Audunn Ludviksson

Atomic layer etching chemistry of Cl2 on GaAs(100)

Abstract The reaction of Cl2 on GaAs(100) was studied under UHV conditions using metastable quenching electron spectroscopy (MQS), AES, LEED and TPD. Chlorination of the Ga-rich c(8 × 2)Ga surface at 85 K by Cl2 forms a GaCl layer, where each surface Ga has one Cl bond and two backbonds to As. The reaction saturates with an exposure of about 1 L Cl2. Formation of the GaCl layer increases the work function by 1.8 ± 0.2 eV. The GaCl desorbs in a doublet at 620 and 650 K, uncovering the underlying As layer, which desorbs in three peaks at 670, 730, and 810 K. The arsenic 670 K peak is due to desorption of As2 and As4, and the 730 and 810 K peaks are due to desorption of As2. Annealing at 820 K returns the surface to the original c(8 × 2)Ga condition. Flashing the chlorinated c(8 × 2)Ga surface to 650 K removes all the GaCl, leaving a clean As-rich surface. Chlorination of this surface at 85 K followed by TPD gives desorption of AsCl3 at 170 K, GaCl2 at 240 K, and GaCl at 620 and 770 K. The data indicates that the sequence of chlorinating the c(8 × 2)Ga surface at 85 K, flashing it to 650 K to remove the GaCl, chlorinating the resulting As-rich surface at 85 K, and heating to 820 K removes 2 ML of Ga and 2 ML of As, and returns the surface to c(8 × 2)Ga. The surface GaCl and GaCl2 give separate bands in the MQ spectrum, with binding energies of about 12.0 and 6.2 eV, respectively, below the GaAs valence band maximum. Repetitive cycles of exposure of the chlorinated c(8 × 2)Ga surface at 85 K to Cl2 followed by warming to 130 K or above to desorb the excess Cl2 gradually converts GaCl to GaCl2. The data suggests that partially chlorinated arsenic formed at 85 K is converted by heating to GaCl and GaCl2. The present results are consistent with studies of the desorption products from steady-state continuous etching of GaAs(100) by beams of Cl2.

Surface oxidation of GaAs(100) by adsorption of NO2

Abstract The adsorption and reaction of NO 2 on n-type Ga-rich GaAs(100) was studied using AES, TPD, HREELS, and MQS. At 78 K all the adsorbed NO 2 up to 4 L exposure reacts to oxidize the surface and desorb NO. At higher exposures NO 2 accumulates on the surface as the N 2 O 4 dimer. Saturation of the oxidation reaction, corresponding to monolayer oxide formation, occurs at a NO 2 exposure of about 30 L. A small amount of NO 3 is also formed, which can be increased by repeated cycles of dosing and heating to 150 K to desorb unreacted NO 2 . Annealing the sample to 800 K develops a HREELS oxide band centered at 84 meV, which is assigned as a gallium oxide structure. All of the oxide is desorbed at Ga 2 O in a narrow peak at about 800 K. Exposure of the clean surface to NO 2 at 300 K is found to oxidize the surface as efficiently as at 78 K.

Time-of-flight measurements of single rovibrational states of carbon monoxide

A new technique has been developed for the measurement of the translational energy of molecules in single rovibrational quantum states. Molecules from a given rotation–vibration level are excited to a long‐lived electronic state by a pulsed, tunable ultraviolet (UV) laser and are allowed to collide with the surface of a low work function metal. Since the energy of the metastable state exceeds that of the metal’s work function, collisions result in the ejection of electrons from the metal surface, which may be detected with high efficiency. This technique has been applied successfully to the carbon monoxide system, where measurements of molecular beam velocities and extremely weak forbidden electronic transitions have been made. The detection efficiency of this technique is estimated to be 2.0×10−4, comparable with electron impact ionization and mass‐selected detection. Possible future applications of the technique in spectroscopy, photodissociation, and photon stimulated desorption experiments are discussed.

Reaction of Li with CO on Ru(001) and Ag/Ru(001) substrates

Abstract Lithium is found to behave quite differently from other alkali metals in its coadsorption with CO on transition metal substrates, and also in its multilayer interaction with CO. Coadsorbed CO and Li on Ru(001) interact or react to give several chemically different species, which depend on surface stoichiometry and surface temperature. At low Li coverage ( n (CO) 2 n is formed and produces an extremely narrow CO desorption peak at 730 K corresponding to the reaction Li n ( CO ) 2 n ( ad ) → Li n ( CO ) n ( ad ) + nCO ( g ). CO adsorbs readily on multilayer Li surfaces at 80 K and forms a compound which is tentatively assigned the stoichiometry Li 2 n (CO) n . Annealing to 500 K desorbs the Li multilayer and transforms this compound to Li n (CO) n . The Li n (CO) n compound decomposes on the surface with further heating to give a Li + CO codesorption TPD peak at 850 K, which is much higher than other alkali + CO codesorption peaks on Ru(001). The LiCO chemical interaction depends strongly on the substrate chemical activity. Li-CO compounds form at lower Li coverage on Ag than on Ru, due to the weaker interaction of Ag with the adsorbates. In addition, a more strongly bound compound with TPD codesorption peak at 1020 K and stoichiometry Li m (CO) n , where n > m , is observed on Ag but not on Ru.

