D. J. Friedman

Computer program for photoemission data analysis and display

A computer program for photoemission data analysis which operates using the VAX computer or the Macintosh personal computer is described. It includes functions which give a summary of the data such as centroid and area, performs curve fitting for extracting chemical information from core levels, and it prepares figures for publication, as well as many other functions. A brief description of the program and its capabilities is given.

Optically enhanced low temperature oxygen chemisorption on GaAs(110)

Optically enhanced oxidation of GaAs is promising both for insight into fundamental mechanisms of oxygen chemisorption and for possible application in GaAs direct write. For atomically clean GaAs(110), we have found that sample temperature is a key parameter in determining the increase of oxygen uptake under low‐intensity argon ion laser illumination (1 W/cm2 at 2.41 eV) relative to the ‘‘dark’’ uptake. Although cooling to 185 K suppresses the dark oxygen adsorption rate, the oxygen uptake with optical irradiation is the same at both 185 and 400 K. The ratio of laser‐assisted to dark uptake for 108 L exposure then approaches the enhancement necessary for spatially selective oxidation, showing an increase from 2.5 at room temperature to well over 10 at 185 K. After ruling out sample heating and direct oxygen gas excitation, we suggest that the light‐induced increase in uptake is due to electronic excitations in the GaAs. To explain the enhancement, one also needs to assume a weakly adsorbed precursor state...

The electrical properties of metallic contacts on Hg1−xCdxTe

A combination of theory and experimental data are used to predict the intrinsic electrical properties (tunnel contacts are considered extrinsic) of metals on Hg1−xCdxTe. It is found that, for x 0.4, Schottky barriers are expected on both n‐ and p‐type material. Possible methods for forming extrinsic Ohmic contacts on p‐type material are also explored.

Electron beam induced Hg desorption and the electronic structure of the Hg depleted surface of Hg1−xCdxTe

Auger electron spectroscopy (AES), x‐ray photoemission spectroscopy (XPS), low energy electron diffraction (LEED), and angle‐resolved ultraviolet photoemission spectroscopy (ARPES) were used to study the electron beam induced Hg desorption from a cleaved (110)Hg1−xCdxTe surface and the electronic structure of the Hg depleted surface. Solid state recrystallized Hg1−xCdxTe single crystals were used. It was found that the electron beam heating dominated the electron beam induced Hg desorption on Hg1−xCdxTe. At the electron beam energy used, the electron beam heating extended several thousand angstroms deep. However, the Hg depletion saturated after a few monolayers were depleted of Hg atoms. At the initial stage of Hg loss (only 3%), the surface band bends upward (more p type). The ARPES spectrum showed the loss of some E vs k dispersion after 22% Hg atoms were removed from the surface region, and no dispersion was observed after 43% Hg atoms were removed. These results have important implications on the ele...

Use of low temperature to reduce intermixing at metal:HgCdTe contacts

The metal/HgCdTe interfaces formed by evaporating Ag and Al onto cleaved HgCdTe substrates held at 170 K in ultrahigh vacuum (p<1×10−10 Torr) exhibit greatly reduced intermixing between the metal and substrate to the extent, in the case of the Ag/HgCdTe interface, that no detectable intermixing occurs. The incremental metal depositions used in this study ranged from tenths of monolayers at submonolayer coverages to several monolayers at thicker coverages. The evolution of these interfaces was investigated using the surface sensitive techniques of photoemission spectroscopy and low‐energy electron diffraction. In addition to the reduced intermixing, unusual band bending behavior in the semiconductor at very low metal coverages is observed for these interfaces formed at low temperature. In the low‐coverage regime (<5 monolayers), the bands are seen to bend from the position after metal deposition back towards the initial cleaved position after time for the Ag case and after exposure to x rays in the Al case...

The Ag/(Hg,Cd)Te and Al/(Hg,Cd)Te interfaces

The interfaces of UHV‐cleaved Hg0.77Cd0.23Te with Ag and with Al have been studied with x‐ray and ultraviolet photoelectron spectroscopy at coverages from submonolayer to tens of monolayers. These two metals were chosen because they lie at the extremes of chemical reactivity with Te: Al is very reactive, while Ag is nonreactive. As expected, little chemical reaction is seen for the Ag/(Hg,Cd)Te interface. The Ag diffuses 102–103 A into the bulk, displacing Hg in the process. No such deep diffusion is seen for Al. For the Al/(Hg,Cd)Te interface, the first 3–5 ML are incorporated into an Al–Te reacted layer, with further Al deposition being incorporated as metallic Al. A significant depletion of Hg in the first few semiconductor layers occurs even at submonolayer coverages. Our results are compared with other work on metal/(Hg,Cd)Te and related interfaces.

Systematics of metal contacts to Hg1−xCdxTe

We discuss the morphology and band bending properties of metal contacts to the surface of Hg1−xCdxTe (MCT) cleaved in ultrahigh vacuum. The dependence of these interface properties upon the overlay metal is seen to correlate with the bulk thermodynamic and doping properties of the overlayer metal with MCT.

Resonant photoemission at the 5p threshold in La, Pr, Sm and Tb

Abstract The 5 p -valence band interaction in the rare earth metals La, Sm, Pr and Tb is studied with resonant ultraviolet photoelectron Spectroscopy in the photon energy range hv = 10–40 eV. For all four metals the valence band (5 d 6 s ) photoionization cross-section has a minimum at the 5 p 3 2 threshold, with a strong delayed, 10 eV wide, maximum above threshold. An O 3 VV Auger channel opens at the 5 p 3 2 threshold.

Fermi‐level movement at metal/HgCdTe contacts formed at low temperature

In an effort to reduce the disruption to the HgCdTe surface upon metal overlayer formation over that observed at room temperature, interfaces formed at 100 K between HgCdTe and the three overlayer metals, Ag, Al, and Pd, are investigated. The metals were deposited onto the [110] cleaved HgCdTe surfaces in UHV in increments ranging from submonolayer at low coverages to tens of monolayers at high coverages, and the formation of the interfaces were monitored using photoemission spectroscopy subsequent to each deposition. The primary experimental observation of this study is that deposition of all three of these metals onto HgCdTe substrates held at reduced temperatures causes the surface Fermi level (Ef) to move from its initial cleaved position, typically located at or near the conduction‐band minimum, into the conduction band, and this movement is correlated with the absence of the movement of the overlayer metal into the semiconductor. This phenomenon is observed at the lowest coverages (submonolayer) for...

Photoemission studies of the room‐temperature Si/Hg1−xCdxTe, Si/HgTe, and Si/CdTe interfaces

The room‐temperature interface between Si and Hg1−xCdxTe (x=0, 0.30, 0.39, 1.0) was investigated using the surface sensitive techniques of photoemission spectroscopy and low‐energy electron spectroscopy (LEED). Elemental Si was electron beam evaporated onto atomically clean cleaved (110) surfaces in ultrahigh vacuum and the chemistry and morphology of the interface was monitored as a function of Si overlayer growth. The LEED pattern disappeared at ∼1 monolayer (ML) coverage Si for all substrates, indicating that the overlayer coverage was fairly uniform. At submonolayer Si coverages on the Hg containing alloys, the substrates exhibited Hg loss with Te coming out of the substrate into the overlayer. At moderate coverages (<6 ML), Te continued to come to the surface. Chemical shifts of the Si and Te core levels suggest that Te interacts with Si in the overlayer, with the possible formation of Si–Te phases in the overlayer. At thicker coverages of Si, Hg and Cd are seen to intermix within the Si overlayer, b...