Issue
Eur. Phys. J. Appl. Phys.
Volume 88, Number 3, December 2019
Disordered Semiconductors: Physics and Applications
Article Number 30102
Number of page(s) 6
Section Semiconductors and Devices
DOI https://doi.org/10.1051/epjap/2020190298
Published online 19 February 2020

© EDP Sciences, 2020

1 Introduction

As it is known, in each pixel of the existing active matrix organic light emitting diode (AMOLED) display systems the light emitting part (OLEDs) and electronic devices (thin film transistors (TFTs)) are produced from separate materials, resulting in a hybrid pixel characteristic. In other words, TFTs are produced from Si-based materials while LEDs make use of organics, since organic LEDs have superior radiative properties over Si-based LEDs. However, if the radiative properties of Si-based LEDs are improved, high performance, easy to manufacture and cheap monolithic optoelectronic systems can be obtained. With this idea in mind, we first produced Si-based LED by plasma enhanced chemical vapor deposition (PECVD) technique, also used for TFT fabrication, so that it will be possible to manufacture both LED and TFT (i.e. the whole pixel) in a single PECVD cycle for monolithic display purpose. The produced LED structure (glass/Cr/p+-nc-Si:H/i-SiNx:H/n+-nc-Si:H/ITO (Fig. 1)) was then subjected to a high forward voltage stress for one time only (Fig. 2). At this step, the diode experiences a type of soft electrical breakdown with a sudden increase in the current density at random filamentary regions where the local temperature sharply increases due to the Joule effect [1]. This procedure, the so-called electroforming process, was previously patented by our research group [2]. The importance and novelty of this LED production method lies in the outstanding properties that LED gained after being electroformed: the intensity of the emitted light increased by ∼30 times [3] and reached a visible level with the naked eye (see inset of Fig. 2), thus offering a great promise for monolithic optoelectronic systems; at the same time, since this electroformed LED exhibits the memory effect [4], it is a suitable candidate for optical memory devices with high speed and capacity. Since these outstanding characteristics are obtained owing to the electroforming process applied to the diode after its thin film production is over, the physical changes occurring in the diode structure during the electroforming are of scientific importance. The exact determination of the factors that enhance the performance characteristics of this LED is of great significance in determining the conditions under which the diode should be produced in order to further increase the light intensity of the LED.

In the literature up to now different indirect characterization tests were conducted in order to understand the physical changes occurring in the Si-based LEDs during the electroforming process [58]. Reports on the electroformed a-Si:H homojunction pin diodes include Raman measurements, by which the authors concluded that there was increase in the local temperature during the electroforming process and it in turn lead to the crystallization of a-Si:H into nc-Si islands [6,7]. Similar Raman measurements were obtained for polymorphous silicon carbon (pm-SiC:H) homojunction pin diode, which exhibited a strong orange-red light emission after electroforming [8]. The authors of this work also examined previously in details the electroformed SiN-based pin diode via indirect tests including temperature-dependent current-voltage and electroluminescence measurements [5], and as a result a model has been proposed to explain the changes taken place in the diode caused by the electroforming process [5]. This model includes the formation of a few nanometer sized Si nanocrystallites in the intrinsic SiN layer of the diode due to the electroforming process. However, the characterization tests which the proposed model is based on are indirect, therefore in order to determine directly the effects of the electroforming on the LED structure, the cross-section of the electroformed diode was imaged with the direct method, i.e. transmission electron microscope (TEM). Since electroforming process caused local heating within the diode and the crystallization of ITO layer (verified previously by XRD measurements [3]) accompanied by ITO breakup in some parts of the diode surface, the TEM analysis was conducted for both the cross-sections that include ITO layer and those that do not have ITO layer above n+-nc-Si:H film. So, in this work we report for the first time the evidence of the nanocrystallization which occurs during electroforming in SiN-based pin diodes by using the direct TEM imaging of the intrinsic layer.

thumbnail Fig. 1

Cross-sectional scanning electron microscope (SEM) image of the plasma grown electroformed a-SiNx:H heterojunction pin diode.

thumbnail Fig. 2

Current density versus time plot of the a-SiNx:H heterojunction pin diode under a constant forward voltage stress of 12 V. Arrows at the beginning of the measurement and at about 400 s correspond to the times at which electroforming starts and mostly ends, respectively. The inset photograph is taken by the camera of a standard mobile phone during electroformed diode's light emission under application of forward bias. The bright circle under the probe corresponds to the contacted top ITO electrode of the electroformed LED, whereas the dark circles are uncontacted top ITO electrodes of the other separate LED structures (mesas). The corresponding electroluminescence (EL) spectrum is also provided in the inset.

