Polarization of stacking fault related luminescence in GaN nanorods

Linear polarization properties of light emission are presented for GaN nanorods (NRs) grown along [0001] direction on Si(111) substrates by direct-current magnetron sputter epitaxy. The near band gap photoluminescence (PL) measured at low temperature for a single NR demonstrated an excitonic line at ∼3.48 eV and the stacking faults (SFs) related transition at ∼3.43 eV. The SF related emission is linear polarized in direction perpendicular to the NR growth axis in contrast to a non-polarized excitonic PL. The results are explained in the frame of the model describing basal plane SFs as polymorphic heterostructure of type II, where anisotropy of chemical bonds at the interfaces between zinc blende and wurtzite GaN subjected to in-built electric field is responsible for linear polarization parallel to the interface planes.

Linear polarization properties of light emission are presented for GaN nanorods (NRs) grown along [0001] direction on Si(111) substrates by direct-current magnetron sputter epitaxy.The near band gap photoluminescence (PL) measured at low temperature for a single NR demonstrated an excitonic line at ∼3.48 eV and the stacking faults (SFs) related transition at ∼3.43 eV.The SF related emission is linear polarized in direction perpendicular to the NR growth axis in contrast to a non-polarized excitonic PL.The results are explained in the frame of the model describing basal plane SFs as polymorphic heterostructure of type II, where anisotropy of chemical bonds at the interfaces between zinc blende and wurtzite GaN subjected to in-built electric field is responsible for linear polarization parallel to the interface planes.© 2017 Author(s).All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).[http://dx.doi.org/10.1063/1.4974461]III-nitride semiconductors (GaN, related alloys with Al and In) have many advantages such as direct bandgap materials, high electrical breakdown field, high electron mobility, good thermal and chemical stability.A reduction of structures size to a nanoscale allows even more exciting applications that cannot be realized with conventional planar bulk design.Besides optoelectronics, nanostructures can be used in such applications where the surface is important, for example, for biochemical sensing or systems for water splitting. 1,24][5] Today, there is a strong need for integration of photonics with electronics on a single platform.Clearly, there is a great interest in the development of novel inexpensive techniques to grow high quality III-nitride structures on available and relatively low-cost templates like silicon.The use of nanostructures like nanowires may be a solution to this problem, if the structural defects can be confined to the stem of the wires facing the substrates, and then the wires themselves are dislocation free.Despite a rapidly increasing interest in applications based on nanostructures, the fundamental physics in GaN nanostructures is still sparsely investigated.One reason can be a lack of high quality GaN nanostructured material.Previously, we have shown that using magnetron sputtering GaN(0001) nanorods (NRs) of high quality can be fabricated on Si(111) substrates. 6,7The GaN NRs have demonstrated a strong near band gap photoluminescence (PL) even at room temperature.However, besides a typical donor bound exciton transition at ∼3.47 eV, an additional characteristic emission at ∼3.42 eV related to basal plane stacking faults (SFs) have been observed at 5 K. 6 Indeed, a bunch of basal plane SFs forming a rather periodic structure in the growth direction of NRs was confirmed by transmission electron microscopy (TEM).In this work, we present recent results on polarization properties of PL obtained in the GaN(0001) NRs grown on Si(111) substrates.We show that the SF related PL has a significantly different polarization response compared to the GaN exciton line.Low temperature SF emission demonstrates a strong polarization perpendicular to the NR axis, while the GaN exciton emission was not polarized.The effect vanishes at enhanced temperatures.The polarization of the SF emission is explained in terms of the model considering SFs as a multiple quantum well (MQW) structure possessed inbuilt electric field in [0001] direction.The observed polarization property is important for modern photonic applications involving control and manipulation of light polarization and phase.For example, individual nanostructures can be used as polarization sensitive photodetectors or optically gated switches.Moreover, polarization properties of nanostructures are important for an exciting research field of nanoantennas, the key optical components for light harvesting. 8he growth of nominally undoped GaN NRs was done on Si(111) substrates by reactive DCmagnetron sputter epitaxy (MSE) using a liquid Ga target of high purity (99.99999 %).The growth was done at 1000 • C under a pressure of 2.667 Pa with nitrogen as a reactive gas.Transmission electron microscopy (TEM) imaging was done with a high resolution FEI Tecnai G2 200 keV FEG microscope.Details about growth procedure and structural characterization have been published previously. 6,9,10L excitation was done using the third harmonics (λ e = 266 nm) from a Ti:sapphire femtosecond pulsed laser with a frequency of 75 MHz.The laser power was kept to 10 mW for the 266 nm line.Polarization properties of the light emission were analyzed by micro-photoluminescence (µ-PL) spectroscopy performed on a single NR.Note that the result was insensitive to the laser polarization.In this case, the laser was focused to a spot with the diameter of ∼1 µm.The single GaN NR was studied in geometry when the light wave vector was perpendicular to the c-axis (i.e.growth direction [0001]) of NR.For that, the NRs were mechanically relocated on a special copper grid, where an exact position and orientation of studied NR could be confirmed by scanning electron microscopy (SEM).For comparison, we have done measurements on the top of as-grown NR sample, in that case the excitation and the detection direction was parallel to the c-axis.The samples were placed inside a variable temperature (5-300 K) Oxford Microstat allowing XY translation with a high precision better than 0.5 µm.Polarization of PL was measured using an analyzer consisting of the achromatic rotating half-wave plate for the wavelength range of 260 -410 nm and the Glan-Taylor prism (linear polarizer) placed before the monochromator.
