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Abstract
Crystal-phase control and crystalline quality improvement of GaAs nanowires (NWs) have been realized by dopant (Be) incorporation in GaAs NWs. We demonstrate the improvement of crystalline quality by X-ray diffraction (XRD) spectra combined with high-resolution transmission electron microscopy (HRTEM). The crystal-phase control from the wurtzite (WZ)/zinc blende (ZB) mixed phase to the pure ZB phase under the effect of Be doping is clearly revealed by Raman spectra combined with HRTEM. The photoluminescence (PL) revealed the free exciton and WZ/ZB type-II emission peaks of undoped GaAs NWs transform into Be impurity-related emission peak of Be-doped GaAs NWs.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Due to III-V materials have higher carrier mobility and absorption coefficient than Si, III-V compound semiconductors are widely considered as promising candidates for the next generation semiconductors [1]. Especially, III-V nanomaterials, such as nanowires (NWs), have been evidently demonstrated with unique optical, electronic and novel mechanical properties originating from their one-dimensional (1D) configuration [2,3]. Moreover, an effective ability of nanomaterials to emit and detect light means that they are often used in photodetectors [4], lasers [5], light-emitting diodes [6] and single-electron memory devices [7]. In bulk III-V materials, cubic zinc blende (ZB) structure is stable phase, however, with the decrease of dimension, the hexagonal wurtzite (WZ) phase appears. The presence of WZ/ZB mixed phase structure significantly degrades the carrier mobility and recombination efficiency of photoelectric devices, which leads to a catastrophic effect on the electrical and optical properties of the NWs [8–10]. In addition, the stacking faults (SFs) and polytype boundaries (PBs) between WZ and ZB may act as the scattering center of electrons and phonons, which will potentially affect electrons and phonons transport [10,11]. Clearly, uncontrolled and random mixed crystal structure will be detrimental for device applications [8,10], thus, the crystal structure control of NWs is imperative.
The research on structure-control of III-V NWs is still hot and difficult. The exchange between ZB and WZ phases was correlated with the growth parameter and surface energy, such as growth temperature [12], V/III ratio [13], droplet shape [14] and doping [15]. In-situ doping of beryllium (Be) has shown strong effects in particle-seeded NWs growth [16–18], which is beneficial for photoelectronic devices [16]. For III-V NWs, the dopants are incorporated with either via vapor–liquid–solid (VLS) growth mechanism by catalyst droplets, or via vapor–solid (VS) growth mechanism on the exposed sidewalls of the nanowires. Recently, the incorporation of dopant (Be) into GaAs NWs during self-catalyzed growth has been investigated by several groups. Zhang et al. reported the incorporation of dopant (Be) was incorporated into NWs predominantly through the Ga droplets [17]. Moreover, Casadei et al. found that the incorporation of dopant (Be) incorporates via the NWs side facets [19]. Raman analysis of local vibrational modes suggests that the Be was found to be incorporated as an acceptor on Ga sites [17,20]. However, the optical properties of Be-doped GaAs NWs are still a poorly investigated subject. In this paper, we report on the effect of Be doping on the crystal structure and the optical properties of self-catalyzed GaAs NWs grown on Si (111) substrate by MBE.
In this study, the undoped and Be-doped GaAs NWs were grown on Si (111) substrates using self-catalyzed growth mechanism by molecular beam epitaxy (MBE). A detailed investigation of the structural and optical properties of the undoped and Be-doped GaAs NWs were presented. The effect on the crystalline quality of the incorporation of dopant (Be) into GaAs NWs is investigated by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The crystal-phase control under the effect of Be doping is clearly revealed by Raman spectra combined with HRTEM. The optical properties of undoped and Be-doped GaAs NWs were studied by photoluminescence (PL) spectra.
