Progress on development of UV photocathodes for

Progress on development of UV photocathodes for photon-counting applications at NASA GSFC J. Stocka,*, G. Hiltonb, T. Nortonb, B. Woodgatec, S.Aslamd, M.Ulmere a

Swales Aerospace 5050 Power Mill Rd., Beltsville, Md 20705 b Science Systems and Applications, Inc. / NASA Goddard Space Flight Center, Code 667 Greenbelt, MD 20771 c NASA Goddard Space Flight Center, Code 667 Greenbelt, MD 20771 d Muniz Engineering / NASA Goddard Space Flight Center, Code 553 Greenbelt, MD 20771 e Dept. of Physics and Astronomy, Northwestern University Evanston, IL 60208

ABSTRACT The development of high quantum efficiency photoemissive detectors is recognized as a significant advancement for astronomical missions requiring photon-counting detection. For solar-blind NUV detection, current missions (GALEX, STIS) using Cs2Te detectors are limited to ~10 % DQE. Emphasis in recent years has been to develop high QE (>50%) GaN and AlGaN photocathodes (among a few others) that can then be integrated into imaging detectors suitable for future UV missions. We report on progress we have made in developing GaN photocathodes and discuss our observations related to parameters that affect efficiency and stability, including intrinsic material properties, surface preparation, and the vacuum environment. We have achieved a QE in one case of 65% at 185 nm and are evaluating the stability of these high QEs. We also discuss plans for incorporating GaN photocathodes into imaging and non-imaging sealed devices in order to demonstrate long term stability. Keywords: Gallium Nitride, GaN, Ultraviolet, UV, Photon-counting, Photocathode

1. INTRODUCTION The development of UV photocathodes for use in photon-counting detectors for space-based astronomical missions is a goal that is being pursued by a number of researchers. In particular, improvement in quantum efficiency (QE) is strongly emphasized as modest increases in detection efficiency provide mission planners with significant leverage in balancing instrument design (collecting area), science requirements, and cost. For photon-counting applications in the UV, the microchannel plate (MCP) detector is currently the technology of choice for space-based missions owing to the advantages of zero read noise, large formats, ruggedness, high resolution, flexible anode configurations, and photocathode choices, among others. MCP detectors are currently used on the FUSE, GALEX, STIS, SWIFT, and SOHO missions, just to name a few. MCP detectors typically have efficiencies that are ~10-40%, depending on photocathode and stimulating wavelength. In near UV (NUV) band, ~150 nm-400 nm, the Cs2Te photocathode is used (GALEX, STIS) and is limited to QE’s of ~10%. Gallium Nitride is a semiconductor material that has received considerable interest as a UV photocathode. Advances in the growth technology of GaN (MBE, MOVPE, MOCVD) have resulted in high quality p-doped GaN films produced by a number of suppliers. GaN is an intrinsic semiconductor with a band gap of 3.4 eV (~370 nm), having a cutoff

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electronic mail: [email protected] UV, X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XIV edited by Oswald H. W. Siegmund, Proceedings of SPIE Vol. 5898 58980F, (2005) · 0277-786X/05/$15 · doi: 10.1117/12.617517 Proc. of SPIE Vol. 5898 58980F-1

