Acceptor-hydrogen Complexes Research Articles

Local vibrational mode study of carbon-dopedInAs

Carbon-doped $\mathrm{InAs}$ samples grown by organometallic vapor phase epitaxy were studied by Raman and IR spectroscopy. Local vibrational modes (LVMs) related to isolated substitutional carbon acceptors, carbon acceptor-hydrogen complexes, and dicarbon centers were detected in samples doped with two isotopes of carbon. The energies of the observed carbon-hydrogen modes are in close agreement with carbon acceptor-hydrogen modes in $\mathrm{GaAs}$ and $\mathrm{InP}$, and are consistent with hydrogen occupying a bond-center position in the complex. No sign of substitutional carbon donors was observed. The $n$-type conductivity of carbon-doped $\mathrm{InAs}$ can be explained by the presence of dicarbon centers that are believed to be deep donors. The stretch mode of this complex was detected at $1832\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ by Raman spectroscopy in as-grown and annealed samples.

Ab-initio modeling of acceptor–hydrogen complexes in CdTe

We have investigated the hydrogenation of group-V impurities ( N Te , P Te and As Te ) in CdTe by computer modeling, using a pseudopotential density-functional method. For these complexes, we found that the lowest energy location for hydrogen is near the bond-center, closer to the acceptor atom, between a Cd atom and the acceptor. Metastable states by 0.5 eV above the ground state where found for structures where H is anti-bonding to N Te , P Te and As Te . The calculated local vibrational mode (LVM) frequencies for the bond centered configurations agree within 3% of the experimental values, i.e. 3210 cm - 1 for N Te –H and 2022 cm - 1 for As Te –H. Deuterium LVM induced shifts for As Te –D fall within 4% of the experimental data. The observed wag mode for N Te –H is reproduced within 8%. A wag mode for As Te –H and LVM frequencies for P Te –H are predicted.

Stability, diffusivity, and vibrational properties of monatomic and molecular hydrogen in wurtzite GaN

than in zinc-blende GaN, with values of 1.99 eV and 2.17 eV for uuc and ’c, respectively. The diffusion barriers for H2 are relatively high ~2.0 eV for uuc and 2.2 eV for ’c), and we propose that diffusion of H2 is more likely to proceed by dissociation followed by diffusion of monatomic H 1 . The vibrational frequency of the molecule in wurtzite GaN is redshifted from the free molecule; for wurtzite GaN the frequency is 129 cm 21 lower than in free H2. Finally, we find that the H 2 complex is only slightly higher in energy than interstitial H2, and we calculate its vibrational frequencies. I. INTRODUCTION Hydrogen plays a very important role in GaN. It is abundantly present in many of the techniques used for growth of GaN, including metal-organic chemical vapor deposition ~MOCVD!, hydride-vapor-phase epitaxy ~HVPE!, and molecular-beam epitaxy ~MBE! when NH3 is used as the nitrogen source. Hydrogen is known to be responsible for the passivation of Mg acceptors, resulting in the lack of p-type conductivity in Mg-doped layers grown in the presence of hydrogen. A post-growth anneal that removes H from the vicinity of the acceptors is necessary to activate the Mg. Detailed knowledge about the behavior of hydrogen is essential to optimize this process. Previous studies 1‐3 have addressed some aspects of the diffusion of monatomic hydrogen, but not of molecular hydrogen. In this paper, we report the first comprehensive study of the diffusion of hydrogen in wurtzite GaN. This includes explicit investigations of the migration paths of monatomic hydrogen, with particular attention to potential anisotropies. Indeed, the symmetry of the wurtzite structure is such that differences may exist between motion along @0001# (uuc) versus motion in planes perpendicular to @0001# (’c) ~which we will also refer to as ‘‘inplane’’ diffusion !. Our recent study of beryllium in GaN indicated that Be interstitials exhibit very anisotropic diffusion. 4 Given the importance of removing hydrogen from acceptor-doped layers, knowledge about any anisotropy in the hydrogen diffusion characteristics is very important. Although we find that molecular hydrogen is only stable over a limited range of Fermi levels, information about its migration is essential to complete our picture of hydrogen motion in GaN. In other semiconductors, H2 can be formed when H is released from acceptor-hydrogen complexes. Although the formation energy of H2 is relatively higher ~when compared with the monatomic species! in GaN than in other semiconductors such as Si or GaAs, the possibility exists that H2 is formed. If it is, then means of detecting it also need to be explored. Vibrational spectroscopy has been a powerful tool for detecting hydrogen-related configurations, including H2, and in order to facilitate interpretation of experiment we will present calculated vibrational frequencies for interstitial H2 molecules. Due to the light mass of the hydrogen atoms the anharmonicity is very large, and it is fully included in our calculations. We also address other diatomic hydrogen complexes, in particular H 2 , which we find to be only slightly higher in energy than H2. Vibrational frequencies for other hydrogen-related configurations relevant for p-type GaN have been reported elsewhere. 5

Isotope effects on the rate of electron-beam dissociation of Mg–H complexes in GaN

