Eric A. Armour

Large scale manufacturing of compound semiconductors by MOVPE

As more compound semiconductor devices reach large volume manufacturing levels, a trend toward the use of the MOVPE technique is clear. In this paper we examine the criteria needed for MOVPE equipment suitable for large scale manufacturing. We find that although uniformity and device performance are necessary, reproducibility is also critical, along with high throughput and low operating costs. These points are illustrated by actual examples including MMIC power amplifiers, HB-LEDs, and solar cells. A realistic COO model provides a tool for evaluating MOVPE systems of different capacities. In situ control of key parameters during growth is now feasible, and will become an important method for increasing reproducibility and throughput. Lastly we look at the prospects for automation, for decreasing labor costs as well as wafer handling. This is likely to first have an impact on systems for the growth of electronic device structures on large (100 and 150 mm) wafers.

Analysis of SAW properties in ZnO/Al/sub x/Ga/sub 1-x/N/c-Al/sub 2/O/sub 3/ structures

Piezoelectric thin films on high acoustic velocity nonpiezoelectric substrates, such as ZnO, AlN, or GaN deposited on diamond or sapphire substrates, are attractive for high frequency and low-loss surface acoustic wave devices. In this work, ZnO films are deposited on Al/sub x/Ga/sub 1-x/N/c-Al/sub 2/O/sub 3/ (0 /spl les/ x /spl les/ 1) substrates using the radio frequency (RF) sputtering technique. In comparison with a single Al/sub x/Ga/sub 1-x/N layer deposited on c-Al/sub 2/O/sub 3/ with the same total film thickness, a ZnO/Al/sub x/Ga/sub 1-x/N/c-Al/sub 2/O/sub 3/ multilayer structure provides several advantages, including higher order wave modes with higher velocity and larger electromechanical coupling coefficient (K/sup 2/). The surface acoustic wave (SAW) velocities and coupling coefficients of the ZnO/Al/sub x/Ga/sub 1-x/N/c-Al/sub 2/O/sub 3/ structure are tailored as a function of the Al mole percentage in Al/sub x/Ga/sub 1-x/N films, and as a function of the ZnO (h/sub 1/) to Al/sub x/Ga/sub 1-x/N (h/sub 2/) thickness ratio. It is found that a wide thickness-frequency product (hf) region in which coupling is close to its maximum value, K/sub max//sup 2/, can be obtained. The K/sub max//sup 2/ of the second order wave mode (h/sub 1/ = h/sub 2/) is estimated to be 4.3% for ZnO/GaN/c-Al/sub 2/O/sub 3/, and 3.8% for ZnO/AlN/c-Al/sub 2/O/sub 3/. The bandwidth of second and third order wave modes, in which the coupling coefficient is within /spl plusmn/0.3% of K/sub max//sup 2/, is calculated to be 820 hf for ZnO/GaN/c-Al/sub 2/O/sub 3/, and 3620 hf for ZnO/AlN/c-Al/sub 2/O/sub 3/. Thus, the hf region in which the coupling coefficient is close to the maximum value broadens with increasing Al content, while K/sub max//sup 2/ decreases slightly. When the thickness ratio of AlN to ZnO increases, the K/sub max//sup 2/ and hf bandwidth of the second and third higher wave modes increases. The SAW test devices are fabricated and tested. The theoretical and experimental results of velocity dispersion in the ZnO/Al/sub x/Ga/sub 1-x/N/c-Al/sub 2/O/sub 3/ structures are found to be well matched.

Effect of Growth Temperature on GaAs Solar Cells at High MOCVD Growth Rates

Increasing epitaxial growth rate is an important path toward III-V solar cell cost reductions; however, photovoltaic device performance has been shown to degrade with increasing growth rate. In this study, gallium arsenide (GaAs) material has been deposited via metal-organic chemical vapor deposition (MOCVD) at growth rates varying between 14 and 60 μm/h. Deep-level transient spectroscopy is utilized to elucidate an exponential rise in EL2 trap density as a function of growth rate when all other growth conditions are held constant. Evidence is provided that this EL2 defect is responsible for limiting the Shockley-Read-Hall (SRH) lifetime of very high growth rate solar cells. The effect of growth temperature on devices at high growth rate is subsequently investigated as a strategy to reduce trap density and improve solar cell performance. From this investigation, EL2 trap density is suppressed, and single-junction on-substrate GaAs solar cells grown at 60 μm/h are reported with 1.01 V 1-sun open-circuit voltage and 23.8% AM1.5G efficiency.

