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Structure and electrical properties of boron doped hydrogenated mixed-phase silicon films for uncooled microbolometer
Accepted Manuscript Structure and electrical properties of boron doped hydrogenated mixed-phase silicon films for uncooled microbolometer Chonghoon Shin, Duy Phong Pham, Jinjoo Park, Sangho Kim, Youn-Jung Lee, Junsin Yi PII: DOI: Reference:
S1350-4495(18)30696-0 https://doi.org/10.1016/j.infrared.2018.10.015 INFPHY 2729
To appear in:
Infrared Physics & Technology
Received Date: Revised Date: Accepted Date:
19 September 2018 11 October 2018 11 October 2018
Please cite this article as: C. Shin, D. Phong Pham, J. Park, S. Kim, Y-J. Lee, J. Yi, Structure and electrical properties of boron doped hydrogenated mixed-phase silicon films for uncooled microbolometer, Infrared Physics & Technology (2018), doi: https://doi.org/10.1016/j.infrared.2018.10.015
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Structure and electrical properties of boron doped hydrogenated mixed-phase silicon films for uncooled microbolometer Chonghoon Shin1, Duy Phong Pham2, Jinjoo Park2*, Sangho Kim2, Youn-Jung Lee2 and Junsin Yi1,2*
1
Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea 2
College of Information and Communication Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea
________________________ *Corresponding author: 1. Jinjoo Park, E-mail: [email protected], Tel.: +82-31-290 7139, Fax.: +82-31-290 7139 2. Junsin Yi, E-mail: [email protected], Tel.: +82-31-290 7139, Fax.: +82-31-290 7139
Abstract Boron doped hydrogenated silicon films used as thermo-sensing layers in infrared detectors or uncooled micro-bolometers are prepared by radio-frequency plasma-enhanced chemical vapor deposition (PECVD). In this work, we investigated TCR (higher the better) and sheet resistance, Rsheet (lower the better), which are important factors for thermos-sensing layer used in uncooled microbolometer. The crystalline volume fraction (Xc) of films is controlled to get silicon films that satisfy the characteristics of high TCR and low Rsheet. Through the control, amorphous, mixed- and microcrystalline phases were identified. As a result, the best TCR and Rsheet were obtained in the mixed-phase. For the films, TRC is around 1 ~ 3 %/K, Rsheet is around 3 ~ 61.4 MΩ/□ and Xc is around 7~17 %. The 1/f noise is measured for various phases. It is found that 1/f noise of boron doped hydrogenated mixed-phase silicon (BMP-Si:H) is smaller than that of the amorphous phase. The results of BMP-Si:H films show that they are more suitable as thermossensing layers than boron doped hydrogenated amorphous silicon films.
Keywords: micro-bolometers; infrared detectors; boron doped hydrogenated silicon films; mixed-phase silicon films.
1. Introduction Hydrogenated silicon films have been much studied as they can be used for various devices such as solar cells and infrared detectors [1, 2]. Especially, boron doped hydrogenated silicon films with a high thermal coefficient of resistance (TCR) and moderate resistance are used as thermos-sensing layers for uncooled micro-bolometers [3-5]. Since the boron doped hydrogenated silicon films are deposited by plasma enhanced chemical vapor deposition (PECVD), it can be deposited uniformly in large areas. Although the boron doped hydrogenated amorphous silicon (a-Si:H) is used as a thermo-sensing layer in uncooled micro-bolometers, it has a high value of 1/f noise [6]. In recent years, polymorphous silicon, a nanostructure inclusion material, and silicon materials alloyed with germanium and carbide have been investigated extensively to reduce the 1/f noise [3, 4, 5, 7]. The amorphous, mixed- and microcrystalline phases of hydrogenated silicon films using PECVD can be easily obtained by controlling the plasma conditions [8, 9]. Since the phase can be shifted, the TCR and resistance can be controlled. Microcrystalline phase has low resistance but it is not suitable for the thermo-sensing layer due to the low TCR. The amorphous phase has high TCR but has high 1/f noise due to the high resistance. For the mixed phase, the microcrystalline phase and the amorphous phase are mixed. In other words, nano- or microcrystalline grains are embedded in the amorphous matrix. For this structure, the electrical properties are improved as the defect density is low and it is very stable [10]. It can be considered that boron doped hydrogenated mixed-phase silicon films (BMP-Si:H) are potential materials as hydrogenated silicon films with low 1/f noise while satisfying TCR and resistance as a thermo-sensing layer.
In this work, the power density, B2H6/SiH4 ratio and H2 /SiH4 which are parameters that affect the phase shift of the hydrogenated silicon films are studied. By varying the parameters, the formation region of BMP-Si:H is confirmed through the formation map. The phase shift of the films is characterized by Raman. The most important parameters for uncooled microbolometers, TCR, sheet resistance and 1/f noise are analyzed.
2. Experiment Boron doped hydrogenated mixed-phase hydrogenated silicon (BMP-Si:H) films were deposited by radio frequency plasma-enhanced chemical-vapor deposition (RF-PECVD), on Corning Eagle XG glass (5 cm × 5 cm, 0.63 mm-thick) substrates. The glass substrate was ultrasonically cleaned, by dipping in acetone, isopropyl alcohol, and de-ionized (DI) water for 10 min. BMP-Si:H films of a thickness of about 100 nm were deposited onto the glass. Mixtures of silane (SiH4), hydrogen (H2), and 1% diborane (B2H6) diluted in hydrogen were used as the source gases. The primary variation was the ratio of B2H6 to SiH4 gas flows in large range from 0.2 to 1.2, power density (0.04, 0.07 and 0.21 W/cm2) and the gas ratio of H2/SiH4 was 80, 160 and 200. The substrate temperature was 200 ℃ and the working pressure was 1.5 torr. For electrical measurements, Al coplanar electrodes (300 nm-thick) were evaporated on the samples deposited on the glass substrate. The electrical characteristics were studied by the coplanar method, using a programmable Keithley 617 electrometer, using samples grown on glass substrates. The activation energy (E a) was obtained from the temperature-dependent dark conductivity (σd), by the Arrhenius relation. The 1/f noise characteristics of the films have been
studied by using an Agilent 89410A vector signal analyzer. The crystallite of BMP-Si:H films was measured by Raman spectroscopy. The crystalline volume fraction (X c) was obtained as Xc = (I520 + I500)/(I520 + I500 + I480) where, I500 and I520 are attributed to the crystalline grain boundary and grains, and are the intensities of crystalline-like peaks, whereas, I480 is attributed to the disorder regions of the amorphous phase-like peak of the transverse optic Si-Si vibrations in the Raman spectra [11].
