3.1. Crystalline Structure, Quality, and Morphology of (InAs)m/(GaAs)n Superlattices
In this work, a 15-cycle (InAs)
2/(GaAs)
2 short-period superlattice structure was grown on a semi-insulating InP substrate at a low temperature (200 °C) using the MEE growth mode of MBE. The superlattice structure of the sample is shown in
Figure 1a.
Figure 1b–d shows the HAADF, bright-field, and dark-field images of (InAs)
2/(GaAs)
2 superlattices, respectively. The results show a short-period superlattice with a period of 15, which agrees with the expected growth structure (
Figure 1a). The (InAs)
2/(GaAs)
2 SLs, composed of alternating layers of InAs and GaAs, each with a thickness of 2 monolayers (ML), exhibit a total thickness of 19 nm.
The TEM results show a distinct and well-defined periodicity for the (InAs)2(GaAs)2 SLs, characterized by a superior crystal quality with no observable crystal defects. Remarkably, the interface of the superlattice exhibits a steep profile, indicating a well-defined structural arrangement. Usually, MBE grown at low temperatures yields sub-optimal crystal quality when compared to the growth performed at standard temperatures. This is particularly true for III-V materials grown below 200 °C, such as LT-GaAs and LT-InGaAs, which often exhibit a polycrystalline or amorphous crystalline structure with unavoidable crystal quality degradation. As a result, the relevant TEM investigations of low-temperature-grown InGaAs-based THz PCAs remain largely unexplored within the existing literature, owing to the degraded crystallization with low-temperature growth techniques.
In this study, a 15-cycle (InAs)
4/(GaAs)
3 short-period superlattice structure was grown on a semi-insulating InP substrate at a low temperature of 200 °C using the MEE growth mode. The superlattice structure of the sample is shown in
Figure 2a. The HAADF, bright-field, and dark-field images of the (InAs)
4/(GaAs)
3 superlattice are shown in
Figure 2b,
Figure 2c and
Figure 2d, respectively. The TEM results reveal a well-defined periodicity of the (InAs)
4(GaAs)
3 superlattice, steep interfaces of the superlattice, and a good crystalline quality with no observable crystalline defects.
Subsequently, the (InAs)
4/(GaAs)
3 short-cycle superlattice samples, which were grown at a low temperature using the MEE mode, were annealed at 580 °C for 10 min. The HAADF, bright-field, and dark-field images of the annealed (InAs)
4/(GaAs)
3 superlattice are presented in
Figure 3a–c. The TEM results indicate that the (InAs)
4/(GaAs)
3 superlattice maintains a well-defined periodicity after annealing, with no apparent degradation of crystal quality and no observable crystal defects.
Figure 4 illustrates a 2D AFM image of the superlattice sample with a scanning range of 1 × 1 μm by using tapping mode imaging. The image shows a remarkably smooth surface of the epitaxial film, characterized by a low root-mean-square roughness (
Rq) value of 0.167 nm.
Figure 5a shows a representative 2D AFM image of a superlattice sample with a larger scanning range of 10 × 10 μm obtained also under tapping mode. The
Rq value of this image is slightly elevated to 0.299 nm, and the surface roughness is slightly increased compared to the case of the same sample with a 1 × 1 μm scanning range in
Figure 4, which is due to the presence of bumps 1, 2, and 3, marked in
Figure 5a.
Figure 5b displays the cross-sectional profiles along the scan lines corresponding to the three specific bumps (1, 2, and 3). Notably, these bumps exhibit a maximum horizontal distance of approximately 600 nm and a maximum height of around 3.5 nm. In a previous work [
17], it was shown that LT-InGaAs samples grown on InP(100) substrates with a Ⅴ/III growth ratio of 29 exhibited a substantially higher
Rq value of 3.9 nm by employing an AFM examination with a scanning range of 10 × 10 μm. Additionally, the surface of these samples displayed pronounced roughness, characterized by the presence of numerous pits. By comparing these results with our studies, it is evident that the superlattice samples grown in this study exhibit a superior surface quality with a much-improved smoothness.
