using atom probe tomography, secondary ion mass

Characterization of AlN/AlGaN/GaN:C heterostructures grown on Si(111) using atom probe tomography, secondary ion mass spectrometry, and vertical current-voltage measurements Martin Huber, Ingo Daumiller, Andrei Andreev, Marco Silvestri, Lauri Knuuttila, Anders Lundskog, Michael Wahl, Michael Kopnarski, and Alberta Bonanni Citation: Journal of Applied Physics 119, 125701 (2016); doi: 10.1063/1.4944652 View online: http://dx.doi.org/10.1063/1.4944652 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/119/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Study on GaN buffer leakage current in AlGaN/GaN high electron mobility transistor structures grown by ammonia-molecular beam epitaxy on 100-mm Si(111) J. Appl. Phys. 117, 245305 (2015); 10.1063/1.4923035 Impact of residual carbon on two-dimensional electron gas properties in AlxGa1−xN/GaN heterostructure Appl. Phys. Lett. 102, 193506 (2013); 10.1063/1.4804600 Atom probe analysis of AlN interlayers in AlGaN/AlN/GaN heterostructures Appl. Phys. Lett. 102, 111603 (2013); 10.1063/1.4798249 III-nitride heterostructure field-effect transistors grown on semi-insulating GaN substrate without regrowth interface charge Appl. Phys. Lett. 92, 133513 (2008); 10.1063/1.2906372 Secondary-ion-mass spectrometry and high-resolution x-ray diffraction analyses of GaSb–AlGaSb heterostructures grown by molecular beam epitaxy J. Vac. Sci. Technol. B 19, 836 (2001); 10.1116/1.1372926

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JOURNAL OF APPLIED PHYSICS 119, 125701 (2016)

Characterization of AlN/AlGaN/GaN:C heterostructures grown on Si(111) using atom probe tomography, secondary ion mass spectrometry, and vertical current-voltage measurements Martin Huber,1,a) Ingo Daumiller,2 Andrei Andreev,2 Marco Silvestri,2 Lauri Knuuttila,2 Anders Lundskog,2 Michael Wahl,3 Michael Kopnarski,3 and Alberta Bonanni4

1 Infineon Technologies Austria AG, Siemensstrasse 2, A-9500 Villach, Austria and Institute of Semiconductor and Solid State Physics, Johannes Kepler University, Altenbergerstrasse 69, A-4040 Linz, Austria 2 Infineon Technologies Austria AG, Siemensstrasse 2, A-9500 Villach, Austria 3 IFOS Institut fuer Oberflaechen- und Schichtanalytik GmbH, Trippstadter Strasse 120, D-67663 Kaiserslautern, Germany 4 Institute of Semiconductor and Solid State Physics, Johannes Kepler University, Altenbergerstrasse 69, A-4040 Linz, Austria

(Received 22 December 2015; accepted 9 March 2016; published online 22 March 2016) Complementary studies of atom probe tomography, secondary ion mass spectrometry, and vertical current-voltage measurements are carried out in order to unravel the influence of C-doping of GaN on the vertical leakage current of AlN/AlGaN/GaN:C heterostructures. A systematic increment of the vertical blocking voltage at a given current density is observed in the structures, when moving from the nominally undoped conditions—corresponding to a residual C-background of 1017 cm3—to a C-content of 1019 cm3 in the GaN layer. The value of the vertical blocking voltage saturates for C concentrations higher than 1019 cm3. Atom probe tomography confirms the homogeneity of the GaN:C layers, demonstrating that there is no clustering at C-concentrations as high as 1020 cm3. It is inferred that the vertical blocking voltage saturation is not likely to be C 2016 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4944652] related to C-clustering. V I. INTRODUCTION

