enhancement in the strength of the surface‐optical (SO) modes in double‐barrier heterostructures as the heterojunction‐to‐heterojunction separation is reduced. There is currently no consensus on the nature of the electron‐SO‐phonon coupling interaction. In this work, the ratio of electron scattering by the SO‐phonon modes to that by the confined longitudinal‐optical (LO) phonon modes is calculated for a GaAs/AlAs short‐period superlattice based on the assumption that the electron‐SO‐phonon interaction may be described by a scalar potential. The scaling of the ratio of electron‐SO‐phonon scattering to electron‐LO‐phonon scattering as a function of the superlattice period provides a sensitive test of the appropriateness of the scalar‐potential model.

Experimental evaluation of low‐frequency oscillations in undoped GaAs to probe deep level parameters

A new method for characterizing undoped, semi‐insulating GaAs is presented. The low‐frequency current oscillations on undoped liquid encapsulated Czochralski GaAs substrates are used to determine deep level energies, cross sections, and concentrations. A Fourier transform is used to resolve the various frequency components of the signal. The frequency is seen to vary almost four orders of magnitude in a 50‐K temperature range. Plotting the temperature dependence of each frequency component in an Arrhenius plot gives activation energies and cross sections corresponding to the deep levels responsible for the oscillations. It is seen that two closely spaced levels cause the low‐frequency oscillations. In addition, this technique allows the determination of trap parameters as a function of electric field at values much less than the critical field for intervalley transfer in GaAs.

SELECTIVE POSITIONING OF INAS SELF-ORGANIZED QUANTUM DOTS ON SUB-250 NM GAAS FACETS

We have selectively grown GaAs mesas with (100) top facets that range from several μm to less than 250 nm in width on an oxide-patterned GaAs substrate, and InAs self-organized quantum dots (SOQDs) on top of the facets. For a given amount of InAs deposited, the dot density varies with the facet width, and is higher than the density on a planar, nonpatterned substrate. The SOQDs also tend to form at the facet edges. These observations are indicative of a strong surface diffusion effect of In-containing species adsorbed on the various crystalline facets of the GaAs mesas.