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CHARACTERIZATION OF AN AlGaAs/GaAs METAL-SEMICONDUCTOR-METAL
PHOTODETECTOR
M. Zirngibl, R. Sachot, M. Ilegems
To cite this version:
M. Zirngibl, R. Sachot, M. Ilegems. CHARACTERIZATION OF AN AlGaAs/GaAs METAL-
SEMICONDUCTOR-METAL PHOTODETECTOR. Journal de Physique Colloques, 1988, 49 (C4),
pp.C4-325-C4-328. �10.1051/jphyscol:1988468�. �jpa-00227966�
JOURNAL DE PHYSIQUE
Colloque C4, supplQrnent au n09, Tome 49, septembre 1988
CHARACTERIZATION OF AN AlGaAs/GaAs METAL-SEMICONDUCTOR-METAL PHOTODETECTOR
M. ZIRNGIBL, R. SACHOT and M. ILEGEMS
Institut de Micro- and Opto6lectronique. Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland
Resume: Nous reportons la caracterisation d'un photodetecteur metal-semiconducteur-metal en fonction de sa geometric, de la tension appliquee et d e la puissance optique incidente. De I'oxyde d'etain et d'indium (ITO) est depose sur des substrats GaAs et des couches MBE afin de dkfinir les contacts Schottky interdigites qui forment la surface active. Les detecteurs montrent une efficacite quantique externe elevee et une reponse rapide (temps de montbe < 70 ps, largeur a mi-hauteur < 185 ps).
Abstract: The characteristics of metal-semiconductor-metal photodetectors as a function of layout geometry, applied voltage and optical input power are reported. The structures are fabricated using interdigitated indium tin oxide Schottky contacts deposited by sputtering on GaAs substrates and MBE-layers to form the active area. The detectors show high external quantum efficency and fast response (risetime < 70 ps, FWHM < 185 ps).
In an optical telecommunication or read-out system, sensitive and broadband photodetectors are necessary as interface elements between an optical waveguide and a electronic circuit. There is an increasing interest in metal-semiconductor-metal photodetectors (MSM) because of their high speed, their planar structure which is compatible with integration with electronic devices and their large active area which facilitates the coupling of the light into the detector. The capacitance of their structure is detemined by the lateral distance between the contact fingers at a given active area of the detector so that the RC product is small even for detectors with large areas (typical 200 f F for 200x200 pm2 active area at punch through voltage). Finally, the fabrication of the MSM's is very simple compared to that of vertical detectors like p-i-n diodes. In spite of these promizing features, the explanation of the internal gain mechanism is still a subject of r e s e a r ~ h l - ~ .
We have fabricated MSM's with an indium tin oxide (ITO) metallization on undoped and Cr doped GaAs substrates (referred to as GaAs-MSM and GaAs(Cr)-MSM) and on undoped AIGaAs/GaAs MBE-- layers (referred to as MBE-MSM) using a simple one-level lift-off technology. ITO, which acts as an anticoating layer at 820 nm, is sputter-deposited on the semiconductor surface to form the interdigitated Schottky contacts (Fig.]) so that the whole detector area can be illuminated. The size of the active area is 50x50 pm2 and 200x200 pm2. The geometry is adapted to be mounted into a 50 Ohm coplanar microstri- pline. The distance between the fingers, L, and the finger width, d, varies from 1 to 16 pm. The MBE-layer consists of an undoped 1 pm thick GaAs active layer and an underlying 0.5 pm thick A10.3Ga0.7As buffer layer to limit the light collection zone.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988468
JOURNAL
DE
PHYSIQUEMeasurements of sensitivity are made by focusing the beam of a laser diode which emits at 820 nm on the detector so as to achieve an uniform illumination over the entire active area. The optical input power is measured with a calibrated Si photodiode. In Fig. 2 we plot the photo- and dark current against bias voltage for a MBB-MSM with finger width and finger spacing of 3 gm. The dark current is probably governed by structural defects rather than by the Schottky barrier heigth as evidenced by the asymmetry of the I-V characteristics. Contrary to photoconductors, the photocurrent is not proportional to the bias voltage but determined by the reverse Schottky diode caracteristics. The sensitivity is 0.46 A/W (Fig. 3) independent of the optical input power. Fig. 4 shows the detector response to a short laser pulse of 60 ps risetime and 100 ps full width at half maximum (FWHM). The detector risetime of 72 ps is limited by the optical pulse while the FWHM of 184 ps is probably limited by the hole transit time (120 ps at 5 x 1 0 ~ cm/s saturation velocity taking into account the carriers generated under the fingers)
