Chapter 3: Experiment techniques
3.1 Molecular beam epitaxy
Our molecular beam epitaxy (MBE) apparatus comprises of two linked VEECO (or Varian) MBE systems, Fig 3.1 showing the front view. One is the GEN-II type on left hand side (called Lm MBE) without antimony source, and the other one is the modified GEN-II on right hand side (called Rn MBE) equipped with antimony source. The two systems are similar to each other and linked with a trail-included extension chamber for wafer manipulation. The samples studied here are grown with the Rn MBE. Fig 3.2(a) and 3.2(b) respectively shows the side view and rear view of the Rn MBE. A sketch is depicted in Fig. 3.3 for better understanding of the configuration. The system comprises of three parts: entry/exit chamber, buffer chamber, and growth chamber. They are joined with 6-inch VAT gate valves as seen in the sketch. Each chamber is connected to the vent system via a metal-sealed angle valve, which can lead to a pure nitrogen gas line and a pump station (not shown in the sketch) for purposes of system purge and roughly pumping down from atmosphere. The pump station consists of an oil-free diaphragm pump and a turbo-pump. Each chamber has its own function described in the following section.
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Fig. 3.1 Front view of the Lm MBE and Rn MBE linked with a center extension chamber.
Fig. 3.2(a) side view and (b) rear view of the Rn MBE.
(a) (b)
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3.1-1 Chambers’ configurations, utilities and functions
The entry/ exit (EE) chamber is also called load-lock chamber, where the trolley moves in or out through an oring-sealed quick door. Wafers are mounted by holders made from Mo, called uni-block, and hanged on the trolley, which moves on the trolley track inside the
Fig. 3.3 The configuration sketch of the Rn MBE. Three parts of the system, Entry/exit, buffer, and growth chambers, are divided and separated by gate valves. The Entry/exit chamber functioned for sample loading is equipped with a cryo pump for ultra-high vacuum (UHV) maintenance and a heater for the 200oC bake. The buffer chamber functioned for wafer preparation is equipped with a heated station for wafer baking and an ion pump for UHV maintenance. The growth chamber functioned for wafer epitaxy is equipped with shutter controlled effusion cells contained sources of As, Sb, Al, Ga, In, and with dopant cells of Si, Be, and Te. The CAR holds the wafer during growth process, which could control the growth temperature and rotate continuously for the deposition uniformity. The RHEED system uses high energy electron beams hitting the wafer surface to in-situ monitor growth condition. The pyro meter monitors wafer surface temperature via optical detection. The growth chamber is equipped with a residue gas analyzer to analyze kinds of molecules inside the chamber and to detect leakage. The UHV environment is maintained by an ion pump and a cryo pump, and assisted by the liquid nitrogen flow in cry-panel during growth.
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chamber. The EE chamber equipped with a low vibration cryo-pump to maintain ultra-high vacuum (UHV) environment around an order of 10-9 torr. The EE heater could provide 200oC baking for driving out the vapor accompanied with wafers and holder when reloading. As the vacuum below 1e-8, the trolley can be transferred into buffer chamber.
In the buffer chamber, wafers are pre-baked around ~4 hours at a heated station and stand by for growths. The purpose of baking is to remove organic contaminations of the wafer. Baking temperatures of 400oC, 350oC, and 300oC are used respectively for substrates of GaAs, InP, and InAs. The UHV environment can be as low as an order of 10-10 torr, which is maintained by an ion pump integrated with a mini titanium sublimation pump (TSP). The titanium can be sublimated to help vacuum pumping by solidifying molecular gas.
Growth chamber is where substrates perform layer growths. Three kind sources of group-III: gallium (Ga), indium (In), and aluminum (Al) are charged in the effusion cells. The type of the cell has a brand name called sumo cell in VEECO company’s products, the bulk and tip temperatures of which can be PID controlled and monitored. Except for the cold-lip design of Al cell, other cells are operated at a higher tip temperature than the bulk temperature.
Group V species of arsenide (As) and antimonite (Sb) are supplied with two cracking cells, molecular flux of which can be adjusted by their step-motor-positioned needle valves.
