Evidence for a shallow muonium acceptor state in Ge-rich Cz-Si1-xGex

ARTICLE IN PRESS Physica B 404 (2009) 5113–5116

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Evidence for a shallow muonium acceptor state in Ge-rich Cz  Si1x Gex B.R. Carroll a,, R.L. Lichti a, Y.G. Celebi b, K.H. Chow c, P.J.C. King d, I. Yonenaga e a

Department of Physics, Texas Tech University, Lubbock, TX 79409-1051, USA Department of Physics, Istanbul University, Beyazit, 34459 Istanbul, Turkey Department of Physics, University of Alberta, Edmonton, Canada T6G 2G7 d STFC ISIS Facility, Rutherford Appleton Laboratory, Chilton OX11 0QX, UK e Institute of Materials Research, Tohoku University, Sendai, Miyagi 980-8577, Japan b c

a r t i c l e in fo

PACS: 71.55.  i 76.75. þ i 61.72.uf Keywords: Hydrogen Muonium Shallow acceptors

abstract We have observed muon spin rotation ðmSRÞ features that are consistent with a shallow muonium (Mu) acceptor state for several Ge-rich silicon germanium alloy compositions. The MuT ½0= defect level crosses into the valence band at a Ge content of roughly 92%, indicating that a shallow acceptor state should be formed for muons that stop in the acceptor related T-site. For Si0:09 Ge0:91, the difference between the diamagnetic amplitudes from RF-driven resonance measurements compared to TFmSR indicates a slowly formed state at temperatures below 50 K, consistent with a valence-band resonant Mu that would form the core of a shallow Mu acceptor. We report a summary of hyperfine characterization and transition energies of the various T-site muonium states in Ge-rich alloys and discuss options for the conditions under which shallow acceptor states may be observed by mSR. & 2009 Elsevier B.V. All rights reserved.

1. Introduction The role of the hydrogen defect in pure Ge and Si1x Gex has been of particular interest in an effort to engineer high mobility transistors and effective optoelectronic devices where hydrogen passivation plays a crucial role. For instance, the amphoteric H defect effectively passivates charged defects in silicon to drastically improve carrier mobility otherwise limited by Coulombic scattering from such charged centers. Hydrogen is predicted to be a shallow acceptor in Ge implying that it would only act as a source of conductivity [1], thus an examination of the compositional dependence of acceptor features in silicon germanium alloys may elucidate the source of such behavior. We are interested in observing the muonium analogue in bulk silicon germanium to better understand the curious behavior of H in this technologically important alloy system. In Ge-rich alloys, we have observed several features consistent with the predicted shallow Mu acceptor [2–4]. The Mu (H ) core of such a shallow acceptor would be unable to passivate negatively charged defects if that is the equilibrium defect state. H½ þ = defect level is predicted to be below the valence band in Ge so that H would occur exclusively as an acceptor [4]. Mu½ þ = level results suggest that only the MuT ½0= acceptor level is in the valence band so that shallow acceptor properties are not necessarily forced. An accurate

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E-mail address: [email protected] (B.R. Carroll). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.08.245

determination of the MuT ½0= acceptor level from our data requires the separation of competing ionization processes and Mu0 site changes. We attempt to characterize this shallow Mu acceptor in Si1x Gex by means of the Muon Spin Research ðmSRÞ technique. Our experiments probe the spin dynamics of isolated, interstitial muonium in semiconductor hosts. Muonium ðm þ e Þ is a light pseudo-isotope of hydrogen formed in our experiments by the implantation of spin-polarized positive muons. The muon spin polarization behaves according to its local crystalline environment and charge state. Detection of positrons, emitted preferentially in the muon spin direction at the time of decay, provides the time-evolved muon spin polarization. We have used this technique to characterize the muon’s local environment (by HF measurements of neutral charge states) and transition energies between various stable sites and charge states. Since mSR measures muon behaviors on a time scale of several muon lifetimes, we are inherently observing non-equilibrium defect states and cannot directly discern the thermodynamically stable configuration of Mu or H.

