Si-based metal–semiconductor–metal photodetectors with various design modifications

Solid-State Electronics 51 (2007) 94–101 www.elsevier.com/locate/sse

Si-based metal–semiconductor–metal photodetectors with various design modifications Meiya Li *, Wayne A. Anderson University at Buffalo, Electrical Engineering Department, 332 Bonner Hall, Buffalo, NY 14260, United States Received 24 April 2006; received in revised form 30 September 2006; accepted 7 November 2006 Available online 17 January 2007

The review of this paper was arranged by Prof. Y. Arakawa

Abstract We have designed and fabricated interdigitated metal–semiconductor–metal photodetectors (MSM-PD’s) on n-type amorphous Si/ crystalline Si (a-Si:H/c-Si). Both thick and thin silicon dioxide (SiO2) layers were grown to reduce dark current and passivate the surface. Au, Cr, Ni, and Pd metals were used for metallization. The dark current for the detector (0.75 · 0.5 cm2) with the added a-Si film was reduced from 0.137 mA to 2.61 lA at 5 V when compared with that of the conventional Si-based film. Its magnitude was found to be at least two orders lower than that of the conventional sample. Simple metal/Si Schottky diodes were fabricated with substrates at RT and low temperature (LT). It was found that Schottky barrier height was improved with cryogenic metallization processing. Both dark current and speed were significantly improved as metallization temperature decreased. The full width at half maximum (FWHM), rise time, and fall time at 800 nm reduced from 0.47 ls to 6.2 ns, 49.7 ns to 23.9 ns, and 2.07 ls to 0.41 ls, respectively, as substrate temperature during metallization decreased from room temperature (RT) to 210 K. Schottky barrier height and ideality factor for the LT samples were increased from 0.399 eV to 0.481 eV, and 3.76 to 4.64, respectively, compared to that of the RT sample at 150 K. The current–voltage–temperature (I–V–T) analysis showed that thermionic field emission dominated the current transport in the forward current region.  2006 Elsevier Ltd. All rights reserved. Keywords: Metal–semiconductor–metal photodetector; Cryogenic processing; Diodes

1. Introduction Over the past few years, there has been extensive research on metal–semiconductor–metal photodetectors (MSM-PD’s) on III–V compound semiconductors due to their wide spectral coverage and direct energy bandgap, which allows the ensuing devices to achieve high speeds and good quantum efficiency [1,2]. However, it is difficult and expensive to incorporate them in conventional Si ICs [3]. Silicon, which has an indirect bandgap, is still widely used as the material of choice for the fabrication of photo-devices in Si-based integrated optoelectronic circuits (OEICs), due to its lower cost and the availability of well*

Corresponding author. E-mail address: [email protected] (M. Li).

0038-1101/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2006.11.006

established processing technology [4]. Conversely, most Sibased MSM-PD’s have a higher dark current due to the large thermionic emission current with lateral current flow between two contiguous finger electrodes. Various MSMPD designs using different compromises to reduce leakage dark current and enhance the responsivity of the devices have been reported in the literature. Kim and Hargis have reported the back illumination bandwidth which showed a decrease of about 50% compared to the top illumination condition [5,6], but it encounters several obstacles in practical applications due to its complicated and critical processes on both sides of the wafer. Ho and Wong have developed a vertical MSM-PD with trench electrodes, fabricated by using reactive ion etching (RIE) technology [7]; however, this sharp vertical trench structure would not let the top metal layer be uniformly deposited onto the

