D X centers, deep levels associated with donors in III‐V semiconductors, have been extensively studied, not only because of their peculiar and interesting properties, but also because an understanding of the physics of these deep levels is necessary in order to determine the usefulness of III‐V semiconductors for heterojunction device structures. Much progress has been made in our understanding of the electrical and optical characteristics of D X centers as well as their effects on the behavior of various device structures through systematic studies in alloys of various composition and with applied hydrostatic pressure. It is now generally believed that the D X level is a state of the isolated substitutional donor atom. The variation of the transport properties and capture and emission kinetics of the D X level with the conduction‐band structure is now well understood. It has been found that the properties of the deep level when it is resonant with the conduction band, and is thus a metastable state, are similar to its characteristics when it is the stable state of the donor. And it has been consistently found that there is a large energy difference between the optical and thermal ionization energies, implying that this deep state is strongly coupled to the crystal lattice. The shifts in the emission kinetics due to the variation in the local environment of the donor atom suggest that the lattice relaxation involves the motion of an atom (the donor or a neighboring atom) from the group‐III lattice site toward the interstitial site. Total energy calculations show that such a configuration is stable provided that the donor traps two electrons, i.e., has negative U. Verification of the charge state of the occupied D X level is needed as well as direct evidence for its microscopic structure.

Deep donor levels (DX centers) in III‐V semiconductors

A thorough discussion of the various features of the photoluminescence spectra of undoped, p‐doped and n‐doped AlxGa1−xAs (0≤x≤1) alloys is given. This review covers spectral features in the energy region ranging from the energy band gap down to ≂0.8 eV, doping densities from isolated impurities to strongly interacting impurities (heavy‐doping effects) and lattice temperatures from 2 to 300 K. The relevance of photoluminescence as a simple but very powerful characterization technique is stressed also in comparison with other experimental methods. The most recent determinations of the Al concentration dependence of some physical properties of the alloy (energy gaps, carrier effective masses, dielectric constants, phonon energies, donor and acceptor binding energies, etc.) are given. The main physical mechanisms of the radiative recombination process in semiconductors are summarized with particular emphasis on the experimental data available for AlxGa1−xAs. The effects of the nature of the band gap (direct ...

Photoluminescence of AlxGa1−xAs alloys

The mechanism of silicon diffusion in GaAs, Al0.3Ga0.7As, and the silicon diffusion-induced layer disordering of multiquantum wells have been studied by photoluminescence, secondary-ion-mass spectroscopy, and transmission electron microscopy across a corner of a wedge-shaped sample. The diffusion source was a grown in highly Si-doped layer. The main photoluminescence properties of point defects in GaAs and Al0.3Ga0.7As are reviewed to interpret the experimental data. The depth profile of the photoluminescence allows the spatial correlation between the luminescence spectra and the Si concentration profile obtained from secondary-ion-mass-spectroscopy measurements. On the basis of the photoluminescence results, the physical processes occurring during the Si diffusion are discussed. Frenkel defects (pairs of element-III vacancies and interstitials) are generated in the highly Si-doped region. The element-III interstitials rapidly diffuse towards the surface where they react with the element-III vacancies generated at the surface when annealing is performed in an external As pressure. This induces a supersaturation of element-III vacancies in the Si-doped region which drives the Si diffusion. Annealing in vacuum reduces the oversaturation of element-III vacancies and, hence, reduces the Si diffusion. A domination of the Si donor-element-III vacancy complex emission band was found in the spectra taken in the Si-diffused region. This gives evidence for the vacancy-assisted mechanism in the Si diffusion and in the impurity-induced disordering.

Role of point defects in the silicon diffusion in GaAs and Al0.3Ga0.7As and in the related superlattice disordering

We summarize a large body of experimental and theoretical work, especially in Si-doped GaAs and Al x Ga1-x As, regarding the bistability of theDX center. There is good evidence that theDX center is just the simple donor, and that each donor can exist in either of two distinct lattice configurations, each with its own spectrum of bound electronic states. Generally, the substitutional configuration binds electrons in shallow hydrogenic states, but many observations also indicate a deep (highly localized) state ofA 1 symmetry. These states are to be distinguished from bound states of a lattice-distorted configuration, the lowest-lying of which is the deepDX level. The occupation of theDX level in thermal equilibrium with the states of the conduction band can be reasonably well modeled by assuming thatDX is either a one-electron or a two-electron state, and we discuss the reasons for this ambiguity. However, we then show that such thermal equilibrium results are consistent with thermal capture and emission kineticsonly if we assume thatDX is a two-electron state. Our results thus support the model of Chadi and Chang in which the distorted configuration is stabilized by capture of two electrons. In other words, the defect exhibits negative effective correlation energy (negativeU).

Bistability of the DX center in GaAs and Al x Ga 1-x As, and experimental tests for negative U of the DX level

Deep‐level photoluminescence (PL) studies were performed on Si‐doped, metal organic chemical vapor deposition grown AlxGa1−xAs as a function of the most important growth parameters. The SiH4 input mole fraction, the V/III ratio, and the Al fraction were varied over a wide range, resulting in net charge carrier concentrations n ranging between 1.8×1016 and 4.5×1018 cm−3, Hall mobilities μH between 220 and 2400 cm2/V s, and a solid Al fraction x between 0 and 0.72. Two novel PL emissions for AlxGa1−xAs in the energy range of 1.05–1.35 eV were recorded. By a systematic analysis of the growth conditions these emissions were attributed to SiGa–SiAs and SiGa–VGa complexes. The behavior of the broad PL emission at 0.8 eV as a function of the growth parameters was studied systematically. It was demonstrated that this emission is not related to the DX center. Instead, there are strong indications that it should be attributed to an internal transition within a native, or oxygen‐related defect.

Deep‐level photoluminescence studies on Si‐doped, metalorganic chemical vapor deposition grown AlxGa1−xAs

A photoluminescence study at low temperatures of Si-doped AlxGa1-xAs (0<x<0.6) reveals the existence of four donor levels D1, D2, D3, D4, in order of increasing ionisation depth. The number of donor levels and the dependence of their ionisation energies on the composition parameter x accords qualitatively with theoretical expectations. D1 can be identified with the common 'shallow donor'. D2 and D3 have been reported in an earlier PL study by Dingle and co-workers (1977). The position of the D4 level suggests an identification with the DX centre. Its luminescence properties have been the subject of a previous publication.

P J Roksnoer

Papers

A photoluminescence study of the donor structure in AlxGa1-xAs