FIGURE 3.7 Schematic of an metal organic chemical vapor deposition system. Panel (b) is a three-dimensional diagram of the gas flow around and under the wafer stage. Panel (b) image courtesy of Veeco Instruments Inc. NOTE: TMGa = trimethyl gallium; TMIn = trimethyl indium.
each package and placing it into a specific performance bin” (DOE, 2009, p. 15). The technology of growth of LED devices and the choices of substrates for that growth form the early components of LED manufacturing and can have a profound “lever effect” with long-term implications for device yield and reliability (DOE, 2011b, p. 14).
There are two principal approaches to mitigation or elimination of the materials-related cost and performance issues. The first approach is to improve the uniformity of the epitaxial growth process. In the second approach, a fundamental breakthrough in native GaN substrate technology would allow the elimination of a vast number of crystal defects and would revolutionize the materials growth process. The two approaches are discussed below and build on an understanding of the MOCVD growth process, which is one of the steps in forming the LED devices.
As was made evident in the preceding section, the formation of the materials that comprise the LED plays a critical role in determining the color output and the efficiency of the device. Sensitive control over the composite layers of the crystalline LED device structure, some of which are only nanometers in thickness, is achieved through the use of epitaxial growth processes. In these processes, a single-crystal material (the overlayer) is grown on a crystalline substrate, and there is a registry, or relationship, between the structure of the overlayer and the substrate. The most commonly used process is MOCVD. These complex MOCVD machines are basically very sophisticated “ovens” used to produce the wafers that are later fabricated into individual LED chips. The typical MOCVD machine costs more than $2 million and can carry out growth on 60 2-inch wafers at a time.
Technology leadership in this field is still based in the United States (VEECO, Applied Materials) and Europe (AIXTRON). In MOCVD technology, ammonia gas and trimethylgallium (called a metal organic gas) are combined in a stainless steel growth chamber. In order for the reaction between ammonia and trimethylgallium to occur, forming the GaN material, the sapphire substrate must be heated to temperatures of about 1,000°C. This is done using a heated metal plate (called a susceptor). There are several possible configurations for this growth system; however, MOCVD growth based on a vertical rotating disk design has had broad acceptance. The generic rotating disk design is shown in Figure 3.7. The sample sits on the rotating disk, which is also a susceptor. Gases are injected vertically into and through a showerhead, and the high-speed rotating disk produces stable gas flow, aiding the uniformity of the material composition produced. The MOCVD technology can be used for all of the III-nitride materials (Ga, In, Al) utilizing a specific metal organic gas for each element (for example, trimethyindium for indium compounds and trimethyaluminum for aluminum compounds). Therefore, all of the elements of the LED structure can be grown in a single run. MOCVD technology has the following several advantages: (1) the ability to grow all of the III-nitride materials and alloys, (2) the ability to produce abrupt junctions between dissimilar regions of materials, and (3) the ability to produce thin (almost single atom layer) quantum well regions.
Prior research on MOCVD technology has established the fundamental understanding of reactor design and scale-up. Excellent numerical codes are available to simulate the gas flow and gas chemistry in the reactor. Therefore, scale-up in reactor size to accommodate larger substrates (and potentially lower-cost manufacturing) should be straightforward. The major challenge in MOCVD technology is control of this complicated growth process over the entire area of the substrate. Complicating the issues of MOCVD control and monitoring for the III-nitride materials is the substantial material differences between the overlayers (GaN LED structure) and the substrate (sapphire). The low thermal conductivity of the sapphire means that substrate and overlayer might not be at the same temperature during the growth process. It is therefore important to accurately measure the temperature of the surface of the growing material. The substrate and the overlayer have different thermal coefficients of expansion,