by Debra Vogler, Senior Technical Editor, Solid State Technology
Oct. 30, 2008 – TDI, an Oxford Instruments company, recently announced that it has developed a hydride vapor phase epitaxy (HVPE) technology that is based on a GaCl3-InCl3-NH3 system. The new process is able to control the growth rate of InGaN down to 1-2μm/hr and In content up to 43%. According to the company, HVPE is known for its capability to grow low-defect, crack-free, high-quality quasi bulk GaN and AlN materials at a significantly high growth rate of up to 100μm/hour.
The technique of X-ray diffraction reciprocal space mapping (RSM) was used to study the strain relaxation of the InGaN layers. TDI said its studies showed that low In-content InxGa1-xN (x~0.08 to 0.15) layers were either fully strained, or partially relaxed, with relaxation strongly depending on layer thickness and full relaxation for higher In-content layer (x~0.2 to 0.4). The results were recently presented at the Second International Symposium on Growth of III-Nitrides in Izu, Japan and the 2008 International Workshop on Nitride Semiconductors in Montreux, Switzerland. “This study further confirms the ability of HVPE to grow high-quality InGaN layers and extend its capability for blue-green LEDs production in the near future,” observed Alexander Syrkin, technology deputy director and team leader for the InGaN project at TDI.
The new process is of interest because green-blue-violet light emitters based on III-nitride compounds are typically fabricated utilizing InGaN alloys in the active region of optoelectronic devices. Most of these materials are grown by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE).
Syrkin told SST that the particular structural characteristics of InGaN materials grown at TDI and cited in the presentations at IWN-2008 and ISGN-2008 represented the material parameters of the same level or exceeding those demonstrated by other methods. “The demonstration of InGaN materials with indium content in the range 10%-40% opens the opportunity to use this material both for the lattice-matched template substrate, reducing stress in the LED structure and for active regions of blue and green LEDs,” said Syrkin.
The high growth rate has been known since the HVPE technology’s introduction for III-V compound materials in the early 1960s, explained Syrkin. It’s defined by the thermodynamics, with the process usually held at atmospheric pressure and close to thermodynamic equilibrium; therefore, the efficiency of materials usage is higher than that for processes held at non-equilibrium conditions (e.g., MOCVD). “The use of atmospheric pressure also enables a higher concentration of active species participating in the growth (e.g. as compared to MBE also held at nearly thermodynamic equilibrium but in deep vacuum), thus creating a greater deposition rate,” he said. Due to lattice mismatch and the difference in thermal expansion coefficients between the substrate and the growing material, the latter tends to crack during the growth and thereafter. Higher growth rates therefore reduce cracking.
3D AFM image of the InGaN on GaN on sapphire layer surface, layer grown by HVPE. The scan area is 1×1μm2, the vertical scale is 5nm per division. (Source: TDI Inc., an Oxford Instruments company.
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How does the higher growth rate delivered by HVPE correlate with increased product quality? “It is well known that the defect density of material grown on foreign substrates (e.g., GaN on sapphire) decreases exponentially with the layer thickness, simply because the further away the growth face is from the foreign substrate, the less it is influenced by it,” Syrkin noted.
The company cannot fully disclose the mechanism responsible for the InGaN growth results, but Syrkin told SST that “it contains non-traditional approaches to the HVPE process.” No definitive answer has been discerned that can explain why the thin InGaN layers grown by HVPE can be either strained or relaxed, he added.
Because HVPE is a relatively straightforward process, it has a limited number of parameters that can be controlled and influenced, according to Syrkin. “TDI has, over the years, fully characterized each of these parameters, understood their influence on the growth process and the product quality,” he noted. “We have established the optimal operating conditions for runs of different material configurations and can repeatedly produce the same quality wafer to wafer within the run and between runs.”
The company also reported that its study on the growth of InGaN by HVPE was rewarded with substantial funding from a US government agency. — D.V.