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IEEE ELECTRON DEVICE LETTERS, VOL. 31, NO. 11, NOVEMBER 2010
Relation Between Low-Frequency Noise and Subgap Density of States in Amorphous InGaZnO Thin-Film Transistors Sungchul Kim, Yongwoo Jeon, Je-Hun Lee, Byung Du Ahn, Sei Yong Park, Jun-Hyun Park, Joo Han Kim, Jaewoo Park, Dong Myong Kim, Member, IEEE, and Dae Hwan Kim, Member, IEEE
Abstract—The relation between the low-frequency noise (LFN) and subgap density of states (DOS) of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) is investigated by changing the postannealing temperature from 150 ◦ C to 300 ◦ C. It is found that the density of the tail states in the TFT annealed at 300 ◦ C (showing the lowest LFN) is prominently lower than those in the TFTs annealed at 250 ◦ C and 150 ◦ C. The densities of the tail states in the TFTs annealed at 250 ◦ C and 150 ◦ C (indicating similar LFN) are almost the same. In addition, it is clearly observed that the increased DOS of the a-IGZO TFT subjected to ac gate voltage stress results in a higher LFN compared with one without electrical stress. Hooge’s parameters αH ’s are extracted to be ∼4.5 × 10−3 (for the TFT annealed at 300 ◦ C) and ∼1 × 10−2 (for the TFTs annealed at 250 ◦ C and 150 ◦ C as well as for the TFT annealed at 300 ◦ C after the application of electrical ac stress). Therefore, the role of an a-IGZO subgap DOS on a LFN characteristic seems to be originated from the generation–recombination noise-induced carrier number fluctuation (via trap centers in the DOS tail states) while its correlation with the carrier mobility fluctuation is not clear except for the slope close to −1 in the logarithmic curve with the normalized power spectral density versus the gate overdrive voltage. Index Terms—Amorphous oxide thin-film transistor (TFT), density of states (DOS), InGaZnO, low-frequency noise (LFN).
I. I NTRODUCTION
M
ULTICOMPONENT amorphous oxide semiconductor (AOS)-based thin-film transistors (TFTs) have been under active research and development due to the advantages of the room temperature (RT) fabrication process, large area uniformity, high carrier mobility, low cost, and compatibility with transparent and flexible display applications. Among them, the amorphous InGaZnO (a-IGZO) TFT is known to be a promising
Manuscript received July 7, 2010; accepted July 18, 2010. Date of publication September 7, 2010; date of current version October 22, 2010. This work was supported by the MEST through the Mid-career Researcher Program under NRF Grant 2009-0080344. The review of this letter was arranged by Editor J. K. O. Sin. S. Kim, Y. Jeon, D. M. Kim, and D. H. Kim are with the School of Electrical Engineering, Kookmin University, Seoul 136-702, Korea (e-mail:
[email protected];
[email protected];
[email protected];
[email protected]). J.-H. Lee, B. D Ahn, S. Y. Park, J.-H. Park, J. H. Kim, and J. Park are with the LCD R&D Center, Samsung Electronics, Yongin 449711, Korea (e-mail:
[email protected];
[email protected];
[email protected];
[email protected]; joohan.kim@ samsung.com;
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2010.2061216
device for active matrix liquid crystal display (AMLCD) and/or active matrix organic light-emitting diode display backplanes. In order to make AOS TFTs applicable to innovative circuits and systems for transparent and flexible electronics in the near future, their operations and related physical mechanisms as analog and digital switching devices should be fully investigated [1]. In particular, for practical applications in flexible amplifiers and photonic sensors, the low-frequency noise (LFN) should be fully characterized as is the case in CMOS image sensors. Although the LFN in the top-gate a-IGZO TFT is reported very recently [2], the physical mechanism has not yet been fully characterized. In addition, in spite of its importance, there