Recombination of minority carriers in heavily doped n-InP wafers has been investigated using spectral and time-resolved photoluminescence at different temperatures. Studies of the transmitted luminescence were enabled by the partial transparency of the samples due to the Moss-Burstein effect. Temporal evolution of the transmitted luminescence shows virtually no effect of surface recombination but is strongly influenced by photon recycling. Temperature dependence of the decay time suggests Auger recombination as the dominant non-radiative process at room temperature. Radiative quantum efficiency has been evaluated at different doping levels and at 2×10 18 cm -3 it is found to be as high as 97%, which makes n-InP suitable for scintillator application.

We study the photoluminescence spectra of n-doped InP bulk wafers, both in the reflection and the transmission geometries relative to the excitation beam. From the observed spectra we estimate the spatial distribution of minority carriers allowing for the spectral filtering due to re-absorption of luminescence in the wafer. This distribution unambiguously demonstrates a non-exponential drop-off with distance from the excitation region. Such a behavior evidences an anomalous photonassisted transport of minority carriers enhanced owing to the high quantum efficiency of emission. It is shown that the transport conforms very well to the so-called Lévy-flights process corresponding to a peculiar random walk that does not reduce to diffusion. The index γ of the Lévy flights distribution is found to be in the range γ = 0.64 to 0.79, depending on the doping. Thus, we propose the highefficiency direct-gap semiconductors as a remarkable laboratory system for studying the anomalous transport.

The shape of the photoluminescence line registered from a side edge of InP wafer is studied as function of the distance from the excitation spot. The observed red shift in the luminescence maximum is well described by radiation filtering and is consistent with the absorption spectra. Our method provides an independent and accurate determination of the Urbach tails in moderately doped semiconductors.Studies of the optical absorption spectra near the interband absorption edge are widely used for characterization of semiconductor materials. [1][2][3] The spectral dependence at low-energy absorption edge is well approximated by the Urbach exponential decay. 4 However, the interband absorption usually overlaps with residual absorption by free carriers, masking the true dependence of the absorption tail. In doped samples, the absorption tail is additionally broadened and this effect is also masked by the residual electronic absorption.Available theories 5-8 do not provide description of the interband absorption tailing in the entire energy range from interband to deep tails. Still, they give an insight on the nature of the bandgap fluctuations causing the tailing. Tailing with Gaussian-like asymptotics is characteristic of classical potential fluctuations, 5 whereas exponential decrease with square-root energy dependence in the exponent is indicative of quantum effects in the band tailing. 6,7 Temperature variations of the tailing are accounted for by the adiabatic potential of thermally excited phonons. 8 In a limited experimental range near the fundamental edge the observed absorption spectra may not differ noticeably from the Urbach law, 7 but the temperature and the concentration dependence of the tailing parameters can be very informative. Therefore, accurate studies of the tailing dependence are highly desirable.In this letter we describe an alternative experimental method for studying the semiconductor absorption edge by measuring the red shift of the peak of the luminescence line, registered from the side edge of the wafer. This shift is sensitive to the sample transparency at the peak wavelength, which is in the region where residual absorption dominates. We show that for moderately doped n-InP wafers this technique provides an accurate determination of the Urbach tailing energy.We used 350 µm-thick InP wafers, doped n-type (S) in the range n = 2 × 10 17 to 6 × 10 18 cm −3 and measured the reflection and the transmission spectra to evaluate the absorption coefficient α. 9 For all moderately doped samples, n ≤ 2 × 10 18 cm −3 , in the temperature range 78 to 320 K, the absorption edge exhibits an Urbach-type energy dependence in the range α = 10 to 100 cm −1 . For lower doped samples, n < 10 18 cm −3 , the observed a) Electronic