NCORRECTED PROOF. High-temperature high-humidity and electrical static discharge stress effects on GaN p i n UV sensor

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1 Materials Science and Engineering B xxx (2005) xxx xxx High-temperature high-humidity and electrical static discharge stress effects on GaN p i n UV sensor Su-Sir Liu a,, Pei-Wen Li a, W.H. Lan b, Wen-jen Lin c a Department of Electrical Engineering, National Central University, Taoyuan, Taiwan, ROC b Department of Electrical Engineering, National University of Kaohsiung, Kaohsiung, Taiwan, ROC c Chung-Shan Institute of Science and Technology, Lung-Tan, Taoyuan 325, Taiwan, ROC Received 13 December 2004; accepted 22 February Abstract We report the operation tests of GaN p i n photodetectors under the conditions of high-temperature (HT), high-temperature high-humidity (HTHH), and electrical static discharge (ESD). It is found that the ESD stress plays the dominant role for the degradation of dark current and the responsivity or rejection ratio of GaN p i n photodetectors. The GaN p i n diodes exhibited similar photoelectrical characteristics after HT or HTHH test but failed after ESD breakdown test at reverse bias of 4500 V. The surface morphologies are not affected even after ESD and HTHH tests Elsevier B.V. All rights reserved. Keywords: High-temperature high-humidity; Electrical static discharge; p i n photodetectors 1. Introduction Motivation to use GaN-based material system for ultraviolet (UV) photon detection is strong in light of its intrinsic visible-blindness and large direct band-gap energy [1 6]. Based on GaN-based semiconductors, photodiodes (PDs) can achieve high UV selectivity without optical filters and take advantages of this material system including high breakdown fields and chemical/thermal stability, which offers great promises for high-temperature device operation in extreme conditions [7 8]. In the past years, various nitride-based UV PDs have been demonstrated, such as p i n PDs, schottkybarrier PDs, and metal semiconductor metal (MSM) PDs, and some superior device characteristics have been reported [9 17]; however, few works have truly discussed the reliability or robustness of the GaN PDs after operations in harsh environments. In this work, we report the growth, fabrication, and characterization of GaN p i n PDs. The optical and electrical Corresponding author. address: susir.liu@msa.hinet.net (S.-S. Liu) /$ see front matter 2005 Elsevier B.V. All rights reserved. 2 doi: /j.mseb properties of the fabricated devices after high-temperature 19 (HT), high-temperature high-humidity (HTHH), and electri- 20 cal static discharge (ESD) stress will be discussed Experiments 22 The p i n structures of GaN device layers were grown by 23 metalorganic chemical vapor deposition system on (0001) 24 sapphire (Al 2 O 3 ) substrates, using trimethylgallium (TMG) 25 and ammonia (NH 3 ) as gallium and nitrogen sources, respec- 26 tively. H 2 was used as the carrier gas. Growth began with a 27 GaN buffer layer on which a 2 m n-gan template layer 28 (n = cm 3 ) was grown in order to reduce the de- 29 fect density of subsequent layers. Device layers for the p i n 30 structure were grown starting with a 1 m heavy doped n- 31 GaN layer (n = cm 3 ), followed by 1 m undoped 32 GaN layer, and then terminated with a 250 nm Mg-doped p- 33 GaN layer. Mesa patterning was subsequently defined with an 34 inductively-coupled plasma reactive-ion etching (ICP-RIE), 35 using Cl 2 and Ar as etching sources. The device consists 36 of two inter-digitated contact electrodes with Ti/Al/Ti/Au 37

