Evolution of monolayer terrace topography on (100) GaAs annealed under an arsine/hydrogen ambient

J. E. Epler, T. A. Jung, H. P. Schweizer
1993 Applied Physics Letters  
The topographical evolution of the (100) GaAs surface annealed under an arsine/hydrogen ambient is studied by in situ orientation-resolved light scattering and ex situ atomic force microscopy (AFM). The light scattering system provides real-time monitoring of the magnitude and crystal orientation of topographical features of 0.3 ,um scale. The AFM images of the GaAs surface, quenched at various annealing temperatures, vividly depict the randomly oriented high density monolayer steps evolving
more » ... o an atomically smooth terracelike structure. An essential preparatory process of semiconductor epitaxial growth is an in situ thermal annealing that desorbs surface contaminants and smooths the surface topography. For the case of GaAs under arsenic impingement, the annealing process and growth initiation have been studied in a molecular beam epitaxy (MBE) system where optical scattering,' ** electron beam,314 and x-ray photoelectron5-7 techniques are applicable. Based on these studies, Ga203 is the most stable component of GaAs native oxide, desorbing near 640°C. Concurrent with the conclusion of the Ga203 desorption, chemical, and topographical restructuring of the surface is observed' with reflection high energy electron diffraction and diffuse optical scattering, respectively. In this letter, we report the application of in situ diffuse optical scattering8 in metalorganic chemical vapor deposition (MOCVD) system to study topographical restructuring from ( 100) GaAs under an arsine/hydrogen ambient. Uniquely, the in situ scattering signal is resolved with respect to crystal (substrate) orientation. Ex situ high quality atomic force microscope (AFM) images of the GaAs surface quenched at four stages of the microstructuring process are also shown. The results provide a greatly improved visualization of the topographical evolution from a high density of randomly oriented steps into a terracelike structure of monolayer steps. This topographical restructuring begins near 500 " C! and reaches a maximum rate of change near 640 " C. Our results suggest that the process of topographical reconstruction is relatively independent of the presence or absence of a naturally occurring surface oxide. The MOCVD system used here is an Emcore GS-3200 with three optical ports added to the growth chamber. All three ports are inclined 73" from the substrate normal and are located at 0 =o" , 90" , and 180" in the plane of the substrate. A 10 mW HeCd laser beam (442 nm) enters the 0" port, reflects off the center of the substrate, and exits the chamber through the opposing ( 180" ) port. The diffusescattered light is collected through the 90" optical port, passed through a narrow band interference filter and pinhole spatial filter, and detected with a photomultiplier. The ' )Present address: IBM T. J. Watson Research Center, Yorktown Heights, NY 10598. signal is amplified ( 1 kHz bandwidth) and digitized. The optical system selectively detects scattering from topographical Fourier components, K, of magnitude 19 pm-' corresponding to a feature size of 0.3 pm. Also, the symmetry of the optical system restricts the angle of K to 0=:45" , thus creating a detection " window." Since in this growth system the substrate is rotated (1000 rpm), we obtain in real time the orientation dependence of the topographical scattering. The substrates are on-axis ( 100) nf GaAs with " epitaxial ready" surface finish and, based upon the data below, are slightly misoriented (0.032" ) toward the [OOl] direction. The substrates are loaded directly from the Fluoroware container (as received) into the loadlock of the MOCVD system. Alternatively, they are deoxidized in 10% HCl/ethanols and loaded with a minimum of air exposure. A 1% arsine in hydrogen annealing ambient (pressure=90 mbar) is sufficient to prevent arsenic loss from the GaAs surface during the anneal. In Fig. 1 is shown the average scattering intensity (all orientations), the [ml] scattering, and the sample temperature as a function of time during a typical pregrowth anneal of an asreceived wafer. [OOl] scattering refers to the momentary signal intensity when the substrate rotation brings K parallel to [OOl] into alignment with the detection window. For example, steps running along the [OlO] direction would have a large K parallel to [OOl]. The sample temperature (calibrated at the Si-Al eutectic) is increased at 0.96 " C per s. A steady decrease in average scattering begins near 500 " C! and continues to over 720 " C. Throughout this range the surface is becoming smoother as Ga mass transport' and evaporation' 0-14 reduce the density of monolayer steps. The steep drop in average scattering at 640 " C! is accompanied by the emergence of the dominant [OOl] component. As discussed below, at this characteristic restructuring temperature, T, the surface topography is evolving into a terracelike structure with monolayer steps of OS-pm width, running along the [OlO] direction. In Fig. 2 , the orientation dependence of the in situ scattering of an as-received wafer is shown for several temperatures. For each curve the data from approximately 35 revolutions (2 s) have been digitally averaged. A gradual evolution is observed from isotropic scattering, with a small enhancement near the [OlO] direction, towards a 143
doi:10.1063/1.109352 fatcat:ulhnav5e5rgf3lux43fm6rzpsm