Title: GaN-ready aluminum nitride substrates for cost-effective, very low dislocation density III-nitride LEDs

Final Scientific/Technical Report

Reporting period: June 15, 2008 - October 15, 2010

Prepared by: Sandra B. Schujman and Leo J. Schowalter

Report issued: January 2011

DOE Award number: DE-FC26-08-NT01578

Crystal IS, Inc.

70 Cohoes Ave.,

Green Island, NY, 12183

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Disclaimer: “This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.”

Abstract The objective of this project was to develop and then demonstrate the efficacy of a cost-effective approach for a low defect density substrate on which AlInGaN LEDs can be fabricated. The efficacy of this “GaN-ready” substrate would then be tested by growing high efficiency, long lifetime InxGa1-xN blue LEDs. The approach used to meet the project objectives was to start with low dislocation density AlN single-crystal substrates and grow graded AlxGa1-xN layers on top. Pseudomorphic AlxGa1-xN epitaxial layers grown on bulk AlN substrates were used to fabricate light emitting diodes and demonstrate better device performance as a result of the low defect density in these layers when benched marked against state-of-the-art LEDs fabricated on sapphire substrates. The pseudomorphic LEDs showed excellent output powers compared to similar wavelength devices grown on sapphire substrates, with lifetimes exceeding 10,000 hours (which was the longest time that could reliably be estimated). In addition, high internal quantum efficiencies were demonstrated at high driving current densities even though the external quantum efficiencies were low due to poor photon extraction. Unfortunately, these pseudomorphic LEDs require high Al content so they emit in the ultraviolet. Sapphire based LEDs typically have threading dislocation densities (TDD) > 108 cm-2 while the pseudomorphic LEDs have TDD ≤ 105 cm-2. The resulting TDD, when grading the AlxGa1-xN layer all the way to pure GaN to produce a “GaN-ready” substrate, has varied between the mid 108 down to the 106 cm-2. These inconsistencies are not well understood. Finally, an approach to improve the LED structures on AlN substrates for light extraction efficiency was developed by thinning and roughening the substrate.

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Table of contents Disclaimer 2 Abstract 2 Executive Summary 4 Report Details 6 Introduction 6 1. Low defect density and device performance 7 Experimental methods 7 Results and discussion 9 a) Device performance 9 b) IQE determinations 10 c) Electroluminescence (EL) measurements 11 2. GaN on AlN substrates 14 Experimental methods 15 3. Photon extraction 18 Aluminum Nitride optical absorption 18 Photon loss in the substrate 19 Substrate removal 20 a) Laser lift-off 21 b) Chemical etching 21 Photonic crystal 21 4. Large diameter AlN substrate development 23 Experimental methods 23 5. AlN roadmap in Solid State Lighting 26 Conclusions 28 References 29

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Executive Summary The main goals of this project were to test the relationship between threading dislocation density and device performance and, the development of a cost-effective method to generate a low defect substrate on which a visible light-emitting diode (LED) could be grown. The first goal was achieved, and a cross-cutting benefit of this research was the understanding of the main parameters affecting the performance of pseudomorphic ultraviolet light emitting diodes. The second goal was not achieved because of the failure to consistently obtain substrates with a diameter compatible with the tools used in the industry to fabricate InGaN-based LEDs. At the beginning of this project, the connection between low defect density and device performance was not clear for nitride-based LEDs, although it was well established that the lifetime of nitride laser diodes improved with decreasing defect density. Prior to the beginning of this work, a discovery was made by Crystal IS about the possibility of growing pseudomorphic epitaxial layers of AlxGa1-xN on AlN substrates, with thicknesses exceeding by orders of magnitude those predicted by the equilibrium theories. In particular, the growth of up to 1µm thick Al0.7Ga0.3N and 0.5µm thick Al0.6Ga0.4N are possible without relaxation. This made these layers, and LEDs fabricated based on these, the natural vehicle to test the relationship between low defect density and device performance. The first step was to determine the defect density on these layers, and analyze its influence in the performance of devices fabricated on Al0.7Ga0.3N pseudomorphic structures. Measurements of etch pit density done in-house showed excellent agreement with cathodoluminescence measurements carried out at Philips-Lumileds Lighting Company. The defect density determined by both methods was in the order of low 104 cm-2 to low 105 cm-2. Light emitting diodes, with emission wavelength in the mid-UV range, were fabricated using these pseudomorphic structures. Determinations of Internal Quantum Efficiency (IQE) were carried out in devices by two different methods: through photoluminescence (PL) at room and low temperature, and through electroluminescence (EL) measurements and posterior modeling. PL measurements conducted with a pumping power bracketing equivalent driving current density resulted on IQE in the order of 60% to 70%. The electroluminescence measurements allowed for the development of a model to estimate the Internal Efficiency (IE), defined as the product of the IQE and the injection efficiency. According to the results of this model, the IE varies between roughly 40% and 60% for the best devices and the injection efficiency is estimated in about 65%. Measurements of the device performance at high current density (up to 400 A/cm2) have shown high output powers (over 14 mW under pulsed mode). The External Quantum Efficiency (EQE) as a function of driving current density is relatively flat, showing a decay of less than 0.1%. The estimated lifetime of these devices, modeled based on thousands of hours of measurements at a current density of 100 A/cm2, exceeds 10,000 hours. At a current density of 20 A/cm2, the fitted lifetime exceeds 100,000 hours. The EQE vs. current density for these devices shows a similar trend than that of commercial InGaN-based LEDs, and is both higher and flatter at all currents than the EQE of devices with similar

