The TF-VLS process is schematically illustrated in Fig. 1a. Indium films (tunable thickness of 0.2–2 μm) are deposited on electropolished molybdenum foils (thickness of ~25 μm) by either electron-beam (e-beam) evaporation or electroplating, followed by e-beam evaporation of a 50 nm silicon oxide (SiO x ) cap. The Mo/In/SiO x stack is then heated in hydrogen to a growth temperature of 450–800°C, which is above the melting point of indium (~157°C). The thin SiO x capping layer enables the liquid indium to maintain a planar geometry by preventing it from dewetting. After temperature stabilization, phosphorous vapor is introduced into the chamber, either by PH 3 gas or a heated red phosphorous solid source. The diffusion of phosphorous vapor through the capping layer and dissolution in the liquid indium results in the precipitation of solid InP crystals as predicted by the indium-phosphorus phase diagram. This process closely resembles the self-catalyzed VLS growth of nanowires2, but instead produces continuous polycrystalline thin films. Figure 1b shows a tilt-view cross-sectional scanning electron microscope (SEM) image of a TF-VLS InP film on Mo foil. This image is representative of the film across the growth substrate. The as-grown InP film thickness is roughly double the original indium thickness (Fig. S1), matching the expected volume expansion from In to InP and implying near unit utilization of the indium film.

Figure 1 Growth technique and resulting InP films. (a), Schematic view of the thin-film VLS growth technique for planar and textured InP films. (b), 30° tilt view false-color SEM of planar InP film on Mo foil, showing the InP surface, cross-section and the Mo foil surface. (c), Tilt view false-color SEM image of contoured InP grown via pre-texturing the Indium film. Full size image

Interestingly, the morphology of the grown InP films can be tuned by the morphology of the starting In film and its corresponding template. As an example, an evaporated indium thin film was coated with closely packed silica beads (~1 μm in diameter) through a Langmuir-Blodgett (LB) process (see Methods) followed by a mechanical press to embed the beads into the indium film. After subsequent capping by SiO x and phosphorization, a nanotextured InP thin film with a hermispherical morphology was obtained (Fig. 1c). The ability to readily control the shape and morphology of the semiconductor film presents a unique feature of the TF-VLS process with important implications for light management15,16 and carrier collection17 in future devices.

The structural characteristics of the TF-VLS InP were probed by x-ray diffraction (XRD), electron backscatter diffraction (EBSD) and SEM. Both as-grown InP films on Mo and free standing InP films, which were obtained by peeling off the InP layer from the substrate, were examined. XRD analysis (Fig. 2a) establishes three points. First, the films are zinc blende InP. Second, the lack of indium peaks indicates that the film has turned into InP within the detection limit of the XRD (InP:In ratio >104 based on the peak ratio method). Third, the films are polycrystalline and slightly textured as evident by the larger 111 peak intensity as compared to that of 200. EBSD mapping of the InP films was used to determine the grain size. The maps (Fig. 2b) show that the grain sizes vary between 10 μm to greater than 100 μm, despite a film thickness of ~3 μm. These grains are 10–100 times larger than those previously reported for vapor phase growth of InP thin films on metal foils using metal organic chemical vapor deposition (MOCVD)12 and close spaced sublimation13. The large crystal grain size obtained with TF-VLS leads to excellent optoelectronic properties as discussed in detail below. A plan view SEM image is shown in Fig. 2c; faceted edges of the grains are visible, providing further evidence of the large grain size.

Figure 2 Structural characterization. (a), XRD spectrum of an InP film grown at 750°C. (b), EBSD image of the backside of a peeled off TF-VLS InP film, indicating large grain sizes of ~10–100 μm. (c), Top-view SEM image of InP peeled off from Mo foil, partially etched in 1% HCl to highlight grain boundaries. Full size image

Important features of the TF-VLS growth process are highlighted through a qualitative model (Fig. 3a). The process involves P diffusion through the SiO x cap into the liquid indium film, increasing the P concentration, [P], until the concentration slightly exceeds saturation, [P] Sat , enabling nucleation of the solid InP phase on the Mo substrate. It should be noted that InP does not nucleate on the SiO x surface (Fig. S2) due to the high surface energy18. Once InP nuclei are formed, they grow via diffusion of nearby P to the In/InP interface and subsequent incorporation into the solid phase. This diffusion/incorporation process creates a depletion zone near each nucleus, limiting subsequent nucleation events allowing large grain sizes. Figure 3b shows optical microscope images at various stages of the film growth (i.e. different growth times). Starting with separate InP nuclei formation, spaced ~100 μm–1 mm apart, the separate islands begin to converge, followed by the completion of film growth. The dendritic morphology is indicative of the rapid diffusion of phosphorous towards the nuclei relative to the rate at which the solid phase relaxes towards its equilibrium shape19.