Reaction of HCl with the GaAs(100) surface

Abstract We have studied the interaction of HCl with Ga-rich and As-rich GaAs(100) surfaces, using AES, TPD, HREELS, and MQS. HREELS data show that when the surface is dosed with H atoms they bond to both As and Ga, but when HCl reacts with the surface the H atoms bond only to As, on both the Ga-rich and As-rich surfaces. Therefore, HCl does not add across Ga dimer bonds on the Ga-rich surface, as is found with Cl 2 . A reaction model is proposed in which HCl adds across Ga-As backbonds at Ga dimer vacancies, with H bonding to As and Cl bonding to Ga. When the sample temperature is raised after reacting HCl with GaAs at 85 K, the only etch products desorbed are GaCl and As 2 . H 2 and HCl are also desorbed due to combination of surface H and Cl atoms. The TPD data for the etch products from reaction of HCl versus Cl 2 agree with the observations that Cl 2 etches GaAs at temperatures as low as 320 K, whereas etching by HCl requires temperatures above 600 K.

Formation of N2O from NO adsorbed on silver layers on ruthenium, studied by MQS and TDS

Abstract NO adsorption on monolayer and multilayer Ag/Ru(0001), and on a Ag(111) single crystal, was studied by metastable quenching spectroscopy (MQS) and thermal desorption spectroscopy (TDS). We find that NO adsorbs and dissociates at 90 K, forming N2O. TDS shows that somewhat more N2O is formed on unannealed Ag/Ru surfaces, but the N2O is much more visible by MQS on the annealed surface. The chemical behavior of NO adsorbed on annealed monolayer and multilayer Ag/Ru(0001) and on an annealed Ag(111) crystal surface is the same. However, two layers of Ag on Ru(0001) are necessary for the surface to approach the properties of the multilayer limit

Reaction of NO to form N2O and surface oxide on GaAs(100)

Abstract Metastable quenching spectroscopy (MQS), temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), and Auger electron spectroscopy (AES) were used to study the reaction of NO on GaAs(100) at 74–77 K. Adsorbed NO reacts nearly completely to form N 2 O and surface oxide. A NO dose of about 200 L saturates the reaction, forming a complete oxide layer. The MQ spectra show that the product N 2 O is adsorbed predominately with the O atom down on the oxide layer, resulting in a 30% higher saturation coverage than for N 2 O adsorbed on the clean surface. Dosing the clean surface at 300 K with 200 L NO gave no measurable NO adsorption, N 2 O production, or surface oxide formation.

THE SEQUENTIAL ADSORPTION OF NOBLE-METAL AND ALKALI-METAL LAYERS ON RU(001) : A COMBINED MQS AND TDS STUDY

Abstract The deposition of Ag or Cu layers on K or Li monolayers on Ru(001) was studied to investigate the effect of underlying alkali layers on noble metal properties. The surface electronic structure was probed using metastable quenching spectroscopy (MQS). CO was also used as a chemical probe of the surface properties, using MQS and thermal desorption spectroscopy (TDS). It was found that large amounts ( ∼ 8 ML, referenced to clean Ru) of Ag or Cu are needed to cover a K monolayer at 88 K, showing that K is displaced from Ru and rapidly diffuses into the noble metal during deposition. Li behaves quite differently; 1 ML of Ag on 1 ML Li/Ru is found to nearly eliminate the Li MQ peak at 88 K. Annealing Ag-covered Li and K monolayers on Ru causes diffusion of the alkali metal atoms to the Ag surface. For coadsorbed K and Cu monolayers on Ru, MQS detects no significant difference in the electronic structure of surfaces prepared by dosing K on Cu versus Cu on K. However, a substantial difference between these surfaces is observed with both MQS and TDS by using CO as a surface probe. Four different KCO codesorption states as well as a low-temperature CO desorption feature are observed from the Cu/Ru system.

Performance and Production of Micron-Sized Spherical Powders for Display Applications

SMP is producing spherical, micron-sized, controlled size distribution powders that can be deposited by conventional and advanced deposition techniques to produce densely packed powder layers. This presentation focuses on the production, deposition methods, characteristics and performance of these phosphor powders used in miniature CRT displays.