Mn Acceptor Level Research Articles

Enhancing the efficiency of the intermediate band solar cells by introducing: carrier losses, alloying and strain

A detailed balance model is used to determine the efficiency of intermediate band solar cell including carrier losses from the intermediate band. The effect of the energy gap of the host semiconductor is examined as a function of the intermediate band position in the energy gap and the host semiconductor energy gap. Generally the optimal intermediate band level decreases within the energy gap to mitigate the carrier losses, and carrier losses are less detrimental to small energy gap materials. We therefore focus on the role of carrier losses in wide bandgap semiconductor intermediate band solar cell systems, such as the GaN semiconductor with an Mn impurity band. Experimentally Mn acceptor level in the GaN energy gap is 1.8 eV above the valence band, which is 199 meV off the ideal intermediate band and reduces the efficiency to 21.36%. We demonstrate how carrier losses can be introduced into the system to shift the optimum IB position. Introducing carrier losses shifts the optimal intermediate band position to 1.8 eV above the valence band and increases the efficiency to 23.41%. We compare this to the effect of alloying GaN and introducing biaxial strain to shift the effective position of the Mn impurity band on the efficiency.

Two acceptor levels and hopping conduction in Mn-doped GaAs

By analysing the experimental data of the temperature-dependent Hall-effect measurements, an additional acceptor level has been confirmed to exist in Mn-doped p-GaAs besides the isolated substitutional Mn acceptor level. It is found that, in most of the investigated samples, the room-temperature hole concentration is governed by the additional acceptor level rather than the isolated substitutional Mn acceptor level. The concentration of the additional acceptor level is found to increase almost in proportion to the square of the concentration of the isolated substitutional Mn acceptors, suggesting that the additional acceptor level is related to Mn dimers. This suggests that the ferromagnetism observed in more heavily Mn-doped GaAs may be attributed to Mn clusters. For some of the samples in which the characteristic of nearest-neighbour hopping conduction in the substitutional Mn acceptor impurity band is evident, the hopping activation energy is deduced and is proved to increase in proportion to the cube root of the concentration of the substitutional Mn acceptors.

Carrier-induced ferromagnetism in the insulating Mn-doped III-V semiconductor InP

Although InP and GaAs have very similar band-structure their magnetic properties appear to drastically differ. Critical temperatures in (In,Mn)P are much smaller than that of (Ga,Mn)As and scale linearly with Mn concentration. This is in contrast to the square root behaviour found in (Ga,Mn)As. Moreover the magnetization curve exhibits an unconventional shape in (In,Mn)P contrasting with the conventional one of well annealed (Ga,Mn)As. By combining several theoretical approaches, the nature of ferromagnetism in Mn doped InP is investigated. It appears that the magnetic properties are essentially controlled by the position of the Mn acceptor level. Our calculations are in excellent agreement with recent measurements for both critical temperatures and magnetizations. The results are only consistent with a Fermi level lying in an impurity band, ruling out the possibility to understand the physical properties of Mn doped InP within the valence band scenario. The quantitative success found here reveals a predictive tool of choice that should open interesting pathways to address magnetic properties in other compounds

Influences of post-heat treatment on microstructures, optical and magnetic properties of unintentionally doped GaN epilayers implanted with Mn ions

In this study, GaN:Mn thin films are fabricated by implementing Mn ions into the undoped GaN material. The effects of annealing temperature on microstructures, optical and magnetic properties of the thin films are investigated. The Raman spectra measured from Mn-implanted GaN samples at different annealing temperatures show that new phonon modes, which are related to macroscopic disorder or vacancy-related defects caused by Mn-ion implantation and the local vibrational mode of Mn atoms in the (Ga, Mn)N, are created. The results of photoluminescence measurement show that new peaks appear at 2.16, 2.53, and 2.92 eV. Among these, the new emission around 2.16 eV, besides some contributions from optical transitions from the conduction band or shallow donor to a deep acceptor, cannot exclude the contribution from optical transitions of free electrons in the conduction band to Mn acceptor level. The Hall test shows that the annealed samples are of n type. Ferromagnetism is observed in the Mn doped GaN thin film at 300 K and found to be sensitive to the density of holes that mediate the Mn-Mn magnetic exchange interaction in this Mn-related impurity band.

