Shallow Band-tail States Research Articles

Sub-50 picosecond to microsecond carrier transport dynamics in pentacene thin films

Carrier transport dynamics from sub-50 ps to ∼μs over five temporal decades in pentacene films was studied by transient photoconductivity. The behavior of the temperature independent photocurrent peak suggests that the photogenerated carriers exhibit pre-trapping transport in extended states upon pulsed laser excitation. From 300 ps to ∼30 ns, the carriers thermalize and fall into shallow band tail states, and multiple-trapping and release transport dominates. From ∼30 ns to ∼μs, the weak temperature dependence of power-law photocurrent decay and the linear relation of logarithmic photocurrent with T−1/4 suggest a variable range hopping transport in deep trap states.

Effect of localized tail states in InGaN on the efficiency droop in GaN light-emitting diodes with increasing current density

The mechanism of the internal quantum efficiency droop in InGaN/GaN structures with multiple quantum wells at current densities of up to 40 A cm−2 in high-power light-emitting diodes is analyzed. It is shown that there exists a correlation between the efficiency droop and the broadening of the high-energy edge of the emission spectrum with increasing current density. It is also demonstrated that the efficiency is a spectrum-dependent quantity and the emission of higher energy photons starts to decrease at higher current densities. The effect of tunneling and thermally activated mechanisms of thermalization of carriers captured into shallow band-tail states in the energy gap of InGaN on the efficiency and the emission spectrum’s shape is considered. Analysis of the results obtained suggests that the efficiency droop occurs at high current densities because of the relative rise in the contribution from nonradiative recombination via defect states as a result of the increasing occupancy of deep band-tail states in InGaN. It is shown that power efficiency close to the theoretical limit can be obtained in the case of low-voltage tunnel injection into localized band-tail states in the InGaN active region.

Studies of carrier dynamics in unintentionally doped gallium nitride bandtail states

Ultrafast carrier dynamics of bandtail states in an unintentionally doped gallium nitride sample was investigated using femtosecond transient transmission measurements. The transient responses of shallow bandtail states resemble those of above band gap extended states. The transient responses of the deep bandtail states are, on the other hand dominated by carrier transfer into the lower energy states through phonon assisted tunneling suggesting that the deep bandtail states are localized states.

Semiclassical model of electrically detected magnetic resonance in undoped a-Si:H

A simple model for spin-dependent photoconductivity is presented based on rate equations for the density of spin pairs in singlet and triplet configuration. The pairs are formed by electrons localized in the conduction band tail and neutral dangling bonds (e—D°). We take into consideration thermally induced pair dissociation, spontaneous and microwave-induced spin flips and recombination by non-radiative tunneling from band tails into dangling bonds. Our model explains, satisfactorily, changes in line shape and the dependence of the signal intensity on defect density and microwave power. We conclude that tunneling dominates recombination even at 300 K. This recombination predominantly involves shallow band tail states.

Optically detected ESR studies of recombination processes in a-Si:H

A study of recombination processes in high-quality (intrinsic) and light-soaked hydrogenated amorphous silicon (a-Si:H) using the technique of optically detected electron spin resonance (ODESR) is reported. A time-domain detection scheme is employed, instead of the conventional lock-in technique, to record the ODESR lineshapes, which effectively minimizes the well-known interference between the ‘enhancing’ and ‘quenching’ signals. Monitoring the photoluminescence (PL), below 1.1 eV, in the intrinsic sample, a nearly symmetric enhancing signal ( g = 2.008) related to radiative recombination of spin pairs in shallow band-tail states and an asymmetric quenching signal ( g = 2.005) related non-radiative recombination of spin pairs in deep band tail states are observed. A quenching signal at half-field ( g ≌ 4) is also observed. The light-soaked sample exhibits an additional signal ( g = 2.006) which is related to the dangling bonds. The PL excitation energy, E x, is varied from above to below the optical gap to study possible changes in the recombination processes. It is found that the ODESR lineshapes are essentially the same for E x > 1.5 eV and some subtle changes occur for E x < 1.5 eV. The signal intensity, however, depends strongly on the excitation energy for a constant electron-hole pair generation rate.

