First observation of electronic trap levels in freestanding GaN crystals extracted from Si substrates by hydride vapour phase epitaxy

The electronic deep level states of defects embedded in freestanding GaN crystals exfoliated from Si substrates by hydride vapour phase epitaxy (HVPE) is investigated for the first time, using deep level transient spectroscopy (DLTS). The electron traps are positioned 0.24 eV (E1) and 1.06 eV (E2) below the conduction band edge, respectively. The capture cross sections of E1 and E2 are evaluated to be 1.65 × 10−17 cm2 and 1.76 × 10−14 cm2 and the corresponding trap densities are 1.07 × 1014 cm−3 and 2.19 × 1015 cm−3, respectively. The DLTS signal and concentration of the electronic deep levels are independent of the filling pulse width, and the depth toward the bottom of the sample, evidenced by the fact that they are correlated to noninteracting point defects. Furthermore, Photoluminescence (PL) measurement shows green luminescence, suggesting that unidentified point defects or complex, which affect the optical characterisitics, exhibit. Despite the Si-based materials, the freestanding GaN exhibits deep level characteristics comparable to those of conventional freestanding GaN, suggesting that it is a desirable material for use in the next generation optoelectronic devices with the large-scalibilityand low production costs.

In this paper, we report the origin and electrical properties of electronic deep trap levels incorporated in the HVPE FS-GaN crystals extracted from Si substrates, using deep-level transient spectroscopy (DLTS). We expect that this study will shed light on the characterization of electronic states embedded in FS-GaN crystals peeled from Si substrates by HVPE, thus providing impetus to the performance optimization of optoelectronic devices that use FS-GaN crystals based on Si substrates.

experimental
The FS-GaN crystals used in this study were grown from Si substrates, using an in-situ removal method by HVPE. The details of the growth process are described elsewhere 15,16 . A home-built vertical type HVPE system with an upstream HCl gas channel was used to achieve the FS-GaN crystal from a Si substrate. To form Si-based HVPE GaN layers, AlN/Al 0.4 Ga 0.6 N buffer layers were deposited on 2 inch Si (111) substrate, using metalorganic chemical vapour deposition (MOCVD). Next, HVPE GaN films were grown in atmosphere pressure with V/ III ratio of 20, followed by the in-situ etching of the substrate at 1273 K. Finally, we could obtain the Si-based FS-GaN. To investigate the electrical characterization of the FS-GaN crystals, Schottky diodes were formed. First, 3-mm-diameter Al metal (150 nm) was deposited on one Ga-face of the FS-GaN to form Ohmic contacts, using an electron beam evaporator. Subsequently, 300-µm diameter Pd Schottky contacts were formed on the other Ga-face surface by the electron beam evaporator, followed by rapid thermal annealing in ambient Ar at 823 K to improve the contact formation. The overall process to form S-based FS-GaN Schottky diodes is illustrated in Fig. 1. The measurements of deep levels in the FS-GaN were performed in a system developed in-house, using a 100 mV signal at 1 MHz within a temperature range of 100-420 K. Moreover, the optical characteristics of the FS-GaN crystals were measured by photoluminescence (PL) analysis, which was excited using a He-Cd laser of 325 nm wavelength at room temperature.

