Growth of InGaAsP Alloy Nanowires for Emission from Visible to Mid-Infrared Wavelengths

III-V compound semiconductor nanowires (NWs) have attracted considerable attention during the past decades as the building block of next generation of optoelectronic devices. Their tunable direct band gap, high carrier mobility and excellent mechanical properties make them superior candidates to other semiconductor materials (‎1). Having direct band gaps, most of the III-V NWs can be rationally designed and synthesized to cover a wide range of the band gap energy by tuning the composition. Full composition growth of In_xGa_1-xN on a single substrate covering the band gap from near UV to mid-IR was achieved for the first time through composition gradient of source materials by Kuykendall et al.(2). Recently we have been able to grow CdSSe and ZnCdSSe NWs alloys using dual gradient method (3,4). Even though alloy nanowires with tunable optical properties for visible wavelengths have received extensive attention, similar NWs for infrared applications have received much less attention. In this contribution, we present successful Au-catalyzed Vapor-Liquid-Solid (VLS) growth of In_xGa_1-xAs, Ga_xAs_1-xP and In_xGa_1-xAs_yP_1-yNWs on a single substrate. Figure 1(a) is 2D composition plane demonstrating different paths for (In,Ga)(As,P) alloy systems. The four corners represent the four binaries, InAs, GaAs, InP, and GaP, while the four edges represent the corresponding four possible ternaries with varying composition, and each  given (x,y) point in the interior represents a quaternary alloy of given composition.

     We report a chemical vapor deposition (CVD) of spatially composition-graded ternary and quaternary alloy semiconductor NWs based on (In,Ga)(As,P) material system with potential applications in optoelectronics, e.g. solar cells and photodetectors. Using the dual gradient method (DMG) we have achieved the continuous spatial composition gradient over a single substrate with a band gap spanning from InAs (0.35 eV) to GaP (2.26 eV) for different (In,Ga)(As,P) alloyed system. The current systematic study based on DMG growth methodology opens a new horizon on structural-optical study of different alloys in III-V systems for various optoelectronic device applications.

     Different alloys were grown on Si(111) coated with 2-3 nm Au film as catalyst for VLS growth. The growth time varies between 30-60 min for different samples, and also growth temperature was adjusted between 550-680◦C for various materials. For example, highly crystalline GaP is preferred to be grown at temperature around 600◦ C while GaAs around 650-680◦C. 40-45 sccm Ar+5%H₂ was used as carrier gas and the pressure of the reactor was kept 1-10 Torr during the process. Based on the difference in enthalpy of formation for different binary materials, Au-coated substrate was positioned downstream facing the opening of the minitubes, each of which is loaded with different source materials. By tilting the substrate along the axial direction of hot-wall furnace tube it was possible to achieve spatially composition gradient growth of NWs over the single substrate. Figure 1(b) shows the representative SEM image of InGaAsP NWs grown on a Si(111) substrate. Optical properties of the as-grown NWs were investigated by photoluminescence (PL) measurements at room temperature using a Ti-Sapphire laser (810 nm) as an excitation source for NIR range and Nd-YLF (349 nm) for visible ranges. PL spectrum of InGaAs NWs shows a red shift from 1100 to 1900 nm from InAs to GaAs rich along the substrate (Fig 1(b)). NW size dependence on time evolution and temperature is also studied. Photoluminescence (PL) emission results agree with both EDS and XRD results on alloy compositions.

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3. D. Caselli, Z. Liu, D. Shelhammer, and C.Z. Ning, Nano letters14(10), 5772 (2014)
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