GRAPHENE

Engineered tunneling layer with enhanced impact ionization for detection improvement in graphene/silicon heterojunction photodetectors


The machine construction and energy-band diagram of the graphene/insulator/silicon (GIS) heterojunction photodetectors had been schematically proven in Fig. 1. On this machine (Fig. 1a and b), a 3 mm × 3 mm silicon window on the SiO2/n-Si substrate was fabricated by means of the lithography, adopted by the insulation layer deposition, 3–5 layers’ graphene transferring, and phone steel’s deposition (Fig. S1, Supporting Data). The Raman spectra point out the well-maintained high quality for the transferred graphene on silicon with the standard traits of few layers (Fig. S2). For the traditional GS heterostructure photodetector (Fig. 1c), because of the completely different work features between graphene and n-type silicon, Schottky junction is shaped within the interface of graphene/n-Si7,34. Underneath gentle illumination, the optical absorption primarily takes place in silicon substrate, whereas graphene primarily acts as a clear electrode resulting from its excessive service mobility and excessive transparency. And the photo-generated carriers could be separated by the built-in electrical discipline: the holes transfer to high electrode by means of graphene whereas the electrons transfer to the underside electrode by means of silicon. After introducing a skinny WBI layer between the graphene and silicon (Fig. 1d), the tunneling construction could be shaped and the energy-band diagram of the photodetector below reverse bias could be aligned as proven in Fig. 1d. On this case, the darkish present is anticipated to be suppressed because of the elevated Schottky barrier peak (SBH), and the photo-generated extra holes would accumulate on the insulation-semiconductor interface. Additional, if the insulating layer is skinny sufficient for carriers’ quantum tunneling, the influence ionization pushed by excessive electrical felid would subsequently happen to understand the photocurrent multiplication28.

Fig. 1: System construction and proposed working mechanism for the gadgets.
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a Schematic and b photograph picture of the fabricated wafer-scale graphene/insulator/Si (GIS) heterojunction photodetectors. The machine dimension was 8 × 8 mm2 with a middle lively area of three × 3 mm2. c, d Reveals the corresponding power band diagram of the traditional GS heterojunction photodetector and the GIS heterojunction photodetector below reverse bias and illumination, respectively.

With a purpose to display the proposed tunneling impact enhanced photo-responsivity by introducing the wide-bandgap tunneling layer, aluminum nitride (AlN) was selectively used because the interlayer materials firstly. Right here, the AlN movie used on this work was grown by ALD at a temperature of 380 °C and the detailed deposition parameters are proven in Fig. S3. The thickness of the AlN movies was decided by the expansion parameter of about 0.09-nm-thick per cycle (Desk S1, Supporting Data). The scanning electron microscope (SEM) photos of the AlN movies earlier than and after deposition on Si substrate could be seen in Fig. S4. Determine 2a reveals the cross-section picture of the SiO2 protected AlN/Si construction by the high-resolution transmission electron microscopy (HR-TEM), by which polycrystalline traits could be properly resolved for the AlN layer. The interplanar spacing in AlN and Si grains had been measured to be 0.268 nm and 0.267 nm, respectively, which correspond to the AlN (100) and Si (200) lattice constructions. Additional X-ray diffraction (XRD) evaluation on the 65-nm AlN movie grown on Si substrate additionally inferred the existence of AlN (100) aircraft with the primary diffraction peak resolved at 33.1°. The transmission spectra proven in Fig. 2d indicated that the deposited AlN movies have a glad crystal high quality with a great optical transmittance. Right here, the estimated band-edge (Eg) by utilizing the Tauc’s methodology35 from the absorption spectra was about 5.64 eV, exhibiting a much less worth than the majority AlN36. Understandably, sorts of crystal defects within the ALD deposited AlN movie had been the primary causes. Additional XPS measurement properly indicated the standard O associated defects (Al–O bond at binding power of 74.6 eV and N-Al-O bond at a binding power of 398.8 eV) for the ALD deposited AlN movie, which reveals comparable outcomes as that reported within the earlier work37,38, as proven in Fig. 2e and f for the Al 2p and N 1s spectra. It’s believed that these defects would contribute to the carriers’ tunneling in addition to the influence ionization, and this can be mentioned later.

