FJELLVAAG HELMER (NO)
ULYASHIN ALEXANDER (NO)
NILSEN OLA (NO)
FJELLVAAG HELMER (NO)
ULYASHIN ALEXANDER (NO)
| US20070238311A1 | 2007-10-11 |
| C l a i m s
1.
A method for growing a doped zinc oxide film on a surface of a substrate by an ALD method characterised in that the method comprises a) reacting a precursor vapour mixture comprising a zinc precursor and a dopant precursor with the surface; b) purging with an inert gas; c) reacting a pulse OfH 2 O with the surface; and d) purging with an inert gas, thereby obtaining a doped zinc oxide film, where the dopant is different from boron.
2. Method according to claim 1, characterised in that it further comprises heating the surface of the substrate to a temperature below 300 °C.
3.
The method according to claim 2, characterised in that the substrate is heated to a temperature in the range from 150 to 200 °C.
4.
Method according to claim 1 or 2, characterised in that the precursor vapour mixture is formed by premixing the zinc precursor and the dopant precursor.
5.
Method according to claim 1 or 2, characterised in that the precursor vapour mixture is formed by evaporating a technical grade zinc precursor with a purity of about 90 to 99.8 at %.
6.
Method according to any one of the claims 1-4, characterised in that the zinc precursor is diethyl-zinc.
7.
Method according to any one of the claims 1-5, characterised in that the dopant is selected among Al, Li, H, Ti, Ga or a mixture comprising two or more of these compounds.
8.
Method according to claim 7, characterised in that the dopant precursor comprises trimethyl-aluminium.
9.
Method according to any one of the previous claims, characterised in that the obtained zinc oxide film is highly doped.
10.
Use of a method according to claim 1 for preparing a transparent conductive oxide layer.
11.
Use according to claim 10, where the transparent conductive oxide layer forms part of an optoelectrical device.
12. Use according to claim 11, where the transparent conductive oxide layer forms part of a transparent electrode in a screen or display, a transparent electrode in a water gas splitting cell, a transparent resistive material in a resistive heating element, a transparent material electrode for a sensor or similar or as a transparent electrode for a solar cell, here it can also function as an anti-reflective coating.
13.
Use of a method according to claim 1 for preparing an active semiconducting material.
14. Use according to claim 13, where the active semiconducting material forms part of a photo-electrochemical water splitting cell, a magnetic semiconductors, or a LED.
15.
Transparent conductive doped zinc oxide thin film, characterised in that the thin film is doped with Al and that the Al atoms are substantially equally distributed on an atomic level. |
Method of depositing a doped zinc oxide film, a conductive zinc oxide film and use of the doped zinc oxide film.
The present invention relates to a method of growing conductive zinc oxide films, doped zinc oxide thin films, use of the doped zinc oxide films and a transparent conductive doped zinc oxide film.
Pure ZnO is a direct band gap semiconductor material, which has the band gap of about 3.5 eV. When doped with suitable elements it becomes a transparent conductive oxide (TCO) with a good conductivity. Thus, applications of the ZnO semiconductors to optoelectronic devices, such as light emitting or receiving (solar cells, detectors etc.) devices, base material for transparent electronics are widely studied recently. The ZnO TCO layers can be used as the transparent conductive electrodes for several of the above-mentioned devices, which require both high conductivity and high transmittance. Electronic devices, such as solar cells, light emitted diodes, displays etc. are constructed by stacking thin films and substrates composed of different substances. ZnO is in this case used for forming a highly conducting film. The conductivity of such films should be as high as possible in order to use as thin a film as possible and thus limit the absorbance. For the same reason, the transparency of such films should be as high as possible. Further a possibility to be able to fabricate such films at a low temperature would open up for the possibility to use a wide range of other materials and substrates in the same devices and also to influence the important properties of the interfaces of such devices.
Known methods for forming a ZnO semiconductor layer include magnetron sputtering, laser ablation, molecular beam epitaxy, e-beam evaporation, sol-gel, spray pyrolysis, metal organic chemical vapour deposition (MOCVD) and atomic layer chemical vapour deposition (ALCVD) also referred to as ALD. The later method is based on sequential introduction of reacting precursors into the reaction chamber. The precursor reacts with the surface of the substrate in a self limiting way and any surplus of precursor is removed before the next precursor is introduced. Since the growth proceeds via surface reactions, the technique allows for a lower deposition temperature (below 200 °C) as compared to classical chemical vapour deposition (CVD), which often demands a temperature of around 400 °C. The usual precursors for deposition of ZnO are diethyl zinc and water. As the ALD technique is based on self limiting surface reactions, the technique can be used to apply films on surfaces with both flat and complex geometries.
