J. Randall Creighton

Highly aligned, template-free growth and characterization of vertical GaN nanowires on sapphire by metal–organic chemical vapour deposition

We report the growth of exceptionally well aligned and vertically oriented GaN nanowires on r-plane sapphire wafers via metal–organic chemical vapour deposition. The nanowires were grown without the use of either a template or patterning. Transmission electron microscopy indicates the nanowires are single crystalline, free of threading dislocations, and have triangular cross-sections. The high degree of vertical alignment is explained by the crystallographic match between the oriented nanowires and the r-plane sapphire surface. We find that the degree of alignment and size uniformity of the nanowires are highly dependent on the nickel nitrate catalyst concentration used, with the highest degree of uniformity and alignment occurring at concentrations much more dilute than typically employed for vapour–liquid–solid-based nanowire growth. Additionally, we report here a strong dependence of the optical and electrical properties of the nanowires on the growth temperature, which we hypothesize is due to increased carbon incorporation at lower growth temperatures.

The influence of adsorbate–absorbate hydrogen bonding in molecular chemisorption: NH3, HF, and H2O on Au(111)

Molecular beam and temperature programmed desorption (TPD) techniques are used to study the low temperature (85 K) adsorption and subsequent thermal desorption of NH3, HF, and H2O from Au(111). At 85 K the molecular sticking coefficients are near unity and are coverage independent. The TPD spectra are qualitatively different for the various species. Simple hydrogen bonding arguments based on absorbate–absorbate interactions are used to explain the differences in the TPD spectra.

Decomposition of trimethylgallium on the gallium-rich GaAs (100) surface: Implications for atomic layer epitaxy

The decomposition of trimethylgallium (TMGa) on the gallium‐rich (4×6) and (1×6) GaAs (100) surface was studied with temperature programmed desorption, Auger electron spectroscopy, and low‐energy electron diffraction. TMGa was found to dissociatively chemisorb on the gallium‐rich surfaces, apparently at the gallium vacancies that exist on these surfaces. We have unambiguously identified methyl radicals desorbing from the surface with the maximum rate at ∼440 °C following a saturation TMGa exposure. Since TMGa was shown to decompose on the clean, gallium‐rich GaAs (100) surfaces, the self‐limiting deposition of gallium during atomic layer epitaxy must be due to the presence of surface methyl groups which inhibit further TMGa dissociative chemisorption.

Metal CVD for microelectronic applications: An examination of surface chemistry and kinetics

Abstract We review the surface chemistry and kinetics relevant to the chemical vapor deposition (CVD) of metals used for microelectronic applications. Our efforts focus on the surface chemistry of aluminum, tungsten, and copper CVD, which have received the most recent interest for metallization. We first briefly review a variety of topics concerning the applications and the chemistry and kinetics of metal CVD. We also give a brief overview of the application of surface science techniques to the study of CVD-related surface chemistry.

The adsorption and reaction of triethylgallium on GaAs(100)

The surface chemistry of triethylgallium (TEGa) on both Ga-rich and As-rich GaAs(100) was studied using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and temperature programmed desorption (TPD). TEGa desorbs molecularly from two states with peak temperatures of about 323 and 523 K after TEGa exposure at 123 K on the Ga-rich surface. These conclusions of molecular desorption are in contrast to previous work which has been interpreted in terms of both TEGa and diethylgallium (DEGa) desorption above 150 K. The major products of TEGa decomposition, in agreement with previous work, are ethylene, hydrogen, and a small amount of ethyl radical. The saturation coverage of ethylene molecules and hydrogen atoms was measured to be ∼ 1 × 1014 cm−2. The total ethyl radical yield is estimated to be 8% of the saturation ethylene coverage (∼ 9 × 1012 molecules cm−2). On the As-rich surface, more TEGa desorption is observed in comparison to the Ga-rich conditions. The decomposition products are identical to those observed for the Ga-rich surface; however, the relative amounts of ethyl radicals to ethylene and hydrogen are significantly different.

Organometallic vapor phase epitaxy (OMVPE)

Abstract Organometallic vapor phase epitaxy (OMVPE) has emerged in this past decade as a flexible and powerful epitaxial materials synthesis technology for a wide range of compound–semiconductor materials and devices. Despite its capabilities and rapidly growing importance, OMVPE is far from being well understood: it is exceedingly complex, involving the chemically reacting flow of mixtures of organometallic, hydride and carrier-gas precursors. Recently, however, OMVPE technologies based on high-speed rotating disk reactors (RDRs) have become increasingly common. As fluid flow in these reactors is typically cylindrically symmetric and laminar, its effect on the overall epitaxial growth process is beginning to be unraveled through quantitative computer models. In addition, over the past several years, a combination of well-controlled surface science and RDR-based growth-rate measurements has led to a richer understanding of some of the critical gas and surface chemistry mechanisms underlying OMVPE. As a consequence, it is becoming increasingly possible to develop a quantitative and physically based understanding of OMVPE in particular chemical systems. In this article, we review this understanding for the important specific case of AlGaAs OMVPE in an RDR under conditions used for growing typical device heterostructures. Our goal is to use typical growth conditions as a starting point for a discussion of fundamental physical and chemical phenomena, beginning with the fluid flow through an RDR and ending with the chemical reactions on the surface. By focusing on one particularly important yet relatively simple specific case, this review differs from more comprehensive previous reviews. Viewed as a case study, though, it complements these previous reviews by illustrating the wide diversity of research that is related to OMVPE. It can also serve as a good starting point for the development and transfer of insights into other more complex cases, such as: OMVPE of materials families containing Sb, P or N species, of other devices types, and in other more complex reactor geometries.

