High-Performance Solar-Blind AlGaN Schottky Photodiodes
With
its wide bandgap and intrinsic visible-blind absorption spectrum,
AlxGa1-xN ternary alloy is the most promising material
system for high-performance photodetectors operating in the ultraviolet (UV)
spectrum [1]. Their cut-off wavelength (
c) can be tuned from
365 nm to 200 nm, which covers the solar-blind spectrum (<280 nm)
[2]. Solar-blind detectors play important roles in a variety of military and
civil applications including missile plume detection, flame/engine monitoring,
chemical/biological agent sensing, and covert space-to-space communications
[3]. AlGaN-based solar-blind photodetectors with high responsivity [4]
[5], low dark current [6] [7], high solar rejection [8]
[9], high bandwidth [10], and high detectivity [11] [12]
performance were demonstrated using p-i-n, MSM and Schottky type detector
structures.
Compared with p-i-n photodiodes, AlGaN Schottky photodiodes have several
advantages. Growth of p+ AlGaN layers and formation p+ ohmic contacts with low
contact resistance are two challenging issues for p-i-n photodiodes, while
Schottky photodiodes do not face these problems. High-quality Schottky and n+
ohmic contacts can be achieved using widely known process techniques. Moreover,
since the photocarriers are generated and efficiently collected within the
depleted active layer, high-frequency operation of Schottky photodiodes is not
affected by minority carrier diffusion [13] [14] [15] [16]. Successful
solar-blind operation of AlGaN Schottky photodiodes were demonstrated by
several research groups [17] [18]. However, the reported dark current and
detectivity performances were poor with respect to AlGaN-based p-i-n and MSM
type photodiodes. In terms of high-speed performance, GaN-based visible-blind
photodiodes with 3-dB bandwidths in the GHz regime were successfully
demonstrated [19] [20] [21]. Reported AlGaN-based solar-blind detectors had
lower bandwidths. A maximum high-speed performance of 100 MHz was reported with
an AlGaN MSM photodiode structure [10]. For solar-blind AlGaN Schottky
photodiodes the best detector bandwidth demonstrated was below the MHz level
[17]. Recently we had demonstrated solar-blind AlGaN Schottky photodiodes with
very low dark current and high detectivity [22]. In this paper, we report on
the design, fabrication, and characterization of high-performance solar-blind
AlGaN Schottky photodiodes with high-speed performance close to the GHz level.
The
epitaxial structure of the AlGaN Schottky detector wafer was designed to
achieve low cut-off wavelength (c < 280 nm), high visible
rejection, and low dark current. Figure 1 shows the details of the photodiode
structure. The AlxGa1-xN/GaN epitaxial layers of our
hetero-junction Schottky PD wafer were grown by MOCVD on a 2-inch single-side
polished (0001) sapphire substrate. The 0.8 µm thick undoped AlGaN layer
with 38% Al concentration was the active absorption region of the detector. The
ohmic contacts were formed within the n+ GaN layer. The 0.2 µm thick n+
Al0.38Ga0.62N layer was designed as a diffusion barrier
for the holes generated in the n+ GaN layer to increase the visible rejection
of the detector.
Fabrication
process of the AlGaN Schottky photodiodes was accomplished in a class-100
clean-room facility using a microwave compatible five mask-level standard
semiconductor process [23] [24]. Fabrication started with the formation of n+
ohmic contacts. The ohmic contact regions were etched down to the n+ GaN ohmic
layer using dry etch. Reactive ion etching (RIE) process was utilized under 20
sccm CCl2F2 plasma at 100 W RF power. Then a ~100 nm
thick Ti/Al contact metal was deposited using thermal evaporation. The metal
was lifted off in acetone solution and the samples were annealed at 700
°C for 30 s afterwards. Then the device mesas with 30-200 µm
diameters were defined using RIE with the same process parameters. Mesa
isolation of the devices was completed with a ~1.8 µm deep etch, down to
the undoped GaN layer. This was followed by the evaporation of ~100 Å
thick Au film as the Schottky contact metal. Afterwards, a ~100 nm thick
Si3N4 layer was deposited using plasma enhanced chemical
vapor deposition to passivate the sample surface and protect the thin Schottky
contactfilms. In the final step, interconnect pads were formed by a ~0.7
µm thick Ti/Au metallization. A schematic cross-section figure of the
completed AlGaN Schottky photodiode is shown in Figure 2.
After
device fabrication, current-voltage (I-V), spectral responsivity, noise, and
high-speed characterizations were carried out. All measurements were made
on-wafer, using a microwave probe station. To observe the electrical diode
characteristics and the leakage current, we first performed the I-V
measurements. The I-V setup consisted of a Hewlett Packard HP4142B DC
parametric measurement instrument, DC probes with triax output and low-noise
triax cables. DC current was measured as voltage was applied to the devices.
