[^1]

*Abstract*\--Different silicon avalanche photodiode structures are
compared for the effects of 51-MeV protons on dark current,
photocurrent, and noise. Large differences in depletion region volumes
contributed to differences in sensitivity to bulk dark current
increases. At high fluences, ionization damage appeared to be the
dominant mechanism for dark current increases in some devices. Increases
in 1/f-type noise and supplemental gamma ray testing indicate that these
high dark current increases are due to surface damage effects. A
discussion of structural parameters that may heighten radiation
sensitivity is presented, including doping levels and p-n junction
termination techniques.

# INTRODUCTION

[The]{.smallcaps} ongoing interest in space-based light detection and
ranging (LIDAR) and optical communications continues to create a demand
for highly sensitive and radiation tolerant photodetectors. Avalanche
photodiodes (APDs) are often chosen for such systems due to their low
noise and high gain compared to conventional detectors. For space
applications requiring high sensitivity, radiation-induced changes in
device parameters such as dark current need to be quantified so that
intensity dependent data are correctly interpreted. Radiation testing of
an APD with electrons and gamma rays has been studied by Swanson, et al.
\[1\]. However, radiation effects on differing avalanche photodiode
structures have not been widely presented. This study examines three
different silicon avalanche photodiode structures: conventional APDs
from Advanced Photonix and Pacific Silicon Sensor, and an IR-enhanced
APD from Perkin Elmer.

Fig. 1 shows a typical APD structure and the processes that occur in
different regions of the device. Other device configurations are
possible, but the basic principles are the same. APDs use a reverse bias
applied to a p-n junction. They operate in a fully depleted mode in that
the reverse bias creates a depletion region in the diode that extends
all the way from the junction to the p+ region at the other side of the
device. Light is absorbed and creates electron-hole pairs in the
depletion region. Electrons are swept via drift toward a very high field
region at the p-n junction called the avalanche (multiplication) region.
It is here that the electrons become energetic enough to create
additional electron-hole pairs by impact ionization, starting a chain
reaction where additional electron-hole pairs are created. Avalanche
multiplication is the internal gain mechanism of APDs.

![](media/image1.png){width="3.4972222222222222in"
height="1.4694444444444446in"}

Fig. 1 Schematic of the avalanche process in an APD

Dark current in APDs has two components: surface currents which are
unaffected by gain, and bulk leakage current which passes through the
avalanche region and is therefore gain multiplied. A common approach to
controlling surface current is the incorporation of one or more guard
rings \[2\].

# EXPERIMENTAL PROCEDURE 

Three silicon APD structures were studied to determine how 51-MeV proton
irradiation affects their characteristics: two "standard" APDs, the
036-70-62-531 by Advanced Photonix (AP) and the AD-800-9 by Pacific
Silicon Sensor (PSS), and the RCA Type C30954E IR-enhanced structure by
Perkin Elmer (PE). All are high speed APDs with active area diameters of
0.9, 0.8, and 0.8 mm, respectively. However, there are important
differences. The PE structure is enhanced for near infrared wavelengths,
and has similar responsivity at 800 nm and 1 micrometer. The AP and PSS
APDs have more typical responsivity curves, for silicon detectors, which
peak near 800 nm and fall off rapidly for longer wavelengths. The
IR-enhanced APD has a much larger active collection depth because of the
long absorption depth near the silicon bandgap edge. A more complete
description of the three structures is presented in Section IV, however
Table 1 identifies the dominant characteristic of each APD.

Table I

Silicon Avalanche Photodiodes in the Study

![](media/image2.png){width="3.4166666666666665in"
height="0.8486111111111111in"}