2 Experimental

The three active layers (p+-, i- and n+-) of SiN-based diodes (glass/Cr/p+-nc-Si:H/i-SiNx:H/n+-nc-Si:H/ITO (Fig. 1)) were grown in a capacitively-coupled PECVD chamber at 13.56 MHz on Cr-coated glass substrate in a single PECVD cycle. The optimized deposition conditions of the p+- and n+-nc-Si:H doped layers of ∼90 nm thicknesses, are described elsewhere [3,9]. These doped layers are responsible for the carrier injection into the luminescent intrinsic a-SiNx:H layer. Sandwiched between doped layers, a-SiNx:H layer of thickness ∼30 nm was deposited using 40 sccm of NH3, 10 sccm of SiH4, chamber pressure of 0.5 Torr, RF power density of 22 mW/cm2 and substrate temperature of 523 K. Under these deposition conditions, the x-value in the a-SiNx:H film was previously estimated to be 0.94 [5,10]. ∼250 nm thick indium tin oxide (ITO) dot contacts of 1 mm diameter were then coated above n+-nc-Si:H doped layer as a window electrode through a shadow mask by sputtering at room temperature. Finally, the diodes were separated from each other by reactive ion etching of n+-, i- and p+-layers to form mesa structures (inset of Fig. 2). Other details of SiN-based diode production conditions can be found in reference [11]. The as-grown mesas, i.e. diodes, were then electroformed by applying a high forward voltage stress (12 V) for one time only (Fig. 2). These electroformed diodes' light emission was remarkably increased and easily detected by naked eye (inset of Fig. 2).

Then, in order to prepare the cross-section of this electroformed diode for TEM analysis, firstly SEM cross-sectional images of these diodes were obtained at the closest level allowed by the microscope (Carl Zeiss Ultra Plus Gemini FE-SEM). From these SEM images the diode appropriate for the TEM analysis was selected. In addition, SEM images showed that after electroforming process the ITO film was partially disappeared leaving spots empty of ITO material on the surface of the diode, while Si layers below ITO were still there after this partial ITO breakup. Therefore, it was decided to conduct TEM imaging of the diode cross-sectional regions for both containing ITO layer and without ITO above n+-nc-Si:H film.

To prepare the cross-section of the chosen diode for the TEM imaging a focused ion beam (FIB) technique was utilized, which allowed extraction of the 100 nm thick cross-section directly from the central part of the electroformed diode (Fig. 3). The whole FIB process took approximately 1 working day. First, the sample containing electroformed diode was covered with several nanometers of conductive film and then placed into the JEOL JIB-4601 MultiBeam FIB-SEM system. Then a slice of about 100 nm thickness was cut by a focused beam of Ga ions from the region of the electroformed diode previously selected by SEM. During the cutting process the slice was ensured to contain the regions with and without ITO layer (Fig. 3a). After cutting of a slice was almost over (Fig. 3b and c), the slice was glued to the probe via focusing Ga ions between the probe and the slice and coating this region with Ga (Fig. 3d). Then, the slice was completely cut from the glass substrate, carried by the probe (Fig. 3d and e) and placed on the TEM sample holder (Fig. 3 and f). During the placement of the slice on the TEM holder, a similar gluing process was followed, Ga was coated on the lower part of the slice from the front and back sides, so that the slice was glued onto the TEM holder (Fig. 3f and g). As the final stage of the FIB process, the high-energy Ga ions were focused on the contact place between slice and the probe in order to cut the probe from the slice (Fig. 3g), so that the diode cross-section containing slice was ready for TEM imaging (Fig. 3h).

High resolution TEM images of the FIB prepared diode cross-section were obtained using JEOL JEM-ARM200CFEG UHR-TEM system for p+-, i- and n+-layers at three different regions. From these examined three regions the first region contains ITO layer while from the other two regions ITO escaped during the electroforming process.

thumbnail Fig. 3

SEM images taken during FIB extraction of the electroformed a-SiNx:H heterojunction pin diode slice.

3 Results and discussion

The first TEM image of the FIB prepared cross-section showed that there was no deformation of the p+-, i- and n+-layers in the regions where ITO escaped after electroforming process (Fig. 4). In order to better understand the nano-structural changes, which were previously proposed by authors to occur during the electroforming process, the 3 different regions of p+-/i- and i-/n+-interfaces were examined by TEM in details: the region 1 contains ITO layer above n+-layer, whereas there is no ITO layer in regions 2 and 3 (Fig. 4).

