Using AFM to enhance MOCVD-grown OE devices
12/01/2005
Doru I. Florescu, Veeco Instruments Inc., Somerset, NJ
Metal-organic chemical-vapor deposition (MOCVD) has steadily progressed from small-scale materials research to a mass production technology, enabling such complex optoelectronic (OE) devices as laser diodes and ultrahigh-brightness light-emitting diodes (UHB-LED). These devices support today’s consumer electronics in a wide range of applications, including optical storage for CD/DVD media and backlighting for cell phone keypads and LCD screens.
The emerging solid-state lighting market, where low-power, long-lasting LEDs are beginning to replace standard light bulbs in high-usage applications, requires performance improvements to drive down the cost of devices while maintaining superior material quality. In the case of gallium nitride (GaN)-based LEDs, brightness and uniformity have become key challenges for epi wafer manufacturers while they simultaneously seek to reduce the cost per die.
As industry specifications continue to tighten, advances in epilayer characterization techniques provide a key means to address these challenges. Atomic force microscopy (AFM) is currently being used as an analytical indicator of ideal process conditions, enabling manufacturers to optimize the surface morphology of constituent layers and ultimately improve device performance.
In addition, increased use of photoluminescence (PL) mapping has led to further improvements in wafer uniformity and device characteristics [1]. These technologies are proving to be vital building blocks as MOCVD transitions to a more fully realized, high-volume manufacturing industry.
Measuring/controlling properties
AFM involves the use of small scanning probes to obtain detailed, high-resolution images of epitaxial surfaces, typically of subangstrom vertical and nanometer-scale lateral resolution. In a representative example of this technique, a probe with an end radius of several nanometers is raster-scanned across material samples in a nondestructive manner, directly measuring surface topography and feeding data back to analysis software. The resulting images allow MOCVD equipment users to monitor the effects of dopant levels for p-side layers and examine the nature of such surface features as “V-groove” pits, a typical trait of indium gallium nitride (InGaN) growth.
Employing a multistage optimization process based on AFM feedback, engineers can modify growth conditions in an MOCVD system to improve the quality of quantum heterostructures (used as light-emitting layers in LED structures) through reducing and controlling materials defects and smoothing layer surfaces. Applying this method to MOCVD-grown GaN LEDs has resulted in improvements to chip-level output power and a decrease in operating voltage [2].
To attain this optimization, the associated material deposition platform must exhibit a highly tunable growth environment with the flexibility to precisely adjust and maintain process parameters, including gas flows and epi growth temperature. Since surface morphology directly affects device performance, AFM is well suited for use with high-speed rotating disc reactors that provide the gas-switching regime needed for tight interface quality control. Also, ideal conditions demand the elimination of turbulence and memory effects and require an extremely stable, repeatable process. Such advantages as in situ temperature control, rapid switching between layers, fast thermal ramping, and strict control of layer thickness become crucial for achieving state-of-the-art-materials.
Optimizing GaN-based LEDs
As demonstrated by recent studies, material features linked to device performance can be found at the nanoscale for GaN LED constituent layers (buffer, n-type, active region, and p-type) and examined at a range of scales. The series of images shown in Figs. 1 and 2 display surface morphologies for MOCVD-grown epitaxial material used to fabricate blue (460nm) and green (520nm) UHB-LED devices. Several design-of-experiments studies have been initiated using these and other AFM topography scans as visual indicators leading to proper selection of MOCVD growth conditions. All images were generated using a Veeco Dimension 3100 AFM and are taken from layers deposited using Veeco TurboDisc reactors preconfigured for both development and production-scale growth of GaN materials.
Figure 1a shows a 1×1μm representation of a GaN buffer layer. This layer is typically employed to address the large lattice mismatch (16%) between the sapphire substrate and GaN epilayer where large grain boundaries (~150-350nm wide) appear during their initial stages of coalescence. In the case of buffer layers, AFM has proven to be an effective tool for simplifying process transfer between different substrate types, such as sapphire and silicon. Duplicate surface morphologies can be generated at this level, easing transfer without further significant changes to developed LED recipes.
