GaN MSM UV Photodetector With Sputtered AlN ... - IEEE Xplore

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Chun-Kai Wang, Yu-Zung Chiou, Shoou-Jinn Chang, Fellow, IEEE,. Wei-Chih Lai, Sheng-Po Chang, Cheng-Hsiung Yen, and Chun-Chi Hung. Abstract— GaN ...
IEEE SENSORS JOURNAL, VOL. 15, NO. 9, SEPTEMBER 2015

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GaN MSM UV Photodetector With Sputtered AlN Nucleation Layer Chun-Kai Wang, Yu-Zung Chiou, Shoou-Jinn Chang, Fellow, IEEE, Wei-Chih Lai, Sheng-Po Chang, Cheng-Hsiung Yen, and Chun-Chi Hung

Abstract— GaN metal–semiconductor–metal (MSM) ultraviolet photodetectors (PDs) with ex situ sputtered AlN nucleation layer were investigated and demonstrated. The crystal quality, electrical, and optical properties of GaN PDs were improved using ex situ sputtered AlN nucleation layer. Compared with in situ AlN nucleation layer, it was found that the X-ray rock curve widths and yellow or blue bands of cathodoluminescence spectra of the PDs prepared by ex situ sputtered AlN nucleation layer were significantly reduced and smaller due to the improved crystal quality. It was also found that the dark current and responsivity of PDs with ex situ sputtered AlN nucleation layer were more effectively reduced and enhanced. Moreover, GaN MSM PDs with ex situ sputtered AlN nucleation layer could achieve the higher quantum efficiency and detectivity. Index Terms— GaN MSM UV PDs, sputtered AlN, nucleation layer, responsivity, detectivity.

I. I NTRODUCTION

L

IGHT detection in the ultraviolet (UV) spectral range has drawn much attention in the recent years. The various fields such as civil and military industries demand high-performance visible-blind UV photodetectors (PDs) for applications such as solar UV monitoring, UV light source calibration, UV astronomy, flame sensors, detection of missile plumes, and secure space-to-space communications. Currently, the most primary means of UV light detecting was the use of silicon (Si) PDs. However, the most sensitive wavelength of Si PDs is not located in the UV region since room temperature bandgap energy of Si is only 1.2 eV. Thus, the responsivity of Si PDs is low in the UV region. To maximize the detector responsivity in the UV spectral region, one should choose wide bandgap materials, such as SiC, GaN, and ZnSe, for the

Manuscript received February 14, 2015; revised April 16, 2015; accepted April 21, 2015. Date of publication April 23, 2015; date of current version July 7, 2015. This work was supported by the National Science Council of Taiwan under Contract NSC 101-2221-E-218-023-MY2 and Contract NSC 101-2632-E-218-001-MY3. The associate editor coordinating the review of this paper and approving it for publication was Dr. M. N. Abedin. C.-K. Wang, Y.-Z. Chiou, and C.-C. Hung are with the Department of Electronic Engineering, Southern Taiwan University of Science and Technology, Tainan 71005, Taiwan (e-mail: [email protected]; [email protected]; [email protected]). S.-J. Chang and S.-P. Chang are with the Department of Electrical Engineering, Institute of Microelectronics, National Cheng Kung University, Tainan 70101, Taiwan (e-mail: [email protected]; [email protected]). W.-C. Lai and C.-H. Yen are with the Department of Photonics, National Cheng Kung University, Tainan 70101, Taiwan (e-mail: weilai@mail. ncku.edu.tw; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2015.2425657

