Observation and tunability of room temperature photoluminescence of GaAs/GaInAs core-multiple-quantum-well shell nanowire structure grown on Si (100) by molecular beam epitaxy
© Park et al.; licensee Springer. 2014
Received: 2 October 2014
Accepted: 6 November 2014
Published: 22 November 2014
We report the observation of room temperature photoluminescence (PL) emission from GaAs/GaInAs core-multiple-quantum-well (MQW) shell nanowires (NWs) surrounded by AlGaAs grown by molecular beam epitaxy (MBE) using a self-catalyzed technique. PL spectra of the sample show two PL peaks, originating from the GaAs core NWs and the GaInAs MQW shells. The PL peak from the shell structure red-shifts with increasing well width, and the peak position can be tuned by adjusting the width of the MQW shell. The GaAs/GaInAs core-MQW shell NW surrounded by AlGaAs also shows an enhanced PL intensity due to the improved carrier confinement owing to the presence of an AlGaAs clad layer. The inclined growth of the GaAs NWs produces a core-MQW shell structure having a different PL peak position than that of planar QWs. The PL emission by MQW shell and the ability to tune the PL peak position by varying the shell width make such core-shell NWs highly attractive for realizing next generation ultrasmall light sources and other optoelectronics devices.
81.07.Gf; 81.15.Hi; 78.55.Cr
Since the first demonstration by Wagner and Ellis , nanowires (NWs) have attracted extensive attention and have found application in solar cells , nano-lasers , and light-emitting devices . The increasing research attention towards NWs stems from their interesting properties, such as low dimensionality and high surface-to-volume ratio . It is also possible to integrate lattice-mismatched materials during NW formation  a feature which paves the way to fabricating optoelectronic devices with improved device performance. The possible wavelength range of operation of NWs is decided by the band gap of its constituent material, which therefore restricts the use of homogeneous single material NWs. On the other hand, heterogeneous NWs made of a core material encapsulated by a shell (also known as core-shell NWs) can operate at a broader wavelength range since they are comprised of materials with different band gaps. Furthermore, surface states which are detrimental to photovoltaic applications, present in homogeneous NWs, can be alleviated by the presence of a shell material in core-shell NWs. The shell material can also act as a gain medium for light-emitting applications. All these features make core-shell NWs highly attractive for optoelectronic device applications, and hence, core-shell NWs have been extensively studied in various material systems such as Ge-Si , nitride [3, 8], arsenide [9, 10], phosphide [11, 12], antimonide  based compound material systems, and so on.
Until now, various growth methods such as metal-organic chemical-vapor deposition (MOCVD) , molecular beam epitaxy (MBE) [9, 10, 15], chemical beam epitaxy (CBE) , chemical vapor deposition (CVD) , and laser ablation , and various materials such as GaInAs, AlInAs, and GaAs have been used to realize homogeneous NW and their epitaxial alloys to form heterogeneous NWs. Among the various growth methods, MBE stands out due to its ability to provide nanostructures having high crystalline quality and sharp atomic interfaces. Of the various materials, GaInAs and GaAs are extremely attractive for making various optoelectronic devices due to their high electron mobility, low effective mass, and the ability to generate higher photocurrent, as well as for photonic devices due to their direct band gap and high-gain and -absorption coefficients. Heterogeneous NWs formed of an epitaxial alloy of these materials would therefore be promising for realizing future electronic/photonic devices with enhanced properties and novel applications.
Despite the technological importance of GaInAs, there are very few reports in the literature describing the growth and characterization of GaInAs NWs. This is due to the fact that GaInAs NW growth is complicated: the Ga and In elements have different growth behaviors, such as solubility, diffusion properties, source decomposition efficiencies, and so on [19, 20]. Overcoming these obstacles would pave the way for the rapid development and deployment of GaInAs-based nanowire electronic/photonic devices.
