Spectroscopic Study of Wide-Bandgap Semiconductors toward Integrated Quantum Circuits

Vivian Zhou, Yanan Wang, Jaesung Lee, Philip X.-L. Feng Department of Electrical Engineering & Computer Science, Case School of Engineering,

Case Western Reserve Unversity Cleveland, Ohio 44106, USA

Emails: vivian.zhou@case.edu; yanan.wang@case.edu; jaesung.lee@case.edu; philip.feng@case.edu

Abstract—Recent advances in solid-state materials science and nanotechnology have led to increasing interest in atom-like defect centers in wide-bandgap materials with controllable quantum coherence that makes them promising quantum emitters and essential building blocks of quantum systems. In this work, we have characterized three leading material platforms – diamond, silicon carbide (SiC), and hexagonal boron nitride (h-BN), by using Raman and photoluminescence (PL) spectroscopy. These three materials not only host coherent and optically addressable defect states at room temperature, but also possess great potential in integration and scaling. A better understanding of the material and photophysical characteristics will serve as a stepping stone toward developing quantum devices and circuits for future computation and communication applications.

Keywords—wide-bandgap semiconductors; quantum emitters; qubits; diamond; silicon carbide; hexagonal boron nitride

I. INTRODUCTION

Non-classical light sources such as single photon emitters (SPEs) are critical components in the emerging field of quantum technology, encompassing optical quantum computing [1], quantum cryptography [2], secure quantum communication [3], quantum metrology [4], and quantum sensing [5] (Fig. 1). Distinct from thermal light sources (such as incandescent light bulbs) and coherent light sources (such as lasers), an ideal SPE can produce only one photon within its fluorescence lifetime, and all successive photons are indistinguishable with controllable quantum correlations. Over the years, SPEs have been extensively explored in the forms of cold atoms, trapped ions, and organic molecules at extreme conditions. There has been growing interest in implementing SPEs in solid-state material platforms which combine desirable optical properties with the convenience of operation and potential for scalability [6]. Recent focus has been on developing such quantum emitter systems based on fluorescent crystal defects in wide-bandgap (WBG) semiconductors [7,8]. Since the defect states are confined deep in the bandgap, far away from the host crystal’s valence and conduction bands, these atom-like defect centers can emit robust single photons even at room temperature. Moreover, via spin-photon interaction, the defect centers can facilitate initialization and readout of individual electron spins, along with their proximal nuclear spins, with optical fields.

In this paper, three WBG semiconductor materials we study are diamond, silicon carbide (SiC), and hexagonal boron nitride (h-BN), an important layered two dimensional (2D) material. These three material systems are chosen, because they not only host coherent and optically addressable defect states at room

temperature, but also possess great potential in chip-scale integration and scaling in fabrication and production [911]. The basic photophysical properties of these leading sources will be reviewed. Detailed synthesis methodology and optical spectroscopic study will be presented with the aim to understand the defect behavior in each material. We will conclude with a comparison among these three materials and highlight the potential in enabling integrated quantum circuits.

Fig. 1. Schematic illustration of quantum emitters based on wide-bandgap materials as central building blocks for quantum computing and communication.

II. EXPERIMENTAL METHODS

The diamond samples we study are grown by hot filament chemical vapor deposition (HFCVD) heated using four tungsten filaments above the substrate holder [12]. The operational gases (hydrogen, methane, and trimethylboron) are mixed and directed to the chamber where the hot filaments and the sample stage are mounted. The substrate used is SiO2/Si/SiO2 that is soaked in a diamond powder slurry and sonicated. Sonication helps evenly disperse the diamond particles in the slurry and aid the polymorphic crystallization process by making the surface of the substrate rough. The rough surface facilitates a firm adhesion between the SiO2 substrate and grown diamond layer. An automatic pressure control valve regulates the pressure at the exhaust until the pressure stabilizes to 20 Torr. The filaments are turned on with approximately 8001000 W AC voltage drive. The temperature is gradually increased until the final temperature of the chamber reaches 2050 °C. Samples are heavily doped to increase nitrogen-vacancy (NV) center defect concentrations. Total growth time for CVD diamond is around 24 hours. The thickness of our polycrystalline diamond layer is measured by scanning electron microscopy (SEM) to be 6 μm.

