Low Hole Concentration Research Articles

IV‐VI Ferromagnetic Semiconductors — Recent Studies

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Nature of Perpendicular-to-Parallel Spin Reorientation in a Mn-doped GaAs Quantum Well: Canting or Phase Separation?

It is well known that the magnetic anisotropy in a compressively strained Mn-doped GaAs film changes from perpendicular to parallel with increasing hole concentration p. We study this reorientation transition at T=0 in a quantum well with delta-doped Mn impurities. With increasing p, the angle theta that minimizes the energy E increases continuously from 0 (perpendicular anisotropy) to pi/2 (parallel anisotropy) within some range of p. The shape of E(min)(p) suggests that the quantum well becomes phase separated with regions containing low hole concentrations and perpendicular moments interspersed with other regions containing high hole concentrations and parallel moments. However, because of the Coulomb energy cost associated with phase separation, the true magnetic state in the transition region is canted with 0<theta<pi/2.

Nitride-Based Green Light-Emitting Diodes With Various p-Type Layers

The performance characteristics of green light-emitting diodes (LEDs) grown by metal-organic chemical-vapor deposition were investigated to study the dependence of the device performance on the materials and the growth conditions of p-type layer grown after the InGaN multiple-quantum-well active region. The electrical and structural qualities of Mg-doped p-In0.04Ga0.96N and p-GaN layers grown under different growth conditions were studied to optimize the growth conditions of p-type hole injection layers of green LEDs. A free-hole concentration of p=1.6times1018 cm-3 of with a resistivity of 0.33Omegamiddotcm was achieved for p-GaN:Mg layers grown at 1040degC. Lower hole concentrations and mobilities and rough surfaces were obtained when the growth temperature was decreased to 930degC in H2 ambient. In the case of p-In0.04Ga0.96N grown at 840degC in N2, a significant improvement of the hole concentration was achieved due to the reduced ionization activation energy of Mg acceptors in InGaN. Also we observed that as-grown p-GaN layers grown in N2 ambient showed p-type properties without Mg dopant activation. The electrical and optical properties of In0.25Ga0.75 N/GaN multiple-quantum-well green LEDs with such different p-layers were investigated. The electroluminescence intensity was improved for the LEDs with p-In0.04Ga0.96N layers grown at 840degC as compared to the LEDs with p-GaN layers grown at higher temperatures due to the reduced thermal damage to the active region, high hole injection, and low piezoelectric field induced in the active region. p-InGaN layers are very attractive candidates for the p-layer in green LED structures. The low temperature and N2 ambient used during the growth of InGaN layers are beneficial to protect the InGaN active region containing high-indium composition quantum-well layers in addition to the advantage of providing a higher hole concentration. However, the LEDs with p-In0.04Ga0.96 N layer showed a slightly higher turn on voltage which could originate from the potential barrier for hole transport at the interface of the p-InGaN layer and the last GaN quantum-well barrier. to reduce this problem, we designed and characterized an LED structure having a graded indium composition in the p-In0.04Ga0.96N layer in order to improve hole transport into the active region. Optimized LEDs with p-InGaN layers grown in a N2 ambient showed much brighter electroluminescence due to low damage to the active region during p-InGaN layer growth

Dependence of Magnetic Anisotropies and Critical Temperatures on the Hole Concentration in Ferromagnetic GaMnAs Thin Films

The magnetic anisotropy constants and critical temperatures of a series of epitaxial GaMnAs thin films on GaAs with 7% Mn concentration in which the free hole concentration is progressively modified by hydrogen passivation is investigated by ferromagnetic resonance (FMR) spectroscopy. Ferromagnetism (FM) is lost for fully passivated films. In the FM films with the lowest hole concentration, the easy axis of magnetization is oriented perpendicular to the film plane and switches to in-plane [100] for higher hole concentrations. Temperature-dependent switching is equally observed. The numerical values of the uniaxial and cubic anisotropy constants are determined as a function of temperature and Tc 3

