CA1272107A

CA1272107A - Methods for cds-based film and zno film deposition

Info

Publication number: CA1272107A
Application number: CA000545363A
Authority: CA
Inventors: Shu Nong Qui; Cindy Xing Qiu; Ishiang Shih
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-08-26
Filing date: 1987-08-26
Publication date: 1990-07-31
Anticipated expiration: 2007-07-31

Abstract

ABSTRACT OF THE DISCLOSURE

The present invention relates to a method for depositing a CdS-based thin film on a substrate. The method consists of depositing the film from mixed sources in a vacuum system. By varying the stoichiometry of the film, the lattice constants are conveniently controlled to values of other semiconductors for the formation a junction electronic device. The present invention also relates to a method for improving the thermal stability of low resistivity ZnO thin film. The processes can be advantageously used to prepare a double-layer transparent film with low electrical resistivity and with good lattice matching to form good quality optoelectronic devices.

Description

BAC:I~GROUND OF THE INVENTION

For optoelectronic applications, it is often require to have polycrystalline thin film materials with high optical transmission for photons in most part of the solar spectrumO
The high optical transmission can be achieved using semiconductors with large energy gaps. In addition to the high optical transmission, it is necessary to have low electrical resistivity. The low electrical resistivity is needed in order to reduce ohmic loss of electricity when an electron device is formed. For example, in a photovoltaic solar cell, the low resistivity is essential to obtain high solar ~o electrical energy conversion efficiency. For display application, the low resistivity is also needed in order to reduce the power dissipation during the operation.

Apart from the above described requirements, it is necessary to prepare the large band gap thin films with a specific crystal orientation and with pre-determined lattice constants so that the films match well to another semiconductor (usually absorbing layer with a smaller band ~ap) forming the junction. In general, there is a lattice mismatch between two different semiconductors. This lattice mismatch is very important in determining the electronic quality of junction devices fabricated using the two semiconductors. The lattice mismatch in the heterojunctions , ~, i /.

will lead to a large density of interface states which act as recombination centers for charge carriers. The excess charge carrier recombination in the interface region often lead to poor devices. For example, in photovoltaic heterojunction solar cells, the excess interface recombination will result in a low open circuit voltage and therefore low solar to electrical energy conversion efficiency It is therefore beneficial to produce heterojunction devices with a minimal lattice mismatch.
~mong several other large band gap semiconductors, the ma~erial CdS has been developed and used with two of the promising polycrystalline materials CdTe and CuInSe2 for photovoltaic devices. ~ finite lattice mismatch exists in devices made using CdS-CdTe and CdS-CuInSe2. In order to improve the device performance, it is obvious that the lattice mismatch has to be reduced. The lattice mismatch can be reduced by adjusting the lattice constants of the absorbing semiconductor or/and the window material by adjusting the composition. However, due to the strict requirements of the carrier concentration and mobility, the lattice constant adjustment of the absorbing layer appears to be more difficult to achieve. For the window layer in the heterojunctions, the requirements of carrier concentration and mobility are not as critical. Therefore, it is advantageous to minimize the lattice mismatch by adjusting the lattice constant o~ the window layer.

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For the optical device applications, it is especially important to reduce the resistivity of the top window material. This can be obtained by incorporating a second low resistivity and large band gap semiconductor on the first lattice-matched window layer. This second layer is needed in order to reduce the surface series resistance of the devices. The second window layer, when fabricated under appropriate conditions using a suitable material, allows more photons in the incident light to penetrate through and to reach the absorbing layer. The increased transmission will increase the solar to electrical energy conversion efriciency. Ideal candidates for such applications include the following materials: indium tin oxide (ITO), indium oxide (In2O3) and zinc oxide (ZnO). Although low resistivity indium tin oxide thin films have been very well studied and used in many optoelectronic devices and display devices, it consists of about 20% indium. The material indium is a rare and expensive metal, making the large scale application of indium tin oxide to be expensive. In order to reduce the cost of device fabricakion, alternative materials must be daveloped. The other potential useful large band gap semiconductor suitable for devices is ZnO. The semiconductor 7nO has an energy band gap value of 3.3 eV~ Therefore most of the photons in the visible region are allowed to penetrate through this material. Low resistivity ZnO thin . ~

