Ultra-low-cost near-infrared photodetectors on silicon

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Ultra-low-cost near-infrared photodetectors on silicon

M. Amin Nazirzadeh

a,b

, Fatih B. Atar

a,b

, B. Berkan Turgut

a,b

, Ali K. Okyay*

a,b,c

a

_{Department of Electrical and Electronics Engineering, Bilkent University, Bilkent, Ankara, Turkey;}

b

_{National Nanotechnology Research Center (UNAM), Bilkent University, Bilkent, Ankara, Turkey;}

c

_{Institute of Materials Science and Nanotechnology, Bilkent University, Bilkent, Ankara, Turkey}

ABSTRACT

We demonstrate Silicon-only near-infrared (NIR) photodetectors (sensitive up to 2000 nm) that meet large-scale ultra-low-cost fabrication requirements. For the detection of infrared photons, we use metal nanoislands that form Schottky contact with Silicon. NIR photons excite plasmon resonances at metal nanoislands and plasmons decay into highly energetic charge carriers (hot electrons). These hot electrons get injected into Silicon (internal photoemission), resulting in photocurrent. Several groups have studied plasmonic nanoantennas using high resolution lithography techniques. In this work, we make use of randomly formed nanoislands for broad-band photoresponse at NIR wavelengths. We observe photoresponse up to 2000 nm wavelength with low dark current density about 50 pA/µm2. The devices exhibit photoresponsivity values as high as 2 mA/W and 600 µA/W at 1.3 µm and 1.55 µm wavelengths, respectively. Thin metal layer was deposited on low-doped n-type Silicon wafer. Rapid thermal annealing results in surface reconstruction of the metal layer into nanoislands. Annealing conditions control the average size of the nanoislands and photoresponse of the devices. An Al-doped Zinc Oxide (AZO) layer was deposited on the nanoislands using thermal atomic layer deposition (ALD) technique to acts as a transparent conductive oxide (TCO) and patterned using photolithography. AZO film creates electrical connection between the nanoislands and also makes a heterojunction to Silicon. Simple and scalable fabrication on Si substrates without the need for any sub-micron lithography or high temperature epitaxy process make these devices good candidates for ultra-low-cost broad-band NIR imaging and spectroscopy applications.

Keywords: Silicon, Plasmonics, hot electrons, infrared detectors, Schottky diodes.

1. INTRODUCTION

Sub-bandgap photodetection via the internal photoemission mechanism is an attractive candidate for near-infrared (NIR) photodetection on Si. A metal in contact with Si creates a Schottky junction and a potential energy barrier forms between the metal and Si. This junction operates as a diode (Schottky diode) and can be used as a sub-bandgap photodetector. The sub-bandgap photons incident on the metal layer excites the electrons of the metal to higher energy levels, enabling them to traverse the Schottky barrier and be collected as photocurrent1_{. The interest in this approach has received a significant}

boost with the recent studies on the use of metallic nanoantennas to capture the incident light by exciting surface plasmons2-4_.

Metals are normally highly reflective at NIR wavelengths and can poorly absorb the incident light. This had been the main drawback of using Si Schottky barrier diodes for NIR photodetection. However, when made into nanostructures, some metals, such as Au, can resonantly interact with the incident light and confine the electromagnetic field in very small volumes in the form of surface plasmons. The surface plasmons decay due to the optical losses of the metal and as they decay their energy is transferred to the electrons of the metal, exciting the electrons to higher energy levels, which are now called “hot electrons”. These hot electrons can be emitted over the Schottky barrier and generate photocurrent2_.

*aokyay@ee.bilkent.edu.tr

Proc. of SPIE Vol. 9367 93671I-1

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by

1

V

AZO

Au

n -type Si

Figure 1. Energy band diagram of a Schottky junction. Photodetection of a sub-bandgap photon via the internal photoemission is shown on the energy band diagram1_.

Several studies have investigated the plasmon assisted hot electron generation process and its utilization for NIR photodetectors by using high resolution lithography techniques to fabricate metallic nanoantennas2, 3_{. Very strong,}

tunable, narrow-band plasmonic resonances have been demonstrated and photoresponsivity values as high as 0.6 mA/W have been reported 3_{. In this study, we first investigate the fabrication process of metallic nanoantennas and propose a}

method to form randomly sized and randomly distributed nanoantennas on Si surface. We then demonstrate the use of these random nanoantennas in a Schottky contact photodetector for broad-band plasmon enhanced NIR photodetection.

2. FABRICATION

We used 4-inch n-Si (100) wafers with resistivity of 2-5 Ω-cm throughout this study and diced the wafers into 15 mm × 15 mm pieces prior to fabrication. Cleaning of the wafers was done in two consecutive steps. In the first step, the wafers are kept in piranha solution ((4:1) H2SO4:H2O2) at 80°C for 5 minutes to remove possible organic and metallic

contaminants. Piranha solution also oxidizes the wafer surface and forms a thin SiO2 layer, making the surface

hydrophilic. The wafers are rinsed with deionized water after the piranha cleaning. In the second step of the cleaning process, the wafers are dipped in buffered hydrofluoric acid (BHF) solution for about 10 seconds until the surface oxide was completely etched and the sample surface became hydrophobic again. The wafers are then rinsed with DI water and dried with N2 gun.

