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Probing the Internal Electric Field in GaN/AlGaN Nanowire Heterostructures Jan Müßener, Jörg Teubert, Pascal Hille, Markus Schäfer, Jörg Schörmann, María de la Mata, Jordi Arbiol, and Martin Eickhoff Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl501845m • Publication Date (Web): 12 Aug 2014 Downloaded from http://pubs.acs.org on August 16, 2014
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Probing the Internal Electric Field in GaN/AlGaN Nanowire Heterostructures Jan M¨ußener,† J¨org Teubert,∗,† Pascal Hille,† Markus Sch¨afer,† J¨org Sch¨ormann,† Maria de la Mata,‡ Jordi Arbiol,‡,¶ and Martin Eickhoff† I. Physikalisches Institut, Justus-Liebig-Universit¨at Gießen, Heinrich-Buff-Ring 16, D-35392 Gießen, Germany, and Institut de Ciencia de Materials de Barcelona, ICMAB-CSIC, Campus de la UAB, 08193 Bellaterra, CAT, Spain E-mail: [email protected]
Abstract We demonstrate the direct analysis of polarization-induced internal electric fields in single GaN/Al0.3 Ga0.7 N nanodiscs embedded in GaN/AlN nanowire heterostructures. Superposition of an external electric field with different polarity results in compensation or enhancement of the quantum-confined Stark effect in the nanodiscs. By field-dependent analysis of the low temperature photoluminescence energy and intensity we prove the [000¯ 1]-polarity of the nanowires and determine the internal electric field strength to 1.5 MV/cm. ∗
To whom correspondence should be addressed I. Physikalisches Institut, Justus-Liebig-Universit¨at Gießen, Heinrich-Buff-Ring 16, D-35392 Gießen, Germany ‡ Institut de Ciencia de Materials de Barcelona, ICMAB-CSIC, Campus de la UAB, 08193 Bellaterra, CAT, Spain ¶ Institucio Catalana de Recerca i Estudis Avan¸cats (ICREA), 08010 Barcelona, CAT, Spain †
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Keywords quantum-confined Stark effect, internal polarization fields, group III-nitride, GaN/AlGaN, single nanowire, micro-photoluminescence
Introduction GaN nanowires (NWs), grown by plasma-assisted molecular beam epitaxy (PAMBE) 1,2 with embedded nanowire heterostructures (NWHs) 3,4 provide a material platform for nanoscale optoelectronic devices as photodetectors, 5–9 LEDs, 10–14 lasers, 15–18 optochemical sensors 19–21 or even quantum-photonic devices in the ultraviolet regime. 22 The properties of polar group III-nitride (III-N) heterostructures are strongly affected by the presence of polarization induced charges. The corresponding internal electric fields are beneficial in certain cases, as e.g. in GaN/AlGaN high electron mobility transistor structures where a polarization-induced two dimensional electron gas is formed. 31,32 On the other hand, polarization-induced internal electric fields are considered to negatively impact the optical properties of heterostructures, such as quantum wells, via the quantum confined Stark effect (QCSE) 23–26 and significant effort is made to overcome these problems e.g. by growth on semi- or non-polar surfaces 33–35 or, in the case of NWs by realization of nanowire core-shell structures on non-polar surfaces. 36 The presence of polarization-induced internal electric fields was also evidenced for PAMBE grown III-N NWHs in different works. 4,27–30 However, for high Al-content in the barriers the magnitude of the internal electric field was found to be smaller than those reported for comparable thin film quantum wells. 4,28 The reason for this experimental result is still under vivid discussion and strain relaxation of the freestanding structure, 28,37 compressive stress exerted by the lateral shell of the barrier material, 29 or the presence of misfit-dislocations 4 are discussed as possible reasons. In those works the internal electric fields were determined only indirectly from steady 2 ACS Paragon Plus Environment
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state and time-resolved photoluminescence (PL) analysis by a variation of the NWH geometry and alloy composition and a comparison of the transition energies with numerical simulations. New insight could be obtained by a modification of the QCSE by controlled superposition of external electric fields as direct experimental access to strength and direction of internal fields in NWHs. Related experiments were performed for group-III-arsenide and III-nitride single and coupled quantum dots as well as quantum wells embedded in thin-film diode structures. 38–43 The authors report Stark shifts of the PL emission from the quantum confined states induced by external bias. However, similar experiments have not been shown for NWHs up to now. Here we present such an experimental approach and probe the internal electric fields in single NWHs by analyzing the impact of an external bias on the PL emission energy and intensity of single GaN nanodiscs (NDs) embedded in Al0.3 Ga0.7 N barriers.
Experimental Details GaN/AlGaN NWHs were grown by PAMBE on Si(111) substrate under nitrogen-rich conditions using a self-assembled and catalyst-free growth process. 4 The NWs show wurtzite crystal structure and grow along the polar [000¯1]-axis. 44 A schematic of the overall sample structure is depicted in Fig. 1a. For realization of carrier confinement the center part of the NWH consists of a single GaN ND (nominal thickness 1.7 nm) embedded in Al0.3 Ga0.7 N barriers (nominal length 50 nm). Outside the GaN/Al0.3 Ga0.7 N barrier region AlN sections (thickness of 16 nm) serve as current blocking layers (CBLs) required for the application of an axial external electric field across the ND region. The long bottom and top sections (≈1 µm) of the NWs are heavily n-type doped using Ge as a donor (concentration of approximately 1020 cm−3 ) 45 and act as electrical contacts (all other sections are not intentionally doped). A scanning electron microscope (SEM) image of the as-grown NW ensemble is shown in Fig. 1b with dark contrast corresponding to the barrier region in the middle of the NW. An annular
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dark field (ADF) scanning transmission electron microscope (STEM) image of a NWH is depicted in Fig. 1c showing the GaN ND in the middle of the two Al0.3 Ga0.7 N barriers and AlN CBLs. 46 a)
GaN:Ge contact
AlGaN GaN AlN current barrier nanodisc blocking layer top
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Figure 1: a) Schematic of the NWH. b) SEM image of the as-grown NW ensemble with the AlGaN barriers and AlN CBLs appearing in dark contrast in the center of the NWs c) ADF STEM image of the inner heterostructure showing the GaN ND, Al0.3 Ga0.7 N barriers and AlN CBLs. 46 d) SEM image of a contacted single NWH.