Fig. 2: Morphology, crystallinity, and optical characterization of the AlN movie.
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a Cross-sectional HR-TEM picture of the ALD deposited AlN movies on Si substrate and b the corresponding EDX elemental mappings. c XRD patterns of the naked Si substrate and 65 nm AlN movie grown on Si. d Optical transmission spectra of AlN movies grown on sapphire substrates with completely different thicknesses. The Tauc plot for the estimation of the bandgap of AlN was proven in inset. e, f Reveals the XPS spectra for Al 2p and N 1s of the AlN movie, respectively.

The photodetection performances within the proposed tunneling gadgets had been firstly evaluated by the current-voltage (I-V) characterization below the darkish and lightweight illumination (365 nm) situations, as proven in Fig. 3a, with the comparability to the traditional Schottky machine. It may be seen that because of the Schottky barrier, each the GIS gadgets with AlN and GS management photodetectors work as a common rectifier diode below darkish situation. Nonetheless, as proposed above, the inserted AlN movie served as an ideal barrier layer to considerably suppress the darkish present of the machine, exhibiting not less than one order of magnitude decrement on the bias of −10 V.

Fig. 3: Photograph-detection efficiency of the GIS gadgets.
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a The I-V characterizations of GIS tunneling photodetector with 15.3-nm AlN evaluating with the traditional graphene/n-Si heterojunction photodetector below darkish and lightweight illumination. b Time-dependent photo-response for the GIS machine below −10 V and 365-nm gentle illumination (12.2 mW cm−2). c Optical power-dependent responsivity within the GIS tunneling photodetector with the comparability to the management machine with out tunneling layer below 365-nm gentle illumination at a bias of −10 V. d Spectra-dependent photocurrent responsivity of the GIS tunneling photodetector with 15.3-nm AlN tunneling layer at a bias of −10 V, evaluating with the management and reference gadgets.

Whereas below the sunshine illumination, for the traditional GS photodetector, a light-induced photocurrent with a present saturation area could be noticed within the reverse bias. On this standard case, the photocurrent primarily is dependent upon the incident gentle simply as that in a typical Schottky photodiode, throughout which the photo-generated minority holes transfer to graphene induced by the built-in electrical discipline or the utilized reverse bias whereas the bulk electrons cross by means of the depletion area to generate the photocurrent. Nonetheless, when a skinny AlN movie was inserted, carriers tunneling happens in GIS photodetector by the utilized reverse voltage offering the driving drive for the carriers’ multiplication. Thus, considerably enhanced photocurrent was noticed below the reverse bias over −2.8 V. Understandably, the shaped boundaries would suppress the carriers’ transport when the bias is within the ahead bias case, ensuing the lowered photocurrent evaluating with the management GS machine. The time-related photo-response proven in Fig. 3b clearly demonstrated the tunneling-effect induced outstanding traits within the GIS machine: a lot stronger photocurrent and decrease darkish present had been obtained exhibiting a considerably elevated detectivity.

