We report on the influence of various plasma species on the growth and structural properties of indium nitride in plasma-assisted metalorganic chemical vapor deposition. Atomic emission spectroscopy was used to quantify the molecular, neutral, and ionized nitrogen species concentrations above the growth surface. Reflectance and Raman spectroscopy and X-ray diffraction techniques were used to characterize the grown InN films. It has been found that ionized rather than molecular or neutral nitrogen species is positively correlated with the InN growth rate. We conclude that InN formation in the present case is due to the chemical combination of atomic nitrogen ions with indium.

III-nitrides (AlN, GaN, and InN) are prominent direct bandgap semiconductor materials used in optoelectronic applications.1–3 InN has a bandgap energy of 0.7 eV.4 It has attracted much attention as its alloying with AlN and GaN allows the continuous bandgap control from the deep-UV to near IR region.5 It makes InN a promising candidate for IR emitters, sensors, and tandem solar cells.6 InN based nanostructures7 can also be used in devices operating in the terahertz spectral range.8 

InN growth is the most challenging among III-nitrides9 due to the low InN dissociation temperature10 and higher nitrogen equilibrium pressure over the grown InN film surface.11 Growing InN at low temperatures (∼500 °C) further restricts the MOVPE and high pressure chemical vapor deposition12 growth of InN due to the low decomposition rate of ammonia.

Furthermore, growth at temperatures below 400 °C is dominated by indium droplet formation due to the shortage of active nitrogen species. A lower growth temperature also reduces the migration of the deposited material over the substrate, inhibiting semi-2D growth in favor of island growth.11 The problem of the low number density of active nitrogen species can be addressed by using nitrogen plasma as a nitrogen precursor. Plasma-assisted MBE (PA-MBE)13–16 and plasma-enhanced atomic layer deposition (ALD)17 have demonstrated InN growth using nitrogen plasma. Plasma-assisted metalorganic chemical vapor deposition (MOCVD) growth of InN could provide higher growth rates without the need for ultrahigh vacuum, and it has been used to grow InN nanopillars and wires at typically low growth temperatures.18,19

In this work, we analyze the effect of the nitrogen plasma composition on the InN growth at temperatures above its decomposition temperature. InN thin films were grown by plasma-assisted MOCVD. The change in the energy and concentration of various plasma species (neutral and ionized, atomic and molecular) was extracted from the in situ optical emission spectra measured at the growth surface, while the RF power was varied. The plasma emission spectra were analyzed using atomic emission spectroscopy.20 A strong correlation has been found between the growth rate of InN films and atomic nitrogen ion (N+, N+2, N+3) flux. Numerous investigations of the RF-nitrogen plasma have been previously done by optical emission spectroscopy21–23 and quadrupole mass spectrometry.21,24–26 These investigations indicate that the outputs of the RF-sources contain atomic nitrogen, metastable molecular nitrogen, and molecular nitrogen ions. The molecular nitrogen ions have been proposed as the responsible species for the growth of GaN under the metastable growth conditions used in PA-MBE.27 In this work, we demonstrate that the atomic nitrogen ions have a dominant effect on the PA-MOCVD growth of InN films and their structural properties.

InN films were grown using PA-MOCVD on single side polished, c-plane (0001) Al2O3 wafers offcut at 0.2° toward the m-plane. Trimethylindium (TMI) and nitrogen plasma were used as group-III and group-V precursors, respectively. The nitrogen plasma was produced using an RF hollow cathode plasma source. Nitrogen gas was used as the TMI carrier gas. The sapphire wafers were cleaned in a hydrogen plasma at 450 °C in the growth chamber. An InN buffer layer was deposited at 550 °C. The sample was then heated to the growth temperature of 775 °C. Three sample sets were grown at different reactor pressures. The RF power vas varied within each set. The power range for sample sets A (2.2 Torr, V/III = 57), B(3.8 Torr, V/III = 805), and C (4.2 Torr, V/III = 822) was 100–300 W, 300–425 W, and 350–550 W, respectively. Nitrogen flow through the plasma source was kept the same within each set. Room temperature Raman spectroscopy in backscattering geometry was used to investigate the structural composition of grown films. Phonon mode positions and FWHM were extracted using a multipeak fit of the experimentally acquired Raman spectra of the samples in this study. The growth rate was determined by fitting interference fringes measured by Fourier-transform infrared (FTIR) spectroscopy. X-ray diffraction (XRD) spectra were recorded using a Panalytical X'Pert PRO X-ray diffractometer with a Cu tube running at 45 kV and 40 mA.

