Ming L. Yu

Doping reaction of PH3 and B2H6 with Si(100)

The reaction of phosphine PH3 and diborane B2H6 on Si(100) surfaces was studied by surface analytical techniques in relation to the in situ doping process in the chemical vapor deposition of silicon. Phosphine chemisorbs readily either nondissociatively at room temperature or dissociatively with the formation of silicon–hydrogen bonds at higher temperatures. Hydrogen can be desorbed at temperatures above 400 °C to generate a phosphorus layer. Phosphorus is not effective in shifting the Fermi level until the coverage reaches 2×1014/cm2. A maximum shift of 0.45 eV toward the conduction band was observed. In contrast, diborane has a very small sticking coefficient and the way to deposit boron is to decompose diborane directly on the silicon surface at temperatures above 600 °C. Boron at coverages less than 2×1014/cm2 is very effective in shifting the Fermi level toward the valence band and a maximum change of 0.4 eV was observed.

Pyrolysis of trimethylgallium on GaAs(100) surfaces

We have used a combination of pulsed molecular beam and time‐resolved mass spectrometry to study the kinetics of the pyrolysis of trimethylgallium on GaAs(100) surfaces. We found that CH3 is the major reaction product. Two CH3 desorption channels were observed, with activation energies 37.9±1.6 and 45.0±1.4 kcal/mole. An arsine ambient significantly accelerates the CH3 desorption, but no CH4 was observed. A model for the reaction of trimethylgallium on the GaAs(100) surface is proposed.

Reaction of trimethylgallium in the atomic layer epitaxy of GaAs (100)

The initial reaction of the As‐rich c(2×8) GaAs(100) surface with trimethylgallium (TMGa) has been studied by x‐ray and ultraviolet photoemission. TMGa was found to chemisorb dissociatively at temperatures below 300 °C with many of the methyl radicals remaining on the surface. Bonding requirements and steric effects limit the saturation coverage to about 0.1 monolayer. At temperatures above 300 °C, TMGa was observed to dissociate on the surface, release all the methyl groups, and deposit Ga up to a self‐limiting coverage. This process is most effective at high temperatures. We propose a model for the temperature dependence of the chemisorption which explains and unites many reported observations in the atomic layer epitaxy of GaAs using TMGa and arsine.

Secondary ion emission from binary alloy systems. Part I: O+2 bombardment

We have studied the emission of positive atomic ions from five binary alloys: Cr‐Ni, Fe‐Ni, Cu‐Ni, Pd‐Ni, and Ag‐Pd, all under O+ 2 bombardment. Significant enhancement and suppression of ionization probabilities due to the presence of the second alloy component were observed. The alloying effect follows two general rules. The first rule states that for an alloy A‐B, where A forms a stronger oxide bond than B, the presence of A enhances the emission of B+, while the presence of B suppresses the emission of A+. The second rule states that the presence of A sharpens the energy distribution of B+, while the presence of B broadens the energy distribution of A+. X‐ray photoemission studies of the O+ 2 bombarded alloy surfaces connect the general rules with the enhanced oxidation of B and the suppressed oxidation of A in the alloys. All the cases that deviate from the rules originate from insufficient oxygen for oxidation resulting from the self‐limiting nature of oxidation by O+ 2 bombardment.

The effect of oxygen on the sputtering of metastable atoms and ions from Ba metal

By means of the Doppler-Shift-Laser-Fluorescence technique we have measured the effect of oxygen coverage on the velocity distributions and relative yields of sputtered Ba atoms and ions in both the ground and metastable excited states. Our measurements show no evidence that the excitation probabilities are correlated with the magnitude of the electric-dipole-moment matrix element to the ground state. We also find that the excitation probability of the Ba(1 D) state from a clean metal surface depends strongly on the emission velocity υ and approaches the functional form exp(−Aav) at higher velocities.