The APDs were irradiated up to 10${}^{\text{12}}$ p/cm${}^{2}$ at
Crocker Nuclear Laboratory, UC Davis using 51-MeV protons. Four to six
samples of each structure were irradiated and evaluated under reverse
bias, with the voltage required for a pre-irradiation gain of
approximately 100. It is important to note that we did not adjust the
gain after each dose to match pre-irradiation values, but rather
maintained a constant bias throughout testing and characterization that
matched operational voltages. This voltage was approximately 400V for
the deep IR-enhanced structure, and 200V for the shallow and
medium-thickness structures. One to two more parts of each type were
irradiated unbiased and tested after each irradiation under reverse
bias. Some additional samples were irradiated to 160 krad(Si) with
Cobalt-60 gamma rays at the Jet Propulsion Laboratory total dose
facility in order to compare proton and gamma radiation effects. The
APDs were irradiated at equivalent total dose rates of 100 rad/s (51-MeV
protons) and 50 rad/s (gammas). All irradiations were conducted at room
temperature, and pre- and post-irradiation characterization was done at
22C using device mounts placed on thermoelectric cooling (TEC) modules.
The temperature of the TEC modules was stable to within
$\pm$`<!-- -->`{=html}0.1C. Approximately ten minutes of time passed
between irradiation and characterization of the devices. This amount of
time was necessary in order for the devices to reach thermal equilibrium
with the TEC modules.

In Geiger mode, an APD is operated at a bias above its breakdown
voltage, resulting in extremely high gains (as high as 10${}^{6}$ or
more). It is important to note that the data presented in this study
relate to the case of linear mode operation only, with the APDs biased
at voltages below breakdown.

800 nm LEDs were used as a light source for cw photocurrent
measurements. 800 nm is near the peak of the responsivity curves for
these detectors and close to 815 nm, a water absorption line that is
important for certain LIDAR atmospheric studies \[3\]. The 800 nm
optical power incident on the active areas of the devices was
approximately 0.15 microwatts. The entire active area surface was
uniformly illuminated; this was accomplished by using opal diffusers
between the LEDs and the APD mounts. Three additional samples of the
deep IR-enhanced structure were evaluated at 1064 nm following biased
proton irradiation, using a diode-pumped cw Nd:YAG laser as the light
source. For this light source, an integrating sphere was used in order
to illuminate APD active areas uniformly. The 1064 nm optical power
incident on active areas was approximately 1.6 microwatts.

Pre- and post-irradiation noise measurements were taken using a special
circuit that incorporated a low-noise transimpedance amplifier and was
connected to a dynamic signal analyzer. Dark current noise spectral
density was evaluated from 10 Hz to 1.6 kHz for the deep and shallow
structures irradiated in biased and unbiased conditions. The Pacific
Silicon Sensor APD (medium thickness) was not included in the noise
study. This was due to the pin out of this APD being different than the
Advanced Photonix and Perkin Elmer devices. Our noise circuitry required
very short, air-bridged leads in order to reduce stray capacitance, and
accommodating the pin out of the Pacific Silicon Sensor device would
have required building a second circuit. At the time of this study,
funding was not available for a second circuit.

# EXPERIMENTAL RESULTS

## Dark Current

Significant increases in dark current (leakage current measured at
operational voltages under unilluminated conditions) were observed in
all three structures. Fig. 2 shows changes in dark current
($\Delta$I${}_{d}$) for representative devices of all three structures.
Pre-irradiation dark currents were approximately 40 nA (deep) and 1 to 4
nA (shallow and medium). After a fluence of 10${}^{\text{12}}$
p/cm${}^{2}$, I${}_{d}$ in most devices was observed to increase by two
orders of magnitude above pre-irradiation values. However,
post-irradiation I${}_{d}$ was approximately an order of magnitude
higher in the deep IR-enhanced structure than in the shallow or medium
thickness structures. On a log/log plot there is a linear relationship
between dark current increases and fluence.

There was an important exception to this linear trend. Half of the
shallow devices irradiated under reverse bias exhibited unusually high
shifts in I${}_{d}$ at the higher fluence levels (5 $$
10${}^{\text{11}}$ and 10${}^{\text{12}}$ p/cm${}^{2}$).
Post-irradiation I${}_{d}$ was approximately three orders of magnitude
above pre-irradiation values for these samples. The response of these
devices is also shown in Fig. 2; the circled data points show the
departure from linearity of the abnormal samples. This behavior is
discussed further in Section IV. It is also important to note the very
similar behavior of the shallow and medium thickness devices at lower
fluences, since these two structures have quite different depletion
region volumes and doping levels. This is expanded upon in Section IV.

![](media/image3.png){width="3.348611111111111in" height="2.5in"}

Fig. 2 Changes in dark current from exposure to 51-MeV protons
(representative devices irradiated under reverse bias).

Samples of all three structures were irradiated unbiased and tested
after each irradiation under bias. Dark current shifts for these parts
closely resembled the data for biased irradiations, however no
"abnormal" dark current increase was observed at high fluences for the
unbiased shallow device.

