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InGaN light-emitting diode (LED) structures get an air-void structure and a tapered GaN structure at the GaN/sapphire interface through a laser decomposition process and a lateral wet etching process. The light output power of the treated LED structure had a 70% enhancement compared to a conventional LED structure at 20 mA. The intensities and peak wavelengths of the micro-photoluminescence spectra were varied periodically by aligning to the air-void (461.8nm) and the tapered GaN (459.5nm) structures. The slightly peak wavelength blueshift phenomenon of the EL and the PL spectra were caused by a partial compressed strain release at the GaN/sapphire interface when forming the tapered GaN structure. The relative internal quantum efficiency of the treated LED structure (70.3%) was slightly increased compared with a conventional LED (67.8%) caused by the reduction of the piezoelectric field in the InGaN active layer.
©2011 Optical Society of America
1. Introduction
GaN-based light emitting diodes (LEDs) have attracted interest because of their wide applications in a variety of products such as traffic signals, as backlights in liquid crystal displays, and as solid-state white lighting sources [1], etc. The lower external quantum efficiency of the InGaN-based LEDs is due to a larger refractive index difference between the GaN layer and the surrounding air (Δn~1.5). Photonic crystal structure formations [2,3], periodic deflector embedded structures [4], periodically oriented embedded air protrusion structures [5], polydimethylsiloxane concave microstructures [6], inverted micropyramid structures [7], lateral epitaxial overgrowths using pyramidal-shaped SiO2 [8], and roughened N-face GaN surfaces at GaN/Al2O3 interfaces [9], embedded rhombus-like air-void structures [10], hexagonal conelike surfaces using laser-lift-off technique [11], and ZnO nanotips prepared by aqueous solution deposition [12] have all been used to increase light-extraction efficiency in InGaN-based LEDs on Al2O3 substrates.
In this paper, a tapered GaN structure and an air-void structure at the GaN/sapphire interface were fabricated in the InGaN-based light-emitting diode (LED) structure through a laser decomposition process, a lateral etching process, and a wet etching process. The lower temperature grown AlN/GaN buffer bi-layers acted as a lateral wet etching sacrificial layer and a laser decomposition layer. The laser-decomposed air void structure and the lateral wet etched tapered GaN structure can increase the light extraction efficiency of LED devices. The compressed strain of the InGaN-based LED structure was partially released from the forming of the tapered GaN and the air-void structures at the GaN/sapphire interface. Here, the optical properties of the InGaN LED structures are analyzed in detail.
2. Experiments
InGaN-based LED structures were grown on two-sided polished optical-grade c-face (0001) 2”-diameter sapphire substrates by using a metalorganic chemical vapor deposition (MOCVD) system. These LED structures consisted of low-temperature grown 15nm-thick AlN/15nm-thick GaN buffer bi-layers, a 10.4μm-thick n-type GaN layer, 10 pairs of the InGaN/GaN multiple quantum wells (MQWs) active layers, and a 0.2μm-thick magnesium-doped p-type GaN layer. The active layers consisted of a 35Å-thick InGaN-well layer with a 15% indium content and a 120Å-thick GaN-barrier layer. The growth temperatures of the AlN and GaN buffer layers were 770°C and 550°C, respectively. The growth temperatures of the AlN/GaN buffer bi-layers were lower than the high temperature (at 1150°C) growth of the GaN epitaxial layer. The GaN buffer layer can be decomposed as Ga metal and N2 gas by absorbing the 355nm pulse laser. The air-void structure was formed at the laser scanning lines through the wet etching process in a hot KOH solution. The lateral wet etching process occurred at the AlN buffer layer as a sacrificial layer to form the tapered GaN structure.