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emission wavelength grown on sapphire substrates. This is a demonstration of the direct impact that defect reduction effects on device performance. To extend these results to visible LEDs, the approach proposed by Crystal IS consists on the epitaxial growth of a graded AlxGa1-xN layer topped by a pure GaN layer on top of a low-defect-density aluminum nitride substrate (AlN). The epitaxial growth of GaN on AlN was carried out and characterization proceeded. The lowest measurements of the etch pit density by atomic force and optical microscopy resulted on 2.5×105 cm-2 < EPD < 4 ×106 cm-2. These results were extremely difficult to repeat, mostly due to grain boundaries and domains in the AlN substrates. GaN growth on the 10mm by 10mm AlN substrates resulted in EPD ranging from low 106 cm-2 to high 108 cm-2. In particular, EPD at a grain boundary may be an order of magnitude higher than the EPD further away from it. Photon extraction is usually improved in sapphire-based devices through substrate removal, generally carried out using laser lift-off (LLO). The adoption of this solution for the case of AlN substrates is not straightforward because of the low transparency of AlN at the wavelength range of the lasers commonly used for LLO. For this reason a plan was thought out to ensure 90% of photon extraction and is detailed in this report. Part of this plan involves the thinning of the device substrate down to 10 μm or 20 μm. A provisional patent application, 61/360,136, “Method of Aluminum Nitride Wafer Thinning” was filed. In order to be commercially viable, these substrates need to not only show low defect density, but also have a size agreeable to an industry with tools optimized to handle 2-inch diameter sapphire wafers. Because of this constraint, part of this project was devoted to the growth and fabrication of 2-inch diameter AlN substrates. Crystal IS has demonstrated crack free, single crystal 2-inch AlN substrates, but with a very low yield. It has also developed a method to grow AlN boules from which it can extract and fabricate 10mm by 10mm AlN substrates with very low defect density. The process currently in use involves the use of small AlN seeds and expansion during crystal growth. This process is inefficient for 2-inch substrates, as it is very difficult to attain proper seed replication while avoiding crack formation. In order to increase this yield, several issues have to be resolved: growth temperature, axial and radial temperature gradients have to be optimized, and a large diameter seed has to be developed. The first two items were achieved through a combination of experimental and modeling work. A crucial part of the radial gradient tailoring is the design of radial shields to control the gradient. This development has been captured in a provisional patent application 61/360,142, “Seeded growth of large Aluminum Nitride single crystals with balanced thermal gradients”. The development of a large diameter seed is still ongoing. Two different types of experiments are being carried out: a) use of a SiC seed and b) in-situ expansion of an AlN seed. Finally, an analysis of the roadblocks and promises of AlN use as substrate for Solid State Lighting uses is presented.

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Report Details

Introduction Misfit dislocations generated in the epitaxial layers that compose a light-emitting diode affect its performance in several ways:

• They may act as photon traps. • They may act as charge carrier traps, increasing the lateral resistivity and thus,

lowering the overall performance of the device. • An increase in resistivity translates into an increase in operating temperature, which

may decrease the lifetime of the device. At the beginning of this project, the experimental evidence for improved internal quantum efficiency (IQE) of LEDs due to lower defect density was not clear. However the evidence for improved lifetime due to lower defect density had been substantially documented in the case of laser diodes. At the time, it had been well established that nitride laser diode lifetimes were very significantly impacted by dislocation densities through the active device layer. [Takeya, 2003]

Present commercial III-N LED epitaxial growth relies on sapphire or SiC substrates and such structures exhibit relatively high densities of extended defects (over 1×108 cm-2). Current InxGa1-xN LED technology used for visible LEDs loses efficiency at high current densities. Prior to the beginning of this project, Crystal IS had developed pseudomorphic structures on AlN substrates that, presumably, had very low defect density. [Grandusky 2008, Grandusky2009]. Light emitting diodes based on these structures, with emission wavelength in the mid-ultraviolet range, were used to test the correlation between low defect density and high performance. Crystal IS alternative to extend these results in the visible range was to analyze the possibility of growing low-defect density GaN substrates (with up to ~ 105 cm-2 dislocations) based on very low dislocation density (less than 104 cm-2) AlN substrates on which a graded AlxGa1-xN layer would be epitaxially grown, topped by a pure, low defect, GaN layer. Part of this project was devoted to the development of large diameter (2-inch) AlN substrates to insure that the substrates are compatible with manufacturing technologies that use 2-inch diameter sapphire wafers. Another important step in the manufacturing of sapphire-based visible LEDs is substrate removal to improve photon extraction. The techniques employed (mainly laser lift-off) to remove sapphire do not necessarily work when the substrate is AlN, so a plan was designed to deal with this issue. The following is a description of Crystal IS achievements, and lessons learned during this project.

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1. Low defect density and device performance Experimental methods When epitaxial layers are grown with slightly different lattice parameters (such as GaN or AlxGa1-xN on AlN substrates), the growth can be initiated with the layer growing under strain up to a certain critical thickness over which the layer cannot sustain the strain and relaxes through some mechanism. A common mechanism for strain relief is the generation of misfit dislocations along the interface of the lattice-mismatched layers. Unfortunately, the generation of misfit dislocations typically happens by forming dislocation loops which thread through the epitaxial layer from the growing surface which results in threading dislocations even if the substrate has a very low density of these dislocations at the start. Thus, the introduction of new threading dislocations can be controlled either by keeping the epitaxial layer thin enough so that strain relaxation does not initiate or by trying to control the introduction of the dislocation loops so that the misfit dislocation part of the loop is increased in size so as to provide more strain relief with fewer threading dislocations. The initial growth of an epitaxial layer which is strained to match the lattice parameter of the substrate is referred to as pseudomorphic growth. Typically the thickness allowed for such pseudomorphic growth before strain relaxation sets in decreases very rapidly as the amount of lattice mismatch increases. In the case of AlxGa1-xN alloys grown on AlN substrates, the critical thickness predicted by the classical energy balance theories (such as Matthews and Blakeslee, [Matthews, 1974]) is too thin to be useful for making LED devices. However, just before the start of this project, Crystal IS found that pseudomorphic layers more than an order of magnitude thicker than predicted by theory could be grown, as indicated in the Crystal IS US Patent Application 12/020,006. In the AlN substrate, it is possible to grow pseudomorphic 70% AlxGa1-xN layers as thick as 1µm while maintaining a low defect density. Through the growth of pseudomorphic layers of AlxGa1-xN grown on AlN, consistency in the growth of low defect structures was successfully achieved. Since this low defect, pseudomorphic layer is a necessary first step for any low threading dislocation density (TDD) structure on the AlN substrate, the decision was made that it was appropriate to use light emitting diodes (LEDs) based on pseudomorphic growth on AlN substrates as a test vehicle. These pseudomorphic LEDs can be used to determine the influence of crystal defects on light emitting devices of any wavelength. If indeed it can be proven that lower TDD significantly improve the performance of LEDs, then it can be attempted to either extend the pseudomorphic approach to longer wavelengths or continue to develop strain relaxation techniques which consistently minimize TDD. Thus, in order to achieve this goal within the timeframe of this project, the influence of defect reduction in the Internal Quantum Efficiency and lifetime of devices was tested on ultraviolet (UV) LEDs grown in CIS OMVPE (Organo-Metallic Vapor Phase Epitaxy) reactors and fabricated by Crystal IS.

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Wafers with nominally 70% Al AlxGa1-xN alloy were grown. Photoluminescence measurements were used to confirm that the layers corresponded to an approximate 70% aluminum content. Pieces from three of the wafers were etched in order to reveal defects. The measured etch pit density (EPD) varied within each sample, and from wafer to wafer, from ~ 5 × 104/cm2 to ~ 3 × 106/cm2. Figure 1 shows measurements on two regions of the samples with lowest and highest EPDs.