Recently, the MOCVD growth of GaAs/GaInAs core-shell nano-needle structures was reported  although the study mainly focused on structural analysis, and little attention was given to studying their optical properties at room temperature. Understanding the optical properties of NWs, particularly at room temperature, is crucial for designing NW-based optoelectronic devices, since practical NW-based devices are expected to operate at room temperature. Moreover, multiple shell NWs studied by previous researchers were grown using gold-catalyst  which unfortunately introduces deep-level carrier traps [23, 24] degrading device performance. This limits their usage for optoelectronic device applications or requires silicon dioxide pre-deposition prior to epitaxial growth, involving additional process steps and complicating the NW formation. Since the first demonstration of catalyst-free selective-area MBE growth of GaAs/AlGaAs core-shell NWs on substrates pre-deposited with a thin layer of silicon dioxide by Morral et al.  in 2008, several researchers over the last few years have grown the simple core-shell [9, 25, 26] to the complex core-shell structure including quantum dot embedded nanowire [27, 28] as well as InAs nanotubes  formed by selective etching of GaAs/InAs core-shell NWs; however, the optical and structural analysis on the GaInAs-related multiple core-shell NWs has not been reported yet.
In this work, we report the room temperature photoluminescence (PL) emission and the room temperature optical characterization of MBE-grown GaAs/GaInAs core-multiple-quantum-well (MQW) shell NWs. GaAs/GaInAs MQW shells having different well widths surrounded by AlGaAs clads were grown on self-catalyzed GaAs NW cores without using any gold catalyst or requiring any pre-deposition of oxide materials or further post-processing. We demonstrate the growth of GaAs NW core on (100) silicon substrate, and the optical characterization of GaInAs MQWs with various quantum-well widths grown on the GaAs NW core. The samples were structurally characterized by conventional transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), field emission scanning electron microscopy (FE-SEM), and energy dispersive X-ray (EDX) spectrometer measurements. Cross-sections of the grown NWs were analyzed by using cross-sectional TEM measurements which revealed the formation of GaInAs quantum-well layers, GaAs barriers, and AlGaAs clad layers. For optical characterization of the grown NWs, PL measurement system (Nanometrics RPM2000) was used. We find that the PL peak position of GaAs/GaInAs core-MQW shell NWs can be tuned by changing the GaInAs shell width, and the presence of an AlGaAs layer assists in enhanced carrier confinement leading to room temperature PL emission.
Samples studied in this work were grown on p-type (100) silicon substrate using VG V80H-10 K MBE system equipped with a valved cracker effusion cell for arsenic dimer (As2) source. For group-III elemental sources, conventional K-cells with linear motion shutters were employed. Growth temperature was measured by an infrared pyrometer which can measure temperature over a range of 250°C ~ 2000°C with an accuracy of ±0.3%. Prior to GaInAs MQW shell growth, GaAs NW cores were grown which acted as the base for depositing the MQW shell structure. For the GaAs NW growth, native oxide on the epi-ready (100) silicon substrate was thermally cleaned at 650°C for 10 min, followed by Ga droplet deposition for 1 min at 650°C without As2. The amount of Ga molecular beam flux corresponded to a growth rate of 2.27 Å/s. Then, GaAs NWs were grown immediately while the MBE growth chamber pressure was kept at 8.0 × 10-7 mbar for 1 h until the NW growth was finished. Throughout the growth, the substrate was rotated at 9 rpm, and the V/III ratio was kept around 8 to initiate self-catalyzed growth of nanowires . To investigate the effect of growth temperature on GaAs NW formation, growth temperature was varied from 600°C to 670°C.
Results and discussion
Prior to the structural and compositional investigation of the complete GaAs/GaInAs core-MQW shell NWs surrounded by AlGaAs layer, the compositional and structural properties of the constituent AlGaAs and GaInAs shells were investigated by TEM. To prepare the sample for TEM measurements, the cleaved as-grown samples were dipped in isopropyl alcohol (IPA) and sonicated for 5 to 10 s in order to separate the nanowires from the substrate. The droplet that contains the mixture of nanowires and IPA was dropped on the carbon film covered copper grid followed by drying of residual IPA to obtain nanowires suitable for TEM evaluation.
