NON-CESIATED SOLID STATE ELECTRON EMITTERS (COLD CATHODES) & THEIR APPLICATIONS IN VACUUM MICROELECTRONICS

INTERIM PROGRESS REPORT

Professor Umesh K. Mishra

Robert D. Underwood

December 31, 1997

U.S. ARMY RESEARCH OFFICE

DAAH04-95-1-0157

UNIVERSITY OF CALIFORNIA, SANTA BARBARA

Department of Electrical & Computer Engineering

APPROVED FOR PUBLIC RELEASE;

DISTRIBUTION UNLIMITED.

THE VIEWS, OPINIONS, AND/OR FINDINGS CONTAINED IN THIS REPORT ARE THOSE OF THE AUTHOR(S) AND SHOULD NOT BE CONSTRUED AS AN OFFICIAL DEPARTMENT OF THE ARMY POSITION, POLICY, OR DECISION, UNLESS SO DESIGNATED BY OTHER DOCUMENTATION.

List of Manuscripts

· D. Kapolnek, R. D. Underwood, B. P. Keller, S. Keller, S. P. DenBaars, and U. K. Mishra, “Selective area epitaxy of GaN for electron field emission devices,” J. Crystal Growth, vol. 170, pp. 340-343, 1997.

· R. D. Underwood, D. Kapolnek, B. P. Keller, S. Keller, S. P. DenBaars, and U. K. Mishra, “Selective-area Regrowth of GaN Field Emission Tips,” Solid-St. Electron., vol. 41, pp. 243-245, 1997.

· R. D. Underwood, D. Kapolnek, S. Keller, B. P. Keller, S. P. DenBaars, and U. K. Mishra, “GaN FEA diode with integrated anode,” in Technical Digest of the 10th IVMC. Seoul: EDIRAK, 1997, pp. 132-136. [also submitted to Journal of Vacuum Science and Technology B]

· R. D. Underwood, D. Kapolnek, B. P. Keller, S. Keller, S. P. DenBaars, and U. K. Mishra, “Selective-Area Regrowth of GaN Field Emission Tips,” in Proceedings of the Topical Workshop on III-V Nitrides (TWN'95), I. Akasaki and K. Onabe, Eds. Nagoya: Pergamon, 1997, pp. 181-183.

(1) Scientific Personnel

· Professors Umesh K. Mishra and Steven P. DenBaars.

· Post-doctoral researcher Stacia Keller.

· Graduate Students Robert Underwood, David Kapolnek, and Peter Kozodoy.

(3) Inventions

· Submitted patent disclosure on InGaN-Coated Field Emitters.

(4) Scientific Progress and Accomplishments

Nitride-based semiconductors are showing great promise in the area of cold cathode emitters. Cold cathodes based on GaN diodes with low work function coatings[1] and planar AlN emitters have been recently reported[2-4]. The primary limitation of the above mentioned devices is their low efficiency. GaN can also be selectively grown to form pyramidal structures that may be used as field emitters in field emitter arrays(FEAs). FEAs will have potentially higher efficiency if operating voltage can be lowered sufficiently. The growth of GaN has been optimized in previous work of this program. Work over the past year has concentrated on fabrication of various device structures for evaluating the effectiveness of GaN for field emission. Fabrication of the following device types has been achieved in the past year:

· GaN field emitter array diodes with integrated anode

· Gated field emitter arrays

· Diamond coated field emitter arrays

· InGaN/GaN field emitter array diodes with integrated anode.

Each of these device types will be discussed in turn in terms of design, fabrication, and available measurements.

Lowering the Operating Voltage by Device Structure

In our previous work, we fabricated GaN field emitters with no other structures and used an external anode to measure the emission current. This approach has several problems. First, it was difficult to determine the anode-cathode separation. This distance is a critical parameter for field emitters as the emission is determined strongly by the field at the tip surface. Secondly, only large separations (~0.5 mm) were possible with this method. Thus, high voltages were necessary in order to achieve field emission[5]. The high voltages made the arrays susceptible to damaging arcs that tended to destroy the cathodes. One method of lowering the operating voltage is by bringing the extracting electrode (positively biased with respect to the tips) closer to the top of the pyramids. One common method in vacuum microelectronic field emitter is to add a gate structure[6]. Another method is to fabricate an anode on-wafer and control the spacing using microelectronic processing methods. Devices of both types have been fabricated and will be discussed below.