Observations of gas-phase nanoparticles during InGaN metal-organic chemical vapor deposition

Using in situ laser light scattering, we have directly observed the formation of gas-phase nanoparticles during InN and InGaN metal-organic chemical vapor deposition. The angular dependence of the light scattering intensity suggests that the nanoparticles are metallic In or InGa alloys. From the angle-resolved scattering profile, we determined that the particle diameters were in the range 20–50nm, and particle densities were mostly in the 108–109cm−3 range. Results indicate that for growth temperatures near 800°C nearly 100% of the indium near the surface is converted into gas-phase nanoparticles and is no longer available for InGaN growth.

The surface chemistry and kinetics of GaAs atomic layer epitaxy

Abstract We discuss the chemical and kinetic aspects of GaAs atomic layer epitaxy (ALE). Explanations of the stoichiometry problems of GaAs ALE are proposed. We review the proposed ALE mechanisms that deal with trimethylgallium exposure. Results conclusively invalidate the selective adsorption ALE mechanism. Kinetic results indicate that the GaAs surface is covered with CH 3 groups during typical ALE conditions, as proposed by the adsorbate inhibition mechanism. Measurements of excess gallium deposition at typical ALE partial pressures are in good agreement with predictions of a unimolecular reaction mechanism using kinetic parameters determined by surface science techniques. Two simple ALE kinetic models based on unimolecular surface reaction mechanisms are constructed and the predictions are in good agreement with atmospheric pressure ALE results. The possible role of the flux balance mechanism is discussed.

A thermal desorption investigation of arsine chemisorption on Ga-rich and As-rich GaAs(100) surfaces

Abstract We have studied the low temperature chemisorption of arsine on the Ga and As-rich surfaces of GaAs(100) using temperature programmed desorption. Arsine desorbing from the “(4 × 6)” Ga-rich surface desorbs from both molecular ( T p ≈165 K ) and recombinative states ( T p =290 and 380 K) with an estimated saturation coverage of 0.09 ML, where 1 ML = 6.26 × 10 14 atoms / cm 2 . A very small amount of arsine is irreversibly adsorbed (approximately 0.01 ML) as evidenced by hydrogen desorption at about 520 K. This small arsine coverage suggests that most of the Ga atoms on the Ga-rich surface are not available for arsine chemisorption. On the c(2 × 8)/(2 × 4) As-rich GaAs(100) surface roughly two times more arsine desorbs compared to the Ga-rich surface and new desorption states are seen around 230 and 460 K. This increased arsine coverage suggests that most of the exposed Ga atoms on the As-rich surface are available for chemisorption. Approximately the same amount of arsine irreversibly decomposes on the c(2 × 8)/(2 × 4) As-rich surface (compared to the Ga-rich surface), but H 2 desorption occurs from a broad peak that is centered around 600 K. On the c(4 × 4) As-rich surface, arsine decomposition is not observed and only a small amount of arsine desorption is detected. This result is to be expected since there are no exposed Ga atoms on the c(4 × 4) surface.

Surface stoichiometry and the role of adsorbates during GaAs atomic layer epitaxy

Several questions regarding the stoichiometry of atomic layer epitaxy (ALE) arise because most polar compound semiconductor surfaces reconstruct into structures terminated with less than monolayer coverage at the outermost layer. In this paper we briefly discuss how to self-consistently account for stoichiometry changes on surfaces that are not ideally terminated. The key aspect of the methodology is that surface steps are allowed to act as a reservoir where atoms may be added or removed. The methodology shows that ideal ALE of GaAs(100) cannot occur by cycling between the known adsorbate-free Ga-rich and As-rich surface reconstructions, because no such transition would yield the observed 1 monolayer (ML) per cycle growth rate. In fact, ideal ALE (1 ML/cycle) must involve at least one adsorbate induced surface reconstruction. Adsorbates may stabilize ideally terminated (i.e. vacancy free) III–V surfaces because of their ability to passivate dangling bond states. For example, methyl groups adsorbed on GaAs(100) exhibit a (1 x 2) LEED pattern, which is not seen for the clean GaAs(100) surface reconstructions. By using the electron counting model we interpret this structure as 12 ML CH3 adsorbed on a complete layer (1 ML) of dimerized Ga atoms. The GaAs(100)-(1 x 2)-Ch3 surface was also examined using surface infrared spectroscopy (SIRS) using a multiple internal reflection geometry. This surface exhibits relatively sharp infrared linewidths suggestive of a well ordered structure. The polarization dependence of the symmetric stretching and bending CH3 modes also supports our proposed structure of the GaAs(100)-(1 x 2)-Ch3 surface. The ideal termination of the Ga-rich GaAs(100)-(1 x 2)-CH3 surface for a plausible ALE mechanism which yields 1 ML deposition per cycle.