I. Motivation
In the field of fiber optical communication, it is desirable to have monochromatic,
fast, and inexpensive light sources. At the moment, available options include Laser
Diodes, Vertical Cavity Surface Emitting Lasers (VCSELs), and Light-Emitting Diodes
(LEDs). While LEDs have broad line width and are not monochromatic, the device has
some advantages that are worthwhile for further investigation. Specifically, LEDs are
comparatively cheaper and are relatively easier to make. These devices also allow for
simpler integration. Furthermore, the outputs of LEDs are linear with the input current, a
fact that is not true for the other devices. In the future, high speed GaAs LEDs will be
beneficial to chip-to-chip, board-to-board, and platform-to-platform optical interconnects.
In order to develop optically efficient high speed LEDs, the effects of radiative
and non-radiative minority carrier recombination as a function of doping level must be
understood. Previous studies have shown that as the doping concentration increases, the
internal quantum efficiency also decreases as the minority carrier lifetime decreases.
Since it is desirable to have a device that is both bright and fast, the decrease in
brightness as the doping concentration increases is very negative effect. We are aware
that the degradation of performance is due to non-radiative recombination mechanisms
(Auger recombination effects, impurity trappings, and etc.), as well as self-absorption
effects. Therefore, we are motivated to design a few optical characterization techniques
to probe possible mechanisms of intensity degradation.
II. Objective
As mentioned in Part One, the output light of the LEDs originates from the
spontaneous emission in the active layer that is a product of the injected electrons and
holes (Figure 1). Since electric energy can be converted directly into photons, the
injection of minority carriers across the junction give rise to efficient radiative
recombination, making the injection process the most important method of excitation for
electroluminescence. The speed at which the LED could be turned on or off depends on
the average time an excess minority carrier will exist in a sea of majority carriers, or the
16
minority carrier lifetime . The lower the value for , the faster the device will become.
The question is how much can we lower ?
Figure 1: Energy band diagram and injected carrier distribution for a double
heterostructure under a forward bias (Graphics courtesy of Fukuda).
As minority carrier lifetime  is dependent upon doping concentration,
temperature, and pressure, increasing the doping concentration of the sample is one way
to lower . In the experiments performed for this project, p-type GaAs highly doped with
Beryllium was used. LEDs samples are generally doped p-type because electron
injection is more efficient compared to hole injection. The objective of this project is to
study of the effect of doping concentration on the minority carrier lifetimes, and to
understand recombination mechanism in compound semiconductors by measurement and
analysis, then relating the result to the optimization of the selected optoelectronic
compound semiconductor devices for telecommunication applications.
III. Previous Studies
A. Minority Carrier Lifetime and Internal Quantum Efficiency of Surface-Free GaAs
In this paper published by R.J. Nelson and R.G. Sobers, minority carrier lifetimes,
internal quantum efficiencies, and values of the radiative recombination coefficient B are
determined from photoluminescence time decay and external quantum efficiency data
taken for LPE GaAs samples (germanium doped or undoped) in the doping range
17
1.9  1015  po  1  1019 cm-3 at 300 K. The result showed that measurements on
samples with higher doping levels give smaller values for . The authors explained that
at lower doping levels, the minority carrier lifetimes are significantly larger due to the
higher quality of the samples (less efficient nonradiative recombination). The radiative
lifetimes determined from the data and calculations are then plotted in conjunction with
lifetime values calculated by Stern and Casey and Stern. At high doping levels, excellent
agreement is found with the calculation of Stern and Casey and Stern11.
0.9
1.00E-06
0.8
1.00E-07
0.7
Tau (sec)
0.6
1.00E-09
0.5
1.00E-10
0.4
1.00E-11
1.00E-12
1.00E-13
1.00E+15
0.3
Nelson and Sobers (tau)
Ahrenkiel et al
Nelson and Sobers (IQE)
Internal Quantum Efficiency
1.00E-08
0.2
0.1
1.00E+16
1.00E+17
1.00E+18
1.00E+19
1.00E+20
0
1.00E+21
NA- (cm-3)
Figure 2: Data from Nelson and Sobers, and Ahrenkiel.