Secondary ion emission from binary alloy systems. Part II: Ar1 bombardment with O2 absorption

The emission of positive atomic ions from Cu‐Ni, Pd‐Ni, and Ag‐Pd alloys was studied under Ar+ bombardment with saturated O2 adsoprtion. In line with other published data on Cr‐Fe, Cr‐Ni, and Fe‐Ni alloys, these studies all agree well with two general rules porposed previously for O+2 bombardment. The first rule states that for an alloy A‐B, where A forms a stronger oxide bond than B, the presence of A enhances the emission of B+, and the presence of B suppresses the emission of A+. The second rule states that the presence of A sharpens the energy distribution of B+ while the presence of B broadens the energy distribution of A+. The experimental data fit the rules with better consistency than for the case of O+2 bombardment except in cases of low oxygen sticking coefficients on the alloy surfaces. The success of the general rules is linked with the enhanced oxidation of B and the suppressed oxidation of A in the alloy. X‐ray photoemission studies of the oxidized surfces support this explanation. Further e...

Comparison between Si(100) and Si(111) in the reaction with oxygen at high temperatures

In this Letter, both the dynamics and kinetics of the reaction of oxygen molecules on Si(100)p2 × 1 and Si(111)7 × 7 and 1× 1 surfaces are compared. In all three cases, two distinct adsorption channels were observed. For oxygen molecules with translational energies less than 0.08 eV, the initial sticking is not sensitive to the energy or the angle of incidence, but displays a high sensitivity to the surface structure. At higher energies, a second channel becomes effective. The initial sticking coefficient increases rapidly and scales with the normal component of the translational energy, but the dependence on surface structure is greatly diminished. The kinetics of SiO formation are qualitatively similar on all surfaces with slightly higher rates on Si(111).

Gas/surface reactions in the chemical vapor deposition of tungsten using WF6/SiH4 mixtures

We have studied the surface reactions of WF6/SiH4 mixtures on Si(100) that are relevant to the chemical vapor deposition of tungsten using x‐ray photoemission and molecular‐beam reactive scattering. The overall reaction is found to be 3SiH4+2WF6→2W+3SiF4+6H2 with very little production of HF. The reaction proceeds by repeating the cycle of tungsten deposition by the reaction of WF6 with silicon, and the deposition of silicon by the reaction of SiH4 with the fluorinated tungsten surface.

Mechanisms of atomic layer epitaxy of GaAs

We have studied the surface chemistry of trimethylgallium and diethylgallium chloride on GaAs(100) surfaces as related to the atomic layer epitaxy (ALE) of GaAs. We have observed that during the Ga deposition, the reaction pathway of trimethylgallium changes such that there is significant CH3Ga emission at high Ga coverages. An examination of the Ga coverage dependence reveals that this stoichiometry dependent CH3Ga desorption can lead to self‐limiting Ga deposition which is a prerequisite for ALE. Numerical simulation of the reaction shows reasonable agreement with low‐pressure ALE growth data. Diethylgallium chloride was found to deposit GaCl on the GaAs surfaces, but with residence time decreasing rapidly with increasing Ga coverage. Again a numerical examination of this stoichiometry dependent phenomenon indicates that it can significantly contribute to the self‐limiting Ga deposition. Both systems showed that stoichiometry dependent reactions can be important mechanisms for the atomic layer epitaxy p...

Charging effects in the secondary ion mass spectrometric analysis of targets containing low‐conductivity regions

Common ion sputtering techniques for depth profiles can cause buildup of charge in low‐conductivity regions such as p‐n junctions in semiconductors, leading to erroneous results. We analyzed Ga0.1Al0.9As liquid phase epitaxial (LPE) layers on GaAs with secondary ion mass spectrometry (SIMS) and observed that the profiles of 69Ga+ and 27Al+ in the low‐conductivity region of these Zn‐diffused LPE layers were a function of the sample bias. We noticed that this artifact was due to the shift and broadening of the secondary ion energy distributions caused by positive charging of the specimen surface. We also found that this could be effectively remedied by integrating the secondary ion yields over a wide energy range. The use of an electron flood gun reduced this charging effect, but complete charge compensation was not achieved even at electron current densities far exceeding the ion current density. This can be explained by the fact that the electron‐induced secondary electron coefficient of GaAlAs is larger ...