Pulsed Laser Deposition
Update November 2007
this narrative is outdated - for a recent review article see Publication 185.

We study growth during Pulsed Laser Deposition (PLD) and compare it with Molecular Beam Epitaxy (MBE). In MBE there is now a solid baseline of knowledge about surface structures, stress effects, atomistic mechanisms, and growth modes; additionally there is some information on dopant or impurity incorporation in growth. PLD uses an excimer laser to ablate a target to produce the depositing flux, as shown in Fig. 1 below. It has several distinct features advantageous for the study of non-equilibrium growth from the vapor, discussed in more detail below. (Additionally it has a number of practical advantages which we won't distract you with here.) One particularly dramatic difference between crystal growth in MBE and in PLD is the instantaneous deposition rate. In MBE a typical growth rate might be only 1 monolayer (ML) per second; in fact, there is recent evidence that MBE growth may take place under conditions much closer to equilibrium than has been believed [1] . In PLD, one can grow films at these rates, but it is also possible (and typical) to grow films at instantaneous rates as high as 1 ML/microsecond, six orders of magnitude faster! The average growth speed is limited only by the repetition rate of the laser. Hence, PLD growth is an area of opportunity for a variety of fundamental kinetic studies that are difficult or impossible in MBE growth.

The PLD process has a number of characteristics that are fundamentally advantageous for the study of the kinetics of crystal growth far from equilibrium:

1. PLD consists of periodic bursts of highly driven growth followed by relatively long periods of uninterrupted surface relaxation, permitting these two competing processes to be isolated from each other and studied separately.

2. In the proper ablation regime, ionized and neutral ablation products having kinetic energies in the range from less than one to a few hundred eV can be produced [2]. The ionization and increased kinetic energy can be used to study a variety of phenomena, e.g. adatom mobility, surface reactions, and enhanced low-temperature epitaxy.

3. The instantaneous deposition flux can be tremendous and can be varied (target-substrate distance; laser fluence; target temperature) independently of either the average growth rate (laser repetition rate) or the kinetic energy of the ablated species (ambient gas mass and pressure; laser fluence).
 
 

 There has been much valuable PLD research centered on questions such as "what new material can be grown?" and "what are the processing conditions for optimizing the properties of the grown film?". The primary focus of this research clearly has been centered on the development of the PLD technology (e.g., elimination of particulates in deposition [3] ), to the extent that research on the fundamental issues of crystal growth in PLD is sorely lacking. This is perhaps surprising given the unique features of PLD. To illustrate, consider item #1 above. Simulation has been an important tool to isolate the effects of deposition and relaxation [4] , but experimentally research involving modulating the deposition rate in MBE [5] is difficult. By the nature of the technique, in MBE these processes occur concurrently during deposition of material. With judicious selection of deposition conditions during PLD we can study these two processes independently. Additionally, using PLD there is the potential for independent control of the kinetic energy of the depositing species.

 

Our current and planned research on on this topic [6-9] is aimed along these general lines of inquiry:

• What can a systematic variation in the kinetic energy of the depositing species tell us about its role in the deposition, relaxation, and growth processes?

• Can we understand growth in PLD in terms of basic mechanisms such as island nucleation and growth (see Fig. 2), and surface diffusion-induced relaxation (see Figs. 3 and 4)?

• Can we understand segregation, trapping and alloying in PLD in terms of basic kinetic processes as we have for rapid solidification?

 References 1. J. Tersoff, M.D. Johnson and B.G. Orr, "Adatom Densities on GaAs: Evidence for Near-Equilibrium Growth", Phys. Rev. Lett. 78, 282 (1997).

2. J.T. Cheung and J.S. Horwitz, MRS Bulletin 17, 30 (1992).

3. T. Venkatesan, X.D. Wu, R. Muenchausen and A. Pique, MRS Bull. 17, 54 (1992).

4. Z.-W. Lai and S.D. Sarma, Phys. Rev. Lett. 69, 3762 (1992).

5. M.I. Larsson, W.-X. Ni and G.V. Hansson, "Manipulation of Nucleation by Growth Rate Modulation", J. Appl. Phys. 78, (1995).

6. J.D. Erlebacher and M.J. Aziz, "Morphological Equilibration of Rippled and Dimpled Crystal Surfaces: the Role of Terrace-Width Fluctuations", Surface Science 374, 427 (1997).

7. J.D. Erlebacher and M.J. Aziz, "Ion-Sputter Induced Rippling of Si(111)", Materials Research Society Symposia Proceedings 440, 461 (1997).

8. J.D. Erlebacher and M.J. Aziz, "Surface Relaxation Mechanisms in the Morphological Equilibration of Crystal Surfaces", Materials Research Society Symposia Proceedings 440, 59 (1997).

9. J.W. McCamy and M.J. Aziz, "Time-resolved RHEED Studies of the Growth of Epitaxial ZnSe Films on GaAs by Pulsed Laser Deposition", Materials Research Society Symposia Proceedings 441, 621 (1997).

10. J.D. Erlebacher, "Kinetic Rate Law Issues in the Morphological Relaxation of Rippled Crystal Surfaces", in Dynamics of Crystal Surfaces and Interfaces, eds. P.M. Duxbury and T. Pence (Plenum, New York, 1997).
 


Mike Aziz