B. Auger Recombination in Heavily Carbon-Doped GaAs
This paper described an experiment that used time-resolved photoluminescence
and quantum-efficiency techniques in parallel to measure the very short lifetimes
expected at high doping. The samples were isotype double heterostructures, and the
doping level was varied from 5  1018 to 1  1020 cm-3. It was found that for doping
levels greater than 1  1019 cm-3, the lifetime decreased as the inverse of the cube of the
hole density. The authors interpreted the result as an indication that phonon and
R.J. Nelson and R.G. Sober, “Minority-carrier Lifetime and Internal Quantum Efficiency of Surface-Free
GaAs,” Applied Physics Journal, Volume 49, (December 1978)
11
18
impurity-assisted Auger processes are dominant. In comparison with theoretical
calculations, the Auger coefficient of heavily carbon-doped GaAs with hole density for
values greater than 1  1019 cm-3 increases much more than in theory. The authors
tentatively concluded that the minority carrier lifetimes decrease in the 1  1019 to
1  1020 cm-3 hole density range is controlled by the Auger effects, or at least by the
impurities induced Auger effects12.
Revisiting Auger Recombination:
The Auger recombination is the direct recombination between an electron and a
hole, accompanied by the transfer of energy to another free hole. Such a process
involving two holes and one electron occurs when electrons are injected into a heavily
doped p+ region. The Auger lifetime A is given by 1/Gpp2 where p is the majority carrier
concentration, and Gp is the recombination rate. Similarly, recombination in a heavily
doped n+ region can occur by involving two electrons and one hole with A = 1/Gpn2. The
minority (electron) lifetime  in a p-type emitter is given by
1


1
p

1
A
As the carrier concentration increases, the Auger recombination becomes dominant,
causing a reduction of the emitter minority lifetime, which in turn reduces the emitter’s
diffusion length, resulting in the degradation of the emitter efficiency13.
R.K. Ahrenkiel, R. Ellington, and W. Metzger, “Auger Recombination in Heavily Carbon-Doped GaAs,“
Applied Physics Letters, Volume 78, (March 2001).
13
S.M. Sze, Physics of Semiconductor Devices, (New York: John Wiley & Sons, 1981) 144-145.
12
19
IV. Experiments and Data
A. Doping Profiling
In order to determine the doping concentration of the samples, we used the
Electrochemical Capacitance Voltage (ECV) profiler. To measure carrier concentration
by Capacitance-Voltage methods, the conditions are such that a depletion region
(depleted of carriers) is formed. In this apparatus, a conductive electrolyte solution
replaces the traditional metal Schottky-metal ohmic contact for the C-V measurement. As
an example, for p-type semiconductor, a depletion region could be formed with a solution
of positive ions. By making the semiconductor more negative than its equilibrium value,
positive ions would attract to the interface and holes are repelled. Variation of the
voltage on the electrolyte cell leads to dissolution of the semiconductor, so that the
semiconductor may be repetitively etched and measured, leading to an accurate
measurement of carrier concentration versus depth in the sample. For p-type materials,
forward biasing the semiconductor/electrolyte junction readily achieves dissolution. This
apparatus permits the continuous determination of doping concentration in a single
semiconductor sample.
(a)
(b)
Figure 3: (a) Movement of electrons from the semiconductor; (b) a depletion region is
formed in the semiconductor.
20
Figure 4: (a) The ECV system, (b) the ECV cell.
The samples used in our experiment ranges from 3  1018 to 6  1019 cm-3. At
these very high doping concentrations, we found that our minority carrier lifetimes are
higher than those Ahrenkiel presented. However, we have also determined that these
samples do not give off very much light. The experiments in this project are designed to
investigate some possible causes for this phenomenon.
Ga0.6Al0.4As: 0.2m; p=5*1018
1E+20
-3
082500a
083100a
071200a
1E+19
-
GaAs: 1 m
S1: p=2.0x1018 to p=6.0x1019
NA (cm )
Grading: 500Å
Grading: 500 Å
Ga0.6Al0.4As: 0.2 m; p=5*1018
1E+18
0.000
0.200
0.400
0.600
Depth (m)
GaAs: Substrate
Figure 5: Sample structure and doping profiles.
