The performance of AlGaN metal-semiconductor-metal (MSM) photodetectors grown on Si(111) substrates is presented. Three key points: 1) An adequate AlN buffer layer is critical to achieve visible-blind photodetectors and electrically insulate the epitaxial film from the conductive substrate. 2) Increasing the Al content produced a transition from photoconductor to MSM photodiode behavior, as seen from responsivity, temporal response, and UV/visible contrast. 3) Contact metal affects photoconductive gain and UV/visible contrast. Pt/Ti/Au contacts showed higher contrast than Ti/Al contacts due to lower dark currents.

1. Materials Science and Engineering B93 (2002) 159 Á/162
www.elsevier.com/locate/mseb
AlGaN ultraviolet photodetectors grown by molecular beam epitaxy
on Si(111) substrates
J.L. Pau *, E. Monroy, M.A. Sanchez-Garcıa, E. Calleja, E. Munoz
´ ´ ˜
Departamento de Ingenierıa Electronica, ETSI Telecomunicacion, Universidad Politecnica de Madrid, Ciudad Universitaria 28040 Madrid, Spain
´ ´ ´ ´
Abstract
The performance of AlGaN metal Á/semiconductor Á/metal (MSM) photodetectors grown on Si(111) is presented in this article. It is
shown that the growth of an adequate AlN buffer layer is critical to achieve visible-blind devices, and that its role as an effective
electrical insulator of the conductive substrate was found to be more efficient for N-excess AlN growth. The increase of Al content
produced a transition from photoconductor to MSM photodiode behaviour, as determined from the detector responsivity, temporal
response, and UV/visible contrast. The effect of the contact metal on photoconductive gain and UV/visible contrast was also
studied. # 2002 Elsevier Science B.V. All rights reserved.
Keywords: Metal Á/semiconductor Á/metal; III-Nitrides; UV photodetectors; Molecular beam epitaxy
Research and development of GaN-based materials usually been fabricated on sapphire substrates and
have been an important focus of attention during the grown by metalÁ/organic chemical vapour deposition
last decade, which has led to industrial devices such as (MOCVD) [1,2].
light-emitting diodes, laser diodes, UV photodetectors, Due to their simplicity and the unnecessary p-type
and heterojunction transistors. The possibility of tuning doping, metal Á/semiconductor Á/metal (MSM) structures
the semiconductor bandgap from 1.9 eV for InN and 3.4 and photoconductors are very attractive devices for
eV for GaN, to 6.2 eV for AlN, makes these alloys very short wavelength photodetection. In GaN photocon-
attractive for a number of applications, such as flame ductors (ohmicÁ/metal Á/ohmic), responsivities as high as
sensing, missile warning, UV biological effects, UV 1000 A W(1 have been reported, but they showed very
astronomy, water purification, pollution monitoring, long time decays, which reduces the detectivity drasti-
high-density optical storage, engine and nuclear reactor cally [3]. In contrast, ideal MSM (or back-to-back
monitoring, and space-to-space communication. The Schottky) photodiodes are devices specially adequate
lack of lattice-matched substrates has forced the use of for high-speed applications, and their maximum respon-
foreign substrates for III-nitride growth. Following the sivity is limited by an external quantum efficiency of
development of arsenides, Si(111) was one of the first 100% (292 mA W(1 for GaN and 161 mA W(1 for
AlN). The frontier between MSM photodiodes and
substrates used due to the availability of high-quality,
photoconductors is still unclear, since many of the
large-area and low-cost wafers. However, its high
devices presented in the literature as MSM photodiodes
lattice- and thermal-mismatch with III-nitrides and the
show an obvious photoconductive gain contribution. If
high diffusion-coefficient of Si at growth temperatures
these intermediate or hybrid devices are characterised
have delayed the progress in the fabrication of efficient
under constant illumination (DC), persistent effects
optoelectronic devices on this substrate. The use of
become evident.
proper buffer layers, which attenuate these inconve- In this work, we present the fabrication of AlGaN
niences, is required. Thus, AlGaN photodetectors have MSM photodetectors on Si(111) substrates. The optimal
growth conditions of the buffer layer for the fabrication
* Corresponding author. Tel.: '34-91-549-5700x420; fax: '34-91-
of these photodetectors will be analysed. The hybrid
336-7323. behaviour of these photodevices will be studied for
E-mail address: jlpau@die.upm.es (J.L. Pau). different Al contents and different contact metals.
