Impurity Cyclotron Resonance Research Articles

Impurity cyclotron resonance of anomalous Dirac electrons in graphene

We have investigated a new feature of impurity cyclotron resonances common to various localized potentials of graphene. A localized potential can interact with a magnetic field in an unexpected way in graphene. It can lead to formation of anomalous boundstates that have a sharp peak with a width R in the probability density inside the potential and a broad peak of size magnetic length ℓ outside the potential. We investigate optical matrix elements of anomalous states and find that they are unusually small and depend sensitively on the magnetic field. The effect of many-body interactions on their optical conductivity is investigated using a self-consistent time-dependent Hartree–Fock approach. For a completely filled Landau level we find that an excited electron-hole pair, originating from the optical transition between two anomalous impurity states, is nearly uncorrelated with other electron-hole pairs, although it displays substantial exchange self-energy effects. This absence of correlation is a consequence of a small vertex correction in comparison to the difference between renormalized transition energies computed within the one electron-hole pair approximation. However, an excited electron-hole pair originating from the optical transition between a normal and an anomalous impurity state can be substantially correlated with other electron-hole states with a significant optical strength.

Impurity cyclotron resonance in InGaAs/GaAs superlattice and InGaAs/AlAs superlattice grown on GaAs substrates

Various temperature measurements of cyclotron resonance (CR) under pulsed ultra-high magnetic field up to 160 T were carried out in InGaAs/GaAs superlattice (SL) and InGaAs/AlAs SL samples grown by molecular beam epitaxy on GaAs substrates. Clear free-electron CR and impurity CR signals were observed in transmission of CO 2 laser with wavelength of 10.6 μm. A binding energy of impurities in these SLs was roughly estimated based on the experiment as result, and we found it was smaller than the previous experimental result of GaAs/AlAs SLs and theoretical calculation with a simple model.

Impurity cyclotron resonance in InGaAs/AlAs superlattice under ultra high magnetic fields

We have carried out cyclotron resonance (CR) measurements of (InGaAs) 8/(AlAs) 8 superlattice (SL) to investigate electronic properties of the SL under pulsed ultra-high magnetic fields. The magnetic fields up to 160 T were generated by using the single-turn-coil technique. Clear CR signals were obtained in the transmission of far-infrared laser through the SL at room temperature and lower temperature. We observed a shift of CR peak to lower magnetic field caused by transition from free-electron CR to impurity CR below ∼90 K. Compared with the previous works of GaAs/AlAs SL, the peak shift was small and the transition temperature was low. This result suggests that a binding energy of the impurity in the InGaAs/AlAs SL is smaller than the GaAs/AlAs SL.

MOCVD n‐InAs thin layers compared with MBE samples – far infrared magnetophotoconductivity

A comparison of the properties of n-InAs thin layers grown by the MOCVD method and, reported elsewhere, similar samples obtained by MBE processing is presented. The measured photoconductivity spectra exhibit a variety of peaks, among them – very narrow peaks (with the halfwidths of ∼20 mT) due to cyclotron resonance (CR) and impurity cyclotron resonance (ICR) transitions. Measurements with tilted magnetic field allowed to correlate peaks with the CR or ICR transitions in the different sample regions: with 2D and 3D electron gas. The detailed analysis shows, that we observe peaks with positions more strongly dependent on the magnitude of the tilt angle than the standard 1/cos Θ (Θ is a sample tilt angle) dependence. These lines were not observed in MBE samples. Results are discussed in terms of a correspondence between the distribution of carriers and the spatial structure of the sample. As a result one can state, that the MOCVD samples seem to have the same quality as samples obtained by the MBE method.

