Paul E. Gregory

The oxidation of Cs—uv photoemission studies

The oxidation of cesium has been studied using ultraviolet photoemission spectroscopy. Upon exposure of a fresh cesium film to oxygen, a very narrow peak appears in the energy distribution curves (EDC’s) about 2.6 eV below the Fermi level Ef and grows with increasing exposure. This peak is associated with oxygen ions dissolved in the cesium metal below the surface. After 3×10−5 Torr sec of exposure, additional structure begins to appear. This is associated with the precipitation of cesium oxides. The structure associated with the oxides changes with increasing oxygen exposure indicating the appearance of different oxides. The oxide penetrates appreciably to the surface only after strong oxide buildup has taken place beneath the surface. A sharp minimum of 0.7 eV is found in the work function over a relatively small range of oxygen exposure.

Valence band studies of clean and oxygen exposed GaAs(100) surfaces

We found, by correlating band bending, ultraviolet photoemission spectroscopy, and partial yield spectroscopy measurements, that Fermi level pinning at midgap of n-type GaAs(110) is caused by extrinsic states. The exact nature of these states is not yet clear, but the surfaces with Fermi level pinning were strained as evidence by a smeared valence band emission. This smearing was removed by as little as one oxygen per 104 to 105 surface atoms. This implies that the oxygen has very long range effects in causing spontanesous but small rearrangement of the surface lattice and removing surface strains. When about 5% of a monolayer of oxygen is adsorbed, a major change in the electronic structure takes place. Again, the oxygen coverage is very small, which suggests long range effects now leading to a fairly large rearrrangement of the surface lattice. Finally, from comparing the oxygen induced emission for exposures greater than 107 L O2, with the spectra from gas photoemission measurements on molecular oxygen, we suggest that the oxygen is chemisorbed as a molecule on the (110) surface of GaAs.

A W-band monolithic amplifier

Monolithic amplifiers at 90 GHz have been fabricated using 75- mu m GaAs MESFET and pseudomorphic high-electron mobility transistor (PHEMT) devices. The below 0.2- mu m gate-length PHEMT devices have demonstrated an F/sub t/ of 100 GHz and an F/sub max/ of 200 GHz. Monolithic MESFET and PHEMT single-stage amplifiers have achieved 3.5-dB and 7-dB gain, respectively, at 90 GHz.<<ETX>>

Photoemission study of the adsorption of O2, CO and H2 on GaAs(110)☆

Abstract Ultraviolet photoemission spectroscopy with hv 2 , CO, and H 2 adsorption on the cleaved GaAs(110) face. It was found that O 2 exposures above 10 5 L(1LM = 10 −6 Torr sec) were required to produce changes in the energy distribution curves. At O 2 exposures of 10 6 L on p-type and 10 8 L on n-type an oxide peak is observed in the EDCs located 4 eV below the valence band maximum. On p-type GaAs, O 2 exposures cause the Fermi level at the surface to move up to a point 0.5 eV above the valence band maximum, while on n-type GaAs O 2 exposures do not remove the Fermi level pinning caused by empty surface states on the clean GaAs. CO was found to stick to GaAs, but to desorb over a period of hours, and not to change the surface Fermi level position. H 2 did not affect the EDCs, but atomic H lowered the electron affinity and raised the surface position of the Fermi level on p-type GaAs. A correlation is found in which gases which stick to the GaAs cause an upward movement of the Fermi level at the surface on p-type GaAs, while gases which stick only temporarily do not change the surface position of the Fermi level.

Photoemission study of the formation of Schottky barriers

For the first time, changes in electronic structure have been studied during Schottky barrier (Cs on GaAs or InP) formation. Strong changes occur near the valence band maximum; however, these do not overcome a dominant role of intrinsic surface states in Fermi‐level pinning.

Photoemission studies of the electronic structure of III–V semiconductor surfaces

Abstract The photoemission technique using synchroton radiation in the photon energy range 5–450 eV has been applied to the study of the electronic structure of some III–V semiconductor surfaces, prepared by cleavage in situ under ultrahigh vacuum conditions, ≲ 10 −11 Torr. For p-type GaAs(110), the Fermi level is pinned at the top of the valence band and thus no filled surface states extend into the band-gap. The situation is more complicated for n-type GaAs(110), where band bending easily can be introduced by extrinsic effects (impurities, cleavage quality, etc.) and push the Fermi level down to about midgap. Chemical shifts of inner core levels (3d for Ga and As) are used to obtain information on the bonding site of oxygen on the (110) surface. GaAs(110) can be exposed to atmospheric pressure of molecular oxygen without breaking the bonds between the surface atoms and the bulk. Oxygen is predominantly bonded to the As atoms on the surface. The oxidation behavior is strikingly different for GaSb(110) with formation of gallium and antimony oxides on the surface directly upon oxygen exposure. Heavier oxidation of GaAs(110) and breaking of the surface bonds will also be reported.

Monolithic 40 to 60 GHz LNA

A monolithic broadband low-noise amplifier (LNA) using MESFET technology has been demonstrated at U-band. This monolithic chip has demonstrated better than 7-dB gain with less than 1.5-dB ripple and a worst-case noise figure of 7.5 dB across the 40-60-GHz band. This chip has also demonstrated better than +8 dBm of output power across the U-band. Lower noise figure numbers are expected when such MMICs (monolithic microwave integrated circuits) are processed using optimized low-noise MESFET devices. These are state-of-the-art results for MESFETs and millimeter-wave amplifier technologies.<<ETX>>

Ultraviolet photoemission study of cesium oxide films on GaAs

Ultraviolet photoemission energy distribution curves (EDC’s) have been measured from a GaAs (110) surface covered with Cs oxide layers of varying thickness. There is no evidence of emission from Cs oxide in the EDC’s from GaAs with a surface treatment that produces optimum yield, but structure characteristic of the GaAs is present in the EDC’s. However, EDC’s characteristic of bulk Cs oxide were measured from GaAs with a thick (at least several molecular layers) Cs oxide surface layer, but no structure characteristic of the GaAs was present in the EDC’s. Compared to the GaAs with the optimum surface treatment, this thick Cs oxide film produced yield spectra having a higher photoemission threshold and a lower yield throughout the photon energy range studied (1.4⩽ hν⩽11.6 eV). These measurements indicate that the Cs oxide layer required for activation of GaAs to negative electron affinity is so thin that the Cs oxide layer does not have bulk properties which can be detected by photoemission. Measurements si...

Oxygen adsorption and the surface electronic structure of GaAs (110)

Abstract The surface electronic structure of cleaved GaAs (110) is found to be very sensitive to small amounts of adsorbed oxygen. For example, adsorbing oxygen on only a few percent of the surface Ga or As atomic sites can produce changes of a factor of two in the surface electronic structure. Thus, long range effects must be involved, and these are associated with rearrangement of the surface atoms.