2. Experimental
The undoped and Be-doped GaAs NWs were grown by self-catalyzed VLS growth mechanism on a Si (111) substrate. The GaAs NWs growth was performed by DCA P600 MBE system. Undoped GaAs NWs were grown with a Ga beam equivalent pressure, V/III ratio and substrate temperature of 6.2 × 10−8 Torr, 25.8 and ∼620 °C, respectively. The Be-doped GaAs NWs were grown under the same conditions as the undoped GaAs NWs, and the Be source temperature is 820 °C. The substrate temperature was measured by pyrometer.
The morphology of undoped and Be-doped GaAs NWs was characterized by scanning electron microscope (SEM, Hitachi S-4800). The crystallization of GaAs NWs was confirmed by XRD (Model ULTIMA IV, Rigaku, Japan). The Raman spectra of GaAs NWs were conducted by a Raman spectrometer (Raman, LabRAM HR Evolution, HORIBA Scientific, Japan) with a laser excitation wavelength of 532 nm. NWs crystal structure was determined by TEM using a FEI Tecnai G2-F20 with selected area electron diffraction (SAED) patterns. The optical properties of GaAs NWs were recorded by a HORIBA iHR550 spectrometer with an InGaAs detector. A 655 nm semiconductor diode laser was used as the excitation source. The spot diameter of the laser is approximately 7 mm. Both the Raman and PL spectra are performed on as-grown state of GaAs NWs.
3. Results and discussions
Both undoped and Be-doped GaAs NWs were grown on Si (111) substrates. Figure 1(a) and 1(b) show the typical 45° tilted SEM image of undoped and Be-doped GaAs NWs. The undoped NWs show irregular distribution, and the Be-doped NWs have better perpendicularity to the Si substrate, indicating that the Be doped NWs have higher crystalline quality. The diameters of more than 200 nanowires are counted, the average diameters of undoped and Be-doped GaAs NWs are about 100 nm and 115 nm, respectively. In addition, the average densities of undoped and Be-doped GaAs NWs are about 2.6×108/cm2 and 1.2×108/cm2, respectively. Moreover, XRD measurements were carried out in order to further determine the crystallization of the undoped and Be-doped GaAs NWs. Figure 1(c) shows the XRD patterns of undoped and Be-doped GaAs NWs. The XRD standard spectra of GaAs (cyan line) is verified using the Joint Committee on Powder Diffraction Standards (JCPDS, GaAs: #32-0389) standard database. The Bragg diffraction peak intensity of Be-doped GaAs NWs is higher than that of undoped GaAs NWs. Normalized intensity of GaAs (111) diffraction peak is magnified, as shown in Fig. 1(d). The full width at half maximum (FWHM) of undoped and Be-doped GaAs NWs are 0.169° and 0.147°, respectively. Therefore, Be-doped GaAs NWs have higher crystalline quality than undoped GaAs NWs.
Fig. 1. SEM images of the 45° tilted view of (a) undoped and (b) Be-doped GaAs NWs. (c) XRD patterns of undoped GaAs NWs (red line) and Be-doped GaAs NWs (yellow line). (d) Normalized intensity of GaAs (111) Bragg diffraction peak.
Figure 2(a) shows the Raman spectra of the undoped and Be-doped GaAs NWs. The Raman spectra of semi-insulating (SI) GaAs substrate as a reference, and the corresponding transverse optical (TO) and longitudinal optical (LO) mode peaks are 267.9 cm-1 and 292.2 cm-1, respectively. Three predominant peaks were observed at ∼259 cm-1, ∼267 cm-1 and ∼291 cm-1, corresponding to E2, TO and LO mode peaks of GaAs NWs, respectively [21,22]. The peaks in the Raman spectra of undoped and Be-doped GaAs NWs were fitted for E2, TO, surface optical (SO), and LO mode peaks, as shown in Fig. 2(b) and Fig. 2(c). It’s worth noting that the E2 mode is absent in Be-doped GaAs NWs, which implies the disappear of WZ structure of Be-doped GaAs NWs. Figure 2 (d) shows the FWHM of the GaAs LO in Be-doped GaAs NWs broadens and the ratio of IGaAs-LO/IGaAs-TO strength decreases [23], thus indicating the effective doping of the Be.
