Effect of annealing treatment on
electroluminescence from GaN/Si
nanoheterostructure array
Chang Bao Han, Chuan He, Xiao Bo Meng, Ya Rui Wan, Yong Tao Tian, Ying Jiu
Zhang, and Xin Jian Li*
Department of Physics and Laboratory of Materials Physics, Zhengzhou University, Zhengzhou 450052, China
回转窑烧嘴*lixj@zzu.edu
Abstract:A GaN/Si nanoheterostructure array was prepared by growing
GaN nanostructures on silicon nanoporous pillar array (Si-NPA). Based on
as-grown and annealed GaN/Si-NPA, two light-emitting diodes (LEDs)
were fabricated. It was found that after the annealing treatment, both the
turn-on voltage and the leakage current density of the nanoheterostructure
varied greatly, together with the electroluminescence (EL) changed from a
yellow band to a near infrared band. The EL variation was attributed to the
radiative transition being transformed from a defect-related recombination
in GaN to an interfacial recombination of GaN/Si-NPA. Ours might have
provided an effective approach for fabricating GaN/Si-based LEDs with
different emission wavelengths.
©2012 Optical Society of America
OCIS codes: (160.4236) Nanomaterials; (230.0230) Optical devices; (230.3670) Light-emitting
diodes.
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1. Introduction
In the past decade, gallium nitride (GaN) has been widely used in fabricating ultraviolet, blue and green light-emitting diodes (LEDs) or laser diodes (LDs) because of the merits of its direct and wide bandgap (3.4 eV), high carrier mobility, and good thermal and chemical stability [1–3]. Although GaN/Si heterostructures were also deemed to be promising candidates for making integrated high-speed or high-power photoelectronic devices [4–6], the practical course was badly baffled by the infe
rior interface quality resulted from the large lattice mismatch between the two semiconductors [7], because an inferior interface quality would bring serious damage to both the inner quantum efficiency (IQE) and the performances of as-constructed devices. To reduce the lattice mismatch and thereby improve the interface #161177 - $15.00 USD Received 10 Jan 2012; revised 9 Feb 2012; accepted 9 Feb 2012; published 22 Feb 2012 (C) 2012 OSA27 February 2012 / Vol. 20, No. 5 / OPTICS EXPRESS 5637
quality, two main approaches have been developed in the past years [8, 9]. One was the usage of an intermediate layer to accommodate most of the lattice mismatch and enhance the metallurgical compatibility between GaN and silicon substrates. This was usually realized by introducing a specially designed multilayer with a lattice constant gradient or an N-ion implanted substrate surface with partially distorted crystal lattices [10, 11]. The other was the adoption of nanoheteroepitaxy method, which was often carried out by utilizing various nanopatterned substrates to release the interfacial stress, reduce the dislocation density and improve the IQE [12–14]. For example, the GaN-Si interfacial residual stress could be greatly reduced by growing GaN film on a silicon nanopore array [15], and good rectifying properties could be obtained by growing p-GaN nanowires on n-Si crystal wafers [16]. These results greatly promoted the confidence for making GaN/Si-based LEDs or LDs with high efficiency and broad emission band range.
Encouraged by these experiments and utilizing silicon nanoporous pillar array (Si-NPA) [17] as functional substrates, we have constructed a GaN/Si nanoheterostructure array (GaN/Si-NPA) by growing GaN nanograins onto Si-NPA, in which an effective yellow or infrared (NIR) electroluminescence (EL) tuned by the applied voltages was obtained [18]. This indicates that GaN/Si-NPA might be a promising material system for fabricating practical GaN/Si-based LEDs. According to the basic theory of luminescence, the adjustability of the EL wavelength inferred that there might have different radiative recombination paths in GaN/Si-NPA, such as the band-band transition or the transitions relating with the high-density defects formed in GaN or at the interfaces. Clearly, the co-existence of multi-recombination paths would produce strong effect on the EL qualities, both the EL intensity and monochromaticity. On the other hand, thermal treatments have been proved to be an effect approach to promote the EL properties of a semiconductor heterojunction through improving the interfacial quality and changing the electronic structures. For instance, the carrier concentration of n-ZnO/p-Si could be changed through annealing treatment and the J-V curve as well as the EL properties could be adjusted notably [19]. The EL intensity and peak position of n-ZnO nanorods/p-GaN LED could be tuned through controlling the concentration and sorts of the defect states by performing annealing treatments at different temperatures and in different atmospheres [20]. As a result, a systematic study of the annealing effect on the EL propertie
s is necessary for both clarifying the luminescent mechanism and promoting the emission qualities of GaN/Si-NPA.
