p-Si heterojunction solar-blind ultraviolet photodetector with enhanced photoelectric responsivity

Journal of Alloys and Compounds 660 (2016) 136e140

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

b-Ga2O3/p-Si heterojunction solar-blind ultraviolet photodetector with enhanced photoelectric responsivity X.C. Guo a, b, N.H. Hao a, D.Y. Guo a, b, Z.P. Wu a, b, **, Y.H. An a, b, X.L. Chu a, b, L.H. Li c, P.G. Li a, b, M. Lei a, b, W.H. Tang a, b, * a b c

Laboratory of Optoelectronics Materials and Devices, School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, China State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing 100876, China Department of Physics, The State University of New York at Potsdam, Potsdam, NY 13676-2294, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 October 2015 Received in revised form 19 November 2015 Accepted 20 November 2015 Available online 2 December 2015

In this work, ð201Þ oriented b-Ga2O3 thin films have been grown on p-type silicon (100) substrates by laser molecular beam epitaxy. b-Ga2O3/Si pen heterojunctions are formed as a deep ultraviolet (UV) solar-blind photodetector. Those heterojunctions exhibit obvious rectifying characteristics and excellent solar-blind UV photoresponse. The responsivity reaches 370 A/W at 3 V reverse bias under 254 nm UV irradiation. The corresponding external quantum efficiency is over 1.8  105%. The combination of wide bandgap semiconductor with silicon might open up possibilities for future generation deep UV solarblind optoelectronic devices. © 2015 Elsevier B.V. All rights reserved.

Keywords: Solar-blind Ultraviolet photodetector Ga2O3 Heterojunction

1. Introduction Due to the strong deep ultraviolet (UV) absorption by stratospheric ozone, the solar irradiation between 200 nm and 280 nm cannot reach the surface of the earth. Since there are almost no interference sources, solar-blind UV detection makes it superior advantages with lower false alarm rate, and all-weather work environment compared with infrared detection [1,2]. In fact, solarblind photodetectors have a vast and ever growing number of military and civil surveillance applications such as missile tracking, secure communication, fire detection, ozone holes monitoring, chemical/biological analysis, and corona detection, etc. [3,4]. Unfortunately, the traditional solar-blind photodetectors, for example photomultiplier tubes, are usually bulky and fragile, which limiting their practical applications [5]. Recently, solar-blind solid

* Corresponding author. Laboratory of Optoelectronics Materials and Devices, School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, China. ** Corresponding author. Laboratory of Optoelectronics Materials and Devices, School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, China. E-mail addresses: [email protected] (Z.P. Wu), [email protected] (W.H. Tang). http://dx.doi.org/10.1016/j.jallcom.2015.11.145 0925-8388/© 2015 Elsevier B.V. All rights reserved.

photodetectors based on wide bandgap semiconductors such as AlGaN, ZnMgO, diamond and b-Ga2O3 have attracted intensive attentions [4,6e10]. However, high quality epitaxial AlGaN film is difficult to be prepared due to the high growth temperature, single wurtzite phase ZnMgO and diamond are not possible to be used to detect entire deep ultraviolet region due to their inappropriate bandgap. Therefore, b-Ga2O3, with ~4.9 eV direct bandgap is considered as one of ideal candidates to fabricate solar-blind photodetector [4,7,11]. To date, many techniques have been employed to prepare the b-Ga2O3 thin films on various substrates [12e16]. High epitaxial b-Ga2O3 thin films based metal-semiconductoremetal photoconductor have been fabricated via pulsed laser deposition (PLD) by our group [17,18]. Compared to photoconductor detector, pen junction detector based on photodiode are expected to exhibit extraordinary behavior such as large photoelectric responsivity and quick response speed. However, b-Ga2O3 behaves as an n-type semiconductor due to the presence of a donor band related to intrinsic oxygen deficiency [19,20]. One possible solution is to fabricate heterojunction with another p-type material. Considering the desire of combining multifunctional devices with commercial silicon technologies, we deposited n-type b-Ga2O3 thin films on p-type Si(100) substrates to fabricate vertically structured pen junction. The responsivity of the formed pen junction is over 370 A/W at 3 V reverse bias under 254 nm UV irradiation.

