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SEMICONDUCTOR SCIENCE AND TECHNOLOGY

doi:10.1088/0268-1242/23/4/045008

Semicond. Sci. Technol. 23 (2008) 045008 (4pp)

Fabrication and characterization of ultrathin double dielectric mirror GaN microcavities K Bejtka1,2, P R Edwards1, R W Martin1, F Reveret3, A Vasson3, J Leymarie3, I R Sellers2, M Leroux2 and F Semond2 1

Department of Physics, SUPA, University of Strathclyde, Glasgow G4 0NG, UK CRHEA-CNRS, Rue Bernard Gregory, Parc Sophia Antipolis, Valbonne, France 3 LASMEA, Universit´e Blaise Pascal, Clermont-Ferrand, France 2

E-mail: [email protected]

Received 15 November 2007, in final form 7 February 2008 Published 4 March 2008 Online at stacks.iop.org/SST/23/045008 Abstract The optical properties and fabrication of ultrathin GaN-based microcavities grown on silicon substrates are described. The epitaxial part of the optical cavities, consisting of a λ/2 GaN layer above a 3-period epitaxial Bragg mirror, is sandwiched between two silica/zirconia mirrors. At a suitable point in the fabrication process the silicon substrate was selectively removed using via holes. The cavity mode and excitonic resonance are observed by reflectivity at low and room temperature, demonstrating a quality factor of ∼125. The dispersion of the modes and their linewidth is measured using angle-resolved reflectivity and successfully modelled using transfer matrix simulations.

1. Introduction

set of five quantum wells (QWs). Duboz et al [4, 5] used dry etching for both substrate removal and epitaxial layer thinning. The thickness of the remaining structures was in the range 400–700 nm and suffered from thickness variations or roughness. The thinnest double dielectric mirror structure reported so far utilized a sacrificial layer in order to achieve the required cavity length with an optical thickness of 3λ/2, for QWs emitting at a wavelength λ of ∼400 nm [6]. This paper presents a new approach involving local removal of silicon by dry etching. Optical characteristics of double dielectric λ/2 III-nitride MCs fabricated from material grown on Si substrates are described. The processing utilizes the selectivity in dry etch recipes used to locally remove the silicon. The etch is stopped at the first interface of a AlN/Al0.2Ga0.8N DBR grown immediately above the silicon substrate. The epitaxial growth defines the cavity length and no further thinning is required. A small number of repeat periods are used in the DBR in order to ensure good crystalline quality in the active region grown above the mirror. For three mirror pairs the epitaxial structure is completely crack free, as required for high-quality devices.

Strong light-matter coupling has recently been observed from bulk III-nitride microcavities (MCs), grown on silicon substrates, incorporating lower semiconductor (Al, Ga)N/AlN and upper dielectric distributed Bragg reflectors (DBRs) [1]. Dielectric DBRs are very attractive because of their high reflectance and broad stop bands, and their use for both mirrors decreases the effective cavity length and should lead to stronger photon–exciton coupling [2]. However, the inclusion of a second, lower dielectric DBR is challenging as it requires removal of the substrate. Silicon can be selectively removed from III-nitrides by various techniques such as chemical or plasma etching [3–7]. The epitaxial layers on top of the silicon in the structures reported in these works were relatively thick (more than 1.5 µm) and dry etching was applied to obtain the necessary optical cavity length. Achieving control of the etching distance and maintaining interface quality are difficult. Previous reports discuss cavities with a final length in the range 400 nm to 1.1 µm. Kim et al [3] report a MC structure in which the silicon was removed by wet etching and the buffer layers by inductively coupled plasma etching. The remaining structure was 1.1 µm thick and contained one 0268-1242/08/045008+04$30.00

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Semicond. Sci. Technol. 23 (2008) 045008