GaN Field Emitter Array Diode with Integrated Anode

Text Box: Figure 1. Process flow for integrated extractor FEA. (a) Selective growth of tips. (b) Etching of mesas. (c) Air-bridge anode. (d) Remove sacrificial bridge support. GaN FEAs with integrated anodes are useful devices for determining the usefulness of an emitter material for field emission. Emitter material effects FEA device operation mainly through its work function and reaction with the residual vacuum gases. A low work function provides lower operating voltage and an inert surface should provide stable emission. Another advantage of our structure is that it has an extremely simple fabrication process flow. There are no critical alignments and the anode-cathode spacing is determined quite simply by a resist layer thickness. Figure 1 shows the processing steps involved. First, the GaN pyramids tips are grown selectively using a SiO₂ masking layer. Then, mesas are etched to isolate the devices. Next, an air-bridge process defines the anode over the cathode tips. Finally, the air-bridge support sacrificial photoresist is etched from under the bridge to leave a freestanding structure. An SEM of a completed device is shown in Figure 2.

Figure 2. SEM of completed FEA diode with integrated extractor.

Figure 2. SEM of completed FEA diode with integrated extractor.

The first measurements of these devices produced emission at substantially reduced voltages compared to the external anode devices. Emission in the microampere range was achieved by a 10-tip array at only 500 V at an anode to cathode separation of about 2 mm as shown in Figure 3. The voltage is about half of the turn-on voltage achieved by the external anode arrays. An aspect that is very encouraging is that the measured device had rounded pyramid tips. Thus, tips with sharper profiles should emit at an even lower applied voltage. Sharper tip arrays have been fabricated but not measured as of yet.

Gated GaN FEAs

The diode structure is a good test vehicle for determining the field emission properties of GaN and other nitride semiconductors but is itself not useful from a device standpoint. A three-terminal version can be used in applications such as electron sources, cathodes for high power tube amplifiers, and displays. In the gated FEA structure the extracting electrode surrounds but does not cover the field emitter. Ideally, it would not intercept any of the current and could be used to modulate the current extracted from the field emitter with a low voltage swing. A schematic of the structure is shown in Figure 4.

The fabrication of this device structure reliably over large arrays has been difficult. Several large arrays were fabricated but showed shorting during testing. An effort was made this year to scale down the array size for increased yield. A successfully fabricated three-terminal device has been fabricated and is awaiting testing.

Lowering the Operating Voltage by Lowering the Work Function

Another method of lowering the operating voltage of a field emitter is to use a material with a lower work function. Work function is a measure of the size of the energy barrier that an electron must penetrate in order to tunnel into vacuum. In general, wider band gap semiconductors show lower electron affinities than narrower band gap semiconductors. GaN has a work function that is estimated to be between 2.1-4.3 eV[7,8]. If the lower range is found to be the true of the work function then a reduction in voltage can be expected with the use of GaN over such conventional materials as tungsten or silicon which have work functions from 4.2 to 4.5 eV. Coating with an electropositive adsorbate can also lower the work function of an emitter. The adsorbate layer produces a dipole that counteracts and lowers the surface workfunction (provided the adsorbate layer is sufficiently thin). The problem with most such adsorbates (Cs is an example) is that they do not form a stable surface in any practical vacuum environment. Coating emitters with materials possessing negative electron affinity can lower the operating voltage. One such material is diamond and has been the object of some study in the field emission community[9]. Diamond also has the advantage of having a very inert surface and thus may form a stable field emitter at high vacuum. The primary disadvantage of diamond is that no satisfactory n-type material has yet been produced thus limiting the electron supply for emission.

Diamond Coated FEAs (external anode)

Although not a main focus of our study of GaN field emitters, an opportunity to coat our emitters with diamond was presented to us by Dr. Shlomo Rotter of SOREQ in Israel. Dr. Rotter was able to coat some of our large arrays with diamond at his laboratory at SOREQ. The coating process seems promising and the GaN may be able to withstand the process without degradation. Measurements of the diamond coated GaN FEA is expected soon.

InGaN/GaN Field Emitters – Piezoelectric Effect

As discussed above, coating a surface with an electropositive adsorbate can produce a work function lowering. The fundamental aspect is the formation of a dipole with its positive end pointing out of the surface. Another crystal effect that can cause the formation of a dipole is the piezoelectric effect. In the piezoelectric effect, a mechanical force (tensile or compressive) in certain crystal directions can produce an electric field in a crystal, or conversely, an applied electric field can distort the crystal shape. The hexagonal nitride semiconductors have been calculated to have large piezoelectric coefficients that determine the magnitude of the effect. By coating a GaN field emitter with a thin layer of InGaN, we produce a structure in which the top InGaN layer is strained by lattice mismatch (also known as pseudomorphic growth). This strain induces a dipole in the thin InGaN layer which has the same effect as an adsorbate provided the thickness of the InGaN is kept so small that the electrons can travel ballistically through the InGaN. In addition the layer must be thin enough to ensure that the strain in the top film can not be relaxed by dislocation formation. Thus, the InGaN acts to lower the surface work function but otherwise does not effect the electron transport significantly.