21
0.800
1.000
1.200
B. Time-Resolved Photoluminescence
We did not have the equipments to set up an experiment using this method of
measurement. However, we did send our samples to NREL to have the minority carrier
lifetime measured. Time-resolved photoluminescence is one example of minority-carrier
lifetime spectroscopy, a nondestructive way of studying the recombination mechanisms
of materials. Specifically, TRPL is used to measure the very short minority carrier
lifetime of a certain sample. To begin with, short pulses of laser light are used to create
an initial distribution of photo-excited “carriers,” or electrons and holes. The sample’s
photoluminescence (PL) is then monitored to give the desired data. When the laser pulse
reached the sample, the photon monitoring begins. The increasing photon energy is
coupled with an increase in a certain voltage that would later be converted into time
through the Time-Amplitude Converter. This process ends when a single photon has
been emitted. Time-Resolved Photoluminescence then correlates the time between
individual excitation pulses and the detection of a single emitted photon. By sending
short laser pulses and measuring a single photon of the emitted PL numerous times, this
sampling technique creates a histogram that represents the transient process that occurs
after excitations.
Setup:
Spectrometer
> 108 samples
MCP-PMT
14000
082500a
083100a
71300
Pulse Height
Discriminator
1 ps laser
Intesity (A.U.)
12000
10000
8000
6000
4000
2000
0
0.E+00 2.E-09
4.E-09
6.E-09
8.E-09
Time (sec)
Photodiode
Stop
TIME AMPLITUDE CONVERTER
Start
22
1.E-08
1.E-08
1.E-08
14000
082500a
083100a
71300
Intesity (A.U.)
12000
10000
8000
6000
4000
2000
0
0.E+00 2.E-09
4.E-09
6.E-09
8.E-09
1.E-08
1.E-08
1.E-08
Time (sec)
Figure 7: Minority carrier lifetimes for GaAs samples doped with Beryllium
Intesity (A.U.)
10000
082500a
083100a
71300
1000
100
0.E+00
1.E-09
2.E-09
3.E-09
4.E-09
5.E-09
6.E-09
7.E-09
8.E-09
9.E-09
1.E-08
Time (sec)
Figure 7: Intensity v. Time Plot
If we compare the data above with Ahrenkiel’s data, our minority carrier lifetimes
are longer for highly doped samples. However, these longer lifetimes does not improve
the photoluminescence of our samples. Our highly doped samples are very dark.
Ahrenkiel claimed that the reason for the decreased lifetime at higher doping
concentration is due to Auger recombination. Comparing our results, Ahrenkiel’s results,
and the theoretical calculations, we have the following plot:
23
1.00E-06
1.00E-07
(sec)
1.00E-08
1.00E-09
1.00E-10
Nelson and Sobers Ge-doped LPE
1.00E-11
Dumpke Theoretical Auger Limit
Takashima theoretical B and C
Ahrenkiel et. al. C-doped MBE
1.00E-12
Ito Be-doped MBE
Yale/NREL Be-doped MBE
1.00E-13
1.00E+15
1.00E+16
1.00E+17
1.00E+18
1.00E+19
1.00E+20
1.00E+21
NA- (cm-3)
Figure 8: Comparison of minority carrier lifetimes from various sources, including theoretical
calculations.
Analyzing the graph above, we observe that Ahrenkiel’s data goes beyond the
theoretical Auger limit, while the last of our lifetimes lies directly on the limit. In
addition, Ahrenkiel’s lifetimes are much shorter in comparison to the other results. In the
following experiments, our objective is to investigate the plausible causes of this
phenomenon, and to find some evidences to prove that perhaps Auger recombination is
not the only force behind the drastic decrease in internal quantum efficiency.
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C. Experiment 1 : Excitation Photoluminescence (DCPL)
Setup :
Laser Pulse
CPU
Spectrometer
Excitation photoluminescence measures the intensity of the sample’s light output.
During the process, a laser pulse is directed onto a sample where it is absorbed and
imparts excess energy into the material. For each photon of light the material absorbed,
an electron-hole pair is created. In our p-type samples, the electrons are the minority
carriers. The recombination between the electrons and the holes (the majority carriers)
may result in the emission of light. In other words, photo-excitation causes electrons
within the material to move into permissible excited states in the conduction band. When
these electrons return to their equilibrium states, the excess energy is released and may
include the emission of light (a radiative process) or may not (a nonradiative process).
The light emitted will have energy equivalent to the bandgap energy and the quantity of
the emitted light is related to the relative contribution of the radiative process.