Inhomogeneity of InGaN quantum wells in GaN‐based blue laser diodes

AbstractWe investigated the inhomogeniety of InGaN quantum wells (QWs) with high In content in the GaN‐based blue laser diodes (LDs). The 2QWs LD structure show the twice higher photoluminescence intensity and the slightly higher electroluminescence intensity than single quantum well (SQW) LD. However, we can obtain the high power blue InGaN SQW LDs with the low threshold current density of 2.19 kA/cm2 and the high sloped efficiency of 0.8 W/A at the cw condition by reducing the number of QWs. From carrier distribution by simulation, blue InGaN 2QWs LDs represented the non‐uniformed hole distribution of n‐side 1st and p‐side 2nd well in InGaN/InGaN 2QWs region due to the low hole concentration and mobility of p‐type nitrides. From time‐resolved photoluminescence (TRPL), we obtained that 2QWs LD shows the some spatial localization states due to the multiple component PL decay process. Therefore, we can guess that the inhomogeniety of InGaN QWs would be quite restricted in deep InGaN QWs of GaN based blue LDs which results in significantly in homogeneities of QWs. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effect of annealing temperature and ambient gas on phosphorus doped p-type ZnO

The authors report on the thermal activation of phosphorus doped p-type ZnO thin films grown by radio frequency magnetron sputtering. The p-type ZnO was produced by activating phosphorus doped ZnO thin films in N2, Ar, or O2 ambients. The hole concentration of the p-type ZnO, prepared in an O2 ambient, showed a lower hole concentration compared to samples annealed in N2 and Ar ambients. The activation energies of the phosphorus dopant in the p-type ZnO under different ambient gases indicate that phosphorus atoms replace oxygen atoms in the ZnO to form PO which acts as an acceptor.

Optimization of the Active-Layer Structure for the Deep-UV AlGaN Light-Emitting Diodes

The dependence of the active-layer structure on the performance of the deep-UV AlGaN light-emitting diodes (LEDs) was theoretically investigated with an APSYS simulation program. Several structure parameters such as well width, well number, barrier height, barrier width, and doping type were employed to study how these parameters change the band structures as well as the carrier distributions. The band offset and bowing parameter used in the theoretical analysis were extracted from the experimental results. Theoretical analysis shows that the nonuniform carrier distributions as well as the low hole concentrations, which caused by polarization-induced tilted band structures, play important roles in improving the performance of the AlGaN LEDs. Compensating this asymmetric band structure and increasing the hole density are the important keys to improve the AlGaN LED performance. Numerical simulation results suggest that the higher output power can be obtained when the active layer consists of only one quantum well with a width of 1-3 nm and two thicker n-doped barriers with a small Al composition

Resonance peak and incommensurability in cuprate perovskites

The magnetic susceptibility of yttrium and lanthanum cuprates is interpreted based on the self-consistent solution of the t-J model of Cu-O planes. To take a proper account of strong electron correlations inherent in cuprates, the calculations were carried out in the formalism of the Hubbard operators and Mori’s projection operator technique. The calculations correctly reproduce the frequency and momentum dependences of the resonance peak in YBa2Cu3O7 − y and its variation with doping and temperature in the normal and superconducting states. In lanthanum cuprates, spin excitations near the antiferromagnetic wave vector Q = (π, π) are overdamped and, therefore, a broad maximum is observed in the susceptibility instead of a sharp peak. For low frequencies, temperatures, and hole concentrations x ≥ 0.02, the susceptibility is peaked at incommensurate wave vectors (π ± 2πδ, π), (π, π ± 2πδ). The incommensurability is connected with the minimum in the magnon damping at Q in the crystal with a short-range antiferromagnetic order. Generally, the incommensurate magnetic response is not accompanied by an inhomogeneity of the charge-carrier density.