Simulations of the InGaN/GaN structure have shown that the effect could produce large enough work function lowering to effect the electron emission. Figure 5 shows the effect of InGaN layer thickness and In mole fraction on the effective work function. The effective work function is the electron affinity of the InGaN minus the effect of the dipole. It is seen that the work function can be potentially reduced to the range of 1-2 eV. Simulations are also in progress to study the fact that we are coating a tip and not a flat surface using full three-dimensional numerical modeling. Already, devices incorporating the InGaN coatings of various thickness have been fabricated and will be measured in the near future.

Summary

The past year has seen the focus of this program change from a study of the optimal growth of GaN field emitters to fabrication of device structures using the GaN FEAs. Diode and triode type structures have been fabricated to meet various purposes. Diode structures are simple and facilitate initial measurement of GaN field emission. The triode structures are similar to the proposed structure of practical devices but have been difficult to measure as of yet. Measurements of the integrated anode diode GaN FEAs have shown emission at 500 V and promise further lowering of the operating voltage. Diamond coated GaN FEAs have been produced and may offer another avenue of exploration into the physics of field emission from diamond-coated surfaces. Finally, simulations of the piezoelectric effect of pseudomorphically grown InGaN on GaN have shown a lowering of the effective work function of the surface. The effect may lower further the operating voltage of nitride-based field emitter arrays.

References

[1] A. I. Akinwande, R. D. Horning, P. P. Ruden, D. K. Arch, B. R. Johnson, B. G. Heil, and J. M. King, “Non-Thermionic Cathodes—Solid State Electron Emitters based on GaN and LaB₆,” in Tech. Digest of the 1997 International Electron Devices Meeting. New York: IEEE, 1997, pp. 729-732.

[2] J. A. Christman, A. T. Sowers, M. D. Bremser, B. L. Ward, R. F. Davis, and R. J. Nemanich, “Nitride Based Thin Film Cold Cathode Emitters,” Mat. Res. Soc. Symp. Proc., vol. 449, pp. 1121-1126, 1997.

[3] E. W. Forsythe, J. A. Sprague, B. A. Khan, S. Metha, D. A. Smith, I. H. Murzin, B. Ahern, D. W. Weyburne, and G. S. Tompa, “Study of IBAD Deposited AlN Films for Vacuum Diode Electron Emission,” Mat. Res. Soc. Symp. Proc., vol. 449, pp. 1233-1238, 1997.

[4] A. T. Sowers, J. A. Christman, M. D. Bremser, B. L. Ward, R. F. Davis, and R. J. Nemanich, “Thin films of aluminum nitride and aluminum gallium nitride for cold cathode applications,” Appl. Phys. Lett., vol. 71, pp. 2289-2291, 1997.

[5] R. D. Underwood, D. Kapolnek, B. P. Keller, S. Keller, S. DenBaars, and U. Mishra, “Field Emission From Selectively Regrown GaN Pyramids,” presented at 54th Device Research Conference, Santa Barbara, California, 1996.

[6] see for example C. A. Spindt, “A Thin-Film Field-Emission Cathode,” J. Appl. Phys., vol. 39, pp. 3504-3505, 1968.

[7] S. Strite and H. Morkoç, “GaN, AlN, and InN: A review,” J. Vac. Sci. Technol. B, vol. 10, pp. 1237-1266, 1992.

[8] S. N. Mohammad, Z. Fan, A. E. Botchkarev, W. Kim, O. Aktas, A. Salvador, and H. Morkoç, “Near-ideal platinum-GaN Schottky diodes,” Electronics Letters, vol. 32, pp. 598-599, 1996.

[9] see for example M. W. Geis, J. C. Twichell, and T. M. Lyszczarz, “Diamond emitters fabrication and theory,” J. Vac. Sci. Technol. B, vol. 14, pp. 2060-2067, 1996.

(5) Technology Transfer

· None at this time.

ACCOMPLISHMENT SUMMARY REPORT

1. TITLE OF PROJECT:

2. GRANT NUMBER: DAAH04-95-1-0157

3. PERIOD COVERED BY REPORT: 1 JAN 1997-31 DEC 1997

4. NAME OF INSTITUTION: University of California, Santa Barbara

5. PRINCIPAL INVESTIGATOR: Umesh K. Mishra

6. MAJOR ACCOMPLISHMENTS:

7. TECHNOLOGY TRANSFER / NEW INITIATIVES:

8. CONFERENCES / WORKSHOPS: Oral presentation at the 10^th International Vacuum Microelectronics Conference held at Kyongju, South Korea, August 17-21, 1997.

9. PAPERS: