The existing electron beam lithography (EBL) system consisting of a slightly modified JEOL 6400F with beam steering soft- and hardware was enhanced by an additional lithography control system. The combined EBL system yields higher resolution and is capable of marker recognition, correction of field distortion, size and rotational corrections implemented in fast hardware. It was used to realize nanostructured resist masks on GaAs substrates produced by G. Straßer, Prof. M. Heiblum (Weizmann Institute of Science, Israel) and Dr. J. Walker (Laboratorio TASC, Italy). These masks were wet chemically etched to form semiconductor quantum wire arrays and single quantum wires. Investigations of lateral transport of the wire arrays revealed new correlation effects. On single wires high field transport was investigated. The EBL system was also used as a tool for the optimization of vertical emitting heterostructure lasers, which led to strongly enhanced far field patterns.
The EBL system at the Institute of Solid State Electronics was used for miscellaneous applications, ranging from low temperature lateral transport investigations of quantum wire arrays and single quantum wires to air bridged high speed Schottky diodes for THz devices and fs-correlation detectors operating at room temperature. It was also found to be an essential analysis tool used in the optimization process for surface gratings used in vertical emitting laser diodes.
Magnetophonon resonances (MPR) have been clearly resolved on nanoscale arrays of quasi one dimensional quantum wires and were used for the first time to systematically study the characteristic properties of such systems.
At temperatures around 100 K, MPR result from resonant scattering of electrons by LO phonons between electronic sublevels induced by the confining potential. The resulting resonant structures were used to investigate subband spacings and the polaron mass of electrons in these one dimensional systems. This is particularly useful for quantum wires near the quantum limit when traditional methods for the characterization of 1D systems fail due to the low number of occupied subbands. By variation of the electron density in the quantum wire systems a situation can be achieved where a decreasing number of subbands are occupied. It was found that the MPR is strongest in cases where the electron density is too low for classical magnetic depopulation experiments to yield reliable information. We found that the subband spacing in these quantum wire systems increases steeply when the electron density is decreased. Compared to the 2DES, the polaron mass in 1D systems is larger and increases with decreasing 1D electron density, which is caused by stronger electron-LO phonon coupling due to reduced screening.
In systems containing only a few (up to five) ballistic quantum wires, which were prepared by electron beam lithography and wet chemical etching (Fig. 1), additional structures in the magnetoresistance at low magnetic fields could be observed (Fig. 2). Channel resistance peaks with a superimposed fine structure develop at relatively high current densities. They can be explained by the assumption that electrons exiting one quantum wire into the 2DES are magnetically focused into the adjacent wire after specular scattering at the boundary of the etched region forming the spacing between the 1D wires (Fig. 3). This coherent focusing causes interference effects which lead to fine structured peaks in the magnetoresistance. In further experiments the dependence of this effect on temperature and on the length of the quantum wires is investigated. It is expected that the results of forthcoming measurements can be used to determine the phase coherence lengths in quantum wires.
Fig. 1: Five wet chemically etched quantum wires (optical microscope, magnification = 2500x). The quantum wires are 2 µm long. The dark areas to the left and to the right are 2DEG used as 'contacts' to the quantum wires. The annealed contacts to the 2DEG are not shown.
Fig. 2: Magnetophonon resonances at a temperature of 115 K in an array of quantum wires.
Fig. 3: Additional structures in the magnetoresistance of an array of quantum wires for three different currents (magnetofocusing peaks).
Investigations of high field transport measurements on single wires etched into the 2DEG of standard GaAs/AlGaAs heterostructures a reduction of the channel resistance at high fields was found. It can be attributed to the reduced dimension of the charge carriers due to strongly reduced electron-electron scattering and a grossly changed interaction of electrons and phonons in one dimension [2].
Fig. 4: Single quantum wire 20 µm long (optical microscope). The mesa is 60 µm x 20 µm. The resist mask used to produce the quantum wire by wet chemical etching is clearly distinguished, since the electron beam resist is removed in a later processing step.
Performance of GaAs based Schottky diodes for the THz regime is strongly influenced by the capacity of the Schottky contact and therefore its area. A reduction of the size of the contact pad has been shown to result in very fast diodes suitable for THz applications. A new method for the fabrication of such diodes is being developed. It is based on an air bridged Schottky contact with extremely small capacity. The size of the contact pad is in the range of 100 nm x 100 nm or smaller. The stray capacity of the leads is further reduced by air bridging.
A tri-level PMMA-based electron beam resist system is employed to form one lead of the THz diode. By variation of the exposure dose the lift off behavior of this system can be changed. Regions with higher dose form contacts to the GaAs substrate, whereas lower dose areas lead to bridges remote of the substrate. This can clearly be seen in Fig. 5 showing part of an array of rather large sized Au-air bridges on a GaAs substrate. This test pattern was used during development of the EBL air bridge technology. Further optimization of this process together with standard optical lithography steps is necessary to develop fully functional THz diodes.
