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Structure of Arsenic-Treated Indium Phosphide „001… Surfaces During Metalorganic Vapor-Phase Epitaxy

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Structure of Arsenic-Treated Indium Phosphide „001… Surfaces During Metalorganic Vapor-Phase Epitaxy PHYSICAL REVIEW B 66, 045314 ͑2002͒ Structure of arsenic-treated indium phosphide „001… surfaces during metalorganic vapor-phase epitaxy D. C. Law, Y. Sun, C. H. Li, S. B. Visbeck, G. Chen, and R. F. Hicks* Department of Chemical Engineering, University of California, Los Angeles, California 90095 ͑Received 4 February 2002; published 22 July 2002͒ We have studied the initial stages of heterojunction formation during the metalorganic vapor-phase epitaxy of indium arsenide on indium phosphide. Exposing an InP ͑001͒ film to 10 mTorr of tertiarybutylarsine below 500 °C results in the deposition of a thin InAs layer from 1.5 to 5.0 atomic layers thick ͑2.3–7.5 Å͒. The surface of this epilayer remains atomically smooth independent of arsenic exposure time. However, in an overpressure of tertiarybutylarsine at or above 500 °C, the arsenic atoms diffuse into the bulk, creating strained InAsP films. These films form three-dimensional island structures to relieve the built-up strain. The activation energy and pre-exponential factor for arsenic diffusion into indium phosphide have been determined to be ϭ Ϯ ϭ Ϯ ϫ Ϫ7 2 Ed 1.7 0.2 eV and Do 2.3 1.0 10 cm /s. DOI: 10.1103/PhysRevB.66.045314 PACS number͑s͒: 68.35.Bs, 68.35.Dv, 68.35.Fx, 68.35.Ct INTRODUCTION In this work, we have characterized the initial stages of InGaAs/InP heterojunction formation during MOVPE Compound semiconductors and their alloys are the key growth by x-ray photoelectron spectroscopy ͑XPS͒, low- materials for high-speed electronic and optoelectronic de- energy electron diffraction ͑LEED͒, and scanning tunneling vices, such as heterojunction bipolar transistors, lasers, and microscopy ͑STM͒. The surface roughness, atomic structure, photodetectors.1–3 Indium phosphide materials have gained and composition of the indium phosphide films during interest recently due to their application in high-frequency growth interruptions, t1 and t2 , have been determined as a devices used for fiber-optic communication.4–6 These opto- function of temperature and time. We find that arsenic atoms electronic devices consist of alternating thin films of lattice- incorporate into only the top two to three bilayers of indium matched InGaAs, InGaAsP, and InP.3,4,7 The interface struc- phosphide below 500 °C, whereas at or above 500 °C, they ture between the alloy layers and indium phosphide have a diffuse into the bulk crystal as well. direct impact on device performance.2,6,8–10 In particular, the interfacial layers may contain defects and exhibit composi- EXPERIMENTAL METHODS tion gradients that reduce electron mobility, enhance nonra- Indium phosphide films, 0.20 ␮m thick, were grown on diative carrier recombination, or alter quantum confinement n-type vicinal InP ͑001͒ substrates in a horizontal MOVPE energies.8,11 Thus, in order to fabricate high-performance de- reactor.15 The growth temperature and total reactor pressure vices, one must control the interface structure on the atomic were 535 °C and 20 Torr. Hydrogen carrier gas was passed ͑ ͒ scale. through a SAES Pure Gas H2 purifier PS4-MT3-H to re- Metalorganic vapor-phase epitaxy ͑MOVPE͒ is widely move oxygen, nitrogen, water, and other impurities. Trimeth- used to fabricate InP-based materials.2 During the growth of ylindium ͑TMIn͒ and tertiarybutylphosphine ͑TBP͒ were heterojunctions, e.g., InGaAs on InP, the group-V source used to deposit the films with partial pressures of 6.5 Ϫ Ϫ must be switched from phosphorus to arsenic. However, si- ϫ10 4 and 1.3ϫ10 1 Torr, respectively. After deposition, multaneous adsorption, desorption, and bulk diffusion of ar- the InP crystals were heated in flowing hydrogen at 500 °C senic and phosphorus atoms have made it challenging to fab- for up to 20 min creating an In-rich surface. The samples ricate atomically abrupt interfaces.2,3 Switching procedures were then either transferred directly to the UHV system for with growth interruptions have been implemented in an at- surface analysis, or remained in the reactor for TBAs expo- tempt to solve this problem. This has led to many studies that sure. focus on the effect of precursor switching procedures on ma- terial quality and device properties.1,2,6,12–14 Most switching sequences consist of a hydrogen annealing step, t1 , a brief exposure to arsine, t2 , and then addition of the group-III precursors. This sequence is illustrated in Fig. 1. The flowing H2 gas sweeps any remaining phosphine out of the MOVPE reactor, so that no phosphorus is incorporated into subse- quently deposited layers. Introduction of arsine or tertiarybu- tylarsine prior to the group-III precursors ensures that the surface is group-V rich and prevents the formation of metal droplets at the onset of film growth. However, little is known about the evolution of the semiconductor surface structure FIG. 1. Schematic of the precursor switching sequence during during the hydrogen and arsine exposure steps. the growth of an InGaAs/InP heterojunction by MOVPE. 