The AFM image presented in this work illustrates the surface morphology of an (InAs)2/(GaAs)2 multi-period superlattice grown on an InP substrate. The growth mode employed involved the alternate switching on and off of the shutters of Group In(Ga) and Group As, thereby enhancing the migration of atoms at low temperatures and facilitating the growth of the (InAs)2/(GaAs)2 multi-period superlattice at 200 °C. However, it is important to note that the adatom’s migration velocity is also affected by the decrease in temperature. We attribute the observed bumps in the AFM image to the localized stress relaxation at individual points during the low-temperature growth process. Specifically, at reduced temperatures, the limited mobility of the adatoms restricted their ability to reach equilibrium positions on the growing surface, leading to inhomogeneous nucleation sites. As the growth progressed, these nucleation sites evolved into three-dimensional nanostructures, which manifest as the bumps observed in the AFM image.
Figure 6 shows the AFM images of the (InAs)
4/(GaAs)
3 samples before and after annealing, with the
Rq value of 0.565 nm for the as-grown samples, and 1.23 nm after annealing at 580 °C. The increased surface roughness of samples annealed at high temperatures is mainly attributed to surface desorption and atomic migration.
3.2. HRXRD of (InAs)m/(GaAs)n Superlattice Structures
As shown in
Figure 7, the 2
θ diffraction peak of the InP substrate in the (InAs)
4/(GaAs)
3 and (InAs)
2/(GaAs)
2 superlattice samples is at 63.337°, which is consistent with the standard diffraction peak position of the InP (004) crystal plane, as seen in the HRXRD curves [
17]. The XRD pattern in
Figure 7 presents the original, unnormalized test results, showing some differences in the intensity values of the InP (004) crystal plane peak between the (InAs)
4/(GaAs)
3 and (InAs)
2/(GaAs)
2 short-period superlattices.
The (InAs)4/(GaAs)3 short-period superlattice has an equivalent In composition of 0.537, closely matching the InP (001) substrate, resulting in a diffraction peak that falls within the envelope of the InP (004) plane’s peak at 63.337°, making it indistinguishable in the graph. No additional diffraction peaks were observed for the (InAs)4/(GaAs)3 superlattice samples, indicating that the (InAs)4/(GaAs)3 superlattice is lattice-matched to the substrate.
The broad peaks between 63.4° and 63.7° are attributed to the (InAs)
2/(GaAs)
2 short-period superlattice layer, with a thickness of approximately 19 nm. The equivalent In composition of the (InAs)
2/(GaAs)
2 superlattice is 0.465, corresponding to the XRD peaks within the mentioned broad peak range. The lower intensity of these peaks is attributed to the thinness of the film, resulting in a weaker XRD signal. According to Scherrer’s formula, thinner films correspond to broader peaks. The observed broadening of the diffraction peak in
Figure 7 for the (InAs)
2/(GaAs)
2 superlattice is ascribed to the relatively thin film thickness, based on the following considerations. In our analysis, we have utilized the Scherrer formula, which is commonly employed to estimate the crystallite size and understand the broadening of X-ray diffraction (XRD) peaks in thin film materials. According to the Scherrer formula, when two-dimensional film materials are relatively thin, the XRD peaks are expected to exhibit broadening due to the finite size of the crystallites in the direction perpendicular to the film surface. For the (InAs)
2/(GaAs)
2 superlattice presented in
Figure 7, we have calculated the full width at half-maximum (FWHM) of the XRD peak using the Scherrer formula. Our calculations yield a FWHM of 0.4°, which is in agreement with the peak width observed in the XRD pattern. Furthermore, the TEM analysis presented in
Figure 1 confirms the high crystalline quality of the (InAs)
2/(GaAs)
2 superlattice. This analysis reveals the absence of significant crystal dislocations and defects within the superlattice, indicating a high level of structural integrity and crystalline quality.
Therefore, the broad diffraction peak observed in the HRXRD curve, as shown in
Figure 7 for the (InAs)
2/(GaAs)
2 superlattice, is attributed to its relatively thin thickness, rather than the crystal dislocations and defects.
The HRXRD results of the annealed (InAs)
4/(GaAs)
3 superlattice samples, shown in
Figure 8, exhibit peak shapes similar to those observed in the HRXRD curves of the as-grown sample, with no significant broadening evident in the diffraction peaks. This indicates that the crystal quality of the annealed (InAs)
4/(GaAs)
3 superlattice samples remains relatively high, with no apparent degradation.
