Valentin G. Panayotov

Energetics of pulsed laser ablation species as determined by quadrupole and time-of-flight mass spectrometry

Time-of-flight data from an electron impact quadrupole mass spectrometer (QMS) are used to determine the composition and energy of a neutral plume created by pulsed laser ablation of ZnTe. Velocities of the ablated species are extracted by taking spectra at two distances and measuring the change in arrival time. Results from the QMS are compared to those obtained by 193-nm laser ionization time-of-flight mass spectrometry.

Algebraic determination of thin film optical constants from photometric (T,Rf,Rm) and (T,Rb,Rm) measurements

In the present work we offer two novel computational methods, defined as (T,Rf,Rm) and (T,Rb,Rm), for the simultaneous determination of the optical constants, n and k, and the thickness, d, of a thin film from three experimental photometric quantities. The basic experimental configuration is a thin film deposited onto a nonabsorbing substrate, half covered with an opaque metal film. An algebraic inversion technique is developed involving a numerical interpolation procedure in the last step. The methods give all mathematical solutions, and according to the specific case, the physical solution can be isolated by the combination of the two methods or by some estimates of the thin film thickness. When the photometric measurements are available in a spectral range, the (n,k,d) solutions, for which the thickness is one and the same, can be easily isolated as correct. The (T,Rf,Rm) and (T,R-b),Rm) methods can be applied without restrictions to a wide range of n and k values. A numerical example illustrates the applicability and the good overall accuracy of the methods.

Multipurpose high vacuum laser ablation/thin film deposition system

An instrumental system is described that combines an apparatus for pulsed laser deposition (PLD) with a vacuum “suitcase” for transport of air-sensitive compounds. Laser ablation, plume diagnostics, and thin-film deposition are readily accomplished through a flexible design. The PLD apparatus consists of two cubes coupled via a bellows assembly and whose distance is easily changed pneumatically. For both the PLD apparatus and the vacuum suitcase, compact-design considerations have been implemented. Consequently, the footprint of the PLD apparatus is relatively small, and the vacuum suitcase can be transported easily between sites.

Thin-Film Deposition Via Pulsed Laser Ablation

We report on a novel experimental approach for thin-film deposition via pulsed laser ablation. A combination of design features including the incorporation of original sample manipulation methods as well as enhanced analytical capabilities allow for experimental control and flexibility for thin-film deposition studies of a variety of materials. Here, the application is illustrated with a study of 532 nm pulsed laser ablation and subsequent deposition of CdTe.Conditions have been found that produce a mildly energetic ablation plume, and the average kinetic energies for the neutral and ionic ablation species in the plume have been determined to be approximately 2 and 11 eV, respectively. It has been shown that in the presence of an electric field the ionic species can be diverted away from the detector (and subsequently from the eventual deposit). Thin film deposits clearly reveal an increase in the number of sub-micron particles when direct ablation ions contribute to the deposition. The possibility for extraction of ablation ions and preparation of ablation plumes of neutrals only has been clearly demonstrated.

Selective removal of potassium from K4In4Sb6 via laser ablation/ionization

Results are reported on the Zintl phase material, K4In4Sb6, with respect to laser ablation and subsequent laser ionization/removal processes. A 308 nm laser pulse is used to ablate the Zintl compound, while a second laser ionizes ejected species within the extraction region of a time‐of‐flight mass spectrometer. With the second laser operating at 248 nm, selective ionization and removal of the potassium is clearly demonstrated. Such a strategy takes advantage of the different ionization potentials of K, In, and Sb, and implications for possible applications of this research to film growth are discussed.

Development of a method for investigating carbon removal processes during photoassisted film growth using organometallic precursors: Application to platinum

A method is used to investigate carbon removal pathways during metal film growth using organometallic precursors. The approach combines a time-of-flight mass spectrometer with a growth chamber from which substrates can be removed during real-time film growth. Consequently, evolving mass spectral signatures can be correlated with changing film properties. Although more general as a technique, results are presented for the photoassisted growth of Pt from CH3CpPt(CH3)3 in a D2 atmosphere. Here, a marked increase in deuterium/hydrogen exchange is clearly correlated with an increase in the Pt:C ratio for the metal films, as determined by x-ray photoelectron spectroscopy. However, results for growth with CH3CH2CpPt(CH3)3 as well as (CH3)3CCpPt(CH3)3 suggest that while extensive D∕H exchange can be a feature of the growth process, it is not a prerequisite for producing films with relatively high Pt:C ratios.

