Eric S. Harmon

Macroelectronics: Perspectives on Technology and Applications

Flexible, large area electronics - macroelectronics - using amorphous silicon, low-temperature polysilicon, or various organic and inorganic nanocrystalline semiconductor materials is beginning to show great promise. While much of the activity in macroelectronics has been display-centric, a number of applications where macroelectronics is needed to enable solutions that are otherwise not feasible are beginning to attract technical and/or commercial interest. In this paper, we discuss the application drivers and the technology needs and device performance requirements to enable high performance applications to include RF systems.

Carrier lifetime versus anneal in low temperature growth GaAs

The photoexcited carrier lifetimes in ex situ‐annealed low temperature growth GaAs are measured with a femtosecond transient absorption experiment. The study encompassed two low temperature growth GaAs films with approximately 0.3% and 0.9% excess arsenic incorporated during growth. The observed lifetimes are found to be a function of the spacing of arsenic precipitates formed during the 30 s anneals to temperatures between 650 and 1000 °C. The carrier lifetime for unannealed films was found to be less than ∼200 fs. The carrier lifetimes increased from ∼2 to ∼10 ps as the average precipitate spacing was increased from ∼400 to ∼900 A. These results are in sharp contrast to recent reports of subpicosecond lifetimes in similar GaAs annealed at 600 °C.

The role of point defects and arsenic precipitates in carrier trapping and recombination in low‐temperature grown GaAs

GaAs epilayers were grown with a wide range of excess arsenic concentrations and subjected to various anneals to study the role of the point defects and arsenic precipitates in carrier trapping and recombination. Prior to anneal, the point defects rapidly trap photogenerated electrons and holes—usually on subpicosecond time scales. However, full electron‐hole recombination occurs on a significantly longer time scale. After anneal, the full electron‐hole recombination lifetime appears to be greatly reduced, indicating that the arsenic precipitates play a significant role.

Properties of InAs metal-oxide-semiconductor structures with atomic-layer-deposited Al2O3 Dielectric

InAs is very attractive as a channel material for high-speed metal-oxide-semiconductor (MOS) field-effect transistors due to its very high electron mobility and saturation velocity. We investigated the processing conditions and the interface properties of an InAs metal-oxide-semiconductor structure with Al2O3 dielectric deposited by atomic-layer deposition. The MOS capacitor I-V and C-V characteristics were studied and discussed. Simple field-effect transistors fabricated on an InAs bulk material without source/drain implantation were measured and analyzed.

Molecular Beam Epitaxy of Nonstoichiometric Semiconductors and Multiphase Material Systems

Abstract When arsenides are grown by molecular beam epitaxy at low substrate temperatures, as much as 2% excess arsenic can be incorporated into the epilayer. This excess arsenic is in the form of antisites, but there is also a substantial concentration of gallium vacancies. With anneal, there is a significant decrease in the arsenic antisite and gallium vancancy concentrations as the excess arsenic precipitates. With further anneal, the arsenic precipitates coarsen. This combination of low substrate temperature molecular beam epitaxy and a subsequent anneal results in a broad spectrum of materials, from highly defected epilayers to a two-phase system of semimetallic arsenic precipitates in an arsenide semiconductor matrix. These materials exhibit some very interesting and useful electrical and optical properties.

Ultrafast‐lifetime quantum wells with sharp exciton spectra

Sharp quantum‐confined excitons in semi‐insulating low‐temperature‐growth AlAs/GaAs quantum wells with 15 ps carrier lifetimes are demonstrated. High‐quality well‐barrier interfaces can be grown at low substrate temperatures and annealed up to temperatures of 700 °C, beyond which interface mixing broadens the exciton transitions. Electroabsorption from the quantum‐confined Stark effect in as‐grown modulators approaches 10 000 cm−1, which is comparable to traditional high‐temperature growth quantum wells. The low‐temperature growth quantum well structures eliminate the need for postgrowth processing, such as ion implantation for photorefractive quantum wells, ultrafast saturable absorption, or electro‐optic sampling applications.

Use of nonstoichiometry to form GaAs tunnel junctions

A tunnel diode was formed from GaAs containing excess arsenic incorporated by molecular beam epitaxy at reduced substrate temperatures. The incorporation of excess arsenic during growth results in a more efficient incorporation of silicon on donor sites and beryllium on acceptor sites. The better dopant incorporation, along with trap assisted tunneling through deep levels associated with the excess arsenic, results in a tunnel junction with record peak current density of over 1800 A/cm2, zero-bias specific resistance of under 1×10−4 Ω cm, and a room-temperature peak-to-valley current ratio of 28.

Effective band‐gap shrinkage in GaAs

Electrical measurements of the equilibrium np product (n2ie) in heavily doped n‐ and p‐GaAs were performed. The n2ieD product (where D is the diffusivity) was measured by fitting the collector current‐voltage characteristic of a homojunction bipolar transistor to an ideal diode equation modified to account for transport in thin base transistors. The n2ie product was then extracted from n2ieD by utilizing diffusivity results obtained with the zero‐field time‐of‐flight technique. Our results show significant effective band‐gap shrinkage in heavily doped p‐GaAs, and very little effective band‐gap shrinkage in heavily doped n‐GaAs. At extremely heavy dopings, an effective band‐gap widening is observed for both n‐ and p‐GaAs and is attributed to the effects of degeneracy.

Minority‐carrier mobility enhancement in p+ InGaAs lattice matched to InP

Minority electron mobilities in p+‐In0.53Ga0.47As have been measured with the zero field time‐of‐flight technique. The room‐temperature (297 K) minority electron mobilities for p+‐In0.53Ga0.47As doped 0.9 and 3.1×1019 cm−3 are found to be 2900 and 3300 cm2 V−1 s−1, respectively. These are the first measurements to demonstrate enhancement in minority‐carrier mobility as doping is increased for heavily doped In0.53Ga0.47As. This enhancement in mobility as doping is increased is similar to that observed in p+‐GaAs, which has been attributed to reductions in plasmon and carrier–carrier scattering between minority electrons and majority holes.

Precipitation in Fe‐ or Ni‐implanted and annealed GaAs

We report the formation of metal/semiconductor composites by ion implantation of Fe and Ni into GaAs and a subsequent anneal to nucleate clusters. Electron diffraction experiments and high resolution transmission electron microscopy images indicate that these precipitates are probably hexagonal and metallic Fe3GaAs or Ni3GaAs with orientation relationship to GaAs of (1010)pp∥(422)m, (0002)pp∥(111)m, and [1210]pp∥[011]m. Correlation of the electrical and structural properties of the samples annealed at different temperatures shows that the buried Schottky‐barrier model has general applicability.