Article pubs.acs.org/crystal
Suppression of Rotational Twin Formation in Virtual GaP/Si(111) Substrates for III−V Nanowire Growth Christian Koppka,* Agnieszka Paszuk, Matthias Steidl, Oliver Supplie, Peter Kleinschmidt, and Thomas Hannappel* Institute of Physics, Technical University of Ilmenau, Gustav-Kirchhoff-Straße 5, D-98693 Ilmenau, Germany ABSTRACT: Planar GaP epilayers on Si(111) are considered as virtual substrates for III−V-related optoelectronic devices such as highefficiency nanowire-based tandem absorber structures for solar energy conversion, next generation LEDs, and fast photodetectors. Rotational twin domains in such heteroepitaxial epilayers are found to strongly impede vertical nanowire growth. We investigate the twin-induced defect density and surface morphology of B-type GaP/Si(111) virtual substrates in dependence on the GaP nucleation process by metalorganic chemical vapor deposition. By employing quantitative high-resolution X-ray diffraction (HR-XRD)), scanning electron and atomic force microscopy (SEM and AFM), we reveal the significant influence of nucleation temperature and substrate miscut direction on the formation of rotational twin domains during a two-step GaP growth approach. The epilayer defect density is drastically decreased by low temperature GaP nucleation on Si(111) misoriented 3° toward [1̅1̅2], where rotational twin domains are suppressed below 5% and the layers exhibit a smooth surface morphology. We demonstrate that these virtual substrates are highly suitable for vertical GaP nanowire growth.
1. INTRODUCTION The integration of tunable III−V materials with well-established Si technology is a highly relevant issue in the field of optoelectronics.1−6 (111)-oriented substrates are preferred for the growth of III−V nanowire (NW) structures, which are appropriate for the development of highly efficient optoelectronic devices. For precise control of the doping level in such NW structures, Au-mediated vapor liquid solid (VLS) growth is a commonly used technique. In VLS growth of III−V nanowire structures on Si-based substrates, III−V buffer layers can prevent Au-induced deep-level defects in the Si substrate7,8 as well as the possibility of uncontrolled Si doping of the III−V nanowires. In addition, the direct VLS growth on Si(111) substrates is difficult9 due to the challenging control of the Si surface treatments,10−12 the strong Au−Si chemical interaction,13,14 and the strong tapering of the wires.15,16 Due to the small lattice mismatch, GaP/Si is a suitable virtual substrate to link nonpolar silicon substrates and polar III−V epilayers.10,17−19 Low defect densities in the GaP buffer layer are required for superior optoelectronic properties in highperformance devices.20 Recently, we demonstrated that an Asmodification of the GaP/Si(111) heterointerface enables growth of so-called B-type GaP layers, which exhibit suitable crystal polarity for subsequent NW epitaxy.11 Because of this fact, all GaP epilayers used in this work are grown on Asmodified Si(111) substrates. Independent of the polarity, epitaxially grown III−V films tend to form rotational twin domains (RTDs) on (111)-oriented substrates, which result in multicrystalline layers with poor surface quality.9,21 Figure 1 © XXXX American Chemical Society
schematically displays RTDs for B-type GaP/Si(111) heterostructures, which are addressed in this article. These 2D defects may propagate into subsequently grown epitaxial structures, such as NWs, and adversely affect their performance. The formation of coherent rotational twins results from two possible epitaxial arrangements of the first GaP monolayer. Accordingly, the twin domain content is initiated during the nucleation phase of the buffer growth. RTDs are characterized by the stacking sequence with respect to the substrate. The domains that maintain the AABBCC sequence along the [111] direction are referred to as α-domains in this work (see Figure 1b), and the rotated twins are referred to as β-domains. The latter are rotated by 60° along the [111] axis with respect to the α-domains (see Figure 1a), which results in a so-called cis bonding configuration at the heterointerface compared to the standard trans-configuration (see Figure 1b, marked in red). The energetic difference between both configurations should be comparable to stacking faults in terms of changes in bond length and angle. Thus, the possibilities for suppressing the twin domain formation seem limited. However, Proessdorf et al.22 were able to increase the α-domain content of GaSb films on Si(111) from ∼52 to ∼57% by varying the growth temperature during molecular beam epitaxy (MBE). This implies that the energy supplied to the system in the form of Received: April 8, 2016 Revised: September 21, 2016
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DOI: 10.1021/acs.cgd.6b00541 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
Figure 1. Schematic of B-type GaP/Si(111). (a) Top view: GaP α- (red triangle) and β- (blue triangle) domains separated by the grain boundary (red dotted line). (b) Side view in [1̅10] projection: cis- and trans-bonding configuration with their corresponding stacking sequence across the heterointerface (see text for As modification). The difference in lattice constant between GaP and Si is neglected.
