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Solar-blind Photodetector with High Avalanche Gains and Bias-tunable Detecting Functionality Based on Metastable Phase #-GaO3/ZnO Isotype Heterostructures 2

Xuanhu Chen, Yang Xu, Dong Zhou, Sen Yang, Fang-Fang Ren, Hai Lu, Kun Tang, Shulin Gu, Rong Zhang, Youdou Zheng, and Jiandong Ye ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09812 • Publication Date (Web): 04 Oct 2017 Downloaded from http://pubs.acs.org on October 8, 2017

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ACS Applied Materials & Interfaces

Solar-Blind Photodetector with High Avalanche Gains and Bias-Tunable Detecting Functionality Based on Metastable Phase α-Ga2O3/ZnO Isotype Heterostructures

Xuanhu Chen1, Yang Xu1, Dong Zhou1,2, Sen Yang1,2, Fang-fang Ren1,2*, Hai Lu1,2, Kun Tang1, Shulin Gu1,3*, Rong Zhang1,3, Youdou Zheng1,3, Jiandong Ye1,3* 1

Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, and School of Electronic Science

and Engineering, Nanjing University, Nanjing 210093, China 2

3

Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China Collaborative Innovation Center of Solid-State Lighting and Energy-Saving Electronics, Nanjing University, Nanjing

210093, China

*

Authors to whom correspondence should be addressed. Electronic mails: [email protected], [email protected] and [email protected] 1 / 33

ACS Paragon Plus Environment


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Abstract The metastable α phase Ga2O3 is an emerging material for developing solar-blind photodetectors and power electronic devices towards civil and military applications. Despite of its superior physical properties, high quality epitaxy of metastable phase α-Ga2O3 remains challenging. _

To this end, single crystalline α-Ga2O3 epilayers are achieved on nonpolar ZnO (1120) substrates for the first time and a high performance Au/α-Ga2O3/ZnO isotype heterostructures based Schottky barrier avalanche diode is demonstrated. The device exhibits self-powered functions with a dark current lower than 1 pA, a UV/visible rejection ratio of 103 and a detectivity of 9.66×1012 cm Hz1/2/W. Dual responsivity bands with cutoff wavelengths at 255 nm and 375 nm are observed with their peak responsivities of 0.50 and 0.071 A/W at -5 V, respectively. High photoconductive gains at low bias is governed by barrier lowing effect at Au/Ga2O3 and Ga2O3/ZnO hetero-interfaces. The device also allows avalanche multiplication processes initiated by pure electron and hole injections under different illumination conditions. High avalanche gains over 103 and a low ionization coefficient ratio of electrons and holes are yielded, leading to a total gain over 105 and a high responsivity of 1.10×104 A/W. Such avalanche heterostructures with ultra-high gains and bias-tunable UV detecting functionality holds the promise for developing high performance solar-blind photodetectors. Keywords: solar-blind photodetector; avalanche breakdown; isotype heterostructures; wide-bandgap semiconductors; gain mechanism

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1. Introduction Solar-blind photodetectors based on wide bandgap semiconductors (e.g. AlxGa1-xN,1-5 MgxZn1-xO,6-8 diamond9-10) have attracted much attention due to its wide range of civil and military applications. Especially for the solar-blind UV region (200-280 nm), alloying engineering with high composition of Al and Mg in the respective AlxGa1-xN, MgxZn1-xO alloys is inevitable, which would limit the detectivity due to serious alloying composition fluctuation or even phase segregation.11 Apart from them, a great effort has been devoted in the past decade to develop photodetectors based on various β-phase gallium oxide (β-Ga2O3) films12-24 or nanostructures25-29. Most of the reported photodetectors are photoconductive type15-20,

27-29

and in spite of achieved high photoconductive

gains, the response speed is sacrificed owing to persistent photoconductivity effect which hinders their practical applications. In practical, for the detection of weak UV signals, the avalanche photodetector (APD) is an ideal candidate to break the trade-off between responsivity and response speed25, in which, high multiplication gain is achieved through impact ionization processes.5 In APDs, high electric field is necessary for the avalanche process, which is often delivered by the formation of p-n junction.1, 4 Unfortunately, as an intrinsic n-type semiconductor, it is difficult to realize p-type conduction of Ga2O3 material. Alternatively, control of band discontinuities at n-n isotype heterostructures also allows independent manipulation of carrier injection, carrier confinement and ionization thresholds in high speed optoelectronic devices.12,

25

Recently, high gain β-Ga2O3/n-SnO212 and

β-Ga2O3/n-ZnO25 n-n isotype APDs have been reported. Nevertheless, the breakdown fields in these structures were much lower than the critical value for avalanche multiplications effects and the physical mechanisms of impact ionization of electrons and holes are still under debate. 3 / 33