3. Result and Discussion Silicon materials are usually composed of hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (µc-Si:H) films. The materials for which nano- or microcrystalline grains are embedded in an amorphous matrix are known as mixed-phase hydrogenated silicon (MP-Si:H) films. MP-Si:H films through PECVD can be obtained by controlling the plasma parameters such as gas (SiH4 and H2), power density, pressure, substrate temperature and electrodes distance. By using a doping gas such as B2H6, p-type MP-Si:H films can be obtained [9]. There are two mechanisms for MP-Si:H films. The two mechanisms of deposition of plasma from SiH4 gas are surface diffusion [12, 13], and selective etching [14-16]. Both have been proposed to explain the deposition of µc-Si:H thin films. The surface diffusion was considered to improve the mobility of the deposition precursors, by many hydrogen atoms that impinge on the surface. For selective etching, amorphous and crystalline phases are assumed to be deposited simultaneously; atomic hydrogen impinges on the film surface, and the amorphous material is selectively etched, leaving behind a crystal film. Raman spectroscopy is the most used method for analyzing the phase of MP-Si:H. The Raman spectra signal at 480 cm-1
corresponds to the amorphous phase and the range of 500 – 520 cm-1 corresponds to the crystalline phase. Fig. 1 (a) shows Raman spectra when B2H6/SiH4 is 0.2, 0.5 and 0.8 with power density of 0.04 W/cm2. When B2H6/SiH4 is 0.2, 0.5 and 0.8, the crystalline volume fraction (Xc) is 62.4, 17.4 and 0 %, respectively. When B2H6 / SiH4 is 0.2, the crystalline phase peak at 510 cm-1 is sharp while B2H6/SiH4 is 0.8, the amorphous phase peak at 480 cm-1 is wide. As the B2H6/SiH4 increases, it shifts from the crystalline phase to the amorphous phase. This phase shift can increase the defect density of the films and may reduce the stability as well as electric properties [17]. Particularly, in the case of B2H6/SiH4 is 0.5, the amorphous peak 480 cm-1 is broadly present and the intensity is small, but crystalline phase peak at 500 cm-1 also exists. It is boron doped hydrogenated mixed-phase hydrogenated silicon (BMP-Si:H) [18]. Fig. 1 (b) shows the crystalline volume fraction (Xc) of the BMP-Si: H films as the formation map for the relationship between power density and B2H6/SiH4. As the power density increases, Xc increases. When B2H6/SiH4 is 0.7, Xc increases from 0% to 10.2% as the power density increases. With a high power density of 0.21 W/cm2, the crystalline phase is formed when B2H6/SiH4 is lower than 0.7 and amorphous phase appeared when B2H6/SiH4 is higher than 0.8. With a low power density of 0.04 W/cm2, the crystalline phase is formed when B2H6/SiH4 is lower than 0.6 and amorphous phase appeared when B2H6/SiH4 is higher than 0.7. When B2H6/SiH4 is 0.5 and 0.6, BMP-Si:H films are formed with Xc of 17.4 and 7.4%, respectively. At low power density, a number of H atoms produced by electron impact dissociation are annihilated by SiH4 molecules, and the surface hydrogen coverage at the surface was low. The growth precursors from the plasma directly reacted with crystalline phase or amorphous phase, thus leading to lower Xc. At high power density, the surface hydrogen coverage was increased,
due to increased hydrogen radicals in the plasma. Strong Si-Si bonds were formed, leading to higher Xc. As B2H6/SiH4 is increased, Xc is reduced. At any power density, X c is decreased from 60% to 0% as B2H6/SiH4 is increased. Increased B2H6/SiH4 reduced the hydrogen coverage at the surface, impeding the selective etching, and the structural deviation changed from microcrystallinity, towards amorphous phase [19]. Boron doped a-Si:H network, probably three-fold coordinated B atoms, introduced additional strain. The surface diffusion coefficient of SiH 3 radicals, which were assumed to be the growth precursors of BMP-Si:H, increased, causing more strained Si-Si, and structural defects. At higher B2H6/SiH4, some boron atoms may segregate at the grain boundary in electrically inactive sites, resulting in a decrease of electrical properties [20-22]. Fig. 2 shows sheet resistance of BMP-Si: H films as formation map of power density and B2H6/SiH4. The power density was 0.04, 0.07 and 0.21 W/cm2. B2H6/SiH4 is varied from 0.2 to 1.2. As B2H6/SiH4 is increased, sheet resistance (Rsheet) is increased. With a high power density of 0.21 W/cm2, Rsheet is 1 ~ 40 MΩ/□from 0.2 to 0.7 of B2H6/SiH4. It is over 40 MΩ/□ when B2H6/SiH4 is more than 0.7. With a low power density of 0.04 W/cm2, Rsheet is under 1 MΩ/□
when B2H6/SiH4 is lower than 0.4. When B2H6/SiH4 is 0.5 and 0.6, Rsheet is 3.4 and 56 MΩ/□, respectively. For both high and low power density, Rsheet is increased with increasing B2H6/SiH4. When B2H6/SiH4 is 0.6, Rsheet is increased as the power density decreased. It shows a similar trend with Fig. 1. For B2H6/SiH4 lower than 0.5 where X c is high i.e., crystalline phase, the value of Rsheet is low (< 3.4 MΩ/□). For B2H6/SiH4 higher than 0.8 where there is no Xc i.e., amorphous phase, Rsheet