The electric properties of pure ZnO films can be improved by incorporating dopants into the films. To grow doped ZnO layers using ALD it is necessary to use special precursors for the doping, which are different from those for the ZnO material growth. To avoid direct reactions between precursors in the vapor phase, which may lead to a 5 decrease in the quality of the films, it has been considered necessary to purge the reaction chamber after each pulse of precursors, hi other words, different precursors have to be used separately, which prolongs the processing time, hence the fabrication cost. Therefore there is a need to develop a process within the ALD approach in which preferably a single source precursor is used for the deposition of doped ZnO films.
10
Yamada et al in "Atomic layer deposition of ZnO transparent conducting oxides" in Applied Surface science 112 (1997) 216-222 discloses the use of atomic layer deposition of ZnO using diethylzinc and H 2 O as reactant gases. Further the publication describes growth of B-doped ZnO films by use of the ALD technique.
I 5
Yamamoto et al. "Preparation of boron-doped ZnO thin films by photo-atomic layer deposition" Solar energy materials & solar cells 65 (2001) 125-132 discloses a study of the effect of the combination of different introduction schemes for a boron dopant precursor B 2 H 6 and UV irradiation intensities on the resulting ZnO film produced by 20 photo- ALD.
The same authors as in the first of the above-mentioned publications have published a study of the growth of boron doped ZnO thin films and the influence of the doping mode. Sang et al. "Growth of Boron-doped ZnO thin films by atomic layer deposition" 5 in Solar energy materials & solar cells 49 (1997) 19-26. Sang et al. conclude that the electrical properties of B-doped ZnO films can be improved when the dopant gas B 2 H 6 is supplied after H 2 O instead of diethyl zinc.
This proposed method of obtaining B-doped ZnO films requires three different 0 precursors and equipment for handling and controlling these. Further when the dopant is introduced instead of the zinc precursor the production time is increased by the dopant pulse and purging time, and the H 2 O pulsing and purging time for each doping sequence needed to obtain the desired dopant concentration. According to Sang et al. the dopant B 2 H 6 is preferably added instead of every sixth Zn precursor pulse. This will, dependant 5 of the dopants ability to diffuse, result in a higher concentration of dopant in the adjacent film layers than in the layers further away.
K. Saito et al. disclose in Superlattices and Microstructures 42 (2007) 172- 175, "Atomic layer deposition and characterisation of Ga-doped ZnO thin films" the preparation of Ga-doped ZnO films. The study is focused on having an oxygen insufficiency to improve conductivity.
5
US 2007/023831 IAl discloses a method for obtaining a continuesly ALD process where the substrate is transported under a special shower-head with multiple channels with alternating reactants. Production of ZnO doped with Al is mentioned; here the dopant is added as a separate stream.
10
V. Lujala et al. describe in Applied Surface Science 82/83 (1994) 34-40, "Atomic layer epitaxy growth of doped zinc oxide thin films from organometals" the preparation of ZnO films doped with Al. The dopant is considered necessary and is added by exchanging some of the dimethyl-zinc or diethyl-zinc pulses with trimethyl-alurninium.
I 5
The object of the present invention is to provide a method for obtaining doped transparent ZnO films using ALD technique which can be performed using only two different precursor sources.
20 A further object of the invention is to provide a method for forming a ZnO thin film layer having good conductivity at a low material cost.
It is also an object of the present invention to provide a method which provides ZnO thin films with desirable electrical properties at relatively low temperatures for the 25 fabrication of various electronic devices, which contain layers unstable at high temperatures, such as i.e. a-Si:H.
Another object of the invention is to provide a method for fabricating a semiconductor optoelectronic device comprising a ZnO window having a good transparency and high 0 conductivity at the same time at a reduced material and processing cost.
Another object of the invention is to provide a method for fabricating a semiconductor optoelectronic device comprising a ZnO highly conductive transparent layer processed at low temperatures, that is to say below 300 °C, preferably below 200 °C and more 5 preferably between 200 and 150 °C and due to this with good interface properties.