The photograph responsivity below particular incident optical energy was calculated utilizing the equation R = (IpId)/P, the place Ip is the photocurrent, Id is the darkish present and P is the utilized optical energy. The calculated particular responsivity and detectivity for the standard machine below the UV gentle (365 nm) was 1.03 A W−1 and a pair of.94 × 107 Jones with relative low NEP worth of ~1.02 × 10−8 W Hz−1/2. Extra responsivity metrics for a batch of gadgets proven that common values of 1.02 ± 0.08 A W−1 and 0.22 ± 0.10 A W−1 had been obtained for the GIS and management GS gadgets, respectively, indicating the glad uniformity in machine efficiency (Fig. S5). The reverse bias dependent responsivity (Fig. S6) properly reveals the avalanche multiplication traits the place the utilized electric-field (bias) throughout the AlN layer dominated influence ionization39. Right here, the photocurrent multiplication issue (Mph) as a operate of reverse bias (V) has been estimated by Mph = [Iph (V)− Idark (V)]/[Iph(unity)Idark(unity)] (Fig. S7), the place Iph (V) and Idarkish (V) are multiplied photo- and darkish currents, respectively; Iph(unity) and Idarkish(unity) are major (unmultiplied) photo- and darkish currents on the unity achieve area39,40. And the unity photocurrent and darkish currents had been evaluated utilizing the photocurrent at bias of −2.8 V of which the photocurrent for GIS machine surpass the traditional GS Schottky photodetector. At this bias (−2.8 V), the calculated corresponding vital electrical discipline for AlN multiplication layer is about 1.8 MV cm−1, which is in keeping with the reported operation electrical discipline of 1.9 MV cm−1 for AlN avalanche multiplication39. With a commerce of the responsivity, detectivity and ON/OFF ratio together with the reverse voltage (Figs. S6 and S7), the bias of −10 V has been used for the investigation.

With a purpose to additional perceive the thickness-dependent tunneling impact of the inserting layer and obtain the utmost enchancment, the photo-responses within the tunneling constructions with completely different thicknesses of the AlN movies had been characterised with evaluating to the management one, as proven in Fig. S8. It may be seen that with growing the thickness of AlN, the I-V traits of the gadgets progressively modified from typical Schottky photodiode to the Schottky tunneling photodiode (Fig. S8a)33,41. The enlarged view of the photocurrent below reverse bias was proven in Fig. S8b, and the photocurrent worth of the gadgets below the -10 V bias with the AlN-thickness elevated was statistically plotted in inset. It may be inferred that the thickness of about 15 nm was the optimized one to attain one of the best efficiency within the corresponding gadgets, whereas the too skinny AlN (5.4 nm) couldn’t kind the efficient tunneling distance to understand the multiplication clearly and the thicker AlN (24.3 nm) would additional suppress the tunneling present.

The photo-response traits for the optimized GIS machine with 15.3-nm AlN below the bias of −10 V and 365-nm gentle illumination with completely different gentle powers was proven in Fig. S9, evaluating to the management machine with out AlN inserting layer. The obtained optical power-dependent responsivity was plotted in Fig. 3c. It may be seen that the photocurrent for the traditional GS photodetector is roughly linear with the incident gentle energy, comparable as a typical Schottky kind photodetector34,42. The calculated responsivity reveals the next worth within the weak incident gentle area of μW degree and a comparatively secure worth about 0.25 A W−1 because the incident gentle energy elevated from μW to mW. Nonetheless, within the GIS construction photodetector the responsivity initially elevated as the sunshine depth rising after which reached a comparatively secure worth of ~1.03 A W−1 on the illumination energy bigger than ~5 mW cm−2. Evaluating with the as-fabricated standard GS heterojunction photodetectors and typical business silicon PIN photodetectors (Hamamatsu S2836-44K), respective ~4.2 and ~7.2 occasions enhancements in photo-responsivity had been facilely achieved within the GIS gadgets. It must be famous that resulting from incident gentle power-dependent response characteristic, the linear dynamic vary (LDR) carried out on this GIS machine has not been elevated clearly in comparison with the management GS machine (72.5 db vs 36.1 db), the place LDR is outlined by equation LDR = 20 log (I*ph/Idarkish)43, and I*ph is the photocurrent measured at an incident optical energy of 1 mW cm−2.