X.C. Guo et al. / Journal of Alloys and Compounds 660 (2016) 136e140

137

2. Experimental

3. Results and discussion

b-Ga2O3 thin films with thickness ~250 nm were deposited directly on (100) p-type silicon substrates by the PLD technique. Si(100) substrates were cleaned sequentially by alcohol, acetone, and deionized water. Before depositing b-Ga2O3 films, the p-Si(100) substrates were dipped into a HF solution (~4%) for 60 s to remove the amorphous SiO2 layer. Subsequently, the Si(100) substrates were immediately transferred into the chamber. The initial two atomic layers of b-Ga2O3 film were deposited under the base pressure of 1  106 Pa at 200  C to prevent the formation of the SiO2 interface layer. Then the substrate temperature was raised to 750  C. The laser ablation was carried out at a laser fluence of 2 J/ cm2 with a repetition rate of 1 Hz using a KrF excimer laser with a wavelength of 248 nm. The distance between target and substrate was 5 cm. The substrates were rotated during the deposition to improve the film uniformity. Reflection high-energy electron diffraction (RHEED) was used to monitor the growth process. After that the deposition, Ti/Au and Au electrodes were deposited on the b-Ga2O3 thin film and the back of p-Si(100) substrates using a shadow mask by radio frequency magnetron sputtering to form omhic contact respectively. The schematic illustration of the device structure is shown in the Fig. 1 (a). The film structure was determined by a Bruker D8 Advance X-ray diffractometer (XRD) using Cu Ka (l ¼ 1.5405 Å) radiation. The currentevoltage (IeV) and timedependent photoresponse of the b-Ga2O3/p-Si photodetector were measured by Keithely 2450.

Fig. 1 (b) shows the typical XRD patterns from b-Ga2O3 thin films on p-Si(100) substrates and the bare Si(100) substrate. It is found that only ð201Þ and its higher order diffraction peaks of b-Ga2O3 were observed apart from the diffraction peaks from Si(100) substrate, indicating that the b-Ga2O3 thin films are grown with the ð201Þ plane parallel to the Si(100) substrate. The inset of Fig. 1(b) is the in situ RHEED pattern of the Ga2O3 thin film. Conspicuous streak patterns imply that the Ga2O3 film has a good crystallized structure and smooth surface. To check the solar blind UV responsivity of the b-Ga2O3/p-Si photodetector, we measure its currentevoltage (IeV) in the dark and under irradiations of 254 nm and 365 nm, respectively. The UV lights with power densities of 12 mW/cm2 and 17 mW/cm2 for 254 nm and 365 nm are vertically irradiated on the sample. And the effective irradiated area was ~0.18 cm2. Fig. 2(a) shows the IeV curves with logelinear scale. The b-Ga2O3/p-Si photodetector presents typical rectifying characteristics. The asymmetric ratio, I(3V)/ I(3V), is over 30 with no irradiation. Under 254 nm irradiation, the current increased from 8.5  107 A to 8.0  104 A at 3 V reverse bias (p-Si connect the cathode), corresponding the I254/Idark ratio of 9.4  102. In contrast, the current is only 5.3  106 A under 365 nm irradiation, and the I365/Idark ratio is 6.2. These data indicate that the b-Ga2O3/p-Si photodetector is much more sensitive for 254 nm

(a)

dark

10-3

365nm

Current(A)

10-4

254nm

10-5 10-6 10-7 10-8 10-9 10-10 10-11

-3

-2

-1

0

1

2

3

2

3

Voltage(V)

(b) 103

365nm 254nm EXP Growth Fitting

Responsivity(A/W)

102 101 100 10-1 10-2 10-3 10-4 10-5 -3

-2

-1

0

1

Voltage(V) Fig. 1. (a) The schematic diagram of the fabricated b-Ga2O3/p-Si heterojunction structure, (b) a typical XRD pattern of b-Ga2O3 film on Si(100) substrate, and the inset is the RHEED patterns for as-grown b-Ga2O3 film.

