Nitrogen Cluster States Research Articles

Substitutionality of nitrogen atoms and formation of nitrogen complexes and point defects in GaPN alloys

Nitrogen substitution and formation of point defects in GaP(1−x)Nx layers (x ranging from 0.01 to 0.04) grown on GaP substrates are characterized by channelling Rutherford backscattering, nuclear reaction analysis and positron annihilation spectroscopy measurements. It is observed that the substitutionality of nitrogen into GaP decreases from a value of 0.91 to that of <0.1 with increasing nitrogen content from x = 1.7% to x = 4.0%. In addition to substitutional nitrogen atoms, GaPN layers have nitrogen interstitials, nitrogen clusters and defect complexes composed of multiple nitrogen atoms. Positron annihilation spectroscopy of GaPN layer shows positron trapping not only in vacancies but also trapping due to nitrogen clusters. In addition, the footprint of different nitrogen cluster states and point defects is observed in temperature dependent photoluminescence measurements.

Mobility in gated GaNxAs1−xheterostructures as a probe of nitrogen-related electronic states

Calculations are presented of $n$-type carrier mobility in a gated, double-quantum-well InGaAs/GaN${}_{x}$As${}_{1\ensuremath{-}x}$ heterostructure (with $x\ensuremath{\approx}0.01$), where transport occurs in the lowest two subbands (one primarily in the InGaAs layer and the other primarily in the dilute nitride layer) and carrier confinement is provided by surrounding AlGaAs layers and gating fields from front and back gates. Such a device allows a controlled interaction between carriers and localized nitrogen cluster states near the conduction band edge in the dilute nitride. Varying the In composition in the GaInAs layer and the gating fields is predicted to alter the Fermi level, relative to the localized nitrogen cluster states, and cause a strong modulation of the $n$-type mobility. For appropriately designed heterostructures, the mobility is predicted to vary by up to a factor of three when the Fermi level is resonant with nitrogen cluster electronic states. Thus, the mobility measurements in such a heterostructure can be used as a spectroscopic probe of the localized states in the dilute nitride layer.

Zero-phonon lines of nitrogen-cluster states in GaN x As1-x : H identified by time-resolved photoluminescence.

The incorporation of only a few percentage of nitrogen atoms into a III–V host material, e.g., GaAs or GaP, leads to dramatic changes in the electronic and optical properties of these compounds [1]. These materials show a very strong band gap bowing with increasing nitrogen concentration [2, 3]. Post-growth annealing of these dilute nitrides induces a noticeable blue-shift of the band-gap [1], while post-growth hydrogenation effectively passivates the incorporated nitrogen atoms due to the formation of various N–H complexes [4, 5]. Both post-growth effects have been studied intensively [1]. Numerous experimental results have shown that the nitrogen-induced disorder leads to the formation of various localized states related to isolated N atoms, NN-pair states, and nitrogen-clusters (NC) even in the very dilute regime (nitrogen concentrations lower than 0.3%) [6, 7]. The modification of the electronic properties, e.g., the electron effective mass and the gyromagnetic factor of electrons [8, 9], by these localized states cannot be explained in the framework of k p models taking into account only the interaction between the host conduction-band (CB) and the localized state of a single nitrogen atom [10, 11]. The theoretical description of the band-formation process in dilute nitrides must be modified to include various N cluster states [12, 13]. In former photoluminescence (PL) experiments the various optical transitions corresponding to different NC states were often attributed to LO-phonon replicas [6, 14]. In this article, we use time-resolved photoluminescence (TRPL) measurements to clarify that actually only zerophonon lines are observed in our PL spectra. Two GaNxAs1-x epitaxial layers of 0.5 lm thickness grown by metal-organic vapor-phase epitaxy are studied. The effective nitrogen concentration xeff is determined by the free exciton recombination energy (see Ref. [8]) to be 0.049% and 0.111%, respectively. The sample containing x = 0.111% of nitrogen is irradiated by a low-energy ion gun with different hydrogen doses (3.5 10–5 10 ions/cm) at 300 C. This reduces the effective nitrogen concentration xeff of the hydrogenated samples to about 0.083% and 0.002%, respectively [15, 16]. The samples are mounted onto a cold finger of a helium cryostat. A 100 fs Ti/Sapphire laser centered at 760 nm with a repetition rate of 80 MHz is used for optical excitation. The emitted light is collected in a backscattering geometry and is dispersed by a spectrometer with a spectral resolution of 0.5 nm. TRPL measurements are performed at low excitation densities (qexc * 8 W/cm ) and low temperatures (T B 70 K) since only at these experimental conditions the emission of various NC states are clearly resolved. The time resolution of the cooled S1 streak camera in these experiments is 15 ps. K. Hantke S. Horst (&) S. Chatterjee P. J. Klar K. Volz W. Stolz W. W. Ruhle Faculty of Physics and Material Sciences Center, Philipps-Universitat Marburg, Renthof 5, 35032 Marburg, Germany e-mail: swantje.horst@physik.uni-marburg.de