is high (> 40 MΩ/□). It shows that the phase of films can determine the electric properties. Low Rsheet can be attributed to efficient carrier transport because of the crystalline phase of the films. Conversely, high Rsheet can be attributed to less efficient carrier transport, driven by the amorphous phase of the films. Undesirable high Rsheet of the thermos-sensing layer can cause a mismatch for the input impedance of the read-out circuits [23]. The temperature coefficient of resistance (TCR) is one of the important parameters that indicate the thermo-sensing ability of films. TCR is related to the activation energy (Ea) of the films. TCRs are defined by following formula: TCR ≈ E a/kT2, where k is the Boltzmann constant and T is temperature. For thermo-sensing layers, materials with high TCR is generally used. It means that a small variation in temperature results in a large change in resistance [24]. Fig. 3 shows the formation map of TCR of MP-Si:H films in relation with power density and B2H6/SiH4. It shows that TCR is dependent on B2H6 / SiH4 and power density. At a higher power density of 0.21 W/cm2, TCR is less than 1 %/K when B2H6/SiH4 is 0.2 ~ 0.5 and it is more than 2 %/K when B2H6/SiH4 is over 0.6. At a low power density of 0.04 W/cm2, TCR is about 1.0 %/K when B2H6 / SiH4 is 0.5 and it is more than 2.1 %/K when B2H6 / SiH4 is over 0.6. These results show the similar trend to that of Xc and Rsheet. At each power density, TCR is increased as B2H6/SiH4 is increased. For B2H6/SiH4 less than 0.5, the value of TCR is low (<1 %/K) even at high power density and it is high wend B2H6/SiH4 is over 0.6. It is confirmed that the TCR is low under the condition that the crystallization phase is dominant and that it has a high TCR when the amorphous phase is dominant. Fig. 4 shows the crystalline volume fraction (Xc) of BMP-Si: H films for changes in B2H6/SiH4 and H2/SiH4. The power density is 0.04 W/cm2. Xc is decreasing with increasing
B2H6/SiH4 in all H2/SiH4. When H2/SiH4 is 80, B2H6/SiH4 shows an amorphous phase above 0.4 and a mixed- and microcrystalline phase (Xc = 9 and 54.3%) below 0.4. As the H2/SiH4 increases from 80 to 200, the Xc increases at the same B2H6/SiH4 (except for B2H6/SiH4 above 0.7). The increase in H2/SiH4 is equal to the increase in hydrogen atom. The increase in the number of hydrogen atoms is due to the increase of Xc, which is caused by breaking of weak Si-Si bonding chaning to a strong one, by selective etching at the surfae causing the increase of crystallization (increase of Xc). In addition, the decomposition of H2 covers the growing film surface with hydrogen. This process enhances the radicals absorbed at the growing surface to have enough time to find their energetically suitable sites that decreased the strutural defect in the films [25]. For this reason, high H2 /SiH4 films with better quality can be structurely obtained even if they have similar Xc. Fig. 5 shows the temperature coefficient of resistance (TCR) of BMP-Si: H films for changes in B2H6/SiH4 and H2/SiH4. The power density is 0.04 W/cm2. When H2/SiH4 is 80, the TCR is increased from 0.41 to 7.15%/K as B2H6/SiH4 increases. The same trend is shown for high H2/SiH4. In particular, it is shown that H2/SiH4 decreases from 6.49 to 1.81 at the same B2H6/SiH4 (0.7). When H2/SiH4 is 80 (B2H6/SiH4: more than 0.4) and 160 (B2H6/SiH4: more than 0.7), high TCR (> 5%/K) is observed in the amorphous phase. When H2/SiH4 is 200, TCR is 1.81 and 3.57%/K. On the other hand, the condition of H2/SiH4 and B2H6/SiH4 with Xc shows low TCR (<3.42%/K). These results confirm that TCR is determined by the phase of silicon films, and high TCR can be applied as a thermo-sensing layer. Fig. 6 shows the sheet resistance (Rsheet) of BMP-Si: H films for changes in B2H6/SiH4 and H2/SiH4. The power density is 0.04 W/cm2. When H2/SiH4 is 80, as B2H6/SiH4 increases, Rsheet ranges from 0.09 159000 MΩ/□. H2/SiH4 show similar trends at 160 and 200 degrees. In
amorphous phase, high Rsheet of 58800 to 600000 MΩ/□ is seen, and microcrystalline phase
(when Xc is more than 20%) shows low Rsheet of less than 0.1 MΩ/□. On the other hand, the
mixed-phase (when Xc is 20% at 0%) shows 1.1 to 61.4 MΩ/□. It was confirmed that Rsheet was determined by the phase of silicon films. A low Rsheet value is required for use as a thermosensing layer, but a high TCR is also required. If we match the TCR and Rsheet shown in Fig. 5, it can be seen that it is a trade-off relationship. Silicon films with values that satisfy the condition are mixed-phase. Fig. 7 shows the correlation between TCR, Xc and Rsheet of BMP-Si:H films. It is confirmed that the characteristics of the films vary greatly depending on the power density and B2H6/SiH4. The amorphous phase is slightly different depending on the power density, but the B2H6/SiH4 is 0.7 or more, and the TRC is 5%/K or more. Although the TCR is high, Rsheet shows a high value of more than 105 MΩ/□. Conversely, the Crystalline phase shows a low Rsheet (<10 MΩ/□) at low B2H6/SiH4 (<0.5) but low TCR (<2%/K). Considering 1/f noise, the crystalline phase has a lower Rsheet than the amorphous phase, so electrical conduction or mobility is better, but TCR is more serious than the amorphous phase [7]. Thus, it is not suitable for the application of thermosensing layers. Mixed-phase has Rsheet of 3 ~ 40 MΩ/□ and TCR of 1~3 %/K. Xc is about 10 ~ 17%. These values are similar to those of boron doped hydrogenated amorphous silicon films (a-Si:H,B) used as a thermos-sensing layer for commercial uncooled micro-bolometers [23]. BMP-Si:H with