2. Experimental details Several mSR techniques have been used to examine muonium defect behavior in order to determine charge state transition energies in Si1x Gex . Our mSR experiments have focused on p-type, Czochrolski-grown silicon germanium alloys provided by Yonenaga of Tohoku University. In particular, we have focused on the

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germanium end of the alloy system. These Ge-rich samples are roughly circular wafers and vary in diameter from 7 to 20 mm with thicknesses between 1 and 2 mm. Though many impurities (O, C, vacancies, etc.) are introduced by the Czochralski growth process, the dominant source of p-type conductivity in these samples is not yet established. Transverse field mSR experiments were performed on the EMU beamline at ISIS (Oxfordshire, UK). This technique observes the time-dependent asymmetry in muon decay counts between front and back scintillating detectors with an applied magnetic field perpendicular to the incident muon spin. Temperatures were controlled with an Oxford Instruments Variox cryostat and range from 5 K to room temperature. Applied fields of 10.0 mT were used to examine diamagnetic Mu states. These states may be Mu þ , Mu , or diamagnetic Mu-impurity complexes. Paramagnetic states with near-zero hyperfine (HF) frequencies would likewise contribute to a TFmSR spectrum but can be easily obscured by a strong diamagnetic signal. Since all diamagnetic states precess at the same frequency, the charge state identification is not directly obtained by TFmSR. This technique is sensitive only to the promptly formed states due to phase coherency restrictions inherent in the measurement of spin precession. The EMU beamline was also used for radio frequency mSR (RFmSR) experiments. These measurements use the standard magnetic resonance configuration to externally excite muon spinflip transitions. This uses an applied field parallel to the incident muon spin polarization with a weak RF field perpendicular to the main axis. Resulting muon spin resonances are obtained from the differences in time-dependent asymmetry between front and back counters with and without RF excitation. RF-driven resonances therefore provide a characterization of the Mu HF distribution for several Mu configurations. Copper coils were wound around our samples to provide an external RF field with frequencies of 500 MHz and below. This technique has been used to examine diamagnetic and paramagnetic muonium states in an effort to determine Mu donor and acceptor energies. Phase coherency is not a requirement for RFmSR, thus it is sensitive to both prompt and slowly formed states. Differences between RF- and TFmSR can then indicate the delayed formation of diamagnetic or shallow Mu states. Microwave muon spin resonance (m-wave mSR) experiments were performed on the M20 beamline at TRIUMF (Vancouver, BC, Canada). m-wave mSR uses the same basic magnetic resonance configuration as RFmSR but at much higher frequencies. This technique is useful to resolve Mu0 states with similar HF parameters by separating such states with higher RF excitation. Our measurements were taken at temperatures below 75 K provided by a cold finger cryostat. This magnetic resonance configuration was used to examine the T-site Mu0 hyperfine distribution and transition energies in Si0:09 Ge0:91 .

observed this split Mu0T signal in similar alloys, we sought to observe such features in Si0:09 Ge0:91 with microwave excitation to increase the splitting between the Si and Ge related T-site muonium. With 811 MHz excitation, we obtained field-swept resonance curves of T-site muonium features in Si0:09 Ge0:91 . Each m-wave mSR resonance consists of a broad HF distribution fit to three dominant features: a 2055 MHz signal at the low field end and two broad resonances from a single state with a HF frequency of 1900 MHz. Since the resonance criteria for the n12 transition has two solutions within the experimentally available field range for HF frequencies below 1900 MHz, such features would contribute two resonances with significantly larger widths than signals with HF frequencies between those found in pure Si and Ge. This effectively obscures much of the details about the germaniumrelated HF signal. Fig. 1 shows fitted amplitudes for the siliconrelated HF signal which yield a transition energy of 18 7 8 meV. Not enough detail is available to obtain a transition energy for the 2055 MHz signal with m-wave mSR, however, we have obtained an energy for this state from lower frequency RF measurements. The T0 to BC0 site change energy in pure Ge was determined to be 18:8 7 2:0 meV [5]. Disappearance of these T-site Mu states is then likely due to the small energy barrier for a site change to the þ bond-centered configuration where Mu0BC further ionizes to MuBC at temperatures above 100 K and likely undergoes cyclic transitions as seen in Ge. A small increase in diamagnetic amplitude (Fig. 2) with the disappearance of the neutral signals suggests that some of the muons end up as some diamagnetic complex. Different energy barriers for the disappearance of Mu0T are likely due to transitions into two preferable bond-centered configurations: one in a Ge–Ge bond and the other in a bond strongly influenced by Si. Another possible transition to account for the loss of Mu0T would be the formation of Mu via scattering from an impurity; the shallow Mu acceptor state we seek to characterize could be an intermediary step in such a process at low temperatures. We therefore have examined prompt and slowly formed diamagnetic amplitudes in Si0:09 Ge0:91 with RF- and TFmSR. RFmSR measurements of the T-site related HF distribution at 500 MHz excitation shows a single peak with a width of 5 mT as opposed to the m-wave resonances with a total width of 1.2 T. No

