**Capabilites and Reliability of LEDs and Laser Diodes**

Melanie Ott

Technology Validation Assurance Group

Swales Aerospace

NASA Goddard Space Flight Center

301-286-0127

melanie.ott@gsfc.nasa.gov

updated 9/3/97

In general, high temperature testing is used to determine LED and laser
diode lifetimes, even though laser diode failure mechanisms are more
sensitive to increases in current density. As a measured parameter of
degradation, the current density is of great significance when searching
for failure modes in a laser diode. Raising the current density however,
is not really indicative of lifetime since it is more likely a situation
to be avoided than one that simulates normal lifetime degradation. The
reliability of semiconductor sources is very dependent on the
degradation modes. This report intends to summarize some of the
degradation modes and capabilities of typical LEDs and laser diodes
currently used in many communication and sensing systems.

LED's are typically used in multimode transmission systems where data
rates no larger than 50 Mb/s are required. They have larger spectral
widths and can add to the problem of dispersion in communications
systems. Laser diodes are used in systems that require coherent and
often single mode light such as high data rate communications and
sensing applications. In comparison to laser diodes, LED's can generally
be driven harder, are less expensive, have lower power, have larger
emitting regions, and longer lifetimes. Lasers, unlike LED's will not
operate below a threshold current. Meaning, only when the threshold
current is reached will the diode commense lasing (functioning). As
mentioned previously, LEDs and laser diodes are temperature sensitive
when considering overall lifetime, for example, operating a laser diode
at 10 °C higher than rated will half the life of the diode. Also a laser
usually will stop functioning at 100°C. The degradation modes that
result in failures or gradual degradation of these devices can be
modelled using Arrhenius relationships where each degradation mode
carries a specific activation energy. For example in reliability tests
in which lifetime is based on temperature aging the relationship is life
= A e ^Ea/kT^.

**Capabilities:**

**Table 1: Comparison of Typical Parameters of Interest for LEDs and
Laser Diodes**

+------------------+---------------------------+-----------------------+
| **Attribute/     | **LEDs**                  | **Laser Diodes**      |
| Parameter**      |                           |                       |
+------------------+---------------------------+-----------------------+
| Radiative        | Spontaneous Emission      | Stimulated Emission   |
| Recombination    |                           |                       |
+------------------+---------------------------+-----------------------+
| Particle Phase   | Incoherent                | Coherent              |
+------------------+---------------------------+-----------------------+
| Polarization     | Randomly polarized        | linearly polarized    |
| state            |                           |                       |
+------------------+---------------------------+-----------------------+
| Direction        | Random                    | linear                |
+------------------+---------------------------+-----------------------+
| 665 nm           | GaAsP                     | GaAlAs                |
+------------------+---------------------------+-----------------------+
| 800-930 nm       | Ga~1-x~ Al~x~As           | Ga~1-x~ Al~x~As       |
+------------------+---------------------------+-----------------------+
| 1300, 1550 nm    | InGaAsP                   | InGaAsP               |
+------------------+---------------------------+-----------------------+
| Spectral Width   | ∆λ ≈ 1.45 λ^2^ kT         |                       |
|                  |                           |                       |
|                  | λ in μm, kT in eV, k =    |                       |
|                  | Boltsman's constant, T =  |                       |
|                  | junction temp             |                       |
+------------------+---------------------------+-----------------------+
| Spectral Width,  | 10s of nm                 | \< 1.5 nm             |
| GaAlAs           |                           |                       |
+------------------+---------------------------+-----------------------+
| Spectral Width,  | surface emitting, 100 nm  | .1 to 10 nm           |
| InGaAsP          |                           |                       |
|                  | edge emitting, 60 -80 nm  |                       |
+------------------+---------------------------+-----------------------+
| Significant      | BW vs Power               | Threshold current,    |
| Parameters       |                           |                       |
|                  | BW increases at the       | Index guided: 10 to   |
|                  | expense of power          | 30 mA                 |
|                  |                           |                       |
|                  |                           | Gain guided: 60 to    |
|                  |                           | 150 mA                |
+------------------+---------------------------+-----------------------+
| Reliability      | 10^5^ to 10^8^ hours      | 10^5^ hours           |
| lifetimes        |                           |                       |
+------------------+---------------------------+-----------------------+
| Temperature      | increases wavelength by   | wavelength varies by  |
| Effects          | .6 nm/ °C                 | .25 nm/°C             |
|                  |                           |                       |
|                  |                           | threshold current     |
|                  |                           | rises by .5mA/°C      |
+------------------+---------------------------+-----------------------+
| Rise Time        | 1 to 100 ns               | \< 1 to 10 ns         |
+------------------+---------------------------+-----------------------+
| Output Power     | 10 - 50 (highpower) μW    | 1 - 1000 mW           |
+------------------+---------------------------+-----------------------+
| Modulation       | 3 Mhz - 350 Mhz           | \> 350 Mhz            |
+------------------+---------------------------+-----------------------+

In general the bandwidth-rise time relationship is calculated as BW =
.35 / rise time.