similar TCR and electrical properties to those of the existing a-Si:H,B can improve the 1/f noise value even lower [7, 26]. Conventionally, the 1/f noise value of a film can be expressed by Hooge’s experimental formula [27, 28]: Sv = αH.V2/NC.f , where Sv is the noise power density at voltage V, αH is a Hooge’s parameter, f is frequency and NC is the total number of charge carriers. 1/f noise is inversely proportional to the number of total free carriers and is proportional to Hooge’s parameter (αH) which is dependent on the quality of crystalline and mobility of carriers. A thermo-sensing layer with high carrier concentration and high crystalline volume fraction has low 1/f noise value [5]. Fig. 8 shows a log-log plot of power spectra density, which is averaged over 40 measurements, versus frequency for various phases at 300 K. The 1/f noise of mixedphase with Xc is lower than that of the amorphous phase. There is the difference in the 1/f noise values for the mixed-phase with similar Xc. As Rsheet is decreased from 21.1 MΩ/□ (power density: 0.62 W/cm2) to 3.4 MΩ/□ (power density: 0.04 W/cm2), the 1/f noise becomes smaller. At this time, TCR is 1.04 and 1.54 %/K, respectively. The structural disorder of amorphous phase arises from the coordination defects since the amorphous phase has a low density of films and the highest disorder compared to the mixed-phase [26]. The 1/f noise value of the amorphous phase is also affected by metastable defect creation [29]. On the other hand, since hydrogen radicals are increased to make mixed-phase films, the films undergo reformation and network rearrangement due to the increased surface hydrogen coverage. In other words, the strained Si-Si bonds which are defects of the amorphous matrix are changed to Si-Si bonds which are stronger, forming more energetically stable and ordered structure. Also, even in the same mixed-phase, the
1/f noise value varies depending on the plasma condition such as high and low power density. The electrons with energies between 8.75 and 9.47 eV produce SiH 3 radicals which are most favored for high-quality films. In other words, the electron temperature of the plasma should be low [30-33]. High-quality films can be formed with a low electron temperature for which SiH 3 radicals with long lifetime are mostly produced. At low power density, the electron temperature is low. SiH3 radicals that form high-quality films are generated reducing the structural disorder, thus, the value of 1/f noise gets low [34].
4. Conclusion Boron doped hydrogenated mixed-phase hydrogenated silicon (BMP-Si:H) films are deposited using PECVD. The phase of hydrogenated silicon materials deposited by PECVD can be controlled by the plasma condition. The plasma conditions such as power density, B2H6/SiH4 and H2/SiH4 are varied and amorphous, mixed- and crystalline phases are confirmed. BMP-Si:H for which nano- or microcrystalline grains are embedded in the amorphous matrix. The electrical properties of BMP-Si:H, TCR, sheet resistance (Rsheet) and crystal volume fraction (Xc) are measured. TRC is around 1 ~ 3 %/K, Rsheet is around 3 ~ 61.4 MΩ/□ and Xc is around 7~17 %. The values of 1/f noise for various phases are measured. It is found that the 1/f noise of BMPSi:H is lower than that of the amorphous phase. For the same mixed-phase, lower 1/f noise is obtained with lower power density, i.e., lower electron temperature. Therefore, it can be said that BMP-Si:H films are a suitable material to be used as thermos-sensing layers of uncooled microbolometers.
Acknowledgments This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20163010012230) and Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT & Future Planning (NRF – 2016R1C1B1011508).
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Figures
(a)
(b) Fig. 1 (a) is Raman spectra when B2H6/SiH4 is 0.2, 0.5 and 0.8 with power density of 0.04 W/cm2. (b) The crystalline volume fraction (Xc) of boron doped hydrogenated mixed-phase hydrogenated silicon (BMP-Si:H) films is shown as a formation map in relation with power
density and B2H6/SiH4. Power density is 0.04, 0.07 and 0.21 W/cm2. B2H6/SiH4 is varied from 0.2 to 1.2.
Fig. 2 The sheet resistance of boron doped hydrogenated mixed-phase hydrogenated silicon (BMP-Si:H) films is shown as a formation map in relation with power density and B2H6/SiH4. The power density is 0.04, 0.07 and 0.21 W/cm2. B2H6/SiH4 is varied from 0.2 to 1.2.
Fig. 3 The temperature coefficient of resistance values (TCR) of boron doped hydrogenated mixed-phase hydrogenated silicon (BMP-Si:H) films is shown as a formation map in relation with power density and B2H6/SiH4. The power density is 0.04, 0.07 and 0.21 W/cm2. B2H6/SiH4 is varied from 0.2 to 1.2.
Fig. 4 Crystalline volume fraction (Xc) of boron-doped mixed-phase hydrogenated silicon (BMPSi: H) films with respect to changes in B2H6/SiH4 and H2/SiH4. The power density is 0.04 W/cm2.
Fig. 5 Temperature coefficient of resistance (TCR) of boron-doped mixed-phase hydrogenated silicon (BMP-Si: H) with respect to changes in B2 H6/SiH4 and H2/SiH4. The power density is 0.04 W/cm2.
Fig. 6 Sheet resistance (Rsheet) of boron-doped mixed-phase hydrogenated silicon (BMP-Si: H) with respect to changes in B2H6/SiH4 and H2/SiH4. The power density is 0.04 W/cm2.
Fig. 7 The correlation between temperature coefficient of resistance (TCR), crystalline volume fraction (Xc) and sheet resistance (Rsheet) of boron doped hydrogenated mixed-phase hydrogenated silicon (BMP-Si:H) films.
Fig. 8 Log-log plot of power spectra density for various phases at 4.5V
Highlights ● We reported boron doped hydrogenated mixed-phase silicon films ● The Rsheet and TCR are dependent on the plasma parameters ● The relationship between TCR and Rsheet is confirmed through the formation map ● For BMP-Si:H films, TRC is around 1~3%/K and Rsheet is 3~40 MΩ/□.
