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May 17, 2012 - We investigated the Au-assisted growth of alternating InAsP/InP heterostructures in wurtzite InP nanowires on InP(111)B substrates for ...
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VLS Growth of Alternating InAsP/InP Heterostructure Nanowires for Multiple-Quantum-Dot Structures Kouta Tateno,* Guoqiang Zhang, Hideki Gotoh, and Tetsuomi Sogawa NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato- Wakamiya, Atsugi, Kanagawa 243-0198, Japan S Supporting Information *

ABSTRACT: We investigated the Au-assisted growth of alternating InAsP/InP heterostructures in wurtzite InP nanowires on InP(111)B substrates for constructing multiple-quantum-dot structures. Vertical InP nanowires without stacking faults were obtained at a high PH3/TMIn mole flow ratio of 300−1000. We found that the growth rate changed largely when approximately 40 min passed. Ten InAsP layers were inserted in the InP nanowire, and it was found that both the InP growth rate and the background As level increased after the As supply. We also grew the same structure using TBAs/TBP and could reduce the As level in the InP segments. A simulation using a finite-difference time-domain method suggests that the nanowire growth was dominated by the diffusion of the reaction species with long residence time on the surface. For TBAs/TBP, when the source gases were changed, the formed surface species showed a short diffusion length so as to reduce the As background after the InAsP growth. KEYWORDS: VLS growth, heterostructure, nanowire, InAsP, quantum dot, wurtzite

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related emission has been reported in several studies.9,10 InP has a direct band gap and a nonradiative recombination rate that is 4 orders of magnitude lower than that of GaAs, which lead advantages in photonic applications.11,12 [111]B-oriented nanowires tend to become polytype. It has been reported that the free-exciton emissions are 1.42 eV in ZB (zincblende) InP and 1.50 eV in WZ (wurtzite) InP at low temperature, that is, the difference of the bandgap energy is 80 meV.12 From ab initio calculation, the valence band offset between ZB and WZ is 45 meV and the ZB/WZ heterostructure is the so-called typeII.13 Therefore, growing nanowires without stacking faults is very important for designing ideal devices. For the nanowire growth, the Au catalyst’s size, V/III ratio, and growth temperature influence the crystallographic structure as recently reported in detail:14,15 WZ is easily formed for smaller catalysts; the nanowires are ZB at a low V/III ratio of 100−200 but WZ at a high V/III ratio of 700 at the same growth temperature of 400 °C; and they are ZB at 420 °C and WZ at 480 °C at the same V/III ratio of 110. It is said that the nanowire growth is limited by the PH3 pyrolysis so that the amount of In in Au catalyst particles fluctuates with small changes in temperature or supply.14 The formation of abrupt interfaces of the heterostructure is important for growing quantum structures. For GaAs/GaP nanowires, Borgstrom et al. suggested As carryover phenomena, where excess As layers adsorbed on the sidewalls of the wires

anowires grown by means of the vapor−liquid−solid (VLS) mechanism are promising as building blocks for nanoscale devices. Many studies on semiconductor nanowires toward functional devices, such as transistors,1 nanolasers,2 and nanosensors,3 have been reported. VLS growth is a commonly used technique for nanowire fabrication, where a liquid particle such as an Au alloy particle acts as a catalyst at the top of nanowires. By using VLS and general vapor−solid (VS) growth, heterostructures in both axial and radial directions can be formed, and we have demonstrated some three-dimensional structures of III−V semiconductor on Si substrates.4,5 InP-related compounds are used for high-speed electronics and also for optical devices, particularly for those operating in the telecommunications wavelength bands around 1.3 and 1.55 μm where optical losses in fiber are small. AlGaInAs and GaInAsP are the material systems generally researched as InPrelated compounds. For AlGaInAs, the layers can be latticematched to InP by adjusting the composition of the group III elements. On the other hand, for GaInAsP we have to adjust the composition of both the group III and group V elements. For nanowire growth, these quaternary alloy systems are very difficult to use because the composition tends to change in the axial direction and there is no proper method to control it.6,7 An easier way to make quantum dots (QDs) in an InP nanowire is to use a ternary alloy. For the InAsP system, As is inserted to form QDs in the base binary InP nanowire. Although the InAsP layer is compressively strained, the stress is compensated in the short range of 10 nm without dislocations in the nanowire.8 For the InAsP/InP system, sharp exciton© 2012 American Chemical Society