As proven in Fig. 3d, a broad spectral response enhancement additionally could be properly acknowledged because of the tunneling impact induced optical achieve within the optimized GIS machine with 15.3-nm AlN at a reverse bias of −10 V. The height optical responsivity was realized on the wavelength round 850 nm as properly acknowledged in Si construction. Additional optical power-dependent photo-response at 850 nm as proven in Fig. S10 demonstrates the same characteristic that the responsivity achieves the next and secure worth when the incident optical energy is elevated to mW degree. A peak optical responsivity of three.96 A W−1 and detectivity of 1.13 × 108 Jones was facilely obtained on the GIS machine, presenting a 5.8-times photogain evaluating with the management machine within the standard construction. Evaluating with the current reported GS herterojuntion photodetectors with an interface insulating layer (Desk S2), the proposed tunneling construction on this work allows the gadgets holding a aggressive detection efficiency.

Right here, the considerably enhanced photo-detection efficiency is proposed to be originated from two elements. Firstly, because of the excessive resistivity and skinny thickness of the insulating layer, the imposed reverse voltage is especially utilized to the insulator, inflicting the excessive electrical discipline within the area and enhanced power band bending for silicon and AlN. In consequence, the influence ionization would occur throughout the carries’ tunneling with excessive kinetics32. The simulated electric-field distribution close to the junction (Fig. 4a and Fig. S11) clearly reveals the considerably enhanced electrical discipline depth about 6.2 × 106 V cm−1 throughout the insulating layer (below −10 V bias), exhibiting about two order enhancement evaluating with the traditional construction (~4.1 × 104 V cm−1) with out insulating layer (Fig. S11). As for the incident power-dependent responsivity proven in Fig. 3c, the change of electrical discipline throughout the insulating layer because of the photo-generated carriers’ accumulation was thought of to be the primary purpose31, as mentioned in Fig. S12.

Fig. 4: Proposed tunneling mechanism of the GIS gadgets with comparability to the traditional metal-semiconductor (MS) construction.
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a Simulated electric-field distribution close to the junction for the gadgets of GS and GIS for a comparability (AlN is 15.3 nm and the bias is −10 V). b Proposed working mechanisms of the GIS heterojunction photodetector exhibiting the tunneling course of from minority carriers of silicon and scorching carriers from graphene with influence ionization. c The I-V curves of MS tunneling photodetector with 15.3-nm AlN evaluating with the traditional MS Schottky photodetector below darkish and lightweight illumination (365 nm and 12.2 mW cm−2). d Comparability of the photocurrent for the MS and GIS/GS gadgets with and with out AlN tunneling layer below the reverse bias. The incident gentle was 365 nm and in an influence of 12.2 mW cm−2.

Secondly, the improved photo-detection ought to have benefitted from the tunneling impact of the recent carriers in graphene, as illustrated in Fig. 4b. Underneath the excessive electrical discipline driving the recent carriers (electrons) from graphene can simply overcome the boundaries to tunnel by means of the AlN insulating layer with the attainable influence ionization beside the commonly thought of minority (holes) from the silicon facet. With a purpose to display this, we’ve got fabricated the traditional gadgets utilizing the skinny steel layer (10-nm Au movie) because the clear conductive electrodes for comparability, as proven in Fig. 4c and d. Comparable because the GIS gadgets with AlN layer, the MS machine additionally reveals clearly decreased darkish present and enhanced photocurrent. Clearly, multiplication has occurred on the MS machine with AlN tunneling layer as anticipated. Nonetheless, a lot low photocurrent enhancement about 2.3-times was obtained on the MS gadgets with AlN in comparison with the ~4.5-times for the GIS gadgets, whereas with comparability to every management machine with out AlN tunneling layer (Fig. 4d). Contemplating the tunneling carriers ought to primarily come from the minority of silicon in addition to the restricted thermionic-field emission below reverse bias for the MS machine with insolating layer, the extra enhancement for GIS machine ought to originate from the presumably existed multiplication of scorching carriers from graphene throughout the tunneling.