M. Li, W.A. Anderson / Solid-State Electronics 51 (2007) 94–101

trench Si surface. In contrast to the above designs, an inexpensive hydrogenated amorphous Si/crystalline Si (a-Si:H/ c-Si) film and oxide layers were used in this study to reduce dark current and enhance response speed. The leakage dark current in MSM-PD’s can also be reduced by Schottky barrier enhancement. Previous studies show that low temperature (LT) metallization processing provides an alternate way to enhance the Schottky barrier height and reduced carrier trapping at the interfaces between metal and semiconductor [8–11]. In this paper, we present various design modifications such as placing a very thin and/or thick interfacial silicon dioxide (SiO2) layer between the semiconductor and the metal contacts, adjusting the barrier height with metal selection, using a cryogenic metallization processing, and adding a hydrogenated amorphous silicon layer to reduce leakage dark current and enhance responses. The performance of Si-based MSM-PD structures were evaluated by dark and continuous wave (CW) photo current–voltage (I–V) characteristics, spectral response and pulse response measurements. We also report the results of metal/Si diodes fabricated with the substrate at room temperature (RT) and LT. The electrical characteristics were also studied by current–voltage–temperature (I–V–T) measurement. 2. Experimental MSM-PD’s were fabricated on double side polished (1 0 0) oriented n- or p-type Czochralski crystal Si with resistivities between 1 and 10 X cm, SiO2 coated wafer, ntype hydrogenated a-Si:H/c-Si thin film, and combining with a-Si:H and SiO2 coated wafer. Prior to the performance of the device fabrication, each substrate was degreased by sequential ultrasonic processing for 3 min in acetone, methanol, rinsed in DI water and blown dry with nitrogen (N2) gas. A 50 nm thick n-type a-Si:H thin film was obtained by using electron cyclotron resonance-chemical vapor deposition (ECR-CVD) at 250 C with 5 mTorr Ar, and approximately 52–68 mTorr silane (SiH4) gases for 15–20 min. An insulating layer of SiO2 with 100 nm thickness was then deposited by plasma enhanced chemical vapor deposition (PECVD) at a substrate temperature of 300 C after the

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ECR-CVD. Prior to the oxide deposition, substrates were dipped in a buffered hydrofluoric acid (BHF) for 30 s to remove a defective layer of the native oxide. Two glass masks were used during the photolithography process. The first mask defined a 50 lm wide window in the SiO2 film. The P-20 (20% hexamethyldisiazane) primer was applied to the sample, allowed to sit for 10 s, and spun at 5000 rpm for 20 s. Next, Shipley 1805 resist was applied to the sample and immediately spun at 5000 rpm for 30 s. The sample was exposed under UV light for 5 s and developed for 30 s after being preheated at 90 C for 10 min, rinsed and dried in N2 gas. The exposed area was etched in buffer HF for 15 s to open the window into SiO2. After that, a second mask was applied to define electrodes and contact pads. At this time, the sample was exposed for 5 or 2.5 s, and developed for 30 or 25 s for critical dimensions greater than or equal to 25 lm and less than 10 lm, respectively. Subsequently, the samples were transferred to a metal evaporation system capable of a 1 · 107 Torr base pressure. The metals were evaporated either under RT or LT conditions. For LT deposition, the substrate temperature was then lowered to the range of 72–238 K using a modified sample holder cooled with liquid N2. Once the temperature stabilized, 50–100 nm of metal was deposited and the substrate holder temperature was raised back to room temperature in a 2-h time period. The metal electrodes and contact pads were formed by using a lift-off process in acetone solution. A 3-D cross section of the MSM-PD structure is shown in Fig. 1. 3. Results and discussion 3.1. MSM-PD’s 3.1.1. Substrate effect Dark and photo I–V characteristics of Si-based MSMPD’s with and without a-Si:H and/or SiO2 layers having electrodes with 50 lm finger width and spacing are shown in Fig. 2. The dark current of Si-based MSM-PD’s with a-Si:H and/or SiO2 layers was reduced by at least two orders in magnitude at 5 V when compared to the bare c-Si MSM-PD. However, the device with an a-Si:H layer

Fig. 1. 3-D cross section of the MSM-PD structure.

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Fig. 2. Room temperature (a) dark I–V comparison for MSM-PD’s having a Cr electrode with 50 lm finger width and spacing on different substrates. Photocurrent of (b) p-Si; (c) SiO2/p-Si and (d) SiO2/a-Si:H/p-Si based MSM-PD’s at different ranges of incident optical power. The active area of the device is 0.75 · 0.5 cm2. The inset shows a detail.

requires more voltage to turn on the current due to higher barrier height. Fig. 2c shows that the I–V curve of the MSM-PD with an SiO2 layer is flatter than that of the bare c-Si device. Both SiO2 and a-Si:H layers act as Schottky barrier enhancement layers and as insulators to reduce the dark current. The response speed of the device with an a-Si:H layer is significantly improved due to defect

states in the a-Si:H film which reduced the lifetime of the photogenerated carrier. As is shown in Fig. 2b–d, symmetric I–V behaviors were observed in all figures. The current increases rapidly at low bias due to incomplete depletion region formation and photogenerated carrier loss at low electric fields. Photocurrent becomes saturated as the bias is increased above the ‘‘flat-band’’ voltage. An excellent

Table 1 Summary of dark current, photocurrent, and responsivity of MSM-PD’s Fig.