are very few results on the LFN in AOS TFTs. In this letter, we report the relation between the LFN and subgap density of states (DOS) in a-IGZO TFTs. The subgap DOS is intentionally modulated by changing the postannealing temperature and applying electrical ac stress. The role of the DOS on the LFN characteristics seems to be originated from the generation–recombination (G-R) noise-induced fluctuation of the carrier number while its correlation with the mobility fluctuation is not clear except for the logarithmic curve [the normalized power spectral density (PSD) versus the gate overdrive voltage (VGS –VT )] with a slope close to −1. II. D EVICE FABRICATION A schematic cross section of the integrated AOS TFTs with the most commonly used staggered bottom gate structure for AMLCDs is shown in Fig. 1(a). The fabrication procedure for the a-IGZO TFTs is as follows: On a glass substrate, the first sputtered deposition at RT and patterning of the molybdenum (Mo) gate are followed by the plasma-enhanced chemical vapor deposition (PECVD) of SiNX and SiO2 at 370 ◦ C as a gate dielectric (TOX : the equivalent oxide thickness (EOT); Cox = εox /TOX : the gate capacitance per unit area). The a-IGZO layer is then sputtered by dc sputtering at RT in a mixed atmosphere of Ar/O2 (35:21 at sscm) and wet etched with a diluted HF (TIGZO : the thickness of an a-IGZO active layer). The etch stopper SiOX layer is deposited by PECVD at 150 ◦ C and patterned by wet etching. For the formation of the source/drain (S/D) electrodes, the Mo is sputtered at RT and patterned by dry etching. A passivation layer (SiOX and SiNX , each having a thickness = 100 nm) is followed by deposition. Finally, in order to intentionally make different DOSs in the TFTs, the fabricated a-IGZO TFTs are annealed at three different temperatures (Sample 1: 300 ◦ C, Sample 2: 250 ◦ C, and Sample 3: 150 ◦ C) for 1 h in the air. The geometrical parameters are
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KIM et al.: RELATION BETWEEN LOW-FREQUENCY NOISE AND SUBGAP DENSITY OF STATES
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Fig. 2. (a) Measured frequency dependences of normalized PSDs with the gate overdrive voltage (VGS −VT ) = 1 and 11 V in the linear operation (VDS = 1 V). (b) Log-log plot of the normalized PSD versus (VGS –VT ) under VDS = 1 V.
Fig. 1. (a) Schematic view of the fabricated a-IGZO TFT with an inverted staggered bottom gate structure. (b) Measured transfer characteristics (IDS –VGS ) of a-IGZO TFTs for three different postanneal temperatures (Sample 1: 300 ◦ C, Sample 2: 250 ◦ C, and Sample 3: 150 ◦ C) with the open symbol corresponding to Sample 1 with ac stress. TABLE I MEASURED DC PARAMETERS IN a-IGZO TFTs FOR DIFFERENT POSTANNEALING TEMPERATURES AND ELECTRICAL AC STRESS
as follows: SiNX /SiO2 = 400/50 nm [TOX = 258 nm (EOT)], W = 50 μm, L = 10 μm, Lov (gate-to-S/D overlap length) = 10 μm, and TIGZO = 50 nm. III. E XPERIMENTAL R ESULTS AND D ISCUSSION Fig. 1(b) shows the measured transfer curves of the a-IGZO TFTs for four cases. The closed symbols correspond to different postanneal temperatures (without electrical stress) and the open one corresponds to Sample 1 with the electrical ac stress (VG = −20 ∼ +20 V, VD = VS = 0 V, f = 50 kHz, rising/falling time tr /tf = 0.1 μs, duty ratio = 50%, and stress time = 2.5 × 104 s). The subgap DOS of the a-IGZO TFT after the application of electrical ac stress is known to be significantly increased [3]. The measured device parameters are summarized in Table I. Fig. 2(a) and (b) shows the frequency and gate overdrive volt2 age (VGS –VT ) dependences of the normalized PSD (SiD /ID )