suitable for solar-blind applications which can be tuned by alloying with Aluminum. Like other III-V semiconductor materials such as GaAs, GaN can be p-doped to produce a negative electron affinity (NEA) surface in which photoelectrons are freely emitted from the surface. For a photon-counting imager, the photocathode surface normally serves as the imaging plane and so these photoelectrons are then imaged by a readout. GaN photocathodes can be use in detectors such as proximity-focused MCP tubes and electron bombarded CCDs and have the potential to be applied directly on silicon MCPs. To date, imaging detectors using photoemissive GaN (or AlGaN) have not been fabricated and so we are motivated towards that goal. Photoemissive GaN results from the past few years by Siegmund1, Ulmer2, Leopold3, Norton4 (our group), and Uchiyama5 are encouraging; GaN photocathodes are being produced with quantum efficiencies from ~40% to >50%, and Uchiyama more recently reports QEs of 72% at 230 nm. Achieving a high QE photocathode is not only strongly dependent on the electronic properties of the bulk semiconductor itself, but also the “processing” of the photocathode, which we will loosely refer to as the cleaning of the surface and “activation” through the application of cesium. In order for a photoelectron to overcome the potential barrier at the surface and be ejected, the photocathode must be made to have an NEA surface. The energy levels near the surface, including the vacuum level, are lowered to below the conduction band minimum of the bulk semiconductor. This is referred to as band bending and is accomplished by two methods: 1) doping with a p-type material, usually Mg in the case of GaN, and 2) application of cesium (other materials are possible, but Cs is very common and effective). The creation of space-charge effects near the surface and dipoles on the surface lower the energy bands near the surface so that photoelectrons that have diffused to the surface encounter a lower (negative) potential energy barrier for emission. This process is described in more detail by Bell.6 In order to maximize the probability for electron escape (and achieve high QE), relatively high acceptor doping concentrations (a1x1017 to ~1x1018 to cm-3) are necessary and this is somewhat challenging for GaN, particularly AlGaN alloys. One method of increasing the p-type conductivity is by co-doping GaN with other elements such as O2, H2, and Be. This method has been demonstrated to increase the hole concentration by compensating for donor impurities that otherwise limit the doping level.7,8 Our collaborators (Ulmer, Han) at Northwestern University have provided us with co-doped (O2) GaN that we have processed and we report those results here. In addition to high doping levels it is desirable for the electron mobility to be high, leading to longer escape depths favorable for electron transport to the surface. These topics and the optimization of electronic properties are discussed in more detail by Bell6 and Ulmer.2 The second method of lowering the electron affinity further is to apply a layer of cesium or a combination of cesium with oxygen. For the GaN photocathodes we have processed to date, activation with cesium is essential to achieve the maximum possible QEs, although we often measure a non-trivial QE (up to 7%) without cesium. The role of cesium and oxygen are described in more detail by Bell6 and more recently by Machuca.9 Photoemissive cathodes are generally run in one of two modes, “opaque” and “semitransparent”. In opaque mode, the photoelectrons are emitted back from surface incident by the photons; in this case the photocathode may be made relatively thick. In semitransparent mode the photon is absorbed on the backside after passing through the substrate, commonly sapphire. Due to the finite electron escape length the photocathode cannot be made too thick; in that case photoelectrons would be generated far from the emitting surface and would have a low probability of emission. The thinner photocathode, then, limits the absorption of photons and leads to a lower QE in semitransparent mode, especially at lower wavelengths. Both modes have advantages with regard to fabricating real detectors and so both are pursued. Field-assisted photoemission provides another mechanism in which to improve the quantum efficiency and has been studied by a number of workers.10,11 In field-assisted photoemission, a high electric field is applied near the surface to further enhance electron escape. One method demonstrated by Nemanich12 is to enhance the field by creating pointed surface structures conducive to stimulate field emission These can be made by etching existing material or by growing. The size and shape of these structures are important as they need to be shaped to enhance the localized field, but not so long so that the electron escape depth is increased too much. Bertness13 has demonstrated the ability to grow and control the types of nanostructures on GaN and AlGaN on silicon substrates for our group at GSFC and we have begun to process some of these materials. In this paper we present results from cesiation and calibration of GaN photocathode material we have obtained from SVT, Nanosciences (mfg. ATMI), and Northwestern University. We will conclude with a discussion of our current efforts to seal GaN photocathodes into sealed devices, and further work towards achieving higher QE GaN and AlGaN photocathodes.

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2. SET UP AND PROCEDURE GaN preparation is done in a semiconductor processing facility at GSFC. Here individual samples are diced from wafers after the application of contacts, and then cleaned. Cleaning consists of a pirana acid (H2SO4:H2O2) bath followed by an HF dip. Samples are then loaded into the UHV vacuum chamber, shown in Fig. 1. The UHV system is baked out to 300-350C for 12-24 hrs; base pressures are 1 >1

9.4 2.9 16 3.0 3.3 7.1 1.9 1.9

Peak Cesiation QEa 254 nm % 25 26 30 23 28 27 35 25 23 25 28 27

Stabilized QEb 254 nm % 24 24 29 26 27 29 25

Peak Cesiation QEa 185 nm %

65

50 45

27

Table 1. Quantum efficiency of cesiated GaN measured in opaque mode. aCesiation QE is the peak response during Cs/O2 cycling. b Stabilized QE is obtained after the cesiated GaN has had a chance to stabilize. cPreviously reported4. dAlso previously reported, except a subsequent processing of the sample.

