Al gan ultraviolet photodetectors grown by molecular beam epitaxy on si(111) substrates
Materials Science and Engineering B93 (2002) 159 Á/162 www.elsevier.com/locate/msebAlGaN 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 isshown that the growth of an adequate AlN buffer layer is critical to achieve visible-blind devices, and that its role as an effectiveelectrical insulator of the conductive substrate was found to be more efficient for N-excess AlN growth. The increase of Al contentproduced a transition from photoconductor to MSM photodiode behaviour, as determined from the detector responsivity, temporalresponse, and UV/visible contrast. The effect of the contact metal on photoconductive gain and UV/visible contrast was alsostudied. # 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 andhave been an important focus of attention during the grown by metalÁ/organic chemical vapour depositionlast 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-typeand heterojunction transistors. The possibility of tuning doping, metal Á/semiconductor Á/metal (MSM) structuresthe semiconductor bandgap from 1.9 eV for InN and 3.4 and photoconductors are very attractive devices foreV 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 assensing, missile warning, UV biological effects, UV 1000 A W(1 have been reported, but they showed veryastronomy, water purification, pollution monitoring, long time decays, which reduces the detectivity drasti-high-density optical storage, engine and nuclear reactor cally . In contrast, ideal MSM (or back-to-backmonitoring, and space-to-space communication. The Schottky) photodiodes are devices specially adequatelack 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 ofdevelopment 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 andsubstrates used due to the availability of high-quality, photoconductors is still unclear, since many of thelarge-area and low-cost wafers. However, its high devices presented in the literature as MSM photodiodeslattice- and thermal-mismatch with III-nitrides and the show an obvious photoconductive gain contribution. Ifhigh diffusion-coefficient of Si at growth temperatures these intermediate or hybrid devices are characterisedhave delayed the progress in the fabrication of efficient under constant illumination (DC), persistent effectsoptoelectronic devices on this substrate. The use of become evident.proper buffer layers, which attenuate these inconve- In this work, we present the fabrication of AlGaNniences, 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 hybrid336-7323. behaviour of these photodevices will be studied for E-mail address: firstname.lastname@example.org (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
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 plasmasource. 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 roleof this layer is not only to improve the crystalline qualityof the latter AlGaN layer, but also to electricallyinsulate the epitaxial film from the conductive substrate.Growth rate and layer thickness were measured by insitu optical interferometry using an IRCON infraredpyrometer with a narrow-band filter centred at 0.94 mm.The resolution of this technique is 95 nm, even for thethinnest layers. Four types of AlN buffer layers were grown, bychanging 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 SEMconduction 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 inTable 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 conductivitychange from two-dimensional to three-dimensional found for AlN layers grown under Al-rich conditionsgrowth mode (Stranski Á/Krastanov mode), as described could be due to leakage currents associated to threadinglater. This transition is clearly identified by the appear- dislocations, as suggested by Hsu et al. for MBE-GaNance of a 2 ) 2 reconstruction in the RHEED monitor- samples grown under Ga-rich conditions . Consider-ing and corresponds to a thickness of 30 Á/40 nm, with ing both the insulating characteristics and the surfacehigh reproducibility . 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 mmsample M564, which was grown under the highest III Á/V and Al mole fractions up to x 00.39 were deposited onratio, little droplets of 2 mm in average were observed, the AlN buffer layer. The growth temperature was in thetogether with larger stains of about 10 mm diameter. 730 Á/760 8C range, depending on the nominal AlSample M573 showed very small stains, with diameters composition. Two-dimensional growth was observedlower 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 comparisonindicated in Table 1. As seen in Fig. 1c for sample M563, to MOCVD, layer-cracking problems were not observedsurface roughness starts to degrade after the transition in any sample. For GaN, a residual n-type doping ofto 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 dependenceunder 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 scansresults, as shown in sample M590 (Fig. 1d). No with a wide open detector, showing full-width at halfremnants of metal were detected in the surface of both maximum (FWHM) values of 8.5 and 15 arcmin forsamples. 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 aistics 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 andTable 1 500 ) 500 mm2. Two different metal systems were usedAlN 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 rectifyingThickness (nm) 200 200 200 35 behaviour, with a higher resistivity as the Al contentIII 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-