**x** is between 0 and 1 in Ga~1-x~ Al~x~As

Table 1 summarizes available information on a wide range of LEDs and
laser diodes. Specialty devices are not included in this summary and the
parameters specified are highly generalized. This data is included as a
reference when considering which device, due to attribute or parameter,
is most useful for the application.

**Degradation Modes**

The main degradation modes are: dislocations that affect the inner
region, metal diffusion and alloy reaction that affect the electrode,
solder instability (reaction and migration) that affect the bonding
parts, separation of metals in the heat sink bond, and defects in buried
heterostructure devices. These modes are enhanced by current during
ambient temperature operations. Facet damage due to oxidation is
enhanced by light or moisture and is particular to laser diodes.

**Degradation of the Inner Region:**

**Point Defects lead to Dark Line Defects**

The main causes of degradation in the inner region is directionally
dependent on the crystal structure as well as dependent on the material
used for the source. In particular dislocations along the 100 direction
grow as a result of interstitial atom or vacancy point defects.
AlGaAs/GaAs show a much higher rate of dislocation growth than sources
fabricated in InGaAs(P)/InP. In general, the longer the wavelength
response of the material, the less sensitive it is to this point defect.
Point defects can also lead to a slow degradation or a rapid degradation
when the defect leads to a plane defect in the crystal structure.
Improving crystal growth techniques is the only way of making them less
likely.

**Other Types of Dark Line Defects**

Dislocations along the 110 crystal axis will grow and form as a result
of mechanical bond stresses. The result of both types of dislocations
are Dark Line Defects (DLD) and induce rapid degradation of the device.
Another degradation in InGaAs(P)/InP sources is precipitation of host
atoms that result in an elevation in pulse threshold current (driving
current required for lasing). The higher this current is driven the
faster degradation of other mechanisms will occur as a result as well as
dark line defects. In looking for these types of degradation mechanisms
it is more revealing to monitor the threshold current as opposed to the
output power. The threshold current is more sensitive to defects than
the output power. As the current is driven to saturation, noise will
develop in the laser signal.

**Surface Degradation**

**Facet Damage From Oxidation in Laser Diodes**

Oxidation of a source facet can lead to slow degradation. Sources that
contain higher concentrations of aluminum tend to inhibit the oxide
growth. Alumium particles are active and inhibit diffusion to the facet
by decreasing the junction temperature. AlGaAs/GaAs sources will develop
oxide thickness proportional to their output power levels when operating
at low power and will grow thickness proportional to the square of the
output power levels when operating at higher power levels.

**Catastrophic Optical Damage (COD)**

COD occurs as a result of facet melting due to current concentration and
optical absorption. Optical absorption that encourages nonradiative
recombination results in heating and melting at the facet. The heat
generated will also cause the bandgap to shrink and therefore as a
result the current concentration increases creating more heat and the
cycle continues.

The AlGaAs/GaAs sources are much more sensitive to this type of damage
than the InGaAs(P)/InP sources. Where the first is considered unstable
against oxidation and has high rates of facet oxidation, the second has
a much lower rate of oxidation with respect to time and output power.
The same is true for COD as the first will experience this at levels
less than 1 MW/cm^2^ the second will experience not until power
densities of tens of MW/cm^2^ have been reached. One solution to this is
a non-absorbing mirror structure or NAM. The Japanese are working on
fabricating this type of technology but it is difficult to manufacture
at present.

**Alloy Electrodes**

In sources (and photodetectors) with alloy electrodes, there develops
degradation as a result of the metal diffusing in towards the inner
region. One example of an alloy type electrode is AuZnNi. During
operation the metal will diffuse creating spikes along with the
direction of current flow. The result is dark spot defects in the inner
region of the semiconductor. The Schottky-type electrodes such as
Ti/Pt/Au do not seem to cause the same degradation. The metal forms an
inert interface with between the electrode and the semiconductor
surface.

**Bonding Parts**

Soft-solders can reduce mechanical stresses on the bonding surface but
tend to add to early degradation of the device. In, Sn, and Sn-rich
Au-Sn are among the type of soft (low melting point) solders that are
attributable to solder instabilities like whisker growth, thermal
fatigue, void formation at the bonding part, and diffusion similar to
what occurs with the alloy electrodes when in contact with the
semiconductor surface. The instabilities directly lead to sudden
premature failures. The higher melting point solders, or hard solders
which include such materials as Au-rich AuSn eliminate many of the
instabilities that plague devices that have problems with soft solders.

**Buried Heterostructure**

In Buried Heterstructure diodes (BH), the configuration and index of
refraction changes nearby the active region of the laser diode creates a
waveguide for light emerging from the interactions. This type of laser
is considered an "index-guided" laser. The n-type InGaAsP active
emitting region is surrounded on both sides by the p-type InP. The
degradation mode in these lasers is associated with a breakdown or
degradation of the active region due to a decrease in injected carriers.
The degradation of the BH interface is considered a wear out failure and
is not a sudden type failure.

Above is a summary of the most generally common characteristics and
degradation modes of laser diodes and LEDs. Facet degradation is
specific to laser diodes. Some degradation modes can be eliminated
through redesign of the semiconductor structures or through packaging
techniques. For more information please review the references at the end
of this paper.