~ilms can be prepared by controlling the Zn/O ratio or by adding impurities. However, unlike CdS and indium tin oxide films, the low resistivity ZnO films have been found previously to be relatively unstable and can not be e~fectively used in devices which require heat treatment steps during the fabrication. The electrical resistivity of a undoped low resistivity ZnO film was found to increase by more than 6 orders of magnitude after a short heat treatment at 400C in air.

From the above comments, it is quite obvious that a process is needed in order to prepare oriented thin films ~ased on CdS with predetermined lattice constants in order to be used with chalcopyrite semiconductor compounds like CuInSe2. Furthermore, a process is also needed in order to prepare thermally stable low resistivity ZnO films for the optoelectronic applications to replace completely or partly the widely used CdS and indium tin oxide films.

OBJECTS AND STAT~MENT

An object of the invention is to provide an improved method to produce CdS-based polycrystalline thin ~ilms with controlled lattice constants and with increased energy band gaps.

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Another object of the present invention is to provide an improved method for the deposlting of low resistivity ZnO
films.

Yet another object is to provide a method for thermally stable ZnO films.

Still another object is to provide a method for depositing sandwiched films with a low resistivity top layer ~nd a bottom thin layer with controlled lattice constants.

In the present invention, we present methods to prepare CdS-based semiconductors with variable lattice constants which match to other semiconductors like CuInSe2 for thin film iunction devices. The lattice constants of the layer are controlled by controlling the stoichiometry of the CdS-b~sad thin films.

This invention also present a process which allow one to produce low resistivity and thermally stable ZnO thin ilms. The low resistivity and thermal stability are achieved by introducing impurities during the deposition.
The processes can be advantageously used for the preparation o conducting double-layer thin films with lattice constant well matched to other semiconductors for optoelectronic devices or display devices.

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BRIEF D~SCRIPTION OF T~E DRAWING

Fig. 1 is a cross section of photovoltaic device with a lattice matched first window layer and a thermally stable low resistivity ZnO layer.

Fig. 2 diagrammatically illustrates the relationship of lattice spacing of CdS~ films with CdS content in the tal ~e~ .

Fig. 3 diagrammatically illustrates the relationship of lattice spacing of CdZnSO films with CdS content in the target.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to polycrystalline CdS
thin films for optoelectronic devices. Referring now to Fi~.
1, the electronic device comprises a supportive substrate ~1), coated with a metal film (2) and a polycrystalline absorbing material ~CuInSe2 for example) (3) with a preferred (112) orientation, a (002) oriented CdZnSO or CdSO

layer (4) with lattice constants very close to that of the CuInSe2 layer, a thermally stable, doped, low resistivity ZnO layer (5) and a counter electrode (6).

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The important requirements in producing an efficient thin film photovoltaic solar cell are: producing a CuInSe2 layer with (112) pre~erred orientation, producing a CdZnSO
or CdSO high resistivity layer with lattice constants very close to that of the absorbing CuInSe2 layer, producing thermally stable low resistivity ZnO layer and providing low resistivity contacts.