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In fact, the most investigated β-Ga2O3 exhibits the monoclinic structure30-32, which is quite different with high symmetry hexagonal systems, like 4H-SiC, GaN, ZnO and sapphire materials. It has been shown that α-phase Ga2O3 with hexagonal structure are stabilized at the interface of c-plane sapphire and β-Ga2O333. The phase transition of α-β inevitably introduce large density of defects or dislocations at hetero-interface or even spreading throughout the whole epilayer.33 To achieve high device performance, high interface quality of heterostructure is essential and lattice matching with foreign materials are of great importance. In comparison to β-Ga2O3, the less-studied metastable, corundum-like α-phase Ga2O3 exhibits similar hexagonal structure and relatively small lattice mismatch with GaN, ZnO and sapphire substrates34. It also offers relatively large bandgap (5.1 eV), small electron effective mass, higher breakdown field and larger Baliga’s figure of merit.34-36 The superior physical properties of α-Ga2O3, together with the easy integration with other corundum structure functional oxides, such as Al, Cr, Fe oxides34, 37, allows ones designing and delivering high performance solar-blind photodetector and power electronic devices.34 Herein, we reported on the _

_

design and fabrication of Schottky photodetector based on Au/α-Ga2O3 (3030)/n-ZnO (1120) isotype heterostructures. The device exhibits an ultra-low dark current ( 103) and novel functionality of bias tunable UV photodetection. At zero bias, it exhibits self-powered characteristics with a sharp cutoff at 255 nm and high UV/visible rejection ratio over 103. In particular, the mechanisms of carrier transport, photoconductive gains and avalanche-launched multiplication processes have been investigated in terms of the unique energy band structures. The delivered novel functionality and high performance allow the realization of simple and inexpensive high sensitivity and low-noise photon counters based on all-oxide heterostrutures. 4 / 33


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2. Experimental The α-Ga2O3 epitaxial layers were grown by laser molecular beam epitaxy technique using a KrF laser (λ = 248 nm) with a repetition rate of 2 Hz and a laser fluence of 1.5 J cm-2. The commercial _

a-plane ZnO (1120) from Tokyo Denba with sizes of 1 cm×1 cm×500 µm are used as substrates, which shows atomically smooth surface with flat terraces. The ZnO substrates were attached to a SiC heater and positioned 55 mm from the high purity Ga2O3 (99.999%) ceramic target. The growth temperature was 600 oC and the oxygen pressure was 10-3 mbar. After deposition, the samples were cooled down to room temperature at the deposition pressure. The thickness of Ga2O3 epilayer on _

ZnO (1120) is determined to be 180 nm by the ellipsometry spectrum measurement. The 2θ-ω x-ray _

diffraction pattern in Figure 1a reveal that single crystalline α-phase Ga2O3 (3030) was formed on _

ZnO (1120) substrates. The atomic force microscopic image shown in Figure 1b indicates a smooth surface morphology with mean roughness of 1.8 nm. The ZnO substrate exhibited a carrier concentration of 1.2×1017 cm-3 and a mobility of 220 cm2/Vs at room temperature, determined by the Hall effect measurements. The intrinsic Ga2O3 epilayer shows high resistivity and a carrier concentration is about 2×1016 cm-3 estimated from the capacitance-voltage profile. Prior to device fabrication, Indium was evaporated on the backside of ZnO followed by metallization at 100 oC for 30 min to ensure the Ohmic contact. Subsequently, the Au (50 nm) Schottky contacts were formed on Ga2O3 surface by consequent processes of lithography, electron beam evaporation and lift-off. Finally, a vertical Schottky diode based on n--n+ isotype heterostructure of Au/α-Ga2O3/ZnO was fabricated, as illustrated by the cross-sectional schematic in Figure 1a. The current-voltage (I-V) characteristics were performed using a Keithley source meter model 2636A in dark or under illumination of 254 nm and 365 nm lamps (Model ENF-240C/FE) 5 / 33



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with an incident power density of 0.5 mW/cm2. A probe station (Model cascade Microtech Tesla) was used to control the measurement temperature ranging from 25 oC to 100 oC. The spectral photoresponse was measured using a monochromator (Model iHR320) equipped with a 500 W xenon-arc lamp as the optical excitation source. The incident power density was calibrated by a Si reference photodiode. A KrF excimer laser (248 nm) with a pulse width of 20 ns at a repetition rate of 20 Hz was employed as the excitation source for transient photoresponse measurements, and a digital oscilloscope (TBS 1102) for data collection. The detailed setup has been described in the supporting information.

