Nobukazu Ohnishi

Photoluminescence of Mg‐doped GaAs grown by molecular beam epitaxy using Mg3As2 as a Mg source: A comparison with Mg+ ion implantation

Photoluminescence spectra of Mg‐doped GaAs grown by molecular beam epitaxy (MBE) are for the first time reported. Mg was introduced during MBE growth by using a synergic reaction of Mg3As2. Two near‐band‐edge emissions, g and [g‐g], were observed below bound exciton emissions which were originally obtained in Mg+ ion‐implanted GaAs. It is explicitly demonstrated that there is no essential difference of photoluminescence spectra between Mg+ ion‐implanted GaAs and the present material. These results conclusively indicate that the above two emissions are definitively related with the acceptor levels and exclusively not associated with crystalline defects produced by ion implantation. It is additionally presented that [g‐g] is principally established also in Mg‐doped GaAs prepared by liquid phase epitaxy, where [g‐g] is significantly suppressed due to the unintentional incorporation of Si donor atoms. For moderately Mg‐doped GaAs prepared by MBE a new fine emission with a doublet structure, temporarily denote...

Observation of new common emissions in GaAs produced by ion implantation of four acceptor impurities

Photoluminescence studies of C+, Mg+, Zn+, and Cd+ ion‐implanted GaAs layers were carried out at 2 K. Two conspicuous emissions denoted by g and [g‐g] were observed to be situated between bound exciton and band to acceptor emissions. It was found that these two are common emissions among the above acceptor impurities, and that they can be explicitly observed only when the background impurity concentration is extremely small.

A theoretical explanation for the red energy shift of a newly discovered, exclusively acceptor‐associated emission in GaAs

Radiative mechanism of a newly discovered near‐band‐edge emission, [g‐g], exclusively associated with acceptor impurities in GaAs, was theoretically discussed. Since the emission energy of [g‐g] for the lowest acceptor concentration limit is nearly identical to that of the excited state of the acceptor impurities and it shows a noticeable shift towards lower‐energy levels with increasing acceptor concentration, it was suggested that the energy of [g‐g] corresponds to the 2pσ bond state of the acceptor‐acceptor pair formed by the overlapping of the 2p excited state. The calculated energy of the 2pσ bond as a function of the distance between the acceptor‐acceptor pair shows good agreement with the experimentally obtained value of [g‐g] in Mg‐doped GaAs grown by molecular‐beam epitaxy (MBE). From a comparison of binding energies in the acceptor‐doped GaAs samples prepared by ion implantation (Zn+ or Mg+) and by MBE, the activation ratio of the implanted atoms was estimated.

Concentration ratio dependence of selective optical compensation effect in dually Zn+ and Se+ ion‐implanted GaAs

The selective optical compensation effect in which exclusively acceptor‐associated emissions g and [g‐g] are selectively quenched by the simultaneous presence of acceptor and donor atoms, was investigated in dually Zn+ (acceptor)‐implanted and Se+(donor)‐implanted GaAs as a function of Se to Zn concentration ratio, [Se]/[Zn], at a fixed Zn concentration of 1×1017 cm−3. It was revealed for the first time that Se atoms with one‐tenth of the concentration of Zn have the ability to significantly suppress the [g‐g] emission. However, the g emission is not significantly suppressed by the presence of even an equal concentration of Se atoms.

INFLUENCE OF THE IMPLANTATION OF C+ IONS ON PHOTOLUMINESCENCE AND ELECTRICAL PROPERTIES OF GAAS

Ion implantation of carbon (C) into extremely pure GaAs grown by molecular‐ beam epitaxy is carried out over a wide range of C atomic concentrations [C], from 1×1016 to 5×1019 cm−3. The impurity levels in the implanted layer are investigated by using photoluminescence (PL) and Hall‐effect measurements. Below the well‐defined exciton luminescence lines, one broad emission band, namely [g‐g], is found to be exclusively attributable to acceptor impurities and is dominant for [C] lower than 3×1017 cm−3. However, a decrease of its intensity and a ‘‘locking’’ of its emission energy shift is observed for [C] higher than this value. A carrier transport mechanism is found to be relevant to the C acceptor level for C atoms at As sites, and deep acceptors caused by residual radiation defects. The concentration of substitutional C atoms is nearly coincident with [C] up to 1×1017 cm−3, but the activation efficiency for [C]=1×1018 cm−3 decreases by about 13%. The decrease of overall PL intensity and the locking of the ...