(in terms of the drain current (ID ) fluctuations in the linear operation regime). In Fig. 2(a), for the frequency dependence, two different effective gate voltages were employed 2 versus (VGS −VT = 1 and 11 V). In the measured SiD /ID γ the frequency characteristic, the 1/f spectrum with γ ≈ 1 is clearly observed in Fig. 2(a). Furthermore, in Fig. 2(b), the characteristic curve of the PSD versus the (VGS –VT ) on a double-logarithmic scale at a fixed frequency (20 Hz) shows a slope ≈ −1. According to the well-accepted LFN theory [4], the slope = −1 indicates that the carrier mobility fluctuation is the dominant mechanism in the LFN. Therefore, it is concluded that the main origin of the 1/f noise in a-IGZO TFTs may be the carrier mobility fluctuation as is the case in [2]. In this letter, it is worthwhile to note that the normalized PSD of Sample 1 (annealed at T = 300 ◦ C) is about a half order of magnitude lower than those of the others (annealed at T = 150 ◦ C and 250 ◦ C). In addition, the normalized PSD of Sample 1 with ac stress increases up to the level comparable to those of Samples 2 and 3. The classic indicator of the carrier mobility fluctuation is Hooge’s parameter (αH ) described as [5] SV SI SR αH q αH = = = = R2 R V2 I I 2 V f N f W LCox (VGS −VT ) (1) with N as the total carrier concentration. A higher αH indicates a higher LFN after de-embedding the structure and current drivability dependences. Hooge’s parameters are extracted to be αH ∼ 4.5 × 10−3 in Sample 1 and αH ∼ 1.0 × 10−2 in the other three samples as shown in Fig. 3, which are consistent with those in [2]. Consequently, αH in the a-IGZO TFTs is observed to be sensitive to both the postannealing temperature and the electrical ac stress. In addition, the G-R (via a large number of traps in an a-IGZO thin film) noise-induced fluctuation of the carrier density can be also the other origin of the 1/f noise. In the case of a-IGZO TFTs with complex traps,
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Fig. 3. Extracted Hooge’s parameters (αH ) versus the gate overdrive voltage (VGS –VT ) in four cases of a-IGZO TFT samples.
IEEE ELECTRON DEVICE LETTERS, VOL. 31, NO. 11, NOVEMBER 2010
G-R noise-induced carrier number fluctuation. This is because the G-R process would occur more frequently under the condition of more subgap trap centers. In addition, the influence of the deep states on the LFN is relatively insignificant in comparison with that of the tail states. This is because the Fermi level EF would be located close to the EC in the a-IGZO TFTs. On the other hand, the role of the DOS on the carrier mobility fluctuation is not physically clarified yet. Most probably, however, the carrier mobility fluctuation would be strongly correlated with the subgap DOS, and this is because the fieldeffect mobility (μFE ) of the AOS TFTs is dependent on the localized charge density trapped in the subgap DOS. IV. C ONCLUSION
Fig. 4. Subgap DOSs in four cases of a-IGZO TFTs extracted by using the multifrequency C–V method. Inset shows the DOS over a half range of bandgap.
the G-R noise from the traps with different time constants may be added up to a 1/f γ spectrum with γ close to one. In other words, the superposition of many Lorentzians may result in the total spectrum with ∼ 1/f dependence over several decades of frequency [6]. In order to investigate the relation between the LFN and the DOS, as shown in Fig. 4, the subgap DOSs near the conduction band edge (EC ) were extracted by using the multifrequency C–V method with three different frequencies (10, 100, and 1 MHz) [7]. The inset shows the DOS over a half range of the bandgap. The DOS parameters are extracted to be NTA = 5 × 1018 /6.0 × 1018 /6.8 × 1018 [cm−3 · eV−1 ], kTTA = 0.012/0.018/0.018 [eV], NDA = 3 × 1016 /4 × 1016 /8 × 1016 [cm−3 · eV−1 ], and kTDA = 0.58/0.3/0.5 [eV] for Samples 1, 2, and 3, respectively. NTA = 5 × 1018 [cm−3 · eV−1 ], kTTA = 0.016 [eV], NDA = 4 × 1016 [cm−3 · eV−1 ], and kTDA = 0.6 [eV] are obtained from Sample 1_Stress. After the application of ac stress, the subgap DOS of Sample 1 is significantly