The next generation of high-power high-electron-mobility-transistors (HEMTs) based on GaN is expected to be developed mostly by metalorganic vapor phase epitaxy (MOVPE) on Si(111), provided that appropriate strainengineered multilayers are deposited between the Si substrate and the GaN buffer1,2 to compensate the lattice- and thermal-mismatch. In order to minimize the leakage current in the devices, the electrical resistance of the GaN buffer should be maximized by compensating the unavoidable n-type conductivity of the MOVPE GaN with the introduction of acceptor-like impurities like Fe or C.3–5 While Fe and other transition metals are to be avoided in front-end-of-line Si technology, C is considered an acceptable source of donor compensation centers also in the Si production lines. The role played by C as amphoteric impurity in the compensation processes of GaN is still a matter of debate, and various compensation mechanisms were proposed. For instance, in a number of studies, it was suggested that C substitutional of N (CN) acts as a shallow acceptor,6–8 and the semi-insulating (SI) behavior of GaN:C results from autocompensation through the interplay of CN and CGa states introduced in comparable concentrations during the epitaxial process.6,9,10 In this case, the Fermi level is predicted to be found around the midgap, making the C-doped GaN layer to behave as a quasi-ideal SI. Presuming equal densities, the two C defects would be neutral, and the Fermi level will actually be pinned by any other defect whose level lies a)

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between the levels of CN and CGa. In this perspective, C doping was explored as a viable way to obtain p-type GaN(0001) layers.11 Alternatively, compensation by a dominant CN acceptor level9,12 was considered, where the Fermi level is assumed to be pulled towards the bottom half of the GaN bandgap, turning the GaN:C layer into a weakly p-type region. Theoretical studies had also reported deep levels arising from interstitial C with a high enthalpy of formation.13,14 Irrespective of the nature of the dominant compensation mechanism, a quite high concentration of C is required in order to ensure an efficient amount of donor compensation centers. This leads to a number of undesirable effects, like the formation of various structural defects,15 and possibly the segregation and clustering of C atoms, which are in turn likely to hamper an effective compensation. Similar adverse effects had been recently observed in highly Mg-doped GaN layers.16 For an efficient GaN-based device, it is of paramount importance to minimize trapping related failures such as current collapse and dynamic on-resistance (RDSON) degradation. The location of the physical trapping sites and the mechanisms accounting for dynamic instabilities in C-compensated HEMT structures have been widely treated.10,17 It was discussed that these effects are generally enhanced through the implementation of low leakage current C-compensated GaN:C buffers.10,18–20 On the other hand, in a recent work, it was shown that a high leakage current is not necessarily needed in order to achieve low dynamic shifts.21 This investigation points at understanding the influence of the C concentration and of the GaN:C layer thickness on the

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vertical blocking strength solely. However, in our previous work on structures analogous to those presented here, we found out that the impact of the GaN:C layer on the dispersion behavior can be suppressed by improving the quality and the thickness of the unintentionally doped GaN layer forming the channel.22,23 In this work, we employ a protocol of complementary characterization techniques in order to elucidate the relationship between vertical leakage current and C concentration in AlN/AlGaN/GaN:C heterostructures. In particular, 3D atom probe tomography (APT) provides access to 3D compositional mapping of carbon with atomic lateral resolution and detection sensitivity down to 10 ppm.24 Measurements of 3D-APT have already given insight into Mg doping of GaN layers, where a lateral clustering of the Mg in the nm range was found for high Mg concentrations.16 II. EXPERIMENT

The AlN/AlGaN/GaN:C heterostructures considered in this work are grown on 150 mm Si(111) substrates by MOVPE in a multi-wafer AIXTRON planetary reactor. The architecture of the epitaxial stack is sketched in Fig. 1(a). It consists of: (i) the same 500 nm thick AlN/AlGaN multilayer grown on the Si substrate for all investigated samples and (ii) a GaN:C layer with thickness between 4.5 lm and 7 lm and a C concentration between 1017 cm3 and 1020 cm3 over the samples’ series. The C concentration in the GaN:C layer has been varied from sample to sample using a hydrocarbon gas source similar to the one reported in Ref. 15 and by tuning the basic epitaxial growth parameters.9 In order to investigate the electrical properties of the different samples, current-voltage (I-V) measurements have been carried out. The measurements have been performed on a semi-automatic probe station with a Keithely 2657A high power system source meter. The substrate is grounded during all measurements, and the top side contact is connected to a positive potential as indicated in the configuration outlined

