- Email: [email protected]
AlGaN ultraviolet photodetectors grown by molecular beam epitaxy on Si(111) substrates
Materials Science and Engineering B93 (2002) 159 /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. Sa´nchez-Garcı´a, E. Calleja, E. Mun˜oz Departamento de Ingenierı´a Electro´nica, ETSI Telecomunicacio´n, Universidad Polite´cnica de Madrid, Ciudad Universitaria 28040 Madrid, Spain
Abstract The performance of AlGaN metal /semiconductor /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 /semiconductor /metal; III-Nitrides; UV photodetectors; Molecular beam epitaxy
Research and development of GaN-based materials have been an important focus of attention during the last decade, which has led to industrial devices such as light-emitting diodes, laser diodes, UV photodetectors, and heterojunction transistors. The possibility of tuning the semiconductor bandgap from 1.9 eV for InN and 3.4 eV for GaN, to 6.2 eV for AlN, makes these alloys very attractive for a number of applications, such as flame sensing, missile warning, UV biological effects, UV astronomy, water purification, pollution monitoring, high-density optical storage, engine and nuclear reactor monitoring, and space-to-space communication. The lack of lattice-matched substrates has forced the use of foreign substrates for III-nitride growth. Following the development of arsenides, Si(111) was one of the first substrates used due to the availability of high-quality, large-area and low-cost wafers. However, its high lattice- and thermal-mismatch with III-nitrides and the high diffusion-coefficient of Si at growth temperatures have delayed the progress in the fabrication of efficient optoelectronic devices on this substrate. The use of proper buffer layers, which attenuate these inconveniences, is required. Thus, AlGaN photodetectors have
* Corresponding author. Tel.: 34-91-549-5700x420; fax: 34-91336-7323. E-mail address: [email protected] (J.L. Pau).
usually been fabricated on sapphire substrates and grown by metal/organic chemical vapour deposition (MOCVD) [1,2]. Due to their simplicity and the unnecessary p-type doping, metal /semiconductor /metal (MSM) structures and photoconductors are very attractive devices for short wavelength photodetection. In GaN photoconductors (ohmic/metal /ohmic), responsivities as high as 1000 A W 1 have been reported, but they showed very long time decays, which reduces the detectivity drastically [3]. In contrast, ideal MSM (or back-to-back Schottky) photodiodes are devices specially adequate for high-speed applications, and their maximum responsivity is limited by an external quantum efficiency of 100% (292 mA W 1 for GaN and 161 mA W 1 for AlN). The frontier between MSM photodiodes and photoconductors is still unclear, since many of the devices presented in the literature as MSM photodiodes show an obvious photoconductive gain contribution. If these intermediate or hybrid devices are characterised under constant illumination (DC), persistent effects become evident. In this work, we present the fabrication of AlGaN MSM photodetectors on Si(111) substrates. The optimal growth conditions of the buffer layer for the fabrication of these photodetectors will be analysed. The hybrid behaviour of these photodevices will be studied for 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
J.L. Pau et al. / Materials Science and Engineering B93 (2002) 159 /162
160
The structures were grown in a MECA 2000 molecular beam epitaxy system. Active nitrogen was produced 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 /V ratio and the thickness of the layer. The resulting effectiveness in avoiding the parallel conduction through the substrate is thus assessed for each type of buffer. Growth conditions are shown in Table 1. The end of the sample M590 growth corresponds to a change from two-dimensional to three-dimensional growth mode (Stranski /Krastanov mode), as described later. This transition is clearly identified by the appearance of a 2 2 reconstruction in the RHEED monitoring and corresponds to a thickness of 30 /40 nm, with high reproducibility [4]. Samples M564 and M573 showed surface features related to Al-excess during the growth (Fig. 1a,b). In sample M564, which was grown under the highest III /V ratio, little droplets of 2 mm in average were observed, together with larger stains of about 10 mm diameter. Sample M573 showed very small stains, with diameters lower than 2 mm, indicating that the III /V relative ratio was very close to the stoichiometry point (III/V 1), as indicated in Table 1. As seen in Fig. 1c for sample M563, surface roughness starts to degrade after the transition to the three-dimensional mode, the scenario becoming harsh for the subsequent growth of AlGaN. However, under the same growth conditions, if we stop when the 2 2 reconstruction appears, a very smooth surface results, as shown in sample M590 (Fig. 1d). No remnants of metal were detected in the surface of both samples. To compare the electrical insulation provided by the above AlN layers, the current /voltage characteristics between 400 mm diameter Ti/Al contacts separated Table 1 AlN buffer layer characteristics Sample
Thickness (nm) III /V Ileakage (mA) at 10 V
M564
M573
M563
M590
200 1.2 1.4 104
200 1 160
200 0.85 5.0
35 0.85 54
Fig. 1. Nomarski views of samples (a) M564 and (b) M573, and SEM photographs of samples (c) M563 and (d) M590 are shown.