As4 to Ga flux ratio dependence of the optical and electrical properties of Ge‐doped GaAs grown by molecular‐beam epitaxy

A drastic change of the conduction type from p to n with an increase of the As4 to Ga flux ratio, γ, was observed for the first time in the photoluminescence spectra of amphoteric impurity (Ge)‐doped GaAs made by molecular‐beam epitaxy. The sample with the lowest γ (γ=1.0) presented a purely p‐type emission associated with pairs between the excited states of acceptors. The sample with the highest γ (γ=10.6) indicated a totally n‐type emission reflecting an increase of quasi Fermi energy. Results show that by precisely controlling the flux ratio, γ, one can reliably make use of substantially amphoteric atoms of Ge both as p‐ and n‐type impurities for the fabrication of GaAs by molecular‐beam epitaxy.

Selective self‐optical compensation effect for a newly discovered acceptor‐associated emission in Zn+ ion‐implanted GaAs

Photoluminescence measurements were carried out at 2 K for Zn+ ion‐implanted GaAs, where the concentration of Zn was widely varied from 3×1016 to 1×1021 cm−3. Two Zn+‐associated emissions were formed. One emission is at 1.512 eV, g, and the other emission [g‐g] is just below g and this moves towards the lower energy level with increasing Zn concentration, [Zn]. The intensity of [g‐g] was enhanced with increasing [Zn], up to [Zn]=3×1017 cm−3, and was gradually suppressed for [Zn] beyond that concentration. This selective self‐optical compensation effect (SSOC) for [g‐g] was found to occur for moderately heavy ion acceptor species such as Zn and Cd, although [g‐g] is a common emission among many other acceptor impurities. In addition two new emissions were observed between g and [g‐g] at certain [Zn]. Preliminary theoretical explanations are presented for this SSOC effect.

Selective optical compensation effect of two new near‐band‐edge emissions in simultaneously acceptor (Zn+) and donor (Se+) ion‐implanted GaAs

Newly discovered Zn‐associated emissions in GaAs, ‘‘g’’ and [g‐g], were found to be completely suppressed by the simultaneous presence of Zn and Se atoms. This selective optical compensation (SOC) effect observed when acceptor (Zn) and donor (Se) atoms exist in combination was established by simultaneously implanting Zn+ and Se+ ions into almost carbon‐free ultrapure GaAs made by molecular‐beam epitaxy. It is suggested that the failure to observe ‘‘g’’ and [g‐g] emissions in acceptor‐incorporated GaAs prepared by conventional epitaxial methods may be ascribed to this SOC effect, in which donor atoms were unintentionally introduced into the epitaxial layer.

Formation of a new deep emission in Si+, S+, Se+, and Te+ ion‐implanted GaAs

Photoluminescence studies of Si+, S+, Se+, and Te+ ion‐implanted GaAs made by molecular beam epitaxy were carried out at 2 K. A new emission denoted by [D] was commonly obtained at 1.408 eV. It was also found that the controversial near band‐edge emissions, ‘g’ and [g‐g], which were originally produced by the ion implantation of acceptor impurities, were not formed by donor ion implantation.

Conditions for the formation of defect‐induced bound exciton emissions in GaAs grown by molecular beam epitaxy

Conditions for the formation of defect‐induced bound exciton (DIBE) emissions in GaAs were investigated by molecular beam epitaxial method. Growth was made on both A‐ and B‐polarity substrates with (321), (221), and (211) orientations. For A‐polarity samples, (321)A and (211)A presented pronounced DIBE emissions. (221)A, however, exhibited no DIBE emission, instead it presented a dominant carbon donor‐carbon acceptor pair emission together with a small hump due to carbon donor‐related bound exciton emissions. For B‐polarity specimens, DIBE was completely vanished in all the three samples. It was theoretically demonstrated that DIBE is formed only when double‐handed Ga adatom site is existing.