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Luminescence imaging of semiconductor surfaces in nanometric resolution is a key to novel optoelectronic nano-devices, which requires local carrier excitation and local luminescence collection within the nanometric areas at the surfaces. However, there have not been a practical nanospectroscopies applicable to wide range of specimens. STM-cathodoluminescence (STM-CL) nanospectroscopy offers both high spatial resolution (of the order of 10 nm) and novel high carrier excitation power (up to ~1 mW), which enables local luminescence imaging of less-luminescent nano-structures. In this study, we advanced STM-CL technique by introducing a novel optical fiber probe with Cr thin film coating (Cr-FP), which was found to work as a STM probe, as an electron field-emitter for local carrier excitation, and as an alignment-free efficient local STM-CL collector which blinds luminescence after the minority carrier diffusion.
©2013 Optical Society of America
1. Introduction
Luminescence imaging of semiconductor surfaces in nanometric resolution is a key to novel optoelectronic nano-devices [1,2], which requires local carrier excitation and local luminescence collection within the nanometric areas at the surfaces. Scanning probe microscopy-based luminescence nanospectroscopies in combination with optical fiber probes are promising for this purpose. For improving their spatial resolutions, near-field optical fiber probes have been applied to scanning near-field optical microscope photoluminescence (SNOM-PL) [3–7] and scanning tunnelling microscope light emission (STM-LE) [8–22] nanospectroscopies. However, since SNOM-PL and STM-LE nanospectroscopies excites few carrier pairs per photon and tunnelling electron (with a few eV) and their near-field optical apertures has low optical transmittance (~1%), their low S/Ns unpractically limit their specimens. On the contrary, STM-cathodoluminescence (STM-CL) nanospectroscopy [9,23–28] realizes both high spatial resolution (of the order of 10 nm) and novel high carrier excitation power (up to ~1 mW), which enables local luminescence imaging of less-luminescent nano-structures.
In STM-CL technique, an STM probe is relatively distant (d ~ 100 nm) and negatively voltage-biased (~-100 V) from a specimen surface. Electron beam with ultra-low energy and high beam current is radially field-emitted from the probe apex and is focused at the specimen surface by short working distance d and positive image potential at the surface, which excites a large number of carriers at the surface small volume where an intense STM-CL is locally emitted. To advance STM-CL technique further with a novel conductive optical fiber probe, it needs to be an electron field-emitter and efficient local STM-CL collector. We advanced STM-CL technique by introducing a novel optical fiber probe with Cr thin film coating (Cr-FP), which was found to work as a STM probe, as an electron field-emitter for local carrier excitation, and as an alignment-free efficient local STM-CL collector which blinds luminescence via minority carrier diffusion.
2. Experimental
For an FP material, we used a step-index guide single-mode optical fiber (Oxford Electronics Inc.) with uniformly GeO2-doped silica core and pure silica cladding (Media 1-Supplementary Information S1). In order to achieve efficient local STM-CL collection by the conductive optical fiber probe, we first theoretically optimized the half cone angle θ of glass optical fiber probes for efficient STM-CL collection (Media 1-Supplementary Information S2) and investigated corresponding conditions of the optical fiber etching in buffered hydrofluoric acid solution (Media 1-Supplementary Information S3). We found that FP with θ = 51.0° gives maximum light collection solid angle ΩFP of 1.94 sr, which is almost independent of the FP-specimen distance d and the light wavelength λ0. The bare FP also needs to have a sharp apex both for STM probe and for electron field emitter.
To obtain such FPs with high axial symmetry under precise shape control, we performed buffered hydrofluoric acid etching in the following manner. We etched a silica optical fiber for the duration of t min in the buffered hydrofluoric acid kept at 22.0 °C in a thermostat bath, which was a mixture of NH4F (40 wt%): HF (50 wt%): H2O = X: 1: 1 (volume ratio) [29,30]. The etching is rate-limited by the solubilities of poorly-soluble reaction products on the silica surface, (NH4)2SiF6 and (NH4)2GeF6. Since the solubility of (NH4)2GeF6 is lower than that of (NH4)2SiF6, the etching rate of GeO2-doped silica core is slower than that of pure silica cladding, resulting in a tapered core with a cone shape (bare FP): the ratio of the core etching rate to the cladding etching rate is sin θ. Since the solubilities of (NH4)2SiF6 and (NH4)2GeF6 depend on X, the θ is controlled by X. We found X = 2.0 and t = 145 min to be optimal. We give further details of this in Media 1-Supplementary Information S3.