As respectively.These self-assembled NRs were grown randomly on Si substrate without any in-plane ordering, as shown in the SEM image.The growth direction is also confirmed to be [0001].TEM studies performed at higher magnifications verify a high quality of GaN NRs revealing that NRs are single crystalline and are almost free from threading dislocations.However, a number of GaN NRs have demonstrated a presence of structural defects such as basal plane SFs arranged in self-organized periodic structures; see Fig. 1(c).
As aforementioned, previously we have found that near band gap PL spectra measured in the top-view geometry on the as-grown GaN NRs are dominated at 5 K by two lines associated with the excitonic emission and with the basal plane SFs of type I 1 at 3.42-3.43eV, respectively. 6At low temperatures, the 3.48 eV peak is related to the excitonic recombination, likely to the donor bound excitons (DBE) since unintentionally doped GaN is n-type.Even though not all NRs have demonstrated the presence of SFs as revealed by TEM, the PL emissions taken on as-grown sample was rather uniform over the surface due to the contribution of numerous NRs (the excitation spot exceeds a single NR diameter).Using µ-PL, a single NR placed on the grid could be examined, and in this case, the PL signal can vary depending on the chosen NR.For example, we could distinguish between two typical PL spectra measured at 5 K for different single NRs: in one case, the PL spectrum shows clearly a strong line at ∼3.43 eV associated with localization of charge carriers on SFs and a weaker DBE emission at ∼3.48 eV; in the second case, the PL spectrum is dominated by only the DBE emission, see Fig. 2(a) and (b), respectively.In the following, we refer corresponding NRs as with or without SFs.The PL intensity for the SF related recombination reduces compared with the DBE line with increasing temperature; thus, at room temperature there is no difference between PL spectra measured for different single NRs.Spectra shown in Fig. 2 are taken without regard to polarization direction of the emission light.