In-situ controls for MOVPE manufacturing

Abstract Much of the early research that established MOVPE was done with simple, single-wafer systems. Uniformity and reproducibility were not show-stoppers — developing the precursors, growth processes and device fabrication techniques were more important. This situation has changed so that now the emphasis is on lower cost devices opening new applications or replacing older technologies. In this paper we will examine the major factors that affect manufacturing costs in the high volume production of MOVPE-grown material for compound semiconductor devices, emphasizing the importance of reproducibility.

Epitaxial growth technologies for HB-LED manufacturing

The bulk of LEDs sold today are still fabricated using older epitaxial techniques such as LPE and VPE, but have relatively low brightness and a limited color range. The newer high brightness LEDs are fabricated from the InGaAlP and III-Nitride systems, with MOCVD being the preferred growth technique for manufacturing. While these new materials represent a significant increase in performance, they are also more expensive to grow. In this paper we consider the reasons for this, which include a less mature growth technology, lower production volumes, expensive starting materials, process efficiency, equipment throughput and cost, and safety and environmental concerns. Addressing each of these issues in turn, we examine what has already been accomplished, and what may be improved by further advances in equipment and process. A realistic COO model is of great utility in comparing product cost for different device structures, staffing schemes, reactor sizes, etc. We demonstrate that for a dedicated LED manufacturing facility, the lowest epitaxial cost is achieved by running around the clock with the highest throughput reactor that is fully utilized for the desired production level. When maintenance tasks such as cleaning and test or calibration runs are minimized, then materials costs will dominate the epi cost, which leads to the desirability of achieving both the best reproducibility and increasing the process efficiency. We show how in-situ control techniques are now capable of increasing preproducibility and thereby lowering product costs for manufacturing scale MOCVD reactors.

Optical and crystal quality improvement in green emitting InxGa1-xN multi-quantum wells through optimization of MOCVD growth

We report on green-emitting In0.18Ga0.82N/GaN multi-quantum well (MQW) structures over a variety of metalorganic chemical vapor deposition (MOCVD) growth conditions to examine the morphology, optical quality, and micron-scale emission properties. The MOCVD growth parameter space was analyzed utilizing two orthogonal metrics which allows comparing and optimizing growth conditions over a wide range of process parameters: effective gas speed, S*, and effective V/III ratio, V/III*. Optimized growth conditions with high V/III, low gas speed, and slow growth rates resulted in improved crystal quality, PL emission efficiency, and micron-scale wavelength uniformity. One of the main challenges in green MQWs with high Indium content is the formation of Indium inclusion type defects due to the large lattice mismatch combined with the miscibility gap between GaN and InN. An effective way of eliminating Indium inclusions was demonstrated by introducing a small fraction of H2 (2.7%) in the gas composition during the growth of high temperature GaN quantum barriers. In addition, the positive effects of employing an InGaN/GaN superlattice (SL) underlayer to crystal quality and micron-scale emission uniformity was demonstrated, which is of special interest for applications such as micro-LEDs.

Direct AFM Observation of Strain Effects on MOCVD-Grown GaN Epilayer Surface Morphology

In this study, we investigate the dependence of GaN surface morphology on the absolute strain values for thin ( c -plane sapphire substrates of various miscut angles towards the m -plane. Results indicate an excellent correlation between the surface roughness observed employing an AFM tool and epilayer strain values. An overall increase of surface roughness (decrease of atomic terrace width) is found with decreasing compressive strain (epilayer vs. bulk value). In addition, sapphire substrates with increasing miscut angle (0.30 deg) appear to relax the inherent, built-in strain differently in the vertical (growth) direction when compared to just (0.00 deg) substrates. Strain relaxation by typical V-shaped, hexagonal pits is directly imaged through the comparison of surface features inside and outside of pits in the thin GaN epilayer films.