Electromotive force generation due to electron and impurity cyclotron resonances in compound semiconductors

We report on the resonant-photoelectromagnetic (R-PEM) effect in semiconductor, that is the electromotive force generation associated with electron cyclotron resonance (ECR) and impurity cyclotron resonance (ICR). This can be explained with a model that the carriers excited by the resonant absorption of the radiation induce the DC voltage through the thermomagnetic process. In this study, we have applied this technique to the n-InGaAs thin film and observed the negative voltage peak in addition to the usual positive ICR one. From the temperature variation and the resonant magnetic field dependence of these signals, we have concluded that the negative signal is induced by the ECR absorption. In addition, we have confirmed that the inversion of polarity for these signals originates in the inhomogeneous distribution of conduction electrons inside the film, and electrons localized at a fraction of the sample mainly cause the resonant absorption. Through the detailed observation and analysis of the generated DC voltage, we can investigate the dynamics of hot carriers inhomogeneously excited in the sample by the ECR as well as the ICR. Moreover, if we pay special attention to the polarity of the generated voltage, we can determine the distribution of electrons and impurities inside the film.

Intensive far-infrared optical study on the donor impurity band in the presence of a magnetic field for n-GaAs

We have intensively studied donor impurity bands formed by the ground and excited states in the presence of a magnetic field by means of the far-infrared optical technique. With this view, we have carried out measurements of the impurity cyclotron resonance absorption (ICRA) for hydrogenic donors in n-GaAs. Concerning the optical transition between the 1s and 2p +1 states, the variation of the full-width at half-maximum (FWHM) for the ICRA has been investigated as functions of the donor concentration and the magnetic field. Those dependencies of the FWHM have been discussed in consideration of energy bandwidths of the 1s and 2p +1 states for donors in the presence of a magnetic field. The energy bandwidths have been estimated by use of the tight binding approximation for the linear combinations of atomic orbitals, assuming the simple cubic configuration of donors. The qualitative agreement between experimental results and numerical estimation has been obtained.

Impurity cyclotron resonance lines of barrier donor electrons in quantum wells

We have investigated by numerical simulation the zero-temperature line broadening of the cyclotron resonance transitions of a dilute system of randomly situated barrier donors, with centers consisting of positive donor ions fixed in the barrier region of a quantum well to which are bound electrons in the adjacent well. Inhomogeneous line shapes due to interdonor Coulomb interactions are obtained. Discussion is restricted to the case in which magnetic fields are sufficiently high that the electrons can be treated semiclassically (exchange can be neglected). The simulations indicate that a monolayer of donor ions produces a single highly asymmetric and relatively broad line, with the excitation induced by each absorbed photon being shared among a number of donors. However, if the same donor ions are distributed among several monolayers the absorption predicted is a finely spaced comb of well-resolved relatively narrow transition lines, each line deriving from a different monolayer. In this case the excitation induced by an individual absorbed photon is more localized. Parameters chosen for the simulation are realistic for ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}\ensuremath{-}\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum wells.

Impurity cyclotron resonance in type-I (GaAs) n/(AlAs) n superlattices

We have carried out measurements of cyclotron resonance (CR) in type-I (GaAs) n /(AlAs) n superlattices (SLs) applying pulsed high magnetic fields up to 150 T. Two types of peaks have been observed in CR signals; one originates from a transition between Landau levels of free electrons, and the other from transition between impurity levels. The free-electron CR signals are dominant at room temperature, and the impurity CR signals become large as temperature decreases. From the peak position of CR spectra, at room temperature, the effective mass of the electron in (GaAs) n /(AlAs) n SL is deduced. To analyze the impurity transition in high magnetic fields, we have calculated the impurity levels in the SLs using the variational method assuming a single quantum well for simplicity. In spite of a very simple model, the calculated results are in good agreement with the CR results of type-I (GaAs) n /(AlAs) n SLs.

Optically Detected Far-Infrared Magnetoabsorption in InGaAs

InGaAs epitaxial layers lattice-matched to InP substrates are studied by photoluminescence measurements modulated by far-infrared radiation under the condition of cyclotron resonance or impurity cyclotron resonance. Based on the experimental results, the ionization energy of shallow donors is estimated to be 2.3 meV, which is lower than the value of 3.0 meV obtained from the effective mass approximation. In order to explain the difference between the two values, we propose a model of the existence of a Ga-rich region in samples. In addition to electron cyclotron resonance, hole cyclotron resonance is observed. The effective masses of electron and light hole are estimated to be 0.041m0 and 0.051m0, respectively.

