DE19723387C2 - Selective hydrogen ion beam etching of binary foreign phases in chalcopyrite semiconductor thin layers - Google Patents

Description

field of use

The invention relates to a method for the low-damage removal of binary, in particular copper-containing phases from chalcopyrite semiconductor thin layers and for the targeted modification of the near-surface composition of these thin layers. This can be used in particular in the production of polycrystalline thin-film solar cells based on absorber layers, for example made of CuInSe ₂ , CuInS ₂ , and their mixtures and alloys with gallium. The low-damage hydrogen passivation of defects in the volume and on the surface of the semiconductor layer that occurs simultaneously when the method is used can be used to reduce the concentration of recombination centers and thus to improve parameters of the thin-film solar cells.

State of the art:

It is known that hydrogen is used in semiconductor technology not only for passivation or deep implantation but also for cleaning the surface of semiconductors. In a large number of semiconductors, hydrogen can passivate substitution defects as well as self-defects. This was demonstrated both for element semiconductors (DE 36 16 101 A1, US 4605447) and for compound semiconductors (US 5510272, US 5059551). The literature on the prior art is quite completely summarized in several overview volumes [JI Pankove and NM Johnson, Hydrogen in Semiconductors, Semiconductors and Semimetals Series vol. 34, ed. By RK Willardson and AC Beer, (Academic Press, New York, 1990)], and [SJ Pearton, JW Corbett, and M. Stavola, Hydrogen in Crystalline Semiconductors, (Springer, Berlin, 1992)]. For surface cleaning (predominantly removal of the oxide) of A ^III -B ^V semiconductors, a hydrogen plasma is predominantly accelerated (PG Hofstra et. Al., J. Vac. Sci. Technol. A 13 (4), 1995, page 2146) or higher Hydrogen ions (<1 keV) from an ion source used (D. Ballutaud et al., App. Sur. Sci. 84, 1995, page 187). However, this treatment leads to the enrichment of the surface of the semiconductor with the A ^III component.

For CuInSe ₂ there are studies on the influence of a hydrogen plasma on surfaces of single crystals [AJ Nelson, SP Frigo, R. Rosenberg, J. Appl. Phys. 73 (1993) 8561] and [MV Yakushev, A. Zegadi, H. Neumann, PA Jones, AE Hill, RD Pilkington, MA Slifkin, RD Tomlinson, Cryst. Res. Technol. 29 (1994) 427], in which damage to the CuInSe ₂ surfaces is reported, which can be explained by selenium loss. No work is known about the use of atomic hydrogen from ion sources for dry etching or for surface cleaning of chalcopyrite semiconductor surfaces. The generation of lattice defects by ion implantation in the 10 keV range and passivation effects by implanted atomic hydrogen were also investigated on single-crystal CuInSe ₂ surfaces [MV Yakushev, G. Lippold, AE Hill, RD Pilkington, RD Tomlinson, Cryst. Res. Technol. 31 (1996) 357] and [MV Yakushev, H. Neumann, RD Tomlinson, P. Rimmer, G. Lippold, Cryst. Res. Technol. 29 (1994) 417].

In the case of polycrystalline chalcopyrite semiconductor layers, the removal of secondary phases by etching processes can be a crucial process step in the production of thin-film solar cells. This applies in particular to secondary phases such as copper sulfide (CuS) in CuInS ₂ layers, where this semimetallic, highly electrically conductive phase can produce short circuits in the absorber layer or to the rear side contact, and due to its high optical absorption, even with a thin covering of the surface negatively affects the efficiency of a solar cell structure.

The use of wet chemical etching processes for removing secondary phases and surface layers is state of the art for chalcopyrite semiconductors. The removal of the semi-metallic CuS phase from CuInS ₂ layers and the associated improvements in the optical and electrical properties by etching in an aqueous KCN solution is described, for example, in [R. Scheer, HJ Lewerenz, J. Vac. Sci. Technol. A12 (1994) 56] and in [Y. Ogawa, A. Jager-Waldau, Y. Hashimoto, K. Ito, Jap. J. Appl. Phys. pt. 2 33 (1994) L1775]. The wet chemical surface treatment using KCN or Br ₂ methanol of Cu (In, Ga) Se ₂ layers is described, inter alia, in [JH Schon, V. Alberts, E. Bucher, J. Appl. Phys. 81 (1997) 2799] in [C. Guillen, J. Herrero, Solar Energy Materials and Solar Cells 43 (1996) 47] and in [R. Klenk, R. Menner, D. Schmidt, HW Schock, D. Cahen, Proceedings of the Ninth EC Photovoltaic Solar Energy Conference, ed. By W. Palz, GT Wrixon, P. Helm, Kluwer Academic Publishers, Dortrecht, 1998, p . 469] examined.