Measurements following periods of unbiased annealing were taken for up
to six days following irradiation (unbiased annealing at room
temperature was required by project specifications). All three
structures maintained much of the high I${}_{d}$ induced by radiation,
and changes following the first 48 hours were minimal. Dark current
levels for the deep IR-enhanced and medium-thickness devices were still
two orders of magnitude above pre-irradiation values following the
annealing period (Fig. 3). The more well-behaved samples of the shallow
structure also retained an I~d~ that was two orders of magnitude above
pre-irradiation levels. Although more recovery was observed in the
"abnormal" shallow devices, the dark current levels were still three
orders of magnitude above pre-irradiation levels after annealing.

![](media/image4.png){width="3.4319444444444445in"
height="2.3201388888888888in"}

Fig. 3 Dark current annealing data for representative devices irradiated
under reverse bias with 51-MeV protons and annealed unbiased at room
temperature (one exception noted).

One deep device was annealed under bias at room temperature using the
voltage that was required for a pre-irradiation gain of 100. An unusual
"reverse annealing" effect was observed with this device. Several hours
following irradiation, I${}_{d}$ was 20 percent higher than it had been
immediately following irradiation. After the initial increase, the dark
current slowly decreased, reaching a level 20 percent below the
post-irradiation value by the sixth day of annealing. One of the deep
devices that was annealed unbiased exhibited the same behavior. Although
this effect is not understood, a similar effect was reported by Swanson
et al. \[1\], who observed a 33 percent increase in I${}_{d}$ 28 minutes
after irradiation in APDs tested with electrons and gamma radiation.

Reverse annealing was also observed with all samples of the
medium-thickness devices. However, I${}_{d}$ levels following the
annealing period never dropped below the post-irradiation value with
this structure.

## Photocurrent

Photocurrent changes at 800 nm are presented in Fig. 4 for
representative devices. The photocurrent of all structures decreased
consistently with fluence. Losses by 10${}^{\text{12}}$ p/cm${}^{2}$
were approximately 65 percent (deep and medium) and 50 percent
(shallow). Data taken at 1064 nm with the deep (IR-enhanced) structure
is also presented. Note that losses at 1064 nm were twice as severe than
at 800 nm for this structure. Results for unbiased irradiations were
similar, however, at higher fluences, unbiased irradiations yielded
photocurrent losses that were 10 to 20 percent less than with biased
irradiations.

![](media/image5.png){width="3.448611111111111in"
height="2.3993055555555554in"}

Fig. 4 Changes in photocurrent from exposure to 51-MeV protons
(representative devices irradiated under reverse bias).

![](media/image6.png){width="3.3722222222222222in"
height="2.3631944444444444in"}

Fig. 5 Photocurrent annealing data for representative devices exposed to
51-MeV protons (biased irradiations).

Fig. 5 presents photocurrent recovery data for representative devices.
After approximately two days of unbiased annealing at room temperature,
photocurrent losses were about 25 percent (shallow), 32 percent
(medium), and 35 percent (deep) compared to pre-irradiation values. Most
of this recovery occurred within the first few hours of annealing. The
deep device that was annealed under bias showed greater recovery, having
only a 20 percent loss of photocurrent after 45 hours of annealing. Its
recovery was also more gradual compared to the unbiased annealing data.
Recovery for unbiased irradiations was 2 to 5 percent less than for
biased irradiations.

All devices in the study were irradiated with their TO-5 packaging and
cover glass intact. It is unlikely that the presence of the glass is
influencing our data, since irradiation with Co-60 yielded no
photocurrent changes in the devices.

## Noise

Our three APD structures have specified dark current noise spectral
densities of 0.4 to 2 pA/Hz${}^{\frac{1}{2}}$ at 10 kHz. Noise spectra
from 10 Hz to 1 kHz were evaluated before and after irradiation with
51-MeV protons and Co-60 for deep and shallow samples. This measurement
bandwidth was chosen to emphasize changes in 1/f-type noise. Fig. 6
shows data for a shallow structure taken prior to irradiation, following
biased irradiation with protons, and after one week of annealing. Data
from another shallow device following biased irradiation with gammas is
also included. Power line harmonics are evident in initial measurements,
but disappear after the noise increases from radiation damage. After
10![](media/image7.wmf) p/cm${}^{2}$, dark current noise across our
measurement spectrum increased by approximately one decade. There was a
slightly higher increase in the lower frequency (1/f) component of the
noise. Although all noise components exhibited annealing, there was less
annealing at higher frequencies.

