Optoelectronic Semiconductor Devices Principals and Characteristics

 

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The disadvantage of broad area lasers : 
• 
since material non homogeneities and fabrication imperfections are inevitable, the current 
distribution is not necessarily uniform throughout the width in the y direction. As a result, the 
local current density may exceed the threshold level in some regions and may be below that level 
elsewhere. 
• 
there may be excessive strain near imperfections, which leads to an increase in the refractive 
index, which in turn causes fields to concentrate in the high index region. As a result, light appears 
in a filamentary form. This filaments are unstable and may vary with the injection current. 
As a result, broad area ILDs are inefficient, unstable and obsolete. 
3.2 STRIPE GEOMETRY LASERS 
Instead of trying to suppress the instabilities associated with filaments, which are typically 10 to 20 µm 
wide, it is better to forcibly control filament formation. This can be done by restricting the current to a 
narrow strip 10 to 20 µm wide. ILDs with this type of lateral current confinement are known as stripe 
geometry injection lasers. 
One way to achieve this objective is to use a narrow conducting strip 10 to 20 µm in width as one of the 
electrodes. With the current confined to the emission region, recombination of the injected electrons with 
holes is more efficient and threshold current density is reduced. 
3.2.1 
GAIN-GUIDED STRIPE GEOMETRY LASERS
 
There are few different schemes to restrict current in a narrow region: 
- the laser with an oxide insulating stripe; 
- ion implantation techniques; 
- internally striped structure. 
 
Figure 13.: Schematic cross section of three gain-guided stripe geometry ILDs. 
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If the stripe is much narrower than 10 µm, the fraction of current confined in the stripe region will be too 
small to be effective. 
For stripes much narrower than 10 µm, the threshold current density increases due to current spreading. 
Since the current is confined to the stripe region, gain in the narrow region is high. Examples of these 
gain-guided structures are shown on Figure 
13.
The optical gain is related to the imaginary part of the 
refractive index.