Steady-state conduction in linear low-density polyethylene with Poole-lowered trap depth

A theory of steady-state conduction of space-charge-limited carriers injected into linear low-density polyethylene with Poole-lowered trap depth is presented. Injected carriers are deeply trapped at crystalline-amorphous boundaries before the steady state is reached. At elevated temperatures, they are thermally excited to the conduction band in the crystalline regions, and then hop with field-independent mobility through the shallow band-tail states of the amorphous regions. A deep-trap-site separation of 2.8 nm, corresponding to 4.5*10/sup 19//cm/sup 3/, yields good agreement with experimental current-field characteristics for fields up to 5*10/sup 5/ V/cm in the temperature range 49 to 82.5 degrees C. The final equation for current I versus field F and temperature T equation shows that the activation energy of the detrapping and transport processes are additive on a semilog plot of I/T/sup 2/ versus 1/T. The total activation energy is 0.83 eV at 2*10/sup 5/ V/cm, in agreement with the results of other methods. >

Initial stages of trapping in a-Si:H observed by femtosecond spectroscopy.

We report the first femtosecond spectroscopic investigation of ultrafast electronic processes in $a$-Si:H. By tuning the probe wavelength in a wide range around the optical gap, we are able to follow the time evolution of carrier trapping in band-tail states. At moderate injected carrier density, the band-tail states are populated by multiple trapping. As more carriers are injected, the shallow band-tail states saturate and deeper band-tail states capture carriers by direct trapping, which is a faster process (1-2 ps) than multiple trapping (10 ps).

Microstripline transient photocurrents in a-Se: Structure resolved in shallow band-tail states

The measurement of transient photocurrents in amorphous semiconductors has been shown in recent years to be a powerful probe of carrier thermalization within the manifold of localized states adjacent to the band edges. Here we discuss recent measurements of transient photocurrents in a-Se made using high-speed stripline techniques in order to observe thermalization within shallow band-tail states. The initial photocurrent decay is power-law like however a sharp change in the slope occurs at 11 nsec at 295K and 22 nsec at 270K. These results provide the first direct evidence for prominent structure superimposed upon the disorder-induced band-tail in a-Se.

Dopant Incorporation and Doping Efficiency in a-Si:H and a-Ge:H

ABSTRACTDoping of hydrogenated amorphous silicon and germanium with boron, phosphorus, and arsenic is investigated. The incorporation of the dopants from the gas phase into the solid film is found to differ strongly for the various dopant-host systems. The doping efficiency is calculated from measurements of the density of dangling bond defects and of shallow band-tail states as a function of the doping level. A common square root dependence of the efficiency on dopant gas concentration is obtained.

Subnanosecond transient photocurrents in a-Si:H: A probe of thermalization within shallow band-tail states

The measurement of transient photocurrents in amorphous semiconductors has been shown to be a powerful way of exploring the thermalization of excess carriers within the manifold of localized states adjacent to the band edges. The extent to which thermalization within the shallowest states can be resolved in controlled by the experimental observation time. Reported here are measurements of transient photocurrents, using high-speed stripline techniques, in glow-discharge a-Si:H carried out in the time regime (0.6 ↔ 100 nsec) and the temperature range 150 ↔ 320K. We find an effective electron drift mobility of < μ D( t)> = 1–3 cm 2/ Vsec at t=0.6 nsec and T=300K and a time-dependent activation energy, ϵ(t), for the photocurrent magnitude. The initial photocurrent decay (t < 100 nsec) can be fit to a power-law, t −n, and the temperature dependence of the dispersion parameter, α=1−n, is given by α= α 0+ βT. In view of these results the nature of the band-tail near the conduction band edge is discussed.

RECOMBINATION OF PHOTOGENERATED CARRIERS IN DOPED HYDROGENATED AMORPHOUS SILICON

Bulk recombination of photogenerated carriers is studied by transient photocurrent measurements in phosphorus doped a-Si:H. With light doping, the monomolecular recombination lifetime exceeds 10 ms and only a slow bimolecular recombination from extended states is observed. For very high initial carrier densities, a fast decay is suggested to be direct recombination from shallow band tail states.