The processes for CuInSe2 film deposition have been described in other literature and will not be described here~

To produce CdS films with pre-determined lattice constants, the deposition is carried out using a high purity source (99.999~) consisting of CdS powder and ZnO or CdO
powder. ~lternatively, two or more sources of CdS and ZnO or Cd~ may be used . The deposition is carried out in a high vacuum system using an rf sputtering method. Using the specific method, a target of CdS~ZnO or CdS-~CdO with a diameter of about 5 cm and a thickness of 0.3 cm is formed~
The vacuum chamber is first evacuated to a pressure of 10-5 torr and maintained at this pressure for several minutes.
High purity argon gas is then introduced into the chamber to a pressure of about 30 mtorrO The rf power supply is turned on and the power adjusted to 60 watts. The chamber argon "` 8 pressure is reduced to a value in the range from 1 to 5 mtorr and the deposition is continued for a period of about 30 minutes. A~ter the deposition, the rf power is switched off and the sample is left in the chamber for about 20 minutes to cool~

The crystalline quality of the films is determined by X-ray diffraction, optical transmission measurements and ~all effect measurements. The results show that the lattice constants of the CdS-rich films can be adjusted in a range to match the absorbing semiconductors like CuInSe2. It is important to point out that the CdS-rich films have a ~urzite structure and still maintain the (002) preferred orientation from the X-ray diffraction results. The lattice constant adjusting effect of the CdS-rich CdSO films is shown in Fig. 2. Here it is seen that the lattice constant (d) for the (002) planes decreases from 3.4 angstroms to about 3.1 angstroms as the CdS content (in the target) is decreased from 100 to 50%. Below 50% CdS, a rapid decrease in the lattice constant is seen from the figure.

Similar lattice constant adjustment effect is also seen in Fig. 3 for CdZnOS filmsO Here, it is seen that the lattice constant (d) for the (002) planes decreases from 3.4 angstroms to 3.3 angstroms as the CdS content is decreased from 100 to 40%. For CdS content less than 40%, there is an 7~ t~

drastic decrease in the lattice constant to about 2.6 angstroms for ZnO~

It is important to emphasize again that all of the CdZnSO films deposited using targets with more than 40% CdS
and the CdSO films using targets with more than 50~ CdS are polycrystalline with a (002) preferred orientation.
Therefore, the present process allows very well oriented transparent ~ilms to be deposited. Furthermore, by adjusting tl1e content of CdS in the films, a very small lattice mismatch can be achieved between the window layer and the bottom absorbing layer. For example, if the bottom absorbing layer is polycrystalline CuInSe2 with a (112) orientation, then films of CdZnSO with a ZnO concentration of about 30%
or films of CdSO with 20% CdO can be used.

Another aspect of the invention is based on a discovery that the thermal stability of ZnO thin films can be greatly improved by introducing suitable impurities and by increasing the film thickness. The impurities are added to the target and incorporated in the films during the deposition process. According to the present invention, the impurities added to the ZnO material create donor centers which are more difficult to remove by the heat treatment in an oxygen containing atmosphere than those created by zinc or oxygen defects.

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The preferred process for the deposition o-f low resistivity and thermalLy stable ZnO films is as follows. A
target consisting of zinc oxide (ZnO) and indium o~ide (In2O3) is first prepared by mixing high purity ZnO and In203 powder and pressing in an aluminum target holder. The target is then mounted to a target support of a conventional vacuum sputtering system with a radio frequency sputtering power source. Glass substrates are selected and mounted on an aluminum substrate holder placed parallel to the target surface~ The sputtering is performed by first evacuating the deposition chamber and filling the chamber with high purity argon. After this step, the RF power supply is turned on and argon ions are created in the vacuum chamber and accelerated and directed towards the ZnO target. The atoms Zn, O and In in the target are knocked out by the accelerated argon ions and deposited on the glass substrates in a polycrystalline thin film orm. The deposition is allowed for a period of time beore the RF power is switched off. After the deposition, the films are removed for subsequent device abrication processing. Alternately, plates already deposited with metal or semiconductor films like CuInSe2 and Cd~SO may be used as the substrates.

