D. C. Law

Structure-sensitive oxidation of the indium phosphide (001) surface

The oxidation of anion- and cation-rich indium phosphide (001) has been investigated by exposure to unexcited molecular oxygen. Indium phosphide oxidation is an activated process and strongly structure sensitive. The In-rich δ(2×4) surface reacts with oxygen at 300 K and above. Core level x-ray photoemission spectra have revealed that the O2 dissociatively chemisorbs onto the δ(2×4), inserting into the In–In dimer and In–P back bonds. By contrast, the P-rich (2×1) reconstruction does not absorb oxygen up to 5×105 Langmuir at 300 K, as judged by the unperturbed reflectance difference spectrum and low energy electron diffraction pattern. Above 455 K, oxygen reacts with the (2×1) inserting preferentially into the In–P back bonds and to a lesser extent into the phosphorus dimer bonds.The oxidation of anion- and cation-rich indium phosphide (001) has been investigated by exposure to unexcited molecular oxygen. Indium phosphide oxidation is an activated process and strongly structure sensitive. The In-rich δ(2×4) surface reacts with oxygen at 300 K and above. Core level x-ray photoemission spectra have revealed that the O2 dissociatively chemisorbs onto the δ(2×4), inserting into the In–In dimer and In–P back bonds. By contrast, the P-rich (2×1) reconstruction does not absorb oxygen up to 5×105 Langmuir at 300 K, as judged by the unperturbed reflectance difference spectrum and low energy electron diffraction pattern. Above 455 K, oxygen reacts with the (2×1) inserting preferentially into the In–P back bonds and to a lesser extent into the phosphorus dimer bonds.

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.

Reflectance difference spectroscopy of mixed phases of indium phosphide (001)

Reflectance difference spectra of mixed (2×1) and (2×4) phases of indium phosphide (001) have been recorded and benchmarked against scanning tunneling micrographs of the surface. The line shapes are found to be linear combinations of the spectra of the pure (2×1) and (2×4) structures, Δr/rmixed=xΔr/r(2×4)+(1−x)Δr/r(2×1), where x is the weighting factor. Thus, in the absence of adsorbates, the reflectance difference spectra can be used to estimate the surface composition, i.e., the fractional coverage of phosphorous is ΘP=1−0.81x±0.06x.

Kinetics of tertiarybutylphosphine adsorption and phosphorus desorption from indium phosphide (001)

Abstract The kinetics of tertiarybutylphosphine adsorption and phosphorus desorption from indium phosphide ( 0 0 1 ) have been determined using reflectance difference spectroscopy for real-time monitoring of the phosphorus coverage. The precursor adsorption rate depends linearly on the coverage, and the initial sticking coefficient varies from 0.007 to 0.001 as the temperature increases from 420 to 520 °C. The phosphorus desorption rate is first order in the coverage and exhibits an activation energy and pre-exponential factor of 2.4 ± 0.2 eV and 10 14.7 ± 1.5 s −1 . These reaction kinetics play an important role in the growth of phosphide-based alloys by metalorganic vapor-phase epitaxy.

Analysis of the growth modes for gallium arsenide metalorganic vapor-phase epitaxy

The surface roughness of gallium arsenide (001) films produced by metalorganic vapor-phase epitaxy has been studied as a function of temperature and growth rate by in situ scanning tunneling microscopy. Height–height correlation analysis reveals that the root-mean-height difference follows a power-law dependence on lateral separation, i.e., Γ(L)=kLa, up to a critical distance Lc, after which it remains constant. For layer-by-layer growth, the roughness exponent, α, equals 0.25±0.05, whereas the critical distance increases from 50 to 150 nm as the substrate temperature increases from 825 to 900 K. The roughness exponent jumps to 0.65±0.1 upon transitioning to three-dimensional island growth. By relating the height–height correlation analysis to the Einstein diffusivity relationship, the activation energy for gallium surface diffusion has been estimated: Ed=1.35±0.1 eV.