We used the Saha–Boltzmann equation (1) in order to analyze the nitrogen plasma composition.28 This equation relates electron number density to number densities of the neutral and ionized species

Nen(Xi+1)n(Xi)=2Qi+1Qi(2πmkBTeh2)32expχikBTe,
(1)

where Ne is the electron number density, n(Xi) is the concentration of the atoms in the ionization state i,Qi is the partition function that includes both the degeneracy and probability of the atomic states,29,30χi is the ionization energy of the neutral atom, m is the electron mass, and Te is the electron temperature.

Te was determined from the plasma emission spectra measured above the sample surface during growth [the typical spectrum is shown in Fig. 1(a)]. The Boltzmann equation (2) relates spectral line intensities to the electron temperature Te

lnImnλmnhcAmngm=EmkBTe+ln(NU(Te)).
(2)

Imn is the intensity, λmn is the wavelength of the spectral line corresponding to the mn state transition, gm = 2Jm + 1 is the statistical weight, Amn is the Einstein transition probability of spontaneous emission, and Em/kB is the normalized energy of the upper electronic level. λmn, gm, Amn, and Em values for selected N+ spectral atomic lines were obtained from the NIST Atomic Spectra Database.31–36 The Boltzmann plot37 for the spectrum in Fig. 1(a) is shown in Fig. 1(b). Electron temperatures Te were extracted from the linear fit of the Boltzmann plot for each spectrum.

FIG. 1.

(a) Nitrogen emission spectrum (solid line) and its multipeak fit (dashed line) measured above the sample surface. N+ spectral transitions are labeled. (Growth conditions: 4.2 Torr, V/III ratio 822) (b) Boltzmann plot for the N+ spectral lines (dots) and its linear fit (dashed line) for the sample grown at 4.2 Torr.

FIG. 1.

(a) Nitrogen emission spectrum (solid line) and its multipeak fit (dashed line) measured above the sample surface. N+ spectral transitions are labeled. (Growth conditions: 4.2 Torr, V/III ratio 822) (b) Boltzmann plot for the N+ spectral lines (dots) and its linear fit (dashed line) for the sample grown at 4.2 Torr.

Close modal

The neutral nitrogen species concentration n(X0) was determined using the one-line calibration-free method utilizing Eq. (3) using an optically thin line (884 nm)38–42 

FnX0=ImnQ0TAmngmexpEmkBTe.
(3)

The normalization factor F was determined by normalizing the ionized nitrogen concentrations obtained from Eq. (1) and n(X0).

The electron density Ne in Eq. (1) was calculated from the linewidth of a Stark broadened line (1043 nm) using the following equation:43,44

Ne=ΔλFWHM×10162ωs,
(4)

where ωs=4.1×103nm is the Stark broadening parameter.45 The state of local thermal equilibrium (LTE)20 was verified to be fulfilled for each spectrum using the McWhirter criterion.41,46

The results of the plasma analysis for each of the grown samples are summarized in Fig. 2. The variation in the density of atomic nitrogen ions is found to be in good correlation with the change in the growth rate for the three sets of samples used in this study. However, no correlation is found between the variation in atomic neutrals or molecular plasma species and the growth rate.

FIG. 2.

InN growth rate and nitrogen plasma species concentration dependence on the plasma power level for sample sets A (a), B (b), and C (c). The dashed spline lines are included as guides for the eye.

FIG. 2.

InN growth rate and nitrogen plasma species concentration dependence on the plasma power level for sample sets A (a), B (b), and C (c). The dashed spline lines are included as guides for the eye.

Close modal

For the set of samples grown at 2.2 Torr (Set A), the molecular nitrogen species density, consisting of both metastable and ionized molecular nitrogen, increases with increasing RF power up to 200 W as more nitrogen can be excited by electrons. However, at high RF power, molecular nitrogen species density decreases due to the collisional quenching with other N2 molecules. The collisional quenching becomes the dominant de-excitation process at higher pressures.24 In contrast, both neutral and ionized atomic nitrogen loss through N–N gas-phase recombination require three-body collisions, which do not begin to compete with the loss by diffusion to the chamber walls until higher pressures.24 Since diffusion losses decrease with increasing pressure, ionized atomic nitrogen density increases with plasma power at lower pressures.