Figure 3 Growth schematic. (a), Qualitative diagram of the TF-VLS growth process, showing the phosphorous vapor diffusing through the cap layer, initial InP nucleus and phosphorus concentration [P] as a function of distance from the nucleus. The depletion zone is defined as the area where [P] < [P] Sat . (b), Optical microscopy images of the growth of the InP films. Initially, separate InP nuclei/islands form, followed by growth outwards in a dendritic fashion. Finally, separate InP islands converge together and growth completes as the In film turns into InP. (c), Nucleation density as a function of incident P flux for two different starting In film heights. Full size image

A simple model helps to identify the factors that determine the density of nuclei (see Supplementary Information for details). The model suggests that the density of nuclei scales as , where F is the flux of P into the liquid indium, D is the diffusivity of P within the liquid phase, h is the initial thickness of the indium film, α is a positive constant less than one related to the critical nucleus size and A is a unitless constant related to the capture cross section of P atoms by other atoms and existing islands. Based on this simple scaling law, the key to producing a small number of nuclei and thus large grains is to insure that the flux of the incoming P is slow in comparison to the rate at which P diffuses within the liquid phase. To examine this simple model, InP thin films were partially grown (i.e., the growth was stopped prior to the full convergence of the islands) with varied PH 3 partial pressure and thereby P flux, F. The nucleation density for each sample was then obtained by optical microscopy. Two different indium film thicknesses of h ~ 500 nm and 3 μm were used. Details of the growths are reported in the supplementary information. The results of this study (Fig. 3c) depict the strong dependence of the nucleation density on the incident flux. Specifically, the nucleation density is tuned by approximately two orders of magnitude for the explored flux range. The lowest obtained nuclei density is ~50 cm−2 for F ~ 4 × 1014 cm−2s−1, corresponding to a grain size on the order of ~1 mm. A good fit between the experimental data and the model described above is obtained (Fig. 3c) when using α ~ 0.6 and A ~ 9.25 × 10−9 as the fitting parameters for both initial indium film thicknesses (see SI for details).

Next, we focus on the detailed electrical and optical characterization of InP thin films (~3 μm in thickness) as a function of growth temperature (T Growth = 450–800°C). After growth, the SiO x cap was etched away in HF. Surface cleaning and passivation was then carried out by a 30 second treatment of 1% HCl followed by a 30 second treatment of 1% HNO 3 The HCl treatment removes the native oxide, while the HNO 3 treatment results in a dense surface oxide layer which was previously shown to improve the surface properties20. The resulting films were characterized via Hall measurements, steady state photoluminescence (SSPL), time resolved PL (TRPL) and external luminescence efficiency measurements (η ext ).

Hall measurements (Fig. 4a) were carried out on InP films peeled off from the Mo substrate to extract carrier concentration and mobility. InP films were found to be n-type with an unintentional doping concentrations between 4 to 8 × 1016 cm−3, regardless of growth temperature. Notably, this relatively low carrier concentration is obtained without the use of ultrahigh purity Mo foil and indium source. Electron mobility across multiple-grains (over an area of ~1 cm2), however, exhibits a strong dependence on the growth temperature, increasing from ~12 cm2/V-s for T Growth = 450°C, to ~500 cm2/V-s for T Growth = 750°C. The electron mobility value for the sample grown at 750°C is respectable compared that of single crystal InP, which range from ~1500–4000 cm2/V-s depending on doping and compensation ratio21.

Figure 4 Optoelectronic characterization. (a), Mobility and carrier concentrations as a function of growth temperature obtained from Hall measurements carried out on peeled off InP films. (b), Steady state photoluminescence characterization of a TF-VLS InP film grown at 750°C (red line) and a similarly doped single-crystal wafer as a reference (black line). (c), Representative TRPL curve for a TF-VLS InP sample grown at 750°C. The dashed line represents 1/e of the initial peak intensity. (d), Average time-resolved photoluminescence lifetimes as a function of InP growth temperature. All measurements were performed at room temperature. Full size image