Figure 1b presents an optimal 5×5μm GaN epilayer surface, used as the n-type (GaN:Si) contact for both blue and green UHB-LEDs. The surface root mean square (rms) for the sample shown is 0.17nm, among the best values published to date and indicative of a very flat surface achieved using an AFM-based optimization process.
Figure 1c shows the optimized 50×50μm AFM surface morphology of GaN layers used as p-type (GaN:Mg) contacts in standard UHB-LED devices. Top LED surface flatness is among the most critical device design parameters, since it is not only the surface where contacts are being constructed (post-epi), but also the layer through which the emitted light from the active region exits the device. In addition to being an ideal indicator from a geometrical device design standpoint (surface flatness, interface quality), surface morphology is also very informative about the quality of typical epilayers of p-type materials used throughout UHB-LEDs, especially from a doping perspective.
While Fig. 1 focuses on the GaN buffer and epilayers used to form n- and p-type contacts, Fig. 2 addresses surface morphology optimization of InGaN-based active (light-emitting) layers at the nanoscale based on proper selection of MOCVD growth parameters. Presented are two examples of topography for the very thin (25Å), high indium-content (20%) InGaN layer, normally used as the quantum well region in UHB-LEDs. Device wavelength and brightness outputs can be adjusted in such layers by controlling the amount and uniformity of the solid-phase indium incorporated, as validated by the PL mapping technique. A smooth, defect-free surface in the quantum well region is crucial for optimal light output.
Figure 2a reveals the negative effects of a specific in situ procedure (gas ambient change). As observed, certain materials of the III-Nitride system are very unstable under different growth ambients, which is often not the case for other III-V materials. Investigation indicates that a nonoptimal, post-growth ambient choice has affected several top monolayers of the InGaN material, which will ultimately be reflected in poor device characteristics.
Figure 2b shows a significant improvement in surface morphology under a separate set of ambient conditions. This morphological change observed for similar InGaN layers translates to an ~2× increase in PL intensity and a nearly 20% decrease in full-width half-maximum between the specimens. This example highlights the importance of rapid, optimal transitions between growth conditions for the barrier and active region in a typical UHB-LED, for instance. Production of high-quality materials requires specific MOCVD growth parameters (temperature ramp, gas ambient, and switching sequence) to be fast and precisely controlled, with exact ambient composition resulting from low residence times in the growth chamber.
Conclusion
AFM improves the quality of MOCVD-grown heterostructures. Based on nanoscale surface morphology response, various growth schemes (fast changes in gas ambient, temperature ramps) can be applied during process development and commercial UHB-LED growth stages to enhance device characteristics, including optical efficiency and electrical properties.
Given the level of precision needed at the active region of GaN-based LEDs, reactor performance becomes critical, exemplified by the improvement or deterioration of layer quality from minute process alterations. Use of AFM reveals the importance of fast gas switching between epilayers and low residence times to secure ideal, stable growth conditions. In addition, AFM correlates well with other characterization techniques (x-ray diffraction, PL mapping) for device quality analysis. These findings have been reproduced across multiple TurboDisc platforms.
Acknowledgments
The author would like to thank his colleagues J.C. Ramer, S.M. Ting, D.S. Lee, D. Lu, V.N. Merai, A. Parekh, J. Cruel, M. Youngers, E.A. Armour, W.E. Quinn, and D. Bellina for their continued support. TurboDisc is a registered trademark of Veeco Instruments Inc.
References
- D.I. Florescu, J.C. Ramer, D.S. Lee, E.A. Armour, “InGaN/GaN Single-quantum-well Light-emitting Diodes Optical Output Efficiency Dependence on the Properties of the Barrier Layer Separating the Active and p-layer Regions,” Appl. Phys. Lett., Vol. 84, p. 5252, 2004.
- D.I. Florescu, “Atomic Force Microscopy Helps Improve GaN LED Performance,” Compound Semiconductor, p. 23, July 2004.
Doru I. Florescu received his PhD in physics from Ohio U., and is a staff scientist with the Advanced Technology Group at Veeco Instruments Inc., TurboDisc Division, 394 Elizabeth Ave., Somerset, NJ 08873; ph 732/560-5300, e-mail [email protected].