The Co-60 data is noticeably different than the proton data. There was a
very high increase in 1/f noise following gamma radiation, and a much
smaller increase in high frequency noise. It is important to note that
post-irradiation DC dark current was almost 50 percent greater in the
sample irradiated with Co-60. Since we have shown that the shallow
structure exhibits different behavior on a part-to-part basis, what is
important to appreciate is not the relative noise amplitudes, but rather
the relative slopes of the noise spectra. Swanson et al. \[1\] observed
similar increases in 1/f noise in an APD biased for a gain of 100 and
irradiated with 1.5-MeV electrons to 300 krad (Si). They attributed
increases in high frequency noise to increases in bulk damage and
gain-multiplied dark current. Increased 1/f noise was linked to
increased surface leakage currents, most likely caused by trapped holes
in the oxide layer creating an n-type channel in the lightly doped
p-region of their structure.

![](media/image8.png){width="3.25in" height="2.515277777777778in"}

Fig. 6 Noise results for the Advanced Photonix (shallow) structure after
irradiation to 10${}^{\text{12}}$ p/cm${}^{2}$ or 160 krad(Si)
\[equivalent total dose\].

Increases in noise were smaller (about a factor of 2 less) and
relatively flat across the measurement spectrum for unbiased
irradiation. Several devices (both shallow and deep) that had increases
in dark current of 1 microampere or more, saturated our noise
measurement circuitry, so comparison of the data from typical and
"abnormal" shallow devices is not possible.

The deep structure irradiated with protons showed smaller noise
increases than the shallow structure. The noise spectrum was relatively
flat following irradiation and was approximately 10
pA/Hz${}^{\frac{1}{2}}$ at all frequencies. After one week of annealing
the spectrum was still flat but had decreased to approximately 8
pA/Hz${}^{\frac{1}{2}}$. Results were similar for unbiased irradiations.
The results from Co-60 testing of the deep structure were marred by the
same saturation issue reported above.

# Analysis and discussion

## Structural Differences in the APDs

Spreading resistance measurements were used to determine the doping
profiles of the three APD structures (Fig.'s 7 to 9). Table 2 lists key
structural parameters of the depletion regions of each device.

![](media/image9.png){width="3.111111111111111in"
height="2.292361111111111in"}

Fig.7 Doping profile of shallow Advanced Photonix APD

![](media/image10.png){width="3.375in" height="2.4034722222222222in"}

Fig. 8 Doping profile of medium-thickness Pacific Silicon APD

![](media/image11.png){width="3.2125in" height="2.3402777777777777in"}

Fig. 9 Doping profile of deep (IR-enhanced) Perkin Elmer APD

Table II

Apd depletion region characteristics

![](media/image12.png){width="3.4319444444444445in"
height="1.5416666666666667in"}

The lightly doped, near intrinsic region of the Perkin Elmer APD is
approximately 130 microns deep, twice the depth of the medium-thickness
Pacific Silicon APD and five times that of the shallow Advanced Photonix
APD. This difference can be explained because the Perkin Elmer device is
enhanced for wavelengths up to approximately 1 micron. Since the 1/e
absorption depth of photons at 1 micron in silicon is over 200 microns
(compared to approximately 15 microns at 800 nm), a depletion region
approaching this depth is necessary to achieve efficiency at long
wavelengths \[2\]. Since the APD structures were deliberately chosen so
as to have similar active region areas, the depletion region volumes for
the three structures scale approximately with depth. Note also that the
carrier concentration of the i-region of the Perkin Elmer device is very
low (4 $$ 10${}^{\text{12}}$ cm${}^{- 3}$) and approximately an order of
magnitude less than the other two devices.

## Commentary Regarding Photocurrent Losses

Changes in responsivity are expected for conventional photodiodes where
diffusion (and hence minority carrier lifetime) contribute to charge
collection. However, because APDs operate in a fully depleted mode with
carriers being transported via drift, we do not believe that we can
legitimately correlate photocurrent degradation to the familiar
mechanism of reduced minority carrier lifetime from displacement damage
\[4\], \[5\]. The field in the p- depletion region is high enough that
carrier velocities approach saturation, which makes carrier transit
times as low as 2 nanoseconds or less \[6\].

