Photoluminescence in hydrogenated amorphous carbon

DiAMON@ AND RELATED TERIAL$ Diamon I and Related Materials 6 (1997) 700. 703

ELSEVIER

Photoluminescence in hydrogenated amorphous Carbon R u s l i ~, J. R o b e r t s o n ~, G A.J. A m a r a t u n g a

b

Department of Enghzeerhlg, Cambridge University, CmnbrMge, CB2 I PZ, UK b Department of Electrical Enghleerhzg and Electronics, UniversiO, of Liverpool. Liverpool, L69 3BX, UK

Abstract Photoluminescence (PL) is studied in a-C:H as a function of excitation energy Eex and optical gap. The PL specu'um is a broad peak whose energy increases then saturates with increasing Ee,, because of the presence of wid~ tails of localized suites. The relative quantum efficiency does not sharply decrease for Eex above the optical gap, indicating that the mobility edges lie well above the optical band edges. The PL shows polarization memory, indicating that PL occurs from highly localized anisotropic centres, with the depolarization rate related to the Urbach energy. The results are consistent with PL occurring by the excitation and recombination of electron-hole pairs within n bonded clusters. © 1997 Elsevier Science S.A.

Keywords: Photoluminescence; a-C:H

1. Introduction

Hydrogenated amorphous carbon (a-C:H) has attracted wide interest because of its useful properties such as mechanical hardness, chemical inertness and low friction [1 ]. The wider band gap, more "'polymeric" types of a-C:H exhibit a photoluminescence {PL) whose quantum etticiency can exceed 10% at room temperature [3-9]. Understanding the luminescence properties of t,-C:H films is of practical interest because of their possible application in thin film electroluminescence devices [5] and of fundamental interest because the luminescence provides valuable information on the nature of localized states and the carrier recombination mechanism in a material with mixed bonding, with atoms in sp 3, sp 2 and sometimes sp 1 hybridization. The luminescence of a-C:H and C-rich a-Sil_~,C~,:H alloys differs substantially from those of other amorphous materials such as a-Si:H [10]. For example, the PL of a-C:H is fast, weakly temperature dependent and not quenched by electric fields of 106V cm -1 [4], in contrast to a..Si:H. The PL spectra possesses a high energy tail, with a PL energy above the excitation energy [4]. In addition, the PL peak energy in some a-C:H is found to be higher than the optical gap [3]. To understand the PL in a-C:H, we first consider its electronic structure, a-C:H contains both sp 2 and sp 3 sites [11,12]. The sp'-" sites tend to pair up and form clusters within an sp3-bonded amorphous matrix [ 11 ]. 0925-9635/97/$17,00© 1997 Elsevier Science S.A. All rights reserved. Pii S0925-9635{ 96 }00665-6

The clusters are believed to be small: 2-10 sites for the a-C:H films of interest here [12]. The sp 2 sites possess n states so their local band gap is very much smaller than that of sp 3 sites. This gives rise to strong fluctuations of the band edges [13]. Therefore a-C:H possesses band tails which are wider and longer ranged than those of a-Si:H. It has been proposed that the PL in a-C:H films arises from the geminate recombination of electron-hole pairs within the sp-'-bonded clusters [ 13,14]. Optical excitation creates electron-hole pairs in the n and n* states of the clusters and these recombine to give the PL (Fig. 1). The carriers are localized within the n clusters by the

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Ev Fig. !. Schematic of photoluminescence in a-C:H. Luminescence occurs by photoexcitation and recombination in an sp z cluster. Nonradiative recombination occurs by the tunnelling or hopping of a carrier to a defect.

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wider gap of the surrounding s p 3 sites but they can hop or runnel out ~o find a defect and recombine nonradiativdy. The strong nocaBfization provided by clusters accounts naturally for many features of the PL. such as ~he short lifetime [4], the absence of eiectric field quenching [15~ and the observation of a polarization mcmor~ [] 6, 17]. Polarization memory has been observed in o~,her amorphous semiconductors, such as a-As:zSe~ and a-Si,_.~Cx:H, and in porous S~ [ib-::',~]. !t requirez PL to originate from an optically aniso, ropic centre, with the electron-hole pair bound together so that absorption and emission occur from the same dipole [21].

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3. Results and discussion

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The a-C:H films of 200-300 nm thickness were deposfled by plasma-enhanced chemical vapour deposition (PECVD) from methane at I Tort, at a range of r.f. powers, to give optical (Eo4) gaps ranging fl'om 2.7 to 4.3 eV [15]. The PL was measured on films deposited on roughened glass substrates to minimize the effecl of interference fringes. The PL was excited by 2.41, 2.71 and 3.41 eV linearly polarized light from an Ar ion laser and tl'~e PL was dispersed by a double monochromator and detected by a scanning double monochromator photomultiplier by the standard lock-in technique. The polarization of the PL was measured by a Spindler and Hoyer polarizer.

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E~ (eV) Fig. 4. The relative PL q u a n t u m efficiency of a-C:H films o f different optical gap. The straight lines are fits to the data.

