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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 perform the investigations of the resonant tunneling via impurities embedded in the AlAs barrier of a single GaAs/AlGaAs heterostructure. In the I(V) characteristics measured at 30 mK, the contribution of individual donors is resolved and the fingerprints of phonon assistance in the tunneling process are seen. The latter is confirmed by detailed analysis of the tunneling rates and the modeling of the resonant tunneling contribution to the current. Moreover, fluctuations of the local structure of the DOS ͑LDOS͒ and Fermi edge singularities are observed.

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 effect of hydrostatic pressure on the paramagnetic -ferromagnetic phase transition has been studied in (Ga,Mn)As. The variation of the Curie temperature (T C ) with pressure was monitored by two transport methods: (1) -measurement of zero field resistivity versus temperature ρ(T), (2) -dependence on temperature of the Hall voltage hysteresis loop. Two specimens of different resistivity characteristics were examined. The measured pressure-induced changes of T C were relatively small (of the order of 1K/GPa) for both samples, however they were opposite for the two.(Ga,Mn)As is one of the most intensively investigated diluted magnetic semiconductors during last decades. The understanding of physical phenomena governing its magnetic properties is crucial for increasing Curie temperature (T C ) and thus for possible application of this material in spintronic devices. The origin of ferromagnetism in (Ga,Mn)As was quantitatively explained within the p-d Zener model assuming magnetic interaction between the localized magnetic moments of Mn 2+ ions mediated by holes in the valence band [1][2][3] . This model, in the case of semiconductors, where the carrier density is smaller than the magnetic ion concentration is equivalent to the Ruderman-Kittel-Kasuya-Yosida (RKKY) approach employed in the diluted magnetic metals 4 . Within this picture the ferromagnetic ordering temperature, T C depends in particular on the local p-d exchange interaction and a free hole concentration. It was demonstrated that indeed an increase of the hole concentration in a field effect transistor structure led to an enhancement of the ferromagnetic state 5. On the other hand it was found that the exchange energy scales with the lattice constant as, N 0 β ~ a 0 -3 , 1 and therefore an external hydrostatic pressure could influence the exchange coupling. Although the studies of (In,Mn)Sb diluted magnetic semiconductor under hydrostatic pressure provided an evidence for an increase in carrier-mediated magnetic coupling 6,7 , giving rise to higher Curie temperature, the effect of hydrostatic pressure on (Ga,Mn)As semiconductor is not as clear.6 Therefore additional study was performed in order to clarify the role of external hydrostatic pressure in the paramagneticferromagnetic phase transition in (Ga,Mn)As. p-type Ga 1-x Mn x As layers were grown by molecular beam epitaxy (MBE) on (100) GaAs substrate. In our studies two different samples were used: A777 and A963. The former sample had 20 nm thick layer of (Ga,Mn)As with Mn content x = 7%. After the MBE growth this sample was capped with amorphous As and annealed in the MBE growth chamber at the temperature of 210 ºC (controlled by the IR pyrometer) for two hours (see Ref. 8 for details). The Curie temperature determined from SQUID magnetometry was close to 85 K (Fig. 1, open symbols). The latter sample had a (Ga,Mn)As layer of 50 nm and x = 6%. This sample was not annealed after the MBE growth. The Curie temperature for A963 sample was approximately 50 K (Fig. 1, solid symbols). Since the amount o...

We present the results of electrical-conductivity and low-temperature deep-level transient spectroscopy (DLTS) measurements performed with use of monochromatic light at hydrostatic pressures up to 0.7 GPa on n-type GaAs samples containing the EL2 defect. Based on our results we give clear experimental evidence that there exists an acceptorlike level of the metastable EL2 configuration. Without pressure this level is resonant with the conduction band and therefore unoccupied, but under pressure it enters the energy gap capturing free electrons and leading to the negative charge state of the metastable EL2. The electrical activity of the level manifests itself in optically induced persistent changes of the electrical conductivity and DLTS signal, driven by EL2 photoquenching and extremely efficient EL2 photorecovery processes. These eff'ects vanish at the temperature at which EL2 thermally recovers its normal configuration. We exclude the "negative-U" case for the level and determine its energetic position and the pressure shift. We claim that the negative charge state of the metastable EL2 is responsible for the optical accessibility of the metastable configuration. Finally, we refer the experimentally confirmed existence of this level to the predictions of the EL2 theoretical models.

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