3. Results and Discussion A vertical Schottky diode based on n-n isotype heterostructure of Au/α-Ga2O3/ZnO was fabricated, as illustrated by the cross-sectional schematic in Figure 1a. Figure 2 shows the spectral responsivity of a 400 µm-diameter device at different reverse biases under front illumination. The

(

photoresponsivity is calculated in terms of the relation R = I ph − I dark

)

Pλ A , where Pλ is the

calibrated incident light power density, and A is the effective illuminated area.38 For the vertical structured device with the opaque Schottky contact, the contribution of illumination to the photocurrent is primarily from the photogenerated carriers in the lateral depletion region around the periphery of the contact. The small amount of excess carriers in flat band region within the diffusion length can be also collected. Thus, the effective illuminated area herein is defined as the ring area with its width of 2.08 µm by taking the sum of depletion width (1.90 µm at a reviser bias of 40 V) and diffusion length (0.18 µm). 39 The detailed description on the effective illuminated area is shown in Supporting Information. Under zero bias, the device has a maximum responsivity of 3.42 mA/W 6 / 33


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at 230 nm with a sharp cutoff at 255 nm that exactly corresponds to the absorption edge of Ga2O3 shown by the transmittance spectra in Figure 2 and the UV/visible rejection ratio (R230 nm/R450 nm) is more than 103, which qualifies α-Ga2O3 photodetectors to be solar blind and self-powered. The thermal-noise-limited detectivity (D*) is 9.66 × 1012 cm Hz1/ 2 W −1 given by D * = R ( r0 A 4kT )

1/ 2

where R is the zero bias responsivity, the differential resistance (r0) at zero bias is 8×10

14

,

Ω, k is

Bolzmann’s constant, A is the detector active area, and T is the absolute temperature.40 The responsivity R can be expressed as R = Gqηext / hν , where q is the electron charge, ηext is the external quantum efficiency, hυ is the photon energy and G is the internal gain.14 With the reverse bias arising to -5 V, the measured responsivity (R) to UVC increases exponentially to 0.50 A/W, yielding a Gηext product of 2.67 at 230 nm, and the thermally limited detectivity is as high as D * = 2.30 × 1015 cm Hz1/ 2 W −1 , which implies a high internal gain and strong detection capability.

Meanwhile, an additional response cutoff edge is observed at 365 nm (3.4 eV), which is a bit higher than the optical bandgap of ZnO substrate (3.3 eV) shown in the transmittance spectrum. The broadening of bandgap is caused by the diffusion of gallium into underneath ZnO layer at the interface.41 By increasing the bias, the depletion region is expanded into ZnO by punch through of the electric field, leading to the efficient collection of carriers generated in the ZnO absorption region. Two distinct response bands at UVA and UVC demonstrated the capability for bias-tunable selective detections with wide UV spectral bandwidth. Figure 3a shows the dependence of dark current and photocurrent on the applied bias for the device with diameter of 400 µm measured under illumination of 254 nm and 365 nm UV lights at room temperature (298 K), respectively. A typical rectifying characteristic is shown with a dark current lower than 1 pA at low reverse bias. The resistance at forward and reverse bias is about 105 Ω 7 / 33



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and in the range of 1013-1015 Ω, respectively. It means that the current transport is dominated by the Au/Ga2O3 Schottky junction. At about -30 V, non-destructive breakdown feature is observed, which is repeatable for the tested device. Temperature-dependent I-V characteristics for the same device are also illustrated in Figure 3b. It is found that the feature of avalanche breakdown is softened at high temperatures due to the dramatically increased parasitic leakage current components. As described in Fig. S1 in the Supporting Information, at high temperature and high electric field, the bulk limited conduction mechanism, like Poole-Frenkel (PF) emission due to the presence of dislocations and deep donor like oxygen vacancies, makes the dominant contribution to the leakage current till the onset of breakdown point

42-43

. Nevertheless, the avalanche breakdown and PF emission exhibits

different slope in ln(I)-V curves. Therefore, we employed the differentiation of ln(I) on the applied bias ( d (ln( I )) dV ) to get the kink point, which is corresponding to the onset of breakdown, as indicated by blue arrows in Figure 3b. Figure 3c shows that the breakdown kinks shift from 29.5 to 34.0 V as the temperature increases from 298 K to 373 K, yielding a positive temperature coefficient of 0.065 V/K. It confirms that breakdown is dominated by avalanche multiplication mechanism rather not the Zener tunneling process.25 In the low forward bias region, the dark current follows exponential relationship. It is well fitted by thermionic-field emission mechanism and the Schottky barrier is estimated to be 1.1 eV, consistent with the previous results.44-45 A current kink is noticeable at a forward bias of 10 V, beyond which, the rapid arising of current occurs again. It is mainly attributed to the blocking effect of electron injections from ZnO into Ga2O3 due to the presence of an additional barrier induced by the conduction band offset (∆EC=0.35 eV) at α-Ga2O3/ZnO interface 46-48

