Giant Bandgap Research Articles

Experimental evidence for N-induced strong coupling of host conduction band states inGaNxP1−x: Insight into the dominant mechanism for giant band-gap bowing

Direct evidence for N-induced strong coupling of host conduction band (CB) states in ${\mathrm{GaN}}_{x}{\mathrm{P}}_{1\ensuremath{-}x}$ is provided by photoluminescence excitation. It is manifested as: (1) a drastic change in the ratio of oscillator strengths between the optical transitions involving the CB minimum (CBM) and the high-lying \ensuremath{\Gamma} CB state; (2) a strong blueshift of the \ensuremath{\Gamma} CB state with increasing $x$ accompanying a redshift of the CBM, (3) pinning of the localized N states and a newly emerging ${t}_{2}$ $(L$ or ${X}_{3})$ CB state. These findings shed new light on the issue of the dominant mechanism responsible for the giant band-gap bowing of dilute nitrides.

Evolution of electronic states inGaAs1−xNxprobed by resonant Raman spectroscopy

Two distinct maxima ${E}_{W}$ and ${E}_{W}^{\ensuremath{'}}$ are observed in the resonant Raman-scattering profile for the LO phonon asymmetric linewidth broadening in ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ and are attributed to states arising from a splitting of the quadruply degenerate conduction band near the L point. The data provide further insight into the physics underlying the giant band-gap bowing observed in ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x},$ as well as reveal asymmetric linewidth broadening to be a powerful signature for studying strongly localized impurity states in semiconductors.

Hydrogen as a probe of the electronic properties of (InGa)(AsN)/GaAs heterostructures

We report on the effects of N incorporation on the electronic properties of (InGa)(AsN)/GaAs heterostructures as investigated by photoluminescence (PL) spectroscopy. PL under a magnetic field shows an increase in the electron effective mass in the N-containing material. In order to address this effect as well as the giant band gap reduction induced by N in (InGa)As, we exploit the ability of hydrogen to passivate the electronic activity of N in (InGa)(AsN). Such passivation is due to the formation of N–H bonds and manifests itself as: (i) a quenching of the exciton recombination in N-related complexes in the N dilute limit; (ii) a blueshift of the (InGa)(AsN) band gap toward that of the N-free material in the alloy limit. A thermal annealing leads to a complete recovery of the electronic properties (InGa)(AsN) had before H irradiation in both limits.The activation energy for the thermal dissociation, E D, of the N–H complexes follows a Gaussian distribution with a mean value increasing with the N concentration, y. Values of E D similar to those found in the alloy limit are found in the case of very dilute N concentrations (impurity limit), where different N–H complexes are singled out. These results show that different N complexes are responsible for the puzzling effects exerted by N on the electronic properties of (InGa)(AsN).

Role of N clusters inInxGa1−xAs1−yNyband-gap reduction

We study the origin of the giant band-gap bowing observed in ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{As}}_{1\ensuremath{-}y}{\mathrm{N}}_{y}$ alloys upon nitrogen incorporation by exploiting the capability of atomic hydrogen to form bonds with nitrogen atoms. The formation of these bonds pushes the band gap of ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{As}}_{1\ensuremath{-}y}{\mathrm{N}}_{y}$ alloys toward that of ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As},$ while subsequent thermal annealing restores the band gap of the untreated ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{As}}_{1\ensuremath{-}y}{\mathrm{N}}_{y}.$ The activation energy for thermal dissociation, ${E}_{D},$ of the N-H complexes follows a Gaussian distribution with a mean value increasing with y. Values of ${E}_{D}$ similar to those found in the alloy limit are found in the case of very dilute N concentrations (impurity limit), where different N-H complexes are singled out. These results show that the giant band-gap bowing should be accounted for by different N complexes rather than by a single N complex.

Dilute nitride based III–V alloys for laser and solar cell applications

The dilute nitrogen alloy GaAs 1− x N x is of recent technological importance for high efficiency solar cells and vertical cavity surface emitting diode lasers used in fiber optic communications. The giant band gap lowering and the abnormal effective mass phenomena observed in this material have been extensively researched. There exist many inconsistencies between the results of various theoretical models and experimental techniques used to probe the above phenomena. It appears that these arise because GaAs 1− x N x should be viewed not as an abnormal alloy but rather as a heavy isoelectronically doped semiconductor.

Overcoming limitations in semiconductor alloy design

The phenomenon of giant band gap ‘bowing’ recently observed in several III–V dilute nitride alloys is promising for increasing the flexibility in choice of semiconductor band gaps available for specified lattice constants. However, the poor electrical transport properties that these materials exhibit seriously limit their usefulness. It is proposed that these materials behave as heavily nitrogen-doped semiconductors rather than dilute nitride alloys and that the abnormal or irregular alloy behavior is associated with impurity band formation that manifests itself in the giant bowing and poor transport properties. The potential for regularizing the alloy behavior using isoelectronic co-doping is discussed.

Electronic Properties of Ga(In)NAs Alloys

A brief review on the present knowledge of the electronic properties of the Ga(In)NAs ternary and quaternary alloys is given mainly from an experimental perspective. The discussion is focused on Ga(In)NAs with low N composition (< 10 %), where a large amount of experimental work has been done. Important fundamental electronic properties of the material system are analyzed with the emphasis on the nature of the giant band gap bowing in the alloy and nitrogen-induced modifications of the electronic structure of the conduction band. The current knowledge of the key material parameters, relevant for the device applications, such as electron effective mass, recombination processes and band alignment in Ga(In)NAs/GaAs heterostructures, is also reviewed.