Field-dependent charge carrier dynamics in GaN: Excitonic effects

Abstract

The electric-field dependence of the charge-carrier dynamics in GaN was studied by measuring excitation spectra of the sub-band-gap (yellow) luminescence as a function of bias using a Schottky junction formed at the interface between the semiconductor and an electrolyte solution. At large bias, the contribution of free electrons and holes to the photoluminescence is significantly reduced due to the dead-layer effect. As a result, striking features are revealed in the spectra close to the fundamental absorption. These features are attributed to exciton decay via yellow luminescence Electrons and holes, generated by light within the depletion layer of a Schottky diode, are effectively separated by the electric field and the minority carriers are collected at the interface. Minority carriers created within a diffusion length of the inner edge of the depletion layer can also reach the junction; 1 if they are transferred across the interface one observes photocurrent in the external circuit. Only those carriers created by light with a penetration depth ͑1/␣͒ which is larger than the sum of the minority carrier diffusion length L and the thickness of the space charge layer W will recombine, either nonradiatively or radiatively. The intensity of the emitted light I PL is given by:where 0 is the absorbed photon flux and is the ratio of the rate of radiative recombination to the total recombination rate. Many systems have been shown to conform to the "dead-layer model." 2-4Among the III-V semiconductors GaN is rather exceptional in that its exciton binding energy is large ͑ജ25 meV͒. [5][6][7][8] The formation of strongly bound electronhole pairs might be expected to influence the charge-carrier dynamics within the space-charge layer of an illuminated Schottky diode. In the present work we have used photoluminescence and photocurrent measurements on the Schottky junction formed at a semiconductor/solution interface to study excitonic effects in the field-dependent dynamics of carriers in n-type GaN epitaxial layers. We report an anomalous peak in the onset of the excitation spectrum of the subband-gap luminescence indicating enhanced emission at large band bending. We argue that this effect is due to a contribution from exciton absorption which is observed because of two reasons: the large binding energy of excitons and the suppression of emission from free carriers as a result of the dead-layer effect.The GaN epitaxial layers were grown by MOCVD on sapphire substrates using a 30 nm AlN buffer layer to reduce stress. 9 The layers were co-doped with Si. The density of uncompensated donors was 6 ϫ 10 17 cm −3 . Ohmic contacts were deposited by microwave sputtering of Ti/ Al/ Ni/ Au ͑15/ 300/ 40/ 200 nm͒. The ͑0001͒ Ga-terminated surface was exposed to solution.9 A standard three electrode cell was used for the electrochemical experiments. All potentials are reported with respect to the standard calomel electrode. Reproducible clean electrode surfaces were obtained by dipping in HCl before each experiment. The electrolyte fo...

Electric-field-induced band bending on GaN: in situ effects of electron beam irradiation on time-dependent cathodoluminescence

“…Free charges within the sample could be enhancing recombination efficiency of electron hole pairs generated by the e-beam. This is consistent with Lagemaat's findings [43], where enhanced photoemission was seen upon electron hole pair generation in regions beyond the contact diffusion length [1,43]. In effect, nominal leakage current leads to a narrow DW [ Fig.…”

Abnormal electronic structure of chemically modified n-InP(100) surfaces

“…49 A simple dead-layer model was used to analyze the dependence of PL intensity on the thickness of the depletion layer width varied by the bias applied to Schottky and metal-insulatorsemiconductor (MIS) diodes, 48 as well as to semiconductor/ electrolyte interfaces. 50,51 In accordance with this model, 48,49 the PL intensity of the semiconductor should be proportional to exp(Àa  d), whereas the larger surface band bending should cause PL quenching. This can explain qualitatively the results shown in Fig.…”

“…10-fold increase in the PL intensity (Table 1) in the framework of the dead-layer model, the space charge layer should be reduced by more than 100 nm after passivation, which is not possible in the present case. Therefore, the simple dead-layer model [48][49][50][51] cannot explain the increase in PL intensity obtained after passivation (Fig. 1 and Table 1).…”

Defect Luminescence at n-GaN Electrodes

“…Very often, a broad sub-bandgap emission band centered at around 2.2 eV, the so-called yellow luminescence band, is observed. On yellow luminescence in aqueous solutions, only few reports have been published, [1][2][3] despite the fact that performing luminescence experiments in aqueous solutions as a function of the electrode potential may yield useful information on the relative contribution of bulk and surface properties in the consumption of minority carriers.…”