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Shubnikov-de Haas and persistent-photoconductivity measurements are used to study the mobility, the free-electron density (n) y and the occupancy of the DX center in heavily doped «-type GaAs [Si,Sn] as a function of doping level and hydrostatic pressure. The results show that the DX center produces a resonant donor level between the T and L conduction-band minima at a concentration comparable with the doping level. For the Si-doped samples, comparison with local vibration-mode measurements indicates that the DX level can be identified with SiGa-

In this paper we present the first direct and unambiguous evidence that the total number of electrons which can be trapped on GQ-DX centers is exactly twice as large as the number of electrons which can be bound on Gc-A \ donor states. It should be emphasized that our reasoning does not require any technological information, about either doping or the compensation of a sample.PACS numbers: 71.55. Eq, 61.72.Vv It is well known that many substitutional donors in semiconducting compounds, beside normal hydrogenlike donor states, can also form highly localized electronic states whose energy relative to the T conduction band minimum is highly pressure dependent. For some of these states there exist potential barriers for both electron emission and electron capture processes, leading to nonequilibrium occupation of these states by electrons at low temperatures-i.e., so-called metastable effects. These are called DX states. There are also other localized states with no apparent metastable effects present-see, e.g., the results of anitcrossing between a shallow donor state and a highly localized one, both originating from the same impurity, observed for w-InSb [l] or GaAs:Ge [2]. For simplicity, following some authors, such highly localized states will throughout this paper be called "A\ states," just to distinguish them from DAMike ones-no matter whether the name is justified or not. There are some rare but very interesting cases for which both types of highly localized states (originating from the same impurity or defect) coexist and can both be populated by electrons in the same experiment. Such a population can be induced, e.g., by hydrostatic pressure which shifts the appropriate electronic levels down with respect to the conduction band minimum and the Fermi level. However, at low temperatures (i.e., well below a certain temperature T c ) due to potential barriers for emission and capture of electrons the population of DX-likc states is nonequilibrium (frozen) and the pressure shift of the corresponding energy level does not influence its occupation. At T<^T C its population can be changed only in a persistent way by means of illumination. While the pressure shift of the corresponding energy level can change the population of ZXY-like states only at T> T c , for A\ states there is no such restriction. Accordingly, in such materials as nominally undoped /2-InSb [3], GaSb:S [4], or GaAs:Ge [5], in which both ZXY-like and A \ states are present, hydrostatic presure measurements of the Hall coefficient reveal that at low temperatures the pressureinduced capture of free electrons is governed by a level whose energy position as well as pressure shift differ from those observed for temperatures high enough to populate DX states.One of the most exciting problems related to DX centers was the charge state of the defect occupied with electrons. Since the first suggestions of Khachaturyan, Weber, and Kamiriska [6] and Chadi and Chang [7] that DX is a negatively charged donor (i.e., occupied with two electrons) subjected t...

We report evidence of the transition from n- to p-type conduction of InN with increasing Mg dopant concentration by using photoconductivity (PC) measurement at room temperature. This transition is depicted as a conversion from negative to positive PC under above-bandgap optical excitation. The n- to p-type transition in InN:Mg is further confirmed by thermopower measurements. PC detection method is a bulk effect since the optical absorption of the surface electron accumulation is negligibly low due to its rather small thickness, and thus shows advantage to detect p-type conduction. This technique is certainly helpful to study p-type doping of InN, which is still a subject of discussions.

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We measured lateral ac transport (up to 20 MHz), thermopower, as well as resistivity and Hall effect in InN:Mg samples with various Mg content. The sign of the Hall effect for all the samples was negative (electrons), however, the thermopower (α) measurements have shown the p-type sign of α for moderate Mg content—in the window centered around 1×1019 cm−3. Further overdoping with Mg yields donor type of defects and the change of thermoelectric power sign. The ac measurements performed as a function of frequency revealed that in both samples exhibiting and nonexhibiting p-type sign of thermopower, the n-type inversion layer at the surface does not prevent the electric contact to the bulk layer. Therefore we conclude that the n-type Hall effect invariably reported for all the Mg-doped samples originates from electron domination in mobility-weighted contributions of both types of carriers.

The pressure coefficient of resistance of manganin sensors was determined from isothermal pressure dependences measured at various temperatures in the range 77-352 K in fully hydrostatic conditions using a He-gas pressure system. The pressure coefficient decreases with decreasing temperature, reaches a minimum at about 110 K with a deviation of 5.0% from its room temperature value and then increases. Both below and above K the temperature dependence of can be approximately treated as linear with the rate equal to and , respectively.

The pressure dependence of the electronic band structure of InN is investigated with emphasis on the conduction band effective mass and its dependence on free-electron concentration. Transport measurements are performed under hydrostatic pressure on three n-type samples of InN with different electron concentrations. The effective mass extracted from the electron mobility is found to increase with pressure, however with a pressure coefficient, which is lower for the samples with higher electron concentration. Calculations of the InN band structure are performed within the density functional theory by means of the linear muffin-tin-orbital method, including an empirical adjustment to reproduce known band edges at ambient pressure. The calculated variations of the highest occupied conduction band energy and the electron effective mass with free-electron concentration are compared to available experimental information. The results show a pronounced deviation from parabolic behavior of the lowest conduction band, and for k&gt;0 this induces large differences between the values of the optical and curvature masses. Both the fundamental band gap and the electron effective mass increase with pressure, but due to the nonparabolic character of the conduction band, the pressure coefficient of the effective mass decreases with electron concentration, in agreement with the experimental observation.