An iron-doped indium phosphide or gallium arsenide semiconductor laser. The emiconductor material is doped when formed by uniformly distributing transition metal ions such as iron throughout the semiconductor. The concentration of the iron ions is from about 1.times.10.sup.15 to about 1.times.10.sup.18 ions per cubic centimeter, but is limited only by the solubility of iron indium phosphide or gallium arsenide. It has been determined that the greater the concentration of ions, the easier the laser emission is obtained. At liquid helium temperature, the iron-doped semiconductor laser will operate at a wavelength near 3 microns.

BACKGROUND OF THE INVENTION 
This invention relates to lasers, more particularly, to semiconductor 
lasers which have been doped with a transition metal such as iron. 
The operation of different types of lasers, such as gas, solid-state and 
semiconductor, is well known in the prior art. Each of these must be 
excited by some means of radiation or, in the case of some semiconductors, 
an electrical potential is required for operation. The prior-art lasers 
are pulsed or continuous and some operate at low temperature, room 
temperature, or high temperature. Semiconductor lasers may be excited by 
the application of an electrical current, by electron-beam bombardment, 
and by optical excitation such as by a laser or any other suitable light 
source. 
Semiconductor lasers usually generate light with photon energy very nearly 
equal to the forbidden gap energy of the particular semiconductor compound 
or alloy of which the laser is constructed. Thus, to obtain light of a 
given photon energy one must construct a laser based on a semiconductor 
which has the specific value of the energy gap, for example, GaAs laser 
light at a wavelength of about 9100.degree. A at room temperature. In 
order to obtain visible radiation it is necessary to alloy GaAs with the 
higher band gap energy semiconductor GaP; correspondingly, to shift the 
laser wavelength farther into the near infrared it is necessary to alloy 
GaAs with a narrow-gap semiconductor, such as InAs, or use the narrow-gap 
semiconductor itself as the laser material. In this manner, large numbers 
of semiconductor lasers have been produced with band gaps and photon 
energies ranging from the visible to the infrared spectral regions. 
However, each new wavelength requires a new compound or alloy and often 
the development of an entirely new production technology. Crystal growth 
procedures must be developed and perfected, doping techniques for type 
conversion, diode growth, epitaxial growth techniques, and contacting 
methods are some examples of the developmental efforts and costs that may 
be encountered in the development/production of new semiconductor lasers. 
In the face of these deterrents to the development of many laser 
wavelengths, commercial development has understandably been concentrated 
on the visible and very-near infrared spectral ranges which are vital to 
civilian application such as visible displays and fiber-optic 
communications. Much less effort has been devoted to development of 
semiconductor lasers for the near infrared and infrared spectral ranges of 
high atmospheric transparency such as from 2 microns to 3 microns. 
SUMMARY OF THE INVENTION 
An iron-doped III-V compound semiconductor laser operable in the 2 .mu.m to 
3 .mu.m wavelength range. Crystalline GaAs or InP semiconductors are doped 
with iron or another transition metal during their formation such that the 
iron ions are uniformly dispersed throughout the semiconductor. The iron 
ions are excited so that the semiconductor emits light at an output 
wavelength which depends upon the characteristic excited state 
configuration of the iron ions. Such a structure is simple to produce 
since the iron ions are added during the growth of the semiconductor 
crystal. It has been determined that the iron ion concentration in InP may 
be as high as 2.times.10.sup.17 iron ions per cubic centimeter.

DETAILED DESCRIPTION 
FIG. 1 shows the basic components of an iron-doped, III-V compound, 
semiconductor laser 10, such as a GaAs or InP laser, excited by raditaion 
of energy greater than the semiconductor band gap from a high-peak-power 
pulsed laser 11. For the purposes of discussion, an iron-doped InP 
semiconductor laser will be referred to throughout the specification. The 
iron-doped semiconductor is secured by one of the flat faces to the end of 
a cold finger Dewar 12 which is maintained at a temperature of about 
4.2.degree. K. 