Ultraviolet Laser SQUID Microscope for GaN Blue Light Emitting Diode Testing

We carried out non-contacting measurements of photocurrent distributions in GaN blue light emitting diode (LED) chips using our newly developed ultraviolet (UV) laser SQUID microscope. The UV light generates the photocurrent, and then the photocurrent induces small magnetic fields around the chip. An off-axis arranged HTS-SQUID magnetometer is employed to detect a vector magnetic field whose typical amplitude is several hundred femto-tesla. Generally, it is difficult to obtain Ohmic contacts for p-type GaN because of the low hole concentration in the p-type epitaxial layer and the lack of any available metal with a higher work function compared with the p-type GaN. Therefore, a traditional probecontacted electrical test is difficult to conduct for wide band gap semiconductors without an adequately annealed electrode. Using the UV-laser SQUID microscope, the photocurrent can be measured without any electrical contact. We show the photocurrent vector map which was reconstructed from measured magnetic fields data. We also demonstrate how we found the position of a defect of the electrical short circuits in the LED chip.

IV-VI ferromagnetic semiconductors recent studies

In some IV-VI semimagnetic semiconductors, the RKKY interaction can dominate over the standard d-d superexchange and become the driving mechanism for ion-ion coupling. In effect, for low hole concentrations the Mn ion system is in a paramagnetic phase, whereas for higher ones it reveals typical ferromagnetic behavior. In this paper, recent work on IV-VI ferromagnetic (SnMnTe, PbSnMnTe and GeMnTe) systems will be presented. In particular, the influence of the presence of two types of magnetic ions (transition metal: Mn and rare earth metal: Eu or Er) incorporated into a semiconductor matrix on magnetic properties of resultant semimagnetic semiconductor will be described.

Impurity effects in ZnO and nitrogen-doped ZnO thin films fabricated by MOCVD

We studied the role of impurities in nitrogen-doped ZnO thin film to understand the difficulty of producing p-type ZnO via nitrogen doping. The ZnO:N films were fabricated by low-pressure metal-organic chemical vapor deposition (MOCVD) using diethylzinc (DEZ) and nitric oxide (NO) precursors. Although very high levels of nitrogen incorporation were observed (∼10 21 cm −3), acceptor concentrations were typically low (10 14–10 17 cm −3). The investigation suggests that the low carrier concentrations are possibly due to compensation and passivation effects by hydrogen and carbon impurities unintentionally incorporated into the films from the metal-organic precursor. X-ray photoelectron spectroscopy (XPS) demonstrated that carbon was a bulk impurity in MOCVD-grown films. Secondary-ion mass spectrometry (SIMS) analysis confirmed the presence of carbon and indicated that hydrogen was also a bulk impurity. The concentration of carbon contaminant was found to increase with nitrogen doping. Both XPS and Fourier transform infrared spectroscopy (FTIR) data indicated that defect complexes (CH x , NH x , and NC x ) are likely present in MOCVD-grown ZnO films. First-principles calculations predict that the N O–H and (NC) O defect complexes are neutral and 1 + charge state; therefore, the existing carbon and hydrogen passivate the nitrogen acceptor species. Thus, we believe a low hole concentration in MOCVD-fabricated ZnO:N films are partially due to inadvertent passivation by hydrogen and carbon.

Effect of thermal annealing induced by p-type layer growth on blue and green LED performance

We investigated the electrical and structural qualities of Mg-doped p-type In 0.01Ga 0.99N and GaN layers grown under different growth conditions by metalorganic chemical vapor deposition. It was found that we obtained a lower hole concentration and rough surface by reducing the growth temperature down to 930 °C in the case of p-GaN. A slight improvement of the hole concentration was obtained for Mg-doped In 0.01Ga 0.99N grown at 840 °C, but this was accompanied by pit formation on the surface. In 0.19Ga 0.81N/GaN and In 0.25Ga 0.75N/GaN multiple-quantum-well (MQW) light-emitting diodes (LED) with such different p-layers were also grown. It was found from photoluminescence studies that the optical and structural properties of the MQW in the LED structure were improved by reducing the growth temperature of the p-layer due to reduced thermal annealing effect of the active region during p-layer growth. However, it was also found the electroluminescence intensity was lower for the LEDs with p-In 0.01Ga 0.99N layers grown at 840 °C.

