DOPED ZnO-ALD FILMS

The doping procedure in ALD technology is relatively simple. This is accomplished through alternative introduction of several precursors during the growth process.

Donor Doping

As can be seen in Figure 8.10, ZnO films show higher carrier concentration for growth temperature of 200°C. Electron mobility of these films is at the level of 28-30 cnr V-1 s_l, thus resistivity is ~ 1 (Г3 £2 cm. Such values belong to the lowest reported in the literature for undoped ZnO films grown by various methods, that is, ZnO films obtained by radio-frequency magnetron sputtering (Schropp, 1989) or pulsed laser deposition (Lorenz et al., 2003). For some applications, such as inorganic solar cells, these resistivity values can still be too high for efficient device operation (Ellmer, Klein, & Rech. 2008). In those cases, additional doping of ZnO films, for example, by aluminum or another group 111 element, is necessary. In this way the ZnO films resistivity as low as 2-3 x Ю’4 Q cm can be obtained. These values are comparable with those reported for Indium-Tin-Oxide films (1 x НГ4 Q cm).

Acceptor doping

Obtaining p-type ZnO is a very challenging task, because this semiconductor compound is naturally «-type and self-compensation phenomena are likely to occur, nullifying the doping effect. After almost two decades of intensive search for the best acceptor dopant, the initial idea of nitrogen as the best p-type dopant in ZnO has returned.

In the ALD technology, nitrogen doping can be easily realized by replacing deionized water with an ammonia water precursor in a part of the ALD cycles.

At temperature of the ALD process, which remains in majority of cases not lower than 100°C, ammonia water decomposes into ammonia and water: NH4OH —* NH3 + H20. Then, the chemical reactions at the surface can be expressed as follows:

The SIMS measurements show that the amount of nitrogen that can be introduced in this way into the ZnO lattice depends on the number of ammonia water versus deionized water cycles and can vary up to ~1021 at. cm-3 in case when all cycles are performed with ammonia water with NH3 concentration of 25% (Snigurenko, Kopalko, Krajewski. Jakiela, & Guziewicz,

2015). As a result of nitrogen doping, the ZnO films exhibit the concentration of free electrons much lower than that of the undoped ZnO films, which means that the ZnO layers with higher nitrogen content are more compensated. It has been reported that carrier concentration can be lowered from 2 x Ю19 to 5.4 x Ю15 cm-3 for ~2 pm thick ZnO:N films doped with every second ammonia water versus ionized water ALD cycle (Snigurenko et al., 2015).

According to Eq. 8.9, nitrogen is introduced as the -NH group onto the oxygen site of the ZnO lattice, so it can be regarded as a kind of co-doping process. This fact explains a surprisingly high nitrogen concentration that can be realized with ammonia water precursor and DEZn. According to theoretical calculations, nitrogen and hydrogen co-doping is expected to be effective, because (NH) as a whole has six valence electrons, identical to oxygen, so the formation of hole-killer defects will be suppressed (Zhang. Wei, & Zunger, 2001).

When the nitrogen doping described above is applied to the ZnO-ALD films deposited under oxygen-rich conditions (Ts - 100°C), conductivity conversion is achieved after the RTP annealing process. The highest hole mobility, 17.3 cm2 V-1 s-1, has been observed when nitrogen concentration was at the level 1019 cm’3. The related hole concentration was measured as 4.5 x Ю16 cm-3 (Guziewicz et al., 2017).

Low temperature PL (see Figure 8.17) and cathodoluminescence (CL) measurements confirmed activation of p-type conductivity after the RTP annealing process (Guziewicz et al., 2017).

Conductivity conversion is also independently seen in low temperature (10 K) microphotoluminescence spectra measured from a cross-section of the ZnO film (see Figure 8.18), in which a He-Cd laser beam emitting at 325 nm was focused to a spot with a diameter of about 1 pm. The signal was collected with the same objective and focused onto an entrance slit of a 500 mm focal length monochromator coupled with a charge-coupled device detector.

Low temperature PL spectra of the “as-grown” and annealed (a) ZnO:N and (b) ZnO films. The intensities are normalized to the most intensive line (Guziewicz et al., 2017 reproduced with permission)

FIGURE 8.17 Low temperature PL spectra of the “as-grown” and annealed (a) ZnO:N and (b) ZnO films. The intensities are normalized to the most intensive line (Guziewicz et al., 2017 reproduced with permission).

A few important conclusions can be derived from the presented CL images. The first one concerns a spatial distribution of acceptor- and donor-related regions in undoped and N-doped samples. It is clearly seen that acceptor and donor emissions are derived from different spatial regions of the samples. It is also evident that acceptor-related luminescence does not derive from grain boundaries. The latter conclusion is also supported by the XRD study (Guziewicz et al., 2017), which reveals a larger grain size in the nitrogen-doped sample. This means that an area of grain boundaries in the ZnO:N film is lower than in the undoped ZnO film, so grain boundaries cannot decide on the observed enhancement of acceptor luminescence. Moreover, in the annealed ZnO film both acceptor- and donor- related regions occupy similar spatial areas that are randomly distributed over the sample cross-section, while in the ZnO:N film the regions with acceptor emission prevail, are arranged along the columns of growth, and average acceptor-related PL and CL are considerably more intensive, pointing at much higher concentration of acceptor states.