Spectral responsivity measurements were done using a 175 W xenon light-source,
1/4 m Digikrom DK240 monochromator, multi-mode UV fiber, DC voltage source,
SR830 DSP lock-in amplifier and a Newport model 1830-C calibrated optical
power-meter. Xenon lamp output was fed into the monochromator. The
monochromator output was chopped and coupled to the fiber using a UV-enhanced
focusing lens. The detectors were illuminated by the optical output coming out
from the fiber which was calibrated using the calibrated photodetector. The
detectors were biased with a DC voltage source upto 50 V, and the resulting
photocurrent was measured using the lock-in amplifier.
Low-frequency (< 100 KHz) noise characteristics of the solar-blind Schottky
detectors were measured with a SR785 dynamic signal analyzer, which measured
the fast Fourier transform spectrum of the detector noise. The detectors were
reverse biased with a DC voltage source.
Temporal high-frequency measurements were done at the solar-blind wavelength of
267 nm. Ultrafast UV pulses were generated using a laser set-up with two
nonlinear crystals. A Coherent Mira 900F model femtosecond
mode-locked Ti:sapphire laser was used to generate the pump beam at 800 nm. The
pump pulses were produced with 76 MHz repetition rate and 140 fs pulse
duration. These pulses were frequency doubled to generate a second harmonic
beam at 400 nm using a 0.5 mm thick type-I 
-BaB2O4 (BBO) crystal. The
second harmonic beam and the remaining part of the pump beam were frequency
summed to generate a third harmonic output beam at 267 nm using another type-I
BBO crystal with thickness of 0.3 mm. The resulting 267 nm pulses had
pulsewidths below 1 ps and were focused on-to the devices using UV-enhanced
mirrors and lenses. The detectors were biased using a DC voltage source and a
40 GHz bias-tee. Using 50 GHz microwave probe and cables, the electrical pulses
were transferred to a 50 GHz sampling oscilloscope where the temporal pulse
responses were observed.
The
fabricated AlGaN Schottky detectors exhibited low dark currents and high
breakdown voltages. Figure 3 shows the I-V curve of a 150x150
µm2 device with >50 V breakdown voltage. Excellent low-leakage
performance was achieved with a dark current below 400 fA under reverse bias
voltages as high as 25 V. The dark current fluctuated between 150-400 fA in the
0-25 V reverse bias range (see inset figure). These leakage values correspond
to dark current density values of 0.7-1.8 nA/cm2. The differential
resistance (R = dV/dI) of these devices was calculated and their dark
impedance was in excess of 1013 
in the 0-25 V range. These
measurement results correspond to the best I-V performance reported for
solar-blind AlGaN Schottky photodiodes. To indicate the performance variation,
Figure 4 shows the I-V characteristics of a 200 µm diameter photodiode
fabricated on a different wafer piece. Few pA dark current at 5 V reverse bias
and a breakdown around 20 V was observed. I-V measurement under UV illumination
at 256 nm showed a clear UV photoresponse when compared with the detector dark
current (see inset figure).
Spectral photoresponse measurements of the AlGaN Schottky photodiodes were
carried out in the 250-350 nm spectral range. A bias-dependent spectral
responsivity with true solar-blind characteristic was observed. Figure 5 shows
the measured spectral responsivity of a photodiode with ~20 V breakdown
voltage. The peak responsivity increased with applied reverse bias. The
photovoltaic peak responsivity of 36 mA/W increased upto 78 mA/W under 15 V
reverse bias. At higher reverse biases, photocurrent fluctuations increased and
signs of photoconductive gain were observed as the detector operated near the
breakdown regime. Peak wavelength of 250 nm did not change with bias. The peak
responsivity value corresponds to a peak quantum efficiency of 39% at 250 nm.
The corresponding spectral quantum efficiency curves are plotted in Figure 6.
True solar-blind characteristic was achieved with cut-off wavelengths smaller
than 280 nm. The cut-off wavelength changed from 262 nm to 266 nm for no bias
and 15 V reverse bias respectively. The device responsivity dropped sharply
around 270 nm. A UV/VIS rejection of 3 orders of magnitude was already obtained
around 280 nm. For > 280 nm, the responsivity decay continued with a
lower rate and a 4 order of magnitude rejection was achieved before 350 nm. The
maximum visible rejection at 350 nm was recorded as 2.4x104
while the detectors were operating in the photovoltaic mode. As applied reverse
bias increased, the rejection showed a descending behavior. The measured
c and UV/VIS rejection values correspond to the lowest
c and highest visible rejection reported for solar-blind
AlGaN Schottky photodiodes. Table 1 summarizes the spectral responsivity
measurement results.