The symmetric design of the NWH allows for bias application in both polarities. However, due to a small lateral growth rate 4 the deposition of Alx Ga1−x N and AlN leads to the formation of a lateral shell rendering the structure slightly asymmetric. This becomes evident in the STEM image in Fig. 1c) which shows a slightly larger NW diameter above the (Al rich) heterostructure section in the middle of the NW compared to the region below the heterostructure. For contact formation, the NWHs were detached from the substrate in isopropanol using an ultrasonic bath. The NW-suspension was dispersed onto an oxidized Si carrier-chip featuring a finder-grid metallization. Isolated NWHs were contacted at the highly Ge-doped sections (cf. Fig. 1d) using electron beam lithography and thermal evaporation of Ti/Au (25 nm/200 nm) with a final annealing step for 60 s at 500 ◦C under vacuum conditions. Micro-photoluminescence (µPL) and electrical measurements on single NWs were performed at T = 4 K. The 325 nm excitation light from a HeCd-laser was focused by a 20-fold UV objective (NA of 0.4) onto the sample and the emitted PL was collected with the same 4 ACS Paragon Plus Environment
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objective, dispersed using a 3600 lines/cm grating in a 250 mm spectrometer, and detected with a cooled charge-coupled device (CCD) camera.
Quantum Mechanical Simulations In order to identify the design parameters, we performed numerical simulations of the carrier confinement for zero external bias using nextnano3 47 in analogy to Ref. 4. The calculations are based on a complete three dimensional model of the NWH with a core diameter of 30 nm 30 and account for N-face crystal polarity as well as the lateral growth of AlGaN material with a growth rate linearly interpolating between 0 % (GaN) and 11 % (AlN) of the axial growth rate. 4 In the simulations pseudomorphic growth is assumed, which is justified for thin NDs and low Al-content in the barriers. Polarization-induced interface charges due to spontaneous and piezoelectric polarization are also included. To mimic the role of surface states, we assume a surface Fermi-level pinning at mid-gap of the shell material according to Ref. 4 as reliable values of the surface potential are not known in case of m-plane AlGaN surfaces. However, by variation of the surface potential in the simulations we approved its negligible impact on the band profiles and transition energies (as discussed in Ref. 19) in comparison with parameters affecting the strain state close to the ND (e.g. geometry, alloy composition, or the thickness of the lateral shell). The results for the NWH design of Fig. 1a are depicted in Fig. 2. A thickness of 1.7 nm for the ND was chosen to obtain an emission energy above the near band edge (NBE) emission of the GaN contact regions. For the alloy composition of the barrier xAl = 0.3 was chosen to avoid lateral separation of electron and hole due to strain-induced lateral electric fields. 4 The length of the Al0.3 Ga0.7 N barriers on both sides of the ND was 50 nm resembling a compromise between axial carrier confinement and effective strength of the superimposed axial external electric field in the ND. In the simulations presented in Fig. 2 the thickness of the AlN CBL and the GaN:Ge contact regions on both sides of the central NWH section
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Figure 2: Numerical simulation results of a NWH with a 1.7 nm thick GaN ND, 50 nm Al0.3 Ga0.7 N barriers, and 3 nm thick AlN CBLs. a) Schematics of the simulated structure (without shell). b) Band profile of CB and VB (hh) along the NW center axis. c) Magnified view of the ND region showing the axial confinement potential, the energetic levels of electron and hole one-particle ground states and the corresponding axial probability distribution (insets depict the lateral probability distribution). d) Corresponding band gap energy along the NW center axis. A dashed line indicates the GaN D0 X transition energy of strain free GaN NWs. 48 The inset magnifies the vicinity of the CBL indicating the strain induced increase of the GaN band gap under pseudomorphic conditions.
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were 3 and 25 nm, respectively. 49 The calculated band profiles of the conduction band (CB) and valence band (VB, heavy hole (hh)) along the NW center axis in Fig. 2b show strong axial electric fields in the vicinity of the ND and the CBLs due to polarization induced interface charges. The field strength inside the ND amounts to 2.4 MV/cm and causes a pronounced axial separation of the confined one-particle ground states of electron and hole (cf. Fig. 2c). Lateral electric fields in the ND are small enough to yield maximum lateral probability density in the ND center for both carrier types (cf. insets in Fig. 2c). Fig. 2d shows the corresponding band gap profile, indicating a strain induced increase of the GaN band gap in the vicinity of the AlN CBLs.