, as shown in the energy band diagram calculated with Silvaco ATLAS program in Figure 3d. Under front illumination at λ = 254 nm (hυ = 4.88 eV) and λ = 365 nm (hυ = 3.40 eV), the front 8 / 33


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Ga2O3 and underneath ZnO serve as the absorbing layers, respectively. Consequently, carriers are excited at different regions and their transport paths are therefore different as well, resulting in the different photoresponse characteristics. Under 254 nm illumination, a remarkable photovoltaic effect is observed with the enhanced photocurrent at zero bias. By increasing reverse bias, the photocurrent initially increases exponentially and then the curve slope becomes gentle before breakdown point. An abrupt arising of photocurrent is observed once the bias approaching the breakdown voltage, which is a signature of avalanche multiplication. Meanwhile, a remarkable photocurrent component in positive direction is also obtained at forward bias, indicating high internal gain that is not expected in an ideal Schottky diode.49 The exact physical origin is related to unique energy band structure of isotype heterostructure and will be discussed later on. Next, under 365 nm illumination, photocurrent exhibits different dependence on the applied bias. At zero and low reverse bias below 10 V, there is no pronounced photocurrent component, while a sudden jumping of photocurrent with the magnitude change of almost 104 is observed above -10 V. The further bias increase does not lead to exponential change of photocurrent until 90% of breakdown voltage. Beyond the breakdown point, a steep increase of photocurrent is also observed like in an avalanche multiplication process. In the forward bias region, a large positive-direction photocurrent increases exponentially with bias up to 10 V, followed by an approximate linear increase above 10 V. To aid understanding the above-mentioned complicated carrier transport properties under different illumination conditions, the energy band diagrams under different forward and reverse biases have been calculated by Silvaco ATLAS program and illustrated in Figure 3d and e. For the simulation, the conduction band offset ( ∆EC ) and valance band offset ( ∆EV ) is taken as 0.35 eV and 1.18 eV, respectively, on the basis of the difference in the electron affinity and bandgaps of Ga2O3 (χ 9 / 33



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= 4.0 eV, Eg = 4.9 eV) and ZnO (χ = 4.35 eV, Eg = 3.37 eV) materials.

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46-48

The Schottky barrier

height of Au/Ga2O3 is set to be 1.2 eV, well consistent with the barrier height estimated from dark I-V curve. As shown in Figure 3e, owing to high dielectric constant and low electron background concentration of α-Ga2O3, most of Schottky potential drops on the fully depleted α-Ga2O3 layer at zero bias. As a result, the carriers generated in Ga2O3 layer under 254 nm illumination are driven to sweep out of the device, giving rise to self-powered photoresponse. However, under 365 nm illumination, due to very limited built-in field distributed in ZnO layer at zero bias, it is difficult to separate photo-excited electron-hole pairs confined at ZnO side near Ga2O3/ZnO interface. As high reverse bias, the depletion region is expanded into ZnO layer and thus the absorption of 365 nm light at ZnO side contributes the photocurrent. It is noticed that the quantum efficiency is much lower than that under 254 nm illumination. It is because the built-in electric field are almost confined near hetero-interface and most of remaining ZnO substrates are in the flat-band status. Photon-excited electrons are difficult to drift towards the bottom electrodes while photon-excited holes are confined at hetero-interface due to the presence of large blocking barrier for holes induced by the valence band offset of 1.18 eV. The further increasing of reverse bias leads to the reduction of hole barrier width and tunneling transport of holes becomes dominant, resulting in significant current increasing as shown in Figure 3a. As a result, in spite of majority carriers are electrons, the photocurrent under 365 nm illumination are mainly contributed by the hole injections from ZnO into Ga2O3 layer. In similarity, as indicated in Figure 3d, the applied forward bias is mainly dropped across the Ga2O3 layer and photon-excited carriers within Ga2O3 under 254 nm illumination are smoothly sweep out of the junction driven, while injection of electrons generated by 365 nm light at ZnO need to overcome the barrier of about 0.35 eV at Ga2O3/ZnO interface by thermionic emission or tunneling process. 10 / 33


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The transient photoresponse properties of the APD device has been measured under different biases and shown in Figure 4a. The rise time from the experimental data of the APD is less than 50 µs, while the decay curves are well fitted using a biexponential relationship 8, 24

I (t ) = I 0 + I1 exp ( −t τ 1 ) + I 2 exp ( −t τ 2 )

(1)

where I0 is the steady-state dark current contribution, I1 and I2 are photocurrent components, and τ1 and τ2 represent the fast and slow decay lifetimes, respectively. the best fitting yields

τ 1 = 563µs,τ 2 = 12.2ms; and τ 1 = 238µs,τ 2 = 3.04ms for the decay characteristics at -20 V and -34.8 V, respectively. The results confirmed that there are two gain channels in the photodetector. The fast decay is resulted from the avalanche impact ionization process and the slow one from the photoconductive gain.