increased. A considerable difference on the deep and tail states in the four cases reasonably accounts for the difference on the turn-on voltage (VON ) and the subthreshold slope (SS) as summarized in Table I. Furthermore, comparing Fig. 4 with Figs. 2 and 3, it is noticeable that the correlation between the LFN and the DOS is obvious. However the subgap DOS is changed (either by postannealing temperature or electrical stress), a sample with low DOS tail states shows a significantly lower LFN characteristic in comparison with those of the samples with high DOS tail states. In the a-IGZO TFTs, therefore, the role of the tail states in the subgap DOS on the LFN characteristic is very important, which is characterized to be originated from the
The postanneal temperature and electrical stress dependences of the LFN characteristics in a-IGZO TFTs are investigated and explained by using the difference of the subgap DOS extracted from the multifrequency C–V method. It is found that the density of the tail states in the TFT annealed at T = 300 ◦ C (showing the lowest LFN) is significantly lower than those in the TFTs annealed at T = 150 ◦ C and 250 ◦ C. In addition, a higher LFN and DOS are observed in the TFT annealed at 300 ◦ C, only with the application of electrical ac stress. The densities of the tail states in the TFTs (annealed at 300 ◦ C) with electrical stress and/or in the TFTs annealed at 150 ◦ C and 250 ◦ C (indicating a high LFN) are almost the same. Hooge’s parameters are extracted to be αH ∼ 4.5 × 10−3 (for the TFT annealed at 300 ◦ C) and αH ∼ 1 × 10−2 (for the TFTs annealed at 250 ◦ C and 150 ◦ C as well for the TFT annealed at 300 ◦ C after the application of electrical stress) . Conclusively, the role of the a-IGZO subgap DOS on the LFN characteristic seems to be originated from the G-R noiseinduced carrier number fluctuation (via trap centers in the DOS tail states) while its correlation with the carrier mobility fluctuation is not clear except for the slope close to −1 in the logarithmic curve with the normalized PSD versus the gate overdrive voltage. R EFERENCES [1] M.-J. Lee, S. I. Kim, C. B. Lee, H. Yin, S.-E. Ahn, B. S. Kang, K. H. Kim, J. C. Park, C. J. Kim, I. Song, S. W. Kim, G. Stefanovich, J. H. Lee, S. J. Chung, Y. H. Kim, and Y. Park, “Low-temperature-grown transition metal oxide based storage materials and oxide transistors for high-density non-volatile memory,” Adv. Funct. Mater., vol. 19, no. 10, pp. 1587–1593, May 2009. [2] I.-T. Cho, W.-S. Cheong, C.-S. Hwang, J.-M. Lee, H.-I. Kwon, and J.-H. Lee, “Comparative study of the low-frequency-noise behaviors in a-IGZO thin-film transistors with Al2 O3 and Al2 O3 /SiNx gate dielectrics,” IEEE Electron Device Lett., vol. 30, no. 8, pp. 828–830, Aug. 2009. [3] S. Lee, K. Jeon, J.-H. Park, S. Kim, D. Kong, D. M. Kim, D. H. Kim, S. Kim, S. Kim, J. Hur, J. C. Park, I. Song, C. J. Kim, Y. Park, and U.-I. Jung, “Electrical stress-induced instability of amorphous indiumgallium-zinc oxide thin-film transistors under bipolar ac stress,” Appl. Phys. Lett., vol. 95, no. 13, p. 132 101, Sep. 2009. [4] L. K. J. Vandamme and F. N. Hooge, “What do we certainly know about 1/f noise in MOSTs?” IEEE Trans. Electron Devices, vol. 55, no. 11, pp. 3070–3085, Nov. 2008. [5] F. N. Hooge and L. K. J. Vandamme, “Lattice scattering causes 1/f noise,” Phys. Lett. A, vol. 66, no. 4, pp. 315–316, May 1978. [6] M. Surdin, “Fluctuations in the thermionic current and the ‘flicker effect’,” J. Phys. Radium, vol. 19, no. 4, pp. 188–189, Apr. 1939. [7] S. Lee, K. Jeon, S. Park, S. Kim, Y. Jeon, K. Jeon, J.-H. Park, J. Park, I. Song, C. J. Kim, Y. Park, D. M. Kim, and D. H. Kim, “Extraction of subgap density of states in amorphous InGaZnO thin-film transistors by using multifrequency capacitance-voltage characteristics,” IEEE Electron Device Lett., vol. 31, no. 3, pp. 231–233, Mar. 2010.