Electrical Properties The influence of the AlN CBLs on the current-voltage (I-V) characteristics of single NWHs with a CBL thickness of 0 nm (a), 8 nm (b) and 16 nm (c) at room temperature is shown in Fig. 3. Voltage was applied to the NW top contact with the bottom contact connected to ground. NWHs without CBLs are characterized by non-destructive break through at low bias values (|Uext | < 10 V). The introduction of AlN CBLs significantly enhances the resistivity and allows the application of an external electric field (I < 1 nA) for up to |Uext | < 30 V for most of the investigated NWs with 16 nm CBLs. This is crucial in order to avoid PL quenching by electric heating 50 or carrier tunneling extraction 43 (cf. supporting information). Comparable measurements on homogeneously Ge-doped GaN NWs with the same doping concentration as the contact regions revealed negligible ohmic resistivity (several 10 kΩ, cf. inset Fig. 3a) compared to the NWH structure, confirming that the applied voltage drops almost completely between the CBLs. Furthermore, the annealing step did not significantly influence the I-V curves of the NWHs in Fig. 3a-c, stating that the contact resistance is
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Figure 3: I-V characteristics of single NWHs at room temperature containing: (a) no, (b) 8 nm, and (c) 16 nm AlN CBLs. For each case seven different NWHs are shown (indicated by different colors) in order to account for wire-to-wire fluctuations. Inset in a) I-V characteristics of a homogeneous Ge-doped GaN NW without heterostructure (note different plot scale).
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small compared to the NWH resistivity.
Optoelectrical Properties Fig. 4a depicts a low temperature (4 K) PL spectrum of a single NWH with 16 nm CBLs. The spectrum is dominated by the NBE emission of the strain-free Ge-doped GaN contact regions between 3.4 and 3.5 eV. 45 The emission at 3.55 eV is assigned to the GaN ND, identified by its dependence on external bias as shown below. It is superimposed by a broad luminescence band between 3.5 to 3.65 eV attributed to a strain induced increase of the GaN band gap in the vicinity of the CBLs (cf. Fig. 2d). It has to be noted that the spectral features of this emission strongly fluctuate between individual NWs due to strain relaxation at the CBLs and fluctuations in the thickness of the lateral AlGaN shell. 4 In case of NWHs with a CBL thickness of 0 and 8 nm we observed bias-induced quenching of the PL intensity in the whole spectral range, attributed to electric heating 50 (cf. supporting information). In contrast, bias-dependent PL spectra obtained from NWHs with 16 nm CBLs do not show this effect, indicating that an external electric field across the ND can be stabilized (Fig. 4b). A reproducible and reversible shift of the ND emission energy of up to 40 meV is found for an applied bias between −10 and +20 V, indicated by the dashed line labeled by number 1 in Fig. 4b. The extracted dependence of the emission energy on the applied bias Uext is depicted in Fig. 4c. For negative Uext the ND peak shifts to lower energies and merges with the GaN NBE emission. For positive Uext the ND peak shifts to higher energies and reaches a maximum at approximately Uext = +20 V. Furthermore, an increase (decrease) of the ND-related PL intensity is found for positive (negative) voltage as evidenced by the non-normalized spectra in Fig. 4d. This correlation of a blue-shift (red-shift) of the emission energy to an increase (decrease) of the PL intensity is a clear evidence for a modification of the QCSE – a positive external
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a ) P L In te n s ity [a r b .u .]
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Figure 4: a) Low temperature (4 K) PL spectrum of a single NWH with thick (16 nm) CBLs. b) PL spectra under the influence of an external bias focusing on the energy range above the GaN NBE. The shifting (ND related) and fixed (strained GaN related) spectral features are labeled with numbers 1 and 2, respectively. The spectra are normalized to the ND emission and vertically shifted for clarity. c) Energetic position of the ND peak (peak 1) as a function of bias voltage Uext . d) Same spectra as in b) for Uext = −10 V, 0 V, +20 V but not normalized and not shifted to indicate the bias dependence of the PL intensity.
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voltage counteracts the internal electric field in the NDs whereas for negative Uext the effect of internal fields is enhanced (cf. schematic in supporting information Fig. S2b). In contrast, the GaN-related NBE and strain-induced emission band do not show a spectral shift upon bias-application (cf. dashed line labeled by number 2 in Fig. 4b). However, the intensity of the latter shows a similar bias-dependence as the ND PL (cf. Fig. 4d) which can be attributed to an external modification of the strong polarization-induced electric fields in the regions close to the GaN/AlN interface (cf. band profile in Fig. 2a, for further explanation see supporting information). However, as no confined states are involved, the transition energy is not affected. The correlation between a positive Uext and a suppression of the QCSE (and vice versa) yields an axial electric field in the ND pointing towards the NW top, i.e. the NWHs exhibit N-face polarity (cf. Fig. S2b) in accordance to the STEM analysis in Ref. 44. From the results in Fig. 4c the magnitude of the internal axial electric field can be estimated. Considering that the maximum in ND emission energy at Uext = +20 V represents flat band conditions in the ND and assuming a homogeneous potential drop in the region between both CBLs (130 nm in length) the electric field is calculated to 1.5 MV/cm, slightly lower than the value of 2.4 MV/cm obtained by the simulations (cf. discussion of Fig. 2b). This reduction could be explained by screening effects of free carriers introduced by residual doping, the external current, or photoexcitation, which lower the internal fields. Although a low excitation intensity was chosen (≈100 W/cm2 ) to minimize the influence of the latter a (photo)current of ≈5 nA (0.25 nA) was observed under illumination (without illumination) at Uext = +20 V. Another reason could be interface dislocations which are not considered in the nextnano3 simulations and which reduce the strain-induced piezoelectric contribution to the internal fields. 4
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Conclusion In conclusion we have measured the evolution of the photoluminescence emission energy and intensity in single NWHs containing a GaN/Al0.3 Ga0.7 N ND upon application of an external axial electric field. In order to stabilize the electric field AlN current blocking layers were inserted between the highly Ge-doped contacts and the Al0.3 Ga0.7 N barriers. A blue-shift (red-shift) in PL emission energy and an increase (decrease) in PL intensity was observed for positive (negative) bias, i.e. for compensation (enhancement) of the polarization-induced internal electric field and the resulting quantum-confined Stark effect. The results revealed the NWHs crystal polarity (N-face) and allowed determination of the strength of the internal electric fields to 1.5 MV/cm. Compared to the simulations, this value is slightly lower, which can be attributed to screening by free carriers or a strain relaxation by interface dislocations. These results demonstrate the suitability of III-N nanowire heterostructures for studying the influence of static or alternating electric fields on the optical properties of single quantum structures.