As stated earlier, project requirements dictated experimentation with a
constant bias, not a constant gain. We have deliberately not referred to
"responsivity" losses, but rather "photocurrent" losses, because
responsivity is gain dependent in APDs. What we may be observing is a
radiation mechanism where displacement damage changes the electric field
distribution within the diode causing a change in the effective gain or
in the multiplication probabilities in different regions of the p- layer
\[7\]. This is an area of ongoing investigation. However, unless losses
were extreme, the magnitude of an APD's signal in many applications may
be relatively unimportant, especially if internal calibration is part of
the system in question.

## Volume Dependence of Bulk Dark Current Increases

Our data show irradiation from 51-MeV protons causing dark current
increases of two orders of magnitude after 10${}^{\text{12}}$
p/cm${}^{2}$ in the three silicon APDs studied. Kalma and Hardwick \[8\]
observed similar dark current changes in fully depleted silicon PIN
diodes after irradiation with 1-MeV neutrons. Because APDs operate in a
fully depleted mode, this is an appropriate comparison. Previously,
leakage current increases in neutron irradiated silicon devices have
been attributed primarily to the creation of carrier generation centers
in the depletion region bulk by displacement damage. Our data agrees
well with calculations of dark current changes ($\Delta$I${}_{d}$) based
on the displacement damage coefficients for silicon depletion regions of
Srour, et al. \[9\] according to

$\frac{{\Delta I}_{d}}{V}$ =
$\frac{\text{qn}_{i}\varphi}{\text{2K}_{\text{gn}}}$ (1)

where V is the depletion region volume, n${}_{i}$ is the intrinsic
carrier density, $\varphi$ is the neutron fluence, and K${}_{\text{gn}}$
is the damage coefficient. Bulk dominated dark current can also be
considered to be gain multiplied. After applying the appropriate NIEL
ratio \[10\], \[11\] and correcting for APD gain, our
$\frac{{\Delta I}_{d}}{V}$ from 51-MeV protons are within a factor of 2
of that reported by Kalma and Hardwick in silicon PIN diodes after
neutron irradiation (one exception was that the medium thickness
structure was a factor of 4 less at 10${}^{\text{12}}$ p/cm${}^{2}$).

As is evident from (1), $\Delta$I${}_{d}$ from displacement damage is
directly proportional to depletion region volume. It is evident from our
data that displacement damage is contributing to the observed dark
current increases in our three structures. However, displacement damage
considerations alone are not sufficient to explain the observed
behaviors of the structures.

The depth of the medium-thickness APD is twice that of the shallow APD.
However, a factor of two difference in $\Delta$I${}_{d}$ was not
observed. In fact, the behavior of the two structures was virtually the
same at lower fluences. A general guideline for bulk leakage current
sensitivity cannot be reduced to volume considerations alone. The
shallow structure has a carrier concentration that is twice that of the
medium-thickness structure. Although damage constants for very high
resistivity Si are not as widely known, damage coefficients for lower
resistivity Si tend to increase with carrier concentration \[12\]. The
medium-thickness device may be showing less damage than expected due to
having a lower carrier concentration in the p- layer than the shallow
device. In other words, the difference in volume of the medium and
shallow structures may be offset by the differences in their carrier
concentrations.

The depletion region volume of the deep APD is 5 times greater than the
shallow APD. Although the respective changes in dark current have a
ratio close to 5 to 1 at lower fluences, this ratio decreases to
approximately 2 to 1 by 10${}^{\text{12}}$ p/cm${}^{2}$. In addition,
the "anomalous" shallow devices, whose dark current changes far exceed
those of the deep devices at higher fluences, change this ratio at
10${}^{\text{12}}$ p/cm${}^{2}$ to *1 to 7*! It is evident that another
mechanism is responsible for the high leakage currents in the shallow
structure.

## Effect of Ionization on Surface Dark Current Increases

Supplemental testing with Co-60 was performed to determine the total
ionizing dose (TID) contribution to the dark current changes observed
with protons. Although both types of radiation cause ionization damage,
gamma radiation primarily causes ionization with minimal displacement
damage, while protons produce both ionization and displacement effects.