3. Results 3.1. Muonium resonance measurements Unlike pure crystalline Ge or Si, the local strain in SiGe alloys distorts the lattice so that interstitial Mu has several possible environments in the T-site configuration. Prior RFmSR results [2] indicate two possible T-site configurations for Mu0. The two T-site HF signals observed in Si0:16 Ge0:84 and Si0:10 Ge0:90 indicate the possibility of multiple acceptor levels for alloys in this composition range given their differing temperature dependences, as discussed later. Both show a feature that approaches the HF frequency of Mu0T in Ge (Mu0T -I) with increasing Ge fraction and a feature with HF frequency near that of Mu0T in Si (Mu0T -II). Having

Fig. 1. Temperature dependence of T-site and bond-centered muonium states from RF- and mwavemSR. Fitted transition energies for Mu0T -I and -II are 4:5 7 1:0 and 18:3 7 8:4 meV, respectively.

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Fig. 2. RFmSR field-swept resonance of the diamagnetic signal in Si0:09 Ge0:91 at 25 K with 25.5 MHz excitation.

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Fig. 3. Diamagnetic amplitudes in Si0:09 Ge0:91 from TFmSR (circles) and RFmSR (diamonds). Fast and slowly relaxing signals from TF measurements are noted by open and filled circles.

obvious 1900 MHz signal is discernible under these conditions. The resonant peak occurs for a HF value of 2055 MHz. The temperature dependent amplitude of this signal (Mu0T -I) is displayed in Fig. 1 along with Mu0BC from RFmSR. This Ge-related T-site feature disappears with a transition energy of 4:57 1:0 meV. Bond-centered Mu grows with the disappearance of Mu0T -I though it does not appear to be a full T-to-BC transition. We observed a time-delayed feature in the diamagnetic RFmSR polarization that suggests the formation of a shallow Mu state. These data together indicate multiple processes for Mu0T disappearance that include the formation of Mu T via neutral scattering with a shallow acceptor intermediate state. 3.2. Diamagnetic muonium in Si0:09 Ge0:91 Fig. 3 compares the normalized diamagnetic amplitudes obtained by RF- and TFmSR. The differences in amplitudes are related to slowly formed states seen by RFmSR. The feature at low temperatures is a time-delayed state that disappears around 50 K. No direct observation indicates the charge state of this feature, however, it is likely Mu T formed from a neutral precursor. Unlike the rise in amplitude at higher temperatures that results from Mu0 in a combination of ionization and site change processes, the loss of the low temperature signal is likely due to the disappearance of an unidentified precursor. The charge state of the two TF features above 100 K in Si1x Gex have not been confirmed directly from prior optical mSR experiments, but are similar to features identified in pure Ge [6]. Mu0T , unlike Mu which fills a large portion of T-site cage, is highly mobile in pure Ge as well as Ge-rich SiGe [5]. One possible situation for the slow formation of Mu T would be scattering from an impurity. Since a host of possible scattering centers are present in our samples, we can only speculate at candidates since no Muimpurity complexes have been directly observed. Mu T is effectively immobile in the crystal, thus its formation from Mu0T needs some scattering center to reduce the kinetic energy and provide the charge transfer. The acceptor level in Si0:09 Ge0:91 is predicted to be shallow and it is unclear from our current analysis whether it is in the bandgap or is band-resonant [2,3]. One explanation of our observations is that Mu0T scatters from a neutral acceptor and forms a shallow state. If the electron becomes localized in the Mu 1s orbital and the hole becomes weakly bound, this shallow Mu acceptor can

