Rectifying Single GaAsSb Nanowire Devices. Based on Self-Induced Compositional Gradients

Transcription

1 Supporting Information Rectifying Single GaAsSb Nanowire Devices Based on Self-Induced Compositional Gradients Junghwan Huh 1, Hoyeol Yun 2, Dong-Chul Kim 1,3*, A. Mazid Munshi 1,3, Dasa L. Dheeraj 1,3, Hanne Kauko 4, Antonius T. J. van Helvoort 4, SangWook Lee 2, Bjørn-Ove Fimland 1, and Helge Weman 1* 1 Department of Electronics and Telecommunications, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway 2 School of Physics, Konkuk University, Seoul , Korea 3 CrayoNano AS, Otto Nielsens vei 12, NO-7052, Trondheim, Norway 4 Department of Physics, Norwegian University of Science and Technology (NTNU), NO- 7491, Trondheim, Norway Correspondence and requests for materials should be addressed to D.-C. K ( dc.kim@crayonano.com) and H. W. ( helge.weman@iet.ntnu.no).

2 Section 1. Effect of the Ga-catalyst and annealing of the contacts It has been reported that Au, which is a commonly used metal catalyst for NWs, can significantly affect the electrical performance of GaAs NW devices or contaminate in subsequent processing steps. Tambe et al. 1 reported that Au diffusion from the catalyst tip gives rise to a rectifying NW device behavior, while the Au catalytic tip/nw contactinterface can be used as a rectifying junction. 2, 3 To check whether the rectifying behavior alters when the Ga-catalyst is included in the contact, we have also fabricated GaAsSb NW devices where the NW top electrode also covers the Ga droplet. The I-V characteristics of these NW devices show similar consistent rectifying behaviors (Figure S1), demonstrating that the rectifying behavior in the GaAsSb NW device is independent on whether the NW top metal electrode covers the Ga droplet or not. Hence, all measured devices (over 40 devices) produce similar rectifying I-V characteristics. We have also compared the I-V curves measured from the NW device before and after a rapid thermal annealing (RTA, under N 2 ambience at 400 C for 30 s) process. Even after such RTA process, the I-V curves exhibit a similar rectifying behavior (Figure S1(d)).

3 Figure S1. Investigation on the effect of external factors (the Ga-catalyst and annealing of the contacts). (a) Representative I-V characteristic of a GaAsSb NW device that incorporates the Ga droplet in the metal electrode. (b) SEM image showing the polarity of the applied voltage for a forward bias current. The Ga droplet is marked by a dashed circle and the scale bar is 1µm. (c) Distribution of the rectification ratio (at ±3 V) of all NW devices. The distribution is fitted with a Gaussian curve. (d) Comparison of I-V characteristics of a GaAsSb NW device before and after a RTA process.

4 Figure S2. Variation of Sb concentration along the GaAsSb NW. (a) High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) image of a GaAsSb NW (NW 4), indicating point scan positions along the NW for energy dispersive X-ray spectroscopy (EDX). (b) Sb concentration based on the EDX point scans for four NWs. (c) Self-induced Sb compositional variation model: Schematics of the As for Sb exchange reaction at the surface and near-surface diffusion process taking place during NW growth, causing a near-surface depletion of Sb and thus a lowering of the average Sb concentration as probed by EDX. 4