Received: February 6, 2012 Revised: April 25, 2012 Published: May 17, 2012 2888

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Figure 1. Typical nanowire length as a function of growth time. Dotted red lines with red circles shows how the nanowire length was changed by changing the V/III ratio at a certain time indicated by the blue arrow. (a) V/III ratio was changed from 1000 to 300 at 39 min. (b) V/III ratio was changed from 300 to 1000 at 30 min. Blue dotted lines and triangles are references at the constant V/III ratio. The blue solid lines are calculated in order to represent the change in the growth rate when the residence time of surface reactants changed. The insets are SEM images of nanowires. The schematic image in (b) explains the change of the diffusion length with changing V/III ratio.

and/or physisorbed As on the bare SiO2 substrate contribute to the As memory effect.16 In this paper, we describe the growth of WZ InP nanowires without stacking faults and the growth of alternating InAsP/InP heterostructure for multiple-QD structures. We observed the As memory effect for the structures, and thus simulated the nanowire growth by supposing the surface diffusion mechanism of reactants. Finally, we discuss the effectiveness of changing the group-V source materials from AsH3/PH3 to TBAs/TBP with tertiary-butyl ligands. The wire growth was carried out in a low-pressure (76 Torr) horizontal metalorganic vapor phase epitaxy (MOVPE) reactor.4−6 Trimethylindium (TMIn) was the group III sources. Phosphine (PH3), tertiary-butyl phosphine (TBP), arsine (AsH3), and tertiary-butyl arsine (TBAs) were the group V sources. The substrates were InP(111)B. The catalysts were Au particles (20 nm in diameter) obtained from Au colloids. For the growth using AsH3/PH3, the flow rate of the group III sources was kept at 1.6 × 10−6 mol/min and the V/III ratio was 300 or 1000 at 420 °C. For the growth using TBAs/TBP, which was carried out in a different MOVPE system, the flow rate of TMIn was 2.3 × 10−6 mol/min and the V/III ratio was 1000 at 380 °C. The structures of the nanowires were observed with a scanning electron microscope (SEM) (Hitachi, S-5200) operated at 15 kV and a transmission electron microscope (TEM) (JEOL, JEM2100F) operated at 200 kV. The observation of selected area electron diffraction (SAED) patterns and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectrometry (EDS) analyses were also performed in the TEM chamber. To evaluate the elemental distribution in the nanowires, compositions of As, P, and In were evaluated from X-ray peaks from P-K, As-K and In-L lines

in the EDS analysis. Electron energy- loss spectroscopy (EELS) was also performed for evaluation of elemental distribution. To clarify the nanowire growth mechanism, we adopted a calculation based on the diffusion-deposition model.17 We used a cylindrically symmetric nanowire: the bottom of the nanowire on the substrate surface is set to the origin as zero, the longitudinal axis (z) is along the nanowire axis, and the radial direction (r) is perpendicular to the nanowire axis. The area number densities of adatoms on the substrate surface (ns) and on the wall of the nanowire (nw) can be described as ⎛ ∂ 2n ∂ns 1 ∂ns ⎞ ns ⎟− = Ds⎜ 2s + + Rs ∂t r ∂t ⎠ τs ⎝ ∂r

(1)

∂n w ∂ 2n n = Dw 2w − w + R w ∂t τw ∂z

(2)

where Ds and Dw are the surface diffusivity of adsorbed reactants on the substrate surface and the sidewall, τs and τw are the average residence times of adsorbed reactants that diffuse on the substrate surface and the sidewall, and Rs and Rw are the effective supply rates for reactants on the substrate surface and the sidewall. The growth rate is described as ∂n dL = −Dw w dt ∂z

