About the 8th Biennial Workshop on OMVPE: Technical Program, Tuesday Afternoon Sessions (April 15)

The following papers will be presented at the 8th Biennial Workshop on OMVPE, on Tuesday evening, April 15th, 1997. The calendar of events describes the entire technical program.

SESSION VI: GROWTH MECHANISMS & ORDERING

Impurity Incorporation and the Surface Morphology of MOVPE Grown GaAs: Jiang Li and T.F. Kuech, Department of Chemical Engineering, University of Wisconsin, 1415 Engineering Drive, Madison, WI 53706

A comprehensive study of impurity incorporation in GaAs in metal-organic vapor phase epitaxy (MOVPE) process, combining growth, electrical and morphological studies, is presented. The impact of impurity incorporation on the development of surface morphology has been systematically investigated. According to our investigation the surface morphology of the epilayers was determined by both the properties of the impurity itself and the surface growth chemistry. A variety of different doping materials including Mg, Zn, C, Si, O and Se, were used to study the influence of the interaction between the impurity atoms and the host material on the surface morphology. Impurity atoms with smaller atomic weight belonging to group II and VI has a larger influence on the surface morphology than the other doping materials. Different doping sources for the same dopant were also used to explore the effect of surface growth chemistry on the formation of surface feature. The surface adsorbed methyl group can impede the lateral motion of surface steps thus lead to the modification of surface features. Models for impact of impurity on the growth front evolution based on surface chemistry are presented.

7:40 pm

Growth of Order/Disorder Heterostructures in GaInP using a Variation in the V/III Ratio: Y.S. Chun*, S.H. Lee*, I.H. Ho*, G.B. Stringfellow*, C.E. Inglefield**, M.C. DeLong**, P.C. Taylor**, J.H. Kim***, and T.Y. Seong***, *Department of Materials Science and Engineering and Electrical Engineering, and **Departments of Physics, University of Utah, Salt Lake City, UT 84112, ***Departments of Materials Science and Engineering, Kwangju Institute of Science and Technology, Kwangju, Korea

CuPt ordering is known to result in a shrinkage of the bandgap of GaInP lattice matched to nominally (001) GaAs substrates. This phenomenon has been used to produce heterostructures having a bandgap energy difference of 160 meV by varying the temperature during the growth process. This type of heterostructure contains absolutely no change in the solid composition. Both layers are precisely lattice matched to the GaAs substrate. However, changing the temperature during growth is an awkward process not suitable for general use. Much more convenient would be to use a change in the flow rate of the phosphorus precursor to change the degree of order for heterostructures. This can be done rapidly and simply. Initial experiments using PH3 as the P precursor failed. Both transmission electron microsoopy (TEM) and photolumincscence (PL) showed that the change in order parameter was graded over distances of thousands of è. Surprisingly, a change of the P precursor to tertiarybutylphosphine (TBP) results in abrupt heterostructures for both order/disorder and disorder/order heterostructures. TEM images and transmission electron diffraction patterns demonstrate the abrupt change in order parameter. PL and PL excitation spectroscopy were used to verify the change in bandgap energy obtained for both types of heterostructures. The reason for the striking and unexpected difference in behavior between PH3 and TBP has been investigated by examining the dynamics of the change in surface reconstruction, using surface photo absorption (SPA), and the step structure, using atomic force microscopy, induced by a change in the TBP flow rate. For TBP both the change in surface reconstruction and the change in the step structure (from predominately bilayer at high TBP flow rates to monolayer at low TBP flow rates) caused by the change in V/III ratio are extremely rapid, resulting in the observed abrupt heterostructures.

8:00 pm

Surface Chemistry of Doping Gallium Arsenide with Carbon: Haihua Qi, Byung Kwon Han and Robert Hicks, Chemical Engineering Department, 5531 Boelter Hall, University of California, Los Angeles, CA 90095-1592

The adsorption and decomposition of carbon tetrachloride (CCl4), a new precursor, trisdimethylaminomethane (HC(Nme2)3, TDMAM) and dimethylamine (DMA) on GaAs(001) surfaces has been studied in ultrahigh vacuum. Dimethylamine adsorbs non-dissociatively on As-rich surfaces at 300 K. Polarized infrared spectroscopy reveals that DMA adsorbs on second-layer Ga atoms with surface N-Ga bonds oriented along the [110] direction. Upon heating to 353 K, all the DMA desorbs intact. By contrast, TDMAM dissociatively adsorbs on As-nch GaAs(100) at 300 K, as evidenced By the appearance of C-H stretching vibrations for CHx groups bonded to Ga and As atoms. During heating, some of these species remain on the surface, and decompose into adsorbed carbon OD both As and Ga sites. On the other hand, it was discovered that CCl4 adsorbs only on the Ga dimers exposed on Ga-rich surfaces. A fraction of these molecules decompose during heating and deposit carbon on the gallium sites. At the meeting, we will describe the pathways for carbon incorporation into the GaAs lattice from CC14 and TDMAM during organometallic vapor-phase epitaxy.

