InP laser diode

The radiative recombination of electron-hole pairs can be used for the generation of electromagnetic radiation by the electric current in a p-n junction. This effect is called electroluminescence.  In a forward-biased p-n junction fabricated from a direct band gap material, such as GaAs or GaN, the recombination of the electron-hole pairs injected into the depletion region causes the emission of electromagnetic radiation. Such a device is called a Light Emitting Diode (LED).  If mirrors are provided (usually by cleaved crystallographic surfaces of the semiconductor) and the concentration of the electron hole pairs (called the injection level) exceeds some critical value, this device may function as a semiconductor laser that emits a coherent electromagnetic radiation with all photons in phase with each other.  LEDs fabricated from different semiconductors cover a broad range of wavelengths, from infrared to ultraviolet.

The electrical conductivity of a semiconductor can be increased by adding doping elements, or small percentages of impurity elements, to the semiconductor. The presence of the small traces of impurity elements can yield extra charge carriers which are free to move through the material.

In the compound gallium arsenide, each gallium atom has three electrons in its outermost shell of electrons and each arsenic atom has five. This gives an average of four electrons per atom in the compound. When a trace of an impurity element with two outer electrons, such as zinc, is added to the crystal, the result is the shortage of one electron from one of the pairs. This shortage sets up an imbalance in which there is a place in the crystal for an electron but there is no electron available. This is commonly called a "hole." This forms the so-called p-type semiconductor in which the conduction of electricity is by motion of the hole from one atom to another.

When a trace of an impurity element with six outer electrons, such as selenium, is added to a crystal of GaAs, it provides on additional electron which is not needed for the bonding. This electron can be free to move through the crystal. Thus, it provides a mechanism for electrical conductivity. This type is called an n-type semiconductor, where n stands for negative because the carrier of electricity is the negatively charged electron.

When p-type and n-type regions are grown side by side in a semiconductor material the result is a p-n junction. On one side of the junction conduction is by electrons, and on the other side it is by positive holes. Such a device is called a diode, and it allows current flow in only one direction.

When a forward voltage is applied to the junction, the density of the carriers, both p-type and n-type, increases around the junction. Electrons move into the junction from the n-type side, and holes move into the junction from the p-type side. The battery applies energy to the electrons and holes, so that they are freed into a higher energy state. Because a hole is essentially the lack of electron in a bond, the hole may be filled with one of the extra electrons. When a reverse voltage is applied to the junction, the charge carriers move away from the junction to create a depletion region with no charge carriers. Thus, only a very small current flows when the diode is reverse biased.

Emission of Light by Semiconductor Diodes

Figure 1 shows the energy-level diagram of a semiconductor diode.

Fig. 1. Band structure near a semiconductor p-n junction.
Left: No forward-bias voltage. Right: Forward-bias voltage present

Figure 1a shows the relative populations of the energy bands or both sides of a p-n junction with no voltage applied to the diode. The n-type material contains electrons which behave as the current carriers in its conduction band, whereas the p-type material has holes for carriers in its valence band. When a forward voltage is applied to the diode, the energy levels are caused to shift as illustrated in Figure 1b. Under these conditions there is a significant increase in the concentration of electrons in the conduction band near the junction on the n-side and the concentration of holes in the valence band near the junction on the p-side.

The electrons and holes recombine and energy is given off in the form of photons. The energy of the photon resulting from this recombination is equal to that associated with the energy gap. In light-emitting diodes (LED) this light energy is transmitted out through the sides of the junction region. For the LED, all of the light is created by spontaneous emission due to electron and hole recombination. In semiconductor lasers the junction forms the active medium, and the reflective ends of the laser material provide feedback. Because of this feedback in diode lasers, most of the light is created by stimulated emission.

Basic Semiconductor Laser Design

Figure 2 is a diagram of the simplest (and earliest) type of gallium arsenide laser. GaAs cleaves easily along certain crystal planes, leaving flat parallel surfaces.

Fig. 2. Sketch of semiconductor laser device

Usually, the mirrors for feedback and output coupling are formed by the cleaved ends of the laser diode, with no further coating. The reflectivity at the interface between gallium arsenide and air is approximately 36%. If output is desired from only one end of the device, or if mirrors of higher reflectivity are desired to reduce the threshold for laser operation, the reflectivity may be increased by coating the surfaces with metal films. Two sides are purposely roughened to reduce reflection and prevent lasing "across" the diode cavity.

Because the diode laser is not 100% efficient, not all of the electrical energy input goes into creating photons. Some input energy into the semiconductor crystalline lattice itself. This elevates the bulk temperature of the semiconductor, but does not contribute to lasing.

The output power available from this laser is limited by the loop gain available within the laser cavity. The amplifier gain of the active medium is dependent on the current density through the junction. Higher currents produce greater power, but higher currents also increase heating effects that can damage the device.

Structures of Laser Diodes

Modern diode lasers are formed of structures that contain several thin layers of material of varying composition. One example is shown in Figure 9. The material is grown in thin layers, typically with thickness of the order of one or a few micrometers, starting on a substrate of semiconductor material. The growth is accomplished by carefully controlled epitaxial growth techniques, such as vapor phase epitaxy. Such growth techniques deposit very thin layers of material of specified composition as single crystalline layers of high perfection.