· z=L

2Ω + 2ΩR top rw

(3)

where Ω is the atomic (or molecular) volume of the reactants and Rtop is the effective supply rate for reactants. The nanowire growth rate was calculated by using the implicit finite-difference time-domain (FDTD) method, where the length of the nanowire and the distribution of nw are calculated at each time step. We set the same flux for both sides of the boundary between the nanowire sidewall and the substrate surface. 2889

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Figure 2. InP capped InP nanowires containing ten InAsP layers. (a) SEM view 38° tilted from the normal direction. (b) Room-temperature PL spectra. The InAsP growth time was 10, 20, and 30 s.

Growth Rate of InP Nanowires. First, we investigated the growth of WZ InP nanowires without stacking faults in a high PH3 ratio condition (Figure S1 in Supporting Information). In order to understand the InP growth at high PH3 flow rate, we further investigated the growth time dependence of nanowire length. The density of the nanowires was 2.9 μm−2 for this sample and within 2−3 μm−2 for each sample. Hereafter, the height value is taken from a typical nanowire with a height similar to that of several main nanowires seen in the SEM image. Figure 1 shows the nanowire length versus growth time for a series of growths. In Figure 1a,b, triangles represent the data obtained as a reference under a constant flow rate with the V/III ratios of 1000 and 300, respectively. Circles represent the data obtained when the V/III ratio was changed during the growth from 1000 to 300 at 39 min (Figure 1a) and from 300 to 1000 at 30 min (Figure 1b) by changing the PH3 flow rate. The timing of the flow rate change is denoted by blue arrows. Around eleven minutes after the V/III ratio was changed from 1000 to 300 at 39 min, the growth rate increased abruptly, and then the growth saturated and the length slightly reduced at around 4.5 μm. On the other hand, when the V/III ratio was changed from 300 to 1000 at 30 min, the growth rate was reduced and then the length seemed to saturate at around 4.5 μm. Moreover, we found that a bulge formed in the middle of several nanowires as shown in the inset images. From these results and the results of additional experiments in which we changed the V/III ratio (not shown here), we found that both the growth rate and the maximum length of the nanowires tend to decrease with increasing PH3 supply. The tendency for a nanowire to reach a certain length and then increase in diameter in the middle indicates the axial growth was diffusionlimited, so that the high density of P reaction species on the surface prevented the diffusion of adsorbed In reaction species from arriving at the Au catalyst particle. When the density of P reaction species was decreased, the In incorporation rate increased due to the increase of the diffusion length of In compounds on the surface. When the PH3 supply was changed, the growth rate tended not to change promptly, which indicates that it takes several minutes to change the surface state. We suspect that adduct (CH3)3In/PH3 or (CH3)2InPH2 molecules, which are thought to exist during InP growth at low temperature,18 diffused on the surface slowly to the Au particle. Also, the form of the compounds sticking to the surface may be changed by the PH3 supply, which influences the diffusion

