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Kinetics of Phosphine Adsorption and Phosphorus Desorption from Gallium and Indium Phosphide (001)

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Kinetics of Phosphine Adsorption and Phosphorus Desorption from Gallium and Indium Phosphide (001) Surface Science 540 (2003) 12–22 www.elsevier.com/locate/susc Kinetics of phosphine adsorption and phosphorus desorption from gallium and indium phosphide (001) Y. Sun, D.C. Law, R.F. Hicks * Department of Chemical Engineering, University of California (UCLA), 405 Hilgard Avenue, Los Angeles, CA 90095-1592, USA Received 11 October 2002; accepted for publication 5 June 2003 Abstract The kinetics of phosphine adsorption and phosphorus desorption from gallium and indium phosphide (0 0 1) has been determined using reflectance difference spectroscopy to monitor the phosphorus coverage in real time. Assuming a Langmuir adsorption mechanism, phosphine exhibited an initial reactive sticking coefficient at 500 °Cof 8.7 ± 1.0 · 10À2, 3.5 ± 1.0 · 10À2 and 1.0 ± 0.2 · 10À3 on the GaP (2 · 4), GaP (1 · 1) and InP (2 · 4) reconstructions, respectively. The sticking coefficient increased with temperature on the gallium phosphide surfaces, exhibiting an ac- tivation energy of 0.5 ± 0.2 eV, while on indium phosphide, no temperature dependence was observed. The desorp- tion of phosphorus from the GaP (2 · 1) surfaces was first-order in coverage with rate constants of 5:0  1015 (sÀ1) exp()2.6 ± 0.2 (eV)/kT). These results may be used to estimate the feed rate of phosphine relative to the group III precursors during the metalorganic vapor-phase epitaxy of gallium and indium phosphide. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Gallium phosphide; Indium phosphide; Phosphine; Adsorption kinetics; Sticking 1. Introduction achieve lattice matching of the alloy films to their binary counterparts. This is particularly true for Compound semiconductor devices, such as InxGa1ÀxAsyP1Ày, where it is found that the y value edge-emitting lasers, heterojunction bipolar tran- in the alloy follows a nonlinear dependence on the sistors and multi-junction solar cells, are com- ratio of the group V precursors fed to the reactor posed of thin films of gallium arsenide, gallium [6,8–10]. For example, when only a small amount phosphide, indium arsenide, indium phosphide, of arsenic is required in the film, the alloy com- and alloys thereof [1–4]. These materials are position is very sensitive to the phosphine addition deposited by metalorganic vapor-phase epitaxy rate. (MOVPE), using volatile precursors of the group The nonlinear dependence of the alloy compo- III and V elements [5–7]. The feed rates of the sition on the concentrations of the group V pre- precursors are carefully controlled in order to cursors in the gas is due to the fact that the decomposition rates of these species are controlled * Corresponding author. Tel.: +1-310-206-6865; fax: +1-310- by heterogeneous chemical reactions [5,6]. The prin- 206-4107. cipal surface reactions occurring during MOVPE E-mail address: [email protected] (R.F. Hicks). are (1) the irreversible adsorption of arsine and 0039-6028/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0039-6028(03)00834-3 Y. Sun et al. / Surface Science 540 (2003) 12–22 13 phosphine; and (2) the desorption of arsenic and Inside the UHV chamber the gallium phosphide phosphorus. It has been found that group V-rich samples were heated to 300 or 550 °C for 20 min to surfaces prevent the incorporation of undesired create either a (2 · 1), or a (2 · 4) reconstruction. impurities in the films, so to insure this condition For indium phosphide, the (2 · 1) and (2 · 4) during growth, V/III feed ratios between 10 and structures were generated by annealing at 300 and 500 are employed [7,8,11,12]. 450 °C for 20 min. After cooling the samples to 25 In this work, we have determined the kinetics °C, the long range ordering on the surfaces was of phosphine adsorption and phosphorus desorp- characterized by low-energy electron diffraction tion from gallium and indium phosphide (0 0 1) (LEED). In addition, reflectance difference spectra at surface coverages ranging from 0.13 to 1.00 of the surface reconstructions were recorded with monolayers (ML) of phosphorus. These experi- an Instruments SA J-Y Nisel RD spectrometer [13– ments were carried out on MOVPE-grown sur- 15]. The measured signal, ReðDR=RÞ, corresponded faces in well-controlled ultrahigh vacuum (UHV) to the difference in the real part of the optical conditions. Changes in the P coverage during iso- reflectance measured along the [1 1 