The use of conventional sputtering processes (1 keV Ar ⁺ ions) to remove or modify a surface layer of polycrystalline Cu (In, Ga) Se ₂ has also been reported [C. Heske, R. Fink, E. Umbach, W. Riedl, F. Karg, Cryst. Res. Technol. 31 (1996) 919]. In addition to producing a metal-rich surface, this sputtering process also leads to damage to the crystal structure near the surface.

Disadvantages of the prior art:

Currently the selective removal of disruptive secondary phases and surface layers used wet chemical processes have a number of disadvantages. The for Chemicals used for etching (aqueous solution of KCN "cyanide") are highly toxic. Appropriate safety precautions are required. The waste products of these processes are also toxic and problematic in terms of disposal.

Another disadvantage is the need for the wet chemical etching step Leave vacuum system in which the layer deposition or the Sulfurisierung or Selenization of the metal layer systems took place. This brings the possibility of Contamination or undesirable chemical processes on the surface of the layer with themselves, which can only be achieved through a very complex design of the process steps, for example High-purity chemicals, clean room conditions or inert gas purging.

Conventional sputtering processes using noble gas ions can be used to remove surface layers under high vacuum conditions. However, there are significant disadvantages to the application for eliminating the disruptive secondary phases in or on chalcopyrite semiconductor thin layers. The sputtering effect is naturally not strictly selective for a specific phase in the semiconductor thin film. The removal of the crystalline chalcopyrite semiconductor thin film, which also takes place, leads to the formation of metal-rich surfaces due to the different sputtering rates of the elements contained. Because of the high kinetic energy of the sputtering ions required for sputtering, the crystal lattice is also damaged in a surface area of the layer. However, both effects must be avoided in order to achieve good electronic and optical properties for use as absorbers in a solar cell structure. Special plasma etching processes using chemically reactive gases or gas mixtures could also be used. The selectivity of the wet chemical KCN etching suggests that CN compounds could also be used here. However, this would again have the disadvantages mentioned above. The action of a hydrogen plasma on compound semiconductor surfaces also makes it possible to selectively remove certain foreign phases if these form volatile hydrogen compounds. However, this has the disadvantage that the direct contact with the low-pressure plasma results in a high thermal energy input into the uppermost atomic layers of the semiconductor surface. Especially with certain compound semiconductors, such as InP or CuInSe ₂ , this leads to severe damage to the surface. For example, with InP, phosphorus depletion of the near-surface sample area occurs due to the formation of hydrogen phosphide, which passes into the gas phase. Similar effects were also observed with CuInSe ₂ [MV Yakushev, A. Zegadi, H. Neumann, PA Jones, AE Hill, RD Pilkington, MA Slifkin, RD Tomlinson, Effect of plasma hydrogenation on the defect properties of CuInSe ₂ single crystals, Cryst . Res. Technol. 29 (1994) 427], where plasma hydrogenation likewise leads to decomposition of the semiconductor material until the surface is metallized, damage depths of up to several micrometers being able to occur.

Modifying the method by increasing the vapor pressure of the volatile connection component in the system, by spatially separating the sample from the immediate plasma area by means of "downstream" geometries or by extracting the ions from the plasma by means of an electrical field, although this leads to a reduction in damage, this can however, not to be avoided entirely [p. Balasubramanian, V. Kumar, N. Balasubramanian, Reduced phosphorus loss from InP surface during hydrogen plasma treatment, Appl. Phys. Lett. 64 (1994) 1696]. For example, the use of a Ga _x In _1-x As protective layer on the InP substrate is proposed in US Pat. No. 5,059,551. However, this has the disadvantage of an additional process step of deposition (e.g. MOCVD) and later removal.

Object of the invention:

The object of the invention is to provide a method for the removal of binary  Secondary phases in chalcopyrite semiconductor thin layers largely without damage or Contamination allowed on the surface or in volume. The sample surface should be are in a high vacuum. In connection with vacuum-based processes for Deposition of the additional window and contact layers of the solar cell should do this Open up the possibility of carrying out the deposition and etching processes in one chamber. Furthermore, an energy transfer to atoms of the semiconductor crystal lattice is necessary for the generation sufficient of lattice defects can be avoided. This alternative to KCN etching The process is intended for the cost-effective etching of large-area thin layers in one Process step can be used. No toxic compounds should be used. The possibility, by choosing the process parameters, also the stoichiometry near the surface to vary the chalcopyrite layer in a targeted manner as well as existing defects in volume and on the Switching off the surface as recombination centers by passivation is extraordinary desired and should be used in addition to the secondary phase etching process.

