Optoelectronic Semiconductor Devices Principals and Characteristics

8.1.3 
MOLECULAR BEAM EPITAXY
 
Molecular Beam Epitaxy (MBE) also occurs far from the thermal equilibrium state. Figure 
38.
shows a 
basic idea for the MBE. Atoms or molecules in the effusion cells are evaporated by heating and shot up to 
the substrate, where they are directly deposited on the heated substrate in a high vacuum. 
Figure 38.: Basic setup for MBE. 
[4]
The high vacuum is needed to lengthen the mean free path of the atoms or molecules and the base 
pressure is controlled at about 10
-7
Pa. The epitaxial layers grow on the substrate, and the doping and the 
composition of the layers are changed by controlling deposited atoms or molecules - opening or closing 
shutters near the effusion cells. 
8.1.3.1 Advantages 
• 
MBE can produce sharp interfaces and thin layers with a precision of atomic monolayer. 
• 
Since MBE is carried out under high vacuum conditions, observations can be done by using Auger 
electron spectroscopy, electron probe microanalysis, reflection high energy electron diffraction, 
etc. 
When compared with LPE, rest of advantages and disadvantages of MBE are similar to those of MOVPE. 
8.2 DEVICE FABRICATION PROCESSES 
After creation of the active region for the device by the epitaxial growth, we start the fabrication process. 
The damage introduced during the process often determines device performance and reliability. 
 

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8.2.1 
ELECTRODE FORMATION
 
Optoelectronic devices need the ohmic contact as p-side and n-side electrodes. The electrical and thermal 
resistance of those contacts must be low as to maintain the little electrical input power and have a high 
frequency response, and there is little Joule's heating (suppression of the heating in the active region). 
Difference between the work functions of the metal and semiconductor determines the potential barrier 
between the metal and the semiconductor. 
(51) 
B
m
φ
φ
φ
=
−
s
m
s
If 
φ
, there is no potential barrier at the heterojunction for n-type semiconductors, and we can form 
an ideal ohmic contact. 
s
φ
>
If 
φ
, we get the condition for an ideal ohmic contact for the p-type semiconductor. 
m
φ
>
Unfortunately, most of semiconductors for optoelectronic devices do not satisfy the above conditions 
because of the surface states. For this reason we have to use some treatment in order to reduce the size of 
the potential barrier between the metal and the semiconductor. That is where we introduce the alloy type 
and the nonalloy type (or Schottky type) ohmic contacts.