As well as, the particular defects in AlN shaped throughout the ALD deposition38,44, had been believed to be chargeable for the multiplication of the transported carriers because of the considerably elevated tunneling present through the mechanism of trap-assisted tunneling33,45. Usually, the substitutional oxygen for nitrogen (ON) and aluminum emptiness (VAl) are the dominated defects within the ALD ready AlN movies38, and would kind the shallow degree defects with the corresponding power degree about 0.8 and 1.0 eV, respectively. In consequence, these level defects may additional fascinate the recent carriers’ tunneling from graphene in addition to the minority from silicon to generate extra photocurrent with multiplication.

Clearly, the band construction of the interfacial insulating layer must be the dominate components for the tunneling in addition to the present multiplication processes. With a purpose to display the universality of this proposed tunneling mechanism, different generally used interfacial insulating supplies, the native oxidized SiO229,34 and Al2O332,46, even have been employed because the tunneling layer within the GIS heterostructure as comparisons (Figs. S13 and S14). It may be seen that the entire three sorts of insulator of their individually optimized thicknesses (15.3 nm, 5.0 nm, and 1.4 nm for AlN, Al2O3 and SiO2, respectively) can successfully suppress the darkish present from 10−6 A to the virtually identical degree about 10−8 A below the −10 V bias, as proven in Fig. 5a. Determine 5b reveals the I-V curves of the corresponding three kinds of GIS gadgets with Al2O3, SiO2 and AlN tunneling layer below the 365 nm illumination (12.2 mW cm−2), evaluating with the management machine with out tunneling layer. From the enlarged view of the photocurrent within the reverse bias area proven in Fig. 5c, it may be seen that the entire tunneling constructions present an apparent photo-induced multiplication impact and have considerably enhanced photocurrent in comparison with these within the management machine below the bias of −10 V. Amongst them, the AlN-tunneling layer achieved the best-performed photo-response and present enhancement adopted by the Al2O3 and SiO2 (Fig. 5d). For Al2O3 movie, because of the bigger bandgap shaped boundaries for tunneling and the standard glad crystallinity with much less defects for the movies fabricated by ALD, much less thickness was wanted for this sort of insulating materials. Whereas for the native oxidized SiO2 movie, the best dense and insulation properties of it make the fabricated gadgets want a lot vital thickness in nanoscale (<2 nm). Additionally, the restricted thickness strongly decreased the potential of influence ionization throughout the tunneling course of, thus not appropriate for the detection enhancement. Subsequently, applicable band hole and materials high quality within the tunneling layer, that’s well-engineered band construction and SBH are vital for reaching glad performances within the GIS tunneling construction.

Fig. 5: Comparability of the photo-response for the GIS gadgets utilizing completely different insulating supplies.
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I-V characterizations of the three kinds of graphene/insulator (SiO2, Al2O3, and AlN)/silicon (GIS) photodetectors below a darkish and b 365-nm gentle illumination (12.2 mW cm−2), with the comparability to the traditional GS heterojunction photodetector. The thickness of the insulators utilized in (ac) for the three kinds of GIS photodetectors all have been optimized to the best-performed one. c The enlarged illustration of (b) within the reverse bias area. d Tunneling layer thickness-dependent photocurrent enhancement for GIS gadgets with the three sorts of insulator supplies.

The dynamic response within the GIS gadgets additionally has been investigated at a bias of −10 V and below 365-nm gentle illumination, as proven in Fig. S15. The measured response time for the rise time (tON) is 1.9 ms and the decay time (tOFF) is 1.4 ms. Totally different from the service drift-diffusion course of within the standard graphene/n-Si photodetector, with the employment of the insulating layer a quicker optical response (1.9 ms vs 2.3 ms in GS construction) was realized resulting from this photo-assisted tunneling course of at the next electrical discipline. Right here, the presumably existed interface trapping defects because of the wet-transferring ought to have restricted the response dynamics for the gadgets9. Moreover, a great environmental stability was additionally revealed within the fabricated GIS photodetectors with the detection efficiency remaining practically unchanged even after being saved in ambient air for 128 days, as proven in Fig. S16.



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