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Chemical Beam Etching Reactions of PCl3 on GaAs(100) Nagindar K. Singh* and Stefan Oerlemans School of Chemistry, The University of New South Wales, Sydney 2052, Australia Received September 30, 1998. In Final Form: January 28, 1999 We report the surface reactions of PCl3 on the gallium-rich GaAs(100)-(4 × 1) reconstruction, studied using thermal desorption spectroscopy, Auger electron spectroscopy, and low-energy electron diffraction. At room temperature, PCl3 was found to adsorb molecularly. Desorption of adsorbed PCl3 competes with its thermal dissociation into PCl and Cl species. Cl selectively etches the surface gallium atoms as GaCl, which subsequently desorbs at 610 K. The adsorbed PCl undergoes further dissociation to form additional surface Cl and elemental P, and this step competes with the desorption of PCl into the gas phase. The adsorbed P desorbs from the surface as P4 at 600 K. Removal of GaCl and P4 exposes the second-layer arsenic atoms which desorb as As2 at 625 K and returns the surface to the original gallium-rich GaAs(100)-(4 × 1) reconstruction. We propose a reaction scheme to account for this digital or layer-by-layer etching of GaAs(100) with PCl3. We also discuss the implications of this reaction scheme pertaining to the use of PCl3 as a chemical beam etchant.
1. Introduction In some device fabrication applications, several etching and regrowth steps are required. To date, the etching steps have always been accomplished outside the growth chamber, which necessiates the transfer of samples to and from the growth chamber during the fabrication process. This can become a time-consuming exercise. A solution to this problem would be to use molecules that could do both, and in situ growth/etching steps could be performed without the need for switching beam fluxes or moving samples from one chamber to another. The work of Tsang et al.1 has shown that molecules such as PCl3 and AsCl3 can be used as chemical beam etchants in a semiconductor growth chamber for etching and regrowth of InP and GaAs, respectively. These molecules are made up of halogen atoms, required for etching compound semiconductors, and a group V element, making them ideal as chemical beam etchants inside a semiconductor growth chamber. Using these halides in the form of chemical beams in a chemical beam epitaxial (CBE) growth chamber has two major advantages. First, high-temperature etching can be performed since the molecules themselves provide a slight overpressure of the respective group V element during etching. An overpressure of the group V element is important during high-temperature etching in order to minimize surface damage due to preferential evaporation of this element at high temperatures. Second, the growth chamber will be set up to enhance the overpressure for another application should a higher overpressure be required to maintain the surface stoichiometry. Group V halides are preferred over conventional etchants such as Cl2, electron cyclotron resonance SiCl4,2-4 HCl,5 and some alkyl halides in chemical beam etching (CBET) processing because these molecules are less * Corresponding author. Fax: +61 2 9385-6141. E-mail: N.Singh@ unsw.edu.au. (1) Tsang, W. T.; Kapre, R.; Sciortino, P. F. J. Cryst. Growth, 1994, 136, 42. (2) Miyamoto, H.; Furuhata, N.; Noshino, H.; Okamoto, A.; Ohata, K. In GaAs(100) and Related Compounds 1988; Harris, J. C., Ed.; Inst. Phys. Conf. Ser 96; Inst. Phys. Press: Bristol, 1989; Chapter 2. (3) Hamm, R. A.; Feygenson, A.; Ritter, D.; Wamg, Y. L.; Temkin, H.; Yadvish, R. D.; Panish, M. B. Appl. Phys. Lett. 1992, 61, 592. (4) Choquette, H. D.; Hong, M.; Freund, R. S.; Chu, S. N. G.; Mannaerts, P. P.; Wetsel, R. C.; Leibenguth, R. E. Unpublished results.
