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Chemical characterization of extra layers at the interfaces in MOCVD InGaP/GaAs junctions by electron beam methods.

Frigeri C, Shakhmin AA, Vinokurov DA, Zamoryanskaya MV - Nanoscale Res Lett (2011)

Bottom Line: A CL emission peak different from that of the QW was detected.By using HAADF, it is found that the GaAs QW does not exist any longer, being replaced by extra interlayer(s) that are different from GaAs and InGaP because of atomic rearrangements at the interface.The nature and composition of the interlayer(s) are determined by HAADF.

View Article: PubMed Central - HTML - PubMed

Affiliation: CNR-IMEM Institute, Parco Area delle Scienze 37/A, 43010 Parma, Italy. frigeri@imem.cnr.it.

ABSTRACT
Electron beam methods, such as cathodoluminescence (CL) that is based on an electron-probe microanalyser, and (200) dark field and high angle annular dark field (HAADF) in a scanning transmission electron microscope, are used to study the deterioration of interfaces in InGaP/GaAs system with the GaAs QW on top of InGaP. A CL emission peak different from that of the QW was detected. By using HAADF, it is found that the GaAs QW does not exist any longer, being replaced by extra interlayer(s) that are different from GaAs and InGaP because of atomic rearrangements at the interface. The nature and composition of the interlayer(s) are determined by HAADF. Such changes of the nominal GaAs QW can account for the emission observed by CL.

No MeSH data available.


Related in: MedlinePlus

Sketch of the three possible mechanisms of atom rearrangement at the inverted GaAs-on-InGaP interface. 1): indium segregation in the growth direction, 2): P/As exchange across the interface, and 3): P/As intermixing in the growing GaAs QW (see text).
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Figure 7: Sketch of the three possible mechanisms of atom rearrangement at the inverted GaAs-on-InGaP interface. 1): indium segregation in the growth direction, 2): P/As exchange across the interface, and 3): P/As intermixing in the growing GaAs QW (see text).

Mentions: The TEM results indicating the formation of InGaAsP at the location of the nominal GaAs QW are in qualitative agreement with an analogous conclusion drawn by CL in refs. [12,13], where a quaternary with the In composition in the 0-0.15 range and the P one a little above zero was proposed. Both the TEM and CL results suggest that at the inverted GaAs-on-InGaP interface there is the formation of an extra quaternary layer of InGaAsP inside the nominal GaAs QW (and partially replacing it), as also suggested in several studies [5,6,8-10,16,21]. The formation of just InGaAs as sublayer 1 might be less likely because it might easily happen that residual P atoms, which remained in the reactor after the PH3 flow had been switched off, are incorporated in the first monolayers of the GaAs QW, since Ga prefers to bond to P rather than to As [22], as long as P atoms are available (P/As intermixing mechanism, see later). Moreover, the absence of P in sublayer 1 would contradict its presence in sublayer 2. On the other hand, the sequence inside the nominal GaAs QW such as layer 1 = In0.15Ga0.85As0.80P0.20 and layer 2 = In0.05Ga0.95As0.84P0.16 or GaAs0.91P0.09 is congruent. In fact, it matches the reasonable expectation that [In] and [P] decrease by moving away from InGaP, i.e. by going deeper into the nominal GaAs QW, while [Ga] and [As] increase. The stoichiometry of the sublayers 1 and 2 as determined by STEM-HAADF thus indicates a slight In and P enrichment of the