Ultrathin Broadband GermaniumâGraphene Hybrid Photodetector with

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Ultrathin broadband germanium-graphene hybrid photodetector with high performance Fan Yang, Hui Cong, Kai Yu, Lin Zhou, Nan Wang, Zhi Liu, Chuanbo Li, Qiming Wang, and Buwen Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16511 • Publication Date (Web): 31 Mar 2017 Downloaded from http://pubs.acs.org on April 1, 2017

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ACS Applied Materials & Interfaces

Ultrathin Broadband Germanium-graphene Hybrid Photodetector with High Performance Fan Yang, †‡ Hui Cong, †‡ Kai Yu, †‡ Lin Zhou, †‡ Nan Wang, †‡ Zhi Liu, †‡ Chuanbo Li, †‡Qiming Wang, †‡ Buwen Cheng*†‡ †State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R. China, ‡College of Materials Science and OptoElectronic Technology, University of Chinese Academy of Sciences

*Address correspondence to [email protected] KEYWORDS: germanium·graphene·hybrid structure·photodetector·broadband

ABSTRACT Germanium-based photodetector is a key component in silicon based photonics because of its unique properties of response at telecommunication band and compatibility with CMOS techniques. However, the limitations of low quantum efficiency and high surface recombination in ultrathin germanium film, especially in the near-infrared range, put huge obstructions on the road towards applications. Nowadays, practical applications require more nano-scale devices with lower power consumption as well as higher responsivity and response speed. In this work, we firstly demonstrate a germanium-graphene hybrid structure photodetector

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that consists of an ultrathin 20 nm germanium layer and a monolayer graphene. The photodetector can achieve a broadband detection from ultraviolet to near-infrared range. A conductive gain of 155 and a responsivity of 66.2 A W-1 are achieved, which is about three orders of magnitude higher than pure graphene photodetectors and about 4 times larger than pure germanium photodetectors. Such enhancement owes to effective generation, separation and transfer of photo-generated carriers at material interface. The photodetector based on germanium-graphene hybrid structure presents a new paradigm for the realization of small but high performance device in the process of integration in silicon-based optical chips. And it offers new opportunities for imaging, sensing and other optoelectronic field applications.

INTRODUCTION Silicon-based photonics have achieved many impressive progresses in integration with microprocessor chips1-5. As a key element in optical interconnection and according to prediction of Moore’s law in photonics, small scale germanium photodetectors with low power consumption that can be integrated with silicon transistor technology attract more and more researchers’ interests6-15. Trying to achieve a series of matches between optical and electronic devices such as device size, capacitance and impedance, pursuing for high performance Ge photodetector especially in nanoscale, vertically and horizontally, naturally, becomes one essential step to realize more mature optical processors16-18. However, once the size of optical devices evolves in nanoscale, some critical problems such as low quantum efficiency and responsivity appears, pulling the development of Ge photodetector of nano-scale standing still in a circle. Various efforts have been made trying to jump out of the cage in past few years7, 19-23, but few breakthrough comes true in solving these essential problems. In addition, due to the effect of “dead region”, carriers excited by visible light in conventional p-i-n Ge detectors are


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difficult to be extracted by electrodes, which becomes a lion in the way for application of Ge photodetectors in visible light communication, not to mention a broadband absorption. To overcome such challenges, the introduction of a hybrid junction structure at surface of germanium, can avoid such problem effectively24-27. The bottommost demand for the material on germanium to obtain a junction is transparent for ultraviolet, visible and NIR range light. Graphene resist blocking the incident light, satisfying the basic need, and its property of all-band absorption makes it a more promising candidate for fabricating hybrid structure with ultrathin germanium material, sequentially, to realize high responsivity and broadband absorption28-33. In the mean time, the relatively high carrier mobility of graphene (highest up to 200 000 cm2 v-1 s-1) could be an urge to push germanium material into a higher speed, larger bandwidth optical integration chip27, 34-35. And the characteristic of semi-metal can lead graphene to be a contact material between germanium and metal electrodes because a Schottky junction can form at the interface of germanium and graphene36. Moreover, fast response time of graphene not only assists germanium devices such as photodetector or phototransistor in fast switching between on and off states37-38, but also does good to improve respond speed for optical detection. Plus, the talent of high thermal conductivity overshadows other competitors in achieving operation at room temperature39. Integration of such a device into chips40-41 surely broaden its applications in imaging, sensing, optical logical circuits, secure quantum key distribution and electronic and optical chips. In this article, we introduced an effective and promising pathway for germanium devices in vertical nanoscale that is compatible with CMOS techniques to break ice in imaging and sensing fields. We firstly proposed and fabricated a high performance broadband photodetector with an ultrathin germanium-graphene hybrid structure. We not only take advantages of the excellent