● 1/f noise of BMP-Si:H is smaller than that of the amorphous phase.
S1350-4495(18)30696-0 https://doi.org/10.1016/j.infrared.2018.10.015 INFPHY 2729
To appear in:
Infrared Physics & Technology
Received Date: Revised Date: Accepted Date:
19 September 2018 11 October 2018 11 October 2018
Please cite this article as: C. Shin, D. Phong Pham, J. Park, S. Kim, Y-J. Lee, J. Yi, Structure and electrical properties of boron doped hydrogenated mixed-phase silicon films for uncooled microbolometer, Infrared Physics & Technology (2018), doi: https://doi.org/10.1016/j.infrared.2018.10.015
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Structure and electrical properties of boron doped hydrogenated mixed-phase silicon films for uncooled microbolometer Chonghoon Shin1, Duy Phong Pham2, Jinjoo Park2*, Sangho Kim2, Youn-Jung Lee2 and Junsin Yi1,2*
1
Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea 2
College of Information and Communication Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea
________________________ *Corresponding author: 1. Jinjoo Park, E-mail: [email protected], Tel.: +82-31-290 7139, Fax.: +82-31-290 7139 2. Junsin Yi, E-mail: [email protected], Tel.: +82-31-290 7139, Fax.: +82-31-290 7139
Abstract Boron doped hydrogenated silicon films used as thermo-sensing layers in infrared detectors or uncooled micro-bolometers are prepared by radio-frequency plasma-enhanced chemical vapor deposition (PECVD). In this work, we investigated TCR (higher the better) and sheet resistance, Rsheet (lower the better), which are important factors for thermos-sensing layer used in uncooled microbolometer. The crystalline volume fraction (Xc) of films is controlled to get silicon films that satisfy the characteristics of high TCR and low Rsheet. Through the control, amorphous, mixed- and microcrystalline phases were identified. As a result, the best TCR and Rsheet were obtained in the mixed-phase. For the films, TRC is around 1 ~ 3 %/K, Rsheet is around 3 ~ 61.4 MΩ/□ and Xc is around 7~17 %. The 1/f noise is measured for various phases. It is found that 1/f noise of boron doped hydrogenated mixed-phase silicon (BMP-Si:H) is smaller than that of the amorphous phase. The results of BMP-Si:H films show that they are more suitable as thermossensing layers than boron doped hydrogenated amorphous silicon films.
Keywords: micro-bolometers; infrared detectors; boron doped hydrogenated silicon films; mixed-phase silicon films.
1. Introduction Hydrogenated silicon films have been much studied as they can be used for various devices such as solar cells and infrared detectors [1, 2]. Especially, boron doped hydrogenated silicon films with a high thermal coefficient of resistance (TCR) and moderate resistance are used as thermos-sensing layers for uncooled micro-bolometers [3-5]. Since the boron doped hydrogenated silicon films are deposited by plasma enhanced chemical vapor deposition (PECVD), it can be deposited uniformly in large areas. Although the boron doped hydrogenated amorphous silicon (a-Si:H) is used as a thermo-sensing layer in uncooled micro-bolometers, it has a high value of 1/f noise [6]. In recent years, polymorphous silicon, a nanostructure inclusion material, and silicon materials alloyed with germanium and carbide have been investigated extensively to reduce the 1/f noise [3, 4, 5, 7]. The amorphous, mixed- and microcrystalline phases of hydrogenated silicon films using PECVD can be easily obtained by controlling the plasma conditions [8, 9]. Since the phase can be shifted, the TCR and resistance can be controlled. Microcrystalline phase has low resistance but it is not suitable for the thermo-sensing layer due to the low TCR. The amorphous phase has high TCR but has high 1/f noise due to the high resistance. For the mixed phase, the microcrystalline phase and the amorphous phase are mixed. In other words, nano- or microcrystalline grains are embedded in the amorphous matrix. For this structure, the electrical properties are improved as the defect density is low and it is very stable [10]. It can be considered that boron doped hydrogenated mixed-phase silicon films (BMP-Si:H) are potential materials as hydrogenated silicon films with low 1/f noise while satisfying TCR and resistance as a thermo-sensing layer.
In this work, the power density, B2H6/SiH4 ratio and H2 /SiH4 which are parameters that affect the phase shift of the hydrogenated silicon films are studied. By varying the parameters, the formation region of BMP-Si:H is confirmed through the formation map. The phase shift of the films is characterized by Raman. The most important parameters for uncooled microbolometers, TCR, sheet resistance and 1/f noise are analyzed.
2. Experiment Boron doped hydrogenated mixed-phase hydrogenated silicon (BMP-Si:H) films were deposited by radio frequency plasma-enhanced chemical-vapor deposition (RF-PECVD), on Corning Eagle XG glass (5 cm × 5 cm, 0.63 mm-thick) substrates. The glass substrate was ultrasonically cleaned, by dipping in acetone, isopropyl alcohol, and de-ionized (DI) water for 10 min. BMP-Si:H films of a thickness of about 100 nm were deposited onto the glass. Mixtures of silane (SiH4), hydrogen (H2), and 1% diborane (B2H6) diluted in hydrogen were used as the source gases. The primary variation was the ratio of B2H6 to SiH4 gas flows in large range from 0.2 to 1.2, power density (0.04, 0.07 and 0.21 W/cm2) and the gas ratio of H2/SiH4 was 80, 160 and 200. The substrate temperature was 200 ℃ and the working pressure was 1.5 torr. For electrical measurements, Al coplanar electrodes (300 nm-thick) were evaporated on the samples deposited on the glass substrate. The electrical characteristics were studied by the coplanar method, using a programmable Keithley 617 electrometer, using samples grown on glass substrates. The activation energy (E a) was obtained from the temperature-dependent dark conductivity (σd), by the Arrhenius relation. The 1/f noise characteristics of the films have been
studied by using an Agilent 89410A vector signal analyzer. The crystallite of BMP-Si:H films was measured by Raman spectroscopy. The crystalline volume fraction (X c) was obtained as Xc = (I520 + I500)/(I520 + I500 + I480) where, I500 and I520 are attributed to the crystalline grain boundary and grains, and are the intensities of crystalline-like peaks, whereas, I480 is attributed to the disorder regions of the amorphous phase-like peak of the transverse optic Si-Si vibrations in the Raman spectra [11].