Fig. 5: Two air-bridges made of 100 nm thick gold (electron microscope). It can clearly be seen that the gold, later to be used as Schottky contact for THz diodes, clears the substrate at a distance of about 2 µm. The bridge is about 250 nm high. The size of the contact pads at the right hand side is about 100 nm x 1 µm. For application in a high speed device, further optimization is obviously needed.
In semiconductor laser technology one is usually faced with the fabrication of very small structures such as gratings with periods varying from 200 - 750 nm used as couplers in DFB/DBR and SME laser diodes.
Effective techniques to fabricate these small structures are laser holography and EBL, since conventional optical lithography techniques are limited in fabricating structures of such small sizes. Obviously, standard optical microscopy can not be employed for the analysis of such small sized structures. Therefore, EB microscopy was used as a vital analysis tool (Figs. 6 and 7). Many repeated steps of photolithography, ion milling and analysis by electron beam microscopy led to optimized surface gratings [3, 4].
Laser holography is based on the interference of two laser beams used to expose standard photoresists. The period of the interference pattern depends on the angle between the two incident beams and the laser wavelength and is therefore limited to half of the laser wavelength. In our setup, the angle between the sample and the reflecting mirror is chosen to be 90°, and the distance between the source and the sample holder assembly is very large compared to the laser wavelength, so that the curvature of the interference pattern is practically zero. The laser source is a He-Cd laser operating at 325 nm with 28 mW of output power. The grating period can be calculated as , where and are grating period and laser wavelength and is the incident angle.
Photo resists used to fabricate surface gratings must satisfy the following conditions in order to fulfill the requirements for successful grating fabrication:
Unfortunately, standard photo resists are not optimized for requirements such as stated above.
In order to achieve a reasonable grating depth by wet chemical etching the resist must reveal steep edges. Positive resist characteristics make it possible to fabricate such structures. The optimized process used for fabrication of 425 nm surface gratings is given in the following:
The etching rate is about 20 nm/min and is fully reproducible.
To fabricate gratings with other periods and duty ratios one should change the exposure duration and development time, i.e., longer exposures decrease the bridge to trench ratio. For smaller grating periods the exposure time should be increased to achieve a one to one duty ratio. The developing time should be kept minimum provided the exposed surface is completely developed. Figure 6 shows a grating of period 425 nm fabricated by positive photo resist.
Fig. 6: Photoresist grating analyzed by electron beam microscopy. The grating consists of positive photo resist, later used for wet chemical etching.
The image reversal resist AZ 5206 is suitable for fabrication of gratings with periods under 500 nm and subsequent lift off processes. The side walls of this resist form undercut edges and are therefore not suitable for wet chemical etching. On the other hand wet chemical etching is known not to be appropriate for such small structures, since the etchant cannot reach the substrate due to surface tension. The etching mechanism used with this resist is ion milling. A grating period of 425 nm with steep side walls is achieved with the optimized parameters following:
The fabrication process is very sensitive to exposure time, post-exposure bake, flood exposure and developing time. To produce smaller periods with image reversal resist the exposure time should be kept shorter and the flood exposure must be longer to achieve a one to one duty ratio. Figure 7 shows an etched grating processed as described above.
Fig. 7: Surface grating etched by ion milling.
EBL was used to produce arrays of quantum wires for transport measurements showing interference effects between carriers emerging out of adjacent wires. Single wires fabricated by EBL were used for high field measurements. Technology for air bridged Schottky contacts to be used in THz diodes has been developed. Surface gratings for emitting laser diodes have been optimized using parts of the EBL system.
[1] M. Hauser, G. Strasser: "Elektronenstrahllithographie von niedrigdimensionalen GaAs/AlGaAs-Heterostrukturen”, proceedings Fortbildungsseminar: Grundlagen und Technologie elektronischer Bauelemente, Großarl 1995, ISBN 3-901578-01-3.
[2] C. Resch, J. Lutz, F. Kuchar: "Transport in AlGaAs/GaAs-Quantendrähten in hohen elektrischen Feldern”, proceedings Fortbildungsseminar: Grundlagen und Technologie elektronischer Bauelemente, Großarl 1995, ISBN 3-901578-01-3.
[3] A. Köck, S. Freisleben, C. Gmachl, A. Golshani, E. Gornik; M. Rosenberger, L. Korte: "Single-mode surface-emitting laser diodes based on surface mode emission”, Vortrag, Photonics West, OE/LASE'96, 27. January - 2. February 1996, San Jose, CA, USA, 1996.
[4] A. Köck, S. Freisleben, C. Gmachl, A. Golshani, E. Gornik; M. Rosenberger, L. Korte: "Single-mode surface-emitting laser diodes based on surface mode emission”, Proceedings Nr. 2682, Laser Diodes and Applications II of Photonics West, OE/LASE'96, USA, SPIE Optical Engineering Press (1996).
Mag. Dr. Gottfried STRASSER
Institute of Solid State Electronics, Technical University of Vienna, Floragasse 7, A1040 Vienna
Last Name First Name Status Remarks Strasser Gottfried postdoc Hauser Markus postdoc 75% GMe funding Ploner Guido dissertation Zotl Ernst student Köck Anton postdoc Golshani Alirezah student Gornik Erich full professor