We have followed several approaches to form plasmonic nanoantennas on Si surface. Electron beam lithography and nanoimprint lithography were used to fabricate nanoantennas with desired dimensions. A third method, involving annealing of a metal layer to form randomly shaped nanoislands, was also investigated.

Nanoimprint lithography is a low-cost, fast, and high resolution lithography method. The desired pattern is first fabricated on a master stamp using a high resolution lithography technique. The pattern on the master stamp is then transferred to a mold which is Polydimethylsiloxane (PDMS) for our case. PDMS consists of a base Silicone elastomer and its corresponding curing agent. The base and the curing agent are mixed carefully in 10:1 volume ratio for about 10 minutes to achieve uniform distribution in the mixture. The mixture is then kept in a desiccator for an hour to remove the air bubbles that may form during the mixing process. The PDMS mixture is poured on the master stamp and cured at 150°C. The PDMS mold takes the shape of the master stamp and must be peeled off carefully after the curing process. The pattern on the PDMS mold is transferred to the sample by coating a photoresist layer on the sample and pressing the PDMS mold on the photoresist. The pressure is applied conformally and at a temperature higher than the glass transition temperature of the photoresist. The photoresist takes the shape of the stamp and hardens with this process. A final dry etching step is applied to remove the residual photoresist. The flow diagram of the nanoimprint lithography process is

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ULTS & DIS

ectors, we us nd light from 3 ble filter (AO hrough an opti light normally is probed w plies the bias ock-in amplifi aracterization ctronic charac ated with a m generated curr plotted in Fig. rence exhibit r devices. Th The device her temperatur cid (HNO3) et nal metallic co fferent temper nce only had t e second refer

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SCUSSION

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the AZO laye rence only had ft-off method hs, the AZO r r hand, Au ref Au reference . The photore e plasmonic e Ltd, supercon 00 nm wavele ochromate the h a mechanica odetector. The anipulators an measures the D ptical chopper wn in Fig. 5. tup. Supercon hopper. The li quency of the ference does n onse at the N ealed at 450° 300°C has l sponsivity of evices were ri C, 450°C and er forming a h d a 10-nm-thi (Au referenc reference is no ference is exp is expected to esponses of the enhancement o ntinuum laser ength. The las e laser light a al chopper an e beam waist o nd electrically DC current. P r frequency as ntinuum laser ight is focuse mechanical c not respond to NIR wavelen °C exhibits th arger and m this device is insed with DI 600°C). Two heterojunction ck Au film on ce). Since both ot expected to pected to show o be low since e devices with of hot electron source (WL ser light is fed at the desired nd a half-wave of the focused y biased with Photocurrent is s the reference light and ed on the chopper is

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pectra of the re temperatures. at different tem DTD simulatio with the Fowle

ion. CONCLUS the excitation ub-bandgap p a thin metal f nd resonance iencies were at 600°C has hibit lower resp h theoretically anced internal e-Domain sol pectra of the A o hot electron ion efficiency rnal photoemi 2 he incident ph he experimen d from such ju ement with the lasmons to ho um efficiency curs since the

nealed at 450 mple annealed horter wavele eference devic . (b) Simulate mperatures. A ons. Quantum er function an SION n of surface p photons on Si

film stand out s at the NIR observed at smaller partic ponse in NIR calculated qu l photoemissio lver (FDTD Au nanoislan generation rat of hot electro ssion process hotons, q is th ntal results wit unctions6_{. Sim} e experimenta ot carriers and spectra of the e sample anne 0°C. Hence, a at 450°C. The engths.

ces and the de ed quantum ef Absorption spe m efficiency sp nd fitting to th plasmons on m . Among vari t as a fast an wavelengths. 1300 nm an cles compared region. uantum efficie on process. SE Solutions, Lu ds with differ tes in the Au n ons in order to , which is exp he electron cha th CF and φB mulated quant al results, indi d the collectio e devices anne ealed at 300°C at this cross-o e sample anne evices with na fficiency spec ectra of the na pectra are calc he experiment

metals is inve ious technique nd ultra-low-c

With the pro nd 1550 nm

d to the other ency spectra in EM images o umerical Inc) rent sizes and nanoislands o o calculate the pressed by the (1) arge and φB is used as the fi tum efficiency

cating that the on of these ho

ealed at 300°C C absorbs the over point, the ealed at 600°C anoislands ctra of the anoislands culated by tal results estigated as an es to fabricate ost method to oposed device wavelengths r n f ), d f e e s it y e ot C e e C n e o e s,

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respectively. FDTD simulations and theoretical calculations were used to fit to the experimental results and successfully predict the quantum efficiency spectrum.

ACKNOWLEDGMENTS

This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK), grant numbers 109E044, 112M004, 112E052, and 113M815. A. K. O. acknowledges support from European Union FP7 Marie Curie International Reintegration Grant (PIOS, Grant # PIRG04-GA-2008-239444). A. K. O. acknowledges support from the Turkish Academy of Sciences Distinguished Young Scientist Award (TUBA GEBIP). F. B. A. acknowledges TUBITAK-BIDEB national PhD Fellowship.

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