1Songshan Lake Materials Laboratory, Dongguan 523808, China
2Guangzhou Wide Bandgap Semiconductor Innovation Center, Guangzhou Institute of Technology, Xidian University, Guangzhou 510555, China
3School of Physical Sciences, Great Bay University, Dongguan 523808, China
4Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, Bautzner Landstrasse 400, 01328, Dresden, Germany
5Dongguan Institute of Opto-Electronics Peking University, Dongguan 523808, China
6Sinopatt. Technology Co., Ltd, Songshan Lake, Dongguan 523808, China
7State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Nano-Optoelectronics Frontier Center of Ministry of Education (NFC-MOE), Peking University, Beijing 100871, China
8State Key Laboratory of Wide Bandgap Semiconductor Devices and Integrated Technology, School of Microelectronics, Xidian University, Xi’an 710071, China
In the present work, the high uniform 6-inch single-crystalline AlN template is successfully achieved by high temperature annealing technique, which opens up the path towards industrial application in power device. Moreover, the outstanding crystalline-quality is confirmed by Rutherford backscattering spectrometry (RBS). In accompanied with the results from X-ray diffraction, the RBS results along both [0001] and reveal that the in-plane lattice is effectively reordered by high temperature annealing. In addition, the constant Φepi angle between [0001] and at different depths of 31.54° confirms the uniform compressive strain inside the AlN region. Benefitting from the excellent crystalline quality of AlN template, we can epitaxially grow the enhanced-mode high electron mobility transistor (HEMT) with a graded AlGaN buffer as thin as only ~300 nm. Such an ultra-thin AlGaN buffer layer results in the wafer-bow as low as 18.1 μm in 6-inch wafer scale. The fabricated HEMT devices with 16 μm-LGD exhibits a threshold voltage (VTH) of 1.1 V and a high OFF-state breakdown voltage (VBD) over 1400 V. Furthermore, after 200 V high-voltage OFF-state stress, the current collapse is only 13.6%. Therefore, the advantages of both 6-inch size and excellent crystallinity announces the superiority of single-crystalline AlN template in low-cost electrical power applications.
【AIGC One Sentence Reading】:A 6-inch high-quality AlN template enables E-mode HEMT with high breakdown voltage and low current collapse, suitable for low-cost power applications.
【AIGC Short Abstract】:A high-quality 6-inch single-crystalline AlN template, produced via high temperature annealing, enables the growth of enhanced-mode HEMT devices with an ultra-thin AlGaN buffer. The devices exhibit a high threshold voltage, excellent breakdown voltage, and minimal current collapse, highlighting the template's potential for low-cost power applications.
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In the past decades, allium nitride (AlN) has been prosperously developed in deep ultraviolet (DUV) emitters and high frequency micro-electromechanical system (MEMS), by benefitting from its ultrawide bandgap (~6.2 eV) and c-axis high-frequency piezoelectric performance[1−4]. However, the high Al−N bonding energy results in necessary high growth temperature (≥1300 °C) which is challenging for most of preparation methods towards bulk substrates, particularly in the wafer-scale, which acts the main obstacle to prohibit the preparation of wafer-scaled single-crystalline AlN[5, 6]. As a result, the application of single-crystalline AlN is also limited. In order to solve the above-mentioned bottleneck, plenty of strategies about replacing single-crystalline bulk AlN substrate with single-crystalline AlN template were proposed, e.g., epitaxial lateral overgrowth (ELOG)[1, 7, 8], 3D−2D AlN growth[9, 10], as well as high temperature annealing[11, 12]. Of those methods, the physical vapor deposition combined with high temperature annealing technique has been confirmed as an effective and low-cost approach[13, 14] to obtain single-crystalline AlN template on hetero-substrate. In addition to preparation efficiency, the high-temperature process induced compressive-strain in AlN region also inhibits the generation of surface crack[11, 12], acting as the prerequisite of achieving high yield during chip process. Subsequently, the successful growth of 4-inch non-cracked UVC-LED wafer indeed announces the superiority of high temperature annealed AlN template[15]. In recent years, promoting the understanding and application[16, 17] of single-crystalline AlN template beyond the conventional UV-optoelectronics has become one of the most urgent missions in the field. In particularly, the advantages of high breaking voltage and high working temperature approve single-crystalline AlN as the ideal candidate in power electronics[18].