The detectors fabricated directly on the substrates show the same risetime but a higher FWHM (about 250 ps). Their photocurrent as a function of the finger spacing L for different values of the average electric field is represented in Fig. 5. The data are taken on diodes of 200x200 gm2 active area with 4 um and 8 p m nominal finger width. For the detectors on undoped GaAs substrates, the dependence on geometry and electrical field is weak, contrary to those on GaAs(Cr) which show an increase of the photocurrent with L. The sensitivity of the GaAs(Cr)-MSM seems to depend only on L and not on the finger width. Unity external quantum efficiency is situated at 290 PA. Thus, the GaAs(Cr)-MSM's have external efficiencies up to 500% for large L. To our knowledge, these are the highest values reported so far for this kind of detector. Taking into account reflexion losses the GaAs-MSM's show also a small internal gain. The measured dark currmts at 2 V/pm average field are 10 nA arid 123 a A for the MSM's on GaAs(Cr) and on the undopcd GaAs, respectively and do r o t show any c l e ~ r dependence on the device geometry. The GaAs(Cr)-MSM with a small finger spacing show a linear degendence of the photocurrent with the input power (Fig. 6) while the sensitivity of those with a large L va:ies at first sublinearly with input power and then increases sharply above 100 pW for the structures operated at 29 V bias applied (Fig. 7). At lower optical input power, the photocurrent is independent of the finger spacing L. Focusing the laser spot alternatively on the two electrodes, we have observed that the photocurrent is maxirnurn on the negative electrode and minimum on the positive electrode. The ratio of these currents is iO for the GaAs(Cr)-MSM and 2 for the GaAs-MSM.
The interpretation of the measurements of the GaAs- and GaAs(Cr)-MSM's is not clear. The observed gain is not believed to be due to a photoconductive effect because the contacts are not ohmic. Avalanche multiplication may be involved in areas of locally enhanced electric field, although the average lateral field is as low as 2 V/pm. There may be an optical induced lowering of the Schottky barrier heigth, but no clear dependence of the photocurrent on the contact area is observed. Measurements of the barriers under illumination and comparisons between MSM's with different metallizations are being carried out to further elucidate the observed behavior.
In conclusion, we have fabricated and characterized a very simple MSM-photodetector with interdigitated I T 0 Schottky contact. This type of detector could find widespread applications in optical telecommunication- and data systems because of its simplicity and low cost, its high speed (risetime < 70
ps, FWHM < 185 ps) and sensitivity (up to 500% external quantum efficiency), and its planar structure.
Acknowledeements: We thank A. Nerini for her excellent technical assistance and J. M. Breguet and P. Marechal for measurements. This work was supported by the Swiss National Science Fondation, research program 13.
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Fig.1: Top view and cross section of a MSM on a Fig. 2: I-V characteristics in the dark and under MBE layer: The dimensions are W = 50 pm and 200 illumination of a MBE-MSM with an active area of um, L = 1,2,3,4,8,12 and 16 pm and d = 1,2,3,4 50x50 um2 and 3 pm finger width and spacing.
and 8 pm (nominal values on the mask, exact values are measured by SEM). The total area of the Schottky contacts measures between 9000 and 30000
p2
for the 200x200 um2 MSM's and 1250 pm2 for the 50x50 pm2 MSM's. The MSM is designed to be integrated into a 50 Ohm microstripline.C4-328 JOURNAL
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PHYSIQUEFig. 3: Photocurrent versus incident optical power for the same MSM as shown in Fig. 2.
0.00 3.00 6.00 9.00 12.00 : 5.00 Distance between the fingers L [micro~s]
Fig. 5: Photocurrent as a function of finger spacing L for different values of the average electric field (Vbias/L) between the fingers. The Schottky contact area, which is different for the 7 structures, does not influence the sensitivity.
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Fig. 4: Response of a MBE-MSM deiector to a laser pulse of 60 ps risetime and 100 ps FWHM. The structure tested is the same as shown in Fig. 2 and 3
.
The data are taken under 16 V applied bias to the detector. The MSM is mounted on a coplanar microstripline to assure 50 Ohm matching and the output signal is measured with a S4 Tektronix sampling head.I G' jv 13' 1G' 1 v 1 @
Incident optical power [W]
Diotonce between fingers 1 .i urn A applied voltage 2.5 V
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Fig. 6: Photocurrent versus incident optical power for a GaAs(Cr)-MSM with 1.7 pm finger spacing measured at 2.5 V bias.
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Fig. 7: Photocurrent versus incident power of a GaAs(Cr)-MSM with 14.8 pm fingerspacing measu- red at 14.8 and 29.6 V bias.
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