Several of the above-mentioned prior art is limited to the use of boron as dopant. If a person skilled in the art were to study other dopants he or she would by Sang et al. be directed towards an introduction scheme where the zinc precursor gas, usually diethyl zinc is substituted at regular intervals with the dopant precursor gas. Further there has been a general expectation among the persons skilled in the art that to obtain films with a controlled quality by the ALD technique the precursors should be of high quality and purity. The properties of transparent conductive oxide films depend strongly on the composition of the film and there has therefore been a common understanding that to obtain transparent conductive zinc oxide films of high quality by ALD technique the zinc precursor should be of high purity so that the quality of the deposited ZnO can be controlled.
Surprisingly the work of the present inventors have led to the conclusion that doped ZnO films can be obtained when the dopant-precursor is introduced at the same time as the Zn-precursor. This was achieved with dopants that are different from boron and where the dopant is different from gallium.
The present invention provides a method for growing a doped zinc oxide film on a surface of a substrate by an ALD method characterised in that the method comprises a) reacting a precursor vapour mixture comprising a zinc precursor and a dopant precursor with the surface; b) purging with an inert gas; c) reacting a pulse of H 2 O with the surface; and d) purging with an inert gas, thereby obtaining a doped zinc oxide film, where the dopant is different from boron.
hi preferred embodiments of the present invention the method further comprises heating the surface of the substrate to a temperature below 300 °C, or in the range from 150 to 200 °C.
In one aspect of the method according to the invention the precursor vapour mixture is formed by premixing the zinc precursor and the dopant precursor.
In another aspect of the method the precursor vapour mixture is formed by evaporating a technical grade zinc precursor with a purity of about 90 to 99.9 at %.
In one aspect the zinc precursor is diethyl-zinc.
According to one aspect of the invention the dopant is selected among Al, Li, H, Ti, Ga or a mixture comprising two or more of these compounds. In another embodiment the dopant is selected among Al, Li, H, Ti.
Li a further aspect the dopant precursor comprises trimethyl-aluminium.
The zinc oxide film obtained according to one aspect of the present invention is highly doped.
The present invention further provides for the use of the method according to the invention for preparing a transparent conductive oxide layer which may form part of an optoelectrical device, a transparent electrode in a screen or display, a transparent electrode in a water gas splitting cell, a transparent resistive material in a resistive heating element, a transparent material electrode for a sensor etc., as a transparent electrode for a solar cell, here it can also function as an anti-reflective coating or for preparing an active semiconducting material which forms part of a photo- electrochemical water splitting cell, a magnetic semiconductors, or a LED.
The present invention also provides a transparent conductive doped zinc oxide thin film, characterised in that the thin film is doped with Al and that the Al atoms are substantially equally distributed on an atomic level.
By incorporating the dopant into the film as a part of every layer of deposited metal atoms, the dopant becomes equally distributed with in each atomic layer. This is advantageous compared to including the dopant in a separate pulse and purge sequence, which usually will result in a layered structure. This advantage is especially important when including dopants with a larger size as the tendency to diffuse is reduced by increased atomic radius. The distribution of the dopant in the direction orthogonal to the surface may be analysed using Secondary Ion Mass Spectrometry (SIMS) analysis
The term "doped" as used in this context describes a material wherein some of the Zinc atoms in the crystal lattice are substituted with other types of cationic atoms such as metal atoms or hydrogen; or some of the oxygen atoms in the crystal lattice are substituted with other types of anionic atoms. The degree of doping will depend on the desired properties of the material.
The term "highly doped" and similar expressions are used with in this context to describe a material that has a higher doping content than what is usual for this compound. ZnO is said to be highly doped when the doping level is above 0.5 atomic %, often in the range 0.5 to 30 atomic %, and more often in the range 1.5 to 3.5 atomic %.
The dopant in the present invention can be Al, Ga, Li, H, Ti or other elements like Y, In, Sc, Si, Ge, Zr or Hf. At present Al, Ga, Li, H, and Ti are considered to be interesting dopants. Doping with Li will provide a p-doped ZnO material, whereas Al and Ga provide a n-doped ZnO material. The dopant can be transition metals (d-elements) for synthesis of semiconducting materials with magnetic properties (e.g. ferro-, ferri-, paramagnetic), or to introduce intermediate stages in the band gap of the ZnO material.