The cubic silicon carbide (3C-SiC) films used in this study are epitaxially grown on single-crystal silicon (Si) substrates in a customized atmospheric-pressure chemical vapor deposition (APCVD) reactor [13]. The reactor is able to accommodate a

DISTRIBUTION STATEMENT A. Approved for public release: distribution is unlimited.

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SiC-coated graphite susceptor which can hold two 4-inch wafers. The process gases are propane (15% in hydrogen) and silane (5% in hydrogen); ultrahigh purity hydrogen is used as the carrier gas. Prior to deposition, the reaction chamber is pumped down to 200 mTorr, then backfilled and purged with ultrahigh purity argon to 760 Torr. Following the purge, argon flow is stopped, and hydrogen is introduced into the chamber. Upon complete displacement of argon by hydrogen, the susceptor is heated from room temperature to 1000 °C over a period of 5 min. At 1000 °C, the temperature is held constant for 5 min. During this step, hydrogen etches the native oxide, as well as any metallic and organic contaminants. The wafer surface is then carbonized by heating the susceptor to 1360 °C under a stable flow of 15% propane in hydrogen. The carbonization step lasts for 90 s, after which the deposition process is initiated by simultaneously decreasing the 15% propane in hydrogen flow rate to 25 sccm and introducing 5% silane in hydrogen at a flow rate of 100 sccm. The deposition time has ranged between 60 and 90 min, with deposition rates of about 166 Å/min.

The 2D h-BN crystals are prepared via a suite of completely dry exfoliation and transfer methods. A few nanometer thick h-BN layers are mechanically exfoliated from a high-quality bulk h-BN crystal and pressed onto polydimethylsiloxane (PDMS) stamps. After exfoliation, we transfer the h-BN nanosheets onto Si substrate with 290 nm thermally oxidized layer (SiO2-on-Si) for best visualization of these 2D layers. Following the recipe reported in [6], high-temperature annealing at 850 °C under 1 Torr of nitrogen is performed to activate the color centers and to desorb surface contaminants.

To better understand the crystal quality and defect behaviors in these WBG materials, we use spectroscopic tools to probe the vibrational modes and photoluminescence (PL) in the materials. Raman measurement is a spectroscopic technique based on inelastic scattering of photons — when photons interact with atomic/molecular vibrations, phonons or excitations, their energies or frequencies can be shifted up or down. Our system consists of a green laser (532 nm), high-magnification optical objectives (50× or 100×), and a spectrometer (Horiba iHR550 or Princeton Instruments 2500) with a detector sensitive to record the weak scattered light signals for Raman measurement. We select a microscopic system configuration that is capable of acquiring detailed and local information with high spatial resolution (~ 1 µm) and collection efficiency. Combined with a motorized stage and digital imaging, it can reveal the spatial distributions of crystalline properties over a certain range. PL measurement and spatial mapping are performed, by utilizing the same apparatus. Without specification, all the optical spectroscopic measurements are conducted at room temperature, and all the samples are sitting in a vacuum chamber with pressure below 50 mTorr.

III. GENERAL MATERIAL PROPERTIES

Relevant materials properties of diamond, SiC, and h-BN are compared together with the quintessential semiconductor Si in Table I. Because the analogy between an isolated atomic system and a point defect in a semiconductor relies on the fact that an electrically active defect has an energy level located within the forbidden semiconductor bandgap. Therefore, the wide bandgap and high Debye temperature of diamond, 3C-

SiC, and h-BN are essential to host stable, optically active defects as they are robust against lattice phonons. The photophysical properties and defect behaviors of each material will be elaborated in the following sections.

TABLE I. GENERAL MATERIAL PROPERTIES OF THE SELECTED WIDE-BANDGAP (WBG) SEMICONDUCTORS AND SI.