In this paper, two GaN/Si LEDs were prepared based on as-grown and annealed GaN/Si-NPA. The structural and physical properties, including the X-ray diffraction (XRD) patterns, surface morphologies, current density-voltage (J-V) curves, EL and photoluminescence (PL) spectra, were measured and comparatively studied. Based on the experimental results, the EL mechanisms of the LEDs were put forward through building up the corresponding electronics structures. Our results might indicate a novel approach for designing and fabricating high-performance LEDs based directly on GaN/Si nanoheterostructures.
2. Experimental details
Si-NPA was prepared by hydrothermally etching (111) oriented, boron-doped single crystal Si (sc-Si) wafers in a solution of hydrofluoric acid containing ferric nitrate [17]. A thin layer of platinum (~3 nm), which acted as catalyst in the subsequent GaN growing process, was pre-deposited on freshly prepared Si-NPA samples by a magnetron sputtering technique. Using high-purity metal Ga (99.999%, 0.8 g) and NH3 gas (99.999%, introduced with a flow rate of 20 sccm) as the sources for t
he two elements, GaN were grown on Si-NPA by a chemical vapor deposition (CVD) method. The deposition was carried out in a vacuum tube furnace equipped with multichannel gas inlets and a gas mixing chamber at 1050 °C for 20 min. Here two kinds of GaN/Si-NPA were prepared, one was the as-grown sample and the other was annealed at 800 °C for 3 hours in nitrogen atmosphere afterwards. Layers of indium tin oxide (ITO, ~100 nm) acting as top electrode and Al (~500 nm) acting as back electrode were deposited by magnetron sputtering and vacuum evaporation methods, respectively. As-constructed LEDs have a device structure of ITO/n-GaN/p-Si-NPA/sc-Si/Al. For the #161177 - $15.00 USD Received 10 Jan 2012; revised 9 Feb 2012; accepted 9 Feb 2012; published 22 Feb 2012 (C) 2012 OSA27 February 2012 / Vol. 20, No. 5 / OPTICS EXPRESS 5638
convenience of narration, the two LEDs here were named as as-grown LED and annealed LED, respectively. The two LEDs were both annealed at 300 °C for 1 hour in Ar atmosphere to realize ohmic contact between the electrodes and the semiconductors. The active areas of the diodes were specified as 10 mm × 10 mm. The surface morphology and the crystal structure of GaN/Si-NPA were characterized by a field emission scanning electron microscope (FESEM, JSM 6700F) and an X-ray diffractometer (Panalytical X' Pert Pro). The electrical and luminescent properties of the devices were measured at room temperature through an electrical group system consisted of Sourcemeter-2400 (Keithley) and a fluorescence spectrometer (Spex Fluorolog-3), respectively.
3. Results and discussion
The XRD patterns of as-grown and annealed GaN/Si-NPA are shown in Part A of Fig. 1(a), in which all the diffraction peaks were indexed to crystalline hexagonal wurtzite GaN (JCPDS card: No. 50-0792). The obvious difference between the two curves is the reduction of the full width at half maximum (FWHM) for all the corresponding diffraction peaks after annealing treatment, as could be seen more obviously in Part B of Fig. 1(a). The typical cross-sectional FESEM image of as-grown GaN/Si-NPA is given in Fig. 1(b), in which GaN layers characterized by two different morphologies were observed. The upper layer was composed of two kinds of quasi one-dimensional GaN nanostructures, straight nanowires with an average diameter of ~30 nm and pencil-like nanorods with an average diameter of ~300 nm. Both the nanowires and the nanorods were well separated and nearly aligned locally perpendicular to the substrate surface, with an average length of ~1.5 µm. Between the nanowire/nanorod layer and Si-NPA substrate was a granular layer consisted of large quantities of GaN nanocrystallites (nc-GaN). The layer thickness and the average grain size were ~150 nm and ~20 nm, respectively. No apparent morphological variation was found by comparing the FESEM images of the samples before and after annealing treatment. Therefore, it was reasonable to think that the reduction of the FWHM of the diffraction peaks observed in Fig. 1(a) sho
uld result from the growing up of nc-GaN, which might have been formed in the GaN granular layer. This indicates that the crystallinity of as-deposited nc-GaN might have been greatly improved after the annealing treatment. As a consequence, the density of crystal defects should have been largely reduced.