ZnO it is a wide band gap semiconducting material, its properties can be imagined used in all places where wide band gap semiconducting, or simply semiconducting, material is used today. This is as radiance detectors, gas sensors, photo-electrochemical cells, etc.
Transition metals are doped into the ZnO material in order to perturb the magnetic properties of the material, or simply to make intermediate band gap positions in order to tailor the properties such as sensor, photo-electrochemical, properties. Also there is a possibility that this approach of doping may lead to better p-type conducting material.
The term "technical grade" as applied here means that the material has a purity below fine chemicals, reagent chemicals, and high purity chemicals. Technical grade diethyl- zinc will normally have a purity of around 96 to 99.8 at %. Traditionally, high purity compounds are used with a purity of 99.999 at%. However in the present invention it is considered possible to utilize chemicals with a lower quality, hi commercial available technical grade diethyl zinc the most common impurity is trimethyl-aluminium (TMA) and/or triethyl-aluminium (TEA). The TMA and/or TEA impurity is thought to be a result of the production method and the starting materials.
The term "optoelectrical device" as used within this text refers to devices such as combined anti-reflection coatings, electrodes in solar cells, transparent conductive materials for use in electrodes in devices such as light emitting diodes, OLED, flat screens, displays, etc., and similar.
Transparent conductive doped ZnO has many possible applications, some of these are as:
• Transparent electrodes in screens and displays • Transparent electrode in water gas splitting cells (photo-electrochemical water splitting).
• Transparent resistive material in resistive heating elements. (Transparent ovens).
• Transparent material electrode for sensors etc. • Transparent electrode for solar cells. Here it can also function as an anti- reflective coating.
Another interesting application of a doped ZnO film is as an active semiconducting material for instance in: • Photo-electrochemical water splitting cells. The doping level would be different than for ZnO exploited as electronic conductive material. For such cells the doping type and level would be designed in order to optimize the band gap for adsorption of sun light and coherency with the other materials in the cell. • Magnetic semiconductors. By doping the ZnO with magnetic materials
(transition materials - d-elements (most commonly Mn, Fe, Co, (Ni, Cr))) one is theoretically able to produce a ferromagnetic semiconducting material. Such a material is most desired for production of spintronic systems. • Light emitting diodes (LED). A LED is constructed of n- and p-type conducting materials. So far, n-type conducting ZnO easily produced with sufficient properties, however, p-type ZnO is not yet available with suitable properties. This is visioned possible to achieve if doped with Li, H (on Zn site) or by N, P (on O site).
In one embodiment of the present invention the method does not require introduction of special precursors for the doping. Instead the precursor for the dopant is included in the Zn-precursor. In a special embodiment this precursor is technical grade diethyl-zinc.
In one aspect of the present invention the substrate is heated to an optimal temperature, which promotes reaction of the precursor vapour on the surface of the substrate such that a film of highly doped ZnO is formed.
Preferably, the reaction temperature of the precursor vapor is low enough to use the developed process for the fabrication of various electronic devices, which contain layers unstable at high temperatures, such as a-Si:H, or polymeric substrates. The temperature is preferably below 200 °C, more preferably between 150 and 200 °C.
In one embodiment the precursor forms the highly doped and at the same time the highly transparent film without introduction of any other reactants than technical grade diethyl-zinc and water.
In one embodiment no tempering of the film produced by the process according to the present invention is needed to obtain the desired properties.
The present invention will be described in more detail with reference to the enclosed figure, where:
Figure 1 shows an ALD pulse sequence for depositing a doped transparent ZnO layer.
Figure 2 illustrates the transparency of ALD and magnetron sputtered highly conductive ZnO films. Figure 3 illustrates one embodiment of the claimed invention, where a silicon surface is passivated by a thin amorphous silicon or amorphous silicon carbide layer combined with a low-temperature ALD ZnO layer.
Figure 4 illustrates a solar cell employing ZnO layer produced according to the present invention, where the claimed method for achieving front surface passivation is introduced in a conventional silicon solar cell structure.