Mean-field study ofFe2+- andCo2+-doped diluted magnetic semiconductors

Based on a modified mean-field model, we calculate the Curie temperatures of ${\mathrm{Fe}}^{2+}$- and ${\mathrm{Co}}^{2+}$-doped diluted magnetic semiconductors (DMSs) and their dependence on the hole concentration. We find that the Curie temperatures increase with an increase in hole concentration and the relationship ${T}_{C}\ensuremath{\propto}{p}^{1∕3}$ also approximately holds for ${\mathrm{Fe}}^{2+}$- and ${\mathrm{Co}}^{2+}$-doped systems with moderate hole concentration. For either low or high hole concentrations, however, the ${p}^{1∕3}$ law is violated due to the anomalous magnetization of the ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Co}}^{2+}$ ions, and the nonparabolic nature of the hole bands. Further, the values of ${T}_{C}$ for ${\mathrm{Fe}}^{2+}$- and ${\mathrm{Co}}^{2+}$-doped DMSs are significantly higher than those for ${\mathrm{Mn}}^{2+}$-doped DMSs, due to the larger exchange interaction strength.

Scaling behavior in the optical conductivity of two-dimensional systems of strongly correlated electrons based on the U(1) slave-boson approach to thet−JHamiltonian

The U(1) holon-pair boson theory of Lee and Salk [Phys. Rev. B 64, 052501 (2001)] is applied to investigate the quantum scaling behavior of optical conductivity in the two-dimensional systems of strongly correlated electrons. We examine the role of both the gauge field fluctuations and spin pair excitations on the $\ensuremath{\omega}∕T$ scaling behavior of the optical conductivity. It is shown that the gauge field fluctuations but not the spin pair excitations are responsible for the scaling behavior in the low-frequency region $\ensuremath{\omega}∕T⪡1$. The importance for the contribution of the nodal spinons to the Drude peak is discussed. It is shown that the $\ensuremath{\omega}∕T$ scaling behavior is manifest in the low-frequency region at low hole concentrations close to a critical concentration at which superconductivity arises at $T=0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

The Special Features of the Hall Effect in GaMnSb Layers Deposited from a Laser Plasma

Epitaxial GaMnSb films with Mn contents up to about 10 at. % were obtained by deposition from a laser plasma in vacuum. The growth temperature T s during deposition was varied from 440 to 200°C, which changed the concentration of holes from 3 × 1019 to 5 × 1020 cm−3, respectively. Structure studies showed that, apart from Mn ions substituting Ga, the GaMnSb layers contained ferromagnetic clusters with Mn and shallow acceptor defects of the GaSb type controlled by the T s value. Unlike single-phase GaMnSb systems studied earlier with negative anomalous Hall effect values and Curie temperatures not exceeding 30 K, the films obtained in this work exhibited a positive anomalous Hall effect, whose hysteresis character manifested itself up to room temperature and was the more substantial the higher the concentration of holes. The unusual behavior of this effect was interpreted in terms of the interaction of charge carriers with ferromagnetic clusters, which was to a substantial extent determined by the presence of Schottky barriers at the boundary between the clusters and the semiconducting matrix; this interaction increased as the concentration of holes grew. The absence of this effect in semiconducting compounds based on III–V Group elements with MnSb or MnAs ferromagnetic clusters was discussed in the literature; we showed that this absence was most likely related to the low hole concentrations in these objects.