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however, and were able to achieve results very similar to material from SVT and NWU. The poor results from the previous run were likely due to the fact that our sample preparation process was not optimized at that time. We report two different QEs in table 1. The “cesiation QE” is the peak value attained during the Cs/O2 cycling. The “stabilized” QE is measured after a suitable period of time has elapsed (usually >1 day) and gives a measure of the photocathode in a quasi-stable state. During this time the photocathode is stored with its holder in a face down position on a flat surface. We compare the peak QE with the stabilized QE to illustrate that the peak QE may be a reasonable indicator of the long term QEs attainable for a photocathodes once sealed into tubes capable of maintaining very low vacuum conditions. Although not convenient in our set up, we have peaked a few GaN samples while monitoring the 185 nm line and have recorded a QE as high as 65% for one of the 0.2 um SVT samples. The QEs that we obtain during calibration in the demount tube are generally lower than the quasi-stabilized QEs due to a 5-20% fall after a short period of time in the demount tube used for calibration. The calibration QEs at 185 nm are typically ~40%. Recently we have been able to adjust our procedure so that the QE obtained during calibration in the demount tube is closer to the peak obtained during cesiation. NWU BH089 was run 3 times and responded favorably to multiple Cs/O2 cycles and has recorded the highest QE at 254 nm, but it did not seem to stabilize very well following its 3rd processing run. Interestingly, of all of the NWU samples, BH089 is reported to have the highest resistivity in contrast to the high QE measured. The SVT samples provide the best opportunity to explore QE vs. GaN thickness due to the many number run and the fact that the samples are processed in the same facility by the same method. We tabulate the QE obtained during the first cesiation of the material (no O2 cycles) so that the comparison is not complicated with the O2 cycle enhancements which was not always performed to a same method (although the trend is the same). There are many factors not taken into account such as 1) unknown surface cleanliness, 2) unknown variance in the material electronic properties (different locations in wafer), and 3) effects of GaN morphology are not considered. However, there is enough data to show the trend that the material thicknesses of ~0.15 Pm to ~0.20 Pm perform better at 254 nm. A lower QE at 254 nm with the thinner 0.1 Pm material could be explained (partially) by transmission losses. We measured ~6% transmission through the GaN corresponding to an absorption depth of ~38 nm after taking into account the sapphire and AlN buffer layer. This is in good agreement with measurements reported by Muth.15 An internal electrostatic potential near the AlN/GaN interface would also promote the diffusion of electrons to the surface (described by Leopold3). This enhancement would not be a factor if this interface is far from where the photoelectrons are generated as is the case with thick films. This may partially explain the lower QE with the 0.3 Pm GaN. SVT GaN (N samples)

SVT 0.1 Pm (6) SVT 0.15 Pm (3) SVT 0.2 Pm (11) SVT 0.3 Pm (5)

QE (%) at 254nm First Cs Peak Value Exposure (avg.) (avg.) 19.1 21.0 24.3 25.0 24.4 27.0 17.1 20.5

Our results do not indicate a correlation between QE and doping level or conductivity. However, except for the NWU material we do not have reliable data and comparing the samples would have to be done with caution as there are other changing parameters between them. The co-doped samples provided by NWU did not show any improvement in QE although the p-doping levels were not significantly higher than the other NWU samples.

Table 2. An informal trend comparison of QE vs. sample thickness for SVT GaN.

Fig. 3 shows calibration curves for a few of the samples, measured face up in the demount tube. As discussed earlier, these QEs are generally lower than the peak or stabilized QE. The exception is the SVT 0.15 Pm sample; at 254 nm the QE was similar to the peak attained during cesiation (~24%). Unlike the other samples (in this plot) it was not cycled with O2 and so there is potential that it might have been higher by a factor of 10-20%. Note the significant improvement of GaN over the Cs2Te curve of STIS (HST). The increase in QE with decreasing wavelength is consistent with the model of decreasing absorption depth vs. wavelengthbased upon the Spicer 3-step model of photoemission. In contrast with Siegmund1, we do not observe a roll off in the QE below 200 nm. Of the 5 curves we measured that include a 160 nm data point (only 3 are shown), 4 out of 5 had a QE increase of ~4% (absolute) from 185 nm. We measure ~2-3 orders magnitude rejection from 200 nm to 400 nm, in fair agreement with the more-detailed curve reported by Shahedipour.16 The ATMI GaN performed better in this respect, both samples measuring ~0.04% at 404 nm. Remarkably, both ATMI samples measured to within 3% (relative) of each other at almost every data point (only one curve is shown).

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1.E+02

1.E+01

1.E+00

BH162 Upper Limit

ATMI (NS); 3 um SVT; 0.15 um NWU BH162; 1 um NWU BH089; 0.1 um STIS CsTe 1.E-01 150

200

250

Quantum Effieciency (%)

Quantum Effieciency (%)

1.E+02

1.E+01

1.E+00

SVT 0.1um; Opaque 1.E-01

SVT 0.1um; Semitransparent, uncorrected SVT 0.1um; Semitransparent, corrected for AlN and sapphire substrate SVT 0.3um; Opaque

ATMI, 0.04% at 404nm 300

350

SVT 0.3um; Semitransparent

400

450

1.E-02 150

Wavelength (nm)

200

250

300

350

400

450

Wavelength (nm)

Figure 3. Calibration curves for selected cesiated GaN samples measured in opaque mode. Errors (measurement + calibration) are estimated to be r5% except r10% at 160 nm and 404 nm.

