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Al0.5Ga0.5N nanostructures
Journal of Luminescence 183 (2017) 291–298
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Low temperature non radiative process's effects on the optical emission of GaN/Al0.5Ga0.5N nanostructures Abdelkarim Kahouli Université de Nice Sophia Antipolis, Parc Valrose, 28 avenue Valrose, 06108 Nice Cedex 2, France
art ic l e i nf o
a b s t r a c t
Article history: Received 18 July 2016 Received in revised form 8 November 2016 Accepted 18 November 2016 Available online 22 November 2016
The optical properties of self-assembled (0001) polar and (11–22) semipolar GaN nanostructures embedded in Al0.5Ga0.5N matrix and grown by molecular beam epitaxy are reported. A statistical analysis of the nanostructure's height dispersion is done by transmission electron microscopy (TEM) in order to have a good estimation of the electric field inside the structure and also to be able to give an explanation for the saturation of the emission energy for polar orientation when the height exceed some values. It is found that the non-radiative processes are not negligible at low temperature and should be taken into account as the radiative ones. The combined comparison between experimental results and our model has allowed us to determine the non-radiative lifetimes at low temperature. We show that the use of epitaxial lateral overgrowth, in addition to the growth of GaN nanostructures, will improve the radiative efficiency that can reach 60%. & 2016 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, GaN based materials present a great interest from worldwide semiconductor research community. The ability to tune the bandgap across the entire solar spectrum allows III-nitrides semiconductors to be the potential candidate used for optoelectronic devices and photovoltaic applications. Due to the lack of suitable substrates, hetero-epitaxial growth on foreign substrates as sapphire or silicon is needed. The large lattice mismatch between substrate and III-Nitrides layers reveals a large dislocation's density in the matrix. Threading dislocations act as non-radiative recombination centers and reduce light emission's efficiency. Thus, it is of highest importance to develop some growth strategies that minimise the effect of structural defects. Growing GaN nanostructures is a possible approach to overcome the effect of the reduced crystal quality [1–4]. The reduced dimensionality of self-assembled GaN nanostructures allows it to be very efficient light emitters. It act as traps for charge carriers. Therefore it hinder their diffusion towards non radiative defects. Molecular beam epitaxy is able to produce nanostructures with high quality by using a nitrogen plasma cell [5–7] or ammonia [2,8,9] as a nitrogen source. Despite the promising future of the optoelectronics devices based on GaN nanostructures [10], the improvement of the optical properties is still limited. One of the major difficulties to realise E-mail address: [email protected] http://dx.doi.org/10.1016/j.jlumin.2016.11.051 0022-2313/& 2016 Elsevier B.V. All rights reserved.
these devices is related to the presence of a strong internal electric field along (0001) orientation. The resulted quantum confined stark effect (QCSE) generates a strong red-shift of the emission energy and a low radiative efficiency characterized by a high radiative lifetime [11–14]. It is thus a challenge to reach the UV range. An alternative strategy to surpass this constraint is the growth along a semipolar orientation where the polar axis (0001) is inclined corresponding to the growth axis by 58° [15–18]. In this case, the effects of the internal electric field can be drastically reduced. As a consequence a shift to higher energy and an increase of the radiative efficiency have been obtained [19]. The value of the internal electric field is reduced by a factor 8 compared to the polar orientation [20]. The problem is not solved yet because semipolar nitrides suffer from a high density of defects, mainly the basal stacking faults (BSFs) and misfit dislocations (MDs) [21–23]. The higher density of defects for semipolar nitrides compared to the polar orientation can reduce the radiative improvement brought by GaN nanostructures. Epitaxial lateral overgrowth (ELO) can be used to filter the spreading of BSFs inside GaN matrix [24–27]. The aim of the present letter is to show that we are able to calculate the PL linewidth and the PL intensity by using the dispersion of the nanostructure's height extracted from the transmission electronic microscopy (TEM). The estimation of the internal electric field is also conducted. In order to improve our model and to explain some features at high nanostructures, it is necessary to introduce the non-radiative processes. It is found that the non-radiative processes are not negligible and should be taken
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into account even at low temperature. Finally, we have demonstrated that the use of the epitaxial lateral growth in addition to the growth of semipolar GaN nanostructures will improve the radiative efficiency of such structures.
2. Experimental details The polar structure was grown by molecular beam epitaxy on c-plane sapphire substrates using a 30 nm GaN low temperature buffer layer followed by a 100 nm AlN and a 1 mm thick Al0.5Ga0.5N base layer. Next, the active region was grown consisting of three GaN nanostructure's planes spaced by 30 nm Al0.5Ga0.5N barriers. The structure was completed with a fourth surface plane of GaN nanostructures for the atomic force microscopy (AFM) investigation [9]. GaN nanostructures were synthesized by the deposition of a 2D GaN layer followed by a growth interruption in vacuum [2]. A schematic illustration of the nanostructures growth method is presented in Fig. 1.a. The nominal thickness presents the thickness of the 2D-GaN layer deposited before the growth interruption. The semipolar structure was grown first by metalorganic vapour phase epitaxy on m-plane sapphire substrates using 2 mm thick semipolar (11–22) GaN templates. Then the growth was performed by molecular beam epitaxy using ammonia as nitrogen precursor [19]. We started by depositing 100 nm AlN followed by an 800 nm Al0.5Ga0.5N thick layer. The growth sequence of the active region is similar to the polar case. The schematic illustration of the structure is presented in Fig. 1. b. Two series of samples were grown with varying the amount of the nominally deposited GaN. The first series is composed by p-GaN9, p-GaN12, p-GaN14 and p-GaN19 which are the polar samples. sp-GaN6, sp-GaN9, sp-GaN12 and sp-GaN16 represent the second series corresponding to the semipolar samples. It is essential to note here that those samples are used before in our publications [20]. Asymmetric epitaxial lateral overgrowth was performed by metalorganic vapour phase epitaxy on 2 mm thick semipolar (11–22) GaN templates [26,27]. Then the growth was performed by
molecular beam epitaxy. The same growth sequence of semipolar nanostructures was repeated. 12 monolayers was the amount of the nominally deposited GaN to form GaN nanostructures. The sample will be referred as spELO-GaN12. Cross-sectional transmission electronic microscopy samples were prepared by mechanical polishing followed by ion milling. A statistical analysis of the nanostructure's height is done. The number of probed nanostructures exceeds 70. The optical properties were investigated by time integrated photoluminescence (PL). PL measurements were performed in a helium closed-cycle cryostat by exciting the samples with a frequency-doubled continuous wave Ar laser (λ ¼244 nm) focused in a 150 mm diameter spot with an excitation power of 20 mW/cm2. It should be noted here that all samples (polar and semipolar samples) were placed together in the same cryostat. We placed a reference sample in order to have a comparative study between the different samples.
3. Results and discussion The structural morphology of the GaN nanostructures is studied by AFM and TEM (Fig. 2). It is clear that polar nanostructures present an isotropic morphology (Fig. 2.a) whereas the semipolar nanostructures have an anisotropic shape and repartition. The reasons of this difference on morphology between the two orientations are discussed in our last work [19]. Based on TEM images taken from different regions of the sample and different nanostructures planes, a statistical analysis of the nanostructure's dimensions is done. Fig. 3 presents schematic histograms done for two samples (p-GaN19 and sp-GaN16) where Gaussian fits are plotted. We should note that such values are obtained from the measurement of a large number of nanostructures (more than 70) from different regions. Table 1 shows the height's dispersion obtained after the Gaussian fit of the distribution. In Fig. 4.a, typical time integrated PL spectra are shown for both polar and semipolar nanostructure's samples. All spectra have been recorded at 12 K under low injection power. We note that the emissions from polar nanostructures (p-GaN) are found in energy
Fig. 1. Schematic illustration of: (a) the GaN nanostructure’s growth method, (b) the epitaxial structure.
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Fig. 2. AFM micrographs and TEM images of : (a) polar GaN QDs with 9 MLs as a nominal thickness, (b) semipolar GaN nanostructures with 12 MLs as a nominal thickness [19].
range lower than the strained GaN band gap (3.6 eV). Besides, the PL energy for semipolar nanostructures (sp-GaN) remains above the strained GaN band gap. On another hand, the PL spectra of semipolar nanostructures are found much narrower than the spectra obtained for polar case. In Fig. 4.b, the integrated PL intensity is plotted as a function of the nanostructure's height. It is clear that the PL intensity decreases when the nanostructure's height increases for the polar samples. This behaviour is related to the enhancement of the stark effect that reduces the electron-hole wave function overlap. The opposite effect is observed in the case of semipolar samples. As shown in Fig.4.b, the increase of PL intensity with the height of the nanostructures reflects an enhancement of the oscillator strength. Thus the effect of the internal electric field is low in the case of semipolar nanostructures. Fig. 4.c presents the variation of the emission energy versus the nanostructure's height. In case of small polar nanostructures, we remark that the calculated energy fits well the experimental points. Besides, the PL energy tends to saturate when the height of polar nanostructures exceeds 4.05 nm. Brault et al. attribute this feature to partial screening of the internal electric field [28]. When the nanostructure's height is large enough, the effect of the internal electric field is important by enhancing the separation of the electron-hole wavefunction. Due the excitation, a certain number of carriers are generated inside the nanostructures. Because of the large value of the radiative lifetime, these carriers stay a long time inside the nanostructures before their radiative recombination. During this time, the next carriers come inside the nanostructures. This feature induces an accumulation of the carrier's density. A partial screening of the internal electric field is then produced which begets a blueshift of the emission energy and a small decrease of the radiative lifetime. Contrary, in the case of the semipolar nanostructures, the calculated energy fit well the experimental data even for high nanostructures. This remark is shot from the evolution of the calculated energy versus the nanostructure's height. The slope of the calculated energy is much lower in the case of the semipolar orientation. This fact is simply due to the weaker electric field inside semipolar samples.
3.1. Estimation of the internal electric field In Fig. 4.a, the PL spectra present broad peaks. This inhomogeneous broadening has two principal origins: the inhomogeneous nanostructure's width (height) and the fluctuation of the aluminium composition inside AlxGa1-xN barrier (the value of the internal electric field). To illustrate this observation, the nanostructure's height distributions are done by using a statistical method which is based on TEM images. Each nanostructure emits at such energy corresponding to it dimension (especially the nanostructure's height). Then, the PL spectrum of the nanostructure's ensemble is the superposition of the individual nanostructure's PL response. Since the dispersion of the nanostructure's height is Gaussian, the PL spectra should be Gaussian. TEM is known to be a local technique probing at the scale of the nm however the PL probes at the scale of the micron. One can says that the correlation between TEM and PL results is not suitable. Moreover, based on the AFM and scanning electron microscopy (SEM) images (results are not shown) of the samples for a large scale, macroscopic homogeneities of the samples are confirmed. In that case, TEM study can be used in order to explain the PL results. Due to the larger aspect ratio between the nanostructure's diameter and the nanostructure's height and also the large values of effective masses in III-nitrides, lateral confinement's effects can be minimized or neglected [20,29–32]. Thus, the system that we examine will take in consideration only the nanostructure's height, i.e. the case of equivalent GaN/Al0.5Ga0.5N quantum wells. It should be noted also that the electronic structure of the nanostructure is dominated by the effects of the internal electric field F as reported by Bretagnon et al. [32] Using the envelop function formalism, including both the excitonic binding energy, the effects of confinement along the growth axis and the built-in electric field, the excitonic ground state transition energies is given by.
EPL ( x, h) = EG (GaN ) + E1 ( x, h) + H1 ( x, h) − R d ( x, h) − qF ( x) h
(1)
Where x is the aluminium composition inside AlxGa1-xN barriers, h
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Fig. 3. Statistical analysis of the nanostructure’s height measured by TEM: (a) polar sample p-GaN19 and (b) semipolar sample sp-GaN16. The solid lines are the Gaussian fits.
Table 1 Nanostructure's average height and variance measured from TEM images for polar and semipolar samples. The PL energy, FWHM and the value of the internal electric field extracted from PL spectra and calculation are also shown.
p-GaN9 p-GaN12 p-GaN14 p-GaN19 sp-GaN6 sp-GaN9 sp-GaN12 sp-GaN16
Height (nm)
Variance (nm2)
PL energy (eV)
FWHM (meV)
Field (kV/cm)
4.05 4.25 5.22 6.2 2.32 2.84 3.35 4.31
0.34 0.38 0.32 0.36 0.25 0.77 0.36 0.7
2.9 2.77 2.65 2.57 3.76 3.71 3.66 3.61
322 337 365 334 88 91 95 93
2800 2950 3200 2900 – – 430 450
is the nanostructure's height, EG(GaN) is the GaN band gap, E1 and H1 are the ground state electron and hole confinement energies, Rd is the excitonic Rydberg, q is the elementary charge and F is the built-in electric field. It should be noted here that the excitonics energies and the oscillator strength are calculated in the envelope function approximation using the transfer matrix method and material parameters given in Ref 13. As expressed below, the transition energy is composed by three components: the nanostructure material gap (Eg), the confinement energies (the sum of the electron and hole confinement
Fig. 4. (a) PL spectra at 12 K of polar and semipolar GaN/Al0.5Ga0.5N nanostructures for different deposited GaN amounts under low injection power. (b) The evolution of integrated PL intensity for polar and semipolar samples as a function of the nanostructure’s average height. (c) The estimation of the internal electric field by plotting the calculated transition energies versus the GaN/Al0.5Ga0.5N nanostructure’s average height. Red circles and blue stars present the experimental data for semipolar and polar samples, respectively [20]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
energies minus the exciton Rydberg constant: E1 þH1-Rd) and the Stark term (qFh). In order to determine the inhomogeneous broadening of the transition energy, it is essential to work under independent variables condition. The carrier confinement and the Stark effect have opposite actions on the transition energies. It is found that these transition energies are independent of the barrier
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composition when the nanostructure's height is near to L0 2.6 nm [13,33]. When h 4L0, the confinement energies become almost independent of the nanostructure's height. Also, when h¼ L0, the slope of the Stark term qFh is much larger than that of the confinement energies. The inhomogeneous broadening is then determined for independent variables (when hZL0), the Gaussian distribution of the nanostructure's height, and the aluminium composition.
⎛ ∂E ⎞2 ⎛ ∂E ⎞2 ΔE2 = ⎜ PL ⎟ Δx2 + ⎜ PL ⎟ Δh2 + Δ02 ⎝ ∂x ⎠ ⎝ ∂h ⎠
(2)
Δ0 is the inhomogeneous broadening of the GaN excitonic gap, Δh is the standard deviation of the nanostructure's height, and Δx is the width of random alloy distribution. Referring to Natali et al. [33] the variance of alloy disorder is given by 1
⎛ x ( 1 − x) ⎞ 2 ⎟⎟ σx = ⎜⎜ ⎝ dIII 3πρ2 ⎠
(3) 15
-2
Where dIII (1.36 10 cm ) is the areal density of cations sites in a (0001) plane, and ρ is an in-plane excitonic Bohr raduis. Considering polar samples, the four samples present similar variances. For simplicity, we can use 0.36 nm2 as an average variance value. The calculation of the PL linewidth is done for a fixed height's variance (0.36 nm2) by varying the nanostructure's height from
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1 to 6 nm and for different electric field. The result is summarized in the Fig. 5.a. Taking into account the PL data and TEM results, it is then possible to plot the experimental values of the PL linewidth versus the nanostructure's height. The superposition of the theoretical curves and the experimental data gives 3 MV/cm as the value of the correspondent electric field. The same work is done for semipolar samples (Fig. 5.b). It is possible to estimate the value of the electric field only for spGaN12 and sp-GaN16. 450 kV/cm is the electric field value extracted from the model applied on PL results of sp-GaN12 and spGaN16 (only the case of sp-GaN16 is shown in Fig. 5.b). This value is in agreement with the field extracted in our previous work for semipolar GaN nanostructures [20]. As stressed in previous works [13,33] this model is not valid for small nanostructures where hr L0. Note that even for sp-GaN9 where the height is equal to 2.84 nm, the calculation does not let us to extract a suitable value of the electric field. As expressed below, we need to work under independent variables. This condition is satisfied when the nanostructure's height h ZL0. The value of L0 is given approximatively. By Grandjean and Natali [13,33], L0 is near to 2.6 nm. It means that it can be less or more but it still around this value. The uncertainty is not given on these works [13,33]. 2.84 nm is very close to 2.6 nm. Only, 2.4 Å of difference. So, it can be situated in the range of the standard deviation of L0. The second reason is that the nanostructure's height average is determined by using a statistical analysis based on the HRTEM images. Due the spherical abberation, the resolution limit of the instrument can be greater than or near to 2.4 Å. Third, the studied sample (sp-GaN9) contains a high density of nanostructures (more than 3.1011 cm-2) but the number of probed nanostructures is 70. The standard deviation of the nanostructure's height for sp-GaN9 is 0.9 nm. The nanostructure's height is then situated between 2.39 nm and 3.29 nm. The problem of the saturation of PL energy for higher nanostructures, especially for polar samples, is not solved yet. When the height of nanostructures increases, the lateral dimensions also increase. This behavior enhances the probability of the crossover of the nanostructures by the surrounding defects. Adding to this, the radiative lifetime increases (the radiative efficiency decreases) strongly with the nanostructure's height. In that case, the nonradiative processes should be taken into account. 3.2. Estimation of the non-radiative lifetime In order to explain the saturation of the PL energy, we will introduce the non-radiative lifetime on the calculation. To simplify this model, we assume a single value of the non-radiative lifetime for polar samples. The intensity of the PL spectrum can be plotted as a function of the nanostructure's height h, using
I (h) α G (h). n (h). fosc (h)
(4)
where G(h) is the nanostructure's height distribution (Gaussian function), n(h) is the carrier's density, and fosc is the oscillator strength (the squared electron hole overlap integral)
⎛ h − h 2⎞ ( 0) ⎟ G (h) = exp ⎜⎜ − 2σ 2 ⎟⎠ ⎝
(5)
where h0 is the nanostructure's average height and s2 is the variance. The carrier's density is determined using Fig. 5. The evolution of FWHM as a function of the nanostructure’s height for different electric field values and for: (a) polar case where the variance is fixed to 0.36 nm2, (b) sp-GaN16 where the variance is fixed to 0.7 nm2. Experimental data are added.
n = g. τ
(6)
where g is the generation rate and τ is the decay time that is given by
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Fig. 7. Calculated transition energies at 3 MV/cm versus the GaN/Al0.5Ga0.5N nanostructure’s average height for different non radiative lifetimes (13.5 ns, 1.35 ms, 135 ms, 1.35 s and 1). Black stars present the experimental emission energies extracted from the PL spectra of polar samples.
