In Situ Bipolar Doping via Mechanically Controlled Dipole Under Water

Polycrystalline silicon (poly‐Si) is essential in integrated circuits and microelectromechanical systems. In addition, poly‐Si is gaining attention for next‐generation display research with high thermal conductivity, stability, and versatile applications. Conventional fabrication methods for doping patterns involve complex lithography and chemical usage, which have raised environmental concerns. The study of novel methods is necessary for environmental friendliness and a significant simplification of the manufacturing processes. This study introduces a novel bipolar work function control technology utilizing deionized water (DI‐W) and nanonewton‐scale mechanical force using an atomic force microscope. The method is implemented with a mechanically induced SiOx layer on poly‐Si in DI‐W. The induced Si─OH and Si─O bonds decreases the work function, whereas a thicker SiOx layer with a high oxidation state increases the work function. Based on the magnitude of the applied force (26.73–75.24 nN) and additional DI‐W immersion, the induced bond and thickness of the SiOx layer are controlled. Therefore, bipolar work function control is achieved in the range of −0.25–+0.103 eV. In addition, the electrical characteristics of the fabricated p‐ and n‐type poly‐Si diodes are investigated. This method is eco‐friendly and enables bipolar doping patterns in a single process with high efficiency.


Introduction
Polycrystalline silicon (poly-Si) is widely used in integrated circuits as a gate layer and in micro-electromechanical systems as a thin film. [1]Recently, low-temperature poly-Si (LTPS) has garnered significant attention as a channel layer for next-generation displays.This material is a potential alternative to metal-oxide DOI: 10.1002/admi.202301092[4][5] Furthermore, thinfilm transistors (TFTs) utilizing LTPS exhibit a high carrier mobility of 50-100 cm 2 V −1 s −1 .This high mobility enables faster response times and the ability to use higher currents compared with those in metal-oxide TFT devices with the same area.In addition, LTPS can be applied to flexible substrates, offering high mobility, driving capability, and stability.][9][10] However, the implementation of a carrier gradient or control of carrier transportation faces various problems when only undoped intrinsic silicon is used.For the industrialization of silicon, doping is a significantly necessary process for effectively controlling the density of charge carriers.For example, p-or n-doped poly-Si has the potential to achieve highly efficient solar cells and streamlined cell processes when used as various passivation contacts. [11,12]However, despite the high level of interest in poly-Si, research on novel processes for doping in local areas is limited.[15][16][17] These doping processes serve as a foundation for the precise manipulation of the Si work function. [18][21] This approach has its limitations, such as a high level of process complexity, the need for vacuum facilities, and extreme environmental control. [22,23]Additionally, the severity of the environmental issues that emerge during the doping and patterning processes reaches a non-negligible level.[26] Hence, research into technologies that are not only environmentally friendly but can also simplify semiconductor-manufacturing processes are required.
In this study, we introduce a bipolar work function control technology called mechanical force-based bipolar doping (MFB doping).The MFB-doping method relies solely on deionized water (DI-W) and nanonewton-scale mechanical force and uses the atomic force microscope (AFM).AFM is widely employed as a versatile and easily accessible tool.[29][30][31][32] Some studies have used an electric field to fabricate doping patterns using AFM. [33,34]This doping method based on an electric field has some disadvantages such as corrosion during a wet process, a limitation of the device thickness, and the need for different types of electrodes to induce p-and n-type doping. [35,36]However, the developed MFB doping method can significantly simplify complex doping processes and is environmentally friendly without relying on electric fields.To evaluate the doping effect, we measured the work function changes of poly-Si using the Kelvin probe force microscope (KPFM) mode of atomic force microscope.The MFB doping method exhibited bipolar work function control where the work function variation was from −0.250 to +0.103 eV.Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analyses revealed that the Si─O/─OH bond was reduced and a thick SiO x layer with a high SiO 2 content increased the work function of poly-Si.The p-and n-type poly-Si diodes were implemented using the MFB-doping method.The increase and decrease in the current in the diodes were controlled by the magnitude of the applied mechanical force.Therefore, the MFB-doping method offers the capability of implementing bipolar doping patterns in a single process by utilizing only DI-W.Furthermore, this approach results in a remarkable enhancement in process efficiency and ecofriendliness.The simple doping procedure and unique functionality of bipolar doping provide great compatibility with the current silicon-based industry compared to the conventional doping process.