Exact Ground States of a Frustrated 2D Magnet: Deconfined Fractional Excitations at a First-Order Quantum Phase Transition

We introduce a frustrated spin 1/2 Hamiltonian which is an extension of the two dimensional J1-J2 Heisenberg model. The ground states of this model are exactly obtained at a first-order quantum phase transition between two valence bond crystals. At this point, the low energy excitations are deconfined spinons and spin-charge separation occurs under doping in the limit of low concentration of holes. In addition, this point is characterized by the proliferation of topological defects.

Highly resistive p-PbTe films with carrier concentration as low as 1014 cm−3

We propose here a model according to which a high density of semiconductor–insulator interface states can deplete practically the whole film volume, provided that the film thickness is of the order of Debye screening length. We demonstrated this experimentally by showing that thin p-PbTe films, thermally deposited on mica substrate, have an unusually low concentration of free holes, as low as 1014 cm−3 at 100 K, resulting in a very high value of resistance, Hall constant, and Seebeck coefficient, respectively. Such low concentration of free carriers allows an investigation of a whole series of phenomena in AIVBVI semiconductors, such as injection currents, injection electroluminescence, electronic memory phenomena, electric field effect control of thermopower, and more.

Influence of carrier concentration on magnetic phase transition in Ge1−xMnxTe

We have studied the influence of the carrier concentration on the phase transition from ferromagnetic to paramagnetic in IV–VI diluted magnetic semiconductor Ge 1− x Mn x Te. While the magnetic phase transition of Ge 1− x Mn x Te with the high hole concentration (∼10 21 cm −3 ) is second order, Ge 1− x Mn x Te with the low hole concentration (∼10 19 cm −3 ) shows anomalous behavior compared to the theoretical model based on an uniformly ferromagnetic system. This suggests that the non-ferromagnetic region remains in Ge 1− x Mn x Te with lower hole concentration due to lack of long-range ferromagnetic order.

Mechanism investigation of NiOx in Au/Ni/p-type GaN ohmic contacts annealed in air

Recently, Au/Ni/p-type GaN ohmic contacts annealed in an air ambient have been widely investigated. However, to obtain a low specific-contact resistance, the annealing window is limited. In this study, to understand the oxidation function of metallic Ni, the Au/Ni/p-type GaN structure was annealed in an air ambient for 10 min at various temperatures. Using x-ray photoelectron spectroscopy (XPS) analysis, the metallic Ni was oxidized into NiO and NiO1.3 compositions at annealing temperatures of 500°C and 600°C, respectively. However, metallic Ni still existed on the interface of the Ni/p-type GaN annealed at 400°C. The associated barrier heights of 0.42 eV, 0.21 eV, and 0.31 eV were obtained with p-type GaN for the Ni, NiO, and NiO1.3 contacts, respectively. The hole concentrations of p-type NiO and p-type NiO1.3 were 2.6×1016 cm−3 and 2.0×1018 cm−3, respectively. The lower hole concentration of the p-type NiO would lead to reducing the valence-band bending of the p-type GaN, as well as the barrier height for holes crossing from the p-type NiO to the p-type GaN. The formation of NiO was thus an important issue for lowering the specific-contact resistance of the Au/Ni/p-type GaN ohmic contacts annealed in an air ambient.

Hall coefficient of the oxygen-intercalated La214 superconductor in the low hole concentration region

The temperature dependence of the Hall coefficient ( R H) of La 2CuO 4+ δ was studied in the low oxygen concentration range δ=0.015–0.030 where the phase separation into the antiferromagnetic and the superconducting states has been observed. The Hall number derived from the R H at room temperature agreed with the hole concentration of 0.03–0.06/Cu in the system, which is similar to the La 2− x Sr x CuO 4. However, in the lowest hole concentration region, the R H increased rapidly below around 250 K with lowering the temperature to 90 K where the R H started to decrease on further cooling. The drop in the R H below 90 K indicates a development of the one-dimensional charge transport in the system. The results are quite different from those observed in the similarly low-doped La 2− x Sr x CuO 4 system and discussed in terms of the phase separation, staging and charged-stripe formation.