We transferred the bare FP to a vacuum evaporator (base pressure ~1 × 10−3 Pa) and uniformly deposited a thin metal film. At first we examined coating with 100 nm-thick indium tin oxide (ITO) film which has a work function of ~4.7 eV, 860 nm light transmittance of ~80%, and melting point of ~900 °C. However, the ITO-coated FP apex did not yield electron field emission due to its instability under a high electric field. We have chosen a 10 nm-thick Cr film because of its (1) high wettability to silica and other metals with surface plasmon polariton (SPP) enhancement, which maintains the sharpness of bare FPs, (2) hardness for stable STM imaging, (3) sufficiently low work function (~4.5 eV) for electron field-emission at VT = 100 V, and (4) relatively high optical transmittance from the visible to near-infrared range (860 nm light transmittance of ~33%).
We transferred the Cr-coated FP to the UHV FP STM-CL system within 10 min to minimize oxidation of the Cr film surface by air exposure. Then the apex of the Cr-coated FP was side-heated in the UHV chamber to outgas the surface adsorbent by placing joule-heated (~500 °C) clean Si wafer at the distance of 0.5 mm from the apex.
Figure 1 is a schematic of our UHV FP STM-CL system based on an ultra-high-vacuum scanning tunnelling microscope (UHV-STM: JEOL JSTM-4500XT, base pressure ~1 × 10−8 Pa), which consists of a home-made specimen holder, a conductive optical FP, FP-STM scanner (preproduct by JEOL Ltd.), an optical fiber cable for luminescence transfer, a monochromator (Horiba Ltd. Triax 320 with a blazed grating: grooves of 150 mm−1 and blazed wavelength of 1200 nm), and a (Cs)GaAs photomultiplier tube (Hamamatsu Photonics Ltd. R943-02) with photon counting units [31,32]. The conductive FP is coaxially connected to the FP-STM scanner, which has a larger core diameter to minimize optical loss. A conventional twin-lens optics system with far-field light collection solid angle ΩLens = 0.157 sr is also equipped at an axis angle of 60° from the surface normal for referential luminescence collection,. For referential PL measurements, a He-Ne laser (633 nm, beam power ~5 mW, beam diameter ~0.3 mm, not shown in Fig. 3) and optics are also equipped. Scanning electron microscope (SEM: JEOL JSM-5410) is equipped for the coarse alignment between the conductive FP and the specimen surface.
Fig. 1 Schematic of FP STM-CL system equipped with conventional twin-lens optics. We collected the luminescence at the specimen surface either via the twin-lens optics and the quartz viewport or via the conductive optical FP and the optical fiber STM scanner. Then it was transferred through an optical fiber cable to a monochromator and finally detected by a (Cs)GaAs photomultiplier tube with photon counting units. Our system also equipped a He-Ne laser (633 nm, beam power ~5 mW, beam diameter ~0.3 mm) and optics, which enabled referential PL spectroscopy (not shown). SEM is also equipped for the coarse alignment between the conductive FP and the specimen surface.
For each STM-CL measurement, we kept a sample-probe distance, d, of the order of 100 nm by retracting the STM probe vertically from the sample surface with a voltage applied to the STM piezoelectric tube. We determined d from the piezoelectric displacement constant (20 nm/V) and the voltage applied to the STM scanner. The nominal FEEB excitation position was determined to be the centre of the STM image obtained before each STM-CL measurement. In referential PL measurements, we adjusted the PL excitation positions to light-emitting junctions by using micrometre gauges for PL optics.
We collected the luminescence at the specimen surface either via the twin-lens optics and the quartz viewport or via the conductive optical FP and the optical fiber STM scanner. The luminescence was then transferred through an optical fiber cable to a monochromator and finally detected by a (Cs)GaAs photomultiplier tube (PMT) with photon counting units. Note that all PL and STM-CL spectra in this report are raw spectra taken with the (Cs)GaAs PMT and show band-edge emission peaks (λ0 = 860 nm) blue-shifted from the GaAs bandgap wavelength (870 nm) due to (Cs)GaAs PMT spectral sensitivity.
3. Results and discussion
We fabricated FPs with θ = 51°, small tip curvature radii of r < 20 nm, and cladding diameters of ~20 μm as shown in the SEM micrographs of Figs. 2(a) and 2(b). The Cr-FPs in STM mode yields topograph of Si (111) 7 × 7 surface structure under typical conditions (Fig. 2(c)): a bias (VT) of 2.0 V and beam current (IT) of 0.1 nA in constant-current mode. The Cr-FPs in field-emission mode emit a stable field-emitted electron beam (FEEB) current (Fig. 2(d)): IT = 60 nA for d = 113 nm and VT = 156 V. IT jumps are significantly suppressed by side heating in UHV, though still observable. These results show that 10 nm-thick Cr film coated on the small FP apex has low enough resistance for STM mode and stable under the high electric field and joule heating in field-emission mode. The side-heating of the Cr-FP apex outgases the surface adsorbent, which stabilizes IT and the resulting STM-CL intensity.