Cyclotron resonance in high mobility CdTe/CdMgTe 2D electron system in the integer quantum Hall regime

We performed the cyclotron resonance experiment in high mobility CdTe/CdMgTe 2D electron system (2DES) in quantizing magnetic fields. We observed the sharp single cyclotron resonance and the weak temperature dependence of this resonance. This single cyclotron resonance reminds us about the coupling of two resonances through the electron–electron interaction. The small oscillation of the effective mass was observed at the filling factor ν=2, 3. This oscillation seems to originate from the polaron effect. Moreover, the effective mass increases abruptly near the LO phonon energy owing to the resonant polaron effect. The impurity cyclotron resonance between 1s-2p+ and 1s-2p− level was also observed. The resonant polaron effect was also conspicuous in impurity cyclotron resonance.

Resonant-Photoelectromagnetic Effects in Far-Infrared Regions for Compound Semiconductors

The dc voltage generation associated with cyclotron resonance (CR) and impurity cyclotron resonance (ICR) in n-InGaAs and n-InSb has been studied. In both materials, it is found that the observed voltage decreases rapidly with increasing temperature and the line shape has similar structure to the absorption coefficient. Using a thermomagnetic model where a heat current induces a voltage through the Nernst-Ettingshausen effect, we have explained the experimental results. This technique allows us to investigate resonant absorption for a sample in which transmission measurements are difficult to perform, as well as to study the mechanism of phonon transport through the sample.

Polaron cyclotron resonance and impurity effects in II–VI semiconductors at ultra-high magnetic fields up to 210 T

We have observed cyclotron resonance and impurity cyclotron resonance in II–VI semiconductors, n-type CdS, n-type ZnSe and n-type ZnS over a wide far-infrared wavelength range between 9.25 μm and 119 μm under pulsed high magnetic fields up to 210 T. Since the electron-phonon interaction is significantly large in these materials, a very large resonant polaron effect was observed near the longitudinal-optical (LO) phonon energy. The experimental results of cyclotron resonance were found to be in good agreement with a theoretical calculation in n-type ZnSe and CdS. Moreover, in n-type CdS, many unidentified absorption peaks were observed at low temperature. In n-type ZnS, we present the first observation of cyclotron resonance.

Polaron cyclotron resonance observed for n-type ZnSe in high magnetic fields up to 180 T.

We have observed cyclotron resonance and impurity cyclotron resonance (1s-2${\mathit{p}}^{+}$ transition) in a wide far-infrared wavelength range between 9.27 and 70.5 \ensuremath{\mu}m under pulsed high magnetic fields up to 180 T. A very large resonant polaron effect reflecting a large electron-phonon coupling constant was observed in both resonances, as well as a large nonparabolicity. The experimental results of cyclotron resonance were found to be in good agreement with a theoretical calculation based on the Wigner-Brillouin second-order perturbation theory taking account of the nonparabolicity. The impurity cyclotron-resonance data can also be well explained by a variational calculation. The donor ionization energy was found to increase by a factor of 2.39 at 180 T due to the magnetic freeze-out effect.

Experimental and theoretical study of magnetodonors in GaAs and InP at megagauss fields.

Donor states in GaAs and InP at megagauss magnetic fields are studied experimentally and theoretically. The data allow us to investigate the magnetodonors for values of \ensuremath{\gamma}=\ensuremath{\Elzxh}${\mathrm{\ensuremath{\omega}}}_{\mathit{c}}$/2 ${\mathrm{Ry}}^{\mathrm{*}}$\ensuremath{\approxeq}10, unattainable until now in these materials. The observations are described by a theoretical model, which uses variational atomic-magnetic wave functions and accounts for the band nonparabolicity within an effective two-band k\ensuremath{\cdot}p formalism. The theory describes well the absolute values and field dependences of energy shifts between the cyclotron resonance (CR) transitions and the corresponding magnetodonor impurity cyclotron resonance (ICR) transitions in both materials. Spin doublets of CR and ICR transitions are considered; they are shown to be related to a variation of the spin g value with the energy. It is argued that the spin-doublet splittings at high fields should be considerably larger in GaAs than in InP. This conclusion agrees with the observations.