Dopants cells of silicon (Si), beryllium (Be), and tellurium (Te) are installed. Molecular beam source of every cell can be open or closed by a computer controlled shutter. UHV environment of growth chamber is maintained by an ion pump and a cryo pump. A TSP is also equipped and functions occasionally to help vacuum pumping. The cryo-panel is cooled down to a temperature of 77K and continuously flowed with liquid nitrogen during growth. Cold cryo-panel surface traps molecular beam and keeps a background pressure order of 10-9~10-10 torr. The residual gas analyzer, operated as pressure under ~10-8 torr, is to analyze the amount and the species of molecule inside the chamber. It also provides the sensitive helium leakage
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test mode (a sensitivity of ~10-12) to detect a leakage place by monitoring helium partial pressure as purging helium from the outside of the chamber at the same time. The reflection high energy electron diffraction (RHEED) system is a very important utility; the high energy (~10KeV) electron beam emits from RHHED gun, hits the wafer surface at a small angle, and reflects to the RHEED screen. It can be used to monitor wafer surface reconstruction diffraction pattern during growth, and also can be used to check the growth rate, which will be elucidated later. The car holds the epi ready substrate on one side, which can control temperature, and has a beam flux ion gauge to monitor beam equilibrant pressure (BEP) on the other side. The CAR can rotate toward transfer direction or growth direction, and the substrate holder also can continuously spins during growth. The car holder can heat the wafer up to a maximum temperature of 850oC. A pyrometer can measure the wafer surface temperature through a heated view port.
3.1-2 Calibrations of growth rate and doping concentration
Growth rate can be decided by RHEED oscillation as shown in Fig.3.4. A cycle of RHEED on screen goes from bright to dark and back to bright when growth surface goes from intact plane to half-grown surface, which causes lower electron beam reflectivity due to scattering from random occupied sites of atom, and back to a smooth surface after growing a monolayer.
The time period thus corresponds to growing a monolayer thickness. Except for In growing on an InAs substrate, others are performed on a GaAs substrate. The growth rate is proportional to group-III molecular beam flux. By correlating to the measured BEP, growth rate versus BEP can be drawn as in Fig. 3.5.
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The doping concentrations are verified by etched C-V measurements of several layers of GaAs grown with various doping temperatures of cells as shown in Fig. 3.6. The Ga growth rate is fixed at 1m/hr and each layer thickness is 700nm for every temperature. The n-type of Si, Te and P-type of Be are doing by the same method. The doping rate of material has an exponential relation with cell temperature which leads to a plot as shown in Fig. 3.7. The cell temperature than can be deduced for a target doping concentration.
0.2 0.4 0.6 0.8 1.0 1.2
Fig. 3.4 REED intensity oscillation trace for the case of InAs growth.
Fig. 3.5 Gallium BEP linearly depends with growth rate for the case of growing GaAs.
0 1 2 3 4
Fig. 3.6 Etched C-V measurement for doping check of the Be case, which is the result of 5 layers of GaAs, each 700nm thick, doped with various Be concentrations using different cell temperatures.
Fig. 3.7 The relationship of doping concentration and doping temperature of the Be cell.
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3.1-2 Sample growth procedure
After baking at the heated station and waiting the buffer chamber pressure below 10-8 torr, substrate can be transfer to the LN2 cooled growth chamber. Cells warm up and stay steady to the set temperatures. As2 or As4 is provided by using a crack zone temperature of 840oC or 450 oC. Sb2 is normally used with a crack zone temperature of 1050 oC. The car rises to an appropriate temperature. Then the substrate is rotated and desorbs the surface native oxide under enough group-V BEP. Once the oxide layer has been removed, RHEED paten becomes streaky as shown in Fig. 3.8, which implies surface reconstruction situation under group-V stable condition. The desorption temperatures are ~620 oC for GaAs and ~520 oC for InP and InAs substrates. Group-V species are usually at over pressure to maintain a good growth condition. Typical values for As/III ratio is ~10-20 and Sb/III is ~3-5. After the desorption process, substrate can be set to an appropriate temperature and proceeded to epi-layer growth. All the structures can be programmed and shutter sequences can be precisely controlled by the computer.
Fig. 3.8 The streaky RHEED pattern randomly taken during desorption process on the (001) GaAs substrate. The pattern evolves periodically as the substrate rotates and the substrate direction paralleled to electron beam varies between [100] and [010] directions.
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