Single Image Spectral Electroluminescence (Photon Emission) of GaN HEMTs P. Scholz, A. Glowacki, U. Kerst, C. Boit, P. Ivo, R. Lossy*, H.-J. Würfl* and Y. Yokoyama** TUB Berlin University of Technology, Einsteinufer 19, D-10587 Berlin, Germany *Ferdinand Braun Institute, Gustav-Kirchhoff-Straße 4, D-12489 Berlin, Germany **Hamamatsu Photonics GmbH, Arzbergerstr. 10, D-82211 Herrsching, Germany Phone: (49)-(030)-314 26803, E-mail:
[email protected] Abstract — Continuous spectra of GaN HEMT photon emission were detected with prism-based optical path. Full spectra are obtained with single emission images by expansion of the emission spot to a spectral tail. Therefore, multi-finger HEMTs require FIB inactivation of excess fingers to ensure single finger operation. The extracted parameter is electron temperature correlated to kinetic energy of the 2DEG electrons. It is fieldrelated and scales with gate voltage, in agreement with device simulation. No spectral peaks were detected.
system using a set of neutral density filters and pinholes. The detected PE spectrum was then compared to the known spectrum of the light source. This comparison yielded the spectral transmission / sensitivity function of the system. This knowledge is necessary in order to be able to obtain a device’s intrinsic PE spectrum from the recorded raw spectral data.
GaN; HEMT; PEM; prism; spectral analysis; FIB; device modification
I.
INTRODUCTION
Spectral electroluminescence measurements (Photon emission PE) of AlGaN/GaN high electron mobility transistors (HEMTs) has so far been performed with time consuming techniques like spectrometers [1, 2] or sequential use of filters [3], with the risk of device degradation during measurement. This paper presents PE spectra obtained quickly in a single image by making use of a prismatic optical path. II.
PRISM IN A PE MICROSCOPE AS SPECTROMETER
The prism is a dispersive unit inserted in the optical path of the microscope. In our configuration a commercially available PEM system, Hamamatsu Phemos 1000, was upgraded and equipped with the prism unit. The prism was inserted between magnification lens turret and tube lens. The magnification lens used for the spectroscopic analysis was typically the 20x. The prism has been intentionally designed to work with this particular objective lens. As a result, using this lens offers the best compromise between the resulting spectral resolution and the spectral range coverage. Two reference calibration experiments were performed. The aim of the first experiment was to obtain the relation between the pixel number in the PEM image and the wavelength of the light. This was necessary in order to be able to convert the raw PE spectrum as read out from the image (PE intensity as a function of the pixel number) into the real spectrum (PE intensity as a function of the wavelength). The aim of the second experiment was to obtain the spectral transmission / sensitivity function of the system, where light of known spectral response was used. The light intensity and spatial signature was adjusted to the requirements of our
Figure 1. How a prism is used in a photon emission microscope to analyze an emission signal spectrally. Local x-y point source information (left) is spread into spectral λ-y information (right).
The prism unit disperses the light in one direction. Therefore, original PE signal distribution in an image must offer one dimension of the image for the spectrum. The schematic of prism-based spectral PE microscopy (SPEM) and principle of what the prism does to an ideal point light source are described in Fig. 1. Ideally, other light sources than the emission source to be spectrally analyzed should be avoided as they can produce artifacts in the spectrum. To be specific, the allowable distances between emission sources in the image should not be smaller than a critical distance which is a function of the magnification used for imaging, the dispersive ability of the prism and also the spectra characteristic of the emission. If the distance between the PE sources is smaller than the mentioned critical value the signals of various emission spots will overlap producing a spectrum that is difficult to
analyze. Typical devices used in this study consist of multiple fingers. For such devices and the 20x magnifying lens typically used for SPEM, the extension of the spectrum in an image ranges to ~130 µm, whereas the distance between the device fingers is ~55 µm. This is illustrated in Fig. 2, where a device emitting only from one of its fingers (see chapter V) is shown, producing spectra relatively free from distortions. However, the SPEM image of the device in its original form with multiple finger emissions would include an overlap of respective spectra. Generally speaking, the device topology may produce artifacts in the spectrum of the emitted light.
condition. The extractable device performance parameter of this exponential function is the electron temperature Te, equivalent to the average kinetic energy of the carriers before relaxation. The Te extracted from the PE spectrum presented in Fig. 4 is 1020K [4]. This method of analyzing PE spectra can be applied to MOSFET and also to HEMT devices.
