Transient evolution of surface roughness on patterned GaAs(001) during homoepitaxial growth

We have investigated the length scale dependence of the transient evolution of surface roughness during homoepitaxial growth on GaAs(100), patterning the surface lithographically with an array of cylindrical pits of systematically varied sizes and spacings. Our atomic force microscopy measurements show that the amplitude of the surface corrugation has nonmonotonic behavior in both the length scale dependence and time evolution. This behavior allows us to rule out a number of existing continuum models of growth.     

Structure and homoepitaxial growth of GaAs(6 3 1)

We have studied the surface atomic structure of GaAs(6 3 1), and the GaAs growth by molecular beam epitaxy (MBE) on this plane. After the oxide desorption process at 585 °C reflection high-energy electron diffraction (RHEED) showed along the [−1 2 0] direction a 2× surface reconstruction for GaAs(6 3 1)A, and a 1× pattern was observed for GaAs(6 3 1)B. By annealing the substrates for 60 min, we observed that on the A surface appeared small hilly-like features, while on GaAs(6 3 1)B surface pits were formed. For GaAs(6 3 1)A, 500 nm-thick GaAs layers were grown at 585 °C. The atomic force microscopy (AFM) images at the end of growth showed the self-formation of nanoscale structures with a pyramidal shape enlarged along the [5−9−3] direction. Transversal views of the bulk-truncated GaAs(6 3 1) surface model showed arrays of atomic grooves along this direction, which could influence the formation of the pyramidal structures.     

Vibrational study of the initial stages of the oxidation of Si(111) and Si(100) surfaces

Using high-resolution electron energy loss spectroscopy (EELS) the vibrations of Si(111) and Si(100) surfaces in the early stages of oxidation have been investigated. Three different stages of oxidation, the last being the formation of a thin layer of vitreous SiO₂ are identified when the surfaces are held at a temperature of 700K during the exposure with molecular oxygen. We show that also the first two stages involve atomic oxygen in bridging positions between silicon atoms. Small exposures at low temperatures (100 K) produce vibrational features of a different, possibly molecular, species. For higher exposures at the same temperature the spectrum again develops the characteristics of atomic oxygen and the molecular species eventually disappears. Exposure at room temperature leads to a mixture of atomic and molecular oxygen for smaller exposures and to purely atomic oxygen for exposures greater than 10² L. At room temperature even exposures as high as 10¹¹ L do not produce the spectrum of vitreous SiO₂. The same is found for the natural, room temperature grown, oxide layer on silicon wafers which we have studied by introducing the sample into the spectrometer through an air-lock. Annealing of the wafer to 700 K produced the characteristic spectrum of vitreous SiO₂. The results are discussed in comparison with previous work.     

Strain relaxation in InAs/GaAs(111)A heteroepitaxy

We have studied strain-relaxation processes in InAs heteroepitaxy on GaAs(111)A using rocking-curve analysis of reflection high-energy electron diffraction. Strain relaxation in the direction parallel to the surface occurs at approximately 1.5 bilayers (BL) thickness. On the other hand, the lattice constant in the direction normal to the surface remains almost unchanged below approximately 3 BL thickness and is estimated to be approximately 3.3 A. This value, slightly larger than that of bulk GaAs (3.26 A), does not quite reach the value predicted by classical elastic theory, 3.64 A. The present result has been supported by the first-principles total-energy calculations.     

Langmuir evaporation from the (100), (111A), and (111B) faces of GaAs

We have measured the Langmuir evaporation of Ga and As from the (100), (111A), and (111B) faces of GaAs above and below the congruent evaporation temperature T_c. We have found that T_c is lowest for the (111B) face and highest for the (111A) face. These differences can be understood in terms of the different lifetimes of surface Ga on these faces. Furthermore, we have deduced that the evaporation processes are the rate limiting steps in the decomposition of GaAs. Below T_c, decomposition is controlled by the evaporation of Ga; above T_c it is controlled by the evaporation of As.     

GaAs(111)A/B surface orientation effects on electron density in normal and inverted pseudomorphic HEMTs

We present calculations of the two-dimensional electron density in a Si -doped AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor (P-HEMT), at low temperature, for three different growth directions (001) and (111)A/B. The calculations are made using a self-consistent resolution of Schrödinger and Poisson equations. The presence of a strong built-in piezoelectric field in (111)A/B growth directions causes changes of the confining potential shape and the carrier distribution in the InGaAs channel. We discuss the influence of GaAs substrate orientation on the conduction-band structure and thereafter on the two-dimensional electron gas (2DEG) concentration in the channel. Our results show that the calculated 2DEG concentration in the normal P-HEMT structure grown on a (111)A GaAs substrate is significantly higher than those grown on (001) and (111)B GaAs substrates. We also note an increase of the average separation between the ionized donors and the carriers. On the other hand, the GaAs (111)B substrate orientation appears as inadequate for the type of structure (normal P-HEMT) on account of the charge-transfer reduction in the channel compared with the (001) orientation. In contrast, we demonstrate that the calculated 2DEG Si -doped GaAs/InGaAs/AlGaAs pseudomorphic inverted high electron mobility transistor (PI-HEMT) grown on aGaAs (111)B substrate is appreciably higher than that grown on (001) and afterwards an enhancement of the spatial separation between confined electrons in the channel and ionized dopants occurs. These effects might result in considerably improved devices of great interest regarding high electron mobility. PACS 73.20.Dx; 73.20.At; 73.90.+f; 73.63.Hs; 72.20.Dp