Fig. 2. (a) Raman spectra of undoped GaAs NWs, Be-doped GaAs NWs with the reference spectra of semi-insulating GaAs substrate. Multi-Lorentzian fitting of undoped GaAs NWs (b) and Be-doped GaAs NWs (c). (d) Integrated intensity ratio (IGaAs-LO/IGaAs-TO) and FWHM of GaAs LO for undoped and Be-doped GaAs NWs.
In order to further determine the crystal quality of the undoped and Be-doped GaAs NWs, TEM is characterized, as shown in Fig. 3. Figure 3(a) shows a low-resolution TEM image of a typical undoped GaAs NW. The results of higher-magnification TEM investigations are shown in Fig. 3(b) and 3(c). The presence of lines with dark contrast perpendicular to the growth direction (<111>ZB or <0002>WZ) confirms the presence of PBs and SFs in undoped GaAs NWs (Fig. 3(b)). Furthermore, in the middle region of undoped GaAs NW, a segment containing ZB-WZ polytypes, twin plane (TP) and SFs can be observed in Fig. 3(c). This region has PBs and high-density defects, which was again confirmed by SAED pattern (Fig. 3(d)). Figure 3(e) shows a low-resolution TEM image of a typical Be-doped GaAs NW. Two Be-doped GaAs NWs exhibit uniform bright-dark contrast and have a single pure phase structure. The results of HRTEM (Fig. 3(f)) and SAED (Fig. 3(g)) show that Be-doped GaAs NWs have distinct ZB structure with excellent crystalline quality. The reason is that the dopant of Be accumulation to form a Be-Ga alloy droplet can lower the droplet supersaturation or surface energy, leading to an increase of the barrier for WZ nucleation, so that WZ nucleation was prohibited [16,17,24]. Consequently, Be doping can achieve entire GaAs NW phase control and perfect single phase.
Fig. 3. TEM and SAED images of undoped and Be-doped GaAs NWs. (a) low resolution TEM image of undoped GaAs NW; (b) Higher-magnification TEM image of the NW middle. (c) HRTEM image taken along [111] zone axis from the (b) middle of undoped GaAs NW; (d) the SAED pattern of the NW middle (b) shown in panel (d). (e) low resolution TEM image of Be-doped GaAs NWs; (f) HRTEM image of the area marked by square in (e). (g) SAED pattern from the middle region of panel (f).
To investigate the optical properties of GaAs NWs before and after Be doping, power- and temperature-dependent photoluminescence (PL) spectra were characterized. Figure 4(a) shows a comparison of the PL spectra of undoped and Be-doped GaAs NWs acquired under the similar experimental conditions: the temperature of 10 K and the laser power excitation of ∼300 mW. Figure 4(a) shows that the PL spectra of undoped and Be-doped GaAs NWs are normalized according to the density of GaAs NWs. The luminous intensity of Be-doped GaAs NWs is 13.4 times that of undoped GaAs NWs. In addition, Fig. 4(b) shows a comparison of the PL spectra of undoped and Be-doped GaAs NWs at room temperature (300 K). Undoped GaAs NWs have no signal, and Be-doped GaAs NWs is observed obvious optical signal. Therefore, Be doping can improve the optical quality of GaAs NWs, which is attributed to the reduction of PBs and defects in NWs [16].
Fig. 4. (a) PL spectra of undoped and Be-doped GaAs NWs are normalized according to the density of GaAs NWs at 10 K. The inset of PL spectra of undoped and Be-doped GaAs NWs. (b) PL spectra of undoped and Be-doped GaAs NWs at room temperature (300 K).