In the present work, with the help of the Rutherford backscattering spectrometry (RBS)[19, 20] technique, the depth-profiled structure of 6-inch single-crystalline AlN template obtained by high temperature annealing is investigated in real space. The channeling feature along the direction has been strongly enhanced after the annealing, confirming the full recrystallization of twist column structures, which are predominant in as-sputtered AlN. The angular scan of channeling measurements also verifies the uniform compressive strain at different depths in AlN. Making full use of excellent crystalline quality, the 6-inch E-mode HEMT wafer with low bow has been successfully prepared with only 300-nm-thick buffer layer. The device with 16 μm-LGD exhibits a VTH of 1.1 V and a high OFF-state breakdown voltage VBD over 1400 V. Moreover, the 13.6% current collapse after 200 V high-voltage OFF-state stress indicates the nice dynamic characteristic.
Experiments
6-inch AlN template was prepared on c-plane sapphires (c to m/a direction 0.2°/0.0°) by reaction magneto-sputtering technique using aluminum (purity ~ 99.999%) as the target, and the sputtering ambient was set as the mixture of argon and nitrogen as a ratio of 1 : 4. The AlN thickness was set as 500 nm by calibrating the growth speed and the sputtering power was 3000 W. Afterwards, as-sputtered AlN wafers were annealed by utilizing a tube furnace at 1700 °C for over 5 h, and the annealing ambient was nitrogen with a flow rate of 0.5 SLM. The AlGaN/GaN HEMT structure was epitaxially grown using a metalorganic chemical vapor deposition (MOCVD) system on the 6-inch AlN templates. The epitaxy structure comprises a 50 nm AlN nucleation layer, a 300 nm graded AlGaN buffer layer with various thickness and Al content, a 250 nm GaN channel, a 0.7 nm AlN spacer, a 15 nm Al0.2Ga0.8N barrier layer, and finally a cap layer of 80 nm p-GaN layer doped with a Mg concentration of 4 × 1019 cm−3.
The crystal structure of AlN thin films was investigated by performing X-ray diffraction (XRD, the wavelength is 1.54 Å, Cu-Kα radiation source). For Rutherford backscattering spectrometry (RBS), a collimated 1.7 MeV He+ beam with a 10−20 nA beam current was used, and the scattered ions were collected at a backscattering angle of 170°. The channeling spectra were recorded by gradually tilting the sample to make the impinging He+ beam parallel to the GaN [0001] and axes. The rocking curves of AlN (002) and (102) planes were measured by X-ray diffraction (XRD, Brucker D8 Discovery) to evaluate the AlN and GaN crystallinity. The scanning electron microscopy (SEM, Hitachi Regulus 8100) and atomic force microscopy (AFM, Veeco Dimension TM-3100) with a typing mode were used to explore the surface morphology. The characteristics of the enhancement-mode p-GaN gate HEMT device were characterized by Agilent 1500 A.
Results and discussion
Fig. 1. shows the RBS results of as-sputtered AlN before and after annealing operation. Both of random spectra are collected with the He+ beams along and directions. The random spectra at both directions present signals predominately from aluminum element of AlN and sapphire regions, which stand for the AlN surface and AlN/Al2O3 interface, respectively. Therefore, the signal whose Channel No. from around 300 to 550 are from the AlN layer, and the depth decreases upon increasing Channel No. On the contrary, signals from oxygen and nitrogen are less-visible due to their light atomic mass. In addition to random spectra, the channeling spectra are also measured along the crystal-axis to quantitatively evaluate the crystalline quality. The figure of merit of the crystalline quality is evaluated by the ratio χmin between backscattering yields of channeling spectrum and the random spectrum at the same Channel No. The smaller χmin presents a better crystalline quality[21]. The χmin at different No. stands for the crystallization at different depth. For the as-sputtered sample, χmin is much larger along than along [0001], which indicates the high-disorder in the in-plane lattice. After post-annealing, both directions present excellent channeling feature. Therefore, such a result in real-space confirms the structure model deduced from XRD results in reciprocal space, the twist crystalline-columns are responsible for the ordered and disordered lattices along out-of-plane and in-plane directions, respectively. Fig. 1(h) presents the depth dependent χmin of as-sputtered and post-annealed AlN samples along [0001] and directions. Overall, the average values of χmin along [0001] and directions are calculated as ~2% and 5%, respectively, which are comparable to those values for single-crystalline GaN[22, 23]. Particularly, the χmin value gradually increases when approaching Al2O3/AlN interfaces, which means the lattice distortion is more serious near the interface due to the lattice mismatch. The result presented here is different from the deduction in previous study[24], which is probably due to that the different sputtering targets leads to different statues in as-sputtered AlN, which further influences differently on AlN-crystallinity during the annealing process. For the post-annealed samples, the χmin value along [0001] and directions at depths of 50, 150, 250, 350, and 450 nm are 1.6%, 1.7%, 1.9%, 2.3%, 4.3% and 4.4%, 4.4%, 5.1%, 6.6%, 8.6%, respectively, whereas the values for the as-sputtered sample are 6.4%, 4.8%, 7.3%, 9.0%, 20.1% and 19.4%, 32.9%, 49.9%, 59.5%, 69.8%, respectively. It is concluded that the annealing treatment effectively reduces χmin along both directions, but it mainly contributes to the lattice along the in-plane direction, which is consistent with the XRD results present in Fig. 2(a).