Figure 1: Optical image of etch pits in nominally 70% AlxGa1-xN pseudomorphically grown on AlN substrates. The top two images show different areas in the sample with lowest EPD, while the bottom two show regions in the sample with highest EPD. Cathodoluminescence (CL) measurements were carried out at Philips-Lumileds facility on pieces of the very same wafers. The results for the CL measurements agree very well with the EPD measurements. The dislocation density measured by CL varies from 2.2×104 cm-2 to1.1×105 cm-2. Figure 2 shows CL images of these samples.

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The defect density as determined by both cathodoluminescence and etch pit count is extremely low for an epitaxial layer of 70% AlxGa1-xN and it would be impossible to achieve on a foreign substrate such as sapphire or silicon carbide. Results and discussion: a) Device performance These very low defect density AlxGa1-xN layers were used to grow light emitting diode structures and test their performance. The goal is to show an improved behavior of the LED due to the lower defect density in the epitaxial layers. It is the expectation that lower threading dislocation density (TDD) in the epitaxial layers that compose a device leads to a better performance for light emitting diodes (LEDs). Pseudomorphic layers, with low defect density, will allow for devices that work at higher currents and show less degradation than conventional devices with high TDD. The main hurdle in the determination of pseudomorphic LEDs performance at high current density is the fact that contact-layers and other thermal barriers not related to the epitaxial quality cause heating of the device, and poor thermal management, especially in non-packaged devices, interferes with quantifications of device performance at high current density. In order to circumvent heating effects, the analysis of the performance was carried out under

Figure 2: Cathodoluminescence images of three different substrates with 70% AlGaN pseudomorphically grown on bulk AlN substrates. The defect density for each sample determined from these images are a) 1.1×105 cm-2, b) 2.2×104 cm-2 and, c) 2.8×104 cm-2.

a b

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pulsed conditions. In Figure 3, the comparison between a device performance when driven in pulsed mode (1% duty cycle) and continuous mode is shown: the output power exceeds 15 mW at an injection current density of 400 A/cm2 when heating effects are avoided. The right side axis shows the behavior of the external quantum efficiency (EQE), which degrades less than 0.1% when heating effects are minimized. Instead, the device that was run in continuous mode shows a steep decrease in EQE at injection current density below 100 A/cm2 and a saturation of the output power.

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Figure 3: a) Comparison between continuous excitation and pulsed (1% duty cycle) for a pseudomorphic LED. The pulsed bias, which helps ameliorate heating effects, allows the device to reach up to 15 mW output power at a current density of 400 A/cm2. b) Measurements for a hero device. b) IQE determinations Photoluminescence (PL) measurements were used to estimate the IQE by taking the ratio of the integrated PL intensities at low temperature (8K or 13K) to the integrated PL intensities at room temperature (293K). This approach is based on the assumption that the low temperature PL, under appropriate conditions, represents 100% IQE. Structures similar to full LEDs were grown except that the epitaxial structure was terminated after the growth of the quantum well layers. The quantum wells were pumped from the quantum well side and PL was collected from the same surface of the structure so the PL did not have to pass through the underlying n-AlxGa1-xN layer or AlN substrate. The measurement conditions were carefully chosen to ensure that the low temperature PL was directly proportional to the pump power which avoids saturation effects due, for instance, to overfilling of the quantum wells. In addition, the pump power was selected to prevent over estimations of the IQE at room temperature due to saturation of defects by photo excited carrier densities much higher than would present in the actual LED under typical drive currents. The PL approach to measuring IQE showed the best estimated IQE (over 60%) at shorter wavelengths with a decrease in IQE at longer wavelengths. Internal Quantum Efficiency determinations through PL measurements were carried out at two different locations: Army Research Laboratory (ARL) and SUNY Albany. The ARL measurements estimated the fluences as a function of photoexcitation pump power. Based on these estimates, the results correspond to a current density of ~26 A/cm2 (87 nJ/cm2) and ~ 15 kA/cm2 (51 μJ/cm2) which bracket our electroluminescence (EL)

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measurements which are between 20 and 100 A/cm2. The IQE estimated from these measurements is in the 60% to 70% level. In Figure 4, a distribution of the IQE estimated based on PL measurements as a function of emission wavelength, determined at SUNY Albany, is shown. The values obtained from SUNY are systematically lower (same sample comparison) than those determined at ARL. The reason for this difference is a combination of better equipment and more stable experimental conditions at ARL than at SUNY Albany. However, the SUNY results are extremely useful for relative comparison in order to gauge structures, and its proximity to Crystal IS makes SUNY Albany a very convenient resource.

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c) Electroluminescence (EL) measurements The output power of the LED, measured through the AlN substrate, was used to develop a model to estimate the internal efficiency (IE) of the device. The IE is defined as ratio of photons, due to radiative recombination in the quantum wells, to the number of electrons injected into the device, IE ≡ IQE×ηinj where ηinj is the injection efficiency (ratio of the number of electrons reaching the active area of the device to the number of electrons injected into the device). In visible LEDs, this injection efficiency is very close to unity and the distinction between IE and IQE is typically unnecessary. Output power was measured on-wafer using a driving current of 20 mA (current density of about 20 A/cm2) before and after thinning. After thinning down the substrate in order to increase the light extraction, a recalculation of the absorption coefficient is made based on the change in average output power, determining how much light is actually lost due to substrate absorption, and recalculating the power generated in the active area. The model uses the measured output power Pout and emission wavelength of the device, the measured substrate absorption coefficient, and assumes:

o Lambertian distribution of the photons generated in the active region o All the photons emitted in the direction of the p-GaN layer are absorbed

Figure 4: Distribution of IQE vs. emission wavelength. Measurements carried out at SUNY Albany.

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o Only those photons impinging on the back surface of the substrate with an angle smaller than the escape cone will be able to leave the device.

o The substrate absorption is calculated using the absorption loss in a single pass The modeled parameters are:

o ηext: Extraction Efficiency: defined as the ratio of the number of photons that leave the device to the number of photons generated in the active region.

o Pgen: Power generated in the active region of the device, calculated as the ratio of the measured Pout to the ηext.

o IE ≡IQE × ηinj: This product is calculated as the ratio of Pgen and the ideal, maximum, output power of the device. The ideal output power would be achieved if each electron injected into the device recombines in the active region and the generated photon leaves the device.