Based on the HRXRD results, it is evident that the (InAs)4/(GaAs)3 superlattice samples are lattice-matched to the InP (100) substrate and are suitable for growing thicker layers. This makes them suitable for the fabrication of PCA antennas. Therefore, the rest of the paper will focus on this class of superlattice samples.
3.3. Raman Spectra of (InAs)m/(GaAs)n Superlattice Structures
It is known that the Raman spectrum can reveal the characteristic phonon information and interface perfection of the superlattice [
18]. The longitudinal acoustic (LA) modes can be observed in the center of the Brillouin zone, providing information about the periodicity of the superlattice. The localization of the longitudinal optical (LO) phonons at the superlattice interfaces can help researchers study the composition of the individual layers [
19]. To further characterize the fabricated (InAs)
4/(GaAs)
3 superlattice grown in this study, the Raman spectra were acquired for both the as-grown and annealed samples using a 532 nm laser excitation with a scanning range from 50 to 400 cm
−1, as shown in
Figure 9. The Raman peaks in
Figure 9 indicate the presence of superlattices. The Raman peaks at 345 cm
−1 (labeled as 4) and 269 cm
−1 (labeled as 3) are attributed to the LO mode of the InP substrate and the transverse optical (TO) phonon mode of GaAs, respectively. Another Raman peak labeled as 2 is attributed to a propagating optical mode primarily associated with InAs [
20]. These peaks are affected by the strain (GaAs and InAs strain-matched InP) and quantum confinement in the (InAs)
4(GaAs)
3 short superlattice. In addition, there is a low-energy Raman peak at 75 cm
−1 attributed to the LA mode, which is related to the periodicity of the superlattice. This peak’s position is close to that of the LA mode of a (InAs)
4(GaAs)
3 superlattice measured at 77 K, as reported in the literature [
14]. This implies that we obtained the same periodicity of the superlattice as that in [
14]. This is because the LA mode, corresponding to the lowest frequency mode of the lattice vibrations at the center of the Brillouin zone, can be used to evaluate the periodic structure of the superlattice. Moreover, the peak positions of the LA modes are mostly stable over a typical temperature range without significant temperature dependence, since the LA modes are mainly affected by the elastic properties of the lattice and its geometrical configuration, with relatively minor temperature variation. Nevertheless, our TEM and Raman results indicate the sharp interfaces and excellent crystal quality for the fabricated superlattices. The individual peak positions of the annealed sample are basically the same compared to those of the as-grown sample, and there is no obvious change in the FWHM, indicating that the crystal quality of the (InAs)
4/(GaAs)
3 superlattice does not change significantly after annealing, and there still exists an obvious periodicity with an unchanged period, which is consistent with the TEM results after annealing (
Figure 3). The weakening of the 1, 2, and 3 peak signals after annealing may be related to the decrease in Raman signal collection efficiency due to the increase in surface roughness or the slight degradation of crystal quality with high-temperature annealing.
3.4. Hall Effect and Carrier Lifetime Measurements of (InAs)m/(GaAs)n Superlattices
The resistivity, mobility, and carrier lifetime, as the key material parameters for making a superior photoconductive antenna, depend greatly on the crystal quality of the epitaxial structure. To evaluate the electrical properties of the samples, a 15-cycle (InAs)
4/(GaAs)
3 superlattice structure was annealed at 580 °C in an H
2-protecting environment for 10 min. Hall measurements at room temperature showed that the as-grown sample had a mobility of 843 cm
2/(V·s) and a square resistance of 1648 ohm/sq, while the annealed sample exhibited a reduced mobility of 766 cm
2/(V·s) and a significant increase in square resistance to 53,887 ohm/sq. The room temperature mobility was reported to be only 425 cm
2/(V·s) for the Be-doped LT-InGaAs annealed at 580 °C [
21]. The enhanced mobility observed in this study can be attributed to the utilization of (InAs)
4/(GaAs)
3 short-period superlattices instead of InGaAs, so that the lattice scattering resulting from the inherent disorder in the ternary alloy can be effectively reduced. The increase in the squared resistance is attributed to the formation of As clusters precipitated during the high-temperature annealing process, resulting in a Schottky barrier around the As precipitate, which leads to an elevation in the resistivity.
