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Studies of ambient-pressure and high-pressure behavior of photoluminescence (PL) for series of InxAl1-xN layers are presented. The measured evolution of PL energy (E-PL) with x is characterized by a clear decrease of E-PL and exhibits a strong bowing. This dependence corresponds to the predictions of ab initio calculations of the band-gap energy changes E-G with x. However, values of E-PL are clearly lower than E-G, for 0

Pressure-dependent transport and photoluminescence (PL) measurements were performed on InN. In the transport measurements on the sample with a low electron concentration (ne lower than about 1018 cm–3) a Localized Donor State (LDS) was seen to cross the Fermi level at elevated pressure magnitude. It was accompanied by the transfer of electrons from the Conduction Band to the LDS and a significant increase in electron mobility. In a sample with a higher concentration of electrons no transport anomaly was observed. Application of pressure helps to confirm that the disputed photoluminescence mechanism in InN consists in band-to-band recombination. No evidence of the participation of this LDS in radiative recombination processes was found in the pressure-dependent PL measurements. On the contrary, the dependence of the PL intensity on excitation power suggested the involvement of the LDS in nonradiative recombination processes. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Reported hydrostatic pressure measurements of Hall electron concentration and Hall mobility of InN suggest the presence of a localized donor state in the conduction band. Since localized-state emission is rather inefficient, the involvement of such a localized state would be highly inadvantageous for device applications. Pressure-dependent photoluminescence (PL) experiments of InN epilayers show PL pressure coefficients dE E /dp of 30.7 +2.0 meV/GPa and 21.4 ± 3.0 meV/GPa for InN epilayers with electron concentration of ∼5 x 10 18 cm -3 and ∼5 x 10 17 cm -3 , respectively. These values are comparable with the InN band gap pressure coefficient dE G /dp of ∼25 -30 meV/GPa (indicating regular band-to-band emission), and differ significantly from the value of 5-10 meV/GPa that should be expected in the case of localized-state emission. Therefore, in the presented experiments no evidence of the involvement of localized donor states in radiative recombination processes in InN is found.

Localization phenomena related to the fluctuating band potential profile of the InGaN alloy were investigated via hydrostatic pressure dependent measurements of luminescence of an InGaN quantum structure. We examine the suitability of the hydrostatic pressure coefficient as a tool for evaluation of the degree of localization of recombining carriers in the InGaN alloy. This is done by experimental investigation of the correlation between the pressure coefficient of the luminescence peak and the Stokes shift between luminescence and the absorption edge, which constitutes a measure of the degree of localization of recombining carriers in the InGaN alloy.

We studied influence of annealing in water vapour and proton exchange on high-pressure low-temperature luminescence spectra of near stoichiometric LN : Cr, MgO crystals, containing both Cr Li and Cr Nb centers by using diamond-anvil cell technique. The observed changes of the spectra of Cr 3+ ions are associated with the larger inhomogeneous broadening and increase of the splitting of the 2 E level of the so called δ Cr Nb center. This shows that neither increase of hydrogen concentration in LN by almost two orders of magnitude (from 10 18 to almost 10 20 cm -3 ) nor proton-exchange procedure do not create any new optically active Cr 3+ centers in those crystals. This does not support the hypothesis about the close compensation of Cr 3+ Nb sites in LiNbO 3 crystals by hydrogen ions.

Electronic band structure calculations of InAlN and InGaN under pressure are presented for two different arrangements of the In atoms, uniform and clustered. The band gap pressure coefficients exhibit strong bowing, and the effect is especially large when indium atoms are clustered. The theoretical results are compared with the results of photoluminescence measurements performed at high hydrostatic pressures on InAlN and InGaN quasi-bulk epilayers. We discuss the modification of the uppermost valence band due to formation of In clusters which, together with the related lattice relaxations, may be a possible source of the observed phenomena.