B.-K. Han

Observation of the atomic surface structure of GaAs(001) films grown by metalorganic vapor-phase epitaxy

We present atomically resolved scanning tunneling micrographs of the surfaces of GaAs(001) films grown by metalorganic vapor-phase epitaxy (MOVPE). Thin films deposited in an MOVPE reactor were transferred to an (ultra high) vacuum system without air exposure. After heating the samples from 480 to 580°C, high-quality images of the (2 × 4)/c(2 × 8), (1 × 6)/(2 × 6) and (4 × 2)/c(8 × 2) reconstructions were obtained. In addition, a new Ga-rich (3 × 2)/(3 × n) phase was observed that forms during annealing at 540°C. This structure consists of single dimer rows running along the [110] direction with a spacing of 12 A. The rows vary in length, and are separated by line defects which occur on average every 20 A (n = 5). A model is proposed for the (3 × 2) which consists of rows of Ga dimers alternating between the first and third layers. Since this structure exhibits a deficit of one electron, line defects are required to expose As dimers in the second layer and neutralize the surface charge.

A phosphorous-rich structure of InP (001) produced by metalorganic vapor-phase epitaxy

A phosphorous-rich structure is generated on the InP (001) surface during metalorganic vapor-phase epitaxy. It consists of phosphorous dimers, alkyl groups, and hydrogen atoms adsorbed onto a layer of phosphorous atoms. The adsorbed dimers produce c(2×2) and p(2×2) domains, with total phosphorous coverages of 2.0 and 1.5 ML. The alkyl groups and hydrogen atoms adsorb onto half of the exposed phosphorous atoms in the first layer. These atoms dimerize producing a (2×1) structure. It is proposed that the first layer of phosphorous atoms is the active site for the deposition reaction, and that the organometallic precursors compete with phosphorous dimers, alkyl radicals, and hydrogen for these sites during growth.

Structure and composition of the c(4×4) reconstruction formed during gallium arsenide metalorganic vapor-phase epitaxy

Surfaces of GaAs (001) were prepared by metalorganic vapor-phase epitaxy and characterized by scanning tunneling microscopy, x-ray photoelectron spectroscopy, infrared spectroscopy, and low-energy electron diffraction. Upon removal from the reactor, the gallium arsenide surface exhibits a (1×2) reconstruction, which is a disordered variant of the c(4×4). The disorder arises from the presence of adsorbed alkyl groups. Heating the sample to 350 °C desorbs the hydrocarbons and produces a well-ordered c(4×4) structure. A model is proposed for the alkyl-terminated (1×2) reconstruction.

STEP STRUCTURE OF ARSENIC-TERMINATED VICINAL GE (100)

Germanium (100) crystals, 9° off-axis towards the [011] were exposed to 2.0 Torr of tertiarybutylarsine and 99.0 Torr of hydrogen at 650 °C, then heated to between 450 and 600 °C in vacuum or H2. The resulting surfaces consist of narrow dimer-terminated terraces, with (1×2) and (2×1) domains, that are separated by steps between one and eight atomic layers in height. The distribution of (1×2) and (2×1) domains changes with temperature, exhibiting a pronounced maximum in the (1×2) fraction at 510 °C. These results suggest that the arsenic passivation of germanium is a critical step in gallium arsenide heteroepitaxy.

The reaction of carbon tetrachloride with gallium arsenide (001)

Carbon tetrachloride dissociatively adsorbs on the Ga-rich (4×2) reconstruction of GaAs (001) at 200 °C. Upon heating to 440 °C, the chlorine desorbs as GaCl, which etches the surface. Scanning tunneling micrographs reveal that this reaction transforms the (4×2) into a Ga-rich (3×2) structure that is interlaced with As-rich (2×4) phases. The (3×2) is well ordered, while the (2×4) phases exhibit a high degree of disorder. This work establishes the surface reaction pathway for carbon doping of GaAs with CCl4.

Surface phases of GaAs and InAs (001) found in the metalorganic vapor-phase epitaxy environment

We have characterized the (2×4) and (4×2) reconstructions of GaAs and InAs (001) that are present in a metalorganic vapor-phase epitaxy (MOVPE) reactor. Scanning tunneling micrographs show that these surfaces are terminated with arsenic and gallium (or indium) dimers. The (2×4) dimer row exhibits a mottled appearance, which is ascribed to the adsorption of alkyl groups on some of the sites. On the (4×2), <10% of the surface is covered with small (2×4) islands. These results show that, in the MOVPE environment, the GaAs and InAs surface structures are nearly the same as those found in ultrahigh vacuum molecular beam epitaxy.

Hydrogen adsorption on GaAs (001) reconstructions

Hydrogen adsorption on the c(4×4), (2×4), (2×6), and (4×2) reconstructions of GaAs (001) have been characterized by internal-reflection infrared spectroscopy. The infrared spectra contain up to 15 bands due to the stretching vibrations of arsenic hydrides (2150–1950 cm−1), terminal gallium hydrides (1950–1800 cm−1), and bridging gallium hydrides (1800–950 cm−1). These features arise from hydrogen adsorption on arsenic and gallium dimers, and second-layer arsenic and gallium atoms. The large number of peaks observed indicates that the surface atoms exist in a variety of different chemical environments.

Formation of etch pits during carbon doping of gallium arsenide with carbon tetrachloride by metalorganic vapor-phase epitaxy

Scanning tunneling microscopy was used to examine the effects of carbon tetrachloride concentration and temperature on the morphology of carbon-doped gallium arsenide films grown by metalorganic vapor-phase epitaxy. Deposition was carried out at 505–545 °C, a V/III ratio of 75, and IV/III ratios between 0.5 and 5.0. The growth rate declined monotonically with increasing carbon tetrachloride concentration. Step bunching and pinning was observed at a IV/III ratio of approximately 2.5. Increasing this ratio further resulted in the formation of pits ranging from 20 to 50 nm in diameter. These results can be explained by two competing processes that occur at the step edges: (1) the reaction of chlorine with adsorbed gallium from the group III precursor, and (2) the reaction of chlorine with gallium arsenide. Both reactions desorb gallium chlorides and reduce the growth rate, but only the latter reaction produces pits.

Composition and Structure of Vicinal Ge(100) Surfaces Exposed to Tertiarybutylarsine

Germanium (100) substrates, 9{degree} off axis towards the [011] direction, were heated in 2.0 Torr tertiarybutylarsine and 97.0 Torr hydrogen for 30 min at 650 C, then transferred to an ultrahigh vacuum system and annealed at several temperatures between 400 and 700 C. The arsenic coverage remained constant at 1.0 monolayer during heating in vacuum to 400 C. Further heating to 700 C caused the As coverage to gradually fall to zero. A mixed domain (1x2)+(2x1) dimer-terminated surface was observed by scanning tunneling microscopy after annealing at 400 to 510 C. Between 510 and 580 C, a single domain (1x2) reconstruction was recorded that contained equally spaced terraces separated by steps four atomic layers in height. Above 600 C, a (2x1) Ge(100) surface was produced that contained uniform terraces separated by steps two atomic layers in height.