Fig. 2. (a) The currentevoltage (IeV) characteristics of the b-Ga2O3/p-Si junction in dark, under 254 nm and 365 nm illumination, (b) the responsivity and fitting of the bGa2O3/p-Si junction at different bias for 254 nm and 365 nm illumination.

138

X.C. Guo et al. / Journal of Alloys and Compounds 660 (2016) 136e140

than for 365 nm. Fig. 2(b) shows the responsivity of the photodetector at different bias, calculated by:

Rl ¼ ðIil  Id Þ=ðPl $SÞ

(1)

EQE ¼ ðhcRl =elÞ  100%

(2)

where Rl is the reponsivity for the wavelength of l irradiation, EQE corresponding external quantum efficiency, Iil is the light current, Id is the dark current, P is the irradiation power density, S is the effective area of the photodetector, respectively. R254 is 370 A/W at 3 V reverse bias, and the corresponding EQE is 1.8  105%. Table 1 shows the comparison of the characteristics parameters of the photodetectors based on b-Ga2O3 material. The Ga2O3 single crystal photodetector has the best performance with the responsivity of 1000 A/W, EQE of 5  105% and I254/Idark ratio of 104 [24], followed by our work with the R254 of 370 A/W, EQE of 1.8  105% and I254/Idark of 9.2  102. On the other hand, the photodetectors based on Ga2O3 reported by other groups have much lower responsivities and EQE value. There are three mainly mechanisms which may contribute to the high responsivity and quantum efficiency of a photodetector: photoconductive gain, Zener tunneling, and avalanche multiplication. As for Zener tunneling, it usually occurs in highly doped semiconductors under small bias [25], while in our case, the bGa2O3 film has not been doped at all, so the high Rl and EQE caused by Zener tunneling may not exist in the b-Ga2O3/p-Si structure. With reference to the photoconductive gain, the responsivity tends to saturate at elevated bias [26]. However, seen from the Fig. 2(b), the fitting curves show that the responsivity increases exponentially with increasing the applied voltage to 1.25 V, indicating that the photoconductive gain cannot be the dominant factor in our case. Large gain of EQE could result from the carrier multiplication in the b-Ga2O3 layer. To further analyze, according to Ref. [27], the multiplication factor M for holes depends on the depletion region thickness Land the ionization coefficient h:

M ¼ expðhLÞ

(3)

hfF

(4)

The ionization coefficient h increases monotonically with the field F, which can be attributed to the energy obtained from higher energy, thus increase the probability of multiplication for carriers. Photogenerated holes contribute to multiplication mainly through impact ionization. Thus, the responsivity of the b-Ga2O3/p-Si junction has an exponential relationship with the applied voltage. The plot of the differential of the responsivity vs voltage shows in Fig. 3(a). We can see that the differential of the responsivity is a constant until the voltage increased to 1.25 V, then increases sharply when the voltage higher than 1.25 V. That is well consistent with the carrier multiplication. The multiplication process can be visualized as Fig. 3 (b). The carrier concentration of b-Ga2O3 film is much lower than that of p-Si, thus an abrupt pen junction is

Fig. 3. (a) The plot of the differential of the responsivity vs voltage, (b) The simplified diagrams of carrier multiplication of the b-Ga2O3/p-Si heterojunction.