by 200 mm were measured. The leakage currents at 10 V bias are also shown in Table 1. The high conductivity found for AlN layers grown under Al-rich conditions could be due to leakage currents associated to threading dislocations, as suggested by Hsu et al. for MBE-GaN samples grown under Ga-rich conditions [5]. Considering both the insulating characteristics and the surface morphology, we decided to use the buffer structure corresponding to sample M590. Undoped AlGaN layers with a thickness of 1 /2 mm and Al mole fractions up to x 0.39 were deposited on the AlN buffer layer. The growth temperature was in the 730 /760 8C range, depending on the nominal Al composition. Two-dimensional growth was observed by RHEED after 5 min in all the samples under study. Due to the lower deposition temperatures in comparison to MOCVD, layer-cracking problems were not observed in any sample. For GaN, a residual n-type doping of around 1 1017 cm 3 is determined from C /V measurements, whereas for AlGaN, the 1/C 2 dependence versus reverse voltage becomes non-linear. X-ray diffraction (XRD) patterns were obtained from u /2u scans with a wide open detector, showing full-width at half maximum (FWHM) values of 8.5 and 15 arcmin for GaN and AlGaN (x 0.30) layers, respectively. Detectors consist of two interdigitated electrodes on a planar structure, with finger widths and gap spacings of 2, 4, and 7 mm, and active areas of 250 250 mm2 and 500 500 mm2. Two different metal systems were used ˚ )/Al (700 A ˚ ) and Pt (400 A ˚ )/Ti (50 for contacts: Ti (300 A ˚ )/Au (1000 A ˚ ), corresponding to extreme values of A their metal workfunction. All current /voltage characteristics for AlGaN photodiodes presented a rectifying behaviour, with a higher resistivity as the Al content increased. Spectral responsivity studies were performed by using a 150 W xenon arc lamp. The photodetector responsiv-
J.L. Pau et al. / Materials Science and Engineering B93 (2002) 159 /162
ity was measured by excitation with a non-focused He / Cd laser (325 nm) for GaN devices, whereas the 514 nm Ar laser line coupled into a second harmonic generator (257 nm) was used for AlGaN photodiodes. These measurements were performed under constant (DC) illumination. Time response characterisation was made using the fourth frequency of a Nd /YAG laser (266 nm), with 10 ns Gaussian pulses. Typical spectral responses of AlGaN MSM photodetectors with Ti/Al contacts are shown in Fig. 2. The optical response above the bandgap drops more markedly as the Al content increases. The buffer layer efficiently insulates the AlGaN, preventing any contribution from the silicon substrate to the detector optical response. The cut-off wavelength reached 290 nm for x 0.39, demonstrating the capability of these photodetectors for solar-blind applications. Below the bandgap, we fitted the quantum efficiency (h ) by the expression hn h 8exp (1) Eurb where Eurb is the Urbach parameter, which varied from 24 meV for GaN to 90 meV for AlGaN (x 0.39) [6]. This parameter measures the cut-off abruptness and is related to the presence of levels inside the bandgap or to alloy disorder. As indicated in Fig. 2, the spectral response of AlGaN devices presents a shoulder below the bandgap, which might be related to regions with different compositions in the ternary alloy or to absorption in defects. The rotation of the layer during the growth and the different positions of the III-element sources could produce alloy inhomogeneities, as already reported [7]. However, in our sample, from XRD measurements, we have not seen any evidence of
161
