Injected carrier density

FIGURE 1 The maximum gain of GaN and GaAs as a function of injected carrier density. 

A more direct route to making polymer lasers would be to excite the polymers electrically in a diode structure. Although polymer LEDs have been made in which the injected carrier density matches the threshold excitation density of the best photopumped polymer lasers, a polymer diode laser has not yet been made. The threshold for diode lasers is higher because the electrodes that must be incorporated to the structure increase the waveguiding loss. In addition, for some... 

The TE mode optical gain per dilute-N "W" and "M" period were calculated for injected carrier densities N2D (><10 cm ) in the active region. As an example, we report in Fig. 2 the optical gain calculated for the "M" system at RT. 

This confinement yields a higher carrier density of elections and holes in the active layer and fast ladiative lecombination. Thus LEDs used in switching apphcations tend to possess thin DH active layers. The increased carrier density also may result in more efficient recombination because many nonradiative processes tend to saturate. The increased carrier confinement and injection efficiency faciUtated by heterojunctions yields increasing internal quantum efficiencies for SH and DH active layers. Similar to a SH, the DH also faciUtates the employment of a window layer to minimise absorption. In a stmcture grown on an absorbing substrate, the lower transparent window layer may be made thick (>100 /tm), and the absorbing substrate subsequendy removed to yield a transparent substrate device. 

If the dephasing time of the coherent phonons depend critically on the carrier density, photo-injection of carriers with the second pump pulse can annihilate them partially or completely, depending on its fluence but not on its relative timing. Such incoherent control was demonstrated for the LO phonons of GaAs [37],... 

TSDC experiments are customarily analyzed assuming the sample behaves ohmic, i.e., the contacts do not introduce an inhomogeneous distfibution of the electric field or carrier density and a uniform bulk density of carriers extend through the entire sample. Experiments were carried out in such a way as to minimize injection effects. Contact configuration was typical for TSDC experiments. Because the currents through the sample are, in almost aU cases, extremely small, we have used a sensitive DC ammeter (model Ul-15, detection limit <10 A) with a hnear output signal. The simplest way to obtain a record of TSDC is an X-Y recorder that displays I(T) and the temperature. The equipment for the extraction of trap-spectroscopic information may be connected with devices for electronic data processing. The experimental errors in determination are less than 2%. 

Injection-type laser diodes based on GalnN active layers have now been demonstrated by a number of different groups [2,25-30], The threshold current densities range from 1.5 kA/cm2 [31] to about 50 kA/cm2 [26], From measurements of the tum-on delay time Nakamura et al [32] have also estimated the threshold carrier density to be about 1 - 2 x 1019 cm 3. 

Both injection-type and optically pumped nitride-based semiconductor laser structures exhibit fairly high threshold pump levels compared to other III-V or II-VI semiconductors. This is fundamentally due to the specific band structure of the nitrides, i.e. the extremely large effective masses of both electrons and holes. The carrier densities needed to achieve transparency are of the order 2 x 1019 cm 3. 

The other problem of current injection is electron overflow [29], The carrier densities shown in FIGURE 4 were simulated for an A1 composition of 0.1 when the bandgap difference between the InGaN well and... 

The charge carrier density decreases with increasing distance from the injection contact according to... 

At very low voltage the injected effective carrier density may be lower than the background thermal carrier density and the current is then ohmic. The voltage at which SCLC begins to dominate is given by,... 

Several authors have observed that log J versus log V plots are straight lines. This result is consistent with the conventional power law (3.42). It is not clear how this result is modified for non-zero Schottky barriers. The J-V characteristics in our theory depend on the value of the constant of integration C. The Schottky barriers 4>b define P 0), and C depends on P 0), where P(0) is the injected charge carrier density at the contact. 

Fig. 3.14. Carrier density as a function of distance x from the injecting contact for different values of P(0). The values of the other parameters used in this calculation are the same as given in Fig. 3.11. Note that for small value of P(0), P(0) = 1016 cm-3 (

---