Fig. 10 revisits the 51-MeV proton data for the shallow structure and
includes data from gamma radiation testing of this structure. At lower
equivalent total dose levels, gamma radiation yielded a relationship
between damage and dose that was similar to that of the 51-MeV proton
results. The proton and gamma plots also appear to be approximately
separated by the expected NIEL ratio for displacement damage from 51-MeV
protons compared to gamma-induced Compton electrons. This indicates that
displacement damage (bulk damage) was the major radiation effect
observed with protons at lower fluences and that ionization (surface
effects) was far lower. At the higher dose levels, gamma radiation also
caused a more rapid increase in dark current, with the "knee" in the
data occurring at the same level of approximately 25 krad(Si). Dark
current increases were not quite as high as what was observed with
protons at higher dose levels. However, gamma testing was done with
devices from a different lot than that used for proton testing, and
lot-to-lot part variation may be responsible for this difference.

![](media/image13.png){width="3.448611111111111in"
height="2.4972222222222222in"}

Fig. 10 Comparison of changes in Dark Current due to 51-MeV protons and
Co-60 (shallow Advanced Photonix APD)

Isochronal annealing was performed on representative proton-irradiated
samples of the three structures, including one "abnormal" shallow device
(see Fig. 11). Devices were placed in a nitrogen-purged oven for 30
minute periods before being recharacterized. The temperature for the
first period was 80 C, and was increased by 20 C for each subsequent
period. This technique was chosen in the event there was a temperature
threshold for annealing (which did not prove to be the case). The
maximum temperature used for the shallow and medium thickness devices
was 100 C, because the maximum storage temperature was 85 C for the
shallow parts and unknown for the medium thickness parts.

![](media/image14.png){width="3.25in" height="2.4868055555555557in"}

Fig. 11 Isochronal anneal of APDs irradiated under bias with 51-MeV
protons

A gradual reduction in dark current was observed in all our structures
during isochronal annealing. However, very little change in photocurrent
was observed at any temperature. Although data for temperatures above
100 C is not available for the shallow and medium thickness structures,
the recovery rates for the three structures appear similar at 100 C and
below.

After the initial 30 minutes at 100 C, the shallow and medium thickness
devices underwent a 24-hour anneal at 100 C. A 57 percent reduction in
dark current was observed in these devices following the 24-hour anneal.
Similar reductions may also have occurred in the deep (IR-enhanced)
device at 100 C if the annealing periods for this device had been longer
than 30 minutes. The abnormal shallow device showed the largest
reduction in I${}_{d}$, losing 67 percent after the 24-hour anneal. This
is a further indication that surface damage (ionization) was a greater
issue for the "abnormal" shallow devices, as this type of damage tends
to anneal more readily with temperature than displacement damage \[13\].

Our supplemental testing with gamma radiation suggests that a TID effect
is a significant contributor to the dark current in the shallow
structure, especially at equivalent total doses above 25 krad(Si). The
increase in 1/f noise observed after irradiation with the shallow
structure also suggests that surface effects are very important \[1\].
This effect also seems to have both a lot-to-lot and part-to-part
variability. The shallow structure uses four guard rings to control
leakage current along the junction edges caused by the junction
curvature effect. Previous work with high voltage power MOSFETs \[14\]
has shown that charge trapping in the oxide above and between field
rings can change their effectiveness by inducing n layers in the lightly
doped p- region allowing surface currents to flow. Variations or
unevenness in the quality of the oxide used for the shallow structure
could be leading to the part-to-part variations in radiation response
that we observed.

Recall that when the shallow devices were irradiated unbiased no
"abnormal" increase in dark current was observed. There also was less of
an increase in 1/f-type noise in unbiased shallow devices. This is
consistent with the effect of bias on hole trapping in oxide. If
irradiations are biased, electrons are swept to the positive contact,
leaving the less mobile holes trapped in the oxide. When irradiations
are unbiased, electron-hole pairs created during ionization will have
more of a tendency to recombine.

The fact that very little increase in dark current was observed with
Co-60 until 25 krad(Si) confirms that the increase in dark current
following proton irradiation is primarily due to bulk damage at lower
fluences. We attribute the relatively small increase in dark current
prior to 25 krad(Si) of total dose to displacement damage from Compton
electrons produced by gamma ray irradiation.