Results and Discussion
In this section, the results and analysis of the MFB-doping process are discussed.In Section 2.1, the mechanism of the MFB-doping method is elucidated.Using the MFB-doping method, the work function change by the various magnitudes of force and immersion in DI-W is investigated in Section 2.2.To analyze the different work function changes, the transmission electron microscopy analysis on poly-Si cross-sections is conducted in Section 2.3.In addition, X-ray photoelectron spectroscopy analysis is investigated on the surface of poly-Si to analyze the formed bonds and layers.Lastly, the electrical characteristics of poly-Si diode fabricated by the MFP-doping method are investigated in Section 2.5.

Mechanism of Bipolar Work Function Control
MFB-doping is a technique that precisely controls the work function of poly-Si by applying a mechanical force in DI-W.Two types of MFB-doping processes exist n-type doping (MFN-doping) and p-type doping (MFP-doping) process.As depicted in Figure 1, the application of a mechanical force in DI-W induces Si─OH or Si─O bonds on the surface, resulting in a reduction of the work function on poly-Si via MFN-doping process.On the other side, MFP-doping occurs when the material is kept in the DI-W after applying the mechanical force.This process promotes the growth of an oxidation layer in the force-applied region, thereby increasing the work function.Therefore, the MFB-doping method implements doping using only mechanical force and DI-W in a single process.This approach is highly advantageous because of its simplicity and environmental friendliness compared to conventional doping methods.In this study, the mechanical force was applied to selected areas using the tapping mode of the AFM, exerting small forces on the order of tens of nanonewtons.The magnitude of the applied force was controlled by adjusting the drive of the AFM cantilever using a silicon tip, and it was estimated based on Equation (1), where k represents the spring constant, Q denotes the quality factor, and A 0 and A sp are the free and set point amplitudes of the AFM cantilever, respectively. [37]

MFB-Doping Process on Poly-Si in DI-W Using AFM
The proposed MFB doping method can be defined as a kind of remote doping method.Few researchers have explored inducing n-type doping using a hydroxyl group at the surface.The mechanical force was applied using the tapping mode of the AFM, which involved two actions: up-and-down tapping of the hammer and horizontal movement of the cantilever.This horizontal movement applied a force to the surface irregularities or asperities, inducing material removal.Furthermore, the motion of the AFM tip can aid in cleaning surface debris, leading to a reduction in roughness. [38]Scanning electron microscopy (SEM) images of the new and used tip after applying a force to a 300 × 300 μm 2 region of poly-Si are shown in Figure S4 (Supporting Information).Figure 2c shows p-and n-type doped 10 × 10 μm 2 square patterns on a poly-Si surface, which were obtained via the MFB-doping process.This indicates that p-and n-type doping processes can be conducted simultaneously in situ.Unlike conventional doping processes that require separate doping procedures for different types of doping, efficient process simplification is possible with the proposed MFB-doping process.Figure 2d  Mechanical forces alone cannot lead to changes in the air environment.[41] Consequently, the work function change in poly-Si is a result of the synergistic combination of extremely small nanonewton-scale mechanical forces and DI-W.