length and the growth rate. The SEM images in Figure 1a show that many microstructures formed on the surface. The density of the microstructures varies with the V/III ratio; the density for V/III = 1000 is twice that for V/III = 300. This indicates that the source materials were consumed at the microstructures on the surface, which reduce the diffusion length of surface reaction species. In Figure 1, the FDTD simulation results are plotted by solid lines. We used one-dimensional rate equations as described above. VLS growth proceeds by means of the difference between the chemical potentials of grown materials in the gas phase and solid phase as the Gibbs−Thomson effect.19 However, in our case the transport of the reaction species from the sidewalls and substrate surface seems to determine the growth rate rather than the nucleation determined by the Gibbs−Thomson effect. Thus, we assumed that all reaction species that reach the top are consumed for the growth. For simplicity, we also did not include the shell growth. Considering Ga diffusion for GaAs growth at 400 °C20 and the lack of diffusion data for InP and InAs growth, we set the diffusion coefficient to 0.01 μm2/s. In our calculation, residence time for reaction species on the surface was varied in order to express the diffusion property for different species. The rate of the source supply was set to 0.9 nm−2 s−1. We found the residence time for V/III = 300 increased from 2 to 200 s after 39 min, which corresponds to the increase of diffusion length of 1 order of magnitude. For V/III = 1000, it decreased from 1 to 0.05 s, which corresponds to a decrease of diffusion length of one-fifth. Our calculation shows that the residence time on the substrate surface did not affect the growth rate so much. On the other hand, the change of the residence time on the wall of the nanowire influenced the growth rate considerably. Considering this, with a change in the PH3 flow rate during the growth, in addition to the variation of the possible adducts described above, the state of the nanowire surface may change largely. Possibly, the density or the adsorbed state of P reaction species was varied. Since the simulation did not include the surrounding situations, such as the influence of the growth of neighboring nanowires and the formation of microstructures on the surface, we will also have to consider them as reasons for the change in the diffusion properties. Thus, from the calculation results we confirmed that the change in the diffusion properties occurs as the growth proceeds and when the V/III ratio is changed. 2890

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Figure 3. Ten InAsP layers in an InP nanowire (a) HAADF-STEM images. A: 20 s for each InAsP growth time. B: 10 s. (b) Variation of each layer. The number corresponds to the InAsP/InP pairs counted from the bottom. (c) EELS mapping images for In and As elements for sample B in (a). (d) EDS line-scan results for As and P elements for sample B.

Alternating InAsP/InP Heterostructure in Nanowires: Growth Rate and As Composition. In our previous experiments using a low V/III ratio of 60, the InP nanowires showed ZB structure with many twins, and we did not succeed in making a heterosturcture by introducing the AsH3 source gas. On the other hand, at a high V/III ratio we obtained InAsP/InP heterostructure nanowires with the WZ structures without stacking faults (Figure S1 in Supporting Information). The observed V/III-ratio-sensitive growth behavior should be related to the surface state and the surface diffusion of the reactants. The smooth surface of the WZ nanowires without stacking faults seems to deliver the As surface reactants smoothly to the Au alloy particle on the top. Under this condition, we confirmed that the thickness of the single InAsP layer formed in the InP nanowire showed linear dependence on the growth time (Figure S2 in Supporting Information). By changing the growth time from 5 to 35 s, the InAsP layer increased from 2 to 12 nm as observed by TEM. Further, we grew alternating InAsP layers in the nanowire. In optical devices, the gain is increased by forming multiple-QD structures. By performing the capping growth to form core− shell structures, nonradiative recombination at the surface is reduced due to effective carrier confinement. Figure 2a shows an SEM image of ten InAsP layers in InP nanowires capped with InP. The capping growth was performed at 470 °C for 35 min. For InAsP growth, the AsH3 supply was 10% for the group V sources. At 470 °C, the VLS growth also occurred, so that a taper structure is seen at the top region. The alternating InAsP/ InP layers were located in the core−shell region. Figure 2b shows PL spectrum measured at room temperature for the samples obtained by changing the InAsP growth time. Here, we measured about 3 × 104 nanowires from the as-grown sample.