fabrication of solid state UV PDs. Among these wide bandgap materials, GaN has a wide and direct bandgap and a high saturation velocity. This material is also remarkably tolerant to aggressive environments, due to its excellent thermal stability, high breakdown electric field, and radiation hardness. These properties make GaN a promising candidate for UV PDs applications. In the past few years, various types of GaN-based UV PDs have been proposed, such as p-n junction PD [1]–[4], p-i-n PD [5]–[10], p-π-n PD [11], Schottky barrier PD [12]–[15], and metal–semiconductor–metal (MSM) PD [16]–[23]. Among them, MSM PD is an attractive choice for UV detectors, given its easier growth, fabrication simplicity and suitability for monolithic integration of optical receivers. Although nitride-based devices prepared on sapphire (Al2 O3 ) substrates are already commercially available, GaN grown on Al2 O3 substrates always contains a high density of threading dislocations (TDs) embedded in the GaN epitaxial films owing to the large mismatch in lattice constant and thermal expansion coefficient between GaN and Al2 O3 substrates [24], [25]. For MSM PDs, TDs can result in leakage current paths at the Schottky interface between metal and GaN. As a result, GaN MSM PDs have a higher leakage current in general, which can influence the characteristics of responsivity, quantum efficiency, and detectivity. Thus, it is extremely important to reduce the density of TDs in GaN. Previously, many techniques, such as those involving an interlayer, an epitaxial lateral over growth (ELOG), a patterned sapphire substrate (PSS), and the use of patterned dielectrics structure along with regrowth process, have been developed to reduce the TD density [26]–[29]. However, all these techniques should take extra efforts in fabrication and growth steps, which might result in a higher production cost. In our study, we report the effects of the simple and effective method by introducing ex-situ sputtered aluminum nitride (AlN) nucleation layer before the growth of undoped GaN (u-GaN) bulk material. Moreover, the detailed physical, electrical, and optical properties of the fabricated GaN MSM PDs will also be discussed. II. E XPERIMENT GaN bulk materials of all samples used in this study were grown by using metal-organic chemical vapor deposition (MOCVD) system. During the epitaxy, trimethylgallium (TMGa), trimethylaluminium (TMAl), and ammonia (NH3 ) were used as the source materials of Ga, Al, and N, respectively. Before the epitaxial growth,

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Fig. 1. The schematic structures of: (a) PD I, (b) PD II, and (c) the top-view of the fabricated PDs.

a 25-nm-thick ex-situ AlN nucleation layer was first deposited onto the c-plane (0001) Al2 O3 substrate by RF sputter system with AlN sputtering targets, called the ex-situ sputtered AlN nucleation layer. After that, for the epitaxial growth by MOCVD, the reactor temperature was ramped up from room temperature to 1050 °C in ammonia and carrier gas ambient. Then, a 2-μm-thick u-GaN bulk material was grown onto the ex-situ sputtered AlN nucleation layer. In contrast, a 10-nm-thick in-situ AlN nucleation layer by MOCVD was grown at 900 °C on the PSS without the ex-situ sputtered AlN nucleation layer. Then, a 3-μm-thick u-GaN bulk material was also grown on in-situ AlN nucleation layer. The pattern diameter, spacing, and height of the PSS were 3.5, 2, and 1.3 μm, respectively, which was fabricated by inductively coupled plasma reactive ion etching to etch sapphire coated the cone-shaped photoresist layer. Figures 1(a) and (b) show the schematic structures of the fabricated PDs. The PDs with the PSS and in-situ AlN nucleation layer by MOCVD were called PD I. The PDs with the normal sapphire substrate and ex-situ sputtered AlN nucleation layer were called PD II. For the process of MSM PDs, the as-grown u-GaN wafers were first dipped in a diluted hydrochloric acid water solution (HCl: H2 O = 1:1) for 5 min to remove the contaminants from the wafer surface. Then, a Ni/Au (500/1500 Å) metal layer was evaporated as the Schottky contact electrodes of the devices. As shown in Fig. 1(c), the devices consisted of two inter-digitated contact electrodes. Such inter-digitated contact electrodes were formed through standard lithography and etching. The fingers of the Schottky contact electrodes were 7 μm wide with a spacing of 5 μm. The active region of the fabricated PDs was 400 × 400 μm2 . An HP-4155C semiconductor parameter analyzer was then used to measure the dark current characteristics of these PDs. Spectral responsivity measurements by a JOBIN-YVON SPEX system were also performed with a 300 W xenon arc lamp light source and a standard synchronous detection scheme. Furthermore, noise characteristics of the fabricated PDs were also measured in the frequency range from 1 Hz to 12.9 kHz by using a SR570 low-noise current pre-amplifier and a 35670A dynamic signal analyzer. III. R ESULTS AND D ISCUSSION Figure 2 shows the (002) and (102) ω-scan DCXRD spectra of the u-GaN bulk materials for PD I and II. It was found that the intensity of (002) and (102) spectra for PD II was larger than that of PD I. The full-width at half-maximum (FWHM)

Fig. 2.