The effect of low-energy electron-beam irradiation on the stability of acceptor–hydrogen complexes in Mg-doped GaN has been directly examined with infrared optical spectroscopy. Consistent with prior electrical transport data, we find that Mg–H pairs begin to break apart under 25 keV electron-beam exposure at doses of a few mC/cm2. However, we find that, even after long exposures, roughly 12 of the acceptor–hydrogen pairs remain unaffected by the electron exposure. Using Mg-doped samples that have been vacuum annealed and D2 gas exposed, we demonstrate that there is a large (∼×5) isotopic shift in the beam-induced debonding rate of these acceptor–hydrogen complexes. H and D remain in the material during these treatments, and Mg–H or Mg–D reforms during postirradiation annealing. The implications of these observations for understanding the nature of the debonding process and the subsequent reactions of the detached H/D in the GaN lattice are discussed.

Acceptor–hydrogen complexes in semiconductors under pressure

The structure of acceptor–hydrogen complexes is a subject of fundamental and technological interest. To probe the interactions between hydrogen, the acceptor, and the surrounding host atoms, hydrostatic pressure may be applied over a wide range. Using infrared spectroscopy at liquid-helium temperatures, we have observed carbon and carbon–hydrogen local vibrational mode (LVMs) in InP at hydrostatic pressures as high as 5.5 GPa. For pressures beyond 4.5 GPa, the carbon–hydrogen mode was not observed, perhaps as a result of a transformation of the complex into a different configuration. The LVM arising from carbon substitutional impurities varies linearly with pressure, whereas the shift of the carbon–hydrogen mode has a positive curvature. Both of these observations are in qualitative agreement with the pressure dependence of LVMs in GaAs. While the substitutional carbon impurities show very similar pressure shifts in the two materials, the linear pressure coefficient of the carbon–hydrogen stretch mode in InP is nearly three times that in GaAs.

Activation of Acceptors in Mg-Doped, p-Type GaN

AbstractThe activation of acceptors was investigated for Mg-doped, heteroepitaxial layers of GaN grown by metalorganic chemical vapor deposition. After growth the samples were exposed to isochronal rapid thermal anneals in the temperature range from 500°C to 775°C. The samples were studied by variable temperature Hall effect measurements and photoluminescence (PL) spectroscopy in the as-grown condition and after each temperature step. The thermal treatment leads to the formation of acceptors which are characterized by an activation energy for ionization in the range between 165 meV and 182 meV. These acceptors are attributed to Mg atoms substituting for Ga in the GaN lattice. The experimental results for the acceptor activation are consistent with the dissociation of electrically inactive acceptor-hydrogen complexes. The reversibility of this process is investigated by the exposure of the activated, p-type GaN samples to atomic hydrogen in a remote-plasma hydrogenation system at 600°C and reactivating at 850°C.

Ground-state energy shift of acceptor-hydrogen complexes in Si and GaAs under uniaxial stress

Several low-symmetry, H-containing complexes in semiconductors can be preferentially aligned by an applied stress. Here, the coupling of the ground-state energy to stress, i.e., the defect's piezospectroscopic tensor, has been determined for the acceptor-hydrogen complexes Si:B-H, GaAs:${\mathrm{Be}}_{\mathrm{Ga}}$-H, and GaAs:${\mathrm{C}}_{\mathrm{As}}$-H.

Dissociation energies of acceptor-hydrogen complexes in InP

The dissociation energies of Zn-H, Cd-H, and Mg-H complexes in p-InP have been determined from the reactivation kinetics of these passivated dopants in reverse-biased Au Schottky diodes. The reactivation process is first order under these conditions, yielding thermal dissociation energies of 1.20±0.10 eV for Zn acceptors, 1.40±0.10 eV for Cd acceptors, and 1.35±0.10 eV for Mg acceptors. These results are consistent with the model in which the hydrogen passivates the acceptor by attaching to a neighboring P atom, leaving the acceptor essentially threefold coordinated. They also indicate that acceptor-hydrogen retrapping during cool down after epitaxial growth of p-InP layers is the primary cause of apparently stable acceptor passivation seen in such layers.

Investigation of hydrogen in semiconductors by nuclear techniques

The interaction of hydrogen with shallow and deep level impurities in crystalline semiconductors results in a passivation of their electrical activity by the formation of complexes. Nuclear techniques, like the perturbed γγ angular correlation (PAC) and the channeling of charged particles, have contributed to the understanding of the behaviour and microscopic structure of these complexes. A brief introduction to these techniques will be given.The results on the behaviour of hydrogen, in both elemental and compound semi-conductors, will be reviewed. For the case of Si, the information obtained on the formation, geometry and stability of acceptor-hydrogen complexes will be discussed more extensively.