Fig. 9. Example of a typical type of structure for a semiconductor laser diode,
showing the different types of semiconductor layers commonly employed

The structures may contain several layers of material of varying composition and doping levels. At some position, a p-n junction region is formed. The junction region is the location where the laser operation occurs. The notations p+ and n+ refer to heavily doped conductive regions of p- and n-type material, respectively. We shall describe the functions of the regions of varying composition below. The figure also indicates some other features, such as the use of proton-bombarded areas that have low electrical conductivity. The proton-bombarded areas essentially form a stripe that confines the current and defines the area where laser operation will occur. The incorporation of a stripe, either in this fashion or by selective metallization of the surface so that the electrical contact is in the form of a stripe, is a very common feature for semiconductor lasers. The current flows only in the region where the metallization contacts the semiconductor.

The heavily doped layers are electrically conducting, facilitating making the electrical contacts. The sequence of layers AlGaAs-GaAs-AlGaAs forms what is called a double heterostructure, in which there are two changes of composition of the material as one goes through the active light-emitting region.

Figure 11 shows a homojunction structure and how the index of refraction of the material changed as one went through it. The variation in doping provided a small step in the index of refraction, as indicated on the right side of the figure. This tended to provide some confinement of light in the region of the junction, because of total internal reflection. However, the step in the index of refraction was small and the confinement was relatively poor. The losses due to spreading of light out of the active region were large, so that the drive currents had to be large, and these devices were short-lived and subject to damage. Homojunction diode lasers are now extinct.

Fig. 11. Structure and index of refraction for various types of junction
in the aluminum gallium arsenide system. Top: Homojunction.
Middle: Single heterojunction. Bottom: Double heterojunction

The use of a single heterojunction (also called a single heterostructure) as shown in the middle portion of the figure provided better confinement. In this structure, there is one change in composition of the material as one goes through the junction, so the device is called a single heterojunction. The structure provides a large change in index of refraction, according to the data shown in Figure 7. The heterostructure reduces the light that leaks into the p+ region because of waveguiding effects. This in turn leads to lower losses, lower current requirements, reduced damage, and longer lifetime for the diodes.

The bottom of the figure shows the double heterojunction (or double heterostructure), so called because there are two changes in the composition of the material as one goes through the junction. This confines the light from both sides by waveguiding effects and reduces the current requirements even further.

There is an extremely wide variation in the structures of semiconductor diode lasers and in the way the layers are actually configured. It is impossible to cover them all here. Figure 12 represents a simplified classification scheme showing some of the major subdivisions of diode lasers and their relationship.

Fig. 12. Simplified classification of major types of semiconductor laser structures

The first major division is into edge-emitting and surface-emitting devices. So far we have described only edge-emitting structures. They are of the form shown in Figure 9, in which the light emerges from the edge of the device, where the junction intersects the surface. The light emerges in the plane of the junction. The configuration is simple and easy to fabricate. Most diode lasers are edge-emitters. But they do suffer from the drawback that the volume of material that can contribute to the laser emission is limited and they are difficult to package as two-dimensional arrays.

In recent years considerable research activity has been aimed at development of surface-emitting diodes, in which the light emerges from the surface of the chip rather than from the edge. This feature is attractive because devices could be packed densely on a semiconductor wafer and it would be possible to fabricate two-dimensional arrays easily.

Temperature Dependence of Laser Output

The current threshold for lasing in GaAs is strongly temperature-dependent, as shown in Figure 5. At low temperature (up to approximately 30 ° K) the threshold is fairly constant. Above 100 ° K, the threshold current density for laser operation increases rapidly with increasing temperature. At the cryogenic temperature of 77 ° K, the threshold current in a gallium arsenide laser is about one tenth that of the room temperature value. This means that cooling to cryogenic temperatures changes the operating and performance characteristics of the laser.

Fig. 5. a) Schematic sketch of the output of a typical laser diode as a
function of drive current for three different operating temperatures. b) Temperature dependence of threshold current.

Gallium arsenide lasers emit radiation in the near infrared portion of the spectrum. The exact wavelength depends on the temperature at which the laser is operated. This is shown in Figure 6 which gives the wavelength of a gallium arsenide laser as a function of temperature. Gallium arsenide lasers have been operated over the range of temperatures from liquid helium temperature to room temperature.

Fig. 6. Temperature dependence of lasing wavelength.

The wavelength at which the laser emits is strongly influenced by the temperature of operation.

Finally, because the lifetime decreases exponentially with increasing temperature, the laser must be in good thermal contact with a heat sink capable of dissipating the thermal load generated by the laser.

Mounting and Cooling of Laser Diodes

Laser diodes are purchased not as bare semiconductor chips, but mounted in housings that allow easy handling and mounting to heat sinks for heat removal. They usually have provision for direct attachment to a heat sink. A typical view of a packaged semiconductor laser on a mount is shown in Figure 17.