Acknowledgement Authors from JLU acknowledge financial support within the LOEWE program of excellence of the Federal State of Hessen (project initiative STORE-E) and from the Federal Ministry for Education and Research (BMBF) within the project SINOMICS. The work was further supported by the Laboratory of Materials Research (LaMa) of JLU. J.A. acknowledges the funding from the Generalitat de Catalunya 2014 SGR 1638. MdlM thanks CSIC Jae-Predoc program.
Supporting Information Available Samples with thin CBL, exhibiting asymmetric PL quenching with external bias. This material is available free of charge via the Internet at http://pubs.acs.org/.
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(12) Lu, Y.-J.; Lin, H.-W.; Chen, H.-Y.; Yang, Y.-C.; Gwo, S. Appl. Phys. Lett. 2011, 98, 233101. (13) Brubaker, M. D.; Blanchard, P. T.; Schlager, J. B.; Sanders, A. W.; Herrero, A. M.; Roshko, A.; Duff, S. M.; Harvey, T. E.; Bright, V. M.; Sanford, N. A.; et al., J. Electron. Mater. 2013, 42, 868–874. (14) Li, S.; Waag, A. J. Appl. Phys. 2012, 111, 071101. (15) Johnson, J. C.; Choi, H.-J.; Knutsen, K. P.; Schaller, R. D.; Yang, P.; Saykally, R. J. Nat. Mater. 2002, 1 . (16) Qian, F.; Li, Y.; Gradecak, S.; Park, H.-G.; Dong, Y.; Ding, Y.; Wang, Z. L.; Lieber, C. M. Nat. Mater. 2008, 7, 701–706. (17) Schlager, J. B.; Sanford, N. A.; Bertness, K. A.; Roshko, A. J. Appl. Phys. 2011, 109, 044312. (18) Gradecak, S.; Qian, F.; Li, Y.; Park, H.-G.; Lieber, C. M. Appl. Phys. Lett. 2005, 87, 173111. (19) Teubert, J.; Becker, P.; Furtmayr, F.; Eickhoff, M. Nanotechnology 2011, 22, 275505. (20) Paul, S.; Helwig, A.; M¨ uller, G.; Furtmayr, F.; Teubert, J.; Eickhoff, M. Sensor. Actuat. B-Chem. 2012, 173, 120–126. (21) Maier, K.; Helwig, A.; M¨ uller, G.; Becker, P.; Hille, P.; Sch¨ormann, J.; Teubert, J.; Eickhoff, M. Sensor. Actuat. B-Chem. 2014, 197, 87 – 94. (22) Arbiol, J.; Magen, C.; Becker, P.; Jacopin, G.; Chernikov, A.; Sch¨afer, S.; Furtmayr, F.; Tchernycheva, M.; Rigutti, L.; Teubert, J.; Chatterjee, S.; Morante, J. R.; Eickhoff, M. Nanoscale 2012, 4, 7517–7524. (23) Bernardini, F.; Fiorentini, V.; Vanderbilt, D. Phys. Rev. B 1997, 56, R10024–R10027. 14 ACS Paragon Plus Environment
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(24) Miller, D. A. B.; Chemla, D. S.; Damen, T. C.; Gossard, A. C.; Wiegmann, W.; Wood, T. H.; Burrus, C. A. Phys. Rev. Lett. 1984, 53, 2173–2176. (25) Grandjean, N.; Massies, J.; Leroux, M. Appl. Phys. Lett. 1999, 74, 2361–2363. (26) Leroux, M.; Grandjean, N.; La¨ ugt, M.; Massies, J.; Gil, B.; Lefebvre, P.; Bigenwald, P. Phys. Rev. B 1998, 58, R13371–R13374. (27) Zamfirescu, M.; Gurioli, M.; Vinattieri, A.; Ristic, J.; Calleja, E. phys stat sol (c) 2005, 2, 3847–3850. (28) Renard, J.; Songmuang, R.; Tourbot, G.; Bougerol, C.; Daudin, B.; Gayral, B. Phys. Rev. B 2009, 80, 121305. (29) Songmuang, R.; Kalita, D.; Sinha, P.; den Hertog, M.; Andr, R.; Ben, T.; Gonzlez, D.; Mariette, H.; Monroy, E. Appl. Phys. Lett. 2011, 99, 141914. (30) Jacopin, G.; Rigutti, L.; Teubert, J.; Julien, F. H.; Furtmayr, F.; Komninou, P.; Kehagias, T.; Eickhoff, M.; Tchernycheva, M. Nanotechnology 2013, 24, 125201. (31) Asif Khan, M.; Bhattarai, A.; Kuznia, J. N.; Olson, D. T. Appl. Phys. Lett. 1993, 63, 1214. (32) Ambacher, O.; Smart, J.; Shealy, J. R.; Weimann, N. G.; Chu, K.; Murphy, M.; Schaff, W. J.; Eastman, L. F.; Dimitrov, R.; Wittmer, L.; Stutzmann, M.; Rieger, W.; Hilsenbeck, J. J. Appl. Phys. 1999, 85, 3222–3233. (33) Speck, J. S.; Chichibu, S. F. MRS Bulletin 2009, 34, 304–312. (34) Gil, B., Ed. III-Nitride Semiconductors and their Modern Devices; Oxford University Press, 2013. (35) Seong, T.-Y., Han, J., Amano, H., Morkoc, H., Eds. III-Nitride Based Light Emitting Diodes and Applications; Springer Netherlands, 2013. 15 ACS Paragon Plus Environment
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(36) Mandl, M.; Wang, X.; Schimpke, T.; Klper, C.; Binder, M.; Ledig, J.; Waag, A.; Kong, X.; Trampert, A.; Bertram, F.; Christen, J.; Barbagini, F.; Calleja, E.; Strassburg, M. phys. status solidi-R 2013, 7, 800–814. (37) Landr´e, O.; Camacho, D.; Bougerol, C.; Niquet, Y. M.; Favre-Nicolin, V.; Renaud, G.; Renevier, H.; Daudin, B. Phys. Rev. B 2010, 81, 153306. (38) Takeuchi, T.; Wetzel, C.; Yamaguchi, S.; Sakai, H.; Amano, H.; Akasaki, I.; Kaneko, Y.; Nakagawa, S.; Yamaoka, Y.; Yamada, N. Appl. Phys. Lett. 1998, 73, 1691. (39) Raymond, S.; Reynolds, J. P.; Merz, J. L.; Fafard, S.; Feng, Y.; Charbonneau, S. Phys. Rev. B 1998, 58, R13415–R13418. (40) Stinaff, E. A. Science 2006, 311 . (41) Robledo, L.; Elzerman, J.; Jundt, G.; Atature, M.; Hogele, A.; Falt, S.; Imamoglu, A. Science 2008, 320 . (42) M¨ uller, K.; Bechtold, A.; Ruppert, C.; Zecherle, M.; Reithmaier, G.; Bichler, M.; Krenner, H. J.; Abstreiter, G.; Holleitner, A. W.; Villas-Boas, J. M.; et al., Phys. Rev. Lett. 2012, 108 . (43) Weidemann, O.; Kandaswamy, P. K.; Monroy, E.; Jegert, G.; Stutzmann, M.; Eickhoff, M. Appl. Phys. Lett. 2009, 94, 113108–3. (44) de la Mata, M.; Magen, C.; Gazquez, J.; Utama, M. I. B.; Heiss, M.; Lopatin, S.; Furtmayr, F.; Fernndez-Rojas, C. J.; Peng, B.; Morante, J. R.; Rurali, R.; Eickhoff, M.; Fontcuberta i Morral, A.; Xiong, Q.; Arbiol, J. Nano Lett. 2012, 12, 2579–2586. (45) Sch¨ormann, J.; Hille, P.; Sch¨afer, M.; M¨ ußener, J.; Becker, P.; Klar, P. J.; KleineBoymann, M.; Rohnke, M.; de la Mata, M.; Arbiol, J.; et al., J. Appl. Phys. 2013, 114, 103505.
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(46) Due to the weak contrast between GaN nanodisc and Al0.3 Ga0.7 N barrier we show a nanowire heterostructure with thicker nanodisc for clarity. (47) nextnano3 Homepage. http://www.nextnano.de/nextnano3. (48) Furtmayr, F.; Vielemeyer, M.; Stutzmann, M.; Laufer, A.; Meyer, B. K.; Eickhoff, M. J. Appl. Phys. 2008, 104, 074309. (49) The reduced length compared to the experiment was chosen to reduce the computational costs and is justified by a negligible influence on the confinement potential in the ND. (50) Prades, J. D.; Jimenez-Diaz, R.; Hernandez-Ramirez, F.; Barth, S.; Cirera, A.; RomanoRodriguez, A.; Mathur, S.; Morante, J. R. Appl. Phys. Lett. 2008, 93, 123110.
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Probing the Internal Electric Field in GaN/AlGaN Nanowire Heterostructures Jan Müßener, Jörg Teubert, Pascal Hille, Markus Schäfer, Jörg Schörmann, María de la Mata, Jordi Arbiol, and Martin Eickhoff Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl501845m • Publication Date (Web): 12 Aug 2014 Downloaded from http://pubs.acs.org on August 16, 2014
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Probing the Internal Electric Field in GaN/AlGaN Nanowire Heterostructures Jan M¨ußener,† J¨org Teubert,∗,† Pascal Hille,† Markus Sch¨afer,† J¨org Sch¨ormann,† Maria de la Mata,‡ Jordi Arbiol,‡,¶ and Martin Eickhoff† I. Physikalisches Institut, Justus-Liebig-Universit¨at Gießen, Heinrich-Buff-Ring 16, D-35392 Gießen, Germany, and Institut de Ciencia de Materials de Barcelona, ICMAB-CSIC, Campus de la UAB, 08193 Bellaterra, CAT, Spain E-mail: [email protected]
Abstract We demonstrate the direct analysis of polarization-induced internal electric fields in single GaN/Al0.3 Ga0.7 N nanodiscs embedded in GaN/AlN nanowire heterostructures. Superposition of an external electric field with different polarity results in compensation or enhancement of the quantum-confined Stark effect in the nanodiscs. By field-dependent analysis of the low temperature photoluminescence energy and intensity we prove the [000¯ 1]-polarity of the nanowires and determine the internal electric field strength to 1.5 MV/cm. ∗
To whom correspondence should be addressed I. Physikalisches Institut, Justus-Liebig-Universit¨at Gießen, Heinrich-Buff-Ring 16, D-35392 Gießen, Germany ‡ Institut de Ciencia de Materials de Barcelona, ICMAB-CSIC, Campus de la UAB, 08193 Bellaterra, CAT, Spain ¶ Institucio Catalana de Recerca i Estudis Avan¸cats (ICREA), 08010 Barcelona, CAT, Spain †
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Keywords quantum-confined Stark effect, internal polarization fields, group III-nitride, GaN/AlGaN, single nanowire, micro-photoluminescence
Introduction GaN nanowires (NWs), grown by plasma-assisted molecular beam epitaxy (PAMBE) 1,2 with embedded nanowire heterostructures (NWHs) 3,4 provide a material platform for nanoscale optoelectronic devices as photodetectors, 5–9 LEDs, 10–14 lasers, 15–18 optochemical sensors 19–21 or even quantum-photonic devices in the ultraviolet regime. 22 The properties of polar group III-nitride (III-N) heterostructures are strongly affected by the presence of polarization induced charges. The corresponding internal electric fields are beneficial in certain cases, as e.g. in GaN/AlGaN high electron mobility transistor structures where a polarization-induced two dimensional electron gas is formed. 31,32 On the other hand, polarization-induced internal electric fields are considered to negatively impact the optical properties of heterostructures, such as quantum wells, via the quantum confined Stark effect (QCSE) 23–26 and significant effort is made to overcome these problems e.g. by growth on semi- or non-polar surfaces 33–35 or, in the case of NWs by realization of nanowire core-shell structures on non-polar surfaces. 36 The presence of polarization-induced internal electric fields was also evidenced for PAMBE grown III-N NWHs in different works. 4,27–30 However, for high Al-content in the barriers the magnitude of the internal electric field was found to be smaller than those reported for comparable thin film quantum wells. 4,28 The reason for this experimental result is still under vivid discussion and strain relaxation of the freestanding structure, 28,37 compressive stress exerted by the lateral shell of the barrier material, 29 or the presence of misfit-dislocations 4 are discussed as possible reasons. In those works the internal electric fields were determined only indirectly from steady 2 ACS Paragon Plus Environment