K Bejtka et al

cyclotron resonance (ECR) etching. The mechanical polishing is done globally using a diamond-embedded disc (14 µm final grain size) with the aim of removing over 2/3 of the substrate. Removal of the thin remaining layer is achieved by ECR etching, using an SF6 plasma. Circular apertures of 0.5–2 mm were defined using a UV-curable optical cement. The etch rate and depth of etch were monitored using in situ reflectometry with a 633 nm laser at an angle of incidence of 60◦ . The AlN etch rate is ∼160 times slower than for silicon. The high etch selectivity in combination with the end-point detection allows the dry etching to be stopped abruptly at the silicon/AlN interface. No sacrificial layer is necessary. A second dielectric DBR, consisting of five pairs of SiO2/ZrO2, was then deposited on the exposed lower surface with the same central wavelength and thicknesses as the previously deposited one. This number of pairs was chosen to balance the reflectivities of the upper and lower mirrors and to facilitate measurement of the excitonic features.

Figure 1. Schematic process flow for the ‘double dielectric’ MC fabrication.

2. Experimental details The samples were grown on silicon (1 1 1) substrates in a Riber Compact 21 MBE reactor using NH3 and elemental Al and Ga sources. AlN/Al0.2Ga0.8N DBRs, with three repeat periods and target thicknesses of 41.3 nm and 36.5 nm, respectively, were grown directly on the substrate followed by the GaN active region. The GaN layer is targeted to have an optical thickness of λ/2 (at 356 nm), corresponding to a total physical thickness of approximately 66 nm. MCs were fabricated from this structure using the processing sequence shown schematically in figure 1. A 7-period dielectric DBR is deposited by electron beam evaporation, using quarter wavelength layers of SiO2 and ZrO2 to give an optimum reflectivity of 96% at a centre wavelength at 359 nm. The structure is then bonded to a sapphire carrier. In order to deposit the second dielectric mirror the underside of the thin semiconductor DBR was exposed by removing the substrate using a combination of polishing and electron

3. Results The resulting MCs and structures at different stages in the fabrication process were characterized by optical reflectance (OR) and photoluminescence (PL) spectroscopy. PL spectra were excited at either 325 nm or 244 nm, with the samples cooled to around 10 K. OR spectra were measured using a 100 W halogen lamp focused on the sample to a 300 µm spot. The OR spectra were corrected by the reflectivity spectrum of a thick aluminium layer (assumed to be perfect although this will not be exactly the case, as seen in figure 2(b)) deposited on silicon wafer. The MC was also characterized using angle-resolved reflectivity, with the sample mounted

(a)

(b)

(c)

(d)

Figure 2. PL (10 K) and OR (RT) spectra from (a) the as-grown MC structure, (b) RT OR after deposition of first dielectric mirror, (c) RT OR and LT PL after removal of silicon, (d) and OR after deposition of the second dielectric DBR.