We did not observe the PL emission from the as-grown sample with a single InAsP layer at room temperature, although we did confirm a sharp exciton emission from a single QD in one nanowire at low temperature (Figure S2 in Supporting Information). On the other hand, the as-grown sample with 10 InAsP layers showed an apparent PL peak at room temperature. As shown in Figure 2b, the PL peak widely shifts by changing the InAsP layer thickness, where the growth times of each InAsP layer were 30 (red), 20 (green), and 10 s (purple). This PL blue shift with decreasing thickness confirms the quantum size effect, and the estimated As composition was roughly around 50% of the group V elements (Figure S2 in Supporting Information). The broad peaks shown here should originate mainly from the variation of the thickness of the InAsP layers, each wire’s width, and the compositions of the Ascontaining region in one wire as described below. The tail of the emission wavelength is extended up to 1.55 μm, which covers the telecommunications wavelength bands. Increasing the As is expected to elongate the emission wavelength further. In order to characterize the detail structures of the 10 InAsP layers in an InP nanowire, we performed the HAADF-STEM, EELS and EDS analyses. Figure 3a shows the HAADF-STEM images for the 10 InAsP layers in an InP nanowire, where the growth proceeds from right to left as indicated by a yellow arrow. For the InAsP layer growth, the flow ratio of AsH3 in the group V sources was 10%. For the InP layer growth, the growth time was 30 s. The first InAsP layer started to grow after the InP growth for 39 min. After the 10-InAsP-layer growth, InP growth was performed again for 3 min. The V/III ratio was kept at 1000. Vertical bright lines are highly As-incorporated InAsP layers. The InAsP layers were grown 20 s for sample A and 10 s for sample B. The contrast between the InAsP and InP 2891

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Figure 4. EDS mapping images for As and P elements for sample B in Figure 3a. The top InP region, alternating InAsP/InP heterostructure region and bottom InP region in the nanowire were sliced perpendicular to the nanowire axis and thinned to about 100 nm thickness.

was formed. This seems to be the reason for the As memory effect as seen Figure 3c,d. FDTD Simulation of InAsP/InP Nanowires. To theoretically analyze the growth properties in regard to the As incorporation during the InP growth after the InAsP growth, we performed a FDTD simulation using one-dimensional rate equations. Figure 5 shows the EDS line scan result for 10 InAsP

layers became blurred as the number of InAsP/InP pairs increased. Figure 3b shows the dependence of the thickness of each layer on the number of the InAsP/InP pairs. The thickness was evaluated from the results of the image analysis. Although the thickness values include a large error due to the blurry contrast at the interface between the InAsP and InP layers, we could find certain tendencies, that is, an almost constant InAsP layer thickness and an increase in the InP layer thickness with the increase in the number of the InAsP/InP pairs. Figure 3c shows the In and As EELS mapping images at the 10-InAsPlayer regions for sample B in Figure 3a. We could confirm each InAsP layer from the contrast for As. Moreover, we found that the As level gradually increased toward the growth direction according to the increase in the brightness. Figure 3d shows the EDS line scan for the As and P level along a nanowire containing ten InAsP layers. Clearly, there was an increase of the As level after the first growth of the InAsP layers and a large amount of As was incorporated into the InP regions. This means that the As content was not completely reduced at the InP segments after the InAsP growth. Considering the VLS mechanism, As must be reduced during the growth interruption under the PH3 supply. However, our supplemental experiments showed that the growth interruption did not change the tendency of As incorporation. Note that the Au particle did not contain much As in the EDS data of Figure 3d. Thus, we think that the Au particle did not act as an As supplier after the InAsP growth. Figure 4 shows the EDS mapping results for sliced samples. A nanowire was sliced and three parts were extracted; one part at the top, one around the InAsP layers, and another at the bottom. The samples were 100 nm thick. In the top region, the estimated composition was InAs0.28P0.72, where relatively high As incorporation in the InP region was confirmed. In the InAsP-layer region, it was InAs0.34P0.66, which corresponds to the averaged value among several InAsP layers and InP barrier layers. In the bottom region, an unintended core−shell structure was formed, where the core region was InAs0.07P0.93 and the shell region was InAs0.26P0.74. The high-As-containing shell region was formed after the AsH3 had been introduced to grow the InAsP layers. These results suggest that the residence time of the adsorbed As reaction species on the nanowire and substrate surface was long so that the high-As-containing layer

Figure 5. As content around the alternating InAsP/InP heterostructure region in the nanowire. Blue line shows the EDS line-scan data for 10 InAsP layers in an InP nanowire. Solid red and green lines are results calculated by changing the residence time of the InAs reaction species. The EDS line-scan data for the sample using TBAs/ TBP are also included in the figure.

