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Electronic Imaging & Signal Processing
Visible and infrared light-emitting diodes provide stable broadband output
By incorporating a transverse p-n junction with multiple-quantum-wells that have varying center wavelengths, LEDs can generate white-light spectra that stay stable across various of bias currents.
8 August 2007, SPIE Newsroom. DOI: 10.1117/2.1200708.0695
Broad optical spectra at IR and visible wavelengths are useful for a wealth of applications including optical coherence tomography (OCT),1 solid-state lighting,2 back-light sources for displays,3 and optical fiber communications.4 As a result, techniques for broadening the optical spectra of phosphor-free GaN-based visible LEDs,5,6 InP-based IR superluminescent diodes (SLDs),7 or semiconductor optical amplifiers (SOAs) that can be driven by injected current, are all very attractive. The main problem with such techniques, however, is that the devices’ optical bandwidths are limited by non-uniform carrier distributions in the multiple-quantum-well (MQW) layers.7
Our recent work on transverse-junction (TJ) LEDs seeks to overcome this problem. Both GaN-based TJ-LEDs for visible white-light and InP-based TJ-LED for infrared ‘white’ light (broadband IR)) exhibit tremendously wide and extremely flat optical spectra with a shape that doesn't change even when the bias varies from low to high currents.8,9Infrared white-light TJ-LED
Figure 1 (a) and (b) show cross-sectional and top views of the InP-based TJ-LED. We use InxAlyGa1-x-yAs layers with a 0.5% compressive-strain, different mole fractions (x), and a fixed thickness (10nm) to serve as the well layers in our MQW regions. The p-type region of the transverse p-n junction is created via Zn diffusion and a disordering process.8 We then trench the topmost In0.53Ga0.47As layer, as shown in Figure 1(a), and metallize the p- and n- contact layers. The major difference between traditional LEDs or SLDs and the device we demonstrated is that in our structure the p-n junction is located in the transverse direction and the lateral injection current will result in a uniform distribution of injected carriers in each quantum well (QW) layer. The problems of a non-uniform gain spectrum due to the accumulation of injected carriers near the p-side QW7 can thus be eliminated.
Figure 1. (a) The conceptual cross-sectional view of a transverse-junction (TJ) LED. (b) Top-view of a demonstrated infrared TJ-LED. MQWs: multiple quantum wells.
Figure 2 show the measured optical spectra for the devices when operated under a range of bias currents at room temperature. We can clearly see that when the current exceeds 40mA, the spectra start to flatten and broaden. To the best of our knowledge, this is the widest optical spectrum reported to date for InP-based SLDs or LEDs that covers all the available wavelengths for long-haul communications. Figure 3 shows the full-width-half-maximum (FWHM) spectra that we measured from these three devices vs. the bias current. The behavior we observed from our device is very different from that reported for the SLD, for which the FWHMs of the spectra increase monotonically with the bias current.7 The inset to Figure 3 shows the total collected output optical power from two cleaved facets vs. the bias current. For this measurement, devices were mounted on a thermoelectric cooler and the temperature was fixed at 20°C. The maximum total collected optical power of device was around 30μW with a 3dB optical bandwidth around 520nm. Higher power can be expected by minimizing the leakage current in our device and further increasing its active length.8
Figure 2. The measured electroluminescent (EL) spectra of our device remains remarkably stable under different bias currents. FWHM: full width at half maximum.
Figure 3. The FWHM of measured optical spectra vs. bias current. The inset shows the collected output optical power (P) vs. bias current (I) of the device when held at 20°C.
Figure 4. The conceptual band diagram and the epi-layer structure of demonstrated near-white-light LED. CB:conduction band. VB: valence band.
Figure 5. Near-white-light LED: (a) top-view; (b) conceptual cross-section.
Figure 6. The output power vs. current of three devices at room temperature clearly shows that the most efficient device has the narrowest trench. The propagation loss (3dB) of the optical fibers has been corrected.
Near visible white-light TJ-LED
Figure 7. The EL spectra of our device under different bias currents. The inset shows the same spectra after normalization.
The main difference between fabrication processes for the InP-based and GaN-based TJ-LEDs lies in forming the transverse p-n junction. For the GaN-based material, it is difficult to create a p-type region with high material quality and high ionized doping density using either thermal diffusion or ion-implantation. We thus changed our process flow and created the n-type region on the topmost p-type GaN layer by Si implantation.9 Figure 4 shows the conceptual band diagram and the adopted epi-layer structure of our GaN TJ-LED. The active MQW layers were composed of interlaced blue (∼450nm) and green (∼560nm) InxGa1-xN/GaN-based QWs. These are sandwiched between the top p-type GaN and bottom n-type GaN layers. Figure 5 (a) and (b) shows the top-view and conceptual cross-section of the fabricated devices, respectively. After the implantation process, a ring shape was trenched into the upper p-type GaN layer, as shown in Figure 5(a). This forces the injected current through the buried MQWs. Finally, we metallized the contact layers. Figure 6 shows the output power versus current of three devices with different trench widths. The inset to Figure 6 shows the measured I-V curves of these devices. All three show rectifying behaviors with turn-on voltages of around 4V. The output power and external efficiency of device C is the highest because it has the largest emitting area and smallest trench-width of the three devices. Figure 7 shows the EL spectra of device C measured at different bias currents. The inset to Figure 7 shows the same spectra after normalization; each trace is normalized to its maximum. We can clearly see that the shape of each spectrum does not change with the bias current and exhibits only a broad peak with an optical bandwidth that ranges from about 440nm to 560nm wide, even under highest bias current (60mA). The performance of the electroluminescent (EL) spectra achieved is superior to that reported for phosphor-free white-light GaN LEDs.5,6 The latter devices usually exhibit much more serious bias dependence in their EL spectra, or the blue or or green-emission lines dominate the measured spectra, due to the accumulation of injected holes near the p-side QWs under high bias current.5,6 Furthermore, the blue-shift in center wavelengths that is observed in typical GaN LEDs—due to the screening of the piezo-electric field under high bias current—doesn't appear in our device.Summary
We demonstrated a novel light-emitter structure with a transverse p-n junction and several MQWs with different center wavelengths designed to effectively maximize the optical bandwidth performance. Both the devices at infrared and visible wavelengths exhibit almost-invariant and flat white-light spectra with tremendously wide optical bandwidths under a wide range of bias currents without using an additional wavelength converter. Higher output optical power can be expected by improving the p-n junction quality and increasing the active areas of our devices.
Jin-Wei Shi, J.-I. Chyi
Department of Electrical Engineering
National Central University
Jin-Wei Shi received his BS a in Electrical Engineering from National Taiwan University, Taipei in 1998 and his PhD from the Graduate Institute of Electro-Optical Engineering at National Taiwan University in 2002. He is an associate professor in the Electrical Engineering Department of National Central University. His current research interests include ultra-high speed/power optoelectronic devices and white-light LEDs.
J.-K. Sheu, W.-C. Lai
Institute of Electro-Optical Science and Engineering
National Cheng-Kung University
Department of Electrical Engineering
National Taiwan University
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