STUDY OF THE FABRICATION OF SEMICONDUCTOR LASER DIODES

Done At

SOLID STATE PHYSICS LABORATORY(DRDO)

PREPARED BY:-Uttam Singh Thakran

09196504912Electrical and Electronics Engineering

TOPICS TO BE COVERED Introduction to lasersPrinciple and theory of lasersTypes of lasersProperties of lasersApplications of lasers Introduction to semiconductor lasersOperation of semiconductor laser diodeTypes of semiconductor lasersApplicationsOverview of fabrication processSteps for fabrication processconclusion

Introduction to lasersLASER is an acronym for “Light

Amplification by Stimulated Emission of Radiation”, coined in 1957 by the laser pioneer Gordon Gould.

The term “Optical Maser” (MASER= Microwave Amplification by Stimulated Emission of Radiation) was initially used, but later replaced with “laser”.

Laser technology is at the core of the wider area of photonics, essentially because laser light has a number of very special properties.

Principle of LASERThe principle of laser is based on the

stimulated emission of light.The components of a typical laser are:- 1. Gain Medium for Population energy

2. Laser Pumping energy3. Cavity4.Reflector5.Laser Beam

Theory of LASERSpontaneous EmissionStimulated EmissionPopulation Inversion

Spontaneous Emission Stimulated Emission

Types of LASERGas LasersChemical LasersExcimer(Excited Dimer) LasersDye LasersSemiconductor lasers

Properties of LASERLasers show three important properties:-MonochromaticityCoherence (Properties of Lasers)Directionality

Applications of LASERManufacturing Medical applicationsMilitary applicationsCommunicationData storage

Introduction to semiconductor laser diodes A semiconductor laser diode is formed when a

crystal is doped to produce to produce an n-type region and a p-type region, one above the other resulting in a p-n junction or diode.

A laser diode is powered by injecting electric current and therefore, they are sometimes referred to as injection laser diodes.

Laser diodes find a wide use in telecommunications. Infrared and red laser diodes are used in CD players, CDROMs and in DVD technology.

Violet lasers are used in HD DVD and blu-ray technology.

Operation of semiconductor laser diode

A typical Laser diode consists of two semiconductors, one sandwiched above the other.

Top semiconductor is GaAs- Provides holes. It is a P-type semiconductor.

At Bottom we use GaAs and Se(Gallium Arsenide and Selinium). It acts as N-type semiconductor.

P-N junction is between both the semiconductor.When current is passed through semiconductors then

Electrons and Holes starts moving towards P-N junction.

Electrons from N-type semiconductor and Holes from P-type semiconductor combines.

Since, Holes exists at lower level, hence free Electron can combine with it only after radiating energy in the form of Photon.

The top and bottom of P-N junction is coated by a mirrored material in order to trap the Photon of light.

This Photon encourages other Electrons and Holes to generate Photons which will be in same Phase and the process will continue until the P-N junction is filled with Laser light.

Some of Laser light exits in rear side which will fall on Photodiode and uses this information to regulate the voltage to Laser Diode.

Large amount of diffracted light exits through front of laser diode and the diffracted light is then made into a single beam by using Collimating Lens.

Types of semiconductor laser diodesDouble hetero-structure laserQuantum well laserQuantum cascade laserSeparate confinement hetero-junction

laserVCSEL(Vertical Cavity Surface Emitting

Lasers)

During our fabrication process of semiconductor laser diode at SSPL, we have only used Quantum well lasers.

Applications of semiconductor laser diodesLaser Range Finder(LRF)……..(1)Proximity Fuses............................

(2)Dazzler weapon……………....(3)

(1) (2) (3)

Overview of the fabrication processThe fabrication process for Quantum well

semiconductor laser is shown here.Steps are performed in cleanroom facility

under contamination control because this laser deals with micro-size features and minor contaminations can be a critical risk.

More attention is required in dealing with a piece of wafer than to the whole wafer.

Since, whole wafer is expensive therefore a piece of wafer is suitable to develop the process.

Steps for the fabrication processThere are several steps involved in the fabrication process which are mentioned below.

EPITAXIAL GROWTH

(GaAs wafer)

Cap

Cladding

Waveguide

Active Region

Waveguide

Cladding

Substrate

PHOTOLITHOGRAPHY FOR MESA

(spin coating photoresist on wafer)

(MESA patterning by photolithography process)

Photoresist

Photoresist

MASK ETCHING

(wet MESA Etching of GaAs wafer)

(Removing photoresist after etching process)

Photoresist

Cap

Cap

Substrate

DIECTRIC(SiO2) DEPOSITION

(oxide layer deposition on E-beam evaporator)

(stripe patterning on oxide by photolithography)

SiO2

Photoresist

SiO2

METALLIZATION

(Metal deposition on P-side)

(Polishing the bottom side of GaAs Wafer)

P contact

P contact

SiO2

(Metal deposition on N-side)

For P-side ohmic contact, Chromium(Cr) and Gold(Au) are deposited by E-beam evaporator.

The next step is the polishing of the bottom side of GaAs substrate for N-side contact.

For N-side ohmic contacts Ge(Germanium), Ni(Nickel) and Au(Gold) are deposited by E-beam evaporator.

N contact

CLEAVING PROCESS

In this step, each laser is cleaved by hand. After that, the facets of Lasers are coated with Asymmetric Reflectivities.

One facet is Partially Reflecting(PR), from which Laser comes out, whereas the other facet is High Reflecting(HR) which reflects Laser beam towards the PR facet.

Coating the facet

BONDING AND PACKAGING

(Die attach and wire bond on heat sink)The last step is the Bonding process between heat-sink

with Indium solder. Indium solder is widely used to bond Semiconductor Lasers due to its simplicity and it can bond directly to Copper(Cu).

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