Electronic Stimulation of Acceptor Reactivation IN p - Type Hydrogenated GaAs

ABSTRACTThe mechanism of light-induced reactivation (LIR) of shallow substitutional acceptors in high-purity p-type hydrogenated GaAs has been investigated. Photoluminescence was used to determine the dependence of the rate and extent of this effect on photon energy, illumination intensity, as well as on sample temperature and chemical composition. At a sample temperature of 1.7 K a sharp threshold in the photon energy, Et, has been observed at about 7.5 meV below the bandgap energy of GaAs. This energy corresponds approximately to the onset of acceptorbound exciton absorption in the material. For photon energy E < Et, only a weak reactivation effect is observed. The efficiency of reactivation increases dramatically for E > Et, and for sufficiently large values of (light intensity).(illumination time) product the LIR process saturates. Both the extent of the subthreshold effect and the saturation level that is attainable with E > Et are independent of the photon energy, excitation power and exposure time in the investigated range of these quantities. For E > Et the initial LIR rate depends on the square of the light intensity, indicating a bimolecular reaction in terms of the photo-generated carrier densities. The observed strong dependence of the saturation level on the sample temperature during LIR is found to be consistent with the relative binding energies of different acceptor-hydrogen passivating complexes in GaAs. Based on these results, it is proposed that LIR of acceptors is electronically stimulated via recombination-enhanced vibrational excitation of acceptor-hydrogen complexes.

Hydrogen Passivation of Shallow Acceptors in Silicon

The common observation that point defects in semiconductors can be passivated by hydrogen is illustrated for the case of shallow acceptors in silicon. Earlier results of experimental and theoretical studies are briefly reviewed and recent progress in the understanding of the microscopic structure of acceptor-hydrogen complexes in crystalline Si is presented, with special emphasis on substitutional boron passivation as studied by nuclear techniques and Raman scattering.

Vibrational spectroscopy of acceptor-hydrogen complexes in silicon: Evidence for low-frequency excitations.

The infrared spectra of acceptor-H centers in passivated Si for B, Al, and Ga provide evidence for an unexpected low-frequency excitation of the complexes. The broad vibrational bands observed near 2000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at room temperature shift to higher frequency and narrow dramatically upon cooling to He temperature. While the B-H--related band remains structureless at intermediate temperatures, the vibrational bands observed for Al-H and Ga-H complexes show thermally populated sidebands to the low-energy side of the main vibrational bands. The sidebands indicate the presence of a low-frequency excitation of the complex. We have determined Boltzmann energies of 78 and 56 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ from the intensity of the sidebands as a function of temperature for Al-H and Al-D complexes, respectively. The absence of an anomalously large isotope shift is consistent with this excitation being due to an ordinary vibration rather than a tunneling splitting as is sometimes observed for hydrogen-containing complexes.

Vibrational-mode theory of acceptor-hydrogen complexes in silicon.

The vibrational properties of the acceptor-hydrogen complexes, for the acceptors B, Al, and Ga, in Si have been investigated within the framework of the Assali and Leite model proposed for the microscopic structure of the complexes [Phys. Rev. Lett. 55, 980 (1985)]. The ${A}_{1}$ and E vibrational modes of the complexes were studied by using a molecular cluster embedded in a rigid Si lattice, where H at an interstitial site is connected to its three Si and the acceptor nearest neighbors by spring force constants of variable strength. It is shown that the model reproduces all the vibrational energies of the hydrogen- and deuterium-related complexes measured so far. Indeed, the model is used to predict several new values of vibrational energies associated to these complexes.

Passivation of shallow acceptors by H in Si: A microscopic study by perturbed angular correlations.

The formation of acceptor-hydrogen complexes in silicon is studied via the electric field gradients caused by H atoms at the site of radioactive $^{11}\mathrm{In}$ acceptor atoms, and is measured by means of the perturbed \ensuremath{\gamma}\ensuremath{\gamma} angular correlation technique. The identical In-H pairs which are oriented along 〈111〉 lattice directions are found in samples hydrogenated by different methods, well known for the passivation of shallow acceptors. They dissociate around 420 K and are more stable than B-H complexes. There is evidence for an influence of the free-hole concentration on the actual structure of the In-H pairs.

Vibrational characteristics of acceptor-hydrogen complexes in silicon

Acceptor-hydrogen complexes for the group III acceptors, B, Al, and Ga, in Si have been studied with low-temperature infrared spectroscopy. The Si-H stretching band narrows dramatically upon cooling to low temperature thereby aiding the detection of the vibrations of the Al and Ga acceptor-H complexes. The frequency 2201 cm−1 we have measured for the Al-H complex is in reasonable agreement with the prediction made by G. G. DeLeo and W. B. Fowler [Phys. Rev. B 31, 6861 (1985)] (2220 cm−1 for a 〈111〉 interstitial configuration for the H). Assignment of the new vibrational bands is confirmed by isotopic substitution. The strength of the absorption provides evidence that the passivation is not the result of compensation alone and that a major fraction of the passivated acceptors result in acceptor-H complexes. A new, low-energy excitation of the acceptor-hydrogen complexes gives rise to a sideband to the main stretching vibration and explains the pronounced energy shift and narrowing of the spectra upon cooling to He temperature.