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state and time-resolved photoluminescence (PL) analysis by a variation of the NWH geometry and alloy composition and a comparison of the transition energies with numerical simulations. New insight could be obtained by a modification of the QCSE by controlled superposition of external electric fields as direct experimental access to strength and direction of internal fields in NWHs. Related experiments were performed for group-III-arsenide and III-nitride single and coupled quantum dots as well as quantum wells embedded in thin-film diode structures. 38–43 The authors report Stark shifts of the PL emission from the quantum confined states induced by external bias. However, similar experiments have not been shown for NWHs up to now. Here we present such an experimental approach and probe the internal electric fields in single NWHs by analyzing the impact of an external bias on the PL emission energy and intensity of single GaN nanodiscs (NDs) embedded in Al0.3 Ga0.7 N barriers.
Experimental Details GaN/AlGaN NWHs were grown by PAMBE on Si(111) substrate under nitrogen-rich conditions using a self-assembled and catalyst-free growth process. 4 The NWs show wurtzite crystal structure and grow along the polar [000¯1]-axis. 44 A schematic of the overall sample structure is depicted in Fig. 1a. For realization of carrier confinement the center part of the NWH consists of a single GaN ND (nominal thickness 1.7 nm) embedded in Al0.3 Ga0.7 N barriers (nominal length 50 nm). Outside the GaN/Al0.3 Ga0.7 N barrier region AlN sections (thickness of 16 nm) serve as current blocking layers (CBLs) required for the application of an axial external electric field across the ND region. The long bottom and top sections (≈1 µm) of the NWs are heavily n-type doped using Ge as a donor (concentration of approximately 1020 cm−3 ) 45 and act as electrical contacts (all other sections are not intentionally doped). A scanning electron microscope (SEM) image of the as-grown NW ensemble is shown in Fig. 1b with dark contrast corresponding to the barrier region in the middle of the NW. An annular
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dark field (ADF) scanning transmission electron microscope (STEM) image of a NWH is depicted in Fig. 1c showing the GaN ND in the middle of the two Al0.3 Ga0.7 N barriers and AlN CBLs. 46 a)
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Figure 1: a) Schematic of the NWH. b) SEM image of the as-grown NW ensemble with the AlGaN barriers and AlN CBLs appearing in dark contrast in the center of the NWs c) ADF STEM image of the inner heterostructure showing the GaN ND, Al0.3 Ga0.7 N barriers and AlN CBLs. 46 d) SEM image of a contacted single NWH.
The symmetric design of the NWH allows for bias application in both polarities. However, due to a small lateral growth rate 4 the deposition of Alx Ga1−x N and AlN leads to the formation of a lateral shell rendering the structure slightly asymmetric. This becomes evident in the STEM image in Fig. 1c) which shows a slightly larger NW diameter above the (Al rich) heterostructure section in the middle of the NW compared to the region below the heterostructure. For contact formation, the NWHs were detached from the substrate in isopropanol using an ultrasonic bath. The NW-suspension was dispersed onto an oxidized Si carrier-chip featuring a finder-grid metallization. Isolated NWHs were contacted at the highly Ge-doped sections (cf. Fig. 1d) using electron beam lithography and thermal evaporation of Ti/Au (25 nm/200 nm) with a final annealing step for 60 s at 500 ◦C under vacuum conditions. Micro-photoluminescence (µPL) and electrical measurements on single NWs were performed at T = 4 K. The 325 nm excitation light from a HeCd-laser was focused by a 20-fold UV objective (NA of 0.4) onto the sample and the emitted PL was collected with the same 4 ACS Paragon Plus Environment
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objective, dispersed using a 3600 lines/cm grating in a 250 mm spectrometer, and detected with a cooled charge-coupled device (CCD) camera.
