Surface Characterization of GaN Formation on GaAs(100) Using Ammonia

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EXPERIMENTAL In this experiment, an n-type GaAs(100) wafer doped with Si (3 x 1018cm-3) was used as a substrate. The substrates were cleaned ultrasonically in organic solvents and etched with an etchant of H 2SO4:H20 2:H20=8:l:l at 50°C for 30s. The nitridation of the GaAs samples was carried out in a preparation chamber attached to a photoemission analysis chamber installed on the 2B11 SGM (Spherical Grating Monochromator) beamline at the Pohan 0 Synchrotron Light Source. The photoemission chamber had a base pressure of 1 x 10 Torr. In order to remove the native oxides on GaAs surface, the substrates were heated up to 600°C under an ultrahigh vacuum (UHV) condition. The nitridation temperature range measured by an optical pyrometer was from 550 to 736°C. The NH 3 dose controlled by the exposure time and the NH 3 partial pressure ranged from 2.52X 105 to 5X 108 L (1L=I0- Torr - sec). After the required substrate temperature was attained, NH 3 was introduced to a preparation chamber through a variable leak valve for the desired exposure. After the nitridation process was finished, the compositional changes in the nitridated layer on the GaAs samples was analyzed by a synchrotron radiation photoemission spectroscopy at room temperature. The surface morphological changes depending on the nitridation temperature were observed by an atomic force microscope (AFM). RESULTS AND DISCUSSION It is well known that the dissociation rate of chemisorbed molecules is strongly dependent on the substrate temperature. To study the effect of chemisorbed NH 3 on nitridation, NH 3 was introduced on the GaAs surface at 550'C and 7361C. First, ammonia was introduced into the preparation chamber through a variable leak valve when the substrate temperature reached 550'C. The NH 3 exposure dose was 5 x 108 L. Figure 1 shows the photoemission spectrum taken on the surface of GaAs after nitridation using a photon energy of 800 eV. A N1 , peak with very low intensity was observed from the sample surface after nitridation. This indicates that a small amount of NH 3 molecules are adsorbed on GaAs(100) at 5501C.

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Fig. 1. Photoemission spectrum taken on the GaAs surface after nitridation using a photon energy of 800 eV. Next, the GaAs surface was exposed to ammonia at 7361C. 64

The amount of NH 3

exposure was 2.52 X 108 L. Figures 2 and 3 show the Asd and Ga3d spectra from (a) the clean GaAs surface and (b) GaAs surface nitridated at 7361C, respectively. The spectra in fig. 2 show that the AS3d peak intensity obtained from the GaAs surface largely decreased after nitridation compared to that from the clean GaAs sample. This result suggests that As atoms on the GaAs surface evaporated efficiently at 7361C. The spectra also show that the binding energy (BE) of AS3d (40.3 eV) is not different from that of clean GaAs surface. Both the intensity decrement and a lack of chemical shift in the AS3d peak indicate that the GaAs surface is deficient in As atoms, and t