corrosive. Investigations of PCl3 chemical beam etching of InP in a CBE chamber1 have already shown that excellent etched and regrown surface morphologies can be obtained at high temperatures (g530-570 °C) with etching rates of e 6 Å s-1. It was proposed that etching occurred by a reaction between the impinging PCl3 and substrate InP to form volatile InCl, InCl2, or InCl3, although no experiments were performed to confirm the formation of these etch products. Previous surface investigations of III-V semiconductor etching with Cl2,7-9 HCl,10 and alkyl halides11,12 have shown that it is the formation and subsequent desorption of the volatile group III halide that is the limiting step in the etching process. Following the desorption of the halide, the surface becomes arsenic enriched, and if the surface is heated further, then desorption of arsenic dimers commences, giving rise to a digital or layer-by-layer etching phenomenon. In the case of Cl2 etching of GaAs(100),7-9 both GaCl and GaCl3 were formed, whereas in the case of HCl,10 bromochloroethane,11 and ethyl iodide,12 monohalides of gallium were only formed. In these latter studies, it was proposed that the lack of formation of higher halides was because of site blocking by the hydrogen or organic fragments, which inhibits further reaction. The choice of alkyl halides used for etching is crucial if levels of carbon contamination are to be minimized, as it is known from previous investigations that it is the surface reactions of the adsorbed organic fragments that determine the levels of carbon incorporated into films. Hence, an added advantage that group V halides will have over alkyl halides as chemical beam etchants is that problems with carbon incorporation during etching will not arise. (5) Caneau, C.; Bhat, R.; Koza, M.; Hayes, J. R.; Esagui, R. J. Cryst. Growth 1991, 107, 203. (6) Chakrabarti, U. K.; Pearton, S. J.; Katz, A.; Hobson, W. S.; Abernathy, C. R. J. Vac. Sci. Technol. 1987, B10 (6), 2378. (7) French, C. L.; Balch, W. S.; Foord, J. S. J. Phys.: Condens. Matter, 1991, 3, S351. (8) Su, C.; Hou, H.; Lee, G. H.; Gai, Z.; Luo, W.; Vernon, M. F.; Bent, B. E. J. Vac. Sci. Technol. 1993, B11 (4), 1222. Su, C.; Xi, M.; Gai, Z.-G.; Vernon, M. F.; Bent, B. E. Surf. Sci. 1993, 282, 357. (9) Ludviksson, A.; Xu, M.; Martin, R. M. Surf. Sci. 1992, 277, 282. (10) Su, C.; Hou, H.; Gai, Z.-G.; Luo, W.; Sun, D. H.; Vernon, M. F.; Bent, B. E. Surf. Sci. 1994, 312, 181. (11) Singh, N. K.; Bolzan, A. Surf. Sci. 1996, 357/358, 656. (12) Singh, N. K.; Bolzan, A.; Foord, J. S.; Wright, H. Surf. Sci. 1998, 409, 272.
10.1021/la981324s CCC: $18.00 © 1999 American Chemical Society Published on Web 03/23/1999
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Although PCl3, AsCl3,1,13,14 and AsBr315 have been shown to be good chemical beam etchants for compound semiconductors, to date, no detailed surface mechanistic studies of the layer-by-layer etching process of these group V halides have been performed. Tsang et al.1,13,14 used RHEED oscillations to demonstrate the layer-by-layer etching with molecular beams of PCl3 and AsCl3, while Zhang et al.15 used modulated molecular beam studies to derive kinetic information and identify reaction products during etching of GaAs(100) with AsBr3. Further experiments on AsBr3 etching of GaAs(100) using RHEED oscillations to monitor the etching and computer simulations of the atomistic (Monte Carlo) models of the etching process16,17 have revealed that the process is temperature dependent, and physical effects such as surface morphology and kinetics of the atomistic processes play important roles in the overall etching mechanism. The results of AsBr3 etching of GaAs(100) identified two etching regimes, a supply-rate-limited regime at high temperatures (>460 °C) and a reaction-rate-limited regime at low temperatures (530 °C) and low etch rates. The surface reaction mechanisms elucidated in this study show that PCl3 reacts with the GaAs surface to produce volatile GaCl species and, hence, can also be utilized in the etching of GaAs under optimum conditions. Perhaps PCl3 will be a better chemical beam etchant during GaAs(1-x)Px growth by CBE in view of the formation of elemental phosphorus on the surface during the dissociation process. Although caution must be exercised in comparing adsorption processes at room temperature or even high temperatures but under static conditions to continuous etching under dynamic equilibrium and hightemperature conditions, some insights into thermal etching with PCl3 can be gleaned from our results: (i) PCl3, GaCl, P4, and As2 are the major thermal desorption products obtained on adsorption at room temperature. These products all desorb below 650 K. This is consistent with the results of Tsang et al.,1 who found that etching was optimum for temperatures g 530 °C (800 K). (ii) The activation energies for desorption of the two etch products GaCl and As2 are 153 and 160 kJ mol-1,
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respectively. The activation energy for desorption of P4 is 148 kJ mol-1. These values are all quite high, and hence, at moderate temperatures, the desorption steps of these products will limit the etch rate, and below 600 K, their desorption rate will be very slow, giving low etch rates. From our results, we conclude that if high rates are to be achieved, then etching will have to be performed at high temperatures (>700 K). (iii) Adsorption at high temperatures shows dissociative adsorption, and an adlayer consisting of adsorbed chlorine and elemental phosphorus forms on the surface. Without site blocking by PCl3, we would expect etching to be considerably higher than that at room temperature. This is consistent with chemical beam etching studies of this molecule on InP, where excellent surface morphologies were obtained for etching at high temperatures. 6. Conclusion At room temperature, PCl3 adsorbs onto gallium-rich GaAs(100)-(4 × 1) and undergoes thermal decomposition on the surface to form predominantly surface chlorines and PCl. The PCl species could either desorb, which it did at 540 K, or undergo further reactions on the surface to form chlorine or elemental phosphorus. The adsorbed chlorines are removed from the surface as GaCl at 610 K. The adsorbed phosphorus desorbs from the surface as P4. Following GaCl desorption, the surface becomes arsenicenriched, and if heated to sufficiently high temperatures (>600 K), As2 desorption commences, returning the surface to its initial gallium-rich state. The reaction mechanisms proposed in this paper provide insights into how etching of GaAs(100) with PCl3 occurs and also confirm its suitability as a chemical beam etchant for this compound semiconductor. LA981324S