Transmission Electron Microscopy Analysis of SiO x Layer Formation
To reveal the proposed doping mechanism in further detail, we observed the surface variation in terms of morphology and atomic coordination via TEM cross-sectional analysis.Specifically, the poly-Si cross-sections were analyzed using TEM before and after immersion for two specific cases: the largest work function decrease, which occurred when a force of 47.41 nN was applied without dipping, and the most significant work function increase observed after dipping, which resulted from an applied force of 75.24 nN.The cross-sectional TEM image and energy dispersive X-ray spectroscopy (EDS) line spectrum of the force applied region with 47.41 and 75.24 nN without immersion in DI-W are shown in Figure 3a,b, respectively.Similarly, the crosssectional TEM image and EDS line spectrum of the force applied region with 47.41 and 75.24 nN after 1 h of dipping in DI-W were also examined as shown in Figure 3c,d, respectively.First, a SiO x layer was observed in all cases.As mentioned previously, this SiO x layer formation on the poly-Si surface was induced by the reaction with DI-W owing to the applied mechanical forces.The smaller force (47.41 nN) formed a thinner SiO x layer on the poly-Si surface compared with that formed by the larger force (75.24 nN) as shown in Figure 3a,b.When comparing the EDS line spectra, a higher O-to-Si ratio was observed in the region where the larger force was applied.This indicated that a larger mechanical force promotes a more substantial SiO x layer formation on the poly-Si surface.In the case of MFP-doping with the same mechanical force, the obtained SiO x layer was thicker than the SiO x layer generated without DI-W immersion, as shown in Figure 3c,d.The growth rate of the SiO x layer was extremely low at the initial stage but increased rapidly as the SiO x layer became thicker under DI-W. [42,43]Initially, when the larger force (75.24 nN) was applied, the SiO x layer formed before immersion was more than twice as thick as the SiO x layer formed with a force of 47.41 nN.After immersion, a significantly thicker SiO x layer of ≈7.48 nm was formed on the region subjected to the larger force, as shown in Figure 3d; however, the SiO x layer only grew from 1.01 to 2.19 nm (Figure 3c).Therefore, the thickness of the SiO x layer is considered an important factor in the work function modulation between MFN and MFP-doping.Based on our previous results, we confirmed that exerting a mechanical force on the poly-Si surface under DI-W can activate the surface, generating several oxidation bonds (Si─O bonds) and hydroxyl groups (Si─OH bonds). [39,41]The formed surface bonds strongly influence the work function of poly-Si owing to their strong polarity.Many studies have been conducted to explain the polarity of the Si─O/Si─OH bond and work function control.Hadi et al. conducted density functional theory studies on Si and demonstrated that changes in the work function occurred owing to the coverage of ─OH groups on the surface.The ─OH groups possess strong polarity, which can increase the dipole moment, resulting in a decrease in the work function. [44] This phenomenon is related to the Si─H groups, which form a negative dipole and increase the work function.When Si─OH and Si─O bonds are formed on the Si surface, they replace the Si─H bonds, resulting in a decrease in the work function. [45]oreover, the charge-sufficient SiO x layer can provide a large amount of formation position for the OH groups, promoting the MFN doping effect.However, since the thickness of SiO x is only 1 nm at maximized MFN-doping conditions, characteristic of the doped poly-Si is more dominant in the KPFM result.Reversely, after the MFP doping, the thick SiO 2 layer is expected to screen the dipole effect of the surface bond to the poly-Si, diminishing the reduction in the poly-Si work function.

