Optoelectronic Semiconductor Devices Principals and Characteristics

8.2.2 
DIELECTRIC FILM DEPOSITION
 
The dielectric films are widely used as masks for impurity diffusion and for chemical and physical 
etching during the process of fabrication, as an electrical insulator between an electrode and a 
semiconductor, as passivation films for device surfaces, and as film for controlling reflectivity. 
Popular materials for dielectric films used for optoelectronic devices are SiO
2
and SiN
x
. Al
2
O
3
is also 
used as passivation film as well as antireflective and highly reflective films. 
There are several kinds of methods used to deposit dielectric films: 
• 
radio-frequency sputtering: 
(i) 
a cation such as Ar
+
is formed by grow discharge excited with radio-frequency 
waves in a vacuum chamber; 
(ii) 
the ions collide with the target composed of the dielectric material and the target 
material is sputtered; 
(iii) 
the sputtered material is deposited on the semiconductor. 
The use of radio-frequency waves is required in order to prevent the dielectric target from 
being positively charged. 
• 
chemical vapor deposition (CVD): 
dielectric film deposition with CVD is carried out by the chemical reaction of material 
gases in a vacuum chamber. The chemical reaction is enhanced by excitations such as 
heating, light incidence, and electrical discharges. The material gases for SiN
x
films, for 
example, are SiH
4
and N
2
. 
• 
plasma CVD. 
Mechanical stress due to the deposited dielectric film has to be taken to consideration. If the stress is 
severe, slip dislocation or cracks are induced and degrade device performance. 
The reflectivity of the optical input and output ports is easily controlled by coating them with dielectric 
films. By adjusting the thickness of the dielectric film, we can control the reflectivity. 
The minimum reflectivity gives the condition for the antireflective film coating. The minimum 
reflectivity would be strictly zero at
(
)
1 2
1
2
rf
r
r
n
n n
=
(52) 
The dielectric material having a refractive index close to that given by formula (52) is therefore chosen 
for an antireflective film. For example, an Al
2
O
3
film with thickness of mλ
0
/4n
rf
and refractive index of 
about 1.7 (n
rf
) is a very popular antireflective coating for AlGaAs and InGaAsP surfaces exposed to air 
(n
r2
=1) because each of these materials has a refractive index of about 3.5 (n
r1
). 

---

The highly reflective films are formed by multilayers of two kinds of dielectric materials, (see Figure 
39.
), in which the thickness of each layer is usually set at the quarter-wavelength, λ
0
/4n
rf
, to increase the 
tolerance for thickness fluctuation.