We report photomixer devices fabricated on a material consisting of self-assembled ErAs islands in GaAs, which is grown by molecular beam epitaxy. The devices perform comparably and provide an alternative to those made from low-temperature-grown GaAs. The photomixer's frequency response demonstrates that the material is a photoconductor with subpicosecond response time, in agreement with time-resolved differential reflectance measurements. The material also provides the other needed properties such as high photocarrier mobility and high breakdown field, which exceeds 2×10^5 V/cm. The maximum output power before device failure at frequencies of 1 THz was of order 0.1 µW. This material has the potential to allow engineering of key photomixer properties such as the response time and dark resistance

Blake, G. A.

Gossard, A. C.

Jackson, A. W.

Kadow, C.

Matsuura, S.

English

Caltech Authors - Main

APPLIED PHYSICS LETTERS VOLUME 76, NUMBER 24 12 JUNE 2000Self-assembled ErAs islands in GaAs for optical-heterodyne
THz generation
C. Kadow,a) A. W. Jackson, and A. C. Gossard
Materials Department, University of California–Santa Barbara, Santa Barbara, California 93106-5050
S. Matsuura and G. A. Blake
Division of Geological and Planetary Science, California Institute of Technology MS150-21, Pasadena,
California 91125
~Received 9 February 2000; accepted for publication 17 April 2000!
We report photomixer devices fabricated on a material consisting of self-assembled ErAs islands in
GaAs, which is grown by molecular beam epitaxy. The devices perform comparably and provide an
alternative to those made from low-temperature-grown GaAs. The photomixer’s frequency response
demonstrates that the material is a photoconductor with subpicosecond response time, in agreement
with time-resolved differential reflectance measurements. The material also provides the other
needed properties such as high photocarrier mobility and high breakdown field, which exceeds 2
3105 V/cm. The maximum output power before device failure at frequencies of 1 THz was of order
0.1 mW. This material has the potential to allow engineering of key photomixer properties such as
the response time and dark resistance. © 2000 American Institute of Physics.
@S0003-6951~00!01924-0#A promising approach for the generation of submillime-
ter ~THz! waves is optical-heterodyne conversion, or photo-
mixing, by photoconductive materials with response times in
the picosecond ~ps! range.1 The response time is a crucial
parameter for device design and performance, and engineer-
ing control over this time is desirable. Brown et al.2 demon-
strated photomixer devices based on low-temperature-grown
~LTG! GaAs, which is the most widely used ultrafast
photoconductor.3–5
Here we report on the performance of photomixers made
from an alternative material consisting of layers of ErAs is-
lands separated by GaAs, as shown schematically in part ~a!
of Fig. 1. The use of this material was motivated by the
recent finding that carrier trapping in GaAs:ErAs occurs on a
subps time scale, similar to LTG GaAs.6 We believe that this
material is particularly promising because one can engineer
its microstructure, which in turn controls the electronic prop-
erties. The superlattice period, for example, controls the car-
rier trapping time.6 With molecular beam epitaxy ~MBE! it is
straightforward to control this period and hence to achieve a
desired carrier trapping time. Other properties that strongly
depend on the microstructure are the dark resistance and the
thermal conductivity.
The material used for the photomixer devices in the
present work is grown by MBE on a semi-insulating ~100!
GaAs substrate. 60 periods of 1.2 monolayers of ErAs and
20 nm of GaAs are deposited on the substrate at a tempera-
ture of 535 °C, resulting in a 1.2 mm thick film. The material
is nominally identical to one of the samples discussed by
Kadow et al.,6 who also present details of the MBE growth.
The response time measured by time-resolved differential