The results are shown in Table 1 in the columns labeled “refined”. For comparison, the modeled results prior to thinning (using the measured values for the absorption coefficient, as opposed to actual variations in output power) are also shown. The comparison is only approximate for the “hero” devices because the “hero” devices pre and post thinning are not necessarily the same device. The table shows values for the best device per wafer before thinning and values for the best device after thinning. Table 1: Estimation of performance parameters for pseudomorphic LEDs. The model uses measured on-wafer output power and takes into account losses due to substrate absorption, p-GaN layer absorption and total internal reflection to estimate the extraction efficiency, and the internal efficiency. For each pair of consecutive rows of the same color, the upper one shows the average of measurements for the devices on a wafer and the lower shows the maximum value measured on one single device on that wafer.

Wafer Wvl. (nm)

Pout (mW)

Pout (mW) thinned

ηext (%)

refined ηext (%)

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refined Pgen (mW)

IQE× ηinj (%)

refined IQE× ηinj (%)

Wafer A average 259 0.23 0.57 0.62 2.11 37.25 27.04 39 28

Wafer A best 259 0.44 0.94 0.62 1.85 69.67 50.91 73 53 Wafer B average 254 0.51 0.78 1.60 3.22 31.93 24.25 33 25

Wafer B best 254 0.78 1.06 1.60 2.86 48.85 37.10 50 38 Wafer C average 249 0.35 0.66 0.74 2.63 47.07 25.06 47 25

Wafer C best 249 0.52 0.88 0.74 2.38 69.34 36.91 69 40 Wafer D average 260 0.57 0.9 1.11 2.90 51.26 31.00 54 32

Wafer D best 260 0.99 1.32 1.11 2.47 88.76 53.53 93 56 In the table,

o Wvl: measured emission wavelength o Pout: on-wafer output power measured through AlN substrate thinned down to

~ 200 µm for an injection current of 20 mA o ηext: Extraction efficiency, calculated taking into account photon loss due to

substrate absorption, p-GaN absorption and probability of the photon

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impinging the back surface of the substrate with an angle smaller than that of the escape cone.

o Pgen: Calculated power generated in the active region of the device: ratio of the measured output power and the calculated ηext.

o IQE × ηinj: calculated based on the ratio of Pgen and (not shown on the table) the ideal power that would be measured if each electron injected in the LED generated a photon that is extracted from the device.

The recalculation of the absorption coefficient after thinning is systematically lower than the absorption coefficient measured on the wafers prior to epitaxial growth. A possible reason for this may be an actual decrease of the substrate optical absorption coefficient due to annealing during epitaxial growth of the LED structures. In addition, the lack of uniformity in the absorption coefficient along the thickness of the wafer and across the wafer may account for large variations between values for the wafer average and the best device, as well as variations between best devices before and after thinning. The main issue with this model is that it is very difficult to decouple the two components of the Internal Efficiency, that is, the Internal Quantum Efficiency (IQE) from the injection efficiency (ηinj). The differences in room temperature efficiency (as determined by PL and EL) could be due to the EL measurements actually measuring the product of the current injection efficiency and IQE whereas the PL measurement does not take into account the injection efficiency. This is an indication that the injection efficiency in these LEDs is lower than unity. The estimated lifetime of these devices, modeled based on thousands of hours of measurements at a current density of 100 A/cm2, exceeds 10,000 hours. At a current density of 20 A/cm2, the fitted lifetime exceeds 100,000 hours. Finally, a comparison between performance of pseudomorphic LEDs grown on AlN substrates and LEDs grown on sapphire can be seen in Figure 5. The plot shows the Internal Efficiency as a function of current density for two different pseudomorphic ultraviolet diodes grown on AlN substrates compared to those of an ultraviolet diode emitting at a similar wavelength grown on sapphire and a commercial, InGaN-based blue LED. Compared to devices emitting at similar wavelengths grown on sapphire substrates, pseudomorphic devices grown on AlN substrates display higher IE at every current density, slower decay in performance at higher current density and a more uniform (“flatter”) efficiency over the entire operational current density range. This improved efficiency is due to the lower defect density achieved through pseudomorphic growth.

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~444 nm LED [APL 91, 243506 (207)] 80% next 247 nm LED [APEX 3, 062101 (2010)] 6% next 248 nm CIS LED [APEX 3, 072103 (2010)] 4.1% next 255 nm CIS LED 3.6% next

The IE data underscores the similarity in efficiency trends for pseudomorphic LEDs and commercial blue InGaN light emitting diodes.

2. GaN on AlN substrates The pseudomorphic ultraviolet LEDs provide an excellent tool to test the relationship between light-emitting diode performance and defect density in the active region of the device, but are not useful in their current form for the Solid State Lighting effort. In order to extend the results achieved with pseudomorphic light emitting diodes into the visible range, different paths are possible. One is to use the very same ultraviolet LEDs developed on AlN substrates in combination with phosphors, given that they can be designed to emit at wavelengths very similar to those of the regular fluorescent lamps based on mercury plasma. However, the efficiency of this approach is fundamentally limited by Stokes loses, that is, the difference in energy between the pump photon and the photon emitted by the phosphor. Additional disadvantages of phosphors are low quantum efficiency (for some), broad band emission spectra, which reduces the luminous efficacy of the lamp, and losses due to photon scattering [DOE SSL MYPP 2010]. An alternative path, chosen for this project, is the development of a low defect substrate on which InGaN-based devices can be grown with low dislocation density for improved performance. Epitaxial growth of a graded layer of AlxGa1-xN topped by pure GaN on a substrate with similar lattice and thermal match such as CIS AlN substrates is expected to render a GaN layer with lower defect density than those obtained on sapphire. Thus, the proposed approach for this project was to develop techniques for growing relaxed GaN on AlN where the threading dislocation density (TDD) was controlled during the strain relaxation phase.

Figure 5: Comparison of Internal Efficiency for pseudomorphic devices, and devices grown on sapphire substrates. The performance of the pseudomorphic devices is very similar to that of commercial blue devices, and superior to that of UV devices grown on sapphire

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Experimental methods: Different concentration grading regimes (from AlN through AlxGa1-xN up to GaN) and the influence of the grading on the presence/type of threading dislocations at the GaN surface were analyzed. The grading was controlled by linearly increasing the Ga precursor flow while linearly decreasing the Al precursor. X-ray rocking curves (symmetric and asymmetric) were used to monitor the quality of the layer. There is an optimum thickness for the grading that will give low full widths at half maximum (FWHM) for the rocking curves. Thicker grading does not help improving the quality of the GaN layer. Figure 6 shows an Atomic Force Microscope (AFM) scan of the GaN layer grown on AlN that show smooth step flow growth.

Figure 6: 10µm × 10µm AFM scan of GaN grown on AlN. The roughness for the layer is lower than 5Å, and step flow growth can be observed. Transmission Electron Microscopy (TEM) was used to estimate the threading dislocation density of the GaN layers. However, this technique is costly and can only measure small areas. Thus, effort was focused on obtaining a correlation between TEM results and those of other techniques, such as cathodoluminescence (CL), Etch-Pit Density (EPD) and X-ray diffraction that, while being indicative of the presence of threading dislocations, could provide faster results in a less costly manner. GaN layers with low EPD have been demonstrated, as shown in Figure 7.