different alloy compositions. Room temperature PL measurements showed two emissions whose positions coincided with the cut-off wavelength and the shoulder observed in the spectral response (see inset Fig. 2). The lower energy transition does not follow Varshni’s law for bandgap energy dependence on temperature, which seems to indicate that the transition corresponds to a DA emission [8]. Detector peak responsivity and dark current values can be found in Table 2. As seen, the responsivity decreases with increasing Al mole fractions. On the other hand, the increase of the Al produces a reduction of the observed photoconductive gain, and persistent effects (see photocurrent decays in Fig. 3). These data, together with the increase of the UV/visible contrast, indicate that the increase of aluminium in the ternary alloy provokes a transition from photoconductive to MSM photodiode behaviour. In AlGaN (x 0.39) MSM photodiodes with Ti/Al contacts, the time constant, tp, value for different load resistances was obtained from transient photoresponse measurements. The photocurrent decays were exponential, with the time constant corresponding to the RC product of the measuring system. The dependence of photocurrent response time on load resistance has been analysed in Fig. 4, and the extrapolation to zero-load allows to obtain a minimum tp value of 150 ns. Finally, a comparative spectral response of MSM photodiodes for the two metal systems used can be observed in Fig. 5. The UV/visible contrast is around a factor 10 higher in the case of Pt/Ti/Au due to the lower value of the dark current. The value of the dark current is dominated by the quality of the Schottky contacts. Pt contacts are known to produce barrier heights of 1.0 / 1.1 eV on GaN [9], whereas barriers of 0.1 /0.5 eV have been reported for Ti contacts [10]. The I /V characteristics 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 superior 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 conductive substrate is avoided. The photoconductive gain Table 2 Responsivities and dark current of 3 V biased AlGaN MSM photodiodes
Fig. 2. Spectral responses of MSM AlGaN photodiodes grown on Si(111). Inset: room temperature photoluminiscence of AlGaN (x 0.39).
%Al Rpeak (mA W 1) Id (nA) at 3 V
2 5400 4100
15 58 1.3
39 12 0.015
162
J.L. Pau et al. / Materials Science and Engineering B93 (2002) 159 /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 different Al contents (x 0, 0.15, and 0.39). Observe the different time scales for GaN and AlGaN photodiodes.
Acknowledgements Thanks are due to J. Sa´nchez 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. 95-0466OP.
References
Fig. 4. Time response dependence of an AlGaN (x 0.39) photodiode with load resistance.
observed in these devices has been found to depend strongly on the Al content. The effect of the contact metal on the responsivity and UV/visible contrast has also been evaluated. The recombination time for the photodetectors with the highest Al content (x 0.39) has been extrapolated to 150 ns for zero-load resistance.
[1] F. Omne`s, N. Marenco, B. Beaumont, Ph. De Mierry, E. Monroy, F. Calle, E. Mun˜oz, J. Appl. Phys. 86 (1999) 1. [2] J.C. Carrano, T. Li, D.L. Brown, P.A. Grudowski, C.J. Eiting, R.D. Dupuis, J.C. Campbell, Appl. Phys. Lett. 73 (1998) 2405. [3] E. Monroy, F. Calle, E. Mun˜oz, F. Omne`s, Semicond. Sci. Technol. 14 (1999) 685. [4] M.A. Sa´nchez-Garcı´a, E. Calleja, E. Monroy, F.J. Sa´nchez, F. Calle, E. Mun˜oz, A. Sanz-Herva´s, C. Villar, M. Aguilar, Mater. Res. Soc. Internet J. Nitride Semicond. Res. 2 (1997) 33. [5] J.W.P. Hsu, M.J. Manfra, D.V. Lang, S. Richter, S.N.G. Chu, A.M. Sergent, R.N. Kleiman, L.N. Pfeiffer, Appl. Phys. Lett. 78 (2001) 1685. [6] J.I. Pankove, Optical Processes in Semiconductors, Dover, New York, 1971, pp. 43 /46. [7] S. Ferna´ndez, F.B. Naranjo, F. Calle, M.A. Sa´nchez-Garcı´a, E. Calleja, P. Vennegues, A. Trampert, K.H. Ploog, Appl. Phys. Lett. 79 (2001) 2136. [8] F. Calle, F.J. Sa´nchez, J.M.G. Tijero, M.A. Sa´nchez-Garcı´a, E. Calleja, R. Beresford, Semicond. Sci. Technol. 12 (1997) 1396. [9] Q.Z. Liu, S.S. Lau, Solid-State Electron. 42 (1998) 677. [10] S.N. Mohammad, Z.F. Fan, W. Kim, O. Atkas, A.E. Botchkarev, A. Salvador, H. Morkoc, Electron. Lett. 32 (1996) 598.