present the calculated luminescence spectra using three values of
τnr (13.5 ns, 135 ms, and 1.35 s). The suitable fit to the experimental
Fig. 6. Calculated PL spectra for three non-radiative lifetime values corresponding to 13.5 ns (dark line), 135 ms (red line) and 1.35 s (green line). (a) For polar GaN/Al0.5Ga0.5N nanostructures with an average height 6.2 nm corresponding to p-GaN19 sample. (b) For semipolar GaN/Al0.5Ga0.5N nanostructures with an average height 4.31 nm corresponding to sp-GaN16 sample. Blue spectra are the experimental PL spectra done at 12 K for low injection power (20 mW/cm2). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
1 1 1 = + τ τr τnr
(7)
where τr and τnr are the radiative and non-radiative lifetime, respectively. These two parameters can be expressed by
τr =
τ0 fosc
(8)
τnr = β. τ 0
(9)
where τ0 is the hypothetical flat-band radiative lifetime (i.e. for zero internal electric field). β is a proportionality factor. For simplicity, we use a constant value for β. This value will be adjusted later. Referring to our last work, τ0 is estimated to 270 ps [20]. The carrier's density is then expressed by:
n = g . τnr . τ 0
1 τ 0 + τnr . fosc
(10)
The luminescence spectra can be calculated for various nonradiative lifetime τnr values. The value of the internal electric field is fixed to 3 MV/cm. Fig. 6.a shows the PL spectrum obtained for the case of GaN nanostructures with an average height 6.2 nm corresponding to the sample p-GaN19. In the same figure, we
data corresponds to 135 ms. The same results are obtained for p-GaN9 (h0 4.05 nm) and p-GaN14 (h0 5.22 nm). From the Fig. 6.a, we remark that the PL peak blueshifts when the non-radiative lifetime decreases. It means that the PL energy obtained after including the non-radiative lifetime corresponds to a nanostructure's height smaller than the average nanostructure's height in the studied sample. It should be noted here that an energy shift more than 500 meV is obtained when the value of the radiative lifetime is varied between 13.5 ns and 1.35 s. In order to have an overall observation, the evolution of the PL energy versus the nanostructure's height for different values of the non-radiative lifetime τnr is shown in Fig. 7. It is clear that, for a fixed electric field, the PL energy depends, in addition to the nanostructure's height, in the non radiative lifetime. As seen in Fig. 7, if the non radiative processes are neglected (the case of τnr ¼1), the evolution of the PL energy as a function of the nanostructure's height is linear. When the non-radiative processes enter in consideration (by reducing the value of the non radiative lifetime), the PL energy shifts to higher values. It is important to note that this behaviour is low or absent for small nanostructures. For example, when the radiative lifetime exceeds 135 ms, the evolution PL energy is independent to the non radiative lifetime (Fig. 7) in the range of smaller nanostructures (ho4 nm). From Fig. 7, the best fit of the experimental data corresponds to a τnr value of 135 ms. The saturation of the emission energy is reached when the non-radiative process become influent. Back to the semipolar samples, Fig. 6.b demonstrates that the incorporation of the non-radiative recombination in the calculation does not change the position of the PL peak. The reason is that this orientation presents a low internal electric field. The oscillator strength decreases slowly as a function of the nanostructure's height. As a consequence the radiative lifetime remains at low values (few hundred of ps) [20,34]. The effect of the non radiative process still negligible in this case. Now, for big nanostructures, if we compare between the nonradiative lifetime extracted from this work for polar GaN/Al0.5Ga0.5N nanostructures (few hundred of ms) and the other one determined in our last work for semipolar GaN/Al0.5Ga0.5N nanostructures (few hundred of ps) [20], we found that it is lower by a factor of 10-5 for semipolar case. This feature can be explained by two reasons. The first one is that the semipolar GaN matrix suffers from a high density of defects compared to the polar one
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in Fig. 8.b. We observe a little decrease of the integrated PL intensity by less than one decade between 15 K and 300 K for both samples. This behaviour is related to the carrier localization inside the nanostructures. At low temperature and until 40 K, the variation of the PL intensity presents a plateau for the case of spELOGaN12. However, this plateau is absent for sp-GaN12 and the PL intensity starts to decrease from 15 K. Another point is PL intensities ratio between 15 K and 300 K is about 1.68 for spELOGaN12 and 2.64 for sp-GaN12, as indicated in Fig. 8.b. This means that the internal quantum efficiency is better for nanostructures grown on GaN buffer where the ELO is used to filter the BSFs.
4. Conclusion In summary, time integrated photoluminescence has been used to study the optical properties of (0001) polar and (11–22) semipolar GaN/Al0.5Ga0.5N nanostructures. The PL emission energies for polar orientation display a saturation when the nanostructure's height exceeds some value 4 4.05 nm. An impressive number of more than 70 structures are probed by transmission electron microscopy (TEM) for statistical analysis of their height distribution. Next, these data are used to analyse the peak energy and linewidth of the measured PL spectra. From this analysis the values of the internal electric field along the growth direction of the nanostructures as well as the non-radiative lifetimes are extracted. Furthermore, it is confirmed that the use of a semi-polar substrate prepared by asymmetric epitaxial lateral overgrowth improves the radiative efficiency of the produced nanostructures.
References Fig. 8. (a) PL spectra at 15 K of the semipolar GaN/Al0.5Ga0.5N nanostructures grown on GaN templates (sp-GaN12) and on GaN buffer layer after the use of ELO method (spELO-GaN12). (b) Integrated PL intensity versus temperature for spGaN12 and spELO-GaN12.