Figure 4. PE spectrum of Si MOSFET operated in saturation (VG = VD = 1.2V) as measured with SiCCD. Te ≈ 1020K [4] Figure 2. Micrograph of the typical AlGaN / GaN HEMT device (left), PEM image (middle) and prismatic PEM image (right). It depicts that the spectrum takes more distance of the image than the distance between the device fingers.
III.
ANALYSIS OF PE SPECTRA
PE spectra of Si devices have been studied since the ‘80s. Related to PE spectra of MOSFETs in saturation there is a general agreement that scattering of field accelerated electrons in the pinch-off region of the transistor results in a pure exponential decay of the PE intensity with increasing photon energy. A typical image of a pure MOSFET PE and an image of spectrally resolved PE are shown in Fig. 3.
Figure 3. Comparison of pure PE image and prismatic PE image of Si MOSFET operated in saturation condition. Images acquired at 20x magnification.
The typical PE spectrum once corrected for the optical function of the optical path, spectral sensitivity of the detector and absorption in the remaining Si layer is shown in Fig. 4. The resulting exponential decrease of PE intensity over photon energy has been explained by the Maxwell-Boltzmann character of the relaxation energy distribution produced by hot electron scattering in the MOSFET channel under saturation
IV.
DEVICES
Aiming at an optical inspection from the front side, the three different HEMT devices presented in this work can roughly be divided in two groups: without and with covering polymer layer. This is due to their field of application. The first device, referred to as “noBCB” is a microwave transistor, allowing for high-speed field effect devices with simultaneous high-charge carrier concentration and relatively high breakdown voltage. It is grown on semi-insulating SiC substrate. The other two devices (“BCB1”, “BCB2”) are devices with the same lateral dimensions (layout), however fabricated using a process towards GaN power electronic devices and are grown on n-type SiC substrate. Their high electron density and mobility in the transistor channel results in low on-state resistances combined with high breakdown voltages. In order to avoid surface arcing at high operation voltage, the latter two are partially covered with an insulating polymer layer (Benzocyclobutene - BCB). As this work later reveals, BCB coated devices have optically unique behavior. A more detailed comparison of the samples can be obtained from Fig. 5 and Tab. 1. The epitaxial structure between substrate and source-gate-drain layer of “noBCB” starts with a GaN buffer followed by an AlGaN spacer, an AlGaN supply doped with Si (5x1018 cm-3), an AlGaN barrier and ends in a GaN cap layer. The samples “BCB1” and “BCB2” are mostly similar to each other and differ only in the middle layers: The epitaxial structure is either a standard GaN buffer (BCB1) or an Al0.05Ga0.95N back-barrier in combination with a GaN channel layer (BCB1). The latter epi structure is especially designed for high voltage device operation. The top barrier layer consists of Al0.23Ga0.77N. Further comparison of “BCB1” and “BCB2” can be found in [5].
Figure 5. Simplified structure of the three presented samples - “noBCB” (left), “BCB1” (middle) and “BCB2” (right) with individual layers (see Tab. 1), source (S), gate (G), drain (D) and two-dimensional electron gas (2DEG). Dimensions of the layers and proportions are not to scale.
TABLE I. Device
DIFFERENCES IN SAMPLES NOBCB, BCB1, BCB2. noBCB (D)
BCB1 (B)
BCB2 (A)
Thickness (in nm) - Material
Layer BCB
-
4300 - C8H8
Cap
5 - GaN
-
-
Barrier
10 - AlGaN
30 Al0.23GaN0.77N
30 Al0.23GaN0.77N
Supply
12 - AlGaN: Si 5x1018cm-3
-
-
4300 - C8H8
Spacer
3 - AlGaN
-
-
Channel layer
-
15 - GaN
-
Buffer/ Backbarrier
2500 - GaN
1840 Al0.05GaN0.95N
2400 - GaN
SiC substrate
Semi-insulating
n-type
n-type
V.