AlGaN ultraviolet photodetectors grown by molecular beam epitaxy on Si(111) substrates J.L. Pau *, E. Monroy, M.A. Sa´nchez-Garcı´a, E. Calleja, E. Mun˜oz Departamento de Ingenierı´a Electro´nica, ETSI Telecomunicacio´n, Universidad Polite´cnica de Madrid, Ciudad Universitaria 28040 Madrid, Spain
Abstract The performance of AlGaN metal /semiconductor /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 /semiconductor /metal; III-Nitrides; UV photodetectors; Molecular beam epitaxy
Research and development of GaN-based materials have been an important focus of attention during the last decade, which has led to industrial devices such as light-emitting diodes, laser diodes, UV photodetectors, and heterojunction transistors. The possibility of tuning the semiconductor bandgap from 1.9 eV for InN and 3.4 eV for GaN, to 6.2 eV for AlN, makes these alloys very attractive for a number of applications, such as flame sensing, missile warning, UV biological effects, UV astronomy, water purification, pollution monitoring, high-density optical storage, engine and nuclear reactor monitoring, and space-to-space communication. The lack of lattice-matched substrates has forced the use of foreign substrates for III-nitride growth. Following the development of arsenides, Si(111) was one of the first substrates used due to the availability of high-quality, large-area and low-cost wafers. However, its high lattice- and thermal-mismatch with III-nitrides and the high diffusion-coefficient of Si at growth temperatures have delayed the progress in the fabrication of efficient optoelectronic devices on this substrate. The use of proper buffer layers, which attenuate these inconveniences, is required. Thus, AlGaN photodetectors have
* Corresponding author. Tel.: 34-91-549-5700x420; fax: 34-91336-7323. E-mail address: [email protected] (J.L. Pau).
usually been fabricated on sapphire substrates and grown by metal/organic chemical vapour deposition (MOCVD) [1,2]. Due to their simplicity and the unnecessary p-type doping, metal /semiconductor /metal (MSM) structures and photoconductors are very attractive devices for short wavelength photodetection. In GaN photoconductors (ohmic/metal /ohmic), responsivities as high as 1000 A W 1 have been reported, but they showed very long time decays, which reduces the detectivity drastically [3]. In contrast, ideal MSM (or back-to-back Schottky) photodiodes are devices specially adequate for high-speed applications, and their maximum responsivity is limited by an external quantum efficiency of 100% (292 mA W 1 for GaN and 161 mA W 1 for AlN). The frontier between MSM photodiodes and photoconductors is still unclear, since many of the devices presented in the literature as MSM photodiodes show an obvious photoconductive gain contribution. If these intermediate or hybrid devices are characterised under constant illumination (DC), persistent effects become evident. In this work, we present the fabrication of AlGaN MSM photodetectors on Si(111) substrates. The optimal growth conditions of the buffer layer for the fabrication of these photodetectors will be analysed. The hybrid behaviour of these photodevices will be studied for 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
J.L. Pau et al. / Materials Science and Engineering B93 (2002) 159 /162
160
The structures were grown in a MECA 2000 molecular beam epitaxy system. Active nitrogen was produced 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 /V ratio and the thickness of the layer. The resulting effectiveness in avoiding the parallel conduction through the substrate is thus assessed for each type of buffer. Growth conditions are shown in Table 1. The end of the sample M590 growth corresponds to a change from two-dimensional to three-dimensional growth mode (Stranski /Krastanov mode), as described later. This transition is clearly identified by the appearance of a 2 2 reconstruction in the RHEED monitoring and corresponds to a thickness of 30 /40 nm, with high reproducibility [4]. Samples M564 and M573 showed surface features related to Al-excess during the growth (Fig. 1a,b). In sample M564, which was grown under the highest III /V ratio, little droplets of 2 mm in average were observed, together with larger stains of about 10 mm diameter. Sample M573 showed very small stains, with diameters lower than 2 mm, indicating that the III /V relative ratio was very close to the stoichiometry point (III/V 1), as indicated in Table 1. As seen in Fig. 1c for sample M563, surface roughness starts to degrade after the transition to the three-dimensional mode, the scenario becoming harsh for the subsequent growth of AlGaN. However, under the same growth conditions, if we stop when the 2 2 reconstruction appears, a very smooth surface results, as shown in sample M590 (Fig. 1d). No remnants of metal were detected in the surface of both samples. To compare the electrical insulation provided by the above AlN layers, the current /voltage characteristics between 400 mm diameter Ti/Al contacts separated Table 1 AlN buffer layer characteristics Sample
Thickness (nm) III /V Ileakage (mA) at 10 V
M564
M573
M563
M590
200 1.2 1.4 104
200 1 160
200 0.85 5.0
35 0.85 54
Fig. 1. Nomarski views of samples (a) M564 and (b) M573, and SEM photographs of samples (c) M563 and (d) M590 are shown.
