Accepted: September 07, 2023 | Published Online: September 09, 2023

Citation: Wang D (2023) Micro-Raman and Photoluminescence Characterization of Doped Gallium Nitride Bulk Crystals. Int J Exp Spectroscopic Tech 8:033

Copyright: © 2023 Wang D. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Abstract

Micro-photoluminescence (μ-PL) and micro-Raman scattering (μ-Raman) techniques are employed to analyze the structural uniformity and electrical properties of bulk Si-doped gallium nitride (GaN) crystal on a sapphire substrate. The major macroscopic defects observed from the surface were pits and microcracks. The crystal has a very small biaxial stress of 0.07 GPa, and a slightly larger tensile stress (0.17 GPa) was found at the microcracked region. The concentration of Si dopant and the electron concentration were on the order of 10 ¹⁷ cm ^-3 and 10 ¹⁶ cm ^-3 , respectively.

Keywords

Optical spectroscopy, Raman scattering, Photoluminescence, Gallium nitride

Introduction

Gallium nitride (GaN) is a promising wide-band gap semiconductor for fabricating devices such as short wavelength semiconductor lasers and light emitting diodes. Its prominent position in power semiconductor devices is becoming increasing important in the backdrop of electrical vehicles application and the electrification of power systems [1,2]. The majority of commercial GaN devices are heteroepitaxially grown on either sapphire or silicon carbide substrates [3,4]. However, it is desirable to utilize single-crystal GaN substrates with low dislocation densities for higher quality device fabrication, because homoepitaxial growth of GaN epilayers eliminate the stress and defects induced by the mismatch in lattice constant and thermal expansion properties between the substrate and the GaN epilayer. Gallium nitride-based laser diodes, for example, have benefited from the low dislocation density achieved through GaN substrates [5]. It has been demonstrated that high quality bulk GaN crystal can also be directly used to fabricate electronic devices such as vertical diode and transistor for high power applications [6,7].

Hydride vapor phased epitaxy (HVPE) is commonly utilized to grow bulk GaN crystals, where the group III and V sources are provided in the form of halides and hydrides, respectively [8,9]. The HVPE growth of GaN on foreign substrates without a buffer layer induces an initial columnar growth mode, which results in stacking faults, dislocations, and inversion domains. As the columns coalesce and the growth terminates, surface pits are formed on the surface [10]. The pit density depends on both the growth time and layer thickness, and usually thicker layers have the advantage of lower pit density. During the growth micro cracks often form due to biaxial tensile strain in the film, and after cracking the strain is relaxed in the vicinity of the crack [11].

To fabricate highly functional devices or to be used as free standing substrates, the materials characteristics of the thick GaN boule need to be elucidated. Of special interest is the spatial distribution of the residual stress, especially the stress near macro defects, such as surface pits and microcracking. The electrical properties of the crystal, such as the free-carrier concentration, are also important characteristics that need to be determined, especially for doped GaN samples. Understanding the stress distribution and uniformity of electrical properties helps to identify critical factors that lead to macro-defects and electrical non-uniformities, and can help in the optimization of a GaN boule growth process for fabricating improved substrates sliced from large crystals.

Methods

Micro-Raman (μ-Raman) and micro-photoluminescence (μ-PL) spectroscopy allow contactless and non-destructive analysis without the need of further sample preparation. The high spatial resolution that they provide makes them ideal analytical tools to study the microscopic defects on the sample surface. In this work, we have obtained important electrical and structural properties of a HVPE-grown GaN bulk crystal by using micro-Raman and μ-PL spectroscopy. As revealed in the SEM image (Figure 1), the bulk GaN crystal had a smooth, step-flow surface morphology across the majority of the sample with multiple surface pits observed on the growth surface (Ga-face).

Micro-Raman spectroscopy was carried out at room temperature using the 441.6 nm line from a He-Cd laser. Backscattering geometry was used for the Raman measurement. The polarization states of the incident and the scattered light were not analyzed. The laser beam (80 mW) was focused onto a spot with ~5 μm in diameter on the sample surface. μ-PL spectroscopy was performed using the 325 nm line from the He-Cd laser at room temperature.