Figure. 5 (figure 5a-5e) shows part of the results of a Secondary Ion Mass
Spectrometry (SIMS) analysis of ALD ZnO films prepared according to the present invention.
Figure 6 shows the growth rate and specific conductivity of doped ZnO as function of deposition temperature.
The present invention provides a new method for obtaining highly conductive transparent ZnO films at a reduced material and production cost. The principle of the invention can be applied equally well to any ZnO layer containing optoelectronic device, which is dependent on high quality transparent electrodes.
FIG. 1 shows the ALD pulse sequence, which were applied for the processing of highly doped transparent ZnO layer according to one embodiment of the present invention.
The typical pulse and purge lengths are 0.6s diethyl-zinc + dopant precursor, 1.0 s purge, 2.0 s H 2 O, 1 s purge. However these intervals can vary to a great extend depending on different parameters such as equipment, substrate, temperature, desired film properties, etc.
According to the present invention, only two precursor sources are used. This is illustrated on figure 1 by the use of technical grade diethyl-zinc; however the zinc and dopant precursor may also be prepared in a different way. The precursor mixture can be obtained by premixing a zinc precursor with a dopant precursor either as solids, liquids, or as gasses that are mixed before they react with the surface of the substrate. Accordingly the mixture can be obtained by premixing before the evaporation of the precursor or after the evaporation has taken place.
If a technical grade zinc precursor is used as the sole or part of the zinc and dopant precursor mixture, it might be necessary to analyse the technical grade material prior to the use thereof to make sure that the produced films will have the desired properties. In one embodiment of the present invention the composition of the precursor mixture is adjusted by adding a stream of at least one of the precursors prior to the contact with the surface to control and adjust the composition.
The zinc precursor used in the present invention can be diethyl-zinc, but also other types of organic zinc compounds such as dimethyl-zinc, zinc chloride, zinc bromide, zinc iodine, zinc acetate, zinc β-diketonates, can be utilized. In one embodiment of the present invention the zinc precursor and the dopant precursor are preferably in the same physical state at the applicable temperatures. They are either both gases or liquids at room temperature and they can both be brought into the gas phase at a temperature below 300 °C
FIG. 2 shows the transparency of the a doped zinc oxide film produced by ALD according to an embodiment of the present invention and for comparison shows the transparency of a magnetron sputtered highly conductive ZnO films. From Fig. 2 it can be concluded that the averaged transparency of ALD ZnO film is comparable with that for magnetron sputtered one.
FIG. 3 contains an illustration of one embodiment of the claimed invention. A silicon sample Ia of either n- or p-type conductivity receives a deposition of a layer 2 of intrinsic hydrogenated amorphous silicon or hydrogenated amorphous silicon carbide of thickness 1-150 run. On top of layer 2 a low-temperature ALD ZnO thin film 3 of 5 thickness 10-200nm is deposited, which does not change passivation properties of the a- Si :H layer. The complete structure can then optionally be annealed for a short time to optimize the surface passivation at a suitable temperature.
FIG. 4 contains an illustration of employment of the claimed invention in a solar cell
I 0 structure. A silicon wafer Ia of monocrystalline or polycrystalline (including multi- crystalline) nature of one type conductivity (n- or p-type) has a silicon layer Ib of the other type of conductivity, processed by e.g. in-diffusion of a suitable dopant into the silicon wafer Ia. On top of the silicon layer Ib an intrinsic hydrogenated amorphous silicon or hydrogenated amorphous silicon carbide film 2 of thickness 1-150 run is is deposited. On top of layer 2 the ALD ZnO thin film 3 of thickness 10-200nm is deposited. A current collection grid 4 for the one polarity carrier is then deposited on top of the ZnO film 3. On the back-side surface a current collection region 5 for the other polarity carrier is deposited, as well as soldering pads 6 for interconnection of individual solar cells in a module. Methods for forming the contact regions 4, 5, and 6
20 include, but are not limited to, screen-printing metal containing pastes or evaporation of appropriate metals. The complete structure can then be heated for a short time in order to optimize the contact properties and also passivation properties of the surface passivation stack consisting of layers 2 and 3. The temperature for combined contact firing and passivation optimization is preferably in the range 200-900°C, most
25 preferably in the range 300-600°C.