optical reflectance for this material is 120 fs. The island di-
a!Author to whom correspondence should be addressed; electronic mail:
kadow@engineering.ucsb.edu3510003-6951/2000/76(24)/3510/3/$17.00
Downloaded 08 Mar 2006 to 131.215.225.174. Redistribution subject ameters are between 1 and 2 nm, and the density is about
731012 cm22.
Interdigitated electrodes with a finger width of 0.2 mm
and a finger spacing of 1.8 mm were fabricated on the mate-
rial surface. These electrodes are located at the vertex of a
planar self-complementary log-spiral antenna. The interdigi-
tated electrodes and the inner part of the spiral antenna are
defined by electron beam lithography, the outer part of the
spiral antenna by optical lithography. The metalization con-
sists of 140 nm of gold on 40 nm of titanium. One of the
devices is shown in part ~b! of Fig. 1. We tested five devices
from the same wafer. There is little scatter among the data
from different devices.
The experimental setup is schematically shown in Fig. 2.
FIG. 1. ~a! Schematic cross section of the samples showing the superlattice
structure. ~b! Top view of the metalization of the photomixer device. Shown
are the interdigitated electrodes at the vertex of the spiral antenna.0 © 2000 American Institute of Physics
to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
3511Appl. Phys. Lett., Vol. 76, No. 24, 12 June 2000 Kadow et al.Two single-frequency diode lasers operating near 850 nm are
used. The first is a grating-tuned external cavity laser, the
second is a nearly fixed wavelength distributed Bragg reflec-
tor ~DBR! laser locked to an external cavity. Both lasers emit
approximately 30 mW. The light from both laser diodes is
coupled into individual single-mode optical fibers and com-
bined in a 50–50 fiber coupler. Half of the power is coupled
into a wave meter where the difference frequency f is deter-
mined. The other half is outcoupled and amplified in an 850
nm tapered semiconductor optical amplifier. The maximum
available power after amplification is 500 mW. This signal is
focused onto the interdigitated finger region of the photo-
mixer devices. The focused spot is measured to be 6 mm in
diameter. We apply a dc bias UDC to the interdigitated fin-
gers and measure the dc photocurrent IDC . The submillime-
ter waves are predominantly emitted through the substrate
and subsequently collimated with a hyperhemispherical Si
lens. The output power is directed onto a Si bolometer,
which is operated at liquid He temperature. We denote the
detected power as Pout . The units are the rms voltage on the
detector. From thermal blackbody calibrations, 650 mV rms
corresponds to a power level of approximately 100 nW.
In the dark, the current–voltage (I – V) characteristic of
the devices is linear for biases below 2 V. At higher bias the
current increases superlinearly. The dark resistance in the
linear region is 5 MV. In the nonlinear region the data follow
a power law with exponent 1.6. We expect the conductivity
of the material to be strongly anisotropic due to its layered
nature. We are currently not in a position to explain the
details of the transport from a microscopic point of view.
The exact behavior of the metal contacts is another unknown
in these devices. Schmidt et al.7,8 performed transport mea-
surements on similar samples with ohmic contacts to all
ErAs containing layers to investigate the in-plane component
of the dark conductivity. They find that the transport is ther-
mally activated and strongly dependent on growth condi-
tions. Different growth temperatures, for example, result in
changes of several orders of magnitude in the sheet resis-
tance. This can be understood in qualitative terms if the in-
plane transport is dominated by hopping between the ErAs
islands, resulting in a strong dependence on island separa-
tion. From the I – V measurements we also know that the
breakdown field exceeds 23105 V/cm.
FIG. 2. Experimental photomixer test setup. Difference frequencies of over