Fig. 4. Time-dependent muon spin polarization under conditions for diamagnetic Mu resonance. Solid line indicates the total fit with components displayed as dotted lines. Additional oscillating signal indicates the presence of a paramagnetic Mu state.

further ionize to leave behind Mu T . Above the hole ionization temperature, the neutral would transition directly to Mu T . Either situation is difficult to identify spectroscopically since the shallow acceptor HF splitting (160 kHz in Si0:16 Ge0:84 [2]) is extremely small compared to the diamagnetic linewidth. Time-evolved RF-driven spin polarizations at the conditions for diamagnetic resonance show a modulation in amplitude due to the B1 -field in the RF coil. An additional frequency suggests the presence of an additional state that is necessarily paramagnetic (Fig. 4). Fits to two components show a low-frequency oscillation for the on-resonance diamagnetic state and a higher frequency due to the possible shallow acceptor. Only a few oscillations can be discerned on the time scale of several muon lifetimes, so this additional feature can only be weakly resolved. Higher RF fields and longer run-times in future experiments would increase the number of observable oscillations and allow for a highly accurate fit to this combination of diamagnetic and paramagnetic states. Only shallow states with HF frequencies close to zero would

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contribute a significant signal with a slightly higher frequency. Assuming this state is equally split in HF frequency from the Larmor frequency, both muon spin-flip transitions (n12 and n34 ) would oscillate with the a higher effective B1 -field. Bond-centered muonium HF frequencies are typically several orders of magnitude larger than that of shallow muonium states, so evidence of a shallow paramagnetic resonance suggests this is Mu in a T-site configuration rather than Mu þ which prefers the BC location. The low spin contact between the Mu core and weakly bound hole of the shallow Mu acceptor yield the HF frequency needed to explain our low temperature data. Such time-dependent RF features may be the most likely candidate for measuring shallow muonium since TF spectra are often obscured in the diamagnetic linewidth. Observed temperature-dependent dynamics involve a combination of site and charge state transitions that make acceptor levels difficult to discern.

resonant. Since this places Mu½ þ = in the band gap, our observations do not exhibit defect behavior analogous to theoretical predictions of H; our results would indicate that H is an acceptor only for a certain range of Fermi energies in Si1x Gex . The non-equilibrium nature of mSR facilitates the measurement of acceptor and donor energies due to the presence of metastable Mu configurations; comparison to H, however, is obfuscated by a lack of experimental observations of isolated H and obvious discrepancies between theoretical calculations under equilibrium conditions. Though mSR cannot determine equilibrium muonium or hydrogen configurations, further characterization of muonium acceptor states in Si1x Gex would at least provide experimental observations analogous to hydrogen charge state transition energies.

Acknowledgments 4. Conclusion Our current measurements show a feint HF signal consistent with a shallow Mu state in Si0:16 Ge0:84 as well as a slowly formed state that may be a combination of diamagnetic and shallow muonium states in Si0:09 Ge0:91 . Future experiments will focus on examining the compositional dependence of this slowly formed state in order to test our model that Mu0T scatters from Si-related centers to form a band-resonant shallow acceptor at low temperatures for Ge fractions above 90%. Preliminary results from RFmSR time spectra provide more substantiating evidence to our prior observation of phase differences in TF time spectra that indicate a slowly formed shallow muonium state. These muonium results differ from the theoretical predictions of H in that the acceptor level is exclusively valence band

We greatly appreciate both the ISIS and TRIUMF facilities for experiment time and assistance as well as financial support from the National Science Foundation [RLL], the Welch Foundation [RLL], the Natural Sciences and Engineering Research Council of Canada [KHC] and the Science and Technology Facilities Council of UK [PJCK]. References [1] [2] [3] [4] [5] [6]

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