8

Furthermore, the dependence of decay lifetimes on the applied biases is

summarized and present in Figure 4b. In comparison, at low bias of -20 V far from the avalanche breakdown, the component weight ratio is I1 / I 2 = 0.06 , indicating that the photoconductive gain induced by the reduced Schottky barrier (and/or P-F emission barrier) is dominant. With increasing bias, the slow-decay lifetime remains several ms level while the fast-decay component decreased dramatically. As the bias beyond the avalanche breakdown point, the fast-decay component becomes dominant with a component weight ratio of I1 / I 2 = 1.61 and a short lifetime of τ 1 = 238µs , indicative of dominant gain driven by avalanche multiplication. When taking the RC delay (~ 20 µs) of the circuit into account, the rise and decay lifetime would be shorter. For a better comparison, a comprehensive survey of Ga2O3 based solar blind photodetectors with critical parameters has been summarized in Table 1. Both photoresponsivity and response speed of the APD device developed herein is better as compared to many other Ga2O3 photodetectors based on film-type or low dimensional nanostructures. The device performance is also comparable to other reported 11 / 33



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Ga2O3-based APDs, which confirm the avalanche process in the device. 12, 18 Note that the forward bias characteristics in the studied device have been assigned as the results of avalanche multiplication of carriers in a reverse biased Ga2O3/SnO2 and Ga2O3/ZnO APDs, in which, Ga2O3 and ZnO (or SnO2) act as anode and cathode, respectively12, 18, exactly opposite to the denotation herein. Therefore, the exact physical origin of the huge enhanced photoresponse in such isotype heterostructures are still problematic. The dependence of photoresponsivity on the applied bias is shown in Figure S2 in the supporting information. Therefore, the total gains for the device under 254 nm illumination as a function of applied bias was extracted and presented as a black line in Figure 5a, given that the incident light power density is 0.5 mW/cm2 and assuming that external quantum efficiency to be unity. The photoconductive gain increases exponentially to 102 at -10 V, corresponding to a responsivity of 2.1 A/W. At 10 V, a huge gain of 104 is yielded and the corresponding responsivity is 1.05×103 A/W, which is comparable to that obtained in the reported Ga2O3/SnO2 and Ga2O3/ZnO APDs.12, 18 However, the reported breakdown voltages (~5 V) in these two isotype APDs almost equal to the Ga2O3 bandgap (Eg), which is much lower than the electron ionization threshold defined by EIe = Eg ( 2 + mh me ) (1 + mh me ) .50 Thus it is not expected that the accelerating fields built by such low bias can initiate impact ionization of electron-hole pairs. In this work, upon the reverse bias over 30 V, an accelerating field with a maximum strength over 2 MV/cm is built within Ga2O3 around the vicinity of Schottky contact as shown in Figure 5b, which is critical to facilitate the impact ionization of carriers and initiate the avalanche multiplication, thus resulting in the additional enhancement of gains in the magnitude of three orders. The resultant total gain of 2.35×105 is obtained at -40 V under 254 nm illumination and the corresponding responsivity is 1.10

×104 A/W, which is comparable the performance of reported Ga2O3 based APDs.12-29 12 / 33


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The photoresponse enhancement (and its bias dependence) phenomena described above tell out that the total gains are the product of two possible gain mechanisms, including (1) barrier lowering due to trapping of minority carrier at low bias region15, 51 and (2) carrier multiplication through avalanche processes at high reverse bias region1,

25, 52

. Indeed, the enhanced photoresponse

characteristics at low reverse and/or forward bias regions have been widely observed in various Schottky photodetectors based on wide bandgap semiconductor materials, such as GaN49, 53, ZnO54 and Ga2O315. To reveal the transport mechanism of non-equilibrium photo-excited carriers under low biases, the schematic energy band diagram under reverse and forward biases are illustrated in Figure 6a and 6b respectively. Note that an absorption coefficient of Ga2O3 is about 105 cm-1 for hv > 5 eV13, 55-56