Si Diamond SiC h-BN

Bandgap (eV) 1.1 5.5 2.4 – 4 5.9

Refractive Index 3.5 2.39 2.6 1.8

Lattice Constant (Å)

5.43 3.57 3C: 4.36

4H: 3.07 (a) 10.05 (c)

2.50 (a) 6.62 (c)

Density (g/cm3) 2.33 3.53 3.17 2.10

Hardness (Mohs) 6.5 10 9.2 – 9.5 2

Young Modulus (GPa)

120 – 200 1220 392 – 748 800

Debye Temperature (K)

640 2200 1200 1700

IV. DIAMOND

Nitrogen-vacancy (NV) centers in diamond are among the most advanced solid-state color centers due to well-defined optical transitions along with optically controlled electronic spin states [9]. These NV centers can form naturally in diamond or can be generated by ion implantation. The NV center has a non-zero electronic dipole moment that causes its optical frequencies to be susceptible to linewidth broadening from local strain and electric fields. Therefore, it is of interest to establish spectroscopic understanding of emission from the NV centers and further to develop engineering ways to activate and control quantum emission.

Fig. 2. Naturally formed nitrogen-vacancy (NV) defect centers in a diamond thin film grown by chemical vapor deposition (CVD) (a). By mapping a 70 µm 70 µm area over 550 nm to 650 nm spectral range (b), clean defect emissions can be resolved, along with the Raman peak of the diamond (c).

Figure 2a exhibits the typical surface morphology of the

diamond thin film grown via the HFCVD method. PL characterization of NV centers in our devices requires finding locations that have undergone relatively high nucleation and therefore have significant diamond density for defect detection.

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As CVD growth does not facilitate defect implantation with micrometer or nanometer precision, we locate NV complexes through the microscope where there appears to be high diamond density. We employ a confocal laser scanning microscope to detect fluorescence from NV centers during laser excitation. PL mapping continuously rasters a designated area of the sample to locate NV centers. The interpolation of the combined PL data is plotted in an intensity map. Figure 2b presents PL mapping of a 70 µm×70 µm area over 550 nm to 650 nm spectral range. The color coding from blue to red represents the spectrum intensity varying from low to high. Strong emission can be clearly resolved around the spots labeled as 5 and 6. The corresponding spectra of these numbered spots are plotted in Fig. 2c. The emissions at 579.5 nm well match the zero-phonon lines of NV centers. The measured linewidth of NV zero-phonon line can be as small as ~1 nm, which is among the smallest of reported at room temperature. It is worthwhile to mention that the characteristic Raman lines of diamond can also be detected and provide more information about the crystalline quality and local strain environment of the diamond film. Correlating the defect emission properties attained from PL measurement with lattice vibration behaviors revealed by Raman characterization will be helpful for developing a comprehensive understanding of photophysics in these defect color centers.

V. SILICON CARBIDE

SiC, another important material of particular interest, has a large bandgap of around 2.4 to 4 eV. A major advantage of SiC is that it can be epitaxially grown into wafers compatible with industrial-scale manufacturing and production [13,14]. SiC color centers also have long coherence times due to having a nuclear spin state of zero [10]. A conclusive study to correlate the emission and spin properties with the crystallographic structure of each defect are crucial to improving the single photon emitter efficiency. To do so, we employ Raman spectroscopy to study material properties such as initial strain and crystal quality of cubic (3C) SiC films [15]. These material subtleties can be directly related to quantum emitter efficiency, revealed by a combined Raman and PL spectroscopic study.

3C-SiC, a zinc-blende crystal, has two first-order Raman active modes, the transverse optical (TO) and the longitudinal optical (LO) modes. The frequencies of TO and LO modes are observed from the phonon dispersion curves using ab initio calculations, as 796.5 cm-1 and 974.8 cm-1 at Γ point (bulk), respectively. TO and LO modes can be clearly resolved in the experimentally measured Raman spectra from SiC diaphragms as two well-defined Lorentzian peaks, and the peak positions agree very well with the theoretical predictions as depicted in Fig. 3c. By performing one-dimensional Raman line scan across the diaphragm over 1 mm length (Fig. 3b), we notice consistent peak position and intensity, which indicates very uniform and high crystal quality of the SiC films.