Quantum Mechanical Simulations In order to identify the design parameters, we performed numerical simulations of the carrier confinement for zero external bias using nextnano3 47 in analogy to Ref. 4. The calculations are based on a complete three dimensional model of the NWH with a core diameter of 30 nm 30 and account for N-face crystal polarity as well as the lateral growth of AlGaN material with a growth rate linearly interpolating between 0 % (GaN) and 11 % (AlN) of the axial growth rate. 4 In the simulations pseudomorphic growth is assumed, which is justified for thin NDs and low Al-content in the barriers. Polarization-induced interface charges due to spontaneous and piezoelectric polarization are also included. To mimic the role of surface states, we assume a surface Fermi-level pinning at mid-gap of the shell material according to Ref. 4 as reliable values of the surface potential are not known in case of m-plane AlGaN surfaces. However, by variation of the surface potential in the simulations we approved its negligible impact on the band profiles and transition energies (as discussed in Ref. 19) in comparison with parameters affecting the strain state close to the ND (e.g. geometry, alloy composition, or the thickness of the lateral shell). The results for the NWH design of Fig. 1a are depicted in Fig. 2. A thickness of 1.7 nm for the ND was chosen to obtain an emission energy above the near band edge (NBE) emission of the GaN contact regions. For the alloy composition of the barrier xAl = 0.3 was chosen to avoid lateral separation of electron and hole due to strain-induced lateral electric fields. 4 The length of the Al0.3 Ga0.7 N barriers on both sides of the ND was 50 nm resembling a compromise between axial carrier confinement and effective strength of the superimposed axial external electric field in the ND. In the simulations presented in Fig. 2 the thickness of the AlN CBL and the GaN:Ge contact regions on both sides of the central NWH section
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Figure 2: Numerical simulation results of a NWH with a 1.7 nm thick GaN ND, 50 nm Al0.3 Ga0.7 N barriers, and 3 nm thick AlN CBLs. a) Schematics of the simulated structure (without shell). b) Band profile of CB and VB (hh) along the NW center axis. c) Magnified view of the ND region showing the axial confinement potential, the energetic levels of electron and hole one-particle ground states and the corresponding axial probability distribution (insets depict the lateral probability distribution). d) Corresponding band gap energy along the NW center axis. A dashed line indicates the GaN D0 X transition energy of strain free GaN NWs. 48 The inset magnifies the vicinity of the CBL indicating the strain induced increase of the GaN band gap under pseudomorphic conditions.
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were 3 and 25 nm, respectively. 49 The calculated band profiles of the conduction band (CB) and valence band (VB, heavy hole (hh)) along the NW center axis in Fig. 2b show strong axial electric fields in the vicinity of the ND and the CBLs due to polarization induced interface charges. The field strength inside the ND amounts to 2.4 MV/cm and causes a pronounced axial separation of the confined one-particle ground states of electron and hole (cf. Fig. 2c). Lateral electric fields in the ND are small enough to yield maximum lateral probability density in the ND center for both carrier types (cf. insets in Fig. 2c). Fig. 2d shows the corresponding band gap profile, indicating a strain induced increase of the GaN band gap in the vicinity of the AlN CBLs.
Electrical Properties The influence of the AlN CBLs on the current-voltage (I-V) characteristics of single NWHs with a CBL thickness of 0 nm (a), 8 nm (b) and 16 nm (c) at room temperature is shown in Fig. 3. Voltage was applied to the NW top contact with the bottom contact connected to ground. NWHs without CBLs are characterized by non-destructive break through at low bias values (|Uext | < 10 V). The introduction of AlN CBLs significantly enhances the resistivity and allows the application of an external electric field (I < 1 nA) for up to |Uext | < 30 V for most of the investigated NWs with 16 nm CBLs. This is crucial in order to avoid PL quenching by electric heating 50 or carrier tunneling extraction 43 (cf. supporting information). Comparable measurements on homogeneously Ge-doped GaN NWs with the same doping concentration as the contact regions revealed negligible ohmic resistivity (several 10 kΩ, cf. inset Fig. 3a) compared to the NWH structure, confirming that the applied voltage drops almost completely between the CBLs. Furthermore, the annealing step did not significantly influence the I-V curves of the NWHs in Fig. 3a-c, stating that the contact resistance is
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Figure 3: I-V characteristics of single NWHs at room temperature containing: (a) no, (b) 8 nm, and (c) 16 nm AlN CBLs. For each case seven different NWHs are shown (indicated by different colors) in order to account for wire-to-wire fluctuations. Inset in a) I-V characteristics of a homogeneous Ge-doped GaN NW without heterostructure (note different plot scale).
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small compared to the NWH resistivity.
