Appl. Phys. B 88, 551–555 (2007)
Applied Physics B
DOI: 10.1007/s00340-007-2758-8
Lasers and Optics
e. esposito1,u fu-jen kao2,3 g. mcconnell1
Confocal optical beam induced current microscopy of light-emitting diodes with a white-light supercontinuum source 1
Centre for Biophotonics, Strathclyde Institute for Pharmacy and Biomedical Sciences, University of Strathclyde, 27 Taylor Street, Glasgow G4 0NR, UK 2 Institute of Biophotonics, National Yang-Ming University, Taipei 11221, Taiwan, R.O.C. 3 Institute of Electro-Optics Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, R.O.C.
Received: 17 May 2007 Published online: 22 August 2007 • © Springer-Verlag 2007 ABSTRACT To improve the efficiency of confocal optical beam induced current (OBIC) and the non-destructive, highresolution analysis of semiconductor media we report the application of a white-light supercontinuum laser source capable of confocal OBIC across a wide spectral range. To demonstrate the capability of this source, we performed confocal OBIC of light emitting diodes with varying absorption and emission properties in the visible spectrum. Using the wavelength flexibility afforded by the broadband laser source, we were able to determine and apply the optimum excitation wavelength range for efficient confocal OBIC instead of applying inferior fixed wavelength laser sources. PACS 87.64.Tt;
1
85.30.De
Introduction
Confocal optical beam induced current (OBIC) microscopy is a powerful and non-destructive method of analysing semiconductor devices such as integrated circuits and light emitting diodes in three dimensions and at high spatial resolutions [1]. The concept of confocal OBIC is similar to confocal fluorescence imaging where a single-mode laser source is focused using an objective lens into the sample and the laser beam position is then moved in a line or raster scan relative to the sample. In confocal OBIC imaging, the sample is typically a semiconductor device. Under irradiation with a laser source with an energy equivalent to or exceeding the bandgap of the device, the exciting photons generate electron-hole pairs in the sample. This resultant current flow can be detected using an appropriate external electronic amplifier, synchronised with the scan-head raster scan and subsequently used to build contrast images of the active regions within the device [1, 2]. Analysis of the resultant high-resolution OBIC images can then be used to visualise the active material within the device [3, 4], study structural defects and electrical processes [5] and to gauge and monitor device performance [6]. Confocal OBIC can be employed as a complementary technique to laser scanning fluorescence microscopy and u Fax: 44 (0)141 548 4887, E-mail:
[email protected]
hence OBIC images are typically captured using a standard laser scanning microscope and laser system. The laser system most frequently employed is a krypton-argon laser [7, 8], Argon ion laser [9] or He-Ne laser [10, 11] but these gas-based sources have several shortcomings in comparison with solidstate lasers. These gas lasers generally require more maintenance, are not typically energy efficient and have shorter operational lifetimes. Additionally, an increasingly important limitation in confocal OBIC microscopy is the lack of suitable wavelengths for excitation of the growing range of semiconductor devices. The gas-based laser systems emit only at discrete wavelengths and for confocal OBIC, it is clearly necessary to supply an excitation wavelength that is matched to the semiconductor sample [6]. Hence for truly efficient and responsive confocal OBIC imaging of a progressively diverse range of semiconductor samples and structures, improvements in laser technology are necessary. To facilitate improved confocal OBIC, we have employed a white-light supercontinuum laser source with a conventional laser scanning microscope. The white-light laser source can possess a wavelength span in excess of an optical octave [12] and by employing subsequent wavelength filtering methods is an extremely wavelength-flexible source [13–15]. Through this wavelength tunability, confocal OBIC imaging of semiconductor devices with emission from the ultraviolet through to the infrared regions of the spectrum would be possible. To demonstrate the power and capability of this source, we performed confocal OBIC of light emitting diodes with varying absorption and emission properties across the visible spectrum. Using the wavelength flexibility afforded by the whitelight supercontinuum, we were able to determine and apply the optimum excitation wavelength range for efficient confocal OBIC imaging instead of applying inferior fixed wavelength laser sources. 2
Experiment