3. Result and Discussion Silicon materials are usually composed of hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (µc-Si:H) films. The materials for which nano- or microcrystalline grains are embedded in an amorphous matrix are known as mixed-phase hydrogenated silicon (MP-Si:H) films. MP-Si:H films through PECVD can be obtained by controlling the plasma parameters such as gas (SiH4 and H2), power density, pressure, substrate temperature and electrodes distance. By using a doping gas such as B2H6, p-type MP-Si:H films can be obtained [9]. There are two mechanisms for MP-Si:H films. The two mechanisms of deposition of plasma from SiH4 gas are surface diffusion [12, 13], and selective etching [14-16]. Both have been proposed to explain the deposition of µc-Si:H thin films. The surface diffusion was considered to improve the mobility of the deposition precursors, by many hydrogen atoms that impinge on the surface. For selective etching, amorphous and crystalline phases are assumed to be deposited simultaneously; atomic hydrogen impinges on the film surface, and the amorphous material is selectively etched, leaving behind a crystal film. Raman spectroscopy is the most used method for analyzing the phase of MP-Si:H. The Raman spectra signal at 480 cm-1
corresponds to the amorphous phase and the range of 500 – 520 cm-1 corresponds to the crystalline phase. Fig. 1 (a) shows Raman spectra when B2H6/SiH4 is 0.2, 0.5 and 0.8 with power density of 0.04 W/cm2. When B2H6/SiH4 is 0.2, 0.5 and 0.8, the crystalline volume fraction (Xc) is 62.4, 17.4 and 0 %, respectively. When B2H6 / SiH4 is 0.2, the crystalline phase peak at 510 cm-1 is sharp while B2H6/SiH4 is 0.8, the amorphous phase peak at 480 cm-1 is wide. As the B2H6/SiH4 increases, it shifts from the crystalline phase to the amorphous phase. This phase shift can increase the defect density of the films and may reduce the stability as well as electric properties [17]. Particularly, in the case of B2H6/SiH4 is 0.5, the amorphous peak 480 cm-1 is broadly present and the intensity is small, but crystalline phase peak at 500 cm-1 also exists. It is boron doped hydrogenated mixed-phase hydrogenated silicon (BMP-Si:H) [18]. Fig. 1 (b) shows the crystalline volume fraction (Xc) of the BMP-Si: H films as the formation map for the relationship between power density and B2H6/SiH4. As the power density increases, Xc increases. When B2H6/SiH4 is 0.7, Xc increases from 0% to 10.2% as the power density increases. With a high power density of 0.21 W/cm2, the crystalline phase is formed when B2H6/SiH4 is lower than 0.7 and amorphous phase appeared when B2H6/SiH4 is higher than 0.8. With a low power density of 0.04 W/cm2, the crystalline phase is formed when B2H6/SiH4 is lower than 0.6 and amorphous phase appeared when B2H6/SiH4 is higher than 0.7. When B2H6/SiH4 is 0.5 and 0.6, BMP-Si:H films are formed with Xc of 17.4 and 7.4%, respectively. At low power density, a number of H atoms produced by electron impact dissociation are annihilated by SiH4 molecules, and the surface hydrogen coverage at the surface was low. The growth precursors from the plasma directly reacted with crystalline phase or amorphous phase, thus leading to lower Xc. At high power density, the surface hydrogen coverage was increased,
due to increased hydrogen radicals in the plasma. Strong Si-Si bonds were formed, leading to higher Xc. As B2H6/SiH4 is increased, Xc is reduced. At any power density, X c is decreased from 60% to 0% as B2H6/SiH4 is increased. Increased B2H6/SiH4 reduced the hydrogen coverage at the surface, impeding the selective etching, and the structural deviation changed from microcrystallinity, towards amorphous phase [19]. Boron doped a-Si:H network, probably three-fold coordinated B atoms, introduced additional strain. The surface diffusion coefficient of SiH 3 radicals, which were assumed to be the growth precursors of BMP-Si:H, increased, causing more strained Si-Si, and structural defects. At higher B2H6/SiH4, some boron atoms may segregate at the grain boundary in electrically inactive sites, resulting in a decrease of electrical properties [20-22]. Fig. 2 shows sheet resistance of BMP-Si: H films as formation map of power density and B2H6/SiH4. The power density was 0.04, 0.07 and 0.21 W/cm2. B2H6/SiH4 is varied from 0.2 to 1.2. As B2H6/SiH4 is increased, sheet resistance (Rsheet) is increased. With a high power density of 0.21 W/cm2, Rsheet is 1 ~ 40 MΩ/□from 0.2 to 0.7 of B2H6/SiH4. It is over 40 MΩ/□ when B2H6/SiH4 is more than 0.7. With a low power density of 0.04 W/cm2, Rsheet is under 1 MΩ/□
when B2H6/SiH4 is lower than 0.4. When B2H6/SiH4 is 0.5 and 0.6, Rsheet is 3.4 and 56 MΩ/□, respectively. For both high and low power density, Rsheet is increased with increasing B2H6/SiH4. When B2H6/SiH4 is 0.6, Rsheet is increased as the power density decreased. It shows a similar trend with Fig. 1. For B2H6/SiH4 lower than 0.5 where X c is high i.e., crystalline phase, the value of Rsheet is low (< 3.4 MΩ/□). For B2H6/SiH4 higher than 0.8 where there is no Xc i.e., amorphous phase, Rsheet