2 2 S.-S. Liu et al. / Materials Science and Engineering B xxx (2005) xxx xxx Fig. 1. (a) Schematic structure of GaN PIN UV detector. (b) Top view of fabricated device. (20/100/20/150 nm) and Ni/Au (20/150 nm) as n and p contact metals, respectively. Finally, a 600 C furnace anneal in N 2 ambient for 10 min was performed to complete the device fabrication. A schematic of a typical p i n PD is shown in Fig. 1(a) and the top view of the completed device is shown in Fig. 1(b). The reliability of p i n PDs were investigated in three different stress modes: (1) ESD test with a human body mode of 3 pulse/3 s at different pulse voltages, (2) high-temperature (HT) test at 90 C for a duration of h, and (3) hightemperature high-humidity (HTHH) stress at 90 C with a relative humidity of 90% for a duration of h. The current voltage characteristics of the GaN p i n PDs with an active region of 1 mm 1 mm were analyzed using an HP4156 parameter analyzer; the interface state capacitance with frequency were analyzed by using HP 4284 parameter analyzer company with light shield model Hg-402L mercury probe. Atomic force microscopic (AFM) and a surface profiler (Dektak3) were used to characterize the surface morphology of PDs. In addition, the spectral response of the p i n UV detector was studied using a monochrometer with a uniform 75 W Xenon illumination. A standard Si-based UV enhanced photodetector was used for calibration. 3. Results and discussion The behaviors of dark current with stress time for GaN p i n PDs after HT (sample A2) and HTHH (sample A3) operation tests are compared in Fig. 2(a). It is clearly to see that the dark current of sample A2 is almost insensitive to temperature stress (changing from to cm 2 ) for a duration of 168 h, while in spite that the dark current of sam- 67 ple A3 remains almost the same during the initial 84 h stress 68 but gradually increases about one order in magnitude after h HTHH stress. The reliability of device performance 70 was also analyzed with ESD breakdown test (sample A1), 71 in which a human body mode of 3 pulses/3 s with different 72 pulse amplitude (reverse bias voltage ranging from 500 to V) were performed shown in Fig. 2(b). The dark cur- 74 rent of sample A1 increases dramatically as the ESD bias 75 voltage is larger than 500 V and finally reaches to the de- 76 vice breakdown regime at ESD bias voltage of 4500 V. These 77 experimental results indicate that HT stress has very little 78 impact on the degradation of device dark current, but ESD 79 stress plays a dominant role for the failure of the measured 80 GaN p i n diodes. 81 The current voltage (I V) characteristics of GaN p i n 82 PDs under the stress conditions of ESD at 4000 V (sample 83 A1), HT for a duration of 48 h (sample A2), and HTHH for a 84 duration of 48 h (sample A3), respectively, are summarized in 85 Fig. 2(c). Sample A1 after EDS stress has the largest turn-on 86 voltage of 1.45 V with an ideality factor n 2.7, while both 87 samples A2 and A3 after HT or HTHH have a lower turn-on 88 voltage of 1.15 V with an ideality factor n 1.67 indicates 89 that sample after ESD stress test has more recombination cen- 90 ter or induced contact degrade which presented more defect 91 density than HT or HTHH stress test. Meanwhile, experi- 92 mental results also show that sample A1 after EDS stress has 93 the highest dark current density of A/cm 2 at a re- 94 verse bias of 10 V, which is about two to three orders higher 95 than that of sample A2 ( A/cm 2 ) and sample A3 96 ( A/cm 2 ). The dark current of a p i n PD is usu- 97 ally attributed to the hopping of charged carriers via localized 98 defect-related traps in the depletion region. The measured I V 99 characteristics indicate that, after ESD stress, more defects 100 resulting from severe lattice mismatch between GaN epilay- 101 ers and sapphire substrate are generated and furthermore, 102 the field-assisted charge hopping via localized defect-related 103 traps, tunneling across the gap, or detrapping from the defect 104 states in the depletion region are enhanced. This explains 105 why the dark current flow of sample A1 is much higher than 106 the others. The highest turn-on voltage and highest dark cur- 107 rent observed in sample A1 indicates that ESD is the most 108 dominant factor for device performance degradation. 109 The film quality of a 2 m GaN film on sapphire substrate 110 after stress conditions of ESD at 4000 V (sample A1), HT 111 for a duration of 48 h (sample A2), and HTHH for a duration 112 of 48 h (sample A3), respectively, was also characterized by 113 capacitance frequency as shown in Fig. 3. It was observed 114 that at (100 Hz/30 mv), the related interfacial states capac- 115 itance C P, produced by defect extension into the surface of 116 GaN bulk, were measured to be , , 117 and F for sample A1, sample A2, and sample 118 A3, respectively. For sample A1, after ESD test, local de- 119 fects or interface states hopping is enhanced by high electric 120 field and that result in a higher interfacial-state capacitance 121 than that of sample A2 and A3. There is no big difference in 122

3 S.-S. Liu et al. / Materials Science and Engineering B xxx (2005) xxx xxx 3 Fig. 2. (a) Time-dependent dark current density analysis of GaN p i n diode under HT and HTHH stress tests, respectively. (b) Device dark current density analysis after ESD test at different pulse voltage. (c) I V characteristics of GaN p i n diode under ESD, HT, and HTHH stress tests, respectively. Fig. 3. Interface state capacitance frequency of GaN film after ESD, HT, and HTHH stress tests, respectively. Fig. 4. Device spectrum responsivity of GaN p i n diodes under ESD, HT, and HTHH stress tests, respectively.

4 4 S.-S. Liu et al. / Materials Science and Engineering B xxx (2005) xxx xxx Fig. 5. AFM surface morphology on top p-layer of devices after (a) ESD test (mean roughness R a = nm) and (b) HTHH test (mean roughness R a = nm). the interfacial-state capacitance for samples A2 and A3. The result of interfacial-state capacitance spectrum agrees well with that of dark current analysis. Fig. 4 shows the spectrum response of visible-blind UV photodiodes at zero bias. All samples exhibit a large average responsivity in the nm visible-blind windows and have an abrupt roll-off at wavelengths larger than 360 nm since they are transparent in that region. A slight decrease of the responses at shorter wavelengths (<360 nm) can be explained by a higher absorption in the p-gan layer. The response tail for photon energies below the band gap can be explained by the mechanisms of central photoionization in various defects such as dislocations during grain boundaries, or interface. These defect centers induce energy levels in the semiconductor band gap, which could be ionized by high-energy photons and provoke band-to-band transition in the bulk. That explains why a lower UV vis contrast is observed. The maximum responsivity of sample A1 is measured to be 0.07A/W at 360 nm, which is lower than that of sample A2 (0.13 A/W), and sample A3 (0.11 A/W). Moreover, the rejection ratio of sample A1 is about (the ratio of responsivity at nm), which is one order of magnitude lower than that of sample A2 and sample A3 (about ). ESD test enhanced more recombination centers like via lattice defect and impurity energy levels within the 147 energy gap, which minimized the photo generated carrier and 148 presented lower responsivity or rejection ratio, however, the 149 responsivity and rejection ratio presented almost identical 150 value between sample A2 and sample A3 (HT or HTHH test) 151 were also observed. 152 Fig. 5(a and b) shows the AFM roughness analysis on top 153 p-layer as samples tested after ESD at 4000 V and HTHH 154 test; the corresponding mean roughness (R a ) is and nm, respectively, which presented almost the same 156 value. The surface morphologies are not affected even after 157 ESD or HTHH test which is not the dominant factor of the 158 dark current flow, however, at the high electric field-assisted 159 hopping charge carriers via localized defect or related inter- 160 facial states by ESD test, which is the dominant factor of 161 the dark current flow and truly affected the responsivity or 162 rejection ratio on our devices Conclusions 164 In conclusion, we reported the growth, fabrication, and 165 characterization of GaN p i n PDs stressed in ESD, HT, and 166 HTHH conditions. The dark current and interfacial states ca- 167

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