Results and Discussion
A typical current-voltage (I-V) measurements of Pd/Si-based FS-GaN Schottky diode were employed at room temperature, as shown in Fig. 2(a). The diode exhibits a clear rectifying characteristics, implying that the Schottky junctions are well formed at metal/Si-based FS-GaN. Furthermore, the inset of Fig. 1(a) obviously represents that the forward and reverse leakage current increase exponentially with the bias, accompanying the shape of a thermionic field emission and trap-assisted tunneling 17 . This indicates that deep traps are present in the Si-based FS-GaN 18 . The detailed analysis of the I-V curves will be elsewhere. To investigate the deep trap states incorporated in the Si-based FS-GaN crystals, DLTS measurements and their Arrhenius plots were conducted, respectively, as illustrated in Fig. 2(b,c). The DLTS spectrum was obtained at an emission rate (e n ) of 0.90 Hz under an applied filling pulse width of 20 ms, within a temperature range of 100-420 K. Two distinct deep level states, which are labelled as E1 and E2, can be seen.
The electronic states of E1 and E2 were centered at 0.24 eV and 1.06 eV below the conduction band edge, respectively. It is apparent that the trap E2 behaves as the dominant defect with a trap density of 2.19 × 10 15 /cm 3 www.nature.com/scientificreports www.nature.com/scientificreports/ and a capture cross section of 1.76 × 10 −14 /cm 2 . A trap density and a capture cross section of E1 were estimated to be 1.07 × 10 14 /cm 3 and 1.65 × 10 −17 /cm 2 , respectively, as shown in Table 1. Compared to the properties of deep levels in conventional HVPE FS-GaN crystals, the fingerprints of electronic states, which indicate a trap density and capture cross section, embedded in the Si-based FS-GaN are comparable [19][20][21][22] . (See Table 2) Furthermore, this is also comparable to the characteristics of the deep levels in the GaN layers grown using metalorganic chemical vapour deposition (MOCVD) [23][24][25] . Notwithstanding HVPE with a high growth rate, which can cause a deterioration of crystal quality in the GaN layers, the properties of deep trap levels in the Si-based HVPE FS-GaN exhibit relatively nondegraded characteristics, compared to those of the preexisting GaN films. This strongly suggests that the Si-based FS-GaN crystals can realize large-scale high-performance opto-electronic devices without any electronic degradation.
To shed light on the origin of the trap states in the Si-based FS-GaN, we measured the deviation of the DLTS signals as a function of filling pulse time (t p ), as depicted in Fig. 3. It is clear that the variations of the signals for the two deep levels are negligible. This implies that the two electronic deep levels originate from noninteracting defects, namely point ones. It is well known that the noninteracting defects depend exponentially on t p . On the contrary, interacting defects related to dislocations and stacking faults, are proportional to ln(tp). Because the outlines of a DLTS signal as a function of ln(tp) in Fig. 2 are independent of ln(t p ), we can define the two defects as noninteracting point defects. As mentioned above, the E1 and E2 traps have been commonly observed in other GaN layers. E1 was considered as nitrogen or nitrogen vacancy (V N )-related defects, denoted as V Ga -V N pair 24,26,27 , and E2 is interpreted as nitrogen interstitial (N I ) 28 or line defects 29,30 . Figure 4 illustrates the depth profiles of the trap densities of E1 and E2 and carrier concentration in the Si-based FS-GaN, obtained from the DLTS signal and capacitance-voltage profile, respectively. The depth from the surface of the Si-based FS-GaN became intense with increasing reverse bias voltage. As shown in Fig. 4(a), the behaviors of the two defect densities are similar, and these were almost unchanged throughout the sample depth. Slight increases in the lines at a depth of 200 nm were found. Surface defects due to electron-beam evaporation are responsible for such a feature 31 . Considering the nonlinear relationship between the DLTS signal and ln (t p ) and concentration depth profiles of E1 and E2, we concluded that these defects is inherited from the noninteracting point defects without any extended defect-related ones. Additionally, we implemented PL analysis of the studied materials, as presented in Fig. 4(b). Strong band-edge emission peaks of the Si-based FS-GaN crystals   www.nature.com/scientificreports www.nature.com/scientificreports/ appeared near 3.375 eV 32 . The full width at half maximum (FWHM) of the edge peak was approximately 8.5 nm. The PL peaks of the samples exhibited a red-shift of 96 meV with respect to those of completely relaxed bulk GaN (3.471 eV), indicating the presence of tensile stress in the FS-GaN 33,34 . This is attributed to the crystal quality difference between the Ga-and N-face surfaces of the FS-GaN 35 . It is essential to note that the slight plateau around wavelength of 514.5 nm can be clearly observed. Indeed, the typical yellow bands are positioned at 2.1-2.3 eV. However, broad band with maxima at 514.5 nm in the Fig. 4 can simulate green band rather than yellow luminescence. Even if a number of the exploration for the green emission from the GaN materials were implemented, its origin has not been clearly discovered up to now 36,37 . Indeed, some literatures stated that it may be associated with V N , and V Ga 38 . Based on PL and DLTS analyses, the Si-based FS-GaN may involve high nitrogen concentration. Note that the two defects are related to V Ga , V N , V Ga -V N pair, N I -related defects, indicating high nitrogen fraction in the GaN films. Lymperakis et al. also stressed that the high strain field of the dislocation in the GaN layers can induce a metal-like structure of the Ga-Ga bonds 39 . Moreover, they addressed that these strain-induced metallic bond states can be observed in the GaN crystals grown by MOCVD or HVPE, which contain the abundant stress and nitrogen concentration in the layer 39 . It is noticeable that Si-based FS-GaN underwent a huge tensile strain during the HVPE growth 16 . Furthermore, we also observed metallic Ga-Ga bonding in GaN template on a Si substrate, which underwent high tensile strain during the growth stage 40 . However, we cannot observe any evidence on the electronic deep level states related to the metallic Ga bonding in Si-based FS-GaN. Consequently, the characteristics of the electronic states of FS-GaN crystal extracted from a Si substrate is comparable to that of conventional freestanding GaN layers, despite of using a Si material. This indicates that Si-based FS-GaN can be a promising material to achieve the high performance opto-electronic devices with large scalability and low cost. To verify the energy states of the metallic bonds, electron energy loss spectroscopy (EELS) combined with scanning transmission electron microscopy (STEM) analysis is under investigation.

Conclusion
The electronic states of the deep levels in Si-based FS-GaN were investigated using DLTS analysis. Two deep trap levels were observed at E c − 0.24 eV and E c − 1.06 eV with capture cross sections of 1.65 × 10 −17 cm −2 and 1.76 × 10 −14 cm −2 and trap densities of 1.07 × 10 14 cm 3 and 1.19 × 1015 cm 3 , respectively. The DLTS signals as a function of filling pulse time and trap density depth profiles prove that the two deep levels are involved in noninteracting point defects. The PL measurement revealed that the green emission was observed, inherited from  www.nature.com/scientificreports www.nature.com/scientificreports/ unknown sources. Despite of originating from a Si substrate, The properties of the electronic deep level states of Si-based FS-GaN is comparable to those of conventional one. We believe that the Si-based FS-GaN exhibits the desirable characteristics for large-scale opto-electronic applications with low production costs.

Methods
The fingerprints of electronic deep centers including the activation energy (E a ) and capture cross section (σ n ) of trap states, extracted from Arrhenius plots, can be evaluated as follows 41  where e n , T, k, m*, and h indicate the emission rate, the absolute temperature, Boltzmann's constant, the effective mass of the carrier, and Plank's constant, respectively. The trap parameters are presented in Table 1. Furthermore, the trap density can be determined as a following equation 42 : where N d , ∆C, and F indicate the donor concentration, the capacitance change during relaxation, and the spectrometer function of 3.5 used in this work, respectively.