8:20 pm

Fermi Level Pinning during OMVPE of GaAs: R.M. Cohen, Department of Materials Science and Engineering, University of Utah, Salt Lake City, UT 84112

The electrochemical potential relates the concentration of neutral dopant atoms in the vapor to the concentration of the corresponding ions and electronic carriers in the solid. Thus, if the carrier concentration is pinned at the surface, the equilibrium ion concentration at the surface will deviate significantly from its bulk equilibrium value. Using the grown-in dopant concentration as a probe of the carrier concentration at a growing surface, it is shown that the surface Fermi energy does not change even if more than lEl9 (cm3) of dopant (n- or p-type) is incorporated into the GaAs. Using Zn, a transition between a metastable supersaturated concentration and a near-equilibrium dopant concentration can be observed as diffusion causes the dopant concentration to relax to its equilibrium bulk value at low growth rates. Results from OMVPE on (100)-oriented wafers are consistent with a Fermi energy pinned approximately 150 meV below the intrinsic Fermi energy for both n- and p-type GaAs.

8:40 pm

Effect of Growth Rate on Step Structure and Ordering in GaInP: Y.S. Chun, S.H. Lee, I.H. Ho, and G.B. Stringfellow, Departments of Materials Science and Engineering and Electrical Engineering, University of Utah, Salt Lake City, UT 84112

CuPt ordering is widely observed in GaInP epitaxial layers grown by organometallic vapor phase epitaxy. The formation of this ordered structure spontaneously during epitaxial growth is intimately related to the atomic-scale physical processes occurring on the surface, specifically surface reconstruction and the attachment of atoms at steps. For growth on singular (001) GaAs substrates the surface structure, measured using atomic force microscopy, is seen to consist of small islands surrounded by either monolayer or bilayer steps. An increase in the growth rate from 0.25 to 2.0 mm/hr with a constant partial pressure of the P precursor (either tertiarybutylphosphine (TBP) or phosphine) has been studied. At a growth temperature of 670C using TBP, the growth rate is found to have no effect on either the degree of order or the step structure. Only the step spacing is observed to change. It decreases systematically as the growth rate is increased, following an approximate 1/(growth rate)1/2 dependence. As the growth rate increases, the time atoms have to rearrange before being frozen due to coverage by the next layer decreases. This leads directly to the dependence observed. These observations are consistent with previous observations which appear to show a close correlation between step structure (monolayer vs. bilayer) and ordering, although the causative factor has not been determined.

9:00 pm

Low temperature (~600°C) OMVPE Growth of High Quality GaAs/AlGaAs Heterostructures on (111)A Substrates: E. Mao, S.A. Dickey, and A. Majerfeld, Department of Electrical and Computer Engineering, CB425, University of Colorado, Boulder, CO 80309, A. Sanz-Hervas, Dpto. Tecnologia Electronica, E.T.S.I. Telecomunicacion, U.P.M. Ciudad Universitaria, 28040 Madrid, Spain, B.W. Kim, Electronics and Telecommunications Research Institute, P.O. Box 106, Yusong, Taejon, 305-600 Korea

Novel surface orientations such as the (111) offer the possibility of exploiting the piezoelectric effect for a variety of optoelectronic devices and also show enhanced optical properties in comparison with structures grown on conventional (100) substrates. The surface morphology, crystalline quality, electrical and optical properties of atmospheric pressure OMVPE GaAs and AlGaAs layers grown on exact and misoriented (111)A, (111)B were investigated and compared to (100) layers. The growth parameters to obtain specular surfaces on (111) substrates for GaAs and AlGaAs single layers and heterostructures were established. We will report the first GaAs/AlGaAs quantum well structures fabricated on (111)A substrates by OMVPE. Detailed characterization by high resolution x-ray diffractometry, photoluminescence and photoreflectance indicate that the quantum well structures have interfaces of the order of one monolayer even for those grown at temperatures as low as 600C. This temperature is significantly lower than that recently reported for MBE grown (111)A quantum wells. The low growth temperature achieved for (111)A surfaces should be suitable for fabrication of InGaAs/AlGaAs heterostructures.