F. Yun

Ferromagnetism of ZnO and GaN: A Review

AbstractThe observation of ferromagnetism in magnetic ion doped II–VI diluted magnetic semiconductors (DMSs) and oxides, and later in (Ga,Mn)As materials has inspired a great deal of research interest in a field dubbed “spintronics” of late, which could pave the way to exploit spin in addition to charge in semiconductor devices. The main challenge for practical application of the DMS materials is the attainment of a Curie temperature at or preferably above room temperature to be compatible with junction temperatures. Among the studies of transition-metal doped conventional III–V and II–VI semiconductors, transition-metal-doped ZnO and GaN became the most extensively studied topical materials since the prediction by Dietl et al., based on mean field theory, as promising candidates to realize a diluted magnetic material with Curie temperature above room temperature. The underlying assumptions, however, such as transition metal concentrations in excess of 5% and hole concentrations of about 1020 cm−3, have not gotten as much attention. The particular predictions are predicated on the assumption that hole mediated exchange interaction is responsible for magnetic ordering. Among the additional advantages of ZnO-and GaN-based DMSs are that they can be readily incorporated in the existing semiconductor heterostructure systems, where a number of optical and electronic devices have been realized, thus allowing the exploration of the underlying physics and applications based on previously unavailable combinations of quantum structures and magnetism in semiconductors. This review focuses primarily on the recent progress in the theoretical and experimental studies of ZnO- and GaN-based DMSs. One of the desirable outcomes is to obtain carrier mediated magnetism, so that the magnetic properties can be manipulated by charge control, for example through external electrical voltage. We shall first describe the basic theories forwarded for the mechanisms producing ferromagnetic behavior in DMS materials, and then review the theoretical results dealing with ZnO and GaN. The rest of the review is devoted to the structural, optical, and magnetic properties of ZnO- and GaN-based DMS materials reported in the literature. A critical review of the question concerning the origin of ferromagnetism in diluted magnetic semiconductors is given. In a similar vein, limitations and problems for identifying novel ferromagnetic DMS are briefly discussed, followed by challenges and a few examples of potential devices.

Deep centers in a free-standing GaN layer

Schottky barrier diodes, on both Ga and N faces of a ∼300-μm-thick free-standing GaN layer, grown by hydride vapor phase epitaxy (HVPE) on Al2O3 followed by laser separation, were studied by capacitance–voltage and deep level transient spectroscopy (DLTS) measurements. From a 1/C2 vs V analysis, the barrier heights of Ni/Au Schottky contacts were determined to be different for the two polar faces: 1.27 eV for the Ga face, and 0.75 eV for the N face. In addition to the four common DLTS traps observed previously in other epitaxial GaN including HVPE-grown GaN a new trap B′ with activation energy ET=0.53 eV was found in the Ga-face sample. Also, trap E1 (ET=0.18 eV), believed to be related to the N vacancy, was found in the N-face sample, and trap C (ET=0.35 eV) was in the Ga-face sample. Trap C may have arisen from reactive-ion-etching damage.

Energy band bowing parameter in AlxGa1−xN alloys

Molecular-beam epitaxy grown AlxGa1−xN alloys covering the entire range of alloy compositions, 0⩽x⩽1, have been used to determine the alloy band gap dependence on its composition. The Al chemical composition was deduced from secondary ion mass spectroscopy and Rutherford backscattering. The composition was also inferred from x-ray diffraction. The band gap of the alloy was extracted from low temperature optical reflectance measurements which are relatively more accurate than photoluminescence. Fitting of the band gap data resulted in a bowing parameter of b=1.0 eV over the entire composition range. The improved accuracy of the composition and band gap determination and the largest range of the Al composition over which our study has been conducted increase our confidence in this bowing parameter.

Backilluminated GaN/AlGaN heterojunction ultraviolet photodetector with high internal gain

We report on a backilluminated GaN/Al0.18Ga0.82N heterojunction ultraviolet (UV) photodetector with high internal gain based on metal-semiconductor-metal structures. A narrow band pass spectral response between 365 and 343 nm was achieved. When operating in dc mode, the responsivity reaches up to the order of 102 A/W under weak UV illumination, which is due to enormous internal gain up to 103. The linear dependence of photocurrent on bias and its square root dependence on optical power are found and explained by a trapping and recombination model. The high photocurrent gain is attributed to trapping and recombination centers with an acceptor character induced by dislocations in GaN.