Tunable InSb detectors under uniaxial and hydrostatic pressure

We report on n-InSb photoconductive detectors, which are either mounted on different substrates (metals and synthetic materials) or embedded in a special glue. If these detectors are cooled to 4.2 K, the different coefficients of expansion provide either a uniaxial or a hydrostatic pressure. This pressure causes a shift of the resonances (for the mounted detectors up to 5.5%, for the embedded detector 5.8% at 2.3 T) and a freeze out of electrons, which results in a decrease of the cyclotron resonance (CR) signal in comparison to the impurity cyclotron resonance (ICR) signal for the embedded detector.

Electronic properties of silicon δ-doped InSb

We have investigated magneto-transport and cyclotron resonance (CR) of two-dimensional electron gas in silicon δ-doped p-InSb under a magnetic field of up to 12 T at 4.2 K. Because there are multiple subbands occupied, Shubnikov–de Haas oscillations show a beating behavior. The CR spectra also display several peaks originating from different subbands. Effective masses of electrons associated with the lowest three subbands can therefore be directly determined, and they are in excellent agreement with a self-consistent calculation, which takes into account the electrostatic Poisson equation, the Schrödinger equation, and realistic sample parameters. Furthermore, we observed an absorption peak, whose resonance position has anomalous angle dependence. It is attributed to impurity CR where donors are in the vicinity of the δ-doped sheet.

Properties of 2D electrons and associated bound donors in delta-doped InSb

We have investigated magneto-transport and cyclotron resonance (CR) of a two-dimensional electron gas (2DEG) and associated bound donors in silicon delta-doped p-InSb, under a magnetic field up to 12 T at 4.2 K. Due to the high mobility of the 2DEG (∼ 20 000 cm 2/V · s), the CR peaks originating from several subbands in the delta-doped region are observed for the first time. We have also observed an additional absorption peak, whose resonant position has an angular dependence opposite to that of the 2DEG. According to its resonant position and its dependence on the Si concentration, we attribute this additional peak to impurity CR where donors are in the vicinity of the delta-doped sheet.

Magnetic-field-induced localization in narrow-gap semiconductors Hg1-xCdxTe and InSb.

Magnetotransport measurements on n-type ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te (x\ensuremath{\simeq}0.2) and InSb and far-infrared magnetooptical spectroscopy in ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te are reported. The transport data at magnetic fields below and near the magnetic-field-induced metal-insulator (M-I) transition indicate clear similarity of ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te and InSb. At fields well above the M-I transition and at low temperatures, the magnetotransport coefficients for ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te show anomalously weak dependences on field and temperature. This is attributed to shorting of the bulk by a conducting surface layer. Below the M-I transition field, an anomalous Hall effect is observed in both ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te and InSb. We interpret this effect within a model in which the M-I transition takes place in the donor impurity band. The impurity cyclotron resonance observed in ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te provides conclusive evidence for donor-bound electrons in this semiconductor and further confirms its similarity to InSb. The cyclotron-resonance data are in agreement with theoretical predictions for hydrogenic donors in a strong magnetic field. These observations provide strong evidence against the Wigner crystallization of electrons in ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te.

CO2 laser spectroscopy of shallow and deep levels in n-InSb by photo-Hall techniques

We report the first use of laser-induced photo-Hall techniques to investigate shallow and deep levels in n-InSb. From these measurements we show that photo-Hall techniques provide a very sensitive measure of the photoinduced changes in the free-electron concentration of InSb due to weak transitions from impurity or defect levels. As a result a new class of impurity transitions, referred to as LO phonon-assisted impurity cyclotron resonance harmonics, have been identified. In addition, time-resolved and intensity-dependent photo-Hall studies performed for temperatures between 3.6 and 20 K have resulted in the determination of absorption coefficients and recombination rates associated with the weak absorption of CO2 laser radiation by shallow and deep levels in n-InSb.

Observation of impurity cyclotron resonance in Hg1-xCdxTe.

Observation de la resonance cyclotron d'impurete (transition induite par rayonnement IR lointain) mettant en evidence l'existence d'electrons lies a des centres donneurs dans Hg 1−x Cd x Te (X∼0,2); bon accord avec la separation theorique entre la resonance d'impurete et la resonance des electrons de conduction. Determination de la duree de vie des electrons dans le niveau de Landau le plus bas a partir de la saturation de l'absorption