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inside a rotatable helium cryostat. The incident halogen lamp light is linearly polarized. The source and polarizer are fixed on a mobile rail with the same rotation axis as the cryostat allowing adjustment of the angle of incidence. The surface morphology was analysed during and after the fabrication steps using scanning electron microscopy (SEM) and atomic force microscopy (AFM). At each stage the reflectance spectra were modelled using standard transfer matrix methods [1] with refractive index values based on those reported by AntoineVincent et al [8]. The GaN excitonic feature was included in the simulation with an oscillator strength of 20 000 meV2, somewhat reduced from the values in excess of 30 000 meV2 reported in [1], in order to obtain good agreement with our data. Absorption within the GaN was included in the simulation of the angle-resolved measurements, with the values used for the absorption coefficient described below. Figure 2(a) shows the OR and PL spectra measured from the as-grown sample at room and low temperature (RT and LT), respectively, in order to assess the quality of the top, thin GaN layer. The GaN PL spectrum, dominated by neutral donor-bound excitons, peaks at 3.475 eV with a full width at half maximum (FWHM) of about 25 meV. This is at a higher energy than in conventional µm-thick GaN on silicon [9], which typically emits near 3.46 eV at low temperature, and at a lower energy than for a λ/2 thin GaN layer on a similar 7-period epitaxial mirror (as used to demonstrate strong light-matter coupling in a GaN microcavity [1]), emitting at ∼3.51 eV. The energy of the exciton is the result of a trade-off between the tensile strain due to growth on Si and the compressive strain resulting from the (Al,Ga)N/AlN DBR, which is smaller for a 3-period mirror compared to a 7-period one. The overall strain remains slightly compressive in the as-grown structure at low temperature. The FWHM of the excitonic PL band is broader in our sample than in these other structures, indicating a lower quality of GaN. This is due to the GaN being only 250 nm above the substrate compared to 1 µm or more in the other structures. The OR spectrum shows the shape expected for the stop band, with low reflectivity (approximately 60%) and a width of approximately 0.4 eV in agreement with simulations of a three pair epitaxial DBR. The OR spectrum recorded after deposition of the first DBR shows a broad high reflectivity stop band with a width of over 0.5 eV (figure 2(b)). The reflectivity dip near the centre of the stop band is the cavity mode. After the removal of the silicon the sample was characterized by PL, OR, AFM and SEM. AFM shows that the surface roughness of the exposed underside of the epitaxial DBR was locally very smooth with rms roughness values as low as 0.3 nm over 1 µm × 1 µm scan areas. However on a larger scale some raised areas (typically 50–100 nm in diameter, 10–100 nm in height and with separation of around 1 µm) were also observed in between the smooth regions. Figure 2(c) shows OR and PL spectra at this stage. The removal of the silicon has resulted in relief of the tensile strain, shifting the excitonic emission towards higher energies (3.493 eV at 10 K). No significant change in the FWHM of the excitonic line was observed. After deposition of the second dielectric DBR into the via holes optical characterization was performed from this side.

Figure 3. Experimental angle resolved reflectivity spectra in TE and TM polarization at 77 K.

Figure 4. FWHM of the TE polarized cavity mode as a function of energy, together with the transfer matrix simulation.

The RT reflectivity (measured in this case using a commercial spectrophotometer under normal incidence) shows a wellpronounced stop band with a negatively detuned cavity mode as well as a weak excitonic feature, as can be seen in figure 2(d). Angle-resolved reflectivity experiments were performed at 77 K and RT on the fabricated microcavity, as shown in figure 3 for both transverse electric (TE) and transverse magnetic (TM) polarizations. Both show similar features and only the TE polarization will be described in detail. At an angle of 5◦ two features are clearly observed in the reflectivity spectrum. The uncoupled, negatively detuned cavity mode is at 3.276 eV and a broad dip, corresponding to the GaN-related excitons, is seen at a higher energy (3.521 eV). As the angle of incidence increases a clear shift of the optical mode towards the excitonic feature is observed and the two features cross at an angle of 50◦ . When the angle increases beyond that corresponding to resonance the FWHM of the cavity modes increases rapidly. This arises mainly from the high absorption in the GaN layer and inhomogeneous broadening of the GaN excitonic feature. This is confirmed by the transfer matrix simulation which provides a fit to the linewidth by adjustment of the absorption in the GaN layer and 3

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demonstrated. The fabrication procedure involves only removing the silicon substrate. The cavity length is controlled exclusively by the epitaxy and not by processing as in earlier reports. This removal of the silicon substrate by combination of polishing and dry etching, finished by deposition of dielectric mirror, results in Q factors which exceed 120, comparing favourably with the theoretical value and that achieved in thicker cavity III-N structures on Si.