In order to obtain low resistivity ZnO films with good thermal stability, it is preferred to placed the substrates in positions away from the vertical projection of the target ~7~

center. These positions are preferred in order to reduce the surface temperature during the deposition. Furthermore, it is preferred to adopt a deposition time so that a ~ilm thickness of at least 0.~ micron is achieved. This ZnO film thickness is important for maintaining the thermal stability when the film is subjected to post deposition heat treatment in air or other oxygen containing atmospheres~

Low resisti~ity and thermally stable II-VI compound thin ~ilms also can be achieved by adding In203 or A1203 in the sputtering target and sputtering in a ~acuum system with argon gas on a substrate placed on a substrate holder mountèd parallel to the target surface and controlling the thickness value of the films. The film electrical resistivity can be further reduced by positioning the substrate so that the angle between the normal of the target and the line through the center of the substrate and the center of the target is about 40 degrees.

High purity ZnO (99.999~) and In203 (99.999) are weighted (ZnO 25 gm and In203 0.5 gm) and mixed and then pressed in an aluminum holder (diameter 5 cm) to form a target. The target is mounted on the target support o~ a ~.

high vacuum system with a diffusion pumpO Clean glass substrates with a dimension 2.5x3x0.1 cm3 are mounted in a substrate holder placed in a position parallel to the tar~et surface. The distance between the target and the substrates is about 5 cm and the horizontal distance between the vertical projection of the target center (on the substrate holder) and the center of the substrate is about 3 cmO The deposition chamber is first evacuated by the pumping system to a pressure of about 10-5 torr. This pressure is malntained for several minutes before introducing high purity argon into the chamber. ~he argon pressure is about 30 mtorr and the RF power is switched on in order to initiate plasma in the chamber. After this stage, the argon pressure is reduced to a value in the range from 1 to 5 mtorr ~or the film deposition~ The incident RF power is maintained at about 60 watts and the deposition time is about 1 hour. During the deposition, no intentional substrate heating is used. However, a substrate heater may be added in order to improve further the film quality by controlling the substrate temperature. After the deposition, the substrates are allowed to cool in the vacuum system for a period of about 20 minutes. The average thic]cness of the deposited ZnO films is about 1 micron using the above described conditions and the resistivity about 10-3 ohm-cm.

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The deposited ZnO thin films using the above conditions ~re thermally stable in an oxygen containing environment at temperatures below 250C. Therefore they are useful for device fabrication where a post fabrication heat treatment is needed.

While the invention has been described with reference to CdS and ZnO thin film deposition, deposition of other II-VI compounds like CdO may also be achieved as well. The techni~ue can be used to create donor states by incorporating impurities in CdO to produce much better thermal stability of electrical properties than undoped CdO
films.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for fabricating a (002) oriented CdS-based thin film with tailored lattice constants, said method comprising the steps of:

- co-depositing of CdS and other II-VI compounds on a substrate in a vacuum system;
- controlling the lattice constants and energy band gap of said film by controlling the stoichiometry.

2. A process as defined in claim 1 wherein said other II-VI
compounds are binary compounds and being chosen in the group consisting of CdO and ZnO.

3. A process as defined in claim 1 further comprising a step of controlling the substrate temperature and deposition rate to improve the crystalline quality of said film.

4. A process as defined in claim 1 further comprising a step of adding impurities to said film to reduce the electrical resistivity.

5. A process as defined in claim 4 wherein said film is of the n-type.

6. A method of preparing a thermally stable polycrystalline II-VI compound thin film of low electrical resistivity comprising the steps of: preparing a target of said compound with In2O3 or Al2O3 added, sputtering a compound film in a vacuum system with argon gas on a substrate placed on a substrate holder mounted parallel to the target surface, controlling the thickness value of said film.

7. The method in claim 6 wherein said II-VI compounds are chosen in a group consisting of ZnO and CdO.

8. The method in claim 7 wherein said ZnO film and CdO film are of n-type.

9. The method in claim 6 wherein said thickness value is greater than 0.6 micron.

10. The method in claim 6 wherein the content of said Al2O3 or In2O3 in said target is in a range from about 1 to 4 weight percents.

11. The method in claim 6 further comprising a step of positioning said substrate in a position. The angle between the normal of said target and the line through the center of said substrate and the center of said target is about 40 degrees.