R. Z. Bachrach

The SSRL ultrahigh vacuum grazing incidence monochromator: Design considerations and operating experience

Abstract Considerable experience has now accumulated with the “grasshopper” monochromator installed on the four degree line at the Stanford Synchrotron Radiation Laboratory. This is one of the first bakeable high vacuum instruments for use with a storage ring source in the photon energy range 25 to 1000 eV. The unique features of this instrument will be discussed from a general point of view, including the source emittance and the transforming properties of the beam line plus monochromator. Actual performance figures will be given in order to better appraise the limits of focusing optics and gratings at two degree grazing incidence. Improvements such as post-monochromator optics, isolation valves and provisions for adjustment will be briefly discussed.

Fine structure above the carbon K-edge in methane and in the fluoromethanes

Abstract Synchroton radiation has been utilized to obtain high resolution soft X-ray absorption spectra above the carbon K-edges in CH4, CD4 and the fluoromethanes. The results near threshold are in close agreement with forward scattering electron energy loss data in the cases where that comparison can be made. With increasing fluorination the Rydberg-like threshold spectra of methane shift to higher energy and evolve into a complex band in CF4. Well above threshold extended fine structure (EXAFS) is observed in CF4.

Initial formation of the interface between a polar insulator and a nonpolar semiconductor: CaF2 on Si(111)

The initial stages of interface formation between CaF2 and Si(111) have been studied with both core‐level and angle‐resolved valence band photoemission spectroscopy. Both the Si 2p and Ca 3p core levels indicate that a Si–Ca bond is present at the CaF2–Si(111) interface. Annealing of thin CaF2 films at 700–800 °C results in preferential evaporation of F, and the surface undergoes a number of reconstructions until a stable Si(111):Ca 3×1 reconstruction is obtained on which there is calcium, but no fluorine. This surface exhibits a surface state with an upward dispersion of 0.4 eV towards the K point of the 1×1 surface Brillouin zone. Our results show that the CaF2 molecule can be dissociated on the Si(111) surface at typical epitaxial growth temperatures, with the Ca being more strongly adsorbed to the Si than is the F.

Excimer‐laser‐induced crystallization of hydrogenated amorphous silicon

The electronic transport properties and structural morphology of fast‐pulse excimer‐laser‐ crystallized hydrogenated amorphous silicon (a‐Si:H) thin films have been measured. The room‐temperature dark dc conductivities and Hall mobilities increase by several orders of magnitude at well‐defined laser energy density thresholds which decrease as the impurity concentration in the films increases. The structural morphology of the films suggests an impurity‐induced reduction of the a‐Si:H melt temperature as the origin of this behavior.

Formation of the interface between GaAs and Si: Implications for GaAs‐on‐Si heteroepitaxy

Results of photoemission core‐level spectroscopy measurements for coverages of around one monolayer of As, Ga, and GaAs on Si substrates are presented. The interfaces were formed on on‐axis Si(100) and Si(111) substrates using molecular beam epitaxy techniques. The bonding between As and the substrate surface leaves the As atoms fully coordinated and thus extremely unreactive. This causes the GaAs films to form islands at average coverages of less than one monolayer. The surface between the islands is found to be terminated by a single atomic layer of As. Use of a Ga predeposition technique shows evidence of decreasing the area between islands. Results for As interaction with stepped Si(100) surfaces and the implications for avoidance of antiphase domain boundaries are discussed.

Electronic and atomic structure of arsenic terminated Si(100)

A comparison between angle resolved photoemission data and ab initio pseudopotential calculations for a structural model of the As covered Si(100) surface is presented. After a coverage of one monolayer the Si(100):As surface shows a 2×1 low energy electron diffraction pattern. The arsenic acts as an effective passivating layer and the adsorption of further arsenic is highly reduced. We propose a symmetric As–As dimer model for the Si(100):As 2×1 surface derived from energy minimization calculations. The major feature of the electronic structure of this model is the occurrence of occupied π and π* surface state bands derived from bonding and antibonding combinations of the dangling hybrids on the As atoms. The calculated dispersions for the π and π* bands are found to be in very good agreement with the experimental surface state dispersions. The results discussed provide an example of how angle resolved photoemission in combination with ab initio pseudopotential calculations can lead to a conclusive deter...