X-Ray Photoelectron Spectroscopy Analysis on SiO x Layer
XPS analysis was conducted to investigate the formed Si─O/─OH bonds and SiO x layer based on the magnitude of the applied mechanical force and the utilization of an additional DI-W immersion as shown in Figure 4. To analyze only the localized changes on the force-applied region, the mechanical force was applied on the marked region within a 150 × 150 μm 2 area to carry out the doping process.An XPS apparatus with a 100 μm spot size was used to examine the localized surface compositional changes.Mechanical forces of 47.41 and 75.24 nN were applied during MFN-doping.The Si 2p and O 1s spectra (insets) are shown in Figure 4a,b, respectively.The deconvolution results show a Si elemental peak (2p 3/2, 2p 1/2 peak), the suboxide peaks related to SiO x (x < 2, Si 1+ , Si 2+ , Si 3+ ), and a fully bonded SiO 2 peak (Si 4+ ). [46]As shown in Figure 4a, when a mechanical force of 47.41 nN was applied to the poly-Si, both the suboxide and full oxidation peaks were observed, and their peak intensities appeared similar.When the applied mechanical force was increased to 75.24 nN, the suboxide peak intensity decreased, whereas the full oxidation peak intensity increased.This trend was also observed in the O 1s XPS spectrum, which was fitted to three peaks (corresponding to SiO 2 , SiO x , and Si─OH bond). [47]As the applied mechanical force increased, a noticeable decrease in the SiO x peak was evident compared with the SiO 2 peak.In Figure 4c,d, the XPS analysis was conducted on poly-Si after the MFP doping process with 47.41 and 75.24 nN forces.In comparison with the results in Figure 4a,b for the Si 2p oxidation peaks, the size of the Si 4+ peak relatively increased relatively after the dipping even when the same mechanical force was applied.However, the sizes of the Si 1+ , Si 2+ , and Si 3+ peaks showed an overall decrease.Similarly, in the O 1s XPS spectra, the size of the SiO x peak was noticeably decreased compared to the sample after the MFN-doping process under the same magnitude of applied force.Comparing Figure 4c,d, a larger Si 4+ peak was observed when a stronger mechanical force was applied.For a quantitative comparison of the deconvoluted peaks variation corresponding to different oxidation states under the experimental conditions, the composition ratios of the O1s and Si2p XPS spectra were plotted, as shown in Figure 4e,f.As shown in Figure 4e, the SiO 2 peak ratio increased from 17.03% to 21.15% when an additional DI-W dipping process was conducted after the application of the mechanical force (47.41 nN).However, the SiO x peak ratio remained almost unchanged.Furthermore, as the magnitude of the applied mechanical force increased from 47.41 to 75.24 nN during the MFP-doping process, the SiO 2 peak ratio increased from 21.15% to 24.19%, and the SiO x peak ratio decreased from 12.77% to 8.58%.Consequently, the SiO 2 peak ratio tended to increase relative to the SiO x peak ratio with an increase in the magnitude of the applied force and additional DI-W immersion.This trend is also observed in the O1s XPS spectra shown in Figure 4f.Therefore, as the applied mechanical force increased and an additional dipping treatment was performed, the SiO 2 peak ratio increased with a decrease in the SiO x peak ratio.
In previous studies, the role of a high Si oxidation state in the doping effect has been described as follows.Goldstein et al. showed that the sign of the work function variation changed from negative to positive as the Si oxidation progressed.The initial decrease in the work function was attributed to the occupation of the first "cave" site by oxygen during adsorption.Subsequent oxygen adsorption filled the cave sites, leading to an increase in the work function. [48]Ghosalya et.al reported that the dissociation of Si into Si─O bonds, which have a Si 2+ oxidation state, decreased the work function.In addition, continuous oxidation resulted in bulk Si oxidation with a high oxidation state (Si 3+ and Si 4+ ) leading to an increase in the work function. [49]Furthermore, the work function of the poly-Si surface increases with the increasing SiO x layer thickness.This phenomenon was caused by the interface trapping of the formed SiO 2 layer. [50]In this context, our results are in good agreement with previous results.The Si 2p spectrum, O 1s spectrum (XPS result), and Si oxidation layer thickness variation during immersion (TEM result) indicated that the SiO 2 /SiO x ra-tio increased during the DI-W immersion process.According to Finster et al., during the formation of an oxide layer on the Si surface, when the thickness is less than 1 nm, a SiO x layer is formed, and as the thickness increases, a SiO 2 layer is developed. [51]Because the dipole effect derived from the Si─O/Si─OH bond in the SiO x layer and surface is weakened with an increase in the SiO 2 layer thickness, the work function of poly-Si is bent upward upon contact with the silicon oxide layer.The details are shown in Figure S6 (Supporting Information).[54] In this regard, the proposed doping method is actually very close to kind of remote doping method rather than the conventional ion implementation method.Based on the proposed mechanism, the trend shown in Figure 2d can be precisely elucidated.When only small amounts of the oxidation layer were deposited on poly-Si with an applied mechanical force of 26.73-47.41nN, the Si─O/Si─OH bond in the SiO x layer increased, and a considerable decrease in the work function was observed.Moreover, with MFP-doping, the SiO 2 thickness increased simultaneously.A small decrease in the work function was confirmed.However, when the intensity of the mechanical force was over 47.41 nN, both MFN and the MFP-doping showed an increase in the work function owing to a sufficiently thick SiO x layer to screen the dipole effect.