Solution of the task:

The interaction of the atomic hydrogen with the sample surface leading to surface damage is eliminated according to the invention in that the hydrogen partial pressure in the vicinity of the sample is kept in the pressure range 10 ^-5 Torr or less. This is achieved according to the invention in that the atomic hydrogen is generated in a region with a higher pressure (ion source) and is guided from there to the sample in an ion beam. The ion source can either be operated with pure hydrogen, deuterium or, if equipped with a device for separating the ion masses, also with a hydrogen-containing gas mixture. In the latter case, the total pressure in the vacuum chamber can be chosen so low that the hydrogen partial pressure does not exceed the limit specified above and the free path length of the H ions corresponds at least to the distance between the ion source outlet opening and the sample.

The task of damage-free selective etching is achieved according to the invention by that in a vacuum chamber a hydrogen ion beam at sufficiently low energies strikes the sample surface in order to collide with atoms of the crystal lattice Generation of Frenkel defect pairs to fall below the necessary energy transfer. Depending on the respective material system, these energies are below about 500 eV. In a few nanometers thick surface layer becomes a high concentration of atomic, very reactive hydrogen. An essential aspect of the solution is that  Etching at an increased target temperature. Through this targeted temperature increase of the Machining material during the etching process is the removal of the binary phase of volatile hydrogen compounds.

The task of targeted modification of the layer stoichiometry near the surface is accomplished by the choice of suitable beam current densities and target temperatures. This can also excess copper selenide or sulfide from the surface area of the chalcopyrite layer be removed. Amount and depth profile of the richer in indium or gallium Layer composition can be set using the parameters beam current density, etching time and Sample temperature can be influenced.

Advantages of the invention:

The method allows the selective removal of binary foreign phases in chalcopyrite semiconductor Thin films largely without generating additional damage to the surface or in the Volume. A major advantage of the method described is that Using low energy hydrogen from a distant indoor source immediate interaction of a plasma with the surface is avoided, however at the same time damage to the surface due to the low kinetic internal energies not possible. The combination of the etching process with a process step for Defect passivation, the hydrogen passivation through low-energy implantation with increased Target temperature (DE 196 52 471.7) is easily possible. Another advantage is that Compatibility of the process with the usual in many semiconductor coating systems High vacuum conditions. Therefore, it is sufficient to insert a hydrogen etching step Installation of a suitable internal source on a flange of such a system. The Ion beam-assisted dry etching processes thus allow the integration of Elimination of foreign phases as a process step in the processes of deposition or Selenization / sulfurization of the layers. Conventional wet chemical processes that also frequently associated with the use of toxic substances could be replaced and surface contamination risks are avoided.

It is advantageous to use internal sources, which in principle can also be used with other gases or Gas admixtures can be operated to hydrogen. This enables the Combination of surface cleaning or removal by sputtering and Hydrogen passivation in one process step.

Another advantage is the possibility in the surface area of the chalcopyrite layer  to subsequently vary the stoichiometry. The amount and depth profile of the Changes in the layer composition can be made using the parameters beam current density, Ion energy, etching time and sample temperature can be influenced.  

Example description

In the following, the procedure for the low-damage removal of secondary phases from polycrystalline chalcopyrite semiconductor thin layers is presented in general. According to the invention, this object is achieved, inter alia, by a controlled amount of hydrogen or deuterium in the order of magnitude of 1 to 100 scc / min ("standard cm ³ / min" = "cm ³ / min. Under standard conditions" exceeding the background level) ) is introduced into an ion source used for the etching. This ion source is preferably operated at a pressure in the range between approximately 1 × 10 ^-2 and approximately 10 × 10 ^-2 mbar (hydrogen). The potential between the ion source and the substrate is mainly 10 to 500 volts DC and the beam current density on the substrate is in the range between approximately 0.05 and 500 μA / cm ² . Irradiation time in the range of about 10 to 90 minutes has been found to be adequate to allow the process to run effectively. The substrate is generally 5 to about 30 cm from the ion source and is heated to a temperature of about 100 ° C to about 300 ° C during the etching process. Preferably, but not necessarily, the ion beam can be z. B. a tungsten filament can be neutralized. An additional built-in mass separator enables the selection of hydrogen ions (H ⁺ , H ₂ ⁺ , H ₃ ⁺ ).

Example 1:

As an example, the etching of a 2 µm thick CuInS ₂ thin film on a glass substrate by the above method is described. Processing is carried out by exposing the specimens to a hydrogen ion beam from a Kaufman-type ion source for 60 minutes. The ion energy was 300 eV, the beam current densities between 0.5 µA / cm ² and 2 µA / cm ² . The selected sample temperatures were between 200 ° C and 250 ° C. The inflow of hydrogen into the ion source was 15 scc / min. the pressure in the source is about 3.5 × 10 ^-2 mbar.

After the etching process, the samples were processed using Raman and photoluminescence Spectroscopy for possible near-surface crystal damage and the removal of the binary phase CuS checked. Parts of the same sample that were not used were used as a comparison Hydrogen were implanted.