Tunable Control over the Ionization State of Single Mn Acceptors in GaAs with Defect-Induced Band Bending

A scanning tunneling microscope was used to study the ionization of single Mn acceptors in GaAs(110). The ionization state switches when the GaAs valence band is bent across a Mn acceptor level. This produces a ringlike feature in STM images, whose diameter depends on the tunneling conditions and distance to charged arsenic vacancies. By varying the latter, we could tune the ionization switching, as well as quantify the contributions from tip- and vacancy-induced band bending.

Room Temperature Ferromagnetism of Mn Implanted AlInN

Ferromagnetic properties of Mn-implanted wurtzite AlxIn1-xN/GaN thin films grown by metal organic chemical vapor deposition (MOCVD) were observed using a quantum design superconducting quantum interference device (SQUID) magnetometer. Hysteresis behavior with a reasonably high saturation magnetic moment at room temperature for all the samples was noted. Two optical thresholds were observed at 1.58 and 2.64 eV, which are attributed to internal transition (5E→5T2) of Mn3+ (d4) and hole emission from the neutral Mn acceptor level to the valence band respectively. Bound magnetic polaron formation is considered to be the origin of ferromagnetism in our samples.

Growth and characterization of manganese-doped InAsP

We produced Mn-doped InAsP in metal-organic vapor phase epitaxy (MOVPE) grown InAsP. Good quality samples could be only prepared by heteroepitaxy on InP(0 0 1), while layers on GaAs showed very little smoothing after the initial roughening. The samples were characterized by X-ray diffraction, photoluminescence and Hall effect measurements. By varying the As content from 0% to 70% the ionization energy of the Mn-acceptor level varied from 220 to < 30 meV and drops into the valence band for higher concentrations.

The optical absorption edge of nanocrystallineGa1−xMnxN films deposited by reactive sputtering

We have focused on the optical absorption edge of nanocrystallineGa1−xMnxN (0.00≤x≤0.18) films deposited by reactive RF magnetron sputtering. The films obtained are nanocrystallinewith grain sizes of about 25 nm, having wurtzite structure and strong orientation texture in thec-axis direction. The optical characterizations of the absorption edges were obtained in the190–2600 nm spectral range. The increase of the Mn content causes an increase ofthe absorption coefficient which can be clearly noticed at low energies, and aquasi-linear decrease of the optical gap. Broad absorption bands observed around∼1.3 and ∼2.2 eV were associated with transitions between the Mn acceptor level and the valence andconduction bands, respectively. The observed changes in the optical properties due to theMn incorporation observed in these nanocrystalline films are similar to those reported forferromagnetic GaMnN single-crystal films.

Ferromagnetic Properties in Diluted Magnetic Semiconductors (Al,Mn)N grown by PEMBE

We present the structural, magnetic, and electrical properties in the (Al,Mn)N films with various Mn concentrations grown by plasma-enhanced molecular beam epitaxy. X-ray diffraction analyses reveal that the (Al,Mn)N films have the wurtzite structure without secondary phases. All (Al,Mn)N films showed the ferromagnetic ordering. Particularly, (<TEX>$Al_{1-x}Mn_{x}$</TEX>)N film with x = 0.028 exhibited the highest magnetic moment per Mn atom at room temperature. Since all the films exhibit the insulating characteristics, the origin of ferromagnetism in (Al,Mn)N might be attributed to either indirect exchange interaction caused by virtual electron excitations from Mn acceptor level to the valence band within the samples or a percolation of bound magnetic polarons arisen from exchange interaction of localized carriers with magnetic impurities in a low carrier density regime.

Analysis of magnetic field dependent mobility in ferromagnetic Ga 1− xMn xAs layers

The magneto-transport properties of ferromagnetic Ga 1− x Mn x As epilayers with Mn mole fractions in the range of x≈2.2–4.4% were investigated through Hall effect measurements. The magnetic field-dependent Hall mobility for a metallic sample with x≈2.2% in the temperature range of T=0–300 K was analyzed by magnetic field-dependent mobility model including an activation energy of Mn acceptor level. This model provides outstanding fits to the measured data up to T=300 K. It was found that the acceptor levels with activation energies of 112 meV at B=0 Oe decreased to 99 meV at B=5 kOe in the ferromagnetic region. The decrease in acceptor activation energy was due to the spin splitting of the Mn acceptor level in the ferromagnetic region, and was responsible for increase in carrier concentration.