Electronic structure of Si(111) surfaces

Abstract We report on new angle-resolved photoemission studies of Si(111) 2 × 1 and 7 × 7 surfaces. The emission from the 2 × 1 surface shows much structure. For normal emission the energy positions are insensitive to the photon energy in the range 19–27 eV. The emission has been interpreted as a probe of the surface density of states, SDOS, including both surface states, resonances and bulk-like states. The SDOS was also calculated as a function of parallel momentum k ∥ for a model of the Si(111) 2 × 1 surface obtained from energy minimization considerations. We identify emission from the dangling bond band, which has a positive dispersion of 0.6 eV, and also emission from surface resonances which have some character of the compressed and stretched back bonds. There are also other predicted surface resonances that correspond to experimental peaks which have not been identified in previous work. Except for the dangling bond band, the surface resonances are limited in k ∥ space, so that it is not possible to follow these resonance bands over all angles. Maximum intensity for the normal emission from the dangling bond is obtained at 23 eV, while the emission from the lowest s-like states monotonically increases towards 30 eV photon energy. When annealing the cleaved 2 × 1 surface to the 7 × 7 reconstructed surface, the spectra broaden significantly. The intensity of the dangling bond decreases and we see a very small metallic edge.

Diffraction efficiencies of holographic transmission gratings in the region 80–1300 eV

Experimental measurements of diffraction efficiencies for holographic transmission gratings have been performed. The transmitted intensity distribution is characterized by an overall efficiency of about 10% into first order and a strong resonance enhancement, up to 20% efficiency, around the region of anomalous dispersion. The intensity distribution is well described by a grating model which predicts the overall efficiency, as well as the detailed behavior of the grating around the regions of anomalous dispersion. The model can be used to predict the efficiencies for an arbitrary grating material and thickness, and thus aid in the determination of grating structure for a specific experimental application.

Atomic interdiffusion at Au‐Al/GaAs interfaces

A new technique has been developed to probe metal‐semiconductor interdiffusion on an atomic scale. With reacted Al atoms as markers at microscopic Au/GaAs (110) interfaces, soft x‐ray photoemission studies reveal both Au indiffusion and nonstoichiometric Ga and As outdiffusion at room temperature.

Soft-X-ray electroreflectance: Final-state effects on Si (2p) optical transitions

SummaryElectric-field modulation techniques have been employed for the first time in the spectral region above 30 eV. A novel analog detection scheme is described which achieves sensitivities of 10−4 with the low-duty-cycle, pulsed synchrotron radiation source used. Application to the sharp SiL2,3 edge at 99.9 eV yielded no detectable electroreflectance for 3·105V/cm modulation. We conclude that thep-core exciton, binding energy must be at least 300 meV, and thus its final state cannot be described by the previously employed effective-mass approximation. Photoemission and absorption measurements are presented to support this finding.RiassuntoTecniche di modulazione di campo elettrico sono state impiegate, per la prima volta, nella regione spettrale al di sopra di 30 eV. Si descrive un nuovo sistema analogico di rivelazione che raggiunge sensibilità di 10−4 con la sorgente di radiazione di sincrotrone che è impulsata con basso «duty-cycle». La tecnica è stata applicata alla stretta soglia di assorbimentoL2,3 del Si a 99.9 eV. Non si sono osservati segnali di elettriflettanza nonostante una modulazione di 3·105V/cm. Si conclude che l’energia di legame dell’eccitonep profondo deve essere almeno 300 meV, e quindi il suo stato finale non può essere descritto dall’approssimazione della massa effettiva precedentemente impiegata. Si presentano misure di assorbimento e di fotoemissione a sostegno di questo risultato.РезюмеВпервые техника модуляции электрического поля используется в спектральной области выше 30 эВ. Описывается схема детектирования, в которой достигается чувствительность 10−4, когда используется импульсный синхротронне источник излучения. Применение к резкому краю SiL2,3 при 99,9 эВ дает ие детектируемое электроотражение для модуляции 3·105 В/см. Мы заключаем, что энергия связи для экситоновp-остова должна быть, по крайней мере, 300 мэВ. Таким образом, конечное состояние не может быть описано с помощью ранее использованного приближения эффективной массы. Приводятся результаты изме-рений по фотоэмиссии и поглощению.