**Reliability Equations:**

There are several methods of extrapolating source lifetime including
methods of calculating lifetime given power output operating
temperature, device drive currents, and decrease in output power. Below
are several of the exprapolation equations for predicting source
lifetime.

**Output Power:**

The lifetime of a laser diode or LED can be approximated by the
following relationship. Given an initial power output of the device
*P~o\ ~*and the exponential lifetime τ, the power output over time *t*,
can be extrapolated.

$P_{\text{out}}(t) = P_{o}e^{- tt}$.

Assume that for a given time *t*, the power output of the device has
dropped to a percentage from the initial power level such that Power
ratio, *P~R~ = P~out~/P~o~* and solve for *τ* such that,

$t\text{=-}t\text{/ln}(P_{R})$.

Now with *τ* known, as well as the initial power output *P~o~*, the
power output *P~out~(t)* can be extrapolated over time *t*.

**Drive Current:**

Another way of predicting source lifetime is by extrapolation of the
current density. Elevated currents can bring out many of the degradation
mechanisms associated with point defects in devices such as AlGaAs. If
*J* is defined as the current density, the lifetime of the device is
defined as *t*, and the empirical value parameter is defined as *n*,
then there exists a relationship such that ${tµJ}^{- n}$. Therefore if
the lifetime of the device, *t~o~* is known for a given operating
current, *I~o\ ~*then a relationship between drive current and device
lifetime can be deduced from

$\frac{t_{o}}{t_{2}} = \frac{J_{o^{- n}}}{J_{2^{- n}}} = \left( \frac{J_{o}}{J_{2}} \right)^{- n} = \left( \frac{I_{o}}{I_{2}} \right)^{- n}$.

Solving for *t~2~* such that a relationship exists where lifetime can be
predicted as a result of elevated or decreased operating drive current,
*I~2.~*

$t_{2} = t_{0}\left( \frac{I_{2}}{I_{0}} \right)^{- n}$.

The values of *n* range from 1.5 to 2.0, with the larger n indicating
more of a reduction in operational lifetime or greater sensitivity of
the device to increased currents.

**Temperature**:

For determining the relationship between temperature of the device to
predict lifetime an Arrhenius relationship can be expressed as,

$$t = \text{ce}^{\frac{\text{Ea}}{\text{kT}}}$$

where

E~a~ is the activation energy for the device in units of eV,

k is Boltzman's constant = 1.38 \* 10^-23^ Joules/Kelvin,

T is absolute temperature, (273.2 + °C ) in units of Kelvin,

c is the device constant in units of time, and

e is electron charge = 1.6 \* 10^-19^ joules/eV.

The lifetime in this relationship is defined as unexceptable drive
currents for lasers where the drive currents elevate to 1.2 to 1.5 times
the rated drive current and for LEDS can be output power loss below that
of the rated value due to point defects. For life time versus
temperature calculations the following E~a~, activation energies can be
used:

AlGaAs/GaAs lasers \~ .7 eV

AlGaAs LEDs \~.5 eV

InGaAsP/InP (longer wavelength) \~ .16 eV

InGaAsP/InP Buried Heterostructure \~ .9 eV

GaAlAs Double Heterstructure LED \~ .56 eV.

Given a known activation energy *E~a~*, operating temperature *T~o~* and
lifetime of the device *t~o~*, the constant can be calculated by

$c = t_{o}e^{\frac{- \text{Ea}}{\text{kT}_{o}}}$.

Or as a ratio, *t~2~* can be solved for in terms of *T~2~* given
*T~o\ ~*and *t~o~* such that

$\frac{t_{o}}{t_{2}} = \frac{e^{\frac{\text{Ea}}{\text{kT}_{o}}}}{e^{\frac{\text{Ea}}{\text{kT}_{2}}}}$.

Simplifying to solve for *t~2~* as a function of the temperature for
accelerated life testing,

$t_{2} = t_{o}e^{- \frac{\text{Ea}}{k}\left\lbrack \frac{1}{T_{o}} - \frac{1}{T_{2}} \right\rbrack}$.

Note that for photodetectors the degradation mechanisms are different
but the same Arrhenius relationship can be used to determine lifetime of
the device given different operating temperatures. The same relationship
holds with the activation energy being \~ .7 eV for infrared detectors.
Also it is important to note that the criteria for detector lifetime
degradation is based on receiving an unacceptable signal to noise ratio
output as a result of the accelerated temperature life test.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

**References**

1.  Mitsuo Fukuda, "Semiconductor Laser and LED Reliability," OFC
    Tutorial 1996, San Jose, California.

2.  Mitsuo Fukuda, "Laser and LED Reliability Update," Journal of
    Lightwave Technology., vol.6, pp.1488 - 1495, 1988.

3.  Mitsuo Fukuda, "Degradation Modes of Semiconductor Lasers used in
    Optical Fiber Transmission Systems," SPIE vol. 1634, pp. 184-191,
    1992.

4.  John P. Powers, *An Introduction to Fiber Optic Systems*, Aksen
    Associates Inc.,1993.