The close TEM view of the region 1 is provided in Figure 5. The thicknesses of the doped layers are measured to be around 90 nm (Fig. 5a and b), while the intrinsic SiNx:H layer is found to be of 30 nm thickness (Fig. 5c). The TEM images taken from the n+-nc-Si:H film lying under ITO layer revealed high nanocrystallinity formed by ∼7 nm sized Si nanocrystallites, which are distributed side by side and have different orientations (Fig. 5d). Similar results are found for p+-nc-Si:H film lying under the intrinsic layer (Fig. 5e). This high nanocrystalline phase in the doped layers is expected since the plasma deposition conditions were arranged intentionally to form nanocrystallites that increase carriers' mobility, so that their injection into the intrinsic layer is highly enhanced. However, the TEM imaging of the doped layers is done after the electroforming process, so the as-grown average Si nanocrystallite size might be slightly smaller than the one found from the TEM images due to the possible increase of the crystallite size under the Joule effect occurring during the electroforming. On the other hand, the as-grown intrinsic SiN layer is amorphous in its nature, but after the electroforming process high resolution TEM images show the formation of nanocrystallites randomly oriented in the amorphous SiN tissue with the size distributed in the range of 2.5 ‒ 7 nm (Fig. 5e and f). The interplanar distance of 0.31 nm measured from the nanocrystallites in the doped layer (Fig. 5d) and those that were formed after electroforming within the intrinsic 30 nm a-SiNx:H layer (Fig. 5e and f) was found to be similar, in addition it is consistent with the interplanar spacing of (111) lattice planes obtained for the crystal Si structure [12]. On the other hand, the literature search indicated that the interplanar distance for SiN nanocrystallites is higher than 0.31 nm (0.43 nm, 0.56 nm and 0.67 nm [13]), so the nature of the formed nanocrystallites within a-SiNx:H layer corresponds to a pure Si. This is reasonable since the intrinsic a-SiNx:H layer at hand is Si-rich film. On the other hand, the average Si nanocrystallite size within the intrinsic SiN layer is observed to be smaller than that within the doped layers, which is an expected result since p+- and n+-films are only Si-based and as-grown doped films are of nanocrystalline phase whereas the SiN intrinsic film contains small amount of N atoms not contributing to the Si nanocrystallite formation and it is deposited in the amorphous phase.

Although Si nanocrystallization during the electroforming process was previously proposed by authors, the Si nanocrystallites' formation within the amorphous intrinsic 30 nm thick layer after electroforming is now experimentally proven. It should be noted that in our model it was also proposed that the size of Si nanocrystallites gradually decreases from n+/i toward i/p+ interface [5], since the influence of the Joule effect is expected to be maximum at ITO/n+ interface with the decrease toward the glass substrate. The gradual decrease of the Si nanocrystallite size in the intrinsic layer is observed in one of closest views (Fig. 5f), while in the other one the Si nanocrystallites are dispersed almost randomly in size (Fig. 5e). Therefore, it is concluded that the Joule effect during the electroforming did not cause a systematic gradient in the Si nanocrystallite size, so they are dispersed almost randomly in the amorphous SiN tissue. In general, TEM analysis have shown that there is no need for a drastic change in our previously proposed model on electrical transport and light emission mechanism in the electroformed diode; that is, the size distributions of the nanocrystallites that are imaged in this work are consistent with the modified band-tail hopping model [5]. This model also explains the memory effect, which is observed in the electroformed diode during forward and reverse bias applications [4].

The TEM examination of the regions without ITO layer (regions 2 and 3 in Fig. 4) shows that the p+- and n+-films are similar to those with ITO layer (region 1), that is they are almost entirely composed of Si nanocrystals; whereas the number of Si nanocrystals in the intrinsic layer of the regions 2 and 3 is much lower and less evident than that of the region 1 (Fig. 6). This difference in the formation of Si nanocrystallites under the regions that include ITO layer or not is probably related to the nature of the electroforming process, in which the presence of ITO layer supplies local heat so that the Si nanocrystallites are formed in the intrinsic layer (region 1) whereas in the regions without ITO layer (regions 2 and 3) this Si nanocrystallite formation is not promoted. Since the electroforming process occurs via electrical transport, the current in the ITO containing regions (region 1) may be transmitted more effectively so that these regions may have undergone more severe electroforming than the regions without ITO layer (region 2 and 3).