Such a spatial distribution of acceptor regions would allow, at least partially, to explain the problem of ambiguous results of Hall effect measurements, which often provide results depending on the sample geometry. In fact, in Hall measurements, the related electrical characteristics are taken across the samples, where acceptor and donor regions are randomly distributed, and hence one might obtain ambiguous results. Similar situation was already reported in scanning capacitance spectroscopy measurements of ZnO samples co-doped with nitrogen and arsenic (Dadgar et al., 2005; Krtschil et al., 2005). On the other hand, contrary to these results, good electrical characteristics can be obtained for ZnO-based homojunction obtained with ALD, as will be shown in the next paragraph. In case of such a homojunction, the carriers’ transport is realized across the ZnO films, that is, along the columns.

Cross-section view of annealed ZnO:N (top panel) and ZnO (bottom panel) samples

FIGURE 8.18 Cross-section view of annealed ZnO:N (top panel) and ZnO (bottom panel) samples: (a) and (d) SEM images; (b) and (e) low temperature CL maps - red color represents CL at 375 nm (3.305 eV) and the green one at 370 nm (3.36 eV); (c) and (0 two-dimensional maps of CL (5 kV) come from the points along the blue lines marked in (b) and (e), the color scale shows intensity of the PL signal (Guziewicz et al., 2017, reproduced with permission).

DEVICES BASED ON ZnO-ALD Films

The correlation between free electron concentration and growth temperature observed for undoped zinc oxide films grown by ALD with DEZn and water precursors is of great importance from the application point of view. This relation means that one can control electron concentration of ZnO films without any external doping but only by choosing appropriate parameters of the ALD process. In fact, different types of applications require various electrical parameters of the ZnO films. Theoretical calculations show (Pra et al., 2008) that electron concentration of zinc oxide film dedicated to Schottky junction should not exceed 2 x Ю17 cm-3 while electron mobility should not be lower than 10 cm2 V-1 s-1. When ZnO is dedicated to a p-n junction, electron concentration at the level of 101S cm-3 is needed, and for the ZnO films used as a transparent electrode for solar cells a very high n, even at the level of 1021 cm-3, is required.

Below', we present examples which show that zinc oxide obtained at a temperature of 100°C by ALD process can really work as an active element of rectification junctions, so that this material can be successfully applied in microelectronics.

Rectification junctions based on ZnO deposited at low' temperature are elements that can be used as selectors in highly integrated nonvolatile memories based on 3D crossbar architecture (Huby, Tallarida, Kutrzeba et al., 2008; Katsia et al., 2009; Lee, Park, et al., 2007, Lee, Seo, et al., 2007; Oh et al., 2011; Park, Cho, Kim, & Kim. 2011). Low temperature processing has to be implemented for both selector and storage elements in order to allow integration with the back-end-of-line technology (Guziewicz et al., 2010).

Schottky Diode

The selector element in the 3D crossbar memory needs to fulfil specific requirements such as a high rectification ratio and a high forward current. The rectification ratio should be high enough to ensure proper selection of the appropriate memory cell without disturbance due to leakage current flowing through the remaining memory cells. A high forward current density is necessary to perform various operations (reading and writing) within the storage element. According to theoretical calculations (Pra et ah, 2008), electron concentration in ZnO dedicated to a Schottky junction should not exceed 1017cm- , so taking into account temperature dependencies described in section 8.3.1 and shown in Figure 8.10 (right), ZnO-ALD film dedicated to a Schottky junction was grown at temperature 100°C. In the ALD process with DEZn and deionized water precursors a polycrystalline ZnO film with an intrinsic free carrier concentration n - 1017cm-3 and mobility of 17 cm2 V-1 s_1are obtained.

A few possibilities of Schottky and ohmic contacts have been tested. Pt, Au, and Ag have been checked as Schottky contacts and Al and Ti/Au as ohmic contacts. The choice of appropriate metal for a Schottky barrier is an important issue in a ZnO-based Schottky junction, because the barrier height at the ZnO/metal interface does not follow' the work function values (Ip et ah, 2006). This is because of the ZnO surface states that strongly modify the barrier height. The best results were obtained for Ag as a Schottky and Ti/Au as an ohmic contact. Aluminum does not work properly as an ohmic contact, because it leads to a high leakage current as it is shown in Figure 8.19 (left). Al is a known donor dopant in zinc oxide (Fan & Freer, 1995), so probably Al diffuses from the contact into a ZnO layer leading to a higher electron concentration and thus to a high leakage current due to increased tunneling through a thinner barrier. The change of an ohmic contact from Al to Ti/Au leads to the decrease of leakage current by almost tw'o orders of magnitude and subsequent increase of the junction rectification ratio from 3 x Ю3 to 105 (at 2V) (Figure 8.19, left). The oxidation of silver at the Ag/ZnO interface causes raised series resistance. In order to minimize this effect, a rectification Ag contact was deposited at the

The I-V characteristics of the Ag/ZnO-ALD Schottky junction obtained at 100°C

FIGURE 8.19 The I-V characteristics of the Ag/ZnO-ALD Schottky junction obtained at 100°C: contact optimization (left) and optimization of ALD growth parameters (right).

bottom of a structure and an ohmic contact at the top. Both I-V characteristics presented in Figure 8.19 are obtained for such a structure. Details of the contact optimization have been described elsewhere (Allen & Dubin, 2008; Fan & Freer, 1995; Tallarida et al., 2009).