The (002) and (102) XRD spectra for PD I and II.

Fig. 3.

The CL spectra for PD I and II.

values of (002) and (102) spectra for PD II were 0.0982° and 0.0911°, respectively, which were narrower and better than those of PD I of 0.1062° and 0.1262°, respectively. The smaller of FWHM values for PD II implies the lower TD density [30]. And, the etch pit densities (EPD) formed by a mixed acid of H3 PO4 +H2 SO4 on the GaN surface of PD I and II were be estimated to be about 7.87×108 and 5.7×108/cm2 , respectively (not shown here). These results indicated that the significant reduction in TD density can be attributed to the abatement of lattice mismatch effect between the u-GaN layer and the Al2 O3 substrate. Therefore, the TD density of the devices can be more effectively suppressed by the ex-situ sputtered AlN nucleation layer on normal Al2 O3 substrate than that by the in-situ MOCVD AlN nucleation layer on the PSS. In other words, the u-GaN layer of PD II can achieve a better performance of crystal quality than that of PD I. Figure 3 shows the cathodoluminescence (CL) spectra of the u-GaN bulk materials for PD I and II, which are normalized to the intensity of near-band-edge (NBE) emission around 365 nm. Two broad yellow (YL) and blue (BL) bands are observed, which is widely believed that the transitions between donors and acceptors were induced by impurities or native point defects [31]–[38]. It can be seen obviously that the integrated intensities of these luminescence bands are very different for the two samples. The relative intensity of YL and BL for PD II was smaller than that of PD I. On the other hand, the inset of Fig. 3, it was also found that the FWHM of NBE emission peak of PD II was narrower than PD I. Such these results in the CL measurements indicated again that PD II had lower defect density and better crystal quality.

WANG et al.: GaN MSM UV PD WITH SPUTTERED AlN NUCLEATION LAYER

Fig. 4.

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I–V characteristics of PD I and II in the dark condition. Fig. 5.

Figure 4 shows current-voltage (I–V) characteristics of PD I and II in the dark condition. It was found that the dark currents of PD II under a bias voltage of 3 V were around one order of magnitude lower than those of PD I. Such a smaller dark current of PD II suggested that the ex-situ sputtered AlN nucleation can more effectively reduce the leakage current of defect-related nonradiative recombination. This result would be confirmed once again that the u-GaN quality was improved and enhanced the dark current characteristics of the PD II. Furthermore, the dark I-V relationship of the fabricated PDs can be expressed by the thermionic emission model for MSM PDs [39], which can be written as −q(φbn − φn ) ) kT −q(φbp − φ p ) ) + A∗p T 2 exp( kT

J = A∗n T 2 exp(

(1)

where A∗n and A∗p are the effective Richardson constants for electrons and holes, T is the absolute temperature, q is electron charge, k is the Boltzmann constant, φbn,bp are the respective barrier height, and φ n, p are the respective imageforce-induced lowering of the potential barrier (i.e. Schottky effect). The effective Richardson constants for electrons and holes can be given by 4πqm ∗p k 2 4πqm ∗n k 2 ∗ , A = (2) p h3 h3 where m ∗n and m ∗p are the effective mass of electrons (i.e. 0.22m o) and holes (i.e. 0.8 m o ) for GaN, respectively, and h is the Planck constant. Therefore, the effective Richardson constant was estimated to be 26.4 and 96 Acm−2 K−2 for electrons and holes, respectively. From I-V measurements and Eq. (1) & (2), it was calculated and found that Schottky barrier heights for electrons on u-GaN bulk materials of PD I and II were around 0.74 and 0.8 eV, respectively. Such a result showed that PD II can accomplish a better performance of the interface between contact metal and u-GaN bulk. It is because that the interface trap state density at metal and semiconductor interface generated from the defect or dislocation can result in the lowering of Schottky barrier height [40]. Therefore, it is more useful for PD II with higher barrier height to prevent the electrons from overcoming the barrier to create a flow of leakage current. A∗n =

The responsivity and quantum efficiency of PD I and II.

