Molecular Beam Epitaxy
 
MBE is a material growth technique that allows near atomic level precision of device dimensions. An ultra-high vacuum of around 10-10 torr is necessary for this precise control of material deposition. This high vacuum is accomplished by loading and unloading the sample through a small entry chamber, which can be quickly pumped down to low pressures. The ultra-high vacuum growth chamber is only opened to the loading chamber after the loading chamber has reached a relatively high vacuum(10-8 torr); thus the growth chamber is never opened to air or other contaminents. The sample is transferred from the loading chamber to the growth chamber by a magnetically controlled mechanical system. Effusion cells adjacent to the growth chamber contain ultra-high purity materials, such as indium and gallium. These cells are heated in order to release material. The amount of material emitting from these cells is controlled by measuring the material flux at different temperatures. The cells can be opened or closed by fast shutters. Growth is monitored by Reflection High Energy Electron Diffraction(RHEED). The sample can be rotated for uniform deposition and/or heated to alter growth properties. Thus, by MBE we can control a very exact growth of materials by controlling the flux of material in an ultra-high vacuum chamber and monitoring in situ growth properties by RHEED. We currently use MBE for the precision growth of III-nitride devices.
 
  Eiko MBE
 
   GENII MBE
 
 
  Eiko style Molecular Bean Epitaxy    Gen II style Molecular Beam Epitaxy  
 The Eiko system faces the sample downwards toward cells mounted on the bottom of the growth chamber. This system has an rf nitrogen plasma source as well as a nitrogen cluster source(not shown in picture).    The GENII system has a horizontally mounted sample, so that it faces cells on the side of the growth chamber. It has an intermediate chamber between the loading chamber and growth chamber where further outgassing of the substrate is done before loading to the growth chamber. It is equipped with an rf nitrogen plasma source as well as an ammonia source.  

 
Hydride Vapor Phase Epitaxy
 
HVPE is a growth technique involving thermochemical reactions at atmospheric pressure. Three concentric quartz tubes split the thermochemical reactions into two different phases. The gases flowing through the tubes, the materials inside the tube, and the temperature at the point of the reaction determine the reaction's products. The final reaction leaves the desired material, which is deposited onto a sample that is rotated at the end of the tube. The temperatures of the reactions and deposition can be independently controlled. We currently use HVPE for the growth of GaN templates. By flowing HCl over Ga, we create GaCl, which is transported toward the substrate and reacts with NH3 near the substrate to create GaN.
 
  Horizontal HVPE
 
   Vertical HVPE
 
 
  Horizontal Hydride Vapor Phase Epitaxy    Vertical Hydride Phase Vapor Epitaxy  
 The horizontal HVPE is currently being used by Adrian Williams. The advantage of a horizontal reactor is the ease of altering the system, cleaning the system, and reloading material into the system.    The vertical HVPE is currently being used by Jasper Cabalu. The advantage of a vertical system is the neglection of gravity in the reaction profile, theoretically producing better uniformity. Our vertical system is partially automated.  

 
Cluster Source
 
  Nitrogen Cluster Beam Source    The nitrogen cluster beam source is a relatively new technique for MBE growth of nitrides. Because nitrogen gas not reactive, special steps must be taken to create nitrides. Normally, a nitrogen rf plasma is created, making the nitrogen very energetic and reactive. The nitrogen cluster source has potential to be less damaging, because each atom of nitrogen is much less energetic. Clusters of nitrogen atoms are created by having a chamber of high pressure connected to a chamber of low pressure and temperature by a small nozzle. This creates a bottleneck effect, which creates clusters of nitrogen bonded by Van der Walls forces at a high velocity. The stagnation pressure in the first chamber can be used to control the size of the cluster. After the nozzle, the clusters are charged by electrons emitting thermionically from a hot tungsten filament. They are then accellerated towards the sample by a large electric field. The voltage of this field determines the energy of the cluster. When the clusters hit the sample, each molecule will have the total energy of the cluster divided by the amount of molecules, which can be in the range of 10,000 per cluster. Because of the low energy of the molecules, they will not damage the surface of the sample because they will not penetrate deeply into the sample. However, if enough energy is given to the cluster, some nitrogen molecules will break into nitrogen atoms, which will be reactive. The sample will be highly locally heated, which will also encourage the formation of nitrides.  

 
Inductively Coupled Plasma etching system
 
  Inductively Coupled Plasma etching system    This system is a piece of vacuum equipment that we use to etch materials which are not easily etched by chemical reagents. A radio frequency coil is placed around a set of tubes containing flowing gases. By having an oscillating current in the coil, an intense electromagnetic field is produced in the tubes. A high voltage spark initially strips electrons from the gas, and these electrons are accelerated by the EM field to collide with more atoms and strip off more electrons. Thus, a high density plasma is created. This plasma is used to collide with and vaporize the atoms of the sample, etching it.  

 
Electron Beam Evaporator
 
  Electron Beam Evaporator    We have an electron beam evaporator for the deposition of materials. E-beam evaporation is particularly suitable for the evaporation of high melting point metals and materials such as oxides. Free electrons are created by thermionic emission from a hot tungsten coil. The large electric potential accelerates these electrons towards an anode target. These electrons are formed into a beam and directed towards an evaporant by permanent magnets and electromagnets. The kinetic energy of the electrons transforms into heat upon impact with the evaporant, causing large local heating. The evaporant is kept in a crucible that is in a water-chilled hearth so only the evaporant gets evaporated. The amount of evaporation can be monitored by use of a deposition rate monitor. In our evaporator, we can heat the chamber and substrate by use of a lamp heater. We can store up to 20 3" wafers in a rotatable planetary. Gas can flow through the chamber during deposition, allowing proper deposition of oxides and the creation of plasmas in the chamber.  

 
Sputtering Machine
 
  Sputtering Machine    Another thin film deposition technique is the sputtering machine. The sample to be deposited onto is placed in a chuck at the bottom of the chamber, and the wafer target to be deposited is placed at top. In vacuum, an rf plasma is created in an inert gas, usually Argon. A bias voltage is put between the plasma and the target, creating an electric field in the chamber. This field accelerates the nuclei of the plasma toward the target, which collide with the target and remove atoms that deposit themselves onto the substrate.  

 
Hall Effect
 
  Hall Effect    The Hall Effect machine can be used to measure carrier concentration as well as sheet resistance and conductivity. A sample is placed inside a vacuum kept in between a large magnet. We attach small contacts to four corners of this sample, allowing us to flow current accross the sample or measure voltage across the sample. By flowing a current accross the sample in x, and using a large magnetic field accross the sample in y, we create an electric field accross the sample in z because of F=q(v x B). The strength of this field can be used to calculate the amount of carriers in the sample.  

 
Monochrometer/Optics
 
  Inductively Coupled Plasma    Our lab is equipped with a dark room, in which we keep various pieces of optical characterization equipment. We have two monochrometers, 2 PMTs, 2 lasers-one HeCd at 325nm and one HeAg at 220nm, lock-in amps, box-car integrators, low temperature cryostats for nitrogen and helium, one optical microscope, and a xenon lamp. With all these pieces of equipment, we can perform photoluminescence measurements(time dependent and steady state) and transmission and reflection measurements. Our lab will soon also be capable of measuring cathodoluminescence as well.