MOCVD (Metal Organic Chemical Vapor Deposition) – ICT

MOCVD (Metal Organic Chemical Vapor Deposition)

  • Many materials have very low vapour pressure and thus are difficult to transport via gases. For these type of materials, MOCVD is used.
  • In MOCVD, metal (for example Ga, Al, Cu, etc…) is chemically attached to an organic compound that has a very high vapour pressure.
  • The organic-metal bond is very weak and can be broken via thermal means on the wafer, depositing the metal with the high vapour pressure organic being pumped away.
  • An MOCVD system must be capable of the controlled growth of thin, atomically abrupt, extremely perfect epitaxial layers.

The growth system can be divided into three sub-components:

  1. Gas Delivery
  2. Reaction Chamber
  3. Gas Exhaust

(1) Gas Delivery System:

  • The gas chamber provides well mixed and well-controlled flow of a number of gases.
  • For growing GaAs,
    • H2, TMG (Trimethyl Gallium) or TEG, AsH3
    • For p-type doping – DMZ (Dimethyl zinc)
    • For n-type doping – SiH4
  • For this process, we use OM (organometallic) arsenic source i.e. TBA (tertiary butyl arsine).
MOCVD OM Sources
Fig: MOCVD Process (Upper) and OM Sources (Lower)
  • The OM sources consist of a bubbler immersed in a controlled temperature bath of glycerin and water.
  • OM vapour is obtained by heating the liquid in a constant temperature bath and bubbling H2 through the bath.
  • The temperature of the bubbler must be carefully controlled since the vapour pressure depends exponentially on the temperature of the bath.
  • The back pressure regulator is used along with the temperature of the bath to establish the mole fraction of the OM in the gas that is sent to the reactor.
  • The OM source also includes valves so that the source can be isolated from the line and flushed with N2 when it is to be changed.
  • The details of the gas delivery system depend on the particular OM being used.
  • Some OM requires substantial heating in order to obtain a sufficient vapour pressure for deposition. For example TEAL (triethyl aluminium)

(2) Reaction Chamber:

  • The vapour of MO formed are deposited on the wafer. The gas switching is used to avoid large gas transients.
  • Two common reactors are used:
    • Wedge-shaped horizontal reactor
    • Vertical impinging jet reactor or stagnation point flow reactor

MOCVD Reactors - Horizontal Reactor and Vertical Reactor

  • The gas enters the chamber in a fully developed flow. Gas velocity is zero at the walls and it increases periodically towards the centre of the inlet tube.
  • In MOCVD process, the flow through the chamber must be laminar, with no turbulence or recirculation. Non-laminar and re-circulative flow trap gases in the chamber.
  • The plot of growth of GaAs in MOCVD as a function of substrate temperature:
    • At low temperature, the growth rate increases exponentially with temperature.
    • At high temperature, the growth rate begins to drop, because H2 release and causes gallium to deposit on the walls of the chamber and deplete TMG source.
    • Most GaAs growth is done in the intermediate temperature regime where the growth rate is determined by the arrival rate of the TMG.

plot of growth of GaAs in MOCVD as a function of substrate temperature

(3) Gas Exhaust:

  • It allows the system to operate at a controlled pressure.
  • Growth at reduced pressure can improve uniformity and layer abruptness.

Problems in MOCVD system:

Carbon contamination and unintentional hydrogen incorporation are a major problem in MOCVD.

  • This results in unintentional p-type doping. So for proper doping, the proper V/III ratio is chosen and the suitable amount of carbon gas is injected to remove out CH3 or C2H5 groups for removing contamination.
  • Also, as the human body absorbs organic compounds very easily, the metal organics are very easily absorbed by humans. Once in the body, the weak metal-organic bond is easily broken, thus, poisoning the body with heavy metals that often cannot be easily removed by normal bodily functions.

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