Influence of nitrogen-cluster states on the gyromagnetic factor of electrons inGaAs1−xNx

The effective gyromagnetic factor of electrons, ${g}_{e}^{*}$, has been determined by Zeeman splitting measurements in a large number of $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}∕\mathrm{Ga}\mathrm{As}$ $(x&lt;0.7%)$ samples. Upon $N$ incorporation, ${g}_{e}^{*}$ shows first a sign reversal with respect to that of GaAs, then increases abruptly for a nitrogen concentration of order of 0.04%, and finally displays a nonmonotonic dependence on composition for higher $x$ values. This behavior is well reproduced by a modified $\mathbf{k}\mathbf{∙}\mathbf{p}$ model taking into account a nonmonotonic loss of $\ensuremath{\Gamma}$ character of the conduction band minimum due to multiple crossings between the redshifting conduction band edge and $N$-cluster states.

Alloy scattering ofn-type carriers inGaNxAs1−x

A tight-binding model of the electronic structure of substitutional nitrogen in GaAs, together with a variational description of quasilocalized nitrogen-induced electronic states near the conduction band edge, is used to calculate the nitrogen-related alloy scattering of conduction band electrons in the dilute nitride alloy, $\mathrm{Ga}{\mathrm{N}}_{x}{\mathrm{As}}_{1\ensuremath{-}x}$. The electron mobility in the nondegenerate and degenerate doping regimes is calculated for bulk and quantum well geometries from the energy-dependent scattering rate using the Boltzmann transport equation in the relaxation-time approximation. Nitrogen cluster states are found to dominate the scattering near the conduction band edge and play a crucial role in limiting the electron mobility. In the experimentally relevant regime of degenerate doping and at nitrogen concentrations of 1 to 2%, the room-temperature mobility is found to be limited to values less than $300\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}{(\mathrm{V}\phantom{\rule{0.2em}{0ex}}\mathrm{s})}^{\ensuremath{-}1}$, in agreement with experimental measurements.

Unification of the Band Anticrossing and Cluster-State Models of Dilute Nitride Semiconductor Alloys

We show that a quantitative description of the conduction band in Ga(In)NAs is obtained by combining the experimentally motivated band anticrossing model with detailed calculations of nitrogen cluster states. The unexpectedly large electron effective mass values observed in many GaNAs samples are due to hybridization between the conduction band edge E- and nitrogen cluster states close to the band edge. Similar effects explain the difficulty in observing the higher-lying E+ level at low N composition. We predict a decrease of effective mass with hydrostatic pressure in many GaNAs samples.

Failure of nitrogen cluster states to emerge into the bandgap of GaAsN with application of pressure

The electronic structure of GaAsN alloys was previously described in terms of nitrogen “cluster states” (CS) that exist in the dilute alloy in the bandgap, and “perturbed host states” (PHS) inside the conduction band. As the nitrogen concentration increases, the PHS plunge down in energy overtaking the CS. We show theoretically that the CS respond to the application of pressure in two different ways: the highly localized deep CS emerge (or remain) in the gap, because their pressure coefficient is lower than that of the conduction band minimum. In contrast, the shallow CS (first to be overtaken) hybridize so strongly with the conduction band that their pressure coefficient becomes comparable to that of the conduction states. These states fail to emerge into the gap upon application of pressure because they move, with application of pressure, at a similar rate with conduction states.