[35]. The second reason is that the morphology of the semipolar nanostructures is more elongated along the [1–100] direction [19,20]. When the GaN deposited amount is increased, the semipolar nanostructures became comparable to wires. Contrary, the polar nanostructures present a homogeneous morphology characterized by a circular form. Thus, the intersection between nanostructures and structural defects is more favourable for the semipolar orientation. In order to improve the radiative efficiency, it is primordial to reduce the defect density inside the GaN matrix. 3.3. Improvement of the internal quantum efficiency of semipolar GaN nanostructures An Asymmetric Epitaxial Lateral Overgrowth is used for this purpose. The growth method is developed by Kriouche and all [26,27]. The Principe is based on the filtration of the BSFs and then to limit their spreading inside the higher matrix. Fig. 8.a reports the PL spectra at 15 K for semipolar GaN/Al0.5Ga0.5N nanostructures presenting the same GaN deposited thickness (12 MLs). The peaks positions are situated at the same energy which confirms that we have the same nanostructure's average height for the two samples. The PL spectrum is intense for spELO-GaN12. The GaN band edge situated at 3.47 eV is detected for this sample but not for the other one. Another point is that the emission of donor- acceptor pairs is also observed at low energy (3.27 eV) when ELO is performed. PL measurements as a function of the temperature are performed on both samples. The temperature dependence of the integrated PL intensity is reported
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Low temperature non radiative process's effects on the optical emission of GaN/Al0.5Ga0.5N nanostructures Abdelkarim Kahouli Université de Nice Sophia Antipolis, Parc Valrose, 28 avenue Valrose, 06108 Nice Cedex 2, France
art ic l e i nf o
a b s t r a c t
Article history: Received 18 July 2016 Received in revised form 8 November 2016 Accepted 18 November 2016 Available online 22 November 2016
The optical properties of self-assembled (0001) polar and (11–22) semipolar GaN nanostructures embedded in Al0.5Ga0.5N matrix and grown by molecular beam epitaxy are reported. A statistical analysis of the nanostructure's height dispersion is done by transmission electron microscopy (TEM) in order to have a good estimation of the electric field inside the structure and also to be able to give an explanation for the saturation of the emission energy for polar orientation when the height exceed some values. It is found that the non-radiative processes are not negligible at low temperature and should be taken into account as the radiative ones. The combined comparison between experimental results and our model has allowed us to determine the non-radiative lifetimes at low temperature. We show that the use of epitaxial lateral overgrowth, in addition to the growth of GaN nanostructures, will improve the radiative efficiency that can reach 60%. & 2016 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, GaN based materials present a great interest from worldwide semiconductor research community. The ability to tune the bandgap across the entire solar spectrum allows III-nitrides semiconductors to be the potential candidate used for optoelectronic devices and photovoltaic applications. Due to the lack of suitable substrates, hetero-epitaxial growth on foreign substrates as sapphire or silicon is needed. The large lattice mismatch between substrate and III-Nitrides layers reveals a large dislocation's density in the matrix. Threading dislocations act as non-radiative recombination centers and reduce light emission's efficiency. Thus, it is of highest importance to develop some growth strategies that minimise the effect of structural defects. Growing GaN nanostructures is a possible approach to overcome the effect of the reduced crystal quality [1–4]. The reduced dimensionality of self-assembled GaN nanostructures allows it to be very efficient light emitters. It act as traps for charge carriers. Therefore it hinder their diffusion towards non radiative defects. Molecular beam epitaxy is able to produce nanostructures with high quality by using a nitrogen plasma cell [5–7] or ammonia [2,8,9] as a nitrogen source. Despite the promising future of the optoelectronics devices based on GaN nanostructures [10], the improvement of the optical properties is still limited. One of the major difficulties to realise E-mail address: [email protected] http://dx.doi.org/10.1016/j.jlumin.2016.11.051 0022-2313/& 2016 Elsevier B.V. All rights reserved.
these devices is related to the presence of a strong internal electric field along (0001) orientation. The resulted quantum confined stark effect (QCSE) generates a strong red-shift of the emission energy and a low radiative efficiency characterized by a high radiative lifetime [11–14]. It is thus a challenge to reach the UV range. An alternative strategy to surpass this constraint is the growth along a semipolar orientation where the polar axis (0001) is inclined corresponding to the growth axis by 58° [15–18]. In this case, the effects of the internal electric field can be drastically reduced. As a consequence a shift to higher energy and an increase of the radiative efficiency have been obtained [19]. The value of the internal electric field is reduced by a factor 8 compared to the polar orientation [20]. The problem is not solved yet because semipolar nitrides suffer from a high density of defects, mainly the basal stacking faults (BSFs) and misfit dislocations (MDs) [21–23]. The higher density of defects for semipolar nitrides compared to the polar orientation can reduce the radiative improvement brought by GaN nanostructures. Epitaxial lateral overgrowth (ELO) can be used to filter the spreading of BSFs inside GaN matrix [24–27]. The aim of the present letter is to show that we are able to calculate the PL linewidth and the PL intensity by using the dispersion of the nanostructure's height extracted from the transmission electronic microscopy (TEM). The estimation of the internal electric field is also conducted. In order to improve our model and to explain some features at high nanostructures, it is necessary to introduce the non-radiative processes. It is found that the non-radiative processes are not negligible and should be taken
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into account even at low temperature. Finally, we have demonstrated that the use of the epitaxial lateral growth in addition to the growth of semipolar GaN nanostructures will improve the radiative efficiency of such structures.
2. Experimental details The polar structure was grown by molecular beam epitaxy on c-plane sapphire substrates using a 30 nm GaN low temperature buffer layer followed by a 100 nm AlN and a 1 mm thick Al0.5Ga0.5N base layer. Next, the active region was grown consisting of three GaN nanostructure's planes spaced by 30 nm Al0.5Ga0.5N barriers. The structure was completed with a fourth surface plane of GaN nanostructures for the atomic force microscopy (AFM) investigation [9]. GaN nanostructures were synthesized by the deposition of a 2D GaN layer followed by a growth interruption in vacuum [2]. A schematic illustration of the nanostructures growth method is presented in Fig. 1.a. The nominal thickness presents the thickness of the 2D-GaN layer deposited before the growth interruption. The semipolar structure was grown first by metalorganic vapour phase epitaxy on m-plane sapphire substrates using 2 mm thick semipolar (11–22) GaN templates. Then the growth was performed by molecular beam epitaxy using ammonia as nitrogen precursor [19]. We started by depositing 100 nm AlN followed by an 800 nm Al0.5Ga0.5N thick layer. The growth sequence of the active region is similar to the polar case. The schematic illustration of the structure is presented in Fig. 1. b. Two series of samples were grown with varying the amount of the nominally deposited GaN. The first series is composed by p-GaN9, p-GaN12, p-GaN14 and p-GaN19 which are the polar samples. sp-GaN6, sp-GaN9, sp-GaN12 and sp-GaN16 represent the second series corresponding to the semipolar samples. It is essential to note here that those samples are used before in our publications [20]. Asymmetric epitaxial lateral overgrowth was performed by metalorganic vapour phase epitaxy on 2 mm thick semipolar (11–22) GaN templates [26,27]. Then the growth was performed by
molecular beam epitaxy. The same growth sequence of semipolar nanostructures was repeated. 12 monolayers was the amount of the nominally deposited GaN to form GaN nanostructures. The sample will be referred as spELO-GaN12. Cross-sectional transmission electronic microscopy samples were prepared by mechanical polishing followed by ion milling. A statistical analysis of the nanostructure's height is done. The number of probed nanostructures exceeds 70. The optical properties were investigated by time integrated photoluminescence (PL). PL measurements were performed in a helium closed-cycle cryostat by exciting the samples with a frequency-doubled continuous wave Ar laser (λ ¼244 nm) focused in a 150 mm diameter spot with an excitation power of 20 mW/cm2. It should be noted here that all samples (polar and semipolar samples) were placed together in the same cryostat. We placed a reference sample in order to have a comparative study between the different samples.