is high (> 40 MΩ/□). It shows that the phase of films can determine the electric properties. Low Rsheet can be attributed to efficient carrier transport because of the crystalline phase of the films. Conversely, high Rsheet can be attributed to less efficient carrier transport, driven by the amorphous phase of the films. Undesirable high Rsheet of the thermos-sensing layer can cause a mismatch for the input impedance of the read-out circuits [23]. The temperature coefficient of resistance (TCR) is one of the important parameters that indicate the thermo-sensing ability of films. TCR is related to the activation energy (Ea) of the films. TCRs are defined by following formula: TCR ≈ E a/kT2, where k is the Boltzmann constant and T is temperature. For thermo-sensing layers, materials with high TCR is generally used. It means that a small variation in temperature results in a large change in resistance [24]. Fig. 3 shows the formation map of TCR of MP-Si:H films in relation with power density and B2H6/SiH4. It shows that TCR is dependent on B2H6 / SiH4 and power density. At a higher power density of 0.21 W/cm2, TCR is less than 1 %/K when B2H6/SiH4 is 0.2 ~ 0.5 and it is more than 2 %/K when B2H6/SiH4 is over 0.6. At a low power density of 0.04 W/cm2, TCR is about 1.0 %/K when B2H6 / SiH4 is 0.5 and it is more than 2.1 %/K when B2H6 / SiH4 is over 0.6. These results show the similar trend to that of Xc and Rsheet. At each power density, TCR is increased as B2H6/SiH4 is increased. For B2H6/SiH4 less than 0.5, the value of TCR is low (<1 %/K) even at high power density and it is high wend B2H6/SiH4 is over 0.6. It is confirmed that the TCR is low under the condition that the crystallization phase is dominant and that it has a high TCR when the amorphous phase is dominant. Fig. 4 shows the crystalline volume fraction (Xc) of BMP-Si: H films for changes in B2H6/SiH4 and H2/SiH4. The power density is 0.04 W/cm2. Xc is decreasing with increasing
B2H6/SiH4 in all H2/SiH4. When H2/SiH4 is 80, B2H6/SiH4 shows an amorphous phase above 0.4 and a mixed- and microcrystalline phase (Xc = 9 and 54.3%) below 0.4. As the H2/SiH4 increases from 80 to 200, the Xc increases at the same B2H6/SiH4 (except for B2H6/SiH4 above 0.7). The increase in H2/SiH4 is equal to the increase in hydrogen atom. The increase in the number of hydrogen atoms is due to the increase of Xc, which is caused by breaking of weak Si-Si bonding chaning to a strong one, by selective etching at the surfae causing the increase of crystallization (increase of Xc). In addition, the decomposition of H2 covers the growing film surface with hydrogen. This process enhances the radicals absorbed at the growing surface to have enough time to find their energetically suitable sites that decreased the strutural defect in the films [25]. For this reason, high H2 /SiH4 films with better quality can be structurely obtained even if they have similar Xc. Fig. 5 shows the temperature coefficient of resistance (TCR) of BMP-Si: H films for changes in B2H6/SiH4 and H2/SiH4. The power density is 0.04 W/cm2. When H2/SiH4 is 80, the TCR is increased from 0.41 to 7.15%/K as B2H6/SiH4 increases. The same trend is shown for high H2/SiH4. In particular, it is shown that H2/SiH4 decreases from 6.49 to 1.81 at the same B2H6/SiH4 (0.7). When H2/SiH4 is 80 (B2H6/SiH4: more than 0.4) and 160 (B2H6/SiH4: more than 0.7), high TCR (> 5%/K) is observed in the amorphous phase. When H2/SiH4 is 200, TCR is 1.81 and 3.57%/K. On the other hand, the condition of H2/SiH4 and B2H6/SiH4 with Xc shows low TCR (<3.42%/K). These results confirm that TCR is determined by the phase of silicon films, and high TCR can be applied as a thermo-sensing layer. Fig. 6 shows the sheet resistance (Rsheet) of BMP-Si: H films for changes in B2H6/SiH4 and H2/SiH4. The power density is 0.04 W/cm2. When H2/SiH4 is 80, as B2H6/SiH4 increases, Rsheet ranges from 0.09 159000 MΩ/□. H2/SiH4 show similar trends at 160 and 200 degrees. In
amorphous phase, high Rsheet of 58800 to 600000 MΩ/□ is seen, and microcrystalline phase
(when Xc is more than 20%) shows low Rsheet of less than 0.1 MΩ/□. On the other hand, the
mixed-phase (when Xc is 20% at 0%) shows 1.1 to 61.4 MΩ/□. It was confirmed that Rsheet was determined by the phase of silicon films. A low Rsheet value is required for use as a thermosensing layer, but a high TCR is also required. If we match the TCR and Rsheet shown in Fig. 5, it can be seen that it is a trade-off relationship. Silicon films with values that satisfy the condition are mixed-phase. Fig. 7 shows the correlation between TCR, Xc and Rsheet of BMP-Si:H films. It is confirmed that the characteristics of the films vary greatly depending on the power density and B2H6/SiH4. The amorphous phase is slightly different depending on the power density, but the B2H6/SiH4 is 0.7 or more, and the TRC is 5%/K or more. Although the TCR is high, Rsheet shows a high value of more than 105 MΩ/□. Conversely, the Crystalline phase shows a low Rsheet (<10 MΩ/□) at low B2H6/SiH4 (<0.5) but low TCR (<2%/K). Considering 1/f noise, the crystalline phase has a lower Rsheet than the amorphous phase, so electrical conduction or mobility is better, but TCR is more serious than the amorphous phase [7]. Thus, it is not suitable for the application of thermosensing layers. Mixed-phase has Rsheet of 3 ~ 40 MΩ/□ and TCR of 1~3 %/K. Xc is about 10 ~ 17%. These values are similar to those of boron doped hydrogenated amorphous silicon films (a-Si:H,B) used as a thermos-sensing layer for commercial uncooled micro-bolometers [23]. BMP-Si:H with