The typical pump–probe transient reflectivity result, measured under an excitation wavelength of 1450 nm with a pumping power of 6 mW at room temperature, is shown in
Figure 10. The decay curve represents the carrier lifetime (
τ) of the superlattice, providing insights into the recombination dynamics of carriers within the structure. The carrier lifetime of the (InAs)
4(GaAs)
3 superlattice samples was determined to be approximately 1.2 ps by fitting with a single exponential decay function. In the study conducted by Namje Kim et al. [
15], the carrier lifetime of a Be-doped LTG-InGaAs sample grown on a semi-insulating InP substrate at 250 °C was measured to be 7.8 ps. It is clear that the (InAs)
m/(GaAs)
n superlattice structure prepared in this work exhibits a significantly shorter carrier lifetime.
The typical pump–probe transient reflectivity result for the (InAs)
4/(GaAs)
3 superlattice sample after annealing is shown in
Figure 11, measured under an excitation wavelength of 1450 nm with a pumping power of 6 mW at room temperature. The carrier lifetime of the (InAs)
4/(GaAs)
3 superlattice sample was determined to be approximately 7.1 ps by fitting with a single exponential decay function. The increased carrier lifetime in the (InAs)
4(GaAs)
3 superlattice samples after annealing is attributed to the high-temperature annealing process, which helps remove defects in the crystal such as As
Ga antisite defects that act as non-radiative recombination centers. During the annealing process, the migration and elimination of crystal defects is facilitated, leading to a reduction in these non-radiative recombination centers and thus to a longer carrier lifetime.
3.5. Electrical Properties of (InAs)m/(GaAs)n Superlattices at Different Temperatures
The electrical properties of the as-grown and annealed (InAs)
4/(GaAs)
3 superlattices at different temperatures are shown in
Figure 12.
Figure 12a,b represents the current–voltage (I-V) characteristics of the vertical current transport through the as-grown and annealed (InAs)
4/(GaAs)
3 superlattices, respectively, as a function of the applied voltage under different temperatures. The I-V curves were obtained by applying a voltage across the top surface of the superlattice sample and the bottom of the InP substrate at various temperatures. The results in the figure show that the current through both the as-grown and annealed samples decreases as the temperature decreases. The current saturates at 230 K for the as-grown sample, while it saturates at a slightly lower 210 K for the annealed sample. The elevated current observed in the as-grown sample, as compared to the annealed sample at equivalent voltages, can be attributed to the increased resistance in the latter.
Figure 12c,d presents the temperature-dependent resistance behaviors for the as-grown and annealed superlattice samples, respectively, under an applied sample surface gate voltage (Vg) of +20 V. For the applied electric field (Vg ± 20 V), the resistivity curves have one distinct slope in the high-temperature region, and the activation energies corresponding to this slope are 294 meV and 332 meV for as-grown and annealed samples, respectively. This value closely approximates the position of the electron level in the conduction band of the (InAs)
4/(GaAs)
3 superlattice (about 0.28 eV). No significant slope is found in the low-temperature region, and the situation is similar in the range of applied voltages between −10 V and +20 V. The thermal activation energies in the high-temperature region of the as-grown and annealed samples are close to each other, and the thermal activation energy of the annealed sample is slightly higher, probably due to the formation of clusters from excess As in the low-temperature-grown (InAs)
4/(GaAs)
3 superlattice after annealing and an increase in resistance. Aggregated arsenic atoms may introduce localized states that impede charge transport, necessitating additional thermal energy to excite charge carriers across the enhanced activation gap and hence increase resistance.
Tunneling effects in nanoscale multilayer electrical transport were investigated by V Osinniy et al. [
16]. In their study, three distinct tunneling mechanisms were discussed: photon-assisted tunneling, direct tunneling, and Poole–Frenkel tunneling. In this paper, voltages were applied across the samples consisting of an (InAs)
4/(GaAs)
3 superlattice and a semi-insulating InP substrate. Compared to the electric field applied to the samples in reference [
16], the electric field imposed on the (InAs)
4/(GaAs)
3 superlattices here was lower. Therefore, the vertical current measurements were performed at relatively low electric field conditions. As reported in the literature [
16], it is suggested that phonon-assisted and direct tunneling mechanisms are significant contributors to the vertical transport of charge carriers under the conditions of low electric fields.