by 200 mm were measured. The leakage currents at 10 V bias are also shown in Table 1. The high conductivity found for AlN layers grown under Al-rich conditions could be due to leakage currents associated to threading dislocations, as suggested by Hsu et al. for MBE-GaN samples grown under Ga-rich conditions [5]. Considering both the insulating characteristics and the surface morphology, we decided to use the buffer structure corresponding to sample M590. Undoped AlGaN layers with a thickness of 1 /2 mm and Al mole fractions up to x 0.39 were deposited on the AlN buffer layer. The growth temperature was in the 730 /760 8C range, depending on the nominal Al composition. Two-dimensional growth was observed by RHEED after 5 min in all the samples under study. Due to the lower deposition temperatures in comparison to MOCVD, layer-cracking problems were not observed in any sample. For GaN, a residual n-type doping of around 1 1017 cm 3 is determined from C /V measurements, whereas for AlGaN, the 1/C 2 dependence versus reverse voltage becomes non-linear. X-ray diffraction (XRD) patterns were obtained from u /2u scans with a wide open detector, showing full-width at half maximum (FWHM) values of 8.5 and 15 arcmin for GaN and AlGaN (x 0.30) layers, respectively. Detectors consist of two interdigitated electrodes on a planar structure, with finger widths and gap spacings of 2, 4, and 7 mm, and active areas of 250 250 mm2 and 500 500 mm2. Two different metal systems were used ˚ )/Al (700 A ˚ ) and Pt (400 A ˚ )/Ti (50 for contacts: Ti (300 A ˚ )/Au (1000 A ˚ ), corresponding to extreme values of A their metal workfunction. All current /voltage characteristics for AlGaN photodiodes presented a rectifying behaviour, with a higher resistivity as the Al content increased. Spectral responsivity studies were performed by using a 150 W xenon arc lamp. The photodetector responsiv-
J.L. Pau et al. / Materials Science and Engineering B93 (2002) 159 /162
ity was measured by excitation with a non-focused He / Cd laser (325 nm) for GaN devices, whereas the 514 nm Ar laser line coupled into a second harmonic generator (257 nm) was used for AlGaN photodiodes. These measurements were performed under constant (DC) illumination. Time response characterisation was made using the fourth frequency of a Nd /YAG laser (266 nm), with 10 ns Gaussian pulses. Typical spectral responses of AlGaN MSM photodetectors with Ti/Al contacts are shown in Fig. 2. The optical response above the bandgap drops more markedly as the Al content increases. The buffer layer efficiently insulates the AlGaN, preventing any contribution from the silicon substrate to the detector optical response. The cut-off wavelength reached 290 nm for x 0.39, demonstrating the capability of these photodetectors for solar-blind applications. Below the bandgap, we fitted the quantum efficiency (h ) by the expression hn h 8exp (1) Eurb where Eurb is the Urbach parameter, which varied from 24 meV for GaN to 90 meV for AlGaN (x 0.39) [6]. This parameter measures the cut-off abruptness and is related to the presence of levels inside the bandgap or to alloy disorder. As indicated in Fig. 2, the spectral response of AlGaN devices presents a shoulder below the bandgap, which might be related to regions with different compositions in the ternary alloy or to absorption in defects. The rotation of the layer during the growth and the different positions of the III-element sources could produce alloy inhomogeneities, as already reported [7]. However, in our sample, from XRD measurements, we have not seen any evidence of
161