of a two finger device reveals a reasonably thin line source for each finger (see Fig. 6). Each source individually could be used for prism analysis (when put into vertical direction), but both of them at once would interfere. The amount of pixels between them is smaller than what is needed to extract the full spectrum of interest during prism analysis (see Fig. 2). Using a higher magnification of the microscope for a smaller field of view would cut off the interference, but also part of the spectrum. Another idea could be to use a weaker prism, which doesn’t spread out the spectrum as much. This might solve the issue of interfering emission sources, but would require a recalibration of the system and result in a reduced spectral resolution. Rotating the sample in order to increase the distance between emission sites is in some cases a feasible solution. The resulting spectra need to be analyzed carefully to avoid spectral superposition. This method was also applied to the presented samples, but did not lead to reliable results on its own. Covering the second source by inking or using an aperture was also considered. It was, however, not carried out, due to the small dimensions of the samples and additional optical effects that could not have been avoided: It is shown later on that a top layer of BCB can act as planar light guide, which means that the influence of a single emission site affects a larger area and could not be prevented by covering the surface. Because of all this, the two-finger device needed to be turned into a one-finger device.
DEVICE READINESS FOR SPECTRAL PE MEASUREMENT
As discussed in II. the ideal case for spectral PE measurement with prism is a point source. While measuring, the prism spreads this signal in horizontal direction (see Fig. 13). Sources above or below have little or no influence. Therefore it is within reason to neglect any interference in ydirection, meaning a vertical line source can also be used. Based on this, the first key to successful measurements is to use corresponding emission sources (ideally single finger devices). In multi-finger devices excess fingers have to be inactivated. The samples used in this work are HEMTs with two gate fingers, which run close to the source region. The devices are asymmetric, which means that the gate-drain distance is much larger than the gate-source distance. Hence, as the PE is mainly generated at the drain-side edge of the gate, the emission image
Figure 6. Image of a two-finger GaN HEMT device from an optical microscope (left) and an emission image superimposed onto an LSM image with vertically aligned gate fingers (right). The emission occurs in the region of the gate fingers stretching from the gate to the drain contact.
Modifying the coplanar probers, which were used for the measurements, was not an option since consistent measuring conditions needed to be guaranteed. Furthermore, the two source pads are connected electrically, meaning contacting one would also supply the other. Therefore the samples needed to be modified. Laser cutting turned out to be too destructive, therefore excess source and gate regions were cut off by focused ion beam technique (FIB). Fig. 7 illustrates the FIB cuts on a sample without BCB. Two large cuts are needed on the source pad to isolate the contacts of the coplanar prober and another separating the upper and lower source pad. The fourth cut is directly on the gate contact, disconnecting the gate finger. The FIB process uses the assistance of an iodine-based chemistry to cut the metal more efficiently. Every modified site is covered with FIB-deposited insulator right after cutting to avoid reconnection. Working on the samples without BCB is comparable to a standard FIB job, since the removal of material can be observed in the FIB image by the change in material contrast. However, this is not the case for the samples with
BCB, because the surface of these samples is well isolated and can not be grounded. The FIB operator then has to deal with the absence of different material contrasts and needs to avoid punch throughs. Despite of this, it was possible to successfully modify samples of type “BCB1” and “BCB2”. The needed depths of the cuts could be derived from technology parameters and was achieved after FIB process development using an atomic force microscope for verification. The overall FIB processing time leads up to five hours, not counting set up.
Figure 7. FIB images before (left) and after (right) modification with FIB. The two-finger device is turned into a one-finger by separating the upper gate finger of these images. Four cuts (arrows right image) are needed for this procedure: three on the source and one on the gate itself.
Figure 8. ID(VDS, VGS) measurements before and after cutting one of the two gate fingers with FIB. Data of two-finger device is multiplied by 0.5 for direct comparison and shows similar behavior as the one-finger device.
three based on the electrical behavior of the device (-7 V OFF, -5 V pinch off, -1 V ON) and two at the maximum intensity of PE (-4 V, -3.5 V).
Figure 9. Defining the parameters for the emission analysis of the device from its transfer characteristic (ID over VGS) and the intensity of electroluminescence (EL over VGS) at VDS = 10 V. Chosen parameters of VGS are -7 V (OFF-state), -5 V (threshold), -4 V, -3.5 V (both close to EL max.) and -1 V (ON-state).
The PE spectra acquired for device “noBCB” for mentioned operating conditions are presented in Fig. 10 and Fig. 11.
