OE-14 Laser Technology

Unit 4 Part 4

Master of Technology Semester I

Dr. Vivek Kant Jogi LASER STRUCTURE Homojunction laser : which uses a single junction. These are fabricated of a single junction between two direct-bandgap materials of the same type, one p-type and one n-type, that is called a homojunction since both materials are of the same type. Light is emitted by electron–hole pair recombination's in the thin active region formed by the junction of the two materials (the depletion region). Mainly gallium arsenide (GaAs) is used, with each part of the device doped slightly differently: one part with an electron donor and one part with an electron acceptor. Mirrors for the laser cavity are fabricated simply by cleaving the crystal at right angles to the laser axis. Having an index of refraction of 3.7, the reflectivity of each mirror may be calculated to be 33% by using the Fresnel equations. This represents a large loss in the cavity; Improved performance may be achieved by fabricating a single dielectric mirror, composed of alternating quarter-wavelength-thick layers of high- and low-index-of-refraction materials, at the HR end of the laser diode.

Heterojunction Lasers : two interfaces of different indexes of refraction, one on top and one below the active region, so two junctions are formed in what is called a heterostructure laser diode, or a double heterostructure, since there are two confining interfaces. The double-heterostructure arrangement confines intracavity light in only one direction (top and bottom) of the GaAs layer, further improvement in performance can be made by manufacturing the device so that a confining dielectric interface exists on all four sides of the active region in a buried heterostructure laser

Single Heterojunction Laser under forward bias

Double Heterojunction Laser under forward bias Use of single Heterojunction for carrier confinement AlGaAs Heterojunction grown on thin p-type GaAs layer A double-heterojunction laser structure , multi layers used confine injected carriers and provide wave guiding for light.

A strip geometry designed to restrict the current injection to a narrow stripe along the lasing direction.

One of many methods for obtaining the strip geometry , this example is obtain by proton bombardment of the shaded region which converts the GaAs and AlGaAs to semi-insulating form. In order to fabricate a double heterostructure, it is necessary to find materials that can be grown on these substrates, which have bandgaps different from the substrate material, and which have a lattice constant compatible with the substrate. The simplest example is AlxGa1-xAs, which consists of two group III elements (Al and Ga) and one group V element. The bandgap increases with the Al fraction (x), but the lattice constant remains nearly unchanged. A constant lattice constant is important in order to avoid the formation of defects during the growth of the material. A double heterostructure semiconductor laser is formed by having an active layer with a low Al content, and confinement layers with a high Al content. The difference in Al content must be sufficiently high to ensure sufficient carrier confinement. The photon energy for the laser is slightly higher than the bandgap of the active layer, and by changing the Al fraction in the active layer, this photon energy can be changed, resulting in lasers with wavelengths in the 800 – 900 nm range. Because the photon energy is higher than the bandgap of the substrate (GaAs), the lower cladding layer must be sufficiently thick to avoid absorption losses in the substrate. More design flexibility is possible by using two group III and two group V materials. The prime example is In1-xGaxAsyP1-y. Lattice matching to an InP substrate is achieved by having x ~ 0.47y and the second degree of freedom in the composition can be used to vary the bandgap, and hence the wavelength. This makes it possible to fabricate lasers with wavelengths from around 1100 nm to nearly 1700 nm, covering the important ‘telecommunications’ wavelengths around 1300 and 1550 nm. The most commonly used technique for growth of heterostructures is metal-organic chemical vapour phase epitaxy (MOVPE). Hydrides such as arsine (AsH3), phosphine (PH3) and organometallics such as tri-methyl-gallium Ga(CH3)3 and tri-ethylindium In(CH3)3 are carried by hydrogen and react on the surface of the wafer. The material composition is controlled by adjusting the flow rate of the various sources. Large wafers can be grown, and some reactors allow multi-wafer handling, making this technique suitable for large volume manufacturing. MOVPE requires very stringent safety measures due to the toxicity of the hydrides. Micrograph of widely tunable laser from Bookham Technology. This particular structure, known as the ‘digital supermode structure’ has multiple individually contacted gratings in the front section. Courtesy Bookham Technology.

Laser Stripe Structures The simplest semiconductor laser stripe structure is called an oxide stripe laser. The metallic contact on the n - doped side of a semiconductor laser is normally applied with no definition for current confinement ; current confinement is introduced on the p side of the device . For a wide-stripe laser , a dielectric coating (usually SiO 2 or Si3N4 ) is evaporated on the p side of the laser . Contact openings in the dielectric are made through photolithography combined with etching of the dielectric . The p metallic contact is then applied across the whole device , but makes electrical contact only at the dielectric openings . since the active region extends outside of the stripe , there is no mechanism to prevent optical leakage in a contact-stripe laser . Lasers like this , which provide electrical confinement , but no optical confinement are called gain - guided lasers . Gain-guide laser is an ion bombardment stripe. The material outside the stripe is made highly resistive by ion bombardment or implantation which produces lattice defects . Implantation causes optical damage , so implantation should not be heavy enough to reach the active region . Complicated stripe structure with electrical and optical confinement is required for an efficient narrow-stripe laser . A number of structures which accomplish the necessary confinement have been developed. These structures are called index - guided lasers , since optical confinement is achieved through a change in refractive index. The buried heterostructure laser (BH), a planar laser structure is first grown . Stripe mesas of the laser structure are formed by photolithography combined with etching For a GaAs-based BH laser , AlGaAs is then regrown around the lasing stripe . Since the active region is completely surrounded by AlGaAs , a BH has tight optical confinement . If the regrown layers are doped to produce a reverse-biased junction or are semi-insulating , a BH laser can also provide good current confinement There are many variations on the BH structure . In some cases the active region is grown in the second growth step. The tight optical confinement of BH lasers allows practical fabrication of very narrow stripes , on the order of 1 to 2 μm. other stripe structures that provide weaker optical confinement than a buried heterostructure . One of the simplest and most widely used of these is the ridge waveguide laser (RWG). After epitaxial growth , most of the p -cladding layer is etched away , leaving a mesa where the lasing stripe will be . Only this mesa is contacted , which provides electrical confinement . Another type of laser stripe is one in which confinement is provided by the p – n junction . The best-known laser of this type is the transverse junction stripe. In order to fabricate a TJS laser , both cladding layers are grown as n -AlGaAs. Zn diffusion is then used to create a p - n junction and contacts are applied on either side of the junction. In this laser the current flows parallel to the substrate rather than perpendicular to it . In a TJS laser the active region is limited to the small region of GaAs in which the Zn diffusion front ends.