Reaction Equations
The MOCVD process for the growth of compound semiconductor materials and devices originated in the pioneering work of H. M. Manasevit [1]. This process is also called organometallic chemical vapor deposition (OMCVD), metalorganic vapor phase epitaxy (MOVPE), and organometallic vapor phase epitaxy (OMVPE). The simplest case in MOCVD [1] involves a pyrolysis reaction of the vapors of a volatile organometallic compound and a gaseous hydride, given by:
(3-1) nRH
AD DH
A
Rn + n → +
where R is an organic radical of some unspecified form but generally of lower order, such as a methyl- or ethyl-radical, and A and D are the constituent species for the
deposited solid. Many organometallic compounds have been studied as sources for the MOCVD processes, with the most important being trimethylgallium (TMGa), trimethylaluminum (TMAl), and trimethylindium (TMIn). In general, the organometallic constituents are transported to a heated substrate by passing a carrier gas, usually hydrogen, over or through the compound contained in a constant-temperature bubbler vessel. Most MOCVD growth of III-V compound semiconductors and alloys involves the use of hydrides, such as arsine or phosphine, for the column V species. In principle, these are the simplest of column V sources to use because they are already gaseous and supplied from simple cylinder-based delivery systems. The growth of semiconductor alloys by MOCVD is easily accomplished by mixing the vapors from the different alloy constituents in the appropriate vapor phase ratio to form the desired composition. A general equation for a ternary alloy is given by:
(3-2) which applies, for example, to ternary InGaAs
(3-3)
A general equation for a quaternary alloy is given by
(3-4)
Gas Blending Systems
Generally, there are four major components in the modern MOCVD system as shown in Figure 3-1: the gas blending unit, the reactor chamber, the vacuum system and the scrubbing system, all controlled by a programming logical unit. Figure 3-2 describes the basic function of these components. The gas blending unit is responsible for transporting and mixing the precursors. The reactor chamber with a heated susceptor determines the crystal quality, the layer thickness, and the uniformity. The
vacuum system is responsible for exhausting the gas after reaction and balancing the pressure in reactor chamber. The scrubbing system deals with the toxic gas after reaction for the safety considerations. The MOCVD system used in this study is D180 series manufactured by EMCORE Co. Ltd (now with Veeco Instruments). The gas blending system is illustrated in Figure 3-3.
The gas blending system [2-6] for D180 MOCVD system is a very clean, leak-free network of stainless-steel tubing, automatic valves, and electronic mass flow controllers. The carrier gas, hydrodren, is purified with the Palledium cell. Hydride delivering modules generally require a few valves and an electronic mass flow controller, since these sources are already provided as dilute, high pressure gases in gas cylinders. Alkyl delivery modules are more complicated. These high-vapor-pressure sources are contained in stainless-steel bubblers and held in a temperature-controlled bath to maintain a stable vapor pressure over the liquid or solid source.
An important part of the gas blending system is the supply of carrier gases within a vent-run configuration, show in Figure 3-3 as the injection block. The design of the injection block can avoid transients from switching or dead space. The vent-run valves couple the individual source modules to the supply line The stable and controllable source flow rate can then be established and stabilized while the valve is vented to a waste line and prior to injection into the run supply line as shown in Figure 3-3.
Reactor Chamber
The reactor chamber is the vessel in which the source gases are mixed, introduced into a heated zone where an appropriate substrate is located, and the basic pyrolysis reactions take place. The reactor geometry in D180 MOCVD system is the
vertical type chamber as shown in Figure 3-4. The alkyl and hydride sources inject into the reactor via regulated mass flow controllers. The flow rate ratio between mass flow controllers labeled 1 to 3 for alkyl injection determines the growth rate and the uniformity of the epitaxial layers. Alkyl and hydride sources do not mix together until they reach the wafer surface in order to prevent the pre-reaction. The wafer carrier for D-180 type MOCVD contains six two-inch wafer recesses and is driven by the susceptor with high rotation speed. The reactor pressure for the epitaxial growth maintains at 70 torr controlled by the throttle valve. To achieve uniformly laminar gas flow near the wafer surface at relatively low pressure, the rotation speed is 900 round per minute. Two-zone heater locating just beneath the susceptor serves the heating source of the reactor. Adjusting the heating temperature settings of inner and outer filaments can modify the temperature distribution along the wafer surface.
Uniformity Issue
The uniformity of the epitaxial layer in terms of thickness, composition and dopant concentration is very important for the MOCVD growth. Except for the reactor pressure, total gas flow rate, the rotation speed of the wafer carrier, two adjustable parameters in D180 type MOCVD system will influence the uniformity issues. The first parameter is the ratio between the settings of alkyl-injection mass flow controllers. This ratio determines the basic growth rate distribution across the entire wafer surface. A simple examination can be done is to grow a stack of DBRs and measure its peak reflectance distribution. Figure 3-5 shows examples grown in D180 type MOCVD reactor with different ratios between the settings of alkyl-injection mass flow controllers. The standard deviation for the peak reflectance can be obtained as small as 0.57% suggesting the growth rate across the entire wafer is only 0.57% in deviation. The second parameter is the ratio between the temperature
settings of inner and outer filaments. Limited to the configuration of high-speed rotation disk, however, temperature uniformity over the entire wafer surface is quite difficult to achieve. The heat is carried away mainly from the spindle by conduction and from the carrier gas by convection. Two-zone heating helps to adjust the temperature profile over the wafer surface. The temperature deviation over 20°C can be observed with the compositional differences over the wafer by measuring the X-ray peak or photoluminescence peak distributions. However, The temperature deviation smaller than 10°C is not easy to observe. The empirical solution is to correlate the characteristics of the optoelectronic devices, such as the slope efficiency of the laser diode, to the temperature distribution.
In-situ Monitoring
The in-situ monitoring is very useful for growing optoelectronic devices such as VCSELs due to the stringent requirement of phase matching. The laser reflectometry is applied as the in-situ monitoring in D180 type MOCVD system. The schematic of configuration is shown in Figure 3-6(a). Two optical heads are mounted on the top flow flange of the reactor and monitoring the growth conditions for the upper and lower points of the wafer surface. An optical head couples two fibers; one is from the laser source with the emission wavelength at 905 nm, and the other is to the detector.
By collecting the reflection of a 905 nm laser light perpendicularly impinging on the wafer surface, the growth rate can be obtain by
GR = λTd/2n (3-5)
where λ is 905 nm, Td is the oscillation period of the reflectivity curve, n is the refractive index of the epitaxial layer at the growth temperature. It’s very convenient to grow calibrated layer with 5λ/4n thickness before the thick DBR is grown. Figure 3-6(b) shows the example for the measured reflectivity of a DBR structure. The use
of 5λ/4n thickness in calibration layers will not interfere the phase conditions in DBR layers. The accurate growth rate of the DBR layers can be obtained and modified first by observing the calibration layers. Not only the growth rate can be monitored with this technique, but also the crystal quality can be monitored. If the average reflectivity value tends to decrease, the wafer surface becomes rough. It is better to stop the growth and check the growth parameters.