Figure 1.(Color online) RBS random and channeling spectra of (a), (b) as-sputtered and (c), (d) post-annealed AlN along (a), (c) and (b), (d) directions; (e) angular scan in the AlN plane at different depths, and the normalized channeling signals along and directions are zoomed in (f) and (g), respectively; (h) the χmin values of as-sputtered and post-annealed AlN along [0001] and directions as a function of depth.
Figure 2.(Color online) (a) X-ray rocking curves (XRCs) of AlN-(002) and -(102) crystalline planes after annealing; (b) optical photograph of 6-inch single crystalline AlN template; (c) FWHM values of XRC-(002) and -(102) crystalline planes at positions marked in (b); (d) XRCs of (002) and (102) crystalline planes of GaN epitaxially grown on 6-inch single crystalline AlN template; (e) bow curves of 6 inch single-crystalline AlN template and HEMT device wafers; (f) atomic force microscopy (AFM) image of HEMT epilayer on single crystalline AlN template.
Beside measuring the crystalline quality, RBS along two crystalline directions can be used to evaluate the elastic strain in epitaxial films along their depth. Figs. 1(e) and 1(g) present the angular scans of RBS channeling in the AlN plane at different depths, particularly towards AlN [0001] and dips. The angle where the channeling dip locates along [0001] and directions is −0.2° and 31.34°, respectively. A Φepi = 31.54° is calculated from 31.34° + 0.2° at five different depths. By considering the relaxed AlN, 31.98°. As schematically shown in the inset of Fig. 1(e), the smaller Φepi than Φ0 means a compressed a and elongated c, indicating that the AlN film is under the compressive strain. Moreover, the Φepi are the same at five different depths, indicating that the whole film is under the uniform compressive strain from interface to surface.
Fig. 2(a) shows the X-ray rocking curves (XRCs) of (002) and (102) planes of the 6-inch AlN template after annealing. It is clear that the full-width at half-maximum (FWHM) of rocking curves for (002) and (102) planes are 55 and 215 arcsec, respectively, again confirming the high quality of single-crystalline AlN region. According to the mosaic model[25, 26], the densities of screw/edge-type dislocations are calculated as below:
where the Ddis stands for the dislocation density, b is the length of Burgers vector, and the b values are 0.4982/0.3112 nm for screw/edge-type dislocations, respectively; β stands for the tilt angle or twist angle of the mosaic structure, which are calculated by analyzing the FWHM values of XRCs on different planes. Accordingly, the densities of screw/edge-type dislocations are calculated as 6.59 × 106 and 5.31 × 108 cm−2, respectively. In order to check the uniformity in 6-inch scale, nine positions were selected to measure the rocking curves of AlN-(002) and -(102) crystalline planes, and the results are plotted in Fig. 2(c). Both values of (002) and (102) crystalline planes appear constantly around 50 and 200 arcsec, respectively, which verifies the excellent uniformity of 6-inch single-crystalline AlN template. According to the equation of , the uniformities/average values of FWHM-RC(002) and FWHM-RC(102) are 23%/55 arcsec and 9%/204 arcsec respectively. In addition to the XRC results, the ellipsometer mapping (37 points) was also measured to confirm the uniformity of thickness and refractive index. As shown in the Figs. S1 and Figs. S2 of Supplementary Information, the average thickness and refractive index are 478.8 nm and 1.99, respectively. In addition, the standard deviation (STD) of thickness and refractive index are 4.45 and 0.009, which also confirms the high uniformity in 6-inch wafer scale. Afterwards, the crystalline quality of graded AlGaN buffer was also evaluated by XRC, and the values of (002) and (102) crystalline planes are 206 and 481 arcsec, respectively. As well known, the wafer bowing is always treated as one of the most important obstacles for chip process. Therefore, the bow values of AlN template and HEMT epilayers are measured as 15.9 and 18.1 μm, respectively, which are smaller enough for subsequent device fabrication process. The smaller wafer bow is a result of the ultra-thin AlGaN buffer layer, which is only possible by using high quality AlN templates. The surface morphology was investigated by atomic force microscopy, and the result is shown in Fig. 2(e). The atomically flat step-flow morphology is observed, and the RMS is calculated as 0.38 nm. The excellent morphology confirms the outstanding crystalline quality of HEMT epitaxial region.