Fig. 2 Cr-FP micrographs and its properties as a STM probe and an electron field-emitter. (a) low- and (b) high-magnification SEM micrographs showing half-cone angle θ = 51° and tip curvature radius of r < 20 nm; (c) STM image of Si (111) 7 × 7 surface structure using Cr-FP at VT = 2.0 V and IT = 0.1 nA under STM constant-current mode; (d) time sequence of IT at d = 113 nm and VT = 156 V under electron field-emission mode; (e) FEEB irradiation assuming radial electron emission from point emitter at Cr- FP apex; (f) STM image for IT = 50 nA with (d, VT, tESD) = (70 nm, 122 V, 41.2 s); (g) STM image for IT = 50 nA with (d, VT, tESD) = (133 nm, 160 V, 95.5 s).
In our previous studies, the size of FEEB irradiation area, DEB, at the specimen surface was a key parameter in STM-CL spatial resolution [27,28]. We then evaluated DEB with varying d from STM image contrast due to electron-stimulated desorption (ESD) of the local oxide layer at the Si (111) substrate surface. Assuming radial electron field emission from the point emitter at the Cr-FP apex (Fig. 2(e)), we varied the FEEB irradiation on the SiOx layer under constant doze density, (IT/d2)∙tESD, where tESD is the irradiation duration. The IT is 50 nA for any condition and the set (d, VT, tESD) is (70 nm, 122 V, 41.2 s) in Fig. 2(f) and (133 nm, 160 V, 95.5 s) in Fig. 2(g). Each STM image shows a circular area corresponding to the ESD area in the surface oxide layer, whose diameter, DESD, was evaluated to be 60 nm in Fig. 2(f) and 112 nm in Fig. 2(g). The decrease of VT with d indicates imperfect electric field concentration at the FP apex, however, DESD/d is invariant at ~0.85 in both Fig. 2(f) and 2(g). In our geometric setup, the electron field emission from the Cr-FP to the specimen surface is largely characterized by radial FEEB emission from the point emitter at the apex with FEEB opening angle of ~23°, as we assumed, (Fig. 2(e)). Thus, DEB can be reduced in proportion to d, which also improves the lateral spatial resolution of STM-CL nanospectroscopy.
We also evaluated the agreement between DEB and DESD by profiling GaAs STM-CL (λ0 = 860 nm) across p-GaAs / p-AlGaAs hetero-junction at DESD = 100 nm as shown in Fig. 3(a). As found in our previous study [28], the STM-CL intensity profile across this hetero-junction is contrasted mainly due to the gap of carrier generation and recombination properties at the hetero-junction and slightly due to the minority carrier diffusion in AlGaAs layer. The broadening of STM-CL profile at the hetero-junction, DEB = 93 nm measured from 20% to 80% of the intensity in Fig. 3(b), agreed with DESD = 100 nm.
Fig. 3 (a) Evaluation of the agreement between DEB and DESD by profiling GaAs STM-CL (λ0 = 860 nm) across p-GaAs / p-AlGaAs hetero-junction at DESD = 100 nm; (b) STM-CL intensity profile; (c) Evaluation of θ = 51° FP light collection area with p-AlGaAs / i-GaAs / n-AlGaAs double-heterostructure cross-section with 100-nm-thick active layer under LED forward voltage and Cr-FP in STM mode; (d) EL intensity profile; (e) Room temperature luminescence spectra of Zn-doped p-GaAs (110) surface normalized by excitation power: He-Ne laser PL spectra collected by twin-lens optics [PL (Lens)], STM-CL spectra collected by twin-lens optics [STM-CL (Lens)] and Cr-FP [STM-CL (FP)]. Both STM-CL spectra were taken with the same Cr-FP in the inset of (e). (f) STM-CL peak intensity for varying FEEB excitation power; (g) STM-CL intensity normalized by FEEB excitation power.
In order to achieve universally high spatial resolution and high S/N ratio in STM-CL nanospectroscopy, STM-CL collection needs to be more local but efficient. Thus, the size of FP light-collection regime must be smaller than the minority carrier diffusion length but larger than that of minority carrier excitation regime whose lateral size is nearly equal to DEB. Figure 3(c) shows how we evaluated the lateral size of the Cr-FP light-collection regime in STM mode by using the 860 nm electroluminescence (EL) from the 100-nm-thick GaAs active layer in the p-AlGaAs / i-GaAs / n-AlGaAs double-heterostructure cross-section under the LED forward voltage [31]. The profile of FP light collection regime was evaluated from the broadened EL intensity profile across the GaAs active layer, which is explained both by the FP light collection angle and by the optical diffraction limit as detailed in Fig. S2(d) in Media 1-Supplementary Information S2. This FP light collection area, 1.1 μm in Fig. 3(d), is large enough to cover the lateral size of minority carrier generation regime, typically DEB < 100 nm, and is smaller than the minority carrier diffusion length, typically a few μm in bulk AlGaAs. Thus, the STM-CL at surface under the Cr-FP is efficiently collected blinding STM-CL via long minority carrier diffusion and STM-CL lateral resolution is given by DEB, which is proportional to d.