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properties of germanium and graphene, but also realize a series of enhancements with achievement of a hybrid junction. The detection spectrum starts from ultraviolet range, transits through visible range and moves forward a single step to near infrared telecommunication band. Moreover, the heretostructure between germanium and graphene effectively prevents carrier combination at device surface and interface. As a result of effective separation and transfer of photo-generated carriers at the interface between germanium and graphene, responsivity and conductive gain all achieve a large boost.

RESULTS AND DISCUSSION The schematic diagram of germanium-graphene hybrid structure MSM photodetector is shown in Figure 1a. Then the grown graphene was transferred to the surface of GOI that consists of a 100nm SiO2 burier layer and an ultrathin 20 nm germanium film (more details in Experimental Procedures)42-44. Optical microscopy of the device is shown in Figure 1b. A continuous film of monolayer graphene can be observed at the gap between two electrodes. Figure 1c shows a typical Raman spectrum of the germanium-graphene hybrid structure measured with a 530nm laser. The locations and relative intensity of Ge peak and graphene peaks (G, 2D) are depicted in the figure. In low frequency region, the characteristic peak of Ge at 300 cm-1 is observed. At high frequency region, there are two main peaks at 1580 cm-1 and 2700 cm-1, which are corresponding with the G and 2D, respectively. A large intensity ratio (I2D/IG > 2) of G and 2D peaks conforms the monolayer property of graphene grown in the CVD system36,

45-47

. The relatively small

intensity of defected peak (D) (comparative with background noise) also indicates excellent quality of graphene. The G-band and 2D-band peaks are consistent with the stretching of the C-C bond and a second-order two-phonon process in sp2 carbon systems. The amplified 2D peak that


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is well fitted by a sharp and symmetric Lorentz curve is shown in the inset, which is in good agreement with the results reported in the previous studies on high quality monolayer graphene28, 48-49

. The optical absorption spectrum from ultraviolet to near infrared range, as shown in Figure

1d, is investigated. Compared with pure germanium material, there is an apparent absorption enhancement of the hybrid structure device at ultraviolet, visible and NIR (λ > 1000 nm) range. Figure 2a shows the difference of photocurrent between two devices, one is based on pure germanium and the other on germanium-graphene heterostructure. The light from the laser transmits onto devices through a fiber. When 0.8 V bias voltage is applied under illumination of a 650 nm laser, the photocurrent of Ge-graphene hybrid structure device is about 3.5 times higher than which of the device without monolayer graphene on the surface. At the bias of 1 V, an approximate 4 times enhancement of photocurrent is also observed at infrared range, as shown in Figure 2b. The response spectrum curve is measured to demonstrate a broadband response of Ge-Graphene hybrid structure. (Specific measurement details can be seen in Experiment Procedures.) From the Figure 2c, a wide range photoresponse from ultraviolet to visible range and then to NIR wavelength range can be observed. Because different incident light wavelengths are corresponding to different gratings in spectrograph, three curves at corresponding wavelength region are obtained (350 nm-550 nm, 550 nm-1300 nm, 1300 nm1650 nm). Two peaks, one at 940 nm, another at 1000 nm are observed, which results from different incident light powers of xenon light source at specific wavelength propagating through different gratings and variable absorption coefficient. From the wavelengths of 1200 nm to 1650 nm, there is a section of relative flat curve, which is due to weak absorption of thin germanium and graphene at a long wavelength band of large penetration depth. Measurement setup and response intensity with different bias can be seen in Figure S1 and Figure S2 in Supporting