5 Section 2. Rectifying behavior and Sb variation in GaAsSb NWs grown using As 2 flux. It is known that the kinetics of NW growth is affected by As species (As 2 or As 4 ) and that the As-Sb exchange is somewhat dependent on the As-species. 5, 6 In order to test the consistency of the self-induced variation in the Sb concentration and carrier density and thus the reproducibility of the rectifying behavior under different growth conditions, GaAsSb NWs were also grown using As 2 flux instead of As 4 flux. These NWs were grown on a Si(111) substrate at 640 C for 60 min. The Ga flux was 0.7 ML/s. The beam equivalent pressures of the As 2 and Sb 2 fluxes used were and Torr, respectively. The results presented in Figure S3 reveal the electronic and compositional features of these GaAsSb NWs grown under As 2 growth conditions. Qualitatively, the trend is similar to the GaAsSb NWs discussed in the Article despite the different growth conditions. The I-V characteristics shown in Figure S3 (a) for a representative NW device show a rectifying behavior. The rectifying direction is determined by the growth direction (i.e. Ga droplet position), as for the NWs grown with As 4. To investigate the Sb variation of the NW, confocal Raman spectroscopy was carried out. As shown in Figure S3 (b), all optical phonon modes show a red shift along the NW from the base to the top, which is consistent with the results shown in Figure 3. From the Raman shifts, the average Sb mole fraction in the probed volume was estimated to be around 29 % at the NW base and 35 % at the NW top (Figure S3 (c)). These results indicate that both the rectifying behavior and the variation in the average Sb mole fraction along the NW axis are similar for the two different sets of GaAsSb NWs independent of which As species was used for the growth.

6 Figure S3. Representative I-V curves and Raman spectra of GaAsSb NWs grown using As 2 flux. (a) Current plots as a function of bias voltage for a representative GaAsSb NW device. Inset: SEM image of the GaAsSb NW device, where the length and the diameter of the NW were estimated to be around 2.3 μm and 200 nm, respectively. (b) Raman spectra of the GaAsSb NW grown under the As 2 growth condition, indicating optical phonon modes corresponding to GaSb-like TO, GaAs-like TO, and GaAs-like LO. The Raman spectra were measured along the NW, where the distance between the measurement positions was around 200 nm. (c) Raman peak shifts of optical phonon modes as a function of the position, indicating that the average Sb mole fraction at each position decreases along the NW from the top to the base. The standard deviation of results from eight NWs is used as an error bar.

7 Section 3. Device model for the GaAsSb NW For bulk it is well known that Sb-based semiconductors generally exhibit p-type characteristics due an unintentional doping by vacancies and Sb-related defects. 7-9 This can for example already be inferred from the relatively high dark current (~ μa at a few V) measured in GaAsSb NW devices, compared to that of intrinsic GaAs NWs which are generally in the pa range. 10, 11 Holes are confirmed to be the majority carrier in our GaAsSb NWs as observed from field effect dependence measurements (Figure S4). In addition, GaAsSb is known to have a high surface defect state density Independent of the work function of metal contacts, a Schottky contact is generally formed by surface Fermi level pinning at the metal/nw interface. 15 Thus, GaAsSb NW devices can be considered as a metal/p-type semiconductor/metal (M-S-M) configuration with an equivalent circuit model 16, 17 composed of a back-to-back Schottky diode and a series resistor. However, I-V characteristics of GaAsSb NW four-probe devices show that the Schottky contact near the NW top dominates the current through the NW (Figure 4 (b) and Figure S5). This indicates that the Schottky contact near the NW base can be neglected as compared to that near the NW top, and thus an equivalent circuit model composed of a Schottky diode and a resistor in series can be used.

8 Figure S4. Gate dependence of a GaAsSb NW device with two contacts approximately 2 μm apart. (a) Output characteristics as a function of back gate voltage (V g ), indicating a p-type characteristic. (b) Transfer characteristics at V ds = -0.5 V. The effective field-effect hole mobility ( h ) was estimated using the following equation: 18 2 L h gm (1) CV g ds where g is the transconductance ( I / V ), L is the channel length (~2.0 μm), and C g is m ds ds the gate capacitance. C g is calculated from the following equation: C g 2 0L t r 1 cosh (1 ) (2) where is the relative dielectric constant of the gate insulator, 0 is the permittivity of the vacuum, t is the thickness of the gate insulator (corresponding to a SiO 2 thickness of 300 nm), and r is the diameter of the GaAsSb NW. h was then calculated to be around 11.4 cm 2 /Vs at V g = -50 V. This is somewhat lower than the literature value of 50 cm 2 /Vs reported before. As mentioned above, however, the GaAsSb NW device has a Schottky barrier limiting the

9 current flow through the NW. Assuming that the mobility is underestimated in the above calculation due to the contact barrier, the real average carrier concentration is less than the estimated carrier concentration based on the 4-probe resistivity measurement ( cm -3 ).