In order to check the performance of HEMT epilayer, the E-mode HEMT device was subsequently fabricated. The device structure is shown in Fig. 3, the CMOS-compatible process in our pilot line starts with the deposition of a 40 nm TiN layer on the p-GaN surface, followed by device isolation through nitrogen implantation[27]. Then, high-selectivity Cl2/BCl3/SF6-mixed gas plasma etching of p-GaN was carried out, where a surface root mean square (RMS) roughness of 0.35 nm was achieved. Afterwards, a thin Al2O3 passivation layer and a 260 nm SiO2 were deposited sequentially using atomic layer deposition (ALD) and plasma enhanced chemical vapor deposition (PECVD), followed by gate window opening through reactive ion etching (RIE), gate metal TiN/Ti/Al/TiN (40/20/250/30 nm) deposition by physical vapor deposition (PVD) and patterning by inductively coupled plasma (ICP) etching. Further steps include the deposition of a SiO2 interlayer, Ohmic contact window opening by RIE, and deposition of Ohmic metal stack Ti/Al (10/200 nm) and rapid thermal annealing at 565 °C for 90 s in N2. Finally, a 300 nm SiO2 layer was deposited. The fabricated devices have a gate length (LG) of 4 μm, gate−source distance of 1.5 μm, and various gate−drain distance (LGD) from 6 to 30 μm.
Figure 3.(Color online) (a) SEM photograph and (b) cross-sectional diagram of enhancement-mode HEMT device on single-crystalline AlN template.
The static and dynamic performance of E-mode HEMT device were measured and the results are shown in Fig. 4. In Figs. 4(a) and 4(b), the output and transfer characteristics of the E-mode-HEMTs with LGD of 16 μm prepared on 6-inch single crystalline AlN template are presented, which the on-state resistance RON and threshold voltage (VTH) reach 21.71 Ω·mm and 1.2 V, respectively. Fig. 4(c) illustrates the breakdown characteristics for the as prepared E-mode p-HEMTs. The OFF-state breakdown voltage (VBD) basically depends on the LGD. In our design, devices with LGD of 22 μm and a quite simple device structure exhibit an OFF-state VBD of up to 1400 V, which demonstrates the potential of 6-inch single crystalline AlN templates for 650−1200 V applications. In order to evaluate the dynamic performance of the devices with the advanced Al2O3/SiO2 passivation, high-voltage OFF-state stress was applied to the p-GaN gate HEMTs with LGD of 16 μm as shown in Fig. 4(d). During the measurements, the devices were first stressed for 10 s in OFF-state. Afterwards, their output and transfer curves were recorded. It can be directly seen that the HEMTs on AlN template exhibit the current collapse with 13.6%. The excellent dynamic performance of the enhancement-mode p-GaN gate HEMTs verifies the high crystal quality of the GaN on the 6-inch AlN template.
Figure 4.(Color online) (a) and (b) Transfer, output characteristics and (c) ID−VDS curves of the enhancement-mode p-GaN gate HEMTs on single crystalline AlN template with LGD of 16 μm after various OFF-state stress. (d) OFF-state breakdown characteristics of the enhancement-mode p-GaN gate HEMTs with LGD from 10 to 22 µm.
By considering the viewpoint of industrial application, on one hand, the outstanding performance of device mainly benefits from the crystalline quality of single-crystalline AlN region. On the other hand, the wafer scale as large as 6 inch allows its access to the semiconductor chip process at industrial level. The above-mentioned two advantages announce the prospect of 6-inch single-crystalline AlN template in power electronics.
Conclusion
As a summary, with the help of RBS measurements, the crystalline quality of high temperature annealed 6-inch AlN is confirmed in the real space by comprehensive channeling measurements along both [0001] and directions. Moreover, the constant Φepi angle between [0001] and of 31.54° reveals the uniform compressive strain in the AlN layer at different depths. On the basis of such excellent template, series of E-mode HEMT devices are fabricated, and the one with 16 μm-LGD exhibits a high OFF-state breakdown voltage VBD over 1400 V. Moreover, the current collapse is only 13.6% after 200 V high-voltage OFF-state stress, indicating the nice dynamic characteristic of device.
Appendix A. Supplementary material
Supplementary materials to this article can be found online at https://doi.org/10.1088/1674-4926/24100041.