The constituent elements of the outermost AlGaAs shell were investigated using high-resolution spot EDX measurements and are shown in Figure 4d. The measurements were carried out from ten different locations of the AlGaAs layer that was situated at a distance of 10 nm away from the nanowire edge. The interspacing between each point was also 10 nm. In the case of Al, the average atomic composition is 13.8% with the error range of 2.7%, while the average atomic compositions for Ga and As are 34.2% and 52% with the error ranges of 2.2% and 3.3%, respectively. These values are in agreement with the composition measured from Al0.30Ga0.70As thin film. As seen from Figure 4d, the measured data reveals the presence of Al, Ga, and As elements while In is absent. The absence of In is not surprising because the outermost layer of the NW is made of AlGaAs layer and hence, In should not be detected in EDX scan. This confirms that the outermost shell of the grown nanowire is indeed an AlGaAs layer; its role is to enhance the carrier confinement.
To further investigate the structural details and to confirm that the growth of the complete structure of GaAs/GaInAs MQW shell surrounded by AlGaAs clad layer was achieved, cross-sectional TEM analysis of the complete NW structure was performed. The sample preparation for obtaining cross-sectional TEM image was done as follows. The sample having nanowires was sonicated in order to detach the nanowires from the mother substrate. The detached nanowires were then scattered on the carrier substrate, and one of them was selected by SEM for performing cross-sectional TEM measurement. The selected nanowire was then thinned by focused ion beam (FIB) milling, and the cross-sectional image was obtained using TEM.
As shown in Figure 6, when the GaInAs shell width is increased from 8 to 16 nm, there is no significant change in the position of the PL peak at around 1.42 eV (which corresponds to the PL from GaAs core), and this is due to fact that all the GaAs NW core samples were grown under the same growth conditions. However, the PL peak situated below 1.42 eV which originates from GaInAs is red-shifted from 1.35 to 1.30 eV with the increase of GaInAs shell width from 8 to 11 nm. The PL peak intensity of the 8-nm-thick GaInAs shell was smaller than the PL intensity from GaAs NW core. However, the intensity of the PL from GaInAs became larger than that from the GaAs NW core as the width of the GaInAs shell was increased from 8 to 11 nm due to an increase in the gain volume. With a further increase in the GaInAs shell width from 11 to 16 nm, the PL peak continued to red-shift from 1.30 to 1.25 eV. The PL peak intensity of the 16-nm-wide GaInAs shell, on the other hand, decreased and became comparable with the PL peak intensity of GaAs NW core. This is presumably due to the onset of plastic relaxation  for a shell thickness far beyond the critical layer thickness of GaInAs on GaAs . The critical layer thickness of individual layers could be much larger compared to the case of planar film growth, enabling coherent growth of layers in core-shell NWs as the core diameter becomes sufficiently small . Note that, in our case, the core NW diameter of the core-shell NWs is in the range of hundred nanometer scale, and hence, the concept of critical layer thickness in the planar film growth regime still holds valid .
The red-shift of the smaller energy peak with the increase in the GaInAs shell width can be directly explained from elementary quantum mechanics . However, the PL peak positions of the GaInAs MQWs grown on GaAs substrate, and that of the GaInAs shell structure grown on GaAs NWs are not identical even though both were grown with the same sequences and conditions. The PL peak of 8-nm-thick planar GaInAs MQWs grown on GaAs substrate was around 1.27 eV; however, the PL peak of GaInAs MQWs grown on GaAs NWs with the same growth duration was around 1.35 eV, indicating an increase in band gap of approximately 80 meV due to variation in well width.