formed. When a positive voltage was applied on b-Ga2O3, the depletion region which mostly falls in the b-Ga2O3 layer is broadened, and almost the whole voltage is applied on b-Ga2O3 due to the larger resistance of b-Ga2O3. Under 254 nm UV illuminations, the photo-generated carriers accelerated by the large electric field and get a high energy to multiplication, so the light current shows an exponential growth with the applied voltage. The energy band diagrams of the b-Ga2O3/p-Si pen junction were interpreted using Anderson model (as shown in Fig. 4). The values of band offsets are estimated using the electron affinity 4.05 eV [28], 4.0 eV [29], and band gap 1.12 eV, 4.9 eV [17,18,20] for p-Si, and b-Ga2O3, respectively. Under 365 nm illumination, it is not able to excite the carriers of b-Ga2O3, but may be absorbed by the pSi and the few defects of b-Ga2O3. On the other hand, the resistance of b-Ga2O3 is much higher than that of p-Si, which means b-Ga2O3 layer dominates performance of the device. However, for 254 nm, it is able to excite the carriers of b-Ga2O3, the resistance of b-Ga2O3 is reduced significantly, and when the negative voltage is applied on b-Ga2O3 (Fig. 4 (c)), the depletion layer is narrowed, and the holes cannot be transported to the p-Si. When the positive voltage is applied on b-Ga2O3 (Fig. 4 (d)), the holes can be transported to the p-Si easily and the multiplication occurs, thus, high responsivity for 254 nm can be gained. Fig. 5(a) shows the time-dependent photoresponse of the

Table 1 Comparison of the characteristic parameters of the photodetectors based on b-Ga2O3. Photodetectors

Reponsivity (A/W)

EQE (%)

I254/Idark (3 V)

Response time

Ref.

Ga2O3 film MSM Ga2O3 nanosheets Ga2O3 nanowire Ga2O3 Schottky photodetectors Ga2O3 single crystal Ga2O3/SiC heterojunction Ga2O3/p-Si heterojunction

0.037 3.3 3.43  103 2.6e8.7 1000 0.068 370

18 1600 17 4350 5  105 34 1.8  105

103 <103 e e 104 103 9.4  102

e 2s 4s e e 1.2 ms 1.79 s

[21] [22] [23] [7] [24] [20] This work

X.C. Guo et al. / Journal of Alloys and Compounds 660 (2016) 136e140

139

Fig. 5. (a)Time-dependent photoresponse of the junction under 254 nm illumination, (b) enlarged view of the rise/decay edges and the corresponding exponential fitting for 254 nm illumination.

with a fast response component and a slow-response component [17,18]. For a more detailed comparison of the response time of the b-Ga2O3/p-Si junction, the quantitative analysis of the process of the current rise and decay process involves the fitting of the photoresponse curve with a bi-exponential relaxation equation of the following type [17,18]. t= t1

I ¼ I0 þ Ae

Fig. 4. The energy-band diagrams of b-Ga2O3/p-Si heterojunction. (a) Before contact (b) After contact (c) Under forward bias and light illumination, (d) Under reverse bias and light illumination.

detector to 254 nm illumination by on/off switching. At are verse bias of 3 V, the photocurrent increases instantaneously to approximately 1.01  103 A when illumination is on, and reduces to 7.5  105 A when illumination is off. The response (rising) edges and the recovery (fall) edges usually consist of two components

þ Be

t= t2

(5)

where I0 is the steady state photocurrent, t is the time, A and B are constant, t1 andt2 are two relaxation time constants. As shown in Fig. 5(b), the photoresponse processes are well fitted. t1 and t2 are the time constants for the rising edge and fall edge, respectively. We note that the decay time constants tr1 and tr2 are estimated to be 1.79 s and 4.54 s, td1 and td2 are 0.27 s and 2.72 s. Generally, the fast-response component can be attributed to the rapid change of the carrier concentration as soon as the light is turned on or off, while the slow-response component is caused by the carrier trapping/releasing owing to the existence of oxygen vacancies defects in b-Ga2O3 thin films and the capacitance junction. In this case, tr1 (1.79 s) caused by the rapid addition of the carrier concentration as soon as the light is turned on and the ResistanceeCapacitance (RC) constant of junction, tr2 caused by the low pressure mercury lamp brightness enhancement process, td1 caused by the rapid reduction of the carrier concentration, td2 caused by the RC constant of