RMS: 0.482 nm

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These results are difficult to repeat even under the same growth conditions. Although more work is needed in order to understand this, the following are possibilities for the low yield:

• Miscut direction: several growth runs with different results were carried out on substrates with a nominal total miscut of about 0.3 degrees with respect to the c-axis, but no effort was carried out in order to control the miscut direction. In Figure 8, the possible effect of miscut direction is shown.

Figure 8: Influence of miscut direction in the formation of TDD. The upper grain shows steps that are parallel to the a-plane, the lower grain shows steps that are twisted 40 degrees with respect to the a-plane.

Figure 7: AFM characterization of GaN grown on AlN substrates after KOH etching. All the areas scanned show smooth morphology and step flow growth. The upper left image shows an area without etch pits. The upper right image shows a single etch pit. The lower left image shows a smaller scan area around that very same etch pit. We can obtain an upper boundary for the etch pit density of 4 × 106 cm-2.

Etch pit

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Grain boundaries: it is possible that small angle grain boundaries provide a stress relief region with higher TDD, leaving cleaner areas in between. Figure 9 shows this case.

Figure 9: The TDD along the grain boundary is about an order of magnitude higher than in adjacent areas A good correlation (within an order of magnitude) was found between results obtained through TEM and EPD for samples with high density of threading dislocations (between 108 cm-2 and 109 cm-2). CL is difficult to use at this defect level because of lack of resolution. It is known that thermal expansion differences between substrate and epi-layer cause bow of the substrates, and that the thermal conductivity non-uniformity in the wafers affects the composition of the epilayers. AlN has a very high thermal conductivity, so for a good quality single crystal, the thermal uniformity on the wafer should not create a problem. Given that, relative to other possible substrates, AlN is very well matched to GaN in terms of thermal expansion, it is expected that these issues will be minimized. However, there is still need to consider substrate shape (mainly bow) and its variations with temperature during the epitaxial deposition process to understand defect generation on the GaN layer. Although most of the epitaxial growth was carried out on 10 mm by 10 mm square substrates, experiments were run in 2” substrates in collaboration with the Penn State Electro-Optics Center (EOC). They analyzed the effects of temperatures such as achieved during epitaxial deposition on the substrate shape. The results are shown in Figure 10. In both cases, the temperature effects are smaller than the initial bow of the samples. The 10mm × 10mm AlN substrates recover their original shape after thermal cycling, while the change in shape of the 2” diameter substrates is permanent. This difference may be due to the better crystal quality of the smaller substrates. It will be possible to understand the importance of size in the shape change when the usable area of the 2” diameter AlN substrates is improved.

TDD along grain boundary

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Figure 10: In-situ measurements of bow during thermal cycling for 10mm by 10mm AlN substrates (left) and 2” diameter AlN substrates (right). The 10mm by 10mm samples return to their initial shape after the thermal variation, while the change in the 2” diameter substrates is permanent.

3. Photon extraction It is common practice in devices based on sapphire to add a top reflective contact and to remove the substrate in order to improve the photon extraction efficiency. The latter is usually done with laser lift-off (LLO), using an ultraviolet laser. The main barriers for light extraction of visible photons in AlN-based LEDs are absorption in the substrate and total internal reflection at the interface between substrate and air. Steps to reduce both effects include substrate thinning, roughening, and eventually, substrate removal. However, alternatives need to be analyzed for this last step because of the AlN absorption band in the same wavelength range of the lasers used for substrate removal (the addition of a top reflective contact would follow the same procedures as those commonly used for devices grown on sapphire). To this effect, CIS developed a plan to raise the photon extraction efficiency to 90%. Aluminum Nitride optical absorption The AlN single crystal absorption shows bands at different parts of the optical spectrum, as shown in Figure 11. CIS progress on reducing the optical absorption of the AlN substrates, especially in the ultraviolet range, has been remarkable, but it is still high for LED applications. Of importance for photon extraction from visible LEDs are those in the 400 nm to 500 nm, which hinders the performance of blue LEDs, and absorption in the ultraviolet range, because commonly the laser wavelength used for substrate removal (laser lift-off) lies in the 240 nm - 260 nm range.

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Figure 11: Typical absorption spectrum of an AlN substrate. Absorption bands in the 240 nm - 260 nm (of importance for substrate LLO), 400 nm - 500 nm (of importance for violet/blue LEDs) and longer wavelengths can be observed. The relatively high background absorption of 15 to 20 cm-1 in the sample shown is believed to be due to light scattering by macroscopic defects such as so-called microvoids [see, for example, Bondokov 2007]. Photon loss in the substrate The influence of the substrate thickness in the percentage of transmitted light is shown in Figure 12.

Prior to packaging, AlN-based diodes are “flip-chip” bonded to a submount. The thickness of the substrate, through which the generated photons leave the active area, plays a significant role in the extraction efficiency. According to Figure 12, for the visible range, where the substrate absorption coefficient typically varies between 10 cm-1 and 35 cm-1, between 20% and 50% of the generated photons can be lost through absorption

0%

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Figure 12: Percentage of light absorbed in the substrate for different absorption coefficients and substrate thickness. AlN absorption in the 240 nm - 260 nm ranges from 15 cm-1 to 70 cm-1. In the visible range, the absorption is typically in the 10 cm-1- 35 cm-1 range.

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in the substrate. Reducing the substrate thickness down to 20μm or 10μm can reduce the loss to less than 5%. Another important photon loss mechanism is total internal reflection at the flat interface between the substrate back surface and air. Because the AlN refraction index (roughly varying between 2.4 at 260 nm and 2.1 at 500 nm) is high compared to air, the escape cone is small, with an angle of about 25 º. This causes the total reflection of about 89% of the photons that reach the flat surface, that are subsequently lost through absorption. Back surface roughening has been shown to alleviate this problem. In particular, previous studies carried out at CIS on surface roughening through wet etching have shown that cones are generated, with an angle of about 20 º to 30 º, and that the extraction efficiency of the devices (measured on-wafer on 400 μm thick substrates) increased by 30% to 78%. Crystal IS developed a method that allows for the simultaneous thinning and roughening of the device back-surface after the fabricated wafer is singulated and the individual die are flip-chip bonded to a submount. This development was captured in a provisional patent application, 61/360,136, filed on June 30th, 2010. In Figure 13, a picture of an AlN-based LED with a thin and rough substrate is shown. This method looks very promising, but actual measurements of output power have not been carried out yet on devices subjected to this process.