As described in the ‘Method’ section, the GaAs NW grown along the (111) direction was inclined with respect to the substrate surface. As a result, the molecular beam flux which incidents vertically on the substrate surface does not result in the growth of conventional epitaxial layers, in contrast with the growth of planar quantum wells. We presume that the incident flux is initially deposited on the NW facets that are directly exposed to the flux, and the adatoms of the incident flux then diffuse along the other facets that are not directly exposed, covering the whole NW surface. With the GaAs NW core (larger than 300 nm in diameter), the surface energy of the adatoms deposited on the NW is very close to that deposited on a planar substrate . As a result, the adatoms deposited on the NW facets that are not directly exposed to the molecular beam flux can possibly form a layer that is slightly thinner than that formed at facets directly exposed to flux. Furthermore, the incident angle of the molecular beam flux was tilted at an angle of 35° with reference to the substrate surface, due to the change in the growth direction . Due to the hexagonal cross-sectional geometry along with the tilted growth, the thickness of the deposited shell layer is expected to be smaller by a factor of Sin(90° - 35°) × Sin(90° - 30°) compared to that of a QW structure grown in planar mode. The spectral bandwidth of the PL emission from the GaInAs MQW shell changed from 0.04 to 0.12 eV and to 0.11 eV with the increase of the MQW shell width from 8 to 11 nm and to 16 nm, respectively. This is larger than the spectral bandwidth of the PL emission from planar GaInAs MQWs (0.03 eV). The wider and rather an irregular variation in the spectral bandwidth of the PL emission from MQW shell as compared with the planar MQWs might be, in part, attributed to the irregular shell width due to the inclined growth and poor uniformity of the NWs. On the other hand, the spectral width of the PL emission from GaAs core NW remained the same at around 0.06 eV revealing that they are grown under the same growth condition.
From this, the thicknesses of the grown quantum wells of the three shell structure samples are expected to be around 6.0, 8.5, and 12 nm, which are different from the well widths of 8, 11, and 16 nm of the planar quantum wells, respectively. As shown in Figure 5, the actual thickness of the shell which corresponding to 16 nm was around 12 nm as expected. From these values, we can observe that the obtained PL peak energies of the grown shell structures are well matched to that of thin film GaInAs quantum-well structures of the same thicknesses . As an example, it is seen that the PL peak position of the GaAs/GaInAs MQW shell with a thickness of 8.5 nm is very close to that of a planar 8-nm-thick GaAs/GaInAs MQWs. These results indicate that the PL peak position of the core-MQW shell NWs can be tuned, and the PL intensity can be increased by simply changing the GaInAs shell width of the GaAs/GaInAs MQW shell structure, making them a potential building block for realizing ultrasmall light sources.
In summary, we have reported the self-catalyzed growth of GaAs NWs on (100) silicon substrate and the subsequent growth of GaAs/GaInAs MQW shells surrounded by AlGaAs clad layer using MBE. The NWs were characterized using FE-SEM, EDX and room temperature PL measurements. EDX measurements confirmed the successful growth of self-catalyzed GaAs NW cores on silicon substrate, while FE-SEM measurements showed that the NW diameter increased with increasing growth temperature. GaAs/GaInAs MQW shell surrounded by AlGaAs clad was then grown on the previously grown GaAs NW core which acts as the base for growing the core-shell NW. The compositional and structural properties of the grown samples were investigated with TEM, STEM, and EDX measurements. The cross-sections of the grown NWs were analyzed by using cross-sectional TEM measurements which confirmed the formation of GaInAs quantum-well layers, GaAs barriers, and AlGaAs clad layers.
Room temperature PL was observed from the grown GaAs/GaInAs core-MQW shell NWs surrounded by AlGaAs, and the PL intensity was enhanced and broadened due to enhanced carrier confinement by the AlGaAs clad layer. Room temperature PL measurement of the core-shell structure revealed two peaks; one originated from the GaAs NW core and the other from the GaInAs MQW shells. Furthermore, the PL peak position of the GaAs core remained the same, while the PL peak of the GaInAs shell red-shifted when the GaInAs shell width was increased. In addition, PL measurements also confirmed that the well widths of a GaInAs shell grown on a GaAs core was slightly different from the expected well widths of planar quantum wells, due to the tilted growth of the GaAs NWs. The observation of photoluminescence emission at room temperature and the possibility of tailoring the room temperature PL peak by varying the GaInAs well width of core-shell NW structures makes the grown GaAs/GaInAs core-MQW shell NW an interesting and versatile candidate for realizing next generation optoelectronic devices.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2011–0017606). BJ Kim acknowledges the support from the Research Institute for Solar and Sustainable Energies (RISE) at Gwangju Institute of Science and Technology (GIST) and the Ministry of Trade, Industry and Energy (MTIE) through the industrial infrastructure program under Grant No. 10033630.
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