## Carrier Removal Concerns

Another concern when choosing an APD is whether carrier removal effects,
which become important at higher fluences, may become involved. For
example, the depletion region of the deep structure uses a relatively
lightly doped p- layer. If carrier removal effects in this region are
large enough to cause dopant inversion, turning the p- region into a
more neutral layer, then n-type guard rings would lose the ability to
control surface leakage currents effectively. Carrier removal has also
been shown to cause device failure in solar cells by increasing the
resistivity of the base layer to the point where short circuit current
abruptly decreases \[15\]. The change in carrier concentration in p-type
silicon as a function of proton fluence is given by:

$\Delta$p = R${}_{c}$$\varphi$ (2)

where R${}_{c}$ is the carrier removal rate and $\varphi$ is fluence
\[16\]. Carrier removal rates for 7 to 14 Ohm-cm p-type silicon have
been shown to range from 50 to 100 cm${}^{- 1}$ for 3 and 10-MeV protons
\[15\]. The resistivities of the depletion regions in our APD structures
are one to two orders of magnitude higher, and we do not know the exact
carrier removal rates for such materials. However, with such low carrier
concentrations, one can assume that carrier removal will eventually
become an issue at high enough fluences, possibly leading to APD
failure. The contribution of carrier removal effects to our data is an
area of continuing study.

## Selection Considerations

It is evident that care must be used when choosing an APD structure for
sensitive space applications. Note that these detectors may be used with
light levels as low as several femtowatts and have peak responsivites of
50 to 60 A/W. We observed a $\Delta$I${}_{d}$ of over 1 microamp in the
deep (IR-enhanced) structure, and nearly 9 microamps for some shallow
samples, after 10${}^{\text{12}}$ p/cm${}^{2}$. For space applications
requiring light levels near the lower limit of these detectors, shifts
in dark current of 1 microamp or more could be quite significant.

At lower fluences there appears to be a relatively straightforward trade
off between high responsivity at long wavelengths and sensitivity to
bulk damage due to the necessarily long depletion width.

However, the choice of an APD based on bulk damage considerations is
further complicated by issues related to the doping level of the
depletion region and how these doping levels may influence damage
constants. Also, some devices may be inherently sensitive to total dose
effects due to the nature of their surface structures. Therefore, device
architecture and possible surface effects must also be considered,
including the potential for carrier removal effects which can cause type
conversion in surface layers thereby weakening the effectiveness of
guard rings intended to control leakage currents. We would also caution
designers that relative radiation hardness can have a lot-to-lot, or
part-to-part variability, as we have shown with the shallow structure.

# conclusion

There is a continuing need for highly sensitive detectors in space
applications. This study examines three avalanche photodiode structures
with very different internal structures. Decreased photocurrent and
increased dark current were observed after irradiation with 51-MeV
protons for all three devices. However, the long-wavelength-enhanced
structure with the wider depletion region showed a much larger
sensitivity to bulk dark current increases.

Comparison of proton and gamma ray data indicate that radiation-induced
increases in dark current are due primarily to displacement damage in
the depletion region and are therefore directly proportional to the
volume of this region, at low fluences. Because silicon detectors
intended for long wavelength applications require wide absorption
regions in order to efficiently collect light, there is a trade off
between the desire for high gain in the near infrared and sensitivity to
bulk damage in the depletion region which leads to increased bulk dark
current. For near IR applications, it may be desirable to chose a
detector with a smaller depletion region and sacrifice some initial
responsivity at wavelengths near 1 micron for the sake of decreasing
vulnerability to bulk damage (the importance of dark current increases
depends on the application in question). However, our results also show
that shallow devices may come with architectural drawbacks that can
contribute to surface currents at high enough fluences. It is therefore
important to consider volume, doping, and junction termination methods,
along with application requirements (such as wavelength and gain), when
choosing an APD for use in a radiation environment. We have also shown
that part-to-part variability can be significant enough that radiation
testing with small sample sizes is not advisable.

# Acknowlegments

The authors are indebted to Eric Swanson for very helpful discussions
regarding noise measurements. We acknowledge the valuable assistance of
Bernard G. Rax in the development of our noise measurement circuitry.