Figure 5(a) shows PL spectra of undoped GaAs NWs, two major emission peaks and a long tail (low energy region) can be observed. Therefore, more than one type of emission mechanism exists in the undoped GaAs NWs. Three gauss peaks labeled as P1, P2, and P3 (from higher energy to lower energy) were used to fit this emission which completely coincides with the experimental results. The dominated peaks are located at 1.519 eV (P1 peak) and 1.484 eV (P2 peak), respectively. The emission peak of P1 derived from exciton emission of the GaAs ZB structure (GaAs $E_g^{ZB}$). The dominated P2 peak represents a type-II like behavior transition that electrons from the ZB GaAs recombined with holes in the WZ GaAs (GaAs$E_g^{ZB - WZ}$). Schematic band structure of the undoped GaAs NWs is shown in the inset of Fig. 5(a). Figure 5(b) shows the PL spectra of Be-doped GaAs NWs. The results show that one major emission peak and it is not a perfect Gaussian peak. Two gauss peaks labeled as P4 and P5 (from higher energy to lower energy) were used to fit this emission, which is completely coincides with the experimental results. The bandgap energy difference (∼21 meV at 10 K) between undoped and Be-doped GaAs NWs is attributed to the doping-induced narrowing of the bandgap [25]. Therefore, it is supposed that the main emission of P4 (1.498 eV) may come from conduction-band-acceptor transition (CB-Be°) with beryllium acting as acceptor [26,27]. Schematic band structure of the Be-doped GaAs NWs is shown in the inset of Fig. 5(b). The band tail of P3 (undoped) and P5 (Be-doped) can be attributed to defect-related luminescence [27].
Fig. 5. Low temperature (10 K) PL spectra of undoped (a) and Be-doped (b) GaAs NWs, which is well fitted by Gaussian peaks. Inset: Schematic of the relevant bands involved in undoped (a) and Be-doped (b) GaAs NWs transitions.
To further confirm the origin of three peaks P1, P2 and P4, power-dependent PL measurement at low temperature (10 K) was carried out. The PL spectra of undoped and Be-doped GaAs NWs under different excitation power (10–300 mW) are shown in Fig. 6. The type-II of WZ-ZB peak of undoped GaAs NWs shows a significant blueshift with the increase of excitation power in Fig. 6(a), the result can be attributed to the band-bending effect [28]. In contrast, the FE peak is fixed at 1.519 eV without any shift. In order to investigate the evolution detail of P1 and P2, the integrated PL intensity of P1 and P2 (Fig. 6(b)), and peak positions of P2 (Fig. 6(c)) under various excitation power were analyzed. The excitation power-related PL intensity is widely used to determine the origin of emission. It has been established that the PL intensity (I) can be expressed as [29,30]:
Here I0 is the excitation power, η is the emission efficiency, and the exponent α is related to the radiative recombination mechanism. For excitons recombination, 1 < α < 2; For band-to-band transition, α ≈ 2; For impurity- or defect-related emission, such as free-to-bound recombination, donor– acceptor transitions, α < 1 [29,30].
Fig. 6. PL spectra of the undoped (a) and Be-doped (d) GaAs NWs under different excitation power at 10 K. The integrated intensity PL of P1 and P2 (b), P4 (e) under different laser excitation power, the purple solid lines are theoretical fitting curves. The fits to the experimental points were obtained with Eq. (1). Dependence on the excitation power of the PL peak energy of the Gaussian components that describes the P2 (c), P4 (f) luminescence of undoped and Be-doped GaAs NWs. The fits to the experimental points were obtained with Eq. (2). Inset: the normalization WZ-ZB (a) and Be-related emission (d) peaks.
Figure 6(b) shows the integrated intensity P1 and P2 under different laser excitation power. According to the fitting Eq. (1), the parameter α1 and α2 can be obtained to be 1.48 and 0.76. To clarify the origin of the P2 peak, the relationship between peak positions and ${P^{{1 / 3}}}$ is considered, as shown in Fig. 6(c). The relationship between P2 position and excitation powers can be expressed as [31,32]:
This trend is the characteristic of type-II radiative transitions [27], which can be understood that the electrons and holes are localized on different sides of the interface between ZB/WZ segments.