3. Results and discussion The structural morphology of the GaN nanostructures is studied by AFM and TEM (Fig. 2). It is clear that polar nanostructures present an isotropic morphology (Fig. 2.a) whereas the semipolar nanostructures have an anisotropic shape and repartition. The reasons of this difference on morphology between the two orientations are discussed in our last work [19]. Based on TEM images taken from different regions of the sample and different nanostructures planes, a statistical analysis of the nanostructure's dimensions is done. Fig. 3 presents schematic histograms done for two samples (p-GaN19 and sp-GaN16) where Gaussian fits are plotted. We should note that such values are obtained from the measurement of a large number of nanostructures (more than 70) from different regions. Table 1 shows the height's dispersion obtained after the Gaussian fit of the distribution. In Fig. 4.a, typical time integrated PL spectra are shown for both polar and semipolar nanostructure's samples. All spectra have been recorded at 12 K under low injection power. We note that the emissions from polar nanostructures (p-GaN) are found in energy
Fig. 1. Schematic illustration of: (a) the GaN nanostructure’s growth method, (b) the epitaxial structure.
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Fig. 2. AFM micrographs and TEM images of : (a) polar GaN QDs with 9 MLs as a nominal thickness, (b) semipolar GaN nanostructures with 12 MLs as a nominal thickness [19].
range lower than the strained GaN band gap (3.6 eV). Besides, the PL energy for semipolar nanostructures (sp-GaN) remains above the strained GaN band gap. On another hand, the PL spectra of semipolar nanostructures are found much narrower than the spectra obtained for polar case. In Fig. 4.b, the integrated PL intensity is plotted as a function of the nanostructure's height. It is clear that the PL intensity decreases when the nanostructure's height increases for the polar samples. This behaviour is related to the enhancement of the stark effect that reduces the electron-hole wave function overlap. The opposite effect is observed in the case of semipolar samples. As shown in Fig.4.b, the increase of PL intensity with the height of the nanostructures reflects an enhancement of the oscillator strength. Thus the effect of the internal electric field is low in the case of semipolar nanostructures. Fig. 4.c presents the variation of the emission energy versus the nanostructure's height. In case of small polar nanostructures, we remark that the calculated energy fits well the experimental points. Besides, the PL energy tends to saturate when the height of polar nanostructures exceeds 4.05 nm. Brault et al. attribute this feature to partial screening of the internal electric field [28]. When the nanostructure's height is large enough, the effect of the internal electric field is important by enhancing the separation of the electron-hole wavefunction. Due the excitation, a certain number of carriers are generated inside the nanostructures. Because of the large value of the radiative lifetime, these carriers stay a long time inside the nanostructures before their radiative recombination. During this time, the next carriers come inside the nanostructures. This feature induces an accumulation of the carrier's density. A partial screening of the internal electric field is then produced which begets a blueshift of the emission energy and a small decrease of the radiative lifetime. Contrary, in the case of the semipolar nanostructures, the calculated energy fit well the experimental data even for high nanostructures. This remark is shot from the evolution of the calculated energy versus the nanostructure's height. The slope of the calculated energy is much lower in the case of the semipolar orientation. This fact is simply due to the weaker electric field inside semipolar samples.
3.1. Estimation of the internal electric field In Fig. 4.a, the PL spectra present broad peaks. This inhomogeneous broadening has two principal origins: the inhomogeneous nanostructure's width (height) and the fluctuation of the aluminium composition inside AlxGa1-xN barrier (the value of the internal electric field). To illustrate this observation, the nanostructure's height distributions are done by using a statistical method which is based on TEM images. Each nanostructure emits at such energy corresponding to it dimension (especially the nanostructure's height). Then, the PL spectrum of the nanostructure's ensemble is the superposition of the individual nanostructure's PL response. Since the dispersion of the nanostructure's height is Gaussian, the PL spectra should be Gaussian. TEM is known to be a local technique probing at the scale of the nm however the PL probes at the scale of the micron. One can says that the correlation between TEM and PL results is not suitable. Moreover, based on the AFM and scanning electron microscopy (SEM) images (results are not shown) of the samples for a large scale, macroscopic homogeneities of the samples are confirmed. In that case, TEM study can be used in order to explain the PL results. Due to the larger aspect ratio between the nanostructure's diameter and the nanostructure's height and also the large values of effective masses in III-nitrides, lateral confinement's effects can be minimized or neglected [20,29–32]. Thus, the system that we examine will take in consideration only the nanostructure's height, i.e. the case of equivalent GaN/Al0.5Ga0.5N quantum wells. It should be noted also that the electronic structure of the nanostructure is dominated by the effects of the internal electric field F as reported by Bretagnon et al. [32] Using the envelop function formalism, including both the excitonic binding energy, the effects of confinement along the growth axis and the built-in electric field, the excitonic ground state transition energies is given by.
EPL ( x, h) = EG (GaN ) + E1 ( x, h) + H1 ( x, h) − R d ( x, h) − qF ( x) h
(1)
Where x is the aluminium composition inside AlxGa1-xN barriers, h
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Fig. 3. Statistical analysis of the nanostructure’s height measured by TEM: (a) polar sample p-GaN19 and (b) semipolar sample sp-GaN16. The solid lines are the Gaussian fits.
Table 1 Nanostructure's average height and variance measured from TEM images for polar and semipolar samples. The PL energy, FWHM and the value of the internal electric field extracted from PL spectra and calculation are also shown.
