 |
We will now try to find some answers
to our fifth question: How can we change the
wavelength of the light produced by radiative recombination? |
|
 |
The recipe coming to mind is: Mix two similar
(direct) compound semiconductors with different bandgaps. |
|
 |
Luckily, most III-V compounds are completely
miscible in ternary or even quaternary crystals. |
|
 |
In other words: From the 2 compounds GaAs and
AlAs we can make ternary GaAl1-xAsx for o<=
x<=1, from GaAs and InP we can produce quaternary
Ga1-xInxAs1-yPy. |
 |
This gives a lot of options. What
happens upon mixing, which changes of properties are useful and which are not?
Are there guidelines or do we have to try it out? |
|
 |
Generally, all properties of interest as given in
a table in subchapter 5.1.1 will change while
x and y run through the accessible range, but not necessarily
smoothly or monotonously with the composition. |
|
 |
Here we focus on just a few of the especially
important properties:
- Bandgap
- Band type (direct or indirect)
- Lattice constant
- Thermal expansion coefficient
|
|
 |
The two last properties will be of overriding
technical importance as soon as we learn how to make heterostructures, i.e.
combinations of two different semiconductors. |
 |
There are some standard diagrams
plotting properties the most important combinations. |
|
 |
The first and most important one
shows the bandgap vs. the lattice constant plus information about the band
type. It is shown below, with the II-VI compounds included for good
measure: |
|
|
|
|
|
 |
There is a tremendous amount of
information in this diagram (note that "X-gap" and L-gap" both
denote indirect band gaps a the respective positions in the band
diagram): |
|
 |
Most III-V compounds radiate at wavelength above
the visible region. However, adding some Al to GaAs producing
AlxGa1-xAs, will shift the wavelength into the red region
of the spectrum - here are our red luminescence diodes and Lasers! |
|
 |
Very fortunate: GaAs and AlAs have almost the
same lattice constant; we can thus combine any combinations of these materials
without encountering mechanical stress. |
|
 |
Very unfortunate: There are no III-V compounds in
the diagram that emit blue light - this is a severe problem for many potential
applications. While SiC (not contained in the diagram) could be used to some
extent, it was only with the recent advent of GaN that this problem was solved.
SiC and GaN crystals, however, are not of the "zinc-blende" type
common to all the III-Vs in the diagram but have a hexagonal unit cell.
They therefore do not easily mix with the
others! |
|
 |
If we want to radiate at 1.3 µm or 1,5
µm - infrared wavelength of prime importance for optical communications -
we should work with combinations of InAs, GaAs, and AlSb |
|
 |
Most interesting: The II-VI compounds are all
direct semiconductors and span a much larger range of wavelengths than the
III-V's. The fact that they are not much used for products tells us that there
must be big problems in utilizing these compounds for mass products. |
 |
Let us now look a bit more closely at
some other properties for the more important systems. |
|
|
The following diagrams show the direct and
indirect bandgap and the refractive index for Ga1-xAlxAs
as a function of x. |
|
|
|
|
|
 |
|
|
|
 |
Mixing does not only affect the band
gap and the lattice constant, but also the quantum
efficiency of light production. The next graphic shows the system GaAs -
GaP. |
|
 |
The quantum efficiency decreases rapidly as the
systems approaches the indirect bandgap region. |
|
 |
If an isoelectronic center - N in this
case - is added, the GaP side obtains a strong radiative recombination channel
via bound excitons and the quantum efficiency is
two orders of magnitude larger. |
|
|
|
|
|
 |
|
|
|
 |
Next the lattice parameters of
various mixtures as a function of x are shown. |
|
 |
This is easy to calculate, for complete mixing it
simply changes linearly with the composition index x between the values for
x = 0 and x = 1. |
|
|
|
|
|
 |
|
|
|
|
 |
Finally, the technically most important systems
are listed together with some key properties |
|
 |
We see that all kinds of ternary and quaternary
compounds are used, and that the external or total efficiency - the relation of
light out to total power in - is relatively
small in most cases. The external efficiency should not be confused with the
quantum efficiency (relation of light produced to total power minus ohmic losses), since
some of the light produced may never leave the device - remember the
fourth question! |
|
 |
Also remember that the total efficiency of a
light bulb is just a few percent. The semiconductor values don't look so bad in
this context, and that we can get up to 30 % in extreme cases is encouraging.
Note that the somewhat exotic exciton process
can account for an efficiency of 15 %!! |
|
|
|
|
|
Material(Doping) |
Wavelength
[nm] |
Transition |
External efficiency
[% of power] |
color |
SiC (Al, N) |
480 |
??? |
0.01 - 0,05 |
blue |
GaP (N) |
565 |
Exciton |
0,1 - 0,7 |
green - yellow |
GaAs0,15P0,85 (N) |
590 |
Exciton |
0,1 - 0,3 |
yellow-orange |
GaAs0,3P0,7 (N) |
630 |
Exciton |
0,4 - 0,6 |
orange-red |
GaAs0,35P0,65 (N) |
640 |
Exciton |
0,2 - 0,5 |
red |
GaAs0,6P0,4 |
650 |
Band-band |
0,2 - 0,5 |
Infrared |
Ga0,6Al0,4P (N) |
650 |
Band-band |
1 - 3 |
GaP (ZnO) |
690 |
Exciton |
4 - 15 |
GaAs |
870 |
Band-band |
0,1 |
GaAs (Zn) |
900 |
Band-acceptor |
0,5 - 2 |
GaAs (Si) |
940 |
Deep level |
12 - 30 |
In0,73Ga0,27As0,58P0,42 |
1310 |
Band-band |
1 - 2 |
In0,58Ga0,42As0,9P0,1 |
1550 |
Band-band |
|
|
|
|
|