nominal GaAs QW, which therefore changes its nature. Three mechanisms can cause such In and P enrichment, namely, In segregation in the growth direction, P/As exchange across the interface and P/As intermixing in proximity of the inverted interface, as discussed in other studies [5,6,8-10,16,21]. The three mechanisms are sketched in Figure 7. Indium surface segregation has been shown for other In-containing systems such as InGaAs/GaAs [23,24]. For the InGaP/GaAs system, the action of In segregation has been proven by experiments, showing that the growth of a thin GaP layer on the top of InGaP, before GaAs is grown, is effective in preventing the formation of the quaternary interlayer because In segregates into the interposed GaP layer and cannot reach the GaAs [5]. In segregates into the growing GaAs layer as soon as the latter starts to grow. In segregation is a kinetically driven process and depends strongly on the growth temperature [5]. It may occur within the first few monolayers of the layer grown next [5,23,24]. P/As exchange across the interface should be excluded according to our results. In fact, this mechanism would entail the incorporation of As in the bottom InGaP with the formation of some InGaAsP alloy inside the nominal InGaP layer, with the consequent broadening of the interface towards both the nominal InGaP and GaAs layers. These detailed investigations by chemically sensitive methods in a TEM right of the inverted interface do not confirm such symmetrical broadening and allow excluding the P/As exchange mechanism. The interface broadening towards only the top GaAs layer was observed by TEM also in other MOVPE-grown InGaP/GaAs samples [16]. P/As intermixing occurs at the beginning of GaAs growth after the growth of InGaP has finished. It consists in the fact that when the Ga and As fluxes are switched on to grow GaAs, some of the incoming Ga atoms bond to residual P atoms that are still remaining in the MOCVD chamber in contact with the sample surface after the PH3 flux has been switched off. This is because the chemical bond strength of Ga-P is greater than that of Ga-As [22], which results in As substitution by P [9,22]. Such intermixing is limited to the first monolayers of the growing nominal GaAs because the residual P atoms vanish out very quickly as no PH3 flux is active. As for In segregation, P/As intermixing also depends on the substrate temperature which affects, e.g., the diffusion length of the P, As and Ga atoms on the growing surface. It also depends on the gas fluxes, on the application or non-application of a PH3-purging procedure or growth interruption [6]. Although the formation of an extra layer at the inverted interface during growth has been reported in a majority of the literature [5-10, 21 and references therein], its composition was seen to vary depending on the growth conditions used, as summarized above. In fact, it has been seen by photoluminescence that the emission associated with the extra layer spans quite a wide range, i.e. from 862 to 914 nm [5-10,21]. A majority of the published articles concluded that the extra layer is InGaAsP albeit with different compositions. Our results agree with this hypothesis. They also show that a finer structure may exist in the modified nominal GaAs QW, i.e. the presence of two sublayers: one more In- and P-rich layer closer to the undergrown InGaP layer and a second one that is less In and P rich farther from it. This structure is certainly due to the expected reduction of P/As intermixing and In segregation as the distance from the inverted interface increases.