The PL of a-C:H con.~ists of a broad band wnc, se peak lies at Ee,, as shown in Fig. 2(a). The position of Epj. for a particular film varies with the excitation energy Ec,,, as shown ill Fig. 3. It t~ seen fllat E,,L increases with Ec, and then saturates when Era, approaches the optical gap Eo4 of that a-C:H. This behaviour is typical of a semiconductor with a wide band of localized tail states and occurs in a-SiN,,:H alloys [22]. Fig. 4 shows the relative quantum efficiency (RQE) as a function of the optical gap and excitation energy. The RQE is the total PL intensity corrected for absorption, reflection and thickness of the films. We see that the RQE increases slowly with Ecx for each film, with no sudden fall when E~,, exceeds E04. This is a critical result and contrasts with the behaviour in a-Si:H and a-chalcogenides [23,24]. There the RQE is constant for subgap excitation and shows a sudden fall for E~, above the mobility gap, which lies close to Eo4. This behaviour in a-Si:H is attributed to excitation of carriers in extended states, so the carriers can now diffuse apart and are less likely to recombine radiativeb to give PL. The steady increase of RQE for the a-C:H with 2.7 eV

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gap, even for E,,, of 3.41 eV, is su'ong evidence that the mobility gap of a-C:H lies well above the optical gap. In a-bonded amorphous semiconductors such as a-Si:H or a-S°, the mobility edges lie roughly at the optical band edges, as defined by the optical gap Eo4. The RQE spectra of Fig. 4 suggest that the longer range disorder arising from clustering in a-C:H may cause the mobifity edges to move beyond the optical band edges, deeper into each band [13]. Fig. 2 shows that the PL retains some of the polarization of the exciting light. This so-called polarization memory is another feature that requires PL to originate from closely correlated carriers. The degree of polarization (DP) is defined as [19] DP=~

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where 1~ and Ip are the intensity of the PL with poiarization parallel and perpendicular to that of the excitation source respectively. Fig. 5(a) shows the variation of the DP with the PL energy EpL for films of each band gap, for an excitation energy E=~ of 2.71 eV. The DP, reaching 0.4 for the 2.71 eV excitation, is quite high, given the theoretical limit of 0.5 for a rando,n distribution of dipoles [18]. The DP for the 3.41 eV excitation also increases as the emission energy approaches the excitation energy and would extrapolate to about 0.4 for EpL--Eex~0 V. To analyse the dependence of DP on excitation energy and optical gap, we have plotted the DP against the energy difference EeL-E~ in Fig. 5(b). It is clear fi'om Fig. 5(b) that the DP varies strongly with EpL but has a weaker dependence on Eo4 or E~,,. For a given fihn, the DP depends primarily on the difference in the excitation energy and PL energy, EpL-E©x, and only weakly on E,, or the optical gap. Thus the DP for three different excitation energies falls almost on a common curve in Fig. 5(b). This is an important observation. It is generally believed that the polarization memory is lost as the carriers hop apart and thermalize down the band tails. Each hop is accompanied by a phonon emission [19], a loss of energy equal to the phonon energy and a small loss of DP. The loss of DP is then given by the number of phonons emitted multiplied by the loss of polarization per phonon, q, i.e. hQ

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Eo4- EpL,

for films with a larger optical gap. An interesting feature of Fig. 5 (c) is that the rate of decrease of the DP varies with ~he PL energy. The DP is seen to decrease in approximately linear fashion on EpL as expected from Eq. (2). The slope of these lines, ~:= q/'h~, the "depolarization rate" is plotted against the optical gap in Fig. 6 and compared to the Urbach slope of the optical absorption edge, 1/Eo. We see that ~ varies in a similar fashion to l/Eo. The inverse of Eo is a measure of the density of tail states. This suggests that the depolarization rate and the coupling factor q in Eq. (2) are proportional to the density of tail states available for thermalization transitions. This is confirmed by the observation that is higher for an Eex of 3.41 eV than 2.71 eV, for excitation higher up the tail. Chernyshov et al. [4] observed a PL above E~x, the anti-Stokes PL. The anti-Stokes PL has polarization

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ceous solid ~ike organic polymers whose PL behaviour is dominated by intra-molecular exchafion due to strong electron electron repulsions [26]. The broad band of Pk in a-C:H and the continuous spectrum of tail states needed for Pk depoh~rizafion, however, sugge;t that while e~ectron-electron repulsion may be much ~arger than in a-Si:H, so is disorder and disorder still dominates.

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memory (Fig. 2) but the DP decreases more sharply with increasing emission energy Anti-Stokes PL is attributed to the thermal excitation of photo-excited carriers to states of higher energy [4]. We attribute the much faster decrease of the DP for anti-Stokes PL to the lower energy of the thermal phonons available for excitation. A carrier loses a typical phonon energy of, say, 1500 cm- t or 0.18 eV per interaction for downward thermalization but gains a thermal phonon energy of 0.025 eV for upward thermalization. The ratio of the slopes of Stokes and anti-Stokes DP is ~, 9, close to the ratio phonon energies 0.18/0.025 = 7.2, supporting this interpretation. Let us now consider how the resuhs relate 1o the model of PL in a-C:H [14]. The constant RQE spectrum and the observation of polarization rnenlory are each strong evidence of the wide range of band tail states at each band edge. The present results are consistent with the dominance of one-electron disorder in a-C:H over other forms of localization such as electron-electron repulsion or electron-lattice coupling. The low coordination and floppy network of polymeric a-C:H could allow electron-lattice coupling to dominate PL as in a-Se [25]. However, PL in the harder, narrower gap, more highly coordinated a-C:H behaves similarly to that of polymeric a-C:H [2] but is much less efficient so we argue that network coordination is not the critical issue. Similarly, polymeric a-C:H is a low dielectric constant, carbona-

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[181 K. [19] Y.