The LED wafer was treated by using a triple frequency ultraviolet Nd:YVO4 (355nm) laser for a front-side laser isolation process and for a backside GaN decomposition process. The conditions of the 355nm Nd:YVO4 laser, with 1KHz repetition rate, were 500μJ (with a 5μm-diameter laser spot size) for a front-side laser isolation process and 10μJ (with a 10μm-diameter laser spot size) for a backside GaN decomposition process, respectively, that were controlled through a neutral density filter. The dimension of the chip was 570×240 µm2 in size defined through a front-side laser isolation process. The mesa region of 540×210 µm2 with a 1.0µm-depth was defined through an inductively coupled plasma (ICP) etcher. The treated rectangular pattern with an area of 277×210μm2 was fabricated through a backside laser scan/decomposition process. The ratio of the roughened area divided by the mesa area was measured at 51% for the Tapered-LED (TP-LED) structure. The schematic of the TP-LED structure is shown in Fig. 1 . The GaN buffer layer decomposed as Ga metal and N2 gas through the laser scanning process, and the lateral etching process occurred on the AlN buffer/sacrificial layer to form the tapered GaN structure at the GaN/sapphire interface. The fabricated procedures consisted of a laser isolation process around the mesa (step 1), a step-scanning backside laser decomposition process (step 2), an N-face crystallographic etching process (step 3), and a lateral etching on the AlN buffer layer (step 4). The treated LED wafer was immersed in a hot KOH solution (KOH, 80°C) for a 15-minute bottom-up crystallographic wet etching process [13]. The N-face crystallographic etching process (step 3) occurred at the laser treated region to form an N-face cone-shaped GaN structure and an air-void structure at the GaN/sapphire interface through a wet etching process in a KOH solution. The N-face cone-shaped GaN structure didn’t come in contact with the sapphire substrate in the step 3 process. Next, the lateral etching on the AlN buffer layer (step 4) occurred from the laser treated air-void structure to the un-treated mesa region between the two laser scanning line patterns. The N-face cone-shaped GaN structure came in contact with the sapphire substrate through the lateral wet etching process (step 4). Both step 3 and step 4 were the wet etching processes in the hot KOH solution. The N-face cone-shaped GaN structure without (step 3) and with (step 4) contact with the sapphire substrate was observed in the TP-LED devices.
Fig. 1 The schematic diagrams and fabricated procedures of the TP-LED structure with a tapered-GaN structure at GaN/sapphire interface were shown here.
The decomposition processes consisted of a 0.5mm/s laser scanning speed, a 355nm pulse laser with 1KHz repetition rate, a 10μm-diameter laser spot size, and a 40μm-spacing between the laser scanning lines. The GaN buffer layer absorbed the 355nm laser energy to decompose as Gallium metal and Nitrogen gas, because the 355nm laser energy is larger than the GaN energy bandgap (365nm). The residual Ga metal and the N-face GaN epitaxial layer have been etched in a hot KOH solution to form the air void structure at the laser scanning patterns. A 240nm-thick indium-tin-oxide layer (ITO) was deposited on the mesa region as a transparent contact layer (TCL). The Cr/Au metal layers were deposited as n-type and p-type contact pads. The LED device fabricated through the standard process flow without a laser decomposition process was defined as a standard LED (ST-LED). Both of the LED structures were fabricated on the same LED epitaxial wafer. In Fig. 3 , the chosen TP-LED and ST-LED are located side by side at the 2” wafer center, with and without laser treatment, to allow for analysis of the optical and electrical properties in similar material properties. The geometric morphology of these LED structures was observed through a scanning electron microscopy (SEM). The optical properties of the LED samples were measured on the basis of the photoluminescence (PL) spectra using a 100 mW, 405 nm InGaN laser diode as the excitation source. The PL spectra, electroluminescence (EL) spectra, and light output power were characterized by an optical spectrum analyzer (Ando-6315), an Agilent 4156C precision semiconductor parameter analyzer, and a beam profiler (Spiricon: effective pixels: 1600 × 1200 pixels), respectively.
Fig. 3 Microscopy images of the ST-LED and the TP-LED structures (a) with front-side light illumination (b) with back-side light illumination were observed. The light-intensity profiles of (c) the ST-LED structure (d) the TP-LED structure at a 20mA operation current are measured by a beam profiler.
3. Results and discussion
The laser decomposition regions of the TP-LED structure were observed on the cross-section SEM micrographs shown in Figs. 2(a) and 2(b). The laser scanning region with the air-void structure and the lateral wet etching region with a continuous tapered GaN structure were also observed. The period width of the laser treated region and the lateral etching region was measured at a value of 40μm as shown in Fig. 2(a). The bottom-up etching process occurred on the N-face GaN layer vertically along the [0001] direction.
Fig. 2 (a) The cross-sectional SEM micrographs of the laser scanning region and the lateral wet etching region at GaN buffer layer with a 40μm period width. (b) The cross-sectional SEM micrograph of the air-void structure at the laser scanning region through the N-face crystallographic wet etching process.