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Information. And the optical measurements at specific wavelength will be discussed in the later section. In order to explain the great enhancement of photocurrent, an energy band of this hybrid structure is presented to illustrate the transfer process of photo-generated electrons and holes, which is shown in Figure 2d. As a semi-metal, graphene has no band gap between conduction band and valence band. However, in the process of growing graphene in a typical CVD system, the existences of doping effect of substrates, defects and absorption of water and oxygen molecules in the air all lead the position of Fermi level drop below Dirac point, forming a p-type graphene32, 50. Because of metal property of graphene, a junction similar to Schottky junction formed arising from metal-semiconductor contact appeared at the interface between Ge and graphene when electrons are injected from graphene into Ge side. And thus, photo-generated carriers in graphene can be driven towards Ge’s conduction band by a build-in electrical field with the light illumination. At the same time, germanium also absorbs photons and excites photogenerated carriers. Electrons will accumulate in conduction band of Ge because of Schottky barrier, while photo-generated holes will be transported into graphene’s valence band until a steady state forms. The mechanism of generation and transportation inhibits effectively recombination of carriers, and in turn, increases the photocurrent. For the purpose of verifying the ability of broadband detection of germanium-graphene photodetector, some photoelectric measurements are performed under the illumination according to several lasers of different wavelengths. At first, a 532 nm laser is used to study the optical response of the device at visible region. Figure 3a shows the results of pulse light of 532 nm at different incident power density. The photocurrent can be modulated when laser power changes, and be effectively turned on and off as the light is switched on and off. A rise time of less than 10 ms indicates a relatively fast time response and a good respond speed. Responsivity, an


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important parameter of photodetector, is also studied under different illuminated light power, as shown in Figure 3b. It should be noted that the photocurrent increases nonlinearly as the incident power density changes from 0.318 mW/cm2 to 1.452 mW/cm2. The responsivity even reaches to 66.2 A W-1 with the incident power density of 0.318 mW/cm2. On the one hand, it is easy to understand the increasing photocurrent because there will generate more photo-excited carriers in the device as incident light power increases. On the other hand, the corresponding responsivity decreased exponentially with the increasing power density. And the decreasing responsivity can be explained by the following points. Firstly, increased photo-generated carrier concentration will result in an appended electrical field in inner device, whose direction is opposite to the build-in electrical field that arises from germanium-graphene junction, which will lead to electron and hole separation and transport decelerating, kinetic energy to cross barrier reducing and recombination increasing. Specifically, as incident power increases, electrons accumulate more and more on the side of germanium (photo-generated electrons on Ge side and transfer from graphene side). Fermi level of Ge rises up and that of graphene goes down, which causes the rise of bottom of conduction band and top of valence band of Ge. Therefore, it is harder for excited electrons in graphene moving to Ge side because of smaller difference of concentration, and for excited holes in Ge transporting to graphene side than cases of weaker light power. Secondly, the increase of incident power density will contribute to fill the unoccupied states in the graphene’s valence band51. This decrease of responsivity can also be attributed to traps and defects in graphene and germanium52. The Figure 3c shows the time response when the light power density continues to increase at the wavelength of 635 nm. Similarly, in Figure 3d, the device shows the same characteristic of on-off at this wavelength. Responsivity drops from 3.92 A W-1 to 1.06 A W-1 when incident power density increases from 13.7 mW/cm2 to 127.3