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 0 5 1 - X

2. 160 J.L. Pau et al. / Materials Science and Engineering B93 (2002) 159 Á/162
The structures were grown in a MECA 2000 mole-
cular beam epitaxy system. Active nitrogen was pro-
duced by an Oxford HD25 radio-frequency plasma
source. After degassing the silicon substrate at 820 8C,
a few monolayers of Al were deposited at 800 8C,
followed by the growth of an AlN buffer layer. The role
of this layer is not only to improve the crystalline quality
of the latter AlGaN layer, but also to electrically
insulate the epitaxial film from the conductive substrate.
Growth rate and layer thickness were measured by in
situ optical interferometry using an IRCON infrared
pyrometer with a narrow-band filter centred at 0.94 mm.
The resolution of this technique is 95 nm, even for the
thinnest layers.
Four types of AlN buffer layers were grown, by
changing the III Á/V ratio and the thickness of the layer.
The resulting effectiveness in avoiding the parallel Fig. 1. Nomarski views of samples (a) M564 and (b) M573, and SEM
conduction through the substrate is thus assessed for photographs of samples (c) M563 and (d) M590 are shown.
each type of buffer. Growth conditions are shown in
Table 1. by 200 mm were measured. The leakage currents at 10 V
The end of the sample M590 growth corresponds to a bias are also shown in Table 1. The high conductivity
change from two-dimensional to three-dimensional found for AlN layers grown under Al-rich conditions
growth mode (Stranski Á/Krastanov mode), as described could be due to leakage currents associated to threading
later. This transition is clearly identified by the appear- dislocations, as suggested by Hsu et al. for MBE-GaN
ance of a 2 ) 2 reconstruction in the RHEED monitor- samples grown under Ga-rich conditions [5]. Consider-
ing and corresponds to a thickness of 30 Á/40 nm, with ing both the insulating characteristics and the surface
high reproducibility [4]. morphology, we decided to use the buffer structure
Samples M564 and M573 showed surface features corresponding to sample M590.
related to Al-excess during the growth (Fig. 1a,b). In Undoped AlGaN layers with a thickness of 1 Á/2 mm
sample M564, which was grown under the highest III Á/V and Al mole fractions up to x 00.39 were deposited on
ratio, little droplets of 2 mm in average were observed, the AlN buffer layer. The growth temperature was in the
together with larger stains of about 10 mm diameter. 730 Á/760 8C range, depending on the nominal Al
Sample M573 showed very small stains, with diameters composition. Two-dimensional growth was observed
lower than 2 mm, indicating that the III Á/V relative ratio by RHEED after 5 min in all the samples under study.
was very close to the stoichiometry point (IIIÁ/V Â 1), as Due to the lower deposition temperatures in comparison
indicated in Table 1. As seen in Fig. 1c for sample M563, to MOCVD, layer-cracking problems were not observed
surface roughness starts to degrade after the transition in any sample. For GaN, a residual n-type doping of
to the three-dimensional mode, the scenario becoming around 1) 1017 cm (3 is determined from C Á/V mea-
harsh for the subsequent growth of AlGaN. However, surements, whereas for AlGaN, the 1/C 2 dependence
under the same growth conditions, if we stop when the versus reverse voltage becomes non-linear. X-ray dif-
2 )2 reconstruction appears, a very smooth surface fraction (XRD) patterns were obtained from u /2u scans
results, as shown in sample M590 (Fig. 1d). No with a wide open detector, showing full-width at half
remnants of metal were detected in the surface of both maximum (FWHM) values of 8.5 and 15 arcmin for
samples. To compare the electrical insulation provided GaN and AlGaN (x 0 0.30) layers, respectively.
by the above AlN layers, the current Á/voltage character- Detectors consist of two interdigitated electrodes on a
istics between 400 mm diameter Ti/Al contacts separated planar structure, with finger widths and gap spacings of
2, 4, and 7 mm, and active areas of 250 )250 mm2 and
Table 1 500 ) 500 mm2. Two different metal systems were used
AlN buffer layer characteristics ˚ ˚ ˚
for contacts: Ti (300 A)/Al (700 A) and Pt (400 A)/Ti (50
˚ ˚
A)/Au (1000 A), corresponding to extreme values of
Sample
their metal workfunction. All current Á/voltage charac-
M564 M573 M563 M590 teristics for AlGaN photodiodes presented a rectifying
Thickness (nm) 200 200 200 35
behaviour, with a higher resistivity as the Al content
III Á/V 1.2 1 0.85 0.85 increased.