Figure 10. PEM images acquired for sample “noBCB” in the condition of maximum PE intensity (VG = -3.5V).
The ohmic properties of the cuts were verified. Similarly the performance of the device before and after FIB was analyzed. Fig. 8 compares the output characteristics of the twofinger and the modified one-finger device (dashed and solid line). The data of the two-finger device were multiplied by 0.5 for better comparison (“x” data points). It shows that the FIB cut devices drive half of the current and behave just like it would be expected from “half a two-finger device”. The FIB modification can therefore reliably prepare two-finger devices for spectral prism analysis. The results of the upcoming chapters further underline this fact. VI.
PE SPECTRA OF DEVICE WITHOUT POLYMER
The proper parameters for an emission analysis are derived from the transfer characteristic of the device (drain current over gate-source voltage) and the PE intensity also dependent on gate-source voltage (Fig. 9). Five different voltages were chosen for the device without BCB in most measurements:
Figure 11. PE spectra for sample “noBCB” acquired in several operating conditions of gate voltage: -5V, -4V, -3.5V and -1V. Drain voltage was kept constant: VD = 10V
Drain voltage was always set to 10V. Presented spectral PE characteristics show an exponentially decreasing PE intensity as a function of the photon energy with slight slope change at
the different gate voltage conditions. The extracted electron temperatures plotted as a function of the VG are presented in Fig. 12. The Te is increasing as the negative VG is elevated.
One simple action that could be taken to check how the artifacts affect the resulting spectrum of the light originating from the area of interest is to rotate the sample 180° and compare the corresponding spectra. The cases with the two device orientations (0° and 180°) will be called P1 and P2. The spectral PEM images taken for sample “BCB1” for both sample orientations (P1 and P2) are shown in Fig. 14. The effect of the rotation is similar for sample “BCB2” (no additional illustration is shown).
Figure 12. Electron temperature as a function of gate voltage for sample “noBCB“. Te extracted from PE spectra acquired using 20x magnifying objective lens.
VII. PE SPECTRA OF DEVICES WITH POLYMER Samples “BCB1” and “BCB2” have been tested at similar operating conditions as sample “noBCB” in order to cover the OFF and ON states and the condition of the maximum PE intensity. They were analyzed in previous works [5], though not yet spectrally with prism. The exemplary PEM images of both samples are shown in Fig. 13.
Figure 14. PEM and SPEM images acquired for sample BCB1 operated at VG = -1V for two orientations (P1 and P2).
The PE spectra have been acquired for the locations as indicated by the red rectangles in the spectral PEM images of the Fig. 14. The comparison of the PE spectra acquired for both device orientations is shown in Fig. 15.
Figure 13. PEM images acquired for sample BCB1 and BCB2 in the condition of maximum PE intensity (VG = -3V)
Unlike in sample “noBCB”, one can observe several parasitic emission locations. The expected emission around the connected transistor finger is still the strongest emission in the images. However, there are some other emissions with approximately an order of magnitude weaker intensity. A possible explanation of these additional signals involves the transmission of the light in the passivating layer and reflection from some topological structures. It can be expected that for part of the original emission the passivating layer of polymer on the devices with BCB creates total internal reflection. The signal then travels inside the BCB layer until it hits a topological feature, like the disconnected second gate finger. This diverted part of the initial emission is also detected during analysis and creates the artifacts. These artifacts need to be avoided or dealt with properly to achieve reliable spectral results.
Figure 15. PE spectra acquired for sample BCB1 operated at VG = -1V for the two device orientations (P1 and P2).
The difference is clearly visible. A closer look at the spectra leads to the conclusion that none of them is free from distortions originating from the parasitic emission sites over the entire spectral regime. For the P1 orientation the impact is severe in the low wavelength regime (high photon energies), whereas for the P2 orientation the high energy regime seems to be unaffected. Only at the very low energy end of the spectrum, the slope is steeper than for the rest of the spectral range. Hence, from the two approaches, the analysis using the P2 orientation provides much more reliable results. They remain
consistent with other measurements acquired for other samples that do not have the issue of parasitic emission locations, and they are also in agreement with the expectations. The P2 orientation has been chosen for final analysis of the sample types “BCB1” and “BCB2” as it delivered more reliable and consistent results. These results will be presented now. The extraction of parameters from the light spectra has been done in the range above 1.4 eV to avoid the distorted part of the spectrum. The PE spectrum has been acquired for two operating condition for sample “BCB1”: VG = 0 V and VG = - 1 V (Fig. 16).
