Selective-Area Growth of AlInAs Nanowires (2020)

Abstract High performance transistors are required for the beyond CMOS in terms of low drive voltage, high drain current, high I ON /I OFF 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 I ON /I OFF 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. Al x In 1-x As NWs were grown by selective-area MOVPE. First, 20-nm-thick SiO 2 film was formed on InP(111)B substrates by RF sputtering. Mask openings with various opening diameter (D op : 50 - 160 nm) and pitch (P op : 350 - 1000 nm) were formed using the electron beam lithography, dry and wet chemical etching. Next, Al x In 1-x As was grown in a horizontal MOVPE systems with working pressure of 0.1 atm. Trimethylindium (TMIn), trimethylaluminum (TMAl), arsine (AsH 3 ), and tertiarybutylphosphine (TBP) were used as source precursors. The growth temperature (T G ) were varied from 550°C to 650°C. Al composition P Al (P Al = [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 P Al . Figure 2 exhibits the T G dependence of the AlInAs growths at P Al = 30%. P op was 350 nm. Figure 3 shows average height of the NWs with the variation of the NW diameter with respect to P Al [Fig.3(a)] and T G [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 P Al , 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 SiO 2 mask. Since the In atoms rarely reach on the top of the (111)B plane of the NWs under high P Al , the volume of the NWs was decreased. Figure 2 indicated that some AlInAs abnormal deposition occurred on the SiO 2 mask at T G = 620°C. In Fig. 3 (b), the volume of the NWs was decreased with increasing T G , and eventually {-110} vertical flat facets disappeared at T G = 650°C, as shown in Fig. 2. This was because that the desorption process of group-III atoms was dominant at high T G . Thus, the optimum T G window to suppress the lateral over growth was thought to be in between 620°C and 650°C at the P Al . After the growth, NWs were etched to characterize D op and the lateral over growth rate. We determined the diameter difference between the average D NW and D op as Δdiameter. Figure. 4 shows the Δdiameter as a function of P Al or T G (P op = 350 nm) . In Fig. 4(a), the Δdiameter was gradually increased with P Al,, and, especially, was rapidly enhanced at P Al = 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 P Al . In Fig. 4(b) at P Al = 30%, the lateral over growth rate was decreased with increasing T G . Simultaneously, the volume of the grown AlInAs was rapidly decreased. Similar properties were observed in case of P Al = 20%. This indicated that the desorption process became dominant at higher T G . Further characterization of the growth mechanism for the selective-area growth of AlInAs will be presented. References: [1] P. Long et al. , IEEE EDL, 35 (2014) p. 1212-1214. [2] F. Glas, Phys. Rev. B, 74 , (2006) 121302(R). [3] H.Takato et al. , IEEE TED, 38 , (1991) p. 573-578. Figure 1