Toward making layered films using selective ionization in InSb and GaSb laser ablation plumes

The 308 nm laser ablation of InSb and GaSb has been investigated with the goal being to exert macroscopic control over the ablation plume. By taking advantage of the lower ionization potential of the group III element, In or Ga can be selectively ionized within the ablation plume by a 193 nm (6.4 eV) photon. The ionized species are removed from the plume with an electric field. As shown by x-ray photoelectron spectra, films subsequently deposited are diminished in In or Ga. A three-cycle deposition study demonstrates that a depleted layer of InSb can be deposited between two nondepleted layers of InSb.

Selective ionization of group I elements from laser ablated plumes of Rb⋅Ga⋅Sb,K3Ga3As4, and K4In4As6

Experimental results are reported on the I–III–V Zintl compounds Rb⋅Ga⋅Sb, K3Ga3As4, and K4In4As6 with respect to laser ablation and subsequent laser ionization/removal processes. The approach takes advantage of the low ionization potentials of the group I elements to achieve selectivity and exert a measure of control over neutral mixtures. A 308 nm laser pulse is used to ablate a I–III–V Zintl compound, while a second laser is used to selectively ionize the ejected species within the extraction region of a time-of-flight mass spectrometer. With the second laser operating at 248 nm (in the case of Rb⋅Ga⋅Sb) and at 266 nm (in the case of K3Ga3As4 and K4In4As6), selective gas-phase ionization and removal of the group I elements is clearly demonstrated.

III-V semiconductor thin films via laser ablation/ionization of I-III-V Zintl-phase materials

Experimental results, related to a novel laser-assisted technique for deposition of III-V semiconductor thin-films, are presented. The method involves precursors in the form of I-III- V Zintl-phase materials. While such compounds exhibit a variety of useful properties, the presence of the group I element is a major concern--in terms of the desired final product it can be considered a major impurity that has to be removed. To address this situation, we employ a strategy, based on the difference in the ionization potentials for the constituent elements, that can be described in brief as (1) laser ablation of a I-III-V compound, (2) removal of the group I element by selective gas-phase laser ionization and extraction in an electric field, and (3) subsequent re-deposition of the III-V compound. For a particular I-III-V Zintl-phase compound, potassium indium antimonide, K4In4Sb6, time-of-flight mass spectra clearly demonstrate high yield selective gas phase ionization and removal of potassium from the ablation plume. Deposition conditions have been found for scaling of the above removal process macroscopically to thin film growth. The deposits have been studied by Auger electron spectroscopy and the results confirm significant depletion of potassium from thin films deposited via the proposed ablation/ionization removal technique. An assessment is made as to the viability of the process and implications for possible applications of this research as related to film growth are discussed.

Using lasers to understand and control the chemistry of semiconductor-related precursors

We present results on the laser-induced photochemistry, ablation, and deposition of a variety of precursor compounds used for III-V semiconductor growth. With compounds such as monoethylarsine, triethylarsenic, and triethylgallium, our efforts are focused on using lasers to generate and detect atomic hydrogen, a species that is known to be a good radical scavenger in certain III-V semiconductor growth environments. We also present results on the laser ablation of inorganic salts that may be useful as precursors for III-V thin-film growth. Here, K3Ga3As4 and K2Ga2Sb4 are irradiated with various excimer laser wavelengths, and we report on the ablation and deposition chemistry induced by such radiation. The prognosis for viable film growth using this approach is also discussed. Finally, recent efforts involving the photochemistry and subsequent deposition characteristics of a Pt- containing organometallic compound, C5H5Pt(CH3)3, are presented.