10 THz are possible with this system at a maximum power level of 500 mW.Downloaded 08 Mar 2006 to 131.215.225.174. Redistribution subject Figure 3 shows the dc photocurrent IDC and the THz
output power Pout at a frequency f of 0.9 THz as a function
of bias UDC . IDC increases superlinearly if UDC is smaller
than 25 V, following a power law dependence with an expo-
nent of approximately 1.7. Within our experimental error,
this is the same exponent as for the dark currrent. The con-
duction mechanisms for dark and photocurrent are, however,
not necessarily identical. At higher bias (UDC.25 V) the dc
photocurrent saturates. The output power Pout increases in a
monotonic fashion with applied bias. In an intermediate bias
regime it appears to follow a power law dependence with an
exponent around 3.0. At high bias, roughly at values larger
than 25 V, the output power saturates. Similar behavior is
seen for devices made from LTG GaAs. Superlinear I – V
characteristics have been described by Brown et al.9 One
explanation for the superlinear I – V curve involves space
charge limited currents.9 Another possible explanation is an
electric field dependent carrier lifetime t as observed by
Zamdmer et al. in LTG GaAs.10 Saturation of the photocur-
rent has not been observed in LTG GaAs. The reason for the
saturation behavior reported in this letter is not understood.
Saturation of the output power has been observed by
Jackson11 at similar THz frequencies. Under illumination the
dc photocurrent increases linearly with the optical power. At
30 V bias the slope is 731023 A/W, which corresponds to
an external quantum efficiency of 1%. A small external
quantum efficiency is expected from the short carrier life-
time. The value is similar to those measured on LTG GaAs.11
Some improvement of the external quantum efficiency could
be achieved by incorporating an antireflection coating and by
either increasing the thickness of the absorbing layer or in-
corporation of a DBR beneath the active region.11
The output power Pout is expected to increase quadrati-
cally with optical power for heterodyne mixing. Fitting the
data with Pout5AP laser
2 yields A5831026 at f 5946 GHz.
When fitted with a power law, a slightly better fit is achieved
with an exponent of 1.8. Figure 4 shows the frequency re-
sponse from one device biased at 30 V for two different laser
powers. The lower laser power of 50 mW results in a dc
photocurrent of 340 mA, while the higher laser power of 90
mW leads to a dc photocurrent of 680 mA. A frequency
corner can clearly be seen around 1.7 THz in the data. At
lower frequencies the power decreases by 3 dB/octave, but at
higher frequencies the power decays by roughly 15 dB/
FIG. 3. Photocurrent IDC and output power Pout as a function of dc bias UDC
for one of the devices investigated. The laser power is 50 mW and the
difference frequency is 900 GHz.to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
3512 Appl. Phys. Lett., Vol. 76, No. 24, 12 June 2000 Kadow et al.octave. The latter number is only an estimate because the
decay could only be observed over one octave as the power
levels approach the thermal background. These two slopes
are indicated in Fig. 4.
Brown et al. introduced a model for the frequency re-
sponse of this type of device.12 The model contains two cor-
ner frequencies f 1 and f 2 . f 1 is due to the RC time constant
of the electrical circuit ( f 151/2pRC), and f 2 originates
from the carrier lifetime in the photoconductor ( f 2
51/2pt). The frequency dependence of our devices can be
fit with this model if we assume the following: First, that the
two frequencies f 1 and f 2 are equal (t5RC) resulting in the
corner at 1.7 THz and second, the existence of an additional
decay with 1/f throughout the frequency range investigated.
This modifies the reduced form of Eq. ~9! in Ref. 12 to
P f5
P0
f @11~2pt f !2#2 .
The corner at 1.7 THz gives a carrier lifetime of 100 fs,
which is in good agreement with the 120 fs measured by