similar TCR and electrical properties to those of the existing a-Si:H,B can improve the 1/f noise value even lower [7, 26]. Conventionally, the 1/f noise value of a film can be expressed by Hooge’s experimental formula [27, 28]: Sv = αH.V2/NC.f , where Sv is the noise power density at voltage V, αH is a Hooge’s parameter, f is frequency and NC is the total number of charge carriers. 1/f noise is inversely proportional to the number of total free carriers and is proportional to Hooge’s parameter (αH) which is dependent on the quality of crystalline and mobility of carriers. A thermo-sensing layer with high carrier concentration and high crystalline volume fraction has low 1/f noise value [5]. Fig. 8 shows a log-log plot of power spectra density, which is averaged over 40 measurements, versus frequency for various phases at 300 K. The 1/f noise of mixedphase with Xc is lower than that of the amorphous phase. There is the difference in the 1/f noise values for the mixed-phase with similar Xc. As Rsheet is decreased from 21.1 MΩ/□ (power density: 0.62 W/cm2) to 3.4 MΩ/□ (power density: 0.04 W/cm2), the 1/f noise becomes smaller. At this time, TCR is 1.04 and 1.54 %/K, respectively. The structural disorder of amorphous phase arises from the coordination defects since the amorphous phase has a low density of films and the highest disorder compared to the mixed-phase [26]. The 1/f noise value of the amorphous phase is also affected by metastable defect creation [29]. On the other hand, since hydrogen radicals are increased to make mixed-phase films, the films undergo reformation and network rearrangement due to the increased surface hydrogen coverage. In other words, the strained Si-Si bonds which are defects of the amorphous matrix are changed to Si-Si bonds which are stronger, forming more energetically stable and ordered structure. Also, even in the same mixed-phase, the
1/f noise value varies depending on the plasma condition such as high and low power density. The electrons with energies between 8.75 and 9.47 eV produce SiH 3 radicals which are most favored for high-quality films. In other words, the electron temperature of the plasma should be low [30-33]. High-quality films can be formed with a low electron temperature for which SiH 3 radicals with long lifetime are mostly produced. At low power density, the electron temperature is low. SiH3 radicals that form high-quality films are generated reducing the structural disorder, thus, the value of 1/f noise gets low [34].
4. Conclusion Boron doped hydrogenated mixed-phase hydrogenated silicon (BMP-Si:H) films are deposited using PECVD. The phase of hydrogenated silicon materials deposited by PECVD can be controlled by the plasma condition. The plasma conditions such as power density, B2H6/SiH4 and H2/SiH4 are varied and amorphous, mixed- and crystalline phases are confirmed. BMP-Si:H for which nano- or microcrystalline grains are embedded in the amorphous matrix. The electrical properties of BMP-Si:H, TCR, sheet resistance (Rsheet) and crystal volume fraction (Xc) are measured. TRC is around 1 ~ 3 %/K, Rsheet is around 3 ~ 61.4 MΩ/□ and Xc is around 7~17 %. The values of 1/f noise for various phases are measured. It is found that the 1/f noise of BMPSi:H is lower than that of the amorphous phase. For the same mixed-phase, lower 1/f noise is obtained with lower power density, i.e., lower electron temperature. Therefore, it can be said that BMP-Si:H films are a suitable material to be used as thermos-sensing layers of uncooled microbolometers.
Acknowledgments This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20163010012230) and Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT & Future Planning (NRF – 2016R1C1B1011508).
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Figures
(a)
(b) Fig. 1 (a) is Raman spectra when B2H6/SiH4 is 0.2, 0.5 and 0.8 with power density of 0.04 W/cm2. (b) The crystalline volume fraction (Xc) of boron doped hydrogenated mixed-phase hydrogenated silicon (BMP-Si:H) films is shown as a formation map in relation with power
density and B2H6/SiH4. Power density is 0.04, 0.07 and 0.21 W/cm2. B2H6/SiH4 is varied from 0.2 to 1.2.
Fig. 2 The sheet resistance of boron doped hydrogenated mixed-phase hydrogenated silicon (BMP-Si:H) films is shown as a formation map in relation with power density and B2H6/SiH4. The power density is 0.04, 0.07 and 0.21 W/cm2. B2H6/SiH4 is varied from 0.2 to 1.2.
Fig. 3 The temperature coefficient of resistance values (TCR) of boron doped hydrogenated mixed-phase hydrogenated silicon (BMP-Si:H) films is shown as a formation map in relation with power density and B2H6/SiH4. The power density is 0.04, 0.07 and 0.21 W/cm2. B2H6/SiH4 is varied from 0.2 to 1.2.
Fig. 4 Crystalline volume fraction (Xc) of boron-doped mixed-phase hydrogenated silicon (BMPSi: H) films with respect to changes in B2H6/SiH4 and H2/SiH4. The power density is 0.04 W/cm2.
Fig. 5 Temperature coefficient of resistance (TCR) of boron-doped mixed-phase hydrogenated silicon (BMP-Si: H) with respect to changes in B2 H6/SiH4 and H2/SiH4. The power density is 0.04 W/cm2.
Fig. 6 Sheet resistance (Rsheet) of boron-doped mixed-phase hydrogenated silicon (BMP-Si: H) with respect to changes in B2H6/SiH4 and H2/SiH4. The power density is 0.04 W/cm2.
Fig. 7 The correlation between temperature coefficient of resistance (TCR), crystalline volume fraction (Xc) and sheet resistance (Rsheet) of boron doped hydrogenated mixed-phase hydrogenated silicon (BMP-Si:H) films.
Fig. 8 Log-log plot of power spectra density for various phases at 4.5V
Highlights ● We reported boron doped hydrogenated mixed-phase silicon films ● The Rsheet and TCR are dependent on the plasma parameters ● The relationship between TCR and Rsheet is confirmed through the formation map ● For BMP-Si:H films, TRC is around 1~3%/K and Rsheet is 3~40 MΩ/□.
● 1/f noise of BMP-Si:H is smaller than that of the amorphous phase.