The PL spectra of Be-doped GaAs NWs under different excitation power is shown in Fig. 6(d). In order to investigate the evolution detail of P4, the integrated PL intensity (Fig. 6(e)) and peak positions (Fig. 6(f)) of P4 under various excitation power were analyzed. Figure 6(e) shows the integrated intensity P4 under different laser excitation power. According to the fitting Eq. (1), the parameter α4 can be obtained to be 0.95 for P4. Therefore, it is believed that the peak at ∼1.498 eV is associated with the band acceptor (BA) transition. The band-acceptor transition energy of Be-doped GaAs is 1.493 eV [27], which is 5 meV different from our luminescence peak position, which is attributed to the quantum confinement effect of NWs. Furthermore, the P4 peak positions of Be-doped GaAs NWs under different laser excitation power are shown in Fig. 6(f). The relationship between P4 positions and excitation power cannot be well fitted with Eq. (2). Hence, the origin of luminescence of P4 is not the type-II of WZ-ZB but the impurity-related luminescence.
Figure 7 plots the temperature-dependent PL emission under an excitation power of 300 mW. Figure 7(a) shows the PL emission of undoped GaAs NWs exists at temperature from 10 to 180 K, when the temperature is above 180 K, the PL emission disappears. Figure 7(b) shows that the PL emission of Be-doped GaAs NWs can be found for the whole temperature range. The peaks position change with temperature of undoped and Be-doped GaAs NWs has been plotted in the inset of Fig. 7(c). The temperature dependence of semiconductor bandgap shrinkage can be well described by Varshni [33,34] model, shown as:
where Eg(0) is the bandgap at 0 K, α is the temperature coefficient, and β is a parameter related to the Debye temperature. For P1, Eg(0) = 1.52 eV, α=6.25×10−4 eV/K, β=415 ± 35 K, For P4, Eg(0) = 1.50 eV, α=5.25×10−4 eV/K, β=478 ± 21 K. Figure 7(c) shows the relationship between the integrated intensity ratio (I/I0) of P1, P4 with increasing temperature, the I0 is the integral strength of the peak at 10 K. The integrated intensity attenuation of P4 is faster than that of P1 with increase of temperature, which is because the defect of temperature rise cannot bind the carrier.Fig. 7. Temperature-dependent PL spectra of the undoped (a) and Be-doped (b) GaAs NWs. (c) Temperature dependent I/I0 (P1 and P4) of the undoped and Be-doped GaAs NWs. Insert of temperature-dependent emission peak position of the P1 and P4, the gray solid line is fitting curve.
4. Conclusions
In this study, the structural and optical properties of the Be doping GaAs NWs grown on Si (111) substrate were investigated. XRD revealed that the GaAs (111) Bragg diffraction peaks of undoped and Be-doped GaAs NWs are 0.169° and 0.147°, respectively, indicating the crystallization quality improvement of Be-doped NWs. The Be doping realized crystal-phase control from WZ/ZB mixed phase to ZB phase in GaAs NWs, which was clearly revealed by Raman spectra and HRTEM. Raman spectra revealed the disappearance of E2 mode peak (GaAs WZ structure) in Be-doped GaAs NWs. The structure of WZ/ZB mixed phase and defects exist in undoped GaAs NWs, while Be-doped GaAs NWs are pure ZB structure, which is further proved by HRTEM. PL spectra revealed the free excitons and WZ-ZB type-II emission peaks of undoped GaAs NWs transform into Be impurity-related emission peaks of Be-doped GaAs NWs. These results provide a stepping stone to harvest the full potential of high-crystalline quality III-V NWs in optoelectronic devices.
Funding
Changchun University of Science and Technology (XQNJJ-2018-18); Education Department of Jilin Province (JJKH20200763KJ); Department of Science and Technology of Jilin Province (20200301052RQ); National Natural Science Foundation of China (11674038, 11804335, 12074045, 61674021, 61704011, 61904017, 62027820).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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