Discussion

Hexagonal GaN belongs to the C6v4 space group, and group theory predicts A ₁ (z) + 2B ₁ + E ₁ (x,y) + 2E ₂ optical modes at the Γ point of the Brillouin zone of GaN [12]. Giving the Z(-,-) Z scattering geometry used in our experiment, only A ₁ (LO) and E ₂ modes are expected to be observed in the Raman spectrum according to the selection rule [13]. Figure 2 shows the different Raman spectra of the bulk GaN crystal collected from the smooth surface, microcracked region and a surface pit. The Raman spectrum collected at the smooth surface of the crystal shows only the A ₁ (LO) and E ₂ modes as predicted by the selection rule. However, the Raman spectrum collected from the surface pit shows the forbidden TO phonon modes besides the allowed modes. Similar behavior was also observed for the spectrum collected from the microcracked region. This violation of selection rule can be explained by the Raman scattering induced the reflected laser beam. As illustrated in Figure 3, when the incident laser beam strikes the side-wall of the pit or crack, part of the beam is reflected laterally towards the other side of the pit, thus effectively creating an X(-,-) Z scattering geometry.

The peak position and the full-width at half-maximum (FWHM) of the Raman E2(2) mode were determined by fitting the Raman peaks in Figure 2 with Lorentzian functions, and the results are summarized in Table 1. The E2 phonon modes collected from the smooth surface and the surface pit were located at the identical peak positions; while the E2 mode collected from the microcracked region was slightly shifted towards the lower frequency as a result of the residual tensile stress which is not completely released via cracking. According to Kisielowski, et al. a biaxial stress of 1 GPa shifts the E2(2) Raman peak of GaN by 4.2 + 0.3 cm ^{-1 [14].} Using the E2 mode of a stress-free single crystal GaN sample as the baseline, we have obtained 0.17 GPa biaxial tensile stress at the microcracking region, and a negligible 0.07 GPa biaxial tensile stress for other regions on the sample. The FWHM of E2 modes collected from the surface pit and microcracked region were slightly broader than that collected from the smooth surface, indicating a slightly shorter phonon life-time in those regions with structural defects. With the exception of the slight peak shift at microcracks and peak broadening at microcracks/pits, the E2 modes collected from multiple locations on different crystal planes of the sample manifest negligible deviations within the system resolution reflecting a uniform distribution of the residual stress on the surface of the crystal.

In polar semiconductors with sufficient free-carrier concentrations such as doped GaN, LO phonon-plasmon coupled modes (LPP) will arise from the resonant interaction between LO phonon and Plasmon [15,16]. Increasing free-electron concentrations cause the coupled mode to shift to higher frequency and broaden asymmetrically, a fact that was exploited to extract the carrier concentration and mobility from Raman scattering experiment [15,17]. The intensity of the LPP mode is given by [18]:

$I(\omega )\text{ = }Const.•A(\omega )•\mathrm{Im}\left[-\epsilon {(\omega )}^{-1}\right]\text{ (1)}$

Where $\omega$ is the phonon frequency, is the dielectric function, and $\epsilon (\omega )$ takes into account of the effect of electro-optic mechanism and deformation potential. Once the plasmon frequency ${\omega }_P$ was obtained by fitting the Raman spectrum of the LPP mode with the functional form given in Eq. 1, the carrier concentration n can be determined via:

$${\omega }_P\text{ = }\sqrt{\frac{4\pi n{e}^2}{{\epsilon }_{\infty }{m}^{*}}}\text{ (2)}$$

 

Where $m^{*}$ is the effective mass of the free carrier. $m^{*}\text{ = }0.22m_e$ in GaN [19].

Figure 4 shows the Raman spectrum of A1(LO) phonon-plasmon coupled mode collected from the smooth surface of the crystal. The curve fitting by using Eq. 1 shows that the GaN crystal has a free-carrier concentration lower than 3 × 10 ¹⁶ cm ^-3 . This is in accord with the concentration of the ionized dopant (1.5 × 10 ¹⁶ cm ^-3 ) measured by EPR. Because of the low free-carrier concentration, the up-shift and broadening of A1(LO) phonon-plasmon coupled mode collected from the smooth surface are relatively small.