The experimental geometry is shown schematically in Fig. 1. To create the white-light supercontinuum source, we employed a 76 MHz repetition rate, fs-pulsed Ti:sapphire laser source (Coherent Mira-900) pumped by a continuous wave frequency-doubled Nd3+ :YVO4 source with 10 W average power (Coherent Verdi). The Ti:sapphire laser source was tuned to a wavelength of λ = 783 nm. To
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Applied Physics B – Lasers and Optics
FIGURE 1 Schematic diagram of the OBIC signal detection experimental setup. A/D is the analogue to digital converter, 1 and 2 are the input and output coupling lenses to the PCF
minimise feedback from optical elements used outwith the Ti:sapphire laser, a Faraday isolator (Leysop) with 30 dB isolation was used. The average power at the output of the Faraday isolator was measured to be 950 mW. This light was then focused using a single anti-reflection coated spherical lens f = 4.2 mm (Thorlabs) into a 2 m long section of photonic crystal fibre (NL-1.8-750, Crystal Fibre). This highly nonlinear PCF was used to generate the white-light supercontinuum, as described in detail in previous experiments [16–19]. The white-light supercontinuum emitted by the PCF was collimated using another single anti-reflection coated spherical lens of f = 6.4 mm (Thorlabs). To ensure reliable performance, two high-precision fibre translation stages were used to mount the input and output ends of the PCF. The available white-light supercontinuum spectrum was filtered using an appropriate band pass filter prior to reaching the sample under investigation. The filters available were λ = 488 ± 10 nm, λ = 500 ± 20 nm, λ = 600 ± 20 nm and λ = 650 ± 20 nm (Comar), with a transmission of approximately T = 75% for each filter. The chosen wavelength band was then coupled into a galvo-mirror based laser scanning microscope (Fluoview FV-300+IX-71, Olympus) to provide beam scanning and focusing into the sample. A current mode pre-amp (SR570, Stanford Research) was used to manipulate the OBIC signal for use with the applied A/D converter (working range 0 ∼ 10 V) within the scanning unit. The gain of the circuit was fixed such that the relative strength of OBIC signal from various wavelengths could be easily compared. The objective lens was a 10-times magnifying lens with a 0.25 numerical aperture, corrected for flat field operation in the visible spectral region. The supercontinuum output spectrum was coupled into a conventional optical fibre connected to a spectrometer (Ocean Optics USB 2000). A typical supercontinuum output spectrum is presented in Fig. 2. The total power measured (including white-light supercontinuum and undepleted Ti:sapphire pump radiation) at the output of the fibre was 800 mW. OBIC images were produced by exciting the LEDs with these discrete wavelength bands extracted from the white-light supercontinuum. Two types of LED samples were studied, one with λ = 600 nm emission wavelength and the other with λ = 630 nm emission wavelength (C5-434-80 and C5-435-80SOL, Daina Electronics, Taiwan). Figure 3 shows the electroluminescent spectrum produced by the two LEDs under investigation as well as the wavelength bands covered by the filters used to
FIGURE 2
Typical unfiltered supercontinuum spectrum emitted from the
PCF
FIGURE 3 Electroluminescence spectra of the LEDs under investigation and wavelength bands covered by the chosen filters
extract the chosen wavelength ranges from the white-light supercontinuum source during the experiment. Throughout the experiment, the emission spectra of the LEDs were characterised by a fibre coupled spectrometer (USB2000, Ocean Optics) with a 16-bit dynamic range. The OBIC contrast image produced by the system had a resolution of 512 × 512 pixels and a depth of 12 bits. To measure and compare the OBIC signal from different devices, we employed freely available image analysis software (ImageJ, NIH). Regions of interest (ROI) within the contrast images were prescribed and a histogram was constructed from each ROI to count the number of pixels at a given pixel intensity. With the sample held in a fixed position and by varying the optical bandpass filter (hence changing the excitation wavelength applied to the sample), wavelength-dependent behaviour of the OBIC signal could be studied. To allow direct comparison between images taken at different amplification before analogue to digital converter, the pre-amplifier current gain was selected at a few fixed values in the linear working regime. The linearity of the electronic response was ensured by varying the gain and the incident laser intensity while monitoring the signal strength accordingly. Care was taken to ensure homogeneity within the region of interest to allow suitable and direct comparison; this was verified by considering the standard deviation provided with the mean for each measurement in the selected ROI.