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in Fig. 1(b) by using a Ti/Pt/Au circle multi-metal contact with the diameter of 1 mm fabricated by shadow masking without passivation of the surface of the heterostructures. The room temperature (RT) I-V curves are acquired stepwise between 0 V and the maximum voltage at a current limit of 1 mA. A leakage current threshold of 10 lA/mm2 has been introduced to determine the blocking voltage, which at the given current density is divided by the total sample thickness as determined by Fourier transformed infrared (FTIR) spectroscopy carried out in a BIORAD QS-500 system. This procedure allows to adjust the data with respect to the total thickness of the epitaxial structure. The maximum normalized blocking voltage per micron (BV/lm) is taken as benchmark parameter in comparing the various heterostructures considered BV voltage at 10 lA=mm2 ¼ : thicknesstotal lm

(1)

The blocking voltage and the layer thickness have been collected from five points using a center to edge line pattern on all the investigated 6 in. wafers. An edge exclusion of 5 mm has been adopted. In order to determine the total C concentrations in the GaN:C layers,25 secondary ion mass spectrometry (SIMS) has been performed in a Phi-Evans quadrupole secondary ion mass spectrometry system equipped with a Cs primary ion source. The SIMS measurements have been performed at the center, half-radius, and edge (10 mm of edge exclusion) on all the investigated 6 in. wafers. Measurements of 3D-APT have been carried out using a LEAP 4000X HR instrument. It is equipped with a reflectron-type time-of-flight mass spectrometer, combined with a double multichannel plate and a triple delay line detector. A pulsed UV laser (k ¼ 355 nm, pulse length 10 ps, and repetition rate 250 kHz) enables the analysis of semiconducting or non-conductive specimens. During the measurements, the chamber pressure has been kept at 1011 mbar, and the temperature of the specimens in the range of (25–30) K. The data reconstruction is accomplished with the software package IVAS3.6.8. The needle-shaped specimens for APT have been prepared by applying the cut-and-lift-out method26 in an ALTURA 875 dual-beam focused ion beam (FIB) workstation with an in situ Kleindiek micromanipulator. The samples have been Cr coated to protect them during the ion beam treatment. A wedge-shaped specimen is cut out by FIB and attached to a Si microtip coupon containing an array of 36 tips. The final sharpening of the tip has been obtained by ion milling. III. RESULTS AND DISCUSSION A. Vertical leakage measurements and secondary ion mass spectrometry

FIG. 1. (a) Architecture of the investigated structures. (b) Sketch of the setup for vertical leakage (blocking voltage) measurements.

Considering that the voltage drop caused by the Schottky behavior of the employed metal layers is negligible compared to the voltage blocking of the epitaxial stack, an ohmic behavior of the contact scheme is not required for the vertical leakage current or blocking voltage analysis. As


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FIG. 2. RT current-voltage plots at RT for three samples with carbon concentration of 5 1017 cm3, 2.8 1018 cm3, and 3.5 1019 cm3, respectively.

seen in Fig. 2, the C concentration significantly influences the I-V behavior of the samples, leading to an onset in the voltage blocking capabilities similar to the one reported in Ref. 27. In particular, the higher the C content in the GaN layer, the more pronounced the tendency towards a SI behaviour of the samples. The I-V curves reported in Fig. 2 are characteristic for GaN HEMTs grown on Si substrates, and currently a considerable effort is being devoted to the understanding of these trends, as witnessed by, e.g., Refs. 28–31. In Fig. 3, for all investigated samples, the BV/lm is plotted as a function of the C concentration in the GaN:C layer, as determined via SIMS measurements. In the lower C content range between 1017 cm3 and 1018 cm3, the GaN:C buffer is considerably leaky due to uncompensated impurities. In the C doping interval 1018 cm3 to 1019 cm3, an enhancement of the vertical blocking voltage is detected. Finally, the values of BV/lm saturate for C concentrations greater than 1019 cm3, suggesting that all intrinsic donors are likely to be compensated from this concentration on. In Fig. 4, the radial distribution on a representative 6 in. wafer is shown for C-concentration, BV/lm, and XRD-FWHM data. The C-concentration and the BV/m are very uniform across the wafer, whereas the XRD-FWHM diffraction peaks are increasing from center towards the wafer edge. In Fig. 5, the BV is plotted as a function of the thickness of the GaN:C layer. For a C concentration between 1017 cm3 and 5 1018 cm3, there is very low thickness

