Use of hydrostatic pressure in determining the nature of DX centers in GaAiAs-Si

Abstract

We report on activated capture and emission experiments obtained under hydrostatic pressure. The observed structures in the thermostimulated capture curves are interpreted as a competition between capture and emission from the different configurations of the DX center, which are related to the local environment of the Si atom. The analysis of the capture kinetic experiments supports the hypothesis of the negative charge state of the DX center.     

DX centers in selectively Si-doped GaAs---AlAs superlattices

DX center has been characterized in four GaAs---AlAs superlattices grown by MBE at 580°C. The structures are uniformly Si-doped or selectively Si-doped in the AlAs layers or in the middle of the GaAs layers or on both sides of the interfaces. Deep level transient spectroscopy measurements (DLTS) put in evidence one dominant electron trap, with an activation energy for thermal emission of about 0.42eV for all the superlattices. This defect shows a thermally activated capture cross section and a large concentration except for the case where the only GaAs layers are doped. For the first time, a study of the capture reveals a capture activation energy of 0.36-0.37 eV, which allows us to locate the DX level nearly 60 meV below the conduction miniband. From these results, we show that the observed DX center is related to silicon in the AlAs layers. For the case when the AlAs barriers are not doped, the DX level is due to the Si diffusion from the middle of the wells towards the barriers, the Si atoms having diffused during the growth.     

Intrinsic DX centers in ternary chalcopyrite semiconductors

In III-V and II-VI semiconductors, certain nominally electron-donating impurities do not release electrons but instead form deep electron-traps known as "DX centers." While in these compounds, such traps occur only after the introduction of foreign impurity atoms, we find from first-principles calculations that in ternary I-III-VI2 chalcopyrites like CuInSe2 and CuGaSe2, DX-like centers can develop without the presence of any extrinsic impurities. These intrinsic DX centers are suggested as a cause of the difficulties to maintain high efficiencies in CuInSe2-based thin-film solar-cells when the band gap is increased by addition of Ga.     

Electrical properties of DX centers in GaAs and AlGaAs

Abstract

The DX center, the lowest energy state of the donor in AIGaAs with x < 0.22, is responsible for the reduced conductivity as well as the persistent photoconductivity observed in this material at low temperature. Extensive studies of the properties of this deep level in Si-doped AIGaAs are reviewed here. Data are presented showing that the characteristics of the DX center remain essentially unchanged when it is resonant with the conduction band (x < 0.22) and that, independent of other compensation mechanisms, the DX center therefore limits the free carrier concentration in Si-doped GaAs to a maximum of about 2 × 10¹⁹ cm^?3. Recent measurements suggesting that the lattice relaxation involves the motion of the Si atom from the substitutional site toward an interstitial site are also presented. Evidence for the negative U model, that the DX level is the two electron state of the substitutional donor, is discussed.     

Si DX centers in GaAs at large hydrostatic pressures

A number of experimental and theoretical studies indicate that DX centers in GaAs, its alloys and other III–V semiconductors have negative U properties. Using far infrared localized vibrational mode (LVM) spectroscopy of Si donors in GaAs under large hydrostatic pressure in a diamond anvil cell we have discovered an LVM of the Si DX center. From the ratio of the LVM absorption lines of Si_Ga and Si_DX and the compensation in our GaAs samples, we show unambiguously that two electrons are trapped when the ionized shallow Si donors transform into negatively charged DX centers, in full agreement with the negative U model.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Donor impurities and DX centers in the ionic semiconductor CdF2

Group-III impurities in the wide-gap ionic crystal CdF₂ are examined. After being heated in a reducing atmosphere, crystals with these impurities acquire semiconductor properties, which are determined by electrons bound in hydrogen-like orbitals near an impurity. Besides these donor states, nontransition impurities form “deep” states accompanied by strong lattice relaxation, i.e. they are strongly shifted along the configuration coordinate. These states are a complete analog of DX centers in covalent and ionic-covalent semiconductors. The difference of the behavior of nontransition impurities from that of transition and rare-earth impurities is analyzed. This difference is attributed to the character of the filling of their valence shells by electrons. A deep, multilevel analogy is drawn between the properties of deep centers in typical semiconductors with an appreciable fraction of a covalent bond component and in predominantly ionic crystal CdF₂ with semiconductor properties. Fiz. Tverd. Tela (St. Petersburg) 39, 1050–1055 (June 1997)     