Figure 2. (a) Stereographic projection of {242} planes in [111] direction for the Si substrate (green), non-twinned α-GaP domains (red), and rotationally twinned β-GaP domains (blue). (b) Measured HR-XRD Φ-scan of GaP/Si(111) of the {242} reflections shown in (a). (c) HR-XRD ω2Φ scans of each {242} reflection of a GaP/Si(111) sample with given nucleation parameters. The β-domain amount in this sample is ∼19%.
an effective suppression of RTD formation to a β-GaP amount below 5%.
the nucleation temperature can have an influence on the RTD ratio. The importance of the nucleation sequence was also evidenced by Suzuki et al.,23 who showed a reduction of the RTD amount in MBE-grown GaAs layers by pre-evaporation of In. In contrast, Ga pre-evaporation slightly enhances RTD formation. Suzuki et al. also show a reduction of the RTD ratio with increased V/III ratio during growth. To our knowledge, published work regarding RTD suppression was done using MBE, and the best value yet reported is an RTD amount of 8%.23 In this paper, we establish X-ray diffraction for precise RTD quantification, which we apply to study the influence of nucleation temperature on the RTD ratio and surface morphology of metal organic vapor phase epitaxy (MOVPE)grown B-type GaP/Si(111) virtual substrates. We demonstrate
2. EXPERIMENTAL PROCEDURE GaP epilayers were grown by MOVPE in an Aixtron 200 reactor using triethylgallium (TEGa) and tertiarybutylphosphine (TBP) as Ga and P precursors, respectively. Si(111) substrates with 3° miscut in [112̅] as well as [1̅1̅2] direction were used. After a wet-chemical treatment (RCA 1 + buffered oxide etching (BOE) + RCA 2) to remove adhered particles, the substrates were thermally deoxidized in the MOVPE reactor at 1000 °C (950 mbar) for 30 min under H2, which leads to a surface free of oxygen and other contaminants.24 For B-type GaP layers to be achieved, the Si(111) surface was terminated with As11 by annealing for 3 min at 670 °C under tertiarybutylarsine (TBA) source flow. B
DOI: 10.1021/acs.cgd.6b00541 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
Figure 3. β-domain amount of GaP/Si(111) in dependence of the nucleation temperature as well as the substrate miscut direction. All samples are grown with a nucleation time of 15 min, and their epilayer thickness is between 81 and 104 nm. Samples with 3° miscut in the [112̅] direction are indicated by blue triangles, and substrates with 3° miscut in the [11̅ 2̅ ] are indicated by orange circles. For the GaP epilayers, we applied a two-step process with a lowtemperature nucleation step (Tnuc = 400−500 °C; tnuc = 2−30 min; nominal V/III = 433; dnuc ≈ 5−22 nm) and a subsequent growth step at 660 °C (nominal V/III ratio = 867; linear heat-up from Tnuc in 2.5 min; dgrowth ≈ 80−85 nm) for 45 min. Because this study primarily focuses on the formation of rotational twins caused by the heterointerface between GaP and Si(111), we deposited approximately 100 nm of GaP to yield a higher diffraction signal. Both nucleation and buffer growth were carried out via codeposition at a pressure of 50 mbar. Low growth rates were intended to reduce the kinetics compared to that of the thermodynamic influence during growth. Note that the effective V/III ratio during the nucleation step strongly depends on the nucleation temperature due to the partial dissociation of TBP at temperatures