140

X.C. Guo et al. / Journal of Alloys and Compounds 660 (2016) 136e140

junction. Compared the photoresponse time with other b-Ga2O3 based photodetectors (Table 1), the Ga2O3/Si heterojunction photodetector is faster than the photoconductors, but is slower than the Ga2O3/SiC heterojunction photodetector, indicating that pen junction based photodetector exhibit prominent performance than photoconductors. It should be noted that the performance of the photodetector will be optimized if we minimize the effect of the junction capacitor and optimize the fabrication parameters of the b-Ga2O3 thin film photodetectors. 4. Conclusions In conclusion, we fabricated high-oriented b-Ga2O3 films on ptype silicon (100) substrates for b-Ga2O3/p-Si pen junction solarblind photodetectors by PLD. The R254 is 370 A/W, EQE 1.8  105% and I254/Idark 9.2  102 under 254 nm UV irradiations, which are comparable with those from single crystal photodetector. The highresponsivity could be attributed to the carrier multiplication. Acknowledgments This work was supported by the National Natural Science Foundation of China (51572033, 11404029, 51172208, 61274017), Beijing Natural Science Foundation (2154055), Fund of State Key Laboratory of Information Photonics and Optical Communications (BUPT), the Fundamental Research Funds for the Central Universities (Grant No. 2014RC0906), China Postdoctoral Science Foundation Funded Project (Grant No. 2014M550661), and National Basic Research Program of China (973 Program) (2010CB923202). References ^co, E. Fortunato, R. Martins, New UV-enhanced solar blind op[1] A. Malik, A. Se tical sensors based on monocrystalline zinc sulphide, Sens. Actuators A 67 (1998) 68e71. [2] L. Li, E. Auer, M. Liao, X. Fang, T. Zhai, U.K. Gautam, A. Lugstein, Y. Koide, Y. Bando, D. Golberg, Deep-ultraviolet solar-blind photoconductivity of individual gallium oxide nanobelts, Nanoscale 3 (2011) 1120e1126. [3] P. Feng, J. Zhang, Q. Li, T. Wang, Individual b-Ga2O3 nanowires as solar-blind photodetectors, Appl. Phys. Lett. 88 (2006) 153107. [4] R. Suzuki, S. Nakagomi, Y. Kokubun, Solar-blind photodiodes composed of a Au Schottky contact and a b-Ga2O3 single crystal with a high resistivity cap layer, Appl. Phys. Lett. 98 (2011) 1114. [5] Y. Li, T. Tokizono, M. Liao, M. Zhong, Y. Koide, I. Yamada, J.J. Delaunay, Efficient assembly of bridged b-Ga2O3 nanowires for solar-blind photodetection, Adv. Funct. Mater. 20 (2010) 3972e3978. [6] Z.-D. Huang, W.Y. Weng, S.J. Chang, C.-J. Chiu, T.-J. Hsueh, S.-L. Wu, AlGaN/GaN heterostructure ultraviolet three-band photodetector, IEEE Sens. J. 13 (2013) 3462e3467. [7] T. Oshima, T. Okuno, N. Arai, N. Suzuki, S. Ohira, S. Fujita, Vertical solar-blind deep-ultraviolet Schottky photodetectors based on b-Ga2O3 substrates, Appl.

