Selective-Area Growth of AlInAs Nanowires

Abstract

High performance transistors are required for the beyond CMOS in terms of low drive voltage, high drain current, high ION/IOFF ratio, and scalability. A superlattice (SL) resonance tunneling field-effect transistor (RTFET) achieves a steep subthreshold slope (SS < 60 mV/decade) while enhancing drain currents with high ION/IOFF ratio as compared with conventional TFETs [1]. However, the difficulty remains in forming high quality SL with various lattice mismatched system. NW structures enable us to maintain lattice strain and increase the critical thickness as compared with planar structures [2]. Thus, NWs can avoid formation of the misfit dislocations in highly lattice mismatched systems. In addition, vertical NWs are suitable for the vertical gate-all-around FETs (VGAA-FETs), which have superior scalability as compared with lateral FETs [3]. The lattice mismatched InGaAs/AlInAs SL is promising candidate for the RTFET [1]. There are, however, no insights regarding nanometer-scaled selective-area growth of AlInAs. Here, we characterized the selective-area growth of AlInAs NWs for the InGaAs/AlInAs NW RTFETs.AlxIn1-xAs NWs were grown by selective-area MOVPE. First, 20-nm-thick SiO2 film was formed on InP(111)B substrates by RF sputtering. Mask openings with various opening diameter (Dop: 50 - 160 nm) and pitch (Pop: 350 - 1000 nm) were formed using the electron beam lithography, dry and wet chemical etching. Next, AlxIn1-xAs was grown in a horizontal MOVPE systems with working pressure of 0.1 atm. Trimethylindium (TMIn), trimethylaluminum (TMAl), arsine (AsH3), and tertiarybutylphosphine (TBP) were used as source precursors. The growth temperature (TG) were varied from 550°C to 650°C. Al composition PAl (PAl = [TMAl] / [TMIn]+[TMAl]) in the vapor phase was changed from 0 to 30%. The partial pressure V/III ratio and growth time (t) were constant (V/III: 411, t: 12 min).Figure 1 shows results of selective-area growth of AlInAs with various PAl. Figure 2 exhibits the TG dependence of the AlInAs growths at PAl = 30%. Pop was 350 nm. Figure 3 shows average height of the NWs with the variation of the NW diameter with respect to PAl [Fig.3(a)] and TG [Fig. 3(b)]. As shown in Fig.1, the NWs had hexagonal pillars surrounded with the {-110} vertical facets and the (111)B plane. The lateral over growth along the <-110> directions were enhanced with increasing the PAl, while vertical NW growth along the <111>B direction was suppressed. The volume of the NWs was also decreased as shown in Fig. 3 (a), meaning that Al atoms disturbed surface diffusion of In atoms on NW sidewalls and SiO2 mask. Since the In atoms rarely reach on the top of the (111)B plane of the NWs under high PAl, the volume of the NWs was decreased. Figure 2 indicated that some AlInAs abnormal deposition occurred on the SiO2 mask at TG = 620°C. In Fig. 3 (b), the volume of the NWs was decreased with increasing TG, and eventually {-110} vertical flat facets disappeared at TG = 650°C, as shown in Fig. 2. This was because that the desorption process of group-III atoms was dominant at high TG. Thus, the optimum TG window to suppress the lateral over growth was thought to be in between 620°C and 650°C at the PAl. After the growth, NWs were etched to characterize Dop and the lateral over growth rate. We determined the diameter difference between the average DNW and Dop as Δdiameter. Figure. 4 shows the Δdiameter as a function of PAl or TG (Pop = 350 nm). In Fig. 4(a), the Δdiameter was gradually increased with PAl,,and, especially, was rapidly enhanced at PAl = 30%, while the NW volume was decreased. This indicated that the Al atoms enhanced the absorption of In atoms on the {-110} sidewalls under higher PAl. In Fig. 4(b) at PAl = 30%, the lateral over growth rate was decreased with increasing TG. Simultaneously, the volume of the grown AlInAs was rapidly decreased. Similar properties were observed in case of PAl = 20%. This indicated that the desorption process became dominant at higher TG. Further characterization of the growth mechanism for the selective-area growth of AlInAs will be presented.