Fig. 1. (a) Part A: the XRD patterns of as-grown and annealed GaN/Si-NPA; Part B: the
comparison of the FWHM variation for all the corresponding XRD peaks before and after
annealing treatment. (b) The cross-sectional FESEM image of as-grown GaN/Si-NPA.
The dark J-V curves of as-grown and annealed LEDs measured at room temperature are depicted in Fig. 2. The inset of Fig. 2 shows the schematic structure of the LEDs. Both of the J-V curves exhibited rectifying characteristic. Because the contact between Si-NPA and sc-Si has been proved to be ohmic [21], the observed rectification behaviors confirmed the formation of heterojunctions for both as-grown and annealed GaN/Si-NPA. But all the junction parameters for annealed LEDs, including the turn-on voltage, breakdown reverse #161177 - $15.00 USD Received 10 Jan 2012; revised 9 Feb 2012; accepted 9 Feb 2012; published 22 Feb 2012 (C) 2012 OSA27 February 2012 / Vol. 20, No. 5 / OPTICS EXPRESS 5639
voltage and leakage current density, have changed largely compared with those for as-grown ones. For example, the turn-on voltage (for obtaining a current density of 1 mA/cm2) increased from ~1.6 V to ~3.9 V, and the leakage current density (at an applied voltage of −4 V) reduced from ~3.2 mA/cm2to ~0.04 mA/cm2. The rectiðcation ratios for the two LEDs were calculated to be ~9 (at ± 3.9 V) and ~36 (at ± 4.8 V), respectively. According to the basic theory of heterojunctions [22], the leakage current density of a heterojunction was generally attributed to the defect-mediated tunneling effect caused by a high defect or trap concentration at the interface. Therefore, the distinct reduction of the leakage current density for the annealed LED might indicate an improvement of interfacial qua
lity and a decrease of the defect state density, just as what occurred in the annealing process of ZnO nanorods/Si heterojunctions [23].
Fig. 2. The room-temperature J-V curves of as-grown and annealed GaN/Si-NPA. Inset: the
schematic diagram of the LEDs.
For clarifying the underlying transportation mechanism of the variation, the log-log plot of the J-V data is presented in Fig. 3. It was found that both the curves for as-grown and annealed LEDs could be fitted by two straight lines. For as-grown LED, the J-V curve exhibits firstly a linear relation at a low forward voltage region (V < 0.9 V, region I). This indicates that the transportation of the carriers obeying the Ohmic law. With the applied voltage increased over 0.9 V (region II), the J-V curve exhibits an exponential relationship (J~V3.3), which infers a typical space charge limited current (SCLC) mechanism [24]. The SCLC mechanism was usually observed in wide bandgap p-n diodes, such as ZnO/Si [25, 26] and ZnO/SiC [27]. As for annealed LED, the J-V curve also exhibits a linear
relation before the inflection point of ~1.9 V (region I′), but the current density is about three orders of magnitude lower than that of as-grown device. With the applied voltage increased beyond ~1.9 V (region II′), the transportation mechanism also transferred to the SCLC model, but with a relationship of J~V10. Clearly, the exponent varied largely from ~3.3 for as-grown LED to ~10 for annealed LED, and the increment of the exponent in SCLC model indicated a narrowed distribution of the localized states and a lowered defect state density in the annealed LED [28].
#161177 - $15.00 USD Received 10 Jan 2012; revised 9 Feb 2012; accepted 9 Feb 2012; published 22 Feb 2012 (C) 2012 OSA27 February 2012 / Vol. 20, No. 5 / OPTICS EXPRESS 5640
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