Phys. Express 1 (2008) 011202. [8] M. Razeghi, Short-wavelength solar-blind detectors-status, prospects, and markets, Proc. IEEE 90 (2002) 1006e1014. [9] X. Du, Z. Mei, Z. Liu, Y. Guo, T. Zhang, Y. Hou, Z. Zhang, Q. Xue, A.Y. Kuznetsov, Controlled growth of high-quality ZnO-based films and fabrication of visibleblind and solar-blind ultra-violet detectors, Adv. Mater. 21 (2009) 4625e4630. [10] W. Weng, T. Hsueh, S. Chang, G. Huang, H. Hsueh, A b-Ga2O3/GaN Schottkybarrier photodetector, IEEE Photonics Technol. Lett. 23 (2011) 444e446. [11] Y. Kokubun, K. Miura, F. Endo, S. Nakagomi, Sol-gel prepared b-Ga2O3 thin films for ultraviolet photodetectors, Appl. Phys. Lett. 90 (2007) 1912. [12] G. Sinha, K. Adhikary, S. Chaudhuri, Sol-gel derived phase pure a-Ga2O3 nanocrystalline thin film and its optical properties, J. Cryst. Growth 276 (2005) 204e207. [13] Z. Ji, J. Du, J. Fan, W. Wang, Gallium oxide films for filter and solar-blind UV detector, Opt. Mater. 28 (2006) 415e417. [14] M. Fleischer, W. Hanrieder, H. Meixner, Stability of semiconducting gallium oxide thin films, Thin Solid Films 190 (1990) 93e102. [15] Y. Lv, J. Ma, W. Mi, C. Luan, Z. Zhu, H. Xiao, Characterization of b-Ga2O3 thin films on sapphire (0001) using metal-organic chemical vapor deposition technique, Vacuum 86 (2012) 1850e1854. [16] T. Oshima, T. Okuno, S. Fujita, Ga2O3 thin film growth on c-plane sapphire substrates by molecular beam epitaxy for deep-ultraviolet photodetectors, Jpn. J. Appl. Phys. 46 (2007) 7217. [17] D. Guo, Z. Wu, Y. An, X. Guo, X. Chu, C. Sun, L. Li, P. Li, W. Tang, Oxygen vacancy tuned Ohmic-Schottky conversion for enhanced performance in bGa2O3 solar-blind ultraviolet photodetectors, Appl. Phys. Lett. 105 (2014) 023507. [18] D. Guo, Z. Wu, P. Li, Y. An, H. Liu, X. Guo, H. Yan, G. Wang, C. Sun, L. Li, Fabrication of b-Ga2O3 thin films and solar-blind photodetectors by laser MBE technology, Opt. Mater. Express 4 (2014) 1067e1076. [19] J. Varley, J. Weber, A. Janotti, C. Van de Walle, Oxygen vacancies and donor impurities in b-Ga2O3, Appl. Phys. Lett. 97 (2010) 142106. [20] S. Nakagomi, T. Momo, S. Takahashi, Y. Kokubun, Deep ultraviolet photodiodes based on b-Ga2O3/SiC heterojunction, Appl. Phys. Lett. 103 (2013) 072105. [21] T. Oshima, T. Okuno, S. Fujita, Ga2O3 thin film growth on c-plane sapphire substrates by molecular beam epitaxy for deep-ultraviolet photodetectors, Jpn. J. Appl. Phys. 46 (2007) 7217. [22] W. Feng, X. Wang, J. Zhang, L. Wang, W. Zheng, P. Hu, W. Cao, B. Yang, Synthesis of two-dimensional b-Ga2O3 nanosheets for high-performance solar blind photodetectors, J. Mater. Chem. C 2 (2014) 3254e3259. [23] Y.L. Wu, S.-J. Chang, W. Weng, C.H. Liu, T.Y. Tsai, C.L. Hsu, K. Chen, Nanowire photodetector prepared on template, IEEE Sens. J. 13 (2013) 2368e2373. [24] R. Suzuki, S. Nakagomi, Y. Kokubun, N. Arai, S. Ohira, Enhancement of responsivity in solar-blind b-Ga2O3 photodiodes with a Au Schottky contact fabricated on single crystal substrates by annealing, Appl. Phys. Lett. 94 (2009), 222102e222101. [25] L. Esaki, New phenomenon in narrow germanium p-n junctions, Phys. Rev. 109 (1958) 603. [26] S.M. Sze, Physics of Semiconductor Devices, second ed., John Wiley & Sons, New York (, 1981. [27] S. Kasap, J. Rowlands, S. Baranovskii, K. Tanioka, Lucky drift impact ionization in amorphous semiconductors, J. Appl. Phys. 96 (2004) 2037e2048. [28] Y. Hou, Z. Mei, H. Liang, D. Ye, S. Liang, C. Gu, X. Du, Comparative study of nMgZnO/p-Si ultraviolet-B photodetector performance with different device structures, Appl. Phys. Lett. 98 (2011) 263501. [29] M. Mohamed, K. Irmscher, C. Janowitz, Z. Galazka, R. Manzke, R. Fornari, Schottky barrier height of Au on the transparent semiconducting oxide bGa2O3, Appl. Phys. Lett. 101 (2012) 132106.