J.L. Pau et al. / Materials Science and Engineering B93 (2002) 159 Á/162 161ity was measured by excitation with a non-focused He Á/ different alloy compositions. Room temperature PLCd laser (325 nm) for GaN devices, whereas the 514 nm measurements showed two emissions whose positionsAr laser line coupled into a second harmonic gen- coincided with the cut-off wavelength and the shouldererator (257 nm) was used for AlGaN photodiodes. observed in the spectral response (see inset Fig. 2). TheThese 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, whichmade 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 . Typical spectral responses of AlGaN MSM photo- Detector peak responsivity and dark current valuesdetectors with Ti/Al contacts are shown in Fig. 2. The can be found in Table 2. As seen, the responsivityoptical response above the bandgap drops more mark- decreases with increasing Al mole fractions. On theedly as the Al content increases. The buffer layer other hand, the increase of the Al produces a reductionefficiently insulates the AlGaN, preventing any contri- of the observed photoconductive gain, and persistentbution 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 ternarydetectors for solar-blind applications. Below the band- alloy provokes a transition from photoconductive togap, 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 hnh 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 RC24 meV for GaN to 90 meV for AlGaN (x 00.39) . product of the measuring system. The dependence ofThis parameter measures the cut-off abruptness and is photocurrent response time on load resistance has beenrelated to the presence of levels inside the bandgap or to analysed in Fig. 4, and the extrapolation to zero-loadalloy 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 MSMthe bandgap, which might be related to regions with photodiodes for the two metal systems used can bedifferent compositions in the ternary alloy or to observed in Fig. 5. The UV/visible contrast is around aabsorption in defects. The rotation of the layer during factor 10 higher in the case of Pt/Ti/Au due to the lowerthe growth and the different positions of the III-element value of the dark current. The value of the dark currentsources could produce alloy inhomogeneities, as already is dominated by the quality of the Schottky contacts. Ptreported . 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 , whereas barriers of 0.1 Á/0.5 eV have been reported for Ti contacts . 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 39Fig. 2. Spectral responses of MSM AlGaN photodiodes grown on Rpeak (mA W(1) 5400 58 12Si(111). Inset: room temperature photoluminiscence of AlGaN Id (nA) at 3 V 4100 1.3 0.015(x0 0.39).
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 Acknowledgementsdifferent Al contents (x 0 0, 0.15, and 0.39). Observe the different timescales 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. References  F. Omnes, N. Marenco, B. Beaumont, Ph. De Mierry, E. Monroy, ` F. Calle, E. Munoz, J. Appl. Phys. 86 (1999) 1. ˜  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.  E. Monroy, F. Calle, E. Munoz, F. Omnes, Semicond. Sci. ˜ ` Technol. 14 (1999) 685.  M.A. Sanchez-Garcıa, E. Calleja, E. Monroy, F.J. Sanchez, F. ´ ´ ´ Calle, E. Munoz, A. Sanz-Hervas, C. Villar, M. Aguilar, Mater. ˜ ´ Res. Soc. Internet J. Nitride Semicond. Res. 2 (1997) 33.Fig. 4. Time response dependence of an AlGaN (x 0 0.39) photodiode  J.W.P. Hsu, M.J. Manfra, D.V. Lang, S. Richter, S.N.G. Chu,with load resistance. A.M. Sergent, R.N. Kleiman, L.N. Pfeiffer, Appl. Phys. Lett. 78 (2001) 1685.  J.I. Pankove, Optical Processes in Semiconductors, Dover, New York, 1971, pp. 43 Á/46.  S. Fernandez, F.B. Naranjo, F. Calle, M.A. Sanchez-Garcıa, E. ´ ´ ´observed in these devices has been found to depend Calleja, P. Vennegues, A. Trampert, K.H. Ploog, Appl. Phys.strongly on the Al content. The effect of the contact Lett. 79 (2001) 2136.  F. Calle, F.J. Sanchez, J.M.G. Tijero, M.A. Sanchez-Garcıa, E. ´ ´ ´metal on the responsivity and UV/visible contrast has Calleja, R. Beresford, Semicond. Sci. Technol. 12 (1997) 1396.also been evaluated. The recombination time for the  Q.Z. Liu, S.S. Lau, Solid-State Electron. 42 (1998) 677.photodetectors with the highest Al content (x 00.39)  S.N. Mohammad, Z.F. Fan, W. Kim, O. Atkas, A.E. Botchkarev,has been extrapolated to 150 ns for zero-load resistance. A. Salvador, H. Morkoc, Electron. Lett. 32 (1996) 598.