different alloy compositions. Room temperature PL measurements showed two emissions whose positions coincided with the cut-off wavelength and the shoulder observed in the spectral response (see inset Fig. 2). The lower energy transition does not follow Varshni’s law for bandgap energy dependence on temperature, which seems to indicate that the transition corresponds to a DA emission [8]. Detector peak responsivity and dark current values can be found in Table 2. As seen, the responsivity decreases with increasing Al mole fractions. On the other hand, the increase of the Al produces a reduction of the observed photoconductive gain, and persistent effects (see photocurrent decays in Fig. 3). These data, together with the increase of the UV/visible contrast, indicate that the increase of aluminium in the ternary alloy provokes a transition from photoconductive to MSM photodiode behaviour. In AlGaN (x 0.39) MSM photodiodes with Ti/Al contacts, the time constant, tp, value for different load resistances was obtained from transient photoresponse measurements. The photocurrent decays were exponential, with the time constant corresponding to the RC product of the measuring system. The dependence of photocurrent response time on load resistance has been analysed in Fig. 4, and the extrapolation to zero-load allows to obtain a minimum tp value of 150 ns. Finally, a comparative spectral response of MSM photodiodes for the two metal systems used can be observed in Fig. 5. The UV/visible contrast is around a factor 10 higher in the case of Pt/Ti/Au due to the lower value of the dark current. The value of the dark current is dominated by the quality of the Schottky contacts. Pt contacts are known to produce barrier heights of 1.0 / 1.1 eV on GaN [9], whereas barriers of 0.1 /0.5 eV have been reported for Ti contacts [10]. The I /V characteristics 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 superior 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 conductive substrate is avoided. The photoconductive gain Table 2 Responsivities and dark current of 3 V biased AlGaN MSM photodiodes
Fig. 2. Spectral responses of MSM AlGaN photodiodes grown on Si(111). Inset: room temperature photoluminiscence of AlGaN (x 0.39).
%Al Rpeak (mA W 1) Id (nA) at 3 V
2 5400 4100
15 58 1.3
39 12 0.015
162
J.L. Pau et al. / Materials Science and Engineering B93 (2002) 159 /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 different Al contents (x 0, 0.15, and 0.39). Observe the different time scales for GaN and AlGaN photodiodes.
Acknowledgements Thanks are due to J. Sa´nchez 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. 95-0466OP.
References
Fig. 4. Time response dependence of an AlGaN (x 0.39) photodiode with load resistance.
observed in these devices has been found to depend strongly on the Al content. The effect of the contact metal on the responsivity and UV/visible contrast has also been evaluated. The recombination time for the photodetectors with the highest Al content (x 0.39) has been extrapolated to 150 ns for zero-load resistance.
[1] F. Omne`s, N. Marenco, B. Beaumont, Ph. De Mierry, E. Monroy, F. Calle, E. Mun˜oz, J. Appl. Phys. 86 (1999) 1. [2] J.C. Carrano, T. Li, D.L. Brown, P.A. Grudowski, C.J. Eiting, R.D. Dupuis, J.C. Campbell, Appl. Phys. Lett. 73 (1998) 2405. [3] E. Monroy, F. Calle, E. Mun˜oz, F. Omne`s, Semicond. Sci. Technol. 14 (1999) 685. [4] M.A. Sa´nchez-Garcı´a, E. Calleja, E. Monroy, F.J. Sa´nchez, F. Calle, E. Mun˜oz, A. Sanz-Herva´s, C. Villar, M. Aguilar, Mater. Res. Soc. Internet J. Nitride Semicond. Res. 2 (1997) 33. [5] J.W.P. Hsu, M.J. Manfra, D.V. Lang, S. Richter, S.N.G. Chu, A.M. Sergent, R.N. Kleiman, L.N. Pfeiffer, Appl. Phys. Lett. 78 (2001) 1685. [6] J.I. Pankove, Optical Processes in Semiconductors, Dover, New York, 1971, pp. 43 /46. [7] S. Ferna´ndez, F.B. Naranjo, F. Calle, M.A. Sa´nchez-Garcı´a, E. Calleja, P. Vennegues, A. Trampert, K.H. Ploog, Appl. Phys. Lett. 79 (2001) 2136. [8] F. Calle, F.J. Sa´nchez, J.M.G. Tijero, M.A. Sa´nchez-Garcı´a, E. Calleja, R. Beresford, Semicond. Sci. Technol. 12 (1997) 1396. [9] Q.Z. Liu, S.S. Lau, Solid-State Electron. 42 (1998) 677. [10] S.N. Mohammad, Z.F. Fan, W. Kim, O. Atkas, A.E. Botchkarev, A. Salvador, H. Morkoc, Electron. Lett. 32 (1996) 598.