The dark current–voltage characteristics of GaAs(Cr) samples of n‐type are found to be linear at 300 °K up to fields of ≈︁ 104 V/cm; the corresponding characteristic of photocurrent in the main maximum for relatively small photoconductivity is N‐shaped, being stable in all parts. This shape of the characteristics is explained by a field dependence of the recombination rate under photoexcitation due to recharging of localized levels with participance of minority charge carriers (holes). The shape of the main maximum of photocurrent is essentially affected by the Franz‐Keldysh effect. A comparison is made of the effects of the uniform field in the sample and of the field applied to the surface.
The theory of the Auger processes of capture of charge carriers with the energy transfer to the bound charge carriers is extended to the case of a semiconductor with different valleys of the C‐band. Using hydrogen‐like Ψ‐functions for the charge carriers in the impurity centres separated with distances higher than the sum of the effective radii of the interacting states, expressions are obtained describing the dependences of the cross‐sections on the characteristic parameters for the four types of Auger processes: hole capture by the “donor—donor” complex, and the capture of the electron from the upper valley by the complexes “donor—donor”, “donor—acceptor”, “acceptor—acceptor”. It is shown that in many cases the cross‐section of the process examined can surpass the effective size of the bound electron state and reach a value ≈ 10−12 cm2. On the basis of the data obtained an interpretation is given of a number of experimental facts connecting the effects of capture and ionization in GaAs doped with deep‐level impurities (O, Fe, Cr) and InP:Fe.
A model is developed for an isolated Cr impurity in GaAs assuming the presence of only one acceptor level at (Ec – 0.6 eV) which shifts down to ≈ (Ec – 0.8 eV) when an ionized donor is neighboured. An effect of longwave excitation of photocurrent is observed (77 to 150 °K), connected with the accumulation of electrons by the donors mentioned and the „transformation”︁ of the level (Ec – 0.8 eV) into the level (Ec – 0.6 eV) with much smaller capture cross‐section for electrons. The energy position of the donor is found to be (Ec – 0.35 eV) in case of a neutral acceptor and (Ec – 0.17 eV) when the acceptor is ionized. In the latter case the capture cross‐section of electrons for the donor is ≈ 10−19 cm2. Considering the mutual influence of the donors and the acceptors permits the prediction of a new scheme of the origin of the negative differential conductance; an increase of the recombination rate with the electric field because of the ionization of donors and the corresponding variation of the total electron capture cross‐section of the acceptors. This point of view is confirmed by the experiment.
Energy spectrum and capture cross sections of local electronic states have been investigated in half-insulating (oxygen-doped) GaAs. The resulting spectrum of energy levels gave ns a possibility t o offer a general scheme of arising of negative differential conductance (DNC) for recombination non-linearity ; this scheme includes in particular two mechanisms discussed earlier [I, 21. It was shown that in the case investigated the DNC appears as a result of recharging of impurity levels under the action of electric field. To study the effect of a uniformly distributed electric field upon semiconductor, high frequency field was employed; with the help of it we have obtained a falling region on volt-ampere characteristic. If constant electric field is applied to the sample, a domain instability can be observed under conditions when mean field corresponds to the ohmic part of I -U characteristic of a uniform sample; the smallest field corresponding to instability could vary within wide limits, depending on the situation a t the cathode. Conceptions are developed about creation of a high-field space charge region next to the cathode, where field reaches the value sufficient for displaying of DNC mechanism. Some information is obtained on the initial phase of creation of such a region under different conditions.
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