Substrates

Metal

Area (cm2)

w · s (lm)

IDark (A)

IPhoto (A)

.R (mA/W)

2

p-Si SiO2/p-Si SiO2/a-Si:H/p-Si

Cr

0.75 · 0.5

50 · 50

1.37 · 104 6.36 · 106 2.61 · 106

3.51 · 103 1.85 · 103 4.41 · 103

93.6 49.3 118

4

n-Si

Cr (4.5a) Au (5.1) Pd (5.12) Ni (5.15)

0.5 · 0.055

50 · 25

2.41 · 103 3.62 · 106 8 · 105 7.03 · 106

3.99 · 103 2.21 · 104 2.55 · 104 9.41 · 105

1451 80.4 92.7 34.2

5

SiO2/n-Si

RT-Pd LT-Pd

0.052 · 0.02

4·4

4.35 · 109 2.77 · 1010

1.02 · 107 5.15 · 108

0.981 0.495

a

Metal work function in eV.

M. Li, W.A. Anderson / Solid-State Electronics 51 (2007) 94–101 0.16

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Fig. 3. Pulse response of MSM-PD’s having a Cr electrode with 50 lm finger width and spacing on (a) p-Si and (b) SiO2/a-Si:H/p-Si substrates.

linear relationship was observed between the photocurrent and incident light power levels. The a-Si:H layer, in Fig. 2d delays the turn-on due to its high barrier height. Dark current, photocurrent, and responsivity are summarized in Table 1. The value of responsivities for the device with an a-Si:H layer was increased from 93.6 mA/W to 118 mA/W at the applied bias of 5 V when compared with the device without the a-Si:H layer. Pulse response measurements were done using a modelocked Ti-sapphire laser tuned to 800 nm with an attenuated average power of about 0.2 mW, emitting 400 ns pulses at a repetition rate of 250 KHz. The rise and fall time of the laser pulse itself is 200 fs. The photocurrent was passed through a load resistor (50 X), and the detector signal is collected by a Tektronix 380 digital sampling oscilloscope. Fig. 3 shows the pulse response for Cr electrodes with 25 lm finger width and spacing on p-Si or SiO2/aSi:H/p-Si substrates. The detector was biased at 10 V and active area of the device was 0.75 · 0.5 cm2. The full width at half maximum (FWHM), rise time, and fall time of the device with an a-Si:H layer was reduced from 1.8 to 0.56 ls, 0.6 to 0.22 ls, and 2.2 to 1.01 ls, respectively, compared with the device without the a-Si:H layer. This may be due to the shorter lifetime in a-Si:H. FWHM, rise time, and fall times of MSM-PD’s are summarized in Table 2. 3.1.2. Metal effect The room temperature dark and photo I–V characteristics of MSM-PD’s with different metal electrodes (Au, Cr, Table 2 Summary of FWHM, rise time and fall time of MSM-PD’s Figure

Area (cm2)

w · s (lm)

FWHM (ls)

Trise (ns) Tfall (ls)

3a 3b

0.75 · 0.5

50 · 50

1.8 0.56

600 220

2.2 1.01

6a 6b 6c

0.038 · 0.02

0.47 0.0785 0.0672

49.7 28.1 23.9

2.07 1.66 0.41

2·4

Ni and Pd) having 50 lm finger width and 25 lm spacing on n-type Si-based substrates were measured and are shown in Fig. 4. Dark currents of 2410 lA, 80 lA, 36.2 lA, and 7.03 lA at applied bias of 5 V were obtained for the Cr, Pd, Au, and Ni, respectively, corresponding to higher metal work function. The dark current reduced with higher metal work function and higher Schottky barrier height due to the thermionic emission. However, higher barrier height decreased quantum efficiency or responsivity. The quantum efficiency of the detector can be expressed by a Fowler equation [12]: g/

