Cross-sectional scanning tunneling microscopy and spectroscopy of InGaP/GaAs heterojunctions

Abstract

Compositionally abrupt InGaP/GaAs heterojunctions grown by gas-source molecular beam epitaxy have been investigated by cross-sectional scanning tunneling microscopy and spectroscopy. Images inside the InGaP layer show non-uniform In and Ga distribution. About 1.5 nm of transition region at the interfaces is observed, with indium carryover identified at the GaAs-on-InGaP interface. Spatially resolved tunneling spectra with nanometer spacing across the interface were acquired, from which band offsets (revealing that nearly all of band offset occurs in the valence band) were determined.Heterojunctions between In x Ga 1-x P (hereafter InGaP) and GaAs have attracted attention recently because of their device applications. It has been predicted and confirmed that this system should have a large fraction of the total energy gap discontinuity occurring in the valence band, which improves the electron injection efficiency of an heterojunction bipolar transistor (HBT) [1]. However, there exist discrepancies between band offsets results obtained by different techniques for samples grown by different methods/groups. It is known that properties of the interface depend sensitively on the detailed growth conditions [2][3][4][5][6]. Prior experimental techniques used in this system are not spatially resolved and in order to better understand this material system a study of the atomic-scale structural and electronic properties of the junctions is useful.In this work we use scanning tunneling spectroscopy (STS) to study the spatial variation of the InGaP/GaAs electronic band structure at nanometer length scales. The advantage of such work is that band offsets can be measured while simultaneously imaging the atomic-scale structural properties of the interfaces. Due to the experimental challenges of measuring band offsets by STS, there are only a few prior examples of such work, most notably on AlGaAs/GaAs [7]. Liu et al. have used cross-sectional STM to study the ordering effect of InGaP [8]. In this paper, we report cross-sectional STM and STS studies of compositionally abrupt InGaP/GaAs heterojunctions prepared by gassource molecular beam epitaxy (GSMBE). Images inside the InGaP layer reveal a random arrangement of In and Ga atom. This result is consistent with PL results and growth conditions for similar samples that indicate a nearly fully disordered InGaP layer [9]. It is found that GaAs-on-InGaP interface has a slightly wider transition region and more interface intermixing than the InGaP-on-GaAs interface. Both interfaces exhibit InGaAs-like properties. Indium outdiffusion from InGaP into GaAs at the GaAs-onPublished in Appl. Phys. Lett. 84, 227 (2004).

Differences between GaAs/GaInP and GaAs/AlInP interfaces grown by movpe revealed by depth profiling and angle-resolved X-ray photoelectron spectroscopies

“…It thus means that, after GalnP and AHnP layer growth, the In carry-over effect takes place and In atoms tend to segregate at the surface, in such way that they will likely incorporate into the GaAs outer layer. This is a well-known phenomenon described by several authors for the interface GaAs/GalnP [4, 6,22,23]. According to our results, In atoms in the GaAs/AllnP interface would exhibit the same behavior.…”

“…Studies of the GaAs/GalnP interface have revealed the formation of GalnAs, mainly caused by the incorporation of In atoms into the subsequent GaAs layer [6,7]. The presence of quaternary phases like GalnAsP is also feasible; however XPS is not able to assert its presence since Ga 2p, As 3d, In 3d and P 2p core levels do not show contributions different to those appearing in the GaAs and the GalnP layers.…”

“…The reasons are related to the possibility of cation segregation and interdiffusion between the two adjacent layers and with As/P anion exchange during the formation of the layers. Several GaAs/GalnP and GalnP/GaAs interface chemistry studies can be found in literature [4][5][6][7][8]. Most of them report the formation of GalnAs, GaAsP, or quaternary phases like GalnAsP, whose exact compositions and thicknesses would be strongly dependent on growth conditions.…”

Metalorganic chemical vapor deposition growth and characterization of InGaP/GaAs superlattices

“…In spite of the extensive studies of the interface control, there is no general agreement upon the dominant mechanism governing the interface formation, probably due to differences in reactor configuration. 2,6,8,9 It has been shown that the interface layers at the two interfaces, i.e., ''GaAs on InGaP'' and ''InGaP on GaAs,'' formed during the metalorganic chemical vapor deposition (MOCVD) growth are not equivalent with respect to their structural, electrical, and optical properties and that the layer formed at the interface of ''GaAs-onInGaP'' has a much smaller bandgap. The reason has been thought to be the In carry-over effect and the lower incorporation efficiency of PH 3 as compared with AsH 3 .…”

“…In order to improve the performance of InGaP/GaAs-based devices, research on controlling the interface quality has been reported since the 1980s. 2,[6][7][8][9] Through controlling the switching sequence of source gases at the interfaces during the growth interruption between layers, In and As carry-over and arsenic-phosphorus exchange at the interfaces have been extensively investigated. In spite of the extensive studies of the interface control, there is no general agreement upon the dominant mechanism governing the interface formation, probably due to differences in reactor configuration.…”