Substrate removal The best way to improve photon extraction is to completely remove the substrate (see, for example, Krames et al. [Krames, 2007]). In addition to photon extraction, the lack of substrate would allow for vertical integration of the devices, improving the electrical characteristics (efficiency), as well as heat sinking. There are different possible methods for substrate removal in the case of AlN-based light emitting diodes:

Figure 13: 100× magnification photograph of an individual die after thinning down to 20 μm and back - surface roughening.

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a) Laser lift-off Laser lift-off (LLO) for substrate removal is successfully applied to separate nitride devices from sapphire substrates. A short pulse of an ultraviolet laser (in the range of 248 nm for excimer KrF or 355 nm for third harmonic of a Nd:YAG) is shined through the UV-transparent substrate and locally heats the AlxGa1-xN or GaN layer, decomposing it into metal and nitrogen. In the case of AlN-based devices, the relatively low transparency of AlN at short wavelengths will require that the substrate be thinned prior to the LLO. Another consideration is the behavior of the strain in the structure once the substrate is removed: it would not be an issue for GaN-ready AlN substrates, because the GaN layer should be relaxed, but it should be studied in the case of pseudomorphic structures. b) Chemical Etching In order to carry out substrate removal by chemical etching, an etchant has to be used that discriminates between the AlN substrate and the AlxGa1-xN or GaN layers that are supporting the device. Ide et al. [Ide, 2001] showed that using hot phosphoric acid did etch AlN at a temperature higher than 160 ºC, while for GaN the temperature had to exceed 210 ºC. Mileham et al. [Mileham, 1995] showed preferential etching of AlN over GaN/InGaN after being etched with an AZ400K solution (KOH is the active ingredient) for half an hour at 85 ºC. The solution attacked the AlN buffer layer in between the GaN/InGaN and sapphire substrate, leaving both the substrate and the epitaxial layer intact. Most important, they demonstrated that the etching acted preferentially without regard for crystallographic orientation. For light emitting diodes based on GaN on AlN, the buffer layer consists of a graded AlxGa1-xN layer that terminates on pure GaN. Thus, the process to remove the AlN substrate should include, for example, a temperature variation during the etching in order to remove a varying Ga content in the alloy until the pure GaN layer is reached, or some sort of chemical detection system to measure the Ga concentration of the removed material and stop before the device structure is affected. Another chemical etching technique that can be implemented is Wet Chemical Jet Etching, where the chemical etchant is circulated through the system and sprayed through a nozzle to provide a mechanical component to the etching. The sample is mounted on a rotating stage that prevents liquid etchants from accumulating on the sample surface. This may be a way to make the process more manufacturable. Photonic crystal Another approach to increasing light extraction from LEDs is to employ a photonic crystal, a material with a periodic variation in the local index of refraction n(r). A

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photonic crystal suitable for light extraction can be fabricated by etching a periodic array of air holes into the top surface of the LED, as illustrated schematically in Figure 14. The periodicity of the hole array may be described by lattice vectors a and b. The spacing of the holes is determined by the wavelength of the light to be extracted. The spacing between the holes (i.e. the magnitude of a and b) is typically on the order of the wavelength of the light. A rectangular pattern of holes is shown in the figure, but other patterns may also be employed. The photonic crystal increases the light extraction efficiency by coupling modes with wave vector k that suffer total internal reflection to modes k’ = k + G that couple to free space. Here G is a reciprocal lattice vector of the photonic crystal.

Figure 14: Periodic hole array on the top surface of the LED In terms of increasing the photon extraction efficiency of AlN-based visible LEDs exceeding 90%, the best road to follow is that of a thin-film, flip-chip configuration with a reflective contact, substrate removal and subsequent roughening/patterning of the exposed layer. Encapsulating the device to reduce the refraction index ratio, and thus, the total internal reflection effects, in addition to the previous steps, has proven to give extraction efficiencies between 80% and 90% [Wierer, 2009]. The use of photonic crystals to control the propagation of light from the device into air has shown extraction efficiencies in the order of 65% [Krames, 2007] and 68% to 78% [Wierer, 2009], and although the values are still lower than those for encapsulated LEDs, the eventual degradation of the encapsulant material at high photon flux that turns it opaque make photonic crystals a very important and interesting path to develop.

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4. Large diameter AlN substrate development A low defect substrate for the growth of visible LEDs has to comply with manufacturability constraints in order to be commercially viable. The tools used for LED fabrication in manufacturing plants are optimized for a 2-inch wafer diameter, which implies that a low defect, GaN on AlN substrate ideally should come in 2-inch diameter flavor in order to be appealing to the industry. Crystal IS developed and demonstrated crack-free, single-crystal 2-inch diameter AlN substrates. However, the yield is extremely low. For this reason, part of this project was devoted to the development of a process to consistently fabricate 2-inch AlN substrates, while low defect density GaN layers were grown on smaller, 10mm by 10mm substrates. The growth of good quality bulk AlN is far from simple because of the complications introduced by the very high deposition temperature (requiring temperatures in excess of 2200 ºC), the chemical reactivity of aluminum at those temperatures, that severely limit the range of materials that can be used, and its affinity for oxygen, that requires a very pure (in terms of oxygen-free) atmosphere. Crystal IS has been very successful on growing very high crystalline quality, high purity, crack-free AlN boules from which it can extract 10 mm by 10 mm substrates with a reasonable (albeit low) yield, but a huge effort has and is still being carried out in order to expand both the boule diameter and the process yield. Experimental methods: The main issues analyzed during this project are: • Seed mounting: seed backing and seed attachment materials and techniques are

crucial steps in the crystal growth setup. The seed backing material needs to be chemically compatible with AlN growth conditions and provide a diffusion barrier to aluminum vapor behind the seed wafer. The diffusion barrier property is important to prevent the formation of voids within the boule via loss of aluminum vapor. It is also important that the thermal expansion coefficient mismatch between seed backing material and AlN is minimized. The reason for this is that the stress induced by this difference during cool down has to remain below the critical cracking stress level of the AlN boule.

• Temperature gradient control during growth: given the reactive atmosphere and the high temperatures involved during the crystal growth process, temperature and temperature gradient determinations are extremely important and complex. A combination of thermal modeling followed by the installation of a system of thermocouples to determine temperatures at locations that are not readily observable through optical pyrometry helped increase the understanding of the crystal growth process.

• Large-diameter seed quality or good quality seed diameter expansion: an important issue in the development of a process for consistent production of 2-inch AlN substrates is the generation and mounting of large diameter seeds. The process used in most of this work consists of using small AlN seeds and expanding during crystal growth in order to obtain a large diameter substrate. Maintaining crystal quality (seed

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replication) and at the same time obtaining crack-free substrates with this method is very time consuming, and the process has had a very low yield.