layers formed in an InP nanowire [blue solid line, the result used here is different from that of Figure 3d] and the FDTD simulation results (green and red solid lines). The growth direction is from left to right in this figure. Growth time for each InAsP layer was 5 s and that for each InP barrier layer was 30 s. There are three regions as shown in this figure. Region I corresponds to the InP growth prior to the introduction of As, region II to the growth of the ten InAsP layers and region III to the final InP growth for three minutes. Here, A and B denote the InP reaction species on the surface and the InAs ones, respectively. For the simulation, we assumed a constant diffusion coefficient of 0.01 μm2/s for both A and B. The rate of the source supply for A was 0.9 nm−2 s−1 in region I and 0.8 nm−2 s−1 in regions II and III. That for B was 0.1 nm−2 s−1 2892

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Nano Letters in region II. The residence time τ(A) = 1 s in region I and 0.05 s in regions II and III. The results for τ(B) = 1000 (red) and 100 s (green) are compared in Figure 5. For the simulation, we did not consider the layer deposition on the sidewalls. Actually, as seen in Figure 4, a large amount of As was incorporated on the sidewalls, although the AsH3 gas was just 10% of all the group V sources and the total growth time was 50 s during the following InAsP growth. The total growth time for region I was 39 min and that after region I was 9.3 min. Our simulation can demonstrate the increase in the As content and the long tail of each InAsP peak. In region II, for each InP growth the thickness increased as the number of growths increased. By increasing the τ(B), the thickness increased in our calculation. This suggests that the long residence time for the InAs reaction species influenced the increase of the thickness in region II. Our simulation did not reproduce the almost constant As level and the growth rate in region III. The increased growth rate in region III seems to originate from the increased residence time for InP reaction species, τ(A). However, simulation results obtained by varying the τ(A) in region III (not shown here) showed that the As level drastically decreased with increasing τ(A). To give a consistent explanation, there seems to be another reason for the As level in region III. For instance, it could be due to As supplied from the microstructures on the substrate surface. For the simulation, it seems that the decomposition of these microstructures will have to be considered. However, this kind of simulation should be performed for a three-dimensional structure, which we plan to do in the future. Finally, we show the recent results obtained by using TBAs and TBP instead of AsH3 and PH3 in Figure 5, as represented by an orange dotted line. For the InAsP growth, the TBAs flow rate was 10% for all group V source gases. From regions II to III, it was found that the As level was drastically reduced. Considering the formation of adducts or complicatedly coupled compounds as described above, we think the composition of the As reaction species on the surface was different. Moreover, the residence time for the InAs reaction species became shorter. Unfortunately, this nanowire was based on WZ containing about 200−400 stacking faults in one micrometer length. Since the growth conditions in using TBP and TBAs to obtain WZ nanowire structures were different from those in using PH3 and AsH3, it is difficult to say what mainly influences the crystal structure. It is possible that the concentration of impurities like carbon was changed by using different source gases. We have to investigate the single-crystal growth conditions for further refinement. We have investigated the growth of alternating InAsP/InP heterostructure nanowires for multiple-QD structures. Vertical InP nanowires without stacking faults as well as ten InAsP layers grown in nanowires were obtained at a high-V/III growth condition. The diffusion property of InAs reaction species influences the InAsP/InP heterostructures. The high As level and variation of the growth rate after the InAsP growth can be explained by the diffusion properties of the reacion species on the surface. By using TBAs/TBP instead of AsH3/PH3, we could reduce the As level in InP segments after the InAsP growth. Further refinement is necessary for controlling QD size, content, and position for use in realistic devices.

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AUTHOR INFORMATION

Letter

S Supporting Information *

Additonal information and figures. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*Phone: +81-46-240-3107. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Drs. H. Sanada and T. Tawara for their fruitful discussions and Drs. H. Yamaguchi and Y. Tokura for their continuous encouragement throughout this work. We also thank Drs. T. Mitate, T. Iizuka, and S. Mizuno of NTT Advanced Technology Corporation for their help in the TEM analysis. This work was partly supported by KAKENHI (23310097).

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REFERENCES

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