ðhm  /B Þ hm

2

ð1Þ

The dark current of the Ni MSM-Pd was reduced by a factor of at least one–three orders in magnitude compared to the case of Au, Cr, and Pd in Fig. 4a. In contrast, both photocurrent and responsivity were increased by a factor of least one–two orders in magnitude. Dark current, photocurrent, and responsivity are summarized in Table 1. Device speed was quite independent of metal work function. 3.1.3. Cryogenic processing Fig. 5 shows the room temperature dark and photo I–V characteristics for MSM-PD’s having Pd electrodes with 4 lm finger width and spacing, processed at RT and LT (227 K and 210 K). Dark current was reduced for the lower temperature cases. The LT-Pd MSM-PD from Fig. 5a showed a dark current of 4.58 pA at 1 V measured at 300 K. In contrast, the dark current of the RT-Pd MSMPD was at least three orders of magnitude greater, with a value of 1.69 nA at 1 V when compared with the LT-Pd MSM-PD. In addition, the RT-Pd MSM-PD showed a continuing increase of the dark current with bias voltage, and a soft breakdown voltage as applied bias increased at around 3 V and 4.5 V for RT- and LT-Pd, respectively.

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Fig. 4. Room temperature (a) dark and (b) photo I–V comparison for n-Si based MSM-PD’s having different electrode metals with 50 lm finger width and 25 lm spacing. The active area of the device was 0.5 · 0.055 cm2. The intensity of incident optical power was 100 mW/cm2.

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Fig. 5. Room temperature (a) dark I–V comparison for SiO2/n-Si MSM-PD’s having Pd metallization at different temperature with 4 lm finger width and spacing. Photocurrent of (b) RT and (c) LT at 227 K MSM-PD’s at different ranges of incident optical power. The active area of the device was 0.052 · 0.02 cm2.

The behavior of soft breakdown may be caused by surface defects. Dark currents obtained from LT processed MSM-

PD’s are low compared to the other reports on MSM-PD’s [13,14].

M. Li, W.A. Anderson / Solid-State Electronics 51 (2007) 94–101

The photo I–V curve, in Fig. 5b, consists of two regions; the photocurrent first rapidly increases as bias increases, and then saturates. This rapid increase of photocurrent in the low bias region is due to extension of the depletion region in the reverse biased Schottky junction and the improved internal quantum efficiency [15]. An excellent linear relationship was observed between the photo-current and incident light power for the employed light power range. The LT sample shown in Fig. 5c gave a more linear behavior with applied voltage whereas the RT device had a softer voltage dependence. Fig. 6a–c show pulse response for SiO2/n-Si MSM-PD’s with Pd metallization at RT, 227 K, and 210 K, respectively, with 2 lm finger width and 4 lm spacing. The laser wavelength was 800 nm with power of 100 mW. The detector was biased at 15 V. Comparing detectors having Pd deposited at different substrate temperatures (temperature decreased from RT to 210 K), the FWHM, rise time, and fall time at 800 nm reduced from 0.47 ls to 6.2 ns, 49.7 ns to 23.9 ns, and 2.07 ls to 0.41 ls, respectively. The speed of the device was strongly improved as temper-

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ature decreased due to higher barrier height and fewer surface defects. FWHM, rise time, and fall times of MSMPD’s are summarized in Table 2. 3.2. Diodes Fig. 7 shows a typical set of experimental forward and reverse dark current–voltage–temperature (I–V–T) curves for diodes formed using (a) RT- and (b) LT-Pd with the measuring temperature range from 150 to 400 K. At high forward bias (region I, V > 0.5 V), the series resistance dominated the region for both RT and LT films. A linear relation between ln(I) and V is shown in region II, which indicated that thermionic emission (TE) or thermionic field emission (TFE) mainly control the current transport mechanism in this region. The reverse saturation current Is, Schottky barrier height, and ideality factor, n, could then be found. In region III, a symmetric curve is observed in the low voltage region of forward and reverse bias for the RT sample, showing I / Vm with m  3. Such behavior may indicate space charge limited current (SCLC), with

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Fig. 6. Pulse response for SiO2/n-Si based MSM-PD’s with Pd metallization at (a) RT; (b) LT at 227 K and (c) LT at 210 K with 2 lm finger width and 4 lm spacing.

