Peter Mark

Polar surfaces of wurtzite and zincblende lattices

Abstract Madelung potentials are calculated at lattice sites along the [0001] wurtzite and [111] zincblende axes, both in the bulk and at the polar surfaces. The potentials are then used to compute the electrostatic surface energies of various (0001) wurtzite and (111) zincblende surface arrangements. All arrangements considered satisfy the ionic stability criterion, which requires a net charge at the polar surface. The surface energy results predict reconstructed and faceted surface structures that agree well with experimental low energy electron diffraction work reported in the literature.

The role of chemisorption in current flow in insulating CdS crystals

Abstract From a study of the dependence on the illumination level of the change in the photocurrent during oxygen adsorption, a semi-quantitative model for photo-induced chemisorption on insulating CdS crystals is given. The examination of atmosphere sensitive trapping is extended to thermally-stimulated-current measurements which show, in agreement with Bube, that a crystal with an oxygen free surface can store 10 5 times more trapped charge than a crystal on whose surface oxygen had been photo-adsorbed. A model is proposed to explain this effect. The dark current is similarly atmosphere dependent; both its magnitude and activation energy are sensitive to oxygen exposure. It is found that the adsorbed oxygen ions lie 0.91 eV below the conduction band and, in the dark, act as deep-lying donors which surface compensate the shallower bulk donors. Photoinduced adsorption of nitrous oxide is described and its relevance to barrier catalysis is briefly discussed.

EFFECTS OF CHEMISORBED OXYGEN ON THE ELECTRICAL PROPERTIES OF CHEMICALLY SPRAYED CdS THIN FILMS

The Hall mobility μ and conduction electron concentration n of semiconducting, spray‐deposited CdS thin films are both reduced by chemisorbed oxygen, the former by a larger factor than the latter. The temperature dependence of n is not affected by chemisorption whereas that of μ is changed from practically no dependence on temperature to a thermally activated dependence. This change is attributed to additional scattering introduced by the chemisorbed ions.

Correlation of electronic, leed, and auger diagnostics on ZnO surfaces

Abstract Surface electrical properties of ZnO single crystals were measured and were correlated with complementary LEED and Auger measurements. For the crystal faces (0001), (000 1 ), (11 2 0), and (10 1 0) that were prepared by ion bombardment and anneal in ultrahigh vacuum, the above surfaces were found to be: (a) atomically regular as judged by bright LEED spots, (b) free of chemisorbates as judged by Auger spectroscopy, and (c) electrically inert, as judged by the inability of the surface to become charged when immersed in a flux of thermalized negative ions. Similar observations were made on the (0001) face, prepared only by chemical polishing in HCl. All the above phenomena persisted for remarkably extended periods without reprocessing, even after exposure to room air for 3 months. On the other hand, the (000 1 ) (11 2 0), and (10 1 0) surfaces that were prepared only by chemical polishing were found to be: (1) atomically irregular as judged by LEED, (2) nominally flat as judged by SEM, (3) contaminated with strongly bound chlorine as judged by Auger, and (4) electrically active in the following sense; when negative thermalized ions were deposited from a corona source in air, the ZnO surfaces easily became charged and a giant band bending of ≈ 70 V was sustained in the dark. On these chargeable surfaces, photodischarge spectroscopy was carried out with less-than-bandgap-light, and discrete energy levels within the ZnO bandgap of the extrinsic surface states were inferred. Depending on crystal face, the surface state levels below the conduction band were from 1.1 to 1.7 eV. On the basis of these and other observations, it was possible to deduce various bulk and surface discharge mechanisms and to correlate the charging ability with the presence of strongly bound chlorine.

Photo-induced chemisorption on insulating CdS crystals

Abstract Substantial reversible changes in the photoconductive gain and response time have been observed in certain insulating CdS crystals as a result of varying the partial pressure of an electronegative ambient (oxygen and iodine vapor). Only crystals showing no red sensitivity display the effect. Both the gain and the response time increase as the partial pressure of the gas is lowered. The atmosphere dependence of the gain is ascribed to a photo-induced chemisorption process similar to the mechanism proposed for semiconductors. The rate of adsorption of oxygen follows the Elovich equation d q d t = a exp(−bq) where q is the adsorbate concentration and a and b are constants. The change in the response time, which can be as large as a factor of 105 for an accompanying change in the gain by a factor of 35, is related to chemisorption as well as to the existence of traps which increase in number as the partial pressure of the gas is reduced. Although these traps are too numerous to be associated with the geometric surface, their number is determined by surface conditions. Their physical origin is not understood. The absence of the atmosphere effect in red sensitive crystals is related to the migration of excitation energy away from the illuminated surface by reabsorption in the bulk of the luminescence produced in the surface region by the band-gap light.

The energy of desorption of photo-chemisorbed oxygen and nitrous oxide on insulating CdS crystals

Abstract The energy of desorption of photo-chemisorbed oxygen and nitrous oxide on the surfaces of thin insulating CdS crystals is obtained in two independent ways: (1) From thermal desorption time measurements where the time required to release a certain fraction of the adsorbed ions by thermal desorption is measured and (2) from an examination of the temperature dependence of the dark current. The two methods yield the self-consistent results 0.91 and 0.74 eV for oxygen and nitrous oxide, respectively. The results are used to illustrate certain features of the model for reversible photo-induced chemisorption on insulators.

Evidence for subsurface atomic displacements of the GaAs(110) surface from LEED/CMTA analysis

Abstract The low-energy electron diffraction constant momentum transfer average (LEED/CMTA) technique is applied to the GaAs(110) surface. In addition to verifying our earlier model of the top layer atomic displacements, atomic displacements in the first subsurface layer, opposite in direction to those in the top layer, are suggested. Best estimates for atomic positions in the surface and first subsurface layers are given.

Analytical and experimental investigation of the effects of oxygen chemisorption on the electrical conductivity of CdS

Abstract An investigation of the effects of surface states on the equilibrium conductivity of thin, wide bandgap semiconductors is presented. A comparison of measured conductivities with those computed from different surface state models (discrete or distributed), chosen a priori, reveals that the oxygen chemisorption states of natural surfaces of CdS (single crystals and evaporated films) are distributed in energy. An exponential distribution of the form D s (E) = N 0 exp[ (E−E c ) kT c ] give the best agreement between theory and experiment.

The influence of surface structure on the chemisorption of oxygen by CdS single crystals

Abstract Atomically ordered surfaces of CdS single crystals are found to be relatively insensitive to oxygen chemisorption. For such surfaces prepared by argon bombardment and high vacuum annealing, the effect of ambient oxygen on the equilibrium conductivity is drastically reduced. This indicates that active sites associated with surface imperfections may be necessary to achieve chemisorption.

Photodesorption from CdS

Abstract Photodesorption was investigated by the mass spectrometric analysis of the ambient surrounding powdered samples of CdS. It was found that greater-than-bandgap illumination causes large and spontaneous desorption which is linearly proportional to the illumination intensity, whereas smaller-than-bandgap energies show little or no desorption. From the illumination wavelength and intensity dependence of desorption it can be concluded that neutralization of the negative surface charge by photoholes is the dominant photo-desorption mechanism in CdS.