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FIG. 4. Representative radial distribution of BV/lm, XRD-FWHM, and C concentration on the 6 in. wafer.

dependency of the BV. For C in the range of 5 1017 cm3 to 5 1018 cm3, the BV increases weakly with the thickness of the GaN:C layer. For C concentrations higher than 5 1018 cm3, a significant increase of BV with the GaN:C thickness is observed. A linear fitting and extrapolation of all three curves gives a value of 110 V for BV at thickness of the GaN:C layer equal to 0 lm. This residual voltage is considered as due to the contribution from AlN/AlGaN multilayer between Si(111) substrate and GaN:C layer. It is worth noting that with these premises, a further increment of the vertical blocking voltage can only be achieved by increasing the thickness of the GaN:C layer, arising issues related to strain and crack management during the epitaxial growth. The understanding and control of the BV/lm saturation level are therefore of uttermost relevance for the development of GaN:C-based high voltage HEMT structures. GaN grown on Si is a very defective material, and previous work on GaN based light emitting diodes (LEDs) had evidenced that reverse biased leakage is linked to the density of screw and mixed dislocations.32,33 Although the LEDs are based on different structures and dopants, it has often been seen as likely that dislocations in GaN:C are also electrically active under high field conditions.34 Remarkably, the saturation effect in our sample set is not critically dependent on the crystal quality of the GaN:C, as evidenced by the full width at half maxima (FWHM) of the (002) and (102) diffraction peaks, which can be correlated to the presence of screw and edge type dislocations.35 The peaks have been obtained from

FIG. 3. Blocking voltage per lm (BV/ lm) as a function of the C concentration. The data points represent the median value of each 6 in. wafer, the error bars represent the minimum and maximum of each 6 in. wafer. The solid line is a guide to the eye.


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FIG. 5. Blocking voltage obtained from I-V measurements as a function of the GaN:C layer thickness. The data points represent the median value of each 6 in. wafer, the error bars represent the minimum and maximum of each 6 in. wafer.

high resolution x-ray diffraction and are shown in Fig. 6. The total dislocation density has been estimated from TEM bright field imaging for a sample with the structure sketched in Fig. 1(a) and a C-concentration of 1019 cm3. As a comparative number, the dislocation density has been found to be in the range of 5 109 cm2 in the GaN:C layer. Thus, at first glance, it appears that there is no correlation between the crystal quality and the BV/m of the material; however, one cannot rule out potential interdependencies between the variables. For example, if C generates dislocations which act as leakage paths, but also suppress the leakage along those at the same time. Further investigations are required for an indepth understanding. One possible mechanism which could limit the vertical leakage strength is the clustering of C atoms for high C concentrations. B. Atom probe tomography

In order to rule out C-clustering, 3D-APT has been carried out on samples with different total concentrations of C in the GaN:C layers.

FIG. 6. XRD-FWHM in (002) and (102) directions as a function of the C concentration. The data points represent the median value of each 6 in. wafer, the error bars represent the minimum and maximum of each 6 in. wafer.

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FIG. 7. Mass spectrum of a sample with a C concentration of 1020 cm3 in the GaN:C layer.

In Fig. 7, the mass spectrum of a specimen with a GaN:C layer with a C concentration of 1020 cm3 is reported. Besides the ions related to the GaN matrix, different peaks corresponding to C atoms and CN and CN2 molecules are detected at 6, 12, 20, 26, and 40 u, respectively. Other background impurities typical for GaN, such as Si and O22 (with concentrations in the range of 1016 up to 1017 cm3), are not observed in the mass spectrum, their concentrations being lower than the APT detection limit, which is 10 ppm (equivalent 8 1017 cm3 for GaN) with a mass resolution m/DmFWHM 1.0 and a detection efficiency of 36% of all emitted ions. A six-fold symmetry of the detector ion event histogram (not shown here), due to the GaN crystal symmetry, rules out amorphization of the specimen upon preparation. Based on the peak assignment in the mass spectrum of Fig. 7, a thorough 3D reconstruction of the analyzed tip volume can be carried out. The projection of the lateral distribution of all C-atoms obtained from the APT measurement of a sample with a nominal C concentration of 1020 cm3 is shown in Fig. 8(a), where every dot corresponds to a detected C atom. The mean concentration of the C dopants determined from four individual tips of the same sample is (1.017 6 0.067) 1020 cm3. The zoom into the directions perpendicular to the X-Z plane for the analyzed tip volume as reported in Fig. 8(c) provides an accurate lateral distribution of the C-atoms. It is worth noting that in all presented reconstruction maps the distribution of C is homogeneous, and no evidence of C-clusters can be found. The measurements have been repeated for three different tips from each sample, and the results could be systematically reproduced. In order to prove the sensitivity of the method to inhomogeneities, the results obtained on a GaN:Mg sample doped with a Mg concentration of 1020 cm3 (Ref. 16) are shown in Figs. 8(b) and 8(d) for a specimen prepared according to the procedure described above. In contrast to the C case, the distribution of the Mg atoms is here inhomogeneous, pointing to a Mg-clustering evidenced in Fig. 8(d). The main issue related to the APT results lies in whether we face a random lateral distribution or a tendency to