of the broadening of the excitonic feature. The dependence of the FWHM of the cavity mode as a function of its energy position, together with the results of the calculation, is shown in figure 4. Good agreement is obtained using a GaN absorption coefficient that reaches 13.5 × 104 cm−1 above the bandgap and a very large broadening for the GaN excitonic feature (70 meV). This high absorption coefficient and large broadening are attributed to the poor quality of GaN due to non-optimized conditions for the growth so close to the substrate. Reducing the number of periods in the epitaxial mirror, from 7 to 3, will increase the background doping in the GaN, which will also contribute to the broadening. The inhomogenous broadening of the excitons is too large to see polariton coupling effects and only conventional dispersion of the cavity mode is observed [10]. The cavity quality factor, Q, is estimated using the lowangle spectra of figure 3 in the conventional way, dividing the energy of the cavity mode by its width at half maximum (E/E). This energy (E ≈ 3.28 eV) lies well below the GaN band-gap energy and it is reasonable to assume there to be negligible absorption, such that this Q equates to that of the ‘empty cavity.’ The value obtained, Q ≈ 125, is similar to the highest value reported for MCs on silicon substrates (∼110 in [3]) but lower than the values of 400–1000 quoted in some reports of MCs with double dielectric Bragg mirrors [11–13]. The highest value was reported by Song et al [11] for a thick MC structure grown on sapphire with an active region containing of 21 (Ga, In)N QWs. The highest value for QW structures grown on silicon carbide is 460 [12] and for an empty cavity is ∼750 [13]. These reports described thicker structures which are easier to control. It is important to mention that the observed quality factor is in very good agreement with the theoretical value of ∼130, simulated for this not fully optimized structure. Future work will focus on the improvement of the GaN quality as well as optimizing the etching process in order to obtain interfaces which are more uniform and smoother after silicon removal.

Acknowledgments The authors acknowledge funding from the EU projects CLERMONT2 (MRTN-CT-2003–503577) and STIMSCAT (STREP Contract 517769).

References [1] Sellers I R, Semond F, Leroux M, Massies J, Disseix P, Henneghien A-L, Leymarie J and Vasson A 2006 Phys. Rev. B 73 033304 [2] Benisty H, De Neve H and Weisbuch C 1998 IEEE J. Quantum Electron. 34 1612 [3] Kim T K, Yang S S, Hong J K and Yang G M 2006 Appl. Phys. Lett. 89 041129 [4] Duboz J-Y, Dua L, Glastre G, Legagneux G P, Massies J, Semond F and Grandjean N 2001 Phys. Stat. Sol. A 183 35 [5] Duboz J-Y et al 2003 Japan. J. Appl. Phys. 42 118 [6] Rizzi F, Edwards P R, Bejtka K, Semond F, Kang X N, Zhang G Y, Gu E, Dawson M D, Watson I M and Martin R W 2007 Appl. Phys. Lett. 90 111112 [7] Park S H, Liu C, Gu E, Dawson M D, Watson I M, Bejtka K, Edwards P R and Martin R W 2006 Phys. Stat. Sol. C 6 1949 [8] Antoine-Vincent N, Natali F, Mihailovic M, Vasson A, Leymarie J, Disseix P, Byrne D, Semond F and Massies J 2003 J. Appl. Phys. 93 5222 [9] Semond F, Damilano B, V´ezian S, Grandjean N, Leroux M and Massies J 1999 Phys. Stat. Sol. B 216 101 [10] Christmann G, Butt´e R, Feltin E, Carlin J F and Grandjean N 2006 Phys. Rev. B 73 153305 [11] Song Y K, Zhou H, Diagne M, Nurmikko A V, Schneider R P, Kuo C P, Krames M R, Kern R S, Carter-Coman C and Kish F A 2000 Appl. Phys. Lett. 76 1662 [12] Tawara T, Gotoh H, Akasaka T, Kobayashi N and Saitoh T 2003 Appl. Phys. Lett. 83 830 [13] Martin R W, Edwards P R, Kim H-S, Kim K-S, Kim T, Watson I M, Dawson M D, Cho Y, Sands T and Cheung N W 2001 Appl. Phys. Lett. 79 3029

4. Conclusions In summary, a fabrication route for ultrathin double dielectric III-nitride MC structures grown on silicon (11 1) has been

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