The anisotropic etching occurs with a continuous consumption of the () N-face and the gradual exposure of the stable {} terminal faces of the GaN layer [14,15]. The direction of the laser scanning line is perpendicular to the primary flat plane at the <> direction of the sapphire substrate. The periodical void structures as a line-pattern were formed at the laser treated line region where the cone-shaped GaN structure didn’t come in contact with the sapphire substrate. The GaN buffer layer was decomposed by the 355nm pulse laser. The lateral wet etching region between the two laser treated line-patterns was formed as the cone-shaped GaN structure that did come in contact with the sapphire substrate. Then, the periodical void structure was observed at the laser treated line-patterns on the mesa region. In Fig. 2(b), a 10μm-width and a 3μm-height air-void structure with a top cone-shaped GaN structure was fabricated through a laser decomposition process and an N-face crystallographic wet etching process. The lateral etching process occurred from the laser-scanning air-void region and formed a 30μm-width tapered structure at GaN/sapphire interface with a 1μm/min lateral etching rate. The treated LED wafer was immersed in a hot KOH solution (KOH, 80°C) for a 15-minute bottom-up crystallographic wet etching process. The wet etching process occurred from the sample edge to the center region along to the laser scanning line to form the air-void structure. Then, the lateral wet etching process on the AlN sacrificial buffer layer occurred from the laser treated region to the non-treated region. The N-face GaN surface at the sample edge region had been exposed more time in the KOH solution that had a larger cone-shaped structure compared with the sample center region. The optical microscopy image of the ST-LED and the TP-LED structures are shown in Figs. 3(a) with front-side light illumination and Fig. 3(b) with back-side light illumination. The slightly bright region labeled on the mesa region has a backside roughened tapered-GaN structure shown in Fig. 3(a). The dark regions are observed at the metal pads and the treated region for the TP-LED structure shown in the OM image with back-side light illumination.
A small size, high density, and uniformly distribution tapered GaN structure was formed at the treated region (277×210μm2) when compared to our previous report [13]. The laser-treated induced air-void structures with the line patterns along the laser scanning direction are observed in Fig. 3(b) with the darkened straight-line patterns. To analyze the light-intensity distribution over the entire LED chip at a 20mA operating current, the EL light intensity profiles are observed in Fig. 3(c) for the ST-LED and in Fig. 3(d) for the TP-LED. The uniform light emission intensity is observed at the mesa region of the ST-LED structure. The light emission intensity of the TP-LED structure is almost saturated in the beam profiler image at the treated region compared to the untreated region and the ST-LED structure.
In Fig. 4(a) , the peak wavelengths of the EL spectra are measured at 464.3nm for the ST-LED and at 464.1nm for the TP-LED at a 20 mA operating current. The EL emission intensity of the TP-LED structure is higher than the ST-LED structure. In the ST-LED structure, the interference phenomenon of the EL spectrum indicated that there were flat surfaces at the top p-type GaN:Mg layer and at the bottom GaN/sapphire interface. There was no interference phenomenon for the TP-LED structures because of the high light scattering process that occurred on the roughened tapered-GaN structure at the GaN/sapphire interface.
Fig. 4 (a) The EL spectra of the both LED structures were measured at 20 mA. (b) The current-voltage (I-V) characteristics and the light-output power as a function of the operating current are measured.
In Fig. 4(b), the operating voltage and the light-output power of the LED structures are measured by varying the injection current. At a 20mA operation current, the operating voltages of both LED structures were almost the same at the value of 3.6 V. The laser-treated regions were located at the GaN/sapphire interface without having any effect on the top InGaN active layer. The light output power of the TP-LED structure had a 70% enhancement compared to the ST-LED structure at a 20 mA operation current. So, the TP-GaN structure was continuously tapered from a film layer to a sharp cone-tip structure to have higher light reflectance [16].