Crack-free, 1-inch diameter AlN substrates were demonstrated, with high crystalline quality area exceeding 50% of the wafer area. Images taken through crossed-polarizers are shown in Figure 15.

Figure 15: Crossed-polarizer images of two different 1-inch diameter crack-free AlN substrates. There is very good seed replication, although some polycrystalline material is also present within the seeded area in (b) The manufacture of 2-inch diameter AlN substrates requires larger diameter backing material, as well as an optimization of growth temperature and temperature gradients. The crystal growth reactors are RF-heated furnaces that use optical pyrometers for temperature determination, as well as feedback for the power control system. Given the geometry of the RF-heating coil, not all the spots needed for proper control of the temperature are accessible to a pyrometer. For that reason, a system of thermocouples that were capable of working under the growth conditions (temperatures over 2200 ºC, and Al-vapor atmosphere) was set. Thermal modeling was used to determine the spots where the temperature readings were more important, and to situate the thermocouples. The response of this thermocouple system was compared to the results of thermal modeling, helping to refine the calculations and understand the crystal growth process. Growth temperature and temperature gradient optimization are crucial in order to obtain good crystalline quality, insuring that nucleation will start and continue following the seed, as opposed to self-nucleation centers that will progress to form different grains, and to reduce stress in the crystal that can be relieved through defect formation or cracking. In particular, the presence of low-angle grain-boundaries has been a consistent problem in the development of 2-inch diameter AlN substrates. Optimization of the growth temperature involved observations of the source material sintering. Color and uniformity can give clues about the temperature being enough or not for the crystal growth to proceed. In terms of temperature gradients, and given the

a) b)

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symmetry of the reactor, both axial and radial gradients need to be carefully tailored to achieve a crack-free, single crystal substrate in a time that makes commercial sense. Axial gradient tailoring was used in order to optimize mass transfer and thus, growth rate. In Figure 16, simulations for three different configurations are shown. In the plot, T/Tmax is a normalized temperature and Z/Zo is the normalized axial distance. In order to carry out the simulations, the heater power was assumed constant and the heat transfer was modeled. The objective was to optimize the axial profile to obtain a favorable mass transport condition. Raising the crucible position with respect to the coil increases the mass transfer. Although this behavior was expected, having the specific data is extremely useful in order to decide the best “starting position” in order to improve crystalline quality.

Figure 16: Simulation of axial thermal field for different initial configurations. T/To represents normalized temperature while Z/Zo represents normalized axial distance Radial gradient tailoring was analyzed in the same way with the objective to control the interface shape. The simulation results are shown in Figure 17. In particular, a slightly convex interface is preferred in order to avoid spurious nucleation that generates polycrystalline areas and, to improve boule expansion.

Figure 17: Simulation of radial thermal field for different initial configurations. R/Ro represents normalized radial gradient (R=ΔT/ΔR) and d/dmax represents the normalized hotzone position with respect to the heater.

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A crucial part of the radial gradient tailoring is the design of radial shields to control the gradient. This development has been captured in a provisional patent application 61/360,142, “Seeded growth of large Aluminum Nitride single crystals with balanced thermal gradients”. An important issue in the development of 2-inch AlN substrates is the generation and mounting of large diameter seeds. The process used in prior work consists of using small AlN seeds and expanding during crystal growth in order to obtain a large diameter substrate. Maintaining crystal quality (seed replication) and at the same time obtaining crack-free substrates with this method is very time consuming, and the process has had a very low yield. This process was successful in the development of the 1-inch diameter substrates, but, although crack free, single crystal 2-inch diameter AlN substrates were demonstrated, the process showed an even lower yield. For this reason, most of the epitaxial growth referred to in this report was carried out in 10 mm by 10 mm AlN wafers. Meanwhile, CIS has initiated a high risk approach to circumvent the problem of seed expansion by testing the use of a large diameter seed. Work was begun using silicon carbide (SiC) wafers as a seed crystal over which AlN can nucleate. The goal is to obtain a 2-inch single crystalline AlN seed crystal with a thickness of at least 2 mm. If AlN seeds of high enough quality are produced this way, the experiment will be a success. Even if the final decision is against the use of SiC wafers as a tool for seed generation, insight will be gained on mounting a large diameter seed prior to crystal growth. Currently, tests are being carried out using different backing materials and adhesives in order to test seed survival that is, that the seed remains attached to the backing plate after the growth cycle. The goal is to have a survival yield exceeding 80%.

5. AlN roadmap in Solid State Lighting While the use of AlN substrates for SSL clearly has potential as an improved substrate material, there remain several significant hurdles. This project had been initiated because of the potential advantages of having a substrate with the same crystal structure as AlN, GaN, InN and their alloys. In addition, there is a close thermal expansion match over the relevant temperature range between these alloys and AlN so that strain (or lack thereof) obtained at the epitaxial growth will be preserved once the epi-structure is cooled to room temperature without the need for complicated strain-control layers. Finally, the thermal conductivity of AlN (measured to be 2.8W/cm-K at room temperature) will allow better device performance particularly when combined with either the elimination or substantial reduction of buffer layers which can considerably increase the thermal resistance to the actual device junction. The AlN will be a potentially appealing substrate if these advantages can translate into better performance and/or yield at an attractive cost. Of course, how much of a premium the market would be willing to pay for an alternative substrate technology will depend on the degree of performance and/or yield and how important these advantages are to a particular application. During the course of this project, we have demonstrated that:

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1. It is possible to obtain smoother epitaxy of GaN with lower defect density than equivalent layers grown on c-face sapphire. In particular, TDD of mid-107 cm-2 or better were demonstrated routinely in relaxed GaN layers. TDD lower than 106 cm-2 were also demonstrated but with low yield and, we anticipate, that novel surface structures and/or epitaxial recipes will need to be developed that will reproducibly allow the formation of misfit dislocations at the GaN/AlN interface to occur while minimizing the density of TDD through the strain relaxed GaN layer.

2. Pseudomorphic layers of AlxGa1-xN (and LEDs based on these layers) were demonstrated on AlN with TDD of less than 105 cm-2. However, new epitaxial and/or processing techniques would need to be developed which would allow this approach to extended to the fabrication of LEDs with wavelengths longer than 300nm since Ga concentrations greater than 50% and/or In concentrations greater than zero will be required, which are larger than what the technology used during this project will allow for pseudomorphic growth.

3. 2” diameter AlN substrates were demonstrated but with low yield and inconsistent quality across the substrate diameter. This size or larger must be achieved with consistent quality before cost-effective manufacturing of LEDs will be possible.