thumbnail Fig. 4

(a) SEM image of the electroformed a-SiNx:H heterojunction pin diode slice taken after FIB process and (b) its corresponding TEM image with indicated 3 regions examined closely by TEM in Figures 5 and 6.

thumbnail Fig. 5

TEM images of the electroformed a-SiNx:H heterojunction pin diode cross-section taken from the region 1 of Figure 4.

thumbnail Fig. 6

TEM images of the electroformed a-SiNx:H heterojunction pin diode cross-section taken from (a) the region 2 and (b) the region 3 of Figure 4.

4 Conclusions

The FIB procedure is successfully applied on the PECVD grown electroformed a-SiNx:H heterojunction pin diode to extract a slice appropriate for the TEM imaging of the diode's cross-section.

TEM imaging of the electroformed a-SiNx:H heterojunction pin diode cross-section revealed that the thickness of the intrinsic SiN-based layer is 30 nm, while the doped nc-Si:H layers are of 90 nm thickness.

In all TEM analyzed cross-sectional regions (region 1, 2 and 3) n+-nc-Si:H and p+-nc-Si:H layers are found to contain a high number of ∼7 nm sized Si nanocrystallites dispersed side by side with different orientations, so that the doped layers are composed almost completely of Si nanocrystallites.

The Si nanocrystallites within the amorphous SiN matrix of the intrinsic layer were directly detected by TEM imaging in the cross-sectional region that contains ITO layer (region 1). The Si nanocrystallites are found to be randomly distributed along the cross-sectional direction without any gradient in size and their size is found to be in the range of 2.5 ‒ 7 nm (smaller than in the doped layers).

For the cross-sections that do not contain ITO layer (region 2 and 3) the Si nanocrystallite number within the intrinsic layer is found to be much lower (region 3) and less obvious (region 2) than the one detected for the cross-sections containing ITO layer (region 1). In other words, the intrinsic layer nanocrystallization via Joule effect during electroforming is more prominent in the regions containing ITO layer.

In summary, TEM analysis supported our previously proposed model on Si nanocrystallite formation within the intrinsic layer during the electroforming process.

Author contribution statement

All the authors were involved in the preparation of the manuscript. All the authors have read and approved the final manuscript.

Acknowledgments

This work was supported by Karabuk University Scientific Research Projects Coordination Unit. Project Number: KBÜBAP-18-DS-030. The electron microscopy analysis leading to these results was performed at the Sabanci University Nanotechnology Research and Application Center − SUNUM.

References

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Cite this article as: Tamila Anutgan, Mustafa Anutgan, İsmail Atilgan, Transmission electron microscope imaging of plasma grown electroformed silicon nitride-based light emitting diode for direct examination of nanocrystallization, Eur. Phys. J. Appl. Phys. 88, 30102 (2019)

All Figures

thumbnail Fig. 1

Cross-sectional scanning electron microscope (SEM) image of the plasma grown electroformed a-SiNx:H heterojunction pin diode.

In the text
thumbnail Fig. 2

Current density versus time plot of the a-SiNx:H heterojunction pin diode under a constant forward voltage stress of 12 V. Arrows at the beginning of the measurement and at about 400 s correspond to the times at which electroforming starts and mostly ends, respectively. The inset photograph is taken by the camera of a standard mobile phone during electroformed diode's light emission under application of forward bias. The bright circle under the probe corresponds to the contacted top ITO electrode of the electroformed LED, whereas the dark circles are uncontacted top ITO electrodes of the other separate LED structures (mesas). The corresponding electroluminescence (EL) spectrum is also provided in the inset.

In the text
thumbnail Fig. 3

SEM images taken during FIB extraction of the electroformed a-SiNx:H heterojunction pin diode slice.

In the text
thumbnail Fig. 4

(a) SEM image of the electroformed a-SiNx:H heterojunction pin diode slice taken after FIB process and (b) its corresponding TEM image with indicated 3 regions examined closely by TEM in Figures 5 and 6.

In the text
thumbnail Fig. 5

TEM images of the electroformed a-SiNx:H heterojunction pin diode cross-section taken from the region 1 of Figure 4.

In the text
thumbnail Fig. 6

TEM images of the electroformed a-SiNx:H heterojunction pin diode cross-section taken from (a) the region 2 and (b) the region 3 of Figure 4.

In the text

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