Further improvement of the leakage current has been achieved by optimization of the ZnO-ALD growth parameters. As it is presented in Figure 8.19 (right), for the same growth temperature various electron concentrations can be obtained depending on the used ALD parameters such as pulses and purging times. The optimization procedure is based on an unique approach that relies on the correlation of optical and electrical properties. For the ZnO deposition process, such growth parameters that lead to ZnO film with very low defect-related photoluminescence (Tallarida et ah, 2009).

Optimization of ZnO parameters results in a further decrease of leakage current. In this way, a rectification ratio at the level of 10s at 2V has been achieved as is presented in Figure 8.19 (right). Such a very high Iq^Hovy value fulfils requirements for a switching element dedicated to a crossbar memory. It assures the proper functioning of the crossbar array at very large integration scale. Moreover, the diode exhibit a high current density of 104 A cm-2. These Schottky junction properties are among the best results published so far for the diodes obtained at low temperature regime (Allen & Dubin, 2008). The 10 kb crossbar memory array with a NiO-based M1M (metal-insulator-metal) memory element and a Ag/ZnO- ALD/TiAu switching element was successfully constructed and showed appropriate switching properties (Fan & Freer, 1995). It should be noted that the high rectification ratio might be further increased by deposition of a 2-3 nm thick НГО2 layer, which positively influences the /oNi/foFF ratio as was reported (Krajewski. Luka, Gieraltowska et al., 2011).

8.7.2 Homojunction

Based on the /Муре ZnO:N, we constructed a ZnO-based homojunction, which has been fully prepared by the ALD technique (Snigurenko et al., 2015, Figure 8.20). The full ALD- ZnO rectifying structure was built on a highly resistive silicon substrate, covered with an approximately 40 nm thick ZnO buffer layer followed by the 8 nm “bottom” dielectric (АЬОз) film. These steps, made prior to the /Муре ZnO deposition, were aimed at the elimination of the possible substrate-related influence on the junction I-V characteristics. Before growing of the second part of the homojunction (/;-ZnO obtained at 130°C), the /Муре ZnO:N layer was capped with 4 nm of АЬОз (“upper dielectric”) to prevent nitrogen out- diffusion. Ohmic contacts to the structure were formed by the Ti/Au bilayer. The /qn//off ratio of the ZnO-ALD homojunction obtained in this way was 4 x Ю4 at ±2 V. It was more than twice higher than for the ZnO-ALD homojunction without any АЬОз protective layer (Snigurenko et al., 2015).

Other Electronic Devices

II—VI semiconductors are nowadays studied as prospective candidates for thin-film transistors dedicated to applications that require low processing temperature, such as transparent electronics or active matrix displays where the silicon-based technology becomes less popular. Polycrystalline ZnO presents a few advantages over amorphous silicon such as much higher electron mobility and transparency. ALD is beneficial in this case, as not only the ZnO films, but also the best quality dielectric layers, especially high-к oxides, can be obtained by this method. In that way, few parts of field effect transistor, channel, and gate dielectric, can be deposited using the same technology.

High-quality films of high-dielectric oxides such as НЮ2 and АЬОз can be grown by ALD at temperature that does not exceed 100°C (Gieraltowska et al., 2011). The ALD method provides unique possibility to control thickness of the films at the nanometer scale.

The I-V characteristics of ZnO-based homojunctions

FIGURE 8.20 The I-V characteristics of ZnO-based homojunctions: without АЬОз ultrathin films (I0N/I0ff - 102) and with the ultrathin A1203 film (/on/^off = 4 x 104), (Snigurenko et al., 2015, with permission).

This is especially important for gate dielectrics used in MOSFET dedicated to highly integrated circuits where dielectric thickness is 3-5 nm. The MOSFET transistor with ZnO- ALD as a channel material and АЬОз as a gate dielectric has been reported (Huby, Ferrari, Guziewicz, Godlewski, & Osinniy, 2008). The ZnO layer deposited by ALD, which acts as

i о _1

a channel there, was obtained at 100°C and has carrier concentration below 10 cm . The device features a high /on^off ratio of 107.

Transparent thin-film transistor where gate, gate dielectric, and channel were obtained by ALD at low temperature regime was also reported (Gieraltowska, Wachnicki, Witkowski, Godlewski. & Guziewicz, 2012). Transistor structure was deposited on a glass substrate and two ZnO films with various conductivities were used as a gate and as a channel, whereas a dielectric composite layer АЬОз/НЮз/АЬОз was used as a gate dielectric.

 
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