For the sample set grown at 3.8 Torr (Set B), the molecular nitrogen species density decreases upon increasing power up to 375 W as more N2 molecules are dissociating into individual atoms and atomic ions as evident from the increase in atomic neutrals and atomic ions. The increase in molecular nitrogen species after 375 W might be caused by diffusion induced recombination of atomic ions into N2 molecules.24 This indicates the loss of atomic ions through N-N gas-phase recombination as evident in Fig. 2. The further increase in molecular nitrogen species seen in the sample set grown at 4.2 Torr (Set C) after 400 W is the continuation of the diffusion induced recombination of atomic ions into N2 molecules observed in the sample set grown at 3.8 Torr (Set B), resulting in an increased density of molecular nitrogen species.

Correlation of the InN growth rate with atomic nitrogen ions is summarized in Fig. 3. The analysis of the in situ optical emission spectra along with the variation in the growth rate indicates that the most dominant chemical combination resulting in InN formation in the present case is due to the chemical combination of atomic nitrogen ions with indium. The contribution of other nitrogen species to InN growth is negligibly small compared to that of the atomic nitrogen species.

FIG. 3.

The film growth rate as a function of atomic nitrogen ion flux for three different sets of samples.

FIG. 3.

The film growth rate as a function of atomic nitrogen ion flux for three different sets of samples.

Close modal

The influence of plasma species on the structural properties of InN has been studied via Raman spectroscopy. Raman spectra for the three sets of InN samples grown at various pressures are shown in Fig. 4. The E2-high (489 cm−1) and A1-LO (590 cm−1) phonon modes were observed in each of the sample sets.

FIG. 4.

Raman spectra measured on InN films plotted for set A (a), set B (b), and Set C (c). Numbers indicate the RF power levels. (d) The E2-high phonon mode relaxation time dependence on nitrogen ion flux.

FIG. 4.

Raman spectra measured on InN films plotted for set A (a), set B (b), and Set C (c). Numbers indicate the RF power levels. (d) The E2-high phonon mode relaxation time dependence on nitrogen ion flux.

Close modal

The phonon relaxation time was estimated using the FWHM and peak position of the E2-high and A1-LO phonon modes extracted from a multipeak fit of the Raman spectra.47Figure 4(d) shows the E2-high phonon mode relaxation time as a function of atomic nitrogen ion flux for each set of samples. The E2-high phonon relaxation time increases with the increase in atomic nitrogen ion flux within each of the sample sets. It is likely to be caused by a decrease in nitrogen vacancies in the grown InN, which are primary contributors to the E2-high mode.47 No significant correlation has been observed between the A1-LO phonon mode relaxation time and the atomic nitrogen ion flux.

XRD spectra were recorded on several samples from Set A grown at 2.2 Torr with V/III = 57. Figure 5 shows θ-2θ scans for three samples grown at RF power levels of 100, 200, and 300 W. For all the analyzed samples, peaks located at 31.38°, 64.90°, and 41.85° were identified as (0002) InN, (0004) InN, and (0006) Al2O3 substrates, respectively. Lower intensity peaks of (101¯1) InN and (202¯2) InN located at 33.0° and 69.22°, respectively, indicate the polycrystalline structure of the grown film mostly crystallographically locked to the substrate. In order to achieve the monocrystalline film, most likely, the InN growth initiation on the sapphire surface should be optimized. The obtained XRD results are consistent with the results previously reported by Gao et al.48 for InN grown at 550 °C by MBE. Peaks labeled as “*” are likely artifacts of the 0.2°-offcut c-plane sapphire substrate as described by Mukherjee et al.49 Increasing (0002) InN peak intensity with the increasing RF power level is correlated with the increasing growth rate, resulting in a thicker InN film.

FIG. 5.

(a) XRD spectra measured on InN films grown at 2.2Torr and an V/III ratio of 57, at varying RF powers indicated in the legend.

FIG. 5.

(a) XRD spectra measured on InN films grown at 2.2Torr and an V/III ratio of 57, at varying RF powers indicated in the legend.

Close modal

In summary, InN films were grown by PA-MOCVD. The composition of nitrogen plasma, used as the nitrogen precursor, was analyzed. The atomic nitrogen ion species were shown to have a dominant impact on the InN growth rate compared to atomic nitrogen neutrals and molecular nitrogen species. The structural quality of the grown films was characterized by the E2-high phonon mode relaxation time estimated via Raman spectroscopy and XRD. InN structural quality evaluated by Raman spectroscopy was also found to depend significantly on the flux of atomic nitrogen ions. This clarifies the role of plasma in PA-MOCVD, allowing a further study to focus on PA-MOCVD growth of III-V semiconductors, in particular, and PA-CVD, in general.

This work was supported by the DOE Grant No. NA-22-WMS-#66204 via PNNL subcontract and NSF Grant No. EAR-1029020. The authors are thankful to Pete Walker for help with growth facility maintenance.

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