Ileakage (mA) at 10 V 1.4) 104 160 5.0 54 Spectral responsivity studies were performed by using
a 150 W xenon arc lamp. The photodetector responsiv-

3. J.L. Pau et al. / Materials Science and Engineering B93 (2002) 159 Á/162 161
ity was measured by excitation with a non-focused He Á/ different alloy compositions. Room temperature PL
Cd laser (325 nm) for GaN devices, whereas the 514 nm measurements showed two emissions whose positions
Ar ' laser line coupled into a second harmonic gen- coincided with the cut-off wavelength and the shoulder
erator (257 nm) was used for AlGaN photodiodes. observed in the spectral response (see inset Fig. 2). The
These measurements were performed under constant lower energy transition does not follow Varshni’s law
(DC) illumination. Time response characterisation was for bandgap energy dependence on temperature, which
made using the fourth frequency of a Nd Á/YAG laser seems to indicate that the transition corresponds to a
(266 nm), with 10 ns Gaussian pulses. DA emission [8].
Typical spectral responses of AlGaN MSM photo- Detector peak responsivity and dark current values
detectors with Ti/Al contacts are shown in Fig. 2. The can be found in Table 2. As seen, the responsivity
optical response above the bandgap drops more mark- decreases with increasing Al mole fractions. On the
edly as the Al content increases. The buffer layer other hand, the increase of the Al produces a reduction
efficiently insulates the AlGaN, preventing any contri- of the observed photoconductive gain, and persistent
bution from the silicon substrate to the detector optical effects (see photocurrent decays in Fig. 3). These data,
response. The cut-off wavelength reached 290 nm for together with the increase of the UV/visible contrast,
x 00.39, demonstrating the capability of these photo- indicate that the increase of aluminium in the ternary
detectors for solar-blind applications. Below the band- alloy provokes a transition from photoconductive to
gap, we fitted the quantum efficiency (h ) by the MSM photodiode behaviour.
expression In AlGaN (x0 0.39) MSM photodiodes with Ti/Al
contacts, the time constant, tp, value for different load
hn
h 8exp (1) resistances was obtained from transient photoresponse
Eurb measurements. The photocurrent decays were exponen-
where Eurb is the Urbach parameter, which varied from tial, with the time constant corresponding to the RC
24 meV for GaN to 90 meV for AlGaN (x 00.39) [6]. product of the measuring system. The dependence of
This parameter measures the cut-off abruptness and is photocurrent response time on load resistance has been
related to the presence of levels inside the bandgap or to analysed in Fig. 4, and the extrapolation to zero-load
alloy disorder. As indicated in Fig. 2, the spectral allows to obtain a minimum tp value of 150 ns.
response of AlGaN devices presents a shoulder below Finally, a comparative spectral response of MSM
the bandgap, which might be related to regions with photodiodes for the two metal systems used can be
different compositions in the ternary alloy or to observed in Fig. 5. The UV/visible contrast is around a
absorption in defects. The rotation of the layer during factor 10 higher in the case of Pt/Ti/Au due to the lower
the growth and the different positions of the III-element value of the dark current. The value of the dark current
sources could produce alloy inhomogeneities, as already is dominated by the quality of the Schottky contacts. Pt
reported [7]. However, in our sample, from XRD contacts are known to produce barrier heights of 1.0 Á/
measurements, we have not seen any evidence of 1.1 eV on GaN [9], whereas barriers of 0.1 Á/0.5 eV have
been reported for Ti contacts [10]. The I Á/V character-
istics of both samples under constant illumination are
shown in the inset of Fig. 5. The increase of the
photocurrent with the applied bias is more pronounced
in samples with Ti/Al contacts, indicating a higher
photoconductive gain contribution. In addition, the
responsivities for Ti/Al contacts are a factor 100 super-
ior to those of Pt/Ti/Au.
We have reported the fabrication and characterisation
of AlGaN MSM photodetectors grown on Si(111), with
Al mole fractions up to 0.39. By using a proper AlN
buffer, the photoresponse contribution from the con-
ductive substrate is avoided. The photoconductive gain
Table 2
Responsivities and dark current of 3 V biased AlGaN MSM
photodiodes
%Al 2 15 39
Fig. 2. Spectral responses of MSM AlGaN photodiodes grown on Rpeak (mA W(1) 5400 58 12
Si(111). Inset: room temperature photoluminiscence of AlGaN Id (nA) at 3 V 4100 1.3 0.015
(x0 0.39).

4. 162 J.L. Pau et al. / Materials Science and Engineering B93 (2002) 159 Á/162
Fig. 5. Comparative spectral response for two different contact metals
(Ti/Al and Pt/Ti/Au) at 5 V.
Fig. 3. Time decay measurements for MSM AlGaN photodiodes with Acknowledgements
different Al contents (x 0 0, 0.15, and 0.39). Observe the different time
scales for GaN and AlGaN photodiodes. Thanks are due to J. Sanchez Osorio and A. Fraile for
´
their technical support and to Professor Jaque for his
assistance in time response measurements. This work
has been partially supported by Comunidad de Madrid,
Project No. 07M/0008/1999 and PETRI No. 95-0466-
OP.
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