To further study the polarization properties of PL emissions from NRs, the polar plots of PL intensity measured for different polarization angles at 5 and 295 K, respectively, are illustrated in Fig. 3 (a) and (b).The insets shows a geometry between the NR orientation and the polarization of light.We denote by E // and E ⊥ the polarization detected parallel and perpendicular to the NR axis, i.e. c-axis, respectively.An unusual polarization behavior of the NR is found.At low temperature for the SF related line the PL intensity has a strong dependence on the analyzer angle, while for the DBE line we have not observed such behavior.It is clearly seen that at 5 K the maximum PL intensity for the SF emission is observed at polarization E ⊥, when the electric field of light is perpendicular to the NR growth axis.In contrast, the DBE line has no noticeable polarization dependence of the PL intensity on the polarization angle.Similarly, the emission light of the NR without SFs showing only the DBE band was not preferentially polarized.At 295 K, when PL is dominated mainly by the GaN excitonic emission, there is no preferable polarization of the emitted light.The dependence of the polarization ratio on the emitted photon energy at 5 K and at room temperature is presented in Fig. 4 together with PL spectra measured for a single NRs at the light polarization parallel (E // ) and perpendicular (E ⊥ ) to the NR axis.The degree of polarization is calculated according to where I // and I ⊥ correspond to the PL intensity measured in polarization parallel and perpendicular to the NR axis, respectively.Fig. 4(a) and (c) shows PL spectra detected for two polarizations for the same NR for 5 K and 295 K, respectively.The maximum degree of polarization (P = 48 %) at low temperature (Fig. 4 (b)) has been observed at the energy of 3.43 eV corresponding to the SF emission, while P decreases rapidly for the energies corresponding to the GaN excitonic emission.At the lower energy side, the degree of polarization shows additional increase to ∼30 % around 3.35 -3.36 eV, i.e. the energy close to the emission of the basal plane SF of type I 2 ; 11 however, the PL line was weak for the studied NR to be discussed.At room temperature, practically no difference in the PL intensity has been detected for the perpendicular and parallel polarizations as shown in Fig. 4(c) and (d).For comparison, we have done similar polarization measurements for the as-grown sample in the top-view geometry.In this case, the NR axis is oriented parallel to the direction of the detection.We denote the polarization as parallel (E // ) and perpendicular (E ⊥ ), when the electric field of light lies in the incidence plane or perpendicular to it, respectively.It is seen from Fig. 5(a) that PL spectra measured in parallel or perpendicular polarizations are almost identical at both low and room temperature.Polar figures in Fig. 5(b) and 5(c) confirm that there is no preferential polarization either for the SF emission or for the DBE line even at low temperature.Note that for comparison, we have chosen the point where the shape of the PL spectra measured in top-view geometry are similar as the PL spectra measured for a single NR shown in Fig. 2. The DBE emission and the SF emission are peaking at 3.48 and 3.43 eV at 5 K, respectively, while at room temperature only 3.42 eV emission related to free exciton is observed.As expected, the PL emission detected in the top-view geometry is not polarized due to a low in-plane (0001) anisotropy and a random orientation of recombination centers (excitons) in GaN NRs.
To explain our results, first we refer to the work by Chen et al. 12 For a single GaN NR measured at room temperature in the geometry when the direction of the detection is perpendicular to NR axis, the polarization of emission was, as expected for thin nanowires, parallel to the c-axis corresponding to the long axis of GaN NR.Chen et al. 12 found the degree of polarization > 80% for the thinnest NRs with diameters not exceeding 40 nm.For thicker NRs, however, the degree of polarization was negligible.The effect was explained by optical confinement in the direction of NR axis.According to the theoretical model, 13 the degree of polarization for cylindrical nanowire can be expressed through effective dipole moment oriented along (d 2 z ) and perpendicular (d 2 x ) long axis: For isotropic internal emission polarization will depend only on the properties of dielectric media, such as dielectric constant (ε) and the ratio of the NR diameter to the emission wavelength in vacuum.For NRs with diameters exceeding the emission wavelength the polarization anisotropy will be negligible.For anisotropic internal emission, optical confinement can be also ignored for NRs with diameters larger than the emission wavelength and the degree of polarization would be then determined mainly by the anisotropy of internal emission.Since the anisotropy of the dielectric constant in wurtzite GaN is small, less than 10%, the anisotropy of internal emission cannot be used to explain the polarization effects in GaN NRs.At low temperatures, Chen et al. 12 have found no preferential polarization for the excitonic emission; however, they observed an appearance of the shoulder at ∼3.44 eV in the PL polarization perpendicular to the NR axis.Though they could not explain this result, the shoulder peak was assigned to the defect emission at the GaN/Si interface.Based on our previous results, 6 the explanation of unusual polarization in GaN NRs at low temperature can be well communicated.The origin of the SF line is associated with the most common basal plane stacking fault of type I 1 having the lowest formation energy and corresponding to stacking sequence ABABCBC in c-direction.Thus, the layer sequence can be presented as 3 monolayers of cubic phase (ZB) inserted in the wurtzite (W) GaN matrix.Polytype-like NRs with mixed W-ZB character have been observed for other materials such as, for example, GaAs. 14For GaN, firstprinciples calculations of the electronic structure of I 1 have shown that the ZB/W interface exhibits a type-II lineup with the valence and conduction band offset of ∆E v = 0.07 eV and ∆E c = 0.27 eV, respectively. 15Thus, a single SF forms a QW of type II, which can confine electrons, while confinement for holes is weak.However, as we have shown in GaN NRs, the bunch of SFs form a rather periodic polymorphic heterostructure in c-direction, which plays a role of multiple quantum wells (MQW), where both electrons and holes can be confined.Closely spaced bundles of basal plane SFs occurring in GaN are not unusual and have been discussed by Lähnemann et al. 16 From this point of view, the SF emission observed in GaN NRs is comparable with the MQW emission.It is known, that for the GaN-based MQW epitaxial structures grown along [0001] direction, the side emission is highly polarized even at room temperature with the electric field in the plane of the MQW. 17This is completely coincide with our model of SFs forming MQW in GaN NRs and with our observation of high polarization degree for the SF emission at low temperature.Since the confinement for holes is weak, the excitons at room temperatures can not be confined anymore inside the SF formation and should be considered as free excitons, thus, explaining the absence of preferable PL polarization at enhanced temperature.In frame of our model, we can even explain the low temperature polarization results presented by Chen et al., 12 assuming that the shoulder at ∼3.44 eV is related to SF emission.
In our case, the linear polarization of low temperature PL associated with SF is caused by anisotropy of chemical bonds on the interface between zinc-blende and wurtzite GaN subjected to in-built electric field in c-direction.The linear polarization properties for type II heterostructures was theoretically studied by Ivchenko and Nestoklon. 18Probability of emission is proportional to the square modulus of dipole matrix elements.At the interfaces exposed to the in-built electric field (as in case of polar III-nitrides), these elements for the dipole oriented perpendicular (M ⊥ ) and parallel (M // ) to the interfaces differ substantially.The degree of linear polarization can be theoretically expressed as: Where the interband matrix elements M ⊥ , M // (M j ) were introduced in Ref. 18 according to: Here, the transfer integrals V l j represent the contributions to Mj from the interatomic transition between corresponding anion and cation planes and a 0 is the interatomic distance.According to the calculation in Ref. 18 for an ideal type II interface, the intensity of emission related to the radiative recombination of an electron and a heavy hole in the polarization parallel to the growth axis is almost by one order of magnitude less than for polarization perpendicular to the growth axis.This theoretical model correlates well with our experimental finds of the polarization degree of ∼50 % for the SFs emission with light polarized mainly in-plane of SFs.
In conclusion, we have studied linear polarization properties of near band gap emission in the MSE-grown GaN(0001) NR grown on Si(111) substrate.We have found a strong anisotropy for the SF related emission measured at low temperature for a single NR in detection geometry perpendicular to NR axis.The emission was linear polarized mainly in-plane of SFs with almost 50 % degree of polarization, at the same time excitonic emission was not preferentially polarized for any temperature or detection geometry.The results are explained in terms of model considering SFs as MQW of type II with anisotropy of chemical bonds at the interfaces between zinc-blende and wurtzite GaN subjected to in-built electric field.

FIG. 2 .
FIG. 2. PL spectra measured at different temperatures for a single GaN NR with SFs (a) and without SFs (b) in geometry shown in the insets.Detection of PL is done without polarization analyzer.PL spectra are normalized and shifted vertically for clarity.

FIG. 3 .
FIG. 3. (a) Polar figures showing PL intensity vs the polarization angle measured for a single NR at 5 K (a) and at room temperature (b).PL intensity at 5 K for the peak of the SF emission (3.43 eV) and DBE (3.48 eV) is shown by circles and squares, respectively.At room temperature, the PL intensity was taken at the peak at ∼3.42 eV (circles).The geometry of excitation and detection is schematically shown in the inset.

FIG. 4 .
FIG. 4. PL spectra measured at 5 K (a) and 295 K (c) for the single NR in parallel (dashed lines) and perpendicular (solid lines) linear polarization to the NR growth axis as shown in the inset in Fig. 3. Degree of polarization is shown as function of photon energy at 5 K (b) and 295 K (d).

FIG. 5 .
FIG. 5. (a) PL spectra measured in two light polarization: parallel (P) and perpendicular (S) polarization to the incident plane for the GaN NRs in the top-excitation and top detection geometry.Spectra are shown for 5 and 295 K. Polar figures measured at 5 K (b) and at 295 K (c) in the top-view geometry as shown in the inset.PL intensity for the SF related emission is shown by circles and for the DBE line -by open squares in (b).PL peak intensity at 295 K measured for the same place in the sample is shown in (c) by circles.