Magnetotransport properties of ferromagnetic Ga1−xMnxAs layers on a (100) GaAs substrate

The magnetotransport properties of ferromagnetic Ga1−xMnxAs epilayers with Mn mole fractions in the range of x≈2.2%–4.4% were investigated using Hall effect measurements. The temperature-dependent Hall carrier concentration for a metallic sample with x≈2.2% was analyzed assuming an activation energy from two acceptor levels. It was found that the two acceptor levels with activation energies of 129.4 and 31.6 meV at B=0Oe decreased to 87.6 and 30.7 meV, respectively, at B=5kOe. The decrease in acceptor activation energy from 129.6 to 87.6 meV was due to the spin splitting of the Mn acceptor level in the ferromagnetic region, and was responsible for the increase in carrier concentration. From magnetic-field-dependent Hall resistance data, the Curie temperature was estimated to be TC=60 and 70 K for Ga1−xMnxAs samples with x≈2.2 and x≈4.4%, respectively. The magnetoresistance measurements confirmed that the anomalous Hall effect existed in these samples that showed metallic and insulating behavior, respectively.

Observation of a manganese acceptor level in ferromagnetic Ga1−xMnxN layers

We have observed the high-temperature (T=300K) photoluminescence (PL) spectra from free to Mn-related acceptor pair transitions in Ga1−xMnxN layers (where x≈0.2%–0.6%) grown on sapphire (0001) substrates using the plasma-enhanced molecular-beam epitaxy technique. Luminescence from an Mn acceptor level in Ga1−xMnxN layer with a low Mn content was observed at around E=3.08eV in the PL spectra. When compared to the PL from undoped GaN, the excitation, temperature-dependent PL at E=3.08eV provides convincing evidence for a neutral Mn acceptor-bound hole character. The binding energy of the Mn acceptor-bound holes of E=330meV is in good agreement with an ionization energy of E=402meV obtained using effective mass theory of shallow impurity states for the hydrogen model. In addition, analysis of the PL intensities of an acceptor-bound hole in a Ga1−xMnxN layer with x≈0.6% gave an activation energy of E=310meV, which indicates that the acceptor-bound hole with a binding energy of E=310meV was dissociated due to thermal quenching.

Observation of room-temperature ferromagnetism in (Al,Mn)N thin films

We present the structural, electrical and magnetic properties in the (Al,Mn)N thin films with various Mn concentrations grown by plasma-enhanced molecular beam epitaxy. The X-ray diffraction patterns reveal that the (Al,Mn)N films have the wurtzite structure without secondary phases. The (Al 1− x Mn x )N films with x = 1.7 % and 2.6% were obviously found to exhibit the ferromagnetic ordering with Curie temperature exceeding room temperature while the film with x = 3.0 % was observed to show the ferromagnetic ordering at 5 K, indicating that a critical Mn concentration exists. Since all the films exhibit insulating characteristics, the origin of ferromagnetism in (Al,Mn)N might be attributed to indirect exchange interaction caused by virtual electron excitations from Mn acceptor level to the valence band within the samples.

Optical and magnetic properties of Mn in bulk GaN

We report results of electron paramagnetic resonance, magnetization, and optical absorption studies of bulk GaN crystals doped with Mn and, for some samples, codoped with Mg acceptor. The experiments performed show that the charge state of the Mn ion in GaN depends on the Fermi level position in the crystal. In n-type samples Mn stays as an ionized acceptor A - center of Mn 2 + (d 5 ), while in the investigated highly resistive samples with lowered Fermi level, in a different charge state, most probably corresponding to neutral configuration A°. Optical absorption spectra of GaN:Mn and GaN:Mn,Mg show characteristic absorption bands related to Mn, which may be interpreted as resulting from photoionization of Mn 2 + (d 5 ) to GaN conduction band (in n-type samples) and photoionization of neutral Mn A 0 to GaN valence band (in highly resistive samples). The absorption spectra are discussed in terms of a configuration coordinate model. The location of the Mn acceptor level is derived as 1.8 eV below the bottom of the GaN conduction band. The magnetization experiments performed reveal Brillouin-type magnetization with Mn spin S=5/2 in n-type crystals, while the investigated highly resistive samples show magnetic anisotropy characteristic for nonspherical transition-metal configurations. The nature of the Mn ion in GaN is discussed.

