Photochemical Dynamics on Semiconductor Surfaces
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surfaces: (i) direct photo-excitation of gas molecules; (ii) direct photo-excitation of adsorbates and adsorbate-surface complexes; (iii) substrate excitation followed by the attachment of excited carriers to the adsorbate; and (iv) thermal reactions due to transient heating by the laser pulse. Mechanism (i) is based on the creation of reactive species in the gas phase for enhanced deposition. Difficulties associated with mass transport in this mechanism usually limits its applications in patterned deposition. Mechanism (iv) is essentially thermal in nature. The surface photochemical mechanisms, (ii) and (iii), are restricted to the adsorbate/substrate interface and are particularly desirable for spatially selective growth and stringent control of ultra-thin films, such as quantum well structures. In this account, I will address the nonthermal surface mechanisms using examples related to III-V compound semiconductor growth: Ga(CH 3)3 /GaAs [2], AsH 3 /GaAs [3], NH 3/GaAs [4-7]. MECHANISMS A typical surface photochemical system usually involves an optically thin adsorbate layer on a strongly absorbing substrate. Direct photo-excitation of adsorbed molecules is, in most cases, overwhelmed by excitations in the substrate. This has lead to the observation of substratemediated charge-transfer excitations in the majority of semiconductor surface photochemical systems. While the common feature in substrate-mediated surface photo-excitation on semiconductors is the first step, i.e., photon absorption by the substrate in the creation of electron-hole pairs, there is a rich spectrum of relaxation, transport, and attachment mechanisms for excited carriers. On the other hand, direct photo-excitations on a semiconductor surface can be different from that in an isolated molecule and may involve various surface states. 543 Mat. Res. Soc. Symp. Proc. Vol. 354 01995 Materials Research Society
Before going into the details of these mechanisms, it is important to note that, under pulsed laser irradiation, transient heating of the surface region may result in thermal processes. A genuine electronic excitation should be easily distinguishable from a thermal process based on either of the following results: unique wavelength response, nonthermal final state distribution, and linear power dependence. The latter is demonstrate in figure 1 for the photodissociation of Ga(CH 3 )3 adsorbed on GaAs(100) [2]. The linear relationship clearly establishes a single photon nonthermal excitation mechanism, as discussed in detail below. In the following, I restrict discussion to nonthermal surface photochemical processes. 200 -Figure 1: Intensity of CH3
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photodesorption from monolayer Ga(CH3)3/GaAs as a function of pulse energy of the ArF excimer laser light (193 nm, 20 ns pulse width). The solid line is a least-square linear fit to the
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Pulse enei gy (mJ/cm
Substrate Mediated Excitations Figure 2 illustrates schematically the dynamics of charge-transfer photo-excitation processes on an
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