Figure 5 shows spectra response of GaN MSM UV PD I and II. It can be seen that extremely sharp cutoff characteristics of PD I and II occurred at around 360 nm with a rejection ratio higher than 2 orders of magnitude. Such a rapid decrease in responsivity suggests a good quality of our GaN epitaxial layer so that almost no long wavelength light was absorbed. It was also found that the peak responsivity and rejection ratio for PD II were better than those of PD I. This phenomenon can be attributed to the less TD density and lower dark current for PD II. Then, the peak responsivity values of PD I and III at a bias voltage of 1 V were 0.01286 and 0.01682 A/W, respectively. The quantum efficiency of the fabricated PDs from the spectra response can be calculated by: qλ (3) hc where R is the measured responsivity, η is the quantum efficiency, q is the electron charge, λ is the incident light wavelength, h is the Planck constant, c is the speed of light. The calculated quantum efficiency of the fabricated PDs with the incident light wavelength of 360 nm at different bias voltages was shown in the inset of Fig. 5. It can be seen clearly that the quantum efficiency values of PD II at different bias voltages were all better than those of PD I. The maximum quantum efficiency for PD II can reach 59.6%. Such a high quantum efficiency of PD II can be attributed to the less TD density which can capture the electron-hole pairs generated by the incident photos. Figure 6 shows the noise power spectra of the fabricated MSM PD I and II. For analysis of low frequency noise at room temperature, the noise floor of our measurement system limited by the preamplifier was about 10−31 A2 /Hz. From low frequency noise measurements, it was found that noise power spectra measured from the detector could be fitted reasonably well by the following equation. R =η×

Sn ( f ) = S0 (

Iβ ) fα

(4)

where Sn ( f ) is the spectral density of the noise power, S0 is a constant, I is the dark current, α and β are two fitting parameters. From the measured curves, it was found that the value of α was almost two throughout the measured

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AlN nucleation layer to reduce the dark current and enhance the responsivity, thus achieve the higher quantum efficiency and detectivity of GaN MSM UV PDs. R EFERENCES

Fig. 6.

The noise power spectra of PD I and II.

frequency range. In other words, the low frequency noise in our devices was dominated by 1/f-type noise (i.e. flicker noise). Furthermore, the values of β of PD I and II were extracted around 3 and 2.8, respectively, while the values of S0 of PD I and II were about 4.88×10−3 and 0.69×10−3, respectively. These two parameters of flicker noise related to material defects of PD II were lower than those of PD I. The total noise current power can be estimated by integrating Sn ( f ) over the frequency range.  < i n >2 = Sn ( f )d f (5) Thus, the noise equivalent power (NEP) can be calculated by  < i n >2 NEP = (6) R where R is the responsivity of the photodiodes. The normalized detectivity (D∗ ) can then be determined by √ √ A B ∗ D = (7) NEP where A is the area of the photodetector and B is the bandwidth. For a given bandwidth of 1 kHz and the detector area of 400×400 μm2 , thus the noise equivalent power and detectivity can be determined. Finally, It was found that we achieved the D∗ of 4.62×1010 cmHz0.5 W−1 for PD II, which was more sensitive than that of 3.56×1010 cmHz0.5 W−1 for PD I. IV. C ONCLUSION In summary, we reported the fabrication and characteristics of GaN MSM UV PDs with ex-situ sputtered AlN nucleation layer. Replacing the in-situ AlN nucleation layer with ex-situ sputtered AlN nucleation layer improved the crystal quality, electrical, and optical properties of GaN PDs. The FWHM values of (002) and (102) spectra of the u-GaN bulk decreased from 0.1062° to 0.0982° and 0.1262° to 0.0911°, respectively, by introducing the technology of ex-situ sputtered AlN nucleation layer. Furthermore, the FWHM of NBE emission peak and the performances of broad YL and BL bands of CL spectra were also remarkably improved. This is the reason that the lattice mismatch effect between the u-GaN layer and the sapphire substrate was abated and the TD density was more effectively suppressed. Moreover, it was also found that we indeed can use the ex-situ sputtered

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WANG et al.: GaN MSM UV PD WITH SPUTTERED AlN NUCLEATION LAYER

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Yu-Zung Chiou was born in Taiwan. He received the B.S., M.S., and Ph.D. degrees from the Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, in 1997, 1999, and 2003, respectively. From 1999 to 2001, he was with Taiwan Semiconductor Manufacturing Company, Ltd., as an Engineer, specializing in CMOS image sensors and high-voltage devices. He is currently with the Department of Electronics Engineering, Southern Taiwan University of Science and Technology, Tainan, as a Full Professor, where he was named Manager of the Optoelectronics and Microwave Device Laboratory. He has authored over 60 papers in thin-film materials and optoelectronic devices. His current research interests include optoelectronic devices of III–V compound semiconductors.