0] and [1 1 0] thermal adsorption and desorption were recorded crystal axis, i.e., ½ðR½1110 À R½110Þ=hRi. Baseline using reflectance difference spectroscopy. By solv- drift was subtracted from the data by taking the ing the mass balances for the adsorbed phospho- average of two spectra collected with the polarizer rus on the different reconstructions, we were able oriented +45° and )45° relative to the [1 1 0]. to extract the heterogeneous reaction kinetics Reflectance difference signals were recorded at from these data. It was found that the sticking specific energies (see below) during the isothermal probability of phosphine is higher on gallium adsorption of phosphine or desorption of phos- phosphide than on indium phosphide, while the phorus. First the samples were annealed to pro- rate of phosphorus desorption is lower on the duce the group III-rich (2 · 4) reconstructions. former surface compared to the latter one. These Then the temperature was adjusted to a desired data can explain why relatively high V/III feed value, and phosphine was introduced through a ratios are used during the MOVPE of these ma- leak valve at pressures ranging from 10À7 to 10À4 terials. Torr for times ranging from 5 to 15 min. The pressures were measured using an ion gauge fila- ment that was corrected with a known calibration 2. Experimental methods factor [16]. Conversely, isothermal desorption data were taken immediately after closing the leak valve Gallium and indium phosphide films, approxi- at background pressures below 10À9 Torr. Peri- mately 0.5 lm thick, were deposited on nominally odically during these measurements, the surface flat GaP and InP(0 0 1) substrates in a horizontal structure was checked by LEED. All the experi- MOVPE reactor. The hydrogen carrier gas was ments were conducted at phosphorus coverages passed through a SAES Pure Gas hydrogen puri- between 0.13 and 1.00 ML [13,14]. fier PS4-MT3-H to remove any oxygen, nitrogen or hydrocarbon contamination. The total reactor pressure was 20 Torr and the substrate tempera- 3. Results ture was 585 and 565 °C for GaP and InP growth. Trimethylgallium, trimethylindium, and tertiary- 3.1. Data analysis butylphosphine (TBP) were introduced to the re- actor at partial pressures of 1.3 · 10À4, 6.5 · 10À4, Previous studies have shown that the gallium and 1.3 · 10À1 Torr, respectively. After deposition, phosphide (0 0 1) surface exhibits three stable re- the TBP flow and the H2 flow were maintained constructions, the d(2 · 4), (1 · 1) and (2 · 1) until the samples were cooled to 300 and 40 °C, [13,17]. As shown in Table 1, these structures are respectively. Then the crystals were transferred covered with 0.13, 0.67 and 1.00 ML of phos- directly to an UHV system for surface analysis. phorus. On the other hand, the indium phosphide 14 Y. Sun et al. / Surface Science 540 (2003) 12–22 Table 1 Surface structures observed on GaP and InP(0 0 1) [13,15,18] Surface phases GaP InP dð2  4Þ (1 · 1) (2 · 1) dð2  4Þ rð2  4Þ (2 · 1) P coverage 0.13 0.67 1.00 0.13 0.25 1.00 Ga/In coverage 0.87 0.33 0.00 0.87 0.75 0.00 (0 0 1) surface may be terminated with dð2  4Þ, rð2  4Þ,or(2· 1) phases with P coverages of 0.13, 0.25, or 1.00 ML, respectively [18–20]. Under InP(2x1) 0.005 super-P-rich conditions, a mixed (2 · 1) and (2 · 2) reconstruction is formed with between 1.00 and 1.50 ML of phosphorus [19]. In this study, the R/R) (2 · 2) phase is not examined. ∆ 0.000 InP(2x4) Shown in Fig. 1 are reflectance difference spec- Re ( tra of the (2 · 1), (1 · 1) and (2 · 4) reconstructions of GaP(0 0 1) recorded at 500 °C. The spectrum of -0.005 the (2 · 1) contains a series of weak negative bands between 1.7 and 2.7 eV, and two positive peaks at 2.9 eV 3.4 and 4.9 eV. The spectrum of the (1 · 1) exhibits 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 several weak negative bands between 1.7 and 2.5 Photon Energy (eV) eV, and two positive peaks at 3.3 and 4.9 eV. For the (2 · 4), one observes an intense negative peak Fig. 2. Reflectance difference spectra of the InP(0 0 1)-(2 · 1) at 2.2 eV, a shoulder at 2.5 eV, and positive bands and (2 · 4) reconstructions at a substrate temperature of 400 °C. at 3.2, 3.8 and 4.9 eV. Shown in Fig. 2 are RD spectra of the (2 · 1) and (2 · 4) reconstructions of metric band centered at 4.0 eV. By contrast, the InP(0 0 1) recorded at 400 °C. The spectrum of the spectrum of the (2 · 4) exhibits a broad negative (2 · 1) contains a negative peak at 1.7 eV, an in- band at 1.8 eV and three positive peaks at 2.7, 3.6, tense positive peak at 2.9 eV, and a broad asym- and 4.5 eV.
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