One of the lessons learned during this research was that being able to control the crystal quality of the substrate is of utmost importance for having a consistent, reproducible platform on which to experiment and develop a viable visible light-emitting diode structure. In particular, the presence of grain boundaries and crystal domains has shown to be the main source of variability in the quality of the GaN layers grown on top. Another issue is that of substrate size and geometry. Until AlN substrates are consistently available in 2-inch diameter, any development regarding their use in Solid State Lighting will be sporadic. It is our expectation that approximately an 18 month, $5M effort is needed to achieve a reliable manufacturable substrate. It is also our expectation that there is sufficient interest in shorter wavelength LEDs (λ<280nm) that this substrate development effort will occur the next 4 years if not sooner. Visible LEDs may be able to take advantage of these developments. It is our belief, based on the results of this project, that any future SSL funding of AlN substrate or LED development based on AlN substrates will need to address the following questions:

1. How much of a performance advantage will achieving active LED device regions with <107 cm-2 TDD give? Can a manufacturing process for large diameter (> 2”) AlN substrates be developed which will allow this performance advantage to be exploited cost effectively?.

2. While lower TDD are possible, new inventions will need to be proposed to consistently achieve <106 cm-2 in InGaN layers needed for near UV (λ>365nm) or longer wavelength LEDs. Again, the relative advantage of lower TDD will need to be weighed against the cost of developing and manufacturing these lower TDD structures.

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Conclusions The use of pseudomorphic epitaxial growth for the fabrication of light emitting diodes allowed for improved performance of the device. This improvement is due to the considerably lower defect density determined on pseudomorphic layers as opposed to defect density measured on relaxed layers. This attribution is confirmed by comparing the performance of similar emission wavelength devices grown on AlN and sapphire at different drive current density, reaching up to 400 A/cm2. The external quantum efficiency for pseudomorphic light emitting diodes is higher and flatter than that determined for sapphire-based devices with similar composition, and shows a very similar trend to that of commercial InGaN-based light emitting diodes. The goal of developing a cost-effective approach to generate low-dislocation density GaN substrates based on AlN was hindered by two drawbacks: Crystal IS has demonstrated high quality, crack free, 2-inch AlN substrates, but has not developed a consistent, high yield product. The second critical barrier is the question of how high the Ga concentration in the AlxGa1-xN buffer layer can be pushed while maintaining a low threading dislocation density (TDD). Higher Ga concentrations will be needed for near UV or visible blue LEDs. While Crystal IS work on this project suggests that low TDD are possible in epitaxial layers grown on AlN substrates even with pure GaN, these results have been difficult to reproduce across the large areas needed for cost-effective LED manufacture. Once these hurdles are surmounted, the use of a platform conducive to lower defect density in the epitaxial layers will be reflected in the performance of LEDs with emission wavelength in the visible, as well as they are already reflected in the performance of UV LEDs.

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References [Bondokov 2007] R.T. Bondokov, K. Morgan, G.A. Slack, and L.J. Schowalter, “Large Aluminum Nitride Crystals with Reduced Defects and Methods of Making Them,” US patent application 2007/0134827. [DOE SSL MYPP 2010] “Solid State Lighting Research and Development: Multiyear Program Plan”, March 2010; US Department of Energy, Energy Efficiency and Renewable Energy [Gardner, 2007] Gardner, N.F., Müller, G.O., Shen, Y.C., Chen, G., Watanabe, S., Götz, W., Krames, M.R.; Applied Physics Letters, 91, 243506, (2007); “Blue-emitting InGaN–GaN double-heterostructure light-emitting diodes reaching maximum quantum efficiency above 200 A/cm2” [Grandusky, 2008] Grandusky, J.R., Smart, J.A., Mendrick, M.C., Schowalter, L.J., Chen, K., Schubert, E.F.; “Pseudomorphic Growth of Thick n-type AlxGa1-xN Layers on Low Defect Density Bulk AlN Substrates for UV LED Applications”; International Symposium on Semiconductor Light Emitting Devices - April 27-May 2, 2008 - Phoenix, Arizona [Grandusky, 2009] Grandusky, J.R., Smart, J.A., Mendrick, M.C., Schowalter, L.J., Chen, K.X., Schubert, E.F.; Journal of Crystal Growth, 311, 2864, (2009); “Pseudomorphic growth of thick n-type AlxGa1-xN layers on low-defect-density bulk AlN substrates for UV LED applications” [Grandusky, 2010] Grandusky, J.R., Gibb, S.R., Mendrick, M.C., Schowalter, L.J.; Applied Physics Express, 3, 072103, (2010); “Properties of mid-ultraviolet light emitting diodes fabricated from pseudomorphic layers on bulk aluminum nitride substrates” [Ide, 2001] Ide, T., Shimizu, M., Suzuki, A., Shen, X-Q., Okumura, H., and Nemoto, T.; Japanese Journal of Applied Physics, 40, 4785, (2001); “Advantages of AlN/GaN Metal Insulator Semiconductor Field Effect Transistor using wet chemical etching with hot phosphoric acid” [Krames, 2007] Krames, M.R., Shchekin, O.B., Mueller-Mach, E., Mueller, G.O., Zhou, L., Harbers, G., Craford, M.G.; Journal of Display Technology 3, 160, (2007); “Status and future of high-power light-emitting diodes for Solid-State Lighting” [Matthews, 1974] Matthews, J.W. and Blakeslee, A.E.; Journal of Crystal Growth, 27, 118, (1974); “Defects in epitaxial multilayers” [Mileham, 1995] Mileham, J.R., Pearton, S.J., Abernathy, C.R., MacKenzie, J.D., Shul, R.J., and Kilcoyne, S.P.; Applied Physics Letters, 67, 1119, (1995); “Wet chemical etching of AlN”

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[Shatalov, 2010] Shatalov, M., Sun, W., Bilenko, Y., Sattu, A., Hu, X., Deng, J., Yang, J., Shur, M., Moe, M., Wraback, M., Gaska, R.; Applied Physics Express, 3, 062101, (2010); “Large chip high power deep ultraviolet light-emitting diodes” [Takeya, 2003] Motonobu Takeya, Takashi Mizuno, Tomomi Sasaki, Shinro Ikeda, Tsuyoshi Fujimoto, Yoshio Ohfuji, Kenji Oikawa, Yoshifumi Yabuki, Shiro Uchida, and Masao Ikeda, “Degradation in AlGaInN lasers,” phys. stat. sol. (c) 0, 2292–2295 (2003) [Wierer, 2009] Wierer, J.J., David, A., Megens, M.M.; Nature Photonics, 3, 163, (2009); “III-nitride photonic-crystal light-emitting diodes with high extraction efficiency”