Shoou-Jinn Chang (F’14) was born in Taipei, Taiwan, in 1961. He received the B.S.E.E. degree from National Cheng Kung University (NCKU), Tainan, Taiwan, in 1983, the M.S.E.E. degree from the State University of New York, Stony Brook, in 1985, and the Ph.D.E.E. degree from the University of California, Los Angeles, in 1989. He was a Research Scientist with NTT Basic Research Laboratories, Musashino, Japan, from 1989 to 1992. In 1992, he became an Associate Professor with the Department of Electrical Engineering, NCKU, where he was promoted to Full Professor in 1998. He was a Royal Society Visiting Scholar with the University of Wales, Swansea, U.K., in 1999, and a Visiting Scholar with the Research Center for Advanced Science and Technology, University of Tokyo, Japan, from 1999 to 2000, the Institute of Microstructural Science, National Research Council, Canada, in 2001, the Institute of Physics, Stuttgart University, Germany, in 2002, and the Faculty of Engineering, Waseda University, Japan, in 2005. He currently serves as the Deputy Director of the Center for Micro/Nano Science and Technology and the Director of the Semiconductor Research Center, NCKU. He is also an Honorary Professor with the Changchun University of Science and Technology, China. His current research interests include semiconductor physics, optoelectronic devices, and nanotechnology. He was a recipient of the Outstanding Research Award from the National Science Council, Taiwan, in 2004.

Wei-Chih Lai was born in Taiwan in 1970. He received the B.S. degree in electrical engineering from Feng-Chia University, Taichung, Taiwan, in 1993, and the M.S. and Ph.D. degrees in electrical engineering from the National Cheng Kung University (NCKU), Tainan, Taiwan, in 2001. In 2001, he was a Post-Doctoral Associate with the Department of Electrical Engineering, NCKU. He is currently an Associate Professor with the Department of Photonics, NCKU. His research interest includes the growth and characterization of III–V nitride semiconductors and devices.

Chun-Kai Wang was born in Tainan, Taiwan, in 1978. He received the B.S. degree from the Department of Electrical Engineering and the Ph.D. degree from the Institute of Microelectronics, National Cheng Kung University, Tainan, in 2001 and 2005, respectively. From 2006 to 2011, he was a member of the Technical Staff with Epistar Corporation, Tainan, specializing in nitride-based light-emitting diodes. He is currently with the Department of Electronics Engineering, Southern Taiwan University of Science and Technology, Tainan, as an Associate Professor. His current research interests include optoelectronics device of III–V compound semiconductors.

Sheng-Po Chang was born in Taichung, Taiwan, in 1982. He received the B.S. degree from the Department of Electronic Engineering, Southern Taiwan University, Tainan, Taiwan, in 2004, the M.S. degree from the Institute of Nanotechnology and Microsystems Engineering, National Cheng Kung University (NCKU), Tainan, in 2006, and the Ph.D. degree from the Institute of Microelectronics, NCKU, in 2009. He is currently a Post-Doctoral Researcher with the Institute of Microelectronics, NCKU. His main research focus on optoelectronics device of II–VI and III–V compound semiconductors.

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Cheng-Hsiung Yen was born in Tainan, Taiwan, in 1984. He received the B.S. degree in physics from Tung-Hai University, Taichung, Taiwan, in 2007, the M.S. degree in optoelectronic sciences from National Taiwan Ocean University, Keelung, Taiwan, in 2009, and the Ph.D. degree from the Department of Photonics, National Cheng Kung University, Tainan, Taiwan, in 2014. His current research is focused on the growth and fabrication of III–V optoelectronic semiconductors.

Chun-Chi Hung was born in Taiwan. He received the B.S. and M.S. degrees from the Department of Electrical Engineering, Southern Taiwan University of Science and Technology, Tainan, Taiwan, in 2011 and 2014, respectively.