Fig. 17. Example of mounting for diode laser

Infineon Technologies, Agilent Technologies, Hitachi, Intelite, Laser Components Instrument Group, Alcatel, Furukawa Electrics, Mitshubishi

Output Characteristics

Operation of a laser diode is characterized by a threshold current. Figure 7 shows the output characteristics of a laser diode as a function of input current. At low values of the input, the device acts as a light-emitting diode (LED), producing a relatively small amount of incoherent light. At a threshold value, where the population inversion is large enough so that gain by stimulated emission can overcome the losses, the laser threshold is reached. As current increases above the threshold value, the light output increases much more rapidly than in the LED region. The light is now coherent laser light. Ideally, the light output should increase linearly with current, as shown in the figure.

One defines a slope efficiency to characterize the laser diode output in this region. It is an important figure of merit that is used to characterize the laser performance. Diode lasers with slope efficiencies around 30% are available.

 Figure 7b shows the peak pulsed power of a typical GaAs laser as a function of the peak current input. The threshold current for this diode is about 10 amperes. When the current through the device is relatively low, a broad spectrum of spontaneous emission with a bandwidth of around 100 nanometers is observed. When the current through the junction is increased stimulated emission will begin when the optical gain exceeds the losses. The threshold current density will depend on the temperature, on the absorption losses in the material, on the reflectivity of the diode surface, and on the doping of the material.

Fig. 7. Peak power output of laser diode as a function of peak input current.
a) Schematic sketch of the output of a laser diode as a function of drive current.
The thresholds for laser operation and for catastrophic optical damage are indicated.

Spectral Characteristics

When the threshold current density is exceeded, the emission spectrum narrows dramatically and the intensity of the emission increases considerably. Figure 8 shows the emission spectrum of a laser diode below and also above threshold. At higher currents the linewidth of the laser output decreases.

The width of the spectral band represented by the spontaneous emission is much greater than that of the stimulated emission. However, stimulated emission produced by the laser is still much broader than that of conventional gas and crystalline lasers. It is of the order of two or three nanometers, as compared to a typical spectral width around 10^–3 nanometers for a HeNe laser.

The emission spectrum is relatively complex and typically contains a number of longitudinal modes of the optical cavity. The spacing between longitudinal modes is relatively large, because of the short length of the optical cavity. However, the relatively large spectral width of the GaAs laser allows several modes to be present.

Fig. 8. Output spectrum of a gallium arsenide laser at various input current densities for continuous operation of a double heterojunction device at cryogenic temperatures.

Spatial Characteristics

The beam emitted from a semiconductor laser typically has an elliptical spatial profile, as illustrated in Figure 9. The profile is caused by diffraction. Light is emitted through the aperture defined by the small junction. Diffraction through the narrow dimensions of the junction causes the beam to spread into a broader angle than is observed with other types of lasers.

Fig. 9. Beam profile from a stripe geometry heterojunction  laser.

In the direction perpendicular to the junction, the beam is confined by the narrow junction, typically of width around one micrometer. The beam in that direction is spread by diffraction to an angle often as large as several tens of degrees. In the direction parallel to the junction, the beam is not confined so stringently and spreads less, perhaps to around ten degrees or so. The result is a fan-shaped beam, as indicated in the figure. The beam is said to be astigmatic.

Laser Lifetime

Two different types of failure mechanisms have been identified in gallium arsenide lasers. One is a catastrophic decrease in the power output. This catastrophic damage may occur within a single pulse of the laser, and it is associated with damage of the end surfaces of the laser. The damage is produced by the light output of the laser itself. Tiny cracks or grooves in the junction are produced. To avoid this type of damage, peak power output of the laser must be limited.

There is also a gradual increase in power, which is manifested by increasing threshold current. This damage is produced by the current flowing through the junction. This is a complex phenomenon that is complicated by random variations in the laser life. To extend the life of the laser diode, current density through the junction should be limited.

Other Types of Semiconductor Lasers

In addition to gallium arsenide lasers, a variety of other semiconductor lasers have been developed. Most of these lasers are in the laboratory development and have not reached commercial status. Table 3 gives some other semiconductor laser materials, and their wavelengths of operation.

Table 3. Semiconductor Laser Materials.

Fig. 10. Picture of GaAs Semiconductor diode Laser with a Pencil Lead. (diameter of pencil lead is 0.7 mm)

Fig. 11. Wavelength ranges covered by a number of semiconductor
lasers of mixed composition.

Gallium Arsenide Lasers and Emitters, RCA Tech. Publ. OPT-100, RCA Electronic Components, Harrison, NJ.

O’Shea, Donald C.; Callen, Russell W.; Rhodes, William T. Introduction to Lasers and Applications. Reading, MA: Addison-Wesley Publishing Co., 1977.

Ready, John F. Industrial Applications of Lasers. New York: Academic Press, 1978.

Solid-State Infrared Emitting Diodes, Injection Lasers and Silicon Photodetectors, RCA Tech. Publ. OPT-100B, RCA Electronic Components, Harrison, NJ.