10 Figure S5. Current plots as a function of bias voltage for each GaAsSb NW device segment shown in Figure 4 (a). (a)-(c): The direction of the rectifying behavior for each device segment is consistent. The NW device segment near the Ga droplet is more rectifying than device segments near the NW base. This indicates that the Schottky contact closer to the Ga droplet dominates the NW rectifying behavior. (d): I-V characteristics of NW devices containing the contact A (Schottky contact near the top of the NW) show similar rectifying behaviors, indicating that the contact A is dominant for the rectifying behavior. Each of the measured I-V curves could be explained by several effects such as variations of surface depletion due to surface roughness 22, surface oxidation 23, and NW geometry 24. However, these effects depend on the NW diameter. As discussed in the Letter, all NWs studied show a non-tapered structure, which is confirmed by FE-SEM and TEM studies. Thus, these effects cannot explain the variation of the I-V curves along the NWs and can

11 therefore be eliminated as origins of the rectifying behavior and the variation of the I-V curves along the NW.

12 Section 4. Thermionic field emission (TFE) model for the metal/gaassb NW contact under reverse bias According to the reverse-biased TFE model, the total current can be expressed as: 17 q 1 I SJ SJ S exp[( ) V ] k T (3) E B 0 where S is the contact area, J is the current density through the Schottky contact, J S is a slowly varying function of applied bias, q is the electron charge, V is the applied voltage, k B is the Boltzmann constant, T is the absolute temperature, and E qe kt 00 0 E coth( ) 00 (4) B q a 1/2 where E00 ( ) (5) * 2 m N S 0 E 00 is the Padovani Stratton parameter associated with the tunneling, is the reduced Planck constant, N a is the acceptor concentration at the metal/nw interface, m* and S are the effective hole mass and relative permittivity at the contact near the top of the NW (here we use the Sb mole fraction as estimated from the micro-raman measurements as described in the Article: Sb ~37%), respectively, and 0 is the vacuum permittivity. Since the effective mass and the relative permittivity depend on the Sb mole fraction (y) 31 ( m * kg ( y) and y ) 25, Eq. 5 is taking this into account. s

13 Section 5. Deduction of carrier concentration using micro-raman spectra The correlation between the intensity of the LO phonon mode and the carrier concentration (acceptor concentration) can be expressed by the following equations. I LO I0 LO L d ( ) ( )[1 exp( 2 )] (6) 2 s 0 where L d qn a s (7) I ( ) 0 LO is the intensity of the LO phonon mode without coupling to the plasmon, is the absorption coefficient for the excitation wavelength used in this work (532 nm), L d is the surface depletion layer width, s is the surface potential barrier, 0 is the vacuum permittivity, S is the relative permittivity of GaAsSb, and N a is the acceptor concentration near the surface. Since the intensity of the TO phonon mode ( I( TO )) is independent of the surface space charge layer width, 26 equation (6) can be re-expressed as, I ( LO ) I ( ) 0 LO [1 exp( 2 Ld )] (8) I( TO) I( TO) From a linear interpolation between the absorption coefficients 27 of GaAs and GaSb, the absorption coefficient at each measurement point was determined. Since the value of s for the GaAsSb NW is not available, we used the value for the GaAsSb bulk structure, 14 where s for GaAsSb depends on the Sb mole fraction (y). For the relative permittivity, the equation s y 25 I0 is used in the calculation. To determine the value of ( LO ) I( TO ), N a ( cm -3 ) is estimated by the thermionic field emission (TFE) model and used as for

