Stages in the catalyst-free InP nanowire growth on silicon (100) by metal organic chemical vapor deposition
© Miao and Zhang et al.; licensee Springer. 2012
Received: 27 April 2012
Accepted: 20 June 2012
Published: 20 June 2012
Catalyst-free InP nanowires were grown on Si (100) substrates by low-pressure metal organic chemical vapor deposition. The different stages of nanowire growth were investigated. The scanning electron microscopy images showed that the density of the nanowires increased as the growth continued. Catalyzing indium droplets could still be fabricated in the nanowire growing process. X-ray diffraction showed that the nanowires grown at different stages were single crystalline with <111 > growth direction. The photoluminescence studies carried out at room temperature on InP nanowires reveal that the blueshift of photoluminescence decreased as the growing time accumulates, which is related to the increase in the diameter, rather than the length. Raman spectra for nanowires at different growing stages show that the quality of the nanowire changes. The growth of InP nanowires at different growing stages is demonstrated as a dynamic process.
KeywordsInP nanowire catalyst-free growth MOCVD photoluminescence Raman
Semiconductor nanowires of different materials have received increasing attention in recent years for their nanodevice application as one-dimensional structures and building blocks [1–6]. III-V nanowires grown on Si substrate have superior electronic properties of direct-bandgap III-V materials combined with the low cost and well-known properties of silicon. One way of fabricating III-V nanowires is by vapor–liquid–solid (VLS) method  using metal organic chemical vapor deposition (MOCVD) . In the typical fabrication of III-V semiconductor nanowires by the VLS method, gold nanoparticle has been used as catalyst . However, gold nanoparticle-mediated growth produces unwanted effects in nanowires, such as introduction of deep levels  and gold migration  on the semiconductor surface. In order to avoid these impacts, catalyst-free growth is demonstrated by using in situ deposited indium droplets as seeds for nanowire growth .
Catalyst-free InP nanowire growth has been realized by MOCVD, and the influence of different growing conditions has been discussed . Vertical InP nanowires have been grown on Si (111) using indium droplets as the catalysts . Individual bulk-like wires of wurtzite InP have been investigated by photoluminescence, photoluminescence excitation spectroscopy, and transmission electron microscopy . Size-dependent photoluminescence from single indium phosphide nanowires has been discussed . If production of nanowires is to reach a technologically relevant scale, exact control of the structure is necessary, and this entails a greater understanding and control of the growth process. In this paper, we study the entire forest of InP nanowires at different growing stages during the MOCVD growth process for a single set of growth conditions. In the following, we first present that new nanowires could grow in a catalyst-free InP nanowire growing process. We then proceed to describe the nanowires during different stages of the growth process by X-ray diffraction. Finally, we identify that the photoluminescence (PL) of the nanowires has direct relations with the diameter and the crystal quality of InP nanowires at different growing stage changes.
InP nanowires were grown by low-pressure MOCVD at the pressure of 10 kPa, with trimethylindium (TMIn) and phosphine (PH3) as precursor materials, transported in H2 as carrier gas. For In droplet fabrication, TMIn was introduced to the reactor at a mole flux of 4.78 × 10−6 mol/min for 30 s at 330 °C. Then, PH3 was introduced into the reactor immediately to start the nanowire growth. The nanowire was grown at 330 °C. For the precursors, the mole flux of TMIn is 4.78 × 10−6 mol/min and that of the PH3 is 1.4 × 10−4 mol/min; the V/III ratio for nanowire growth is 30. After the growth was completed, TMIn was turned off and PH3 was continuously inputted to protect the substrate until the temperature drops to 250 °C. The time for nanowire growth changed from 30 s, 1 min, 2 min, 3 min, 5 min, 7 min, till 15 min. The InP nanowires were characterized by scanning electron microscopy (SEM), X-ray diffraction, PL, and Raman. For PL, the 532-nm wavelength of a laser and a liquid N2-cooled Ge detector were used. The 488-nm wavelength of a laser is used as excitation source for Raman diffraction.
Results and discussion
There are two possible reasons for that. One is the physical condition especially the temperature. The catalyst indium droplets are fabricated by the thermal decomposition of TMIn. For catalyst-free InP nanowire growth, it has been tested in our experiments before that the temperature is in the range of 330 °C to 370 °C [17, 18]. In this range, indium droplets can be fabricated . This provides physical possibility for indium droplet fabrication during the nanowire growing step.
The other is the TMIn on the surface of the substrate which provides material for the fabrication of new indium droplets. In the model of the nanowire growing process on the sidewall, the precursor material on the surface of the substrate can be absorbed by the nanowires in the way of adatom diffusion and every single nanowire can only absorb a limited area around itself . Figure 1a shows that the catalyst droplet, which provides the initial seed for nanowire growth, is scatter distributed on the substrate and there is large space between the droplets. In the nanowire growing process, the TMIn in these spaces is decomposed and accumulates to fabricate indium droplets. These droplets are the seeds for new nanowire growth.
By comparing the images of Figure 1, it is found that the density of nanowires had almost no change in the first 3 min. After 5 min, the density obviously increased. This means that the indium droplets take more than 3 min to be fabricated, as the space and the material TMIn for indium droplet fabrication are not as sufficient as the catalyst-fabricating step. Since the density of the nanowires in Figure 2a is quite low, the new nanowires fabricated in the growing process take a limited part in the catalyst-free nanowire growth.
PL blueshift of the nanowires with different diameters and lengths
Nanowires for trace
The catalyst-free MOCVD growth of the InP nanowires on Si (100) substrates is demonstrated as a dynamic process. The density of the nanowires is increasing as time accumulates. The catalyst fabricated in the nanowire growing process is caused by two reasons. One is the temperature region for indium droplet fabrication and InP nanowire growth matched together. The other is the existence of the space for accumulating TMIn and decomposed on the surface. The new nanowires fabricated in the nanowire growing process have the same crystal structure with the nanowires grown on the indium droplets. The nanowires exhibited that room-temperature photoluminescence blueshifted from the bulk zinc blende InP bandgap energy. The bandgap of the nanowires has direct relations with the diameter. In the nanowire growth, the bandgap of the nanowires can be modulated by adjusting the diameter of the nanowires. Controlling the nanowire growing time or the size of the catalyst will be possible. The crystal quality of the InP nanowires at different growing stages is a dynamic process. These results verify and supplement the research on catalyst-free InP nanowire growth.
GQM is a professor and DWZ is an assistant professor in the State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, 3888 Dongnanhu Road, Changchun 130033, People’s Republic of China.
longitudinal optical phonon
metal organic chemical vapor deposition
scanning electron microscopy
transverse optical phonon.
This work was partly supported by the National Key Basic Research Program of China (Grant No. 2012CB619200) and the National Natural Science Foundation of China (Grant No. 50972141).
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