At the lattice and atomic scale, the Raman peaks red-shift as the SiC film thickness increases, indicating the reduction of residual stress (Fig. 3d). The linewidth decreases as the film grows thicker, which suggests improvement in crystalline quality with increasing thickness [15].

Fig. 3. Representative (a) optical image and (b) line scan Raman intensity map of the thin 3C-SiC diaphragms on Si. (c) Measured Raman signatures of the two vibrational modes in 3C-SiC. (d) Thickness dependence of measured SiC TO mode position and linewidth.

VI. HEXAGONAL BORON NITRIDE

Hexagonal boron nitride (h-BN) is a van der Waals material featuring a large bandgap of ~5.9 eV and has been frequently used for applications in 2D heterostructures as an atomically smooth, ultrathin dielectric or passivation layer. Similar to diamond and SiC, h-BN can also incorporate color centers through electronic states within the bandgap. Quantum emitter energy values are predicted by the density functional theory (DFT) to be in the range of 1.52.2 eV. The defect center in h-BN is characterized by a zero-phonon line and dominant optical phonon sidebands [16]. Broad spectral distribution presents a central problem for developing single photon sources. The influence from the dielectric environment and local strain on the quantum emission properties can be investigated via Raman and PL measurements, which can provide important insight for improving the spectral purity of quantum emissions from h-BN. It is worth mentioning that the 2D geometry of h-BN offers advantages in integration with photonic and plasmonic structures to enable on-chip devices. As these single-photon emitters (SPEs) are embedded in atomic layers, total internal reflection can be avoided and the light extraction efficiency can be very high.

An example of h-BN flake on perforated ~290 nm SiO2-on-Si substrate is depicted in Fig. 4a. The lateral sizes of the flakes

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are typically in tens to hundreds of micrometer and the thicknesses are smaller than a hundred of nanometer. Isolated emitters can be directly identified under a fluorescence microscope equipped with a 100× objective. Sharp emission lines can be detected as shown in the pixel image (Fig. 4b) and PL spectra (Fig. 4c), along with the Raman peaks of h-BN.

Fig. 4. (a) A typical optical image of h-BN flake on 290nm SiO2-on-Si substrate. (b) Emission intensity map from an exfoliated h-BN flake with a thickness of ~200 nm, and the corresponding measured photoluminescence spectra.

VII. CONCLUSION

TABLE II. PHOTOPHYSICAL PROPERTIES OF SPES IN THE SELECTED WIDE-BANDGAP (WBG) SEMICONDUCTORS [7-9].

Diamond SiC h-BN

Emission Position NV: ~575 nm NV: ~635 nm SiV: ~738 nm

CSiVC in 4H: 648 – 710 nm 570 – 820 nm

Maximum Count Rate (counts/s)

106 106 107

Spectral Linewidth (nm)

NV: >100 nm (Side Band) SiV: ~4 nm

> 100 nm 1 – 10 nm

Emission Lifetime (ns)

NV: 12 – 22 ns SiV: ~1 ns 1 – 4 ns 1 – 4 ns

Spectral / Spatial Controllability

Yes Yes, but not deterministic

Yes, but not deterministic

Integrability Yes Yes Yes

By employing Raman and PL spectroscopy, we have studied the material properties and quantum emission of defects in diamond, SiC, and h-BN. Measured emission properties, along with some figures-of-merit from the literature for each system, are summarized in Table II, which can serve as a reference for

ongoing and future studies. This work will also facilitate our effort on developing integrated functional devices and circuits for quantum computing, sensing, and communication.

ACKNOWLEDGMENT

We thank the financial support from the National Science Foundation via EFRI ACQUIRE and REM programs (Grant: EFMA #1641099), and technical support from collaborators.

REFERENCES [1] E. Knill, R. Laflamme, and G. J. Milburn, “A scheme for efficient

quantum computation with linear optics”, Nature, vol. 409, pp. 46-52, January 2001.

[2] N. Gisin, G. Ribordy, W. Tittel, and H. Zbinden, “Quantum cryptography”, Rev. Mod. Phys., vol. 74, pp. 145-195, March 2002.