, and thus, most of excess carriers are generated near the topmost Ga2O3 surface. The excess

electrons are rapidly swept out of the junction, leaving behind excess holes localized at metal-induced sub-gap defective states at the metal-semiconductor interface, as explained in most of high gain Schottky photodiode.49, 51, 57 Recently, A. Armstrong et al. made a strong argument that the photo-excited holes are self-trapped in the depletion region near Schottky interface.51 Regardless of the different trapping manners, the remaining holes near interface produce an excess positive space-charge in the junction, giving rise to the photo-capacitance and the reduction of Schottky barrier ( ∆φb ) as shown in Figure 6a.15 Thus, the responsivity under illumination can be alternatively described as53, 58

  ∆φb   exp  kT  − 1 I dark − I λ    R= P

(2)

where P is the light intensity, kT is the thermal energy, and Iλ is the primary photocurrent at zero bias. There are two parts contributing to the photoresponse. The first part is Iλ, which does not depend on 13 / 33



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the gain mechanism, which is maintained by carriers optically generated in the depletion region and flows in the negative direction. The second part is originated from the lowered barrier by hole trapping effect.53 Thus, the reduction of Schottky barrier leads to the thermionic component of the reverse current exponentially increasing as exp ( ∆φb kT ) and produces high photoconductive gain. As compared to the reverse bias case, the forward bias leads to much larger gain, which cannot be fully explained by the reduction of Schottky barrier by hole trapping mechanism. R. Suzuki et al. has reported similar huge photoconductive gain under forward bias for the Au/Ga2O3 Schottky diode with a high resistive capping layer.15 In our device, due to the cation exchanges or inter-diffusion during high temperature growth, the Zn-doped Ga2O3 interfacial layer would be insulating while ZnO layer near interface is more conductive due to Ga doping.41 Thus, the studied Ga2O3/ZnO isotype heterostructure can be regarded as an n--n+ heterojunction and the real electron barrier is larger than the calculated conduction band offset (0.35 eV) due to downward/upward band bending at ZnO/Ga2O3 sides. Upon 254 nm illumination, the resistivity of Ga2O3 layer decreases, and considerable forward bias is consequently applied to the interfacial n--n+ junction, leading to a reduction of barrier ( ∆φb′ ) at ZnO/Ga2O3 and facilitating electron injection from ZnO into Ga2O3 [Figure 6b]. The gain mechanism described by Equation (2) is also applicable to interpret the huge enhancement of photoresponsivity under 365 nm illumination at forward bias. Next, we turn to the carrier transport and avalanche photo-multiplication mechanisms above breakdown voltages. We designate the unmultiplied photocurrent at -10 V as the unity gain reference. The multiplication gains (Mph) are determined from the illuminated and dark current in Figure 3a by the relationship of Mph = (Iph-Id) / (Iph0-Id0),25 where Iph and Id are the multiplied photocurrent and dark current, while Iph0 and Id0 are the unmultiplied photocurrent and dark current (Mph =1) at -10 V, 14 / 33


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respectively. Figure 5a shows the dependence of the avalanche gains on the reverse bias. It is found that the carrier multiplication launches above the threshold voltage of -30 V and the onset avalanche field is about 2 MV/cm as shown in Figure 5b. At -40 V, the edge field strength is about 3 MV/cm and the reproducible avalanche gains of 5224 and 1404 are yielded under 254 nm and 365 nm illumination, respectively. It implies that, with the marriage of high unmultiplied gains at low reverse biases, the studied Ga2O3/ZnO isotype heterostructure Schottky diode demonstrates an unparalleled ability in detecting ultra-weak UV signals with high gain-bandwidth product. Due to the differences in excitation locations and transport paths of excess carriers under 254 nm and 365 nm light irradiation, it permits the electrons and holes to be selectively injected into the active region to initiate the avalanche process in the same device. As illustrated in Figure 6c, excess holes generated by 254 nm illumination near Ga2O3 surface are rapidly sweep out almost without acceleration while excess electrons are driven towards ZnO side by high accelerating field in the Ga2O3 layer. Thus, it can be treated as the nearly pure electron injection to launching the impact ionization process, and the avalanche gains under 254 nm illumination is the electron multiplication factor, Mn.5 On the other hand, the 365 nm illumination generates excess carriers at ZnO side, which consequently changed the interfacial layer from the deep depletion into the inversion status as indicated by the quasi-Fermi levels in Figure 6d. Due to most of bias dropped on Ga2O3 layer, the strength and width of electric field for the acceleration of electrons in ZnO layer are limited. In contrast, the accumulated holes at interface are injected into Ga2O3 accelerating region by tunneling or thermionic field emission at high reverse bias. Thus, the yielded gain is contributed to hole-initiated avalanche process by the purely hole injection, denoted as Mp.5 It is worth noting that for UVA-UVB spectral region, the Au/α-Ga2O3 /n-ZnO isotype heterostructure is working like a 15 / 33