In Fig. 5 , the peak wavelength and the linewidth (the full width at the half maximum, FWHM) of the EL spectra are measured by varying the injection current. At 20mA, the peak wavelength and the linewidth of the EL spectra were measured at 464.3nm/21.6nm and 464.1nm/21.6nm for the ST-LED and the TP-LED, respectively. By increasing the injection current, the wavelength blueshift phenomenon occurring in the InGaN-based LED structure is due to the band filling effect that increases the injection carriers into the tilted band diagram in the InGaN well structure. The tilted band diagram in the InGaN layer is caused by a compression strain-induced piezoelectric field. The slightly wavelength blueshift phenomenon of the EL spectra can be observed in the TP-LED when compared with the ST-LED by varying the injection current from 1mA to 68mA. The linewidths of the EL spectra are 23.6nm and 24.1nm for the ST-LED and the TP-LED, respectively, at a 68mA operation current. The broadened EL linewidth of the TP-LED structure is caused by reducing the heat dissipated area at the GaN/sapphire interface with the tapered-GaN and air void structures at a high injection current that doesn’t affect the device’s performance at a typical 20mA operation current.
Fig. 5 The peak wavelength and the FWHM of the EL spectra were measured by varying the injection current. The peak wavelength and the FWHM of the EL spectra were measured at 464.3nm/21.6nm and 464.1nm/21.6nm for the ST-LED and the TP-LED, respectively, at a 20mA operation current. The thermal heat of the EL spectra were observed in the TP-LED structure with the peak wavelength red-shifted and line-width broadened phenomena.
The PL spectra of both LED structures were measured at 300K and 10K as shown in Fig. 6(a) for the TP-LED and 6(b) for the ST-LED. The laser spot size (200μm diameter) and the excitation power density (0.32kW/cm2) were the PL measuring conditions. The peak wavelengths of the PL spectra were measured at the values of 462.4nm/465.2nm and 463.8nm/465.9nm at 10K/300K for the TP-LED and the ST-LED structure, respectively. The PL intensity of the TP-LED structure was higher than the ST-LED structure when forming the tapered GaN and the air-void structures at GaN/sapphire interface. At 300K, the peak wavelength of the TP-LED structure (465.2nm) had a slight wavelength blueshift phenomenon compared to the ST-LED structure (465.9nm). The TP-LED and the ST-LED devices were located side by side on the same LED wafer. The peak wavelengths of the ST-LED devices surrounding the treated TP-LED were very close, almost the same, at 465.9nm. The wavelength deviation from the center to the edge of the two-inch LED wafer was about 3nm containing 42 pieces of LED chips. The wavelength difference from one LED chip to its neighboring LED chip was about 0.07nm (3nm/42pieces LED chips), smaller than the 0.7nm peak wavelength blueshift phenomenon of the TP-LED structure. The slight peak wavelength blueshift phenomenon of the TP-LED structure has been repetitively measured compared with the surrounding ST-LED devices. The detailed peak wavelength blueshift phenomenon of the treated mesa region was analyzed in a line-scanning PL emission intensity profile. In the TP-LED structure, the slight PL peak wavelength blueshift phenomenon of the treated region on the mesa region was observed to be affected by the partially compressed strain release from the GaN/sapphire interface. The piezoelectric field in the InGaN well structure was partially reduced by forming the tapered GaN structure at GaN/sapphire interface.
Fig. 6 The PL spectra of (a) the TP-LED and (b) the ST-LED structures are measured at 10K and 300K. (c) The integral PL intensities of both LED structures are measured by varying the measurement temperatures. (d) The line-scanning PL emission intensity profile, scanning from non-treated region to treated region shown in the inserted OM image, is measured by the μ-PL measurement, and the periodic peak intensities and wavelengths of the μ-PL spectra are observed in the TP-LED structure.
Figure 6(c) shows that the Arrhenius plot of the integrated PL intensity obtained from the MQW active layer emission by varying the temperature range from 10 to 300 K has a clearly observable thermal quenching at the higher temperature. The relative internal quantum efficiencies in the MQW active layers are calculated by dividing the integrated PL intensity measured at 300 K (I300K, room temperature) and by the integrated PL intensity measured at 10K (I10K, low temperature). The PL intensities of both LED structures were almost saturated at the measured temperatures below 50K, because the recombination efficiency of the photo-excited electrons and holes was almost 100% at 10K [17]. The integrated PL intensity ratios (I300K/I10K) are defined as the relative internal quantum efficiency in the MQW active layer. The relative internal quantum efficiencies are calculated at 67.8% for the ST-LED and at 70.3% for the TP-LED structure, respectively. The slightly higher internal quantum efficiency and the PL wavelength blueshift phenomenon are measured in the TP-LED structure with the tapered GaN at the GaN/sapphire interface when releasing the compressed strain in the treated LED structure.