Characterization of ferromagnetic Ga1−xMnxN layers grown on sapphire (0001) substrates

Ga 1−x Mn x N epilayers with a well-ordered ferromagnetic structure were grown on sapphire (0001) substrates, using the plasma enhanced molecular-beam epitaxy technique. Ga1−xMnxN films were found to be homogeneous, and to exhibit n-type conductivity and ferromagnetic ordering with a Curie temperature (TC) above room temperature. This was confirmed by transmission electron microscopy, x-ray diffraction, and by magnetometry using a superconducting quantum interference device. The high-temperature (T=300 K) photoluminescence (PL) spectra exhibited Mn-related free-to-acceptor pair transitions for Ga1−xMnxN layers with x≈0.2% and x≈0.6%. A Mn acceptor level of a Ga1−xMnxN layer with low Mn content was found to be located around 330 meV above the top of the valence band, suggesting that Mn-bound holes in group-III nitrides exhibit effective-masslike behavior. The excitation and temperature-dependent PL provided convincing evidence of a band-edge exciton to acceptor transition.

Mn Impurity in GaN Studied by Electron Paramagnetic Resonance

We present the results of electron paramagnetic resonance investigations of GaN bulk crystals doped with Mn. The EPR experiment shows the Mn 2 + resonance in all the investigated n-type crystals, while in highly resistivesamples extra doped with Mg acceptor the Mn 2 + resonance decreases. This is a consequence of the location of Mn acceptor level in GaN band gap. The analysis of the spin relaxation times reveals the Korringa scattering as the dominating spin relaxation mechanism in n-type GaN:Mn crystals. The effective exchange constant determined from spin relaxation rate temperature dependence is of the order of 14 meV.

Charge Transfer at the Mn Acceptor Level in GaN

MBE-grown GaN : Mn layers with Mn doping concentrations around 1020 cm−3 were investigated by photoconductivity measurements. From electron spin resonance (ESR), Mn is known to be mostly present in the neutral Mn3+ or Mn2+ + h+ state, which leads to a reassignment of the known optical absorption features to charge transfer from Mn3+, either by direct photoionization at about 1.8 eV or by a photothermal ionization process via an excited state (Mn3+)* at 1.42 V higher internal energy than the Mn3+ ground state. It is proposed that the Mn3+/Mn2+ acceptor level is located about 1.8 eV above the valence band edge of GaN so that the nature of the acceptor wavefunction is very different from an effective-mass-like state such as the Mn2+ + h+ complex in GaAs : Mn. According to these experimental results, the realization of carrier-mediated ferromagnetism becomes rather unlikely in not co-doped GaN : Mn.

Mn-related absorption and PL bands in GaN grown by metal organic vapor phase epitaxy

In this paper, the optical absorption and photoluminescence spectra of semi-insulating Mn-doped GaN films were studied. Two characteristic bands were observed in the absorption spectra of Mn-doped epilayers at 300 K. The integrated intensities of these bands increased with increasing Mn concentration indicating that they were Mn-related. An analysis of the temperature behavior of the absorption band with a maximum at 1.5 eV indicated that it involved a free to bound transition from the valence band to the deep Mn-acceptor level. Photoluminescence measurements of Mn-doped films indicated the presence of an intra 3d-shell transition of the Mn ion. The luminescence band at 1.25±0.02 eV is tentatively attributed to the 4T 1(G)→ 6A 1(S) transition.

The band offset measurement at the In0.5Ga0.5P/GaAs heterojunction by using properties of transition metal

Abstract It is well known that the energy level of a transition metal (TM) such as manganese is tied to the vacuum level rather than to a particular band extreme in semiconductors. Therefore, TM can be used as a common reference energy level in all the semiconductors. We have investigated for the first time the photoluminescence spectrum of Mn-doped In0.5Ga0.5P/GaAs heterojunction and by using the Mn acceptor level in photoluminescence (PL) spectra, the fractional band offsets Qc and Qv for the In0.5Ga0.5P/GaAs heterojunction are determined as 0.63 and 0.37, respectively.

Hole ionization of Mn-doped GaAs: Photoluminescence versus space-charge techniques.

The thermal quenching of the Mn-related emission in GaAs is studied by measuring both the drop in intensity and the reduction in the luminescence time decay as a function of temperature. It is shown that both these quantities are governed by a temperature-activated process with an activation energy equal to the hole-emission activation energy, which is, in turn, equal to the ground-state energy of the Mn acceptor level at ${E}_{v}$+0.11 eV. However, a large reduction is found in the thermal emission rates of holes in bulk material compared to thermal measurements made with deep-level transient spectroscopy in space-charge layers. This discrepancy is discussed in terms of a recapture mechanism which strongly affects measurements in neutral material. It is also demonstrated that great care has to be taken when the detailed-balance relationship is used for the determination of thermodynamic parameters.