Chemical characterization of extra layers at the interfaces in MOCVD InGaP/GaAs junctions by electron beam methods.

Frigeri C, Shakhmin AA, Vinokurov DA, Zamoryanskaya MV - Nanoscale Res Lett (2011)

Sketch of the three possible mechanisms of atom rearrangement at the inverted GaAs-on-InGaP interface. 1): indium segregation in the growth direction, 2): P/As exchange across the interface, and 3): P/As intermixing in the growing GaAs QW (see text).
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Figure 7: Sketch of the three possible mechanisms of atom rearrangement at the inverted GaAs-on-InGaP interface. 1): indium segregation in the growth direction, 2): P/As exchange across the interface, and 3): P/As intermixing in the growing GaAs QW (see text).
Mentions: The TEM results indicating the formation of InGaAsP at the location of the nominal GaAs QW are in qualitative agreement with an analogous conclusion drawn by CL in refs. [12,13], where a quaternary with the In composition in the 0-0.15 range and the P one a little above zero was proposed. Both the TEM and CL results suggest that at the inverted GaAs-on-InGaP interface there is the formation of an extra quaternary layer of InGaAsP inside the nominal GaAs QW (and partially replacing it), as also suggested in several studies [5,6,8-10,16,21]. The formation of just InGaAs as sublayer 1 might be less likely because it might easily happen that residual P atoms, which remained in the reactor after the PH3 flow had been switched off, are incorporated in the first monolayers of the GaAs QW, since Ga prefers to bond to P rather than to As [22], as long as P atoms are available (P/As intermixing mechanism, see later). Moreover, the absence of P in sublayer 1 would contradict its presence in sublayer 2. On the other hand, the sequence inside the nominal GaAs QW such as layer 1 = In0.15Ga0.85As0.80P0.20 and layer 2 = In0.05Ga0.95As0.84P0.16 or GaAs0.91P0.09 is congruent. In fact, it matches the reasonable expectation that [In] and [P] decrease by moving away from InGaP, i.e. by going deeper into the nominal GaAs QW, while [Ga] and [As] increase. The stoichiometry of the sublayers 1 and 2 as determined by STEM-HAADF thus indicates a slight In and P enrichment of the nominal GaAs QW, which therefore changes its nature. Three mechanisms can cause such In and P enrichment, namely, In segregation in the growth direction, P/As exchange across the interface and P/As intermixing in proximity of the inverted interface, as discussed in other studies [5,6,8-10,16,21]. The three mechanisms are sketched in Figure 7. Indium surface segregation has been shown for other In-containing systems such as InGaAs/GaAs [23,24]. For the InGaP/GaAs system, the action of In segregation has been proven by experiments, showing that the growth of a thin GaP layer on the top of InGaP, before GaAs is grown, is effective in preventing the formation of the quaternary interlayer because In segregates into the interposed GaP layer and cannot reach the GaAs [5]. In segregates into the growing GaAs layer as soon as the latter starts to grow. In segregation is a kinetically driven process and depends strongly on the growth temperature [5]. It may occur within the first few monolayers of the layer grown next [5,23,24]. P/As exchange across the interface should be excluded according to our results. In fact, this mechanism would entail the incorporation of As in the bottom InGaP with the formation of some InGaAsP alloy inside the nominal InGaP layer, with the consequent broadening of the interface towards both the nominal InGaP and GaAs layers. These detailed investigations by chemically sensitive methods in a TEM right of the inverted interface do not confirm such symmetrical broadening and allow excluding the P/As exchange mechanism. The interface broadening towards only the top GaAs layer was observed by TEM also in other MOVPE-grown InGaP/GaAs samples [16]. P/As intermixing occurs at the beginning of GaAs growth after the growth of InGaP has finished. It consists in the fact that when the Ga and As fluxes are switched on to grow GaAs, some of the incoming Ga atoms bond to residual P atoms that are still remaining in the MOCVD chamber in contact with the sample surface after the PH3 flux has been switched off. This is because the chemical bond strength of Ga-P is greater than that of Ga-As [22], which results in As substitution by P [9,22]. Such intermixing is limited to the first monolayers of the growing nominal GaAs because the residual P atoms vanish out very quickly as no PH3 flux is active. As for In segregation, P/As intermixing also depends on the substrate temperature which affects, e.g., the diffusion length of the P, As and Ga atoms on the growing surface. It also depends on the gas fluxes, on the application or non-application of a PH3-purging procedure or growth interruption [6]. Although the formation of an extra layer at the inverted interface during growth has been reported in a majority of the literature [5-10, 21 and references therein], its composition was seen to vary depending on the growth conditions used, as summarized above. In fact, it has been seen by photoluminescence that the emission associated with the extra layer spans quite a wide range, i.e. from 862 to 914 nm [5-10,21]. A majority of the published articles concluded that the extra layer is InGaAsP albeit with different compositions. Our results agree with this hypothesis. They also show that a finer structure may exist in the modified nominal GaAs QW, i.e. the presence of two sublayers: one more In- and P-rich layer closer to the undergrown InGaP layer and a second one that is less In and P rich farther from it. This structure is certainly due to the expected reduction of P/As intermixing and In segregation as the distance from the inverted interface increases.

Bottom Line: A CL emission peak different from that of the QW was detected.By using HAADF, it is found that the GaAs QW does not exist any longer, being replaced by extra interlayer(s) that are different from GaAs and InGaP because of atomic rearrangements at the interface.The nature and composition of the interlayer(s) are determined by HAADF.

View Article: PubMed Central - HTML - PubMed

Affiliation: CNR-IMEM Institute, Parco Area delle Scienze 37/A, 43010 Parma, Italy. frigeri@imem.cnr.it.

ABSTRACT
Electron beam methods, such as cathodoluminescence (CL) that is based on an electron-probe microanalyser, and (200) dark field and high angle annular dark field (HAADF) in a scanning transmission electron microscope, are used to study the deterioration of interfaces in InGaP/GaAs system with the GaAs QW on top of InGaP. A CL emission peak different from that of the QW was detected. By using HAADF, it is found that the GaAs QW does not exist any longer, being replaced by extra interlayer(s) that are different from GaAs and InGaP because of atomic rearrangements at the interface. The nature and composition of the interlayer(s) are determined by HAADF. Such changes of the nominal GaAs QW can account for the emission observed by CL.

No MeSH data available.


Related in: MedlinePlus