Figure 4. Calibration curves for selected cesiated GaN samples measured in semitransparent mode. Errors (measurement + calibration) are estimated to be r5% except r10% at 404 nm.

3.2. Quantum efficiency in semitransparent mode Results from our measurements in semitransparent mode are shown in Fig. 4 for an SVT 0.1 Pm sample and an SVT 0.3Pm sample, both on sapphire substrates polished both sides. These measurements were made with the sample in the cesiation position but illuminated from above. An opaque mode curve for the same sample is shown for comparison. The transmission correction for the 100 nm AlN buffer layer is unknown but significant for the 185 nm data point as it is beyond the 6.2 eV (200 nm) band edge for AlN. As expected, the semitransparent mode QE is lower than opaque mode; we intend to repeat this measurement on more samples to verify the higher QE beyond 340 nm. We also measured a thicker sample in semitransparent mode (SVT 0.3Pm) and confirmed a low QE at 254 nm. Although this was to be expected, GaN films grown under different conditions or methods may produce different results; Siegmund1 has reported high semitransparent QEs (~30%) with thick (1.1 Pm) GaN from NWU. 3.3. Stability 1.1

Sample stored on "flat", ~1cc, left undisturbed

1.0

O2

0.9 O2

Sample stored on "flat", QE checked multiple times

Cs

0.8 Cs

Relative QE

0.7 0.6

Sample quasi-sealed into "demount tube", ~150 cc

Typical Cs/O2 Activiation

0.5 0.4 0.3

Sample fully exposed to UHV system 500C) are required to remove Cs from the surface in a reasonable time period, in agreement with Machuca. One property of these cesiated GaN films that we have studied is the threshold for photocurrent saturation. Factors which we have loosely determined to be contributing are the electric field bias, the geometry of the ohmic contact, the “age” of the photocathode and possibly the termination of the Cs activation. We hypothesize the two latter points are related to conductivity near the surface due to equilibrium condition of cesium. Other factors may include the surface morpholology and the electronic characteristics of the GaN bulk (such as the resistivity). We have measured saturation currents ~20 pA/mm2 in some cases. When present, photocurrent saturation typically results in a 5%-25% drop in photocurrent in 1 minute after the light source is unshuttered. Even though it may not be an issue for photon-counting applications it will need to be considered and better understood and characterized. It is more of a nuisance for our current work and we have had to take steps to ensure it does not confuse our results.

4. FUTURE WORK In the near term, our emphasis is to demonstrate stable GaN photocathodes in sealed diode tubes. We have diode tube hardware in house and are beginning to prepare for assembly and sealing. We then intend to follow with an EBCCD device as a demonstration of an imaging GaN detector. The EBCCD fabrication will require a significant upgrade of our UHV system to include a load lock system to transfer the CCD assembly, which has a limited bakeout temperature, into the main chamber following its bakeout. The load lock will also allow faster testing of GaN samples. In addition to sealing GaN into tubes we plan to continue testing on GaN and other photocathode materials. We intend to do the following: x x x x x x

Test GaN with higher p-doping and co-doping Test Al GaN alloys and heterostructured alloys Continue with processing optimization Test for field-assisted photoemission with various structured GaN samples Obtain more results in semitransparent mode Evaluate alternate UV/VIS photocathodes including MgZnO, InP, and GaAsP

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5. CONCLUSIONS With our baseline procedures we are able to consistently produce QEs ~25%-45% with GaN photocathodes having an estimated p-doping around 1-5x1017 cm-3. Material we have received from 3 different suppliers (with 3 different growth methods) all show remarkably similar QEs, ranging from 24%-29% at 254 nm. Higher QEs are targeted for the next phase of testing using higher p-doped GaN and with structured surfaces conducive for field-assisted photoemisson. We have shown that the stability of the cesiated GaN photocathode is sensitive to the vacuum environment and have quantified this in different vacuum conditions. We have also established that a quasi-stable photocathode can be achieved and we hope to demonstrate long term stability with a diode tube in the near future.

ACKNOWLEDGMENTS The authors would like to thank Nanosciences, Inc. for providing the ATMI GaN and Bing Han from Northwestern University for growing GaN samples. Many thanks to Dave Franz at GSFC for chemical cleaning of the GaN samples (usually on short notice). We would also like to thank Dr. Kris Bertness of NIST for useful discussions on growth of textured GaN for field emission studies. This work was supported by GSFC IRAD Program 51IRAD053.

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