p-GaN9 p-GaN12 p-GaN14 p-GaN19 sp-GaN6 sp-GaN9 sp-GaN12 sp-GaN16
Height (nm)
Variance (nm2)
PL energy (eV)
FWHM (meV)
Field (kV/cm)
4.05 4.25 5.22 6.2 2.32 2.84 3.35 4.31
0.34 0.38 0.32 0.36 0.25 0.77 0.36 0.7
2.9 2.77 2.65 2.57 3.76 3.71 3.66 3.61
322 337 365 334 88 91 95 93
2800 2950 3200 2900 – – 430 450
is the nanostructure's height, EG(GaN) is the GaN band gap, E1 and H1 are the ground state electron and hole confinement energies, Rd is the excitonic Rydberg, q is the elementary charge and F is the built-in electric field. It should be noted here that the excitonics energies and the oscillator strength are calculated in the envelope function approximation using the transfer matrix method and material parameters given in Ref 13. As expressed below, the transition energy is composed by three components: the nanostructure material gap (Eg), the confinement energies (the sum of the electron and hole confinement
Fig. 4. (a) PL spectra at 12 K of polar and semipolar GaN/Al0.5Ga0.5N nanostructures for different deposited GaN amounts under low injection power. (b) The evolution of integrated PL intensity for polar and semipolar samples as a function of the nanostructure’s average height. (c) The estimation of the internal electric field by plotting the calculated transition energies versus the GaN/Al0.5Ga0.5N nanostructure’s average height. Red circles and blue stars present the experimental data for semipolar and polar samples, respectively [20]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
energies minus the exciton Rydberg constant: E1 þH1-Rd) and the Stark term (qFh). In order to determine the inhomogeneous broadening of the transition energy, it is essential to work under independent variables condition. The carrier confinement and the Stark effect have opposite actions on the transition energies. It is found that these transition energies are independent of the barrier
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composition when the nanostructure's height is near to L0 2.6 nm [13,33]. When h 4L0, the confinement energies become almost independent of the nanostructure's height. Also, when h¼ L0, the slope of the Stark term qFh is much larger than that of the confinement energies. The inhomogeneous broadening is then determined for independent variables (when hZL0), the Gaussian distribution of the nanostructure's height, and the aluminium composition.
⎛ ∂E ⎞2 ⎛ ∂E ⎞2 ΔE2 = ⎜ PL ⎟ Δx2 + ⎜ PL ⎟ Δh2 + Δ02 ⎝ ∂x ⎠ ⎝ ∂h ⎠
(2)
Δ0 is the inhomogeneous broadening of the GaN excitonic gap, Δh is the standard deviation of the nanostructure's height, and Δx is the width of random alloy distribution. Referring to Natali et al. [33] the variance of alloy disorder is given by 1
⎛ x ( 1 − x) ⎞ 2 ⎟⎟ σx = ⎜⎜ ⎝ dIII 3πρ2 ⎠
(3) 15
-2
Where dIII (1.36 10 cm ) is the areal density of cations sites in a (0001) plane, and ρ is an in-plane excitonic Bohr raduis. Considering polar samples, the four samples present similar variances. For simplicity, we can use 0.36 nm2 as an average variance value. The calculation of the PL linewidth is done for a fixed height's variance (0.36 nm2) by varying the nanostructure's height from
295
1 to 6 nm and for different electric field. The result is summarized in the Fig. 5.a. Taking into account the PL data and TEM results, it is then possible to plot the experimental values of the PL linewidth versus the nanostructure's height. The superposition of the theoretical curves and the experimental data gives 3 MV/cm as the value of the correspondent electric field. The same work is done for semipolar samples (Fig. 5.b). It is possible to estimate the value of the electric field only for spGaN12 and sp-GaN16. 450 kV/cm is the electric field value extracted from the model applied on PL results of sp-GaN12 and spGaN16 (only the case of sp-GaN16 is shown in Fig. 5.b). This value is in agreement with the field extracted in our previous work for semipolar GaN nanostructures [20]. As stressed in previous works [13,33] this model is not valid for small nanostructures where hr L0. Note that even for sp-GaN9 where the height is equal to 2.84 nm, the calculation does not let us to extract a suitable value of the electric field. As expressed below, we need to work under independent variables. This condition is satisfied when the nanostructure's height h ZL0. The value of L0 is given approximatively. By Grandjean and Natali [13,33], L0 is near to 2.6 nm. It means that it can be less or more but it still around this value. The uncertainty is not given on these works [13,33]. 2.84 nm is very close to 2.6 nm. Only, 2.4 Å of difference. So, it can be situated in the range of the standard deviation of L0. The second reason is that the nanostructure's height average is determined by using a statistical analysis based on the HRTEM images. Due the spherical abberation, the resolution limit of the instrument can be greater than or near to 2.4 Å. Third, the studied sample (sp-GaN9) contains a high density of nanostructures (more than 3.1011 cm-2) but the number of probed nanostructures is 70. The standard deviation of the nanostructure's height for sp-GaN9 is 0.9 nm. The nanostructure's height is then situated between 2.39 nm and 3.29 nm. The problem of the saturation of PL energy for higher nanostructures, especially for polar samples, is not solved yet. When the height of nanostructures increases, the lateral dimensions also increase. This behavior enhances the probability of the crossover of the nanostructures by the surrounding defects. Adding to this, the radiative lifetime increases (the radiative efficiency decreases) strongly with the nanostructure's height. In that case, the nonradiative processes should be taken into account. 3.2. Estimation of the non-radiative lifetime In order to explain the saturation of the PL energy, we will introduce the non-radiative lifetime on the calculation. To simplify this model, we assume a single value of the non-radiative lifetime for polar samples. The intensity of the PL spectrum can be plotted as a function of the nanostructure's height h, using
I (h) α G (h). n (h). fosc (h)
(4)
where G(h) is the nanostructure's height distribution (Gaussian function), n(h) is the carrier's density, and fosc is the oscillator strength (the squared electron hole overlap integral)
⎛ h − h 2⎞ ( 0) ⎟ G (h) = exp ⎜⎜ − 2σ 2 ⎟⎠ ⎝
(5)
where h0 is the nanostructure's average height and s2 is the variance. The carrier's density is determined using Fig. 5. The evolution of FWHM as a function of the nanostructure’s height for different electric field values and for: (a) polar case where the variance is fixed to 0.36 nm2, (b) sp-GaN16 where the variance is fixed to 0.7 nm2. Experimental data are added.
n = g. τ
(6)
where g is the generation rate and τ is the decay time that is given by
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Fig. 7. Calculated transition energies at 3 MV/cm versus the GaN/Al0.5Ga0.5N nanostructure’s average height for different non radiative lifetimes (13.5 ns, 1.35 ms, 135 ms, 1.35 s and 1). Black stars present the experimental emission energies extracted from the PL spectra of polar samples.