FIG. 5 shows the results of a Secondary Ion Mass Spectrometry (SIMS) analysis of ALD ZnO films prepared according to the present invention using technical grade diethyl-zinc and water as precursor sources. Figure 5 is split into figures 5a to 5e each
3 o showing different parts of the graphs as indicated on the x-axis. The graphs compare the result of a ZnO film according to the invention (broken line) with the analysis of a hydro thermally grown ZnO test sample (full line). The analysis shows that the sample prepared according to the invention comprises a high concentration of Al as well as noticeable amounts of H, B, F, Cl and As. The Al and H present in this material provide
3 5 formation of donors. Therefore the high conductivity of the ALD grown ZnO films using a technical grade diethyl-Zinc is provided by the presence of Al in this low-cost precursor.
FIG. 6 shows the growth rate as obtained for ca. 70 nm thick films versus deposition temperature (open circles). The films was grown with a pulsing sequence of 0.6 s Zn- source , 1.0 s purge, 2.0 s water, 1.0 s purge. The zinc source utilized in this example was technical grade diethyl zinc. The growth rate diminishes somewhat with increasing deposition temperature, but is on overall relatively constant. FIG. 6 also shows the specific resistivity measured at room temperature versus deposition temperature for as deposited films at different positions (closed triangles). The specific resistivity decreases somewhat with increasing deposition temperature providing the best conductivity of 1.0 10 "3 Ohm cm at a deposition temperature of 300 0 C. However, on overall the specific resitivity is relatively constant.
While preferred embodiments of the invention have been described, it is understood that various modifications to the disclosed process and its implementation for the processing of various optoelectronic devices may be made without departing from the scope of the invention as defined in the subsequent claims.
Example
Thin films have been deposited using a F-120 Sat (ASM Mirochemistry) reactor by using differently prepared diethyl zinc sources, hereafter termed DEZ, and H 2 O (distilled) as precursors. The temperature of the DEZ and H 2 O precursors was held at 20 0 C during film growth.
Four different DEZ precursors were prepared as follows:
DEZ#1 was prepared using diethyl zinc with a purity of 52.5 wt% Zn and virtually no
Al.
DEZ#2 was prepared using technical grade diethyl zinc with a purity of 51.8 wt% Zn and also containing 0.33 at% Al (with respect to at% of Zn). DEZ#3 was prepared using DEZ#2 and adding 0.05ml TMA (trimethyl aluminium) with a purity of 98.7 % to 25 ml DEZ#2. This resulted in a total Al content of 0.43 at%
(with respect to at% of Zn).
DEZ#4 was prepared using DEZ#2 and adding 0.5ml TMA with a purity of 98.7 % to
25 ml DEZ#2. This resulted in a total Al content of 1.4 at% (with respect to at% of Zn).
Nitrogen was produced in house using a Schmidlin Nitrox 3001 generator (99.999% as to N2+Ar) and used as purging and carrier gas. The pressure of the reactor during growth was maintained at ca. 2 mbar by employing an inert gas flow of 300 cm 3 min "1 .
A four-point probe using tips of osmium with a radius of 0.5 mm and in connection with a Keithley 2400 was used to measure the resistivity of the as-deposited samples. A Siemens D5000 diffractometer in θ-θ mode, equipped with a gδbel mirror producing parallel Cu Ka radiation, was used for x-ray profilometry thickness measurements.
The films were deposited on soda lime, SiO 2 (OOl) and Si(111) substrates by sequentially pulsing of the DEZ and H 2 O precursors. The substrate material is not limited to the type of materials used here but can be any material reactive towards at least on of the precursors.
The films were deposited using the typical pulse and purge lengths of 0.6s DEZ pulse, 1.0 s purge, 2.0 s H 2 O pulse, 1 s purge, at a deposition temperature of 175 °C. A total of 330 cycles were used to deposit films of ca. 70 nm thickness.
The specific conductivity of the as deposited films were measured to be: DEZ#1 - 0.63 ω cm DEZ#2 = l.M0 ~3 ω cm DEZ#3 = 2.3 10 "2 ω cm DEZ#4 > 1000 ω cm
This indicates that the optimum level of Al in a diethyl zinc precursor is near the 0.3 at% as employed for the DEZ#2 composition.
The optical transparency of as deposited films on SiO 2 (OOl) was measured using a Helios γ (ThermoSpectronic) to be 77-88% in the visible range.