Referring to the setup above, we changed the direction of the laser pulse through
the beamsplitter so that the laser would incident upon the sample. A lens placed in front
of the sample collects the scattered photoluminescence and collimates it into a beam
directing at spectrometer. The light beam enters the spectrometer through a slit. It is
then collimated by another mirror, directing at a grating. The grating separates the beam
into light of different wavelengths. Afterwards, a second mirror focuses the spread of
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light with different wavelengths into lines located at various positions representing the
numerical value of the wavelengths. A detector transmits the data to the computer, where
we could view the spectra.
Using Excitation Photoluminescence, we were able to generate the following
spectra for our samples with doping concentrations 3  1018, 4  1019, and 6  1019 cm-3:
4000
3500
NA=3E18
NA=4E19
NA=6E19
Intensity (A.U.)
3000
2500
2000
1500
1000
500
0
750
800
850
900
950
1000
Wavelength (nm)
Figure 10: Intensity and wavelength of different samples.
Self-Absorption Effects:
Self-absorption describes the situation when the photoluminescence generated by
the device is being absorbed within the material as it travels to the outside. In order to
determine the self-absorption effect, we used two different laser lights, namely red and
blue lasers, to pump our sample. Since red light has longer wavelengths and lower
energy than blue light, red light will be able to penetrate deeper into the sample. If selfabsorption were indeed a problem, then the sample’s photoluminescence (PL) intensity
would be less when we pump with the red laser because this PL would have to travel a
farther distance within the material than the PL generated by the blue light. The
following data is what we found when we performed the experiment.
26
4000
3500
Intensity (A.U.)
3000
2500
2000
660 nm pump 13.3 mW
405 nm pump 2.1 mW
1500
1000
500
0
750
800
850
900
950
1000
Wavelength (nm)
Figure 11: Self-absorption effects.
D. Experiment 2: Pump Intensity (input) vs. PL Intensity(ouput)
Setup:
Figure 12: Experimental Setup
27
This experiment was designed to investigate the relationship between the nonradiative and radiative recombination mechanisms as a function of minority carrier
concentration. Specifically, we wanted to know how this relationship changes as we
inject more carriers into the sample through an increase in the power of the pumping
laser.
Referring to the setup above, we used the Argon-48 laser to pump our samples.
First, the laser passed through a neutral density filter that is situated on a slider. This
apparatus allowed us to change the power of the laser. The laser was coupled to a fiber
after it went though the neutral density filter. To measure the power of the laser, we
used an integrating sphere. The integrating sphere admitted the beam inward, then a
detector, baffled from directed reflected radiation, measures the spatially integrated beam
power.
Figure 13: Integrating sphere.
With the help of a microscope objective, we pumped several small samples individually.
The fiber probe picked up the resulting photoluminescence as well as some of the laser
light. This light passed through a filter that has the function of removing light of
wavelength above that of the PL. The PL was transferred to a spectrometer and then to
the computer. Our data is shown in Figure 14.
We generated the graphs in Figure 14 from the data gathered in this experiment.
The vertical axis represents the PL intensity and the horizontal axis the pump intensity.
The two graphs at the top present data from brighter samples than the two below.
Examining the graphs, we observe a relatively linear relationship between the two
parameters, especially at higher power intensity, the region where we operated in most of
the time. At low power density, the non-linearity is probably due to impurity trappings.
As we increase the power, all the traps would be filled and would no longer interfere with
the generation of photoluminescence. Therefore output power intensity is more linear
28
with respect to the input power intensity at in at higher power range. This result implies
that the samples are not dark because we are not putting in enough power. As we
increase the pump intensity, the PL intensity also increases.
Figure 14: PL Intensity vs. Pump Intensity
V. Conclusion
Much research effort had been devoted into the study of minority carrier lifetimes
as a function of doping concentration in order to improve the quality and speed of light
emitting diodes, with hopes that these devices may be able to contribute to the growing
field of optical communications. The results of these previous studies formed the
motivation for this project, namely to investigate the recombination mechanisms of
highly doped (with Beryllium) p-type GaAs. Although the experiments performed in this
project eliminated a few failure mechanisms, it is still unclear why our samples are not
brighter despite the fact that they have longer minority carrier lifetimes. Therefore, much
more work must still be done in order to realize fast and bright light emitters that are
desirable for optical communications.
29