The electron temperatures for sample “BCB2” have been extracted in the spectral energy regime between 1.4 eV and 2 eV. The results are presented in Fig. 18.
Figure 18. Electron temperature as a function of gate voltage for sample BCB2. Te extracted from PE spectra acquired using 20x magnifying objective lens.
The PE spectra have also been compared for all three available samples for the operating condition VG = -1 V and VD = 10 V. The comparison is presented in Fig. 19. Figure 16. PE spectra acquired for sample BCB1 operated at VG = -1V and VG = 0V for the P2 orientation.
The electron temperatures for “BCB1” (Fig. 16) have been extracted in the range of photon energies between 1.4 eV and 2.4 eV and the numbers are: 1864 K for VG = 0 V and 2020 K for VG = -1 V. The PE spectra for sample “BCB2” have been analyzed for following gate voltages: 0 V, -1 V, -2 V, -3 V, -5 V and -7 V. The location within the homogenous part of the finger-related emission has been chosen for the spectral analysis. P2 orientation has been used. The results are presented in Fig. 17. Figure 19. Comparison of PE spectra for all analyzed samples (noBCB, BCB1 and BCB2) for the operating condition: VG = -1V, VD = 10V.
Figure 17. PE spectra acquired for sample BCB2 operated at several gate voltages for the P2 orientation.
It can be noticed that some spectra in Fig. 15-17 show ripples in the 2.2 to 2.4 eV regime. This can be explained by interference of light due to total internal reflection inside the passivating layer or within the device [6].
PE spectra for samples “BCB1” and “BCB2” show distortion from the artifact emission, especially in the low energy end of the spectrum. There is also minor distortion at the high energy end of the spectrum starting at different energies depending on the sample. This is later limiting the useful range for the Te extraction. Sample “noBCB” shows no distortion below 1.8 eV as there are no artifact emissions. However, the overall PE intensity is lower and therefore the PE spectrum is prone to noise at the high energy end of the spectrum above 1.8 eV. This also limits the useful range in terms of parameter extraction such as Te. As the electron temperatures have been extracted for all the analyzed sample types operated in various electrical conditions, the results are presented in one comparison plot shown in Fig. 20. It was expected that the increase of the extracted Te with the gate voltage is the result of the electrical field increase. To find a proof of this statement the simulated maximum of the electrical field in sample “BCB2” [7] has been compared to the Te. This comparison is shown in Fig. 21. Electron temperature
and electrical field show a qualitatively similar dependence on gate voltage.
While the main focus of this work is to illustrate the applicability of prism-based spectral analysis, the recorded spectra furthermore showed no peaks from possible point defect related radiant transitions [1, 8]. The electron temperatures extracted from the spectra indicate that this parameter is a quantitatively matching indicator of the electric field in the channel as induced by the gate voltage. ACKNOWLEDGMENT
Figure 20. Electron temperature as a function of gate voltage for samples noBCB, BCB1 and BCB2 (threshold voltages are -5 V, -2 V, -3 V respectively). Te extracted from PE spectra acquired using 20x magnifying objective lens.
The authors thank DCG Systems Inc. for support of OptiFIB at TUB. We thank the Berlin semiconductor devices research team, namely Andreas Eckert for meticulous device preparation, Parts of this work have been funded by the space agency of France, CNES, filed under n° 4700033802 / DCT094 of 12.07.2011. We thank Philippe Perdu (CNES) for facilitation of this project and very productive meetings. REFERENCES [1]
[2]
[3]
[4]
Figure 21. Electrical field [7] and electron temperature for BCB2 as a function of gate voltage.
VIII. CONCLUSION It was shown that dispersing an emission signal by inserting a prism into the optical path of a photon emission microscope offers reliable spectral analysis while needing only single image acquisition. This method was applied to different types of HEMT devices, which illustrated the challenges of turning a laterally resolved image into a spectrally resolved one: The geometry of the sample needs to be taken into account in detail. This was dealt with accordingly, partially by editing the sample with FIB and repositioning the sample.
[5]
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