The lattice vibration modes of the binary phase no longer occur after the hydrogen etching without a lattice damage in the chalcopyrite layer being detectable. Rather, the intensity of the band-band photoluminescence emission grows by about an order of magnitude, from which the passivation of recombination centers can be concluded. Electron microscopic examinations also showed no damage to the surface of the chalcopyrite crystallites. Only the CuS foreign phase, which occurs as a thin surface layer or in the form of crystalline grains in the nanometer to micrometer range, was eliminated by the hydrogen ion beam process and left cavities or pits of the appropriate size in the layer. Measurements of the surface resistance of the layers showed an increase in this resistance by more than an order of magnitude after tempering in the hydrogen ion beam, which can be explained by the removal of the highly conductive semi-metallic CuS phase. EDX measurements confirmed the removal of the complete binary phase without leaving any component. They also showed a change in the originally copper-rich composition in the surface area of the chalcopyrite crystallites. In these samples this was typically In: Cu = 0.83, S: Cu = 1.62, S: In = 1.96 and (Cu + In): S = 1.13. The removal of copper sulfide in conjunction with diffusion processes at the elevated target temperature led to the establishment of an In-rich composition, which is desirable for the surface area of the solar cells. The corresponding composition data after the one-hour tempering at 200 ° C in a hydrogen ion beam with 1 µA / cm ² : In: Cu = 1.18, S: Cu = 1.97, S: In = 1.67, (Cu + In): S = 1.11. The information depth for these EDX measurements was in the order of 1 µm due to the electron energy of 12 keV.

Example 2:

Another example describes the etching of a 3 µm thick CuInS ₂ layer on a glass substrate similar to Example 1. In addition to the ion source, a mass separator was used which only allowed the etching with H ⁺ . The hydrogen ion beam had an acceleration potential of 300 eV and a beam current density of 2 µA / cm ² for a period of one hour on the sample surface. The temperature of the substrate during the entire etching process was 250 ° C. The pressure in the recipient was 3 × 10 ^-4 mbar during the etching. Raman and photoluminescence measurements, similar to example 1, showed that the binary phase of CuS had been removed without lattice damage in the chalcopyrite layer being detectable.

The preferred method described above can be modified in several ways without departing from the scope of the invention. So z. B. the Ion beam generated by a single-hole ion source and scanned over the sample or a particle accelerator ion beam generated by deceleration be used.

Of course, the method according to the invention is not limited to the passivation of CuInS ₂ . For example, other ternary or multinary compound semiconductors, such as I-III-VI ₂ compounds (e.g. CuInSe ₂ for thin-film solar cells) can be processed with the method illustrated for the purpose of removing secondary phases with the hydrogen ion beam.

Claims (15)

1. A method for removing binary phases in chalcopyrite semiconductors and for modifying the composition of thin layers of these semiconductors, characterized in that hydrogen with an energy below the energy barrier for damage generation in the chalcopyrite semiconductor material is implanted in the surface thereof combined with an increase in temperature of the treated material and that the process takes place in a vacuum chamber with a hydrogen partial pressure near the semiconductor surface of less than 10 ^-5 Torr. 2. The method according to claim 1, characterized in that the temperature of the Material or parts of the material during the etching for a short time or during the entire implantation process is increased. 3. The method according to claim 1, characterized in that the temperature of the Material or parts of the material after the etching is increased. 4. The method according to claim 1, characterized in that the temperature of the Material or parts of the material is increased during and after the etching. 5. The method according to claim 1, 2, 3 or 4, characterized in that the Hydrogen ion beam is generated using a broad beam ion source. 6. The method according to claim 1, 2, 3 or 4, characterized in that the Hydrogen ion beam is generated using a single-hole ion source. 7. The method according to claim 1, 2, 3 or 4, characterized in that the Hydrogen ion beam decelerating an ion beam from a particle accelerator is produced. 8. The method according to claim 5, 6 or 7, characterized in that a Ion beam consists of hydrogen and / or a hydrogen-containing compound and afterwards is separated by a mass separator. 9. The method according to claim 5, 6, 7 or 8, characterized in that the Ion beam remains constant over time. 10. The method according to claim 5, 6, 7 or 8, characterized in that the The intensity of the ion beam is varied over time. 11. The method according to claim 5, 6, 7 or 8, characterized in that the  Ion beam is pulsed. 12. The method according to claim 9, 10 or 1 l, characterized in that the Ion beam is scanned over the sample. 13. The method according to claim 12, characterized in that at the same time Defect passivation takes place in the volume or on the surface of the target material. 14. The method according to claim 13, characterized in that a change in Initial stoichiometry of the chalcopyrite layer or of parts of this layer takes place. 15. The method according to claim 14, characterized in that instead of the hydrogen Deuterium is used.