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transport in the ballistic regime, or a tunneling component. This was not seen for the LT sample. A strong dependence of the I–V characteristics on the temperature is shown in region IV on both RT and LT samples, which indicates that thermionic emission dominates the current transport mechanism in this region. A weak dependence of the I–V characteristics for the reverse voltage region, in the low temperature range (150–250 K), was observed for the RT sample. Such behavior may indicate the effect of tunneling; however, this was not seen for the LT sample. The current value for reverse bias was at least four orders lower than the forward current at the same bias voltage and temperature. The reverse saturation current density for the LT diode was about one order smaller than that of the RT diode at the same measuring temperature, due to the higher value of barrier height. Another linear relation between ln(I) and V is shown in region V for the LT sample, which may indicate that the current transport is affected by recombination of carriers. Schottky barrier heights, /B, and ideality factors, n, were obtained from Fig. 7 and are listed in Table 3. Schottky barrier height and ideality factor for the LT sample were increased from 0.399 eV to 0.481 eV, and 3.76 to 4.64, respectively, compared to that of the RT sample at

Table 3 Schottky barrier height and ideality factor for RT and LT films at various temperatures Temperature (K)

150 200 250 300 350

RT

LT

Region II

Region II

Region V

/B (eV)

n

/B (eV)

n

/B (eV)

n

0.399 0.537 0.642

3.76 2.35 1.72

0.481 0.573

4.64 4.31

0.424 0.543 0.633 0.719 0.834

8.74 5.3 4.64 3.85 2.65

150 K. Both barrier height and ideality factor should decrease with increasing temperature according to thermionic emission theory [16]. However, the barrier height increased, while the ideality factor decreased as the temperature increased for both RT and LT samples. This may indicate that the current transport in the region II is dominated by thermionic field emission.

4. Conclusions We have successfully designed and fabricated Si-based MSM-PD’s with conventional linear interdigitated fingers using various methods to improve the performance of the devices. From I–V measurements, the dark leakage current and sensitivity of MSM-PD’s were significantly improved by adding a-Si:H and/or SiO2 layers on top of c-Si. Schottky barrier height was improved with cryogenic processing. The dark current for the cryogenic film was found to be one to three orders lower in magnitude compared to the film deposited at room temperature. However, the quantum efficiency or responsivity was reduced with higher Schottky barrier height. I–V–T analysis showed that thermionic field emission dominates the current transport in the forward current region. Dark current density for the LT diode was about one order smaller than that of the RT diodes at the same measuring temperature. Combing the a-Si:H top layer, judicious use of oxides and cryogenic processing will lead to a low dark current and a subsequent lower noise. Lower intensity optical signals can then be detected.

Acknowledgements The authors would like to thank Dr. Fei Chen for the pulse response measurements. This research was partially

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supported by the National Science Foundation, Grant number ECS-0324893. References [1] Hong WP, Chang GK, Bhat RH. IEEE Trans Electron Dev 1989;36:659. [2] Moons JH, Li SS, Lee JH. Electron Lett 2001;37:1249. [3] Herrscher M, Grundmann M, Droge E, Kollakowski S, Bottcher EH, Bimberg D. Electron Lett 1995;31:1383. [4] Stiff AD, Krishna S, Bhattacharya P, Kennerly SW. IEEE Trans Electron Dev 2001;48:1747. [5] Kim JH, Griem HT, Friedman RA, Chan EY, Ray S. IEEE Photon Technol Lett 1992;4:1241.

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[6] Hargis MC, Ralph SE, Woodall JW, McInturff D, Negri AJ, Haugsjaa PO. IEEE Photon Technol Lett 1996;8:110. [7] Ho JYL, Wong KS. IEEE Photon Technol Lett 1996;8:1064. [8] Lee HJ, Anderson WA. Appl Phys Lett 1993;63:1939. [9] Palmer JW, Anderson WA. IEEE Photon Technol Lett 1997;9:1385. [10] Hong H, Anderson WA. IEEE Trans Electron Lett 1999;46:1127. [11] Li M, Anderson WA. Mater Res Soc Symp Proc 2005;864:E9.39. [12] Liu MY, Chou SY. Appl Phys Lett 1995;66:2673. [13] Hirata D, Sugino T, Shirafuji J. Jpn J Appl Phys 1996;35:1779. [14] Chan PT, Choy HS, Shu C, Hsu CC. Appl Phys Lett 1995;67:1715. [15] Soole JBD, Schumacher H. IEEE J Quantum Electron 1991;27:737. [16] McCafferty PG, Sellai A, Dawson P, Elabd H. Solid-State Electron 1996;39:583.