Heating In Ultrahigh Vacuum Research Articles

Surface science studies of semiconductor growth processes

A new approach to the study of semiconductor growth mechanisms is described which utilizes surface science and related photon-based techniques. It is possible to study growth processes over the pressure range 5×10−11 to 103 mbar through the use of an isolatable atmospheric pressure reactor described briefly. As an illustration of the potential of the surface science approach, the effectiveness of the reducing hydrogen atmosphere employed in the pregrowth bake of a GaAs (100) metal-organic vapor phase epitaxy (MOVPE) substrate in removing surface carbon and oxygen is determined using Auger electron spectroscopy. It is shown that at a pressure of 0.5 mbar of H2, temperatures as low as 600 K are sufficient to remove the surface carbon and oxygen contamination present on the substrate following wet chemical etching and heating in ultrahigh vacuum. This result implies that the conventional MOVPE sample bake at high temperatures (>850 K), in H2 and AsH3 is not necessary to produce clean substrates. Following the H2 treatment the GaAs(100) surface gives rise to a (1×1) low-energy electron diffraction pattern suggesting that it has been stabilized towards reconstruction via hydrogen chemisorption.

Binding and decomposition of organic dye molecules on ZnO crystals

Prism surfaces of ZnO crystals are prepared by various pretreatments as heating in ultrahigh vacuum (UHV), annealing in oxygen at pressures up to 10 5 Pa, argon bombardment, cleavage in UHV. An organic dye is deposited by sublimation in UHV and the exposure (10 12−10 15 cm −2 is recorded by means of a vibrating quartz. Adsorption and desorption studies including thermal desorption spectroscopy (TDS) are performed with variation of crystal temperature during deposition and of coverage. In these experiments the spectral distribution of absorption is used for the recording of coverage down to about 10 12 cm −2, for the observation of ordering processes within the dye layer and for the detection of dye decomposition. By the various treatments the catalytic activity of the ZnO surface for decomposition of the dye molecules at temperatures above 350 K can be increased or decreased in a wide range. Possible sources of the activity are discussed. Molecules bound to a clean inactive ZnO surface desorb above 600 K. In contrast they come off already at 350 K if they are bound only to other molecules. A sticking coefficient is derived as a function of coverage at 470 K and compared with calculations after several adsorption models. The best fit is obtained by a precursor state model.

LEED-AES Study of Iron Film Deposited on MgO (001) Surface

The orientation and the surface structure of the epitaxially grown iron film is studied by a four grid type LEED-AES system. The substrate of MgO (001) surface is prepared by cleaving in air and heating in ultra high vacuum (5 × 10-9 Torr) at 500°C. No contaminant is detected by AES from this surface.Iron atoms are evaporated from an iron wire (99.95%) which is heated by electric current directly, and the film grows epitaxially on the surface. The orientations of the film decided by LEED are denoted as (001) Fe// (001) MgO[100] Fe // [110] MgOWhen the substrate is kept at room temperature, the surface of deposited film offeres fuzzy spots in LEED, and they become sharp by annealing. As the annealing temperature is kept below 400 °C, only small amounts of carbon and oxygen are detected and the P (1 × 1) pattern is maintained. But when it is higher than 500°C, a distinctive peak of sulphur appears in AES curve and the pattern indicates that the C (2× 2) -S structure is formed. By the heating in the oxygen atomosphere (1 × 10-9 Torr) at 500°C. the sulphur peak vanishes away.

A LEED study of the Ir (100) surface

The clean Ir (100) surface was obtained by heating in ultra-high vacuum and shows no atomic rearrangement. However after argon ion-bombardment and mild annealing an Ir (100) 5×1 structure was obtained. On further annealing the diffraction pattern reverted to the unreconstructed arrangement. Auger electron spectroscopy measurements indicate that the 5×1 structure might be due to oxygen. Reactions of the Ir (100) surface with CO, CO 2 and O 2 are also reported.