ESPOSITO et al.
OBIC microscopy using white light supercontinuum
Given that the supercontinuum source did not have a flat amplitude across the broad wavelength range, it was also necessary to compensate for the amount of light applied to the sample when quantifying and comparing the OBIC signal for each sample. Thus, in order to weight the OBIC signal against the amount of light available after a particular filter, the transmitted spectrum was measured for each applied filter. Each spectrum was integrated and normalised to the total unfiltered spectrum to obtain a relative value of the energy available for each filter. We refer to this ratio as the wavelength weight factor (WWF) and this is defined as: Wavelength weight factor (WWF) =
=
Filtered light intensity Total light intensity λ2 I(λ) dλ λ1 λ R
.
(1)
I(λ) dλ
λB
Where
λ2
I(λ) dλ corresponds to the irradiance profile ac-
λ1
quired by the spectrometer and integrated between the two extreme wavelengths λ1 and λ2 for a particular filter; and λR I(λ) dλ corresponds to the total irradiance of the superconλB
tinuum spectrum when no filter is applied integrated between λB and λR corresponding to the two extreme detectable wavelengths of the spectrometer. A comparable value for the OBIC signal was obtained by taking the product of the mean of the region of interest of a particular sample and the amplifier gain divided by the WWF: OBIC Signal(λ) =
MeanHistogram × Gain . WWF
(2)
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By applying this weighting factor and through analysis of the OBIC images, our aim was to determine the optimum excitation wavelength for any given sample and hence limit the dose of radiation to the sample through more effective excitation. 3
Results
Reflection and confocal OBIC images of two AlGaInP LEDs are displayed in Figs. 4a–c and 5a–c. Figure 4 shows a λ = 600 nm emitting LED (C5-434-80, Daina Electronics, Taiwan) while Fig. 5 shows a λ = 630 nm emitting LED (C5-435-80SOL, Daina Electronics, Taiwan). The reflection and merge images showing the active region of the LEDs shown in Figs. 4b and c and 5b and c were obtained using the standard chromatic reflector and source blocker filters located within the scanhead. This was done to reveal the structure of the active layer. However, in order to maximise the OBIC signal generated all the light coupled into the scanhead was directed towards the sample by replacing the chromatic reflector with a mirror that was highly reflecting across the visible spectral range. Using standard freely available image analysis software to define multiple ROI as previously described we were able to accurately measure the OBIC signal intensity within that region. In processing, the 12-bit image was reduced to 8 bits, i.e. a regular TIFF format. Given the wavelength flexibility of the excitation source, we could then determine the dependence of the resultant OBIC signal upon the incident wavelength extracted from the white-light supercontinuum. Figure 6a and b respectively reveal the wavelength dependence on OBIC signal for the 600 nm LED and 630 nm LED for multiple ROI and at various excitation wavelengths. We could then apply the WWF as described by (2) to quantify and compare the OBIC signal across a range of excitation wavelengths. Given that the emission wavelength of the LED was approximately λ = 600 nm, we anticipated that the optimum excitation wavelength for the maximum OBIC signal would
FIGURE 4 (a) OBIC image of a 600 nm emitting LED marked with three ROI (b) reflection image and (c) merge of image (a) and image (b) for the same device. The scale bars correspond to 100 µm
FIGURE 5 (a) OBIC image of a 630 nm emitting LED marked with two ROI (b) reflection image and (c) merge of image (a) and image (b) for the same device. The scale bars correspond to 100 µm
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Applied Physics B – Lasers and Optics
FIGURE 6 OBIC signal intensity measured across a range of wavelengths and for multiple ROI for (a) the 600 nm emitting LED and (b) the 630 nm emitting LED. In both (a) and (b), the error bars correspond to ±5% error
be approximately λ = 600 nm. Indeed, it is clear from Fig. 6a that this sample behaves as anticipated, with the maximum OBIC signal at least twice that obtained for other excitation wavelengths. However, similar measurements and analysis of the 630 nm LED as shown in Fig. 6b clearly indicates that the optimum excitation wavelength for maximum OBIC signal generation was approximately λ = 550 nm. 4