Optoelectrical Properties Fig. 4a depicts a low temperature (4 K) PL spectrum of a single NWH with 16 nm CBLs. The spectrum is dominated by the NBE emission of the strain-free Ge-doped GaN contact regions between 3.4 and 3.5 eV. 45 The emission at 3.55 eV is assigned to the GaN ND, identified by its dependence on external bias as shown below. It is superimposed by a broad luminescence band between 3.5 to 3.65 eV attributed to a strain induced increase of the GaN band gap in the vicinity of the CBLs (cf. Fig. 2d). It has to be noted that the spectral features of this emission strongly fluctuate between individual NWs due to strain relaxation at the CBLs and fluctuations in the thickness of the lateral AlGaN shell. 4 In case of NWHs with a CBL thickness of 0 and 8 nm we observed bias-induced quenching of the PL intensity in the whole spectral range, attributed to electric heating 50 (cf. supporting information). In contrast, bias-dependent PL spectra obtained from NWHs with 16 nm CBLs do not show this effect, indicating that an external electric field across the ND can be stabilized (Fig. 4b). A reproducible and reversible shift of the ND emission energy of up to 40 meV is found for an applied bias between −10 and +20 V, indicated by the dashed line labeled by number 1 in Fig. 4b. The extracted dependence of the emission energy on the applied bias Uext is depicted in Fig. 4c. For negative Uext the ND peak shifts to lower energies and merges with the GaN NBE emission. For positive Uext the ND peak shifts to higher energies and reaches a maximum at approximately Uext = +20 V. Furthermore, an increase (decrease) of the ND-related PL intensity is found for positive (negative) voltage as evidenced by the non-normalized spectra in Fig. 4d. This correlation of a blue-shift (red-shift) of the emission energy to an increase (decrease) of the PL intensity is a clear evidence for a modification of the QCSE – a positive external
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Figure 4: a) Low temperature (4 K) PL spectrum of a single NWH with thick (16 nm) CBLs. b) PL spectra under the influence of an external bias focusing on the energy range above the GaN NBE. The shifting (ND related) and fixed (strained GaN related) spectral features are labeled with numbers 1 and 2, respectively. The spectra are normalized to the ND emission and vertically shifted for clarity. c) Energetic position of the ND peak (peak 1) as a function of bias voltage Uext . d) Same spectra as in b) for Uext = −10 V, 0 V, +20 V but not normalized and not shifted to indicate the bias dependence of the PL intensity.
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voltage counteracts the internal electric field in the NDs whereas for negative Uext the effect of internal fields is enhanced (cf. schematic in supporting information Fig. S2b). In contrast, the GaN-related NBE and strain-induced emission band do not show a spectral shift upon bias-application (cf. dashed line labeled by number 2 in Fig. 4b). However, the intensity of the latter shows a similar bias-dependence as the ND PL (cf. Fig. 4d) which can be attributed to an external modification of the strong polarization-induced electric fields in the regions close to the GaN/AlN interface (cf. band profile in Fig. 2a, for further explanation see supporting information). However, as no confined states are involved, the transition energy is not affected. The correlation between a positive Uext and a suppression of the QCSE (and vice versa) yields an axial electric field in the ND pointing towards the NW top, i.e. the NWHs exhibit N-face polarity (cf. Fig. S2b) in accordance to the STEM analysis in Ref. 44. From the results in Fig. 4c the magnitude of the internal axial electric field can be estimated. Considering that the maximum in ND emission energy at Uext = +20 V represents flat band conditions in the ND and assuming a homogeneous potential drop in the region between both CBLs (130 nm in length) the electric field is calculated to 1.5 MV/cm, slightly lower than the value of 2.4 MV/cm obtained by the simulations (cf. discussion of Fig. 2b). This reduction could be explained by screening effects of free carriers introduced by residual doping, the external current, or photoexcitation, which lower the internal fields. Although a low excitation intensity was chosen (≈100 W/cm2 ) to minimize the influence of the latter a (photo)current of ≈5 nA (0.25 nA) was observed under illumination (without illumination) at Uext = +20 V. Another reason could be interface dislocations which are not considered in the nextnano3 simulations and which reduce the strain-induced piezoelectric contribution to the internal fields. 4
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Conclusion In conclusion we have measured the evolution of the photoluminescence emission energy and intensity in single NWHs containing a GaN/Al0.3 Ga0.7 N ND upon application of an external axial electric field. In order to stabilize the electric field AlN current blocking layers were inserted between the highly Ge-doped contacts and the Al0.3 Ga0.7 N barriers. A blue-shift (red-shift) in PL emission energy and an increase (decrease) in PL intensity was observed for positive (negative) bias, i.e. for compensation (enhancement) of the polarization-induced internal electric field and the resulting quantum-confined Stark effect. The results revealed the NWHs crystal polarity (N-face) and allowed determination of the strength of the internal electric fields to 1.5 MV/cm. Compared to the simulations, this value is slightly lower, which can be attributed to screening by free carriers or a strain relaxation by interface dislocations. These results demonstrate the suitability of III-N nanowire heterostructures for studying the influence of static or alternating electric fields on the optical properties of single quantum structures.
Acknowledgement Authors from JLU acknowledge financial support within the LOEWE program of excellence of the Federal State of Hessen (project initiative STORE-E) and from the Federal Ministry for Education and Research (BMBF) within the project SINOMICS. The work was further supported by the Laboratory of Materials Research (LaMa) of JLU. J.A. acknowledges the funding from the Generalitat de Catalunya 2014 SGR 1638. MdlM thanks CSIC Jae-Predoc program.
Supporting Information Available Samples with thin CBL, exhibiting asymmetric PL quenching with external bias. This material is available free of charge via the Internet at http://pubs.acs.org/.
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(46) Due to the weak contrast between GaN nanodisc and Al0.3 Ga0.7 N barrier we show a nanowire heterostructure with thicker nanodisc for clarity. (47) nextnano3 Homepage. http://www.nextnano.de/nextnano3. (48) Furtmayr, F.; Vielemeyer, M.; Stutzmann, M.; Laufer, A.; Meyer, B. K.; Eickhoff, M. J. Appl. Phys. 2008, 104, 074309. (49) The reduced length compared to the experiment was chosen to reduce the computational costs and is justified by a negligible influence on the confinement potential in the ND. (50) Prades, J. D.; Jimenez-Diaz, R.; Hernandez-Ramirez, F.; Barth, S.; Cirera, A.; RomanoRodriguez, A.; Mathur, S.; Morante, J. R. Appl. Phys. Lett. 2008, 93, 123110.
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Graphical TOC Entry
_ U
Photoluminescence
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