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Page 16 of 33

separate absorption and multiplication (SAM) APD, in which, low dark current can be accomplished without sacrificing the quantum efficiency.59 By comparing the multiplication-voltage characteristics induced by nearly pure electron and hole injections, the ionization coefficients of electrons (α) and holes (β) can be separately estimated by the simplified relationship of α =

and β =

1 W

1 W

 M n −1   M n     ln   M n − M p   M p 

 M p − 1   M p  60 5 -1   ln   . It is found that the ionization coefficient α is about 10 cm , one  M p − M n   M n 

order higher than β (~104 cm-1) at the electric field of 3 MV/cm. The low ratio of hole and electron ionization coefficients k ≈ 0.1 holds the promise for high speed and low noise avalanche photodetection in the ultraviolet spectral range.

4. Conclusions In summary, single crystalline α-Ga2O3 epilayers were achieved on a-plane ZnO substrates by laser MBE technique for the first time and a Schottky type solar-blind avalanche photodetector has been constructed based on α-Ga2O3/ZnO isotype n--n+ heterostructures. The device has low dark current below 1 pA and exhibits efficient self-powered characteristics with sharp cutoff at 255 nm and UV/visible rejection ratio over 103. In addition, the device has novel functionality of bias tunable response at solar blind (255 nm) and visible blind (365 nm) UV regions. The photoresponse enhancement (and its bias dependence) phenomena have been investigated in terms of barrier lowering effect and carrier avalanche multiplication mechanisms. Especially, by changing illumination condition, nearly pure electrons and holes injection can be realized to initiate the avalanche process in the same device and a low ratio of hole and electron ionization coefficient is achieved for the first time in α-Ga2O3. The marriage of avalanche multiplication and enhanced 16 / 33


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photoresponse results in a reproducible gain over 105 and a high responsivity of 1.10×104 A/W. The results herein provide a facile route to deliver low-cost photodetector based on α-Ga2O3/ZnO heterostructure with high performance and novel functionality.

Acknowledgments This research was supported by the National Key Research and Development Project of China (No. 2017YFB0403003) and the Natural Science Foundation of Jiangsu Province (Nos. BK20130013, and BK20161401), the Six Talent Peaks Project in Jiangsu Province (2014XXRJ001), the Priority Academic Program Development of Jiangsu Higher Education Institutions.

Supporting Information Determination of avalanche breakdown voltages, dependence of photoresponsivity on the applied biases, the transient response properties, determination of effective illuminated area in opaque Schottky photodiode provided.

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Temperature-Dependent Capacitance-Voltage and Current-Voltage Characteristics of Pt/Ga2O3 (001) Schottky Barrier Diodes Fabricated on n-Ga2O3 Drift Layers Grown by Halide Vapor Phase Epitaxy. Appl. Phys. Lett. 2016, 108 (13), 133503. 45. He, Q.; Mu, W.; Dong, H.; Long, S.; Jia, Z.; Lv, H.; Liu, Q.; Tang, M.; Tao, X.; Liu, M., Schottky

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Phys., Part 1 1965, 4 (7), 473-484.

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10

20

30

40

50

60

α-Ga2O3 (300)

(b)

ZnO (11-20)

(a)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

70

80

2θ (deg)

_

_

Figure 1 a) 2θ-ω x-ray diffraction pattern of the as grown α-phase Ga2O3 (3030) on ZnO (1120) substrates. The inset shows the schematic of device structure. b) The atomic force microscopic image of the as grown α-phase Ga2O3.

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Page 27 of 33

Photon Energy (eV) 6

R (A/W)

10

5

4

3

0

10-1

QE=100%

10-2

QE=10%

10-3 10

-4

Bias

10-5

T(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0V -1V -5V

100 50 0 200

TGa2O3

TZnO

300

400

500

Wavelength (nm)

Figure 2. The spectral photoresponse of a 400 µm-diameter Schottky diode under different biases and the transmittance spectra of Ga2O3 epilayer and ZnO substrate.