Discussion
It is clear that the white-light supercontinuum source is well-suited to confocal OBIC. Although we only present the results from a few samples to demonstrate the technique, the wavelength flexibility provided by the source enables the excitation of a very wide range of samples. The results obtained for the λ = 600 nm emitting LED were entirely as anticipated, with a strong confocal OBIC signal measured in the three regions of interest using the configuration as described. However, the results for the λ = 630 nm LED were unexpected, with the optimum excitation wavelength of the source observed to be much shorter than that anticipated in each of the regions of interest. From previous investigations, this discrepancy is not attributed to amplitude instability of the white-light supercontinuum source [20]. Instead, the discrepancy between the absorption OBIC spectrum and the emission spectrum of the 630 nm LEDs is likely to be a consequence of energy relaxation of injected carriers, in a manner similar to the excitation and emission of fluorophores. Before the light emission, the internal energy relaxation pathways (such as carrier-lattice or phonon scattering, or vibrational relaxation in the case of molecules) dissipate the energy of the carriers (or excited fluorophores) until they reach the energy minimum in the excited states. The emission thus takes place at a lower energy. If there is no such discrepancy, it seems the process of energy relaxation may be slow when compared with emission. From this we can conclude that it is crucial for optimum confocal OBIC to employ a wavelength tunable source in order to closely match the excitation wavelength to the sample under investigation. The white-light supercontinuum source easily provides such wavelength flexibility with ample average power for confocal OBIC studies of semiconductor media. Furthermore, the simple experimental design could be easily adapted to perform high-throughput, high-resolution
and nondestructive morphological analysis of an extremely wide range of samples and structures. 5
Conclusion
In summary, we employed a white-light supercontinuum laser source to improve the capacity of confocal OBIC. To demonstrate the capability of this source, we performed confocal OBIC of light emitting diodes with varying absorption and emission properties in the visible spectrum. Using the wavelength flexibility afforded by the reported laser source, based on white-light supercontinuum generation, we were able to determine and apply the optimum excitation wavelength range for efficient confocal OBIC instead of applying inferior fixed wavelength laser sources. Contrary to our expectations, the optimum excitation wavelength did not consistently match the anticipated excitation wavelength corresponding to the bandgap of the material, even when we compensated for the differences in average power available from the excitation source at different wavelength regions. This confirmed that for optimum confocal OBIC it is crucial to employ a wavelength tunable source in order to closely match the excitation wavelength to the sample under investigation. The white-light supercontinuum is capable of providing an extremely wide range of excitation wavelengths suited to a diverse range of semiconductor media and hence this light source offers significant advantages over traditional fixed wavelength laser sources. Also, with the white-light supercontinuum sources now extending to even shorter wavelengths [21], excitation of UV LEDs through the same methods could now potentially be performed. We have demonstrated the technique using visible-wavelength emitting LEDs, however it is clear that the white-light supercontinuum source could easily be applied to alternative semiconductor structures and devices, e.g. laser diodes, solar cells and we will investigate this in due course. Furthermore, the ultrashort pulsed nature of the supercontinuum output is ideally suited for time-resolved OBIC measurements. This will be the subject of further study. ACKNOWLEDGEMENTS The authors are grateful to the Royal Society, the Ministry of Education and National Science Council of Taiwan (under the grants “Aim for Top University Plan”, NSC95-2112-M-010-
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OBIC microscopy using white light supercontinuum
001, and NSC 95-3112-B-010-015) and Research Councils UK for financial support.
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