0163-1829/2002/66͑4͒/045314͑7͒/$20.0066 045314-1 ©2002 The American Physical Society LAW, SUN, LI, VISBECK, CHEN, AND HICKS PHYSICAL REVIEW B 66, 045314 ͑2002͒ FIG. 2. The mass spectrometer response to a reaction by- product, propylene, during the introduction of tertiarybutylarsine to FIG. 3. The dependence of the phosphorus coverage on time the MOVPE reactor. during exposure of the InP ͑001͒ surface to 20 Torr hydrogen at 500 °C. Arsenic was introduced to the indium phosphide surface by exposing the crystal to 10.0 mTorr of tertiarybutylarsine time at 500 °C. The coverage is assessed from LEED pat- at 350–600 °C for 5–600 s. A mass spectrometer was at- terns and STM images, which correspond to known surface tached to the exhaust of the MOVPE reactor to monitor the 16 structures as described in the following paragraph. The as- arsenic exposure in real time. Figure 2 shows the instanta- grown InP film is terminated with 1.5 monolayers ͑ML’s͒ of ͑ neous increase in concentration of pyrolysis by-products in P atoms. Annealing for 3, 15, and 20 min decreases the phos- ͒ this case, propylene when TBAs was introduced into the phorus coverage to 1.0, 0.25, and finally 0.125 ML’s, respec- reactor. After the exposure, the samples were cooled to 30 °C tively. Holding the InP film for longer times in flowing H2 in an H2 ambient at a rate of 4 °C/s. results in surface degradation and indium droplet formation. Periodically during the TBAs exposure, the indium phos- Hicks and co-workers have determined the surface struc- phide samples were analyzed by XPS, LEED, and STM. ture of InP ͑001͒ as a function of the phosphorus Core-level photoemission spectra of the As 3d,P2p, and In 20–22 ␪ ϭ ϫ ϫ coverage. At P 1.5 ML’s, a (2 2)/(2 1) recon- 3d lines were collected by a PHI 3057 XPS spectrometer, struction is observed that consists of half a monolayer of ␯ϭ using aluminum K␣ x-rays (h 1486.6 eV). The binding phosphorus dimers adsorbed on top of a complete layer of P energies of the As 3d,P2p, and In 3d levels are 41.5, 17 atoms. Desorbing the excess phosphorus yields a pure (2 133.0, and 444.0 eV, respectively. The As 3d and P 2p, ϫ ␪ ϭ ϫ 1) phase with P 1.0 ML. The (2 1) is covered with a photoelectrons exhibit the same inelastic mean free path of complete layer of buckled phosphorus dimers, which are re- ϳ 18,19 25 Å. All XPS spectra were taken in small area mode vealed in STM images as zigzagging rows of gray spots.21 At with a 7° acceptance angle and 23.5-eV pass energy. The ␪ ϭ ␴ ϫ P 0.25 ML, an indium-rich (2 4) reconstruction is re- take-off angle with respect to the surface normal was 25°. corded. The unit cell of this structure contains a single phos- The indium phosphide surface structure was characterized phorus dimer sitting astride four indium dimers. As observed using a Princeton Instruments low-energy electron diffracto- in the STM, the P dimers from adjacent unit cells line up in meter. In addition, filled-states scanning tunneling micro- straight rows extending along the ͓¯110͔ direction. At a graphs were obtained with a Park Scientific Instruments Au- Ϫ slightly lower P coverage of 0.125 ML, the ␦(2ϫ4) phase is toProbe VP at a bias of 3.6 V and a tunneling current of 1.0 ␴ ϫ nA. Images were acquired on different areas of each sample observed. This structure is the same as the (2 4) except that the P-P dimer in the top layer is replaced with a mixed for comparison. Height data were acquired from the STM 22 images at a resolution of 256ϫ256 pixels. These data were In-P dimer. used to calculate the root-mean-square ͑rms͒ surface rough- The indium phosphide surface remains atomically smooth ness: during the phase transition from a P-rich to an In-rich sur- face. Figure 4 shows an STM micrograph of the In-rich sur- 1 face obtained after annealing in 20 Torr flowing H2 for 20 ϭͱ ͚ ͒Ϫ¯ 2 ͑ ͒ Rq ͓h͑x,y h͔ , 1 min at 500 °C. The surface exhibits atomically flat terraces N with an rms roughness of 1.1 Å. A sharp (2ϫ4) LEED pat- where h(x,y) is the surface height at location (x,y), h is the tern is recorded for this sample. In addition, the reflectance average surface height, and N is the total number of data difference spectrum is indicative of a ␦(2ϫ4) reconstruction 23,24 points.
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