Dependence of GaN polarity on the parameters of the buffer layer grown by molecular beam epitaxy

The polarity of GaN films grown using GaN and AlN buffer layers on sapphire substrates by molecular beam epitaxy were investigated by atomic force microscopy, hot wet chemical etching, and reflection high-energy electron diffraction. We found that the GaN films grown on high temperature AlN (>890 °C) and GaN (770–900 °C) buffer layers invariably show Ga and N polarity, respectively. However, the films grown using low temperature (∼500 °C) buffer layers, either GaN or AlN, could have either Ga or N polarity, depending on the growth rate of the buffer layer.

Systematic measurement of AlxGa1−xN refractive indices

Dispersion of the ordinary and extraordinary indices of refraction have been measured systematically for wurtzitic AlxGa1−xN epitaxial layers with 0.0⩽x⩽1.0 throughout the visible wavelength region. The dispersion, measured by a prism coupling waveguide technique, is found to be well described by a Sellmeier relation. Discrepancies among previous measurements of refractive index dispersion, as a consequence of different growth conditions and corresponding band gap bowing parameter, are reconciled when the Sellmeier relation is parameterized not by x but by band gap energy.

Effectiveness of TiN porous templates on the reduction of threading dislocations in GaN overgrowth by organometallic vapor-phase epitaxy

We report on the reduction of threading dislocations in GaN overlayers grown by organometallic vapor phase epitaxy on micro-porous TiN networks. These networks were obtained by in situ annealing of thin Ti layers deposited in a metalization chamber, on the (0001) face of GaN templates. Observations by transmission electron microscopy indicate dislocation reduction by factors of up to 10 in GaN layers grown on TiN networks compared with the control GaN. X-ray diffraction shows that GaN grown on the TiN network has a smaller (102) plane peak width (4.6 arcmin) than the control GaN (7.8 arcmin). In low temperature photoluminescence spectra, a narrow excitonic full-width-at-half-maximum of 2.4 meV was obtained, as compared to 3.0 meV for the control GaN, confirming the improved crystalline quality of the overgrown GaN layers.

Structural analysis of ferromagnetic Mn-doped ZnO thin films deposited by radio frequency magnetron sputtering

We report on the structural analysis of ferromagnetic Mn-doped ZnO thin films deposited by radio frequency magnetron sputtering, using transmission electron microscopy (TEM), high-resolution x-ray diffraction, and Rutherford backscattering spectroscopy (RBS) measurements. The ferromagnetic Mn-doped ZnO film showed magnetization hysteresis at 5 and 300K. A TEM analysis revealed that the Mn-doped ZnO included a high density of round-shaped cubic and elongated hexagonal MnZn oxide precipitates. The incorporation of Mn caused a large amount of structural disorder in the crystalline columnar ZnO lattice, although the wurtzite crystal structure was maintained. The observed ferromagnetism is discussed based on the structural characteristics indicated by TEM and the behavior of Mn when it is substituted into a ZnO lattice derived from RBS measurements.

Characterization of free-standing hydride vapor phase epitaxy GaN

A free-standing GaN template grown by hydride vapor phase epitaxy has been characterized by transmission electron microscopy (TEM). The TEM investigation was augmented by x-ray diffraction, defect delineation etching process followed by imaging with atomic force microscopy and variable temperature photoluminescence. The density of dislocations near the N face was determined to be, in order, 3±1×107, 4±1×107, and about 1×107 cm−2 by cross-sectional TEM, plan-view TEM, and a defect revealing etch, respectively. The same methods on the Ga face revealed the defect concentration to be, in order, less than 1×107 cm−2 by plan-view TEM, less than 5×106 cm−2 by cross-sectional TEM, and 5×105 cm−2 by defect revealing hot H3PO4 acid, respectively. The full width at half maximum of the symmetric (0002) x-ray diffraction peak was 69 and 160 arc sec for the Ga and N faces, respectively. That for the asymmetric (101_4) peak was 103 and 140 arc sec for Ga and N faces, respectively. The donor bound exciton linewidth was a...

Investigation of inversion domains in GaN by electric-force microscopy

Inversion domains in III-nitride semiconductors degrade the performance of devices fabricated in them. Consequently, it is imperative that we understand their electrostatic manifestation, the growth conditions under which such domains form, and an effective means of their identification. In what is nominally referred to as Ga-polarity samples, N-polarity domains have a polarization that is reversed with respect to the remainder of the surface, and therefore, have a different potential under strain. We have used surface-potential electric-force microscopy (SP-EFM) to image the electrostatic surface potential of GaN grown on sapphire, which is strained due to the thermal mismatch between the substrate and GaN. Employing a control sample with side-by-side Ga- and N-polarity regions, we have established the EFM mode necessary to identify inversion domains on GaN samples grown by molecular-beam epitaxy. This method is not sensitive to topology and has a spatial resolution of under 100 nm. The measured surface ...