Electron statistics in CdF2 semiconductor crystals with DX centers

The degree of compensation and ionization energy of two-electron DX centers in CdF₂: In and CdF₂: Ga semiconductors are determined by studying the statistical distribution of electrons localized on impurity levels. The sharp temperature dependence of the concentration of neutral donors observed in CdF₂: Ga over the temperature range T = 250–400 K is explained by a high compensation degree, K ≥ 0.996. Thus, all Ga ions introduced into a CdF₂ crystal lattice during crystal growth form shallow donor levels. However, the concentration of Ga ions that can form bistable DX centers is rather low and is close to the concentration of electrons injected into the crystal during additive coloring. In CdF₂: In crystals, the degree of compensation is smaller than that in CdF₂: Ga but is sufficiently high and the number of bistable DX centers is not limited. It is concluded that an extremely narrow impurity band forms in the CdF₂: Ga semiconductor. For a total charged-impurity concentration of ～10²⁰ cm^?3, the width of the impurity band in CdF₂: Ga is not likely to exceed ～0.02 eV.     

Photoluminescence from deep centers in GaAs

Sharp line structure attributable to phonon assisted radiative emission has been observed in the 6 K photoluminescence spectra from deep centers in bulk samples of chromium doped GaAs. Two luminescence bands at 0.56 and 0.8 eV have been observed and both bands exhibit evidence of phonon assisted radiative recombination. An exploration of these luminescence bands in terms of excited state to ground state transitions of Cr³⁺ and Cr²⁺ ions is proposed.     

Boron-related deep centers in 6H-SiC

6H-silicon carbide layers are grown by a liquid phase epitaxy (LPE) process. The layers are doped with boron either by ion implantation or during the LPE process from a B-doped silicon melt. Deep-level transient spectroscopy (DLTS), admittance spectroscopy and photoluminescence (PL) are used to investigate deep impurity centers. Two electrically active defect centers are detected: the isolated boron acceptor at E _B=E _v+0.3eV and the boron-related D-center at E _D=E _v+0.58eV. The yellow luminescence observed in these layers is proposed to be due to pair recombination via D-center and nitrogen donor. Formation and origin of the D-center are discussed.     

AlGaAs∶Sn中DX中心电子俘获势垒的精细结构

采用定电容电压法,测量了n型Al026Ga074As∶Sn中DX中心电子热俘获瞬态,以及不同俘获时间后的电子热发射瞬态;并对瞬态数据进行数值Laplace变换,得到其Laplace缺陷谱(LDS).通过分析LDS谱,确定了电子热俘获和热发射LDS谱之间的对应关系,从而得到热俘获系数对温度依赖关系,以及与Sn相关的DX中心部分电子热俘获势垒的精细结构;通过第一原理赝势法计算表明,Sn附近的AlGa原子的不同配置是电子热俘获势垒精细结构产生的主要原因     

AlGaAs:Sn中DX中心电子俘获势垒的精细结构

采用定电容电压法,测量了n型Al0.26Ga0.74As:Sn中DX中心电子热俘获瞬态,以及不同俘获时间后的电子热发射瞬态;并对瞬态数据进行数值Laplace变换,得到其Laplace缺陷谱（LDS）。通过分析LDS谱,确定了电子热俘获和热发射LDS谱之间的对应关系,从而得到热俘获系数对温度依赖关系,以及与Sn相关的DX中心部分电子热俘获势垒的精细结构;通过第一原理赝势法计算表明,Sn附近的Al/Ga原子的不同配置是电子热俘获势垒精细结构产生的主要原因。     

High pressure transormations of pyrope (garnet) and pyrope (pyroxene)

Abstract

Clinopyroxenes with higher contents of calcium and alumina in pyroxenites from inclusions in serpentinites of the central part of the granulite complex near Waldheim/Saxony contains several different exsolutions as producte of diffusive transformations under high pressures. The interpretation of the chemical composition of the exsolutions and the pyroxene host in application to the Published thermobarometers in the CMAS-system shows very different values between 685–1400°C temperature and 8 to above 30 kbar pressure, indicating a polymetamorphic high pressure history of the pyroxenites. The partial transformation of the pyrope (garnet) to pyrope (pyroxene) is strong kinetic influenced.