Semiconductor Optical Communication Components and Devices Lecture 8: Epitaxial growth - I (Techniques)

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1 Semiconductor Optical Communication Components and Devices Lecture 8: Epitaxial growth - I (Techniques) Prof. Utpal Das Professor, Department of Electrical Engineering, Laser Technology Program, Indian Institute of Technology, Kanpur

2 Synthesis & Processing: Much of the processing and synthesis of semiconductor lasers parallels that used in conventional Integrated Circuits. Patterning, as in IC s, is done primarily through photolithography. Ion implantation and metallization are also similar to that in IC s, but different temperature constraints and material properties of GaAs or InP. A more significant departure is required for growing thin epitaxial alloy films often required for compound semiconductor devices. Techniques such as Liquid Phase Epitaxy (LPE), Molecular Beam Epitaxy (MBE), and Metal-Organic Chemical Vapor Deposition (MOCVD) are employed in growing epitaxial heterostructures for compound materials including GaAs, AlGaAs, InGaAs, InP, InGaAsP, etc. For Si mostly VPE is used. EPITAXY: Epi (top)-taxis (to deposit) Growing a crystalline layer over a crystalline substrate.

3 Overview of the competing technologies

4 Epitaxial Growth Techniques

5 Epitaxial Growth (CBE) LPE VPE MBE MOCVD CBE A)Excellent Quality, Simple Setup. B)Thin Structure is not possible. Surface uneven. A)Quality not great but moderate, Rugged B)High temperature Growth. Sharp interfaces not possible. A)Extremely narrow Layers, Quality is good, Characterizing instruments can be mounted on the chamber. B) Expensive, Limited Source, Not production oriented. P Compounds very difficult. A)Good Quality not as thin a layer as MBE but close, Infinite Source B) Highly toxic gases, Precautions needed A)Best compromise between MBE & MOCVD B) Expensive, Carbon Contamination, Not many Disadvantages.

6 Liquid-Phase Epitaxy (LPE) Gasket Seal Mo Push Rod Mo Fixed Rod Thermocouple H 2 A crystalline layer of material is grown from a saturated liquid solution. The process takes place in a high-purity carbon boat, which contains the substrate in the lower part and a series of bins with Quartz Growth Furnace O-Ring Seal Furnace Control Mo Push Rod Temperature ramp Graphite Cover Graphite Crucible Furnace Heater Etch and Growth Solution Liquids Substrate melted materials in each bin. For example, one bin might hold gallium (Ga), another aluminum (Al), and another gallium arsenide (GaAs). The substrate is placed in contact with each bin in turn, the temperature is reduced, and an exact thickness of material is crystallized. H 2 Graphite Substrate Carrier

7 Typical VPE Epitaxial Growth systems Gas Showerhead Substrates Gas Inlet Carrousel Carrousel rotation Water cooled RF Induction Heating Pumped Gas Outlet

8 VPE In vapor-phase epitaxy (VPE), the material to be deposited is transported as part of a gaseous compound, a halogen such as gallium chloride (GaCl3), or an organometallic compound, such as trimethyl gallium [(CH3)3Ga]. When the vapor touches the substrate, it reacts, depositing the material to grow the crystal. When the vapor touches the substrate, it reacts, depositing the material to grow the crystal.

9 Growth Rate (Log) Kinetics of Growth for III-V compounds Desorption Mass Transport Limited Limited By Chemical Kinetics ~ 1/850 ~ 1/500 Inverse Temperature

10 Review questions 1. Which system should one use for the growth of In x Ga (1-x) As y P (1-y)? What are the disadvantages of this growth system? 2. Why is VPE system not popular for the growth of In x Ga (1-x) As y P (1-y) communication device applications? 3. Which growth system is suitable for obtaining highest quality material? What are the disadvantages of this system? 4. How should one select the growth temperature for a particular semiconductor? 5. Between MBE and MOCVD, which is more suitable in obtaining better electronic devices (Should have less non-compensated unintentional doping) 6. Which growth system should one choose if both optical devices and electronic devices are to be integrated on the same chip?