The LPP mode collected from the surface pit and microcracked region, on the other hand, displays up-shift in peak position and peak broadening (Figure 4). We have found that the peak shift and broadening in LPP mode collected from different locations on the crystal was always accompanied by the appearance of the forbidden TO phonon modes. This correlation strongly suggests that the change in the LPP mode should be ascribed to the deviation from Z(-,-)Z backscattering geometry rather than to the increase in free-carrier concentration. The angular dispersion of mixed A1(LO) and E1(LO) (quasi-LO) phonon modes in wurtzite crystal can be written in the form [20]:

${\omega }_{QLO}^2(\theta )\text{ = }{\omega }_{A_1(LO)}^2{\cos }^2\theta +{\omega }_{E_1(LO)}^2{\sin }^2\theta \text{ (3)}$

Where θ is the angle between phonon wave vector q and the c-axis of the crystal (Figure 3). The quasi-LO phonon mode is formed unless the q vector is strictly parallel or perpendicular to the c-axis, and the frequency of quasi-LO phonon lies between the frequency of the A1(LO) and that of the E1(LO) mode. According to Shi, et al. [21], the intensity of the measured Raman line shape can be described as a summation of contributions from all possible quasi-LO modes that can be collected:

$I(\omega )\propto {\displaystyle {\int }_0^{\pi /2}d\theta P(\theta ){\cos }^2\theta \frac{1}{{\left[\omega -{\omega }_{QLO}(\theta )\right]}^2+{\Gamma }^2}}\text{ (4)}$

Where ${\omega }_{QLO}(\theta )$ was defined in Eq. 3, and Γ is the FWHM of A1(LO) mode. The $P(\theta )$ is the angular distribution function which weights quasi-LO modes propagating along different directions. In an ideal backscattering geometry, the finite collection angle subtend by the lens is neglected, and the angular distribution function is replaced by Dirac δ function, reducing the Raman line shape in Eq. 4 to a simple Lorentzian function.

μ-PL spectra were collected from the smooth surface and microcracks of the crystal. Both the PL spectra show a near-band-edge (NBE) emission peak at ~3.4 eV and a broad visible emission band at ~2.5 eV (Figure 5). The LO phonon replicas can also be seen at the long-wavelength side of the NBE emissions. The intensity ratio of the phonon replicas to the zero-phonon-lines are ~ 0.4 for both spectrums. The relative intensity of the visible emission band is much stronger in the PL collected from microcracks than that collected from smooth surface, indicating a higher concentration of gallium vacancy in the microcracked regions.

Due to the potential fluctuation induced by impurities, the NBE emission peak red-shifts and broadens as the Si doping concentration increases [22,23]. According to Yang, et al . [24], the red-shift in NBE ($\Delta E_{UV}$ ) can be related to the Si doping concentration, $N_{Si}$ , through the following empirical formula:

$\Delta E_{UV}\text{ = }K{N}_{Si}^{1/3}\text{ (5)}$

Where the coefficient K is taken as -2.17 × 10 ^-8 eV/cm. Using the NBE peak position (3.39 eV) obtained from Figure 5 and the NBE peak position of an undoped stress-free bulk GaN sample, the Si doping concentration is calculated to be 7.8 × 10 ¹⁷ cm ^-3 . Despite the small amount of tensile stress (which should lead to a small red-shift, if any.) presented at the microcracked region, the NBE emission peak of the PL collected from this region has a higher energy (3.41 eV) than that collected from the smooth surface, implying a lower dopant concentration at microcracked regions.

Conclusion

In summary, bulk GaN crystals grown by HVPE was studied by optical spectroscopy. The smooth surface and the microscopic defect structures such as microcracks and pits on the bulk crystal were analyzed by micro-Raman and PL spectroscopy. It was found that the bulk crystal has a negligible biaxial tensile stress of 0.07 GPa at regions free of microcracks, and a small biaxial tensile stress (0.1 GPa) at the microcracked region. The free-carrier concentration was determined to be lower than 3 × 10 ¹⁶ cm ^-3 by the line-shape analysis of A ₁ (LO)-plasmon coupled mode, which is in agreement with the concentration of the ionized dopant measured by EPR. The peak shift and broadening in A ₁ (LO) mode collected from microcracked and pitted regions were attributed to the deviation from the backscattering geometry: Z(-,-) Z . The measured Raman line-shape was analyzed by considering the angular dispersion of quasi-LO phonons. The doping concentration was estimated to be 7.8 × 10 ¹⁷ cm ^-3 via the position of the NBE emission peak.

---