We also experimentally evaluated the STM-CL collection efficiency of a Cr-coated FP as well as conventional twin-lens optics (Fig. 3(e)–3(g)) with the p-GaAs (110) surface. In advance we confirmed that the p-GaAs (110) surface had no other luminescence peaks than its band-edge emission in the range 800–1700 nm. This means the density of deep-level states in the p-GaAs (110) surface is too low to provide competing radiative carrier recombination, which breaks the linearity of band-edge emission intensity with respect to the FEEB power. Figure 3(e) shows room-temperature luminescence spectra from a Zn-doped p-GaAs (110) surface normalized by their specimen excitation power: He-Ne laser PL spectra collected with PL twin-lens optics [PL (Lens)], STM-CL spectra collected with twin-lens optics [STM-CL (Lens)], and STM-CL spectra collected with the Cr-coated FP [STM-CL (FP)]. Both STM-CL spectra were taken with the same Cr-coated FP as shown in the inset of Fig. 3(e), whose cladding diameter was thinned so that the cladding would not block STM-CL in the twin-lens setup. Both the STM-CL spectra and reference PL spectrum showed a GaAs band-edge emission peak; however, STM-CL (FP) showed about 4 times higher luminescence intensity than STM-CL (Lens).
To investigate the difference in light collection efficiencies between the Cr-FP and conventional twin-lens optics, we studied the STM-CL peak intensity at different FEEB excitation powers, ITVT, as shown in Fig. 3(f). The linear relation with FEEB excitation power indicates that p-GaAs STM-CL emission is governed by minority electron concentration, and that its gradient is proportional to the STM-CL collection efficiency. The collection efficiency of the Cr-FP was 4.1 times higher than that of conventional twin-lens optics, which agrees well with the theoretical factor of 4.1 calculated from ΩLens = 0.157 sr, ΩFP = 1.94 sr (refer to Media 1-Supplementary Information S2), and 860 nm light transmittance of the 10 nm-thick Cr film (~0.33). We also studied the d-dependence of the FP light-collection efficiency to see the principle of STM-CL collection. The STM-CL intensity normalized by FEEB excitation power shows almost no d dependence at d = 50–150 nm, as shown in Fig. 3(g).
The efficiency of STM-CL collection by Cr-FP has no noticable d-dependence. The STM-CL collection is explained by far-field light collection and not by near-field light collection, which is featured by the intensity decrease with d ~dNF (≡ 2π/λ0) = 137 nm. STM-CL enhancement by SPP resonance is also absent when the FP is simply Cr-coated, which is featured by the decrease with d ~dSPP (≡ √(2rd) [33]) = 63 nm when r = 20 nm and d = 100 nm. Au-coating except FP apex may enhance 860 nm STM-CL through its SPP resonance without degrading the field-emission properties.
4. Conclusions
We developed a conductive glass optical fiber probe coated with a Cr thin film to obtain alignment-free optics that collects local STM-CL with high efficiency and high spatial resolution. Our geometric-optics-based model predicts that the FP has a maximum light collection solid angle of 1.94 sr when its half-opening angle is θ = 51.0°. A Cr-FP with θ = 51° was fabricated by optimized buffered hydrofluoric-acid etching of a glass optical fiber followed by 10 nm-thick Cr film coating. We demonstrated the following: (1) Cr-FP works both as a stable STM probe and a stable electron field-emitter with radial field-emission property. (2) Cr-FP works as far-field STM-CL collector whose light collection area is about 1.1 μm in diameter and smaller than typical minority carrier diffusion length: efficient local STM-CL collection is achieved and the lateral resolution is given by the FEEB diameter; (3) Cr-FP was proved to have higher STM-CL collection efficiency than conventional optics due to its large light collection solid angle and high optical transmittance of Cr thin film; and (4) SPP enhancement of 860 nm STM-CL by Cr thin film was absent, which can be achieved by additional Au coating on Cr-FP.
Acknowledgments
The authors are grateful to Dr. Shin-ichi Kitamura of JEOL Ltd. for FP-STM scanner preproduction, to Prof. Koji Maeda and his colleagues at The University of Tokyo for the electrochemical etching device and fruitful discussions, and to Prof. Motoichi Ohtsu and his colleagues at The University of Tokyo for their technical advice on optical fiber probe etching. This work was partially supported by JSPS KAKENHI (19710088, 20360015, 23760022).
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