During production of the InP semiconductor, iron ions are added so that the 
concentration of iron ions is in the range of about 1.times.10.sup.18 iron 
ions per cubic centimeter. Once the iron-doped InP crystal has been grown, 
the semiconductor laser element is formed. In forming the laser element, 
the crystal is cleaved along the (100) lattice plane which forms the face 
that is secured to the cold finger Dewar 12 and the face which receives 
the excitation radiation from the laser. Two of the opposing side faces 13 
and 14 are cleaved perpendicular to the (100) lattice faces along the 
(110) lattice plane to form a Fabry-Perot resonator. The other two 
opposing side faces may be roughened to prevent laser emission in a 
direction perpendicular to the laser output between the two (110) lattice 
faces. 
In operation the semiconductor laser is secured to the face of the cold 
finger Dewar by any suitable means, such as solder, so that the faces 
forming the Fabry-Perot resonator are in the direction in which the output 
is to be directed. A high-peak-power laser having an output photon energy 
greater than the band gap irradiates the crystal to excite electronhole 
pairs therein. Some of these carriers then recombine with the emission of 
photons having energy very near that of the energy gap of the InP. 
However, since the semiconductor has been doped with iron ions the 
excitation also produces photons with energy less than half the band gap 
with a spectral distribution as shown in FIG. 2. This energy is 
characteristic of the optical transitions from excited states to the 
ground state of the iron ions in the InP lattice. In iron doped InP 
semiconductors, the optical tranistions are intracenter transitions 
between the .sup.5 T.sub.2 and 5.sub.E crystal field levels of Fe.sup.2+ 
in tetrahedral coordination. For sufficiently high intensities of the 
exciting light, a population inversion will be produced in the excited 
state of the iron ions and stimulated emission of 0.35 eV light will 
occur. In the case of iron-doped GaAs, stimulated emission of 0.37 eV 
light will occur. 
The operation above has been obtained using a Krypton gas laser with an 
output of 6471.degree. A. Semiconductors of III-V compounds may be excited 
optically by a high energy electron beam, by injecting electron-hole pairs 
across a PN-junction, by injecting through PNP and NPN structures, by 
injection across heterojunction or Schottky barriers or by any other 
methods known in the art for injecting electron-hole pairs into 
semiconductors. 
The addition of transition metal ions to III-V compounds to form 
semiconductors permits one to achieve laser action at wavelengths much 
longer than those obtainable by semiconductors of III-V compounds without 
transition metal ions. It has been determined that transition metals other 
than iron, such as chromium, cobalt, nickel and manganese may be added to 
semiconductors to obtain longer laser wavelengths than those 
characteristic of the band gap of the semiconductor. The output energy is 
determined primarily by the transition metal dopant and not by the energy 
gap of the host lattice alone. Thus, lasers formed as described above 
behave in a manner analogous to the behavior of rare earth dopants in 
insulating hosts where the laser wavelength is also determined by the 
energy levels of the dopant. The major difference between the doped 
semiconductor and the solid state lasers is that the solid state lasers 
can only be excited optically by high intensity radiation sources whereas 
transition metal-doped, semiconductors, III-V compounds can be excited by 
different means as set forth above. 
The device has been described above as being optically excited. The 
iron-doped III-V semiconductor compound may be made as a PN junction and 
excited electrically as in prior art P-N junctions which are without iron 
doping. Furthermore an iron-doped semiconductor laser may be made in a 
hybrid construction in which a conventional heterostructure III-V 
semiconductor laser is grown on a suitable III-V compound, 
iron-doped-semiconductor substrate. The heterostructure laser is then 
excited electronically and the emitted radiation from the heterostructure 
optically excites the iron-doped, III-V compound, semiconductor substrate. 
It has also been demonstrated that the iron ion transition can be excited 
by the absorption of extrinsic light, that is, light with photon energy 
less than the band gap. This makes it possible to excite optically a 
larger volume of the semiconductor than can be excited by the strongly 
absorbed high energy interband light. 
Obviously many modifications and variations of the present invention are 
possible in light of the above teachings. It is therefore to be understood 
that within the scope of the appended claims the invention may be 
practiced otherwise than as specifically described.