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-3

25 oC

10-5

10-8 10-10 10-12 10-14 -40

10

Current (A)

10-6 Current (A)

(b)

dark 254 nm 365 nm

10-4

10

avalanche regime

10

-10

0

-13

-30 -15

10

(d) Electron Energy (eV)

33 32 31 30 29 320

340

360

Temp. (K)

PF emission

PF emission

0 -30 -15

0 -30 -15

0 -30 -15

0

Rverse Bias (V)

34

300

PF emission

-11

Bias (V)

(c)

avalanche regime

-7

PF emission

-20

100 oC

avalanche regime

avalanche regime

10-9 10

-30

75 oC

50 oC

380

14 12 10 8 6 4 2 0 -2 -4

ZnO

Ga2O3 0V 5V

1V 10 V

Forward Bias

Ga2O3

(e) Electron Energy (eV)

(a)

BV (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ZnO

0V -5V

3 0 -3 -6 -9

Reverse Bias

-12

0.0

0.2

0.4

0.6

0.8

Depth Profile (µm)

-1V -10V

-15 0.0

0.3

0.6

0.9

Depth Profile (µm)

Figure 3. a) The I-V characteristics of the photodetector in dark and under illumination with 254 nm and 365 nm light, respectively; b) temperature-dependent dark current characteristics in the reverse bias; c) the avalanche breakdown voltage as a function of measurement temperatures; d) and e) the energy band diagrams under forward and reverse bias calculated by Sivaco ATLAS program.

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Page 29 of 33

(b)

1.2

exp. at -20 V exp. at -34.8 V fitting data

1.0

700

τ1 τ2

600

20

15 τ1 = 563 µs

500

I1/I2=0.06

0.6 τ2 = 12.2 ms

0.4

200

0.0 0

5

10

10

5 τ2 = 3.04 ms

I1/I2=1.61

400 300

τ1 = 238 µs

0.2

τ2 (ms)

0.8

τ1 (µs)

(a) Normalized intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15

20

100 0 18 20 22 24 26 28 30 32 34 36

25

Time (ms)

Reverse bias (V)

Figure 4. a) the normalized transient photoresponse characteristics measured under different reverse biases at room temperature and b) The fitted decay lifetimes as a function of applied biases.

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106 105

Total Gain @ 254 nm Gain results from Fig.2

104

Mn @ 254 nm Mp @ 365 nm

6000

4000

103 102 101 10

2000

0

10-1 10-2

(b)

Multiplication Gains (M n, Mp)

(a)

Total Internal Gain

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 33

0 -40

-30

-20

-10

0

10

Voltage (V)

Figure 5. a) The photoconductive gain for the device under 254 nm illumination as a function of applied bias and the avalanche multiplication gains for 254 nm and 365 nm light irradiation, respectively; b) the spatial distribution of built-in fields under reverse bias of 30 V calculated by Sivaco ATLAS program.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. a) and b) the schematic energy band diagram under 254 nm illumination at low reverse and forward biases, respectively; c) and d) the schematic energy diagram at high reverse bias under 254 nm and 365 nm illumination, respectively.

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Page 32 of 33

Table 1. Comparison of the main parameters for the reported Ga2O3 photodetectors. Material

Structure

Responsivity [A W-1]

Decay time - τd [µs] τ1

τ2

Reference

ZnO/α-Ga2O3

Film-based APD

1.10×104

238

3.04×103

This work

β-Ga2O3/SnO2

Film-based APD

6.3×103

48

102

12

β-Ga2O3/ZnO

Film-based vertical

0.35

> 105

> 106

18

β-Ga2O3/SiC

Film-based vertical

0.18

> 106

> 106

21

β-Ga2O3

Film-based MSM

0.903

< 3×106

22

β-Ga2O3/NSTO

Film-based vertical

43.31

7×104

23

amorphous-Ga2O3

Film-based MSM

0.19

Au/β-Ga2O3

Nanowire array

6×10-4

ZnO/β-Ga2O3

Core-shell heterojunction APD

1.3×103

ZnO/β-Ga2O3

Core-shell heterojunction

9.7×10-3

900

26

β-Ga2O3

Nanobelts

851

< 3×105

28

β-Ga2O3

quasi-two-dimensional flake

1.8×105

> 106

29

19.10

80.70

27

64 42

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24

815

25

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Table of Contents (TOC) graphic

10-1

-2

10

10-3 200 250 300 350 400 450 500

Mn @ 254 nm Mp @ 365 nm

104 10

2000

10-2

0 -40

-30

-20 -10 Voltage (V)

0.6 0.4 0.2 0.0 0

5

10

15

20

4000

100

bias= -34.8 V; Pulsed laser λ=248 nm

0.8

6000

2

Wavelength (nm) 1.0

Total Gain @ 254 nm

QE=100%

Total Internal Gain

R (A/W)

106

Bias -5V

25

Time (ms)

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0

10

Avalanche Gains

0

10

I Normalized (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Solar-Blind Photodetector with High Avalanche ... - ACS Publications

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