Electrical Characteristics of Poly-Si Diode
To validate the effect of the MFB-doping process, we analyzed the I-V curve of the poly-Si diode according to the type of doping, as shown in Figure 5a.MFB-doping was conducted between the source and drain electrodes, which were horizontally deposited with a width of 10 μm. Figure 5b shows the electrical characteristics of the p-doped poly-Si and pristine Si diodes.Three different poly-Si diodes were prepared using the MFP-doping process and subjected to mechanical forces of 0, 66.01, and 75.24 nN.As the force intensity increased, at V D = 5 V, a gradual decrease was observed in the current value from 20.7 to 6.3 nA.Conversely, in Figure 5c, when mechanical forces of 0, 35.89, and 47.41 nN were applied to the Si diode in the MFN-doping process, the current exhibited a gradual increase from 19.5 to 100.3 nA.This tendency verified that the MFB-doping process enables precise control of semiconductor characteristics and device performance.Additionally, the symmetric current behaviors at both the MFP and MFN doping processes indicate that the channel area has uniformly doped via the proposed doping method.To confirm the feasibility of the proposed doping process, we fabricated the intrinsic/ptype Si homojunction (PIJ) and intrinsic/n-type Si homojunction (NIJ), as shown in Figure S6 (Supporting Information).A homojunction was prepared with the MFP-doping process on a 200 nm thick poly-Si layer.The vertically structured homojunction was probed using CAFM and exhibited an evident rectification curve.The drain current of the pristine Si diode exhib- ited a positive linear relationship with the drain voltage.Moreover, the PIJ diode showed a typical rectification curve, exhibiting a smaller offset drain voltage (V off ) of 1 V than the breakdown voltage (V break ) = −1.5 V shown in Figure 5d.Furthermore, V off was larger than V break in the case of the NIJ diode, with values of 0.6 and −0.3 V, respectively, shown in Figure 5e.In addition, the overall current level of the NIJ diode was higher than that of the PIJ diode at the same drain voltage.We propose that the band alignment explains the difference in the voltage parameter and current level between the PIJ and NIJ diodes, as shown in Figure 5f,g.
In the case of the MFP-doping process, the dipole generated between the poly-Si and SiO 2 layers increased the hole concentration in the poly-Si layer, consequently, bending the conduction and valence bands in the upward direction, as shown in Figure 5f. [55,56]Under a positive drain voltage, the carriers were transported through the tunneling between the valence band of the p-doped poly-Si and the conduction band of the poly-Si, showing a small V off value.However, under a negative drain voltage, V break was confirmed to have a larger value than V off , because a higher voltage bias was needed to overcome the built-in potential between the intrinsic and p-doped poly-Si layers.
When only mechanical force was applied to poly-Si without additional exposure to DI-W, the thin SiO x layer was insufficient to screen for the dipole effect of Si─O and Si─OH at the surface.According to the dipole effect of Si─O and Si─OH, the electron concentration increased, and the work function decreased, as shown in Figure 5g.In this case, the thermionic current prevailed across built-in potential in the positive bias state, and a tunneling current generated throughout the tunneling window between the valence band of the poly-Si and the conduction band of the n-doped poly-Si when a negative drain voltage was applied.Additionally, the overall current of the NIJ diode was higher than that of the PIJ diode because the thin SiO x layer was more penetrable.The CAFM results and electrical characteristics of the fabricated diodes perfectly reflected the effect of the MFB-doping method and verified their feasibility.

Conclusion
In this study, a novel bipolar work function control technology was developed to overcome the complex lithography techniques, chemical usage, and environmental concerns associated with conventional fabrication methods for doping patterns.A bipolar work function control was achieved in variation from −0.25 to +0.103 eV by controlling the magnitude of the mechanical force using AFM and DI-W immersion.A nanonewton-scale mechanical force from 26.73 to 75.24 nN was applied to induce a SiO x layer with Si─OH and Si─O bonds.The induced Si─OH and Si─O bonds formed dipole moments, which decreased the work function.In addition, additional DI-W immersion grew the SiO x layer with SiO 2, which caused an interface trap resulting in an increase in the work function of the force-applied region on the poly-Si surface.The TEM and XPS analyses confirmed the formation of Si─O and Si─OH bonds on the poly-Si when the work function was decreased by applying a force in DI-W.In addition, the growth of a SiO x layer with a high oxidation state and SiO 2 was confirmed when the work function increased with the application of a mechanical force and additional dipping treatment.Moreover, we verified the feasibility of the proposed reconfigurable MFB-doping process by modulating the electrical characteristics of the channel materials in the poly-Si diodes.Based on the magnitude of the mechanical force, the current increased in the directly dried sample, whereas it decreased in the sample dipped in DI-W.The results confirmed the n-/p-doping effect, which was caused by the mechanically induced bonds (Si─O and Si─OH) and the growth of the SiO 2 layer, respectively.The performance of the novel doping process was confirmed with p-and n-doped poly-Si diodes, representing a current decrease (from 20.7 to 6.3 nA) and a current increase (from 19.5 to 100.3 nA), respectively.Consequently, the MFB doping method can be applied across various fields, offering a high process efficiency enhancement and an environmentally friendly approach.