present the calculated luminescence spectra using three values of
τnr (13.5 ns, 135 ms, and 1.35 s). The suitable fit to the experimental
Fig. 6. Calculated PL spectra for three non-radiative lifetime values corresponding to 13.5 ns (dark line), 135 ms (red line) and 1.35 s (green line). (a) For polar GaN/Al0.5Ga0.5N nanostructures with an average height 6.2 nm corresponding to p-GaN19 sample. (b) For semipolar GaN/Al0.5Ga0.5N nanostructures with an average height 4.31 nm corresponding to sp-GaN16 sample. Blue spectra are the experimental PL spectra done at 12 K for low injection power (20 mW/cm2). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
1 1 1 = + τ τr τnr
(7)
where τr and τnr are the radiative and non-radiative lifetime, respectively. These two parameters can be expressed by
τr =
τ0 fosc
(8)
τnr = β. τ 0
(9)
where τ0 is the hypothetical flat-band radiative lifetime (i.e. for zero internal electric field). β is a proportionality factor. For simplicity, we use a constant value for β. This value will be adjusted later. Referring to our last work, τ0 is estimated to 270 ps [20]. The carrier's density is then expressed by:
n = g . τnr . τ 0
1 τ 0 + τnr . fosc
(10)
The luminescence spectra can be calculated for various nonradiative lifetime τnr values. The value of the internal electric field is fixed to 3 MV/cm. Fig. 6.a shows the PL spectrum obtained for the case of GaN nanostructures with an average height 6.2 nm corresponding to the sample p-GaN19. In the same figure, we
data corresponds to 135 ms. The same results are obtained for p-GaN9 (h0 4.05 nm) and p-GaN14 (h0 5.22 nm). From the Fig. 6.a, we remark that the PL peak blueshifts when the non-radiative lifetime decreases. It means that the PL energy obtained after including the non-radiative lifetime corresponds to a nanostructure's height smaller than the average nanostructure's height in the studied sample. It should be noted here that an energy shift more than 500 meV is obtained when the value of the radiative lifetime is varied between 13.5 ns and 1.35 s. In order to have an overall observation, the evolution of the PL energy versus the nanostructure's height for different values of the non-radiative lifetime τnr is shown in Fig. 7. It is clear that, for a fixed electric field, the PL energy depends, in addition to the nanostructure's height, in the non radiative lifetime. As seen in Fig. 7, if the non radiative processes are neglected (the case of τnr ¼1), the evolution of the PL energy as a function of the nanostructure's height is linear. When the non-radiative processes enter in consideration (by reducing the value of the non radiative lifetime), the PL energy shifts to higher values. It is important to note that this behaviour is low or absent for small nanostructures. For example, when the radiative lifetime exceeds 135 ms, the evolution PL energy is independent to the non radiative lifetime (Fig. 7) in the range of smaller nanostructures (ho4 nm). From Fig. 7, the best fit of the experimental data corresponds to a τnr value of 135 ms. The saturation of the emission energy is reached when the non-radiative process become influent. Back to the semipolar samples, Fig. 6.b demonstrates that the incorporation of the non-radiative recombination in the calculation does not change the position of the PL peak. The reason is that this orientation presents a low internal electric field. The oscillator strength decreases slowly as a function of the nanostructure's height. As a consequence the radiative lifetime remains at low values (few hundred of ps) [20,34]. The effect of the non radiative process still negligible in this case. Now, for big nanostructures, if we compare between the nonradiative lifetime extracted from this work for polar GaN/Al0.5Ga0.5N nanostructures (few hundred of ms) and the other one determined in our last work for semipolar GaN/Al0.5Ga0.5N nanostructures (few hundred of ps) [20], we found that it is lower by a factor of 10-5 for semipolar case. This feature can be explained by two reasons. The first one is that the semipolar GaN matrix suffers from a high density of defects compared to the polar one
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in Fig. 8.b. We observe a little decrease of the integrated PL intensity by less than one decade between 15 K and 300 K for both samples. This behaviour is related to the carrier localization inside the nanostructures. At low temperature and until 40 K, the variation of the PL intensity presents a plateau for the case of spELOGaN12. However, this plateau is absent for sp-GaN12 and the PL intensity starts to decrease from 15 K. Another point is PL intensities ratio between 15 K and 300 K is about 1.68 for spELOGaN12 and 2.64 for sp-GaN12, as indicated in Fig. 8.b. This means that the internal quantum efficiency is better for nanostructures grown on GaN buffer where the ELO is used to filter the BSFs.
4. Conclusion In summary, time integrated photoluminescence has been used to study the optical properties of (0001) polar and (11–22) semipolar GaN/Al0.5Ga0.5N nanostructures. The PL emission energies for polar orientation display a saturation when the nanostructure's height exceeds some value 4 4.05 nm. An impressive number of more than 70 structures are probed by transmission electron microscopy (TEM) for statistical analysis of their height distribution. Next, these data are used to analyse the peak energy and linewidth of the measured PL spectra. From this analysis the values of the internal electric field along the growth direction of the nanostructures as well as the non-radiative lifetimes are extracted. Furthermore, it is confirmed that the use of a semi-polar substrate prepared by asymmetric epitaxial lateral overgrowth improves the radiative efficiency of the produced nanostructures.
References Fig. 8. (a) PL spectra at 15 K of the semipolar GaN/Al0.5Ga0.5N nanostructures grown on GaN templates (sp-GaN12) and on GaN buffer layer after the use of ELO method (spELO-GaN12). (b) Integrated PL intensity versus temperature for spGaN12 and spELO-GaN12.
[35]. The second reason is that the morphology of the semipolar nanostructures is more elongated along the [1–100] direction [19,20]. When the GaN deposited amount is increased, the semipolar nanostructures became comparable to wires. Contrary, the polar nanostructures present a homogeneous morphology characterized by a circular form. Thus, the intersection between nanostructures and structural defects is more favourable for the semipolar orientation. In order to improve the radiative efficiency, it is primordial to reduce the defect density inside the GaN matrix. 3.3. Improvement of the internal quantum efficiency of semipolar GaN nanostructures An Asymmetric Epitaxial Lateral Overgrowth is used for this purpose. The growth method is developed by Kriouche and all [26,27]. The Principe is based on the filtration of the BSFs and then to limit their spreading inside the higher matrix. Fig. 8.a reports the PL spectra at 15 K for semipolar GaN/Al0.5Ga0.5N nanostructures presenting the same GaN deposited thickness (12 MLs). The peaks positions are situated at the same energy which confirms that we have the same nanostructure's average height for the two samples. The PL spectrum is intense for spELO-GaN12. The GaN band edge situated at 3.47 eV is detected for this sample but not for the other one. Another point is that the emission of donor- acceptor pairs is also observed at low energy (3.27 eV) when ELO is performed. PL measurements as a function of the temperature are performed on both samples. The temperature dependence of the integrated PL intensity is reported
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