# References

1.  E. A. Swanson, E. R. Arnau, and F. D. Walther, "Measurements of
    natural radiation effects in a low noise avalanche photodiode,"
    *IEEE Trans. on Nucl.. Sci.*, vol. 34, no. 6, pp. 1658-1661, Dec.
    1987.

2.  S. M. Sze, *Physics of Semiconductor Devices*, 2nd ed., John Wiley &
    Sons, 1981.

3.  T. F. Refaat, G. E. Halama, and R. J. DeYoung, "Characterization of
    advanced avalanche photodiodes for water vapor lidar receivers,"
    *NASA/TP-2000-210096*, July 2000.

4.  M. Yamaguchi, S. J. Taylor, S. Matsuda, and O. Kawasaki, "Mechanism
    for the anomalous degradation of Si solar cells induced by high
    fluence 1MeV electron irradiation," *Appl. Phys. Lett*., vol. 66,
    no. 22, pp. 3141-3143, 1996.

5.  M. Yamaguchi, et al., "A detailed model to improve the
    radiation-resistance of Si space solar cells," *IEEE Trans. Elec.
    Dev. Lett.*, vol. 46, no.10, pp. 2133-2137, 1999.

6.  R. P. Webb, R. J. McIntyre, and J. Conradi, "Properties of avalanche
    photodiodes," *RCA Rev.*, vol. 35, pp.234-278, 1974.

7.  R. J. McIntyre, "The distribution of gains in uniformly multiplying
    avalance photodiodes: theory," *IEEE Trans. Elec. Dev*., vol. ED-19,
    no. 6, pp. 703-713, 1972.

8.  A. H. Kalma and W. H. Hardwick, "Radiation testing of PIN
    photodiodes," *IEEE Trans. on Nucl. Sci*., vol. 25, no. 6, pp.
    1483-1488, 1978.

9.  J. R. Srour, S. C. Chen, S. Othmer, and R. A. Hartmann, "Radiation
    damage coefficients for silicon depletion regions," *IEEE Trans. on
    Nucl. Sci.*, vol. 26, no. 6, pp. 4784-4791, 1979.

10. G. P. Summers, et al., "Correlation of particle-induced displacement
    damage in silicon," *IEEE Trans. on Nucl. Sci*., vol. 34, no. 6, pp.
    1134-1139, 1987.

11. G. P. Summers, E. A. Burke, P. Shapiro, S. R. Messenger, and R. J.
    Walters, "Damage correlations in semiconductors exposed to gamma,
    electron and proton radiations, *IEEE Trans. on Nucl. Sci*., vol.
    40, no. 6, pp. 1372-1379, 1993.

12. J.R. Srour, S. Othmer, and K.Y. Chiu, "Electron and proton damage
    coefficients in low-resistivity silicon," *IEEE Trans. on Nucl.
    Sci*., vol. 22, no. 6, pp. 2656-2662, 1975.

13. V.A.J. van Lint, et al., *Mechanisms of Radiation Effects in
    Electronic Materials*, Vol. 1, John Wiley & Sons, 1980.

14. R.D. Pugh, A.H. Johnston, and K.F. Galloway, "Characteristics of the
    breakdown voltage of power MOSFETs after total dose irradiation,"
    *IEEE Trans. on Nucl. Sci.*, vol. 33, no. 6, pp. 1460-1464, 1986.

15. M. Yamaguchi, et al., "High-energy and high-fluence proton
    irradiation effects in silicon solar cells," *J. Appl. Phys*., vol.
    80, no. 9, pp. 4916-4920, 1996.

16. R. L. Pease, E. W. Enlow and G. L. Dinger, "Comparison of proton and
    neutron carrier removal rates," *IEEE Trans on Nucl. Sci*., vol. 34,
    no. 6, pp. 1140-1146, 1987.

[^1]: ^^Manuscript received August 22, 2003. This research in this
    paper was carried out at the Jet Propulsion Laboratory, California
    Institute of Technology, under contract with the National
    Aeronautics and Space Administration, Code AE as part of the NASA
    Electronic Parts and Packaging Program.

    The authors are with the Jet Propulsion Laboratory, California
    Institute of Technology, Pasadena, CA 91109. (telephone:
    818-393-5491, e-mail: <Heidi.N.Becker@jpl.nasa.gov>,
    <Testsuo.F.Miyahira@jpl.nasa.gov>, and
    Allan.H.Johnston@jpl.nasa.gov).