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mW/cm2. And the difference of photocurrent between incident power density of 75.6 mW/cm2 and 127.3 mW/cm2 is much smaller than increasing photocurrent with the power density changes from 13.7 mW/cm2 to 35.0 mW/cm2. Although absorption characteristics may be different for specific wavelengths, the trend of above results is consistent with that of 532 nm laser. We further investigate the performance of Ge-Graphene hybrid structure photodetector at near infrared (NIR) region. A 1310 nm laser and a 1550 nm laser, which are mainly light sources used in NIR optical telecommunication, are engaged for optoelectronic measurements. Figure 4a shows illumination of a 1310 nm laser at different optical powers. As shown in Figure 4b, optical response at 1550 nm is also studied. It is noted that photocurrent can be sensitively turned on and off, similar to the situations under visible lights. The rise and fall times are calculated to be 5.6 ms and 3.5 ms, respectively, indicating high time resolution. What’s more, we calculate relationship between photocurrent curves and applied bias voltage, which is measured under optical responsivity and optical gain at visible and NIR range, respectively, and Figure 4c, 4d show the comparison results. The highest responsivity at visible region (532 nm) is 66.2 A W-1, which is, over 1000 times more than that of photodetectors based on pure monolayer graphene29, 37-38

. The responsivity at 635 nm is also large enough, and the decrease may attribute to the

increasing incident light density. Nevertheless, we obtain relatively low responsivity at telecommunication wavelength, which may arise from low absorption coefficient of Ge at 1310 nm and 1550 nm. And the thinner Ge layer (20 nm) leads to low responsivity because of large penetration depth of NIR light source. Low excited photon energy maybe also a main cause. Then, we calculate photoconductive gain of the photodetector by equation (1)53-54. It is wellknown that definition of photoconductive gain means, in unit time, the number of excited photogenerated hole and electron pairs by a single absorbed photon.


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Gሺλሻ =

ூ ௉೔೙

∙

௛ఔ ௘

=ܴ∙

௛௖ ఒ௘

(1)

where I is the photocurrent, Pin is the incident light power, R is responsivity of the device, hν is the energy of an incident photon and λ is the incident light wavelength. And the gain results are shown in Figure 4d. The highest conductive gain is 155 under illumination of a 532 nm laser at power density of 0.318 mW/cm2, which means there will generate as many as 155 electron-hole pairs excited by an individual photon in a unit time.

CONCLUSION In summary, we fabricated and characterized a broadband germanium-graphene photodetector in nano size that can achieve photo detection from ultraviolet (350nm) to NIR (1650nm) region. The photocurrent in such hybrid structure enlarges by about 3-4 times compared with pure germanium photodetector and about 1000 times with reported pure graphene device. Effective separation and transfer of photo-generated carriers at the junction build the foundation for such enhancement. This device shows a large responsivity (up to 66.2 A W-1) and conductive gain (up to 155). Our work demonstrates that germanium-graphene hybrid structure is a prospective and attractive structure in improving the performance of ultrathin semiconductor bulk material. And such hybrid structure opens up a new way for small size but high performance photodetectors in communication applications and many other sensing and imaging fields.

Experimental Procedures：：


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Synthesis, characterization and fabrication of germanium-graphene hybrid structure photodetector. A 30 nm germanium layer was grown in a typical CVD system. A 50nm SiO2 film deposited on the surface by PECVD. Another silicon wafer with 50nm SiO2 on the surface was used to bond with the former sample (germanium and SiO2). Then the bonding wafer was put into tetramethylammonium hydroxide (TMAH) solution to remove the top silicon layer and an ultrathin germanium layer (~10nm). A copper foil of 25 µm (Alfa Aesar, item no. 046365) was first cleaned up and annealed at 1030 ℃ under the flow of H2 for 30 minutes so as to remove all the impurities and particles, importantly, to deoxidise a thin copper oxide layer on the copper foil surface. Graphene was grown at the atmosphere of CH4 and H2 in a typical chemical vapor deposition chamber. After growth, PMMA was spun coated on the top of graphene. The grown graphene was then transferred onto GOI after the copper foil was etched in FeCl3. The UV-NIR absorption spectra of germanium-graphene were collected with a Shimadzu UV3600PLUS UV-VIS-NIR SPECTROPHOTOMETER over an excitation range from 350 nm to 1650 nm with 1 nm intervals. Raman measurements were performed using a LabRam HR 800 Raman instrument at room temperature with a 530 nm laser and a Si photodetector. The device fabrication process consisted of ICP etching to form a mesa, UV lithography to define the device pattern, and electron-beam evaporation to deposit nickel and gold electrodes (Ni/Au 20 nm/50 nm). Optoelectronic Measurements. The response spectrum is measured using a monochromator (Zolix, Omni-λ500) supplied with a Tungsten-Halogen Light Source (Zolix, LSH-T150) and a Stanford Research Systems SR830 lock-in Amplifier to collect respond signal. Specific details