[3] H.-K. Lo, M. Curty, and K. Tamaki, “Secure quantum key distribution”, Nat. Photonics, vol. 8, pp. 595-604, July 2014.

[4] V. Giovannetti, S. Lloyd, and L. Maccone, “Advances in quantum metrology”, Nat. Photonics, vol. 5, pp. 222-229, March 2011.

[5] J. R. Maze, P. L. Stanwix, J. S. Hodges, S. Hong, J. M. Taylor, et al., “Nanoscale magnetic sensing with an individual electronic spin in diamond”, Nature, vol. 455, pp. 644-647, October 2008.

[6] G. Grosso, B. Lienhard, H. Moon, D. Scarabell, et al., “Quantum emission from atomic defects in wide-bandgap semiconductors”, Tech. Digest, IEEE Photonics Society Summer Topical Meeting, San Juan, Puerto Rico, July 10-12, 2017.

[7] I. Aharonovich, D. Englund, and M. Toth, “Solid-state single-photon emitters”, Nat. Photonics, vol. 10, pp. 631-641, September 2016.

[8] M. Atatüre, D. Englund, N. Vamivakas, S.-Y. Lee, and J. Wrachtrup, “Material platforms for spin-based photonic quantum technologies”, Nat. Rev. Mater., vol. 3, pp. 38-51, April 2018.

[9] I. Aharonovich and N. Elke, “Diamond nanophotonics”, Adv. Opt. Mater., vol. 2, pp. 911-928, July 2014.

[10] A. Lohrmann, B. C. Johnson, J. C. McCallum, and S. Castelletto, “A review on single photon sources in silicon carbide”, Rep. Prog. Phys., vol 80, no. 3, art. no. 034502, January 2017.

[11] T. T. Tran, K. Bray, M. J. Ford, M. Toth, and I. Aharonovich, “Quantum emission from hexagonal boron nitride monolayers”, Nat. Nanotech., vol. 11, pp. 37-41, October 2016.

[12] S. Amponsah, “Optical characterization of nitrogen-vacancy centers and resonance analysis of CVD grown diamond MEMS devices”, Master Thesis in Electrical Engineering, Case Western Reserve University, Cleveland, OH, May 2018.

[13] C. A. Zorman, A. J. Fleischman, A. S. Dewa, M. Mehregany, C. Jacob, et al., “Epitaxial growth of 3C–SiC films on 4 in. diam (100) silicon wafers by atmospheric pressure chemical vapor deposition”, J. Appl. Phys., vol. 78, pp. 5136-5138, June 1995.

[14] J. W. Palmour, “Silicon carbide power device development for industrial markets”, Tech. Digest, 2014 IEEE Int. Electron Devices Meeting (IEDM’14), Paper No. 1.1 (Plenary Paper), pp. 1-8, San Francisco, CA, December 15-17, 2014.

[15] Y. Wang, J. Lee, V. Zhou, P. X.-L. Feng, et al., “Correlating macroscopic and microscopic vibrations in silicon carbide (SiC) micromechanical resonators”, Tech. Digest, Frontier in Optics / Laser Science (FiO + LS), JTu2A.86, Washington DC, September 16-20, 2018.

[16] X. Li, G. D. Shepard, A. Cupo, N. Camporeale, et al., “Nonmagnetic quantum emitters in boron nitride with ultranarrow and sideband-free emission spectra”, ACS Nano, vol. 11, pp. 6652-6660, May 2017.

1 2 3 4 5 6 7 8 9 10

575

600

625

650

675

700

725

750

775

Inte

nsi

ty (

a.u.)

Pixel Number

Wave

length

(nm

)

600.00

13.60k

26.60k

39.60k

52.60k

65.60k

560 580 600 620 680 700 720 740

2.21 2.14 2.07 2.00 1.82 1.77 1.72 1.68

0E+00

1E+04

2E+04

3E+04

4E+04

5E+04

6E+04

585.9 nm

577.8 nm

718.5 nm

Photon Energy (eV)

Inte

nsi

ty (

a.u

.)

Wavelength (nm)

Pixel 1 Pixel 8

FWHM ~3.16 nm

10 m

(a) (b)

(c)

41