Experimental Section
AFM Fabrication: An undoped poly-Si wafer with 200 nm thickness was used.Surface fabrication on poly-Si was conducted using AFM (NX10, Park Systems Co., Republic of Korea) in DI-W.A silicon-based probe, NCHR, was used for fabrication.The nominal resonant frequency of the tip was 320 kHz with a force constant of 42 N m −1 .The scan rate for the fabrication of all the 10 × 10 μm 2 regions was 1.0 Hz.
Characterization: The work functions of poly-Si were measured using the KPFM mode of AFM with a gold-based probe, NCSTAu.In addition, the CAFM mode of AFM was used to obtain the I-V characteristics of the poly-Si diode.In the CAFM mode, a CDT-CONTR which was an electrically conductive diamond-coated probe, was used.XPS (NEXSA, Thermo Fisher Scientific Co., USA) was used to investigate the surface variations of the poly-Si.The electrical properties of the MFB-doped poly-Si diodes were characterized using a semiconductor characterization system (SCS, 4200-SCS, Tektronix Inc., USA).FIB (JIB-4601F, JEOL, Japan) and high-resolution TEM (JEM ARM 200F, JEOL Co., Japan) were used to obtain the crosssectional images of the wafer.A field-emission SEM (JSM-IT800, JEOL Co., Japan) was used to investigate the AFM tip.

Figure 1 .
Figure 1.Schematic of bipolar work function control technology through mechanically induced bonds in DI-W using AFM.

Figure 2 .
Figure 2. Work function image of the poly-Si surface after five different magnitudes of mechanical forces (35.89, 47.41, 57.08, 66.01, and 75.24 nN) were applied in DI-W a) without additional DI-W immersion and b) after 1-h of DI-W immersion.c) Work function image of the fabricated p-and n-type square patterns with the in situ doping process.d) Work function change of (a) and (b) by a mechanical force of 26.73 nN.The work function values are listed in TableS1(Supporting Information).
shows the work function reduction trend of poly-Si during the variation in the intensity of the mechanical force and the presence of the additional immersion step.The work function values shown in Figure 2d are listed in Table S1 (Supporting Information).Also, the work function change over time in an exposed state to the atmosphere was investigated which showed good reliability.The results of poly-Si fabricated by the MFN and MFP-doping process with 47.41 and 75.24 nN are shown in Figure S5 (Supporting Information).As shown in Figure 2a,b, the work function reduction difference between MFN and MFP-doping at the same force gradually increased in proportion to the applied force intensity.The largest work function change shows a variation from −0.191 to +0.103 eV with a force of 75.24 nN.The observed work function variation is considered to be caused by the applied mechanical force and surrounding DI-W.

Figure 3 .
Figure 3. Cross-sectional TEM image and EDS line spectrum of the fabricated region of poly-Si.Results obtained after the MFN-doping process on poly-Si with a) 47.41 nN and b) 75.24 nN.Results obtained after the MFP-doping process on poly-Si with c) 47.41 nN and d) 75.24 nN.

Figure 4 .
Figure 4. Results of XPS analysis conducted on the doping processed region (150 × 150 μm 2 area).Results of poly-Si after MFN-doping process with a) 47.41 nN and b) 75.24 nN.Results of poly-Si after the MFP-doping process with c) 47.41 nN and d) 75.24 nN.The XPS analysis was performed with a 100 μm spot size.(x < 2 in SiO x ).

Furthermore,
Yang et al. investigated the reduction in the work function of Si through the formation of Si─OH and Si─O bonds.

Figure 5 .
Figure 5. Electrical characteristics of the poly-Si diode doped with the proposed MFB method.a) Schematic of the poly-Si diodes with p-type and n-type doping.b) Current behavior of the p-doped poly-Si diode fabricated using the MFP-doping process with 66.01 and 75.24 nN.c) Current behavior of the n-doped poly-Si diode fabricated using the MFN-doping process with 35.89 and 47.41 nN.CAFM voltage sweeping results of d) pristine poly-Si and p-doped poly-Si, e) pristine poly-Si and n-doped poly-Si.Band alignment of the f) PIJ and g) NIJ diodes.