INDUSTRY MOCVD


system, these gas mixtures are kept separated until they enter the chamber through a multi- zone, multi-port injector array with a diameter considerably less than that of the wafer carrier. The gas streams enter separate chambers, and while confined there, are laterally diffused.


Gases then pass through an array of micro- tubes, where the temperature is stabilized, before entering the main volume of the reactor chamber. The multi-zone injection design allows the delivery of reactants to be adjusted proportionally to the surface area of the wafer carrier, offsetting the radial depletion that is characteristic of horizontal- flow reactors. This enables excellent uniformity, all the way to the outer edge of the wafer carrier.


The carrier is heated with a three-zone, radiant heating system that achieves temperatures above 1200 °C. The walls of the chamber are lined with a thin exterior layer, with a fluid gap immediately behind. Through appropriate choice of heat transfer fluid, it is possible to control the temperature of the walls and injector over a wide range. One of the attractive features of our MOCVD reactor is that virtually no reaction products are deposited on the injector – so regularly scheduled cleaning is not needed. This is possible because the wafer carrier is located far enough away from the injector array to prevent the deposition of reaction products on the injector.


The chamber lid is a top-loading, clamshell design, allowing easy access for loading and unloading of the wafer carrier and routine maintenance. Perhaps more importantly, the top-loading design minimises the internal volume of the chamber and maintains the chamber symmetry. This attribute, which is not present in systems that use a gate valve for wafer carrier transfer, helps realise a fully-symmetric flow field and abrupt switching of gases between layers.


Proven capability Our current version, the GaN-500 MOCVD reactor, released in summer 2013, accommodates 52 2-inch wafers in three concentric rings, and is easily expanded to 59 wafers with no significant design changes. This tool’s high degree of capability is revealed by photoluminescence mapping of a typical LED structure, which we have grown using a gas flow rate of around 120 standard litres per minute – some 40 percent lower than normal flow rates in ‘widely available’ MOCVD reactors of comparable capacity. The chosen structure comprises 4 µm of GaN, a 20-period strain-relieving superlattice and an eight- period multi-quantum well, capped by a standard p-type GaN layer.


This heterostructure, which was formed using a GaN growth rate of 5 µm/hr, has an average peak wavelength of 460 nm (see Figure 2). The full-platter standard deviation (1σ) is less than 1.8


nm, and the difference between the maximum and minimum values of the average wavelength emitted by the wafers is less than 4 nm. Within wafer uniformity is also excellent: Standard deviations are less than 1 nm, with many wafers showing less than 0.5 nm (see for example, Figure 3, which shows a photoluminescence map of a representative middle-ring wafer with a uniformity, in terms of 1σ, of 0.417 nm).


We attributes these excellent values of uniformity to: good macroscopic temperature and gas composition uniformity across the whole wafer carrier; excellent strain compensation at the multi-quantum well growth temperature, which ensures that the wafer lies in good thermal contact


The VPE GaN-500 features a novel injector/chamber assembly. One can see the profiled wall and the multi-port injector


Figure 2. A high degree of wafer-to-wafer photoluminescence uniformity is possible with the VPE GaN-500


August / September 2013 www.compoundsemiconductor.net 39


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