Laser Stimulated Desorption from Compound Semiconductors, Dimerization Enhanced Phase Transition

Abstract

Laser stimulated desorption (LSD) of neutral particles has been investigated in compound semiconductors MX of GaN, GaP, GaAs and CdS. No effects of crystallographic orientations on the characteristics of LSD, desorption yields, mean kinetic energies, and speed ratios have been recognized among (111) and (100) orientations in GaP and GaAs. This fact suggests that LSD does not follow the unimolecular dissociation of locally excited surface bonds, but occurs via a bulky event involving a phase transtion. The threshold laser powers for LSD were correlated to the crystal bond ionicities f i, which are good measures for the structural stability of tetrahedral covalent solids. It was found that with increasing f i of the solids, threshold laser powers tend to decrease. The dominant species ejected from MX were found to be in the forms of monomer M and X as well as dimer X2. The ratio of the yields of X2 to those of X increases as the single-bond energy of X2 increases. Considering these experimental results, we propose a model of a non-local electronic excitation mechanism for LSD; namely, LSD occurs via the dimerization enhanced phase transition in a high-density electron-hole plasma.

Excitation-induced structural instability of semiconductor surfaces

“…Therefore, equation ( 16) can be applied for the present analysis. Also, since the maximum value of N(0, t) is less than 2Z h for the highest excitation intensity, we can use the approximated form of J of equation (19) to calculate the bond-rupture rate J (t). The result of J (t) for I ex = 0.9 MW cm −2 , at constant J 0 , is shown in figure 22(b).…”

“…On the other hand, laser-stimulated desorption (LSD) below the melt threshold was studied for several MX compound semiconductors. In these studies, it was shown that the monatomic and diatomic nonmetallic species are emitted as major products, together with monatomic metallic species [18,19]. The yields of these neutral species often show a highly superlinear dependence on the laser intensity.…”

“…The yields of these neutral species often show a highly superlinear dependence on the laser intensity. Several models of the electronic mechanisms were proposed based on these results [18][19][20]. However, for establishing a basic understanding of the electronic mechanism of LSD, the following aspects needed clarification: (1) the initial bonding properties of atoms to be desorbed, (2) the electronic transition that triggers the desorption, and (3) the origin of the instability that causes bond breaking and desorption, as often pointed out in studies of desorption induced by electronic transitions [21].…”