14 measurement position 1 (i.e. near the NW top). By calculating equation (7) and (8) for I0 measurement point 1 (Sb mole fraction ~37%), ( LO ) I( TO) is estimated to be Based on this, equation (8) can be rewritten in terms of N a : N a 2 s 0 s 1 I( LO) / I( TO) q[ ln(1 )] (9)

15 Section 6. Image force barrier lowering at the Schottky contact The barrier height reduction ( b ) due to the image force barrier lowering effect is given by 28 qe max b (10) 4 s 0 where E max 2 qna ( b Va ) (11) s 0 and V a is the applied voltage. Taking into account the difference in acceptor concentration along the GaAsSb NW, the lowering effect is more prominent in the Schottky contact near the NW base. Assuming that b is 0.3 ev with zero applied voltage, b at the metal/nw base contact is estimated to be ~ 83 mev, whereas that at the metal/nw top contact is found to be ~ 37 mev.

16 Section 7. Example device: Simple logic circuit based on GaAsSb NWs. The unique self-induced rectifying behavior of the GaAsSb NWs can also be utilized directly for NW-based logic devices (Figures S6 and S7). Figure 6(a) and (b) show the output voltage (V out ) of the OR/AND logic circuits versus the four possible configurations of the two voltage inputs (V1, V2). In all the logic circuits described here, voltages of 3 V and 0 V correspond to logic 1 and logic 0, respectively. In the OR logic circuit, a high input voltage allows a forward-biased GaAsSb NW device. Thus, the output is low (~0 V) when both input voltages are 0 V, while the output is high (~3 V) when either or both of the input voltages are 3 V. As it is a high input voltage that makes a reverse-biased condition in the AND logic configuration, the output of logic 0 is observed when either or both of the inputs are 0 V. The logic 1 is produced only when both input voltages are high. The characteristics of these simple GaAsSb NW devices are comparable to that of asymmetric Schottky contact-based NW devices, where the asymmetric contacts are intentionally fabricated with selective 29, 30 metallization.

17 Figure S6. Simple logic circuit based on GaAsSb NWs: (a) and (b): GaAsSb NW-based OR/AND logic gates. The output voltages (V out ) of OR/AND logic gates versus the logic address level input (V 1, V 2 ): (0, 0), (0, 1), (1, 0), and (1, 1) where logic input 0 is 0 V and logic input 1 is 3 V. Inset: Schematics of logic OR/AND gates: the OR/AND gates are composed of two GaAsSb NWs and one resistor, where the resistor is 100 MΩ and V c is biased at 3 V.

18 Figure S7. Schematics and SEM images of two different logic circuits composed of GaAsSb NWs (a): OR gate and (b): AND gate.

19 Section 8. Performance of GaAsSb NW photodetector. In addition to the photoresponsivity ( R ), detectivity (D*) and external quantum efficiency (EQE) are also important parameters to evaluate the performance of photodetectors. If the shot noise from the dark current at the reverse biased Schottky contact is predominantly responsible for the noise limiting detectivity, D* of the GaAsSb NW photodetector can be 31, 32 calculated from: R D* (12) 0.5 (2 qi / S) dark where q is the electric charge. D* at -3 V bias was estimated to be Jones, which is smaller than for the GaAs/AlGaAs NW photodetector. 31 This can be attributed to the high dark current in the present case. The EQE can be expressed by the following equation: 32 hc EQE R (13) q where h is Plank s constant, c is the velocity of light, and is the wavelength of incident light. The EQE was calculated to be %, which is comparable to recently reported values of nanostructure-based photodetectors (Table S1).

20 Table S1. Comparison of important parameters of nanostructure-based photodetectors. Materials R (A/W) D* (Jones) EQE (%) r (s) d (s) Ref GaAs/AlGaAs NW InAs NW GaN NW Sb-Bi-Se NW Sb 2 Se 3 NW CdTe NW ~20 HfS 3 nanobelt < 0.4 < 0.4 GaSe nanosheet Monolayer MoS Multilayer MoS 2 ~0.12 ~10 GaAsSb NW This work

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