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can be seen in Figure S1 in Supporting Information. A power supply (Keithley 2611A SYSTEM SourceMeter) can provide bias to the device. All characterizations are kept in a dark, shielded environment in ambient air. Photoelectric measurements and time response at visible and NIR range were conducted at room temperature in ambient conditions using a probe station (Cascade M150) equipped with a semiconductor property analyzer (Keithley 2400). Responsivity (R) was calculated by measuring the photocurrent Iph and dividing this value by the incident power Pinc, such that R=Iph / Pinc. The light area was estimated as the area of the mesa with visible laser, while with NIR laser, light area, S, was the product of diameter of single mode fiber (SMF), numerical aperture (NA) and distance to device surface. ACKNOWLEDGMENT This work was supported in part by the Major State Basic Research Development Program of China (Grant No. 2013CB632103), the National Natural Science Foundation (Grant No. 61435013 ， 61534005 ， 61534004 and 61411136001), Beijing Science and Technology Commission (Grant No. Z151100003315019). Supporting Information: Response spectrum measurement setup, Response spectrum of Germanium-graphene hybrid structure device and Comparison of some performances of state-ofthe-art photoconductive detectors based on different materials.

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Figure 1. Material characteristic of Ge-Graphene hybrid structure device. (a) 3D schematic illustration of photodetector. (b) Microscope image of Ge-Graphene photodetector, top view. (c) Raman spectrum of Ge-Graphene hybrid structure. The inset: blue dots for experiment data, red solid line for Lorentz fit of 2D peak of graphene. (d) UV-NIR absorption spectra of pure germanium and germanium-graphene hybrid structure.


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Figure 2. (a) Dependence of photocurrent on applied voltage at visible range with a 650 nm laser. The black and red dots are experiment data for pure Ge device and Ge-Graphene heterostructure device, respectively, red and black solid line are linear fit lines. (b) Dependence of photocurrent on applied voltage at NIR range with a 1310nm laser. The black and red dots are experiment data for pure Ge device and Ge-Graphene heterostructure device, respectively, red and black solid line are linear fit lines. (c) Voltage intensity of device at different photo-excitation wavelength from 350 nm to 1650 nm. (d) Energy band diagram showing the process of generation and transfer of electron and hole at junction between Ge and Graphene.


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Figure 3. Optical electronic measurements at visible region. (a) Photo switching characteristic: by a 532 nm laser with different incident powers at the bias voltage of 1 V. (b) Responsivity and photocurrent measured with a 532 nm laser at the bias of 1 V with different incident power density. The black and blue dots are experiment data, black solid line and blue solid line are exponential fit curves. (c) Photo switching characteristic: photocurrent excited by a 635 nm laser with different incident powers at the bias voltage of 1 V. (d) Responsivity and photocurrent measured with a 635 nm laser at the bias of 1 V with different incident power density. The black solid square and blue solid triangle are experiment data, black solid line and blue solid line are exponential fit curves.


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Figure 4. Optical electronic measurements at NIR region. (a) Photocurrent as a function of bias voltage: the currents are measured at a 1310 nm laser with different incident powers. Colorful hollow dots are experiment data; solid colorful lines are linear fit curves. Inset: enlarged view of


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the curves. (b) Photo switching characteristic of Ge-Graphene hybrid photodetector illuminated by a 1550 nm laser at the incident power of 2.5mW with a bias voltage of 1 V. (c) The responsivity of the photodetector as a function of incident power density at four different wavelengths. (d) The photoconductive gain of this device as a function of incident power density at 532 nm, 635 nm, 1310 nm and 1550 nm, respectively.


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Table of contents: (3.6 cm × 8.4 cm)


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