Detector noise characteristics were measured in the 0.1 Hz - 100 KHz
frequency range. Solar-blind AlGaN Schottky photodiodes with low dark current
exhibited spectral noise density values below the noise floor of the
measurement setup. Noise floor of our setup was around 3x10-29
A2/Hz for frequencies higher than 1 KHz, and increased for lower
frequency values. Therefore we were not able to measure the noise density
values of our low-leakage devices. To understand the dominant noise mechanism
and its bias dependence, we measured a device with high leakage (few µA dark
current at 5 V, breakdown at ~14 V). The detectors displayed 1/f-noise
characteristic with noise densities proportional to reverse bias. Figure 7
shows the dark current noise power density values measured at 1 Hz as a
function of applied reverse bias voltage. With increasing bias from 0 V to 10
V, the noise density increased by more than 4 orders of magnitude.
Time-domain high-speed characterization of the fabricated solar-blind Schottky
photodiodes resulted in very fast pulse responses with high 3-dB bandwidths.
The detector pulse response was bias dependent. Faster pulses were obtained for
higher reverse bias values as the thick n- AlGaN absorption layer was fully
depleted under high reverse bias voltages. As the bias tee limit was 16 V, we
were not able to increase the bias voltage above this limit. The fastest pulse
response was measured with an 80 µm diameter device (Figure 8). Under 15 V
reverse bias, it produced a pulse response with a very fast rise time of 12 ps,
and a full-width-at-half-maximum (FWHM) of 216 ps. Using FFT, the corresponding
3-dB bandwidth of the detector was calculated as 870 MHz. Pulse responses with
narrower pulse-widths were also measured, but their calculated 3-dB bandwidth
was lower due to longer decay parts. Figure 9 shows such a response of a 30
µm diameter solar-blind detector with 13 ps rise time, 74 ps FWHM, and 300
MHz 3-dB bandwidth.
To analyze this unexpected longer decay time for a smaller diode area, curve
fitting with exponential decay functions were applied to the measured pulse
responses shown in Figure 8 and Figure 9. A simple first order exponential fit
with a single time constant could not be achieved. Instead, both responses were
fitted well with second order exponential decay functions, i.e. with a sum of
two exponential decay functions with two different time constants. The fit for
80 µm diameter device response had time constants of 157 ps and 2.85 ns,
while the 30 µm diameter device response fit exhibited time constants of 120
ps and 5.45 ns. These time constants agree with our bandwidth calculations: The
response with the shorter FWHM but lower 3-dB bandwidth (30 µm diameter
diode) had a shorter initial decay time constant along with a longer subsequent
decay time constant. The reverse conditions were valid for the other device
response giving rise to a longer FWHM but higher 3-dB bandwidth. The physical
reason for the multi-exponential relaxation mechanism was not clear. Evenmore,
the initial decay time constants were much longer than the RC time constants of
4 ps and 27 ps for 30 µm and 80 µm device respectively. These results
convinced us that mechanisms other than RC and transit time limitations were
responsible for the decay part of the high-speed response of these solar-blind
detectors. We postulate that the long and multi-exponential decay times are
related to issues such as carrier trapping effect in AlGaN layers, carrier
diffusion component originating from low field regions close to the center of
the mesa, and growth/process related effects. A detailed study on the
relaxation mechanisms of solar-blind AlGaN PDs will be made in the near future.
Although there were exceptions, as discussed above, in general, an inverse
proportionality between device area and detector bandwidth was observed. FFT
curves of 4 pulse responses from detectors with different device areas are
plotted in Figure 10. 3-dB bandwidths of 220, 280, 510, and 780 MHz were
obtained for devices with 200, 150, 100, and 60 µm diameter respectively.
These results correspond to the best high-speed performance reported for any
type of solar-blind photodetectors.
High-performance
solar-blind AlGaN Schottky photodiodes have been successfully designed,
fabricated and tested. The solar-blind Schottky detectors displayed dark
current densities as low as 7x10-10 A/cm2 under
reverse biases as high as 25 V. A bias dependent spectral responsivity was
observed with a maximum peak responsivity of 78 mA/W at 250 nm. The
corresponding peak quantum efficiency obtained from these devices was 39%. True
solar-blind detection was demonstrated with a sharp cut-off at 266 nm. The
AlGaN Schottky photodiodes exhibited over 4 orders of magnitude visible
rejection within 100 nm, with a maximum rejection of 2.4x104
at zero bias. Low frequency noise was 1/f limited with noise power densities
below the measurement setup noise floor of 3x10-29 A2/Hz.
Time-domain high-frequency measurements resulted in very short rise times and
3-dB bandwidths as high as 870 MHz were achieved. The recorded values for dark
current, cut-off wavelength, visible rejection, and high-speed measurements
correspond to the best detector performance results reported for solar-blind
AlGaN Schottky photodiodes.
This work was supported by NATO Grant No. SfP971970, Turkish Department of Defense Grant No. KOBRA-002, and FUSAM-03.
last updated Tuesday, February 10, 2004 5:40:28 PM.© 2003-2004 The Materials Research Society