The EL spectra of both LED structures were measured in an integrating sphere where the emission light from all directions was collected. The PL emission light was collected at the normal direction of the LED chip. By forming the tapered GaN structure at GaN/sapphire interface, the light extraction efficiency of the TP-LED structure was higher at the normal direction compared to that of the ST-LED structure similar to the AlGaInP-based LED structure [18]. The relative PL intensity and the intensity enhancement ratio of both LED structures cannot be compared to that of the PL emission light from all directions. In the PL measurement, the apparent temperature invariance of the InGaN LED structure was caused because the top p-type layer and the n-type GaN layer didn’t absorb the 405nm laser. The InGaN active layer was excited by the lower energy and the continuous-wavelength 405nm laser compared to the 355nm pulse laser source for the laser decomposition process. During the temperature dependent PL measurement, the LED wafers were mounted on the copper stage to control the measured temperature.
The peak wavelengths of the PL spectra were measured without any dc operating bias. The peak wavelength of the EL spectra were measured at 20mA operation current with a bias voltage applied on the InGaN active layer. The EL peak wavelength of both LED structures had a blueshift phenomenon when increasing the operating current that was caused by the band filling effect of the tilted InGaN quantum well structure. Both the EL and the PL peak wavelengths for the TP-LED structure had a slight wavelength blueshift phenomenon when compared with the ST-LED structure.
The line-scanning micro-photoluminescence (μ-PL) spectra of the TP-LED structure are measured with a 1μm-diameter laser spot excited on the mesa region as shown in Fig. 6(d). The excitation laser power density is calculated as 12.7MW/cm2. The line-scanning μ-PL intensities have a 1.8 and a 3.6 times enhancement at the air-void region (region A), and the laterally etched region (region B), respectively, compared to the non-treated mesa region (region C) on the same mesa. From the PL intensity profile, a higher PL intensity is observed at the interface of the regions A and B located at the air-void boundary region. The peak μ-PL wavelengths were measured as the values of 461.8nm, 459.5nm, and 462.5nm for the laser-treated air-void structure (region A), the laterally-etched tapered GaN structure (region B), and the non-treated region (region C), respectively. In the TP-LED structure, the shorter μ-PL emission wavelength is observed at the InGaN active layer with the bottom treated structure (region A and B) when compared with the non-treated region (region C), possibly caused by the partial release of the compressed strain at the GaN/sapphire interface [19]. The periodical variation of the line-scanning μ-PL peak wavelength is measured as 2.3nm corresponding to the tapered GaN structure (region B with 30μm-width) and to the air-void structure (region A with 10μm-width) in the TP-LED. Compared to the laser treated air-void region, in the TP-LED structure, the slight wavelength blueshift phenomenon of the PL spectra is observed at the tapered GaN region through the lateral wet etching process that has a lower piezoelectric field in the InGaN active layer. The TP-LED structure has a slight wavelength blueshift phenomenon, a larger light scattering process, and slightly higher internal quantum efficiency when compared with the ST-LED structure.
4. Conclusion
The TP-LEDs with the air-void structure and the tapered GaN structures were fabricated through a laser decomposition process on the GaN buffer layer, a lateral wet etching process on the AlN sacrificial/buffer layer, and a bottom-up N-face crystallographic wet etching process. The higher light output power of the TP-LED structure was observed to have a higher light scattering process occurring on the tapered-GaN structure at the GaN/sapphire interface. The slightly peak wavelength blueshift phenomena of the EL and the PL spectra of the InGaN active layer were caused by the partial release of the compressed strain from the GaN/sapphire interface and thus forming the tapered GaN structure. The LED devices with the tapered-GaN and the air-void structures at the GaN/sapphire interface had high external quantum efficiency for high efficiency nitride-based LEDs applications.
Acknowledgement
The authors gratefully acknowledge the financial support for this research from the National Science Council of Taiwan under grant No. NSC 98-2221-E-005-007-MY3, and No. NSC 98-2622-8-005-002-A2, NSC 100-3113-E-005-002-CC2, and 99-EC-17-A-07-S1-158
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