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 650u2009°C, then heated to between 450 and 600u2009°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 510u2009°C. These results suggest that the arsenic passivation of germanium is a critical step in gallium arsenide heteroepitaxy.

Kinetics of phosphine adsorption and phosphorus desorption from gallium and indium phosphide (0 0 1)

Abstract The kinetics of phosphine adsorption and phosphorus desorption from gallium and indium phosphide (0xa00xa01) has been determined using reflectance difference spectroscopy to monitor the phosphorus coverage in real time. Assuming a Langmuir adsorption mechanism, phosphine exhibited an initial reactive sticking coefficient at 500 °C of 8.7xa0±xa01.0xa0×xa010 −2 , 3.5xa0±xa01.0xa0×xa010 −2 and 1.0xa0±xa00.2xa0×xa010 −3 on the GaP (2xa0×xa04), GaP (1xa0×xa01) and InP (2xa0×xa04) reconstructions, respectively. The sticking coefficient increased with temperature on the gallium phosphide surfaces, exhibiting an activation energy of 0.5xa0±xa00.2 eV, while on indium phosphide, no temperature dependence was observed. The desorption of phosphorus from the GaP (2xa0×xa01) surfaces was first-order in coverage with rate constants of 5.0xa0×xa010 15 (s −1 )xa0exp(−2.6xa0±xa00.2 (eV)/kT). These results may be used to estimate the feed rate of phosphine relative to the group III precursors during the metalorganic vapor-phase epitaxy of gallium and indium phosphide.

Reflectance difference spectroscopy of gallium phosphide(001) surfaces

Gallium phosphide(001) surfaces have been prepared by metalorganic vapor-phase epitaxy, and characterized in situ by low-energy electron diffraction, x-ray photoemission spectroscopy, and reflectance difference spectroscopy. Three stable phases were observed: (2×1), (1×1), and (2×4) with phosphorus coverages of 1.00, 0.67, and 0.13 ML, respectively. Reflectance difference spectra obtained at coverages intermediate between these three values were found to be linear combinations of the spectra of the pure phases. In particular, ΔR/R(mixed)=mΔR/R(1×1)+(1−m)ΔR/R(2×1)u2009oru2009(2×4), where m is a weighting factor. The weighting factors were used to estimate the phosphorus coverage, and these results agreed to within 5.0% of the values measured by x-ray photoelectron spectroscopy.

Hydrogen atoms as a probe of the optical anisotropy of indium phosphide (001)

The reflectance difference spectra of the InP(0 0 1) (2×1) and δ(2×4) reconstructions have been characterized using hydrogen as a probe of the surface bonds. Bands observed at 1.9, 3.1, 4.1, and 4.6 eV on the (2×1) and at 2.8, 3.7, and 4.6 eV on the δ(2×4) decrease in direct proportion to the hydrogen coverage. By comparing the changes in the reflectance difference spectra to the changes in the atomic structure of the surfaces, it is possible to relate the peaks to transitions involving specific valence bond states.

Gallium arsenide and indium arsenide surfaces produced by metalorganic vapor-phase epitaxy

Abstract Thin films of GaAs and InAs were deposited on GaAs(0xa00xa01) substrates by metalorganic vapor-phase epitaxy (MOVPE), and their surfaces were characterized by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). Gallium arsenide surfaces produced in the MOVPE reactor at 570°C and a V/III ratio of 50 exhibit a (1×2) reconstruction, and are covered with weakly bound alkyl groups. Heating this material in flowing hydrogen in the reactor produces a variety of surface phases, depending on the sample temperature. These phases include (2×4) and (4×2) reconstructions, all of which are terminated with As or Ga dimers. The surfaces of InAs films grown by MOVPE exhibit these same phases. This study demonstrates that compound semiconductor surfaces formed in the MOVPE environment are nearly the same as those produced by molecular-beam epitaxy under ultrahigh vacuum conditions.