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higher concentrations, where a significant deviation from the binomial distribution is assigned to Mg-clustering. C. Saturation effect

FIG. 8. 3D-APT reconstruction of the distribution of the C (a) and Mg (b) atoms in the X-Z plane; zoomed 3D distribution of C (c) and Mg atoms (d) in the X-Z plane.

clustering of the C atoms in the analyzed volume. In this perspective, a statistical approach based on the calculation of the frequency distribution36 has been applied. The data from the 3D reconstruction have been binned in 100, 500, and 1000 atom bins, respectively, and the number of C atoms in each bin has been calculated. A regime of 100, 500, and 1000 atoms would have a spherical real space dimension of 1, 1.5, and 2 nm in diameter, which is of the same order of magnitude as the one of the clusters found for a comparable Mg doping of GaN, as reported in Figs. 8(b) and 8(d). In Fig. 9, the frequency distribution of the C and Mg atoms for two samples with a carbon concentration 8 1018 cm3 and 1020 cm3 and two samples with a Mg concentration of 5 1019 cm3 and 1020 cm3 is provided. The theoretical binomial distribution expected for the case of a random alloy is given in Fig. 9. For both C-doped samples, an agreement of the experimental data with the binomial distribution is obtained, in contrast with the Mg distribution for

The compensation effect of C in GaN may be seen in the context of the two mechanisms previously mentioned, namely: (i) auto compensation of CN CGa and (ii) compensation through a CN acceptor level. One interesting question independent on the mechanism in place is whether the saturation level of the blocking voltage can be shifted to higher C concentrations. Considering the theoretical equilibrium formation energy for different species,6,12 a lowering of the V/III ratio (Ga-rich) during GaN:C growth is expected to decrease the enthalpy for incorporation of CN and to increase the one of CGa. Whether the origin of the saturation effect resides mainly in the fact that at 1019 cm3 all intrinsic donors of GaN are compensated, or whether it depends on the number of substitutional CN and CGa lattice sites or even on the dependence of the CN/CGaN ratio on the C concentration, it must be clarified. On the other hand, the above results rule out C-clustering as responsible for the blocking voltage saturation. IV. SUMMARY AND CONCLUSION

In conclusion, the influence of the C concentration on the vertical leakage current of AlN/AlGaN/GaN:C heterostructures has been investigated by means of vertical I-V measurements, SIMS, and APT. An increase of the vertical blocking voltage at a given current density from undoped conditions (1017 cm3) up to C concentrations of 1019 cm3 in the GaN:C layer is observed. For C concentrations higher than 1019 cm3, the vertical blocking voltage saturates. Atom probe tomography results provide no indication of C-clustering in the GaN lattice C concentrations up to 1020 cm3, ruling out the clustering of C as responsible for the vertical blocking voltage saturation. From these findings, we can remarkably infer that the blocking

FIG. 9. Frequency distribution to confirming the homogeneous distribution of C with concentrations of (a) 1019 cm3 and (b) 1020 cm3 and—for comparison—of Mg with concentrations of (c) 5 1019 cm3 and (d) 1020 cm3.


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capability of C-compensated GaN HEMT structures grown on Si is limited by saturation, and higher vertical blocking voltages can be achieved by increasing the thickness of the homogeneous GaN:C layers. 1

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