pump–probe techniques.6 The radiation resistance of the an-
tenna structure is 72 V. This yields a capacitance of 1.3 fF
for the observed corner, close to the capacitance of 1.6 fF
expected from calculations. The origin of the additional 1/f
rolloff is currently not understood. It may be related to the
fact that the pump–probe data from nominally identical
samples do not show a simple exponential decay,6 but con-
tain a secondary decay, which indicates that a fraction of the
carriers need longer than 120 fs to relax to their original
state. These carriers may degrade the frequency response and
lead to the 1/f decay.
FIG. 4. Frequency response of one photomixer device biased at 30 V for
two different laser powers. Circular symbols are for P laser590 mW, trian-
gular symbols are for P laser550 mW.Downloaded 08 Mar 2006 to 131.215.225.174. Redistribution subject The maximum output power of the tested devices was
limited by device failure as in the case for photomixers made
from LTG GaAs. Each of the five tested devices was driven
toward failure by increasing the laser power while keeping
dc bias and frequency constant. The dc bias ranged between
25 and 30 V, and the frequencies f were between 0.9 and 1
THz. In order to compare the results, we scale the output
power with the known frequency response to 1 THz. At 1
THz the average maximum output power was 85 nW, with a
minimum of 55 nW and a maximum of 106 nW. The laser
power at failure was roughly 100 mW, the dc photocurrent
between 0.6 and 0.7 mA, and the dissipated ohmic power
about 17 mW.
In conclusion, we have demonstrated that self-assembled
ErAs islands in GaAs can be used to fabricate photomixer
devices. The photomixer’s frequency response demonstrates
that the material is a photoconductor with subpicosecond re-
sponse time, in agreement with time resolved differential re-
flectance measurements. It also provides the other needed
properties such as high photocarrier mobility and high break-
down field. The breakdown field exceeds 23105 V/cm. The
maximum output power before device failure at frequencies
of 1 THz was of order 0.1 mW. The ability to reliably engi-
neer important properties of the ErAs based material, for
example the response time and the dark resistance, should
result in devices with improved performance.
This research was supported by Jet Propulsion Lab under
Contract No. 960776.
1 For a review, see for example: S. Verghese, K. A. McIntosh, and E. R.
Brown, IEEE Trans. Microwave Theory Tech. 45, 1301 ~1997!.
2 E. R. Brown, K. A. McIntosh, K. B. Nichols, M. J. Manfra, and C. L.
Dennis, Proc. SPIE 2145, 200 ~1994!.
3 S. Matsuura, M. Tani, and K. Sakai, Appl. Phys. Lett. 70, 559 ~1997!.
4 S. Verghese, K. A. McIntosh, and E. R. Brown, Appl. Phys. Lett. 71, 2743
~1997!.
5 S. Matsuura, G. A. Blake, R. A. Wyss, J. C. Pearson, C. Kadow, A. W.
Jackson, and A. C. Gossard, Appl. Phys. Lett. 74, 2872 ~1999!.
6 C. Kadow, S. B. Fleischer, J. P. Ibbetson, J. E. Bowers, A. C. Gossard, J.
W. Dong, and C. J. Palmstrøm, Appl. Phys. Lett. 75, 3548 ~1999!.
7 D. R. Schmidt, J. P. Ibbetson, D. E. Brehmer, C. J. Palmstrøm, and S. J.
Allen, Mater. Res. Soc. Symp. Proc. 475, 251 ~1997!.
8 D. R. Schmidt, A. G. Petukhov, M. Foygel, J. P. Ibbetson, and S. J. Allen,
Phys. Rev. Lett. 82, 823 ~1999!.
9 E. R. Brown, K. A. McIntosh, F. W. Smith, K. B. Nichols, M. J. Manfra,
C. L. Dennis, and J. P. Mattia, Appl. Phys. Lett. 64, 3311 ~1994!.
10 N. Zamdmer, Q. Hu, K. A. McIntosh, and S. Verghese, Appl. Phys. Lett.
75, 2313 ~1999!.
11 A. W. Jackson, Ph.D. thesis, University of California at Santa Barbara,
1999.
12 E. R. Brown, F. W. Smith, and K. A. McIntosh, J. Appl. Phys. 73, 1480
~1993!.to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp


Self-assembled ErAs islands in GaAs for optical-heterodyne THz generation

https://authors.library.caltech.edu/2148/1/KADapl00b.pdf

Self-assembled ErAs islands in GaAs for optical-heterodyne THz generation

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main